Surface acidity of supported vanadia catalysts

Abstract

The surface acidity of SiO2, γ-Al2O3 and TiO2 supported vanadia catalysts has been studied by the microcalorimetry and infrared spectroscopy using ammonia as the probe molecule. The acidity in terms of nature, number and strength was correlated with surface structures of vanadia species in the catalysts, characterized by X-ray diffraction and UV-Vis spectroscopy. It was found that the dispersion and surface structure of vanadia species depend on the nature of supports and loading and affect strongly the surface acidity. On SiO2, vanadium species is usually in the form of polycrystalline V2O5 even for the catalyst with low loading (3%) and these V2O5 crystallites exhibit similar amount of Bronsted and Lewis acid sites. The 25%V2O5/SiO2 catalyst possesses substantial amount of V2O5 crystallites on the surface with the initial heat of 105 kJ mol-1 and coverage of about 600 mmol g-1 for ammonia adsorption. Vanadia can be well dispersed on g-Al2O3and TiO2 to form isolated tetrahedral species and polymeric two-dimensional network. Addition of vanadia on γ-Al2O3 results in the change of acidity from that associated with g-Al2O3 (mainly Lewis sites) to that associated with vanadia (mainly Bronsted sites) and leads to the decreased acid strength. The 3%V2O5/TiO2 catalyst may have the vanadia structure of incomplete polymeric two-dimensional network that possesses the Ti-O-V-OH groups at edges showing strong Bronsted acidity with the initial heat of about 140 kJ mol-1 for ammonia adsorption. On the other hand, the 10%V2O5/TiO2 catalyst may have well defined polymeric two-dimensional vanadia network, possessing V-O-V-OH groups that exhibit rather weak Bronsted acidity with the heat of 90 kJ mol-1 for NH3 adsorption. V2O5 crystallites are formed on the 25%V2O5/TiO2 catalyst, which exhibit the acid properties similar to those for 25%V2O5 on SiO2 and γ-Al2O3.