Photocatalytic and electrocatalytic transformations of C1 molecules involving C–C coupling

Abstract

Selective transformation of one-carbon (C1) molecules, which are abundant or easily available and inexpensive carbon feedstocks, into value-added multi-carbon (C₂₊) compounds is a very attractive but highly challenging research target. Photocatalysis and electrocatalysis have offered great opportunities for the activation and controllable C–C coupling of C1 molecules under mild and environmentally benign conditions. This article provides a critical review on recent advances in photocatalytic and electrocatalytic conversions of major C1 molecules, including CO, CO₂, CH₄, CH₃OH and HCHO, into C₂₊ compounds, such as C₂H₄, C₃H₆, ethanol and ethylene glycol, which play essential roles in the current chemical or energy industry. Besides the photocatalysts and electrocatalysts reported for these conversions, the structure–performance relationships and the key factors that control the activity and product selectivity are analysed to provide insights into the rational design of more efficient catalysts for the synthesis of C₂₊ compounds from C1 feedstocks. The active species, reaction intermediates and reaction or catalyst-functioning mechanism are discussed to deepen the understanding of the chemistry for the activation and selective C–C coupling of C1 molecules in the presence of solar energy or electrical energy.

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Shunji Xie

Shunji Xie received his BS and MSc degrees from Hunan University of China in 2008 and 2011, and obtained his PhD degree from Xiamen University in 2014. He then carried out postdoctoral research at the Collaborative Innovation Center of Chemistry for Energy Materials (iChEM). He is currently a senior engineer in State Key Laboratory of Physical Chemistry of Solid Surfaces of Xiamen University. His research interest focuses on photocatalysis and electrocatalysis for C1 and sustainable chemistry, including CO2 reduction, CH4 oxidation, biomass conversion and ethylene glycol synthesis.

Wenchao Ma

Wenchao Ma received his BSc degree from Xiamen University in June 2016. He is currently a PhD candidate in Professor Ye Wang's group in the Collaborative Innovation Center of Chemistry for Energy Materials (iChEM) at Xiamen University. His research interests focus on photo- and/or electro-catalytic upgrading of C1 molecules.

Xuejiao Wu

Xuejiao Wu received her BSc degree from Xiamen University in June 2015. She is currently a PhD candidate in Professor Ye Wang's group in the Collaborative Innovation Center of Chemistry for Energy Materials (iChEM) at Xiamen University. Her research interest focuses on the valorisation of lignocellulose, in particular by photocatalytic approaches.

Qinghong Zhang

Qinghong Zhang received her BSc and MSc degrees from Nanjing University in 1989 and 1992, and obtained her PhD degree from Hiroshima University of Japan in 2002. She joined Xiamen University in October 2002 and was promoted to a full professor in 2010. Her research interests include the synthesis and characterization of novel materials with advanced catalytic properties.

Yangdong Wang

Yangdong Wang received his PhD degree in the Department of Physical Chemistry from Nanjing University in 2000. After that he joined the faculty at SINOPEC Shanghai Research Institute of Petrochemical Technology, where he worked as a research director of Chemistry. His research fields include the heterogeneous catalysis of porous materials and their application in petrochemical processes such as methanol conversion, and the production and transformation of olefins.

Ye Wang

Ye Wang received his BS degree from Nanjing University and obtained his PhD degree in 1996 from Tokyo Institute of Technology. He then worked at Tokyo Institute of Technology, Tohoku University and Hiroshima University, and was promoted to associate professor at Hiroshima University in 2001. He became a full professor of Xiamen University in the August of 2001. He serves as an associate editor of ACS Catalysis and is a council member of International Association of Catalysis Societies. The research interest of Prof. Ye Wang's group is catalysis for C1 and sustainable chemistry, including C–H activation and C–C coupling of C1 molecules and C–O/C–C cleavage chemistry for cellulose/lignin valorization.

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Broader context

Under the current background of looking for alternative carbon resources to replace crude oil for the production of chemicals and liquid fuels, the transformation of C1 molecules, which are typically either abundant in nature or can be readily produced from non-petroleum carbon resources, into important C₂₊ compounds such as ethylene, propylene, ethanol and ethylene glycol, has attracted much recent attention. The activation of stable C1 molecules (e.g., CH₄ and CO₂) and controllable C–C coupling are two of the most challenging research goals in chemistry. As compared to thermocatalysis, which is usually performed under harsh conditions and has difficulty in controlling product selectivity, photocatalysis and electrocatalysis hold great potential to activate and selectively convert C1 molecules to C₂₊ compounds under mild conditions with solar or electrical energy. This article highlights the state-of-the-art advances in photocatalytic/electrocatalytic transformations of C1 molecules into important C₂₊ compounds. The challenges and opportunities in this field will be analysed. The present article may stimulate further research effort to develop efficient photocatalytic/electrocatalytic systems for C1 chemistry and to deepen the understanding of mechanisms for activation and selective C–C coupling of C1 molecules in the presence of solar or electrical energy.

1. Introduction

C1 chemistry, i.e., the chemistry for the transformation of C1 molecules into value-added products, is experiencing a renaissance because of the rapidly growing demand for the harvesting of carbon-based energy and chemicals from alternative resources other than petroleum, such as the emerging shale gas in the US, coal in China, renewable biomass and CO₂.^1–4 Traditional homogeneous and heterogeneous catalysis has played crucial roles in C1 chemistry by transforming C1 molecules such as CO, CO₂, CH₄ and CH₃OH into liquid fuels (e.g., gasoline, diesel and jet fuel) or building-block chemicals (e.g., C₂H₄, C₃H₆, aromatics and ethanol).^5–26 Several C1-chemistry-based commercial processes such as the carbonylation of methanol to acetic acid,⁴ the conversion of natural gas to liquid fuels (GTL) or the conversion of coal to liquid fuels (CTL) via synthesis gas (CO/H₂) by Fischer–Tropsch synthesis,⁷ and the conversion of methanol to gasoline (MTG) or the conversion of methanol to lower olefins (MTO)^23–26 have contributed largely to the chemical and energy industries. Most of these reactions are performed under harsh conditions such as high temperatures and/or high pressures, and the selectivity of the products, in particular, a specific C₂₊ compound, is difficult to control. Some developments based on the design of homogeneous and heterogeneous catalysts have been achieved for selective conversions of C1 molecules into C₂₊ compounds. For example, the design of bi(multi)functional catalysts or catalytic systems that work in tandem have shown great potential for the synthesis of C₂–C₄ olefins, aromatics and C₂₊ oxygenates by hydrogenation of CO and CO₂.^{8,13–15,27–38}

Nevertheless, the activation of stable C1 molecules (e.g., CO₂ and CH₄) under mild conditions and the precise C–C coupling of C1 molecules remain two of the most challenging research targets in chemistry.

Photocatalysis and electrocatalysis have been emerging as promising tools for the activation and selective conversion of C1 molecules into various types of products under mild conditions.^39–42 The transformation of C1 molecules by photocatalysis/electrocatalysis has become a booming research field, and the number of academic publications in the field has increased enormously over the past decade (Fig. 1). At the same time, many review articles have been published on either photocatalytic or electrocatalytic conversions of a single C1 molecule, in particular CO₂.^43–56 However, to the best of our knowledge, so far few critical reviews have been contributed to photocatalytic/electrocatalytic conversions of multiple C1 molecules with a focus on C–C coupling.

Fig. 1 Numbers of annual publications and percentages of electrocatalysis and photocatalysis for different C1 molecules based on Web of Science with keywords of C1 molecules plus “catal*”, “electrocatal*” and “photocatal*”. (a) CO₂. (b) CO. (c) CH₄. (d) CH₃OH and HCHO.

The present article is devoted to reviewing recent advances in the photocatalytic/electrocatalytic conversions of typical C1 molecules including CO₂, CO, CH₄, CH₃OH and HCHO into C₂₊ compounds involving C–C coupling (Fig. 2). The products include both C₂₊ hydrocarbons (e.g., C₂H₄, C₃H₆ and C₂₊ alkanes) and C₂₊ oxygenates (e.g., ethanol, acetate, n-propanol and ethylene glycol), which are important building blocks/synthetic intermediates in the current chemical industry or fuel additives/alternative fuels. The photocatalytic/electrocatalytic systems that are capable of catalysing the formation of C₂₊ compounds from C1 molecules and the active catalyst structures are summarized and analysed to provide insights into the design of efficient catalysts. The mechanistic insights such as reactive species, reaction intermediates and reaction pathways are discussed to offer in-depth understanding of the chemistry for the activation of C1 molecules and selective C–C coupling with solar or electrical energy. The challenges, opportunities and future trends in photocatalytic/electrocatalytic transformations of typical C1 molecules are further described to advance the emerging research area of solar energy- or electrical energy-driven C–C coupling of abundant C1 feedstocks to form high-value C₂₊ compounds.

Fig. 2 Major topics of the present article.

2. Fundamental aspects of photocatalysis and electrocatalysis

The essence of chemical reactions is the rearrangement of chemical bonds, i.e., the breaking of some chemical bonds in reactants and the formation of new bonds in the products. This process usually needs to overcome an activation barrier. Thus, a sufficiently high temperature is required to overcome the activation energy without supply of external energy. For traditional thermocatalysis, the activation energy can be decreased by addition of a catalyst, which can chemically interact with the reactants, and thus can activate the reactants and change the reaction path. On the other hand, for photocatalysis and electrocatalysis, the supplied photo or electrical energy can be applied to the reactants via the photocatalyst/electrocatalyst and the reactants are then excited, and thus the reaction can proceed without the need to overcome a high activation energy. Furthermore, photocatalysis and electrocatalysis can also drive the thermodynamically unfavourable reactions that are impossible for thermocatalysis. Therefore, photocatalysis and electrocatalysis hold great potential to bring about breakthroughs toward new reactions or more efficient catalytic processes.

The matching of the energy-band position in semiconductor photocatalysis or the electrode potential in electrocatalysis with the redox potentials of reactants is the key to accomplishing photocatalytic or electrocatalytic reactions due to the thermodynamic requirement. For a semiconductor-based photocatalytic reaction, the conduction-band edge should be higher (more negative) than the reduction potential of the reactant for the reduction half reaction, while the valence-band edge should be lower (more positive) than the oxidation potential of the reactant for the oxidation-half reaction (Fig. 3a). Similarly, for an electrocatalytic reaction, the cathode potential should be higher than the reduction potential and the anode potential should be lower than the oxidation potential of the reactants for the reduction- and oxidation-half reactions, respectively (Fig. 3b). The energy-band position of the semiconductor or the electrode potential can be tuned by choosing a suitable semiconductor or changing the applied potential. The redox potentials of some possible reactions related to the conversions of typical C1 molecules to some C1 and C₂ compounds are displayed in Fig. 3c. They are all located in the range of −0.2 to 0.6 V versus normal hydrogen electrode (NHE) at 298 K and can be easily matched by controlling the energy-band structure of the semiconductor catalyst or the electrode potential. However, selectivity control would be difficult from a thermodynamic consideration, because the redox potentials for different products are very close. Therefore, it can be expected that kinetic control by designing proper photocatalysts/electrocatalysts is crucial to accomplish highly selective synthesis of specific C₂₊ compounds.

Fig. 3 (a) Positions of semiconductor energy-band edges (E_c, E_v) and substrate redox potentials (E_redox), and the thermodynamics of electron transfer between a semiconductor and substrate. (b) Positions of electrode applied potentials (E₁, E₂, E₃) and substrate redox potentials (E_redox), and the thermodynamics of electron transfer between an electrode and a substrate. (c) The redox potentials of some reactions related to the conversion of C1 molecules to other C1 or C₂ products. The redox potentials are obtained from the thermochemical software “HSC Chemistry”.

3. Photocatalytic conversions of CO₂ and CO

3.1. Photocatalytic conversion of CO₂

CO₂ is one of the most stable molecules and also the most probable terminal state of carbon after full utilization of carbon resources. The natural photosynthesis using solar energy to convert CO₂ and H₂O to biomass is the principal step for the carbon cycle in nature.⁴⁴ Many approaches have been proposed and developed for establishing anthropogenic chemical recycling of CO₂ to make the carbon cycle more controllable.⁴⁴ However, the chemical transformation of CO₂ is very challenging due to the thermodynamic stability and kinetic inertness of CO₂. Photocatalytic reduction of CO₂ with H₂O to organic compounds, also known as artificial photosynthesis, is an ideal route for the transformation of CO₂ and the storage of solar energy in chemical bonds.^45–50

Since the pioneering work reported by Fujishima, Honda and their co-workers in 1979,⁵⁷ a lot of semiconductors, such as TiO₂, g-C₃N₄, BiVO₄, Bi₂WO₆, CdS, SrNb₂O₆, NaTaO₃, Zn₂GeO₄ and Zn₂GaO₄, have been reported for photocatalytic reduction of CO₂ with H₂O.^45–50 The products in these systems are often C1 compounds, such as CO, HCOOH, CH₃OH and CH₄. The direct photocatalytic reduction of CO₂ to longer carbon-chain products involving C–C coupling is more challenging not only due to the multi-electron/multi-proton transfer but also because of the requirement of C–C coupling active sites.^58–61 Nevertheless, some advances have been achieved for the synthesis of C₂₊ compounds such as C₂H₄, C₂H₆ and C₂H₅OH by photocatalytic reduction of CO₂. Some semiconductors, such as TiO₂,⁶² g-C₃N₄,⁶³ BiVO₄,⁶⁴ Bi₂WO₆,⁶⁵ InTaO₄,⁶⁶ and CdS,⁶⁷ have been reported to be capable of catalysing the formation of C₂₊ compounds, and some factors are known to play key roles in determining the catalytic performances. Here, we will analyse the effects of doping and co-catalysts, hybridisation of different semiconductors, hybridisation of semiconductors and carbon materials and the presence of plasmonic components on photocatalytic reduction of CO₂ with H₂O to C₂₊ compounds. Some typical photocatalysts and performances are summarized in Table 1.^{62–64,66–87}

Table 1 Typical photocatalytic systems for reduction of CO₂ to C₂₊ compounds

Photocatalyst	Formation rate (μmol g^−1 h^−1)	Light source, reaction mode	Ref.
Effects of semiconductor types and doping
TiO2 (effect of electrolytes)	NaOH: CH4, 0.23; CH3OH, 2.5; HCOOH, 0.23; C2H5OH, 11; CH3CHO, 29	Xe lamp, solid–liquid	62
	H2O: CH4, 0.32; CH3OH, 0.04; HCOOH, 1.7; C2H4, 0.06
Mesoporous flake-like g-C3N4 (u-g-C3N4)	u-g-C3N4: CH3OH, 6.3; C2H5OH, 4.5; O2, 21	Xe lamp (>420 nm), solid–liquid	63
Non-porous flaky g-C3N4 (m-g-C3N4)	m-g-C3N4: C2H5OH, 3.6; O2, 10
BiVO4	Monoclinic: C2H5OH, 101	Xe lamp (>400 nm), solid–liquid	64
	Tetragonal: C2H5OH, 5.6
1 wt% Co-doped TiO2	H2, 63; CH3OH, 26; C2H5OH, 18; CH3CHO, 19	Halogen lamp (>380 nm), solid–vapour	68
Cu^2+-Doped TiO2	CH3OH, 24; C2H5OH, 47	365 nm LED lamp, solid–vapour	69
TiO2	CH3OH, 6.7; C2H5OH, 6.3
Hybridisation of semiconductors and a semiconductor with a nanocarbon material
4 nm PbS QD–Cu/TiO2	CH4, 0.58; CO, 0.82; C2H6, 0.31	Xe lamp (>420 nm), solid–vapour	70
AgBr–TiO2	CH4, 26; CH3OH, 16; CO, 6; C2H5OH, 2.7	Xe lamp (>420 nm), solid–liquid	71
AgBr–NG–g-C3N4	CH3OH, 21; C2H5OH, 51	Xe lamp (>420 nm), solid–liquid	72
CNT–TiO2	CH4, 12; HCOOH, 19; C2H5OH, 30	UV lamp (365 nm), solid–vapour	73
GO–TiO2	CH3OH, 12; C2H5OH, 145	Hg lamp, solid–liquid	74
2% G–TiO2	CH4, 8.0; C2H6, 16.8	Xe lamp, solid–vapour	75
0.5G–TiO2−x	CH4, 7.4; C2H6, 1.2	Solar simulator (AM 1.5), solid–vapour	76
1% Pt–0.5G–TiO2−x	CH4, 37; C2H6, 11
Carbon-coated Cu2O nanorods	CH4, 0.014; C2H4, 0.019; O2, 0.15 μmol h^−1	Xe lamp (>420 nm), solid–liquid	77
Effect of co-catalyst
1% Pd/TiO2	CH4, 0.37; CO, 0.033; C2H6, 0.067	Hg lamp, solid–liquid	78
Nafion-modified Pd/TiO2	CH4, 2.2; C2H6, 1.0 μmol h^−1	UV lamp (>300 nm), solid–liquid	79
Pd/TiO2	CH4, 2.1; C2H6, 0.47 μmol h^−1
Rh nanowires/TiO2	H2, 11; CH4, 4.5; CO, 14; C2H5OH, 12	Xe lamp (<400 nm), solid–vapour	80
2.6% NiO/InTaO4	CH3CHO, 0.3	Xe lamp (AM 1.5), solid–vapour	66
NiO/Na1−xLaxTaO3+x	H2, 0.2; CH4, 0.02; CH3OH, 60; C2H6, 0.01; C2H4, 0.14; C2H5OH, 19; CH3CHO, 2.2; C3H6, 0.4	Hg lamp (300–700 nm), solid–liquid	81
CdS/(Cu–Na1.5H0.5Ti3O7)	CH4, 28; C2H6, 17; C2H4, 0.1; C3H8, 9.7; C3H6, 0.8 μL g^−1 h^−1	Xe lamp (>420 nm), solid–liquid	67
5% Cu/TiO2	CH4, 0.29; C2H6, 0.02; C2H4, 0.54 μL g^−1 h^−1	Xe lamp, solid–liquid	82
Cu–Pt/nitrogen-doped TiO2 nanotube array	H2, 160; CH4, 75; other alkanes, 25; olefin, 8.5; branched paraffin, 2.5 ppm cm^−2 h^−1	Outdoor sunlight (AM 1.5), solid–vapour	83
Cu0.33Pt0.67/PMTiNT	CH4, 2.6; C2H6, 0.47; C2H4, 0.24 mL g^−1 h^−1	Solar simulator (AM 1.5), solid–vapour	84
Cu(0.5 wt%)–Fe(0.5 wt%)/TiO2	Glass plate: CH4, 0.06; C2H4, 0.05	Hg lamp (320–580 nm), solid–vapour	85
	Optical fiber: CH4, 0.91; C2H4, 0.58
Plasmonic effect
Au/TiO2	CH4, 231; CH3OH, 87; HCHO, 135; C2H6, 162 μmol m^−2	Hg lamp (254 nm), solid–vapour	86
Au nanoparticle in 5% EMIM-BF4 aqueous solution	TOF: CH4, 4.5; C2H6, 0.99; C2H4, 1.1; C3H8, 0.56; C3H6, 0.93 NP^−1 h^−1	532 nm laser, solid–liquid	87

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3.1.1. Effects of semiconductor types and doping. TiO₂ is the most intensively studied semiconductor for photocatalytic reduction of CO₂ to C₂₊ compounds, but the yield of C₂₊ products (e.g., C₂H₄, C₂H₆ and C₂H₅OH) is usually lower than that of C1 products (e.g., CO, CH₄ and CH₃OH).^58–61 An early work showed that the increase in CO₂ pressure and the presence of an alkali medium in aqueous solution favoured the formation of C₂₊ products in the pure TiO₂-catalysed CO₂ reduction under UV-light irradiation.⁶² Despite low yields, C₂H₅OH and CH₃CHO were claimed to be formed as major products at a CO₂ pressure of 2.5 MPa in the presence of 0.2 M NaOH.

Visible-light responsive semiconductors have also been investigated for photocatalytic reduction of CO₂ to C₂₊ compounds.^63–67 For example, g-C₃N₄ was reported to catalyse the formation of C₂H₅OH together with CH₃OH during the reduction of CO₂ in 1.0 M NaOH solution under visible-light (λ ≥ 420 nm) irradiation.⁶³ The yield of C₂H₅OH increased proportionally to the irradiation time together with those of CH₃OH and O₂ (Fig. 4). The morphology of g-C₃N₄ was found to influence the catalytic performance; a mesoporous flake-like g-C₃N₄ (denoted as u-g-C₃N₄) showed higher activity than a non-porous flaky g-C₃N₄ (denoted as m-g-C₃N₄). After 12 h of reaction, the yields of C₂H₅OH, CH₃OH and O₂ with u-g-C₃N₄ were 10.8, 15.1 and 51.2 μmol, respectively, whereas the yields of C₂H₅OH and O₂ over m-g-C₃N₄ were 8.7 and 24.6 μmol, respectively. The mesoporous structure and large surface area of u-g-C₃N₄ might lead to higher charge separation ability and photocatalytic activity than m-g-C₃N₄. It is surprising that CH₃OH and C₂H₅OH were both formed on u-g-C₃N₄, whereas only C₂H₅OH was observed on m-g-C₃N₄. It is speculated that the H⁺-deficient feature of the system, in particular m-g-C₃N₄, might favour the CO₂ reduction and dimerization to form C₂H₅OH as mentioned in an early work.⁸⁸ Considering that the formations of one mole of CH₃OH and C₂H₅OH require 6 and 12 moles of electrons, respectively, and the formation of 1 mole of O₂ from H₂O requires 4 moles of holes, the ratio of the consumed electrons to holes in the photoredox reactions is nearly 1. This evaluation implies that the results may be reliable. Nevertheless, the confirmation of the source of products by using for example ¹³CO₂ as the reactant is necessary for the carbon-containing photocatalysts or photocatalytic systems.

Fig. 4 Photocatalytic conversion of CO₂ with H₂O to CH₃OH, C₂H₅OH and O₂ over u-g-C₃N₄ and m-g-C₃N₄ under visible-light irradiation.⁶³

The doping of a metal or non-metal element is a useful strategy to tune the energy-band structure of semiconductors for CO₂ photoreduction.^48,49 Generally, the metal doping can create intra-band states below the conduction band, whereas the non-metal doping is capable of forming intra-band states above the valence band. Both may narrow the bandgap and extend the light absorption range. Moreover, the metal or non-metal doping may also generate surface vacancies or new active sites, thus affecting the CO₂ activation and reduction. In this case, the metal dopant may also function like a co-catalyst.

Some studies have investigated the effect of metal or metal cation dopants, such as Cu, Fe, V, Cr, Co and Rh, in TiO₂ on the photocatalytic reduction of CO₂ to C₂ compounds.^68,69 As compared to the UV-light responsive TiO₂, the cationic V-, Cr- and Co-doped TiO₂ catalysts extended the absorption spectrum into the visible-light region, thus enabling the photocatalytic reduction of CO₂ under visible-light irradiation.⁶⁸ It was found that the Co²⁺-doped TiO₂ showed higher photocatalytic activity than the other metal cation-doped catalysts, but the product selectivities for the V-, Cr- and Co-doped TiO₂ were similar, and the major reduction products were H₂, CH₃OH, C₂H₅OH and CH₃CHO.⁶⁸ The performance was also strongly dependent on the doping amount of Co²⁺, and the formation rates of H₂, CH₃OH, C₂H₅OH and CH₃CHO over the 1 wt% Co-doped TiO₂ reached 63, 26, 18 and 19 μmol g⁻¹ h⁻¹, respectively.⁶⁸ A Cu²⁺-doped TiO₂ nanorod thin film catalyst was also claimed to be efficient for the gas-phase reduction of CO₂ with H₂O vapour to CH₃OH and C₂H₅OH under UV-light irradiation.⁶⁹ The reaction was carried out in a flow-type optofluidic planar microreactor, and the formation rates of CH₃OH and C₂H₅OH with Cu²⁺-doped TiO₂ reached 24 and 47 μmol g⁻¹ h⁻¹, respectively at a CO₂ flow rate of 2 mL min⁻¹ and 60 °C, whereas they were only 6.7 and 6.3 μmol g⁻¹ h⁻¹, respectively, with TiO₂ alone. The increase in temperature to 80 °C could further increase the formation rates of CH₃OH and C₂H₅OH to 36 and 79 μmol g⁻¹ h⁻¹, respectively.⁶⁹ Besides the decreased band-gap energy and the extended light absorption, the doped Cu²⁺ ions were supposed to serve as active sites of electron traps and suppress the electron–hole recombination. Thus, the doped metal cation centre may also play a role as a co-catalyst, which will be discussed later.

3.1.2. Hybridisation of semiconductors and a semiconductor with a nanocarbon material. The combination of two different semiconductors can form heterojunctions at the interface of the two semiconductors. The construction of a hybrid photocatalyst may not only enhance the light absorption and charge separation but also drive the transfer of photogenerated electrons and holes between the two semiconductors, thus affecting the catalytic activity and selectivity.⁴⁶ The charge transfer between different semiconductors proceeds through three main kinds of mechanisms including sensitisation, p–n junction and Z-scheme mechanisms. As for photocatalytic reduction of CO₂ with H₂O, a semiconductor may either have high ability for O₂ evolution with photogenerated holes or be good at the reduction of CO₂ to C₂₊ compounds with the photogenerated electrons. The hybridisation of the O₂-evolution semiconductor A with the CO₂ reduction semiconductor B through p–n junction or Z-scheme can drive the transfer of photogenerated electrons to semiconductor B for reducing CO₂ and photogenerated holes to semiconductor A for oxidizing H₂O (Fig. 5).

Fig. 5 Schematic illustrations of hybridisation of two different semiconductors for photocatalytic CO₂ reduction to C₂₊ compounds. (a) p–n junction. (b) Z-scheme.

Some hybrid photocatalysts were reported for the photocatalytic reduction of CO₂ to products including C₂₊ products such as C₂H₅OH and C₂H₆ together with C1 compounds.^70,71,89 For example, the combination of PdS quantum dots (QDs) with TiO₂ could increase the rate of CO₂ photocatalytic reduction by a factor of 5 because of the sensitisation effect of PbS QDs, which made the utilisation of visible and near-infrared light possible.⁷⁰ CO, CH₄ and C₂H₆ were the major products over the PbS–TiO₂ nanocomposite. Similarly, AgBr–TiO₂ could catalyse the reduction of CO₂ to CH₄, CH₃OH, CO and C₂H₅OH under visible-light irradiation because of the sensitisation effect of AgBr.⁷¹ The light-harvesting complex II (LHCII), which was extracted from spinach and functioned as a sensitizer, was combined with Rh-doped TiO₂ for photocatalytic reduction of CO₂ in aqueous solution under visible-light irradiation, offering acetaldehyde, CO and methyl formate as major products.⁸⁹ The formation rates of CO, acetaldehyde and methyl formate were 0.57, 3.2 and 0.26 μmol g⁻¹ h⁻¹. Han, Niu and their co-workers loaded AgBr nanoparticles onto g-C₃N₄-decorated N-doped graphene (NG) and found that this nano-composite showed excellent performance for the photocatalytic reduction of CO₂ to C₂H₅OH under visible-light irradiation.⁷² The reaction was proposed to proceed by the scheme of Fig. 5a and the reduction of CO₂ took place on AgBr nanoparticles. The rate of C₂H₅OH formation reached 51 μmol g⁻¹ h⁻¹, 2.4 times that of CH₃OH over the ternary composite.⁷²

Nanocarbon materials, such as graphene (G), graphene oxide (GO) and carbon nanotubes (CNTs), have been widely used to fabricate hybrid photocatalysts, owing to their high conductivity, good electron mobility, favourable Fermi levels and large surface areas.¹⁷ The nanocarbon can serve as an electron collector or transporter to enhance the charge separation, an active surface to facilitate the adsorption of reactants or a protector to inhibit semiconductor photo-corrosion. Some nanocarbon–semiconductor hybrid photocatalysts, such as CNT–TiO₂, GO–TiO₂, G–TiO₂, G–TiO_2−x and carbon–Cu₂O, could offer C₂₊ compounds during photocatalytic reduction of CO₂.^73–77

A CNT–TiO₂ nanocomposite was reported to provide CH₄, HCOOH and C₂H₅OH as the major products during the reduction of CO₂ with H₂O vapour under UV-light (λ = 365 nm) irradiation, and the formation rate of C₂H₅OH reached ∼30 μmol g⁻¹ h⁻¹, higher than those of CH₄ (12 μmol g⁻¹ h⁻¹) and HCOOH (19 μmol g⁻¹ h⁻¹).⁷³ A GO–TiO₂ hybrid suspended in an aqueous medium was also claimed to offer C₂H₅OH together with CH₃OH during the reduction of CO₂ under UV-vis-light irradiation.⁷⁴ It was found that the pH value of the aqueous solution influenced the products; CH₃OH was the major product at a lower pH value, whereas C₂H₅OH became the major product at a higher pH value. The formation rate of C₂H₅OH reached ∼145 μmol g⁻¹ h⁻¹ at a pH of 11.⁷⁴ It was speculated that the pH value of the aqueous solution might affect the point of zero charge of the catalyst surface and thus the interaction between carbonate species and the surface.⁷⁴ It is, however, surprising that C₂H₅OH was also reported to be the major product with a formation rate of ∼110 μmol g⁻¹ h⁻¹ over TiO₂ (P25) alone. Such a high C₂H₅OH formation rate for TiO₂ alone has not been reported by other groups.

On the other hand, Zou and co-workers showed that a G–TiO₂ hybrid photocatalyst with 2D sandwich-like nanosheet morphology could function for the reduction of CO₂ with H₂O vapour to C₂H₆ along with CH₄ under UV-light irradiation.⁷⁵ The G–TiO₂ nanocomposite was fabricated by a simultaneous reduction-hydrolysis strategy in ethylenediamine/H₂O solvent, by which the reduction of GO to graphene by ethylenediamine and the hydrolysis of Ti⁴⁺ to TiO₂ took place simultaneously. The pure TiO₂ synthesized by the same procedure showed a better performance for CO₂ reduction than P25 probably owing to the abundant Ti³⁺ sites on TiO₂, and the formation rates of CH₄ and C₂H₆ were 10 and 7.2 μmol g⁻¹ h⁻¹, respectively. Upon increasing the content of graphene in G–TiO₂ from 0 to 2 wt%, the formation rate of CH₄ somewhat decreased but that of C₂H₆ increased significantly (Fig. 6a).⁷⁵ Although a further increase in the content of graphene decreased the formation rate of the total products, the molar ratio of C₂H₆ to CH₄ increased monotonically with the content of graphene to 5 wt%, suggesting that the introduction of graphene favoured the formation of C₂H₆. The 2% G–TiO₂ offered the highest total product formation rate, and the formation rates of CH₄ and C₂H₆ reached 8.0 and 16.8 μmol g⁻¹ h⁻¹, respectively. Both Ti³⁺ sites on the TiO₂ surfaces and the presence of graphene are proposed to play key roles in the photosynthesis of C₂H₆ from CO₂. It is speculated that the abundant surface Ti³⁺ sites can work for trapping photogenerated electrons and the electrons may be further accumulated on graphene, both favouring the multi-electron reduction of CO₂ to CH₃˙. The electron-rich graphene may also be beneficial for stabilising CH₃˙, the reaction intermediate, and thus limit the combination of CH₃˙ with H⁺ and e⁻ into CH₄. At the same time, the accumulation of CH₃˙ on graphene may contribute to increasing the probability of coupling of CH₃˙ to form C₂H₆.⁷⁵

Fig. 6 (a) Photocatalytic reduction of CO₂ to CH₄ and C₂H₆ over a graphene–TiO₂ hybrid under UV-light irradiation.⁷⁵ (b) Schematic illustration of photocatalytic reduction of CO₂ over a graphene–TiO₂ hybrid.⁷⁶

In and co-workers fabricated graphene-wrapped reduced blue titania (G–TiO_2−x) loaded with a small amount of Pt nanoparticles and found that this nanocomposite showed a high formation rate of C₂H₆ and CH₄ in the photocatalytic reduction of CO₂ in a continuous flow-through (CO₂, H₂O) photo-reactor under one sun AM1.5G illumination.⁷⁶ P25 was inactive under such circumstances and TiO_2−x catalysed the formation of CH₄ with a rate of 1.0 μmol g⁻¹ h⁻¹. The presence of graphene not only enhanced the formation of CH₄ but also induced the formation of C₂H₆, and the loading to a small amount of Pt could further accelerate the formations of both CH₄ and C₂H₆. The formation rates of CH₄ and C₂H₆ could reach 37 and 11 μmol g⁻¹ h⁻¹, corresponding to CH₄ and C₂H₆ selectivities of 77% and 23%, respectively.⁷⁶ This work provided solid evidence for the formation of CH₄ and C₂H₆ from CO₂ by ¹³CO₂ isotopic control experiments. A CH₃˙ radical was proposed to be the intermediate for the formations of both CH₄ and C₂H₆. In other words, CH₄ is formed by the reaction of CH₃˙ with H⁺ and e⁻, whereas the coupling of two CH₃˙ radicals gives C₂H₆. However, different from Zou and co-workers,⁷⁷ In and co-workers proposed through transient absorption spectroscopy studies that photogenerated holes rather electrons migrate to graphene, where the oxidation of H₂O occurs, whereas the reduction of CO₂ takes place on TiO_2−x surfaces, where the electrons accumulate.⁷⁶ Nevertheless, In and co-workers also speculated that graphene might stabilise CH₃˙ radicals, enabling the increased probability for C₂H₆ formation (Fig. 6b).

Yu et al. reported that the coating of a carbon layer onto mesoporous Cu₂O nanorods deposited on Cu foils could also enhance the photocatalytic reduction of CO₂, but they observed the formation of CH₄ and C₂H₄.⁷⁷ The photocatalytic reaction was carried out in KHCO₃ aqueous solution under visible-light irradiation. Cu₂O nanorods on Cu foils alone could offer CH₄ and C₂H₄ with almost the same amount, but the amounts of CH₄ and C₂H₄ did not increase with time after ∼6 h of reaction because of the photo-corrosion of Cu₂O. The coating of a carbon layer enhanced the formation of C₂H₄, the amount of which became ∼1.3 times that of CH₄, and the amounts of C₂H₄ and CH₄ kept increasing with time at least in 12 h over the carbon-coated Cu₂O nanorods. The total amounts of C₂H₄ and CH₄ were 0.037 μmol after 12 h of reaction. The carbon layer was proposed to work as a protective layer to quench the photo-corrosion of Cu₂O and to extract photogenerated electrons, thus accelerating the separation of electron–hole pairs and the transfer of a multitude of electrons for the formation of C₂H₄. Although the formation rates of C₂H₄ and CH₄ reported in this work were quite low, it is noteworthy to point out that the ¹³C-labelling isotopic experiment using ¹³CO₂ and the measurement of the evolved O₂ provided solid evidence that C₂H₄ and CH₄ were produced by the reduction of CO₂ in such a carbon-containing catalyst system.⁷⁷

3.1.3. Effect of a co-catalyst. The design of a suitable co-catalyst is one of the most important strategies to improve the efficiency of a photocatalytic reaction.^49,90,91 The co-catalyst can provide trapping sites for photogenerated charges and promote their separation, thus enhancing photocatalytic activity. The fabrication of suitable co-catalysts on semiconductors can also offer active sites/reaction sites and facilitate oxidation and reduction reactions by lowering the activation energy. Because of the inertness of the CO₂ molecule and various possible products, the co-catalyst plays crucial roles not only in improving the activity through promoting the charge separation and the adsorption/activation of CO₂ but also in tuning the product selectivity. In this section, we will focus on the role of co-catalysts in the semiconductor-based photocatalytic reduction of CO₂ and will demonstrate the effects of co-catalysts on promoting the activity for CO₂ reduction and improving the selectivity toward C–C coupling products.

Noble and coinage metals (such as Pt, Pd, Rh, Au, Ag and Cu) have been used as co-catalysts for TiO₂-based photocatalytic reduction of CO₂ with H₂O.^49,92 Among the noble and coinage metal co-catalysts, Pt is known to be the most efficient in extracting electrons from a semiconductor and in promoting CO₂ reduction, but the reduction of H₂O to H₂ is also enhanced at the same time because of the fast recombination of H atoms on Pt. A co-catalyst that can efficiently extract conduction-band electrons but has lower ability of H atom recombination to H₂ is beneficial for CO₂ reduction.

Pd is a good candidate of such a co-catalyst because of the retarded recombination of H atoms on Pd surfaces. Ishitani and co-workers found that the photocatalytic reduction of CO₂ in aqueous solution with TiO₂ suspended provided CO as the major product and the loading of Pd onto TiO₂ remarkably enhanced the formation of CH₄ (Fig. 7a).⁷⁸ It is noteworthy that the formation of C₂H₆ was observed over the Pd/TiO₂, although the rate of C₂H₆ formation was about 6 times lower than that of CH₄ (Fig. 7a).⁷⁸ The acid/base property of the support, which was used to load the semiconductor, was found to influence the selectivity of photocatalytic reduction of CO₂ to C1 or C₂₊ products.⁹³ This result implied that the introduction of an acidic support favoured the formation of C1 products, whereas a basic support such as MgO or basic promoter (e.g., Li⁺) was beneficial for the formation of C₂₊ products such as C₂H₆, which were supposed to be formed by dimerization of radical intermediates (e.g., CH₃˙).⁹³ On the other hand, Choi and co-workers demonstrated that the introduction of a thin layer of acidic Nafion (perfluorinated polymer with sulfonated groups) onto Pd/TiO₂ could significantly enhance the photocatalytic reduction of CO₂ in aqueous solution with different pH values not only to CH₄ but also to C₂H₆ (Fig. 7b).⁷⁹ In particular, the effect of Nafion was still remarkable for the formation of C₂H₆ even at a pH value of 1, whereas the enhancement in CH₄ formation due to the Nafion layer was insignificant at such a low pH value. A small amount of C₃H₈ was detected over the Nafion-modified Pd/TiO₂ but not over the Pd/TiO₂. The formation of C₂H₆ and C₃H₈ requires more protons and electrons than that of CH₄. Choi and co-workers strengthened the crucial role of proton-coupled multi-electron transfer in the formation of C₂H₆ and C₃H₈. It is speculated that at a lower pH value (pH = 1), protons may be sufficient at the surface active site for the formation of CH₄ without the need of Nafion.⁷⁹ The presence of a Nafion layer accelerated the formation of C₂H₆ and C₃H₈ even at an acidic environment (pH = 1) because the Nafion layer might enhance the local proton activity to facilitate the proton-coupled multi-electron transfer processes. The Nafion layer might also stabilise the intermediates (probably radicals) involved in the formation of C₂H₆ and C₃H₈.

Fig. 7 (a) Photocatalytic reduction of CO₂ over TiO₂ and 1% Pd/TiO₂.⁷⁸ (b) Effect of Nafion (Nf) layer deposited on Pd/TiO₂ on photocatalytic reduction of CO₂.⁷⁹

The morphology of a co-catalyst may affect the interface between the co-catalyst and semiconductor, thus influencing the photocatalytic reduction of CO₂. Rh and Pd nanowires loaded onto TiO₂ nanosheets were found to be more efficient co-catalysts than the corresponding nanoparticles for photocatalytic reduction of CO₂ with H₂O under UV-light irradiation.⁸⁰ The use of nanowire co-catalysts reduced the H₂ evolution and enhanced the reduction of CO₂. The larger density of grain boundaries between the co-catalyst and TiO₂ in the nanowire-based photocatalyst was proposed to be the key. CO, CH₄ and C₂H₅OH were the major products, and the rates of CO, CH₄ and C₂H₅OH formation reached 4.5, 14 and 12 μmol g⁻¹ h⁻¹ over the Rh-nanowire-loaded TiO₂ nanosheet catalyst.⁸⁰ The molar ratios of CO, CH₄ and C₂H₅OH were comparable between Rh nanowire- and Rh nanoparticle-based catalysts.

