Wa
Gao
*a,
Haonan
Li
a,
Jianqiang
Hu
f,
Yong
Yang
*e,
Yujie
Xiong
h,
Jinhua
Ye
g,
Zhigang
Zou
cd and
Yong
Zhou
*bcd
aSchool of Physical Science and Technology, Tiangong University, Tianjin 300387, P. R. China
bSchool of Chemical and Environmental Engineering, Anhui Polytechnic University, Wuhu 241000, P. R. China
cSchool of Physics, Jiangsu Key Laboratory of Nanotechnology, Eco-materials and Renewable Energy Research Center (ERERC), National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, P. R. China
dSchool of Science and Engineering, The Chinese University of Hongkong (Shenzhen), Shenzhen, Guangdong 518172, P. R. China
eKey Laboratory of Soft Chemistry and Functional Materials (MOE), Nanjing University of Science and Technology, Nanjing 210094, P. R. China
fJiangxi Normal Univ., Inst. Adv. Mat. IAM, Coll. Chem. & Chem. Engn., Nanchang 330022, P. R. China
gNational Institute for Materials Science (NIMS), International Center Materials Nanoarchitecture MANA, 1-1 Namiki, Tsukuba, Ibaraki 305-0044, Japan
hSchool of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230036, Anhui, P. R. China
First published on 15th August 2024
Photocatalytic CO2 reduction captures solar energy to convert CO2 into hydrocarbon fuels, thus shifting the dependence on rapidly depleting fossil fuels. Among the various proposed photocatalysts, systems containing metal active sites (MASs) possess obvious advantages, such as effective photogenerated carrier separation, suitable adsorption and activation of intermediates, and achievable C–C coupling to generate multi-carbon (C2+) products. The present review aims to summarize the typical photocatalytic materials with MAS, highlighting the critical role of different formulations of MAS in CO2 photoreduction, especially for C2+ product generation. State-of-the-art progress in the characterization and theoretical calculations for MAS-containing photocatalysts is also emphasized. Finally, the challenges and prospects of catalytic systems involving MAS for solar-driven CO2 conversion are outlined, providing inspiration for the future design of materials for efficient photocatalytic energy conversion.
CO2 capture and conversion by artificial photosynthesis can simultaneously address the rising global CO2 emissions and produce hydrocarbon fuels to accomplish carbon neutrality.4–6 It is proving to be an elegant and promising solution, attracting the interest of scientists and making great strides in the efficiency of CO2 conversion.7–11 Specifically, the photocatalytic reduction of CO2 achieves a sustainable alternative to conventional fossil fuels, which is carried out in relatively mild conditions-room temperature and pressure, driving the conversion of CO2 to hydrocarbons directly without additional energy supply or harmful substance release. Photocatalytic CO2 reduction consists of three main processes:12 the catalyst generates electron–hole pairs under sunlight;13–15 carriers separate and migrate to the surface;16 CO2 is activated and converted at the surface active sites, involving C–O bond breaking, C–H bond formation, and C–C coupling.17,18 In fact, the CO2 molecule possesses a rather stable structure with high dissociation energy to cleave the CO bond (750 kJ mol−1), which poses a challenge for the activation and conversion of CO2.19 In addition, solar-driven CO2 reduction involves complex multi-step conversions that, depending on the number of electrons and protons transferred, produces CO,20–22 CH3OH,23–25 CH4,26,27 C2H4,28–30 C2H6,31,32 C2H5OH,33etc. Due to the diversity of products of CO2 photoreduction, achieving highly selective CO2 conversion to generate specific target products is extremely challenging.
Various strategies have been employed to improve the efficiency and selectivity of CO2 photoreduction, including enhancing light absorption,34,35 promoting carrier separation,36 constructing active sites,37 and promoting C–C coupling.38 According to the current findings, MAS possesses significant advantages in improving the performance of photocatalytic reduction of CO2 and promoting C–C coupling to obtain C2+ products.39–41 This review mainly introduces the research progress of MAS in driving photocatalytic CO2 conversion (Fig. 1). Photocatalytic systems containing MAS including metal oxides, metal sulfides, layered double hydroxides (LDHs), metal organic framework (MOF), covalent organic frameworks (COF), single atom catalysts, and metal complexes are summarized. The achievement of MAS with different formulations in photocatalytic CO2 conversion is systemically explored while mechanisms that drive performance improvement are discussed in detail. The development of characterization techniques and theoretical calculation to determine the structure and function of MAS in CO2 photoreduction is also highlighted. Finally, the difficulties, challenges, and novel viewpoints of feasible solutions are addressed regarding the design of advanced photocatalysts containing MAS for efficient solar energy conversion.
Photocatalytic CO2 reduction for the generation of C2+ products was achieved in metal–oxide systems. The vacancy-rich TiO2 with Cu single atoms loading produced C3H8, which involved an overall 20 e− reduction and two sequential C–C coupling processes.46 The Cu–Ti–VO unit in the Ti0.91O2 matrix was formed through the modulation of the electronic coupling interaction between the Cu atoms and adjacent Ti atoms by Vo. This unique unit lowered the energy levels of the key *CHOCO and *CH2OCOCO intermediates, thereby tuning the C1–C1 and C1–C2 couplings to thermodynamically favourable exothermic processes (Fig. 2a). Moreover, the metastable hexagonal WO3 (h-WO3), offering a suitable bandgap, exhibited efficient photoreduction of CO2 to C2H4.47 The blue color of the metastable state of WO3 was attributed to the reduction of W6+ to W5+ or W4+ with the increase of the reduced state, which indicated the existence of Vo. The surface Vo enhanced the light absorption capability and promoted the photogenerated carrier separation. More importantly, the W–S–W sites formed by the S atom, which replaced oxygen atoms and bridged the adjacent W atoms, benefited the adsorption of *CH2 intermediates and promoted the C–C coupling to generate C2H4 (Fig. 2b).
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Fig. 2 (a) Gibbs free energy diagrams of CO2 reduction on the Cu–Ti–VO unit. This figure has been reproduced from ref. 46 with permission from Springer Nature, copyright 2023. (b) The C–C coupling process of Ov–WO3 and S–Ov–WO3. This figure has been reproduced from ref. 47 with permission from Elsevier, copyright 2023. |
Based on the above analysis, the MAS constructed in the metal oxide system is structurally stable, successfully achieving efficient photocatalytic CO2 conversion with C2+ generation. In fact, MAS in metal oxides often synergizes with defect and doping atoms to achieve performance enhancement. Therefore, MAS introduced in metal oxides usually needs multi-step experimental treatment processes.
The reduction potential of ZnS is −1.04 V vs. NHE, which is relatively negative and particularly suitable for the photocatalytic reduction reaction.48 However, the wide band gap (3.6–3.8 eV), fast recombination of the charge carriers, and low selectivity toward CO2 reduction competing with the hydrogen evolution reaction (HER) limit the efficiency of ZnS. Cu-doped ZnS, containing abundant sphalerite and wurtzite phase (S–W) junctions, enhanced the photocatalytic activity with high CO selectivity.49 The S–W phase junction containing abundant sulfur vacancies (Vs) effectively facilitated the separation of charge carriers and the localization of photoelectrons for surface catalysis. The addition of Cu increased the electron density at the Fermi energy level, promoted the reactivity of the Cu sites, and enhanced the bonding of the catalyst surface to the *CO intermediate, thereby improving the CO selectivity. Furthermore, the dual functional ZnS coupled with g-C3N4 simultaneously enhanced the surface and bulk carrier separation of g-C3N4 for the highly selective reduction of CO2 to CH4.50 ZnS acted as a cocatalyst to capture the photogenerated electrons of g-C3N4. It contributed a polarization electric field, which was created inside the ZnS nanoparticle along the direction of spontaneous polarization, to prompt the migration and separation of photogenerated charges from bulk to surface.
ZIS, belonging to the AB2X4 group of ternary compounds, has been used in photocatalysis with visible light absorption.53 The metallic ZnIn2S4, which is rich in indium vacancies (VIn), exhibited full-spectrum responsiveness for high CO2 photoreduction efficiency.51 The VIn defect state captured the excited hole (h+) and increased the minority carrier diffusion length, resulting in a large number of carriers moving to the surface and participating in the conversion of CO2. Theoretical calculations revealed that VIn lowered the energy barrier of the rate-limiting step (the formation of COOH*, Fig. 3a and b), leading to the high rate of CO evolution. In addition, anchoring single Au atoms to ultrathin ZIS (Au1/ZIS) nanosheets with Vs resulted in the Au1–S2 low coordination structure, which enabled exceptional photocatalytic CO2-to-CH4 conversion.52 The introduction of Au single atoms enhanced the efficiency of carrier separation and transfer. Moreover, the low-coordinated single Au atom significantly enhanced CO2 activation, lowered the energy barrier for *CO protonation, and stabilized the *CH3 intermediate, leading to the selective generation of CH4 by CO2 photoreduction.
