Optimal Icosahedral Copper-Based Bimetallic Clusters for the Selective Electrocatalytic CO2 Conversion to One Carbon Products

Electrochemical CO2 reduction reactions can lead to high value-added chemical and materials production while helping decrease anthropogenic CO2 emissions. Copper metal clusters can reduce CO2 to more than thirty different hydrocarbons and oxygenates yet they lack the required selectivity. We present a computational characterization of the role of nano-structuring and alloying in Cu-based catalysts on the activity and selectivity of CO2 reduction to generate the following one-carbon products: carbon monoxide (CO), formic acid (HCOOH), formaldehyde (H2C=O), methanol (CH3OH) and methane (CH4). The structures and energetics were determined for the adsorption, activation, and conversion of CO2 on monometallic and bimetallic (decorated and core@shell) 55-atom Cu-based clusters. The dopant metals considered were Ag, Cd, Pd, Pt, and Zn, located at different coordination sites. The relative binding strength of the intermediates were used to identify the optimal catalyst for the selective CO2 conversion to one-carbon products. It was discovered that single atom Cd or Zn doping is optimal for the conversion of CO2 to CO. The core@shell models with Ag, Pd and Pt provided higher selectivity for formic acid and formaldehyde. The Cu-Pt and Cu-Pd showed lowest overpotential for methane formation.


Introduction
The rising carbon dioxide (CO 2 ) level and overall concentrations in the atmosphere due to fossil fuel combustion, a major cause of global warming, pose a serious threat to humankind [1]. One of the most promising solutions to mitigating this risk is via the chemical conversion of gaseous CO 2 into value-added chemicals and materials [2]. The electrochemical CO 2 reduction reaction (eCO 2 RR) has emerged as a potential strategy for converting CO 2 because if coupled with electricity from renewable sources (wind, solar, or hydropower plants), the eCO 2 RR could achieve a carbon-neutral energy cycle [3,4]. The main challenges in eCO 2 RR lie in the activation of competitive CO 2 -minimizing pathways such as the hydrogen evolution reaction (HER, H + + e − → 1 2 H 2 ) [5,6] and the conversion Computational characterizations of clusters in the size range of 10 ≤ n ≤ 55 showed that (Cu) n adopted the icosahedral structure [38] derived from 13-and 55-atom icosahedra, built by adding or removing atoms. In addition, a comparison of icosahedral and cuboctahedral (n = 55, 147 and 309) clusters confirmed the icosahedral copper clusters to be more stable. Experimental verification of the formation of copper clusters using microemulsion techniques revealed Cu 55 to be one of the most abundant clusters followed by Cu 13 , Cu 147 and Cu 309 [39]. According to a recent DFT investigation, Cu 55 exhibits highly degenerate states [40]; a direct outcome of its icosahedral symmetry. Therefore, study on nanoclusters such as the highly symmetric 55-atom icosahedral structures would give a deeper understanding than stepped surfaces. This has been attributed to their larger surface-to-volume ratio and higher proportion of coordinatively unsaturated surface atoms (corner or edge) in comparison to bulk materials, resulting in a narrowing of the d-band, an upward shift of the band's energy, and consequently, a stronger adsorption of the reaction intermediates [41]. Investigation on the adsorption of CO 2 on icosahedral 55-atom Cu-based bimetallic clusters [42] found that for the Cu 55−x Zr x systems (x = 1-12), the formation of the CO 2 -activated state (linear to bent transition and elongation of C-O bonds) was endothermic on the pure copper cluster but barrierless and exothermic on the Zr-decorated system. Similarly, DFT calculations of Cu 55−x Zr x systems (x = 0, 12, 13, 42, 43 and 55) with a core@shell and decorated distribution of Cu and Ni atoms showed the presence of Ni on the clusters was crucial to the activation of CO 2 [43]. Although previous computational studies of icosahedral Cu-based bimetallic nanocatalysts considered the adsorption, activation and gas-phase dissociation of CO 2 , in the context of eCO 2 RR, the focus should be on the concerted proton-electron transfer (CPET) steps [44].
Here, we present a computational investigation based on DFT calculations of the effect of nano-structuring and alloying in Cu-based catalysts on the activity and selectivity of the eCO 2 RR. Starting from the icosahedral Cu 55 structure, we generated Cu 54 M 1 , Cu 43 M 12 and Cu 30 M 25 decorated architectures and Cu 13 M 42 core@shell models (M = Ag, Cd, Pd, Pt, and Zn) (Figure 1), with the metals located at three different coordination sites (6, 8 and 12). We provide a thorough analysis of the structural, thermodynamic and electronic properties of these nanoclusters and their ability to activate CO 2 . The computational hydrogen electrode (CHE) model [45] was then applied to compute the mechanism of eCO 2 RR to the C1 products carbon monoxide (CO), formic acid (HCOOH), formaldehyde (CH 2 O), methane (CH 4 ) and methanol (CH 3 OH). We have focused our attention to C1 products because a recent techno-economic assessment of low-temperature CO 2 electrolysis shows the production costs of C1 products such as HCOOH and CO are competitive to conventional processes compared to C2 products such as ethylene and ethanol, which production has substantially higher costs [46] We compare the free energy profiles for the electrocatalytic CO 2 conversion to these C1 products to the competitive HER. The relative binding strength of the intermediates involved is used to identify catalysts for the selective CO 2 conversion. For comparison purposes, calculations of the eCO 2 RR and HER were also conducted on the (100), (110), (111) and (211) facets of pure copper.

