Theoretical study of the catalytic performance of Fe and Cu single-atom catalysts supported on Mo2C toward the reverse water–gas shift reaction

The reverse water–gas shift (RWGS) is an attractive process using CO2 as a chemical feedstock. Single-atom catalysts (SACs) exhibit high catalytic activity in several reactions, maximizing the metal use and enabling easier tuning by rational design than heterogeneous catalysts based on metal nanoparticles. In this study, we evaluate, using DFT calculations, the RWGS mechanism catalyzed by SACs based on Cu and Fe supported on Mo2C, which is also an active RWGS catalyst on its own. While Cu/Mo2C showed more feasible energy barriers toward CO formation, Fe/Mo2C presented lower energy barriers for H2O formation. Overall, the study showcases the difference in reactivity between both metals, evaluating the impact of oxygen coverage and suggesting Fe/Mo2C as a potentially active RWGS catalyst based on theoretical calculations.

. Nevertheless, an emerging class of catalysts enabling optimal metal utilization is single-atom catalysts (SACs) (Qiao et al., 2011;Lin et al., 2013;Wei et al., 2014;Yang et al., 2015;Li et al., 2016;Wang et al., 2016;Lu et al., 2018;Mondelli et al., 2018;Li et al., 2020a;Li et al., 2020b;Kaiser et al., 2020;Gao et al., 2021;Xiong et al., 2021;Zhu et al., 2021). SACs are based on an isolated metal atom anchored on a solid support. Several studies have shown that SACs can exhibit superior catalytic performance in thermocatalytic processes, such as selective hydrogenation (Wei et al., 2014;Wang et al., 2016), CO oxidation (Qiao et al., 2011;Lu et al., 2018), CO 2 conversion (Li et al., 2020a;Zhu et al., 2021), and water gas-shift (WGS) and RWGS reactions, C-C coupling, and electrocatalytic and photocatalytic processes (Wang et al., 2016;Mondelli et al., 2018;Kaiser et al., 2020) (Lin et al., 2013;Yang et al., 2015;Li et al., 2020b), with high activity, selectivity, metal atom utilization, and stability (Li et al., 2016;Gao et al., 2021;Xiong et al., 2021). For example, Lin et al. (2013) synthesized Ir/FeO x SAC, having exceptionally high activity for WGS, where the Ir center greatly enhanced the reducibility of the FeO x support by generating oxygen vacancies, leading to the excellent catalytic performance. Currently, the development of SACs is a highly active research field . Other metal-based catalysts are also promising candidates for the RWGS reaction (Kim et al., 2015;Juneau et al., 2020). Nevertheless, they have drawbacks, that is, their poor natural abundance and high cost. Other alternative materials have also been considered as possible catalysts (Kim et al., 2015). In this context, MXene materials, a family of twodimensional (2D) carbides, nitrides, and carbonitrides with the general formula of M n+1 X n T x (where M is an early transition metal; n = 1, 2, and 3; X is C; and/or N and T are surface -O-, -OH, and/or -F groups), are currently emerging in thermocatalytic applications as catalysts or supports with reactive metal-support interactions (Li et al., 2018a;Li et al., 2018b;Diao et al., 2018;Zhao et al., 2019;Kurlov et al., 2020). As a member of MXene materials, transition metal carbides (TMCs) have attracted particular attention (Reddy et al., 2019;Lin et al., 2021) as they are cheap, potentially selective, and efficient catalysts.
