(72h) Aqueous-Phase Phenol Hydrogenation over Platinum and Rhodium: Electrocatalysis Versus Thermal Catalysis

A major future need is providing renewable energy without emitting greenhouse gases. Catalysis can play an integral role in the chemical processes required to produce transportation fuels and store energy. One method to make renewable fuels is to convert biomass to hydrocarbon fuels. Bio-oil (modeled here using phenol) is an especially promising feedstock, which can be hydrogenated and then deoxygenated to yield fuel grade hydrocarbons.¹ Although hydrogen generally comes from steam methane reforming, which emits carbon dioxide, renewable electricity can be used to drive carbon-neutral (i) electrochemical hydrogen production which is then used for thermal hydrogenation (TH), or (ii) direct electrochemical hydrogenation (ECH) of the organic substrate. Both of these routes require high rates for industrial application, and have the advantage that greenhouse gases are not emitted, and renewable energy can be stored as transportation fuels. ECH has the further advantage that there is no need to store hydrogen as a gas (other than as a side-product of ECH). TH and ECH of phenol on rhodium and platinum have similar activation barriers due to a shared rate determining step of adsorbed H adatoms reacting with adsorbed phenol.² The conditions (hydrogen pressure or applied potential) influence the reaction rate, but how they affect the rate and how the conditions of TH and ECH compare is not well-understood. Additionally, the high rates needed for industrial application are not currently achievable due to catalyst deactivation at high temperatures.² Similar behavior for gas-phase benzene TH is due to the poisoning of surface sites by dehydrogenated benzene.³ We show here that a similar effect exists for TH of phenol, and that ECH and TH rates are controlled by the surface coverages of the reactants, which are in turn controlled by either applied potential or hydrogen pressure. We also discuss differences in ECH and TH and propose solutions to improve reaction rates.

On Pt, TH activity decreases from 60 to 80 °C, when the hydrogen pressure (P_H2) is below 1 bar. At 10 bar, the rate of reaction is greatly increased and the reaction takes on Arrhenius behavior from 60 to 80 °C. Based on the third-order in hydrogen, the decreased rate from 60 to 80 °C must be from poisoning species derived from dehydrogenated phenol, which are removed when the hydrogen pressure is sufficiently high (between 1.5 to 6 bar in this work) to rehydrogenate them. At pressures sufficiently high to remove the dehydrogenated phenol, the order in hydrogen is first-order, indicating a dependence on the hydrogen coverage. At even greater high pressures the hydrogen order decreases to zero, indicating further changes in the hydrogen coverage do not greatly influence the rate. The applied potential has a similar effect on the reaction rate for ECH, where initial increases in potential (from 0.05 to -0.05 V vs. RHE) greatly increase the rate of reaction, but from -0.05 to -0.45 V vs. RHE the rate increases slowly, after which it plateaus. The similarities in the maximum rate achieved for TH and ECH at a given temperature indicate the rate is controlled by the coverage and surface reaction of adsorbed phenol and an adsorbed hydrogen adatom. By comparing the rates of ECH and TH we are able to see conditions where the coverages must be similar, and thus can empirically relate applied potentials to pressure. For example, under this study it appears that 6 bar P_H2 results in similar coverages as -0.45 V vs. RHE, and 1 bar P_H2 is similar to -0.3 V vs. RHE.

We also present in situ characterization using X-Ray Absorption Spectroscopy (XAFS) on Pt nanoparticles in the presence and absence of phenol under applied potential. Through analysis of the near-edge (XANES) portion of XAFS we detect H-adsorption on Pt nanoparticles at potentials known to fully saturate the Pt surface with hydrogen. We see that the addition of phenol to the electrolyte solution lowers the H-adsorption signal, indicating phenol displaces hydrogen, lowering the hydrogen coverage. As higher applied potentials are applied, in the presence of phenol, the surface coverage of hydrogen increases. This supports our hypothesis from the reaction data that we see increased ECH rates at higher applied potentials because of an increase in hydrogen coverage, which reacts with adsorbed phenol. From the extended X-ray absorption fine structure (EXAFS) we see that the introduction of phenol also results in interaction between Pt and carbon atoms, which we believe to be adsorbed phenol. The Pt-C distance matches calculations for phenol adsorption on platinum,⁴ and cannot be attributed to oxygen due to the absence of surface oxide from our XANES results. The strength of the Pt-C interaction is inversely related to the H-adsorption signal from XANES, supporting our conclusion that phenol displaces hydrogen on the surface, and that the applied potential can control the H-adsorption, and indirectly control the phenol coverage. Because the H-adsorption appears to affect phenol adsorption, we believe that phenol and hydrogen compete for sites on platinum, which is supported by electrochemical results that show phenol inhibits the H adsorption from protons at low applied potentials.²

References

(1) Zhao, C.; He, J.; Lemonidou, A. A.; Li, X.; Lercher, J. A. J. Catal. 2011, 280 (1), 8.

(2) Song, Y.; Gutiérrez, O. Y.; Herranz, J.; Lercher, J. A. Appl. Catal. B Environ. 2016, 182, 236.

(3) Chou, P.; Vannice, M. A. J. Catal. 1987, 107, 140.

(4) Yoon, Y.; Rousseau, R.; Weber, R. S.; Mei, D.; Lercher, J. A. J. Am. Chem. Soc. 2014, 136 (29), 10287.