Catalyst Development for Biogas Dry Reforming: A Review of Recent Progress

Abstract

Amidst the rapid expansion of the global economy, the demand for energy has escalated. The depletion of traditional energy sources coupled with environmental pollution concerns has catalyzed a shift towards the development and utilization of clean, renewable energy. Biogas, as a renewable energy source, provides diverse applications and holds the potential to alleviate energy shortages. Recently, biogas dry reforming technology has garnered substantial attention as a significant pathway for renewable energy utilization, particularly in the development and optimization of catalysts. Contemporary research predominantly focuses on enhancing the activity and stability of catalysts, with particular emphasis on their resistance to coking and sintering. This review delineates the classification of biogas dry reforming catalysts, their catalytic activity, and issues related to carbon deposition, contrasting biogas dry reforming with traditional dry reforming in catalyst design. It synthesizes numerous studies from recent years aimed at mitigating carbon deposition during the biogas dry reforming process and boosting catalytic activity via active components, carriers, and promoters in both precious and non-precious metal catalysts. Furthermore, it discusses the current challenges of biogas dry reforming technology and outlines prospective future development trends. This discussion provides an in-depth understanding of biogas dry reforming technology and catalyst design, offering insights and recommendations for future research and industrial applications.

1. Introduction

Biogas, a combustible gas, is produced from organic wastes (including crop residues, animal manure, discarded fruits and vegetables, kitchen waste, and agricultural by-products) via microbial fermentation under anaerobic conditions, requiring specific temperatures and pH levels [1,2,3,4]. Despite biogas’s origin from biodegradable materials classifying it as a renewable energy source, its potential is frequently underestimated [5]. Even with the advancement of the shale gas revolution, traditional shale gas may have cost advantages in certain regions. However, utilizing biogas can create additional economic value for the agriculture and waste management industries, promoting their sustainable development. For example, in 2019, the biogas industry created approximately 335,000 temporary construction jobs and 23,000 full-time operational positions. In China, the biogas industry employs 209,000 workers [6]. And recent years have seen biogas technology’s rising potential in addressing global energy shortages, evidenced by a surge in global production from 9519 MW in 2010 to 20,108 MW in 2020, more than doubling the installed capacity [7]. Therefore, from the perspective of sustainable development and environmental impact, biogas possesses distinct advantages over traditional shale gas for utilization.

Biogas primarily finds application in electricity generation, heating, fueling, transportation, chemical production, and biogas upgrading [8,9,10]. However, biogas’s direct combustion may contribute to environmental pollution by emitting NO_x and particulates [11]. Enhancing biogas’s value involves converting its methane (CH₄) content into chemical raw materials (like methanol, ethanol, and formaldehyde) or into synthesis gas (H₂ and CO) via dry reforming [12,13]. This technology not only mitigates greenhouse gas emissions but also yields significant economic benefits, underscoring its substantial application potential [14,15]. Unlike classical dry reforming, which relies heavily on natural gas as a feedstock, biogas dry reforming leverages renewable resources, making it more environmentally friendly. Additionally, the integration of biogas, which is often produced from organic waste, can help mitigate greenhouse gas emissions while providing a consistent and localized energy source. As the shale gas revolution has indeed made classical dry reforming more competitive, biogas dry reforming offers a unique advantage by transforming waste into valuable energy, aligning with global efforts to promote circular economy practices and reduce reliance on fossil fuels.

Current review literature primarily examines the synthesis, characterization, and reactivity of catalysts for biogas dry reforming, notably nickel-based catalysts, while often neglecting a detailed categorization of catalysts and the carbon deposition issues encountered during the reaction [16,17]. This review systematically categorizes catalysts for biogas dry reforming into noble and non-noble metal catalysts, delves into the catalytic activity of different catalyst types, and emphasizes addressing carbon deposition issues. It concludes by outlining the current challenges biogas dry reforming technology faces and discussing potential future development trends.

2. Basis of Biogas Dry Reforming Technology

Biogas consists mainly of CH₄ and CO₂, with minor traces of impurities including H₂S, H₂, N₂, NH₃, H₂O, and O₂ [18]. Dry reforming of biogas primarily targets CH₄ and CO₂, with other impurities effectively removed by current technologies [19]. Extensive research has focused on enhancing the conversion rates of reactants and product selectivity. The high bond dissociation energies of CH₄ and CO₂ (435 and 526 kJ mol⁻¹, respectively) make the dry reforming process energy-intensive [17]. Thermodynamic analyses show that higher reaction temperatures promote biogas dry reforming’s forward conversion, activating CO₂ and CH₄ at temperatures above 700 °C (Equation (1)) [20]. Several side reactions accompany this process, including the reverse water–gas shift reaction that converts CO₂ and H₂ into CO and H₂O (Equation (2)) [21]. Furthermore, below 800 °C, CH₄ decomposition (Equation (3)) and CO disproportionation (Equation (4)) can lead to carbon formation and catalyst deactivation [22,23].

