Facet-controlled palladium nanocrystalline for enhanced nitrate reduction towards ammonia.

Electrochemical nitrate reduction reaction (NO3-RR) is considered an appealing way for producing ammonia (NH3) under ambient conditions and solving environmental problems caused by nitrate, whereas the lack of adequate catalysts hampers the development of NO3-RR. Here, we firstly demonstrate that the Pd nanocrystalline with a well-desired facet can act as a highly efficient NO3-RR electrocatalyst for ambient ammonia synthesis. Pd (1 1 1) exhibits excellent activity and selectivity in reducing NO3- to NH4+ with a Faradaic efficiency of 79.91% and an NH4+ production of 0.5485 mmol h-1 cm-2 (2.74 mmol h-1 mg-1) in 0.1 M Na2SO4 (containing 0.1 M NO3-), which is 1.4 times higher than Pd (1 0 0) and 1.9 times higher than Pd (1 1 0), respectively. Density functional theory (DFT) calculation reveals that the superior NO3-RR activity of Pd (1 1 1) originates from its optimized activity of NO3- adsorption, smaller free energy change of the rate-limiting step (*NH3 to NH3), and poorer hydrogen evolution reaction activity (HER, competitive reaction). This work not only highlights the potentials of Pd-based nanocatalysts for NO3-RR but also provides new insight for the applications in NO3-RR of other facet-orientation nanomaterials.