FTIR study of low-temperature adsorption and co-adsorption of (CO)-C-12 and (CO)-C-13 on a TiO2-SiO2 mixed oxide

Adsorption of (CO)-C-12 on a TiO2-SiO2 sample prepared via the homogeneous precipitation method (Ti: Si molar ratio 1 : 1) is negligible at ambient te mperature. At 85 K, however, different sites are monitored by CO: two kinds of Ti4+ cations (upsilon(CO) at 2185-2199 and 2173-2177 cm(-1), respective ly) and CO attached to surface silanol groups (2156 cm(-1)). Although the h igher frequency carbonyls are more stable, all species disappear after evac uation at 85 K. The Ti4+ -CO band positions are coverage dependent: the fir st band is shifted from 2199 cm(-1) at low coverage to 2185 cm-l at high co verage, while the second band shifts from 2177 cm(-1) at low coverage to 21 73 cm(-1) at high coverage. Co-adsorption of (CO)-C-12-(CO)-C-13 isotopic m ixtures reveals that these shifts are entirely static in character. Some co nclusions about the acidity and distribution of the Ti4+ cations on the sam ple surface are drawn. (C) 1999 Elsevier Science B.V. All rights reserved.