**3. Plasma-assisted catalytic conversion of CO2**

Nonthermal plasma technology provides an attractive alternative to the other (classical) technologies for converting inert carbon emissions. Different types of plasmas have already been used for CO2 reduction, including dielectric barrier discharges (DBDs), glow discharges, radio frequency (RF) discharges, microwave (MW) discharges and gliding arc plasma (GAP) and corona discharges [8–39]. In this section, the most widely used discharges for CO2 conversion are presented.

**65**

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

DBDs have been known for more than a century. They were first reported in 1857 by Siemens for the use in ozone production and were originally called 'silent' discharges [64]. The DBD is the most widely used discharge type for CO2 conversion among the variety of other plasma sources because it is easy to handle with relatively cheap equipment and it operates at atmospheric pressure [34]. Even though the conversion efficiencies obtained in DBDs are generally quite low [1, 4, 5, 8, 9, 12, 13], the possibility to work at atmospheric pressure under non-equilibrium conditions is still a very strong advantage of these discharges. Combined with plasma catalysis, these discharges should also improve the selective production of the targeted compounds. An atmospheric pressure GAP discharge can be formed between two flat knifeshaped electrodes with a gas flowing between them. These discharges are suitable for applications that require relatively large gas flows (several l/min). The gliding arc plasma can be operated in the thermal and nonthermal regime depending on the applied power and flow rate. Furthermore, the arc can be operated in the transition regime, which is an evolving arc starting in the thermal regime going to the nonthermal regime. This transition regime makes the discharge energy efficient for gas treatment. An energy efficiency of 43% was reported by Nunnally et al. for the decomposition of CO2 in a reverse vortex flow gliding arc discharge, which is quite high compared to the efficiency obtained with DBDs (about 10%) [31]. The high level of efficiency can be attributed to non-equilibrium vibrational excitation of CO2 and a high-temperature gradient between the gliding arc and the surrounding gas that results in fast quenching. Plasmas generated by the injection of microwave power, i.e. electromagnetic radiation in the frequency range of 100 MHz–10 GHz, are called MW plasmas [65]. MW discharges are commonly generated using frequencies of 2.45 and 0.915 GHz. They can be operated over a wide pressure range (from few mTorr to the atmospheric pressure). The properties of the MW discharges operating at atmospheric pressure are close to those of thermal plasma. However, the MW discharges are far from thermodynamic equilibrium at low pressure. The performance of a microwave discharge in terms of efficiency of CO2 dissociation process depends heavily on the plasma parameters such as power and operating pressure. The highest energy efficiency (about 90%) for pure CO2 conversion was reported in a MW plasma operating with supersonic gas flows [22]. The ability to create a strong non-equilibrium environment in microwave discharges possesses highly vibrational states of CO2 molecules, which are energy-efficient for CO2 decomposition [3]. In general, the high efficiency of microwave plasmas is attained due to the high absorption of the applied power by electrons as well as relatively high excitation of the CO2 asymmetric mode [24], which plays a key role for CO2 decomposition [22]. In the low-pressure case, the microwave plasmas are typically characterized by an electron temperature around 1–2 eV and a gas temperature below 1500 K. Under these conditions, it has been estimated that about 95% of all the discharge energy is transferred from the plasma electrons to the CO2 molecules, mostly to their asymmetric vibrational mode [3, 24]. Bogaerts et al. has presented some insights into how the electron energy is transferred to different channels of excitation, ionization or dissociation of the CO2 molecules [1, 66]. **Figure 4** illustrates the fractional energy transferred from electrons to different channels of excitation, ionization and dissociation of CO2, as a function of the reduced electric field (E/n) in a discharge. This plot is calculated based on the cross sections of the corresponding electron impact reactions [1, 37, 66]. In microwave plasma, the reduced electric field is typically around 50 Td, which is most appropriate for the vibrational excitation of CO2. Fridman has shown that up to 97% of the total nonthermal discharge energy can be transferred from the plasma electrons to vibrational excitation of CO2 molecules at an electron temperature around 1–2 eV or a reduced electric field (E/n) of about 20–40 Td [3, 36]. This is indeed indicated by the calculated curve referred to as the 'sum of all vibrations'

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

#### *Progress in Plasma-Assisted Catalysis for Carbon Dioxide Reduction DOI: http://dx.doi.org/10.5772/intechopen.80798*

*Plasma Chemistry and Gas Conversion*

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.

