Non-Thermal Plasma-Catalytic Conversion of Biogas to Value-Added Liquid Chemicals via Ni-Fe/Al₂O₃ Catalyst

Abstract

This study investigates the transformation of biogas (methane and carbon dioxide) into high-value liquid products using Ni/Al₂O₃, Fe/Al₂O₃, and Ni-Fe/Al₂O₃ catalysts in a non-thermal plasma (NTP)-assisted process within a dielectric barrier discharge (DBD) reactor, operating at room temperature and atmospheric pressure. We compared the effectiveness of these three catalysts, with the Ni-Fe/Al₂O₃ catalyst showing the highest enhancement in conversion rates, achieving 34.8% for CH₄ and 19.7% for CO₂. This catalyst also promoted the highest liquid yield observed at 38.6% and facilitated a significant reduction in coke formation to 10.4%, minimizing deactivation and loss of efficiency. These improvements underscore the catalyst’s pivotal role in enhancing the overall process efficiency, leading to the production of key gas products such as hydrogen (H₂) and carbon monoxide (CO), alongside valuable liquid oxygenates including methanol, ethanol, formaldehyde, acetic acid, and propanoic acid. The findings from this study highlight the efficacy of combining NTP with the Ni-Fe/Al₂O₃ catalyst as a promising approach for boosting the production of valuable chemicals from biogas, offering a sustainable pathway for energy and chemical manufacturing.

1. Introduction

Human activities, like burning fossil fuels, cutting down forests, and using land in unsustainable ways, are broadly recognized as the leading causes of climate change. To tackle this, various strategies have been proposed, including soil restoration and capturing and recycling greenhouse gases [1]. The mitigation of climate change necessitates addressing the accumulation of methane and carbon dioxide, primary contributors to the greenhouse effect. Transforming these atmospheric pollutants into useful chemical feedstocks and energy sources is paramount for the establishment of an environmentally sound, carbon-neutral economic framework. Scientific inquiry has focused on diverse methodologies for the activation of methane, employing novel strategies. Among these, microwave-mediated methane transformation has emerged as a prominent area of investigation. This technique harnesses microwave radiation, wherein a catalyst either absorbs or deflects the electromagnetic energy, subsequently facilitating the transfer of this energy to methane molecules, thereby initiating chemical reactions [2,3]. However, challenges such as excessive coke formation and inefficient energy transfer have been identified with this technique. Another promising technology is non-thermal plasma (NTP), which allows chemical reactions that are typically unfavorable at lower temperatures to occur. This is made possible by generating highly energetic electrons that can excite inert molecules, enabling chemical reactions to take place at much lower temperatures than those required in traditional thermal catalysis [4,5].

The utilization of plasma-enhanced catalytic reactions has garnered significant interest, as it enables the interaction of highly reactive species, created via non-equilibrium electrical discharges, with the active regions of a catalyst. This interaction promotes the generation of desired products with enhanced effectiveness and selectivity. This cooperative effect between non-thermal plasma and the catalyst is especially notable in reactions such as the conversion of methane and carbon dioxide into synthesis gas (syngas) at ambient temperatures, a process known as dry reforming [6,7,8,9]. Research findings indicate that the integration of a catalytic agent within a plasma-driven chemical transformation significantly enhances the preference for the formation of hydrocarbons containing two to four carbon atoms during methane processing. Likewise, investigations have demonstrated favorable outcomes in the direct, single-stage synthesis of liquid chemical compounds from the combined reaction of carbon dioxide and methane [10,11]. A recent investigation conducted by Fulcheri et al. revealed that the inclusion of Fe/SiO₂ or Co/SiO₂ catalysts within a plasma-mediated co-reaction of carbon dioxide and methane significantly amplified the formation of oxygen-containing organic compounds. This catalytic intervention resulted in a notable enhancement of selectivity towards these products, achieving values up to 40% [12]. In an experiment conducted by Shao et al., the introduction of a NiAl-LDH/NF catalyst into a plasma environment containing carbon dioxide and methane led to an 18% preference for alcohol production, with methanol constituting 12% of this fraction [13]. Their research also highlighted the role of oxygen deficiencies in facilitating methanol synthesis, while cobalt metal species were shown to favor the creation of ethanoic acid. Further studies using a P-CoMgAl/NF catalyst demonstrated an augmentation in alcohol selectivity, reaching approximately 15% [14]. In a separate investigation involving plasma-driven dry reforming of methane utilizing Ni/BN catalysts, Liu et al. observed the presence of trace quantities of methanol, ethanoic acid, and ethanol within the resultant products [15].

