Abstract: Methane (CH₄), as both a greenhouse gas and a crucial energy source, plays an important role in achieving China's carbon peaking and carbon neutrality goals. The significant concentration differences of CH₄ from various sources influence the selection of relevant conversion technologies. However, little research has addressed the impact of CH₄ concentration variation on catalytic performance, and studies focusing on the catalytic pyrolysis of methane for carbon material production are especially scarce. In this work, molten salt catalytic pyrolysis was employed as the core strategy to systematically investigate the catalytic decomposition behavior of CH₄ with varying concentrations (20%-100%) and the morphology control mechanisms of carbon products in a CuCl₂-NaCl molten salt system. The results revealed that the formation of graphene films was attributed to the two-dimensional assembly of carbon atoms on bubble surfaces at high CH₄ concentrations, followed by subsequent film growth. High CH₄ concentration in the CuCl₂-NaCl system favored the formation of well-ordered graphene structures, while low concentrations primarily produced fragmented carbon. Furthermore, various molten salt systems yielded different carbon morphologies, including graphite sheets, short rod-like carbon nanotubes, and film-like carbon. Comprehensive characterizations demonstrated that the CH₄ concentration determined both the nucleation rate and the growth mode of carbon products. This study elucidates the morphology control mechanisms of carbon products driven by the CH₄ concentration gradient in molten salt systems, providing a theoretical basis for the environmentally friendly synthesis of high-value-added carbon materials and the development of low-carbon technologies.

Key words: CH₄ pyrolysis, molten medium, concentration gradient, CuCl₂-NaCl, morphology control