Research Progress on Plasma-Assisted Catalytic Dry Reforming of Methane

Abstract

With the significant consumption of traditional fossil fuels, emissions of greenhouse gases such as methane (CH₄) and carbon dioxide (CO₂) continue to rise, requiring effective treatment methods. The dry reforming of methane (DRM) offers a promising pathway for greenhouse gas mitigation by converting CH₄ and CO₂ into high-value syngas. However, traditional thermal catalysis is prone to catalyst deactivation due to high-temperature sintering and carbon deposition caused by side reactions. The introduction of non-thermal plasma (NTP) provides a mild reaction environment, effectively mitigating catalyst sintering and carbon deposition, extending catalyst lifespan, reducing energy consumption, and significantly enhancing reaction performance and energy efficiency. This paper reviews recent progress in plasma-assisted DRM, focusing on different plasma discharge types and catalyst materials. The synergistic effects between plasma and catalysts and the challenges and prospects of plasma-assisted DRM technology are discussed.

1. Introduction

With the continuous development of society, energy has become increasingly important in the overall social system. The world’s energy supply is still dominated by traditional fossil fuels such as coal, oil, and natural gas. The combustion of coal and other fossil fuels produces large amounts of carbon dioxide; in contrast, activities such as fossil fuel extraction, landfill operations, and agriculture lead to significant emissions of non-CO₂ greenhouse gases like methane. The greenhouse effect caused by these emissions has intensified global warming [1,2,3,4]. Therefore, the reduction and reutilization of greenhouse gases such as carbon dioxide and methane have become urgent priorities.

Methane, the simplest hydrocarbon, is widely found in natural gas, coal mine gas, and biogas. It is a high-quality fuel and an essential raw material for producing syngas and various chemical products. Since the Industrial Revolution, large-scale activities such as coal development and oil and gas extraction have steadily increased methane emissions. According to the International Energy Agency, global methane emissions reached approximately 356 million tons in 2022, making it the second-largest greenhouse gas source. Studies have shown that due to its unique properties, methane can be converted into a variety of high-value chemicals, providing strong support for global energy utilization and contributing to the realization of a circular economy [5,6,7,8,9]. The emission reduction and utilization of methane have increasingly drawn researchers’ attention. The dry reforming of methane (DRM) provides an effective pathway to convert greenhouse gases such as carbon dioxide and methane into syngas. The syngas produced has a low H₂/CO molar ratio (H₂/CO = 1), making it a versatile feedstock for synthesizing high-value chemical products [10,11,12]. Therefore, DRM enables carbon dioxide utilization and offers a promising approach for methane conversion. This technology is significant for efficiently using abundant natural gas resources, reducing carbon dioxide emissions, and mitigating the greenhouse effect.

The dry reforming of methane (DRM), also known as methane–carbon dioxide reforming, was first proposed by Fischer and Tropsch in 1928.

Table 1. Reactions related to dry reforming of methane.

Reaction Process	Reaction Equation	ΔH298K (kJ/mol)
Dry reforming of methane	CH4 (g) + CO2 (g) = 2CO (g) + 2H2 (g)	247	(1)
Reverse water–gas shift reaction	CO2 (g) + H2 (g) = H2O (g) + CO (g)	41	(2)
Methane cracking	CH4 (g) = 2H2 (g) + C (s)	75	(3)
CO disproportionation	2CO (g) = CO2 (g) + C (s)	−172	(4)

shows DRM-related reactions, and Equation (1) is the main reaction, which can simultaneously convert two greenhouse gases into syngas (H₂ + CO), which can be used for the synthesis of various industrial chemicals such as methanol and dimethyl ether (DME) [13,14,15,16,17]. It is also a key feedstock for the Fischer–Tropsch process, making it an ideal pathway for greenhouse gas conversion [18,19]. However, CO₂ and CH₄ molecules are highly stable and inert. The C-H bond energy in CH₄ is 412 kJ/mol, and the C=O bond energy in CO₂ is 799 kJ/mol, requiring high reaction temperatures (≥700 °C) to overcome their thermodynamic barriers. Adding catalysts can enhance the reaction activity [20]. Still, the high temperatures often lead to catalyst sintering, resulting in a decline in catalytic performance [21]. In Table 1, the reverse water–gas conversion reaction (Equation (2)), methane cracking reaction (Equation (3)), and carbon monoxide disproportionation reaction (Equation (4)) are various side reactions accompanied by DRM. The progress of these reactions will cause the carbon deposition of the catalyst and lead to catalyst inactivation. Therefore, the key challenge lies in effectively activating CH₄ and CO₂ under mild conditions to address these catalytic issues.

Compared to traditional DRM reactions, plasma-assisted DRM offers advantages such as faster reaction rates and lower operating temperatures, making it a promising approach to reducing the reaction temperature [22,23,24]. In addition, efficient catalysts are indispensable. As confirmed in the review by Chen et al., the combination of plasma with suitable catalysts can overcome thermodynamic limitations, allowing the reaction to occur under mild conditions while achieving high catalytic activity and selectivity [25]. The main reaction pathways of plasma-assisted catalytic DRM are shown in Figure 1. A possible plasma-assisted DRM pathway is illustrated in Figure 1, where the dissociation of CH₄ and CO₂ produces CH_x (x = 1, 2 and 3) and O radicals. The excited Ar state can participate in the dissociation of CO₂ and CH₄ through the Penning dissociation phenomenon, resulting in the formation of CH₃ and O radicals. Reactive radicals form syngas and light hydrocarbons from the gas phase. Low electron energy (<6 eV) promotes the formation of ethane (C₂H₆) and propane (C₃H₈), while high electron energy (>13 eV) promotes the selective formation of acetylene (C₂H₂). C2H6 reacts with O, H, and OH radicals to form C₂H₅, and then it recombines with CH₃ to form C₃H₈. Despite numerous studies on plasma-assisted DRM, improving catalytic stability, activity, and selectivity, as well as enhancing the catalyst’s resistance to carbon deposition, remains a significant challenge [26].

Plasma is a mixture of particles, including electrons, ions, radicals, and excited gas atoms and molecules, and it is quasi-neutral overall. Substances in the plasma state exhibit extremely high chemical reactivity, enabling the activation of many chemically stable substances. The particle temperatures in non-thermal plasma vary significantly, in which gas temperatures typically remain below 1000 K, while electron temperatures can reach 10⁴–10⁵ K. The energy in non-thermal plasma is primarily used to generate highly reactive species such as radicals, excited atoms, ions, and molecules [28]. It can effectively activate chemically stable molecules like methane and carbon dioxide, offering excellent chemical selectivity. The non-equilibrium nature of non-thermal plasma overcomes thermodynamic barriers in chemical reactions, such as DRM, allowing reactions to proceed under atmospheric pressure and low temperatures. Thus, NTP provides an attractive method for converting greenhouse gases into syngas and other valuable chemicals. It has become a research hotspot in the plasma field and is widely applied across various industries.

NTP is a promising alternative to traditional thermally activated catalysis, especially for thermodynamically limited reactions [29,30]. NTP-assisted DRM has been demonstrated using various types of plasma [31,32,33]. This review summarizes recent studies on plasma-assisted catalytic DRM and explores the application of non-thermal plasma in synergistic catalytic DRM reactions, providing a comprehensive analysis of the performance of different non-thermal plasma reactors, including conversion rates, product selectivity, and energy efficiency. The synergistic effects between plasma and catalysts are introduced, focusing on the latest research progress in catalyst-active components and supports. Finally, the future development trends in non-thermal plasma-assisted catalytic DRM are discussed.