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Fig. 3 Free energy diagrams for the reduction of CO2 to CO over the (001) facets of VIn-rich-ZIS (a) and VIn-poor-ZIS (b). These figures have been reproduced from ref. 51 with permission from American Chemical Society, copyright 2022. Free energy diagrams of photocatalytic CO2 to CH4 for Au1–S2/ZIS (c). This figure has been reproduced from ref. 52 with permission from Wiley-VCH, copyright 2022. |
Transition metal phosphorous trichalcogenides have been well-explored in the photoreduction of CO2 to valuable fuels. The introduction of Vs in AgInP2S6 regulated the CO2 photoreduction reaction pathway to steer the dominant generation of C2H4.54 The VS led the charge accumulation on the Ag atoms near VS, which effectively captured the forming *CO. This phenomenon enriched the catalyst surface with key reaction intermediates and promoted C–C coupling to generate C2 species with low binding energy barriers. In addition, the tandem synergistic effect of the charge-enriched Cu–In dual site, which was confined on the lateral edge of the CuInP2S6 monolayer, became the main reason for the efficient conversion and high selectivity of C2H4.55 In the presence of light, the limbic In site of the CuInP2S6 monolayer converted CO2 mainly to *CO, which was transferred to the neighbouring Cu site for the subsequent C–C coupling reaction to C2H4.
By reason of the foregoing, MAS in metal chalcogenides shows unique advantages in the generation of C2+ products. However, due to the limitation of the stability of the metal sulfide, the stability of MAS in them may be lacking. Therefore, in photocatalytic applications, attention should be paid to improving the stability of MAS in metal sulfides.
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Fig. 4 (a) Scheme showing CO2 and H2O photoreduction on the different ZnM-LDH photocatalyst (carbon: dark gray; oxygen: red; hydrogen: white). DFT calculations of the elementary steps of (b) H2 evolution, (c) CO2 reduction to CO, and (d) CO2 reduction to CH4 involving a methanol intermediate over ZnTi-LDH, ZnAl-LDH, and ZnFe-LDH. These figures have been reproduced from ref. 56 with permission from Elsevier, copyright 2020. |
Obviously, MAS can take advantage of the two-dimensional (2D) structure, as well as the highly flexible adjustability in the morphology and ionic composition of LDH. However, the synergies of different metals and their roles in C2+ generation remain to be explored.
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Fig. 5 (a) Schematic atomic interface model of CA@PCN-222. (b) Proposed mechanism for the CO2 photoreduction reaction over CA@PCN-222. These figures have been reproduced from ref. 59 with permission from Wiley-VCH, copyright 2023. |
The modulation of the coordination environment around the MAS in MOFs helps to reveal the relationship between structure and activity during CO2 photoreduction. In UiO-type MOFs bearing bipyridine linkers, the number of coordinated N atoms around a single Co site was tuned to provide UiO-Co-Nx (x = 2, 3 and 4) for photocatalytic CO2 reduction.61 UiO-Co-N3 exhibited superior activity to the other counterparts, which was mainly attributable to the difference in the number of coordinating N atoms around the Co site. Particularly, UiO-Co-N3 endowed the lowest energy barriers of the rate-determining step (CO2 → COOH*) and the desorption of CO* among all UiO-Co-Nx samples, accounting for the optimized CO2 photoreduction activity.
The MAS in MOF enables liquid C2+ generation. The specific NH2–Cu–NH2 triple atom site was constructed by incorporating Cu sites into the connected nodes of defective UiO-66-NH2, realizing photocatalytic CO2-to-acetone conversion.62 Specifically, one of the N active sites on the NH2–Cu–NH2 adsorbed CO2 and converted CO2 to form *CH3 (CO2 → COOH* → CO* → *CH3). Meanwhile, the Cu site activated its adsorbed CO2 to CO*. *CH3 and *CO underwent the first C–C coupling on CuN2O2 to generate *CO–*CH3. At the same time, another N site on the CuN2O2 fragment activated the adsorbed CO2 into CO*, which underwent a second C–C coupling process with the *COCH3 intermediate to generate the crucial *COCOCH3. Due to the synergistic interaction between the Cu site and the N site, the C2 intermediate on the CuN2O2 ultimately generated C3.
In summary, the MAS in MOF connected with organic ligands exhibits good dispersion and high utilization, which can make full use of the porous property of MOF to optimize the photocatalytic CO2 reduction performance. However, its synthesis process involves complex chemical reactions and utilizes expensive organic ligands, making practical production applications difficult.
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Fig. 6 Gibbs energy profiles of the CO2RR-to-CO reaction over Co-2,3-DHTA-COF (A, blue line) and Co-TP-COF (B, orange line). This figure has been reproduced from ref. 65 with permission from American Chemical Society, copyright 2023. |
Particularly, MAS can utilize the π-conjugated framework in COF to promote the photogenerated carrier separation and enhance the photocatalytic CO2 reduction. However, MAS needs to be grafted onto the COF structure with fine regulation. In addition, the construction of multiple MAS in COF and the study of their catalytic mechanism still need to be continuously explored.
Loading rare-earth La single atoms on carbon nitride (O/La–CN) constructed the active centers of La–N charge-transfer bridges for photocatalytic CO2 reduction.67 Electronic state changes induced by the hybridization of the 4f and 5d orbitals of La single atoms and the p–d orbitals of La–N atoms established charge-transfer channels for La–N bridges to promote carrier separation. Bader charge and differential charge distributions suggested that electrons were transferred through La atoms into connected N atoms, and eventually through C atoms to O atoms. The O/La–CN strengthened CO2 adsorption, endowed the high capacity for CO2 uptake, and reduced the activation energy barrier for COOH* formation. Moreover, the desorption of CO from the surface of O/La–CN required the lowest energy (0.2 eV) compared with the dissociation reaction (COH*, 2.96 eV and C* + H2O, 4.59 eV) or hydrogenation (HCO*, 1.53 eV), contributing CO formation with high selectivity (Fig. 7a). Cu loading on BiOBr nanosheets (Cu1@BiOBr) established a strong built-in electric field with isolated Cu sites that acted as electron traps to promote charge transfer and stabilize charge carriers.68 The high selectivity of methanol within this photocatalytic system could be ascribed to the energy-favorable hydrogenation of the *CO intermediate into *CHO (Fig. 7b). Furthermore, the unfavorable adsorption of CH3OH on Cu1@BiOBr relative to H2O prevented methanol from being oxidized by photogenerated holes.
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Fig. 7 (a) Calculated free energy for the photocatalytic CO2 reduction reaction of the selectivity-determining step. This figure has been reproduced from ref. 67 with permission from American Chemical Society, copyright 2020. (b) The DFT-calculated free energy diagram for the hydrogenation of CO2 to CH3OH and CH4 over Cu1@BiOBr. This figure has been reproduced from ref. 68 with permission from Wiley-VCH, copyright 2023. |
In addition to single MAS loading, dual MAS incorporation has been explored to drive CO2 photoreduction with diverse functions. The dual MAS of cobalt (Co) and ruthenium (Ru) supported on a conjugated porous carbon nitride polymer exhibited efficient photocatalytic CO2 reduction.69In situ characterizations and theoretical calculations revealed that the active Co sites facilitated dynamic charge transfer, along with Ru working as adsorption sites for CO2 photoactivation. In detail, the atomic Co facilitated dynamic electron transfer from the carbon nitride polymer to the atomic Ru site, resulting in the COOH* intermediate being effectively stabilized by the charged-rich Ru site. Moreover, the smaller Gibbs free energy of CO formation than that of the protonation of CO* to CHO* resulted in high CO selectivity. Similarly, Ru and Cu single atoms simultaneously incorporated into polymeric carbon nitride (poly-CN) exhibited higher selectivity (95%) for CH4 production than that decorated with Ru or Cu individually.70 The atomically dispersed Ru–N4 and Cu–N3 sites tuned the electronic structure of poly-CN, and were identified as active centers. During CO2 photoreduction, the Ru single atom was essential for proton production, while the Cu single atom played a dominant role in the reduction process. Due to the thermodynamically more favorable conversion of *CO to *CHO, the coexistence of the Ru and Cu single atoms resulted in the efficient conversion of CO2 to CH4.
Remarkably, the MAS in single-atom catalysts exhibits high metal atom utilization and achieves synergistic effects of multiple MASs. However, the metal is easily aggregated to form particles during the construction process. Thus, a delicate experimental design is required to successfully construct MAS in single-atom form.
