The Role of Alkali and Alkaline Earth Metals in the CO₂ Methanation Reaction and the Combined Capture and Methanation of CO₂

Abstract

CO₂ methanation has great potential for the better utilization of existing carbon resources via the transformation of spent carbon (CO₂) to synthetic natural gas (CH₄). Alkali and alkaline earth metals can serve both as promoters for methanation catalysts and as adsorbent phases upon the combined capture and methanation of CO₂. Their promotion effect during methanation of carbon dioxide mainly relies on their ability to generate new basic sites on the surface of metal oxide supports that favour CO₂ chemisorption and activation. However, suppression of methanation activity can also occur under certain conditions. Regarding the combined CO₂ 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 efforts 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 CO₂ methanation catalysts, as well as DFMs for the combined capture and methanation of CO₂.

1. Introduction

Antropogenic CO₂ 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 CO₂ from emission sources and then depositing it to underground geological formations [4]. However, CO₂ utilization after the initial capture step (Carbon capture and utilization (CCU)) through conversion of CO₂ 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 offered by the intense research and development in the field of novel photovoltaics and electrolyzers, aiming to drastically lower the costs for the production of renewable hydrogen [7].

CO₂ methanation, also known as the Sabatier reaction, is an exothermic reaction that converts CO₂ to methane using hydrogen as the reducing agent [8]:

CO₂ + 4H₂ ⇌ CH₄ + 2H₂O, ΔH_298K = −164 kJ/mol

This reaction has gained increased traction in recent years, since it can be employed to reduce our carbon footprint by recycling existing carbon sources, and it is also considered vital in the facilitation of long term space exploration missions and manned missions to Mars [8,9]. Other possible products besides CH₄ during the hydrogenation of CO₂ include CO via the reverse water-gas shift (RWGS) reaction [10], methanol (CH₃OH) and C₂₊ hydrocarbons with two or more carbon atoms through a promotion of the C–C coupling step [11]. CO₂ methanation targets the exclusive production of CH₄ over the other possible hydrogenation products [6].

According to Vogt et al. [12], three different pathways can be conceived during the hydrogenation of CO₂ to methane i.e., the carbide pathway through a CO intermediate, the formate pathway and the carboxyl pathway. The first two pathways have been observed for Ni-based catalysts through operando spectroscopic techniques (in-situ FTIR and ambient-pressure XPS), while the carboxyl pathway, though observed for Cu and Fe, is considered to be inactive for Ni-based catalysts [9,12]. Furthermore, catalyst supports can often play a major role in the methanation mechanism and the overall catalytic activity [13]. For example, Cárdenas-Arenas et al. [14] showed that oxygen species generated via CO₂ dissociation on Ni/CeO₂ catalysts could migrate throughout the ceria lattice and then be reduced at other sites, while the accumulation of formate intermediates at specific sites on Ni/Al₂O₃ catalysts delayed the chemisorption and activation of further CO₂ molecules. These different reaction routes caused by the oxygen ion mobility and lability of CeO₂-based supports could explain the greatly enhanced methanation activity of Ni catalysts supported on CeO₂ compared to other metal oxide supports [14,15,16].

The initial capture of CO₂ can be achieved by using liquids, such as aqueous amines, or solid materials, such as alkali and alkaline earth metal carbonates and oxides [17]. CaO solids promoted with different stabilizers are some of the most common CO₂ sorbents and the cyclical process that integrates CO₂ adsorption and desorption through CaO carbonization and calcination is often referred to as calcium looping [18]. Hydrogen from renewable sources can then be used to convert the captured CO₂. This whole process (CCU) is more energy efficient, since an exothermic CO₂ reduction step can provide the desired thermal energy for the endothermic desorption of captured CO₂ [5,19].

The combined CO₂ capture and conversion to methane can be realized either via the utilization of two different types of solids, namely one solid sorbent for the CO₂ capture and one methanation catalyst, or via the use of a single solid known as a dual-function material (DFM) that incorporates both the sorption and methanation functions [20]. The Farrauto group have been pioneers in this new and dynamic field, being the first to report a DFM consisting of CaO and Ru phases supported on a γ-Al₂O₃ support. The CaO and Ru phases functioned as CO₂ sorption and methanation active sites, respectively [19]. A recent review focusing exclusively on the combined CO₂ capture and conversion to methane has been published in early 2019 [20].

A common feature in the design of such DFMs is the incorporation of an alkali or alkaline earth metal oxide/carbonate phase (e.g., CaO, Na₂CO₃, K₂CO₃ and MgO) on a high surface area support (e.g., Al₂O₃ and ZrO₂) in order to be combined with adjacent catalytically active metal sites for CO₂ methanation [20,21]. Ru has been a favourite active metal due to its easier reducibility and coke-resistant propensity, but Ni is also studied in an attempt to curb the material’s costs [22,23]. Various properties, such as the choice of the right support, the type of the CO₂ sorption phase and the appropriate sorbent loading, as well as the choice of the active metal or the right combination of metals, have an important role to play in enhancing the CO₂ sorption capacity and CH₄ yield of the designed material formulations [23,24,25].

Hereby we aim to summarize the most recent progress made in the field of alkali and alkaline earth metal promotion of the CO₂ methanation reaction and provide a comprehensive overview over the development of novel dual-function materials for the integrated capture and methanation of CO₂. The latter materials, by functioning as both catalysts and adsorbents towards the combined CO₂ capture and methanation, appear capable of improving the process and as a result have attracted intense interest in the last five years.

2. Alkali and Alkaline Earth Metals as Promoters of the CO₂ 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 CO₂ 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 CO₂ methanation catalysts by introducing some kind of metal or metal oxide promoter [27].

Alkali and especially alkaline earth metal promotion of the CO₂ methanation reaction has been a quite popular route in recent years. These types of metals are cheap, abundant and can potentially promote the CO₂ methanation reaction by increasing support basicity and by introducing oxygen vacancies, thus favouring the initial CO₂ 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.