Nonthermal plasma technology provides an attractive alternative to the other (classical) technologies for converting inert carbon emissions. Different types of plasmas have already been used for CO2 reduction, including dielectric barrier discharges (DBDs), glow discharges, radio frequency (RF) discharges, microwave (MW) discharges and gliding arc plasma (GAP) and corona discharges [8–39]. In this section, the most widely used discharges for CO2 conversion are presented.

**3. Plasma-assisted catalytic conversion of CO2**

**64**

**Figure 3.**

*Interaction between catalyst and plasma.*

DBDs have been known for more than a century. They were first reported in 1857 by Siemens for the use in ozone production and were originally called 'silent' discharges [64]. The DBD is the most widely used discharge type for CO2 conversion among the variety of other plasma sources because it is easy to handle with relatively cheap equipment and it operates at atmospheric pressure [34]. Even though the conversion efficiencies obtained in DBDs are generally quite low [1, 4, 5, 8, 9, 12, 13], the possibility to work at atmospheric pressure under non-equilibrium conditions is still a very strong advantage of these discharges. Combined with plasma catalysis, these discharges should also improve the selective production of the targeted compounds.

An atmospheric pressure GAP discharge can be formed between two flat knifeshaped electrodes with a gas flowing between them. These discharges are suitable for applications that require relatively large gas flows (several l/min). The gliding arc plasma can be operated in the thermal and nonthermal regime depending on the applied power and flow rate. Furthermore, the arc can be operated in the transition regime, which is an evolving arc starting in the thermal regime going to the nonthermal regime. This transition regime makes the discharge energy efficient for gas treatment. An energy efficiency of 43% was reported by Nunnally et al. for the decomposition of CO2 in a reverse vortex flow gliding arc discharge, which is quite high compared to the efficiency obtained with DBDs (about 10%) [31]. The high level of efficiency can be attributed to non-equilibrium vibrational excitation of CO2 and a high-temperature gradient between the gliding arc and the surrounding gas that results in fast quenching.

Plasmas generated by the injection of microwave power, i.e. electromagnetic radiation in the frequency range of 100 MHz–10 GHz, are called MW plasmas [65]. MW discharges are commonly generated using frequencies of 2.45 and 0.915 GHz. They can be operated over a wide pressure range (from few mTorr to the atmospheric pressure). The properties of the MW discharges operating at atmospheric pressure are close to those of thermal plasma. However, the MW discharges are far from thermodynamic equilibrium at low pressure. The performance of a microwave discharge in terms of efficiency of CO2 dissociation process depends heavily on the plasma parameters such as power and operating pressure. The highest energy efficiency (about 90%) for pure CO2 conversion was reported in a MW plasma operating with supersonic gas flows [22]. The ability to create a strong non-equilibrium environment in microwave discharges possesses highly vibrational states of CO2 molecules, which are energy-efficient for CO2 decomposition [3]. In general, the high efficiency of microwave plasmas is attained due to the high absorption of the applied power by electrons as well as relatively high excitation of the CO2 asymmetric mode [24], which plays a key role for CO2 decomposition [22]. In the low-pressure case, the microwave plasmas are typically characterized by an electron temperature around 1–2 eV and a gas temperature below 1500 K. Under these conditions, it has been estimated that about 95% of all the discharge energy is transferred from the plasma electrons to the CO2 molecules, mostly to their asymmetric vibrational mode [3, 24].

