(750c) Controlling Hydrogenation Rates Via Applied Potential and Brønsted Acidity in Water

We aim to control hydrogenation of biogenic, oxygen-containing, molecules in water at near ambient temperature. We hypothesize that in aqueous environments, the concentration of hydronium ions and applied electrical potential help control and maximize reaction rates while otherwise maintaining mild reaction conditions. Our research focuses on understanding the hydrogenation of model molecules enabled by an applied potential (electrocatalytic hydrogenation) or by externally added H₂ (thermocatalytic hydrogenation, TCH). The catalytic performance of a series of C-supported metals have been studied at varying pH values or varying concentrations of acid functional groups neighboring the metal sites to investigate the influence of positive charges on reductive conversions. Kinetic experiments at varying applied potential together with electrochemical and in-situ spectroscopic studies have allowed us to differentiate the reaction mechanisms for hydrogenation of aromatic rings and carbonyl groups in ketones and aldehydes.

Decreasing pH of the aqueous phase enhances the rates of carbonyl hydrogenation in benzaldehyde and acetophenone as well as of C-O bond cleavage in benzyl alcohol regardless of the origin of the reduction equivalents. We have correlated this effect with the potential difference with respect to the potential of zero total charge, i.e., the potential at which the total charge of the surface is zero. Varying the pH towards acidic conditions shifts the potential towards the point of total zero charge, which facilitates the charge redistributions associated with reactions at the metal-liquid interface. This includes weakening of H-metal binding, which also correlates hydrogenation rates.

In contrast, acid functional groups on the support and co-adsorbed proton-donating molecules (both providing protic hydrogen adjacent to metal sites) facilitate ECH but not TCH. Detailed kinetics and theoretical simulations point to this effect originating from the PCET mechanism that allows for multiple ways of proton delivery to the carbonyl groups.