Unraveling the reaction mechanisms for furfural electroreduction on copper

Abstract

Electrochemical routes for the valorization of biomass-derived feedstock molecules offer sustainable pathways to produce chemicals and fuels. However, the underlying reaction mechanisms for their electrochemical conversion remain elusive. In particular, the exact role of proton-electron coupled transfer and electrocatalytic hydrogenation in the reaction mechanisms for biomass electroreduction are disputed. In this work, we study the reaction mechanism underlying the electroreduction of furfural, an important biomass-derived platform chemical, combining grand-canonical (constant-potential) density functional theory-based microkinetic simulations and pH dependent experiments on Cu under acidic conditions. Our simulations indicate the second PCET step in the reaction pathway to be the rate- and selectivity-determining step for the production of the two main products of furfural electroreduction on Cu, i.e., furfuryl alcohol and 2-methyl furan, at moderate overpotentials. We further identify the source of Cu's ability to produce both products with comparable activity in their nearly equal activation energies. Furthermore, our microkinetic simulations suggest that surface hydrogenation steps play a minor role in determining the overall activity of furfural electroreduction compared to PCET steps due to the low steady-state hydrogen coverage predicted under reaction conditions, the high activation barriers for surface hydrogenation and the observed pH dependence of the reaction. As a theoretical guideline, low pH (<1.5) and moderate potential (ca. -0.5 V vs. SHE) conditions are suggested for selective 2-MF production.

Fig. 1. A summary of reported major products of aqueous-phase furfural reduction reaction on metal electrodes at moderate potentials. Note that the ratios of colored regions in the box suggest the relative selectivity under similar reaction conditions and the minor products are omitted. Cu is the only metal that is highly selective towards both furfuryl alcohol and 2-methyl furan at different acidic conditions. The details for reported experiments are summarized in Table S1 (ESI†).

Scheme 1. Proposed reaction mechanisms for the furfural electroreduction reaction towards furfuryl alcohol (FAL) and 2-methylfuran (2-MF) on Cu surfaces. (H⁺ + e⁻) and H* denote proton-coupled electron transfer (PCET) and surface hydrogenation (where H* is produced via the Volmer reaction), respectively. The solid or dashed arrows represent elementary reduction step without or with H₂O production.

Fig. 2. The calculated constant-potential free energy diagrams of furfural reduction on Cu(111) surface. (a) The competition of reaction thermodynamics and kinetics in the PCET steps from furfural (FCHO*) to the adsorbates FCHOH* and FCH₂O*; (b) the complete free energy profiles to both products: furfuryl alcohol (FCH₂OH, red) and 2-methyl furane (FCH₃, black). Color code in the insets: brown-Cu, dark grey-C, red-O, and light grey-H.

Fig. 3. Activity for furfural reduction reaction on Cu. (a and b) Simulated and experimental total polarization curves with furfural reduction (in black) and without furfural (HER, in red); (c) and (d) simulated and experimental partial current densities (on log scale) towards furfuryl alcohol (FAL) and 2-methyl furan (2-MF). Simulated reaction conditions: 100 mM furfural, 300 K, pH = 1. We applied ±0.2 eV error estimates for the barrier of selectivity (rate)-determining step for 2-MF formation i.e., FCHOH* → FCH* to account for the intrinsic uncertainty in DFT calculations and electrochemical interface simulations, which is represented by the orange and green shade areas in (c). Reaction conditions: 0.1 M HClO₄ electrolyte (pH 1), 8 mM furfural, constant potential was applied for three hours. Error bars were produced using the results of two separate experiments, where each point warranted a fresh experiment. All experimental potentials were reported with iR corrections. No repeats were performed for −0.60 and −0.65 V vs. RHE as mass transport limitations were already reached at these potentials.

Fig. 4. Degree of rate control (DRC) analysis of furfural electroreduction reaction on Cu. (a) and (b) are the DRC for FAL and 2-MF production respectively. Note that the number of actual reaction steps that determine the activity towards the respective products is 20. In order to increase readability, we only include the steps with DRC values > 0.1 in the plot, as the rest of the steps play a minor role in determining the overall activity.

Fig. 5. Theoretical results of furfural reduction reaction on Cu following an ECH-based mechanism. (a) The calculated constant-potential free energy diagram for furfural reduction to FAL (in red) and 2-MF (in black) at 0 and −0.5 V vs. RHE; (b) the simulated partial current densities (solid lines) assuming an ECH-based mechanism in comparison with experimental results (dots, cf.Fig. 3(d)); (c) the simulated coverages of the main surface adsorbates in the ECH-based mechanism.

Fig. 6. Measured and simulated partial current densities of FAL and 2-MF at varying acidic pH at −0.5 V vs. RHE. The solid lines represent the fitted current densities from our measurement, while the dashed and dash-dotted lines are the simulated current densities for the PCET and ECH based mechanisms, respectively. Reaction conditions: HClO₄ electrolyte adjusted in concentration for varying pH, 8 mM furfural, potential applied was −0.50 V vs. RHE for 3 hours. Error bars were produced using the results of two separate experiments.