Surfaces of doped nanophase cerium oxide catalysts

Abstract

Solid solutions of nanophase cerium oxides have been prepared and the relationship between their bulk crystal structure and surface characteristics has been studied at room temperature with X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). Dopants with a valence lower than +4, such as Ca²⁺, Nd³⁺, and Pb²⁺, introduce structural defects (oxygen vacancies) in the cerium oxide lattice, which has been found to affect the redox catalytic activity of the materials. The introduction of oxygen vacancies leads to the appearance of an O1s core level peak with a shift of 2.0–2.5 eV to higher binding energies as compared to the core level peak of the lattice oxygen in CeO₂. The intensity of this high binding energy O1s peak varies with the expected concentration of oxygen vacancies in the surface, the type of dopant cation and its concentration in the solid solution. It was found that carbonate and hydroxide species are responsible for the appearance of this O1s peak, presumably as a result of capping of oxygen vacancies at the surface. The implications of these surface groups on the catalytic activity of the materials are discussed.