The role of metal accessibility on carbon dioxide electroreduction in atomically precise nanoclusters

Abstract

Atomically precise nanoclusters (NCs) can be designed with high faradaic efficiency for the electrochemical reduction of CO₂ to CO (FE_CO) and provide useful model systems for studying the metal-catalysed CO₂ reduction reaction (CO₂RR). While size-dependent trends are commonly evoked, the effect of NC size on catalytic activity is often convoluted by other factors such as changes to surface structure, ligand density, and electronic structure, which makes it challenging to establish rigorous structure–property relationships. Herein, we report a detailed investigation of a series of NCs [Au_nAg_46−n(C≡CR)₂₄Cl₄(PPh₃)₂, Au₂₄Ag₂₀(C≡CR)₂₄Cl₂, and Au₄₃(C≡CR)₂₀/Au₄₂Ag₁(C≡CR)₂₀] with similar sizes and core structures but different ligand packing densities to investigate how the number of accessible metal sites impacts CO₂RR activity and selectivity. We develop a simple method to determine the number of CO₂-accessible sites for a given NC then use this to probe relationships between surface accessibility and CO₂RR performance for atomically precise NC catalysts. Specifically, the NCs with the highest number of accessible metal sites [Au₄₃(C≡CR)₂₀ and Au₄₂Ag₁(C≡CR)₂₀] feature a FE_CO of >90% at –0.57 V vs. the reversible hydrogen electrode (RHE), while NCs with lower numbers of accessible metal sites have a reduced FE_CO. In addition, CO₂RR studies performed on other Au–alkynyl NCs that span a wider range of sizes further support the relationship between FE_CO and the number of accessible metal sites, regardless of NC size. This work establishes a generalizable approach to evaluating the potential of atomically precise NCs for electrocatalysis.

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Introduction

Heterogeneous catalysts composed of metal nanoparticles (NPs) dispersed on high-surface-area supports have been studied for more than a century,^1–3 and these catalysts are of increasing interest for the electrochemical reduction of CO₂ into chemical fuels and feedstocks.^4–6 Gold- and silver-based NPs are particularly effective for the selective reduction of CO₂ to CO.⁷ Though the effects of nanoparticle size,^8–10 shape,^11,12 and surface ligands^13,14 on the CO₂ reduction reaction (CO₂RR) have been widely studied, the nonuniformity of metal NP catalysts is a long-standing challenge in the investigation of fundamental catalytic mechanisms.^1,15 In particular, it is often difficult to identify the specific active sites that drive catalysis because of the wide distribution of local microenvironments in ligand-protected NPs that adopt varying sizes, shapes, and surface structures.^16,17 For example, although functionalization with larger organic ligands has been shown to enhance the CO₂RR activity of Au NPs,^18,19 uncertainty over the exact arrangement of surface ligands makes it difficult to determine how bulky ligands impact selectivity and catalytic activity. Such molecular-level insight is, however, possible when atomically precise nanoclusters (NCs) are used as catalysts, since their uniformity allows the entire particle structure—including the ligand shell—to be resolved crystallographically.^20–23

Soon after the canonical Au₂₅(SR)₁₈ (SR = aryl or alkylthiolate) NC was first reported, it was shown to be effective for CO₂RR, featuring a high faradaic efficiency for CO (FE_CO) at –1 V vs. the reversible hydrogen electrode (RHE).²⁴ Though the CO₂RR has since been studied for many other atomically precise Au NCs,^25–28 there still remains much to be understood about how NC size, structure, and surface ligand identity influence catalytic activity and selectivity. For instance, relationships between NC size and CO₂RR activity are challenging to identify because the ligand-to-metal ratio typically increases for smaller NCs,^29–31 resulting in higher surface coverage. Changes to the arrangement of surface ligands and metal atoms—as well as the electronic structure of the NC—may also affect the outcome of catalytic reactions, further convoluting structure–property relationships.³² Indeed, differing trends have been reported for how NC size affects CO₂RR activity. For example, in the series Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, and Au₁₄₄(SR)₆₀ (SR = SC₂H₄Ph), CO₂RR activity increases with increasing NC size,³³ while other studies have found that the FE_CO of Au-SR NCs is not directly affected by NC size.³⁴ Given the different size-dependent trends that have been observed for atomically precise NCs, the number of active sites is often a better predictor of catalytic behavior but is difficult to manipulate in a predictable fashion.

