Looking for an Optimal Composition of Nickel-Based Catalysts for CO₂ Methanation

Abstract

A detailed critical analysis of the scientific literature data concerning catalysts for CO₂ methanation based on nickel supported over oxides was performed. According to the obtained information, it seems that an ionic support is necessary to allow a good nickel dispersion to produce very small nickel metal particles. Such small metal particles result in being very active toward methanation, limiting the production of carbonaceous materials. The use of support and/or surface additives gives rise to medium surface basicity, allowing medium-strong adsorption of CO₂, and it is also advisable to increase the reaction rate. A medium nickel loading would allow the free support geometric surface to be covered densely by small nickel metal particles without the production of larger Ni crystals. It is also advisable to work at temperatures where Ni(CO)₄ formation is not possible (e.g., >573 K). The promising properties of systems based on doped Ni/Al₂O₃, doped with basic and re-active oxides such as MnO_x or/and CeO₂, and those based on Ni/CeO₂ were underlined.

1. Introduction

The methanation of captured CO₂ using green hydrogen is considered a possible way to valorize and reuse it, also in view of the so-called “power to Gas” technologies [1,2]. The new process to be designed for CO₂ methanation [3] is strictly related to ones developed decades ago for Substitute Natural Gas (SNG) production from CO-rich syngases [4]. It possibly implies a CO₂ purification step followed by a catalytic reaction step. Depending on the performances and design of the catalytic reactor and on the final use of the produced methane, the following steps may be needed: condensation and removal of coproduced water, product separation from unreacted CO₂ and/or hydrogen (that can be recycled to the reactor), and separation from byproducts, such as CO and/or other hydrocarbons or oxygenated compounds. Thus, as always for industrial chemical plants, the key step for an efficient process is to develop high activity and high selectivity catalysts, just to reduce the onerousness of the separation and recycling steps and to maximize productivity. Although artificial intelligence-based methods are under development for discovering new and better catalysts for industrial chemical reactions [5], up to now, empirical methods based on human intelligence still represent the most reliable approach, in our opinion.

2. Thermodynamics of CO₂ Hydrogenation

The CO₂ methanation reaction,

CO₂ + 4 H₂ ⇆ CH₄ + 2 H₂O

also denoted as the “Sabatier reaction”, is an exothermic equilibrium reaction: thus, it is most favored at low temperatures and high pressures. However, according to its large exothermicity (ΔH°₂₉₈ = −165 kJ/mol), the theoretical thermodynamic methane yield approaches totality at temperatures up to 473 K, also at 1 atm [6,7]. At higher temperatures, CO₂ conversions and methane yields allowed by thermodynamics decrease. However, methane synthesis is competitive with the production of CO through the so-called “reverse Water Gas Shift” reaction, rWGS,

CO₂ + H₂ ⇆ CO + H₂O

which is weakly endothermic (ΔH°₂₉₈ = +41.16 kJ/mol) and almost independent from reaction pressure; thus, its equilibrium is shifted to products the higher the temperature is. Thus, thermodynamically allowed CO₂ conversion will increase again, and at higher temperatures, CO will become the predominant product [6]. However, given the strong dependence of methanation on, and the almost independence of rWGS from, total pressure, at higher pressures the temperature range where methanation is predominant will increase to higher temperatures. In Figure 1, the resulting CO₂ conversions and CH₄ and CO yields at thermodynamic equilibrium are reported, as a function of temperature, at 0.1, 2.03, and 20.3 MPa.

The production of higher hydrocarbons (paraffins and olefins), as it occurs in the so-called CO₂-Fischer Tropsch process, through the following reactions,

n CO₂ + (3n + 1) H₂ = C_nH_{2n + 2} + 2n H₂O     n > 1

m CO₂ + 3m H₂ = C_mH_2m + 2m H₂O     m > 1

also represent competitive reactions, in principle. These reactions, which are also exothermic equilibrium reactions most favored at low temperature and high pressure, are less favored than methanation and may be realized in conditions where rWGS is still not favored. The higher hydrocarbon yield, forecasted at thermodynamic equilibrium at atmospheric pressure, is extremely low, and methane also remains the most favored hydrocarbon product at higher pressures [9].

According to Gao et al. [6] and our own thermodynamic calculations, carbon formation theoretically does not occur when the starting molar ratio is H₂/CO₂ = 4, while it should occur at T < 873 K when H₂/CO₂ = 2. In contrast, the calculations of Schmider et al. [10] report the formation of significant amounts of carbon by CO₂ hydrogenation with H₂/CO₂ = 4 at atmospheric pressure, with a maximum amount at around 800 K, which almost disappears if reaction pressure is raised to 2 MPa.

Thus, the overall available thermodynamic data indicate that, in principle, the best conditions for an industrial CO₂ methanation process are to realize the reaction in the presence of a selective catalyst for methanation, with low or no activity for reverse water gas shift and higher hydrocarbon synthesis, with a moderate H₂ excess (H₂/CO₂ > 4) working at T < 650 K or less at a moderately high pressure (1–2 MPa) to push conversion and selectivity to methane, limiting coproduction of CO and carbon deposition. In addition, the adoption of high space velocities will limit the significant production of higher hydrocarbons, achieving high CH₄ productivity.

To test and develop optimal catalysts, however, conditions in which possible limits become evident may give more information. To evidence and compare intrinsic catalytic activity and selectivity toward methane, as well as stability with respect to carbon deposition phenomena, studies realized at atmospheric pressure are more informative than those realized at high pressures [11], where the production of CO and carbon are essentially thermodynamically not possible.

