Low-Temperature Redox Properties of Nanocrystalline Cerium (IV) Oxides Revealed by in Situ XANES

Abstract

Time-resolved, in situ XANES studies of some cerium titanate pyrochlores and a fluorite-type bismuth-cerium oxide under reducing and oxidizing gas atmospheres reveal that reduction of cerium occurs at temperature below 500 °C in each of the materials. Cerium oxidation state during these treatments is quantified by deconvolution of the Ce LIII-edge XANES signal using fitted Lorentzians and comparison to data from reference materials of known average oxidation state. In the case of the cerium titanate pyrochlore, complementary XANES experiments at the Ti K-edge show that average titanium oxidation state is largely unaffected on reduction of the sample, although small changes in titanium local environment are apparent that are most likely due to thermal expansion effects as well as an expansion of the structure due to the reduction of cerium. Reoxidation of the pyrochlore is extremely facile, occurring even under hydrogen, which may be attributed to an increase in crystallite size and loss of oxide-rich surfaces, which is backed up by TEM observations.