Heterogeneous Catalysts for Carbon Dioxide Methanation: A View on Catalytic Performance

Abstract

CO₂ methanation offers a promising route for converting CO₂ into valuable chemicals and energy fuels at the same time as hydrogen is stored in methane, so the development of suitable catalysts is crucial. In this review, the performance of catalysts for CO₂ methanation is presented and discussed, including noble metal-based catalysts and non-noble metal-based catalysts. Among the noble metal-based catalysts (Ru, Rh, and Pd), Ru-based catalysts show the best catalytic performance. In the non-noble metal catalysts, Ni-based catalysts are the best among Ni-, Co-, and Fe-based catalysts. The factors predominantly affecting catalytic performance are the dispersion of the active metal; the synergy of the active metal with support; and the addition of dopants. Further comprehensive investigations into (i) catalytic performance under industrial conditions, (ii) stability over a much longer period and (iii) activity enhancement at low reaction temperatures are anticipated to meet the industrial applications of CO₂ methanation.

1. Introduction

Fossil fuels have been considered the world’s principal energy resource since the early 1970s [1,2,3]. After the industrial revolution, using different energy sources led to global warming and climate change [4,5]. Oil industries, power production plants, cement factories, steel, building constructions, and iron manufacturers are regarded as major contributors to the rise in the emission of carbon dioxide (CO₂), which is a well-known greenhouse gas [6,7,8,9]. The sixth report of the Intergovernmental Panel on Climate Change (IPCC) states that global net anthropogenic emissions include a considerable amount of CO₂, equivalent to 75%, which is the byproduct of the world energy sector [10]. Moreover, anthropogenic CO₂ is a promoter of many other pollutants and a predominant global warming precursor [5,11,12]. Fossil fuels, industrial processes, land use change, and forestry are significantly responsible for releasing CO₂ into the environment, as shown in Figure 1a. Region-wise global cumulative net anthropogenic CO₂ is shown in Figure 1b [13]. The cumulative net anthropogenic analysis reveals North America is the lead emitter, releasing 23% of CO₂ into the environment from 1850 to 2019.

CO₂ emissions into the atmosphere are increasing every second, directly affecting global warming and climate change [6,10]. A record increase in CO₂ was reported at 424 parts per million (ppm) on a global average in May 2023, reported by the Mauna Loa Observatory, as shown in Figure 1c. If this ongoing upward trend persists, it could have a detrimental impact on the global average temperature [14]. The recorded average growth in CO₂ concentration in the last decade was 2 ppm per year [15]. The current and projected trends are identical, as many underdeveloped countries still produce 65% of their energy from the combustion of fossil fuels [16]. It is a harsh reality that we—mankind—are solely responsible for climate change and global warming [3]. Suppose that considerable actions are not taken right now to regulate and stabilize the emission of CO₂ into the atmosphere. In that case, a disastrous threat will come from ecosystem destruction due to heavy rain spells, tornadoes, and storms; flooding from low coastal glaciers melting; and a rise in sea levels [3,10]. Recently, there have been many severe ecological catastrophes in various regions of the globe, including Cyclones Idai and Biparjoy, lethal heatwaves and ambient pollution in the Indo-Gangetic Plain [6] and Europe, and flooding in Southeast Asia. From Mozambique to Bangladesh, millions of people have already lost their homes, livelihoods, and loved ones because of more dangerous and frequent extreme weather events [17].

Figure 1. Global trend in CO₂ emissions from 1850 to 2019: (a) net anthropogenic emissions [10], (b) net anthropogenic emissions per region [13], (c) global monthly CO₂ measured by Mauna Loa Observatory [18].

Figure 1. Global trend in CO₂ emissions from 1850 to 2019: (a) net anthropogenic emissions [10], (b) net anthropogenic emissions per region [13], (c) global monthly CO₂ measured by Mauna Loa Observatory [18].

Several international conferences, treaties, and summits have been held to propose a solution to mitigate this situation, such as the 1997 International Agreement on Climate Change and the 2005 Kyoto Protocol in collaboration with the UN, which was held to reduce CO₂ emissions [19]. At the UN Climate Change Conference in Paris in 2017, an agreement was reached to limit the global temperature rise to within 2 °C by minimizing CO₂ emissions [20]. In 2019, the Climate Change Action Summit committed to zero carbon emissions by 2050 [21]. In 2021, the G7 Climate Summit and the annual G20 Summit were conducted with the objective of tackling climate change and advancing the pursuit of carbon neutrality.

Numerous strategies have been proposed in the literature to reduce CO₂ emissions and combat climate change [22,23,24,25,26]. There are two prominent available routes: (i) carbon capture, utilization, and storage (CCUS) and (ii) substituting conventional fuels with renewable energy sources. Both routes can be combined by using CO₂ with renewable power sources, i.e., solar and wind to produce fuel [24,27]. Wind and solar energy are intermittent and variable energy sources, posing challenges to long-term storage. However, renewable energy can produce hydrogen via water electrolysis, for which the storage of hydrogen is also a crucial task. Thus, to address these issues effectively and promote environmentally friendly solutions, utilizing CO₂ alongside renewable energy for fuel production, such as methane (CH₄), emerges as a compelling green alternative. Fuel can consist of hydrocarbons and oxygen (O₂)-containing hydrocarbons, among which, CH₄ holds particular significance.

CH₄ can be produced by utilizing CO₂ and hydrogen (H₂) through CO₂ methanation, also known as the Sabatier process, which is an efficient and appropriate procedure for CO₂ utilization [28,29,30]. The extant natural gas infrastructure may be used for the CH₄ produced from CO₂ methanation. Thus, this strategy has the potential for large-scale deployment. By integrating CO₂ capture, H₂ from renewable energy, and CO₂ methanation, we can make significant strides in reducing CO₂ emissions, transitioning to a low-carbon economy, and mitigating the impacts of climate change [28,31,32].

For the conversion of CO₂ into CH₄, the largest impediment is developing a highly active and stable catalyst suitable for a cost-effective and large-scale industrial chemical process. To the best of our knowledge, there has been no comprehensive review that covers recent advancements in catalytic performance for the catalysts of CO₂ methanation, including activity, selectivity, and stability. Therefore, the primary focus of this review is to address the roadmap of catalytic performance for CO₂ methanation catalysts by tackling the challenges associated with developing industrial catalysts for CO₂ methanation. The first section provides a basis for understanding the CO₂ methanation reaction by explaining the thermodynamic equilibrium and reaction mechanisms. The second and third sections review the catalytic performance of noble and non-noble metal catalysts and factors that influence the catalytic performance.

2. Reaction Mechanism and Thermodynamic Equilibrium

2.1. CO₂ Methanation Reaction

In 1872, renowned scientist Brodie demonstrated the reduction of CO₂ into CH₄ [16]. Paul Sabatier and Jean Baptiste Senderens analyzed the same results using a heterogeneous catalyst in 1902. Later, in 1912, the Nobel Prize was awarded to Sabatier for CO₂ hydrogenation into CH₄ (the Sabatier reaction) with a well-dispersed catalyst [23,28,33].

The Sabatier reaction is a highly exothermic reaction with eight electron processes with considerable kinetic limitations [34,35]. Catalysts are necessary to overcome the activation barriers and achieve maximum CO₂ conversions at low enough temperatures, which is recommended based on the equilibrium conditions [34,35]. To prepare an effective and stable catalytic system, it is crucial to understand the reaction mechanism and its intermediate process [34]. For CO₂ methanation, the reaction step and the nature of the reaction intermediate are still being discussed in the literature [36,37,38,39,40,41,42].

The CO₂ methanation reaction, characterized by several elementary steps, displays varying reaction mechanisms that can be investigated through diverse in situ spectroscopic techniques and DFT theoretical calculations, contingent on the catalyst type. The process initiates with CO₂ adsorption onto the carrier, leading to interactions with hydroxyl groups, forming bicarbonate species. Concurrently, H₂ adsorbs onto metal sites, dissociating into H species. This step is followed by the hydrogenation of bicarbonate to yield formate species [43,44]. As depicted in Figure 2a, these formate species undergo hydrogenation into CH₄ via three distinct pathways.

The first pathway involves the further reduction of formate into adsorbed CO, aided by H in facilitating CO dissociation, thereby forming the (HCO_ad) intermediate. This intermediate is then subjected to a series of consecutive hydrogenation reactions, culminating in CH₄ formation. Zhang et al. [43] identified the cleavage of the C=O bond in the (HCO_ad) intermediate on the Ni/CeO₂ (111) surface as the critical step in this process. Macroscopically, the primary reaction can be described as CO₂ undergoing RWGS to convert into CO, followed by hydrogenation into CH₄; hence, it is termed the RWGS + CO Hydro pathway.

