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• Catalyst-free approach for activating potent greenhouse gas methane. • Nanosecond pulsed plasma enables new reaction pathways at low temperature (300 K). • Time-resolved and in-situ measurements for the first time reveal reaction kinetics. • Energetic argon species function as an energy pooling for methane activation. 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. [ABSTRACT FROM AUTHOR]

[Display omitted] • Without any chemical agent or rare gas, microwave plasma surface modification only consuming low-power electricity and ambient air. • Rapid hydrophobicity recovery of seriously contaminated silicone rubber on the order of 10 s, with a nice long-term aging performance. • Competition between plasma-enabled bond-breaking/etching and oxidization dictates hydrophobicity. • Targeted at power system in severe environment, highly-efficient, environmentally-friendly, and on-line treatment to reduce the risk of pollution flashover. The loss of hydrophobicity of silicone rubber (SR) insulators due to contamination accumulation will increase the risk of flashover and threaten the stability of electrical power system. To solve this problem, a highly efficient and environmentally friendly microwave plasma surface treatment method is developed, which only consumes low-power electricity and ambient air without any chemical agent or rare gas. The hydrophobicity of seriously contaminated SR can be recovered on the order of 10 s, with a nice long-term aging performance under natural transfer. The underlying mechanism is revealed via carefully designed control experiments and systematic surface analysis. It is illustrated that the plasma-generated active species break the long chain of SR into non-crosslinked low-molecule-weight siloxanes (LMWs) and accelerate their transfer to contamination layer, while the bombing and etching effects of plasma manufacture a porous structure and increase the surface roughness. All of these factors will contribute to the increase of hydrophobicity. On the contrary, the oxidation effect of plasma will turn LMWs into polar groups and destroy the hydrophobicity, which should be restricted in practice. The microwave plasma treatment method developed in this work has the on-line application potential in power system for hydrophobicity recovery under severe contamination conditions. [ABSTRACT FROM AUTHOR]