Methane cracking in a low-pressure RF-ICP plasma: pressure and flow effects on hydrogen formation

Methane dissociation in low-pressure radio-frequency inductively coupled plasmas (RF-ICP) is governed by the coupled effects of electron energy distribution, excitation kinetics, and residence time. In this work, methane cracking is investigated in a low-pressure RF-ICP reactor with systematic variation of pressure and methane flow rate, while plasma behavior and reaction products are characterized using optical emission spectroscopy, residual gas analysis, and X-ray photoelectron spectroscopy. The results reveal an optimal operating regime near 1 × 10−2 mbar, where sufficient electron density is maintained without excessive collisional damping, enabling efficient vibrational excitation, methane dissociation, and hydrogen formation. Under these conditions, enhanced dissociation pathways lead to a methane conversion of approximately 63%, maximized hydrogen formation, and an estimated minimum specific energy cost of ∼64 kJ/mol H2, while the formation of C2 hydrocarbons is suppressed. Increasing methane flow rate reduces residence time and vibrational excitation, resulting in weakened plasma activation and lower hydrogen yield despite increased feed availability. Surface analysis confirms the formation of hydrogenated amorphous carbon (a-C:H) with mixed sp2/sp3 bonding, consistent with plasma-assisted methane dissociation and solid-carbon formation. These findings demonstrate how pressure- and flow-controlled energy transfer mechanisms govern methane dissociation and product selectivity in RF-ICP plasmas.