Role of WO₃ in mixed V₂O₅-WO₃/TiO₂catalysts for selective catalytic reduction of nitric oxide with ammonia

A series ofTiO₂-supported V₂O₅ and V₂O₅+WO₃ catalysts were prepared to study the role of WO₃ in the commercial selective catalytic reduction (SCR) catalysts. The activities of these catalysts were measured under both dynamic and steady-state conditions. The catalysts were characterized by temperature-programmed reaction (TPR), ammonia chemisorption and proton magic angle spinning nuclear magnetic resonance (MAS NMR) measurements. The following effects of WO₃ on V₂O₅/TiO₂ in the commercial WO₃-V₂O₅/TiO₂ (coimpregnated) SCR catalyst are observed; (1) It increases the activity and widens the temperature window for SCR. (2 ) It significantly increases the poison resistance to both alkali metal oxides and arsenious oxide. (3) It reduces ammonia oxidation (as well as SO₂ oxidation which is known from the published literature). These results have been correlated directly with ammonia chemisorption and proton MAS NMR measurements. These results suggest that the Brønsted acid sites are the active sites for SCR. Addition of WO₃ in V₂O₅/TiO₂ increases the Brønsted acidity (both Brønsted acid site density and strength of the acid, i.e., proton donicity), whereas the alkali poison decreases the Brønsted acidity.