Lignocellulose, the non-edible part of biomass, is considered an attractive source of renewable carbon.[1] Its breakdown can produce a range of platform compounds, including furfural. Two of the main valorization products of furfural are furfuryl alcohol (FOH; a binder for the foundry industry) and, to a smaller extent, 2-methylfuran (2MF), which has been proposed as a drop-in biofuel[2] and high-density biofuel precursor[3]. Electrocatalytic hydrogenation appears to be a promising synthesis route for 2MF owing to the direct use of (renewable) electricity, mild operating conditions (ambient temperature and pressure) and eased separation of the product thanks to a low solubility of 2MF in water. Through preliminary techno-economic analysis, we recently demonstrated the beneficial aspects of this approach compared to other decarbonization pathways.[4]

Still, there are challenges to this approach, including (i) achieving sufficient faradaic efficiency (FE) toward furfural conversion (vs the competing Hydrogen Evolution Reaction) and (ii) reaching high selectivity toward 2MF at high current densities. Recent notable studies managed to improve FE_2MF up to 75% at pH 2.9 and -0.58 V vs RHE through the use of a bimetallic CuPd catalyst while the use of a surfactant like butyltrimethylammonium bromide in a pH 1 electrolyte switched selectivity from S_FOH : 83.8% to S_2MF : 80.1% at 15 mA.cm^-2 on electrodeposited Cu on Cu foam.

To address these challenges, we investigated the performance of Cu₂O nanocubes as well-defined model pre-catalysts (Cu₂O-p) for the electrocatalytic hydrogenation of furfural toward 2MF. Well-dispersed particles with diameters of 62.5±39.2 nm were produced on the carbon support via immersion in an acidic electrolyte followed by a redeposition step. In light of a recent report, the morphology of the particles was monitored in situ by transmission electron microscopy (TEM) to help establish structure-activity relationships in our system.[5]

In various electrocatalytic systems, an effective strategy to steer the selectivity of a reaction is through electrolyte engineering.[6] From this perspective, we conducted an evaluation of the effects of cations (Li⁺, Na⁺, K⁺, Cs⁺) and anions (SO₄^2-, PO₄^3-) in acidic conditions (pH 2) using Cu₂O-p. At a fixed potential of -0.5 V vs RHE, in 0.5 M KH₂PO₄ (pH 2) and 40 mM furfural, a selectivity toward 2MF of 72.1±3.4% (FE_2MF: 82.3±3.3%) was measured as opposed to a lower selectivity of 54.8±4.5% in 0.5 M K₂SO₄ (FE_2MF: 65.1±5.4%). In addition to a favorable selectivity toward 2MF in phosphate, we found (i) a noticeable influence of cations on FE_2MF in sulfate and (ii) a limited influence of cations on selectivity in both electrolytes.

To examine the interplay between anions and the observed selectivity change, we utilized time-resolved in-situ surface-enhanced Raman spectroscopy (TR-SERS). In 0.5 M KH₂PO₄ + 40 mM furfural (pH 2) and at open-circuit potential (OCP), a stronger redshift of the adsorbed carbonyl bond was observed in comparison to the same experiment in sulfate. We associated this shift with a stronger interaction of phosphate anions with furfural through hydrogen bonding. Upon the application of cathodic potentials, a local increase in pH was linked to the proton donor behavior of phosphate. We posit those interactions allow for an enhanced cleavage rate of the C=O bond under operating conditions, resulting in selectivity toward 2MF comparable to state-of-the-art results.