Supported Co-based perovskites as catalysts for total oxidation of methane

Supported LaCoO3 perovskites with 2, 5, 10, 15, 20 and 30 wt.% loading were prepared by impregnation of a Ce0.8Zr0.2O2 support (40 m(2) g(-1)) with: (i) a solution of La and Co nitrates and (ii) a "citrate" solution, namely containing La and Co nitrates, and citric acid. All precursors were decomposed and calcined at 700 degreesC for 5 h. XRD investigations indicated the formation of a pure perovskite phase only if citrates were used. These materials were tested as catalysts for methane combustion in the temperature range 300-700 degreesC. All catalysts showed a lower T-50 (the temperature at which the conversion level of methane is 50%) than the Ce0.8Zr0.2O2 support or non-supported LaCoO3. The activity increased continuously with the perovskite loading. The samples prepared from citrates were slightly more active than from nitrates. This is due to a more homogeneous surface, as indicated by XPS measurements. The presence of a well-characterized perovskite phase (as opposed to highly dispersed elements) seems necessary for good activity. A higher reaction rate per perovskite weight is observed for low loadings when compared to bulk LaCoO3, but the variation with perovskite loading presents a breakpoint, suggesting complex interactions in the catalysts or in the oxidation mechanism. In spite of the experimental impossibility to evaluate the area developed by the supported perovskite, an approximative approach strongly suggests a synergy between the support and supported species. (C) 2004 Elsevier B.V. All rights reserved.