Local structural environment of single-atom catalysts

Abstract

Single-atom catalysts (SACs) provide opportunities for bridging the gap between homogeneous and heterogeneous catalysis, facilitating the precise structural identification of the catalytically active sites, and offering new opportunities for interpreting the structure–performance relationship from the atomistic perspective. In view of this, the background of catalysis, catalysts and the history of the development of SACs are introduced in sequence. Subsequently, the correlation between the structure and performance of SACs is reviewed with respect to aspects of their local structural environment including metal single-atoms (MSA), amount of MSA loaded, electronegativity, oxidation state, coordination atom, and coordination number. By combining theoretical and experimental analyses, a deep understanding of the correlation between the structure and catalytic mechanism will shed light on the optimization and design of SACs toward high-efficiency chemical energy conversion. Finally, the whole review of SACs is summarized and future prospects are outlined.

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Zheng Chen

Zheng Chen obtained his PhD degree from RWTH Aachen University (Germany) in 2021, and currently works as a post-doctoral researcher at Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences (CAS). His research focuses on the design and synthesis of novel inorganic materials and their applications in (photo)electrochemical energy conversion.

Lili Han

Lili Han obtained her PhD degree from Tianjin University. After that, she worked at Tianjin University of Technology, Brookhaven National Laboratory and the University of California, Irvine, before transferring to Research on the Structure of Matter, CAS, as a group leader in 2021. Her research interests are focused on transmission electron microscopy, three-dimensional reconstruction of electron tomography, and single-atom/nano-sized materials for energy conversion applications.

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Introduction

Catalysis, as the soul of chemistry, is one of the essential cornerstones of modern materials science and industrial production.^1,2 It works as the engine of the chemical industry, being employed in more than 80% of the energy, chemical, medicine, and food production processes required for life in modern society.^3,4 The realization of these processes is inseparable from recognizing catalytic reactions and understanding the essential properties of catalysts to develop new catalysts.

Nanotechnology has flourished in the past decades, driving the development of nanocatalysis science and technology and resulting in nanocatalysts emerging as the times require.^5–8 Compared with traditional catalysts, the size of the active species of nanocatalysts is nanoscale, therefore nanocatalysts have higher specific surface areas with more exposed reaction sites.^9–11 Due to the unique physicochemical properties of catalysts at the nanoscale, such as crystal face effects, size effects and synergistic effects, nanocatalysts exhibit excellent catalytic performances in many chemical reactions.^12–14 Sub-nanoscale clusters outperform nanoscale particles in terms of catalytic activity and/or selectivity as demonstrated by theoretical and experimental findings.¹⁵ Therefore, downsizing the catalyst particles or clusters to single-atoms is extremely desirable for catalytic reactions because atomically isolated sites often function as active sites.^14–21

Single metal atoms are ubiquitous in natural biological enzymes as active centers;²²e.g. the active centers of chlorophyll, heme and nitrogenase are Mg, Fe and Mo atoms, respectively.^23–25 These active single atoms combine and cooperate with surrounding biomolecules, and play a crucial catalytic role in life activities.^26,27 The design and fabrication of isolated metal atoms as active species can be traced back to the pioneering work of Thomas et al. in 1995.^28,29 They reported an oxygen-coordinated Ti-catalyst with a very high dispersion of isolated Ti atoms and they quantitatively determined the atomic environment of the active sites.²⁹ In 2000, Heiz et al. prepared Pd clusters supported on MgO substrate as a catalyst and determined that one Pd atom was enough for the catalytic cyclotrimerization of acetylene.³⁰ In 2003, Flytzani-Stephanopoulos et al. developed catalysts with nonmetallic gold or platinum species as the active species loaded on CeO₂.³¹ A significant milestone was established by John Meurig Thomas, who introduced the concept of the single-site heterogeneous catalyst (SSHC) by immobilizing well-defined organometallic compounds on uniform supports like zeolites in 2005.³² In 2007, Lee et al. fabricated a single-site mesoporous Pd₁/Al₂O₃ catalyst that was confirmed by atomic-level-resolution electron microcopy and extended X-ray absorption fine structure (EXAFS) analysis.³³ Based on pioneering works, which include those of Heiz and Flytzani-Stephanopoulos, Thomas further proposed the hypothesis of single metal atoms as active sites for heterogeneous catalysis in 2011.^31,34,35 In the same year, a SAC that consisted of isolated single Pt atoms anchored to FeO_x was synthesized by Zhang et al.³⁶ This catalyst has extremely high atom-efficiency, exhibiting excellent stability and high activity for CO oxidation and attracting extensive research interests.³⁶ After that, numerous SACs,^37–46 including non-noble metal and multiple-metal SACs,^47–56 have been developed as excellent catalysts with special catalytic properties for application in photocatalysis,^57–60 electrocatalysis,^61–66 and thermocatalysis^67–76 (Fig. 1).

Fig. 1 Schematic illustration of the timeline for the history of the monatomic catalyst.

SACs with atomically dispersed metal atoms anchored on supports offer a fundamental but powerful platform for probing and determining their structure–performance relationships at the atomic scale.^37,77–85 However, there is still a lack of a universal design principle that can provide a universal interpretation of the relationship between the intrinsic properties of the active sites and the catalytic activity of supported SACs, as well as a set of unified guiding principles that govern the formation of SACs.^28,86–94 Thus, analyses of individual elements of SACs not only provide unified principles to understand the nature of active sites in different kinds of SAC, but also inspire deeper insights into the SAC formation mechanism and shed light on carefully rationally designing SACs with effective targeting performance.

