Supported Bimetallic Trimers Fe2M@NG: Triple-Atom Catalysts for CO2 Electroreduction

Excessive accumulation of carbon dioxide in the atmosphere has become a serious environmental problem due to the increasing consumption of fossil fuels in modern society. Reasonably reducing CO2 in the atmosphere has become a new research hotspot. Electrocatalytic CO2 reduction reaction (CO2RR) offers an appealing strategy to reduce the atmospheric CO2 concentration and to produce value-added chemicals simultaneously. In this paper, two-dimensional (2D) N-decorated graphene (NG)-supported bimetallic trimers (Fe2M@NG) were designed as triple-atom catalysts (TACs). Theoretical calculations showed that Fe2M@NG can effectively activate CO2, and among the 23 TACs examined, Fe2Ir@NG not only has a good catalytic activity for CO2RR (limiting potential is 0.49 V for CH4 formation) but also limits the competing side reaction of the hydrogen evolution reaction (HER). Our theoretical study not only further extends the triple-atom catalysts, but also opens a new door to boost the sustainable CO2 conversion.


INTRODUCTION
For centuries, the utilization of fossil fuels, coal, oil, and natural gas has led to the rapid development of human society. 1−3 However, the ever-increasing energy demands and serious environmental problems require the "optimization" of these traditional energy sources. 4 The massive consumption of these carbonaceous substances results in the continuous increase and accumulation of carbon dioxide (CO 2 ) in the atmosphere. 5 There are mainly three strategies to prevent increasing atmospheric CO 2 concentration: prohibition of CO 2 release, CO 2 storage, and CO 2 conversion. 6 Among them, the CO 2 conversion technology has attracted the attention as the most promising approach to slow down or even reverse the rising trend of atmospheric carbon dioxide concentration. 7,8 The electrochemical CO 2 reduction reaction (CO 2 RR) can directly convert CO 2 9 into high value-added chemicals and fuels, 10,11 and renewable electricity can be used for this electrocatalytic process, resulting in a "net-zero emission" sustainable development. 12 As early as the 1990s, Hori et al. demonstrated the ability of different pure metal catalysts for the electrochemical reduction of CO 2 , providing a solid foundation for subsequent CO 2 RR. 13 However, CO 2 molecules are chemically inert and the hydrogen evolution reaction (HER) competes fiercely with CO 2 RR, and it is challenging to design suitable CO 2 RR electrocatalysts with low limiting potential, high current density, high selectivity, and low cost. 14 Recent studies showed that nitrogen-doped carbon (N-C)supported transition-metal atom catalysts can effectively reduce CO 2 to single-carbon and multicarbon species. 15−18 N-C-based materials have attracted attention because of their high-temperature resistance, acid and alkali resistance, poison resistance, and environmental protection. 19 Compared to the low activity of pure carbon materials, 20 on the one hand, the N-doped carbon materials as a support can provide stable anchoring sites for the metal atoms and thus endow the system with the characteristics of high stability, selectivity, and low coordination state; 21 on the other hand, its strong interactions with the supported metal atoms can regulate the electronic structure of the catalyst 22 and thus enhance the adsorption of CO 2 and facilitate CO 2 RR. 23 The single-atom catalysts (SACs) were considered as the minimum use of metal, 24−27 and expansively, double-atom catalysts (DACs) 27−29 and tripleatom catalysts (TACs) 17,30−33 have gained enormous attention from both theoretical and experimental aspects owing to their synergistic effect and tunable composites in the metal dimers or trimers. For example, the experimentally obtained uniform Ru 3 clusters stabilized by nitrogen species (Ru 3 /CN) were found to exhibit excellent catalytic activity for the oxidation of alcohols. 32 Ma and collaborators fabricated Pt 3 clusters on a core−shell nanodiamond@graphene (ND@G) hybrid support, which inhibits the side reactions and enhances catalytic performance in the direct dehydrogenation of n-butane at a low temperature (450°C) toward olefin products with a selectivity >98%. 33 Inspired by the theoretical finding that the stable Fe 3 @NG (iron trimer-embedded N-decorated graphene) exhibits high catalytic activity to convert CO 2 through the C 2 and C 3 pathways 17 and considering the large space of mediating the combination and electronic coupling of the bimetallic trimers, we explored the stability and catalytic performance of 23 Febased bimetallic TACs (Fe 2 M@NG) for CO 2 reduction to C 1 products by means of first-principles calculations and elucidated that the Fe 2 Ir@NG has excellent catalytic activity for the electroreduction of CO 2 to CH 4 with a small limiting potential of 0.49 V and can also inhibit the competing hydrogen evolution reaction (HER). Our theoretical explorations provide an effective approach for designing bimetallic trimers of high-performance for electrocatalyzing CO 2 reduction.

