Ce_2/3Cr_1/3O_2+y: a new oxygen storage material based on the fluorite structure

Abstract

In an exploratory search for new oxygen storage materials (OSMs), we have identified a fluorite-type solid solution series, Ce_1-xCr_xO_2+y (0 < x ≤ 1/3), of which the x = 1/3 member, Ce_2/3Cr_1/3O_2.11 (oxide I), exhibits excellent reversible oxygen release/storage properties at relatively low temperatures (~350 °C). Oxide I prepared by a hydrothermal route crystallizes in a metastable nanocrystalline (~4 nm) fluorite type structure (α = 5.352 Å) where the cerium and chromium are randomly distributed. Characterization of oxide I by electron microscopy, powder X-ray diffraction, X-ray photoelectron spectroscopy, and thermogravimetry for the composition, crystal structure, oxidation states of metal atoms, and oxygen stoichiometry yields the formula Ce^IV_2/3Cr^VI_1/9Cr^IV_2/9O_{2.10 ± 0.02} for oxide I. Investigation of oxygen release/intake by temperature programmed hydrogen reduction and thermogravimetric studies has revealed that oxide I possesses a high oxygen storage capacity of 0.175 moles of O₂/mole up to 550°C; this value is better than or comparable to the best OSMs based on CeO₂-ZrO₂. The temperature (~350°C) at which facile oxygen release/intake occurs with oxide I is also lower than that of CeO₂-ZrO₂ based oxides. Formation of oxide I with a cation-disordered fluorite structure, unlike other lanthanide analogs, and its oxygen release/storage properties are discussed in terms of the redox chemistry of Ce^IV/III and Cr^VI/IVin the solid state.