Metal complexes could drive C–C coupling to generate C2+ products. The synergistic dual sites of rhenium(I) bipyridine fac-[ReI(bpy)(CO)3Cl] (Re-bpy) and copper porphyrinic triazine framework [PTF(Cu)] working in tandem strategy achieved the photocatalytic conversion of CO2 to C2H4 (Fig. 8).74 The CO generated at the Re-bpy sites was adsorbed by the nearby Cu single sites in PTF(Cu), followed by a synergistic C–C coupling process to ultimately produce C2H4. Critically, Rebpy-*CO could enter the porous PTF(Cu) and move to the nearby Cu–*CO, leading to practicable free energy for the CO–CO coupling between Cu–*CO and Re-bpy-*CO (Re-bpy-*CO-*CO-PTF(Cu)). However, PTF(Cu) catalysts alone produced CO under similar conditions. This was caused by the large distance between the adjacent Cu centers in PTF(Cu) hindering C2+ formation via the coupling of Cu–*CO–*CO–Cu.
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Fig. 8 Tandem catalysis mechanism of Re-bpy/PTF(Cu). This figure has been reproduced from ref. 74 with permission from American Chemical Society, copyright 2023. |
As mentioned above, the alteration of the MAS coordination environment can be achieved by modulating the skeleton in metal complexes, which in turn regulates the photocatalytic CO2 reduction performance. It is conducive to revealing the in-depth reaction mechanism. However, the typical problems in photocatalysis, such as the high separation cost and low chemical stability, still limit their practical applications.75
Catalyst | Reactive sites | Light source | Reaction condition | Performance (μmol g−1 h−1) | Ref. | |
---|---|---|---|---|---|---|
Single MAS | Cu-doped ZnS containing abundant S–W junctions | Cu | 300 W Xe lamp | 18 mL solution containing chloroform + 2 mL 2-propanol + CO2 | CO: 68.9 (99.9%) | 49 |
VIn-rich-ZIS | VIn | 300 W Xe lamp AM 1.5 cut-off filter | 45 mL acetonitrile (MeCN) + 5 mL trolamine (TEOA) + CO2 | CO: 298 | 51 | |
ZnM-LDH (M = Ti4+, Ga3+, Al3+) | M | 300 W Xe lamp | 0.1 mL H2O + 0.08 MPa CO2 | CH4: 1.2 (68% M = Ti), CO: 1.3 (90% M = Al), CO: 1.6 (78% M = Ga) | 56 | |
CA@PCN-222 | ZrIII-cluster | 300 W Xe lamp (800 nm > λ > 400 nm) | 60 mL mixed solution (MeCN/TEOA = 30/1) + CO2 | HCOO−: 281.6 | 59 | |
UiO-Co-N3 | Co–N3 | 300 W Xe lamp (λ ≥ 420 nm) | 20 mL MeCN + 30 mg [Ru(bpy)3]Cl2·6H2O + 0.025 M 1,3-dimethyl-1,3-dihydro-2-phenyl-2H-benzimidazole (BIH) + CO2 | CO: 179.3 (99.3%) | 61 | |
Incorporating Cu sites into the connected nodes of defective UiO-66-NH2 | NH2-Cu-NH2 | 300 W Xe lamp (λ > 420 nm) | 2 mL H2O + 6 mL N,N-dimethylformamide (DMF) + CO2 | CH3COCH3: 70.9 (97%) | 62 | |
2,2′-Bipyridine-based COF bearing non-noble single Cu sites | Cu2+ sites | 300 W Xe lamp (λ > 420 nm) | 10 mL DMF or H2O + 1 mL triethylamine + CO2 | CH4: 17.5; CO: 1.6 (H2O), CO: 10.2 (100%; DMF), CO: 22.5 (66.4%; DMF/H2O = 50%) | 63 | |
Cu-COF | Cu–O/N sites | 300 W Xe lamp (λ > 420 nm) | 12 mL mixed solution + 600 torr CO2, MeCN/H2O/TEOA (8 mL/2 mL/2 mL) | CO: 206 (94%) | 64 | |
Co-2,3-DHTA-COF | Co–O4 sites | 300 W Xe lamp (λ > 420 nm) | 46 mL mixed solution + 10 mg [Ru(bpy)3]Cl2·6H2O + 1 atm CO2, MeCN/H2O/TEOA (32 mL/8 mL/6 mL) | CO: 18![]() |
65 | |
Mo-COF | MoN2 sites | 300 W Xe lamp (λ ≥ 420 nm) | 5 mL H2O + 1 mL CO2 | CO: 6.19, C2H4: 3.57, CH4: 1.08 | 66 | |
O/La–CN | La–N charge-transfer bridge | Xe lamp | 12 mL mixed solution + 1 atm CO2MeCN/H2O/TEOA (6 mL/4 mL/2 mL) | CO: 92 (80.3%) | 67 | |
Cu1@BiOBr | Isolated Cu sites | 300 W Xe lamp | 5 mL H2O + 4 MPa CO2 | CH3OH: 627.66 (86.6%) | 68 | |
[CoN3O]ClO4 | Co(II) catalytic center | 300 W Xe lamp (λ > 420 nm) | 5 mL mixed solution + [Ru(bpy)3]Cl2·6H2O + BIH + CO2 MeCN/H2O (4 mL/1 mL) | CO: 88.14% ± 2.09% | 72 | |
CuNi-L2 | Active NiII site | LED light (λ > 420 nm) | 25 mL mixed solution + [Ru(bpy)3]Cl2·6H2O (0.85 mmol L−1)+ Triisopropanolamine (TIPA 0.8 mol L−1) + CO2, MeCN/H2O (20 mL/5 mL) | CO: (93.5%) | 73 | |
Cu single atoms on the nitrogen-doped carbon anchored on TiO2 | Cu SAs | 300 W Xe lamp | H2O vapor + CO2 | CO: 65.8 | 77 | |
2Re-In2O3 | Odef | 300 W Xe lamp | 100 mL H2O + 1.01 bar CO2 | CH3OH: 265.6 (100%) | 78 | |
Cu1/TiO2 | Cu1–O2+1 | Xe lamp | 20 mL H2O + TEOA + CO2 | C2H4: 60.4 (75.2%) | 79 | |
MIL-88B-NS40 | Fe–N coordinated sites/uncoordinated S | 300 W Xe lamp (λ > 420 nm) | 2 mL H2O + CO2 | C2H4: 17.7 | 80 | |
B-Ni1/WO2.72 | Ni site | 300 W Xe lamp (λ > 420 nm) | 6 mL mixed solution + 15 mg [Ru(bpy)3]Cl2·6H2O + CO2, MeCN/H2O/TEOA (3 mL/2 mL/1 mL) | CO: 80![]() |
82 | |
Isolated Mn atoms over the multi-edged TiO2 nano-pompons | Mn atoms | 300 W Xe lamp (λ > 400 nm) | 6 mL mixed solution + 10 mg [Ru(bpy)3]Cl2·6H2O + 1 atm CO2 MeCN/H2O/TEOA (3 mL/2 mL/1 mL) | CO: 80![]() ![]() |
83 | |
In-bonded N-atom (Inδ+-N4) in the (002) crystal planes of g-C3N4 | Inδ+-N4 | 300 W Xe lamp | 400 μL H2O + CO2 | CO: 398.87 | 84 | |
CuACs/PCN | Cu–N4 sites doped P | 300 W Xe lamp | 5 mL TEOA + [Ru(bpy)3]Cl2·6H2O + 45 mL H2O + CO2 | C2H4: 10.17 (53.2%), CH4: 9.55 | 85 | |
3Er-ZnIn2S4 | Er | 300 W Xe lamp (λ ≥ 420 nm) | TEOA + 100 mL H2O + 80 kPa CO2 | CH4: 6.68 (>90%) | 86 | |
Cu1N3@PCN | Different coordination structures Cu-atom | 300 W Xe lamp (λ > 420 nm) | 1 mL H2O + CO2 | CO: 49.8 | 87 | |
Ag1@PCNT | Ag–N3, Ag single atoms | 300 W Xe lamp AM 1.5 cut-off filter | 3 mL H2O + 70 kPa CO2 | CO: 160 (>94%) | 88 | |
Ultra-high Pr loading CN | Pr1–N4O2 sites | 300 W Xe lamp | 1.0 mL H2O + 4.0 mL CO2 | CH3OH: 511.1 | 89 | |
MCOF-Ti6Cu3 | Cu cluster | Xe lamp AM 1.5 cut-off filter | 30 mL H2O + CO2 | HCOOH: 169.8 | 90 | |
Dual MAS | Mn1Co1/CN | Two compatible active centers of Mn and Co | 300 W Xe lamp (λ > 400 nm) | 2 mL H2O + 80 kPa CO2 | CO: 47 | 17 |
Cu–Ti–VO/Ti0.91O2-SL | Cu–Ti–VO unit | 300 W Xe lamp | 12.5 mL MeCN + 2.5 mL H2O + 1 atm CO2 | C2H4: 7.6, C3H8: 13.8, CO: 18.6 | 46 | |
