Nature of Active Sites and Surface Intermediates during SCR of NO with NH₃ by Supported V₂O₅–WO₃/TiO₂ Catalysts

Time-resolved in situ IR was performed during selective catalytic reduction of NO with NH₃ on supported V₂O₅–WO₃/TiO₂ catalysts to examine the distribution and reactivity of surface ammonia species on Lewis and Brønsted acid sites. While both species were found to participate in the SCR reaction, their relative population depends on the coverage of the surface vanadia and tungsta sites, temperature, and moisture. Although the more abundant surface NH₄⁺_,ads intermediates dominate the overall SCR reaction, especially for hydrothermally aged catalysts, the minority surface NH_3,ads intermediates exhibit a higher specific SCR activity (TOF). The current study serves to resolve the long-standing controversy about the active sites for SCR of NO with NH₃ by supported V₂O₅–WO₃/TiO₂ catalysts

Reaction Pathways and Kinetics for Selective Catalytic Reduction (SCR) of Acidic NO_<i>x</i> Emissions from Power Plants with NH₃

Selective catalytic reduction (SCR) of NO_<i>x</i> with NH₃ by supported vanadium oxide catalysts is an important technology for reducing acidic NO_<i>x</i> emissions from stationary sources and mobile diesel vehicles. However, rational design of improved catalysts is still hampered by a lack of consensus about reaction pathways and kinetics of this critical technology. The SCR fundamentals were resolved by applying multiple time-resolved in situ spectroscopies (ultraviolet–visible light (UV-vis), Raman and temperature-programmed surface reaction (TPSR)) and isotopically labeled molecules (¹⁸O₂, H₂¹⁸O, ¹⁵N¹⁸O, ND₃). This series of experiments directly revealed that the SCR reaction occurs at surface V⁵⁺O₄ sites that are maintained in the oxidized state by O₂ and the rate-determining step involves the reduction of V⁵⁺O₄ sites by NO and NH₃, specifically the breaking of N–H bonds during the course of formation or decomposition of the NO–NH₃ intermediate