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

Abstract

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 N₂ 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 VO_x 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 TiO_x 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.