Insights into the electrochemical hydrogenation of phenol

Brianna Markunas

doi:10.17918/00011056

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 sourced from the aqueous electrolyte near ambient conditions. As a renewable energy source, lignocellulose is an abundant yet underutilized feedstock comprised of complex phenolic compounds that can be valorized via ECH. Phenol is a simple model compound for lignocellulose that has been the focus of recent ECH studies. The added complexity of aqueous organic electrochemical systems from the electrolyte and applied potential 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), lower turnover rates compared to TCH, and varying degrees of hydrogenation. Furthermore, the wide range of reaction conditions employed in the phenol ECH literature makes comparison of the ECH performance difficult. Compared to the decades of research that have been devoted to small molecule electrocatalysis, our understanding of electro-organic conversions like phenol ECH is still in its infancy. The goal of this work is to advance insight into what parameters of the electrode/electrolyte interface govern phenol ECH activity and selectivity, in order to identify strategies for overcoming limitations that hinder rates and efficiency. We employ low-volume electrochemical systems with well-controlled mass transport combined with model catalysts/molecules to develop fundamental insights and extract kinetic parameters. In Chapter 3 we employ rotating disk electrode voltammetry to study phenol and hydrogen adsorption processes at the electrode surface. Our work using platinum single crystal electrodes reveals unique phenol/H adsorption behavior, showing slowed kinetics with increasing electrolyte pH. Comparison of Pt(111) and Pt(110) electrodes in phenol voltammetry and extended phenol electrolysis reveal that phenol adsorption strength is highly sensitive to atomic structure. In Chapter 4, we use chrono-amperometry on higher surface area platinum and rhodium electrodes combined with product analysis to investigate phenol ECH in alkaline pH, where we expect higher faradaic efficiency due to lower rates of the parasitic HER. Surprisingly, we find that phenol ECH on platinum and rhodium exhibit inverted pH trends. On platinum, increasing pH leads to significant decay in phenol ECH activity, becoming negligible above pH 9. On rhodium, however, phenol ECH activity is lowest in acidic electrolyte and highest near pH 10. Using suppression of H_[UPD] in the presence of varying phenol concentration as a proxy for phenol coverage, we use a Temkin adsorption isotherm to extract phenol adsorption energies and coverages across both acidic and alkaline pH. Our results point to slightly higher phenol coverage on rhodium than platinum, possibly explaining the overall higher rates of phenol ECH on rhodium, but could not sufficiently explain their opposite pH trends. With this, we turn to investigating the mechanistic progression for phenol ECH on both metals. Most of the existing phenol ECH literature assumes that like TCH, phenol ECH progresses through a Langmuir Hinshelwood (LH) mechanism in which surface adsorbed phenol reacts with neutral, surface adsorbed hydrogen in a hydrogen atom transfer step. It is possible in aqueous electrochemical systems, however, for phenol ECH to progress through an Eley-Rideal (ER) mechanism in which surface adsorbed phenol reacts with a proton abstracted directly from solution in a proton-coupled electron transfer. Our kinetic analysis constructing reaction order plots for phenol ECH suggested that an LH mechanism dominates on platinum while an ER mechanism dominates on rhodium. The high rates and faradaic efficiency observed for rhodium in base are likely a combination of its progression through an ER mechanism, the slower kinetics of hydrogen adsorption on rhodium than platinum, and the higher coverage of phenol on rhodium in base. We hypothesize that the peak activity near pH 10 is due to operating near the pKa of phenol where it is at dissociative equilibrium, where it can act to mediate and lower the barrier for proton transfer to surface adsorbed phenol. This mechanistic understanding pointed to the proton environment at the interface as a critical parameter space for controlling phenol ECH through an ER mechanism. In Chapters 5 and 6, we investigate the role of electrolyte components, namely buffer and cation species, that could potentially affect this environment. In Chapter 5, we identify the role of the buffer in not only regulating bulk and interfacial pH, but also as a low-barrier proton donor in ER kinetics unique to rhodium in alkaline electrolyte. The positive dependence of phenol ECH on buffer acid concentration supports our earlier evidence of an ER mechanism on rhodium in alkaline media and demonstrates how tuning the buffer composition can significantly improve phenol ECH activity. Operating near the pKa of the buffer acid species enhances this effect, showing how we can exploit the acid/base equilibria of both the buffer and phenol as proton-mediators. In Chapter 6 we investigate the role of alkali metal cations (Li⁺, Na⁺, K⁺, Cs⁺) with an understanding that the negative electric field present at ECH relevant potentials drives their accumulation at the interface. We find that cations can promote, inhibit, or exert no significant effect on phenol ECH activity depending on catalyst identity, pH, applied potential, and cation size: all factors that we identify to affect the degree of their interaction with the interface. Increasing cation size and concentration promotes phenol ECH exclusively on rhodium at pH 9, which we attribute to the ER mechanism unique to these conditions. We further propose mechanisms by which cations act to promote or inhibit phenol ECH under these varied conditions. In this work, rhodium serves as an ideal model catalyst for understanding fundamental reaction parameters and how to tune them. However, rhodium is inconveniently one of the most rare and expensive earth metals available with little viability as an industrial catalyst. In Chapter 7, we fabricate platinum/rhodium alloys in an effort to enhance phenol ECH rates with very low rhodium compositions. We employ an isotopic labeling method developed by Zhang et al. (2025), using the H/D ratio of phenol ECH products to determine the dominant ECH mechanism, LH or ER. as a function of rhodium composition. Future work to further elucidate why an ER mechanism is enabled on rhodium but not platinum, along with structural sensitivity analysis, will inform the design of cheaper yet highly efficient catalysts and electrolytes for economically viable ECH systems.

Insights into the electrochemical hydrogenation of phenol

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Insights into the electrochemical hydrogenation of phenol

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