Raman Spectroscopic Study of V/θ-Al2O3 Catalysts: Quantification of Surface Vanadia Species and Their Structure Reduced by Hydrogen

Abstract

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-Al2O3 for the self-absorption effect. On the basis of the ability to selectively detect monovanadate (UV-excited), polyvanadate (visible-excited), and V2O5 (visible-excited) in the Raman measurements, the distribution of monovanadate, polyvanadate, and V2O5 present on dehydrated V/θ-Al2O3 samples was successfully quantified as a function of surface VOx density. It is shown that monovanadate species are present at all surface VOx densities studied but are the dominant species at low surface VOx density. Polyvanadate and V2O5 are also present and predominate on the surface at intermediate and high surface VOx density. The UV- and visible-excited Raman studies of the V/θ-Al2O3 samples reduced in hydrogen show that polyvanadate and V2O5 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/θ-Al2O3 with bulk vanadium oxide compounds suggests that reduced VOx species can assume a V2O3-like form. The reduced VOx species redisperse on the support surface upon reoxidation.