As shown in previous studies, plasma-assisted catalysis has recently gained attention for its ability to convert methane and carbon dioxide—major components of biogas—into valuable chemicals at low temperatures. Unlike traditional thermal processes that rely on high heat (typically above 773 K), non-thermal plasma (NTP) uses energetic electrons and reactive species to activate these otherwise stable molecules under near-ambient conditions [14,16]. This capability has made NTP-catalyst systems an exciting area of research, as the reactive species generated in the plasma can interact with catalyst surfaces to drive selective chemical transformations, including the formation of C-C and C-O bonds. Among the catalysts studied, nickel supported on gamma-alumina (Ni/γ-Al₂O₃) has shown strong potential. The addition of iron not only improves the dispersion and electronic properties of nickel but also enhances resistance to coke formation—one of the key challenges in plasma catalysis [16]. The investigation herein focused on the evaluation of catalysts composed of nickel and iron deposited onto gamma-alumina (γ-Al₂O₃), specifically examining their efficacy in enhancing the synthesis of oxygen-containing compounds during plasma-assisted dry reforming of biogas. This process was carried out under ambient temperature and atmospheric pressure conditions. These catalytic materials, recognized for their robust performance and specificity in both biogas transformation and the generation of oxygenates from synthesis gas, underwent meticulous characterization. The objective was to elucidate their inherent characteristics and to determine their impact on the plasma-mediated catalytic conversion of biogas via dry reforming.

2. Experimental Section

2.1. Catalyst Preparation

Commercial γ-Al₂O₃ was purchased from Sigma-Aldrich (St. Louis, MO, USA). A series of catalysts, including 10 wt.% Ni/Al₂O₃, 10 wt.% Fe/Al₂O₃, and Ni-Fe bimetallic catalysts with varying metal loadings, were prepared using the incipient wetness impregnation method. For the Ni-Fe/Al₂O₃ samples, appropriate amounts of Ni(NO₃)₂·6H₂O and Fe(NO₃)₃·9H₂O were dissolved in deionized water. The resulting solution was slowly added to the Al₂O₃ support under continuous stirring for 4 h at room temperature. The mixture was then filtered, and the resulting paste was dried at 120 °C overnight. Finally, the dried samples were calcined at 550 °C for 4 h in air. Catalysts were denoted as xNi–yFe/Al₂O₃, where x and y represent the weight percentages of nickel and iron, respectively.

2.2. NTP-Catalytic Conversion of Biogas

The experimental setup used for biogas conversion is illustrated in Figure 1. Reactions were carried out in a coaxial dielectric barrier discharge (DBD) reactor made of quartz, with an inner diameter of 8 mm, outer diameter of 10 mm, and wall thickness of 1 mm. A central tungsten rod (4 mm diameter) served as the high-voltage electrode, while a silver mesh wrapped around the outer surface acted as the ground electrode. Plasma was generated using a LEAP100© power supply (PlasmaLeap Technologies, Marrickville, Australia), delivering up to 700 W with a peak-to-peak voltage of 80 kV. The discharge frequency was set at 500 Hz, and the resonance frequency was automatically adjusted to approximately 56.99 kHz by the generator. For each run, 0.5 g of finely powdered catalyst was loaded into the center of the reactor. Quartz wool plugs were placed at both ends of the catalyst bed to keep it fixed and prevent displacement by the gas flow. The experiments were conducted at a discharge power of 23 W, with a total gas flow rate of 20 cm³/min with gas residence time of 0.3 min.

The gas products were analyzed using a four-channel micro-GC (Agilent 490, Agilent Technologies, Inc., Santa Clara, CA, USA), which was equipped with a thermal conductivity detector. The first channel, fitted with a 10 m molecular sieve 5A column, facilitated the precise analysis of H₂, O₂, CH₄, and CO. The second channel, utilizing a 10 m PPU column, was employed for the analysis of CO₂, C₂H₂, C₂H₄, and C₂H₆. Hydrocarbons ranging from C₃ to C₆, were quantified in the third and fourth channels, which were equipped with a 10 m alumina column and an 8 m CP-Sil 5 CB column, respectively. Argon (Ar) served as the carrier gas for the first channel, while hydrogen (H₂) was used for the remaining three channels. The gaseous products resulting from the reaction were analyzed using micro-gas chromatography, enabling the quantification of methane and carbon dioxide conversion percentages, as well as the determination of gaseous product yields, utilizing the principles of the ideal gas law. Additionally, a soap-film flowmeter was used to assess the difference in gas flow rates before and after the process. To collect and condense the liquid products, a cooling unit maintained at −15 °C was attached to the condenser. Then, 1 mL of carbon disulfide (99.9%, Sigma-Aldrich) was introduced to aid in the extraction of the condensed liquids from the condenser. The chemical makeup of the liquid products was then determined using a previously calibrated gas chromatography-mass spectrometer (Agilent 6890N/5973 GCMS). The conversion of carbon dioxide and methane was measured via following equations.

