CO₂ Methanation Routes on Ni, Co, and NiCo (111) and (100) Surfaces

Methanation of CO₂ can decrease its emission and produce energy carriers. This study probes catalytic routes for CO₂ activation and CO-H₂ reactions on (111) and (100) facets of Ni, Co, and NiCo, using density functional theory. C and O binding strengths capture stability trends for surface species and demonstrate a strong structure sensitivity on NiCo surfaces. Direct *CO₂ dissociation to *CO and *O is facile on all surfaces and exhibits the highest barriers on Ni(111). CH₄ formation is limited by *CO activation and *CH_x hydrogenation steps. On (111) surfaces, the preferred pathway is limited by *HCO formation steps, with barriers trending Co < NiCo < Ni. On (100) surfaces, the direct *CO dissociation is slightly favored over the *COH route for NiCo and Co, while the *COH formation is favored for Ni. The highest free energy barriers are for *CH_x hydrogenations on Ni(100) and Co(100), but for *CO activation on NiCo(100). The (100) barriers are lower than (111) but NiCo(100) exhibits higher barriers than both Ni(100) and Co(100), a consistent trend with experimental reaction rates. These results suggest that the (100) facets can contribute significantly to measured rates, but higher surface converages and contributions from other facets should also be considered.