Current Research Status and Future Perspective of Ni- and Ru-Based Catalysts for CO₂ Methanation

Abstract

Using anthropogenic carbon dioxide (CO₂) as a feedstock for the production of synthetic fuel has gained significant attention in recent years. Among the various CO₂ conversion pathways, the production of synthetic natural gas via CO₂ methanation holds promise because of its potential for both carbon recycling and renewable energy storage. Nickel (Ni) and ruthenium (Ru) are the dominant metals employed as catalysts in the CO₂ methanation reaction. This review summarizes the research landscape of Ni- and Ru-based catalysts over the last ten years. Bibliometric analysis revealed that China has the highest number of publications, the Chinese Academy of Sciences is the foremost academic institution, and the International Journal of Hydrogen Energy is the leading journal in this area of research. The publication trend revealed that research on Ni-based catalysts is published at almost four times the rate of Ru-based catalysts. Despite growth in research, problems with catalyst stability and kinetics still exist. The latest research on various catalytic systems, including supported, bimetallic, and single-atom catalysts and the fundamental challenges associated with the CO₂ methanation process are reviewed. This review provides a new angle for future studies on catalysts based on non-noble Ni and noble Ru metals and opens the way for additional research in this area.

1. Introduction

A net-zero world can be achieved by combining many strategies, such as producing power from renewable energy sources, storing and distributing eco-friendly hydrogen, and lowering CO₂ emissions [1]. CO₂ is a significant greenhouse gas and air component, accounting for almost 0.04% of the atmosphere’s total volume [2]. Short waves are emitted from the sun that may penetrate the atmosphere and reach the ground. In contrast, the ground releases long-wave radiation when it warms up, absorbed by compounds such as CO₂ in the atmosphere. This process leads to atmospheric warming. CO₂ in the atmosphere functions as an insulating layer, resembling a substantial sheet of glass, transforming the Earth into an immense greenhouse. Tens of millions of people are in immense danger because of greenhouse gas effects such as global warming, melting glaciers, extreme weather, and increasing sea levels [2]. Estimates indicate that a 75% reduction in CO₂ release to the environment by 2050 will be crucial to keep global warming below 2 °C [3]. With atmospheric CO₂ concentrations currently hovering around 420 ppm and projected to climb to 600–1550 ppm in the coming decades, we are still a long way from our net-zero emission target [4]. Significant contributors to anthropogenic CO₂ emissions include a rapidly expanding industrial sector and a rising world population [5]. CO₂ is primarily produced by the combustion of fossil fuels [6]. The two primary processes that produce over 830 million metric tons of CO₂ per annum are the burning of conventional coal and natural gas steam reforming [7].

Since the emission of CO₂ has adverse effects on both humans and the environment, there is a high demand for methods to convert it into chemicals with additional value [8]. Valuable chemicals that can be generated using captured CO₂ are displayed in Figure 1 [9]. Using CO₂ as a reactant will be crucial in the near future [10]. Several options for CO₂ valorization have been developed and proposed, including biological conversion [11,12], electrocatalysis [13,14], photochemical catalysis [15], thermo-chemical processes [2,16] and their combination [17,18]. Thermo-catalysis is a better choice for increasing CO₂ conversion on an industrial scale [19]. The classes of heterogenous catalysis that have been primarily investigated are metal oxides [20,21,22,23], metal–organic frameworks [24,25,26], metal sulfides [27,28,29,30], covalent organic frameworks [31,32], and carbonaceous materials [33,34,35]. The main focus of this review is metal and metal oxide-based thermal heterogeneous catalysts for CO₂ methanation.

Hydrogenation of CO₂ to CH₄, also known as CO₂ methanation, is one promising strategy that takes advantage of the numerous existing systems for transporting and utilizing methane as chemical feedstock and fuel [36]. The main product of this process, methane, finds widespread application in industries and daily life. Moreover, the most widely used method of methane conversion is the Fischer–Tropsch process, which turns syngas into methanol or liquid hydrocarbons [37,38]. CO₂ methanation is a key component of power-to-gas (PtG) technology. In this process, excess renewable energy would be converted into synthetic methane and transported through existing natural gas infrastructure for many valuable applications [39]. Figure 2 presents a brief schematic of this process. Hence, Synthetic Natural Gas (SNG) production with captured CO₂ through a methanation reaction using hydrogen obtained by using renewable energy, i.e., solar and wind, is widely recognized as a competent and promising technique for valorizing anthropogenic CO₂ [40].

Various catalysts have been used for CO₂ methanation reactions in recent decades. Metals that showed activity in CO₂ hydrogenation to methane reaction include noble metals such as rhodium (Rh), platinum (Pt), palladium (Pd), and ruthenium (Ru), and different non-noble metals such as nickel (Ni), iron (Fe), and cobalt (Co) [41,42,43].

The two metals that have been utilized the most are Ni and Ru. When it comes to dissociating hydrogen at low temperatures, Ru, in its reduced state, is more effective than Ni. This dissociated hydrogen then combines with CO₂ that has been adsorbed on the catalyst surface. Based on the findings in the previously published literature, we can make orders of different metals used in CO₂ methanation, such as Ru > Ni > Co ~ Rh (for activity) and Ni > Rh > Co > Ru (for selectivity) [4,44]. Nevertheless, this order may vary based on different reaction temperatures and metal–support interactions [16,45].

Among noble metals, Ru stands out for its exceptional process stability, low-temperature activity with small loading, and lower cost than other noble metals [43,46]. Nickel-based catalysts have become the favoured alternative because of their low price, high methane selectivity, and good compatibility [47]. Both Ni- and Ru-based catalysts have some challenges and limitations. While recent reviews [2,40,44,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65] on active catalyst systems for CO₂ methanation have been covered in the literature in the past few years, a comprehensive analysis of Ni- and Ru-based catalysts over the last decade is still needed. Such a review would provide valuable insights into the current state of research, publication trends, key research areas, and future directions in this field. In this work, we tried to cover this gap, and we hope that these findings will help better understand the way forward in this research domain.

2. Research Status with Literature Measurement

Literature measurement using bibliometric study involves a systematic review of publications within a specific academic discipline, providing current and comprehensive information on the subject matter [2]. Here, the primary purpose of adding a part of this analysis is to provide current research status (based on the number of publications) and possible future directions of Ni- and Ru- metals for CO₂ methanation (based on authors’ keywords).

