Comparative studies of redox behaviors of Pt-Co/SiO₂ and Au-Co/SiO₂ catalysts and their activities in CO oxidation

Silica supported Pt-Co and Au-Co nanoparticles (NPs) were subjected to various redox processes and characterized by X-ray diffraction, X-ray absorption near edge structure, and X-ray photoelectron spectroscopy. We found that most of the Co oxide (CoO_x) species on Pt NPs can be reduced at 100 °C forming an alloy structure with Pt at elevated temperatures. Oxidation of Co in the reduced sample takes place gradually with increasing temperatures. In contrast, temperatures higher than 400 °C are needed to reduce CoO _x on Au NPs and Co atoms hardly form an alloy with Au even at 600 °C. The Co species in the reduced Au-Co/SiO₂ sample were quickly oxidized in an O₂ atmosphere at room temperature. High CO oxidation activity was observed in the Pt-Co/SiO₂ catalyst reduced below 300 °C; however this necessitated reduction at 600 °C of the Au-Co/SiO ₂ catalyst. The results illustrate a stronger interaction of Co (CoO_x) with Pt than with Au. In both systems, the optimum treatment conditions are to produce a similar CoO-on-noble metal (NM) active structure and maximize the density of interface sites between the surface CoO structure and the NM support.