Selectivity of Diphenylacetylene Semi-Hydrogenation on Pt1Pd99/C Using PEM Electrolyzer
Authors: Nakamura Hiroki , Harada Juri , Shida Naoki , Nagasawa Kensaku , Kuroda Yoshiyuki , Atobe Mahito , Mitsushima Shigenori 
Published: September 2023

Introduction

For carbon neutralization, organic synthesis should be electrified with renewable electricity besides energy synthesis. The Lindlar catalyst has been a common means for stereoselective Z-alkene synthesis from alkynes, which need hydrogen as a reactant and lead as an additive on the catalyst and has substrate limitation. By contrast, a proton exchange membrane (PEM) reactor is one of the alternative ways of Z-alkene formation, enabling the synthesis under mild conditions without lead use and substrate restriction, using renewable electricity and water instead of hydrogen.
The previous research figured Pt1Pd99/C (at%) catalyst had the highest selectivity among various compositions of Pt-Pd catalysts for semi-hydrogenation of diphenylacetylene (DPA) to cis-stilbene. Pd sites selectively adsorb the DPA, while even a small amount of Pt sites quickly reduces protons to Had, which hydrogenate the DPA. The Pt1Pd99 catalyst indicated the best synergy effect of Pt and Pd.1,2) For an optimal PEM electrolyzer on a larger scale, its flow field structure and dimension must be decided with the selected electrocatalyst property. Therefore, a better understanding of the electrocatalyst activity is essential.
In this study, we have investigated the relationship among catalyst loading, reactant flow rate, overall current density, and selectivity to understand the nature of the Pt1Pd99 catalyst.

Experimental

DSE® (De Nora Permelec Ltd) was used for the oxygen evolution as the anode and Nafion® 117 (DuPont) as the PEM, respectively. Carbon paper (39BB, SGL Carbon), loaded with 0.2–1.6 mg cm-2-precious metal of Pt1Pd99 (ISHIFUKU Metal Industry Co., Ltd.) and cut into 3.3 cm × 3.5 cm, was employed as the cathode. The cathode was hot-pressed on the PEM at 120 ℃, 3 MPa, for 3 min to make a membrane cathode assembly. In the anode compartments, 1 M (= mol dm-3) sulfuric acid circulated, and 1 M cyclohexane solution of DPA in the cathode compartments.
As electrochemical measurements, electrochemical impedance spectroscopy at 100 kHz–0.1 Hz with an amplitude of 10 mV, chronoamperometry at 1.3–2.0 V for catalyst loading dependency experiments, chronopotentiometry at 9–36 mA cm-2 for flow rate dependency were conducted. The concentrations of cis-Stilbene and dibenzyl in electrolytes were quantified with high-performance liquid chromatography (HPLC) after collection during the last 2 min out of 5 min of electrolysis at 60 ℃. Each partial current was derived by the multiplications of each concentration, the stoichiometric coefficient of electrons (2 for cis-stilbene and 4 for dibenzyl), the Faraday constant, and the flow rate.

Results and Discussion

Figure 1 shows anode and cathode potentials as functions of current density. The anode potential was stable around 1.6 V vs. RHE, while the cathode potential appeared to get higher with more catalyst loading, considering the variation of the anode potential.
Figures 2 and 3 show the current efficiencies of cis-stilbene and dibenzyl as functions of cathode potential. The current efficiency of cis-stilbene decreased with the cathode potential increase, while that of dibenzyl increased and peaked at approximately -0.2 V vs. RHE, where cis-stilbene current efficiency steeply dropped.
Figures 4 and 5 show both current efficiencies at various cathode potentials as functions of the catalyst loading. The current efficiency of cis-stilbene decreased with the catalyst loading increase, while that of dibenzyl increased. Both absolute values of the slopes of Figs. 4 and 5 are the highest at -0.2 V vs. RHE, where over-hydrogenation to dibenzyl overwhelmed cis-stilbene formation. Since at a more negative potential, reducing current was almost consumed by hydrogen evolution, the effect of the catalyst layer thickness on the selectivity was maximum here.
The current efficiencies of cis-stilbene and dibenzyl and the selectivity at various flow rates are shown in Figure 6. As the flow rate rose, the current efficiency and selectivity of cis-stilbene increased.
With a thin catalyst layer, the substrate quickly moved away from the reaction field, and the same happened at a high flow rate. Therefore, produced cis-stilbene is less over-hydrogenated, and the selectivity increased.
According to these results, since the reaction of cis-stilbene formation is quick, speedy product elimination would be the key to further selectivity improvement.

Acknowledgment

This work was financially supported by CREST (JST Grant 18070940). The authors are grateful to ISHIFUKU Metal Industry Co., Ltd. for providing the cathode catalysts.

References

1) S, Nogami, K. Nagasawa, A. Fukazawa, K. Tanaka, S. Mitsushima, M. Atobe, J. Electrochem. Soc.,
167, 155506 (2020).
2) S. Nogami, N. Shida, S. Iguchi, K. Nagasawa, H. Inoue, I. Yamanaka, S. Mitsushima, M. Atobe, ACS Catal.,
12, 5430 (2022).

Figure 1