Highly Selective Nitrite Hydrogenation to Ammonia over Iridium Nanoclusters: Competitive Adsorption Mechanism
Huimin Xu, Xiaoqiang Wang, Yaoyu Zhang, Le Shi, Xuanhao Wu, Yue Liu, Zhongbiao Wu
Abstract
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* → NH 2 * → NH 3 * 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.