1：Experimental Design and Method Selection:

A resonant cavity reactor was used to generate and sustain a plasma discharge at atmospheric pressure within an axial dielectric (quartz) tube. A custom ceramic flow distributor was mounted above the axial dielectric to impart a rotational component to the methane/hydrogen/argon mixture to promote plasma stability and prevent carbon deposition on the inner walls of the dielectric. Copper cooling coils were wrapped around the reactor cavity to maintain bulk temperatures below 300 °C for the duration of all tests.

2：Sample Selection and Data Sources:

Tests were conducted by adding H2 to the CH4/Ar feed stream, for which the first test point had no additional H

3：By CH

H2 ratio, tests are designated as follows: (RCA) 1:0, (RCB)

4：

1 and (RCC) 1:1. The Ar carrier was adjusted to maintain approximately the same overall flow velocity through the reactor as the CH4 and H2 concentrations were varied.

5：The Ar carrier was adjusted to maintain approximately the same overall flow velocity through the reactor as the CH4 and H2 concentrations were varied. List of Experimental Equipment and Materials:

3. List of Experimental Equipment and Materials: The input conditions for each test included net power, CH4 concentration, H2 concentration, and Ar concentration. Materials characterization was performed using transmission electron microscopy (TEM), thermogravimetric analysis (TGA), Raman spectroscopy, X-ray diffraction (XRD), Fourier-transform infra-red (FTIR) spectroscopy, and electron energy loss spectroscopy (EELS).

6：Experimental Procedures and Operational Workflow:

Samples were dispersed and sonicated in methanol before being dropped onto 300 mesh C/Cu lacey TEM grids. Thermogravimetric analysis used a TA Instruments Q600 SDT. Raman spectra were obtained using a Horiba LabRam Raman microscope. XRD measurements were carried out in a PANalytical Empyrean X-ray Diffractometer. FTIR spectroscopy was performed using a Bruker Vertex 70v FTIR Spectrometer.

7：Data Analysis Methods:

Data analysis included TEM image quantification by fringe analysis, deconvolution of XRD profiles, and comparison of EELS spectra to assess sp2/sp3 hybridization.