2.1. Method

Publication data were obtained from the “science citation index expanded” collection of “Web of Science (WoS)”. For bibliometric analysis, a topic search was conducted using a specific term found within the document’s title, abstract, author keywords, and keyword-plus categories. The methodology employed followed the approach described in the referenced study [66]. The topic search term used in this analysis was composed of four parts, as shown in Figure 3. In the case of Ruthenium, the last part of the topic search term was changed to: “Ru” or “Ruthenium” or “ruthenium” or “ruthenium-based catalyst” or “Ruthenium-based catalyst” or “Ruthenium-based Catalyst”. Quantitative analysis of publication trends and authors’ keywords was executed on all relevant documents from 1 January 2013 to 31 December 2023. These data were retrieved on 1 August 2024. Microsoft Excel was used to perform quantitative analysis. The VOSviewer software (version 1.6.20) was used for visualization and network mapping of authors’ keywords.

It is important to note that the bibliometric approach applied in this work could have shortcomings and limitations in data accuracy, errors in publication trends (by considering only the Web of Science database), and the inability to capture the qualitative aspects of research [67,68].

2.2. Findings

A total of 4461 total documents were retrieved by applying the search stream shown in Figure 3. After a thorough and careful screening, e.g., excluding papers related to syngas, steam reforming, methanol, pyrolysis, methane decomposition, and chemical looping combustion, 2054 documents were found related to nickel catalysts used in CO₂ methanation and used for further investigations. Among them, 1870 were original research works, and 89 were review papers. Similarly, in the case of the Ru search, a total of 751 documents were obtained, and then after screening, 503 papers were found relevant. Among these 503 publications, 454 were original research works, and 23 were review papers. Keywords plus are automatically generated keywords derived from the title of a publication. The elevated document counts in reviews and research papers may be due to “keywords plus” in the TOPIC search stream. Nickel-based catalysts showed almost four times more research work than Ru-based catalysts. Further comprehensive analysis was conducted only on research papers and reviews.

2.3. Publication Trends

The publication volume in a particular field (such as journal articles, conference papers, or book chapters) is an important metric for assessing the activity and growth of a research domain [2]. The annual number of articles and reviews on Ni- and Ru-based CO₂ methanation catalysts from 2013 to 2023 is presented in Figure 4. In general, the number of articles published on Ni-based catalysts has steadily increased, while annual Ru-based publications showed fluctuations notably before 2020. Each year, publications about Ru metal-based catalysts remain below 50. The growth rate was slow before 2016 and experienced a hike after that. In 2016, the Paris Agreement was formed by 178 countries, focused on keeping the rise in the average world temperature to 1.5 °C from pre-industrial levels. Countries have committed to reducing global emissions by 45% from 2010 to 2030 to meet carbon emission reduction targets [69]. It might be one of the reasons for the fast growth in the overall publication volume in this research field between 2016 and 2023, mainly on nickel metal as a promising catalyst. In 2020, there was a significant surge in review papers published on catalysts based on Ni and Ru. This can be attributed to the restrictions on physical work imposed during the COVID-19 pandemic. It can be seen from Figure 4 that between 2021 and 2023, there was an upsurge in reviews for Ni catalysts and a fall for Ru catalysts. According to the numbers, CO₂ reduction research is rising, but the problem is still severe. Therefore, it is realistic to say that the research on these catalysts for the methanation of anthropogenic CO₂ will remain popular.

There are many important reasons why there has been less research on catalysts based on Ru for CO₂ methanation than Ni catalysts. Ni-based catalysts experienced new directions to increase the catalytic activity, including metal–support interactions [46,70,71]. On the other hand, ruthenium catalysts have potential but are frequently hindered by high costs and difficulties in attaining identical catalytic performance. Studies incorporating confinement effects and multi-metal synergistic interactions show how hard optimizing ruthenium-based systems may be, which discourages researchers from pursuing the use of Ru as a catalyst [72,73]. Economic considerations, well-established techniques for nickel catalysts, and the intrinsic difficulties of ruthenium systems all explain why these two types of catalysts are published at different rates. As an added complication, several synthetic techniques are still unable to create Ru-based heterostructure catalysts that are both cost-effective and have enough catalytic activity [74].

2.4. Countries, Institutes, and Journals Analysis

According to bibliometric analysis, 91 countries published articles about nickel-based catalysts for CO₂ methanation. Of them, 58 have published 10 or more articles. From 2013 to 2023, the top five countries contributing to WoS publications were China, the United States, Spain, France, and South Korea, accounting for over 65% of the total publications. On the other hand, 25 countries have published at least 10 publications on Ru catalysts, while 66 countries are actively participating in the research. Five countries account for about half of all publications: China, the United States, Japan, Italy, and Germany. Regarding research on Ni- and Ru-based catalysts for methanation of CO₂, Chinese universities rank well in terms of article and citation counts. Among articles about Ru, the Chinese Academy of Sciences has 3547 citations, and the Beijing University of Chemical Technology has 1747 citations. The most prominent journals published extensively on Ni- and Ru-based catalysts for carbon dioxide methanation are the International Journal of Hydrogen Energy (impact factor: 8.1), Applied Catalysis B Environmental (impact factor: 20.2), and Catalysts (impact factor: 3.8).

2.5. Keyword Analysis

Keyword analysis is a powerful tool to identify the author’s objective and core idea of a paper. Researching keyword frequency is necessary to identify popular subjects and patterns in an area [75]. In this review, the relationship between the author’s keywords that appeared in the publications of Ni- and Ru-based metal catalysts for carbon dioxide methanation was investigated by utilizing VOSviewer software. Out of 7075 authors’ keywords obtained by applying co-occurrence analysis in VOSviewer on Ni-based publications, 236 authors’ keywords appeared at least 10 times. In the case of Ru, out of a total of 1559, 37 met the threshold limit of at least 10 times appearance. Among these high-frequency keywords used by many authors, the top 10 are presented in

Table 1. Leading authors’ keywords appeared in publications from 2013 to 2023.

	Position	Authors’ Keyword	Count
Ni	1	Nickel	401
	2	CO2 Methanation	393
	3	Hydrogen	364
	4	Methane	261
	5	Carbon Dioxide	198
	6	CO2 Hydrogenation	155
	7	Catalyst	134
	8	Biogas	133
	9	Ni catalyst	123
	10	Carbon Deposition	97
Ru	1	CO2 Methanation	93
	2	Ruthenium	70
	3	CO2 Hydrogenation	53
	4	Carbon Dioxide	40
	5	Sabatier Reaction	22
	6	CO2 Reduction	21
	7	Power to Gas	19
	8	Density Functional Theory	17
	9	Ceria	12
	10	Kinetics	11

. Links between these terms in different clusters via visualization map are presented in Figure 5 and Figure 6. Each circle represents a keyword, while the connections between the nodes signify the relationships among the keywords. The diameter of the circle and the width of the connecting line signify the extent of the relationship between the frequency of the term’s occurrence and the keyword itself.