Regulation of the metal center of SACs

Metal single-atoms

Although metals are less abundant elements on Earth and expensive, their presence in catalysts is inherent to modulate chemical reactions through electrocatalysis, thermocatalysis and photocatalysis for energy conversion and storage, environmental treatment, and organic electrosynthesis (Table 1).^95–99 Thus, there is longstanding interest in fabricating metal-containing catalysts, especially SACs featuring atomically dispersed metal atoms as powerful catalytically active centers with 100% atom-utilization efficiency (Fig. 2).¹⁰⁰

Fig. 2 Schematic illustration of SACs across the periodic table.

Table 1 Various representative metal-based SACs with unique advantages towards high-efficiency electrochemical, photochemical and thermochemical applications

Metal	Support	Application	Performance	Ref.
Mg	Graphitic carbon nitride	Electrochemical CO2 reduction reaction (CO2RR) to CO	CO faradaic efficiency (FE): >90%, turnover frequency (TOF): 18 000 h^−1,	124
Al	Nitrogen-doped carbon	Photochemical CO2 cycloaddition	Conversion efficiency: ≈95%, reaction rate: 3.52 mmol g^−1 h^−1	125
Ca	N- and O- coordinated carbon matrix	Electrochemical oxygen reduction reactions (ORRs)	Half-wave potential (E1/2): 0.77 V, power density: 218 mW cm^−2	126
Sc	Nitrogen-doped carbon	Electrochemical nitrogen reduction reaction (NRR) to ammonia	Yield rate (YR): 20.4 μg cm^−2 h^−1, FE: 81.3% at −0.68 V	127
Ti	Reduced graphene oxide	Cathodic reduction in hybrid photovoltaics	Power conversion efficiencies: 20.6%	128
V	3D nitrogen-doped hierarchical carbon	Na–S batteries	Reversible capacity: 445 mA h g^−1 over 800 cycles; rate capability: 224 mA h g^−1	129
Cr	Nitrogen-doped carbon	Electrochemical ORR	E 1/2: 0.773 V	130
Mn	Graphitic carbon nitride	Electrochemical CO2RR to CO	Current density: 14.0 mA cm^−2, FE: 98.8%,	131
Fe	Nitrogen-doped carbon	Electrochemical ORR	E 1/2: 0.78 V	129
Co	nitrogen-doped carbon	Electrochemical ORR	Current density: 0.022 A cm^−2 at 0.9 ViR-free, power density: 0.64 W cm^−2 in 1.0 bar H2/O2	132
Ni	MoS2	Electrochemical oxygen evolution reaction (OER)	Overpotential at 20 mA cm^−2 decreases from 465 to 174 mV	133
Cu	N- and O- coordinated carbon	Selective oxidation of benzene	Conversion efficiencies: 83.7%, selectivity of phenol: 98.1%	134
Zn	Nitrogen-doped carbon	Electrochemical ORR	E 1/2: 0.746 V in an acidic medium	135
Ga	N-, P- and S-coordinated carbon	Electrochemical CO2RR to CO	FE: 92%	136
Y	Nitrogen-doped carbon	Electrochemical CO2RR to CO	YR: 23.2 μg cm^−2 h^−1, FE: 88.3% at −0.58 V	127
Zr	N- and O- coordinated carbon matrix	Zinc–air battery	power density: 324 mW cm^−2	137
Nb	Graphitic carbon nitride	Photocatalytic degrading amoxicillin	Enhanced from 15.3% to 69.1% compared to pure g-C3N4	138
Mo	Nitrogen-doped porous carbon	Electrochemical NRR to NH3	YR: 34.0 ± 3.6 μg h^−1 mgcat.^−1, FE: 14.6 ± 1.6% in 0.1 M KOH	139
Ru	Nitrogen-doped carbon	Thermochemical propane dehydrogenation	Selectivity of propylene: around 92%	140
Rh	Nitrogen-doped carbon	Electrochemical formic acid oxidation	28- and 67- fold Enhancements compared with state-of-the-art Pd/C and Pt/C	141
Pd	TiO2	Hydrogenation of C=C bonds	100% styrene conversion	142
Ag	C3N4 nanotubes	Photochemical CO2RR to CO	YR: 0.32 μmol h^−1, selectivity: >94%	143
Cd	N- and S- coordinated carbon matrix	Electrochemical CO2RR to CO	Current density: 182.2 mA cm^−2, FE: 99.7%, TOF: 73 000 h^−1	144
In	Nitrogen-doped carbon	Electrochemical CO2RR to formate	Current density: 8.87 mA cm^−2 FE: 96%, TOF: 12 500 h^−1	145
Sn	N- and O- coordinated carbon fibre	Electrochemical CO2RR to CO	Current density: over 240 mA cm^−2, FE: 92.1%	146
Sb	Nitrogen-doped carbon	Electrochemical CO2RR to CO	TOF: 16 500 h^−1 at −0.9 V	147
Ba	Pd1/Al2O3	Hydrogenation reaction	Least increased 4-fold than Pd1/Al2O3 in activity	148
La	MoO3−x	Photocatalytic NRR	YR: 209.0 μmol h^−1 g^−1	149
Hf	Two-dimensional extended porphyrin	Electrochemical NRR to ammonia	Onset potential: −0.53 V	150
Ta	Nitrogen-doped carbon	Electrochemical NRR to ammonia	YR: 19.97 mg h^−1 mgcat.^−1, FE: 8.52%	151
W	Nitrogen-doped carbon	Photochemical water vapor oxidation	YR of C1 oxygenates: 4956 μmol gcat.^−1	152
Re	Carbon black	Electrochemical ORR	E 1/2: 0.72 V	153
Os	sulfur-functionalized MXene Ti2C	Low-temperature CO oxidation	CO oxidation with a reaction barrier energy of only 0.74 eV, the highest stability	154
Ir	Nickel–iron sulfide nanosheet arrays	Electrochemical OER	Overpotential: ∼170 mV in 1.0 M KOH, TOF: 9.85 s^−1	155
Pt	Nitrogen-doped carbon	Electrochemical HER	Overpotential: 19 mV in 0.5 M H2SO4	156
Au	CdS	Photochemical CO2RR	160- and 113-fold enhancement compared with pristine CdS for CO and CH4 generation	157
Hg	Three-dimensional Ag aerogel	Oxidase-like reaction	A specific enhancement of the surface-enhanced Raman spectroscopy effect	158
Pb	Polydopamine	Thermal decomposition of cyclotrimethylenetrinitramine	4.8 times faster than that without polydopamine-supported Pb-SACs	159
Bi	Nitrogen-doped carbon networks	Electrochemical CO2RR to CO	FE: 97%, current density: 3.9 mA cm^−2, TOF: 5535 h^−1 at 0.39 V	160

