Cooperative oxidation of NH3 and H2O to selectively produce nitrate via a nearly barrierless N–O coupling pathway†
Abstract
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 ∼12 mA cm−2 at a low bias of 1.38 VRHE under AM 1.5G illumination. This results in a NOx− production rate of 59 μmol h−1 cm−2 and simultaneously a H2 production rate of 214 μmol 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 NiIVO 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 O2 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.