Combined non-thermal plasma and catalytic oxidation of low concentration methane

COMBINED NON-THERMAL PLASMA AND CATALYTIC OXIDATION OF LOW CONCENTRATION METHANE Irzam Javed Lappeenranta–Lahti University of Technology LUT Master´s degree program in Chemical and Process Engineering Examiner: Docent, Arto Laari, LUT University M.Sc (Tech.), Johanna Kihlman, VTT 1 ABSTRACT Lappeenranta-Lahti University of Technology LUT LUT-School of Engineering Science Master´s degree program in Chemical and Processing Engineering Irzam Javed Combined non-thermal plasma and catalytic oxidation of low concentration methane Master´s Thesis 2023 74 Pages, 40 Figures, 14 Tables and 05 Appendices Examiner: Docent Arto Laari M.Sc (Tech.) Johanna Kihlman Key words: Methane oxidation, Non-thermal plasma, Cold plasma, Dielectric barrier discharge. This master´s thesis studies the effect of non-thermal plasma and the hybridized plasma-catalysis system on the methane abatement by using Dielectric Barrier Discharge (DBD) reactor. The catalysts in the hybridized plasma-catalysis system are found to enhance the selectivity towards carbon dioxide. The literature part of the thesis presents methane sources, various methods for methane abatement and successful examples related. The effect of different catalysts on methane conversion is also reviewed. Plasma technology specifically the non-thermal plasma chemistry and the plasma effect has been provided in detail. In the experimental part, the effect of plasma power alone and in the presence of catalysts was studied. Total of seven catalysts were used. The experiments were conducted in DBD reactor at atmospheric pressure having the feed flowrate of 200 ml/min while changing the plasma power and catalysts. According to the experimental results, the system with only plasma achieved the highest methane conversion 95 %, encompassing both partial and complete oxidation reactions. In contrast, the hybridized system, specifically with 1 % Pd/Al2O3, exhibited a slightly lower methane conversion 88 % than the plasma alone system. However, the only reaction that took place in this case was complete oxidation because there was no any carbon monoxide formation. Additionally, there was no noticeable difference observed in the formation of NOx throughout the study. 2 Acknowledgements This master´s thesis was completed at VTT technical Research Center of Finland. This thesis was funded under Horizon Europe project Carbon Neutral Milk (CANMILK, Grant number 101069491). First, all the praises and gratitude to Almighty Allah. I would like to extend my gratitude to Alexander Reznichenko for granting me the opportunity to pursue my master´s thesis at VTT. My sincere appreciation to my supervisors Johanna Kihlman and Arto Laari for their unwavering cooperation, valuable feedbacks, and patience throughout the work. Special thanks to my advisor Jasmiina Palo, whose continuous guidance and insightful comments have been a great asset. Additionally, I want to sincerely thank Bibesh Gauli for his assistance during experimental work and catalyst characterization. Finally, I would like to express my sincere gratitude to my family and friends for being there for me throughout the thesis. Words may fall short, but my heart is filled with gratitude for the support received from each person involved. Irzam Javed 3 Table of Content Abstract Acknowledgments Symbols and abbreviations Appendices 1. Introduction .............................................................................................................................. 6 LITERATURE PART ........................................................................................................................ 8 2. Methane ..................................................................................................................................... 8 2.1. Atmospheric methane sources ............................................................................................ 8 2.2. Methane emissions from livestock ..................................................................................... 9 2.3. Methane abatement .......................................................................................................... 10 2.4. Methane oxidation ............................................................................................................ 10 3. Plasma technology .................................................................................................................. 12 3.1. What is a Plasma? ............................................................................................................ 12 3.1.1. Thermal plasma ........................................................................................................ 13 3.1.2. Non-thermal plasma ................................................................................................. 13 3.2. Non-thermal plasma chemistry ........................................................................................ 14 3.3. Dielectric barrier discharge .............................................................................................. 16 4. Catalysts in plasma processes ............................................................................................... 18 4.1. Catalyst for methane complete oxidation ......................................................................... 18 4.2. Effect of supports on methane activation ......................................................................... 19 5. Plasma-catalysis ..................................................................................................................... 20 5.1. Coupling of plasma and catalyst ...................................................................................... 20 5.2. Hybridization effect of the plasma-catalysis system ........................................................ 21 6. Plasma operation .................................................................................................................... 24 6.1. The effect of input power ................................................................................................. 24 6.2. The effect of gas composition .......................................................................................... 25 6.3. The effect of temperature ................................................................................................. 25 EXPERIMENTAL PART ................................................................................................................ 26 7. Materials and methods .......................................................................................................... 26 7.1. Catalysts ........................................................................................................................... 26 7.2. Catalyst characterization .................................................................................................. 27 7.3. Test rig ............................................................................................................................. 28 7.4. Packing of the catalyst inside the DBD reactor ................................................................ 29 4 .................................................................................................................................................. 30 7.5. Experimental conditions................................................................................................... 30 7.6. Execution of the experiments ........................................................................................... 31 7.7. Product analysis ............................................................................................................... 31 7.7.1. Plasma power calculation ............................................................................................. 31 7.7.2. Analytical methods ....................................................................................................... 32 7.7.3. Calculation methods ..................................................................................................... 32 8. Results and discussions .......................................................................................................... 34 8.1. Catalyst characterization .................................................................................................. 34 8.2. Results for only plasma system ........................................................................................ 35 8.2.1. Effect of discharge gap ............................................................................................. 38 8.3. Results for plasma-catalysis system ................................................................................. 41 8.3.1. Hybridization effect of plasma and γ-Al2O3 ................................................................. 41 8.3.2. Hybridization effect of plasma and 1 % Pd/γ-Al2O3 .................................................... 43 8.3.3. Hybridization effect of plasma and 3 % Pd/ γAl2O3 .................................................. 44 8.3.4. Hybridization effect of plasma and 1 % Pd-0,5 % Cu/ γAl2O3 .................................. 46 8.3.5. Hybridization effect of plasma and 1 % Cu/ γAl2O3.................................................. 47 8.3.6. Hybridization effect of plasma and 1 % Pt/ γAl2O3 ................................................... 49 8.3.7. Hybridization effect of plasma and 1 % Pd-NH4-ZSM-5 ............................................ 50 9. Error estimation ..................................................................................................................... 58 10. Conclusion and future perspective ................................................................................... 