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Review

Ni Catalysts for Thermochemical CO2 Methanation: A Review

by
Jungpil Kim
Carbon & Light Materials Group, Korea Institute of Industrial Technology (KITECH), 222 Palbok-ro, Deokjin-gu, Jeonju 54853, Republic of Korea
Coatings 2024, 14(10), 1322; https://doi.org/10.3390/coatings14101322
Submission received: 19 September 2024 / Revised: 10 October 2024 / Accepted: 11 October 2024 / Published: 16 October 2024
(This article belongs to the Special Issue Advanced Research on Energy Storage Materials and Devices)
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Abstract

:
This review underscores the pivotal role that nickel-based catalysts play in advancing CO2 methanation technologies, which are integral to achieving carbon neutrality. This study meticulously examines various aspects of catalyst design, including the significance of support materials and co-catalysts in enhancing catalytic activity and selectivity. This discussion reveals that while nickel catalysts offer a cost-effective solution due to their availability and high performance, challenges such as sintering and carbon deposition at high temperatures remain. These issues necessitate the development of catalysts with superior thermal stability or those capable of maintaining high activity at lower temperatures. This review also highlights the innovative use of three-dimensional fiber deposition technology in fabricating catalysts, which has shown promising results in improving reaction efficiency and stability over prolonged operation. Moving forward, this research emphasizes the importance of optimizing catalyst structure and fabrication techniques to overcome existing limitations. The ongoing development in this field holds great promise for the industrial application of CO2 methanation, contributing significantly to global efforts in reducing greenhouse gas emissions and promoting sustainable energy use.

1. Introduction

Global warming poses one of the most significant threats to modern civilization, with the continuous rise in atmospheric greenhouse gas concentrations, particularly the high levels of carbon dioxide (CO2), being a primary contributing factor [1,2,3,4]. The increase in average global temperatures has led to rising sea levels, the melting of polar ice caps, and a surge in extreme weather events worldwide, resulting in serious environmental and societal challenges. These changes manifest in various forms, including reduced agricultural productivity [5], ecosystem destruction, and an increase in natural disasters [6], all of which pose severe threats to the sustainable development of humanity. Consequently, research and development aimed at reducing CO2 emissions and ultimately achieving carbon neutrality have garnered significant global attention [7,8,9].
Carbon neutrality aims to balance the amount of carbon dioxide emitted with the amount captured, stored, and converted, effectively reducing net emissions to zero. Various methods are being explored to achieve this goal, with the CO2 methanation reaction emerging as a crucial technology [10,11,12,13]. The CO2 methanation reaction is a process in which CO2 and hydrogen (H2) are reacted to produce methane (CH4) and water, as described by Reaction 1. By converting CO2 into methane, a valuable fuel, this process not only reduces greenhouse gas emissions but also enables the utilization of methane as an energy source. Additionally, the CO2 methanation reaction also produces carbon monoxide (CO) and water as byproducts in the reverse water–gas shift (RWGS) reaction (Reaction 2). Therefore, it is essential to enhance the selectivity for CH4 to improve the efficiency of the primary reaction while suppressing the byproduct reaction [14,15,16]. The methane generated through this process can be integrated into existing natural gas infrastructure, offering both energy efficiency and economic benefits.
CO2 + 4H2 → CH4 + 2H2O, ΔH298K = −165.4 kJ/mol
CO2 + H2 → CO + H2O, ΔH298K = 41.17 kJ/mol
The CO2 methanation reaction can be facilitated by various catalytic systems, including photocatalysts [12], electrochemical catalysts [17], biocatalysts [18], plasma catalysts [11], and thermocatalysts [19,20,21]. Among these, the conversion methods using photocatalysts, electrochemical catalysts, and plasma catalysts generally exhibit lower CO2 conversion rates and methane selectivity compared with thermocatalysts, rendering them less efficient for practical applications. In contrast, the methanation reaction using thermocatalysts is recognized for its high reaction efficiency and suitability for large-scale processes, making it a technology of considerable interest for industrial applications [22]. However, it requires significant energy consumption due to the high temperatures (300–400 °C) needed for efficient operation [23,24]. To address this issue, recent research has focused on the development of thermocatalysts that can maintain stable activity in low-temperature processes, reducing operational costs while enhancing CO2 conversion rates and CH4 selectivity [25,26]. The ability to reduce energy consumption and simplify reaction systems through low-temperature processes makes research on thermocatalysts that can sustain high activity and selectivity at lower temperatures essential [27].
In CO2 methanation using thermocatalysts, precious metals such as Ru, Rh, Pt, Pd, and Au are commonly employed due to their high activity. However, their high cost presents limitations for large-scale industrial applications [28,29]. As a result, nonprecious metal catalysts, such as Ni, Co, Fe, and Mo, have gained attention as more cost-effective alternatives [30]. Among these, nickel is particularly advantageous due to its low cost and abundant availability. Nickel also exhibits excellent hydrogen adsorption capacity, high catalytic activity, and methane selectivity, making it the most widely studied nonprecious metal catalyst for CO2 methanation [31,32]. However, nickel catalysts can suffer from performance degradation due to sintering or carbon deposition at high temperatures. To overcome these challenges, there is a need to either (1) develop catalysts with high thermal stability for high-temperature processes or (2) develop catalysts capable of maintaining high activity at low temperatures [33]. To achieve these goals, comprehensive catalyst screening and optimization, considering factors such as catalyst morphology and fabrication methods, are crucial. Additionally, thorough analysis and optimization of the physical and chemical properties of the catalysts are required to design catalysts that can perform efficiently under both high- and low-temperature conditions [34].
This review focuses on Ni-based catalysts with high CO2 conversion rates, CH4 yield, and selectivity, aligned with recent trends in industrial development and practical process applicability. It examines recent studies that have improved the catalytic activity and methane selectivity of nickel-based catalysts with high thermal stability through control of Ni active metal dispersion [35], the application of supports and co-catalysts [36], and the optimization of reaction conditions such as appropriate fabrication methods, temperature, pressure, space velocity, and H2/CO2 ratios [37]. Furthermore, it analyzes the impact of different catalyst forms (monoliths, 3D structures, etc.) on reaction efficiency compared with conventional powder catalysts, providing guidelines for improving catalyst lifespan and designing high-efficiency reactors with industrial feasibility. Through this, it aims to review the recent advances in practical catalyst technologies that contribute to optimizing low-temperature CO2 methanation processes and achieving carbon neutrality.

