The Role of Alkali and Alkaline Earth Metals in the CO 2 Methanation Reaction and the Combined Capture and Methanation of CO 2

: CO 2 methanation has great potential for the better utilization of existing carbon resources via the transformation of spent carbon (CO 2 ) to synthetic natural gas (CH 4 ). Alkali and alkaline earth metals can serve both as promoters for methanation catalysts and as adsorbent phases upon the combined capture and methanation of CO 2 . Their promotion e ﬀ ect during methanation of carbon dioxide mainly relies on their ability to generate new basic sites on the surface of metal oxide supports that favour CO 2 chemisorption and activation. However, suppression of methanation activity can also occur under certain conditions. Regarding the combined CO 2 capture and methanation process, the development of novel dual-function materials (DFMs) that incorporate both adsorption and methanation functions has opened a new pathway towards the utilization of carbon dioxide emitted from point sources. The sorption and catalytically active phases on these types of materials are crucial parameters inﬂuencing their performance and stability and thus, great e ﬀ orts have been undertaken for their optimization. In this review, we present some of the most recent works on the development of alkali and alkaline earth metal promoted CO 2 methanation catalysts, as well as DFMs for the combined capture and methanation of CO 2 .


Introduction
Antropogenic CO 2 emissions have skyrocketed in the recent century due to the rapidly expanding energy demands of our society [1]. Since increasing concentrations of greehouse gas emissions have the potential to cause irreversible damage to our planet's climate, it is clear that this trend eventually has to be reversed [2,3]. Carbon capture and storage (CCS) technologies aim to mitigate this problem by capturing CO 2 from emission sources and then depositing it to underground geological formations [4]. However, CO 2 utilization after the initial capture step (Carbon capture and utilization (CCU)) through conversion of CO 2 into fuels and useful chemicals, such as methane, syngas, methanol and higher hydrocarbons, should be considered as a more prudent methodology to recycle our carbon sources [5]. Among the conversion products, methane or synthetic natural gas (SNG) is often favoured as it is an energy source with a high caloric content and also because the already existing and extensive natural gas grid can be utilized for its distribution [6]. However, the hydrogen used during the hydrogenation step should be produced using renewable energy sources in order to achieve carbon neutrality. Hope is Hereby we aim to summarize the most recent progress made in the field of alkali and alkaline earth metal promotion of the CO 2 methanation reaction and provide a comprehensive overview over the development of novel dual-function materials for the integrated capture and methanation of CO 2 . The latter materials, by functioning as both catalysts and adsorbents towards the combined CO 2 capture and methanation, appear capable of improving the process and as a result have attracted intense interest in the last five years.

Alkali and Alkaline Earth Metals as Promoters of the CO 2 Methanation Reaction
There has been extensive effort in trying to achieve high catalytic activity, especially in the low-temperature region (ca. 100-250 • C), for the CO 2 methanation reaction. Rh and Ru are two of the most common precious metal catalysts studied in this reaction, since they achieve a substantial low-temperature activity and exhibit superior stability [26,27]. Ni-based catalysts, on the other hand, remain the favourite due to their low cost and large natural abundance, but they present some serious drawbacks, including inferior low-temperature activity when compared with Rh and Ru-based catalysts, as well as Ni nanoparticle sintering and coke formation [28,29]. Extensive research effort has been devoted in an attempt to enhance the low-temperature activity of Ni-based catalysts or decrease the loading of precious metal CO 2 methanation catalysts by introducing some kind of metal or metal oxide promoter [27].
Alkali and especially alkaline earth metal promotion of the CO 2 methanation reaction has been a quite popular route in recent years. These types of metals are cheap, abundant and can potentially promote the CO 2 methanation reaction by increasing support basicity and by introducing oxygen vacancies, thus favouring the initial CO 2 chemisorption [28,30,31], or even by strongly modifying the work function and thus the chemisoptive properties of the metal active phase [32]. Hereby, we categorize the different alkali and alkaline earth metal promoters starting from alkalimetals. Works that perform a screening of different promoters will be presented at first, followed by a categorization of the works focussing on a single metal promoter, depending on the type of metal used.

Screenings of Various Alkalimetals
One of the few works reporting a promoting effect of various alkalimetals during the CO 2 methanation reaction was conducted by Petala et al. [33]. The authors employed quite a small loading of alkalimetal promoters (0.06-0.4 wt.% of Li, Na, K and Cs) by impregnating the corresponding alkali precursor salts on the TiO 2 support. A low (0.5 wt.%) and a high (5 wt.%) Ru loading was achieved via a subsequent impregnation step of Ru(NO)(NO 3 ) 3 . The lower active metal loading resulted in a smaller Ru crystallite size. Interestingly, it was found that alkali addition had a greater impact on the catalyst with the low Ru loading (0.5 wt.%), in comparizon to that with the high Ru loading (5 wt.%), and an optimal Na loading of 0.2 wt.% was obtained for the former (i.e., 0.5 wt.% Ru/0.2 wt.% Na-TiO 2 ). These results agree with the general features of alkali promotion on several catalytic systems, which typically obey a volcano-type behavior with respect to the promoter/metal ratio; low promoter/metal ratios are inefficient to induce succesful promotion, high promoter/metal ratios lead to over-promotion (poisoning) [32]. The catalytic activity and CH 4 selectivity were promoted according to the following order with respect to the alkali identity: TiO 2 (unpromoted) < Li ≈ K < Cs < Na, as shown in Figure 1 [33]. In a continuation of this work, Panagiotopoulou [34] employed in-situ DRIFTS and transient-mass spectrometry in order to shed light on the reaction mechanism. Alkalimetal addition on the low Ru loading catalyst appeared to favour the formation of Ru-bonded carbonyl species (Ru x -CO), which are key intermediates towards the production of CH 4 .  Table  0. Ru/X-TiO 2 catalysts of the same alkali loading (0.2 wt.%). Experimental conditions: mass of catalyst: 100 mg; particle diameter: 0.18 < d p < 0.25 mm; feed composition: 5% CO 2 , 20% H 2 (balance He); total flow rate: 150 cm 3 min −1 . Reproduced with permission from ref. [33]. Copyright 2018 Elsevier.
The impact of alkalimetal addition on a Ru-based catalyst towards the methanation of CO 2 was also studied by Cimino et al. [35] in a recent study. In this case, a higher alkali promoter loading was chosen and the impregnation of the alkali precursor salts took place mostly after the impregnation of the Ru active metal. All alkali modified catalysts, except the one modified with LiNO 3 , exhibited lower CO 2 conversions and CH 4 selectivities compared to the unmodified 1% Ru/Al 2 O 3 one. The lower catalytic activity was attributed to the coverage of some Ru active sites, while Li could promote the Ru-based catalyst activity by forming Li-aluminate spinel-type phases with the Al 2 O 3 support. The LiNO 3 and NaNO 3 promoted materials were also tested for cyclic CO 2 capture and methanation and the results will be further discussed in the second part of this review.
Liang et al. [36] and Zhang et al. [37] studied the effect of K and Na modification (amongst other metals) on Ni/Al 2 O 3 and Co/Al 2 O 3 catalysts, as well as the alteration of the CO 2 methanation mechanism via in-situ DRIFTS. In the first study [36], a total of 16 different metal additives were first impregnated on a γ-Al 2 O 3 support, followed by a subsequent impregnation of Ni. Na and K additives gravely impacted the physicochemical properties of the catalysts, leading to lower activity. The lower catalytic activity of the K-modified catalyst was attributed to the increased population of H 2 CO* intermediates. In the second study [37], the modification of the alumina support in Co/Al 2 O 3 by K, Na and Mg led to the formation of Al(OH) 3 and spinel-type structures. The addition of Na and K followed by the increase in support alkalinity appeared to stir the reaction towards HCOO* and CO 3 2− intermediates, lowering the catalytic activity and CH 4 selectivity, as well as favouring coke formation. The promotion of the CO 2 chemisorption by alkalimetal addition did not appear to correlate with the catalytic activity.
In regards to the electrocatalytic CO 2 methanation, the Vayenas group employed the electrochemical promotion of catalysis (EPOC) concept, an effective in-situ method of catalyst promotion, also known as non-faradaic electrochemical modification of catalytic activity (NEMCA) [32,38] to enhance CO 2 methanation performance of a Ru electrocatalyst [39,40]. Taking advandage of the high potential of the method, the authors comparatively investigated a variety of electropositive or electronegative promoters, namely Na + , K + , H + and O 2− . They found that in all cases, increasing catalyst work function (induced by the promoters) enhances the methanation rate and selectivity. In the case of the present interest (alkali promotion), Na + and K + supply to Ru surface caused a remarkable promotion of the methanation rate, which was found to pass through a maximum upon increasing alkali loading (volcano-type promotion). This result is in qualitative agreement with the typical features of alkali-promotion [32] and with the aforementioned results of Petala and Panagiotopoulou on CO 2 Catalysts 2020, 10, 812 5 of 36 methanation over traditional supported Ru catalysts upon varying Na-loading by an impregnation method [33].