2. Types of Plasma

The plasma application in DRM research began in the late 20th to early 21st century. M. Kraus and B. Eliasson conducted one of the earliest studies around 2000. They explored using non-thermal plasma technology to facilitate the conversion of methane and carbon dioxide. They discussed the application of thermal plasma, dielectric barrier discharge (DBD), and other techniques in DRM in their 2001 study [34]. Subsequently, many researchers focused on this field, investigating various plasma technologies, such as DBD and gliding arc discharge, to improve DRM efficiency. Over time, plasma-assisted DRM technology has undergone further development and optimization.

Plasma-assisted DRM reactions offer significant advantages over traditional DRM, including faster reaction rates and lower temperature requirements, with the potential to operate at room temperature. This makes plasma-assisted DRM a promising approach for reducing the temperature demands of the process [22]. In plasma-catalyzed DRM, reactions are typically carried out under high-power and low-flow-rate conditions, which can enhance the conversion rates of CH₄ and CO₂ [35]. However, higher discharge power can lead to excessive cracking of reactants, resulting in significant carbon deposition, negatively impacting DRM’s long-term stability [24]. Therefore, maintaining appropriate discharge intensity and a stable discharge environment is crucial for ensuring the reaction’s stability. In addition, an efficient catalyst is essential. Combining plasma with a catalyst to form a hybrid plasma–catalyst reactor can improve DRM performance. Currently, there are two approaches for integrating plasma and catalysts. One involves placing the catalytic process after the plasma discharge, known as plasma post-catalysis (PPC); the other involves placing the catalyst within the plasma reactor, referred to as in-plasma catalysis (IPC) [36,37]. In PPC, the catalyst can strongly interact with the intermediates generated by the plasma, enhancing catalytic performance. In IPC, the synergistic effect between the plasma and catalyst is more complex, offering the potential for further improvement in DRM efficiency.

Over the past few decades, various types of plasma reactors have been extensively tested for DRM. Among them, dielectric barrier discharge (DBD), microwave plasma (MW), and gliding arc (GA) are commonly used technologies. DBD plasma, in particular, has been widely studied in DRM. Despite the progress made, many plasma reactors still face specific challenges. For example, conversion rates are sometimes suboptimal because not all feed gases pass effectively through the plasma zone, a common issue in many gliding arc (GA) plasma reactors [38].

2.1. Dielectric Barrier Discharge (DBD)

Dielectric barrier discharge (DBD) is a technology in which a dielectric material (insulating material) is inserted between the discharge electrodes or within the discharge space. The gas breaks down and discharges when the applied voltage is sufficiently high. During the discharge process, many highly reactive free radicals are produced, readily reacting with other free radicals, atoms, or molecules to form new substances. Therefore, DBD can be used in applications such as ozone generators, volatile organic compound (VOC) treatment, and automobile exhaust. It is now primarily applied in environmental protection industries. Common materials for the dielectric barrier include glass or quartz glass, while ceramics, thin enamel, or polymer layers can also be used in exceptional cases.

Dielectric barrier discharge (DBD) offers several advantages, including low energy consumption, high electron density, a simple design, easy scalability, and low installation costs. The efficient generation of high-energy electrons and the uniform distribution of the effective electric field also demonstrate that DBD is a superior technology for the DRM reaction. Different configurations of DBD reactors are shown in Figure 2. Numerous studies have tested DRM using DBD plasma under various experimental conditions [39,40], revealing that the performance of DBD plasma depends on factors such as gas flow rate, feed gas molar ratio, input power, temperature, and the catalyst used [41,42].

Different reactor configurations have a significant impact on the performance of DBD plasma. Asif Hussain Khoja and colleagues evaluated the effect of various dielectric materials on reaction efficiency [44]. The results showed that compared to quartz reactors alumina reactors exhibited higher conversion rates, CO/H₂ ratios, and yields under the same experimental conditions. Alumina demonstrated good DRM activity at high discharge gaps, with less carbon deposition and higher energy efficiency, making it a better choice for reactor dielectric materials. In addition to alumina, some materials with high dielectric constants, such as ferroelectric ceramics and polyethylene, which can suppress coke formation, are also considered suitable for DBD reactors.

Md Robayet Ahasan and colleagues investigated the application of DBD plasma on Ru/CeO₂ catalysts with different shapes [45]. They found that compared to conventional thermal catalysis, DBD plasma significantly improved the conversion rates of reactants and the yield of syngas. In DBD-assisted DRM reactions, the Ru/CeO₂ nanorod catalyst exhibited superior catalytic performance due to its surface chemistry and defect structure, which provided more active sites and oxygen vacancies.

2.2. Gliding Arc Discharge (GA)

The high-voltage power supply is connected to two separate electrodes, and gas is broken down where the electrode gap is smallest. The resulting arc spreads outward until it breaks, generating a new arc and continuously producing plasma. This discharge method is known as sliding arc discharge. It is mainly used for the surface treatment of materials and hazardous waste disposal.

The high temperature of GA plasma enables DRM to achieve the most optimal energy efficiency and cost results. In recent years, most DRM studies using sliding arc plasma have focused on improving energy efficiency by adjusting operational parameters such as gas temperature to increase reactant conversion rates or product selectivity [46,47,48].

In addition to adjusting operational parameters, researchers have improved DRM efficiency by optimizing the geometry of GA reactors. Dinh et al. designed a GA reactor with a nozzle and evaluated its performance [49]. Experimental results showed that the novel reactor significantly improved CH₄ and CO₂ conversion rates and energy efficiency compared to conventional GA reactors, achieving a maximum energy efficiency of 53%, much higher than traditional reactors. This improvement is attributed to the nozzle structure, which concentrates heat within a confined reaction volume, enhancing heat transfer efficiency and reducing heat loss. As a result, more energy is utilized in the reaction, boosting its overall efficiency.

The effect of quenching on DRM has not yet been studied, and whether quenching can suppress the reverse water–gas shift reaction, improve energy efficiency, and reduce energy costs remains an open question. To investigate this, Hyoungjoon Kwon and colleagues developed a quenching device (QR) applied downstream of the reactor, using a rotating gliding arc (RGA) as the plasma source to suppress the reverse water–gas shift reaction [50]. Experimental results showed that while the conversion rates of CH₄ and CO₂ decreased with the use of QR, the selectivity and yield of H₂ significantly increased, leading to improved energy efficiency and reduced syngas production costs. This demonstrated the feasibility of using quenching as a plasma parameter to enhance DRM efficiency.

2.3. Atmospheric Pressure Glow Discharge (APGD)

Glow discharge typically operates at low currents (a few to several milliamps) and high voltages (a few to several tens of kilovolts). Compared to other types of plasma reactors, atmospheric pressure glow discharge (APGD) offers relatively low energy costs while achieving comparatively high conversion rates, making it highly promising for DRM applications. However, it has yet to be extensively studied [51,52]. Moreover, existing experimental results on APGD have raised concerns about the accuracy of low-power measurements, which could potentially lead to an underestimation of energy costs. Therefore, the impact of APGD on DRM performance remains unclear and requires further investigation.

