(307f) Relationship between Aerobic Oxidation Catalysis and Electrochemical O2 Reduction on Heterogeneous M–N–C Catalysts

Heterogeneous catalysts mediate the reduction of O₂ (ORR) and the oxidation of organic molecules both separately in electrocatalytic redox steps on-electrode, and in concert in thermocatalytic aerobic oxidations off-electrode. These disparate reactivity modes have been proposed to share key mechanistic features [1,2], but the generality of this concept remains unclear. Our group recently reported a fuel cell wherein electrochemically generated hydroquinone (H₂Q) molecules mediate thermocatalytic ORR on a heterogeneous metal–nitrogen–carbon catalyst (M–N–C) off-electrode [3]. Here, we interrogate the kinetics of H₂Q-mediated ORR on M–N–C to compare its mechanism with that of electrocatalytic ORR.

The ORR TOF (303 K, per total M) mediated by a tetra-sulfonated thioether-substituted H₂Q (E = 0.6 V vs. RHE) on a representative Co–N–C catalyst in the aqueous phase was quantified as a function of [O₂], [H⁺], [H₂Q], and [Q]. These kinetic dependencies ([O₂]¹, [H⁺]⁰, [H₂Q] saturation, [Q] inhibition), together with an H/D KIE of 2.4 ± 0.4 in D₂O solution, are consistent with a cooperative mechanism (Panel (a)) that involves inner-sphere O₂ reduction by H₂Q [4,5], which is physisorbed on the surface near saturation coverages during catalysis, as corroborated by equilibrium adsorption isotherms. The inner-sphere mechanism of H₂Q-mediated ORR leads to a lack of correlation between on- and off-electrode ORR TOFs on a suite of M–N–C (Panel (b)), where Co–N–C catalyzes off-electrode ORR with higher TOFs than Fe–N–C, but not on-electrode ORR. ORR TOFs mediated by a series of H₂Q with varying redox potentials establish a linear free energy relationship (Panel (c)) whose slope deviates from the on-electrode ORR Tafel slope, further supporting the kinetic relevance of inner-sphere H₂Q-mediated O₂ reduction steps off-electrode. These insights provide guidance to design mediators and catalysts for ORR and aerobic oxidations.

References

[1] Wieckowski and Neurock, Advances in Physical Chemistry 2011, 2011, 1–18.

[2] Ryu, Surendranath, et al. ChemRxiv 2021. 10.26434/chemrxiv.13830920.v1

[3] Preger, Stahl, et al. Joule 2018, 2 (12), 2722–2731.

[4] Anson, Hammes-Schiffer, Stahl, et al. J. Am. Chem. Soc. 2016, 138 (12), 4186–4193.

[5] Adams, Neurock, Flaherty, et al. Science 2021, 371 (6529), 626–632.