Sodium (Na)
Among the works focusing on the support modification with Na, Le et al. [41] investigated the CO 2 methanation reaction over Ni catalysts supported on Na (0.1-1 wt.%) doped SiO 2 and CeO 2 supports. Na was incorporated (via impregmation) in the previously made 33% Ni/SiO 2 and 13% Ni/CeO 2 catalysts in a second impregnation step using a NaNO 3 precursor. The available active metal surface area was shown to decrease upon Na addition, following H 2 chemisorption experiments. Subsequently, CO 2 conversion decreased upon Na modification of the initially highly active Ni/CeO 2 catalyst, while the increase of the amount of moderately strong basic sites and CO 2 chemisorption sites following the incorporation of Na in Ni/SiO 2 appeared to offset the negative effect caused by the decrease of the catalytically active surface area.
Moreover, Beierlein et al. [42] employed various preparation methods in order to prepare Ni/Al 2 O 3 catalysts with high Ni loadings (14-88 wt.% Ni). The primary focus of their work was to establish a linear correlation between the surface area of the active metal (Ni) and the CO 2 conversion, arguing that CO 2 methanation over highly loaded Ni/Al 2 O 3 catalysts is a structure-insensitive reaction. Two of the preparation methods applied, namely deposition precipitation and coprecipitation, required the use of large quantities of NaOH and Na 2 CO 3 reagents, thus introducing a relatively high content of Na on the final catalysts. A high Na content was shown to decrease CH 4 selectivity, while boosting CO production. This behaviour was attributed to the lower hydrogen coverage of the active metal and the hindrance of the H-assisted CO dissociation, a key intermediate step (also reported to be the rate determining step [9]) during the conversion of CO 2 to CH 4 .

Pottasium (K)
Pottasium-doping of the catalyst support has been performed by various groups with most of them reporting a negative effect on the CO 2 methanation. Büchel et al. [43] studied the CO 2 hydrogenation over K-and Ba-modified 1% Rh/Al 2 O 3 catalysts prepared by flame spray pyrolysis. The K-modified catalyst produced only CO over the entire temperature range. This extreme CO selectivity over CH 4 was attributed to a weak interaction of CO with the catalyst, as shown during CO chemisorption experiments. This could cause the intermediate CO species to desorb, rather than be further hydrogenated into CH 4 . Iloy et al. [44] further studied the modification of 15% Co/SiO 2 with 1-5 wt.% K. K-dopant was shown to enhance CO 2 chemisorption, but favour CO selectivity and C-C coupling along with chain growth reactions. For the production of C 2+ hydrocarbons, 1 wt.% K appeared to be optimal, while a higher potassium loading favoured CO as a main product.
Modification of the Al 2 O 3 support with a KOH strong base was performed by Zhang et al. [45] and the reaction intermediates were followed via in-situ DRIFTS. KOH was first dissolved in d-H 2 O and then introduced into the Al 2 O 3 support, while Ni was impregnated during a second step. KOH addition led to a dramatic decrease of the Al 2 O 3 surface area and after calcination, K 2 CO 3 and K 2 O phases were also detected. The introduction of the strong KOH base led to the increased presence of adsorbed formate intermediates during CO 2 hydrogenation, which was validated via in-situ DRIFTS ( Figure 2) [45]. This strong interaction with the formate intermediate species apparently did not allow for their further hydrogenation to CH 4 and instead led to a high CO selectivity. The Ni-KOH/Al 2 O 3 catalysts, on the other hand, could be considered as more suitable candidates for the RWGS reaction and the steam reforming of acetic acid, in an antithesis to other works: He et al. [46] reported a promoting effect of KOH addition towards higher CO 2 conversions and CH 4 selectivities, when impregnated on a Ni-Al hydrotalcite precursor. This was attributed to the introduction of new strong basic sites in the support. The different behaviour in this case could be likely due to the small loading of the KOH base (0.3 wt.%), similarly to the work of Petala et al. [33]. This also demonstrates the key role of the alkali promoter/active metal ratio in alkali promotion efforts and the nessecity of a thorough screnning for the determination of the optimal alkali promoter loading for each specific catalytic system, alkali indentity and reaction conditions used [47,48].
Another interesting study that includes an in-situ KOH modification of a Ni/SiO 2 methanation catalyst has been conducted by Vogt et al. [49]. The authors first created a setup that produced hydrogen through solar driven H 2 O electrolysis from a KOH-based alkaline electrolyzer. Produced H 2 was then transferred downstream to a methanation unit that contained a Ni/SiO 2 methanation catalyst. During operation, KOH aerosols were transferred to the methanation reactor and part of them was deposited on the Ni-based catalysts. Interestingly, this in-situ alkali modification promoted the Ni/SiO 2 methanation catalyst, whereas a similar ex-situ 0.6 wt.% K addition via a common KOH impregnation resulted to catalyst deactivation. LaNiO 3 -based perovskite oxides can serve as catalyst precursors for the preparation of Ni/La 2 O 3 catalysts following a reductive pretreatment. The fine and well-dispersed Ni nanoparticles formed following the perovskite lattice decomposition retain a strong interaction with the La 2 O 3 support and are expected to exhibit a different behaviour compared to the Ni/La 2 O 3 catalysts prepared via Catalysts 2020, 10, 812 7 of 36 a mere impregnation step [50]. Tsounis et al. [51] presented a way to tune the product selectivity of LaNiO 3 -based perovskites during CO 2 hydrogenation, via partially substituting La on the perovskite A-site with K. KNO 3 was mixed together with La and Ni nitrates during the perovskite citrate sol-gel synthesis, in order to prepare La 1-x K x NiO 3 precursors. Following reduction, the catalysts exhibited a drastically different behaviour during CO 2 hydrogenation depending on the amount of K-doping. The K-free catalyst had a high CO 2 conversion and an almost complete selectivity towards CH 4 , while with increased amounts of K-substitution CO 2 conversion gradually decreased and CO selectivity spiked (Figure 3). A possible explanation given was that easy-to-desorb CO intermediates were formed after K-doping and they in turn stirred the reaction towards the RWGS route. Figure 3. (a) CO 2 hydrogenation product selectivity for the catalysts derived from LaNiO 3 and K-substituted (La 1-x K x NiO 3 ) perovskites; (b) rising CO selectivity towards the RWGS at T 50% for the catalysts containing increased amounts of K. Reproduced with permission from ref. [51]. Copyright 2020 MDPI.

