Flame aerosol synthesis of vanadia-titania nanoparticles: Structural and catalytic properties in the selective catalytic reduction of NO by NH3

Flame aerosol synthesis has been used to prepare vanadia-titania nanopartic les with high activity for the selective catalytic reduction of NO by NH3. The mixed oxides were prepared from vanadium and titanium alkoxides which w ere evaporated into an argon stream and burned in a methane oxygen diffusio n flame. Silica-containing samples were produced in a similar way by mixing hexamethyldisiloxane vapor into the precursor stream. Different flame stru ctures were investigated for the effect of temperature and residence time o n particle morphology, vanadia surface species, and overall catalytic activ ity. By changing the oxygen flow rate into the flame, particles with specif ic surface areas between 23 and 120 m(2)/g could be produced. High-resoluti on transmission electron microscopy (HRTEM) revealed that nanoparticles wer e spherical with diameters of 10 to 50 nm. X-ray photoelectron spectroscopy analysis indicated that vanadia was dispersed on the surface of the titani a spheres. No indication for the presence of crystalline V2O5 could be foun d by X-ray diffraction or HRTEM. Catalysts with a vanadia surface loading o f 10 mu mol/m(2) showed high activity with less than 1% N2O formation up to 350 degreesC. Catalytic activity strongly depended on the vanadia loading; an increase from 2.5 to 7 mu mol/m(2) resulted in a 30 times higher activi ty per vanadium. Addition of silica lowered the overall activity but did no t change the activation energy. Raman spectroscopy indicated the presence o f vanadate clusters. Temperature-programmed reduction corroborated that no significant amount of vanadia entered the titania lattice to form an inters titial solution. The selective catalytic reduction activity of as-prepared vanadia-titania is comparable to the best catalysts obtained by wet chemica l methods. (C) 2001 Academic Press.