The ceramic tube in an APGD reactor directly impacts glow discharge. To prevent some gases from passing through the reactor without conversion—similar to the typical issue with sliding arc plasma reactors—Bart Wanten and colleagues inserted a ceramic tube with an inner diameter matching the plasma width [53]. This setup maximized the proportion of gas passing through the active plasma region, achieving CO₂ and CH₄ conversion rates of 63% and 94%, respectively. Additionally, it was found that hydrogen atoms play a crucial role in CO₂ and CH₄ conversion. Conversely, a high concentration of hydroxyl radicals was detrimental to the reaction, as these radicals promote H₂O formation and recombine with CO, regenerating CO₂, thereby reducing the net CO₂ conversion rate.

The addition of oxygen helps suppress the reverse water–gas shift reaction, thereby improving the selectivity and yield of hydrogen while reducing the formation of solid carbon. This was confirmed in the research by Stein Maerivoet and others [54]. The study found that an inlet ratio of 42.5% CO₂, 42.5% CH₄, and 15% O₂ achieved optimal performance, with CO₂ and CH₄ conversion rates of 50% and 74%, respectively. The inclusion of oxygen enhanced the stability of the plasma, and the partial oxidation reaction between oxygen and methane increased the reaction pathways, thereby improving the overall conversion rate of the response.

2.4. Microwave Discharge (MW)

Microwave discharge (MW) is considered an energy-efficient discharge method due to its high energy transfer efficiency, high effectiveness, and ease of automation, making it particularly suitable for DRM. Compared to traditional discharge technologies, microwave discharge provides shorter reaction times and specific effects on reactant molecules, allowing the DRM reaction to proceed at lower pressures and temperatures while offering better uniformity and a more extensive discharge space than DBD reactors. In addition, microwave discharge helps remove carbon deposited on the catalyst surface, thereby enhancing the catalyst’s lifespan and improving the overall performance of the reactor. Currently, research on the application of microwave plasma in DRM is still limited, with potential for further exploration and optimization [55,56].

Microwave plasmas of different frequencies can effectively promote the DRM reaction, as confirmed in the research by Seán Kelly [57] and Dariusz Czylkowski [58]. In their studies, a higher methane (CH₄) ratio was found to lead to the formation of visible carbon particles in the plasma region, which coated the quartz tube of the reactor. The significant increase in the luminescence observed in the plasma region supports this finding. Most of the carbon particles are amorphous carbon, which has some impact on the reaction stability. Understanding the effects of carbon formation, especially under high CH₄ input conditions, is helpful for processes like Fischer–Tropsch synthesis and methanol production with higher syngas ratios.

2.5. Spark Discharge

The two electrodes are positioned very close in a spark discharge plasma reactor. During plasma generation, a high alternating voltage is applied to one of the electrodes, creating a “spark” between the electrodes. In this type of reactor, the catalyst does not directly contact the plasma. The catalyst bed in this system is positioned close enough to utilize the substances or heat generated by the plasma for the DRM reaction. Compared to other non-thermal plasmas, spark discharge plasmas offer advantages such as high density, high discharge channel temperature, high reactant conversion rates, and low energy consumption, making it an effective method for activating CO₂ and CH₄ to produce syngas [59,60].

Ferroelectrics have the unique property of spontaneous polarization, and they can be polarized by an external electric field, giving them a higher dielectric constant than insulators. The polarization mechanism of ferroelectrics is shown in Figure 3. When the external electric field is removed, the ferroelectric material’s polarization remains at a certain intensity. The surface charges generated by polarization can interact with free electrons in the plasma, thereby increasing the energy density in the reactor [61]. Wei-Chieh Chung and others explored the synergistic effects between ferroelectric materials (BaZr_0.05Ti_0.95O₃, abbreviated as BZT) and plasma [62]. The study found that after adding BZT, the CO₂ conversion rate increased from 49.4% to 79.0%, and the CH₄ conversion rate rose from 52.5% to 84.2%. Regarding energy efficiency, the BZT-filled bed reactor also exhibited lower specific energy consumption. Characterization analysis revealed that during the discharge process, the presence of BZT significantly increased the charge density in the plasma reactor, facilitating the dissociation of CO₂ and CH₄. The plasma affected the morphology, surface structure, and chemical properties of BZT, and this modification further promoted the positive synergistic effect between the two, thereby improving the overall reforming efficiency.

Bin Zhu and others established the DRM kinetic rate equation for spark discharge plasma and verified the reliability of the equation [63]. Experimental results showed that adjusting parameters such as SEI, electrode gap, and voltage frequency could improve reactant conversion rates, with CH₄ and CO₂ conversion rates reaching up to 74% and 63%, respectively. At higher CH₄ ratios, carbon deposition issues, similar to those observed in microwave discharge, occurred. These carbon particles, composed of pure carbon nanoparticles, affected the stability of the plasma. Recent studies on using non-thermal plasmas for DRM are summarized in

Table 2. Different types of plasma-assisted catalytic dry reforming of methane.

Types of Plasma	Catalysts	Feed Ratio	Flow Rate	Power	Conversion (%)		Selectivity (%)		H2/CO	Refs
			mL/min	W	CH4	CO2	CO	H2
Dielectric Barrier Discharge (DBD)	Ni-CeO2 NR	CH4:CO2 = 100:250	150	23.8	63.5	48	60	58	0.68	[64]
	Ni-Co3O4/TiO2	CH4:CO2 = 1:1	20	100	86.4	84.9	49.0	50.1	1.01	[65]
	Ni-CeO2 NO	CH4:CO2 = 100:250	350	23.8	56	45	62	56	0.6	[66]
	Ni/NiZnAl-LDHs	CH4:CO2 = 1:1	30	/	68.9	54.3	74.5	62.5	/	[67]
	Ru/CeO2 NR	CH4:CO2 = 100:250	350	10.2~13.6	51	37	62	44	0.48	[45]
	NiFe/MgAlO	CH4:CO2:Ar2 = 3:3:2	50	52.2	73.8	80.5	60	42	0.72	[68]
	NiO/CeO2 NR	CH4:CO2 = 100:250	350	24.9~25.8	66	48	51	46	/	[69]
	Ni/SiO2	CH4:CO2 = 1:1	20	25	55	44	61	49	/	[70]
Gliding Arc Discharge (GA)	NiO/SiO2	CH4:CO2 = 2:3	4500	64	10.2	9.0	92.0	84.9	0.80	[71]
	NiO/Al2O3	CH4:CO2 = 2:3	3700	136	11.8	11.2	88.1	75.3	0.82
	Ni/MgAl-LDH	CH4:CO2:N2 = 1:1:8	8000	508	91	79	95	92	0.9	[72]
Atmospheric Pressure Glow Discharge (APGD)	Ni-Co/Al2O3-ZrO2	CH4:CO2 = 1:1	/	/	99	99	/	/	0.98	[73]
Spark Discharge	D-Ni NP/γ-Al2O3	CH4:CO2 = 1:1	60	2.1~3.9	65	43	/	/	1.35	[74]
	F-NiNP/γ-Al2O3	CH4:CO2 = 1:1	60	2.1~3.9	52	33			1.41
	S-NiNP/γ-Al2O3	CH4:CO2 = 1:1	60	2.1~3.9	26	16			1.91
	Ni/Al2O3	CH4:CO2 = 20/80~70/30	100~200	30~70	85	75			/	[75]
Microwave Discharge (MW)	CsRu/CeO2	CH4:CO2 = 1:1	/	<40	84.6	85.7	99	99	/	[76]
	NiFe/MgAl2O4	CH4:CO2 = 1:2	450	600	85	62	/	/	≈1	[77]
	Ni-La/AC	CH4:CO2:N2 = 15:15:70	160~320	80~100	≈100	≈100			≈1	[78]

.

