Atomically precise alkynyl-protected Ag₁₉Cu₂ nanoclusters: synthesis, structure analysis, and electrocatalytic CO₂ reduction application

Abstract

We report the synthesis, structure analysis, and electrocatalytic CO₂ reduction application of Ag₁₉Cu₂(C≡CAr^F)₁₂(PPh₃)₆Cl₆ (abbreviated as Ag₁₉Cu₂, C≡CAr^F: 3,5-bis(trifluoromethyl)phenylacetylene) nanoclusters. Ag₁₉Cu₂ has characteristic absorbance features and is a superatomic cluster with 2 free valence electrons. Single-crystal X-ray diffraction (SC-XRD) revealed that the metal core of Ag₁₉Cu₂ is composed of an Ag₁₁Cu₂ icosahedron connected by two Ag₄ tetrahedra at the two terminals of the Cu–Ag–Cu axis. Notably, Ag₁₉Cu₂ exhibited excellent catalytic performance in the electrochemical CO₂ reduction reaction (eCO₂RR), manifested by a high CO faradaic efficiency of 95.26% and a large CO current density of 257.2 mA cm⁻² at −1.3 V. In addition. Ag₁₉Cu₂ showed robust long-term stability, with no significant drop in current density and FE_CO after 14 h of continuous operation. Density functional theory (DFT) calculations disclosed that the high selectivity of Ag₁₉Cu₂ for CO in the eCO₂RR process is due to the shedding of the –C≡CAr^F ligand from the Ag atom at the very center of the Ag₄ unit, exposing the active site. This study enriches the potpourri of alkynyl-protected bimetallic nanoclusters and also highlights the great advantages of using atomically precise metal nanoclusters to probe the atomic-level structure–performance relationship in the catalytic field.

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1. Introduction

Thanks to the alloying effect, bimetallic nanoclusters usually display different physicochemical properties and functionalities from their monometallic counterparts.^1–5 Specifically, in the catalytic field, upon introducing another metal atom into the core, on the one hand, the geometric configuration of the metal nanocluster is changed and new catalytically active sites can be generated, and on the other hand, it can modulate the electronic structure of the metal nanocluster hence tuning the adsorption capacities to the reaction substrates and intermediates. For instance, the Jiang and Lee group reported that the Au₂₄Pt(SC₆H₁₃)₁₈ cluster possesses much higher hydrogen evolution reaction (HER) activity than Au₂₅(SC₆H₁₃)₁₈, as the central Pt atom can more easily form an M–H chemical bond to lower the energy barrier.⁶ In another study, Deng et al. found that Cu doping can significantly boost the electrochemical CO₂ reduction reaction (eCO₂RR) of atomically precise Au nanoclusters. In a gas diffusion electrode-based membrane electrode assembly cell, the Au₁₅Cu₄ nanocluster exhibited a high CO faradaic efficiency (over 90%), markedly larger than that of the Au₁₈ cluster (∼60%). Density functional theory (DFT) calculations disclosed that Cu doping induced catalytic synergistic effects, where the exposed pair of AuCu dual sites can accelerate the eCO₂RR process.⁷ As seen above, atomically precise bimetallic nanoclusters have good catalytic activity and selectivity for the eCO₂RR, and more importantly, the atomically precise structure of bimetallic nanoclusters is conducive to the in-depth analysis of the reaction mechanism and the elucidation of the catalyst's structure–performance relationship at the atomic level.