$${\mathit{C}\mathit{H}}_4\mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{s}\mathit{i}\mathit{o}\mathit{n} \left(\%\right)={\displaystyle \frac{\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}\mathit{H}}_4 \mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{t}\mathit{e}\mathit{d} }{\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}\mathit{H}}_4 \mathit{i}\mathit{n} \mathit{f}\mathit{e}\mathit{e}\mathit{d}}}\times 100$$

$${\mathit{C}\mathit{O}}_2\mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{s}\mathit{i}\mathit{o}\mathit{n} \left(\%\right)={\displaystyle \frac{\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}\mathit{O}}_2 \mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{t}\mathit{e}\mathit{d} }{\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}\mathit{O}}_2 \mathit{i}\mathit{n} \mathit{f}\mathit{e}\mathit{e}\mathit{d}}}\times 100$$

$${\mathit{H}}_{\mathit{2}}\mathit{Y}\mathit{i}\mathit{e}\mathit{l}\mathit{d} \left(\%\right)={\displaystyle \frac{\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{H}}_2 \mathit{p}\mathit{r}\mathit{o}\mathit{d}\mathit{u}\mathit{c}\mathit{e}\mathit{d} }{2\times \mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}\mathit{H}}_4 \mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{t}\mathit{e}\mathit{d}}}\times 100$$

$$\mathit{C}\mathit{O} \mathit{Y}\mathit{i}\mathit{e}\mathit{l}\mathit{d} \left(\%\right)={\displaystyle \frac{\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} \mathit{C}\mathit{O} \mathit{p}\mathit{r}\mathit{o}\mathit{d}\mathit{u}\mathit{c}\mathit{e}\mathit{d} }{\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}\mathit{H}}_4 \mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{t}\mathit{e}\mathit{d}+\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}\mathit{O}}_2 \mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{t}\mathit{e}\mathit{d} }}\times 100$$

$$C_x{\mathit{H}}_y \mathit{Y}\mathit{i}\mathit{e}\mathit{l}\mathit{d} \left(\%\right)={\displaystyle \frac{{\sum }_{x}x\times \mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}}_x{\mathit{H}}_y \mathit{p}\mathit{r}\mathit{o}\mathit{d}\mathit{u}\mathit{c}\mathit{e}\mathit{d} (x=\mathrm{2,3}, \mathit{a}\mathit{n}\mathit{d} 4) }{\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s}\mathit{o}\mathit{f} {\mathit{C}\mathit{H}}_4 \mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{t}\mathit{e}\mathit{d}\mathit{+}\mathit{m}\mathit{o}\mathit{l}\mathit{e}\mathit{s} \mathit{o}\mathit{f} {\mathit{C}\mathit{O}}_2 \mathit{c}\mathit{o}\mathit{n}\mathit{v}\mathit{e}\mathit{r}\mathit{t}\mathit{e}\mathit{d} }}\times 100$$

$$\mathit{Y}\mathit{i}\mathit{e}\mathit{l}\mathit{d} \mathit{o}\mathit{d} \mathit{l}\mathit{i}\mathit{q}\mathit{u}\mathit{i}\mathit{d} \mathit{p}\mathit{r}\mathit{o}\mathit{d}\mathit{u}\mathit{c}\mathit{t}\mathit{s} \left(\%\right)=100-\left({\mathit{Y}}_{\mathit{C}\mathit{O}}+{\mathit{Y}}_{{\mathit{C}}_x{\mathit{H}}_y}\right)-\mathit{C}\mathit{o}\mathit{k}\mathit{e}$$

2.3. Catalyst Characterization Techniques

The crystalline phases of the prepared samples were characterized by X-ray diffraction (XRD) using a Rigaku ULTIMA III diffractometer (Rigaku Corporation, Tokyo, Japan). Copper Kα radiation, generated at 40 kV and 44 mA, served as the X-ray source. Diffractograms were acquired across a 2θ range of 10° to 80°, with a scan rate of 2°/min. The functional bonds and groups in the catalyst samples were identified using Fourier-transform infrared (FT-IR) spectroscopy (Nicolet iS 50, Thermo Fisher Scientific, Waltham, MA, USA). Nitrogen adsorption analysis was conducted using a Micromeritics ASAP 2020 Plus surface area and porosimeter system (Micromeritics Instrument Corporation, Norcross, GA, USA). The sample was degassed at 350 °C for 4 h with a temperature ramp of 10 °C per minute under a vacuum of 10 mmHg. Liquid nitrogen was used to obtain a 28-point adsorption isotherm, with the total surface area calculated via the Brunauer–Emmett–Teller (BET) method and the total pore volume determined at a relative pressure of 0.995. The elemental composition of the active metals, specifically the quantification of iron and nickel content, was determined utilizing a Thermo Scientific ICAP 7200 SERIES inductively coupled plasma-optical emission spectrometer (ICP-OES). Calibration curves were meticulously constructed using certified reference materials: a 10,000 mg/L iron standard solution of Fe(NO₃)₂ in 10% HNO₃ (Sigma-Aldrich) and a 1000 mg/L nickel standard solution of Ni(NO₃)₂ in 2–3% HNO₃ (Sigma-Aldrich). To prepare samples for analysis, a precisely measured aliquot of both fresh and spent catalyst material was subjected to a digestion process involving incubation in aqua regia at a controlled temperature of 80 °C for an extended period of overnight duration. This digestion procedure, coupled with vigorous agitation, ensured the complete dissolution of all metallic constituents within the catalyst matrix, thereby facilitating accurate quantification via ICP-OES. To examine the acidic properties of the catalyst, temperature-programmed desorption of ammonia (NH₃-TPD) was performed using a Finesorb-3010 instrument designed for chemisorption studies (Finetec Instruments Company, Zhejiang, China). Ammonia was chosen as the test molecule because of its uncomplicated structure, small dimensions, and capability to quantify both robust and feeble acidic locations on the catalytic surface. Initially, roughly 200 mg of the newly prepared material underwent a heat treatment in a helium stream at a flow rate of 30 standard cubic centimeters per minute (sccm). This calcination process reached 600 °C over 30 min, with the temperature increasing at a rate of 20 °C/min. Subsequently, the sample’s temperature was reduced to 120 °C to facilitate the adsorption of ammonia. This adsorption was achieved by exposing the sample to a 25 sccm flow of a mixture containing 10% ammonia in helium for a duration of 30 min. To eliminate any ammonia molecules that were merely held by weak physical forces, a helium flow of 25 sccm was maintained for 10 min at 120 °C. The TPD analysis involved heating the sample to 600 °C at a rate of 10 °C/min, followed by a 30 min hold at that temperature, all under a 25 sccm helium flow. A thermal conductivity detector (TCD) was employed to measure the quantity of ammonia released during the heating process.

