Decrypting Catalytic NO_<i>X</i> Activation and Poison Fragmentation Routes Boosted by <i>Mono</i>- and <i>Bi</i>-Dentate Surface SO₃^2–/SO₄^2– Modifiers under a SO₂-Containing Flue Gas Stream

Abstract

SOA2– (A = 3–4; B–) functionalities are anchored on metal oxides used to catalyze NH3-assisted selective NOX reduction (SCR) for a SO2-bearing feed gas stream. SOA2– species act as conjugate bases of Brönsted acidic bonds (B––H+) and modifiers of redox sites (M(n–1)+–O–), both of which are combined to dictate the activities of SCR (−rNOX) and ammonium (bi) sulfate (AS/ABS) poison degradation (−rAS/ABS) at low temperatures. Nonetheless, their pathways have been barely clarified and underexplored, while questioning catalytic significance of mono-dentate or bi-dentate SOA2– species in dominating −rNOX and −rAS/ABS. While using Sb-promoted MnV2O6 as a reservoir of SOA2– functionalities with distinct binding arrays, elementary stages for the SCR and AS/ABS degradation were proposed, thermodynamically assessed, and analyzed using kinetic control runs in tandem with density functional theory calculations. These allowed for the conclusions that the reaction stage between B––H+•••NH3•••O––M(n–1)+ and gaseous NO and the liberation stage of H2O/SO2 from B–•••H2O•••SO2•••H2O via dissociative desorption are endothermic and dominate −rNOX and −rAS/ABS as the rate-determining steps of the SCR and AS/ABS degradation, respectively. In addition, mono-dentate and bi-dentate SOA2– species are verified central in directing −rNOX and −rAS/ABS by elevating collision frequency between B––H+•••NH3•••O––M(n–1)+ and NO and declining the energy barrier required for dissociative H2O/SO2 desorption for the SCR and AS/ABS degradation, respectively. In particular, mono-dentate SOA2– functionalities can improve the overall redox trait of the surface, thereby substantially promoting its low-temperature SCR performance under a SO2-excluding feed gas stream. Meanwhile, bi-dentate SOA2– functionalities can slightly improve the overall redox trait of the surface, yet, can readily degrade AS/ABS by accelerating the endothermic fragmentation of S2O72– innate to ammonium pyrosulfate, while compensating for the moderate efficiency in fragmenting NH4+ of ammonium pyrosulfate via Eley–Rideal-type SCR. This can significantly elevate the SCR performance of the bi-dentate SOA2–-containing surface under a SO2-including feed gas stream alongside with the promotion of its long-term stability at low temperatures. These can be adaptable and exploited in discovering/amending a host of metal oxides (or vanadates) imperatively functionalized with SOA2– or poisoned with AS/ABS under low thermal energies.

Topics & Concepts