**4.1 The effects of catalyst on plasma**

With the packed catalyst in the plasma zone, gaseous species adsorbed on the catalyst surface increase the concentration of surface species. In addition, the electric field is enhanced near the catalyst surface due to catalyst nanofeatures [50, 51]. Moreover, the packed catalyst also enables the discharge type change, as well as microdischarge generation.

Without packed materials, discharge mode is the "free-standing" filamentary discharge propagating across the gas gap (**Figure 7(a)**). With a packed catalyst,

### **Figure 7.**

*(a) Filamentary discharge without catalyst pellet, (b) time-resolved images of surface streamers propagating on the surface of γ-Al2O3 (reproduced with permission from ref 54. Copyright 2016 IOP Publishing), and (c) distributions of the electron density and total ion density with a 100 μm pore (reproduced with permission from ref 56. Copyright 2016 Elsevier).*

**45**

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

catalytic process.

following aspects:

(*Ea*

**4.2 The effects of plasma on catalyst**

plasma-enhanced reactivity.

or catalyst only reaction [63].

the surface streamer is propagated with the close contact with the catalyst surface, and intensive partial discharges occur between the contact area of catalysts [52, 53]. The time-resolved visualization of surface streamer propagation and partial discharge were detected by Kim et al. [54] with an intensified charge-coupled device (ICCD) camera. **Figure 7(b)** shows time-resolved images of surface streamers (i.e., primary surface streamer and secondary surface streamer) propagating on the surface. Enhanced catalytic performance in the presence of a catalyst is closely linked with the propagation of surface streamers [55]. Moreover, with packed catalyst, microdischarge is generated inside catalyst pores (when the pore sizes >10 μm) [56, 57]. Zhang et al. [56] investigated microdischarge formation inside catalyst pores by a two-dimensional fluid model in the μm range (**Figure 7(c)**), indicating that the plasma species can be formed inside pores of structured catalysts in the μm range and affect the plasma

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

(I)Modification of physicochemical properties of the catalyst by plasma, which is widely used in catalyst preparation processes [58]. With plasma preparation, the catalyst obtains a higher adsorption capacity [59], higher surface area, and higher dispersion of the catalyst material [60–62], leading to a

(II) It is possible that plasma makes changes in the surface process with the catalyst. As for the CH4 reforming process, the deposited carbon from Ni catalysts can be removed effectively by plasma-excited CO2 and H2O [8]. The other example of the synergism is NH3 decomposition for the application of fuel cell, where NH3 conversion reached 99.9% when combining plasma and catalyst, although the conversion was less than 10% in the case of either plasma

(III)Based on Arrhenius plot analysis, plasma can decrease the activation barriers (overall activation energy), attributed to the vibrational excitation, which is schematically depicted in **Figure 8**. The net activation barrier will be

(IV)The excited species or dissociated species might create other pathways with the presence of catalyst, e.g., during the CO2 plasma oxidation process, plasmaenhanced CO2 oxidized Ni/Al2O3 catalyst to form a NiO layer, which could drive an oxidation–reduction cycle in dry methane reforming reaction. The same NiO layer was found when specific energy input (*SEI*) was sufficiently high: the O2 that dissociated from CO2 plays the key role in the oxidation pro-

nonadiabatic barrier crossing case, respectively [64]. The activation barrier decrease was reported in toluene destruction process [65]. In steam methane reforming, the preexponential factor was increased clearly by plasma, attributing to plasma-activated H2O removes adsorbed carbon species, which

*<sup>v</sup>*=0 − *Evib* − *E*<sup>∗</sup>

) in a

*<sup>v</sup>*=0 − *Evib*) in an adiabatic barrier crossing case and (*Ea*

regenerate active sites for subsequent CH4 adsorption [11].

cess. The details will be interpreted in Section 5.2.

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

*Plasma Chemistry and Gas Conversion*

**4. The synergy of plasma and catalyst**

plasma, and plasma affects catalyst.

well as microdischarge generation.

**4.1 The effects of catalyst on plasma**

could be further explained by the promoted overall reaction order, which plays

Synergism in plasma catalysis in the single-stage reactor is not fully understood due to the complex interaction between the various plasma-catalyst interaction processes [45–49]. Kim et al. [27] discussed the criteria for interaction between nonthermal plasma and the porous catalyst. The chemical species in nonthermal

50 μs; and OH, 100 μs: their one-dimensional diffusion length is limited from 0.7 to 65 μm. Plasma generated species within diffusion length from the external surface of pellets would contribute to the plasma-induced reaction pathways. The interdependence of plasma and catalyst can be discussed as two aspects: catalyst affects

With the packed catalyst in the plasma zone, gaseous species adsorbed on the catalyst surface increase the concentration of surface species. In addition, the electric field is enhanced near the catalyst surface due to catalyst nanofeatures [50, 51]. Moreover, the packed catalyst also enables the discharge type change, as

Without packed materials, discharge mode is the "free-standing" filamentary discharge propagating across the gas gap (**Figure 7(a)**). With a packed catalyst,

*(a) Filamentary discharge without catalyst pellet, (b) time-resolved images of surface streamers propagating on the surface of γ-Al2O3 (reproduced with permission from ref 54. Copyright 2016 IOP Publishing), and (c) distributions of the electron density and total ion density with a 100 μm pore (reproduced with permission from ref* 

D), 10 ns; O (3P),

the key role in the estimation of the rate-determining step [44].

plasma are highly reactive; the lifetime is very short, e.g., O (1

**44**

**Figure 7.**

*56. Copyright 2016 Elsevier).*

the surface streamer is propagated with the close contact with the catalyst surface, and intensive partial discharges occur between the contact area of catalysts [52, 53]. The time-resolved visualization of surface streamer propagation and partial discharge were detected by Kim et al. [54] with an intensified charge-coupled device (ICCD) camera. **Figure 7(b)** shows time-resolved images of surface streamers (i.e., primary surface streamer and secondary surface streamer) propagating on the surface. Enhanced catalytic performance in the presence of a catalyst is closely linked with the propagation of surface streamers [55]. Moreover, with packed catalyst, microdischarge is generated inside catalyst pores (when the pore sizes >10 μm) [56, 57]. Zhang et al. [56] investigated microdischarge formation inside catalyst pores by a two-dimensional fluid model in the μm range (**Figure 7(c)**), indicating that the plasma species can be formed inside pores of structured catalysts in the μm range and affect the plasma catalytic process.
