Surface properties of LaNiO3: Kinetic studies of reduction and of oxygen adsorption

The kinetics of reduction and the kinetics of oxygen adsorption on LaNiO3 were studied in a wide range of temperatures. Temperature-programmed reduction showed two reduction steps: (a) of 1 e− per molecule between 425 and 675 K and (b) of 3 e− per molecule between 675 and 900 K. In reduction step (a) a distorted LaNiO3 (because of the presence of anion vacancies) was identified by X-ray diffraction; after sintering in He at 1073 K the perovskite structure was destroyed and La2NiO4 and NiO were formed. In reduction step (b) La2O3 and Ni were formed, these species being better crystallized after sintering at 1173 K. Steps (a) and (b), in the nonsintered samples, were reversible, i.e., after oxidation at 973 K the perovskite structure was newly formed. Reduction in step (a) takes place according to the contracting sphere model while reduction in step (b) is controlled by formation and growth of reduction nuclei. Experimental data in step (b) fit Avrami-Erofeev′s equation. Activation energies for reduction steps (a) and (b) were 108 and 221 kJ mol−1, respectively. The kinetics of oxygen adsorption obeyed Elovich′s equation, t0 being calculated by extrapolation to Z(t) = 0 of t vs Z(t) plots. The activation energy of adsorption was 38 kJ mol−1. Within the LaMeO3 series, the more easily reducible oxides (LaNiO3, LaCoO3, LaMnO3) were found to be better adsorbents of oxygen and better catalysts for oxidation than the less reducible One (LaCrO3).