The unique properties of the monomolecular surface layer of reduced ceria

Abstract

Experimental results of the defect concentrations in highly reduced, monomolecular surface layer of ceria-based oxides, are discussed. The data is XPS vs. oxygen pressure relations of reduced Pr_0.1Ce_0.9O_2.-x (PCO), CeO_2-x and Sm_0.2Ce_0.8O_1.9-x (SDC). In PCO the analysis predicts that the surface layer is negatively charged and the concentration of Pr³⁺ ions is higher than in the bulk. A double layer exists between the surface layer and the bulk. In CeO₂ and SDC the concentration of Ce³⁺ ions and oxygen vacancies in the surface is higher than in the bulk. The surface is neutral. The analysis predicts that the surface layer is metallic, i.e. the electrons on Ce³⁺ are delocalized and not localized small polarons. The bulk is a semiconductor. The Ce³⁺ ions are randomly distributed on the Ce sublattice and not in the boundary of oxygen vacancies. The latter are doubly ionized vacancies ${{\text{V}}_{\text{O}}}^{••}$despite the presence of a high concentration of quasi free electrons. It is also predicted that the surface of ceria or SDC has a phase diagram of temperature vs. oxygen vacancy concentration, at T > 450 °C, like that of ceria bulk or SDC bulk, respectively. Further, the phase diagram of SDC bulk (and surface) is like that of ceria bulk shifted to a higher oxygen vacancy concentration and doping has no other effect at elevated temperature. Both in ceria and SDC the difference between the surface and the corresponding bulk is only in the oxygen pressure at which a level of reduction is reached, with the surface more easily being reduced than the bulk.