(687g) Quantification of Catalytically Relevant Fe Species in Nitrogen-Doped Carbon

Heterogeneous catalysts consisting of iron cations incorporated into nitrogen-doped carbon (“Fe-N-C”) have received extensive attention as leading alternatives to Pt for the electrochemical oxygen reduction reaction (ORR), and additionally catalyze the thermal oxidation of organic molecules with O₂ as the oxidant. Fe-N-C catalysts host mononuclear nitrogen-ligated active centers, FeN_x, that frequently coexist with agglomerated Fe species. Although FeN_x are the dominant active centers for ORR electrocatalysis and many aerobic oxidations, the accuracies of methods to quantify them are still debated. Here, we develop a kinetic probe-reaction approach to quantify FeN_x centers and compare it with spectroscopic and probe-molecular methods.

Initial rates of the aerobic oxidation of a water-soluble hydroquinone (HQ) were measured under kinetically controlled and mechanistically well-defined conditions (303 K, 50 mM HQ, 1.1 atm O₂, 0.5 M aqueous H₂SO₄). Fe-N-C catalysts were synthesized to contain mononuclear FeN_x species at low loadings (0.1–0.4 wt% bulk Fe) on solvent-accessible surfaces using a postsynthetic metalation approach and their Fe speciation was confirmed by low-temperature ⁵⁷Fe Mössbauer spectroscopy. The HQ oxidation rate (r_HQ, per g_catalyst) catalyzed by these well-defined materials correlates linearly with the density of FeN_x centers (N_FeNx), reflecting their intrinsic HQ oxidation turnover frequency (TOF_FeNx). Assuming negligible contributions to r_HQ from other Fe species, HQ oxidation was evaluated as a kinetic probe to estimate the density of FeN_x species (N_FeNx = r_HQ / TOF_FeNx) in a suite of Fe-N-C catalysts with diverse synthetic origins and Fe speciation (0.3–8.4 wt% bulk Fe). Kinetically determined FeN_x site densities are compared with those estimated by low-temperature ⁵⁷Fe Mössbauer spectroscopy, pulse CO chemisorption, and electrochemical stripping of NO derived from NO₂^−. Kinetic quantifications of FeN_x centers correlate well with probe-molecular methods and do not require pretreatments that may alter active-site distributions or specialized equipment, offering an attractive complementary approach.