A Review Paper on Non-Thermal Plasma Catalysis for CH₄ and CO₂ Reforming into Value Added Chemicals and Fuels

Abstract

The global reliance on fossil fuels, particularly natural gas, underscores the urgency of developing sustainable methods for methane (CH₄) and carbon dioxide (CO₂) conversion. Methane, which constitutes 95% of natural gas, is a critical feedstock and fuel source. However, its high bond dissociation energy and volatility pose challenges for large-scale utilization and transport. Current research emphasizes the catalytic and plasma-assisted conversion of CH₄ and CO₂ into value-added products such as methanol, higher hydrocarbons, and organic oxygenates. Advancements in these technologies aim to overcome obstacles such as high operating temperatures, coking, and low product selectivity while addressing methane’s environmental impact, as leakage during extraction and distribution significantly contributes to global warming. Plasma-assisted conversion has emerged as a promising approach, leveraging electron impact processes to generate reactive species that facilitate CH₄ and CO₂ transformation at near-room temperatures. The integration of catalysts within plasma environments enhances reaction pathways, product yields, and selectivity by modifying plasma properties and surface interactions. This review comprehensively discusses the various methods investigated for CH₄ conversion and energy efficiency. We attempt to highlight the recent progress in plasma-assisted catalytic processes for CH₄ and CO₂ valorization, with a focus on the mechanisms of product formation, catalyst modifications, and their impact on plasma discharge characteristics. The insights gained could pave the way for scalable, energy-efficient solutions to produce sustainable fuels and chemicals, thereby contributing to global efforts in carbon cycle fixation and climate change mitigation.

1. Introduction

Recent reports indicate that approximately 80% of the global energy demand is met by fossil fuels such as oil, natural gas, and coal. Natural gas has witnessed a significant increase in consumption over the past few decades due to its extensive availability and relatively lower emissions compared to oil and coal. Estimates suggest that natural gas reserves, encompassing both conventional and unconventional sources, amount to approximately 6330 trillion m³ [1].

The methane cycle diagram illustrates the various sources and sinks of CH₄ in the environment. It includes processes such as fossil fuel combustion, emissions from landfills, CH₄ release from wetlands, plant decomposition, methane release due to permafrost melting, and digestion by termites and animals, among others. These processes contribute to the natural flux of CH₄ in the atmosphere, impacting its concentration and ultimately influencing climate change and global warming, and are illustrated in Figure 1 [2].

According to studies, using natural gas reduces CO₂ emissions by 25–45% when compared to burning coal or oil. Crucially, using natural gas results in significantly lower emissions of genotoxic substances, aromatic hydrocarbons, and different oxides (NOx, Sox, and CO). Methane (CH₄) makes up most of the natural gas (approximately 95%) and is often acquired as a byproduct of petroleum processing [3]. Shale gas stands out as a major unconventional source of natural gas, where methane is trapped within the micropores of sedimentary rocks, namely, shales. Hydraulic fracturing combined with horizontal drilling techniques is employed to extract the gas from shale reserves. The challenges, potential opportunities, and associated environmental concerns related to the hydraulic recovery of CH₄ from shale gas have been extensively discussed [4]. Another potentially abundant source of CH₄ is natural gas hydrates [5].

CH₄ serves dual roles as a feedstock chemical and a fuel source across various sectors including residential, electric power generation, industrial processes, and transportation. However, its liquefaction is crucial for facilitating long-distance transportation from remote natural gas reserves to market centers. Yet, the energy-intensive process of compressing CH₄ into a liquid state poses economic challenges for commercial viability [6]. Transporting CH₄ in its gaseous form is hindered by its extremely low boiling point (−161.6 °C at 1 atm) and high flammability, making it challenging and limiting its applications, especially from remote sources [7]. Consequently, research emphasis has shifted towards controlled oxidation of CH₄ to produce liquid fuels such as methanol (CH₃OH) or converting it into easily liquefiable hydrocarbons. Advancements in this technology would enable the efficient storage, transportation, and utilization of CH₄ on a larger scale [8,9].

The demand for CH₃OH production is rising due to its application in fuel cells for energy generation. However, activating CH₄ poses significant challenges due to the covalent nature of its C–H bonds [10]. Only strong superacid, as demonstrated by Olah et al. [11,12], can induce hydride ion abstraction to generate methyl carbocation. Overcoming these challenges is imperative for advancing the cost-effective conversion of CH₄ into liquid fuels, thereby expanding its utilization and impact across various sectors.

The release of CH₄ into the atmosphere poses a significant environmental challenge due to its high volatility. Approximately 8% of CH₄ escapes during various stages of shale gas extraction, transportation, storage, and distribution. This leakage can be mitigated by converting methane into less volatile or liquid forms [13]. The escalating emissions of both CH₄ and CO₂ have drawn increased scrutiny in recent decades due to the widespread reliance on fossil fuels and the extensive exploitation of natural gas resources.

