INFRARED-SPECTROSCOPY OF SUPPORTED METAL-OXIDE CATALYSTS

In situ infrared spectroscopy can provide much fundamental information about the nature of the surface metal oxide species present in supported metal oxide catalysts. The molecular structure of the surface metal oxide species can be obtained from M=O vibrations (number of different species and the number of terminal M=O bonds present in each species). The location of the surface metal oxide species on the oxide supports can be determined by directly monitoring the specific surface hydroxyls of the support being titrated by the surface metal oxide species. The surface coverage of the surface metal oxide species on the oxide supports can only be qualitatively determined by monitoring the surface hydroxyl region and the chemisorption of CO2 because these measurements vary nonlinearly with surface metal oxide coverage. The nature of surface Lewis and Bronsted acid sites present in supported metal oxide catalysts is readily determined by adsorbing basic probe molecules such as pyridine. Only surface Lewis acid sites are present at low surface coverages and both surface Lewis and Bronsted acid sites can be present at high surface coverages. The appearance of Bronsted acid sites is related to the surface density of the surface metal oxide species. Specific examples are presented for supported rhenium oxide catalysts.