Atomically dispersed metals incorporated into nitrogen-doped carbon (“M-N-C”) are promising alternatives to precious-metal catalysts for fuel cell and (electro)chemical synthesis applications. The ability of M-N-Cs to catalyze electrochemical O2 reduction and chemical aerobic oxidations of organic molecules suggests that the latter could involve electrochemical and/or thermochemical elementary steps. We explored these mechanistic concepts in the context of the aerobic oxidation of hydroquinone (HQ) catalyzed by M-N-C (M = Co, Fe) particles suspended in solution in the absence of an electrochemical driving force. Kinetic and adsorption studies reveal HQ molecules saturate M-N-C surfaces and undergo direct hydrogen-atom transfer to an O2 species bound to an active metal center. This alternative mechanistic pathway circumvents the rate–potential relationship observed for electrocatalytic O2 reduction, opening new opportunities to design fuel cell systems that reduce O2 at lower overpotential by using high-potential mediators to drive thermocatalytic pathways. In a complementary effort, molecular concepts for macrocycle preparation and metalation were adapted to synthesize Fe-N-C catalysts. Solution-phase conditions form FeNx moieties via metalation with FeCl2 facilitated by a Brønsted base at 150 °C, contrasting conventional pyrolysis strategies that employ high temperatures (600–1100 °C). The removal of Zn2+ ions from the N-C support prior to metalation leads to higher FeNx site densities, resembling the transmetalation strategy commonly employed for the preparation of molecular macrocycles. These well-defined Fe-N-C catalysts demonstrate that increased Fe dispersion leads to improved aerobic HQ oxidation reactivity.

Chemical and electrochemical reactivity of heterogeneous M-N-C catalysts and molecular concepts for their preparation

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