Temperature-independent, nonoxidative methane conversion in nanosecond repetitively pulsed DBD plasma

Currently, about 90% of methane (CH4) is used in various combustion processes, releasing carbon dioxide into the atmosphere. In order to optimize the use of fossil-fuel energy and reduce greenhouse gas emissions, finding effective ways to convert CH4 into fuels and chemicals has become a research focus in academic circles and industry. Activating the strong C(sp3)–H bond (434 kJ mol−1) in CH4 under mild conditions is one of the most rigorous challenges. The use of a plasma for dissociating CH4 to CH3 free radicals has advantages over conventional pyrolysis approaches. This work aims to increase the CH3 production rate at low temperature by use of a nanosecond repetitively pulsed dielectric barrier discharge plasma with optimization of several parameters, including pulse rise time, pulse width and pulse repetition frequency. The maximum CH4 conversion reaches 31.9% when the pulse repetition frequency is increased to 9 kHz, while the dominant C2 hydrocarbon is always C2H6 (from CH3 coupling reaction). A chemical kinetic model confirms that H and CH3 are the critical radicals produced from electron-impact CH4 dissociation, with their peak densities of the order of 1015 cm−3 and 1014 cm−3, respectively. The results indicate that the improved plasma technology can continuously produce abundant H and CH3 radicals, which would enhance the surface reactions on the catalyst in the plasma-catalytic CH4 conversion process.