Reversible Thermochemical Routes for Carbon Neutrality: A Review of CO₂ Methanation and Steam Methane Reforming

Abstract

This review explores CO₂ methanation and steam methane reforming (SMR) as two key thermochemical processes governed by reversible reactions, each offering distinct contributions to carbon-neutral energy systems. The objective is to provide a comparative assessment of both processes, highlighting how reaction reversibility can be strategically leveraged for decarbonization. The study addresses methane production via CO₂ methanation and hydrogen production via SMR, focusing on their thermodynamic behaviors, catalytic systems, environmental impacts, and economic viability. CO₂ methanation, when powered by renewable hydrogen, can result in emissions ranging from −471 to 1076 kg CO₂-equivalent per MWh of methane produced, while hydrogen produced from SMR ranges from 90.9 to 750.75 kg CO₂-equivalent per MWh. Despite SMR’s lower production costs (USD 21–69/MWh), its environmental footprint is considerably higher. In contrast, methanation offers environmental benefits but remains economically uncompetitive (EUR 93.53–204.62/MWh). Both processes rely primarily on Ni-based catalysts, though recent developments in Ru-based and bimetallic systems have demonstrated improved performance. The review also examines operational challenges such as carbon deposition and catalyst deactivation. By framing these technologies through the shared lens of reversibility, this work outlines pathways toward integrated, efficient, and circular energy systems aligned with long-term sustainability and climate neutrality goals.

1. Introduction

Climate change is a global challenge with profound environmental, economic, and societal consequences. The European Union (EU) ranked as the fourth-largest greenhouse gas (GHG) emitter in 2023 after China, the United States, and India [1]. The top five countries in the EU as emissions contributors were Germany, France, Italy, Poland, and Spain, and the energy supply sector contributed 27.4% of the total GHG emissions [1]. The European Green Deal has set the EU on a path toward ambitious targets, including a 55% reduction in emissions by 2030 and achieving climate neutrality by 2050 [2]. These efforts already reduced emissions from 15.2% in 1990 down to 6.0% in 2023 [1].

Further improving climate objectives calls for a shift toward the use of renewable energy sources. In this regard, low-carbon energy carriers like methane (when used sustainably) and hydrogen have gained popularity when it comes to realizing carbon neutrality.

The aim of this review is to comparatively analyze CO₂ methanation and steam methane reforming as two thermochemical routes that contribute to carbon neutrality, with particular emphasis on the role of reaction reversibility, as illustrated in Figure 1. The scope includes methane production via CO₂ methanation and hydrogen and syngas production via SMR, with a focus on their integration within a circular carbon framework. Both methane and hydrogen are versatile energy carriers with applications in transportation, residential energy use, and energy storage. Additionally, syngas generated from SMR serves as a crucial feedstock for various chemical synthesis processes. By evaluating these two processes side by side, the review seeks to clarify their thermochemical relationship and access their respective roles in enabling sustainable and reversible energy systems.

Several reviews have been conducted on CO₂ methanation and steam methane reforming, as summarized in

Table 1. Summary of the reviews about CO₂ methanation and steam methane reforming.

Process	Topic	Authors	Year	Ref.
CO2 Methanation	Review of catalysts used over the past five decades	Frontera et al.	2017	[3]
	Overview of low-temperature CO2 methanation	Lee et al.	2021	[4]
	Bimetallic Ni-based catalysts	Tsiotsias et al.	2020	[5]
	Nickel-based catalysts for low-temperature applications	Li et al.	2022	[6]
	Thermodynamic analysis and assessment of catalysts and reactors	Ghaib et al.	2018	[7]
SMR	Steam reforming of natural gas for hydrogen production	Boretti et al.	2021	[8]
	Reforming and partial oxidation of hydrocarbons for hydrogen production	Sengodan et al.	2018	[9]
	Catalytic processes for hydrogen production	Kumar et al.	2024	[10]
	Bimetallic catalysts for steam methane reforming	Yusuf et al.	2024	[11]
	Membrane reactor technology	Iulianelli et al.	2016	[12]
		Habib et al.	2021	[13]

.

CO₂ methanation has been extensively investigated in recent decades. Frontera et al. presented, in their review, the evolution of studies conducted over the last 50 years on catalysts used in the CO₂ methanation process [3]. Lee et al. provided an overview of low-temperature CO₂ methanation [4]. Tsiotsias et al. summarized and discussed recent advances in the development of nickel-based bimetallic catalysts for the CO₂ methanation reaction [5]. Li et al. analyzed nickel catalysts for low-temperature application in their review [6]. Ghaib et al. performed an analysis of the thermodynamics of the methanation process, complemented by an evaluation and description of the catalysts and reactors used [7].

For hydrogen production via SMR, an extensive body of literature exists, as the process has been widely studied. Boretti et al. reviewed hydrogen production through steam reforming of natural gas [8]. Sengodan et al. examined hydrogen generation via reforming and partial oxidation of hydrocarbons, highlighting the significance of SMR within these pathways [9]. Kumar et al. provided a broader overview of catalytic processes for hydrogen production, including SMR, and discussed various catalysts used in this context [10]. Yusuf et al. focused specifically on catalysts for SMR, emphasizing the superior performance of bimetallic catalysts over monometallic ones [11]. Additionally, Iulianelli et al. and Habib et al. presented reviews on membrane reactor technology, which is increasingly being applied in SMR processes [12,13].

Catalysts play a crucial role in CO₂ methanation and steam methane reforming. However, several mechanisms can negatively affect their performance, leading to deactivation. Common causes of the catalyst deactivation include coke deposition, sintering, metal segregation or agglomerate, poisoning, and oxidation, as outlined in

Table 2. Description of the catalyst deactivation mechanisms.

Mechanism	Description	Ref.
Coke (or carbon) Deposition	Deposition of carbonaceous materials on the catalyst surface	[14,15]
Sintering	Loss of active surface area due to migration and growth of metals particles on the catalyst support	[16]
Metal segregation or agglomerate	Material particles may segregate or agglomerate, leading to reduced dispersion and catalyst deactivation	[17]
Poisoning	Irreversible chemical deactivation caused by deposition of impurities (e.g., sulfur, ammonia, etc.) on the active sites	[16,18]
Oxidation	Catalyst degradation due to exposure to oxidative environments	[8,11,16]

.

1.1. Hydrogen Production Processes

Hydrogen production takes place from multiple sources and processes, including fossil fuels, water electrolysis, and biomass conversion [19]. The production pathways can be categorized at a general level according to the energy source and the used feedstock, and a color coding system, shown in

Table 3. Hydrogen color code explained.

Hydrogen Color	Feedstock	Energy Used	Processes
Green	Water	Renewable sources	Electrolysis
Blue	Natural gas/methane	Fossil fuel	Reforming
Grey	Renewable natural gas/methane	Fossil fuel	Reforming
Brown/black	Coal	Fossil fuel	Gasification
Turquoise	Natural gas	Renewable sources	Reforming with carbon solidification or Pyrolysis
Purple	Water	Nuclear energy	Electrolysis
Yellow	Water	Grid energy	Electrolysis
Orange	Waste plastic	Fossil fuel	Gasification in carbon capture
White	Natural	Natural	Fracking

, has also been used to differentiate between them [20,21]. The key hydrogen categories are green, blue, grey, brown/black, turquoise, purple, yellow, orange, and white hydrogen [20,21].

Brown or black hydrogen is generated by coal gasification, and grey hydrogen comes from natural gas, which goes through steam methane reforming (SMR) [20]. Both involve high levels of GHG emissions because they utilize non-renewable fossil fuels. SMR, used most for industrial hydrogen production, emits large quantities of CO₂ to the atmosphere [20].

Though cleaner production processes exist, hydrogen production remains dominated by fossil processes worldwide [22]. In 2017, about 48% of hydrogen was generated from natural gas, 30% from heavy oil and naphtha, and 18% from coal [22].

Blue hydrogen is made from fossil fuels like natural gas but uses carbon capture and storage (CCS) methods to suppress CO₂ emissions [20,23]. Hydrogen from renewable natural gas sources, e.g., biomethane reforming, also belong to this group [20,23].

Among all forms, green hydrogen has generated the most interest due to its environmentally positive aspects. It can be generated via the electrolysis of water by means of electricity obtained from renewable sources like solar, wind, or hydro power, producing little or even zero carbon emissions [20,21].

At a process classification level, hydrogen can be generated through several hydrocarbon reforming methods, including steam reforming (SR), dry reforming (DR), partial oxidation (POx), auto-thermal reforming (ATR), and plasma reforming (PR) [20,23]. All the processes differ in operation conditions, energy efficiency, and distribution of the final product but share the same goal of recovering hydrogen from hydrocarbons [20,23].

A standard hydrogen reforming plant has several integrated sections: a desulfurization stage for removal of sulfur compounds from the feed material and a reforming and gas purifying section for the production and purification of hydrogen and auxiliary systems ranging from pumps and compressors through expanders and heat exchangers and coolers and combustors and other related equipment used for the processing and energy integration in the system [22].

Hydrogen may also be generated from biomass by means of thermochemical or biochemical processes and from water through a variety of splitting processes [19]. As far as the present situation goes, electrolysis remains the most advanced renewable-based method of hydrogen production, although continuous research strives for less expensive and efficient production methods on all the production pathways [19].

1.2. Methane Production Processes

Methane is a particularly important energy carrier for the industrial and energy sectors as well as for the transport industries worldwide. Its distribution system in numerous countries makes it an essential part of contemporary economies in most countries. Fossil resources of natural gas supply most of the methane used industrially. Nevertheless, the argument on the limited nature of fossil resources and global warming has prompted a high level of investment in research on catalytic and biotechnological production of methane from carbon oxide gases (methanation) in recent years [24].

The production of methane may be from two processes: chemical (catalytic) methanation or biological methanation [25]. The two processes differ primarily in the temperature and pressure conditions of the reaction and the catalyst employed [25].

During the bioproduction of methane (biogas), two processes can be differentiated: acetolactic methanogenesis and hydrogenotrophic methanogenesis. A positive trait can be identified as moderate operation temperature (30–60 °C), pressure at regular atmospheric levels, and high resistance to contaminants in the feeding gas. The slow kinetics, low mass transfer, and lack of flexibility unfortunately act as challenges of its broader application [7].

The catalytic methanation synthesis proceeds in reactors usually operated at elevated temperature (200–550 °C) and pressures of 1–100 bar. Some of the metals used in methanation reactors include Ni, Ru, Rh, and Co, and preferably, the latter’s use has been recommended because of its relatively high activity and good selectivity toward CH₄ at a low cost [26].

2. Methane Production from Carbon Dioxide Methanation

Numerous reactions take part in the methanation of CO₂, and even after extensive research work on the system of CO₂ methanation, the system remains extraordinarily complex. This complexity also implies that some of the thermodynamic features of the reactions remain unclarified so far. The principal reactions involved in the methanation of CO₂ are listed in

Table 4. Reactions of the process.