To address the urgent issue of global warming and associated climate changes, it is imperative to focus on the transformation of CH₄ and CO₂ into more useful feedstocks. Ongoing research in the catalytic conversion of CO₂ and CH₄ plays a pivotal role in fostering sustainable development and carbon cycle fixation [14,15,16]. Figure 2 shows the various methods of the CH₄ conversion process, including photochemical, electrochemical, biological, catalytic, and plasma-assisted techniques. The large-scale production of valuable products, such as higher hydrocarbons, hydrogen (H₂), synthesis gas (CO + H₂), alkanes, CH₃OH, carboxylic acids, alkenes, and aromatics, was accomplished through the effective use of thermal and/or catalytic pyrolysis, oxidative coupling, biological techniques, and plasma activation [17,18,19,20,21]. Notably, C-5+ hydrocarbons and liquid organic oxygenates are the most desired products. Additionally compared to CH₄, the C-2 to C-4 hydrocarbons are readily compressible and hold promise for more cost-effective transportation.

The most promising avenue for significant advancement in CH₄ catalytic conversion technology lies in the identification of a pathway leading to the formation of oxygenates such as CH₃OH and formaldehyde (HCHO) [19,20,22]. Current theories propose that H₂, released as a byproduct during the dehydrogenation of CH₄, combines with oxygen (O₂) on the catalyst surface, giving rise to a surface peroxide species. This peroxide species is believed to play a crucial role in activating CH₄ and facilitating its conversion into oxygenated compounds [23]. Some studies have investigated the oxidation of CH₄ using oxygen over molybdenum-based catalysts, demonstrating low CH₄ conversions (<10%) with a notable selectivity (50%) toward HCHO. Additionally, research involving zeolite-based catalysts has explored their capability to convert CH₄ into CH₃OH. Notably, Cu-exchanged zeolites have been found to effectively convert CH₄ to CH₃OH at temperatures ranging from 130 to 200 °C, utilizing molecular oxygen as the oxidant [21,22,23].

As previously mentioned, the catalytic conversion of CH₄ and CO₂ poses significant challenges, including the need for high operating temperatures, issues such as coking and sintering, and low conversion rates and limited product selectivity. A significant challenge in the thermal activation of CH₄ for liquid product formation is that many of these products begin to decompose at the high temperatures necessary for effective CH₄ conversion. This issue complicates the ability to achieve both high conversion rates and selectivity simultaneously, ultimately restricting single-pass product yields.

Recent advancements in plasma-assisted conversion techniques present promising solutions to many of these challenges. As a result, there has been growing interest and extensive research in the plasma-assisted valorization of CH₄ and CO₂ [24,25,26]. Within a plasma environment, processes such as electron impact excitation, dissociation, and ionization produce a diverse array of reactive species. These reactive species subsequently engage in recombination reactions, facilitating the formation of value-added neutral molecules.

Controlling product formation within a plasma environment involves adjusting the density of generated species, achievable through regulating parameters such as the gas flow rate, discharge power, and feed gas composition. Introducing appropriate catalysts into the discharge zone often enhances product yield and selectivity. Catalysts play multiple roles in plasma environments, facilitating efficient adsorption of reactive species on their surfaces and thus altering reaction pathways. Furthermore, catalysts within the discharge zone can significantly impact plasma properties, inducing effects such as surface discharge [27]. These effects vary depending on the physical and chemical properties of the catalyst material.

This review extensively explores the interaction between catalysts and plasma properties. It focuses on the latest advancements in plasma-assisted processes, both catalytic and non-catalytic, for the activation and conversion of CH₄ and CO₂ into valuable chemicals. Special attention is given to the mechanisms of product formation, particularly the synthesis of higher hydrocarbons and organic oxygenates. Moreover, summary tables are presented to highlight the complexity of methane conversion processes and underscore the importance of catalyst design and reaction engineering in controlling product selectivity and yield. These tables provide a comprehensive overview of the various pathways, catalysts, and reaction conditions that influence the formation of desired products from methane conversion. Additionally, it delves into important surface processes and the role of plasma in modifying various catalyst materials, both physically and chemically.

2. Methane-to-Energy Conversion

Converting CH₄ to energy is an important part of sustainability since it tackles environmental problems related to CH₄ emissions, which is a powerful greenhouse gas. In the upcoming sections, we will explore various sustainable technologies developed for CH₄ conversion, discussing their efficiency as well as their merits and demerits (Figure 3).

2.1. Sustainable Technologies Developed for CH₄ Conversion

2.1.1. Solar-Energy-Derived Methane Conversion

Solar-energy-driven CH₄ conversion is a potential green technique for reducing the environmental effect of CH₄ emissions and decreasing dependency on fossil fuels. Solar energy can transform CH₄ into other useful products via numerous chemical processes [28]. There are several pathways for solar-energy-derived CH₄ conversion such as solar–thermal-energy-derived CH₄ conversion [29,30], solar–photovoltaic-derived CH₄ conversion [31,32], and photocatalytic CH₄ convention [33,34].

