Research Progress and Reaction Mechanism of CO 2 Methanation over Ni-Based Catalysts at Low Temperature: A Review

: The combustion of fossil fuels has led to a large amount of carbon dioxide emissions and increased greenhouse effect. Methanation of carbon dioxide can not only mitigate the greenhouse effect, but also utilize the hydrogen generated by renewable electricity such as wind, solar, tidal energy, and others, which could ameliorate the energy crisis to some extent. Highly efﬁcient catalysts and processes are important to make CO 2 methanation practical. Although noble metal catalysts exhibit higher catalytic activity and CH 4 selectivity at low temperature, their large-scale industrial applications are limited by the high costs. Ni-based catalysts have attracted extensive attention due to their high activity, low cost, and abundance. At the same time, it is of great importance to study the mechanism of CO 2 methanation on Ni-based catalysts in designing high-activity and stability catalysts. Herein, the present review focused on the recent progress of CO 2 methanation and the key parameters of catalysts including the essential nature of nickel active sites, supports, promoters, and preparation methods, and elucidated the reaction mechanism on Ni-based catalysts. The design and preparation of catalysts with high activity and stability at low temperature as well as the investigation of the reaction mechanism are important areas that deserve further study.


Introduction
With the continuous advancement of social economy and the unceasing enhancement of human living standards, the overuse of fossil fuels has resulted in an energy crisis while the excessive emission of CO 2 has exacerbated the greenhouse effect, which has induced global climate problems [1][2][3][4][5]. One of the most effective strategies is to replace fossil fuels with clean, renewable, carbon-neutral alternatives. Photocatalytic hydrogen production by solar energy has attracted the extensive attention of researchers, which was considered to be one of the most promising ways of utilizing solar energy because the combustion of H 2 would release a large quantity of energy and the product would not pollute the environment [6][7][8][9]. However, it is impossible to achieve an industrial application due to the cost and operational limitations of this technology at present. Moreover, capture and geological sequestration strategies have also been used to remove excessive CO 2 in the atmosphere [10][11][12]. However, several issues such as energy consumption, unsustainability, and leakage risks prevent these two methods from a large-scale application [13,14]. As the main component of industrial waste gas, CO 2 could also be used as an abundant and cheap chemical feedstock for renewable fuels [15]. Therefore, converting CO 2 into value-added chemicals is considered to be one of the most promising strategies to mitigate the energy crisis and reduce the greenhouse effect. CO 2 reform of methane (DRM) is becoming an interesting strategy to convert the two greenhouse gases into syngas, which can be further converted to value-added products such as methanol or fuels through Fisher−Tropsch synthesis to replace traditional fossil fuels [16][17][18][19][20]. Many studies have also In light of the importance of the CO 2 methanation reaction, it has been widely investigated. This reaction was proposed by Sabatier and Senderens in 1902, which was also called the Sabatier reaction (Equation (1)) [72,73]. The reaction is exothermic, and can be carried out at low temperature to achieve high CO 2 conversion [74][75][76][77]. However, CO 2 is the upmost oxidized state of carbon and the activation of the C-O bond in CO 2 faces many challenges. The hydrogenation of CO 2 to methane is an eight-electron process with high kinetic barrier that requires a catalyst to achieve acceptable rates and selectivity [78][79][80]. More and more researchers are interested in designing a CO 2 methanation catalyst. As reported, the sintering of metal particles and carbon deposition would result in the deactivation of catalysts at high temperature. At the same time, the reverse water-gas shift reaction (RWGS) (Equation (2)) would also occur at high temperature, and produce CO as a byproduct [46]. The presence of CO might induce the Boudouard reaction, resulting in the formation of coke. Furthermore, the Tammann temperature of nickel is about 590 • C, so the sintering would occur at high temperature. Therefore, sintering-resistant and coke-resistant characteristics would be facilitated at low temperature (below 300 • C), where the life of the catalysts could also be prolonged [77]. The active metals usually affect the catalytic activity and selectivity of the catalysts. Many noble metals such as Rh [2,49], Ru [81][82][83], and Pd [84,85] have been widely applied in CO 2 methanation due to their excellent activity and CH 4 selectivity at low temperature. The catalytic performance of some noble metal catalysts on CO 2 methanation is summarized in Table 1. It can be seen that noble metal catalysts with low metal loading exhibited high catalytic activity, especially excellent CH 4 selectivity at low temperature. However, their large-scale industrial applications are limited due to the high costs. In addition to noble metal catalysts, many non-noble metals such as Co and Ni have also been widely used in CO 2 methanation due to their low cost. Co-based catalysts exhibit excellent low temperature catalytic performance and could effectively avoid catalyst sintering deactivation. However, the CH 4 selectivity of the catalysts is generally low. Among the results reported thus far, only a few Co-based catalysts could achieve high CO 2 conversion and high CH 4 selectivity [61,[97][98][99]. Therefore, many researchers have turned their attention to Ni-based catalysts with relatively high activity in this reaction because of their comparable price and abundance [75,79,[100][101][102]. Some Ni-based catalysts also exhibit high catalytic activity and CH 4 selectivity. Zhou et al. [103] prepared Ni/CeO 2 catalyst by using the hard template method and the catalyst exhibited high CO 2 methanation activity with 91.1% CO 2 conversion and 100% CH 4 selectivity at 340 • C, atmospheric pressure. However, Ni-based catalysts still face several challenges in the application of CO 2 methanation at present. First, the activity of Ni-based catalysts at low temperature needs to be improved. Second, the sintering of nickel particles due to the exothermic methanation reaction and the formation of carbon deposition could all result in deactivation of the catalyst during the reaction process [104]. Third, the precise elucidation of the CO 2 methanation mechanism is still a challenging task.
Many papers have reviewed the progress of the CO 2 methanation reaction. Fan et al. [46] mainly reviewed various active metals and heterogeneous catalyst supports used for the thermal hydrogenation of CO 2 to methane. Lee et al. [105] mainly focused on the recent work around low temperature CO 2 methanation on a wide range of catalysts including reaction thermodynamics and kinetics, catalyst materials including supports and promoters, and suitable reactor technologies. Ashok et al. [51] mainly reviewed the research progress of three kinds of CO 2 methanation reaction systems including thermo-catalytic systems, CO 2 methanation in a catalytic membrane reactor, and the CO 2 hydrogenation under plasma process in detail. Insightful guidance was provided in these review papers. The present work highlighted, in particular, CO 2 methanation on Ni-based catalysts and elaborated the CO 2 methanation mechanism, which was focused on discussing the research progress and present status of CO 2 methanation with an emphasis on designing catalysts with high activity and stability at low temperature. We summarized the recent development of Nibased catalysts used in CO 2 methanation including the effects of nickel active sites, supports, promoters, and preparation methods on CO 2 methanation as well as the two kinds of reaction mechanisms. Several suggestions for the future development of Ni-based catalysts for CO 2 methanation at low temperature are also proposed.

