Catalyst Potential Prescribes Intermediate Coverages in Thermocatalytic Gluconic Acid Oxidation on Pt Nanoparticles

Thermocatalytic aqueous aerobic oxidation reactions occur as kinetically coupled processes that resemble two electrocatalytic half-reactions operating at open circuit. We leverage this concept to reveal a single set of elementary steps that describe not only the fundamental relationships between steady-state rates but also catalyst electrode potentials at open circuit (E_cat) during reactions among gluconic acid (GNA) and O₂ on Pt surfaces at relevant conditions (0.03-0.95 M GNA, 20-2800 kPa O₂, 333-363 K). Irreversible O₂ reduction and GNA oxidation half-reactions couple via cycles that produce and consume surface hydroxyl moieties (HO*) on Pt sites, with kinetically relevant C-H bond scission that proceeds through heterolytic transition states (e.g., [*^-O(H)···H⁺···C(H)(C₅H₉O₆)-O(H)*]^⧧). Observed turnover rates depend sensitively on the fractional coverages of both the O₂- and GNA-derived intermediates. In situ measurements of E_cat provide needed mechanistic insight because these values appear uniquely sensitive to HO* coverages, which increase monotonically with increasing molar ratios of O₂ to GNA. Notably, E_cat does not vary with the coverage of other surface intermediates. Intermittent exclusion of O₂ from the reactor decreases E_cat to values below the point of zero charge (400 mV_RHE) and, nonintuitively, yields increased reaction rates upon reintroduction of O₂ due to the desorption of inhibiting GNA-derived carboxylate species. This strategy controls E_cat (and reactant coverages) in ways that increase reactor productivities by 50% or more when they are averaged over modulation cycles. These findings reveal the critical role of E_cat in setting the coverages of reactive intermediates and the ensuing impacts on rates during redox thermocatalysis.