4.1.1.1 DBD conditions

The dominant reaction pathways for CO2 splitting in a DBD plasma, as predicted from the model in [7], are plotted in Figure 1. As a DBD is characterized by relatively highly reduced electric field values (typically above 200 Td), and thus relatively high electron energies (several eV), electron impact reactions with CO2 ground-state molecules dominate the chemistry. The most important reactions are electron impact dissociation into CO and O (which proceeds through electronically excited CO2, that is, the so-called electron impact excitation-dissociation), electron impact ionization into CO2 <sup>+</sup> (which recombines with electrons or O2 ions into CO and O and/or O2), and electron dissociative attachment into CO and O (cf. the thick black arrow lines in Figure 1). These three processes account for about 50%, 25%, and 25%, respectively, to the total CO2 conversion [28]. Because these processes require more energy than strictly needed for breaking the C=O bond (i.e., 5.5 eV), the energy efficiency for CO2 splitting in a DBD plasma is quite limited, that is, up to maximum 10% for a conversion up to 30% [1].

The CO molecules are relatively stable, but at very long residence time, they will recombine with O ions or O atoms, to form again CO2 (cf. thin black arrow lines in Figure 1). This explains why the CO2 conversion typically saturates at long residence times. Furthermore, the O atoms created upon CO2 splitting also recombine quickly into O2 or O3, based on several processes (see also Figure 1).

## 4.1.1.2 MW and GA conditions

While our calculations predict that ca. 94% of the CO2 splitting in a DBD plasma arises from the ground state, and only 6% occurs from the vibrationally excited levels [28], the situation is completely different in a MW or GA plasma. These plasmas are characterized by much lower reduced electric field values (in the order of 50–100 Td), creating lower electron energies (order of 1 eV), which are most suitable for vibrational excitation of CO2. Therefore, the CO2 splitting in MW and GA discharge is mainly induced by electron impact vibrational excitation of the

#### Figure 1.

Dominant reaction pathways of CO2 splitting and the further reactions between O, O2, and O3 in a DBD plasma, as obtained from the model in [7], where the labels are also explained. Adopted from [119] with permission.

lowest vibrational levels, followed by vibrational-vibrational (VV) collisions, gradually populating the higher vibrational levels, leading to dissociation of CO2. This stepwise vibrational excitation, or the so-called ladder climbing, is illustrated in Figure 2. As this process only requires 5.5 eV for dissociation, that is, exactly the C=O bond energy, this explains why MW and GA discharges exhibit a much better energy efficiency than a DBD, where the dominant dissociation mechanism is electron impact excitation-dissociation, as explained above, which requires 7–10 eV (see Figure 2).

> higher vibrational levels of CO2. Indeed, as the GA operates at atmospheric pressure, the vibrational distribution function (VDF) is too much thermal, that is, there is no significant overpopulation of the higher CO2 vibrational levels. This was predicted both in a classical GA at a temperature around 1200 K [45] and in a RVF GA, operating at temperatures around 2500–3000 K [47]. The CO2 dissociation even proceeds mainly from the ground state or the lowest vibrational levels. Indeed, based on these models, the major dissociation process was electron impact dissociation [45] or thermal dissociation [47] of the lower CO2 vibrational levels, and the chemical reactions of the higher vibrational levels (with either O atoms or any arbitrary molecules in the plasma), which theoretically provide the most energyefficient process for CO2 conversion, were found to be of minor importance. Just like in the MW plasma, the model predicts that a significant overpopulation of the VDF, and thus a more energy-efficient CO2 conversion, can be realized by decreas-

> Schematic illustration of some CO2 electronic and vibrational levels, illustrating the energy-efficient dissociation process through electron impact vibrational excitation, followed by vibrational-vibrational collisions, which gradually populate the higher vibrational levels, that is, the so-called ladder climbing (5.5 eV), compared to direct dissociation through electronic excitation (above 7 eV). Adopted from [119] with permission.

When adding an H-source, such as CH4, to the CO2 plasma, a variety of mole-

cules can be formed, with a mixture of H2 and CO (or syngas) as the major compounds, but also smaller fractions of higher hydrocarbons and oxygenates can be formed. Figure 4 illustrates the dominant pathways in a CO2/CH4 mixture, as predicted by the model in [66]. The thickness of the arrow lines is correlated to the rate of the reaction. CH4 dissociation is initiated by electron impact, forming CH3

ing the temperature or by increasing the power density [45].