Dong, Chunyang et al. [35] have demonstrated a revolutionary photocatalytic reaction route that allows for the direct production of CH₃COOH from CH₄, CO, and H₂O over Pt/polyoxometalate/TiO₂ at room temperature. Under ideal circumstances, they produced a phenomenal yield of 5.7 mmol·L⁻¹ CH₃COOH within 60 h, with exceptional selectivity surpassing 90%. Using isotopic labeling and in situ spectroscopy, the researchers discovered that CH₄ is activated by ·OH radicals and then reacts with surface-bound CO molecules on Pt single atom sites to form acetyl groups. Further oxidation resulted in the production of CH₃COOH. Despite impressive advances in catalyst development and the investigation of new reaction routes for photocatalytic CH₄ conversion, the problem of selectively activating and functionalizing CH₄ into useful molecules remains tough.

Ma et al. [36] created a unique heat pipe tubular reactor including a heat transfer cavity and a reaction cavity. This system offered a constant hybrid energy source by combining concentrated solar energy with photovoltaic power. Stability tests over 24 h found that the system maintained about 90% CH₄ conversion and 72% H₂ selectivity. This emphasizes the importance of combining thermal and photovoltaic systems to completely utilize solar energy, rather than depending simply on one method. Such integration may also be applied to processes such as CH₄ breakdown and other high-energy-demand endothermic reactions.

2.1.2. Microbe-Driven CH₄ Conversion

Microbe-driven methane conversion uses microorganisms such as bacteria or archaea to convert CH₄ into various chemicals. This process is often carried out by bacteria that contain enzymes capable of metabolizing CH₄. Microbial CH₄ conversion occurs naturally in a variety of habitats, including wetlands, rice paddies, and the digestive tracts of some animals. Biological conversion has emerged as an innovative and sustainable approach to carbon sequestration and the conversion of CH₄ into valuable compounds such as liquid fuels, bioplastics, and chemicals, in contrast to thermochemical processes. Bacteria, including different types of bacteria such as ammonia-oxidizing bacteria, can activate the stable C–H bond in CH₄ molecules under normal circumstances. This activation is most likely due to catalytic mechanisms assisted by particular enzymes. This biological technique functions in milder conditions than standard thermochemical procedures, providing an ecologically benign and energy-efficient option for exploiting methane’s potential in a variety of industrial applications [37,38,39].

In the study by Patel et al. [40], several support materials, such as chitosan modified with glutaraldehyde (GLA), 3-aminopropyltriethoxysilane (APTES), polyethyleneimine, and APTES followed by GLA (APTES-GLA), were investigated to improve methanol synthesis from biogas. Among these materials, chitosan-APTES-GLA showed the greatest improvement in immobilization yield and relative efficiency of Methylomicrobium album, reaching 56.4% and 97.7%, respectively. Notably, the maximal cell loading of M. album was 236 mg dry cell mass per g-support, representing a remarkable 7.7-fold increase over chitosan alone. The immobilized M. album produced significantly more methanol, approximately 23.9 times that of free cells.

2.1.3. Nuclear-Driven Methane Conversion

For the conversion of CH₄ (methane), nuclear energy is an attractive and sustainable source of thermal energy. This is due to processes such as steam reforming and methane breakdown, which are characterized by endothermic reactions that produce H₂. Nuclear energy provides an alternative to the traditional Steam Methane Reforming (SMR) method, which produces significant CO₂ emissions from the burning of CH₄. In this case, CH₄ not only acts as a reactant to produce H₂, but it also helps the process by releasing heat during combustion.

By using nuclear heat in these processes, as opposed to traditional combustion techniques, greenhouse gas emissions and other toxic pollutants are reduced. Furthermore, nuclear-driven systems that convert thermal energy directly to H₂ show a far faster rate of H₂ generation than indirect conversions that include thermal–electrical–H₂ energy pathways. This highlights the environmental benefits and efficiency improvements linked to nuclear H₂ generation via steam CH₄ reforming, thus establishing it as a very advantageous choice in the context of sustainable energy supplies [41,42].

2.1.4. Tribochemical-Driven Methane Conversion

Tribochemical-driven methane conversion is the process by which methane (CH₄) is converted into various compounds using chemical reactions sparked by the mechanical forces produced during frictional contact. This method does not require high temperatures or outside energy sources to drive chemical reactions since it uses mechanical energy, which is normally produced by rubbing or friction. “Tribology”, the study of friction, and “chemical” are combined to form the phrase “tribochemical”.