Atomistic Models of Clusters and Surfaces
The icosahedral (Ih) 55-atom monometallic Cu cluster was generated using the ab initio random structure searching (AIRSS) code [47]. The decorated Cu54M clusters were then generated by replacing one surface Cu with a dopant metal atom M, where M = Ag, Cd, Pd, Pt and Zn. As shown in Figure 1a, there are three possible coordination sites: CN6 is the edge site, CN8 is the corner site and CN12 is the center of the nanocluster. The Cu43M12 model in Figure 1b was generated by replacing 12 Cu atoms with M located at CN6. The Cu25M30 model in Figure 1c was generated by replacing 12 Cu atoms with M located at CN8. The Cu13M42 core@shell model in Figure 1d was generated by replacing all 13 surface Cu atoms with M. We also considered four-layer (3 × 3) slab models of Cu(100), Cu(110), Cu(111) and Cu(211) [20] with the (100), (110) and (111) being the dominant surfaces of copper. The Cu (211) facet was considered because of its good selectivity towards C1 formation. This was linked to the Cu (211) morphology characterized by step-edge sites with a coordination number equal to 7 (CN7) [48]. Here, we have also compared the catalytic conversion of CO2 to C1 chemicals on Cu(211) to that on 55-atoms icosahedral Cu-M nanoclusters with M located at CN6 and CN8.

Density Functional Theory Calculations
Calculations of energies and structures were conducted at the spin-polarized DFT level using the "Vienna ab initio simulation package" (VASP Software GmbH, version

Atomistic Models of Clusters and Surfaces
The icosahedral (I h ) 55-atom monometallic Cu cluster was generated using the ab initio random structure searching (AIRSS) code [47]. The decorated Cu 54 M clusters were then generated by replacing one surface Cu with a dopant metal atom M, where M = Ag, Cd, Pd, Pt and Zn. As shown in Figure 1a, there are three possible coordination sites: CN6 is the edge site, CN8 is the corner site and CN12 is the center of the nanocluster. The Cu 43 M 12 model in Figure 1b was generated by replacing 12 Cu atoms with M located at CN6. The Cu 25 M 30 model in Figure 1c was generated by replacing 12 Cu atoms with M located at CN8. The Cu 13 M 42 core@shell model in Figure 1d was generated by replacing all 13 surface Cu atoms with M. We also considered four-layer (3 × 3) slab models of Cu(100), Cu(110), Cu(111) and Cu(211) [20] with the (100), (110) and (111) being the dominant surfaces of copper. The Cu(211) facet was considered because of its good selectivity towards C1 formation. This was linked to the Cu(211) morphology characterized by step-edge sites with a coordination number equal to 7 (CN7) [48]. Here, we have also compared the catalytic conversion of CO 2 to C1 chemicals on Cu(211) to that on 55-atoms icosahedral Cu-M nanoclusters with M located at CN6 and CN8.

Density Functional Theory Calculations
Calculations of energies and structures were conducted at the spin-polarized DFT level using the "Vienna ab initio simulation package" (VASP Software GmbH, version 6.3.1, Vienna, Austria) [49] using the following computational settings: the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional with the Grimme's-D3 dispersion correction; a plane-wave basis set within the framework of the projector augmented wave method with a kinetic energy cutoff (E cut ) set to 400 eV; a single k-point (1 × 1 × 1) for the nanoclusters and a (5 × 5 × 1) k-point mesh for the surface model to sample the Brillouin zone of the simulation supercell; a 0.18 eV width for the smearing. Energies, zero-point energies, and entropies of H 2 (g), CO 2 (g) and CO(g), and H 2 O used to compute the free energy corrections are reported in Supplementary Information (Table S1).