The most prominent one (in the red colour of Figure 5) consists of terms related to catalysts for CO₂ methanation, i.e., nickel, supported catalysts, titania, ceria, alumina, basicity, CaO, MgO, oxygen vacancy, stability, catalyst deactivation, etc. the second most influential group (in green colour) of authors’ keywords contained terms related to hydrogen, CO₂ utilization, gasification, kinetics, thermodynamic analysis etc. While coloured lines help differentiate between distinct keyword categories, they are not isolated. Instead, they are indissolubly connected and reliant on one another. These results imply that there is a great deal of research being performed on carbon dioxide catalysts, including everything from manufacturing methods to reaction control, source analysis to catalyst characteristics, and deactivation. Their impact on scientific inquiry is still increasing. The research in nickel-based catalysts for CO₂ methanation is mainly directed toward metal oxide-supported catalysts, catalyst stability, carbon deposition, bimetallic catalytic systems, and biogas methanation [4].

In the case of Ru catalysts, a comprehensive keyword analysis revealed four clusters of authors’ keywords. The leading cluster (blue colour of Figure 6) contains Ru catalysts, CO₂ methanation, kinetics, and DRIFT terms. The appearance of these terms shows that research has been focused more on the structure of Ru catalysts, reaction mechanisms and in situ techniques for catalyst mechanisms [76]. The research direction of Ru-catalysts for CO₂ methanation is directed mainly towards the morphology of Ru [77,78,79,80,81], metal–oxide-supported Ru [82,83,84,85,86], and kinetics [41,87,88,89,90]. The various keyword categories are not really separate from each other, even though they are shown by different coloured lines. On the contrary, they form connected with others. It demonstrates that research on additional active metals, such as Ni, is mostly linked to Ru catalysts.

The findings of the publication trend and authors’ keyword analysis via bibliometric analysis on this subject indicate that studies on active catalysts for CO₂ methanation, particularly those involving Ni-based catalytic systems, continue to catch significant attention. We can infer from the bibliometric analysis that the structure of Ni- and Ru-based catalysts, along with supported Ni/Ru catalysts, are among the main focus areas of current research in CO₂ methanation. In view of these considerations, the subsequent section of this paper provides a concise overview of the primary obstacles in the CO₂ methanation reaction, including thermodynamic limitations and deactivation mechanisms in nickel and ruthenium-containing catalysts. Several in-demand catalytic systems based on nickel and ruthenium metals are also reviewed to view contemporary research on these prominent non-noble and noble metals for the methanation of CO₂.

3. Fundamental Challenges in CO₂ Methanation

Several drawbacks and fundamental challenges affect the CO₂ methanation process. One major obstacle to the widespread use of CO₂ methanation technology is the lack of economically feasible renewable hydrogen [44]. The electrolysis of water is the current developing method for producing hydrogen, which is an essential reactant in methanation. Current electrolysis technologies are energy intensive [91,92,93]. We can only hope that in the near future, there will be an abundance of surplus renewable energy accessible for the creation of hydrogen, as substantial research is being conducted on H₂ production from renewable sources [91]. Methane synthesis cannot begin without the CO₂ capture from the emission source. CO₂ capture and storage necessitate more energy for compressing and separating CO₂ [94], which would increase the total energy requirement and cost of the methanation process significantly.

There is a significant role for catalyst structure as well [95]. Support materials and catalyst preparation methods significantly impact the performance of catalysts, even though non-noble metal catalysts such as Ni are promising because of their activity and low cost [96]. Equally critical is the availability of stable catalysts that can keep their activity and selectivity levels high even when subjected to extreme operating conditions [97]. High temperatures and pressures are necessary for the reaction to have favourable kinetics [4]. Though cleaner than other fossil fuels, methane is also a greenhouse gas. Leaking methane during production, storage, or transport can add to global warming [98]. In order to make CO₂ methanation a practical carbon recycling process, these obstacles must be overcome. A quick review of thermodynamic constraints and catalyst deactivation is given here.

3.1. Thermodynamic Limitations

CO₂, which is produced as a result of burning carbon-based fuels, is highly stable from a thermodynamic point of view. It has a formation enthalpy (ΔH_f) of −394 kJ·mol⁻¹. Thus, the majority of reactions that transform CO₂ into any other product necessitate an energy input unless a high-energy molecule is employed as a co-reactant. Any chemical conversion of CO₂ would inevitably demand substantial energy since all potentially valuable compounds formed from CO₂ are inherently less stable. Nevertheless, its stability does not imply that CO₂ is “unreactive”. CO₂ bond can be broken because of the presence of O atom Lewis basicity [99]. The catalytic reduction of CO₂ using H₂ is considered the most promising strategy for reducing CO₂ emissions because of the high reducing power of hydrogen and research on renewable hydrogen [100]. Additionally, hydrogenation reactions exhibit superior efficiency compared with less advanced electrochemical or photochemical techniques [101].

Equation (1) reports the Sabatier reaction (hydrogenation of CO₂ to CH₄).

CO₂ + 4H₂ ⇄ CH₄ + 2H₂O
ΔH_298K = −165 kJ mol⁻¹

(1)

From an industrial standpoint, scaling up this type of operation is extremely challenging due to its strong exothermicity, reactor-side restrictions, and the requirement to remove the massive quantity of heat generated. The reaction follows two possible pathways which are the formate (COOH) route and the carbon monoxide (CO) route (Figure 7). Other possible reactions during CO₂ methanation are the reverse water gas shift (RWGS) reaction, a CO methanation (Equation (3)) and the reverse dry reforming reaction (Equation (4)) [4,102].

CO₂ + H₂ ⇄ CO + H₂O
ΔH_298K = 41 kJ mol⁻¹

(2)

CO + 3H₂ ⇄ CH₄ + H₂O
ΔH_298K = −206 kJ mol⁻¹

(3)

2CO + 2H₂ ⇄ CH₄ + CO₂
ΔH_298K = −247 kJ mol⁻¹

(4)

The RWGS (Equation (2)) is only an endothermic reaction among these four possible reactions, and the overall reaction (Equation (1)) has ΔG°_298K = −114 kJ mol⁻¹.