Reaction Formula	$\Delta {\mathbf{h}}_{298}^0$ (kJ/mol)	Reaction Type	Equation No.
${\mathrm{C}\mathrm{O}}_2+4{\mathrm{H}}_2\rightleftarrows {\mathrm{C}\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O}$	−165	CO2 methanation	(1)
$\mathrm{C}\mathrm{O}+3{\mathrm{H}}_2\rightleftarrows {\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}$	−206	CO methanation	(2)
${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\rightleftarrows \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}$	41	RWGS	(3)
$2\mathrm{C}\mathrm{O}+2{\mathrm{H}}_2\rightleftarrows {\mathrm{C}\mathrm{H}}_4+{\mathrm{C}\mathrm{O}}_2$	−247	RDM	(4)
$2\mathrm{C}\mathrm{O}\rightleftarrows \mathrm{C}+{\mathrm{C}\mathrm{O}}_2$	−172	CO disproportionation	(5)
${\mathrm{C}\mathrm{H}}_4\rightleftarrows \mathrm{C}+2{\mathrm{H}}_2$	75	CH4 cracking	(6)
$\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\rightleftarrows \mathrm{C}+{\mathrm{H}}_2\mathrm{O}$	−131	CO reduction	(7)
${\mathrm{C}\mathrm{O}}_2+2{\mathrm{H}}_2\rightleftarrows \mathrm{C}+2{\mathrm{H}}_2\mathrm{O}$	−90	CO2 reduction	(8)

[4].

Four reactions take place in the process: CO₂ methanation (Equation (1)), CO methanation (Equation (2)), reverse water–gas shift (RWGS) (Equation (3)), and dry reforming of methane (DRM) (Equation (4)). The first reaction is mildly endothermic, and the latter ones are highly exothermic. As a result, the process is highly exothermic. Efficient removal of heat and a strict necessity for it keep the CO₂ methanation in the optimum temperature zone. The RWGS reaction converts the available CO₂ into CO, and the latter undergoes methanation through the subsequent reactions and the operation of the CO methanation. The CO₂ methanation reaction proceeds in parallel [27].

The latter four reactions (Equations (5)–(8)) cause fouling of the catalyst surface, plugging of the catalyst pores, and/or physical destruction of the catalytic support because of carbon deposition and thus must be inhibited. Generally, each of the above reactions is dependent on the catalyst employed and on the chemical equilibrium of the catalyst [27]. All the above reactions are temperature- and pressure-dependent.

2.1. Operational Conditions Effects

The performance of CO₂ methanation reactions is influenced by several parameters, including temperature, pressure, the H₂/CO₂ ratio, and the presence of water vapor. This subsection analyzes the most favorable conditions for the methanation process, based on thermodynamic calculations and the chemical equilibrium of the involved reactions.

2.1.1. Effect of Temperature and Pressure

Two independent studies based on Gibbs reactor models identified carbon deposits formed at low temperature in the reactor [28,29]. Carbon deposition also occurred at low temperature when methanation was conducted below 500 °C and at H₂/CO₂ ratio levels less than 3. Coke deposition did not take place at H₂/CO₂ levels above 4 [28,29].

Gao et al. showed through their work that CH₄ and CO₂ are the preferred products in the methanation reaction when the stoichiometric H₂/CO₂ ratio is 4 and when the temperature ranges from 200 to 250 °C, as illustrated in Figure 2 [29].

The reaction takes preference at low temperature and high pressure, as illustrated in Figure 3, since it is a highly exothermic reaction and involves a reduction in the number of moles [29]. At a temperature above 600 °C and a pressure of 1 atm, the reaction has a gradual rise in the conversion of CO₂ due to the high activity of RWGS, a catalyst that consumes CO₂ [29]. RWGS activity increases above 450 °C and thus rises as a by-product of CO and consequently increases in quantities of unreacted CO and H₂, with a reduction in the selectivity of CH₄ [29]. These results are in line with experimental results given in other studies [30,31].

Thermodynamic calculations without considering the addition of steam also reveal no carbon formation. It can be concluded from a thermodynamic point of view that the water generated as a by-product in methanation is adequate to not cause any carbon deposition [29].

2.1.2. Effect of the H₂/CO₂ Ratio

Minimization of the H₂ in the feed stream reduces the cost of the process since hydrogen constitutes a significant cost component of the first- or second-generation biogas upgrading processes. Nevertheless, this method proves tough since CO₂ conversion and selectivity of CH₄ depend very much on the H₂/CO₂ ratio. High H₂/CO₂ ratios provide good conversion and selectivity at any pressure. At low H₂/CO₂ ratios, CO₂ conversion and CH₄ production will be lesser. In addition to this, at a ratio of 2, carbon deposition occurs at lower temperature below 500 °C [32].

As a result, both the activity of the reactions and carbon deposition depend heavily on the H₂/CO₂ ratio. A high H₂/CO₂ ratio has a positive effect on the conversion of CO₂ and selectivity towards CH₄. For the H₂/CO₂ ratio of 2, at 1 atm and 30 atm conditions, the maximum selectivity is 73% and 88%, and CH₄ yield is 40% and 45%, respectively. Under both conditions, there is a maximum of 50% carbon deposits below 500 °C. The H₂/CO₂ ratio has a considerable effect on improving the yield of CH₄ when increased under the same conditions. At H₂/CO₂ ratios above 4, carbon does not deposit during the reaction. From this, it can be inferred that for high yield of CH₄ and carbon deposition to not take place, the H₂/CO₂ ratio must not be less than 4 even at 30 atm [29]. Figure 4 illustrates the influence of the H₂/CO₂ ratio on the reaction performance [29].

Another study showed that for an H₂/CO₂ ratio of 4, total selectivity is achieved at temperatures below 500 °C [33]. Furthermore, as indicated by other researchers, higher pressures can also increase the temperature at which carbon deposition becomes significant [34].

2.1.3. Effect of Adding H₂O

The addition of water vapor to the CO₂ methanation process has both positive and negative effects, as reported by some studies [29,35]. First, on the negative side, it has a slight effect in reducing the conversion of CO₂ since H₂O is a product of the reaction and has the effect of driving the equilibrium toward the reactants when present in excess. Secondly, the addition has a positive effect in suppressing the production of solid carbon and hence the deactivation of catalysts. Additionally, the decreases in yield and selectivity of CH₄ are minimal, and thus, the approach proves useful in enhancing the long-term stability of the catalytic system.

Thus, low temperature and high pressure and a suitable H₂/CO₂ ratio are all favorable conditions for efficient methanation of CO₂.

2.2. CO2 Methanation Catalysts

Noble and non-noble metals have been utilized as catalysts for methanation of CO₂. At first, the catalyst that has been used and purified most extensively is the non-noble metal nickel (Ni), but in recent times, the trend has moved toward catalysts made from group 8, 9, and 10 periodic table metals, e.g., cobalt (Co), iron (Fe), rhodium (Ru), and molybdenum (Mo), which give promising results. The activity, selectivity, and stability of these varied catalysts differ from each other. Selectivity determines the yield of methane obtained from the catalysts, activity describes the interaction between the catalyst and CO₂, and stability accounts for resistance against thermal and chemical conditions. Additionally, the availability and cost of the metal utilized as a catalyst matter, affecting the applicability of the catalyst for industrial settings and profits [36].

Among the catalysts, ruthenium (Ru) has proven highly active, highly selective for CH₄, and resistant to oxidation conditions but suffers from the costliness of the metal, restricting actual application. Iron (Fe) is inexpensive but not highly selective for CH₄. The most economical and general metal used for CO₂ methanation is nickel (Ni), but it oxidizes and forms toxic compounds. The catalyst activity is also dependent on the carrier material employed and materials include aluminum oxide (Al₂O₃), silicon dioxide (SiO₂), zirconium dioxide (ZrO₂), and so on. In heterogeneous catalyzed reactions like gas–solid methanation of CO₂, catalyst deactivation modes may be present in the form of sintering, fouling, poisoning, and mechanical stressing. The loss of active surface area of the catalyst upon exposure to high-temperature conditions is known as sintering and may be prevented under low-temperature conditions. The active catalyst surface becomes obstructed by a solid upon fouling, generally in the form of carbon deposition, and may be restored by suitable combustion operations. Poisoning involves deposition at the active center of the catalyst irreversibly by a substance and has the form of sulfur poisoning, where catalyst activity may experience a sharp diminution. This mechanically deforms catalyst activity by means of resistance and/or heat stress, and the latter has originated in fluid bed reactors and owes its origin to thermal cycling of the catalyst bed, where it results from variations in temperature. These conditions explain the diversity in catalyst activity preservation in a course of operation during CO₂ methanation and highlight the need for the employment of multiple metals and multiple deactivations coupled with multiple materials used as a carrier and as important considerations in catalyst design and operation [16,37].

2.2.1. Comparative Performance of Metal Catalysts for CO₂ Methanation

Systematic comparisons between noble metals (such as Ru, Rh, Pt, and Pd) and non-noble metals (including Ni, Co, Fe, and Mo) reveal that Ru exhibits the highest levels of activity and selectivity for methane formation, particularly at lower reaction temperatures [38]. Among the noble metals, Ru and Rh are the most effective; however, Ru is generally preferred due to its relatively lower cost [39]. On the other hand, Ni stands out among the non-noble metals, offering high activity and selectivity combined with low cost and wide availability, which makes it the preferred catalyst for industrial-scale applications [24,39].

Direct comparative studies further reinforce these trends. For example, Panagiotopoulou et al. compared noble metals (Rh, Ru, Pt, and Pd) supported on TiO₂ and Al₂O₃, finding that Ru and Rh delivered the highest selectivity for CH₄, with Ru being favored on economic grounds [38]. Quindimil et al. showed that Ru-based catalysts outperformed their Ni-based counterparts at lower temperatures, achieving 85% CO₂ conversion and approximately 98% CH₄ selectivity at 375 °C using a 4% Ru/Al₂O₃ catalyst [40]. Furthermore, Stangeland et al. demonstrated that incorporating Ru into Ni/Al₂O₃ catalysts significantly enhanced both activity and selectivity, reaching 100% CH₄ selectivity and 82% CO₂ conversion at 350 °C, thereby highlighting the synergistic effect of Ru in such systems [41].

Broader screenings of group 8–10 transition metals support these observations; although Fe and Co are less costly, they tend to exhibit lower selectivity for CH₄ and are more prone to deactivation or the formation of undesirable by-products [36]. Ru remains the most active and selective, albeit limited by its cost, whereas Ni offers the most favorable compromise between catalytic performance and industrial scalability [42].

In light of these findings, the focus on Ni and Ru catalysts in this article is clearly justified. Ru delivers outstanding performance at low temperatures and under dynamic conditions, while Ni remains the most commonly used catalyst due to its low cost, adequate activity, and scalability. Together, they represent the most promising and practical options for CO₂ methanation and therefore merit the detailed discussion presented in this work.

