(468a) Postsynthetic Routes to Atomically Dispersed M–N–C Catalysts Inspired By Surface Organometallic Chemistry

Earth-abundant metals incorporated into nitrogen-doped carbon (M–N–C) as mononuclear MN_x species catalyze a variety of electrochemical reactions relevant to energy and the environment, and thermochemical reactions relevant to organic synthesis. Yet, common methods to synthesize M–N–C catalysts lead to a low practical limit for the density of mononuclear MN_x, because their high temperatures (>600 °C) mobilize M species whose aggregation competes with their stabilization at N_x binding sites on the support. Here, methods are developed to selectively form MN_x species on N-doped carbon supports, inspired by surface organometallic chemistry (SOMC).

N–C supports were synthesized by treating N, C precursor mixtures, including ZIF-8 metal-organic framework, polyaniline (PANI), and 1,10-phenanthroline (Phen), under flowing N₂ to high temperatures (800–1050 °C). In a second step, solution-phase metal precursors were grafted to N–C, likely as ML_nN_x, followed by thermal treatments intended to remove L_n species and afford MN_x sites. Hydroquinone oxidation turnover rates (per total M) were quantified on Co–N–C as a probe reaction sensitive to mononuclear MN_x sites, such that TOF values independent of metal loading identify catalysts with similar CoN₄ speciation (shaded area, panel C), inferred to be predominantly mononuclear. One-step pyrolysis produces Co–N–C with low active site utilization unless the metal loading is sufficiently low to minimize aggregation, as indicated by TOF values that systematically decrease with metal loading (>0.6 wt%). SOMC-inspired postsynthetic methods using N–C derived from the pyrolysis of ZIF-8 increase the range of achievable weight loadings at TOF values consistent with predominant speciation as CoN_x. These approaches are extendable to relevant metals such as Fe and Mn, which are competent O₂ reduction catalysts according to measured electrochemical polarization curves. Quantitative characterizations of metal speciation and strategies to design N–C precursors will also be discussed.