**2.1 CO2 dissociation chemistry**

As mentioned in Introduction, nonthermal plasma shows a great potential for an efficient CO2 utilization. Different routes for CO2 conversion have been investigated using plasma-catalytic process. **Table 2** summarizes some of the main reactions


**Table 2.**

*Chemical reactions related to CO2 reduction and their enthalpies.*

usually considered in plasma chemistry for CO2 reduction using different pathways (such as dry reforming of methane, hydrogenation of CO2). Significant attention has been given to plasma-catalytic dry reforming of methane (DRM) using supported Ni catalysts. However, most of these studies focused primarily on identifying plasmacatalytic chemical reactions to maximize process performance. Optical emission spectroscopy and plasma chemical kinetic modeling should be used to achieve a better understanding on the formation of a wide range of reactive species in this plasmacatalytic reforming process. Recently, Chung et al. had described the mechanisms of catalysis promotion, elucidated the synergistic effects between catalyst and plasma and proposed possible approaches to optimize DRM process performance [2]. As explained by Fridman [3], cumulative vibrational excitations of the CO2 molecule can result in a highly energy-efficient stepwise dissociation. Thus, CO2 splitting using nonthermal plasmas has been considered as another promising pathway to produce synthetic fuels via CO, as an intermediate product. As well-accepted in the literature, dissociation of a CO2 molecule in plasma is represented by the following global reaction [3]:

$$\text{CO}\_2 \rightarrow \text{CO} + \frac{1}{2}\text{O}\_2\\\Delta\text{H} = 2.9\,\text{eV/molecule}\tag{1}$$

The main pathways for decomposition of CO2 molecule include the electron impact dissociation:

$$\text{CO}\_2 \rightarrow \text{CO} + \text{O}, \Delta\text{H} \ = \text{ 5.5 eV/molecule} \tag{2}$$

which is often accompanied by the further recombination of atomic O:

$$\text{M} \star \text{O} \star \text{O} \rightarrow \text{O}\_2 \star \text{M} \text{ (M is a particle)}\tag{3}$$

In addition to this, the vibrationally excited CO2 molecules may also undergo decomposition via the collisions with atomic O:

$$\text{O} \star \text{CO}\_2^{\text{virr}} \rightarrow \text{CO} \star \text{O}\_2 \Delta \text{H} \ = \text{ 0.3 eV/molecule} \tag{4}$$

as well as with the plasma electrons:

$$\mathbf{e} \star \mathbf{CO}\_2 \overset{\text{vibr}}{\rightarrow} \mathbf{CO} + \frac{1}{2} \mathbf{O}\_2 \text{ (the energy required is } \ll \mathbf{1 eV)}\tag{5}$$

Traditionally, to characterize the process efficiency, two main parameters reflecting the *conversion* efficiency and *energy* efficiency are used. The conversion efficiency (χ) and energy efficiency (η) of CO2 are defined as follows: <sup>χ</sup> <sup>=</sup> moles of CO2 input <sup>−</sup> moles of CO2 output \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ moles of CO2 input (6)

$$\chi = \frac{\text{moles of CO}\_2 \text{ input - moles of CO}\_2 \text{ output}}{\text{moles of CO}\_2 \text{ input}} \tag{6}$$

$$
\eta\_{\perp} = \frac{\chi \ast 2.9 \text{ eV}}{\text{SEI}} \tag{7}
$$

**63**

stage system.

**Figure 2.**

*number.*

*Progress in Plasma-Assisted Catalysis for Carbon Dioxide Reduction*

the catalyst [59, 60]. These three configurations are illustrated in **Figure 2**. In all cases, the plasma can be used to supply energy for catalyst activation, and it can also provide the reactive gas species needed for reactions on the catalyst surface. The single-stage type is constructed by coating catalyst on the surface of electrode(s) or packing catalyst within the plasma zone, which is also called in-plasma catalysis (IPC). The catalysts could completely or just partially overlap with the plasma zone. In this manner, the plasma and catalysis could directly interact with each other. This single-stage system is also easy to combine with the UV irradiation, which is known as plasma photo catalysis, as shown in **Figure 2**. For the two-stage type, the catalyst is placed after the plasma discharge region; it is also called post-plasma catalysis (PPC). The plasma provides chemically reactive species for catalysis or pre-converts reactants into the easier-to-convert products to accelerate the catalysis. In the nonthermal plasma catalysis system, the long-lived reactive species produced by plasma, e.g. vibration-excited species, radicals, and ionized molecules, can react with the catalyst to induce catalytic reactions via either the Eley-Rideal mechanism or Langmuir-Hinshelwood mechanism [2, 59]. The multistage plasma catalysis system is a promising option for the industrial use in the future. Different functions of the catalysts can be combined to achieve certain expected reaction in the multi-

*Schematic diagram of different plasma-catalyst configurations according to the catalyst bed position and* 

In the context of plasma catalysis, the synergy is referring to a surplus effect of combining the plasma with catalyst, namely, when the resulting effect has a higher

*DOI: http://dx.doi.org/10.5772/intechopen.80798*

Here the specific energy input (SEI) per molecule is given by the ratio of the discharge power (P) to the gas flow rate (F) through the discharge volume.