2. Mechanism of CO2 Methanation

The CO2 methanation reaction on catalysts primarily proceeds through two well-known mechanisms: the formate intermediate pathway (Reaction 3) [38] and the carbon monoxide (CO) intermediate pathway (Reaction 4) [37], as illustrated in Figure 1. In the formate pathway, the selection of the main reaction mechanism is influenced by the stability and reactivity of the formate intermediate on the catalyst surface, which is determined by the interaction between the formate and the catalyst [38]. Meanwhile, in the CO pathway, the direct interaction between CO2 and H2 on the catalyst, the adsorption of CO, its ability to dissociate, and the hydrogenation of adsorbed carbon species are key factors in determining the reaction’s progress [37]. Beyond these two mechanisms, other pathways also contribute, which are elucidated through various analytical methods such as density functional theory (DFT) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) [39].
Formate intermediate mechanism (Reaction 3):
  • Step 1: H2 + 2* ⇌ 2H*; CO2 + 2H* → HCOOH*
  • Step 2: HCOOH* → HCO* + OH*; OH*+ H* → H2O* → H2O
  • Step 3: HCO* → *CH + *O or HCO* + H* → HCOH* → *CH + OH*
  • Step 4: CH* + H* → CH2* + H* → CH3* + H* → CH4* → CH4
  • Step 5: O* + H* → OH* + H* → H2O* → H2O
In the formate intermediate mechanism for CO2 methanation, the reaction begins with the adsorption of CO2 and hydrogen (H2) molecules onto the catalyst surface [40]. Following this, the hydrogenation of CO2 with H2 leads to the formation of a surface-bound formate intermediate, which is considered the rate-determining step in the formate pathway. This surface-bound formate then decomposes into surface-bound HCO* species and surface hydroxyl (OH*) groups. As the reaction continues, the HCO* species undergo further hydrogenation with additional H* atoms to form surface-bound CH3* species, which ultimately results in the production of methane (CH4) (Reaction 3). The stability and reactivity of the formate intermediate play a critical role in determining the overall catalytic activity and selectivity for methane production from CO2 on the catalyst [38].
Direct evidence for the CO2 methanation mechanism was obtained using in situ infrared techniques [40]. Aldana et al. investigated CO2 methanation on a Ni–ceria–zirconia catalyst using in situ Fourier transform infrared spectroscopy (FTIR) [40]. At 150 °C, the FTIR spectra revealed carbonate (CO3(ad)) species on the catalyst support and carbonyl (CO(ad)) species on the Ni surface (Figure 1a-1). As the temperature increased, the carbonate species were hydrogenated to form bicarbonate (HCO3(ad)), which rapidly dehydrated to produce formate (HCOO(ad)), while the carbonyl species on the Ni surface remained unchanged (Figure 1a-1). It was suggested that the hydrogen atoms (H(ad)) required for this reaction originated from the Ni particles. The researchers proposed that methane and carbon monoxide were produced via different mechanisms: CH4 was formed through stepwise hydrogenation of the formate species, while CO was generated as a byproduct of CO2 reduction at Ce3+ sites (Figure 1a-2). The overall reaction pathways for methane production through the hydrogenation of carbonate and formate species are illustrated (Figure 1a-3) [39]. Additionally, Schild et al. observed a correlation between the decrease in formate signal and increased methane production, further supporting the role of formate as a key intermediate in the methane formation process [39] (Figure 2).
The carbon monoxide intermediate mechanism in CO2 methanation involves the formation of a formate (HCOO*) intermediate, which activates and directly converts CO2 and hydrogen into methane (CH4) on the catalyst surface [39]. Initially, CO2 and hydrogen molecules adsorb onto the active sites of the catalyst. CO2 is bound to the catalyst surface as CO2* species, while hydrogen forms surface-bound H* species. The surface-bound CO2* reacts with H* to form the formate intermediate (HCOO*), which subsequently decomposes into carbon monoxide (CO*) and hydroxyl (OH*) species. The CO* species undergoes hydrogenation, resulting in surface-bound intermediates such as CH*, CH2*, and CH3*. These intermediates continue to undergo hydrogenation until methane (CH4) is produced as the final product (Reaction 4). In energy calculations of the reaction steps, studies have shown that the potential energy barrier for CO2 dissociation into CO and O on the Ni (111) surface is approximately 41.8 kJ/mol, which is significantly lower than the barrier of 84.9 kJ/mol on the Ni (200) plane [43]. This highlights the importance of the surface structure in determining the activity of the catalyst. The carbon monoxide intermediate mechanism provides a distinct pathway for CO2 methanation compared with the formate intermediate mechanism, and it explains the overall catalytic activity and selectivity observed during the reaction [37,39,43].
Carbon monoxide intermediate mechanism (Reaction 4):
  • Step 1: CO2 ⇌ CO2*; H2 + 2* ⇌ 2H*
  • Step 2: CO2* + H* → HCOO*
  • Step 3: HCOO* → CO* + *OH
  • Step 4: CO* + 2H* → CH* + OH*
  • Step 5: CH* + H* → CH2* + H* → CH3* + H* → CH4* → CH4
  • Step 6: OH* + H* → H2O* → H2O
The methanation process via the carbon monoxide intermediate has been observed not only on Ni-based catalysts but also on Ru-based catalysts. Eckle et al. used Steady-State Isotope Transient Kinetic Analysis (SSITKA) to identify intermediates during CO and CO2 methanation on Ru–Al2O3 under hydrogen-rich conditions [41]. Their findings showed that isotope exchange from 12CO2 to 13CO2 led to a decrease in 12CO(ad) bands and an increase in 13CO(ad) bands, confirming carbonyl (C=O) as a key intermediate in the CO2 methanation process. In contrast, formate bands exhibited a slower response, suggesting that formate is not a major intermediate in this pathway (Figure 1b). These results align with temperature-programmed desorption studies. De Leitenburg et al. demonstrated that reduced ceria (Ce3+) enhances CO2 adsorption in the form of carbonyl species, while the metal particles provide the H(ad) necessary for methanation, with both reduced and oxidized forms of ceria (Ce3+ and Ce4+) playing roles in the process [39].
The thermal stability of catalysts plays a crucial role in both the formate and carbon monoxide intermediate mechanisms of CO2 methanation. In the formate mechanism, thermal stability influences the stability and reactivity of the formate intermediate (HCOO*) on the catalyst surface. A more thermally stable catalyst enhances resistance to the decomposition of the formate intermediate, allowing for prolonged catalytic activity through this pathway. Higher thermal stability also prevents sintering and surface restructuring, which helps maintain the active surface necessary for formate formation and subsequent hydrogenation reactions [38]. In the carbon monoxide intermediate mechanism, thermal stability is essential for the direct decomposition of CO2 and the subsequent hydrogenation steps on the catalyst surface. A stable catalyst surface facilitates CO2 adsorption and activation, promoting the efficient formation and hydrogenation of CO* species [37]. Additionally, thermal stability affects the stability of the CO* intermediate on the catalyst surface. A more thermally stable catalyst can maintain the CO* species as an active intermediate, enabling efficient CH4 production at high temperatures without decomposition or deactivation [43]. Understanding the relationship between thermal stability and reaction mechanisms is therefore critical for the rational design and optimization of catalysts for CO2 methanation.