3. Results and Discussion

3.1. Catalyst Characterization

XRD pattern of the as-prepared Ni-Fe/Al₂O₃ catalyst is illustrated in Figure 2A. The peaks at around 45.0° and 65.6° are assigned to spinel NiAl₂O₄. In addition, the peaks at 30.3° and 43.4° are attributed to spinel NiFe₂O₄ and the peaks at 37.4°, 46.1°, and 66.9° are related to γ-Al₂O₃ [17].

Figure 2B represents the FTIR spectra of Ni-Fe/Al₂O₃ catalyst. The absorption bands at 3406 cm⁻¹ and 1643 cm⁻¹ are related to the stretching vibration of the physically adsorbed water on the sample surface [18]. A strong infrared absorption band appears in the 400 to 1000 cm⁻¹ region, which is attributed to the vibrational absorption of Al–O in the carrier Al₂O₃ [17,18].

N₂ adsorption–desorption analysis was conducted to examine the textural characteristics of the synthesized catalyst. The catalyst displays type IV isotherms accompanied by a H3-type hysteresis loop (Figure 2C), indicating the presence of a mesoporous structure within the composite [19]. As a result, the texture of the prepared catalysts consists of plate-like structures featuring slit-shaped pores. Additionally, the significant rise in adsorption near P/P₀ = 1 suggests the presence of a large-sized mesoporous structure within the catalysts. Furthermore, the pore size distribution for each sample, determined using the BJH method, is displayed in Figure 2D. As demonstrated, all the synthesized catalysts exhibit a narrow pore size distribution primarily ranging from 2 to 30 nm, confirming their mesoporous nature. The BET surface area of the Ni-Fe/Al₂O₃ was 161.5 m²/g with total pore volume and average pore diameter of 0.56 cm³/g and 13.9 nm, respectively. The obtained high surface area of prepared catalyst can effectively improve the adsorption of reactant on the surface of the catalyst. Therefore, there will be more active sites to proceed the reaction.

The nominal and experimentally determined metal loadings for Fe/Al₂O₃, Ni/Al₂O₃, and Ni-Fe/Al₂O₃ catalysts are comprehensively presented in

Table 1. ICP-OES results of the prepared catalysts.

Catalyst	Nominal Loading (wt/wt) %		Experimental Loading (wt/wt) %
	Ni	Fe	Ni	Fe
Ni/Al2O3	10	0	7.1	0
Fe/Al2O3	0	10	0	6.9
Ni-Fe/Al2O3	5	5	3.1	3.0

, providing a direct comparative analysis of intended and realized metal incorporation.

The minimal discrepancy between the nominal and experimental metal loadings, as determined by ICP-OES, underscores the effectiveness of the synthesis method.

To evaluate the acidic characteristics of the newly prepared catalytic materials, temperature-programmed desorption of ammonia (NH₃-TPD) was performed. The quantity of ammonia retained by the acidic locations on each catalyst was quantified, with the results presented in

Table 2. NH₃-TPD surface acidity of the fresh catalysts.

