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 °C. The high activity is attributed to the presence of Ag⁰ at the surface of SiO₂, which enhances the generation of superoxide O₂⁻, 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 °C). XPS analysis performed on the catalysts before and after their use in DPM oxidation revealed that a fraction of Ag⁰ 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.