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The electrical power dissipated in the discharge was calculated by the Lissajous method Falkenstein and Coogan, Figure 1 shows the SIE as a function of the amplitude of the applied voltage. The SIE depends approximately linearly on the voltage. When increasing the applied voltage from 10 to 20 kV, the average power increased from 0. Since the power is very low, thermal effect can be excluded. The formation of nitrogen oxides is also avoided at these low values of discharge power. Figure 1. Specific input energy as a function of amplitude of the discharge voltage.
Figure 2 shows toluene conversion and the selectivity to carbon dioxide and carbon monoxide as a function of SIE for the experiments performed in plasma, in the absence of catalysts. The only gaseous reaction products of toluene oxidation in the plasma were CO and CO 2. Figure 2. In a first set of experiments 2. Initially, the gas was passed through the reactor for several hours with plasma off, so that toluene was adsorbed on the catalyst bed until saturation was reached.
After the plasma was turned on, a large amount of toluene was desorbed from the catalyst. Therefore, a much higher toluene concentration is actually present in the plasma-catalytic reactor as compared to the input concentration. Since it is well known that conversion decreases with increasing concentration, poorer toluene conversion as compared to that obtained in the DBD is expected. Figure 3. CO 2 selectivity as a function of SIE in the plasma and in the plasma—catalytic system.
In the following experiments only 0.
In this case the toluene conversion was similar with that obtained with plasma alone. The amount of toluene removed is actually higher, because part of the toluene adsorbed on the catalyst is also oxidized. Another experiment was performed with 0. This behavior, together with the conversion decrease are most likely due also to the desorption of toluene from the catalyst.
Even after the conversion stabilizes, the concentration of COx is almost two times higher than the maximum possible value, which suggests that the amount of oxidized toluene is actually higher than the 50 ppm introduced. Therefore, plasma and ozone generated in the DBD have the ability to oxidize toluene on the catalyst surface.
Figure 4. Temporal evolution of CO and CO 2 concentrations and toluene conversion. A further advantage of using this configuration is the complete removal of ozone in the effluent gas. Figure 5 shows the ozone concentration generated in the DBD in the absence of catalysts, with the plasma reactor fully packed with catalyst and with 0. In plasma, in the absence of catalysts, the O 3 concentration increased with increasing SIE in the range 0.
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The O 3 concentration was reduced to 0. It is therefore obvious that ozone generated in the plasma plays an important role in toluene oxidation. The most likely mechanism is ozone decomposition on the catalyst surface, forming highly reactive atomic oxygen, which reacts with toluene and also shifts the product distribution toward total oxidation.
Figures 6 — 9 present results using spent catalysts collected from inside the plasma reactor. However, similar characterizations were obtained with catalysts inside the plasma, downstream of plasma reactor and both inside and after the reactor.
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Figure 6. Spent catalyst was collected from inside the plasma reactor. Figure 7. Figure 8. Figure 9.
None of them exhibited diffraction lines associated with palladium that accounts firstly for the good dispersion of the metal, with a particle size under the detection limit of XRD technique, and secondly for the fact that working under plasma irradiation for more than 24 h induced no agglomeration. DRIFTs spectra provided additional arguments concerning the stability of the catalyst under the reaction conditions Figure 7.
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Working under plasma did not affected the presence of the OH groups, but only induced a decrease of the intensity of these bands that was caused by the deposition of a small amounts of carbonaceous deposits. Thermal analysis was not able to clearly detect the presence of these deposits neither from the TGA nor from the DTA profiles Figure 8.
However, elemental analysis of the fresh and spent samples identified, indeed, small amounts of carbon on the spent catalyst Table 1. While it showed no difference in the nitrogen and sulfur content, a very slight increase in the carbon content was detected that is typical for radicalic processes where a polycondensation of the aromatic ring can occur.
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TEM pictures of the fresh and spent catalyst also reveal that the exposure to the reaction conditions described above did not generate changes in the morphology of palladium Figure 9. Fresh catalysts exposed Pd particles with sizes in the range 4. Spent catalysts exposed Pd particles with sizes in a narrower range 3.
Accordingly these measurements suggest only a redispersion of the large particles. The efficiency of a plasma-catalyst hybrid system has already been demonstrated by other studies Magureanu et al. However, up to date no clear evidence of the stability of the catalyst has been provided in the literature. Our studies demonstrate that the investigated catalyst was not affected by the plasma conditions or by a deposition of soot. Except a very small change in the intensity of the OH bonds belonging to the support no other changes have been evidenced.
These results are well correlated to the catalysts characterization. The presence of small palladium particles limited a redispersion of the metal during the plasma-catalytic oxidation of toluene and conserved the catalytic behavior with a good stability. Obviously, the radicalic pathway, in addition to the oxidation of the hydrocarbon, is also matter of polymerization of small derived entities with the formation of large polyaromatic structures, namely, soot.
In addition, the characterization results using elemental and thermal analysis, XRD, and DRIFT provided evidence about the stability of this catalyst under the investigated conditions. Noteworthy, these characterizations evidenced the formation of soot only in a very small amount that is also in agreement with previous reports Holzer et al.
The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest. Da Costa, P. Plasma catalytic oxidation of methane on alumina-supported noble metal catalysts. B Environ. CrossRef Full Text. Delagrange, S. Combination of a non-thermal plasma and a catalyst for toluene removal from air: Manganese based oxide catalysts.
Falkenstein, Z. Microdischarge behaviour in the silent discharge of nitrogen - oxygen and water - air mixtures. D Appl. Harling, A. Temperature dependence of plasma catalysis using a nonthermal, atmospheric pressure packed bed; the destruction of benzene and toluene. C , — Holzer, F. Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds: Part 1. Accessibility of the intra-particle volume. Horikoshi, S. A novel environmental risk-free microwave discharge electrodeless lamp MDEL in advanced oxidation processes.