TMCs have similar properties as precious metals (Zhang et al., 2020;Morales-Salvador et al., 2021), being active in many reactions, such as CO hydrogenation, water-gas shift (WGS), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), methanol oxidation reaction, and methane reforming Wang et al., 2020). As a key member of TMCs, Mo 2 C is particularly interesting for CO 2 conversion because of its low cost, dual functionality for H 2 dissociation, and C=O bond scission capability (Porosoff et al., 2014). Many studies have shown that Mo 2 C is highly active in activating CO 2 in various processes, especially for RWGS reactions. Therefore, combining enriched SACs with Mo 2 C as support is an appealing way to balance catalytic activity, selectivity, and stability effectively (Wang et al., 2022). Theoretical calculations can provide detailed insights into the energetics of the catalytic processes (Geiger and López, 2022). The catalytic cycle, energy barriers catalytic sites, and obtained structure-reactivity relationships of each elementary step can be calculated using DFT-based methods with a good compromise between accuracy and computational cost. Mo 2 C has also been used as a catalyst as the oxygen coverage was a key aspect determining the catalytic activity of the material toward the dry reforming of methane, another CO 2 conversion process (Kurlov et al., 2020). However, the effect of changing the oxygen coverage on the catalytic activity was not evaluated in depth. In our previous work, we found that Cu SACs on Mo 2 C are highly active catalysts toward the CO 2 hydrogenation to methanol, showing higher catalytic activity than that of unsupported Cu and Cu/ZnO catalysts. We found that a Cu SAC supported on Mo 2 C and surrounded by O has a high cationic character in agreement with the experiment (Zhou et al., 2021). We proposed feasible reaction mechanisms for the CO 2 hydrogenation and the RWGS reaction.
In the present article, we address, using theory, that is, DFT calculations, the study of the RWGS reaction catalyzed by SACs supported on TMC (Mo 2 C) with different surface O coverages, particularly Cu-and Fe-based SACs. We focus on the CO 2 activation and the H 2 O formation, which involve the adsorption of reactants, direct CO 2 dissociation through the redox mechanism (CO 2 * → CO* + O*), H 2 dissociation (H 2 * → H* + H*), and water formation (2H* + O* → H 2 O*) (Alonso et al., 2021).

Computational details
We studied Fe's and Cu's catalytic performance supported on Mo 2 CT x with different oxygen coverages, which we denote as Fe/ Mo 2 C and Cu/Mo 2 C, respectively. Spin-polarized density functional theory was used for the energetics as implemented in the Vienna Ab initio Simulation Package (VASP) (Kresse and Hafner, 1993;Kresse and Furthmüller, 1996a;Kresse and Furthmüller, 1996b). We used the BEEF-vdW (Wellendorff et al., 2012) as the exchangecorrelation functional and projected-augmented wave (PAW)based pseudopotentials for all calculations. A plane-wave basis set with the kinetic energy cutoff of 500 eV was employed to expand the wave functions. We set the convergence criteria for minima calculations to have a lower force than 0.02 eV/A. A vacuum layer of 10 Å, which is perpendicular to the surface of Fe/Mo 2 C and Cu/Mo 2 C, was added to avoid spurious interactions between periodic images. For gas-phase calculations of molecules, we employed a cubic supercell of 15 Å × 15 Å × 15 Å. We included dipole corrections along the z-direction due to the asymmetry of the M/Mo 2 C surface with co-adsorbed oxygen atoms. We used nudgedelastic band (NEB) methods to locate the transition states until the atomic forces were less than 0.05 eV/Å. Finally, we constructed the energy profile for the RWGS for all evaluated systems referencing all minima and transition states against the sum of energies of the given evaluated catalyst and initial reactants (CO 2 and H 2 ) as the origin of energies.

Results and discussion
We selected our former model for the Cu/Mo 2 CT x system (Zhou et al., 2021), hereafter Cu/Mo 2 C, to evaluate their activity toward the RWGS reaction for different oxygen coverages (O ML); 0, 0.33, 0.67, and 0.78, and performed an analogous study for the hypothetical Fe/Mo 2 C one. Figure 1 shows the structures of Fe/Mo 2 C and Cu/Mo 2 C. CO 2 and H 2 adsorption minima close in energy were considered as initial structures. From their most stable configurations, other minimums and transition states were localized. For each adsorbate (CO 2 , CO, O, and H), four highsymmetry sites were explored (Figure 1), namely, top (T), bridge (B), and two types of threefold hollow sites, either with an X atom (H x ) or a metal (H m ) atom beneath. The adsorption energy for each adsorbate (CO 2 , CO, O, and H) on each site for both systems is provided in Supplementary Table S1 of the Supporting Information.

