Decomposition of Methane Diluted with Inert Gas in an RF Discharge Cell

TL;DR

This study investigates how different plasma discharge modes in RF cells affect methane decomposition, revealing that contracted modes significantly enhance conversion efficiency and produce valuable hydrocarbons and carbon materials.

Contribution

It demonstrates the influence of discharge mode transitions on methane decomposition efficiency and product selectivity in RF plasma, highlighting the role of plasma properties in optimizing chemical conversion.

Findings

Contracted discharge mode achieves 99.7% methane decomposition.

Discharge mode transitions influence product selectivity.

He and Ne plasmas remain uniform and diffuse under conditions.

Abstract

Decomposition of methane using non-thermal plasmas is an attractive route for producing hydrogen-rich gases and valuable carbon nanomaterials. Understanding how plasma discharge modes influence methane decomposition in optimizing plasma-assisted chemical conversion remains unexplored. This study explores the coupling between the discharge structure and product selectivity in RF capacitively coupled discharges operating in methane/inert gas mixtures in the pressure range of 2 to 3 torr. The discharge exhibits mode transitions from uniform to striated in Ar and Kr and from diffuse to contracted in Ar and Kr with 5 percent or less CH4. The discharges in He and Ne remained uniform under our operating conditions, and their mixtures with CH4 remained diffuse. A 0-D model for Ar/CH4 discharge established a threshold for contraction while also asserting the importance of Ar metastable in the dissociation and ionization processes. The highest degree of methane decomposition, 99.7% with the main products of acetylene and graphitized solid carbon was achieved in the contracted discharge mode for both Kr or Ar with 5% or less CH4. We demonstrate that contraction can play a crucial role in the effective decomposition of methane with value-added products and that both the electronic and thermal properties of plasma gas are responsible for this effect.