3. Synergistic Effect Between Plasma and Catalyst

In plasma-assisted catalytic DRM, the reaction efficiency is greatly influenced by the size and geometry of the reactor and the catalyst filling the reactor. Therefore, the study of plasma-assisted catalytic DRM must focus on the reaction process between the feed gas and the catalyst and consider the interaction between the plasma and the catalyst.

A major challenge in studying plasma-assisted catalytic DRM is that the reaction mechanism of the catalyst is much more complex than in thermal reactors, with a complicated interaction between the catalyst and the plasma. This includes not only the effect of high-energy electrons from the plasma on the active components of the catalyst but also the influence of the catalyst on the discharge form of the plasma. Md Monir Hossain and others found that NC waste catalysts exhibited larger grain sizes after plasma-assisted DRM reactions than the original catalysts [66], while NO catalysts showed the opposite effect. This indicates that the interaction between the plasma and each catalyst is different. The interaction between the plasma and the catalyst is shown in Figure 4.

Plasma establishes a strong electric field and generates highly reactive species, including ions, electrons, and photons, while the catalyst itself helps reduce the activation energy barriers of specific key reactions. The catalyst’s presence in this process affects the plasma’s discharge characteristics and the reactor’s temperature distribution. Additionally, plasma can alter the catalyst’s morphology, surface area, and oxidation state, further reducing the activation energy barrier by optimizing the surface reaction pathways. The catalyst’s involvement also regulates the electric field’s distribution, thus affecting the system’s discharge efficiency and reaction kinetics [43].

Due to the inherent properties of the catalyst, reactant gases are adsorbed onto the catalyst’s surface. During the plasma discharge process, electrons are accelerated from the cathode to the anode, which excites the gas molecules on the catalyst surface, promotes the diffusion of gas molecules, and optimizes the interaction between high-energy electrons and gas molecules. Additionally, the physicochemical properties of the catalyst may change, such as an increase in porosity. This change helps active species diffuse more effectively within the pores, enhancing collisions between gas molecules and high-energy active species. These collisions trigger a series of reaction processes, such as adsorption, excitation, dissociation, and ionization, leading to the formation of intermediates and final products. This process significantly improves the conversion rate of reactants and optimizes the overall reaction efficiency.

In a plasma environment, the bombardment of high-energy electrons can excite the catalyst’s surface, forming active particles with abundant active sites, typically accompanied by significant lattice defects. These active sites can selectively adsorb reactant molecules and promote their conversion into target products. Meanwhile, those active particles that are not selectively adsorbed may further participate in reactions through free bonding, leading to the formation of the desired products. This process not only enhances the activity of the catalyst but also improves the selectivity of the reaction. The active species generated by plasma help oxidize and gasify the carbon deposits on the catalyst surface, reducing catalyst deactivation caused by carbon buildup and enabling in situ regeneration of the catalyst. Plasma can generate short-lived and long-lived active species, significantly affecting the catalyst’s adsorption properties and reaction mechanisms. For example, one of the short-lived active species, the free radical, can adsorb onto the catalyst surface and alter its reaction pathway. Specifically, CH_x radicals (where x = 1–3) adsorbed on the catalyst are more susceptible to attack by oxygen atoms, leading to dissociation. This process effectively inhibits the recombination of CH_x radicals, thus preventing the formation of C₂₊ hydrocarbons [79].

The dielectric properties and surface characteristics of the catalyst can also influence the discharge mode and electron energy distribution of the plasma, thereby affecting the chemical reactivity of the plasma. Taking a DBD reactor as an example, filamentary discharge is the primary discharge mode in DBD, composed of countless micro-discharge filaments. After the addition of a catalyst, the discharge mode in the filler section changes to surface discharge, which may promote plasma-assisted catalytic DRM. However, the filler also reduces the discharge volume, thereby decreasing the micro-discharge space and reducing the residence time of the feed gas in the reactor, which is unfavourable for the reaction. Ferroelectric materials with high dielectric constants can improve the electric field, thus enhancing the electron energy distribution in the discharge region. This results in a good synergistic effect with the plasma, further promoting the conversion of CH₄ and CO₂ [39].

Plasma discharge alters the catalyst’s physicochemical properties, improving the efficiency of adsorbing reactant gases. The catalyst changes the plasma discharge characteristics and reduces the activation barriers of specific gas conversion processes. The interplay between the two has a complex effect on the DRM process. Additionally, reactant gases in the plasma state may exhibit completely different properties from those in thermal reactions. Therefore, the synergistic effect between the plasma and catalyst remains not fully understood.

4. Types of Catalysts

4.1. Active Components

In DRM, supported noble metal catalysts, such as Ru, exhibit high catalytic activity and resistance to coking. However, due to their high cost and limited availability, noble metals are unsuitable for industrial applications. Transition metal catalysts are considered the most common catalysts in DRM, with Ni being particularly notable for its excellent activity, strong redox capability, and relatively low cost. Catalysts containing Ni particles also feature a higher proportion of lattice planes and surface structures at the nanoscale, making Ni a frequently used active metal in catalyst development. However, severe carbon deposition leads to rapid catalyst deactivation. Therefore, efforts have been focused on developing efficient and stable Ni-based catalysts with enhanced activity and resistance to coking, such as bimetallic catalysts combining Ni with noble metals.

4.1.1. Transition Metal Active Components

In DRM, transition metal catalysts perform well in suppressing carbon deposition and enhancing syngas yield, with more flexible standards for reaction conditions and catalyst support selection. Ni-based catalysts are the most commonly used transition metal catalysts, but other metals such as Co, Fe, Cu, and Zn are also employed. However, their effectiveness in converting CH₄ and CO₂ is generally lower than Ni-based catalysts.

Kristy Stanley et al. studied three different morphologies of Ni nanoparticles (Figure 5a–c) [74]. Regarding reactivity, dendritic and layered catalysts exhibited higher activity than spherical catalysts, thanks to their more significant number of (111) planes and smaller pores. Potential micro-discharges occurring within these small pores enhanced plasma density and reactant conversion. Pure plasma showed higher conversion rates than spherical catalysts but significantly lower yields of CO and H₂, indicating a synergistic effect between the plasma and catalyst. However, pure plasma also exhibited a higher degree of competitive side reactions. Therefore, the presence of a catalyst can improve the selectivity of the reaction products.