3. Nickel Catalysts for CO₂ Hydrogenation Reactions

As is well known, transition metals are the most catalytically active materials for hydrogenation reactions, at least in sulfur-free environments [12]. As mentioned above, CO₂ hydrogenation can produce CO by reverse water gas shift, methane, and any other hydrocarbon and oxygenated organic compound. According to the literature, while Cu- and Pd-based catalysts are very active for methanol and higher alcohol synthesis, and Fe, Co, and Ru catalysts are active for higher hydrocarbon synthesis, Ni and Ru are the most active for CO_x methanation reactions. In fact, commercial catalysts are available for methanation in CO-rich syngases [12]: they are essentially based on Ru/Al₂O₃ for low-temperature applications, mainly for the methanation of CO_x residues in hydrogen and ammonia synthesis gas. Ni/Al₂O₃ is used for the same application and for high-temperature applications for the production of SNG [13].

An enormous amount of research work was recently carried out concerning catalysts for CO₂ methanation. Several reviews collect such studies [14,15,16,17]. Most of these papers, however, when concerning catalyst composition, appear to be mere collections of data with poor criticism. According to the literature, the commercial catalysts for CO-rich syngas methanation are also active for pure CO₂ methanation, although the optimal composition for these quite different applications can be remarkably different. In fact, although CO methanation and CO₂ methanation are certainly related reactions, the two reactants CO and CO₂ have very different chemical and adsorption properties. CO adsorbs strongly, also at very low temperatures, on metal surfaces, including nickel [18], with which it can even react in mild conditions, producing the volatile carbonyl compound Ni(CO)₄ [19]. Instead, the interaction of CO with oxides, such as catalyst carriers, is essentially weak [20]. In contrast, CO₂ interaction with metal surfaces may be relatively weak [21] and may dissociatively occur at moderate temperatures, such as on Ni [22]. Instead, CO₂ adsorption on metal oxides occurs at room and even lower temperatures and may be very strong with basic oxides, producing a number of surface and subsurface species [23,24] and sometimes bulk carbonates [21]. The stronger reactivity of CO₂ with oxide carriers suggests that the carrier can have a more relevant role in the kinetics of CO₂ hydrogenation than of CO hydrogenation.

Taking into account that nickel is by far cheaper than ruthenium and other noble metals, and that the reaction temperature for CO₂ methanation may not necessarily be so low, a large part of the current research is centered on nickel catalysts [25,26]. The main approach is to increase the low-temperature activity of Ni-based catalysts to retain total selectivity to methane, limiting deactivation phenomena.

3.1. Unsupported Nickel Catalysts

Unsupported Ni (nano)particles can be produced in several ways, as summarized by Abboud et al. [27]. The preparation of ferromagnetic Ni particles [28] can help in the recovery of the catalyst by magnetic methods or by using induction heating systems. Thermal stability against sintering is a main point concerning unsupported metal particles. Lucchini et al. [29] reported relevant sintering and consequent deactivation of differently prepared unsupported nickel nanoparticles upon CO catalytic hydrogenation experiments in the 550–800 K range.

CO₂ methanation was investigated on unsupported Ni metal particles produced by the reduction of NiO with H₂ by Garbarino et al. [30], Pandey et al. [31], Varvoutis et al. [32], and Kristiani and Takeishi [33], while Lee et al. tested Raney nickel [34]. Ni particles sized at 100–200 nm show moderate catalytic activity, definitely lower than for supported nickel [30,32]. Low Turn Over Frequency [33] but also low selectivities to methane with both coproduction of CO and a quite quick deactivation was associated with the production of encapsulating carbon and carbon nanotubes [30]; see Figure 2.

Similarly, Hao et al. [35] reported low activity and low selectivity to methane over “bare nickel”. According to Pandey [31], the moderate activity of pure nickel is significantly increased by the addition of small amounts of iron. On Raney nickel, selectivity to methane is reported to be not far from 100% for the purest Ni sample, which is also the most active [34]. Very small (d < 8 nm) unsupported nickel nanoparticles produced by the reduction of NiCl₂*6H₂O with NaBH₄ were found almost inactive in CO₂ methanation by Riani et al. [36]. However, it must be taken into account that such a preparation method may leave B impurities on the metal samples [37], which can affect catalytic behavior. Lan et al. [38] reported that the increase in the (111) to (200) crystal planes, obtained by exposing the catalyst to a magnetic field during preparation, enormously increases the catalytic activity of unsupported nickel at 453 K, with total methane selectivity.