60 References ....................................................................................................................................... 62 Appendices Appendix 1. Experiment list Appendix 2. Electrical circuit for DBD Appendix 3. FTIR results Appendix 4. FTIR spectrum Appendix 5. Plasma power calculation software 5 Abbreviations GHG GreenHouse Gas GWP Global warming potential IPCC Intergovernment Panel on Climate Change NTP Non-Thermal Plasma DBD Dielectric Barrier Discharge BET Brunauer-Emmett-Teller BJH Barrett-Joyner-Halenda E-R Eley-Rideal FTIR Fourier Trasnformed Infrared Spectroscopy GC Gas Chromatograph VOC Volatile Organic compounds SIE Specific Input Energy Symbols ΔH Standard enthalpy of reaction (kJ/mol) Ub Gas breakdown voltage (V) P Plasma power (W) p Pressure (bar) T Temperature (℃) Q Volumetric flowrate (ml/min) V1 Molar volume of the ideal gas at STP (dm3/mole) ṅ𝑖 Molar flow of a component (mol/min) Ci Molar fraction of a component 𝑋𝐶𝐻4 Conversion of methane (%) Si Selectivity of a component (%) 6 1. Introduction Methane, recognized as a strong greenhouse gas, is widely acknowledged as most influential contributor to the global warming after carbon dioxide (Edenhofer et al., 2015). Starting from 2011, the concentration of methane in the atmosphere has consistently risen, reaching an annual average of 1866 parts per billion in 2019 (Zhai, Panmao. et al., 2021). The increasing trend of methane emissions is a major cause of concern, leading to a heightened interest in comprehending and mitigating its impact (Jackson et al., 2021). Almost 33 % of the overall anthropogenic methane emissions are from the livestock (Zhang et al., 2022). Several methods are commonly employed to mitigate the atmospheric methane emissions like catalytic oxidation (Epling & Hoflund, 1999). It stands out as a widely adopted and established method for mitigating the methane emissions from coal mines and vehicle exhaust (Monai et al., 2018). Nevertheless, these catalysts are inclined to be deactivated caused by the poisoning and sintering under elevated temperatures (Wang et al., 2015). Meanwhile, to control the emissions from livestock, various livestock management practices are in used like optimizing feed regimes and dietary supplements (Becker et al., 2023). Another option to oxidize the methane is plasma technology, which has also been investigated by (Da Costa et al., 2008; Pham Huu et al., 2015a, 2015b; Whitehead, 2016, 2019). Specifically cold plasma or non-thermal plasma used in the presence of catalyst in the system to enhance the conversion rate of methane and to refine the product selectivity (Gholami et al., 2022). This thesis was funded under Horizon Europe project Carbon Neutral Milk (CANMILK), which endeavors to develop an innovative approach utilizing cold plasma and catalysis system to abate methane emissions emanating from cow barns. The focus of this process lies in the conversion of methane into carbon dioxide, since methane has 30 times greater potential of global warming than carbon dioxide (Wang et al., 2023). The principal objective of this thesis revolved around investigating the impact of plasma alone and various catalysts within the plasma system on methane conversion. The literature part of the thesis diligently expounded upon the comprehensive understanding of methane, methane sources, plasma technology, specifically cold plasma or non-thermal plasma, and reviewing the impact of different catalysts on the conversion of methane. 7 The experimental phase, on the other hand, was designed to examine the effect of plasma power alone on methane abatement and the effect of catalysts inside the plasma system on methane conversion and subsequent selectivity towards carbon dioxide, while striving to minimize NOx production. In the experiments, seven catalysts i.e., γ-Al2O3, 1 % Pd/γ-Al2O3, 3 % Pd/γ-Al2O3, 1 % Cu/γ-Al2O3, 1 % Pd-0,5 % Cu/γ-Al2O3, 1 % Pt/γ-Al2O3 and 1 % PdNH4 ZSM-5 zeolite were examined. 8 LITERATURE PART 2. Methane Methane is a simplest hydrocarbon and the primary component of the natural gas. It is odorless, colorless, and highly flammable. Migeotte in 1948 discovered the methane in atmosphere (Migeotte, 1948). Methane has a much greater warming potential as a greenhouse gas (GHG) than carbon dioxide over a shorter period (Pizzolitto et al., 2019). Globally, the concentration of methane is increasing, and this is not just affecting the climate but also the life on Earth. Over the past 200 years, anthropogenic activities have contributed significant increase in atmospheric methane concentration (Jang et al., 2019). This implies that 0,5 ℃ of the 1,1 ℃ increase in atmospheric temperature is because of the methane emissions. The IPCC's sixth assessment report states (Intergovernmental Panel on Climate Change, 2023), an increase in global temperature has resulted in notable modifications to the water cycle, including changes in precipitation patterns, increased evaporation rates, and shifts in the distribution of water resources. Furthermore, it also causes the rise in the sea levels and severity of weather events such as heatwaves, hurricanes, droughts, and heavy rainfalls. 2.1.Atmospheric methane sources Methane emissions arise from both natural and man-made sources. While natural sources of methane have existed for millions of years, human activities have significantly contributed to the amount of methane in the atmosphere over the past century. The annual global methane emissions are 580 Metric tons CO2 eq. and emissions from natural sources such as wetlands account for 40 % while the emissions from human activities account for 60 % of the annual emissions globally (Saunois et al., 2020). 15 In this case, the gas atoms are C and D, its molecules are C2 and D2, its energized electron is e*, and its transient collision species is N. The ionic species are marked with negative and positive charges, and excited species are marked with (*). The initial stage involves electron collisions, which induce the activation of atoms and molecules present in the gas. Subsequently, these excited species engage in reactions and facilitate the transfer of charge and energy, thereby stimulating and exciting the other species. This cascade effect perpetuates the chain of activation and excitation. Non-thermal plasma technology has been viewed as an attractive substitute for traditional thermal or catalytic methods due to its non-equilibrium nature, low energy consumption, and exceptional capacity to start physical and chemical reactions at low temperatures for gas purification and atmospheric pollution removal (Bruggeman et al., 2010) (Vandenbroucke et al., 2011). The configuration of plasma setup usually consists of an electrode, the injected gas, and a power source. The change in the gas composition and altering the power of the plasma can have noticeable effect on the energy yield of these devices (Chandana et al., 2018b). NTP can be categorized into groups based on the method employed to generate them. Some important plasma discharges and their typical parameters are provided in the Table 1. As the focus of this study revolves around the utilization of silent discharge which is also named as dielectric barrier discharge. Table 1 Typical parameters of different NTP discharges (Eliasson et al., 1991) Parameters Glow Discharge Corona Discharge Silent Discharge Pressure (bar) < 0,01 1 1 Electric-field (kV/cm) 0,01 0,5-50 0,1-100 Electron energy (eV) 0,5-2 5 1-10 Ionization degree 10-6-10-5 N.A 10-4 Electron density (cm-3) 108-1011 1013 1014 16 3.3.Dielectric barrier discharge Theodose du Moncel was the first to realize that plasma could be created at atmospheric pressure utilizing dielectric barriers spanning two electrodes. It has history dating back to its use in ozone synthesis (Kogelschatz, 2003). Dielectric barrier discharge or DBD is a highly versatile, efficient technology to produce non-thermal plasma and it offers scalability, flexibility, and stable plasma generation. Additionally, its fundamental chemical and physical properties are well understood. Usually, two types of DBD configurations, planar and cylindrical, are used. In Figure 3, a basic planar and cylindrical configurations of a dielectric barrier discharge reactor is shown. Both configurations consist of one ground electrode, live electrode, and dielectric barrier. The barrier is used to avoid the formation of sparks and arcs. The flow is applied in the discharge gap between the electrode and dielectric barrier. It ranges from 1 mm to 1 cm without any loss in the plasma efficiency (Hammer, 2000). Mostly, DBD operates at lower temperature and pressure is atmospheric. Plasma is generated by breakdown of the gas, and it depends on the applied voltage, Ub which is determined by the Paschen´s law as shown in Eq. 3 (Snoeckx & Bogaerts, 2017). Ub=Dpd ln(Cpd ln1 γ) (3) Figure 3 Planar (top) and coaxial (bottom) reproduced from (Snoeckx & Bogaerts, 2017) 17 In Equation 3, D is related to the ionization and excitation energy, γ is the electron emission coefficient, C is the saturated ionization in the gas, p is the pressure of the gas and d is the gap between the electrodes. There are three steps in the growth of discharge (Wong, 2016): • Pre-breakdown: A negative space charge as electrons gets collected at anode. The enough generation of electric field to breakdown the gas. It lasts for 0,5 ms. • Propagation: This depends on the wave of the ionization from anode to cathode. Ions and electron pairs are formed during the wave. It takes 1-2 ns. • Decay: This is the charge build up on dielectric material which balances the external applied voltage. Usually, the micro discharge only lasts for few nanoseconds. As the amount of energy given and the charge provided to the micro discharge are restricted by dielectric material. That’s why it produces cold plasma. Dielectric barrier discharge broadly studied for various applications like reduction of NOx, removal of odor, and air pollution control (Quoc An et al., 2011). While plasma technology offers numerous advantages, it is important to address the challenges of high energy consumption and non-selectivity of products. Researchers have been concentrating on integrating plasma with various techniques over the past two decades (Magureanu & Bradu, 2022). One such combination that has garnered significant attention is plasma-catalysis, which is the focus of the current study. By leveraging plasma-catalysis, attempts are being made to overcome the challenges and enhance the energy efficiency and selectivity. In the study by (De Rosa et al., 2022), demonstrated that the addition of a catalyst to the plasma system can lower the specific input energy (SIE) needed to achieve the same level of carbon dioxide yield or methane conversion. It has been noted that the specific input energy of a plasma-catalytic system is two to three times lower when catalyst is present than in a plasma alone system. 