Description of the reverse water-gas shift reaction mechanism
We studied the RWGS reaction catalyzed by the M(Cu and Fe)/Mo 2 C system, which we previously evaluated for a 0.67 O ML coverage as a side reaction of the CO 2 hydrogenation to methanol reaction for the Cu/Mo 2 C system (Zhou et al., 2021;Geiger and López, 2022). In the present work, we systematically assess the oxygen coverage's effect on the energetics of the RWGS reaction for Cu and Fe SACs supported on Mo 2 C. Thus, we first evaluated the clean M/Mo 2 C system, that is, without oxygen being adsorbed, and 0.33, 0.67, and 0.78 O ML systems. We optimized the system's minima and transition states to evaluate the RWGS mechanism for both catalysts at several oxygen coverages to assess the latter's effect and compare the intrinsic activity of Fe and Cu on the RWGS activity ( Figure 2).
We split the RWGS reaction into two key steps, namely, CO 2 activation (CO*+O*) and water formation (H 2 O*). Concerning CO 2 activation, hydrogen-assisted routes via formate (HCOO*) and carbonyl (COOH*) are an alternative to direct CO 2 activation. Nevertheless, forming HCOO* and COOH* species for both evaluated catalysts is more demanding than just directly splitting CO 2 (see Figure 3). Thus, assessing the subsequent C-O bond cleavage of HCOO* and COOH* is not needed to conclude that the redox pathway by direct activation of CO 2 is preferred over the hydrogen-assisted routes.
We will now describe in detail the CO 2 activation step. First, the CO 2 molecule adsorbs on the metal atom (M = Cu, Fe)/Mo 2 C interface, forming a δ-CO 2 * intermediate. Subsequent CO 2 pre-activation is exoenergetic or slightly endoenergetic, depending on the oxygen coverage. In the resulting structure, the carbon atom and one oxygen atom from CO 2 carbon bind directly to the metal center, while the second oxygen of CO 2 coordinates to a Mo atom. From the δ-CO 2 * structure, CO 2 can split via TS1 into CO* and O* in an endoenergetic step for all the cases. This transition state allows activating CO 2 and cleaving one of the C-O bonds. Oxygen bonds on the highly oxophilic Mo-hollow sites, while CO* remains coordinated to the metal center (M = Cu/Fe). The next step we evaluated is the desorption of the CO molecule to the gas phase. This step is endoenergetic for all cases. Given the high temperature of the RWGS reaction (200°C-500°C for maximum conversion of CO 2 ranging from 10% to 50%) (Porosoff et al., 2016), both the CO 2 cleavage and the CO* desorption seem feasible at both the kinetic and thermodynamic levels.
The second part of the mechanism corresponds to the H 2 O molecule formation. This process is endoenergetic in all cases. This pathway starts with the adsorption of the H 2 molecule on the surface, which is exoenergetic in all cases. Next, the H-H bond cleaves (TS2), giving rise to a proton (H + ) and a hydride ion (H − ). The latter transition state can be understood as a heterolytic TS, producing formally a metal hydride (M-H) and the proton bonded to the cleaved O*. The resulting formal metal hydride remains at the interface (H*-M/Mo 2 C) and the hydroxy group on a Mo-hollow site (HO*-Mo). The subsequent migration of the H* to the OH* group and the O-H bond formation to produce the H 2 O* molecule has a high energy barrier (TS3). This transition state is the most energy demanding along the energy profile for all the evaluated oxygen coverages. After forming H 2 O*, its desorption is endoenergetic for all systems.

