Structure Sensitivity of Pd Facets for Enhanced Electrochemical Nitrate Reduction to Ammonia

Abstract

The design of electrocatalysts capable of selectively reducing nitrate to ammonia is gaining interest as a means of transforming waste into fertilizers. However, most prior investigations of prototypical electrocatalysts, such as polycrystalline Pd and Pt, have focused on unraveling the mechanisms responsible for the selective reduction of nitrate to nitrogen gas. Such polycrystalline noble metals demonstrate notoriously low activity for nitrate reduction (nitrate to nitrite) and high activity for nitrite reduction (nitrite to nitrogen). Here, we aim to elucidate the effect Pd surface structure has on nitrate and nitrite reduction and to determine what role catalyst structural design can play in enabling selective reduction of nitrate to ammonia. Through synthesizing nanocatalysts with controlled facets (e.g., nanocubes, cuboctahedrons, octahedrons, and concave nanocubes), we demonstrate that Pd(111) > Pd(100) > Pd( hk 0) for nitrate reduction activity and Pd(100) > Pd( hk 0) > Pd(111) for nitrite reduction activity in an alkaline electrolyte. Octahedrons without Pd (100) facets exhibited nearly selective production of NO 2 – with little to no measurable NH 3 or N 2 . However, nanocubes that expose only the Pd(100) facet exhibited high activity for NO 2 – reduction to NH 3 . Cuboctahedrons that expose both Pd(111) and Pd(100) facets demonstrated the highest production of ammonia (306.8 μg h –1 mg Pd –1 ) with a faradaic efficiency of 35%. Density functional theory (DFT) simulations reveal that *NO 3 dissociation to *NO 2 + O* is more favorable on Pd(111) than Pd(100), explaining the faster nitrate reduction kinetics on the Pd(111) facet observed in the experiments. The simulations also show that *NO 2 binds less strongly to Pd(111) compared to Pd(100). Thus, nitrite formed via nitrate dissociation readily desorbs from the Pd(111) surface, which explains why Pd(111) selectively reduces nitrate to nitrite. The results show that cuboctahedron is bifunctional in nature, with the (111) facet catalyzing the conversion of NO 3 – to NO 2 – and the (100) facet catalyzing the conversion of NO 2 – to NH 3 .