COMPUTATIONAL METHODS
All of the spin-polarized density functional theory (DFT) calculations were carried out using the Vienna Ab initio Simulation Package (VASP). 34 The exchange−correlation functional was described by the Perdew−Burke−Ernzerhof (PBE) parameterization of the generalized gradient approximation (GGA). 35 The cutoff energy of 500 eV was adopted. The van der Waals interactions were described using the empirical correction in the Grimme scheme (DFT + D3). 36 The convergence parameters of geometric optimization of the maximum force and energy were designated as 0.01 eV/Å and 10 −4 eV, respectively. The Brillouin k-point grid was sampled using a 2 × 2 × 1 γ centered Monkhorst−Pack scheme. 37 To weaken the interaction between the layers, a 20 Å vacuum layer was applied. The binding energy (E b ) of Fe 2 M@NG based on Fe 2 @NG was determined by the following equation where E M represents the energy of a single metal atom, E Fe2M@NG and E Fe2@NG are the energies of Fe 2 M@NG and Fe 2 @NG, respectively. The free energy G was calculated based on the hydrogen electrode (CHE) model proposed by Nørskov and colleagues 38 where E is the reaction energy, which can be directly obtained from the DFT calculation. G(T) = E ZPE − TS, where E ZPE and S are the zero-point energy and entropy, respectively, which can be obtained by calculating the vibration frequencies. 39 T is 298.15 K. 40 The calculated values of gas molecules and intermediates are presented in Tables S1 and S2, respectively. Free energy change (ΔG) of the elementary reaction represents the free energy difference between the product and the reactant (ΔG = G product − G reactant ). 41 The limiting potential (η) of the entire reduction process is determined by the potential limiting step, which has the most positive (ΔG Max ), as computed by η = ΔG Max /e. 42

RESULTS AND DISCUSSION
3.1. Structure and Stability of Fe 2 M@NG. A 7 × 7 graphene supercell doped with six N atoms, forming a hole structure similar to that of a pyridine hole, as shown in Figure  1a, was taken as the support for bimetallic trimers. 17 The optimized configurations of the 23 TACs examined are displayed in Figure S1, and the geometric parameters are given in Table S3. The binding energies were in the range of 1.25−7.70 eV ( Figure 1b). With similar treatment in ref 17, we performed a first-principles molecular dynamics (FPMD) simulation for Fe 2 Zn@NG, whose binding energy is the smallest among the 23 Fe-based TACS, in the NVT ensemble with a temperature of 500 K, a time step of 0.5 fs, and the total time scale of 10 ps. The Fe 2 Zn@NG can maintain its original structure through the 10 ps's FPMD simulation at 500 K ( Figure S2), indicating the high thermal stability in Fe 2 Zn@ NG and other bimetallic TACs.
3.2. CO 2 Adsorption. The capture of CO 2 molecules on the catalyst is the first step in the CO 2 RR process. 43 Due to the strong interaction between CO 2 and the catalyst, the C−O−C angle of the adsorbed CO 2 in the most stable configuration on these TACs is reduced to 121.36−139.78°compared to 180°o f the isolated CO 2 molecule. We denoted the O of CO 2 closer to the TAC as O2, and the other as O1. The lengths of the C− O1 and C−O2 bonds were elongated to 1.20−1.26 and 1.29− 1.45 Å, respectively, compared with 1.18 Å of free CO 2 . For comparison, C−O1 and C−O2 bonds are 1.21 and 1.33 Å for the adsorbed CO 2 on Fe 3 @NG. The adsorption energy (E ad ) of CO 2 was estimated by the following formula   Figure S3) clearly shows the couplings between the d orbitals of the transition metal and the molecular orbitals of CO 2 near the Fermi level. The above results indicate that the TACs well activate the adsorbed CO 2 , which is beneficial to facilitate CO 2 activation and reduction. 3.3. CO 2 Reduction Pathways. To find the optimal CO 2 RR pathway, the free energies of the reaction over these TACs were calculated. 44 The reaction pathways of all catalysts are given in Table S3, and the ΔG of each elementary step is illustrated in free energy diagrams (Figures 3 and S4−S8). The CO 2 electroreduction reaction involves multiple electron

ACS Omega
http://pubs.acs.org/journal/acsodf Article transfer paths, and the general form is CO 2 + nH + + ne − → products + yH 2 O. 45 In the formula, n is usually equal to 2, 4, and 8, and the corresponding products are CO/HCOOH, methanol, and methane. 46 The C 1 products and pathways are discussed in the following subsections, and the potential limiting steps (PLSs) and limiting potentials (η) of our examined TACs are illustrated in Table S4.