CO 2 Methanation at Low Temperature
As reported, Ni-based catalysts have been widely studied in CO 2 methanation. Many key parameters including the essential nature of nickel active sites, supports, promoters, and preparation methods could influence the catalytic performance of Ni-based catalysts for CO 2 methanation [106].
We summarized the catalytic activity and selectivity of most of the previously reported Ni-based catalysts for CO 2 methanation with different reaction conditions, as listed in Tables 2 and 3. Many studies added the balance gas in the reaction mixture such as Ar, N 2 , and He, which could improve heat and mass transfer during the reaction process, thus improving the CO 2 conversion. Tables 2 and 3 summarize the catalytic performance of different Ni-based catalysts for CO 2 methanation without the balance gas and with balance gas, respectively. Here, we also review the recent research progress of CO 2 methanation and discuss the effect of different factors through some detailed representative examples.   [141] nd: no data.

The Nature of Active Sites on CO 2 Methanation
The catalytic performance of Ni-based catalysts can be influenced by characteristics of Ni active sites including the content of Ni [142,143], the size of Ni [125,[144][145][146][147], the structure and chemical state of Ni [127,[148][149][150][151], the different nickel precursor salts [152], and the location of Ni active sites [153].
Vita et al. [142] designed Ni/GDC (gadolinium-doped-ceria) catalysts containing different contents (15-50 wt%) of Ni. They found that the catalytic activity increased with an increase in Ni content because of the enhanced Ni-support interaction, basicity, and oxygen vacancies. However, some Ni-based catalysts with low metal loading also exhibited excellent catalytic performance, which are summarized in Table 4. For example, the CO 2 conversion of the 2 wt %Ni/CeZrO 4 catalyst was 63%, and the CH 4 selectivity could reach 100% at 350 • C [154]. Both the catalytic activity and CH 4 selectivity were proposed to also be related to the size of the Ni particles, and Ni-based catalysts can be prepared by controlling the nickel size to achieve higher catalytic performance [144,145,155,156]. Many researchers have devoted themselves to exploring the role of nickel size in CO 2 methanation when designing catalysts with high activity. Citric acid was added during the preparation of the Ni/Y 2 O 3 catalyst, which could affect the nickel particle size and the Ni-support interaction [125]. Ni/CeO 2 with different sized Ni particles (2, 4 and 8 nm) were prepared to explore the size effect on CO 2 methanation performance, and Ni/CeO 2 with 8 nm nickel particles exhibited the highest catalytic activity. According to DRIFTS study, the larger Ni over CeO 2 efficiently promoted the hydrogenation of the formate intermediates, which accounted for the excellent CO 2 methanation performance [146]. Hao et al. [147] also explored Ni particle size in CO 2 methanation over Ni/CeO 2 catalyst. Through TGA measurements, the small Ni nanoparticles suffered a temporary loss of activity due to the carbon deposition. Ni-based catalysts with special structure and chemical state also affect the performance of the catalyst [148]. Sponge Ni exhibited excellent CO 2 methanation performance, and the CO 2 conversion could achieve 83% under a high space velocity (0.11 mol CO2 g cat −1 h −1 ) at 250 • C [149]. Hongmanorom et al. [127] designed Ni nanoparticles encapsulated in mpCeO 2 using strong electrostatic adsorption with 0.183 s −1 TOF, but the TOF of the conventional Ni/CeO 2 catalyst was only 0.057 s −1 . The higher activity resulted from more oxygen vacancies provided by the encapsulated structure on the Ni-CeO 2 surface. The Ni@HZSM-5 catalyst was synthesized by the hydrothermal method with a special embedment structure that showed high activity and stability in CO 2 methanation, and Ni@HZSM-5 could retain a structure and content of nickel similar to that of the fresh catalyst due to its special structure, while the conventional Ni/SiO 2 and Ni/HZSM-5 catalysts changed after a long period of reaction [150]. Hu et al. [151] discovered three distinct nickel active phases on Ni/Al 2 O 3 obtained by reduction at different temperatures, that is, the catalysts reduced at different temperatures (535, 573, and 673 K, respectively) exhibited different CO 2 conversion and CH 4 yield.
Ni-based catalysts prepared by different nickel salt precursors exhibited different catalytic performance. The Ni-AA catalyst (nickel acetylacetonate precursor) with special coordinating anion showed the best catalytic performance, but the Ni-S catalyst (nickel sulfate precursor) deactivated rapidly due to the presence of Ni 3 S 2 after the reduction pretreatment [152]. In addition, Yan et al. [153] found that the promising catalytic performance was also associated with the location of the nickel. They controlled the location of the nickel by employing the terminal groups of siloxene and varying the solvent used. When nickel was on the interior of adjacent siloxane nanosheets, the activity of Ni@Siloxene was higher, with above 90% CH 4 selectivity. When the location of the nickel was different, there were two disparate reaction pathways accordingly.