Screenings of Various Alkaline Earth Metals
In the works discussed previously, some alkaline earth dopants were also used and compared with the effect caused by the doping with alkalimetals. Zhang et al. [37] concluded that the effect of Mg and Ca addition on Co/Al 2 O 3 was less pronounced compared with Na and K and that, although the activity of the modified catalysts was inferior to Co/Al 2 O 3 , alkaline earth metal doping still provided a benefit by preventing Co nanoparticle sintering during prolonged exposure under reaction conditions. Liang et al. [36] also mentions that 5% Mg and Ca doping on Ni/Al 2 O 3 led to a slight increase in support alkalinity, but did not appear to benefit the methanation activity.
Guo et al. [52] were among the first to conduct a comparative study on the impact of alkaline earth metal addition on Ni/SiO 2 catalysts. During the catalyst preparation, alkaline earth metal nitrates were first impregnated on the SiO 2 support, followed by wet impregnation of the Ni active phase. The authors reported that MgO had a negative effect on the catalytic activity, while CaO did not have a pronounced effect. The heavier alkaline earth metal oxides SrO and BaO led to better results, both enhancing the catalytic activity. SrO was shown to be the most suitable additive by providing the highest promotion of the CO 2 methanation reaction and by increasing the stability of the modified catalyst.
Liang et al. [53] studied the modification of Ni/Al 2 O 3 catalysts with alkaline earths for the CO 2 methanation reaction and reported similar results. The catalysts were also prepared via subsequent impregnation of alkaline earth metal nitrates and then nickel nitrate on the support. The lighter alkaline earth metals Mg and Ca were found to promote the formation of carbonate intermediates and the CO selectivity, while the heavier alkaline earth metals Sr and Ba were the ones being beneficial for the Catalysts 2020, 10, 812 8 of 36 CH 4 yield by promoting the formation of *CO and H 2 CO* intermediates. Another important feature, namely the generation of oxygen vacancies upon Sr-addition, appeared to play a major role via the creation of Lewis basic sites for the adsorption and activation of CO 2 .
Ni/CeO 2 catalysts are among the most active for CO 2 methanation and are a main research focus. CeO 2 has a high oxygen storage capacity (OSC) that, together with its redox properties due to the Ce 4+ /Ce 3+ redox couple, promotes the stability of the active metal particles and suppresses the formation of coke. Furthermore, CeO 2 facilitates a strong interaction with the supported metal phase [54,55]. Liu et al. [30] conducted a comprehensive study, where they doped the CeO 2 support of Ni/CeO 2 catalysts with various alkaline earth metals, with the aim to further enhance their low-temperature catalytic activity. Alkaline earth metal nitrates were introduced during the citrate sol-gel synthesis of the CeO 2 support with a molar ratio of 1/9, compared with Ce(NO 3 ) 3 ·6H 2 O. Mg 2+ and Ca 2+ appeared to fully dissolve into the CeO 2 lattice, while SrCO 3 and BaCO 3 secondary phases were detected in the other two samples. The Ca-doped catalyst induced the highest promotion effect, measured via the CO 2 conversion and turnover frequency (TOF) increase at low temperatures, with the Sr-doped one being second-to-best. The higher turnover frequency could be linearly correlated with two related factors, namely the increase in the population of oxygen vacancies (Ce 3+ /(Ce 3+ + Ce 4+ ) molar ratio) and moderate alkaline sites (200-450 • C peaks in the CO 2 -TPD profiles), as shown in Figure 4. The studies discussed up to this point, which focused on the screening of different alkaline earth metals as dopants, appear to provide conflicting conclusions on their effect, since some of them Catalysts 2020, 10, 812 9 of 36 report enhancement of the CO 2 methanation activity, while others a suppression of it (for Mg and Ca). One must take into account that these works differ on the type of support, the alkaline earth metal loading and the preparation procedure. Notably, almost all works focusing on a single alkaline earth metal additive, especially concerning Mg and Ca doping, report a promoting effect for the CO 2 methanation. Generally, this promotion is attributed to a better active metal dispersion, as well as to the generation of surface oxygen vacancies and the enhancement of support alkalinity, factors that substantially favour the initial chemisorption and activation of CO 2 .

Magnesium (Mg)
Mg in the form of MgO has been studied as a type of support during the CO 2 methanation reaction. However, Ni/MgO catalysts have been shown to lack the activity of other Ni catalysts supported, for example, on TiO 2 , Al 2 O 3 and CeO 2 [15]. The CO 2 methanation mechanism over Ni/MgO has been reported to proceed via a H 2 COO* intermediate and to follow the formate route, due to the strong metal-support interaction (SMSI) between the Ni nanoparticles and the MgO support [56]. Ni/MgO catalysts encapsulated by a SiO 2 shell were prepared by Li et al. [57] via chemical co-precipitation and a modified Stöber method, followed by in-situ reduction of the initial SiO 2 encapsulated NiO-MgO solid solution. Interestingly, it was found that the confinement induced by the SiO 2 shell helped to control the size of Ni nanoparticles, whereas the MgO phase also contributed to the improved Ni dispersion.
Besides its function as a catalyst support, MgO has also been thoroughly studied as a promoter for the CO 2 methanation reaction in an attempt to improve the dispersion of the active metal phase and enhance the support basicity. Park et al. [58,59] reported a bifunctional mechanism of CO 2 methanation over a Pd-MgO/SiO 2 catalyst. The addition of MgO in Pd/SiO 2 led to improved catalytic activity and CH 4 selectivity. A mechanism was proposed where MgO and Pd phases functioned as CO 2 and H 2 sorption active sites, respectively. The adsorbed hydrogen atoms on the Pd surface were then proposed to spillover to Mg carbonates, hydrogenating the CO 2 -adsorbed phases and forming the CH 4 final product. This bifunctional character of CO 2 methanation reaction that requires the presence of a catalyst componentent capable of activating the CO 2 (Supporting Material: even the commonly considered "inert" Al 2 O 3 acts in this way; support doping is often used for further enhancing this role), and a metallic component typically active in H 2 dissociation has been well established in the literature [60][61][62].
Guo et al. [63] prepared Ni/SiO 2 catalysts with different MgO loadings and with two different preparation methods, namely co-impregnation and sequential impregnation. Co-impregnation was shown to be superior to sequential impregnation and 1% MgO loading to be sufficient for the reaction promotion, whereas a higher MgO content risked masking some of the Ni active sites. The promotion was reportedly achieved via the enhancement of Ni dispersion and via Ni particle sintering prevention. The beneficial effect of MgO in the Ni nanoparticle dispersion was also reported by Tan et al. [64]. Ni and Mg nitrates were co-impregnated on a ZrO 2 support with the simultaneous addition of citric acid and glycol during the impregnation step, applying the citric complexing method. An optimal Mg/Ni molar ratio of 0.25 led to the desired Ni nanoparticle confinement and resistance to sintering and coke formation. Moreover, Xu et al. [65] included Mg in the synthesis of ordered mesoporous Ni/Al 2 O 3 catalysts prepared via evaporation-induced self-assembly (EISA). Mg increased the surface basicity of the catalysts, thereby facilitating the chemisorption and activation of CO 2 . The optimized Mg content of 5% and the confinement created by the mesoporous alumina structure provided significant advantages for the employment of these materials in the CO 2 methanation reaction.
Further studies include the promotion of Ni-based catalysts supported on coconut shell carbon (CSC) [66] and MCM-41 [67] with Mg, as well as the Mg-modification of catalysts comprised of Ni supported on ultrastable Y (USY) and MFI (Mobil-type five) nanosponge zeolites [68,69]. All these studies reported an improvement of the CO 2 methanation catalytic performance. Additionaly, a second promoter could also be introduced together with Mg to investigate co-promotion effects [70,71]. La-promoted Ni/MgO-Al 2 O 3 catalysts showed improved low-temperature activity via the increase in the population of moderately strong basic sites [60], while Mg and Mn co-promoted Ni/α-Al 2 O 3 catalysts were shown to be superior compared to the ones prepared with the use of a single promoter [71].
Finally, Mg has been indicated as a suitable promoter for Fe catalysts during CO 2 methanation [72]. Fe catalysts, commonly known to be active during the Fischer-Tropsch synthesis, could also be viable candidates for the CO 2 hydrogenation into CH 4 under high pressure, after the incorporation of a suitable promoter [73]. Baysal et al. [72] incorporated a series of 15 different promoters on a commercial α-Fe 2 O 3 powder via incipient wetness. Improved CO 2 adsorption over the Mg-modified Fe catalyst led to highly active Fe carbides, bulk carbon moieties and an overall improved CO 2 methanation performance.