2.1. Alkalimetals (Li, Na, K and Cs)

2.1.1. Screenings of Various Alkalimetals

One of the few works reporting a promoting effect of various alkalimetals during the CO₂ 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 TiO₂ support. A low (0.5 wt.%) and a high (5 wt.%) Ru loading was achieved via a subsequent impregnation step of Ru(NO)(NO₃)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-TiO₂). 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 CH₄ selectivity were promoted according to the following order with respect to the alkali identity: TiO₂ (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 CH₄.

The impact of alkalimetal addition on a Ru-based catalyst towards the methanation of CO₂ 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 LiNO₃, exhibited lower CO₂ conversions and CH₄ selectivities compared to the unmodified 1% Ru/Al₂O₃ 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 Al₂O₃ support. The LiNO₃ and NaNO₃ promoted materials were also tested for cyclic CO₂ 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/Al₂O₃ and Co/Al₂O₃ catalysts, as well as the alteration of the CO₂ methanation mechanism via in-situ DRIFTS. In the first study [36], a total of 16 different metal additives were first impregnated on a γ-Al₂O₃ 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 H₂CO* intermediates. In the second study [37], the modification of the alumina support in Co/Al₂O₃ 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 CO₃²⁻ intermediates, lowering the catalytic activity and CH₄ selectivity, as well as favouring coke formation. The promotion of the CO₂ chemisorption by alkalimetal addition did not appear to correlate with the catalytic activity.

In regards to the electrocatalytic CO₂ 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 CO₂ 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 O²⁻. 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 CO₂ methanation over traditional supported Ru catalysts upon varying Na-loading by an impregnation method [33].

2.1.2. Sodium (Na)

Among the works focusing on the support modification with Na, Le et al. [41] investigated the CO₂ methanation reaction over Ni catalysts supported on Na (0.1–1 wt.%) doped SiO₂ and CeO₂ supports. Na was incorporated (via impregmation) in the previously made 33% Ni/SiO₂ and 13% Ni/CeO₂ catalysts in a second impregnation step using a NaNO₃ precursor. The available active metal surface area was shown to decrease upon Na addition, following H₂ chemisorption experiments. Subsequently, CO₂ conversion decreased upon Na modification of the initially highly active Ni/CeO₂ catalyst, while the increase of the amount of moderately strong basic sites and CO₂ chemisorption sites following the incorporation of Na in Ni/SiO₂ 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/Al₂O₃ 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 CO₂ conversion, arguing that CO₂ methanation over highly loaded Ni/Al₂O₃ 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 Na₂CO₃ reagents, thus introducing a relatively high content of Na on the final catalysts. A high Na content was shown to decrease CH₄ 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 CO₂ to CH₄.

2.1.3. Pottasium (K)

Pottasium-doping of the catalyst support has been performed by various groups with most of them reporting a negative effect on the CO₂ methanation. Büchel et al. [43] studied the CO₂ hydrogenation over K- and Ba-modified 1% Rh/Al₂O₃ catalysts prepared by flame spray pyrolysis. The K-modified catalyst produced only CO over the entire temperature range. This extreme CO selectivity over CH₄ 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 CH₄. Iloy et al. [44] further studied the modification of 15% Co/SiO₂ with 1–5 wt.% K. K-dopant was shown to enhance CO₂ chemisorption, but favour CO selectivity and C–C coupling along with chain growth reactions. For the production of C₂₊ hydrocarbons, 1 wt.% K appeared to be optimal, while a higher potassium loading favoured CO as a main product.

Modification of the Al₂O₃ 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-H₂O and then introduced into the Al₂O₃ support, while Ni was impregnated during a second step. KOH addition led to a dramatic decrease of the Al₂O₃ surface area and after calcination, K₂CO₃ and K₂O phases were also detected. The introduction of the strong KOH base led to the increased presence of adsorbed formate intermediates during CO₂ 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 CH₄ and instead led to a high CO selectivity. The Ni-KOH/Al₂O₃ 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 CO₂ conversions and CH₄ 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/SiO₂ methanation catalyst has been conducted by Vogt et al. [49]. The authors first created a setup that produced hydrogen through solar driven H₂O electrolysis from a KOH-based alkaline electrolyzer. Produced H₂ was then transferred downstream to a methanation unit that contained a Ni/SiO₂ 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/SiO₂ methanation catalyst, whereas a similar ex-situ 0.6 wt.% K addition via a common KOH impregnation resulted to catalyst deactivation.

LaNiO₃-based perovskite oxides can serve as catalyst precursors for the preparation of Ni/La₂O₃ catalysts following a reductive pretreatment. The fine and well-dispersed Ni nanoparticles formed following the perovskite lattice decomposition retain a strong interaction with the La₂O₃ support and are expected to exhibit a different behaviour compared to the Ni/La₂O₃ catalysts prepared via a mere impregnation step [50]. Tsounis et al. [51] presented a way to tune the product selectivity of LaNiO₃-based perovskites during CO₂ hydrogenation, via partially substituting La on the perovskite A-site with K. KNO₃ was mixed together with La and Ni nitrates during the perovskite citrate sol-gel synthesis, in order to prepare La_1-xK_xNiO₃ precursors. Following reduction, the catalysts exhibited a drastically different behaviour during CO₂ hydrogenation depending on the amount of K-doping. The K-free catalyst had a high CO₂ conversion and an almost complete selectivity towards CH₄, while with increased amounts of K-substitution CO₂ 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.

2.2. Alkaline Earth Metals (Mg, Ca, Sr and Ba)

2.2.1. 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/Al₂O₃ was less pronounced compared with Na and K and that, although the activity of the modified catalysts was inferior to Co/Al₂O₃, 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/Al₂O₃ 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/SiO₂ catalysts. During the catalyst preparation, alkaline earth metal nitrates were first impregnated on the SiO₂ 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 CO₂ methanation reaction and by increasing the stability of the modified catalyst.

Liang et al. [53] studied the modification of Ni/Al₂O₃ catalysts with alkaline earths for the CO₂ 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 CH₄ yield by promoting the formation of *CO and H₂CO* 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 CO₂.

