UC Irvine

Interfacial proton-coupled electron transfer (I-PCET) is typically viewed as a single elementary reaction despite general recognition that analogous solution-phase reactivity requires proton donor-acceptor pre-association. Herein, we examine the role of pre-association in I-PCET to a graphite-conjugated carboxylic acid (GC-COOH) surface by quantifying electrolyte pH and I-PCET kinetics as a function of NaClO4 concentrations up to 17 mol kg−1. In acidic and acetate-buffered media, we observed attenuations in the I-PCET rate relative to those expected given the solution pH. To account for the influence of electrolyte concentration on I-PCET rate, we propose a multiple-step model wherein the exchange of interfacial Na+ for H3O+ to form a hydrogen-bonded pre-association complex precedes rate-limiting concerted proton-electron transfer. In this model, the increased electrolyte concentration inhibits H3O+ pre-association, a phenomenon that is recovered in molecular dynamics simulations. These studies demonstrate the non-innocence of supporting electrolyte and expose the key role that pre-association equilibria play in I-PCET mechanisms.