Special issue: Renewable energies

This special issue highlights renewable energies (REs). Among them, renewable electricity is becoming the most accessible and flexible low-carbon energy source. It can potentially allow achieving a drastic reduction of CO₂ emissions that will contribute to our future sustainable society. RE is not limited to the development of high-performance energy devices, such as photovoltaics, fuel cells, and secondary batteries. Importantly, the utilization of RE in sustainable transformation and valorization of widely available yet hard-to-activate small carbon and hydrogen-containing molecules, such as CO₂, CH₄, and H₂O, are vital for the production of low-carbon e-fuels and sustainable chemicals.

The transition to a low-carbon footprint using RE is known as the Power-to-X concept. Photochemistry, electrochemistry, and a combination of these technologies have been heavily studied and explored.^[1] Microwave and resistive heating is also studied as an alternative low-carbon high-temperature heat source used in chemical processes.^{[2, 3]} Further, thermal plasma technology attracts keen attention for cracking methane to (turquoise) hydrogen and carbon black. Thermal plasma powered by RE minimizes carbon emission, equivalent to CH₄ steam reforming combined with CCS.^[4]

More recently, plasma catalysis has become an emerging low-carbon footprint technology that can benefit from the efficient use of RE to control chemical reactions such as CH₄ reforming, CO₂ conversion, and N₂ fixation.^[5] Plasma-generated reactive species initiate chemical reactions at much lower temperatures than conventional thermal catalysis. In the meantime, plasma is generating simultaneously activated species (e.g., radicals) and heat, enabling operation of a catalytic reactor without an additional external heat source. This ability to perform endothermal reactions at relatively low temperatures is in contrast to an electrochemical reaction, such as a solid electrolyte, where the reaction temperature is limited in a narrow window due to the charge transport properties of electrolyte materials. Plasma catalysis is not limited by the combination of nonthermal plasma and heterogeneous catalysts but is closely related to standalone plasma technology for CO₂ splitting and N₂ fixation, which is also known as plasma conversion. Plasma catalysis has gained recognition as the key research topic in the Gordon Research Conference (Plasma Processing Science) over the decades. Highly cited review articles on plasma catalysis have also been accessible since late 2010.^[6-10]

This special issue focuses on plasma–catalyst coupling technology for gas conversion and consists of one review, one perspective paper, and eight original research papers. The review paper introduces the concept of a fluidized-bed DBD reactor for the conversion of CH₄ and CO₂.^[11] Plasma–surface interaction in terms of increased radical flux and heat transfer augmentation is discussed. The perspective article describes the importance of product separation from the plasma–catalyst reaction field for improving energy efficiency.^[12] In addition to the appropriate catalyst selection, a product separation strategy is also important to maximize the plasma-induced synergistic effect. In addition, hydrocarbon reforming technologies by plasma–catalyst coupling such as CH₃OH synthesis using DBD^[13] and C₂H₅OH and CH₄ reforming using warm plasma are presented.^[14] CO₂ conversion using the plasma–liquid interface is presented,^[15] which is an ideal reaction system to validate the product separation concept presented in Rouwenhorst and Lefferts.^[12] Plasma catalysis of CH₄ reforming is studied numerically by the chemical kinetic model of nanosecond-pulsed plasma^[16] and process simulation of thermal and DBD integrated systems.^[17] CO₂ separation from the dry methane reforming reactor effluent is generally energy intensive but necessary to increase the product yield as high CO₂:CH₄ ratios are necessary; the paper provides insightful information from the application perspective. Additionally, alloyed catalyst preparation by glow discharge,^[18] industrial-scale exhaust gas clearing by plasma ozonizer,^[19] and N₂ fixation by the warm plasma combined with catalysts^[20] are presented.

Finally, we would like to thank all contributors to this special issue, the reviewers, and the editorial staff of Plasma Processes and Polymers for their outstanding and continuous support. We hope that this special issue will enhance the recognition of plasma catalysis as an emerging electrification technology. Also, we hope that readers gain mechanistic insights as well as find stimulation to contribute to technology transfer from laboratory to industrial scale which may well involve multiple dispersed relatively small units.