ACIDIC PROPERTIES OF ALUMINA-SUPPORTED METAL-OXIDE CATALYSTS - AN INFRARED-SPECTROSCOPY STUDY

The infrared spectra of the hydroxyl region and that of surface chemisorbed CO2 Species for Re2O7/Al2O3, CrO3/Al2O3, MoO3/Al2O3, V2O5/Al2O3, TiO2/Al2O3, and Nb2O5/Al2O3 catalytic systems have been investigated. A sequential consumption of the alumina OH groups upon deposition of the supported metal oxide has been found for all the investigated catalytic systems. A possible relationship between Bronsted acidity and a new low-frequency band in the hydroxyl region observed at high loadings of the supported metal oxide systems is postulated. The various chemisorbed CO2 surface species formed on the uncovered parts of the exposed surface of alumina are identified. Furthermore, the applicability of the infrared CO2 chemisorption technique as a general method to determine the monolayer coverage for alumina-supported metal oxides has been confirmed because CO2 adsorption is suppressed as monolayer coverage is approached. Infrared pyridine chemisorption data for selected alumina-supported metal oxide catalysts are quantified, and a simple model for the Bronsted acid site is proposed. Comparison with the molecular structures of the surface metal oxide overlayer, determined by Raman spectroscopy, reveals that there is no correlation between the surface metal oxide structures and the corresponding surface hydroxyl chemistry, Bronsted acidity, and CO2 chemisorption.