Md Monir Hossain et al. explored the potential application of Ni-based catalysts supported on CeO₂ nanorods (NR) in plasma–thermal synergistic catalysis [64]. XRD and Raman spectroscopy analyses (Figure 6a,b) revealed that introducing Ni increased the specific surface area, enhanced particle dispersion, prevented particle aggregation, and increased the concentration of oxygen vacancies. Performance tests showed that thermal catalysis resulted in relatively high CO₂ conversion rates, and CeO₂ NR support significantly reduced C₂H₆ production due to the abundance of oxygen vacancies. In contrast, with plasma introduced, CH₄ conversion rates consistently exceeded those of CO₂. This was primarily because the plasma provided sufficient energy to break the lower bond dissociation energy of CH₄, enhancing its dissociation. The plasma-assisted thermal catalytic reaction performed best at 350 °C. Although CO yield decreased, H₂ yield increased, indicating that the Ni-CeO₂ NR catalyst exhibited good performance in plasma synergistic catalysis, highlighting the role of plasma coupling and oxygen vacancy effects in improving catalytic performance.

Although Ni-based catalysts offer advantages such as high conversion rates, good activity, and low cost, rapid deactivation due to surface carbon deposition and sintering of Ni remains the biggest challenge. The deactivation mechanism involves carbon deposition on the Ni surface through methane cracking and carbon monoxide disproportionation reactions [80,81]. This surface carbon increases the size of Ni nanoparticles, reducing the exposed Ni surface area and diminishing the activity of the metallic component. Additionally, it exacerbates further carbon deposition, resulting in a rapid deactivation of the Ni-based catalyst. To prevent this, extensive research has been conducted to mitigate carbon deposition [82,83]. Two practical approaches to reduce carbon formation include suppressing carbon monoxide disproportionation/methane cracking reactions and accelerating the gasification of deposited carbon. These issues are typically addressed by optimizing the support and active materials to solve the coking problem.

The stability of Ni-based catalysts can also be improved by adding promoters and altering the acidity of the support. Adding basic elements such as Na or Mg can reduce carbon deposition. Alipour et al. reported that adding MgO changed the acidity of the support, reducing coke formation on the Ni/Al₂O₃ catalyst and enhancing its catalytic activity [84]. Ping Wu et al. introduced a CaO promoter into the Ni-based catalyst, increasing CO₂ adsorption on the composite catalyst and allowing more CO₂ molecules to participate in DRM. The oxygen generated at the interface reduced carbon deposition, contributing to the catalyst’s high efficiency and long-term stability [85].

In addition to Ni-based catalysts, Jinxin Wang et al. synthesized three-dimensional catalysts containing only Cu and CuO with different pore sizes to explore the effect of pore size on plasma-assisted catalytic DRM [86]. SEM and pore size calculations found that compared to particle size, particle gaps, and stacking density, changes in pore size had a more significant impact on plasma performance. Although larger pore sizes are typically considered favourable for catalyst performance, the actual study results showed that catalysts with smaller pore sizes also exhibited good activity in the reaction. Furthermore, microstructural analysis of the catalyst revealed that the effect of pore size on plasma catalytic reactions and the enhancement of the electric field on the catalyst surface are key factors. Future research could focus on optimizing catalyst pore size to maximize its advantages in plasma reactions.

In the study by K. Krawczyk et al. [40], the Fe/Al₂O₃ catalyst exhibited a high CH₄ conversion rate, enhancing the conversion of CH₄ into high-value-added products (such as C₂–C₄ hydrocarbons) while being less affected by temperature. This indicates that Fe/Al₂O₃ can direct the reaction toward a more favourable pathway, improving product selectivity. Additionally, the Fe/Al₂O₃ catalyst exhibited the lowest carbon deposition on its surface, which helps extend the catalyst’s lifespan and maintain reaction stability and efficiency.

Furthermore, in the work of Yuxuan Zeng et al. [36], a Mn/γ-Al₂O₃ catalyst was developed, which demonstrated good activity. The CH₄ conversion rate and the yields of CO and H₂ were significantly improved, while the selectivity of C₂H₆ was markedly reduced, contributing to a higher purity of the target products, CO and H₂. Additionally, no carbon deposition was observed on the catalyst surface, enhancing the overall energy efficiency of the process. Although its performance was slightly lower than that of the Ni/γ-Al₂O₃ catalyst, it still exhibits high application potential.

Different transition metals have distinct characteristics. Ni is relatively low in cost and easily available. It exhibits good activity in catalytic reactions and high methane conversion rates. Still, it is prone to catalyst poisoning, especially by sulphur and chlorine, and it may undergo sintering at high temperatures, leading to catalyst deactivation [21]. Co-based catalysts show more substantial selectivity for CH₄, can effectively produce hydrogen, and have high thermal stability, making them suitable for high-temperature reactions. However, they are more expensive than Ni and have lower activity. Fe is abundant and extremely low in cost, but its stability is poor and prone to deactivation. Its reaction selectivity is also inferior to that of Ni and Co. In summary, choosing an appropriate transition metal catalyst requires a comprehensive consideration of factors such as catalytic activity, selectivity, cost, and stability to meet the specific reaction conditions and economic requirements.

4.1.2. Noble Metal Active Components

Due to their high activity and selectivity, expensive noble metals such as Pt, Ru, and Rh are reliable alternatives to Ni catalysts. Unlike Ni, Ru and Rh do not deactivate due to coke deposition and sintering. Especially in cases where the metal content is lower and catalyst regeneration is not required, the performance of Ru catalysts is promising. Ashvin L. Karemore and others studied the effect of Ru/Al₂O₃, Pt/Al₂O₃, and Pd/Al₂O₃ on the DRM process by altering the gas composition [87]. As the CO₂/CH₄ ratio in the feed gas increased, the CO₂ conversion rate decreased, while the CH₄ conversion rate increased. All the catalysts showed stability. The H₂ and CO production trends were similar to the trends in reactant conversion, with minimal change as the operating time increased, maintaining a constant syngas ratio.

Danhua Mei and others developed a water-cooled DBD reactor for plasma-assisted DRM reactions using supported catalysts (Ni/γ-Al₂O₃, Ag/γ-Al₂O₃, and Pt/γ-Al₂O₃) [26]. XRD characterization results showed that the active metal components were highly dispersed on the catalyst supports. Among them, Ag/γ-Al₂O₃ and Pt/γ-Al₂O₃ catalysts exhibited more stable performance. After plasma modification, the grain size increased, and the specific surface area decreased. In terms of conversion rates, after introducing plasma, the CO₂ and CH₄ conversion rates of the supported noble metal Ag/γ-Al₂O₃ catalyst were the highest, reaching 21.4% and 27.6%, respectively. Regarding the products, the supported noble metal catalysts achieved high H₂ selectivity (34.5%) and CO selectivity (61.1%) and also showed higher energy efficiency and more stable gas conversion rates.

The presence of noble metal catalysts affects the reaction efficiency of plasma-assisted DRM. Still, their specific effect depends on various factors, including the type of catalyst, loading amount, reduction degree, and plasma conditions. In some cases, noble metal active components may enhance the selectivity of specific products, but they do not necessarily significantly improve overall energy efficiency or conversion rates. Andersen and others studied the effects of Ag, Cu, and Pt supported on γ-Al₂O₃ on conversion rates and energy efficiency [24]. Without the catalyst, the conversion rates of CH₄ and CO₂ were approximately 33% and 22%, respectively. However, after filling the catalyst, the conversion rates were similar to those with pure plasma, mainly due to the geometry of the filling material shortening the gas residence time. This suggests that although noble metal components were introduced to enhance reaction efficiency, their impact on conversion rates and yields was limited under these experimental conditions. This may be due to the catalyst’s effect not significantly exceeding the plasma effect.