Catalyst	Weak (<250 °C) mmol/g	Medium and Strong (>250 °C) mmol/g	Total Acid Sites (mmol/g)
Al2O3	0.31	0.59	0.90
Ni/Al2O3	0.35	0.13	0.48
Fe/Al2O3	0.32	0.11	0.43
Ni-Fe/Al2O3	0.33	0.09	0.42

. It was observed that the incorporation of metals onto the alumina (Al₂O₃) support led to a substantial decrease, exceeding 50%, in the overall number of acidic sites. This reduction followed a specific sequence: Al₂O₃ exhibited the highest acidity (0.90 mmol/g), followed by Ni/Al₂O₃ (0.48 mmol/g), Fe/Al₂O₃ (0.43 mmol/g), and finally bimetallic Ni-Fe/Al₂O₃ (0.42 mmol/g), which displayed the lowest acidity. Additionally, a significant decline in the density of medium and strong acidic sites was detected, while conversely, the population of weak acidic sites increased as metal loading progressed.

3.2. Plasma-Catalytic Conversion of Biogas

As indicated in Figure 3, no conversion of CH₄ or CO₂ takes place in the “catalyst only” run without any plasma discharge in the system, which uses the 5Ni-5Fe/Al₂O₃ catalyst. This lack of reaction can be attributed to the relatively low reaction temperature (<100 °C). Nevertheless, upon applying NTP, the conversion of both CH₄ and CO₂ enhances substantially, resulting in the production of gas, liquid, and coke by-products, according to

Table 3. Products analysis in plasma-catalytic experiments for biogas conversion.

Run	H2 Yield (%)	CO Yield (%)	CxHy Yield (%)	Liquid Yield (%)	Coke (%)
Catalyst only	0	0	0	0	0
Plasma only	48.9	29.2	26.7	0	44.1
10Fe/Al2O3	44.2	28.4	25.9	30.9	14.8
10Ni/Al2O3	43.3	27.8	25.1	34.0	13.1
2.5Ni-7.5Fe/Al2O3	42.4	27.5	24.6	35.6	12.3
7.5Ni-2.5Fe/Al2O3	41.7	27.3	24.3	36.7	11.7
5Ni-5Fe/Al2O3	40.1	27.1	23.9	38.6	10.4

. The major gas products, hydrogen (H₂) and carbon monoxide (CO), are detected by the micro-GC, while the incondensable hydrocarbons (C_xH_y) are mainly composed of C₂H₆, along with trace amounts of C₂H₄, C₃H₈, and C₄H₁₀. The liquid products collected were analyzed using GC-MS, and the results identified them as C₁-C₃ oxygenates, including methanol, ethanol, formaldehyde, propanoic acid, acetic acid, and isopropanol, as shown in Figure S1.

In the “plasma only” run without using any catalyst powder in the reactor, the conversion rates for CH₄ and CO₂ are 30.3% and 15.4%, respectively. Due to the lower dissociation energy of CH₄ (4.57 eV) in comparison with CO₂ (5.52 eV) [14], the conversion of methane is higher than that of carbon dioxide. However, a significant drawback of this purely plasma-driven process was the complete absence of any condensable liquid products, coupled with a substantial formation of solid carbonaceous deposits, or coke, which reached a yield of 44.1%. This high coke formation posed a significant limitation to the practical application of the plasma-only process. In stark contrast, the introduction of metal-supported catalysts, namely 10Fe/Al₂O₃, 10Ni/Al₂O₃, 2.5Ni-7.5Fe/Al₂O_3, 7.5Ni-2.55Fe/Al₂O₃ and 5Ni-5Fe/Al₂O₃, into the plasma discharge region profoundly altered the reaction pathway. These catalytic interventions not only enabled the formation of liquid products but also dramatically mitigated the undesirable deposition of coke. Notably, the CH₄ conversion rates exhibited a consistent increase across the catalytic series, reaching 31.6% for 10Fe/Al₂O₃, 33.1% for 10Ni/Al₂O₃, and culminating at 34.8% for the bimetallic 5Ni-5Fe/Al₂O₃ catalyst. Similarly, CO₂ conversion rates demonstrated a corresponding upward trend, registering 15.9%, 17.5%, and 19.7% for the respective catalysts. Moreover, the yields of liquid products also showed a progressive enhancement, attaining values of 30.9%, 34.0%, and 38.6% for 10Fe/Al₂O₃, Ni/Al₂O₃, and Ni-Fe/Al₂O₃, respectively. The observed superiority of the bimetallic 5Ni-5Fe/Al₂O₃ catalyst, characterized by the highest liquid product yield and the lowest coke formation, can be rationalized by considering its surface acidity. High surface acidity promotes coke formation by facilitating undesired side reactions, such as methane cracking and polymerization of hydrocarbons. In contrast, catalysts with lower acidity, especially with a higher proportion of weak acid sites, tend to exhibit better coke resistance. The NH₃-TPD analyses revealed that the 5Ni-5Fe/Al₂O₃ catalyst exhibited a lower density of acidic sites compared to the monometallic counterparts. This reduction in surface acidity likely contributed to the suppression of coke formation, which is often associated with acid-catalyzed reactions. These results strongly suggest the considerable potential of plasma-mediated catalytic processes for the efficient conversion of biogas, a mixture of CH₄ and CO₂, into valuable liquid products, thereby offering a sustainable route for resource utilization.

