ROLE OF WO3 IN MIXED V2O5-WO3/TIO2 CATALYSTS FOR SELECTIVE CATALYTIC REDUCTION OF NITRIC-OXIDE WITH AMMONIA

A series of TiO2-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 catalysts 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 Bronsted acid sites are the active sites for SCR. Addition of WO3 in V2O5/TiO2 increases the Bronsted acidity (both Bronsted acid site density and strength of the acid, i.e., proton donicity), whereas the alkali poison decreases the Bronsted acidity.