Silver Valence State Determines the Water Tolerance of Ag/Al₂O₃ for the H₂–C₃H₆–SCR of NO<i>_x</i>

Abstract

The influence of the silver valence state on Ag/Al₂O₃ on the water tolerance of H₂–C₃H₆–SCR of NO<i>_x</i> 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 °C. Oxidized silver species were predominant on Ag/Al₂O₃ calcined at temperatures below 600 °C (LT-catalysts), while further increasing the calcination (temperatures above 600 °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 H₂–C₃H₆–SCR. Activation energy measurements confirmed that the mechanism of NO<i>_x</i> 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<i>_x</i> performance and water tolerance