To prevent the bimetallic core from aggregation, there are various types of small molecules that can be used as capping agents or ligands for preparing bimetallic nanoclusters.⁸ The ligand molecules include but are not limited to thiolate molecules,⁹ N-containing molecules,¹⁰ carbenes,¹¹ hydrides,¹² alkynyl molecules,^13–15 and so on. The ligand molecules can not only tune the physical and chemical properties of the metal nanoclusters,¹⁶ but also affect the catalytic performance drastically.^17–19 The most widely employed ligand is thiolate molecules; however, alkynyl molecules as a protecting ligand have been gaining more and more research attention.^13–15 In a quite recent study, the Wang and Zhou groups reported the photophysics of Au₂₂(^tBuPhC≡C)₁₈ and Au₁₆Cu₆(^tBuPhC≡C)₁₈, where the latter exhibited >99% photoluminescence quantum yield (PLQY) in de-aerated solution at room temperature with an emission maximum at 720 nm tailing to 950 nm and 61% PLQY in an oxygen-saturated solution.²⁰ This is due to the fact that the Cu doping suppressed the non-radiative decay (∼60-fold less) and promoted the intersystem crossing rate. Moreover, in the catalytic regime, alkynyl ligands can perturb the electronic structure of metal nanoclusters by forming σ and/or π bonds, thereby promoting the catalytic performance of some specific reactions.²⁰ In 2017, the Wang group fabricated two isostructural [Au₃₈L₂₀(Ph₃P)₄]²⁺ (L = alkynyl or thiolate) clusters with the ligand being the only variable.²¹ The alkynyl-protected Au₃₈ cluster is very active in the semi-hydrogenation of alkynes with the substrate conversion ratio over 97% while the thiolate one possesses negligible activity (less than 2%). In another study, our group disclosed that, in the electrochemical CO₂ reduction reaction (eCO₂RR), alkynyl-protected Ag₃₂ clusters can outcompete the phosphine and thiolate co-protected Ag₃₂ clusters in terms of FE_CO, as the former has a much lower energy barrier for forming the key intermediate *COOH.²² Similar phenomena have also been documented for alkynyl-protected Au₂₅ nanoclusters to catalyze the HER,²³ and alkynyl-protected Au₂₈ nanoclusters supported by NiFe-LDH for catalyzing the oxygen evolution reaction.²⁴

So far, significant progress has been achieved on the synthesis, electronic properties, luminescent behaviours, and catalytic applications of alkynyl-protected bimetallic nanoclusters, yet the family or potpourri of alkynyl-protected bimetallic nanoclusters is still limited. For AgCu bimetallic nanoclusters, molecules with atomically precise structures and alkynyl protection only include Ag₉Cu₆,²⁵ Ag₂₀Cu₁₂,²⁶ Ag₁₅Cu₆,²⁷ Ag₂₂Cu₇,²⁸ Ag₁₄Cu₂,²⁹ and so on. Inspired by the above studies, herein, we report the synthesis, structural analysis, and eCO₂RR application of Ag₁₉Cu₂(C≡CAr^F)₁₂(PPh₃)₆Cl₆ nanoclusters (hereafter referred to as Ag₁₉Cu₂, C≡CAr^F: 3,5-bis(trifluoromethyl)phenylacetylene). Ag₁₉Cu₂ has characteristic absorbance features, and its overall structure was analyzed by SC-XRD. Ag₁₉Cu₂ exhibited excellent catalytic performance, evidenced by a high CO faradaic efficiency (FE_CO) of 95.26%, a high current density of 257.2 mA cm⁻² at −1.3 V, and robust stability with no significant current decay during 14 h of continuous operation.

2. Experimental section

See all the experimental details in the ESI.†

3. Results and discussion

The UV-visible absorption spectrum of Ag₁₉Cu₂ is shown in Fig. 1a, and two characteristic absorbance peaks at 498 nm and 703 nm can be easily identified. The overall structure of Ag₁₉Cu₂ was then analyzed by single-crystal X-ray diffraction (Fig. 1b). Specifically, the Ag₁₉Cu₂ nanocluster is positively charged, with BF₄⁻ as the counter ion. Ag₁₉Cu₂ crystallizes in the P1̄(2) space group. Ag₁₉Cu₂ has 21 metal atoms in the core, which is protected by twelve –C≡C–Ar^F ligands, six PPh₃ molecules and six chlorine ions. The calculated free valence electron number is 2, indicating that it is a superatomic cluster with a closed-shell electronic structure. In fact, after being stored at room temperature for 3 months, the absorbance features of the Ag₁₉Cu₂ clusters remained almost unchanged (Fig. S5†), confirming the excellent chemical stability of Ag₁₉Cu₂ under ambient conditions.

