## 3 Results and Discussion
### Analysis of Liquid and Solid By-products  
In the case of the DBD reactor with the Rh catalyst and the highest discharge energy applied (7542 J/L), no liquid by-products were found. This means complete conversion of all tar components into gases. A similar result was found by [PERSON] et al. [51] during oxidation of benzene in a packed-bed DBD reactor after adsorption on a catalyst. In all other cases, i.e., at a lower discharge energy, with an Ni catalyst and without a catalyst, a wide range of liquid by-products were detected. Those of a single aromatic ring (phenol, hydroquinone, 1,4-benzoquinone, 4-phenoxyphenol, o-phthalaldehyde, benzaldehyde, and benzyl), and an aliphatic or cyclic structure (maleic anhydride, methanol, heptanoic acid, acetic acid and formic acid) can be attributed to the oxidation of all three tar components introduced to the reactor. Four compounds derived exclusively from naphthalene have also been detected: phthalic anhydride, 1,4-naphthquinone, 1-naphthol, and 2-naphthol. They were also  
Figure 8: Concentrations of (**a**) CO\({}_{2}\) and (**b**) CO in gas leaving the DBD reactor. Error bars represent standard deviation. Figure 7: Concentrations of (**a**) H2 and (**b**) CH4 in gas leaving the DBD reactor. Error bars represent standard deviation. all detected by other researchers in different studies concerning tar removal in plasma-catalytic systems [4; 26; 27; 28; 29; 33; 51]. In the case of the Ni catalyst incorporated in the inner ceramic tube, no aliphatic products were detected. When an Ni or catalyst-free reactor was used, char formation on the inner ceramic tube was observed. It caused fast deactivation of the catalyst. No char was observed only when the Rh catalyst was used, even at lower discharge energies. Then, also no deactivation was observed for 6 h of total use of Rh catalyst. In the case of the reactor with the Ni catalyst, an interesting result concerning char composition was obtained. The amount of char was enough to perform FTIR-ATR analysis. Samples were taken at five equally spaced points along the inner ceramic tube. Usually, as in the work of [PERSON] et al. [28], the identification of the exact decomposition by-products of naphthalene alone by using FTIR spectroscopy, especially those in the solid phase, is not trivial. Then, often only functional groups can be determined. However, as can be seen in Figure 9, the spectra are not very complex which allowed to identify all components basing on FTIR spectra libraries. This was carried out using Thermo Scientific(tm) OMNIC(tm) Spectra Software with library analysis tool. It was found that spectra presented in Figure 9 is the sum of at most seven components. The first point of sampling, named Char1, was located just a few centimeters from the inlet to the reactor, thus it is not surprising that the spectra is dominated with naphthalene. In this sample, small amounts of naphthols and 1,4-naphthquinone were also recognized. In the spectra of the second sample, named Char2, bands originated from naphthalene are lower whereas those from naphthols and 1,4-naphthoquinone are higher and a band from phthalic anhydride appeared. The third sample, named Char3, shows a further decrease in naphthalene concentration, but also a decrease in the concentration of naphthols. Bands derived from 1,4-naphthoquinone and phthalic anhydride increased and additional bands from maleic anhydride and 1,4-benzoquinone appeared in the spectra. In the next sample, named Char4, there is no more naphthalene, and the concentration of naphthols continue to decrease. Moreover, it is seen that the band from 1,4-naphthoquinone is lower than in the previous sample and bands typical for 1,4-benzoquinone, phthalic anhydride, and maleic anhydride are higher. In the last sample, named Char5, there are only traces of naphthols and 1,4-naphthoquinone, a decreased amount of phthalic anhydride and an increased amount of maleic anhydride and 1,4-benzoquinone. Figure 9: IR spectra of five char samples from the DBD reactor containing Ni catalyst.