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By preparing the samples through different precursor reactions, SACs can be formed with various channels and anchoring groups as well as distribution of metals. One type of metal precursor is anchored on the support and forms atomic dispersions of monometallic active species, i.e. monometallic SACs.^101–109 If an insufficient amount of a metal precursor is injected, some uncoordinated organic ligands will be retained and enable the coordination of another species of metal atoms, thus resulting in the formation of SAC with multi-metallic active species, i.e. multi-metallic SAC.¹¹⁰ Catalysts with an atomically bimetallic structure can deliver superior catalytic performance to that of monometallic SAC due to cooperation between the two metal atoms.^111–114 The reported atomically bimetallic catalysts mostly feature supported diatomic-pair structures with M–M bonds or diatomic-ensemble structures with M–X–M (M′) linkages.^115–123 For example, Hu et al. reported the synergistic effect of two sets of single-atom sites, specifically for Ni₁ and Ru₁ anchored on CeO₂ for the dry reforming of CH₄.⁵⁵ Complementary theoretical analysis revealed that Ni₁ activated CH₄ while Ru₁ dissociated CO₂ in the catalytic reaction. Han et al. developed SACs based on 37 monometallic elements, including 1-, 2-, 8- and 12-metal SACs.⁵⁴ Each metallic atom in the 12-metal SAC with Sc, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Pd, Hf and W overcame aggregation with not only itself but also any of the other metallic atoms. This provides a demonstration of complex multimetallic phase for SACs and demonstrates that employing single atom anchor sites as structural units to construct concentration-complex SAC materials can be achievable and has no fundamental limit when the complex concentrations of down to 12 different elements.

These results demonstrate that metals especially the presence of MSAs in catalysts drive superior catalytic activities, as confirmed by complementary analyses of density functional theory (DFT) calculations and experiments. Meanwhile, regulating the metal centers of SACs, spanning from monometallic to concentration-complex multi-metallic materials, offers the opportunity to diversify the design of SACs.

Amount of MSA loaded

The amount of MSA loaded is a key factor as it dominates the density of active sites for targeted catalytic applications. Therefore, increasing the loading of MSA is one of the aspects to increase the number of active sites to boost their activity. It is well-accepted that the amount of MSA loaded is positively correlated to the ratios of metal precursor in all precursor and end up to the largest ratios of that, indicating that SACs in preparation have the available metal precursor ratios and the amount of metal precursors employed as the active centers can be adjusted to upper limit in a certain extent.^99,120

In general, the amount of MSA in each SACs is usually near 0.1 wt%, because the obtainable SACs have a low loading of MSA to avoid aggregation due to the high surface energy of atomically dispersed atoms. To a large degree, this occurrence results from the lack of anchoring groups on the support to atomically stabilize the metal atoms or the relatively weak interactions between the anchoring groups of the support and the metal atoms. Karim et al. successfully synthesized an Ir-SAC with various amounts of Ir. The Ir single-atoms supported on MgAl₂O₄ exhibited outstanding catalytic behavior for CO oxidation when the amount of Ir loaded was 0.0025 wt%.¹⁶¹ Whilst Ir nanoparticles were clearly observed when the amount of Ir loaded was increased up to 1 wt%. In 2016, Li and co-workers made a significant breakthrough to accomplish an order-of-magnitude improvement in metal loading up to 4 wt% via rationally designing and preparing suitable supports to form structures with numerous anchoring groups and strong coordination environments; this holds great promise for achieving the practical application of SACs.¹⁶² Zeng et al. demonstrated that the amount of Pt supported on MoS₂ obviously influenced the catalytic activity and selectivity for CO₂ hydrogenation.⁴⁵ Neighboring Pt monomers can be achieved by increasing the amount of Pt loaded up to 7.5 wt% while maintaining atomically dispersed Pt. Notably, for the basis of kinetic experiments, the activation energy for 7.5 wt% loading of Pt supported on MoS₂ is 72.3 kJ mol⁻¹, much lower than that of 124.7 kJ mol⁻¹ for 0.2 wt% loading of Pt supported on MoS₂. These results supposed that the higher loading of atomically isolated Pt monomers was favorable for CO₂ hydrogenation compared to those with low loading.

A high amount of MSA is essential to maximize their potential in the catalytic industry.^{26,110,162–166} It is noteworthy that Lu et al. reported a versatile approach combining impregnation and two-step annealing to synthesize ultra-high-density SACs with up to 23 wt% loading of MSA for 15 SACs utilizing a standardized, automated protocol (Fig. 3). Drastically enhanced reactivity was observed upon increasing the amount of MSA loaded. Surprisingly, Li et al. successfully developed a Fe-SAC that could be easily synthesized at the gram scale with Fe loaded up to a record value of 30 wt%, which overcame the aggregation of single-atom sites, showing unprecedented catalytic performance and meeting the requirement for practical applications.¹⁶⁵ Additionally, Han et al. developed 12-metal SACs with high metal loadings.⁵⁴ The total metal loading for the 12-metal SAC reached a high value of 7.53 wt%, exhibiting higher activity than those of the mixed and individual monometallic SACs for the OER as well as demonstrating that there was no fundamental limit to assembling concentration-complex multi-metallic SAC with a high metal loading.

