**5.2 Oxidation behavior of Ni/Al2O3 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 without solid carbon deposition (R7):

$$\text{Ni} \star \text{CO}\_2 \rightarrow \text{NiO} \star \text{CO} \tag{\text{R6}}$$

$$\text{NiO} \star \text{CH}\_4 \rightarrow \text{Ni} \star \text{OH} \star \text{CH}\_3 \tag{\text{R}\%}$$

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

$$\begin{array}{l} \text{Semiconductor} \\ \text{fits dramatically with SEI:} \\\\ \text{SEI} & \text{= C} \times \frac{\text{Discharge power (W)}}{\text{Total flow rate (cm}^3/\text{min})} \end{array} \text{ (eV/molelecule)} \tag{2}$$

*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).

$$\text{Direct oxidation route: Ni} + \text{CO}\_2 \rightarrow \text{NiO} + \text{CO} \tag{\text{R8}}$$

$$\text{Indirect oxidation route:}\,\text{CO}\_2\text{ + e}\,\text{ }\rightarrow\,\,\mathbf{1}/2\,\text{O}\_2\text{ + CO}\,\text{ + e}\tag{\text{R9}}$$

$$\text{Ni} + \text{1/2 O}\_2 \rightarrow \text{NiO} \tag{R10}$$

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 the oxidation time were controlled as 1000 cm3 /min, 600°C, and 70 min, respectively.

After DBD-enhanced oxidation and thermal oxidation, the formation of NiO and its distribution over the cross-section of 3 mm spherical pellet were investigated by Raman spectroscopy and optical microscope. Results showed that the NiO layer was recognized clearly with the thickness of ca. 20 μm. In contrast, the NiO layer was not identified after thermal oxidation. We should point out that the plasmaexcited CO2 has a strong oxidation capability of Ni catalyst. In addition, the effect of DBD is inhibited in the internal pores beyond 20 μm from the pellet surface.

In the thermal oxidation, CO2 is most likely adsorbed at the perimeter between Ni nanocrystals and Al2O3 interfaces [74–76] (**Figure 10(a)**). Subsequently, the adsorbed CO2 oxidize Ni to NiO near the perimeter. It is clear that the reaction sites for thermal oxidation are limited in the perimeter. The Ni oxidation reaction terminates after the reaction sites are fully oxidized by adsorbed CO2. In plasma-enhanced oxidation reaction, CO2 is firstly excited by electron impact. The vibrationally excited CO2 plays the key role to enhance adsorption process and subsequent oxidation reaction of Ni catalyst, leading to an extensive Ni nanoparticle oxidation, which occurs not only in the perimeter but also in the terrace, step, and kink (**Figure 10(b)**). **Figure 10(c)** and **(d)** show hemispherical catalyst pellets after thermal and plasma oxidation. After thermal oxidation, the external surface and cross-section of catalyst pellets remained black. In contrast, after plasma oxidation, the external surface was oxidized and has showed whitish color change (oxidized stage); in the meantime, the cross-section of the hemispherical pellet has been kept black (unoxidized stage).

The vibrationally excited CO2 by DBD would induce Ni oxidation to form the oxygen-containing active species (i.e., NiO) rather than simple adsorption, leading to oxygen-rich surface beyond Langmuir isotherm. Incoming plasma-excited CO2 would carry a few eV internal energy due to the gas phase vibration-to-vibration energy transfer [77, 78], which is the main source of energy for NiO formation. Plasma-excited CO2 could promote the adsorption flux; however, the adsorbed CO2 is finally desorbed by the equilibrium limitation unless it forms NiO. In addition, the plasma-induced nonthermal heating mechanism plays another key role in the

**Figure 10.**

*Ni oxidation pathways: (a) thermal oxidation including CO2 adsorption near the perimeter of Ni catalysts and (b) plasma-enhanced oxidation. Hemispherical catalyst pellets: (c) thermal oxidation and (d) plasma oxidation.*

**49**

minor role.

1019 cm−<sup>3</sup>

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

rather than precursor-type adsorption enhancement.