Calcium (Ca)
Ca is also one of the favourite promoters for the CO 2 methanation reaction, with its main functions being the increase in the population of surface basic sites and the alteration of support defect chemistry by introducing oxygen vacancies. Similarly to the Mg doping of Ni-based catalysts supported on ordered mesoporous alumina [65], Ca was also employed as a basic modifier by Xu et al. [74]. Ca(NO 3 ) 2 ·4H 2 O was introduced during the evaporation-induced self-assembly (EISA) catalyst synthesis. The surface alkalinity was greatly increased upon Ca-doping and that led to an enhancement of CO 2 chemisorption and a lowering of the activation energy for CO 2 methanation. The spike in the population of new strong alkaline sites was confirmed by CO 2 -TPD. The catalytic activity was the highest, when the Ca/Al molar ratio was fixed at 8%, with a roughly 4 times CO 2 conversion increase at low temperatures (250 • C).
Liu et al. [75] tried to further optimize these types of catalysts by an additional introduction of 1% Ru. The 10N1R2C-OMA or 1% Ru, 10% Ni, 2% CaO/Al 2 O 3 (ordered mesoporous alumina) catalyst aimed at combining the beneficial effects of: (a) the nanoparticle confinement inside an ordered mesoporous alumina framework, (b) the increase of surface basicity and CO 2 chemisorption using a CaO promoter and (c) the introduction of 1% noble metal Ru, forming a bimetallic NiRu phase and creating a synergy between the two catalytically active metals. A step increase in the CO 2 conversion and CH 4 selectivity was reported following the subsequent introduction of each modifier. Furthermore, the ordered mesoporous structure and the fine dispersion of the active metal phase were retained in the spent catalysts, highlighting their superior stability. The methanation performance of the prepared catalysts, as well as the stability test for the 10N1R2C-OMA one, are shown in Figure 5.  Furthermore, Do et al. [76] reported a promoting effect of CaO addition on a bimetallic PtNi/Al 2 O 3 catalyst during methanation of a CO 2 -rich gas, while the group of Chu also published a a number of works focussing on the introduction of Ca-dopant as a promoter for the methanation of CO 2 [77][78][79]. A 20% CaO addition on 15% Ni/Al 2 O 3 was reported to modify the interaction between Ni and Al 2 O 3 and inhibit the incorporation of Ni into the alumina structure [77], while Ca addition on carbon supported Ni catalysts (carbon nanotubes [78] and activated carbon [79]) also appeared to be beneficial by increasing Ni reducibility and by limiting the growth of the Ni nanoparticles.
Ca 2+ cations are soluble into the crystalline lattice of ZrO 2 and the corresponding solid solution is commonly known as calcia stabilized zirconia (CSZ). Oxygen vacancies (V o 2 ) are created in order to compensate for the negatively charged Ca-induced substitutional impurities (Ca Zr 2 '), thereby enhancing the material's ionic conductivity and altering its defect chemistry [80]. Takano et al. [81] impregnated Ni and Ca nitrates on ZrO 2 and studied the effect of Ca/(Zr + Ca) molar ratio on the CO 2 methanation activity. The authors argued that the presence of medium-strength basic sites caused by the Ca-induced oxygen vacancies could facilitate CO 2 chemisorption, which then further reacted with atomic hydrogen adsorbed on Ni adjacent sites. An optimized Ca/(Zr + Ca) molar ratio between 16.7% and 20% was shown to maximize the population of oxygen vacancies, while preventing the formation of undesirable Ca-rich and CaZrO 3 phases. More recently, Everett et al. [82] showed that a small amount of Ca 2+ substitution (0.7 wt.%) in the lattice of ZrO 2 generated pairs of oxygen vacancies and coordinately unsaturated sites (cus). They concluded that these pairs could facilitate the C-O bond scission on adsorbed intermediate CO, reportedly the rate determining step during CO 2 methanation. Finally, perovskite oxide titanates containing Ca have been also used in the CO 2 methanation reaction. Jia et al. [83] employed a common perovskite oxide that is also found as a natural mineral, CaTiO 3 , as a catalyst support and compared it with γ-Al 2 O 3 . The 10% Ni/CaTiO 3 catalyst exhibited CO and CO 2 methanation activity superior to 10% Ni/Al 2 O 3 , due to the perovskite oxide's exceptional stability, increased support basicity and the limited interaction between Ni particles and the CaTiO 3 support. Do et al. [84] used Ca as a promoter in order to substitute part of the A-site cations in the NiTiO 3 lattice and the whole perovskite was loaded on γ-Al 2 O 3 . They performed extensive characterizations on the composite materials and concluded that upon 5% molar substitution of Ni 2+ with Ca 2+ , the catalytic activity could be maximized. Ca-addition promoted the formation of oxygen vacancies in close proximity to Ni active phases and thereby the CO 2 chemisorption. The proposed reaction mechanism, as summarized in Figure 6, involved a combination of the CO and formate routes. The initial CO 2 adsorption and hydrogenation steps resulted into formate species, which in turn formed adsorbed CO. The carbonyl species were then further reduced to form other hydrogenated intermediates, with CH 4 being the final product. Reproduced with permission from ref. [84]. Copyright 2020 Elsevier.

Strontium (Sr) and Barium (Ba)
Despite the fact that the works dealing with the screenings of different alkaline earth metal promoters report a quite beneficial contribution upon Sr and Ba addition [30,52,53], little further research has been conducted on the individual promotion of either of these heavier elements, especially when compared to their lighter alkaline earth metal counterparts. Toemen et al. [85] modified a Ru/Mn/Al2O3 catalyst with Sr and Ce. The Sr-modified catalyst initially adsorbed increased amounts of CO2 as monodentate carbonates, which in turn formed bridged bidentate carbonates before being further hydrogenated. The enhanced catalytic activity of the Sr-doped catalyst was, however, overshadowed by the Ce-doped one, where the methanation reaction followed a different mechanism.
Steiger et al. [86] prepared La1-xSrxFe0.8Ni0.2O3 perovskite oxides with increasing amounts of A-site Sr substitution. Pre-reduction of the initial perovskite caused the lattice Ni to be exsolved on the perovskite surface in the form of fine and well-dispersed Ni nanoparticles that exhibited strong interaction with the perovskite support. The amount of exsolved Ni, as well as the size and dispersion of the Ni nanoparticles, could be tailored by altering the A-site perovskite composition. La substitution on the A-site by Sr caused a destabilization of the perovskite lattice and an increase in the amount and population of exsolved Ni nanoparticles. This in turn promoted the CO2 methanation reaction by increasing the CH4 yield, though a possible modification of the surface chemistry due to the increased presence of Sr could also affect the reaction.
Regarding Ba, it was mentioned in a previously discussed work [43] that K-modified 1% Rh/Al2O3 was highly selective towards CO. The authors also prepared a Ba-modified Rh/Al2O3 catalyst, whose catalytic performance was similar to the unpromoted Rh/Al2O3 one. However, the CH4 yield was lower, especially at lower temperatures, and the enhanced CO2 affinity due to the Ba-addition did not benefit the overall methanation activity. The results appear to contrast the findings of previously discussed works [52,53] that report a promoting role of Ba for the CO2 methanation reaction, probably as a result of different types of catalysts, preparation methods, reaction conditions and Ba loadings used.

Strontium (Sr) and Barium (Ba)
Despite the fact that the works dealing with the screenings of different alkaline earth metal promoters report a quite beneficial contribution upon Sr and Ba addition [30,52,53], little further research has been conducted on the individual promotion of either of these heavier elements, especially when compared to their lighter alkaline earth metal counterparts. Toemen et al. [85] modified a Ru/Mn/Al 2 O 3 catalyst with Sr and Ce. The Sr-modified catalyst initially adsorbed increased amounts of CO 2 as monodentate carbonates, which in turn formed bridged bidentate carbonates before being further hydrogenated. The enhanced catalytic activity of the Sr-doped catalyst was, however, overshadowed by the Ce-doped one, where the methanation reaction followed a different mechanism.
Steiger et al. [86] prepared La 1-x Sr x Fe 0.8 Ni 0.2 O 3 perovskite oxides with increasing amounts of A-site Sr substitution. Pre-reduction of the initial perovskite caused the lattice Ni to be exsolved on the perovskite surface in the form of fine and well-dispersed Ni nanoparticles that exhibited strong interaction with the perovskite support. The amount of exsolved Ni, as well as the size and dispersion of the Ni nanoparticles, could be tailored by altering the A-site perovskite composition. La substitution on the A-site by Sr caused a destabilization of the perovskite lattice and an increase in the amount and population of exsolved Ni nanoparticles. This in turn promoted the CO 2 methanation reaction by increasing the CH 4 yield, though a possible modification of the surface chemistry due to the increased presence of Sr could also affect the reaction.
Regarding Ba, it was mentioned in a previously discussed work [43] that K-modified 1% Rh/Al 2 O 3 was highly selective towards CO. The authors also prepared a Ba-modified Rh/Al 2 O 3 catalyst, whose catalytic performance was similar to the unpromoted Rh/Al 2 O 3 one. However, the CH 4 yield was lower, especially at lower temperatures, and the enhanced CO 2 affinity due to the Ba-addition did not benefit the overall methanation activity. The results appear to contrast the findings of previously discussed works [52,53] that report a promoting role of Ba for the CO 2 methanation reaction, probably as a result of different types of catalysts, preparation methods, reaction conditions and Ba loadings used.

