Figure 5. Plasma diagnostics for “plasma only”, “plasma+MOR” and “plasma+Cu-MOR IE-3” at the same conditions as in Figure 1. (a) Lissajous curves; (b) Discharge voltage; (c) Discharge current; (d) In-situ OES results; (e) Calculated mean electron energy as a function of reduced electric field (E/N); (f) Electron energy distribution function (EEDF).
The Lissajous curves depicting the CH4/O2 plasma are presented in Figure 5a. Notably, variations in equivalent capacitance result in distinct discharge powers when employing different packing materials. The discharge power for “plasma only,” “plasma + MOR”, and “plasma + Cu-MOR” is 21 W, 14 W, and 11 W, respectively. The corresponding discharge voltage and current as a function of time are shown in Figures 5b and 5c. It is evident that the packing material exhibits virtually no influence on the discharge voltage, but it does affect the discharge current. Indeed, the “plasma + Cu-MOR” yields lower current peaks but a higher number of pulses. Filamentary discharges facilitate the generation of reactive species, a localized electric field and surface charge accumulation at the catalyst surface and pores, thereby influencing the reactivity of the potential reactions.26,27 The influence of current and voltage amplitudes on the catalytic performance is not significant in this work, indicating that the catalyst rather than the gas-phase is the main reaction area.
Figure 5d shows the optical emission spectra (OES) of the CH4/O2 plasmas. OES lines of CH (431.4 nm, A2Δ→X2П) and O (777.4 nm, 3s5S0→3p5P and 844.7 nm, 3s3S0→3p3P) are detected, indicating the presence of a significant amount of CH and O radicals in the CH4/O2plasmas.10 Notably, the intensities of the above lines attributed to CH and O species vary with different reaction conditions. Compared with the “plasma only”, the OES intensity significantly weakens after packing the CH4/O2 plasma with Cu-MOR catalyst. This phenomenon is attributed to the light shielding effect of the catalyst particles or the adsorption of active species by the catalyst sites.28,29
The mean electron energy (MEE) and the electron energy distribution function (EEDF) for the CH4/O2 plasma were calculated using Bolsig+, as shown in Figure 5e and f. The MEE in both packing systems is significantly higher than in the “plasma only” system, indicating the enhanced reactivity of the plasma after packing (Figure 5e). This higher reactivity is attributed to the catalyst packing, which increases the E/N values. However, the MEE for MOR support and Cu/MOR catalyst are quite close, primarily determined by their differences in relative dielectric constants. Similar trends are observed in Figure 5f, where high-energy electrons are more likely to be generated in the packing systems. Consequently, the catalyst packing systems are more likely to improve the production of reactive species through electron impact dissociation, excitation, and ionization of the feedstock molecules, as well as their further reactions. The reactive species in the plasma could facilitate catalytic reactions over the MOR surface.30 The computational details for calculating the MEE and EEDF are provided in the Supporting Information (section 5.4).