According to Massa et al. [103], adding water vapours to the process slightly lowers the CH₄ production but moderately prevents the build-up of carbon deposits. The effect of temperature, pressure, feed ratio, and the presence of steam with reactants on CO₂ conversion, CH₄ selectivity, and carbon deposition were studied by Gao et al. It was determined that a high feed ratio (H₂/CO₂), low temperature, and elevated pressure are favourable conditions for the methanation process [104].

3.2. Deactivation

One of the primary issues with any heterogeneous catalysis application is catalyst deactivation. There are two methods for deactivation: chemical and physical. The second category includes processes such as sintering and fouling [105]. The former happens when particles agglomerate, pores close, and active metal clusters grow in size as a result of high temperatures [106,107]. In the case of chemical deactivation, the catalyst is usually poisoned by the deposition of carbon atoms from byproducts on active sites. In heterogeneous catalysts containing Ni and/or Ru metals, it is necessary to limit these side reactions (Equations (5)–(8)) that lead to the production of carbon deposits [16,108].

2CO → C + CO₂
ΔH_298K = −172 kJ mol⁻¹

(5)

CH₄ → C + 2H₂
ΔH_298K = 75 kJ mol⁻¹

(6)

CO + H₂ → C + H₂O
ΔH_298K = −131 kJ mol⁻¹

(7)

CO₂ + 2H₂ → C + 2H₂O
ΔH_298K = −90 kJ/mol⁻¹

(8)

Active phases, such as Ni, mix with gases in the vapor-solid reaction to produce inactive species, such as Ni(CO)₄, generated at temperatures < 230 °C [109]. The formation of these volatile species can cause the reactor to lose its active phase and release potentially harmful species. Toxic gas causes the active sites of the catalyst surface to be chemisorbed with gas contaminants, rendering the catalyst inactive [110]. It is imperative to employ desulfurization units when biogas is utilized as a feedstock because sulfur and organic-sulfur compounds provide the most significant threat of catalyst poisoning, especially in the case of nickel-containing catalysts [111]. Equation (9) [112] shows that when inorganic Sulphur compounds such as H₂S are present in the biogas, they lead to the synthesis of NiS, which deactivates the Ni active sites.

NiO + H₂S → NiS + H₂O
ΔG_300K = 48.5 kJ mol⁻¹

(9)

The impact of the H₂/CO₂ ratio on coke generation during low temperature methanation was revealed by Miguel et al. [113]. The catalyst surface showed signs of coke formation at a reactor temperature below 250 °C and a feed gas ratio (H₂/CO₂) below 3. On the other hand, when operating at an H₂/CO₂ > 4 ratio, practically little coke was produced [114,115].

4. Ni- and Ru-Based Catalytic Systems

In this section, major catalytic systems, including Ni- and Ru-based supported catalysts, Bi-metallic catalysts, and Single Ni or Ru atom catalysts, are concisely reviewed.

4.1. Supported Catalysts

In the Ni- and Ru metal-based catalysts that drive CO₂ reduction, solid supports are essential. A catalyst that is active needs appropriate support. In addition to providing CO₂ adsorption sites, the support regulates the catalyst’s dispersion and stabilizes the distributed active metal species [59]. The interaction of the metal with the support material affects activity, selectivity, and stability differently.

4.1.1. Role of Supports in Ni-Based Catalysts

Metal oxides, including SiO₂, SiO₂-Al₂O₃, CeO₂, Al₂O_3, MgO, TiO₂, and ZrO₂, are frequently used as supports for Ni catalysts. Zeolites, carbon-based materials (less regularly), and MOFs are also used as potential supports. The basicity of the support has a major impact on the formation of monodentate formate species, which are necessary to increase the selectivity of the Ni-based catalyst [51].

CeO₂ offers supplementary basic sites for CO₂ activation, creates oxygen vacancies, and enhances adsorption. At 250 °C, the Ni/CeO₂ catalyst demonstrated 81.2% CO₂ conversion and 100% CH₄ selectivity [116]. Magnesium oxide (MgO) is recognized for its significant basicity, which promotes CO₂ adsorption and improves the catalytic efficacy of nickel-based catalysts. For example, Ni-Mg/C₃N₄ catalysts exhibited a CO₂ conversion rate of 77% and methane selectivity of 99%, attributable to the formation of Lewis basic sites [117]. A study detailed in [118] compares 10wt% Ni on different metal oxides, including CeO₂, Al₂O₃, TiO₂, and MgO. CeO₂ was identified as the most advantageous support, with 90% CO₂ conversion and 100% CH₄ selectivity at 300 °C. The intriguing activity has been ascribed to the substantial quantity of adsorbed CO₂, as proven by CO₂ Temperature Programmed Desorption (TPD) analysis. Liu et al. [119] studied the impact of CeO₂ addition on the catalytic performance of a 15% wt Ni/Al₂O₃, revealing a significant correlation between CeO₂ level and catalytic efficacy. The catalyst containing 2% Ce exhibited excellent performance because of the diminished contact between Ni and Al₂O₃ [119]. Evaluation of a 10% wt Ni/(Al₂O₃-ZrO₂) synthesized using the epoxide-driven sol-gel method revealed that the addition of ZrO₂ enhanced the nickel dispersion and also led to the development of oxygen vacancies, which are crucial for the catalyst’s activity. With a conversion rate of 77% at 340 °C, complete selectivity towards methane was accomplished [120].

The size and configuration of Ni particles on the support surface influence catalytic activity. A lower Si/Al ratio promotes Ni particle dispersion and CO₂ adsorption, improving CO₂ adsorption and higher methane selectivity [121]. A 3D organized mesoporous silica, FDU-12, has pore cages of 12–42 nm. Because of its cage-like structure, FDU-12 could demonstrate better resistance to pore obstruction compared with SBA-15. Liu and Dong studied the impact of the solvent on the synthesis of 10wt% Ni on an FDU-12 support. They found that smaller Ni particles formed when ethylene glycol was employed as the solvent for impregnation instead of water (5.9 nm vs. 10 nm) [122]. The conversion rate was 74% and the selectivity towards CH₄ was 97% at 425 °C and high WHSV (60,000 mLg⁻¹h⁻¹). In the case of water, CO₂ conversion and CH₄ selectivity were 50% and 93%, respectively.