Fe/Mo 2 C system
The energy profiles of the RWGS catalyzed by Fe/Mo 2 C with 0 ML, 0.33 ML, 0.67 ML, and 0.78 ML surface oxygen coverages are  Frontiers in Chemistry frontiersin.org shown in Figure 4. Table 1 summarizes the energy barriers for all evaluated steps. The adsorption of CO 2 is exoenergetic for all coverages, by 176.6, 131.2, 91.5, and 88.5 kJ mol -1 going from lower to higher oxygen coverages. All adsorbed CO 2 molecules have a bent structure, in which one oxygen is bound to the Fe center, while the other one is bound to a Mo-top site, and the carbon atom is bonded to the Fe and the two Mo-top sites, as shown in the initial state (IS) on Figure 5. The oxygen coverage effect can explain the energy differences in CO 2 pre-activation. On the one hand, lower oxygen coverage means less repulsion between the adsorbed surface species and the catalytic system. Thus, higher CO 2 adsorption energies are obtained for the 0 and 0.33 O ML systems shown in Table 1. Overall, the δ-CO 2 * intermediate is more stable when decreasing the oxygen coverage. On the other hand, the resulting bent CO 2 angles of the resulting intermediate (∠O-C-O) are 122°(0 ML), 132.3°(0.33 ML), 136.1°( 0.67 ML), and 136.4°(0.78 ML), confirming the relationship between the bending angle and the energy gain upon adsorbing CO 2 on the catalytic surface.
The subsequent CO 2 cleavage step (TS1) has energy barriers equal to 68.7, 73.6, 31.5, and 27.4 kJ mol -1 , from lower to higher oxygen coverages. These energy barriers are related to the δ-CO 2 pre-activation and stability. The energy barriers for CO 2 cleavage slightly decrease when increasing the ∠O-C-O angle and decreasing the energy stability of the δ-CO 2 * intermediate. Overall, reaction energy differences are exoenergetic, so all catalysts are favorable for the formation of CO, as shown in Table 1. The desorption of the resulting CO* species on Fe (FS; Figure 5) is endoenergetic in all cases, that is, by 148.7, 166.9, 93.8, and 120.7 kJ mol -1 (see Figure 4). For the first two coverages (0 and 0.33 ML), the energy for the CO* desorption includes slight Fe movement (Supporting Information Supplementary Table S2 reports the energy difference involved in both Fe displacements). The 0 ML coverage has the most favorable adsorption energy, confirming a more significant interaction between CO 2 with Mo sites and the iron center. Among all the coverages evaluated, 0.67 ML has a lower CO release energy, but 0.78 ML allows a better rate of CO formation due to its affordable reaction barrier and moderate releasing energy.
The H 2 O formation starts via H 2 adsorption, which is exoenergetic in all cases. The adsorption energies are 63.2, 56.4, 9.8, and 37.3 kJ mol -1 for oxygen coverages equal to 0, 0.33, 0.67, and 0.78 ML, respectively. Again, the 0 O ML system has the most favorable adsorption energy, in which the location of the H 2 molecule coordinates to Fe but is closer to the co-adsorbed oxygen coming from the CO 2 activation than the other oxygen coverages just above the Fe, and the co-adsorbed oxygen favors the adsorption. It means that the more in the middle it is, the better the energy absorption. The energy barriers for the subsequent heterolytic H 2 cleavage (TS2, Figure 6) are equal to 73.2, 53.4, 64.7, and 69.8 kJ mol -1 for 0, 0.33, 0.67, and 0.78 ML coverages, respectively. The H 2 cleavage forms an OH* species adsorbed on a Mo-hollow site and a metal hydride intermediate (H*-M + OH*-Mo, Figure 6).
Finally, H 2 O forms by reaction of OH* and H* with energy barriers equal to 147.7, 100.4, 53.9, and 66.6 kJ mol −1 for oxygen coverages of 0, 0.33, 0.67, and 0.78 ML, respectively. TS3 geometries differ only in the proximity of the H 2 O* formed to the Mo-hollow and in the migration step of the H* atom from the interface to the OH* group (TS3, Figure 6). The position of the OH* group, the migration site of the H* atom, and the bond lengths Fe-H and H-OH on the TS3 differ depending on the oxygen coverage, as is summarized in Table 2.
