Preparation,characterization and activity testing of vanadia catalysts deposited onto silica and alumina supports by atomic layer deposition

摘要

A gas phase preparation technique,atomic layer deposition(ALD),was applied to synthesize supported vanadium oxide catalysts.The growth of vanadia on silica and alumina was achieved by chemisorption of the volatilized vanadium precursor onto surface hydroxyl groups of the support,followed by oxidative treatment in air.The influence of the preparative method on surface structure,redox and acidic properties and catalytic activity was demonstrated by examining also the corresponding conventionally impregnated catalysts.The catalysts were characterized by chemical analysis,BET surface area measurements,X-ray diffraction(XRD),electron paramagnetic resonance(EPR),X-ray photoelectron spectroscopy (XPS) and ~(51)V solid state unclear magnetic resonance (~(51)V NMR).Tetracoordinated surface vanadia species were detected as the principal vanadium species in all the catalysts studied.The ALC catalysts,compared to the analogous impregnated ones,were found to be highly dispersed and more easily reduced,as revealed by temperature programmed reduction(TPR).The results form measurements using ammonia adsorption microcalorimetry and pyridine adsorption infrared (IR) spectroscopy indicated that the number,strength and strength distribution of the surface acid sites were affected by the preparative mode and the support.The same Lewis and Br#PHI#nsted acid sites were detected for all impregnated and ALD catalysts,but those prepared in the gas phase showed slightly stronger acidic characteristics.In propane dehydrogenation(DH),the rate of propene formation by the ALD catalysts was higher than that achieved by the conventionally impregnated catalysts.