IR STUDY OF CO ADSORPTION ON CU-ZSM-5 AND CUO SIO2 CATALYSTS - SIGMA-COMPONENT AND PI-COMPONENT OF THE CU+-CO BOND/

Adsorption of carbon monoxide on CuO/SiO2 (1 wt.% CuO) and Cu-ZSM-5 (1 1 wt.% CuO) catalysts has been studied by IR spectroscopy. CO adsorpti on on CuO/SiO2 leads to formation of: (i) three kinds of unstable Cu2-CO species detected only under equilibrium CO pressure and characteri zed 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 remo ved upon evacuation. Water replaces CO preadsorbed on the Cu+ ions. Te sting the surface of Cu-ZSM-5 with CO reveals the existence of two typ es of sites: (i) associated Cu+ cations, monitored by a CO band at 213 7 cm(-1) whose intensity is reduced during evacuation, and (ii) isolat ed Cu+ sites, which form, at high CO equilibrium pressures, dicarbonyl s (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 to wards evacuation. Water is coadsorbed with CO on the isolated Cu+ site s, 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 r estored after subsequent evacuation. The results show that the state o f Cu+ is quite different in Cu-ZSM-5 and CuO/SiO2 catalysts. It is ass umed 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 dicarbon yls or to coordinate water and CO simultaneously. On the basis of the results obtained it is concluded that the participation (underestimate d up to now) of the sigma component in the Cu+-CO bond plays a decisiv e role with respect to the frequency of CO adsorbed on Cu+ ions and th e stability of the corresponding carbonyls.