FTIR study of low-temperature CO adsorption on pure Al2O3-TiO2 and V/Al2O3-TiO2 catalysts

Adsorption of CO at low temperature (77 K) on Al2O3-TiO2 and V2O5(12 wt.%)/Al2O3-TiO2 samples was studied by FTIR spectroscopy. IR spectroscopy of CO adsorbed at low temperature on Al2O3-TiO2 samples suggests an enrichment of TiO2 on the catalyst surface. The IR spectra of the calcined (at 773 K) V2O5(12 wt.%)/Al2O3-TiO2 sample exhibits two kinds of vanadium species, monomeric tetrahedral V5+ = O species (band at 1035 cm(-1)) and an amorphous VOx monolayer phase (band at 1002 cm(-1)). CO adsorption at low temperature on this sample shows the presence of V4+ <-- CO carbonyl species (band at 2197 cm(-1)). As reported in the literature no V5+ carbonyl species are observed. Reduction of the V2O5(12 wt.%)/Al2O3-TiO2 sample at 773 K induces a rearrangement of the amorphous VOx phase, as indicated by the disappearance of the band at 1002 cm(-1). Reduced vanadium species, mainly V3+ species, were observed by CO adsorption at low temperature. This V3+ <-- CO carbonyl complex (monitored by a band at 2185 cm(-1)) is highly resistant to evacuation which suggests some pi-back donation. The V3+ species are highly stable towards reoxidation at low temperature. Reoxidation at room temperature leads to a restoration of the V5+ and V4+ species as observe in the oxidized samples. Only a small amount of V3+ species can not be completely oxidized remaining in an oxidation state of 4+. Hence, two bands at 2197 and 2194 cm(-1) were observed and assigned to two different V4+ Lewis acid sites.