Thermochemical aerobic oxidation reactions can be conceptualized as kinetically coupled O
2 reduction and substrate oxidation electrochemical half reactions (Figure 1a) where the reduction and oxidation currents are equal and the catalyst potential equals the open circuit potential (E
oc).
[1] With this conceptual framework, thermochemical rates and selectivities are predictable from independent electrochemical measurements,
[2] however, these analyses are limited at elevated temperatures and in packed bed reactors, especially for complex substrates such as gluconic acid (GNA, C
6H
12O
7). Here, we bridge this gap by introducing working and reference electrodes to a packed bed reactor for
in situ measurements of E
oc (Figure 1b), demonstrated for the selective oxidation of GNA to guluronic acid on Pt/C catalysts. This system accurately reaches the O
2 reduction equilibrium potentials (1200–1250 mV vs. RHE, 0.2–30 bar O
2, 353 K) in the absence of GNA. Kinetic dependencies give low reaction orders for GNA (0–0.3) and O
2 (0–0.2), corresponding to high surface coverages of both intermediates. The corresponding E
oc values scale as a single-valued function of the O
2-to-GNA ratio, with a Tafel-like slope of 120 (±20) mV per decade, arising from the additional electron required to evolve GNA C-H scission transition states relative to O
2 O-H formation transition states. Furthermore, E
oc prescribes the GNA oxidation rates (Figure 1c) over a wide range of GNA concentrations (0.03–0.5 M) and O
2 pressures (0.2–30 bar), where fixing the GNA concentration gives the effective Butler Volmer kinetic expression for O
2 reduction, accounting for site coverages, and vice versa. Taken together, these findings reveal the inherent electrochemical half reactions embedded in aqueous aerobic oxidation catalysis.
[1] J. Ryu et. al., Nat. Catal. 2021, 4, 742.
[2] J. S. Adams, M. L. Kromer, J. Rodríguez-López, D. W. Flaherty, J. Am. Chem. Soc. 2021, 143, 7940.
