Decrypting Catalytic NO_XActivation and Poison Fragmentation Routes Boosted by Mono- and Bi-Dentate Surface SO₃^2-/SO₄^2-Modifiers under a SO₂-Containing Flue Gas Stream

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.