Carbon-based support materials, including carbon nanotubes (CNTs), graphene, biochar, activated carbon (AC) and carbon nanofibers, have also been used in Ni catalysts for CO₂ methanation [123]. The confinement effects of Ni-supported multi-walled CNTs for CO₂ methanation were clarified by Wang et al. [124]. Ni/CNTs improved the dispersion and reducibility of the Ni species, which in turn improved the resistance to carbon deposition, in contrast to Ni/Al₂O₃ catalysts. Methanation of CO₂ was investigated with Ni-ZrO₂/CNTs prepared by two distinct methods: co-impregnation and sequential impregnation [125]. CNT-supported Ni-ZrO₂ catalyst prepared by sequential impregnation showed better CO₂ conversion and CH₄ selectivity. The increased accessibility of the Ni-ZrO₂ interface to the reaction gases, which promotes the coupling of hydrogen atoms with activated CO₂, is responsible for the increased catalytic activity of Ni-ZrO₂/CNT-sequential impregnation [125]. Research on Ni supported on AC with 10 wt% Ni was carried out by Wang et al. [126]. The catalyst showed a 69% conversion of CO₂ and a 96% selectivity for CH₄ at 340 °C. Although there have been positive outcomes, carbon-supported Ni catalysts are not often used for catalytic processes that occur at high temperatures. This is because they are not as thermally stable as other common materials, including silica and alumina [123].

The development of a supported nickel catalyst for CO₂ methanation has improved low-temperature activity, and a well-defined mechanism is an area of intense research and development [127].

4.1.2. Role of Support in Ru-Based Catalysts

Ru-based catalysts for CO₂ hydrogenation to CH₄ reactions have been widely supported by reducible oxides such as CeO₂, ZrO₂, TiO₂, and In₂O₃. The presence of oxygen vacancy, which notably impacts the catalytic characteristics for CO₂ methanation, is a distinctive feature of the reducible oxide [128]. Wang et al. [129] discovered that the presence of oxygen vacancies in CeO₂ is crucial for the improved performance of the Ru/CeO₂ catalyst in CO₂ methanation at low temperatures. A sample of Ru/α-Al₂O₃ (without any oxygen vacancy) was also synthesized to provide additional evidence of the significance of oxygen vacancy. Through the formate pathway, the Ru/CeO₂ catalyst’s oxygen vacancy and surface hydroxyl are involved in the catalytic process. The specific process that sets the rate of the reaction is the dissociation of formate to methanol, which is catalyzed by the oxygen vacancy. Methane is produced through the successive hydrogenation of methanol. Xu et al. [130] found that Cr doping in Ru/CeO₂ boosted the number of surface oxygen vacancies and hydroxyl groups, promoted the formate pathway, and significantly improved catalyst activity. The presence of oxygen vacancy in CeO₂-supported Ru catalysts is favourable in achieving 99% CH₄ selectivity at low temperatures (150 °C) [131]. Ru/α-Al₂O₃ utilizes the carboxylic pathway for CO₂ methanation without oxygen vacancies, in contrast to Ru/CeO₂. Oxygen vacancies are also crucial in the Ru/ZrO₂ catalyst for CO₂ methanation. Chen et al. [132] synthesized a catalyst consisting of Ru supported on ZrO₂, which exhibited a significant interaction between the Ru metal and the ZrO₂ support. This connection was attributed to oxygen vacancies at or near the interface between the metal and oxide. Consequently, a charge transfer occurs from the O-vacancy to the Ru atoms, particularly those near the O-vacancy at the interface. The primary cause for the increased activity and exclusive production of methane in CO₂ methanation is the alteration of the electronic state in the surrounding atoms near the interface [133].

In addition to oxygen vacancies, Ru’s interaction with variously shaped supports is another significant feature of Ru-supported catalysts. Liao et al. [134] investigated the impact of the Ru-ZnO interfacial structure by using different shapes of ZnO, i.e., nanoplates, nanorods, and nanoparticles. Catalytic activity is ranked as follows: Ru/nanoplate ZnO > Ru/nanorod ZnO > Ru/nanoparticle ZnO, according to their findings. A Ru supported on a monoclinic ZrO₂ (m-ZrO₂) catalyst was developed by a selective deposition approach by Tada et al. [133]. At 250 °C, with product selectivity > 99%, an 82% CO₂ conversion was accomplished. There was no change in the catalytic performance after 70 h. Ru and m-ZrO₂ have a strong association that boosts CO₂ activation, which is responsible for their outstanding low-temperature performance. Amorphous ZrO₂ (am-ZrO₂) was another material used as a support [135]. For CO₂ methanation, Ru/am-ZrO₂ exhibits the same high methane selectivity (>99%) as Ru/m-ZrO₂. At 250 °C, the CO₂ conversion on Ru/m-ZrO₂ was 73% lower than Ru/am-ZrO₂ [135].

Ru-based catalysts for the CO₂ methanation process also employ nonreducible metal oxides such as Al₂O₃, SiO₂, and TiO₂ as suitable supports [128]. Al₂O₃-supported ruthenium catalysts have been widely used in comparison studies with other supported catalysts. Jabotra et al. [136] compared the performance of Ru and Ni catalysts supported on γ-Al₂O₃ for the process of CO₂ methanation. It has been confirmed that Ru/γ-Al₂O₃ exhibits more activity. CO₂ conversion on Ru/γ-Al₂O₃ catalyst begins at 200 °C and reaches its peak of 100% at 325 °C. The CH₄ selectivity was 100% at a temperature of 325 °C. At temperatures exceeding 350 °C, the RWGS reaction initiates the production of carbon CO. On the other hand, the CO₂ conversion in Ni/γ-Al₂O₃ started at 250 °C. At 375 °C, the maximum CO₂ conversion was 59%. Once this happens, CO₂ conversion starts to decline above 375 °C [136]. Findings of some high-performing supported Ni- and Ru-based catalysts are listed in

Table 2. Findings of some of the best performing Ni- and Ru-based catalysts in thermal CO₂ methanation at temperatures ≤200 °C.

