Direct reduction of SO2 to elemental sulfur by methane over ceria-based catalysts

The catalytic reduction of SO2 to elemental sulfur by methane was studied over ceria-based catalysts. Both La-doped and undoped ceria were found to catalyze the SO2 reduction by CH4 in the temperature range of 550-750 degrees C at atmospheric pressure and with feed gases containing a molar ratio of CH4/SO2 = 0.5-3. At temperatures below similar to 550 degrees C, the catalyst surface is capped by SO2. The reaction light-off coincides with the threshold temperature for sulfate decomposition. Various SO2/CH4/H2O gas mixtures were used to study the catalyst activity and selectivity to elemental sulfur. The incorporation of nickel and copper improves the wet activity of Ce(La)Ox catalysts. Also, the addition of a low amount (5 at.%) of copper or nickel into La-doped ceria, Ce(La)Ox, had a markedly different effect on the catalyst selectivity under fuel-rich conditions. The Cu-Ce(La)Ox catalyst has higher selectivity to elemental sulfur and complete oxidation products (H2O, CO2), while Ni-Ce(La)Ox favors the formation of H2S and partial oxidation products (CO). The catalyst activity/selectivity studies were complemented by SO2 uptake experiments and by reduction of the as-prepared and pre-sulfated catalysts in methane, both isothermally and in the TPR mode. The initial rate of reduction of pre-sulfated catalysts with methane is similar for Ce(La)Ox, Cu-Ce(La)Ox and Ni-Ce(La)Ox in the temperature range of 500-700 degrees C. Activation of methane on these surfaces requires partial sulfate decomposition. (C) 1999 Elsevier Science B.V. All rights reserved.