To further elucidate the composition of the liquid products obtained, the selectivity of various oxygenated compounds generated using the 5Ni-5Fe/Al₂O₃ catalyst under plasma discharge was meticulously analyzed and presented in Table 3. A detailed examination of the data, as presented in

Table 4. Selectivity of different oxygenate chemicals in liquid products.

Chemical	Formaldehyde	Isopropyl Alcohol	Ethanol	Methanol	Propanoic Acid	Acetic Acid
Selectivity (%)	10.5	15.7	14.5	19.1	16.3	23.9

, revealed that acetic acid exhibited the highest selectivity, registering 23.9%, while formaldehyde displayed the lowest, at 10.5%. Additionally, other oxygenates, including methanol, propanoic acid, ethanol, and isopropyl alcohol, were detected with selectivity of 19.1%, 16.3%, 14.5%, and 15.7%, respectively. These findings provide valuable insights into the product distribution and selectivity of the plasma-catalytic process, highlighting its potential for the production of a diverse range of oxygenated chemicals.

When biogas is converted into liquid chemicals in a plasma environment, it likely happens through a radical chain reaction. In this process, radicals like CO, CH₃, and OH are key players in converting CH₄ and CO₂ [10,20]. In a plasma setting, CO₂ becomes excited through electron collisions, breaking it down into CO and O radicals. Similarly, methane breaks apart to form H and CH_x radicals (where x can be 1 to 3), with CH₃ being the most common. These CH_x radicals can come together to form C₂-C₄ hydrocarbons, as seen in the experiments [21]. High-energy electrons can also break down the CH_x radicals further, causing carbon to accumulate on the catalyst surface. OH radicals form when H and O radicals combine, and H radicals from methane can interact with excited CO₂ to create more OH and CO radicals [10]. When CH₃ and OH radicals react, they form methanol, while C₂H₅ radicals can combine with OH radicals to make ethanol. The reaction between O/OH radicals and CH_x may lead to formaldehyde (HCHO). CO radicals also react with CH₃ to form acetyl radicals, which then react with CH₃ to produce acetone. Using a catalyst in the DBD plasma system changes the plasma discharge, shifting it from gas microdischarges to surface discharges [22]. This shift can help prevent the breakdown of CH_x into carbon byproducts. Plus, the catalyst’s porous structure provides a larger surface area, which helps increase the production of liquid products [22].

Among the various liquid products formed during plasma-assisted biogas conversion, acetic acid consistently exhibited the highest selectivity. This observation aligns with previous studies that have explored direct oxygenate synthesis from CH₄ and CO₂ under non-thermal plasma (NTP) conditions. In NTP environments, highly reactive species such as CH₃, CO, and OH radicals are generated in abundance. The coupling of methyl (–CH₃) and carboxyl (–COOH) radicals—formed through the reaction of CO with OH—has been identified as a key pathway for the formation of acetic acid (CH₃COOH). This mechanism is energetically favorable and has been supported by both experimental results and density functional theory (DFT) calculations [16]. Additionally, the role of the catalyst in stabilizing intermediate species through appropriate oxygen adsorption energies further promotes the selective formation of acetic acid over other oxygenates. As demonstrated in earlier studies, such as those by Zou et al. [23] and Wang et al. [10], acetic acid has emerged as a dominant product in similar plasma-catalytic systems.

Figure 4A illustrates the impact of altering the carbon dioxide to methane (CO₂/CH₄) molar ratio on the catalytic conversion of these gases and the resulting product composition. By systematically adjusting the reactant proportions from a CO₂/CH₄ ratio of 2:1 to 1:2, a steady decrease in methane conversion was observed, falling from 36.8% to 19.7%. In contrast, carbon dioxide conversion followed a curvilinear pattern, reaching a maximum of 19.7% at a CO₂/CH₄ ratio of 1:0.5. The selectivity for total liquid products displayed a similar trend to carbon dioxide conversion, indicating a direct correlation with the reactant feed ratio. Notably, a peak liquid product selectivity of 38.6% was achieved at a stoichiometric CO₂/CH₄ ratio of 1:0.5. Moreover, an excess of carbon dioxide (CO₂/CH₄ ratio of 2:1) led to a preference for gaseous product formation, resulting in a lower liquid selectivity of 13.7%. This implies that an abundance of methane molecules enhances carbon–carbon bond formation within the plasma environment. Consequently, subsequent experiments were performed using a CO₂/CH₄ molar ratio of 1:0.5, based on these experimental findings.