2.2.2. Ru-Based Catalysis

Xu et al. studied the catalytic activity of ruthenium (Ru) catalysts supported on TiO₂-Al₂O₃ binary oxides and compared their performance with the Al₂O₃ support in the methanation of carbon dioxide (CO₂). The Ru/TiO₂-Al₂O₃ catalyst has a catalytic activity 3.1 times greater than the Ru/Al₂O₃ catalyst and hence is a highly promising material for application in processes of CO₂-to-CH₄ conversion. The catalytic performance improvement in the rate of the reaction is due to the size of Ru particles on the TiO₂-Al₂O₃ support being smaller compared to Ru particles on the Al₂O₃ support due to a strong Ru and rutile–TiO₂ interaction, which prevents Ru nanoparticles from aggregating together. Additionally, TiO₂ modification of the support has a positive effect on the catalyst’s structure and stability and hence on its activity [43].

The work of Panagiotopoulou et al. involved an examination of the utilization of noble metals (Rh, Ru, Pt, and Pd), at various loadings (0.1–5.0 wt.%), as the active phase for methanation. TiO₂ and Al₂O₃ served as the supporting materials used, and the catalysts were prepared via the method of impregnation. The research indicated that catalytic activity and the selectivity of the obtained reaction products of methanation heavily depend on the type of the metal phase. The best performance of the catalysts in selectivity for CH₄ was shown by Rh and Ru; however, considering the high cost of Rh, utilization of Ru has gained extensive popularity and acceptance [38].

Garbarino et al. investigated the methanation of CO₂ on a commercial 3% Ru/Al₂O₃ catalyst in a catalytic flow reactor system with continuous detection of the products by IR. It was established that 3% Ru/Al₂O₃ catalyzes the CO₂ methanation well, showing 96% selectivity to produce CH₄ and not producing any CO at 573 K and 15,000 h⁻¹. It was also found that even after multiple stop–start operation procedures, the catalyst did not lose its high and stable activity and thus proved suitable for intermittent operation conditions [44].

The work of Quindimil et al. explored the effect of metal loading on the catalytic activity of alumina-supported catalysts for the methanation of CO₂. The research compares the active metals nickel (Ni) and ruthenium, considering the effect of varying percentages of loading on activity, selectivity, and stability of catalysts. Ru catalysts exhibited better catalytic performance at low temperature compared to the case for Ni catalysts, and a 4% Ru/Al₂O₃ loading obtained a maximum of 85% conversion of CO₂ and approximately 98% selectivity of CH₄ at 375 °C [40].

The research in Stangeland et al.’s paper explored the evaluation of activity and stability of the Al₂O₃-supported nickel catalysts, both without and with ruthenium promotion, at various loadings of Ni, such as 12 and 20 wt.%. The findings showed an improvement in the activity of the catalysts as a function of increasing the metal loading and Ru proportion, and the optimum performance of the catalyst 20Ni0.5Ru/Al₂O₃ was at 100% CH₄ selectivity and 82% conversion of CO₂ at 350 °C [41].

Liang et al. conducted a study on Ru nanoparticle-based catalysts supported on TiO₂/Pal, aiming to optimize the CO₂ methanation reaction. The optimized Ru (4%)- TiO₂/Pal sample exhibited good catalytic activity, achieving up to 88.7% CO₂ conversion and nearly 100% CH₄ selectivity at 360 °C. Furthermore, the catalyst also demonstrated excellent stability, maintaining consistent performance during 60 h of continuous operation at 450 °C, with no carbon deposition [45].

For studying the performance of methanation at low-temperature conditions, Wang et al. employed CeO₂ as a supporting material and prepared Ru catalysts by the impregnation method at loadings of 0.25%, 0.5%, 1%, and 1.5%. Out of the catalysts examined by them, 1%Ru/CeO₂ exhibited the largest methanation activity at low temperature due to the availability of plenty of active metal sites and good metal–support interactions. To determine catalyst durability, the 0.5%Ru/CeO₂ material was subjected to operation for 30 h at 300 °C and recorded 76% conversion of CO₂ and 100% selectivity for CH₄, thus verifying its superior stability [46].

Wang et al. studied the performance of a nitrogen-doped activated biochar catalyst. The Ru/N-ABC-600 catalyst showed superior catalytic activity compared to Ru/ABC-600, with a CO₂ conversion of 93.8% and a CH₄ selectivity of 99.7%. This study demonstrated the positive effect of introducing nitrogen into a carbon-based support in the methanation of CO₂ [47].

Roldán et al. analyzed the use of nitrogen-doped carbon nanofibers (NCNF) in Ru-based catalysts for CO₂ methanation. The results demonstrated remarkable CH₄ productivity and stability in CO₂ hydrogenation, showing competitive performance compared to commercial Al₂O₃-supported catalysts. After 20 h of testing, these catalysts exhibited a CH₄ selectivity of 99% and a CO₂ conversion of 66% [48].

Lippi et al. studied the use of a zirconium-based metal–organic framework (MOF) material impregnated with Ru. The Ru/ZrO₂ catalyst showed high stability, with a CO₂ conversion and CH₄ selectivity of 96% and 99%, respectively. This catalytic activity was due to the final composition of the catalyst, consisting of a mixture of Ru nanoparticles supported on ZrO₂ nanoparticles in the monoclinic and tetragonal phases [49].

A summary of the studies analyzed on Ru-based catalysts used in CO₂ methanation is presented in

Table 5. Performance summary of supported Ru-based catalysts for CO₂ methanation.

Catalyst	Active Metal, wt.%	T (°C)	X CO2 (%)	S CH4 (%)	Ref.
Ru/TiO2	2	250	20	100	[38]
Ru/Al2O3	3	300	96	96	[44]
Ru/Al2O3	4	375	85	98	[40]
Ru/Ni/Al2O3	0.5 (Ru)–20 (Ni)	350	82	100	[41]
Ru/TiO2/Pal	4	450	88.7	100	[45]
Ru/CeO2	0.5	300	76	100	[46]
Ru/N-ABC-600	1.7	380	94	100	[47]
Ru/NCNF	5	350	66	99	[48]
Ru/ZrO2	1	-	96	99	[49]

.

2.2.3. Ni-Based Catalysts

Ni-based catalysts are another type of catalyst commonly used for methanation. In recent years, scientific research has focused on these catalysts due to their satisfactory catalytic performance in terms of activity, selectivity, and stability at low temperatures. In addition, Ni can be easily obtained at low cost due to its abundance, which makes its application in industrial scale methanation processes particularly attractive [50,51].

Muroyama et al. conducted a study in which they investigated the methanation activity of CO₂ and the species formed on its surface during the reaction on various Ni catalysts supported on metal oxides [52]. The catalyst that showed the best results was Ni/Y₂O₃, with a CO₂ conversion of 77% and CH₄ selectivity and yield of 99.5% and 80% at 300 °C, respectively.

In a similar study, Italiano et al. also concluded that the catalysts studied, namely Ni supported on Y₂O₃, had greater activity and good resistance to thermal shock and sintering after 200 h of testing. This is due to the moderate Ni–support interactions, for which higher Ni content correlates with increased CO₂ conversion and CH₄ yield [53].

A study by Wu et al. analyzed the behavior of SiO₂ as a support material in Ni-based catalysts. It proved to be more selective towards CH₄ at 10 wt.% Ni loading (90%) but showed low CO₂ conversion (10%). These results can be explained by the larger size of the Ni particles at higher loadings, which favors CH₄ formation [54].

Le et al. studied the influence of different supports, such as Al₂O₃, SiO₂, TiO₂, CeO₂, and ZrO₂, on Ni catalysts. Of those studied, the most active for CO₂ methanation was Ni/CeO₂ due to the small size of the Ni particles [55].

High-oxygen valences promote an increase in CO₂ methanation activity in catalysts, as shown by Zhou et al. with a study using Ni/CeO₂ catalysts that exhibited excellent performance and stability in the methanation process, with CO₂ conversion and CH₄ selectivity values of 91.1% and 100% at 340 °C, respectively, after 10 h of continuous reaction [56].

Jia et al. investigated the effect of Ni/ZrO₂ prepared via dielectric barrier discharge (DBD) plasma decomposition. The results obtained with this catalyst showed almost complete selectivity for CH₄ (97%) and a CO₂ conversion of 80% at 350 °C, which, according to the authors, was due to the formation of Ni-ZrO₂ interstitial sites with more oxygen vacancies, playing a crucial role in the activation and methanation of CO₂ [57].

In another study conducted by Martínez et al., the Ni/ZrO₂ catalysts produced the most remarkable results, showing CO₂ conversion close to 60% and CH₄ selectivity of 100%, with high stability after 250 h of operation [58].

Although Ni-based catalysts supported on Al₂O₃ exhibit relatively high activity due to their high surface area and excellent stability, they require further investigation [59]. Lin et al. conducted a study to analyze Ni catalysts modified with Al₂O₃-ZrO₂. The addition of ZrO₂ to Ni/Al₂O₃ promoted the reduction and dispersion of Ni particles, improving catalytic stability in the CO₂ methanation reaction. Nearly 100% CH₄ selectivity with 77% CO₂ conversion at a temperature of 300 °C was achieved by the 20Ni/Al₂O₃-ZrO₂ catalyst [60].

Abahussain et al. investigated the effect of strontium (Sr) promotion on nickel (Ni) catalysts supported on zirconia-alumina. Their findings showed that the 5Ni4Sr/10ZrO₂-Al₂O₃ catalyst achieved a CO₂ conversion of 80% and a CH₄ selectivity of 70%. These results indicate that catalytic activity is significantly influenced by the dispersion of active Ni sites, which was improved by Sr impregnation [61].

Luo et al. investigated the performance of Ni-based catalysts supported on metal oxides and silica-based porous materials. Their results demonstrated that the choice of support significantly affected both catalytic activity and stability. The Ni/CeO₂ catalyst exhibited superior performance, attributed to its abundant oxygen vacancies, high nickel dispersion, and the excellent redox properties of CeO₂. In comparation, the activity of the Ni/NS-MFI catalyst, where NS-MFI is a porous silica-based material, was primarily associated with mesoporous confinement and microporous penetration provided by the NS-MFI framework. Notably, both catalysts maintained excellent stability over 50 h of continuous operation at 400 °C, achieving approximately 80% CO₂ conversion and 98% CH₄ selectivity [62].

The addition of CeO₂ to Ni/Al₂O₃ catalysts containing 15 wt.% of nickel, prepared via impregnation, was studied by Liu et al. The results showed that the catalytic performance was strongly dependent on the CeO₂ content in the Ni-CeO₂/Al₂O₃ catalysts, with those containing 2 wt.% CeO₂ exhibiting the highest catalytic activity among those evaluated at 350 °C [63].

The addition of La to Ni/γ-Al₂O₃ catalysts was extensively studied by Garbarino et al. In this study, it was concluded that the addition of La to the catalyst significantly enhances the activity for CO₂ methanation. Furthermore, CH₄ selectivity increases to values close to 100% at low temperatures. The authors proposed that these improvements were due to the basicity of La, which promotes stronger CO₂ adsorption [64].

Tan et al. studied the influence of MgO addition on the thermal stability of 6 wt.% Ni/ZrO₂ catalysts. Improved performance was observed when the Mg/Ni molar ratio was 1/4, with conversion values of 95% and total selectivity towards CH₄. Highly dispersed Ni nanoparticles are stabilized at elevated temperatures by the doping of structural MgO additives, thereby explaining the enhanced performance [65].

