Microscopic-Level Insights into the Mechanism of Enhanced NH₃ Synthesis in Plasma-Enabled Cascade N₂ Oxidation–Electroreduction System

Abstract

Integrated/cascade plasma-enabled N2 oxidation and electrocatalytic NO x – (where x = 2, 3) reduction reaction (pNOR-eNO x –RR) holds great promise for the renewable synthesis of ammonia (NH3). However, the corresponding activated effects and process of plasma toward N2 and O2 molecules and the mechanism of eNO x –RR to NH3 are unclear and need to be further uncovered, which largely limits the large-scale deployment of this process integration technology. Herein, we systematically investigate the plasma-enabled activation and recombination processes of N2 and O2 molecules, and more meaningfully, the mechanism of eNO x –RR at a microscopic level is also decoupled using copper (Cu) nanoparticles as a representative electrocatalyst. The concentration of produced NO x in the pNOR system is confirmed as a function of the length for spark discharge as well as the volumetric ratio for N2 and O2 feeding gas. The successive protonation process of NO x – and the key N-containing intermediates (e.g., −NH2) of eNO x –RR are detected with in situ infrared spectroscopy. Besides, in situ Raman spectroscopy further reveals the dynamic reconstruction process of Cu nanoparticles during the eNO x –RR process. The Cu nanoparticle-driven pNOR-eNO x –RR system can finally achieve a high NH3 yield rate of ∼40 nmol s–1 cm–2 and Faradaic efficiency of nearly 90%, overperforming the benchmarks reported in the literature. It is anticipated that this work will stimulate the practical development of the pNOR-eNO x –RR system for the green electrosynthesis of NH3 directly from air and water under ambient conditions.

Near-unity electrochemical conversion of nitrate to ammonia on crystalline nickel porphyrin-based covalent organic frameworks

“…[4][5][6][7] By comparison, NH 3 synthesis from the nitrate/nitrite reduction reaction (NO 3 RR or NO 2 RR, respectively) is more kinetically favorable, and has recently attracted rapidly growing attention. [8][9][10] The nitrate or nitrite feed sources can be derived from N 2 via plasma treatments, thereby enabling the indirect fixation of atmospheric N 2 ; 11,12 in addition, the reaction can be powered using renewable electricity from solar and wind sources, and is potentially emission-free. Moreover, the NO 3 RR or NO 2 RR offers a viable solution for removing nitrates and nitrites in eutrophic water bodies produced via overfertilization or other anthropogenic activities and may mitigate the global water security challenge that we face today.…”

Potential Dependence of Ammonia Selectivity of Electrochemical Nitrate Reduction on Copper Oxide

“…Recent studies have shown that electrocatalytic nitrate reduction to ammonia (eNO 3 RR to NH 3 ) is a promising strategy to remove nitrate, while yielding a high value-added ammonia product. − Ammonia is known to play a significant role in agriculture, textile, plastics, and pharmaceutical industries. It has a high energy density (4.3 kWh kg –1 ) and can make ammonia economy, as used in ammonia fuel cell. , Conventionally, ammonia is mainly produced at high temperature and high pressure via Haber-Bosch (H–B) process, which consumes ∼1% of global energy and releases ∼1.5% of global CO 2 emission. − In this regard, eNO 3 RR to NH 3 was considered to not only solve nitrate pollution, but also produces ammonia in benign conditions.…”

Emerging p-Block-Element-Based Electrocatalysts for Sustainable Nitrogen Conversion

“…When the electrocatalysts are employed as the cathode for assembling metal (Li, Na, Zn, or Al)–N 2 batteries, it not only expands the field of metal–gas batteries, but also offers a desirable route for realizing high-efficiency NH 3 production. − Electrocatalysis approaches can be coupled with a nonthermal plasma technique to solve the problem in activating N 2 . The facile plasma-assisted activation of N 2 to NO x – (nitrate or nitrite) allowed for much easier conversion into NH 3 than direct N 2 reduction. − These approaches can deliver relatively high selectivity and efficiency for NH 3 synthesis, which requires further exploration of p-block-element-based catalysts.…”