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

Comparisons of the oxygen-handling properties of calcined CeO2, CeO2-Al2O3, 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 temperatures (ca. 550 K) for pairwise heterophase oxygen isotope-exchange between gas-phase O-18(2) , and oxygen-16 species from the oxide. Those oxygen equilibration/exchange processes were enhanced over precalcined 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 each 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 equilibration/exchange by residual RhOx species at the RhOx-CeO2 surfaces, but also to ways in which bulk-incorporated, variable-valency rhodium ions can indirectly facilitate those processes.