Redox behaviour of transition metal ions in zeolites. Part 2.—Kinetic study of the reduction and reoxidation of silver-Y zeolites

Abstract

The consumption of hydrogen over Ag⁺-Y and of oxygen over Ag°-Y zeolites was followed kinetically. I.r. spectroscopy and X-ray spectrometry were used to characterize the products obtained. The results revealed the presence of a high and a low temperature reduction mechanism. Quantitative reduction of the Ag⁺ ions requires higher temperatures and the migration of cations from sites SI becomes rate controlling. Reduction at low temperatures results in the formation of highly dispersed Ag clusters containing unreduced Ag (Ag⁺ ₃). The reduction is fully reversible. At low temperatures only highly dispersed silver in the zeolite cages is reoxidized. Reoxidation of external Ag clusters (21 nm) requires a temperature of at least 500 K and results in the migration of Ag ions into the zeolite matrix.