Reduction and Oxidation of Cu Species in Cu-Faujasites Studied by IR Spectroscopy

Abstract

The process of reduction (by hydrogen and ethanol) and oxidation (by oxygen and NO) of Cu sites in dealuminated faujasite-type zeolites (of Si/Al = 31) was studied by infrared (IR) spectroscopy with CO (for Cu⁺) and NO (for Cu²⁺) as probe molecules. Two zeolites were studied: one of them contained mostly Cu⁺_exch., whereas another one contained mostly Cu²⁺ and Cu⁺_ox. The susceptibility of various forms of Cu for reduction were investigated. IR experiments of CO sorption evidenced that Cu⁺_ox. was more prone for the reduction than Cu⁺_exch. According to NO sorption studies, Cu²⁺_exch. was reduced in the first order before Cu²⁺_ox. Ethanol reduced mostly Cu²⁺ and, also, some amounts of Cu⁺. The treatment with oxygen caused the oxidation of Cu⁺ (both Cu⁺_exch. and Cu⁺_ox.) to Cu²⁺. The adsorption of NO at 190K produced Cu⁺(NO)₂ dinitrosyls, but heating to room temperature transformed dinitrosyls to mononitrosyls and increased the Cu²⁺ content.

Keywords: CO/NO-IR spectroscopy; Cu sites; ethanol; hydrogen; reduction.