3. Influence of Catalyst Supports on CO2 Methanation

In the CO2 methanation reaction using nickel-based catalysts, the role of the support material is crucial. Catalyst supports improve the dispersion of nickel particles, increasing the reactive surface area and thus providing more active sites for the reaction. Additionally, the support enhances the thermal stability of the nickel catalyst, helping it maintain its activity and structural integrity at high temperatures. Supports also influence the adsorption and reactivity of both CO2 and hydrogen, which in turn affects the selectivity and efficiency of the methanation reaction. Therefore, the chemical and structural properties of the support—such as surface area, porosity, reducibility, and activity—directly impact the performance of the catalyst and serve as key factors in determining the overall efficiency of the CO2 methanation process (Table 1) [31].
Ilsemann et al. summarized the effects of various supports in CO2 methanation based on their type [42]. For instance, supports like SiO2 do not exhibit significant support effects, and the methanation proceeds through the formation of C=O intermediates via CO2 hydrogenation. In contrast, supports such as Al2O3 and MgO can form bicarbonate species by interacting with CO2 through Brønsted basic OH groups. Additionally, surface oxygen atoms with sufficient Lewis basicity (e.g., defect sites) can act as adsorption sites, where monodentate carbonates may form. Other supports, such as TiO2, ZrO2, and Sm2O3, exhibit different mechanisms due to the oxygen vacancy effect, which provides additional pathways for the reaction to proceed.
In studies analyzing the thermal stability and resistance to sintering at higher activation temperatures [31], it has been shown that different supports—such as alumina (Al2O3), zirconia (ZrO2), and ceria (CeO2)—exhibit varying catalytic activities in high-temperature environments. For nickel-based catalysts used in CO2 methanation, the support material plays a crucial role in enhancing the overall catalytic performance and stability, especially under thermal stress. For alumina-supported catalysts, the reduction process occurs over a wide temperature range (250–800 °C). At temperatures above 600 °C, Ni and Al interact to form a spinel structure (NiAl2O4), which helps prevent sintering of the Ni particles. This interaction provides resistance to sintering, making 600 °C the optimal reduction temperature for alumina-supported nickel catalysts. As the nickel content increases, the number of active reaction sites grows, while the reduction temperature shows minimal impact on CO2 conversion efficiency. For zirconia-supported catalysts, the reduction typically occurs around 300 °C, with the optimal catalytic performance achieved at nickel loadings of 20–30 wt.%. The primary reduction occurs between 200 °C and 600 °C, involving the reduction of nickel oxide (NiO). Although reduction at 500–600 °C is common, larger nickel oxide crystallites formed at these temperatures may promote sintering, indicating that 300 °C is the ideal reduction temperature to minimize sintering. Ceria-supported catalysts exhibit a broader reduction range, from 100 °C to 800 °C. At lower temperatures (100–600 °C), the reduction primarily involves NiO, whereas at higher temperatures (600–800 °C), both NiO reduction and partial reduction of the ceria support (CeO2 → Ce2O3) occur. The structural changes in the ceria support and sintering of Ni particles at higher temperatures suggest that 400 °C is the most appropriate reduction temperature to prevent structural deterioration.
X-ray diffraction (XRD) studies [31] further highlight the distinct characteristics of these supports. For alumina-supported catalysts, the reflection peaks of Ni partially overlap with those of the metal nickel phase. The average crystallite size of NiO is very small (near detection limits) for 10 wt.% Ni/Al2O3 but increases slightly as the nickel content rises to 40 wt.%, with an average size of 6.3 nm. After reduction at 600 °C, the Ni (100) reflection peak decreases, indicating that alumina-supported catalysts exhibit high resistance to sintering at elevated temperatures. For instance, at 600 °C, the average Ni crystallite size for 20 wt.% Ni/Al2O3 is 4.2 nm, and this increases to 6.9 nm at 800 °C. Zirconia-supported nickel catalysts, reduced at 400 °C, exhibit strong tetragonal (t-ZrO2) phase peaks in the XRD patterns. Heating leads to phase transitions from t-ZrO2 to cubic (c-ZrO2), stabilizing the structure by forming cuboid or tetragonal polymorphs [57]. The average Ni crystallite size increases significantly with nickel content, ranging from 9.5 nm at 10 wt.% to 83.2 nm at 40 wt.%. This size increase is more pronounced than in alumina-supported catalysts. Ceria-supported catalysts also show strong CeO2 phase peaks in the XRD curves. For 20 wt.% Ni/CeO2, the average ceria crystallite size is around 25 nm. After reduction at 400 °C, this increases to 26.0 nm, 29.1 nm at 600 °C, and 61.2 nm at 800 °C. Under reduction conditions, Ce4+ is partially converted to Ce3+, leading to the formation of oxygen vacancies, changes in the Ce-O bond length, and an increase in the ionic radius of Ce ions. This behavior contributes to the enhanced dispersion of nickel particles. In 20 wt.% Ni/CeO2 catalysts, the average size of Ni crystallites is 18.2 nm after reduction at 600 °C and 28.4 nm after reduction at 800 °C.
