Influence of Potassium Doping on the Formation of Vanadia Species in V/Ti Oxide Catalysts

The influence of potassium on the formation of surface vanadia species on V/Ti oxide catalysts containing from 0.2 to 5 monolayers of vanadia (K/V atomic surface ratio ≤1) has been investigated by temperature programmed reduction in hydrogen and by FT-Raman spectroscopy under dehydrated conditions. In the pure catalysts, monomeric and polymeric (metavanadate-like) species, “amorphous” and bulk crystalline V2O5 were detected depending on the surface vanadia loading. In the K-doped catalysts, vanadia species formed on the surface depend also on the K/V atomic ratio. Even at small K/V ratios, K inhibits the formation of the polymeric species in favor of the “K-doped” and/or “K-perturbed” monomeric species. These species possess lengthened VO bonds with respect to the monomeric species in the undoped V/Ti oxides. At K/V = 1, the “K-doped” monomeric species and “amorphous” KVO3 are mainly present on the surface. Reduction of vanadia forms in the K-doped catalysts takes place at higher temperatures than in the catalysts where potassium was absent. The monomeric and polymeric species, which are the active sites in partial catalytic oxidation, have the lowest reduction temperature. Vanadia species formed on the commercial titania, containing K, were also elucidated. The catalysts were characterized via X-ray photoelectron spectroscopy, high-resolution transmission electron microscopy, and Brunauer−Emmett−Teller surface area measurements.