Molecular Copper Bipyridine Complex as a Catalyst for Electrochemical Nitrate Reduction.

Area of Science:

• Electrochemistry
• Catalysis
• Sustainable Chemistry

Background:

• Nitrate electroreduction to ammonia is a sustainable alternative to conventional nitrogen fixation.
• Developing molecular model systems for mechanistic understanding is crucial but challenging.

Purpose of the Study:

• To report a copper bipyridine chloride complex, [Cu(bpy)2Cl]+, for efficient nitrate electroreduction.
• To investigate structure-activity relationships and elucidate reaction mechanisms.
• To explore the catalytic scope for organonitro substrate reduction.

Main Methods:

• Electrochemical reduction in aqueous phosphate buffer.
• Synthesis and characterization of copper bipyridine chloride complexes with varying substituents.
• In situ FTIR, electrochemical mass spectrometry, and UV-vis spectroscopy for mechanistic studies.

Main Results:

• [Cu(bpy)2Cl]+ achieved ~100% Faradaic efficiency for ammonia at -0.89 V vs RHE with high current density.
• Sterically accessible and electron-deficient copper centers enhanced catalytic activity.
• Mechanistic studies revealed stepwise N-O bond cleavage and proton-coupled electron transfer via NO2, NO, and hydroxylamine intermediates.
• [Cu(bpy)2Cl]+ selectively reduced nitrobenzene to aniline with 90% Faradaic efficiency.

Conclusions:

• The copper bipyridine chloride complex is a highly effective catalyst for nitrate electroreduction.
• Molecular design, particularly copper center accessibility and electron deficiency, is key for enhanced activity.
• Detailed mechanistic insights were gained, advancing the understanding of electrocatalytic nitrogen and nitro group reduction pathways.