Atomically precise metal nanoclusters as catalysts for electrocatalytic CO₂ reduction

Abstract

Electrochemical carbon dioxide (CO₂) reduction can be used to convert CO₂ into various compounds at room temperature and ambient pressure using electricity generated from renewable energy sources. This technology is indispensable in establishing an environmentally responsible and sustainable society. However, further improvements in the activity and selectivity require the development of electrocatalysts that can directly serve as the actual reaction sites. In recent years, metal nanoclusters, which are metal particles with a size of approximately 1 nm, have been reported to be capable of electrochemical CO₂ reduction with high activity and selectivity, owing to their unique geometric/electronic structure. This review summarizes the synthesis methods of atomically precise metal nanoclusters and their application in electrochemical CO₂ reduction. We expect that this review will help clarify the current status of these studies and further accelerate the research on highly active and selective CO₂ reduction catalysts using metal nanoclusters.

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Tokuhisa Kawawaki

Tokuhisa Kawawaki is a Junior Associate Professor in the Department of Applied Chemistry at Tokyo University of Science. He received his Ph.D. degree in applied chemistry from the University of Tokyo in 2015. In 2016, he worked as a Japan Society for the Promotion of Science (JSPS) postdoctoral fellow at the University of Melbourne. He then worked as a JSPS super postdoctoral fellow at Kyoto University. In 2019, he moved to his current University. His current research topics include the synthesis of ligand-protected metal nanoparticles and nanoclusters and their application in photoelectrochemistry and photocatalysis.

Tomoshige Okada

Tomoshige Okada is a masters student in the Negishi group at Tokyo University of Science. He received his B.Sc. degree in chemistry from Tokyo University of Science in 2022. His main research interest is on the development of high-performance water-splitting electrocatalysts by using atomically precise metal clusters.

Daisuke Hirayama

Daisuke Hirayama is a masters student in the Negishi group at Tokyo University of Science. He received his B.Sc. degree in chemistry from Tokyo University of Science in 2022. His main research interest is on the development of high-performance water-splitting photocatalysts by using atomically precise metal clusters as cocatalysts.

Yuichi Negishi

Yuichi Negishi is a professor at the Department of Applied Chemistry at TUS. He received his Ph.D. degree in chemistry (2001) from Keio University under the supervision of Prof. Atsushi Nakajima. Before joining TUS in 2008, he was employed as an assistant professor at Keio University and the Institute for Molecular Science. His current research interests include the precise synthesis of stable and functionalized metal nanoclusters, metal nanocluster-connected structures, and covalent organic frameworks.

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1. Introduction

1.1 Atomically precise metal nanoclusters as catalysts

Among nano-sized materials, an aggregate of metal atoms with a size of approximately 1 nm is called a metal nanocluster (NC).^1–28 Metal NCs have different physical/chemical properties than bulk metals and relatively larger (>5 nm) metal nanoparticles (NPs), which are attributed to quantum size effects (Fig. 1A).^29–31 Furthermore, the properties and functions of NCs show a remarkable dependence on the number of constituent atoms, unlike metal NPs.^32–36 Therefore, if the number of constituent atoms can be precisely controlled in the metal NCs, it is possible to create a wide variety of properties and functions with the difference of a single atom. It has become possible not only to control the size of metal NCs but also to control heterometal doping^37,38 and geometric structures^39–42 at the atomic level. Thus, metal NCs are expected to be applied as key components in next-generation materials. In particular, for metal NCs protected by ligands, unlike gas-phase generated clusters, their geometric structures have been revealed by single-crystal X-ray diffraction (SC-XRD), and the combination of density functional theory (DFT) calculations and SC-XRD can reveal the electronic structure of metal NCs and their origin. Because of these characteristics, metal NCs have attracted significant attention in the development of materials based on luminescence control^43–47 and chiral control,^48,49 environmental fields,^50–54 energy harvesting,^51,55–59 and biological sensor^45,60,61 and catalytic reactions.^62–64 Metal NCs are highly active catalysts because of their unique geometric/electronic structure and small size. Moreover, theoretical calculations of the adsorption/desorption energies of reacting molecules based on the geometric structure obtained by SC-XRD can help elucidate the mechanisms of various reactions.

Fig. 1 Schematic of this review. A next-generation energy system based on electrocatalytic CO₂ reduction reaction using metal-cluster catalysts. (A) Size dependences of metals for electronic structure; (B) kinds of metal clusters and each parameter for electrocatalytic CO₂ reduction reaction; and (C) a flow reaction system for electrocatalytic CO₂ reduction using electricity from renewable energy source.

The studies on metal NCs began after the 1950s, with efforts to synthesize metal NCs consisting of group 8–10 elements and ligands such as phosphine, halogen (X), and carbon monoxide (CO).⁶⁵ In 1994, Brust and Schiffrin et al. reported the synthesis of thiolate (SR)-protected gold (Au) NCs (Au_n(SR)_m).⁶⁶ Because these NCs have extremely high thermal and chemical stability owing to the strong bonding between Au and sulfur (S), and can be synthesized by simply mixing reagents under atmospheric conditions, research on Au_n(SR)_m increased dramatically after 2000. At that time, Au_n(SR)_m could only be treated as a mixture with a distribution in the number of constituent atoms; however, with the establishment of highly resolved separation techniques by Whetten et al. and Murray et al., the chemical composition of Au_n(SR)_m could be highly controlled.^67–69 In 2005, Tsukuda and Negishi were the first to systematically separate Au_n(SR)_m with a well-defined chemical composition.^70–72 In 2007, Kornberg et al. determined the geometric structure of Au₁₀₂(p-MBA)₄₄ (p-MBA: 4-mercaptobenzoic acid) by SC-XRD,⁷³ and since then, the chemical compositions and geometric structures have been determined for several Au_n(SR)_m. In addition, SR-protected silver (Ag) and copper (Cu) NCs have been synthesized in numerous studies, and the research has been extended to metal NCs capped by other ligands (Fig. 1B).

1.2 Electrochemical carbon dioxide reduction

Recently, interest in carbon dioxide (CO₂) reduction using electrocatalysts has been growing. In 1967, Manabe presented a “coupled atmosphere-ocean model” showing that CO₂ has a significant impact on long-term climate change, which bolstered global efforts to reduce CO₂ emissions.⁷⁴ In 2015, the Paris Agreement was adopted to solve the global warming problem, and “achieving carbon neutrality” is now a long-term goal shared among numerous countries.⁷⁵ Herein, carbon neutral means the reduction of the total greenhouse gases, including CO₂, to net zero. To achieve this, various efforts are being actively pursued worldwide. In particular, the transformation of CO₂ into useful organic compounds by reduction has been demonstrated as a promising method for directly decreasing CO₂ emissions.^75–77

The most notable CO₂ reduction reaction (CRR) is the Sabatier reaction (methanation), in which hydrogen (H₂) and CO₂ are converted to methane (CH₄) and water under high temperature and high pressure using a metal catalyst. This method has been used for industrially synthesizing methane since its discovery by Sabatier and Senderens in 1902.^78,79 Additionally, the reverse water gas shift reaction that produces CO and water (H₂O) from CO₂ and H₂ is often used in industry. However, the current system produces H₂ from fossil fuels, such as natural gas reforming or coal gasification, which emit a large amount of CO₂ during the production process. Alternatively, if CO₂ can be directly reduced by electrochemical reduction using water as an electron and proton source and electricity derived from renewable energy sources, then CO₂ can be directly converted into other organic compounds without using H₂ derived from fossil fuels (Fig. 1C). Electrochemical CRR is becoming easier to industrialize because it is possible to divert the water-electrolysis hydrogen-production technology currently under development.^75,80

Research on electrochemical CRRs began in the 1950s.⁸¹ In 1985, Hori et al. quantitatively analyzed the amount of liquid- and gas-phase products produced by electrolytic reduction, which corresponded to the amount of applied electricity (i.e., performed with a Faraday efficiency (FE) of 100%).⁸² Furthermore, their study revealed that the products differed depending on the metal electrodes:^83–86 (1) nickel (Ni), iron (Fe), platinum (Pt), and titanium (Ti) produce H₂; (2) lead (Pb), mercury (Hg), thallium (Tl), indium (In), tin (Sn), cadmium (Cd), and bismuth (Bi) primarily produce formic acid (HCOOH); (3) Au, Ag, zinc (Zn), palladium (Pd), and gallium (Ga) mainly produce CO; and (4) Cu produces various hydrocarbons, aldehydes, and alcohols. For these metal catalysts, increases in specific surface area, crystal facet control, alloying, and support interactions for CRR catalysts have been investigated, along with advances in nanotechnology, since the 2000s.^87–89 Thereby, overvoltage suppression and improvements in product selectivity and durability have been achieved. However, further improvements in the activity, selectivity, and durability are required, and research in this area remains active. In this review, we discuss the use of atomically precise metal NCs for electrochemical CRRs. Therefore, readers interested in studying electrochemical CRRs using nanomaterials other than metal NCs (metal NPs,^89–92 single atoms,^93–97 and molecular catalysts^98–100) are invited to read the review articles that describe them in detail. Gas-phase generated clusters are excluded from this review because there are few reports on CO₂ reduction.

1.3 Purpose and contents of this review

Metal NCs have significant potential not only for use as various thermal catalysts^51,52,54,101 and photocatalysts^102–114 but also as electrocatalysts,^{101,115–117} particularly in hydrogen evolution reactions (HER),^118–123 oxygen evolution reactions (OER),^{120,124–126} oxygen reduction reactions (ORR),^{58,120,127–134} and alcohol and HCOOH oxidation reactions.^135–137 In addition, the application of metal NCs as electrochemical CRR catalysts^{1,55,57,113–115,138–150} has been widely studied. However, CRR activity is often evaluated on accidentally synthesized metal NCs, and there are not many examples of metal NCs synthesized based on standard design principles. The reason for this is the lack of deep understanding regarding the synthetic methods and geometric/electronic structures of metal NCs and their resulting correlation with CRR activity. In this review, we summarize the synthesis methods and geometric/electronic structures of metal NCs, and the representative studies on electrochemical CRRs using these materials. Furthermore, on the basis of this knowledge, we discuss (1) the synthesis and geometric/electronic structure of metal NCs as high-performance CRR catalysts, (2) the appropriate experimental conditions (e.g., support and metal NCs) for the application of high-performance CRR catalysts, and (3) the CRR mechanism of the metal NC catalysts. Through these discussions, we hope to promote the establishment of clear guidelines for the design of metal NCs that are suitable for electrochemical CRRs, thereby enhancing the development of metal NCs and electrochemical CRR catalysis.

In section 2, we first describe the synthesis methods and geometric/electronic structures of the predominant metal NCs that have been applied to CO₂ reduction in previous studies, categorized by the metal species. Then, in section 3, we discuss the catalytic activities of metal NCs in electrochemical CRRs and their mechanisms. In section 4, we summarize the conclusions of this review, and in section 5, we briefly describe the outlook for the future.

2. Synthesis of atomically precise metal nanoclusters

Metal NCs have high surface energy owing to their small size and can easily aggregate or degrade in air if not specially handled. Thus, metal NCs are often protected using organic ligands, which facilitates handling and modification. Furthermore, by controlling the ligands, metal species, and reaction rates, metal NCs with various numbers of constituent atoms, chemical compositions, and structures can be synthesized. In this section, we describe the synthesis of mainly Au, Ag, and Cu NCs, among those applied to electrochemical CRRs, which have relatively high stability and can be synthesized with atomic precision, as well as the effects of ligands and alloying. Multiple reviews have focused on the synthesis of metal NCs, and thus readers who would like to know more details should refer to those articles.^32–36

2.1 Synthesis of gold nanocluster

Au_n(SR)_m have relatively high stability and more examples have been reported at an early stage, compared with SR-protected Ag and Cu NCs.^{68,71,151,152} Au_n(SR)_m also possess useful physicochemical properties such as photoexcited luminescence, thermocatalysis, electrocatalysis, and magnetism, and many studies have reported on their application in electrochemical CRRs. In this section, we describe the synthesis of Au NCs, particularly those protected by SR, phosphine, alkynyl (C≡CR), and N-heterocyclic carbene (NHC).

2.1.1 Thiolate-protected gold 25 nanoclusters. Among Au_n(SR)_m, one of the most widely studied is Au₂₅(PET)₁₈ (PET = 2-phenylethanethiolate). There are two common synthetic methods: (i) two-phase synthesis and (ii) in situ single-phase synthesis.¹⁵³ In the two-phase synthesis, tetrachloroauric(III) acid (HAuCl₄·3H₂O) dissolved in water is mixed with tetraoctylammonium bromide (TOAB) dissolved in toluene to induce a phase transition, and then cooled to 0 °C and 2-phenylethanethiol is added while slowly stirring to reduce Au(III) to Au(I). Next, sodium borohydride (NaBH₄) is added, and Au₂₅(PET)₁₈ is obtained by washing/separation.¹⁵⁴ For the in situ single-phase synthesis, HAuCl₄·3H₂O and TOAB are dissolved in tetrahydrofuran (THF) and 2-phenylethanethiol, NaBH₄ is added, and Au₂₅(PET)₁₈ is obtained by washing/separation.¹⁵⁵ These methods can produce Au₂₅(PET)₁₈ in relatively high yields. The highly stable Au₂₅(PET)₁₈ is essentially Au_n(SR)_m with an entire −1 valence of ionic charge ([Au₂₅(PET)₁₈]⁻). However, it is also possible to synthesize neutral or +1 valent Au₂₅(PET)₁₈ with the same chemical composition ([Au₂₅(PET)₁₈]⁰ and [Au₂₅(PET)₁₈]⁺, respectively).^156,157 Notably, there are some differences in their geometric structures. The basic Au₂₅(PET)₁₈ structure consists of an icosahedral Au 13 core surrounded by six SR–Au–SR–Au–SR staples with quasi-octahedral symmetry (Au–S staples) protecting the structure (Fig. 2A).^158,159 The Au–S staple structure is distorted with regard to the σ_h symmetry plane in [Au₂₅(PET)₁₈]⁻, whereas the staples are aligned with the σ_h symmetry plane in [Au₂₅(PET)₁₈]⁰ (Fig. 2B). This might be caused by the presence or absence of counter ions and might be one of the reasons why metal NCs exhibit different properties depending on their valence, even if they have the same chemical composition. In addition, there are differences in the electronic structures of [Au₂₅(PET)₁₈]^z (z = −1, 0, +1) (Fig. 2C). [Au₂₅(PET)₁₈]⁻ is an 8-electron system corresponding to a noble gas-like 1S²1P⁶ superatomic electron configuration, whereas [Au₂₅(PET)₁₈]⁰ and [Au₂₅(PET)₁₈]⁺ have 1S²1P⁵ and 1S²1P⁴ electron configurations, respectively. This significantly affects their optical absorption spectra.^157,160,161

Fig. 2 (A) (a) Au₁₃ core and (b) Au–S staple structure of [Au₂₅(PET)₁₈]⁻. (B) Comparison of the crystal structures of (a, c) [Au₂₅(PET)₁₈]⁻ and (b, d) [Au₂₅(PET)₁₈]⁰. Both Au₂₅(PET)₁₈ in (A) and (B) are capped by 18 PET ligands (for clarity C and H atoms are omitted); for [Au₂₅(PET)₁₈]⁻, the counterion is tetraoctylammonium (TOA⁺; only N (in blue) is shown for clarity). (C) The optical absorption spectra of [Au₂₅(PET)₁₈]⁻ (blue profile) and [Au₂₅(PET)₁₈]⁰ (red profile) in solution. (B) and (C) are reproduced with permission from ref. 156. Copyright 2008 American Chemical Society.

Owing to its relatively stable electronic/geometric structure, Au₂₅(SR)₁₈ can be synthesized with a variety of SR ligands (including hydrophilic SR ligands). For example, in 2016, Li and Jin et al. synthesized 1-naphthalenethiolate (SNap)-protected Au₂₅(SNap)₁₈ using a two-step method. First, they synthesized hexanethiolate (SC₆H₁₃)-protected Au₂₅(SC₆H₁₃)₁₈, then dissolved it in toluene and reacted with naphthalene thiol to synthesize Au₂₅(SNap)₁₈.¹⁶² For the structure of Au₂₅(SNap)₁₈, the SC-XRD results revealed that Au₂(SNap)₃ is coordinated as a staple around an icosahedral Au₁₃ core.

Many studies have attempted to alloy Au₂₅(SR)₁₈ because of its high stability.¹⁶³ As a result, many metals, such as Ag,^164–167 Cu,^168–173 Pt,^174–177 Pd,^{175,178–181} Hg,^182,183 Cd,^183–185 and Ir,¹⁸⁶ can be doped into Au₂₅(SR)₁₈. In 2015, Jiang and Lee et al. reported the synthesis of MAu₂₄(SC₆H₁₃)₁₈ (M = Pd or Pt) using HAuCl₄·3H₂O and hydrogen hexachloroplatinate(IV) (H₂PtCl₆·6H₂O) or Na₂PdCl₄·6H₂O as precursors, and their electronic structures were elucidated (Fig. 3Aa).¹⁸⁷ [MAu₂₄(SC₆H₁₃)₁₈]⁰ has a 6-electron superatomic configuration (1S²1P⁴), whereas [MAu₂₄(SC₆H₁₃)₁₈]²⁻ has an 8-electron superatomic configuration (1S²1P⁶), which is similar to the trend observed in [Au₂₅(PET)₁₈]^z (z = −1, 0, +1). Furthermore, the Jahn–Teller-like distortion of [MAu₂₄(SC₆H₁₃)₁₈]⁰ results in 1P orbital splitting, which causes strong absorption in the near-infrared region (Fig. 3Ab). These results agree well with the predicted optical absorption spectra from simulations.

Fig. 3 (A) (a) Cartoon depicting Jahn–Teller-like distortion in the core (e.g., PdAu₁₂) predicted for the 6-electron [PdAu₂₄(SR)₁₈]⁰ (left), which undergoes a structural change to nearly spherical 8-electron [PdAu₂₄(SR)₁₈]²⁻ upon reduction (right). The vertical compression for [PdAu₂₄(SR)₁₈]⁰ (left) is exaggerated. (b) UV/Vis/NIR absorption spectra of Au₂₅(SC₆H₁₃)₁₈ (black), PdAu₂₄(SC₆H₁₃)₁₈ (red), and PtAu₂₄(SC₆H₁₃)₁₈ (blue) in trichloroethylene. (B) (a) Total crystal structures and (b) UV/Vis absorption spectra of Au₂₅(PET)₁₈, Au₂₄Cd₁(PET)₁₈, Au₁₉Cd₃(S-tol)₁₈, and Au₃₈Cd₄(d-MBT)₃₀. Au, green; Cd, red; S, yellow; C, gray. H atoms are omitted for clarity. (C) (a) Schematic of active-site engineering and crystal structure of the [Ag_xAu_25−x(PET)₁₈]⁻. (b) UV/vis absorption spectra of the [Ag₂₅(SPhMe₂)₁₈]⁻, [Au₂₅(PET)₁₈]⁻ and [Ag_xAu_25−x(PET)₁₈]⁻ (x = 10–15) in dichloromethane. (A) is reproduced with permission from ref. 187. Copyright 2015 American Chemical Society. (B) is reproduced with permission from ref. 188. Copyright 2021 American Chemical Society. (C) is reproduced with permission from ref. 190. Copyright 2022 American Chemical Society.

Especially among Au_n(SR)_m applied in CRRs, Cd doping supports high activation. In this case, [Au₂₅(PET)₁₈]⁻ is dissolved in CH₃CN, and Cd(NO₃)₂ is added and stirred/extracted to synthesize Au₂₄Cd₁(PET)₁₈ in which one Au atom is replaced by a Cd atom (Fig. 3Ba). Significant differences can be observed in the electronic structure after doping (Fig. 3Bb). In addition, by dissolving Au₂₄Cd₁(PET)₁₈ in toluene as a precursor, mixing it with different SR ligands, and adding methanol and stirring, a ligand exchange reaction can be used to synthesize Au₂₄Cd₁(TBBT)₁₈ (TBBT = 4-tert-butylbenzenethiolate) and Au₂₄Cd₁(d-MBT)₁₈ (d-MBT = 3,5-dimethylbenzenthiolate) from Au₂₄Cd₁(PET)₁₈.¹⁸⁸ Similarly, Cd-doped Au_n(SR)_m, such as Au₁₉Cd₃(S-tol)₁₈ and Au₃₈Cd₄(d-MBT)₃₀ (S-tol = p-toluenethiolate), can be synthesized using Au₂₅(PET)₁₈ and Au₄₄(d-MBT)₂₈ as precursors, respectively.¹⁸⁸ In the case of Au₂₄Cd₁(PET)₁₈, Cd atoms are substituted for Au atoms on the surface of the Au₁₃ core, but for Au₁₉Cd₃(S-tol)₁₈ and Au₃₈Cd₄(d-MBT)₃₀, Cd atoms are substituted at the outer staples of the Au₁₃ and Au₂₆ cores, respectively (Fig. 3Ba). These differences can significantly alter the electronic structure (Fig. 3Bb).

Ag can be doped into [Au₂₅(SR)₁₈]⁻ with a relatively large number of substitutions.^{165,166,170,189} In 2022, Kim, Yoo, and Lee et al., synthesized [Au_xAg_12−x@Au₁₂(PET)₁₈]⁻ (x = 10–15) with an Au₁₂(PET)₁₈ shell (Fig. 3Ca).¹⁹⁰ In this synthesis, they referred to the previously reported synthesis of [Ag₂₅(SPhMe₂)₁₈]⁻ (SPhMe₂ = 2,4-dimethylbenzenethiolate)¹⁹¹ and alloyed [Au₂₅(PET)₁₈]⁻ by introducing silver acetate (CH₃COOAg) during the process. In [Au_xAg_12−x@Au₁₂(PET)₁₈]⁻, absorption peaks were observed at 1.9, 2.6, and 3.8 eV (Fig. 3Cb), and SC-XRD revealed that the AuAg₁₂ core was covered by an Au₁₂(PET)₁₈ shell (Fig. 3Ca).

