Applied Science and Convergence Technology 2024; 33(5): 135-139
Published online September 30, 2024
https://doi.org/10.5757/ASCT.2024.33.5.135
Copyright © The Korean Vacuum Society.
Hyeokjun Kanga , Hyun Jun Kangb
, Hyeyoung Koa
, Yong Hee Leec
, and Sooseok Choia , ∗
aFaculty of Applied Energy System, Jeju National University, Jeju 63243, Republic of Korea
bUNISEM Co. Ltd., Hwaseong 18510, Republic of Korea
cInstitute for Nuclear Science and Technology, Jeju 63243, Republic of Korea
Correspondence to:sooseok@jejunu.ac.kr
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (https://creativecommons.org/licenses/by-nc-nd/4.0/) which permits non-commercial use, distribution and reproduction in any medium without alteration, provided that the original work is properly cited.
Many researchers have suggested various catalysts and synthesis methods to enhance hydrogen (H2) production efficiency and replace the noble catalyst in the methane (CH4) pyrolysis process. In particular, nanocomposites incorporating nickel (Ni) have shown outstanding pyrolysis performance due to their high activity. In the present study, a triple thermal plasma system is employed to synthesize Ni nanocomposites, Ni-cobalt (Ni-Co) and Ni-plated carbon nanotube (Ni-CNT). We then examine the catalytic performance of the synthesized Ni nanocomposite using a CH4 pyrolysis system. A field-emission scanning electron microscopy analysis with energy-dispersive X-ray spectroscopy mapping demonstrates that Ni-Co and Ni-CNT are successfully synthesized through the thermal plasma system. From the results of CH4 pyrolysis using the synthesized Ni composite, the Ni-Co and Ni-CNT show lower CH4 conversions of 29.3 and 45.7 %, respectively, than that of pure Ni at 45.8 % because of a thin oxygen layer formed on the particle surface. The Ni nanocomposite meanwhile exhibits more efficient performance than pure Ni in terms of H2 selectivity. In particular, Ni-Co has higher H2 selectivity of about 45 % due to its lower surface area compared to Ni-CNT. This work demonstrates the possibility of synthesizing Ni nanocomposites using triple thermal plasma for CH4 pyrolysis.
Keywords: Thermal plasma, Methane pyrolysis, Nanocomposite, Nickel, Catalysts
The rapid increase in global energy demand, coupled with environmental issues arising from the use of fossil fuels, is becoming more severe. Furthermore, carbon dioxide emissions from fossil fuel combustion have been identified as a significant cause of global warming, highlighting the growing need for clean energy alternatives. Among such alternatives, hydrogen (H2) energy is gaining attention as a nextgeneration energy source due to its high energy density and environmental friendliness [1–4]. H2 can be produced through various methods, such as grey H2, blue H2, turquoise H2 [5], and green H2, each differing in greenhouse gas emissions and economic viability. Specifically, the production of turquoise H2 through methane (CH4) pyrolysis is notable for its ability to reduce greenhouse gas emissions by not releasing carbon dioxide [6].
The CH4 pyrolysis for producing turquoise H2 follows the endothermic reaction CH4 → C + 2H2, with ΔH = 74.52 kJ/mol at 298 K, requiring significant energy to break the strong carbon–H2 (C–H) bond, which holds an energy of 435 kJ/mol [7,8]. Due to the high reaction energy requirement, various studies have been conducted utilizing different catalysts to improve this process. Catalysts, substances that alter the reaction rate without being consumed in the process, are commonly used in turquoise H2 production to reduce the reaction energy and enhance process speed [4].
The catalysts that can be used in CH4 pyrolysis include noble metals, transition metals, and non-metals. Noble metal catalysts include platinum and ruthenium. These catalysts exhibit excellent performance but are expensive due to their scarcity. Consequently, more affordable and efficient transition metal and non-metal catalysts are predominantly employed. Transition metal catalysts include nickel (Ni), cobalt (Co), and iron (Fe), which possess high electrical and thermal conductivity. Additionally, their partially filled 3d orbitals can accept electrons from the C–H bonds of CH4, which facilitates CH4 pyrolysis [4,9]. Non-metal catalysts include activated carbon, carbon fibers, and carbon nanotubes (CNTs) [10]. Although these catalysts exhibit lower performance than metal catalysts, they are cost-effective and stable.
A single catalyst has limitations in matching the performance of noble metal catalysts. To overcome this issue, extensive research has been conducted on synthesizing two or more catalysts that can achieve performance comparable to noble metal catalysts [4,11–16]. Among transition metal catalysts, Ni, Co, and Fe are widely used for CH4 pyrolysis, with Ni demonstrating the highest performance, followed by Co and Fe [4,9,11–16]. In this study, we synthesized a catalyst based on Ni combined with Co as a transition metal and CNTs as a nonmetal.