CuInP2S6 monolayer | Cu–In dual site | 300 W Xe lamp AM 1.5 cut-off filter | 0.4 mL H2O + CO2 | C2H4: 20.89 (56.4%), CO: 8.59, CH4: 6.22 | 55 | |
V into NiAl-LDH | V; lower-valence Ni | 300 W Xe lamp (λ > 400 nm) | 10 mL mixed solution + 0.005 mmol [Ru(bpy)3]Cl2·6H2O + 1.8 bar CO2 MeCN/H2O/TEOA (6 mL/2 mL/2 mL) | CH4: 217 (78.9%) | 57 | |
Dispersed Co–Ru bimetal into conjugated porous carbon nitride polymer | Co sites; Ru sites | 300 W Xe lamp | 500 μL H2O + 70–80 kPa CO2 | CO: 27.3 | 69 | |
Ru Cu single atom incorporated into polymeric carbon nitride | Ru–N4 sites, Cu–N3 sites | 300 W Xe lamp | 9 mL NaHCO3 (3 M) + 1 mL TEOA + CO2 | CH4: 154 (95%), CO: 2, H2: 6 | 70 | |
Re-bpy/PTF(Cu) | Re-bpy sites; Cu single sites in PTF(Cu) | 300 W Xe lamp (1100 nm > λ > 400 nm) | 5 mL mixed solution + 1 atm CO2 MeCN/TEOA (2.5 mL/2.5 mL) | C2H4: 73.2 | 74 | |
Cu–N2–V | Cu+/Cu2+ | 300 W Xe lamp | 21 mL DMF + 3 mL H2O + 0.8 MPa CO2 | C2H5OH: 69.8 | 81 | |
LaNi-Phen/COF-5 | Optically active (La site) catalytically active (Ni site) | 300 W Xe lamp | 10 mM BIH + 48 mL MeCN + 2 mL H2O + 80 kPa CO2 | CO: 605.8 (98.2%) | 91 | |
CuIn5S8 | Cu–In dual sites | 300 W Xe lamp (λ > 400 nm) | 2 mL H2O + 0.1 atm CO2 | CH4: 8.7 | 92 | |
Ag2Cu2O3 | Cu–Ag Lewis acid–base dual sites | 300 W Xe lamp | 5 mLH2O or 5 mL 0.2% methanol aqueous solution was injected + CO2 | CH4: 3.6 | 93 | |
In/TiO2-Vo | Vo-regulated In–Ti dual sites | Xe lamp (320 nm < λ < 780 nm) | H2O + CO2 | CH4: 35.49 (91.3%) | 94 | |
Au/Co dual single atom loaded CdS NPs | Au/Co DSA | 300 W Xe lamp (λ > 400 nm) | CO2 | CO: 64.1 μmol g−1, CH4: 7.7 μmol g−1 | 95 | |
Vo-rich Zn2GeO4 nanobelts | Asymmetric Zn–O–Ge sites | Xe lamp light | 12 mL H2O + 0.03 vol% CO2/Ar gas mixture | CH3COOH: 12.7 (29.95%) | 96 | |
Au–CeO2 | Au–O–Ce sites | 300 W Xe lamp | 5 mL H2O + 80 kPa CO2 | CO: 11.07, C2H6: 11.07 (65.3%) | 97 | |
InCu/PCN | In–Cu dual sites | 300 W Xe lamp | 24 mL DMF/H2O (12.5 vol% H2O) + 0.8 MPa CO2 | C2H5OH: 28.5 (92%) | 98 | |
Bi2WO6 nanosheets with Vo anchoring Au and Cu dual single atoms | Au–Cu dual-single-atom sites; Vo | 300 W Xe lamp (780 nm > λ > 320 nm) | 1.0 mL H2O + (CO2-1%, N2-99%) | Total amount of production (CO, CH4, C2H4, C3H6): 83.9 | 99 | |
2% Pd–Cu/TiO2 | Pd–Cu; Vo | 300 W Xe lamp | 15 mL H2O + 0.5 MPa CO2 | CH3OH: 71.84 (91.71) | 100 | |
Single atoms of Ni/Co loaded on TiO2 | Ni Co single atoms | 300 W Xe lamp | 50 mL H2O (0.1 M Na2SO3 + 0.2 M CsOH) + CO2 | CH3COOH: 22.6, 71% | 101 | |
Incorporated the redox-active Co2+/Ni2+ centers into the chemically stable layered lead iodide hybrids | Co2+/Ni2+–O–Pb bimetallic catalytic sites | 300 W Xe lamp AM 1.5 G filter | 1 mL H2O + CO2 | C2H5OH: 24.9–31.4, 89.5–93.6% | 102 | |
g-C3N4/UiO-66(Zr/Ce) | N–Zr/Ce–O | 300 W Xe lamp | 15 mL MeCN + 15 mL H2O + 101 kPa CO2 | CH3OH: 54.71, C2H5OH: 38.10 | 103 | |
Cu-CuTCPP/g-C3N4 | Cu1+δ | 300 W Xe lamp (800 nm > λ > 360 nm) | 400 μL H2O + 200 μL TEA + 0.1 MPa CO2 | CO: 12.3, CH4: 11.6, C2H6: 18.5 | 104 | |
MAS combined with vacancies | P and Cu dual sites anchored on graphitic carbon nitride | P–N and Cu–N4 dual sites | 300 W Xe lamp | 3 mL TEOA + H2O + 0.65 atm CO2 | C2H6: 616.6 | 31 |
1% Ru–TiO2−x | Ru; Vo | 300 W Xe lamp | 0.14 g NaHCO3 + N2 + H2O + H2SO4 (2 M) | CH4: 31.63 (90.93%) | 45 | |
A series of metastable WO3 photocatalysts with the coexistence of S doping and Vo | W–S–W bridge; Vo | 300 W Xe lamp | 0.4 mL H2O + CO2 | C2H4: 224.278, 87.6% | 47 | |
Au atoms to ultrathin ZnIn2S4 nanosheets with Vs | Au1–S2 | 300 W Xe lamp (λ > 420 nm) | 5 mL mixed solution + 10 mg [Ru(bpy)3]Cl2·6H2O + 1 atm CO2 MeCN/H2O/TEOA (3 mL/1 mL/1 mL) | CH4: 275 (77%) | 52 | |
Vs-AgInP2S6 single atomic layer | Ag atoms; Vs | 300 W Xe lamp | 0.4 mL H2O + CO2 | C2H4: 44.3 (73%) | 54 | |
Mn dopants and Vo were engineered in Zn2GeO4 nanorods | Mn dopants and Vo | 300 W Xe lamp | 1 mL H2O + CO2 | CO: 40.02 (82.9%) | 105 | |
Cu1/N2CV-CN | Cu single atoms; N2C vacancies | 300 W Xe lamp AM 1.5 cut-off filter | 200 μL H2O + 105 kPa CO2 | CO: 11.12 | 106 | |
CuGaS2/Ga2S3 containing Vs | Cu–Ga metallic bond | 300 W Xe lamp (λ > 420 nm) | 3 mL H2O + TEOA + 70 kPa CO2 | C2H4: 335.67 (93.87%) | 107 | |
Cu1.95S1−x | Dual Cu and Vs | 300 W Xe lamp AM 1.5 cut-off filter | 1 atm CO2 | CH4: 12.42 | 108 | |
MAS synergizing with nanoparticles | CA/Ni1CuNP N–C | Cu nanoparticles; Ni single-atom sites; asymmetric Ni–N4 sites | 300 W Xe lamp (λ > 420 nm) | 95 mL H2O + 5 mL TEOA + 100 kPa CO2 | CH4: 35.245, CO: 32.067, C2H6: 25.328 | 109 |
Co1Ag(1+n)-PCN | Co–N6–P bonds, Ag–N2C2 SA sites, Ag NPs | 300 W Xe lamp | 6 mL CH3CN + 4 mL H2O + 80 kPa CO2 | CO: 46.82 (70.1%) | 110 | |
Cu single atoms and nanoclusters supported on defective TiO2 | Cu SAs, Cu NCs | 300 W Xe lamp | 100 μL H2O + CO2 | CH4: 19.63 (98%) | 111 | |
single Pd atoms and Pd nanoparticles on graphitic carbon nitride | Pd1 sites; PdNPs sites | Xe lamp | 10 mL H2O + 1 atm CO2 | CH4: 20.3 (97.8%) | 112 | |
MAS within heterostructure | g-C3N4/CoCo-LDH | Co–N2 bonds | 300 W Xe lamp | 3 mL H2O + 1 atm CO2 | CO: 71.39 | 58 |
CuO/Cu2V2O7 | Two metal atomic sites of Cu and V | 300 W Xe lamp | 20 mL TEOA + 80 mL H2O + 1.01 bar CO2 | CO: 118, C2H4: 29.57 | 113 | |
2D/2D Ni-doped CsPbBr3/Bi3O4Br Z-scheme heterojunction | Ni2+ | 300 W Xe lamp | 2 mL H2O + 101 kPa CO2 | CO: 96.89 (98.2%) | 114 | |
Co-TCPP/Bi3O4Br | Bi–O bridge bond; Co atoms | 300 W Xe lamp | 50 mL H2O + 0.08 MPa CO2 | CO: 71.3 | 115 | |
Cu-SAEB | N–Cu1–S single-atom electron bridge | 300 W Xe lamp | 0.1 mL H2O + CO2 | CO: 236.0, O2: 120.1 | 116 | |
In2O3/CdSe-diethylenetriamine | In–O–Cd bonds; Vo | 300 W Xe lamp | 120 mg NaHCO3 + 0.5 mL H2SO4 | CO: 70.08, CH4: 27.92 | 117 | |
2D/0D g-C3N4/Cu2SnS3 | Cu–C and Cu–N dual chemical bond; Cu sites | 100 W Xe lamp AM 1.5 filter | H2O + CO2 | CO: 18.2 | 118 | |
