Surface-Controlled Gas-Phase Deposition and Characterization of Highly Dispersed Vanadia on Silica

Highly dispersed V 2 O 5 /SiO 2 materials were successfully synthesized using the atomic layer deposition (ALD), by sequentially applying surface-saturating reactions of volatilized vanadyl triisopropoxide and oxygen with the silica surface. The controlled and reproducible chemisorption-based growth of vanadia on silica occurred in a temperature range from 363 to 393 K, as confirmed by elemental analyses and DRIFTS measurements. The precursor deposition progressed via both mono- and bidentate surface species, depending on the silica pretreatment temperature. Upon increase of the silica preheating temperature from 473 to 1023 K, i.e., decreasing the number of isolated OH groups, the vanadium densities diminished from 2.0 to 1.1 V at/ . The maximum dispersion of vanadia (∼2.3 V at/ ) was attained by two consecutive precursor binding-oxidation cycles on silica pretreated at 873 K. Analogous liquid-phase-impregnated catalysts were prepared for comparison, and the structure and dispersion of the catalysts were studied by N 2 adsorption, XRD, XPS, and Raman spectroscopy. The results showed that the properties of the silica-supported vanadia catalysts were strongly affected by the preparative method. The ALD catalysts with loadings between 1.0 and 2.3 V at/ consisted of highly dispersed isolated VO 4 species, whereas in the corresponding impregnated catalysts, V 2 O 5 crystallites were formed, in addition to the monomeric vanadia species. The vanadia deposition either by ALD or impregnation created both Lewis and Brønsted surface acid sites of weak and medium strength, as detected by ammonia adsorption XPS and microcalorimetry. However, the better dispersion of the surface vanadia species in the catalysts prepared by the gas-phase route led to a 30% higher number of surface acid sites.