For the clean Fe/Mo 2 C system, the H* atom comes from the interface, while the OH* group has more available adsorption sites as  Figure 6. The H 2 O* structure at 0.67 and 0.78 O ML, that is, with the high oxygen coverage, is more stable due to the formation of two hydrogen bonds between H 2 O* and a co-adsorbed oxygen atom (FS, Figure 6). The desorption of H 2 O is endoenergetic for all systems by 66.5, 76.7, 92.3, and 93.1 kJ mol -1 , from lower to higher oxygen coverages. High oxygen coverages, likely present under reaction conditions, provide the most feasible energy barriers, and therefore, high catalytic activity is expected. In contrast, for low oxygen coverages, the strong adsorption of the intermediates increases the key energy barriers for the RWGS reaction, suggesting a lower catalytic activity. The most active system along the evaluated series is the Fe/Mo 2 C surface with a 0.67 O ML coverage as it presents the lowest energy barrier, with the highest energy barrier being the H 2 O formation step, amounting to 53.9 kJ mol -1 .  Table 3.

Cu/Mo 2 C system
The CO 2 adsorption is exoenergetic by 121.9, 25.9, and 7.6 kJ mol -1 for 0 , 0.33 , and 0.78 ML coverages, respectively, while for 0.67 ML, it is slightly endothermic, which is about 5.1 kJ mol -1 . On the Cu/Mo 2 C catalyst, a low oxygen coverage decreases the repulsion of the adsorbed CO 2 and therefore results in a more favorable CO 2 adsorption energy. For Cu/Mo 2 C 0 ML, CO 2 binds mainly on Mo rather than on Cu in comparison to the other coverages, in a very exothermic adsorption step of 121.9 kJ mol -1 , as mentioned earlier (CO 2 *-Cu, Figure 8). In this structure, CO 2 bends the most, with an ∠O-C-O angle equal to 121.4°. For the rest coverages (0.33, 0.67, and 0.78 ML), the carbon and one oxygen atom of CO 2 are bonded to the Cu center, and the oxygen of CO 2 is connected to the top Mo site (shown as IS in Figure 9).
The energy barriers for the subsequent CO 2 splitting (TS1) are 105.8, 32.4, 2.4, and 4.7 kJ mol -1 . As found for the Fe/Mo 2 C system (vide supra), a more stable δ-CO 2 intermediate implies a high energy barrier for CO 2 cleavage; that is, a high oxygen coverage favors CO 2 activation. Overall, the reaction energies are exoenergetic, so all catalysts favorably form CO, as shown in Table 3. Once CO* is obtained, the CO* desorption is endoenergetic in all the cases, arising from the strong bond between CO* and the Cu atom, as we can see in the optimized minimum CO*-Cu + O*-Mo shown in Figure 8 and as FS in Figure 9. The 0 ML coverage has the most significant adsorption energy and the highest reaction barrier for CO 2 activation, suggesting a more substantial interaction between the CO 2 and the catalyst increases the energy barrier. In contrast, 0.78 ML has the highest CO desorption energy (83.2 kJ mol -1 ), but all oxygen coverages present CO desorption values within 70.9-83.2 kJ mol -1 . When the oxygen coverage is equal to 0.67 O ML, the lowest energy barrier toward CO* + O* is obtained: 2.4 mol -1 . After CO desorbs, the adsorption of H 2 is exoenergetic by 3.7, 7.9, 13.9, and 10.8 kJ mol -1 , from lower to higher oxygen coverages. Next, H 2 splits in a heterolytic way. The hydride ion (H − ) remains on the Cu/Mo 2 C interface, while the proton (H + ) bonds to the O* atom arising from the CO 2 cleavage, forming an OH* group bonded to Mo, as shown in Figure 10. The reaction barriers for H 2 splitting (heterolytic TS2, Figure 10) are 83.2, 69.3, 73.1, and 68.8 kJ mol -1 for 0 ML, 0.33 ML, 0.67 ML, and 0.78 ML, respectively. We can observe that when the H 2 adsorption is higher, the energy barrier for the H-H bond cleavage decreases (Table 3).
Finally, the energy barriers to forming H 2 O (TS3, Figure 10) are 141.4, 144.1, 100.7, and 80.2 kJ mol -1 from lower to higher oxygen coverages. This step has the highest energy barriers for all evaluated oxygen coverages. The related transition states correspond to the formation of the second O-H bond of water by the hydrogen transfer of the H* atom at the Cu/Mo 2 C interface to the OH*  Figure 10.