NiO was reported to be an efficient co-catalyst for photocatalytic reduction of CO₂ in a few studies.^66,81 NiO/InTaO₄ photocatalysts with different NiO contents were investigated for photocatalytic reduction of CO₂ with H₂O vapour in a monolith reactor with optical fibres inserted inside, i.e., an internally illuminated monolith reactor.⁶⁶ It is of interest that acetaldehyde was formed with a considerable amount under UV-light or simulated-sunlight irradiation. The formation rate of acetaldehyde increased with the content of NiO and reached 0.21 μmol g⁻¹ h⁻¹ over the 2.6 wt% NiO/InTaO₄ catalyst at 30 °C under AM1.5G irradiation.⁶⁶ The increase in temperature to 70 °C slightly increased the formation rate of acetaldehyde to 0.30 μmol g⁻¹ h⁻¹. A NiO/Na_1−xLa_xTaO_3+x catalyst was reported to offer CH₃OH and C₂H₅OH during photocatalytic reduction of CO₂ in 0.2 M NaOH aqueous solution under UV-light irradiation.⁸¹ The rates of CH₃OH and C₂H₅OH reached 59.6 and 18.8 μmol g⁻¹ h⁻¹, respectively over the NiO/Na_1−xLa_xTaO_3+x catalyst, whereas they were 35.0 and 6.4 μmol g⁻¹ h⁻¹, respectively in the absence of NiO. Thus, NiO might play a crucial role in accelerating the formation of C₂ oxygenates by increasing the charge separation and C₂ oxygenate selectivity.

Among noble or coinage metals, Cu is a very attractive co-catalyst that can accelerate the formation of C₂₊ hydrocarbons.^58,67,94 An early study showed that Cu-loaded TiO₂ powders suspended in aqueous solution, which was pressurised CO₂ (2.8 MPa), could catalyse the reduction of CO₂ into CH₄, C₂H₄ and C₂H₆ with yields of 21.8, 26.2 and 2.7 μL g⁻¹ under optimised conditions.⁸² A Cu²⁺-doped TiO₂ nanorod thin film catalyst was recently reported to offer CH₃OH and C₂H₅OH as the major products during the gas-phase reduction of CO₂ with H₂O vapour under UV-light irradiation in a flow-type optofluidic planar microreactor.⁶⁹ The formation rates of CH₃OH and C₂H₅OH reached 24 and 47 μmol g⁻¹ h⁻¹, respectively. For a nitrogen-doped TiO₂ nanotube array, the presence of Cu as a co-catalyst enhanced the formation of hydrocarbons including CH₄ and C₂–C₆ paraffins as well as minor olefins during the photocatalytic reduction of CO₂ with H₂O vapour under outdoor sunlight.⁸³ On the other hand, the employment of a Pt co-catalyst mainly enhanced the formation of H₂ by accelerating the reduction of H₂O under the same reaction circumstance.

Hoffmann and co-workers reported an interesting CdS/Cu–TNTs photocatalyst fabricated by coating CdS QDs on Cu-deposited sodium trititanate nanotubes (TNTs, Na_xH_2−xTi₃O₇) and found that this ternary photocatalyst was capable of working for photocatalytic reduction of CO₂ in aqueous solution to C₁–C₃ hydrocarbons under visible-light (λ ≥ 420 nm) irradiation.⁶⁷ Over the ternary catalyst, photogenerated electrons can transfer from the CdS QDs to the Cu particle surface through the titanate nanotube framework and the reduction of CO₂ to C₁–C₃ hydrocarbons may mainly proceed on the Cu surfaces. The formation rates of CH₄, C₂H₆ and C₃H₈ were 28, 17 and 9.7 μL g⁻¹ h⁻¹, and minor amounts of C₂H₄ (0.1 μL g⁻¹ h⁻¹) and C₃H₆ (0.8 μL g⁻¹ h⁻¹) were also formed. The isotopic labelling experiments using ¹³CO₂ confirmed that most of the hydrocarbons were formed by the reduction of CO₂, and a part of CH₄ may be formed by organic compound contamination.⁶⁷ Cu was found to be crucial in the formation of C₂ and C₃ hydrocarbons. Hoffmann and co-workers performed a mechanistic study using the CdS/Cu–TNTs catalyst. The formation of CH₃˙ was confirmed through the EPR characterization of the formed radicals using DMPO as a spin trapping reagent. It is speculated that CO₂ reduction is initiated most likely by a one-electron reduction to CO₂˙⁻, which then reacts with H˙ to produce hydrocarbons through CH₃˙ radicals (Fig. 8).⁶⁷ The Fischer–Tropsch route for the formation of hydrocarbons may not occur in the present system because no evolution of H₂ and CO was observed at the low temperature. Lower-intensity of signals ascribed to CH₃˙ was observed in the absence of Cu, suggesting that Cu played a key role in accelerating the formation of CH₃˙.

Fig. 8 Proposed reaction pathways for photocatalytic reduction of CO₂ to hydrocarbons over the CdS/Cu–TNTs catalyst.⁶⁷

As compared to a single metal co-catalyst, a bimetallic or alloy co-catalyst may have potential to further improve the catalytic activity and/or selectivity of a semiconductor photocatalyst. Grimes and co-workers co-loaded Cu and Pt nanoparticles on nitrogen-doped titania nanotube arrays (NT), and conducted photocatalytic reduction of CO₂ under outdoor sunlight irradiation.^58,83 The formation rates of H₂ and hydrocarbons were 61 and 104 ppm cm⁻² h⁻¹ over the Cu/NT, respectively, whereas they were 190 and 82 ppm cm⁻² h⁻¹ over the Pt/NT.⁸³ The formation rate of hydrocarbons over the Cu–Pt/NT was 111 ppm cm⁻² h⁻¹, which was close to that over Cu/NT, while the formation rate of H₂ was 160 ppm cm⁻² h⁻¹ over the bimetallic catalyst, close to that of Pt/NT. The co-loading of Cu and Pt nanoparticles did not show a significant synergistic effect on tuning the selectivity of C₂₊ hydrocarbons. The fabrication of a core–shell structured Pt@Cu co-catalyst with Pt as the core and Cu as the shell to cover Pt could promote both the extraction of photogenerated electrons by Pt and the reduction of CO₂ to CH₄.⁹⁵ Shankar and co-workers synthesized CuPt alloy co-catalysts with different Cu/Pt molar rations on a periodically modulated double-walled TiO₂ nanotube (PMTiNT) array and found that the Cu_0.33Pt_0.67/PMTiNT showed the highest performance for photocatalytic reduction of CO₂ to CH₄, C₂H₄, and C₂H₆ under simulated one-sun AM1.5 illumination.⁸⁴ It is of interest that the present system could work for the reduction of not only concentrated CO₂ (99.9%) but also diluted CO₂ (0.998%). The formation rates of CH₄, C₂H₄ and C₂H₆ over Cu_0.33Pt_0.67/PMTiNT reached 2.60, 0.24 and 0.47 mL g⁻¹ h⁻¹, respectively.⁸⁴ In contrast, the formation rates of hydrocarbons over the Cu/PMTiNT and Pt/PMTiNT were both quite a bit lower (0.61 and 0.20 mL g⁻¹ h⁻¹, respectively), further suggesting the role of the CuPt alloy co-catalyst in photocatalytic reduction of CO₂. A recent work by In and co-workers further adopted the bimetallic Cu–Pt nanoparticles as co-catalysts of blue titania (BT) for photocatalytic reduction of CO₂ with H₂O vapour in a flow-through reactor under AM1.5 illumination.⁹⁶ It was found that the configuration of bimetallic nanoparticles had a significant effect on catalytic performance; the deposition of Cu atop Pt particles directly attached on blue titania was optimal. Photogenerated electrons were believed to be extracted from blue titania to Pt, due to the interfacial charge transfer phenomenon, which was confirmed by photoluminescence spectroscopy. All extracted photogenerated electrons were then transferred to Cu, resulting in an enriched electron density within Cu, and CO₂ was activated and reduced on Cu surfaces. Over a 6 h period, the Cu_1.00%–Pt_0.35%–BT catalyst offered CH₄ and C₂H₆ with yields of 3.0 and 0.15 mmol g⁻¹, respectively, and the sunlight to fuel (CH₄ and C₂H₆) photoconversion efficiency reached 1% with an apparent quantum yield of 86%.⁹⁶ This sunlight to fuel (CH₄ and C₂H₆) photoconversion efficiency is significantly higher than those reported before (∼0.03%).⁵⁸

In addition, a Cu and Fe co-doped TiO₂ photocatalyst was studied for photocatalytic reduction of CO₂ with H₂O vapour under UV-light irradiation.⁸⁵ The photocatalyst was intriguingly coated on optical fibres, which were assembled into a flow reactor. This configuration of reactor showed significant advantages in improving the CO₂ reduction activity. The presence of Fe together with Cu was found to suppress the formation of CH₄ but enhanced that of C₂H₄, and the Cu(0.5 wt%)–Fe(0.5 wt%)/TiO₂ catalyst offered a C₂H₄ formation rate of 0.58 μmol g⁻¹ h⁻¹.

3.1.4. Plasmonic effect. The Au and Ag nanoparticles have superior surface plasmon resonance (SPR) or localised surface plasmon resonance (LSPR) effect, and thus can work as alternative sensitizers to promote the visible-light absorption of photocatalysts. Some studies have demonstrated that Au- and Ag-containing photocatalysts could work for photocatalytic reduction of CO₂ to C1 and C₂₊ compounds.^{86,87,97–101} Cronin and co-workers prepared a Au/TiO₂ photocatalyst, and found that the wavelength of the light source could significantly affect the activity and selectivity of the catalyst (Fig. 9a).⁸⁶ Under 532 nm-light irradiation, only Au nanoparticles could be excited because of the SPR effect. Au/TiO₂ has remarkably higher activity than TiO₂ and Au alone, and CH₄ was the only product. Under irradiation of 365 nm light, only TiO₂ could be excited, and the products over TiO₂ and Au/TiO₂ were CH₄. Under 254 nm-light irradiation, the photon energy is high enough to excite the d band electronic transition in Au nanoparticles. Besides CH₄, C₂H₆, CH₃OH and HCHO were formed over Au/TiO₂ and Au nanoparticles. However, over TiO₂, only CH₄ was produced, indicating that C₂H₆, CH₃OH and HCHO were formed on Au nanoparticle surfaces. Several other studies also reported the formation of C₂₊ products during photocatalytic reduction of CO₂ using plasmonic Au nanoparticle-based catalysts. For example, a structured montmorillonite (MMT)-dispersed Au/TiO₂ catalyst was claimed to catalyse the reduction of CO₂ by H₂ under not only UV-visible but also simulated sunlight irradiation, offering CO, CH₄ and C₂H₆.⁹⁷ The LSPR effect of Au nanoparticles significantly enhanced the formation of CO and hydrocarbons. The formation rates of CO, CH₄ and C₂H₆ were 199, 42 and 2.1 μmol g⁻¹ h⁻¹ under simulated one-sun irradiation at 100 °C.

Fig. 9 (a) Photocatalytic reduction of CO₂ over TiO₂, Au/TiO₂ and Au under light irradiation with different wavelengths.⁸⁶ (b) Hydrocarbon selectivity and total hydrocarbon turnover frequency in photocatalytic reduction of CO₂ over plasmonic Au nanoparticle photocatalysts in aqueous solutions with different concentrations of EMIM-BF₄.⁸⁷

The alloying effect such as the loading of Au–Cu or Au–Pd alloy onto a semiconductor could further accelerate the photocatalytic reduction of CO₂.^98,99 For example, Kuang and co-workers found that the Au nanoparticles loaded on {101} facets of the well-defined truncated bipyramidal TiO₂ nanocrystals offered mainly CO during the photocatalytic reduction of CO₂ with H₂O vapour.⁹⁹ The use of an Au–Pd alloy instead of Au significantly decreased the selectivity of CO and increased that of hydrocarbons. Over the Au₆Pd₁/TiO₂ {101} catalyst, the selectivities of CH₄ and C₂ hydrocarbons (C₂H₆ and C₂H₄) were 71% and 14%, respectively. It was argued that selectivity depended on the synergistic functions of several parameters including: (i) the LSPR effect of Au, (ii) the hydrogenation performance of Pd and (iii) the adsorption–desorption behaviour of the reaction intermediate (CO_ads) on the Au and Pd sites.⁹⁹ These parameters enable the optimisation of activity and selectivity by tuning the elemental composition of the alloy co-catalyst.

Jain and co-workers reported the use of semiconductor- or support-free colloidal Au nanoparticles with diameters of ∼12 nm and a characteristic LSPR band centred at 520 nm for photocatalytic reduction of CO₂ under visible-light irradiation in aqueous solution.¹⁰⁰ In the presence of isopropanol, which acted as a sacrificial reagent to scavenge holes, the reduction of CO₂ by the photo-excited electrons generated on the oxide- or support-free Au nanoparticles could offer CH₄ and C₂H₆ as major products. The formation rates of CH₄ and C₂H₆ were 0.68 and 0.56 NP(nanoparticle)⁻¹ h⁻¹. Both the photon energy and the light intensity play crucial roles in C–C coupling. No C₂H₆ was observed using 532 nm-light irradiation, and under 488 nm-light irradiation, there existed an onset of C₂H₆ generation at a light intensity of 300 mW cm⁻², above which the formation rate of C₂H₆ increased with an increase in light intensity.¹⁰⁰ CO₂˙⁻ or its hydrogenated form is proposed to be the intermediate, which undergoes a series of hot-electron- and proton-transfer steps to form products. The C–C coupling was proposed to occur between the two CO₂˙⁻ pair. The occurrence of C–C coupling requires simultaneous harvesting of two electrons, and thus higher rate of hot electron transfer and higher light intensity.

Considering that an ionic liquid, i.e., 1-ethyl-3-methylimidazolium tetra-fluoroborate (EMIM-BF₄) could stabilise the high-energy CO₂˙⁻ intermediate, Yu and Jain further adopted aqueous solution for photocatalytic reduction of CO₂ using the plasmonic Au nanoparticle under green light (532 nm) irradiation.⁸⁷ As shown in Fig. 9b, the activity and selectivity of CO₂ reduction depended on the concentration of EMIM-BF₄.⁸⁷ The activity was very low in pure water or EMIM-BF₄. The presence of EMIM-BF₄ in water significantly accelerated the photocatalytic reduction of CO₂ and the highest activity was found in 5 mol% EMIM-BF₄ solution. Besides CH₄, C₂ (C₂H₄ and C₂H₂) and C₃ (C₃H₈ and C₃H₆) were also formed with considerable amounts. The EMIM-BF₄ concentration affected the selectivity of C–C coupling products. The selectivity of C₂₊ hydrocarbons was ∼50% in 1–10 mol% EMIM-BF₄ solutions, and it decreased to ∼20% in 25 mol% EMIM-BF₄ solution. The ¹³C isotope labelling experiment and the fluorogenic test confirmed that the hydrocarbons were generated from CO₂ and that the photogenerated holes oxidized H₂O to H₂O₂ instead of O₂. The studies further showed that EMIM-BF₄ enhanced the activation of CO₂ and promoted the electron transfer from Au nanoparticles to CO₂, thus improving the performance for CO₂ reduction. Moreover, the CO₂˙⁻ intermediate formed at the Au nanoparticle/solution interface may be stabilized due to the complexation by EMIM⁺ ([EMIM*–CO₂]) and the higher concentration of the activated [EMIM*–CO₂] complex would lead to higher probability of C–C coupling. The drops in activity and selectivity at higher EMIM-BF₄ concentrations might arise from the fact that the adsorption of BF₄⁻ on Au nanoparticles inhibited the adsorption of CO₂ and the transfer of electrons to CO₂.

3.1.5. Mechanism for C–C coupling in photocatalytic reduction of CO₂ to C₂₊ compounds. The mechanism for photocatalytic reduction of CO₂ to C₂₊ compounds, which involves CO₂ activation and multi-electron/multi-proton transfer, is quite complicated. The addition of the first electron to activate a free CO₂ molecule is difficult, because the potential for reduction of CO₂ to anion radical CO₂˙⁻ is highly negative (−1.4 V vs. NHE, pH = 0).¹⁰² The surface adsorption of CO₂ can lower the barrier for accepting electrons by changing the geometry of the CO₂ molecule.^103,104 Take the most widely studied TiO₂ as an example. The possible surface structures of adsorbed CO₂ on TiO₂ surfaces are displayed in Fig. 10a.¹⁰⁵ These structures can be grouped into: (i) CO₂ with linear structure (L) and bent structure (B); (ii) monodentate carbonates (MC) that include a carbonate coordinated to a surface Ti centre and a carbonate with one of the oxygens fixed at the lattice position; (iii) bidentate carbonates (BC) that include bridging BC bound to two adjacent Ti centres and chelating BC bound to a single Ti centre; (iv) monodentate and bidentate bicarbonates (MB and BB) derived from the corresponding carbonate structures. The activation of CO₂ is strongly dependent on the adsorption structure of CO₂ on surfaces, which can lead to different reaction intermediates and pathways to different products. For instance, a bridging bidentate configuration of CO₂ favours the attachment of a hydrogen atom to the carbon atom, leading to the formation of a bidentate formate anion intermediate and the formation of HCOOH.¹⁰⁶

Fig. 10 (a) Possible adsorption structures of CO₂ on TiO₂ surfaces.¹⁰⁶ (b) Proposed mechanisms for photocatalytic CO₂ reduction involving C–C coupling.

After CO₂ activation, a series of elementary steps may take place including the transfer of electrons and protons, formation of intermediates, the cleavage of the C–O bond, and the creation of C–H and C–C bonds, resulting in C₂₊ products. Many studies have been devoted to studying the mechanism for CO₂ photocatalytic reduction, but efforts focusing on C–C coupling to C₂₊ compounds are limited. Most studies reported to date have proposed that the adsorbed CO₂ first accepts an electron to form surface CO₂˙⁻. Several possible routes have been proposed for the subsequent conversion of CO₂˙⁻ and C–C coupling. These routes include the oxalic acid pathway, glyoxal pathway and methyl radical pathway (Fig. 10b). In the oxalic acid pathway, the C–C coupling is proposed to occur between the CO₂˙⁻ pair to oxalate (C₂O₄²⁻), and oxalic acid is a primary C₂ product, which can further be reduced to other C₂₊ compounds.^{59,100,107,108} Eggins and co-workers found the formation of C₂O₄²⁻ as the major product together with glyoxylate, glycolate and tartrate during the photocatalytic reduction of CO₂ on ZnS in the presence of tetramethylammonium chloride, providing evidence for the oxalic acid pathway.^107,108 In the glyoxal pathway, CO₂˙⁻ is converted to formyl radicals (˙CHO), which can not only be reduced to C1 derivatives but also be dimerized into glyoxal, followed by further reduction to other C₂₊ compounds. Shkrob and co-workers investigated the intermediates on TiO₂ for CO₂ photocatalytic reduction by ESR spectroscopy and the result suggested that ˙CHO is the intermediate of glyoxal.¹⁰⁹ Glyoxal can be reduced to glycolaldehyde and acetaldehyde, which may undergo oxidation and decarbonylation to CH₃˙, the intermediate of CH₄ or C₂H₆. CH₃˙ radicals may also be generated through other paths,^45,104 and the formation of CH₃˙ has been confirmed through the ESR characterization.⁶⁷ CH₃˙ has been proposed as the key intermediate for C–C coupling to form C₂H₆ in many studies, although it may also react with H˙ or ˙OH to yield CH₄ or CH₃OH (Fig. 10b).

3.2. Photocatalytic conversion of CO

CO can be transformed into C₂₊ chemicals and fuels by thermocatalysis via hydrogenation.^6–8 Fischer–Tropsch synthesis is the most intensively studied heterogeneous reaction for hydrogenation of CO to C₂₊ hydrocarbons, but the reaction should be performed at high temperatures and pressures, and suffers from the limited selectivity of a specific C₂₊ hydrocarbon.⁷ The most accepted scenario for C₂₊ formation on Fischer–Tropsch catalyst surfaces is schematically illustrated in Fig. 11. In brief, the adsorption and dissociation of CO and H₂ on catalyst surfaces lead to the generation of CH_x (x = 1–3) species, which undergo C–C coupling to form surface C_nH_m intermediates (n ≥ 2), followed by the dehydrogenation or hydrogenation to olefins or paraffins, respectively.⁷ The uncontrollable C–C coupling of CH_x species on Fischer–Tropsch catalysts causes a wide distribution of hydrocarbon products.⁷ Developing novel methods or new catalytic systems with enhanced activity and selectivity for the transformation of syngas to C₂₊ products has drawn great research interests.^6–8

Fig. 11 Simplified reaction mechanism for photo-driven CO hydrogenation.

Solar energy-driven hydrogenation of CO or Fischer–Tropsch synthesis has recently attracted research attention.⁴² The solar light can either trigger the photo-thermal effect or induce free charge carriers on a catalyst, which may have potential to enhance the adsorption/activation of CO and H₂ to increase the activity or to regulate the growth of C–C bonds and the termination of reactions to control the selectivity (Fig. 11). Co, Fe and Ru, which are typical Fischer–Tropsch catalysts owing to their balanced capacities for CO/H₂ activation and C–C coupling,⁷ have mainly been studied for the hydrogenation of CO under light irradiation (Table 2).^110–115

Table 2 Photocatalytic systems for the transformations of CO into C₂₊ chemicals

Catalyst	External heating (°C)	Light wavelength (nm)	CO conv. (%)	CO2 selectivity (%)	Selectivity without CO2 (%)				Ref.
					CH4	C^=2–4	C^02–4	C5+
Co–Co3O4/ZnO–Al2O3	No	200–800	15.4	47.6	47.6	36.0	5.9	10.1	110
Fe–Fe3O4/ZnO–Al2O3	No	200–800	20.9	11.4	28.6	42.4	9.0	8.6	111
Fe5C2	No	200–1100	49.5	18.9	33.1	55.5	5.1	6.3	112
Ru/graphene	150	400–800	1.44	—	2.6	—	15.7	83.9	113
20% Co/TNT	220	UV	63.9	17.3	34.6	0.3	22.4	42.7	114
Ni-Based catalyst	No	200–800	27.7	2.7	38.4	—	21.7	39.9	115
Ni-Based catalyst	No	400–800	20.9	2.0	31.5	—	23.3	45.2	115

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Zhang and co-workers reported that under UV-vis-light irradiation without external heating, a Co-based catalyst, Co–Co₃O₄/ZnO–Al₂O₃, which was fabricated by H₂ reduction of ZnCoAl-layered double-hydroxide (LDH) nanosheets at 450 °C, showed a CO conversion of 15.4% and C₂–C₄ olefin selectivity of 36.0% (ratio of olefins to paraffins, 6.1).¹¹⁰ This Co-based catalyst was found to exhibit strong light adsorption ability across the UV-vis region, and the temperature of the catalyst bed could increase rapidly from 25 to 195 °C by irradiating the catalyst under UV-vis light irradiation without external heating. It is proposed that the solar-driven CO hydrogenation with the present Co catalyst involves a photo-thermal rather than a photocatalytic pathway.¹¹⁰ Control experiments for thermocatalytic hydrogenation of CO at 195 °C without light irradiation showed a similar conversion of CO and distribution of hydrocarbon products, confirming that the photo-thermal effect contributed to the CO hydrogenation. The catalytic performance depended on the temperature adopted for H₂ reduction, and it was clarified that the co-existence of Co₃O₄ and Co played a crucial role in hydrogenation of CO to lower olefins. The oxide-decorated metallic Co nanoparticle heterostructure was proposed to weaken the hydrogenation capability of metallic Co, leading to the formation of lower olefins over Co-based catalysts.

Using a similar strategy, Zhang and co-workers further fabricated a Fe-based catalyst, denoted as Fe–Fe₃O₄/ZnO–Al₂O₃, by H₂ reduction of ZnFeAl-LDH nanosheets at 500 °C for photo-thermal catalytic hydrogenation of CO.¹¹¹ This catalyst, which was composed of Fe⁰ and FeO_x nanoparticles, showed a CO conversion of 21% and C₂–C₄ olefin selectivity of 42% at a syngas pressure of 0.18 MPa under UV-vis irradiation without external heating. The reaction could also be performed under visible-light irradiation, offering a CO conversion of 11% and C₂–C₄ olefin selectivity of 41%. It is of interest that the selectivity towards CO₂ using this system is quite low (11% under UV-vis light irradiation) as compared with the traditional thermocatalysis. The heterostructure of Fe⁰ and FeO_x may also play a crucial role in the hydrogenation. The temperature of the catalyst bed for Fe–Fe₃O₄/ZnO–Al₂O₃ increased to 230 °C under UV-vis light irradiation. The control experiments with external heating and without light irradiation showed a lower CO conversion (9.9%) compared to the light-driven hydrogenation of CO (21%).¹¹¹ Furthermore, the selectivity of C₂–C₄ olefins was 37.2% under a thermocatalytic reaction, lower than that obtained in a solar-driven reaction, while the selectivity of CO₂ was 60.8%, much higher than that under light-driven CO hydrogenation (Fig. 12a). DFT calculations revealed that the catalyst was in an excited state under light irradiation and had different ability for CO₂ formation and adsorption/hydrogenation of olefin compared to the ground state under thermocatalytic conditions (Fig. 12b). Thus, the Fe-based system may involve a photocatalytic mechanism.

Fig. 12 (a) Product distributions in photo-driven CO hydrogenation over Fe–Fe₃O₄/ZnO–Al₂O₃ and Fe₅C₂. The filled bars are for the reaction performed under light irradiation, while the bars with slashes are for the reaction without light irradiation but with external heating.^111,112 (b) The potential energy profile for CO₂ production and olefin hydrogenation over ground-state and excited-state Fe–Fe₃O₄/ZnO–Al₂O₃.¹¹¹ (c) Distributions of hydrocarbon products over Ru/graphene with or without light irradiation at different temperatures. The filled bars and bars with slashes are for the reactions performed under light irradiation and without light irradiation, respectively.¹¹³ (d) Proposed roles of photogenerated electrons in tuning the selectivity of photo-driven CO hydrogenation over Ru/graphene and Co/TNT.^113,114

Ma and co-workers further employed Fe₅C₂ for photo-thermal hydrogenation of CO at atmospheric pressure under UV-vis light irradiation in the absence of external heating.¹¹² The conversion of CO and the selectivity of C₂–C₄ olefins were 49.5% and 55.5%, respectively. This photo-thermal catalytic system was particularly favourable for the production of olefins and the ratio of C₂–C₄ olefins to paraffins reached 10.9. At the same time, the selectivity of CO₂ was 18.9%, relatively lower than that reported in the conventional FTO process. This performance is quite surprising because usually heavy modification of Fe₅C₂ is required to gain considerable selectivity of lower olefins in the conventional thermocatalytic reaction operated under a high pressure. Fe₅C₂ was found to exhibit a remarkable photo-thermal effect, which might result from excitation of the surface plasmon band and the consequential fast thermal relaxation of the excitation, and the catalyst bed could reach 490 °C under UV-vis-light irradiation. The control experiments for thermocatalytic hydrogenation of CO over Fe₅C₂ at 300–500 °C showed that the CO conversion was low at a low temperature, and upon increasing the temperature gradually to 500 °C, CO conversion reached 80.5%, but CH₄ was the predominant product with a CO₂ selectivity of 36% and no C₂–C₄ olefins were formed (Fig. 12a).¹¹² It was clarified that the Fe₅C₂ surface was oxidized to oxides at the high temperature under traditional thermal catalytic reaction conditions. On the other hand, the characterisation indicated that the surface of the Fe₅C₂ catalyst was partially decorated by O atoms under light irradiation. The DFT calculations further revealed that the O-decorated Fe₅C₂ surface could accelerate the desorption of olefin molecules, thus increasing the selectivity of C₂–C₄ olefins.

Photo-irradiation of a Ru/graphene catalyst was found to accelerate the hydrogenation of CO to C₅₊ hydrocarbons.¹¹³ The nanostructured worm-like Ru dispersed on graphene sheets could catalyse the hydrogenation of CO under visible-light irradiation under relatively mild reaction conditions (150 °C, 2.0 MPa CO and 1.0 MPa H₂), and the catalytic activity reached 14.4 mol_CO mol_Ru⁻¹ h⁻¹, significantly higher than that in the absence of photo-irradiation (7.8 mol_CO mol_Ru⁻¹ h⁻¹).¹¹³ The reaction rate was found to be dependent on the light intensity or the light wavelength, confirming that light irradiation played a key role in the enhancement of CO conversion activity. Furthermore, at the same temperature, the selectivity of the C₅₊ hydrocarbons was higher under irradiation than that without irradiation (Fig. 12c). It was claimed that the enhancement in CO conversion to C₅₊ hydrocarbons mainly resulted from the light-excited hot electrons in Ru nanoparticles. The injecting of photo-excited hot electrons into the 2π* orbital of CO molecules can accelerate the activation of CO molecules, thus increasing the rate of CO dissociation. Furthermore, it is speculated that the excited electrons under light irradiation may increase the unsaturation of the d-band of Ru metal, which results in the donation of electrons from surface CH_x species to the metal centre and decreases the Pauli repulsion between the CH_x species and C_nH_m intermediates (Fig. 12d).¹¹³ The decrease in the Pauli repulsion may increase the chance of chain growth, resulting in high selectivity of the C₅₊ product.

TiO₂ nanotube (TNT)-supported Co catalysts were examined for the hydrogenation of CO under UV-light illumination at 220 °C and syngas pressure of 2.0 MPa.¹¹⁴ Under such conditions, the thermocatalytic conversion of syngas could also occur, but the UV-light illumination was found to enhance the CO conversion. For example, the CO conversion was increased from 9.2% to 64%, while the C₅₊ selectivity was kept at 42–43%, when 20 wt% Co/TNT was illuminated under UV light.¹¹⁴ The selectivity of CH₄ was enhanced from 25% to 35% and the ratio of C₂–C₄ olefins to paraffins decreased from 11 to 1.4 at the same time. It was speculated that the transfer of photogenerated electrons from TNTs to Co sites might occur, thus promoting the adsorption and activation of CO molecules to enhance activity and induce the hydrogenation of olefin to paraffins or hydrogenolysis of heavier hydrocarbons to shorter-chain paraffins (Fig. 12d).¹¹⁴

Ni-Based catalysts showed an unexpected activity in the hydrogenation of CO to C₂₊ hydrocarbons under photo-irradiation without external heating. The traditional thermocatalytic conversion of syngas usually leads to the formation of CH₄. Zhang, Ma and their co-workers found that the irradiation of a Ni-based catalyst, which was synthesized by H₂ reduction of NiAl-LDH nanosheets at 525 °C, under UV-vis illumination could convert syngas (0.8 MPa, H₂/CO = 3/1) into C₂–C₇ hydrocarbons with a selectivity of 60% at a CO conversion of 27.7%.¹¹⁵ The temperature of the catalyst bed increased to 150 °C under UV-vis illumination. The control experiment carried out at 150 °C without photo-irradiation only afforded a CO conversion of 6% with CO₂ as a predominant product. The catalyst could also work for the formation of C₂–C₇ hydrocarbons under visible-light (λ ≥ 400 nm) irradiation; the selectivity of C₂–C₇ hydrocarbons was 67% at a CO conversion of 21%. Under visible-light illumination, the temperature of the catalyst bed only increased to around 81 °C. It is proposed that the CO hydrogenation to C₂₊ hydrocarbons is photo-activated rather than a photo-thermal process. The characterisation suggested that the catalyst was composed of Ni and NiO_x. The DFT calculations indicated that the modification of metallic Ni by surface oxides changed the reaction path of the surface CH_x species by increasing the barrier for CH₄ formation and decreasing that for C–C coupling.¹¹⁵ This is similar to the phenomena observed for the Co- and Fe-based catalysts,^110–112 forming a concept that the oxide-decorated surfaces modulate the product selectivity in these photo-enhanced CO hydrogenation systems.

Photo-driven Fischer–Tropsch reaction enables the production of C₂₊ hydrocarbons through hydrogenation of CO in the absence of external heat source. Photo-thermal effect induces temperature increment at the catalyst bed, thus providing energy required for CO/H₂ activation and C–C coupling. Photo-excited charge carriers may modify the oxidation state and electronic structure of the active site on catalyst surfaces, thus regulating the product selectivity and making the product distribution different from that in thermocatalysis. High yields of C₂–C₄ olefins have been obtained over oxide-decorated Co- and Fe-based catalysts under light irradiation.^110–112 Decoration with the corresponding oxides can change the hydrogenation ability of the metal centre^110,111 or accelerate the desorption of olefins from the surface,¹¹² thus contributing to the high selectivity of lower olefins. C₅₊ hydrocarbons could be formed as major products using Ru/graphene¹¹³ or Co/TNT.¹¹⁴ Photo-generated electrons have been proposed to play a key role in the activation of CO/H₂ and the favourable production of these C₅₊ paraffins. A Ni-based catalyst, which usually catalyses the formation of CH₄ in thermocatalysis, has shown an unexpectedly high selectivity of C₂–C₇ hydrocarbons in the photo-driven hydrogenation of CO.¹¹⁵ The modification of Ni by NiO_x is proposed to contribute to the selectivity switching by changing the energy barriers for CH₄ formation and C–C coupling.

4. Electrocatalytic conversions of CO₂ and CO

Electrocatalytic conversions of CO₂ and CO to C₂₊ products such as olefins and alcohols have become one of the most promising routes to utilize the abundant CO₂ and CO feedstocks under ambient conditions, owing to recent progress in generating electricity from renewable energy sources, such as solar, wind and hydropower.^40,116–118 An analysis comparing the electrocatalytic, biocatalytic and fossil fuel-derived chemical-production routes has shown that the electrocatalytic route has great potential to yield the greatest reduction in CO₂ emission, provided that a steady supply of clean electricity is available.⁴⁰ Different from photocatalytic reactions, in which the reduction and oxidation reactions usually take place in the same cell, the electrocatalytic reaction typically separates reduction and oxidation half reactions into two cells, and thus is capable of avoiding the re-oxidation of reduction products. The activity and selectivity of electrocatalytic reactions can be modulated and optimised by regulating the applied potential. Furthermore, the use of a gas diffusion electrode in a flow cell has technically pushed the current densities to industrially relevant levels (>200 mA cm⁻²).^119–121 Thus, electrocatalytic reductions of CO₂ and CO into high-value C₂₊ compounds have become a very promising and appealing research area in chemistry.

Metal catalysts have been typically employed in an electrocatalytic CO₂ reduction reaction (CO₂RR) with high activity and selectivity. It is generally accepted that the metal–CO binding strength, expressed by ΔH_CO, plays an important role in determining the selectivity of a metal catalyst for electrocatalytic CO₂RR (Fig. 13).^122,123 This is known as the Sabatier principle in electrocatalytic CO₂RR.¹²³ In brief, metals with too weak CO binding energy, such as In, Sn, Pb and Hg, tend to be beneficial for the formation of formate,^{116,123–125} while those with relatively weak CO binding energy, such as Zn, Au, and Ag, offer CO as the main product.^116,122,126 The metals with too strong CO adsorption (such as Fe, Ni, Co, and Pt) can cause the poisoning of electrocatalysts, and thus usually favour H₂ evolution.¹²⁶ In the single-metal electrocatalysts, only Cu with moderate CO binding energy is able to catalyse the formation of C₂₊ products with considerable efficiency.^{53,117,118,127}

Fig. 13 General relationship between the primary product formed and ΔH_CO over different metals for electrocatalytic CO₂RR.¹²² Reproduced from ref. 122, with permission of Wiley-VCH Verlag GmbH&Co. KGaA, copyright 2017.

For the synthesis of C₂₊ products, it is generally believed that CO is the key reaction intermediate in electrocatalytic CO₂RR.^{53,118,127,128} The study of an electrocatalytic CO reduction reaction (CORR) would be beneficial for developing more efficient electrocatalysts for CO₂RR to C₂₊ products and understanding the C–C coupling mechanism.^127,128 Furthermore, because better selectivity to C₂₊ products and improved stability may be expected for electrocatalytic CORR than for direct CO₂RR, electrocatalytic CORR would be useful for implementing a tandem strategy to synthesize C₂₊ products from CO₂via CO.^43,129,130 An analysis demonstrated that a two-step tandem electrocatalytic reactor, in which the first reactor converts CO₂ to CO or formate and the second reactor converts CO or formate to C₂H₅OH or C₂H₄ may offer higher optimal solar-to-fuel conversion efficiencies.¹³¹

Here, we highlight recent advances in developing Cu-based catalysts for the synthesis of four C₂₊ products, i.e., ethylene, ethanol, acetate and n-propanol, by electrocatalytic CO₂RR and CORR. It is noteworthy that the synthesis of a single product is still very difficult. Therefore, we mainly review the studies that can offer one of these C₂₊ compounds as the major product. The electrocatalysts other than Cu that are capable of forming C₂₊ products are also briefly touched upon. The possible reaction mechanism will be discussed with an emphasis on C–C coupling.

