Surface properties and catalytic performance for ethane combustion of La1-xKxMnO3+δ perovskites

The influence of K substitution in LaMnO3 perovskite on the surface properties and catalytic oxidation of ethane has been studied. La1-xKxMnO3+delta perovskites (x=0.05, 0.10, 0.15, 0.20 and 0.25) with BET surface area of 20-26 m(2) g(-1) have been prepared by the freeze-drying method and characterized by X-ray diffraction patterns (XRD), TPR, TPD and XPS. The XRD analyses showed the presence of a single crystalline phase (x less than or equal to0.15,) with the linear increase of the lattice parameter corroborating the incorporation of potassium in the perovskite structure. The XPS studies revealed an enrichment of the perovskite surface with potassium accompanied by an increase in the surface Mn4+/Mn3+ ratio. The TPR and chemical analyses suggested the presence of an oxygen non-stoichiometry (delta) of +0.15 for LaMnO3 which decreases to -0.04 for La0.85K0.15MnO3. Oxygen temperature programmed desorption experiments showed similar results with the amount of beta oxygen desorption decreasing in the presence of potassium in the perovskite structure. The catalytic activity for the total oxidation of ethane decreased for K containing perovskite and was closely related to the oxygen non-stoichiometry (delta). The effect of oxygen partial pressure and temperature suggests the predominance of an intrafacial mechanism at higher temperature with a possible contribution of the suprafacial reaction at lower temperatures. (C) 2001 Elsevier Science B.V. All rights reserved.