4.1.3 CO2/H2 mixture

The dominant reaction pathways for the conversion of CO2 and H2 in a 50/50 CO2/H2 DBD plasma are illustrated in Figure 6, as predicted by the model in [75]. The conversion starts again with electron impact dissociation of CO2, yielding CO and O atoms. Simultaneously, electron impact dissociation of H2 results in the formation of H atoms, and this reaction seems more important (cf. the thickness of the arrow line). The O and H atoms recombine into the formation of OH radicals and further into H2O. The model thus predicts that H2O is produced at relatively high density [75]. The CO molecules will partially react back into CO2, mainly through the formation of CHO radicals. This pathway appears to be more important than the direct three-body recombination between CO and O atoms into CO2, which is the dominant pathway in a pure CO2 plasma. The H atoms thus contribute significantly to the back reaction of CO into CO2, and this explains why the

#### Figure 6.

Dominant reaction pathways for the conversion of CO2 and H2 into various products, in a 50/50 CO2/H2 DBD plasma, as obtained from the model in [75]. The thickness of the arrow lines corresponds to the rates of the net reactions. The stable molecules are indicated with black rectangles. Reproduced from [75] with permission.

#### Modeling for a Better Understanding of Plasma-Based CO2 Conversion DOI: http://dx.doi.org/10.5772/intechopen.80436

calculated CO2 conversion is quite limited in a CO2/H2 mixture [75]. Electron impact dissociation of CO yields the formation of C atoms, which react further into CH, CH2, C2HO, and CH3 radicals in several successive radical recombination reactions. The CH2 radicals react with CO2 into CH2O, while the CH3 radicals easily form CH4. The latter reaction is more favorable than CH3OH formation out of CH3. Finally, CH4 partially reacts further into higher hydrocarbons (CxHy).

Figure 6 clearly illustrates that several subsequent radical reactions are required for the formation of (higher) hydrocarbons and oxygenates. This explains the very low calculated yields and selectivities of these end products [75]. In summary, the lack of direct formation of CH2 and CH3 in the CO2/H2 mixture, which is important in CO2/CH4 gas mixtures (see Figure 4), combined with the very low conversion of CO2, which is again attributed to the absence of CH2 as important collision partner for the loss of CO2, makes the CO2/H2 mixture less interesting for the formation of higher hydrocarbons and oxygenates than a CO2/CH4 mixture at the conditions under study. Furthermore, as H2 is a useful product by itself, while CH4 is also a greenhouse gas (besides a fuel), the simultaneous conversion of CO2 and CH4, that is, two greenhouse gases, is considered to be of higher value, also because it represents a direct valorization of biogas.

### 4.1.4 CO2/H2O mixture

thickness of these arrow lines, indicating the importance of these reactions) and CO2. Furthermore, formaldehyde is also partially converted into H2O. Note that this pathway is illustrated for a 70/30 CH4/O2 mixture, which obviously leads to nearly full oxidation of CH4, rather than partial oxidation, where the major end products should be the higher oxygenates. When less O2 would be present in the mixture, our model predicts that methanol and methyl hydroperoxide are formed in nearly equal amounts as CO and H2O [66]. Figure 5 also illustrates that the O2 molecules are mainly converted into CO, O atoms, and HO2 radicals. Some O3 is also formed out of O2, but the reverse process, that is, the production of two O2 molecules out of O3 and O atoms, is more important, explaining why the arrow points from O3 toward O2. The O atoms are converted into CH3O and OH radicals, producing methanol and water, respectively. The latter reaction (from OH to H2O) appears to be very important (cf. thick arrow line in Figure 5), and thus, significant amounts of H2O

In summary, comparing Figures 4 and 5 clearly indicates that the chemical pathways in CH4/O2 and CH4/CO2 plasma are quite different, even at the same mixing ratios. Finally, in both mixtures a large number of different chemical compounds can be formed, but due to the reactivity of the plasma, there is no selective production of some targeted compounds. To reach the latter, the plasma will have

