**2. Catalytic reforming**

CO2 and CH4 are stable molecules under atmospheric pressure, thus the temperature required for inducing spontaneous dissociation of CO2 and CH4 is comparatively high. **Figure 1** shows thermodynamic equilibrium for CO2 /CH4 reforming without catalyst achieved with Gibbs free energy minimization algorithm [14]. In indicates thermodynamic equilibrium of reactants (CO2 and CH4 ) and products (CO, H2 , C(s) and H2 O(g)) with the assumption that carbon formation is inhibited. Assuming that carbon formation is inhibited, the temperature required for effective conversion of CO<sup>2</sup> /CH4 is comparatively high (> 550°C) while water vapor can be generated simultaneously. Actually, carbon stands for the major byproduct during reforming and influences thermodynamics as well. At a lower operating temperature, water vapor and carbon are the major products and their formation can be inhibited when operating temperature is increased to over 700°C. Generally speaking, to generate syngas efficiently, operating temperature should be higher than 700°C, which is energy-consuming.

Catalyst is required to reduce the operating temperature of DRM since both CO2 and CH4 are stable and a great amount of thermal energy is needed to induce reforming. Noble metalbased catalysts including Pt, Pd, Ir, Rh and Ru have been investigated for their activities. They possess great activities toward DRM and good resistivities for coke deposition. The

However, high operating temperature is required for effective conversion. Moreover, coke deposition leads to subsequent catalyst deactivation. Thus, how to effectively reduce operating temperature and coke deposition remains the big challenge for catalytic reforming [6, 7]. On the other hand, nonthermal plasma stands for an energy-saving reforming for GHGs reduction and many kinds of nonthermal plasma reactor have been designed and developed

operating temperature since the driving force of nonthermal plasma is electric energy instead of thermal energy [8]. Even so, nonthermal plasma has some limitations including low GHGs conversions, low syngas selectivity and byproduct formation, e.g. carbon soot. The above disadvantages reduce the applicability of nonthermal plasma for DRM [9]. To overcome the shortcomings of catalytic reforming and nonthermal plasma reforming, combining catalyst and nonthermal plasma as a hybrid reactor can be a solution since various interactions can be induced between catalyst and nonthermal, including the change of physicochemical properties of catalyst, enhancement of electric field and activation of catalysis [10]. Based on the interactions between catalysis and nonthermal plasma, limitations of catalytic reforming and nonthermal plasma reforming including catalyst deactivation and byproduct formation can

be resolved due to enhancement of reforming performance toward DRM [11–13].

/CH4

) and products (CO, H2

/CH4

temperature should be higher than 700°C, which is energy-consuming.

Catalyst is required to reduce the operating temperature of DRM since both CO2

In this chapter, application of three types of DRM system, i.e. catalysis, nonthermal plasma and hybrid plasma catalysis will be introduced and discussed for their fundamental concepts, including reaction mechanism, state-of-the-art development, opportunities and shortcomings. Some important features for various reactors will also be highlighted in this chapter as

are stable molecules under atmospheric pressure, thus the temperature required

is comparatively high. **Figure 1** shows

O(g)) with the assumption that carbon

and CH4

reforming without catalyst achieved with Gibbs

is comparatively high (> 550°C) while water vapor can be

and CH4

, C(s) and H2

free energy minimization algorithm [14]. In indicates thermodynamic equilibrium of reac-

formation is inhibited. Assuming that carbon formation is inhibited, the temperature required

generated simultaneously. Actually, carbon stands for the major byproduct during reforming and influences thermodynamics as well. At a lower operating temperature, water vapor and carbon are the major products and their formation can be inhibited when operating temperature is increased to over 700°C. Generally speaking, to generate syngas efficiently, operating

are stable and a great amount of thermal energy is needed to induce reforming. Noble metalbased catalysts including Pt, Pd, Ir, Rh and Ru have been investigated for their activities. They possess great activities toward DRM and good resistivities for coke deposition. The

conversion efficiency. Nonthermal plasma can generate syngas at a lower

to enhance CO2

a reference.

and CH4

CO2

tants (CO2

**2. Catalytic reforming**

for inducing spontaneous dissociation of CO2

thermodynamic equilibrium for CO2

and CH4

for effective conversion of CO<sup>2</sup>

/CH4

86 Carbon Dioxide Chemistry, Capture and Oil Recovery

**Figure 1.** Thermodynamic equilibrium plots for DRM at 1 atm, from 0–1000oC and at inlet feed ratio of CO2 /CH4 =1 (a) Assuming no carbon formation occurs, (b) assuming carbon formation occurs. These plots were created by using Gibbs free energy minimization algorithm on HSC Chemistry 7.1 software [14].

activity order of the above metal catalysts also depends on support and preparation method. Generally, Rh and Ru catalysts are good candidates since they have better catalytic activities and durabilities than other noble metals [15, 16]. However, their costs are also high which limits their industrial applicability. Hence, transition metal-based catalysts such as nickel, cobalt-based catalysts are frequently developed and investigated. Ni-based catalysts are most applied for DRM since they have high adsorption capacities toward CO2 and CH4 and many researches have been conducted for the purpose of increasing selectivity of syngas and stability of catalyst in terms of resistivity of coke deposition [17,18].

