Origin of enhanced reducibility/oxygen storage capacity of Ce_1-xTi_xO₂ compared to CeO₂ or TiO₂

Abstract

We determine chemical origins of increase in the reducibility of CeO₂ upon Ti substitution using a combination of experiments and first-principles density functional theory calculations. Ce1-xTixO2 (x = 0.0-0.4) prepared by a single step solution combustion method crystallizes in a cubic fluorite structure, confirmed by Rietveld profile analysis. Ce_1-xTi_xO₂ can be reduced by hydrogen to a larger extent compared to CeO₂ or TiO₂. Temperature programmed reduction of CeO₂, TiO₂, Ce_0.75Ti_0.25O₂ and Ce_0.6Ti_0.4O₂ up to 700 °C in H₂ gave CeO_1.96, TiO_1.92, Ce_0.75Ti_0.25O_1.81, and Ce_0.6Ti_0.4O_1.73, respectively. An extended X-ray absorption fine structure (EXAFS) study of mixed oxides at the Ti K-egde showed that the local coordination of Ti is 4:4, with Ti-O distances of 1.9 and 2.5 Å, respectively, which are also confirmed by our first-principles calculations. Bond valence analysis of the microscopic structure and energetics determined from first principles is used to evaluate the strength of binding of different oxygen atoms and vacancies. We find the presence of strongly and weakly bound oxygens in Ce_1-xTi_xO₂, of which the latter are responsible for the higher oxygen storage capacity in the mixed oxides than in pure CeO₂.