Insight into the origin of sulfur tolerance of Ag/Al2O3 in the H2-C3H6-SCR of NOx

The sulfur tolerance of Ag/Al2O3 catalysts in H2-assisted C3H6-SCR was investigated by UV–vis, TPR, TPSR, DRIFTS-MS, and DFT calculations. Ag/Al2O3 with higher silver loadings exhibited better deNOx performance and sulfur tolerance, especially the 4% Ag/Al2O3 catalyst. UV–vis and H2-TPR revealed that highly dispersed Ag+ cations were predominant on 2% Ag/Al2O3, while more metallic Ag clusters with large sizes were present on the 4% Ag/Al2O3. After exposure to SO2, large amounts of sulfates were adsorbed on the Ag sites and Al sites of the Ag/Al2O3 surface. The sulfates were reduced to H2S and SO2 in a reducing atmosphere, while they showed little decomposition under real SCR reaction conditions. DRIFTS-MS experiments showed that sulfate species transferred rapidly between Ag sites and Al sites on the Ag/Al2O3 catalysts with higher amounts of Ag clusters. DFT calculations revealed that Ag1 cations show stronger affinity for sulfate species than Ag clusters, thus resulting in blockage by sulfates at the Ag-O-Al interface. Such blocking by sulfates suppressed the activation of C3H6 as well as the formation of -NCO species, and thus severely inhibited the deNOx performance of 2% Ag/Al2O3. In contrast, the rapid mobility of sulfate species on 4% Ag/Al2O3 made more active sites available for the formation of key intermediates of HC-SCR, finally contributing to its excellent sulfur tolerance.