The Support on CO 2 Methanation
The nature of supports plays a critical role in the catalytic process by influencing the morphology, dispersion, stability, and reducibility of the active sites [160,161]. The nickel-support interaction can potentially modify the electronic state of active sites and suppress nickel sintering, which is important in promoting CO 2 methanation at low temperature [82,111,162]. Therefore, synthesis of highly efficient supported Ni catalysts is one of the major directions of CO 2 methanation. According to the reported study, we divided the supports of Ni-based catalysts into single supports, composite oxide supports, and other supports (MOF, zeolite, and activated biochar), as shown in Figure 2.

Ni-Based Catalysts Supported on Single Oxide Supports
Various oxides such as Al 2 O 3 [143,[163][164][165], TiO 2 [158,166], ZrO 2 [110,167], CeO 2 [145,168], and SiO 2 [25,101] have been explored for Ni-based CO 2 methanation catalysts. Muroyama and co-workers [124] compared the catalytic activity of different metal oxide supported Ni catalysts and found that Ni/Y 2 O 3 exhibited the highest catalytic activity with 80% CH 4 yield at 300 • C. Italiano et al. [162] also found that Ni/Y 2 O 3 exhibited the highest activity and no deactivation was observed after 200 h of testing, resulting from its good anti-coking and anti-sintering ability among Ni/CeO 2 , Ni/Al 2 O 3 , and Ni/Y 2 O 3 catalysts. As reported, the oxygen vacancies on the support played a significant role in the adsorption/activation of CO 2 species. CeO 2 supported Ni-based catalysts have been widely used in CO 2 methanation due to the high concentration of oxygen vacancies on the surface [103]. Le et al. [169] reported that Ni/CeO 2 was the most active for CO 2 methanation due to the smallest Ni particle size among Ni-based catalysts on different supports. The Ni/ZrO 2 catalyst is also one of the most active systems for CO 2 methanation, and ZrO 2 is very relevant for the generation of active sites of the methanation reaction, and the high concentration of oxygen vacancies on ZrO 2 also play an essential role in CO 2 methanation [110,170,171]. Therefore, ZrO 2 has also received extensive attention in CO 2 methanation because of its high mechanical and thermal stability [172]. Martínez et al. [76] reported the outstanding catalytic performance of Ni/ZrO 2 , where the conversion of CO 2 was close to 60% and CH 4 selectivity was 100% at 500 • C, with high stability after a 250 h reaction.

