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