Reaction Pathways and Kinetics for Selective Catalytic Reduction (SCR) of Acidic NO_x Emissions from Power Plants with NH₃

Selective catalytic reduction (SCR) of NO_x with NH₃ by supported vanadium oxide catalysts is an important technology for reducing acidic NO_x 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.