2.1.2 Thiolate-protected gold 38 nanoclusters. Au₃₈(PET)₂₄ is another Au_n(SR)_m with many reported cases,¹⁹² and the structural isomers Au_38Q^193,194 and Au_38T ¹⁹⁵ can be synthesized (Fig. 4Aa). Au_38T is relatively unstable and converts to Au_38Q, a more stable structure, by heating to approximately 50 °C. Au_38Q is composed of two icosahedral Au₁₃ cores connected by sharing an Au3 face, and its electronic structure differs significantly from that of Au_38T (Fig. 4Ab). The protected shell of Au_38Q consists of the remaining 15 Au atoms and contains six Au₂(SR)₃ and three Au(SR)₂ staples. Furthermore, Au_38Q has a pair of enantiomers, also known as Au_n(SR)_m with chirality.^196–198 In 2022, Zhou, Gao, and Zhu et al. synthesized [Au₃₈(SCH₂Ph^tBu)₂₄]⁰ (SCH₂Ph^tBu = 4-tert-butylbenzylthiolate) using the ligand exchange method, in which 4-tert-butylbenzylthiol is added to Au_38Q and stirred.¹⁹⁹ They also synthesized Au₃₈(SCH₂Ph^tBu)₂₄ doped with Pt and Pd. In this synthesis, HAuCl₄·4H₂O and H₂PtCl₆·6H₂O are mixed in THF, and then TOAB is added and stirred. Next, they added 4-tert-butylbenzylthiol and NaBH₄ and reacted for 6 h. After washing and separation, [Pt₂Au₃₆(SCH₂Ph^tBu)₂₄]⁰ and [Pt₁Au₃₇(SCH₂Ph^tBu)₂₄]⁰ were obtained (Fig. 4Ba). Although [Au₃₈(SR)₂₄]⁰ and [Pt₂Au₃₆(SR)₂₄]⁰ have closed-shell electron configurations with 14- and 12-electron superatomic configurations, respectively,^200,201 [Pt₁Au₃₇(SCH₂Ph^tBu)₂₄]⁰ has a 13-electron configuration, and electron spin resonance measurements indicate that only [Pt₁Au₃₇(SCH₂Ph^tBu)₂₄]⁰ is magnetic. The optical peaks are blue-shifted with increasing Pt doping, compared with [Au₃₈(SCH₂Ph^tBu)₂₄]⁰ (Fig. 4Bb).

Fig. 4 (A) (a) Crystal structures and (b) UV/Vis/NIR absorption spectra of Au_38T (blue) and Au_38Q (black) in toluene (measurement temperature: 0 °C). (B) (a) Crystal structures and energy band diagrams, and (b) UV/Vis/NIR spectra of [Au₃₈(SCH₂Ph^tBu)₂₄]⁰, [Pt₁Au₃₇(SCH₂Ph^tBu)₂₄]⁰ and [Pt₂Au₃₆(SCH₂Ph^tBu)₂₄]⁰. The inset is the picture of the three clusters in CHCl₃. (A) is reproduced with permission from ref. 195. Copyright 2015 Springer Nature. (B) is reproduced with permission from ref. 199. Copyright 2022 Wiley-VCH GmbH.

2.1.3 Other thiolate-protected gold nanoclusters. In 2022, Yang, Yang, and Wu et al. successfully synthesized TBBT-protected Au–Pd alloy and Au–Ag–Pd trimetallic alloy NCs.²⁰² First, they dissolved TOAB in THF and then added HAuCl₄·4H₂O palladium acetate (Pd(CH₃COO)₂) and TBBT, and stirred. After that, they added NaBH₄, followed by washing and separation to obtain Au₄Pd₆(TBBT)₁₂. Additionally, they dissolved Au₄Pd₆(TBBT)₁₂ in dichloromethane, added silver nitrate (AgNO₃) dissolved in acetonitrile, stirred for 1 h, and performed separation to obtain Au₃AgPd₆(TBBT)₁₂. SC-XRD revealed that Au₄Pd₆(TBBT)₁₂ has a pair of enantiomers (Fig. 5A). The structure has an outer shell consisting of two Pd₃(TBBT)₃ and three Au(TBBT)₂ and a core framework with one Au atom in the center, indicating that a difference of one atom can result in distinct electronic structures (Fig. 5B).

Fig. 5 (A) (a) Total structure of the Au₄Pd₆(TBBT)₁₂. (b) One Au kernel of Au₄Pd₆. (c) Two Pd₃(TBBT)₃ staple motifs. (d) AuPd₆(TBBT)₆. (e) Three Au(TBBT)₂ staple motifs. (f) Au₄Pd₆(TBBT)₁₂ (green, Au; purple, Pd; yellow, S; gray, C; white, H). (B) Experimental UV/Vis/NIR spectra of (a) Au₄Pd₆(TBBT)₁₂ and (b) Au₃AgPd₆(TBBT)₁₂. (A) and (B) are reproduced with permission from ref. 202. Copyright 2022 American Chemical Society.

Many studies have reported on Au_n(SR)_m of different sizes, whose crystal structures are also known. For example, Au₁₈(SR)₁₄,²⁰³ Au₂₀(TBBT)₁₆,²⁰⁴ Au₂₁(S^tBu)₁₅,²⁰⁵ [Au₂₃(SC₆H₁₁)₁₆]⁻ (SC₆H₁₁ = cyclohexanethiolate),²⁰⁶ Au₂₄(TBBT)₂₀,²⁰⁷ Au₂₈(TBBT)₂₀,²⁰⁸ Au₃₀S(S^tBu)₁₈,²⁰⁹ Au₃₆(SR)₂₄,²¹⁰ Au₄₄(TBBT)₂₈,²¹¹ Au₅₂(TBBT)₃₂,²¹² Au₉₂(TBBT)₄₄,²¹³ Au₁₃₀(p-MBT)₅₀ (p-MBT = 4-methylbenzenethiolate),²¹⁴ and Au₁₄₄(SR)₆₀ ^215,216 have been reported, as well as many other Au_n(SR)_m. Notably, their systematic syntheses and size dependence have also been investigated.^32–36 In 2017, Ramakrishna and Lee et al. used SC₆H₁₃ as a ligand to synthesize Au₂₅(SC₆H₁₃)₁₈, Au₃₈(SC₆H₁₃)₂₄, Au₆₇(SC₆H₁₃)₃₅, Au₁₀₂(SC₆H₁₃)₄₄, Au₁₄₄(SC₆H₁₃)₆₀, and Au₃₃₃(SC₆H₁₃)₇₉.²¹⁷ Their chemical compositions were confirmed by matrix-assisted laser desorption/ionization mass spectrometry (MALDI-MS) (Fig. 6Aa). Furthermore, these Au_n(SR)_m exhibit size-dependent optical properties (Fig. 6Ab), and the optical gap of the electronic transitions decreases as the cluster size increases (Fig. 6B).

Fig. 6 (A) (a) MALDI mass spectra and (b) UV/Vis/NIR absorption spectra of Au₂₅, Au₃₈, Au₆₇, Au₁₀₂, Au₁₄₄, and Au₃₃₃ clusters. The absorption spectra of clusters were obtained in tetrachloroethylene and were offset for clarity. (B) Plot of ln(k_nr) versus HOMO–LUMO gap for Au_n(SR)_m. The solid line is the best fit straight line for data (Au₂₅–Au₁₄₄). (A) and (B) are reproduced with permission from ref. 217. Copyright 2017 American Chemical Society.

2.1.4 Phosphine-protected gold nanoclusters. In addition to the SR ligands described above, a number of phosphine-protected Au NCs have been reported.^218–222 From the late 1960s to 1980s, Malatesta et al.,²²³ the groups at Nijimegen,^224–226 and Mingos et al.^227–229 synthesized phosphine-protected Au NCs and determined their geometric structures. In general, these phosphine-protected Au NCs can be synthesized by adding a reducing agent, such as NaBH₄, to a phosphine-gold complex dispersed in a solvent, such as dichloromethane or ethanol. In some cases, washing operations have been conducted through an etching/growth process using hydrochloric acid or other chemicals.²³⁰ For example, in 2022, Jiang, Sun, and Wang et al. synthesized Au₁₁(DPPE)₅ (DPPE = 1,2-bis(diphenylphosphino)ethane) and Au₂₂H₃(DPPE)₃(PPh₃)₈ (PPh₃ = triphenylphosphine) by liquid-phase synthesis, and other phosphine-protected Au NCs.²³¹ In 2023, Pei and Wang et al. synthesized [Au₇(PPh₃)₇H₅]²⁺ using a one-pot synthesis method.²³² For this synthesis, they first added PPh₃ and cyclohexanethiol to a HAuCl₄·3H₂O solution and stirred. After that, NaBH₄ was added, and the mixture was stirred for 14 h and then washed and separated to obtain [Au₇(PPh₃)₇H₅]²⁺. For [Au₇(PPh₃)₇H₅]²⁺, the corresponding peaks decayed after 12 minutes of irradiation with 408 nm light, and peaks at 370 and 460 nm from Ultraviolet–visible-near-infrared (UV/Vis/NIR) spectroscopy appeared. Electrospray ionization-MS (ESI-MS) results indicated that this final product was [Au₈(PPh₃)₇]²⁺, which changes its structure under light irradiation. In 2021, Wang et al. reported the synthesis of phosphine and SR co-protected [Au₅₅(p-MBT)₂₄(PPh₃)₆]³⁺.²³³ For this synthesis, gold thiolate and gold phosphine compounds, which are halide-free Au precursors, were prepared first, followed by reduction with NaBH₄. SC-XRD revealed that these Au NCs have a face-centered cubic (fcc) structure and an Au₅₅ core with four layers stacked on top of each other, and the [111] planes are exposed, especially in the lateral direction (Fig. 7).

Fig. 7 (a) Anisotropic growth of the Au fcc lattice into a triangle prism in [Au₅₅(p-MBT)₂₄(PPh₃)₆]³⁺. The Au atom packing of four layers (green, orange, light blue and green) stacked along the [111] direction in an A–B–C–A manner. (b) Top view; (c) side view; (d) the ligand binding positions; (e) simplified facets (Au green/orange/light-blue, S yellow, P pink). Note: the red arrow refers to the missing Au atom in the real Au₅₅ cluster. These figures are reproduced with permission from ref. 233. Copyright 2021 Wiley-VCH GmbH.

2.1.5 Other ligand-protected gold nanoclusters. In addition to these ligands, Au NCs protected by C≡CR,^23,234–236 NHC,^237–241 and nitrogen donor ligands²⁴² have been reported in recent years. In 2016, Wang and Zheng et al. synthesized Au₃₄Ag₂₈(C≡CPh)₃₄ by adding silver acetate (CH₃COOAg) and tert-butylamineborane complexes to a solution containing Au precursors and phenylacetylene (HC≡CPh) while stirring. The NCs function as highly active catalysts for the hydrolytic oxidation of triethylsilanes by removing the ligands more readily at relatively low temperatures than the previously reported Au_n(SR)_m.²⁴³ In 2021, Pei and Tang et al. synthesized an alloy cluster of Ag and Cu, [Ag₉Cu₆(C₆H₉)₁₂]⁺, and an alloy cluster of Au and Ag, [Au₇Ag₈(C≡C^tBu)₁₂]⁺ (HC≡C^tBu = tert-butylacetylene), using antigalvanic reactions.²⁴⁴ These two alloy clusters have body-centered cubic (bcc) structures and exhibit different optical properties (Fig. 8Aa). The differences in optical properties were attributed to slightly different structures of the M₁ kernel (M = Ag or Au), M₆ octahedron (M = Cu or Au), and Ag₈ cube. Subsequently, in 2022, Q. Tang and Z. Tang et al. synthesized [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺, an alloy cluster composed of Au, Ag, and Cu, by adding chlorodimethylsulfide gold(I) ((CH₃)₂SAuCl) as an Au precursor to [Ag₉Cu₆(C₆H₉)₁₂]⁺.²⁴⁵ Although many reports have focused on the synthesis of alloy NCs,³⁸ [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺ is the first report of a trimetal NC protected with only a C≡CR ligand. The optical absorption spectrum of [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺ reveals a completely different electronic structure compared with those previously described for [Ag₉Cu₆(C₆H₉)₁₂]⁺ and [Au₇Ag₈(C≡C^tBu)₁₂]⁺ (Fig. 8A). Photoluminescence measurements showed that [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺ has stronger emission than the other two NCs in the near-infrared region (Fig. 8Ab). [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺ also consists of an M@M₈@M₆ metal core, similar to the other two NCs, but each NC exhibits different CRR selectivity (see section 3).

Fig. 8 (A) (a)Absorbance spectra of [Au₇Ag₈(C≡C^tBu)₁₂]⁺, [Ag₉Cu₆(C₆H₉)₁₂]⁺ and [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺. (b) The emission spectra of [Au₇Ag₈(C≡C^tBu)₁₂]⁺ (λ_ex = 482 nm), [Ag₉Cu₆(C₆H₉)₁₂]⁺ (λ_ex = 580 nm) and [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺ (λ_ex = 490 nm) in dichloromethane. Inset: photographs of the three NCs in dichloromethane under room light (left) and 365 nm UV-light (right), respectively. (B) Production of different NHC-stabilized Au clusters depending on the ancillary ligand (halide or pyridine). (a) Use of halide ligands gives typical Au₁₃ clusters ([Au₁₃(NHC)₉Cl₃]²⁺), while (b) labile ancillary ligand leads to hydride bridged Au₂₄ clusters ([Au₂₄(NHC)₁₄Cl₂H₃]³⁺). (C) (a) The overall structure of [Au₂₈(Ph-form)₁₂]²⁺. (b) The T symmetrical Au₂₈ kernel with concentric tetrahedral Au₄ core (black) and truncated tetrahedron Au₂₄ shell divided into four {111} faces. (c) The bridge mode of formamidinate ligands; all Ph rings are omitted for clarity. (d) The experimental (black) and simulated (red) absorption spectra of Au₂₈. (Inset) The NTO picture of the first absorption peak. (A) is reproduced with permission from ref. 245. Copyright 2022 Royal Society of Chemistry. (B) is reproduced with permission from ref. 249. Copyright 2022 American Chemical Society. (C) is reproduced with permission from ref. 250. Copyright 2021 Wiley-VCH GmbH.

Au NCs using NHC as a ligand are generally synthesized by the reduction of precursors, such as NHC–Au(I)–X complexes (X = Cl, Br),^246,247 using NaBH₄. In 2022, Dinh, Tsukuda, Häkkinen, and Crudden et al. dissolved the [NHC–Au–Pyr]⁺ complex (Pry = pyridine)²⁴⁸ in toluene, added NaBH₄, stirred the mixture, separated the products, and then purified them by chromatography to obtain [Au₂₄(NHC)₁₄Cl₂H₃]³⁺ (Fig. 8B).²⁴⁹ [Au₂₄(NHC)₁₄Cl₂H₃]³⁺ consists of two icosahedral Au₁₂ cores with distorted structures and three bridging hydride ligands, and the hydride ligands are coordinated to the Au₁₃ core, which has been confirmed by SC-XRD, ESI-MS, and nuclear magnetic resonance (NMR) spectroscopy.

Studies have also reported on amidinate-protected Au NCs. In 2021, Wang et al. mixed (CH₃)₂SAuCl, N,N′-diphenylformamidinate (Ph-form), and sodium trifluoromethanesulfonate (NaOTf) in chloroform, stirred, and then added sodium methanolate (MeONa). Next, they synthesized [Au₂₈(Ph-form)₁₂]²⁺ by adding NaBH₄ and vigorously stirring the mixture under various temperature conditions.²⁵⁰ The [Au₂₈(Ph-form)₁₂]²⁺ exhibited high stability owing to its superatomic configuration of 1S²1P⁶2S²1D⁴, and theoretical calculations reproduced the measured electronic structure (Fig. 8C).

As mentioned above, numerous synthetic examples of Au NCs protected by organic ligands, such as SR, phosphine, C≡CR, and NHC, have been reported. Moreover, many of the Au NCs described in this section have been applied in electrochemical CRRs, which are described in detail in section 3.3.

2.2 Synthesis of silver nanocluster

Ag NCs, which are mainly composed of Ag, have been widely studied in recent years because of their relatively high quantum yields in terms of luminescence.⁴³ Similar to Au NCs, Ag NCs can be synthesized with atomic precision using ligands such as SR, phosphine, and C≡CR. In this section, we describe several examples.

2.2.1 Thiolate-protected silver nanoclusters. Regarding SR-protected Ag NCs (Ag_n(SR)_m), the geometry of Ag_n(SR)_m was determined using [Ag₄₄(SR)₃₀]⁴⁻ in 2013 for the first time.^251,252 The general synthetic method involves dissolving AgNO₃ in methanol or another solution, adding a thiol ligand, and generating an Ag(I)–SR complex. Then, the Ag(I)–SR complex is dissolved in solution, NaBH₄ is added and stirred, and Ag_n(SR)_m is obtained by washing/separation. For example, in 2018, Zhu and Jin et al. synthesized Ag₁₄₆Br₂(TIBT)₈₀ (TIBT = 4-isopropylbenzenethiolate).²⁵³ First, Ag(I)–TIBT was synthesized by mixing AgNO₃ and 4-isopropylbenzenethiol in a mortar. Next, NaBH₄ was added to this mixture and reacted for 72 h. Then, the mixed solution was separated, and the precipitate was washed to obtain pure Ag₁₄₆Br₂(TIBT)₈₀. From SC-XRD, Ag₁₄₆Br₂(TIBT)₈₀ was found to have an Ag₅₁ core structure protected by a shell of Ag₉₅Br₂(TIBT)₈₀ (Fig. 9Aa). The optical absorption spectrum showed four peaks in the visible region, indicating an optical gap of 0.5 eV (Fig. 9Ab).

Fig. 9 (A) (a) Top views of the total structure of the Ag₁₄₆Br₂(TIBT)₈₀. Color labels: dark gray/green = Ag, yellow = S, purple = Br; the carbon tails are shown in wire-frame mode. (b) UV/Vis/NIR absorption spectra of the Ag₁₄₆Br₂(TIBT)₈₀ in CCl₄. Inset: Photograph of 5.08 g of Ag₁₄₆Br₂(TIBT)₈₀ in a vial, and the spectrum plotted on the photon energy scale (the y-axis is transformed from the wavelength scale spectrum by Abs × λ² to preserve the oscillator strength). (B) Size and slicing of the M₅₀ kernel in [Ag₃₉Cu₁₁(SC₇H₇O)₃₂]²⁺. Color codes: pure blue, Ag; pink, Cu; green, Ag/Cu, yellow, S; red, O; gray, C. For clarity, all H atoms are omitted. (C) Structural analysis of [AuAg₂₆(S-Adm)₁₈S]⁻. (a) The framework structure of AuAg₂₆, (b) the AuAg₁₂ icosahedron kernel, (c) the Ag₁₄(SR)₁₈S open shell, (d) three-AgS₃ motif, (e) Ag₆(SR)₃S motif, (f) Ag₅(SR)₆ motif. Color code: azure, Au; magenta/blue/orange/violet, Ag; red, single S atom; green, other S. The C and H atoms are omitted for clarity. (A) is reproduced with permission from ref. 253. Copyright 2018 American Chemical Society. (B) is reproduced with permission from ref. 254. Copyright 2023 American Chemical Society. (C) is reproduced with permission from ref. 255. Copyright 2020 American Chemical Society.

Alloy NCs based on Ag_n(SR)_m were also reported. In 2023, Wang, Yan, and Wu et al. synthesized fcc-structured bimetallic NCs ([Ag₄₁Cu₉(SC₇H₇O)₃₂]²⁺) using a one-pot method and achieved surface substitution of alloy clusters with an increased Cu/Ag atomic ratio ([Ag₃₉Cu₁₁(SC₇H₇O)₃₂]²⁺), without changing the reaction conditions.²⁵⁴ In this synthesis, AgNO₃ and copper(II) acetylacetonate (Cu(acac)₂) were added as precursors in the appropriate ratios for the respective NCs, 3-methoxythiophenol was added and stirred, and the mixture was co-reduced with NaBH₄. These NCs have an fcc structure consisting of four layers, with different atoms exposed on the surface depending on the Ag/Cu ratio (Fig. 9B). In 2021, Liu and Huang et al. synthesized S-Adm (1-adamantanethiolate)-protected bimetallic NCs ([AuAg₂₆(S-Adm)₁₈S]⁻) using a one-pot method.²⁵⁵ For this synthesis, AgNO₃ and AuCl₃ solutions were mixed, and 1-adamantanethiol was added to the mixture. Then, PPh₃, tetraphenylphosphonium bromide (PPh₄Br), and NaBH₄ were added and kept to form [AuAg₂₆(S-Adm)₁₈S]⁻, in which the Ag₁₂ core covers the central Au atom and the Ag₁₄ shell covers the core (Fig. 9C). These differ from common 25-mer NCs in that the outer Ag₁₄ shell has a large open shell structure consisting of three different motifs.

2.2.2 Phosphine-protected silver nanoclusters. Ag NCs co-protected by SR and phosphine have also been reported. In 2022, Wang and Zang et al. reported the synthesis of [Ag₃₉(PFBT)₂₄(PPh₃)₈]²⁻ (PFBT = pentafluorobenzenethiolate) (Fig. 10A).²⁵⁶ In this synthesis, pentafluorobenzenethiol, PPh₃, and NaBH₄ were added to a solution of AgNO₃ and vigorously stirred. They also synthesized [Ag₃₇Cu₂(PFBT)₂₄(PPh₃)₈]²⁻ by dissolving AgNO₃ and copper(I) chloride (CuCl) and then adding PFBT, PPh₃, and NaBH₄ while stirring. However, if trifluoroacetate copper is used as the Cu precursor instead of CuCl, the co-crystal of [Ag₃₇Cu₂(PFBT)₂₄(PPh₃)₈]²⁻·[Ag₁₄(PFBT)₆(PPh₃)₈] can be obtained. In the structure of [Ag₃₉(PFBT)₂₄(PPh₃)₈]²⁻ (Fig. 10Aa), the Ag₁₃@Ag₁₈ core is covered by Ag₈(PFBT)₆(PPh₃)₈, but when alloyed with Cu, the two outer Ag atoms are replaced with Cu atoms (Fig. 10Ab). In addition, the optical absorption spectra show that [Ag₃₉(PFBT)₂₄(PPh₃)₈]²⁻ exhibits peaks at 426 and 524 nm, which are caused by charge transfer from the ligand to the metal or from the metal to the ligand. In addition, Cu doping shifts the peaks to the lower energy side of these peaks (Fig. 10B).

Fig. 10 (A) Overall structure of (a) [Ag₃₉(PFBT)₂₄(PPh₃)₈]²⁻ and (b) [Ag₃₇Cu₂(PFBT)₂₄(PPh₃)₈]²⁻. (B) UV/Vis absorbance spectra of [Ag₃₉(PFBT)₂₄(PPh₃)₈]²⁻ (red), [Ag₃₇Cu₂(PFBT)₂₄(PPh₃)₈]²⁻ (blue), and [Ag₃₇Cu₂(PFBT)₂₄(PPh₃)₈]²⁻·[Ag₁₄(PFBT)₆(PPh₃)₈] (purple) in solution. (A) and (B) are reproduced with permission from ref. 256. Copyright 2022 American Chemical Society.