18 4. Catalysts in plasma processes 4.1.Catalyst for methane complete oxidation Numerous catalysts have been reported to be used in various plasma-assisted catalysis processes; however, the best catalyst to choose for a given plasma-catalysis system is still unknown. In short, catalyst should adsorb the reagents and create the pathways that will get the required conversion and products and avoid the formation of byproducts. So far, after reading the literature and studies, it has been seen that all the catalysts, that are proved to be suitable for a hybrid system, are used in the thermal catalytic systems for the same processes. The metals such as palladium, platinum, rhodium, has been reported for the oxidation of methane in both systems, thermal as well as in plasma. Palladium metal is well known for the methane oxidation at lower temperatures. In Eq. 4, the mechanism of the methane oxidation on a catalyst having the Pd metal is explained by (Nkinahamira et al., 2023). The pathway to produce carbon dioxide: CH4 adsorbs on the surface and gives CH3 radical, then the formation of formate happens and oxidation of formates into carbonates by surface oxygen and finally gives carbon dioxide. CH3 ∗→CH3O∗→CH2O∗→OCH2O∗→OCHO∗→OCHOO∗→CO2 (4) Pd supported on alumina (Pd/Al2O3) has been used to convert methane (Pham Huu et al., 2015a). Different amounts of loading of Pd were used. For an inlet methane concentration of 500 ppm, it was found that the methane conversion increased from 35 % to 55 % when the loading of metal increased from 0,5 to 1%. The availability of more reactive sites on the catalyst surface could be the underlying reason. In the study by Yao, at room temperature in plasms-catalysis system, when Al2O3 was used as catalyst, the methane conversion was prevented by the generation of carbonates on the surface of the catalyst but when Au metal was introduced, the bidentate carbonates decomposed into gaseous CO2 (Yao et al., 2019). In a study by (Dong et al., 2023), a dielectric barrier discharge reactor was used having the Pd/ZrO2/Al2O3 catalysts to oxidize the lean concentration of methane (0,3 %). Various loadings (5 %, 10 % and 30 %) of ZrO2 were used to check the effect and it was found that the ZrO2 loading enhanced the CH4 oxidation (85% to almost 98%) and the CO2 selectivity by converting surface formate. 19 In this thesis, the primary objective is to address the mitigation of methane, but also taking in account the minimization of byproducts such as NOx. Consequently, the emphasis lies on identifying the catalysts that exhibit dual functionality. 4.2.Effect of supports on methane activation The activation of the methane is noticeably impacted by the particle size and characteristics of the supporting material (Nilsson et al., 2015). Some commonly used support material for oxidation of methane are alumina, silica, zeolites, and mixed metal oxides. To decrease the amount of metal present and to improve the dispersion of the metal and stability of the metal, the catalyst active sites are deposited onto supports (Jiang et al., 2020). This improved metal dispersion is crucial for catalyst performance, as it ensures a higher number of metal atoms are exposed to the reactant species. The effect of various zeolite supports on the oxidation of methane has been reviewed by (H. Y. Chen et al., 2022). Typically, the principal species to activate the methane in a palladiumbased catalyst is the PdO. The production of PdO can be enhanced by selecting support materials with high tendency of adsorbing oxygen. Zeolites with high Si/Al ratio proved to be fell under this category (Friberg et al., 2021). These are crystalline aluminosilicate materials having a well-defined microporous structure. The high surface area, shape selectivity, and acidity (adjustable) properties make it suitable support for methane activation. That´s why, when Pd is dispersed and stabilized on the zeolites, the catalytic activity increased. The relationship between silica and alumina ratio, Pd supported on different zeolites frames and the oxidation of methane is explained by (Friberg et al., 2021). Its noteworthy that the specific effects of increasing the silica and alumina ratio can vary depending on the kind of zeolite. According to their findings, the selection of zeolite support is important for both promoting the generation of well-dispersed Pd particles and minimizing the generation of ion exchanged Pd species. Small pore zeolites and a high Si/Al ratio can be used to achieve this. 20 5. Plasma-catalysis Plasma-catalysis, also known as plasma-assisted catalysis, involves combining a gaseous discharge or plasma with catalysts. This can be done by various methods. This hybridization can give results with improved outcomes which cannot be possible to get while using plasma system and catalytic system separately. You can see the various applications of this hybridized system in the Figure 4. Unlike thermal catalysis, where reactive species form exclusively on the surface of the heated catalyst, in plasma-catalysis, these reactive species can be generated in the gas phase through dissociation within the plasma or subsequent reactions of plasma-excited species. (Whitehead, 2016). This highlights a distinct mechanism that extends beyond traditional thermal catalytic processes. 5.1.Coupling of plasma and catalyst Generally, plasma and catalyst can be coupled in one of two ways as mostly described in different studies (Khoja et al., 2019). When the catalyst material is introduced straight into the plasma zone, it is known as “In-plasma catalyst system” as shown in Figure 5. The catalyst is inserted after the plasma zone, it is called “post-plasma catalyst system” as shown in Figure 6. NTP + Catalyst Figure 5 One stage setup Figure 4 Various applications of the plasma-catalysis system (Kim et al., 2016b) 21 After applying the voltage, the formation of radicals and active species happened as already describe in Chapter 3.2. The lifetime of these reactive species or radicals is the important parameter to examine the feasibility of these systems. Since a ground state oxygen atom in an NTP has a lifetime of about 14 μs (Holzer, 2002). This implies that the species will only be able to react in a very small area or boundary layer. Many studies have employed both configurations to examine the feasibility and have consistently demonstrated that the combination of one stage plasma-catalysis is the more favorable option (Drake et al., n.d.; Kim et al., 2016a). For example, in the work by (Quoc An et al., 2011), a plasma-catalysis system was used to abate toluene from the atmosphere with both arrangements, post-plasma, and in-plasma. In-plasma configuration showed up to 96 % toluene conversion while post plasma showed 80 % conversion. 5.2.Hybridization effect of the plasma-catalysis system In the investigation conducted by (Costa et al., 2008), Pd supported on alumina (Pd/Al2O3) was tested in a DBD reactor having the mixture of gases (CH4, O2, CO2, and NO) and nitrogen as a balance gas. It was observed that the methane conversion started when temperature was 300 ℃ in the absence of catalyst but when catalyst was present the methane conversion was almost 100 % at the same temperature. The integration of plasma and catalysis offers unique advantages by harnessing the active species formed by plasma and the specific reactivity of catalyst for selectivity. The possible effects of plasma-catalysis system are shown in Figure 7. Figure 6 Two stage setup 22 There have been conflicts about the synergy effects of this modified system like in one study, it is argued that the catalyst only promotes the CO oxidation (Baylet et al., 2012). DBD reactor was used to breakdown the mixture of 0,3 % to 1 % CH4 (CO2/N2/O2/H2O) and to generate the plasma. No significant difference was observed in the methane conversion but the CO2 selectivity upon introducing the catalyst was increased. While in other study, by (Lee et al., 2015), the effective synergy of the plasma coupled with catalyst for CH4 conversion was observed. They used a dielectric barrier discharge reactor to perform the complete oxidation of the methane in an in-plasma arrangement. Palladium as an active metal supported on different materials was used as a catalyst. Even when there was no any catalyst placed the methane beginning to convert into CO and CO2 but the selectivity for CO2 was lower. When catalyst was introduced, the selectivity of the CO2 increased, and it showed the complete oxidation of the methane. The reaction mechanism inside plasmacatalysis configuration is shown Eq. 5 to 10. e∗+O2→O∗+O∗+e (5) e∗+CH4→CH3 ∗+H∗+e (6) CH4+3O∗→CO2+2H2O (7) CH4+O∗→CO+2H2 (8) 2H∗+O∗→H2O (9) CO+O∗→CO2 (10) Equations 5 and 6 illustrate how the collisions between the energy-rich electrons and methane caused the two elements to dissociate, forming methyl and oxygen radicals. CO, H2O, and CO2 were produced by the reaction of oxygen radicals