Tribochemical reactions involving sliding surfaces and additives such as lubricants, ambient air, and some tribomaterials are frequently triggered at sliding interfaces by high contact pressures [43], frictional heating [44], and shear stresses [45]. At the contact interface, this causes a protective boundary layer or tribofilm to form. For instance, atmospheric pressures of CH₄ are used during the production of carbon-based compounds such as graphene [46], polycrystalline diamond [47], or diamond-like carbon [48]. It has been shown that the methyl group (–CH₃) with a diamond structure plays a critical role in the deposition of diamond films. Methyl groups with sp3 hybrid orbitals engage with the surface of the substrate to produce C–C covalent bonds, and they nucleate on the surface of the matrix to form the nucleus of a diamond crystal. Active methyl groups progressively replace H atoms in the crystal nucleus by continuous impact with high-energy particles, promoting the connection and creation of a diamond film. Consequently, it is more advantageous to use carbon sources such CH₄, acetone, methanol, etc., that have a diamond-like structure for diamond film deposition [49].

2.1.5. Geothermal-Driven Methane Conversion

Geothermal energy, a clean and sustainable resource with a widespread distribution and significant reserves, is still in its early phases of development. The concept of utilizing geothermal heat to convert CH₄ was introduced, with an improved geothermal system (EGS) reservoir. This technique used two wells, one of which injected water for chemical reactions and heat transmission, resulting in smaller, more cost-effective wells [50]. Geothermal energy may also be transformed into electricity using thermoelectric generators. Li et al. [51] report on successful field testing that produced 500 W of electrical energy from a geothermal–thermoelectric generator with a temperature differential of 152 °C. The study of thermoelectric materials in geothermal energy conversion is predicted to progress quickly, with the possibility of using the produced electric power for industrial production and CH₄ conversion in the future [52].

2.2. Energy Efficiency

Table 1. Different energy-driven CH₄ conversion technologies.

Sustainable Energy Source		Sustainability	Energy Efficiency	Product	Ref.
Solar energy	Photothermal	Low	11.4%	H2, CO	[55]
	Photocatalysis	High	14.1%	CH3OH, CH3OOH, HCHO, CO2	[56]
Bioenergy		High	80%	CH3OH, H2O, H2	[57]

compares the energy efficiency of various processes employed in energy production using sustainable technology. Researchers have been exploring methane (CH₄) conversion utilizing renewable energy sources, specifically solar and nuclear energy. Early efforts used heat or electricity-based techniques to produce hydrogen (H₂), with considerable CO and CO₂ byproducts and 11.4% energy efficiency [53]. Photocatalytic technology has also evolved, which uses semiconductor materials to convert CH₄ into different C1/C2 oxygenates using daylight, and about 14.1% energy efficiency was reached [54]. Despite development, difficulties remain, including kinetic constraints and the generation of byproducts.

Microbial-driven methane conversion has an amazing 80% energy efficiency [55], outperforming other approaches. The direct use of sustainable energy is essential to boosting efficiency, exhibiting the potential of previously untapped sources such as wind energy and ocean waves. Furthermore, unused energy, such as leftover heat and electrochemical neutralization energy, offers chances for driving or helping CH₄ conversion. Further research into these driving forces is required for sustained progress in sustainable methane conversion [56,57].

Although there are not many papers on sustainable energy-driven methane (CH₄) conversion, it is clear that developing effective, well-controlled systems in moderate climates is still a difficult task. It is possible to partially alleviate the energy crisis and solve global environmental issues by incorporating alternative energy sources into existing CH₄ technology. Thankfully, continued research has advanced fundamental knowledge, reactor design, catalyst production, characterization methods, and theoretical explorations. However, converting CH₄ just using renewable energy sources will not cut it in the long run. Research on more efficient catalysts is now underway, as is the development of sustainable energy-driven reactors and the seamless integration of these systems with current CH₄ converters to improve overall conversion performance and energy utilization efficiency. To make significant advancements in the sector moving forward, researchers should investigate a variety of pathways in sustainable energy-driven CH₄ conversion [58,59].

3. Plasma Technology for CH₄-to-Energy Conversion

Non-thermal plasma (NTP) can be generated through a straightforward method involving the placement of two electrodes in a gas medium and the application of an electrical potential difference. This potential can be in the form of direct current (DC), alternating current (AC), or pulsed electrical signals. Alternatively, induction coils or microwaves can also be employed to provide the necessary electrical energy. Depending on the chosen gas and the method of plasma generation, electrons in NTP can achieve an average temperature ranging from 10,000 to 100,000 K (1–10 eV), despite the gas temperature remaining as low as room temperature [60,61].

Elevated electron temperatures are pivotal in the distinctive chemistry of NTP, enabling the activation and dissociation of stable atoms and molecules, thus facilitating diverse reactions. NTP encompasses several primary categories as depicted in Figure 4 [62].