Catalyst	T (°C)	P (bar)	WHSV (mL min−1 g−1)	Activity (μmolCH4 gcat−1·s−1)	CH4 Select (%)	Ref.
Ru/TiO2	160	1	70	0.7	>99	[137]
Ru/TiO2	180	10	50	1.2	>95	[138]
Ru/CeO2	190	0.3	800	0.4	98	[139]
Ru/Ce0.9Cr0.1Ox	190	1	600	3.6	>99	[130]
Ru/ZrO2	190	1	208	3.1	>99	[132]
Ru-Na/TiO2	200	1	1500	5	>99	[140]
NiAl-MO/CeO2	170	0.5	70	1	99	[141]
Ni/MgAl-MMO	170	0.75	80	1.4	>95	[142]
Ni/CeO2	180	0.05	240	18.8	-	[143]
Ni/Al2O3	200	0.5	70	1.5	99	[144]
Ni-La/Mg-Al	200	0.8	750	18.2	94	[145]

,

Table 3. Summary of various highly active supported Ni catalysts in thermal CO₂ methanation at temperatures 200–400 °C.

Catalyst	Preparation	T (°C)	WHSV (mL h−1 g−1)	X.CO2 (%)	S.CH4 (%)	Stability (h)	Ref.
Ni/CeO2	sol-gel method	250	40,000	82.5	94.8	106	[146]
Ni/CeO2-NR	hydrothermal	250	24,000	23	98	-	[147]
20Ni/CeO2-La2O3	wet impregnation	300	30,000	89	100	100	[148]
Ni/La2O3	wet impregnation	300	6000	25	90	-	[95]
Ni-La2O3/SBA-15	citrate complex method	320	6000	90.7	99.5	160	[149]
Ni-Ru/Ce-Zr	sequential wet impregnation	350	24,000	53	93	80	[150]
Ni-Fe/Ce-Zr	sequential wet impregnation	350	24,000	13	60	-	[150]
10%Ni-1%MgO/SiO2	co-impregnation	350	15,000	67	98	50	[151]
Ni/Sm2O3–CeO2	microwave-assisted sol-gel method	350	25,000	44.9	100	-	[152]
40%Ni/SiO2	ammonia-evaporation	370	30,000	82.4	95.5	60	[153]
Ni-Ce/Al2O3	simultaneous impregnation	375	21,500	68	>99	90	[154]
20%Ni-CeO2/MCM-41	deposition–precipitation	380	9000	85.6	99.8	30	[155]
Ni/SiO2-Al2O3	impregnation	400	10,000	70	100	30	[156]
Ni-5%Ru/SiO2	wet impregnation	400	6000	71	92	24	[157]
Ni/Ca/Si	sequential impregnation	400	15,000	73.3	98.9	50	[158]

and

Table 4. Performance of various supported Ru-based catalysts in CO₂ methanation: H₂/CO₂ = 4; X = conversion; S = selectivity.

Catalyst	Preparation	T (°C)	GHSV/WHSV	X.CO2 (%)	S.CH4 (%)	Stability (h)	Ref.
2.5%Ru/TiO2	TiO2 mixing with RuO2	325	6000 mL h−1 g−1	>80	100	50	[86]
Ru/Ce0.9Cr0.1Ox	impregnation	325	36,000 mL h−1 g−1	70	100	55	[130]
Ru/Al2O3	Evaporation and dryness			86	100	30	[159]
10Ru-10Ni/Sm2O3	wet impregnation	350	6000 mL h−1 g−1	33	94	13	[160]
0.89%Ru−5%Li/Al2O3	incipient wetness	320	25,000 h−1	>45	99.5	-	[161]
Ru-Ni/CeO2-Al2O3	co-impregnation	350	24 L g −1 h−1	82	99	20	[162]
4%Ru/Al2O3 a	impregnation	375	10,000 h−1	85	98	24	[163]
Ru/Ce3PrOx	incipient wetness impregnation	270	9000 h−1	55	100	-	[164]
Ru/CeO2-rod	hydrothermal	300	72,000 mL h−1 g−1	72	99	24	[165]
3%Ru/Al2O3	(commercial)	350	55,000 h−1	93	100	-	[166]
Ru/m-ZrO2	selective deposition method	250	10,000 mL h−1 g−1	82	>99	70	[133]
5%Ru/CeO2	spray pyrolysis	300	7640 h−1	83	99	-	[167]

^a H₂/CO₂ = 5.

.

4.2. Bimetallic Ni- and Ru-Based Catalysts for CO₂ Methanation

Recently, there has been a growing emphasis on conducting experiments that specifically investigate using bimetallic catalysts for CO₂ reduction [128]. For catalysts to enable CO₂ hydrogenation, CO₂ adsorption and H₂ dissociation are critical. Synergistic interactions between certain noble metals and transition metals can decrease activation energies [168]. Metal oxide-based bimetallic nanoparticles prevent carbon deposition and particle agglomeration in reactions due to their improved reducibility and metal dispersion [169]. More efficient CO₂ hydrogenation is possible as a result of reduced energy barriers and improved catalytic performance in the case of bimetallic catalysts in comparison to single-metallic catalysts [170]. Compared with monometallic catalysts, bimetallic catalysts, especially RuNi, improve CO₂ methanation efficiency by increasing the reducibility of metal oxides, increasing active sites for CO₂ and H₂ dissociation, and improving dispersion of active metal on the surface of support [171]. This ultimately leads to higher catalytic activity. Several studies [169,171,172] have made use of noble metals in this context because Ni-based catalysts that are prompted by noble metals have shown better catalytic activity and durability in a range of thermocatalytic processes. This allows for the possibility of using nickel-based catalysts with low noble metal loadings (≤1 wt%), avoiding the need for larger loadings of Ni [85,173,174]. Elliott et al. [175] synthesized catalysts using Ru-Ni/zeolite and Ru-Cu/zeolite. They found that incorporating an excess amount of Ni in the catalyst might mitigate the issue of carbon deposition during the methanation reaction and enhance the stability of the catalyst. This study discovered that although Ru catalysts outperformed Ni catalysts in nearly all activity tests, the combination of Ni as the first active metal and Ru as the second metal demonstrated higher catalytic activity than the opposite order. A bimetallic Ru−Cu catalyst for CO₂ methanation was studied by Sun et al. [176]. They discovered that combining Ru with Cu increases CO₂ conversion from 15% to 80% at 700 °C with 60% CO selectivity, particularly due to the highly active metal dispersion on the CNS support.

