Electrocatalytic Coupling of Nitrate and Formaldehyde for Hexamethylenetetramine Synthesis via C-N Bond Construction and Ring Formation.

Hexamethylenetetramine (HMTA) is extensively used in the defense industry, medicines, food, plastics, rubber, and other applications. Traditional organic synthesis of HMTA relies on ammonia derived from the Haber process at high temperatures and pressures. In contrast, electrochemical methods enable a safe and green one-pot synthesis of HMTA from waste NO3-. However, HMTA synthesis through the electrochemical method is challenging owing to the complex reaction pathways involving C-N bond construction and ring formation. In this study, HMTA was efficiently synthesized over electrochemical oxidation-derived copper (e-OD-Cu), with a yield of 76.8% and a Faradaic efficiency of 74.9% at -0.30 VRHE. The catalytic mechanism and reaction pathway of HMTA synthesis on e-OD-Cu were investigated through a series of in situ characterization methods and density-functional theory calculations. The results demonstrated that the electrocatalytic synthesis of HMTA involved a tandem electrochemical-chemical reaction. Additionally, the results indicated that the presence of Cu vacancies enhanced substrate adsorption and inhibited the further hydrogenation of C═N. Overall, this study provides an electrocatalytic method for HMTA synthesis and an electrochemical strategy for constructing multiple C-N bonds.

In situ Activation of Molecular Oxygen at Intermetallic Spacing-Optimized Iron Network-Like Sites for Boosting Electrocatalytic Oxygen Reduction.

The oxygen reduction reaction (ORR) catalyzed by transition-metal single-atom catalysts (SACs) is promising for practical applications in energy-conversion devices, but great challenges still remain due to the sluggish kinetics of O═O cleavage. Herein, a kind of high-density iron network-like sites catalysts are constructed with optimized intermetallic distances on an amino-functionalized carbon matrix (Fe-HDNSs). Quasi-in situ soft X-ray absorption spectroscopy and in situ synchrotron infrared characterizations demonstrate that the optimized intermetallic distances in Fe-HDNSs can in situ activate the molecular oxygen by fast electron compensation through the hybridized Fe 3d-O 2p, which efficiently facilitates the cleavage of the O═O bond to *O species and highly suppresses the side reactions for an accelerated kinetics of the 4e- ORR. As a result, the well-designed Fe-HDNSs catalysts exhibit superior performances with a half-wave potential of 0.89 V versus reversible hydrogen electrode (RHE) and a kinetic current density of 72 mA cm-2 @0.80 V versus RHE, exceeding most of the noble-metal-free ORR catalysts. This work offers some new insights into the understanding of 4e- ORR kinetics and reaction pathways to boost electrochemical performances of SACs.