Highly efficient single-step microwave plasma conversion of hazardous aromatic hydrocarbons into few-layer graphene.

Plasma conversion of hazardous aromatic hydrocarbons into carbon nanomaterials is both feasible and increasingly attractive - particularly when paired with waste valorization strategies. Benzene, toluene and xylene (BTX) were decomposed and converted into few-layer graphene (FLG) using reactive gas admixture, hydrogen or oxygen, in an atmospheric pressure dual-channel microwave plasma torch in Ar. Gas flow rates and delivered microwave power are investigated as main parameters of FLG synthesis. Decomposition of pure BTX vapors produced carbon nanoparticles, with up to 53 % yield, whereas decreasing precursor flow rate and increasing delivered microwave power promoted the formation of planar structures. Hydrogen admixture strongly enhanced selectivity toward FLG and suppressed spherical carbon nanoparticles formation. Highest FLG yield, 20 % ≈ 360 mg/h, was achieved using 7.5 sccm of benzene, 20-40 sccm H2, 350 W. Comparable yield was also obtained using hydrogen admixture to toluene and xylene. The use of oxygen admixture led to the FLG formation for benzene and toluene precursors at lower power, but with lower yield 4-18 %. No FLG were synthesized using oxygen admixture to xylene. Controlled addition of reactive gases to aromatic hydrocarbons can be used in a new synthesis route - single-step, catalyst-free, microwave plasma process, highlighting its potential for scalable production of high-value graphitic nanomaterials by controlled decomposition of hazardous pollutants.