Fig. 1. Diagram of ¹⁴CH₄ plasma-catalytic experimental system

Fig. 2. Typical Lissajous figure

Fig. 3. Lissajous figures and current-voltage waveforms corresponding to different discharge voltages

Fig. 4. Variation of plasma discharge power with discharge voltage

Fig. 5. Effect of discharge voltages on CH₄ processing efficiency (a) and CO₂ selectivity (b) before and after plasma introduction into the catalysts (color online)

Fig. 6. Effect of gas flow rate on CH₄ processing efficiency (a) and CO₂ selectivity (b) before and after plasma introduction into the catalysts (color online)

Fig. 7. Effect of reaction temperature on CH₄ processing efficiency (a) and CO₂ selectivity (b) before and after plasma introduction into the catalysts (color online)

Fig. 8. Isothermal N₂ adsorption and desorption curves and pore size distribution of catalyst before and after the reaction (color online)

Fig. 9. XRD patterns of the catalyst before and after the reaction (color online)

Fig. 10. XPS patterns of the catalyst before and after the reaction (color online)

Fig. 11. Mechanism of plasma-catalysis oxidation of CH₄