Ni-Based Catalysts Supported on Composite Oxide Supports
Composite supports could modify Ni-support interaction more easily and exhibited better properties compared with single oxide support. Zhu et al. [129] prepared a Y 2 O 3promoted NiO-CeO 2 catalyst and compared the catalytic activity of NiO-CeO 2 and NiO-CeO 2 -Y 2 O 3 catalysts. They found that the introduction of Y 2 O 3 to CeO 2 greatly facilitated the generation of surface oxygen vacancies during the reaction, which promoted the dissociation of CO 2 and thus improved the catalytic activity. Siakavelas et al. [107] also reported binary CeO 2 -based oxides Sm 2 O 3 -CeO 2 , Pr 2 O 3 -CeO 2 , and MgO-CeO 2 supported Ni catalysts used in CO 2 methanation. The addition of Sm 3+ and Pr 3+ increased the amount of oxygen vacancies of the catalysts, which improved CO 2 methanation activity. The strong nickel-support interaction on the Ni/MgO-CeO 2 increased the anti-sintering ability of the catalyst. The oxygen vacancies and coordinatively unsaturated sites formed cation pairs on the Ca doped Ni/ZrO 2 . The number of these pairs on Ni/CaZrO 2 was higher than the Ni/ZrO 2 catalyst, which increased the CO 2 methanation rate [110].
Although Al 2 O 3 supported Ni-based catalysts exhibited relatively high catalytic activity because of their high surface area and excellent hydrothermal stability, it needed further improvement [112]. Much research has been conducted on the modification of the Al 2 O 3 support by the employment of rare-earth oxides [74]. Furthermore, La 3+ doping could also enhance the surface basicity of the catalysts [173]. A variable amount of La-doped Ni/γ-Al 2 O 3 catalysts [164] was used in CO 2 methanation, and the activity and selectivity of Ni/La-γ-Al 2 O 3 increased compared with Ni/γ-Al 2 O 3 . The basicity of the support increased because of the addition of La, which enhanced the adsorption of CO 2 . ZrO 2 , as a promoter, is also widely used in CO 2 methanation catalysts because of its excellent hydrothermal stability and high oxygen defects. Lin et al. [119] designed a Ni/Al 2 O 3 -ZrO 2 catalyst with CH 4 selectivity of about 100%, CO 2 conversion of 77% at a lower temperature of 300 • C. The formed Al 2 O 3 -ZrO 2 solid solution could promote the reduction and dispersion of NiO, and increase the number of nickel active sites and oxygen vacancies because of the higher Zr loading, which results in significant improvement of the catalytic activity at low temperature.

Ni-Based Catalysts Supported on Other Supports
In addition to various oxide carriers, there are many other materials such as zeolite [108,113,131,174], MOF [101,175], and activated biochar [176] supported Ni catalysts, which have also been used in CO 2 methanation. Sholeha and co-workers [108] synthesized zeolite NaY from dealuminated metakaolin and used as a support of the Ni catalyst. The prepared Ni/NaY exhibited significant CO 2 conversion (67%) and CH 4 selectivity (94%) in CO 2 methanation, which could be attributed to the combination of well-defined crystalline structures and the large surface area of NaY. The Ni catalyst with porous zirconia obtained from a Zr-based metal-organic framework with UiO-66 as the support exhibited a turnover frequency of 345 h −1 space−time yield of 5851 mmol·g Ni −1 ·h −1 with CH 4 selectivity of over 99%, showing only a 4% decrease in activity after testing for 100 h on stream [175]. Ni/zeolite X [131] derived from fly ash was also applied in CO 2 methanation, where around 50% CO 2 conversion could be obtained at 450 • C. Wang et al. prepared a Ni/Ce-ABC (where ABC refers to activated bio-char) catalyst using biomass as the raw material, and the bio-char was modified by highly dispersed CeO 2 [176]. This catalyst showed fantastic catalytic performance in CO 2 methanation, achieving 88.6% CO 2 conversion and 92.3% CH 4 selectivity at 360 • C.

Promoter Effect on CO 2 Methanation
To obtain satisfactory catalytic performance, doping a second metal into Ni-based catalysts as promoters is also a good idea. The second metal and nickel might form an alloy structure, which tends to modify the geometric and electronic structure of Ni-based catalysts. Based on the literature, these promoters contain alkaline earth oxides [109,126,174,177,178], noble metals [120,137], rare-earth metals [121,130,134,136,163,[179][180][181][182][183], and some other typical transition metals and non-metallic elements [3,13,75,80,118,157,166,[184][185][186], as displayed in Figure 3. These promoters could affect the dispersion of nickel active sites, the acid-base properties of the support, the nickel-support interaction, and thus to the catalytic activity and stability of the catalyst.

Alkaline Earth Oxides Promoted Ni-Based Catalysts
Various studies have been conducted where alkaline-earth oxides have been widely used in CO 2 methanation catalysts as promoters. As alkaline-earth oxides are cheap, abundant, and can improve the basicity of the catalysts, they can promote the adsorption, activation, and reduction of CO 2 [61,74,130,177]. Ca-doped Ni-based catalysts have shown great potential in CO 2 methanation. The influence of alkaline-earth oxides (Ca, Mg, Sr, and Ba) as promoters on Ni/γ-Al 2 O 3 has also been investigated [126], where Ni and promoters were found to be uniformly dispersed into the pore structure of the support, and the Ca-promoted Ni/Al 2 O 3 catalyst exhibited the highest catalytic activity, which could be attributed to the enhanced CO 2 adsorption and the reducibility of Ni active sites. Do et al. [109] applied Ca-inserted NiTiO 3 in CO 2 methanation and found that the addition of Ca generated oxygen vacancies on the catalyst, and 84.73% CO 2 conversion and 99.95% CH 4 selectivity were observed. Xu and co-workers [178] reported that the Ca promoter increased the surface basicity of Ni-Al composite oxide catalysts, which enhanced the adsorption and activation of CO 2 . Bacariza et al. [174] prepared a Mg-promoted Ni/USY zeolite catalyst using different incorporation methods. They found that lower contents of Mg could enhance the catalytic performance because of the increase in nickel dispersion, while a higher amount of Mg would decrease CO 2 conversion due to the formation of a NiO−MgO solid solution, and the dispersion and stability of nickel on the Ni/USY zeolite catalyst obtained by ion exchange were higher than that of the impregnated catalysts.