2.2.3 Other ligand-protected silver nanoclusters. Reports have also focused on the synthesis of Ag NCs protected by C≡CR ligands, similar to Au NCs. In 2017, Li and Zhang et al. reported the synthesis of [Ag₇₄(C≡CPh)₄₄]²⁺.²⁵⁷ In this synthesis, 1,3-bis(diphenyphosphino)propane (DPPP) and PhC≡CH were first added to a solution of dissolved AgNO₃ and stirred, and then NaBH₄ was added. In addition to C≡CPh ligands, many reports have considered the synthesis of Ag NCs protected by C≡C^tBu, which is a C≡CR ligand with a tert-butyl group. In 2021, Tang, Jin, and Tang et al. reported the synthesis of C≡C^tBu-protected Ag₁₅ NCs ([Ag₁₅(C≡C^tBu)₁₂]⁺) and their application in CRRs.²⁵⁸ In this synthesis, HC≡C^tBu, MeONa, and sodium cyanoborohydride (NaBH₃CN) are first added to a silver trifluoroacetate solution and stirred. Subsequently, [Ag₁₅(C≡C^tBu)₁₂]⁺ is synthesized by cooling and extraction with a mixture of dichloromethane and CH₃CN (Fig. 11A). The optical absorption spectrum of [Ag₁₅(C≡C^tBu)₁₂]⁺ shows five peaks at 360, 480, 508, 542, and 580 nm (Fig. 11B). Additionally, [Ag₁₅(C≡C^tBu)₁₂]⁺ has a core/shell structure of Ag@Ag₈@Ag₆, with an irregularly shaped cubic Ag8 in the second layer and an octahedron of Ag₆ in the outermost shell (Fig. 11A). In 2022, Tang, Wang and Tang et al. reported the synthesis of Ag₃₂(C≡CPh(CF₃)₂)₂₄ (HC≡CPh(CF₃)₂ = 3,5-bis(trifluoromethyl)phenylacetylene) based on Chen's and Tang's synthetic method²⁵⁹ for Au₃₆(C≡CPh)₂₄ reported in 2020 (Fig. 12A).²⁶⁰ In this synthesis, MeONa, HC≡CPh(CF₃)₂, and NaBH₃CN are added to a solution of silver trifluoroacetate (CF₃COOAg) and stirred. The purple-black crystals are synthesized by centrifuging the mixed solution to remove the precipitate, followed by washing/extraction. Ag₃₂(C≡CPh(CF₃)₂)₂ has an Ag₅ core and Ag₅@Ag₁₂ kernel in the center, according to SC-XRD, and the surface is protected by four different ligands (Fig. 12A). The optical absorption spectra reveal peaks at 518 and 656 nm (Fig. 12B).

Fig. 11 (A) (a) The overall structure of [Ag₁₅(C≡C^tBu)₁₂]⁺. (b)–(d) Shell-by-shell representations of the metal framework, Ag@Ag₈@Ag₆ (note that there is no bond between the orange silver atom and the adjacent three silver atoms (yellow) in the frame of the Ag₈ cube; similarly, the six capping Ag atoms form an octahedron with no bonds between them). (e) Metal framework of [Ag₁₅(C≡C^tBu)₁₂]⁺. (f) and (g) Side and top views of the six Ag(C≡C^tBu)₂ staple units capping a silver square of the Ag₈ cube. Ag gray/yellow/orange/light-green, C dark green; all hydrogen atoms are omitted for clarity. (B) UV/Vis absorbance spectrum of [Ag₁₅(C≡C^tBu)₁₂]⁺. (A) and (B) are reproduced with permission from ref. 258. Copyright 2021 Wiley-VCH GmbH.

Fig. 12 (A) (a) Four types of ligand–metal bonding motifs in Ag₃₂(C≡CPh(CF₃)₂)₂₄. (b) Four motifs in Ag₃₂(C≡CPh(CF₃)₂)₂₄ coordination modes for alkynyl ligands. Color code: Ag, blue/pink/purple/yellow; C, gray; and F, green. (B) UV/Vis absorbance spectrum of Ag₃₂(C≡CPh(CF₃)₂)₂₄. Inset: photograph of the cluster in CH₂Cl₂. (A) and (B) are reproduced with permission from ref. 260. Copyright 2022 Springer Nature.

Many examples of Ag-based alloy NCs protected by C≡CR have also been reported. In 2021, Pei and Tang et al. reported the synthesis of an Ag–Cu alloy cluster ([Ag₉Cu₆(C≡C^tBu)₁₂]⁺) (Fig. 13A).²⁴⁴ In this synthesis, ^tBuC≡CAg was used as the silver precursor, and bis(triphenylphosphine)copper borohydride ((PPh₃)₂CuBH₄) was used as the copper precursor. First, ^tBuC≡CAg was mixed with sodium hexafluoroantimonate (NaSbF₆), to which (PPh₃)₂CuBH₄ was added, and [Ag₉Cu₆(C≡C^tBu)₁₂]⁺ was obtained by stirring and reducing the mixture in the dark. Notably, [Ag₈Au₇(C≡C^tBu)₁₂]⁺, which has a similar geometric structure to [Ag₉Cu₆(C≡C^tBu)₁₂]⁺, can also be synthesized.²⁶¹ Both NCs are composed of an M₁ kernel@Ag₈ cube@M₆ octahedral structure with a bcc basis (Fig. 13A), but they have slightly different geometric structures depending on the atomic diameters and argentophilic Ag–Ag interaction (Fig. 13B), which affect their optical properties and stability. In 2017, Mizuta et al. reported the synthesis of Pt₅Ag₂₂(C≡CPh)₃₂, obtained by mixing PhC≡CAg and H₂PtCl₆·6H₂O, and then adding NaBH₄ and stirring in the dark.²⁶² In 2022, Tang, Hwang and Hyeon et al. synthesized an alloy NC of Ag and Cu co-protected by C≡CPh(CF₃)₂ and DPPE ([Ag₁₅Cu₆(C≡CPh(CF₃)₂)₁₈(DPPE)₂]⁻).²⁶³ In this synthesis, a silver complex and C≡CPh(CF₃)₂ ligand were stirred with (PPh₃)₂CuBH₄ as a Cu precursor, then washed and extracted from the organic phase, which produced black crystals. The structure of [Ag₁₅Cu₆(C≡CPh(CF₃)₂)₁₈(DPPE)₂]⁻ is composed of three layers, Ag@Ag₈@Ag₂Cu₄ (Fig. 13C), with a single Ag atom at the center, followed by an Ag₈ cube in the second layer and an Ag₂Cu₄ octahedron in the third layer, with two Ag₂(DPPE) capping the Ag–Cu or Ag–Ag interactions to form [Ag₁₅Cu₆(C≡CPh(CF₃)₂)₁₈(DPPE)₂]⁻ (Fig. 13C).

Fig. 13 (A) Anatomy of (a) [Ag₉Cu₆(C≡C^tBu)₁₂]⁺ and (b) [Ag₈Au₇(C≡C^tBu)₁₂]⁺. Color legend: Au, yellow; Ag, purple; Cu, orange. (B) Experimental absorbance spectra plotted in the energy axis of (a) [Ag₉Cu₆(C≡C^tBu)₁₂]⁺ and (b) [Ag₈Au₇(C≡C^tBu)₁₂]⁺. (C) Structure analysis of the [Ag₁₅Cu₆(C≡CPh(CF₃)₂)₁₈(DPPE)₂]⁻. (a) Structure anatomy of the Ag₁₁Cu₄ metal core. (b) Capping of two opposite Ag₃ faces of the Ag₁₁Cu₄ core forms the structure of Ag₁₁Cu₆. (c) Stabilization of the Ag₁₁Cu₆ metal framework by two Ag₂DPPE motifs forms the structure of [Ag₁₅Cu₆(C≡CPh(CF₃)₂)₁₈(DPPE)₂]⁻. Color legend: turquoise, Ag; dark red, Cu; pink, P; gray, C. (A) and (B) are reproduced with permission from ref. 244. Copyright 2021 Royal Society of Chemistry. (C) is reproduced with permission from ref. 263. Copyright 2022 American Chemical Society.

In addition to ligand-protected Ag NCs, many reports have focused on the synthesis of Ag NCs using anions, such as halide ions, chalcogenide ions, oxoanions, and polyoxometalates (POMs), as templates.^264,265 In 2022, Liu et al. used a solvothermal reaction to synthesize POM-templated Ag NCs (Ag₄₉Mo₁₆) protected by six thiacalix[4]arenes (TC4A).²⁶⁶ In this synthesis, TC4A was mixed with ⁱPrSAg, CH₃COOAg, and ammonium molybdate tetrahydrate ((NH₄)₆Mo₇O₂₄·4H₂O), and then triethylamine was added dropwise. After that, light brown crystals were obtained by subjecting the mixture to heat treatment.

As mentioned above, many synthetic examples of Ag NCs protected by organic ligands, such as SR, phosphine, C≡CR, and template types, have been reported. These Ag NCs are also expected to be applied in electrochemical CRRs. In section 3.4, we describe CRRs using mainly the Ag NCs introduced in this section.

2.3 Synthesis of copper nanocluster

Cu is more abundant than Au and Ag, and thus Cu-based nanomaterials have advantages such as low environmental burden and low cost. Furthermore, Cu-based catalysts have the appropriate adsorption energy for CO₂ and exhibit promising CRR activity, with the ability to produce a variety of organic products from CO₂. Moreover, Cu NCs can be synthesized with atomic precision using ligands such as SR, phosphine, and C≡CR.^{44,56,236,267–271} Here, we describe the existing synthesis methods for Cu NCs protected by the above ligands.

2.3.1 Thiolate-protected copper nanoclusters. Cu NCs partially protected by SR have been synthesized, similar to the reports for Au_n(SR)_m and Ag_n(SR)_m. In 2009, Liu et al. synthesized Cu₈(H)(L1)₆PF₆ (L1 = 9H-carbazole-9-carbodithioate) by dissolving tetrakis(acetonitrile)copper(I) hexafluorophosphate (Cu(CH₃CN)₄PF₆) and NH₄[S₂P(OiPr)₂] in acetone and stirring to remove impurities.²⁷² In 2022, Wang and Zang et al. synthesized three Cu NCs with the same size, namely Cu₈(S^tBu)₄(L1)₄ and Cu₈(S^tBu)₄(L2)₄ (L2 = O-ethyl carbonodithiolate), in addition to Cu₈(H)(L1)₆PF₆, and their CRR activities were compared.²⁷³ The synthesis of Cu₈(S^tBu)₄(L1)₄ used ^tBuSCu dissolved in chloroform as the Cu precursor, and L1 dissolved in methanol was added and mixed. Cu₈(S^tBu)₄(L2)₄ was synthesized by the same procedure but with L2 instead of L1. Notably, Cu₈(H)(L1)₆PF₆ contains a slightly twisted cubic Cu₈ core, but the introduction of ^tBuS in Cu₈(S^tBu)₄(L1)₄ and Cu₈(S^tBu)₄(L2)₄ leads to the breakdown of the Cu₈ core and the formation of a tetrahedral Cu arrangement.

2.3.2 Phosphine-protected copper nanoclusters. As with Au and Ag NCs, phosphine-protected Cu NCs can also be synthesized. In 2015, Scott and Hayton et al. synthesized two Cu NCs by adding diphenyl silane (Ph₂SiH₂) to a slurry containing appropriate amounts of copper(I) acetate (Cu(OAc)), CuCl, and PPh₃ in benzene to obtain [Cu₂₅H₂₂(PPh₃)₁₂]⁺ and [Cu₁₈H₁₇(PPh₃)₁₀]⁺.²⁷⁴ When these were extracted with benzene, only [Cu₁₈H₁₇(PPh₃)₁₀]⁺ could be dissolved and separated by filtration. Most Cu NCs are composed of cationic Cu, but [Cu₂₅H₂₂(PPh₃)₁₂]⁺ was found to contain partially zerovalent Cu(0). This is the first report of such metallic Cu NCs with a Cu₁₃ icosahedral core and four [Cu(PPh₃)]₃ capping structures (Fig. 14A). In 2019, Wang and Zhu et al. reported the synthesis of [Cu₂₅H₂₂(PPh₃)₁₂]⁺, [AuCu₂₄H₂₂(PPh₃)₁₂]⁺, [Cu₂₅H₂₂(P(p-FPh)₃)₁₂]⁺ (P(p-FPh)₃ = tris(4-fluorophenyl)phosphine), and [AuCu₂₄H₂₂(P(p-FPh)₃)₁₂]⁺ with both ligand exchange and Au doping.²⁷⁵ [AuCu₂₄H₂₂(PPh₃)₁₂]⁺ was synthesized by adding Cu(acac)₂ and HAuCl₄·3H₂O to the precursor, followed by stirring and aging with PPh₃ and NaBH₄ as ligands. [Cu₂₅H₂₂(P(p-FPh)₃)₁₂]⁺ was also synthesized by adding P(p-FPh)₃ and NaBH₄ to a dissolved solution of Cu(acac)₂ and stirring. AuCu₂₄H₂₂(P(p-FPh)₃)₁₂ was synthesized by adding HAuCl₄·3H₂O at the beginning of the synthetic scheme for Cu₂₅H₂₂(P(p-FPh)₃)₁₂ described above. All three NCs contain distorted icosahedral MCu₁₂ (M = Au or Cu) structures and are protected by four Cu₃P₃ shells (Fig. 14A). UV/Vis measurements of these materials also showed that [Cu₂₅H₂₂(PPh₃)₁₂]⁺ and [Cu₂₅H₂₂(P(p-FPh)₃)₁₂]⁺ have absorption at 459 and 635 nm, whereas [AuCu₂₄H₂₂(PPh₃)₁₂]⁺ and [AuCu₂₄H₂₂(P(p-FPh)₃)₁₂]⁺ are more blue shifted than the undoped ones. This indicates that the electronic structures of NCs can be changed by an inactive metal doping of Au heteroatoms (Fig. 14B).

Fig. 14 (A) Total structures and (B) UV/Vis spectra of (a) [Cu₂₅H₂₂(PPh₃)₁₂]⁺, (b) [AuCu₂₄H₂₂(PPh₃)₁₂]⁺, (c) [Cu₂₅H₂₂(P(p-FPh)₃)₁₂]⁺ and (d) [AuCu₂₄H₂₂(P(p-FPh)₃)₁₂]⁺. Color labels: gold = Au; blue = Cu; green = F; purple = P; gray = C. All H atoms are omitted for clarity. (A) (a) is reproduced with permission from ref. 274. Copyright 2015 American Chemical Society. (A) (b–d) and (B) are reproduced with permission from ref. 275. Copyright 2019 American Chemical Society.

2.3.3 Other ligand-protected copper nanoclusters. In addition to SR and phosphine protection, many reports have focused on the synthesis of Cu NCs protected with C≡CR and fluoroacetic acid. In 2020, Aikens and Sun et al. synthesized [Cu₂₃(C≡C^tBu)₁₃(CF₃COO)₆]⁰ containing Cu(0) using various reducing agents.²⁷⁶ First, copper(II) trifluoroacetate (Cu(CF₃COO)₂), Cu powder, and ^tBuC≡CH were mixed and stirred in dichloromethane and methanol, and when the solution turned pale yellow, Ph₂SiH₂ was added. After aging, the crystals were filtered and purified at room temperature to give [Cu₂₃(C≡C^tBu)₁₃(CF₃COO)₆]⁰. The Cu NCs consist of a Cu₁₉ outer shell and Cu₄ tetrahedral core (Fig. 15A). Furthermore, UV/Vis measurements of the solid using diffuse reflection spectroscopy revealed that the band gap of [Cu₂₃(C≡C^tBu)₁₃(CF₃COO)₆]⁰ is approximately 1.45 eV, which is consistent with the value calculated based on DFT calculations.

Fig. 15 (A) (a) Total structure and (b) metal framework of the [Cu₂₃(C≡C^tBu)₁₃(CF₃COO)₆]⁰. Color labels: Orange, Cu; green, F; gray, C; red, O. (B) Molecular structure of (a) [Au₁₉Cu₃₀(C≡C-3-SC₄H₃)₂₂(PPh₃)₆Cl₂]³⁺ and (b) [Au₁₉Cu₃₀(C≡CPh)₂₂(PPh₃)₆Cl₂]³⁺. Anatomy of Au₁₉Cu₃₀ kernel structure in Au₁₉Cu₃₀ clusters. (c) Centered icosahedron Au@Au₁₂. (d) Cu₃₀ icosidodecahedron. (e) Au@Au₁₂@Cu₃₀ multi shelled structure. (f) Six outmost Au atoms highlighted in green. (A) is reproduced with permission from ref. 276. Copyright 2020 American Chemical Society. (B) is reproduced with permission from ref. 277. Copyright 2017 American Chemical Society.

Cases of Cu-based alloy NCs protected by C≡CR ligands have also been reported. In 2017, Jiang and Wang et al. reported the synthesis of two types of Au–Cu alloy clusters protected by 3-ethynylthiophene (H₃C₄S-3-C≡CH) or PhC≡CH.²⁷⁷ First, a mixture of the H₃C₄S-3-C≡CAuPPh₃ precursor, chloro(triphenylphosphine)gold(I) (PPh₃AuCl), and copper(II) nitrate (Cu(NO₃)₂) was reduced by adding NaBH₄. Then, the reaction proceeded in the dark, and the resulting solid was washed to obtain [Au₁₉Cu₃₀(C≡C-3-SC₄H₃)₂₂(PPh₃)₆Cl₂]³⁺. They also synthesized [Au₁₉Cu₃₀(C≡CPh)₂₂(PPh₃)₆Cl₂]³⁺ protected with PhC≡C by adding PhC≡CAuPPh₃ as the Au precursor and CuCl and Cu(NO₃)₂ as the Cu precursors in the above synthetic scheme. The NCs have identical internal structures (Fig. 15B). At the center is an Au₁₃ core, followed by a Cu₃₀ icosahedral shell, which has a characteristic structure of six Au atoms bonded to this shell portion (Fig. 15B).

As described above, many synthetic examples of Cu NCs protected by various organic ligands, such as SR, phosphine, and C≡CR, have been reported. These Cu NCs are expected to be applied in electrochemical CRRs, as described in section 3.5.

3. Electrochemical CO₂ reduction reactions of atomically precise metal nanoclusters

3.1 Atomically precise metal nanoclusters as electrocatalysts

As the depletion of fossil resources and the problem of global warming become more serious, there is a growing desire to diversify the industrial sources of energy and reduce greenhouse gases. Electrocatalysts, which induce chemical reactions using electrical energy, have been attracting attention as a means of addressing these problems. Electricity can be produced cleanly using natural energy sources (i.e., without also producing CO₂), and electrocatalysts can be used to convert the unused surplus of those sources into chemical energy. Doing so would reduce the loss of electricity in the transmission and storage processes, promoting a cleaner and more sustainable society. Among the various electrocatalytic reactions, in water electrolysis, fuel cells, and electrochemical CO₂ reduction are the most widely studied.

In recent years, there has been a focus on the development of electrocatalysts that reduce the activation energy of these electrocatalytic reactions and allow them to proceed with less power consumption. In general, an efficient electrocatalyst has many active sites with high reaction rates. The number of active sites is an important factor related to the specific surface area of the catalyst, and the reaction rate is largely related to the adsorption energy of the reacting molecules on the catalyst surface. Chemical reactions at the catalyst surface exhibit the highest activity based on the Sabatier principle, only occurring when the Gibbs energy of adsorption between the catalyst and the reactant is appropriate.²⁷⁸ The reaction cannot occur if there is no adsorption of the reactant on the electrode surface, but if the adsorption is too strong, the reaction cannot proceed. Therefore, depending on the metal species of the electrocatalyst and the adsorption energy of the reactants, the reaction efficiency follows a mountainous trend, called a volcano plot.²⁷⁹

The metal NCs described in section 2 generally have high catalytic activities in a variety of heterogeneous systems based on their large specific surface areas and unique geometric/electronic structures. Recently, many groups have studied the application of metal NCs to electrocatalysts, considering that metal NCs have the potential to outperform conventional materials. Furthermore, the geometric structure of many metal NCs can be revealed by SC-XRD, and the adsorption and reaction processes between the metal NCs as an active site, and reactants can be estimated using theoretical calculations.^{158,280–287} Therefore, metal NCs have been widely studied as model catalysts to elucidate the reaction mechanisms. If we can clarify the correlation between the electronic/geometric structure of metal NCs and their electrocatalytic activity, we may find new ways to enhance the catalytic activity.

Here, we summarize the application of ligand-protected metal NCs in electrochemical CRRs, with a focus on electrocatalysts using ligand-protected metal NCs obtained by liquid-phase reduction, which can be easily synthesized in large quantities. Particularly, we describe the use of metal NCs that have a well-defined geometric structure.

3.2 Electrocatalytic CO₂ reduction reactions using atomically precise metal nanoclusters

Electrochemical CRRs provide a variety of useful reduction products depending on the number of electrons involved in the reaction.²⁷⁹ For example, 2-electron transfer reduction produces CO or HCOOH, 8-electron transfer reduction produces CH₄, 16-electron transfer reduction produces ethylene (C₂H₄), and reduction by more electrons is also possible (eqn (1)–(7)).^89,288 First, CO₂ is adsorbed on the surface of the metal catalyst and then interacts with the transferred electrons or protons to form an intermediate. Next, these intermediates undergo various reactions and finally desorb from the catalyst surface. There are various theories, but (i) *COOH and (ii) *OCHO are the primary intermediates in most cases. Among these, (i) *COOH is the first intermediate in the formation of CO and various hydrocarbons, and (ii) *OCHO is most likely an intermediate in the formation of HCOOH. (iii) *COCHO, produced by C–C coupling of *CHO and CO, has been reported as an important intermediate in the formation of C₂ compounds. Therefore, different metal species also have different binding energies to these intermediates, and these differences change the main products (Fig. 16). For example, using bulk metals as catalysts, (i) CO is obtained as a product for Au or Ag; (ii) HCOOH is obtained for Pb, Sn, and In; and (iii) a variety of organic products, such as aldehydes, alcohols, and C₂ compounds, are obtained for Cu. However, certain metals, such as Pt, Ni, and Pd, which are highly active in HER, are less useful catalysts in CRR because the adsorption of *H prevents the formation of carbon products.^289,290 Additionally, the specific geometric/electronic structures of metal NCs affect the selectivity. As described in section 2, many metal NCs can be synthesized under atmospheric conditions with atomic accuracy, mainly for group 11 metals (Ag, Au, and Cu). Therefore, most applications of electrochemical CRRs have been based on these metal NCs. In this section, we mainly discuss the relationship between the structure of metal NCs and electrochemical CRR activity in terms of (1) size effects, (2) structure effects, (3) ligand effects, and (4) heterometallic doping effects, with respect to the application of Ag, Au, and Cu NCs in electrochemical CRRs.

