pH-Mediated Solution-Phase Proton Transfer Drives Enhanced Electrochemical Hydrogenation of Phenol in Alkaline Electrolyte

The faradaic efficiency (FE) of the electrochemical hydrogenation (ECH) of phenol and other biomass-derived model compounds could potentially be improved by operating in alkaline electrolytes, where the parasitic hydrogen evolution reaction rate is significantly slower due to a higher Volmer step barrier. However, this approach is potentially limited by the impact of the higher barrier for adsorbed hydrogen (H_ad) formation, as hydrocarbon ECH is expected to be limited by a hydrogen atom transfer, progressing through a Langmuir–Hinshelwood-type (LH) mechanism. In this work, we show that there are contrasting pH trends for phenol ECH between Pt and Rh, two common catalysts for ECH reactions. Phenol ECH FE and rate on Pt is highest in acidic electrolytes of pH ≤ 5, while activity on Rh is highest near pH 9–10. While our kinetic analysis supports a LH mechanism for Pt at all pH, phenol ECH on Rh shifts from a LH mechanism at low pH to being limited by a direct proton-coupled electron transfer (Eley–Rideal-type mechanism) in which surface adsorbed phenol is hydrogenated by solution-phase H-transfer. We show that the peak activity on Rh at pH 9–10 is due to the proximity of the pH to the pK_a of phenol (pK_a = 10.0). The reversibility of protonation/deprotonation of phenol when electrolyte pH matches its pK_a helps to mediate H-transfer from solution to adsorbed phenol. We also discuss the role of buffer species in mitigating the local pH change and as a H-donor in phenol ECH on Rh at alkaline pH.