**3. DBD-enabled dry methane reforming**

The pulsed reaction spectrometry using DBD with Ni/Al2O3 catalysts was investigated to develop a reforming diagnostic method [10]. Pulsed reforming enables the transient analyses of both CH4/CO2 consumption and H2 and CO generation. Furthermore, carbon formation was analyzed quantitatively without serious catalyst deactivation. The varied CH4/CO2 ratios between 0.5 and 1.5 were investigated at a fixed catalyst temperature near 600°C. The CH4/CO2 ratio was initially set to 0.5, and then the CH4/CO2 ratio was incremented stepwise until CH4/CO2 = 1.5, consecutively, while total flow rate was fixed at 1000 cm3 /min. De-coking process (R2) was followed up after every pulsed reaction. System pressure was kept at 5 kPa during the reforming process. Discharge power was 85−93 W where specific energy input was ca. 1.2 eV/molecule. Commercially available catalyst pellets (11 wt% Ni-La/Al2O3, Raschig ring type: 3 mm) was packed for 40 mm length (total weight ca. 12 g; Ni 1.36 g; La 0.35 g). **Figure 4** provides an overview of gas component changes in the entire hybrid reforming.

**Figure 4.** *Overview of the entire pulsed hybrid DMR.*

**Figure 5.**

*Effects of CH4/CO2 ratio on DMR at ca. 600°C: (a) CH4 conversion, (b) CO2 conversion, (c) H2 yield, and (d) CO yield.*

Reactant conversion and product yields are shown in **Figure 5**. The definition for conversion and yield were provided in Ref. [8]. CH4 conversion and H2 yield were monotonically increased with the CH4/CO2 ratio. There are two simultaneous routes for CH4 conversion as shown in **Figure 6**. Route (I) is a reforming path: CH4 is chemisorbed on metallic sites (adsorbed species are denoted by \* in reaction). The adsorbed CH4 fragments (CH*x*\*) is oxidized by CO2\* to form CH*x*O\* before complete dehydrogenations to C\* occurs. In route (II), CH4 almost irreversibly dehydrogenates toward carbon atom, and then C\*-rich layer is oxidized slowly by CO2\* (R2), which can be evidenced in the de-coking process in **Figure 4**. When the CH4/CO2 ratio exceeded 1.0, CH4 prefers to dehydrogenate to solid carbon through route (II) due to the low proportion of CO2. Subsequently, a nonnegligible amount of solid carbon is produced, and CO2 conversion and CO yield turned to proportionally decrease.

**43**

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

CH4 → CH4

*Two simultaneous routes for CH4 conversion.*

**Figure 6.**

of plasma-generated reactive species.

C + CO2 → CO + CO (R2)

H2 + CO2 → CO + H2O (R4)

C + H2O → H2 + CO (R5)

The CO2 conversion and CO yield were promoted in hybrid reforming compared to thermal reforming (**Figure 5**). H2O was simultaneously produced as a by-product by reverse water gas shift (RWGS) reaction (R4). Reactivity of plasma-activated H2O was confirmed by Arrhenius plot analysis where reaction order for H2O was doubled by DBD [44]. Plasma-activated H2O promotes reaction with adsorbed carbon; it creates additional pathways (R5) to syngas (H2 and CO). The CO2 conversion and CO yield were promoted in the hybrid reforming, illustrating that the reverse-Boudouard reaction (R2) was enhanced by DBD. The reaction between plasma-activated CO2 and adsorbed carbon increases CO yield. The same result was obtained in the de-coking period [10]. Although excessive production of carbon is detrimental for catalyst activity and lifetime, the presence of adsorbed carbon creates key pathways for emerging plasma-induced synergistic effect. Consequently, plasma-activated CO2 and H2O would promote surface reaction and increase CO and H2 yield. **Figure 5** clearly shows that the slope of each line increased in hybrid reforming compared with thermal reforming, attributing to the nonthermal plasma-excited species. The increase of slope

Compared with thermal reforming, both CH4 conversion and H2 yield were clearly promoted in hybrid reforming (**Figure 5**), and the main pathway of CH4 conversion and H2 yield could be simply described as CH4 dehydrogenation (R3). It is proposed that CH4 dehydrogenation was enhanced by the synergistic effect of DBD and catalyst. Molecular beam study revealed that dissociative chemisorption of CH4 on the metal surface was enhanced by vibrational excitation [42]. The numerical simulation of one-dimensional streamer propagation demonstrated that the vibrationally excited CH4 is the most abundant and long-lived species generated by low-energy electron impact [43]. The reaction mechanism of plasma-enabled catalysis could be explained by the Langmuir-Hinshelwood (LH) reaction scheme. The analysis of overall activation energy is expected to understand the contribution