Fig. 3 Loading of MSA achieved in this study on nitrogen-doped carbon (NC), polymeric carbon nitride (PCN) and CeO₂ supports for 15 SACs. Reproduced with permission from ref. 163. Copyright 2022 Nature Publishing Group.

Regulating the amount of MSA loaded will affect the site density of SACs thus modulating the potential properties of SACs, e.g., the ORR and the cathodic process of fuel cell membrane electrodes. Recent breakthroughs in synthesizing SACs have led to MSA loadings above 10 wt%; these catalysts surpass the performance of the benchmark Pt/C (20 wt%) catalyst for the ORR. Noteworthily, Yu et al. synthesized a series of isolated Fe–N₄ type SACs with different amounts of MSA loaded and demonstrated that strong interactions between adjacent Fe–N₄ moieties altered the electronic structure when the inter-site distance was less than about 1.2 nm with the increased MSA loading, resulting in improved intrinsic ORR activity (Fig. 4).¹⁶⁷ The noteworthy improvement in site performance proceeds until adjacent Fe atoms become as close as about 0.7 nm. Their results highlight the significance of identifying the fundamental mechanism of the inter-site distance effect with the amounts of MSA loaded in Fe–N₄ type catalysts for the ORR, which promotes the development of targeted SACs with a high amount of MSA loaded.

Fig. 4 Correlation of Fe contents and site densities with different d_site values. (a) Calibration of d_site with statistical distributions against the corresponding Fe loading. The error bars correspond to standard deviations of Fe contents (x-axis) and d_site (y-axis) for each sample prepared in five independent trials. (b) The plot of Fe atom density (n_s, Fe atoms per 100 nm⁻²) versus d_site. Error bars indicate standard deviations for n_s in five experiments. The dotted curve shows the values for n_s calculated according to assumed rectangular models. (c) Apparent ORR activity: the mass current density at 0.85 and 0.80 V normalized by the total weight of Fe in different samples. The error bars correspond to standard deviations of mass activity calculations from five groups of data for each sample. Reproduced with permission from ref. 167. Copyright 2021 Nature Publishing Group.

These results indicate that utilizing supports with a low metal loading could realize the atomic dispersion of mononuclear metal and precursors and reducing the metal loading could be a direct and effective approach to resist the agglomeration of metal species into nanoparticles. Furthermore, rationally designing and preparing suitable supports to form structures with numerous anchoring groups and strong coordination environments are beneficial for stabilizing metal atoms to achieve a much higher MSA loading, ensuring abundant active sites to target subsequent catalytic reactions.

Electronegativity

Electronegativity is a chemical property that describes the ability of an atom to attract an electron to itself. It is used to predict whether there is a bond between atoms and quantify the electron affinity of the element when the corresponding bond is formed.¹⁶⁸ The electronegativity difference generated by different metal atoms in the catalyst induces significant charge redistribution around the metal atom, thus generating different interactions with multiple intermediates on various sites of the SAC system.^44,168,169 The construction of controllable multifunctional active sites on the surface of the SAC by tailoring the electronegativity of the metal composition benefits the adsorption of relevant intermediates, stabilizing the intermediates and greatly enhancing the efficiency of the catalytic reaction. Simultaneously, creating highly active sites on the surface of the SAC by tailoring the electronegativity of the metal composition could optimize energy barriers and overcome kinetics issues for multistep reactions, accelerating the rates of reaction pathways during the catalytic process.

For example, regarding the correlation of the adsorption free energies of intermediates (ΔG_OH* and ΔG_H*) for the ORR, OER and HER with the electronegativity of atoms, Xu et al. plotted ΔG_OH* and ΔG_H*versus the descriptors θ_d * E_M/E_O and θ_d * E_M/E_H (Fig. 5), respectively (where θ_d, ΔG_OH* and ΔG_H* represent valence electrons in the d-orbitals of the metal, the free energy for OH_* adsorption and the free energy for H_* adsorption, respectively; E_M, E_O and E_H represent the electronegativity of the metal, oxygen and hydrogen, respectively).¹⁶⁹ The plots show that the descriptors have a strong correlation with ΔG_OH* and ΔG_H* for SACs under certain conditions (Fig. 5), suggesting that electronegativity and valence electrons in the d-orbitals of the metal can be used as response factors to evaluate the adsorption strength between the adsorbate and active metal center and affect the catalytic activity of the SACs for the HER, OER and ORR.

Fig. 5 (a) ΔG_OH* described by θ_d * E_M/E_O. (b) ΔG_H* described by θ_d * E_M/E_H. Reproduced with permission from ref. 169. Copyright 2018 Nature Publishing Group.