Bogaerts et al. has presented some insights into how the electron energy is transferred to different channels of excitation, ionization or dissociation of the CO2 molecules [1, 66]. **Figure 4** illustrates the fractional energy transferred from electrons to different channels of excitation, ionization and dissociation of CO2, as a function of the reduced electric field (E/n) in a discharge. This plot is calculated based on the cross sections of the corresponding electron impact reactions [1, 37, 66]. In microwave plasma, the reduced electric field is typically around 50 Td, which is most appropriate for the vibrational excitation of CO2. Fridman has shown that up to 97% of the total nonthermal discharge energy can be transferred from the plasma electrons to vibrational excitation of CO2 molecules at an electron temperature around 1–2 eV or a reduced electric field (E/n) of about 20–40 Td [3, 36]. This is indeed indicated by the calculated curve referred to as the 'sum of all vibrations'

**Figure 4.** *The fraction of electron energy transferred to different channels of excitation as a function of the reduced electric field (E/n) (adapted from [1]).*

shown in **Figure 4**. Moreover, the purple curve in **Figure 4** has its particular importance as it represents the first vibrational level of the asymmetric vibrational mode of CO2, which represents the most important channel for the dissociation [66]. The energy efficiency for the dissociation of CO2 is quite limited in a DBD plasma [3, 4, 11–13]. The electron temperature in a DBD is about 2–3 eV, which is somewhat high for efficient population of the CO2 vibrational levels. The reduced electric field values are being typically about 200 Td or even higher, indicated as 'DBD region' in the figure. As a result of previous studies on CO2 decomposition in plasma, it was concluded that higher pressures and lower values of reduced electric field make the vibrational excitation mechanism more favorable than the electronic excitation mechanism, explaining the higher energy efficiency of these types of discharges (e.g. MW, GAP) [1, 3, 22, 26, 28, 32, 33, 35, 37, 38].

#### **3.1 MW region**

In this chapter we have summarized the results from the recent publications on plasma set-ups with and without combining a catalyst for CO2 conversion in **Table 2** and discussed the current research status on this topic. Porous Al2O3 (α-Al2O3 and γ-Al2O3) has been investigated in a pulsed corona discharge reactor for CO2 conversion by Wen et al. [39]. γ-Al2O3 was found to enhance CO2 conversion due to its high surface area and strong adsorption capability. Zhang et al. investigated CO2 decomposition to CO and O2 in a DBD reactor packed with a mixture of Ni/SiO2 catalyst and BaTiO3 spheres. In comparison to the reaction in the absence of a Ni/SiO2 catalyst, introducing a Ni/SiO2 catalyst to the plasma reactor packed with BaTiO3 spheres slightly increase the CO2 conversion from 19 to 23.5% at low temperatures [17]. Van Laer demonstrated a packing of ZrO2 beads in a DBD reactor. The best combination of conversion (37.8%) and energy

**67**

**Figure 5.**

*Schematic mechanism of plasma-assisted catalytic process for CO2 conversion.*

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

(6.4%) efficiency was reached at a flow rate of 20 mL min<sup>−</sup><sup>1</sup>

of 60 W [16]. Their simulation results suggest that the increased CO2 conversion is caused by the presence of strong electric fields and thus high electron energies at the contact points, which thereby lowers the breakdown voltage. These findings suggest that the interactions between plasma and packing materials play an important role in the plasma conversion of CO2. Brock et al. studied the catalytic effect of metallic coating on the decomposition of CO2 in fan-type AC glow discharge plasma reactors, using a gas mixture of 2.5% CO2 in He [19]. They showed that an Rh-coated reactor has the highest activity for the CO2 decomposition compared to the reactors coated with Cu, Au, Pt and Pd and mixed rotor/

