A study of the Influence of the Synthesis Conditions upon the Catalytic Properties of LaMnO_3.15 in Methane Combustion in the Absence and Presence of H₂S

Abstract

We report here on the activity and stability of LaMnO_3.15 for the methane combustion, in the absence and presence of H₂S, in a temperature interval of 250–750 °C. Two powders with different specific surface area were prepared by coprecipitation method using ammonia. Precursors calcined at high temperature, in air, for 10 h have led to LaMn-C solid with S_BET = 11 m²/g, while those previously aged in solution (hydrothermal treatment at 200 °C under 20 atm. for 24 h) then calcined at high temperature led to LaMn-HydC with S_BET = 31 m²/g. Temperature programmed reduction (TPR) profile of both samples showed two main peaks; surface and weakly bound oxygen named α-oxygen species and lattice oxygen β-oxygen species. While for LaMn-C the maximum reduction temperature peak corresponding to α-oxygen species was found to be ca. 600 °C, for LaMn-HydC samples this peak was shifted to lower temperature ca. 430 °C. Indeed, LaMn-HydC samples showed higher depletion of surface and weakly bound oxygen species compared to LaMn-C. The superior catalytic performance of LaMn-HydC in methane combustion was attributed to its high BET surface area and to both the high amount of α-oxygen species and their mobility. In the presence of 100 ppm H₂S in the feed this catalyst showed a higher propensity to poisoning by sulphur compounds than LaMn-C. This was attributed to the rapid formation of stable sulphate/sulphite species, the decomposition of which occurs above 800 °C.