PNNL

Abstract

The presence of aqueous phase complicates catalytic reactions significantly, involving phenomena such as solvating the reacting substrates, enabling a proton-coupled-electron transfer pathway, and inducing changed excess chemical potentials because of the environment. These effects are influenced by the open circuit potential (OCP) established on the metal catalyst, by the quasi-equilibrated electrode reaction of H2 and H3O+(hydr.). The rate of aqueous phase hydrogenolytic reduction of benzylic alcohol to toluene on Pd/C is enhanced by hydronium ions, increasing 2-3 orders of magnitude in rate with a pH decrease from 7 to 0.6. The reaction follows a pathway of protonated benzyl alcohol dehydration to a benzylic carbenium ion, followed by a hydride addition to form toluene. The dehydration of protonated benzyl alcohol is the rate determining step, thus, being enhanced by a high concentration of H3O+(hydr.). The OCP stabilizes all cationic species in the elementary steps. In particular, the initial state (benzyl alcohol oxonium ion) is less stabilized than the dehydration transition state and the product (benzylic carbenium), because both are stronger coordinated in the Helmholtz layer, thus, lowering the difference between the standard free energies of the ground and transition state. In accordance, the rate increased with increasingly negative OCP. This is also manifested by the rate enhancement in presence of an external electric potential on Pd.