#### **2.2 Plasma catalysis**

When catalysts are combined with plasmas, they can be classified into three systems, i.e. single stage, two stage, and multistage, depending on the location of *Progress in Plasma-Assisted Catalysis for Carbon Dioxide Reduction DOI: http://dx.doi.org/10.5772/intechopen.80798*

#### **Figure 2.**

*Plasma Chemistry and Gas Conversion*

CO2 → CO + \_1

decomposition via the collisions with atomic O:

vibr → CO + \_1

η = \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_

as well as with the plasma electrons:

O + CO2

e + CO2

**2.2 Plasma catalysis**

impact dissociation:

usually considered in plasma chemistry for CO2 reduction using different pathways (such as dry reforming of methane, hydrogenation of CO2). Significant attention has been given to plasma-catalytic dry reforming of methane (DRM) using supported Ni catalysts. However, most of these studies focused primarily on identifying plasmacatalytic chemical reactions to maximize process performance. Optical emission spectroscopy and plasma chemical kinetic modeling should be used to achieve a better understanding on the formation of a wide range of reactive species in this plasmacatalytic reforming process. Recently, Chung et al. had described the mechanisms of catalysis promotion, elucidated the synergistic effects between catalyst and plasma and proposed possible approaches to optimize DRM process performance [2]. As explained by Fridman [3], cumulative vibrational excitations of the CO2 molecule can result in a highly energy-efficient stepwise dissociation. Thus, CO2 splitting using nonthermal plasmas has been considered as another promising pathway to produce synthetic fuels via CO, as an intermediate product. As well-accepted in the literature, dissociation of a

CO2 molecule in plasma is represented by the following global reaction [3]:

The main pathways for decomposition of CO2 molecule include the electron

CO2 → CO + O,∆H = 5.5 eV/molecule (2)

M + O + O → O2 + M (M is a particle) (3)

In addition to this, the vibrationally excited CO2 molecules may also undergo

Traditionally, to characterize the process efficiency, two main parameters reflecting the *conversion* efficiency and *energy* efficiency are used. The conversion

<sup>χ</sup> <sup>=</sup> moles of CO2 input <sup>−</sup> moles of CO2 output \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ moles of CO2 input (6)

χ ∗ 2.9 eV

Here the specific energy input (SEI) per molecule is given by the ratio of the

When catalysts are combined with plasmas, they can be classified into three systems, i.e. single stage, two stage, and multistage, depending on the location of

discharge power (P) to the gas flow rate (F) through the discharge volume.

efficiency (χ) and energy efficiency (η) of CO2 are defined as follows:

which is often accompanied by the further recombination of atomic O:

<sup>2</sup> O2,∆H <sup>=</sup> 2.9 eV/molecule (1)

vibr → CO + O2,∆H = 0.3 eV/molecule (4)

<sup>2</sup> O2 (the energy required is << 1 eV) (5)

SEI (7)

**62**

*Schematic diagram of different plasma-catalyst configurations according to the catalyst bed position and number.*

the catalyst [59, 60]. These three configurations are illustrated in **Figure 2**. In all cases, the plasma can be used to supply energy for catalyst activation, and it can also provide the reactive gas species needed for reactions on the catalyst surface. The single-stage type is constructed by coating catalyst on the surface of electrode(s) or packing catalyst within the plasma zone, which is also called in-plasma catalysis (IPC). The catalysts could completely or just partially overlap with the plasma zone. In this manner, the plasma and catalysis could directly interact with each other. This single-stage system is also easy to combine with the UV irradiation, which is known as plasma photo catalysis, as shown in **Figure 2**. For the two-stage type, the catalyst is placed after the plasma discharge region; it is also called post-plasma catalysis (PPC). The plasma provides chemically reactive species for catalysis or pre-converts reactants into the easier-to-convert products to accelerate the catalysis. In the nonthermal plasma catalysis system, the long-lived reactive species produced by plasma, e.g. vibration-excited species, radicals, and ionized molecules, can react with the catalyst to induce catalytic reactions via either the Eley-Rideal mechanism or Langmuir-Hinshelwood mechanism [2, 59]. The multistage plasma catalysis system is a promising option for the industrial use in the future. Different functions of the catalysts can be combined to achieve certain expected reaction in the multistage system.

In the context of plasma catalysis, the synergy is referring to a surplus effect of combining the plasma with catalyst, namely, when the resulting effect has a higher

#### **Figure 3.** *Interaction between catalyst and plasma.*

impact than the sum of their individual impacts. In several studies, the combination of plasma and catalysts has been found to have synergistic effects [34, 35, 61, 62]. A highly important synergistic effect of plasma catalysis is promotion of catalyst activity at reduced temperatures, and hence, a significant reduction in the energy cost for activating the catalyst [34]. For example, Wang et al. illustrated such synergy for plasma catalysis of dry reforming methane (DRM) in the single-stage system with Ni/Al2O3 catalyst but did not observe this synergy in the two-stage system or when the catalyst is only placed at the end of the plasma zone [62]. Typical synergistic effect factors of 1.25–1.5 were obtained. Zhang et al. presented the results on the plasma-catalyst synergy in the case of dry reforming methane using different Cu-Ni/γ-Al2O3 catalysts [63]. The effect was observed on the conversions of CH4 and CO2, where the result for the plasma-catalytic reaction was greater than the sum of the catalyst-only or plasma-only results. The selectivity towards H2 and CO production was also enhanced by the use of plasma catalysis. In general, the enhanced performance of plasma catalysis can in part be attributed to vibrational excitation of CO2 in the plasma, which enables easier dissociation at low temperature on the catalyst surface. The plasma electrons in turn affect the catalyst properties (chemical composition or catalytic structure). Synergistic effects in the plasma and catalyst are illustrated in **Figure 3**. Plasma can alter the physicochemical characteristics of catalyst via several routes, which are induced mainly by energetic electron generation. In the meantime, a catalyst can induce electric field concentration due to its pore structure and dielectric properties. Hence, both electric field distribution and catalyst characteristics are modified to have better DRM performance.