4.1. Electrocatalytic CO₂RR and CORR into ethylene over Cu-based catalysts

Ethylene is one of the most important building blocks in the current chemical industry for the production of a wide range of chemicals, in particular plastics.^7,8 It is primarily produced by thermal cracking of crude oil-derived naphtha. C₂H₄ can be produced by hydrogenation of CO via direct Fischer–Tropsch synthesis, an indirect route via methanol synthesis followed by methanol-to-olefins (MTO) or a direct route via a methanol intermediate using a bifunctional catalyst.^7,8 However, the selectivity of C₂H₄ by these thermocatalysis routes is generally not high (<50%) even though the selectivity of C₂–C₄ olefins has recently made a significant step-forward by using the bifunctional catalyst.^{27,28,132,133} The thermocatalytic hydrogenation of CO₂ also suffers from lower selectivity of C₂H₄.^8–15 CO₂RR or CORR with H₂O by electrocatalysis provides a promising avenue for the synthesis of C₂H₄ as a major product under mild conditions. This section contributes to summarizing recent advances in Cu-catalysed CO₂RR and CORR to C₂H₄ with the aim to offer insights into the key factors controlling the CO₂ or CO conversion activity and faradaic efficiency (FE) of C₂₊ compounds, in particular C₂H₄. The effects of different influencing factors such as the exposed facet, chemical state of surface Cu and dopant-modification of Cu catalysts are discussed. Some engineering factors such as cell configuration, electrolyte engineering and reaction conditions will also be analysed. Some typical results for electrocatalytic CO₂ and CORR are displayed in Tables 3 and 4.^134–175

Table 3 Typical electrocatalytic systems for CO₂RR to C₂₊ compounds

Electrocatalyst	Cell	E (V)	j (mA cm^−2)	FE (%)					Ref.
				Ethylene	Acetate	Ethanol	Propanol	C2+
a The potential refers to RHE. b The potential represents the full-cell potential (V).
Cu(100) nanocubes	H-cell	−0.96	−68	32	0	13	15.5	60.5	134
44 nm Cu cubes	H-cell	−1.1	−5.6	41.1	0.2	3.7	2.7	46.4	135
Plasma-activated Cu	H-cell	−0.9	−23	60	0	0	0	60	136
Cu2O derived Cu	H-cell	−0.99	−25	40	0	8.7	0	48.7	137
Nano-defective Cu nanosheets	H-cell	−1.18	−60	83.2	0	0	0	83.2	138
B-Doped Cu	H-cell	−1.1	−70	52	0	27	0	79	139
N-Doped Cu	H-cell	−0.95	−22	39.3	0	18.4	6.0	63.7	140
Cu_I	H-cell	−0.90	−39	45	0.7	27	7.3	80	141
F–Cu	Alkaline GDE	−0.89	−1600	65	2	12	1	80	142
Thermally deposited Cu	Alkaline GDE	−0.67	−750	65	4	11	0	80	143
Cu–Al	Alkaline GDE	−1.8	−600	80	∼2	∼13	∼2	∼97	144
Catalyst:ionomer bulk heterojunction	Alkaline GDE	−0.91	∼−1750	∼75	—	—	—	∼75	145
Arypyridiniums-Cu	Neutral GDE	−0.83	−320	71.5	1.5	10.5	2.1	85.6	146
	MEA	−3.65^b	−120	60	—	—	—	60
Mesoporous Cu	H-cell	−1.3	−14.3	2	46 (ethane)	—	—	48	147
Iodine-derived Cu	H-cell	−1.0	−20.5	0.8	4.0 (ethane)	1.9	—	6.7	148
Cu4Zn	H-cell	−1.05	−8.2	10.7	0.96	29.1	4.4	45.16	149
Phase-blended Ag–Cu2O	H-cell	−1.2	−3	9.5	5.6	34.15	0	49.25	150
Nanoporous CuAg-wire	Alkaline GDE	−0.7	−300	60	0	25	0	85	151
Ag0.14/Cu0.86 alloy	Alkaline GDE	−0.67	−250	35	5	41	0	81	152
Cu2S@Cu vacancy	Alkaline GDE	−0.92	−400	21.2	3.0	24.7	6.9	55.8	153
FeTPP[Cl]/Cu	Neutral GDE	−0.82	−300	39	2	41	3	85	154
Cu/N-doped carbon nanospike	H-cell	−1.2	−2	0	0	63	0	63	155
34% N–C/Cu	Alkaline GDE	−0.68	−300	37.5	2.3	52.3	1.4	93.5	156
N–C/Cu nanorods	Alkaline GDE	−0.9	−281	24	4	45	7.4	80.4	157
Cu single atom/C	H-cell	−0.7	−1.23	0	0	91	0	91	158
N-Doped nanodiamond	H-cell	−1.0	−7	0	91	0	0	91	159
B- and N-co-doped nanodiamond	H-cell	−1.0	−0.5	0	0	93.2	0	93.2	160
Mesoporous N-doped carbon	H-cell	−0.56	−2	0	0	78	0	78	161
N-Doped graphene quantum dots	Alkaline GDE	−0.75	−28	31	6	14	4	55	162

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Table 4 Typical electrocatalytic systems for CORR to C₂₊ compounds

Catalyst	Cell	E (V)	j (mA cm^−2)	FE (%)					Ref.
				Ethylene	Acetate	Ethanol	Propanol	C2+
a The potential refers to RHE. b The potential represents the full-cell potential (V).
Oxide-derived Cu	Alkaline GDE	−0.67	−700	41.8	13.9	26.7	8.3	90.7	163
Oxide-derived Cu (5% CO/N2)	Alkaline GDE	−0.52	−120	72	—	—	—	72	164
Oxide-derived Cu	H-cell	−0.3	−0.285	0.6	13.6	42.9	0	57.1	165
Cu/carbon nanotubes	H-cell	−0.3	−0.37	0	36	36	0	72	166
Cu nanowires	H-cell	−0.30	−0.22	0.66	14	50	0	65	167
Pd0.07Cu	Alkaline GDE	−0.62	−700	37	14.9	33.2	6.3	91.4	168
Cu nanoflower	H-cell	−0.23	−0.21	0.39	23.76	15.38	59.32 (aldehyde)	99	169
Cu–Ag nanoflower	H-cell	−0.335	−0.61	1.2	1.8	2.6	70 (aldehyde)	75.6	170
Cu nanoparticles	MEA	−2.4^b	−144	35.2	30.4	3.6	1.8	71	171
Cu nanosheet	Alkaline GDE	−0.736	−200	16.3	48	2.4	2	68.7	172
Cu nanocavity	Alkaline GDE	−0.56	−37	21	7.8	12.5	21	62.3	173
Fragmented Cu	Alkaline GDE	−0.45	−50	20	7	13	20	60	174
Cu adparticle	Alkaline GDE	−0.44	−48	29.6	4.7	14.6	23	71.9	175

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The control of the exposed facet of a Cu catalyst may modulate its electrocatalytic performance, because different crystalline facets have different surface atomic arrangements and electronic structures, and may have different abilities to adsorb and activate the reactants and intermediates. Model studies using Cu single crystals showed that the product selectivity in electrocatalytic reduction of CO₂ or CO depended strongly on the facet.^176–178 Two early studies by Hori et al. showed that the electrocatalytic reduction of CO₂ in 0.1 M KHCO₃ aqueous solution using Cu(111) and Cu(100) yielded CH₄ and C₂H₄ as the major products, respectively.^176,177 The introduction of (111) or (110) step atoms to the (100) basal plane could significantly enhance the formation of C₂H₄. The Cu(110) facet favoured the formation of C₂₊ oxygenates such as CH₃COOH, CH₃CHO and C₂H₅OH.¹⁷⁷ Koper and co-workers found a similar trend in the electrocatalytic reduction of CO in aqueous solution over Cu single crystals with different facets; CH₄ was favoured on Cu(111), while C₂H₄ was favoured on Cu(100).¹⁷⁸ The reduction of CO on Cu(110) exhibited a similar potential dependence to that on Cu(111), but more C₂H₅OH was observed.¹⁷⁸ A study on electrocatalytic CO₂RR over Cu thin films on single crystals by epitaxial growth with the physical vapour deposition technique demonstrated that the Cu(100) film and the Cu(751) film that has a heterogeneous kinked surface with a (110) terrace were more active and selective for C–C coupling than Cu(111) film, but the Cu(751) film was more selective toward C₂₊ oxygenate formation.¹⁷⁹

Yeo and co-workers performed experimental and computational studies for electrocatalytic CO₂RR on Cu(100), Cu(111) and Cu(110) single-crystal surfaces.¹⁸⁰ It was found that the onset potentials for C₂H₄ and CO were very closely related and Cu(100) showed the earliest onset potentials for CO and C₂H₄ formation, whereas Cu(111) exhibited the latest onset potentials for CO and C₂H₄ formation. The DFT calculations suggested that Cu(100) had the lowest energy barrier for CO dimerization and the energy barrier could be further lowered by increasing the *CO coverage.¹⁸⁰ These results indicate the importance of Cu(100) surfaces and *CO coverage in CO₂RR to C₂H₄. Recently, Wang, Chan and their co-workers performed detailed DFT calculations on the energetics of the initial C–C coupling steps on different Cu facets in CO₂ reduction.¹³⁴ The result suggested that relative to Cu(111) facet, the Cu(100) and stepped (211) facets were more favourable for CO dimerization, and thus were beneficial for the formation of C₂₊ products. They synthesized Cu nanocubes with preferentially exposed (100) facets through a metal ion battery cycling method and found that the 100-cycled Cu nanocubes exhibited a six-fold increase in the ratio of C₂₊ (mainly C₂H₄) to C1 products as compared to the pristine polycrystalline Cu foil primarily with (111) facets (Fig. 14a).¹³⁴ The FEs of C₂₊ products and C₂H₄ could reach 60% and 32%, respectively, over the 100-cycled Cu nanotubes, which were much higher than those over the pristine polycrystalline Cu foil.

Fig. 14 (a) Comparisons of C₂₊ FEs and ratios of C₂₊/C₁ over different catalysts for electrocatalytic CO₂RR. 100-Cycle Cu mainly with (100) facets exhibits a six-fold improvement as compared to the polished Cu foil primarily with (111) facets.¹³⁴ (b) C₂H₄ and CH₄ FEs for plasma-treated Cu foils. From left to right, the insets show SEM images of the low surface area H₂ plasma-treated metallic Cu foil, the O₂ 20 W 2 min plasma-treated Cu foil with optimised C₂H₄ selectivity, and the high surface area nanoneedles on the O₂ 100 W 10 min oxidized sample after the reaction. Scale bars: 500 nm.¹³⁶ Reproduced from ref. 136 with the permissions of Springer-Nature, copyright 2016. (c) Comparison of C₂₊ FEs on Cu(H), Cu(C) and Cu(B).¹³⁹ (d) Comparisons of C₂₊ FEs and ratios of C₂₊/CH₄ on Cu, Cu-on-Cu₂O and Cu-on-Cu₃N.¹⁴⁰

A few studies reported the fabrication of Cu nanocubes by other techniques for electrocatalytic reduction of CO₂ to C₂H₄.^135,181 For example, Buonsanti and co-workers fabricated cube-shaped Cu nanocrystals with three different sizes (edge lengths) of 24, 44 and 63 nm by a colloidal chemistry-based method and found that the cubes with 44 nm edge length showed the best performance for C₂H₄ formation.¹³⁵ The FE of C₂H₄ over the Cu cubes with 44 nm edge length was 41%, significantly higher than those over the smaller or bigger Cu cubes as well as Cu foil. The H₂ evolution reaction was very serious over the smaller Cu cubes.

Many groups found that as compared to the pure metallic Cu catalyst, the oxide-derived Cu catalysts showed enhanced C₂₊ selectivity.^{136–138,182–188} This may result from the differences in surface structures and local environments such as the density of defects, roughness factors, chemical states and oxygen modifiers. Cuenya and co-workers fabricated an oxide-derived Cu catalyst by treating Cu foil with O₂ plasma for electrocatalytic CO₂RR.¹³⁶ After O₂-plasma treatment of Cu foil, the formation of C₂H₄ was significantly enhanced, while the formation of CH₄ was suppressed. The FE of C₂H₄ reached 60% at −0.9 V versus reversible hydrogen electrode (RHE) for the Cu catalyst with 20 W-2 min O₂ plasma-treatment (Fig. 14b). On the other hand, the changes in the FEs of CH₄ and C₂H₄ were limited with H₂-plasma treatment (Fig. 14b). The O₂-plasma treatment could increase both the roughness factor and the surface Cu⁺ sites. It was found that when the O₂ plasma-activated Cu foil was further subjected to H₂-plasma treatment, the FE of C₂H₄ decreased significantly despite similar roughness. The H₂-plasma treatment could deplete the Cu⁺ sites. Thus, it was argued that Cu⁺ species played a key role in enhancing the formation of C₂H₄ on the oxide-derived Cu catalysts.

Through theoretical calculations, Goddard III and co-workers proposed a unique electrocatalyst composed of Cu metal embedded in oxidised matrix for CO₂RR to C₂₊ products.¹⁸² It was shown that the fully oxidised catalyst model could not explain the enhanced electrocatalytic performance and such a catalyst was also unstable under CO₂RR conditions. Both Cu⁺ and Cu⁰ were proposed to contribute to CO₂RR to C₂₊ products and they worked synergistically to improve the kinetics and thermodynamics for both CO₂ activation and CO dimerization.¹⁸² It was proposed that in the initial step, the Cu⁺ site could bind H₂O molecules at the edge of a Cu⁰ region and this H₂O on surface Cu⁺ may form strong hydrogen bonds to CO₂, stabilising both the transition state and the final state of CO₂ activation on nearby Cu⁰, facilitating the formation of CO (Fig. 15a). In the next step, the carbon atom of CO on the Cu⁺ site may be positively charged (Mulliken charge, +0.11), whereas that on the Cu⁰ site may be negatively charged (Mulliken charge, −0.31) because of the back donation (Fig. 15b). The electrostatic attraction between the two carbon atoms would assist C–C coupling to form C₂₊ products (Fig. 15c). The theoretical calculation in combination with ambient-pressure X-ray photoelectron spectroscopy (XPS) studies by Goddard III and co-workers further pointed out the importance of the presence of a thin layer of suboxide below the metallic Cu surface in CO₂ sorption and activation.¹⁸³

Fig. 15 Schematic illustration of the synergistic effects of Cu⁺ and Cu⁰.¹⁸² (a) CO₂ activation. (b) Chemical state of CO. (c) C–C coupling. Reproduced from ref. 182 with the permissions of the National Academy of Sciences, copyright 2017.

The nature of active sites leading to high C₂₊ FE over oxide-derived Cu catalysts is still under debate. In situ ambient-pressure XPS and quasi in situ electron energy loss spectroscopy (EELS) studies showed no residual copper oxide over the oxide-derived Cu catalysts in electrocatalytic CO₂RR, but subsurface oxygen species may exist after oxidation–reduction cycles.¹⁸⁴ The subsurface oxygen species may increase the CO binding energy and contribute to enhanced C–C coupling owing to the increased CO coverage on the catalyst surfaces. It is noteworthy that the reduced oxide-derived Cu catalysts could be re-oxidised rapidly in air or H₂O environments, making the ex situ characterisation to determine the true chemical state of Cu under CO₂RR conditions highly challenging. Lum and Ager prepared an ¹⁸O-enriched oxide-derived Cu catalyst by oxidation/reduction cycling in H₂¹⁸O and the residual ¹⁸O in the Cu catalyst after the electrocatalytic CO₂RR was measured by secondary-ion mass spectrometry (SIMS).¹⁸⁵ The result showed that only a small fraction (<1%) of the original ¹⁸O was sustained after the CO₂RR, whereas the catalyst maintains a high FE (∼60%) towards C₂ and C₃ products.¹⁸⁵ Ager and co-workers further studied the possible active sites of a series of Cu₂O samples with different morphologies for electrocatalytic CO₂RR by in situ Raman in combination with real-time selected-ion flow tube mass spectrometry.¹⁸⁶ The result showed that the reduction of Cu₂O is kinetically and energetically more favourable than the CO₂RR, thus excluding the possibility of Cu₂O as active sites for the electrocatalytic CO₂RR.¹⁸⁶ Although further operando spectroscopy studies are needed, it is generally believed that the Cu^δ+ sites on the catalyst surfaces play pivotal roles in the formation of C₂₊ products.

Xu, Jiao and co-workers recently demonstrated that oxygen-containing surface species are unlikely involved in facilitating the formation of C₂₊ oxygenates in CORR.¹⁸⁹ By using in situ surface-enhanced Raman spectroscopy, they showed that oxygen-containing species do exist under the CORR conditions in an alkaline electrolyte for four commonly used Cu surfaces, i.e., Cu foil, Cu micro/nanoparticles, electrochemically deposited Cu film and oxide-derived Cu. The relative abundance of surface CuO_x on Cu foils is higher than that on other catalysts, while the opposite is true for the surface CuO_x/(OH)_y species. The surface speciation on different catalysts has been correlated with initial degree of oxidation of the Cu surfaces prior to the exposure to negative potentials. However, the CORR experiments showed different trends in product selectivity. Thus, they concluded that the oxygen-containing surface species might not be the active species in the formation of C₂₊ oxygenates.¹⁸⁹

The doping of a heteroatom can modify the structure, and the electronic and chemical properties of Cu catalysts, and thus plays an important role in governing the activity and selectivity for electrocatalytic CO₂RR by influencing the adsorption and activation of reactants and intermediates. Boron- and nitrogen-doped Cu catalysts have been found to show significantly improved performance for electrocatalytic CO₂RR to C₂H₄.^139,140,190 On the other hand, the doping of chalcogen (such as sulfur, selenium and tellurium) has been reported to improve the formation of formate.^191–193 For example, Sargent and co-workers synthesized a boron-doped Cu (Cu(B)) catalyst by reducing CuCl₂ with NaBH₄.¹³⁹ Two reference samples, Cu(C), which consisted of oxidized nano-copper, and Cu(H) that were synthesized by using hydrazine hydrate as the reducing reagent were also prepared. The FE of C₂H₄ over Cu(B) reached 52% at −1.1 V versus RHE, which was 1.6 and 2.4 times higher than those of Cu(C) and Cu(H), respectively (Fig. 14c). The FE of total C₂ products (mainly C₂H₄ and C₂H₅OH) reached 79% with a current density of 70 mA cm⁻² under the same conditions over the Cu(B) catalyst, which was also much better than those over the Cu(H) and Cu(C) catalysts. The DFT calculations and characterisation results revealed that boron was located at the subsurface of the catalyst. In situ X-ray absorption near-edge spectroscopy (XANES) studies indicated that the average oxidation state of Cu was between 0 and +1, and the slightly positive oxidation state of Cu was stable during the electrocatalytic CO₂RR. The electrocatalytic CO₂RR performance of the Cu(B) catalyst kept stable in 40 h of continuous operation, which was much better than other oxide-derived Cu catalysts.¹³⁹ Han and co-workers also reported the superior performance of a B-doped oxide-derived Cu catalyst, which was prepared by reduction of B₂O₃–Cu(OH)₂ film, for electrocatalytic CO₂RR.¹⁹⁰ An FE toward total C₂₊ products (C₂H₄, C₂H₆ and C₂H₅OH) of 48% was achieved at a current density of 33.4 mA cm⁻² at −1.05 V vs. RHE over the B-doped catalyst, better than that over the oxide-derived Cu catalyst without B (FE of C₂ products was 30.5% with a slightly lower current density at −1.05 V).¹⁹⁰ The XPS studies indicated that the doping of boron could stabilise a larger fraction of Cu⁺ on the Cu surfaces.

The group of Sargent further synthesized Cu deposited on Cu₃N (Cu-on-Cu₃N) catalyst, which achieved a 6.3-fold enhancement in the ratio of C₂₊ to CH₄ as compared to Cu deposited on Cu₂O (Cu-on-Cu₂O) catalyst and a 40-fold enhancement relative to a pure Cu catalyst (Fig. 14d).¹⁴⁰ The highest FE towards total C₂₊ products over the Cu-on-Cu₃N catalyst reached 64% at −0.95 V vs. RHE, with the FEs of C₂H₄, C₂H₅OH and C₃H₇OH being 39%, 19% and 6%, respectively. The Cu-on-Cu₃N catalyst also showed a better stability than the Cu-on-Cu₂O and Cu catalysts. The characterisation by XPS and in situ XANES suggested that Cu₃N could stabilise Cu⁺ to a larger extent as compared to Cu₂O during CO₂RR. DFT calculations further indicated that the modulated partial oxidation state by nitride enabled Cu-on-Cu₃N to achieve the lowest CO dimerization barrier energy. Therefore, both the boron and nitride modification contribute to stabilising Cu⁺ sites during electrocatalytic CO₂RR, which likely play crucial roles in CO adsorption and dimerization to form C₂₊ products.^139,140,190

A few groups reported that the modification of Cu by sulphur and other chalcogen modifiers enhanced the formation of formate during electrocatalytic CO₂RR.^191–193 For example, Pérez-Ramírez, López and co-workers found that the FE of formate increased markedly from 26% to 78% after doping of sulfur on Cu surfaces at −0.8 V versus RHE.¹⁹³ It was proposed that the chalcogen modifier on Cu surfaces actively participated in the CO₂RR either by tethering CO₂ or transferring a hydride, thus suppressed the formation of CO and C₂₊ products. The different behaviours of heteroatoms doped on Cu catalysts reveal diversified functioning mechanisms of heteroatoms and offer us opportunities to explore more heteroatom dopants to narrow product distribution for electrocatalytic CO₂RR.

Cuenya and co-workers reported that the addition of a halide ion into KHCO₃ electrolyte could enhance the electrocatalytic CO₂RR into C₂₊ products over a plasma-oxidised polycrystalline Cu foil catalyst.¹⁹⁴ The CO₂ reduction rate was found to increase in the order of Cl⁻ < Br⁻ < I⁻, while the high FE of C₂₊ products kept almost unchanged. The enhancement in CO₂ reduction activity was proposed to arise from the adsorption of halide ions on Cu surfaces, which may facilitate the formation of a carboxyl intermediate (*COOH), which was a precursor of CO, by partial charge donation from halide to CO₂. The effect of electrolytes on electrocatalytic CO₂RR over Cu(100) and Cu(111) surfaces was investigated by Yeo and co-workers.¹⁹⁵ They found that the FEs of C₂H₄ and C₂H₅OH increased in the sequence of KClO₄ < KCl < KBr < KI over both Cu surfaces. The FE of total C₂₊ products reached 74% with those of C₂H₄ and C₂H₅OH being 50% and 16%, respectively, over the Cu(100) surface in the case of KI. KI roughened the Cu surface but the increase in the roughness was not the major reason for the enhancement in performance. It was proposed that the increase in the population of adsorbed CO on Cu surfaces in the presence of KI might contribute to enhancing the formation of C₂₊ products. The effect of alkali metal cations in an electrolyte was also examined for electrocatalytic CO₂RR in MHCO₃ (M = Li, Na, K, Cs) over an O₂-plasma-activated Cu catalyst.¹⁹⁶ The increase in the size of alkali-metal cations increased the formation of C₂₊ products (mainly C₂H₄), and the co-existence of Cs⁺ and I⁻ further enhanced the partial current of C₂₊ compounds (Fig. 16). The highest FE of C₂₊ products reached ∼69% with a C₂₊ partial current density of −45.5 mA cm⁻².

Fig. 16 Effects of cations and anions added in the electrolyte on CO₂RR.¹⁹⁶ Reproduced from ref. 196 with the permissions of American Chemical Society, copyright 2018.

Recently, the modification of Cu by halides has shown promising performances in CO₂RR to C₂₊ products.^141,142 Cuenya and co-workers prepared halide-modified Cu (denoted as Cu_X, X = Cl, Br or I) catalysts by cycling electropolished Cu foils in 0.1 M KX solution and found that the Cu_I catalyst showed the best performance in electrocatalytic CO₂RR to C₂₊ products (mainly C₂H₄).¹⁴¹ The highest FE of a C₂₊ product was ∼80% with a partial current density of ∼31.2 mA cm⁻² at −0.90 V vs. RHE. The nanostructure of Cu changed significantly for the Cu_I catalyst, and it was proposed that the enhancement in the formation of C₂₊ products arose from the reconstructed surface structures and adsorbed I⁻ species, which probably led to unique electronic and chemical effects to stabilise the subsurface oxygen and Cu⁺ species.

A recent work by our group demonstrated that a fluorine-modified Cu catalyst, which was fabricated by electroreduction of a Cu(OH)F precursor synthesized by a solvothermal method, showed excellent performance for electrocatalytic CO₂RR to C₂₊ products in a flow cell with a gas diffusion electrode (GDE).¹⁴² The FE of C₂₊ products (mainly C₂H₄ and C₂H₅OH) reached ∼80% with an ultrahigh current density of 1.6 A cm⁻² at −0.89 V vs. RHE. The FE and formation rate of C₂H₄ reached 65% and ∼3.2 mmol h⁻¹ cm⁻², much better than those reported to date. The carbon-based selectivity and single-pass yield of C_2–4 products (mainly C₂H₄ and C₂H₅OH) reached 85.8% and 16.5%, respectively, outperforming those achieved in high-temperature and high-pressure thermocatalysis.⁸ It was proposed that besides stabilising the Cu⁺ sites to enhance CO adsorption, the fluoride species on Cu surfaces facilitated the activation of H₂O and the hydrogenation of CO to CHO intermediate, which could undergo C–C coupling more facilely.¹⁴²

Most studies for electrocatalytic CO₂RR and CORR have typically been conducted in an H-cell. The limited CO₂ or CO solubility in water (33 mM for CO₂ and 1 mM for CO at 25 °C and 1 atm) usually leads to low reaction rates (<100 mA cm⁻² for CO₂RR and <10 mA cm⁻² for CORR), which are not viable for commercialisation, although the hydrophobic treatment can further enhance the CORR reaction rate in an H-cell.^197,198 As already mentioned, the flow cell with GDE configuration enables the direct feeding of gaseous reactants to the electrode–electrolyte interface to circumvent the mass-transport limitation of CO₂ or CO, resulting in high reaction rates.^119–121 Flow cells with different configurations (Fig. 17) have shown great potential in electrocatalytic CO₂RR to C₂₊ products.^{142–146,199,200}

Fig. 17 Configurations of three types of flow cells, stability of the cell voltage and gas products for 120 min in different flow cells with an applied current density of 150 mA cm⁻².²⁰⁰ (a) Alkaline flow cell. (b) Neutral flow cell. (c) MEA flow cell. Reproduced from ref. 200, with permission of Elsevier, copyright 2019.

One significant merit of the flow cell is that it can be operated in a strong alkaline electrolyte (e.g., KOH) for CO₂RR (Fig. 17a),²⁰⁰ whereas the use of a strong alkaline electrolyte is restricted in the H-cell configuration because of the acidic nature of the CO₂ molecule. The current consensus is that a high local pH can significantly promote the C–C coupling and suppress the formation of H₂ and methane.^{143,163,201,202} Thus, the alkaline flow cell particularly favours the formation of C₂₊ products, offering high FEs of C₂₊ compounds at impressively high current densities.^{142–146,202} For example, Sargent and co-workers reported that upon increasing the concentration of KOH from 1 to 10 M, the onset potential of C₂H₄ for CO₂RR decreased to only −0.165 V versus RHE, close to the CO onset potential, whereas the onset potential of H₂ remained almost unchanged (Fig. 18a).¹⁴³ As such, a Cu catalyst with a thickness of 25 nm deposited on a carbon-based gas diffusion layer could achieve an C₂H₄ FE of 66% with a current density of 275 mA cm⁻² at a potential of −0.54 V versus RHE in a 10 M KOH electrolyte. By further optimising the electrolyte to (3.5 M KOH + 5.0 M KI), the current density could increase to 750 mA cm⁻² with C₂H₄ FE of 65% at −0.67 V versus RHE.

Fig. 18 (a) C₂H₄ and CO FEs versus applied potential during CO₂RR at different KOH concentrations.¹⁴³ (b) Partial current densities of C₂₊ products in CORR in KOH with different concentrations.¹⁶³ (c) Partial current densities of C₂₊ products in CORR in electrolytes with different Na⁺ and OH⁻ concentrations.²⁰³ Reproduced from ref. 143, 163 and 203, with the permissions of American Association for the Advancement of Science, Springer-Nature and Wiley-VCH Verlag GmbH&Co. KGaA, copyright 2018 and 2020.

Very recently, a Cu–Al catalyst, which demonstrated further improved C₂H₄ selectivity in electrocatalytic CO₂RR, was developed through machine learning-accelerated high-throughput DFT studies.¹⁴⁴ The de-alloyed nanoporous Cu–Al catalyst, which was fabricated by both thermal evaporation and co-sputtering followed by chemical etching, on a carbon-based gas diffusion layer showed an FE of C₂H₄ as high as 80% at a current density of 600 mA cm⁻² in a 1 M KOH flow cell.

Similarly, by using the alkaline flow cell, Jiao and co-workers achieved excellent performances for electrocatalytic CORR to C₂₊ compounds.¹⁶³ An oxide-derived Cu catalyst exhibited a C₂₊ FE of 91% and a C₂₊ partial current density of 635 mA cm⁻² at a potential of −0.67 V versus RHE (Fig. 18b).¹⁶³ It is noteworthy that the current density is very low (∼10 mA cm⁻²) in electrocatalytic CORR with H-cell configuration due to the extremely low solubility of CO in aqueous electrolyte solution. Thus, the work by Jiao and co-workers is a significant step-forward in electrocatalytic CORR to C₂₊ compounds.

A recent work revealed that a high concentration of the cation (i.e., Na⁺) rather than OH⁻ might be the determining factor in promoting the formation of C₂₊ products during electrocatalytic CORR.²⁰³ By systematically varying the concentration of cations (Na⁺) and OH⁻ at identical absolute electrode potential, that is on the standard hydrogen electrode (SHE) scale, Lu, Xu and their co-workers found that the partial currents of C₂₊ products, including C₂H₄, CH₃COO⁻, C₂H₅OH and n-C₃H₇OH, all increased with an increase in the concentration of Na⁺ but not OH⁻ (Fig. 18c).²⁰³ The promotional effect of OH⁻ at the same potential on the RHE scale was attributed to the larger overpotential at higher electrolyte pH. The chelation of Na⁺ with a crown ether to form a bulky organic cation while maintaining the concentration of OH⁻ significantly decreased the current density and the FE of C₂₊ products, providing further evidence that Na⁺ played a pivotal role in the formation of C₂₊ products from CO.

To further increase the efficiencies for CO₂RR in an alkaline flow cell, Sargent, Sinton and their co-workers recently designed a catalyst/ionomer bulk heterojunction (CIBH) architecture, which could decouple gas, ion and electron transport.¹⁴⁵ The ionomer layer with hydrophobic and hydrophilic functionalities was assembled into an architecture with differentiated domains that favoured gas and ion transport routes on the metal catalyst surfaces. Gas transport was promoted through a side chain of hydrophobic domains, leading to extended gas diffusion, while water uptake and ion transport took place through hydrophilic domains. The CIBH configuration with a Cu catalyst in a 7 M KOH electrolyte could offer an FE of C₂H₄ in the range of 65–75% and a peak partial current density of ∼1.3 A cm⁻² at a cathodic energy efficiency of 45%.¹⁴⁵ This C₂H₄ partial current density is the highest one reported to date.

The alkaline GDE design successfully circumvents mass-transport limitations associated with the low solubility of CO₂/CO in aqueous solutions and enables superior performances to be achieved at high reaction rates, resulting in a significant step toward future practical applications. However, challenges remain in sustaining the high reaction rate for CO₂RR in the alkaline flow cell over a long period of time. The flooding and the salt accumulation in the triple-phase boundary are two major reasons that can cause decreases in activity and selectivity in electrocatalytic CO₂RR using the alkaline flow cell. To solve the problem of the formation of carbonate salt due to the undesirable consumption of CO₂ by KOH in the alkaline GDE, neutral electrolytes, such as KHCO₃ and K₂SO₄, were employed (Fig. 17b).^146,200 However, the activity in neutral GDE is significantly lower than that of alkaline GDE because of the high Ohmic resistance and overpotential. Moreover, to maintain the stability of catholyte in neutral electrolytes still remains an issue due to the reaction-driven pH increase. A recent work demonstrated that the electrodeposition of N-aryl-substituted-tetrahydro-bipyridine films on a Cu catalyst could achieve 72% FE of C₂H₄ with a current density of 320 mA cm⁻² at a potential of −0.83 V versus RHE during electrocatalytic CO₂RR in a liquid-electrolyte flow cell using 1.0 M KHCO₃ as the electrolyte.¹⁴⁶ It was found that, although the large current densities in the flow cell drove up the local pH, the tetrahydro-bipyridine layer did not create a further pH gradient near the active Cu surface, and the layer was chemically robust to the locally alkaline environment. When the substituents in the organic modifier were changed to tune the electronic structure of organic films, the configuration of CO adsorption changed. A volcano-shaped relationship was observed between the FE of C₂H₄ and the ratio of atop-bound CO to bridge-bound CO. Thus, there is an optimum ratio of atop-bound CO to bridge-bound CO for the formation of C₂H₄. The DFT calculations indicated that the coupling of CO molecules located on atop–bridge pair sites was more facile than that of CO molecules on the bridge–bridge pair sites or top–top sites. The adhered N-aryl-substituted-tetrahydro-bipyridine molecules may help to optimise the pairs of atop- and bridge-bound CO.

The membrane electrode assembly (MEA) cell, which has been widely applied to the H₂–O₂ fuel cell, is emerging as an alternative for CO₂RR to solve the stability issue.²⁰⁰ In the MEA cell, the cathodic GDE contacts directly with an ion-exchange membrane instead of a catholyte (Fig. 17c). The stability issues associated with GDE flooding, electrolyte consumption with CO₂ and electrolyte ohmic loss may be solved. Sargent and co-workers integrated the Cu catalyst sputtered onto a porous polytetrafluoroethylene gas diffusion layer and modified with N-aryl-substituted-tetrahydro-bipyridine films into an MEA device.¹⁴⁶ They found that the system operated at a full-cell voltage of 3.65 V was stable at least for 190 h, offering a stable C₂H₄ FE of 64% and a stable current density of ∼120 mA cm⁻².¹⁴⁶ This is probably the best stability achieved for electrocatalytic CO₂RR with a current density of >100 mA cm⁻². In general, the reaction rate in MEA is still lower than that in alkaline flow cell. Future studies should be focused on improving the ion conductivity of the membrane and the assembly craft.

As already mentioned, excellent performances could also be achieved for electrocatalytic CORR by using an alkaline flow cell.¹⁶³ The electrocatalytic CORR in an alkaline flow cell has advantages over the CO₂RR because of no salt accumulation problem and potentially higher efficiencies towards C₂₊ products.^43,163 Moreover, no formation of carbonates in an alkaline electrolyte can increase the process economics, since the regeneration of KOH and CO₂ from the formed carbonates would be costly in the case of CO₂RR in an alkaline flow cell.

It is noteworthy that the C₂₊ products in electrocatalytic CORR in an alkaline flow cell contained a large fraction of C₂₊ oxygenates (such as C₂H₅OH, CH₃CHO, CH₃COO⁻, n-C₃H₇OH) and the FE of C₂H₄ was not very high (∼40%).^43,163 A recent work suggested that the coverage of CO on Cu surfaces determined the path for C₂H₄ formation or for C₂H₅OH formation.¹⁶⁴ Conventionally, it is generally believed that a higher CO coverage on Cu surfaces is beneficial for the formation of C₂₊ products.¹⁸⁰ Through analysing the possible paths for C₂H₄ and C₂H₅OH formation, Sinton and co-workers proposed that the higher CO coverage may favour the formation of C₂ oxygenates, while the lower CO coverage may favour the formation of C₂H₄.¹⁶⁴ They found that the concentration of CO significantly influenced the product selectivity in electrocatalytic CORR in an alkaline flow cell using KOH as the electrolyte. At a fixed potential of −0.44 V vs. RHE, the CORR in 1 M KOH offered C₂₊ products containing C₂H₄, C₂H₅OH, CH₃COO⁻ and C₃H₇OH, and the FE of C₂H₄ was ∼30%. The decrease in CO concentration to 10% or 5% increases the FE of C₂H₄ to ∼50% at the expense of mainly C₃H₇OH, but a further decrease in CO concentration led to a significant increase in H₂ formation. By tuning the conditions to constrain local CO availability, C₂H₄ FEs of 65–72% could be achieved with high cathodic energy efficiencies (35–44%) and current densities (120–1250 mA cm⁻²).¹⁶⁴ The DFT calculations indicated that a lower CO coverage favoured the dehydroxylation of *CHCOH, which was probably a common intermediate for C₂H₄ and C₂ oxygenates, to *CCH, leading to C₂H₄. On the other hand, the higher CO coverage would be beneficial for the hydrogenation of *CHCOH to *CHCHOH, resulting in oxygenates.

Recently, Strasser and co-workers found that the co-feeding of CO and CO₂ could enhance the formation of C₂H₄ during electrocatalysis over an oxide-derived Cu catalyst.²⁰⁴ There existed an optimum ratio of CO/CO₂ in the gas mixture for C₂H₄ formation. Using an operando differential electrochemical mass spectrometry capillary flow cell with millisecond time resolution, it was demonstrated that the enhanced C₂H₄ formation mainly originated from a cross-coupling between two *CO species derived from CO₂ and CO (the so-called CO₂–CO pathway). It was speculated that the co-fed CO did not compete with CO₂ for adsorption sites, implying non-competent and reactant-specific sites for CO and CO₂ on Cu surfaces. These insights enabled the design of a tandem system of oxide-derived Cu in combination with Ni–N-functionalised carbon (NiNC), which involved NiNC for CO formation from CO₂ and Cu for CO–CO coupling, for electrocatalytic CO₂RR to C₂H₄ with doubled C₂H₄ yields.²⁰⁴

Besides C₂H₄, C₂H₆ could also be obtained as a C₂ hydrocarbon product in electrocatalytic CO₂RR, although the selectivity of C₂H₆ was typically low in most cases. Nam and co-workers found that the selectivities of C₂H₄ and C₂H₆ could be tuned by using mesoporous Cu with different morphologies.¹⁴⁷ The mesoporous Cu catalysts with different pore widths and depths were fabricated by a thermal deposition method on anodized aluminium oxide. As the pore width decreased and the pore depth increased, the FE of C1 products gradually decreased and that of C₂ products increased. The FE of C₂H₄ reached 38% at the pore width and depth of 30 and 40 nm, respectively. As the pore depth increased to 70 nm, the major C₂ product changed to C₂H₆ and the FE of C₂H₆ was 46% (Fig. 19a). Through computational simulations, they proposed that the mesopore can change the local pH and prolong the retention time of key intermediates, thus favouring deep hydrogenation of intermediates and promoting C₂H₆ formation.¹⁴⁷

Fig. 19 (a) FEs of different CO₂RR products at −1.3 V versus RHE over polycrystalline Cu and mesoporous Cu catalysts with different widths and depths (inset: SEM image of mesoporous Cu).¹⁴⁷ (b) Reaction pathway toward the formations of ethane and ethanol via an ethoxy intermediate.¹⁴⁸ Reproduced from ref. 147 and 148, with the permissions of Wiley-VCH Verlag GmbH&Co. KGaA, copyright 2017 and 2020.

Recently, Qiao and co-workers reported that an iodide-derived Cu was more selective for electrocatalytic CO₂RR to C₂H₆ than an oxide-derived Cu.¹⁴⁸ The iodide-derived Cu offered a normalized C₂H₆ selectivity of 72% (normalized to the total C₂ + C₃ products) at −1.0 V versus RHE, whereas that of oxide-derived Cu was only 27%. Through in situ X-ray absorption fine-structure (XAFS) and Raman spectroscopy studies, an O-bound ethoxy (*OCH₂CH₃) intermediate was detected during C₂H₆ formation and *OCH₂CH₃ was also responsible for C₂H₅OH formation (Fig. 19b).¹⁴⁸ The optimized oxidation state of Cu on iodide-derived Cu may stabilize the *OCH₂CH₃ intermediate and steer the reaction into C₂H₆ over C₂H₅OH.

