Unraveling the Active Site in Copper−Ceria Systems for the Water−Gas Shift Reaction: In Situ Characterization of an Inverse Powder CeO2−x /CuO−Cu Catalyst

An inverse powder system composed of CeO2 nanoparticles dispersed over a CuO−Cu matrix is proposed as a novel catalyst for the water−gas shift reaction. This inverse CeO2/CuO−Cu catalyst exhibits a higher activity than standard Cu/CeO2 catalysts. In situ synchrotron characterization techniques were employed to follow the structural changes of CeO2/CuO−Cu under reaction conditions. Time-resolved X-ray diffraction experiments showed the transformation of CuO to metallic Cu via a Cu2O intermediate. Short-order structural changes were followed by pair distribution function analysis and corroborated the results obtained by diffraction. Moreover, X-ray absorption spectroscopy also revealed oxidation state changes from Cu2+ to Cu0 and the partial reduction of CeO x nanoparticles. The activity data obtained by mass spectrometry revealed that hydrogen production starts once the copper has been fully reduced. The strong interaction of ceria and copper boosted the catalytic performance of the sample. The inverse catalyst was active at low temperatures, stable to several reaction runs and to redox cycles. These characteristics are highly valuable for mobile fuel cell applications. The active phases of the inverse CeO2/CuO−Cu catalyst are partially reduced ceria nanoparticles strongly interacting with metallic copper. The nature and structure of the ceria nanoparticles are of critical importance because they are involved in processes related to water dissociation over the catalyst surface.