Highly Selective Nitrite Hydrogenation to Ammonia over Iridium Nanoclusters: Competitive Adsorption Mechanism

Wet denitrification is a promising approach to control nitrogen oxides (NO_x) produced in fossil fuel combustion. Yet, the highly concentrated nitrite (NO₂^-) wastewater generated poses a major threat to the aqueous environment. Here, iridium nanoclusters (d = 1.63 nm) deposited on TiO₂ were applied for NO₂^- reduction to ammonia (NRA), showing an exceptional NH₄⁺ selectivity of 95% and a production rate of 20.51 mg_N·L^-1·h^-1, which held significant potential for NO₂^- wastewater purification and ammonia resource recovery. Notably, an interesting non-first-order NO₂^- hydrogenation kinetics was observed, which was further confirmed to result from the competitive adsorption mechanism between H₂ and NO₂^- over iridium. The NRA pathways on the Ir(111) surface were explored via density functional theory calculations with the NO₂^-* → NO* → HNO* → HNOH* → H₂NOH* → NH₂* → NH₃* identified as the most energetically favorable pathway and the NO* → HNO* confirmed as the rate-determining step. In situ DRIFTS further experimentally verified the generation of HNO* intermediate during NO* hydrogenation on Ir(111). To verify NRA kinetics at varied NO₂^- concentrations or H₂ pressures, a kinetic model was derived based on the Langmuir-Hinshelwood competitive adsorption mechanism. These findings provide mechanistic insights into the NRA pathways on Ir nanocatalysts, which will be beneficial for wet denitrification waste stream decontamination and valorization.