In an effort to decouple the role of metal active sites from NC size, structure, and ligand type, we designed a series of alkynyl-protected atomically precise Au/Ag NCs with similar sizes and core structures but different degrees of surface ligand coverage. We investigated the CO₂RR performance of these NCs and developed a convenient computational method to quantitatively evaluate the accessibility of potential catalytically active sites. Critically, the use of acetylene-based ligands—bearing one rotatable bond—simplifies the conformational landscape at the metal–ligand interface, thereby clarifying the effect of ligand modification on surface coverage and the number of accessible metal sites. In particular, we found that the number and accessibility of surface metal sites is directly correlated to experimental CO₂RR activity.

Results and discussion

Synthesis and characterization of alkynyl-protected Au/Ag NCs

We recently reported the isostructural alkynyl-protected NCs Au₄₃(C≡C^tBu)₂₀ (Au₄₃) and Au₄₂Ag₁(C≡C^tBu)₂₀ (Au₄₂Ag₁), which are synthesized by reducing an oligomeric Au^I–C≡C^tBu or Au^I/Ag^I–C≡C^tBu precursor with borane tert-butylamine then purifying via thin layer chromatography (Fig. S1A†).³⁵ With a nearly identical NC core but an increased density of alkynyl surface ligands, we also selected the previously reported Au₂₄Ag₂₀(C≡CPh^tBu)₂₄Cl₂ NC (Au₂₄Ag₂₀) for comparison (Fig. S1B†).^36,37 To complete a series of NCs with similar sizes and varying surface ligand densities, we also targeted an Au/Ag–alkynyl NC with an even denser organic shell. This was achieved by introducing a bulky triphenylphosphine (PPh₃) ligand through the “hydride-mediated conversion” method.^38,39 Specifically, [Au₉(PPh₃)₈]³⁺ (ref. 40) was reduced with NaBH₄ to furnish a hydride-doped [HAu₉(PPh₃)₈]²⁺ cluster, which was then reacted with CH₃COOAg, meta-substituted phenylacetylene ligands, and triethylamine to yield NCs with a composition of Au_nAg_46−n(C≡CPh–m–X)₂₄Cl₄(PPh₃)₂ (Au_nAg_46−n, n = 16–19, X = H, F, CH₃) (Fig. 1, see ESI† for experimental details). Though attempts to grow single crystals of Au_nAg_46−n(C≡CPh)₂₄Cl₄(PPh₃)₂ suitable for structure determination were unsuccessful, crystal structures were successfully determined for Au_nAg_46−n with X = F and CH₃ (see Fig. 2). The successful crystallization of Au_nAg_46−n with meta-substituted phenylacetylene ligands can be attributed to the additional interparticle C–H⋯π, or C–F⋯π interactions between the meta functional groups on one NC and the phenyl rings of ligands on another NC (Fig. S2 and S3†).^41,42

Fig. 1 (A) UV-vis-NIR absorption spectra and (B) ESI-MS of Au_nAg_46−n(C≡CPh–m–X)₂₄Cl₄(PPh₃)₂ (n = 16–19, X = H, F, or CH₃).

