On the correlation of Ni oxidation states and electronic conductivity of (R,A)NiO_3-δ (R=lanthanides, A=alkaline earths, Th) perovskites with catalytic activity for H₂O₂ decomposition

Abstract

Several Ni-containing perovskite-related oxides of stoichiometries RNiO₃ (R=La, Pr, Eu), La_1-xA_xNiO₃ (A=Sr, Th), LaNiO_2.5, 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°C under 200 bar of O₂ pressure, the catalytic activity at 30°C, measured by the gasometric method, showed a maximum value for La_0.9Sr_0.1NiO₃, 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.

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