For chemical conversion processes, operating at pressures close to atmospheric levels is generally preferred. This preference stems from the considerable expense and technical challenges associated with low-pressure systems, along with their limited favorability for kinetic reactions. Typically, high pressures are utilized in industrial catalytic processes where thermodynamic constraints permit, thereby accelerating reaction rates. Although plasma can operate at higher pressures, it is generally less preferred due to the escalating voltage requirements for sustaining discharge. Addressing voltage limitations may involve reducing the discharge gap, but this approach could render the gap too narrow for accommodating potential catalyst integration within the discharge zone. Consequently, it is unsurprising that most of the research in CH₄ activation chemistry is conducted using atmospheric pressure plasmas [61].

3.1. CH₄ Conversion to Energy or Oxygenated Products

Extensive research on greenhouse gas conversion using plasma has primarily focused on the valorization of CH₄. This is achieved through plasma-assisted non-oxidative coupling, which facilitates the formation of higher hydrocarbons, and oxidative coupling, which leads to the production of organic oxygenates. This section provides an overview of plasma-based CH₄ valorization processes, highlighting key influencing factors. A summary of the findings from previous studies on CH₄ valorization across different plasma reactors is presented in

Table 2. CH₄ valorization in various non-catalytic plasma reactors.

Gas Mixture	Flow Rate	Reactor Setup	Plasma Type	Parameters of Plasma	Conversion	Product Selectivity	Ref.
Ar/CH4	Q = 0.1 L/min	Vaccum chamber cylinder	Nanosecond pulsed non-thermal discharge	Vapp = 1 kV	CH4 (11%)	H2 (70%) C2H2 (21%) C2H4 (20%) C2H6 (6%)	[62]
Ar/CH4	Q = 0.11 L/min	DBD tubular reactor	Nanosecond pulsed non-thermal discharge	Vapp = 12 kV f = 10 kHz	CH4 (87.2%)	H2 (80%)	[63]
CH4	Q = 0.4 L/min	DBD tubular reactor	Nanosecond pulsed non-thermal discharge	Vapp = 21 kV f = 22 kHz	CH4 (60%)	H2 (nd)	[64]
CO2/CH4 1:1	Q = 0.05 L/min	Single DBD tubular reactor	DBD plasma discharge	Power = 75 W	CO2 (24.1%) CH4 (49.2%)	CO (39.8%) H2 (31.3%)	[65]
Ar/CO2/CH4 1:1	Q = 1.92 L/min	Cylindrical multi-electrode DBD reactor	DBD plasma discharge	Power = 60 W f = 17.1 kHz	CH4 (17%) CO2 (12%)	H2 CO C2H4 C2H6	[66]
N2/CH4/CO2	QCH4 = 0.19 L/min QCO2 = 0.38 L/min	Box–Behnken design	Microwave plasma	nd	CH4 (84.91%) CO2 (44.40%)	H2 (51.31%) CO (61%)	[67]
CH4/CO2 1:1	nd	Glass tube reactor (plane to plane electrode)	Nanosecond pulsed discharge	SED = 10 kJ/dm3	CH4 (50%) CO2 (40%)	CO H2 C2H2	[68]
CH4/CO2/O2, N2 (10%, 10%, 9%, 71%)	Q = 10 L/min	Sophisticated shape reactor (with cathode and anode)	Gliding arc plasma	Power = 364 W	CH4 (93%) CO2 (44%)	H2 CO	[69]
CH4/CO2 1:1	Q = 0.05 L/min	Cylindrical quartz DBD reactor	Nanosecond Pulsed DBD	Power = 62.7 W	CH4 (39.6%) CO2(22.9%)	H2 (46.7%) CO (68.6%)	[70]
H2O/CO2/CH4	nd	nd	Electric-arc plasma torch	Power = 90,000 W	CH4 (94–96%)	nd	[71]
CO2/CH4 1:1	Q = 0.6 L/min	Cylinder-wire-type reactor	Corona discharge plasma	Vapp = 12 kV F = 900 Hz	CH4 (30.1%) CO2 (27.8%)	CO (29.1%) H2 (55.1%)	[72]
CO2/CH4 1:1	Q = 30 L/min	Nd	Microwave plasma torch	Power = 6000 W f = 2.45 GHz	CH4 (96.4%) CO2 (68.4%)	H2 CO	[73]
CO2/CH4 1:1	nd	Pin-to-pin configuration reactor	Nanosecond pulsed discharge.	SED = 6.5 kj/dm3	CH4 (40%) CO2 (30%)	CO (68%) H2 (77%)	[74]
CH4/CO2 (25%, 75%)	Q = 0.5 L/min	Cylindrical pin-to-plate reactor	Confined atmospheric pressure glow discharge plasma	SED = 16 kJ/L	CH4 (94%) CO2 (64%)	CO (95%) H2 (10%)	[75]
CO2/CH4 1:1	Q = 0.15 L/min	Tubular reactor	Kilohertz spark-discharge plasma	SED = 725 kJ/mol	CH4 (77%) CO2 (70%)	CO (80%) H2 (78%)	[76]
CO2/CH4 1:1	Q = 12.5 L/min	Cylindrical-shaped reactor	Rotating gliding arc	Power = 471 W	CH4 (36%) CO2 (70%)	CO (29.1%) H2 (27.1%)	[77]
CO2/CH4 1.8:1	-	Tip–tip plasma batch-reactor	Arc discharge reactor at very high pressure	SED = 7450 kJ/mol	CH4 (46%) CO2 (45.2%)	H2 (43.63%) CO (26.99%)	[78]

Nd: Not defined; V_app: Applied voltage; f: Frequency; SED: Specific energy density.