Fig. 1 (a) Absorbance spectra of the Ag₁₉Cu₂ nanocluster in CH₂Cl₂. (b) The overall structure of Ag₁₉Cu₂. Color legend: red, Cu; blue, Ag; green, Cl; purple, P; laurel green, F; white, H; gray, C.

Subsequently, the metal core configuration of Ag₁₉Cu₂ was detailedly examined. As illustrated in Fig. 2a, Ag₁₁Cu₂ forms an icosahedron with two Cu atoms on two diagonal points, and two Ag₄ tetrahedra are connected with two faces of the two opposite sides in the Ag₁₁Cu₂ icosahedron to form the Ag₁₉Cu₂ core. Apparently, the Ag₁₉Cu₂ core is centrosymmetric. Interestingly, the average Ag–Ag bonding length in the Ag₁₁Cu₂ kernel is 2.9300 Å, slightly longer than that in the Ag₄ tetrahedra (see Fig. S2†). Next, the metal–ligand coordination mode between Ag/Cu and the alkynyl molecule was analyzed and is summarized in Fig. 2b. Basically, there are four types of coordination modes: Motif A, B, C, and D. In Motif A, the alkynyl molecule binds with three Ag atoms with σ bonding and one Ag atom with both σ and π bonding; hence Motif A adopts a μ4–η1, η1, η1, η2 coordination mode. In Motif B, the alkynyl molecule binds with four Ag atoms with σ bonding; hence the coordination mode is μ4–η1, η1, η1, η1. In Motif C, the alkynyl molecule binds with two Ag atoms with σ bonding and one Cu atom with both σ and π bonding; hence Motif C adopts a μ3–η1, η1, η2 coordination mode. Motif D has a similar coordination pattern to Motif C, and the alkynyl molecule binding with both σ and π bonding is one Ag atom in Motif D but one Cu atom in Motif C; therefore, Motif D adopts a μ3–η1, η2, η3 coordination mode. There are twelve alkynyl molecules capping the metal core, and the total number of Motif A, B, C, and D is 4, 4, 3, and 3, respectively. In addition, six PPh₃ ligands are coordinated with six Ag atoms on two sides in the cluster (Fig. S3†), and six Cl atoms are also coordinated with Ag₁₁Cu₂ by σ bonds in the cluster (Fig. S4†). Four Cl atoms are connected to three Ag atoms and two Cl atoms are connected to one Cu atom and two Ag atoms. All the detailed structure parameters are summarized in Table S1.†

Fig. 2 (a) The structural anatomy of the Ag₁₉Cu₂ core. (b) Four different coordination modes between Ag/Cu and one alkynyl molecule. Color legend: red, Cu; blue, Ag; green, Cl; purple, P; laurel green, F; white, H; gray, C.

The elemental composition and charge states of Ag₁₉Cu₂ were subsequently analyzed using X-ray photoelectron spectroscopy (XPS). The survey scan spectra shown in Fig. S6a† confirmed the presence of Ag, Cu, C, F, and P atoms. The high resolution XPS spectra of the Ag 3d and Cu 2p electrons in Ag₁₉Cu₂ can be found in Fig. S6b and S6c,† respectively. Notably, the binding energy of the Ag 3d_5/2 electrons is 368.7 eV, suggesting that the valence state of Ag is between (0) and (I). Such a value is also in good agreement with the previously reported Ag₁₅Cu₆ ²⁷ and Ag₂₀Cu₁₂ ²⁶ clusters, whose Ag 3d_5/2 binding energies are 368.1 eV and 368.84 eV, respectively. Meanwhile, the binding energy of the Cu 2p_3/2 electrons is 933.34 eV, indicating that the valence state of Cu is (0, 1). The XPS results further validate that the superatomic cluster of Ag₁₉Cu₂ is successfully obtained.

