Combined DFT and Microkinetic Investigation of CO2 Conversion Reaction Mechanisms and Addressing the Coking Problem: On Ni, Nib and Nib Based Single Atom Alloy Catalysts

Catalytic conversion of CO₂ to methane (via hydrogenation) and syngas (via dry reforming of methane, DRM) has significant importance in controlling global CO₂ emissions. The widely employed nickel-based catalysts have limitations, and gaining fundamental mechanistic insights is pivotal in developing more active and coke resistant nickel-based catalysts. The present work combines Density functional theory (DFT) calculations (employing a benchmarked functional), together with microkinetic modeling, to provide mechanistic insights into CO₂ methanation and DRM reactions. Employing a comprehensive reaction network of 46 elementary reactions, we studied the mechanistic differences for CO₂ methanation on Ru and Ni surfaces. The dominant reaction pathways are CO₂*→HCOO*→HCO*→CH*→CH₂*→CH₃*→CH₄ and CO₂*→CO*→COH*→C*→HC*→CH₂*→CH₃*→CH₄ on Ni (111) and Ru (001) respectively. Even though the CO₂ direct dissociation barrier is low on Ni, the reaction bypasses CO route and progresses via the formate route leading to CO poisoning. Conversely, the CO route is preferred on Ru. For DRM, we investigated boron-doped Ni (NiB) as a potential catalyst and compared the dominant pathway with that on Ni. The dominant reaction pathway is CO₂*→CO*+O*; CH₄→CH₃*→CH₂*→HC*→C*→CO* and CO₂*→CO*+O*; CH₄→CH₃*→CH₂*→CH₂O*→CHO*→CO* on Ni (111) and NiB respectively. Boron doping alters the dominant reaction pathway (no C* intermediate) to kinetically hinder carbon formation. However, the CO₂ activation barrier (124 kJ/mol) on NiB is high resulting in reduced conversion. The strategy is to modify NiB to reduce the CO₂ activation barrier. Based on the ability to break C-O bond, we considered 15 single atom alloy’s (SAA) of NiB and performed a thorough computational screening and found that only 5 of them are thermodynamically stable. Nevertheless, the only candidate on which there is a significant reduction in CO₂ activation barrier (68 kJ/mol) is Mn-SAA. Thus, we believe that Mn-SAA of NiB can be a potential catalyst that can prevent deactivation without compromising the catalytic activity for the DRM process.