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 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 hydrogen comes 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 utilization of biomass feedstocks and could be powered directly by renewable energy, further minimizing its environmental impact.

Aromatic hydrocarbons are of interest as model compounds for lignocellulosic bio-oil [1]. The ECH of aromatic hydrocarbons proceeds with high faradaic efficiency (FE) at low overpotentials, however, the ECH turnover rate remains low at these potentials. There is no simple fix to this because any increase in overpotential to increase ECH rate will reduce the reaction FE due to competition with the hydrogen evolution reaction (HER). In this work we address the need for a more fundamental understanding of ECH systems to promote high turnover rates at low overpotentials. In phenol TCH, it is well known that the hydrogenation is limited by the formation/addition of surface adsorbed hydrogen. The current ECH literature draws on these TCH studies and assumes that phenol ECH is also limited by surface adsorbed hydrogen [2]. Thus, most ECH work is done is acidic media, where the kinetics of surface adsorbed hydrogen formation are fastest on platinum [3]. HER, however, is also faster on platinum in acidic media [4], making it more difficult to achieve high FE.

Because of this limited scope of investigation, the impact of electrolyte chemistry, i.e. pH, on ECH kinetics has only been sparingly investigated. In this work, we investigate a new pathway for enhanced rates at low overpotentials, driven by the acid-base chemistry of reactants and using electrolyte pH to promote dissociative adsorption. By tailoring the electrolyte pH based on the pKa of the reactant molecule, we can lower the barrier associated with the first hydrogenation step, improving overall reaction kinetics. We demonstrate this effect with phenol ECH at various pH electrolyte on low-index single crystal electrodes using rotating disk electrode voltammetry and chrono-amperometry with product analysis. We report on the structural sensitivity, influence of pH and resulting electric field effects, and present new mechanistic insights for phenol ECH. With these insights we propose methods to promote higher ECH rates at lower overpotentials.

References:

[1] J. Yan et al., "Characterizing Variability in Lignocellulosic Biomass: A Review," ACS Sustainable Chemistry & Engineering, vol. 8, no. 22, pp. 8059-8085, 2020, doi: 10.1021/acssuschemeng.9b06263.

[2] N. Singh et al., "Aqueous phase catalytic and electrocatalytic hydrogenation of phenol and benzaldehyde over platinum group metals," Journal of Catalysis, vol. 382, pp. 372-384, 2020, doi: 10.1016/j.jcat.2019.12.034.

[3] S. Intikhab, J. D. Snyder, and M. H. Tang, "Adsorbed Hydroxide Does Not Participate in the Volmer Step of Alkaline Hydrogen Electrocatalysis," ACS Catalysis, vol. 7, no. 12, pp. 8314-8319, 2017, doi: 10.1021/acscatal.7b02787.

[4] I. Ledezma-Yanez, W. D. Z. Wallace, P. Sebastián-Pascual, V. Climent, J. M. Feliu, and M. T. M. Koper, "Interfacial water reorganization as a pH-dependent descriptor of the hydrogen evolution rate on platinum electrodes," Nature Energy, vol. 2, no. 4, 2017, doi: 10.1038/nenergy.2017.31.