The Ag₁₉Cu₂ nanoclusters were then loaded onto carbon nanotubes (CNTs) to investigate the eCO₂RR catalytic performance. As shown in Fig. 3a, in N₂ saturated 1 M KOH, both the CNTs and Ag₁₉Cu₂/CNTs exhibited no activity, while in CO₂ saturated 1 M KOH, both displayed effective activity. At the same applied potential, Ag₁₉Cu₂/CNTs has a much higher current density than CNTs, indicating superior catalytic activity. In the eCO₂RR, the main product is CO, along with H₂ as the side product. Faradaic efficiency (FE) is defined as the amount of product collected relative to the amount that can be produced by the total charge passing through, expressed as a fraction or percentage. In electrochemical reactions, a higher FE means that the actual power of the reaction is closer to the theoretical power and the efficiency of the reaction is higher. The FE_CO for Ag₁₉Cu₂/CNTs and CNTs is presented in Fig. 3b. The FE_CO for CNTs is nearly zero in the whole tested potential window, but for Ag₁₉Cu₂/CNTs, when the applied potential becomes more negative, the FE_CO first increases and then decreases, and the highest FE_CO is achieved at −1.37 V with a value of 95.26%. When the potential becomes quite negative, the hydrogen evolution reaction starts to dominate, which is the main reason why FE_CO decreases. Meanwhile, Ag₁₉Cu₂/CNTs displays a strong capability to suppress hydrogen evolution, as the FE_H2 is below 5% in the whole potential window. However, the HER process dominates the reaction for CNTs, as the FE_H2 increases from ∼51% to nearly 100% (Fig. 3c). To further evaluate the catalytic performance of the Ag₁₉Cu₂/CNT catalyst, we also calculated the energy efficiency (EE), which is defined as the conversion efficiency of the chemical energy from the input electrical energy to the final product, and a higher EE means a higher yield of the reaction product and less energy wasted. From −1.47 V to −0.77 V, the EE first increased and then decreased, reaching a maximal value of 39.76% at an EE of −0.97 V (Fig. 3d). In addition, the single-pass conversion efficiency (SPCE) of the Ag₁₉Cu₂/CNT catalyst increased when the potential changed from −0.77 V to −1.47 V, achieving a maximal value of 7.46% at −1.47 V. Furthermore, long-term stability is another important criterion to further assess the intrinsic catalytic properties of a catalyst. As demonstrated in Fig. S7,† after continuous operation for 14 h, the current density and FE_CO of the Ag₁₉Cu₂/CNT catalyst remained almost unchanged, suggesting excellent long-term stability.

Fig. 3 (a) Polarization curve of CNTs and Ag₁₉Cu₂/CNTs in N₂/CO₂ saturated 1 M KOH. (b) FE_CO of CNTs and Ag₁₉Cu₂/CNTs. (c) FE_H2 of CNTs and Ag₁₉Cu₂/CNTs. (d) EE and SPCE of the eCO₂RR.

The spin-polarized density functional theory (DFT) calculations were subsequently carried out to investigate the electrocatalytic activity and selectivity of Ag₁₉Cu₂ nanoclusters by using the Vienna ab initio simulation package (VASP5.4.4).³⁰ In order to save the computational cost, we simplified the –C≡CAr^F groups to –C≡CR (R = 3,5-C₆H₃F₂) and the –PPh₃ groups to –P(CH₃)₃, respectively. The nanoclusters were placed in a cubic box (26 Å × 26 Å × 26 Å), and their structures were optimized. The interactions of electron exchange–correlation were represented by the Perdew–Burke–Ernzerhof (PBE) functional form of the generalized gradient approximation (GGA).³¹ The projector augmented-wave (PAW) method was utilized to describe the ion–electron interactions,³² and the wave functions of all the computations were extended by a plane-wave cutoff energy of 400 eV. Only the Gamma point was used to sample the Brillouin zone, and the convergence criteria for energy and force were set as 10⁻⁴ eV and −0.05 eV Å⁻¹, respectively. In addition, considering the non-negligible van der Waals interactions between ligands, we used the empirical density functional dispersion (DFT-D3) method.³³