Fig. 16 A volcano plot of CO₂ reduction reactions for (111) bulk metals. This figure is reproduced with permission from ref. 279. Copyright 2017 AAAS.

3.3 Electrocatalytic CO₂ reduction reactions using gold nanoclusters

Many studies of electrochemical CRRs with Au NCs have been reported since 2012 (Tables 1–5). Kauffman et al. reported the first study on electrochemical CRRs using Au_n(SR)_m. They prepared the catalyst ink by sonicating a solution containing [Au₂₅(PET)₁₈]⁻ (Fig. 2AB) dissolved in acetone, carbon black (CB), Nafion, and methanol. Then, the prepared catalyst ink was loaded on a glassy carbon electrode (GCE) and dried naturally to form the working electrode. Evaluation of the CRR activity showed that the current curves were different after saturation with nitrogen (N₂) and CO₂ when the potentials were traced to negative. The onset potential for CO formation was found to be −0.193 V vs. RHE, according to the potential-dependent product analysis (Fig. 17A). Compared with the ideal CO₂ to CO redox potential (−0.103 V vs. RHE), the overvoltage of bulk Au was 200–300 mV, whereas that of [Au₂₅(PET)₁₈]⁻ was only 90 mV (Fig. 17B). The FE of CO formation (FE_CO) was approximately 100% at −1.0 V vs. RHE. This was 7–700 times higher than Au NPs with 2–5 nm particle sizes and bulk Au. Furthermore, the FE_CO for [Au₂₅(PET)₁₈]⁻ was stable at a relatively high selectivity of 80.8–99.6%, compared with 26.9–92.9% for bulk Au and 71.0–96.9% for Au NPs. The turnover frequency of CO (TOF_CO) was calculated to be 87 molecules per site per s. In addition, the absorption spectrum of [Au₂₅(PET)₁₈]⁻ was maintained after electrochemical measurements, further indicating its stability.²⁹¹ The adsorption of CO₂ was also predicted using a simplified model of [Au₂₅(SCH₃)₁₈]⁻ for the DFT calculation (Fig. 17C). As a result, the binding energy is 0.13 eV in the stable structure, where the oxygen atom of CO₂ interacts with the three S atoms located in the shell of Au₂₅ (Fig. 17C). These small perturbations in the internal structure of the molecule were consistent with the physisorption of CO₂. CRRs were also investigated using [Au₂₅(PET)₁₈]^z (z = −1, 0, and +1).²⁹² [Au₂₅(PET)₁₈]⁻ exhibited higher activity for CO₂ reduction to CO than [Au₂₅(PET)₁₈]^z (z = 0 and +1). From DFT calculations, they speculated that this is due to the stronger binding energy for CO₂ and H⁺ co-adsorption.

CO₂ + e⁻ → CO₂*⁻ E = −1.50 V vs. RHE (1)

CO₂ + 2H⁺ + 2e⁻ → CO + H₂O E = −0.11 V vs. RHE (2)

CO₂ + 2H⁺ + 2e⁻ → HCOOH E = −0.25 V vs. RHE (3)

CO₂ + 4H⁺ + 4e⁻ → HCHO + H₂O E = −0.07 V vs. RHE (4)

CO₂ + 6H⁺ + 6e⁻→ CH₃OH + H₂O E = 0.02 V vs. RHE (5)

CO₂ + 8H⁺ + 8e⁻ → CH₄+ 2H₂O E = 0.17 V vs. RHE (6)

2CO₂ + 12H⁺ + 12e⁻ → C₂H₄ + 4H₂O E = 0.06 V vs. RHE (7)

Fig. 17 (A) Potential-dependent H₂ and CO formation rates for Au₂₅/CB. (B) LSVs of various Au catalysts in quiescent CO₂ saturated 0.1 M KHCO₃ (pH = 7). (C) DFT model of stable CO₂ adsorption where an O atom of CO₂ interacts with three S atoms in the shell and Bader charge analysis showing the change in Au₂₅ valence electrons upon CO₂ adsorption. Panels (A)–(C) are reproduced with permission from ref. 291. Copyright 2012 American Chemical Society.

Table 1 Representative references on CRR activity using Au NCs (section 3.3.1 Size effects)

Nanocluster/nanoparticle	Electrolytes	Electrolyzer	Main product	Selectivity (@V vs. RHE)	Current density (mA cm^−2 @V vs. RHE)	Stability (@V vs. RHE)	Ref.
PET = phenylethanethiolate, PPh3 = triphenylphosphine, TBBT = 4-tert-butylbenzenethiolate, SC6H13 = 1-hexanethiolate, DPPE = 1,2-bis(diphenylphosphino)ethane, DPPP = 1,3-bis(diphenylphosphino)propane.a vs. Ag/AgCl.b Au38Q.c Au38T.d Vulcan XC-72R.e Vulcan XC-72.f Carbon black.g Multi-walled carbon nanotube.h Carbon paper.
[Au25(PET)18]^−	0.1 M KHCO3 aq.	H-cell	CO	∼100%@−1.0	N/A	N/A	291 ^d
Au 2 nm NPs				97%@−1.0	N/A	N/A
Au 5 nm NPs				74%@−1.0	N/A	N/A
Bulk Au NPs				27%@−1.0	N/A	N/A
[Au25(PET)18]^−	0.1 M KHCO3 aq.	H-cell	CO	99%@−1.0	N/A	N/A	292 ^d
[Au25(PET)18]^0				82%@−1.0	N/A	N/A
[Au25(PET)18]^+				81%@−1.0	N/A	N/A
Au25(SC6H13)18	1.0 M KOH aq. or 3.0 M KOH aq.	Flow cell	CO	∼90%@−0.56	59@−0.56	100 h@−1.16	295 ^e
Au38(SC6H13)24				∼90%@−0.56	100@−0.56	25 h@−1.16
Au144(SC6H13)60				∼92%@−0.56	230@−0.56	25 h@−1.16
Au137(PET)56	0.1 M NBu4PF6 in DMF	3 neck flask	N/A	N/A	N/A	N/A	294
Au25(PET)18	0.1 M KHCO3 aq.	N/A	CO	∼90%@−1.0	N/A	36 h@−1.0	293 ^f
Au38(PET)24 ^b	0.5 M KHCO3 aq.	H-cell	CO	∼100%@−0.77	32@−0.87	N/A	296 ^f
Au38(PET)24 ^c				∼100%@−0.77	23@−0.87	N/A
Au144(PET)60				∼100%@−0.77	26@−0.87	N/A
Au333(PET)79				∼95%@−0.77	23@−0.87	N/A
Au28(TBBT)20				∼100%@−0.77	28@−0.87	N/A
Au36(TBBT)24				∼100%@−0.77	26@−0.87	N/A
Au279(TBBT)84				∼90%@−0.77	18@−0.87	N/A
Au4(PPh3)4I2	0.5 M KHCO3 aq.	H-cell	CO	∼55%@−0.9	∼3@−0.9	N/A	298 ^g
			H2	∼45%@−0.9	∼3@−0.9	N/A
Au11(PPh3)7I3			CO	∼77%@−0.9	∼5@−0.9	N/A
			H2	∼23%@−0.9	∼2@−0.9	N/A
[Au6(DPPP)4]^2+	0.2 M KHCO3 aq.	Flow cell	CO	∼80%@−1.5^a	−3.3@−1.5^a	4 h@−1.5^a	297 ^h
			H2	∼15%@−1.5^a	N/A	N/A
[Au9(PPh3)8]^3+			CO	∼83%@−1.5^a	−3.1@−1.5^a	N/A
			H2	∼17%@−1.5^a	N/A	N/A
[Au13(DPPE)5Cl2]^3+			CO	∼90%@−1.5^a	−3.3@−1.5^a	N/A
			H2	∼15%@−1.5^a	N/A	N/A
Au101(PPh3)21Cl5			CO	∼76%@−1.5^a	−6.6@−1.5^a	N/A
			H2	∼20%@−1.5^a	N/A	N/A

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Table 2 Representative references on CRR activity using Au NCs (section 3.3.2 Structure effects)

Nanocluster/nanoparticle	Electrolytes	Electrolyzer	Main product	Selectivity (@V vs. RHE)	Current density (mA cm^−2@V vs. RHE)	Stability (@V vs. RHE)	Ref.
PET = phenylethanethiolate, PPh3 = triphenylphosphine, TBBT = 4-tert-butylbenzenethiolate.a Au38Q.b Au38T.c Vulcan XC-72R.d Carbon black.e Vulcan XC-72.
[Au25(PET)18]^−	0.5 M KHCO3 aq.	H-cell	CO	73.5%@−1.07	N/A	N/A	299 ^c
			H2	24.9%@−1.07	N/A	N/A
[Au25(PPh3)10(PET)5Cl2]^2+			CO	∼55%@−1.07	N/A	N/A
			H2	41.2%@−1.07	N/A	N/A
Au38(PET)24 ^a	0.5 M KHCO3 aq.	H-cell	CO	∼100%@−0.77	32@−0.87	N/A	296 ^d
Au38(PET)24 ^b				∼100%@−0.77	23@−0.87	N/A
Au44(TBBT)24	0.5 M KHCO3 aq.	H-cell	CO	83%@−0.57	1.9@−0.57	N/A	300 ^e
			H2	∼17%@−0.57	N/A
Au44(PPh3)(TBBT)26			CO	97%@−0.57	3.8@−0.57	60 h@−0.57
			H2	2.7%@−0.57	N/A
Au44(PPh3)2(TBBT)24			CO	90%@−0.57	3.1@−0.57	N/A
			H2	∼10%@−0.57	N/A

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Table 3 Representative references on CRR activity using Au NCs (section 3.3.3 Ligand effects)

Nanocluster/nanoparticle	Electrolytes	Electrolyzer	Main product	Selectivity (@V vs. RHE)	Current density (mA cm^−2@V vs. RHE)	Stability (@V vs. RHE)	Ref.
PET = phenylethanethiolate, PPh3 = triphenylphosphine, SNap = 1-naphthalenethiolate, SePh = phenylselenolate, NHC^Me = 1,3-dihydro-1,3-dimethyl-2H-benzimidazol-2-ylidene, NHC^iPr = 1,3-dihydro-1,3-diisopropyl-2H-benzimidazol-2-ylidene, Ph-form = N,N′-diphenylformamidinate, PVP = polyvinylpyrrolidone, p-MBT = 4-methylbenzenethiolate, NHC^Bn = 1,3-dibenzyl-1H-benzo[d]imidazol-2-ylidene, DPPE = 1,2-bis(diphenylphosphino)ethane, MEA = membrane electrode assembly.a vs. MEA-cell potential.b The ligand-removed Au25(PET)18 by electrochemical pretreatment.c The ligand-removed Au25(PET)18 by thermal pretreatment.d Multi-walled carbon nanotube.e Carbon paper.f Ketjen black (EC300J).g Carbon gas diffusion layer (TGP-H-060).h Vulcan XC-72R.i Sulfur-doped graphene.j Carbon.
[Au7(PPh3)7H5]^2+	0.5 M KHCO3 aq.	H-cell	H2	10%@−0.67	∼6@−0.97	5 h@−0.77	232 ^d
			CO	∼90%@−0.67	∼0@−0.97
[Au8(PPh3)7]^2+			H2	∼26%@−0.67	∼1@−0.97	5 h@−0.77
			CO	73.5%@−0.77	6.47@−0.97
[Au11(PPh3)7(NHC^Me)Cl2]^+	0.1 M KHCO3 aq.	H-cell	CO	80%@−1.0	N/A	N/A	238 ^e
[Au11(PPh3)8Cl2]^+				80%@−1.0	N/A	N/A
[Au11(PPh3)7(NHC^iPr)Cl2]^+				∼30%@−1.0	N/A	N/A
[Au11(DPPE)5]^3+	0.5 M KHCO3 aq.	H-cell	CO	70.6%@−0.6	1.5@−0.6	N/A	231 ^f
[Au22H3(DPPE)3(PPh3)8]^3+				92.7%@−0.6	3.5@−0.6	10 h@−0.6
[Au24(NHC^Bn)14Cl2H3]^3+	0.1 M KHCO3 aq.	MEA-cell	CO	N/A	10@2.4^a	100 h@3.2^a^,^b	249 ^g
[Au13(NHC^Bn)9Cl3]^2+				N/A	10@2.5^a	N/A
Ag NPs				N/A	10@2.8^a	N/A
[Au25(PET)18]^−	0.5 M KHCO3 aq.	H-cell	CO	∼100%@−0.8	22@−0.8	N/A	301 ^h
[Au25(SNap)18]^−				∼95%@−0.8	20@−0.8
[Au25(SePh)18]^−				∼80%@−0.8	15@−0.8
Au25(PET)18 (Electrochemical)^b	0.1 M KHCO3 aq.	H-cell	CO	82%@−0.59	N/A	3 h@−0.5	305 ^i
Au25(PET)18(Thermal)^c				64%@−0.59		3 h@−0.5
[Au28(C2B10H11S)12(THT)4]^4+	0.5 M KHCO3 aq.	H-cell	CO	98.5%@−0.9	∼8@−0.9	5.5 h @−0.9	303 ^h
[Au28(C4B10H11)12(THT)8]^3+				∼70%@−0.9	∼4@−0.9	5.5 h @−0.9
[Au28(Ph-form)12]^2+	0.5 M KHCO3 aq.	H-cell	CO	96.5%@−0.57	N/A	40 h@−0.69	250 ^d
PVP-coated Au NPs				∼25%@−0.57	N/A	N/A
[Au55(p-MBT)24(PPh3)6]^3+	0.1 M KHCO3 aq.	H-cell	CO	94.1%@−0.6	∼410@−0.9	4 h@−0.6	233 ^j
[Au25(p-MBT)5(PPh3)10Cl2]^2+				75.1%@−0.6	N/A	N/A

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Table 4 Representative references on CRR activity using Au NCs (section 3.3.4 Doping effects)

Nanocluster/nanoparticle	Electrolytes	Electrolyzer	Main product	Selectivity (@V vs. RHE)	Partial current density (mA cm^−2@V vs. RHE)	Stability (@V vs. RHE)	Ref.
PET = phenylethanethiolate, TBBT = 4-tert-butylbenzenethiolate, SC6H11 = cyclohexanethiolate, SC6H13 = 1-hexanethiolate, S-tol = p-toluenethiolate, d-MBT = 3,5-dimethylbenzenethiolate, SCH2Ph^tBu = 4-tert-butylbenzylthiolate, SPhMe2 = 2,4-dimethylbenzenethiolate.a vs. cell potential.b Vulcan XC-72R.c Carbon paper.d Carbon paper (Toray TGP-H090).
[Au19Cd2(SC6H11)16]^−	0.5 M KHCO3 aq.	H-cell	CO	∼95%@∼−0.67	42@−0.9	N/A	308 ^b
[Au23(SC6H11)16]^−				∼65%@∼−0.67	18@−0.9	N/A
Au25(PET)18	1.0 M KHCO3 aq.	H-cell	CO	∼80%@−0.4	8@−0.8	25 h@−0.7	188 ^c
			HCOOH	∼10%@−0.4
Au24Cd(PET)18			CO	∼90%@−0.4	35@−0.8	25 h@−0.7
			HCOOH	∼8%@−0.4
Au19Cd3(S-tol)18			CO	∼60%@−0.4	5@−0.8	25 h@−0.7
			HCOOH	0%@−0.4
Au38Cd4(d-MBT)30			CO	∼60%@−0.4	12@−0.8	25 h@−0.7
			HCOOH	∼10%@−0.4
Au47Cd2(TBBT)31	0.5 M KHCO3 aq.	H-cell	CO	96%@−0.57	3.2@−0.57	20 h@−0.57	306 ^b
Au44(TBBT)28				83%@−0.57	1.6@−0.57	20 h@−0.57
1.5 nm Au NPs				76%@−0.67	1.0@−0.57	20 h@−0.67
Au25(SC6H13)18	0.1 M KHCO3 aq. + 0.4 M KCl	H-cell	H2	∼6%@−0.5	12@−1.0	N/A	309 ^b
			CO	∼90%@−0.5
PtAu24(SC6H13)18			H2	∼80%@−0.5	20@−1.0	N/A
			CO	∼20%@−0.5
Au38(SCH2Ph^tBu)24	0.5 M KHCO3 aq.	H-cell	CO	∼70%@−0.6	7.5@−0.9	N/A	199 ^d
PtAu37(SCH2Ph^tBu)24				∼80%@−0.6	7.5@−0.9	N/A
Pt2Au36(SCH2Ph^tBu)24				∼60%@−0.6	14@−0.9	N/A
Ag4+xAu40−x(C≡CPh(CH3)2)28	0.5 M KHCO3 aq.	H-cell	CO	∼98%@−0.5	18@−0.8	10 h@−0.6	310 ^d
Au44(C≡CPh(CH3)2)28				∼75%@−0.5	6@−0.8	10 h@−0.6
Au44(d-MBT)28				∼80%@−0.5	8@−0.8	10 h@−0.6
[Au25(PET)18]^−	1.0 M KOH aq.	Flow cell or zero-gap cell	CO	∼100%@−0.2	∼40@−0.2	N/A	190
[Ag25(SPhMe2)18]^−				∼95%@−0.6	∼50@−0.6	N/A
[AuAg12@Au12(PET)18]^−				∼90%@−0.2	∼30@−0.2	24 h@−0.5^a
[Au4Pd6(TBBT)12]^−	0.5 M KHCO3 aq.	H-cell	CO	88.1%@−0.57	∼11@−0.9	65 h@−0.57	202 ^b
[Au3AgPd6(TBBT)12]^−				94.1%@−0.57	∼15@−0.9	65 h@−0.57
[Au24Pd(PET)18]^0	0.1 M KHCO3 aq.	H-cell	CO	∼100%@−1.1	35@−1.25	6 h@−0.8	311 ^b
[Au25(PET)18]^0				∼70%@−1.1	20.3@−1.1	N/A

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Table 5 Representative references on CRR activity using Au NCs (section 3.3.5 Other effects)

Nanocluster/nanoparticle	Electrolytes	Electrolyzer	Main product	Selectivity (@V vs. RHE)	Current density (mA cm^−2@V vs. RHE)	Stability (@V vs. RHE)	Ref.
PET = phenylethanethiolate, PVDF = poly vinylidene fluoride.a Vulcan XC-72R.
[Au25(PET)18]^−/Nafion	0.1 M KHCO3 aq.	Two compartment cells	CO	∼90%@−0.8	N/A	N/A	312 ^a
[Au25(PET)18]^−/PVDF				∼55%@−0.8
Au foil				∼4.6%@−0.8
[Au25(SC6H13)18]^−	KOH aq. (1–5 M) or 0.5 M PBS + 0.5 M KCl aq. (pH 8–13)	Flow cell	CO	∼95%@−0.4	∼100@−0.4	12 h@−1.16	313 ^a

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Next, the researchers investigated practical applications.²⁹³ Specifically, they evaluated the relationship between the amount of [Au₂₅(PET)₁₈]⁻ loaded per electrode area (using CB as the support) and their resulting particle size, which was determined by transmission electron microscopy (TEM) (Fig. 18A). As a result, the size of [Au₂₅(PET)₁₈]⁻ was 1.4 ± 0.4 nm when the loading amount was 0.96 μg_Au cm_geo⁻², indicating that the initial particle size was approximately maintained. However, aggregates were observed when loading was increased to 15 μg_Au cm_geo⁻². With an [Au₂₅(PET)₁₈]⁻ loading of 0.96 μg_Au cm_geo⁻², CO₂ flow rate of 50 mL min⁻¹, and applied potential of −1.0 V vs. RHE, electrolytic CO₂ reduction for 1 h produced 810 ± 11 L g_Au⁻¹ h⁻¹ of CO (FE_CO > 90%). Even after 36 h of continuous CO₂ reduction by electrolysis, CO production of 7450 ± 59 L g_Au⁻¹ h⁻¹ and an FE_CO of approximately 86% was maintained (Fig. 18B). In addition, 1–4 × 10⁶ mol_CO2 mol_catalyst⁻¹ of turnover (TON) was obtained after 12 h of CO₂ reduction by electrolysis in combination with a 1.5 W, 6 V solar cell or a 6 V solar rechargeable battery.

Fig. 18 (A) TEM images of carbon-supported Au₂₅ at low and high loadings. Isolated 1.4 ± 0.4 nm Au NC were observed on the carbon black support at low loadings. (B) Day-to-day (a) product formation rates, (b) cumulative TON (mol of CO/(mol of Au₂₅)). Panels (A) and (B) are reproduced with permission from ref. 293. Copyright 2015 American Chemical Society.

3.3.1 Size effects in gold nanoclusters. Studies on the size dependence of Au NCs used in electrochemical CRRs are summarized in Table 1. Dass et al. evaluated CRR activity by dissolving Au₁₃₇(PET)₅₆ in N,N-dimethylmethanamide (DMF) containing 0.1 M tetrabutylammonium hexafluorophosphate (Bu₄NPF₆).²⁹⁴ They observed a rise in current from ∼0.55 V vs. NHE, and the overvoltage of CO₂ reduction to CO for Au₁₃₇(PET)₅₆ was approximately 200 mV lower than that observed for [Au₂₅(PET)₁₈]⁻.

In 2021, Lee and Yoo et al. evaluated CRR activity using three different core sizes of Au_n(SR)_m (i.e., [Au₂₅(SC₆H₁₃)₁₈]⁻, [Au₃₈(SC₆H₁₃)₂₄]⁰ and [Au₁₄₄(SC₆H₁₃)₆₀]⁰).²⁹⁵ As a result, CRR activity increased with decreasing Au_n(SR)_m size, showing much higher CO production current density (j_CO) and selectivity (FE_CO > 90%) than Au NPs (25 ± 4 nm particle size). Electrochemical measurements, X-ray photoelectron spectroscopy (XPS), and DFT calculations indicated that the application of applied potential in Au_n(SR)_m (n = 25, 38, 144) leads to a progressive de-thiolation and the appearance of an active Au site, which is converted to a more active and stable geometry. Furthermore, it was suggested that Au sites de-thiolated at the staple motif promote CRRs by stabilizing the *COOH intermediate.

