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

Mixed oxides of the type LaFexNi1-xO3 with a perovskite structure were prep ared by thermal decomposition of citrate mixtures at temperatures ranging f rom 773 to 1373 K. Temperature-programmed reduction profiles showed a low t emperature 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 est ablished that the samples exhibit oxidative nonstoichiometry LaFexNi1-xO3+d elta with delta = 0.00-0.08. All the catalysts were tested in the heterogen eous decomposition of hydrogen peroxide (H2O2 --> H2O + 1/2 O-2) in alkalin e medium. It was found that the active sites for the reaction are the highl y oxidized transition metals (Ni-III and Felv). The concentration of these sites, as determined by iodometric titration, varied with catalyst composit ion and also with the calcination temperature of the precursors. The compen sation effect was observed in all the catalysts investigated for hydrogen p eroxide decomposition. The appearance of this effect was interpreted in ter ms of the catalytic reaction occurring on a heterogeneous catalytic surface , that is, a surface with active sites bearing different activation energie s. (C) 2001 Academic Press.