Plasma-chemical mechanism and multi-objective optimization of microwave discharge plasma-driven methane pyrolysis enabling scalable carbon-negative hydrogen and high-quality graphene co-production

Methane pyrolysis is emerging as a promising carbon-negative hydrogen production technique, offering an energy-efficient alternative to conventional methane steam reforming. A novel microwave discharge plasma (MDP) system is introduced to enhance methane pyrolysis, enabling the simultaneous production of carbon-negative hydrogen and high-quality carbon materials. Key operational parameters, including microwave power, flow rate, gas composition, and reactor geometry, have been systematically investigated for their significant effects on methane pyrolysis. Optimal system performance is achieved at 300 W microwave power and a flow rate of 0.07 m/s, yielding a methane conversion rate of 99.8 % and a specific energy requirement (SER) of 180 kJ/mol, with few-layer graphene produced as a valuable byproduct. The critical role of energetic electrons, argon species, and methane interactions in C–H bond cleavage and carbon nanostructure formation is elucidated through optical emission spectroscopy (OES) analysis. Additionally, density functional theory (DFT) calculations reveal the influence of microwave fields on methane adsorption energy over tungsten surfaces, shedding light on the mechanisms of methane cracking product formation. This study provides fundamental insights into MDP methane pyrolysis, advancing sustainable methane conversion and utilization strategies.