.

3.2. Non-Oxidative CH₄ Conversion

NTP-assisted non-oxidative CH₄ conversion is widely investigated owing to its simple methodology. This process primarily involves two mechanisms: (i) electron collision activation and (ii) thermal activation. Both mechanisms strip hydrogen from CH₄, creating active species to produce stable products through further reactions. Predominant products include C₂H₆ and C₂H₂, along with hydrogen. While higher hydrocarbons such as C-5+ are detected, their yields are typically low. Importantly, an undesired outcome of this process is the formation of pure carbon or coke, which can hinder energy efficiency. Despite this, if CH₄ needs to be utilized due to transportation challenges, even the production of C₂H₆, C₂H₄, and C₂H₂ can be considered valorization, albeit safety concerns arise, especially with C₂H₂. These gases offer advantages such as cheaper transportation due to greater compressibility and potential transformation into liquids or other valuable products. A few studies even reported on the synthesis of high- graphene nano-flakes and sheets as high-value carbon products. The study reveals that manipulating the pressure can successfully regulate the properties of graphene nanoflakes, although it comes with considerations regarding energy usage. Notably, a significantly low energy consumption of approximately 0.13 kWh/g was attained under 200 kPa, indicating promising opportunities for refining NTP methods to enhance the mass production of graphene nanoflakes on a large scale [79].

3.3. Partial Oxidation of CH₄

Partial oxidation of CH₄ by NTP is a promising avenue for producing oxygenates, initiated by electron collision with CH₄ and O₂ molecules. CH₃ and O are key intermediate species produced by plasma. Interestingly, controlling the complete oxidation of CH₄ to H₂O and CO₂ is crucial to maximize the oxygenate yield [80,81,82,83,84,85,86]. Achieving high selectivity, such as gliding arcs near 100% syngas selectivity at an O₂/CH₄ ratio of 0.6, is vital [81].

Thermal plasma is not favored; instead, DBD plasma is commonly employed. It is essential to manage CH₃OH decomposition and selectivity, especially as the residence time affects conversion and selectivity. Optimizing the residence time is critical for maximizing the oxygenate yield while considering specific energy input adjustments, typically controlled via the total gas flow rate at constant power.

Another crucial aspect to consider is the breakdown of CH₃OH in plasma and the shifting preference towards complete combustion products as the duration of residence increases, a phenomenon extensively discussed in a review by Indarto et al. [82]. It is worth noting that determining the optimal residence time is essential to maximize the yield of CH₃OH or other oxygenates, as there exists a trade-off between CH₄ conversion and the selectivity towards organic oxygenates concerning the specific energy input (SEI). The SEI can be fine-tuned by regulating the flow rate of gas under equivalent power conditions, as demonstrated in previous studies [87].

3.4. CH₄ Conversion to Syngas

Traditional syngas production methods utilize catalytic reformers, which are associated with high economic investment costs and several drawbacks [83,84]. These drawbacks include the need to activate catalytic sites at high temperatures and low flow rates of inlet gas [85]. To address these issues, alternative technologies such as plasma (both thermal and non-thermal) are gaining traction [86]. Luche et al., investigated the behavior of CH₄–air mixtures injected into an NTP reactor at atmospheric pressure and ambient temperature. The research demonstrated successful syngas (H₂ and CO) production across a wide range of fuel flow rates and CH₄ concentrations in air. Notably, the lowest energy cost for H₂ production is approximately 45 kWh/kg (H₂) at the highest mass flow rate. The numerical simulations further validate that the NTP reactor achieves optimal electrical power utilization with high mass flow rates and low inlet CH₄ concentrations in air [87].

3.5. Plasma Coupling of CH₄ and CO₂

While most investigations into the simultaneous conversion of CH₄ and CO₂ have focused on dry reforming processes, some studies have explored the conversion of these gases into liquid chemicals [88,89]. Dry reforming processes are typically conducted in warmer plasmas such as a spark or gliding arcs, whereas research targeting liquid products has predominantly utilized dielectric barrier discharge (DBD) systems [90,91]. Catalysts were strategically positioned around the high-voltage electrode within the central discharge zone of the reactor to capitalize on the synergistic effects of the catalyst and plasma. With an input power of 140 W from DBD plasma and no additional heating, impressive conversion rates of 80.3% for CO₂ and 86.4% for CH₄ are achieved. Furthermore, the catalyst proves advantageous in producing higher-value C₂H₄ products under plasma promotion. These findings hold significant implications for optimizing plasma-promoted dry reforming reactions to yield high-value products and are shown in Figure 5 [92].