$${\mathrm{CH}}_4+{\mathrm{CO}}_2\rightleftarrows 2\mathrm{CO}+2{\mathrm{H}}_2 \to {\mathsf{\Delta }\mathrm{H}}_{298\mathrm{K}}=+247{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(1)

$${\mathrm{CO}}_2+{\mathrm{H}}_2\rightleftarrows \mathrm{CO}+{\mathrm{H}}_2\mathrm{O}\to {\mathsf{\Delta }\mathrm{H}}_{298\mathrm{K}}=+41{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(2)

$${\mathrm{CH}}_4\rightleftarrows \mathrm{C}\left(\mathrm{s}\right)+2{\mathrm{H}}_2\to {\mathsf{\Delta }\mathrm{H}}_{298\mathrm{K}}=+75{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(3)

$$2\mathrm{CO}\rightleftarrows \mathrm{C}\left(\mathrm{s}\right)+{\mathrm{CO}}_2 \to {\mathsf{\Delta }\mathrm{H}}_{298\mathrm{K}}=-171{\mathrm{kJ}\mathrm{mol}}^{-1}$$

(4)

In dry reforming of biogas, CH₄ and CO₂ initially dissociate into reactive intermediates, CH₄ on the active metal and CO₂ at the metal–support interface [24]. CH₄ activation results in C-H bond breakage, yielding active methyl (•CH₃) and hydrogen (H) atoms; CO₂ activation splits the C=O double bond to generate oxygen (O) and carbon monoxide (CO) [25,26]. These intermediates, arising from carbon and hydrogen atom separation, form on the catalyst surface and are crucial for transforming the reaction into the final products, CO and H₂, through complex steps [27,28].

Microbial anaerobic digestion, driven mainly by methanogens, converts organic matter’s carbon and hydrogen into CH₄, yielding a biogas CH₄-to-CO₂ ratio of about 1.5 [29,30]. Excessive CH₄ may worsen side reactions like methane decomposition, causing carbon deposition on the catalyst, obstructing active sites, and altering surface structure and morphology [31,32]. This reduction in specific surface area and reactant adsorption and activation capacity slows the reaction rate, leading to catalyst deactivation [33]. Traditional dry reforming typically employs pure CH₄ and CO₂ as feedstock in a 1:1 ratio, which lessens the impact of carbon deposition [34]. Consequently, this necessitates the use of different catalysts. Biogas dry reforming catalysts emphasize addressing carbon deposition, while traditional dry reforming catalysts, benefiting from feedstock purity and an adjustable CH₄-to-CO₂ ratio, prioritize enhancing catalytic activity and selectivity.

Dry reforming of biogas catalysts comprises active components, supports, and promoters. Active components, anchored on stable supports, enhance surface area and stability [35]. Promoters optimize catalyst performance, enhancing reaction stability and efficiency [35,36]. Catalysts are classified into noble and non-noble metal types based on their active components [37]. Noble metal catalysts, with superior activity and selectivity, face limited application due to high costs [38]. Non-noble metal catalysts show significant activity when well dispersed but suffer from carbon deposition and thermal instability, causing deactivation and sintering [39]. To reduce carbon deposition and enhance activity, specific catalyst supports and promoters are selected. Researchers have explored metal oxides like Al₂O₃, SiO₂, La₂O₃, CeO₂, and ZrO₂ as supports [40,41] and used transition (e.g., Fe, Co, Sn) [37,42], noble (e.g., Ag, Pt, Pd) [43], rare earth (e.g., La, Ce, Pr) [44,45], and alkaline earth metals (e.g., Sr, Ca, Ba) [46] as promoters, creating a carrier-mediated system to enhance the reaction [47,48].

3. Advances in Catalyst Research

3.1. Noble Metal Catalysts

Noble metal catalysts in biogas dry reforming show high-temperature stability and resistance to carbon deposition, extending their lifespan [49]. Researchers improve noble metal utilization and efficiency by designing catalyst structures and compositions, including single-atom and supported variants [50]. Selecting appropriate support materials and optimizing their morphology and surface attributes enhances the catalysts’ thermal stability and anti-sintering features [51,52]. The influence of reactant gas ratios on biogas dry reforming efficiency in different catalyst systems has been studied to boost syngas production and quality.