Based on the computational hydrogen electrode (CHE) model,³⁴ the change in Gibbs free energy (ΔG) for each elementary step of the CO₂ reduction reaction and hydrogen evolution reaction can be calculated as follows:

ΔG = ΔE + ΔZPE − TΔS

where ΔE, ΔZPE, and ΔS represent the changes in the total energy, zero-point energy, and entropy at 298.15 K. The entropy of gas molecules is obtained from the NIST database. The entropy correction at 298.15 K and zero-point energies for CO₂, CO, and H₂ are presented in Table S2.† For the adsorbed intermediates, only the vibrational entropy calculated from the vibrational frequencies is considered. Furthermore, to address the inaccuracies in the description of CO₂ and CO gas-phase molecules by the PBE functional,^35,36 we added corrections of −0.34 eV and +0.10 eV for CO and CO₂, respectively.

To further understand the high eCO₂RR activity and selectivity of Ag₁₉Cu₂ nanoclusters observed in the experiment, DFT calculations were carried out to identify the catalytically active sites for the CO₂RR. To simplify the calculations, we replaced the –C≡CAr^F groups with –C≡CR (R = 3,5-C₆H₃F₂) groups and the –P(Ph)₃ groups with –P(CH₃)₃ groups, using the [Ag₁₉Cu₂(C≡CR)₁₂(P(CH₃)₃)₆Cl₆]⁺ cluster as the computational model. Here, we consider all cases where the removal of a single ligand exposes under-coordinated metal active sites. We found that after the removal of the –Cl ligand, the spatial accessibility around the under-coordinated metal site (highlighted by red circles, Fig. S8†) is too small to act as a catalytic site; thus its catalytic activity is negligible. We therefore focused on the catalytic performance of Ag₁₉Cu₂ clusters after the removal of a single –C≡CR or –P(CH₃)₃ group. For the intact Ag₁₉Cu₂ cluster, –P(CH₃)₃ ligands and part of the –C≡CR ligands are attached to the Ag₄ unit of the Ag₁₉Cu₂ cluster, and the remaining –C≡CR ligands are connected with the Ag₁₁Cu₂ unit of the Ag₁₉Cu₂ cluster. We first consider the case of the removal of a single –P(CH₃)₃ ligand from the Ag₄ unit, as predicted by the free energy profile shown in Fig. 4a, where the formation of *COOH is the potential-limiting step (PDS) of the CO₂RR with a high reaction energy up to 1.82 eV. With the removal of a –C≡CR ligand from the Ag₁₁Cu₂ unit to form Ag₁₉Cu₂(C≡CR)₁₁(P(CH₃)₃)₆Cl₆⁺-(1) clusters, the formation of the *COOH intermediate remains the most uphill step with a relatively high energy barrier of 1.53 eV. It is noteworthy that when a –C≡CR ligand is removed from the Ag₄ unit, the reaction energy for COOH* formation (PDS) on the Ag₁₉Cu₂(C≡CR)₁₁(P(CH₃)₃)₆Cl₆⁺-(2) cluster is lower (1.09 eV). In comparison, the Ag₁₉Cu₂(C≡CR)₁₁(P(CH₃)₃)₆Cl₆⁺-(2) cluster has a better catalytic activity for the CO₂RR. The optimized structures of all substrates and intermediates are depicted in Fig. S9 and S10† and Fig. 4c. We found that for the Ag₁₉Cu₂(C≡CR)₁₁(P(CH₃)₃)₆Cl₆⁺-(2) cluster, the under-coordinated central Ag atom in the Ag₄ unit prefers to act as a catalytically active site for the CO₂RR, with both COOH* and CO* adsorbed to the central Ag atom.

