Origin of enhanced reducibility/oxygen storage capacity of Ce1-xTixO2 compared to CeO2 or TiO2

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. Ce1-xTixO2 can be reduced by hydrogen to a larger extent compared to CeO2 or TiO2. Temperature programmed reduction of CeO2, TiO2, Ce0.75Ti0.25O2 and Ce0.6Ti0.4O2 up to 700 degrees C in H-2 gave CeO1.96, TiO1.92, Ce0.75Ti0.25O1.81, and Ce0.6Ti0.4O1.73, 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 angstrom, 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 Ce1-xTixO2, of which the latter are responsible for the higher oxygen storage capacity in the mixed oxides than in pure CeO2.