First Proton-Coupled Electron Transfer (PCET) Process.
For n = 1, the first proton-coupled electron transfer (PCET) process involves the hydrogenation of the C or O atom. 47 We calculated the free energies of the first hydrogenation of *CO 2 in Figure S4. Therefore, the reaction pathway in this study was along * → *OCO → *H + *(O1/ C/O2), i.e., hydrogenation occurs at different positions (O1, C, O2 atoms). After structural optimization, we noticed that it is easier to form *OCOH on the five Fe 2 M@NG (M = Co, Ni, Rh, Pd, Re), whose free energy changes are 0. 16 3.3.2. CO 2 Reduction to CO. After the first hydrogenation of *CO 2 , the intermediates (*CO + *OH, *OCHO, and *OCOH) became the targets of the next proton−electron pair attack. Based on our computations, the following intermediates may be formed in the subsequent hydrogenation process: *COH + *OH (M = Sc), *CHO + *OH (M = Cr, Cu, W), *CO (after releasing a water molecule, M = Ti, V, Co, Ni, Zn, Y, Zr, Nb, Mo, Ru, Pd, Hf, Ta, Os, Ir, Pt), and *OCHOH (M = Mn, Rh, Re). The free energies of releasing the adsorbed CO were in the range of 0.92−2.62 eV; thus, the strong interaction between CO(g) and the TACs (Ti, V, Co, Ni, Zn, Y, Zr, Nb, Mo, Ru, Pd, Hf, Ta, Os, Ir, Pt) makes these TACs suffer CO poisoning; however, strong adsorption is beneficial to further hydrogenation of *CO on the other hand.
3.3.3. CO 2 Reduction to CH 3 OH(g). Regarding the reduction of CO 2 to CH 3 OH(g), we will continue the discussion based on the above results. For the third hydrogenation, the *COH + *OH intermediate on the  Table S4 and Figure S8), largely because *COH is adsorbed on the hollow position of the Fe 3 cluster, making it unsuitable to attach H to C of *COH.
3.4. Essential Analysis of Catalytic Performance. Previous studies reveal that an ideal catalyst should provide moderate adsorption strength for all reactants, intermediates, and products. 48 Therefore, we evaluated the relationship between the limiting potential of the C 1 pathways over Fe 2 M@NG and the CO 2 adsorption energy (Figure 4a). The results revealed that there is a volcanic relationship between the limiting potentials of the C 1 pathways and the CO 2 adsorption energies. By comparison, we found that six Fe 2 M@NG have moderate E ad for CO 2 (ranging from −1.40 to −1.13 eV) and have smaller η (0.77, 0.75, 0.55, 0.76, 0.70, and 0.49 V for Ni, Cu, Rh, Pd, Re, and Ir, respectively) compared with the Fe 3 @NG (η = 0.79 V, E ad = −1.57 eV). As the main competitive reaction of CO 2 RR, the hydrogen evolution reaction (HER) also consumes proton−electron pairs (H + + e − ), 49 we considered the HER activity of the Fe 2 M@NG. We first calculated the E ad of *H on Fe 2 M@NG and compared with the E ad of *CO 2 ( Figure 4b) and found that on the Fe 2 M@NG (M = Rh, Ni, Pd, Mn, Pt), whose η values are lower than 0.79 V of Fe 3 @NG, the E ad of *H is more negative than that of CO 2 , indicating that it is not conducive for the progress of CO 2 RR on these TACs. 17 However, the Fe 2 Ir@NG has a stronger interaction with *CO 2 than *H, and the ΔG (0.94 eV) for the HER ( Figure S9) is much greater than the maximum ΔG for CO 2 RR toward CH 4 (g) (0.49 eV). Thus, the Fe 2 Ir@NG has high selectivity for CO 2 RR.
According to the density of states ( Figure S3), compared to the case of Fe 3 @NG, in bimetallic TACs, the coupling between M and Fe 2 weakens or strengthens the hybridization with CO 2 near the Fermi level and thereby modulates the catalytic capability of the TACs. Among the 23 bimetallic TACs, the Fe 2 Ir@NG has a medium adsorption strength for CO 2 and a minimum limiting potential and can effectively suppress the HER. All in all, the Fe 2 Ir@NG is a very promising catalyst for CO 2 reduction to CH 4 .

CONCLUSIONS
In summary, by performing comprehensive first-principles calculations, we explored the stability and catalytic behavior of 23 bimetallic triple-atom catalysts (Fe 2 M@NG) for CO 2 reduction to C 1 products (including CO, CH 3 OH, and CH 4 ). Our results showed that the supported bimetallic trimers provide sufficient adsorption sites to adsorb CO 2 , thereby effectively activating the CO 2 and promoting the breaking of the C−O bond. The synergy between Ir and Fe atoms in the outstanding TAC of Fe 2 Ir@NG improves the catalytic activity (low limiting potential 0.49 V for CH 4 formation) and selectivity (reluctant for the competing reaction HER) for CO 2 RR. Our theoretical study offers guidance to design bimetallic TACs with high catalytic performance beyond CO 2 RR. ■ ASSOCIATED CONTENT
The G(T) of the free gas molecules and adsorbed intermediates on Fe 2 M@NG; the binding energy of M, bond lengths, and the adsorption energy of CO 2 for the Fe 2 M@NG; the potential limiting steps of CO 2 RR via C1 pathways and the limiting potentials on Fe 2 M@N; top and side views of optimized Fe 2 M@NG structures; the energy profile and the final structure of FPMD simulation of Fe 2 Zn@NG at 500 K; the density of states of the CO 2 -adsorbed Fe 2 M@NG; the free energy diagram of the first hydrogenation of *CO 2 ; the free energy diagrams of different pathways on Fe 2 M@NG and Fe 3 @NG; the free energy diagram of the HER on