Unveiling the Reaction Mechanism of Nitrate Reduction to Ammonia Over Cobalt-Based Electrocatalysts

Abstract

Electrocatalytic reduction of nitrate to ammonia (NRA) has emerged as an alternative strategy for sewage treatment and ammonia generation. Despite excellent performances having been achieved over cobalt-based electrocatalysts, the reaction mechanism as well as veritable active species across a wide potential range are still full of controversy. Here, we adopt CoP, Co, and Co 3 O 4 as model materials to solve these issues. CoP evolves into a core@shell structured CoP@Co before NRA. For CoP@Co and Co catalysts, a three-step relay mechanism is carried out over superficial dynamical Co δ+ active species under low overpotential, while a continuous hydrogenation mechanism from nitrate to ammonia is unveiled over superficial Co species under high overpotential. In comparison, Co 3 O 4 species are stable and steadily catalyze nitrate hydrogenation to ammonia across a wide potential range. As a result, CoP@Co and Co exhibit much higher NRA activity than Co 3 O 4 especially under a low overpotential. Moreover, the NRA performance of CoP@Co is higher than Co although they experience the same reaction mechanism. A series of characterizations clarify the reason for performance enhancement highlighting that CoP core donates abundant electrons to superficial active species, leading to the generation of more active hydrogen for the reduction of nitrogen-containing intermediates.

Ni2P active site ensembles tune electrocatalytic nitrate reduction selectivity

“…At these potentials, despite a strong driving force toward HER, NO x * intermediates could be strengthening the adsorption of H*, which inhibits the Tafel step of HER and promotes the hydrogenation of NO x *. 45,46 In general, the observed difference in Δ G due to co-adsorbed H* can be rationalized in terms of the electron-donating/withdrawing propensity and steric effects of the co-adsorbates, where in our system, the H* on a Ni 3 -hollow site exhibits electrostatic repulsion effect on species such as NO 3 − and NO 2 − . 32,48,49 The impacts of H* and NO x * on each other's surface energetics imply that the rates of NO 3 RR and HER are inherently dependent, corroborating our conclusions from the rate-order analysis that NO x * and H* are co-adsorbing on Ni 2 P/C's active site ensembles during NO 3 RR.…”

“…This coverage ratio favors the hydrogenation of NO x * species to NH 3 over the formation of NO 2 − and H 2 . 45,46 In the H 2 O-mediated region (≤−0.3 V), we observe near 100% FE toward NO 3 RR at −0.4 V, which suggests a surface coverage that almost completely inhibits HER activity, i.e. , an ideal ratio of H*:NO x * for selectively performing NO 3 RR.…”

Active Site Ensembles on Ni2P Surfaces Tune Electrocatalytic Nitrate Reduction Selectivity

“…This coverage ratio could favor the hydrogenation of NOx* species to NH3 over the formation of NO2and the coupling of H* to form H2. 50,51 Conversely, at high cathodic potentials (≤ -0.4 V), we observe an overall decrease in selectivity toward the NO3RR, and product selectivity is driven toward H2 and NO2 -. We attribute this to H + outcompeting NO3for sites because it is from H2O.…”

“…At these potentials, despite a strong driving force toward HER, NOx* intermediates could be strengthening the adsorption of H*, which inhibits the Tafel step of HER and promotes the hydrogenation of NOx*. 50,51 In general, the observed difference in ΔG due to co-adsorbed H* can be rationalized in terms of the electrondonating/withdrawing propensity and steric effects of the coadsorbates, where in our system, the H* on a Ni3-hollow site exhibits electrostatic repulsion effect on species such as NO3and NO2 -. 40,53,54 The impacts of H* and NOx* on each other's surface energetics imply that the rates of NO3RR and HER are inherently dependent, corroborating our conclusions from the rate-order analysis that NOx* and H* are co-adsorbing on Ni2P/C's active site ensembles during the NO3RR.…”

CuCo2O4/CuO Heterostructure with Oxygen Vacancies Induced by Plasma for Electrocatalytic Nitrate Reduction to Ammonia

“…Nitrate (NO 3 – ) is a widely recognized environmental pollutant that can lead to the eutrophication of water bodies and disturb the natural nitrogen cycle in nature. Nitrate pollution sources include industrial activities, textile manufacturing, domestic wastewater, and the use of chemical fertilizers. − The inefficiency of nitrogen uptake by crops leads to a significant amount of nitrate flowing into surface water and groundwater, leading to various environmental issues . Moreover, the presence of residual nitrate and nitrite pollutants in wastewater discharged from sewage treatment plants further contaminates natural water systems, posing significant risks to human health.…”