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 (NO 2 - ) wastewater generated poses a major threat to the aqueous environment. Here, iridium nanoclusters ( d = 1.63 nm) deposited on TiO 2 were applied for NO 2 - reduction to ammonia (NRA), showing an exceptional NH 4 + selectivity of 95% and a production rate of 20.51 mg N ·L -1 ·h -1 , which held significant potential for NO 2 - wastewater purification and ammonia resource recovery. Notably, an interesting non-first-order NO 2 - hydrogenation kinetics was observed, which was further confirmed to result from the competitive adsorption mechanism between H 2 and NO 2 - over iridium. The NRA pathways on the Ir(111) surface were explored via density functional theory calculations with the NO 2 -* → NO* → HNO* → HNOH* → H 2 NOH* → 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 NO 2 - concentrations or H 2 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.