**4. Hybrid plasma catalysis**

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

#### **4.1. Plasma influencing catalyst**

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

**Figure 7.** Schematic representation of the catalyst-packed dielectric barrier discharge reactor [29].

can be transferred onto catalyst surface. The most important part of transferred energy is thermal energy. Thermal energy can be transferred from electrons to particles on catalyst surface to heat up the particle, forming a hot spot, as shown in **Figure 8**. It is observed that with the packing of catalyst on the electrode, the surface temperature of electrode is increased since catalyst can absorb thermal energy transferred from particles [30]. As a result, catalytic reforming may take place on catalyst surface if local temperature (hot spot) exceeds the temperature required for catalysis. Next, local high temperature may induce restructuring of metal oxide clusters since their internal energy is increased. The result is that physicochemical properties of catalyst can be altered during reforming, such as particle size, pore structure, valence of metal, metal-support interactions, surface area, surface free energy, surface acidity/basicity and oxygen vacancy. In terms of catalysis, the above characteristics influence its catalytic activity well: firstly, particle size influences adsorption heat and thus adsorption and desorption rate are further changed. Electron bombardments can result in smaller average metal cluster size (**Figure 9**) and adsorption heat between CO2 /CH4 and catalyst and further enhance CO2 /CH4 adsorption and H2 /CO desorption rate [31]. Secondly, pore structure also affects CO<sup>2</sup> /CH4 adsorption on surface and inside pores. Larger pore size may be feasible for reforming since the resistance of diffusion can be lower and leads to better desorption. However, the relationship between pore size and operating parameter of nonthermal plasma is still unclear, thus, how to control the pore size remains a challenging task [32]. Thirdly, larger surface area and density of oxygen

vacancy are beneficial toward DRM since the former provides more adsorption sites and the latter provides more oxidizing agent [33]. Moreover, electron and ion bombardments can alter the chemical bonding between metal and oxygen, hence, density of oxygen in catalyst lattice can be increased. Fourthly, nonthermal plasma can affect the surface acidity/basicity since acidic and basic active species can be generated and then collide with catalyst. For catalytic reform-

**Figure 9.** Particle size distribution achieved with TEM of (a) Plasma treated Ni/MgO and (b) conventional Ni/MgO

Lastly, nonthermal plasma generates various stable and active species. Those active species can react with other species or can be adsorbed on catalyst surface, as presented in **Figure 10**. In plasma catalysis system, alternate reaction routes are provided since various active species are generated. Active species such as ions, radicals and electrons can be adsorbed to react

is acidic. In other words, cata-

adsorption and further dissociation [34].

CO2 Reforming with CH4 via Plasma Catalysis System http://dx.doi.org/10.5772/intechopen.73579 93

ing, surface acidity plays an important role since adsorbed CO2

**Figure 10.** Three key steps in (a) thermal catalysis and (b) plasma-catalysis [10].

lyst possesses surface basicity are favorable for CO2

catalysts [31].

**Figure 8.** Absolute temperature distribution in a DBD, showing the occurrence of surface hot spots [30].

**Figure 9.** Particle size distribution achieved with TEM of (a) Plasma treated Ni/MgO and (b) conventional Ni/MgO catalysts [31].

can be transferred onto catalyst surface. The most important part of transferred energy is thermal energy. Thermal energy can be transferred from electrons to particles on catalyst surface to heat up the particle, forming a hot spot, as shown in **Figure 8**. It is observed that with the packing of catalyst on the electrode, the surface temperature of electrode is increased since catalyst can absorb thermal energy transferred from particles [30]. As a result, catalytic reforming may take place on catalyst surface if local temperature (hot spot) exceeds the temperature required for catalysis. Next, local high temperature may induce restructuring of metal oxide clusters since their internal energy is increased. The result is that physicochemical properties of catalyst can be altered during reforming, such as particle size, pore structure, valence of metal, metal-support interactions, surface area, surface free energy, surface acidity/basicity and oxygen vacancy. In terms of catalysis, the above characteristics influence its catalytic activity well: firstly, particle size influences adsorption heat and thus adsorption and desorption rate are further changed. Electron bombardments can result in smaller average metal cluster size (**Figure 9**)

**Figure 7.** Schematic representation of the catalyst-packed dielectric barrier discharge reactor [29].

and catalyst and further enhance CO2

/CH4

/CH4

adsorption

adsorption on

/CH4

/CO desorption rate [31]. Secondly, pore structure also affects CO<sup>2</sup>

**Figure 8.** Absolute temperature distribution in a DBD, showing the occurrence of surface hot spots [30].

surface and inside pores. Larger pore size may be feasible for reforming since the resistance of diffusion can be lower and leads to better desorption. However, the relationship between pore size and operating parameter of nonthermal plasma is still unclear, thus, how to control the pore size remains a challenging task [32]. Thirdly, larger surface area and density of oxygen

