Raman spectroscopic study of V/θ-Al ₂O ₃ catalysts: Quantification of surface vanadia species and their structure reduced by hydrogen

Two interesting topics about supported vanadia catalysts were studied using in situ UV and visible Raman and UV-vis diffuse reflectance spectroscopy (DRS): The quantification of different surface vanadia species and the hydrogen reduction of these vanadia species. Using the diffuse reflectance value as an external standard, we could correct the Raman intensity measurements of V/θ-Al ₂O ₃ for the self-absorption effect. On the basis of the ability to selectively detect monovanadate (UV-excited), polyvanadate (visible-excited), and V ₂O ₅ (visible-excited) in the Raman measurements, the distribution of monovanadate, polyvanadate, and V ₂O ₅ present on dehydrated V/θ-Al ₂O ₃ samples was successfully quantified as a function of surface VO _x density. It is shown that monovanadate species are present at all surface VO _x densities studied but are the dominant species at low surface VO _x density. Polyvanadate and V ₂O ₅ are also present and predominate on the surface at intermediate and high surface VO _x density. The UV- and visible-excited Raman studies of the V/θ-Al ₂O ₃ samples reduced in hydrogen show that polyvanadate and V ₂O ₅ are more easily reduced than monovanadate species. UV Raman is better able to obtain information on reduced vanadia species than visible Raman, mainly because of a decrease in self-absorption and resonance enhancement in the UV region. Comparison of the UV Raman spectra from reduced V/θA1 ₂O ₃ with bulk vanadium oxide compounds suggests that reduced VO _x species can assume a V ₂O ₃-like form. The reduced VO _x species redisperse on the support surface upon reoxidation.