Fig. 2 Structures of (A) Au_nAg_46−n(C≡CPh–m–X)₂₄Cl₄(PPh₃)₂ (n = 16–19, X = F or CH₃), (B) Au₂₄Ag₂₀(C≡CPh^tBu)₂₄Cl₂, and (C) Au₄₃(C≡C^tBu)₂₀ and Au₄₂Ag₁(C≡C^tBu)₂₀, Color code: magenta = Au; violet = Ag; light pink = Au/Ag; orange = P; green = Cl; light green = F; grey = C; white = H. The vacant surface sites in Au₄₃(C≡C^tBu)₂₀ and Au₄₂Ag₁(C≡C^tBu)₂₀ are indicated with a dashed circle.

The solution-phase UV-vis absorption spectra of all three Au_nAg_46−n NCs with different protecting ligands exhibit sharp absorption peaks centered near 400 and 690 nm (Fig. 1A). Moreover, electrospray ionization mass spectrometry (ESI-MS) analysis shows 2+ and 1+ ion peaks for the NCs (Fig. 1B). Combined with the X-ray crystallography data, the 2+ and 1+ charges are attributed to ionization in ESI-MS—not the native charge states of the NCs—and the peaks correspond to NCs that have lost one or two PPh₃ ligands, which has been commonly observed for similar atomically precise NCs (Fig. S4–S6†).^43,44 The ESI-MS spectrum of Au_nAg_46−n(C≡CPh–m–F)₂₄Cl₄(PPh₃)₂ contains peaks corresponding to n = 16, 17, and 18 (Fig. S5†), consistent with the crystallographically refined composition of Au_17.67Ag_28.33(C≡CPh–m–F)₂₄Cl₄(PPh₃)₂ (Tables S1 and S2†). When HC≡CPh–m–CH₃ was used instead, three peaks corresponding to n = 17, 18, and 19 were found, which is consistent with the higher refined Au : Ag ratio in Au₁₉Ag₂₇(C≡CPh–m–CH₃)₂₄Cl₄(PPh₃)₂ (Tables S3 and S4†). The small inconsistency between MS and X-ray crystallography data has also been observed for other heterometallic NCs, which are known to be dynamic—and prone to rearrangement—in solution.^45,46 Regardless, the similar UV-vis and ESI-MS spectra for Au_nAg_46−n(C≡CPh)₂₄Cl₄(PPh₃)₂ and Au_nAg_46−n(C≡CPh–m–X)₂₄Cl₄(PPh₃)₂ (X = F, CH₃) NCs—along with the fact that the same synthesis conditions were used—suggest that the three NCs are isostructural. Note that small differences in absorption features near 400 nm and shifts in MS can be attributed to the different alkynyl ligands used.

The series of five atomically precise Au/Ag NCs exhibit similar core structures. Specifically, Au_nAg_46−n has an icosahedral Au_nAg_12−n kernel (n = 4–7 with Au and Ag randomly distributed), a dodecahedral Ag₂₀ inner shell, and an icosahedral Au₁₂ outer shell with two additional Ag atoms on the surface (Fig. 2A). Each of the additional Ag atoms on the surface of Au_nAg_46−n is bonded to two chloride ligands, one Au atom in the outer shell, and one PPh₃ ligand. Four chloride ligands are necessary for Au_nAg_46−n to adopt a closed-shell superatomic electronic configuration (46 − 24 − 4 = 18 e^–) similar to Au₂₄Ag₂₀ with two chloride ligands (24 + 20 − 24 − 2 = 18 e^–). The structure of Au₂₄Ag₂₀ also consists of an icosahedral Au₁₂ kernel, a dodecahedral Ag₂₀ inner shell, and an icosahedral Au₁₂ outer shell (Fig. 2B), while Au₄₃/Au₄₂Ag₁ has an icosahedral Au₁₂ kernel, a dodecahedral Au₂₀ inner shell, and an incomplete icosahedral Au₁₁ or Au₁₀Ag₁ outer shell with a single vacant surface site (Fig. 2C).

