Using renewable electricity to drive electrochemical reduction reactions is a promising way to sustainably and selectively transform chemicals to desired products (e.g., to make liquid fuels or destroy harmful waste). Active and selective electrocatalysts are necessary for the success of these processes. Here we discuss the hydrogenation of bio-oil for production of transportation fuels, using phenol and benzaldehyde as model reactants over carbon-supported Pt, Pd and Rh. By comparing to rates of aqueous thermal hydrogenation with H2, we show that these electrochemical reactions involve the reduction of aqueous protons to make adsorbed hydrogen atoms (Had), which then add to the adsorbed organic reactant. To examine the adsorption of reactants and their intermediates, we conduct cyclic voltammetry on Pt surfaces, where the underpotential deposition of adsorbed hydrogen atoms (Had) is quantified. The saturation amount of Had decreases with the coverage of adsorbed organics, providing an indirect measure of the fraction of the surface covered by the adsorbed organic at equilibrium versus its aqueous-phase concentration. Such adsorption isotherms provide adsorption equilibrium constants versus coverage, and indirect estimates of the adsorption energy. Comparing to calorimetric adsorption energies measured on Pt(111) in vacuum allows us to assess the role water plays in modifying the adsorption energies. For phenol hydrogenation on Pt/C, the thermal hyrdrogenation rates over a 200-fold change in phenol concentration are well fit by a mechanism where Had adds to adsorbed phenol in the rate-determining step, with an adsorption equilibrium constant for phenol almost identical to that determined independently as described above. We discuss how the differences in binding of reactants can explain some of the observed differences in the kinetics of these hydrogenation reactions, and how the effect of pH on rates can be understood through a BEP relationship to the energy of Had with slope of ~1.

Aqueous-phase hydrogenation of phenol and benzaldehyde on carbon-supported Pt, Pd and Rh

2019 Spring National Meeting

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