Effect of substitution by cerium on the activity of LaMnO3 perovskite in methane combustion

This study concerns the effect of lanthanum substitution by cerium on the catalytic activity of La1-xCexMnO3 catalysts and its relation to their physico-chemical characteristics. Samples of pure and cerium-substituted lanthanum manganese perovskites. La1-xCexMnO3 with x = 0.1-0.5 and LaCexMnO3 with x = 0.1, 0.2 and 0.3, were prepared by the citrate method and calcined 5 h at 973 or 1073 K. All samples were characterized by XRD, XPS and oxygen TPD and had their specific surface area (SSA) determined by nitrogen adsorption. The catalytic activity was determined, using 0.1 g catalyst, 1% methane in air at a flow rate of 75 ml/min (GHSV = 45,000 ml/g(cat) h). Substitution with cerium affects significantly the physico-chemical properties of individual compositions. It slows the rate of perovskite phase formation, increases the SSA, has an effect on thermal stability and modifies the oxygen desorption characteristics. However, these changes do not correlate in the expected way with changes in activity for methane combustion. Substitution with cerium or addition of cerium over the formal stoichiometry were positive only for x = 0.1 in samples calcined 5 h at 973 K. Higher x values resulted in lower activity. (C) 2002 Elsevier Science B.V. All rights reserved

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

Methane and propane combustion over lanthanum transition-metal perovskites: role of oxygen mobility

Catalytic hydrocarbon combustion is a technologically important, but still relatively poorly understood reaction. To shed more light on the role of various physical and chemical characteristics of the catalyst on its activity for hydrocarbon combustion, La1-xSrxM1-yM'O-y(3-delta) perovskites (M and M' represent transition metals) were used as a model system. Four representative compositions were prepared and fully characterized by different methods and their activity was determined in methane and propane combustion. Oxygen desorption reflecting oxygen mobility is mainly a function of composition, more or less independently from specific surface area (SSA). On the other hand, the results confirm that SSA is the important factor determining the high activity. Yet, in these oxide type catalysts (perovskites) the loss of activity due to lower SSA, resulting from aging at elevated temperatures, seems, to a large part, be compensated for by fast oxygen mobility assured in oxygen nonstoichiometric compositions. (C) 2003 Elsevier B.V. All rights reserved

One pot synthesis of mesoporous ternary V2O5-TiO2-SiO2 catalysts

Mixed vanadia-titania-silica catalysts (3 or 6wt% V2O5, and 16-34wt% TiO2) were one pot prepared by sol-gel and hydrothermal methods in the presence of different surfactants. Tetraethylortosilicate (TEOS) was used as precursor for silica, tetraisopropylorthotitanate (TIPOT) for titania, and vanadyl sulfate for vanadia. As surfactants were used cetyltrimethylammonium bromide (CTMABr) or octadecyl-trimethylammonium bromide (ODCABr). The catalysts were characterized by adsorption and desorption curves of N-2 at 77 K, NH3-DRIFTS, H-2-TPR, XRD, in-situ Raman spectroscopy, XPS, and TEM

Effect of LaCoO3 perovskite deposition on ceria-based supports on total oxidation of VOC

Supported LaCoO3 perovskites with 10 wt.% loading were prepared by impregnation of different supports containing ceria with a solution of La and Co nitrates and citric acid. All precursors were calcined at 700 degrees C for 5 h. XRD investigations indicated the perovskite formation via "citrate" precursor only on ceria support. All catalysts were tested for toluene total oxidation in the temperature range 100-600 degrees C. In spite of a large surface area, alumina-supported perovskites showed a lower global activity. It appears then the necessity of the presence of a perovskite phase for good oxidative activity. In terms of reaction rates higher reaction rates per perovskite weight were observed for all supported catalysts when compared to bulk LaCoO3. (c) 2005 Elsevier B.V. All rights reserved