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 H-2 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.