CATALYTIC COMBUSTION OF METHANE OVER BULK AND SUPPORTED LACRO3 PEROVSKITES

LaCrO3 perovskite samples have been prepared in the bulk state as well as in the supported state. Whatever the nature of the precursors, bulk perovskites exhibit low specific areas, leading to a low activity in the catalytic combustion of methane; changes in catalytic activity have been connected with modifications of the B.E.T. areas of the bulk material. The activity of LaCrO3 is strongly enhanced by supporting the perovskite on carriers of high specific area like alumina or magnesium aluminate. In the fresh state, the increase in catalytic activity is due to the increase of the dispersion state of the perovskite phase. In the case of the Al2O3 support, spectroscopic measurements show that ca. 70% of the alumina surface is covered by LaCrO3 and catalytic activity is proportional to the fraction of alumina covered by LaCrO3. Supported perovskites have been aged at 1340 K under a stoichiometric (air-methane) mixture. For the LaCrO3/Al2O3 sample, the loss of activity has been correlated with a sintering of the perovskite phase onto the surface of alumina and does not follow the global area of this material. For the LaCrO3/MgAl2O4 sample, the loss of specific area agrees with the decrease in catalytic activity, suggesting that the coverage of MgAl2O4 by LaCrO3 is preserved, although sintering of the solid occurs. For supported perovskites in both fresh and aged states, no incorporation of Cr3+ ions into the support lattice was observed, probably because of the procedure of preparation.