Nanocatalyst materials can be synthesized using chemical methods and plasma processes. Chemical methods involve producing materials of desired sizes through ion exchange, allowing for nanoparticle synthesis with minimal energy input [17]. However, these processes are often complex, requiring multiple stages and resulting in waste byproducts. This study employs thermal plasma as analternative, overcoming the limitations of traditional methods.
Thermal plasma synthesis is a technique that utilizes the high temperatures of plasma flames to synthesize catalysts efficiently. Thermal plasma has temperatures exceeding several thousand Kelvin, and it can rapidly vaporize any form of raw material, allowing for the synthesis of nanocatalysts in a short time [17–19]. Additionally, thermal plasma synthesis does not produce waste byproducts and supports continuous production, making it an environmentally friendly and economically viable method for catalyst manufacturing [20]. This study evaluates the efficiency and potential of catalysts synthesized using thermal plasma in the CH4 pyrolysis process.
This study used a triple thermal plasma system to synthesize Ni composites for CH4 pyrolysis. Generally, starting materials are injected into a plasma flame in a radial direction in a single thermal plasma system. However, because the fluid is more viscous with increasing temperature, injecting starting materials into the core region of a plasma flame with temperature of a few thousand Kelvin is difficult. In the triple thermal plasma system, the injector for starting materials is located in the center of three plasma torches. Once the three plasma flames gather and form a single enormous flame, starting materials can easily penetrate the core region of the flame in an axial direction. Previous works demonstrated that boron powder with a high boiling point of about 4,300 K evaporated more actively in the triple thermal plasma system. Since the Ni powder used as the starting material in this work also has a high boiling point of about 3,300 K, we synthesized Ni composites using the triple thermal plasma system and examined their characteristics for CH4 pyrolysis.
Figure 1 shows a schematic illustration of the triple thermal plasma system (Plasnix Co., Republic of Korea). This system comprises three thermal plasma torches, a vertical reactor (R-1, R-2, and R-3), horizontal reactors (R-4, R-5, R-6, and R-7), three power supplies, a cyclone filter, cooling water, mass flow controllers, and two injectors and powder feeders. Each torch has a 25° angle from the axis direction, with the three torches arranged at 120° intervals. Table I lists the experimental conditions used in this work. The plasma is discharged using a gas mixture, including 4 L/min of Ar and 8 L/min of N2, and its power is about 7 kW per torch. Thus, the total power supplied to the system is about 21 kW.
Table I. Experimental conditions used in this work..
Experimental conditions | Ni-Co composite | Ni-CNT composite |
---|---|---|
Input power (kW) | 21 | |
Plasma forming gas (L/min) | N2 8, Ar 4 (mixed, each torch) | |
Carrier gas (L/min) | Ar 5 | |
Mass ratio of Ni powder | 0.7 | 0.5 |
Injection position | Ni, Co: co-injector | Ni: co-injector |
CNT: counter-injector | ||
Feeding rate (g/min) | 1.1 |
Ni (50 μm, purity 99.7 %, Sigma Aldrich, USA) and Co (2 μm, purity 99.8 %, Sigma Aldrich, USA) penetrate the merged plasma flames through the injector, called a co-injector, placed in the center of plasma torches. On the other hand, the other injector, a counter injector, is located inside the reactor and supplies CNTs (D: 5 –20 nm, L: 0 –10 μm, Carbon Nano-material Technology, Republic of Korea) in the opposite direction to the mainstream. Since the vaporization of Ni and Co powders is essential for synthesizing the Ni-cobalt (Ni-Co) nanocomposite, a large amount of energy must be transferred into the powders. Thus, Ni and Co powders are supplied together through the co-injector. In comparison, it is necessary to prevent the CNTs from evaporating to put Ni particles on the CNTs without damage. To synthesize the Ni-CNT nanocomposite, CNTs are injected through the counter injector away from the core region of plasma flames. Powder feeders uniformly feed the starting materials at a 1.1 g/min rate using 5 L/min of Ar carrier gas.
The morphology and nanostructure of the products were investigated using field-emission transmission electron microscopy (FETEM) (Talos F200X G2, Thermo Fisher Scientific, USA) at an accelerating voltage of 200 kV. Elemental mapping of the nanoparticles was conducted using energy-dispersive X-ray spectroscopy (EDS) (Talos F200X G2, Thermo Fisher Scientific, USA). In addition, the standard Brunauer-Emmett-Teller (BET) (Tristar-2 plus 3030) with nitrogen adsorption was employed to determine the surface area and pore volume.