NiAl–Fe-TCPP | Fe; Ni | 300 W Xe lamp | 0.005 mmol [Ru(byp)3]Cl2·6H2O + 10 mL mixed solution + 1.8 bar CO2MeCN/H2O/TEOA (6 mL/2 mL/2 mL) | C2H4: 2470 | 119 | |
r-In2O3/InP | O–In–P polarized sites | 300 W Xe lamp (λ > 420 nm) | 100 mL H2O + 80 kPa CO2 | CH3COOH: 9.67 (96.1%) | 120 | |
CdS:Dy3+/g-C3N4 | Dy3+ single atom | 300 W Xe lamp (λ > 400 nm) | 4 mL H2O + CO2 | CH4: 8.06 | 121 | |
MAS as cocatalysts | Bi NCs/Bi2O3 | Bi0 of Bi NCs | 300 W Xe lamp (λ > 420 nm) | 1 mL H2O + 0.08 MPa CO2 | CH4: 7.45 (94.8%) | 122 |
CsCuCl3/Cu NCs | Cu NCs | 150 W Xe lamp AM 1.5 cut-off filter | 4 mL ethyl acetate (EA) + 1 mL propan-2-ol (IPA) + CO2 | CH4: 7.2 (92.7%) | 123 | |
Ultrathin Bi12O17Cl2 nanosheets | Vo; Bi clusters | 300 W Xe lamp | 50 mL H2O + 0.08 MPa CO2 | CO: 64.3 | 124 | |
Bi–BiOCl plasmonic nanoparticles decorated TiO2 nanosheets | Bi | 300 W Xe lamp AM 1.5 cut-off filter | 5 mL 0.2 M NaHCO3 + CO2 | CH3OH: 235.78 (≈90%) | 125 | |
Au/TiO2/W18O49 | Au–O–Ti, W–O–Ti | 300 W Xe lamp (λ > 420 nm) | 0.1 mL H2O + CO2 | CH4: 35.55, CO: 2.57, CH4 (93.3%) | 126 | |
PtAg-2/HNb3O8 | Pt–Ag alloy (7![]() ![]() |
300 W Xe lamp (320 nm < λ < 780 nm) | 2 mL H2O + 0.09 MPa CO2 | CH4: 93.6 (74.3%) | 127 | |
PtCu/TiO2 | Pt/Cu alloy (0.4![]() ![]() |
300 W Xe lamp (300 nm < λ < 400 nm) | Water vapor + CO2 | CH4: 100% | 128 | |
Cu–Ag/TiO2 | Cu–Ag alloy (0.8![]() ![]() |
300 W Xe lamp AM 1.5 cut-off filter | 150 mL H2O + 90 kPa CO2 | C2H4: 1110.6 | 129 | |
AuIr with InGaN nanowires on silicon | Au3Ir1 alloy (0.44![]() ![]() |
300 W Xe lamp | 30 mL H2O + 2 atm CO2 | C2H6: 58![]() ![]() ![]() ![]() ![]() |
130 | |
Au/TZO | Au nanoparticles | 300 W Xe lamp AM 1.5 G filter | 20 mL mixed solution + 1 atm CO2 MeCN/H2O/TEOA (16 mL/2 mL/2 mL) | H2: 271.6, CO: 260.6, C2H4: 6.80, C2H6: 4.05 | 131 | |
Bi nanoparticles grown on the Bi2MoO6 with Vo | Bi nanoparticles Vo | Xe lamp | 0.42 g NaHCO3 + 30 mL H2O + CO2 | C2H5OH: 17.93 (92%) | 132 |
Photocatalytic CO2 conversion mainly involves CO bond cleavage and C–H bond formation, accompanied by the challenging C–C coupling toward the generation of C2+ products. The MAS could reduce electrostatic repulsion between the C1 intermediates, promote C–C coupling, and thus lead to the highly selective formation of the C2+ product. The photogenerated electron transition from TiO2 to atomically dispersed Cu atoms rearranged the energy levels of the Cu 3d orbitals.79 Consequently, the initial four-coordinated Cu1–O4 was distorted into a Cu1–O2+1 structure (twofold normal Cu–O coordination and one stretched Cu–O coordination), which could be reversibly recovered after removing the synergistic light field. The photoinduced metastable intermediate of Cu1–O2+1 delivered an C2H4 yield rate of 60.4 μmol gcat−1 h−1. The energy barrier of the first protonation step from CO2 to *COOH showed an obvious decline over Cu1–O2+1, indicating the stronger activation capability of the metastable asymmetrical structure to reactants than that of Cu1–O4. *CO molecules adsorbed more strongly on the Cu1–O2+1 (−1.49 eV) surface than on the Cu1–O4 (−0.44 eV) surface, leading to subsequent hydrogenation rather than desorption. It was noteworthy that, in contrast to Cu1–O4, Cu1–O2+1 promoted C–C coupling and further formation of C–C bonds, thus facilitating the generation of C2H4. Furthermore, N,S-codoped Fe-MOF MIL-88B with a well-defined bipyramidal hexagonal prism shape was designed.80 The synergistic effect of the Fe–N coordinated sites and reasonable defects from uncoordinated S increased the electron density disorder around Fe, accelerated the migration of photogenerated carriers, benefited electron storage, and effectively promoted the formation of C–C coupling intermediates for C2H4. For liquid products, the carbon nitride-supported Cu single-atom catalyst with a defective low-coordination Cu–N2 motif (Cu–N2–V) exhibited superior photocatalytic activity for CO2 reduction to ethanol relative to Cu–N3 and Cu–N4.81 In particular, Cu in Cu–N2–V existed in both Cu+ and Cu2+ valence states. On the one hand, the Cu+ sites benefited CO2 activation. On the other hand, the coexistence of Cu+/Cu2+ sites contributed to the strong adsorption of *CO and subsequent *CO–*CO dimerization. Finally, ethanol was ultimately produced from *CO–*CO, which underwent a series of hydrogenation processes.
The d-band center tuning strategy influences the carrier transportation and the adsorption of the reactant and intermediate to promote photocatalytic CO2 conversion, which can be achieved by incorporating MAS. Bulk doping of single Ni atoms in WO2.72 (B-Ni1/WO2.72) displayed superior solar-driven CO2 reduction performance to surface anchoring of single Ni atoms on WO2.72 (S-Ni1/WO2.72).82 The introduction of Ni atoms led to an upward shift of the d-band center of W atoms in the WO2.72 host structure due to the overlapping orbital hybridization. Critically, the d-band center of W in B-Ni1/WO2.72 shifted upward to a greater extent relative to S-Ni1/WO2.72. Therefore, the Ni atoms in B-Ni1/WO2.72 exhibited stronger electronic interactions with the WO2.72 host, facilitating the formation of charge-transfer channels that enabled the rapid transfer of photogenerated electrons to the surface Ni atoms. Moreover, the free energies of *CO2, *COOH, and *CO intermediates of B-Ni1/WO2.72 were integrally decreased compared to those on S-Ni1/WO2.72, benefiting the conversion of CO2 to CO. Analogously, the decoration of isolated Mn atoms over the multi-edged TiO2 nano-pompons shifted the d-band center upwards and pushed the antibonding orbital closer to the Fermi level, thus facilitating CO2 adsorption.83 The Mn site acted as an active center for CO2 activation, and significantly reduced the formation energy barriers of *COOH to accelerate the decisive step of the reaction.
