The conversion of methane was investigated in a packed-bed dielectric barrier discharge (DBD) plasma reactor operated at ambient conditions. High dielectric BaTiO3 was utilized as packing in comparison with γ-alumina, α-alumina, and silica-SBA-15. Results show a considerably lower conversion of CH4 and C2 yield for the BaTiO3 packed reactor, which is even less than that obtained for the nonpacked reactor. In contrast, the low dielectric alumina (γ and α) packed reactor improved the conversion of CH4 and C2 yield. Additionally, the alumina packed reactor shifted the distribution of C2 compounds towards C2H4 higher than that obtained for the nonpacked reactor and resulted in a higher energy efficiency compared to the BaTiO3 packed reactor. This is attributed to the small pore size of BaTiO3 (10–200 nm) and its high dielectric constant, whereas the polarization inside small pores does not lead to the formation of an overall strong electric field.

Methane is activated at ambient conditions in a dielectric barrier discharge (DBD) plasma reactor packed with Pd/γ-alumina catalyst containing different loadings of Pd (0.5, 1, 5 wt%). Results indicate that the presence of Pd on γ-alumina substantially abates the formation of deposits, leads to a notable increase in the production of alkanes and olefins and additionally improves the energy efficiency compared to those obtained for the non-packed reactor and the bare γ-alumina packed reactor. A low amount of Pd (0.5 and 1 wt%) favors achieving a higher production of olefins (mainly C2H4 and C3H6) and a higher yield of H2. Increasing Pd loading to 5 wt% promotes the interaction of H2 and olefins, which consequently intensifies the successive hydrogenation of unsaturated compounds, thus incurring a higher production of alkanes (mainly C2H6 and C3H8). The substantial abatement of the deposits is ascribed to the role of Palladium in moderating the strength of the electric and shifting the reaction pathways, in the way that hydrogenation reactions of deposits’ precursors become faster than their deposition on the catalyst.

The deposits formed during the DBD plasma conversion of CH4 were characterized by high-resolution scanning electron microscopy (HRSEM) and energy dispersive X-ray elemental analysis (EDX) for both cases of a non-packed reactor and a packed reactor. For the non-packed plasma reactor, a layer of deposits was formed on the dielectric surface. HRSEM images in combination with EDX and CHN elemental analysis of this layer revealed that the deposits are made of a polymer-like layer with a high content of hydrogen (60 at%), possessing an amorphous structure. For the packed reactor, γ-alumina, Pd/γ-alumina, BaTiO3, silica-SBA-15, MgO/Al2O3, and α-alumina were used as the packing materials inside the DBD discharges. Carbon-rich agglomerates were formed on the γ-alumina after exposure to plasma. The EDX mapping furthermore indicated the carbon-rich areas in the structure. In contrast, the formation of agglomerates was not observed for Pd-loaded γ-alumina. This was ascribed to the presence of Pd, which enhances the hydrogenation of deposit precursors, and leads to a significantly lower amount of deposits. It was further found that the structure of all other plasma-processed materials, including MgO/Al2O3, silica-SBA-15, BaTiO3, and α-alumina, undergoes morphological changes. These alterations appeared in the forms of the generation of new pores (voids) in the structure, as well as the moderation of the surface roughness towards a smoother surface after the plasma treatment.

Chapter 3: The synergistic effect of Pd/γ-alumina catalyst and the DBD plasma reactor in the post-plasma catalysis configuration……………………………………………..54 3.1. Introduction…………………………………………………………………………..55 3.2. Experimental…………………………………………………………………………57 3.3. Results and discussion……………………………………………………………….58 3.4. Conclusions………………………………………………………………………….76 References………………………………………………………………………………..77 Chapter 4: The synergy of DBD plasma and Pd/γ-alumina catalyst integrated in the inplasma catalysis configuration for non-oxidative coupling of methane…………….79 4.1. Introduction………………………………………………………………………….80 4.2. Experimental………………………………………………………………………...82 4.3. Results and discussion………………………………………………………………83 4.4. Conclusions……………………………………………………………………….135 References……………………………………………………………………………..136 Chapter 5: The effect of dielectric property of the packing materials on the performance of the packed-bed DBD plasma reactor for non-oxidative coupling of methane…138 5.1. Introduction……………………………………………………………………….139 5.2. Experimental………………………………………………………………………141 5.3. Results and discussion…………………………………………………………….142 5.4. Conclusions……………………………………………………………………….168 References……………………………………………………………………………..169 Chapter 6: The high resolution scanning electron microscopy (HRSEM) and energy dispersive X-ray spectroscopy (EDX) characterization of the deposits formed during the DBD plasma reaction for non-oxidative coupling of methane…………………….171