Role of bulk and surface structures of La1−xSrxNiO3 perovskite-type oxides in 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 presence of a single perovskite structure for substitutions 0≤x≤0.05, whereas a mixture of the major perovskite phase and minor NiO, SrO and SrCO3 phases were observed for substitutions x≥0.10. Temperature-programmed reduction (TPR) profiles showed: (i) a first reduction step at 520–650, due to reduction of Ni3+ to Ni2+, in which the perovskite structure is preserved although distorted by the oxygen vacancies; (ii) an intermediate reduction step at 589–650 K associated to reduction of the segregated NiO phase; (iii) a final step at 720–800 K assigned to the complete reduction of the Ni2+-containing phases into La2O3 and Nio. The partial substitution of Sr by La alters the oxide stoichiometry, yielding a mixture of NiII/NiIII oxidation states and oxygen vacancies, together with surface enrichment of Sr-containing phases. A close relationship has been found between substitution degree (x) and the oxygen non-stoichiometry. The highest intrinsic activity for the CH4 combustion in sample x=0.10 is associated not only with the largest proportion of oxygen non-stoichiometry but also with the lowest Sr segregation on the surface.