A.L.A. Marinho and his colleagues [53] compared the performance of Pt-encapsulated CeO₂ catalysts, synthesized in a single step via the sol–gel method, against Pt/CeO₂ catalysts prepared by the traditional impregnation method in DRM reactions. Figure 1a,b show the changes in the number of exchanged oxygen atoms during TPOIE and IOIE processes at 400 °C for Pt@CeO₂ and Pt@CeZrO₂ catalysts. The results showed that embedding Pt nanoparticles in Pt@CeO₂ and Pt@CeZrO₂ catalysts balanced carbon’s formation and gasification rates due to the high active oxygen content at the metal–support interface, enhancing carbon deposition resistance. Compared to supported Pt, embedded Pt nanoparticles interact more strongly with Ce, increasing active lattice oxygen on the catalyst surface and facilitating carbon gasification. Stability tests simulated biogas reactions at 800 °C for 24 h under CH₄/CO₂ molar ratios of 1.0 and 1.5. At a CH₄/CO₂ molar ratio of 1.5, all catalysts showed significant stability. Faris Jasim Al-Doghachi et al. [54] enhanced MgO-based catalysts by doping them with varying proportions of NiO to examine the resultant effects on catalyst performance. As demonstrated in Figure 1c through FT-IR analysis, the altered Pt/Mg_1−xNi_xO catalysts revealed the presence of Pt-O, Ni-O, and MgO bonds. The Pt-O and Ni-O bonds likely serve as catalytic active sites, essential for the methane dry reforming reaction’s CH₄ and CO₂ conversion. These bonds are posited to facilitate critical reaction steps, such as methane dissociation and CO₂ activation, thus accelerating the reaction rate. All catalysts achieved CO₂ and CH₄ conversion rates over 95%, with the following efficiency rankings: Pt/Mg_0.85Ni_0.15O > Pt/Mg_0.93Ni_0.07O > Pt/Mg_0.97Ni_0.03O > Pt/MgO. The pore volume to specific surface area ratio (SBET) aligned with the catalytic activity order, indicating that a higher ratio improves performance. Additionally, at a CH₄/CO₂ molar ratio of 2, the catalysts showed enhanced resistance to deactivation from carbon formation and superior H₂ and CO selectivity.

Ainara Morala et al. [49] investigated Rh-based catalysts utilizing γ-Al₂O₃, SiO₂, and CeO₂ as supports, comparing their activities across various gas hourly space velocities (GHSV) and highlighting CH₄ conversion rates. The findings revealed that both laboratory-prepared and commercial Rh/Al₂O₃ catalysts surpassed others in performance, with a catalytic activity ranking of Rh/Al₂O₃ > Rh/SiO₂ > Rh/CeO₂. Specifically, the Rh/Al₂O₃ catalyst showed the highest metal dispersion (approximately 36%), offering superior sintering resistance.

At a GHSV of 30 N L CH₄/(g_cat·h) and 700 °C, dry reforming of biogas with a commercial Rh catalyst reaches equilibrium after 2 h. With higher GHSV values (150 and 300 N L CH₄/(g_cat·h)), catalytic activity declines over time, particularly at the highest flow rates. Even at elevated flow rates and temperatures, coke production stays minimal, underscoring the substantial benefits of noble metal catalysts like Rh in this process.

Recent research has significantly enhanced the carbon gasification process by increasing the active lattice oxygen on catalyst surfaces, which is crucial for boosting long-term stability and catalytic efficiency. Additionally, the optimization of the catalyst’s pore volume and surface area ratio, alongside improvements in metal dispersibility and sintering resistance through the use of diverse supports or element doping, has been achieved. However, the prohibitive cost of precious metals constrains the large-scale industrial deployment of these catalysts. Future studies should focus on ways to sustain catalytic performance while minimizing costs, such as by identifying alternatives to precious metals or refining manufacturing processes. Moreover, the stability and long-term performance of these catalysts in actual applications still require further validation and enhancement. Through these initiatives, we aim to develop commercially viable advanced catalytic technologies that offer practical solutions to global energy challenges.

3.2. Non-Noble Metal Catalysts

Noble metal catalysts are known for their excellent resistance to carbon deposition, activity, and stability, yet their development is constrained by limited resources. Non-noble metal catalysts, like Ni, Co, and Fe, offer economic benefits and are more apt for large-scale industrial use compared to noble metals [45,55]. However, nickel-based catalysts often rapidly deactivate due to carbon buildup and active phase sintering [56]. Cobalt-based catalysts, in comparison to nickel-based ones, exhibit lower CH₄ conversion activity and are costlier, diminishing their economic benefit [57]. Iron-based catalysts are the least expensive, yet they may not match the activity and stability of nickel- or cobalt-based alternatives [38]. Addressing non-noble metal catalysts’ issues involves using diverse preparation methods and modification techniques to optimize performance under various conditions or refining the catalyst composition by incorporating promoters or alloying to boost activity and stability [58,59].

Table 1. Reaction conversion rate and product selectivity on non-noble metal catalysts in biogas dry reforming.

