**4.2 The effects of plasma on catalyst**

The plasma also affects the catalyst properties, which are summarized as the following aspects:


#### **Figure 8.**

*Reduction of the overall activation energy by vibrational excitation of the reactants. (A) Adiabatic barrier crossing case and (B) nonadiabatic barrier crossing case. Reprinted with permission from ref 64. Copyright 2004 AAAS.*

## **5. Discussion**

#### **5.1 Coke formation behavior**

Coke formation behavior was studied as a reaction footprint to track reaction pathways induced by DBD [26]. Coke morphology and their distribution over the 3 mm spherical Ni/Al2O3 catalyst pellets were obtained after 60 min DMR. **Figure 9** shows cross-sectional carbon distribution, where (a)–(c) and (d)–(f) correspond to plasma catalysis and thermal catalysis in low, middle, and high temperatures. For the thermal catalysis with the temperature at 465°C, carbon deposition over the entire crosssection was obvious. With the temperature increased, coke was decreased and finally became nondetectable at ca. 620°C. At low temperature, plasma catalysis suppressed the coke formation significantly over the entire cross-section.

By the analysis of scanning electron micrographs (SEM) and Raman spectrum, fine carbon filaments were detected on the external pellet surface in plasma catalysis [26]. In contrast, thick fibrous carbon deposition was observed on the external surface in thermal catalysis, as well as in the internal pores in both thermal and plasma

#### **Figure 9.**

*Carbon distribution over the 3 mm spherical pellet cross-section after 60 min reforming: plasma catalysis (a)–(c) and thermal catalysis (d)–(f), respectively. Reprinted with permission from ref 26. Copyright 2018 IOP Publishing.*

**47**

*Plasma-Enabled Dry Methane Reforming DOI: http://dx.doi.org/110.5772/intechopen.80523*

**5.2 Oxidation behavior of Ni/Al2O3 catalyst**

without solid carbon deposition (R7):

shifts dramatically with *SEI*:

*SEI* <sup>=</sup> *<sup>C</sup>* <sup>×</sup> *Discharge power* (W) \_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_\_ *Total flow rate* (cm3

the oxidation time were controlled as 1000 cm3

catalyses. The CH4 dehydrogenation on catalyst is enhanced by nonthermal plasma, contributing to the generation of highly filamentous and amorphous carbon. Such nonthermal plasma-enhanced reaction has been demonstrated by carbon nanotube growth [66] and plasma-enabled steam methane reforming [67]. The fine amorphous carbon filaments, deposited in the external surface of catalyst, prove that the interaction of DBD occurs mainly in the external surface. Consequently, DBD generation and plasma-excited species diffusion are inhibited in the internal pores of catalyst.

The nickel (Ni) of Ni/Al2O3 catalyst was oxidized slightly by CO2 in the thermal catalysis [68, 69] . However, the significant Ni oxidation by CO2 (R6) was demonstrated when the catalyst were packed in nonthermal plasma zone. In this case, Ni uptakes surface oxygen beyond the adsorption/desorption equilibrium (i.e., Langmuir isotherm) to form NiO, which further promotes CH4 dehydrogenation

Ni + CO2 → NiO + CO (R6)

NiO + CH4 → Ni + OH + CH3 (R7)

The specific energy input (*SEI*) is a critical operational parameter in plasmaenabled CO2 treatment process due to the fact that dominant reaction pathway

*SEI* expresses energy consumption by discharging per unit volume of the feed gas, which could be further interpreted as average electrical energy (eV) per molecule. In Eq. (2), *C* is the conversion factor of the unit [10]. Two contrasting conditions are demonstrated in plasma-enabled CO2 oxidation: one is designated as the direct oxidation route; with a small *SEI*, CO2 dissociation to CO and 0.5 O2 (R9) is negligible, and then the plasma-excited CO2 dominates the oxidation process (R8). The other is the indirect oxidation route where O2 provides an additional oxidation pathway; with a large *SEI*, CO2 is dissociated into CO and O2 (R9) without heterogeneous catalysts by electron impact [70, 71], followed by Ni oxidation by O2 (R10).

Direct oxidation route: Ni + CO2 → NiO + CO (R8)

Indirect oxidation route: CO2 + e → 1/2 O2 + CO + e (R9)

Ni + 1/2 O2 → NiO (R10)

The plasma-enhanced direct oxidation route (R8) is further investigated because the plasma-enabled synergistic effect was demonstrated distinctly without O2 [8, 10, 26]. Ni oxidation behavior without O2 was studied with *SEI* = 0.46 eV/molecule. The CO2 conversion is far below 1% when the *SEI* was smaller than 0.5 eV/molecule [72, 73]; in the plasma and thermal oxidation, the CO2 flow rate, catalyst temperature, and

/min) (eV/molecule) (2)

/min, 600°C, and 70 min, respectively.

*Plasma-Enabled Dry Methane Reforming DOI: http://dx.doi.org/110.5772/intechopen.80523*

*Plasma Chemistry and Gas Conversion*

**5. Discussion**

**Figure 8.**

*2004 AAAS.*

**5.1 Coke formation behavior**

Coke formation behavior was studied as a reaction footprint to track reaction pathways induced by DBD [26]. Coke morphology and their distribution over the 3 mm spherical Ni/Al2O3 catalyst pellets were obtained after 60 min DMR. **Figure 9** shows cross-sectional carbon distribution, where (a)–(c) and (d)–(f) correspond to plasma catalysis and thermal catalysis in low, middle, and high temperatures. For the thermal catalysis with the temperature at 465°C, carbon deposition over the entire crosssection was obvious. With the temperature increased, coke was decreased and finally became nondetectable at ca. 620°C. At low temperature, plasma catalysis suppressed

*Reduction of the overall activation energy by vibrational excitation of the reactants. (A) Adiabatic barrier crossing case and (B) nonadiabatic barrier crossing case. Reprinted with permission from ref 64. Copyright* 

By the analysis of scanning electron micrographs (SEM) and Raman spectrum, fine carbon filaments were detected on the external pellet surface in plasma catalysis [26]. In contrast, thick fibrous carbon deposition was observed on the external surface in thermal catalysis, as well as in the internal pores in both thermal and plasma

*Carbon distribution over the 3 mm spherical pellet cross-section after 60 min reforming: plasma catalysis (a)–(c) and thermal catalysis (d)–(f), respectively. Reprinted with permission from ref 26. Copyright 2018* 

the coke formation significantly over the entire cross-section.

**46**

**Figure 9.**

*IOP Publishing.*

catalyses. The CH4 dehydrogenation on catalyst is enhanced by nonthermal plasma, contributing to the generation of highly filamentous and amorphous carbon. Such nonthermal plasma-enhanced reaction has been demonstrated by carbon nanotube growth [66] and plasma-enabled steam methane reforming [67]. The fine amorphous carbon filaments, deposited in the external surface of catalyst, prove that the interaction of DBD occurs mainly in the external surface. Consequently, DBD generation and plasma-excited species diffusion are inhibited in the internal pores of catalyst.