4.2. Electrocatalytic CO₂RR and CORR into ethanol over Cu-based catalysts

Ethanol is a key chemical in both chemical and energy industries as well as in our daily life. It has been widely used as a fuel additive, alternative fuel, solvent or disinfectant, and can also serve as a versatile feedstock for the production of various chemicals and polymers.²⁰⁵ Traditionally, C₂H₅OH is primarily produced through the fermentation of sugars, and recently, the catalytic conversions of syngas derived from fossil resources (in particular natural or shale gas and coal) and cellulose have attracted much attention.^38,206–212

As described above, single Cu catalysts (in particular oxide-derived Cu catalysts) and some non-metallic dopant (e.g., boron and halogen)-modified Cu catalysts usually offer C₂H₄ as the major products during electrocatalytic CO₂RR. C₂H₅OH is typically formed as the second largest product, and the ratio of FE of C₂H₅OH to that of C₂H₄ is typically lower than 0.5. DFT calculations suggested that the formation of C₂H₄ is more favourable than the formation of C₂H₅OH from a CH₂CHO* intermediate over a Cu(100) surface.²¹³ It is generally believed that the incorporation of a different atom in the Cu lattice may alter its geometric and electronic structures, thus changing its selectivity and activity during CO₂RR. The design of bimetallic catalysts containing Cu and another guest metal such as Zn, Au or Ag has shown potential to improve the selectivity of C₂₊ oxygenates especially C₂H₅OH in electrocatalytic CO₂RR.^{149–152,214,215}

Oxide film-derived Cu_xZn bimetallic catalysts were reported to exhibit enhanced selectivity towards C₂H₅OH.¹⁴⁹ Cu alone showed twice higher FE of C₂H₄ as compared to that of C₂H₅OH at most potentials studied, whereas CO was the major product along with a small fraction of HCOO⁻ on Zn alone. By increasing the Zn content in the bimetallic catalysts, the maximum FEs of C₂H₅OH increased from 11.3% (on Cu) to 29.1% (on Cu₄Zn) and then decreased to 18.1% (on Cu₂Zn). Meanwhile, the maximum FEs of C₂H₄ decreased monotonically from 26.5% (on Cu) to 4.1% (on Cu₂Zn). The selectivities of C₂H₅OH versus C₂H₄ production, defined as the ratio of their FEs, could be modulated from 0.48 (on Cu) to 6 (on Cu₂Zn). The formation of C₂H₅OH was maximized on a Cu₄Zn catalyst at −1.05 V versus RHE with a FE of 29.1% (Fig. 20a).¹⁴⁹ Although CO was a major product on Zn in the whole potential ranges investigated, the formation of CO was suppressed at potentials more negative than −1.0 V, where C₂H₅OH formation was significantly accelerated, over the Cu₄Zn and Cu₂Zn catalysts. Furthermore, the formation of HCOO⁻ was also inhibited over the bimetallic catalysts. These results suggest that Zn works synergistically with Cu to enhance the formation of C₂H₅OH. A dual-site mechanism has been proposed for the enhanced formation of C₂H₅OH. In brief, both Cu and Zn sites catalyse the reduction of CO₂ to CO, but Cu sites can work further for the formation of CHO or CH_x (x = 1–3) intermediates via CO. Zn sites provide CO, which might undergo spill-over to a nearby Cu site and then insert itself into the bond between Cu and *CHO or *CH_x, eventually forming C₂H₅OH.¹⁴⁹

Fig. 20 (a) Maximum FEs of C₂H₄ and C₂H₅OH as well as the average FE_ethanol/FE_ethylene ratio (calculated on the basis of the ratios measured at different potentials) in CO₂RR on Cu_xZn.¹⁴⁹ (b) Comparison of C₂H₅OH FEs on Ag–Cu₂O_PS, Ag–Cu₂O_PB and Cu₂O.¹⁵⁰

The deposition of Au nanoparticles on Cu foils without forming an alloy was found to enhance C₂H₅OH and C₃H₇OH formation at low overpotentials.²¹⁴ The Au/Cu catalyst had an earlier onset potential for alcohol production than either Cu or Au alone and it showed higher selectivity for alcohols as compared with hydrocarbons at lower overpotentials. Mechanistic studies suggest that Au nanoparticles work for the reduction of CO₂ to CO, driving a high CO coverage on the nearby Cu surface, enhancing the dimerization of CO and further reduction to C₂₊ compounds.

Several groups have studied Cu–Ag bimetallic catalysts for electrocatalytic CO₂RR to C₂H₅OH.^{150–152,215} For example, Lee and co-workers reported another interesting example to enhance the formation of C₂H₅OH by incorporating Ag, a CO formation electrocatalyst, into Cu₂O.¹⁵⁰ Two types of Ag–Cu₂O, phase-separated (Ag–Cu₂O_PS) and phase-blended (Ag–Cu₂O_PB) were fabricated by electrochemical co-deposition. The Ag–Cu₂O_PB catalyst had more intimate contact between Ag and Cu atoms than the Ag–Cu₂O_PS. The maximum FEs of C₂H₅OH on Ag–Cu₂O_PS and Ag–Cu₂O_PB reached 20.1% and 34.2% at −1.2 V versus RHE, being twice and three times higher than that on pure Cu₂O, respectively (Fig. 20b).¹⁵⁰ This result also indicated the importance of intimate contact between Ag and Cu for synergistic formation of C₂H₅OH. Lee also pointed out the CO insertion mechanism for the enhanced C₂H₅OH. In other words, CO formed on Ag sites could migrate to Cu sites to couple with the CH_x intermediates, contributing to the formation of C₂H₅OH. The closer distance between Ag and Cu was vital for facilitating efficient CO transfer to Cu sites, thus affecting the ethanol selectivity. Nanoporous Cu–Ag alloy nanowires, which were fabricated by co-electrodeposition in the presence of 3,5-diamino-1,2,4-triazole (DAT), an additive to inhibit nucleation, and contained Cu and Ag atoms mixed homogeneously, demonstrated excellent performances for electrocatalytic CO₂RR to C₂H₄ and C₂H₅OH in an alkaline flow cell.¹⁵¹ The FEs of C₂H₄ and C₂H₅OH reached ∼60% and 25% and a current density of −300 mA cm⁻² at a cathode potential of only −0.7 V vs. RHE over the Cu–Ag alloy nanowires containing 6% Ag.¹⁵¹ The studies suggest that the presence of Ag may stabilise the Cu₂O overlayer and keep optimal availability of the CO intermediate, contributing to the enhanced formation of C₂₊ compounds. On the other hand, Bell and co-workers proposed a compressive-strain mechanism to explain the role of Ag in enhancing C₂₊ oxygenate formation over bimetallic Cu–Ag catalysts with surface alloy.²¹⁵ Briefly speaking, the formation of Cu–Ag surface alloy induced compressed strain in the Cu lattice and thus modified the electronic structure of Cu, leading to decreases in the binding energies of H and O relative to CO. This results in the enhancement of the formation of C₂₊ oxygenates at the expense of C₂H₄ due to the reduced coverage of adsorbed H and the reduced oxophilicity of the compressively strained Cu.

Recently, Sargent and co-workers proposed through DFT calculations that after the introduction of Ag on Cu surfaces, the intermediates for the formation of C₂H₅OH would become more favourable than the formation of C₂H₄.¹⁵² They fabricated Ag/Cu catalysts with different Ag/Cu ratios by a co-sputtering technique. The morphology and in situ XAS characterization indicated that Cu and Ag were distributed homogeneously in the bimetallic catalysts, forming Cu–Ag alloy phase. The best CO₂RR performance was achieved over the Ag_0.14/Cu_0.86 catalyst, which exhibited a maximum FE of C₂H₅OH of 41% with a current density of 250 mA cm⁻² at −0.67 V vs. RHE in an alkaline flow cell using a 1 M KOH electrolyte.¹⁵² The cathodic energy efficiency reached 25%. Further in situ Raman spectroscopy studies suggest that the introduction of Ag into Cu could lead to multiple different binding configurations. The C₂H₄ formation path would be suppressed due to the unsaturated nature of C₂H₄ formation intermediates.

DFT calculations also suggest that the presence of vacancies on Cu surfaces, in particular on a Cu shell with a Cu₂S core, may increase the activation energy barrier for C₂H₄ formation via a *CH₂CHO intermediate, which is believed as a common intermediate for both C₂H₄ and C₂H₅OH formations, while leaving that for C₂H₅OH formation mostly unaffected.¹⁵³ The Cu₂S@Cu-V catalyst with a Cu₂S core and surface vacancy-enriched Cu shell, which is known as a core–shell-vacancy engineering (CSVE) catalyst, showed enhanced formation of C₂H₅OH and C₃H₇OH. In an alkaline flow cell with 1 M KOH as an electrolyte, the Cu₂S@Cu-V catalyst exhibited maximum FEs of C₂H₅OH and C₃H₇OH of 25 and 7% with a total current density of 400 mA cm⁻², respectively.¹⁵³ Cuenya and co-workers recently investigated the roles of surface defects and Cu(I) sites on Cu(100) surfaces in the formation of C₂₊ compounds.²¹⁶ They controlled the generation of surface defects and Cu(I) species on Cu(100) by designing a pulsed potential sequence applied, i.e., a brief pulse at an anodic potential to reconstruct the Cu(100) surface and generate Cu(I), followed by a pulse at a cathodic potential (typically −1.0 V) for CO₂RR. The result indicated that the continuous regeneration of defects and Cu(I) species synergistically enhanced the C–C coupling especially to C₂H₅OH.²¹⁶

A tandem CO₂ electrocatalysis composed of two working electrodes (Ag and Cu) in a flow cell with 0.1 M CsHCO₃ electrolyte showed high FEs of C₂₊ oxygenates (Fig. 21).²¹⁷ Ag in the upstream worked for electrocatalytic CO₂RR to CO, which was then transported by convective flow to Cu to be converted to C₂₊ products. The ratio of oxygenates to C₂H₄ reached a maximum by operating Ag and Cu electrodes at −1 and −0.8 V, respectively, and the maximum FE of C₂₊ oxygenates was ∼30%.²¹⁷ This tandem-catalysis concept has also been exploited to design and fabricate electrocatalysts such as Cu–Zn, Cu–Au and Cu–Ag bimetallic systems for CO₂RR to enhance the selectivity towards the formation of C₂H₅OH.^{149–152,214} Furthermore, porphyrin-based metallic complex-immobilised Cu catalysts were recently reported to enhance the formation of C₂H₅OH during CO₂RR via the tandem mechanism.¹⁵⁴ A FeTPP[Cl] (5,10,15,20-tetraphenyl-21H,23H-porphine iron(III) chloride)-modified Cu catalyst showed a maximum FE of C₂H₅OH of 41% with a partial current density of >100 mA cm⁻² at −0.82 V in a flow cell with a 1 M KHCO₃ electrolyte. It was confirmed using ¹³CO₂ that the carbon in C₂H₅OH originated from CO₂. DFT calculations and control experiments suggested that the molecular complex mainly functioned for the reduction of CO₂ to CO, generating a rich *CO environment at molecule-metal interface, increasing the coverage of *CO on Cu surfaces.¹⁵⁴ This lowers the energy barrier for C–C coupling and favours the formation of C₂H₅OH.

Fig. 21 Schematic illustration of the tandem-catalysis strategy for CO₂RR to C₂₊ products.²¹⁷ Reproduced from ref. 217 with the permissions of American Chemical Society, copyright 2019.

Carbon materials have also shown potential to modulate the products on Cu/carbon-catalysed CO₂RR.^{155–158,218} An early study reported that Cu nanoparticles loaded on a highly textured N-doped carbon nanospike film (CNS) could achieve high ethanol selectivity in electrocatalytic CO₂RR, whereas there was no ethanol formation over Cu/glassy carbon or bare CNS.¹⁵⁵ It was claimed that the Cu/CNS offered 63% FE of C₂H₅OH at −1.2 V versus RHE, whereas CO and CH₄ were the major CO₂RR products on CNS or Cu/glassy carbon catalysts (Fig. 22a). A synergistic effect between Cu and CNS was speculated to accelerate the formation of C₂H₅OH. Very recently, Sargent and co-workers developed an electrocatalyst composed of Cu nanoparticles surrounded by N-doped carbon layers (denoted as N–C/Cu), which displayed high FEs of C₂H₅OH at high reaction rates.¹⁵⁶ The CO₂RR in a flow cell with 1 M KOH electrolyte using the 34% N–C/Cu (the percentage of N in the N–C layer = 34%) catalyst offered a total C₂₊ FE of 93% and a C₂H₅OH FE of 52% at a current density of 300 mA cm⁻² at −0.68 V vs. RHE.¹⁵⁶ This C₂H₅OH FE was significantly higher than that on Cu alone and the use of a carbon-only layer to cover Cu did not improve the FE of C₂H₅OH as compared to Cu alone (Fig. 22b).¹⁵⁶ It is noteworthy that no C₂H₅OH was formed on the bare 34% N–C layer. These observations indicate the crucial role of the N–C layer in Cu surfaces in the formation of C₂H₅OH. The indispensability of nitrogen might imply that the electron-donating ability of the capping layer is a key. DFT calculations suggest that the confinement effect of Cu by an N–C layer may stabilise the C–O bond of *CH–CHOH, the intermediate for the formation of either C₂H₅OH or C₂H₄, thus facilitating the path toward C₂H₅OH and suppressing that toward C₂H₄.

Fig. 22 (a) FEs of CO₂RR products at different potentials on Cu/CNS, Cu/glassy carbon and CNS catalysts.¹⁵⁵ (b) FEs of C₂H₅OH and the ratios of FE_ethanol to FE_ethylene at 300 mA cm⁻² on different catalysts.¹⁵⁶

It is noteworthy that g-C₃N₄ cannot only be applied to photocatalytic CO₂ reduction,⁶³ but also holds the potential to tune the selectivity of C₂₊ products on a Cu catalyst during the electrocatalytic CO₂RR. From DFT calculations, Qiao and co-workers revealed that g-C₃N₄ could modify the electronic structure of Cu in a Cu–C₃N₄ composite by shifting its d-orbital position toward the Fermi level, thus leading to a stronger adsorption of intermediates.²¹⁸ In addition, the Cu–C₃N₄ composite shows an intramolecular synergistic effect with dual active centres (Cu and C) on electrocatalytic CO₂RR, different from the widely studied bifunctional catalysts composed of a metallic site and a support. Actually, the fabricated Cu–C₃N₄ catalyst was able to produce a wider variety of C₂ products (C₂H₄, C₂H₆ and C₂H₅OH) in electrocatalytic CO₂RR, whereas a Cu/N-doped graphene catalyst only provided C₁ products.²¹⁸

A comparison of CORR and CO₂RR on polycrystalline Cu has demonstrated that there is a large decrease in the overpotential for C–C coupling products during CORR.²⁰² Some studies have demonstrated that the electrocatalytic CORR on single Cu catalysts operated at a low overpotential can afford C₂H₅OH as the major product.^{163–167,219} whereas C₂H₄ is usually formed as the major product in CO₂RR on single Cu catalysts operated at higher overpotentials. For example, Kanan and co-workers found that an oxide-derived nanocrystalline Cu catalyst, which was fabricated by annealing polycrystalline Cu foil at 500 °C in air followed by electroreduction, offered C₂₊ oxygenates (C₂H₅OH, CH₃COO⁻ and C₃H₇OH) with an FE of 57% (C₂H₅OH FE: 43%) at −0.3 V versus RHE for electrocatalytic CORR.¹⁶⁵ On the other hand, a commercial Cu-nanoparticle catalyst provided H₂ as the major product under the same conditions. Characterisation of the oxide-derived Cu catalyst revealed particles with sizes of 30–100 nm and interconnected nanocrystalline networks with distinct grain boundaries between nanocrystallites. The oxide-derived Cu catalyst reduced by H₂ at 130 °C instead of electroreduction also showed nearly 50% FE towards C₂₊ oxygenates (C₂H₅OH and CH₃COO⁻) in electrocatalytic CORR.¹⁶⁵ However, the annealing of the oxide-derived Cu catalyst in N₂ at 350 °C resulted in significant decreases in surface-area corrected current density and FE of CO reduction (<5%) as well as a remarkable increase in grain sizes. It is thus proposed that the excellent C₂₊ oxygenates (mainly C₂H₅OH) selectivity on oxide-derived Cu is not a consequence of nanocrystalline size or morphology. Instead, the grain boundaries play a pivotal role in electrocatalytic CORR to C₂₊ oxygenates.^165,219

Kanan and co-workers further fabricated Cu/carbon nanotube (Cu/CNT) catalysts with different average grain-boundary densities by depositing Cu onto a film of superaligned CNTs using e-beam evaporation.¹⁶⁶ TEM characterisation showed that CNTs were decorated with Cu NPs, most of which consisted of multiple Cu crystallites connected by grain boundaries. The density of grain boundaries, defined as the sum of surface grain boundary lengths divided by the sum of NP surface areas, was evaluated by TEM, and the fabricated Cu/CNT had a grain boundary of 40.6 ± 4.3 μm⁻¹. The density of grain boundaries decreased monotonically by increasing the annealing temperature from 200 to 500 °C in N₂. The Cu/CNT catalyst showed an FE of 72% towards C₂H₅OH and CH₃COO⁻ with a current density of 0.37 mA cm⁻² at −0.3 V during the electrocatalytic CORR.¹⁶⁶ The catalyst after annealing showed increased FE of H₂ evolution at the expense of CO-reduction products. A strong relationship was found between the density of surface grain boundaries and the CO electrocatalytic activity.

Cu nanowires, which were fabricated by reducing CuO nanowires by H₂ at 150 °C, also showed higher selectivity of C₂₊ oxygenates during CORR.¹⁶⁷ The FE of C₂H₅OH reached 50% with 65% FE towards total CO reduction products at a current density of −0.36 mA cm⁻² at −0.22 V vs. RHE.¹⁶⁷ The H₂ reduction of CuO nanowires at higher temperatures led to significantly lower FE of CO reduction. Characterisation and DFT calculations suggested that the coordinately unsaturated (110) surface sites on Cu nanowires might be responsible for CORR to ethanol.¹⁶⁷ Jaramillo, Hahn and co-workers recently demonstrated that the roughness factor of a Cu electrode was a key to determining the product selectivity of CO reduction.¹⁶⁹ By comparing Cu catalysts with different morphologies, such as nanoflowers, nanodendrites, nanowires and nanorods, they found that the activity of CORR and selectivity of C₂₊ oxygenates increased almost linearly with an increase in the roughness factor. It is of interest that although the current density was quite low, the nanoflower Cu catalysts with the highest roughness factor could offer a nearly 100% selectivity to C₂ oxygenates (C₂H₅OH, CH₃CHO and CH₃COO⁻) without detectable H₂ evolution at an applied potential of only −0.23 V versus RHE. Acetaldehyde was the major product with an FE of ∼60% and the current density was ∼0.2 mA cm⁻² at this applied potential. The change in the applied potential to −0.33 V increased the current density to ∼0.6 mA cm⁻², and meanwhile C₂H₅OH became the major product with an FE of ∼60%. It was proposed that the porous mesostructure might contribute to suppressing H₂ evolution and enabling highly selective formation of C₂ oxygenates at a quite low overpotential.¹⁶⁹

Very recently, Jaramillo, Hahn and co-workers demonstrated that a Cu–Ag bimetallic electrocatalyst could achieve a high FE of CH₃CHO during electrocatalytic CORR in a 0.1 M KOH electrolyte.¹⁷⁰ The FE of CH₃CHO reached 50% and the carbon-based selectivity of CH₃CHO reached 90% at a potential of −0.536 V vs. RHE. The FE of CH₃CHO could be further enhanced to 70% by using a porous bimetallic Cu–Ag nanoflower electrocatalyst with an increased roughness factor. DFT calculations suggested that the Ag-adatoms on Cu weakened the binding energy of the reduced acetaldehyde intermediate, thus suppressing its further reduction to C₂H₅OH.

4.3. Electrocatalytic CORR into acetate over Cu-based catalysts

Acetate is a versatile chemical in the current chemical industry, which can be used for the production of vinyl acetate monomer, cellulose acetate, acetate esters and polyhydroxyalkanoate (PHA) or used as a solvent in many processes such as the production of polyethylene terephthalate (PET). Recent studies demonstrated that the electrocatalytic synthesis of acetic acid would also be a very promising route for the utilization of CO₂via CO.⁴³

As described above, although electrocatalytic CORR could offer high FEs of C₂ oxygenates at relatively low overpotentials, the current density in the conventional H-cell was typically very low (<10 mA cm⁻²) mainly due to the low solubility of CO.^{165–167,169,170} Recently, Kanan and co-workers designed a flow cell with GDE to circumvent the problem of low CO solubility.¹⁷¹ It is reported that the electrocatalytic CORR in the flow cell with a Cu catalyst and NaOH electrolyte could achieve not only high current density (>100 mA cm⁻²) and high C₂₊ FE but also high CO single-pass conversion. At a full cell potential of 2.4 V, the CO single-pass conversion reached 43% and the acetate concentration reached 1.1 M over 24 h. Meanwhile, the FE of acetate and current density were 30% and 144 mA cm⁻², respectively. It is noteworthy that in the work reported by Kanan and co-workers, very limited CO flow rate (1.0 mL min⁻¹) and high CO pressure (4 bar) were used.¹⁷¹

At the same time, Jiao and co-workers discovered that after switching the reactant from CO₂ to CO over an oxide-derived Cu catalyst in an alkaline flow cell, the selectivity of C₂₊ oxygenates especially for acetate increased significantly and the FE of acetate could reach ∼20% (Fig. 23a).¹⁶³ The C¹⁸O isotopic-labelling studies revealed that one oxygen of acetate originated from CO and the other oxygen originated from the electrolyte, probably from an OH⁻. The pH-gradient simulations for CO₂/CO reduction under various current densities clarified that the pH at electrode surfaces during CO reduction was much higher than that during CO₂ reduction, because of the carbonate formation through a fast chemical reaction between CO₂ and KOH. They thus attributed the high acetate selectivity in CORR to higher local pH at the electrode–electrolyte interface.

Fig. 23 (a) CO/CO₂ reduction on OD-Cu at 300 mA cm⁻² in 1 M KOH over 2 h.¹⁶³ (b) Acetate molar fraction excluding hydrogen for CO reduction over (111) Cu nanosheet, 25 nm Cu particle and 1 μm Cu nanoparticle catalysts.¹⁷² Reproduced from ref. 163 and 172, with the permission of Springer-Nature, copyright 2018 and 2019.

The same group further reported a freestanding triangle-shaped two-dimensional Cu-nanosheet catalyst, which exhibited an acetate FE of 48% with a current density of 200 mA cm⁻² at −0.736 V vs. RHE for electrocatalytic CORR in a flow cell with a 2 M KOH electrolyte.¹⁷² This value of FE is the highest one reported to date for electrocatalytic synthesis of acetate. The Cu nanosheet catalyst, which was fabricated by chemical reduction of Cu(II) nitrate with ascorbic acid in the presence of hexadecyltrimethylammonium bromide (CTAB) and hexamethylenetetramine (HMTA), selectively exposed the (111) surface. As compared to Cu nanoparticles with polycrystalline surfaces, the Cu-nanosheet catalyst exhibited a similar intrinsic activity towards acetate formation but much suppressed intrinsic activities towards the formations of ethylene and ethanol, thus leading to a higher selectivity of acetate (Fig. 23b). Previous studies indicated that (100) and (110) facets were responsible for the formations of C₂H₄ and C₂H₅OH, respectively.¹⁷⁸ Thus, the preferentially exposed (111) surface of the Cu-nanosheet catalyst was proposed to contribute to the high acetate selectivity.¹⁷² DFT calculations suggested that the formation of acetate may proceed through a ketene intermediate (CH₂CO) with the oxygen from a CO molecule, and the incorporation of another oxygen from H₂O (or OH⁻) resulted in the formation of acetic acid or acetate.

4.4. Electrocatalytic CORR into n-propanol over Cu-based catalysts

Higher alcohols, such as n-propanol, which have impressive volumetric energy densities (27 MJ L⁻¹ for n-C₃H₇OH) and excellent octane numbers (118 for n-C₃H₇OH), can be used as engine fuels. Typically, n-C₃H₇OH can be observed accompanied by C₂H₅OH and/or C₂H₄ in electrocatalytic CO₂RR or CORR on Cu catalysts under suitable conditions, but its selectivity is usually lower. For example, n-C₃H₇OH was formed with an FE of ∼5% in CO₂RR on a Cu catalyst composed of monodisperse Cu nanoparticles deposited on a carbon paper support.²²⁰ The Cu₂S@Cu-V catalyst with a Cu₂S core and surface vacancy-enriched Cu shell offered a n-C₃H₇OH FE of ∼7%.¹⁵³ A high FE (∼13%) of n-C₃H₇OH was once reported during electrocatalytic CO₂RR on a Cu mesh-supported oxide-derived Cu catalyst at a potential of −0.9 V vs. RHE.²²¹ In electrocatalytic CORR, the oxide-derived Cu catalyst provided an FE of n-C₃H₇OH of ∼10%.¹⁶⁵ It is expected that the key step in n-propanol formation may be the coupling between surface-adsorbed C1 and C₂ intermediates. However, the inadequate stabilization of C₂ intermediates on pristine Cu surfaces would result in desorption rather than further intermolecular coupling with C1 intermediates to form C₃ products. A few recent studies have contributed to developing strategies to design selective catalysts for the formation of n-C₃H₇OH.^173–175

The first strategy is the confinement effect to enhance the concentration of C₂ intermediates. Sargent, Sinton and co-workers fabricated open Cu-nanocavity catalysts with tuneable geometry by acidic etching of Cu₂O nanoparticles and found that suitable opening could significantly promote the formation of n-propanol during electrocatalytic CORR.¹⁷³ The FE of n-propanol reached 21% with a partial current density of 7.8 mA cm⁻² at −0.56 V versus RHE in an alkaline GDE, much better than solid and fragment Cu catalysts (Fig. 24a). Through finite element method (FEM) simulations, they found that the nanocavity geometry could concentrate C₂ species via steric confinement and reduce the desorption of C₂ species, thus promoting the coupling to form a C₃ product.¹⁷³ DFT calculations suggest that the coupling between C₂ species and CO is the most likely route for the formation of C₃ species. A lower C₂ surface coverage would reduce the likelihood that CO and C₂ species meet to form C₃ species, and thus the enhancement in the concentration of C₂ species is vital to increasing the C₃ selectivity.

Fig. 24 (a) FEs of C₂₊ products in CORR at −0.56 V versus RHE on Cu-based catalysts with morphologies of solid, cavity I, cavity II and fragment as well as the corresponding SEM images. Scale bars, 100 nm.¹⁷³ (b) FEs of C₂₊ products in CORR at −0.45 V versus RHE on high-fragment Cu (HF-Cu), medium-fragment Cu (MF-Cu) and low-fragment Cu (L-Cu) catalysts.¹⁷⁴ (c) FEs of C₂₊ products in CORR at different potentials on Cu adparticle catalysts.¹⁷⁵ (d) Maximum FEs of n-propanol and ratios of n-propanol FE to C₂₊ FE on Cu nanoparticle (NP), Cu nanobump (NB) and Cu adparticle (AD).¹⁷⁵ Reproduced from ref. 173–175, with the permission of Springer-Nature, copyright 2018 and 2019.

The second strategy is to create interfaces of (100) and (111) facets, which are selective towards C₂ and C1 species, respectively.¹⁷⁴ For this purpose, a highly fragmented (HF) Cu catalyst was fabricated by a slow hydrolysis and oxidation of CuI in aqueous solution to Cu₂O with a variety of crystalline phases, followed by in situ reduction on a GDE during CORR. TEM confirmed that the HF-Cu catalyst possessed Cu(100) and Cu(111) facets that were adjacent and comprised mostly fragments below 200 nm² (all fragments <700 nm²). The electrocatalytic CORR in a flow cell with GDE and 1 M KOH electrolyte showed that the HF-Cu catalyst exhibited a maximum FE of n-C₃H₇OH of 20.3% at −0.45 V vs. RHE (an overpotential of 0.55 V, iR corrected) (Fig. 24b).¹⁷⁴ The partial current density for n-C₃H₇OH formation was 8.5 mA cm⁻² at this electrode potential and the full cell electric power to chemical energy conversion efficiency was 10.8%. The control experiments with medium-fragmented (MF) and low-fragmented (LF) Cu catalysts further demonstrated that the interface between Cu(100) and Cu(111) played a critical role in the formation of n-C₃H₇OH (Fig. 24b).

The third strategy is to load Cu adparticles on Cu surfaces to enhance the adsorption of CO and binding energy of C₂ intermediates.¹⁷⁵ DFT calculations indicated that the presence of adatoms on pristine Cu surfaces, i.e., Cu(100), Cu(111) and Cu(211), could increase the CO adsorption. The adsorption of C₂ intermediates (*CCH₂ and *OCCOH) was stabilised on the Cu(111) surface in the presence of Cu adatoms. The reaction energies for *CO dimerization and the coupling between *CO and *CCH₂ or *OCCOH were lowered in the presence of Cu adatoms on the pristine Cu surfaces. Adparticle-covered Cu nanoparticle catalysts were fabricated by in situ electroreduction of a nanoparticulate copper oxide precursor deposited on a GDE under rich CO conditions, which enabled simultaneous rapid reduction of oxide and growth of adparticles. Electrocatalytic CORR in a flow cell with a 1 M KOH electrolyte showed a maximum FE of n-C₃H₇OH of 23% at −0.44 to −0.47 V vs. RHE (Fig. 24c). The partial current of n-C₃H₇OH reached 11 mA cm⁻² at −0.47 V vs. RHE. The control experiments using adparticle-free Cu nanoparticles and the catalyst by reduction of the nanoparticulate copper oxide under N₂ instead of CO, which contained nanobumps (NBs) on the surfaces, showed lower maximum FEs of n-C₃H₇OH (Fig. 24d). The partial pressure of CO played a key role in the formation of n-C₃H₇OH and it was found that the FE of n-C₃H₇OH increased at the expense of that of C₂H₄ upon increasing the CO partial pressure.¹⁷⁵ This further supports the hypothesis that n-C₃H₇OH is formed by coupling between CO and the C₂ intermediate, which may also be converted to C₂H₄.

4.5. Electrocatalytic CO₂RR and CORR into C₂₊ over catalysts other than Cu

High FEs towards C₂₊ compounds have made Cu a star catalyst for electrocatalytic CO₂RR and CORR. One of the major reasons for the high C₂₊ FEs of Cu-based catalysts is believed to be the appropriate CO-binding ability. If a metal binds CO too weakly, CO would desorb rapidly as a final product, whereas a too strong CO adsorption of a metal would increase the difficulty for the further conversion of surface CO species. The CO₂RR to C₂₊ products on Cu usually require potentials more negative than or close to −1.0 V to achieve high current densities. The search for other electrocatalytic materials other than Cu might enable the CO₂RR to C₂₊ products to proceed at lower overpotential, thus resulting in alternative lower-energy pathways.

Several bimetallic catalysts that do not contain Cu have shown potential to offer C₂₊ products during CO₂RR. Kortlever et al. demonstrated that a Pd–Au bimetallic catalyst fabricated by electrodeposition of Pd, which could bind CO strongly, on Au without strong binding to CO afforded C₁–C₅ hydrocarbons and C₂₊ oxygenates including ethanol and acetate besides HCOOH and CH₃OH.²²² The onset potential for the formation of hydrocarbons was −0.8 V vs. RHE. Electrocatalytic CORR to C₁–C₅ hydrocarbons could also take place on the Pd–Au catalyst with a slightly lower onset potential of −0.6 V, suggesting that CO was a key intermediate. The active sites were proposed to be Pd-rich Au–Pd alloy thin layer on the catalyst surface. The FE of C₂₊ products was <5%. Several Ni-based bimetallic catalysts have also been investigated for electrocatalytic CO₂RR to C₂₊ compounds.^223–225 The Ni–Ga intermetallic compound is an interesting catalyst, since it has similar oxygen adsorption energy to Cu and has shown comparable or even better performance as compared to the well-known Cu–Zn–Al catalyst in the hydrogenation of CO₂ to CH₃OH.²²³ In electrocatalytic CO₂RR, the Ni–Ga intermetallic catalyst could work for the formation of C₂₊ hydrocarbons, mainly C₂H₄ and C₂H₆, in addition to CH₄.²²⁴ The onset potential for the formation of C₂H₄ and C₂H₆ on the Ni–Ga was −0.48 V vs. RHE, even more positive than that reported for Cu catalysts. The Ni₃Al intermetallic compound was reported to show the formation of C₃ oxygenates, including n-C₃H₇OH and acetone, during the electrocatalytic CO₂RR.²²⁵ The FEs of C₂₊ products using these bimetallic electrocatalysts that do not contain Cu are still quite low (<5%).

Nitrogen-doped carbon-based materials have been reported to be candidates for metal-free electrocatalytic CO₂RR to C₂₊ products, in particular C₂ oxygenates.^{159–162,226–228} Quan and co-workers reported that a nitrogen-doped nanodiamond (NDD) catalyst deposited on a Si rod array could work for the electrocatalytic CO₂RR to CH₃COO⁻ and HCOO⁻ in a NaHCO₃ electrolyte.¹⁵⁹ The total FE toward CO₂ reduction was 91.8% with a current density of ∼7 mA cm⁻² at −1.0 V vs. RHE. The FE of CH₃COO⁻ reached ∼78% at the same time. The high overpotential for H₂ evolution on the NDD catalyst was proposed to favour the FE of CO₂ reduction. In situ FT-IR studies suggested the appearance of OOC–COO species, which might be a reaction intermediate. Further studies showed that the presence of nitrogen with suitable content (≥∼2 at%) and its chemical state played crucial roles. The N–sp³C was found to be more active than the N–sp²C. It was speculated that the defect sites and the polarised carbon atoms adjacent to nitrogen might stabilise the CO₂˙⁻ through electronic interactions. The coupling between CO₂˙⁻ was proposed to account for the formation of CH₃COO⁻.¹⁵⁹

A boron- and nitrogen-co-doped nanodiamond (BND) catalyst deposited on a Si substrate was found to be very selective for the formation of C₂H₅OH during the electrocatalytic CO₂RR in H-cell with a 0.1 M NaHCO₃ electrolyte.¹⁶⁰ A surprisingly high C₂H₅OH FE of 93.2% was claimed on a BND catalyst with 2.5 at% boron and 4.9 at% nitrogen at −1.0 V versus RHE in spite of the low current density (∼−1 mA cm⁻²). The increase in nitrogen content in BND catalysts from 3.1 at% to 3.6 at% and further to 4.9 at% (from BND1 to BND2 and further to BND3) while keeping the boron content at 2.4–2.5 at% increased the FE of C₂H₅OH (Fig. 25a). It is of interest that the nitrogen-doped nanodiamond (NDD) mainly provided CH₃COO⁻ and HCOO⁻, whereas HCHO and HCOO⁻ were the major products on the boron-doped nanodiamond (BDD) catalyst (Fig. 25a). Thus, the doping of nitrogen may be the key to C–C coupling. DFT calculations on the (111) facet of BND suggested that the CO₂RR might proceed via the following intermediates: CO₂ → *COOH → *CO → *COCO → *COCOH → *COCHOH → *COCH₂OH → *CHOCH₂OH → *CH₂OCH₂OH → CH₃CH₂OH.¹⁶⁰ The doped nitrogen and boron would work synergistically for the formation of C₂H₅OH.

Fig. 25 (a) FEs for CO₂ reduction on NDD, BDD, BND1, BND2 and BND3 at −1.0 V versus RHE.¹⁶⁰ (b) FEs for CO₂ reduction on cylindrical mesoporous carbon (c-NC) and inverse mesoporous carbon (i-NC) catalysts at various applied potentials.²²⁶ (c) FEs for CO₂ reduction at various applied potentials for NGQDs.¹⁶² Reproduced from ref. 160, 226 and 162, with the permission of Wiley-VCH Verlag GmbH&Co. KGaA and Springer-Nature, copyright 2017 and 2016.

Chen, Sun and co-workers demonstrated that a N-doped ordered cylindrical mesoporous carbon (denoted as c-NC) catalyst showed high selectivity toward C₂H₅OH during CO₂RR in a 0.1 M KHCO₃ electrolyte.²²⁶ The FE of C₂H₅OH reached 77% with a current density of ∼0.2 mA cm⁻² at −0.56 V vs. RHE (Fig. 25b). The presence of nitrogen was also confirmed to play key roles in CO₂ reduction and C–C coupling, since H₂ evolution was the major reaction on the ordered cylindrical mesoporous carbon without nitrogen. The cylindrical mesoporous structure was also important, and the reference N-doped inverse mesoporous carbon only showed a much lower FE of C₂H₅OH (∼44%) at −0.50 V vs. RHE and CO was formed as a major product. DFT calculations suggested that the pure carbon or graphitic N sites were highly unfavourable for the formation of *CO, which was the key intermediate for C₂ formation, whereas both pyridinic and pyrrolic N sites could work for the *CO formation.²²⁶ The formation of *CO proceeded preferentially on the pyridinic N site. The ordered channel surface with high electron density might stabilise *CO, thus favouring the coupling between *CO, and might also facilitate the subsequent multiple electron and proton transfer to form C₂H₅OH. A recent work from the same group demonstrated that the pore-structure engineering by constructing hierarchical micro-/mesoporous N-doped carbon materials could further enhance the formation of C₂H₅OH.¹⁶¹ The creation of medium micropores (∼0.52 nm) in the channel walls of the N-doped ordered mesoporous carbon resulted in an FE of C₂H₅OH of 78% with a current density of ∼2 mA cm⁻² at −0.56 V vs. RHE during CO₂RR in H-cell with a 0.1 M KHCO₃ electrolyte. The formation rate of C₂H₅OH reached 2.3 mmol g⁻¹ h⁻¹ at −0.8 V vs. RHE, which was one order of magnitude higher than that on the corresponding catalyst without micropores. Further experiments and DFT calculations suggested that the microporous structure with the active pyridinic and pyrrolic N sites might lead to fast kinetics of charge transfer and high electric driving potentials to enhance the formation of ethanol.¹⁶¹

N-Doped graphene quantum dots (NGQDs) were also found to be efficient for the electrocatalytic CO₂RR into C₂₊ products.¹⁶² C₂H₄, C₂H₅OH, CH₃COO⁻ and n-C₃H₇OH were all formed in CO₂RR in a flow cell with DGE and 1 M KOH electrolytes. The maximum FE of C₂H₄ was 31% at −0.75 V versus RHE, and the total FE of C₂₊ products was 55% at the same time (Fig. 25c).¹⁶² The maximum FE of C₂₊ oxygenates was 26% at −0.78 V versus RHE with 16% FE of C₂H₅OH. The flow cell with DGE also resulted in high current density, and the partial current densities of CO, C₂H₄ and C₂H₅OH were 23, 46 and 21 mA cm⁻² at −0.86 V vs. RHE. On the other hand, CO and HCOO⁻ were the major products on the graphene quantum dots without nitrogen or the nitrogen-doped reduced graphene at similar applied potentials. Thus, it was confirmed again that the nitrogen sites played crucial roles in the formation of C₂₊ products. Further, the result indicated that the pyridinic N at edge sites were more efficient for C–C coupling than the pyridinic N at basal planes.

Recently, some studies have demonstrated that the addition of a catalyst component that can accelerate the CO₂RR to CO and the N-doped carbon can enhance the formation of C₂ oxygenates (Fig. 26).^229–231 For example, the immobilisation of a molecular catalyst, i.e., Ru(II) polypyridyl carbine complex (RuPC), which was efficient for electrocatalytic CO₂RR to CO, onto N-doped porous carbon (NPC) was found to show synergistic effects in the formation of C₂₊ products.²²⁹ The NPC alone could catalyse the electrocatalytic CO₂RR in H-cell with a 0.5 M KHCO₃ electrolyte, forming C₂H₅OH, CH₃COO⁻, methanol, HCOO⁻ and CO. The immobilisation of RuPC onto NPC significantly enhanced the formation of C₂H₅OH. At −0.97 V vs. NHE, the FEs of C₂H₅OH and CH₃COO⁻ were 27.5% and ∼10%, respectively, on the RuPC/NPS, whereas they were ∼15% and 10% on NPC. It was speculated that the RuPC enhanced the electrocatalytic reduction of CO₂ to CO and the increased coverage of *CO species on NPC or at the interface between RuPC and NPC would facilitate the C–C coupling to form C₂H₅OH. The loading of Ag nanoparticles, which are known to catalyse the CO₂RR to CO, onto a 3D graphene-wrapped nitrogen doped carbon foam (G–NCF) has also been found to enhance C₂H₅OH formation during electrocatalytic CO₂RR in a H-cell with 0.1 M KHCO₃ electrolyte.²³⁰ The FE of C₂H₅OH reached 85.2% at −0.6 V vs. RHE despite the low current density (<0.5 mA cm⁻²). The anchoring of CoO onto N-doped carbon materials composed of mesoporous carbon and carbon nanotube resulted in an efficient catalyst (Co/MC–CNT) for electrocatalytic CO₂RR to ethanol.²³¹ The maximum FE of C₂H₅OH on this catalyst was 60.1% at −0.32 V vs. RHE during the electrocatalytic CO₂RR in H-cell with a 0.5 M KHCO₃ electrolyte. CH₃CHO, probably the precursor of C₂H₅OH, and H₂ were observed as the only by-products under such conditions. The FE of CH₃CHO was 10% and the partial current of (C₂H₅OH + CH₃CHO) was 5.1 mA cm⁻² at −0.32 V vs. RHE. Further studies indicated that CoO facilitated the formation of *CO intermediate that might undergo spill-over to MC–CNT, where the C–C coupling took place.²³¹ The pyridinic N and pyrrolic N on MC–CNT as well as the mesoporous structure may stabilise *CO species and promote C–C coupling.