4.1.3. Bimetallic Active Components

Among the active metals reported for DRM catalysts, transition metals mostly suffer from deactivation. Many practical strategies have been employed in DRM reactions to enhance coke resistance and improve catalyst stability, such as enhancing metal–support interactions, designing bimetallic or polymetallic catalysts, and improving the oxygen transfer ability of catalyst supports [88,89,90].

Ag and Sn have high conductivity and coke resistance properties. Adding appropriate amounts of Ag and Sn can improve the reactor activity and the catalyst’s coke resistance. Suttikul T. et al. reported the effect of Ag and Sn as second metals on Ni-based catalysts [91]. In a DBD reactor, the coke formation rate for Al₂O₃ was 4.7%, and for the Ni/Al₂O₃ catalyst, it was 3.7%. As the amount of Ag loading increased, the output current increased due to its higher dispersion and specific surface area, leading to improved CH₄ and CO₂ conversion rates. When 1.5% wt Ag was loaded, the CH₄ and CO₂ conversion rates were 19% and 21%, respectively, and the coke formation rate was 0.74%. The effect of Sn was similar to that of Ag, but excessive loading caused clustering, which increased coke formation. The best performance was observed with 0.5% wt Sn loading, where the CH₄ conversion rate was 15%, CO₂ was 19%, and the coke formation rate was 2.1%. Stability tests showed that the conversion rates increased as the reaction time was extended.

Thanks to its porous structure, high surface area, and strong metal–support interactions, the Ni/MgAlO catalyst exhibits excellent stability and coke resistance [92]. Song et al. prepared a NiCu alloy structure by adding Cu to the Ni/MgAlO catalyst, confirming that the alloy structure can enhance the activity and stability of Ni-based catalysts [93]. In the DRM reaction, iron is a suitable promoter that improves Ni-based catalysts’ decarbonization ability and stability [94]. Jian-Feng Diao et al. designed two catalysts, Ni/MgAlO and NiFe/MgAlO [68]. Adding Fe increased Ni’s surface oxygen content and dispersion, enhanced CO₂ adsorption, and improved catalyst activity and coke resistance (Figure 7a,b). The NiFe bimetallic catalyst’s CO2 and CH4 conversion rates were 80.5% and 73.8%, respectively (Figure 7c,d), with good stability. The NiFe/MgAlO catalyst also lowered the activation temperature of CH₄ and exhibited stronger CO₂ adsorption ability. Fe and Ni are adjacent at the atomic level, which enhances CO₂ adsorption and activation and promotes the conversion of CH₄ cracking to reduce carbon deposition. Therefore, the NiFe/MgAlO catalyst shows higher activity.

Due to the different issues faced by transition metals and precious metals, another class of catalysts is intermetallic compounds (transition metal carbides, nitrides, and phosphides), which have been widely studied for DRM in recent years. These catalysts exhibit high DRM activity and coke resistance, similar to precious metals. In terms of carbides, various metal carbides (Mo, W, V, Nb, and Ta) have been used as DRM catalysts [95,96]. Among them, molybdenum carbide is the most widely studied carbide, showing good activity and stability under relatively high-pressure conditions. However, under atmospheric pressure, it is rapidly deactivated by CO₂ oxidation.

4.2. Support

The support can provide a high specific surface area for the dispersion of active metals, and its physicochemical properties and structure significantly affect the catalyst’s activity, structural stability, and coke resistance [41]. Different types of support have various advantages. For example, perovskites (ABO₃, such as LaNiO₃ and LaCoO₃) can uniformly distribute active components on the support surface, suppress sintering, and give the catalyst high activity and stability [97]. Additionally, embedding active metals in the pores of ordered mesoporous materials to obtain controllable nanoparticles is a feasible strategy for promoting the DRM reaction.

In plasma-assisted catalytic DRM, porous materials such as zeolites and Al₂O₃ are commonly used as packing materials. The increase in discharge volume generates more plasma, which promotes the conversion of CH₄ and CO₂. Plasma can also alter the morphology of the support surface, create surface defects, and induce local changes in the pore structure, thereby modifying the Fermi level and the density of free charge carriers.

The morphology of the catalyst support also plays an important role in the synergistic effect of plasma catalysis. Metal-supported catalysts with a layered pore structure exhibit higher metal dispersion and better catalytic activity. In addition, supports with high porosity facilitate the generation of filamentary micro-discharges and surface discharges under plasma conditions. Therefore, using an appropriate support with a specific structure to form surface discharges is crucial for improving catalytic performance. The most commonly used supports include oxides, carbon-based materials, and new materials.

4.2.1. Oxides

Oxides (SiO₂, Al₂O₃, La₂O₃, CeO₂, ZrO₂, TiO₂) can provide more active sites, promoting the adsorption and conversion of reactants, thereby improving the catalytic activity of the reaction. Nassim et al.’s research indicates that the coupling strength between plasma and the catalyst is closely related to the nature of the surface-active species and, to some extent, the number of active sites located in the micropores of the oxides [98]. Oxides also have good thermal and chemical stability, allowing the catalyst to maintain stability in high-temperature and complex reaction environments, thus extending its lifespan. In plasma-assisted catalytic DRM, oxide supports can also interact synergistically with plasma, enhancing the electric field effect, activating the reactants, and increasing the reaction rate. Some oxide supports can even alter the reaction mechanism and optimize the reaction pathway. These advantages make oxide support an important research direction in plasma-assisted catalytic DRM.

Shan-Shan Lin et al. prepared an AE-NiO/γ-Al₂O₃ catalyst with small particle size, strong metal dispersion, strong metal–support interaction, and high catalytic activity [23]. Compared to catalysts prepared by the traditional impregnation method, this is more suitable for plasma-assisted DRM reactions. At an input power of 140W, the conversion rates of CO₂ and CH₄ can reach 80.3% and 86.4%, respectively.

Although γ-Al₂O₃ supports are widely used in plasma-assisted DRM reactions, few studies have revealed the impact of γ-Al₂O₃ support on Ni dispersion and discharge performance. Diao et al. investigated the effect of the morphology of γ-Al₂O₃ supports (nano-tube Ni/NR-Al₂O₃, nano-layer Ni/NS-Al₂O₃, and spherical flower Ni/SF-Al₂O₃) on Ni dispersion, metal site stability, and discharge performance [99]. The study found that Ni is highly dispersed on NS-Al₂O₃ and SF-Al₂O₃ supports, and the dispersion mechanism is shown in Figure 8 The average Ni particle size on Ni/NS-Al₂O₃ is the smallest (~4.3 nm), and the smaller the Ni particle size, the higher the activity. During thermal catalysis and plasma-coupled DRM reactions, the Ni/NS-Al₂O₃ catalyst showed the best catalytic activity, with the highest average discharge frequency, average discharge current, average discharge time, and stability. The study suggests that coupling non-thermal plasma with catalysts of specific morphologies can improve synergy, and two-dimensional materials with priority for mass diffusion and electron transfer hold more significant potential.

