Influence of catalyst synthesis method on selective catalytic reduction (SCR) of NO by NH3 with V2O5-WO3/TiO2 catalysts

Abstract

The molecular structures, surface acidity and catalytic activity for NO/NH3/O2 SCR of V2O5-WO3/TiO2 catalysts were compared for two different synthesis methods: co-precipitation of aqueous vanadium and tungsten oxide precursors with TiO(OH)2 and by incipient wetness impregnation of the aqueous precursors on a reference crystalline TiO2 support (P25; primarily anatase phase). Bulk analysis by XRD showed that co-precipitation results in small and/or poorly ordered TiO2(anatase) particles and that VOx and WOx do not form solid solutions with the bulk titania lattice. Surface analysis of the co-precipitated catalyst by High Sensitivity-Low Energy Ion Scattering (HS-LEIS) confirms that the VOx and WOx are surface segregated for the co-precipitated catalysts. In situ Raman and IR spectroscopy revealed that the vanadium and tungsten oxide components are present as surface mono-oxo O=VO3 and O=WO4 sites on the TiO2 supports. Co-precipitation was shown for the first time to also form new mono-oxo surface VO4 and WO4 sites that appear to be anchored at surface defects of the TiO2 support. IR analysis of chemisorbed ammonia showed the presence of both surface NH3* on Lewis acid sites and surface NH4+* on Brønsted acid sites. TPSR spectroscopy demonstrated that the specific SCR kinetics was controlled by the redox surface VO4 species and that the surface kinetics was independent of TiO2 synthesis method or presence of surface WO5 sites. SCR reaction studies revealed that the surface WO5 sites possess minimal activity below ∼325°C and their primary function is to increase the adsorption capacity of ammonia. A relationship between the SCR activity and surface acidity was not found. The SCR reaction is controlled by the surface VO4 sites that initiate the reaction at ∼200°C. The co-precipitated catalysts were always more active than the corresponding impregnated catalysts. The higher activity of the co-precipitated catalysts is ascribed to the presence of the new surface WOx sites associated surface defects on the TiO2 support that increase the ammonia adsorption capacity.