In 2022, Kauffman, Mpourmpakis, and Jin et al. used two different Au_n(SR)_m with a series of sizes ([Au₃₈(PET)₂₄]⁰, [Au₁₄₄(PET)₆₀]⁰, Au₃₃₃(PET)₇₉, [Au₂₈(TBBT)₂₀]⁰, [Au₃₆(TBBT)₂₄]⁰, and Au₂₇₉(TBBT)₈₄) to evaluate CRR activity.²⁹⁶ A catalyst ink of each Au_n(SR)_m loaded at 10 wt% in isopropyl alcohol and 0.2 wt% Nafion on CB was placed on GCE, and CRR measurements were performed in 0.5 M potassium hydrogen carbonate (KHCO₃) aq. As a result, only CO and H₂ were produced, and the PET-protected Au_n(SR)_m ([Au₃₈(PET)₂₄]⁰ and [Au₁₄₄(PET)₆₀]⁰) and TBBT-protected Au_n(SR)_m ([Au₂₈(TBBT)₂₀]⁰ and [Au₃₆(TBBT)₂₄]⁰) showed almost 100% FE_CO at potentials more positive than −0.8 V vs. RHE and −0.77 V vs. RHE, respectively (Fig. 19A). Furthermore, Au₃₃₃(PET)₇₉ and Au₂₇₉(TBBT)₈₄ with larger core sizes showed lower FE_CO and j_co values than Au_n(SR)_m with smaller core sizes (Fig. 19A). This size dependence may have been observed because the S atom of the ligand is the CRR active site and the ratio of S atoms to Au atoms (R_S/Au) is larger for Au_n(SR)_m with a smaller core size (Fig. 19B). The authors also attribute the higher activity of PET-protected Au_n(SR)_m to the difference in ligand conductivity and ligand removal energies compared with TBBT-protected Au_n(SR)_m.

Fig. 19 (A) FE_CO of (a) PET-protected Au₃₈, Au₁₄₄ and Au₃₃₃ and (b) TBBT-protected Au₂₈, Au₃₆ and Au₂₇₉ NCs. (B) Electrochemical performance: j_CO (at −0.87 V vs. RHE) vs. S : Au ratio of Au₃₈, Au₁₄₄, Au₃₃₃ and Au₂₈, Au₃₆, Au₂₇₉ series of NCs (A) and (B) are reproduced with permission from ref. 296. Copyright 2022 Wiley-VCH GmbH.

In 2023, Sharma, Golovko, and Marshall et al. also reported on the size dependence in CRRs using phosphine-protected Au NCs.²⁹⁷ The CRR activities of [Au₆(DPPP)₄](NO₃)₂, [Au₉(PPh₃)₈](NO₃)₃, [Au₁₃(DPPE)₅Cl₂]Cl₃, and [Au₁₀₁(PPh₃)₂₁]Cl₅ were measured and showed high selectivity for CO formation (FE_CO = 75–90%), and [Au₁₃(DPPE)₅Cl₂]Cl₃ showed slightly higher CO selectivity compared with other NCs. Additionally, the calcination process for ligand removal was found to cause aggregation of the Au NCs, thus decreasing the CO selectivity. It was also reported that Au4 clusters (Au₄(PPh₃)₄I₂) stabilized by phosphine and iodide (I) show higher CO selectivity (FE_CO = >60%) than Au₁₁(PPh₃)₇I₃ (FE_CO = <60%).²⁹⁸

From these reports, the size reduction of Au NCs contributes to the improvement of CRR activity and selectivity. This is probably due to the increase in active sites to increase in specific surface area and the change in the electronic state of metal NCs, which is advantageous for CO₂ adsorption.

3.3.2 Structure effects in gold nanoclusters. Studies on the structure dependence of electrochemical CRRs using Au NCs are summarized in Table 2. The geometric structure of metal NCs can have a significant effect on CRR activity. Mpourmpakis and Jin et al. evaluated the CRR activity of [Au₂₅(PET)₁₈]⁻, a spherical form, and [Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺, a rod form (Fig. 20A).²⁹⁹ Au_n(SR)_m loaded at 10 wt% on CB was dropped on GCE (Fig. 20B) and electrochemical measurements were performed in 0.5 M KHCO₃ aq. saturated with CO₂. The FE_CO of [Au₂₅(PET)₁₈]⁻ was 73.7% at −0.57 V vs. RHE, 1.63 times higher than that of [Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺ (FE_CO = ∼28.0%) (Fig. 20C). The FE_CO of [Au₂₅(PET)₁₈]⁻ was also high, 73.5% at −1.07 V vs. RHE, indicating that H₂ formation is suppressed. From the DFT calculations, they speculated that this high selectivity is due to the energetically advantageous ligand desorption that exposes the active site, and the negatively charged [Au₂₅(PET)₁₈]⁻ is presumed to be favorable for stabilization of the adsorbed intermediate COOH (Fig. 20D). Although these NCs cannot be directly compared because they have different ligands, the results demonstrated that the structure of the active site where the substrate is adsorbed is important.

Fig. 20 (A) Atom packing structures of Au₂₅ nanosphere ([Au₂₅(PET)₁₈]⁻) and nanorod ([Au₂₅(PPh₃)₁₀(PET)₅Cl₂]²⁺). (B) STEM images and particle size histograms of (a) Au₂₅ nanosphere and (b) nanorod supported on carbon black. (C) FE_CO and FE_H2 at the potential of −1.07 and −1.17 V over Au₂₅ nanosphere and nanorod, respectively. (D) Free energy diagrams (ΔG) for CO₂ reduction to CO on the ligand-removed NCs at 0 V vs. RHE. The black, blue, green, and red lines represent CO₂ reduction to CO on the nanorod with PH₃ removed, the nanorod with –SCH₃ removed, the nanorod with –Cl removed, and on the nanosphere with –SCH₃ removed, respectively. Panels (A)–(D) are reproduced with permission from ref. 299. Copyright 2018 American Chemical Society.

In 2022, Kauffman, Mpourmpakis, and Jin et al. evaluated CRR activity using two structural isomers of Au₃₈(PET)₂₄ (Fig. 4A; Au₃₈Q and Au₃₈T).²⁹⁶ Au₃₈Q exhibited an FE_CO of 87% at −0.97 V vs. RHE, and Au₃₈T exhibited a lower FE_CO than Au₃₈Q, especially at more negative potentials (67% at −0.97 V vs. RHE). Thus, they investigated the differences in these activities based on DFT calculations. As a result, the average thermodynamic barrier to ligand removal (exposed active site) was similar for the two Au₃₈(PET)₂₄. However, there are significant differences in the average *COOH formation energies, indicating a linear trend between the *COOH formation energy and the ligand removal energy. Specifically, the average *COOH production energy at the two S sites for Au₃₈Q is 0.17 eV, compared with 0.31 eV for Au₃₈T. This indicates that the differences in CRR activity between the two isomers may be due to differences in the ease of *COOH formation. Overall, two Au₃₈(PET)₂₄ with the same size, composition, and number of ligands can have substantially different catalytic selectivity and activities if their underlying structures are different, which can affect the formation energies.

Asymmetric structural defects of metal NCs can induce enhanced activity. Using Au₄₄(PPh₃)(TBBT)₂₆ and Au₄₄(PPh₃)₂(TBBT)₂₄ with fcc metal cores co-protected by PPh₃ and TBBT, and Au₄₄(TBBT)₂₈, changes in CRR activity due to ligand defects have also been reported. Au₄₄(PPh₃)(TBBT)₂₆ with a single Au atom at the bottom of the fcc lattice exhibited high CO selectivity. DFT calculations showed that this active site promotes the electrochemical reduction of CO₂ to CO by lowering the free energy of *COOH formation.³⁰⁰

From this report, even in Au NCs composed of the same chemical composition, the adsorption energy with the CRR intermediate changes due to the difference in the geometric structure, and Au NCs with different geometries showed different CRR activities.

3.3.3 Ligand effects in gold nanoclusters. Studies on the ligand dependence of electrochemical CRRs with Au NCs are summarized in Table 3. Kauffman, Mpourmpakis, and Jin et al. compared the activity of CRRs using [Au₂₅(SR)₁₈]⁻ protected by three different ligands (PET, SNap, and benzeneselenolate (SePh)).³⁰¹ They found that changes in the carbon tail do not significantly affect catalytic selectivity, but the anchoring atom of the ligand binding to Au (X = S/Se) has a strong effect on both selectivity and activity. As a result, [Au₂₅(SR)₁₈]⁻ exhibited higher CO selectivity than [Au₂₅(SeR)₁₈]⁻. According to DFT calculations, they ascribed the higher selectivity of [Au₂₅(SR)₁₈]⁻ to the enhanced electronic interaction between the reaction intermediates and the exposed S active site compared with the Se site. This was interpreted as an increase in the electron density of the S site in [Au₂₅(SR)₁₈]⁻, changing the bonding properties of the reaction intermediates and decreasing the energy penalty between COOH and CO.

In 2022, Wang, Zang, and Mak et al. studied the effect of the protective ligand on the CRR activity using [Au₂₈(C₂B₁₀H₁₁S)₁₂(THT)₄Cl₄]⁰ (THT = tetrahydrothiophe) (Au₂₈–S) and [Au₂₈(C₄B₁₀H₁₁)₁₂(THT)₈]³⁺ (Au₂₈–C),³⁰² which have the same metal core but different ligand shells.³⁰³ As shown in Fig. 21A, Au₂₈–S and Au₂₈–C have almost the same Au core structure, but the ligand group of the carborane ligand changes from a C≡CR group (σ–π donor) to an SR group (σ donor), and the four THT ligands in Au₂₈–C are replaced by four Cl ions in Au₂₈–S. As a result, the total numbers of valence electrons in Au₂₈–C and Au₂₈–S are 12e⁻ and 13e⁻, respectively. As shown in Fig. 21B, Au₂₈–S exhibited higher CRR catalytic activity than Au₂₈–C. Au₂₈–S exhibited a maximum FE_CO of 98.5% at −0.9 V vs. RHE, approximately 1.4 times that of Au₂₈–C. Furthermore, time-resolved in situ Fourier transform infrared (FT-IR) spectroscopy showed that the vibrational peak V(C–O) at 1362 cm⁻¹ and the C=O stretch at 1636 cm⁻¹ of the *COOH intermediate in Au₂₈–S is stronger than that of Au₂₈–C, confirming that Au₂₈–S exhibits higher electrocatalytic activity. In addition, they used DFT calculations to determine the active sites. The Au atom in the linear C₂B₁₀H₁₁C≡C–Au–C≡CC₂B₁₀H₁₁ staple of Au₂₈–C has enough space between the two C≡CR groups to access the CO₂ molecule, but in Au₂₈–S, the Cl atom of the surface ligand is removed, and the CO₂ molecule is not accessible until the Au atom is exposed. The Gibbs free energy calculations of the CRRs for Au₂₈–S and Au₂₈–C and the time-resolved in situ FT-IR results indicate that the pathway of CO₂ reduction to CO is CO₂(g) → *CO₂⁻ → *COOH → *CO → CO(g), with the rate-limiting step being *COOH formation. Notably, Au₂₈–S is energetically more favorable for CO formation than Au₂₈–C.

Fig. 21 (A) The total crystal structures of (a) Au₂₈–S and (b) Au₂₈–C. (B) (a) Linear sweep voltammetry (LSV) curves of Au₂₈–S and Au₂₈–C in a N₂-saturated (dotted line) and a CO₂-saturated (full line) 0.5 M KHCO₃ solution. (b) FE_CO at various applied potentials over two NCs. (c) The j_CO and (d) TOF_CO at the potential of −0.6 to −1.1 V vs. RHE of two NCs. Color codes: orange, green and blue = gold turquoise icosahedrons = carborane; yellow = sulfur; gray = carbon; bright green = chloride. Hydrogen atoms are omitted for clarity. (A) and (B) are reproduced with permission from ref. 303. Copyright 2022 Wiley-VCH GmbH.

Häkkinen, Tsukuda, and Crudden et al. reported on the electrocatalytic activity for CO₂ reduction using NHC-coordinated Au₁₁ clusters.²³⁸ Specifically, [Au₁₁(PPh₃)₇(NHC^Me)Cl₂]Cl (NHC^Me = 1,3-dihydro-1,3-dimethyl-2H-benzimidazol-2-ylidene) showed higher stability and CRR mass activity than [Au₁₁(PPh₃)₈Cl₂]Cl. The activity was greatly enhanced when the PPh₃ ligand was removed by heating the catalyst at 180 °C for 2 h during catalyst preparation. In 2022, Häkkinen, Tsukuda, and Crudden et al. also reported on the electrocatalytic activity of CRRs with NHC-protected Au₂₄ clusters.²⁴⁹Fig. 22A shows the ESI-MS spectrum of [Au₂₄(NHC^Bn)₁₄Cl₂H₃][TFA]₃ (TFA = trifluoroacetate; NHC^Bn = 1,3-dibenzyl-1H-benzo[d]imidazol-2-ylidene) and its geometric structure. [Au₂₄(NHC^Bn)₁₄Cl₂H₃]³⁺ exhibited a high FE_CO of >90% in the current density range of 10 to 50 mA cm⁻² and higher CO selectivity than commercial Ag NP catalysts and [Au₁₃(NHC^Bn)₉Cl₃]²⁺ NCs³⁰⁴ (Fig. 22B). The CRR evaluation at a steady current density of 100 mA cm⁻² showed that the FE_CO and full cell voltage remained stable at approximately 90% and 3.2 V, respectively, even after 100 h, demonstrating the high stability of [Au₂₄(NHC^Bn)₁₄Cl₂H₃]³⁺ (Fig. 22C).

Fig. 22 (A) (a) ESI-MS spectra and (b) molecular structure of [Au₂₄(NHC^Bn)₁₄Cl₂H₃][TFA]₃ as determined by SC-XRD with hydrogen atoms and TFA anions omitted for clarity. (B) (a) FE_CO and FE_H2 at different current densities. (b) Cell voltage vs. current density. (C) Stability of FE_CO and FE_H2, and cell voltage vs. time at an operating current density of 100 mA cm⁻² with the Au₂₄ catalyst. Color codes: yellow = gold; green = chloride; grey = carbon; and blue = nitrogen. (A)–(C) are reproduced with permission from ref. 249. Copyright 2022 American Chemical Society.

In 2021, Wang et al. synthesized the first all-amidinate-protected Au NC, [Au₂₈(Ph-form)₁₂]²⁺, and evaluated its CRR activity.²⁵⁰ The crystal structure of [Au₂₈(Ph-form)₁₂]²⁺ has a core–shell Au₂₈ kernel consisting of a tetrahedral Au₄ core and a twisted truncated tetrahedral Au₂₄ shell (Fig. 8C). Unlike Au NCs protected by SR and C≡CR, [Au₂₈(Ph-form)₁₂]²⁺ does not have a staple. After loading [Au₂₈(Ph-form)₁₂]²⁺ at 10 wt% on multiwalled carbon nanotubes (CNTs), the CRR activity was evaluated, and the highest FE_CO was 96.5% at −0.57 V vs. RHE. These values were higher than those observed for Ph-form Au/CNT and polyvinylpyrrolidone (PVP)-coated Au NPs. In addition, stable potentials at approximately −0.69 V vs. RHE and an FE_CO of >90% were maintained for 40 h in the long-term activity evaluation at −3.5 mA cm⁻². The high stability is attributed to the strong bonding between Au and amidinate ligands, indicating that Au NCs protected by N-containing ligands, such as NHCs and amidinates, can be used as highly stable and active CRR catalysts.

In 2021, Wang et al. synthesized [Au₅₅(p-MBT)₂₄(PPh₃)₆]³⁺, which is co-protected with SR and phosphine and has the fcc structure, and evaluated its CRR activity.²³³ As a result, [Au₅₅(p-MBT)₂₄(PPh₃)₆]³⁺ showed the highest FE_CO of 94.1% at −0.6 V vs. RHE. The Au₅₅ core in [Au₅₅(p-MBT)₂₄(PPh₃)₆]³⁺ is fcc aligned and consists of four layers stacked in ABCA order along the [111] direction. Moreover, [Au₅₅(p-MBT)₂₄(PPh₃)₆]³⁺ may have shown higher CRR activity than the Au NPs with an fcc structure because of the protection of the HER-active corner by the p-MBT, which is a hydrophobic ligand. Thus, CRR activity may be controlled by the appropriate selection of surface ligands.

In 2022, Wang et al. investigated the effect of uncoordinated metal sites in CRRs using [Au₁₁(DPPE)₅]³⁺ and a hydride-coordinated Au NC ([Au₂₂H₃(DPPE)₃(PPh₃)₈]³⁺).²³¹ They presume that [Au₁₁(DPPE)₅]³⁺ is completely capped with a diphosphine ligand, whereas [Au₂₂H₃(DPPE)₃(PPh₃)₈]³⁺ has three H atoms bridging six uncoordinated Au sites, and the remaining 14 surface Au atoms are protected with PPh₃ ligands. They evaluated the CRR activities and found that compared with [Au₁₁(DPPE)₅]³⁺, [Au₂₂H₃(DPPE)₃(PPh₃)₈]³⁺ exhibited higher FE_CO (92.7% at −0.6 V vs. RHE), TOF_CO (488 h⁻¹), and mass activity (134 A g_Au⁻¹). Furthermore, [Au₂₂H₃(DPPE)₃(PPh₃)₈]³⁺ was found to have high stability, with little change in j_CO and FE_CO after more than 10 h of reaction at −0.6 V vs. RHE. The results of DFT calculations suggest that the hydride-coordinated Au atoms in [Au₂₂H₃(DPPE)₃(PPh₃)₈]³⁺ are the active sites, and the bridge H atoms directly hydrogenate CO₂ to adsorbed *COOH. Moreover, it was estimated that the H voids are easily recovered by proton reduction, thus completing the catalytic cycle. These findings suggest that Au–H bonds can improve the stability and CRR catalytic activity of Au NCs.

Pei and Wang et al. evaluated the CRR activity for [Au₇(PPh₃)₇H₅]²⁺ with hydride ligands and [Au₈(PPh₃)₇]²⁺ synthesized by degrading [Au₇(PPh₃)₇H₅]²⁺ using light (300–450 nm) irradiation.²³² As a result, [Au₇(PPh₃)₇H₅]²⁺ showed high selectivity for HER (max FE_H2 = 98.2% at −0.67 V vs. RHE; FE_H2 = FE of H₂ generation), whereas [Au₈(PPh₃)₇]²⁺ was highly selective for CRR (FE_CO = 73.5% at −0.77 V vs. RHE). UV/Vis and ESI-MS measurements of [Au₇(PPh₃)₇H₅]²⁺ and [Au₈(PPh₃)₇]²⁺ before and after CRRs did not change significantly, indicating their stability. In addition, isotope labeling experiments with deuterium indicated that all the constituent hydrogen atoms of H₂ released during the CO₂ reduction process came from the aqueous solution (H₃O⁺) and not from hydrides in the NC. Notably, [Au₂₂H₃(DPPE)₃(PPh₃)]³⁺,²³¹ protected by hydride ligands, showed high CO selectivity, unlike [Au₇(PPh₃)₇H₅]²⁺, indicating that the reactions between these NCs proceeded via different mechanisms. Therefore, assuming that all the hydrogen atoms come from the aqueous solution and not from the hydride ligands, DFT calculations showed that the values of ΔG of the *COOH formation for [Au₇(PPh₃)₇H₅]²⁺ and [Au₈(PPh₃)₇]²⁺ are 1.68 and 0.58 eV, respectively. The small potential barrier to COOH* formation might be responsible for the high CO selectivity of [Au₈(PPh₃)₇]²⁺.

In 2021, Yang et al. investigated the process of ligand removal by thermal and electrochemical treatment of Au₂₅(PET)₁₈ and its effect on the electrolytic reduction of CO₂ to CO.³⁰⁵ Under-potential deposition (UPD) measurements of Cu, Au L₃ edge X-ray absorption fine structure (EXAFS) spectroscopy, and XPS measurements were used to determine the extent of ligand removal. Increasing the annealing temperature during the thermal treatment and the bias voltage in the electrochemical treatment increased the surface area on the catalyst, indicating that the Au–S bonds were broken, and the surface Au atoms became metallic. In particular, when the reducing potential was applied by electrochemical treatment, there was no change in the Au–S bond strength measured by Cu UPD and FT-EXAFS measurements up to −0.30 V vs. RHE, but from −0.35 V vs. RHE ligand, removal was observed, and the aggregation of Au₂₅(PET)₁₈ was observed in the TEM images after applying a higher reducing potential. For the electrochemical evaluation of CRR activity, S-doped graphene (S–G)-loaded Au₂₅(PET)₁₈ (Au₂₅(PET)₁₈/S–G) annealed at 150 °C (Au₂₅/S–G(Thermal)), and electrochemically pretreated Au₂₅(PET)₁₈/S–G at −0.8 V vs. RHE (Au₂₅/S–G(Electrochemical)) were used. Au₂₅/S–G(Electrochemical) exhibited a higher FE_CO (82% at −0.59 V vs. RHE) than Au₂₅/S–G(Thermal). This suggests that ligand removal by thermal and electrochemical pretreatment exposed Au atoms as active sites and improved the CRR catalytic activity.

From these papers, regarding the influence of ligands, it was revealed that (1) the CRR activity changes greatly depending on the element that directly coordinates with the metal (electron-withdrawing or donating ligands), (2) high CRR stability by using ligands with strong binding energy with metals, (3) the hydride contained in the ligand also affects the CRR activity, and (4) elimination of some ligands promotes the CRR activity.

