Water Effect on Preparation of Ag/Al2O3 Catalyst for Reduction of NOx by Ethanol

Alumina-supported silver catalysts (Ag/Al2O3) derived from wet impregnation, a semiwet method, and water-free ball-milling were investigated for the selective catalytic reduction of NOx by ethanol. It was found that Ag/Al2O3 catalysts with the highest NOx reduction efficiency can be obtained via wet impregnation and the semiwet method but not by water-free ball milling. To optimize the preparation parameters and discern the effect of water in the production of Ag/Al2O3 catalysts, the resulting series of alumina-supported silver samples were characterized by means of XRD, BET, XAS (XANES and EXAFS), in situ DRIFTS, and NMR. The results indicated that silver species were oxidized and highly dispersed on the large surface area Al2O3 obtained from wet impregnation and the semiwet method. In situ DRIFTS revealed that the vital intermediate enolic species were correspondingly predominant on the surfaces of catalysts prepared by these methods, while the inactive intermediate acetate species were significantly formed on Ag/Al2O3 catalysts prepared by water-free ball milling. Solid state H-1 NMR suggested that the water employed in the wet methods enhances the exchange of Ag+ with protons from hydroxyl groups on the alumina surface, which leads to silver species being highly dispersed and forming active Ag-O-Al entities.