Energy pooling mechanism for catalyst-free methane activation in nanosecond pulsed non-thermal plasmas

Abstract

Methane activation at low temperature remains a major challenge for its utilization in the field of reduction of greenhouse gases and generation of value-add products, while non-thermal plasmas provide a promising catalyst-free methane activation approach. In this work, an innovative energy pooling mechanism for catalyst-free methane activation at low temperature enabled by the nanosecond pulsed non-thermal plasma in argon and methane gas mixture is investigated by both optical diagnostic and kinetics modelling. Importantly, the evolution of the absolute density of the hydrogen atom at the ground state, as a real-time and in-situ indicator of the methane dissociation degree, is measured using the two-photon absorption laser induced fluorescence (TALIF) method. A simple zero-dimensional reaction kinetics model is built-up to perform a quantitative interpretation on the density evolution of active species. Especially, a good agreement is achieved between the measured hydrogen atom density and that from the kinetics model, validating the reaction pathways of active species. It is demonstrated that, except for the direct electron impact dissociation during the pulse-on period, both the charge transfer between argon ion and methane and the quenching of argon metastable species by methane contribute to a further increase of the hydrogen atom density during the pulse-off period, i.e. the argon ions and metastable species can function as an energy pooling for the formation of hydrogen atom and hydrocarbon radicals. The innovative mechanism proposed in this work may contribute to the catalyst-free and cost-effective strategies for methane utilization at low temperature.