Modeling for a Better Understanding of Plasma-Based CO2 Conversion

DOI: http://dx.doi.org/10.5772/intechopen.80436

4.1.2 CO2/CH4 mixture

15

Figure 2.

Still, it must be realized that the vibrational excitation pathway is not always optimized in a MW or GA plasma. Indeed, as illustrated in detail in [42], the vibrational excitation is higher at lower pressures and higher power densities. The latter give rise to higher electron densities, which yield more vibrational excitation. Higher pressures, on the other hand, result in more vibrational-translational (VT) relaxation collisions, which represent the major loss mechanism of the vibrational energy. Finally, also the gas temperature plays a crucial role, as a higher gas temperature also results in more pronounced VT relaxation. Our models predict that in a MW plasma at atmospheric pressure, the dissociation is too much determined by thermal processes, thus limiting the CO2 conversion and energy efficiency, in agreement with experimental observations. In addition, the recombination of CO and O atoms also becomes gradually more important at high gas temperature and pressures [42], further explaining why the experimental CO2 conversion and energy efficiency drop upon increasing pressure. The main processes occurring in a MW plasma in the two extreme cases, that is, the ideal non-equilibrium conditions of low pressure and temperature and high power density and the near-thermal conditions of high pressure and temperature, are summarized in Figure 3. The model predicts a much higher CO2 conversion and energy efficiency in a pressure range of 200–300 mbar and much lower values at atmospheric pressure, in the near-thermal conditions [42]. Hence, we should exploit as much as possible the non-equilibrium character of a MW plasma, in which the higher vibrational levels of CO2 are overpopulated, to obtain the most energy-efficient CO2 conversion.

The same conclusions can be drawn for a GA plasma, where our models predict that the CO2 conversion could be further enhanced, by exploiting the role of the

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

#### Figure 2.

lowest vibrational levels, followed by vibrational-vibrational (VV) collisions, gradually populating the higher vibrational levels, leading to dissociation of CO2. This stepwise vibrational excitation, or the so-called ladder climbing, is illustrated in Figure 2. As this process only requires 5.5 eV for dissociation, that is, exactly the C=O bond energy, this explains why MW and GA discharges exhibit a much better energy efficiency than a DBD, where the dominant dissociation mechanism is electron impact excitation-dissociation, as explained above, which requires 7–10 eV

Dominant reaction pathways of CO2 splitting and the further reactions between O, O2, and O3 in a DBD plasma, as obtained from the model in [7], where the labels are also explained. Adopted from [119] with

Still, it must be realized that the vibrational excitation pathway is not always optimized in a MW or GA plasma. Indeed, as illustrated in detail in [42], the vibrational excitation is higher at lower pressures and higher power densities. The latter give rise to higher electron densities, which yield more vibrational excitation. Higher pressures, on the other hand, result in more vibrational-translational (VT) relaxation collisions, which represent the major loss mechanism of the vibrational energy. Finally, also the gas temperature plays a crucial role, as a higher gas temperature also results in more pronounced VT relaxation. Our models predict that in a MW plasma at atmospheric pressure, the dissociation is too much determined by thermal processes, thus limiting the CO2 conversion and energy efficiency, in agreement with experimental observations. In addition, the recombination of CO and O atoms also becomes gradually more important at high gas temperature and pressures [42], further explaining why the experimental CO2 conversion and energy efficiency drop upon increasing pressure. The main processes occurring in a MW plasma in the two extreme cases, that is, the ideal non-equilibrium conditions of low pressure and temperature and high power density and the near-thermal conditions of high pressure and temperature, are summarized in Figure 3. The model predicts a much higher CO2 conversion and energy efficiency in a pressure range of 200–300 mbar and much lower values at atmospheric pressure, in the near-thermal conditions [42]. Hence, we should exploit as much as possible the non-equilibrium character of a MW plasma, in which the higher vibrational levels of CO2 are overpopulated, to obtain the most energy-efficient CO2 conversion. The same conclusions can be drawn for a GA plasma, where our models predict that the CO2 conversion could be further enhanced, by exploiting the role of the

(see Figure 2).