Hydrotalcite-Derived and Zeolite Supported Catalysts
Besides commonly used supports such as Al 2 O 3 , ZrO 2 , SiO 2 and TiO 2 , MgO-Al 2 O 3 mixed oxides derived from Mg-containing hydrotalcites, as well as zeolites, have also become popular Catalysts 2020, 10, 812 13 of 36 CO 2 methanation catalyst supports in recent years. Hydrotalcites, or layered double hydroxides, with Ni 2+ and Mg 2+ as divalent cations can be calcined and reduced at high temperatures in order to produce active catalysts that consist of Ni nanoparticles supported on a MgO-Al 2 O 3 mixed oxide. Mg 2+ presence in the initial hydrotalcite facilitates the creation of alkaline sites on the support that favour CO 2 chemisorption and activation. Zeolites can also be impregnated with an active metal phase (e.g., Ni and Rh) to be used as methanation catalysts. These types of catalysts have gained increased traction recently and two review articles have been published summarizing the progress made [87,88]. Since an Mg component is mostly present in hydrotalcite-derived catalysts and alkali, as well as alkaline earth metals, serve as compensating cations in zeolites, we will provide here a brief overview on the influence that these two parameters exert on the CO 2 methanation activity. The cooperation between the Ni catalytically active phase and the MgO alkaline phase in hydrotalcite-derived catalysts has been studied by Liu et al. [90]. The authors varied the Ni 2+ /Mg 2+ molar ratio during the coprecipitation synthesis, while the ratio between trivalent (Al 3+ ) and divalent (Mg 2+ and Ni 2+ ) cations remained constant. After calcination and reduction, the formed Ni nanoparticles were well-dispersed over the MgO-Al 2 O 3 support with an average crystallite size of about 2.5 nm. Two types of basic sites were observed upon CO 2 -TPD, namely weak basic sites for CO 2 adsorbed on Al 2 O 3 phase and moderately strong basic sites for CO 2 adsorbed on MgO phase. Regarding the catalytic performance for CO 2 methanation, the hydrotalcite-derived catalysts presented far superior low-temperature CO 2 conversion compared with other Ni-based catalysts supported on Al 2 O 3 , MgO and carbon nanotubes (CNTs) (Figure 7a). Moreover, a linear correlation could be derived between TOF Ni values and the amount of moderately strong basic sites on the support (Figure 7c). The reaction mechanism proposed involved a CO 2 activation step by the support basic sites to carbonate/hydrocarbonate species and an H 2 dissociation step on the Ni metallic sites. The interplay between these two phases facilitated the excellent low-temperature activity towards CO 2 methanation.  Further works report on the influence of other parameters, as well as on the incorporation of further promoters and metals. Ren et al. [91] changed some variables during the hydrotalcite coprecipitation synthesis that led to platelet-like and rosette-like support structures and emphasised on the effect of support morphology and metal-support interaction over the CO 2 methanation performance. Wierzbicki et al. [92,93] introduced La during the hydrotalcite preparation to further promote the support alkalinity and CO 2 chemisorption, as well as the interaction between the Ni phase and the hydrotalcite-derived support. Additionally, bimetallic catalysts were prepared by Martins et al. [94], with Ru being impregnated over the initial Ni-Mg-Al hydrotalcites, while Mebrahtu et al. [95] varied the amount of added Fe during the hydrotalcite synthesis. The intimate mixing of the Ni 2+ and Fe 3+ metal cations led to the formation of NiFe alloys after reduction and the alloy composition could be correlated with the catalytic performance [95]. We can thereby conclude that hydrotalcite precursors offer a unique possibility to tailor the catalyst composition and morphology, as well as examine the influence of various different factors towards enhancing the CO 2 methanation activity and stability.

Zeolite Supported Catalysts
Zeolites are microporous aluminosilicates with a general formula of [Al x Si 1-x O 2 y-] x and consist of a negatively charged network that requires cationic counterions (e.g., Na + , Ca 2+ ) for charge compensation. Through cation exchange and post-synthetic modification, zeolitic physicochemical properties, such as basicity and hydrophilicity, can be finely tuned. Furthermore, a transition metallic phase, such as Ni or Rh, can be introduced via ion-exchange or impregnation to prepare zeolite supported catalysts suitable for CO 2 methanation applications [88]. Graça et al. [96] were among the first to introduce Ni on ultrastable Y (USY) zeolites with Na + counter-ions for application as CO 2 methanation catalysts. Ni introduction via impregnation proved to be more suitable compared to introduction via ion-exchange because of the higher reducibility of impregnated NiO species, while a latter Ce addition improved the performance further via its synergy with the Ni metallic phase. Westermann et al. [97] studied the mechanism of CO 2 methanation over Ni/USY catalysts and concluded that CO 2 was initially weakly adsorbed over the Na + compensating cation sites on the zeolite. Following the initial chemisorption step, CO 2 hydrogenation proceeded via a CO intermediate adsorbed on the Ni particles. Bacariza et al. [98] then performed an extensive work on the influence that several monovalent and divalent compensating cations exert over the CO 2 methanation performance of Ni/USY catalysts.
The compensating cations were ion-exchanged in the H-USY zeolite in its acidic form and the Ni phase was impregnated subsequently. Regarding the monovalent cations, a trend was observed where the CO 2 methanation performance (CO 2 conversion and CH 4 selectivity) increased for the heavier elements, following the order Cs + > Na + > Li + > K + > H + (with K + being an exception). These differences were attributed to the increase of the CO 2 adsorption capacity through a higher support alkalinity. Regarding the divalent cations, the catalytic activity followed a different trend by decreasing for the heavier elements in the order Mg 2+ > Ca 2+ > Ba 2+ . The better performance for ion-exchange with Mg 2+ was attributed to the higher Ni dispersion and Ni surface area. Thus, ion-exchange of compensating cations could be a correct pathway to tune specific properties of zeolite supported CO 2 methanation catalyst, such as metallic phase dispersion, support alkalinity and water affinity.
Besides the effect of different compensating cations, the optimization of the Si/Al ratio and the introduction of additional promoters (e.g., Mg, La and Ce) via impregnation has been studied. Bacariza et al. [99] modified the USY zeolitic composition in terms of Si/Al ratio and observed that higher Si/Al ratios decreased the water affinity and provided an enhanced methanation activity. Mg [68], La [100] and Ce [101] have also been employed as further promoters and introduced via impregnation over the zeolite support. The introduction of these promoters aimed to improve the dispersion of the metallic phase and promote CO 2 chemisorption via enhancing the surface basicity. Bacariza et al. [88] also presented some proposals for the preparation of active zeolite supported CO 2 methanation catalysts that incorporate properties such as high metallic phase dispersion, increased zeolite support basicity, low water affinity and sufficient steam resistance.
Lastly, Zhang et al. [102] prepared a novel methanation catalyst by using a Na-modified Y zeolite as a support and Al-source for the subsequent formation of a Ni-Al hydrotalcite precursor phase. After reduction, Ni-based, hydrotalcite-derived catalysts supported on the zeolitic structure were created. The catalysts exhibited a unique mosaic structure and the supported Ni nanoparticles were successfully confined by the AlO x phase, thus improving their resistance to sintering and coke formation. Table 1 summarizes the CO 2 methanation performance of some typical catalysts promoted with group I and II metals.

Other Promoters that Favour CO 2 Chemisorption
Besides alkali and alkaline earth metals, lanthanides or rare earths, such as La, Ce and Pr [103], transition metals, such as V, W, Mn, Co and Fe [39,104,105], as well as noble metals, such as Pd, Pt, Rh and Ru [106], are commonly introduced into CO 2 methanation catalysts to enhance their activity. A common Ni-based catalyst promotion with metals, such as Ru, Rh, Pt, Co and Fe, mostly aims at the creation of bimetallic and alloy particles in order to improve the properties of the Ni metallic phase, such as reducibility, metal dispersion and resistance to oxidation, agglomeration and coking [8,28]. On the other hand, lanthanides and some transition metals often target the modification of the metal oxide support to improve properties such as surface basicity and oxygen vacancy concentration, leading to enhanced CO 2 chemisorption similarly to the promotion with alkali and alkaline earth metals [27,28].
Just like the works dealing with screenings performed using alkali and alkaline earth metals, Xu et al. [103] and Guilera et al. [107] screened several rare earth and transition metals (i.e., La, Ce, Sm, Pr, Y and Zr) as promoters over Ni/Al 2 O 3 catalysts. Among them, Pr, Ce and La exhibited the most pronounced promoting effects. La-promotion specifically, has been reported to be beneficial for the CO 2 methanation performance by a number of studies, as it appears to enhance the support alkalinity. La-addition on the support can been performed either via separate impregnation [108,109] or via co-impregnation with Ni that targets the formation of an initial LaNiO 3 perovskite precursor phase [110,111]. Lastly, Mn promotion also targets similar improvements, namely enhancing the CO 2 adsorption and increasing the metal phase dispersion [104]. Easy-to-desorb CO species formed after K-doping stirred the reaction towards RWGS and CO selectivity Step increase in the CO 2 conversion and CH 4 selectivity after adding Ru second metal and CaO promoter [75] 0.7% Ca 5% Ni/Ca x Zr

Alkali and Alkaline Earth Metals as Active Sorbents Phases in the Combined Capture and Methanation of CO 2
Carbon capture and utilization (CCU) revolves around the concept of coupling post-combustion CO 2 capture to a CO 2 utilization step [5]. If this step is an exothermic reduction reaction, then the energy released can be used to mitigate the energy demands arising from the endothermic CO 2 desorption step. Additionaly, CH 4 selectivity for many catalysts during CO 2 hydrogenation is high at temperatures that can coincide with those required for CO 2 desorption, thus rendering CH 4 as a preferable final product. The combination of these two processes, namely CO 2 adsorption and hydrogenation, in a single reactor can be realized via the development of novel dual-function materials (DFMs) that embody both adsorption and catalytic capabilities [19,20,25].
In most DFM materials targeting CH 4 production, Ru is chosen as the preferred metal to provide the catalytically active sites, despite it being significantly less abundant and pricier compared to Ni. Since most flue gases are O 2 -rich, a partial oxidation of the metallic surface takes place during the CO 2 capture step under practical operation conditions. The catalyst then needs to be reactivated under a H 2 -rich gas flow. Ru is most promising in this regard, since the oxidized RuO x layers can be re-reduced under H 2 flow at temperatures where CO 2 capture takes place (< 350 • C), thereby increasing the process energy efficiency by operating both steps isothermally [22]. Oxidized NiO requires more intense reducing environments to be reactivated, depending also on the support used and the type and loading of the adsorbent phase. A higher temperature activation step or the absence of O 2 from the CO 2 -containing gas are thus necessary prerequisites for the employment of Ni-based DFMs [21,112]. By promoting a Ni catalyst with a second metal though, its reducibility could be enhanced in order for it to meet the requirements to be used under more practical flue-gas applications [23].