The catalytic performance, including CO2 conversion and CH4 selectivity, is influenced by the support material and reaction temperature. As temperature increases, CO2 conversion and CH4 selectivity exhibit similar trends across all catalysts. However, due to the thermodynamic limitations of the CO2 methanation reaction—an exothermic reaction with a negative enthalpy value (ΔH298K = −165.4 kJ/mol)—the reaction equilibrium shifts at temperatures above 350 °C, leading to lower CO2 conversion rates and higher CO production. At lower temperatures (200–350 °C), all catalysts reach approximately 90% CO2 conversion, but this decreases to about 60% at 600 °C. CH4 selectivity is nearly 100% at low temperatures but drops to around 60% at 600 °C due to the thermodynamic constraints. Among the supports, alumina and ceria-based catalysts maintain high thermal stability and efficiency at elevated temperatures. Notably, 24 h stability tests at 350 °C showed no significant deactivation in 20 wt.% Ni/Al2O3 and 20 wt.% Ni/CeO2 catalysts. The high performance of alumina-supported catalysts is attributed to the low thermal conductivity of alumina, which helps maintain long-term reaction efficiency.
Other support materials, such as silica (SiO2), are also widely used for Ni catalysts in CO2 methanation. SiO2-based supports, particularly mesoporous structures such as MCM-41 and SBA-15, offer distinct advantages due to their high surface areas and unique pore structures. Studies have shown that Ni/MCM-41 catalysts exhibit excellent CH4 selectivity (90–100%) in the 250–450 °C range (Figure 3) [58]. The small Ni particles on the outer surface of the support enhance accessibility to active sites, improving catalytic performance. The band gap of NiO is influenced by the size of the Ni particles, with larger particles resulting in more vacancies and lower band gaps, which effectively enhances the CO2 methanation process [59]. Adding ceria to Ni/MCM-41 further enhances methanation activity, especially at lower temperatures, by increasing the number of adsorbed formate species and improving the overall reaction mechanism [60].
In addition to MCM-41 and SBA-15, mesoporous silica nanoparticles (MSN) are also widely used as supports for nickel catalysts in CO2 methanation. These catalysts offer high activity and selectivity due to the unique characteristics of MSNs, including their nanoscale size, ordered structure, very high surface area, large pore volume, and tunable pore sizes (1.5–10 nm) [61]. In a study conducted by Aziz et al., the reaction was carried out between 150 and 450 °C, and Ni/MSN exhibited higher catalytic activity compared with Ni/MCM-41, Ni/HY, Ni/SiO2, and Ni/γ-Al2O3 (Figure 4a) [62]. The carbon species detected on the MSN surface were attributed to defect sites and oxygen vacancies within the MSN structure, while the Ni sites dissociated H2 into atomic hydrogen, which then reacted with the surface carbon species to form CH4. In a 200 h thermal stability and durability test, the Ni/MSN catalyst maintained stable activity (Figure 4b).
Catalytic activity measurements of MSN-supported catalysts loaded with various metals (Rh, Ru, Ni, Fe, Ir, Cu, Zn, V, Cr, Mn, Al, Zr) revealed that the oxygen vacancy sites associated with the metals are critical for catalytic performance. At 350 °C, the activity followed the order Rh/MSN > Ru/MSN > Ni/MSN > Ir/MSN > Fe/MSN > Cu/MSN, although on a per-area basis, Ni/MSN demonstrated the highest efficiency [63]. During the reaction, CO2 and H2 adsorbed onto the metal sites, where they dissociated into CO, O, and H atoms, which then migrated across the MSN surface. CO reacted with the oxides on MSN, forming bridged and linear carbonyls, while interacting with atomic hydrogen to generate bidentate formates. Simultaneously, oxygen atoms dissociated from the MSN surface and stabilized in oxygen vacancies near the metal sites. The adsorbed oxygen atoms then interacted with atomic hydrogen to form hydroxyl groups, which reacted with additional hydrogen atoms to form water. As a result, the carbon species were hydrogenated to CH4 [53,62,63].
To validate the efficiency of the Ni/MSN catalyst, Aziz et al. conducted a study [53] where various experimental conditions were modified, revealing that CO2 methanation efficiency depends on multiple factors, including reaction temperature, catalyst treatment time, H2/CO2 ratio, and space velocity. CO2 conversion increased with reaction temperature, but after reaching the thermodynamic limit (614 K), conversion decreased due to carbon deposition and side reactions. Based on these insights, the optimal experimental conditions were determined to be a catalyst treatment time of 6 h, reaction temperature of 614 K, space velocity of 69.1 mL g_cat−1 h−1, and an H2/CO2 ratio of 3.68, achieving a CO2 conversion rate of approximately 82%. These studies highlight the importance of selecting the appropriate support material and considering various factors—such as the interaction between nickel and the support, reaction mechanisms, and the doping of other metals—to design highly efficient Ni catalysts for CO2 methanation (Table 1) [60,64].