Noble Metals Promoted Ni-Based Catalysts
Bimetallic methanation catalysts were prepared by adding small amounts of noble metals into Ni-based catalysts, which showed a significantly enhanced catalytic performance at low temperature. Shang et al. [120] prepared different Ru content doped 30 wt% Ni/Ce x Zr 1−x O 2 catalysts and explored the effects of Ru content on catalytic performance as well as the Ce/Zr molar ratios. 3Ru-30Ni/Ce 0.9 Zr 0.1 O 2 exhibited the best catalytic performance with 98.2% CO 2 conversion and 100% CH 4 selectivity at a low reaction temperature (230 • C). The addition of Ru can improve the Ni dispersion and the basicity of the surface of the catalysts. Noble metals as promoters could enhance the reducibility, the dispersion of nickel, and H 2 chemisorption capacity, thus improving the catalytic performance. Mihet et al. [137] also explored the effect of Pt, Pd, or Rh on the Ni/γ-Al 2 O 3 catalyst, and found that Ni-Pd/γ-Al 2 O 3 exhibited the highest activity with 74.6% CO 2 conversion and 96.6% CH 4 selectivity at 250 • C.

Rare-Earth Metal Promoted Ni-Based Catalysts
Rare-earth elements, mainly, La, Ce, and Y, have been used as promoters to increase the catalytic activity at low temperature because they can enhance the basicity of catalysts, Ni reducibility, and smaller Ni particles. Wang et al. [179] explored different rare-earth metals including La, Y, Ce and Ge promoted Ni/γ-Al 2 O 3 catalyst. They found that the Y promoted Ni/γ-Al 2 O 3 catalyst exhibited the highest catalytic activity, followed by the promoters of Ce, Gd, and La , which could be attributed to the differences in atomic radius, electron layer structure, and oxide basicity of each promoter. Mikhail et al. [173] also found that 4 wt% Gd promoted Ni/CeZrO x exhibited the highest activity with 85% CO 2 conversion and 100% CH 4 selectivity due to its high metal dispersion and the high percentage of medium basic sites. Many studies have reported that the addition of La could enhance the surface basicity and the adsorption of CO 2 , resulting in a significant increase in CO 2 conversion and CH 4 selectivity [136,182]. The effect of La on the catalytic activity was investigated and compared with that of Y and Ce [130]. The highest space velocity (480 L g −1 h −1 ) and CH 4 productivity (101 L CH4 g Ni −1 h −1 ) were obtained on the La-promoted catalyst, which was related to more reduced, highly dispersed Ni nanoparticles and basic sites in the La 2 O 3 -Al 2 O 3 matrix. Zhang et al. [134] also explored La as a promoter on CO 2 methanation. The NiLa 5 /Mg-Al catalyst showed the best catalytic performance, obtaining 61% CO 2 conversion and nearly 100% CH 4 selectivity with a WHSV of 45,000 mL g −1 h −1 at 250 • C, 0.1 MPa. Characterization results showed that La effectively increased Ni dispersion and decreased Ni particle size. In addition, La could significantly increase the amount of moderate basic sites, which could contribute to enhanced CO 2 adsorption capacity.
Ce as a promoter could increase the basicity of the catalyst, thus improving the adsorption and activation of CO 2 . In addition, the rapid reduction of Ce 4+ to Ce 3+ could form oxygen vacancies, which is of great importance for the catalytic activity [163]. Li et al. [180] prepared a Ce-promoted Ni-La 2 O 3 /ZrO 2 catalyst with high Ni dispersion and excellent resistance to sintering, leading to high activity and stability. Daroughegi et al. [163] found that the Ce promoted Ni-Al 2 O 3 catalyst exhibited the highest activity and stability with 76.4% CO 2 conversion and 99.1% CH 4 selectivity at 350 • C. Alarcón et al. [26] also reported a Ce promoted Ni-Al 2 O 3 catalyst with high loading CeO 2 (25 wt%), which was highly stable for 120 h.

Other Transition Metals and Non-Metallic Elements Promoted Ni-Based Catalysts
In addition, other typical transition metals such as Fe [3,13,184], Co [80,185], Mn [3,118,166], and non-metallic elements such as Si [75] promoted Ni-based catalysts can also be used in CO 2 methanation.
Serrer et al. [184] reported that the Ni-Fe catalyst exhibited high activity and long-term stability. They found a synergistic effect between nickel and iron on a bimetallic Ni-Fe catalyst that led to higher fractions of reduced nickel compared to a monometallic Ni-based catalyst. The Fe 0 Fe 2+ Fe 3+ redox mechanism could be observed at the interface of these FeO x clusters, which could promote CO 2 dissociation. Mn was introduced into Ni/bentonite, and the conversion of CO 2 on 2 wt% Mn-Ni/bentonite was 85.2% with 99.8% CH 4 selectivity at 270 • C. On the other hand, CO 2 conversion on Ni/bentonite needed 300 • C to obtain 84.7% CO 2 conversion. The addition of Mn enhanced the nickel-support interaction and the dispersion of nickel, and increased the amount of oxygen vacancies on the catalyst surface, which promoted the CO 2 methanation reaction [118]. Li et al. [75] prepared a series of Ni-xSi/ZrO 2 catalysts and found that the appropriate amount of Si promoter increased the dispersion of nickel, Ni-support interaction, and the number of active sites. The Ni-0.1Si/ZrO 2 catalyst exhibited the highest catalytic activity with 72.5% CO 2 conversion and 72.2% CH 4 yield at 250 • C.

