IR study of CO adsorption on Cu-ZSM-5 and CuO/SiO2catalysts: σ and π components of the Cu+—CO bond

Abstract

Adsorption of carbon monoxide on CuO/SiO2(1 wt.% CuO) and Cu-ZSM-5 (11 wt.% CuO) catalysts has been studied by IR spectroscopy. CO adsorption on CuO/SiO2 leads to formation of: (i) three kinds of unstable Cu2+—CO species detected only under equilibrium CO pressure and characterized by ν(CO) at 2216, 2199 and 2180 cm–1, respectively, and (ii) one kind of Cu+—CO carbonyl manifesting an IR band at 2126.5 cm–1. The latter carbonyls possess moderate stability, and some of them are removed upon evacuation. Water replaces CO preadsorbed on the Cu+ ions. Testing the surface of Cu-ZSM-5 with CO reveals the existence of two types of sites: (i) associated Cu+ cations, monitored by a CO band at 2137 cm–1 whose intensity is reduced during evacuation, and (ii) isolated Cu+ sites, which form, at high CO equilibrium pressures, dicarbonyls (bands at 2177.5 and 2151 cm–1). Decrease in CO pressure leads to destruction of these species according to the reaction Cu+(CO)2→ Cu+—CO + CO and after evacuation only monocarbonyls are detected by a band at 2158.5 cm–1. These monocarbonyls are stable and resistant towards evacuation. Water is coadsorbed with CO on the isolated Cu+ sites, which is accompanied by a ca. 30 cm–1 red shift of the 2158.5 cm–1 band. This shift is reversible and the original band position is restored after subsequent evacuation.