Ni/CeO₂ catalysts are among the most active for CO₂ methanation and are a main research focus. CeO₂ has a high oxygen storage capacity (OSC) that, together with its redox properties due to the Ce⁴⁺/Ce³⁺ redox couple, promotes the stability of the active metal particles and suppresses the formation of coke. Furthermore, CeO₂ facilitates a strong interaction with the supported metal phase [54,55]. Liu et al. [30] conducted a comprehensive study, where they doped the CeO₂ support of Ni/CeO₂ 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 CeO₂ support with a molar ratio of 1/9, compared with Ce(NO₃)₃·6H₂O. Mg²⁺ and Ca²⁺ appeared to fully dissolve into the CeO₂ lattice, while SrCO₃ and BaCO₃ secondary phases were detected in the other two samples. The Ca-doped catalyst induced the highest promotion effect, measured via the CO₂ 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 (Ce³⁺/(Ce³⁺ + Ce⁴⁺) molar ratio) and moderate alkaline sites (200–450 °C peaks in the CO₂-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 report enhancement of the CO₂ 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 CO₂ 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 CO₂.

2.2.2. Magnesium (Mg)

Mg in the form of MgO has been studied as a type of support during the CO₂ methanation reaction. However, Ni/MgO catalysts have been shown to lack the activity of other Ni catalysts supported, for example, on TiO₂, Al₂O₃ and CeO₂ [15]. The CO₂ methanation mechanism over Ni/MgO has been reported to proceed via a H₂COO* 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 SiO₂ 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 SiO₂ encapsulated NiO-MgO solid solution. Interestingly, it was found that the confinement induced by the SiO₂ 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 CO₂ 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 CO₂ methanation over a Pd-MgO/SiO₂ catalyst. The addition of MgO in Pd/SiO₂ led to improved catalytic activity and CH₄ selectivity. A mechanism was proposed where MgO and Pd phases functioned as CO₂ and H₂ sorption active sites, respectively. The adsorbed hydrogen atoms on the Pd surface were then proposed to spillover to Mg carbonates, hydrogenating the CO₂-adsorbed phases and forming the CH₄ final product. This bifunctional character of CO₂ methanation reaction that requires the presence of a catalyst componentent capable of activating the CO₂ (Supporting Material: even the commonly considered “inert” Al₂O₃ acts in this way; support doping is often used for further enhancing this role), and a metallic component typically active in H₂ dissociation has been well established in the literature [60,61,62].

Guo et al. [63] prepared Ni/SiO₂ 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 ZrO₂ 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/Al₂O₃ catalysts prepared via evaporation-induced self-assembly (EISA). Mg increased the surface basicity of the catalysts, thereby facilitating the chemisorption and activation of CO₂. 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 CO₂ 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 CO₂ 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-Al₂O₃ 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-Al₂O₃ 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 CO₂ methanation [72]. Fe catalysts, commonly known to be active during the Fischer–Tropsch synthesis, could also be viable candidates for the CO₂ hydrogenation into CH₄ 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-Fe₂O₃ powder via incipient wetness. Improved CO₂ adsorption over the Mg-modified Fe catalyst led to highly active Fe carbides, bulk carbon moieties and an overall improved CO₂ methanation performance.

2.2.3. Calcium (Ca)

Ca is also one of the favourite promoters for the CO₂ 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(NO₃)₂·4H₂O 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 CO₂ chemisorption and a lowering of the activation energy for CO₂ methanation. The spike in the population of new strong alkaline sites was confirmed by CO₂-TPD. The catalytic activity was the highest, when the Ca/Al molar ratio was fixed at 8%, with a roughly 4 times CO₂ 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/Al₂O₃ (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 CO₂ 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 CO₂ conversion and CH₄ 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/Al₂O₃ catalyst during methanation of a CO₂-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 CO₂ [77,78,79]. A 20% CaO addition on 15% Ni/Al₂O₃ was reported to modify the interaction between Ni and Al₂O₃ 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.

Ca²⁺ cations are soluble into the crystalline lattice of ZrO₂ and the corresponding solid solution is commonly known as calcia stabilized zirconia (CSZ). Oxygen vacancies (V_o²) are created in order to compensate for the negatively charged Ca-induced substitutional impurities (Ca_Zr²’), thereby enhancing the material’s ionic conductivity and altering its defect chemistry [80]. Takano et al. [81] impregnated Ni and Ca nitrates on ZrO₂ and studied the effect of Ca/(Zr + Ca) molar ratio on the CO₂ methanation activity. The authors argued that the presence of medium-strength basic sites caused by the Ca-induced oxygen vacancies could facilitate CO₂ 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 CaZrO₃ phases. More recently, Everett et al. [82] showed that a small amount of Ca²⁺ substitution (0.7 wt.%) in the lattice of ZrO₂ 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 CO₂ methanation.

Finally, perovskite oxide titanates containing Ca have been also used in the CO₂ methanation reaction. Jia et al. [83] employed a common perovskite oxide that is also found as a natural mineral, CaTiO₃, as a catalyst support and compared it with γ-Al₂O₃. The 10% Ni/CaTiO₃ catalyst exhibited CO and CO₂ methanation activity superior to 10% Ni/Al₂O₃, due to the perovskite oxide’s exceptional stability, increased support basicity and the limited interaction between Ni particles and the CaTiO₃ support. Do et al. [84] used Ca as a promoter in order to substitute part of the A-site cations in the NiTiO₃ lattice and the whole perovskite was loaded on γ-Al₂O₃. They performed extensive characterizations on the composite materials and concluded that upon 5% molar substitution of Ni²⁺ with Ca²⁺, the catalytic activity could be maximized. Ca-addition promoted the formation of oxygen vacancies in close proximity to Ni active phases and thereby the CO₂ chemisorption. The proposed reaction mechanism, as summarized in Figure 6, involved a combination of the CO and formate routes. The initial CO₂ 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 CH₄ being the final product.

