The structure of vanadium oxide species on gamma-alumina; an in situ X-ray absorption study during catalytic oxidation

Abstract

The mechanism of catalytic oxidation reactions was studied using in situ X-ray absorption spectroscopy (XAFS) over a 17.5 wt% V2O5/Al2O3 catalyst, i.e., at reaction temperatures and in the presence of reactants. It was found that X-ray absorption near-edge structure (XANES) is a powerful tool to study changes in the local environment and the oxidation state of the vanadium centres during catalytic oxidation. At 623 K, the catalyst follows the associative mechanism in CO oxidation. XAFS revealed that the Mars-van Krevelen mechanism is operative at 723 K for CO oxidation. The extended X-ray absorption fine structure (EXAFS) results showed that the structure of the supported V2O5 phase consists of monomeric tetrahedral (Al-O)(3)-V=O units after dehydration in air at 623 K. However, the residuals of the EXAFS analysis indicate that an extra contribution has to be accounted for. This contribution probably consists of polymeric vanadate species. The structure remains unchanged during steady-state CO oxidation at 623 and 723 K. Furthermore, when oxygen was removed from the feed at 623 K, no changes in the spectra occurred. However, when oxygen is removed from the feed at 723 K, reduction of the vanadium species was observed, i.e., the vanadyl oxygen atom is removed. The V3+ ion subsequently migrates into the gamma-Al2O3 lattice, where it is positioned at an Al3+ octahedral position. This migration process appears to be reversible; so the (Al-O)(3)-V=O units are thus restored by re-oxidation.