Electronic state of silver in Ag/SiO2 and Ag/ZnO catalysts and its effect on diesel particulate matter oxidation: An XPS study

Diesel particulate matter (DPM) oxidation activities of 3%Ag/SiO2 and 3%Ag/ZnO catalysts were investigated. The catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy (XPS) and diffuse reflectance spectroscopy (DRS). 3%Ag/SiO2 showed excellent activity for DPM oxidation below 300 degrees C. The high activity is attributed to the presence of Ag at the surface of SiO2, which enhances the generation of superoxide O-2(-), the highly active species in the oxidation reactions. The activity of the catalyst did not change up to 6 oxidation cycles, indicating there is no change in electronic state of Ag during high temperature oxidation of DPM. However, 3%Ag/ZnO exhibited a very low DPM oxidation activity at the studied temperature range (25-600 degrees C). XPS analysis performed on the catalysts before and after their use in DPM oxidation revealed that a fraction of Ag in the 3%Ag/ZnO converts to Ag1+ state, probably due to the electron transfer from the Fermi level of the low work function metallic Ag to the conduction band of high band gap n-type semiconductor ZnO. We demonstrate a direct relation between the DPM oxidation activity of silver and its electronic state at the catalyst surface. (C) 2017 Elsevier B.V. All rights reserved.