Hydride-doped Ag17Cu10 nanoclusters as high-performance electrocatalysts for CO2 reduction

Summary The atomically precise metal electrocatalysts for driving CO2 reduction reactions are eagerly pursued as they are model systems to identify the active sites, understand the reaction mechanism, and further guide the exploration of efficient and practical metal nanocatalysts. Reported herein is a nanocluster-based electrocatalyst for CO2 reduction, which features a clear geometric and electronic structure, and more importantly excellent performance. The nanocatalysts with the molecular formula of [Ag17Cu10(dppm)4(PhC≡C)20H4]3+ have been obtained in a facile way. The unique metal framework of the cluster, with silver, copper, and hydride included, and dedicated surface structure, with strong (dppm) and labile (alkynyl) ligands coordinated, endow the cluster with excellent performance in electrochemical CO2 reduction reaction to CO. With the atomically precise electrocatalysts in hand, not only high reactivity and selectivity (Faradaic efficiency for CO up to 91.6%) but also long-term stability (24 h), are achieved.


INTRODUCTION
3][14][15] In this regard, atomically precise metal nanoclusters with 100% uniform size, definite composition, and molecular characteristics are evolved as model systems to investigate the structure-performance relationships of metal nanocatalysts in eCO 2 RR.  The 2][53][54] In the continuous exploration of cluster-based catalysts, we thus wonder whether efficient eCO 2 RR catalysts can be developed by doping hydrides into the lattice of Ag/Cu alloy nanoclusters with alkynyl protection.
Herein, we report the first example of hydride-doped Ag/Cu nanoclusters as high-performance catalysts for eCO 2 RR.The cluster with the molecular formula of [Ag 17 Cu 10 (dppm) 4 (PhChC) 20 H 4 ] 3+ (labeled as Ag 17 Cu 10 H 4 hereafter, dppm is bis(diphenylphosphino)methane) has been selectively obtained by dppmCuBH 4 -initiated reduction process in a simple way.The structure of the cluster as revealed by single-crystal ll OPEN ACCESS X-ray analysis is inspiring in terms of metal framework, metal-ligand interfacial structure, and surface motifs.Notably, the cluster displays a quite high selectivity (faradaic efficiency (FE) up to 91.6%) and high stability (24 h) in eCO 2 RR to CO, outperforming most reported Ag/Cu alloy nanoclusters co-stabilized by alkynyl and phosphine ligands.

