Structural Effects on Reaction Rate and Selectivity for Direct Electrochemical Hydrogenation of Aromatic Hydrocarbons at Pt Electrocatalysts
Authors: Wakisaka Mitsuru , Kunitake Masashi 
Published: February 2020

Organic chemical hydrides have been attracting considerable attention as hydrogen carriers.1-3 From the viewpoints of the total energy conversion efficiency, direct electrochemical hydrogenation of aromatic hydrocarbons to obtain hydrides is preferable to the conventional multistep reaction process consisting of water electrolysis and gas-solid catalytic hydrogenation with the produced hydrogen. So far, several groups have reported the direct electrochemical hydrogenation of aromatic hydrocarbons at Hg, Ni or Pt electrocatalysts in quaternary ammonium ion supporting electrolytes,4-6 at Raney Ni electrocatalysts in ethylene glycol solutions,7 at non-catalyzed carbon electrodes in bi-continuous microemulsions,8,9 or using membrane electrode assemblies (MEAs) consisting of Pt or PtRu nanoparticle electrocatalysts and proton or anion exchange membranes.10-12

Very recently, we have demonstrated the direct electrochemical hydrogenation of toluene at Pt electrodes in a microemulsion electrolyte solution.13 We have achieved a Faradaic efficiency of 80% for the toluene/methylcyclohexane conversion at a Pt black electrode with a roughness factor (RF) of 360 even under galvanostatic conditions and in a one-compartment cell filled with the Winsor III type microemulsion, as shown in Figure 1.
However, the reaction rate and selectivity of the toluene reduction were found to depend strongly on the surface structure of the Pt electrodes in the microemulsion, as shown by steady-state polarization curves at the Pt black electrode and a mirror-finished Pt electrode (RF = 2) in Figure 2. Interestingly, cyclohexane rather than methylcyclohexane was found as the predominant product at the mirror-finished Pt electrode. The formation of cyclohexane can be ascribed to the further reduction (hydrogenolysis) of the produced methylcyclohexane, yielding methane. Similar phenomenon, splitting of the methyl group, has been previously reported by Heitbaum and co-workers in their study of toluene reduction at a Pt sputtered on a Teflon membrane (RF = 8) in a 0.5 M H2SO4 solution using differential electrochemical mass spectroscopy (DEMS).14

In the meeting, we will present the latest results of direct electrochemical hydrogenation of other aromatic hydrocarbons such as benzene, xylene and naphthalene, and will discuss the structural effects on the reaction rate and selectivity.
This research was supported by Japan Science and Technology Agency, PRESTO.

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Figure 1