MICROSTRUCTURAL AND OXYGEN-HANDLING CHARACTERISTICS OF CEO2 WITH M(3-ISOTOPE EXCHANGE() PROMOTERS .1. CHARACTERIZATION OF CALCINED POWDERS BY XRD AND OXYGEN)

Comparisons of the oxygen-handling properties of calcined CeO2, CeO2-A l2O3, RhOx-CeO2, RhOx-CeO2/Al2O3 and La2O3-CeO2 materials are made on the basis of their experimentally observed abilities to bring about R( 0)-type homophase oxygen isotope equilibration of an equimolar (O-16(2 ) + O-18(2)) mixture at room temperature, and/or to induce low onset t emperatures (ca. 550 K) for pairwise heterophase oxygen isotope-exchan ge between gas-phase O-18(2) , and oxygen-16 species from the oxide. T hose oxygen equilibration/exchange processes were enhanced over precal cined RhOx-CeO2 materials prepared from chloride-free precursors, but not over La2O3-CeO2. That difference contrasted with expectations for similar concentrations of dopant-related, anion vacancy creation in ea ch material, based upon XRD observations of the incorporation of each dopant into a slightly expanded CeO2 (fluorite) lattice. Consideration is given not only to conventional contribution to oxygen equilibratio n/exchange by residual RhOx species at the RhOx-CeO2 surfaces, but als o to ways in which bulk-incorporated, variable-valency rhodium ions ca n indirectly facilitate those processes.