METHANE COMBUSTION OVER LA0.8SR0.2MNO3+X SUPPORTED ON MAL2O4 (M=MG, NI AND CO) SPINELS

Catalysts containing 20 wt.-% La0.8Sr0.2MnO3+x supported on spinels (MAl2O4), were prepared by impregnation. Three different supports (MgAl2O4, NiAl2O4 and CoAl2O4) were prepared by coprecipitation. X-ray diffraction line broadening indicated that the mean particle size of the deposited perovskite depended on the spinel used as support. Oxygen evolution measurements carried out in helium in the temperature range from 300 to 1360 K showed that the thermal decomposition proceeded similarly on all three supported catalysts, beginning at about 800 K. The amount of oxygen which evolved was found to depend on the particle size of the perovskite. The smaller the perovskite particles were, the more stable they were toward thermal decomposition. Kinetic tests were performed in a fixed-bed microreactor at 600-1200 K and atmospheric pressure using a reactant mixture with a ratio CH4:O2 = 1:4. The reaction rate per gram perovskite, measured at 770 K, was more than ten times higher for La0.8Sr0.2MnO3+x/MgAl2O4 than for unsupported La0.8Sr0.2MnO3+x. The high activity of the MgAl2O4-supported perovskite is attributed to the high dispersion of the perovskite on this catalyst, leading to a larger active surface area compared to the unsupported La0.8Sr0.2MnO3+x.