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).