3.2. Nickel Supported on Simple Metal Oxide Carriers

Several papers report a comparison of the catalytic activity of nickel catalysts supported on different simple oxide carriers for CO₂ methanation, with partially agreeing data. According to Muroyama et al. [39], the CO₂ conversion for most catalysts drastically increases at 498–523 K and reaches a maximal value at 573–623 K, approaching thermodynamically allowed values for the given conditions. The order of CH₄ yield at 523 K was as follows: Ni/Y₂O₃ > Ni/Sm₂O₃ > Ni/ZrO₂ > Ni/CeO₂ > Ni/Al₂O₃ > Ni/La₂O_3. The catalytic activity can be partly explained by the basic property of the catalysts. However, the same authors reported different scales for selectivity, with Ni/Al₂O₃ being the most selective and performant in methanation at 673 K. In at least partial agreement, Hatzisymeon et al. [40] reported the following sequence of activity: Ni/CeO₂ > Ni/Y₂O₃₋ stabilized ZrO₂ > Ni/Al₂O₃ > Ni/TiO₂, all with 5 wt% Ni. These authors attribute the trend to the decreasing particle size of Ni metal particles. According to Ma et al. [41], the CO₂ methanation activity order for 5 wt% nickel catalysts on relatively low surface area supports is Ni/CeO₂ > Ni/Al₂O₃  >  Ni/TiO₂  >  Ni/ZrO₂. According to Gac et al. [42], the activity trend for 10–40 wt% Ni-supported catalysts is Ni/CeO₂ > Ni/ZrO₂ > Ni/Al₂O₃, all with very high methane selectivity. However, these authors underline that Al₂O₃-supported catalysts have the highest resistance against sintering. Lee et al. [43] reported the following activity trend: Ni/CeO₂ ≈ Ni/Y₂O₃ > Ni/Al₂O₃ > Ni/TiO₂. These authors suggest that the reducibility of both metal and support at low temperatures, strong metal–support interaction, and small Ni particle size are important factors for low-temperature CO₂ methanation. Li et al. [44] reported the activity scale of Ni/CeO₂ > Ni/TiO₂ > Ni/SiO₂, with additional lower selectivity to methane for the Ni/SiO₂ catalyst, attributed to the absence of strong metal–support interaction.

3.2.1. Ni/SiO₂ Catalysts

As it is well known, amorphous silicas are largely covalent supports [45] which are usually supposed to be essentially non-dispersive with respect to other more ionic materials. Depending on the preparation method and type of materials, amorphous silicas have high morphological stability towards sintering and loss of surface area up to 873 K or more [46,47]. Silica is active in reversible adsorption, including of CO₂, with an important role of surface silanol groups as hydrogen bond donors [48], but without any Lewis acidity and basicity [49]. This is the reason for poor dispersing ability towards supported phases, as well as for poor adsorption of CO₂ without the formation of carbonate-like species.

Preparations of Ni/SiO₂ catalysts have been described by several authors. In general, it has been found that supporting nickel salts on amorphous silica followed by calcination gives rise to segregated NiO particles, although quite a small part of Ni²⁺ can be highly dispersed on ion exchange sites, i.e., exchanging protons in silanol group nests [50,51,52]. After reduction, such Ni/SiO₂ catalysts usually mainly present large Ni metal particles with poor dispersion [53] and poor interaction with the support, although the copresence of dispersed unreduced nickel has also been reported [52]. Stability towards sintering of the Ni/SiO₂ catalyst is limited [27], although the use of meso/microporous silica supports and opportune preparation methods can improve it [54].

The CO₂ hydrogenation reaction over Ni/SiO₂ catalysts has been the object of several studies. According to Gac et al. [55], small particles (d = 15.8 nm) of nickel on silica can show very high catalytic activity and selectivity to methane at low temperatures, while selectivity to CO increases with increased particle size and reaction temperature above 673 K. Wu et al. [56] reported a different result, showing that low-loading (0.5 wt% Ni/SiO₂) catalysts essentially work as a rWGS catalyst while high-loading and high particle size samples (10% Ni/SiO₂) are effective methanation catalysts. Galhardo et al. [57] reported high selectivity to CO for 15 wt% Ni/SiO₂ at both low and high temperatures, while near 673 K selectivity to methane is slightly higher than that to CO. These authors report data suggesting that the formation of carbon deposits on Ni metal particles produces Ni carbide species favoring the rWGS activity and deactivating methanation. Li et al. [44] reported lower methane selectivity for Ni/SiO₂ with respect to other supported Ni catalysts. Chen et al. [58] investigated CO₂ hydrogenation on Ni nanoparticles supported on surface-modified, cage-type mesoporous silica at 773 K, observing high selectivity to CO and low selectivity to methane. Riani et al. [52] reported that low-loading Ni/SiO₂ catalysts with particle sizes < 20 nm showed high selectivity to CO and moderate activity for the rWGS reaction, likely mainly due to nickel species dispersed in silica exchange sites, as evidenced by visible spectroscopy. The same authors reported that high-loading Ni/SiO₂ catalysts with particle size ~ 100 nm show both methanation and rWGS activity, but there is evident short-term deactivation for methanation (see the decreased methane yield in the decreasing temperature steps at the right of Figure 3), attributed to large, segregated Ni metal particles, covered by a carbon veil.

Hao et al. [35] reported low activity for methanation and high activation energy (125 kJ/mol) for Ni/SiO₂ when compared with Ni/CeO₂ (E_act near 100 kJ/mol). Guo et al. [60] compared the methanation activity of different 10 wt% nickel catalysts supported on amorphous and mesoporous silica (SBA-15-type and MCM-41-type), showing that they have selectivity to methane approaching 100%, Ni/MCM-41 being definitely less active. In contrast, Bakariza et al. [61] found 15 wt% Ni/MCM-41 more active than Ni/SBA-15. Aziz et al. [62] reported high methanation activity and selectivity for 5% Ni/SiO₂ materials, with better performances when using sol–gel prepared mesostructured silica nanoparticles. Yang et al. [63] reported near-equilibrium conversion with 92% methane selectivity on a Ni/MCM-41 silica catalyst, whose activity is improved by zirconium doping. Vogt et al. [64] reported high selectivity to methane for a Ni/SiO₂ catalyst with 6 nm Ni particle size but lower selectivity, decreasing with time on stream, for methane for a Ni/SiO₂ catalyst with 1.8 nm Ni particle size. Zhu et al. [65] emphasized the progressive deactivation of a 20 wt% Ni/SiO₂ catalyst in a 30 h time on stream experiment. However, they also reported a significant increase in activity, selectivity to methane, and stability by further adding, in a second step, additional silica, alumina, or zirconia over Ni/SiO₂. Guo and Lu [66] reported that co-impregnation of 1 wt% magnesium improves the activity and stability of 10% Ni/SiO₂, but the addition of more Mg results in lower activity and selectivity to methane.