The second pathway is analogous to the first in its initial steps and diverges as (CO_ad) dissociates directly into (C_ad) and O, with (C_ad) undergoing direct hydrogenation into CH₄, bypassing other intermediates. This is known as the CO pathway. Xinyu et al. [45] noted that the active sites on the Ni catalyst facilitate H₂ molecule dissociation into H atoms. They proposed that, on Ni/ZrO₂, CO₂ is activated by O₂ vacancies and transformed into adsorbed CO, which, along with H₂ activation on Ni, leads to CH₄ generation. Ren et al. [44] suggested that, on Ni (111) surfaces, the direct hydrogenation of CO₂ is the most efficient methanation route, with the CO* → C* + O* transition as the rate-limiting step.

Figure 2. Reaction mechanism of CO₂ methanation: (a) reaction pathway; (b) thermodynamic equilibrium impact of pressure and temperature on CO₂ methanation [46].

Figure 2. Reaction mechanism of CO₂ methanation: (a) reaction pathway; (b) thermodynamic equilibrium impact of pressure and temperature on CO₂ methanation [46].

The formate pathway, also referred to as the direct pathway, involves the direct interaction of CO₂ and H₂, resulting in the formation of a formate intermediate (HCOO) on the catalyst surface, as illustrated in Figure 2a. Notably, no CO intermediates are formed during this process [41,45]. A study conducted by Pan et al. [47] determined that the CO₂ methanation reaction pathways on Ni/γ-Al₂O₃ and Ni/CeZrO₂ catalysts both followed the formate pathway but exhibited variations in terms of reactive basic sites. On the Ni/CeZrO₂ catalyst, CO₂ adsorption primarily occurred on medium basic sites, resulting in the formation of bidentate formate, whereas CO₂ adsorption on surface O₂ led to the creation of monodentate formate. The faster hydrogenation of monodentate formate suggested its dominance as the primary reaction route on the Ni/CeZrO₂ catalyst. In contrast, on Ni/γ-Al₂O₃, the principal reaction pathway involved the hydrogenation of bidentate formate, with the strong basic sites of Ni/γ-Al₂O₃ not actively participating in the CO₂ methanation reaction. It was proposed that medium basic sites play a crucial role in facilitating the formation of monodentate formate species, thereby enhancing the overall CO₂ methanation activity.

2.2. Thermodynamic Equilibrium Conversion and Selectivity

Several thermodynamic studies have examined the effects of reaction parameters on CO₂ methanation [34,36,48,49,50,51].

$${\mathrm{C}\mathrm{O}}_2+{4\mathrm{H}}_2\to {\mathrm{C}\mathrm{H}}_4+{2\mathrm{H}}_2\mathrm{O}\hspace{1em}∆\mathrm{H}\mathrm{R}=-165 \mathrm{k}\mathrm{J} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$

(1)

$${\mathrm{C}\mathrm{O}}_2+{\mathrm{H}}_2\to \mathrm{C}\mathrm{O}+{\mathrm{H}}_2\mathrm{O}\hspace{1em}∆\mathrm{H}\mathrm{R}=41 \mathrm{k}\mathrm{J} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$

(2)

$$\mathrm{C}\mathrm{O}+{3\mathrm{H}}_2\to {\mathrm{C}\mathrm{H}}_4+{\mathrm{H}}_2\mathrm{O}\hspace{1em}∆\mathrm{H}\mathrm{R}=-206 \mathrm{k}\mathrm{J} {\mathrm{m}\mathrm{o}\mathrm{l}}^{-1}$$

(3)

Understanding the CO₂ methanation reaction significantly relies on thermodynamic equilibrium. The CO₂ methanation reaction is a combination of the deceleration of exothermic (CO methanation) and endothermic (water gas shift) reactions, as shown in Equations (1)–(3) [52,53].

Lower temperatures are more favorable for the methanation reaction, leading to an increase in CO₂ conversion and an improvement in CH₄ selectivity [46,54]. The CO₂ methanation process exhibits the highest productivity at low temperatures, resulting in nearly 100% CO₂ conversion and CH₄ selectivity [55]. However, at higher temperature surpassing 500–600 °C, the dynamics shift toward a reversed water gas shift (RWGS) reaction, which results in a decrease in catalytic efficiency (depicted in Figure 2) due to carbon deposition [50,56].

In CO₂ methanation, the number of molecules decreases from five for reactants to three for products; therefore, the rise in pressure has a positive impact on CO₂ conversion, mainly when operating within temperature ranges of 200–500 °C [57]. High pressure is also favorable for CH₄ selectivity [46]. The effect of pressure can also be seen in Figure 2b, which presents thermodynamic equilibrium results. For the catalytic reaction, when the desorption of the product is the rate-limiting step, high pressure can suppress the reaction. Moreover, high pressure can increase equipment costs. CO₂ conversion and CH₄ selectivity are significantly influenced by the ratio of H₂ to CO₂. Higher ratios of H₂/CO₂ are associated with an increase in both the conversion of CO₂ and the selectivity toward CH₄ at 1 and 30 atm. Specifically, at a H₂/CO₂ ratio of two, the conversion of CO₂ ranges between 50% and 70% at both 1 atm and 30 atm. In these conditions, the selectivity for CH₄ reaches a maximum of 73% at 1 atm and 88% at 30 atm. Moreover, in this stoichiometric ratio, the yield of CH₄ can be observed to be around 40% at 1 atm, increasing marginally to 45% at 30 atm. The optimal conversion of CO₂ into CH₄ is achievable under conditions of low temperature and high pressure, provided that the H₂/CO₂ ratio is meticulously controlled. The enthalpy, entropy, and Gibbs free energy in standard conditions of both reactants and products are provided in

Table 1. Enthalpy, entropy, and Gibbs free energy of reactants and products [57].

Substance	Enthalpy (ΔH)	Entropy (ΔS)	Gibbs Free Energy (ΔG)
	J/mol	J/mol.K	J/mol
CO2	−393,509	214	−394,359
H2	0	130.7	0
CH4	−74,520	186.4	−50,460
H2O	−241,818	188.3	−228,572

.

3. The Catalytic Performance of Noble Metal Catalysts in CO₂ Methanation

Numerous studies on using noble and non-noble metal catalysts to convert CO₂ into CH₄ have been proposed, and many of these have shown high activity and selectivity [58,59]. Ruthenium (Ru) and rhodium (Rh) show high activity with a comparatively small amount of metal loading at low temperatures [60]. Among the noble metals, Ru is the most active and stable catalyst for CO₂ methanation, next to Rh and Pd in order of activity [61]. Results from the representative published literature describing the patterns of activity, selectivity, stability, and corresponding reaction conditions are presented in Table 2.

3.1. Ruthenium-Based Catalysts

The catalytic performance of alumina-supported ruthenium (Ru/Al₂O₃) was investigated by Garbarino et al. [62], who observed that supported Ru catalysts are more active and stable than other metal-based catalysts. The supported Ru catalysts show excellent activity, especially at low temperatures, while the selection of the best catalyst preparation method and activation need to be carefully managed [63]. Ru/Al₂O₃ demonstrates high catalytic activity and good stability at 375 °C for 100 h, exhibiting 91% conversion and standing at 91% CH₄ selectivity. The superior activity and stability were attributed to the high dispersion of Ru nanoparticles (NPs) and interactions between Ru NPs and the Al₂O₃ support. The consistent and stable high activity observed after different shutdown and start-up sequences suggests that the catalyst retains its effectiveness even under intermittent operation [62].

Ru doped with ceria is one of the most promising methanation catalysts owing to the essential nature of the O₂ vacancies on the surface of CeO₂, which can activate CO₂. For Ru/Al₃O₂, adding ceria can improve the activity [64]. For Ce-doped Ru/Al₃O₂ catalysts, initial Ru NPs can be redispersed during oxidative pretreatment into atomically distributed RuOx species owing to interactions between Ru and ceria. The re-dispersion of Ru NPs on CeO₂ was also reported by Aitbekova et al. [65], and the re-dispersion could maintain the high activity of Ru NPs.

This reaction route was proposed to belong to the CO route, with CO₂ activated by O₂ vacancies and then converted into adsorbed CO; the CO and H₂ were activated by Ru to generate CH₄.

Besides CeO₂ and Al₂O₃, TiO₂ is also a promising support for loading Ru [66]. Researchers have demonstrated that TiO₂-supported Ru NP catalysts possess good stability, which is attributable to the unique interaction between metal Ru and support TiO₂. The CO₂ conversion can maintain stability for 34 h running, with 68% of CO₂ conversion and 98% selectivity to CH₄ at 290 °C and 1 atm. For Ru/TiO₂ catalysts, the catalytic performance relies on metal–support interactions, the loading amount of Ru, and the particle size of Ru NPs [67,68,69,70,71].

Table 2. Recent stable noble metal catalyst development for CO₂ methanation.