The Effect of Preparation Methods on CO 2 Methanation
Diverse methods are applied in the preparation of Ni-based catalysts including the impregnation method (IM), precipitation method (PM), sol-gel method (SGM) as well as other methods. Table 5 summarizes the synthesis methods of different Ni-based catalysts. From Table 5, it can be observed that most catalysts were synthesized by the traditional impregnation method and precipitation method. In addition, a small number of catalysts were prepared by other methods including sol-gel, solution combustion, ammonia evaporation (AE), and the mechanochemical ball-milling method. Furthermore, catalysts prepared by different methods also exhibited different structures and properties as well as catalytic activity [128,187]. Combustion method 300 60~97.5 [193] nd: no data.
Zhang et al. [134] found that the urea hydrolysis method was a more efficient approach compared to the coprecipitation method in the preparation of Ni-based catalysts and obtained a Ni-based catalyst with higher Ni dispersion, larger CO 2 adsorption capacity, and therefore better catalytic performance. The Ni/CeO 2 catalyst prepared via decomposition of the nickel precursor by gas discharge plasma exhibited above 99% CH 4 selectivity at reaction temperatures lower than 300 • C, and more active sites were exposed to the CeO 2 surface, which promoted the splitting of H 2 and the activation of CO 2 , thus significantly improving the catalytic activity at low temperature [133]. Bukhari et al. [194] prepared Ni/SBA-15 catalysts using three kinds of hydrothermal treatment techniques (Reflux (R) and Teflon (T)) and without the hydrothermal technique (W), and applied them to CO 2 methanation. The catalytic activity sequence was as follows Ni/SBA-15(R) > Ni/SBA-15(T) > Ni/SBA-15(W). Characterization results showed that Ni/SBA-15(R) possessed excellent catalytic properties due to its high surface area and pore diameter, finest metal particles, strongest metal-support interaction, and highest concentration of basic sites. In addition, Gnanakumar et al. [115] studied the influence of pretreatment method on the catalytic performance of Ni/Nb 2 O 5 by calcining the Nb 2 O 5 support at different temperatures. Ni/Nb 2 O 5 calcined at 700 • C provided higher methanation activity and CH 4 selectivity with good stability in a stream study for 50 h.

The Reaction Mechanism of CO 2 Methanation
It is crucially important to understand the key intermediates and reaction mechanisms in depth when designing catalysts with excellent catalytic performance [81]. Many researchers have made efforts to elucidate the possible CO 2 methanation mechanism by in situ FTIR, mass spectrometry (transient-MS) techniques, and DFT calculations. Although there are many arguments on the intermediates and different reaction pathways of CH 4 formation, two widely accepted pathways have been proposed: (1) the formate pathway where formate species are the main intermediate products formed during CO 2 methanation reaction, also called the CO 2 associative methanation: the chemisorbed *CO 2 species can first be converted to bidentate formates (HCOO*) and then to formic acid (HCOOH), then to CH 4 , and (2) the CO pathway, also called the CO 2 dissociative methanation: the chemisorbed *CO 2 species can dissociate into *CO and *O. The formed *CO species can further dissociate into carbon species (*C), which can then be hydrogenated to CH 4 by dissociated H 2 still on the metal particles, desorbing from the catalyst surface, whereas the *O species can react with hydrogen to produce H 2 O [28,51,128,153,[195][196][197].
The possible reaction pathways are illustrated in Figure 4. CO 2 methanation on different catalysts occur via two different pathways, which are affected by the nature of nickel active sites and the supports [28].