4. Effect of Co-Catalysts on CO2 Methanation

The use of co-catalyst materials alongside nickel significantly influences the CO2 methanation reaction. Bimetallic catalysts often exhibit enhanced catalytic activity compared with monometallic catalysts, as the combination of two metals can generate new active sites or modify existing ones, improving catalytic efficiency. The addition of a co-catalyst metal can alter the reaction pathway, increasing selectivity for the desired products. Bimetallic catalysts may also exhibit greater resistance to deactivation processes such as sintering, coking, and other forms of catalyst degradation, thereby enabling longer catalyst lifespans and stable operation at high temperatures. The interaction between the two metals in bimetallic catalysts can lead to changes in the surface structure of the catalyst, affecting the adsorption and activation of reactants [65,66]. Some bimetallic catalysts perform better under high-temperature conditions, making them useful in processes where temperature control is critical. In certain studies, small amounts (1 wt.%) of noble metals (Ru, Pt, Rh, Pd, Ir) have been combined with non-noble metals such as nickel to maintain or enhance catalytic performance while reducing overall costs [67]. The specific effects of the co-catalyst depend on its selection, its interaction with nickel, and the reaction conditions.
Certain noble metals exhibit lower efficiency in CO2 methanation (Figure 5a–d). A study analyzing the CO2 methanation efficiency of nickel-based catalysts supported on ceria doped with praseodymium (Pr) and loaded with various noble metals showed different outcomes depending on the metal used [67]. Ruthenium (Ru) significantly enhanced the catalytic activity of nickel-based catalysts due to its high hydrogenation reactivity, achieving around 80% CO2 conversion (Figure 5a) and 99.5% CH4 selectivity at 325 °C (Figure 5b). This improvement is attributed to better metal dispersion, increased catalyst reducibility, and the presence of additional active sites for CO2 and H2 dissociation. In contrast, platinum (Pt) and iridium (Ir) maintained similar catalytic performance to monometallic nickel (Figure 5a–d), indicating the presence of active sites, but without a notable improvement in overall performance. Interestingly, the addition of rhodium (Rh) and palladium (Pd) resulted in decreased catalytic performance (Figure 5a,c,d). Both metals acted as inhibitors for the reaction, requiring high temperatures for 50% CO2 conversion. The activation energy for CO2 (Ea,CO2) was determined from the Arrhenius plots in Figure 5c, with values ranging between 100 and 115 kJ/mol−1. Notably, the 1Ir10Ni catalyst showed an exception, with an Ea,CO2 of 102 kJ/mol−1, which can be attributed to the more gradual increase in CO2 conversion as the temperature rises, as discussed previously. This suggests that iridium (Ir) could have a beneficial effect on CO2 methanation at lower reaction temperatures. In summary, except for ruthenium, the addition of noble metals generally reduced the surface activity of the catalyst, making them less favorable for CO2 methanation.
Non-noble metals such as iron (Fe), cobalt (Co), and copper (Cu) are commonly used as co-catalysts alongside nickel in CO2 methanation. Studies examining the impact of these bimetallic catalysts supported on alumina, zirconia, and ceria have shown promising results [66]. Nickel-iron (Ni-Fe) bimetallic catalysts demonstrated higher methane yields compared with monometallic nickel catalysts. Specifically, Ni-Fe alloys exhibited improved thermodynamic stability and activity compared with using nickel alone. The enhanced performance is attributed to the alloy structure, which optimizes active sites for the CO2 methanation reaction. Fe addition modifies the reaction mechanism, promoting more efficient methanation. Nickel–cobalt (Ni-Co) catalysts also improved CO2 methanation performance, with cobalt enhancing nickel’s reducibility and dispersion. This is likely due to cobalt’s proximity to nickel in the periodic table, allowing cobalt to dissolve into nickel’s metal lattice, thereby improving the catalyst’s overall activity and stability. Nickel-rich Ni-Co catalysts, in particular, displayed higher activity, with one study suggesting that a Co-Ni ratio of 0.4 provided the most optimal performance (Figure 5e). High cobalt content promotes CO dissociation and hydrogen mobility, further boosting catalytic activity [68].
On the other hand, copper’s addition to nickel-based catalysts negatively affected CO2 methanation. Copper tended to promote the reverse water–gas shift (RWGS) reaction instead, which led to increased CO production rather than methane. Copper-containing bimetallic catalysts were found to be less effective than monometallic nickel catalysts for CO2 methanation. One study reported that adding just 1% copper significantly reduced CO2 conversion while increasing selectivity towards CO production (Figure 5f). This is attributed to copper’s ability to improve nickel’s dispersion and reducibility while simultaneously hindering hydrogen adsorption on the Ni-Cu alloy, deactivating the catalyst for CO2 methanation [69].

5. Performance Comparison between Powder Catalysts and Monolithic Catalysts

In low-temperature CO2 methanation processes, the differences between powder catalysts and monolithic catalysts have a significant impact on reaction efficiency [70]. Powder catalysts provide a high surface area, which maximizes contact with the reactants, thereby increasing the reaction rate and enabling efficient methanation. However, at the temperatures required for CO2 methanation, sintering caused by particle agglomeration can occur, leading to a reduction in the active surface area of nickel and a decline in catalyst performance. For example, a 50 wt.% Ni/GDC (gadolinium-doped ceria) powder catalyst exhibited a CO2 conversion rate of around 44% at 300 °C, which increased to a maximum of 71% at 450 °C (Figure 6b). While higher temperatures yield better conversion, optimizing the catalyst structure for low-temperature processes is essential to reduce operating costs and prevent catalyst sintering [71].
On the other hand, monolithic catalysts feature a mechanically stable structure with a catalyst layer formed on a coated support, allowing them to maintain uniformity (Figure 6a). These catalysts facilitate efficient heat transfer, ensuring a uniform temperature distribution within the catalyst layer, which is particularly beneficial for maintaining high catalytic activity even at lower temperatures. Experimental results showed that a monolithic catalyst loaded with 0.5 g/cm3 of nickel achieved a CO2 conversion rate of approximately 41% at 350 °C and a maximum of 68% at 450 °C (Figure 6b). Moreover, this catalyst maintained high conversion rates and methane productivity under space velocity conditions of 50,000 h−1, achieving a maximum methane productivity of 10.7 L_CH4/g h at 400 °C—five times higher than that of the powder catalyst—and demonstrating stability for over 200 h [70]. Thus, while powder catalysts are advantageous in low-temperature processes by maximizing contact with reactants and increasing the initial reaction rate, monolithic catalysts outperform in terms of long-term operation and temperature control, demonstrating superior performance in these areas [72,73].
SEM images and EDX mapping were used to analyze the pore structure and particle dispersion of uncoated and coated monolithic nickel catalysts. The SEM image of the uncoated monolith surface revealed distinct macroporosity (Figure 7a) [70]. These large pores could hinder heat transfer during the reaction or interfere with the uniform coating of the catalyst layer. In contrast, the SEM of the coated monolith showed a uniformly distributed catalyst layer along the edges of the monolith surface, increasing the contact area with the reactants and enhancing the efficiency of the CO2 methanation reaction (Figure 7b). At lower magnification, the internal structure of the coated monolith revealed a uniformly formed catalyst layer inside the structure, which helps prevent catalyst layer detachment during the reaction and contributes to maintaining consistent activity [70].
The macroporosity observed in the monolith is thought to have been formed during the heat treatment process when gases escaped, leaving behind pores that increase the surface area. This pore structure positively impacts the initial reaction rate and overall CO2 conversion in methanation. Additionally, the uniform distribution of the catalyst layer along the interior walls of the monolith (Figure 7c) ensures that the flow of reactants is not obstructed while maintaining high reaction efficiency. EDX mapping confirmed the even distribution of nickel across the monolith surface (Figure 7d). This uniform nickel distribution ensures that reactants are evenly activated and converted, maximizing the overall reaction efficiency in CO2 methanation. The results suggest that monolith catalysts, compared with powder catalysts, maintain high activity even at low temperatures, and their optimized structure is well-suited for large-scale processes to implement efficient CO2 methanation [74,75,76].