Zhu et al. demonstrated both theoretically and experimentally that the introduction of Y₂O₃ into an Ni/CeO₂ catalyst facilitates the generation of surface oxygen vacancies during the reaction, which promotes CO₂ dissociation. This resulted in excellent methanation activity three times higher than that of the unmodified catalyst [66].

Siakavelas et al. studied the catalytic performance for CO₂ methanation over Ni catalysts based on CeO₂ and Ni catalysts supported on binary oxides based on CeO₂, namely Sm₂O₃-CeO₂, Pr₂O₃-CeO₂, and MgO-CeO₂. It was demonstrated that the incorporation of Sm³⁺ or Pr³⁺ into the CeO₂ lattice generated oxygen vacancies, thereby enhancing CO₂ methanation activity [67].

Everett et al. investigated the addition of Ca to the Ni/ZrO₂ catalyst. This addition led to an almost threefold increase in the CO₂ consumption rate. Analyses revealed that the presence of Ca²⁺ on the ZrO₂ lattice surface of the Ni/CaZrO₂ catalyst promoted the formation of oxygen vacancies, which in turn enhanced the CO₂ methanation rate [68].

Le et al. studied the use of composites with activated carbon (AC) as support in Ni catalysts, supported on AC and on AC modified with Ce_0.2Zr_0.8O₂. The results for the Ni/Ce_0.2Zr_0.8O₂/AC catalyst were substantially better compared to the Ni/AC catalyst, achieving 85% CO₂ conversion and 100% CH₄ selectivity at 350 °C. The findings indicate that the addition of Ce_0.2Zr_0.8O₂ to the activated carbon support significantly improves Ni particle dispersion and metal–support interaction, resulting in higher activity for CO₂ methanation at low temperatures [69].

Ashok et al. prepared different Ni-based catalysts supported on Ce_xZr_1−xO₂ using various methods, in which 10 wt.% Ni/Ce_xZr_1−xO₂ showed superior catalytic performance at comparatively lower reaction temperatures. At 275 °C, it achieved a maximum CO₂ conversion and methane selectivity of 55% and 99.8%, respectively, and remained stable for almost 70 h of reaction time [70].

Wang et al. compared the activity of the Ni/Ce-ABC catalyst, where ABC refers to activated biochar, with that of Ni/ABC. The catalyst with the best performance was Ni/Ce-ABC, achieving 88.6% CO₂ conversion and 92.3% CH₄ selectivity at 360 °C. The highly dispersed species on the biochar were considered beneficial for the dispersion of nickel species and for enhancing CO₂ adsorption capacity. This study enabled the development of an environmentally friendly and efficient catalyst for achieving CO₂ reduction and valorization [71].

Sholeha et al. studied the influence of NaY in Ni catalysts synthesized from dealuminated metakaolin. The Ni/NaY catalysts showed a CO₂ conversion of 67% and CH₄ selectivity of 94%. The authors identified the large surface area, low sulfur content, and well-defined crystalline structures as the key factors contributing to the enhanced activity [72].

Zeolite X synthesized from residual fly ash was used as a support for Ni catalysts for CO₂ methanation, showing CO₂ conversion results of around 50%. Compared to commercially available zeolite catalysts, this shows lower performance. However, it should be noted that the use of fly ash zeolites allows waste to be used, generating positive economic and ecological impacts [73].

Romero-Sáez et al. studied Ni/ZrO₂ catalysts supported on carbon nanotubes (CNTs) for the CO₂ methanation reaction. They were able to demonstrate that the co-impregnation strategy is more effective in the preparation of Ni/ZrO₂ catalysts supported on CNTs, providing better metal dispersion and greater metal–support interaction, resulting in better catalytic activity [74].

The use of CNTs was also studied by Wang et al., with 12Ni4.5Ce/CNT showing CO₂ conversion results of 83.8% and almost 100% CH₄ selectivity, with no obvious deactivation observed after one hundred hours of stability testing. This catalytic performance can be attributed to the synergistic interaction between the two metals and the exceptional properties of the CNT support [75].

Zhi et al. studied the Ni/SiC catalyst modified with La₂O₃ and prepared by the co-impregnation method for CO₂ methanation. The results obtained showed a significant improvement when compared to the traditional catalyst, exhibiting high activity and excellent stability, operating stably at 360 °C for 70 h with high CH₄ selectivity. These results were explained by the fact that the Ni particles formed on the surface of the SiC were smaller, allowing for a stronger interaction between the NiO and the SiC, favoring the activation of CO₂ [76].

Zhen et al. evaluated the introduction of a well-known metal–organic framework, MOF-5, as a support for the Ni catalyst [77]. MOF-5 is a metal with the chemical formula Zn₄O(BDC)₃, where BDC is 1,4-benzenedicarboxylate [78]. The results obtained in the study revealed that for 10 wt.% Ni loadings, the catalytic activity showed improved values, as the Ni was uniformly distributed and highly dispersed on the MOF-5, giving it high stability, with no significant deactivation observed up to 100 h of testing [77].

Some of the studies on Ni-based catalysts are summarized in

Table 6. Performance summary of supported Ni-based catalysts for CO₂ methanation.

Catalyst	Active Metal, wt.%	T (°C)	X CO2 (%)	S CH4 (%)	Ref.
Ni/Y2O3	10	300	77	99.5	[52]
Ni/Y2O3	35	350	83.5	90.3	[53]
Ni/SiO2	10	350	10	90	[54]
Ni/CeO2	10	340	31.1	100	[56]
Ni/ZrO2	10	350	80	97	[57]
Ni/ZrO2	20	400	50	100	[58]
Ni/Al2O3-ZrO2	20	300	77	100	[60]
Ni4Sr/10ZrO2-Al2O3	5	-	80	70	[61]
Ni/CeO2 and Ni/NS-MFI	-	400	80	98	[62]
Ni/CeO2-Al2O3	15	350	85	100	[63]
Ni/MgO-ZrO2	6	300	95	100	[65]
Ni/Ce0.2Zr0.8O2/AC	7	350	85	100	[69]
Ni/CexZr1−xO2	10	275	55	99.8	[70]
Ni/Ce-ABC	15	360	88.6	92.3	[71]
Ni4.5Ce/CNT	12	350	83.8	98.8	[75]

.

2.3. CO₂ Methanation Environmental Impact

To evaluate the environmental impact of industrial processes, it is important to understand the methodology commonly used for such assessments. One of the most widely applied tools is life cycle assessment (LCA), which considers all stages of a product or process, from raw material extraction and production to use and disposal [79]. The LCA is divided into four phases, which are goal and scope definition, life cycle inventory (LCI), life cycle impact assessment (LCIA), and improvement assessment [79].

The goal and scope definition phase sets the objectives of the study, defines the system boundaries, and establishes the reference unit for inputs and outputs [79]. In the LCI phase, data are collected and quantified for energy and material inputs and outputs across all processes within the system boundaries [79]. The LCIA phase evaluates the environmental impacts of these flows. This evaluation is conducted in three steps: classification of emissions into impact categories, characterization of inventory data by quantifying contributions to these categories, and the application of normalization and weighting [79]. The improvement assessment phase supports the identification and selection of better-performing alternatives among the evaluated systems [79].

Two impact categories commonly considered in LCIA are global warming potential (GWP) and acidification potential (AP) [79]. GWP refers to the impact of human emissions on atmospheric radiative forcing and expressed in kilograms of CO₂ equivalent [79]. AP measures the effect of acidifying emissions such as SO₂, NO_x, and NH_x on ecosystems, reported in kilograms of SO₂ equivalent [79].

A study conducted by Reiter and Lindorfer analyzed the global warming potential for various sources of CO₂ and H₂ for the methanation reaction. Emissions from the methanation process were analyzed, considering CO₂ as a waste product from an industrial activity and when obtained from fossil sources, with H₂ derived from renewable (wind and photovoltaic (PV)) or mixed (renewable and fossil) energy sources [80].

Meylan et al. also studied the GWP of the methanation process, considering various sources of CO₂ and H₂. The scenarios analyzed in this article involved CO₂ methanation using CO₂ captured directly from the air (DAC), along with various sources of renewable H₂ [81].

Another, more recent study by Navajas et al. evaluated the environmental impact of power-to-methane systems with CO₂ supplied by the chemical looping combustion of biomass. In this study, different materials were considered as oxygen carriers depending on the specific operating mode, such as CLOU (a synthetic material based on CuO (Cu60)), iG-CLC_sOC (a synthetic material based on CuO (Cu15)), and iG-CLC_mOC (a mineral material based on ilmenite (FeTiO₃)) [82].

The results of these studies are presented in

Table 7. Comparison of the GWP reported in different studies.

Authors	H2 Production	CO2 Origin	GWP (kg CO2 eq/MWh CH4)	Ref.
Reiter and Lindorfer	Wind	Residue	22	[80]
Reiter and Lindorfer	PV	Residue	108	[80]
Reiter and Lindorfer	Mix	Residue	994	[80]
Reiter and Lindorfer	Wind	Fossil	104	[80]
Reiter and Lindorfer	PV	Fossil	191	[80]
Reiter and Lindorfer	Mix	Fossil	1076	[80]
Meylan et al.	Wind	DAC	54	[81]
Meylan et al.	PV	DAC	134	[81]
Navajas et al. (CLOU)	Wind and PV	Biomass	−341/−10 (a)	[82]
Navajas et al. (iG-CLC_sOC)	Wind and PV	Biomass	−418/−9 (a)	[82]
Navajas et al. (iG-CLC_mOC	Wind and PV	Biomass	−471/−8.5 (a)	[82]

^(a) Values with and without CO₂ storage, respectively.

, which highlights both the source of the electricity used for electrolysis and the origin of the CO₂ used in methanation.

Thus, by analyzing the results of previous studies, it can be observed that, for the same source of CO₂, the source of electricity used for electrolysis significantly affects GHG emissions; in particular, electricity generated from wind power appears to have the most beneficial environmental impact in terms of reducing GWP. For the same energy source used to produce hydrogen, cases in which CO₂ is captured from industrial processes show a higher GWP compared to those where CO₂ originates from waste or as a by-product of another process. It can also be seen that, with more advanced technologies, it is possible to achieve negative CO₂ emissions in the methanation process, effectively removing carbon dioxide from the atmosphere, even without accounting for the storage of surplus CO₂ produced.

2.4. CO₂ Methanation Process Costs

The actual costs of power-to-gas (PtG) plants were given in Leeuwen C. et al. This report presented an overview of the current costs of power-to-gas methane plants, including investment costs, operating costs, and raw material costs. These costs were estimated based on bibliographical sources and data from STORE&GO demonstration plants, currently built in Falkenhagen (Germany), Solothurn (Switzerland), and Troia (Italy) [83].