Adding or removing a single metal atom to or from the surface of NCs has provided insight into the optical and electronic properties of thiolate-capped metal NCs.^47,48 The successful synthesis of our series of NCs with the same core (M₄₆, M₄₄, and M₄₃; M = Au and/or Ag) represents—to the best of our knowledge— the first demonstration of atom-by-atom evolution for alkynyl-stabilized metal NCs structure (Fig. 2). Moreover, the linear directionality of alkynyl ligands in alkynyl-stabilized NCs offer advantages for catalytic studies since the NC/electrolyte interface is dominated by Au–C≡C bonds with similar local arrangements. Note that for Au_nAg_46−n and Au₂₄Ag₂₀, Ag atoms are located in the inner dodecahedral Ag₁₂ shell and/or the icosahedral Au_nAg_12−n kernel which are not accessible to substrates interacting with the surface of the NC (Fig. 2, highlighted by the dashed red box). Importantly, the surface ligand density varies systematically across the series: 24 alkynyl, 4 chloride, and 2 phosphine ligands for Au_nAg_46−n, 24 alkynyl and 2 chloride ligands for Au₂₄Ag₂₀, and only 20 alkynyl ligands for Au₄₃ and Au₄₂Ag₁. Thus, this series of atomically precise NCs provides a powerful platform to study relationships between metal site accessibility and CO₂RR activity.

Evaluation of CO₂RR activity and selectivity

To investigate their efficacy for CO₂RR catalysis, synthesized atomically precise NCs were mixed with carbon black (20 wt% NC loading) and deposited on a carbon paper electrode. For comparison, a carbon electrode was also prepared with spherical Au–SC₂H₄Ph NPs (Au–S NPs) with an average diameter of 3.1 ± 0.4 nm (Fig. S1C†) that were mixed with carbon black at the same mass loading. To provide an additional comparison, an 85 nm thick bulk gold layer was also deposited on one carbon paper electrode (referenced as Au layer) by electron beam deposition. Linear sweep voltammetry (LSV) was then performed for each electrode in CO₂-saturated 0.5 M KHCO₃ solution. For the atomically precise NCs, the current density (j_total) was found to increase in the order of Au_nAg_46−n < Au₂₄Ag₂₀ < Au₄₂Ag₁ < Au₄₃ (Fig. 3A). Larger sized Au–S NPs exhibited a lower j_total than all atomically precise NCs, and j_total for the Au layer was the lowest of all catalysts investigated here (Fig. 3A). We note that the NC catalysts were not activated before electrocatalysis to minimize possible ligand stripping, and the LSV curves taken before and after chronoamperometric CO₂RR catalysis (potential range of –0.47 V to –0.77 V vs. RHE) were in close agreement (Fig. S7†). Moreover, the NC-based catalysts display a steady current density over at least 40 min at each applied voltage (–0.47 V, –0.57 V, –0.67 V and –0.77 V vs. RHE; Fig. 3C), and the absorption spectra of NCs recovered from the electrode after catalysis matched those of as-synthesized NCs (Fig. S8†), confirming the stability of the NCs during CO₂RR catalysis.

Fig. 3 (A) LSV and (B) FE_CO for CO₂RR for the series of NC-based catalysts. (C) Chronoamperometry data for the Au₄₃-based catalyst at different applied potentials. (D) j_CO and (E) TOF_CO for the NC-based catalysts at different applied potentials during CO₂RR. (F) FE_CO for the NC-based catalysts at –0.57 V vs. RHE during CO₂RR over an extended time period. All experiments were conducted in a 0.5 M KHCO₃ solution saturated with CO₂. In panels A, B, D, E, and F, magenta represents Au₄₃(C≡C^tBu)₂₀, purple represents Au₄₂Ag₁(C≡C^tBu)₂₀, indigo represents Au₂₄Ag₂₀(C≡CPh^tBu)₂₄Cl₂, blue represents Au_nAg_46−n(C≡CPh)₂₄Cl₄(PPh₃)₂, sky blue represents Au-SC₂H₄Ph NPs, and cyan represents the Au layer.

