INFRARED SPECTROSCOPIC INVESTIGATIONS OF CARBON-MONOXIDE PHYSICALLY ADSORBED ON DECATIONATED Y-TYPE ZEOLITE

The physical adsorption of CO at low temperature on HY zeolite leads to the formation of three types of physisorbed CO, depending on the CO pressure. At low CO pressure, CO primarily interacts with HF hydroxyls through hydrogen bonding and gives rise to nu(CO) at 2176 cm-1 (species A). Simultaneously, the nu(OH) vibration of the perturbed hydroxyls is shifted by 298 cm-1 toward lower wavenumbers, which provides a measurement of the acid strength of the protons. The saturation of hydroxyl sites is followed by the formation of more weakly physisorbed CO (species C) exhibiting nu(CO) at 2160 cm-1, tentatively ascribed to CO adsorbed on framework oxygens acting as basic sites. At higher CO pressures, a liquid-like CO phase (species B, nu(CO) = 2140 cm-1) forms extensively, exhibiting hindered rotational behaviour. The progressive formation of OH-CO complexes is accompanied by frequency shifts of all nu(OH) and nu(CO) vibrations, whether the corresponding species are directly involved or not in the CO adsorption. In particular, LF hydroxyls, although not interacting with physisorbed CO, experience coverage-dependent downward shifts. These phenomena are demonstrated to correlate better with CO-induced effects than with intrinsic acid strength heterogeneity.