with methyl and hydroxyl Effect of plasma on catalyst •Enlarge surface area •Modify surface reactions •Modify activation barrier Synergy •Prolong catalyst stability •Increase product selectivity •Enhance energy efficiency Effect of catalyst on plasma •Change discharge behaviour •Low activation barrier •Prolong the lives of active species Figure 7 Possible effects of plasma-catalysis system (Khoja et al., 2019) 23 radicals as well as hydrogen. (Kim et al., 2016b) also reported the direct Eley-Rideal mechanism inside the plasma-catalysis system. Radicals generated by plasma directly reacted with the oxygen which is adsorbed onto the surface of the catalyst. C-H bond dissociation was observed to be rate determining step, it was also found that the methane, which is vibrationally excited by plasma, is the key species to promote the methane conversion in the modified system (Nozaki & Okazaki, 2013). As it has the greatest electron collision cross section. It has the potential to boost the catalyst's dissociative chemisorption, which would raise conversion. In the study by (Stere et al., 2020), in NTP system the formation of surface formate over Pd/Al2O3 was observed via IR which subsequently decomposed to form CO and CO2. Unlike thermal catalysis, the formation of surface formate is predominant in a NTP system, and the role of palladium could be to form oxygen active species and increase the decomposition of the species adsorbed on the support such as formate and in result facilitate the carbon dioxide yield. Recently, (Dong et al., 2023) also observed the same phenomenon for the oxidation of lean methane (0,3 %) in a DBD reactor. At lower temperature the dissociation took place and converted methane and oxygen into methyl radicals and O atom by the NTP plasma. O reacted with O2 and formed O3. Methyl radical got adsorbed on to surface of the catalyst and oxidized by the oxygen present on the surface of the catalyst, into surface formate and water. That water reacted with O species at higher temperature and produce OH. The catalyst functioned when temperature is 300 ℃ and promoted the further oxidation of formate present on the surface to CO2 and water. NOx production was also recorded by chromatographic analysis (Baylet et al., 2012). As nitrogen and oxygen were present in the reactant gas mixture, the generation of the O and N species by plasma reacted with each other and formed these byproducts as shown in the Eq. 11 to 16. e∗+O2→ O + O + e∗ (11) N2+ O → N +NO (12) N2+e∗→ N + N +e∗ (13) NO+e∗→ O + N +e∗ (14) O+NO→ NO2 (15) N+O→NO (16) 24 6. Plasma operation Understanding, the impact of plasma operations on the conversion is a complex and multifaceted process. The interplay of various parameters, including plasma power, gas composition, and reactant characteristics needs to be carefully controlled and optimized to achieve desired conversion efficiency and selectivity. 6.1.The effect of input power The impact of input plasma power also studied. Higher input power can enhance the generation of plasma (Nozaki & Okazaki, 2013). This results in a greater number of highly energetic electrons, ultimately facilitating the dissociation of methane and formation of reactive species. Generally, when the power is increased, it can enhance the overall conversion but the relationship between power and conversion can also be nonlinear at higher voltages (Gholami et al., 2022). During the experiments, when the specific input energy (SIE) was increased from 1 to 4 (J/mL) the CH4 conversion improved from 0 to 90 %. In another study (Stere et al., 2020), NTP-assisted oxidation of methane over Al2O3 is investigated by using dielectric barrier discharge reactor. In the mass spectroscopy analysis, it was observed that when plasma voltage was raised from 5 to 6 kV, the methane conversion also increased. It was 27 % at 5 kV and reached to 42 % when voltage was 6 kV. The effect of power on NOx formation was also examined. At higher SIE, the NOx formation is mainly NO, as an increased in the electric impact resulted in the formation of radicals (N, O, CH3). Different catalysts were examined to assess their impact on NOx formation and CH4 conversion. However, it was observed that simultaneous reduction of NOx and oxidation of CH4 proved to be challenging due to the nitrogen and oxygen presence in the reactant gas mixture. The presence of oxygen led to a diminutive effect on NOx removal. This outcome can be attributed to the dissimilar dissociative energies of O2 and N2, combined with the relatively low average electron kinetic energy of the NTP reactor. Oxygen has dissociative energy of 4,8 eV, nitrogen has 9,2 eV (Khacef & Da Costa, 2019). NTP can readily dissociate the O2 molecules into oxygen atoms which can then engage with the nitrogen to generate NOx as shown in the section 5.2 (Equations 11, 12, 15 and 16). 31 Table 4 Experiment conditions. Factor Description Plasma discharge gap size (mm) 4 Plasma discharge length (cm) 2 Tested plasma power (W) 15, 20, 25, 30 Feed gas composition (vol. %) 0,99 % CH4, 20,89 % O2, 78,12 % N2 Gas flow rate (ml/min) 200 Temperature ℃ 40-200 (No external heating) Pressure Atmospheric 7.6. Execution of the experiments Before conducting the reaction experiments, a pressure test was performed using N2 gas. This step was crucial to ensure the system was sealed correctly and that there were no leakages. Blank measurements were also taken prior to the plasma ignition. In the blank measurement, the reactant gas was passed through the system without turning on the plasma system, to get the initial concentration of the reactant gas and the flowrate of the gas. These measurements provided a reference point, allowing for accurate determination of the feed concentration in subsequent experiments. After taking the readings, the plasma system was switched on. All experiments were carried out at atmospheric pressure. No external heating was applied during the experiments, ensuring the observed results were purely due to the plasma and catalyst interactions. The reactor outside temperature was measured with an Infrared thermometer. For each setpoint, the measurements were taken when FTIR and µGC readings were stabilized, to minimize any fluctuation and errors in the readings. Stabilized reading was the indication that the system was now consistent and steady state. For a setpoint to be completed, it took one to one and half hours for the steady state readings. 7.7. Product analysis 7.7.1. Plasma power calculation The evaluation of each experiment was carried out by analyzing data sourced from multiple instruments. When plasma was switched on and adjusted to the desired power with the help of PVM500 power source, TiePie Engineering Handyscope HS6 Oscilloscope was used to monitor the various parameters which include input voltage, voltage over the capacitor and 32 the current. In this work, a 22 nF capacitor was used. The plasma power was measured online and determined by external capacitor method with MATLAB-based calculation software. For each setpoint, plasma power was stabilized around 25 to 40 minutes. In the Figure 12, an example of the stabilization of the plasma power is showed. Figure 12 Plasma power monitored with MATLAB-based calculation software created by VTT. 7.7.2. Analytical methods A Fourier-transform infrared spectroscopy (Gasmet FTIR Analyzer) was employed for realtime analysis of the gas products specifically for NOx and N2O. It offered a comprehensive spectral overview, enabling the identification of various gas compounds present. To further refine the analysis and gain more detailed insights into the product composition including CH4, O2, N2, CO2, CO and H2, an Agilent micro–Gas Chromatograph (490 µGC) was used. The online connection ensured timely data capture. 7.7.3. Calculation methods To quantify and interpret the reaction results, the results of the µGC and FTIR analysis were integrated with quantitative calculations. The ideal gas law was used in the calculations to determine the molar flows of the reactants and products. The molar flow of methane is computed in Equation 17. ṅCH4= CCH4×Qin VI (17) where ṅCH4 is the CH4 molar flow in the feed. 33 CCH4 is the CH4 molar fraction in the feed. VI is the ideal gas molar volume at STP (22,7 dm3/mol) Qin is the flowrate of the reactant gas at normal condition. The molar flowrates of other gasses and the products were calculated in the same way. For products the out flowrate was used. The conversion rate of CH4, which is an indication of how much of the initial CH₄ feed has reacted or been transformed in the experimental process, was calculated by the Equation 18. The equations 19, 20, and 21 provided the selectivity towards CO2, CO, and carbon balance. CH4 conversion (XCH4%)= [ nCH4 in− nCH4 out nCH4 in ×100] (18) CO2 selectivity (SCO2%)= [nCO2 produced nCH4 converted ×100] (19) CO selectivity (SCO%)= [nCO produced nCH4 converted ×100] (20) Carbon balance (%)= [nCH4 out+ nCO2 out+ nCOout nCH4 in ×100] (21) 34 8. Results and discussions This chapter contains, the outcomes and analyses derived from the catalyst characterization and plasma-assisted catalysis experiments. Throughout this chapter, the data, graphs, and the interpretations will explain the interplay between catalyst properties and plasma conditions, painting a picture of using plasma-assisted catalysis for methane abatement. 