Evolution of surface and bulk structures of CexTi1-xO2 oxide composites

A series of CexTi1-xO2 oxide composites were synthesized using a coprecipitation method, and their structures were investigated using X-ray diffraction, N2 adsorption-desorption isotherms, X-ray photoelectron spectroscopy, X-ray absorption spectroscopy, H2 temperature-programmed reduction, selective chemisorption of methyl orange, and isoelectric point measurements. The selective chemisorption of methyl orange and isoelectric point measurements successfully characterized the outmost surface structures of the CexTi1-xO2 oxide composites, and the term “equivalent CeO2 surface coverage” in a monolayer was introduced to describe the outmost surface compositions. CexTi1-xO2 oxide composites with x ≥ 0.7 form a cubic fluorite phase solid solution, the Ce0.3Ti0.7O2 oxide composite is a pure monoclinic compound, and the other oxide composites have mixed phase structures. The outmost surface structure evolves in a different way from the bulk structure. A cubic fluorite Ce0.7Ti0.3O2 solid solution partially undergoes cubic fluorite solid solution-to-monoclinic Ce0.3Ti0.7O2 phase transition on its outmost surface, and Ce0.3Ti0.7O2 on the outmost surface of Ce0.7Ti0.3O2 grows from the surface to the bulk. Cubic fluorite CexTi1-xO2 solid solutions exhibit good reducibilities at relatively low temperatures, whereas Ce0.3Ti0.7O2 exhibits good reducibility at relatively high temperatures. These results provide comprehensive and in-depth structural information for important CexTi1-xO2 oxide composites.

Graphical abstractThe cubic fluorite solid solution-to-monoclinic Ce0.3Ti0.7O2 phase transition initially occurs on the outmost surface of a cubic fluorite Ce0.7Ti0.3O2 solid solution and then grows from the surface to the bulk.Figure optionsDownload full-size imageDownload as PowerPoint slide