Synthesis and atomic structure
The synthesis of Ag 17 Cu 10 H 4 was carried out in one pot and finalized within 1 h (see method details for additional information).The simple synthetic strategy avoided any lengthy preparation steps and thus was beneficial for its later application.The synthetic parameters of the Ag 17 Cu 10 H 4 cluster are summarized in Table 1.The key factor in the successful attainment of the title cluster is the introduction of dppmCuBH 4 reductant in the synthesis.The dppmCuBH 4 was prepared from ligand exchange between (PPh 3 ) 2 CuBH 4 and excess dppm.We note that this is the first time that dppmCuBH 4 is used as reducing agent for the synthesis of atomically precise metal nanoclusters. 60In a typical synthesis of Ag 17 Cu 10 H 4 , PhChCAg suspended in mixed solvent of dichloromethane and methanol was reduced by dppmCuBH 4 .Upon the addition of reductant, the polymeric PhChCAg dissolved gradually, in the meanwhile the solution turned from colorless, pale yellow, pale brown to finally dark brown (Figure S1).The raw product after centrifugation was subjected to the diffusion of ether in the dark, affording black block crystals as the final product (Figure S2).
We first performed X-ray single crystal diffraction to determine the molecular structure of the crystalline products (Figures S3 and S4).The analysis revealed that the products were crystallized in a cubic crystal system with the space group of I-4 3m (Table S1).In each unit cell, 6 cluster moieties are observed (Figure S5).The molecular formula of the cluster is finally determined to be [Ag 17 Cu 10 (dppm) 4 (PhChC) 20 H 4 ] 3+ by high-resolution electrospray ionization mass (HRESI-MS, vide infra), although the four hydrides are difficult to be visualized by X-ray diffraction and the three counterions have not been observed in the single crystal structure analysis.The absence of counterions in the lattice of Ag 17 Cu 10 H 4 can be rationalized by the possibility that the anions are so disordered that cannot be distinguished or that they are not locked in the lattice. 61he size of each Ag 17 Cu 10 H 4 moiety is measured to be $2.0 nm (Figure S6).Shown in Figures 1 and S7 are the total structure of Ag 17 Cu 10 H 4 along the a, b, and c-axis, respectively.The overall structure of the cluster along the c-axis resembles a Chinese knot made up of squares and rectangles.In the knot, the 17 Ag and 10 Cu atoms form the regular metal framework and 4 phosphine and 20 alkynyl ligands construct the shell.We note that a C 2 rather than C 4 symmetric axis is present in the cluster because the rectangles are in different planes (Figure S8, vide infra).
The structure of the title cluster has then been anatomized carefully.Displayed in Figures 2A and S9 is the metal framework of Ag 17 Cu 10 H 4 along different axis, whose structure can be described as the combination of several metal rings.The metal architecture of the cluster can be alternatively portrayed as the interfusion of an Ag-centered Cu 6 polyhedron along the cavity of a Ag 4 /Ag 4 /Cu 4 /Ag 4 /Ag 4 multi-layer partition (Figures 2B and 2C).In the Ag-centered Cu 6 octahedron, the average bond distances of Ag-Cu and Cu-Cu give the value of 2.7405 and 3.9629 A ˚, respectively (Figure 2D).These values are comparable to those in other Ag/Cu alloy nanoclusters co-protected by phosphine and alkynyl ligands. 62,63In the Ag 4 /Ag 4 /Cu 4 /Ag 4 /Ag 4 five-shell structure, the identical rectangles of the first and fifth Ag 4 are arranged orthogonally, with the Ag-Ag bond lengths of the longer side 4.4160 A ˚and shorter 2.7920 A ˚, respectively (Figure 2E, marked as green).Similarly, the Ag 4 rectangles in the second and fourth shells are orthogonal to each other as well, although their bond lengths are slightly different (longer side 9.5351 and shorter 2.9762 A ˚, red in Figure 2E).The remaining four Cu atoms form a perfect square with a side length of 6.473A ˚, which surrounds the middle of the Cu 6 octahedron (Figure 2E, marked as yellow).
As discussed previously, the metal core of the Ag 17 Cu 10 H 4 cluster is stabilized by both dppm and alkynyl ligands.The four dppm ligands are connected to the metal atoms in the same coordination pattern (Figure 2F).The dppm ligands bind the Ag atoms in the outermost ring with strong interaction (average Ag-P of 2.3934 A ˚).Besides the Ag-P interaction, it is observed from the analysis that the dppm ligands can further solidify the metal core by multiple H-M (M = Ag or Cu) interactions (Figure 2G).As suggested by their short bond distances (2.699A ˚for H-Ag and 2.865 A ˚for H-Cu), the hydrogen atoms of the phenyl groups in the dppm ligands bind the Ag and Cu atoms tightly (Figure S10).The 20 alkyne ligands show three coordination modes on the surface (Figure 2H).One is in m 3 (marked as gray), and the other in m 5 (marked as turquoise).The last mode refers to the ''staple'' motif formed by two alkynyl ligands (marked as red).The bond lengths of Ag-C and Cu-C are in the range of 2.431-2.70A ˚and 1.78-2.33A ˚, respectively, similar to those in [Au 13 Ag 16 (C 10 H 6 NO) 24 ] 3À , [Ag 15 (ChC-t Bu) 12 ] + and Cu 53 (ChCPhPh) 9 (dppp) 6 Cl 3 (NO 3 ) 9 . 27,47,646][67][68] Both the experimental (crystallographic data) and theoretical (Density functional theory (DFT) calculation) results suggest the rationale of the identification (Tables S2-S5).As displayed in Figures 2I and S11, the four hydrides are symmetrically doped in the metal framework of the cluster with the same type of coordination mode.Each hydride atom is in an Ag 3 Cu 3 octahedron (m 6 -H), with the average bond distances of Ag-H and Cu-H of 1.9365 and 2.07 A ˚, respectively.