8.1. Catalyst characterization N2 Physisorption These measurements were done for fresh catalysts and few spent catalysts. There was some technical problem with the instrument later, so only few spent catalysts were characterized. Table 5 shows the physisorption results for catalysts, displaying their pore volume, surface area, and pore size. Table 5 Pore size, pore volume and surface area of the catalysts examined by BJH desorption isotherm. Fresh catalyst BET surface area Pore volume Pore size (m2/g) (cm3/g) (nm) γ-Al2O3 231 0,60 10 1 % Pd/γ-Al2O3 263 0,63 10 3 % Pd/γ-Al2O3 228 0,59 10 1 % Cu/γ-Al2O3 227 0,60 10 1 % Pd-0.5 % Cu/γ-Al2O3 225 0,60 10 1 % Pt/γ-Al2O3 228 0,59 10 1 % Pd-NH4 ZSM-5 566 0,30 2 Spent catalyst 1 % Pd/γAl2O3 259 0,63 10 1 % Cu/γAl2O3 224 0,60 10 1 % Pd-NH4 ZSM-5 508 0,27 2 When the metals, such as Pd, Pt and Cu, were added to γ-Al2O3, it showed a decreasing trend in the pore volume and surface area of the catalysts. But 1 % Pd/γ-Al2O3 catalyst exhibited a relatively high pore volume and surface area compared to other catalysts. The zeolite supported 1% Pd-NH4 ZSM-5 displayed a significantly higher surface area but has a much smaller pore size and lower pore volume. To assess the influence of plasma on the catalyst 35 surface, physisorption experiments were conducted on several spent catalysts. The results of these experiments indicated that there was no significant difference in the surface areas of the catalysts suggesting that the plasma treatment had no appreciable effect on the structural characteristics of the catalysts except 1 % Pd-NH4 ZSM-5 for which the surface area reduced to 508 m2/g from 566 m2/g. 8.2. Results for only plasma system To look into how plasma power affects the conversion of methane, experiments were conducted using only plasma, without the presence of a catalyst. Throughout the experiments, the feed flowrate was kept constant at 200 ml/min, while adjusting the plasma power across various levels. This approach ensured that any observed changes in methane conversion were solely attributable to the variations in plasma power, eliminating flow rate as a variable factor. These µGC results in Table 6 from the Experiment 03 and 06 using the Reactor A. The details about all the experiments can be found in the Appendix 1. Table 6 µGC results from experiment 03 and 06 using plasma alone system Power Inlet % Outlet % CH4 O2 N2 CH4 O2 N2 CO2 CO H2 20 W 0,97 20,7 76,7 0,55 20,26 77,3 0,18 0,23 0,05 25 W 0,97 20,7 76,7 30 W 0,97 20,7 76,7 0,06 19,32 78,2 0,61 0,30 0,00 20 W 0,96 20,7 76,8 0,51 20,11 77,5 0,17 0,24 0,03 25 W 0,96 20,7 76,8 0,09 19,26 77,7 0,54 0,30 0 30 W 0,96 20,7 76,8 0,06 19,17 77,9 0,64 0,25 0 The Figure 13 is showing the effect of plasma power on the conversion of methane. At the lowest plasma power of 15 W, there was no conversion of CH4. This suggested that this power level was insufficient to ignite the reaction and activate the methane. At 20 W, there's a noticeable CH4 conversion rate of approximately 59 %. Increasing the plasma power to 25 W significantly enhances the CH₄ conversion to 92 %. At the highest power setting of 30 W, CH4 conversion further increases to 95 %. The results clearly demonstrated the impact of power on the conversion of methane. As the power increases, the conversion of CH4 improves. This phenomenon can be described by the fact that as power increased, there is a corresponding rise in the production of active species. And these active species played important role in promoting the conversion of methane (Lee et al., 2015). 36 Figure 13 Methane conversion while keeping the feed flow rate 200ml/min in plasma alone system. In the Figure 14, the selectivity of CO2 and CO are described. With increasing plasma power from 15 to 30, there is a clear trend of higher selectivity towards CO2 and lower selectivity towards CO. This indicates that higher plasma powers are more effective in achieving complete oxidation of CH4 to CO2, rather than partial oxidation to CO. This phenomenon can be explained by delving into the intricacies of plasma chemistry. Methane and oxygen underwent dissociation upon interacting with the high-energy plasma electrons, resulting in the formation of methyl, hydrogen, and oxygen radicals, respectively (Kolb et al., 2013). Notably, during this phenomenon oxygen radicals were not only more reactive but also more abundant compared to methane. Consequently, these energetic oxygen atoms engaged in reactions with CH3, H, or OH radicals, leading to the production of carbon monoxide, carbon dioxide, water and hydrogen. At lower temperatures, CO was the product of choice, displaying higher selectivity, whereas as the temperature increased, a more abundant generation of CO2 occurred, stemming from the oxidation of CO to CO2 (Zhou et al., 1998). 0 20 40 60 80 100 120 15 W 20 W 25 W 30 W (%) Conversion Plasma power (W) 37 Figure 14 Selectivity of CO2 and CO. Feed flow rate 200ml/min while changing the plasma powers in plasma alone system. In the Figure 15, the impact of plasma power on the production of NOx and N2O showed. Increasing the plasma power enhances the formation of unwanted gasses. Across all plasma powers NO2 and N2O levels escalated while there was not any formation of NO. These outcomes are in line with the findings of prior work by (Penetrante et al., 1997) and (Marques et al., 2008). N2 molecules dissociated into nitrogen active species and then reacted with the active oxygen and form NO and this NO reacted with the oxygen radical and formed NO2 (Nguyen et al., 2019). Figure 15 Formation of NOx and N2O. Feed flowrate 200ml/min 0,00 20,00 40,00 60,00 80,00 100,00 120,00 15 W 20 W 25 W 30 W % Selectivity Plasma power (W) S-CO2 S-CO 0 100 200 300 400 500 600 700 800 900 15 W 20 W 25 W 30 W ppm Plasma power (W) NO NO2 N2O 38 8.2.1. Effect of discharge gap When we conducted experiments using only the plasma-alone system, we observed varying results across the reactors. The reactor A, which had an even gap between electrodes, performed well. The outcomes from this reactor were better than the other. Conversely, the reactors B and C, even when operated under the same conditions as the reactor A, yielded less methane conversion. Moreover, it was visually observed that the strength of the plasma glow inside the reactors B and C was not homogenous and at some parts of the regime the glow was more pronounced than in other parts. At parts where the discharge gap visually larger, the glow was dimmer there. The µGC results for these three reactors are shown in Table 7. Table 7 µGC results for three DBD reactors A, B, C under same experiment conditions. Power Inlet vol. % Outlet vol. % CH4 O2 N2 CH4 O2 N2 CO2 CO H2 20 W 0,96 20,7 76,8 0,51 20,11 77,5 0,17 0,24 0,03 25 W 0,96 20,7 76,8 0,09 19,26 77,7 0,54 0,30 0 30 W 0,96 20,7 76,8 0,06 19,17 77,9 0,64 0,25 0 20 W 0,96 20,7 77,02 0,84 20,55 77,1 0,05 0,06 0 25 W 0,96 20,7 77,02 0,44 19,94 77,7 0,24 0,24 0,02 30 W 0,96 20,7 77,02 0,20 19,49 78,05 0,41 0,30 0,03 20 W 0,96 20,7 76,8 0,96 20,68 76,8 0 0 0 25 W 0,96 20,7 76,8 0,53 20,09 77,4 0,15 0,25 0,03 30 W 0,96 20,7 76,8 0,25 19,73 77,8 0,34 0,33 0,03 In this experimental study utilizing three quartz glass reactors (A, B, and C), the observed trends in methane conversion and product selectivity revealed distinct performance characteristics as shown in Figures 16, 17 and 18. Reactor A consistently demonstrated higher methane conversion and carbon dioxide production compared to Reactor B and Reactor C across all tested power levels. Conversely, Reactor C exhibited the highest selectivity to carbon monoxide 57 % at 25 W among the three reactors. 39 Figure 16 Methane conversion, CO2 and CO selectivity for reactor A (plasma alone system) Figure 17 Methane conversion, CO2 and CO selectivity for reactor B (plasma alone system) 0,00 10,00 20,00 30,00 40,00 50,00 60,00 70,00 80,00 90,00 100,00 15 W 20 W 25 W 30 W % Plasma power (W) Reactor A X-CH4 S-CO2 S-CO 0 10 20 30 40 50 60 70 80 90 15 W 20 W 25 W 30 W % plasma power (w) Reactor B X-CH4 S-CO2 S-CO 40 In the Figure 19, the NOx and N2O formation for these reactors are shown. The trend was same as for the methane conversion. The reactor A generated more unwanted gases than those of other two reactors, which is more likely related to the higher conversion compared to other reactors. The variations in reactor performance underscore the influence of reactor design and power conditions on methane conversion and selectivity to products. As explained by (Van Laer & Bogaerts, 2017), in the DBD reactors with microgap (0,5 mm), the electric field and electron temperature experience a significant augmentation compared to other reactor configurations. This intensified electric field contributes to a higher electron temperature within the reactor. However, there is a trade-off, as this heightened electric field concurrently leads to a reduction in electron density. The lowered electron density has a pronounced impact on the reaction kinetics within the reactor. Specifically, the reaction rate is influenced by the density of the electron. When the density of electron decreases it can consequently affect the overall efficiency and dynamics of the chemical processes occurring in the reactor with smaller gaps. 0 10 20 30 40 50 60 70 80 15 W 20 W 25 W 30 W % plasma power (w) Reactor C X-CH4 S-CO2 S-CO Figure 18 Methane conversion, CO2 and CO selectivity for reactor C (plasma alone system) 47 In the Figures 26 and 27, the conversion of methane and the selectivity of CO2 and CO are shown. The methane conversion trend was same as in the previous experiments done with the catalysts. Almost 65 % of conversion achieved at 30 W. There was no any formation of CO. Figure 27 Selectivity of CO2 and CO using 1 % Pd-0,5 % Cu/γ-Al2O3 as catalyst in plasma-catalysis system 8.3.5. Hybridization effect of plasma and 1 % Cu/ γAl2O3 The 1 % Cu /Al2O3 was used as a catalyst inside the reactor plasma zone. Two experiments were performed by keeping the experimental conditions same. The 1,17 g of catalyst was 0 10 20 30 40 50 60 70 15 W 20 W 25 W 30 W % Conversion Plasma power (W) X-CH4 Figure 26 Methane conversion using 1 % Pd-0,5 % Cu/γ-Al2O3 as catalyst in plasma-catalysis system 48 used in both experiments and the flowrate of the feed kept constant at 200 ml/min. In the Table 12, the results are shown. Table 12 Results from experiment 04 and 05 using 1 % Cu/γ-Al2O3 as catalyst in plasma-catalysis. Power (W) Inlet % Outlet % CH4 O2 N2 CH4 O2 N2 CO2 CO H2 20 0,96 20,80 76,70 0,79 20,50 76,90 0,11 0,03 0,03 30 0,96 20,80 76,70 0,26 19,54 77,82 0,66 0,03 20 0,96 20,74 76,78 0,59 20,11 77,33 0,30 0,03 0,05 30 0,96 20,74 76,78 0,21 19,42 78,06 0,71 0,02 0,02 In the Figures 28 and 29, the methane conversion and CO2 selectivity with respect to various plasma powers is shown. At 20 W, the conversion was 51 % and the CO2 selectivity 54 %. When the power increased to the highest point tested, the conversion was 85 % and the CO2 selectivity increased to 96 %. 