2.2.4. 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 La_1-xSr_xFe_0.8Ni_0.2O₃ 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.

2.3. Hydrotalcite-Derived and Zeolite Supported Catalysts

Besides commonly used supports such as Al₂O₃, ZrO₂, SiO₂ and TiO₂, MgO-Al₂O₃ mixed oxides derived from Mg-containing hydrotalcites, as well as zeolites, have also become popular CO₂ methanation catalyst supports in recent years. Hydrotalcites, or layered double hydroxides, with Ni²⁺ and Mg²⁺ 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-Al₂O₃ mixed oxide. Mg²⁺ presence in the initial hydrotalcite facilitates the creation of alkaline sites on the support that favour CO₂ 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 CO₂ methanation activity.

2.3.1. Hydrotalcite-Derived Catalysts

Hydrotalcite materials, also known as layered double hydroxides, have the general formula of [M_1-x²⁺M_x³⁺(OH)₂](A^n-)_x/n·mH₂O with M representing divalent and trivalent metals and A being an anion. Mg²⁺ and Ni²⁺ can serve as divalent cations, while Al³⁺ is a common trivalent cation. Upon calcination and reduction, supported Ni catalysts are obtained with high activity and selectivity for specific catalytic reactions [87]. Bette et al. [89] claimed to be the first to present an active CO₂ methanation catalyst derived from a hydrotalcite precursor (Ni_0.5Mg_0.17Al_0.33(OH)₂(CO₃)_0.17·mH₂O) synthesized via the coprecipitation method. The resulting catalyst consisted of a high Ni loading (59 wt.%) supported on a spinel-type (Mg,Al)O_x mixed oxide and exhibited stable catalytic performance, as well as Ni particle sintering and coking resistance.

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 Ni²⁺/Mg²⁺ molar ratio during the coprecipitation synthesis, while the ratio between trivalent (Al³⁺) and divalent (Mg²⁺ and Ni²⁺) cations remained constant. After calcination and reduction, the formed Ni nanoparticles were well-dispersed over the MgO-Al₂O₃ support with an average crystallite size of about 2.5 nm. Two types of basic sites were observed upon CO₂-TPD, namely weak basic sites for CO₂ adsorbed on Al₂O₃ phase and moderately strong basic sites for CO₂ adsorbed on MgO phase. Regarding the catalytic performance for CO₂ methanation, the hydrotalcite-derived catalysts presented far superior low-temperature CO₂ conversion compared with other Ni-based catalysts supported on Al₂O₃, 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 CO₂ activation step by the support basic sites to carbonate/hydrocarbonate species and an H₂ dissociation step on the Ni metallic sites. The interplay between these two phases facilitated the excellent low-temperature activity towards CO₂ 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 CO₂ methanation performance. Wierzbicki et al. [92,93] introduced La during the hydrotalcite preparation to further promote the support alkalinity and CO₂ 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 Ni²⁺ and Fe³⁺ 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 CO₂ methanation activity and stability.

2.3.2. Zeolite Supported Catalysts

Zeolites are microporous aluminosilicates with a general formula of [Al_xSi_1-xO₂^y-]_x and consist of a negatively charged network that requires cationic counterions (e.g., Na⁺, Ca²⁺) 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 CO₂ 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 CO₂ 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 CO₂ methanation over Ni/USY catalysts and concluded that CO₂ was initially weakly adsorbed over the Na⁺ compensating cation sites on the zeolite. Following the initial chemisorption step, CO₂ 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 CO₂ 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 CO₂ methanation performance (CO₂ conversion and CH₄ 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 CO₂ 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 Mg²⁺ > Ca²⁺ > Ba²⁺. The better performance for ion-exchange with Mg²⁺ 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 CO₂ 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 CO₂ chemisorption via enhancing the surface basicity. Bacariza et al. [88] also presented some proposals for the preparation of active zeolite supported CO₂ 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. Summary of some typical alkali and alkaline earth promoted catalysts on the methanation of CO₂.