Catalyst	Preparation Methods	Metal Loading (%)	XCO2 (mol%)	SCH4 (mol%)	Period of Stable Running (h)	Reaction Conditions		Ref.
						GHSV h−1/WHSV (mL.g−1h−1)	T (°C)
Ruthenium-based catalysts
Ru/Al2O3	Commercial catalyst	3	91	91	100	55,000 * H2:CO2 = 5:1	375	[62]
Ru/CeO2	Hydrothermal	5	86	100	30	30,000	300	[72]
Ru-Ni/Ce0.6Zr 0.4O2	Deposition precipitation	3/30	98	100	300	24,000	230	[73]
Ru-CeO2/Al2O3	Impregnation	2	60	99	-	10,000	300	[74]
Ru/TiO2	Impregnation	2.5	90	99	50	6000	350	[75]
Ru/TiO2	Impregnation	5	68	98	34	7580	290	[67]
Ru/UiO-66	Impregnation	1	60	100	160	19,000	250 ** 5	[76]
Ru/TiO2 (001)	Solvothermal hydrolysis	2.5	80	100	168	6000	325	[66]
Ru/CeO2/r	Hydrothermal	3.7	75	99	24	72,000	350	[77]
Ru/CeO2	Thermal deposition	-	83	90	14	-	225	[78]
Ru@MIL-101	Hydrothermal	-	90	99	48	-	225	[69]
Rhodium-based catalysts
Rh/TiO2	Impregnation	1	90	96	3	12,000	370 ** 2	[79]
Rh/γAl2O3	Wet impregnation	1	25	100	-	-	125	[80]
RhY	Ion-exchange	6	59	99.8	2	60,000 * H2:CO2 = 3:1	150	[81]
Rh/CeO2	Impregnation	3	~46	~100	-	-	350	[82]
Rh/Al2O3	Impregnation	1	25	100	-	-	250	[57]
Rh/PSAC	Impregnation	2	54	73	2	10,000	207	[83]
PdRuNi/Al2O3	Impregnation	2/8/90	53	40	5		400	[84]
RuRh-γ Al2O3	Impregnation	0.5/0.5	80	100	-	6000	250	[85]
Ni-Rh/Al2O3	Co-impregnation	10/0.5	65	92	4	57,000	300	[86]
Palladium-based catalysts
Pd/UiO-66	Sol–gel	6	56	97.3	-	15,000	340	[87]
Pd/Al2O3	Impregnation	5	-	40	-	45,000	280	[88]
PdO/LaCoO3	One-pot	3	62	99	2	18,000	300	[89]
PdO/LaCoO3	Impregnation	3	32	87	2	18,000	300	[89]
Pd-Mg/SiO2	Microemulsion	6.2	59	95	9	7320	450	[90]
Pd@FeO	Seeded growth	5.2	98	100	20 Rounds	-	180	[91]
NiPd/Al2O3	Co-impregnation	10–0.5	91	99	4	57,000	300	[86]

Notes: Reaction gas composition is H₂:CO₂ = 4:1, otherwise noted as *. Reaction Pressure is 1 bar, otherwise noted as **.

Table 2. Recent stable noble metal catalyst development for CO₂ methanation.

Catalyst	Preparation Methods	Metal Loading (%)	XCO2 (mol%)	SCH4 (mol%)	Period of Stable Running (h)	Reaction Conditions		Ref.
						GHSV h−1/WHSV (mL.g−1h−1)	T (°C)
Ruthenium-based catalysts
Ru/Al2O3	Commercial catalyst	3	91	91	100	55,000 * H2:CO2 = 5:1	375	[62]
Ru/CeO2	Hydrothermal	5	86	100	30	30,000	300	[72]
Ru-Ni/Ce0.6Zr 0.4O2	Deposition precipitation	3/30	98	100	300	24,000	230	[73]
Ru-CeO2/Al2O3	Impregnation	2	60	99	-	10,000	300	[74]
Ru/TiO2	Impregnation	2.5	90	99	50	6000	350	[75]
Ru/TiO2	Impregnation	5	68	98	34	7580	290	[67]
Ru/UiO-66	Impregnation	1	60	100	160	19,000	250 ** 5	[76]
Ru/TiO2 (001)	Solvothermal hydrolysis	2.5	80	100	168	6000	325	[66]
Ru/CeO2/r	Hydrothermal	3.7	75	99	24	72,000	350	[77]
Ru/CeO2	Thermal deposition	-	83	90	14	-	225	[78]
Ru@MIL-101	Hydrothermal	-	90	99	48	-	225	[69]
Rhodium-based catalysts
Rh/TiO2	Impregnation	1	90	96	3	12,000	370 ** 2	[79]
Rh/γAl2O3	Wet impregnation	1	25	100	-	-	125	[80]
RhY	Ion-exchange	6	59	99.8	2	60,000 * H2:CO2 = 3:1	150	[81]
Rh/CeO2	Impregnation	3	~46	~100	-	-	350	[82]
Rh/Al2O3	Impregnation	1	25	100	-	-	250	[57]
Rh/PSAC	Impregnation	2	54	73	2	10,000	207	[83]
PdRuNi/Al2O3	Impregnation	2/8/90	53	40	5		400	[84]
RuRh-γ Al2O3	Impregnation	0.5/0.5	80	100	-	6000	250	[85]
Ni-Rh/Al2O3	Co-impregnation	10/0.5	65	92	4	57,000	300	[86]
Palladium-based catalysts
Pd/UiO-66	Sol–gel	6	56	97.3	-	15,000	340	[87]
Pd/Al2O3	Impregnation	5	-	40	-	45,000	280	[88]
PdO/LaCoO3	One-pot	3	62	99	2	18,000	300	[89]
PdO/LaCoO3	Impregnation	3	32	87	2	18,000	300	[89]
Pd-Mg/SiO2	Microemulsion	6.2	59	95	9	7320	450	[90]
Pd@FeO	Seeded growth	5.2	98	100	20 Rounds	-	180	[91]
NiPd/Al2O3	Co-impregnation	10–0.5	91	99	4	57,000	300	[86]

Notes: Reaction gas composition is H₂:CO₂ = 4:1, otherwise noted as *. Reaction Pressure is 1 bar, otherwise noted as **.

In one study using metal–organic frameworks (MOFs) with Ru metal as a precursor and silica nanofibrous veils (MIL-101) as the support, the resulting Ru@MIL-101 catalyst showed good catalytic performance with a CO₂ conversion of 90% and a selectivity to CH₄ of 99%, and it maintained stability for 48 h under conditions of 1 bar and 225 °C [69]. The high activity is attributable to the high dispersion of Ru NPs and the stability owes to the interaction between Ru and the silica support [69].

The best catalytic performance is shown on Ru-Ni/Ce_0.6Zr_0.4O₂ with 3 wt.% Ru loading: CO₂ conversion reaches 98%, the CH₄ selectivity is 100%, and the stability period is 300 h under reaction conditions of a GHSV/WHSV of 24,000 (mLg⁻¹h⁻¹), 230 °C, and 1 bar [73].

3.2. Rhodium-Based Catalysts

For Rh-based catalysts, the best performance is shown on Rh/TiO₂ with 1 wt.% of Rh loading, which exhibits 90% CO₂ conversion and 96% selectivity to CH₄ with only 3 h of stability running under reaction conditions of a GHSV/WHSV of 12,000 h⁻¹ (mL.g⁻¹h⁻¹), 370 °C, and 2 bar [79]. Considering the high reaction temperature and pressure, the activity of Rh is inferior to that of Ru; furthermore, investigations on the stability of Rh-based catalysts for CO₂ methanation have not been extensively covered in the literature.

Moreover, the activity of Rh-based catalysts has been improved by adding other active metals, such as Ru-Rh/Al₂O₃, showing better activity than mono Rh [85], and Ni-Rh/Al₂O₃ also exhibits better activity than Rh/Al₂O₃ [86].

Catalytic performance may be affected by the O₂ presence; O₂ in a low percentage boosts the catalyst’s performance, whereas a higher concentration leads to a negative effect [56,92]. Martin et al. [82] investigated the effects of support on Rh for CO₂ methanation reactions; Rh NPs were loaded on SiO₂, Al₂O₃, and CeO₂, and they found that CeO₂ and Al₂O₃ exhibited better activity than SiO₂. Furthermore, researchers have discovered that the catalytic performance of Rh/Al₂O₃ catalysts is dependent on Rh particle sizes ranging from 3.6 nm to 15.4 nm [49,59].

TiO₂ is one of the best options for supporting Rh catalysts. The effects of Rh/TiO₂ on catalytic performance at low temperatures were investigated by Alejandro et al. [93], who found that large Rh particle sizes have more active sites and weak CO intermediates, which also affects the order of CO₂ methanation reaction and activation energies. Notably, a slight variation in the activation energy of CO dissociation with Rh particle size was observed. Higher response orders in H₂ were also seen for smaller particles, indicative of reduced H₂ coverage. CH₄ selectivity gradually increased with a change in particle size (from 2 nm to 7 nm), while, beyond this particle size, there was no discernible difference [93].

It has been observed that Rh/TiO₂ catalysts have higher CH₄ selectivity as compared with Rh/Al₂O₃ and Rh/SiO₂. The literature suggests that the breakdown of the C-O bond could be aided by electron interactions between metal and supports or interactions between CO absorbed by a catalyst and Ti 3⁺ ions positioned at the border metal support [79,93].

