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

Abstract

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.