Unlocking the significance of high H2O resistance for nickel vanadate phases to improve the kinetic parameters or consequences of catalytic NOX reduction and poison pyrolysis

Abstract

Anthropogenic flue gases consist of NO_X/SO₂/H₂O, among which H₂O resistance is often underrated in activating the acidic/redox cycles of NH₃-assisted catalytic NO_X reduction (SCR), SO₂/H₂O-induced evolution of ammonium (bi)sulfate (AS/ABS) poisons, or AS/ABS pyrolysis. Herein, TiO₂-supported nickel vanadates (Ni_XV₂O_X+5; X = 1–3) were functionalized with SO_Z²⁻ (Z = 3–4) to simulate the resulting Ni_X–S surfaces under a SO₂-containing wet flue gas, at which mono/bidentate SO_Z²⁻ modifiers transform into Brønsted acidic bonds (B⁻–H⁺) via protonation. Ni₁–S exhibited the highest efficiency in the recurring acidic cycle, as proved by its highest NO_X consumption rate (−r_NOX) among Ni_X–S catalysts. This was enabled by the smallest H₂O binding affinity to the B⁻–NH₄⁺⋯O_L⁻–M⁽ⁿ⁻¹⁾⁺ intermediates involved in the rate-determining step of the SCR, thus revealing the smallest energy barrier needed for SCR on Ni₁–S. Moreover, Ni₁–S provided the largest quantity of labile oxygens and the highest oxygen mobility among Ni_X–S catalysts, leading to the highest efficiency in the recurring redox cycle. Notably, the Ni₁–S surface repelled H₂O markedly upon the inclusion of promotive Sb₂O₅ alongwith the improvement of redox cycling efficiency for the resulting Ni₁–Sb–S. Hence, aside from exhibiting greater −r_NOX values or SCR consequences than Ni_X–S and a commercial catalyst (V₂O₅–WO₃–S) at low temperatures, Ni₁–Sb–S also reduced the number of AS/ABS accumulated by evading H₂O adsorption. H₂O resistance was also crucial to accelerate desorptive B⁻–H₂O⋯SO₂⋯H₂O dissociation on the Ni₁–Sb–S surface. Ni₁–Sb–S thus unveiled a higher AS/ABS degradation rate and a smaller energy barrier required for AS/ABS pyrolysis than V₂O₅–WO₃–S. Importantly, Ni₁–Sb–S significantly enhanced the resistance toward AS/ABS or hydrothermal aging over V₂O₅–WO₃–S and SO_Z²⁻-modified Mn₁V₂O₆ (or Cu₃V₂O₈) on Sb-promoted TiO₂ reported previously by our group.