Jiang et al. [79] further investigated the influence of different oxide supports (TiO₂, Al₂O₃, and ZnO) on the catalytic activity of Rh catalysts in CO₂ methanation and indicated that the selection of a suitable support significantly affects the performance of Rh catalysts. Rh/TiO₂ exhibited the highest catalytic activity over a 6 h duration stability, followed by Rh/Al₂O₃ and Rh/ZnO. Variations in the electronic structure of the metallic Rh and its interaction with oxide supports can be linked to this discrepancy in activity. The TiO₂ support of Rh-based catalysts enhances electron transport to Rh NPs, resulting in more marked CO dissociation and catalytic activity. In contrast, electron transport happens to the support, or it remains localized in Rh/ZnO and Rh/Al₂O₃ catalysts, resulting in a decrease in catalytic efficiency. These findings highlight the significance of electronic structure modulation and support effects in designing efficient CO₂ conversion catalysts.

Research on Rh/TiO₂ by Martnez et al. [68] showed that changes in activity depend on interactions between Rh NPs and the support. The average particle size after H₂ reduction at a high temperature is substantially smaller than the size of calcined Ru in the presence of synthetic air and then further heated at 300 °C, which was confirmed with TEM and XRD [68]. The interaction can make Rh re-disperse, which leads to the high dispersion state of Rh NPs, resulting in higher CO₂ conversion.

Scientists have turned to innovative mesoporous Rh NPs synthesized using a wet chemical reduction technique [59]. The mesoporous Rh catalyst is more active than the nonporous Rh catalyst in the CO₂ methanation reaction. The high density of atomic steps on the mesoporous Rh catalyst is responsible for its higher catalytic activity with 99% CO₂ conversion and 96% CH₄ selectivity [94].

3.3. Palladium-Based Catalysts

For Pd-based catalysts, the best catalytic performance was shown on Pd@FeO with a Pd loading amount of 5.2 wt.%, showing 98% CO₂ conversion and 100% selectivity to CH₄ at a low temperature of 180 °C, and the catalytic performance was maintained for 20 rounds of cyclic running [91]. The high activity was attributed to the face-centered tetragonal structure of the Pd-Fe intermetallic nanocrystal, and it was proposed that Pd-Fe intermetallic nanocrystals aided in maintaining metallic Fe species during CO₂ methanation via a reversible oxidation-reduction mechanism; thus, adding metallic Fe facilitated the direct conversion of CO₂. This study was efficient on a laboratory scale, but for practical applications, the loading amount of Pd is too high, and the stability needs to be investigated.

The bimetallic catalysts of Ni-Pd supported on Al₂O₃ show better activity with 0.5 wt.% and 10 wt.% loadings of Pd and Ni, respectively. In one study, a CO₂ conversion of 91% and 99% CH₄ selectivity was achieved under reaction conditions of a GHSV/WHSV of 57,000 (mL.g⁻¹h⁻¹), 300 °C, and 1 atm, and it remained stable for 4 h [86].

Jiang et al. [87] investigated Pd NPs supported on MOFs and found a synergistic interaction between the metal and the support, which improved the catalyst’s performance and exhibited higher CO₂ conversion and CH₄ selectivity at 340 °C with 6 wt.% Pd loading compared with individual Pd NPs or UiO-66. Apart from the high loading of Pd, the metal–organic compound UiO-66 is unstable for long periods in CO₂ methanation reactions because CO₂ reacts with organics [95,96].

It was reported that, on Pd/Al₂O₃ catalysts, CO₂ methanation is a structure-sensitive reaction; as noted by Wang et al. [93], by stabilizing CO species on the terrace sites of Pd NPs, CH₄ generation can be effectively enhanced. They proposed that the arrangement of Pd atoms on the surface affects the adsorption strength of reactants and CO intermediates, resulting in significantly boosted CH₄ selectivity.

3.4. Summary of Performance of Noble Metal Catalysts

Studies on noble metal catalysts for CO₂ methanation have mainly concentrated on Ru-, Rh-, and Pd-based catalysts, among them Ru based catalysts exhibited the best catalytic performance. For all catalysts, selectivity to CH₄ can be very good, especially at low reaction temperatures, so the challenge is to enhance the activity at low temperatures. When the loading amount of noble metal is as high as 3–6 wt.%, much higher CO₂ conversion can be achieved, although the high price means it has promise in practical applications. Ru-based catalysts have shown comparatively good stability in maintaining stability for 300 h running, but stability investigations of Rh- and Pd-based catalysts are needed.

Research on noble metal catalysts for CO₂ methanation has predominantly focused on Ru-, Rh-, and Pd-based catalysts; among them, Ru-based catalysts demonstrate superior catalytic performance. These catalysts generally exhibit high CH₄ selectivity, particularly at low reaction temperatures. However, enhancing activity at low temperatures remains a challenge. Notably, when the noble metal loading reaches 3–6 wt.%, a significant increase in CO₂ conversion can be observed. Nonetheless, the economic feasibility of such high loadings is questionable given the consequent increase in costs, limiting practical applications. While Ru-based catalysts have shown relatively better stability, maintaining catalytic performance for 300 h, the stability of Rh- and Pd-based catalysts needs to be further investigated.

The catalytic performance of noble metal catalysts has been found to be dependent on metal dispersion, metal–support interactions, and morphology, and bimetallic or intermetallic nanocrystals seem to be a promising route for improving catalytic performance.

4. The Catalytic Performance of Non-Noble Metal Catalysts in CO₂ Methanation

Extensive research has been conducted to identify alternative catalyst materials given the scarcity and high costs of noble metals. Non-noble metals, mainly Ni, Co, and Fe. have acquired considerable attention for their potential in CO₂ methanation [97,98,99]. In the following sections, the application of non-noble metal catalysts, nickel (Ni), cobalt (Co), and iron (Fe), in the Sabatier reaction are investigated. Research is being conducted in order to optimize their catalytic performance, optimize their activity, and investigate the factors affecting their stability.

4.1. Nickel-Based Catalysts

Ni-based catalysts have gained significant attention in CO₂ methanation. The research emphasis has been on enhancing its activity in low reaction temperatures and improving the sintering resistance of Ni NPs because of the inert nature of CO₂ and the strong exothermic activity of the methanation reaction. In designing a promising catalyst for industrial applications, the critical factors of catalytic performance are the properties of Ni NPs, including dispersion and its chemical state, metal–support interactions, and additive materials. Supports such as Al₂O₃ [62,100], SiO₂ [64,101], TiO₂ [102,103], ZrO₂ [104,105], CeO₂ [106,107], and the solid solution Ce-Zr-O [108] are predominantly utilized for loading Ni NPs. The additives mostly employed are alkaline earth metals, such as La, Y, and Ce; the basic element Mg; the transition element Mn; and so on. Catalyst synthesis methods are diverse, with the principal objectives being to increase Ni NP dispersion and/or adjust interactions between Ni NPs and the support or additives. Examples of the catalytic performance of Ni-based catalysts are presented in Table 3.

Al₂O₃ is the most frequently applied catalyst support for CO₂ methanation [109,110]. Al₂O₃ is extensively used owing to its high surface area, adjustable porous structure, and complex chemistry properties [99,111,112]. Therefore, Al₂O₃ is not suitable for loading Ni NPs for CO₂ methanation, but reports on Ni/Al₂O₃ for CO₂ methanation are significant [113,114]. An issue with using Al₂O₃ as a support for the methanation reaction is that it tends to sinter when exposed to water (a byproduct of the process) at high temperatures [99].

The activity of Ni/Al₂O₃ is not good enough; with a loading of 12.5 wt.% Ni on Al₂O₃, CO₂ conversion could reach 71% at a high temperature of 500 °C [115], and the reason for the poor activity can likely be attributed to the poor CO₂ activation ability of Al₂O₃. When adding an additive to activate CO₂, such as ceria, the activity could be improved effectively over Ni/CeO₂-Al₂O₃; 71% CO₂ conversion and 99% selectivity to CH₄ could be achieved at a low temperature of 350 °C, at reaction conditions of a GHSV/WHSV of 15,000 (mL.g⁻¹h⁻¹) and 1 atm [116].

Silica (SiO₂) is another popular choice since it has a large surface area and may adjust its pore diameter to suit a given application [117,118,119]. Ni and SiO₂ form metal–support interactions, which are antagonistic to the growth of Ni carbide, which leads to the improved catalyst’s resistance to coke production and Ni sintering. [120,121]. In the literature, CO₂ methanation with a SiO₂-supported catalyst has shown a CO₂ conversion efficiency of only about 60% to 90% [90,122,123,124,125], which may be due to the inertness of SiO₂; therefore, the additive addition or surface modification of the support needs to be further investigated [126,127].

Moghaddam et al. [128] demonstrated that Ni NPs loaded on a composite support made of Al₂O₃-SiO₂ showed much better catalytic performance at 350 °C. A CO₂ conversion of 82% and a CH₄ selectivity of 98% were achieved, and constant performance was maintained for over 30 h. The good activity and stability of this catalyst can be attributed to the formation of highly dispersed Ni NPs and interactions between Ni NPs and the support, respectively [128].