Entry	Catalysts	Reaction Conditions	CH4 Conversion (%)	CO2 Conversion (%)	Product Selectivity (H2/CO Ratio)	References
1	Ni/MgO	CH4/CO2/N2 = 2:2:1, 750 °C, 200 mg	96.4	91.7	≈1.06	[60]
2	Co(NO3)2 + Al(NO3)3	CH4/CO2 = 1:1, 900 °C	100	86.2	≈1.16	[61]
3	NiLaTi-I	CH4/CO2 = 1.5:1, 800 °C, 200 mg	79	≈79	>1	[62]
4	Ni0.11/Ce0.20 (Al2O3-TiO2)	CH4/CO2 = 1.5:1, 850 °C, 150 mg	84.9	95.6	1.06	[55]
5	LaNiO3	CH4/CO2 = 1.5:1, 850 °C, 0.5 g	97.2	93.8	1.6	[34]
6	0.2Ca-10Co/Al2O3	CH4/CO2 = 1:1, 973 K, 100 mg	84	89	0.74	[63]
7	Gd.Ni/Mg1.3AlOx	CH4/CO2 = 2:1, 750 °C, 50 mg	49	95	>0.9	[64]
8	Ni0.10/(Zn0.1-Ce0.9)	CH4/CO2 = 1.5:1, 900 °C, 150 mg	83.1	97	1.04	[65]
9	Ni/Al2O3	CH4/CO2/N2 = 2:2:1, 750 °C, 200 mg	82.7	/	>0.9	[66]
10	NiO/ZrO2/MgO	CH4/CO2 = 1.5:1, 900 °C, 150 mg	97	100	1.7	[67]
11	Ni-SiO2@SiO2	CH4/CO2 = 1.5:1, 700 °C, 100 mg	72.7	93	0.95	[68]
12	Fe5%Ni5%Al2O3	CH4/CO2 = 1.8:1, 700 °C, 100 mg	22	49	≈1.16	[69]
13	Fe/ZrO2	CH4/CO2 = 1.5:1, 900 °C, 3.5 g	92	89	2.25	[70]
14	NiMo/MgO	CH4/CO2 = 1.5:1, 900 °C, 1 g	95	100	3.1	[71]

summarizes some typical non-noble metal catalysts for biogas dry reforming.

3.2.1. Nickel-Based Catalysts

Nickel-based catalysts activate CH₄ and CO₂ efficiently at lower temperatures, accelerating product formation [72]. As an active component, Ni lowers the energy needed for C-H bond breaking, thus easing CH₄ decomposition [73]. Its surface sites enhance CO₂ adsorption and activation, simplifying reactions with C and H. However, nickel particle sintering and surface carbon formation during reactions reduce active sites, diminishing catalytic activity. Recently, researchers have improved the catalyst’s performance by reducing its size, adjusting its structure, and modifying the carrier [74,75].

Developing nickel-based catalysts with strong metal–support interaction to enhance CO₂ adsorption and conversion is crucial for mitigating metal sintering and minimizing carbon deposition [76]. Pali Rosha et al. [55] synthesized CeO₂-promoted Ni catalysts on TiO₂ and Al₂O₃ supports using the wet impregnation method to investigate biogas dry reforming. Figure 2a shows that H₂-TPR results, after impregnating 11wt% Ni on TiO₂ support, reveal a shift in the NiO reduction peak to a higher temperature, indicating a strong metal–support interaction. Enhanced binding force between the metal and support hinders active metal migration, thereby slowing metal sintering. Additionally, CeO₂’s inclusion boosts CH₄ and CO₂ conversion rates, thanks to its high oxygen storage capacity that facilitates timely C and O combination during reactions, thereby somewhat preventing carbon formation. At 850 °C and a CH₄/CO₂ ratio of 1.5, the Ni_0.11/Ce_0.20 (Al₂O₃-TiO₂) catalyst achieved the highest CH₄ and CO₂ conversion rates (84.9% and 95.5%, respectively), operating stably for 7 h with a carbon deposition rate of 8.8 wt%. Danhua Mei et al. [77] employed Dielectric Barrier Discharge (DBD) technology for plasma catalytic dry reforming of biogas using nickel-based bimetallic catalysts. Figure 2b illustrates that, compared to 10Ni, 10Ni3Co has the highest CO₂ desorption peak temperature. Furthermore, the base site quantity ranks as follows: 10Ni3Co > 10Ni3Mn > 10Ni > 10Ni3Cu. Catalysts with higher basicity more effectively adsorb and dissociate CO₂. Absorption at these sites yields more surface oxygen, facilitating carbon gasification and thus increasing resistance to carbon deposition. After a 150-min reaction at 50 W, 10Ni3Co showed minimal carbon deposition (2.9%). At 60 W, it achieved the highest CH₄ (49.1%) and CO₂ (29.8%) conversion rates.

