Understanding the active catalyst surface structure on Ru-doped Ni/CeO2 catalysts for CO2 methanation

The effect of Ru doping on the CO2 hydrogenation performance of Ni/CeO2 catalysts was investigated using experimental and computational approaches. Five types of catalysts (1 wt% Ni/CeO2, 4 wt% Ni/CeO2, 0.3 wt% Ru/CeO2, 1 wt% Ni + 0.3 wt% Ru/CeO2, and 4 wt% Ni + 0.3 wt% Ru/CeO2) were prepared by calcining CeO2 impregnated with Ni and Ru precursor solutions and shown to be oxide solid solutions formed through the dissolution of Ni and Ru in the CeO2 lattice. Ru doping increased the reducibility of Ni species and decreased the number of oxygen vacancy sites on the catalyst surface. This reducibility enhancement resulted in a markedly increased proportion of metallic Ni among the exposed Ni species in reduced Ru-doped Ni/CeO2, which was identified as a key factor contributing to superior CO2 methanation performance at low temperatures. The number of oxygen vacancy sites on Ru-doped Ni/CeO2 remained low even after reductive pretreatment with hydrogen, which suggested that Ru species remained incorporated into the CeO2 lattice to form a Ru-Ce oxide solid solution. As a result, the interface between this solution and metallic Ni was concluded to be the primary CO2 conversion site.