According to this report, the methane production costs of power-to-gas plants were calculated at EUR 1.30/kg for the full-time production of the plant, and these costs can be broken down into 43% electricity costs, 37% CAPEX (Current Capital Expenditure), 13% OPEX (Operating Expenditure), and 7% CO₂ costs. It was found that operating the installation only during the hours when electricity prices are low does not improve the overall profitability of the installation due to the greater weight of capital and operating expenditure in the cost of methane produced. Water costs were considered negligible in all cases (adding a maximum of 0.3% to total costs) [83]. Since the lower heating value (LHV) of methane is 13.90 kWh/kg, the cost can be converted from kilogram to kilowatt-hour [84].

Comparing the methane production costs of power-to-gas installations with the current costs of natural gas (around EUR 0.30/kg), it is clear that power-to-gas cannot currently compete with fossil gas. Even by halving the investment costs of the electrolyzer, the most expensive component of the power-to-gas installation in terms of investment costs, it was found that methane production costs fall from EUR 1.30/kg to EUR 1.09/kg. Although this is a significant cost reduction (16%), it is by far not enough to make up the difference with the price of natural gas [83].

For example, the Swiss government has implemented several measures to achieve the goal of reducing CO₂ emissions in Switzerland by 50% by 2030 compared to 1990. The article by Baier J. et al. showed the cost estimate of the power-to-gas model applied to carbon dioxide methanation in the Swiss cement industry. Using the technology chain of photovoltaics, alkaline electrolysis, and catalytic methanation to recover the investigated CO₂, it was possible to produce 0.9 million tons or 1.26 billion m³ of synthetic CH₄ per year using CO₂ from the exhaust gases of all the cement plants. At least 32.9% of the 3.82 billion m³ of natural gas imported each year could be offset by the CH₄ produced, thus reducing CO₂ emissions from natural gas by the same amount of 32.9% [85].

The investment costs for the entire methane electricity production infrastructure amounted to a total of 38,579 million Swiss francs. This includes installation costs of CHF 20,703 million for electricity production using photovoltaic panels, CHF 13,471 million for alkaline electrolysis, and a further CHF 4405 million needed to install the methanation units themselves. In addition to these installation costs, annual running costs must also be considered. Separating and converting the 2.5 million tons of CO₂ emitted annually into 0.9 million tons of CH₄ was estimated to cost CHF 3836 million per year. The highest individual costs were CHF 1982 million per year for electricity production using photovoltaics (51.7%), followed by CHF 1397 million per year for alkaline electrolysis (36.4%), and CHF 457 million per year for methanation (11.9%). Thus, taking all costs into account, methane production was estimated to cost CHF 0.30/kWh [85].

The Gorre J. et al. study also presented an overview of the effects of operating strategies and configuration on the economics of power-to-gas plants for the years 2030 and 2050. They used two key financial variables, capital expenditure (CAPEX) and operational expenditure (OPEX), to evaluate the initial investment required to build the plant and the costs of running and maintaining it, respectively. Four electricity supply approaches were evaluated in combination with two strategies for selling synthetic natural gas. The calculations showed that adapting the system configuration can significantly reduce methane production costs. By incorporating an intermediate hydrogen storage tank and adjusting the methanation capacity, the methanation workload can be increased by allowing independent operating of the electrolysis and methanation processes. Thus, for the year 2030, methane production prices were estimated at EUR 33.60/MWh, with an electricity price of EUR 0/MWh and EUR 204.62/MWh with an electricity price of EUR 100/MWh [86].

Another study by Lee et al. aimed to assess the economic viability of PtG technology with surplus electricity, especially from renewable sources. The analysis was carried out considering operating capital costs, resulting in a unit cost of methane production of approximately USD 0.094/kWh for a production capacity of 700 m³/h. This figure is higher than the cost of producing natural gas in South Korea, which varies between USD 0.038 and USD 0.069/kWh [87].

The costs reported in previous studies are summarized in

Table 8. Summary of the CO₂ methanation process costs from the literature.

Costs per kg CH4	Costs per kWh CH4	Costs per MWh CH4	Ref.
EUR 1.30	EUR 0.09	EUR 93.53	[83,84]
-	CHF 0.30	CHF 300	[85]
-	-	EUR 33.60 and EUR 204.62 (a)	[86]
-	USD 0.094	USD 94	[87]

^(a) For electricity costs of EUR 0/MWh and for EUR 100/MWh.

.

2.5. Discussion on CO₂ Methanation

According to thermodynamic analysis, the ideal CO₂ methanation operation should be carried out at moderate temperatures (200–500 °C), high pressure, and with an H₂/CO₂ ratio ≥ 4, and water vapor can be used as a carbon deposition mitigating agent. These conditions ensure the technical and economic viability and long-term stability of the process.

The studies reviewed demonstrate the significant progress made in the development of Ni-based catalysts for CO₂ methanation, highlighting their high activity, selectivity, and thermal stability, especially when supported on metal oxides such as Y₂O₃, CeO₂, and ZrO₂. The incorporation of promoters such as La, Mg, Ca, and rare-earth elements has also been shown to enhance catalytic performance by improving Ni dispersion, increasing oxygen vacancy concentration, and strengthening metal–support interactions. These modifications have contributed to more efficient CO₂ conversion and CH₄ production at lower reaction temperatures, making Ni-based systems promising candidates for large-scale, sustainable methane synthesis.

However, one important aspect that remains insufficiently explored in many of the reviewed works is carbon formation. The accumulation of carbon deposits on catalyst surfaces can lead to deactivation over time, negatively impacting catalyst longevity and industrial feasibility. Despite the promising performance metrics reported, a deeper understanding of the mechanisms of carbon formation and strategies for its suppression is essential. Future research should therefore place greater emphasis on the characterization and control of carbon deposition to ensure its long-term stability and practical application in CO₂ methanation processes.

In terms of economic viability, PtG technology focused on methane production is not yet competitive with natural gas. Intermittent operation of the technology to take advantage of the best electricity price does not improve profitability since fixed costs (CAPEX and OPEX) remain high per unit of methane produced. Therefore, considering the price of natural gas production, the application of CO₂ methanation techniques is not yet mature enough to pay off economically, and continuous investment is needed to improve the technologies so that they have cheaper operating prices that are competitive with natural gas prices.

In terms of environmental impact, methanation shows satisfactory results, as it reduces the amount of CO₂ in the atmosphere. The studies reviewed confirm that assessing the environmental performance of PtG systems depends critically on the origin of the CO₂ and H₂, the energy matrix involved, and the specific technology used to capture and convert the carbon. The LCA approach is therefore essential for a holistic and informed comparison of the different methanation technology configurations.

3. Hydrogen Production from Steam Methane Reforming Process

The process of steam methane reforming (SMR) is the most widely used method for hydrogen production, accounting for approximately 75% of global hydrogen output, and involves two main reactions [20,21].

The first is the reforming reaction (Equation (9)), an endothermic process requiring external energy input. In this reaction, methane reacts with water at high temperatures (700–1000 °C) and pressures (3–25 bar) to produce hydrogen and carbon monoxide [8,20]. This reaction occurs in the presence of a catalyst, which may be based on either non-precious or precious metals, enhancing the process and allowing conversion rates of up to 90% [20,23]. The selection of a suitable catalyst should be based on its high activity in hydrocarbon conversion, stability under operating conditions, effective heat transfer, low pressure drop, high hydrogen selectivity, thermal stability, and mechanical strength [9].

Simultaneously, the water–gas shift reaction (Equation (10)) takes place in the presence of carbon monoxide, which acts as a trigger for the reaction [88]. During this process, steam reacts with the carbon monoxide to form carbon dioxide and hydrogen [20,88,89]. This reaction plays a crucial role in steam methane reforming, as it helps reduce the concentration of carbon monoxide, thereby minimizing environmental impact and increasing the overall hydrogen yield [88,89].

The overall SMR process is also endothermic and is represented by Equation (11).

$$\begin{array}{cc}\mathrm{C}{\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\to \mathrm{C}\mathrm{O}+3{\mathrm{H}}_2& \Delta {\mathrm{h}}_{298}^0=206 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\end{array}$$

(9)

$$\begin{array}{cc}\mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2& \Delta {\mathrm{h}}_{298}^0=-41 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\end{array}$$

(10)

$$\begin{array}{cc}\mathrm{C}{\mathrm{H}}_4+2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{C}\mathrm{O}}_2+4{\mathrm{H}}_2& \Delta {\mathrm{h}}_{298}^0=165 \mathrm{k}\mathrm{J}/\mathrm{m}\mathrm{o}\mathrm{l}\end{array}$$

(11)

Hydrogen can also be produced through steam reforming of other hydrocarbons. The reactions involved in this process are shown in Equations (10), (12) and (13), where Equation (12) represents the reforming reaction, and Equation (13) represents the overall reaction [21].

$${\mathrm{C}}_{\mathrm{m}}{\mathrm{H}}_{\mathrm{n}}+\mathrm{m} {\mathrm{H}}_2\mathrm{O}\to \mathrm{m} \mathrm{C}\mathrm{O}+\left(\mathrm{m}+0.5 \mathrm{n}\right){\mathrm{H}}_2$$

(12)

$${\mathrm{C}}_{\mathrm{m}}{\mathrm{H}}_{\mathrm{n}}+2\mathrm{m} {\mathrm{H}}_2\mathrm{O}\to \mathrm{m} {\mathrm{C}\mathrm{O}}_2+\left(2\mathrm{m}+0.5 \mathrm{n}\right){\mathrm{H}}_2$$

(13)

Hydrogen production in the SMR process is significantly influenced by the steam-to-methane ratio, commonly referred to as the steam-to-carbon ratio (S/C), as well as by pressure and temperature conditions [90]. Optimal hydrogen yields are achieved at high S/C ratios, low pressures, and high temperatures [90]. Pashchenko et al. studied the effects of temperature and the steam-to-carbon ratio, as shown in in Figure 5 [91].

Additionally, the hydrogen-to-carbon ratio (H/C) of the hydrocarbon feedstock affects greenhouse gas emissions [20]. A higher H/C ratio results in lower emissions, making methane a more environmentally favorable option compared to hydrocarbons such as ethane or propane [20].

This method offers several advantages, including the flexibility to use various feedstocks, high hydrogen purity, and high thermal efficiency [21]. However, it also presents some drawbacks, such its strongly endothermic nature and limited catalyst durability [21].

When using an Ni/Al₂O₃ catalyst at high temperatures (750–920 °C) and high pressures (3.5 MPa), the process benefits from being a mature technology with reliable equipment, economic efficiency, and rational resource use. Nevertheless, it also requires substantial equipment investment and suffers from catalyst deactivation over time [92].

Steam methane reforming, while currently the most widely used method for hydrogen production, presents several technical and environmental challenges. The process is highly endothermic and requires elevated temperatures and pressures, which negatively impact overall energy efficiency. Catalyst deactivation, caused by mechanisms such as coke formation, sintering, poisoning, oxidation, and metal agglomeration, remains a major limitation to long-term operation [8,11,16,17,18].

From an environmental perspective, SMR contributes significantly to CO₂ emissions due to its reliance on fossil fuels, reinforcing the urgency of developing cleaner reforming methods and incorporating carbon capture technologies [17].

