Tackling the problem of sulfur poisoning of perovskite catalysts for natural gas combustion

LaMn_0.5Mg_0.5O₃y MgO and LaCr_0.5-xMn_xMg_0.5O₃.y MgO catalysts (x=0-0.25, y=0-17) were prepared, characterized (XRD, BET, SEM-EDS, TEM, FTIR, chemical and atomic absorption analyses), tested for high-temperature methane combustion and aged in presence of SO₂ to investigate sulfur effect on catalytic activity. Different roles of manganese and chromium in the perovskite structure have been assessed: (i) manganese improves the activity of catalysts in the fresh state, whereas chromium worsens it; (ii) owing to its basic nature, manganese makes perovskites more prone to adsorb SO₂ and so less resistant to sulfur poisoning, whereas chromium, owing to its acidic nature, has opposite effects; (iii) the partial solubility of chromium oxide in basic media renders Cr-catalyst regeneration by NH₃ leaching less effective. MgO promotes catalytic activity of fresh LaCr_0.5Mg_0.5O₃.y MgO catalysts and slows down sulfur poisoning of LaMn_0.5Mg_0.5O₃.y MgO and LaCr_0.25Mn_0.25Mg_0.5O₃.17 MgO catalysts only.