Fig. 26 Simplified mechanism on bifunctional catalyst for CO₂RR to ethanol by integrating CO synthesis and C–C coupling.

4.6. Mechanism of C–C coupling for electrocatalytic CO₂RR and CORR to C₂₊ compounds

The most important step for the synthesis of C₂₊ hydrocarbons or oxygenates from CO₂ or CO is the C–C coupling. To understand how the C–C coupling occurs and how to control the C–C coupling as well as the formation of different products would be very helpful for rational design of highly selective catalysts for the formation of C₂₊ compounds.

The well-known catalytic system involving the coupling of C1 molecules is Fischer–Tropsch synthesis, i.e., the hydrogenation of CO to C₂₊ hydrocarbons, which typically proceeds on Fe-, Co- or Ru-based heterogeneous catalysts at high temperatures and pressures.⁷ The hydrogenation of CO₂ to C₂₊ hydrocarbons on related heterogeneous catalysts usually proceeds via CO intermediate, i.e., the reverse water-gas shift reaction (H₂ + CO₂ → CO + H₂O) followed Fischer–Tropsch synthesis. The mechanism for Fischer–Tropsch synthesis is relatively mature. Take the formation of C₂H₄ as an example, it is accepted that the reaction proceeds through the following key elementary steps on catalyst surfaces: (1) adsorption and dissociation of CO and H₂; (2) formation of surface CH_x species; (3) C–C coupling of CH_x species; (4) hydrogenation or dehydrogenation of C₂H_x into C₂H₄ (Fig. 27a).⁷ However, the selective formation of C₂H₄ on the Fischer–Tropsch catalyst is very difficult because the C–C coupling is generally uncontrollable. Actually, the polymerisation of CH_x species to form C_nH_m with different carbon numbers, also known as chain growth, can occur on the Fischer–Tropsch catalyst surfaces. Thus, the selectivity of products generally follows the Anderson–Schulz–Flory distribution, which is determined by the rate of chain growth and that of chain termination.⁷ According to the ideal ASF model, the maximum selectivity of C₂–C₄ (including olefins and paraffins) is only 58%. For the formation of ethanol by hydrogenation of CO over a heterogeneous catalyst, it is generally believed that both dissociated and non-dissociated CO species should exist on the catalyst surface.²⁰⁹ The formation of C₂H₅OH requires the coupling of CH_x species and *CO species, and the formed CH_xCO intermediate undergoes further hydrogenation to ethanol (Fig. 27b).

Fig. 27 (a) Mechanism for the conversion of syngas to C₂H₄ in Fischer–Tropsch synthesis. (b) Mechanism for the conversion of syngas to C₂H₅OH on thermocatalysts. (c) Three possible C–C coupling mechanisms in electrocatalytic CO₂RR. (d) Two possible mechanisms for the formation of C₂H₄, C₂H₅OH and n-C₃H₇OH.

A few studies have claimed that similar to the thermocatalysis, the C–C coupling may take place between surface *CH₂ species to form C₂H₄ on Cu nanoparticles during electrocatalytic CO₂RR, and meanwhile the coupling between *CH₂ and *CO or the insertion of CO contributes to the formation of C₂H₅OH or CH₃CHO.^199,201 However, most studies have pointed out that the electrocatalytic CO₂RR or CORR under mild conditions (typically ambient temperature and pressure) would proceed in different mechanisms.^232–238 The current consensus is that CO is the key intermediate in electroreduction CO₂RR. The direct dimerization of CO₂ or activated CO₂ is very rare.¹⁵⁹ Thus, the electrocatalytic CO₂RR to C₂₊ products proceeds via the same pathway as that for CORR after the reduction of CO₂ to CO.

Several possibilities have been proposed for C–C coupling of CO or CO-derived intermediates. The direct coupling of *CO to form *OCCO species followed by hydrogenation to C₂H₄ and C₂H₅OH has been proposed in most of the studies reported to date (Fig. 27c).^232–234 The pathway of direct *CO dimerization has mainly been supported by the DFT calculations. Recently, Calle-Vallejo, Koper and co-workers observed the appearance of adsorbed species with vibrational bands at 1191 and 1584 cm⁻¹ in in situ FT-IR spectroscopic studies during CO reduction on the Cu(100) surface in a LiOH electrolyte, which were assignable to the C–O–H and C=O stretching vibrations of *OCCOH species based on DFT calculations.²³⁵ The *OCCOH species was a hydrogenated CO dimer intermediate, and thus this provided evidence for the direct dimerization of *CO species. It is of interest that the *OCCOH species could not be observed on the Cu(111) surface, confirming that the coupling of CO is a structure-sensitive reaction. This result also agrees well with the fact that the Cu(100) with square symmetry is more selective for the formation of C₂₊ products.^178,180

On the other hand, some studies have demonstrated the importance and participation of *CHO species in the C–C coupling step (Fig. 27c).^{142,236,237,239} Nørskov and co-workers studied the kinetic barriers to the formation of a C–C bond between adsorbates on the Cu(211) surface derived from CO using DFT calculations.²³⁶ The result demonstrated that the kinetic barriers for C–C coupling decreased significantly with the degree of hydrogenation of reacting adsorbates. It was confirmed that this trend was not affected by the electric field present during the solid–electrolyte interface. The DFT calculations offered energy barriers of 1.619, 0.675, 0.564, 0.364 and 0.203 eV for the coupling steps of (*CO + *CO), (*CO + *CHO), (*CHO + *CHO), (*CHO + *CH₂O) and (*CH₂O + *CH₂O), respectively.²³⁶ Generally, the coupling reaction with a kinetic barrier of <0.7 eV can proceed facilely at ambient temperature. Thus, these coupling steps may occur except for the direct dimerization of *CO. It is believed that the unique feature of the electrochemical environment lies in that the chemical potential of hydrogen (electrons and protons) can be tuned by the applied potential. The computational work by Goddard III and co-workers tended to suggest that the *CHO species can be an intermediate for the formation of both CH₄ and C₂H₄.²³⁷ Bell, Head-Gorden and co-workers performed DFT calculations using an implicit electrolyte model, which enables a better simulation of the effect of an applied potential by varying on the surface to match the Fermi level to the target potential of electrode electrons, for CO₂RR on Cu(100) and Cu(111) surfaces.²³⁹ These results strongly suggest that the coupling between *CO and *CHO plays a pivotal role in the formation of C₂ compounds and *COCHO is the common intermediate for the formation of C₂H₄ and C₂H₅OH.²³⁹ Our recent work demonstrated that the coupling of (*CHO + *CHO) could also play a key role in the formation of C₂ compounds during CO₂RR on a fluoride-modified Cu catalyst, which exhibited excellent FEs and formation rates of C₂₊ compounds (C₂H₄ and C₂H₅OH).¹⁴²In situ electrochemical attenuated total reflection Fourier-transform infrared spectroscopy (ATR-FTIRS) measurements provided evidence for the generation of *CHO species on the fluoride-modified Cu surface, whereas the *CHO could not be detected on the Cu surface without fluoride modification probably due to the very low concentration.¹⁴²

Two possible mechanisms have been proposed for the formation of C₂₊ compounds including C₂H₄, C₂H₅OH and C₃H₇OH (Fig. 27d).^{127,237,239–241} Koper, Bell and co-workers proposed that after a series of protonation and electron transfer steps, the *OCCOH or *OCHCHO* species is converted to *CH₂CHO, which serves as the selectivity-determining intermediate to C₂₊ products.^{213,239–241} The hydrogenolysis of *CH₂CHO intermediate directs a pathway to C₂H₄, while the hydrogenation of *CH₂CHO results in *CH₃CHO and consequently C₂H₅OH. DFT calculations revealed that the barrier for C₂H₅OH formation via *CH₃CHO was ∼0.2 eV higher than that for C₂H₄ formation via *CH₂CHO.²¹³ This may explain why higher C₂H₄ selectivity over C₂H₅OH is usually observed on Cu-based catalysts. Further, n-C₃H₇OH can be formed through a (*CH₃CHO + *CO) coupling step. Alternatively, Goddard III and co-workers suggested that the selectivity-determining intermediate may be formed earlier for the formation of different C₂₊ products.²³⁷ They proposed that the hydrogenation of *OCCOH gave *CHCOH intermediate, which was the selectivity-determining intermediate. After the formation of *CHCOH, the dehydroxylation of *CHCOH to *CCH leads to C₂H₄, whereas the hydrogenation of *CHCOH to *CHCHOH leads to C₂H₅OH.

5. Photocatalytic and electrocatalytic conversions of CH₄

Methane is the major component of natural gas, shale gas, biogas and methane hydrate, and has abundant reserves and vast availability. Nowadays, CH₄ has become a major source for energy and fuels owing to its cleaner feature as compared with other fossil resources, but the chemical use of CH₄ is still insufficient.^20,242,243 The activation and selective conversion of CH₄ is one of the most challenging goals and is regarded a “holy grail” in chemistry because of the much higher stability of CH₄ as compared to a potential target product (such as CH₃OH and C₂H₄).²⁴⁴ In the current chemical industry, the transformation of CH₄ is performed indirectly via syngas (CO/H₂), i.e., the steam reforming of CH₄ to syngas followed by conversion of syngas to CH₃OH or C₂₊ hydrocarbons by CH₃OH synthesis or Fischer–Tropsch synthesis. The steam reforming of CH₄ is a high-cost and energy-intensive process. Although a lot of efforts have been devoted to transforming CH₄ directly into chemicals or liquid fuels via thermocatalysis, the selectivity of the target product is generally low at a reasonably high CH₄ conversion, because the reactive target product can undergo consecutive dehydrogenation or oxidation to carbon or CO₂ under severe conditions. Photocatalysis and electrocatalysis may provide new opportunities for the transformation of CH₄ to chemicals and fuels under mild conditions by introducing photo or electrical energy. Several review articles have been published for photocatalytic and electrocatalytic conversions of CH₄,^{55,56,245–248} but those are devoted to the photocatalytic or electrocatalytic conversion of CH₄ to C₂₊ compounds are still quite limited. Here, we focus on recent advances in the transformation of CH₄ to C₂₊ compounds via photocatalysis or electrocatalysis.

5.1. Photocatalytic conversion of CH₄ to C₂₊ compounds

Photocatalytic conversion of CH₄ alone by a non-oxidative dehydrogenation process mainly offered C₂₊ hydrocarbons. In the presence of H₂O, not only C₂₊ hydrocarbons but also C₂₊ oxygenates could be formed in CH₄ photocatalytic conversions. Some typical systems for photocatalytic coupling of CH₄ in the absence and presence of H₂O are summarized in Table 5.^249–265

Table 5 Typical photocatalytic systems for conversion of CH₄ to C₂₊ compounds

Photocatalyst	Formation rate (μmol g^−1 h^−1)	Quantum efficiency (QE)	Light source	Ref.
Photocatalytic coupling of CH4 alone
SiO2–Al2O3	C2H4, 0.01; C2H6, 0.10; C3H6, 0.005; C3H8, 0.02; C4H8, 0.003; C4H10, 0.002	—	Xe lamp	249
SiO2–Al2O3–TiO2	C2H6, 0.69; C3H8, 0.056	—	Xe lamp (220–300 nm)	250
FSM-16	C2H6, 0.09; C3H8, 0.003	—	Xe lamp (220–300 nm)	251
MgO/SiO2	C2H6, 0.046; C3H8, 0.0008	—	Xe lamp (220–300 nm)	252
GaxO/SiO2	C2H4, 0.005; C2H6, 0.11; C3H6, 0.002; C3H8, 0.002	0.01% (220–270 nm)	Xe lamp (220–300 nm)	253
CexO/Al2O3	C2H4, 0.053; C2H6, 0.24; C3H6, 0.038; C3H8, 0.0049	—	Xe lamp (220–300 nm)	254
Zn^2+-ZSM-5	C2H6, 3.0	0.55% (300–400 nm)	High-pressure Hg lamp	255
Ga^3+-ETS-10	C2H4, 0.61; C2H6, 10.89; C3, 1.18; C4, 0.74	—	High-pressure Hg lamp	256
Au/ZnO	C2H6, 11.5	0.08% (320–2500 nm)	Xe lamp (320–2500 nm)	257
Si-Doped GaN	Benzene, 55.6; toluene, 0.5; ethane, 3.3; ethylene, 0.5	0.72% (290–380 nm)	Xe lamp (290–380 nm)	258
Ag–HPW–TiO2	C2H6, 23; C3H8, 1.2	3.5% (362 nm)	Hg–Xe lamp (280–400 nm)	259
Photocatalytic coupling of CH4 in the presence of H2O
BiVO4	CH3OH, 21; C2H6, 2	—	Medium-pressure mercury lamp (>185 nm)	260
Beta zeolite	CH3OH, 242; HCHO, 117; HCOOH, 122	—	Deep UV irradiation (<200 nm)	261
BiVO4/V2O5/beta zeolite	CH3OH, 10.7; C2H6, 2.46	—	Medium-pressure Hg lamp	262
Pt–TiO2	C2H6, 50.6	3.3% (254 nm)	UV lamp (254 nm)	263
Pd–TiO2	C2H6, 54.7	2.76% (254 nm)	UV lamp (254 nm)	264
Cu-0.5/PCN	CH3OH, 24.5; C2H5OH, 106	—	Xe lamp	265

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5.1.1. Photocatalytic coupling of pure CH₄. The non-oxidative coupling of CH₄ to form C₂₊ hydrocarbons and H₂ is an attractive route for CH₄ utilisation. However, the dehydrogenation of CH₄ to C₂₊ compounds, such as C₂H₆ and benzene (eqn (1) and (2)), is generally thermodynamically unfavourable under mild conditions, and high temperatures are required to promote the equilibrium CH₄ conversion to an acceptable level by thermocatalysis. High temperatures may unavoidably lead to coke deposition. Under such circumstances, the photocatalytic conversion of CH₄ driven by solar energy is a promising route to overcome these problems.

2CH₄ → C₂H₆ + H₂, ΔG_(298K) = 68.6 kJ mol⁻¹ (1)

6CH₄ → C₆H₆ + 9H₂, ΔG_(298K) = 434 kJ mol⁻¹ (2)

A series of studies by Yoshida and co-workers showed that porous SiO₂ or metal oxide-loaded SiO₂ or Al₂O₃ (e.g., MgO/SiO₂, GaO_x/SiO₂, and CeO_x/Al₂O₃) could catalyse the dehydrogenative coupling of CH₄ to C₂H₆ and H₂ under UV light irradiation at mild temperatures.^247–254 A SiO₂–Al₂O₃ composite evacuated at 800 °C was first reported to be active for non-oxidative coupling of CH₄ to C₂₊ hydrocarbons (including C₂H₆, C₂H₄, C₃H₈, C₃H₆, C₄H₁₀ and minor C_5–6) with C₂H₆ as the major product under UV-vis light irradiation using a 250 W Xe lamp at 37 °C.²⁴⁹ A later study found that the ternary composite of SiO₂–Al₂O₃–TiO₂ (Al 10 mol%, Ti 0.5 mol%) after 800 °C evacuation showed higher performance, offering a C₂H₆ yield of ∼2% on a molar carbon basis during the conversion of 200 μmol CH₄ with a 1.0 g catalyst in a closed system for 3 h under UV-vis light irradiation using a 250 W Xe lamp at 37 °C.²⁵⁰ The pair site composed of dispersed AlO₄ and TiO₄ in the silica matrix with Ti⁴⁺–O²⁻–Al³⁺ might function as the active site. Nevertheless, pure SiO₂, in particular mesoporous silica such as MCM-41 or FSM-16 could also work for this reaction in spite of lower activity.²⁵¹ The photocatalytic sites on pure SiO₂ were proposed to be surface defect sites such as non-bridging oxygen hole sites (≡Si–O˙), which were generated during pretreatment at a high temperature (e.g., 800 °C) to remove surface hydroxyl groups. The photoexcitation of this site was believed to be the first step, followed by CH₄ activation to CH₃˙ and H˙ and the coupling of radical intermediates into C₂H₆ and H₂.²⁵¹ The mesoporous silica with a thinner silica wall such as FSM-16 showed better performance than that with a thicker wall (MCM-41) and amorphous SiO₂. The excitation occurred at 258 nm and thus only a part of UV light could be harnessed for this photochemical process. The highest yield of C₂₊ hydrocarbons (∼91% C₂H₆ with 8.8% C₃H₈ and minor C₂H₄) on FSM-16 (0.20 g) after 3 h of the reaction for 200 μmol CH₄ under UV-vis irradiation was ∼0.1%.²⁵¹ The loading of MgO improved the performance of amorphous silica.²⁵² Ga₂O₃ alone exhibited much higher yield of C₂H₆ during the photocatalytic conversion of CH₄, but the formation of H₂ was very low possibly due to the occurrence of other side-reactions such as coke deposition.²⁵³ The dispersion of Ga³⁺ on SiO₂ (0.1 mol% Ga/SiO₂) could smoothly catalyse the dehydrogenation of CH₄ to C₂H₆ and H₂ with enhanced product formation. The yield of C₂H₆ reached 0.14% after 3 h irradiation under UV-vis light, which was ∼35 times higher than the pure amorphous silica.²⁵³ The bulk Ga₂O₃ could work as a semiconductor photocatalyst and thus showed high activity, but side-reactions would also occur. On the other hand, the highly dispersed tetrahedral Ga³⁺ sites on SiO₂ surfaces may function as another type of photoactive sites and the photoexcitation may take place on localised isolated sites, thus leading to the selective dehydrogenation of CH₄ to C₂H₆ and H₂.²⁵³ Similarly, the highly dispersed Ce³⁺ sites on SiO₂ or Al₂O₃ surfaces could also accelerate the non-oxidative dehydrogenation coupling of CH₄ to C₂H₆ and H₂.²⁵⁴

A Zn⁺-modified ZSM-5 catalyst was found to show significantly higher activity for non-oxidative coupling of CH₄ to C₂H₆ and H₂ than SiO₂- or Al₂O₃-based catalysts.²⁵⁵ The Zn²⁺-ZSM-5⁻ sample, which was fabricated by a solid-vapour reaction between dehydrated H-ZSM-5 and metallic Zn vapour, contained delocalised electrons on the zeolite framework. The irradiation of Zn²⁺-ZSM-5⁻ under UV light from a 150 W high-pressure Hg lamp led to the generation of Zn⁺ as confirmed by ESR. The obtained (Zn⁺,Zn²⁺)-ZSM-5⁻ catalyst could work for the non-oxidative coupling of CH₄ to C₂H₆ and H₂ under either UV or sunlight irradiation. The formation rate of C₂H₆ was 9.8 μmol h⁻¹ g⁻¹ under UV-light irradiation from a high-pressure Hg lamp. Almost an equimolar amount of H₂ was formed and the selectivity of C₂H₆ was close to 100%. The conversion of 200 μmol CH₄ with a 1.0 g (Zn⁺,Zn²⁺)-ZSM-5⁻ catalyst offered a 17.5% C₂H₆ yield in 24 h UV-light irradiation, and the quantum efficiency was around 0.55%.²⁵⁵ The highest CH₄ conversion reached 23.8% with 99% C₂H₆ selectivity under the optimised conditions. Further studies suggested a two-stage mechanism (Fig. 28a): (1) the generation of (Zn⁺,Zn²⁺)-ZSM-5⁻ in the first stage that required irradiation by UV light of 278–390 nm; (2) the dehydrogenative coupling of CH₄ by the (Zn⁺,Zn²⁺)-ZSM-5⁻ catalyst that could work under visible light of <700 nm.²⁵⁵

Fig. 28 (a) Schematic energy diagram for the processes of the photocatalytic conversion of CH₄ to C₂H₆ over Zn²⁺-ZSM-5⁻ (inset: optimized geometry of the adsorbed CH₄ on Zn⁺ active site; red: O, blue: Si, pink: Al, gray: C, white: H, and green: the 4s electron of Zn⁺).²⁵⁵ (b) Photocatalytic performance for the conversion of CH₄ over different cation-modified ETS-10 catalysts.²⁵⁶ (c) Proposed reaction mechanism for the photocatalytic conversion of CH₄ over Ga³⁺-ETS-10.²⁵⁶ Reproduced from ref. 255 and 256, with permission of Wiley-VCH Verlag GmbH&Co. KGaA, copyright 2011 and 2012.

A latter study from the same group found that the Ga³⁺-modified zeolite ETS-10, which was a microporous titanosilicate with a framework containing one-dimensional O–Ti–O–Ti–O semiconducting nanowires (diameter, 0.67 nm) insulated from each other by the surrounding SiO₂ matrix, could work for photocatalytic dehydrogenative coupling of CH₄, offering a CH₄ conversion rate of 29.8 μmol g⁻¹ h⁻¹, which was about 3 times higher than that of (Zn⁺,Zn²⁺)-ZSM-5⁻.²⁵⁶ The ETS-10 samples containing exchanged cations of Ga³⁺, Al³⁺, Zn²⁺ and Fe³⁺ were all active, and the Ga³⁺-ETS-10 catalyst was the most efficient for photocatalytic conversion of CH₄ (Fig. 28b). During the conversion of 200 μmol CH₄ with a 0.2 g catalyst in a closed system, CH₄ conversion reached 14.9% with C₂H₆ selectivity of 70% after 5 h irradiation under UV light from a high-pressure Hg lamp.²⁵⁶ Other products included C₂H₄, C₃H₈, C₃H₆, C₄H₁₀ and C₄H₈. The catalyst was also efficient for photocatalytic conversion of C₂H₆, offering C₄H₁₀ with a selectivity of 57% at C₂H₆ conversion of 11% after 5 h of reaction. Ga³⁺ was found to be a key to the reaction, but the Ga³⁺-ZSM-5 and Ga³⁺-Y catalyst only showed very poor performances for photocatalytic conversion of CH₄. Therefore, the Ga³⁺ ions and TiO₂-wire units in the ETS-10 framework both played crucial roles in photocatalytic conversion of CH₄. The in-depth studies by Chen and co-workers indicated a dual active site mechanism (Fig. 28c).²⁵⁶ In other words, the extraframework metal cation (Ga³⁺) and the Ti⁴⁺–OH group on the titanate wire interacted synergistically under UV-light irradiation, leading to the cleavage of a C–H bond in CH₄. Probably, CH₄ was adsorbed and polarized through the interaction between Ga³⁺ and a methyl group to weaken the C–H bond, and then the hydrogen abstraction from CH₄ took place by the ˙OH radical in the vicinity generated by photoexcitation of the surface Ti–OH species. C₂H₆ is formed through the C–C coupling of two CH₃ surface species or radicals.

Semiconductor-based photocatalysts have also been examined for non-oxidative photocatalytic conversion of CH₄ to C₂₊ compounds.^257,258,266 Long and co-workers fabricated a plasmonic photocatalyst consisting of Au nanoparticles and ZnO nanosheets with exposed (001) facets.²⁵⁷ C₂H₆ and H₂ with formation rates of about 11 and 10 μmol g⁻¹ h⁻¹ could be achieved on the optimized Au/ZnO catalyst under simulated solar-light irradiation, and the solar-to-C₂H₆ energy conversion efficiency was 0.08%. It was proposed that the strong polarisation of the (001) facet of ZnO nanosheets would promote the polarization and activation of the C–H bond of CH₄. Meanwhile, the SPR effect of Au nanoparticles offered photogenerated holes and electrons for the reduction of H⁺ to H atoms to give H₂ and the oxidation of CH₃⁻ to CH₃˙, which coupled to produce C₂H₆ (Fig. 29).

Fig. 29 Proposed reaction mechanism for photocatalytic conversion of CH₄ over a plasmonic Au/ZnO catalyst.²⁵⁷

Li and co-workers found that GaN nanowires with a 3% top c-plane and 97% lateral m-plane showed high selectivity of C₆H₆ during non-oxidative photocatalytic conversion of CH₄ under UV light irradiation at room temperature.²⁵⁸ The selectivity of C₆H₆ in hydrocarbon products was about 97% and H₂ was also formed with a ratio of benzene to H₂ of 1 : 9, in agreement with the stoichiometry (eqn (2)). Further studies with GaN samples with different morphologies and different fractions of exposed facets revealed that the reaction was structure sensitive (Fig. 30a). The analysis of results indicated that the lateral m-plane, which was composed of Ga³⁺ and N³⁻ tetrahedrally coordinated with each other, contributed to CH₄ conversions, whereas the c-place made up of only Ga or N atoms had no contribution to the photocatalytic conversion of CH₄ (Fig. 30b). Mechanistic studies suggested that the Ga³⁺ and N³⁻ on the m-plane could interact with CH₄, leading to polarisation of H^δ+–CH₃^δ− and heterolytic dissociation of CH₄. The photogenerated electrons and holes would function for the reduction and oxidation of H⁺ and CH₃⁻ to H˙ and CH₃˙, respectively. The coupling of radical intermediates could give H₂ and C₂H₆, and benzene was formed by the dehydrogenation of C₂H₆ to C₂H₄, followed by cyclization and dehydrogenation processes (Fig. 30b).²⁵⁸

Fig. 30 (a) Photocatalytic conversion of CH₄ over GaN catalysts with different morphologies and different exposed surface areas of the m-plane.²⁵⁸ (b) Schematic diagram for CH₄ C–H bond polarization on the m-plane of GaN.²⁵⁸ Reproduced from ref. 258, with permission of the American Chemical Society, copyright 2014.

Very recently, Khodakov and co-workers reported an interesting photochemical looping strategy for CH₄ conversion to C₂H₆ over a silver–phosphotungstic acid–titania nanocomposite (Ag–HPW/TiO₂) photocatalyst.²⁵⁹ When a batch photo-reactor was used, the selectivity of C₂H₆ could reach 90% with small amounts of C₃H₈ and CO₂, and the C₂H₆ yield was lower than 0.1%. The yield of C₂H₆ could reach 9% after 5 h irradiation in a capillary photo-reactor with a C₂H₆ selectivity of ∼55%. It was proposed that the photochemical looping involved two important steps, i.e., a CH₄ coupling step and silver regeneration step (Fig. 31).²⁵⁹ During the CH₄ coupling step, C₂H₆ was formed through oxidative coupling by photogenerated holes, while the photogenerated electrons were responsible for the reduction of cationic Ag⁺ to metallic Ag⁰. Ag⁰ was then re-oxidized to Ag⁺ under light irradiation in the presence of air in the silver regeneration step. The cationic Ag⁺ dispersed in the HPW layer over Ag–HPW/TiO₂ was essential for the coupling of CH₄ to C₂H₆.

Fig. 31 Schematic illustration of the photochemical looping process for coupling of CH₄ to C₂H₆.²⁵⁹

5.1.2. Photocatalytic coupling of CH₄ in the presence of H₂O. Early studies reported that the photocatalytic conversion of CH₄ in the presence of H₂O or an oxidant could provide C₁ oxygenates, in particular CH₃OH, using WO₃, La³⁺-doped WO₃ and BiVO₄ semiconductor photocatalysts.^{260,267–269} In these systems, photogenerated holes were considered to oxidize H₂O to ˙OH, which then participated in the activation of CH₄ to CH₃˙, eventually giving CH₃OH as the major product. Pure silica zeolite could also catalyse the conversion of CH₄ to CH₃OH under deep ultraviolet light (λ = 165 or 185 nm) irradiation.²⁶¹ Beta zeolite could offer a selectivity of C₁ oxygenates (including CH₃OH, HCHO and HCOOH) of 95% at CH₄ conversion of 13%. It was speculated that the surface ≡Si–OH was activated by deep UV light irradiation to ≡Si–O˙ and H˙, and the ≡Si–O˙ functioned for the activation of CH₄ to CH₃˙, followed by the transformation of CH₃˙ to CH₃OH. Actually, the formation of C₂ products, mainly C₂H₆, which can be formed via the coupling of CH₃˙, have also been observed in some of these systems using semiconductor- or zeolite-based catalysts.^260,261 The formation rate of C₂H₆ could reach 10 μmol g⁻¹ h⁻¹ on a beta zeolite under light irradiation by a medium-pressure Hg lamp, and the loading of V₂O₅ or BiVO₄/V₂O₅ heterojunction could shift the product to CH₃OH.²⁶²

Pt- or Pd-loaded semiconductor TiO₂ was reported to catalyse the conversion of CH₄ to C₂H₆ in an aqueous medium under UV-light irradiation.^263,264 H₂ and CO₂ were also formed in considerable amounts. The studies using electron or hole scavengers suggested that H₂ was formed by photogenerated electrons, while holes participated in the formation of C₂H₆ and CO₂. The selectivity of C₂H₆ was quite low on TiO₂ alone because of the formation of a large fraction of CO₂ from CH₄. The loading of Pt or Pd not only enhanced H₂ formation but also improved the formation rate and the selectivity of C₂H₆ (Fig. 32a).^263,264 It is of interest that although Pd is not as efficient as Pt in promoting H₂ formation, it is more suitable for the formation of C₂H₆. The C₂H₆ selectivity and CH₄ conversion quantum efficiency could reach 73% and 2.8% over the Pd/TiO₂ catalyst, respectively.²⁶⁴ Electron spin resonance (ESR) spectroscopy studies showed the generation of ˙OH and CH₃˙ radicals. Thus, the ˙OH species generated by the oxidation of H₂O by photoexcited holes was proposed to be responsible for abstracting a hydrogen atom from CH₄ to generate CH₃˙, the coupling of which afforded C₂H₆. Although it was assumed that Pd nanoparticles on TiO₂ surfaces might participate in the activation of the C–H bond of CH₄,²⁶⁴ there was no evidence to support this hypothesis.

Fig. 32 (a) Photocatalytic conversion of CH₄ over TiO₂, Pt/TiO₂ and Pd/TiO₂.^263,264 (b) Photocatalytic conversion of CH₄ over Cu-X/PCN photocatalysts with different amounts of Cu.²⁶⁵ (c) Proposed mechanism for photocatalytic conversion of CH₄ with H₂O over Cu-0.5/PCN.²⁶⁵ Reproduced from ref. 265, with the permission of Springer-Nature, copyright 2019.

Designing a photocatalytic system that can generate ˙OH in a controllable manner could be beneficial for the conversion of CH₄. Wang and co-workers recently reported that a Cu-modified polymeric carbon nitride (PCN) catalyst could work for photocatalytic conversion of CH₄ and H₂O to C₂H₅OH under UV-vis light irradiation.²⁶⁵ PCN with a bandgap of 2.97 eV and valence band position of 1.82 eV can work for the oxidation of H₂O to H₂O₂ but not to ˙OH. The control experiment for photocatalytic conversion of H₂O on PCN confirmed the formation of H₂O₂. The photocatalytic conversion of CH₄ on PCN led to the formation of CH₃OH as a major product together with a small amount of C₂H₅OH. The loading of Cu onto PCN from 0 to 0.5% significantly increased the formation rate of C₂H₅OH from 5.5 to 106 μmol g⁻¹ h⁻¹, whereas the formation rate of CH₃OH remained almost unchanged (20–25 μmol g⁻¹ h⁻¹) (Fig. 32b).²⁶⁵ The Cu sites could accelerate the decomposition of H₂O₂ to ˙OH radicals and the oxidised Cu⁺ or Cu²⁺ could be reduced by photogenerated electrons. The Cu sites might also function for the adsorption and activation of CH₄ to help the formation of CH₃˙ by the ˙OH species. The synergistic effect between the Cu sites and the adjacent C atom in PCN has been proposed to play key roles in the formation of C₂ product. Understanding the mechanism for C–C coupling would be very helpful for the design of more efficient photocatalytic systems for the conversion of CH₄ to C₂ compounds. Wang and co-workers considered that C₂H₅OH was formed via a CH₃OH intermediate on the Cu/PCN catalyst and the C–C coupling might proceed through hydroxymethyl and methoxy species formed by interaction of CH₃OH with ˙OH radicals (Fig. 32c).²⁶⁵ However, this mechanism is too speculative and there exist many other possibilities. Considering that Cu-based catalysts show interesting performances also in C–C coupling of C1 intermediates from CO₂/CO during photocatalysis or electrocatalysis, further studies on the Cu-based CH₄ photocatalytic conversion are definitely needed in the future.

5.2. Electrocatalytic conversion of CH₄ to C₂₊ compounds

The electrocatalytic partial oxidation of CH₄ could produce C1 (such as CH₃OH, HCHO) or C₂₊ (such as C₂H₆, C₂H₄) compounds.^{54,55,246,270,271} Previous studies tend to suggest that the major useful products obtained from low-temperature electrocatalytic reaction systems were C1 derivatives, whereas useful C₂₊ compounds could be formed at high temperatures (>600 °C).^270,271 Some typical electrocatalysts and performances are summarized in Table 6.^272–283

Table 6 Typical electrocatalytic systems for conversion of CH₄ to C₂₊ compounds

Catalyst	Reaction condition	E (V)	j (mA cm^−2)	CH4 conv. (%)	C2+ selectivity (%)	C2+ yield (%)	Ref.
a The potential represents the full-cell potential (V). b The potential refers to RHE; RT: room temperature.
Bi2O3–Ag	SOE, CH4|O2, 700 °C	1.3^a	—	—	C2H4 + C2H6: 47	—	272
Porous Ag	SOE, CH4|O2, 800 °C	—	5	97	C2H4 + C2H6: 91	88	273
Mn–Ce–Na2WO4/SiO2	SOE, CH4|O2, 800 °C	—	470	61	C2H4: 36; C2H6: 6.2	23	274
Ag	SOE, CH4|H2O, 820 °C	—	400	26	C2H4: 26; C2H6: 4.3	8	275
Ag + Ce–Na2WO4/SiO2	SOE, CH4|H2O, 800 °C	—	150	23	C2H4: 44; C2H6: 17	13	276
Ag + SrZr0.95Y0.05O3	SOE, CH4|H2O, 720 °C	—	250	18	C2H4: 21; C2H6: 21	7.6	277
Ag + Mn–Ce–Na2WO4/SiO2	SOE, CH4|H2O, 800 °C	—	70	34	C2H4: 36; C2H6: 7.4	15	278
0.075Fe–Sr2Fe1.5Mo0.5O6−δ	SOE, CH4|O2, 850 °C	1.6^a	1000	41	C2H4 + C2H6: 82	33	279
0.075Fe–Sr2Fe1.5Mo0.5O6−δ	SOE, CH4|CO2, 850 °C	1.6^a	750	14	C2H4 + C2H6: 76	10	279
Co3O4/ZrO2	Na2CO3 as the electrolyte, CH4|H2O, RT	2.0^a	8.7	40	1-Propanol: 31; 2-propanol: 30	24	280
ZrO2:NiCo2O4	Na2CO3 as the electrolyte, CH4|H2O, RT	2.0^a	2.3	48	Propionic acid: 65	31	281
NiO@NiHF	NaOH as the electrolyte, CH4|H2O, RT	1.5^b	0.04	C2H5OH: 85 (FE)			282
NiO/Ni	Proton exchange membrane, CH4|H2O, RT	1.4^b	3.0 mA gNiO^−1	C2H5OH: 89 (FE), 25 μmol gNiO^−1 h^−1 (formation rate)			283

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The oxidative coupling of methane (OCM) to C₂ compounds (mainly C₂H₆ and C₂H₄) is a well-known thermocatalytic reaction for CH₄ transformation at high temperatures.^20,242,243 The selectivity of C₂H₆ and C₂H₄ usually decreases with an increase in the conversion of CH₄, thus limiting the C₂ yield. It is generally accepted that the molecular oxygen in the gas phase would lead to the formation of CO and CO₂, whereas the oxygen species on the catalyst surfaces are responsible for the OCM reaction.^20,242,243 To suppress the gas-phase non-selective oxidation of CH₄, the O₂ concentration in the gas phase should be minimized. The solid oxide electrolyte electrochemical reactor, which uses oxygen ion conductors as the electrolyte, has been harnessed for the electrocatalytic oxidation of CH₄ with the aim to improve the selectivity of C₂ products. This electrochemical reactor provides an oxidant through transferring oxygen ions from the cathode side to the anode side, and thus can avoid the co-feeding of CH₄ and O₂. An electrocatalytic reactor with solid oxide electrolyte (SOE) mainly consists of two porous electrodes (i.e., anode and cathode) and a dense SOE to separate the cathode and anode compartments (Fig. 33). The anode compartment is usually fed with CH₄, while the cathode compartment can be fed either with O₂ (Fig. 33a) or another reactant (e.g., H₂O or CO₂) that is capable of providing oxygen ions (Fig. 33b). It is noteworthy that the feeding of H₂O or CO₂ can simultaneously produce H₂ or CO.

Fig. 33 (a) Schematic illustration of the solid oxide electrolyte reactor for O₂/CH₄ conversion. (b) Schematic illustration of a solid oxide electrolyte reactor for H₂O/CH₄ or CO₂/CH₄ conversion.

Otsuka and co-workers reported a pioneering work for CH₄ coupling in an SOE electrocatalytic reactor in 1985.²⁷² A Y₂O₃-stabilized ZrO₂ (YSZ) with high oxygen ion conductivity was used as a solid oxide electrolyte, and Ag and Bi₂O₃–Ag were used as anode catalysts for electrocatalytic conversion of CH₄ at 700 °C. CH₄ and O₂ were fed into the anode and cathode compartments separately (Fig. 33a). The oxygen transfer flux through the YSZ could be controlled by the externally applied potential. The CH₄ conversion rate increased with an increase in oxygen flux over Ag and Bi₂O₃–Ag catalysts, but the selectivity of C₂ compounds (C₂H₄ and C₂H₆) decreased upon increasing the oxygen flux. The Bi₂O₃–Ag catalyst reached a maximum C₂ formation rate of ∼1.85 μmol min⁻¹ (selectivity, 47%) at an oxygen flux of 12 μmol min⁻¹, whereas the C₂ formation rate was only 0.42 μmol min⁻¹ (selectivity, 30%) at the same oxygen flux over the Ag catalyst (Fig. 34a).

Fig. 34 (a) Electrocatalytic conversion of CH₄ over Ag and Bi₂O₃–Ag catalysts at the same oxygen flux of 12 μmol min⁻¹.²⁷² (b) Catalytic performances of porous Ag catalyst without and with C₂ hydrocarbons trapping and gas recycling.²⁷³

After that, many catalysts and reaction systems based on the SOE electrocatalytic reactor have been developed.^{273,274,284,285} Vayenas and co-workers developed a reaction system with product separation and CH₄ recycling abilities for electrocatalytic conversion of CH₄ to C₂ hydrocarbons.²⁷³ The C₂ hydrocarbons could be separated by molecular sieve trap, thus significantly inhibiting the over-oxidation of C₂ hydrocarbons and improving the total yield of C₂ hydrocarbons (Fig. 34b). Over a porous Ag catalyst, at an applied current density of 5 mA cm⁻² at 800 °C, CH₄ was oxidatively coupled to C₂ hydrocarbons with a C₂ hydrocarbon yield of 88% and a C₂H₄ yield of 85%. Some typical OCM catalysts in thermocatalysis, such as Mn/Na₂WO₄/SiO₂ and La-doped SrTiO₃, were also examined for the electrocatalytic conversion of CH₄ to C₂ hydrocarbons.^274,284,285 Tang and co-workers integrated a Mn–Ce–Na₂WO₄/SiO₂ catalyst with SOE tubular membrane reactor for electrocatalytic coupling of CH₄.²⁷⁴ The reaction could be operated at a high applied current density of 470 mA cm⁻² at 800 °C with a CH₄ single-pass conversion of 61% and C₂₊ selectivity of 42%. The C₂₊ selectivity still needs to be improved.

