Electrolyte Effects in Ethanol Electro-Reforming at Anode with Acetaldehyde As Target Product
Authors: Cheng Kai Hung , Kawase Motoaki 
Published: December 2024

Introduction

The development of the electrolysis hydrogen production process has been vigorously promoted due to the widespread use of hydrogen energy. However, the oxygen produced as a byproduct during this process has not been effectively utilized. Therefore, this research focuses on the production of chemical compounds through organic electrosynthesis in place of oxygen generation as the anode reaction, examining selectivity and reaction rates. Specifically, this study targets the synthesis of acetaldehyde from the oxidation of ethanol. Currently, acetaldehyde is produced from ethylene through the Wacker process, but the synthesis of acetaldehyde from ethanol, which can be derived from biomass, is gaining attention due to concerns over the depletion of fossil resources. The energy losses in electrochemical processes are primarily due to Ohmic losses, requiring high electrolyte concentrations to achieve sufficient ion conductivity. Moreover, the adsorption of electrolyte anions on the catalyst surface can decrease catalytic activity. This study examines the relationship between the pH, anion adsorption, and reaction rate in the electrolytic oxidation reaction of ethanol in acid electrolytes.

Experimental

A schematic of the experimental setup is shown in Figs. 1 and 2. Platinum electrodes (De Nora Permelec Ltd, JL-510 coating DSE) were placed on silicone gaskets with flow channels, and the gaskets were sandwiched between silicone gaskets, PTFE plates, and stainless-steel endplates. A cation exchange membrane (Chemours, Nafion® NR-212) was placed in the center to separate the reactions at the two electrodes. An electrochemical measurement system (Hokuto Denko, HZ-pro) was used to measure current and solution resistance. In the experiments, sulfuric acid or perchloric acid was used as the electrolyte; solutions containing either ethanol or acetaldehyde were supplied to the anode, while plain sulfuric or perchloric acid solutions were supplied to the cathode. The reaction was performed at room temperature and pressure. High-performance liquid chromatography (Hitachi High-Tech Science, Chromaster®, ion exclusion column GL-C610-S, Hitachi High-Tech Science 5450 RI Detector) was used for product analysis.

Results and Discussion

The ethanol electro-oxidation reaction is a consecutive reaction in which acetaldehyde is formed and further oxidized to acetic acid. In addition, the parallel reaction of ethanol oxidizing into carbon dioxide also occurs simultaneously. The reaction equations are expressed by Eqs. (1)–(3), and the uncovered fraction of the catalyst active site is expressed as θ
v in Table 1. Assuming the reaction mechanism involving Eqs. (1)–(4), the overall reaction rate equations r
s1,
r
s2 and r
s3 are expressed in Eqs. (5)–(11).
Due to the oxidation of ethanol being a complex reaction, to better understand the entire reaction, the oxidation of acetaldehyde to acetic acid was first investigated using acetaldehyde as the reactant. The relationship between electrolyte concentrations and r
s2 at potential differences of 1.2 V are shown in Figs. 3(a) and 3(b). The results shown in Fig. 3(a) indicate that when the concentration of perchloric acid is low, increasing the concentration of the electrolyte significantly enhances the reaction rate, which is believed to be due to improved proton transport. In contrast, as illustrated in Fig. 3(b), the presence of sulfate ions may lead to decrease in reaction rate due to their adsorption.
In the ethanol electro-oxidation experiment, the impact of electrolyte concentration on reaction rate (r
s1) at 0.6 V and 1.2 V is depicted in Figs. 4(a) and 4(b). At 0.6 V, anion adsorption is minimal, leading to lower OH•σ production from water electrolysis, and increase in r
s1 due to enhanced proton transport. Conversely, at 1.2 V, stronger anion adsorption and greater OH•σ formation occur, reducing r
s1 despite the positive effect of better proton transport. Fig. 5 illustrates the relationship between ethanol conversion and acetaldehyde selectivity for the two electrolytes. The findings show that acetaldehyde selectivity is higher when using perchloric acid compared to sulfuric acid as the electrolyte. The possible reason is that acetaldehyde and sulfate ions undergo co-adsorption, making acetaldehyde more likely to oxidize into acetic acid, resulting in a lower selectivity of acetaldehyde in sulfuric acid solution than in perchloric acid solution.

Conclusions

In the electrolytic reaction of ethanol or acetaldehyde, increasing the sulfuric acid concentration decreases the reaction rate. This may be due to the adsorption of sulfate ions occupying the active site of the catalyst. Higher acetaldehyde selectivity is obtained when perchloric acid is used as the electrolyte. The maximum reaction rate was obtained under 0.1 M perchloric acid, influenced by high proton transport and low anion adsorption. The result of this study illustrates that the selection of an appropriate electrolyte and its concentration is also important in optimizing the electrolytic oxidation reaction of ethanol to produce acetaldehyde.

Figure 1