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.