6. Performance Analysis of Nickel Catalysts Using 3D Fiber Deposition Technology

According to research results on the development of an alumina (Al2O3) macroporous structure using three-dimensional fiber deposition (3DFD) technology to enhance the structure of monolithic catalysts, it was observed that this approach exhibited higher reaction efficiency compared with previously mentioned powder and monolithic catalysts [77]. 3DFD refers to a technique in which a nozzle moves in the x, y, and z directions under computer control, depositing a viscous paste layer by layer (Figure 8a). Several variables, such as nozzle diameter, fiber thickness, spacing between fibers, and layer stacking method, are configured during this process. By adjusting these variables, the pore size and surface roughness of the 3D structure can be precisely controlled. The paste extruded from the nozzle forms the structure as it accumulates layer by layer, and a subsequent high-temperature sintering process completes the structure into a robust porous metal or ceramic framework. Catalysts synthesized by coating Ni/Al2O3 on a stainless-steel surface with a structure controlled using the 3DFD method were tested in the CO2 methanation reaction within the temperature range of 250 °C to 450 °C, achieving a CO2 conversion rate of 74% at 350 °C. In contrast, under the same temperature conditions, the powder catalyst recorded a conversion rate of 61%. Additionally, at 400 °C, the 3D structure catalyst showed a CO2 conversion rate of 87%, demonstrating higher efficiency than the powder catalyst (Figure 8b).
A comparison of the long-term stability of CO2 conversion rates over time between the 3D structure catalyst and the powder catalyst (Figure 8c) revealed that the 3D structure catalyst maintained a CO2 conversion rate of approximately 80% over a reaction period exceeding 50 h, indicating stable performance during extended operation. In contrast, while the powder catalyst initially exhibited a CO2 conversion rate of approximately 75%, its conversion rate gradually decreased over time, dropping to about 65% after 50 h. The 3D structure catalyst’s stable CO2 conversion rate during long-term operation is attributed to its inherent structural and thermal stability. Specifically, the 3D structure catalyst minimizes the aggregation and sintering of nickel catalyst particles, allowing consistent catalytic performance over extended periods. In contrast, the powder catalyst experiences a reduction in active sites due to nickel particle aggregation and sintering over time, leading to a decline in CO2 conversion. These results suggest that the superior heat and mass transfer capabilities of the three-dimensional fiber deposition structure improve reaction rates and optimize the catalyst’s performance [78].
The low-temperature CO2 methanation reaction is characterized by a slow reaction rate, making the surface area of the catalyst and the contact efficiency between the nickel particles and the reactants critically important. From this perspective, the quality, uniformity, and durability of the nickel coating can directly impact the reaction efficiency. An analysis of the durability and adhesion strength of the coated 3DFD structure, conducted using ultrasonic adhesion strength tests and SEM images of the catalyst coating before and after reliability evaluations, showed that the coating adhered uniformly to the stainless-steel structure before ultrasonic treatment, with a thickness measured at approximately 18 μm. This suggests that the coating possesses sufficient thickness to maximize the contact area with the reactants during the reaction [77]. However, after ultrasonic treatment, the coating thickness decreased to about 12 μm, indicating that a small amount of the coating layer had been removed. Nevertheless, the remaining thickness was deemed sufficient to maintain stability, even under low-temperature processing. This implies that the catalyst layer experiences minimal detachment during long-term operation, ensuring consistent performance in the CO2 methanation reaction.
To control the viscosity of the nickel coating slurry in the 3DFD process, various concentrations of polyvinyl alcohol (PVA) were used to coat the catalyst. Analysis of the surface after the CO2 methanation reaction indicated that when 1 wt.% PVA was used, cracks appeared on the coating surface, likely due to shrinkage during the drying process [77]. These cracks could disrupt the continuity of the catalyst layer, potentially hindering sufficient contact between CO2 and H2 with the catalyst surface during the reaction, thereby reducing the efficiency of the methanation process [79]. In contrast, when 3 wt.% PVA was used, the coating was observed to form uniformly without any cracks, indicating that the optimal PVA concentration contributed to controlling the viscosity and uniformity of the catalyst coating, thereby maximizing the continuity and surface area of the catalyst layer. Such a uniform coating plays a critical role in enhancing the efficiency of the CO2 methanation reaction at low temperatures [77]. However, when 5 wt.% PVA was used, the coating formed unevenly, as the increased coating load resulted in a thicker layer, causing imbalance due to internal stresses. This uneven coating can hinder heat transfer and may lead to localized overheating or inefficiencies during the reaction. These findings highlight the importance of the uniformity and adhesion strength of the nickel coating in designing catalysts for low-temperature CO2 methanation reactions using 3DFD. By forming a uniform catalyst layer with the optimal PVA concentration, high CO2 conversion rates can be achieved, and sufficient adhesion strength ensures stable long-term operation.

7. Conclusions

This review article presents an in-depth analysis of the recent advancements in nickel-based catalysts for CO2 methanation, a critical process in the pursuit of carbon neutrality. This study focuses on the mechanisms involved in the reaction, the role of support materials, and the impact of various co-catalysts. By examining the influence of catalyst morphology and the fabrication process, this review highlights the challenges and strategies for improving catalyst performance, particularly at low temperatures. The findings suggest that optimizing the uniformity and adhesion strength of nickel coatings is essential for enhancing methanation efficiency. Continued interdisciplinary research is essential to refine these technologies and fully realize their potential in addressing the pressing challenges of climate change.

