Electron spin resonance study of the interaction of oxygen with Ag/SiO2

Abstract

The adsorption of oxygen on Ag/SiO₂ catalyst prepared by the cation-exchange method was studied by means of the electron spin resonance (e.s.r.) technique. Observed e.s.r. signals include Ag°(g= 2.001, ΔH= 20 G), Ag²⁺(g_⊥= 2.032, g_∥= 2.293, A_⊥= 32 G), O^–₂(g₁= 2.035, g₂= 2.015, g₃= 2.003) and an unidentified peak with g= 1.977. The intensity of these peaks varied with reduction and oxidation treatments. The area of the unidentified peak was found to be maximized as the catalyst was partially reduced. This same condition also stabilized the O^–₂ adsorbed on the silver.

The intensity of these e.s.r. signals obtained on exposing reduced catalysts to oxygen also changed with the systems temperature. O^–₂ increased initially with the temperature, reached a maximum concentration at 65 °C, and decreased at higher temperature. Ag²⁺ also increased with the temperature at T < 150 °C. However, its signal suddenly disappeared above this temperature. The Ag° signal, which was lost on oxygen exposure, reappeared as the temperature was raised above 250 °C. A mechanism is proposed to account for these observed variations.