(72h) Aqueous-Phase Phenol Hydrogenation over Platinum and Rhodium: Electrocatalysis Versus Thermal Catalysis
On Pt, TH activity decreases from 60 to 80 °C, when the hydrogen pressure (PH2) 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 PH2 results in similar coverages as -0.45 V vs. RHE, and 1 bar PH2 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
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