Besides in-plane regulation, MAS can realize interlayer-spacing adjustment, which may significantly influence the carrier transport of the bulk-catalyst, thereby affecting CO2 photoreduction. Single-atom In-bonded N-atom (Inδ+-N4) in the (002) crystal planes of g-C3N4 reduced the (002) interplanar spacing of g-C3N4, benefiting the separation of bulk carriers.84 More charges were transferred to the adsorbed CO2 molecule from the Inδ+-N4 active center (0.116 eV) compared to the bare C3N4 site (0.006 eV), which was favorable for CO2 adsorption. Moreover, the Inδ+-N4-led CO2 hydrogenation to *COOH was downhill by −0.114 eV and evidently surpassed the uphill step by 2.09 eV on the bare C3N4, which optimized the reaction path.
Precise control of the formation of Metal1⋯CO⋯Metal2 (M1⋯C
O⋯M2) intermediates at the photocatalyst interface is one of the critical steps in the formation of hydrocarbons. This is due to the fact that the energy required for simultaneous cleavage of the M1⋯O and M2⋯C bonds is much greater than that required for C–O bond breaking (Fig. 9a).92 Ag2Cu2O3 nanowires with abundant Cu–Ag Lewis acid–base dual sites on the preferentially exposed (110) surface were utilized as a model catalyst to achieve 100% selectivity in the photogeneration of CH4 from CO2.93 The Cu⋯Ag Lewis acid–base dual sites on Ag2Cu2O3(110) regulated the M1⋯C
O⋯M2 intermediate formation, converting CO2 into hydrocarbons. The rate-determining step with a corresponding Gibbs free energy (ΔG(CHO*)) of 0.75 eV led to greater feasibility in CO* forming CHO* than desorbing from the catalyst surface (1.16 eV, Fig. 9b). Similarly, the highly stable Cu–C–O–In intermediate at the Cu–In dual sites of the sulfur-deficient CuIn5S8 converted the endoergic protonation step to an exoergic reaction process, changing the reaction pathway to form CH4 instead of CO.92 Furthermore, the VO-regulated In–Ti dual sites enabled the formation of a stable adsorption conformation of the In–C–O–Ti intermediate, leading to the highly selective reduction of CO2 to CH4.94
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Fig. 9 (a) CO2 photoreduction into fuels such as CH4 and CO through the use of dual-metal-site catalytic systems (M represents the metal site, ‘H+ + e−’ refers to the proton coupled electron transfer process and ‘−H2O’ means the desorption of H2O molecules after the intermediates react with the proton–electron pair). This figure has been reproduced from ref. 92 with permission from Springer Nature, copyright 2019. (b) The rate-determined step of photoreduction of CO2 into CH4 over Ag2Cu2O3 nanowires. This figure has been reproduced from ref. 93 with permission from Wiley-VCH, copyright 2023. |
Catalysts containing dual MAS regulates the electron distribution by creating asymmetric atomic configurations, which significantly affects the photocatalytic performance, especially for multi-electron CO2 reduction. The mechanism of reverse electron transfer over Au and Co bimetallic atom catalysts was reported.95 Electrons were delocalized from Au and accumulated around the Co atoms, resulting in the electron-rich Co atoms adsorbing/activating CO2 molecules readily, which significantly promoted photocatalytic CO2 reduction. In this regard, the Au/Co double single-atom loaded CdS increased the yield of CO and CH4 by nearly 2800% and 700%, respectively, compared to CdS alone. In addition, the disparate electron distributions and valence states of two distinct metal atoms could endow significantly different charge distributions of the neighboring C1 intermediates to effectively suppress the electrostatic repulsion. The asymmetric Zn–O–Ge triatomic sites in the Vo-rich Zn2GeO4 nanobelts induced distinct charge distributions of neighboring C1 intermediates, which facilitated the C–C coupling with a high CO2-to-CH3COOH conversion rate of 29.95%.96 Under similar mechanisms, incorporating redox-active Co2+/Ni2+ cations (TM) into layered lead iodide hybrids (TJU-39(Pb)) achieved efficient photocatalytic CO2-to-C2H5OH conversion (yield of 24.9–31.4 μmol g−1 h−1, selectivity exceeding 90%).102 It was experimentally demonstrated that the interlayer TMs were delocalized to the lead iodide layers to construct TM–O–Pd sites with substantial asymmetric charge distribution, which reduced the reaction barrier for C–C coupling. Specifically, the two-electron reduction of CO2 to CO* occurred simultaneously at the charge-enriched Pb2+ sites and the Ni2+ site, which subsequently underwent C–C coupling to form OC–CO* intermediates. The OC–CO* intermediate coupled multiple e−/H+ pairs, combining the dehydration process to finally yield C2H5OH.
From an overall reaction perspective, the CO2 photoreduction contains oxidation and reduction half-reactions, involving multiple proton-coupled electron transfer processes. The MAS construction not only enhances CO2 reduction, but also influences the corresponding oxidation half-reaction. The existence of an internal electric field pointing from the Ti cluster to the Cu cluster in MCOF-Ti6Cu3 facilitated carrier separation, leading to electron and hole transfer to the Cu cluster and Ti cluster, respectively.90 Consequentially, electrons reaching the Cu cluster drove the reductive reactions, and the oxidative reaction proceeded at the Ti cluster. Analogously, two compatible active centers of Mn and Co were loaded onto carbon nitride (Mn1Co1/CN).17 The active center of Mn promoted H2O oxidation by accumulating photogenerated holes, along with Co-facilitated CO2 activation by increasing the bond length and bond angle of CO2 molecules.
The geometry of MAS regulated by vacancies ultimately affects the catalytic performance to generate C2+. Ultra-thin CuGaS2/Ga2S3 containing Vs realized the selectivity of C2H4, reaching ≈93.87% with the yield of ≈335.67 μmol g−1 h−1.107 Vs induced a highly delocalized electron distribution in the original region of S atoms, forming a Cu–Ga metallic bond. The upshift of the d-band center of Cu–Ga ions induced by Vs could not only enforce the adsorption ability for the intermediates of *CHOH*CO to initiate C–C coupling, but also accumulate electrons to drive the kinetic process of photocatalytic CO2 reduction.
The interface of the heterostructure could natively provide MAS for photocatalytic reactions. For example, the unique Cu–C and Cu–N dual chemical bond at the interface of the fabricated g-C3N4/Cu2SnS3 nanocomposite led to a superior CO formation rate from CO2 photoreduction.118 The Z-scheme carrier conduction was highly activated due to the interfacial nitrogen vacancies via the Cu–C and Cu–N bonding for CO2 reduction on the conduction band (CB) of Cu2SnS3 and water oxidation on the VB of g-C3N4. The more negative overall free energy of Cu sites relative to Sn and S implied that the CO2 conversion reaction occurred mainly via effective dual bonds between Cu sites and the C and N elements in g-C3N4.118
Heterogeneous structures enriched with MAS can generate C2+ products. A novel supramolecular assembly of NiAl–Fe-TCPP obtained by intercalating iron porphyrin (Fe-TCPP) into NiAl-layered double hydroxide (NiAl-LDH) exhibited superior catalytic performance of CO2 photoreduction to derive C2H4 with high selectivity up to 93.4%.119 The strong host–guest interaction between LDH and Fe-TCPP led to the electron transfer from NiAl-LDH to Fe-TCPP. According to the ex/in situ XAS, the valence of Fe decreased along with the increase of Ni valence compared with that of Fe-TCPP and LDH, respectively. The low valence of Fe in NiAl–Fe-TCPP facilitated the CO* hydrogenation and coupling with CHO* to form COCHO*. Particularly, the CO2 activation to CO was accomplished on NiAl-LDH, and then spilled to Fe-TCPP and coupled to generate COCHO*, which was further hydrogenated to produce C2H4. The O–In–P polarized sites at the r-In2O3/InP interface promoted C–C coupling with a productivity of 96.7 μmol g−1 and selectivity >96% for CO2 photoreduction to CH3COOH.120 The interfacial In atoms accumulated electrons and transferred electrons to CO2, thus promoting CO2 adsorption and activation. The quite different carbon Bader charges of OCCO* on r-In2O3/InP caused by the formation of rich O–In–P reactive sites at the interface alleviated dipole–dipole repulsion and promoted C–C coupling (Fig. 10a–c). Ultimately, the C–C coupling energy barrier of r-In2O3/InP (0.5461 eV) was lower than that of pure In2O3 (0.9446 eV), suggesting that the formation of heterostructures by modifying In2O3 with P favored the formation of C2+ products (Fig. 10d).