Similarly, Guan et al. demonstrated that the catalytic activity and selectivity toward the CO₂RR on transition-metals (TMs) among SACs were strongly correlated with the intrinsic properties and could be well described with a single descriptor, ψ, which was calculated based on a formula containing the valence-electron number and the electronegativity of metals:

ψ = (Π_i=1^NS_vi)^2/N/(Π_i=1^Nχ_i)^1/N (1)

where N is the number of metals and neighboring nitrogen atoms; S_vi and χ_i are the sum of the valence-electron number and the electronegativity of the metal and anchored nitrogen dopants, respectively.¹⁷⁰ In the research, TM-doped pyridine-like graphene was chosen as the test system, of which TMs surrounded with four pyridine N atoms, named M–N₄–G SACs. Complementary analyses of DFT calculations and experiments demonstrated that the reactivity and selectivity of M–N₄–G SACs were in good agreement with the trend Co–N₄–G > Fe–N₄–G > Mn–N₄–G > Cu–N₄–G > Ni–N₄–G, suggesting that the catalytic activity of the SACs for the CO₂RR could be predicted with the aid of the corresponding metal electronegativity.

Simultaneously, Han et al. found that the largest SAC metal precursor ratio was related to its metal electronegativity through a parabolic relationship; the ratio initially increases then flattens and finally decreases as the electronegativity increases (Fig. 6a).⁵⁴ A parabolic relationship between electronegativity and MSA loading is not as obvious as the relationship between electronegativity and metal precursor ratio (Fig. 6b), since the amount of MSA loaded is not always proportional to the metal precursor ratio.¹¹⁰ This suggests that tailoring the metal electronegativity can achieve the corresponding SAC regulation.

Fig. 6 (a) Dependence of the largest SAC metal precursor ratios on electronegativity. (b) Dependence of the monometallic SAC loading of MSA on electronegativity. Reproduced with permission from ref. 54. Copyright 2022 Nature Publishing Group.

These results indicate that the local environment of the active metal center can impact on the catalytic activities of SACs, owing to the difference in adsorption free energies among SACs in different coordination environments with different charge distributions. Tailoring the electronegativity of the metal composition can achieve charge redistribution, leading to adjustments of the catalytic activities of SACs; this is also a powerful option for addressing the scaling relationship challenge.

Oxidation state

Although there is still confusion over how the oxidation state affects the catalytic activity for SACs clearly,¹⁶² it is undeniable that the oxidation state of the metal element of SACs is key for the local environment of the active sites and dominates the catalytic performance toward targeted reactions.¹⁷¹ In view of the electronic structure of SACs, the oxidation state of their metal sites can reflect electron transfer between metals and intermediates, which potentially has a relationship with the redox reaction activity.¹⁷²

Gu et al. reported a Fe-SAC with dispersed Fe(III) ions coordinated with pyrrolic N atoms of the N-doped carbon support.¹⁷³ Electrochemical results suggested that the superior activity of Fe³⁺ sites was derived from faster CO₂ adsorption and weaker CO absorption compared to that of conventional Fe²⁺ sites while the iron ions maintained a +3 oxidation state during electrocatalysis. Jin et al. constructed a Ni-SAC with partially oxidized Ni single-atom sites in polymeric carbon nitride. XANES revealed that the oxidation state of Ni single-atom sites increased with the deepening of oxidation, getting closer to the oxidation state of Ni²⁺.¹⁷⁴ X-ray photoelectron spectroscopy (XPS) further revealed that the oxidation state of the Ni single-atom active site was actually an intermediate oxidation state with a ratio of 2 : 1 for Ni²⁺: Ni⁰. Electron paramagnetic resonance spectroscopy (EPR) further found that the oxidized Ni single-atom active site had more d-orbitals with unpaired electrons occupying the valence band, which was positively correlated with the catalytic performance of Ni single-atom active sites, indicating that the unpaired electrons had a significant promoting effect on the photolysis of water for hydrogen production. X-ray absorption near-edge structure (XANES) and XPS revealed the oxidation state changes to the Ni single-atom active sites during the reaction. The catalyst was gradually oxidized during the reaction, affecting the catalytic activity. After the oxidation annealing process of the catalyst, the Ni single-atom active sites returned to the oxidation state before the reaction, and the photocatalytic performance was also restored. These results proved that the Ni single-atom active sites in the partially oxidized state were effective for carbon nitride photolysis with the best catalytic effect of hydrogen production from water.

Cao et al. developed a series of Os-SACs with oxidation states ranging from +0.9 to +2.9 produced by modulation of the coordination environments (Fig. 7).⁹⁴ The Os-SACs showed a volcano relationship between oxidation state and HER activity, with an upsurge of activity achieved at a moderate oxidation state of +1.3 (Os-N₃S₁). Mechanistic analysis illustrated that increasing the oxidation states strengthened H atom adsorption on Os owing to increased energy levels and decreased occupancy of Os–H antibonding states, whereas further increasing the oxidation states weakened H atom adsorption due to the decreased occupancy of the Os–H bonding state (Fig. 7). This demonstrated the essential roles of oxidation states in manipulating catalytic activity, which was conducive to the rational design of SACs.

Fig. 7 (a) The relationship between oxidation state and overpotential obtained from experiments for different Os-SAC. (b) DFT calculations. ΔG_H* as a function of the oxidation state of Os-SACs. (c) Theoretical over-potential at 10 mA cm⁻² as a function of oxidation state for different Os-SACs. Reproduced with permission from ref. 94. Copyright of 2022 Nature Publishing Group.

These results indicate that regulating the oxidation state offers an approach for tailoring SACs, which further provides a significant chance to rationally form SACs with their targeted activities. Furthermore, modulating the oxidation state is the key to understanding the structure–performance relationship and revealing the redox reaction mechanisms.

