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., (CH 2 NH) 3 and N(CH 2 OH) 3 ). Herein, we report a sustainable photoelectrocatalytic (PEC) approach for HMTA synthesis via coupling NO 3 – 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, *NH 2 OH generated on Co sites during NO 3 – reduction is identified as a key intermediate that couples directly with *HCHO to form an oxime (*CH 2 ═NOH), accelerating the formation of (CH 2 NH) 3 . Meanwhile, Cu sites favor the NH 3 –HCHO condensation pathway to generate N(CH 2 OH) 3 . The rational dual-site strategy and photovoltaic compatibility offer a scalable, sustainable platform for solar-driven C–N coupling toward high-value chemicals.