Exposing the role of pre-association in interfacial proton-coupled electron transfer

Interfacial proton-coupled electron transfer (I-PCET) reactions are typically viewed as single elementary reaction steps even though analogous solution-phase reactivity is known to require pre-association of proton donor and acceptor. Herein, we examine the role of pre-association in I-PCET to a molecularly well-defined graphite-conjugated carboxylic acid (GC-COOH) surface site. We quantify electrolyte proton activity and I PCET kinetics in acidic, acetate buffered, and alkaline electrolytes as a function of NaClO4 concentration, ranging from 1 mole kg−1 to 17 mole kg−1. Upon accounting for the previously measured proton activity dependence of I-PCET kinetics to GC-COOH, we find that rate of I-PCET is systematically attenuated by factors of 4.3 and 4.6 over this range of NaClO4 concentration in acidic and acetate buffered media, respectively. In contrast, the rate of I-PCET remains unchanged within error across NaClO4 concentration in alkaline electrolyte. Based on these observations, we propose a multiple-step model for I-PCET in acidic media that invokes quasi-equilibrated displacement of Na+ from the interface to form hydrogen-bonded pre-association complexes prior to rate-limiting concerted proton-electron transfer. Increased NaClO4 concentration is invoked to increase Na+ activity in the bulk vs the interface, inhibiting pre-association complex formation and the overall I-PCET rate. These studies emphasize the non-innocent role of support electrolyte species and expose the key role of pre-association equilibria in I-PCET mechanisms. The work also suggests that control over pre-association equilibria could be used as an additional handle for tailoring the kinetics of interfacial ion transfer reactions.