Free Energy Calculations
Following the computational hydrogen electrode (CHE) method proposed by Nørskov and co-workers [45], the Gibbs free energy of each step involved in the eCO 2 RR to C1 products was computed using the following equation: where ∆E is the reaction energy; ∆E ZPE is the change in zero-point energy; ∆S is the change in entropy and T is the temperature of the reaction (300 K). We determined the latter two quantities within the harmonic approximation by taking the vibrational frequencies of adsorbates and molecules calculated with DFT. The solvation effects to compute the solvation free energy term ∆G solv were included using VASPsol [50]. ∆G U is the free energy correction introduced by the difference of the electrode potential. For reactions involving a concerted proton-electron transfer (CPET) step, the ∆G U term can be computed by applying the formula: where n is the number of electrons transferred, e is the electron charge and U is the applied electrode potential. The limiting potential (U L ) and the overpotential (η) are important factors for evaluating the catalytic activity. The limiting potential is given by the formula: where ∆G max is the relative change of the Gibbs free energy of the rate-determining step. The overpotential (η) can be obtained by calculating the difference between the equilibrium potential (U eq ) and the limiting potential: Thus, η represents the minimum applied potential required to facilitate the formation of relevant intermediates.

Stability, Structure, and Electronic Properties of the Icosahedral 55-Atom CuM Clusters
The segregation energy (SE) was used to determine the preference of the metal dopants (Ag, Cd, Pd, Pt and Zn) to be in the core or shell of Cu 54 M. The SE is defined as [51]: where E[Cu 54 M(surface)] and E[Cu 54 M(core)] are the electronic energies of the fully optimized Cu 54 M 1 cluster obtained by replacing one Cu atom with a dopant metal at a surface (CN6 or CN8) and at the center of the cluster (CN12), respectively. In Figure 2, the SE values are negative for all Cu 54 M, which implies that the metal prefers to be at the surface of the cluster, consistent with DFT calculations of Cu 54 Zr [51]. The metal doping at the CN8 site is more stable than CN6, but because their values of SE were close, the adsorption and reduction of CO 2 on both coordination sites. To gain insights into the relative stability of pure and bimetallic 55-atom systems, we used the binding energy per atom (E B ), defined as [52]: where E(Cu 55−x M n ) is the total energy of the most stable isomer of each Cu 55−x M x cluster and E(Cu) and E(M) are the total energies of the Cu and Sn atoms, respectively. A higher negative value of E B indicates higher thermodynamic stability of the cluster. The calculated E B for pure Cu 55 nanocluster is -2.99 eV, equal to the value obtained using all-electron triplez quality DFT calculations [53]. Table 1 reports the calculated E B and other structural and electronic properties: the average interatomic bonding distance between nearest neighbors, the energy difference between the highest occupied molecular orbital (HOMO), the lowest unoccupied molecular orbital (LUMO) (∆ H−L ), the Bader charge difference between the Cu and M atoms (∆Q M ), and the surface energy (γ). The surface energy was computed using the following equation [54]: where E nanosphere is the energy of a cluster with N atoms (N = 55), E bulk is the energy of the bulk material per one layer of cross-section and R is the radius a spherical incorporating the cluster.  (6) where Enanosphere is the energy of a cluster with N atoms (N = 55), Ebulk is the energy of the bulk material per one layer of cross-section and R is the radius a spherical incorporating the cluster. A descriptor to analyze the global reactivity descriptor is the gap energy ΔH−L, which relates to the energy cost for an electron to jump from the HOMO to the LUMO orbital. Therefore, ΔH−L characterizes the chemical stability of the system, with a higher value corresponding to a more chemically stable (less reactive) cluster. The ΔH−L for pure Cu55 atom is 0.028 eV, consistent with the literature value of 0.03 eV [55]. In Table 1 and Figure S1b of Supplementary  A descriptor to analyze the global reactivity descriptor is the gap energy ∆ H−L , which relates to the energy cost for an electron to jump from the HOMO to the LUMO orbital. Therefore, ∆ H−L characterizes the chemical stability of the system, with a higher value corresponding to a more chemically stable (less reactive) cluster. The ∆ H−L for pure Cu 55 atom is 0.028 eV, consistent with the literature value of 0.03 eV [55]. In Table 1 and Figure S1b of Supplementary (Table S2). Regarding the surface energy, all clusters have negative γ values signifying the stability of the clusters compared to the bulk.