Ru-Based Dual-Function Materials
Duyar et al. [19] introduced the concept of a DFM for the CO 2 capture and conversion to CH 4 ( Figure 8) [113]. The materials were prepared via incipient wetness impregnation, either impregnating Ca(NO 3 ) 2 first and then Ru(NO)(NO 3 ) 2 on γ-Al 2 O 3 , or vice versa. The loadings of the adsorbent (CaO) and the catalyst (Ru) were varied between 1 and 10 wt.%. It was shown that first impregnating the sorbent and then the catalyst was a more suitable route for the material preparation, whereas a high sorbent loading appeared to be beneficial, since it increased the amount of captured CO 2 and the extent of CO 2 spillover towards the methanation active sites. An optimum composition of 5% Ru, 10% CaO/γ-Al 2 O 3 produced the maximum amount of CH 4 per kg of DFM material at 0.5 mol CH 4 /kg DFM (or 0.5 mmol CH 4 /g DFM). The authors also showed that the high dispersion of the CaO sorbent phase, that can facilitate a reversible chemisorption and desorption of CO 2 at moderate temperatures, in combination with the easily reducible and highly active Ru methanation catalyst, could enable the whole process to proceed isothermally at 320 • C. A follow up parametric study [114] reported an effective DFM preparation method, as well as suitable adsorption and methanation conditions in order for it to be applied to industrial-scale applications.
Following the earlier works, Duyar et al. [113] also used Rh as the active metal with CaO adsorbent phase and they showed that a comparatively miniscule amount of Rh loading in the DFM (0.1 wt.%) could generate comparable amounts of CH 4 with a 5% Ru loading. However, the high price of Rh is still a major problem and thus, the less expensive Ru is considered as a more suitable active metal. In the second part of their work, Duyar et al. [113] tested K 2 CO 3 , Na 2 CO 3 and MgO as potential CO 2 sorbent alternatives. K 2 CO 3 and Na 2 CO 3 exhibited better results, with the 5% Ru, 10% Na 2 CO 3 /γ-Al 2 O 3 DFM yielding 1.05 mol CH 4 per kg material at 320 • C operation. Thus, it was shown that the optimization of the CO 2 sorption phase can be of equal importance to that of the catalytically active metal phase in an attempt to lower the catalyst material costs and increase the process efficiency. A parametric study was also conducted for the new optimized 5% Ru, 10% Na 2 CO 3 /γ-Al 2 O 3 DFM [115]. Two parallel reactors, one operating under CO 2 capture and the other under CO 2 methanation conditions, could lead to an optimal use of available resources, while prolonged operation did not appear to be detrimental for the material and its catalytic properties. Figure 8. Schematic summarizing the dual-function materials (DFM) concept (adopted from [113]).
Due to its easy reduction once oxidized, ruthenium's role is crucial for isothermal DFM applications and it can provide stable performance [22]. From a kinetic point of view, Ru-based DFMs (5% Ru) appearred to achieve faster hydrogenation rates, even compared to their Rh-based analogues (0.5% Rh), when Na 2 CO 3 (or "Na 2 O") was used as sorbent. Thus, Ru can be considered a superior DFM catalyst phase based on price and reaction kinetics criteria. Regarding the sorbent's impact on the methanation kinetics, "Na 2 O"-type active sorbents appeared far superior and led to an overall higher CH 4 yield, despite CaO sorbents initially providing a higher CO 2 adsorption capacity [21].
The mechanism behind the two-step CO 2 capture and methanation process over the 5% Ru, 10% Na 2 CO 3 /γ-Al 2 O 3 (or 5% Ru, 6.1% "Na 2 O"/γ-Al 2 O 3 ) DFM was highlighted by Proaño et al. [116] employing in-situ DRIFTS (Figure 9). After calcination and pre-reduction of the materials, Na 2 CO 3 was decomposed and Na-rich surface, as well as Al-O − -Na + sites, were created and denoted as "Na 2 O". On Na-free Ru/Al 2 O 3 , CO 2 was chemisorbed at Ru sites and at -OH groups of the support, whereas on the Na-containing DFM, CO 2 was chemisorbed at Al-O-Na + sites in large quantities. The presence of Na + on the surface caused CO 2 to be primarily adsorbed in the form of chelated carbonato complexes, while the Ru and Al-OH sites could still accommodate adsorbed CO 2 , as in Ru/Al 2 O 3 . The adsorbed bidentate carbonates could then spillover to adjacent Ru catalytically active sites, where they were hydrogenated into CH 4 via formate-type intermediates. Enhanced CH 4 production for the Na-containing DFM was thus attributed to the larger CO 2 capture capacity, due to the additional Al-O-Na + sites that had a high affinity towards CO 2 .
Besides the Farrauto group, Bermejo-López et al. [24] performed a thorough work to describe the mechanism of CO 2 capture and methanation, as well as to correlate the type and loading of the adsorbent phase with the material's performance ( Figure 10). Ru metal dispersion was one parameter dependant on the adsorbent type, with CaO seeming to hinder it, while Na 2 CO 3 promoting it. The basicity of the materials increased with higher adsorbent loading and this promoted the CH 4 yield. CaO provided strong basic sites, where CO 2 was chemisorbed forming stable carbonates (CaCO 3 ) that required higher temperatures to be desorbed. On the other hand, medium-strength basic sites over Na 2 CO 3 led to the formation of less stable carbonates, enabling operation at lower temperatures. The proposed reaction mechanism involved an initial adsorption of CO 2 with simultaneous release of H 2 O from the sorption active phases. During the following methanation step, adsorbed CO 2 spilled over to Ru sites and was hydrogenated to methane, while a delay in the H 2 O signal could be attributed to some H 2 O-product being adsorbed by the basic sorbent sites and forming Ca(OH) 2 and NaOH.
The mechanism behind the two-step CO2 capture and methanation process over the 5% Ru, 10% Na2CO3/γ-Al2O3 (or 5% Ru, 6.1% "Na2O"/γ-Al2O3) DFM was highlighted by Proaño et al. [116] employing in-situ DRIFTS (Figure 9). After calcination and pre-reduction of the materials, Na2CO3 was decomposed and Na-rich surface, as well as Al-O − -Na + sites, were created and denoted as "Na2O". On Na-free Ru/Al2O3, CO2 was chemisorbed at Ru sites and at -OH groups of the support, whereas on the Na-containing DFM, CO2 was chemisorbed at Al-O-Na + sites in large quantities. The presence of Na + on the surface caused CO2 to be primarily adsorbed in the form of chelated carbonato complexes, while the Ru and Al-OH sites could still accommodate adsorbed CO2, as in Ru/Al2O3. The adsorbed bidentate carbonates could then spillover to adjacent Ru catalytically active sites, where they were hydrogenated into CH4 via formate-type intermediates. Enhanced CH4 production for the Na-containing DFM was thus attributed to the larger CO2 capture capacity, due to the additional Al-O-Na + sites that had a high affinity towards CO2. Figure 9. Proposed CO2 methanation mechanism over 5%Ru/Al2O3 (down) and extended (up and down) for over 5%Ru, 6.1%"Na2O"/Al2O3 DFM. Reproduced with permission from ref. [116]. Copyright 2019 Elsevier. Figure 9. Proposed CO 2 methanation mechanism over 5%Ru/Al 2 O 3 (down) and extended (up and down) for over 5%Ru, 6.1%"Na 2 O"/Al 2 O 3 DFM. Reproduced with permission from ref. [116]. Copyright 2019 Elsevier.
Cimino et al. [35] modified the γ-Al 2 O 3 support with various alkali metals (Li, Na, K and Cs) and 1% Ru was chosen as the active metal phase. Regarding CO 2 methanation performance, only Li exhibited a promotion effect and formed Li-aluminate spinel-phases that stabilized the alumina support and its interaction with Ru active sites. During cyclic CO 2 capture and methanation at low temperature (230 • C), the Li-promoted material presented faster methanation kinetics and higher CH 4 production compared to the Na-promoted one. It was noted that the temperature of the cyclic operation for the Li-promoted 1% Ru DFM (230 • C) was considerably lower compared with that for other DFMs that have higher Ru loadings. operation at lower temperatures. The proposed reaction mechanism involved an initial adsorption of CO2 with simultaneous release of H2O from the sorption active phases. During the following methanation step, adsorbed CO2 spilled over to Ru sites and was hydrogenated to methane, while a delay in the H2O signal could be attributed to some H2O-product being adsorbed by the basic sorbent sites and forming Ca(OH)2 and NaOH. Cimino et al. [35] modified the γ-Al2O3 support with various alkali metals (Li, Na, K and Cs) and 1% Ru was chosen as the active metal phase. Regarding CO2 methanation performance, only Li exhibited a promotion effect and formed Li-aluminate spinel-phases that stabilized the alumina support and its interaction with Ru active sites. During cyclic CO2 capture and methanation at low temperature (230 °C), the Li-promoted material presented faster methanation kinetics and higher CH4 production compared to the Na-promoted one. It was noted that the temperature of the cyclic operation for the Li-promoted 1% Ru DFM (230 °C) was considerably lower compared with that for other DFMs that have higher Ru loadings.
Lastly, Sun et al. [25] employed a DFM consisting of a Ru supported on CeO2 nanorods methanation catalyst and an alkali nitrate promoted MgO sorbent mechanically mixed together ( Figure 11). It has been already reported by Wang et al. [117] that the CO2 methanation mechanism over Ru/CeO2 follows a different pathway compared to Ru/Al2O3, with the oxygen vacancy catalyzed formate dissociation being the rate determining step in the former. Sun et al. [25] performed the integrated CO2 capture and methanation process in their DFM at 300 °C. CO2 was initially captured in the form of MgCO3. Subsequently, during the methanation process, CO2 was desorbed and activated by an oxygen vacancy in the CeO2 nanorod support, forming a formate intermediate. The formate was then decomposed into a ruthenium carbonyl and further hydrogenated into CH4 by nearby hydrogen atoms chemisorbed at the Ru surface. It was further noted that, although CH4 yield and CO2 conversion were higher for 10% Ru loading during the 1st cycle, the 5% Ru loaded catalyst showed better performance during the 10th cycle, probably due to the better preservation of the oxygen vacancies in the support. Lastly, Sun et al. [25] employed a DFM consisting of a Ru supported on CeO 2 nanorods methanation catalyst and an alkali nitrate promoted MgO sorbent mechanically mixed together ( Figure 11). It has been already reported by Wang et al. [117] that the CO 2 methanation mechanism over Ru/CeO 2 follows a different pathway compared to Ru/Al 2 O 3 , with the oxygen vacancy catalyzed formate dissociation being the rate determining step in the former. Sun et al. [25] performed the integrated CO 2 capture and methanation process in their DFM at 300 • C. CO 2 was initially captured in the form of MgCO 3 . Subsequently, during the methanation process, CO 2 was desorbed and activated by an oxygen vacancy in the CeO 2 nanorod support, forming a formate intermediate. The formate was then decomposed into a ruthenium carbonyl and further hydrogenated into CH 4 by nearby hydrogen atoms chemisorbed at the Ru surface. It was further noted that, although CH 4 yield and CO 2 conversion were higher for 10% Ru loading during the 1st cycle, the 5% Ru loaded catalyst showed better performance during the 10th cycle, probably due to the better preservation of the oxygen vacancies in the support. Figure 11. Schematic illustration for the CO 2 capture from the MgO sorbent phase and the conversion of captured CO 2 into CH 4 over the Ru/CeO 2 catalyst phase. Reproduced with permission from ref. [25]. Copyright 2020 Elsevier.

