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

Abstract

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+â†�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+â†�CO carbonyl complex (monitored by a band at 2185 cm-1) is highly resistant to evacuation which suggests some π-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.