Infrared spectrometric studies of the surface basicity of metal oxides and zeolites using adsorbed probe molecules

Infrared spectra analysis of species formed by acid probe adsorption on divided metal oxides and alkaline zeolites can lead to information on their surface basicity, particularly on the nature and strength of basic sites. Results obtained from carbon monoxide, carbon dioxide, sulfur dioxide, pyrrole, chloroform, acetonitrile, alcohols, thiols, boric acid trimethyl ether, ammonia and pyridine are critically reviewed. It is concluded that no probe can be universally used. Pyrrole in the case of alkaline zeolites, CO2 for weakly basic metal oxides and for basic OH groups and CO for the characterization of highly basic structural defects on metal oxides activated at high temperature appear quite suitable probes. When other methods are used (TPD, microcalorimetry, volumetry, etc.) IR spectroscopy is a complementary method necessary for the knowledge of the type of sites involved; in particular, data obtained from CO2 and SO2 adsorption have to be carefully used since both probes can lead on highly basic metal oxides to polydentate (bulk) species which do not result from their adsorption but from their reaction with the given oxide. Ammonia and pyridine, generally used as probes for the measure of the acidity of catalysts, also adsorb on basic oxides through a dissociative chemisorption.