and adsorption heat between CO2

92 Carbon Dioxide Chemistry, Capture and Oil Recovery

and H2

vacancy are beneficial toward DRM since the former provides more adsorption sites and the latter provides more oxidizing agent [33]. Moreover, electron and ion bombardments can alter the chemical bonding between metal and oxygen, hence, density of oxygen in catalyst lattice can be increased. Fourthly, nonthermal plasma can affect the surface acidity/basicity since acidic and basic active species can be generated and then collide with catalyst. For catalytic reforming, surface acidity plays an important role since adsorbed CO2 is acidic. In other words, catalyst possesses surface basicity are favorable for CO2 adsorption and further dissociation [34]. Lastly, nonthermal plasma generates various stable and active species. Those active species can react with other species or can be adsorbed on catalyst surface, as presented in **Figure 10**. In plasma catalysis system, alternate reaction routes are provided since various active species are generated. Active species such as ions, radicals and electrons can be adsorbed to react

**Figure 10.** Three key steps in (a) thermal catalysis and (b) plasma-catalysis [10].

with other active species or can react with adsorbed species directly without prior adsorption. Hence, DRM does not necessarily follow L-H mechanism, which requires both two reactants are adsorbed on catalyst surface. In summary, nonthermal plasma can be applied for catalyst modification due to its capability to improve the physicochemical properties of catalyst. Also, nonthermal plasma can be combined with catalyst and the dissipated energy during discharge can possibly induce catalytic reforming. The most important advantage is that nonthermal plasma provides more reaction routes and more active species participating in DRM.

leads to higher polarization, plasma catalysis reactor with the catalyst possessing a higher dielectric constant has higher deposited energy, current density and electron temperature.

Catalysts generally possess various types of pores, e.g. micropore or macropore, the geometry and distribution of pores can also influence discharge properties. Local discharge may take place inside the pore if the size of pore is appropriate (larger than Debye's length), which is called microdischarge. Once discharge takes place inside the pore, species adsorbed inside the pore can be dissociated or excited

During discharge, photons can be generated via excitation-relaxation. Photons may be absorbed by catalyst if the catalyst possesses a band structure similar to photocatalyst, i.e. a valence band (VB) and a conduction band (CB). The photons with kinetic energy higher or equal to the gap between VB and CV can transfer its energy to catalyst to activate electron near VB edge to CB and leave a hole in VB. Hence, electron–hole pair is formed at CB and VB,

tocatalysis has an important obstacle: recombination of electron–hole pair. Excited electron at CB is very stable and tends to return to VB, which is recombination of electron–hole pair. Recombination leads to lower energy utilization rate, as a result, how to reduce recombination

**Figure 12.** Mechanism and pathways for photocatalytic oxidation and reduction processes on the surface of heterogeneous

into CO and H2, as indicated in **Figure 12** [36]. As a result, syngas generation can

/CH4

/CH4

into CO, and electron hole at VB can

can be activated. Unfortunately, pho-

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

reforming.

95

into smaller fragments or active species to further provide alternative routes for CO2

respectively. Electron at CB can induce reduction of CO2

be enhanced if photocatalytic conversion of CO2

oxidize CH4

rate is essential.

photocatalyst [36].

#### **4.2. Catalyst influencing plasma**

Packing catalyst into discharge region can affect plasma properties including electric field, electron density and energy distribution. Most of catalysts are dielectrics, which can be polarized to form electric dipole, i.e. surface electric potential. Electric potential can further interact with external electric field, electron and other charged particles. Thus, discharge behavior of plasma is influenced by the existence of catalyst and its dielectric constant. **Figure 11** shows the dependence of deposited power, current density and electron temperature on the dielectric constant (εp) of packing catalyst [35]. Since the catalyst with a higher dielectric constant

**Figure 11.** Simulation results of (a) time averaged input power, (b) time averaged discharge current and (c) time- and space-averaged electron temperature as a function of applied voltage for various pellet dielectric constant [35].

leads to higher polarization, plasma catalysis reactor with the catalyst possessing a higher dielectric constant has higher deposited energy, current density and electron temperature.

with other active species or can react with adsorbed species directly without prior adsorption. Hence, DRM does not necessarily follow L-H mechanism, which requires both two reactants are adsorbed on catalyst surface. In summary, nonthermal plasma can be applied for catalyst modification due to its capability to improve the physicochemical properties of catalyst. Also, nonthermal plasma can be combined with catalyst and the dissipated energy during discharge can possibly induce catalytic reforming. The most important advantage is that nonthermal plasma

Packing catalyst into discharge region can affect plasma properties including electric field, electron density and energy distribution. Most of catalysts are dielectrics, which can be polarized to form electric dipole, i.e. surface electric potential. Electric potential can further interact with external electric field, electron and other charged particles. Thus, discharge behavior of plasma is influenced by the existence of catalyst and its dielectric constant. **Figure 11** shows the dependence of deposited power, current density and electron temperature on the dielectric constant (εp) of packing catalyst [35]. Since the catalyst with a higher dielectric constant

**Figure 11.** Simulation results of (a) time averaged input power, (b) time averaged discharge current and (c) time- and space-averaged electron temperature as a function of applied voltage for various pellet dielectric constant [35].

provides more reaction routes and more active species participating in DRM.