Fig. 4 Free energy diagrams (ΔG) for CO₂ reduction (a) and hydrogen evolution (b) on Ag₁₉Cu₂ clusters with different ligands removed at zero potential. (c) Schematic presentation of the electrocatalytic CO₂ reduction process on the [Ag₁₉Cu₂(C≡CR)₁₁(P(CH₃)₃)₆Cl₆]⁺-(2) cluster. Colour legend: Ag, baby blue; Cu, dark orange; C, grey; Cl, light green; F, cyan; P, purple; H, white; O, red.

Furthermore, we analyzed the competitive HER process on the surface of [Ag₁₉Cu₂(C≡CR)₁₂(P(CH₃)₃)₅Cl₆]⁺, [Ag₁₉Cu₂(C≡CR)₁₁(P(CH₃)₃)₆Cl₆]⁺-(1) and (2), and the free energy diagrams are shown in Fig. 4b. Our calculations show that the hydrogen-adsorption step is the potential-determining step (PDS) for the HER on the three NCs, with limiting potentials calculated to be 1.02, 0.54 and 1.24 eV respectively. All schematic presentations of the adsorption structures of intermediates are shown in Fig. S11.† Compared with the result of the electroreduction of CO₂ to CO, we found a contrasting product selectivity of Ag₁₉Cu₂ clusters with the removal of different ligands. Ag₁₉Cu₂ clusters after the removal of a single –P(CH₃)₃ group from the Ag₄ unit or a single –C≡CR group from the Ag₁₁Cu₂ unit are more favourable to undergo the HER process. However, the limiting potential of the HER (1.24 eV) significantly exceeds that of the CO₂RR (1.09 eV) on the [Ag₁₉Cu₂(C≡CR)₁₁(P(CH₃)₃)₆Cl₆]⁺-(2) cluster, fully supporting the experimentally observed high CO selectivity. Overall, the DFT results suggest that the high CO selectivity of Ag₁₉Cu₂ clusters is probably due to the removal of the –C≡CR group from the Ag₄ unit, exposing the under-coordinated Ag site as a catalytically active centre to promote electrochemical CO₂ reduction.

4. Conclusions

In summary, we successfully synthesized an atomically precise nanocluster of Ag₁₉Cu₂(C≡CAr^F)₁₂(PPh₃)₆Cl₆, which possesses two valence electrons and characteristic absorbance features. The overall structure of Ag₁₉Cu₂ was analyzed by SC-XRD, and it has a novel Ag₁₉Cu₂ core composed of a Ag₁₁Cu₂ kernel connected by two Ag₄ units and various metal–ligand binding coordination modes. When loading into carbon nanotubes, it exhibited excellent catalytic performance, manifested by a high FE_CO value of 95.26%, a large CO current density of 257.2 mA cm⁻² at −1.3 V, and robust long-term stability for a continuous 14 h test. DFT calculations revealed that the high CO selectivity is due to the shedding of the –C≡CR group from the Ag₄ unit during the catalytic process, which exposes the undercoordinated Ag atom as the catalytically active site. This study enriches the family of alkynyl-protected bimetallic nanoclusters, and also provides atomic-level mechanistic insights into employing bimetallic nanoclusters as catalysts for complex electrochemical reactions.

Data availability

All data supporting the findings of this study are available within the paper and its ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Z. T. acknowledges the funding from the Guangdong Natural Science Funds (No. 2023A0505050107). L. W. acknowledges the financial support from the National Natural Science Foundation of China (No. 21805170). Q. T. acknowledges the funding by the Chongqing Science and Technology Commission (cstc2020jcyj-msxmX0382).

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Footnotes

† Electronic supplementary information (ESI) available: Experimental details, supporting figures and tables, and more calculation results. CCDC 2366717. For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nr02702g

‡ These authors contributed equally.

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