Promoter	Catalyst	Synthesis Method	Conditions	Performance	Comments	Ref.
0.06–0.4% Li, Na, K and Cs	0.5% and 5% Ru/TiO2	Wet impregnation	GHSV = 56,000 h−1 H2/CO2 = 4/1	XCO2 = 60%, SCH4 = 95% at 400 °C (0.2% Na on 0.5% Ru/TiO2)	Catalytic activity improved after addition of small amounts of alkalimetals on 0.5% Ru/TiO2. Less pronounced effect for 5% Ru/TiO2	[33]
10% LiNO3, NaNO3, Na2CO3 and K2CO3	1% Ru/Al2O3	Incipient wetness impregnation	GHSV = 25,000 h−1 H2/CO2 = 4/1	XCO2 = 60%, SCH4 = 99% at 350 °C (10% LiNO3 on 1% Ru/Al2O3)	Only LiNO3 significantly promoted activity. Na2CO3 and K2CO3 masked Ru particles and suppressed methanation activity	[35]
5% Na, K, Mg and Ca	20% Co/Al2O3	Incipient wetness impregnation	H2/CO2 = 4/1	XCO2 = 52%, SCH4 = 67% at 400 °C (5% Na on 10% Co/Al2O3)	Na and K increased the population of basic sites but promoted CO selectivity. Mg and Ca did not favour methanation activity either, but prevented Co nanoparticle sintering	[37]
0.1–1% Na	33% Ni/SiO2 and 13% Ni/CeO2	Wet impregnation	H2/CO2 = 50/1	XCO2 = 85%, SCH4 = 85% at 250 °C (0.1% Na on 33% Ni/SiO2)	Ni active surface area decreased upon Na addition. Negative impact on activity for Ni/CeO2, but slightly positive impact for Ni/SiO2	[41]
16.5% K and Ba	1% Rh/Al2O3	Flame spray pyrolysis	GHSV = 6000 h−1 H2/CO2 = 4/1	XCO2 = 25%, SCH4 = 0% at 375 °C (16.5 % K on 1% Rh/Al2O3)	K-addition led to pure CO production over the entire temperature range. Ba-modified catalyst slightly inferior to Rh/Al2O3 for CO2 methanation	[43]
0.5–20% KOH	0.5–20% Ni/Al2O3	Incipient wetness impregnation	H2/CO2 = 4/1	XCO2 = 40%, SCH4 = 45% at 400 °C (10% K on 12% Ni/Al2O3)	Strongly adsorbed formate intermediates over KOH-modified catalysts favoured CO selectivity	[45]
1.7–7.6% K (x = 0.1–0.4 for La1-xKxNiO3)	24–28.5% Ni/La2O3	Citrate sol-gel La1-xKxNiO3 synthesis and reduction	GHSV = 48,000 h−1 H2/CO2 = 12.5/1	XCO2 = 50%, SCH4 = 45% at 306 °C (for x= 0.3 => La0.7K0.3NiO3)	Easy-to-desorb CO species formed after K-doping stirred the reaction towards RWGS and CO selectivity	[51]
4% MgO, CaO, SrO and BaO	10% Ni/SiO2	Wet impregnation	GHSV = 15,000 h−1 H2/CO2 = 4/1	XCO2 = 71%, SCH4 = 99% at 300 °C (4% SrO on 10% Ni/SiO2)	MgO inhibited activity, CaO had little effect, SrO and BaO enhanced activity, while SrO also promoted the catalyst stability	[52]
1–7.5% Mg, Ca, Sr and Ba	20% Ni/Al2O3	Incipient wetness impregnation	GHSV = 16,000 h−1 H2/CO2 = 4/1	XCO2 = 80%, SCH4 = 100% at 300 °C (5% Ba on 20% Ni/Al2O3)	Lighter elements Mg and Ca promoted CO selectivity, while heavier elements Sr and Ba favoured methanation	[53]
1.4% Mg, 2.2% Ca, 4.7% Sr and 7.2% Ba (Ce0.9M0.1Ox support)	10% Ni/Ce0.9M0.1Ox	Citrate sol-gel modified CeO2 synthesis and Ni incipient wetness impregnation	WHSV = 36,000 mL h−1 g−1 H2/CO2 = 4/1	XCO2 = 70%, SCH4 = 95% at 270 °C (10% Ni/Ce0.9Ca0.1Ox)	Ca-doping led to the highest methanation activity (with Sr being second-to-best). TOFNi could be linearly correlated to the amount of oxygen vacancies and moderately strong basic sites on the support surface	[30]
1–4% MgO	10% Ni/SiO2	Sequential wet impregnations and co-impregnation	WHSV = 36,000 mL h−1 g−1 H2/CO2 = 4/1	XCO2 = 65%, SCH4 = 98% at 400 °C (1% Mg on 10% Ni/SiO2)	1% MgO introduced by co-impregantion led to the highest catalytic activity and stability. Higher MgO contents blocked some Ni active sites	[63]
0.4–2.5% Mg (Change in Ni/Mg molar ratio)	6% Ni/ZrO2	Citrate complexing method	WHSV = 15,000 mL h−1 g−1, p = 0,1 MPa, H2/CO2 = 4/1	XCO2 = 90%, SCH4 = 100% at 250 °C (1% Mg on 6% Ni/ZrO2)	Mg-addition led to Ni nanoparticle confinement, as well as resistance to sintering and coke formation	[64]
0.5–5% Mg	12% Ni/Al2O3 (OMA)	Evaporation-induced self-assembly (EISA)	GHSV = 15,000 h−1 H2/CO2 = 4/1	XCO2 = 65%, SCH4 = 96% at 300 °C (2.5% Mg on 12% Ni/Al2O3)	Mg-dopant increased the surface basicity of the catalysts and facilitated the chemisorption and activation of CO2	[65]
1% Mg	α-Fe2O3	Incipient wetness impregnation	GHSV = 150,000 h−1, p = 8 bar, H2/CO2 = 4/1	XCO2 = 22%, SCH4 = 9% at 400 °C after 18 h (2.% Mg on α-Fe2O3)	Improved CO2 adsorption over the Mg-modified Fe catalyst led to highly active Fe carbides and better methanation performance	[72]
0.8–8% Ca	12% Ni/Al2O3 (OMA)	Evaporation-induced self-assembly (EISA)	GHSV = 15,000 h−1 H2/CO2 = 4/1	XCO2 = 70%, SCH4 = 98% at 300 °C (6.4% Ca on 12% Ni/Al2O3)	Increased surface alkalinity of the support led to enhanced CO2 chemisorption and lower activation energy for CO2 methanation	[74]
2% CaO	1% Ru, 10% Ni/Al2O3 (OMA)	Evaporation-induced self-assembly (EISA)	WHSV = 30,000 mL h−1 g−1 H2/CO2 = 4/1	XCO2 = 80%, SCH4 = 98% at 360 °C (2% CaO on 0.8% Ru, 8% Ni/Al2O3)	Step increase in the CO2 conversion and CH4 selectivity after adding Ru second metal and CaO promoter	[75]
0.7% Ca	5% Ni/CaxZr1-xO2-δ	Wet impregnation	H2/CO2 = 4/1	XCO2 = 72%, SCH4 = 100% at 350 °C (0.7% Ca on 5% Ni/CaxZr1-xO2-δ)	Oxygen vacancies-coordinately unsaturated sites (cus) pairs facilitated the C-O bond scission, a crucial step during CO2 methanation	[82]
0.1–1% Ca (x = 0.01–0.1 for CaxNi1-xTiO3-δ)	30% Ca-NiTiO3/Al2O3 (11.5% Ni)	Coprecipitation Ca-NiTiO3 synthesis and wet impregnation	GHSV = 5000 h−1 H2/CO2 = 4/1	XCO2 = 55%, SCH4 = 99.7% at 350 °C (0.5% Ca on 30% Ca-NiTiO3/Al2O3)	Ca-doping on NiTiO3 A-site promoted the formation of oxygen vacancies in close proximity to active Ni and therefore the chemisorption of CO2	[84]
14% Mg	59% Ni/MgO- Al2O3	Coprecipitation hydrotalcite synthesis and reduction	WHSV = 1100 min-1 g−1 H2/CO2 = 4/1	XCO2 = 70%, SCH4 = 90% at 315 °C(14% Mg on 59% Ni/MgO- Al2O3)	Stable performance, as well as sintering and coking resistance for the hydrotalcite-derived catalysts	[89]
6.1–28.5% Mg	17.2–57.8% Ni/MgO- Al2O3	Coprecipitation hydrotalcite synthesis and reduction	GHSV = 2400 h−1 H2/CO2 = 4/1	XCO2 = 97.9%, SCH4 = 97.5% at 250 °C (17.3% Mg on 37.5% Ni/MgO-Al2O3	Far superior low-temperature CO2 conversion for Mg-containing hydrotalcite-derived catalysts, due to the increased surface alkalinity	[90]
0.3–12% Li, Na, K, Cs, Mg, Ca and Ba	15% Ni/USY zeolite	Ion-exchange for cations and Ni incipient wetness impregnation	GHSV = 43,000 h−1 H2/CO2 = 4/1	XCO2 = 62%, SCH4 = 92% at 375 °C (12.1% Cs in 15% Ni/Cs-exchanged USY)	Greater size for monovalent cations improved CO2 adsorption (Cs+ > Na+ > Li+ > H+), while Mg2+ benefited Ni dispersion	[98]
0.9–13% Mg	4.8–15% Ni/USY zeolite	Ion-exchange or impregnation for Mg and Ni incipient wetness impregnation	GHSV = 43,000 h−1 H2/CO2 = 4/1	XCO2 = 65%, SCH4 = 93% at 400 °C (0.7% Mg in 15% Ni/Mg-exchanged USY)	Finer and better-dispersed Ni nanoparticles after Mg impregnation or Mg ion-exchange	[68]

