Role of V and W Sites in V₂O₅−WO₃/TiO₂ Catalysts and Effect of Formaldehyde during NH₃−SCR of NO_x

Abstract

Dedicated to Professor Matthias Beller on the occasion of his 60 th birthday WO 3 /TiO 2 , V 2 O 5 /TiO 2 and V 2 O 5 À WO 3 /TiO 2 catalysts (1 wt % V, 6 wt % W) were tested in NH 3 -SCR of NO x with/without formaldehyde and water and analyzed by in situ FTIR and operando EPR spectroscopy. Activity decreased in the order VÀ WÀ Ti > VÀ Ti @ WÀ Ti. Two VO 3 + /VO 2 + sites with different redox behavior exist in V-containing catalysts. Sites I were less active with restricted accessibility by reactants while activity is mainly driven by undercoordinated sites II exposed on the surface. In VÀ WÀ Ti, sites II can form W 6 + À O(H)À V 5 + bridges, accelerating surface acidity and V reduction rate. HCHO lowers NO x conversion and N 2 selectivity by reacting with NH 3 to formamide that decomposes to HCN. Formation of formamide is promoted by high surface acidity and redox activity. Water lowers NH 3 conversion by creating Brønsted sites that adsorb NH 3 and suppress oxidation to N 2 O, thereby improving N 2 selectivity.