Regulation of the coordination environment for SACs

Coordination atoms

Except the active central metal is critical to driving a specific catalytic reaction, the coordination components of supports on the surface have essential roles in modulating the catalytic performances of SACs.^175–183 This has prompted intense efforts toward the precise design of tailored structures by engineering the coordination environment with coordination atoms, from the first coordination shell to the second shell or higher for SACs.^184–193 The first coordination shell refers to those atoms directly bonded to the central MSAs, while the second coordination shell refers to atoms bonded to the first-shell atoms but not to the central MSAs. The engineering of the first or higher coordination shells by precisely tailoring the heterogeneity of coordination atoms offers a great opportunity to tune the properties of SACs.^194–202 Compared to engineering the first coordination shell, engineering the second and/or higher coordination shells would alter the distribution of the electron density over the central single metal sites indirectly and more moderately through long-range electron delocalization, thus tuning the catalytic performance of SACs.¹⁸¹ The effects of engineering the coordination shell is not just to redistribute uneven charge and thus optimize the adsorption energies of intermediate species, but also to modify the conductivity of the supported framework and thus improve charge conduction.²⁰³ The numerous types and variety of coordination elements (e.g. N, O, P, and S), as well as dual or multiple heteroatoms being simultaneously employed to coordinate with the MSA, offer a broad range of coordination configurations to rationally build SACs with targeting activities.^44,204

Nitrogen possesses a lone pair of electrons, so its specific electric structure offers the potential to bond and stabilize the MSAs, forming M–N_x configurations and effectively regulating the electronic structure, light absorption, charge transfer, reaction barrier, etc.¹¹⁰ The stabilization effect mainly results from the strong coordination interactions between the lone pair of electrons of N and the d-orbitals of metals.¹⁶⁹ Numerous SACs coordinated to N have been fulfilled. The incorporated N aids in activating the supporting materials for trapping electrons or protons.²⁰⁴ For example, Zhou et al. reported that Pt single-atom-coordinated ultrathin MOF nanosheets showed notable promotion of electron transfer and a decrease in the Gibbs free energy for the adsorbed H atoms, by constructing surface active sites for support materials through M–N_x configurations.^164,204 In addition, Zhang et al. reported that the controllable synthesis of single cobalt atoms could achieve superior electrochemical activity by tuning C/N hybrid coordination precisely, because C/N hybrid coordination could enhance electron transfer to improve the adsorption and reaction kinetics of the intermediates.^205,206 Zhang et al. established high-purity pyrrole-type and pyridine-type Fe–N₄-based SACs, of which the coordinated N atoms were pyrrolic-N and pyridinic-N, respectively (Fig. 8). The pyrrole-type showed obviously better ORR activity than the pyridine-type in an acidic medium.²⁰⁷

Fig. 8 Illustrations of the (a) pyridine-type Fe–N₄ and (b) pyrrole-type Fe–N₄ geometries and their corresponding electron accumulation (yellow) and electron depletion (blue). (c) Free energy diagram of the oxygen reduction reaction on pyrrole-type Fe–N₄ and pyridine-type Fe–N₄. Reproduced with permission from ref. 207. Copyright 2020 Royal Society of Chemistry.

For O-coordinated SACs, their MSAs are anchored by coordinating with O atoms. Metal oxides as typical oxygenated compound supports are employed to stabilize the MSAs, where the coordinating O atoms are generally the lattice oxygen.^208–214 The first report on the concept of SACs was the successful preparation of O-coordinated SACs with a structure in which an isolated single Pt atom was anchored to the FeO_x support by forming a bond with two FeO_x lattice O atoms.³⁶ The strong bonding interaction with negatively charged O in the unsaturated state empowered the formation and stability of a positively charged Pt single-atom possessing partly empty d-orbitals. Distinguishing metal-oxides as support for the formation of SACs, some compounds do not contain oxygen intrinsically, but they can also anchor MSAs through metal–oxygen bonds because they possess incorporated O atoms or oxidized-functional groups after oxide-treated. For example, Wu et al. found that graphene oxide (GO) had dangling oxygen groups that possessed sufficient power to pluck the MSAs (Fe, Co, Ni, and Cu) from their corresponding bulk metal configurations with the assistance of sonication to form the target GO-supported SACs (Fig. 9).²¹⁵

Fig. 9 Schematic illustration of the construction and EXAFS fitting curve of Fe-single-atoms/GO, inset is the proposed Fe–O₄ coordination environment. Reproduced with permission from ref. 215. Copyright 2019 John Wiley and Sons.

Phosphorus, as a group V_A element resembling N, possesses a lone pair of electrons, making it a promising alternative to coordinate the metal atom. Different from N, P has a larger atomic radius and weaker electronegativity, which leads to the metal sites of the M–P_x configuration having a greater electron density than those of the M–N_x configurations.²⁰⁴ Based on the properties of P, the P-ligand might work as a channel to transfer electrons from the supports to metal sites, thus promoting electron accumulation at the single metal sites. This electron-enriching behavior of M–P_x benefits reduction processes at the active metal sites.^216–218 For example, Zhou et al. developed a SAC with P-coordinated Pd single-atoms supported on g-C₃N₄ nanosheets.²¹⁹ Compared to the corresponding N-coordinated SAC, its Pd single-atoms were less oxidized, and its Pd–P bond was longer than the Pd–N bond. Moreover, coordination with P atoms led to a decreased electron density within the supports, thus showing superior photocatalytic HER activity. Similarly, Zhou et al. developed a SAC with P-coordinated Co single-atoms supported on CdS (Fig. 10).²²⁰ The incorporated P can induce the decreased electron density of the CdS support and increase the electron density of CoP₃ sites, tuning the electronic states of the active sites and thus exhibiting an amazing photocatalytic activity toward the dehydrogenation of formic acid.

Fig. 10 (a) Mechanism of formic acid dehydrogenation and hydrogen production for atomically dispersed Co–P₃ species on CdS nanorods (CoPSA-CdS). (b) Geometric structure of CoPSA-CdS at the atomic level from the first-principles simulation. (c–e) The charge density difference maps between the adsorbed formic acid and CdS for P-CdS, sulfur-coordinated Co single atom-loaded CdS nanorods (CoSSA-CdS) and CoPSA-CdS. Reproduced with permission from ref. 220. Copyright 2019 John Wiley and Sons.