Electronic structure and optical properties
The presence of hydrides in the cluster has been unambiguously confirmed by HRESI-MS.The spectrum of the cluster in the positive measurement mode exhibits several prominent peaks at $2011 m/z (Figure 3A).The main peak can be assigned to [Ag 17 Cu 10 (dppm) 4 (PhChC) 20 H 4 ] 3+ , whose experimental isotopic distribution pattern is perfectly consistent with the simulated one (Figure 3A, inset).It is noteworthy that Ag-Cu exchange is observed in the mass spectrum of the cluster, suggesting the disorder of position occupancy in the structure (Figure S12). 63The X-ray photoelectron spectroscopy (XPS) was then performed to confirm the valence state of Ag and Cu of Ag  S13C). 31,69,70Shown in Figure 3B is the experimental UV-Vis absorption spectrum of the Ag 17 Cu 10 H 4 cluster in CH 2 Cl 2 at room temperature.It exhibits three absorption bands at 390, 446 and 524 nm, respectively.The solution behavior of the Ag 17 Cu 10 H 4 cluster has also been investigated by nuclear magnetic resonance (NMR, Figure S14).Proton-decoupled 31 P NMR of Ag 17 Cu 10 H 4 cluster in CD 2 Cl 2 exhibits two groups of heptets centered at 9.387 ppm, suggesting that the cluster retains its molecular moieties in the solution from (Figure 3C). 71FT computations were then performed to gain deep insight into the electronic properties of the Ag 17 Cu 10 H 4 cluster and corroborate the rationality of hydride locations proposed from X-ray crystallographic analysis.The electronic structure calculations of the Ag 17 Cu 10 H 4 cluster were done using the semi-empirical method PM6 of the Gaussian 09 package (see the supplementary information text for details).The geometry optimization was started from the experimentally measured structure, which did not change the atomic arrangement of the cluster.The atomic coordinates and averaged bond lengths of optimized structures are summarized in Tables S2-S4.The obtained data show that the averaged bond lengths are consistent well with the experimental values (Table S5). Figure 4A and Table S6 are the frontier orbital charge densities of the Ag 17 Cu 10 H 4 cluster.It can be seen that the charge densities are primarily localized near Cu atoms.This indicates that Cu atoms are the catalytic active sites in the Ag 17 Cu 10 H 4 cluster.We have also calculated the Bader charge to analyze the charge transfer between elements of Ag 17 Cu 10 H 4 cluster (Table S7). 72,73In general, each of the Ag, Cu, and P atoms in Ag 17 Cu 10 H 4 cluster lose electrons and carry positive charges, with an average amount of 0.309 electrons for Ag, 0.438 for Cu, and 1.286 for P, respectively.For C and H, the situation is different, as they are unevenly charged.For example, the H atoms inside the cluster and around the P atoms hold more negative charge, while the charges carried by other H atoms are positive.This indicates that the H inside the cluster is electro-withdrawing (namely, hydride).From the analysis of the Bader charge, it is reasonable to assume that the electrons of Ag and Cu are transferred to nearby C and H. Cu transfers more electrons, so it is more active than Ag.Normally, the positive-charged site can act as Lewis acid, so the Cu site is the better site for the activation of Lewis bases (such as CO 2 ). Figure 4B are the projected density of states (PDOS) of the Ag 17 Cu 10 H 4 cluster.Seen from the PDOS, the energy gap between HOMO and LUMO is 2.15 eV, and the HOMO and LUMO orbitals of Ag 17 Cu 10 H 4 cluster are mainly provided by the Cu and Ag elements, which agrees well with the orbital charge densities in Figure 4A.

