Nature of Ag Species on Ag/γ-Al2O3: A Combined Experimental and Theoretical Study

The nature of silver species on Ag/Al2O3 catalysts with different silver loadings was studied by photoelectron spectroscopy (XPS) and X-ray absorption near-edge spectroscopy (XANES) and extended X-ray absorption fine structure spectroscopy (EXAFS) combined with theoretical calculation (DFT). On the basis of selective catalytic reduction of NOx by ethanol experiments, it was found that the optimum silver content varies from 1 wt % to 2 wt %. The supported silver species are predominated by +1 oxidation state ions attached to surface oxygen atoms (Ag-O) under low silver loading of 2 wt 96, which play a crucial role during the HC-SCR process. An Ag-Ag shell emerged dearly in analysis of EXAFS data when silver loading was increased to 2 wt 96, which was beneficial for low-temperature activity. The theoretical models for Ag-n(delta+) species (1 <= n <= 4, both ions and oxidized silver clusters) on alumina were consistent with the coordination structure analysis by EXAFS. The predominant silver ions are most likely stabilized at isolated tetrahedral Al sites (Ag-O-Al-IVb) on the gamma-Al2O3 (110) surface. However, the most reactive silver ion seems to be anchored on a tricoordinate Al-III site (Ag-O-Al-III). Density of states analysis revealed that the Ag-O-Al-III, entity might be a very active silver species in terms of the hybridization of Ag, O, and Al orbitals to promote its catalytic activity.