Elucidating the NH ₂ OH–Mediated Pathway for Photoelectrocatalytic C–N Coupling toward Solar-Driven Hexamethylenetetramine Synthesis

The sustainable construction of multiple C-N bonds remains a fundamental challenge, particularly for complex carbon-nitrogen molecules that require multielectron and multistep processes. Hexamethylenetetramine (HMTA) is an essential chemical feedstock, while its conventional synthesis is energy and carbon-emission intensive. Recent studies indicate the promise of Cu-based catalysts for electrochemical HMTA formation, yet a single Cu site is inherently limited in coordinating the multiple hydrogenation and condensation steps necessary for simultaneously forming the two key HMTA precursors (i.e., (CH₂NH)₃ and N(CH₂OH)₃). Herein, we report a sustainable photoelectrocatalytic (PEC) approach for HMTA synthesis via coupling NO₃^- and HCHO using a Si-based photocathode integrated with hierarchical Cu/Co nanoarray cocatalysts (Cu/Co-Si). This architecture decouples light absorption and catalytic reaction, delivering a high HMTA faradaic efficiency of 80.62% at 0.2 V_RHE, over 3.2 times that of Cu-Si, and a yield rate of 5.43 μmol h^-1 cm^-2. Importantly, *NH₂OH generated on Co sites during NO₃^- reduction is identified as a key intermediate that couples directly with *HCHO to form an oxime (*CH₂═NOH), accelerating the formation of (CH₂NH)₃. Meanwhile, Cu sites favor the NH₃-HCHO condensation pathway to generate N(CH₂OH)₃. The rational dual-site strategy and photovoltaic compatibility offer a scalable, sustainable platform for solar-driven C-N coupling toward high-value chemicals.