For all atomically precise NC catalysts evaluated here, CO was the major CO₂RR product, and H₂ was the sole byproduct with no liquid products detected by ¹H NMR spectroscopy. The faradaic efficiency for CO production (FE_CO) was assessed for each catalyst by calculating the percentage of transferred charge that was directed toward CO production (see ESI† for details), and the highest FE_CO were 92.1 ± 1.7% and 90.9 ± 1.4% for Au₄₃ and Au₄₂Ag₁, respectively, at a potential of −0.57 vs. RHE (Fig. 3B). At the same potential, FE_CO for Au₂₄Ag₂₀ and Au_nAg_46−n were just 72.9 ± 1.0% and 59.5 ± 2.5%, respectively (Fig. 3B). These lower efficiencies can be attributed to more densely packed surface ligands, which likely favors the hydrogen evolution reaction (HER) over CO₂RR.^25,26,49 The Au–S NPs have an even lower FE_CO (38.7 ± 0.8%) at the same potential, which is consistent with previously reported studies.^27,33 If FE_CO is related to the density of surface ligands, one might assume that a ligand-free Au layer would have the highest efficiency. However, the FE_CO for the Au layers is only 20.4 ± 1.6% at –0.57 V vs. RHE, highlighting the important role of the microenvironments created by nanostructured catalysts in driving CO₂RR.^3,50,51

To further evaluate catalytic performance, CO partial current densities (j_CO) were determined and compared across the NC series. Generally, j_CO decreases with increasing ligand density on the surface (Fig. 3D): Au₄₃/Ag₄₂Ag₁ (20 surface ligands) > Au₂₄Ag₂₀ (26 surface ligands) > Au_nAg_46−n (30 surface ligands). For Au₄₃, j_CO is slightly higher than for Au₄₂Ag₁, suggesting that substitution of a single surface Au atom for Ag leads to a small decrease in CO₂RR performance, particularly at more negative potentials. Furthermore, Au₄₃ shows a high CO turnover frequency (TOF_CO) of 4718 h⁻¹ at −0.57 V and 15 193 h⁻¹ at −0.77 V vs. RHE (Fig. 3E), which exceeds the values for Au₂₄Ag₂₀ (2597 h⁻¹ at −0.57 V and 10 658 h⁻¹ at −0.77 V) and Au_nAg_46−n (1427 h⁻¹ at −0.57 V and 6400 h⁻¹ at −0.77 V). Note that the same mass loading of NCs was used for all experiments. This makes it reasonable to directly compare TOF values since the NCs in this series have similar molecular weights (Table S5†). In addition, differences in catalytic activity cannot be attributed to differences in NC stability as the FE_CO of all catalysts remained constant for at least 2.5 hours at −0.57 V (Fig. 3F).

While the nature of active sites is regarded as one of the best predictors of catalytic activity,⁵² it is often challenging to experimentally determine the number of catalytically active sites in a nanostructured material, and theoretical models are required.^53,54 Though double-layer capacitance measurements can be used to determine the electrochemically active surface area (ECSA) of catalysts, the ECSA might not reflect the surface area active specifically for CO₂RR since CO₂RR and HER frequently occur simultaneously. For example, previous studies have shown that Au₂₅(SR)₁₈ and Au₃₈(SR)₂₄ exhibit different CO₂RR behavior even though the NCs have almost the same ECSA.^33,49 The ligand-to-metal ratio can serve as a proxy for active site density when the sizes of NCs are similar,⁵⁵ but this ratio is not directly related to the number of active sites owing to the different shapes and surface structures that similarly sized NCs can adopt. Indeed, two isomeric Au₃₈(SR)₂₄ NCs with the same ligand-to-metal ratio have shown significant differences in CO₂RR catalysis.³⁴