Similar to studies that used alumina-supported Ni catalysts, Li et al. used Mg as a promoter for Ni/SiO₂; the resultant catalyst showed obviously improved activity with 82% CO₂ conversion and 99% selectivity to CH₄ under reaction conditions of a GHSV/WHSV of 60,000 (mL.g⁻¹h⁻¹), 250 °C, and 1 atm [129]. SiO₂ with a high surface area and a mesoporosity of MCM-41 is sufficient support for CO₂ methanation. A high surface area favors the high dispersion of Ni NPs, and mesoporosity is beneficial for reactant transfer [110,130].

For example, Ni/MCM-41 showed 56% CO₂ conversion and 98% selectivity to CH₄ under reaction conditions of a GHSV/WHSV of 50,000 (mL.g⁻¹h⁻¹), 250 °C, and 1 atm [131]. At the same time, it has been observed that improved resistance to coke formation can be achieved with a high surface area, and it is also known that carbon deposition formation can be suppressed on highly dispersed Ni NPs supported on SiO₂[132].

Table 3. Recent developments in non-noble catalysts.

Catalyst	Preparation Methods	Metal Loading	XCO2	SCH4	Period of Stability	Reaction Conditions		Ref.
		(%)	(mol%)	(mol%)	(h)	GHSVh−1/WHSV mL.g−1h−1	T (°C)
Nickel-based catalysts
Ni/Al2O3	Impregnation	20	80	100	10	9000	350	[133]
Ni/Al2O3	Improved one-pot EISA	10	60	95	60	10,000	400	[134]
Ni/Al2O3	Impregnation	30	71	95	24	30,000 * H2:CO2:Ar = 61:15: 21	350 ** 2	[135]
Ni/Al2O3-SiO2	Sol–gel	30/0.5	82	98	30	12,000 H2:CO2 = 3.5:1	350	[128]
Ni-Co/Al2O3	Improved one-pot EISA	10/3	70	96	60	10,000	400	[134]
NiFe/Al2O3	One-pot-induction EISA		84.1	100	150		420	[136]
Ni-Pt/γ-Al2O3	Co-impregnation	10–0.5	83	97	60	5700	250	[86]
Ni-Pd/γ-Al2O3	Co-impregnation	10–0.5	91	97	30	5700	250	[86]
Ni/Al2O3-ZrO2	Sol–gel	20	76	100	100	20,000	300	[137]
Mn-Ni/Al2O3	Impregnation	1.71	66	100	-		300	[138]
Ni-Pr/Al2O3	Impregnation	12–5	98	100	48	6000	300	[139]
Ni-Ce/Al2O3	Impregnation	15–15	70	98	80	30,000	350	[116]
NiO/SiO2	Sol–gel	60	86	95	48	10,000	350	[140]
Ni-La2O3/SiO2	Citric complex	7.7	89	90	400	15,000	500	[141]
NiLaMoO3/SiO2	Citric complex	6	87	100	320	15,000	350	[121]
Ni/MSN	Sol–gel	5	64	100	200	50,000	300	[130]
Ni/MOF-5	Impregnation	10	75	100	100	2000	320	[142]
Ni/MSN	Impregnation	5	64	100	100	50,000	300	[131]
Ni@HZSM-5	Hydrothermal	20	64	99	40	36,000	400	[143]
Ni/OMA	Impregnation	15	87	98	150	91,000 * H2:CO2 = 5:1	400	[144]
Ni/CNT	Co-impregnation	12	61	97	100	30,000	350	[145]
Ni/3D-SBA-15	Impregnation	15	86	99	100	60,000	400	[146]
Ni/C.Z.	Pseudo sol–gel	5	80	99	90	43,000	350	[37]
HTNiCu	Co-precipitation	15	86	98	72	12,000 * H2:CO2:Ar = 6:1.5:2.5	350	[147]
Ni-Ce-Y/SBA-15	CTAB-assisted impregnation	15–10	61	97	26	_	350	[148]
NiMg/USY	Ion exchange	13–9	65	92	10	12,000 * H2:CO2:Ar = 36:9:10	400	[149]
Ni-Mn/Bn-U	Solution combustion synthesis		85	100	150	36,000	270	[150]
Ni-La	Urea hydrolysis	15	85	94	150	45,000	350	[151]
Ni-La	Impregnation	13.6–14	90	100	8	55,000 H2:CO2 = 5:1	350	[152]
Ni/TiO2	Deposition precipitation	15	80	96	81	18,000 * H2:CO2:Ar = 12:3:5	340	[153]
Ni/Y2O3	Impregnation	10	80	100	50	20,000	300	[110]
Ni/ZrO2	Impregnation	15	60	100	50	48,000	300	[154]
Ni/ZrO2	Plasma decomposition	5	79	77	70	60,000	350	[45]
Ni/ZrO2	Impregnation	10	74	71	10	60,000	400	[45]
Ni/CeO2	Sol–gel	20	81	96	106	40,000	250	[155]
Ni/CeO2	Hard template	10	91	100	11	22,000	340	[107]
Ni/CeO2-	Impregnation	10	93	100	14	10,000	350	[64]
Ni-Ce0.2Zr0.8O2	Citric Complex	15	71	100	200	15,000	250	[108]
Ni-Ce0.72Zr0.28O2	Sol–gel	15/0.6	90	98	150	21,000	350	[156]
Ni/CeO2-ZrO2	Ammonia evaporation	10	55	99.8	70	20,000	275	[157]
Ni/CeO2-ZrO2	Colloidal dispersion	2	58	98	60	60,000 * H2:CO2:He = 12:3:5	350	[158]
Ni/CeO2-ZrO2	Pseudo-sol–gel	5	68	98		43,000	350	[37]
Ni/Ce0.6O2Zr0.4O2	Co-precipitation	15/0.6	71	86	83	12,500	300	[159]
Ni/Ce0.75Zr0.25		25	85	—	30.		300	[160]
Ni-La2O3CeO2/ZrO2	Citric Complex	-	80	100	160	15,000	350	[161]
Ni/ZrO2-Al2O3	Co-impregnation	12	70	100	100	8100	360	[113]
Ni/CeO2-Al2O3	Co-impregnation	15	71	99	120	15,000	350	[116]
Ni/CeO2-Al2O3	Co impregnation	13	85	99	120	15,000	350	[162]
Cobalt- and iron-based catalysts
Co-Pt/Al2O3	Double flame spray Pyrolysis	0.03	70	98	210	36,000	400	[163]
NiCo/Al2O3	Impregnation	20	90	100	200	13,000	325	[164]
Ni-Co/Al2O3	Solid-phase synthesis	15–12.5	76	96	10	9000	400	[165]
Ni-Co/Al2O3	Impregnation	10–10	61	95	200	13,000	350	[164]
NiCoMgZnMn/Al2O4	Citric complex	6.8/9.8	64	100	320	15,000	350	[166]
Co3O4 nanorod	Co-precipitation		70	99	50	18,000 H2:CO2 = 4:6	330 ** 10	[167]
Co/KIT-6	Excess impregnation	20	48.9	100	-	22,000	280	[33]
CoNR/TiO2	Modular synthesis	-	57	100	72	18,000	250 ** 10	[168]
(Co0.95 Ru0.05)O4	Modified wet chemistry protocol	-	34.2	97.4	50	21,240	420	[167]
PrCoPal	Impregnation		68	95	200	150,000	350	[169]
Co/ZrO2	Wetness impregnation	10	92	99	300	3600	400 ** 30	[170]
Co/ZrO2	Citric complex	2	85	99	300	7200	400 ** 30	[171]
Co-Zr0.1B-O	Liquid-phase synthesis	-	78	98	12	20,000	180	[172]
NiFe/Al2O3	Impregnation	13–9	84.1	100	150	50,000	420	[136]
NiFe/Al2O3	One-pot sol–gel	30–5	71	99	10	9000	350	[173]
NiFe	Single-step co-precipitation	3–0.5	78		30	20,000	200	[174]
Ni8Fe2	Co-precipitation	30	80	95	200	10,000	150	[175]

Notes: Reaction gas composition is H₂:CO₂ = 4:1, otherwise noted as *. Reaction Pressure is 1 bar, otherwise noted as **.

Table 3. Recent developments in non-noble catalysts.