Rare-earth-metal-promoted nickel-based catalysts enhance stability by improving CO₂ adsorption and activation capabilities. Quan Luu Manh Ha and colleagues examined low-nickel-load (2.5 wt%) catalysts modified by Gd, Sc, or La, noting that Gd and Sc modifications maintained CH₄ and CO₂ conversion rates during a 100 h stability test, as depicted in Figure 2c. This stability results from the strong metal–support interaction that stabilizes micro-nickel particles and boosts CO₂ adsorption and activation, thus increasing resistance to carbon accumulation [64].

Optimizing support materials enhances metal–support interactions and supplies active oxygen to the catalyst surface. Pali Rosha et al. [65] explored the impact of 10 wt% nickel catalysts on CeO₂ and ZnO supports in methane dry reforming. For the Ni_0.10/CeO₂ catalyst, adding 10 wt% Zn improved metal–support interaction, Ni particle dispersion at high temperatures, and reduced carbon deposition. CeO₂, offering numerous oxygen vacancies and releasing oxygen, curbs carbon buildup and fortifies metal–support interaction, thereby enhancing the catalyst’s activity and selectivity in the reforming reaction. Zinc oxide’s high melting point (1975 °C) preserves surface area at elevated temperatures, aids nickel particle dispersion, and boosts active sites, thus improving CH₄ and CO₂ conversion rates. Figure 2d demonstrates that the Ni_0.10/(Zn_0.1-Ce_0.9) catalyst, with lower carbon deposition, attained high CH₄ conversion rates (83.1%) and H₂ yield (33.6%). Similarly, Yuchen Gao and colleagues synthesized nickel catalysts supported by Al₂O₃, SiO₂, MgO, CeO₂, and ZnO. Ni/Al₂O₃, characterized by its smaller particle size and strong metal–support interaction, offers more active sites, thereby enhancing the adsorption and surface reaction rates of CH₄ and CO₂, resulting in the highest CH₄ conversion efficiency of 82.7%. The O 1s XPS spectrum, illustrated in Figure 2e, reveals an O_α/O_β ratio, calculated from peak areas, indicative of oxygen activity. Ni/MgO, with an O_α/O_β ratio of 1.46, suggests a surface rich in active oxygen. This active oxygen interacts with CO₂ to form intermediates such as CO₃²⁻, which enhances CO₂ adsorption and activation, speeds up the dry reforming reaction, and helps oxidize surface carbon deposits, thereby reducing carbon accumulation and maintaining catalyst activity and stability. A 12 h stability test indicated a slight decrease in the CH₄ conversion rate of Ni/MgO from 68% to 64%; however, unlike the other four catalysts, it did not experience any blockage [66].

Smaller catalyst particle sizes yield a greater number of required active sites. Studies show that incorporating precious metals reduces catalyst particle size [78], boosting the quantity of highly efficient active sites. S. Carrasco-Ruiz et al. developed a stable Ni-Rh bimetallic catalyst on CeO₂-Al₂O₃ mixed oxides. Adding a small amount of Rh decreased the active metal’s average particle size, increased active sites, and enhanced reducibility, showcasing superior redox capabilities. This improved CH₄ and CO₂ adsorption and conversion, minimized carbon deposition, and prolonged the catalyst’s lifespan, as depicted in Figure 3a [79].

The preparation method significantly affects the particle size of Ni mixed catalysts. Santiago Veiga et al. [62] synthesized Ni-La-Ti catalysts through impregnation, co-precipitation, and an improved Pechini method. Significant differences in Ni particle size were observed among catalysts prepared by different methods, as illustrated in Figure 3b. The improved Pechini method yielded catalysts with smaller active metal particle sizes and stronger metal–support interactions, which mitigated metal sintering and increased the number of active sites. Figure 3c shows that the catalyst prepared using the improved Pechini method demonstrated excellent stability and activity during a 10 h stability test at a CH₄/CO₂ ratio of 1.5.

The stability of the methane dry reforming reaction relies heavily on the catalyst’s structure. Matinee Chuenjai et al. [67] synthesized NiO/MgO/ZrO₂ catalysts using urea carbon surface modification technology. Figure 4a illustrates that adding MgO stabilizes ZrO_2’s tetragonal phase and enhances ZrO₂-MgO interaction, which prevents phase transformation, boosts catalyst stability, and increases alkalinity. This process enhances CO₂ adsorption and reduction on the catalyst surface, leading to CO production. Surendar Moogi et al. [34] prepared perovskite catalysts (LaCoO₃ and LaNiO₃) using both the citrate–gel combustion and co-precipitation methods, observing more severe carbon deposition under CH₄/CO₂ = 1.5 than CH₄/CO₂ = 1.0. Figure 4b shows TEM-EDS images of the reduced catalyst. TEM analysis of perovskite catalysts indicated that synthesis methods influenced particle sizes; notably, the citrate–gel method produced smaller particles than the co-precipitation method, resulting in more active sites. The citrate–gel combustion method enhances internal porosity of the catalysts, facilitating methane diffusion and product desorption at active sites, crucial for minimizing carbon deposition. Consequently, catalysts synthesized via the citrate–gel combustion method showed reduced carbon content.