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Fig. 10 Calculated electron density difference diagrams and the Bader charge values of (a) r-In2O3/InP, (b) In2O3, and (c) p-In2O3/InP with OCCO* absorbed. (d) Free energy diagrams for the reduction of CO2 to CH3COOH over r-In2O3/InP, In2O3, and p-In2O3/InP. These figures have been reproduced from ref. 120 with permission from American Chemical Society, copyright 2023. |
The surface plasmon resonance of MAS plays a crucial role in the collection and conversion of solar energy by the strong local fields. The coexistences of Vo and Bi clusters generated in situ on ultrathin Bi12O17Cl2 nanosheets contributed to the high efficiency of the CO2-to-CO conversion.124 The Bi clusters exhibited a plasmon effect that extended the light absorption and enabled more sunlight harvest. Furthermore, Bi clusters acted as hole trapping centres in synergy with Vo as electron trapping sites, leading to the spatial separation of photogenerated electron–hole pairs. As another example, the attachment of non-noble plasmonic Bi particles with BiOCl shells to self-assembled TiO2 nanosheets created a transformative hybrid plasmonic nanostructure. Bi stabilized by the BiOCl shell generated robust localized surface plasmon resonances, inducing a local field enhancement of 7–9 times, thus enabling the efficient and selective CO2-to-methanol conversion at the TiO2–BiOCl heterointerfaces.125 In addition, it is worth mentioning that the localized surface plasmon resonance effect of Bi nanoparticles could significantly improve the rate and selectivity of the C2H5OH generation.132 The high localized electron density and abundance of hot electrons in the active site drove the multi-electron reduction reaction, favoring the generation of C2+ products.
Plasmon resonance-mediated photocatalysis on precious metal surfaces (mainly Ag and Au) shows great potential for solar energy harvesting and conversion. For instance, plasmonic Au mediated the S-scheme charge transfer, and generated additional energetic hot electrons and holes to inject to the CB of ZnIn2S4 and VB of CuS, respectively, enabling a more thorough separation of carriers for CO2 reduction and H2O oxidation.134 Moreover, a new idea of plasmonic active “hot spot”-confined photocatalysis was proposed to improve the photocatalytic CO2 conversion. Specifically, tiny gaps (<10 nm) between the plasma nanostructures promoted plasma coupling between the nanostructures, resulting in the formation of classical “hot spots”. The Au/TiO2/W18O49 sandwich-like substructures with the short distance (<10 nm) between Au and adjacent W18O49 induced an intense plasmon-coupling to form the active “hot spots” in the substructures.126 These active “hot spots” could gather the incident light to enhance “hot electron” generation and migration, and capture protons and *CO through the dual-hetero-active-sites (Au–O–Ti and W–O–Ti) at the Au/TiO2/W18O49 interface, thus accelerating the protonation of *CO intermediates to derive CH4.
Alloys as MAS enhance light absorption, inhibit charge recombination, and adsorb and activate reactants, thereby improving the selectivity of specific products. The PtAg alloy over HNb3O8 nanosheets trapped the electrons and improved the charge separation efficiency, mitigating the kinetical challenge of the eight-electron transfer process for CH4.127 In addition, the PtAg alloy acted as synergistic sites to reduce CO2 to CO intermediates at the Ag site, which were then spilled over or sequentially adsorbed at neighboring Pt sites for further hydrogenation to CH4. As another example, PtCu alloys with appropriate Pt/Cu ratios were deposited onto TiO2 nanocrystals, which significantly improved CH4 production with 100% selectivity.128 The PtCu alloy effectively facilitated the separation/transfer of photogenerated charges, benefited the adsorption of CO2, and promoted the formation and activation of intermediates (CO2−, *COOH, *CO, and *CHO). The synergistic effect of PtCu lowered the activation energy barriers of *CO2 and *CHO, inhibited the desorption of *CO, and ultimately optimized the efficiency and selectivity of CH4.
The alloy MAS promotes the C–C coupling and desorption of *C2 intermediates from the catalyst surface. Loading Cu–Ag alloy sub-nanoclusters (ASNCs) on TiO2 for CO2 photoreduction produced C2H4 with a record-high formation rate (1110.6 ± 82.5 μmol g−1 h−1).129 The interaction between Cu and Ag in the Cu–Ag ASNCs promoted the C–C coupling of CH2* at the Cu active site, which led to the spontaneous formation of *C2H4 from CH2* on Cu–Ag alloy/TiO2. In addition, the desorption energy of *C2H4 in the Cu–Ag alloy/TiO2 was lower than that in Ag/TiO2. This suggested that the interaction between Cu and Ag promoted the desorption of *C2H4, and thus the selective and efficient production of C2H4. Ag, in conjunction with Ir, achieved C–C coupling by inserting CO2 into –CH3, facilitating the formation of C2+ during CO2 photoreduction.130 Specifically, the assembly of AuIr with InGaN nanowires on silicon achieved a C2H6 activity of 58.8 mmol g−1 h−1 with a turnover number of 54595 over 60 h. The Ir sites in Au–Ir alloys increased CO2 reduction activity by lowering the reaction energy of key elementary steps (for example, CO2 to *COOH on pure Au and *CO to *CHO on Au3Ir1 alloy) and steered the selectivity from the dominant HER to C–C coupling. The insertion of CO2 into *CH3 toward *CH3COO exhibited the lowest reaction energy compared with other C–C coupling forms (CH + *CH → *C2H2, *CH2 + *CH2 → *C2H4, *CH3 + *CH3 → C2H6(g) and CO2 insertion into *CH3 to *CH3COO), revealing the possible mechanism of C–C coupling for C2H6 synthesis.
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Fig. 11 (a) Aberration-corrected HAADF STEM image of the Co-sZIS; single-atomic Co sites are highlighted by yellow circles. (b) Magnified view of the white box in (a) (inset is the simulated atom distribution of Co-sZIS, and the green, gray, and red spheres represent the Zn, In, and Co atoms, respectively). (c) Intensity profile corresponding to the dark cyan arrow in (b). These figures have been reproduced from ref. 136 with permission from American Chemical Society, copyright 2023. Aberration-corrected HAADF-STEM images of (d) Ni–N–C, (e) Fe–N–C, and (f) (Ni, Fe)–N–C. These figures have been reproduced from ref. 137 with permission from Wiley-VCH, copyright 2023. |
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Fig. 12 High-resolution XPS spectra of (a) Cd 3d, (b) S 2p, (c) C 1s, and (d) N 1s of CdS, CTF-HUST-1, and CdS(0.9)@CTF samples. The in situ XPS measurement was performed under light irradiation. These figures have been reproduced from ref. 138 with permission from Wiley-VCH, copyright 2023. |
Cu–N4 sites-anchored phosphorus-modulated carbon nitride (CuACs/PCN) regulated the intermediate energy levels to achieve C2H4 formation during CO2 photoreduction.85 The Cu K-edge XANES of CuACs/PCN was between Cu2O and CuO (Fig. 13a), indicating that the oxidation state of Cu is between +1 and +2. In the EXAFS spectrum (Fig. 13b), CuACs/PCN had a prominent peak near 1.5 Å, corresponding to the scattering of the Cu–N bond in the first coordination shell. The absence of the Cu–Cu coordination (2.2 Å) indicated that the Cu sites were atomically dispersed. From the fitting results (Fig. 13c), Cu atoms in CuACs/PCN were coordinated by four N atoms at a distance of 1.97 Å. The wavelet transform contour plot of CuACs/PCN exhibited a radial distance of 1.5 Å (Y-axis), which was significantly different from that of the Cu foil. These results strongly demonstrated that Cu atoms were dispersed with Cu–N4 coordination in CuACs/PCN. In addition, XAFS was applied to study the coordination environment of MAS in Co1Ag(1+n)PCN.110 In the Ag K-edge XANES spectra, the near-edge absorption peak of Co1Ag(1+n)PCN was located above Ag2O, indicating that the valence state of Ag was near +1 (Fig. 13e). Two main peaks at 1.5 and 2.4 Å in the Fourier-transformed Ag k2-weighted EXAFS spectra (Fig. 13f) corresponded to the first shell layer of Ag–N and the second shell layer of Ag–C, respectively. The presence of Ag–Ag coordination (2.6 Å) indicated that Ag existed in the form of single atoms and nanoparticles. The wavelet transform Ag K-edge EXAFS of Co1Ag(1+n)PCN showed a maximum peak around 4.7 Å, which was different from that of the Ag foil and Ag2O, attributed to Ag–N/C coordination. Specifically, the EXAFS fitting of the shell coordination elucidated that the Ag atom was coordinated to 2.3 N atoms and 1.7C atoms, referred as the Ag–N2C2 site. Based on a similar analytical procedure, the existence of a Co–N6–P configuration of the Co single atom in Co1Ag(1+n)PCN was also revealed.