Funding

This work was supported by the Ministry of Trade, Industry, and Energy (MOTIE) [Grant number 20016789] and the Korea Institute of Industrial Technology (KITECH UR-24-0034).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data are contained within the article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Mechanisms and energy pathways in CO2 methanation: Exploring formate and carbon monoxide intermediates on catalysts. (a-1) FTIR spectra for CO2 methanation on the Ni–ceria–zirconia catalyst (150 to 400 °C, H2:CO2 = 4:1). The formation of bicarbonate and formate intermediates is associated with the production of CH4, while carbonyl species remain as nonreactive observers (spectators) during the reaction [40] (reprinted/adapted with permission from Elsevier, 2013). (a-2) In a transient experiment (at 400 °C) on the Ni–ceria–zirconia catalyst, the formation of CH4 ceased immediately after H2 was turned off (solid line circle), and the formation of CO ceased immediately after CO2 was turned off (dotted circle line). These observations indicate distinct pathways for the production of methane and carbon monoxide [40] (reprinted/adapted with permission from Elsevier, 2013). (a-3) Methane is produced through the continuous hydrogenation of carbonate species adsorbed on the catalyst support, proceeding via formate and methoxy intermediates [39] (reprinted/adapted with permission from the Royal Society of Chemistry, 2016). (b-1,b-2) DRIFT spectra of CO2 methanation on Ru–Al2O3 (190 °C, H2:CO2 = 5.3:1), showing a switch from (b-1) 12CO2 reformate gas to (b-2) 13CO2 reformate gas, indicating the involvement of CO2 dissociation in the reaction [41] (reprinted/adapted with permission from American Chemical Society, 2011). (b-3) CO2 dissociates into CO(ad) and O(ad) species on Ru sites. The adsorbed CO then evolves into methane through an associative pathway, emphasizing the mechanistic role of CO intermediates in methane production [39] (reprinted/adapted with permission from the Royal Society of Chemistry, 2016).
Figure 1. Mechanisms and energy pathways in CO2 methanation: Exploring formate and carbon monoxide intermediates on catalysts. (a-1) FTIR spectra for CO2 methanation on the Ni–ceria–zirconia catalyst (150 to 400 °C, H2:CO2 = 4:1). The formation of bicarbonate and formate intermediates is associated with the production of CH4, while carbonyl species remain as nonreactive observers (spectators) during the reaction [40] (reprinted/adapted with permission from Elsevier, 2013). (a-2) In a transient experiment (at 400 °C) on the Ni–ceria–zirconia catalyst, the formation of CH4 ceased immediately after H2 was turned off (solid line circle), and the formation of CO ceased immediately after CO2 was turned off (dotted circle line). These observations indicate distinct pathways for the production of methane and carbon monoxide [40] (reprinted/adapted with permission from Elsevier, 2013). (a-3) Methane is produced through the continuous hydrogenation of carbonate species adsorbed on the catalyst support, proceeding via formate and methoxy intermediates [39] (reprinted/adapted with permission from the Royal Society of Chemistry, 2016). (b-1,b-2) DRIFT spectra of CO2 methanation on Ru–Al2O3 (190 °C, H2:CO2 = 5.3:1), showing a switch from (b-1) 12CO2 reformate gas to (b-2) 13CO2 reformate gas, indicating the involvement of CO2 dissociation in the reaction [41] (reprinted/adapted with permission from American Chemical Society, 2011). (b-3) CO2 dissociates into CO(ad) and O(ad) species on Ru sites. The adsorbed CO then evolves into methane through an associative pathway, emphasizing the mechanistic role of CO intermediates in methane production [39] (reprinted/adapted with permission from the Royal Society of Chemistry, 2016).
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Figure 2. Conceptual illustration of potential support effects and the classification of the support materials used [42] (reprinted/adapted with permission from the Royal Society of Chemistry, 2021). No support effect (‘CO pathway’), (1) CO2 adsorption as monodentate or hydrogen carbonate, (2) CO2 activation as bidentate carbonate, and (3) positive impact of electronic metal–support interactions; red squares represent oxygen vacancies, and red arrows denote electronic interactions. In each scenario, H2 is presumed to dissociate upon adsorption on the metal (not depicted here for simplicity).
Figure 2. Conceptual illustration of potential support effects and the classification of the support materials used [42] (reprinted/adapted with permission from the Royal Society of Chemistry, 2021). No support effect (‘CO pathway’), (1) CO2 adsorption as monodentate or hydrogen carbonate, (2) CO2 activation as bidentate carbonate, and (3) positive impact of electronic metal–support interactions; red squares represent oxygen vacancies, and red arrows denote electronic interactions. In each scenario, H2 is presumed to dissociate upon adsorption on the metal (not depicted here for simplicity).
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Figure 3. Comparison of CO2 conversions (solid symbols) and CH4 selectivity (hollow symbols) for (a) 15% Ni/SBA-15 CL (circles) versus 15% Ni/SBA-15 MW (triangles); (b) 15% Ni/SBA-15 CL (circles) versus 15% Ni/MCM-41 (squares); (c) 15% Ni/MCM-41 (squares) versus 15% Ni/HNaUSY (diamonds). CL and MW refer to samples synthesized via the classical solution process and microwave-assisted synthesis, respectively [58] (reprinted/adapted with permission from Elsevier, 2018).
Figure 3. Comparison of CO2 conversions (solid symbols) and CH4 selectivity (hollow symbols) for (a) 15% Ni/SBA-15 CL (circles) versus 15% Ni/SBA-15 MW (triangles); (b) 15% Ni/SBA-15 CL (circles) versus 15% Ni/MCM-41 (squares); (c) 15% Ni/MCM-41 (squares) versus 15% Ni/HNaUSY (diamonds). CL and MW refer to samples synthesized via the classical solution process and microwave-assisted synthesis, respectively [58] (reprinted/adapted with permission from Elsevier, 2018).
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Figure 4. (a) Activity of all catalysts at the steady state as a function of the reaction temperature. (b) Long-term stability test of Ni catalysts for CO2 methanation reaction at a temperature of 573 K. GHSV = 50,000 mL/g·h and H2/CO2 = 4:1 [62] (reprinted/adapted with permission from Elsevier, 2014).
