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 NO_x (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.