Electrochemical Hydrogenation of Toluene to Methylcyclohexane over Rh-Modified Pt Nanoparticles: Effect of Rh Coverage and Electrochemical Surface Area
Authors: Imada Toyoki , Chiku Masanobu , Higuchi Eiji , Inoue Hiroshi 
Published: December 2020

Introduction

Organic hydrides are known to be hydrogen carriers to achieve large-scale storage/transportation system of hydrogen. Especially, a couple of toluene (TL) and its hydrogenation product, methylcyclohexane (MCH), is promising because they are liquids at ambient temperature and pressure to use existing infrastructure and MCH has high hydrogen storage density (6.1 wt.%). In general, MCH is regenerated by the hydrogenation of TL with molecular hydrogen on appropriate catalysts, and another chemical or electrochemical process is required for the production of molecular hydrogen. However, electrochemical hydrogenation of TL to MCH is one-pot process, in which the electrochemical generation of atomic hydrogen by one-electron reduction of H+ and the hydrogenation of TL with atomic hydrogen consecutively proceed on the same catalyst surface. The standard potential of the electrochemical TL hydrogenation process is more positive than that of hydrogen evolution reaction, indicating the former is more energy-saving.1, 2

Pt nanoparticles loaded on carbon (Pt/C) are active for electrochemical hydrogenation of TL to MCH.1, 2 Recently, it has been found in our group that Rh-modified Pt/C (Rh/Pt/C) catalysts exhibited higher current for electrochemical hydrogenation of TL than Pt/C. In order to evaluate the specific activity and investigate the relationship between electrochemical hydrogenation activity and Rh loading or Rh coverage (θ
Rh), it is necessary to separately evaluate the electrochemical surface areas (ECSAs) of Pt and Rh, but it is difficult because the potential region of hydrogen adsorption/desorption for Pt and Rh overlaps. In this study, we have succeeded in separately evaluating the ECSAs of Pt and Rh for Rh/Pt/C catalysts with different quantities of deposited Rh by using the difference in CO stripping potential between Pt and Rh, and estimated the Rh coverage (θ
Rh) to discuss the relationship with catalytic activity.

Experimental

The commercial Pt/C (Tanaka Kikinzoku Kogyo, TEC10E50E) powder was dispersed in ultrapure water. Different concentrations (1.2, 2.4, 3.6 and 4.8 mM) of rhodium chloride aqueous solutions were added into the dispersion, followed by hydrogen bubbling for 1 h at 30 °C. The final black powder is named Rh0.19/Pt/C, Rh0.38/Pt/C, Rh0.56/Pt/C and Rh0.76/Pt/C hereafter. For the preparation of a catalyst electrode, each catalyst ink was cast on a GC substrate (diameter: 5 mm) and dried overnight to make a working electrode. The counter electrode, reference electrode and electrolyte were a platinized Pt electrode, reversible hydrogen electrode (RHE) and 0.5 M H2SO4 aqueous solution, respectively. For the ECSA evaluation, CO was saturated into the electrolyte solution at 0.30 V. After removing dissolved CO by Ar-bubbling, the electrode potential was stepped from 0.30 to 0.75 V, and kept at this potential for 15 s to preferentially desorb CO adsorbed on the Rh surface. After that, cyclic voltammogram was measured between 0.05 and 1.2 V.

Results and Discussion

The θ
Rh increased with the Rh/Pt mole ratio for Rh/Pt/C catalysts, and stagnated as the Rh/Pt mole ratio excessed 0.38 (Fig. 1). However, the fraction of ECSA of Rh (ECSARh) to total ECSA of Pt and Rh (ECSARh/Pt/C) continued to increase with the Rh/Pt mole ratio, suggesting that Rh was preferentially accumulated on deposited Rh as θ
Rh reached ca. 0.2. The Tafel slope of the electrochemical hydrogenation of TL on the Rh0.19/Pt/C, Rh0.38/Pt/C, Rh0.56/Pt/C and Rh0.76/Pt/C catalysts was ca. 34, 34, 33 and 32 mV dec-1, respectively. Each Tafel slope was close to that for the Pt/C catalyst, suggesting that electrochemical TL hydrogenation on Rh/Pt/C follows the same reaction mechanism and rate-determining step, which is the addition reaction of adsorbed TL with atomic hydrogen. The change in hydrogenation current per ECSAPt (j
Pt) at 0 V vs. RHE with the Rh/Pt mole ratio showed the similar tendency to that in θ
Rh (Fig. 1), suggesting that the electrochemical hydrogenation of TL preferentially occurs at the interface between Pt and Rh.

Galvanostatic electrolysis of 30 vol.% TL with the Pt/C and Rh/Pt/C catalysts at −200 mA cm−2 was carried out with polymer electrolyte membrane electrolysis cell. It was found by GC/MASS analysis that sole reaction product in the electrochemical hydrogenation of TL was MCH. The faradaic efficiency for MCH production and conversion of TL to MCH were also increased with the increase of θ
Rh, and the side reaction, hydrogen generation, was suppressed. These results clearly indicate that the Rh/Pt/C catalysts are superior to Pt/C.

References

1 Y. Bao, T. W. Nappom, K. Nagasawa, S. Mitsushima, Electrocatalysis, 10, 184 (2019).

2 S. Mitsushima, Y. Takakuwa, K. Nagasawa, Y. Sawaguchi, Y. Kohno, K. Matsuzawa, Z. Awaludin, A. Kato, Y. Nishiko, Electrocatalysis, 7, 127 (2016).

Figure 1