3.2.2. Ni/Al₂O₃ Catalysts

In contrast to silica, spinel-type aluminas are considered to be ionic solids with high surface, Lewis acidity and basicity, and a strong ability to disperse supported species [67,68]. Thus, Ni dispersion on alumina is very high. Impregnation of γ-Al₂O₃ with Ni salts followed by calcination results in the formation of a NiAl₂O₄-like surface layer which is hardly reducible, forming, upon reduction, very small Ni metal clusters [69] which are very selective for CO₂ methanation [52,70], as evident from Figure 3. Instead, even at a high Ni loading, the formation of bulk NiAl₂O₄ does not occur until at least 873 K, given how the reaction between NiO and γ-Al₂O₃ is still slow and incomplete even at 1273 K [71]. However, the surface spinel layer stabilizes the Ni/γ-Al₂O₃ catalysts up to high temperatures [72], preventing bulk transformation and loss of surface area.

Surface characterization studies performed by IR spectroscopy of adsorbed CO allowed for the distinguishing of three types of surface Ni metal sites depending on Ni loading on reduced Ni-γ-Al₂O₃. For a low Ni loading, nearly isolated metal sites are able to adsorb CO in the form of polycarbonyls; at medium loadings, small nickel metal particles adsorb CO on-top (terminal carbonyls) only, and at higher coverages, larger metal particles adsorb CO also as bridging species [73].

In their screening work, among oxide-supported nickel catalysts, Muroyama et al. [39] found Ni/Al₂O₃ to be not very active at low temperatures (473–523 K) with respect to other oxide-supported Ni catalysts, but to be the most performant and selective to methane at 673 K. In an early study, Van Herwijnen et al. [74] reported a kinetic study of methanation of diluted CO₂ (1.4 vol%) in hydrogen, over 33.6 wt% NiO/γ-Al₂O₃ at 473–503 K, with full conversion at sufficiently high contact times already at 500 K at atmospheric pressure. For γ-alumina supports with 180–220 m²/g, the best activity and selectivity are reported for catalysts with 15–25 wt% Ni [75,76], which roughly corresponds to the coverage of the support by a “geometric monolayer” of surface Ni aluminate species, while at a lower loading, both activity and selectivity decrease in favor of rWGS activity to CO. Otherwise, at an even higher loading, where larger but still strongly support-bonded Ni metal particles are formed, activity decreases, but deactivation is significant, while high selectivity is usually retained [77].

On the other hand, Ni-Al₂O₃ catalysts prepared by coprecipitation, such as by coprecipitated Ni-Al hydrotalcite, were reported to be definitely more active and more selective than impregnated Ni/γ-Al₂O₃ at medium-low loadings [78], but may be less active and selective at a near monolayer loading [75]. On the other hand, coprecipitated catalysts with very high Ni loadings (50–90 wt% Ni) were reported by Beierlein [77] to be definitely more active in methanation and stable than high-loading samples (>30 wt% Ni) produced by impregnation. These authors attribute this activity trend to the effect of higher nickel surface area. Indeed, when working with Ni-Al hydrotalcite, the Ni loading is usually very high. When working with a catalyst with a Ni/Al atomic ratio = 3, high selectivity to CO was found at low contact time, with methane selectivity increasing with increasing contact time [79].

3.2.3. Ni/CeO₂ Catalysts

Most authors of screening papers report ceria as the best oxide support for nickel catalysts used in CO₂ methanation. Hao et al. [35] reported excellent activity and near 100% methane selectivity over 7.5% Ni/CeO₂ with 25 m²/g surface area, although the Turn Over Frequency increases by decreasing particle size as a result of decreasing loading. Methanation activity is already significant at 498 K with near 100% selectivity. Similarly, Bian et al. [80] reported activity already in the 473–523 K range for Ni/CeO₂ with a near 100% methane selectivity, which somehow depended on ceria particle morphologies: Ni on ceria nanorods appeared to be more active than Ni on ceria nanocubes. Using ceria nanorods, Varvoutis et al. [32] showed high activity already in the 473–573 K range and equilibrium conversion above 623 K with total methane selectivity for samples with 8 to 33% Ni/CeO₂ (S_BET 72 to 51 m²/g by increasing loading). On the other hand, low-loading Ni/CeO₂, with 1–2% nickel, are reported to work as excellent catalysts for the competitive reverse water gas shift reaction to CO [81]. The role of the preparation method has also been emphasized by several authors: Ye et al. [82] reported much higher activity and selectivity of sol–gel prepared Ni/CeO₂ than from an analogous catalyst produced by impregnation. As a key for explaining the best performances of Ni/CeO₂, the role of the partial reducibility of CeO₂, with the detection of Ce³⁺ ions, has been emphasized [83]. Indeed, among typical oxide carriers, ceria is the most reducible, and also has the ability to absorb hydrogen-forming bulk and surface hydride species [84]. In fact, it has been reported that the Ni/CeO₂ system is quite an unstable one, with the ability to dissolve Ni²⁺ ions in the bulk at higher temperatures and longer times [85]. It is, in fact, known that NiO can dissolve in CeO₂ in significant amounts, forming a solid solution with a fluorite structure [86,87], at least for small particle size nanomaterials. A similar deactivation phenomenon with loss of metal surface area is reported, e.g., for Pd/CeO₂ for the Water Gas shift reaction [88].

