Evidence of plasma-driven nonequilibrium chemistry in graphene formation from gas-phase kinetic modeling

Graphene can be synthesized entirely in the gas phase within microwave-assisted reactors operating at atmospheric pressure. Although these systems are sustained by plasmas with extremely high local temperatures, graphene formation occurs downstream where chemical kinetics govern molecular growth. A one-dimensional plug-flow model coupled with a sectional aerosol framework is used to evaluate how different detailed gas-phase chemical mechanisms influence graphene formation from an ethanol precursor. Five mechanisms commonly used for polycyclic aromatic hydrocarbon (PAH) chemistry—ABF, DLR, CALTECH, KAUST, and CRECK—are compared with experimental measurements of graphene yield and Feret diameter. The mechanisms predict very different onsets of graphene formation. Notably, the KAUST mechanism, despite its unrealistic assumption of irreversible PAH growth, reproduces experimental trends most closely. This outcome suggests that the plasma environment maintains a chemically frozen state where large PAHs behave as effectively irreversible species. Comparison between kinetic and equilibrium calculations confirms that PAH concentrations in the post-plasma region exceed equilibrium predictions by 18–20 orders of magnitude. Because the model itself does not include plasma physics, this kinetic–equilibrium disparity provides indirect, but not exclusive, evidence that plasma-driven processes push the system far from chemical equilibrium and enable the rapid molecular growth required for graphene formation. These findings explain why equilibrium models fail to predict graphene synthesis and demonstrate that model discrepancies can expose hidden nonequilibrium mechanisms.