Martini et al. [93] reported the formation of liquid products such as CH₃OH and C₂H₅OH, along with formic, acetic, propanoic, and butanoic acids. The gas products consisted primarily of syngas (89%), with smaller amounts of C-2 (8.5%) and C-3 hydrocarbons (1.5%). The authors suggested that the plasma-induced dissociation of the CH₄/CO₂ mixture generates the reactive species CH₃, H, CO, and O, which subsequently contribute to product formation (

Table 3. List of catalysts combined with plasma-assisted DRM.

Type of Plasma Source	Plasma Power	Feed Flow Rate	Feed Flow Ratio	CH4 Conversion	CO2 Conversion	H2 Production	CO Production	Catalyst	Reference
	W	mL/min	CH4/CO2	%	%	%	%
Gliding arc	128	2000	1:2	39	25	65	88	None	[94]
Microwave	6000	30,000	1:1	96	68	_	_	None	[95]
Microwave	7500	278	2:3	61	_	63	_	None	[96]
Pulsed DBD	55.7	50	1:1	39.6	22	42	51	None	[97]
DBD	11	100	1:1	56	43	62	64	PtUiO-67	[98]
GAD	128	2670	1:1	94	91	97	95	Ni/CeO2/Al2O3	[99]
DBD	67.5	600	1:1	31	34	60	53	Zeolite	[100]
DBD	100	20	1:1	74.5	73	47	48	10%Ni//γ Al2O3–MgO	[101]
AC DBD	50	50	1:1	56.4	30.2	31	52	10%Ni/Al2O3	[102]
Pulsed corona	30	25	2:1	25	22	_	18	La2O3/γ-Al2O3	[103]

). Additionally, potential mechanisms were proposed through density functional theory (DFT) calculations. For instance, acetic acid could be produced through successive additions of CO and OH to methyl radicals, or via the addition of COOH. It is hypothesized that COOH primarily forms through the recombination of CO and OH radicals.

4. Plasma-Catalysis for Energy Production

NTP technology diverges from conventional thermal catalytic methods by prioritizing the conversion of electrical energy into reactive species and electrons. While this presents a promising alternative for direct CH₄ coupling, the gas-phase non-catalytic plasma reaction typically operates through a free radical mechanism, making product selectivity challenging to control [104,105,106]. To address this limitation and enhance methane conversion, significant research has focused on integrating NTP with heterogeneous catalyst systems. Various plasma types, such as dielectric barrier discharge, corona, glow discharge, and gliding arc, have been explored. Studies indicate that manipulating electron energy through reactor design and catalyst selection significantly impacts product selectivity, particularly in DBD and corona discharge setups [107].

The fusion of NTP with catalytic systems opens a promising avenue for directly upgrading CH₄ into higher-value products, operating at significantly lower temperatures compared to conventional thermal-catalytic methods [108]. Over the past two decades, extensive experimental investigations have delved into plasma-activated CH₄ conversion, both with and without oxidants. These studies, involving a diverse range of catalysts and plasma discharge configurations, consistently showcase the process’s potential [109,110]. However, a common challenge lies in the limited understanding of the mechanistic intricacies underlying the observed performance outcomes. The interplay between plasma and catalyst in plasma-catalysis systems introduces high complexity, emphasizing the need for an increased focus on modeling efforts to elucidate some of the intricate interactions at play [111,112].

Recent research has been directed towards modeling the influence of vibrationally excited species in elucidating the synergistic relationship observed between the plasma and catalyst phases in various chemical processes. For instance, Engelmann et al. [113] conducted a study on the nonoxidative coupling of CH₄ over different transition metals utilizing a surface microkinetic model. Their findings indicated a notable enhancement in the turnover frequency of vibrationally excited CH₄, particularly with low-activity metals characterized by weak binding. Similarly, Loenders et al. [114] employed a similar methodology to investigate the partial oxidation of CH₄ on the Pt surface.

These investigations underscore the necessity for kinetic models that encompass the diverse array of species originating from the plasma phase and the elucidation of their interactions with the catalyst. However, most existing studies have primarily focused on initial rates within a static gas phase framework, where populations of vibrationally excited states were predetermined based on a fixed vibrational temperature distribution. Dynamic studies integrating plasma-kinetic models with catalytic surface models have been limited to the production of ammonia [115,116]. Furthermore, recent research in thermal catalysis by the Dauenhauer group has demonstrated the potential exploitation of dynamical effects to achieve performance surpassing that of steady-state conditions [117].