3.1. Steam Methane Reforming Catalysts

As previously discussed, SMR relies on the use of catalysts, typically comprising an active metal, a support material, and, in some cases, a promoter or dopant [93]. Nickel supported on Al₂O₃ is the most widely used catalyst in SMR, although alternative supports such as SiO₂ and ZrO₂ have also been explored [93].

A review by Iulianelli et al. underscored the predominance of Ni-based catalysts in the literature, with particular emphasis on the effects of different supports and promoters [12]. Incorporating a second metal into Ni-based catalysts has been shown to enhance catalytic activity, selectivity, and durability, thereby mitigating challenges such as coke formation, oxidation, sintering, and metal segregation [8]. The catalyst performance is strongly influenced by the nature of the support material [8].

Ni-based catalysts are considered highly effective for SMR due to their excellent activity and selectivity [9]. However, they are prone to carbon deposition (coking), which impairs long-term performance [9]. This limitation can be addressed by adding noble metals, rare earth elements, or modifying the support, for instance, by incorporating perovskite-type materials [9].

Sengodan et al. presented data on the methane conversion and the selectivity of hydrogen, carbon dioxide, and carbon monoxide during the SMR process over an Ni/Al₂O₃ catalyst, as shown in Figure 6 [9].

Despite their advantages, Ni-based catalysts face challenges including limited thermal stability and sensitivity to oxidative treatments (e.g., regeneration with O₂, air, H₂O, or CO₂) commonly employed to remove carbon deposits [10]. Nevertheless, catalysts remain essential to enhancing reaction kinetics and hydrogen selectivity, contributing to sustainable H₂ production [10].

Nickel is favored in SMR due to its high availability and low cost, but it suffers from deactivation through carbon deposition, sintering, and moderate intrinsic activity [11]. Other group 8 metals, such as iron and cobalt, have been studied; however, cobalt lacks steam resistance, and iron is highly susceptible to oxidation [11]. Noble metals like Pt, Ir, Ru, Pd, and Rh offer superior activity, coke resistance, and stability but are limited by their high cost [11].

To balance performance and cost, noble metals are often employed as promoters alongside base metals like Ni or Co [11]. This approach sustains catalytic efficiency while minimizing the use of expensive materials [11]. Studies have shown that noble metals enhance Ni dispersion, increase active site availability, and improve the reducibility of Ni species, resulting in improved SMR performance [11]. Bimetallic catalysts benefit from synergistic effects, often exhibiting distinct properties compared to their monometallic counterparts, including enhanced sulfur resistance, thermal stability, and coke suppression [11].

One study investigated a packed-bed reactor using a cylindrical Ni/Al₂O₃ catalyst comprising 14.5 wt.% NiO, 0.2 wt.% SiO₂, and a CaO-MgO-La₂O₃-α-Al₂O₃ support [91]. The impact of temperature, S/C ratio, and residence time on carbon deposition and flow behavior was examined [91]. Higher pressure and increased S/C ratios significantly reduced carbon formation, while longer residence times improved methane conversion toward equilibrium [91]. Low S/C ratios (e.g., 0.5) led to greater conversion losses compared to higher ratios (1.0 or 2.0), and both higher temperatures and S/C ratios helped reduce carbon deposition [91].

A separate investigation focused on Rh-, Ir-, and Ru-promoted Ni/Al₂O₃ catalysts [94]. Rh- and Ir-promoted variants showed higher active metal surface area and improved SMR activity, along with reduced Ni sintering [94]. In contrast, Ru-promoted catalysts were less effective due to lower sintering resistance and diminished activity, which were related to differences in alloy structure and thermal stability [94].

Further research into Ni-Mg-Al-based catalysts, with and without Ru, resulted in formulations labelled Ru/Ni_xMg_6−xAl₂ 800 800 (x = 2, 4, and 6), reflecting their composition and dual calcination at 800 °C [95]. The Ru/Ni₆Al₂ 800 800 (x = 6) catalyst exhibited superior methane conversion at 750 °C, achieving more than 95% conversion, suggesting that Mg is not essential in Ru-Ni systems for optimal performance [95].

Another study examined the use of Fe and Cu promoters in catalysts of the form (Ni_xMg_1−x)₂Al and (Ni_0.05M_0.05Mg_0.9)₂Al (M = Fe or Cu) as well as an Mg-free version Ni_0.05Al_0.95O_x [17]. Increasing Ni content enhanced both activity and H₂ yield in unpromoted catalysts. Although the addition of Fe or Cu slightly reduced activity, it improved stability due to the formation of Ni-Fe or Ni-Cu alloys, which suppress Ni agglomeration and coke formation [17]. Mg also improved activity and stability and reduced acidity [17]. Moreover, Fe and Cu aided Ni oxide reduction and enhanced metal dispersion [17].

A study investigating Rh-promoted Ni/Al₂O₄ catalysts aimed to evaluate their stability under both ambient and high-pressure conditions and to identify the optimal Rh loading [96]. Two catalyst series were synthesized: the first with 15 wt.% Ni and varying Rh content (xRh15Ni) and the second with 0.5 wt.% Rh and 15 wt.% Ni/Al₂O₄, calcined at either 600 °C or 850 °C [96]. The latter were designated Cat T1 T2, with T1 and T2 referring to calcination temperatures post-Ni and post-Rh impregnation, respectively [96]. The Cat 600 600 catalyst exhibited the highest activity, H₂ yield, and stability, highlighting the importance of calcination temperature in performance [96].

A related study evaluated Pt-promoted Ni/MgAl₂O₄ catalysts, with Pt content ranging from 0 to 1.0 wt.% under atmospheric and high-pressure conditions [97]. Low Pt loadings significantly enhanced activity and stability, while higher Pt levels led to agglomeration and decreased catalytic activity. Pt addition also increased CO selectivity, thereby reducing the H₂/CO ratio [97].

Hydrogen production from SMR was also studied at 450 °C and 1 atm using Ni catalysts supported on modified alumina with Au or Ag [98]. Ni-Au bimetallic catalysts demonstrated improved activity and stability below 600 °C compared to Ni/Al₂O₃, whereas Ag addition negatively impacted catalytic performance [98].

An additional study compared various modified catalysts, such as Ni/Al₂O₃, Ni-Au/Al₂O₃, Ni-Ag/Al₂O₃, Ni/La₂O₃-Al₂O₃, and Ni/CeO₂-Al₂O₃, for hydrogen production and coke resistance [99]. Consistent with [98], Ag addition reduced catalytic performance. In contrast, Au, LaO₃, and CeO₂ enhanced CH₄ conversion, CO₂ selectivity, and H₂ production at temperatures below 600 °C. At 700 °C, catalytic activity showed minimal variation, except for a slight improvement in the Ni-Au/Al₂O₃ system [99].

A conflicting finding emerged in another study investigating Ag-promoted Ni/Al₂O₃ catalysts for SMR [100]. Catalysts containing > 0.3 wt.% Ag exhibited high coke resistance and stability, contradicting earlier conclusions from [98,99]. These discrepancies may stem from differences in catalyst preparation or testing conditions [100].

A subsequent study developed Ni-based catalysts optimized for millisecond-scale residence times to enhance process throughput and minimize reactor size [101]. Among three candidates, Ni/ZrO₂/Al₂O₃, Ni/La-Ca/Al₂O₃, and Ni_0.5Mg_2.5AlO₉, and the latter demonstrated superior activity and long-term stability at approximately 10 milliseconds residence time, showing comparable performance to Rh-based systems [101].

In another study, nano-NiO/SiO₂ catalysts supported on alumina were tested for SMR at various Ni loadings (5–15 wt.%) and calcination temperatures (350–500 °C) [102]. The 10 wt.% Ni catalyst calcined at 400 °C exhibited the highest CH₄ conversion and excellent stability [102]. Optimal operating conditions were 700 °C, an S/C ratio of 3.5, and a space-time of 11.31 kg cat/kmol CH₄, achieving 95.7% CH₄ conversion and an H₂ production rate of 3.8 mol/mol CH₄ [102].

La-Ni/α-Al₂O₃ catalysts with varying Ni/La ratios (7:3, 8:2, and 9:1) and a conventional 10 wt.% Ni/α-Al₂O₃ catalyst were also evaluated [103]. At 600 °C, the CH₄ conversion values converged. However, at 500 °C, 8Ni–2La/Al showed the highest CH₄ conversion (≈80%), while 7Ni-3La/Al was least active (≈60%) [103]. Interestingly, 7Ni-3La/Al achieved the highest H₂ yield at 800 °C [103].

A study explored the use of pyrochlore Y₂Zr₂O₇, prepared via glycine-nitrate combustion (GNC), hydrothermal synthesis (HT), and co-precipitation (CP), as a support for 10 wt.% Ni catalysts [104]. The Ni/Y₂Zr₂O₇–GNC catalyst exhibited the best performance in terms of activity, stability, and coke resistance [104]. The size of the Ni particles and the active surface area were key determinants of catalyst behavior [104].

The final study compared a homemade methanation catalyst (HMMC) with two commercial variants, i.e., with and without potassium doping, and its composition is presented in

Table 9. Compositions of the catalysts used in the test [105].

Catalyst	Support	NiO wt.%	K2O wt.%	SiO2 wt.%	Al2O3 wt.%
57-4Q	CaAl2O4	18	-	-	-
25-4Q	CaAl2O4	18	1.8	-	-
HMMC	SiO2-Al2O3	33.3	-	3.3	63.3

[105]. Catalysts containing K showed lower activity in SMR but improved CO₂ and H₂ selectivity at low temperatures [105]. The HMMC catalyst achieved an H₂ yield of 95% at 900 K but demonstrated stability only for short-term experiments, limiting its long-term applicability [105].

Table 10. Summary of catalyst used in the SMR process from the literature.

Catalyst	Advantages	Disadvantages	Ref.
Ni/Al2O3	Low carbon deposition	Deactivates easily	[8,91]
Ni/MgAl2O4	Maximum efficacy	-	[8]
Ni/SiO2	-	Lowest efficacy Deactivates easily Carbon deposition	[8]
Ni/SiO2Al2O3	Thermal stability Hydrogen selectivity Coke resistance	-	[106]
Ni2/Al2O5	High and stable activity	-	[106]
Ru/Ni6Al2O9	High CH4 conversion rate at high temperature	-	[95]
(Ni0.5M0.05Mg0.9)2Al (M = Fe or Cu)	High catalyst stability	-	[17]
Rh-Ni/Al2O4	High catalytic stability and activity High hydrogen yield	-	[96]
Ni-Au/Al2O3	Good catalytic stability and activity	-	[98]
Ni/La2O3-Al2O3 and Ni/CeO2-Al2O3 (at low temperature)	Improvement in methane conversion Improvement in hydrogen production Improvement in CO2 selectivity	-	[99]
Ni-Ag/Al2O3	High resistance to coke formation	Lower catalytic stability and activity	[98,99,100]

presents a summary of the advantages and disadvantages of the catalysts reviewed for the SMR process.

