Selective hydrogenation of acetylene to ethylene (HAE) is a crucial process in the chemical industry, traditionally relying on energy-intensive thermocatalytic methods operating at high temperatures (above 200 °C) and pressures (around 5 bar). These conditions lead to high energy consumption and limited control over ethylene selectivity, often resulting in the undesired over-hydrogenation to ethane. The development of a more economical, energy-efficient, and selective HAE process is therefore highly desirable. This paper investigates an electrocatalytic approach to HAE (E-HAE), offering a potential solution to the limitations of thermocatalysis. Electrocatalysis provides the advantages of mild reaction conditions (room temperature and ambient pressure), environmental friendliness (using water as a hydrogen source), and avoidance of external hydrogen supply. The research focuses on optimizing a Cu catalyst to achieve high selectivity and efficiency in the E-HAE process. Specifically, this involves optimizing the catalyst structure to enhance preferential acetylene adsorption and hydrogenation over hydrogen evolution. The study also examines the role of electrode potential in modulating product selectivity, aiming to completely avoid ethane formation.

The existing literature extensively covers thermocatalytic HAE, highlighting the challenges of energy consumption and selectivity. Numerous studies explore different catalysts (e.g., Pd, Ag, Ni, Au) and support materials to improve the process. However, controlling selectivity and minimizing ethane formation remain significant hurdles in thermocatalysis. Recent research is increasingly focusing on electrocatalytic approaches to HAE, demonstrating their potential as a greener alternative. Previous reports have shown some success in E-HAE using various catalysts and electrode materials but often suffer from low Faradaic efficiency due to competitive hydrogen evolution reaction (HER). While studies have explored the use of Cu, Ag, and other metals in electrocatalytic HAE, the achievement of high efficiency and selectivity under mild conditions remains a significant challenge, and this research addresses this gap by significantly improving the performance compared to previously reported catalysts.

The study employed a multi-faceted approach involving catalyst synthesis, characterization, electrochemical testing, and theoretical calculations. Copper microparticles (Cu MPs) were synthesized via a facile solvothermal method using CuSO₄·5H₂O as precursor in ethylene glycol. The morphology and structure of the Cu MPs were characterized using scanning electron microscopy (SEM), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), transmission electron microscopy (TEM), and powder X-ray diffraction (XRD). Electrochemical tests were conducted using Cu MPs loaded on gas diffusion layer (GDL)-coated carbon paper (Cu/GDL-CP) as the working electrode. A three-electrode system was employed, with a Hg/HgO electrode as the reference and a Pt net as the counter electrode. Linear sweep voltammetry (LSV) was used to assess the E-HAE activity, while the reaction products were analyzed using gas chromatography (GC) to determine Faradaic efficiencies. To enhance mass transfer, a microporous GDL was incorporated. In situ characterization techniques, such as X-ray absorption fine structure (XAFS), Raman spectroscopy, and attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, were employed to investigate the electronic properties and reaction intermediates of the catalyst under operating conditions. Density functional theory (DFT) calculations were performed to understand the reaction mechanism and active sites. The DFT calculations examined the adsorption behavior of acetylene and hydrogen on different Cu crystal facets, analyzed the charge transfer between the Cu surface and adsorbed acetylene, and investigated the reaction pathways and energy barriers for both the desired ethylene formation and the undesired side reactions (hydrogen evolution and butadiene formation). The effect of electrode potential on reaction pathways was also assessed.

The study achieved remarkable success in electrocatalytic HAE using a Cu catalyst at room temperature and ambient pressure. A high Faradaic efficiency (FE) of 83.2% for ethylene production was achieved at an applied potential of -0.6 V versus reversible hydrogen electrode (RHE) with an overall current density of 29 mA cm⁻². The geometric current density for ethylene formation reached 26.7 mA cm⁻² at -0.7 V vs. RHE. The use of a microporous GDL significantly improved the reaction efficiency by promoting mass transfer. The electrochemical tests revealed that the product distribution was strongly dependent on the applied electrode potential. At potentials above -0.6 V vs. RHE, the over-hydrogenation of acetylene to ethane was fully avoided. In situ XAFS measurements showed that acetylene adsorption on the Cu surface led to an increase in the oxidation state of the catalyst due to electron transfer from Cu to the acetylene molecule. In-situ Raman and ATR-FTIR spectroscopy confirmed the presence of key reaction intermediates, providing insights into the reaction mechanism. DFT calculations revealed that the strong adsorption of acetylene on the Cu surface, along with the electron transfer from Cu to acetylene, was crucial in suppressing the hydrogen evolution reaction and enhancing ethylene selectivity. Calculations showed a preference for acetylene adsorption over hydrogen adsorption on all three major crystal facets of Cu ((111), (100), (110)). The calculations further supported an electron-coupled proton transfer mechanism, where the rate-limiting step is the first hydrogenation of acetylene to vinyl. The study showed that the selectivity towards ethylene over butadiene was greatly influenced by the applied potential; the more negative potential favoring ethylene formation. A long-term stability test demonstrated that the FE for ethylene remained above 70% for over 100 h.

The findings address the research question by demonstrating a highly efficient and selective electrocatalytic route for ethylene production from acetylene under mild conditions. The high FE and geometric current density for ethylene formation achieved in this study surpass those reported for previously studied catalysts. The avoidance of ethane formation at potentials above -0.6 V vs. RHE significantly improves the selectivity of the process. The combined in situ characterization and DFT calculations provide strong support for an electron-coupled proton transfer mechanism, explaining the enhanced selectivity towards ethylene. The ability to control product selectivity through modulation of the electrode potential offers a new level of control in the HAE process. The results hold significant implications for developing energy-efficient and eco-friendly technologies for industrial ethylene production. This work addresses a critical need for sustainable and cost-effective ethylene production by providing a viable alternative to traditional energy-intensive methods.

This research successfully demonstrated a highly efficient and selective electrocatalytic hydrogenation of acetylene to ethylene using a Cu catalyst under mild conditions. The superior performance achieved, surpassing previous reports, is attributed to a combination of catalyst optimization and in-situ generated hydrogen from water. The insights into the reaction mechanism gained from in-situ characterization and DFT calculations provide a foundation for further improving this process. Future research could focus on exploring different support materials, optimizing the catalyst structure at a nanoscale, and investigating the effects of electrolyte composition on the reaction efficiency and selectivity. Scaling up the process for industrial applications is also a promising avenue for future work.

While the study achieved high ethylene selectivity and efficiency, potential limitations exist. The long-term stability test showed a gradual decrease in current density, potentially due to the formation of a polyacetylene layer on the catalyst surface. The long-term stability over several hundred hours should be further explored to assess the industrial viability of this process. The current study focused on a specific Cu catalyst; investigations with other materials are also warranted. The DFT calculations made certain assumptions regarding the reaction model, and further refining these could provide additional insights. Further study is needed to optimize operating parameters such as flow rate and pressure for improved ethylene conversion and yield in larger scale systems.