Direct Electrochemical Hydrogenation of Naphthalene to Decalin at Pt and Pt-Ru Alloy Electrodes in Liquid Phase
Authors: Wakisaka Mitsuru 
Published: December 2020

Decalin has been recognized as one of organic chemical hydrides, which can be obtained by reversible hydrogenation reaction of naphthalene in the presence of a catalyst. However, because naphthalene is solid under the ambient conditions, the naphthalene/decalin system has been hardly turned to practical use for the hydrogen storage so far. Recently, we have, for the first time, demonstrated a direct electrochemical hydrogenation of toluene to methylcylcohexane at Pt electrodes in a microemulsion solution at room temperature [1]. In a microemulsion system, a hydrophobic organic compound and water (or hydrated ion, e.g. proton) can be supplied simultaneously to an electrode surface, and can react together with electron transfers. Thus, any solid organic compounds that form a microemulsion solution with water can react electrochemically with proton at lower temperatures than their melting or boiling points. In the present study, direct electrochemical hydrogenation of naphthalene was investigated by employing the microemulsion technique with Pt and Pt-Ru electrocatalysts sputtered on a Ti sheet. Author has, for the first time, achieved a high Faraday efficiency and selectivity for the naphthalene/decalin conversion under a galvanostatic condition at 60°C.
Thin Pt-Ru alloy or pure Pt were prepared on a Ti sheet by RF-sputtering a Pt target with or without Ru target. The composition of the alloys was controlling by varying exposed target area ratio, and analyzed by X-ray photoelectron spectroscopy. Microemulsion electrolytes were prepared from naphthalene, a sulfuric acid solution and two surfactants by mixing and sonicating. All the electrochemical measurements were performed with an H type cell at 60°C. A platinum black and a reversible hydrogen electrode were employed as the counter electrode and reference electrode, respectively. The working electrode was immersed into a microemulsion solution while the counter electrode into a sulfuric acid solution. After the electrolysis, products were analyzed and quantified by gas chromatography (GC) with a flame ionization detector for calculating the Faraday efficiency and selectivity. Toluene was employed as internal standard for quantitative GC analysis.
Fig. 1 shows cyclic voltammograms (CVs) at a Pt/Ti electrode taken in 1 M sulfuric acid (black) and a naphthalene-containing microemulsion electrolyte (red) at sweep rate of 1 mV/s. In the sulfuric acid solution, the Pt/Ti electrode exhibited a characteristic reversible hydrogen adsorption wave below 0.3 V. In contrast, the hydrogen adsorption feature completely disappeared from the CV in the microemulsion electrolyte presumably due to an adsorption of naphthalene on the Pt surface. However, reduction current appeared below ca. 0.2 V and steeply increased below 0.05 V. This can be explained by a partial reduction and desorption of the adsorbed naphthalene molecules[2] and subsequent bulk hydrogenation reaction on the vacant sites. It should be pointed out that the steep reduction current was observed at more positive potential than the hydrogen evolution reaction, suggesting a Langmuir-Hinshelwood type reaction with underpotential-deposition (UPD) hydrogen on the electrode surface.
The GC analysis after a constant-current electrolysis at 2 mA cm-2 for 150 min revealed the formation of cis- and trans- decalin via10-electron transfer as well as tetralin via 4-electron transfer. In comparison between the isomers, cis-decalin was the dominant species presumably due to a flat orientation of the adsorbed naphthalene molecule [2], which reacted with UPD hydrogens via the Langmuir-Hinshelwood type mechanism. Surprisingly, the Fraday efficiency for the naphthalene/decalin conversion and selectivity for dacalin/tetralin was much improved at a Pt-Ru/Ti electrode, up to 48% and 93%, respectively. These improvements might be ascribed to change in the adsorption energy of naphthalene on the Pt site due to alloying with Ru, or change in the adsorption site to Ru.
Reference
[1] M. Wakisaka and M. Kunitake, Electrochem. Commun. 64 (2016) 5.
[2] S.-L. Yau, Y-G. Kim, and K. Itaya, J. Phys. Chem. B 101 (1997) 3547.

Figure 1