Methane Decomposition in RF Discharge Cell with the Addition of an Inert Gas

The decomposition of methane using non-thermal plasmas presents an attractive route for producing hydrogen-rich gases and valuable carbon nanomaterials. Understanding how plasma discharge modes influence methane decomposition remains unexplored in optimizing plasma-assisted chemical conversion. This work explores the coupling between discharge structure and product selectivity in RF capacitively coupled discharges operating in methane/inert gas mixtures. The discharge exhibits mode transitions from uniform to striated in Ar, Kr, and H₂ and from diffuse to contracted in Ar and Kr with <5% CH₄. The discharge in He and Ne remains uniform under our operating conditions and their mixtures with CH₄ remain diffuse. A 0-d model for Ar/CH₄ discharge established a threshold for contraction at the same time asserting the importance of 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% CH₄. We demonstrate that contraction can play a crucial role for effective decomposition of methane with value-added products and that both the electronic and thermal properties of plasma gas are responsible for this effect.

We thank Igor Kaganovich (PPPL), Ali Mesbah (UC Berkeley), and Yiguang Ju (PU) for fruitful collaboration, Stanislav Musikhin (PPPL) and Hengfei Gu (PU) for solid material analysis (SEM, TEM/EDS), and Aleksandr Merzhevsky for technical assistance.