Adsorption and Activation of CO 2 on Cu and CuM Clusters
CO 2 is a linear molecule with two equivalent C-O bonds (length = 1.12 Å). 13 Before its dissociation, the first step in the catalytic conversion of CO 2 is its adsorption on the catalyst surface. CO 2 can maintain the geometric properties of gas-phase CO 2 (physisorption) or become activated because of the charge transferred from the metal catalyst to the π* molecular orbitals of the CO 2 molecule (chemisorption) resulting in the elongation of the C-O bonds and decrease in the O-C-O bond angle (linear to bent mode) [56]. Here, we have conducted a detailed characterization of the adsorption and activation of CO 2 on the pure copper cluster Cu 55 Table 2. The adsorption energy was calculated as where the first term is the total energy of the CuM···CO 2 system, and the second and third terms are the energies of the isolated cluster and CO 2 molecules, respectively. CO 2 is physisorbed on all Cu-Ag and core@shell clusters as indicated by the absence of significant deviations of the bond angle, bond elongation of adsorbed CO 2 from the gas-phase values, and small charge transfer between Cu and M (∆Q M~0 .04e). In Figure 3 and Table 2

Mechanism of CO 2 Reduction Reaction to C1 Products on Cu-M Clusters and Cu Surfaces
In this section, we present calculations of the mechanism of electrochemical CO 2 reduction. Scheme 1 shows the pathways and intermediates for the formation of the following C1 products: CO, HCOOH, CH 2 O, CH 4 and CH 3 OH. Depending on the atom coordinated to the catalyst, O or C, the first CPET step leads to two intermediates, *OCHO and *COOH. The second CPET will determine whether the 2e − products HCOOH or CO is formed. Subsequent CPET will lead to 4e − (CH 2 O), 6e − (CH 3 OH) and 8e − (CH 4 ) C1 products. Compared to other catalytic reactions, the pathway of the eCO 2 RR is more complex because of the number of intermediates involved. According to Equation 1, the optimal reaction pathway is determined by the lowest free energy pathway at the applied potential U. the highest ΔG value (a high ΔGPLS corresponds to poor catalytic performance). The e mentary steps leading to the formation of CO are: (i) CO2 adsorption (CO2 → *CO ΔG*CO2); (ii) CPET to convert *CO2 to C-coordinated formate (*CO2 + H + + e − → *COO ΔG*COOH); (iii) CPET to convert formate to adsorbed carbon monoxide (*COOH + H + + → *CO + H2O, ΔG*CO); (iv) the release, from the catalyst surface, of gas-phase CO (*CO CO(g), ΔGCO). The structures of the optimized *COOH and *CO on all Cu and CuM sy tems are reported in Supplementary Information (Figures S2-S8).