The oxygen for electrocatalytic oxidation of CH₄ could be provided by H₂O instead of O₂, enabling the simultaneous formation of H₂ and C₂ hydrocarbons (C₂H₄ and C₂H₆).^{275–278,286,287} A single-chamber reactor configuration with Ag/YSZ/Pt (anode/solid electrolyte/cathode) co-fed with CH₄ and H₂O mixture was reported to show a maximum C₂ yield of ∼8% under the optimised conditions at 820 °C.²⁷⁵ The addition of 5 wt% Ce–5 wt% Na₂WO₄/SiO₂ to the single-chamber Ag/YSZ/Pt reactor could enable thermocatalytic oxidation of CH₄ by the O₂ released to the gas phase, enhancing the C₂ yield to 13% at 800 °C.²⁷⁶ The double-chamber reactor with CH₄ and H₂O vapours fed separately to the anode and cathode chambers has the potential to obtain higher C₂ selectivity. In the double-chamber reactor with Ag/YSZ/Pt configuration (Fig. 35a), Stoukides and co-workers obtained a C₂ yield of 5.7% at 840 °C at CH₄ conversion of 16.2% and C₂ selectivity of 35.1%.²⁷⁷ Although O₂ was not co-fed with CH₄, some excess oxygen ions electrochemically pumped from the cathode to the anode may form O₂ and evolve to the CH₄ stream, leading to the gas-phase non-selective oxidation of CH₄. To enhance the selective oxidation of CH₄ on anode surfaces, a layer of OCM catalyst was deposited on the top of an anodic electrode (Fig. 35b). When a perovskite OCM catalyst (SrZr_0.95Y_0.05O_3−a) was deposited, the C₂ yield was improved to 7.6% with a CH₄ conversion of 18% and C₂ selectivity of 42.2%, and the reaction temperature could decrease to 720 °C.²⁷⁷ In a Ag/YSZ/Ag double-channel reactor, the addition of the Mn–Ce–Na₂WO₄/SiO₂ catalyst could promote the C₂ yield to 15% at 800 °C.²⁷⁸

Fig. 35 Schematic diagram of an O²⁻-conducting solid oxide electrolyte reactor. (a) Without OCM catalyst. (b) With OCM catalyst.

CO₂ could also be used as the source of oxygen for the electrocatalytic oxidation of CH₄. Xie and co-workers reported simultaneous electrocatalytic reduction of CO₂ to CO on the cathode and the oxidation of CH₄ on the anode (Fig. 36a).^279,288 The utilisation of a redox-reversible layered perovskite, i.e., Sr₂Fe_1.5Mo_0.5O_6−δ (SFMO), as the anode and the in situ construction of metal/oxide interfaces at nanoscale using Sr₂Fe_1.5+x–Mo_0.5O_6−δ (x = 0–0.1) to grow iron nanoparticles on the SFMO scaffold (xFe–SFMO) were demonstrated to be an efficient approach for the electrocatalytic oxidation of CH₄ to C₂ compounds by oxygen from CO₂.²⁷⁹ A La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) was used as the solid electrolyte and Ce_0.8Sm_0.2O_2−δ (SDC) was mixed with xFe–SFMO to fabricate a single cell composed of xFe–SFMO–SDC/LSGM/SFMO–SDC (anode/solid electrolyte/cathode). DFT calculations suggested that the Fe–SFMO metal–oxide interface favoured the cleavage of the C–H bond of CH₄ with a lower energy barrier than the SFMO alone (Fig. 36b). Such a metal/oxide interface structure also improved the coking resistance and the thermal stability. The 0.075Fe–SFMO showed the best catalytic performances at an applied voltage of 1.6 V and a temperature of 850 °C (Fig. 36c and d).²⁷⁹ When O₂ was fed to the cathode, the conversion of CH₄, selectivity and yield of C₂ products (C₂H₄ and C₂H₆) could reach 41%, 81.2% and 33%, respectively (Fig. 36c). The conversion of CH₄, selectivity and yield of C₂ products were 13.7%, 75.6% and 10.3%, respectively, as CO₂ was fed to the cathode (Fig. 36d). The decreased performance in the case of electrocatalytic conversion of CH₄–CO₂ as compared to that of CH₄–O₂ may probably arise from the higher thermodynamic and kinetic barriers for CO₂ electrolysis than O₂ splitting to generate oxygen ions on the cathode. The simultaneous electrocatalytic conversion of CH₄ and CO₂ to C₂H₄ and CO is an interesting and challenging direction and is worth further study.

Fig. 36 (a) Schematic diagram of electrocatalytic conversion of CH₄ on the anode and reduction of CO₂ on the cathode. (b) Energy diagrams for adsorption and C–H bond cleavage of CH₄ on SFMO(001) and Fe/SFMO(001) surfaces including their initial, transition and final states. Inset: Electronic charge density difference for the transition state of Fe/SFMO(001). (c) Anodic product concentrations of xFe–SFMO catalysts in the electrocatalytic conversion of CH₄ on the anode and O₂ feeding on the cathode. (d) Anodic product concentrations of xFe–SFMO catalysts in the electrocatalytic conversion of CH₄ on the anode and reduction of CO₂ on the cathode.²⁷⁹ Reproduced from ref. 279, with the permission of Springer-Nature, copyright 2019.

The studies on electrocatalytic conversion of CH₄ to C₂₊ compounds under mild conditions are rare. Park and co-workers claimed that CH₄ could be electrocatalytically oxidized to C₂₊ alcohols, acids and ketones at ambient temperature.^280,281 It is very surprising that 2-propanol and 1-propanol were obtained as major products when a Co₃O₄/ZrO₂ nanocomposite loaded on carbon paper (Alfar) was employed as the anode for CH₄ oxidation with Na₂CO₃ as the electrolyte and Pt as the cathode for H₂O reduction (Fig. 37a).²⁸⁰ Acetaldehyde was formed at shorter times and was proposed to be the precursor for the formation of 2-propanol and 1-propanol. When a ZrO₂:NiCo₂O₄ quasi-solid solution catalyst loaded on graphite foil was used as the anode for the oxidation of CH₄, propionic acid was obtained as the major product along with 2-propanol, 1-propanol, acetone and acetic acid. The conversion of CH₄ could reach 47.5% after 20 h of reaction and the formation rate of propionic acid was 1173 μmol g_cat⁻¹ h⁻¹ (selectivity ∼65%) at ambient temperature operated in a two-electrode system at 2.0 V versus a Pt counter electrode.²⁸¹ The reaction-time-course studies suggested that 1-propanol, acetaldehyde and 2-propanol were the major products in the first 5 h of reaction, and these products were further oxidized to propionic acid, acetic acid and acetone in 20 h of reaction, respectively (Fig. 37b). Although the phenomenon that the electrocatalytic conversion of CH₄ could offer C₂₊ alcohols, acids and ketones (with C₃ alcohols or acids as the major products) at room temperature is very interesting, some solid evidence, such as ¹³CH₄-labelling isotopic experimental results, is expected to further confirm these very surprising results obtained in the oxidation of CH₄ with such kinds of carbon-containing catalyst systems.

Fig. 37 (a) Schematic reaction processes for electrocatalytic conversion of CH₄.²⁸⁰ (b) Catalytic performances of a ZrO₂:NiCo₂O₄ quasi-solid solution catalyst for electrocatalytic conversion of CH₄ to C₂₊ compounds.²⁸¹ Reproduced from ref. 280, with permission of Wiley-VCH Verlag GmbH&Co. KGaA, copyright 2017.

Recently, Chen, Sun and co-workers reported that CH₄ could be converted to C₂H₅OH with high selectivity by electrocatalytic oxidation over a NiO/Ni catalyst.^282,283 They constructed catalysts with a series of NiO/Ni interfaces (Fig. 38a) by controlling the calcination temperature of Ni foam.²⁸³ The 3.0NiO/Ni catalyst calcined at 500 °C with a NiO content of 3.0 wt% showed the best performance. At an applied potential of 1.4 V vs. RHE, the FE and formation rate of C₂H₅OH reached 89% and 25 μmol g_NiO⁻¹ h⁻¹, respectively.²⁸³ The optimised NiO/Ni interface with appropriate NiO content showed balanced charge transfer ability and the highest electrochemically active surface area, thus enabling efficient C–H activation and C–C coupling. DFT calculations further demonstrated a thermodynamically favourable route on the NiO(200)/Ni(111) interface for C₂H₅OH formation with the elementary steps of CH₄* → CH₃* + H*, CH₃* → CH₂* + H*, CH₂* + OH* → CH₂OH* and CH₃* + CH₂OH* → C₂H₅OH* in sequence (Fig. 38b).²⁸³ The NiO(200)/Ni(111) interface favours the dissociation of CH₄* to CH₃* with a relatively low activation barrier (0.30 eV). Although the steps of CH₃* to CH₂* and CH₃OH* had similar activation barriers, the exothermic reaction energy of CH₃* to CH₂* was significantly lower than that of CH₃* to CH₃OH*. This may be the main reason for the low CH₃OH selectivity observed over the NiO/Ni catalyst. The subsequent hydroxylation of CH₂* and the C–C coupling of CH₃* and CH₂OH* to C₂H₅OH* are both more energy favourable steps.

Fig. 38 (a) Schematic illustration of electrocatalytic conversion of CH₄ to C₂H₅OH on the NiO/Ni interface. (b) Reaction energy profiles for electrocatalytic conversion of CH₄ to CH₃OH and C₂H₅OH at the NiO(200)/Ni(111) interface from DFT calculations.²⁸³ Reproduced from ref. 283, with permission of Elsevier, copyright 2020.

5.3. Mechanism of C–C coupling for photocatalytic and electrocatalytic conversions of CH₄ to C₂₊ compounds

Many studies have been devoted to the transformation of CH₄ by thermocatalysis.⁵⁶ For the oxidative coupling of methane (OCM) at high temperatures (typically 873–1073 K), a gas-phase methyl radical (CH₃˙) has long been proposed as the key intermediate for the C–C coupling into C₂H₆, which undergoes dehydrogenation to C₂H₄. The generation of CH₃˙ during OCM was confirmed by synchrotron VUV photoionization mass spectroscopy.²⁸⁹ CH₃˙ has also been proposed as the intermediate for the C–C coupling of CH₄ to C₂H₆ in photocatalysis,^{251,256–258,264} and the formation of CH₃˙ has been detected by in situ electron spin resonance (ESR) spectroscopy.²⁶⁴ The selectivity of C₂H₄ is lower by photocatalysis. This is probably because the dehydrogenation of C₂H₆ becomes difficult at low temperatures used for photocatalysis. C₂H₅OH could also be formed by photocatalytic conversion of CH₄ on a Cu/PCN catalyst, and CH₃OH was proposed to be the intermediate.²⁶⁵ However, this mechanism still lacks solid evidence. On the other hand, in the electrocatalytic CH₄ transformations, C₂H₄ and C₂H₆ could be obtained at high temperatures,^272,273,279 whereas 1-propanol and C₂H₅OH may be obtained at low temperatures.^280–283 The studies on the mechanism of electrocatalytic C–C coupling of CH₄ are very scarce. In the future, the advanced characterization techniques used for thermocatalytic CH₄ studies such as synchrotron VUV photoionization mass spectroscopy should be exploited for mechanistic studies for photocatalytic and electrocatalytic CH₄ conversions to offer deeper understanding of the C–C coupling and other elementary steps.

6. Photocatalytic and electrocatalytic conversions of CH₃OH and HCHO

Methanol is an abundant and key C1 feedstock, which can be produced from either fossil resources (in particular coal and natural/shale gas via syngas) or renewable carbon resources (biomass via syngas or CO₂ hydrogenation).²⁹⁰ Various types of C₂₊ chemicals, including both hydrocarbons and oxygenates, can be produced from methanol. The selective oxidation of methanol to formaldehyde, carbonylation of methanol into acetic acid and the conversion of methanol to olefins (MTO) and gasoline (MTG) are well-established commercial processes. The direct conversion of methanol into a high-value C₂₊ product with high selectivity is still the most attractive and challenging target in methanol chemistry. Here, we highlight recent studies on the conversion of methanol or formaldehyde to C₂₊ compounds under mild conditions using photocatalysis and electrocatalysis (Table 7).^291–306

Table 7 Photocatalytic and electrocatalytic systems for the conversions of CH₃OH and HCHO to C₂₊ compounds

Catalyst	Target product	Reaction conditions	Formation rate (mmol g^−1 h^−1)	Selectivity (%)	Yield (%)	Ref.
a The potential refers to RHE. b The potential represents the full-cell potential (V).
Photocatalytic conversion of CH3OH
ZnS	EG	Hg arc lamp	1.0	75	0.3 (60 h)	291
MoS2/CdS	EG	Xe lamp (420–780 nm)	11	90	16 (100 h)	292
CoP/Zn2In2S5	EG	Xe lamp (AM 1.5)	18.9	90	4.5 (12 h)	293
GaN	C2H5OH	Xe lamp (290–380 nm)	∼4.0	100	∼5.4 (12 h)	294
Ni2P/CdS	1,1-Dimethoxymethane	Xe lamp (>400 nm)	188	83	1.7 (3 h)	295
Pt/TiO2	EG	UV light (320–400 nm)	∼2	80	∼0.4 (10 h)	296
Pt/TiO2	Aliphatic alcohols	Xe lamp (200–400 nm)	0.013–0.094	>90	0.94–6.6 (15 h)	297
CdS	Aliphatic alcohols	Xe lamp (>420 nm)	0.0035–0.0070	>70	0.35–0.70 (20 h)	298
Electrocatalytic conversion of CH3OH (including dielectric barrier discharge, DBD)
Co3ZnC/NC	C2H5OH	j: 10 mA cm^−2, E: 1.54 V^a	24.4 mg cm^−2 h^−1	95 (FE: 12)	3.2 (4.5 h)	299
—	EG	DBD, AC 16.8 kV,^b 300 °C	∼1.7 mmol h^−1	72	11.3	300
—	EG	DBD, AC 16.8 kV,^b 300 °C	∼3.3 mmol h^−1	75	22.5	301
—	n-C4H10	DBD, AC 30 kV,^b 140 °C	—	37.5	15	302
Photocatalytic conversion of HCHO
BiVO4	EG, CH3CHO, HOCH2CHO	Xe lamp (320–780 nm)	11	74	8 (12 h)	303
MnOx–Pt@MoOx/BiVO4	EG, CH3CHO, HOCH2CHO	Xe lamp (320–780 nm)	54	74	21 (12 h)	304
Electrocatalytic conversion of HCHO
Mercury	EG	j: 250 mA cm^−2, E: ∼−1.0 V^a	—	FE: 30	—	305
Graphite	EG	j: 400 mA cm^−2, E: −1.0 V^a	—	FE: 97	∼10.4	306

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6.1. Photocatalytic and electrocatalytic conversions of CH₃OH

6.1.1. Photocatalytic conversion of CH₃OH. CH₃OH has been widely used as a sacrificial agent for photocatalytic H₂ evolution, where CH₃OH serves as a hole scavenger. In most cases, the products of methanol oxidation were C1 derivatives, such as HCHO, HCOOH, and CO₂.³⁰⁷ However, some recent studies have demonstrated that CH₃OH can be converted to high-value C₂₊ compounds such as ethylene glycol and ethanol by semiconductor photocatalysis under light irradiation.^{291–295,308} Furthermore, the photocatalytic coupling of CH₃OH with other molecules such as HCHO and olefins can also proceed with CH₃OH as a hydroxymethylation agent, forming unique higher alcohols.^296–298

The dehydrogenative coupling of methanol to ethylene glycol (EG) (eqn (3), denoted as MTEG) is a very attractive but highly challenging reaction. The product of this reaction, i.e., EG, is an important chemical with many important applications, in particular as a monomer for the production of poly(ethylene terephthalate) (PET). Currently, EG is primarily produced from ethylene by hydration. The development of a methanol-based EG production route would provide an alternative non-petroleum route.

2CH₃OH → HOCH₂CH₂OH + H₂, ΔG_(298K) = 9.3 kJ mol⁻¹ (3)

In an early communication, Yanagida and co-workers reported that the oxidation of methanol over ZnS (E_g = 3.6 eV, corresponding to λ = 345 nm) under UV-light irradiation formed HCHO and EG.²⁹¹ The rate and selectivity of EG were ∼1 mmol g⁻¹ h⁻¹ and ∼75%, respectively.

Recently, our group demonstrated that the dehydrogenative coupling of methanol into ethylene glycol could proceed over CdS (E_g = 2.4 eV, corresponding to λ = 518 nm) under visible-light irradiation.²⁹² ZnS and CdS were found to be unique photocatalysts for the formation of EG and other types of semiconductors such as TiO₂, ZnO, C₃N₄ and CuS provided HCHO, HCOOH, CO and CO₂. Nanorod-shaped CdS showed better performances than CdS nanoparticles. Different types of co-catalysts were examined to enhance the performance of CdS nanorods and MoS₂ was found to be the best co-catalyst. Upon loading 5 wt% three-dimensional MoS₂ nanofoams to CdS nanorods (Fig. 39a), the formation rate of EG increased from 0.46 to 11 mmol g⁻¹ h⁻¹ and the selectivity of EG on a molar carbon basis increased from 71% to 90%. The yield of EG on the molar carbon basis was calculated to be ∼2.5% after 12 h of the reaction under visible-light (λ = 420–780 nm) irradiation. The increase in reaction time to obtain higher yield of EG in the conventional batch-type reactor led to significant decreases in EG selectivity due to the consecutive oxidation. The design of a process-intensified reactor with simultaneous reaction and CH₃OH/EG separation ability could keep the EG selectivity at 90% during 100 h of reaction, affording an EG yield of 16% (Fig. 39b).²⁹² The mechanistic studies revealed that EG was formed by oxidative coupling of CH₃OH by photogenerated holes, while photogenerated electrons contributed to H₂ formation. CdS was found to be quite unique in the preferential activation of the C–H bond in CH₃OH by the holes without affecting the O–H group, generating a ˙CH₂OH intermediate, which coupled to give EG. DFT calculations suggested that the weak absorption of CH₃OH on CdS surfaces offered a possibility of preferential activation of the C–H bond instead of the O–H bond (Fig. 39c). The cleavage of the C–H bond in CH₃OH on CdS surfaces to produce ˙CH₂OH radicals occurred through a concerted proton–electron transfer (CPET) mechanism with a low activation barrier. The weakly adsorbed ˙CH₂OH could readily desorb from CdS surfaces for subsequent C–C coupling. On the other hand, the stronger adsorption of CH₃OH on TiO₂ led to a very easy proton transfer in the first step, forming CH₃O⁻ species on TiO₂ surfaces, which consequently resulted in the formation of HCHO (Fig. 39c).

Fig. 39 (a) Schematic illustration of MoS₂ foam/CdS for photocatalytic synthesis of EG and H₂ from CH₃OH. (b) Catalytic performance of MoS₂ foam/CdS with process intensification. (c) Reaction energy profiles via ˙CH₂OH and CH₃O˙ on CdS(100) and rutile TiO₂(110).²⁹² Reproduced from ref. 292, with the permission of Springer-Nature, copyright 2018.

Very recently, instead of CdS, an environmentally friendly sheet-like Zn₂In₂S₅ semiconductor was further found to be efficient for selective conversion of CH₃OH to EG under visible-light irradiation.²⁹³ The loading of 0.25 wt% CoP onto Zn₂In₂S₅ enhanced the EG selectivity to 90%. Under the simulated sunlight irradiation (AM1.5), the formation rate of EG on the 0.25 wt% CoP/Zn₂In₂S₅ catalyst reached 18.9 mmol g⁻¹ h⁻¹ and the EG yield on the molar carbon basis was 4.5% after 12 h of reaction.²⁹³ In addition to the conversion of CH₃OH to EG, the CoP/Zn₂In₂S₅ catalyst could also work for the photocatalytic dehydrogenative coupling of ethanol to 2,3-butanediol. Under visible-light irradiation, the selectivity and formation rate of 2,3-butanediol were 53% and 3.2 mmol g⁻¹ h⁻¹, respectively.²⁹³ ESR measurements confirmed that the reaction proceeded via the ˙CH(OH)CH₃ intermediate. The photocatalytic dehydrogenative coupling of ethanol to 2,3-butanediol was also reported previously using a TiO₂ catalyst by Zhu and co-workers.³⁰⁸ The pre-treatment of TiO₂ at high temperatures to remove the surface hydroxyl groups was found to be key to obtaining high selectivity of 2,3-butanediol from ethanol, and this might indicate that avoiding the interaction of the –OH of ethanol with the surface was vital.

Photocatalytic conversion of CH₃OH could also afford C₂H₅OH as the major product. Li and co-workers showed that GaN nanowires could catalyse the one-step conversion of CH₃OH to C₂H₅OH under UV-vis-light irradiation at ambient temperature.²⁹⁴ The cylinder-shaped GaN-nanowire catalyst with average diameters of 100 nm and lengths of 800 nm was fabricated and the catalyst exposed two types of surfaces, i.e., the polar surface (c-plane) located at the end of the rod and the non-polar surface (m-place) located at the side of the rod. The conversion of 25 μL CH₃OH over 0.35 mg GaN nanowires doped with a trace amount of Mg²⁺ (p-GaN) under UV-vis-light irradiation offered 17 μmol C₂H₅OH in 12 h, showing a formation rate of C₂H₅OH of ∼4.0 mmol g⁻¹ h⁻¹.²⁹⁴ Further studies indicated that the polar plane (c-plane) was mainly responsible for the formation of C₂H₅OH. Mechanistic studies suggested that a methyl carbene (:CH₂) intermediate might be generated on the polar c-plane of GaN surfaces for subsequent insertion into the C–H bond of CH₃OH to form C₂H₅OH (Fig. 40). Moreover, CH₃OH could be converted to n-C₃H₇OH over GaN nanowires under UV-vis-light irradiation by lowering the reaction temperature from 15 to 0 °C, and this indicated that this reaction system may have potential for the synthesis of a wide range of lower to higher alcohols.

Fig. 40 Proposed mechanism for the conversion of CH₃OH to C₂H₅OH on GaN surface.²⁹⁴

Besides EG and C₂H₅OH, 1,1-dimethoxymethane (DMM) could also be synthesized by photocatalytic conversion of CH₃OH. DMM is a product without a C–C bond but is of importance as the precursor of polyoxymethylene dimethyl ether (POMM), which is an environmentally friendly embalming agent to replace HCHO. When Ni₂P/CdS was used as a photocatalyst for the conversion of CH₃OH under visible-light irradiation, the major product was found to shift from EG to DMM by adding a small amount of sulphuric acid.²⁹⁵ The selectivity and formation rate of DMM reached 83% and 188 mmol g⁻¹ h⁻¹ at a H₂SO₄ concentration of 40 mM. It was speculated that the presence of H⁺ induced the reaction between the formed HCHO and CH₃OH to produce DMM.

Photocatalytic coupling of CH₃OH with other molecules has been reported by Zhong, Sun and co-workers.^296–298 For example, the photocatalytic coupling of CH₃OH and HCHO could proceed over a Pt/TiO₂ catalyst under UV-light irradiation, offering EG with a selectivity of 80%.²⁹⁶ It was proposed that CH₃OH was first oxidised by photogenerated holes to form ˙CH₂OH and then the ˙CH₂OH intermediate reacted with HCHO to give HOCH₂CH₂O˙, which was finally transformed to EG by photogenerated electrons and protons. The coupling of ˙CH₂OH could also afford EG. The photocatalytic coupling of CH₃OH with terminal olefins could proceed efficiently over Pt/TiO₂ or CdS to form higher aliphatic alcohols.^297,298 In the reaction, CH₃OH worked as a hydroxymethylation agent and a wide range of terminal olefins were successfully functionalized to the corresponding longer aliphatic alcohols with high anti-Markovnikov selectivity. Mechanistic studies indicated that CH₃OH was oxidised by photogenerated holes to a ˙CH₂OH radical, which then attacked the terminal olefins to generate a new C–C bond and another radical intermediate for subsequent reduction by photogenerated electrons to aliphatic alcohols.

6.1.2. Electrocatalytic conversion of CH₃OH. Electrocatalytic conversion of CH₃OH has widely been studied in direct CH₃OH fuel cells (DMFCs), where CH₃OH is completely oxidised to CO₂ over most electrocatalysts.³⁰⁹ Only a few papers have reported the electrocatalytic conversion of CH₃OH to C₂₊ products. Recently, Zhan and co-workers demonstrated that CH₃OH could be converted to C₂H₅OH over a carbon-supported metal cathode at 20 °C and atmospheric pressure (eqn (4)–(6)).²⁹⁹ A Co₃ZnC/NC (NC = nitrogen-containing carbon) catalyst was found to show the best performance for C₂H₅OH formation among several carbon-supported metal catalysts. The conversion rate of CH₃OH reached 25.7 mg cm⁻² h⁻¹ with a C₂H₅OH selectivity (molar carbon basis) of 95% and a C₂H₅OH FE of 12%.²⁹⁹ The lower FE of C₂H₅OH was due to the competitive H₂ evolution reaction on the cathode. It is clear that the FE of C₂H₅OH needs to be improved in the future.

Cathode: 4CH₃OH + 2e⁻ → 2OH⁻ + H₂ + 2C₂H₅OH (4)

2H₂O + 2e⁻ → 2OH⁻ + H₂ (5)

Anode: 2H₂O → O₂ + 4e⁻ + 4H⁺ (6)

Dielectric barrier discharge (DBD) is the electrical discharge between two electrodes separated by an insulating dielectric barrier, which can readily generate free radicals in the gas phase for radical reactions. The DBD has been used for methanol conversion to C₂₊ compounds such as EG and n-C₄H₁₀.^{300–302,310} Guo and co-workers have systematically studied the direct synthesis of EG by co-feeding of CH₃OH and H₂ in a double dielectric barrier discharge reactor.^300,301 Under the optimised reaction conditions, under a H₂/CH₃OH molar ratio of ∼7.2 and 300 °C, the CH₃OH conversion and EG selectivity reached 30% and 75%, respectively. The co-feeding of H₂ could improve the CH₃OH conversion and EG selectivity, and played a catalytic role by releasing ˙H radicals for the abstraction of hydrogen from CH₃OH to form ˙CH₂OH intermediates for subsequent C–C coupling to EG (Fig. 41a).³⁰¹ Tu and co-workers demonstrated that CH₃OH could be converted to n-C₄H₁₀ and H₂ in a DBD reactor.³⁰² The optimised selectivities of n-C₄H₁₀, H₂ and CO were 37.5%, 28.9% and 14%, respectively, with a CH₃OH conversion of 40% at a CH₃OH inlet concentration of 18 mol% and a reaction temperature of 140 °C using N₂ as a carrier gas. It was proposed that CH₃˙ radicals were generated by electron impact dissociation and then the CH₃˙ radicals dimerized to C₂H₆, which could further be converted to C₂H₅˙ radicals through hydrogen abstraction and then undergo C–C coupling to n-C₄H₁₀ (Fig. 41b). The presence of several competitive pathways for the formation of CO, CH₄, and C₂H₅OH reduced the selectivity of n-C₄H₁₀.

Fig. 41 Proposed mechanisms for the conversion of CH₃OH in a dielectric barrier discharge reactor. (a) EG formation.³⁰¹ (b) n-C₄H₁₀ formation.³⁰²

6.2. Photocatalytic and electrocatalytic conversions of HCHO

Photocatalysis and electrocatalysis also offer opportunities for the direct conversion of formaldehyde to C₂₊ compounds, in particular EG.^303–306 Our group first developed a photocatalytic route for coupling of HCHO to C₂ oxygenates.³⁰³ BiVO₄ and Bi₂WO₆ were found to catalyse the conversion of HCHO, providing EG, glycolaldehyde, and acetaldehyde as the major products with a total C₂ selectivity of 74%. BiVO₄ nanocrystals with truncated tetragonal bipyramidal morphology and controllable {010} and {110} facets were fabricated. The BiVO₄ nanocrystal catalyst with an equal fraction of {010}/{110} showed a C₂ yield of 12% during the conversion of HCHO aqueous solution under UV-vis-light irradiation for 12 h.³⁰⁴ This catalyst had the highest ability to separate the photogenerated electrons and holes probably because photogenerated electrons and holes migrated separately to {010} and {110} facets, respectively. The effects of co-catalysts were further investigated. The photo-deposition of co-catalysts showed that Pt@MoO_x with a Pt core and MoO_x shell was selectively deposited on the {010} facets, whereas MnO_x was deposited selectively on the {110} facets (Fig. 42a).³⁰⁴ This provided further evidence for the migration of photogenerated electrons and holes towards different facets for reduction and oxidation. The single Pt or MoO_x did not significantly promote the catalytic activity of BiVO₄, whereas the Pt@MoO_x co-catalyst accelerates the formation of C₂ oxygenates, in particular EG (Fig. 42b). The presence of MnO_x as an oxidation co-catalyst further enhanced EG formation. The yields of C₂ compounds and EG over the 3% MnO_x–0.5% Pt@3% MoO_x/BiVO₄ catalyst reached 21% and 11%, respectively, under UV-vis light irradiation for 12 h. Mechanistic studies revealed that Pt worked for the extraction of photogenerated electrons and accelerates the electron–hole separation, while MoO_x provided active sites for the activation of HCHO. In brief, Mo⁶⁺ was proposed to be reduced to Mo⁵⁺ by accepting a photogenerated electron passed from the Pt core, and the Mo⁵⁺ site could adsorb HCHO and transfer the electron (ET) to the adsorbed HCHO (Fig. 42c). Then, the proton transfer (PT) led to the formation of ˙CH₂OH, which coupled to give EG after desorption.

Fig. 42 (a) Model of the MnO_x–Pt@MoO_x/BiVO₄ catalyst. (b) Effect of co-catalysts on the performance of BiVO₄ for photocatalytic conversion of HCHO. (c) Proposed mechanism for the conversion of HCHO to EG over MnO_x–Pt@MoO_x/BiVO₄.³⁰⁴

The electrocatalytic reductive coupling of HCHO is also an important route for the synthesis of EG under mild conditions (eqn (7) and (8)).^305,306 The materials with high H₂ evolution overpotential, such as Cd, Hg, Pb, Ti, and graphite, could be used as a cathode to suppress the competitive H₂ evolution reaction (eqn (9)). Mazur and co-workers found that graphite showed the best performance for the formation of EG.³⁰⁶ A large fraction of HCHO was in the hemiacetal form in the presence of methanol inhibitor, and a high pH could cause the disproportionation of HCHO to CH₃OH and HCOO⁻ (eqn (10)), whereas a lower pH would lead to the polymerization of HCHO by acid catalysis (eqn (11)). Thus, the control of the reaction conditions to obtain a high concentration of free HCHO for C–C coupling and to suppress side reactions is essential to EG formation. The optimum reaction conditions required a high concentration of aqueous HCHO solution, near-neutral pH of 5 to 8, the addition of quaternary ammonium salt and a reaction temperature of about 80 °C. Under the optimized conditions, with an applied current density of 300 mA cm⁻² and HCHO concentration of 52%, the FE of EG was higher than 80% and the EG concentration reached 23%.³⁰⁶ However, with an increase in HCHO conversion, the FE of EG decreased significantly. The separation of EG from the electrolyte solution is also a challenging issue that needs to be solved for commercialisation.

Cathode: 2HCHO + 2H⁺ + 2e⁻ → EG (7)

Anode: 2H₂O → O₂ + 4e⁻ + 4H⁺ (8)

Side reaction: 2H⁺ + 2e⁻ → H₂ (9)

2HCHO + OH⁻ → CH₃OH + HCO₂⁻ (10)

nHCHO → polymer (11)

6.3. Mechanism of C–C coupling for photocatalytic and electrocatalytic conversions of CH₃OH and HCHO to C₂₊ compounds

The thermocatalytic activation of CH₃OH usually leads to the cleavage of its C–O or O–H bond, and the resulting CH_x species may undergo C–C bond formation. The use of zeolite catalysts like in the MTO process can partially control the product selectivity due to their shape-selective effect.²² Interestingly, the preferential activation of the C–H bond of CH₃OH has been successfully achieved by photocatalysis using CdS-based catalysts, and EG could be obtained with a selectivity of 90% via the subsequent coupling of the ˙CH₂OH intermediate.^292,293 To the best of our knowledge, this cannot be accomplished through thermocatalysis. The formation of ˙CH₂OH in photocatalytic conversion of CH₃OH has been confirmed by in situ ESR.^292,293 In a DBD reactor, CH₃OH could also be converted to ˙CH₂OH with electrical energy, which undergoes coupling to EG.^300,301 Furthermore, EG can be produced by photocatalytic or electrocatalytic reduction of HCHO. Similarly, ˙CH₂OH is the key intermediate during photocatalytic/electrocatalytic conversion of HCHO to EG.^303–306 Besides EG, C₂H₅OH was reported to be obtained through photocatalytic or electrocatalytic conversion of CH₃OH.^294,299 It is proposed that C₂H₅OH is formed by insertion of methyl carbine (:CH₂) into the C–H bond of CH₃OH on the polar plane of GaN in photocatalysis.²⁹⁴ On the other hand, CH₃˙ and ˙CH₂OH intermediates were confirmed by in situ ESR on Co₃ZnC/NC in electrocatalysis, and thus C₂H₅OH might be the coupling product between these two radicals.²⁹⁹ In the DBD reactor, CH₃OH may also be transformed into a CH₃˙ intermediate, which then undergoes C–C coupling to C₂H₆. C₂H₆ could be further converted to n-C₄H₁₀via C₂H₅˙.³⁰² Thus, the nature of the intermediate generated from CH₃OH or HCHO in photocatalysis/electrocatalysis has a significant effect on the final C–C coupling product.

7. Conclusions and outlook

The selective transformation of C1 molecules to value-added C₂₊ compounds through controllable C–C coupling is a highly attractive but challenging research goal in chemistry. The control of C–C bond formation in the conversion of C1 molecules by thermocatalysis under harsh reaction conditions is generally difficult. Photocatalysis and electrocatalysis have offered promising routes for activation and selective transformation of C1 molecules to C₂₊ compounds under mild conditions by harnessing photo or electrical energy. The present article has highlighted key advances in photocatalytic and electrocatalytic conversions of typical C1 molecules, including CO₂, CO, CH₄, CH₃OH, and HCHO, to C₂₊ compounds, in particular C₂H₄, C₂H₆, C₂H₅OH, ethylene glycol and n-propanol.

Photocatalytic reduction of CO₂ to C₂₊ compounds requires efficient transfer of multi-electrons/multi-protons and suitable active sites to catalyse not only CO₂ activation but also C–C coupling. Many semiconductors, e.g., TiO₂,⁶² g-C₃N₄,⁶³ BiVO₄,⁶⁴ Bi₂WO₆⁶⁵ and InTaO₄,⁶⁶ have been reported to be capable of working for the reduction of CO₂ to C₂₊ compounds, but the efficiencies were low. Several strategies have been reported to increase the activity and the selectivity of C₂₊ compounds. To increase the light harvesting by semiconductor doping, combination with sensitizer and loading of SPR metals could enhance the activity for CO₂ photocatalytic reduction in particular under visible-light irradiation. The enhancement in the separation of photogenerated electrons and holes by the hybridisation of different semiconductors or a semiconductor with a carbon nanomaterial and loading of co-catalysts may contribute not only to increasing the activity but also improving the selectivity to C₂₊ products by promoting the multi-electron transfer. Several studies demonstrated that the hybridisation of TiO₂ with CNT or graphene could enhance the formation of C₂H₅OH or C₂H₆.^73–76 The nanostructure of a photocatalyst may also affect the C–C coupling. For example, the construction of mesoporous g-C₃N₄ was found to be beneficial to the formation of C₂H₅OH. The surface structure is the key to the surface reaction, and thus the modification of the catalyst surfaces may modify the selectivity of C₂₊ products. For example, the coating of a thin Nafion layer on the catalyst surface may increase the local proton concentration, thus favouring the multi-step proton-coupled electron transfer (PCET) reactions to facilitate the formation of C₂₊ products.⁷⁹ The co-catalyst can not only enhance the electron–hole separation but also provide active sites for CO₂ activation and C–C coupling. Several co-catalysts such as Pd,^78,80 Rh,⁸⁰ NiO,^66,81 Cu,^67,83,94 and bimetallic Cu–Pt^83,84,96 have been reported to hold potential to promote the formation of C₂₊ compounds. Among these co-catalysts, Cu has received particular interest in the formation of C₂₊ hydrocarbons and the enhancing effect of Cu or Cu-based co-catalysts has been confirmed in many studies.^{67,69,82,83,85,94} The loading of co-catalysts on semiconductors has been demonstrated to be one of the most effective strategies for improving the photocatalytic activity and tuning the C–C coupling selectivity in CO₂ reduction. The findings suggest that the co-catalysts are necessary for developing highly efficient photocatalysts for photocatalytic reduction of CO₂ to C₂₊ compounds. In addition, the reaction conditions and reaction medium could also exert influences on the selectivity of C₂₊ compounds. For example, the increase in CO₂ pressure and the use of a basic medium were reported to be beneficial for the formation of C₂₊ products.⁶² The addition of ionic liquid (EMIM-BF₄) into aqueous solution with a proper concentration could promote the selectivity of C₂ and C₃ hydrocarbons during photocatalytic reduction of CO₂ on plasmonic Au nanoparticles under green-light irradiation because of the enhanced CO₂ activation and accelerated electron transfer from Au nanoparticles to CO₂.⁸⁷ Despite the progress achieved, the formation rates of C₂₊ compounds are still quite low, being <100 μmol g⁻¹ h⁻¹ in most cases (Table 1). It is noteworthy that the results with such low rates should be treated carefully, because the carbon sources contained in the reaction system including CO₂ may all have possibilities to be reduced to C₂₊ products. Therefore, to carry out isotopic ¹³C-labelling experiments to confirm that the products are derived from CO₂ as well as the quantification of O₂ formation is crucial to providing solid results. This is particularly necessary when carbon-containing photocatalysts are studied.

Three pathways, i.e., the oxalic acid pathway, glyoxal pathway and methyl radical pathway, have been proposed for C–C coupling in photocatalytic reduction of CO₂ (Fig. 10b).^45,59,104 A deep understanding of how the surface structure and other influencing factors determine the CO₂ activation and the C–C coupling is still lacking. The mechanism for the formation of C₂₊ compounds needs further studies.

Photo-driven Fischer–Tropsch synthesis is the major research area in photocatalytic conversion of CO to C₂₊ compounds. It is demonstrated that the conversion of syngas to C₂₊ hydrocarbons could proceed over several supported metal catalysts under light irradiation without external heating.^110–115 The typical Fischer–Tropsch metal (i.e., Co, Fe and Ru)-based and Ni-based catalysts all showed activities in light-driven syngas conversions. The photo-thermal effect by metal catalysts and the photogenerated charge carriers on catalyst surfaces may both contribute to the light-driven CO hydrogenation to C₂₊ hydrocarbons. The photo-thermal effect could provide sufficient temperature at the catalyst bed for CO hydrogenation. The photogenerated charges on the catalyst surfaces may create a unique oxidation state and electronic structure of the catalyst, thus regulating the C–C coupling and hydrogenation ability to obtain unique product selectivity. For example, lower olefins were formed with higher selectivity in the Co- and Fe (or Fe₅C₂)-catalysed photo-driven syngas conversions.^110–112 Ni-Based catalysts could also work for the formation of C₂₊ hydrocarbons under light irradiation, whereas CH₄ was the only hydrocarbon product over Ni catalysts under thermocatalytic conditions.¹¹⁵ Despite uniqueness, the distribution of hydrocarbon products is still quite broad in the photo-driven CO hydrogenation, indicating that the C–C coupling is still uncontrollable.