Catalyst	Preparation Methods	Metal Loading	XCO2	SCH4	Period of Stability	Reaction Conditions		Ref.
		(%)	(mol%)	(mol%)	(h)	GHSVh−1/WHSV mL.g−1h−1	T (°C)
Nickel-based catalysts
Ni/Al2O3	Impregnation	20	80	100	10	9000	350	[133]
Ni/Al2O3	Improved one-pot EISA	10	60	95	60	10,000	400	[134]
Ni/Al2O3	Impregnation	30	71	95	24	30,000 * H2:CO2:Ar = 61:15: 21	350 ** 2	[135]
Ni/Al2O3-SiO2	Sol–gel	30/0.5	82	98	30	12,000 H2:CO2 = 3.5:1	350	[128]
Ni-Co/Al2O3	Improved one-pot EISA	10/3	70	96	60	10,000	400	[134]
NiFe/Al2O3	One-pot-induction EISA		84.1	100	150		420	[136]
Ni-Pt/γ-Al2O3	Co-impregnation	10–0.5	83	97	60	5700	250	[86]
Ni-Pd/γ-Al2O3	Co-impregnation	10–0.5	91	97	30	5700	250	[86]
Ni/Al2O3-ZrO2	Sol–gel	20	76	100	100	20,000	300	[137]
Mn-Ni/Al2O3	Impregnation	1.71	66	100	-		300	[138]
Ni-Pr/Al2O3	Impregnation	12–5	98	100	48	6000	300	[139]
Ni-Ce/Al2O3	Impregnation	15–15	70	98	80	30,000	350	[116]
NiO/SiO2	Sol–gel	60	86	95	48	10,000	350	[140]
Ni-La2O3/SiO2	Citric complex	7.7	89	90	400	15,000	500	[141]
NiLaMoO3/SiO2	Citric complex	6	87	100	320	15,000	350	[121]
Ni/MSN	Sol–gel	5	64	100	200	50,000	300	[130]
Ni/MOF-5	Impregnation	10	75	100	100	2000	320	[142]
Ni/MSN	Impregnation	5	64	100	100	50,000	300	[131]
Ni@HZSM-5	Hydrothermal	20	64	99	40	36,000	400	[143]
Ni/OMA	Impregnation	15	87	98	150	91,000 * H2:CO2 = 5:1	400	[144]
Ni/CNT	Co-impregnation	12	61	97	100	30,000	350	[145]
Ni/3D-SBA-15	Impregnation	15	86	99	100	60,000	400	[146]
Ni/C.Z.	Pseudo sol–gel	5	80	99	90	43,000	350	[37]
HTNiCu	Co-precipitation	15	86	98	72	12,000 * H2:CO2:Ar = 6:1.5:2.5	350	[147]
Ni-Ce-Y/SBA-15	CTAB-assisted impregnation	15–10	61	97	26	_	350	[148]
NiMg/USY	Ion exchange	13–9	65	92	10	12,000 * H2:CO2:Ar = 36:9:10	400	[149]
Ni-Mn/Bn-U	Solution combustion synthesis		85	100	150	36,000	270	[150]
Ni-La	Urea hydrolysis	15	85	94	150	45,000	350	[151]
Ni-La	Impregnation	13.6–14	90	100	8	55,000 H2:CO2 = 5:1	350	[152]
Ni/TiO2	Deposition precipitation	15	80	96	81	18,000 * H2:CO2:Ar = 12:3:5	340	[153]
Ni/Y2O3	Impregnation	10	80	100	50	20,000	300	[110]
Ni/ZrO2	Impregnation	15	60	100	50	48,000	300	[154]
Ni/ZrO2	Plasma decomposition	5	79	77	70	60,000	350	[45]
Ni/ZrO2	Impregnation	10	74	71	10	60,000	400	[45]
Ni/CeO2	Sol–gel	20	81	96	106	40,000	250	[155]
Ni/CeO2	Hard template	10	91	100	11	22,000	340	[107]
Ni/CeO2-	Impregnation	10	93	100	14	10,000	350	[64]
Ni-Ce0.2Zr0.8O2	Citric Complex	15	71	100	200	15,000	250	[108]
Ni-Ce0.72Zr0.28O2	Sol–gel	15/0.6	90	98	150	21,000	350	[156]
Ni/CeO2-ZrO2	Ammonia evaporation	10	55	99.8	70	20,000	275	[157]
Ni/CeO2-ZrO2	Colloidal dispersion	2	58	98	60	60,000 * H2:CO2:He = 12:3:5	350	[158]
Ni/CeO2-ZrO2	Pseudo-sol–gel	5	68	98		43,000	350	[37]
Ni/Ce0.6O2Zr0.4O2	Co-precipitation	15/0.6	71	86	83	12,500	300	[159]
Ni/Ce0.75Zr0.25		25	85	—	30.		300	[160]
Ni-La2O3CeO2/ZrO2	Citric Complex	-	80	100	160	15,000	350	[161]
Ni/ZrO2-Al2O3	Co-impregnation	12	70	100	100	8100	360	[113]
Ni/CeO2-Al2O3	Co-impregnation	15	71	99	120	15,000	350	[116]
Ni/CeO2-Al2O3	Co impregnation	13	85	99	120	15,000	350	[162]
Cobalt- and iron-based catalysts
Co-Pt/Al2O3	Double flame spray Pyrolysis	0.03	70	98	210	36,000	400	[163]
NiCo/Al2O3	Impregnation	20	90	100	200	13,000	325	[164]
Ni-Co/Al2O3	Solid-phase synthesis	15–12.5	76	96	10	9000	400	[165]
Ni-Co/Al2O3	Impregnation	10–10	61	95	200	13,000	350	[164]
NiCoMgZnMn/Al2O4	Citric complex	6.8/9.8	64	100	320	15,000	350	[166]
Co3O4 nanorod	Co-precipitation		70	99	50	18,000 H2:CO2 = 4:6	330 ** 10	[167]
Co/KIT-6	Excess impregnation	20	48.9	100	-	22,000	280	[33]
CoNR/TiO2	Modular synthesis	-	57	100	72	18,000	250 ** 10	[168]
(Co0.95 Ru0.05)O4	Modified wet chemistry protocol	-	34.2	97.4	50	21,240	420	[167]
PrCoPal	Impregnation		68	95	200	150,000	350	[169]
Co/ZrO2	Wetness impregnation	10	92	99	300	3600	400 ** 30	[170]
Co/ZrO2	Citric complex	2	85	99	300	7200	400 ** 30	[171]
Co-Zr0.1B-O	Liquid-phase synthesis	-	78	98	12	20,000	180	[172]
NiFe/Al2O3	Impregnation	13–9	84.1	100	150	50,000	420	[136]
NiFe/Al2O3	One-pot sol–gel	30–5	71	99	10	9000	350	[173]
NiFe	Single-step co-precipitation	3–0.5	78		30	20,000	200	[174]
Ni8Fe2	Co-precipitation	30	80	95	200	10,000	150	[175]

Notes: Reaction gas composition is H₂:CO₂ = 4:1, otherwise noted as *. Reaction Pressure is 1 bar, otherwise noted as **.

Zhen et al. [142] investigated the effect of O₂ vacancies on the reaction mechanisms of Ni NPs supported on SiO₂ with a high surface area and found that O₂ vacancies could be formed on the surface of the SiO₂ support, which could activate CO₂ to then improve the catalytic performance.

The distinctive catalytic performance of SiO₂ with a high surface area loaded with Ni NPs has naturally inspired researchers to investigate zeolite as a support for CO₂ methanation, as it is known that a key characteristic of silica zeolite is a high surface area. Chen et al. [143] used a conventional Ni/SiO₂ as a precursor-prepared core–shell catalyst for Ni@HZSM-5 via the hydrothermal method. Compared with traditionally prepared Ni/SiO₂ and Ni/HZSM-5 catalysts, Ni@HZSM-5 exhibited superior performance, preserving its Ni content and the structure of the active Ni after a 40 h CO₂ methanation reaction. A key feature of this catalyst is the interaction between the Ni active phase and zeolite, with the former donating more electrons to the latter, thus preventing sintering and enhancing the activity of Ni. At 400 °C, the Ni@HZSM-5 catalyst demonstrated a CO₂ conversion of 64% and near 100% CH₄ selectivity under reaction conditions of a GHSV/WHSV of 36,000 (mL.g⁻¹h⁻¹), 400 °C, and 1 atm [131].

Dong and Liu [146] developed a three-dimensional-ordered (3D) macroporous SBA-15-loaded Ni catalyst. As is known, SBA-15 is a silica zeolite, and making SBA-15 into a 3D macroporous can lead to a high surface area accompanied by macropores for transferring. The resultant catalyst showed high catalytic performance for CO₂ methanation, achieving a CO₂ conversion of 80% and selectivity to CH₄ of 98% under reaction conditions of GHSV/WHSV of 36,000 (mL.g⁻¹h⁻¹), 400 °C, and 1 atm, and it maintained its performance over an extended period of 120 h.

Generally, SiO₂ is an inert support; therefore, adding basic elements could help to activate CO₂. Gong et al. [141] added La₂O₃ to Ni/SiO₂ for CO₂ methanation and achieved a CO₂ conversion of 89% and a selectivity to CH₄ of 90% under reaction conditions of a GHSV/WHSV of 15,000 (mL.g⁻¹h⁻¹), 500 °C, and 1 atm, and it remained stable for 400 h. The addition of lanthanum improved the dispersion and reducibility of the Ni ions, which significantly improved its activity. The SiO₂ support provided good stability and improved resistance to carbon deposition. The addition of SiO₂ as a support, along with lanthanum, considerably increased catalytic activity and stability.

