Predictive Theoretical Model for the Selective Electroreduction of Nitrate to Ammonia.

The electrochemical reduction of nitrate (eNO 3 RR) 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 NH 3 surpasses that of NO 2 - at -0.66 V RHE , which nicely reproduced the experimental value on titania. NH 2 OH* → NH 2 * is the kinetically controlling step at a low overpotential for NH 3 generation, while NO 2 * → HNO 2 has the highest barrier to producing nitrite. Based on these mechanistic insights, we suggest that ΔG 1 (NH 2 OH* → NH 2 *) - Δ G 2 (NO 2 * → HNO 2 ) can serve as a descriptor to predict the S(NO 2 - )/S(NH 3 ) crossover potential. Such a model is verified by the experimental results on Ag, Cu, TiO 2- x , Fe 3 O 4 , and Fe-MoS 2 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 NH 3 .