Phenol-Mediated Proton Shuttling Enables Electrochemical Hydrogenation of Phenol in Alkaline Electrolytes on Rhodium

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Thermal chemical hydrogenation (TCH) is a key reaction for upgrading petroleum and biomass feedstocks to value- added fuels and chemicals. TCH requires high operating temperatures and pressures which results in high energy consumption and CO2 emissions. Electrochemical hydrogenation (ECH) is a mild alternative where applied potential drives the hydrogenation of hydrocarbons with protons from the aqueous electrolyte near ambient conditions. Phenol is a simple representative compound for renewable lignocellulosic feedstocks that has been the focus of recent ECH studies. The added complexity from the electrolyte and applied potential in aqueous ECH systems requires a fundamental understanding of these reaction parameters on ECH activity in-order to overcome current limitations, including faradaic efficiency losses to the competitive hydrogen evolution reaction (HER). Furthermore, the wide range of reaction conditions employed in the phenol ECH literature makes comparison of the ECH performance difficult. Phenol ECH on PGMs is commonly performed in electrolytes in a pH range of 1-5, assuming that the ECH of phenol follows a Langmuir Hinshelwood (LH) mechanism [1, 2]. One study reported a 95% faradaic efficiency (FE) for phenol ECH on rhodium at pH 10 [3], but no further investigation into the source of this promising activity at alkaline pH has been conducted.In this work we study the role of electrolyte pH on phenol ECH on platinum and rhodium. We show that there are differing pH trends for ECH activity between platinum and rhodium, with the highest ECH rates on platinum at pH 1 and the highest rates on rhodium at pH 9, near the pka of phenol. We show using cyclic voltammetry and in-situ electrochemical FTIR that this pH dependent behavior results from the interplay between hydrogen adsorption kinetics, differences in phenol/hydrogen coverage on each metal, and competition with HER. We show that the electrolyte pH dictates the balance between a hydrogen atom transfer (LH) mechanism and a phenol-mediated proton coupled electron transfer mechanism. With this insight, we show that the unique properties of rhodium and the acid-base chemistry of phenol and the electrolyte buffer can be leveraged for higher ECH rates and FE.[1] Singh, N., et al., Aqueous Phase Catalytic and Electrocatalytic Hydrogenation of Phenol and Benzaldehyde over Platinum Group Metals. Journal of Catalysis 2020, 382, 372–384. https://doi.org/10.1016/j.jcat.2019.12.034.[2] Song, Y., et al., Integrated Catalytic and Electrocatalytic Conversion of Substituted Phenols and Diaryl Ethers. Journal of Catalysis 2016, 344, 263–272. https://doi.org/10.1016/j.jcat.2016.09.030.[3] Song, Y., et al., Aqueous Phase Electrocatalysis and Thermal Catalysis for the Hydrogenation of Phenol at Mild Conditions. Applied Catalysis B: Environmental 2016, 182, 236–246. https://doi.org/10.1016/j.apcatb.2015.09.027.