Electrocatalytic reduction has recently been proven to be very promising for the synthesis of C₂₊ compounds from CO₂ or CO. Although Cu is still the only selective catalyst for electrocatalytic reduction of CO₂ to C₂₊ compounds owing to its moderate CO binding energy, significant advances have been achieved in developing active and selective Cu-based electrocatalysts for the formation of various C₂₊ compounds, including C₂H₄, C₂H₅OH, CH₃COOH and n-C₃H₇OH. Model studies using Cu single crystals reveal that the electrocatalytic reduction of CO₂ is structure sensitive; the Cu(111) surfaces favour the formation of CH₄, whereas Cu(100) and Cu(110) surfaces can offer C₂H₄ and C₂₊ oxygenates, respectively.^176–178 As compared to pure metallic Cu, the oxide-derived Cu catalyst showed enhanced C₂₊ selectivity. Although the nature of the active sites leading to high C₂₊ FE over oxide-derived Cu catalysts is still under debate, the current consensus is that the presence of surface Cu^δ+ sites plays a pivotal role in the C–C coupling and the formation of C₂₊ products. Many studies have been devoted to improving the C₂₊ formation activity on Cu-based catalysts and high C₂₊ FEs have been reported. For example, the doping of a heteroatom such as boron, nitrogen or halogen on the surfaces of Cu nanoparticles could offer high FEs (∼80%) towards C₂₊ compounds with C₂H₄ as the major product.^139–142 A nano-defective Cu nanosheet catalyst derived from CuO nanosheets showed an FE of C₂H₄ as high as 83.2%.¹³⁸ Electrolyte engineering could also improve C₂₊ FEs, and larger sizes of anions or cations (e.g., I⁻ or Cs⁺) added into the electrolyte showed significant promoting effects.^194–196 Recent studies using the flow cell with GDE instead of H-cell to circumvent the mass-transport limitation have achieved high C₂₊ FEs at high current densities. Typically, the alkaline GDE design has successfully enabled superior performances to be achieved at commercially-relevant levels (≥200 mA cm⁻²).

Many studies have attempted to improve the FE toward C₂₊ oxygenate formation. The fabrication of bimetallic catalysts containing Cu and another guest metal such as Zn, Au or Ag has shown great promising for the formation of C₂₊ oxygenates, in particular C₂H₅OH, in electrocatalytic CO₂RR. The FE of C₂H₅OH could be enhanced to 41% on an Ag_0.14/Cu_0.86 alloy catalyst at a current density of 400 mA cm⁻² in the alkaline flow cell.¹⁵² The design of carbon materials also holds potential to modulate the selectivity of C₂H₅OH in Cu/carbon-catalysed CO₂RR. For example, Cu nanoparticles surrounded by N-doped carbon layers displayed a C₂H₅OH FE of 52% with a total C₂₊ FE of 93% at a current density of 300 mA cm⁻² in an alkaline flow cell.¹⁵⁶

As compared to electrocatalytic CO₂RR, Cu-catalysed electrocatalytic CORR has typically demonstrated lower overpotentials for C–C coupling and higher selectivity towards C₂₊ oxygenate formation. For example, a nanoflower Cu catalyst with a high roughness factor offered a nearly 100% selectivity to C₂ oxygenates at a potential of only −0.23 V vs. RHE despite the low current density.¹⁶⁹ Not only C₂H₅OH but also CH₃CHO, CH₃COO⁻ and n-C₃H₇OH can be produced with high selectivity during electrocatalytic CORR. CH₃COO⁻ formation was typically enhanced after switching CO₂ reactant to CO, which was attributed to higher local pH at the electrode–electrolyte interface in CORR. A 48% FE of CH₃COO⁻ was achieved at a current density of 200 mA cm⁻² during electrocatalytic CORR over a Cu nanosheet catalyst.¹⁷² It is expected that the key step in n-C₃H₇OH formation is the coupling between C1 and C₂ intermediates, however, the inadequate stabilization of C₂ intermediates on Cu surfaces usually results in desorption rather than further coupling with C1. Several strategies have been developed to stabilize C₂ species and promote the C₁–C₂ coupling to n-C₃H₇OH, including the confinement effect, creating interfaces of Cu(100) and Cu(111) facets and loading Cu adparticles on Cu surfaces. The maximum FE of n-C₃H₇OH achieved to date is 20–25%.^173–175

The moderate CO binding strength is believed to be one major reason for the excellent performance of Cu catalysts in electrocatalytic CO₂RR and CORR to C₂₊ products. The electrocatalytic CO₂RR to C₂₊ products on Cu usually requires potentials more negative than or close to −1.0 V to achieve industrial-relevant current densities. The search for electrocatalytic materials other than Cu may enable the CO₂RR to C₂₊ products at lower overpotentials. The fabrication of Cu-free bimetallic catalysts (e.g., Pd–Au, Ni–Ga and Ni–Al), which combine one metal that binds CO strongly with the other that binds CO weakly, could offer C₂₊ products during CO₂RR. However, the C₂₊ FEs of these bimetallic catalysts are still quite low (<5%). Nitrogen-doped carbon-based materials have been exploited for metal-free electrocatalytic CO₂RR to C₂₊ products. A surprisingly high FE of C₂H₅OH (>90%) has been claimed on a boron- and nitrogen-co-doped nanodiamond catalyst despite of the low current density (∼1 mA cm⁻²).¹⁶⁰ In addition, some studies have demonstrated that the fabrication of a bifunctional catalyst composed of a catalyst component that can accelerate the CO₂RR to CO and the N-doped carbon can enhance the formation of C₂ oxygenates, in particular C₂H₅OH.^229–231

CO is generally believed to be the key intermediate in the electrocatalytic CO₂RR to C₂₊ compounds. Several mechanisms have been proposed for C–C coupling of CO or CO-derived intermediates on Cu-based catalysts. The direct coupling of *CO to form a *OCCO intermediate followed by hydrogenation to C₂H₄ and C₂H₅OH is believed to be the major C–C coupling route in most studies reported to date. Some studies have demonstrated the importance of *CHO species in the C–C coupling step, and the (*CO + *CHO) coupling or (*CHO + *CHO) coupling may participate in the formation of C₂ compounds. Two major mechanisms have been proposed for the formation of C₂H₄ and C₂ oxygenates. One assumes that *CH₂CHO serves as the selectivity-determining intermediate, while the other considers that *CHCOH is the key intermediate. In both cases, it is accepted that the hydrogenolysis of the selectivity-determining intermediate leads to C₂H₄ formation, while the hydrogenation results in C₂₊ oxygenate.

As compared to photocatalytic CO₂RR or CORR, electrocatalytic CO₂RR and CORR have made a significant step-forward in controlling the product selectivity. Excellent selectivity of C₂H₄ or C₂ oxygenates such as C₂H₅OH or CH₃COO⁻ has been achieved at industrially relevant current densities. Nevertheless, challenges still remain for electrocatalytic CO₂ and CO conversions to C₂₊ products. First, the most efficient alkaline GDE suffers from stability issues. More effort should be put into improving the resistance of GDE flooding. Electrocatalytic CORR can avoid the salt accumulation under alkaline conditions, and thus improved stability may be expected for electrocatalytic CORR than for direct CO₂RR. Thus, to implement a tandem strategy to synthesize C₂₊ products from CO₂via CO would be useful. Second, most electrocatalytic CO₂RR and CORR studies have only touched on the cathodic half-reaction, and the oxygen evolution reaction in the anode has seldom been considered. Combining an efficient oxygen-evolution catalyst with CO₂RR or CORR to improve the full-cell energy conversion efficiency would be important for future industrial applications. Third, it still remains challenging to achieve a sufficient CO₂/CO single-pass conversion or single-pass target product yield that is comparable to the commercial Fischer–Tropsch synthesis process.

As compared to the traditional thermocatalytic OCM and CH₄ dehydrogenative coupling processes, the photocatalytic conversion of CH₄ to C₂₊ compounds can be performed under very mild conditions, although the efficiency is quite limited. Metal cations or oxides highly dispersed on SiO₂, Al₂O₃ and zeolites (e.g., Zn⁺-ZSM-5²⁵⁵ and Ga-EST-10²⁵⁶) and some semiconductors (Au/ZnO,²⁵⁷ GaN²⁵⁸ and Ag⁺–HPW/TiO₂²⁵⁹) have been reported for the non-oxidative dehydrogenation of CH₄ to C₂H₆ or C₆H₆ under UV-light irradiation. The active site, such as Zn²⁺ and Ga³⁺ ions, could promote the adsorption of CH₄ and the polarization of C–H bonds to form CH₃ surface species or radicals for C–C coupling to C₂H₆. The formation of C₆H₆ is more difficult, because it needs multi-step dehydrogenation and cyclization processes. The non-oxidative reaction condition may facilitate the C–C bond formation to C₂₊ hydrocarbons. In the presence of H₂O, in addition to hydrocarbons, CH₃OH and C₂H₅OH are also formed. Hydroxyl radicals (˙OH) formed through the oxidation of H₂O by photogenerated holes have been detected and proposed to be the reactive oxygen species for abstracting a hydrogen atom from CH₄ to generate CH₃˙. The ˙OH radical can be easily generated through the decomposition of H₂O₂. Thus, the design of photocatalysts with high H₂O₂ generation activity may be promising for CH₄ conversion.

Electrocatalytic conversion of CH₄ to C₂ hydrocarbons could be performed in solid oxide-electrolyte reactors at high temperatures similar to that used for OCM reactions. However, the gas-phase non-selective oxidation of CH₄ may be inhibited, because CH₄ and O₂ are fed separately in the anode and cathode compartments. Many modified OCM catalysts have been employed as the electrocatalysts for CH₄ conversion. By using a reaction system with product separation and CH₄ recycling abilities, the yields of C₂ hydrocarbons and C₂H₄ could reach 88% and 85%, respectively, over a porous Ag catalyst.²⁷³ Over a Fe-doped layered perovskite anode (Sr₂Fe_1.575–Mo_0.5O_1−δ) for CH₄ oxidation, the single-pass conversion of CH₄ could reach 41% and the yield of C₂ hydrocarbons (C₂H₄ and C₂H₆) reached 33%, when O₂ was fed to the cathode.²⁷⁹ O₂ on the cathode could be replaced by H₂O or CO₂ and H₂ or CO would be formed simultaneously with CH₄ oxidative coupling products. Because of the higher thermodynamic and kinetic barriers for H₂O and CO₂ electrolysis to generate oxygen ions on the cathode, the performance of electrocatalytic conversion of CH₄ to C₂ hydrocarbons using H₂O or CO₂ is still not high. However, the full use of the anode and cathode half reaction to produce useful chemicals is important, and much effort should be directed in the future to developing high efficiency electrocatalysts for both cathode and anode reactions. Studies on electrocatalytic conversion of CH₄ to C₂₊ compounds at moderate temperatures have also emerged in recent years, and this direction is definitely worthy of further study.

The photocatalytic and electrocatalytic conversions of CH₃OH and HCHO to C₂₊ compounds are important targets that have not been widely studied. Some novel reactions, such as the visible-light-driven dehydrogenative coupling of CH₃OH to EG,^292,293 light-driven one-step conversion of CH₃OH to C₂H₅OH,²⁹⁴ photocatalytic coupling of HCHO to EG,^303,304 and electrocatalytic conversion of CH₃OH to C₂H₅OH,²⁹⁹ have been developed in recent years. A high selectivity (90%) and yield (16%) of EG have been achieved through photocatalytic coupling of CH₃OH to EG by using a MoS₂ foam-modified CdS nanorod catalyst under visible-light irradiation.²⁹² It has been demonstrated that the C–H bond of CH₃OH is activated by the photogenerated hole to ˙CH₂OH, which couples to give EG.²⁹² Thus, these studies also offer an opportunity for the preferential activation of the C–H bond of functionalised molecules. It is of interest that ˙CH₂OH is a common intermediate for the photocatalytic and electrocatalytic coupling of CH₃OH or HCHO to EG. Thus, CH₃OH or HCHO could be utilised as a platform for hydroxymethylation reaction to construct new C–C couplings to produce diols or higher aliphatic alcohols in synthetic chemistry.

Some catalysts have shown promising performances for photocatalytic and electrocatalytic conversions of C1 molecules into C₂₊ compounds. For photocatalytic reduction of CO₂, a Pt–0.5G–TiO_2−x catalyst achieved formation rates of CH₄ and C₂H₆ of 37 and 11 μmol g⁻¹ h⁻¹, respectively, under one sun AM 1.5G illumination with solid evidence for the formation of CH₄ and C₂H₆ from CO₂ by ¹³CO₂ isotopic control experiments.⁷⁶ The apparent quantum yield of (CH₄ + C₂H₆) reached 7.9% and the catalyst was stable during 42 h of reaction. A Cu-modified polymeric carbon nitride (PCN) could work for photocatalytic conversion of CH₄ with H₂O to C₂H₅OH under UV-vis light irradiation, and the formation rate of C₂H₅OH reached 106 μmol g⁻¹ h⁻¹.²⁶⁵ Despite the progress, the state-of-the-art performances for photocatalytic conversions of CO₂ and CH₄ are far from commercial consideration. On the other hand, the performances for photocatalytic conversions of CO, CH₃OH and HCHO are usually significantly better than those for photocatalytic conversions of CO₂ and CH₄. For example, without external heating, the Fe₅C₂ for photo-thermal hydrogenation of CO at atmospheric pressure under UV-vis light irradiation provided a CO conversion of 49.5% (conversion rate, 17 mmol g⁻¹ h⁻¹) with C₂–C₄ olefin selectivity of 55.5% (CO₂ selectivity, 18.9%).¹¹² For photocatalytic conversion of methanol, the EG formation rate could reach 11 mmol g⁻¹ h⁻¹ over a MoS₂/CdS catalyst under visible-light irradiation, with EG selectivity, yield and quantum yield of 90%, 16% and 5%, respectively.²⁹² HCHO is more reactive and the formation rate of C₂ compounds (EG, CH₃CHO, HOCH₂CHO) could reach 54 mmol g⁻¹ h⁻¹ from HCHO over a MnO_x–Pt@MoO_x/BiVO₄ catalyst under UV-vis light irradiation.³⁰⁴ Furthermore, as compared to photocatalysis, electrocatalysis has achieved much better performances for the transformations of inert CO₂ and CH₄. For electrocatalytic CO₂RR, the FE and partial current density of C₂H₄ over Cu-based catalysts could reach 83% and 1.3 A cm⁻², respectively.^138,145 A fluorine-modified Cu catalyst offered C_2–4 products (mainly C₂H₄ and C₂H₅OH) with selectivity of 85.8% and single-pass yield of 16.5%, and the formation rate of C₂H₄ reached 3.2 mmol h⁻¹ cm⁻² (12.8 mol g⁻¹ h⁻¹).¹⁴² This performance outperforms those achieved in high-temperature and high-pressure thermocatalysis.⁸ A Fe–Sr₂Fe_1.5Mo_0.5O_6−δ catalyst showed a current density of 1.0 A cm⁻² in the electrocatalytic oxidation of CH₄ at an applied voltage of 1.6 V and a temperature of 850 °C.²⁷⁹ The conversion of CH₄, and the selectivity and yield of C₂ products (C₂H₄ and C₂H₆) reached 41%, 81.2% and 33%, respectively.²⁷⁹ It is noteworthy that recent studies on electrocatalytic conversion of CH₃OH or HCHO to C₂₊ compounds are very limited.

With the flourishing development of photocatalytic and electrocatalytic systems for the transformation of C1 molecules into C₂₊ products, the catalyst stability has become an important issue for future applications. Better stability would reduce the associated maintenance and consumable costs, and is one of the keys for commercial applications of photocatalytic and electrocatalytic conversions of C1 molecules into C₂₊ compounds. Many photocatalysts and electrocatalysts reported for the conversion of C1 molecules showed good stability of tens or hundreds of hours. For a potential photocatalytic process such as H₂ evolution by water splitting, a stability of 1000 h with a solar to chemical efficiency of 10% is required for commercial applications.³¹¹ Considering the comparability, the catalyst stability for photocatalytic conversions of C1 molecules to C₂₊ compounds should exceed 1000 h at a sufficiently high quantum efficiency (at least >10%). As described above, the FE (>80%) and current density (>1 A cm⁻²) reported for the electrocatalytic CO₂RR to C₂₊ products (mainly C₂H₄, C₂H₅OH) could meet the industrial requirement, but the long-term catalyst stability is still a challenge. The best stability achieved to date for electrocatalytic CO₂RR to C₂₊ compounds is 190 h at a current density of ∼120 mA cm⁻².¹⁴⁶ It was once considered that a catalyst stability of about 5000 h is required as the first cornerstone for an electrocatalytic process.³¹² There is large room for the future studies to improve the catalyst stability in photocatalytic and electrocatalytic transformations of C1 molecules to C₂ compounds.

An understanding of the reaction mechanism would not only deepen our knowledge of C1 chemistry but also help to develop more efficient photocatalysts and electrocatalysts for valorisation of C1 molecules. Generally, the photocatalytic and electrocatalytic conversions of C1 molecules to C₂₊ compounds include two major steps: the activation of C1 molecules to C1 intermediates and C–C coupling of C1 intermediates to C₂₊ compounds. The C1 intermediates may also be easily converted into other C1 molecules, and thus controlling C–C coupling of C1 intermediates is the key to obtaining high selectivity of C₂₊ products. Mechanistic studies have pointed out that the typical C1 molecules, including CO₂, CO, CH₄, CH₃OH and HCHO, are transformed into specific C1 surface species or radicals, which then undergo C–C coupling to C₂₊ compounds through multi-electron/multi-proton transfer processes (Fig. 43). For example, CO₂˙⁻, ˙CHO and CH₃˙ have been proposed as the C1 intermediates for C–C coupling to form oxalic acid, glyoxal and C₂H₆, respectively, in photocatalytic CO₂ reduction.^67,107–109 In electrocatalytic CO₂RR and CORR, *CO species has been proposed to be the major intermediate for C–C coupling in most studies,^232–234 but recent studies have demonstrated that the coupling between *CHO species is a more facile pathway.^{142,236,237,239} The ˙CH₂OH radical is the intermediate for C–C coupling in photocatalytic and electrocatalytic conversions of CH₃OH and HCHO to EG.^{292,301,304,306} Meanwhile, the CH₃˙ radical is the key intermediate for the C–C coupling in either photocatalytic or electrocatalytic conversion of CH₄ to C₂H₆.^{251,256–258,264,279} It is noteworthy that the insights into the detailed mechanisms for C1-molecule activation, reaction intermediate formation, C–C coupling and product formation are still very limited. The nature of active sites and the synergy between the catalytically active sites and electrons/holes/protons are not well documented. The state-of-the-art knowledge cannot enable rational design of efficient photocatalysts or electrocatalysts with controlled product selectivity.

Fig. 43 Proposed C1 intermediates in photocatalytic and electrocatalytic conversions of C1 molecules to C₂₊ compounds. “*” in the figure represents surface or radical species.

Advanced characterization techniques have also promoted the progress in photocatalytic and electrocatalytic transformations of C1 molecules, in particular for the elucidation of active sites and reaction intermediates. Take electrocatalytic CO₂RR and CORR as examples. To understand the nature of the active sites leading to high C₂₊ FE over oxide-derived Cu catalysts, multiple in situ characterization techniques have been employed to investigate the oxidation state of Cu under reaction conditions, including secondary-ion mass spectrometry,¹⁸⁵ surface-enhanced Raman spectroscopy¹⁸⁹ and time-resolved X-ray absorption spectroscopy.³¹³In situ spectroscopic studies have also been used to detect the reaction intermediate. Through in situ FT-IR spectroscopic studies, our group observed *CHO species during electrocatalytic CO₂RR,¹⁴² while *OCCOH species were detected during electrocatalytic CORR by Koper and co-workers.²³⁵ These insights provide deeper understanding of the intermediate formation and C–C coupling process. However, it is noteworthy that most of the in situ characterization techniques are performed under conditions with low reaction rates, which are different from those adopted for achieving high reaction rates. In the future, characterization techniques should be developed to improve the capability of working under real reaction conditions.

Theoretical calculations have largely contributed to the understanding of the active sites and reaction mechanisms for the conversion of C1 molecules to C₂₊ products as well as the insights that guide the selectivity control. Elementary steps including the activation of C1 molecules, proton/electron transfer, C1 intermediate formation, C–C coupling of C1 intermediates and product formation for different systems have been studied on molecular or atomic levels by using DFT calculations. However, most of the theoretical calculations are based on gas-phase reaction models, whereas the effects of coverage, solution, electrolyte and electric field have seldom been considered, although these effects are known to be very important for photocatalytic and electrocatalytic reactions. Theoretical studies have recently also been exploited to accelerate the catalyst discovery. For example, Sargent and co-workers developed a machine-learning-accelerated, high-throughput DFT framework to screen electrocatalysts.¹⁴⁴ After studying 144 different Cu-containing intermetallic materials, they discovered that a Cu–Al alloy had multiple sites and surface orientations with near-optimal CO binding. The experimentally fabricated de-alloyed nanoporous Cu–Al catalyst showed an FE of C₂H₄ as high as 80%.¹⁴⁴ More efforts should be put into this area in the future.

Photocatalysis and electrocatalysis have been enriching the area of C1 chemistry. More sustainable routes for the utilisation of C1 molecules including CO, CO₂, CH₄, CH₃OH and HCHO can be developed to produce C₂₊ compounds such as C₂H₄, C₂H₅OH and EG, which play important roles in the current chemical or energy industry. Not only could some existing C1-chemistry reactions, such as the hydrogenation of CO to C₂₊ hydrocarbons, be performed under very mild conditions, offering better selectivity toward target products, but photocatalysis and electrocatalysis also enable new transformations of C1 molecules involving C–C coupling, in particular the thermodynamically limited reactions. With the assistance of photo or electrical energy, the inert CO₂ and CH₄ molecules can be efficiently activated to reactive intermediates for subsequent C–C coupling, selectively offering C₂₊ compounds under mild conditions. It is noteworthy that the electrocatalytic conversion of CO₂ to C₂H₄ and C₂H₅OH has achieved significant progress in recent years. Furthermore, the inert C–H bond in CH₃OH could be preferentially activated by photocatalysis with the OH group keeping intact, generating ˙CH₂OH radicals for C–C coupling to EG. Photocatalysis and electrocatalysis have brought many opportunities and also challenges in the area of C1 chemistry. We are expecting further breakthroughs in this emerging field.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Financial support by the National Key Research and Development Program of Ministry of Science and Technology (2017YFB0602201) and the National Natural Science Foundation of China (No. 91945301, 21972115 and 21503176) is greatly acknowledged.