While a range of other metals, including noble and transition elements such as Rh, Ru, Ir, Pt, Au, Ag, Fe, Co, and Cu, have been investigated for steam methane reforming [12], this discussion focuses on Ni-based systems due to their dominant role in industrial applications and the extensive research dedicated to improving their performance. Enhancements have primarily involved the use of supports, promoters, and structural modifications, such as Rh- and Ru-promoted Ni/Al₂O₃ catalysts [94], Ni–Fe and Ni–Cu bimetallic alloys [17], and Ni–Au/Ni–Ag systems [98,99,100].

3.2. Steam Methane Reforming Technologies

There are plenty of technologies used in the SMR process, such as sorption-enhanced steam methane reforming (SESMR), membrane reactors (MR), wall coating steam methane reformers (WC-SMR), electrified steam methane reforming (eSMR), chemical looping steam methane reforming (CL-SMR), chemical looping sorption-enhanced steam methane reforming (CL-SESMR), and solar-assisted steam methane reforming (SASMR). These technologies are summarized in

Table 11. Summary of SMR technologies from the literature.

Technologies	Process Description	Advantages	Disadvantages	Ref.
SESMR	Use of a solid sorbent to remove carbon dioxide gases in hydrogen production from SMR	Lower GHG emissions	-	[18,107]
MR	Uses a membrane to separate the hydrogen from other composts	Low energy consumption Continuous separation Rigorous process conditions Integration with other separation technologies	Shorter lifetime Low flux or selectivity Fouling tendency	[13]
WC-SMR	Uses wall coating for the fuel processing in hydrogen production	Higher conversion rate Higher hydrogen production rate Increases residence time	-	[108]
eSMR	Uses electrified systems to produce the necessary heat for SMR	Operation flexibility Lower GHG emissions	-	[20,109,110]
CL-SMR	Uses oxidation and reduction reactions to produce hydrogen from SMR	Pure hydrogen at lower temperatures Reduces CO2 emissions	-	[20,111]
CL-SESMR	Combines both chemical lopping reforming and sorption-enhanced to produce hydrogen from SMR	Lower GHG emissions	-	[20,112]
SASMR	Uses solar thermal energy to provide heat for SMR	Lower GHG emissions	-	[20,113,114]

.

3.2.1. Sorption-Enhanced Steam Methane Reforming

To improve hydrogen production from SMR process and minimize carbon dioxide emissions, a sorbent can be used, such as lithium zirconate (Li₂ZrO₃) in its solid form, which was proven in its capability of capturing CO₂ [107,115]. The process, known as sorption-enhanced reaction (SER), with lithium zirconate as sorbent is represented with Equation (14), and it is an exothermic reaction with ${\Delta \mathrm{h}}_{298\mathrm{K}}^0=-160\mathrm{kJ}/\mathrm{mol}$ [107].

$${\mathrm{L}\mathrm{i}}_2\mathrm{Z}\mathrm{r}{\mathrm{O}}_3\left(\mathrm{s}\right)+{\mathrm{C}\mathrm{O}}_2\left(\mathrm{g}\right)\leftrightarrow {\mathrm{L}\mathrm{i}}_2{\mathrm{C}\mathrm{O}}_3\left(\mathrm{s}\right)+\mathrm{Z}\mathrm{r}{\mathrm{O}}_2\left(\mathrm{s}\right)$$

(14)

The SESMR approach presents a promising route for producing hydrogen with minimal carbon monoxide contamination [107]. Lithium zirconate demonstrated high CO₂ adsorption capacity under the tested conditions, although further enhancement of its sorption performance, particularly at lower CO₂ partial pressures, could improve the overall efficiency of the process [107].

Another study with the SER process used commercial dolomite, which was composed of 53.78 wt.% of CaCO₃, 45.89 wt.% of MgCO₃, and 0.01 wt.% of sulfur, in its solid form to obtain the sorbent used in the SMR reaction [18]. The preparation of the dolomite involves the sulfur removal to avoid catalyst poisoning and the calcination of CaCO₃ existent in the dolomite to form CaO, the compost that works as sorbent [18]. The reaction of carbon dioxide removal in the SMR process is presented in the Equation (15), and the overall reaction of the SMR process is presented in the Equation (16) [18].

$$\mathrm{C}\mathrm{a}\mathrm{O}\left(\mathrm{s}\right)+{\mathrm{C}\mathrm{O}}_2\left(\mathrm{g}\right)\leftrightarrow {\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3\left(\mathrm{s}\right)$$

(15)

$${\mathrm{C}\mathrm{H}}_4\left(\mathrm{g}\right)+2{\mathrm{H}}_2\mathrm{O}\left(\mathrm{g}\right)+\mathrm{C}\mathrm{a}\mathrm{O}\left(\mathrm{s}\right)\leftrightarrow 4{\mathrm{H}}_2\left(\mathrm{g}\right)+{\mathrm{C}\mathrm{a}\mathrm{C}\mathrm{O}}_3\left(\mathrm{s}\right)$$

(16)

The study showed that dolomite can be used as sorbent after sulfur removal, because it performed effectively [18].

3.2.2. Membrane Reactors

Membrane technology is increasingly employed in hydrogen production processes to enhance hydrogen yield and reduce the need for extensive downstream purification [13,116]. Membranes are also used in applications such as carbon capture, microfiltration, and reverse osmosis [13].

Among the various membrane types, palladium-based (Pd-) membranes are the most extensively studied due to their exceptional selectivity for hydrogen [13]. Hydrogen transport through these membranes occurs via a sorption–diffusion mechanism, which involves seven steps [117]. First, hydrogen molecules move from the bulk gas phase to the surface of the membrane, where they are adsorbed and dissociated into atomic hydrogen [117]. The atomic hydrogen then dissolves into the Pd membrane, diffuses through the metal lattice to the opposite side, precipitates at the surface, recombines into molecular hydrogen, desorbs, and finally leaves the membrane through the gas layer [117]. This sequence results in high-purity hydrogen production [13,117].

To prepare Pd membranes there are several processes, divided into chemical or physical processes, which consider the geometry and properties of the substrate and the thickness and Pd purity of the membrane [117]. The chemical processes involve various processes like chemical vapor deposition (CVD), electrochemical vapor deposition (EVD), electroplating (EP), electroless plating deposition (ELP), sol–gel technology (sol–gel), molecular layering (ML), spray pyrolysis, and solvated metal atom deposition [13,117]. On the other hand, physical processes involve conventional cold rolling, physical vapor deposition (PVD), sputtering, and magnetron sputtering (MS) [13,117].

These membranes also offer notable advantages, including excellent mechanical strength, high selectivity, and favorable adsorption and desorption characteristics [13].

Nevertheless, several limitations restrict their wider adoption. At temperatures below 300 °C, exposure to hydrogen can lead to embrittlement, compromising mechanical integrity. At temperatures above 450 °C, carbonaceous deposits may form, and exposure to sulfur compounds can cause irreversible poisoning of the membrane. Furthermore, the high cost of palladium significantly limits its commercial viability [13].

To overcome these drawbacks, Pd is commonly alloyed with other elements such as copper (Cu), gold (Au), nickel (Ni), silver (Ag), titanium (Ti), platinum (Pt), tungsten (W), indium (In), rhenium (Re), yttrium (Y), cerium (Ce), molybdenum (Mo), ruthenium (Ru), and rhodium (Rh) [13]. These alloying elements improve mechanical and thermal stability, reduce susceptibility to sulfur poisoning, and, in many cases, enhance or maintain hydrogen permeability while lowering the critical temperature for hydride formation [13,117]. As a result, Pd alloys are increasingly preferred over pure Pd membranes for their greater durability and operational resilience [13].

Among binary alloys, Pd-Ag is the most widely researched, offering good resistance to embrittlement and sulfur poisoning [13]. Pd-Cu alloys also exhibit strong embrittlement resistance even at lower temperatures and maintain performance up to 650 °C, although their properties depend on their phase structure [13]. The incorporation of Au into Pd suppresses hydride formation and enhances resistance to sulfur poisoning, while Pd-Ru alloys offer improved mechanical strength and thermal stability along with favorable hydrogen permeability [13]. Moreover, ternary alloys, such as Pd-Ag-Ru or Pd-Cu-Ag, show even better hydrogen permeability, strength, and resistance to poisoning than binary systems [13].

Other types of membranes have been developed, like porous inorganic membranes [117]. Porous inorganic membranes are valued in separation processes for their excellent stability under thermal and chemical stress as well as their ability to endure large pressure differences [117]. This type of membrane is a viable alternative to Pd-based membranes since they can distinguish gas by molecular size [117]. Several materials have been employed in the fabrication of such membranes, including zeolite, titanium dioxide, metal–organic frameworks, carbon, and silica [117]. Among these, silica membranes are especially promising for high-temperature applications due to their robustness, resistance to contaminants, high selectivity and permeability, and high cost effectiveness [117]. These structures are formed by SiO₄ tetrahedra linked through covalent bonds, creating a continuous 3D network with adjustable pore sizes, typically averaging around 0.3 nanometers [117]. Given the diameters of small molecules, like H₂ with 0.29 nanometers, silica membranes are capable of selectively filtering hydrogen [117].

Carbon capture techniques

There is a study about the application of MR in SMR to do carbon capture with the objective of reducing the emissions [111]. They studied the behavior at operation conditions that could be relevant for processes with CO₂ capture [118]. They concluded that the operating conditional will be a key factor to the design of the power process with CO₂ capture [118]. The utilization of some methane in the feed outlet and burning it with pure oxygen will still give the opportunity for CO₂ capture because the results of the combustion are water and carbon dioxide; the water will condense, and there will be a steam of almost pure carbon dioxide, which can be stored [118].

3.2.3. Wall Coating Steam Methane Reformers

One study investigated the effect of Ni-based catalyst deposition within a 20 mm long parallel-plate channel by evaluating two different configurations, referred to as Model A and Model B [108]. Model A consisted of a single, continuous catalytic zone 4 mm in length, symmetrically positioned in the channel, beginning 8 mm from both the inlet and outlet [108]. In contrast, Model B employed two 2 mm catalytic zones, separated by a 2 mm inert section, with the first catalytic zone starting at 7 mm from the channel inlet [108].

The results demonstrated that Model B, with its segmented catalytic zones, offered superior thermal performance [108]. It achieved a 28.71% improvement in methane conversion and reached 88.574% hydrogen production efficiency compared to Model A [108]. The study concluded that the use of multiple catalytic zones enhances steam methane reforming performance by increasing the residence time and consequently improving the hydrogen production rate [108].

3.2.4. Electrified Methane Reforming

Electrification of steam methane reforming (eSMR) involves the use of electricity to drive the reforming reaction and offers the potential to significantly reduce CO₂ emissions while addressing key limitations of conventional thermal processes [20,109]. Various electrification methods, including induction, microwave, ohmic, and plasma heating, enable the direct delivery of energy to the catalyst or reaction zone, enhancing process efficiency [109]. Among these, ohmic heating stands out due to its ability to support compact reactor designs, potentially reducing reactor volume by up to two orders of magnitude [109]. At small scales, heat transfer ceases to be the limiting factor in reaction kinetics [109].