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Fig. 13 (a) XANES and (b) EXAFS spectra of the Cu K-edge of CuACs/PCN, Cu foil, Cu2O, CuO, and CuPc. (c) The EXAFS fitting curves of CuACs/PCN at R-space. (d) WT EXAFS of the Cu K-edge. These figures have been reproduced from ref. 85 with permission from Wiley-VCH, copyright 2022. Ag K-edge (e) XANES, (f) EXAFS, and (g) WT-EXAFS spectra of Co1Ag(1+n)-PCN. These figures have been reproduced from ref. 110 with permission from American Chemical Society, copyright 2023. |
In situ DRIFTS identifies key intermediates of the C2+ product, contributing to the inference of C–C coupling processes. In this regard, crucial C2 intermediates including COCO* (1374 and 1486 cm−1) and COCOH* (1233 and 1574 cm−1) were observed on the Au–CeO2 nanocomposite, providing strong evidence that the Au–O–Ce sites drove the generation of C2H6 from the C–C coupling.97 In addition, the peaks at 1367 and 1485 cm−1 appearing in the In2O3/InP heterojunction were related to OCCO*, a key intermediate in the generation of CH3COOH.120 The peak at 1332 cm−1 belonged to the C–H vibration of the hydrogenated intermediate after C–C coupling, and the peak at 1433 cm−1 belonged to the COO stretching vibration of CH3COOH, further reflecting the generation of C2+.
As another crucial feature, electrochemical analysis contributes to reveal the change of activation and reduction energy of CO2 conversion. Kang's research group examined the Tafel plots of pure Cu, Cu/PVP and Cu/rGO/PVP/Nafion composite electrodes to estimate the reaction overpotentials of the samples.145 The overpotentials of Cu/PVP (6.16 kJ mol−1) and Cu/rGO/PVP/Nafion (9.00 kJ mol−1) electrodes were significantly lower than those of the pure Cu electrode, revealing a decrease in the CO2 reduction energy.
In addition, the Mott–Schottky measurement can be used to specify the semiconductor type and determine the valence band maximum (VBM) and conduction band minimum (CBM) potentials. For example, according to the results of the Mott–Schottky test, the Zr/Ti bimetallic oxide solid solution integrated with Au nanoparticles (Au/TZO) exhibited the characteristics of n-type semiconductors.131 As the CBM of Au/TZO was estimated as −0.80 V with respect to a normal hydrogen electrode (NHE), the photocatalytic conversion of CO2 to CO (–0.53 vs. NHE), CH4 (–0.24 vs. NHE), C2H4 (–0.34 vs. NHE) and C2H6 (–0.27 vs. NHE) was thermodynamically feasible over Au/TZO. It is also worth mentioning that the electrochemical active surface area (ECSA) can be used to reveal the number of catalytically active sites. A larger ECSA indicates more active sites, which is more favorable for photocatalytic CO2 conversion.146
Electronic property analysis can infer underlying factors affecting the catalytic activity. The d-band centers of Cu in Cu1N3@P(1)CN, Cu1N3@CN, and Cu1P3@PCN were determined to be −1.24 eV, −1.46 eV, and 11.60 eV, respectively, by partial density of states analysis, which depended on the local coordination environment of Cu.87 As the d-band center of Cu approached the Fermi energy level, the catalytic activity for the COOH* formation became higher. It is worth mentioning that the overlap between Cu 3d and P 3p in Cu1N3@P(1)CN was more pronounced than the overlap of Cu 3d with C 2p at the same corner position in Cu1N3@CN. This reflected the more pronounced orbital and electronic interactions of Cu and P in Cu1N3@P(1)CN than that of Cu and C in Cu1N3@CN, which moved the d-band center of Cu1N3@PCN closer to the Fermi energy level. Charge density difference analysis further demonstrated that the electronic interaction between Cu and COOH* in Cu1N3@P(1)CN was the strongest relative to Cu1N3@CN and Cu1P3@PCN, prompting a large number of electrons to be transferred from the Cu center to COOH* in Cu1N3@P(1)CN, benefiting CO2 activation.
Theoretical calculations provide insight into the mechanism of the photocatalytic CO2 reduction reaction. From the free energy analysis, the introduction of Ag single atoms on hollow porous polygonal C3N4 nanotubes (Ag1@PCNT) significantly enhanced the adsorption of *CO2 and weakened the adsorption of *H, suggesting that Ag1@PCNT was more favorable for CO2 conversion than for hydrogen production.88 The COOH* formation energy (the rate-limiting step) of Ag1@PCNT was lower than that of C3N4 nanotubes. This suggests that the strong interaction of the Ag–N3 coordination with *COOH stabilized the *COOH intermediate and reduced the barrier for CO2 photoreduction (Fig. 14a and b). In addition, the generation of CO molecules by desorption of *CO groups on the surface of Ag1@PCNT had a smaller Gibbs free energy than the protonation of *CO into *CHO, which contributed to the highly selective generation of CO.
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Fig. 14 (a) Structure of Ag1@PCNT. (b) Gibbs free energies of the CO2 photoreduction pathways by DFT calculations over the Ag1@PCNT (Ag sites) and PCNT surfaces. Atom key: N (blue), C (purple), H (green), and Ag (orange). These figures have been reproduced from ref. 88 with permission from Wiley-VCH, copyright 2023. Gibbs free energy diagrams and CO2-to-ethanol photoreduction pathways on InCu/poly-CN, Cu/poly-CN, and In/poly-CN (c). This figure has been reproduced from ref. 98 with permission from Wiley-VCH, copyright 2022. |
Theoretical calculations can reveal the specific processes involved in the generation of C2+ products, especially for C–C coupling. Calculation of the Gibbs free energy diagrams for CO2 photoreduction on In and/or Cu-anchored poly-CN (InCu/poly-CN, Cu/poly-CN, and In/poly-CN) (Fig. 14c) revealed that the formation of *COOH was a rate-determining step on InCu/poly-CN (ΔG = +1.20 eV) and In/poly-CN (ΔG = +1.86 eV), while the generation of *COCH2OH was a rate-determining step on Cu/poly-CN (ΔG = +1.98 eV).98 On InCu/poly-CN, *CO underwent a dimerization reaction (ΔG = 0.45 eV) and a subsequent hydrogenation–electron addition process to ultimately generate CH3CH2OH. In the Cu/poly-CN system, *CO was also coupled to form *COCO, but its Gibbs free energy was higher than that of the InCu/poly-CN. Furthermore, the process from *COCHOH to *COCH2OH in Cu/poly-CN was endothermic with the highest gain (ΔG = +1.98 eV), resulting in lower CH3CH2OH yields for Cu/poly-CN relative to InCu/poly-CN. However, the *CO intermediates on In/poly-CN preferred desorption as CO products (exothermic) rather than C–C coupling, resulting in a poor CH3CH2OH yield on In/poly-CN.
(1) The complexity of the preparation process and the difficulty of achieving uniform high-volume production are the major drawbacks of MAS-containing catalysts. In addition, the exposed MAS are susceptible to oxidation, resulting in performance degradation. Therefore, effective technologies should be continuously developed for the large-scale production of MAS-containing photocatalysts with stable and controllable surface states. Elaborate design strategies to improve the catalyst stability (e.g., surface encapsulation and ligand grafting) need to be further explored. Continuous optimization of existing synthesis methods is an effective and viable solution.
(2) Although photocatalysts containing MAS have shown great potential in artificial photosynthesis, their performance is still unsatisfactory for practical production. Particularly, photocatalytic CO2 conversion involves a variety of intermediates and products, making it difficult to regulate the selectivity of the target product. For photocatalytic systems with MAS, the interactions of reactants and intermediates with the MAS affect the activation and conversion of CO2, as well as C–C coupling. Defect design, doping, construction of heterostructures, and introduction of co-catalysts have become widely used strategies to enhance the CO2 photoreduction performance. However, there is still a need to continuously explore effective and novel strategies to improve the performance for real-world production, which remains elusive at the present stage.
(3) Accurate characterization and in-depth understanding of the mechanism of MAS in photocatalysis still face great challenges. It is crucial to carefully reveal the coordination environments of the MAS and its changes during the photocatalytic reaction. Specifically, the working mechanism of MAS in photocatalysis can be explored in-depth by advanced in situ characterisation techniques and accurate theoretical calculations, which guide the rational design and regulation of photocatalysts. Therefore, there is a need for the continuous development of highly sophisticated characterisation techniques, and theoretical computational models close to the actual reaction processes.
(4) In the face of the explosion of research data, the traditional research method of relying on pre-investigation to identify scientific problems has become overwhelming. This approach is costly, time-consuming, and increasingly difficult to meet the research needs of modern materials science. Nowadays, machine learning is changing the traditional research paradigm and bringing new opportunities for the development of related technologies and industrial upgrading. Combining machine learning techniques with the design of photocatalytic systems to rapidly mine effective information from a large amount of data is conducive to the construction of novel materials and the excavation of reaction mechanisms. Specifically, machine learning techniques are becoming a powerful tool for scientific research in the design and construction of photocatalysts, as well as in the prediction of physicochemical properties and mechanisms.
In summary, through continuous exploration and in-depth research, it is found that photocatalytic CO2 reduction technology is expected to realize the establishment of a resource-saving and environmentally-friendly energy system, and improve human life in the near future.
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