Gas-phase synthesis, structure and surface acid-base properties of highly dispersed vanadia/titania/silica catalysts

The atomic layer deposition (ALD) method based on surface-saturating gas-solid reactions was applied to grow vanadia species on highly dispersed titania/silica supports with from submono- to above monolayer levels of titania. The physicochemical properties of the V2O5/TiO2/SiO2 catalysts were examined with N-2 adsorption, XRD, XPS and Raman spectroscopy techniques. The catalysts were compared to the corresponding titania supported catalysts, and besides a series of liquid-phase impregnated samples were characterized for comparison. In the ALD catalysts, isolated and highly dispersed tetracoordinated VOx species were formed on titania/silica supports with only small impact on the surface area and porosity of the support. Vanadia was homogeneously associated with both the surface titania species and the uncovered silica support. However, in the impregnated catalysts, crystalline vanadia at low titania coverage and TiOx agglomeration at high titania level decreased the surface area. Vanadia had also a preferential interaction with the titania phase. Moreover, the better dispersion of the active species in the ALD catalysts was confirmed by XPS of NH3 adsorption and microcalorimetry of NH3 and SO2 adsorption. The number and strength of acid-base sites on the surface of the catalysts were directly related to the V-O-Ti and V-O-Si concentrations as well as to the titania and vanadia dispersion in the samples, in favor of the ALD catalysts.