Use of combination modes and overtones of metal carbonyls for the ir study of cation states in zeolites: copper(I) carbonyls in reduced CuNa Y zeolites

Abstract

Diffuse reflectance FTIR spectroscopy has been applied to the study of fundamental CO 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 °C. The frequencies of the combination modes of copper(I) carbonyls, e.g. fundamental CO bond stretching vibrations plus M—C stretching or ∠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 CO bonds. Their analysis proved the formation of polycarbonylic species Cu⁺(CO)_x(x 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.