3.3.4 Doping effects in gold nanoclusters. In Table 4, we summarized the studies on electrochemical CRR using heterometal-doped Au NCs. In 2019, Yang and Wu et al. reported on CRRs using Au₄₇Cd₂(TBBT)₃₁ and Au₄₄(TBBT)₂₈.^211,306Fig. 23A (inset) shows the structure of Au₄₇Cd₂(TBBT)₃₁, which consists of an Au₂₉ kernel, two Cd(S–Au–S)₃, three Au₂(TBBT)₃, one Au₃(TBBT)₄, and three Au(TBBT)₂ staple motifs in the shell structure. Au₄₇Cd₂(TBBT)₃₁, Au₄₄(TBBT)₂₈, or Au NPs were loaded on GCE and examined by linear sweep voltammetry (LSV) in 0.5 M KHCO₃ aq. saturated with Ar or CO₂. All catalysts exhibited higher current densities in a CO₂ atmosphere than in an Ar atmosphere, indicating that electrocatalytic CRRs occurred (Fig. 23A). Notably, Au₄₇Cd₂(TBBT)₃₁ exhibited the highest current density. The maximum values of FE_CO at −0.57 V vs. RHE were 96% and 83% for Au₄₇Cd₂(TBBT)₃₁ and Au₄₄(TBBT)₂₈, respectively. In comparison, 1.5 nm Au NPs exhibited a maximum FE_CO of 76% at −0.67 V vs. RHE (Fig. 23B). At a constant applied voltage (−0.57 V vs. RHE), the j_CO reached −3.2 mA cm⁻² for Au₄₇Cd₂(TBBT)₃₁, which was higher than that of Au₄₄(TBBT)₂₈ (−1.6 mA cm⁻²). In Au₄₇Cd₂(TBBT)₃₁, the FE_H2 in HER was 3.8% at −0.57 V vs. RHE, indicating that the HER was effectively suppressed. Furthermore, the Tafel slope was lower for Au₄₇Cd₂(TBBT)₃₁ at 151 mV per decade, compared with Au₄₄(TBBT)₂₈ (166 mV per decade) and Au NPs (203 mV per decade). These results indicated that Au₄₇Cd₂(TBBT)₃₁ favored the formation of the *COOH species by the first electron transfer. Subsequently, they used DFT calculations to examine the reaction mechanism. Cd prefers to bond with oxygen (O) atoms, forming Cd–O–C(OH)–Au to stabilize the intermediate *COOH. As a result, the formation of *COOH in Au₄₄(TBBT)₂₈ and Au NPs causes an increase in free energy, whereas the formation of COOH* in Au₄₇Cd₂(TBBT)₃₁ causes the release of free energy (0.39 eV), which favors the formation of intermediates. The DFT calculations also revealed that the release of H₂ in Au₄₇Cd₂(TBBT)₃₁ was the most difficult of the three systems. Overall, the doped NC, Au₄₇Cd₂(TBBT)₃₁, exhibited enhanced CRR activity.

Fig. 23 (A) LSV curves of Au₄₇Cd₂(TBBT)₃₁, Au₄₄(TBBT)₂₈ and the ca. 1.5 nm Au NPs in an Ar-saturated (dotted line) and a CO₂-saturated (full line) 0.5 M KHCO₃ solution; inset: total structure of the Au₄₇Cd₂(TBBT)₃₁. Color labels: yellow = S; gray = C; red = Cd, others = Au. (B). FE_CO for the catalysts examined with different applied potentials. Panels (A) and (B) are reproduced with permission from ref. 306. Copyright 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Kauffman, Mpourmpakis, and Jin et al. used [Au₁₉Cd₂(SC₆H₁₁)₁₆]⁻ and [Au₂₃(SC₆H₁₁)₁₆]⁻ (ref. 206 and 307) to compare CRR activity.³⁰⁸ For [Au₁₉Cd₂(SC₆H₁₁)₁₆]⁻, the CO selectivity of the CRR was ∼90–95% at applied potentials of −0.5 to −0.9 V vs. RHE, approximately twice that of [Au₂₃(SC₆H₁₁)₁₆]⁻. From the DFT calculations, similar to the previous study,¹⁶ it is thermodynamically preferable to remove the –R group from the NCs, compared with the –SR group. Therefore, they simplified the ligand to SCH₃ and calculated the reaction free energy pathways using the two NCs, but with one ligand's R site removed ([Au₁₉Cd₂S(SCH₃)₁₅]⁻ and [Au₂₃S(SCH₃)₁₅]⁻). The thermodynamic energy barrier for CO formation was 0.74 eV lower for [Au₁₉Cd₂S(SCH₃)₁₅]⁻ compared with [Au₂₃S(SCH₃)₁₅]⁻. In addition, comparing the thermodynamic limiting steps of CRR and HER, the calculated results were consistent with the experimental data showing that [Au₁₉Cd₂S(SCH₃)₁₅]⁻ has a higher CRR selectivity. The effect of solvation was also examined, and it was found that the solvent (water) only had a minimal effect on the reaction energy. These results showed that modifying both the NC surface morphology and electronic structure by Cd doping can significantly enhance CRR activity.

In 2021, Zhu and Chen et al. synthesized Au₂₅(PET)₁₈, Au₂₄Cd(PET)₁₈, Au₁₉Cd₃(S-tol)₁₈, and Au₃₈Cd₄(d-MBT)₃₀ and investigated the effect of different substitution positions of Cd doping on the catalytic CRR activity.¹⁸⁸ As shown in Fig. 3Ba, the Cd atoms in Au₂₄Cd(PET)₁₈ replaced the Au atoms located in the core–shell layer, and the Cd atoms in Au₁₉Cd₃(S-tol)₁₈ and Au₃₈Cd₄(d-MBT)₃₀ substituted the Au atoms on the shell staples. Among these four NCs, Au₂₄Cd(PET)₁₈ showed the highest current density, FE_CO, and j_CO, whereas Au₁₉Cd₃(S-tol)₁₈ showed the lowest CRR activity (Fig. 24a–c). Fig. 24d and e shows that a small amount of HCOOH was detected in addition to the main product of CO, for all NCs. Furthermore, the stability measurements in Fig. 24f show that Au₂₄Cd(PET)₁₈ and Au₁₉Cd₃(S-tol)₁₈ were highly stable, with no significant change in current density after 24 h. DFT calculations showed that removing the carbon chain on the NC surface (S–C cleavage) was advantageous for CRR, as observed in previous studies,¹⁶ and exposing the Au site by Au–S cleavage was advantageous for HER. From these results, it was speculated that only S–C cleavage occurs in Au₂₄Cd(PET)₁₈ and that the HER was suppressed by avoiding Cd–S cleavage, resulting in a high FE_CO. These results showed that the CRR activity can be enhanced by controlling the surface state through doping to different sites.

Fig. 24 (a) LSV curves of Au₂₅(PET)₁₈, Au₂₄Cd₁(PET)₁₈, Au₁₉Cd₃(S-tol)₁₈ and Au₃₈Cd₄(d-MBT)₃₀ under an Ar-saturated (dotted line) and a CO₂-saturated (full line) in 1.0 M KHCO₃ solution. (b) FE_CO for the catalysts examined with different applied potentials. (c) The corresponding j_CO. (d) FEs for various CRR products obtained on Au₂₅(PET)₁₈ and Au₂₄Cd₁(PET)₁₈ catalysts. (e) FEs for various CRR products obtained on Au₁₉Cd₃(S-tol)₁₈ and Au₃₈Cd₄(d-MBT)₃₀ catalysts. (f) The stability test conducted at −0.7 V vs. RHE for Au₂₅(PET)₁₈, Au₂₄Cd₁(PET)₁₈, Au₁₉Cd₃(S-tol)₁₈ and Au₃₈Cd₄(d-MBT)₃₀ catalysts. These figures are reproduced with permission from ref. 188. Copyright 2021 American Chemical Society.

In 2021, Lim, Yoo, and Lee et al. investigated the effect of Pt doping on CRR activity using [Au₂₅(SC₆H₁₃)₁₈]⁻ and [PtAu₂₄(SC₆H₁₃)₁₈]⁰.³⁰⁹ [PtAu₂₄(SC₆H₁₃)₁₈]⁰ has a central atom of [Au₂₅(SC₆H₁₃)₁₈]⁻ replaced by Pt, but otherwise, the overall structures are very similar (Fig. 3A). Comparing the electrocatalytic performance, [PtAu₂₄(SC₆H₁₃)₁₈]⁰ was found to promote HER more than CRR, compared with [Au₂₅(SC₆H₁₃)₁₈]⁻ (Fig. 25a and b). As shown in Fig. 25c and d, in the 0.3–0.6 V vs. RHE range where CO formation is not controlled by the solubility of CO₂, the FE_CO of [Au₂₅(SC₆H₁₃)₁₈]⁻ was >95%, but [PtAu₂₄(SC₆H₁₃)₁₈]⁰ showed FE_H2 = ∼80% at 0.4 V vs. the RHE. They also obtained a synthesis gas with a controlled H₂/CO molar ratio of 1 to 4 by mixing [Au₂₅(SC₆H₁₃)₁₈]⁻ and [PtAu₂₄(SC₆H₁₃)₁₈]⁰ on the gas diffusion electrode (GDE) with an appropriate mass ratio (Fig. 25e). Additionally, DFT calculations for the CRR and HER processes were performed with SC₆H₁₃ replaced by SCH₃ for simplicity. As a result, the HER with [PtAu₂₄(SCH₃)₁₈]⁰ was found to have an almost ideal *H bonding energy, which is favorable for HER. For [Au₂₅(SCH₃)₁₈]⁻, the reaction is more active for the CRR because the limiting potential of the CRR (0.09 eV) is higher than that of the HER (0.32 eV). Therefore, it was revealed that the CRR is more likely to proceed for [Au₂₅(SC₆H₁₃)₁₈]⁻ and the HER is more likely for [PtAu₂₄(SC₆H₁₃)₁₈]⁰.

Fig. 25 Results of FE_CO and FE_H2 measured on the (a) Au₂₅/C/GDE and (b) PtAu₂₄/C/GDE in the CO₂-saturated solution of 0.1 M KHCO₃ and 0.4 M KCl at various applied potentials. The corresponding j_total, j_CO, and j_H2 obtained on the (c) Au₂₅/C/GDE and (d) PtAu₂₄/C/GDE at various potentials. (e) Calculated (shaded) and experimentally determined (filled) H₂/CO ratio on formulated Au₂₅ and PtAu₂₄ catalysts. These figures are reproduced with permission from ref. 309. Copyright 2021 AIP Publishing.

In 2022, Zhou, Gao, and Zhu et al. synthesized [Au₃₈(SCH₂Ph^tBu)₂₄]⁰, [PtAu₃₇(SCH₂Ph^tBu)₂₄]⁰, and [Pt₂Au₃₆(SCH₂Ph^tBu)₂₄]⁰ with Pt doping of one or two atoms and investigated the effect of the Pt atomic position and number on the CRR catalytic activity.¹⁹⁹ SC-XRD analysis revealed that [Au₃₈(SCH₂Ph^tBu)₂₄]⁰ consists of a 23-atom kernel composed of two 13-atom icosahedra sharing one triangular plane, three Au(SR)₂ staples around the icosahedra, and six Au₂(SR)₃ staples (Fig. 4B and 26A). In addition, theoretical calculations and X-ray absorption fine structure analysis suggested that the position of Pt in [PtAu₃₇(SCH₂Ph^tBu)₂₄]⁰ and [Pt₂Au₃₆(SCH₂Ph^tBu)₂₄]⁰ is at the center of the icosahedron. From time-dependent DFT (TDDFT) calculations, [PtAu₃₇(SH)₂₄]⁰ has one less total electron than [Au₃₈(SH)₂₄]⁰, and the highest occupied molecular orbital (HOMO) energy increases. In contrast, in [Pt₂Au₃₆(SH)₂₄]⁰, the total number of electrons is decreased by two compared with [Au₃₈(SH)₂₄]⁰, and the HOMO of [Pt₂Au₃₆(SH)₂₄]⁰ changes to an orbital corresponding to the HOMO−1 of [Au₃₈(SH)₂₄]⁰, which results in a narrower HOMO-lowest unoccupied molecular orbital (LUMO) gap. This result suggests that [PtAu₃₇(SH)₂₄]⁰ is more likely to give electrons to reactants and [Pt₂Au₃₆(SH)₂₄]⁰ is less likely to give electrons to reactants compared with [Au₃₈(SH)₂₄]⁰ in the reduction process. Evaluation of CRR activity with these NCs showed that [PtAu₃₇(SCH₂Ph^tBu)₂₄]⁰ had the highest j_CO and FE_CO (80% at −0.6 V vs. RHE) (Fig. 26B).

Fig. 26 (A) Structural anatomy of [Au₃₈(SR)₂₄]⁰, asymmetrical [Pt₁Au₃₇(SR)₂₄]⁰ and symmetrical [Pt₂Au₃₆(SR)₂₄]⁰. Color labels: green/blue = Au, red = Pt, yellow = S. The C and H are omitted for clarify. (B) (a) LSV profiles of [Au₃₈(SCH₂Ph^tBu)₂₄]⁰, [PtAu₃₇(SCH₂Ph^tBu)₂₄]⁰ and [Pt₂Au₃₆(SCH₂Ph^tBu)₂₄]⁰ catalysts in a 0.5 M KHCO₃ solution. (b) FE_CO and (c) j_CO for [Au₃₈(SCH₂Ph^tBu)₂₄]⁰, [PtAu₃₇(SCH₂Ph^tBu)₂₄]⁰ and [Pt₂Au₃₆(SCH₂Ph^tBu)₂₄]⁰ catalysts at different applied potentials. Error bars correspond to the deviations from several independent experiments. (A) and (B) are reproduced with permission from ref. 199. Copyright 2022 Wiley-VCH GmbH.

In 2022, Zhu et al. synthesized Ag_4+xAu_40−x(C≡CPh(CH₃)₂)₂₈ (HC≡CPh(CH₃)₂ = 1-ethynyl-2,4-dimethylbenzene), and they investigated the effect of Ag doping in CRRs using Au₄₄(C≡CPh(CH₃)₂)₂₈, Au₄₄(d-MBT)₂₈, and Ag_4+xAu_40−x(C≡CPh(CH₃)₂)₂₈.³¹⁰ When electrolytic reduction was performed in a 0.5 M KHCO₃ solution saturated with CO₂, the main products of the three NCs were CO and a small amount of H₂. In particular, Ag_4+xAu_40−x(C≡CPh(CH₃)₂)₂₈ exhibited the highest current density, and the FE_CO reached approximately 98% (−0.5 V vs. RHE). They also measured electrochemical impedance and electrochemical surface area to determine the reason for the differences in their catalytic performance. It was suggested that Ag doping decreased the charge transfer resistance and increased the number of active sites, thereby enhancing CRR activity.

In 2022, Kim, Yoo, and Lee et al. used [Au₂₅(PET)₁₈]⁻, [Ag₂₅(SPhMe₂)₁₈]⁻, and bimetallic [AuAg₁₂@Au₁₂(PET)₁₈]⁻ with a core–shell structure for CRR activity.¹⁹⁰ Comparing the CRR and HER activities of [Au₂₅(PET)₁₈]⁻ and [Ag₂₅(SPhMe₂)₁₈]⁻ activated by de-thiolation, the j_CO was found to be higher than j_H2 for both NCs. To investigate the origin of the CRR activity, DFT calculations were conducted (ligands were replaced with SCH₃ for simplicity). As a result, the partially de-thiolated sites, the bridging core metal, and the SR ligand in both NCs were determined to be the CRR active sites. The CRR limiting potential of [Au₂₅(PET)₁₈]⁻ (0.14 V) is lower than that of [Ag₂₅(SPhMe₂)₁₈]⁻ (0.24 V). The reason for the superior CRR activity of [Au₂₅(PET)₁₈]⁻ might be that the stabilization of *COOH promoted more activation of CO₂. Therefore, they replaced the Ag₁₂(SR)₁₈ shell, the active site in [Ag₂₅(SPhMe₂)₁₈]⁻, with the more active Au₁₂(SR)₁₈ shell to improve the CRR activity. After optimizing the synthesis conditions, they obtained [AuAg₁₂@Au₁₂(PET)₁₈]⁻ consisting of an AuAg₁₂ core and an Au₁₂(PET)₁₈ protective shell (Fig. 3C). Then, the CRR activity was evaluated using the alloy NCs activated by de-thiolation. The j_CO of [AuAg₁₂@Au₁₂(PET)₁₈]⁻ was found to be similar to that of [Au₂₅(PET)₁₈]⁻, and the FE_CO was slightly lower than that of [Au₂₅(PET)₁₈]⁻. Furthermore, the long-term stability of [AuAg₁₂@Au₁₂(PET)₁₈]⁻ was confirmed using a zero-gap CO₂ electrolyzer. When galvanostatic electrolysis was performed at a current density of 200 mA cm⁻², the full cell potential of the electrolyzer maintained 2.13 ± 0.03 V and 57% FE_CO for 24 h for [AuAg₁₂@Au₁₂(PET)₁₈]⁻.

In 2022, Yang, Yang, and Wu et al. reported Au–Pd–Ag trimetallic alloy NCs. They showed the effect of Ag doping in CRRs using similar structures protected with TBBT (Fig. 5), namely Au₄Pd₆(TBBT)₁₂ and Au₃AgPd₆(TBBT)₁₂.²⁰² The LSV curves indicated that Au₃AgPd₆(TBBT)₁₂ exhibits a higher current density than Au₄Pd₆(TBBT)₁₂, suggesting that Ag doping enhances the catalytic CRR activity. The maximum FE_CO of Au₃AgPd₆(TBBT)₁₂ was 94.1% (at −0.57 V vs. RHE), which was higher than that of Au₄Pd₆(TBBT)₁₂ (88.1% at −0.67 V vs. RHE). Furthermore, Au₃AgPd₆(TBBT)₁₂ showed high long-term stability, and no decrease in current density or selectivity was observed after 65 h of electrolysis (−0.57 V vs. RHE). They used DFT calculations to investigate the effect of Ag doping on the reaction mechanism. Notably, the increase in the free energy of CO desorption was smaller for Au₃AgPd₆(SCH₃)₁₁ than for Au₄Pd₆(SCH₃)₁₁, which suggests that Ag doping into Au₄Pd₆(SCH₃)₁₁ weakens the binding energy of *CO on the active site and promotes CO desorption, thereby improving the reduction to CO. This may be attributed to the electronic effects caused by the substitution of Ag atoms. From the XPS spectra, the binding energies of Au 4f and Pd 3d of Au₃AgPd₆(TBBT)₁₂ were significantly higher than those of Au₄Pd₆(TBBT)₁₂, which explained why the *CO adsorption on the active site of Au₃AgPd₆(SCH₃)₁₁ was weaker. Furthermore, this electronic effect might suppress the competitive HER. Therefore, Au₃AgPd₆(SCH₃)₁₁ showed high selectivity as an electrocatalyst for the reduction of CO₂ to CO owing to the more difficult desorption of H*. These results demonstrated that Ag doping can affect the electronic structure of NCs and enhance CRR activity.

In 2020, Kauffman, Mpourmpakis, and Jin et al. evaluated the electrocatalytic activity of CRRs using [Au₂₄Pd(PET)₁₈]⁰ and [Au₂₅(PET)₁₈]⁰ (Fig. 27A).³¹¹ [Au₂₅(PET)₁₈]⁰ showed an FE_CO of ∼100% below −0.9 V vs. RHE. However, at more negative potentials, the FE_CO began to decrease (approximately 60% at −1.2 V vs. RHE; Fig. 27A). In contrast, [Au₂₄Pd(PET)₁₈]⁰ maintained ∼100% FE_CO over a much wider potential range and showed almost complete HER suppression up to −1.2 V vs. RHE. The j_CO and mass activity of CO showed that [Au₂₄Pd(PET)₁₈]⁰ was more active than [Au₂₅(PET)₁₈]⁰ at all potentials. The mass activity of [Au₂₄Pd(PET)₁₈]⁰ (∼1770 mA mg⁻¹) was approximately twice as high as [Au₂₅(PET)₁₈]⁰ (∼980 mA mg⁻¹) at −1.2 V vs. RHE. Considering that only one Au atom is replaced by a Pd atom, the above results indicate that the doping effect of different elements plays an important role in CO production. No change in absorption spectrum was observed in [Au₂₄Pd(PET)₁₈]⁰, even after 6 h of electrolysis at −0.8 V vs. RHE, demonstrating its stability. From the DFT calculations, the thermodynamic limiting potentials (U_L) in CRRs and HERs for [Au₂₄Pd(SCH₃)₁₈]⁰ and [Au₂₅(SCH₃)₁₈]⁰ were calculated (Fig. 27B). Fully ligand-protected NCs were electrochemically inactive and had relatively large U_L (CRR) values. However, the NCs were activated when some Au and S atom sites were exposed, such that the Au₁₃ core can act as an electron reservoir. From the calculated ΔU_L, Au₂₄PdS(SCH₃)₁₇ with the active site at the S atom was predicted to show the most selective activity for the CRR.

Fig. 27 (A) Electrocatalytic CO₂ reduction performance (FE_CO) of [Au₂₅(PET)₁₈]⁰ and [Au₂₄Pd(PET)₁₈]⁰; inset: atom packing structures of [Au₂₄Pd(PET)₁₈]⁰ (–R groups omitted for clarity). (B) Free-energy diagrams for electrochemical (a) CRR and (b) HER at U = 0 V vs. RHE of [Au₂₄Pd(SCH₃)₁₈]⁰ and [Au₂₅(SCH₃)₁₈]⁰. Panels (A) and (B) are reproduced with permission from ref. 311. Copyright 2020 American Chemical Society.

From these reports, it became clear that (1) Ag, Cd, and Pd doping to Au NCs tends to improve CRR activity and selectivity, and (2) Pt doping may improve HER selectivity. Such a doping effect is considered to depend also on the geometric/electronic structure of the Au NCs.

3.3.5 Other effects of gold nanoclusters. Other studies on electrochemical CRRs using Au NCs are summarized in Table 5. Flake et al. evaluated CRR activity by changing the binder used in electrode fabrication from non-sulfonate-containing poly(vinylidene fluoride) (PVDF) to sulfonate-containing Nafion using [Au₂₅(PET)₁₈]⁻.³¹² The onset potential for CO₂ reduction to CO was shifted ∼170 mV toward the anode side when Nafion was used compared with that when PVDF was used. Thus, the environment surrounding Au_n(SR)_m plays an important role in CRR activity. Lee and Oh et al. reported on a practical photoelectrochemical cell using GDE-based electrodes for flow electrolysis, which was expected to achieve high current densities by removing the mass diffusion limit of gaseous CO₂ in water.³¹³ They constructed a combined system using a NiFe reverse opal anode for the OER catalyst, an Au₂₅-loaded GDE cathode for the CRR catalyst, and 3 M potassium hydroxide (KOH) as an electrolyte in combination with Ga_0.5In_0.5P/GaAs tandem solar cells. Solar-to-CO (STC) conversion efficiencies of 18.0% were achieved over 12 h under CO₂ gas flow (concentration: 100%), and STC efficiencies of 15.9% were achieved using typical CO₂ concentrations (10% of CO₂) in the flue gas. The STC efficiency was greatly improved considering that previous reports using solar-powered Au₂₅ cluster-based CO₂ reactors had STC efficiencies of less than 1%.²⁹³

3.4 Electrocatalytic CO₂ reduction reactions using silver nanoclusters

Among the precious metals, Ag is relatively inexpensive and abundant. Furthermore, it has attracted attention in CRRs because of its high selectivity for CO formation.^202,314 The reported cases of Ag-based NCs used in CRRs are summarized in Table 6.