Combining catalysts with plasma reactors enhances CH₄ conversion with improved selectivity for desired products compared to plasma alone. This synergy modifies the surface chemistry, activating reactants at mild conditions and facilitating selective reactions on catalyst surfaces. Plasma generates reactive species and alters catalyst properties, promoting catalyst activation and reducing the activation barrier. Catalysts enhance electric fields, change plasma discharge behavior, and increase adsorption rates, leading to higher product selectivity, catalyst stability, and energy efficiency while reducing carbon formation in DRM [118]. Catalyst placement depends on reactor geometry and temperature, with high-thermal-stability catalysts being suitable for simple plasma reactor geometries.

Noble metal catalysts, notably Pt, coupled with plasma reactors show promise for the thermal DRM reaction. Wang et al. utilized Ag, Pt, Pd, Ir, and Re fixed on CZSM5 and UZSM5 catalysts combined with DBD reactors for biogas reforming, achieving significant liquid product yields (60%) with low carbon deposition (5.1%) [100]. Another Pt catalyst, PtUiO-67, exhibited enhanced CH₄ conversion and H₂ yield in a DBD reactor, attributed to increased CH₄ dissociation and dehydrogenation reactions [20]. In situ DRIFTS studies provided direct evidence for surface reactions under plasma conditions.

Metal oxides such as zeolite [102], La₂O₃/Al₂O₃ [119], γ-Al₂O₃ [120], and CaO [121] show limited activity in the thermally driven dry reforming of methane (DRM). However, when combined with plasma, significant enhancements in conversion and selectivity are observed. For instance, Nguyen et al. demonstrated a 55.9% increase in CO₂ conversion and a 67.7% increase in CH₄ conversion using a zeolite-packed dielectric barrier discharge (DBD) plasma reactor [102]. Zeolite also improved CO selectivity by 45.3% by enhancing CO₂ decomposition. Zeolite A was found to suppress carbon deposition and enhance C₂-C₄ selectivity [102]. La₂O₃/Al₂O₃ in a DBD reactor increased CH₄ conversion three-fold due to surface oxygen species aiding in CH₄ activation. La₂O₃/γ-Al₂O₃ combined with pulse corona plasma produced C2 hydrocarbons with notable yields [121]. γ-Al₂O₃ facilitated CH₄ adsorption and activation under plasma, leading to the formation of various hydrocarbon products [122]. Additionally, decreasing the CaO grain size in DBD reactors increased CH₄ and CO₂ conversions by 14% and 22%, respectively, attributed to enhanced gas-phase reactions favoring ethane formation [122].

Photocatalytic DRM typically suffers from low reaction rates and syngas yield due to inefficient solar energy utilization. However, packing photocatalysts into plasma reactors can induce a synergistic effect called SEPC, improving energy efficiency (EE), selectivity, and stability. Chung et al. demonstrated this by packing perovskite photocatalysts into a spark discharge reactor, resulting in a 42% increase in syngas production efficiency compared to plasma alone [101]. The plasma reduces particle size, extends the lifetime of electron-hole pairs, and increases the specific surface area, leading to the faster diffusion of electron-hole pairs and enhanced reduction and oxidation rates. Moreover, the packing of photocatalyst increases the plasma current density, facilitating the dissociation of CO₂ and CH₄. Additionally, Liu’s group showed that DBD plasma can alter the surface structure of catalysts, inhibiting certain reactions and improving carbon resistance in DRM reactions [122].

5. Conclusions

The integration of non-thermal plasma (NTP) with heterogeneous catalytic systems represents a transformative approach for CH₄ conversion, offering significant advantages over conventional thermal catalytic methods. NTP prioritizes the generation of energetic electrons rather than heating gases, enabling reactions at lower temperatures. However, challenges in product selectivity due to the gas-phase non-catalytic plasma reaction’s free radical mechanism have spurred extensive research into combining NTP with catalysts. Studies have demonstrated that this synergy enhances CH₄ conversion and selectivity, leveraging plasma-generated reactive species and catalyst-induced modifications to the surface chemistry. Advanced reactor designs, coupled with active catalysts, have shown significant potential, enabling selective product formation, increased energy efficiency, and reduced carbon deposition in CH₄ reforming processes.

Despite these advancements, the complex interplay between plasma and catalysts remains a critical research focus. Investigations into plasma–catalyst systems have highlighted the importance of developing kinetic models to better understand the dynamic interactions at play, including the influence of vibrationally excited species and plasma-generated radicals. Experimental and modeling studies suggest that optimizing the plasma discharge configurations, catalyst compositions, and placement can further enhance performance. Additionally, incorporating photocatalysts within plasma reactors has revealed a promising avenue for improving energy efficiency and reaction stability. By addressing the current limitations in mechanistic understanding and reactor design, this emerging field holds the potential to revolutionize CH₄ conversion technologies, enabling the sustainable production of high-value chemicals and fuels.