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

Cerium oxide is a pivotal compound in catalysis, bothas a supportand as an active phase. However, its fine features, such as the Ce-(III) to Ce-(IV) ratio, strongly depend on the protocolused to prepare it. While the literature contains tens of protocolsto prepare cerium oxide nanoparticles from Ce-(III) precursors,there is still an open question regarding the time at which the ceriumoxidation 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. Forthis purpose, phase-pure cerium trihydroxide (Ce(OH)(3))nanorods (<20 nm) were first prepared by hydrothermal treatmentof a cerium nitrate salt (Ce(NO3)(3)& BULL;6H(2)O) in the presence of KOH. Aging of this phase in air wasmonitored by X-ray absorption spectroscopy, which confirmed the formationof an intermediate Ce-(IV) crystalline compound, unreferencedso far. Based on X-ray and neutron powder diffraction experimentscombined with DFT calculations, we propose that this new phase isa cerium oxyhydroxide of the chemical formula CeO(OH)(2),for which a structural model is proposed. Thermodiffraction experimentsfinally established that this compound is an observable intermediatein the synthesis of cerium dioxide CeO2 by calcinationof Ce(OH)(3) in air.