Flame Aerosol Synthesis of Vanadia–Titania Nanoparticles: Structural and Catalytic Properties in the Selective Catalytic Reduction of NO by NH3

Abstract

Flame aerosol synthesis has been used to prepare vanadia–titania nanoparticles with high activity for the selective catalytic reduction of NO by NH3. The mixed oxides were prepared from vanadium and titanium alkoxides which were evaporated into an argon stream and burned in a methane oxygen diffusion flame. Silica-containing samples were produced in a similar way by mixing hexamethyldisiloxane vapor into the precursor stream. Different flame structures were investigated for the effect of temperature and residence time on particle morphology, vanadia surface species, and overall catalytic activity. By changing the oxygen flow rate into the flame, particles with specific surface areas between 23 and 120 m2/g could be produced. High-resolution transmission electron microscopy (HRTEM) revealed that nanoparticles were spherical with diameters of 10 to 50 nm. X-ray photoelectron spectroscopy analysis indicated that vanadia was dispersed on the surface of the titania spheres. No indication for the presence of crystalline V2O5 could be found by X-ray diffraction or HRTEM. Catalysts with a vanadia surface loading of 10 μmol/m2 showed high activity with less than 1% N2O formation up to 350°C. Catalytic activity strongly depended on the vanadia loading; an increase from 2.5 to 7 μmol/m2 resulted in a 30 times higher activity per vanadium. Addition of silica lowered the overall activity but did not change the activation energy. Raman spectroscopy indicated the presence of vanadate clusters. Temperature-programmed reduction corroborated that no significant amount of vanadia entered the titania lattice to form an interstitial solution. The selective catalytic reduction activity of as-prepared vanadia–titania is comparable to the best catalysts obtained by wet chemical methods.