Electrocatalytic CO 2 reduction
Excellent catalytic performance (high activity, selectivity, and stability) is highly desired for cluster-based catalysts when driving eCO 2 RR.The following features of the Ag 17 Cu 10 H 4 cluster enable it a promising candidate catalyst for eCO 2 RR: (1) the enhanced stability brought by both large electronic energy gap and multiple coordination interactions between metal framework and dppm ligands; (2) the ready exposure of metal active sites from the removal of labile alkynyl ligands under the electrocatalytic conditions 47,59,74 ; (3) the unique local surface structure of the Ag 17 Cu 10 H 4 cluster and the inclusion of hydride species in its framework that may guide the catalytic processes taken place in a specific way and enhance the selectivity.We thus in the following section set out to explore the catalytic performance of the Ag 17 Cu 10 H 4 cluster in eCO 2 RR.
To enhance the dispersion of cluster catalysts and overcome the conductivity problems, the Ag 17 Cu 10 H 4 cluster was first deposited on carbon black (XC-72R) with a loading of 1 wt % to form the catalysts of Ag 17 Cu 10 H 4 /XC-72R (Figure S15).The eCO 2 RR measurement was conducted in a flow cell equipped with a two-electrode system using iridium oxide sprayed on titanium mesh as a counter electrode (Figure S16).In the cell configuration, the cathode and anode were separated by a Sustainion membrane.CO 2 was passed through the cathode chamber at a flow rate of 40 sccm, while 1 M KOH electrolyte was circulated through the anode chamber and cathode chamber by a peristaltic pump.The produced gas and liquid products were analyzed by gas chromatography and 1 H NMR (Figure S17).
As envisioned, the Ag 17 Cu 10 H 4 cluster exhibits excellent performance in eCO 2 RR to CO.The product distribution of eCO 2 RR on Ag 17 Cu 10 H 4 in the current density range (50-250 mA cm À2 ) is shown in Figure 5A.Impressively, CO FE (FE CO ) in all tested current densities is higher than 80%, with the highest FE CO up to 91.6% at 100 mA cm À2 .In comparison to other Ag/Cu alloy nanoclusters co-stabilized by alkynyl and phosphine ligands but without hydrides, the Ag 17 Cu 10 H 4 cluster exhibits superior performance when used as a catalyst for eCO 2 RR to CO.For example, the mass activity of Ag 17 Cu 10 H 4 cluster is calculated to be 41 A/mg, much higher than that of [Ag 15 Cu 6 (ChCR) 18 (DPPE) 2 ] À reported by Hyeon et al. (0.5 A/mg).Moreover, displayed in Figures 5B and 5C is the comparison of the partial current density to CO (j CO ) and FE values of CO on Ag 9 Cu 6 , Ag 18 Cu 8 , Ag 13-x Cu 6+x , and Ag 17 Cu 10 H 4 clusters, respectively. 58,62,63Evidently, both j CO and FE CO show that the Ag 17 Cu 10 H 4 cluster features much better selectivity than other clusters.For example, Ag 17 Cu 10 H 4 shows the highest j CO among the four clusters at all investigated current densities.The j CO of the Ag 17 Cu 10 H 4 cluster at 250 mA cm À2 can be high up to 206.25 mA cm À2 and is nearly 7, 60, and 140 times than that of Ag 13-x Cu 6+x, Ag 9 Cu 6 , and Ag 18 Cu 8 counterparts, respectively.The Ag 17 Cu 10 H 4 cluster correspondingly exhibits much higher FE CO than the comparative clusters (Figure 5C), indicating that H 2 evolution can be significantly suppressed on the Ag 17 Cu 10 H 4 cluster.The excellent catalytic performance of Ag 17 Cu 10 H 4 cluster may be attributed to multiple factors, including but not limited to, the presence of both Ag and Cu active sites, the inclusion of hydride species in near the surface metal atoms, and the unique geometric and electronic structures.The stability of the Ag 17 Cu 10 H 4 cluster in eCO 2 RR has also been evaluated by monitoring the products at a constant current density of 100 mA/cm 2 over 24 h.As portrayed in Figure 5D, the FE CO remains unchanged ($90%) during the test.In the meanwhile, stable full cell potentials have been observed over the period.The Chronopotentiometry test data of Ag 17 Cu 10 H 4 cluster at various current densities are shown in Figure S18, which show that the Ag 17 Cu 10 H 4 cluster has a stable full cell voltage over the entire test current density range.The unchanged UV-Vis profiles of the Ag 17 Cu 10 H 4 cluster after catalysis is another strong evidence for its high robustness in the eCO 2 RR (Figure S19).
To further investigate the reactive mechanism and rationalize the high selectivity of CO 2 reduction reaction over the Ag 17 Cu 10 H 4 cluster, we have performed the DFT calculations using the optimized structures (Figure 6).Here, we construct two types of clusters, including pristine Ag 17 Cu 10 H 4 cluster (black trace) and Ag 17 Cu 10 H 4 cluster with the absence of hydrides, called Ag 17 Cu 10 delete H À (red trace).The light blue, orange, gray, red, and white balls represent Ag, Cu, C, O, and N atoms, respectively.During the simulations, we considered different adsorption sites (Cu, Ag, Cu-Ag sites) and found that the intermediates (*COOH, *CO) can be stably present and adsorbed on the clusters only when they are on the Cu site (Figures S20 and S21).In the pristine cluster, the determining step of CO 2 RR is the generation of *CO, and the energy barrier is 0.76 eV.In contrast, the rate-determining step of CO 2 RR for the Ag 17 Cu 10 delete H À system is the desorption of CO, and the energy barrier of determining step increase to 1.58 eV, which indicates the presence of hydrides is beneficial for the formation of CO products.54]75 It also inspires the exploration of hydride-doped metal nanoclusters as high-performance electrocatalysts for CO 2 reduction in future studies.