In their review, Xie et al. [89] summarized recent attempts to further improve Ni/CeO₂ catalysts for CO₂ methanation by modifying the preparation method (constructing more efficient structures and producing more efficient morphologies) and adding additional components, such as alkali, alkali–metal, and rare earth oxides and by alloying Ni with other transition metals. According to these authors, from a practical industrial point of view, challenges remain in developing a durable and low-cost CeO₂-based methanation catalyst with considerable low-temperature activity.

3.2.4. Ni/ZrO₂ Catalysts

As shown above, most screening studies report high methanation activity for Ni/ZrO₂. Chen et al. [90] investigated the activity of 15 wt% Ni/ZrO₂ catalysts with predominant monoclinic or tetragonal zirconia support: the monoclinic sample showed lower initial conversion temperature (down to 358 K) and higher basicity than the tetragonal one, both being very selective to methane formation, but with a slight selectivity decrease above 623 K. A similar result was reported by Jia et al. [91] for a 10 wt% Ni/ZrO₂ monoclinic + tetragonal zirconia support. Tan et al. [92] used a monoclinic + tetragonal zirconia support with a surface area of 96.6 m²/g to prepare a series of Ni/ZrO₂ catalysts. While all catalysts showed selectivity to methane near 100%, the most active appeared to be that with a 6 wt% Ni loading. According to these authors, the intrinsic activity of the catalyst is related to the Ni dispersion and the size of metallic Ni; thus, excellent activity was obtained with highly dispersed particles. According to Vargas et al. [93], really highly dispersed 1 wt% Ni/m-ZrO₂ prepared by ultrasound-assisted synthesis is very active and selective in methanation of CO₂. Ma et al. confirmed the high activity of Ni/m-ZrO₂, more active than a catalyst based on cubic zirconia [94], attributing high activity to the high local electron density of nickel sites. Zhao et al. [95] compared the activities of different 15 wt% Ni/ZrO₂ catalysts produced by combustion with different combustion compounds with a sample produced by impregnation of a mainly monoclinic ZrO₂ support. The catalyst produced by the urea combustion method (resulting in being tetragonal) was reported to be the most active and stable, although, in the 573–773 K range, the impregnated sample was reported to be even more selective to methane. Concerns about the use of Ni/ZrO₂ catalytic materials is due to the potential of NiO to enter monoclinic, tetragonal, and cubic zirconia phases forming solid solutions [96,97] which influence the solid state chemistry of zirconia, apparently favoring the metastable tetragonal phase.

3.2.5. Nickel Catalysts Supported on Other Rare Earth Oxides

In a recent review, Spennati et al. [98] emphasized the role of rare earth oxides in the composition of Ni-based catalysts for CO₂ methanation. According to the screening work of Muroyama et al. [39], where pure support effect has been considered for 10 wt% Ni/oxide catalysts, Ni/Y₂O₃ and, to a lesser extent, Ni/Sm₂O₃, show the highest methanation activities at 523 K, although they lose methane selectivity above 573 K in favor of CO. Thus, the methanation activity of Ni/Y₂O₃ is closely similar to that of Ni/Al₂O₃ and Ni/ZrO₂ at 623 K. Other screening studies confirm the high activity of Ni/Y₂O_3, but no more than Ni/CeO₂ [43]. Li et al. [99] confirmed the very high methanation activity of 10 wt% Ni/Y₂O_3, if prepared in the presence of large amounts of citric acid in particular. Ni/Sm₂O₃ catalysts were also reported as interesting, with high low-temperature activity and almost total selectivity to methane, mainly when the sample is highly charged (63 wt% Ni) [100], with a slight deactivation attributed to carbonate formation.

Relatively high-loading Ni/La₂O₃ catalysts [101] are reported to be selective in the methanation of CO_2, while low-loading Ni/La₂O₃ appear to work as rWGS catalysts with high CO selectivity [102]. However, studies report that La₂O₃ is unstable in reaction conditions, suffering from carbonation or hydration with the formation of oxycarbonate and hydroxide production [103], respectively. On the other hand, 10 wt% Ni supported on the oxycarbonate La₂O₂CO₃ has been reported to display enhanced activity and stability with respect to Ni/La₂O₃ [104].

3.2.6. Other Ni/Oxide Catalysts

As commented before, most screening studies find a relatively low activity of Ni/TiO₂ with respect to other oxide-supported Ni catalysts. In their study, Unwiset et al. [105] found catalytic activity only above 573 K, increasing with the Ni loading, and high selectivity to methane for Ni/TiO₂-anatase samples. The XRD study of the catalysts shows that part of the nickel was incorporated in the anatase framework in the solid solution. When looking at solid state stability, the Ni/TiO₂-anatase system is considered to be quite an unstable one, since Ni²⁺ is a promoter of the anatase to rutile phase transformation [106] that usually also occurs at low temperatures with a significant loss of surface area. For example, the TiO₂ phase transformation was reported to affect the performance of the Ni/TiO₂ catalyst for biomass fast pyrolysis volatiles [107]. On the other hand, Messou et al. [108] found Ni/rutile more active than Ni/anatase, although Ni over a mixed phase support was even more active, showing some kind of synergism between the two TiO₂ phases. A few studies also report on the methanation activity of Ni/MgO catalysts [109,110], where high activity and selectivity to methane can also be found. It must be mentioned, however, that the full reciprocal solubility of NiO and MgO (which are isostructural) may represent an element of instability of the system if prepared by impregnation, or of limited efficiency per nickel atom if prepared by coprecipitation.

