Feed-composition control of gas-phase chemistry and carbon-film growth in low-pressure RF-ICP CH4/CO2/N2 plasmas

Low-pressure RF inductively coupled plasmas provide controlled environments for examining how mixed molecular feeds redistribute input energy among excitation, dissociation, gas-phase reactions, and surface-growth pathways. Here, CH4/CO2/N2 mixtures were investigated under fixed RF-ICP conditions using optical emission spectroscopy, residual gas analysis, and X-ray photoelectron spectroscopy to determine how feed composition affects plasma response, product formation, and carbon-film chemistry. Landfill-gas-like compositions are treated as model feeds rather than direct process-scale reforming routes. Increasing CH4 fraction shifted the discharge toward methane-dominated fragmentation, producing stronger hydrogen-related signals and enhanced carbon-film growth. Increasing CO2 fraction promoted oxygen-containing pathways, CO-forming channels, and surface restructuring through oxidation and etching reactions. N2 mainly acted as an energy-transfer and dilution component, with possible vibrational and metastable contributions. Because RGA measurements were uncalibrated for sensitivity factors and fragmentation patterns, mass-spectrometric signals are interpreted comparatively, with overlapping channels treated as composite indicators. XPS confirmed composition-dependent plasma-surface coupling: CH4-rich conditions favoured heteroatom-containing hydrogenated amorphous carbon, whereas CO2-rich mixtures produced more carbon-rich and relatively sp2-enriched films. Overall, feed composition controls coupled gas-phase and surface-growth pathways in CH4/CO2/N2 RF-ICP plasmas under vacuum conditions.