Electron paramagnetic resonance study of the interactions of oxygen with silver/titania

Abstract

A 2% Ag/TiO₂ catalyst has been prepared by the impregnation method. Hydrogen reduction and oxygen adsorption treatments of this sample were studied by electron paramagnetic resonance (EPR) spectroscopy. Signals from Ag⁰(g= 2.001, ΔH= 12 G), Ag²⁺(g_⊥= 2.032, g_‖= 2.293, A_⊥= 32 G), O^–₂(g₁= 2.035, g₂= 2.015, g₃= 2.003), and Ti³⁺(g≈ 1.97) were observed. The intensity of these peaks varied with the temperature of hydrogen reduction and the time of oxygen adsorption. Ag⁰ and O^–₂ signals were maximized in a sample reduced at 473 K. Severe reduction at higher temperatures caused sintering to the supported silver crystallites and consequently, a diminution of the Ag⁰ signal. Ti³⁺ was generated by hydrogen spillover through silver. This species may be destroyed by appropriate calcinations. Two kinds of Ti³⁺ signal (a and b) were observed. The signal a (probably from TiOOH) was generated on the spillover but changed to signal b (probably from Ti₂O₃) on evacuation at T > 573 K. The O^–₂ signal was observed after reduced Ag/TiO₂ samples were partially oxidized by gaseous oxygen. The site of O^–₂ adsorption is at the interface of Ag crystallites with TiO₂.