Electrolyte organization leads to potential-dependence in thermochemical catalysis of nonpolar reactions

Electrochemical polarization is now known to play a key role in thermochemical catalysis at solid-liquid interfaces. However, existing frameworks cannot account for why even nonpolar, non-faradaic reactions are sensitive to interfacial polarization. In order to uncover the molecular basis of this phenomenon, we herein study the potential-dependent reaction kinetics of ethylene and trans-2-butene hydrogenation at Pt-liquid interfaces. Measurements were performed in aqueous and ortho-difluorobenzene (o-DFB) solutions, spontaneously polarizing the Pt-liquid interfaces by respectively varying the pH or dissolving distinct metallocene redox buffers into solution. We find that, at comparable mechanistic regimes, the rates of both ethylene and trans-2-butene hydrogenation are maximized near the same electrochemical potential, E. Moreover, the potential-dependence, defined as as ∂ ln⁡r/∂E, of trans-2-butene hydrogenation is approximately 2.2x greater than that of ethylene hydrogenation across the full potential range studied. These observations are all consistent with a model in which polarization of the Pt surface away from the local potential of zero free charge (E_PZFC) induces electrostatic organization of polar solvent and/or charged ions near the interface, which impedes olefin adsorption and surface reaction because these surface reactions induce electrolyte displacement. Accordingly, interfacial polarization alters the free energy landscape and thus rate of nonpolar heterogeneous catalysis by controlling the degree of electrostatic organization of polar and charged spectators at the interface, which do not in general need to be specifically chemisorbed onto the surface, but could simply be close enough to the surface to be perturbed by the olefin adsorption. These results point towards electrochemical design handles – namely, the electrolyte, catalyst potential, and local E_PZFC of the catalyst – with which to tune interfacial catalysis of thermochemical organic transformations.