Plasma-catalytic methane decomposition: Radical-mediated pathways and controlled CNTs growth over plasma-modified Fe catalyst

Methane decomposition is increasingly recognized as pivotal technology for simultaneous production of H₂ and carbon nanomaterials, yet its industrial implementation is severely constrained by high operating temperatures and rapid deactivation to thermal catalytic systems. By integrating in-situ optical emission spectroscopy with multi-scale characterization, it is demonstrated that plasma serves a four-fold function in the Fe-based catalytic system: (1) excitation of CH₄ into a controllable radical pool, (2) enhancement of radical adsorption energy on Fe surfaces through plasma-induced electronic polarization, (3) implementation of a hydrogen-mediated “pathway pruning” mechanism wherein H* abstracts H from CH_x* species, preventing gas-phase polymerization while simultaneously accelerating surface-catalyzed carbon assembly, and (4) in-situ etching of amorphous carbon and promotion of surface hydrogen-assisted dehydrogenation on M-H sites. These coupled mechanisms synergistically suppress electrode carbon deposition, enabling sustained discharge stability and maintaining the plasma discharge in high-efficiency tip-discharge regime. Consequently, at 700 °C, the plasma-catalytic system achieves methane conversion of 42.67 % (compared to 25 % in pure plasma at 700 °C, and 19.25 % in pure thermal catalytic at 750 °C), hydrogen selectivity of 57.88 %, carbon yield of 105 mg·g_cat⁻¹·h⁻¹, and uniform CNTs with enhanced graphitization. This work provides a quantitative mechanistic blueprint for designing next-generation plasma-catalytic systems that overcome the limitations of conventional thermal processes.