High-resolution transmission electron microscopy (HR-TEM) analysis by Liu et al. [177] revealed that their catalyst consisted of monometallic Ni and Ru rather than a Ni-Ru alloy. Additionally, the presence of Ru significantly increased the capacity for H₂ adsorption. In general, including Ru increased the Ni-based catalysts’ capacity to adsorb H₂. Additionally, the improved reaction pathway led to higher efficiency in converting CO₂ and boosted the antioxidant characteristics. Significantly, the Ni-Ru series yields superior economic advantages compared with monometallic Ru. The synergistic impact resulting from integrating the fundamental preparation technique and several supplementary procedures is particularly effective in synthesizing exceptionally efficient catalysts. The results indicate that the catalytic efficiency of the Ni-Ru/Al₂O₃ catalyst prepared using glycerol-assisted impregnation (GAI) is superior to that of the Ni-Ru/Al₂O₃ catalyst prepared using the standard impregnation approach. The GAI synthesis method enhances Ni’s surface area and dispersion on the catalyst’s surface by reducing the contact between Ni²⁺ and Al₂O₃ [178].

Ni-Pt bimetallic catalysts have also been investigated for CO₂ methanation. This is motivated by platinum’s excellent H₂ dissociation ability despite its known tendency to promote CO formation [179]. Chen et al. [180] found that while CO selectivity was high on both Pt/Al₂O₃ and Ni/Al₂O₃ catalysts, CH₄ selectivity increased on the Ni-Pt/Al₂O₃ bimetallic catalyst. The shift in product selectivity was due to the synergistic action of Ni and Pt, which changes the hydrogenation pathway. Table 5 displays the activity of some bimetallic catalysts.

In conclusion, the bimetal catalyst made by doping the second metal enhances catalytic activity. Two crucial factors for catalytic system optimization are the size and dispersion of the metal particles. Exploring the content of metallic components in bimetallic catalysts has been a significant area of interest. The performance of bimetallic catalysts, which have been infused with varying amounts of metals, exhibits variations. Excessive addition of metal doping might obstruct the active site of the active metal and impede the methanation action [180]. Appropriate metal doping can increase the surface energy of the catalyst, facilitate the degree of reduction of metal oxides, and boost the low-temperature performance of the methanation reaction [181].

Table 5. Activity comparison of some bimetallic catalysts with their counter monometallic catalysts.

Catalyst	Temp (°C)	CO2 Conv (%)	CH4 Select (%)	Ref.
Cu/NMCN	500	<5	0	[175]
Ru-Cu/NMCN		53	90
Ni/Pr-CeO2	300	61	97	[172]
Ni-Ru/Pr-CeO2		80	100
Ni/Al2O3	300	77	98	[182]
Ni-Fe/Al2O3		81	100
Ni/Al2O3	500	5	<10	[179]
Ni-Pt/Al2O3		65	60

NMCN = N-doped Mesoporous Carbon.

Table 5. Activity comparison of some bimetallic catalysts with their counter monometallic catalysts.

Catalyst	Temp (°C)	CO2 Conv (%)	CH4 Select (%)	Ref.
Cu/NMCN	500	<5	0	[175]
Ru-Cu/NMCN		53	90
Ni/Pr-CeO2	300	61	97	[172]
Ni-Ru/Pr-CeO2		80	100
Ni/Al2O3	300	77	98	[182]
Ni-Fe/Al2O3		81	100
Ni/Al2O3	500	5	<10	[179]
Ni-Pt/Al2O3		65	60

NMCN = N-doped Mesoporous Carbon.

4.3. Ni- and Ru-Based Single Atom Catalysts

The scientific world has recently been paying significant attention to single-atom catalysts (SACs) because of their exceptional capabilities as both homogeneous and heterogeneous catalysts. SAC offers a stable and insoluble foundation that allows for facile recovery and possesses excellent chemical stability. Furthermore, it potentially delivers 100% exposed active sites [183]. Significant efforts have been devoted in recent decades to the selective hydrogenation of CO₂ to produce desired products [184,185].

The structural properties of catalysts are crucial for tailoring SACs to a specific reaction, such as the hydrogenation of CO₂ [48]. The primary factor for achieving selective conversion of CO₂ is the desorption of CO from the catalyst’s surface. If the desorption of CO is advantageous, then the reverse water gas shift reaction (RWGS) is the predominant process. Further hydrogenation enables the formation of substances such as methane or formic acid [19]. For this purpose, zeolite-based SACs were engineered into bifunctional materials with high catalytic activity. In order to create SACs, Wei et al. [186] produced 13X zeolite, which contained CeOx particles that separated Ni atoms. The significant function of the Ce loading in adjusting the catalyst’s properties, such as reducibility, activity for the reaction, and basicity, was examined. By maintaining an appropriate equilibrium between basicity and acidity, the catalyst surface was able to interact with CO₂ manageably, preserving the selectivity toward methane. At 360 °C for 200 h, the 5%Ni2.5%Ce13X catalyst exhibited the best conversion and selectivity toward methane, outperforming all of the others that were analyzed in this study.

In the case of Ru metal catalysts, a quantity (4–6 wt%) of valuable metal is included or spread out on an inexpensive metal support in the form of a SAC [127]. This method reduces the overall expenses of metal catalysts while maintaining their catalytic efficiency [187]. The presence of atomically distributed single-atom metals enhances the availability of active sites for the reactant, hence maximizing the catalytic performance [188]. To convert CO₂ into methane with high conversion and selectivity, Fan et al. [189] synthesized a catalyst supported on porous hexagonal boron nitride. The catalyst’s activity and thermal stability can be preserved while supporting material reduces its cost. The valence state of atomic Ru is diminished in a catalyst containing Ru supported over boron nitrate because it aids Ru dispersion and produces defects via coordination between boron and nitrogen. Ru-supported pBN catalysts’ catalytic activity was superior to that of Ru nanoparticles in 110 h at 350 °C, with a selectivity of around 93.5% towards methane. In addition, Rivera-Cárcamo et al. [190] synthesized CNT and titanium dioxide (TiO₂)-supported SACs to enhance the stability of ruthenium (Ru) and nickel (Ni) single atoms. These SACs were then employed for the hydrogenation of CO₂. CNT and TiO₂ support were modified to create oxygen vacancies to stabilize the Ru and Ni single atoms. The SACs based on Ru/CNT exhibited remarkable selectivity for methane production due to charge transfer between the support and metal. They proved that metal surfaces with more electrons created CH₄, whereas those with fewer electrons formed CO more preferentially [190]. Zhang et al. [191] investigated the functioning of a tandem catalyst (Ru1Ni/CeO₂) with two active sites composed of Ru atoms (Ru1) and Ni nanoparticles. After comparing the Ru1CeO₂ and Ni/CeO₂ catalysts to the Ru1Ni/CeO₂ catalyst, the results demonstrated a significant improvement in efficiency. The catalysts made of Ru1Ni and CeO₂ demonstrated a 90% conversion of CO₂ and a ~99% selectivity for CH₄ at 325 °C. The experimental results showed that a single atom of Ru undergoes CO₂ dissociation, and then *CHO is produced by hydrogenation over Ni nanoparticles. After that, the methane was produced when *CHO was exposed to the Ni sites of the Ni nanoparticles. Therefore, Ru single atom species aids in CO₂ conversion to CO, whereas Ni sites carry the process onto CH₄ [191]. The activity performance of some single-atom catalysts is tabulated in

Table 6. Activity comparison of some single atom catalysts with dispersed nanoparticle catalysts.