The catalytic performance of the synthesized Ni composites was investigated using the CH4 pyrolysis system shown in Fig. 2. A quartz tube with an inner diameter of 50 mm and a length of 350 mm is vertically inserted in an electrical furnace for thermal insulation. The quartz tube has two stainless gas pipes for the gas inlet and gas outlet and one thermocouple. The catalyst feeder is installed before the gas inlet pipe to supply the Ni composites with CH4 to the quartz tube. The supplied rate of CH4 and Ni composites is 0.2 L/min and 1.1 g/min, respectively. In addition, the temperature inside the quartz tube is fixed at 800 °C. Gas products at the outlet are diluted with N2 to decrease the gas temperature and are exhausted into the atmosphere. Some of the gas products with N2 are sampled using gas chromatography (YL6500 GC, Youngin Chromass, Republic of Korea) to measure their composition and species volume ratio. Based on the measured volume ratio, we calculated the flow rate of each species and estimated CH4 conversion and H2 selectivity as follows:
The crystal structures of the catalysts synthesized using triple DC thermal plasma were characterized by X-ray diffraction (XRD). Figure 3 presents the XRD results for the synthesized Ni-based catalysts. In the case of the Ni-Co, a prominent peak corresponding to Ni was observed at 44.3 degrees [21]. For the Ni-CNT, a peak associated with Ni was also detected, with no evidence of nickel carbide (NiC) present [22]. These observations indicate that the precursor materials did not undergo transformation into new compounds but rather retained their distinct properties and remained as a separate material. However, since the peaks for Ni, Co, and the Ni-Co appeared at similar degree values, further analytical methods are required to accurately ascertain the presence and composition of the nanocomposite.
Figure 4 shows field-emission scanning electron microscopy (FESEM) images of the synthesized Ni-based catalysts. The FE-SEM analysis revealed that the particles have an average size of less than 100 nm. Furthermore, Fig. 4(a) shows that the nanomaterials exhibit a spherical morphology. This spherical shape is likely the result of complete vaporization of the material followed by rapid condensation during the plasma process. Such spherical particles are expected to enhance the surface area available for CH4 adsorption due to their increased surface area, which could be beneficial for CH4 decomposition when used as a catalyst [23].
Figure 4(b) displays a FE-SEM image of the Ni-CNT. It reveals that spherical Ni particles, with sizes on the order of tens of nanometers, are distributed over the CNT surface. The FE-SEM images confirm that the structure of the CNTs remains intact. This indicates that it is possible to deposit Ni onto the surface of CNTs without damaging their structure using the plasma method.
Figure 5 shows the FE-TEM and EDS mapping analysis results for the synthesized Ni-based catalysts. The FE-TEM analysis in Fig. 5(a) reveals the presence of Ni with a lattice size of 3.50 Å and Co with a lattice size of 2.52 Å. This is corroborated by the EDS mapping, which shows the presence of both Ni and Co. The overlapping mapping images for Ni and Co indicate successful synthesis of Ni-Co. However, the mapping also reveals that the spherical particles are coated with a thin oxide layer, likely formed due to exposure to atmospheric oxygen because of the increased surface area from the nanoparticle synthesis. Additionally, Fig. 5(b) presents a FE-TEM image of Ni-CNT, showing that Ni metal particles, with sizes on the order of tens of nanometers, adhered to the CNT surface. The CNT particles were found to have a multi-walled structure with fewer than 20 layers. In the case of Ni-CNT, a significant oxide layer was also observed on the Ni particles, more so than on the CNTs.
To analyze the catalytic activity, specific surface area measurements were conducted, as shown in Fig. 6. Both materials exhibited an increase in total pore volume with increasing specific surface area. For the Ni-Co catalyst, the specific surface area was measured as 9 m2/g and the total pore volume was 0.06 cm3/g. This is notably higher compared to bulk Ni and Co, which typically have a specific surface area of 1 m2/g, indicating a nine-fold increase in specific surface area. Similarly, for Ni-CNT, a specific surface area of 12 m2/g and a total pore volume of 0.08 cm3/g were observed, which is more than twelve times greater than that of pure Ni. This suggests a positive impact on catalytic activity. An increased specific surface area allows more contact between reactants and active sites, potentially enhancing the catalytic reaction rate and lowering the activation energy for efficient CH4 reactions. However, the relatively high specific surface area may also lead to excessive oxide formation, as observed in the FE-TEM images, which could suppress or alter specific chemical reactions. Moreover, the oxide layer may obstruct reactants from accessing the catalyst’s active sites, potentially reducing reactivity. Therefore, further research on the synthesis conditions of the catalyst is warranted.
