Oxygen desorption and oxidation-reduction kinetics with methane and carbon monoxide over perovskite type metal oxide catalysts

Abstract

Selected perovskite-based, metal oxide catalysts of composition La 1−x Sr xB yB' zO 3 (B: Mn,.Fe; B': Fe or Cu) containing, in some cases, an excess of one or more of the B-position elements (i.e. y+z >1) were investigated for their use in methane oxidation. A flow reactor/mass spectrometer arrangement was used to characterize the supported monolith catalyst samples. The catalysts possess significant activity for methane oxidation and also are able to exchange bulk oxygen in quantities corresponding to several hundred monolayers. Much greater quantities of oxygen could be removed under reducing reaction conditions than by desorption in an inert argon atmosphere. With one catalyst, LaFe 0.84Cu 0.16O 3, oxygen desorption occurred already at 200 ° C; otherwise typical temperatures for desorption onset were 500–600 °C. Lattice oxygen removal under reducing conditions and oxygen uptake by reduced catalysts were both found to exhibit a rather complex temperature dependence. Investigated catalysts with an excess of B-position metals had a greater capacity to exchange oxygen than the stoichiometric perovskites LaMnO 3 and LaFe 0.84Cu 0.16O 3. Reoxidation of the reduced catalysts occurred at lower temperatures than reduction. Oxidation of carbon monoxide consistently occurred at considerably lower temperatures than oxidation of methane. Complete oxidation of methane to carbon dioxide and water was observed under most conditions with little carbon monoxide or higher hydrocarbon formation. Only for strongly reduced catalysts was carbon monoxide production detected.