Silver Valence State Determines the Water Tolerance of Ag/Al2O3 for the H2-C3H6-SCR of NOx

The influence of the silver valence state on Ag/Al2O3 on the water tolerance of H-2-C3H6-SCR of NO was investigated. The valence state of silver species on Ag/Al2O3, which was carefully characterized by XPS, UV vis, and XANES measurements, was adjusted by varying the calcination temperature from 500 to 900 degrees C. Oxidized silver species were predominant on Ag/Al2O3 calcined at temperatures below 600 degrees C (LT-catalysts), while further increasing the calcination (temperatures above 600 degrees C, HT-catalysts) promoted the transformation of oxidized silver species into metallic silver clusters. The samples with higher amounts of oxidized silver species exhibited better water tolerance in the H-2-C3H6-SCR. Activation energy measurements confirmed that the mechanism of NOx reduction on these catalysts was the same. In situ DRIFTS studies demonstrated that metallic silver species promoted the formation of active enolic species and the complete oxidation of formate, thus improving the low-temperature activity of HT-catalysts in the absence of water vapor. Water addition eliminated the formate, releasing the active Ag+ sites for enolic species formation, and thus promoted the low-temperature activity of LT-catalysts. From a comprehensive point of view, 60% oxidized silver species on Ag/Al2O3 catalysts is the optimal percentage for deNO(x) performance and water tolerance.