<sup>∗</sup> +\_\_\_\_\_ (4 − *x*)

<sup>2</sup> H2 (*<sup>x</sup>* <sup>=</sup> <sup>0</sup>–3) (R3)

<sup>∗</sup> → CH*<sup>x</sup>*

*Plasma Chemistry and Gas Conversion*

*Overview of the entire pulsed hybrid DMR.*

**Figure 4.**

**42**

**Figure 5.**

*(d) CO yield.*

Reactant conversion and product yields are shown in **Figure 5**. The definition for conversion and yield were provided in Ref. [8]. CH4 conversion and H2 yield were monotonically increased with the CH4/CO2 ratio. There are two simultaneous routes for CH4 conversion as shown in **Figure 6**. Route (I) is a reforming path: CH4 is chemisorbed on metallic sites (adsorbed species are denoted by \* in reaction). The adsorbed CH4 fragments (CH*x*\*) is oxidized by CO2\* to form CH*x*O\* before complete dehydrogenations to C\* occurs. In route (II), CH4 almost irreversibly dehydrogenates toward carbon atom, and then C\*-rich layer is oxidized slowly by CO2\* (R2), which can be evidenced in the de-coking process in **Figure 4**. When the CH4/CO2 ratio exceeded 1.0, CH4 prefers to dehydrogenate to solid carbon through route (II) due to the low proportion of CO2. Subsequently, a nonnegligible amount of solid carbon is produced,

*Effects of CH4/CO2 ratio on DMR at ca. 600°C: (a) CH4 conversion, (b) CO2 conversion, (c) H2 yield, and* 

and CO2 conversion and CO yield turned to proportionally decrease.

**Figure 6.** *Two simultaneous routes for CH4 conversion.*

$$\text{C} \star \text{CO}\_2 \rightarrow \text{CO} \star \text{CO} \tag{\text{R2}}$$

$$\text{CH}\_4 \rightarrow \text{CH}\_4^\* \rightarrow \text{CH}\_x^\* \star \frac{(4-x)}{2} \text{H}\_2 \text{ (x = 0-3)}\tag{\text{R3}}$$

$$\text{H}\_2\text{+ CO}\_2 \rightarrow \text{CO} + \text{H}\_2\text{O} \tag{\text{R4}}$$

$$\text{C} \star \text{H}\_2\text{O} \quad \text{D} \quad \text{H}\_2\text{+ CO} \tag{\text{R}}$$

Compared with thermal reforming, both CH4 conversion and H2 yield were clearly promoted in hybrid reforming (**Figure 5**), and the main pathway of CH4 conversion and H2 yield could be simply described as CH4 dehydrogenation (R3). It is proposed that CH4 dehydrogenation was enhanced by the synergistic effect of DBD and catalyst. Molecular beam study revealed that dissociative chemisorption of CH4 on the metal surface was enhanced by vibrational excitation [42]. The numerical simulation of one-dimensional streamer propagation demonstrated that the vibrationally excited CH4 is the most abundant and long-lived species generated by low-energy electron impact [43]. The reaction mechanism of plasma-enabled catalysis could be explained by the Langmuir-Hinshelwood (LH) reaction scheme. The analysis of overall activation energy is expected to understand the contribution of plasma-generated reactive species.

The CO2 conversion and CO yield were promoted in hybrid reforming compared to thermal reforming (**Figure 5**). H2O was simultaneously produced as a by-product by reverse water gas shift (RWGS) reaction (R4). Reactivity of plasma-activated H2O was confirmed by Arrhenius plot analysis where reaction order for H2O was doubled by DBD [44]. Plasma-activated H2O promotes reaction with adsorbed carbon; it creates additional pathways (R5) to syngas (H2 and CO). The CO2 conversion and CO yield were promoted in the hybrid reforming, illustrating that the reverse-Boudouard reaction (R2) was enhanced by DBD. The reaction between plasma-activated CO2 and adsorbed carbon increases CO yield. The same result was obtained in the de-coking period [10]. Although excessive production of carbon is detrimental for catalyst activity and lifetime, the presence of adsorbed carbon creates key pathways for emerging plasma-induced synergistic effect. Consequently, plasma-activated CO2 and H2O would promote surface reaction and increase CO and H2 yield. **Figure 5** clearly shows that the slope of each line increased in hybrid reforming compared with thermal reforming, attributing to the nonthermal plasma-excited species. The increase of slope

could be further explained by the promoted overall reaction order, which plays the key role in the estimation of the rate-determining step [44].