summarizes the CO₂ methanation performance of some typical catalysts promoted with group I and II metals.

2.4. Other Promoters that Favour CO₂ 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 CO₂ 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 CO₂ 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/Al₂O₃ catalysts. Among them, Pr, Ce and La exhibited the most pronounced promoting effects. La-promotion specifically, has been reported to be beneficial for the CO₂ 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 LaNiO₃ perovskite precursor phase [110,111]. Lastly, Mn promotion also targets similar improvements, namely enhancing the CO₂ adsorption and increasing the metal phase dispersion [104].

3. Alkali and Alkaline Earth Metals as Active Sorbents Phases in the Combined Capture and Methanation of CO₂

Carbon capture and utilization (CCU) revolves around the concept of coupling post-combustion CO₂ capture to a CO₂ 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 CO₂ desorption step. Additionaly, CH₄ selectivity for many catalysts during CO₂ hydrogenation is high at temperatures that can coincide with those required for CO₂ desorption, thus rendering CH₄ as a preferable final product. The combination of these two processes, namely CO₂ 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 CH₄ 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 O₂-rich, a partial oxidation of the metallic surface takes place during the CO₂ capture step under practical operation conditions. The catalyst then needs to be reactivated under a H₂-rich gas flow. Ru is most promising in this regard, since the oxidized RuO_x layers can be re-reduced under H₂ flow at temperatures where CO₂ 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 O₂ from the CO₂-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].

3.1. Ru-Based Dual-Function Materials

Duyar et al. [19] introduced the concept of a DFM for the CO₂ capture and conversion to CH₄ (Figure 8) [113]. The materials were prepared via incipient wetness impregnation, either impregnating Ca(NO₃)₂ first and then Ru(NO)(NO₃)₂ on γ-Al₂O₃, 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 CO₂ and the extent of CO₂ spillover towards the methanation active sites. An optimum composition of 5% Ru, 10% CaO/γ-Al₂O₃ produced the maximum amount of CH₄ per kg of DFM material at 0.5 mol CH₄/kg DFM (or 0.5 mmol CH₄/g DFM). The authors also showed that the high dispersion of the CaO sorbent phase, that can facilitate a reversible chemisorption and desorption of CO₂ 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 CH₄ 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 K₂CO₃, Na₂CO₃ and MgO as potential CO₂ sorbent alternatives. K₂CO₃ and Na₂CO₃ exhibited better results, with the 5% Ru, 10% Na₂CO₃/γ-Al₂O₃ DFM yielding 1.05 mol CH₄ per kg material at 320 °C operation. Thus, it was shown that the optimization of the CO₂ 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% Na₂CO₃/γ-Al₂O₃ DFM [115]. Two parallel reactors, one operating under CO₂ capture and the other under CO₂ 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.

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 Na₂CO₃ (or “Na₂O”) 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, “Na₂O”-type active sorbents appeared far superior and led to an overall higher CH₄ yield, despite CaO sorbents initially providing a higher CO₂ adsorption capacity [21].

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.

Besides the Farrauto group, Bermejo-Lόpez et al. [24] performed a thorough work to describe the mechanism of CO₂ 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 Na₂CO₃ promoting it. The basicity of the materials increased with higher adsorbent loading and this promoted the CH₄ yield. CaO provided strong basic sites, where CO₂ was chemisorbed forming stable carbonates (CaCO₃) that required higher temperatures to be desorbed. On the other hand, medium-strength basic sites over Na₂CO₃ led to the formation of less stable carbonates, enabling 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.