Catalyst	Particle Size	Temp (°C)	CO2 Conv (%)	CH4 Select (%)	Ref.
Ni-Ce13X	single atom	320	78	100	[186]
Ni-Ce/Na-USY a	2.4 nm	350	23	85	[192]
Ni-Ce13X	single atom	280	69.9	100	[186]
Ni-CeUSY	17–33 nm	300	3	84	[193]
Ru/CeO2	single atom	260	28	100	[139]
Ni/CeO2	7.4 nm	350	57.7	97.5	[194]
Ru/h-BN b	single atom	350	29	93.5	[189]
Ru/TiO2	2.9 nm	300	30	>90	[195]
Ru/TiO2	single atom	210	15.6	97.3	[190]

^a USY = ultra-stable Y zeolite; ^b h-BN = porous hexagonal boron–nitride.

.

In summary, scaling up precious metals is not easy, but single-atom catalysts (SACs) solve multiple problems with an emphasis on efficiency and affordability. Traditional precious metal catalysts, such as Ru, Pt, and Pd, are quite expensive, which is a big obstacle. With the use of SACs, inexpensive metal supports can be doped or dispersed with trace amounts of these valuable metals (usually 4–6 wt%), drastically lowering the cost without sacrificing catalytic activity. Supported materials also keep atoms or particles of valuable metals from clumping together, which keeps them active and makes them more stable during reactions [188].

5. Future Perspectives

Bibliometric analysis revealed that research will focus on catalyst development, coke deposition, catalyst stability, kinetics, production of green hydrogen, and biomass valorization through the CO₂ methanation process. Further research should be performed on the deactivation mechanisms of supported catalysts, particularly Ni, over metal oxide-based catalysts.

Metal oxides, metal–organic frameworks (MOFs), and zeolites are examples of support that can help resolve catalyst activity and long-term stability challenges [57,196]. Each support material has a different thermal stability strength, sintering resistance, and metal dispersion capacity. Supports with a high surface area, moderate basicity, sufficient porosity, and metal–support solid interactions stabilize active Ni particles and prevent agglomeration for optimal catalytic activity [132,197,198]. Incorporating promoters such as Ru in Ni catalysts can improve catalyst performance by lowering deactivation rates and increasing methane yields [128]. Ru improves hydrogenation activity and resistance to carbon formation. To maximize the efficiency of commercial bimetallic catalysts, it is essential to consider factors such as catalysts’ suitability for methanation reactions in industrial processes, especially when severe reaction conditions such as high temperatures, pressure, and sulfur are present.

Transitioning from nanoparticles to nanoclusters and ultimately to atomically active metal dispersed sites enables enhanced activity and selectivity through the full utilization of active sites, alongside improved stability resulting from robust metal–support interactions that impede metal sintering and agglomeration [183]. Theoretical investigations have shown that ordinary SACs can hydrogenate CO₂ into formic acid [48]. The most challenging part of CO₂ hydrogenation is developing SACs, which need an in-depth knowledge of the structural properties that govern product selectivity towards methanation. In order to theoretically analyze the complicated reaction network and determine the precise structure of Ni and Ru catalysts, advanced data technology is necessary. In the case of SACs, there is a lack of knowledge about essential reaction intermediates and the complicated interaction between the many phases of the SAC and the reactants, making it challenging to develop a universal reaction mechanism for CO₂ conversion by SAC [199]. Unfortunately, most articles in the literature do not report carbon balance analysis, which is essential to detect undetectable C-containing products or coke deposition over the catalyst. When choosing a metal, either nickel or ruthenium, carbon balance over a complete CO₂ hydrogenation reaction should be performed. Advanced operando techniques, such as near ambient pressure X-ray photoelectron spectroscopy, supported by low energy electron diffraction and scanning tunnelling microscopy, can provide detailed insights into the intermediate stages that occur during CO₂ methanation [48]. Dual functional catalysts (CO₂ storage and methanation), reactors set up for intermediate water removal, and in situ, techniques for reaction mechanisms analysis are growing research areas in this field [4].

6. Conclusions

Methane (CH₄) is a desirable renewable fuel hydrocarbon among the products of CO₂ reduction reaction due to its well-established storage, transportation, and application capabilities. Based on bibliometric analysis, a synopsis of the catalyst for CO₂ methanation based on Ni and Ru has been developed. The Web of Science database was used to find relevant publications. VOSviewer software was used to visualize the link between different authors’ keywords appearing in Ni- and Ru-related documents from 2013 to 2023. Economic considerations, well-established techniques for Ni catalysts, and the intrinsic difficulties of ruthenium systems explained why these two types of catalysts are published at different rates. Recent studies in this field have focused mostly on catalysts based on Ni because of their extraordinary selectivity towards CH₄ and low cost. This review presents a comprehensive outlook of the current research landscape of Ni and Ru catalysts over the past decade to analyze their current status, publication trends, research pivotal points, and future development patterns.

Although the methanation of CO₂ is thermodynamically favourable, it is constrained by the solid stability of CO₂. It is significant to remember that the efficiency with which CO₂ is converted to methane during the reaction process depends on metal (Ni or Ru) interactions with various supports. The presence of oxygen vacancies in various reducible oxides, for example, CeO₂ and medium strength of Ni/Ru interaction with structured supports, have been active research areas. The stability and catalytic activity of monometallic Ni or Ru catalysts toward CO₂ methanation might be improved by adding other precious and transition metals to the catalyst. Research outcomes of bimetallic Ni and Ru catalysts are reviewed in this work. The development of single-atom catalysts on a global scale removes a significant barrier to scaling up precious metals-based catalysts for efficient CO₂ conversion. Methanation of CO₂ over nickel-based catalysts has the potential to expand further, making significant strides in areas such as sustainable environment, policymaking, and energy systems.