Oxidation-State Control of Cobalt Electrocatalysts Enables Selective Hydrogenation of Nitrogen-Containing Aromatics

Electrocatalytic hydrogenation offers a sustainable route to reductive molecular transformations by generating hydrogen equivalents from water under ambient conditions. However, practical implementation requires earth-abundant heterogeneous electrocatalysts that combine activity, selectivity, and durability under operating conditions. Here, we report a highly selective electrocatalytic hydrogenation of nitrogen-containing aromatic compounds in an anion-exchange membrane electrolyzer using a carbon-supported cobalt catalyst with a finely tuned Co(0)/CoOx ratio. The optimized catalyst, prepared from CoSO4·7H2O and calcined at 750 °C, converts pyridine to piperidine in >99% yield under ambient electrolysis conditions. Ex situ characterization and in situ Co K-edge X-ray absorption spectroscopy revealed that the catalyst retains an intermediate Co(0)/CoOx ratio under reductive electrolysis conditions, whereas catalysts that become either excessively reduced or remain oxide-rich show lower activity. Kinetic analysis and density functional theory calculations indicate that pyridine hydrogenation proceeds through a Langmuir–Hinshelwood-type pathway, in which the mixed Co(0)/CoOx surface provides an electronically suitable environment for pyridine adsorption and hydrogenation. This electronically tuned Co(0)/CoOx surface enables selective hydrogenation of a broad range of nitrogen-containing aromatic compounds, including pyridines, quinolines, pyrazines, nitriles, and nitroarenes, while suppressing undesired hydrogenation pathways observed with Rh-based catalysts. Guided by the operando insight, intermittent electrolysis was introduced to prevent catalyst over-reduction during prolonged operation, enabling gram-scale conversion of pyridine to piperidine in 89% yield with stable cell voltage. These results highlight the importance of maintaining an intermediate Co(0)/CoOx state for selective electrocatalytic hydrogenation with earth-abundant cobalt catalysts.