CH₄ Decomposition over Carbon for CO₂-Free Hydrogen Production: A Combined DFT and Experimental Investigation on the Role of Carbon

Carbon-based self-catalysis for methane pyrolysis is of great interest for producing H2 and carbon materials without CO2 emissions and the need for catalyst regeneration, but atomic-level insights into this process remain limited. Here, building on experimental observations of layered carbon synthesis during CH4 pyrolysis and the catalytic role of carbon in accelerating CH4 decomposition, we employ density functional theory (DFT) and ab initio atomistic thermodynamics to systematically investigate the adsorption, dehydrogenation, and reaction mechanisms involved in CH4 pyrolysis on armchair-edged graphene (Gr_arm). Our study highlights the unique catalytic properties of Gr_arm under varying temperature, pressure, and radical partial pressures. Equilibrium analyses of adsorption–desorption and dehydrogenation–desorption revealed the unique ability of Gr_arm to stabilize and activate key intermediates across varying conditions. The initial dehydrogenation of CH4 is a critical step where the presence of carbon markedly enhances the reaction rate under low-temperature and high-pressure conditions. The reaction mechanism study indicates that the CH* + H* co-adsorbed species corresponds to the most stable state in the overall CH4(g) → 2H2(g) + C* reaction, with CH* dehydrogenation requiring the highest activation free energy (Ga = 3.65 eV), suggesting potential for CH*–CH* coupling reactions. Moreover, varying CHx• and H• radical partial pressures alters the thermodynamic preference between dehydrogenation and desorption, reshaping the most favorable reaction pathways. These findings underscore the significant role of carbon in methane pyrolysis, offering critical insights for designing efficient carbon-based catalysts and advancing their practical applications.