Electrocatalytic CO 2 conversion to CO and HCOOH
We computed the free energy of reactions (∆G) of the elementary steps to the CO 2 conversion to HCOOH and CO on the following systems: icosahedral Cu 55 cluster; decorated and core@shell Cu-M bimetallic clusters; Cu(100), Cu(110), Cu(111) and Cu(211) surfaces. In the context of the CHE model (see Equation 1), we define the potential limiting step (∆G PLS ) as the elementary reaction in the eCO 2 RR to CO or HCOOH (at U = 0 V) with the highest ∆G value (a high ∆G PLS corresponds to poor catalytic performance). The elementary steps leading to the formation of CO are: (i) CO 2 adsorption (CO 2 → *CO 2 , ∆G *CO2 ); (ii) CPET to convert *CO 2 to C-coordinated formate (*CO 2 + H + + e − → *COOH, ∆G *COOH ); (iii) CPET to convert formate to adsorbed carbon monoxide (*COOH + H + + e − → *CO + H 2 O, ∆G *CO ); (iv) the release, from the catalyst surface, of gas-phase CO (*CO → CO(g), ∆G CO ). The structures of the optimized *COOH and *CO on all Cu and CuM systems are reported in Supplementary Information (Figures S2-S8).
Pure copper: CO 2 -to-CO conversion. The Gibbs free energy diagrams for the CO 2 -to-CO conversion on the monometallic 55-atom cluster and the (100), (110), (111) and (211) surfaces are reported in Figure 4a. There is a significant dependence of the stability of the *COOH and *CO intermediates on the surface morphology and coordination environment. The (211) facet has better catalytic performance (lower ∆G PLS ) towards CO formation than any other surfaces but higher than Cu 55 . This cluster was, therefore, taken as the reference system to assess the performance of the bimetallic clusters. The competitive HER (H + + e − → 1 2 H 2 ) in Figure 4b shows a similar morphology dependence: unfavourable on the (110) surface; highly favorable on the (100) surface; moderately favorable on the (211) surface and the Cu 55 cluster. than any other surfaces but higher than Cu55. This cluster was, therefore, taken as the reference system to assess the performance of the bimetallic clusters. The competitive HER (H + + e -→ ½ H2) in Figure 4b shows a similar morphology dependence: unfavourable on the (110) surface; highly favorable on the (100) surface; moderately favorable on the (211) surface and the Cu55 cluster.  Bimetallic clusters: CO 2 -to-CO conversion. The free energy diagrams for the eCO 2 RR to CO and the HER on CuM are reported in Figure 5. In the single metal doped clusters, Cu 54 M, the value of ∆G PLS depends on both the coordination site and nature of the metal. The ∆G PLS is lower when the reaction occurs on CN6 for M equal to Cd (0.16 eV), Pd (0.23 eV) and Pt (0.53 eV) compared to CN8, Cd (0.12 eV), Pd (0.42 eV) and Pt (0.78 eV). However, for Ag (0.27 eV) and Zn (0.18 eV), CN6 shows higher ∆G PLS than CN8, Ag (0.14 eV) and Zn (0.14 eV). Each intermediate shows strong chemisorption with a low ∆G PLS value and vice versa. For both CN6 and CN8 systems, HER is dominant (lower ∆G H ) over eCO 2 RR because of the strong CO binding to the cluster, leading to a large ∆G *CO ; the exception is M = Pt. When the number of metal dopants on the CuM cluster increases, Cu 43  and Pt leads to an increase in the intermediate adsorption energy. Therefore, the core@shell model with low d-band center, Ag (-3.56 eV), Cd (-7.29 eV) and Zn(-6.23 eV), show poor catalytic performance, and Pd (-1.58 eV) and Pt (-1.92 eV) show good catalytic performance for HCOOH. Overall, the core@shell promotes the formation of HCOOH, and single metal-doped clusters show good catalytic performance for CO, except for Pt, which catalyzes HCOOH formation.

Electrocatalytic CO2 conversion to CH2O, CH3OH, and CH4
The free energy diagrams for the eCO2RR to formaldehyde (CH2O), methanol (CH3OH) and methane (CH4) on the CuM clusters are reported in Figure 6.
Bimetallic clusters: CH2O formation. After the eCO2RR reduction to *CO or *HCOOH, further CPET steps generates three distinct intermediates: *CHO, *COH or *OCH. Among them, *CHO is the easiest to generate, as illustrated by the free energy diagram for the formation of these species, where the lowest ∆GPLS is for COH. A subsequent CPET step leads to the formation of formaldehyde: *CHO + H + + e − → *OCH2 → * + Bimetallic clusters: CO 2 -to-HCOOH conversion. For HCOOH formation, the steps are the CPET to convert adsorbed *CO 2 to O-coordinated formate (*CO 2 + H + + e − → *OCOH, ∆G *OCOH ) and the CPET to convert adsorbed formate to liquid phase formic acid (*OCHO + H + + e − → HCOOH(l), ∆G HCOOH ). In Figure 5 (Table 2), which decreases for Ag, Cd and Zn with increasing doping concentration. The adsorption energy of the intermediates involved in the CO or HCOOH reaction pathway also decreases. Similarly, the higher position of the d-band center for Pd and Pt leads to an increase in the intermediate adsorption energy. Therefore, the core@shell model with low d-band center, Ag (-3.56 eV), Cd (-7.29 eV) and Zn (-6.23 eV), show poor catalytic performance, and Pd (-1.58 eV) and Pt (-1.92 eV) show good catalytic performance for HCOOH. Overall, the core@shell promotes the formation of HCOOH, and single metaldoped clusters show good catalytic performance for CO, except for Pt, which catalyzes HCOOH formation. The free energy diagrams for the eCO 2 RR to formaldehyde (CH 2 O), methanol (CH 3 OH) and methane (CH 4 ) on the CuM clusters are reported in Figure 6. Out of these four paths, our calculations predict the last one as the most suitable for CH3OH formation. Similar to CH2O, the Cu25Pd30 and core@shell Cu@Pd show the lowest ΔGPLS, 0.28 eV and 0.33 eV, respectively, and still, these reactions are dominant over HER.
Bimetallic clusters: CH4 formation. The main 8-electron product of eCO2RR is CH4,   Figure 6a-e Bimetallic clusters: CH 3 OH formation. The formation of CH 3 OH involves five CPET steps. The first three reduction steps coincide to the eCO 2 RR to CH 2 O. The *CHO is reduced to *CHOH or *OCH 2 depending on if the O or C atoms ate protonated. This leads to four possible routes to convert CO 2 to CH 3 OH: concentration for Cu-Pd and Cu-Pt catalysts. Consequently, there is strong adsorption of the intermediates involved in the reaction (pathways 1 to 5). However, with M = Ag, Cd and Zn, the values of d-band centers decreases, which leads to weak adsorption of the intermediates and poor catalytic performance towards CH 4 formation.

