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

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 VO_x and WO_x 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 VO_x and WO_x 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 WO_x sites associated surface defects on the TiO₂ support that increase the ammonia adsorption capacity.