The effect of input power on CH₄ and CO₂ conversion was investigated by varying the discharge power from 13 W to 23 W. As shown in Figure 4B, increasing the input power led to a noticeable improvement in conversion efficiency for both gases. At 13 W, CH₄ and CO₂ conversions were 30.4% and 15.1%, respectively. When the power was increased to 18 W, CH₄ conversion rose to 32.1%, while CO₂ conversion reached 17.3%. At the highest power level of 23 W, CH₄ and CO₂ conversions further increased to 34.8% and 19.7%, respectively. This trend can be attributed to the enhanced plasma discharge intensity at higher input powers, which generates a greater number of energetic electrons and reactive species such as radicals, ions, and excited molecules. These species are responsible for breaking the strong C–H and C=O bonds in methane and carbon dioxide, thereby promoting the desired chemical transformations. Thus, increasing the input power effectively enhances the activation of reactants and improves overall conversion performance in the plasma-catalytic system.

Figure 4C presents the stability test of the Ni-Fe/Al₂O₃ catalyst during plasma-assisted biogas conversion over a 5 h period. A gradual decline in both CH₄ and CO₂ conversions is observed during the first three hours of operation, followed by a significant drop in performance by the end of the 5 h run. This deactivation is likely caused by the accumulation of carbon deposits on the catalyst surface, particularly on the active metal sites (Ni and Fe) and the weak acidic sites of the support. These carbon deposits can block access to the active sites and reduce their availability for reaction, ultimately diminishing the catalyst’s effectiveness. The loss of accessible Ni, Fe, and acidic sites due to carbon buildup is a key factor contributing to the observed decline in conversion performance over time. These findings highlight the importance of addressing carbon deposition in future catalyst design to enhance long-term stability.

3.3. Comparison with Previous Plasma Catalytic Studies and Other Conversion Technologies

As shown in

Table 5. Comparative study for different catalysts in plasma-assisted biogas conversion.

Catalyst	Parameters	CH4 Conv. (%)	CO2 Conv. (%)	Remarks	Ref.
SiO2	6 g, 9.25 kV P-P	17.0	10.9	The plasma-assisted catalytic biogas conversion was studied for value-added liquid chemicals; 0% liquid yield was achieved.	[14]
Ag/CZSM5	6 g, 9.25 kV P-P	18.7	13.6	The plasma-assisted catalytic biogas conversion was studied for value-added liquid chemicals; 33.1% liquid yield was achieved.	[14]
Ir/CZSM5	6 g, 9.25 kV P-P	21.2	12.8	The plasma-assisted catalytic biogas conversion was studied for value-added liquid chemicals; 25.0% liquid yield was achieved.	[14]
Ni/Al2O3	0.25 g, 50 W	18	13	The plasma-assisted catalytic DRM was investigated for the discharge behavior characteristics. The Ni-dispersed catalyst contributes to the expansion of discharges and enhancement of DRM activity with enhanced product distribution.	[24]
BaTiO3	0.2 g, 86 W	28	17	While BaTiO3 led to a higher hydrogen (H2) yield, Ni/SiO2 and Ni-Fe/SiO2 catalysts demonstrated greater selectivity for carbon monoxide (CO), indicating the enhancement of the reverse water-gas shift (RWGS) reaction. The catalyst was packed within the 100 cm3 discharge volume of the reactor used in this investigation.	[25]
Ni/SiO2	0.2 g, 86 W	26	16
Ni-Fe/SiO2	0.2 g, 86 W	22	14
5Ni-5Fe/Al2O3	0.5 g, 23 W	34.8	19.7	The plasma-assisted catalytic biogas conversion was studied for value-added liquid chemicals; 38.6% liquid yield was achieved.	This work

, the CH₄ and CO₂ conversions achieved in this study using the 5Ni-5Fe/Al₂O₃ catalyst (34.8% and 19.7%, respectively) are significantly higher than those reported in previous plasma-assisted biogas conversion studies. For instance, inert SiO₂ support showed only 17.0% CH₄ and 10.9% CO₂ conversion, highlighting the limited activity of non-catalytic or passive supports in such systems. Catalysts based on noble metals like Ag/CZSM5 and Ir/CZSM5 demonstrated slightly improved conversion rates, with Ag/CZSM5 reaching 18.7% CH₄ and 13.6% CO₂ conversion, and Ir/CZSM5 achieving 21.2% and 12.8%, respectively. However, these values still fall short of the performance obtained in this work. In addition to higher conversions, the 5Ni-5Fe/Al₂O₃ catalyst also demonstrated superior liquid product yield. While the SiO₂ support resulted in 0% liquid yield and Ag/CZSM5 and Ir/CZSM5 yielded 33.1% and 25.0% liquid products, respectively, the catalyst developed in this study achieved a notably higher yield of 38.6%. This improvement can be attributed to the synergistic interaction between Ni and Fe species on the alumina support under plasma conditions, which effectively promotes both conversion and selectivity toward valuable liquid products. These findings underline the enhanced catalytic performance of the 5Ni-5Fe/Al₂O₃ system, both in terms of reactant utilization and product formation, compared to previously reported plasma-catalytic configurations.

