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 adsorpti on on CuNaY zeolite after reduction with CO at 400 degrees C. The freq uencies of the combination modes of copper(I) carbonyls, e.g. fundamen tal C=O bond stretching vibrations plus M-C stretching or angle MCO be nding vibrations, were found to be much more sensitive to the composit ion and structure of Cu+-CO complexes than those of the fundamental st retching 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 li terature data on the composition, structure and localisation of Cu+ ca rbonyls in Y zeolite. It was concluded that: (1) the reduction by CO o f the Cu2+ ions to Cu+ proceeds without dealumination of the zeolite f ramework and creation of true Lewis acidic sites. (2) During adsorptio n of CO, Cu+ ions migrate from the sodalite cages into the supercages of the zeolite. (3) In the presence of gaseous CO the mean stoichiomet ry of Cu+ carbonyls differs from 1 CO molecule per 1 Cu+ cation. (4) A significant fraction of Cu+ ions is located in cationic positions, pe rhaps 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 su percages, probably in S-II cationic positions.