An initial screening among common noble metals showed that palladium has outstanding catalytic activity and Faradaic efficiency for the electrocatalytic reduction of carbonyl groups. Thus, we explore here a series of C-supported Pd catalysts to electrocatalytically reduce aromatic compounds with carbonyl groups, ranging from benzaldehyde to vanillin as well as bio-oil surrogates. The aim of our work is twofold, i.e., (i) to understand the mechanisms and elementary kinetics that enables the higher rates with reduction equivalents produced electrochemically than from added H₂, and (ii) to translate this knowledge into innovative technology. We used H-cells to study the effect of potential, pH and metal particle size on the rates of electrocatalytic hydrogenation (ECH) of model compounds. On the other hand, bio-oil surrogates were converted in continuous-flow membrane units, which performed as ideal plug-flow reactors. The active metal, Pd, was supported directly in carbon felt electrodes and on powdered activated carbon infiltrated in the felt electrode.

ECH rates of model compounds (e.g., benzaldehyde, in the figure) increase with increasing cathodic potentials and increasing proton concentrations. This is attributed to a coupled proton and electron addition to the adsorbed oxygenated substrate, which is faster than the H₂ evolution reaction (HER). The difference between ECH and HER rates; however, depends on pH and the competitive adsorption of the electrolyte at the Pd cathode. Acetate buffer, for instance, allows reaching Faradaic efficiencies of 99%, whereas H₂SO₄ decrease HER the Faradaic efficiencies to 80%. Radical mechanisms are enabled at high cathodic potentials, which lead to the formation of dimers.

In continuous flow systems, similar to batch H-cells, the aromatic aldehydes are quantitatively converted to aromatic alcohols. That is, the reactivity of aromatic rings is quantitatively suppressed in the presence of a substituting carbonyl. The reactivity of these compounds is strongly affected by decreasing polarity of the reaction media due to lower conductivity and solvation effects (HER is less affected than ECH by increasing the C number of the co-solvent alcohol from methanol to i-propanol). This effect can be compensated by increasing cell potentials (see Figure 2 for the ECH of acetophenone).

Overall, the results help to understand the elementary steps on Pd catalysts of the reductive electrocatalytic conversion of carbonyl functionalities, which participate in undesired condensation reactions in high temperature processes. These processes can be catalyzed with unprecedented Faradaic efficiency that is caused by both the selective adsorption of the reactive substrates and the high rate of hydrogen addition.

Figure caption. Left: Turnover frequency (TOF) and faradaic efficiency (FE) observed in the electrocatalytic hydrogenation of benzaldehyde on Pd/C at varying cathodic potential. Right: Turnover frequency (TOF) and current observed in the electrocatalytic hydrogenation of acetophenone on Pd/C at varying cell potential.