Sulfur, as another p-block element with moderate electronegativity, shows promising coordination and the ability to stabilize MSAs by forming the M–S_x configurations,²⁰⁴ especially when metal sulfides are used as supports to anchor the MSAs.^45,221–225 This type of substitution would tune the electron density and modulate the activity of the catalytic site. For example, Bao et al. developed an S-coordinated SAC, where the Pt atoms substituted Mo sites and were atomically dispersed within MoS (Fig. 11).²²³ A suitable hydrogen adsorption energy was tuned by forming Pt–S–Mo, effectively boosting the HER activity. Additionally, the doped S atoms employed to stabilize MSAs can also be observed to form MS_xH_x (H = heteroatom).^182,226,227 Tang et al. developed a Mo-SAC with isolated Mo atoms dispersed in the O and S codoped graphene matrix to form a mixture configuration (MoO_x, MoS_x, and MoO_xS_x), where the isolated Mo atoms were anchored by O and S co-coordination.¹⁸² After tailoring the coordination environment surrounding the single Mo atoms by incorporating S, excellent catalytic activity toward the OER was exhibited.

Fig. 11 (a) Schematic illustration of the construction. (b) HAADF-STEM images of Pt-MoS₂ showing that the single Pt atoms uniformly disperse in the 2D MoS₂ plane. (c) Magnified domain of the red dashed rectangle in (b) showing a honeycomb arrangement of MoS₂, and the single Pt atoms occupying the exact positions of the Mo atoms (marked by red arrows). Reproduced with permission from ref. 223. Copyright 2015 Royal Society of Chemistry.

In addition to the above-mentioned N, O, P and S doping, some studies have attempted doping with other nonmetallic atoms, like B, F and Cl, and identified a new synergistic mechanism.^228–230 Typically, B, with an atomic radius close to that of carbon as well as the unique tri-electron structure, can be doped into the carbon skeleton without affecting its overall structure and introduce electron-deficient sites on the carbon substrate, achieving B-doped carbon-based SACs with higher conductance and ion adsorption capacity.²³¹

These above results indicate that the catalytic activity of SACs is susceptible to their local coordination environment because the MSAs as the catalytically active centers are stabilized on the supports through coordination interactions from the viewpoint of coordination chemistry. Engineering the coordination environment with various coordination atoms, such as N, O, P and S, makes sense for developing effective SACs with optimal electronic structures for their targeted catalytic applications.

Coordination number

The coordination number of SACs, as a crucial parameter in the modulation of the coordination microenvironment, would significantly affect the electronic and geometric structure of active centers in SACs, playing important roles in regulating catalytic activity and selectivity.^{204,232–236}

Despite tremendous advances in developing SACs, an effective strategy for precisely regulating coordination numbers without interfering with the single dispersed metal centers is lacking.^237–240 MOFs have been well-recognized as one of the most promising candidates for the precise regulation of the coordination environment of SACs.^241–244 The normally adopted strategy used to modulate the coordination number is exclusively restricted to the one-step pyrolysis of metal-doped MOF precursors, for which a higher temperature results in lower N coordination numbers for MOF-derived M–N type SACs. Given that MOF-derived SAC formation is synchronous with pyrolysis, complex synthesis parameters, such as pre-reduction and pyrolysis temperature, have a severe influence on the flexibility and universality of regulating SACs. Thus, it is possible that a promising and manageable solution to manipulate the coordination microenvironment of SACs could come from the decoupling of MOF carbonization and single-atom decoration, which would avoid complicated variables for pyrolysis.¹⁷⁸

Wang et al. developed Mo–N_x–C (x = 2, 3, 4) type Mo-SACs by controlling the Mo–N_x coordination numbers (Fig. 12a).²⁴⁵ These three Mo–N_x–C catalysts containing similar Mo contents but different N contents were obtained by tailoring host–guest templates of Mo-doped ZIF-8 precursor at 800 °C, 900 °C and 1000 °C, and these catalysts were named Mo–N₄–C, Mo–N₃–C and Mo–N₂–C, respectively. The Mo–N₃–C catalyst was identified as the most promising nanozyme candidate with exclusive peroxidase-like behavior, geometrical structure differences and different orientation relationships of the frontier molecular orbitals (Fig. 12b and c). Similarly, Wang et al. developed a series of Co-SACs with different nitrogen coordination numbers of the Co center (Fig. 12d), by regulating the pyrolysis temperature.²⁴⁶ Among the three different types of Co–N_x moieties (x = 2, 3, 4), the CO₂RR activity for Co–N₂ was significantly higher than those for the others (Fig. 12e and f). Jiang et al. developed a facile post-synthetic metal substitution strategy to achieve the tailored synthesis of Ni-SAC with a low coordination number on N-doped carbon support.²⁴⁷ The Ni-SAC was synthesized by pyrolysis of the Zn-MOFs at high temperature to vaporize their low boiling point Zn atoms and then treating them in acidic media to remove Zn atoms from the Zn–N₃–C sites, followed by treatment to refill abundant Zn vacancies with Ni atoms (Fig. 12g). The formed Ni–N₃–C type SAC exhibited superior performance under certain conditions compared with Ni–N₄–C (Fig. 12h and i). Xiao et al. successfully synthesized a Cu-SAC supported on g-C₃N₄ planes with two types of Cu–N_x configurations.²⁴⁸ The Cu–N₃ configuration was constructed between a single Cu atom and three N atoms of the monolayer of carbon nitride, while the Cu–N₄ configuration was constructed between a single Cu atom and four N atoms of two layers of carbon nitride. The simultaneous presence of Cu–N₃ and Cu–N₄ dramatically exhibited superior charge transfer and thus improved the performance of the solar-driven HER.^181,248