Conventional dry reforming of methane (DRM) via thermal catalysis requires high operating temperatures (typically >700 °C) due to the endothermic nature of the reaction and the stability of CH₄ and CO₂. Although effective at high temperatures, these systems often suffer from catalyst deactivation caused by sintering and coke deposition. While noble metals exhibit better resistance, their high cost limits large-scale applications, leading to greater interest in transition metal catalysts like Ni, which are more economically viable but more susceptible to deactivation. Photocatalytic DRM, on the other hand, offers the advantage of operating under milder conditions by utilizing solar energy. However, low light absorption, poor charge separation, and low conversion efficiencies hinder its practical application, and significant advances in photocatalyst design are still required [16]. In contrast, non-thermal plasma (NTP)-assisted DRM enables chemical reactions at near-ambient conditions by producing energetic electrons that can activate stable molecules like CH₄ and CO₂ without the need for bulk gas heating. This not only reduces energy consumption but also allows for rapid startup, operational flexibility, and easier reactor design. When combined with a catalyst, NTP creates a synergistic environment where plasma-generated species enhance reaction rates, while the catalyst promotes selectivity and stability. This hybrid system offers improved performance over stand-alone thermal or photocatalytic processes and presents a promising pathway for efficient, low-temperature biogas conversion. Nonetheless, further work is needed to optimize catalyst design and scale-up strategies to fully harness the potential of NTP technologies.

3.4. In Situ OES in Plasma-Catalytic Biogas Conversion

To pinpoint crucial chemical entities present during the plasma-mediated catalytic co-conversion of carbon dioxide and methane, in situ optical emission spectroscopy (OES) was utilized, as shown in Figure 5. The plasma-driven biogas transformation, when conducted without the introduction of a catalyst, displayed the strongest emission intensity, characterized by numerous discrete spectral lines and two distinct spectral bands. These include the H_α line (656.3 nm, 3d²D → 2p²P⁰), two atomic oxygen spectral lines (777.5 nm, 3s⁵S⁰ → 3p⁵P; 844.7 nm, 3s³S⁰ → 3p³P), a hydrogen molecular band (580–650 nm, d³Π_u → a³∑_g⁺), and a CO molecular band (450–580 nm, B¹∑ → A¹Π) [26,27,28,29]. This observation suggests a significant production of H atoms and CO molecules within the plasma region. A noticeable reduction in emission intensity was observed upon the introduction of the catalyst, a phenomenon likely stemming from both the physical obstruction of light by the catalyst bed and the consumption of reactive plasma species by the catalyst’s active sites [30].

3.5. Sustainability and Scalability Study

The implementation of cold plasma technology in value-added liquid products proffers a constellation of boons, primarily predicated upon its capacity to modulate reaction pathways and engender process intensification. Departing from the traditional reliance on syngas derived from fossil fuels, with their attendant environmental encumbrances, non-thermal plasma (NTP) technology facilitates the direct transformation of CO₂ and CH₄ into valuable liquid products. This direct methodology not only streamlines separation protocols but also holds the potential to substantially augment overall process efficiency. Furthermore, NTP enables reactions that are thermodynamically prohibitive under conventional thermal regimes. The inherent electric field within the plasma environment engenders the formation of a plethora of reactive species, thereby surmounting thermodynamic barriers. This attribute is particularly salient in the direct synthesis of acetic acid and valuable liquid products from biogas, where thermodynamic constraints impede yields under traditional conditions. Moreover, NTP is capable of potentiating catalytic activity. Plasma elicits photocatalytic activity by diminishing metal oxide cluster size, amplifying the specific active surface area, and prolonging electron-hole pair lifetimes. The synergy between plasma and catalyst can precipitate enhanced conversion rates, energy efficiency, and product ratios. In summation, the adoption of cold plasma in liquid fuel and syngas production presents a propitious avenue towards a more sustainable and potentially efficacious process, achieved through the enablement of direct biogas conversion, circumvention of thermodynamic limitations, and potentiation of catalytic activity.

4. Conclusions

In this study, we successfully demonstrated the conversion of biogas into valuable liquid products using a non-thermal plasma-catalytic process with a 5Ni-5Fe/Al₂O₃ catalyst. The addition of the Ni-Fe/Al₂O₃ catalyst not only improved the conversion rates of methane and carbon dioxide, achieving 34.8% and 19.7%, respectively, but also led to the highest observed liquid yield of 38.6% among the catalysts tested. Additionally, this catalyst substantially reduced coke formation to 10.4%, markedly enhancing the overall efficiency of the biogas conversion process. These results highlight the catalyst’s pivotal role in optimizing the production of key gas products like hydrogen and carbon monoxide, as well as valuable liquid oxygenates including methanol, ethanol, and acetic acid. This study not only underscores the effectiveness of the Ni-Fe/Al₂O₃ catalyst in facilitating higher conversion rates and selectivity towards liquid products but also emphasizes the potential of this plasma-catalytic method as an effective strategy for sustainable chemical production from renewable sources like biogas. By improving the efficiency and yield of biogas conversion, this approach contributes to a low-carbon economy and helps in reducing greenhouse gas emissions, providing a viable and eco-friendly solution to the energy and chemical industries.