Faraday efficiency is close to 200% organic substrate electrocatalytic bilateral hydrogenation

Electrocatalytic hydrogenation provides a gentler method than traditional thermocatalytic hydrogenation reactions. However, its widespread application is subject to some inherent limitations, including the solubility of the substrate and the subsequent cumbersome separation of the product from the electrolyte. By physically separating the generation and utilization of active hydrogen atoms, electrocatalytic hydrogenation based on palladium (Pd) membrane reactors can overcome the above limitations.

Recently, the team of Sun Yujie at the University of Cincinnati in the United States found that the active hydrogen atoms produced by the low-potential oxidation of formaldehyde on the Pd membrane anode can penetrate through the membrane electrode to the chemical hydrogenation pool on the other side. If the Yin and Yang electrodes use Pd membrane electrodes at the same time, and water and formaldehyde are used as hydrogen sources, electrocatalytic bilateral hydrogenation of the same organic substrate can be carried out in the chemical pool on both sides, and the theoretical maximum Faraday efficiency can reach 200%.

On February 20, 2023, the study was published in the journal Nature Catalysis under the title “Electrocatalytic dual hydrogenation of organic substrates with a Faradaic efficiency approaching 200%”. The corresponding author of the paper is Sun Yujie, and the first author is Han Guanqun.

Hydrogenation reactions play an integral role in the chemical industry, with about 25% of chemical processes, such as petroleum refining, chemical raw material manufacturing, and pharmaceutical synthesis, including at least one hydrogenation step. At present, the mainstream thermocatalytic hydrogenation strategy usually uses hydrogen (H2) as the hydrogen source, and is carried out at high temperature and pressure, and its energy-intensive and high-energy consumption characteristics have prompted people to develop lower-cost and more environmentally friendly methods. Among them, electrocatalytic hydrogenation has attracted widespread attention due to its advantages of renewable energy drive, water as hydrogen source, and mild reaction conditions.

Electrocatalytic hydrogenation typically uses adsorbed hydrogen (H*) generated on the cathode to hydrogenate with an unsaturated substrate, while the anode undergoes oxygen evolution or oxidation of organic matter (Figure 1a). This process has the following limitations: (1) high overpotential; (2) The market demand for hydrogenation and oxidation products is often different; (3) The use of proton electrolyte greatly limits the solubility and application scope of hydrogenation substrate; (4) Subsequent processes such as separation of hydrogenation products from the electrolyte will increase additional energy consumption. It has been reported that electrocatalytic cathode hydrogenation based on Pd membrane reactor can solve the problems of substrate solubility and product separation. Due to the unique permeability of the Pd membrane to hydrogen, the H* produced by the Pd membrane as a cathode can penetrate from the electrochemical cell side to the adjacent chemical cell for hydrogenation (Figure 1b). However, due to the excellent hydroxidation (HOR) activity of Pd, there have been no reports of using Pd membrane as an anode to achieve H* penetration and hydrogenation reaction. Pd is an excellent formaldehyde oxidation (FOR) electrocatalyst that can be performed by one- or two-electron transfer. It is speculated that if the H* generated by the anode can penetrate into the chemical reaction cell that is not in contact with the electrolyte, according to an electron transfer process, then HOR in the electrochemical cell can be greatly reduced or avoided, and the hydrogenation reaction in the chemical cell can be realized.

In view of this, Yujie Sun’s team at the University of Cincinnati reported for the first time a hydrogenation strategy for a four-chamber device using Pd membrane electrodes as both cathodes and anodes (Figure 1c), with formaldehyde and water as hydrogen sources for the anode and cathode, respectively, to realize the simultaneous bilateral hydrogenation of the same organic substrate in the chemical cells on both sides of the electrochemical cell.

Figure 1: Schematic diagram of different electrocatalytic hydrogenation units

The highlights of the study are: 1) under alkaline conditions, with the Pd membrane as the anode, the adsorbed hydrogen atoms generated by the oxidation of low-potential formaldehyde can penetrate into the chemical pool on the other side through the Pd membrane electrode for hydrogenation of organic substrates; 2) When the PD film is used as the cathode and anode, one electron can form two adsorbed hydrogen atoms and use it for bilateral hydrogenation, and the maximum theoretical Faraday efficiency is 200%; 3) Compared with traditional unilateral electrocatalytic hydrogenation, this bilateral hydrogenation strategy can save at least 1V voltage input, while doubling the hydrogenation rate and Faraday efficiency; 4) Through hydrogen isotope research, it was confirmed that the hydrogen source at the anode of the Pd membrane is formaldehyde, and the hydrogen source at the cathode of the Pd membrane is water.

Figure 2: Comparison of electrocatalytic hydrogen production using the Pd electrode as the cathode and anode in different devices.

Figure 3: Electrocatalytic hydrogenation of bilateral maleic acid to succinic acid

Figure 4: Electrocatalytic bilateral hydrogenation at different formaldehyde and maleic acid concentrations

Figure 5: Electrocatalytic bilateral hydrogenation of different substrates

Figure 6: Hydrogen source exploration for electrocatalytic bilateral hydrogenation

The study shows that low-potential formaldehyde oxidation on the Pd membrane anode can produce hydrogen atoms that can penetrate through the Pd membrane electrode from the electrochemical cell to the chemical cell on the other side for hydrogenation reaction. When using two Pd membrane electrodes as the cathode and anode, using water and formaldehyde as hydrogen sources, this electrocatalytic bilateral hydrogenation strategy can simultaneously hydrogenate the same organic substrate in the chemical cell outside the electrochemical cell. Compared with traditional single-sided electrocatalytic hydrogenation, this bilateral hydrogenation strategy not only saves at least 1V of voltage input, but also improves hydrogenation rate and Faraday efficiency. It is foreseeable that this bilateral hydrogenation strategy will be applicable to many organic hydrogenation reactions using the modifiability of the Pd membrane electrode. (Source: Science Network)

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