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 VO_x 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 VO_x 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.

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