3.3. Effect of Addition of Doping with a Second Oxide on Ni/γ-Al₂O₃ and Ni/CeO₂

The data summarized above indicate that Ni/CeO₂ and Ni/γ-Al₂O₃ are likely the most attractive systems for CO₂ methanation: alumina is usually the preferred support by catalyst producers because of its moderate costs, its well-known chemistry, and its well-proven stability. On the other hand, Ni-Al₂O₃ catalysts are also already largely used in the industry for CO hydrogenation both to produce Synthetic Natural Gas and to remove CO and CO₂ from hydrogen and ammonia synthesis gas, as well as for several hydrogenations in the treatment of hydrocarbon streams [12]. Otherwise, Ni/CeO₂ seems to give more active catalysts, in particular at low temperatures. The use of CeO₂ is increasingly of interest in heterogeneous catalysis [111].

3.3.1. Effect of a Second Oxide Component on Ni/γ-Al₂O₃

Silica is a well-known stabilizer of the γ-Al₂O₃ against phase transition and surface area loss [49,112]. However, the addition of silica in small amounts to γ-Al₂O₃ shows a slightly negative effect on CO₂ hydrogenation activity, although it may have a positive effect when applied in small amounts on the stability of the methanation activity [52]. Liang et al. reported on the effect of Na, K, Mg, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Zr, La, and Ce on the activity of Ni/γ-Al₂O₃ [113], observing that Zn only promotes methanation activity at low temperature, while Cr, Fe, Mn, La, Ce promote stability in prolonged CO₂ methanation experiments. In contrast, Cai et al. [114] reported a marked promotion effect of zirconia on 12 wt% Ni/γ-Al₂O₃, in particular when the 3 wt% ZrO₂/γ-Al₂O₃ carrier was prepared by the impregnation–precipitation method. In their recent review, Spennati et al. [98] emphasized the positive role of doping Ni/Al₂O₃ with rare earth oxide components. Ahmad et al. [115] reported the activity scale Pr > La > Gd > Ce > Eu for 12 wt% Ni/γ-Al₂O₃ doped with 5 wt% rare earth metal. Guilera et al. [116] co-impregnated alumina with nickel and Ce, La, Sm, Y, and Zr compounds and found La and Ce gave the best catalytic performance, while Sm and Zr showed a lower positive effect. Looking at the optimal amount of Ceria, Kim et al. [117] report maximum activation for methanation with 15 wt% CeO₂ for 15 wt% Ni/Al₂O₃ with 92 m²/g surface area. As for the Ni/La₂O₃ –γ-Al₂O₃ system, Garbarino et al. [59] reported 14 wt% La₂O₃ as the best loading for 13.6 wt% Ni/γ-Al₂O₃ (17 wt% as NiO). In Figure 3, the positive effect of La₂O₃ addition to Ni/γ-Al₂O₃ is evident. This effect was in part attributed to the increased strength of absorption of CO₂ in the form of carbonate species over the La₂O₃-Al₂O₃ support, with respect to bicarbonate species on pure Al₂O₃. Riani et al. [118] reported later that the coaddition of silica and lanthana to Ni/γ-Al₂O₃ can give rise to catalysts with even higher low-temperature methanation activity. A catalyst obtained by a precursor with a weight composition of 16.7% NiO, 37.0% La₂O₃, 4.6% SiO₂, and 41.7% Al₂O₃ was found to be very active at 523 K.

Beierlein et al. [77] confirmed the negative effect of sodium impurities in the methanation activity and selectivity of Ni/γ-Al₂O₃. Potassium addition on Ni/γ-Al₂O₃ was reported by Gandara Loe et al. [119] to significantly increase CO selectivity, finally producing an excellent catalyst for rWGS at T > 873 K. As for alkali earth elements, Liang et al. [120] reported that Ba (1–5 wt% loading) doping of Ni/γ-Al₂O₃ results in a remarkable selectivity increase, while Sr is less effective and Mg and Ca enhance rWGS reaction, thus reducing methane selectivity. However, Garbarino et al. [121] reported that Ni/Ca-aluminate industrial methane steam reforming catalyst (18 wt% NiO in the catalyst precursor) has excellent activity and selectivity to methane, which are both decreased by 1.8 wt% K₂O doping. Garbarino et al. [122] found excellent low-temperature activity for Ca-doped 19 wt% Ni/γ-Al₂O₃, further enhanced by the addition of V₂O₅.

A very interesting point concerns the Ni/MgO-Al₂O₃ system, which can be prepared by coprecipitation starting from Ni-Mg-Al hydrotalcite-type double hydroxides. Liu et al. reported that Ni/MgAl catalysts (36.9 wt% Ni, (Ni + Mg)/Al = 2) exhibit outstanding performance in CO₂ methanation, compared with Ni/γ-Al₂O₃ and Ni/MgO [123], associated with a very small Ni metal particle size, allowing better low-temperature CO₂ methanation activity. Very high CO₂ methanation activity and selectivity, higher than that of Ni/γ-Al₂O₃, was recently reported by Fasolini et al. [124] for ex-hydrotalcite 27 wt% Ni/MgO-Al₂O₃.

