Sulphur poisoning of LaMn1-xMgxO3 catalysts for natural gas combustion

Ageing treatments were carried out on LaMn{sub 1-x}Mg{sub x}O{sub 3+{delta}} (x=0, 0.2, 0.5) catalysts for methane combustion to throw light on their deactivation mechanism in the presence of tetrahydrothiophene - used in Italy as odorant of natural gas - or directly in the presence of SO{sub 2}. Various characterisation techniques were used in concert: XRD, BET, TEM, SEM-EDS, FTIR, TPD, chemical analysis and atomic absorption. The effects of treatment temperature and exposure on catalytic activity were evaluated: exposure to 200ppmv SO{sub 2} at 800C for 24h causes a rather drastic deactivation of each LaMn{sub 1-x}Mg{sub x}O{sub 3} catalyst. Metal sulphate formation is the major responsible for deactivation. However, the extent of poisoning as well as the catalyst regenerability depends on the catalyst composition. The LaMn{sub 0.5}Mg{sub 0.5}O{sub 3} catalyst can in fact be regenerated completely by water leaching, while LaMn{sub 0.8}Mg{sub 0.2}O{sub 3} and LaMnO{sub 3} perovskites can be regenerated significantly only by leaching with aqueous NH{sub 3} solutions. An explanation of these results, also based on the values of activation energy for each fresh, poisoned and regenerated catalysts, is suggested and a mechanism for catalyst regeneration by water and NH{sub 4}OH solution is proposed.