SiO₂ is considered to have no catalytic activity in DRM, but due to dielectric polarization, the enhanced electric field around the contact points of filler particles and the surface discharge of the filler particles can promote conversion, which is why it is widely used in DRM. To better understand the interaction between catalyst particles and plasma, Jinxin Wang et al. investigated the impact of submicron and micron-sized SiO₂ fillers on conversion rates [70]. SiO₂ fillers generally increased CH₄ and CO₂ conversion rates compared to no filler. After introducing plasma, the plasma power, micro-discharge frequency, and discharge intensity, all increased with the increase in SiO₂ particle size. However, SiO₂ fillers hindered plasma discharge, and the different plasma powers led to a trend where conversion rates and energy yield initially increased and then decreased with particle size. In contrast, product selectivity initially decreased and then increased. Tests with Cu-, Fe-, and Ni-loaded catalysts showed that the conversion rate of most samples decreased, with only 5%wt Ni/Si-740 showing an increase, possibly due to the interaction between the metal and plasma. Tests with Ni loaded on Si spheres of different diameters showed that as particle size increased, the conversion rate and energy yield initially increased and then decreased.

Given the significant impact of submicron- and micron-sized particles on conversion and energy yield, particle size is an important factor influencing catalyst performance in plasma-catalyzed conversion. For catalysts with different particle sizes, the interaction between the metal and plasma can have either a positive or negative effect on DRM, and the changes caused by metal loading vary. Therefore, when selecting active components, it is essential to consider the interaction between plasma and the catalyst and the discharge effects.

Due to the acidic nature of CO₂ when dissolved in water, the alkalinity of the prepared material is considered one of the important characteristics of DBD plasma DRM catalysts. In the presence of plasma and active species, La₂O₃ as a support significantly increases the basic sites and alkalinity of the catalyst. The overall basicity and the enhanced active Ni promote the activation and adsorption of CO₂. La₂O₃ can also form intermediate carbonates (La₂O₂CO₃), which can further react with carbon on the surface near Ni to generate CO, thereby inhibiting carbon deposition and extending the stability of the catalyst.

CeO₂, as a support or additive in DRM, offers multiple advantages. Its excellent oxygen storage capacity, strong interaction with metal oxides, and good redox properties effectively limit carbon formation and enhance catalyst stability. In addition, certain exposed surfaces of CeO₂ have a lower oxygen vacancy formation energy or more surface defects, which can enhance the metal–support interaction. The high surface area and morphology-controllable CeO₂ support help improve the activity and stability of uniformly distributed metal clusters [100]. Therefore, using morphology-controllable CeO₂ in plasma-assisted DRM can enhance the overall performance of the catalyst. Md Monir Hossain and colleagues prepared two forms of Ni-CeO₂ catalysts to investigate the effect of catalyst morphology on activity and reaction pathways [66]. The study found that after introducing plasma, the catalyst generated different concentrations of surface defects, and the exposed crystal planes enhanced the metal–support interaction, significantly improving conversion and yield. The nan-octahedral (NO) catalyst showed more significant effects due to its higher number of oxygen vacancies and basic sites (Figure 9a). The nan-cubical (NC) catalyst exhibited higher activity in C₂H₆ formation. After the reaction, both catalysts produced easily gasifiable carbon nanotube-like amorphous carbon (Figure 9b,c). This study demonstrates the potential of CeO₂-supported catalysts’ shape effects in improving plasma-assisted DRM process performance.

4.2.2. Carbon Materials

Carbon materials are widely used in catalytic processes in the chemical industry due to their unique properties, such as excellent electrical conductivity, chemical inertness, high surface area and porosity, good mechanical properties, abundant active sites, and low cost. These materials are applied in processes such as the production of chlorine and aluminum, metal refining, batteries, biosensors, the electrochemical production of hydrogen peroxide, and photoelectrochemical water splitting. In the DRM process, carbon-based materials also offer many advantages, such as high specific surface area, which provides more active sites and enhances catalytic activity; good electrical conductivity, enabling effective interaction with plasma and improving reaction rates; and good thermal stability, maintaining catalyst activity and structural stability under high-temperature conditions. Activated carbon, carbon nanotubes (CNTs), g-C₃N₄, and graphene oxide, among others, have shown promising potential for such applications.

Huaqin Wang and colleagues prepared a Ni/AC catalyst using activated carbon as the support [101]. The experimental results showed that the reduction temperature significantly affected the specific surface area, pore structure, and Ni particle size of the Ni/AC catalyst. Comparative experiments revealed that plasma-assisted catalysis achieved the best performance, with the NiC700 catalyst reduced at 700 °C showing the highest CO₂ and CH₄ conversion rates and good stability. This was attributed to its mesoporous structure, high surface area, and strong metal–support interaction. Activated carbon enhanced catalytic efficiency and helped reduce costs, improving the economic benefits of the reaction.

Ma et al. prepared Ni NP catalysts both inside and outside carbon nanotubes (CNTs) to evaluate the DRM performance of different catalytic sites located inside and outside the CNTs [102]. The results showed that internal Ni’s activity was superior to external Ni’s. This was attributed to the confinement effect of the CNTs and the different electron densities inside and outside, with the active species of internal Ni being more stable.

In plasma-assisted catalytic DRM, g-C₃N₄ can interact with high-energy electrons and reactive species generated by the plasma, improving the conversion rates of CH₄ and CO₂ and generating more active species such as OH radicals and excited CO₂. Debjyoti Ray et al. combined g-C₃N₄ with TiO₂, ZnO, and their mixed oxides (TiO₂ + ZnO) to study the acid–base properties of metal oxides and the characteristics of graphitized carbon nitride catalysts [103]. They found that the g-C₃N₄ + TiO₂ catalyst had more acidic sites, the g-C₃N₄ + ZnO catalyst had more basic sites, and the g-C₃N₄ + mixed oxide catalyst had the most acidic and basic sites. In terms of performance, the conversion rates of the reactants increased with increasing electron density. Due to the combination of acid–base properties, the g-C₃N₄ + mixed oxide catalyst exhibited the highest reactant conversion rates (35.5% for CH₄, 13% for CO₂) and CO selectivity, with carbon balance higher than that of pure g-C₃N₄ (Figure 10). In this study, g-C3N4, as a catalyst support, significantly improved the overall performance of the reaction by enhancing reactant conversion rates, improving syngas selectivity, optimizing carbon balance, and boosting energy efficiency after adding mixed oxides.

4.2.3. Other Materials

Zeolites have a well-defined structure, high surface area, high thermal stability, and a strong affinity for CO₂, making them attractive for DRM. Additionally, zeolites are natural minerals that offer good environmental friendliness, and using zeolites as a catalyst helps reduce the environmental impact of the catalytic process. Coupled with excellent coke resistance, Anis H. Fakeeha et al. prepared H-ZSM-5-supported Ni catalysts for DRM. They exhibited low carbon deposition and high stability, with a H₂/CO ratio close to unity. Furthermore, adding metal components has been shown to promote the catalytic performance of zeolite-supported catalysts [104]. Aaron J. Najfach et al. added different amounts of Mn to Ni-based zeolite catalysts. They found that the impact of Mn on catalyst activity depends on the type of zeolite support, but it enhanced the stability of most catalysts and reduced carbon deposition. Zeolites have become an important catalytic support for DRM reactions [105].

