Discerning the Role of Ag–O–Al Entities on Ag/γ-Al₂O₃ Surface in NOx Selective Reduction by Ethanol

Abstract

Alumina-supported silver catalysts (Ag/Al₂O₃) derived from AlOOH, Al­(OH)₃, and Al₂O₃ were investigated for the selective catalytic reduction of NOx by ethanol. In order to discern the role of support Al skeleton in anchoring silver species and reducing NOx, the series of alumina-supported silver catalysts calcined at different temperatures was characterized by means of <i>in situ</i> DRIFTS, XPS, UV–vis DRS, XRD, BET, and NMR. It was found that the NO_<i>x</i> reduction efficiency order as affected by alumina precursors could be generally described as AlOOH > Al₂O₃ ≫ Al­(OH)₃, with the optimum calcination temperature of 600 °C. XPS and UV–vis results indicated that silver ions predominated on the Ag/Al₂O₃ surface. Solid state NMR suggested that the silver ions might be anchored on Al tetrahedral and octahedral sites, forming Ag–O–Al_tetra and Ag–O–Al_octa entities. With the aid of NMR and DFT calculation, Al_octa was found to be the energetically favorable site to support silver ions. However, DFT calculation indicated that the Ag–O–Al_tetra entity can significantly adsorb and activate vital −NCO species rather than the Ag–O–Al_octa entity. A strongly positive correlation between the amount of Al_tetra structures and N₂ production rate confirms the crucial role of Al_tetra in NOx reduction by ethanol