Microstructural and oxygen-handling characteristics of CeO₂with M³⁺promoters. Part 1.—Characterization of calcined powders by XRD and oxygen-isotope exchange

Abstract

Comparisons of the oxygen-handling properties of calcined CeO,, CeO2-AI2O3, RhO,-CeO,, Rh0,-CeO2/AI,O3 and La,O,-CeO, materials are made on the basis of their experimentally observed abilities to bring about R,-type homophase oxygen isotope equilibration of an equimolar (1602 + 1802) mixture at room temperature, and/or to induce low onsettemperatures (ca. 550 K) for pairwise heterophase oxygen isotope-exchange between gas-phase 1802 and oxygen-1 6 species from the oxide. Those oxygen equilibration/exchange processes were enhanced over precalcined Rho,-CeO, materials prepared from chloride-free precursors, but not over La,03-CeO,. 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 CeO, (fluorite) lattice. Consideration is given not only to conventional contribution to oxygen equilibration/exchange by residual Rho, species at the Rho,-CeO, surfaces, but also to ways in which bulk-incorporated, variable-valency rhodium ions can indirectly facilitate those processes.

Ceria‐Based Solid Catalysts for Organic Chemistry

“…[4] Since the pioneering works of Yao and Yu Yao [2] and of Su et al, [5,6] many studies have been devoted to improving knowledge of the kinetics of OSC [7][8][9] and of the mechanisms implicated in surface and bulk oxygen mobility in ceria and related compounds, [10][11][12][13][14] with a special insight into CeZrO x oxides. [15][16][17][18][19] Oxygen diffusivity in these materials can be measured by 18 O/ 16 O isotopic exchange, [20][21][22] while oxygen species (e.g., superoxides, peroxides) that might be involved in the diffusion process can be investigated by electron spin resonance (ESR) [23][24][25][26][27] or FTIR. [18,[28][29][30][31] ESR studies have shown that the presence of a metal (Pt, Rh) drastically changes the nature of the oxygen species; the metal favoring the formation of O À ions instead of superoxide species.…”

Reduction kinetics of ceria surface by hydrogen

“…It is the facile redox cycle existing between Ce 4+ and Ce 3+ that makes ceria so interesting and it has been studied extensively. Of particular interest has been its reduction 11–15 and oxygen diffusion 16. The defect structure and electrical properties of a single crystal and polycrystalline ceria have also been extensively studied 17–22.…”

Kinetics of the high temperature oxygen exchange reaction on 238PuO2 powder

“…Two potential intermediate steps have been proposed for the multiple heteroexchange reaction. The first is an associative mechanism that includes a tetratomic (i.e., ) surface intermediate [17,18].…”

“…Isotopic oxygen exchange reactions are used in the production of 238 PuO 2 based Radioisotope Power Systems (RPS) employed in space exploration to help minimize the (,n) reaction that occurs between the  radiation emitted by 238 Pu and the naturally abundant 17 O (0.037%) and 18 O (0.204%)…”

“…Fortunately, the (,n) reaction does not occur with 16 O, so neutron emission rates can be minimized through use of an oxygen exchange reaction. This reaction reduces the amount of detrimental 17 O and 18 O in the 238 PuO 2 , and replaces it with 16 O. While a number of publications have proven the principle of the oxygen exchange reaction on PuO 2 , there is very little published kinetic or mechanistic information.…”