The Formate Pathway
Many studies have reported that CO 2 methanation follows the formate route on different nickel catalysts such as Ni/MgO [198], Ni-Mn/Al@Al 2 O 3 [199], Ni/Y 2 O 3 [200], Ni/ZrO 2 [196,201], Ni/ultra-stable Y (USY) zeolite [139], and Ni@C [102]. For example, Xu and coworkers [196] discussed the formation and evolution of CO 2 adsorbed species on Ni/c-ZrO 2 by in situ FTIR and DFT calculations. CO 2 methanation on Ni/c-ZrO 2 was dominated by the formate pathway as follows: CO 2 *→ HCOO* → H 2 COO* → H 2 COOH* → H 2 CO* → CH 2 *→ CH 4 *, which is the same as that shown in Figure 4. CO was a by-product instead of a reaction intermediate, which could not further form CH 4 , and the DFT calculations also confirmed the formate pathway, which was highly consistent with the in situ FTIR results. Solis-Garcia et al. [201] also found that CO 2 methanation follows the formate pathway over Ni/ZrO 2 and no CO species were observed during the reaction. The possible reaction pathway of the CO 2 methanation over Ni@C was also investigated by CO 2 -TPD measurements and in situ FTIR characterization. All results demonstrated that CO 2 methanation over Ni@C catalyst proceeded via the formate route without involving CO as an intermediate [102]. Aldana et al. [41] also found that the main CO 2 methanation mechanism on Ni-CZ sol-gel was the formate pathway, which does not require CO as reaction intermediate. They also found that H 2 was dissociated on Ni 0 sites while CO 2 was activated on the ceria-zirconia support to form carbonates and then further into CH 4 , suggesting that a stable metal-support interface is beneficial for the adsorption of CO 2 .
In another study, Pan et al. [202] found that the reaction pathway on Ni/γ-Al 2 O 3 and Ni/Ce 0.5 Zr 0.5 O 2 all followed the formate pathway, only differing in reactive basic sites. On the Ni/Ce 0.5 Zr 0.5 O 2 catalyst, CO 2 adsorption on medium basic sites formed bidentate formate, whereas CO 2 adsorption on surface oxygen resulted in the monodentate formate. Due to the faster hydrogenation of monodentate formate, it was assumed to be the main reaction route on the Ni/Ce 0.5 Zr 0.5 O 2 catalyst. For CO 2 methanation on Ni/γ-Al 2 O 3 , hydrogenation of bidentate formate was the main reaction route as bidentate formate was the main adsorption and intermediate species and CO 2 adsorbed on strong basic sites of Ni/γ-Al 2 O 3 will not participate in the CO 2 methanation reaction. It was assumed that medium basic sites are responsible for promoting the formation of monodentate formate species, thus enhancing CO 2 methanation activity. CO 2 methanation reaction pathways on Ni/Ce 0.5 Zr 0.5 O 2 and Ni/γ-Al 2 O 3 are shown in Figure 5.

The CO Pathway
The CO pathway involves the dissociation of CO 2 to CO prior to methanation, and in the subsequent reaction, CO is converted to CH 4 by reacting with H 2 [203]. Karelovic et al. showed the direct dissociation of CO 2 . The reactions below summarize the reduction process (Equations (3) and (4)). The excess amount of CO generated in the first reaction deposits on the catalyst, which produces coking effects. To avoid this problem, the methanation of CO must proceed much faster than the CO production, and the CO 2 methanation reaction must take place at low temperatures. Therefore, the direct dissociation of CO 2 to CO ads and O ads often occur over a variety of noble metal-based catalysts at low temperature [49,204,205]. In addition, the formation of nickel carbonyls Ni(CO) 4 would cause the deactivation of Ni-based catalysts [162].
Therefore, CO 2 methanation occurred via the CO pathway only over some Ni-based catalysts including Ni/CeO 2 [103], Ni/F-SBA-15 [113], and Ni-sepiolite [206]. The CO pathway over Ni/CeO 2 could be proven by in situ FTIR. The FTIR adsorption bands at 2017 cm −1 were assigned to the CO adsorption state, and the bands at 2120 and 2170 cm −1 were ascribed to gas phase CO, which indicated that CO 2 molecules can be converted to CO molecules on the surface of the Ni/CeO 2 catalyst. Characterization results indicated that CO species generated from the reduction of CO 2 molecules by nickel active sites and surface oxygen vacancies promoted CO 2 methanation [103]. Bukhari et al. found that Ni metals on Ni/F-SBA-15 (Fibrous type SBA-15) contributed to the CO 2 dissociation into CO and O species as well as the dissociation of H 2 into atomic hydrogen species. The linear carbonyl group came from the dissociation of CO 2 , which was an intermediate during CO 2 methanation and could be seen at 2055 cm −1 . Then, the adsorbed CO species interacted with surface oxygen, producing bidentate and unidentate carbonate groups, thus CH 4 [113]. Cerdá-Moreno et al. [206] also found linearly and bridged bonded CO as intermediates during CO 2 methanation over a Ni-sepiolite catalyst.

