From Ce(OH)₃ to Nanoscaled CeO₂: Identification and Crystal Structure of a Cerium Oxyhydroxide Intermediate Phase

Abstract

Cerium oxide is a pivotal compound in catalysis, both as a support and as an active phase. However, its fine features, such as the Ce(III) to Ce(IV) ratio, strongly depend on the protocol used to prepare it. While the literature contains tens of protocols to prepare cerium oxide nanoparticles from Ce(III) precursors, there is still an open question regarding the time at which the cerium oxidation occurs and the role of hydroxide phases in this process. This article identifies an oxyhydroxide phase of Ce(IV) as a key intermediate and proposes a crystal structure for it. For this purpose, phase-pure cerium trihydroxide (Ce(OH)3) nanorods (<20 nm) were first prepared by hydrothermal treatment of a cerium nitrate salt (Ce(NO3)3·6H2O) in the presence of KOH. Aging of this phase in air was monitored by X-ray absorption spectroscopy, which confirmed the formation of an intermediate Ce(IV) crystalline compound, unreferenced so far. Based on X-ray and neutron powder diffraction experiments combined with DFT calculations, we propose that this new phase is a cerium oxyhydroxide of the chemical formula CeO(OH)2, for which a structural model is proposed. Thermodiffraction experiments finally established that this compound is an observable intermediate in the synthesis of cerium dioxide CeO2 by calcination of Ce(OH)3 in air.