Rate enhancement of phenol hydrogenation on Pt by hydronium ions in the aqueous phase

Metal-containing zeolites exhibit remarkable catalytic activity for hydrogenation owing to the synergistic interactions between acid and metal sites. In the aqueous phase, the presence of water and, in particular, hydronium ions, complicate the adsorption of H₂ and organic substrates. It is shown how hydrated hydronium ions formed from zeolite Brønsted acid sites promote the rate of hydrogenation of phenol on Pt by modifying reaction pathways in the aqueous phase. Hydrogen is preferentially added to the ortho-C of phenol at low concentrations of hydronium ions, while at high concentrations of hydronium ions hydrogen adds to both ortho- and para-C of phenol with equal probability. A proton coupled electron transfer (PCET) pathway is hypothesized to occur at metal surfaces associated with large concentrations of hydrated hydronium ions and adsorbed H, establishing a (quasi-) equilibrium open circuit potential. In the presence of lower concentrations of hydrated hydronium ions, the reaction follows a Langmuir-Hinshelwood mechanism in which adsorbed H atoms add to co-adsorbed phenol. DFT calculations show a lower activation energy barrier for the PCET pathway in the presence of hydronium ions compared to the pathway following a Langmuir-Hinshelwood type mechanism.