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°C). 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°C in He. The FTIR spectra of the adsorbed NO+O2 suggested that vanadium oxides were anchored directly on the titania pillars of the catalysts. NH3 molecules adsorbed on the Brønsted acid and Lewis acid sites to form, respectively, NH+4 ions and coordinated NH3 species. These NH3 adspecies were active in reaction with NO and NO+O2. The Brønsted acidity increased with increasing vanadium content, which was consistent with an increase in the SCR activity for low temperatures (e.g., 200°C). 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., N2 originates mainly from the reaction between gaseous or weakly adsorbed NO and NH3 adspecies.