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

Integrated/cascade plasma-enabled N-2 oxidation and electrocatalytic NOx- (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 N-2 and O-2 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 N-2 and O-2 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 NOx in the pNOR system is confirmed as a function of the length for spark discharge as well as the volumetric ratio for N-2 and O-2 feeding gas. The successive protonation process of NOx- 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 similar to 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.

Automated summary

This study systematically investigates the activation and recombination processes of N-2 and O-2 molecules in plasma, and decouples the mechanism of NOx- reduction to NH3, providing a basis for the practical development of ammonia synthesis.