Zirconia is a promising support for industrial applications because of the basic properties of ZrO₂, which can activate CO₂. Moreover, ZrO₂ possesses high thermal stability and can be prepared with high porosity and a large surface area [110]. Ni-supported ZrO₂ aids in forming active sites and increases O₂ vacancies on ZrO₂, which are also crucial for CO₂ methanation [171,176]. According to the literature, the dispersion of Ni NPs and particle reductions are supported by the high specific surface area of ZrO₂. CO₂ methanation reactions will vary depending on the polymorphic structure of the support (monoclinic, tetragonal, or cubic). Tetragonal ZrO₂ is the functional structure as catalyst support [177]. Preparing tetragonal ZrO₂ is challenging given its structure sensitivity. O₂ vacancies in m-ZrO₂ are more abundant, making it an effective catalyst for the CO₂ methanation reaction than t-ZrO₂ [105].

Tan et al. [178] prepared highly dispersed Ni NPs supported by MgO-doped ZrO₂; the resultant catalyst exhibited outstanding activity in CO₂ methanation, a CO₂ conversion of 90%, and a selectivity to CH₄ of 100% under reaction conditions of a GHSV/WHSV of 15,000 (mL.g⁻¹h⁻¹), 250 °C, and 1 atm, and it remained stable for 110 h. Ren et al. showed that 30% Ni supported on ZrO₂ could convert around 90% of CO₂ at 250 °C with 95% selectivity toward CH₄ [179]. High activity and selectivity were observed in Ni supported on t-ZrO₂ for methanation reactions between 200 and 300 °C. Catalytic activity rises with increasing t-ZrO₂ content, and the highest CH₄ selectivity can be achieved at 300 °C using tetragonal ZrO₂ prepared with Ni-Zr alloy [92].

CeO₂ is another extensively investigated support or additive for CO₂ methanation, owing to the fact that O₂ vacancies can be formed on the surface of ceria and the fact that the interaction between Ni NPs and ceria can promote a reduction in Ni ions and the dispersion of Ni NPs [64,80,107,155,156,180,181,182,183,184,185,186]. Ye et al. [155] conducted a comprehensive study on using a nanostructured Ce-supported Ni-based catalyst prepared using the sol–gel method for CO₂ methanation. The findings highlight the extraordinary performance of this catalyst at a relatively modest temperature of 250 °C, yielding a CO₂ conversion ratio of 81% and a CH₄ selectivity of 96%, which could be maintained for 106 h. The good stability was attributed to the interaction between Ni NPs and CeO₂.

Both TiO₂ [64] and MOF [142] materials as support for CO₂ methanation have also gained some attention. The activity of Ni/TiO₂ is generally inferior compared with Ni/ZrO₂. For MOFs and carbon materials, the stability is not good enough because CO₂ can react with these materials.

Tada et al. conducted experiments comparing Ni NPs loaded on different supports made of CeO₂, Al₂O₃, TiO₂, and MgO [64]. They found that Ni/CeO₂ exhibited maximum CO₂ conversion across the usual temperature range, approaching an equilibrium value above 300 °C. Selectivity for CH₄ was nearly maximized at 100%. The catalyst performance of Ni/CeO₂ was attributed to the adsorption of CO₂ on O₂ vacancies in ceria and the high dispersion of Ni NPs [187]. The trend of the activity for different supports is as follows under the same reaction conditions: Y₂O₃ > Sm₂O₃ > ZrO₂ > CeO₂ > Al₂O₃ > La₂O₃.

A solid solution of Ce-Zr-O, which can be obtained via solid reactions between CeO₂ and ZrO₂, is the best support for loading Ni NPs for CO₂ methanation [186,187,188,189,190]. Sun et al. [108] analyzed how Ni/CeZrO₂ is the most predominant catalyst for CO₂ methanation reactions. Ni NPs on Ce-Zr-O solid solutions convert CO₂ more efficiently than pure CeO₂ and ZrO₂, of which Ni/CeO_0.2Zr_0.8O₂ catalyst has the highest CO₂ conversion rate (14% at 200 °C) and CH₄ selectivity (100% at 250 °C). Furthermore, Ni NPs supported on Ce-Zr-O solid solutions showed better stability than Ni NPs on CeO₂ or ZrO₂ [108].

Li et al. [161] investigated CO₂ methanation employing cerium-modified Ni supported on lanthanum oxide and zirconia (Ni-La₂O₃-CeO₂/ZrO₂). Their study focused on improving catalytic activity and stability. The study found that the addition of cerium to Ni/La₂O₃-ZrO₂ can enhance the dispersion and reducibility of Ni NPs, leading to higher catalytic activity and stability for 160 h.

Considering the comparatively high price, rare earth metal oxides such as Y₂O₃, Sm₂O₃, CeO₂, and La₂O₃ are not suitable for use as supports. In view of their characteristics, some of the reported supports, such as La₂O₃ and MgO, are unstable; preparing high surface area CeO₂ is a challenge; etc. However, the oxides of these are likely good additives. Thus, promising catalysts for CO₂ methanation for industrial applications are Ni NPs loaded on Al₂O₃, SiO₂, or ZrO₂, accompanied by some additives.

4.2. Cobalt-Based and Iron-Based Catalysts

Co-based catalysts are the most well-studied active component of Fischer–Tropsch synthesis (FTS), which generates hydrocarbons from syngas (a gas mixture of CO and H₂), as CO₂ can be converted into CO via reverse water gas shift reactions; therefore, studying Co-based catalysts for CO methanation is expected [191,192]. However, according to several studies, only a select few Co-based catalysts can concurrently achieve high CO₂ conversion, selectivity to CH₄, and stability, as shown in Table 3 [167,168,193,194,195].

Tu et al. [172] reported in their investigation that adding a Zr promoter allowed an amorphous Co-Zr_0.1-B-O catalyst to initiate CO₂ methanation at temperatures as low as 140 °C. Maximum catalytic activity was achieved at 180 °C with 78% CO₂ conversion and 98% CH₄ selectivity, but the stability exhibited only lasted for 12 h.

In another study, Co/ZrO₂ showed good catalytic performance with 92% CO₂ conversion and 99% selectivity to CH₄, and this performance was maintained for 300 h under reaction conditions of a GHSV/WHSV of 36,000 (mL.g⁻¹h⁻¹), 400 °C, and 30 atm [170]. Considering the low space velocity, high pressure, and comparatively high temperature in these reaction conditions, the activity is very good but not excellent compared with Ni-based catalysts, although the stability seems very good.

Another study on Co-ZrO₂ for CO₂ methanation indicated that decreasing the particle size of Co, increasing Co dispersion, and strengthening the interaction between Co and ZrO₂. can improve activity, attributable to interactions between Co and ZrO₂, possibly generating more catalytically reduced active sites in Co and more O₂ vacancies, resulting in higher CO₂ adsorption (owing to O₂ vacancies) and high catalytic hydrogenation (owing to Co) [167].

Recent research on CO₂ methanation has revealed that adding a small number of catalytic promoters (Cu, Fe, La, Pr) significantly increases the catalyst’s performance and stability, as determined in experimental and DFT analyses. Using a straightforward impregnation technique, a series of lanthanide-modified Co-palygorskite composites (LnCoPal, Ln–La/Ce/Pr/Sm) for CO₂ methanation under atmospheric pressure were developed. The CO₂ conversion of the PrCoPal catalyst was 68%, the selectivity to CH₄ was 95%, and it exhibited 200 h of stability under reaction conditions of a GHSV/WHSV of 15,000 (mL.g⁻¹h⁻¹), 350 °C, and 1 atm [169].

The most promising Co-containing catalysts for CO₂ methanation are bimetallic Ni-Co-based catalysts. Alrafei et al. [164] investigated a bimetallic 10%Ni-10%Co/Al₂O₃ catalyst prepared according to incipient wetness impregnation, and the catalyst exhibited very good performance in converting CO₂ into CH₄, with a 61% conversion rate and a 95% selectivity achieved at a relatively low temperature of 350 °C, and it maintained stability for 200 h with GHSV/WHSV of 13,000 (mL.g⁻¹h⁻¹) and 1 atm. The good stability was attributed to the interaction between the Ni-Co alloy NPs and the Al₂O₃ support (which inhibited the clustering of active Ni-Co alloy NPs) and to the Ni-Co alloy’s resistance to carbon deposition. Incorporating Co into the Ni catalyst enhanced the reducibility of the Ni species and the distribution of Ni particles across the support, which increased activity.

High-entropy oxides (HEOs), defined as single-phase oxide systems containing a minimum of five distinct cations, exhibit unique properties advantageous to catalytic CO₂ methanation. The incorporation of diverse cations within the same lattice phase leads to a synergistic interaction, significantly enhancing their catalytic activity [165,166]. The cocktail effect leverages the diverse chemical environments created by interactions between cations. Additionally, the high dispersion of these elements within the lattice, thermal stability, an abundance of surface defects, and the synergistic effects of multielement interactions position HEOs as effective catalyst carriers. Significantly, HEOs are proving to be highly effective as precursors for catalysts, primarily because of their exceptional elemental dispersion. Chen et al. [165] incorporated platinum into a CoNiMgCuZnOx-based HEO via co-deposition, resulting in a catalyst with notable activity in CO oxidation and remarkable thermal stability, a testament to the entropic stability inherent to HEOs. The recent research conducted by Liu et al. [166] investigated the application of a remarkably stable bimetallic catalyst comprising Ni-Co supported on alumina-coated spinel oxide, derived from HEO (CoNiMgZnMg/Al₂O₄), for CO₂ methanation. The study revealed the exceptional stability and elevated catalytic efficiency of this Ni-Co/alumina–spinel catalyst for CO₂ methanation.