14

Figure 1.

Plasma Chemistry and Gas Conversion

permission.

Schematic illustration of some CO2 electronic and vibrational levels, illustrating the energy-efficient dissociation process through electron impact vibrational excitation, followed by vibrational-vibrational collisions, which gradually populate the higher vibrational levels, that is, the so-called ladder climbing (5.5 eV), compared to direct dissociation through electronic excitation (above 7 eV). Adopted from [119] with permission.

higher vibrational levels of CO2. Indeed, as the GA operates at atmospheric pressure, the vibrational distribution function (VDF) is too much thermal, that is, there is no significant overpopulation of the higher CO2 vibrational levels. This was predicted both in a classical GA at a temperature around 1200 K [45] and in a RVF GA, operating at temperatures around 2500–3000 K [47]. The CO2 dissociation even proceeds mainly from the ground state or the lowest vibrational levels. Indeed, based on these models, the major dissociation process was electron impact dissociation [45] or thermal dissociation [47] of the lower CO2 vibrational levels, and the chemical reactions of the higher vibrational levels (with either O atoms or any arbitrary molecules in the plasma), which theoretically provide the most energyefficient process for CO2 conversion, were found to be of minor importance. Just like in the MW plasma, the model predicts that a significant overpopulation of the VDF, and thus a more energy-efficient CO2 conversion, can be realized by decreasing the temperature or by increasing the power density [45].

### 4.1.2 CO2/CH4 mixture

When adding an H-source, such as CH4, to the CO2 plasma, a variety of molecules can be formed, with a mixture of H2 and CO (or syngas) as the major compounds, but also smaller fractions of higher hydrocarbons and oxygenates can be formed. Figure 4 illustrates the dominant pathways in a CO2/CH4 mixture, as predicted by the model in [66]. The thickness of the arrow lines is correlated to the rate of the reaction. CH4 dissociation is initiated by electron impact, forming CH3

#### Figure 3.

Dominant reaction pathways of CO2 splitting in a MW plasma, as obtained from the model in [42], for two extreme cases: (a) the ideal non-equilibrium conditions of low pressure and temperature and high power density and (b) the near-thermal condition of high pressure and temperature. Adopted from [116] with permission.

radicals, which recombine into higher hydrocarbons. Moreover, electron impact dissociation of CH4 and of the higher hydrocarbons also yields H2 formation. In addition, the CH3 radicals also create methanol (CH3OH) and CH3O2 radicals, albeit to a lower extent. Furthermore, the CH2 radicals, also created from electron impact dissociation of CH4, react with CO2 to form formaldehyde (CH2O) and CO. Finally, the O atoms, created from electron impact dissociation of CO2 (see also Figure 1), also initiate the formation of higher oxygenates, like acetaldehyde (CH3CHO). This species reacts further into CH3CO radicals and subsequently into ketene (CH2CO), although these pathways are not so important in absolute terms, as indicated by the thin dashed lines in Figure 4.

mixture, in spite of the fact that the same chemical species are included in the models (see Table 1). Electron impact dissociation of CH4 again produces CH3 radicals, which will recombine into methanol or higher hydrocarbons, but the recombination into CH3O2 radicals, which form either CH3O radicals or methyl hydroperoxide (CH3OOH), is now more important. The CH3O radicals produce methanol, which seems a more important formation mechanism than the recombination of CH3 with OH radicals (cf. the arrow line thickness in Figure 5), and methanol can also react further into CH2OH radicals, producing formaldehyde. The latter is also easily converted into CHO radicals and further into CO (note the

Dominant reaction pathways for the conversion of CH4 and O2 into (mainly) higher oxygenates, as well as some full oxidation products, in a 70/30 CH4/O2 DBD plasma, as obtained from the model in [66]. The thickness of the arrow lines corresponds to the importance of the reaction paths. Reproduced from [84] with permission.

Dominant reaction pathways for the conversion of CH4 and CO2 into higher hydrocarbons, H2 and CO, and higher oxygenates, in a 70/30 CH4/CO2 DBD plasma, as obtained from the model in [66]. The thickness of the arrow lines corresponds to the importance of the reaction paths. Reproduced from [84] with permission.