Another element that has been reported as a good activator for methanation on Ni/γ-Al₂O₃ is manganese. Zhao et al. [125] reported maximum activation of 15 wt% Ni/γ-Al₂O₃ by adding 1.7 wt% Mn. These authors attributed the activation effect to the increased number of CO₂ adsorption sites, the improved Ni dispersion, and the weakened interactions between the nickel species and the support. In good agreement, Vrijburg et al. [126] reported significantly higher methanation rates and CH₄ selectivities for Mn-promoted Ni/γ-Al₂O₃. The optimal Mn/Ni atomic ratio was found to be 0.25, showing metallic Ni particles and small oxidic Mn²⁺ species. According to these authors, Mn addition improves Ni dispersion and enhances Ni²⁺ reducibility by weakening the interaction between the Ni-oxide precursor and the support. Wu et al. [127] reported on the excellent methanation activity of a catalyst with 20 wt% Ni/Al₂O₃-ZrO₂ doped with 6.7% Mn. All these studies conclude that Mn oxide species are dispersed on the support surface, favoring nickel dispersion.

3.3.2. Effect of a Second Oxide Component on Ni/CeO₂

A number of studies reported on the further promotion of Ni/CeO₂ for CO₂ methanation. The use of CeO₂-ZrO₂ solid solutions is a common way to stabilize ceria. Pan et al. [128] found Ni/Ce_0.5Zr_0.5O₂ to definitely be more active than Ni/γ-Al₂O₃ and attributed to the stronger basicity of the Ce_0.5Zr_0.5O₂ support, having more and stronger CO₂ adsorption sites than γ-Al₂O₃. Looking at different CeO₂-ZrO₂ support compositions, Ullah et al. [129] found maximum activity for 5 wt% Ni catalysts when the support composition is Ce_0.7Ni_0.3O₂. A number of studies reported the promoting effect of rare earth ions on the activity of Ni/CeO₂, such as Eu³⁺ [130], Sm³⁺ [131], Y³⁺ [132], the couples La³⁺ -Sm³⁺, La³⁺-Pr³⁺, and La³⁺ -Mg²⁺ [133], etc. Looking at the effect of alkali earth metals, Liu et al. [134] found an increase in the methanation activity of Ni/CeO₂ by adding Ca²⁺ and Mg²⁺, with a more limited effect for Sr²⁺ and a deactivation effect for Ba²⁺.

3.4. Effect of Alloying

Several studies report bimetallic Ni-based catalysts being more active than pure nickel ones in CO₂ methanation. This subject has been reviewed recently by Tsiotsias et al. [26]. The addition of small amounts of noble metals (Pt, Ru, Rh, Pd, and Re) to Ni/Al₂O₃ provides several advantages, such as excellent activity at low temperatures, high reducibility, and stability once oxidized [135], producing even new sites or by forming an alloy with nickel as evidenced for Ru and Pt or Pd, respectively. However, the use of very expensive noble metals to activate nickel may make sense only if they are added in extremely small amounts, due to their activity even at small loadings over oxide support. According to Pan et al. [136] the addition of 0.2 wt% Ru causes a 3-fold rate enhancement at low temperature for selective methanation of CO₂ relative to the monometallic 20 wt% Ni/CeO₂ and to 0.2 wt% Ru/CeO₂.

The use of less expensive transition metal to increase Ni activity seems to be more interesting. As already cited, according to Pandey [31], the moderate activity of pure nickel is significantly increased by the addition of small amounts of iron. For example, the addition of 1 wt% cobalt was reported to significantly increase the low-temperature activity of 15 wt% Ni/MgO-Al₂O₃ catalysts [137]. Similarly, the addition of 4.7 wt% iron significantly increases the low-temperature activity of a 20 wt% Ni/MgO-Al₂O₃ catalyst [138]. According to Xu et al. [139], the additions of Fe, Co, and, to a lower extent, Mn on Ni/Ce₀.₈Zr_0.2 enhance activity, while copper addition decreases both activity and selectivity to CH₄. Over low-loading Ni/La₂O₃, copper addition increases rWGS activity and selectivity to CO [102].

4. Discussion

The data summarized above show that, essentially, all Ni-containing catalysts found activity in CO₂ hydrogenation. In

Table 1. Proposed criteria for selecting optimal Ni-based CO₂ methanation catalysts.

Nickel state	reduced
Nickel particle size	very small
Support type	ionic, dispersing
	not dissolving Ni
Nickel loading	near a “monolayer” on the support surface
Support surface chemistry	containing some basicity
Additive(s)	favoring nickel dispersion
	producing medium basicity
	activating nickel by electronic effects
	alloying nickel

, the most relevant criteria for selecting optimal Ni-based CO₂ methanation catalysts are proposed. Catalysts where most of the nickel species remain unreduced, because they strongly interact with the oxide support, usually have predominant rWGS activity to CO, while those where sufficient nickel can be reduced to the metallic state have at least good initial activity to methanation. It seems that good and stable methanation activity is associated mainly with very small nickel metal particles associated with very high nickel dispersion, which can be stabilized mainly on ionic oxides. In fact, for large particle Ni catalysts, fast deactivation by carbon deposition is usually observed, which may also result in switching toward higher CO selectivity by rWGS activity, likely due to carbon or nickel carbide species.

Methanation occurring on unsupported Ni particles and Ni/SiO₂ should fully occur at the nickel metal surface, where carbide or carbon intermediates are reasonably formed and active in methanation [140]. However, mechanisms via oxygenate intermediates (carboxylate, carbonate, formate, formyl, and methoxy) can also occur at nickel surfaces [141]. On ionic oxides, where CO₂ adsorbs and forms different types of carbonate and bicarbonate species [23], a more important role of the support in the methanation mechanism is plausible [142].