A viewpoint proposed in this paper needs to be noted: the active metals Co and/or Ni can be oxidized into metal ions, resulting in the deactivation of the catalyst. Different from CO methanation, in the process of CO₂ methanation, CO₂ can oxidize metallic NPs, and the resulting metal ions may react with the support to form a composite oxide, such as Co ions reacting with alumina to form CoAl₂O₄ spinel.

In the context of Fe-based catalysts, research indicates that pure Fe exhibits limited effectiveness in converting CO₂ into CH₄. However, when Fe is combined with Ni to form a bimetallic or alloy structure, the resulting combinations often perform better than pure Ni catalysts [36,196].

In a study by Moghaddam et al. [173], Ni/Al₂O₃ catalysts were prepared using the one-pot sol–gel method, with small amounts of additional elements such as Fe, Co, Cu, Zr, and La added. The catalyst containing Fe demonstrated exceptional performance, achieving 71% CO₂ conversion and nearly 99% selectivity for CH₄ at 350 °C, a GHSV/WHSV of 9000 (mL.g⁻¹h⁻¹), and 1 atm. This improvement can be attributed to the presence of a Ni-Fe alloy, which enhanced the adsorption of H₂ and the dissociation of CO₂. Interestingly, increasing the Fe content from 5 to 7 wt.% resulted in enhanced activity at lower temperatures and maintained stability over a 10 h period.

Yin et al. [174] investigated the effects of adding Fe to Ni-based catalysts for low-temperature CO₂ methanation. Different amounts of Fe were introduced, and it was found that a small amount (Ni3Fe0.5) significantly improved the catalyst’s performance, achieving a CO₂ conversion of 78% at 200 °C, a GHSV/WHSV of 20,000 (mL.g⁻¹h⁻¹), and 1 atm. However, an excessive amount of Fe (1.5 wt.%) led to the formation of a detrimental NiFe₂O₄ spinel phase. The optimal amount of Fe addition was determined to be 0.5 wt.%, effectively enhancing the reducibility and basicity of the catalytic system. This facilitated CO₂ adsorption and activation, resulting in improved CO₂ methanation activity, which exhibited stability for 30 h.

In a recent study by Lan et al. [175], the Ni-to-Fe ratio was examined in catalysts prepared under an external magnetic field. The catalyst with the highest performance for CO₂ methanation had a Ni-to-Fe ratio of 8/2 (Ni8Fe2). This particular catalyst achieved a conversion rate of over 80% of CO₂ into CH₄ at 150 °C, along with a CH₄ selectivity of 95% under controlled reaction conditions, including a GHSV of 10,000 (mL.g⁻¹h⁻¹) and 1 atm. Furthermore, the Ni8Fe2 catalyst maintained stable activity for 200 h.

The addition of Fe increases the surface area and reduces the size of Ni crystals. Consequently, Fe-promoted catalysts exhibit improved CO₂ conversion and CH₄ selectivity compared with unpromoted catalysts. Notably, these catalysts also show enhanced resistance to carbon formation. However, it is important to note that further research is needed to fully understand the synergistic effects of Ni and Fe in these catalysts and to elucidate the underlying reaction mechanisms over Fe-based catalysts.

4.3. Summary of Performance of Non-Noble Catalysts

Non-noble catalysts for CO₂ methanation include Ni-, Co-, and Fe-based catalysts; among all catalyst systems, Ni-based catalysts show the best catalytic performance. The selectivity to CH₄ is generally high (close to 100%), especially at low reaction temperatures (lower than 400 °C), and they have revealed very good activity. Excellent Ni-based catalysts can reach or come close to equilibrium conversion at around 350 °C and 1 atm with a WHSV of 10,000 mL.g⁻¹h⁻¹ or even higher, and the stability generally extends to several hundred hours. For Co- and Fe-based catalysts, some show very high activity at low temperatures, though the stability and selectivity compared with the corresponding catalyst is possibly not good enough. Co- and Fe-based catalysts are well known for being used as FTS catalysts, which means that hydrocarbons besides CH₄ can be easily generated, thus leading to decreased selectivity to CH₄.

In this study, the catalytic performance of noble metal catalysts was found to be dependent on metal dispersion, metal–support interactions, and additive doping. Various catalyst configurations, including supports made of single-oxide supports (Al₂O₃, TiO₂, ZrO₂, and CeO₂), composite oxide supports (Ce-Zr-O), basic oxide promoters, and bimetallic systems, were investigated to determine their impact on catalytic activity and stability. These results suggest that Ni-based catalysts are the most promising.

The development of an industrially viable catalyst for CO₂ methanation requires rigorous evaluations of its activity, selectivity, and stability. Achieving 100% CO₂ conversion to CH₄, maintaining stability for extended periods (3–4 years), and operating under typical industrial pressures (2–5 MPa) are crucial goals. To assess catalyst performance, we connected experimental data from the literature, presented in Table 2 and Table 3, and corresponding explanations are provided above. For noble metal catalysts, Ru-based catalysts are the best; the activity and selectivity are very good, but the stability lasts only for 300 h in the most stable catalyst. The critical obstacle for industrial application would be the high price and high noble metal loading in these catalysts. For non-noble metal catalysts, Ni-based catalysts are the best; the activity and selectivity are also very good, and as for stability, many studies have shown that stability lasts for 100 to 400 h. It seems that the stability of Ni-based catalysts is promising.

It should be noted that carbon deposition for CO₂ methanation is rare; it is likely that carbon deposition for CO₂ methanation is not severe because CO₂ can eliminate the deposited carbon by reacting with the carbon to produce CO and H₂.

To develop a catalyst for industrial applications of CO₂ methanation, investigations of the following are necessary: (1) A study of catalytic performance under industrial conditions is necessary at pressures from 2.0 to 6.0 MPa, reaction temperatures between 250 and 600 °C, and various space velocities; specific reaction conditions depend on the particular industrial situation. The reported literature primarily conducts experiments at 1 atm; an increased reaction temperature favors a decrease in the number of molecules reacting with CO₂, which may not always be true, as pressure also affects the adsorption and activation of both the targeted reaction and the byproducts’ reactions. (2) An investigation of stability over much longer periods is needed; the stability of the CO₂ methanation reaction has only been studied for hundreds of hours in the literature, whereas an industrial catalyst should be stable for years. (3) The improvement of activity at low reaction temperatures is essential. As can be seen from the thermodynamic equilibrium profile in Figure 2b, the complete conversion of CO₂ into CH₄ is achievable only at low temperatures and comparatively high pressures; therefore, high activity at low temperatures is required.

5. Conclusions and Future Prospective

In combatting the challenges posed by CO₂ emissions, using renewable energy to produce green hydrogen and then hydrogenating CO₂ into valuable chemicals is a promising route, including through the production of CH₄ via CO₂ methanation. Given the thermodynamic equilibrium of the reaction, to convert CO₂ completely into CH₄, low reaction temperatures and high pressure are favorable, and meeting the requirements of developing catalysts is critical. The catalysts that have been extensively reported include noble metal-based catalysts and non-noble metal-based catalysts.

Among the noble metal catalysts, Ru shows the best catalytic performance. In excellent Ru-based catalysts, CO₂ conversion can be achieved close to equilibrium (98%) and with 100% selectivity to CH₄, with 300 h of stability obtained at 24,000 mL.g⁻¹h⁻¹, 230 °C, and 1 atm. Excellent noble metal-based catalysts feature the high dispersion of the noble metal, the strong metal-support interaction with noble metal NPs, and doped with a transitional metal: Ni, Co, or Fe. The disadvantages of noble metal-based catalysts are the high price and high loading, and the stability needs to be improved.

Non-noble metal catalysts include Ni-, Co-, and Fe-based catalysts. Ni-based catalysts show the best catalytic performance and are well studied, while studies on Co- and Fe-based catalysts are much fewer, possibly because other hydrocarbons besides CH₄ can be formed over Co and Fe catalysts. With excellent Ni-based catalysts, CO₂ conversion close to equilibrium (around 90%) and 100% selectivity to CH₄ can be obtained at around 350 °C a WHSV of 10,000 (mL.g⁻¹h⁻¹) or higher, and 1 atm with good stability. Excellent Ni-based catalysts feature high Ni NP dispersion; a support and/or additive to activate or help activate CO₂; and interactions between Ni NPs and the support that resist sintering. Bimetals (Ni-Fe, Ni-Co) are favorable for improving catalytic performance.