0 10 20 30 40 50 60 70 80 90 15 W 20 W 30 W % Conversion Plasma power (W) X-CH4 Figure 28 Methane conversion for plasma-catalysis system using 1 % Cu/Al2O3 49 Figure 29 Selectivity of CO2 and CO using 1 % Cu/γ-Al2O3 as catalyst in plasma-catalysis system 8.3.6. Hybridization effect of plasma and 1 % Pt/ γAl2O3 The 1 % Pt /Al2O3 was used as a catalyst inside the reactor plasma zone. Two experiments were performed by keeping the experimental conditions same. The 1,12 g of catalyst was used in both experiments and the flowrate of the feed kept constant at 200 ml/min. In the Table 13, the results are shown. Table 13 Result from experiment 19 and 20 using 1 % Pt/γ-Al2O3 as catalyst in plasma-catalysis. Power (W) Inlet % Outlet % CH4 O2 N2 CH4 O2 N2 CO2 CO H2 20 0,96 20,79 77,00 0,89 20,69 77,10 0,04 0,03 0 25 0,96 20,79 77,00 0,71 20,36 77,52 0,24 0 0 30 0,96 20,79 77,00 0,61 20,11 77,42 0,34 0 0 20 0,96 20,66 76,58 0,89 20,50 76,59 0,03 0 0 25 0,96 20,66 76,58 0,78 20,35 76,85 0,21 0 0 30 0,96 20,66 76,58 0,61 19,99 76,91 0,32 0 0 In the Figures 30 and 31, the conversion of methane and selectivity is provided. At 20 W the conversion was 22 % at the CO2 selectivity was 45 %. After increasing the power to 30 W the conversion was 57 % and CO2 selectivity was 97%. 50 Figure 31 Selectivity of CO2 and CO using 1 % Pt/γ-Al2O3 as catalyst in plasma-catalysis system 8.3.7. Hybridization effect of plasma and 1 % Pd-NH4-ZSM-5 The 1 % Pd-NH4-ZSM-5 was used as a catalyst inside the reactor plasma zone. Two experiments were performed by keeping the experimental conditions same. The 1,10 g of catalyst was used in both experiments and the flowrate of the feed kept constant at 200 ml/min. In the Table 14, the results are shown. 0 10 20 30 40 50 60 15 W 20 W 25 W 30 W % Conversion Plasma power (W) X-CH4 Figure 30 Methane conversion for plasma-catalysis system using 1 % Pt/Al2O3 51 Table 14 Results from experiment 16 and 18 using 1 % Pd-NH4-ZSM-5 as catalyst in plasma-catalysis system Power Inlet % Outlet % CH4 O2 N2 CH4 O2 N2 CO2 CO H2 20 W 0,97 20,75 77,16 0,96 20,74 77,11 0,00 0 0 25 W 0,97 20,75 77,16 0,88 20,56 77,17 0,08 0 0 30 W 0,97 20,75 77,16 0,84 20,55 77,43 0,12 0 0 20 W 0,96 20,81 77,03 0,96 20,71 77,02 0,00 0 0 25 W 0,96 20,81 77,03 0,87 20,72 77,29 0,09 0 0 30 W 0,96 20,81 77,03 0,83 20,58 77,15 0,12 0 0 In the Figure 32, the conversion of methane is given. At 15 W to 20 W, there was no conversion at all. But when the power increased to 25 W, 31 % conversion was achieved and for 30 W the conversion was 38 %. Regardless of less conversion, the gas that was produced during the reaction was only CO2 as you can see from the Figure 33. 0 5 10 15 20 25 30 35 40 15 W 20 W 25 W 30 W % Conversion Plasma power (W) X-CH4 Figure 32 Methane conversion for plasma-catalysis system using 1 % Pd-NH4-ZSM-5 52 Figure 33 Selectivity of CO2 and CO using 1 % Pd-NH4-ZSM-5 as catalyst in plasma-catalysis system Summary of the results In the Figure 34, the comparison of all the catalysts under the same experimental conditions is presented. The data shows a consistent trend: methane conversion generally improved with escalating plasma power, irrespective of the catalyst. However, the extent of improvement and the starting conversion rates are highly contingent upon the catalyst used. Some catalysts, such as 1 % Pd, 1 % Cu with alumina support displayed a synergy with increased power producing only the carbon dioxide and water. But these conversions were lower of that plasma alone system as plasma alone system converted 95 % of methane at 30 W and 1 % Pd on alumina support converted 88 % methane. The extent of oxidation on the metal surfaces plays noticeable role in examining the overall reaction rate. This phenomenon is characterized by terms like weak and strong adsorption of oxygen, as well as the variations in the surface oxide state (Becker et al., 2007). For example, in the case of Pd metal, the PdO is the principal species for methane oxidation as discussed in the Section 4.2 above. Except Pd metal, all other metals loadings on alumina support resulted in the lower conversion of methane but the selectivity towards carbon dioxide was promising. The active metals could also promote the side reactions like water gas shift reaction, methanation which could result in lower conversion of methane (Gao et al., 2021). 53 Figure 34 Methane conversion while keeping the feed flow rate 200ml/min constant and using the catalysts. When catalyst is used in the plasma system, the excited methane by electron collisions, is the key species to promote the conversion of methane according to (Nozaki & Okazaki, 2013). When a catalyst was placed into a plasma system, the temperature of vibrationally excited methane changed. In the plasma alone system, the temperature of the reactor at 30 W was 197 ℃ while in the presence of the catalyst (1 % Pd/ Al2O3) the bed temperature was 156℃. In the Figure 35, the reactor outside temperature in accordance with plasma power is shown. The trend was in increasing manner, as the power increased the bed temperature increased. The energy from the plasma, which was previously dedicated to exciting methane and oxygen molecules, was now shared to activate the catalyst in the plasma-catalysis system. These results align with the research done by (Nozaki et al., 2004). As the catalyst holds importance in the adsorption of active species such as oxygen atoms and excited methane species. The presence of the catalyst impacted on the reaction pathways by which methane was converted in the plasma alone reaction. When there was no catalyst inside the reactor, for example, the plasma power was used to break down methane, resulting in the generation of oxygen radicals. Following that, these radicals undergo both complete and partial oxidation, producing carbon dioxide and carbon monoxide. The reaction routes, however, were altered when catalyst material was inserted into the reactor. Because the catalyst 0 20 40 60 80 100 15 W 20 W 25 W 30 W % Conversion 1% Pd/Al2O3 3% Pd/Al2O3 1% Pd-0.5% Cu/Al2O3 1% Pt/Al2O3 1% Pd-NH4-ZSM-5 1% Cu/Al2O3 γ-Al2O3 54 allowed for more selective reactions, only carbon dioxide was generated, implying a shift in product distribution. Figure 35 Plasma power vs reactor outside temperature for plasma catalyst system The Figures 36 and 37, show the CO2 and CO selectivity for different catalysts. In the case of plasma alone system CO2 selectivity increased slightly with plasma power while CO selectivity decreased as power increased. The plasma-alone system becomes more selective towards CO2 and less towards CO as power increases. When 1 % Pd/Al2O3 was used with plasma CO2 selectivity improved significantly with power and CO remained at zero across the tested power range. While using 3 % Pd/Al2O3 the CO2 selectivity begins lower but catches up at the highest power. Like the 1% Pd loading, 3% Pd loading is also effective at suppressing CO formation while promoting CO2. These results are similar as from (Marques et al., 2008). When Pd/ Al2O3 is used as catalyst, the active phase of Pd is PdO (Chen et al., 2015). For γ-Al2O3 without any metal became slightly selective towards CO at the highest power tested, while CO2 increased gradually. In comparing the CO and CO2 selectivity, it seems that catalysts with metals, strongly favor CO2 formation over CO as explained by (Nkinahamira et al., 2023). This phenomenon exemplified the complicated interplay between plasma and catalyst, in which the catalyst becomes an active participant in the total reaction scheme, impacting not just product distribution but also plasma energy use as explained by (Cordi & Falconer, 1996). 0 50 100 150 200 250 15 20 25 30 Reactor outside temp. C Plasma power (W) Plasma alone 1% Pd/Al2O3 1% Cu/Al2O3 1% Pt/Al2O3 1% Pd-NH4-ZSM-5 1% Pd-0.5% Cu/Al2O3 3% Pd/Al2O3 γ-Al2O3 55 Figure 36 CO2 selectivity under various catalyst while keeping the Feed flowrate constant 200ml/min and changing the plasma power. Figure 37 CO selectivity under same experiment condition (Feed flowarte 200ml/min) NOx formation This section describes the NOx and N2O formation in the reactor for both systems, when no catalyst was used inside the plasma zone and when catalyst was placed inside the plasma zone. In Figures 38-40, the formation of NOx and N2O gases under plasma alone and plasmacatalysis system is shown. At the lowest plasma power of 15 W, there's a unanimous 0 20 40 60 80 100 120 15 W 20 W 25 W 30 W S-CO2 (%) Plasma Power (W) 1% Pd/Al2O3 3% Pd/Al2O3 1% Pd-0.5% Cu/Al2O3 1% Pt/Al2O3 1% Pd-NH4-ZSM-5 1% Cu/Al2O3 γ-Al2O3 0 10 20 30 40 50 60 15 W 20 W 25 W 30 W S-CO Plasma Power (W) 1% Pd/Al2O3 3% Pd/Al2O3 1% Pd-0.5% Cu/Al2O3 1% Pt/Al2O3 1% Pd-NH4-ZSM-5 1% Cu/Al2O3 γ-Al2O3 56 observation: no NOx production occurred across all catalysts and even with plasma alone system. There was no any NO formation for plasma alone, 1% Pd-NH4-ZSM-5, 1 % Pt/Al2O3, 1 % Pd-0,5 % Cu/Al2O3 and γ-Al2O3 plasma-catalysis systems regardless of plasma powers. However, as the plasma power increased, the NOx formation trend become discernible. By increasing the power, both systems displayed an increase in NOx formation, particularly the plasma alone and 1% Pd/Al2O3 system. This phenomenon was explained by (Baylet et al., 2012). In addition to dissociating methane molecules, the plasma also induced the dissociation of N2 molecules into nitrogen radicals through