Influence of the mobility of oxygen on the reactivity of La1 − xSrxMnO3 perovskites in methane oxidation

Radically different dependences of the activity of La1 − xSrxMnO3 (x = 0−0.5) perovskites in methane oxidation on the degree of substitution of strontium for lanthanum are observed for low and high temperatures. Unsubstituted LaMnO3 exhibits the highest activity in the temperature range from 300 to 500°C, while the sample with the maximum degree of substitution (La0.5Sr0.5MnO3) shows the highest activity at higher temperatures of 700–900°C. In the low temperature region, the activity of La1t - xSrxMnO3 is determined by the amount of weakly bound (overstoichiometric) oxygen, which is formed in cation-deficient lattices and is characterized by a thermal desorption peak with Tmax = 705°C. At higher temperatures (800–900°C), the strongly bound oxygen of the catalyst lattice is involved in the formation of the reaction products under both unsteady- and steady-state conditions. As a consequence, the catalytic activity in methane oxidation correlates with the apparent rate constant of oxygen diffusion in the oxide bulk.