**5.3 Interaction between DBD and catalyst pores**

reaction enhancement by DBD.

enabled synergistic effect.

enhancement of Ni oxidation. Charge recombination and association of radicals can release energy corresponding to 1–10 eV/molecule on catalyst surface. When this excess energy is directly transferred to the adsorbed CO2, Ni oxidation may be enhanced without increasing macroscopic catalyst temperature. This reaction scheme is explained by nonthermal plasma-mediated Eley-Rideal mechanism,

In the DMR process, the oxygen-rich surface (NiO layer) has a capability of oxidizing a large flux of ground-state CH4 efficiently. Consequently, CH4 is not necessarily preexcited. CH4 is almost fully reacted in the NiO layer (20 μm thickness) to inhibit the coke deposition toward the internal pores [26]. However, in the thermal catalysis, NiO is generated in a negligible amount. The ground-state CH4 can diffuse into internal pores and deposit coke as previously confirmed [26]. Formation of NiO shell (**Figure 10(d)**) and the coke formation behavior (**Figure 9**) are well correlated in plasma catalysis as further discussed in the next section.

Although the synergies of plasma and catalyst have been summarized in Section 4, the interaction between DBD and catalyst pores will be further discussed in this section based on the carbon formation and oxidation behavior, as well as DBD-enhanced DMR. For the plasma catalysis, carbon deposition in the internal pores could be remarkably prevented, and fine amorphous carbon filaments were deposited only on the external surface of pellets. A similar trend was observed when NiO was formed in the limited region over the external surface (20 μm depth) only when DBD was superimposed. The results of coke formation behavior and oxidation behavior of Ni-based catalyst in plasma catalysis evidence that the interaction of DBD and catalyst occurs at the external surface of the pellets and the effected thickness is ca. 20 μm. Neither generation of DBD nor diffusion of plasma-generated reactive species in the internal pores is possible. Although DBD and pellet interaction is limited in the external surface, conversion of CH4 and CO2 was promoted clearly compared with thermal catalysis: this is the clear evidence of

For DBD, due to the enhanced physical interaction between propagating streamers and catalysts, plasma and catalyst contact area, as well as the streamer propagation from one pellet to the other, are promoted significantly. Nevertheless, electron density in a narrow filamentary channel is of the order of 1014 cm−<sup>3</sup>

79, 80]; in contrast, molecule density at standard condition is approximately

excited. Consequently, the extremely low proportion of ionized and excited species is inadequate to explain the net increase of CH4 and CO2 conversion and selectivity change by DBD. However, if reactive species are fixed and accumulated on the surface of the catalyst, the gross conversion of materials will be promoted. For this reason, the hetero-phase interface between DBD and the external pellet surface provides the most important reaction sites. In Section 5.2, nonthermal plasma oxidation of Ni to NiO creates a critically important step for plasma-

, indicating that a major part of the gas stream is neither ionized nor

As Section 4.1 mentioned, gas breakdown is hard to occur in a pore smaller than 10 μm. For the pores catalyst with a pore size less than 2 nm, standard Paschen-type gas breakdown is impossible. To sum up, the external surface of pellet plays the key role for the DBD and catalyst interaction; however, the internal pores play a

[6,

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

*Plasma Chemistry and Gas Conversion*

After DBD-enhanced oxidation and thermal oxidation, the formation of NiO and its distribution over the cross-section of 3 mm spherical pellet were investigated by Raman spectroscopy and optical microscope. Results showed that the NiO layer was recognized clearly with the thickness of ca. 20 μm. In contrast, the NiO layer was not identified after thermal oxidation. We should point out that the plasmaexcited CO2 has a strong oxidation capability of Ni catalyst. In addition, the effect of

In the thermal oxidation, CO2 is most likely adsorbed at the perimeter between Ni nanocrystals and Al2O3 interfaces [74–76] (**Figure 10(a)**). Subsequently, the adsorbed CO2 oxidize Ni to NiO near the perimeter. It is clear that the reaction sites for thermal oxidation are limited in the perimeter. The Ni oxidation reaction terminates after the reaction sites are fully oxidized by adsorbed CO2. In plasma-enhanced oxidation reaction, CO2 is firstly excited by electron impact. The vibrationally excited CO2 plays the key role to enhance adsorption process and subsequent oxidation reaction of Ni catalyst, leading to an extensive Ni nanoparticle oxidation, which occurs not only in the perimeter but also in the terrace, step, and kink (**Figure 10(b)**). **Figure 10(c)** and **(d)** show hemispherical catalyst pellets after thermal and plasma oxidation. After thermal oxidation, the external surface and cross-section of catalyst pellets remained black. In contrast, after plasma oxidation, the external surface was oxidized and has showed whitish color change (oxidized stage); in the meantime, the cross-section of

