USE OF COMBINATION-MODES AND OVERTONES OF METAL-CARBONYLS FOR THE IR STUDY OF CATION STATES IN ZEOLITES - COPPER(I) CARBONYLS IN REDUCED CUNAY ZEOLITES

Diffuse reflectance FTIR spectroscopy has been applied to the study of fundamental C=O bond stretching vibrations and their first overtones, and combination modes of copper(I) carbonyls, formed upon CO adsorption on CuNaY zeolite after reduction with CO at 400 degrees C. The frequencies of the combination modes of copper(I) carbonyls, e.g. fundamental C=O bond stretching vibrations plus M-C stretching or angle MCO bending vibrations, were found to be much more sensitive to the composition and structure of Cu+-CO complexes than those of the fundamental stretching vibrations of C=O bonds. Their analysis proved the formation of polycarbonylic species Cu+(CO)(x)(x greater than or equal to 2) at room temperature and CO pressures of several kPa. These species, after removal of gaseous CO, were reversibly transformed into the thermally stable monocarbonyls. This finding prompted reconsideration of the literature data on the composition, structure and localisation of Cu+ carbonyls in Y zeolite. It was concluded that: (1) the reduction by CO of the Cu2+ ions to Cu+ proceeds without dealumination of the zeolite framework and creation of true Lewis acidic sites. (2) During adsorption of CO, Cu+ ions migrate from the sodalite cages into the supercages of the zeolite. (3) In the presence of gaseous CO the mean stoichiometry of Cu+ carbonyls differs from 1 CO molecule per 1 Cu+ cation. (4) A significant fraction of Cu+ ions is located in cationic positions, perhaps at S-I and S-I sites, inaccessible to adsorbed CO molecules even at high CO pressures. (5) Cu+ forming carbonyls are located in the supercages, probably in S-II cationic positions.