3.2. 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-Al₂O₃ carrier that had been previously loaded with a Na₂CO₃ 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% “Na₂O”/γ-Al₂O₃) operated cyclically at a constant 320 °C temperature could only produce substantial amounts of CH₄, if the CO₂ capture step was performed under an O₂-free gas (7.5% CO₂/N₂). On the other hand, if the CO₂ capture step was performed at 320 °C under a simulated flue-gas (7.5% CO₂, 4.5% O₂, 15% H₂O, balance N₂), no CH₄ was produced during the subsequent hydrogenation step at the same temperature. This was explained by the inability of NiO to be reactivated under 15% H₂/N₂ at 320 °C in order for the whole operation to proceed isothermally. Furthermore, Ni-based DFMs retained a certain amount of preadsorbed CO₂ 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% H₂/N₂ 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 H₂ molecules and helped reduce the adjacent NiO sites. For cyclic operation at 320 °C, the 1% Ru, 10% Ni, 6.1% “Na₂O”/γ-Al₂O₃ material exhibited great cyclic stability and the highest CH₄ production, since both Ru and Ni sites are active for CO₂ methanation. Compared to the Ni-free DFM with the same Ru loading (1% Ru, 6.1% “Na₂O”/γ-Al₂O₃), the Ni-containing one (1% Ru, 10% Ni, 6.1% “Na₂O”/γ-Al₂O₃) exhibited an increased CO₂ adsorption capacity and CH₄ 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 CO₂ methanation [75,118].

Other works typically perform the CO₂ capture step with an O₂-free gas mixture of CO₂ balanced with a carrier gas. Bermejo-Lόpez et al. [112] studied the effects of Ni, as well as CaO and Na₂CO₃ 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 Na₂CO₃ decomposed at a lower temperature. Thus, during the following hydrogenation step, CH₄ formation was favoured at higher temperatures when CaO was used as the active sorbent, peaking at 520 °C, while maximum CH₄ was produced for Na₂CO₃ at 400 °C. Increasing the Ni loading resulted in increased CO₂ sorption capacity and CH₄ production. Regarding the proposed mechanism, CO₂ was initially adsorbed over NaOH and Ca(OH)₂ species, with H₂O being simultaneously released. During the subsequent H₂/N₂ inflow, adsorbed CO₂ was reduced after its spillover to Ni sites and detected product H₂O was delayed due to its initial adsorption by the sorbent phases. The DFM with CaO sorbent (10NiCa) exhibited a higher CH₄ selectivity upon hydrogenation during cyclic operation at 400 °C, while the DFM with Na₂CO₃ sorbent (10NiNa) produced more CO arising from the RWGS reaction.

Chai et al. [119] also tested the effects of different CO₂ adsorbents (MgO, CaO, Na₂CO₃ and K₂CO₃) on Ni-based DFMs supported on Al₂O₃. The catalyst and adsorbent loadings were the same, at 10 wt.%, while the CO₂ capture step was performed under an O₂-free 9.5% CO₂/N₂ gas flow. In a similar fashion to the findings of Bermejo-Lόpez et al. [23,102], the authors showed that CaO sorbent led to greater CO₂ sorption capacity. However, the release of adsorbed CO₂ in the material with CaO sorbent was not fully reversible. The material with Na₂CO₃ sorbent exhibited a high CO₂ capture capacity, along with a high release efficiency. Hence, methanation capacity for the material with Na₂CO₃ sorbent was superior, a property attributed to the easier release and spillover of adsorbed CO₂ towards the Ni catalytically active sites.

Hu et al. [120] prepared Ni-based DFMs supported on ZrO₂ and modified with La and K. The use of K and La promoters raised the CO₂ sorption capacity of the materials, with the K-modified material increasing its CO₂ capture efficiency four-fold, compared with the unmodified one. K-promoted Ni/ZrO₂ released CH₄ upon hydrogenation much slower, while the La-promoted material exhibited faster methanation kinetics, with the time required for CH₄ release roughly matching the time required for CO₂ 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/ZrO₂ 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 CO₂ chemisorption and methanation efficiency at higher temperatures (>350 °C).

Finally, Zhou et al. [121] prepared a 2D-layered Ni/MgO-Al₂O₃ material to be used during the integrated CO₂ 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-Al₂O₃ support weakened and Ni species became more reducible. For the cyclic CO₂ capture and methanation process, the authors aimed at optimizing the respective durations, so that CO₂ breakthrough was minimal and a maximum utilization of reactant H₂ was achieved. Complete capture and methanation could be achieved by a 35 s CO₂ 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 CO₂ sorption phase. Following 20% H₂/Ar flow, the observed bands suggested a gradual removal of these species via hydrogenation to CH₄, 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 CO₂ chemisorption [90].

Table 2. Summary of dual-function materials (DFMs) employed in the combined CO₂ capture and methanation process.

