Unveiling a proton-coupled electron-transfer mechanistic library of nitrate to ammonia via ultramicroelectrode-hyphenated mass spectrometry

For valuable ammonia synthesis and green nitrogen recycling, electrocatalytic nitrate reduction reaction (NO₃RR) presents a sustainable alternative to the conventional Haber-Bosch process. The NO₃RR involves intricate, multi-step proton-coupled electron transfers (PCET) featuring multiple nitrogen-oxygen intermediates and reaction branches. Unveiling this complex reaction library is crucial for rational tailoring of NO₃RR for improved practical application, yet it remains a formidable challenge. Herein we present an in situ ultramicroelectrode-hyphenated mass spectrometry technique to systematically investigate the dynamic electrocatalytic NO₃RR, using a cobalt-based molecular catalyst as a model system, and to decipher its mechanistic library under complex reaction environments (potential- and pH-dependent). Several key short-lived CoNO_xH_y intermediates were directly tracked and identified, experimentally revealing that the overall catalytic pathway of NO₃RR proceeds through the intermediary [LCo-NO₃]→[LCo-NO₃H]⁺→[LCo-NO₂]⁺<b>→</b>[LCo-NO₂H]⁺<b>→</b>[LCo-NO]⁺<b>→</b>[LCo-NHOH]⁺<b>→</b>[LCo-NH]⁺ to produce NH₃, which were further validated by isotopic ¹⁵N-labeling and collision-induced dissociation experiments. Combining theoretical simulations, a complete PCET-based mechanistic pathway was elucidated and distinguished from competing hydrogenation-deoxygenation mechanisms. Notably, a systematically interconnected electrochemical mechanistic library for NO₃RR, visualized through heat maps, was constructed to illustrate intermediate selectivity across a broad potential-pH space. This platform underscores the promising potential of navigating the pathway prediction and regulation of complex reaction environments, thereby advancing the mechanistic understanding of NO₃RR and other complicated electrocatalytic processes.