Theoretical Insight into Tuning CO2 Methanation and Reverse Water Gas Shift Reactions on MoOx-Modified Ni Catalysts

Catalytic conversion of CO2 to CO via reverse water gas shift (RWGS) and CH4 via methanation are competing reactions that simultaneously happen on Ni-based catalysts, and selective control of the reactions is of great importance to subsequent applications. Herein, conversion of CO2 on Mo3O5/Ni(111) with varying MoOx coverages was investigated using a combination of density functional theory (DFT) calculation and microkinetic modeling. The overall reaction proceeds through sequential RWGS to CO and CO methanation. The coordinatively unsaturated Mo (Moov) site at the interface of Mo3O5/Ni(111) enhances CO2 adsorption and facilitates C–O cleavage over the bare Ni(111), leading to more favorable CO2 direct dissociation to CO formation than the carboxyl and formate pathways. The oxophilic Moov also facilitates hydrogenation of CO to HCO and decomposition of CHO to CH and O. On Mo3O5/4 × 4 Ni(111), the enhanced hydrogenation and C–O breakage activity resulted in CH4 as the selective product. On Mo3O5/3 × 3 Ni(111), in contrast, the reduced size of the surface Ni ensembles caused the d-band center of surface Ni sites to shift downward, weakened CO adsorption, and reduced the hydrogenation activity, resulting in CO as the dominant product. Microkinetics analysis revealed that direct CO2 dissociation was the dominant path on Mo3O5/4 × 4 Ni(111) and the interfacial sites of Mo3O5/4 × 4 Ni(111) were primarily covered by O and OH. The rate-limiting step on Mo3O5/Ni(111) was the regeneration of Mo3O5. Consistent with the experimental results, the microkinetics also predicts that CH4 is selectively produced on Mo3O5/4 × 4 Ni(111) whereas CO is the primary product on Mo3O5/3 × 3 Ni(111).