Acidity, surface species, and catalytic activity study on V₂O₅-WO₃/TiO₂ nanotube catalysts for selective NO reduction by NH₃

In this work, we report the catalytic activity of V₂O₅/TiO₂, V₂O₅-WO₃/TiO₂ and WO₃/TiO₂-nanotube model catalysts in removing NO with NH₃ via the SCR process. The catalytic activity includes the effect of SO₂ and H₂O. We also analyze the effect of WO₃ and V₂O₅ loading upon the surface acidity of the nanotubes, and the effect of WO₃ on the V⁴⁺/V⁵⁺ ratio, and its correlation with the catalytic activity. TiO₂-nanotubes (NT), employed as support, were prepared by hydrothermal treatment of TiO₂ with NaOH. The catalysts were characterized by X-ray diffraction, HR-TEM microscopy, N₂ physisorption, FTIR, H₂-TPR, Raman and XPS. In general, we found that ternary catalysts (V₂O₅-WO₃/NT) showed a higher NO conversion versus V₂O₅/NT and WO₃/NT model catalysts. In fact, we found a high NO conversion (93%) over 3V-10W/NT catalyst at low temperature (380 °C). In the presence of SO₂ (50 ppm) and H₂O (5 vol.%), NO conversion slightly decreases (from 93 to 80% at 380 °C). The surface acidity (Brønsted and Lewis) of the nanotubes is the main parameter improved by adding V₂O₅. WO₃ preferably modifies the Lewis acid sites of the nanotubes. Additionally, the structure and morphology of the nanotubes as well as the V⁴⁺/V⁵⁺ ratio, which depend on the metal loading, play an important role in the removal of NO at intermediate temperatures even in the presence of SO₂ and H₂O.