Hoang Hai Nguyen et al. studied the relationship between reaction activity and plasma parameters, gas flow rate, and reactant input ratios using zeolite catalysts [106]. They found that the addition of zeolite catalysts reduced the dissociation energy of reactants, and the microporous structure promoted micro-discharges, enhancing the average energy density and strengthening the synergy between the plasma and the catalyst. As the voltage increased and the total gas flow rate decreased, the zeolite catalyst significantly improved the reactant conversion rate and CO selectivity, bringing the syngas ratio closer to one. Compared to the pure plasma process, the zeolite catalyst not only provided many active sites but also enhanced the plasma discharge effect by reducing the dissociation energy of the reactants while simultaneously reducing the formation of by-products, thus improving energy efficiency.

Metal–organic frameworks (MOFs) are versatile porous materials that, in addition to possessing good stability, also exhibit excellent CO₂ and CH₄ adsorption capabilities, which enhance CO₂ conversion. Therefore, MOFs are promising candidate materials for DRM catalysts. Reza Vakili et al. used UiO-67 MOF to support Pt nanoparticles (PtNP@UiO-67) as a catalyst and found that this catalyst demonstrated good performance [27]. The study revealed that compared to pure plasma, the porous structure of UiO-67 MOF facilitated the formation of micro-discharges and surface discharges, improving plasma generation and increasing the reactant conversion rate and stability. Additionally, Pt nanoparticles provided extra active sites, further enhancing the reaction performance and stability. TEM analysis of the catalyst also showed that the sintering of the PtNPs during the reaction was minimal (Figure 11a,b), and the morphology and crystallinity were largely unaffected (Figure 11c,d). The MOF support remained stable under plasma reaction conditions, contributing to the catalyst’s reusability and long-term operation.

Spinel (AB₂O₄, such as MgAl₂O₄, NiFe₂O₄, NiAl₂O₄) supports have excellent high-temperature stability. The ceramic nature of MgAl₂O₄ provides a high melting point and mechanical strength, making it a good support in plasma-assisted DRM. When Ni is added as the active metal, MgAl₂O₄ spinel can also effectively inhibit the phase transition of NiAl₂O₄ and stabilize Ni crystals.

However, MgAl₂O₄ has a low basicity and large surface area, which is unfavourable for DRM reactions. Adding a co-support to increase the basicity of the carrier is a feasible approach. La₂O₃, due to its strong basicity and high metal dispersion ability, exhibits significant anti-coking performance. As a promoter, La₂O₃ can also enhance the interaction between Ni and MgAl₂O₄, making it a good choice. Asif Hussain Khoja and others studied the performance of Ni-based catalysts supported on La₂O₃-MgAl₂O₄ in a DBD reactor for DRM [107]. The Ni/La₂O₃-MgAl₂O₄ catalyst combines the high melting point, mechanical strength, and good chemical stability of spinel with the strong basicity and anti-coking properties of La₂O₃, achieving a CH₄ conversion rate of 86% and a CO₂ conversion rate of 84.5%, with selectivities of H₂ and CO at 50.0% and 49.5%, respectively, and a H₂/CO ratio close to one. When CO₂ undergoes chemical adsorption on the catalyst surface, it promotes the formation of intermediate carbonates (La₂O₂CO₃). The carbon generated from methane decomposition reacts with La₂O₂CO₃ to produce CO₂ and further reacts with Ni to regenerate La₂O₃. This is a key reaction step to suppress carbon formation and ensure catalyst stability (Figure 12a,b). Therefore, the Ni/La₂O₃-MgAl₂O₄ catalyst exhibits low carbon formation and high energy efficiency. These results confirm the significant synergistic effect between the catalyst and plasma, providing a new strategy to enhance the performance of plasma-assisted DRM.

5. Conclusion and Outlook

This paper reviews the progress of research on the plasma-assisted catalytic dry reforming of methane. Combining plasma with traditional catalysts not only overcomes the thermodynamic limitations of the reaction, allowing it to proceed under mild conditions, but also achieves higher reaction efficiency, reduces carbon deposition, and extends catalyst lifespan. DBD (dielectric barrier discharge) holds the most potential among all types of plasma due to its simple operation and excellent conversion efficiency, and it has been widely applied. However, the high energy cost requires the development of efficient catalysts to support its commercial application. APGD (atmospheric pressure glow discharge) and spark discharge can provide high conversion rates in the presence of catalysts. Still, current research is insufficient, and improvements to these discharge modes are needed. GA (glow discharge) has relatively lower energy costs for DRM but is somewhat lacking in its ability to treat greenhouse gases, a limitation that can be addressed by improving reactor design. MW (microwave) discharge offers the advantage of rapid heating, enabling quick temperature stabilization and significantly reducing heating time and energy loss, making it one of the technologies closest to industrial application.

In the plasma-assisted catalytic dry reforming of methane, the presence of a catalyst can enhance the electric field, alter the discharge mode, and lower the activation energy of the reaction, thereby improving energy efficiency. At the same time, plasma can modify the physicochemical properties of the catalyst, increase gas adsorption efficiency, and enhance metal–support interactions, thus improving catalytic activity. Therefore, combining both can induce a synergistic effect, which is much more complex than the traditional catalytic or pure plasma reforming. The synergistic mechanism between plasma and the catalyst is still not fully understood.

Currently, catalysts mainly use supported catalysts, with both non-precious metals and precious metals having their advantages and disadvantages. Non-precious metals such as Ni, Fe, Cu, and Co offer good catalytic performance and are cost-effective, but they suffer from severe sintering and carbon deposition. Precious metals like Pt, Ru, and Rh have good anti-coking properties, but their high cost hinders further industrial application. Efforts are currently focused on developing efficient and stable Ni-based catalysts to enhance their activity and coke resistance, such as the development of bimetallic catalysts.

However, despite the progress made in plasma-assisted catalytic DRM research, there are still some challenges. First, the energy efficiency of plasma reactions needs to be improved, and reducing energy consumption while increasing energy conversion efficiency is an urgent issue to address. Secondly, the selection and design of catalysts still require further study, particularly concerning the stability and lifespan of the catalysts. In addition, factors such as working temperature, catalyst alkalinity, power input, dielectric constant, reactor geometry, and the shape and packing method of the catalyst are also key factors that influence the reaction performance.

Future research directions can focus on the following aspects: first, new catalysts should be developed, especially composite materials with higher stability and activity, to better adapt to the plasma reaction environment; second, in practical application, due to the high power consumption of plasma, it is necessary to optimize the power supply design and discharge mode to reduce energy consumption. The problems such as uneven gas distribution and local overheating of plasma should be overcome when the reactor is scaled up. In industrial applications, high-voltage power supplies and plasma-resistant materials are costly, low-cost reactor components need to be developed, and NOx or ozone that may be generated during discharge also need to be cleaned up; third, the synergistic enhancement mechanisms between plasma and catalysts to better explain the reaction mechanism should be explored. Through these efforts, plasma-assisted catalytic DRM is expected to play a significant role in energy conversion and greenhouse gas reduction, driving more sustainable chemical manufacturing and energy utilization. In conclusion, the prospects of plasma-assisted catalytic DRM are promising, but continued research and innovation are needed to address the challenges and ultimately achieve its widespread industrial application.