processes involving electron impact and vibrational excitation. These nitrogen radicals actively participated in oxidation reactions, which led to the generation of nitrogen oxides and nitrous oxide. The produced NO further undergoes oxidation to NO2 (Equations listed in the section 5.2). Figure 38 NOx and N2O formation for both systems at 20 W power 0 100 200 300 400 500 600 Plasma alone 1% Pd/Al2O3 3% Pd/Al2O3 Pd-Cu/Al2O3 1% Pd-NH4-ZSM-5 1% Pt/Al2O3 1% Cu/Al2O3 γ-Al2O3 20 W Concentration (ppm) Plasma power (W) NO NO2 N2O Total 63 Dong, M., Li, T., Xu, J., Zhang, T., Sun, Y., Li, N., Wu, Z., Li, J., Gao, E., Zhu, J., Yao, S., & Huang, Y. (2023). Pd on anionic conductive ZrO2 for low-concentration methane oxidation: Synergetic effect of plasma and catalysis. Molecular Catalysis, 537. https://doi.org/10.1016/j.mcat.2023.112936 Drake, G. W. F., Babb, J., Bandrauk, A. D., & Joachain, C. J. (n.d.). Springer Series on Atomic, Optical, and Plasma Physics Editor-in-Chief. http://www.springer.com/series/411 Edenhofer, O., Pichs-Madruga, R., Sokona, Y., Minx, J. C., Farahani, E., Kadner, S., Seyboth, K., Adler, A., Baum, I., Brunner, S., Eickemeier, P., Schlömer, S., von Stechow, C., & Zwickel, T. (2015). Climate change 2014: mitigation of climate change. Eliasson, B., & Kogelschatz, U. (1991). Nonequilibrium Volume Plasma Chemical Processing. IEEE Transactions on Plasma Science, 19(6), 1063–1077. https://doi.org/10.1109/27.125031 Eliasson, B., Member, S., & Kogelschatz, U. (1991). Nonequilibrium Volume Plasma Chemical Processing. In IEEE TRANSACTIONS ON PLASMA SCIENCE (Vol. 19, Issue 6). Epling, W. S., & Hoflund, G. B. (1999). Catalytic Oxidation of Methane over ZrO2-Supported Pd Catalysts. Journal of Catalysis, 182(1), 5–12. https://doi.org/10.1006/jcat.1998.2341 Friberg, I., Clark, A. H., Ho, P. H., Sadokhina, N., Smales, G. J., Woo, J., Auvray, X., Ferri, D., Nachtegaal, M., Kröcher, O., & Olsson, L. (2021). Structure and performance of zeolite supported Pd for complete methane oxidation. Catalysis Today, 382, 3–12. https://doi.org/10.1016/j.cattod.2020.11.026 Fulcheri, L. (1995). From methane to hydrogen, carbon black and water. International Journal of Hydrogen Energy, 20(3), 197–202. https://doi.org/10.1016/0360-3199(94)E0022-Q Gao, Y., Dou, L., Zhang, S., Zong, L., Pan, J., Hu, X., Sun, H., Ostrikov, K. (Ken), & Shao, T. (2021). Coupling bimetallic Ni-Fe catalysts and nanosecond pulsed plasma for synergistic low-temperature CO2 methanation. Chemical Engineering Journal, 420, 127693. https://doi.org/10.1016/j.cej.2020.127693 Getabalew, M., Alemneh, T., & Akeberegn, D. (2019). Methane Production in Ruminant Animals: Implication for Their Impact on Climate Change. https://doi.org/10.32474/CDVS.2019.02.000142 Gholami, R., Stere, C., Chansai, S., Singhania, A., Goguet, A., Hinde, P., Millington, P., & Hardacre, C. (2022). Optimization of Non-thermal Plasma-Assisted Catalytic Oxidation for Methane Emissions Abatement as an Exhaust Aftertreatment Technology. Plasma Chemistry and Plasma Processing, 42(4), 709–730. https://doi.org/10.1007/s11090-022-10253-3 Hammer, T. (2000, October 16). Pulsed Electrical Excitation of Dielectric Barrier Discharge Reactors using Semiconductor Power Supplies. https://doi.org/10.4271/2000-01-2894 Harling, A. M., Kim, H.-H., Futamura, S., & Whitehead, J. C. (2007). Temperature Dependence of Plasma−Catalysis Using a Nonthermal, Atmospheric Pressure Packed Bed; the Destruction of Benzene and Toluene. The Journal of Physical Chemistry C, 111(13), 5090–5095. https://doi.org/10.1021/jp067821w Hayashi, K., Yasui, H., Tanaka, M., Futamura, S., Kurita, S., & Aoyagi, K. (2009). Temperature Dependence of Toluene Decomposition Behavior in the Discharge–Catalyst Hybrid Reactor. IEEE Transactions on Industry Applications, 45(5), 1553–1558. https://doi.org/10.1109/TIA.2009.2027101 64 Heberlein, J. (2002). New approaches in thermal plasma technology*. In Pure Appl. Chem (Vol. 74, Issue 3). Holmes, C. D., Prather, M. J., Søvde, O. A., & Myhre, G. (2013). Future methane, hydroxyl, and their uncertainties: key climate and emission parameters for future predictions. Atmospheric Chemistry and Physics, 13(1), 285–302. https://doi.org/10.5194/acp-13-285-2013 Holzer, F. (2002). Combination of non-thermal plasma and heterogeneous catalysis for oxidation of volatile organic compounds Part 1. Accessibility of the intra-particle volume. Applied Catalysis B: Environmental, 38(3), 163–181. https://doi.org/10.1016/S0926-3373(02)00040-1 Huang, L., Nakajyo, K., Hari, T., Ozawa, S., & Matsuda, H. (2001). Decomposition of Carbon Tetrachloride by a Pulsed Corona Reactor Incorporated with In Situ Absorption. Industrial & Engineering Chemistry Research, 40(23), 5481–5486. https://doi.org/10.1021/ie010172k Intergouvernemental panel on climate change. Working group 1., Masson-Delmotte, V. (1971-. . . . )., Zhai, P. (19. .-. . . . )., & Pirani, A. (19. .-. . . . ). (n.d.). Climate change 2021 : the physical science basis : summary for policymakers : working group I contribution to the sixth Assessment report of the Intergovernmental Panel on Climate Change. Intergovernmental Panel on Climate Change. (2023). Weather and Climate Extreme Events in a Changing Climate. In Climate Change 2021 – The Physical Science Basis (pp. 1513–1766). Cambridge University Press. https://doi.org/10.1017/9781009157896.013 Isaksen, I., Berntsen, T., Dalsøren, S., Eleftheratos, K., Orsolini, Y., Rognerud, B., Stordal, F., Søvde, O., Zerefos, C., & Holmes, C. (2014). Atmospheric Ozone and Methane in a Changing Climate. Atmosphere, 5(3), 518–535. https://doi.org/10.3390/atmos5030518 Jackson, R. B., Abernethy, S., Canadell, J. G., Cargnello, M., Davis, S. J., Féron, S., Fuss, S., Heyer, A. J., Hong, C., Jones, C. D., Damon Matthews, H., O’Connor, F. M., Pisciotta, M., Rhoda, H. M., de Richter, R., Solomon, E. I., Wilcox, J. L., & Zickfeld, K. (2021). Atmospheric methane removal: a research agenda. Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, 379(2210), 20200454. https://doi.org/10.1098/rsta.2020.0454 Jagadeesh, P., Varun, Y., Himajaa Reddy, B., Sreedhar, I., & Singh, S. A. (2023). A short review on recent advancements of dry reforming of methane (DRM) over pyrochlores. Materials Today: Proceedings, 72, 361–369. https://doi.org/10.1016/j.matpr.2022.08.107 Jang, W.-J., Shim, J.-O., Kim, H.-M., Yoo, S.-Y., & Roh, H.-S. (2019). A review on dry reforming of methane in aspect of catalytic properties. Catalysis Today, 324, 15–26. https://doi.org/10.1016/j.cattod.2018.07.032 Jiang, D., Khivantsev, K., & Wang, Y. (2020). Low-Temperature Methane Oxidation for Efficient Emission Control in Natural Gas Vehicles: Pd and beyond. ACS Catalysis, 10(23), 14304– 14314. https://doi.org/10.1021/acscatal.0c03338 Khacef, A., & Da Costa, P. (2019). Plasma-Catalytic Removal of NOx in Mobile and Stationary Sources (pp. 115–144). https://doi.org/10.1007/978-3-030-05189-1_5 Khoja, A. H., Tahir, M., & Amin, N. A. S. (2019). Recent developments in non-thermal catalytic DBD plasma reactor for dry reforming of methane. Energy Conversion and Management, 183, 529–560. https://doi.org/10.1016/j.enconman.2018.12.112 65 Kim, H. H., Teramoto, Y., Ogata, A., Takagi, H., & Nanba, T. (2016a). Plasma Catalysis for Environmental Treatment and Energy Applications. In Plasma Chemistry and Plasma Processing (Vol. 36, Issue 1, pp. 45–72). Springer New York LLC. https://doi.org/10.1007/s11090-015-9652-7 Kim, H. H., Teramoto, Y., Ogata, A., Takagi, H., & Nanba, T. (2016b). Plasma Catalysis for Environmental Treatment and Energy Applications. In Plasma Chemistry and Plasma Processing (Vol. 36, Issue 1, pp. 45–72). Springer New York LLC. https://doi.org/10.1007/s11090-015-9652-7 Kogelschatz, U. (2003). Dielectric-barrier Discharges: Their History, Discharge Physics, and Industrial Applications (Vol. 23, Issue 1). Kolb, T., Voigt, J. H., & Gericke, K. H. (2013). Conversion of methane and carbon dioxide in a DBD reactor: Influence of oxygen. Plasma Chemistry and Plasma Processing, 33(4), 631– 646. https://doi.org/10.1007/s11090-013-9448-6 Lee, H., Lee, D. H., Song, Y. H., Choi, W. C., Park, Y. K., & Kim, D. H. (2015). Synergistic effect of non-thermal plasma-catalysis hybrid system on methane complete oxidation over Pd-based catalysts. Chemical Engineering Journal, 259, 761–770. https://doi.org/10.1016/j.cej.2014.07.128 Li, W., He, D., Hu, G., Li, X., Banerjee, G., Li, J., Lee, S. H., Dong, Q., Gao, T., Brudvig, G. W., Waegele, M. M., Jiang, D., & Wang, D. (2018). Selective CO Production by Photoelectrochemical Methane Oxidation on TiO 2. ACS Central Science, 4(5), 631–637. https://doi.org/10.1021/acscentsci.8b00130 Li, X., Wang, C., & Tang, J. (2022). Methane transformation by photocatalysis. Nature Reviews Materials, 7(8), 617–632. https://doi.org/10.1038/s41578-022-00422-3 Magureanu, M., & Bradu, C. (n.d.). Plasma-Catalysis for Environmental and Energy-Related Applications. www.mdpi.com/journal/catalysts Majdinasab, A., & Yuan, Q. (2017). Performance of the biotic systems for reducing methane emissions from landfill sites: A review. Ecological Engineering, 104, 116–130. https://doi.org/10.1016/j.ecoleng.2017.04.015 Marques, R., Da Costa, S., & Da Costa, P. (2008). Plasma-assisted catalytic oxidation of methane. Applied Catalysis B: Environmental, 82(1–2), 50–57. https://doi.org/10.1016/j.apcatb.2007.12.024 Melse, R. W., & van der Werf, A. W. (2005). Biofiltration for Mitigation of Methane Emission from Animal Husbandry. Environmental Science & Technology, 39(14), 5460–5468. https://doi.org/10.1021/es048048q Migeotte, M. V. (1948). Spectroscopic Evidence of Methane in the Earth’s Atmosphere. Physical Review, 73(5), 519–520. https://doi.org/10.1103/PhysRev.73.519.2 Monai, M., Montini, T., Gorte, R. J., & Fornasiero, P. (2018). Catalytic Oxidation of Methane: Pd and Beyond. European Journal of Inorganic Chemistry, 2018(25), 2884–2893. https://doi.org/10.1002/ejic.201800326 Monroe J.A. (1997). Air Quality Inside Livestock Barns. http://omafra.gov.on.ca/english/livestock/swine/facts/93-001.htm 66 Nguyen, V. T., Nguyen, D. B., Heo, I., & Mok, Y. S. (2019). Plasma-Assisted Selective Catalytic Reduction for Low-Temperature Removal of NOx and Soot Simulant. Catalysts, 9(10), 853. https://doi.org/10.3390/catal9100853 Nations, Food and Agriculture Organization of the United Nations. FAOSTAT statistical database. [Rome]: FAO, c1997-. https://search.library.wisc.edu/catalog/999890171702121 Nilsson, J., Carlsson, P.