Sulphur poisoning of LaMn1−xMgxO3 catalysts for natural gas combustion

Ageing treatments were carried out on LaMn1−xMgxO3+δ (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 SO2. 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 200 ppmv SO2 at 800°C for 24 h causes a rather drastic deactivation of each LaMn1−xMgxO3 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 LaMn0.5Mg0.5O3 catalyst can in fact be regenerated completely by water leaching, while LaMn0.8Mg0.2O3 and LaMnO3 perovskites can be regenerated significantly only by leaching with aqueous NH3 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 NH4OH solution is proposed.