**3. Nonthermal plasma reforming**

is noted that CO desorption is an endothermic reaction, thus desorption of CO plays the role of

O atmosphere (b). The reaction proceeds in clockwise direction. Pd dissociates CH4

O into PdOH and PdH, PdOH and PdC were combined into PdCOH. H2

association from two PdH. These CO and H2

effluent gas stream. Moreover, if water vapor is added into the gas stream, water molecules par-

In terms of long-term operation of scaled-up catalytic reforming, coke deposition is the serious problem to shorten the duration of operation. Coke can be generated via several ways during reforming, as described in **Table 2**. Carbon formation can be classified into 5 categories, including the form of surface carbide, amorphous carbon films, metal carbide, whiskers and

Eley-Rideal One of reactants is firstly adsorbed on catalyst surface. Further reaction takes place between

Mars-van Krevelen One of reactants is chemisorbed on catalyst, and then diffuses to lattice to react with the other

, resulting in higher generation rate of H2

Both reactants are adsorbed on catalyst surface firstly. Next, adsorbed reactants can react with each other and form products. The final step is desorption of products and regeneration of

(ME) [20].

and desorb onto

and MgO binds and

association from PdCOH and

production pathways are accessible at low

production. Pd

radicals,

, H\*

reforming mechanism under dry reforming conditions

O byproducts are attributed to H<sup>2</sup>

and COH\*

.

rate-limiting reaction of DRM. Also, H atoms can further recombine to form H2

ticipate in catalysis and more active species can be generated such as OH\*

adsorbed specie and gas phase-specie.

. MgO opens a favorable CO production pathway. H2

providing more formation routes of H2

active sites.

adsorbed reactant.

**Mechanism Description**

**Table 2.** Categories of catalysis mechanisms.

(a) and an H2

activates CO2

dissociates H2

Langmuir-Hinshelwood

PdH is easier than direct H2

**Figure 2.** Results from DFT studies on the multifunctional CH4

88 Carbon Dioxide Chemistry, Capture and Oil Recovery

temperature and assure low-temperature activity of Pd-MgO/SiO2

Nonthermal plasma stands for an alternative to treat GHGs since the driving force of nonthermal plasma is electronic energy instead of thermal energy. With the existence of external electric field, electrons can be accelerated and then collide with gas particles including CO2 , CH4 , intermediates, radicals and ions. When energy is transferred from electron to the above species, chemical reactions take place such as electron impact excitation, dissociation and ionization, Penny ionization and electron attachment. CH<sup>4</sup> and CO2 can be directly dissociated into smaller fractions when the transferred energy exceeds 8.8 and 4.5 eV, respectively [22, 23]. The dissociated products including methyl radical, methylene,

**Figure 4.** Possible reaction mechanisms in a nonthermal plasma reactor [24].

oxygen and hydrogen radical can further react with each other to form H2 , CO and other byproducts, e.g. ethylene, water vapor and methanol. **Figure 4** depicts possible reaction routes for nonthermal plasma reforming [24]. CH4 can be dissociated into various radicals, depending on how much energy is transferred from electron. On the other hand, CO2 can also be dissociated into CO and O simultaneously. The above reactive species including radicals, H and O atoms can further react to form hydrocarbon radicals and molecules. Radicals mentioned beforehand can react with CH4 and CO2 to enhance their dissociation rates. It is noted that electrons can react with those particles to dissociate them into smaller fragments and the more important point is that radicals are unstable to have high activity toward other particles.

**4. Hybrid plasma catalysis**

branching mechanism [27].

**Figure 6.** CH4

**Figure 5.** Reaction pathway of the CO2

**4.1. Plasma influencing catalyst**

Combining nonthermal plasma with catalyst to form a hybrid system is expected to solve the obstacles of catalysis and nonthermal plasma due to the induction of various interactions. **Figure 7** shows a conventional plasma catalysis reactor, catalyst is placed inside the discharge region of plasma reactor [29]. With this manner of reactor designing, various interactions can be induced to enhance the performance of reforming. Up to date, many interactions are discovered while some synergies are still vague. Many works are focusing on elucidation of synergies of plasma-catalysis interactions including experimental and simulation studies. Currently, those known interactions can be divided into two categories: plasma influencing catalyst and catalyst influencing plasma. Some interactions have positive effects on DRM, thus some researches are

kinetic model [25].

CO2 Reforming with CH4 via Plasma Catalysis System http://dx.doi.org/10.5772/intechopen.73579 91

dedicated to modify those synergies. Those synergies will be briefly introduced below.

During discharge, a large amount of particles including electrons, ions, intermediates, excited species and radicals are generated. These particles may collide with catalyst and some energy

Actually, direct dissociation of CO2 /CH4 is difficult to take place in nonthermal plasma due to high energy demand. In terms of CO2 , vibrational excited and electron excited CO2 are more easily generated and participate in DRM since the energy required for excitation is lower than CO2 dissociation. Those excited CO2 possess higher energy, thus, energy required to generate CO, O, CO2 + and O2 + can be reduced as illustrated in **Figure 5**. In terms of CH4 , CHx (x = 1–3) radicals especially CH3 (methyl radical) and CH2 (methylene) are formed with different levels. These species are unstable and tend to react with other species [26]. When two or more radicals react with each other, higher hydrocarbons can be generated [25]. For example, when two CH3 \* radicals react with each other, ethane can be formed as illustrated in **Figure 6** [27]. From the perspective of DRM, hydrocarbons are byproducts since the purpose of DRM is to generate syngas. However, this process can be useful for generating hydrocarbons, such as plasma polymerization [28].

**Figure 5.** Reaction pathway of the CO2 kinetic model [25].

**Figure 6.** CH4 branching mechanism [27].

oxygen and hydrogen radical can further react with each other to form H2

/CH4

(methyl radical) and CH2

routes for nonthermal plasma reforming [24]. CH4

**Figure 4.** Possible reaction mechanisms in a nonthermal plasma reactor [24].

Radicals mentioned beforehand can react with CH4

toward other particles.

+ and O2 +

radicals especially CH3

plasma polymerization [28].

CO2

CO, O, CO2

two CH3 \*

Actually, direct dissociation of CO2

90 Carbon Dioxide Chemistry, Capture and Oil Recovery

high energy demand. In terms of CO2

dissociation. Those excited CO2

byproducts, e.g. ethylene, water vapor and methanol. **Figure 4** depicts possible reaction

also be dissociated into CO and O simultaneously. The above reactive species including radicals, H and O atoms can further react to form hydrocarbon radicals and molecules.

rates. It is noted that electrons can react with those particles to dissociate them into smaller fragments and the more important point is that radicals are unstable to have high activity

easily generated and participate in DRM since the energy required for excitation is lower than

els. These species are unstable and tend to react with other species [26]. When two or more radicals react with each other, higher hydrocarbons can be generated [25]. For example, when

From the perspective of DRM, hydrocarbons are byproducts since the purpose of DRM is to generate syngas. However, this process can be useful for generating hydrocarbons, such as

can be reduced as illustrated in **Figure 5**. In terms of CH4

radicals react with each other, ethane can be formed as illustrated in **Figure 6** [27].

and CO2

, vibrational excited and electron excited CO2

depending on how much energy is transferred from electron. On the other hand, CO2

, CO and other

can

are more

(x = 1–3)

can be dissociated into various radicals,

is difficult to take place in nonthermal plasma due to

(methylene) are formed with different lev-

possess higher energy, thus, energy required to generate

to enhance their dissociation

, CHx
