Methanation of CO₂ and reverse water gas shift reactions on Ni/SiO₂ catalysts: the influence of particle size on selectivity and reaction pathway

Catalytic CO₂ hydrogenation has been studied on both 0.5 wt% and 10 wt% Ni/SiO₂ catalysts with particular focus on the production of CO and CH₄. The large difference in Ni particle size between the 0.5 wt% and 10 wt% Ni loadings strongly affects the kinetic parameters of CO₂ hydrogenation, the formation pathways of CO and CH₄, and the reaction selectivity. The consecutive and parallel reaction pathways show preferences for small Ni clusters and large Ni particles, respectively. At low Ni loading (0.5 wt%), the catalyst shows a comparatively higher catalytic activity for CO₂ hydrogenation with high CO selectivity. With Ni loading increased to 10 wt% (ca. 9 nm particles), the selectivity is switched to favor CH₄ formation. A formate species in a monodentate configuration is intricately involved in CO₂ hydrogenation on both Ni/SiO₂ catalysts, regardless of the Ni loading and particle size. The consecutive pathway, which is favored on small Ni particles, is attributed to low H₂ coverage on the Ni surface, leading to dissociation of formate intermediates resulting in CO formation and high CO selectivity. The reaction of CO₂ hydrogenation on large Ni particles may be controlled by mixed consecutive and parallel pathways, providing the likelihood that the formate intermediate is competitively hydrogenated to CO or CH₄ as part of a parallel reaction pathway. The sites corresponding to kink, corner or step positions on the Ni/SiO₂ surface are proposed as the primary active sites for CO₂ hydrogenation.