In relation to microwave plasmas, Chen et al. reported that placing a NiO/ TiO2 catalyst in the downstream of a low-pressure microwave plasma significantly increased the CO2 conversion efficiency and energy efficiency [25, 28]. They concluded that the oxygen vacancies provide the sites for adsorption of oxygen atoms from CO2. The energetic electrons supplied by the plasma enhance the dissociative electron attachment of CO2 at the surface. Recently, Ray et al. found that CO2 conversion was enhanced upon packing CeO2 into the discharge region of a DBD reactor. They also suggest this enhancement can be mainly attributed to the formation of oxygen vacancy defects on the surface of CeO2, to stabilize the produced atomic oxygen, thereby preventing the revise reaction [18]. Spencer et al. experimentally investigated the conversion of CO2 in an atmospheric pressure microwave plasma-catalytic system [21]. The results showed that Rh/TiO2 coating on a monolithic cordierite structure used as a catalyst actually caused a drop in conversion efficiency due to reverse reactions occurring on the surface. Mei et al. demonstrated that the combination of plasma with BaTiO3 and TiO2 catalysts has a synergistic effect, which significantly enhances the conversion of CO2 and the energy efficiency by a factor of 2.5 compared to the plasma reaction in the absence of a catalyst [7]. The overall synergistic effect resulting from the integration of DBD with catalysis for CO2 conversion can be attributed to the dominant catalytic surface reaction driven by energetic electrons from the CO2 discharge. Theoretical and experimental studies consistently showed that the CO2 adsorption, activation and dissociation processes were significantly enhanced by the presence of oxygen vacancies [7, 23, 28, 67, 68]. The mechanism of plasma-catalytic CO2 conversion can be described by **Figure 5**. The oxygen vacancies provide sites for the adsorption of

and an input power

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

stator systems (Rh/Au and Au/Rh).

*Progress in Plasma-Assisted Catalysis for Carbon Dioxide Reduction DOI: http://dx.doi.org/10.5772/intechopen.80798*

*Plasma Chemistry and Gas Conversion*

shown in **Figure 4**. Moreover, the purple curve in **Figure 4** has its particular importance as it represents the first vibrational level of the asymmetric vibrational mode of CO2, which represents the most important channel for the dissociation [66]. The energy efficiency for the dissociation of CO2 is quite limited in a DBD plasma [3, 4, 11–13]. The electron temperature in a DBD is about 2–3 eV, which is somewhat high for efficient population of the CO2 vibrational levels. The reduced electric field values are being typically about 200 Td or even higher, indicated as 'DBD region' in the figure. As a result of previous studies on CO2 decomposition in plasma, it was concluded that higher pressures and lower values of reduced electric field make the vibrational excitation mechanism more favorable than the electronic excitation mechanism, explaining the higher energy efficiency of these types of discharges

*The fraction of electron energy transferred to different channels of excitation as a function of the reduced* 

In this chapter we have summarized the results from the recent publications on plasma set-ups with and without combining a catalyst for CO2 conversion in **Table 2** and discussed the current research status on this topic. Porous Al2O3 (α-Al2O3 and γ-Al2O3) has been investigated in a pulsed corona discharge reactor for CO2 conversion by Wen et al. [39]. γ-Al2O3 was found to enhance CO2 conversion due to its high surface area and strong adsorption capability. Zhang et al. investigated CO2 decomposition to CO and O2 in a DBD reactor packed with a mixture of Ni/SiO2 catalyst and BaTiO3 spheres. In comparison to the reaction in the absence of a Ni/SiO2 catalyst, introducing a Ni/SiO2 catalyst to the plasma reactor packed with BaTiO3 spheres slightly increase the CO2 conversion from 19 to 23.5% at low temperatures [17]. Van Laer demonstrated a packing of ZrO2 beads in a DBD reactor. The best combination of conversion (37.8%) and energy

(e.g. MW, GAP) [1, 3, 22, 26, 28, 32, 33, 35, 37, 38].