To evaluate the catalysts for CH4 pyrolysis, CH4 pyrolysis tests were conducted using an electric furnace, as shown in Fig. 2. The emitted gases were analyzed using a gas mass spectrometer. Additionally, the CH4 conversion rate and H2 selectivity were respectively assessed using Eqs. (1) and (2).
Figure 7(a) presents the CH4 decomposition rates for each catalyst. The CH4 decomposition experiments were conducted at 800 °C. The results, based on four conditions—non-catalytic, pure Ni, Ni-Co, and Ni-CNT—were 18.2, 45.8, 29.3, and 45.7 %, respectively. Pure Ni exhibited the highest decomposition rate. This is likely because, while pure Ni consists of 50-micrometer-sized particles, the manufacturing process involving plasma leads to nanoparticle formation. This nanoparticle formation results in the creation of an oxide layer, which reduces the number of active sites on the catalyst and consequently lowers the CH4 decomposition rate compared to pure Ni.
However, the Ni-CNT catalyst exhibited similar performance to that of pure Ni. This can be attributed to the CNTs not only providing a higher specific surface area compared to Co but also serving as a support material. This dual role of the CNTs likely contributed to the improved catalytic performance. Additionally, CNTs are generally chemically stable and less prone to oxidation compared to Co, which may further enhance the reaction efficiency with CH4.
Figure 7(b) shows the H2 selectivity results from CH4 decomposition experiments using each catalyst. All conditions exhibited higher H2 selectivity compared to the non-catalytic case. However, contrary to the CH4 decomposition rates, the Ni-Co catalyst demonstrated higher H2 selectivity than Ni-CNT. This can be explained by the fact that while the higher specific surface area of Ni-CNT may increase the overall CH4 conversion rate, it may result in lower H2 selectivity. The need for more active sites for CH4 decomposition with Ni-CNT, combined with potential side reactions facilitated by the CNT support, likely contributes to this lower selectivity.
In this study, metal catalysts, non-metallic catalysts, and supports used for CH4 pyrolysis were synthesized into nanocomposites using a thermal plasma synthesis method. Catalysts are employed to reduce the required temperature and energy for CH4 pyrolysis reactions, and previous research has shown that using two or more catalysts together can enhance catalytic performance more effectively than single catalysts. Nickel-based catalysts, specifically Ni-Co and Ni-CNT, were synthesized and tested at 800 °C in an electric furnace for CH4 pyrolysis, and their performance was compared with that obtained under non-catalytic CH4 pyrolysis conditions.
The Ni-Co, synthesized via plasma, was confirmed through FETEM and EDS mapping to be physically synthesized without structural changes. The particles exhibited sizes in a range of tens of nanometers, resulting in a specific surface area that is nine times greater than that of bulk Ni. This increased specific surface area is expected to enhance interaction with CH4 gas and improve the catalytic reaction rate, positively impacting the pyrolysis process. Similarly, for Ni-CNT, although it underwent high-temperature plasma processing, the absence of NiC was confirmed. FE-TEM and EDS mapping revealed that Ni particles, with size of tens of nanometers, were distributed on the CNT surface. The high specific surface area of the CNTs combined with physical bonding resulted in a twelvefold increase in the specific surface area.
The results confirm that Ni-Co and Ni-CNT were successfully synthesized using thermal plasma, and the performance of these synthesized nano-catalysts was evaluated through CH4 pyrolysis processes conducted in an electric furnace. Performance assessments across four conditions –non-catalytic, pure Ni, Ni-Co, and Ni-CNT –showed that the CH4 decomposition rates were highest for pure Ni, followed by Ni-CNT, Ni-Co, and the non-catalytic case. Although combining two catalysts improves performance, the FE-TEM mapping images indicated that nanoparticle formation and the associated oxide layer reduced the catalytic activity compared to pure Ni. In terms of H2 selectivity, Ni-Co exhibited the highest performance. The high specific surface area of Ni-CNT contributed to a higher overall CH4 conversion rate, but it also resulted in lower H2 selectivity. This is due to the increased number of active sites required for CH4 decomposition with Ni-CNT and the occurrence of various side reactions facilitated by the CNTs, which led to reduced selectivity. This study demonstrates that thermal plasma synthesis can successfully produce dual-component catalysts, and the characteristics of these catalysts were analyzed for CH4 pyrolysis applications. On the basis of all the factors considered in this study, the Ni-CNT catalyst showed the most favorable properties. Future research could focus on optimizing synthesis conditions to enhance and control specific attributes such as surface area and oxide layer formation.
This research was supported by the 2023 scientific promotion program funded by Jeju National University.
The authors declare no conflicts of interest.