Table 6 Representative references on CRR activity using Ag NCs (section 3.4)

Nanocluster/nanoparticle	Electrolytes	Electrolyzer	Main product	Selectivity (@V vs. RHE)	Current density (mA cm^−2@V vs. RHE)	Stability (@V vs. RHE)	Ref.
^iPr = isopropyl, EMIM-BF4 = 1-ethyl-3-methylimidazolium tetrafluoroborate, S-Adm = 1-adamantanethiolate, SPhMe2 = 2,4-dimethylbenzenethiolate, DPPE = 1,2-bis(diphenylphosphino)ethane, TC4A = thiacalix[4]arene, DPPP = 1,3-bis(diphenylphosphino)propane, MEA = membrane electrode assembly.a vs. MEA-cell potential.b Vulcan XC-72R.c Multi-walled carbon nanotube.d Acetylene black.e Ketjen black (EC-600JD).f Hydrophobic carbon paper.g Carbon paper (Toray TGP-H-090).h Carbon paper.
[Ag15(C≡C^tBu)12]^+	0.5 M KHCO3 aq.	H-cell	CO	∼95%@−0.6	13.0@−0.9	10 h@−0.75	258 ^b
Ag32(C≡CPh(CF3)2)24	0.5 M NaHCO3 aq.	H-cell	CO	96.44%@−0.8	9.05@−1.0	15 h	260 ^c
[Ag32(DPPE)5(SC6H4CF3)24]^2−				56.67%@−1.0	N/A	15 h
(Mo6O22)@H3Ag49(MO3)9(MoO4)–(TC4A)6(^iPrS)18(CH3CN)2(H2O)	0.5 M KHCO3 aq.	H-cell	H2	∼54%@−0.8	N/A	5 h@−0.8	266 ^d
			CO	44.75%@−0.8	4@−0.8
[Au7Ag8(C≡C^tBu)12]^+	1.0 M KOH aq.	Flow cell	CO	∼90%@−0.19	∼160@−0.19	10 h @−0.49	245 ^c
			HCOOH	0%@−1.19	0@−0.19
[Ag9Cu6(C≡C^tBu)12]^+			CO	∼25%@−1.19	∼30@−0.19	10 h @−1.19
			HCOOH	∼47%@−1.19	∼50@−0.19
[Au2Ag8Cu5(C≡C^tBu)12]^+			CO	∼25%@−1.19	∼30@−0.19	10 h @−0.99
			HCOOH	∼20%@−1.19	∼25@−0.19
[Ag15Cu6(C≡CPh(CF3)2)18(DPPE)2]^−	0.1 M KHCO3 aq.	H-cell	CO	91.3%@−0.81	−60@−3.25^a	145 h@−3.25^a	263 ^e
[Ag9Cu6(C≡C^tBu)12]^+		MEA-cell		48.5%@−0.89		N/A
[AuAg26(S-Adm)18S]^−	EMIM-BF4/H2O (v/v = 7 : 1) + 0.5 M H2SO4	H-cell	CO	98.4%@−0.97	8@−0.97	11 h @−0.97	255 ^c
[Ag25(SPhMe2)18]^−				∼60%@−0.97	3@−0.97	N/A
Au21(S-Adm)16				∼5.0%@−0.97	1@−0.97	N/A
Ag40.63Cu9.37(SC7H7O)32	2 M KOH aq.	Flow cell	C1 product	∼58%@−0.57	∼3.0@−0.57	N/A	254 ^f
			C2 product	29.5%@−0.57	−2.24@−0.57
Ag36.14Cu13.86(SC7H7O)32			C1 product	∼40%@−0.57	∼2.1@−0.57	14 h@−0.57
			C2 product	47.5%@−0.57	−3.04@−0.57
∼5 nm Ag NP			N/A	N/A	N/A	N/A
Au24Ag20(C≡C^tBu)24Cl2	0.5 M KHCO3 aq.	H-cell	CO	∼90%@−0.5	25@−0.8	N/A	315 ^g
Au24Ag20(C≡CPhC)24Cl2				∼90%@−0.5	17.5@−0.8	N/A
[Au43Ag38(C≡C^tBu)36Cl12]^+				∼80%@−0.5	7.5@−0.8	N/A
[Au43Ag38(C≡CPhC)36Cl9]^+				∼70%@−0.5	5@−0.8	N/A
[Au8Ag55(DPPP)4(SC6H11)34]^2+	0.5 M KHCO3 aq.	H-cell	CO	∼67%@−0.8	∼14@−0.8	9 h@−0.8	316 ^h
			H2	∼25%@−0.8	N/A
[Au8Ag57(DPPP)4(SC6H11)32Cl2]^+			CO	∼62%@−0.8	∼8@−0.8	N/A
			H2	∼25%@−0.8	N/A
[Au12Ag60(DPPP)6(SC6H11)31Br9]^2+			CO	∼46%@−0.8	∼5@−0.8	N/A
			H2	∼45%@−0.8	N/A

---

In 2021, Tang, Jin, and Tang et al. evaluated CRR activity using [Ag₁₅(C≡C^tBu)₁₂]⁺ as an electrocatalyst.²⁵⁸Fig. 28A shows the ESI-MS spectrum and geometric structure of [Ag₁₅(C≡C^tBu)₁₂]⁺, which comprises a bcc structure with a metal core configuration of Ag@Ag₈@Ag₆. [Ag₁₅(C≡C^tBu)₁₂]⁺ was first mixed with the conductive support, CB, in a 2 : 1 ratio before the CRR evaluation. As shown in Fig. 28B(a), the main product of CRRs was CO in the applied voltage range from −0.5 to −1.1 V vs. RHE. FE_CO remained above 90% over the potential range of −0.6 to −0.9 V vs. RHE and reached approximately 95% at −0.6 V vs. RHE. The partial current densities of CO and H₂ increased with overvoltage, and the maximum TOF value was 6.37 s⁻¹ at −1.1 V vs. RHE. Furthermore, constant potential measurements showed that the current density of the [Ag₁₅(C≡C^tBu)₁₂]⁺ catalyst decreased only slightly after 10 h of continuous operation at −0.75 V vs. RHE, indicating high long-term stability (Fig. 28Bb). Next, to elucidate the electrocatalytic mechanism, the optimal catalytic site on [Ag₁₅(C≡CCH₃)₁₂]⁺ was determined by DFT calculations, and the selectivities of the reduction products were compared. As a result, the formation of *COOH was the rate-limiting step in [Ag₁₅(C≡CCH₃)₁₂]⁺, and the formation of H₂ was energetically favorable (Fig. 28Ca). However, with one ligand removed ([Ag₁₅(C≡CCH₃)₁₁]⁺), the appearance of an Ag active site is favorable for the formation of *COOH and significantly lowers the energy barrier for the formation of *COOH and promotes CRR activity (Fig. 28Cb).

Fig. 28 (A) (a) the ESI-MS spectra and (b) the overall structure of [Ag₁₅(C≡C^tBu)₁₂]⁺. (B) (a) Faradaic efficiency at various applied potentials over [Ag₁₅(C≡C^tBu)₁₂]⁺ in aqueous solution of 0.5 M KHCO₃. (b) Stability of [Ag₁₅(C≡C^tBu)₁₂]⁺ for CO₂ reduction at −0.75 V vs. RHE. (C) Comparison of ΔG of electroreduction of CO₂ to CO vs. HER on (a) [Ag₁₅(C≡CCH₃)₁₂]⁺, (b) [Ag₁₅(C≡CCH₃)₁₁]⁺. (A)–(C) are reproduced with permission from ref. 258. Copyright 2021 Wiley-VCH GmbH.

3.4.1 Structure effects in silver nanoclusters. In 2022, Pei and Zhu et al. evaluated CRR activity using monomeric compounds, such as Au₂₄Ag₂₀(C≡C^tBu)₂₄Cl₂ (Au₂₄Ag₂₀-1) and Au₂₄Ag₂₀(C≡CPhC)₂₄Cl₂ (Au₂₄Ag₂₀-2; HC≡CPhC = 2-methylphenylacetylene), and two dimeric compounds, such as Au₄₃Ag₃₈(C≡C^tBu)₃₆Cl₁₂ (Au₄₃Ag₃₈-1) and Au₄₃Ag₃₈(C≡CPhC)₃₆Cl₉ (Au₄₃Ag₃₈-2).³¹⁵ The monomer NCs (Au₂₄Ag₂₀-1 and 2) have a similar core structure containing icosahedral Ag₁₂ surrounded by Ag₂₀. These two Au₂₄Ag₂₀ monomers self-assemble in different growth modes to form two different dimers: Au₄₃Ag₃₈-1 and Au₄₃Ag₃₈-2. The catalytic ability of these monomers and dimers for CRRs was examined, with the Au₂₄Ag₂₀ monomer exhibiting higher j_CO and FE_CO than the Au₄₃Ag₃₈ dimer. Among all four species, Au₂₄Ag₂₀-1 showed the highest CRR activity, and Au₄₃Ag₃₈-2 showed the lowest CRR activity. Electrochemical impedance measurements suggest that the Au₂₄Ag₂₀ monomer has faster charge transfer than the Au₄₃Ag₃₈ dimer. Structural effects have also been investigated using metal NCs with different chemical compositions. Although [Au₈Ag₅₅(DPPP)₄(SC₆H₁₁)₃₄](BPh₄)₂, [Au₈Ag₅₇(DPPP)₄(SC₆H₁₁)₃₂Cl₂]Cl, [Au₁₂Ag₆₀(DPPP)₆(SC₆H₁₁)₃₁Br₉]Br₂ are similar in size and structure, their different assembly structures lead to significantly different CRR catalytic performances. Among the three NCs, [Au₈Ag₅₅(DPPP)₄(SC₆H₁₁)₃₄] were found to have strong CO₂ adsorption capacity and exhibit the best CRR performance due to charge segregation placing Ag atoms on the outer side of Au₈.³¹⁶

3.4.2 Ligand effects in silver nanoclusters. C≡CR ligands have been used by Wang et al. for the semihydrogenation of alkyne³¹⁷ and by Tsukuda et al. for the HER³¹⁸ to enhance the activity of these reactions, and similar enhancements have also been expected for CRRs. In 2022, Tang, Wang, and Tang et al. compared the effect of ligands in CRRs using Ag₃₂(C≡CPh(CF₃)₂)₂₄ and [Ag₃₂(DPPE)₅(SC₆H₄CF₃)₂₄]²⁻.²⁶⁰ In this case, Ag₃₂(C≡CPh(CF₃)₂)₂₄ is composed of an Ag₁₇ kernel. [Ag₃₂(DPPE)₅(SC₆H₄CF₃)₂₄]²⁻ also has an Ag₁₇ kernel, although the overall geometric structure and the number of ligands are different. When catalysts were prepared by mixing both NCs with CNTs in the ratio of 2 : 1, respectively, and evaluated for CRR activity, CO and H₂ were the main products. The FE_CO was 96.44% at −0.8 V vs. RHE for Ag₃₂(C≡CPh(CF₃)₂)₂₄, but it was only 56.67% at −1.0 V vs. RHE for [Ag₃₂(DPPE)₅(SC₆H₄CF₃)₂₄]²⁻ (Fig. 29A(a and b)). Furthermore, as shown in Fig. 29A(c and d), the overall CO and H₂ partial current densities in both NCs showed gradually increasing trends when the potential was more negative. DFT calculations were performed to clarify how the CRR activity was affected by different ligands (C≡CCH₃, P₂C₂H₆, and SCH₃ were used as ligands to simplify the calculations). As a result, Ag₃₂(C≡CCH₃)₂₄ and [Ag₃₂(P₂C₂H₆)₅(SCH₃)₂₄]²⁻ have large free energy barriers (1.28 and 1.23 eV, respectively), but when one C≡CCH₃ or SCH₃ is removed, the thermodynamic barrier to *COOH formation is decreased (0.40 and 0.50 eV, respectively) and CRR activity is enhanced (Fig. 29B and C). Ag₃₂(C≡CCH₃)₂₃ also has a higher thermodynamic barrier to H* for H₂ formation than [Ag₃₂(P₂C₂H₆)₅(SCH₃)₂₃]²⁻. This HER suppression suggests that Ag₃₂(C≡CPh(CF₃)₂)₂₄ has a high selectivity for CO₂ to CO.

Fig. 29 (A) Faradaic efficiency of CO and H₂ for (a) Ag₃₂(C≡CPh(CF₃)₂)₂₄ and (b) [Ag₃₂(DPPE)₅(SC₆H₄CF₃)₂₄]²⁻ at different applied voltages. Partial current densities of CO and H₂ for (c) Ag₃₂(C≡CPh(CF₃)₂)₂₄ and (d) [Ag₃₂(DPPE)₅(SC₆H₄CF₃)₂₄]²⁻ at different applied voltages (electrolyte: 0.5 M NaHCO₃ aq.). (B) Change of reaction free energy (ΔG) at each fundamental step of CRR and HER for two NCs and stripping of one intact ligand. (C) Adsorption structure of COOH*, CO*, and H* intermediates on (a–c) [Ag₃₂(C≡CCH₃)₂₃]⁺ and (d–f) [Ag₃₂(P₂C₂H₆)₅(SCH₃)₂₃]⁻. Colour code: Ag, blue; C, gray; O, red; S, yellow; P, pin; H*, green and other H, white. (A)–(C) are reproduced with permission from ref. 260. Copyright 2022 Springer Nature.

In 2023, Liu et al. synthesized a polymolybdate-templated Ag NC, [(Mo₆O₂₂)@H₃Ag₄₉(MO₃)₉(MoO₄)–(TC4A)₆(ⁱPrS)₁₈(CH₃CN)₂(H₂O)] (Ag₄₉Mo₁₆ NC), by a solvothermal method and evaluated its CRR activity.²⁶⁶ As a result, the polymolybdate-templated Ag₄₉Mo₁₆ NC exhibited higher TOF values and j_CO than (NH₄)₆Mo₇O₂₄, indicating that the Ag site is the active center of the CRR. The highest FE_CO was 44.75% at −0.8 V vs. RHE.

3.4.3 Doping effects in silver nanoclusters. Ag-based alloy NCs have also been used in electrochemical CRRs. In 2021, Liu and Huang et al. used [Ag₂₅(SPhMe₂)₁₈]⁻, Au₂₁(S-Adm)₁₆, and [AuAg₂₆(S-Adm)₁₈S]⁻ as electrocatalysts to evaluate CRR activity.²⁵⁵ [AuAg₂₆(S-Adm)₁₈S]⁻ is composed of an AuAg₁₂ icosahedral kernel and an Ag₁₄(SR)₁₈S shell, where the four triangular facets of the icosahedral kernel are exposed without protection by SR. Electrochemical measurements in an ionic liquid (1-ethyl-3-methylimidazolium tetrafluoroborate/H₂O, v/v = 7 : 1) containing water saturated with CO₂ showed that the main CO₂ reduction products were CO and H₂. Particularly, [AuAg₂₆(S-Adm)₁₈S]⁻ showed the highest CO selectivity in the potential range from −0.82 to −1.12 V vs. RHE. The maximum FE_CO of [AuAg₂₆(S-Adm)₁₈S]⁻ at −0.97 V vs. RHE was 98.4%, approximately 1.8 and 24 times higher than that of [Ag₂₅(SPhMe₂)₁₈]⁻ and Au₂₁(S-Adm)₁₆, respectively. The electrochemical stability of [AuAg₂₆(S-Adm)₁₈S]⁻ was evaluated by chronopotentiometry at −0.97 V vs. RHE, which showed the highest CO selectivity, and the current density remained high with almost no decrease after 11 h of operation. In addition, they investigated the mechanism of catalytic selectivity by performing DFT calculations using SCH₃ as a ligand. The *COOH formation energy barrier of [AuAg₂₆(SCH₃)₁₈S]⁻ (1.06 eV) was lower than that of [Ag₂₅(SCH₃)₁₈]⁻ (1.26 eV), Au₂₁(SCH₃)₁₆ (1.83 eV), and [Ag₂₇(SCH₃)₁₈S]⁻ (1.11 eV). This suggested that Au doping of Ag NCs and the exposed core structure of [AuAg₂₆(S-Adm)₁₈S]⁻ promoted CRR activity.

In 2023, Bootharaju, Tang, Hwang, and Hyeon et al. evaluated CRR activity using [Ag₁₅Cu₆(C≡CPh(CF₃)₂)₁₈(DPPE)₂]⁻ as an electrocatalyst.²⁶³ Ag₁₅Cu₆ has a bcc-based structure with a core of Ag@Ag₈@Ag₂Cu₄, as shown in Fig. 13C. For comparison, [Ag₉Cu₆(C≡C^tBu)₁₂]⁺ (Ag₉Cu₆) and Ag₁₅Cu₆, which also have bcc structures and are similarly protected by C≡CR, were loaded on Ketjen black (Ag₁₅Cu₆/C and Ag₉Cu₆/C, respectively) and evaluated for CRR catalytic activity. As a result, Ag₁₅Cu₆/C showed high FE_CO (>85%) in the potential range of −0.72 to −0.87 V vs. RHE, with the highest FE_CO of 91.3% at −0.81 V vs. RHE. In contrast, Ag₉Cu₆/C showed the highest FE_CO of 48.5% at −0.89 V vs. RHE. Moreover, a membrane electrode assembly (MEA) cell was used to evaluate CRR activity. FE_CO remained at ∼90% and j_CO was −60 mA cm⁻² during the 145 h electrolytic reaction, indicating that Ag₉Cu₆/C has high stability. From DFT calculations, it was revealed that the removal of one C≡CCH₃ exposes the ligand-deficient AgCu dual site and the thermodynamic barrier to *COOH formation was reduced. As a result, Ag₁₅Cu₆ with one desorbed C≡CCH₃ can proceed with a low energy barrier of 0.34 eV, associated with the desorption of *CO to CO. For the competing HER, the higher energy barrier of the H* desorption step (0.61 eV) to H₂ evolution activity was also considered to enhance CO activity and selectivity.

In 2023, Wang, Yan, and Wu et al. evaluated CRR activity using Ag_40.63Cu_9.37(SC₇H₇O)₃₂ [(AgCu)₅₀-1] and Ag_36.14Cu_13.86(SC₇H₇O)₃₂ [(AgCu)₅₀-2].²⁵⁴ This is the first report of C₂ production from CO₂ reduction in the application of metal NCs as CRR catalysts. Both (AgCu)₅₀ NCs also have a bimetallic M₅₀ (M = Ag/Cu) core with fcc topology protected by 32 ligands of 3-methoxythiophenol. The catalytic CRR activity of (AgCu)₅₀, as well as Ag NPs of ∼5 nm for comparison, was measured by flow cell in 2 M KOH electrolyte saturated with Ar and CO₂. In the same potential range, the maximum FE_C2 for (AgCu)₅₀-1 and (AgCu)₅₀-2 reached 29.5% and 47.5%, respectively, indicating a conversion in catalytic selectivity due to Cu substitution (Fig. 30Aa). Comparing the C₂ and C₁ partial current densities of both NCs, (AgCu)₅₀-2 showed higher values (Fig. 30Aa). This improvement in FE was mainly due to ethanol production, with FE_CO in (AgCu)₅₀-2 being 19% lower than that in (AgCu)₅₀-1, and more C₂ than CO was produced in (AgCu)₅₀-2 (Fig. 30Ab). Comparing the FE ratios of C₂/C₁ for both NCs, (AgCu)₅₀-2 reached a C₂/C₁ ratio of 1.2 at −0.57 V vs. RHE, indicating higher selectivity for the C₂ compound. Furthermore, (AgCu)₅₀-2 exhibited high stability, with FE remaining almost the same after long-term CRR activity at −0.57 V vs. RHE (Fig. 30B). DFT calculations showed that the Cu–Cu sites are thermodynamically more favorable than the Ag–Cu sites, such that (AgCu)₅₀-2 with more Cu–Cu sites has relatively high selectivity for C₂ products (Fig. 30C).

Fig. 30 (A) CRR performance in a flow cell reactor. (a) C₁ and C₂ products FE on (AgCu)₅₀-1 and (AgCu)₅₀-2 at different potentials. (b) C₂H₄ and C₂H₅OH FE on (AgCu)₅₀-1 or(AgCu)₅₀-2 at different potentials. (B) Stability test of (AgCu)₅₀-2 over 14 h of CO₂ electrolysis in 2 M KOH at −0.57 V vs. RHE (the arrows indicate the corresponding axes). (C) Reaction pathway diagrams of Ag₄₂Cu₈ for the production of ethylene and ethanol at site 2 for DFT calculations. (A)–(C) are reproduced with permission from ref. 254. Copyright 2023 American Chemical Society.

In 2022, Tang et al. investigated the effect of metal core alloying on CRR selectivity using C≡CR-protected bimetallic [Au₇Ag₈(C≡C^tBu)₁₂]⁺ (Au₇Ag₈),²⁴⁴ [Au₉Ag₆(C≡C^tBu)₁₂]⁺ (Au₉Ag₆),²⁴⁴ and trimetallic [Au₂Ag₈Cu₅(C≡C^tBu)₁₂]⁺ (Au₂Ag₈Cu₅).²⁴⁵ Au₇Ag₈, Ag₉Cu₆, and Au₂Ag₈Cu₅ have a core–shell–shell structures (M_core@M_cube@M_octahedron), with core structures of Au₁@Ag₈@Au₆, Ag₁@Ag₈@Cu₆, and Au₁@Au₁Ag₄Cu₃@Ag₄Cu₂, respectively. Au₇Ag₈ only converted CO₂ to CO, but Ag₉Cu₆ and Au₂Ag₈Cu₅ converted CO and HCOOH, with maximum FE_HCOOH values of 47.0% and 28.3%, respectively. From DFT calculations, the desorption of surface ligands exposed active metal sites, improving the selectivity and activity of the CRRs. Furthermore, the formation of surface hydrides played an important role in the formation and stabilization of HCOO* at the Au–Cu active center. Thus, the inclusion of Cu in the core structure led to a high selectivity of HCOOH. This report indicated that the selectivity of CRR products can be adjusted by controlling the metal core of C≡CR-protected metal NCs at the atomic level.

As mentioned above, it was clarified that Au or Cu doping to Ag NCs can greatly change the activity and selectivity of CRR.

3.5 Electrocatalytic CO₂ reduction reactions using copper nanoclusters

Cu is attracting attention in electrocatalytic CRRs as a unique metal catalyst that can yield a variety of products by multi-electron reduction.^319,320 Several cases of CRRs for Cu NCs,^321–323 which are summarized in Table 7, have been reported.