Conclusion
In conclusion, a simple synthetic approach of using dppmCuBH 4 as a reductant has been developed for the access to hydride-doped Ag/Cu alloy nanoclusters co-protected by phosphine and alkynyl ligands.The co-presence of silver, copper, and hydrides in the framework, along with the stabilization effect from the dppm and phenylacetylene ligands endows the [Ag 17 Cu 10 (dppm) 4 (PhChC) 20 H 4 ] 3+ cluster with unique geometric and electronic structures.The multiple binding interactions from dppm make the cluster robust, while the surface alkynyl ligands are flexible enough to expose Cu active sites.The included hydride species are proposed to help transfer protons and electrons.As a result, the cluster catalyst exhibits high reactivity and selectivity (FE CO up to 91.6%) and long-term stability (24 h) in electrochemical CO 2 reduction reaction to CO.The current work not only provides atomic insights into the composition/structure-performance relationships of metal nanoclusters in electrochemical CO 2 reduction but also inspires the exploration of more underlying metal nanocatalysts with excellent electrocatalytic properties by using the strategies learned from atomically precise nanochemistry.The crystal morphologies of the copper cluster samples were characterized on a transmission electron microscope (Talos F200C G2, 200 KV).The TEM specimens were prepared by dropping an ether suspension of copper nanoparticle sample onto a copper grid.The particle sizes of the copper clusters were directly measured from the TEM images of at least 150 individual particles.
The diffraction data of the single crystals of [Ag 17 Cu 10 (dppm) 4 (PhChC) 20 H 4 ] 3+ cluster was recorded on a Rigaku Oxford Diffraction system X-ray single-crystal diffractometer using Cu Ka (l = 1.54184A ˚) at 100 K.The data were processed using CrysAlis Pro .The structure was solved and refined using Full-matrix least-squares based on F 2 using ShelXT, 77 ShelXL 78 in Olex2,. 79The hydrogen atoms of organic ligands were generated geometrically.Note: the counterions of the cluster could not be observed by X-ray crystallography, which is maybe due to the missing of the anions in the lattice. 61The formula of [Ag 17 Cu 10 (dppm) 4 (PhChC) 20 H 4 ] 3+ was confirmed by mass spectrometry.The thermal ellipsoids of the ORTEP diagram were done at 50% probability.Detailed crystal data and structure refinements for the compound are given in Table S1.CCDC 2256124 contains the supplementary crystallographic data for this paper.Further details can be obtained from the CIF files deposited at the Cambridge Crystallographic Data Center and can be obtained free of charge on request via http://www.ccdc.cam.ac.uk/ data_request/cif.

