Special issue: Plasma Conversion

With growing concern of energy and environmental issues, the combination of plasma and heterogeneous catalysts receives special attention in greenhouse gas conversion, nitrogen fixation and hydrocarbon chemistry. Plasma gas conversion driven by renewable electricity is particularly important for the electrification of the chemical industry, which is currently dominated by water electrolysis, producing CO2-free renewable hydrogen. Meanwhile, electrochemical conversion of CO2, CH4 and other gaseous materials has yet to be successful because the material and energy conversion efficiency as well as productivity of the electrochemical processes greatly depend on the electrolyte material in terms of the solubility of gaseous materials and mass transport capability, which needs to be dramatically improved. As for fuel processing, the majority of reactions are categorized as uphill (endothermic) reactions where energy input is indispensable due to the conservation of energy. Ideally, such energy should be supplied by low temperature heat while plasma-generated reactive species promote initiation reactions. One promising approach is the use of vibrationally excited molecules which enhance the probability of chemisorption onto the catalyst surfaces. The role of vibrational species has been well investigated through molecular beam studies. However, the way to create such species has not been studied yet. Non-thermal plasma creates an oversaturated amount of vibrational species even at low temperature, showing great promises towards low temperature plasma catalysis of stable molecules. Furthermore, vibrational species uniquely create reaction pathways in homogeneous reactions via stepwise energy transfer gradually populating higher vibrational levels which eventually leads to the dissociation of stable molecules such as CO2. The special issue entitled “Plasma Conversion” consists of three review articles and 11 original research papers, focusing on the state-of-the-art plasma technology that is used for enhancing homogeneous/heterogeneous plasma catalytic conversion of greenhouse gases, nitrogen fixation and hydrocarbon chemistry. Review articles include the new catalyst synthesis approach using nonthermal plasma processes and the potential application of such unique catalysts is introduced (Rahimpour et al.) [1]. CH4 and CO2 reforming is one of the hot topics in plasma catalysis and research has been conducted mostly by experimental approaches. Bogaerts et al. discuss CH4 reforming and CO2 splitting and various mixtures from a modeling perspective, paying special attention to plasma chemistry and the role of plasma-generated vibrationally excited molecules [2]. Reactions involving vibrationally excited species need to be explored further and a numerical approach is truly beneficial providing insightful information towards the reaction mechanisms. Liquefaction of CH4 is to produce higher hydrocarbons via CH4 polymerization at ambient conditions (not for cryogenic methane condensation) [3]. Carbon containing liquid fuels such as gasoline and diesels produced via renewable electricity improve transportability and storage of renewable energy. Two research papers are related to heterogeneous plasma catalysis of CH4 and CO2 where nonthermal plasma is combined with Ni-supported reforming catalysts [4,5]. Research was conducted experimentally and plasma induced synergistic effects are highlighted. Van Rooij and Bongers discuss the homogeneous CO2 conversion with warm discharge, or low pressure microwave plasma [6,7]. Special emphasis on the vibrationally excited CO2 chemistry and CO2 dissociation by the latter mechanism is discussed. A novel heterogeneous plasma catalysis approach of CO2, where nonthermal plasma is combined with an oxygen permeating solid electrolyte membrane, is presented by Mori [8]. O2 is extracted directly from the plasma reaction field, and thus the CO2 conversion and energy efficiency were greatly improved. Moreover, CO2 microwave plasma diagnostics by optical emission spectroscopy [9], diagnostics plasma-catalyst interfacial phenomena [10], and atomistic simulations of plasma catalytic interactions [11] are included in the collection of this special issue. Three other papers focus on ammonia synthesis with atmospheric pressure plasma combined with heterogeneous catalysts [12–14]. Finally we would like to thank all contributors to this special issue and the editorial staff of Plasma Processes and Polymers for their great support. We hope that this issue contributes to exploring the new frontiers of plasma chemistry, and also to promoting the interdisciplinary interaction among the plasma science, chemical catalysis and surface science community, as well as other science specialists.