On the correlation of Ni oxidation states and electronic conductivity of (R,A)NiO3-δ (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 cases the samples were single-phase materials characterized structurally by X-ray and neutron power diffraction. In these perovskites, Ni cations exhibited oxidation states between 2+ and 4+, as previously determined by thermogravimetric (TG) analysis under reducing conditions. In a set of samples prepared 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 activity was very poor for BaNiO3, with 100% Ni4+, due to the insulating character of this oxide. The activity rate of PrNiO3 and EuNiO3, showing characteristic metal-insulator transitions as a function of temperature, was much higher for PrNiO3, which is in the metallic regime at the measuring temperature, whereas EuNiO3 remained in the semiconducting (insulating) regime. A linear relationship between the pre-exponential factor and the apparent activation energy, known as the compensation effect, was found for the metallic samples, which suggests that the catalytic surface is heterogeneous, showing active sites with different activation energies.