Plasma-catalytic coupling in a ns pulsed discharge for the DRM reaction

In recent years, a continuous increase in carbon dioxide (CO2) emissions has been observed in both the energy sector and the cement and metallurgical industries. While CO2 emissions from the energy sector can be reduced, the issue of emissions from the cement and metallurgical industries remains unresolved. The dry reforming of methane (DRM) reaction converts equimolar amounts of CH4 and CO2 into syngas, a valuable mixture of carbon monoxide and hydrogen. With an H2/CO ratio close to 1, DRM is an ideal process for the production of synthetic liquid hydrocarbons via the Fischer-Tropsch reaction. Despite its advantages, DRM technology struggles to achieve industrial maturity. High activation temperatures (above 700°C) and the endothermic nature of the process result in low energy efficiency. Furthermore, the high operating temperatures contribute to catalyst deactivation due to sintering and/or carbon deposition. A parallel strategy for reducing CO2 emissions in the energy sector through the use of renewable energy sources is also not without challenges. These sources (e.g., wind turbines, photovoltaics) are dependent on atmospheric conditions and operate intermittently. Improving the efficiency of renewable energy sources is possible through the use of high-performance energy storage systems, which allow for the accumulation of excess energy produced during peak weather conditions and its utilization during periods of increased demand. One of the proposed approaches for managing surplus energy is its utilization in the production of valuable chemicals, such as through an endothermic reaction. One such process is the dry reforming of methane (DRM), which enables the production of syngas with an H2:CO ratio of 1:1, aligning with the carbon capture and utilization (CCU) strategy. Additionally, the application of activation energy-reducing techniques, such as non-thermal plasma (NTP), can enhance the efficiency of the reaction. Plasma can respond to variations in available energy since it can be rapidly switched on and off. NTP operates outside thermal equilibrium, meaning that the kinetic energy of electrons can be significantly higher than that of heavier particles. This enables thermodynamically unfavorable reactions, such as DRM, while maintaining the gas temperature close to ambient conditions. Experimental studies on DRM under plasma conditions have demonstrated higher energy efficiency compared to thermal processes; however, they are characterized by lower conversion rates and reduced selectivity toward syngas. The combination of plasma and catalysis exhibits synergistic effects that help overcome the limitations of both techniques. However, a fundamental understanding of the underlying processes and the interactions between plasma and the catalyst is still lacking. Most studies utilize dielectric barrier discharge (DBD) plasma since it allows the catalyst to be placed directly within the plasma reaction zone. Nevertheless, more efficient plasma sources, such as gliding arc discharge, microwave discharges, and nanosecond repetitive pulsed discharges (NRP), need to be explored. The coupling of these plasma sources with a catalyst presents a complex challenge and constitutes an entirely new research area. This study investigates the effect of NRP discharge in a DRM reaction system using supported nickelbased catalysts. The selected nickel catalysts were previously tested in high-temperature catalytic reactions and demonstrated the highest activity and selectivity [1]. The plasma discharge, previously characterized in DRM studies by Scapinello and Montesano, is a pin-to-pin discharge operating at atmospheric pressure. NRP plasma has gained increasing attention as one of the most energy-efficient methods for promoting chemical reactions, leveraging high electron densities and high electron energies achievable in this process. The mechanism of NRP discharge involves the repetition of very short high-voltage pulses at kHz frequencies between two electrodes. The pulse duration typically ranges from 4 to 10 ns, with voltage amplitudes on the order of tens of kV and an electrode gap of several millimeters. As a result, the gas temperature can easily exceed 4000 K, making catalyst application feasible only in a post-plasma catalytic mode. The objective of this study was to develop a series of supported nickel catalysts, evaluate their performance in NRP plasma-activated DRM, and analyze the efficiency of the process under these conditions. Two types of catalysts were synthesized: nickel catalysts based on hydrotalcite (NiHT) and alumina (Ni/Al2O3), with nickel loadings ranging from 5% to 40% by weight. The active material (Ni) was introduced via co-precipitation (for NiHT) and incipient wetness impregnation or adsorption (for Ni/Al2O3). To investigate the influence of foam composition, two support materials were compared: Y2O3 – ZrO2 and α-Al2O3. Additionally, α-Al2O3 foam was functionalized by impregnation with γ-Al2O3 and 15% by weight of Ni/γ-Al2O3 to assess the contribution of the catalyst on the foam surface. The obtained catalytic materials were characterized using XRD, FTIR, CO2-TPD, low-temperature nitrogen adsorption, and H2-TPR. DRM experiments were conducted under atmospheric pressure at a CH4:CO2 ratio of 1:1. Performance parameters were evaluated as a function of the specific energy input (SEI). The results indicated that the placement of the catalytic bed too far from the plasma source (in a postplasma position) did not lead to improved activity. The distance of the catalyst from the active plasma zone significantly limited the interaction between plasma-activated reactants and the catalyst surface. Thermal activation was not feasible since the catalytic bed remained below 100°C. These findings suggest that a serial arrangement of the catalyst with the plasma reactor is insufficient, and various configurations should be explored to enhance catalyst-plasma interactions. The study of discharge limitations was conducted to address issues observed when positioning the catalyst in the post-plasma region. To this end, catalysts supported on monolithic foam carriers in a coaxial geometry were investigated. This configuration was designed to position the catalyst closer to the plasma discharge, enhancing the interaction between plasma-activated reactants and the catalyst surface. The structure of the ceramic monolithic foam was obtained via polyurethane foam replication, enabling the production of a material with controlled porosity and geometry. The foam formed a hollow cylinder surrounding the discharge in a coaxial configuration. The proximity of the foam to the active plasma zone facilitated exposure to reactants and indirect heating by plasma. It was hypothesized that this dual activation mechanism, combining thermal and plasma effects, would enhance catalytic performance. A critical aspect of the experimental design was the ability to regulate the foam temperature while maintaining a constant SEI. This was achieved by proportionally adjusting the discharge power and reactant flow rate. The external temperature of the foam surface was measured using a calibrated IR thermal camera, allowing for correlation with conversion and selectivity results. Experimental results demonstrated that the presence of foam improved DRM performance compared to plasma alone. Both Y2O3 – ZrO2 and α-Al2O3 supports exhibited a linear increase in CO2 and CH4 conversion with rising temperature, surpassing the performance of plasma-only conditions. However, the foam composition was not a differentiating factor. The presence of foams also affected product selectivity, reducing CO production and shifting toward acetylene formation as a byproduct instead of ethane and ethene. Notably, these effects were observed even in the case of uncoated foam structures, suggesting that the foam itself plays a key role in modifying the plasma-catalytic process. These findings indicate that foam application influences the reaction through: • temperature moderation, • altered gas flow dynamics, • enhanced surface interactions. Further research on discharge limitations was extended to address the constraints of monolithic catalysts in favor of powdered catalysts. A novel "quartz wool nest" structure was developed to integrate powdered catalysts with plasma discharge in a coaxial geometry. This configuration was designed to leverage the high surface area and versatile compositional possibilities offered by powdered catalysts. Experimental results demonstrated that the presence of the "quartz wool nest" structure, regardless of catalyst loading, nearly doubled the CO2 and CH4 conversion rate compared to plasma alone. Future research should focus on optimizing catalyst composition and elucidating the complex interaction between plasma physics and surface chemistry in this novel configuration.