CO2 Sorbent	Catalyst	CO2 Capture Conditions	CO2 Methanation Conditions	CH4 Production [mmol CH4/g DFM]	Comments	Reference
1–10% CaO	1–10% Ru/Al2O3	10% CO2/N2, 320 °C, 30 min	4% H2/N2, 320 °C, 2 h	0.50 (5% Ru, 10% CaO/Al2O3)	High sorbent loading increased CO2 spillover to Ru active sites. 5% Ru, 10%CaO/Al2O3 as best composition. Cyclic operation at 320 °C	[19]
10% CaO, MgO, Na2CO3 and K2CO3	0.1–10% Rh and 5% Ru/Al2O3	10% CO2/N2, 320 °C, 30 min	4% H2/N2, 320 °C, 2 h	0.4 (0.1% Rh, 10% CaO/Al2O3), 1.05 (5% Ru, 10% Na2CO3/Al2O3)	0.1% Rh was quite active but Rh is pricy. Na2CO3 constituted a better adsorbent than CaO for Ru-based DFMs	[113]
10% CaO, MgO, Na2CO3 (6.1% “Na2O”) and K2CO3 (7.01% “Κ2O”)	0.5% Rh and 5% Ru/Al2O3	10% CO2/N2, 320 °C, 30 min	10% H2/N2, 320 °C, 1 h	0.614 (5% Ru, 6.1% “Na2O”/Al2O3)	Ru-based DFMs with Na2CO3 achieved the fastest hydrogenation rates	[21]
5–15% CaO and Na2CO3	4% Ru/Al2O3	11% CO2/Ar, 280–400 °C, 1 min	10% H2/Ar, 280–400 °C, 2 min	0.414 at 400 °C (4% Ru, 15% CaO/Al2O3), 0.383 at 310 °C (4% Ru, 10% Na2CO3/Al2O3)	Higher sorbent loading promoted CH4 yield. More stable carbonates formed on CaO required higher temperature to decompose	[24]
10% LiNO3, NaNO3, Na2CO3 and K2CO3	1% Ru/Al2O3	15% CO2/Ar, 230 °C, 30 min	12% H2/Ar, 230 °C, 30 min	Not reported	Li-promoted DFM presented faster methanation kinetics. Operation at just 230 °C, a lower temperature compared with other DFMs	[35]
200% High-capacity MgO sorbent	2.5–10% Ru/CeO2	65% CO2/N2, 300 °C, 1 h	5% H2/N2, 300 °C,	6.60, 1. cycle 3.36, 10. cycle (5% Ru/CeO2 + 200% MgO)	High CO2 capture capacity due to a large MgO content. Mechanism: oxygen vacancy catalyzed CO2 hydrogenation through a formate intermediate	[25]
10% Na2CO3 (6.1% “Na2O”)	10% Ni/Al2O3	7.5% CO2/N2 and 7.5% CO2, 4.5% O2, 15% H2O/N2, 320 °C, 20 min	15% H2/N2, 320 °C, 1 h	0.276, Without O2 during CO2 capture 0, With O2 during CO2 capture (10% Ni, 6.1% “Na2O”/Al2O3)	Ni-based DFMs required a prereduction/activation step at 650 °C and an O2-free gas during CO2 capture in order to produce CH4	[21]
10% Na2CO3 (6.1% “Na2O”)	0.1–1% Ru, Pt, Pd with 10% Ni/Al2O3	7.5% CO2, 4.5% O2, 15% H2O/N2, 320 °C, 20 min	15% H2/N2, 320 °C	0.38 (1% Ru, 10% Ni, 6.1% “Na2O”/Al2O3)	Pt and Ru addition as second metals promoted NiO reducibility. Ni and Ru synergy led to the highest CH4 yield	[23]
5–15% CaO and Na2CO3	5–15% Ni/Al2O3	10% CO2/Ar, 280–520 °C, 1 min	10% H2/Ar, 280–520 °C, 2 min	0.142 at 520 °C (15% Ni, 15% CaO/Al2O3), 0.185 at 400 °C (10% Ni, 10% Na2CO3/Al2O3)	The adsorbent phase enhanced NiO reducibility. Methanation was favoured at higher temperatures when CaO was used as sorbent	[112]
10% CaO, MgO, Na2CO3 and K2CO3	10% Ni/Al2O3	9.5% CO2/N2, 320 °C, 25 min	10% H2/N2, 320 °C, 20 min	0.140 (10% Ni, 10% CaO/Al2O3) 0.178 (10% Ni, 10% Na2CO3/Al2O3)	Easier CO2 release and higher methanation capacity when Na2CO3 was used as sorbent	[119]
5% K and La	15% Ni/ZrO2	4.7% CO2/He, 250–450 °C, 5 min	Pure H2, 250–450 °C, 5 min	Not reported	K-promotion greatly enhanced CO2 adsorption and promoted methanation at higher temperatures, while La-promotion favoured faster methanation kinetics	[120]
15–30% Mg	17–50% Ni/MgO- Al2O3	15% CO2/N2, 200 and 250 °C, various times	Pure H2, 200 and 250 °C, various times	Not reported	Hydrotalcite-derived layered oxides with high Ni and Mg contents provided a high CH4 yield and favourable methanation kinetics	[121]

summarizes the performance of DFMs for the combined capture and methanation of CO₂.

3.3. Dual-Function Materials for Products Other than Methane

Apart from synthetic natural gas (SNG) obtained during methanation, the concept of DFMs for the integrated CO₂ 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-Al₂O₃. The K-component was added to increase the CO₂ 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/Al₂O₃ catalyst with K and Ba for CO₂ capture and conversion to CO. K-promotion facilitated Cu dispersion and resulted in a very high capacity for CO₂ capture as formate intermediates. These intermediates were then selectively reduced to CO via the RWGS route. The Ba-promoted material captured much less CO₂ 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 CO₂ adsorbing components, in an attempt to greatly increase the CO₂ capture efficiency and sorption kinetics. The K-Ca, Na-Ca, K-Mg and Na-Mg double salts were supported on γ-Al₂O₃ via the impregnation of alkalimetal carbonates and alkaline earth metal nitrates, whereas Ni was impregnated subsequently. After the CO₂ 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 CO₂ 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 CO₂ capture and sequestration (CCS) could be modified through the integration of a CO₂ utilization step by converting CO₂ to CO. The incorporation of Ni facilitated CO₂ reduction, whereas Ce-species provided oxygen vacancies that were active sites for CO₂ dissociation and prevented the agglomeration of individual CaO and Ni nanocrystallites.

3.4. 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 CO₂ 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 K₂CO₃/Al₂O₃ sorbent and a Ru/Al₂O₃ methanation catalyst [126,127,128]. The high CO₂ sorption capabilities of the K₂CO₃-based sorbent enabled it to capture CO₂ 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 H₂ flow and CH₄ was produced at a subsequent catalytic reactor that contained the Ru/Al₂O₃ methanation catalyst. The whole operation was coined as direct air capture and methanation (DACM) process [128].

Finally, Miguel et al. [129] also operated the CO₂ capture and conversion to SNG process by using a single reactor that contained alternating layers of the sorbent and catalyst materials. CO₂ 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 CO₂ could be fully hydrogenated into CH₄ and that CH₄ production under cyclic operation was quite high, peaking at 2.36 mol CH₄ per kilogram material and hour of operation.

4. 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 (CO₂) and lowest (CH₄) 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 CO₂ methanation activity of supported metal catalysts. Their promoting effect mainly rests on the enhancement of support alkalinity that promotes CO₂ 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/CeO₂ and Ni/Al₂O₃) 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 CO₂ 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 CO₂ sorption capacity and CH₄ production (mostly measured in mmol CH₄/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., Al₂O₃, SiO₂, ZrO₂, CeO₂ 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 CO₂ 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).