The Key Factors of CO 2 Methanation Reaction Route
There are also many factors influencing the CO 2 methanation mechanism. The addition of promoters affects the formation of intermediates. Mg or Ca modified Ni/Al 2 O 3 catalysts promote the formation of the carbonate species due to the increased basicity, while Sr or Ba modified catalysts promoted *CO and H 2 CO* formation [177]. The nature of nickel active sites also influence the CO 2 methanation mechanism. Zhou et al. [158] found that CO 2 methanation took the pathway of CO over the Ni/TiO 2 catalyst with Ni (111) as the principal exposing facet, while the catalyst with multi-facets followed the formate route, with which nickel was only functional for hydrogen dissociation. The location of nickel active sites also affects the CO 2 methanation reaction pathways [153]. Controlling nickel being on either the interior or the exterior of adjacent siloxene nanosheets is achieved by employing different solvents in the preparation process, which determines the reaction intermediates and pathways for CO 2 methanation, as shown in Figure 6. CO 2 methanation occurred through the formate pathway over Ni@SiXNS-EtOH with nickel active sites being on the interior of adjacent siloxene nanosheets while CO 2 methanation followed the CO pathway when nickel was at the exterior of adjacent siloxene nanosheets on Ni@SiXNS-H 2 O. The different preparation methods can also influence the reaction pathway of CO 2 methanation. Jia et al. [135] used the operando DRIFT analyses to demonstrate the CO 2 methanation pathway on Ni/ZrO 2 obtained via different preparation methods. CO 2 methanation over the plasma decomposed catalyst follows the Co-hydrogenation route. The exposed high-coordinate Ni (111) facets of the plasma decomposed catalyst facilitate the decomposition of CO 2 and formates into adsorbed CO. The subsequent hydrogenation of adsorbed CO leads to the production of methane. However, the thermally decomposed catalyst with a complex Ni crystal structure and more defects mainly takes the pathway of direct formate hydrogenation.
Although some researchers have reported on the mechanism of CO 2 methanation, it is still hotly debated. Some of the problems are as follows: (1) Why are there two different CO 2 methanation mechanisms on Ni-based catalysts with the same Ni element as the active component? (2) What are the influencing factors of reaction pathways? (3) Are the adsorbed CO 2 species on different Ni-based catalysts are the same? (4) What is the approach and process of the adsorbed CO 2 species evolution on the catalysts? (5) What is the effect of Ni on the activation and dissociation of CO 2 ? (6) Ni-based catalysts are also used in CO methanation, so why is CO only a by-product and does not react further to form CH 4 on some catalysts? and (7) What is the role of CO across the whole CO 2 methanation reaction process? Regarding these problems, no consensus has been reached. Establishing a reaction network to understand the CO 2 methanation mechanism at the molecular-level is extremely important. These mechanistic insights will have great potential to guide the rational design of catalysts with high activity and CH 4 selectivity.

Summary and Perspective
This review encompassed the recent development of Ni-based catalysts used in CO 2 methanation including the effects of nickel active sites, supports, promoters, and preparation methods on CO 2 methanation as well as the two kinds of reaction mechanism, with emphasis on designing catalysts with high activity and stability at low temperature. By discussing the research progress and present status of CO 2 methanation over Ni-based catalysts, some conclusions could be obtained. First, interactions between Ni and the support seem to be a key parameter for the methanation reaction. The appropriate nickel-support interaction could suppress nickel sintering, resulting in high activity and good stability. Second, the promoters could affect the dispersion of the active phase, the acid-base properties of catalysts, and thus the catalytic activity and stability. Adding noble metals into Ni-based catalysts as promoters could improve the catalytic activity at low temperature. In addition, alkaline-earth oxides as promoters could improve the alkaline properties of the catalysts, which could promote the adsorption, activation, and reduction in CO 2 , thus enhancing the catalytic activity at low temperature. Third, CO 2 methanation occurred via the formate pathway and CO pathway over Ni-based catalysts, and the addition of promoters and the nature of active metal affects the formation of intermediates, thus affecting the reaction mechanism. Fourth, appropriate basic sites on catalysts could promote the formation of monodentate formate species, thus enhancing CO 2 methanation activity. Using different basic oxides as supports could adjust the basicity of the catalysts, enhancing the adsorption of CO 2 .
Although Ni-based catalysts have been widely used in CO 2 methanation, many challenges and problems remain. To increase the activity of Ni-based catalysts at low temperature, the activation of CO 2 and H 2 at low temperature is important, and the properties of the nickel metal, support, and promoter would influence this. Some new technologies have been applied in CO 2 methanation such as photo-catalysis, electro-catalysis, and plasma-catalysis. CO 2 could be hydrogenated to produce CH 4 by these methods at low temperature, but there are some challenges including the low CH 4 selectivity and low efficiency. One recommended advancement to overcome these issues is to couple these new technologies with traditional thermo-catalysis such as thermal-photo catalysis, which could significantly improve the selectivity and yield of CH 4 . For example, solar-thermal CO 2 reduction could achieve high-efficient conversion CO 2 at mild conditions, which could overcome the low efficiency and very high operating temperature of photo-catalysis and thermo-catalysis, and achieve higher solar energy utilization efficiency. In addition, it is still a big challenge to experimentally determine CO 2 methanation reaction pathways, and the effects of active sites, supports, and promoters on the reaction mechanism are still being debated. The precise elucidation of the activation of the C-O bond in CO 2 and the relationship between CO 2 activation and H 2 activation are also challenging tasks.
Based on the above discussions, the following suggestions on CO 2 methanation on Ni-based catalysts are proposed.
(1) Design of CO 2 methanation catalysts with high activity at low temperature simultaneously with high carbon deposition resistance and anti-sintering properties; (2) Try new materials and technologies such as MOF, alloy material, and plasma assisted technology for the design and preparation of CO 2 methanation catalysts; (3) Combine photo-catalysis, electro-catalysis, and plasma-catalysis with traditional thermo-catalysis to integrate their advantages; (4) Investigate the mechanism of the activation and cleavage of C-O in CO 2 , and the relationship between CO 2 activation and H 2 activation as well as provide deep insights into the CO 2 methanation reaction pathways and the key factors in the reaction mechanism; and (5) Combine the theoretical calculations with experiments to explore the role of the active metal, support, and the nickel-support surface in the CO 2 methanation reaction process.