Background
The dehydrogenation of biomass ethanol is of great significance for the synthesis of fuels and value-added chemicals. However, due to the limitation of thermodynamic equilibrium, ethanol thermal catalytic dehydrogenation generally requires a high temperature above 260°C to achieve high ethanol conversion efficiency. Electrocatalytic systems with ethanol oxidation coupled with hydrogen evolution are considered a green technology for the production of hydrogen and value-added chemical products, but significant power consumption (>0.8 V) is still required in acidic media to obtain considerable current density. Therefore, it is imperative to develop efficient pathways for the efficient conversion of ethanol to hydrogen and acetaldehyde under mild conditions. To this end, Wang Shuangyin's team of Hunan University proposed a new thermoelectric coupling catalysis system for the first time to achieve efficient green conversion of ethanol, and the relevant results were published in the Proceedings of the American Academy of Sciences PNAS.
Highlights of this article
Highlight 1. For the first time, by using the dual functions of thermocatalysis and electrocatalysis of Ru/C catalysts, a thermal-electrocoupled catalytic system was established in a medium-temperature (120-200°C) proton exchange membrane electrolyzer.
Highlight 2. Specifically, the electrochemical hydrogen pump (EHP) is introduced into the ethanol thermal catalytic dehydrogenation reaction, and the application of low voltage can reduce the partial pressure of hydrogen in the thermocatalytic region, so that the equilibrium of the ethanol dehydrogenation reaction can be shifted in the positive direction, thereby increasing the yield of hydrogen and acetaldehyde.
Highlight 3. Since the anode is a coupling reaction of ethanol thermally catalyzed dehydrogenation and hydroxidation rather than ethanol electrooxidation, the system can achieve a current density of 100 mA cm-2 with only a voltage input of 0.13 V.
Graphic analysis
Figure 1: Specific reaction paths for the efficient conversion of ethanol to acetaldehyde and hydrogen.
In order to solve the problems of high energy consumption and low efficiency of ethanol dehydrogenation, we couple hydrogen electrooxidation and ethanol thermal catalytic dehydrogenation reaction on bifunctional Ru/C catalysts to improve the overall reaction efficiency. Applying a lower voltage can quickly oxidize and remove hydrogen products from the dehydrogenation reaction zone, and the consumption of hydrogen effectively reduces its partial pressure at the anode, shifting the equilibrium of the dehydrogenation reaction in the direction of acetaldehyde and hydrogen generation. H+ is then reduced to high-purity H2 at the cathode through a proton exchange membrane. The anode hydroxidation and cathodic hydrogen evolution processes in this ethanol electrolyzer act as an electrochemical hydrogen pump (EHP), significantly improving the production efficiency of hydrogen and acetaldehyde.
Figure 2. Ethanol thermal catalytic dehydrogenation performance of Ru/C catalyst.
The results of gas phase product detection, density functional theory (DFT) calculation and programmed heating surface reaction (TPSR) show that Ru/C has higher ethanol dehydrogenation catalytic activity than Pt/C catalyst, and obvious dehydrogenation products can be detected above 120°C.
Figure 3. Electrochemical performance of ethanol thermal-electrical coupling system for the production of hydrogen and acetaldehyde.
Since Ru/C has the bifunctional catalytic activity of ethanol thermal dehydrogenation and hydrogen electrooxidation, when Ru/C is used as an anode electrode, higher current densities can be obtained in the low voltage region. It shows that the thermal-electrical coupling system can significantly reduce the input voltage to reach the target current density and greatly reduce the power consumption of hydrogen production.
Figure 4. Yield and selectivity calculation of reaction products.
When Ru/C is used as the anode electrode, the yield of hydrogen and acetaldehyde increases with the increase of potential, and the selectivity of acetaldehyde also increases. When the potential increased from 0.3 V to 0.6 V, the C1 product showed a downward trend, indicating that there was basically no direct ethanol electrooxidation of C1 path on the Ru/C electrode. Combined with the relevant solution electrochemical tests, the electrooxidation behavior of ethanol on the Ru/C electrode was excluded. Therefore, the increase of hydrogen and acetaldehyde is due to the anode electrochemical hydrogen-oxygen reaction reducing the partial pressure of hydrogen, and finally promoting the equilibrium of ethanol dehydrogenation in the positive direction. At a potential of 0.3 V, the hydrogen yield of 1020 mmol G-1 H-1 and the acetaldehyde yield of 1185 Mmol G-1 H-1 were achieved, which was 3 times higher than the thermal catalytic dehydrogenation performance of ethanol alone and better than the performance of ethanol electrolysis.
Summary and outlook
Ethanol dehydrogenation is an important way to develop green energy and produce high-value chemicals. In this study, a thermal-electrical coupling catalytic system capable of performing at moderate temperatures and ultra-low starting voltages (0.06 V) was established. The introduced EHP promotes the process of ethanol dehydrogenation by changing the thermodynamic equilibrium of the thermocatalytic reaction and significantly improves the reaction efficiency. The results showed that at a low voltage of 0.3 V, the yield of hydrogen and acetaldehyde increased by 4 times, and the selectivity of acetaldehyde increased to 97.9%. This work provides an attractive way to drive chemical equilibrium to improve hydrogen and acetaldehyde production efficiency.
Original link: https://www.pnas.org/doi/10.1073/pnas.2300625120