DFT calculations
The geometry optimization of the isolated Ag 17 Cu 10 H 4 cluster is calculated using the semi-empirical method PM6 of the Gaussian 09 package.Based on the optimized Ag 17 Cu 10 H 4 cluster, the single-point energy of the structure is calculated using the PBE method, and the projected density of states (PDOS) is obtained by the Multiwfn package. 80The reaction scheme for CO 2 RR is performed using the Vienna Ab initio Simulation Package (VASP). 81The exchangeÀcorrelation interactions are described via the Perdew-Burke-Ernzerhof (PBE) functional. 82The interactions between the ionic cores and the valence electrons are treated with the projector augmented wave (PAW) method. 83The energy cutoff is set to 500 eV.To save computational cost, we simplified the benzene ring (Ph) of the Ag 17 Cu 10 cluster to H. as done by others. 48,61he Gibbs free energy G is computed using the following equation: Here, E, E ZPE , and S represent the single point energy, zero-point energy and entropy, respectively.U denotes the potential versus standard hydrogen electrode.T is set to 298.15 K.

Working electrode preparation
The CH 2 Cl 2 solutions of nanoclusters were added to pre-dispersed XC-72R in large quantities of ethanol.After stirring overnight, the solvent was removed collection by centrifugation.The XC-72R-supported clusters (1 wt %) were used to prepare catalyst inks.For the flow cell test, 2 mg of XC-72R-supported clusters, 20 mL of Nafion, and 1 mL of ethanol were mixed and sonicated for 5 min.The prepared catalyst ink was sprayed on the carbon paper, and the loading amount was controlled to be 0.5 mg cm À2 .The geometric area of the working electrode was masked to be 1 cm 2 .

Electrochemical CO 2 reduction reaction (eCO 2 RR) measurement
The eCO 2 RR performance in the flow cell was a two-electrode system using IrO 2 sprayed on Ti mesh as a counter electrode.The cathode and anode were separated by an anion exchange membrane (Fumasep, FAA-3-PK-130).CO 2 was passed through the cathode chamber at a flow rate of 40 sccm, while 1 M KOH electrolyte was circulated through the anode chamber and cathode chamber by a peristaltic pump.We used Chronopotentiometry (CP) method to test electrochemical CO 2 reduction performance.The produced gas products from the cathode chamber were injected and analyzed by the GC online.The liquid products were analyzed by 1 H NMR by using d 6 -DMSO as the internal standard.Using a calibration curve of each product, the total FE was confirmed to be close to 100%.
The gaseous products of eCO 2 RR (H 2 , CO, CH 4 , C 2 H 4 , and C 2 H 6 ) were analyzed by online-connected gas chromatography (GC-2014C, SHIMADZU) equipped with a six-port sampling valve and a TDX-01 packed column.Ar (99.999%) was used as a carrier gas.H 2 gas was quantified with a thermal conductivity detector (TCD) and the other gases were quantified with a flame ionization detector (FID).The sampling gas was passed through a mechanized before FID detector when a low concentration of CO gas was quantified.The faradaic efficiency (FE) of a given product was calculated by the following equation.

FE = nFvrP=iRT
where n is the number of electrons transferred, F is the Faraday constant, v is the CO 2 flow rate, r is the concentration of the gas product in parts-per-million (ppm), P is the pressure, i is the total current, R is the ideal gas constant and T is temperature.
17 Cu 10 H 4 cluster.As displayed in Figure S13A, the similar XPS profiles of Ag 17 Cu 10 H 4 with AgNO 3 suggest that all Ag atoms in Ag 17 Cu 10 H 4 cluster are in the +1 state.The Cu atoms in Ag 17 Cu 10 H 4 cluster are also in oxidation state, as evidenced by its similar XPS spectra with (PPh 3 ) 2 CuBH 4 and the presence of two peaks (913.0 and 915.4 eV) in its X-ray-excited Auger electron spectroscopy (Figures S13B and

Figure 2 .
Figure 2. Structure anatomy of the [Ag 17 Cu 10 (dppm) 4 (PhChC) 20 H 4 ] 3+ cluster Color code of atoms: pink spheres, Ag; pale blue spheres, Cu; lavender spheres, P; bright green spheres, H; gray, red, and turquoise spheres, C. (A-E) The metal framework of the Ag 17 Cu 10 H 4 cluster.(F and G) Coordination structures of dppm ligands of the Ag 17 Cu 10 H 4 cluster.(H) Coordination modes of alkynyl ligands of the Ag 17 Cu 10 H 4 cluster.(I) Coordination environment of hydrides of the Ag 17 Cu 10 H 4 cluster.
d METHOD DETAILS B Synthesis B Characterizations B DFT calculations B Working electrode preparation B