**66**

**3.1 MW region**

**Figure 4.**

*electric field (E/n) (adapted from [1]).*

(6.4%) efficiency was reached at a flow rate of 20 mL min<sup>−</sup><sup>1</sup> and an input power of 60 W [16]. Their simulation results suggest that the increased CO2 conversion is caused by the presence of strong electric fields and thus high electron energies at the contact points, which thereby lowers the breakdown voltage. These findings suggest that the interactions between plasma and packing materials play an important role in the plasma conversion of CO2. Brock et al. studied the catalytic effect of metallic coating on the decomposition of CO2 in fan-type AC glow discharge plasma reactors, using a gas mixture of 2.5% CO2 in He [19]. They showed that an Rh-coated reactor has the highest activity for the CO2 decomposition compared to the reactors coated with Cu, Au, Pt and Pd and mixed rotor/ stator systems (Rh/Au and Au/Rh).

In relation to microwave plasmas, Chen et al. reported that placing a NiO/ TiO2 catalyst in the downstream of a low-pressure microwave plasma significantly increased the CO2 conversion efficiency and energy efficiency [25, 28]. They concluded that the oxygen vacancies provide the sites for adsorption of oxygen atoms from CO2. The energetic electrons supplied by the plasma enhance the dissociative electron attachment of CO2 at the surface. Recently, Ray et al. found that CO2 conversion was enhanced upon packing CeO2 into the discharge region of a DBD reactor. They also suggest this enhancement can be mainly attributed to the formation of oxygen vacancy defects on the surface of CeO2, to stabilize the produced atomic oxygen, thereby preventing the revise reaction [18]. Spencer et al. experimentally investigated the conversion of CO2 in an atmospheric pressure microwave plasma-catalytic system [21]. The results showed that Rh/TiO2 coating on a monolithic cordierite structure used as a catalyst actually caused a drop in conversion efficiency due to reverse reactions occurring on the surface. Mei et al. demonstrated that the combination of plasma with BaTiO3 and TiO2 catalysts has a synergistic effect, which significantly enhances the conversion of CO2 and the energy efficiency by a factor of 2.5 compared to the plasma reaction in the absence of a catalyst [7]. The overall synergistic effect resulting from the integration of DBD with catalysis for CO2 conversion can be attributed to the dominant catalytic surface reaction driven by energetic electrons from the CO2 discharge. Theoretical and experimental studies consistently showed that the CO2 adsorption, activation and dissociation processes were significantly enhanced by the presence of oxygen vacancies [7, 23, 28, 67, 68]. The mechanism of plasma-catalytic CO2 conversion can be described by **Figure 5**. The oxygen vacancies provide sites for the adsorption of

**Figure 5.** *Schematic mechanism of plasma-assisted catalytic process for CO2 conversion.*

#### *Plasma Chemistry and Gas Conversion*


#### **Table 3.**

*Summary of the plasma-assisted catalytic CO2 conversion for different discharge types.*

oxygen atoms from CO2. The energetic electrons supplied by the plasma enhance the dissociative electron attachment of CO2 at the surface. Subsequently, CO desorbs or moves from the reactive site while the other O (bridging) atom 'heals' the oxygen vacancy. The oxygen vacancy can be regenerated via the recombination

**69**

conversion in the industry.

No. 7267 (for GC, TG), should be acknowledged.

**Acknowledgements**

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

chemical effects of the catalyst are highly needed (**Table 3**).

on the surface of a bridging oxygen atom with a gaseous oxygen atom. Such regeneration maintains the equilibrium of the active sites in the catalyst and controls the CO2 conversion [23]. If the catalyst is placed in the plasma zone (single stage), the electron–hole pairs can be created by highly energetic electrons from the discharge upon the surface of photocatalysts once plasma can generate electrons of very similar energy (3–4 eV) to the photons. In this case, oxygen vacancy can be regener-

Plasma-catalytic conversion of CO2 is a complex and challenging process involving a large number of physical and chemical reactions. The performance of the process is controlled by means of plasma parameters and the properties of the catalysts as well. This suggests that more systematic studies on both the plasma effects and the

