Observation of dynamic nanostructural and nanochemical changes in ceria-based catalysts during in-situ reduction

We have studied the nanostructural and nanochemical changes occurring in pure ceria (CeC₂) and CeO₂-zirconia (ZrO₂) catalysts during reduction. The reduction is carried out in an environmental transmission electron microscope in dry H₂ at 2 Torr up to 800°C. We use in-situ electron-energy-loss spectroscopy to follow changes in the oxidation state of the Ce at close to the nanometre level. Our measurements show that reduction may or may not be accompanied by significant structural rearrangements in the material. In pure CeO₂ and the mixed oxide, the reduced form can be composed of both crystalline and amorphous material. Our measurements confirm the formation of a superlattice formed due to the ordering of O vacancies in CeO₂ during reduction. The mixed oxide material shows that O vacancy ordering occurs only over 2-3 nm domains.