In previous studies, Maryam Kaviani et al. [68] employed the ammonia evaporation method to synthesize SiO₂-coated Ni-SiO₂@SiO₂ core–shell catalysts, enhancing stability and coking resistance at a CH₄/CO₂ ratio of 1.5, as depicted in Figure 4c. At 700 °C and with 10 wt% Ni loading, Ni-SiO₂@SiO₂ exhibited stable and high CH₄ and CO₂ conversion rates with negligible carbon formation, as Figure 4d shows. The core–shell catalyst’s high stability is due to its silica-coated core, which limits nickel particle mobility and imparts anti-migration and anti-sintering properties. The catalyst’s unique interlayer structure encapsulates Ni particles between the SiO₂ core and shell. During the reduction process, this leads to a Ni-SiO₂ core forming from layered nickel silicate, enhancing Ni particle dispersion and Ni-SiO₂ interaction, thus increasing stable active sites during the reaction.

Employing diverse synthesis methods and designing catalysts with core–shell configurations can effectively restrict nickel particle migration, conferring anti-migration and anti-sintering properties that enhance catalyst stability. Optimizing metal–support interactions boosts CO₂ adsorption and conversion, increases active sites, and reduces metal sintering and carbon deposition. These innovative strategies are essential for developing efficient and environmentally friendly catalysts, especially in the methane dry reforming process for sustainable energy conversion. Continued research and optimization of these methods could enable future industrial applications of these high-performance catalysts, which would significantly lower energy consumption and carbon emissions, guiding the energy sector towards a more sustainable and greener path.

3.2.2. Cobalt-Based Catalysts

The key distinction between cobalt-based catalysts in conventional and biogas dry reforming is the minor variations in CH₄/CO₂ feed ratios and the reactants’ complexity in biogas dry reforming. Research into the application of cobalt-based catalysts is vital given their significance in these domains, yet the challenges they present are non-trivial [80]. Catalyst deactivation primarily results from Co oxidation or carbon deposition, with smaller Co particles more susceptible to oxidation and larger particles predominantly losing activity due to carbon formation [81]. Cobalt-based catalysts exhibit lower conversion rates compared to precious metals and nickel-based catalysts [82]. Adding promoters is an effective strategy to boost Co-based catalysts’ activity and stability [83]. Promoters enhance active metal particle dispersion, electron transfer, carbon gasification, and catalyst reducibility, thereby increasing activity while reducing the reaction’s temperature requirement [84,85]. Promoters encompass alkanes, alkaline earth metals, transition metals, and rare earth metals, with alkanes and alkaline earth metals offering notable advantages in availability and cost [86,87].

Zhang et al. [88] synthesized a series of cobalt-based catalysts using γ-Al₂O₃ as the support and rare earth metals (Y, Tb, Gd) as promoters to mitigate carbon deposition. They introduced a carbon elimination mechanism involving rare earth metal oxides, where a bidentate carbonate intermediate is formed by a reaction with CO₂, as depicted in Figure 5a. This intermediate interacts with carbon deposits on the catalyst surface, yielding CO and metal oxides. Employing an oxidation–reduction cycle process significantly improves the catalyst’s resistance to carbon accumulation. Additionally, the influence of alkalinity on coke formation has not been extensively investigated. Anh Ngoc T. Cao et al. [63] employed γ-Al₂O₃ as a support with the impregnation method to synthesize cobalt catalysts. The reaction mechanism, depicted in Figure 5b, involves the adsorption and subsequent dissociation of CH₄ into hydrogen and hydrocarbons (CH_x). These CH_x intermediates then react with activated oxygen species (O*) from CO₂ to form CH_xO* radicals, which further react to produce CO and H₂. Illustrated in Figure 6c, calcium addition notably enhances the catalyst’s surface alkalinity, thereby improving its CO₂ adsorption capacity and modifying the reaction pathways to prevent carbon buildup at primary active sites. With low calcium dosages, the average crystallite size of Co₃O₄ is reduced, suggesting an improved dispersion of cobalt. This discovery opens new avenues for using earth-abundant catalysts to enhance the performance of simulated biogas dry reforming. Mabkhoot Alsaiari et al. [82] incorporated cost-effective iron into cobalt-based catalysts, as shown in Figure 5d, significantly affecting the material’s alkalinity due to the iron load. The alkalinity peaked at 705.7 μmol CO₂ g⁻¹ in the 2Fe-5Co/Al₂O₃ formulation. They established a correlation between the catalytic activity and the physicochemical properties of the materials. The 0.8Fe-5Co/Al₂O₃ sample demonstrated superior performance, improving the dispersion, reducibility, and alkalinity of cobalt on the support. This additive facilitated carbon gasification and effectively minimized coke deposition.

