Ce_2/3Cr_1/3O_2+y: A New Oxygen Storage Material Based on the Fluorite Structure

In an exploratory search for new oxygen storage materials (OSMs), we have identified a fluorite-type solid solution series, Ce 1− x Cr x O 2+ y (0 < x ≤ 1/3), of which the x = 1/3 member, Ce 2/3 Cr 1/3 O 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 ( a = 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 2/3 Cr VI 1/9 Cr IV 2/9 O 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 O 2 /mole up to 550 °C; this value is better than or comparable to the best OSMs based on CeO 2 −ZrO 2. The temperature (∼350 °C) at which facile oxygen release/intake occurs with oxide I is also lower than that of CeO 2 −ZrO 2 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/IV in the solid state.