Deconvoluting the Role of Electrolyte pH, Structural Sensitivity, and Electric Field Effects on the Electrochemical Hydrogenation of Phenol

Abstract

Traditional thermochemical hydrogenation (TCH) methods that are used in the U.S. chemical and petroleum industries to produce value-added fuels are highly energy intensive. To achieve hydrogenation, they require high temperature and pressure, along with an external source of hydrogen that usually comes from steam methane reformation. Electrochemical hydrogenation (ECH) is an attractive alternative that replaces the thermal driving force with an applied electrical potential, allowing ECH to operate near ambient conditions. The participating protons come from the aqueous electrolyte solution rather than externally produced hydrogen. These features of ECH lower its energy demand compared to TCH and result in less greenhouse gas emissions. ECH is also promising for the processing of biomass feedstocks and could be powered directly by renewable energy, further minimizing the environmental impact.The addition of an aqueous electrolyte and electric field means that insights from TCH should not be directly translated to ECH systems, however, this is often the case in the literature. Fundamental studies on ECH of aromatic hydrocarbons (AHs) are necessary to address current limitations including: competition with the hydrogen evolution reaction (HER) resulting in lower faradaic efficiency, low rates compared to TCH, and little control over selectivity and degree of hydrogenation. For instance, the role of electrolyte pH on phenol ECH remains poorly understood. While some groups have reported negligible phenol turnover rates in electrolytes of pH 10 and above on Pt/C and Pd/C catalysts [1], Song et al. have shown that turnover rates and efficiency are highest on Rh/C at pH 10 [2]. Yet, the idea that low pH (3-5) is best has persisted and the source of the pH dependence remains unknown.In this work we use phenol as a simple representative AH for lignocellulosic bio-oil. We have shown with rotating disk electrode voltammetry that the presence of phenol in electrolytes of pH ~10 enhances HER rates on Pt, Pd, and Au catalysts. Considering that the pKa of phenol is also 10.0, we propose that interfacial phenol molecules channel protons to the catalyst surface by way of facile proton dissociation/reassociation, lowering the barrier for adsorbed H formation in alkaline electrolytes. While this may explain, in part, the low phenol turnover rates observed at higher pH, we further investigate why the opposite is observed on Rh/C catalysts. The role of atomic surface structure is also critical to the adsorption geometry and coverage of AHs. We use well-defined, low-index Pt single crystals to identify the true structural sensitivity for the ECH of AHs. We show with select stepped single crystal surfaces that there is a synergetic role between terrace width/step density that defines activity, balancing coverage of AH molecules, formation of surface adsorbed H, and transfer of that H to AH reactants. We use a low-volume, membrane-separated flow cell for chrono-amperometry followed by product analysis to correlate pH, structural sensitivity, and applied potential to phenol ECH rates and faradaic efficiency. With an understanding of the role of atomic surface structure and the true source of pH dependence, we propose strategies for enhancing phenol turnover rates at lower overpotentials, improving faradaic efficiency, and controlling selectivity.