(280c) Selective Catalytic Reduction of No with Methane over Ag-Alumina Catalysts in the Presence of So2

In this work, we investigated the performance of Ag-alumina catalysts for the SCR of NO with methane in gas streams with a high concentration of SO2, typical of coal-fired power plant flue gas. Ag-alumina catalysts were prepared by co-gelation and nitric-acid leaching was used to remove weakly bound silver species from the catalyst surface [1, 2]. We found that SO2 has a dramatic inhibitory effect on these catalysts, essentially quenching the SCR reaction at T <600oC. At temperatures higher than 600oC, the CH4-SCR of NO occurs without catalyst deactivation even in 1000 ppm SO2, the SO2 effect was reversible, and quick activity recovery was observed after removal of SO2. Further, co-addition of water vapor (6.2%) does not change the above findings.

At low temperatures, SO2 adsorbs strongly on the catalyst surface forming sulfates on both alumina and silver. We determined that sulfates are not active for CH4-SCR of NOx. The reaction light-off coincides with the onset of silver sulfate decomposition, indicating the critical role of silver, which is associated with methane activation [2]. SO2 is reversibly adsorbed on silver above 600oC, while alumina sites remain sulfated, and sulfated alumina decreases the extent of adsoption of NOx. Kinetics results indicate that SO2 increases the apparent activation energy for CH4-SCR of NOx on Ag-alumina catalysts, suggesting the participation of SO2 in the reaction pathway. Our results also indicate a partial contribution of gas phase reactions to the formation of N2 above 600oC.

References: 1. A. Keshavaraja, X. She and M. Flytzani-Stephanopoulos, “Selective catalytic reduction of NO with methane over Ag-alumina catalysts”. Applied Catalysis B: Environmental, 2000. 27(1): p. L1-L9. 2. X. She and M. Flytzani-Stephanopoulos, “The role of AgOAl species in silver-alumina catalysts for the selective catalytic reduction of NOx with methane”. Journal of Catalysis, 2006. 237(1): p. 79-93.