Operando XAS and NAP-XPS studies of preferential CO oxidation on Co3O4 and CeO2-Co3O4 catalysts

Abstract

Co 3 O 4 is a promising catalyst for removing CO from H 2 streams via the preferential CO oxidation (PROX). A Mars-van-Krevelen redox mechanism is often suggested but a detailed knowledge especially of the oxidation state of the catalytically active surface under reaction conditions is typically missing. We have thus utilized operando X-ray absorption spectroscopy to examine structure and oxidation state during PROX, and near atmospheric pressure-XPS at low photoelectron kinetic energies and thus high surface sensitivity to monitor surface composition changes. The rather easy surface reduction in pure CO (starting already at $100 C) and the easy reoxidation by O 2 suggest that molecularly adsorbed CO reacts with lattice oxygen, which is replenished by gas phase O 2 . Nevertheless, the steady state concentration of oxygen vacancies under reaction conditions is too low even for XPS detection so that both the bulk and surface of Co 3 O 4 appear fully oxidized during PROX. Furthermore, the effect of adding CeO 2 (a less active material) to Co 3 O 4 was studied. Promotion of Co 3 O 4 with 10 wt% CeO 2 increases the reduction temperatures in CO and H 2 and enhances the PROX activity. Since CeO 2 is a less active material, this can only be explained by a higher activity of the Co-O-Ce interface.