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

Abstract

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, H2 18O, 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.

Polymeric vanadyl species determine the low-temperature activity of V-based catalysts for the SCR of NO x with NH 3

“…Then, the NH 2 NO intermediate is decomposed into N 2 and H 2 O (C→E, H→J, M→O, and R→T). Gas-phase O 2 replenishes the consumed surface oxygen on the vanadyl species (J→K) ( 8 ). When the V=O groups are regenerated, a catalytic cycle is completed.…”

“…The lower exhaust temperature of advanced combustion engines requires that catalysts be active at lower temperatures ( 5 , 6 ). Therefore, it is highly desirable to develop vanadia-based SCR catalysts with excellent low-temperature activity to meet future emission regulations, which requires an in-depth understanding of the active center and the reaction mechanism ( 2 , 7 , 8 ). …”

“…Therefore, the polymeric vanadyl species may play an important role in the removal of NO x from mobile sources. However, currently proposed NH 3 -SCR schemes over vanadia-based catalysts mainly focus on the reaction process occurring on isolated monomeric vanadyl species ( 1 , 7 , 8 , 17 ), without considering the polymerization of reactive sites and their coupling effects. The working principle of polymeric vanadyl species remains unclear.…”

Research Status and Prospect on Vanadium-Based Catalysts for NH3-SCR Denitration

“…Zhu, M. et al [2] used elemental tagging to study the SCR reaction pathway of vanadium-based catalysis and found that the SCR reaction occurs at the surface V 5+ O 4 site. This is shown in Figure 2, which exhibits a schematic diagram of the standard SCR reaction and indicates that SCR reaction is divided into three steps.…”

“…Currently, there are three kinds of reductant for the selective catalytic reduction of NO x : NH 3 , Urea and HC. NH 3 -SCR (selective catalytic reduction with ammonia) is the most mature industrial technology at present, and its main reaction process [1,2,3] is:4NH 3 + 4NO + O 2 → 4N 2 + 6H 2 O (standard SCR reaction) 4NH 3 + 2NO + 2NO 2 → 4N 2 + 6H 2 O (fast SCR reaction)…”