Figure 4. (a) Activity of all catalysts at the steady state as a function of the reaction temperature. (b) Long-term stability test of Ni catalysts for CO2 methanation reaction at a temperature of 573 K. GHSV = 50,000 mL/g·h and H2/CO2 = 4:1 [62] (reprinted/adapted with permission from Elsevier, 2014).
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Figure 5. Catalytic performance and selectivity of Ni-based catalysts in CO2 methanation reactions: (a) CO2 conversion with increasing reaction temperature for various Ni-based catalysts [67] (reprinted/adapted with permission from Elsevier, 2024). (b) CH4 selectivity with increasing reaction temperature for various Ni-based catalysts [67] (reprinted/adapted with permission from Elsevier, 2024). (c) Natural logarithmic values of the CO2 consumption rate plotted against the inverse of temperature (Arrhenius plots) [67] (reprinted/adapted with permission from Elsevier, 2024). (d) CO2 consumption rate data for all catalysts at three distinct reaction temperatures under kinetic control [67] (reprinted/adapted with permission from Elsevier, 2024). (e) CO2 conversion at 300 °C as a function of Co/Ni molar ratios [68] (reprinted/adapted with permission from Springer, 2014). (f) CH4 selectivity as a function of temperature for Ni5, Ni10, Ni15, Ni9Cu1, and Ni10Cu1 catalysts [69] (reprinted/adapted with permission from Elsevier, 2020).
Figure 5. Catalytic performance and selectivity of Ni-based catalysts in CO2 methanation reactions: (a) CO2 conversion with increasing reaction temperature for various Ni-based catalysts [67] (reprinted/adapted with permission from Elsevier, 2024). (b) CH4 selectivity with increasing reaction temperature for various Ni-based catalysts [67] (reprinted/adapted with permission from Elsevier, 2024). (c) Natural logarithmic values of the CO2 consumption rate plotted against the inverse of temperature (Arrhenius plots) [67] (reprinted/adapted with permission from Elsevier, 2024). (d) CO2 consumption rate data for all catalysts at three distinct reaction temperatures under kinetic control [67] (reprinted/adapted with permission from Elsevier, 2024). (e) CO2 conversion at 300 °C as a function of Co/Ni molar ratios [68] (reprinted/adapted with permission from Springer, 2014). (f) CH4 selectivity as a function of temperature for Ni5, Ni10, Ni15, Ni9Cu1, and Ni10Cu1 catalysts [69] (reprinted/adapted with permission from Elsevier, 2020).
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Figure 6. Differences between Ni-based powder and monolith catalysts [70] (reprinted/adapted with permission from Elsevier, 2018): (a) Images of monoliths before coating (left) and after coating (right). (b) Influence of temperature (300–600 °C) and space velocity (10,000–50,000 h−1) on CH4 production for powder catalysts (50 wt.% Ni/GDC) and monolith catalysts with a density of 0.5 g/cm3.
Figure 6. Differences between Ni-based powder and monolith catalysts [70] (reprinted/adapted with permission from Elsevier, 2018): (a) Images of monoliths before coating (left) and after coating (right). (b) Influence of temperature (300–600 °C) and space velocity (10,000–50,000 h−1) on CH4 production for powder catalysts (50 wt.% Ni/GDC) and monolith catalysts with a density of 0.5 g/cm3.
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Figure 7. SEM images of uncoated and coated monoliths [70] (reprinted/adapted with permission from Elsevier, 2018): (a) Corner surface view of the uncoated monolith. (b) Coated monoliths. (c) Frontal surface view. (d) EDX mapping of the coated monolith.
Figure 7. SEM images of uncoated and coated monoliths [70] (reprinted/adapted with permission from Elsevier, 2018): (a) Corner surface view of the uncoated monolith. (b) Coated monoliths. (c) Frontal surface view. (d) EDX mapping of the coated monolith.
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Figure 8. 3DFD Ni/Al2O3-coated structured catalysts [77] (reprinted/adapted with permission from Elsevier, 2016): (a) Cross-sectional images of structures with 1-1 (top) and 1-3 (bottom) stacking configurations. (b) Methanation reactions conducted at various temperatures (WHSV 1500 h−1). (c) Stability test comparing packed-bed and 4B1 structured catalysts.
Figure 8. 3DFD Ni/Al2O3-coated structured catalysts [77] (reprinted/adapted with permission from Elsevier, 2016): (a) Cross-sectional images of structures with 1-1 (top) and 1-3 (bottom) stacking configurations. (b) Methanation reactions conducted at various temperatures (WHSV 1500 h−1). (c) Stability test comparing packed-bed and 4B1 structured catalysts.
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Table 1. Overview of Nickel-Based Catalysts for CO2 Methanation: According to Support Materials.
Table 1. Overview of Nickel-Based Catalysts for CO2 Methanation: According to Support Materials.
TypeCatalystsCO2 Conv. (%)CH4 Sel. (%)Temp. (°C)Pressure (bar)Ref.
Alumina, zirconia, and ceria25% Ni-CeO2/γ-Al2O3211002506[44]
20% Ni/Al2O382.41003501[45]
23% Ni/CaO-Al2O381994001[46]
6% Ni-MgO/ZrO2901002501[47]
10% Ni/CeO2-ZrO25599.82751[48]
15% Ni/ZrO26097.53001[49]
10% Ni/ZrO2-Al2O3771003001[50]
20% Ni/ZrO279.196.73501[51]
10% Ni/CeO283.31003501[52]
Silica10% Ni/MSN851003501[53]
10% Ni-MgO/SiO273.298.74001[54]
20% Ni-CeO2/MCM-4185.699.83801[55]
10% Ni-CeO2/SBA-1568.899.04001[56]
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Kim, J. Ni Catalysts for Thermochemical CO2 Methanation: A Review. Coatings 2024, 14, 1322. https://doi.org/10.3390/coatings14101322

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Kim J. Ni Catalysts for Thermochemical CO2 Methanation: A Review. Coatings. 2024; 14(10):1322. https://doi.org/10.3390/coatings14101322

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Kim, Jungpil. 2024. "Ni Catalysts for Thermochemical CO2 Methanation: A Review" Coatings 14, no. 10: 1322. https://doi.org/10.3390/coatings14101322

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Kim, J. (2024). Ni Catalysts for Thermochemical CO2 Methanation: A Review. Coatings, 14(10), 1322. https://doi.org/10.3390/coatings14101322

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