Modeling for a Better Understanding of Plasma-Based CO2 Conversion

DOI: http://dx.doi.org/10.5772/intechopen.80436

Figure 4.

Figure 5.

17

We have also compared the chemistry in the CO2/CH4 mixture, used for dry reforming of methane, with that of partial oxidation of methane, that is, a CH4/O2 mixture [66]. The reaction pathways of the latter are depicted in Figure 5. The CH4/O2 mixture clearly leads to a completely different chemistry than the CO2/CH4 Modeling for a Better Understanding of Plasma-Based CO2 Conversion DOI: http://dx.doi.org/10.5772/intechopen.80436

#### Figure 4.

Dominant reaction pathways for the conversion of CH4 and CO2 into higher hydrocarbons, H2 and CO, and higher oxygenates, in a 70/30 CH4/CO2 DBD plasma, as obtained from the model in [66]. The thickness of the arrow lines corresponds to the importance of the reaction paths. Reproduced from [84] with permission.

#### Figure 5.

radicals, which recombine into higher hydrocarbons. Moreover, electron impact dissociation of CH4 and of the higher hydrocarbons also yields H2 formation. In addition, the CH3 radicals also create methanol (CH3OH) and CH3O2 radicals, albeit to a lower extent. Furthermore, the CH2 radicals, also created from electron impact dissociation of CH4, react with CO2 to form formaldehyde (CH2O) and CO. Finally, the O atoms, created from electron impact dissociation of CO2 (see also Figure 1), also initiate the formation of higher oxygenates, like acetaldehyde (CH3CHO). This species reacts further into CH3CO radicals and subsequently into ketene (CH2CO), although these pathways are not so important in absolute terms, as indicated by the

Dominant reaction pathways of CO2 splitting in a MW plasma, as obtained from the model in [42], for two extreme cases: (a) the ideal non-equilibrium conditions of low pressure and temperature and high power density and (b) the near-thermal condition of high pressure and temperature. Adopted from [116] with permission.

We have also compared the chemistry in the CO2/CH4 mixture, used for dry reforming of methane, with that of partial oxidation of methane, that is, a CH4/O2 mixture [66]. The reaction pathways of the latter are depicted in Figure 5. The CH4/O2 mixture clearly leads to a completely different chemistry than the CO2/CH4

thin dashed lines in Figure 4.

Plasma Chemistry and Gas Conversion

Figure 3.

16

Dominant reaction pathways for the conversion of CH4 and O2 into (mainly) higher oxygenates, as well as some full oxidation products, in a 70/30 CH4/O2 DBD plasma, as obtained from the model in [66]. The thickness of the arrow lines corresponds to the importance of the reaction paths. Reproduced from [84] with permission.

mixture, in spite of the fact that the same chemical species are included in the models (see Table 1). Electron impact dissociation of CH4 again produces CH3 radicals, which will recombine into methanol or higher hydrocarbons, but the recombination into CH3O2 radicals, which form either CH3O radicals or methyl hydroperoxide (CH3OOH), is now more important. The CH3O radicals produce methanol, which seems a more important formation mechanism than the recombination of CH3 with OH radicals (cf. the arrow line thickness in Figure 5), and methanol can also react further into CH2OH radicals, producing formaldehyde. The latter is also easily converted into CHO radicals and further into CO (note the

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

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.

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 repre-

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

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

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

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

e� þ CO2 ! CO þ O þ e� (4) e� þ H2O ! OH þ H þ e� (5) CO þ OH ! CO2 þ H (6) H þ O2 þ M ! HO2 þ M (7) HO2 þ O ! OH þ O2 (8) OH þ H ! H2O (9)

2e� þ CO2 þ H2O ! CO2 þ H2O þ 2e� (10)

also the calculated concentrations were only in the ppb level [73].

Finally, CH4 partially reacts further into higher hydrocarbons (CxHy).

Modeling for a Better Understanding of Plasma-Based CO2 Conversion

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sents a direct valorization of biogas.

pathway (see overall reaction (10)).

19

4.1.4 CO2/H2O mixture

following reactions:

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 to be combined with a catalyst.
