On the correlation of Ni oxidation states and electronic conductivity of (R,A)NiO3-delta (R=lanthanides, A=alkaline earths, Th) perovskites with catalytic activity for H2O2 decomposition

Several Ni-containing perovskite-related oxides of stoichiometries RNiO3 (R = La, Pr, Eu), La(1-x)A(x)NiO(3) (A = Sr, Th), LaNiO2.5, BaNiO2 and BaNiO3 were studied as catalysts for hydrogen peroxide decomposition. In all case s the samples were single-phase materials characterized structurally by X-r ay and neutron power diffraction. In these perovskites, Ni cations exhibite d oxidation states between 2+ and 4+, as previously determined by thermogra vimetric (TG) analysis under reducing conditions. In a set of samples prepa red at 1000 degrees C under 200 bar of O-2 pressure, the catalytic activity at 30 degrees C, measured by the gasometric method, showed a maximum value for La0.9Sr0.1NiO3, containing 90% of Ni3+ and 10% of Ni4+. However, the a ctivity was very poor for BaNiO3, with 100% Ni4+, due to the insulating cha racter of this oxide. The activity rate of PrNiO3 and EuNiO3, showing chara cteristic metal-insulator transitions as a function of temperature, was muc h higher for PrNiO3, which is in the metallic regime at the measuring tempe rature, whereas EuNiO3 remained in the semiconducting (insulating) regime. A linear relationship between the pre-exponential factor and the apparent a ctivation energy, known as the compensation effect, was found for the metal lic samples, which suggests that the catalytic surface is heterogeneous, sh owing active sites with different activation energies.