Generally, pathways of catalytic reactions can be divided into three categories: Langmuir-Hinshelwood (L-H), Eley-Rideal (E-R) and Mars-van Krevelen (MVK), as described in **Table 1** [19]. Kinetic studies point out that DRM follows the reaction route of L-H mechanism. The reaction mechanism of DRM can be described as **Figure 2**, density functional theory (DFT) simulation results of DRM kinetics achieved with Pd/MgO catalyst indicate that CH4 and CO2 are firstly adsorbed on Pd and MgO surface and then dissociated into CO, O, C and H atoms [20]. It


**Table 1.** Details of different carbon species formed on the catalyst surface [21].

**Figure 2.** Results from DFT studies on the multifunctional CH4 reforming mechanism under dry reforming conditions (a) and an H2 O atmosphere (b). The reaction proceeds in clockwise direction. Pd dissociates CH4 and MgO binds and activates CO2 . MgO opens a favorable CO production pathway. H2 O byproducts are attributed to H<sup>2</sup> production. Pd dissociates H2 O into PdOH and PdH, PdOH and PdC were combined into PdCOH. H2 association from PdCOH and PdH is easier than direct H2 association from two PdH. These CO and H2 production pathways are accessible at low temperature and assure low-temperature activity of Pd-MgO/SiO2 (ME) [20].

platelet films, depending on the carbon source, temperature, structure and deposition site.

sized via several ways as listed in **Figure 3** [21]. It is noted that carbon can be transferred from one form to another. For example, amorphous carbon film can be transformed into graphite platelet films when the temperature is increased. Another example is that carbon whiskers can be easily formed from many types of carbon at a high temperature. To effectively reduce the formation of Cα, operating temperature is suggested to be high. However, high operating temperature leads to formation of other carbon species. As a result, carbon deposition inevitably takes place since formation routes are various. Many works are conducted to reduce the problem of coke deposition, including catalyst modification via partial metal substitution, introduction of support and surface pre-treatment and reactor designing. Nevertheless, coke deposition still plays an important role in limiting performance and the increase of the cost

Nonthermal plasma stands for an alternative to treat GHGs since the driving force of nonthermal plasma is electronic energy instead of thermal energy. With the existence of external electric field, electrons can be accelerated and then collide with gas particles including

the above species, chemical reactions take place such as electron impact excitation, dis-

directly dissociated into smaller fractions when the transferred energy exceeds 8.8 and 4.5 eV, respectively [22, 23]. The dissociated products including methyl radical, methylene,

sociation and ionization, Penny ionization and electron attachment. CH<sup>4</sup>

, intermediates, radicals and ions. When energy is transferred from electron to

and CH4

and Cc

and this reaction is feasible at

CO2 Reforming with CH4 via Plasma Catalysis System http://dx.doi.org/10.5772/intechopen.73579 89

can be further synthe-

and CO2

can be

Generally, Cα is firstly formed via dissociation of CO<sup>2</sup>

**Figure 3.** Mechanism of carbon formation at the catalyst surface [21].

of catalytic reforming.

CO2

, CH4

**3. Nonthermal plasma reforming**

a lower temperature. Other carbon species including Cβ, Cγ, Cv

is noted that CO desorption is an endothermic reaction, thus desorption of CO plays the role of rate-limiting reaction of DRM. Also, H atoms can further recombine to form H2 and desorb onto effluent gas stream. Moreover, if water vapor is added into the gas stream, water molecules participate in catalysis and more active species can be generated such as OH\* , H\* and COH\* radicals, providing more formation routes of H2 , resulting in higher generation rate of H2 .

In terms of long-term operation of scaled-up catalytic reforming, coke deposition is the serious problem to shorten the duration of operation. Coke can be generated via several ways during reforming, as described in **Table 2**. Carbon formation can be classified into 5 categories, including the form of surface carbide, amorphous carbon films, metal carbide, whiskers and


**Table 2.** Categories of catalysis mechanisms.

**Figure 3.** Mechanism of carbon formation at the catalyst surface [21].

platelet films, depending on the carbon source, temperature, structure and deposition site. Generally, Cα is firstly formed via dissociation of CO<sup>2</sup> and CH4 and this reaction is feasible at a lower temperature. Other carbon species including Cβ, Cγ, Cv and Cc can be further synthesized via several ways as listed in **Figure 3** [21]. It is noted that carbon can be transferred from one form to another. For example, amorphous carbon film can be transformed into graphite platelet films when the temperature is increased. Another example is that carbon whiskers can be easily formed from many types of carbon at a high temperature. To effectively reduce the formation of Cα, operating temperature is suggested to be high. However, high operating temperature leads to formation of other carbon species. As a result, carbon deposition inevitably takes place since formation routes are various. Many works are conducted to reduce the problem of coke deposition, including catalyst modification via partial metal substitution, introduction of support and surface pre-treatment and reactor designing. Nevertheless, coke deposition still plays an important role in limiting performance and the increase of the cost of catalytic reforming.
