REACTION KINETICS OF METHANE COMBUSTION OVER La1-xCaxFeO3 PEROVSKITES

The reaction kinetics of methane combustion over La1-xCaxFeO3 perovskites (x=0.0, 0.2, 0.4) was studied. The results show that the catalytic activity is higher in the calcium-doped perovskites. It is found that this higher catalytic activity is related to a lower activation energy. The value of the activation energy is not affected by the level of Ca-doping. The observed difference in the catalytic activity between the doped perovskites is interpreted as a consequence of the so-called compensatory effect, i.e., a larger pre-exponential factor which results from the higher number of active sites promoted by defect reactions triggered by the substitution of lanthanum by calcium. The substitution of lanthanum by a lower valence cat Cm, as calcium, entails both the formation of oxygen vacancies and oxidation of some iron ions from Fe3+ to Fe4+, in order to preserve the electroneutrality. The oxygen vacancies account for the increasing participation of lattice oxygen in the combustion kinetics as the calcium doping increases. The experimental results were satisfactorily fitted to the Rideal-Elay mechanism.