IR study of CO adsorption on Cu-ZSM-5 and CuO/SiO2 catalysts: sigma and pi components of the Cu+-CO bond

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 v(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. The results show that the state of Cu+ is quite different in Cu-ZSM-5 and CuO/SiO2 catalysts. It is assumed that the Cu+ sites on CuO/SiO2 have one coordinative vacancy each, which. leads to formation, primarily, of Cu+-CO monocarbonyls after CO adsorption. On the contrary, the isolated Cu+ ions on Cu-ZSM-5 each possess two vacancies, which determine their ability to form dicarbonyls or to coordinate water and CO simultaneously. On the basis of the results obtained it is concluded that the participation (underestimated up to now) of the sigma component in the Cu+-CO bond plays a decisive role with respect to the frequency of CO adsorbed on Cu+ ions and the stability of the corresponding carbonyls.