Notes and references

1. C. Mesters, Annu. Rev. Chem. Biomol. Eng., 2016, 7, 223–238.
2. C. B. Roberts and N. O. Elbashir, Fuel Process. Technol., 2003, 83, 1–9.
3. Energy Information Administration (EIA), Annual Energy Outlook 2018, (Washington, DC, July 2018) (https://www.eia.gov/pressroom/presentations/capuano_07242018.pdf).
4. W. Keim, Pure Appl. Chem., 1986, 58, 825–832.
5. W. Cui, G. Zhang, T. Hu and X. Bu, Coord. Chem. Rev., 2019, 387, 79–120.
6. J. Bao, G. Yang, Y. Yoneyama and N. Tsubaki, ACS Catal., 2019, 9, 3026–3053.
7. K. Cheng, J. Kang, D. L. King, V. Subramanian, C. Zhou, Q. Zhang and Y. Wang, Adv. Catal., 2017, 60, 125–208.
8. W. Zhou, K. Cheng, J. Kang, C. Zhou, V. Subramanian, Q. Zhang and Y. Wang, Chem. Soc. Rev., 2019, 48, 3193–3228.
9. T. Sakakura, J. Choi and H. Yasuda, Chem. Rev., 2007, 107, 2365–2387.
10. R. W. Dorner, D. R. Hardy, F. W. Williams and H. D. Willauer, Energy Environ. Sci., 2010, 3, 884–890.
11. W. Wang, S. Wang, X. Ma and J. Gong, Chem. Soc. Rev., 2011, 40, 3703–3727.
12. M. D. Porosoff, B. Yan and J. G. Chen, Energy Environ. Sci., 2016, 9, 62–73.
13. H. Yang, C. Zhang, P. Gao, H. Wang, X. Li, L. Zhong, W. Wei and Y. Sun, Catal. Sci. Technol., 2017, 7, 4580–4598.
14. L. Guo, J. Sun, Q. Ge and N. Tsubaki, J. Mater. Chem. A, 2018, 6, 23244–23262.
15. A. Dokania, A. Ramirez, A. Bavykina and J. Gascon, ACS Energy Lett., 2019, 4, 167–176.
16. J. Zhong, X. Yang, Z. Wu, B. Liang, Y. Huang and T. Zhang, Chem. Soc. Rev., 2020, 49, 1385–1413.
17. X. Jiang, X. Nie, X. Guo, C. Song and J. G. Chen, Chem. Rev., 2020, 120, 7984–8034.
18. M. Ravi, M. Ranocchiari and J. A. van Bokhoven, Angew. Chem., Int. Ed., 2017, 56, 16464–16483.
19. A. I. Olivos-Suarez, À. Szécsényi, E. J. M. Hensen, J. Ruiz-Martinez, E. A. Pidko and J. Gascon, ACS Catal., 2016, 6, 2965–2981.
20. P. Schwach, X. Pan and X. Bao, Chem. Rev., 2017, 117, 8497–8520.
21. N. J. Gunsalus, A. Koppaka, S. H. Park, S. M. Bischof, B. G. Hashiguchi and R. A. Periana, Chem. Rev., 2017, 117, 8521–8573.
22. U. Olsbye, S. Svelle, M. Bjørgen, P. Beato, T. V. W. Janssens, F. Joensen, S. Bordiga and K. P. Lillerud, Angew. Chem., Int. Ed., 2012, 51, 5810–5831.
23. P. Tian, Y. Wei, M. Ye and Z. Liu, ACS Catal., 2015, 5, 1922–1938.
24. S. Xu, Y. Zhi, J. Han, W. Zhang, X. Wu, T. Sun, Y. Wei and Z. Liu, Adv. Catal., 2017, 61, 37–122.
25. I. Yarulina, A. D. Chowdhury, F. Meirer, B. M. Weckhuysen and J. Gascon, Nat. Catal., 2018, 1, 398–411.
26. M. Yang, D. Fan, Y. Wei, P. Tian and Z. Liu, Adv. Mater., 2019, 31, 1902181.
27. F. Jiao, J. Li, X. Pan, J. Xiao, H. Li, H. Ma, M. Wei, Y. Pan, Z. Zhou, M. Li, S. Miao, J. Li, Y. Zhu, D. Xiao, T. He, J. Yang, F. Qi, Q. Fu and X. Bao, Science, 2016, 351, 1065–1068.
28. K. Cheng, B. Gu, X. Liu, J. Kang, Q. Zhang and Y. Wang, Angew. Chem., Int. Ed., 2016, 55, 4725–4728.
29. K. Cheng, W. Zhou, J. Kang, S. He, S. Shi, Q. Zhang, Y. Pan, W. Wen and Y. Wang, Chem, 2017, 3, 334–347.
30. S. Kasipandi and J. W. Bae, Adv. Mater., 2019, 31, 1803390.
31. Z. Li, J. Wang, Y. Qu, H. Liu, C. Tang, S. Miao, Z. Feng, H. An and C. Li, ACS Catal., 2017, 7, 8544–8548.
32. X. Liu, M. Wang, C. Zhou, W. Zhou, K. Cheng, J. Kang, Q. Zhang, W. Deng and Y. Wang, Chem. Commun., 2018, 54, 140–143.
33. P. Gao, S. Dang, S. Li, X. Bu, Z. Liu, M. Qiu, C. Yang, H. Wang, L. Zhong, Y. Han, Q. Liu, W. Wei and Y. Sun, ACS Catal., 2018, 8, 571–578.
34. Y. Ni, Z. Chen, Y. Fu, Y. Liu, W. Zhu and Z. Liu, Nat. Commun., 2018, 9, 3457.
35. Z. Li, Y. Qu, J. Wang, H. Liu, M. Li, S. Miao and C. Li, Joule, 2019, 3, 570–583.
36. C. Zhou, J. Shi, W. Zhou, K. Cheng, Q. Zhang, J. Kang and Y. Wang, ACS Catal., 2020, 10, 302–310.
37. W. Zhou, J. Kang, K. Cheng, S. He, J. Shi, C. Zhou, Q. Zhang, J. Chen, L. Peng, M. Chen and Y. Wang, Angew. Chem., Int. Ed., 2018, 57, 12012–12016.
38. J. Kang, S. He, W. Zhou, Z. Shen, Y. Li, M. Chen, Q. Zhang and Y. Wang, Nat. Commun., 2020, 11, 827.
39. G. Chen, G. I. N. Waterhouse, R. Shi, J. Zhao, Z. Li, L. Wu, C. Tung and T. Zhang, Angew. Chem., Int. Ed., 2019, 58, 17528–17551.
40. P. D. Luna, C. Hahn, D. Higgins, S. A. Jaffer, T. F. Jaramillo and E. H. Sargent, Science, 2019, 364, eaav3506.
41. Y. Jiang, R. Long and Y. Xiong, Chem. Sci., 2019, 10, 7310–7326.
42. Y. Zhao, G. I. N. Waterhouse, G. Chen, X. Xiong, L. Wu, C. Tung and T. Zhang, Chem. Soc. Rev., 2019, 48, 1972–2010.
43. M. Jouny, G. S. Hutchings and F. Jiao, Nat. Catal., 2019, 2, 1062–1070.
44. G. A. Olah, G. K. Prakash and A. Goeppert, J. Am. Chem. Soc., 2011, 133, 12881–12898.
45. S. N. Habisreutinger, L. Schmidt-Mende and J. K. Stolarczyk, Angew. Chem., Int. Ed., 2013, 52, 7372–7408.
46. S. Xie, Q. Zhang, G. Liu and Y. Wang, Chem. Commun., 2016, 52, 35–59.
47. X. Chang, T. Wang and J. Gong, Energy Environ. Sci., 2016, 9, 2177–2196.
48. K. Li, B. Peng and T. Peng, ACS Catal., 2016, 6, 7485–7527.
49. X. Li, J. Yu, M. Jaroniec and X. Chen, Chem. Rev., 2019, 119, 3962–4179.
50. C. F. Shih, T. Zhang, J. Li and C. Bai, Joule, 2018, 2, 1925–1949.
51. C. Chen, J. F. K. Kotyk and S. W. Sheehan, Chem, 2018, 4, 2571–2586.
52. L. Zhang, Z. Zhao, T. Wang and J. Gong, Chem. Soc. Rev., 2018, 47, 5423–5443.
53. D. Gao, R. M. Arán-Ais, H. S. Jeon and B. R. Cuenya, Nat. Catal., 2019, 2, 198–210.
54. K. Elouarzaki, V. Kannan, V. Jose, H. S. Sabharwal and J. M. Lee, Adv. Energy Mater., 2019, 9, 1900090.
55. S. Xie, S. Lin, Q. Zhang, Z. Tian and Y. Wang, J. Energy Chem., 2018, 27, 1629–1636.
56. X. Meng, X. Cui, N. P. Rajan, L. Yu, D. Deng and X. Bao, Chem, 2019, 5, 2296–2325.
57. T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature, 1979, 277, 637–638.
58. S. C. Roy, O. K. Varghese, M. Paulose and C. A. Grimes, ACS Nano, 2010, 4, 1259–1278.
59. M. R. Hoffmann, J. A. Moss and M. M. Baum, Dalton Trans., 2011, 40, 5151–5158.
60. L. Wei, C. Yu, Q. Zhang, H. Liu and Y. Wang, J. Mater. Chem. A, 2018, 6, 22411–22436.
61. F. M. Mota and D. H. Kim, Chem. Soc. Rev., 2019, 48, 205–259.
62. T. Mizuno, K. Adachi, K. Ohta and A. Saji, J. Photochem. Photobiol., A, 1996, 98, 87–90.
63. J. Mao, T. Peng, X. Zhang, K. Li, L. Ye and L. Zan, Catal. Sci. Technol., 2013, 3, 1253–1260.
64. Y. Liu, B. Huang, Y. Dai, X. Zhang, X. Qin, M. Jiang and M.-H. Whangbo, Catal. Commun., 2009, 11, 210–213.
65. W. Dai, J. Yu, Y. Deng, X. Hu, T. Wang and X. Luo, Appl. Surf. Sci., 2017, 403, 230–239.
66. P. Liou, S. Chen, J. Wu, D. Liu, S. Mackintosh, M. Maroto-Valer and R. Linforth, Energy Environ. Sci., 2011, 4, 1487–1494.
67. H. Park, H. Ou, A. Colussi and M. R. Hoffmann, J. Phys. Chem. A, 2015, 119, 4658–4666.
68. O. Ola and M. M. Maroto-Valer, Appl. Catal., A, 2015, 502, 114–121.
69. M. Cheng, S. Yang, R. Chen, X. Zhu, Q. Liao and Y. Huang, Int. J. Hydrogen Energy, 2017, 42, 9722–9732.
70. C. Wang, R. L. Thompson, P. Ohodnicki, J. Baltrus and C. Matranga, J. Mater. Chem., 2011, 21, 13452–13457.
71. M. A. Asi, C. He, M. Su, D. Xia, L. Lin, H. Deng, Y. Xiong, R. Qiu and X. Li, Catal. Today, 2011, 175, 256–263.
72. H. Li, S. Gan, H. Wang, D. Han and L. Niu, Adv. Mater., 2015, 27, 6906–6913.
73. X. Xia, Z. Jia, Y. Yu, Y. Liang, Z. Wang and L. Ma, Carbon, 2007, 45, 717–721.
74. L. M. Pastrana-Martínez, A. M. T. Silva, N. N. C. Fonseca, J. R. Vaz, J. L. Figueiredo and J. L. Faria, Top. Catal., 2016, 59, 1279–1291.
75. W. Tu, Y. Zhou, Q. Liu, S. Yan, S. Bao, X. Wang, M. Xiao and Z. Zou, Adv. Funct. Mater., 2013, 23, 1743–1749.
76. S. Sorcar, J. Thompson, Y. Hwang, Y. H. Park, T. Majima, C. A. Grimes, J. R. Durrant and S. In, Energy Environ. Sci., 2018, 11, 3183–3193.
77. L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, P. Wong, J. C. Yu and Y. Yu, ACS Catal., 2016, 6, 6444–6454.
78. T. Yui, A. Kan, C. Saitoh, K. Koike, T. Ibusuki and O. Ishitani, ACS Appl. Mater. Interfaces, 2011, 3, 2594–2600.
79. W. Kim, T. Seok and W. Choi, Energy Environ. Sci., 2012, 5, 6066–6070.
80. Y. Zhu, Z. Xu, Q. Lang, W. Jiang, Q. Yin, S. Zhong and S. Bai, Appl. Catal., B, 2017, 206, 282–292.
81. V. Jeyalakshmi, R. Mahalakshmy, K. R. Krishnamurthy and B. Viswanathan, Catal. Today, 2016, 266, 160–167.
82. K. Adachi, K. Ohta and T. Mizuno, Sol. Energy, 1994, 53, 187–190.
83. O. K. Varghese, M. Paulose, T. J. LaTempa and C. A. Grimes, Nano Lett., 2009, 9, 731–737.
84. X. Zhang, F. Han, B. Shi, S. Farsinezhad, G. P. Dechaine and K. Shankar, Angew. Chem., Int. Ed., 2012, 51, 12732–12735.
85. T. Nguyen and J. C. S. Wu, Appl. Catal., A, 2008, 335, 112–120.
86. W. Hou, W. Hung, P. Pavaskar, A. Goeppert, M. Aykol and S. B. Cronin, ACS Catal., 2011, 1, 929–936.
87. S. Yu and P. K. Jain, Nat. Commun., 2019, 10, 2022.
88. J. Paul and F. M. Hoffmann, Catal. Lett., 1988, 1, 445–456.
89. C. Lee, R. Antoniou Kourounioti, J. C. S. Wu, E. Murchie, M. Maroto-Valer, O. E. Jensen, C. Huang and A. Ruban, J. CO2 Util., 2014, 5, 33–40.
90. J. Yang, D. Wang, H. Han and C. Li, Acc. Chem. Res., 2013, 46, 1900–1909.
91. Y. Zhang, B. Xia, J. Ran, K. Davey and S.-Z. Qiao, Adv. Energy Mater., 2020, 10, 1903879.
92. S. Xie, Y. Wang, Q. Zhang, W. Deng and Y. Wang, ACS Catal., 2014, 4, 3644–3653.
93. M. Subrahmanyam, S. Kaneco and N. Alonso-Vante, Appl. Catal., B, 1999, 23, 169–174.
94. M. B. Gawande, A. Goswami, F.-X. Felpin, T. Asefa, X. Huang, R. Silva, X. Zou, R. Zboril and R. S. Varma, Chem. Rev., 2016, 116, 3722–3811.
95. Q. Zhai, S. Xie, W. Fan, Q. Zhang, Y. Wang, W. Deng and Y. Wang, Angew. Chem., Int. Ed., 2013, 52, 5776–5779.
96. S. Sorcar, Y. Hwang, J. Lee, H. Kim, K. M. Grimes, C. A. Grimes, J. Jung, C. Cho, T. Majima, M. R. Hoffmann and S. In, Energy Environ. Sci., 2019, 12, 2685–2696.
97. M. Tahir, Appl. Catal., B, 2017, 219, 329–343.
98. Q. Kang, T. Wang, P. Li, L. Liu, K. Chang, M. Li and J. Ye, Angew. Chem., Int. Ed., 2015, 54, 841–845.
99. Q. Chen, X. Chen, M. Fang, J. Chen, Y. Li, Z. Xie, Q. Kuang and L. Zheng, J. Mater. Chem. A, 2019, 7, 1334–1340.
100. S. Yu, A. J. Wilson, J. Heo and P. K. Jain, Nano Lett., 2018, 18, 2189–2194.
101. M. A. Asi, L. Zhu, C. He, V. K. Sharma, D. Shu, S. Li, J. Yang and Y. Xiong, Catal. Today, 2013, 216, 268–275.
102. W. H. Koppenol and J. D. Rush, J. Phys. Chem., 1987, 91, 4429–4430.
103. H.-J. Freund and M. W. Roberts, Surf. Sci. Rep., 1996, 25, 225–273.
104. N.-N. Vu, S. Kaliaguine and T.-O. Do, Adv. Funct. Mater., 2019, 29, 1901825.
105. L. Mino, G. Spoto and A. M. Ferrari, J. Phys. Chem. C, 2014, 118, 25016–25026.
106. H. He, P. Zapol and L. A. Curtiss, Energy Environ. Sci., 2012, 5, 6196–6205.
107. B. R. Eggins, P. K. J. Robertson, J. H. Stewart and E. Woods, J. Chem. Soc., Chem. Commun., 1993, 349–350.
108. B. R. Eggins, P. K. J. Robertson, E. P. Murphy, E. Woods and J. T. S. Irvine, J. Photochem. Photobiol., A, 1998, 118, 31–40.
109. I. A. Shkrob, T. W. Marin, H. He and P. Zapol, J. Phys. Chem. C, 2012, 116, 9450–9460.
110. Z. Li, J. Liu, Y. Zhao, G. I. N. Waterhouse, G. Chen, R. Shi, X. Zhang, X. Liu, Y. Wei, X.-D. Wen, L.-Z. Wu, C.-H. Tung and T. Zhang, Adv. Mater., 2018, 30, 1800527.
111. Y. Zhao, Z. Li, M. Li, J. Liu, X. Liu, G. I. N. Waterhouse, Y. Wang, J. Zhao, W. Gao, Z. Zhang, R. Long, Q. Zhang, L. Gu, X. Liu, X. Wen, D. Ma, L.-Z. Wu, C.-H. Tung and T. Zhang, Adv. Mater., 2018, 30, 1803127.
112. W. Gao, R. Gao, Y. Zhao, M. Peng, C. Song, M. Li, S. Li, J. Liu, W. Li, Y. Deng, M. Zhang, J. Xie, G. Hu, Z. Zhang, R. Long, X.-D. Wen and D. Ma, Chem, 2018, 4, 2917–2928.
113. X. Guo, Z. Jiao, G. Jin and X. Guo, ACS Catal., 2015, 5, 3836–3840.
114. L. Wang, Y. Zhang, X. Gu, Y. Zhang and H. Su, Catal. Sci. Technol., 2018, 8, 601–610.
115. Y. Zhao, B. Zhao, J. Liu, G. Chen, R. Gao, S. Yao, M. Li, Q. Zhang, L. Gu, J. Xie, X. Wen, L.-Z. Wu, C.-H. Tung, D. Ma and T. Zhang, Angew. Chem., Int. Ed., 2016, 55, 4215–4219.
116. L. Zhang, Z.-J. Zhao and J. Gong, Angew. Chem., Int. Ed., 2017, 56, 11326–11353.
117. L. Fan, C. Xia, F. Yang, J. Wang, H. Wang and Y. Lu, Sci. Adv., 2020, 6, eaay3111.
118. D. U. Nielsen, X.-M. Hu, K. Daasbjerg and T. Skrydstrup, Nat. Catal., 2018, 1, 244–254.
119. D. M. Weekes, D. A. Salvatore, A. Reyes, A. Huang and C. P. Berlinguette, Acc. Chem. Res., 2018, 51, 910–918.
120. D. Higgins, C. Hahn, C. Xiang, T. F. Jaramillo and A. Z. Weber, ACS Energy Lett., 2019, 4, 317–324.
121. T. Burdyny and W. A. Smith, Energy Environ. Sci., 2019, 12, 1442–1453.
122. J. He, K. E. Dettelbach, D. A. Salvatore, T. Li and C. P. Berlinguette, Angew. Chem., Int. Ed., 2017, 56, 6068–6072.
123. W. Ma, S. Xie, X.-G. Zhang, F. Sun, J. Kang, Z. Jiang, Q. Zhang, D.-Y. Wu and Y. Wang, Nat. Commun., 2019, 10, 892.
124. W. Ma, M. Xie, S. Xie, L. Wei, Y. Cai, Q. Zhang and Y. Wang, J. Energy Chem., 2021, 54, 422–428.
125. A. Vasileff, X. Zhi, C. Xu, L. Ge, Y. Jiao, Y. Zheng and S.-Z. Qiao, ACS Catal., 2019, 9, 9411–9417.
126. R. P. Jansonius, L. M. Reid, C. N. Virca and C. P. Berlinguette, ACS Energy Lett., 2019, 4, 980–986.
127. Y. Zheng, A. Vasileff, X. Zhou, Y. Jiao, M. Jaroniec and S.-Z. Qiao, J. Am. Chem. Soc., 2019, 141, 7646–7659.
128. X. Liu, J. Xiao, H. Peng, X. Hong, K. Chan and J. K. Nørskov, Nat. Commun., 2017, 8, 15438.
129. H. Zhang, J. Li, M. Cheng and Q. Lu, ACS Catal., 2019, 9, 49–65.
130. N. S. Romero Cuellar, K. Wiesner-Fleischer, M. Fleischer, A. Rucki and O. Hinrichsen, Electrochim. Acta, 2019, 307, 164–175.
131. X. Zhou and C. Xiang, ACS Energy Lett., 2018, 3, 1892–1897.
132. X. Liu, M. Wang, H. Yin, J. Hu, K. Cheng, J. Kang, Q. Zhang and Y. Wang, ACS Catal., 2020, 10, 8303–8314.
133. X. Liu, W. Zhou, Y. Yang, K. Cheng, J. Kang, L. Zhang, G. Zhang, X. Min, Q. Zhang and Y. Wang, Chem. Sci., 2018, 9, 4708–4718.
134. K. Jiang, R. B. Sandberg, A. J. Akey, X. Liu, D. C. Bell, J. K. Nørskov, K. Chan and H. Wang, Nat. Catal., 2018, 1, 111–119.
135. A. Loiudice, P. Lobaccaro, E. A. Kamali, T. Thao, B. H. Huang, J. W. Ager and R. Buonsanti, Angew. Chem., Int. Ed., 2016, 55, 5789–5792.
136. H. Mistry, A. S. Varela, C. S. Bonifacio, I. Zegkinoglou, I. Sinev, Y.-W. Choi, K. Kisslinger, E. A. Stach, J. C. Yang, P. Strasser and B. R. Cuenya, Nat. Commun., 2016, 7, 12123.
137. D. Ren, Y. Deng, A. D. Handoko, C. S. Chen, S. Malkhandi and B. S. Yeo, ACS Catal., 2015, 5, 2814–2821.
138. B. Zhang, J. Zhang, M. Hua, Q. Wan, Z. Su, X. Tan, L. Liu, F. Zhang, G. Chen, D. Tan, X. Cheng, B. Han, L. Zheng and G. Mo, J. Am. Chem. Soc., 2020, 142, 13606–13613.
139. Y. Zhou, F. Che, M. Liu, C. Zou, Z. Liang, P. De Luna, H. Yuan, J. Li, Z. Wang, H. Xie, H. Li, P. Chen, E. Bladt, R. Quintero-Bermudez, T.-K. Sham, S. Bals, J. Hofkens, D. Sinton, G. Chen and E. H. Sargent, Nat. Chem., 2018, 10, 974–980.
140. Z.-Q. Liang, T.-T. Zhuang, A. Seifitokaldani, J. Li, C.-W. Huang, C.-S. Tan, Y. Li, P. De Luna, C. T. Dinh, Y. Hu, Q. Xiao, P.-L. Hsieh, Y. Wang, F. Li, R. Quintero-Bermudez, Y. Zhou, P. Chen, Y. Pang, S.-C. Lo, L.-J. Chen, H. Tan, Z. Xu, S. Zhao, D. Sinton and E. H. Sargent, Nat. Commun., 2018, 9, 3828.
141. D. Gao, I. Sinev, F. Scholten, R. M. Arán-Ais, N. J. Divins, K. Kvashnina, J. Timoshenko and B. R. Cuenya, Angew. Chem., Int. Ed., 2019, 58, 17047–17053.
142. W. Ma, S. Xie, T. Liu, Q. Fan, J. Ye, F. Sun, Z. Jiang, Q. Zhang, J. Cheng and Y. Wang, Nat. Catal., 2020, 3, 478–487.
143. C.-T. Dinh, T. Burdyny, M. G. Kibria, A. Seifitokaldani, C. M. Gabardo, F. P. García de Arquer, A. Kiani, J. P. Edwards, P. De Luna, O. S. Bushuyev, C. Zou, R. Quintero-Bermudez, Y. Pang, D. Sinton and E. H. Sargent, Science, 2018, 360, 783–787.
144. M. Zhong, K. Tran, Y. Min, C. Wang, Z. Wang, C. T. Dinh, P. De Luna, Z. Yu, A. S. Rasouli, P. Brodersen, S. Sun, O. Voznyy, C. S. Tan, M. Askerka, F. Che, M. Liu, A. Seifitokaldani, Y. Pang, S. C. Lo, A. Ip, Z. Ulissi and E. H. Sargent, Nature, 2020, 581, 178–183.
145. F. P. G. de Arquer, C. T. Dinh, A. Ozden, J. Wicks, C. McCallum, A. R. Kirmani, D. H. Nam, C. Gabardo, A. Seifitokaldani, X. Wang, Y. C. Li, F. Li, J. Edwards, L. J. Richter, S. J. Thorpe, D. Sinton and E. H. Sargent, Science, 2020, 367, 661–666.
146. F. Li, A. Thevenon, A. Rosas-Hernández, Z. Wang, Y. Li, C. M. Gabardo, A. Ozden, C. T. Dinh, J. Li, Y. Wang, J. P. Edwards, Y. Xu, C. McCallum, L. Tao, Z.-Q. Liang, M. Luo, X. Wang, H. Li, C. P. O’Brien, C.-S. Tan, D.-H. Nam, R. Quintero-Bermudez, T.-T. Zhuang, Y. C. Li, Z. Han, R. D. Britt, D. Sinton, T. Agapie, J. C. Peters and E. H. Sargent, Nature, 2020, 577, 509–513.
147. K. D. Yang, W. R. Ko, J. H. Lee, S. J. Kim, H. Lee, M. H. Lee and K. T. Nam, Angew. Chem., Int. Ed., 2017, 56, 796–800.
148. A. Vasileff, Y. Zhu, X. Zhi, Y. Zhao, L. Ge, H. M. Chen, Y. Zheng and S.-Z. Qiao, Angew. Chem., Int. Ed., 2020, 59, 19649–19653.
149. D. Ren, B. S.-H. Ang and B. S. Yeo, ACS Catal., 2016, 6, 8239–8247.
150. S. Lee, G. Park and J. Lee, ACS Catal., 2017, 7, 8594–8604.
151. T. T. H. Hoang, S. Verma, S. Ma, T. T. Fister, J. Timoshenko, A. I. Frenkel, P. J. A. Kenis and A. A. Gewirth, J. Am. Chem. Soc., 2018, 140, 5791–5797.
152. Y. C. Li, Z. Wang, T. Yuan, D.-H. Nam, M. Luo, J. Wicks, B. Chen, J. Li, F. Li, F. P. G. de Arquer, Y. Wang, C.-T. Dinh, O. Voznyy, D. Sinton and E. H. Sargent, J. Am. Chem. Soc., 2019, 141, 8584–8591.
153. T. T. Zhuang, Z. Q. Liang, A. Seifitokaldani, Y. Li, P. De Luna, T. Burdyny, F. Che, F. Min, R. Quintero-Bermudez, C. T. Dinh, Y. Pang, M. Zhong, B. Zhang, J. Li, P. N. Chen, X. L. Zheng, H. Liang, W. N. Ge, B. L. J. Ye, D. Sinton, S. H. Yu and E. H. Sargent, Nat. Catal., 2018, 1, 421–428.
154. F. Li, Y. C. Li, Z. Wang, J. Li, D.-H. Nam, Y. Lum, M. Luo, X. Wang, A. Ozden, S.-F. Hung, B. Chen, Y. Wang, J. Wicks, Y. Xu, Y. Li, C. M. Gabardo, C.-T. Dinh, Y. Wang, T.-T. Zhuang, D. Sinton and E. H. Sargent, Nat. Catal., 2020, 3, 75–82.
155. Y. Song, R. Peng, D. K. Hensley, P. V. Bonnesen, L. Liang, Z. Wu, H. M. Meyer, M. Chi, C. Ma, B. G. Sumpter and A. J. Rondinone, ChemistrySelect, 2016, 1, 6055–6061.
156. X. Wang, Z. Wang, F. P. G. de Arquer, C. T. Dinh, A. Ozden, Y. C. Li, D. H. Nam, J. Li, Y. S. Liu, J. Wicks, Z. Chen, M. Chi, B. Chen, Y. Wang, J. Tam, J. Y. Howe, A. Proppe, P. Todorović, F. Li, T. T. Zhuang, C. M. Gabardo, A. R. Kirmani, C. McCallum, S. F. Hung, Y. Lum, M. Luo, Y. Min, A. Xu, C. P. O’Brien, B. Stephen, B. Sun, A. H. Ip, L. J. Richter, S. O. Kelley, D. Sinton and E. H. Sargent, Nat. Energy, 2020, 5, 478–486.
157. C. Chen, X. Yan, S. Liu, Y. Wu, Q. Wan, X. Sun, Q. Zhu, H. Liu, J. Ma, L. Zheng, H. Wu and B. Han, Angew. Chem., Int. Ed., 2020, 59, 16459–16464.
158. H. Xu, D. Rebollar, H. He, L. Chong, Y. Liu, C. Liu, C.-J. Sun, T. Li, J. V. Muntean, R. E. Winans, D.-J. Liu and T. Xu, Nat. Energy, 2020, 5, 623–632.
159. Y. Liu, S. Chen, X. Quan and H. Yu, J. Am. Chem. Soc., 2015, 137, 11631–11636.
160. Y. Liu, Y. Zhang, K. Cheng, X. Quan, X. Fan, Y. Su, S. Chen, H. Zhao, Y. Zhang, H. Yu and M. R. Hoffmann, Angew. Chem., Int. Ed., 2017, 56, 15607–15611.
161. Y. Song, S. Wang, W. Chen, S. Li, G. Feng, W. Wei and Y. Sun, ChemSusChem, 2020, 13, 293–297.
162. J. Wu, S. Ma, J. Sun, J. I. Gold, C. Tiwary, B. Kim, L. Zhu, N. Chopra, I. N. Odeh, R. Vajtai, A. Z. Yu, R. Luo, J. Lou, G. Ding, P. J. A. Kenis and P. M. Ajayan, Nat. Commun., 2016, 7, 13869.
163. M. Jouny, W. Luc and F. Jiao, Nat. Catal., 2018, 1, 748–755.
164. J. Li, Z. Wang, C. McCallum, Y. Xu, F. Li, Y. Wang, C. M. Gabardo, C.-T. Dinh, T.-T. Zhuang, L. Wang, J. Y. Howe, Y. Ren, E. H. Sargent and D. Sinton, Nat. Catal., 2019, 2, 1124–1131.
165. C. W. Li, J. Ciston and M. W. Kanan, Nature, 2014, 508, 504–507.
166. X. Feng, K. Jiang, S. Fan and M. W. Kanan, ACS Cent. Sci., 2016, 2, 169–174.
167. D. Raciti, L. Cao, K. J. T. Livi, P. F. Rottmann, X. Tang, C. Li, Z. Hicks, K. H. Bowen, K. J. Hemker, T. Mueller and C. Wang, ACS Catal., 2017, 7, 4467–4472.
168. J. Li, A. Xu, F. Li, Z. Wang, C. Zou, C. M. Gabardo, Y. Wang, A. Ozden, Y. Xu, D.-H. Nam, Y. Lum, J. Wicks, B. Chen, Z. Wang, J. Chen, Y. Wen, T. Zhuang, M. Luo, X. Du, T.-K. Sham, B. Zhang, E. H. Sargent and D. Sinton, Nat. Commun., 2020, 11, 3685.
169. L. Wang, S. Nitopi, A. B. Wong, J. L. Snider, A. C. Nielander, C. G. Morales-Guio, M. Orazov, D. C. Higgins, C. Hahn and T. F. Jaramillo, Nat. Catal., 2019, 2, 702–708.
170. L. Wang, D. C. Higgins, Y. Ji, C. G. Morales-Guio, K. Chan, C. Hahan and T. F. Jaramillo, Proc. Natl. Acad. Sci. U. S. A., 2020, 117, 12572–12575.
171. D. S. Ripatti, T. R. Veltman and M. W. Kanan, Joule, 2019, 3, 240–256.
172. W. Luc, X. Fu, J. Shi, J.-J. Lv, M. Jouny, B. H. Ko, Y. Xu, Q. Tu, X. Hu, J. Wu, Q. Yue, Y. Liu, F. Jiao and Y. Kang, Nat. Catal., 2019, 2, 423–430.
173. T. T. Zhuang, Y. Pang, Z.-Q. Liang, Z. Wang, Y. Li, C.-S. Tan, J. Li, C. T. Dinh, P. De Luna, P.-L. Hsieh, T. Burdyny, H.-H. Li, M. Liu, Y. Wang, F. Li, A. Proppe, A. Johnston, D.-H. Nam, Z.-Y. Wu, Y.-R. Zheng, A. H. Ip, H. Tan, L.-J. Chen, S.-H. Yu, S. O. Kelley, D. Sinton and E. H. Sargent, Nat. Catal., 2018, 1, 946–951.
174. Y. Pang, J. Li, Z. Wang, C.-S. Tan, P.-L. Hsieh, T.-T. Zhuang, Z.-Q. Liang, C. Zou, X. Wang, P. De Luna, J. P. Edwards, Y. Xu, F. Li, C.-T. Dinh, M. Zhong, Y. Lou, D. Wu, L.-J. Chen, E. H. Sargent and D. Sinton, Nat. Catal., 2019, 2, 251–258.
175. J. Li, F. Che, Y. Pang, C. Zou, J. Y. Howe, T. Burdyny, J. P. Edwards, Y. Wang, F. Li, Z. Wang, P. De Luna, C.-T. Dinh, T.-T. Zhuang, M. I. Saidaminov, S. Cheng, T. Wu, Y. Z. Finfrock, L. Ma, S.-H. Hsieh, Y.-S. Liu, G. A. Botton, W.-F. Pong, X. Du, J. Guo, T.-K. Sham, E. H. Sargent and D. Sinton, Nat. Commun., 2018, 9, 4614.
176. Y. Hori, I. Takahashi, O. Koga and N. Hoshi, J. Phys. Chem. B, 2002, 106, 15–17.
177. Y. Hori, I. Takahashi, O. Koga and N. Hoshi, J. Mol. Catal. A: Chem., 2003, 199, 39–47.
178. K. J. P. Schouten, E. Pérez Gallent and M. T. M. Koper, ACS Catal., 2013, 3, 1292–1295.
179. C. Hahn, T. Hatsukade, Y.-G. Kim, A. Vailionis, J. H. Baricuatro, D. C. Higgins, S. A. Nitopi, M. P. Soriaga and T. F. Jaramillo, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 5918–5923.
180. Y. Huang, A. D. Handoko, P. Hirunsit and B. S. Yeo, ACS Catal., 2017, 7, 1749–1756.
181. F. S. Roberts, K. P. Kuhl and A. Nilsson, Angew. Chem., Int. Ed., 2015, 54, 5179–5182.
182. H. Xiao, W. A. Goddard III, T. Cheng and Y. Liu, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 6685–6688.
183. M. Favaro, H. Xiao, T. Cheng, W. A. Goddard III, J. Yano and E. J. Crumlin, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 6706–6711.
184. A. Eilert, F. Cavalca, F. S. Roberts, J. Osterwalder, C. Liu, M. Favaro, E. J. Crumlin, H. Ogasawara, D. Friebel, L. G. M. Pettersson and A. Nilsson, J. Phys. Chem. Lett., 2017, 8, 285–290.
185. Y. Lum and J. W. Ager, Angew. Chem., Int. Ed., 2018, 57, 551–554.
186. L. Mandal, K. R. Yang, M. R. Motapothula, D. Ren, P. Lobaccaro, A. Patra, M. Sherburne, V. S. Batista, B. S. Yeo, J. W. Ager, J. Martin and T. Venkatesan, ACS Appl. Mater. Interfaces, 2018, 10, 8574–8584.
187. C. W. Li and M. W. Kanan, J. Am. Chem. Soc., 2012, 134, 7231–7234.
188. J.-J. Velasco-Vélez, T. Jones, D. Gao, E. Carbonio, R. Arrigo, C.-J. Hsu, Y.-C. Huang, C.-L. Dong, J.-M. Chen, J.-F. Lee, P. Strasser, B. Roldan Cuenya, R. Schlögl, A. Knop-Gericke and C.-H. Chuang, ACS Sustainable Chem. Eng., 2019, 7, 1485–1492.
189. Y. Zhao, X. Chang, A. S. Malkani, X. Yang, L. Thompson, F. Jiao and B. Xu, J. Am. Chem. Soc., 2020, 142, 9735–9743.
190. C. Chen, X. Sun, L. Lu, D. Yang, J. Ma, Q. Zhu, Q. Qian and B. Han, Green Chem., 2018, 20, 4579–4583.
191. T. Shinagawa, G. O. Larrazábal, A. J. Martín, F. Krumeich and J. Pérez-Ramírez, ACS Catal., 2018, 8, 837–844.
192. Y. Huang, Y. Deng, A. D. Handoko, G. K. L. Goh and B. S. Yeo, ChemSusChem, 2018, 10, 1–8.
193. R. García-Muelas, F. Dattila, T. Shinagawa, A. J. Martín, J. Pérez-Ramírez and N. López, J. Phys. Chem. Lett., 2018, 9, 7153–7159.
194. D. Gao, F. Scholten and B. R. Cuenya, ACS Catal., 2017, 8, 5112–5120.
195. Y. Huang, C. W. Ong and B. S. Yeo, ChemSusChem, 2018, 11, 3299–3306.
196. D. Gao, I. T. McCrum, S. Deo, Y. W. Choi, F. Scholten, W. Wan, J. G. Chen, M. J. Janik and B. R. Cuenya, ACS Catal., 2018, 8, 10012–10020.
197. J. Li, K. Chang, H. Zhang, M. He, W. A. Goddard, J. G. Chen, M.-J. Cheng and Q. Lu, ACS Catal., 2019, 9, 4709–4718.
198. R. Chen, H.-Y. Su, D. Liu, R. Huang, X. Meng, X. Cui, Z.-Q. Tian, D. H. Zhang and D. Deng, Angew. Chem., Int. Ed., 2020, 59, 154–160.
199. S. Ma, M. Sadakiyo, R. Luo, M. Heima, M. Yamauchi and P. J. A. Kenis, J. Power Sources, 2016, 301, 219–228.
200. C. M. Gabardo, C. P. O’Brien, J. P. Edwards, C. McCallum, Y. Xu, C.-T. Dinh, J. Li, E. H. Sargent and D. Sinton, Joule, 2019, 3, 2777–2791.
201. M. Ma, K. Djanashvili and W. A. Smith, Angew. Chem., Int. Ed., 2016, 55, 6680–6684.
202. L. Wang, S. A. Nitopi, E. Bertheussen, M. Orazov, C. G. Morales-Guio, X. Liu, D. C. Higgins, K. Chan, J. K. Nørskov, C. Hahn and T. F. Jaramillo, ACS Catal., 2018, 8, 7445–7454.
203. J. Li, D. Wu, A. S. Malkani, X. Chang, M. J. Cheng, B. Xu and Q. Lu, Angew. Chem., Int. Ed., 2020, 59, 4464–4469.
204. X. Wang, J. F. de Araújo, W. Ju, A. Bagger, H. Schmies, S. Kühl, J. Rossmeisl and P. Strasser, Nat. Nanotechnol., 2019, 14, 1063–1070.
205. V. Subramani and S. K. Gangwal, Energy Fuels, 2008, 22, 814–839.
206. J. Pang, M. Zheng and T. Zhang, Adv. Catal., 2019, 64, 89–191.
207. A. Hira, Energy Policy, 2011, 39, 6925–6935.
208. Q. Liu, H. Wang, H. Xin, C. Wang, L. Yan, Y. Wang, Q. Zhang, X. Zhang, Y. Xu, G. W. Huber and L. Ma, ChemSusChem, 2019, 12, 3977–3987.
209. H. T. Luk, C. Mondelli, D. C. Ferré, J. A. Stewart and J. Pérez-Ramírez, Chem. Soc. Rev., 2017, 46, 1358–1426.
210. H. Song, P. Wang, S. Li, W. Deng, Y. Li, Q. Zhang and Y. Wang, Chem. Commun., 2019, 55, 4303–4306.
211. M. Yang, H. Qi, F. Liu, Y. Ren, X. Pan, L. Zhang, X. Liu, H. Wang, J. Pang, M. Zheng, A. Wang and T. Zhang, Joule, 2019, 3, 1937–1948.
212. C. Li, G. Xu, C. Wang, L. Ma, Y. Qiao, Y. Zhang and Y. Fu, Green Chem., 2019, 21, 2234–2239.
213. F. Calle-Vallejo and M. T. M. Koper, Angew. Chem., Int. Ed., 2013, 52, 7282–7285.
214. C. G. Morales-Guio, E. R. Cave, S. A. Nitopi, J. T. Feaster, L. Wang, K. P. Kuhl, A. Jackson, N. C. Johnson, D. N. Abram, T. Hatsukade, C. Hahn and T. F. Jaramillo, Nat. Catal., 2018, 1, 764–771.
215. E. L. Clark, C. Hahn, T. F. Jaramillo and A. T. Bell, J. Am. Chem. Soc., 2017, 139, 15848–15857.
216. R. M. Arán-Ais, F. Scholtenm, S. Kunze, R. Rizo and B. R. Cuenya, Nat. Energy, 2020, 5, 317–325.
217. Gurudayal, D. Perone, S. Malani, Y. Lum, S. Haussener and J. W. Ager, ACS Appl. Energy Mater., 2019, 2, 4551–4559.
218. Y. Jiao, Y. Zheng, P. Chen, M. Jaroniec and S.-Z. Qiao, J. Am. Chem. Soc., 2017, 139, 18093–18100.
219. A. Verdaguer-Casadevall, C. W. Li, T. P. Johansson, S. B. Scott, J. T. McKeown, M. Kumar, I. E. L. Stephen, M. W. Kanan and I. Chorkendorff, J. Am. Chem. Soc., 2015, 137, 9808–9811.
220. D. Kim, C. S. Kley and P. Yang, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 10560–10565.
221. M. Rahaman, A. Dutta, A. Zanetti and P. Broekmann, ACS Catal., 2017, 7, 7946–7956.
222. R. Kortlever, I. Peters, C. Balemans, R. Kas, Y. Kwon, G. Mul and M. T. M. Koper, Chem. Commun., 2016, 52, 10229–10232.
223. F. Studt, I. Sharafutdinov, F. Abild-Pedersen, C. F. Elkjær, J. S. Hummelshøj, S. Dahl, I. Chorkendorff and J. K. Nøskov, Nat. Chem., 2014, 6, 320–324.
224. D. A. Torelli, S. A. Francis, J. C. Crompton, A. Javier, J. R. Thompson, B. S. Brunschwig, M. P. Soriaga and N. S. Lewis, ACS Catal., 2016, 6, 2100–2104.
225. A. R. Paris and A. B. Bocarsly, ACS Catal., 2017, 7, 6815–6820.
226. Y. Song, W. Chen, C. Zhao, S. Li, W. Wei and Y. Sun, Angew. Chem., Int. Ed., 2017, 56, 10840–10844.
227. X. Zhi, Y. Jiao, Y. Zheng and S.-Z. Qiao, Small, 2019, 15, 1804224.
228. J. Liu, C. Guo, A. Vasileff and S.-Z. Qiao, Small Methods, 2017, 1, 1600006.
229. Y. Liu, X. Fan, A. Nayak, Y. Wang, B. Shan, X. Quan and T. J. Meyer, Proc. Natl. Acad. Sci. U. S. A., 2019, 116, 26353–26358.
230. K. Lv, Y. Fan, Y. Zhu, Y. Yuan, J. Wang, Y. Zhu and Q. Zhang, J. Mater. Chem. A, 2018, 6, 5025–5031.
231. J. Du, S. Li, S. Liu, Y. Xin, B. Chen, H. Liu and B. Han, Chem. Sci., 2020, 11, 5098–5104.
232. K. J. P. Schouten, Y. Kwon, C. J. M. van der Ham, Z. Qin and M. T. M. Koper, Chem. Sci., 2011, 2, 1902–1909.
233. J. H. Montoya, C. Shi, K. Chan and J. K. Nørskov, J. Phys. Chem. Lett., 2015, 6, 2032–2037.
234. X. Liu, P. Schlexer, J. Xiao, Y. Ji, L. Wang, R. B. Sandberg, M. Tang, K. S. Brown, H. Peng, S. Ringe, C. Hahn, T. F. Jaramillo, J. K. Nørskov and K. Chan, Nat. Commun., 2019, 10, 32.
235. E. Pérez-Gallent, M. C. Figueiredo, F. Calle-Vallejo and M. T. M. Koper, Angew. Chem., Int. Ed., 2017, 56, 3621–3624.
236. J. H. Montoya, A. A. Peterson and J. K. Nørskov, ChemCatChem, 2013, 5, 737–742.
237. T. Cheng, H. Xiao and W. A. Goddard III, Proc. Natl. Acad. Sci. U. S. A., 2017, 114, 1795–1800.
238. X. Zhi, Y. Jiao, Y. Zheng, A. Vasileff and S.-Z. Qiao, Nano Energy, 2020, 71, 104601.
239. A. J. Darza, A. T. Bell and M. Head-Gordon, ACS Catal., 2018, 8, 1490–1499.
240. Y. Y. Birdja, E. Pérez-Gallent, M. C. Figueiredo, A. J. Göttle, F. Calle-Vallejo and M. T. M. Koper, Nat. Energy, 2019, 4, 732–745.
241. I. Ledezma-Yanez, E. P. Gallent, M. T. M. Koper and F. Calle-Vallejo, Catal. Today, 2016, 262, 90–94.
242. Z. Guo, B. Liu, Q. Zhang, W. Deng, Y. Wang and Y. Yang, Chem. Soc. Rev., 2014, 43, 3480–3524.
243. P. Tang, Q. Zhu, Z. Wu and D. Ma, Energy Environ. Sci., 2014, 7, 2580–2591.
244. B. A. Arndtsen, R. G. Bergman, T. A. Mobley and T. H. Peterson, Acc. Chem. Res., 1995, 28, 154–162.
245. H. Song, X. Meng, Z. Wang, H. Liu and J. Ye, Joule, 2019, 3, 1606–1636.
246. J. Baltrusaitis, I. Jansen and J. D. Schuttlefield Christus, Catal. Sci. Technol., 2014, 4, 2397–2411.
247. L. Yuliati and H. Yoshida, Chem. Soc. Rev., 2008, 37, 1592–1602.
248. K. Shimura and H. Yoshida, Catal. Surv. Asia, 2014, 18, 24–33.
249. Y. Kato, H. Yoshida and T. Hattori, Chem. Commun., 1998, 2389–2390.
250. H. Yoshida, N. Matsushita, Y. Kato and T. Hattori, J. Phys. Chem. B, 2003, 107, 8355–8362.
251. L. Yuliati, M. Tsubota, A. Satsuma, H. Itoh and H. Yoshida, J. Catal., 2006, 238, 214–220.
252. L. Yuliati, T. Hattori and H. Yoshida, Phys. Chem. Chem. Phys., 2005, 7, 195–201.
253. L. Yuliati, T. Hattori, H. Itoh and H. Yoshida, J. Catal., 2008, 257, 396–402.
254. L. Yuliati, T. Hamajima, T. Hattori and H. Yoshida, J. Phys. Chem. C, 2008, 112, 7223–7232.
255. L. Li, G. Li, C. Yan, X. Mu, X. Pan, X. Zou, K. Wang and J. Chen, Angew. Chem., Int. Ed., 2011, 50, 8299–8303.
256. L. Li, Y. Cai, G. Li, X. Mu, K. Wang and J. Chen, Angew. Chem., Int. Ed., 2012, 51, 4702–4706.
257. L. Meng, Z. Chen, Z. Ma, S. He, Y. Hou, H. Li, R. Yuan, X. Huang, X. Wang, X. Wang and J. Long, Energy Environ. Sci., 2018, 11, 294–298.
258. L. Li, S. Fan, X. Mu, Z. Mi and C. Li, J. Am. Chem. Soc., 2014, 136, 7793–7796.
259. X. Yu, V. L. Zholobenko, S. Moldovan, D. Hu, D. Wu, V. V. Ordomsky and A. Y. Khodakov, Nat. Energy, 2020, 5, 511–519.
260. S. Murcia-López, K. Villa, T. Andreu and J. R. Morante, ACS Catal., 2014, 4, 3013–3019.
261. F. Sastre, V. Fornés, A. Corma and H. García, J. Am. Chem. Soc., 2011, 133, 17257–17261.
262. S. Murcia-López, M. C. Bacariza, K. Villa, J. M. Lopes, C. Henriques, J. R. Morante and T. Andreu, ACS Catal., 2017, 7, 2878–2885.
263. L. Yu, Y. Shao and D. Li, Appl. Catal., B, 2017, 204, 216–223.
264. L. Yu and D. Li, Catal. Sci. Technol., 2017, 7, 635–640.
265. Y. Zhou, L. Zhang and W. Wang, Nat. Commun., 2019, 10, 506.
266. F. Amano, C. Akamoto, M. Ishimaru, S. Inagaki and H. Yoshida, Chem. Commun., 2020, 56, 6348–6351.
267. C. E. Taylor and R. P. Noceti, Catal. Today, 2000, 55, 259–267.
268. C. E. Taylor, Catal. Today, 2003, 84, 9–15.
269. M. A. Gondal, A. Hameed and A. Suwaiyan, Appl. Catal., A, 2003, 243, 165–174.
270. M. Stoukides, J. Appl. Electrochem., 1995, 25, 899–912.
271. M. Stoukides, Res. Chem. Intermed., 2006, 32, 187–204.
272. K. Otsuka, S. Yokoyama and A. Morikawa, Chem. Lett., 1985, 319–322.
273. Y. Jiang, I. V. Yentekakis and C. G. Vayenas, Science, 1994, 264, 1563–1566.
274. K. Liu, J. Zhao, D. Zhu, F. Meng, F. Kong and Y. Tang, Catal. Commun., 2017, 96, 23–27.
275. A. Caravaca, A. de Lucas-Consuegra, J. González-Cobos, J. L. Valverde and F. Dorado, Appl. Catal., B, 2012, 113–114, 192–200.
276. A. Caravaca, V. J. Ferreira, A. de Lucas-Consuegra, J. L. Figueiredo, J. L. Faria, J. L. Valverde and F. Dorado, Int. J. Hydrogen Energy, 2013, 38, 3111–3122.
277. V. Kyriakou, I. Garagounis and M. Stoukides, Int. J. Hydrogen Energy, 2014, 39, 675–683.
278. A. Caravaca, A. de Lucas-Consuegra, V. J. Ferreira, J. L. Figueiredo, J. L. Faria, J. L. Valverde and F. Dorado, Appl. Catal., B, 2013, 142–143, 298–306.
279. C. Zhu, S. Hou, X. Hu, J. Lu, F. Chen and K. Xie, Nat. Commun., 2019, 10, 1173.
280. M. Ma, B. Jin, P. Li, M. Jung, J. Kim, Y. Cho, S. Kim, J. Moon and J. Park, Adv. Sci., 2017, 4, 1700379.
281. M. Ma, C. Oh, J. Kim, J. Moon and J. Park, Appl. Catal., B, 2019, 259, 118095.
282. Z. Guo, W. Chen, Y. Song, X. Dong, G. Li, W. Wei and Y. Sun, Chin. J. Catal., 2020, 41, 1067–1072.
283. Y. Song, Y. Zhao, G. Nan, W. Chen, Z. Guo, S. Li, Z. Tang, W. Wei and Y. Sun, Appl. Catal., B, 2020, 270, 118888.
284. N. Lapeña-Rey and P. H. Middleton, Appl. Catal., A, 2003, 240, 207–222.
285. S. Kodama, R. Kikuchi, N. Fujiwara, S. Tada, Y. Kobayashi and S. T. Oyama, ECS Trans., 2019, 91, 2697–2705.
286. V. Kyriakou, C. Athanasiou, I. Garagounis, A. Skodra and M. Stoukides, Int. J. Hydrogen Energy, 2012, 37, 16636–16641.
287. A. de Lucas-Consuegra, N. Gutiérrez-Guerra, A. Caravaca, J. C. Serrano-Ruiz and J. L. Valverde, Appl. Catal., A, 2014, 483, 25–30.
288. J. Lu, C. Zhu, C. Pan, W. Lin, J. P. Lemmon, F. Chen, C. Li and K. Xie, Sci. Adv., 2018, 4, eaar5100.
289. L. Luo, X. Tang, W. Wang, Y. Wang, S. Sun, F. Qi and W. Huang, Sci. Rep., 2013, 3, 1625.
290. G. A. Olah, Angew. Chem., Int. Ed., 2013, 52, 104–107.
291. S. Yanagida, T. Azuma, H. Kawakami, H. Kizumoto and H. Sakurai, J. Chem. Soc., Chem. Commun., 1984, 1, 21–22.
292. S. Xie, Z. Shen, J. Deng, P. Guo, Q. Zhang, H. Zhang, C. Ma, Z. Jiang, J. Cheng, D. Deng and Y. Wang, Nat. Commun., 2018, 9, 1181.
293. H. Zhang, S. Xie, J. Hu, X. Wu, Q. Zhang, J. Cheng and Y. Wang, Chem. Commun., 2020, 56, 1776–1779.
294. M. Liu, Y. Wang, X. Kong, R. T. Rashid, S. Chu, C. Li, Z. Hearne, H. Guo, Z. Mi and C. Li, Chem, 2019, 5, 858–867.
295. Y. Chao, J. Lai, Y. Yang, P. Zhou, Y. Zhang, Z. Mu, S. Li, J. Zheng, Z. Zhu and Y. Tan, Catal. Sci. Technol., 2018, 8, 3372–3378.
296. Y. Fan, J. Bao, L. Shi, S. Li, Y. Lu, H. Liu, H. Wang, L. Zhong and Y. Sun, Catal. Lett., 2018, 148, 2274–2282.
297. Y. Fan, S. Li, J. Bao, L. Shi, Y. Yang, F. Yu, P. Gao, H. Wang, L. Zhong and Y. Sun, Green Chem., 2018, 20, 3450–3456.
298. J. Bao, Y. Fan, S. Zhang, L. Zhong, M. Wu and Y. Sun, Catal. Lett., 2019, 149, 1651–1659.
299. Y. Li, J. Lu, X. Wang, H. Zhang, X. Wu, K. Zhang, J. Ye and D. Zhan, ChemCatChem, 2019, 11, 2277–2282.
300. J. Zhang, Q. Yuan, J. Zhang, T. Li and H. Guo, Chem. Commun., 2013, 49, 10106–10108.
301. J. Zhang, T. Li, D. Wang, J. Zhang and H. Guo, Chin. J. Catal., 2015, 36, 274–282.
302. L. Wang, S. Liu, C. Xu and X. Tu, Green Chem., 2016, 18, 5658–5666.
303. Z. Shen, S. Xie, W. Fan, Q. Zhang, Z. Xie, W. Yang, Y. Wang, J. Lin, X. Wu, H. Wan and Y. Wang, Catal. Sci. Technol., 2016, 6, 6485–6489.
304. S. Xie, Z. Shen, H. Zhang, J. Cheng, Q. Zhang and Y. Wang, Catal. Sci. Technol., 2017, 7, 923–933.
305. M. I. Montenegro, D. Pletcher, E. A. Liolios, D. J. Mazur and C. Zawodzinski, J. Appl. Electrochem., 1990, 20, 54–59.
306. N. L. Weinberg and D. J. Mazur, J. Appl. Electrochem., 1991, 21, 895–901.
307. Y. Ma, X. Wang, Y. Jia, X. Chen, H. Han and C. Li, Chem. Rev., 2014, 114, 9987–10043.
308. H. Lu, J. Zhao, L. Li, L. Gong, J. Zheng, L. Zhang, Z. Wang, J. Zhang and Z. Zhu, Energy Environ. Sci., 2011, 4, 3384–3388.
309. H. Liu, C. Song, L. Zhang, J. Zhang, H. Wang and D. P. Wilkinson, J. Power Sources, 2006, 155, 95–110.
310. J. T. Franclemont, X. Fan, R. Li, R. K. Singh, T. M. Holsen and S. Mededovic Thagard, Plasma Processes Polym., 2018, 15, e1180019.
311. A. Nawaz, A. Kuila, A. Rani, N. S. Mishra, L. C. Sim, K. H. Leong and P. Saravanan, in Industrial Applications of Nanomaterials, ed. S. Thomas, Y. Grohens and Y. B. Pottathara, Elsevier, 2019.
312. A. J. Martin, G. O. Larrazabal and J. Perez-Ramirez, Green Chem., 2015, 17, 5114–5130.
313. S.-C. Lin, C.-C. Chang, S.-Y. Chiu, H.-T. Pai, T.-Y. Liao, C.-S. Hsu, W.-H. Chiang, M.-K. Tsai and H. M. Chen, Nat. Commun., 2020, 11, 3525.

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