Activity in methane combustion and sensitivity to sulfur poisoning of La1−xCexMn1−yCoyO3 perovskite oxides

The activity in methane combustion and sensitivity to sulfur poisoning of a series of perovskite type oxides La1−xCexMn1−yCoyO3 (x=0, 0.1, 0.2, 0.3 and y=0.3, 0.5, 0.7), prepared by the citrate method were investigated. The citrate method and calcination 5 h at 973 K gave homogeneous oxides exhibiting the XRD pattern typical of perovskite structure, with traces of ceria when x≥0.1. Specific surface areas were from 10 to 31 m2/g, depending on composition. In general, these catalysts were less active than the La1−xCexCoO3 and La1−xCexMnO3 compositions with x=0–0.5, except LaCoO3. No gain in performance was observed by mutual Co and Mn B-site substitution. As expected, all these catalysts are sensitive to sulfur dioxide (or H2S) in the feed, but the rate of poisoning varies significantly with composition. A low degree of substitution by cerium seems to slow the poisoning process. La0.9Ce0.1CoO3, La0.8Ce0.2CoO3 and La0.8Ce0.2MnO3 compositions were the least sensitive. After 15 h exposure at 823 K to 20 ppm SO2 added to the feed, conversion of 1% methane to CO2 remained more than 80% of the original. XPS analysis of spent samples showed the presence of surface S6+ species, even when no bulk sulfate formation was detected by XRD. However, XRD analysis of samples exposed 40 h to 80 ppm SO2 (or H2S) indicates partial catalyst decomposition with the formation of La2(SO4)3, CeO2 and Co3O4.