Catalytic performance and characterization of VO2+-exchanged titania-pillared clays for selective catalytic reduction of nitric oxide with ammonia

VO2+ ion-exchanged TiO2-pillared clays (VO-TiO2-PILC) were investigated for selective catalytic reduction of nitric oxide by ammonia in the presence of oxygen. They were also characterized for surface area and pore size distribution and by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), electron spin resonance (ESR), and Fourier transform infrared (FTIR) spectroscopy. It was found that VO-TiO2-PILC catalysts were highly active for the selective catalytic reduction (SCR) reaction. The maximum activity was obtained with 2.1-3.5 wt% vanadium, which was close to or slightly higher than the activity of the commercial V2O5 + WO3/TiO2 catalyst. The VO-TiO2-PILC catalysts were also resistant to water vapor and sulfur dioxide at high temperatures (>350 degreesC). XRD patterns of VO-TiO2-PILC were similar to that of TiO2-PILC, showing no peaks due to vanadium oxides, even when the vanadium content reached 13.2 wt%. XPS and ESR spectra indicated that vanadium was present mainly as the +5 valence form (probably as V2O5) on the fresh catalysts, but was partially reduced to the +4 form (VO2+) after being heated at 300 degreesC in He. The FTIR spectra of the adsorbed NO + O-2 suggested that vanadium oxides were anchored directly on the titania pillars of the catalysts. NH3 molecules adsorbed on the Bronsted acid and Lewis acid sites to form, respectively, NH4+ ions and coordinated NH3 species. These NH3 adspecies were active in reaction with NO and NO + O-2. The Bronsted acidity increased with increasing vanadium content, which was consistent with an increase in the SCR activity for low temperatures (e.g., 200 degreesC). By comparison, the adsorption of NOx (x = 1, 2) on the catalysts was very weak, especially under reaction conditions. The present results indicate that the reaction path for NO reduction by NH3 on VO-TiO2-PILC is similar to that on V2O5/TiO2; i.e., N-2 originates mainly from the reaction between gaseous or weakly adsorbed NO and NH3 adspecies. (C) 2000 Academic Press.