In situ X-ray and neutron powder diffraction studies of redox behavior in CeO2-containing oxide catalysts

In situ X-ray and neutron diffraction experiments at high temperatures were performed to monitor the lattice response due to conversions between the C e4+ and Ce3+ in CeO2-based oxide catalysts, CeO2, Ce1-xLaxO2-x/2 and Ce1-xZ rxO2 with and without impregnation of Pt metal. The significant difference in the Ce4+ and Ce3+ ionic radius permitted a quantitative measurements of the fraction of Ce3+ in the cubic phases of crystalline CeO2 and the oxide solid solutions during the redox cycle. The reduction kinetics, kt={1-(1-x) (1/3))}(2) (x=fraction of Ce3+, t-time, k=constant) for CeO2 and Ce1-xLaxO2 -x/2 were characterized from an analysis of the X-ray data collected under a CO/N-2 atmosphere at 500-700 degrees C. Pt-impregnated Ce0.1Zr0.9O2 was e xamined by neutron diffraction measurements under both CO/Ar and O-2/Ar atm ospheres up to similar to 700 degrees C. Under a CO/Ar atmosphere, Pt-impre gnation accelerates the reduction of Ce4+ to Ce3+ first on the interface of the metal and oxide particles, generating oxygen vacancies and releasing C O2 molecules. Subsequently, oxygen vacancies migrate to the bulk of the oxi de particles. The significance of the redox behavior of CeO2 in automotive three-way catalysts for purification of exhaust pollutants is discussed. (C ) 1999 Elsevier Science B.V. All rights reserved.