Manapkhan Zhumabek et al. [61] synthesized syngas using catalysts prepared from Co(NO₃)₂–Al(NO₃)₃–urea mixtures with varied compositions. The crystallite sizes of the catalysts, estimated from the widths of the XRD peaks as shown in Figure 5e, decreased as the Al(NO₃)₃ content increased, enhancing the catalysts’ thermal stability and resistance to carbon deposition—attributes vital for prolonged catalytic operations. For CO₂ conversion, the Co_xAl_y phase, with a lattice parameter of 1.423 Å, closely matches the C-O bond length of 1.43 Å in CO₂, suggesting that Co_xAl_y could serve as an active site for C-O bond activation. Additionally, incorporating metals such as Zn is seen as an effective strategy to prevent metal oxidation and manage carbon deposition on catalysts. Previous research has indicated that Zn addition prevents the oxidation of Co in the Co/ZrO₂ catalyst during ethanol steam reforming. Jung-Hyun Park et al. [81] examined the effects of Zn addition on the catalytic performance of CoZn(x)/ZrO₂ in methane dry reforming. Introducing 2 wt% Zn to Co/ZrO₂ notably improved catalyst stability; however, exceeding this amount reversed the effect, with higher oxidation rates and extents observed in CoZn(2)/ZrO₂ compared to Co/ZrO₂. Catalysts enhanced by Zn demonstrated increased stability, as the formation of a Co-Zn alloy impeded Co oxidation.

Adding promoters significantly enhances cobalt-based catalysts’ performance in dry reforming reactions by improving their activity, selectivity, stability, and carbon deposition resistance. Mesoporous carriers, like γ-Al₂O₃, have been shown to improve active metal dispersion and thermal stability in traditional dry reforming [89]. However, their application research in methane dry reforming remains limited. Adding promoters not only optimizes the catalyst’s physical and chemical properties but also boosts catalytic efficiency, opening new avenues for optimizing methane dry reforming and its industrial applications. Nonetheless, further research is needed to determine the optimal amounts, methods of doping, and the mechanisms by which promoters affect catalysts.

3.2.3. Iron-Based Catalysts

Iron, as the second most abundant metal in the Earth’s crust, offers substantial economic benefits, being approximately 250 times cheaper than Ni or Co [69]. With appropriate preparation and use, iron-based catalysts show excellent thermal stability and poison resistance [90]. Despite Fe’s economic advantages in methane conversion, related research remains limited. Additionally, under high temperatures and certain conditions, iron-based catalysts can experience structural changes like particle growth or phase transformation. While iron-based catalysts resist poisoning, trace impurities like sulfur and chlorine in methane can lead to their deactivation [29]. Currently, the deactivation mechanisms and reasons for monometallic iron catalysts remain unclear. Studies on bimetallic catalysts highlight iron’s potential to enhance the performance of other metals [37]. Research has also concentrated on using secondary metals to boost iron’s reactivity, aiming to develop cost-effective, primarily iron-based catalysts for industrial use [91].