The vibrationally excited CO2 by DBD would induce Ni oxidation to form the oxygen-containing active species (i.e., NiO) rather than simple adsorption, leading to oxygen-rich surface beyond Langmuir isotherm. Incoming plasma-excited CO2 would carry a few eV internal energy due to the gas phase vibration-to-vibration energy transfer [77, 78], which is the main source of energy for NiO formation. Plasma-excited CO2 could promote the adsorption flux; however, the adsorbed CO2 is finally desorbed by the equilibrium limitation unless it forms NiO. In addition, the plasma-induced nonthermal heating mechanism plays another key role in the

*Ni oxidation pathways: (a) thermal oxidation including CO2 adsorption near the perimeter of Ni catalysts and (b) plasma-enhanced oxidation. Hemispherical catalyst pellets: (c) thermal oxidation and (d) plasma* 

DBD is inhibited in the internal pores beyond 20 μm from the pellet surface.

the hemispherical pellet has been kept black (unoxidized stage).

**48**

**Figure 10.**

*oxidation.*

enhancement of Ni oxidation. Charge recombination and association of radicals can release energy corresponding to 1–10 eV/molecule on catalyst surface. When this excess energy is directly transferred to the adsorbed CO2, Ni oxidation may be enhanced without increasing macroscopic catalyst temperature. This reaction scheme is explained by nonthermal plasma-mediated Eley-Rideal mechanism, rather than precursor-type adsorption enhancement.

In the DMR process, the oxygen-rich surface (NiO layer) has a capability of oxidizing a large flux of ground-state CH4 efficiently. Consequently, CH4 is not necessarily preexcited. CH4 is almost fully reacted in the NiO layer (20 μm thickness) to inhibit the coke deposition toward the internal pores [26]. However, in the thermal catalysis, NiO is generated in a negligible amount. The ground-state CH4 can diffuse into internal pores and deposit coke as previously confirmed [26]. Formation of NiO shell (**Figure 10(d)**) and the coke formation behavior (**Figure 9**) are well correlated in plasma catalysis as further discussed in the next section.

## **5.3 Interaction between DBD and catalyst pores**

Although the synergies of plasma and catalyst have been summarized in Section 4, the interaction between DBD and catalyst pores will be further discussed in this section based on the carbon formation and oxidation behavior, as well as DBD-enhanced DMR. For the plasma catalysis, carbon deposition in the internal pores could be remarkably prevented, and fine amorphous carbon filaments were deposited only on the external surface of pellets. A similar trend was observed when NiO was formed in the limited region over the external surface (20 μm depth) only when DBD was superimposed. The results of coke formation behavior and oxidation behavior of Ni-based catalyst in plasma catalysis evidence that the interaction of DBD and catalyst occurs at the external surface of the pellets and the effected thickness is ca. 20 μm. Neither generation of DBD nor diffusion of plasma-generated reactive species in the internal pores is possible. Although DBD and pellet interaction is limited in the external surface, conversion of CH4 and CO2 was promoted clearly compared with thermal catalysis: this is the clear evidence of reaction enhancement by DBD.

For DBD, due to the enhanced physical interaction between propagating streamers and catalysts, plasma and catalyst contact area, as well as the streamer propagation from one pellet to the other, are promoted significantly. Nevertheless, electron density in a narrow filamentary channel is of the order of 1014 cm−<sup>3</sup> [6, 79, 80]; in contrast, molecule density at standard condition is approximately 1019 cm−<sup>3</sup> , indicating that a major part of the gas stream is neither ionized nor excited. Consequently, the extremely low proportion of ionized and excited species is inadequate to explain the net increase of CH4 and CO2 conversion and selectivity change by DBD. However, if reactive species are fixed and accumulated on the surface of the catalyst, the gross conversion of materials will be promoted. For this reason, the hetero-phase interface between DBD and the external pellet surface provides the most important reaction sites. In Section 5.2, nonthermal plasma oxidation of Ni to NiO creates a critically important step for plasmaenabled synergistic effect.

As Section 4.1 mentioned, gas breakdown is hard to occur in a pore smaller than 10 μm. For the pores catalyst with a pore size less than 2 nm, standard Paschen-type gas breakdown is impossible. To sum up, the external surface of pellet plays the key role for the DBD and catalyst interaction; however, the internal pores play a minor role.