H 2 O desorption steps are all endoenergetic by 66.8, 78.1, 65.2, and 89.1 kJ mol -1 from lower to higher oxygen coverages. These resulting products with the water molecule adsorbed are more stable for high oxygen coverages (0.67 and 0.78 O ML) than for the lower ones (0 and 0.33 O ML) due to the formation of hydrogen bonds between water and the co-adsorbed oxygen atom (FS, Figure 10). The Cu/Mo 2 C system shows that the more favorable the reaction energy, the lower the energy barrier for forming H 2 O.
Overall, among all the RWGS catalyzed by Cu/Mo 2 C, the systems with high oxygen coverages have the lowest energy barriers for CO 2 activation and H 2 O formation (0.67 and 0.78 O ML) compared to the systems with low oxygen coverages (0 and 0.33 O ML). Overall, the system with the lowest energy barriers is the Cu/Mo 2 C 0.78 O ML one, in which the highest energy barrier is 80.2 kJ mol -1 , corresponding to the water formation step.

Comparison of the RWGS catalytic activity of Cu/Mo 2 C vs Fe/Mo 2 C
The discussion will be divided into two parts: one for 0 and 0.33 O ML coverages and the other for 0.67 and 0.78 O ML coverages, respectively. We first describe the results for the CO formation with the 0/0.33 O ML systems. The adsorption of CO 2 releases energy in all cases. When increasing the amount of coadsorbed oxygen, the (∠O-C-O) angle and the adsorption energy decrease; that is, it is less negative-from −121.9 to −25.9 kJ mol -1 for Cu/Mo 2 C and −176.6 to −131.2 kJ mol -1 for Fe/Mo 2 C. These values indicate that CO 2 interaction is significantly stronger on Fe than on Cu on clean surfaces. All reaction energies are exoenergetic for cleaving CO 2 to CO* and O*. The energy barriers of CO 2 splitting for Cu are 105.8 and 32.4 kJ mol -1 for 0 and 0.33 O ML, respectively, whereas, for Fe, they are equal to 68.7 and 73.6 kJ mol -1 for 0 and 0.33 O ML, respectively. The Cu system presents an essential difference between both coverages since the 0.33 O ML coverage has a much lower energy barrier than the 0 ML one:  Frontiers in Chemistry frontiersin.org 105.8 vs 32 kJ mol -1 . In contrast, the Fe system only shows a difference of 5 kJ mol -1 between 0 and 0.33 O ML. The Cu/Mo 2 C at 0.33 O ML is the most active system toward cleaving CO 2 . The reaction energy of this step becomes more negative and, therefore, more favorable upon increasing the surface oxygen coverage for both Fe/Mo 2 C and Cu/Mo 2 C. Nevertheless, the variation is more significant for Cu/Mo 2 C, indicating that the presence of surface oxygen atoms substantially affects the Cu/Mo 2 C system more than the Fe/Mo 2 C one. Finally, CO desorption is endothermic for both Fe/Mo 2 C (148.6 and 167 kJ mol -1 ) and Cu/Mo 2 C (70.9 and 71.6 kJ mol -1 ). Here, a remarkable difference between both systems is that the energy required to desorb CO is much higher for Fe/Mo 2 C than for Cu/Mo 2 C, regardless of oxygen coverage. This difference means the CO binding energy is much stronger on Fe/ Mo 2 C than on Cu/Mo 2 C. However, at the high temperature of RWGS, desorption is favored entropically, so it should be feasible for both catalysts. Concerning H 2 adsorption, it is more favorable on the Fe/Mo 2 C catalyst than on the Cu/Mo 2 C one. The Fe system has a maximum energy release of 62.9 kJ mol -1 per 0 ML. The splitting of H 2 to OH*+H*-M is endothermic in Fe, while for Cu, it is exothermic. The reaction energy absorbed for Fe or released for Cu energy increases with coverage; 0.33 ML exhibits better OH* formation for both metals due to the lower energy barrier. The second energy barrier for obtaining H 2 O, both Fe/Mo 2 C and Cu/ Mo 2 C, shows the same trend, with the highest energy barriers and endothermic processes. Finally, H 2 O desorption requires similar adsorption energy values on both metal systems and coverages.
