In situ studies of the active sites for the water gas shift reaction over Cu-CeO₂ catalysts: Complex interaction between metallic copper and oxygen vacancies of ceria

New information about the active sites for the water gas shift (WGS) reaction over Cu-CeO₂ systems was obtained using in-situ, time-resolved X-ray diffraction (TR-XRD), X-ray absorption spectroscopy (TR-XAS, Cu K and Ce L₃ edges), and infrared spectroscopy (DRIFTS). Cu-CeO₂ nanoparticles prepared by a novel reversed microemulsion method (doped Ce_1-xCu_xO₂ sample) and an impregnation method (impregnated CuO_x/ CeO₂ sample) were studied. The results from all of the samples indicate that both metallic copper and oxygen vacancies in ceria were involved in the generation of active sites for the WGS reaction. Evidence was found for a synergistic Cu-O _vacancy interaction. This interaction enhances the chemical activity of Cu, and the presence of Cu facilitates the formation of O vacancies in ceria under reaction conditions. Water dissociation occurred on the O _vacancy sites or the Cu-O_vacancy interface. No significant amounts of formate were formed on the catalysts during the WGS reaction. The presence of strongly bound carbonates is an important factor for the deactivation of the catalysts at high temperatures. This work identifies for the first time the active sites for the WGS reaction on Cu-CeO₂ catalysts and illustrates the importance of in situ structural studies for heterogeneous catalytic reactions.