Mechanistic Insights into the Electrochemical Reduction of Crotonaldehyde on Pd Nanoparticle Electrodes

Electrochemical hydrogenation is an attractive method for the decentralized production of fuels and commodity chemicals from biomass. Molecules in biomass often contain a diversity of functional groups, such as allyls and carbonyls. Selective electrochemical hydrogenation of the desired group is therefore often a key challenge. Although significant progress has been made in the electrocatalytic hydrogenation of compounds containing aromatic groups, that of aliphatic unsaturated aldehydes and ketones is an understudied problem. In this work, we studied the electrochemical hydrogenation of crotonaldehyde on Pd nanoparticles. Crotonaldehyde, an α,β-unsaturated aldehyde, is a relevant model compound because it represents the multifunctional oxygenates typically found in processed biomass. We systematically examined the dependence of the reaction rate and selectivity on electrolyte pH and electrode potential. We found that crotonaldehyde hydrogenation on Pd is selective toward C═C hydrogenation to yield butyraldehyde, with the hydrogen evolution reaction (HER) being the major competing process. The yield for C═C bond hydrogenation increases markedly with decreasing pH, indicating that hydronium ions are the main proton donors. The reaction also exhibits a significant H2O/D2O kinetic isotope effect. To gain further mechanistic insights, we examined the elementary hydrogenation steps with density functional theory (DFT). We found that the primary elementary steps of hydrogenation can proceed in a Langmuir–Hinshelwood (LH) fashion or through proton-coupled electron transfer (PCET), with both pathways having comparable activation barriers, suggesting that both mechanisms may be simultaneously operational. Taken together, this work provides a comprehensive examination of the electrocatalytic hydrogenation of crotonaldehyde on Pd electrodes and furthers the mechanistic understanding of electrochemical C═C hydrogenation in this prototypical α,β-unsaturated aldehyde. These insights have implications for improving reaction selectivity and energy efficiency of the hydrogenation of aliphatic unsaturated carbonyl compounds by electrocatalytic means.