With these challenges in mind, we sought to establish a simple method for determining the number of metal sites accessible to CO₂ in atomically precise NCs that does not rely on computationally intensive density functional theory (DFT) calculations. Briefly, the NC structure determined by crystallography is used to generate a series of several thousand conformers accounting for the different ligand conformations that may arise due to ligand rotation in the absence of crystal packing effects (see ESI† for details). The accessible surface area of each conformation within the conformer series was then calculated for every surface atom using a 1.65 Å spherical probe (the kinetic radius of CO₂, Fig. S9†).⁵⁶ Metal atoms with a positive contact area with CO₂ (Fig. S10†) were counted as accessible since these atoms have sufficient space to accommodate a covalent bond with CO₂. Accessible metal atoms for the different alkynyl-protected NCs under investigation are highlighted in Fig. 4. Note that the NC conformation with the greatest number of accessible metal atoms (N) was used to represent the surface accessibility of each NC. Using this approach, the number of accessible metal atoms for each NC can be calculated in ∼1 hour. Notably, the number of accessible metal atoms, N, for Au₄₃/Au₄₂Ag₁, Au₂₄Ag₂₀, and Au_nAg_46−n are 16, 12, and 5, respectively, which is consistent with a greater density of CO₂-accessible surface metal sites driving increased CO₂RR activity. The role of CO₂-accessible metal sites is further supported by the fact that we observe little variation in TOF_CO when it is normalized to the number of accessible metal sites on each NC (TOF_CO/N). Specifically, TOF_CO/N for Au₄₃, Au₄₂Ag₁, Au₂₄Ag₂₀, and Au_nAg_46−n is 294 h⁻¹, 279 h⁻¹, 216 h⁻¹ and 285 h⁻¹, respectively, at –0.57 V vs. RHE (Table 1 and Fig. S11†). This suggests that the number of CO₂-accessible metal sites—rather than the degree of Ag doping,⁵⁷ surface ligand functional groups,⁵⁸ or the electronic structure⁵⁹ of the cluster—is the primary driver of catalytic activity, at least for NCs with relatively similar structures and compositions.

Fig. 4 The metal sites accessible to CO₂ on the surface of different Au/Ag-alkynyl NCs.

Table 1 The calculated number of accessible metal sites (N) for a series of alkynyl-protected NCs, along with their experimentally determined FE_CO, j_CO, TOF_CO, and TOF_CO/N values for CO₂RR electrocatalysis at −0.57 V vs. RHE. All experiments were conducted in a 0.5 M KHCO₃ solution saturated with CO₂

Catalyst	Number of accessible metals (N)	FECO (%)	j CO (mA cm^−2)	TOFCO (h^−1)	TOFCO/N (h^−1)
Au43	16	92.1 ± 1.7	2.5	4718	295
Au42Ag1	16	90.9 ± 1.4	2.4	4458	279
Au24Ag20	12	72.9 ± 1.0	1.3	2597	216
AunAg46−n	5	59.5 ± 2.5	0.8	1427	285
Au44	12	74.5 ± 1.3	1.0	2045	170
Au36	9	69.7 ± 1.6	0.7	1325	147
Au23	6	66.7 ± 2.5	0.7	794	132