The dominant reaction pathways for the conversion of CO2 and H2 in a 50/50 CO2/H2 DBD plasma are illustrated in Figure 6, as predicted by the model in [75]. The conversion starts again with electron impact dissociation of CO2, yielding CO and O atoms. Simultaneously, electron impact dissociation of H2 results in the formation of H atoms, and this reaction seems more important (cf. the thickness of the arrow line). The O and H atoms recombine into the formation of OH radicals and further into H2O. The model thus predicts that H2O is produced at relatively high density [75]. The CO molecules will partially react back into CO2, mainly through the formation of CHO radicals. This pathway appears to be more important than the direct three-body recombination between CO and O atoms into CO2, which is the dominant pathway in a pure CO2 plasma. The H atoms thus contribute

significantly to the back reaction of CO into CO2, and this explains why the

Dominant reaction pathways for the conversion of CO2 and H2 into various products, in a 50/50 CO2/H2 DBD plasma, as obtained from the model in [75]. The thickness of the arrow lines corresponds to the rates of the net reactions. The stable molecules are indicated with black rectangles. Reproduced from [75] with permission.

are formed, as predicted by the model [66].

to be combined with a catalyst.

Plasma Chemistry and Gas Conversion

4.1.3 CO2/H2 mixture

Figure 6.

18

H2O is the cheapest H-source to be added to a CO2 plasma for the direct production of value-added chemicals, and the combined conversion of CO2 and H2O could mimic the natural photosynthesis process. However, adding H2O (in concentrations up to 8%) to a CO2 DBD plasma causes a significant reduction in the CO2 conversion, while no oxygenated hydrocarbons were detected experimentally, and also the calculated concentrations were only in the ppb level [73].

These results can be explained by a kinetic analysis of the reaction chemistry. The latter reveals that the reaction between CO and OH, yielding H atoms and CO2, is crucial, as it has a very high rate constant, and it controls the ratio between the conversions of CO2 and H2O. This can be explained in a very simple way by the following reactions:

$$\text{e}^- + \text{CO}\_2 \rightarrow \text{CO} + \text{O} + \text{e}^- \tag{4}$$

$$\text{e}^- + \text{H}\_2\text{O} \rightarrow \text{OH} + \text{H} + \text{e}^- \tag{5}$$

$$\text{CO} + \text{OH} \rightarrow \text{CO}\_2 + \text{H} \tag{6}$$

$$\rm H + O\_2 + M \rightarrow HO\_2 + M \tag{7}$$

$$\text{HO}\_2 + \text{O} \rightarrow \text{OH} + \text{O}\_2 \tag{8}$$

$$\text{OH} + \text{H} \rightarrow \text{H}\_2\text{O} \tag{9}$$

$$\text{2e}^- + \text{CO}\_2 + \text{H}\_2\text{O} \rightarrow \text{CO}\_2 + \text{H}\_2\text{O} + 2\text{e}^- \tag{10}$$

Reactions (4) and (5) yield the dissociation of CO2 and H2O, but the products, CO and OH, will rapidly recombine into CO2 again (reaction (6)). Moreover, the two H atoms and one O atom formed will also quickly recombine, first into OH (through subsequent reactions (7) and (8)) and subsequently into H2O through reaction (9). Thus, overall, there is no net dissociation of CO2 and H2O in this pathway (see overall reaction (10)).

Of course, there exist also other pathways for the conversion of these molecules, so there will still be some conversion of CO2 and H2O in the plasma, but electron impact dissociation is typically the major loss mechanism for CO2 in a DBD (cf. also above), so the above mechanism explains the drop in CO2 conversion upon addition of H2O, as the OH radicals created upon H2O dissociation give rise to the back reaction, creating CO2 out of CO.

The above mechanism can also explain why no (significant) methanol (or other oxygenated hydrocarbons) is formed in the CO2/H2O mixture, because all the H atoms needed to form CH and CHO fragments for the formation of methanol are steered to OH and subsequently H2O again. Hence, this chemical kinetic analysis indicates that H2O might not be a suitable H-source for the formation of oxygenated hydrocarbons in a one-step process, because of the abundance of O atoms, O2 molecules, and OH radicals, trapping the H atoms.

It should be noted that this fast reaction between H and O atoms was demonstrated to be useful for the O-trapping in the case of pure CO2 conversion, thus providing a solution for the separation of the CO2 splitting products [120], but in the present case, it is clearly the limiting factor for the formation of oxygenated hydrocarbons.
