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

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.