Plasma-assisted catalytic processes used for CO2 reduction are gaining increasing interest worldwide. There is still a room, however, for further improvement of the CO2 conversion and energy efficiencies through the optimization of the plasma parameters (e.g. high pressure and high flow rate) as well as through modification

The plasma-catalytic activities can be controlled by numerous factors such as the nature of the catalyst support, active metal sites, surface area and the nanoparticle size. Let us note that the catalyst preparation (sometime called 'activation') plays a very important role in this regard. In addition to these factors and also due to their existence, the fine-tuning of a given catalyst is inevitable and crucial factor for enhancing plasma-catalytic process efficiency. Several methods, such as loading different metal nanoparticles, using different catalyst preparation schemes (sol gel, co-precipitation, deposition-precipitation or hydrothermal synthesis), using larger surface area of the support, etc., can be mentioned to realize the mentioned tuning. An important factor which cannot be omitted here is that a chosen catalyst material should have rather low costs to be potentially commercialized and implemented in the industrial scale. Moreover, as a result of recent development of the microwave discharges, namely, a possibility to place catalyst packing directly in the discharge zone can be a powerful way to take advantage of the stepwise vibrational excitation on the catalyst surface. In addition, using plasma as a tool for the preparation (activation) of the catalyst surface may be another promising way. To improve its application, a better insight into the underlying mechanisms of the plasma catalysis is desirable. A greater understanding of the plasma chemistry, both by plasma modeling and by coupling with other techniques such as catalysis and membrane materials, will allow this field to expand. We expect that the results presented in this chapter will provide useful insights into the plasma-assisted CO2 conversion in the presence or the absence of catalysts, which may be used for greenhouse gas

The authors acknowledge financial support from the network on the Physical Chemistry of Plasma-Surface Interactions—Interuniversity Attraction Poles phase VII project (http://psi-iap7.ulb.ac.be/), supported by the Belgian Federal Office for Science Policy (BELSPO). The support of the 'REFORGAS GreenWin' project, grant

<sup>−</sup> anions using holes, followed by releasing O2 [7].

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

ated by oxidizing the surface O2

**4. Conclusions and perspectives**

of catalysts.

*Progress in Plasma-Assisted Catalysis for Carbon Dioxide Reduction DOI: http://dx.doi.org/10.5772/intechopen.80798*

on the surface of a bridging oxygen atom with a gaseous oxygen atom. Such regeneration maintains the equilibrium of the active sites in the catalyst and controls the CO2 conversion [23]. If the catalyst is placed in the plasma zone (single stage), the electron–hole pairs can be created by highly energetic electrons from the discharge upon the surface of photocatalysts once plasma can generate electrons of very similar energy (3–4 eV) to the photons. In this case, oxygen vacancy can be regenerated by oxidizing the surface O2 <sup>−</sup> anions using holes, followed by releasing O2 [7]. Plasma-catalytic conversion of CO2 is a complex and challenging process involving a large number of physical and chemical reactions. The performance of the process is controlled by means of plasma parameters and the properties of the catalysts as well. This suggests that more systematic studies on both the plasma effects and the chemical effects of the catalyst are highly needed (**Table 3**).

## **4. Conclusions and perspectives**

*Plasma Chemistry and Gas Conversion*

DBD 10% CO2 in the

MW Supersonic

Gliding arc

Gliding arc

Gliding arc

**Table 3.**

flow

gas mixture

DBD CO2 Ni/

**Comments Gas** 

**mixture**

CO2- H2O-Ar **Catalyst χ**

DBD CO2 — 17 9 5.8 [4] DBD CO2 — 30 1 87 [5] DBD CO2 γ-Al2O3 20 4.9 12 [6] DBD CO2 BaTiO3 38 17 6.5 [7] DBD CO2 — 18 4 13 [8] DBD Low flow rate CO2 — 14 8 5.2 [9]

DBD CO2 — 35 2 50.8 [11] DBD CO2 — 28.2 11.1 7.4 [12] DBD CO2-N2 — 4.5 4.5 2.9 [13] DBD CO2 CaO 39.2 7.1 16 [14] DBD CO2 — 20 10.4 5.6 [15] DBD CO2 ZrO2 2.9 9.6 9.6 [16]

