Role of bulk and surface structures of La1-xSrxNiO3 perovskite-type oxidesin methane combustion

La1-xSrxNiO3 (x = 0.00-0.20) mixed oxides have been prepared and tested in the combustion of methane. X-ray diffraction (XRD) profiles revealed the pr esence of a single perovskite structure for substitutions 0 less than or eq ual to x less than or equal to 0.05, whereas a mixture of the major perovsk ite phase and minor NiO, SrO and SrCO3 phases were observed for, substituti ons x greater than or equal to 0.10. Temperature-programmed reduction (TPR) profiles showed: (i) a first reduction step at 520-650, due to reduction o f Ni3+ to Ni2+, in which the perovskite structure is preserved although dis torted by the oxygen vacancies; (ii) an intermediate reduction step at 589- 650 K associated to reduction of the segregated NiO phase; (iii) a final st ep at 720-800 K assigned to the complete reduction of the Ni2+-containing p hases into La2O3 and Ni-0. The partial substitution of Sr by La alters the oxide stoichiometry, yielding a mixture of Ni-II/Ni-III oxidation states an d oxygen vacancies, together with surface enrichment of Sr-containing phase s. A close relationship has been found between substitution degree (x) and the oxygen non-stoichiometry. The highest intrinsic activity for the CH4 co mbustion in sample x = 0.10 is associated not only with the largest proport ion of oxygen non-stoichiometry but also with the lowest Sr segregation on the surface. (C) 2001 Elsevier Science B.V. All rights reserved.