Fig. 12 (a) Different potential peroxidase-like models with optimized top Mo–C₃, Mo–N₂, Mo–N₃, and Mo–N₄ structures by DFT calculations. (b) Adsorption energies for O₂ and H₂O₂ adsorption on Mo–C₃, Mo–N₂, Mo–N₃ and Mo–N₄ structures. (c) Different values of H₂O₂ and O₂ adsorption energies on Mo–C₃, Mo–N₂, Mo–N₃ and Mo–N₄ structures. Reproduced with permission from ref. 245. Copyright 2021 Elsevier Inc. (d) Schematic illustration of the geometric configurations of Co–N₂, Co–N₃ and Co–N₄. (e) LSV curves of Co–N₂, Co–N₃, Co–N₄ and Co nanoparticles (NPs) and pure carbon paper as a background. (f) CO faradaic efficiencies at different applied potentials for different catalysts. Reproduced with permission from ref. 246. Copyright 2018 John Wiley and Sons. (g) Schematic illustration of the geometric configurations of Ni–N₃–C and Ni–N₄–C. (h) LSV curves and (i) faradaic efficiencies of Ni–N₃−C, Ni–N₄−C and N–C. Reproduced with permission from ref. 247. Copyright 2021 John Wiley and Sons.

These results support the importance of the coordination number. The variation in the number of coordinated atoms in SACs can induce different distributions of local electron densities of the central MSA leading to catalytic activation sites being jointly constructed from coordinated atoms and MSAs, which significantly impacts on the adsorption and desorption of reaction intermediates, thus modulating the catalytic activity and selectivity of SACs.

Conclusions and perspectives

The SAC has become one of the most active research frontiers in the field of catalysis. Herein, the nature of catalysis has been reviewed, for which the catalysts, from bulk materials, nanoparticles, and clusters to single-atoms, play a fundamental role and sequentially possess higher atomic utilization and special activity. Benefiting from the unique physicochemical properties of SACs, important developments for SACs have been reviewed chronologically and their corresponding opportunities have been introduced.

Given that catalytic reactions require catalysts with appropriate active sites, the unique advantages of SACs are obvious in terms of their catalytic activity. The library of SACs includes various SACs that offer a database showing the relationship between their structure and special catalytic activity. Here we systematically elaborated on the catalytic properties of SACs in terms of their structural elements including MSA, amount of MSA, metal electronegativity, oxidation state, coordination atoms, and coordination number. Thus, analyzing individual formation elements of SACs aids in the identification of the real active sites and unveils the catalytic mechanisms for single-atom catalysis, while summarizing the unified principles sheds light on rationally designing SACs with special performances.

With the development of research, the knowledge and understanding of SACs have achieved remarkable progress. Considering the enormous potential of SACs, plenty of opportunities have sprung up and pushed the rapid development further. Specifically, future improvement in this field can be tied to the following aspects:

The success of synthesizing various SACs with unique interactions between the supports and the active sites enables them to exhibit unique catalytic activities, which surpass those of traditional catalysts. However, to further enhance catalytic processes, precisely regulating the electric structures is highly desirable; this holds enormous promise for modifying and optimizing the specific performance of SACs. Apart from precisely regulating SAC structures with dual or multiple metals as active centers and regulating the coordination environment with different types of coordinated atoms and numbers from the first shell to those beyond, the supports should also be considered to achieve the rational regulation of their electric structures and thus can create possible synergistic effects that potentially further enhance atom utilization and reduce energy barriers as well as improve catalytic efficiency for target catalytic applications.

In situ characterization technologies help to understand the catalytic process of SACs, the mechanism of interaction with reactants and the dynamic evolution of the active site. However, understanding their dynamics in practical working states is still inadequate. Therefore, continued efforts should be devoted to investigating the structure–performance relationship and catalytic mechanisms at the atomic level by employing the SAC library as model systems with the assistance of theoretical simulations and analyses combined with more accurate in situ studies including in situ electron microscopy and in situ spectra analyses. These are urgently required to elucidate and provide deep insights into the catalytic mechanism and dynamics of SACs in the reaction system.

To meet the demand for fundamental research and practical applications of catalysis, improving the MSA loading with uniform dispersion and stability while achieving large-scale and green synthesis processes are major vital avenues. Exploring SACs with a high loading of MSA with uniform dispersion is an important pathway to further enhance the catalytic performance. On the other hand, the stability of currently available single-atom-based catalysts is normally unsatisfactory because the MSAs are extremely active, causing them to rapidly react with the surrounding reactants to form other undesired species. In view of this, it is significant to explore and develop novel strategies to controllably synthesize SACs with a high loading of MSA and robustness. Additionally, the synthesis of SACs for large-scale production usually involves preparing precursors and multiple subsequent processes, which may entail harsh reaction conditions, the generation of toxic byproducts and high production costs. Thus, optimizing the chemical production process, alleviating environmental pollution and reducing production costs are highly desired to achieve SACs via “green” technology for large-scale manufacturing.

Overall, SACs hold great potential for advancing the development of various catalytic applications and pushing them toward commercialization. Much effort should be continuously devoted to the SAC area, achieving the design of specific catalysts for the targeted reactions and promoting the atomic-economic green catalytic process leading to an improvement in our daily lives in the future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

The authors gratefully acknowledge the National Key Research and Development Program of China for Young Scientists (2022YFA1505700) and the National Natural Science Foundation of China (22205232) for financial support.

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