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(hk0) for nitrate reduction activity and Pd(100) > Pd­(hk0) > Pd(111) for nitrite reduction activity in an alkaline electrolyte. Octahedrons without Pd (100) facets exhibited nearly selective production of NO2 – with little to no measurable NH3 or N2. However, nanocubes that expose only the Pd(100) facet exhibited high activity for NO2 – reduction to NH3. Cuboctahedrons that expose both Pd(111) and Pd(100) facets demonstrated the highest production of ammonia (306.8 μg h–1 mgPd –1) with a faradaic efficiency of 35%. Density functional theory (DFT) simulations reveal that *NO3 dissociation to *NO2 + 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 *NO2 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 NO3 – to NO2 – and the (100) facet catalyzing the conversion of NO2 – to NH3.

Interfacial Engineering Enhances the Electroactivity of Frame‐Like Concave RhCu Bimetallic Nanocubes for Nitrate Reduction

“…− -RR at PA-RhCu cNCs at 0.05 V applied potential are much higher than most reported values at various electrocatalysts (Table S1, Supporting Information), [29,[44][45][46][47][48][49][50][51][52] proving the high NO 3 − -RR activity of PA-RhCu cNCs. Obviously, the high apparent NO 3 − -RR activity of PA-RhCu cNCs directly benefits from the high ECASA derived from the unusual frame-like concave nanocube structure with hollow, pore-like and dendritic features.…”

“…− -RR at PA-RhCu cNCs and Rh NPs originates from the underpotentially deposited H (H upd ) at Rh surface rather than HER at Rh surface. [28] As a result, the onset reduction potential of NO 3 − -RR at Rh surface is much lower than that at various nonprecious metal based electrocatalysts (Table S1, Supporting Information), [29,[44][45][46][47][48][49][50][51][52] showing an energy-saving electrocatalytic process for NO 3 − -RR. The influence of applied potential on the activity and selectivity of NO 3 − -RR at PA-RhCu cNCs and Rh NPs were investigated by the chronoamperometry measurements at different applied potentials.…”

Subnanometric alkaline-earth oxide clusters for sustainable nitrate to ammonia photosynthesis

“…37 – 39 . b Comparison of NH 4 + production rate and selectivity with different ammonia synthesis routes under ambient conditions 11 , 16 , 24 , 59 – 69 . c Calculated activation energy for NO 3 − RR for NH 4 + synthesis and water splitting for H 2 generation.…”

“…Despite its advantages, NO 3 − RR also suffers from some inevitable difficulties as an active, yet challenging area of current research. In the eight-electron transfer reaction for NO 3 − -NH 4 + synthesis, competitive side reactions may be fierce, mainly ascribed to five-electron transfer for the partial reduction of NO 3 − to N 2 and hydrogen evolution reaction (HER) 22 – 24 . Moreover, it is well established that the yield rate and selectivity for NH 4 + dominantly rely on the development of novel catalytic materials, precise regulation of the reaction parameters and systematic investigation of the reaction mechanism.…”

A TiO2−x nanobelt array with oxygen vacancies: an efficient electrocatalyst toward nitrite conversion to ammonia

“…Some noble metal-based catalysts exhibit decent NO 2 − reduction activities to produce NH 3 electrochemically, 15–17 but the high cost is the main hinderance of their large-scale use. Our group and others are thus focused on identifying cost-effective alternatives such as Cu 3 P, 5 CoP, 18 Ni 35 /NC-sd, 19 Ni-NSA-V Ni , 20 and CF@Cu 2 O, 21 etc.…”