-A., Fouladvand, S., Martin, N. M., Gustafson, J., Newton, M. A., Lundgren, E., Grönbeck, H., & Skoglundh, M. (2015). Chemistry of Supported Palladium Nanoparticles during Methane Oxidation. ACS Catalysis, 5(4), 2481–2489. https://doi.org/10.1021/cs502036d Nkinahamira, F., Yang, R., Zhu, R., Zhang, J., Ren, Z., Sun, S., Xiong, H., & Zeng, Z. (2023). Current Progress on Methods and Technologies for Catalytic Methane Activation at Low Temperatures. Advanced Science, 10(5), 2204566. https://doi.org/10.1002/advs.202204566 Nozaki, T., Muto, N., Kadio, S., & Okazaki, K. (2004). Dissociation of vibrationally excited methane on Ni catalyst. Catalysis Today, 89(1–2), 67–74. https://doi.org/10.1016/j.cattod.2003.11.039 Nozaki, T., & Okazaki, K. (2013). Non-thermal plasma catalysis of methane: Principles, energy efficiency, and applications. Catalysis Today, 211, 29–38. https://doi.org/10.1016/j.cattod.2013.04.002 Oeste, F. D., de Richter, R., Ming, T., & Caillol, S. (2017). Climate engineering by mimicking natural dust climate control: the iron salt aerosol method. Earth System Dynamics, 8(1), 1–54. https://doi.org/10.5194/esd-8-1-2017 Penetrante, B. M., J. Norman Bardsley, J. N. B., & Mark C. Hsiao, M. C. H. (1997). Kinetic Analysis of Non-Thermal Plasmas Used for Pollution Control. Japanese Journal of Applied Physics, 36(7S), 5007. https://doi.org/10.1143/JJAP.36.5007 Pham Huu, T., Gil, S., Da Costa, P., Giroir-Fendler, A., & Khacef, A. (2015a). Plasma-catalytic hybrid reactor: Application to methane removal. Catalysis Today, 257(P1), 86–92. https://doi.org/10.1016/j.cattod.2015.03.001 Pham Huu, T., Gil, S., Da Costa, P., Giroir-Fendler, A., & Khacef, A. (2015b). Plasma-catalytic hybrid reactor: Application to methane removal. Catalysis Today, 257(P1), 86–92. https://doi.org/10.1016/j.cattod.2015.03.001 Pizzolitto, C., Pupulin, E., Menegazzo, F., Ghedini, E., Di Michele, A., Mattarelli, M., Cruciani, G., & Signoretto, M. (2019). Nickel based catalysts for methane dry reforming: Effect of supports on catalytic activity and stability. International Journal of Hydrogen Energy, 44(52), 28065–28076. https://doi.org/10.1016/j.ijhydene.2019.09.050 Quoc An, H. T., Pham Huu, T., Le Van, T., Cormier, J. M., & Khacef, A. (2011). Application of atmospheric non thermal plasma-catalysis hybrid system for air pollution control: Toluene removal. Catalysis Today, 176(1), 474–477. https://doi.org/10.1016/j.cattod.2010.10.005 Rotz, C. A., Montes, F., & Chianese, D. S. (2010). The carbon footprint of dairy production systems through partial life cycle assessment. Journal of Dairy Science, 93(3), 1266–1282. https://doi.org/10.3168/jds.2009-2162 67 Samal, S., & Park, D.-W. (2012). Nano-particle synthesis of titanium oxides from ilmenite in a thermal plasma reactor. Chemical Engineering Research and Design, 90(4), 548–554. https://doi.org/10.1016/j.cherd.2011.08.011 San Wong Rattachat Mongkolnavin, C. (n.d.). SPRINGER BRIEFS IN APPLIED SCIENCES AND TECHNOLOGY. http://www.springer.com/series/8884 Saunois, M., Stavert, A. R., Poulter, B., Bousquet, P., Canadell, J. G., Jackson, R. B., Raymond, P. A., Dlugokencky, E. J., Houweling, S., Patra, P. K., Ciais, P., Arora, V. K., Bastviken, D., Bergamaschi, P., Blake, D. R., Brailsford, G., Bruhwiler, L., Carlson, K. M., Carrol, M., … Zhuang, Q. (2020). The Global Methane Budget 2000–2017. Earth System Science Data, 12(3), 1561–1623. https://doi.org/10.5194/essd-12-1561-2020 Schwach, P., Pan, X., & Bao, X. (2017). Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects. Chemical Reviews, 117(13), 8497– 8520. https://doi.org/10.1021/acs.chemrev.6b00715 Snoeckx, R., & Bogaerts, A. (2017). Plasma technology-a novel solution for CO2 conversion? In Chemical Society Reviews (Vol. 46, Issue 19, pp. 5805–5863). Royal Society of Chemistry. https://doi.org/10.1039/c6cs00066e Stere, C., Chansai, S., Gholami, R., Wangkawong, K., Singhania, A., Goguet, A., Inceesungvorn, B., & Hardacre, C. (2020). A design of a fixed bed plasma DRIFTS cell for studying the NTP-assisted heterogeneously catalysed reactions. Catalysis Science and Technology, 10(5), 1458–1466. https://doi.org/10.1039/d0cy00036a Tubiello, F. N. (2019). Greenhouse Gas Emissions Due to Agriculture. In Encyclopedia of Food Security and Sustainability (pp. 196–205). Elsevier. https://doi.org/10.1016/B978-0-08100596-5.21996-3 Van Laer, K., & Bogaerts, A. (2017). Influence of Gap Size and Dielectric Constant of the Packing Material on the Plasma Behaviour in a Packed Bed DBD Reactor: A Fluid Modelling Study. Plasma Processes and Polymers, 14(4–5). https://doi.org/10.1002/ppap.201600129 Vandenbroucke, A. M., Morent, R., De Geyter, N., & Leys, C. (2011). Non-thermal plasmas for non-catalytic and catalytic VOC abatement. In Journal of Hazardous Materials (Vol. 195, pp. 30–54). https://doi.org/10.1016/j.jhazmat.2011.08.060 Wang, C., Xu, Y., & Tang, J. (2023). Catalytic methane removal to mitigate its environmental effect. In Science China Chemistry. Science Press (China). https://doi.org/10.1007/s11426022-1487-8 Wang, J., Chen, H., Hu, Z., Yao, M., & Li, Y. (2015). A Review on the Pd-Based Three-Way Catalyst. Catalysis Reviews, 57(1), 79–144. https://doi.org/10.1080/01614940.2014.977059 Wang, W., Snoeckx, R., Zhang, X., Cha, M. S., & Bogaerts, A. (2018). Modeling Plasma-based CO 2 and CH 4 Conversion in Mixtures with N 2 , O 2 , and H 2 O: The Bigger Plasma Chemistry Picture. The Journal of Physical Chemistry C, 122(16), 8704–8723. https://doi.org/10.1021/acs.jpcc.7b10619 Wei, H., Wang, M., Ya, M., & Xu, C. (2022). The denitrifying anaerobic methane oxidation process and microorganisms in the environments: A review. In Frontiers in Marine Science (Vol. 9). Frontiers Media S.A. https://doi.org/10.3389/fmars.2022.1038400 68 Wen, W., Che, J.-W., Wu, J.-M., Kobayashi, H., Pan, Y., Wen, W., Dai, Y.-H., Huang, W., Fu, C., Zhou, Q., Lu, G.-L., Tian, H., Liu, J., Yang, P., Chen, X., Sun, T.-L., & Fan, J. (2022). Co 3+ –O Bond Elongation Unlocks Co 3 O 4 for Methane Activation under Ambient Conditions. ACS Catalysis, 12(12), 7037–7045. https://doi.org/10.1021/acscatal.1c05744 Whitehead, J. C. (2016). Plasma-catalysis: The known knowns, the known unknowns and the unknown unknowns. In Journal of Physics D: Applied Physics (Vol. 49, Issue 24). Institute of Physics Publishing. https://doi.org/10.1088/0022-3727/49/24/243001 Whitehead, J. C. (2019). Plasma-catalysis: Is it just a question of scale? In Frontiers of Chemical Science and Engineering (Vol. 13, Issue 2, pp. 264–273). Higher Education Press. https://doi.org/10.1007/s11705-019-1794-3 Yao, S., Chen, Z., Weng, S., Mao, L., Zhang, X., Han, J., Wu, Z., Lu, H., Tang, X., Jiang, B., & Nozaki, T. (2019). Mechanism of CO2-formation promotion by Au in plasma-catalytic oxidation of CH4 over Au/γ-Al2O3 at room temperature. Journal of Hazardous Materials, 373, 698–704. https://doi.org/10.1016/j.jhazmat.2019.04.003 Zhang, L., Tian, H., Shi, H., Pan, S., Chang, J., Dangal, S. R. S., Qin, X., Wang, S., Tubiello, F. N., Canadell, J. G., & Jackson, R. B. (2022). A 130‐year global inventory of methane emissions from livestock: Trends, patterns, and drivers. Global Change Biology, 28(17), 5142–5158. https://doi.org/10.1111/gcb.16280 Zhao, J., Chen, Y., Wang, Y., Li, Z., Nkinahamira, F., Zhu, R., Zhang, J., Sun, S., Zhu, Y., Li, H., & Li, C. (2023). The poisoning mechanism of H2O/SO2 to In/H-Beta for selective catalytic reduction of NOx with methane. Applied Catalysis A: General, 649. https://doi.org/10.1016/j.apcata.2022.118973 Zhou, L. M., Xue, B., Kogelschatz, U., & Eliasson, B. (1998). Partial Oxidation of Methane to Methanol with Oxygen or Air in a Nonequilibrium Discharge Plasma. In Plasma Chemistry and Plasma Processing (Vol. 18, Issue 3). 69 APPENDIX 1 In the Table. 1, the experiment list of this study is showed. Feed composition (1 % CH4 in balanced air) and feed flowrate of 200 ml/min was employed throughout all the experiments. Experiment Description Packing material Catalyst weight (g) Reactor 01 Plasma alone None A 02 Plasma alone None A 03 Plasma alone None A 04 Plasma-catalysis 1 % Cu/γ-Al2O3 1,17 A 05 Plasma-catalysis 1 % Cu/γ-Al2O3 1,17 A 06 Plasma alone None A 07 Plasma-catalysis 1 % Pd/γ-Al2O3 1,20 A 08 Plasma-catalysis 1 % Pd/γ-Al2O3 1,20 A 09 Plasma-catalysis 3 % Pd/γ-Al2O3 1,18 A 10 Plasma-catalysis 3 % Pd/γ-Al2O3 1,18 A 11 Plasma-catalysis 1 % Pd-0,5 % Cu/γ-Al2O3 1,19 A 12 Plasma-catalysis 1 % Pd-0,5 % Cu/γ-Al2O3 1,19 B 13 Plasma alone None B 14 Plasma alone None B 15 Plasma alone None B 16 Plasma-catalysis 1 % Pd-NH4ZSM-5 1,10 B 17 Plasma-catalysis 1 % Pd-NH4ZSM-5 1,10 B 18 Plasma-catalysis 1 % Pd-NH4ZSM-5 1,10 B 19 Plasma-catalysis 1 % Pt/γ-Al2O3 1,20 B 20 Plasma-catalysis 1 % Pt/γ-Al2O3 1,20 B 21 Plasma-catalysis 1 % Pd-0,5 % Cu/γ-Al2O3 1,19 B 22 Plasma alone None C 23 Plasma alone None C 24 Plasma-catalysis γ-Al2O3 1,19 C 25 Plasma-catalysis γ-Al2O3 1,19 C Table. 1 The experiment list for this study 70 APPENDIX 2 Figure.1 is showing the electrical circuit of dielectric barrier discharge reactor which was utilized in this study. Figure. 1 Electrical circuit of DBD reactor (VTT) 71 APPENDIX 3 Figure 2. An example of measured gas composition with FTIR Figure. 2 Stabilized FTIR results from experiment 05 at 30 W plasma power. 72 APPENDIX 4 Figure. 3 presents the example of FTIR spectrum. Figure. 3 FTIR spectrum from experiment 05 at 30 W plasma power