Correlation of oxidation states in LaFexNi1-xO3+δ oxides with catalytic activity for H2O2 decomposition

Mixed oxides of the type LaFexNi1-xO3 with a perovskite structure were prepared by thermal decomposition of citrate mixtures at temperatures ranging from 773 to 1373 K. Temperature-programmed reduction profiles showed a low temperature step (450-500 K) due to the reduction of Ni3+ to Ni2+ and a high temperature step (650-1170 K) associated with the reduction of Ni2+ to Ni-0 and Fe3+ to Fe-0. From the microgravimetric reduction profiles it was established that the samples exhibit oxidative nonstoichiometry LaFexNi1-xO3+delta with delta = 0.00-0.08. All the catalysts were tested in the heterogeneous decomposition of hydrogen peroxide (H2O2 --> H2O + 1/2 O-2) in alkaline medium. It was found that the active sites for the reaction are the highly oxidized transition metals (Ni-III and Felv). The concentration of these sites, as determined by iodometric titration, varied with catalyst composition and also with the calcination temperature of the precursors. The compensation effect was observed in all the catalysts investigated for hydrogen peroxide decomposition. The appearance of this effect was interpreted in terms of the catalytic reaction occurring on a heterogeneous catalytic surface, that is, a surface with active sites bearing different activation energies. (C) 2001 Academic Press.