SiO2 + BaTiO3

DBD CO2 CeO2 (2 mm) 10.6 27.6 1.11 [18] DBD CO2 TiO2 (3–4 mm) 8.2 15.54 1.53 [18] Glow CO2-Ar Rh-coated 30 1.4 62 [19] RF CO2 — 20 3 19 [20] MW CO2-Ar — 10 20 1.4 [21]

MW CO2 NiO/TiO2 42 18 7.0 [23] MW CO2-N2 — 80 6 39 [24] MW CO2 — 20 20 2.9 [25] MW CO2 — 12 45 0.8 [26] MW CO2:H2O = 1:1 CO2-H2O — 12 8.7 4 [27] MW CO2 NiO/TiO2 45 56 2.3 [28] MW CO2 — [29] Corona CO2 — 11 2 16 [30]

**(%)**

**η (%)**

Ni/γ-Al2O3 36 23 4.5 [10]

CO2 — 10 90 0.3 [22]

CO2 — 4.6 43 0.3 [31]

CO2 — 15 19 2.3 [32]

CO2 — 10 34 0.85 [33]

23.5 2.31 29.5 [17]

**SEI eV/ molecule** **Ref.**

**Plasma type**

**68**

oxygen atoms from CO2. The energetic electrons supplied by the plasma enhance the dissociative electron attachment of CO2 at the surface. Subsequently, CO desorbs or moves from the reactive site while the other O (bridging) atom 'heals' the oxygen vacancy. The oxygen vacancy can be regenerated via the recombination

*Summary of the plasma-assisted catalytic CO2 conversion for different discharge types.*

Plasma-assisted catalytic processes used for CO2 reduction are gaining increasing interest worldwide. There is still a room, however, for further improvement of the CO2 conversion and energy efficiencies through the optimization of the plasma parameters (e.g. high pressure and high flow rate) as well as through modification of catalysts.

The plasma-catalytic activities can be controlled by numerous factors such as the nature of the catalyst support, active metal sites, surface area and the nanoparticle size. Let us note that the catalyst preparation (sometime called 'activation') plays a very important role in this regard. In addition to these factors and also due to their existence, the fine-tuning of a given catalyst is inevitable and crucial factor for enhancing plasma-catalytic process efficiency. Several methods, such as loading different metal nanoparticles, using different catalyst preparation schemes (sol gel, co-precipitation, deposition-precipitation or hydrothermal synthesis), using larger surface area of the support, etc., can be mentioned to realize the mentioned tuning.

An important factor which cannot be omitted here is that a chosen catalyst material should have rather low costs to be potentially commercialized and implemented in the industrial scale. Moreover, as a result of recent development of the microwave discharges, namely, a possibility to place catalyst packing directly in the discharge zone can be a powerful way to take advantage of the stepwise vibrational excitation on the catalyst surface. In addition, using plasma as a tool for the preparation (activation) of the catalyst surface may be another promising way. To improve its application, a better insight into the underlying mechanisms of the plasma catalysis is desirable. A greater understanding of the plasma chemistry, both by plasma modeling and by coupling with other techniques such as catalysis and membrane materials, will allow this field to expand. We expect that the results presented in this chapter will provide useful insights into the plasma-assisted CO2 conversion in the presence or the absence of catalysts, which may be used for greenhouse gas conversion in the industry.

### **Acknowledgements**

The authors acknowledge financial support from the network on the Physical Chemistry of Plasma-Surface Interactions—Interuniversity Attraction Poles phase VII project (http://psi-iap7.ulb.ac.be/), supported by the Belgian Federal Office for Science Policy (BELSPO). The support of the 'REFORGAS GreenWin' project, grant No. 7267 (for GC, TG), should be acknowledged.

*Plasma Chemistry and Gas Conversion*
