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Plasma (catalysis) reactors are increasingly being used for gas-based chemical conversions, providing an alternative method of energy delivery to the molecules. In this work we explore whether classical concepts such as equilibrium constants, (overall) rate coefficients, and catalysis exist under plasma conditions. We specifically investigate the existence of a so-called partial chemical equilibrium (PCE), and how process parameters and packing properties influence this equilibrium, as well as the overall apparent rate coefficient, for CO2 splitting in a DBD plasma reactor. The results show that a PCE can be reached, and that the position of the equilibrium, in combination with the rate coefficient, greatly depends on the reactor parameters and operating conditions (i.e., power, pressure, and gap size). A higher power, higher pressure, or smaller gap size enhance both the equilibrium constant and the rate coefficient, although they cannot be independently tuned. Inserting a packing material (non-porous SiO2 and ZrO2 spheres) in the reactor reveals interesting gap/material effects, where the type of material dictates the position of the equilibrium and the rate (inhibition) independently. As a result, no apparent synergistic effect or plasma-catalytic behaviour was observed for the non-porous packing materials studied in this reaction. Within the investigated parameters, equilibrium conversions were obtained between 23 and 71%, while the rate coefficient varied between 0.027 s−1 and 0.17 s−1. This method of analysis can provide a more fundamental insight in the overall reaction kinetics of (catalytic) plasma-based gas conversion, in order to be able to distinguish plasma effects from true catalytic enhancement.

We have found some inadvertent errors in our paper published in Catalysts [...]

We studied the influence of dense, spherical packing materials, with different chemical compositions, on the dry reforming of methane (DRM) in a dielectric barrier discharge (DBD) reactor. Although not catalytically activated, a vast effect on the conversion and product selectivity could already be observed, an influence which is often neglected when catalytically activated plasma packing materials are being studied. The &alpha, Al2O3 packing material of 2.0&ndash, 2.24 mm size yields the highest total conversion (28%), as well as CO2 (23%) and CH4 (33%) conversion and a high product fraction towards CO (~70%) and ethane (~14%), together with an enhanced CO/H2 ratio of 9 in a 4.5 mm gap DBD at 60 W and 23 kHz. &gamma, Al2O3 is only slightly less active in total conversion (22%) but is even more selective in products formed than &alpha, Al2O3. BaTiO3 produces substantially more oxygenated products than the other packing materials but is the least selective in product fractions and has a clear negative impact on CO2 conversion upon addition of CH4. Interestingly, when comparing to pure CO2 splitting and when evaluating differences in products formed, significantly different trends are obtained for the packing materials, indicating a complex impact of the presence of CH4 and the specific nature of the packing materials on the DRM process.

DBD plasma reactors are of great interest for environmental and energy applications, such as CO2 conversion, but they suffer from limited conversion and especially energy efficiency. The introduction of packing materials has been a popular subject of investigation in order to increase the reactor performance. Reducing the discharge gap of the reactor below one millimetre can enhance the plasma performance as well. In this work, we combine both effects and use a packed-bed DBD micro plasma reactor to investigate the influence of gap size reduction, in combination with a packing material, on the conversion and efficiency of CO2 dissociation. Packing materials used in this work were SiO2, ZrO2, and Al2O3 spheres as well as glass wool. The results are compared to a regular size reactor as a benchmark. Reducing the discharge gap can greatly increase the CO2 conversion, although at a lower energy efficiency. Adding a packing material further increases the conversion when keeping a constant residence time, but is greatly dependent on the material composition, gap and sphere size used. Maximum conversions of 50–55% are obtained for very long residence times (30 s and higher) in an empty reactor or with certain packing material combinations, suggesting a balance in CO2 dissociation and recombination reactions. The maximum energy efficiency achieved is 4.3%, but this is for the regular sized reactor at a short residence time (7.5 s). Electrical characterization is performed to reveal some trends in the electrical behaviour of the plasma upon reduction of the discharge gap and addition of a packing material.

This paper demonstrates that the CO2 conversion in a dielectric barrier discharge rises drastically upon addition of Ar or He, and the effect is more pronounced for Ar than for He. The effective CO2 conversion, on the other hand, drops upon addition of Ar or He, which is logical due to the lower CO2 content in the gas mixture, and the same is true for the energy efficiency, because a considerable fraction of the energy is then consumed into ionization/excitation of Ar or He atoms. The higher absolute CO2 conversion upon addition of Ar or He can be explained by studying in detail the Lissajous plots and the current profiles. The breakdown voltage is lower in the CO2/Ar and CO2/He mixtures, and the discharge gap is more filled with plasma, which enhances the possibility for CO2 conversion. The rates of electron impact excitation–dissociation of CO2, estimated from the electron densities and mean electron energies, are indeed higher in the CO2/Ar and (to a lower extent) in the CO2/He mixtures, compared to the pure CO2 plasma. Moreover, charge transfer between Ar+ or Ar2+ ions and CO2, followed by electron-ion dissociative recombination of the CO2+ ions, might also contribute to, or even be dominant for the CO2 dissociation. All these effects can explain the higher CO2 conversion, especially upon addition of Ar, but also upon addition of He.

Plasma catalysis is gaining increasing interest for CO2 conversion, but the interaction between the plasma and catalyst is still poorly understood. This is caused by limited systematic materials research, since most works combine a plasma with commercial supported catalysts and packings. In the present paper, we study the influence of specific material and reactor properties, as well as reactor/bead configuration, on the conversion and energy efficiency of CO2 dissociation in a packed bed dielectric barrier discharge (DBD) reactor. Of the various packing materials investigated, BaTiO3 yields the highest conversion and energy efficiency, i.e., 25% and 4.5%. Our results show that, when evaluating the influence of catalysts, the impact of the packing (support) material itself cannot be neglected, since it can largely affect the conversion and energy efficiency. This shows the large potential for further improvement of packed bed plasma reactors for CO2 conversion and other chemical conversion reactions by adjusting both packing (support) properties and catalytically active sites. Moreover, we clearly prove that comparison of results obtained in different reactor setups should be done with care, since there is a large effect of the reactor setup and reactor/bead configuration.