Table 7 Representative references on CRR activity using Cu NCs (section 3.5)

Nanocluster/nanoparticle	Electrolytes	Electrolyzer	Main product	Selectivity (@V vs. RHE)	Current density (mA cm^−2@V vs. RHE)	Stability (@V vs. RHE)	Ref.
BC = benzoic acid, ^iPr = isopropyl, PPh3 = triphenylphosphine, S^tBu = tert-butylthiolate, L1 = 9H-carbazole-9-carbodithioate, L2 = O-ethyl carbonodithiolate, MBO = 2-mercaptobenzoxazole, DPPE = 1,2-bis(diphenylphosphino)ethane, p-FPPh3 = tris(4-fluorophenyl)phosphine.a Vulcan XC-72.b Multi-walled carbon nanotube.
Cu4Ti9O9(BC)18(O^iPr)3(^tBuC≡C)(CH3CN)	1.0 M KOH aq.	Flow cell	C2H4	∼50%@−1.0	∼200@−1.0	8 h@∼−1.0	330
			H2	∼30%@−1.0	∼120@−1.0
			CO	∼10%@−1.0	∼40@−1.0
			CH4	∼7%@−1.0	∼28@−1.0
			CH3CH2OH	∼2%@−1.0	∼8@−1.0
			CH3COO^−	∼0.5%@−1.0	∼2@−1.0
			HCOO^−	∼0.5%@−1.0	∼2@−1.0
[Cu8(H)(L1)6PF6]^+	0.5 M KHCO3 aq.	H-cell	HCOOH	50%@−1.0	7.5@−1.0	8 h@−0.9	273 ^a
[Cu8(S^tBu)4(L1)4]^+				92%@−1.0	15@−1.0	8 h@−0.9
[Cu8(S^tBu)4(L2)4]^+				∼80%@−1.0	21@−1.0	8 h@−1.0
Cu13(MBO)12	0.1 M KOH aq.	N/A	H2	72.6%@−1.05	N/A	N/A	326
			CO	13.2%@−1.05
			CH4	0.25%@−1.05
Cu25H22((p-FPh)3P)12	0.1 M KHCO3 aq.	H-cell	CO	N/A	N/A	N/A	329 ^a
			H2	∼47%@−0.8	N/A
[Cu26(CF3CO2)8(CH3O)2(C≡C^tBu)4(DPPE)3H11]^+			CO	81%@−0.8	N/A	50 h@−0.8
			H2	∼19%@−0.8	N/A
[Cu53(CF3COO)10(C≡C^tBu)20Cl2H18]^+			CO	∼8%@−0.8	N/A	N/A
			H2	∼92%@−0.8	N/A
[Cu61(S^tBu)26S6Cl6H14]^+			CO	N/A	N/A	N/A
			H2	∼98%@−0.8	N/A
Cu32H20(S2P(O^iPr)2)12	0.1 M KHCO3 and 0.4 M KCl aq.	H-cell	HCOOH	∼90%@−0.55	∼25@−0.55	N/A	325 ^a
[Cu25H22(PPh3)12]^+	0.5 M KHCO3 aq.	H-cell	H2	∼90%@−0.8	∼27@−1.2	12 h@−0.8	327 ^b
			CO	∼1%@−0.8	∼1@−1.2
			HCOOH	∼8.5%@−0.8	∼1@−1.2
[AuCu24H22(PPh3)12]^+			H2	∼48%@−0.8	∼22@−1.2	12 h@−0.8
			CO	∼40%@−0.8	∼8@−1.2
			HCOOH	∼12%@−0.8	∼2@−1.2
[Cu25H22(p-FPPh3)12]^+			H2	∼80%@−0.8	∼45@−1.2	12 h@−0.8
			CO	∼18%@−0.8	∼2@−1.2
			HCOOH	∼2%@−0.8	∼2@−1.2
[AuCu24H22(p-FPPh3)12]^+			H2	∼60%@−0.8	∼25@−1.2	12 h@−0.8
			CO	∼12%@−0.8	∼7@−1.2
			HCOOH	∼28%@−0.8	∼2@−1.2

---

In 2017, Liu, Lee, and Jiang et al. evaluated CRR activity using Cu₃₂H₂₀(S₂P(OⁱPr)₂)₁₂ (S₂P(OⁱPr)₂ = dithiophosphate ligand)³²⁴ as an electrocatalyst.³²⁵ From DFT calculations, they inferred a mechanism for HCOOH formation and subsequent hydride regeneration in Cu₃₂H₂₀(S₂PH₂)₁₂ (S₂PH₂ = dithiophosphine) (Fig. 31A and B). The two O atoms of the HCOO* intermediate bond strongly with Cu atoms on the surface to form a five-membered ring (Fig. 31B(a)). Then, HCOO* reacts to release HCOOH products (Fig. 31B(b)). The obtained Cu₃₂H₁₈(S₂PH₂)₁₂ with two hydride vacancies is expected to revert to Cu₃₂H₂₀(S₂PH₂)₁₂via a proton reduction process (Fig. 31B(c and d)). To verify these theoretical predictions, electrocatalytic activity was examined by constant potential electrolysis (CPE) with Cu₃₂H₂₀(S₂P(OⁱPr)₂)₁₂ in 0.1 M KHCO₃ and 0.4 M potassium chloride (KCl) solution (pH = 6.8) (Fig. 31C). From the cumulative FE of product formation after 90 min of CPE at various overvoltages, H₂, HCOOH, and CO, were detected as the main products, accounting for 90% of the FE. Cu₃₂H₂₀(S₂P(OⁱPr)₂)₁₂ mainly produced HCOOH at low overvoltages (FE_HCOOH = 89% at 0.3 V vs. RHE). In contrast, when the overvoltage exceeded 0.5 V vs. RHE, H₂ was predominantly produced (FE_H2 = 94% at 0.6 V vs. RHE). These experimental results agree well with the predictions from the DFT calculations, implying that HCOOH formation occurred at low overvoltages and that HER was dominant at high overvoltages. The TOF of HCOOH formation was examined and found to yield 1740 mol of HCOOH per 1 mol of Cu₃₂H₂₀(S₂P(OⁱPr)₂)₁₂ during 90 min of CPE. As a comparison, Cu NPs mainly produced CO at low overvoltages, and Cu foil mainly produced H₂ at low overvoltages. These results indicate that Cu NCs have the potential to be used as unique electrocatalysts with higher selectivity than conventional Cu NP catalysts.

Fig. 31 (A) Geometric structure of the Cu₃₂H₁₈(S₂PH₂)₁₂. Color code: orange, Cu; green, hydride; yellow, S; purple, P; white, H on the dithiophosphate ligands. Different types of hydrides are indicated by the arrows. (B) (a–d) Overall mechanism of HCOOH formation from CO₂ reduction on Cu₃₂H₁₈(S₂PH₂)₁₂via the lattice-hydride channel. (C) (a) Average current densities (black circles) and cumulative Faraday efficiencies and (b) product selectivity for H₂, HCOOH, and CO obtained at different overpotentials. Panels (A)–(C) are reproduced with permission from ref. 325. Copyright 2017 American Chemical Society.

In 2020, Robinson et al. reported the CRR activity of Cu₁₃(MBO)₁₂ (MBO = 2-mercaptobenzoxazole), which is composed of 13 Cu atoms and 12 MBO ligands.³²⁶ They reported that the valence of Cu atoms in Cu₁₃(MBO)₁₂ has an oxidation state between 0 and +1 and is highly resistant to oxidation. Cu₁₃(MBO)₁₂ produced 72.6% H₂, 13.2% CO, and 0.25% CH₄ as the main product at −1.04 V vs. RHE in CRR.

In 2022, Wang and Zang et al. evaluated the electrocatalytic activity of CRRs using three types of Cu₈ NCs with different cubic and tetrahedral core structures.²⁷³ For Cu₈(H)(L1)₆PF₆ with a cubic core structure and Cu₈(S^tBu)₄(L1)₄ and Cu₈(S^tBu)₄(L2)₄ with tetrahedral core structures, Cu₈(S^tBu)₄(L1)₄ showed the highest catalytic activity, selectivity, and stability for the conversion of CO₂ to HCOOH, showing an FE_HCOOH of 92% at −1.0 V vs. RHE. From DFT calculations, the difference in CRR catalytic activity was attributed to the cubic form of Cu₈(H)(L1)₆PF₆ having an active site favorable for the competitive reaction HER, whereas the tetrahedral form of Cu₈(S^tBu)₄(L1)₄ had an active site favorable for the formation of *COOH in CO formation and *OOCH in formic acid formation. This suggested that highly active and highly selective CRR catalysts can be created by controlling the core structure and surface morphology of Cu NCs.

Doping and ligand effects in Cu NCs have also been reported. In 2022, Ma, Song, and Wang et al. evaluated the CRR activity using [M@Cu₂₄H₂₂(PR₃)₁₂]⁺ (M = Au or Cu; PR₃ = PPh₃ or P(p-FPh)₃),²⁷⁵ which are protected by two phosphine ligands, as electrocatalysts.³²⁷ [M@Cu₂₄H₂₂(PR₃)₁₂]⁺ has a M@Cu₁₂ (M = Au or Cu) icosahedral kernel core, surrounded by a shell of four Cu₃P₃ units. The M@Cu₁₂ kernel is bonded to the Cu(I) shell via a Cu–Cu metal bond. These [M@Cu₂₄H₂₂(PR₃)₁₂]⁺ were loaded on multi-layered CNTs at 50 wt% to make CRR catalysts. From gas chromatography and ¹H-NMR spectroscopy, only CO, H₂, and HCOOH were observed as products. As shown in Fig. 32(a), [Au@Cu₂₄H₂₂(PPh₃)₁₂]⁺ exhibited the highest FE_CO of 45.6% at −1.0 V vs. RHE, whereas [Cu₂₅H₂₂(PPh₃)₁₂]⁺ mainly produced H₂ (FE_H2 > 80%). Differences in the results are due to Au doping, indicating that a single atom of Au doping can significantly change the selectivity of the CRRs. Moreover, the use of a different ligand, p-FPPh₃, decreased CO selectivity and increased HCOOH selectivity. As a result, for [Au@Cu₂₄H₂₂(P(p-FPh)₃)₁₂]⁺ and [Cu₂₅H₂₂(P(p-FPh)₃)₁₂]⁺, FE_HCOOH was 30.6% and 20.3%, respectively, more than three times higher than that of [M@Cu₂₄H₂₂(PPh₃)₁₂]⁺ (Fig. 32(a)). When P(p-FPh)₃ was used as the ligand, similarly to PPh₃, the Au doping of one atom tended to increase the selectivity of C₁ products and decrease the selectivity of H₂ (Fig. 32(b)). The FE_CO+HCOOH values at −0.8 V vs. RHE were 55.9%, 14.8%, 40.5%, and 20.3% for [Au@Cu₂₄H₂₂(PPh₃)₁₂]⁺, [Cu₂₅H₂₂(PPh₃)₁₂]⁺, [Au@Cu₂₄H₂₂(P(p-FPh)₃)₁₂]⁺ and [Cu₂₅H₂₂(P(p-FPh)₃)₁₂]⁺, respectively (Fig. 32(b)). These results indicate that the metal core has a significant effect on the selectivity of the two competing reactions (CRR and HER) and that the type of ligand changes the selectivity of the C₁ product of CRR (HCOOH and CO). The change in selectivity may be due to the lower density of electron clouds for Cu atoms on the NC surface, caused by the electrophilic Au doping and the electrophilic ligand, P(p-FPh)₃. This report revealed that Au doping can reduce the selectivity of the HER in electrochemical CRRs of Cu NCs containing hydride, and the use of electrophilic ligands not only enhances the stability of Cu NCs but also promotes the selectivity of HCOOH in the CRR. Such high selectivity for HCOOH was also predicted from DFT calculations using [Cu₂₅H₂₂(PH₃)₁₂]Cl.³²⁸ Furthermore, [Cu₂₆(CF₃CO₂)₈(CH₃O)₂(C≡C^tBu)₄(DPPE)₃H₁₁]⁺ is also reported to be a catalyst exhibiting high CO selectivity.³²⁹ Regarding the templated Cu NCs, [Cu₄Ti₉O₉(BC)₁₈(OⁱPr)₃(C≡C^tBu)(CH₃CN)] (BC = benzoic acid) exhibited high selectivity and good catalytic activity for the electrocatalytic reduction of CO₂ to C₂H₄ at 400 mA cm⁻² (FE_C2H4: 47.6 ± 3.4%).³³⁰

Fig. 32 Electrocatalytic performance of the four M@Cu₂₄ (M = Au/Cu) NCs in CRR. (a) [M@Cu₂₄H₂₂(PPh₃)₁₂]⁺ (M = Au or Cu) and (b) [M@Cu₂₄H₂₂(P(p-FPh)₃)₁₂]⁺ (M = Au or Cu). The error bars represent the standard deviation of three tests at the same test potential. (A) and (B) are reproduced with permission from ref. 327. Copyright 2022 Springer Nature.

From these reports, the CRR products of Cu NCs are often not only CO but also HCOOH, CH₄, and C2 compounds.

4. Conclusion

The conversion of CO₂ into useful compounds by electrochemical CRR is a promising strategy for achieving carbon neutrality by directly reducing CO₂ emissions. Unlike suppressing CO₂ emissions, electrochemical catalysis directly supports the sustainable development of an energy-independent society that is considerate of the global environment. Against this background, research on electrochemical CRR has been actively studied by various groups in recent years. However, to date, most studies have focused on metal NPs with particle sizes larger than 2–3 nm. Conversely, the number of reported cases of electrocatalysts using ∼1 nm metal NCs has been increasing since the 2010s and expanded rapidly around 2020 (Fig. 33). The previous studies summarized in this review have revealed the following insights into electrochemical CRRs using metal NCs (Fig. 34).

Fig. 33 Schematic of the synthesis methods of metal NCs using (A) liquid-phase reduction, (B) ligand-exchange-induced size/structure transformation (LEIST), (C) metal exchange, (D) anti-galvanic reaction, (E) metal-atom deposition, (F) molecular surgery.

Fig. 34 Schematic of the relationship between (A) metal species, (B) size, (C) ligands, (D) alloying effect for electrocatalytic CRR performance using metal NCs.

(1) Au, Ag, and Cu NCs can be synthesized as controlling the chemical composition and geometric electronic structure with atomic precision. In particular, there are many synthesis reports for Au and Ag NCs, and examples of synthesis for Cu NCs have also been reported. These can be synthesized with changing the size, geometric structure, type of ligand, and charge state (Fig. 1B).

(2) Au, Ag, and Cu NCs can be easily synthesized by a liquid-phase reduction method (Fig. 33A) in which a reducing agent is added to a solution containing metal salts and ligands.^{32,35,37,38,40,41} Furthermore, metal NCs with different compositions and structures can be synthesized by the ligand exchange method (Fig. 33B) using metal NCs as precursors.^39,42,331 Moreover, these alloy NCs can also be obtained by metal exchange method⁶ (Fig. 33C) or antigalvanic reaction⁸ (Fig. 33D) using specific metal NCs as precursors. It is also possible to add metal atoms (Fig. 33E) by reacting with metal salts under conditions different from metal exchange.³³² There are also reports of molecular surgery (Fig. 33F), which removes some surface Au atoms while maintaining the core structure of the precursor metal NCs.³³³ In these examples, it has become possible to synthesize metal NCs using various methods. However, most of the reported examples using various synthesis methods are limited to Au NCs.

(3) In Au and Ag NCs, the primary CRR product is CO in many cases, and some have high selectivity for the HER, which is a competitive reaction (Fig. 34A).

(4) In Cu NCs and Cu-doped Au and Ag NCs, the CRR products are often not only CO but also HCOOH, CH₄, and C₂ compounds (Fig. 34A).

(5) Decreases in the size of Au_n(SR)_m may promote increased CRR activity and selectivity (Fig. 34B). This is also the case with other electrocatalytic reaction systems such as HER, OER, and ORR.¹²⁰

(6) Even in metal NCs with the same chemical composition, differences in geometric structure can cause changes in CRR activity.

(7) The CRR selectivity changes significantly depending on the type of protective ligand bound to the surface of the metal NCs (Fig. 34C).

(8) Exposure of metal atoms on metal NCs due to desorption of protective ligands may promote increased CRR activity and selectivity. DFT calculations suggest that CRR activity is associated with the desorption of organic ligands in many cases, and such desorption is presumably caused by electrochemical pretreatment or adsorption process on carbon supports.

(9) Doping of Au_n(SR)_m using metal species, such as Ag and Pd, improves CRR selectivity in many cases (Fig. 34D). Such changes in activity and selectivity also depend on the doping position.

(10) Pd and Cd doping in Au₂₅(SR)₁₈ promote enhanced CRR activity, and Pt doping promotes enhanced HER activity in many cases (Fig. 34D).

(11) When the main CRR product is CO, the *COOH formation is the rate-determining step in many cases, and the lowering of the energy barrier is the factor of high activation.

5. Outlook

As summarized in section 4, further development of synthetic methods and evaluation of the activity of noble metal NCs may lead to highly active electrocatalysts that have not been discovered for NPs. In addition, the following initiatives are expected to be implemented in the future.

(1) Highly active and selective synthesis of useful compounds

In many reports, the main CRR products in Au, Ag, and Cu NCs is CO. CO can be used as a raw material for synthesis gas (mixture of CO and H₂), etc., but it may not economically be worth considering the cost of CO₂ capture because of the price competition with CO made from fossil fuels.⁷⁵ Therefore, the development of NCs catalysts that can synthesize other useful compounds (C1 compounds such as HCOOH and CH₃OH or C2–C3 compounds) is required.⁷⁵ Although some metal NCs have been reported that can be synthesized useful compounds from CO₂,^{245,254,273,325} their efficiency and selectivity is still low, and the creation of NCs catalysts with higher activity and selectivity is required. In particular, the bond distance of atoms in Metal NCs is considered to have a great effect on the making C–C coupling to obtain C2–C3 compounds. The effect of the geometric/electronic structure of Metal NCs on the selectivity of CRR products will be further clarified, and it is expected that Metal NCs with highly selective CRR catalyst will be designed and created to obtain useful compounds.

(2) Increased durability

Without proper treatment, ligand-protected metal NCs easily agglomerate due to ligand elimination.¹⁰⁶ Electrochemical reduction has also been reported to desorb ligands from metal NCs, but the detailed mechanism remains unclear.³⁰⁵ In addition, at present, in many cases, metal NCs are supported on a carbon black as a support having a high specific surface area, which prevents the aggregation of metal NCs even if some of the ligands are eliminated. Therefore, it has been reported that some metal NCs with stable structures have relatively high stability.^{188,202,245,255,263,295,314} However, in order to improve the durability of less stable metal NCs, it is necessary to protect them by ligands with strong bonds, load metal NCs on supports with high specific surface area and high conductivity.

(3) Use of various non-precious metal nanoclusters

Precious metals are expensive and rare, and the discovery of base metals that can be substituted in metal NCs is important because it directly leads to lower costs. The use of metal NCs composed of inexpensive metal elements, such as manganese (Mn), Ni, Fe, cobalt (Co), and Cu, is expected to develop further. It has been suggested that these metal NCs may also exhibit high CRR activity due to their different electronic states from the bulk.^334–336 Especially in CRRs, Cu is expected to be a unique catalyst for obtaining various organic compounds. However, reports of Cu NCs applied in CRRs are limited. In the future, the development of highly stable Cu NCs may lead to the practical application of CO₂ recycling using electrocatalytic CRRs.

(4) Control of geometric structure of loaded nanoclusters

The true geometric/electronic structure of metal NCs after adsorption on carbon supports and after ligand desorption remains unclear. Therefore, further advancing the structural analysis/analytical techniques for loaded NCs and elucidating the desorption mechanism of ligands are necessary. If the structure of the loaded NCs is clarified, theoretical calculations using this information may provide an understanding of the reaction mechanism and reveal the key factors for high activity.

(5) Development of support and binder for metal nanoclusters

Electrocatalytic activity is also greatly affected by the support and the ionomer (such as Nafion), which functions as the binder. In the previous reports, there are few examples examining these dependencies for metal NCs. The geometric/electronic structure of metal NCs is significantly different from that of metal NPs, and support interactions and stability are also expected to be different from those of metal NPs. Particularly, supports and binders with higher specific surface area and stronger binding to metal NCs should be developed for electrocatalytic applications. For example, metals NCs such as Au, Ag, and Cu have strong bonds with nonmetallic elements such as S, P (phosphorus), and N (nitrogen).³⁰⁵ Therefore, it is expected that the stability can be improved by using a support and a binder to which these elements are included.

(6) Accurate analysis of liquid-phase CO ₂ reduction reaction products

The improper evaluation of the liquid-phase components may lead to inaccurate assessments of the selectivity of the actual CRR products. For example, HCOOH produced by CRR is not identified by only analysis of gas-phase components. Furthermore, it might be present in the electrolytic solution as formate (HCOO⁻), and appropriate quantitative analysis methods are needed. Therefore, standardized analytical methods must be developed.

(7) Verification of the products derived from CO ₂

Many studies have not demonstrated that the CRR products are derived explicitly from the CO₂ that is being introduced. Therefore, it is necessary to show that the products are not derived from ligands desorbed from metal NCs or carbon supports (e.g., evaluation using the CO₂ isotope).

(8) Activity evaluation according to standards

The selectivity of CRR products varies greatly depending on the catalytic system, such as the structure of the CRR reaction cells (e.g., two-chamber cell or flow cell). Thus, standardized evaluation methods (using a standard electrolyte, Nafion membrane, counter electrode, carbon support type, etc.) are necessary for sharing and comparing data among researchers.

We hope that these issues will be overcome in the future and that efficient electrocatalysts can be developed for practical applications, which can potentially solve multiple energy and environmental problems.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Japan Society for the Promotion of Science (JSPS) KAKENHI (grant number 20H02698, 20H02552, 23H00289), Scientific Research on Innovative Areas “Innovations for Light-Energy Conversion” (grant number 18H05178 and 20H05115), Scientific Research on Innovative Areas “Hydrogenomics” (grant number 21H00027), and JST Adaptable and Seamless Technology transfer Program through Target-driven R&D (A-STEP; grant number JPMJTM20MS). Funding from Nissanken, the Yashima Environment Technology Foundation, the Yazaki Memorial Foundation for Science and Technology, the MIKIYA Science and Technology Foundation, the Kao Foundation for Arts and Sciences, the Sasakawa Scientific Research Grant from the Japan Science Society, Advanced Technology Institute Research Grants 2022, TEPCO Memorial Foundation Research Grant (Basic Research), Takahashi Industrial and Economic Research Foundation and the Kumagai Foundation for Science and Technology are gratefully acknowledged.

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