Very high low-temperature methanation activity is obtained on small Ni metal particles supported on semiconducting and slightly reducible metal oxides characterized by medium surface basicity, such as, in particular, for Ni/CeO₂. On the other hand, it seems also quite evident that the addition of basic, semiconducting, and moderately reducible components to alumina (a non-conducting, non-reducible, and mainly acidic support) is sufficient to significantly increase the methanation activity at 473–523 K, up to values comparable to those obtained with (doped) Ni/CeO₂, and with Ru/Al₂O₃ as well. Such components to be added to alumina support include, e.g., cerium or manganese oxides.

The activating effect of such an intermediate level of basicity is usually attributed to the resulting ability of the support to adsorb CO₂ in the form of carbonate species that, thanks to the spillover of hydrogen from nickel metal particles, is at the origin of a oxygenate intermediate-based hydrogenation mechanism [143]. For example, the activating effect of the addition of La₂O₃ to Ni/γ-Al₂O₃ has been attributed in part to the increased strength in the adsorption of CO₂ [59]; as shown in Figure 4, the adsorbed species of CO₂ are fully modified.

On γ-Al₂O_3, two slightly different forms of bicarbonates are observed (Figure 4, bands at 1644 cm⁻¹; O-C-O asymmetric stretching, 1484, 1442 cm⁻¹, two different O-C-O symmetric stretchings, 1236 cm⁻¹, COH deformation mode), whose band decreased by the addition of La₂O₃ until almost disappearing, while bands due to two forms of carbonates are formed (COx stretchings at 1680 and 1380 cm⁻¹, likely bridging carbonates, and at 1540 and 1435 cm⁻¹, likely monodentate carbonates). However, slight effects in modifying the electronic properties of Ni metal particles by increasing support basicity as well as semiconducting or redox species cannot be excluded [59].

As shown above, the addition of small amounts of another metal, e.g., cobalt or ruthenium, can also produce a significant increase in activity at low temperatures.

On the other hand, to develop industrial catalysts, besides (initial) activity, other factors must be considered and are even more relevant. In their work concerning high-temperature methanation of CO-rich syngases, Nguyen et al. [13] reported that catalysts must be stable against metal particles and support sintering, formation of carbon and gum residues, as well as the formation of nickel tetracarbonyl. Such a volatile compound can favor Ni particle growth through its formation and decomposition. The need to avoid Ni(CO)₄ formation drives the limiting of nickel loading and the use of sufficiently high reaction temperatures, e.g., >573 K, where such gaseous compounds cannot form. Thus, for this reason, the very low-temperature activity of the catalyst cannot be so relevant.

It seems that the limitation of the formation of carbon residue and gum can be obtained using catalysts allowing the production of small to very small nickel metal particles in reaction conditions. This is apparently obtained using high-dispersing ionic oxide supports and a moderate loading of nickel.

Another point is certainly the solid state stability of the support and its reactivity towards nickel species. In fact, most metal oxides allow solubility of Ni²⁺, forming mixed oxides. The penetration of nickel in the bulk of the support, with the help of the slight oxidizing properties of the reactant CO₂ and of the product H₂O, would result in the consumption of the active phase (metallic nickel) and in the deterioration of the morphological properties of the support. From this point of view, γ-Al₂O₃ seems to be optimal, taking into account that it tends to be covered by a NiAl₂O₄-like surface layer upon the preparation of the catalyst that protects the bulk from further Ni penetration. On the other hand, for decades, the use of Ni/Al₂O₃ catalysts in several other hydrogenation processes in relevant industrial conditions has been a confirmation of the stability of this system. In fact, as reported recently by Spennati et al., commercial Ni catalysts for CO_x methanation seem to be exclusively based on Ni/Al₂O₃ [98]. This might be due to its cheapness and well-known chemistry of alumina, coupled with its commercial availability, stability in mild conditions, and versatility, which make it preferred by the industry, e.g., as hydrotreating catalysts, as reported years ago [144]. It must also be taken into consideration that even though coprecipitated Ni/Al₂O₃-based catalysts may be a little more active than impregnated ones at least at initial steps, the less complete use of nickel in coprecipitated samples and the likely simpler preparation of catalysts by co-impregnation of commercial alumina with nickel and dopants can have advantages (also from the environmental point of view) with respect to more complex coprecipitation procedures.

5. Conclusions

A detailed analysis of the scientific literature data concerning catalysts for CO₂ methanation allowed us to evidence the main features needed for such an application. It seems that the necessary features of the best-performing oxide-supported nickel catalyst for CO₂ methanation are as follows:

• The use of an ionic support that allows good nickel dispersion, resulting in very small nickel metal particles. Such small metal particles would be very active toward methanation and would not produce many carbon residues and gums;
• The use of a support and/or surface additives that produce a medium basicity, allowing a medium-strong adsorption of CO₂;
• A medium nickel loading, which allows the free support’s geometric surface to be covered with optimal density by small nickel metal particles, without the production of larger Ni crystals and allows sufficient active sites for CO₂ adsorption on the support surface;
• Working at temperatures where Ni(CO)₄ formation is impossible (e.g., >573 K).

Taking into account the already significant use of Ni/Al₂O₃ systems for catalytic hydrogenations, its ascertained long-term stability, and the common use of γ-alumina as a catalyst support, the application of 15–20 wt% nickel over a typical γ-Al₂O₃ (200 m²/g) with small amounts of slightly reducible and basic promoters, such as Ce⁴⁺ and Mnⁿ⁺, or/and of simply mild basic components, such as Mg²⁺, and maybe small amounts of an alloying metal, such as cobalt or iron, can be recommended. The most likely alternatives are based on Ni/CeO₂ with additives, whose long-term stability, however, should be verified.