Selectivity
The overpotentials (η) to C1 products for all CuM systems, summarized in Figure 8, were computed from the equilibrium (U eq ) and limiting (U L ) potentials (Figures 5 and 6). The values of U eq for CO, HCOOH, CH 2 O, CH 3 OH and CH 4 are 0.12 V, 0.25 V, 0.07 V, 0.02 V and 0.17 V, respectively. For the Cu clusters doped with Ag, Cd and Zn, an increase in metal doping, particularly after 30-atom, leads to the HER becoming dominant over other C1 products. This behaviour is clearly noticeable for the single atom doped clusters, Cu 54 M with M = Ag, Cd and Zn, which shows higher η values for HER than the corresponding core@shell systems. The values of the overpotential also show that a single atom doped system supports either CO or HCOOH. Therefore, small doping does not promote CH 3 OH or CH 4 formation. As metal doping increases, the Cu-Pd and Cu-Pt clusters show lower overpotential for CH 2

Conclusions
In this work, the catalytic properties towards the electrochemical CO2 reduction reaction of a series of icosahedral 55-atom Cu-based clusters doped with Ag, Cd, Pd, Pt and Zn were investigated using density functional theory calculations. The adsorption and

Conclusions
In this work, the catalytic properties towards the electrochemical CO 2 reduction reaction of a series of icosahedral 55-atom Cu-based clusters doped with Ag, Cd, Pd, Pt and Zn were investigated using density functional theory calculations. The adsorption and activation of CO 2 on these clusters and all possible reaction paths that lead to the CO 2 reduction to C1 products (CO, HCOOH, CH 2 O, CH 3 OH and CH 4 ) were considered. Apart from the composition effects, the role of the coordination environment of the metal dopant on the catalytic performance of copper-based clusters was also investigated, with the results showing that nanoclusters with eight-coordinated metal dopants have better catalytic activity towards CO 2 activation. Single-atom doping with Cd and Zn is the best candidate for the CO 2 -to-CO conversion, while core@shell with Ag, Pd and Pt is a good choice for formic acid or formaldehyde formation. The CuPt and CuPd systems show the lowest overpotential for methane formation. This work identifies the influence of size, metal coupling and metal coordination on CO 2 activation and intermediate stability and, consequently, the structure-property relationship in Cu-based mono and bi-metallic clusters for the selective CO 2 conversion to value-added C1 products.
Supplementary Materials: The following supporting information can be downloaded at: https:// www.mdpi.com/article/10.3390/nano13010087/s1, Table S1: The energies (E), zero-point energies (ZPE), and entropies (S) of H 2 (g), CO 2 (g) and CO(g), and H 2 O; Table S2: The atomic, covalent and Van der Waals radii, the electronegativity difference, electronic configuration, and calculated value of segregation energies (in eV) [57,58]; Figure S1: (a) The binding energy and (b) HOMO-LUMO (H-L) gap of Cu-M clusters with increasing doping concentration; Figure S2: The structure and adsorption energies (in eV) of COOH adsorbed on the Cu-M clusters; Figure S3: The structure and adsorption energies (in eV) of CO adsorbed on the Cu54M clusters with CN6 and CN8 nano-catalysts; Figure S4: The structure and adsorption energies (in eV) of CO on the Cu 43   We are grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC (EP/P020194/1). We are grateful to the UK Materials and Molecular Modelling Hub for computational resources, which is partially funded by EPSRC (EP/P020194/1). Via our membership of the UK's HEC Materials Chemistry Consortium, which is funded by EPSRC (EP/L000202), this work used the ARCHER UK National Supercomputing Service.

Data Availability Statement:
The data presented in this study are available on request from the corresponding author.