Ni-Based Dual-Function Materials
Although Ni would be a more suitable choice as a methanation catalyst in DFMs, being considerably cheaper and more abundant compared to noble metals like Ru and Rh, there are a number of limitations that need to be carefully considered. Arellano-Treviño et al. [21] impregnated Ni on a γ-Al 2 O 3 carrier that had been previously loaded with a Na 2 CO 3 adsorbent and the Ni-based DFM underwent a pre-reduction at 650 • C for the catalyst activation. The authors showed that the Ni-based DFM (10% Ni, 6.1% "Na 2 O"/γ-Al 2 O 3 ) operated cyclically at a constant 320 • C temperature could only produce substantial amounts of CH 4 , if the CO 2 capture step was performed under an O 2 -free gas (7.5% CO 2 /N 2 ). On the other hand, if the CO 2 capture step was performed at 320 • C under a simulated flue-gas (7.5% CO 2 , 4.5% O 2 , 15% H 2 O, balance N 2 ), no CH 4 was produced during the subsequent hydrogenation step at the same temperature. This was explained by the inability of NiO to be reactivated under 15% H 2 /N 2 at 320 • C in order for the whole operation to proceed isothermally. Furthermore, Ni-based DFMs retained a certain amount of preadsorbed CO 2 that was not further converted.
Following their earlier work, Arellano-Treviño et al. [23] attempted to enhance the Ni-based material's reducibility by introducing small amounts (0.1 and 1 wt.%) of noble metals Ru, Pt and Pd. Pd-containing samples exhibited comparatively inferior performance, whereas the beneficial effect of Pt and Ru addition rested on the enhancement of NiO reducibility under 13.26% H 2 /N 2 flow at 320 • C. 1 wt.% Pt addition caused 50% of the initially oxidized NiO to be reduced, while this value for the promotion with 1 wt.% Ru reached 70%. The authors speculated that PtO x and RuO x species reduced rapidly and the exposed Pt and Ru active surfaces splitted H 2 molecules and helped reduce the adjacent NiO sites. For cyclic operation at 320 • C, the 1% Ru, 10% Ni, 6.1% "Na 2 O"/γ-Al 2 O 3 material exhibited great cyclic stability and the highest CH 4 production, since both Ru and Ni sites are active for CO 2 methanation. Compared to the Ni-free DFM with the same Ru loading (1% Ru, 6.1% "Na 2 O"/γ-Al 2 O 3 ), the Ni-containing one (1% Ru, 10% Ni, 6.1% "Na 2 O"/γ-Al 2 O 3 ) exhibited an increased CO 2 adsorption capacity and CH 4 yield, at a cost, however, of slower methanation kinetics. As such, the synergy between Ni and Ru metals promoted the material's performance, in agreement with other studies regarding CO 2 methanation [75,118].
Other works typically perform the CO 2 capture step with an O 2 -free gas mixture of CO 2 balanced with a carrier gas. Bermejo-López et al. [112] studied the effects of Ni, as well as CaO and Na 2 CO 3 loadings on the DFM performance ( Figure 12). The addition of the adsorbent phase weakened the metal-support interaction and enhanced the reducibility of the Ni metallic phase. Similarly to their previous work [24], the authors showed that CaO adsorbent led to the creation of highly stable carbonates that required higher temperatures to decompose, while less stable carbonates over Na 2 CO 3 decomposed at a lower temperature. Thus, during the following hydrogenation step, CH 4 formation was favoured at higher temperatures when CaO was used as the active sorbent, peaking at 520 • C, while maximum CH 4 was produced for Na 2 CO 3 at 400 • C. Increasing the Ni loading resulted in increased CO 2 sorption capacity and CH 4 production. Regarding the proposed mechanism, CO 2 was initially adsorbed over NaOH and Ca(OH) 2 species, with H 2 O being simultaneously released. During the subsequent H 2 /N 2 inflow, adsorbed CO 2 was reduced after its spillover to Ni sites and detected product H 2 O was delayed due to its initial adsorption by the sorbent phases. The DFM with CaO sorbent (10NiCa) exhibited a higher CH 4 selectivity upon hydrogenation during cyclic operation at 400 • C, while the DFM with Na 2 CO 3 sorbent (10NiNa) produced more CO arising from the RWGS reaction.
Chai et al. [119] also tested the effects of different CO 2 adsorbents (MgO, CaO, Na 2 CO 3 and K 2 CO 3 ) on Ni-based DFMs supported on Al 2 O 3 . The catalyst and adsorbent loadings were the same, at 10 wt.%, while the CO 2 capture step was performed under an O 2 -free 9.5% CO 2 /N 2 gas flow. In a similar fashion to the findings of Bermejo-López et al. [23,102], the authors showed that CaO sorbent led to greater CO 2 sorption capacity. However, the release of adsorbed CO 2 in the material with CaO sorbent was not fully reversible. The material with Na 2 CO 3 sorbent exhibited a high CO 2 capture capacity, along with a high release efficiency. Hence, methanation capacity for the material with Na 2 CO 3 sorbent was superior, a property attributed to the easier release and spillover of adsorbed CO 2 towards the Ni catalytically active sites. Hu et al. [120] prepared Ni-based DFMs supported on ZrO 2 and modified with La and K. The use of K and La promoters raised the CO 2 sorption capacity of the materials, with the K-modified material increasing its CO 2 capture efficiency four-fold, compared with the unmodified one. K-promoted Ni/ZrO 2 released CH 4 upon hydrogenation much slower, while the La-promoted material exhibited faster methanation kinetics, with the time required for CH 4 release roughly matching the time required for CO 2 capture. The methanation mechanism was modified upon K-promotion and additional less-reducible intermediate bidentate carbonates were formed, whereas the reaction over unpromoted and La-modified Ni/ZrO 2 proceeded over active formate intermediates. It could be concluded, that La-promotion was more suited for operation at lower temperatures (250 • C) by achieving favourable kinetics, while K-modification favoured increased CO 2 chemisorption and methanation efficiency at higher temperatures (>350 • C).
Finally, Zhou et al. [121] prepared a 2D-layered Ni/MgO-Al 2 O 3 material to be used during the integrated CO 2 capture and methanation ( Figure 13). The layered oxides resulted after calcination and reduction of the initial hydrotalcite (or layered double hydroxide) precursors, which were in turn prepared via the coprecipitation method. By increasing the Ni content in the material, the interaction between Ni and the MgO-Al 2 O 3 support weakened and Ni species became more reducible. For the cyclic CO 2 capture and methanation process, the authors aimed at optimizing the respective durations, so that CO 2 breakthrough was minimal and a maximum utilization of reactant H 2 was achieved. Complete capture and methanation could be achieved by a 35 s CO 2 sorption time and a 25 s methanation time. Upon conducting in-situ FTIR experiments, monodentate and bidentate carbonates, as well as formate species were detected during the CO 2 sorption phase. Following 20% H 2 /Ar flow, the observed bands suggested a gradual removal of these species via hydrogenation to CH 4 , with no CO being detected. Further development of materials derived from hydrotalcite precursors containing Mg and Ni could be favourable for DFM applications, since Ni can be highly dispersed and stabilized, while a high Mg content can provide a plethora of surface basic sites for CO 2 chemisorption [90]. Table 2 summarizes the performance of DFMs for the combined capture and methanation of CO 2 .

