Microstructural and oxygen-handling characteristics of CeO2 with M3+ promoters. Part 1.—Characterization of calcined powders by XRD and oxygen-isotope exchange

Abstract

Comparisons of the oxygen-handling properties of calcined CeO₂, CeO₂–Al₂O₃, RhO_x–CeO₂, RhO_x–CeO₂/Al₂O₃ and La₂O₃–CeO₂ materials are made on the basis of their experimentally observed abilities to bring about R₀-type homophase oxygen isotope equilibration of an equimolar (¹⁶O₂+¹⁸O₂) mixture at room temperature, and/or to induce low onsettemperatures (ca. 550 K) for pairwise heterophase oxygen isotope-exchange between gas-phase ¹⁸O₂ and oxygen-16 species from the oxide. Those oxygen equilibration/exchange processes were enhanced over precalcined RhO_x–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 RhO_X species at the RhO_X–CeO₂ surfaces, but also to ways in which bulk-incorporated, variable-valency rhodium ions can indirectly facilitate those processes.