Zahira Yaakob et al. [70] synthesized a Fe/ZrO catalyst via a urea-assisted solid-state combustion technique, applied in the thermal catalytic decomposition of undiluted biogas. This approach promotes the uniform dispersion of catalysts. As evidenced in Figure 6a, at elevated reaction temperatures, such as 800 °C and 900 °C, the peak intensity of iron nanoparticles reaches its maximum, suggesting a more thorough reduction of Fe, thereby enhancing catalytic activity. Concurrently, Figure 6b illustrates that the area of the TPR peaks increases substantially with added Fe content, indicating that higher reaction temperatures and greater iron content can improve CH₄ and CO₂ conversion and the generation of hydrogen-rich syngas by augmenting the number of active sites and enhancing metal–carrier interactions. This methodology, in contrast to the absence of a catalyst or the use of alternative catalyst types, allows for more efficient exploitation of biogas as a feedstock, converting it into higher-value products. Thunyathon Kludpantanapan et al. [71] investigated monometallic Fe and bimetallic Fe-based catalysts on MgO supports for synthesizing carbon nanotubes and hydrogen from biogas. The Fe/MgO catalyst showed poor catalytic activity due to the CO₂-induced oxidation of iron, resulting in significant deactivation. Nonetheless, the addition of Mo significantly improved metal dispersion and strengthened metal–support interactions, as evidenced in Figure 6c, leading to substantial increases in conversion rates and syngas production, thereby boosting the yields of H₂ and CNTs. Additionally, Karam Jabbour and colleagues developed ordered mesoporous Fe_10% and Fe_xNi_(1−x) catalysts within Al₂O₃ using an evaporation-induced self-assembly direct “one-pot” method to generate syngas [69]. Figure 6d illustrates that carbon deposition is directly proportional to the Fe content and inversely proportional to the Ni content in the catalysts, suggesting that higher Ni levels reduce carbon accumulation. The introduction of Ni facilitates the formation of a Fe-Ni alloy, minimizing the segregation of iron species and the accumulation of iron oxides and carbon. Furthermore, Ni catalyzes the activation of CO₂, where the resulting oxygen atoms oxidize deposited carbon (C(s)) to CO, diminishing carbon buildup. Regarding the resistance to poisoning, particularly against sulfide in real-world applications, incorporating additives enhances the resilience of iron-based catalysts and supports their regeneration and reuse. Research indicates that chromium additives notably improve the low-temperature denitrification activity and sulfur resistance of these catalysts.

Adopting innovative synthesis methods or incorporating secondary metals such as Mo significantly enhances metal dispersion and strengthens metal–support interactions. These advancements lead to marked increases in conversion rates and syngas production, consequently enhancing the yields of hydrogen and carbon nanotubes (CNTs). Moreover, introducing nickel facilitates the formation of a Fe-Ni alloy, which minimizes the segregation of iron species and reduces the accumulation of iron oxides and carbon. This improvement promotes CO₂ activation, potentially enhancing the catalyst’s resistance to poisoning and its regenerative capabilities in practical applications. These developments not only underscore the potential of iron-based catalysts in the methane dry reforming process but also create essential technological pathways for developing more efficient and environmentally friendly catalytic systems.

4. Challenges and Future Trends

Biogas dry reforming technology enables the conversion of biogas into syngas, effectively using waste resources and decreasing reliance on traditional fossil fuels, thus promoting sustainable energy use. Furthermore, this technology transforms greenhouse gases into high-value syngas, offering significant environmental and economic benefits, with considerable potential for future expansion. This review classifies noble and non-noble metal catalysts for biogas dry reforming, summarizing recent advances through strategies like smaller particle size design and improved metal–support interactions. The findings suggest that current methods effectively reduce carbon deposition and enhance catalyst activity. Despite laboratory progress, biogas dry reforming technology encounters challenges in industrial production and large-scale applications. Primary challenges encompass the long-term catalyst stability meeting industrial standards and issues in process parameter control, equipment design, and optimization, which are crucial for the technology’s commercial application.

Future research will primarily concentrate on enhancing the electronic and structural properties of catalyst surfaces. This will be achieved through strategic modifications including structural alterations, the introduction of additives, and alloying to improve both the resistance to carbon deposition and reaction efficiency. Additionally, efforts will be directed towards the development of innovative catalysts, such as those featuring layered, porous, or nanostructured oxides, sulfides, and nitrides. These structures are expected to offer a greater number of active sites and superior surface activity. Designing catalysts for improved light interaction is a promising research direction. Research demonstrates that illumination not only diminishes the thermal demands of reactions but also stimulates catalyst surface electrons, resulting in the activated adsorption of reactants. Consequently, this enhances the reaction rate and significantly reduces carbon deposition. Investigating the effects of illumination on the electronic properties of catalyst surfaces, and the control of reaction pathways through illumination, can potentially increase both the conversion rate of methane and the selectivity of products. Generative artificial intelligence (AI), including Transformer, Diffusion models, and Generative Adversarial Nets (GANs), is increasingly pivotal in driving catalyst design innovations. By learning from extensive datasets, these models produce catalyst structures with enhanced predictive capabilities, overcoming the constraints of traditional design methods. In developing biogas dry reforming catalysts, these technologies enable rapid screening and optimization, simulating material reactivity and stability to predict performance. This approach not only accelerates the experimental process but also reduces research and development costs. Additionally, generative AI deepens understanding of catalytic mechanisms, shows how various structures and components influence efficiency and selectivity, and offers a scientific basis for designing more efficient and eco-friendly catalysts. The interplay between theoretical simulation and experimental validation remains vital, uncovering the methane dry reforming reaction’s mechanisms and key steps. Deep insights into the reaction mechanism will inform catalyst design and optimization, enhancing reaction efficiency and reducing carbon deposition. Such research will offer crucial theoretical and technical support for the industrial use of methane dry reforming and sustainable production.