Dual-Function Materials for Products Other than Methane
Apart from synthetic natural gas (SNG) obtained during methanation, the concept of DFMs for the integrated CO 2 capture and conversion has been extended for the production of CO and syngas, that can be used for the further production of other value-added products. Bobadilla et al. [122] prepared a K-promoted trimetallic FeCrCu catalyst supported on MgO-Al 2 O 3 . The K-component was added to increase the CO 2 sorption capacity, while the combination of three earth-abundant metals (Fe, Cr and Cu) favoured CO selectivity via RWGS. Hyakutake et al. [123] promoted a Cu/Al 2 O 3 catalyst with K and Ba for CO 2 capture and conversion to CO. K-promotion facilitated Cu dispersion and resulted in a very high capacity for CO 2 capture as formate intermediates. These intermediates were then selectively reduced to CO via the RWGS route. The Ba-promoted material captured much less CO 2 in comparison and thus exhibited an inferior performance as DFM for CO production.
Al-Mamoori et al. [124] introduced a new feature via the use of K and Na-promoted CaO and MgO-based double salts as the CO 2 adsorbing components, in an attempt to greatly increase the CO 2 capture efficiency and sorption kinetics. The K-Ca, Na-Ca, K-Mg and Na-Mg double salts were supported on γ-Al 2 O 3 via the impregnation of alkalimetal carbonates and alkaline earth metal nitrates, whereas Ni was impregnated subsequently. After the CO 2 capture step, ethane was flowed into the reactor in order to produce syngas via the dry reforming of ethane (DER) reaction.
Additionally, Sun et al. [125] employed a common CO 2 sorbent, CaO, directly as support material for Ni nanocatalysts with adjacent Ce-species. They argued that the already established calcium-looping process that rests on the carbonization and calcination of CaO for CO 2 capture and sequestration (CCS) could be modified through the integration of a CO 2 utilization step by converting CO 2 to CO. The incorporation of Ni facilitated CO 2 reduction, whereas Ce-species provided oxygen vacancies that were active sites for CO 2 dissociation and prevented the agglomeration of individual CaO and Ni nanocrystallites.

Configuartions Using a Seperate Sorbent and Methanation Catalyst
Apart from the use of a single solid that incorporates both sorption and methanation functions, the process of integrated CO 2 capture and conversion to methane can be realized via the combination of two different solids that perform these functions individually. Veselovskaya et al. published a number of works that incorporated the use of a K 2 CO 3 /Al 2 O 3 sorbent and a Ru/Al 2 O 3 methanation catalyst [126][127][128]. The high CO 2 sorption capabilities of the K 2 CO 3 -based sorbent enabled it to capture CO 2 directly from ambient air (direct air capture or DAC), where its concentration is as low as 400 ppm. The sorbent could be regenerated at the mild temperature of 200 • C under H 2 flow and CH 4 was produced at a subsequent catalytic reactor that contained the Ru/Al 2 O 3 methanation catalyst. The whole operation was coined as direct air capture and methanation (DACM) process [128].
Finally, Miguel et al. [129] also operated the CO 2 capture and conversion to SNG process by using a single reactor that contained alternating layers of the sorbent and catalyst materials. CO 2 sorption was performed by a K-promoted hydrotalcite, whereas a commercial Ni-rich material was used as the methanation catalyst. It was shown that the whole amount of captured CO 2 could be fully hydrogenated into CH 4 and that CH 4 production under cyclic operation was quite high, peaking at 2.36 mol CH 4 per kilogram material and hour of operation.

Conclusions
Given the continuous increase of humanity's energy needs and the depletion of our fossil fuel deposits, better management of existing carbon resources via the cyclic conversion between carbon's highest (CO 2 ) and lowest (CH 4 ) oxidation state is of particular importance towards the transition to a renewable energy-based economy.
Alkali and alkaline earth metal addition appears to greatly impact the CO 2 methanation activity of supported metal catalysts. Their promoting effect mainly rests on the enhancement of support alkalinity that promotes CO 2 chemisorption, the generation of oxygen vacancies on the surface of metal oxide supports and the improvement of active metal dispersion. Being extremely cheap and earth-abundant, group I and II metals seem to be a viable choice to for use as catalyst promoters towards enhancing the low-temperature activity of Ni-based catalysts, or allowing for a lower noble metal loading (e.g., Ru), without compromising the catalytic activity.
It is, however, evident that, under specific conditions, alkali and sometimes alkaline earth metal addition can suppress methanation activity. This can be achieved by the masking of some active metal sites, for example when impregnating the alkaline modifier after the active metal, or by a detrimental change in the reaction pathway, for example, through a strong interaction with formate intermediates or the increased production of easy-to-desorb CO. More detailed studies on the effect of different alkali and alkaline earth metal promoters on already active methanation catalysts (such as benchmark Ni/CeO 2 and Ni/Al 2 O 3 ) should be undertaken to further clarify their promotion function. In general, it appears that alkaline earths constitute reliable promoters, while the existing literature on promotion with heavier group II metals (Sr and Ba) is comparatively scarce.
The development of new dual-function materials (DFMs) functioning as both catalysts and adsorbents towards the combined CO 2 capture and methanation appears capable of improving the process and as a result, research efforts towards the preparation of such novel materials have boomed during the last 5 years. The main issues that need to be resolved concern the enhancement of the CO 2 sorption capacity and CH 4 production (mostly measured in mmol CH 4 /g DFM), as well as the active metal stability under industrial-like flue-gas applications.
However, since the field is quite new and has great potential, many possible research directions do exist. These could include the comparative study of different support materials (e.g., Al 2 O 3 , SiO 2 , ZrO 2 , CeO 2 and mesoporous silicas), the employment of additional catalyst synthesis methods (e.g., hydrotalcite-derived catalysts by coprecipitation), the further use of other promoter elements that favour CO 2 adsorption (e.g., La, Sr) and the fabrication of DFMs with stable bimetallic nickel-based catalysts with other transition metals (e.g., NiFe and NiCo alloys). Funding: This research has been co-financed by the European Union and Greek national funds through the operational program 'Regional Excellence' and the operational program Competitiveness, Entrepreneurship and Innovation, under the call Research -Create -Innovate (Project code: T1EDK-00782). The authors are also grateful to the Research Committee of UOWM for financial support through grant no. 70304.

Conflicts of Interest:
The authors declare no conflict of interest.