**4.2. Catalyst influencing plasma**

94 Carbon Dioxide Chemistry, Capture and Oil Recovery

Catalysts generally possess various types of pores, e.g. micropore or macropore, the geometry and distribution of pores can also influence discharge properties. Local discharge may take place inside the pore if the size of pore is appropriate (larger than Debye's length), which is called microdischarge. Once discharge takes place inside the pore, species adsorbed inside the pore can be dissociated or excited into smaller fragments or active species to further provide alternative routes for CO2 /CH4 reforming.

During discharge, photons can be generated via excitation-relaxation. Photons may be absorbed by catalyst if the catalyst possesses a band structure similar to photocatalyst, i.e. a valence band (VB) and a conduction band (CB). The photons with kinetic energy higher or equal to the gap between VB and CV can transfer its energy to catalyst to activate electron near VB edge to CB and leave a hole in VB. Hence, electron–hole pair is formed at CB and VB, respectively. Electron at CB can induce reduction of CO2 into CO, and electron hole at VB can oxidize CH4 into CO and H2, as indicated in **Figure 12** [36]. As a result, syngas generation can be enhanced if photocatalytic conversion of CO2 /CH4 can be activated. Unfortunately, photocatalysis has an important obstacle: recombination of electron–hole pair. Excited electron at CB is very stable and tends to return to VB, which is recombination of electron–hole pair. Recombination leads to lower energy utilization rate, as a result, how to reduce recombination rate is essential.

**Figure 12.** Mechanism and pathways for photocatalytic oxidation and reduction processes on the surface of heterogeneous photocatalyst [36].

**Figure 13.** (a) CO2 and (b) CH4 conversions achieved with various reactors [37].

Chung and Chang (2016) combined BaZr0.05Ti0.05O3 (BZT) catalyst (particle size ranging from 210 to 420 μm) and spark plasma reactor to form a hybrid system [37]. Results show that packing catalyst BZT into discharge region can increase electric field and current density, indicating that more kinetic electrons are generated in hybrid reactor. CO2 and CH4 conversions can be enhanced since the energy and amount of free electrons are increased, as shown in **Figure 13** [37]. Next, the selectivities of H<sup>2</sup> and CO are also increased after packing BZT, and this can be attributed to the fact that catalyst provides formation site for H<sup>2</sup> and CO. Last, in terms of energy efficiency (moles of syngas generated per kilowatt-hour input), packing BZT into plasma reactor leads to higher energy consumption, thus, the energy efficiency achieved

> with the hybrid reactor is not necessarily higher than plasma reactor at a low reactant feeding rate. Increasing feeding rate can result in higher energy utilization rate to generate syngas and

> Overall, interactions between nonthermal plasma and catalysis are presented in **Figure 15**. Since electrons can be generated via nonthermal plasma to attain high kinetic energy. Energetic electrons can hit on catalyst surface to transfer energy and further influence the physicochemical properties of catalyst including particle size, surface area and pore structure. On the other hand, packing catalyst into plasma reactor can alter discharge behavior of plasma, depending on electrical and geometrical properties of catalyst. However, there remains unclear synergies

> Catalysis and nonthermal plasma are two efficient approaches to generate syngas from

nism and CO desorption is the rate-determining step. Coke formation via multiple routes is the most important obstacle to limit the application of catalytic reforming. On the other

and CH4

follows the mechanism of L-H mecha-

CO2 Reforming with CH4 via Plasma Catalysis System http://dx.doi.org/10.5772/intechopen.73579 97

further enhance synergies between plasma and catalyst as shown in **Figure 14** [37].

and requires more works to discover and elucidate detailed interactions.

. Catalytic conversion of CO2

**Figure 15.** Interactions between nonthermal plasma and catalysis [33].

**5. Conclusions**

and CH4

CO2

**Figure 14.** Energy efficiencies achieved with various reactors [37].

**Figure 15.** Interactions between nonthermal plasma and catalysis [33].

with the hybrid reactor is not necessarily higher than plasma reactor at a low reactant feeding rate. Increasing feeding rate can result in higher energy utilization rate to generate syngas and further enhance synergies between plasma and catalyst as shown in **Figure 14** [37].

Overall, interactions between nonthermal plasma and catalysis are presented in **Figure 15**. Since electrons can be generated via nonthermal plasma to attain high kinetic energy. Energetic electrons can hit on catalyst surface to transfer energy and further influence the physicochemical properties of catalyst including particle size, surface area and pore structure. On the other hand, packing catalyst into plasma reactor can alter discharge behavior of plasma, depending on electrical and geometrical properties of catalyst. However, there remains unclear synergies and requires more works to discover and elucidate detailed interactions.
