We investigated the electrocatalytic hydrogenation (ECH)
of model
aldehydes and ketones over carbon-supported Pd in the aqueous phase.
We propose reaction mechanisms based on kinetic measurements and on
spectroscopic and electrochemical characterization of the working
catalyst. The reaction rates of ECH and of the H2 evolution
reaction (HER) vary with the applied electric potential following
trends that strongly depend on the organic substrate. The intrinsic
rates of hydrogenation and H2 evolution are influenced,
in opposing ways, by the sorption of the reacting organic substrate.
Strong interactions, that is, higher standard free energies of adsorption
of the organic compound, induce high hydrogenation rates. The fast
hydrogenation kinetics produces a hydrogen-depleted environment that
kinetically hinders the HER and the bulk phase transition of Pd to
a H-rich bulk Pd hydride, which is triggered by the applied potential
in the absence of reacting organic compounds. As a consequence of
strong organic–metal interactions, hydrogenation dominates
at low overpotential. However, the coverages of organic substrates
on the metal surface decrease, and the rates of H2 evolution
surpass those of hydrogenation with increasingly negative electric
potential. We determined the range of electric potential favoring
hydrogenation on Pd and quantitatively deconvoluted the effects of
the sorption of the organic compound, and of the rates of proton-coupled
electron transfers, on the kinetics of both ECH and HER. The results
indicate that electrocatalysis offers hydrogenation pathways for polar
molecules which are different and, in some cases, faster than those
dominating in the absence of an external electric potential