Concerning the catalytic performance of Fe/Mo 2 C and Cu/Mo 2 C with surface oxygen coverages of 0.67 ML and 0.78 M O ML, the adsorption of CO 2 is more favored for Fe/Mo 2 C than for Cu/Mo 2 C. The energy released is less negative upon increasing oxygen content and the ∠O-C-O angle, which is consistent with the behavior from 0ML to 0.3 ML oxygen coverages. However, Cu/Mo 2 C has a slight endothermic reaction at 0.67 ML instead of 0.78 ML, which is exothermic, and the energy released is less than that of Fe/Mo 2 C with the same oxygen coverage. Again, the overall reaction energy differences are exothermic, so the CO formation is thermodynamically favorable for all catalysts. The energy barriers for the CO 2 splitting at 0.67 and 0.78 O ML are the  In summary, for CO 2 and H 2 adsorption, Fe/Mo 2 C with 0 ML coverage is the most energy-releasing system upon the adsorption of the reactants. For CO formation, Cu/Mo 2 C 0.67 ML has the lowest energy barrier (TS1, 2.4 kJ mol -1 ), and it is among those with the lowest reaction energies (−103.4 kJ mol -1 ). Therefore, it is the system leading most easily to CO. The CO desorption values are higher for Fe catalysts than for Cu ones, indicating a higher interaction of CO on Fe than on Cu/Mo 2 C. Concerning H 2 splitting, Fe/Mo 2 C 0.33 ML presents the lowest energy barrier (TS2, 53.6 kJ mol -1 ) among all systems. However, the energy barriers for both systems and coverages do not differ much, being all within 53.6-83.2 kJ mol -1 . Fe/Mo 2 C 0.67 O ML is the system presenting the lowest energy barrier for forming H 2 O (TS3: 53.9 kJ mol -1 ), followed by the Fe/Mo 2 C 0.78 O ML system (66.6 kJ mol -1 ). Conversely, the energy barriers for this transition state (TS3) for 0 and 0.33 O ML for Fe (147.7 and 100.4 kJ mol -1 , respectively) and Cu (141.1 and 144.1 kJ mol -1 , respectively) present high energy barriers. According to the overall analysis, the coverage of 0.67 O ML is the most effective one in catalyzing the formation of CO and the formation of H 2 O. While Cu more favorably forms CO and Fe H 2 O, the best Fe/Mo2C system (0.67 O ML) presents the lowest energy barriers.

Conclusion
Systematic DFT calculations were performed on Cu/Mo 2 C and Fe/Mo 2 C catalysts to explore the effect of metal and different oxygen coverages through the RWGS reaction. The study indicates that both catalysts can pre-activate and cleave CO 2 , heterolytically split H 2 , and form water by reacting with two adsorbed hydrogen atoms, formally as a proton (H + ) and a Energies are referenced against the sum of the initial reactants' energy in kJ mol -1 (E rel ).
Frontiers in Chemistry frontiersin.org hydride ion (H − ). On the one hand, Cu/Mo 2 C showed more feasible energy barriers for CO formation, while Fe/Mo 2 C presented more feasible energy barriers for H 2 O formation. Overall, the most active Fe/Mo2C system, having oxygen coverage equal to 0.67 O ML, presents lower energy barriers than the Cu/Mo 2 C system, suggesting that it is more active than the latter. The presence of O in the catalysts may explain the previously mentioned favorable trends for high coverages for RWGS reactivity. On the other hand, H 2 activation has similar trends for both metals and all oxygen coverages. In conclusion, the calculated energy barriers and reaction energies suggest that the Fe/Mo 2 C 0.67 O ML catalyst has the potential for being a highly active RWGS catalyst, likely arising from the highly oxophilic and positive character of Fe, in which the high oxygen coverage balances the catalytic activity in agreement with the Sabatier principle.   Frontiers in Chemistry frontiersin.org