Effect of SO2 on the activity of Ag/γ-Al2O3 catalysts for NOx reduction in lean conditions

XPS analysis showed that the dispersion of silver species decreased by exposure to the feed gas containing SO2. Crystallite silver sulfate (Ag2SO4) was observed in the XRD pattern for the 8 wt.% Ag/Al2O3. The catalyst performance and characterization results suggested that the silver sulfate species enhanced the NOx reduction capability via producing more intermediate species (NCO). Eventual decrease in the performance could be attributed to SO2 poisoning of the alumina sites, which are responsible for NOx reduction to N2. A sample containing selectively sulfated silver sites showed higher NOx reduction performance than a sample containing silver oxide. However, when alumina sites were sulfated it demonstrated lower NOx reduction than a pure alumina. With the results obtained from this study the order of apparent catalytic activity for silver species to enhance NOx reduction on alumina is proposed as the following: Ag2SO4 > Ag2O ≫ Ag0.