**Abstract**

Production of chemicals and fuels based on CO2 conversion is attracting a special attention nowadays, especially regarding the fast depletion of fossil resources and increase of CO2 emissions into the Earth's atmosphere. Recently, plasma technology has gained increasing interest as a non-equilibrium medium suitable for CO2 conversion, which provides a promising alternative to the conventional pathway for greenhouse gas conversion. The combination of plasma and catalysis is of great interest for turning plasma chemistry in applications related to pollution and energy issues. In this chapter a short review of the current progress in plasmaassisted catalytic processes for CO2 reduction is given. The most widely used discharges for CO2 conversion are presented and briefly discussed, illustrating how to achieve a better energy and conversion efficiency. The chapter includes the recent status and advances of the most promising candidates (plasma catalysis) to obtain efficient CO2 conversion, along with the future outlook of this plasma-assisted catalytic process for further improvement.

**Keywords:** green energy, plasma-based CO2 conversion, plasma catalysis, oxygen vacancies, synergistic effect

## **1. Introduction**

The utilization of CO2 for production of fuels, energy storage media, chemicals or aggregates is attracting interest worldwide due to the essential contribution of the greenhouse gases to the global warming. CO2 capture and utilization are considered as a promising option for the mitigation of CO2 emissions, which provides a lower carbon footprint for the synthesis of value-added products than those produced by conventional processes using fossil fuels. In spite of the continuously increasing interest for CO2 recycling, there are significant challenges to overcome due to its stable molecular structure and low chemical activity. There are several methods that can be used to convert CO2, including traditional catalysis, photochemical, biochemical, solar thermochemical, electrochemical and plasma chemical. Snoeckx and Bogaerts recently made a detailed comparison of these technologies as shown in **Table 1** [1]. They concluded that the plasma technology fares very well in this comparison and is quite promising. Indeed, nonthermal plasma has attracted much attention of the scientific community as a non-equilibrium medium suitable for CO2


*a Bio- and photochemical processes can also rely on indirect renewable energy when they are coupled with artificial lighting.*

*b Electrochemical cells are turnkey, but generally the cells need to operate at elevated temperatures and the cells are sensitive to on/off fluctuations.*

*c The need for post-reaction separation for the electrochemical conversion highly depends on the process and cell type used. d Biochemical CO2 conversion requires very energy-intensive post-reaction separation and processing steps. <sup>e</sup> The need for post-reaction separation for plasma technology highly depends on the process.*

#### **Table 1.**

*Comparing the advantages and disadvantages of the different technologies for CO2 reduction (adapted from [1]).*

conversion, which provides an attractive alternative to the conventional pathway for CO2 recycling, such as traditional catalysis and solar thermochemical process.

Nonthermal plasmas have been successfully utilized in many applications for the environmental control (such as gaseous pollutant abatement), material science (such as surface treatment) and medical applications (such as wound and cancer treatment) [1–3]. Nowadays, an increasing interest has been focused on examining their use for CO2 utilization [3–54]. In comparison to the other processes, plasma process is fast: plasma has the potential to enable thermodynamically unfavorable chemical reactions (e.g. CO2 dissociation) to occur on the basis of its non-equilibrium properties, low-power requirement and its capacity to induce physical and chemical reactions at a relatively low temperature. In addition, plasma can be ignited and shut off quickly, which enables plasma technology powered by renewable energy to act as an efficient chemical switch for the conversion purposes. Although plasma technology shows great potential, there is always a trade-off between the energy efficiency and conversion efficiency in plasma-only process. Last but not least, the conversion efficiency can be significantly improved by combining plasma with catalyst while maintaining high-energy efficiency.

Plasma catalysis (also referred to as plasma-enhanced catalysis, plasma-driven catalysis or plasma-assisted catalysis) has gathered attention as a way of increasing energy efficiency and optimizing the byproduct distribution [55]. On one hand, the catalyst can increase reaction rates and overall process selectivity. The nonthermal plasma can provide energy to drive highly endothermic processes. Plasma-catalytic processes have great potential to reduce the activation barrier of different reactions and improve the conversion rates. In addition, the nonthermal plasma itself can influence the acid–base nature of the supports, enhance the dispersion of the

**61**

**Table 2.**

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

supported metals and even adjust the microstructure of the metal nanoparticles and metal-support interface [56, 57] and in this way change the catalyst properties. All these factors contribute in different ways to the enhancement of energy efficiency of the plasma process as well as the catalyst stability, due to a synergy that occurs between the catalyst and the plasma [58]. This novel technique combines the advantages of high product selectivity from thermal catalysis and the fast startup from plasma technique. Plasma catalysis has been widely investigated for many applications. **Figure 1** briefly summarizes the main application areas of plasma catalysis. In the domain of energy applications, the use of plasma catalysis for dry reforming, CO2 reduction, hydrogen production, methanation and ammonia (NH3) synthesis has been intensively studied. In this chapter, however, we focus only on their application for CO2 conversion into value-added chemicals and fuels.

