FT-IR ASSESSMENT THROUGH PYRIDINE ADSORPTION OF THE SURFACE-ACIDITY OF ALKALI-DOPED MOO3 TIO2

Adsorption of pyridine on MoO3/TiO2 doped with alkali metal cations (L i, Na, K, and Rb) has been studied by FT-IR spectroscopy in order to a ssess the effect of the nature and concentration of alkali cations on the surface acidity. It has been found that incorporation of the alkal i cations leads to a decrease in the surface acidity and of the ratio between the concentration of Bronsted and Lewis surface acid sites. Th ese decreases are due to (i) formation of weak Lewis acid sites (alkal i cations) with lower electron affinity than Mo6+ and Ti4+ and (ii) ca ncellation of Bronsted sites (Mo-OH) because of formation of alkali mo lybdates. These effects are more pronounced as the alkali content incr eases and/or the polarizing character of the alkali cation is increase d. (C) 1994 Academic Press, Inc.