Comparing the advantages and disadvantages of the different technologies for CO2 reduction (adapted from ).
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 plasma-assisted 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.
- green energy
- plasma-based CO2 conversion
- plasma catalysis
- oxygen vacancies
- synergistic effect
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 . 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 CO2conversion, 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, 2, 3]. Nowadays, an increasing interest has been focused on examining their use for CO2 utilization [3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 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 . 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 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 . 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.
2. Brief theoretical background
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 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 plasma-catalytic 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 plasma-catalytic 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 . As explained by Fridman , 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 :
|CO2 splitting||CO2 → CO + O2||279.8||2.9|
|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|
|Reverse water-gas shift reaction||CO2 + H2 → CO + H2O||41.2||0.4|
|Water-gas shift reaction||CO + H2O → CO2 + H2||−41.2||−0.4|
|Methanation||CO + 3H2 → CH4 + H2O||−205.8||−2.1|
|Water spitting||H2O → H2 + O2||250.9||2.6|
The main pathways for decomposition of CO2 molecule include the electron impact dissociation:
which is often accompanied by the further recombination of atomic O:
In addition to this, the vibrationally excited CO2 molecules may also undergo decomposition via the collisions with atomic O:
as well as with the plasma electrons:
Traditionally, to characterize the process efficiency, two main parameters reflecting the
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 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 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 . 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 . 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 . 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.
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, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39]. In this section, the most widely used discharges for CO2 conversion are presented.
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 . 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 . 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 knife-shaped 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%) . 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 . 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 . 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 . 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 , which plays a key role for CO2 decomposition . 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’ 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 . The energy efficiency for the dissociation of CO2 is quite limited in a DBD plasma [3, 4, 11, 12, 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. . 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 . Van Laer demonstrated a packing of ZrO2 beads in a DBD reactor. The best combination of conversion (37.8%) and energy (6.4%) efficiency was reached at a flow rate of 20 mL min−1 and an input power of 60 W . 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 . 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 . Spencer et al. experimentally investigated the conversion of CO2 in an atmospheric pressure microwave plasma-catalytic system . 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 . 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 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 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 . 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− anions using holes, followed by releasing O2 . 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).
|Plasma type||Comments||Gas mixture||Catalyst||χ (%)||η (%)||SEI|
|DBD||Low flow rate||CO2||—||14||8||5.2|||
|DBD||10% CO2 in the gas mixture||CO2-H2O-Ar||Ni/γ-Al2O3||36||23||4.5|||
|DBD||CO2||Ni/SiO2 + BaTiO3||23.5||2.31||29.5|||
|DBD||CO2||CeO2 (2 mm)||10.6||27.6||1.11|||
|DBD||CO2||TiO2 (3–4 mm)||8.2||15.54||1.53|||
|MW||CO2:H2O = 1:1||CO2-H2O||—||12||8.7||4|||
4. Conclusions and perspectives
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.
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.