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

2

Dry reforming of methane CO2 + CH4 → 2CO + 2H2 247.4 2.6 Methanol synthesis CO2 + 3H2 → CH3OH + H2O −128 −1.3 Methanation CO2 + 4H2 → CH4 + 2H2O −164.8 −1.7

Water-gas shift reaction CO + H2O → CO2 + H2 −41.2 −0.4 Methanation CO + 3H2 → CH4 + H2O −205.8 −2.1

2

**kJ mol<sup>−</sup><sup>1</sup>**

CO2 + H2 → CO + H2O 41.2 0.4

O2 279.8 2.9

O2 250.9 2.6

**Enthalpy (∆H) eV/molecule**

**Process Reaction Enthalpy (∆H)**

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

**2. Brief theoretical background**

CO2 splitting CO2 → CO + \_1

Water spitting H2O → H2 + \_1

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

**2.1 CO2 dissociation chemistry**

Reverse water-gas shift

reaction

**Figure 1.**

*Applications of plasma catalysis.*

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

**Figure 1.** *Applications of plasma catalysis.*

*Plasma Chemistry and Gas Conversion*

conversion, which provides an attractive alternative to the conventional pathway for CO2 recycling, such as traditional catalysis and solar thermochemical process. Nonthermal plasmas have been successfully utilized in many applications for the environmental control (such as gaseous pollutant abatement), material science (such as surface treatment) and medical applications (such as wound and cancer treatment) [1–3]. Nowadays, an increasing interest has been focused on examining their use for CO2 utilization [3–54]. In comparison to the other processes, plasma process is fast: plasma has the potential to enable thermodynamically unfavorable chemical reactions (e.g. CO2 dissociation) to occur on the basis of its non-equilibrium properties, low-power requirement and its capacity to induce physical and chemical reactions at a relatively low temperature. In addition, plasma can be ignited and shut off quickly, which enables plasma technology powered by renewable energy to act as an efficient chemical switch for the conversion purposes. Although plasma technology shows great potential, there is always a trade-off between the energy efficiency and conversion efficiency in plasma-only process. Last but not least, the conversion efficiency can be significantly improved by combining plasma with catalyst while maintaining high-energy efficiency.

*Bio- and photochemical processes can also rely on indirect renewable energy when they are coupled with artificial* 

*Electrochemical cells are turnkey, but generally the cells need to operate at elevated temperatures and the cells are* 

*Biochemical CO2 conversion requires very energy-intensive post-reaction separation and processing steps. <sup>e</sup>*

*Comparing the advantages and disadvantages of the different technologies for CO2 reduction (adapted* 

*The need for post-reaction separation for plasma technology highly depends on the process.*

*The need for post-reaction separation for the electrochemical conversion highly depends on the process and cell type used.*

Plasma catalysis (also referred to as plasma-enhanced catalysis, plasma-driven catalysis or plasma-assisted catalysis) has gathered attention as a way of increasing energy efficiency and optimizing the byproduct distribution [55]. On one hand, the catalyst can increase reaction rates and overall process selectivity. The nonthermal plasma can provide energy to drive highly endothermic processes. Plasma-catalytic processes have great potential to reduce the activation barrier of different reactions and improve the conversion rates. In addition, the nonthermal plasma itself can influence the acid–base nature of the supports, enhance the dispersion of the

**60**

*a*

*c*

*d*

*lighting. b*

**Table 1.**

*from [1]).*

*sensitive to on/off fluctuations.*

supported metals and even adjust the microstructure of the metal nanoparticles and metal-support interface [56, 57] and in this way change the catalyst properties. All these factors contribute in different ways to the enhancement of energy efficiency of the plasma process as well as the catalyst stability, due to a synergy that occurs between the catalyst and the plasma [58]. This novel technique combines the advantages of high product selectivity from thermal catalysis and the fast startup from plasma technique. Plasma catalysis has been widely investigated for many applications. **Figure 1** briefly summarizes the main application areas of plasma catalysis. In the domain of energy applications, the use of plasma catalysis for dry reforming, CO2 reduction, hydrogen production, methanation and ammonia (NH3) synthesis has been intensively studied. In this chapter, however, we focus only on their application for CO2 conversion into value-added chemicals and fuels.
