Cooperative oxidation of NH3 and H2O to selectively produce nitrate via a nearly barrierless N-O coupling pathway

Photoelectrochemical (PEC) direct ammonia oxidation is a sustainable alternative to the industrial production of nitrate or nitrite (NOx-), while the highly selective NOx- synthesis remains challenging due to the intricate kinetics and the inherent competition from the water oxidation reaction (WOR). Herein, we report a high-performance Si-based photoanode modified with Ni-Cu bimetallic nanosheets (NiCuOx/Ni/n-Si), which presents a benchmark faradaic efficiency of 99% for PEC NOx- synthesis together with a record partial photocurrent density of similar to 12 mA cm(-2) at a low bias of 1.38 V-RHE under AM 1.5G illumination. This results in a NOx- production rate of 59 mu mol h(-1) cm(-2) and simultaneously a H-2 production rate of 214 mu mol h(-1) cm(-2) on the counter electrode. Operando PEC spectroscopic measurements combined with theoretical calculations reveal that water molecules activated at Ni sites produce (NiO)-O-IV species, which bond with the activated ammonia at Cu sites via a nearly barrierless N-O coupling pathway. Such a bimetallic synergetic mechanism presents a first-order hole-transfer kinetics, which kinetically circumvents the competing O-2 production and thus significantly promotes the selectivity to NOx- on the NiCuOx/Ni/n-Si photoanode. This work provides a promising strategy for designing advanced photoanodes for cooperative oxidation reactions.