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To further investigate the generalizability of accessible metal site number as a predictor of CO₂RR activity, we also evaluated a wider range of previously reported alkynyl-protected Au NCs: Au₂₃(C≡C^tBu)₁₅ (Au₂₃),⁶⁰ Au₃₆(C≡CPh)₂₄ (Au₃₆), and Au₄₄(C≡CPh)₂₈ (Au₄₄)⁶¹ (Fig. S12†). Within this series, Au₄₄ has the highest number of CO₂-accessible metal sites (N = 12) and the highest CO₂RR activity, while Au₂₃ has the lowest number of CO₂-accessible sites and the lowest CO₂RR activity (Fig. 5). The activity of Au₄₄, however, is much lower than that of Au₄₃, which is consistent with the greater number of CO₂-accessible metal sites (N = 16) for the latter NC (Table 1 and Fig. S13†). The relationship between the number of accessible metal sites and FE_CO, j_CO is plotted in Fig. S14.† This highlights that even though larger sized Au–alkynyl NCs often have increased CO₂RR activities, just like their Au–thiolate counterparts,³³ the number of accessible surface metals tends to be more closely related to catalyst performance. Therefore, we conclude that the number of accessible metal sites provides a useful metric for evaluating the likelihood of CO₂ binding to a particular atomically precise NC and for predicting trends in catalytic activity. Since Au₄₃ and Au₄₂Ag₁ NCs are isostructural, their slight difference in CO₂RR activity could be due to replacing a surface Au atom with a more electropositive Ag atom.³⁵ Though Au₂₄Ag₂₀ and Au₄₄ NCs have the same number of metal atoms (44) as well as the number of accessible metals (12), Au₄₄ exhibits a slightly higher FE_CO and lower j_CO than Au₂₄Ag₂₀. Differences in the geometric and electronic structures of the NCs may influence their CO₂RR performance, but this is likely a less significant effect than the number of accessible metals. For NCs of the same core structure, more valence electrons in the frontier molecular orbitals elevate the energy of the highest occupied molecular orbital (HOMO), thereby favoring electron transfer from the NC catalyst to the substrate and thus improving the CO₂RR activity.⁵⁹ The performance of Au₄₃ and Au₄₂Ag₁ NCs could also be partially attributed to more facile electron transfer during electrocatalysis. However, since the electronic structure of the NCs is determined by the number of metals and ligands, the surface coverage is still of importance. Moreover, since calculating the number of accessible metal sites is straightforward, it can serve as a quick screening tool for identifying the most promising NCs for electrochemical catalysis that is complementary to advanced DFT calculations.

Fig. 5 (A/D) LSV, (B/E) FE_CO at different applied potentials (left), or at –0.57 V vs. RHE for prolonged time (right), and (C/F) j_CO at different applied potentials during CO₂RR for NC-based catalysts conducted in CO₂-saturated 0.5 M KHCO₃ solution. Magenta represents Au₄₃(C≡C^tBu)₂₀, pink represents Au₂₃(C≡C^tBu)₂₀, orange represents Au₄₄(C≡CPh)₂₈, and yellow represents Au₃₆(C≡CPh)₂₄.

Conclusions

A series of alkynyl-stabilized NCs with similar sizes and core structures but different degrees of surface ligand coverage was used to provide insight into the effect of the number of accessible metal sites on electrochemical CO₂RR activity. A simple computational method was developed to calculate the number of metal sites on the NCs that are accessible to CO₂. The highest faradaic efficiencies for CO₂RR were observed for Au₄₃ and Au₄₂Ag₁, which feature the largest number of accessible metal sites. When the TOF_CO of the NC-based catalysts was normalized by the number of accessible sites, the differences between NCs were reduced. Collectively, these trends suggest that the number of substrate-accessible metal sites serves as a useful and generalizable predictor for evaluating the potential of atomically precise NCs for CO₂RR.

Data availability

All the data are shown in the manuscript or the associated ESI.†

Author contributions

J. A. M and Y. L. conceived the project. Y. L. synthesized the NCs and grew the single crystals. Y. L. and A. E. T. performed the electrochemical studies and analysed the data. G. J. S performed the computational studies. R. D. M. and S.-L. Z. solved the crystal structures. Y. L and G. J. S wrote the manuscript with contribution from other authors.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We acknowledge support from the Major Research Instrumentation (MRI) Program of the National Science Foundation under NSF award no. 2216066 for the X-ray facility. This research was partially supported by the Arnold and Mabel Beckman Foundation through a Beckman Young Investigator grant awarded to J. A. M.

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Footnotes

† Electronic supplementary information (ESI) available. CCDC 2260158 and 2260159. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d3sc04085b

‡ These authors contributed equally to this work.

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