Integrated ohmic heating enhances thermal control by establishing direct contact between the heat source and the catalyst bed, enabling rapid thermal response during startup and shutdown, often within minutes [109]. In contrast to conventional fired reformers, electrically heated systems exhibit minimal thermal gradients and can operate closer to thermodynamic limits, making them highly suitable for transient operation [109]. This operational flexibility is particularly advantageous when integrating with intermittent renewable electricity sources, thus supporting low-carbon hydrogen production aligned with variable energy availability [109].

A novel reactor configuration employing direct electrical heating for SMR was demonstrated using a silicon-infiltrated silicon carbide (SiSiC) foam coated with an Rh/Al₂O₃ catalyst [110]. This structure enabled direct Joule (ohmic) heating, resulting in methane conversions approaching thermodynamic equilibrium [110]. At temperatures exceeding 700 °C, nearly complete methane conversion was achieved [110]. The system also demonstrated favorable energy performance, achieving an energy efficiency of 61% and low specific power consumption for hydrogen production [110]. When powered by renewable electricity, this approach presents a promising route for decarbonized hydrogen production [110].

3.2.5. Chemical Looping Reforming

Chemical looping reforming (CLR) has been investigated as a strategy to address challenges associated with partial oxidation, such as the need for air separation and the inherent flammability of the process [111]. In the context of SMR, steam is used as the oxidant instead of oxygen, giving rise to a process known as chemical looping steam methane reforming (CL-SMR) [111]. The CL-SMR process involves cyclic oxidation and reduction steps to convert methane into hydrogen and syngas [20,111]. It offers the potential to produce high-purity hydrogen at relatively low temperatures while also reducing CO₂ emissions [111].

Efficiency improvements can be achieved by integrating CLR with sorption-enhanced reforming (SER), combining the strengths of both approaches [20,112]. This hybrid method has the potential to enhance overall process efficiency, address challenges related to sorbent regeneration, and further mitigate greenhouse gas emissions associated with conventional SMR [20,112].

3.2.6. Solar-Assisted Steam Methane Reforming

Steam methane reforming is an energy-intensive process that typically relies on fossil-fueled furnaces to supply the necessary thermal energy, leading to substantial non-renewable energy consumption and associated CO₂ emissions [113]. Solar-assisted SMR (SASMR) introduces solar energy as the primary heat source, providing a more sustainable and environmentally friendly alternative [20,113]. This approach represents a hybrid solution that preserves the high efficiency and technological maturity of traditional SMR while contributing to a reduction in greenhouse gas emissions [114].

3.3. Steam Methane Reforming Environmental Impact

One analysis showed that the steam methane reforming process had the highest environmental impact among the hydrogen production routes considered, with a GWP of 11.89 kg CO₂-eq and an AP of 14.52 g SO₂-eq per kilogram of hydrogen produced [79]. In another study, a GWP of 4.7 kg CO₂-eq/kg H₂ was found [119]. Due to the low value, the authors referenced additional sources, where a GWP of 10.6 kg CO₂-eq/kg H₂ was reported [119]. A more recent study, published in 2023, reported a GWP of 25 kg CO₂-eq/kg H₂ for SMR [23].

Another work focused on the environmental impact of natural gas steam reforming (NGSR) [120]. The authors initially used values of 11.893 kg CO₂-eq for GWP and 14.516 g SO₂-eq for AP per kilogram of hydrogen produced [120]. They then converted these indicators to classify the sustainability of different processes and concluded that NGSR ranked as the second least sustainable, ahead only of coal gasification [120].

Further studies presented varied GWP results. One reported values between 9.5 and 11.5 CO₂-eq/kg H₂ for SMR [118], while another indicated a GWP of 13.7 kg CO₂-eq/kg H₂ [121]. Lastly, one study reported that NGSR had a GWP ranging from 3 to 7 kg CO₂-eq/kg H₂ [122].

The lower heating value of hydrogen is 33.3 kWh/kg [123]. Based on this, the GWP and the AP can be converted from kg CO₂-equivalent and g SO₂-equivalent per kilogram of hydrogen to per MWh of hydrogen produced, as shown in

Table 12. Summary of environmental impact of SMR (or NGSR) from the literature.

GWP (kg CO2-eq/MWh H2)	AP (g SO2-eq/MWh H2)	Ref.
≈357.06	≈436.04	[79,123]
≈141.14 (or 318.32)	-	[119,123]
≈750.75	-	[23,123]
≈357.15	≈435.92	[120,123]
≈285.29 to 345.35	-	[114,123]
≈411.41	-	[121,123]
≈90.90 to 210.21	-	[122,123]

[123].

3.4. Steam Methane Reforming Process Costs

The production of hydrogen via steam methane reforming involves significant economic factors, including production and operational costs. One study reported that, as of 2019, the cost of producing one kilogram of hydrogen through SMR ranged from USD 1.54 to USD 2.30 [119]. More recent findings from a 2023 study indicated a slightly lower cost of USD 1.50 per kilogram of hydrogen produced [23].

Another analysis examined both the production cost and the social cost of carbon (SCC) associated with NGSR [120]. The SCC represents the estimated economic damage resulting from the emission of one additional unit of CO₂, accounting for long-term climate impacts [120]. This is defined as the marginal external cost linked to those emissions and reflects the difference in expected societal welfare between a baseline scenario and one influenced by increased CO₂ levels [120]. In that study, the production cost of hydrogen via NGSR was USD 11.44 per gigajoule of hydrogen, while the SCC was reported as USD 142,716 per kilogram of hydrogen [120]. The low heating value of hydrogen is 119.9 MJ/kg, so the NGSR production cost can be converted to, approximately, USD 1.37 per kg H₂ produced [123].

Additional studies have reported a range of operational costs. One source estimated USD 1 to USD 2 per kilogram of hydrogen produced via SMR [118]. Another study presented a cost range of USD 0.7 to USD 2.1 per kilogram specifically for NGSR [122].

A summary of the process costs for steam methane reforming (or natural gas steam reforming) is presented in

Table 13. Summary of the SMR (or NGSR) process costs from the literature.

Costs (USD/kg H2)	Costs (USD/GJ H2)	Costs (USD/MWh H2)	Ref.
1.54 to 2.30	-	≈46.25 to 69.07	[119,123]
1.50	-	≈45.05	[23,123]
-	11.44	≈41.19	[120,123]
1.00 to 2.00	-	≈30.03 to 60.06	[114,123]
0.70 to 2.10	-	≈21.02 to 63.06	[122,123]

, including a conversion from kilograms or gigajoule of hydrogen to megawatt-hour of hydrogen [123].

3.5. Discussion on Steam Methane Reforming

Steam methane reforming accounts for approximately 75% of global hydrogen production, reflecting its technological maturity and cost effectiveness. Production costs range from USD 21.02 to USD 69.07 per MWh of hydrogen, while the associated greenhouse gas emissions span from 90.90 to 750.75 kg CO₂-equivalent per MWh, underscoring the significant environmental impact of the process.

To reduce emissions and improve sustainability, several advanced configurations have been proposed. Electrified SMR (eSMR) and solar-assisted SMR (SASMR) substitute conventional combustion with renewable electricity or solar thermal energy, thereby lowering indirect CO₂ emissions from heat generation. Where decarbonizing the heat source is not feasible, sorption-enhanced SMR offers an effective alternative by incorporating in situ CO₂ capture, improving hydrogen yield while reducing environmental burden. Furthermore, SESMR, being an exothermic process, helps the SMR reaction by reducing the energy input needed for the process.

The selection of catalyst systems is equally critical to SMR efficiency. Catalysts must be tailored to process conditions, with considerations including reaction temperature, residence time, coke resistance, and thermal stability. Bimetallic catalysts, particularly those promoted with noble metals such as Rh, Ru, or Pt, have demonstrated superior activity and resistance to deactivation. However, their excessive cost poses a barrier to large-scale deployment, leading to ongoing research into optimized formulations that balance performance with economic viability.

All of this, along with a high S/C ratio, a high temperature, and a low pressure, will improve the hydrogen yield.

4. Comparative Analysis of CO₂ Methanation and SMR

Since the aim of the review is to analyze the processes of CO₂ Methanation and SMR,

Table 14. Comparative summary of the global warming potential caused by each of the processes.

GWP CO2 Methanation (kg CO2 eq/MWh CH4)		Ref.	GWP SMR (kg CO2 eq/MWh H2)	Ref.
H2 production from wind	22 (a)/104 (b)	[80]	≈357.06	[79,123]
H2 production from PV	108 (a)/191 (b)	[80]	≈141.14 (or 318.32)	[119,123]
H2 production from mix	994 (a)/1076 (b)	[80]	≈750.75	[23,123]
H2 production from wind	54	[81]	≈357.15	[120,123]
H2 production from mix	134	[81]	≈285.29 to 345.35	[114,123]
H2 production from wind and PV	−341(c,e)/−10 (d,e) (CLOU)	[82]	≈411.41	[121,123]
	−471(c,e)/−8.5 (d,e) (iG-CLC_mOC)	[82]	≈90.90 to 210.21	[122,123]

^(a) CO₂ with residue origin; ^(b) CO₂ with fossil origin; ^(c,d) Values with and without CO₂ storage, respectively; ^(e) CO₂ with biomass origin.

and

Table 15. Comparative summary of the costs of each process.

Costs of CO2 Methanation (per MWh CH4)	Ref.	Costs of SMR (per MWh H2)	Ref.
EUR 93.53	[83,84]	USD 46.23 to 69.07	[119,123]
CHF 300	[85]	USD 45.05	[23,123]
EUR 33.60 (a)	[86]	USD 41.19	[120,123]
EUR 204.62 (b)	[86]	USD 30.03 to 60.06	[114,123]
USD 94	[87]	USD 21.02 to 63.06	[122,123]

^(a) For electricity cost of EUR 0/MWh; ^(b) for electricity cost of EUR 100/MWh.

provide a comparative overview of the two in terms of global warming potential and production costs. The values reflect variations energy sources and process conditions, offering insight into the environmental and economic performance of each pathway.

The tables show that CO₂ methanation results in lower environmental emissions compared to SMR but is associated with higher production costs.

By presenting a comparative analysis of CO₂ methanation and steam methane reforming, this review offers a unified perspective of their thermochemical behavior, catalyst challenges, and techno-economic performance. This framework supports future research on integrated systems and provides insights into scaling up sustainable fuel production for industrial applications.

5. Conclusions

Due to the growing demand for green fuels, methane and hydrogen production via CO₂ methanation and steam methane reforming, respectively, have emerged as promising thermochemical routes.

CO₂ methanation stands out for its potential to mitigate greenhouse gas emissions by utilizing CO₂ as a feedstock, transforming a pollutant into a valuable fuel. Although the process currently incurs higher costs than conventional natural gas production, it supports a lower carbon footprint, aligning with global sustainability goals.

Conversely, SMR remains the dominant method for hydrogen production due to its lower cost, but it is a major source of CO₂ emissions. Integrating SMR with carbon capture technologies and next-generation catalysts is essential to reduce its environmental impact.

These two processes can be viewed as components of a reversible thermochemical cycle, offering a path towards a circular carbon economy. When combined with renewable energy inputs and effective CO₂ management, they present viable routes to achieving long-term carbon neutrality.