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 contg. from 0.2 to 5 monolayers of vanadia (K/V at. surface ratio ?1) has been investigated by temp. programmed redn. in hydrogen and by FT-Raman spectroscopy under dehydrated conditions. In the pure catalysts, monomeric and polymeric (metavanadate-like) species, "amorphous" and bulk cryst. 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 at. 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. Redn. of vanadia forms in the K-doped catalysts takes place at higher temps. than in the catalysts where potassium was absent. The monomeric and polymeric species, which are the active sites in partial catalytic oxidn., have the lowest redn. temp. Vanadia species formed on the com. titania, contg. K, were also elucidated. The catalysts were characterized via XPS, high-resoln. transmission electron microscopy, and Brunauer-Emmett-Teller surface area measurements. [on SciFinder (R)]