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The electrochemical reduction of nitrate (eNO3RR) emerges as a promising route for decentralized ammonia synthesis. However, the competitive production of nitrite at low overpotentials is a challenging issue. Herein, using the combination of density functional theory and microkinetic modeling, we show that the selectivity for NH3 surpasses that of NO2- at -0.66 VRHE, which nicely reproduced the experimental value on titania. NH2OH* → NH2* is the kinetically controlling step at a low overpotential for NH3 generation, while NO2* → HNO2 has the highest barrier to producing nitrite. Based on these mechanistic insights, we suggest that ΔG1 (NH2OH* → NH2*) - ΔG2 (NO2* → HNO2) can serve as a descriptor to predict the S(NO2-)/S(NH3) crossover potential. Such a model is verified by the experimental results on Ag, Cu, TiO2-x, Fe3O4, and Fe-MoS2 and can be extended to the Au catalyst. Thus, this work sheds light on the rational design of catalysts that are simultaneously energy-efficient and selective to NH3.
Mou et al. (Tue,) studied this question.