Supported Catalysts for CO₂ Methanation: A Review

Abstract

CO₂ methanation is a well-known reaction that is of interest as a capture and storage (CCS) process and as a renewable energy storage system based on a power-to-gas conversion process by substitute or synthetic natural gas (SNG) production. Integrating water electrolysis and CO₂ methanation is a highly effective way to store energy produced by renewables sources. The conversion of electricity into methane takes place via two steps: hydrogen is produced by electrolysis and converted to methane by CO₂ methanation. The effectiveness and efficiency of power-to-gas plants strongly depend on the CO₂ methanation process. For this reason, research on CO₂ methanation has intensified over the last 10 years. The rise of active, selective, and stable catalysts is the core of the CO₂ methanation process. Novel, heterogeneous catalysts have been tested and tuned such that the CO₂ methanation process increases their productivity. The present work aims to give a critical overview of CO₂ methanation catalyst production and research carried out in the last 50 years. The fundamentals of reaction mechanism, catalyst deactivation, and catalyst promoters, as well as a discussion of current and future developments in CO₂ methanation, are also included.

1. Introduction

Several studies [1,2,3,4,5,6,7,8,9] and recent reviews [10,11,12,13,14] have focused on the methanation reaction, mainly due to its important implications for energy and the environment. Methanation processes produce methane (Substitute or Synthetic Natural Gas, SNG) from hydrogen and CO_x. CO and CO₂ methanation processes, discovered in 1902 by Paul Sabatier and Jean-Baptiste Senderens, represent a promising solution for reducing anthropogenic gas emissions [11].

The increasing use of renewable sources, due to their fluctuating character, makes mandatory the development of adequate storage systems in order to overcome the mismatch between power production and instantaneous demand (Figure 1). Water electrolysis is a mature technology to produce hydrogen and CO₂ can be conveniently recovered from several industrial processes, such as biomass combustion and gasification, biogas facilities, power plants, oil refineries, and cement kilns.

The reaction is also considered a key technology able to facilitate future manned space missions by the recycling of CO₂ from breathing or wasted H₂ from water electrolysis on the International Space Station [15,16,17].

The methanation of CO₂ is an exothermic reaction (Equation (1)), typically operating between 200 °C and 450 °C, depending on the catalyst and experimental conditions [8,10]. Although several papers have been published on the subject in the recent past, no general consensus exists on the reaction’s operating mechanism, mainly due to the uncertainty in determining the intermediate compound involved in the rate determining step [10,11].

$${\mathrm{CO}}_2+4{\mathrm{H}}_2\leftrightarrow {\mathrm{CH}}_4+2{\mathrm{H}}_2{\mathrm{O}}_{\left(g\right)}∆H_{298\mathrm{K}}=-164\frac{\mathrm{kJ}}{\mathrm{mol}}$$

(1)

The first and most popular path considers the conversion of CO₂ to CO, which, in turn, is hydrogenated to CH₄ by the same mechanism involved in CO methanation. The second path includes the direct hydrogenation of CO₂ to CH₄, without formation of any CO intermediate (Figure 2). As said above, numerous reviews exist on this argument (i.e., mechanistic aspects, reactor type modeling, simulation); therefore, it is outside the scope of the present paper [10,11].

The first part of the paper is dedicated to noble metal catalysts. These have so far demonstrated a high performance in the reaction, so are of relative interest from an industrial point of view on account of their high cost. Therefore, most of the discussion on the subject is devoted to Ni-based materials that couple a high catalytic activity and affordable cost.

Transition metals were deeply investigated as active catalysts for CO₂ hydrogenation [18,19,20], and affect the CO₂ activation and reduction steps. Iron, cobalt, nickel, and copper show high catalytic properties for CO₂ activation. Chemisorption of carbon dioxide on transition metals is spontaneous, while the surface structure of the metal strongly affects the thermodynamic of the catalytic process [18]. For example, the binding energy of carbon dioxide on the iron surface is stronger than that of platinum, while the CO₂ dissociation proceeds more easily on platinum than on iron [19]. All these works demonstrate that metal plays a pivotal role in CO₂ hydrogenation (both in the activation and reduction steps).

In order to preserve and improve metal activity, reduction technology focused on the metal-supported catalyst development [21]. The physico-chemical characteristics (structure, chemical composition, defect groups, and thermal stability) of supports are fundamental aspects to consider in metal-supported catalyst tuning because they affect the activity, productivity, and lifetime of the final heterogeneous catalyst. The interactions between support and metal were studied by different researchers in many reactions of environmental interest, such as dry reforming, steam reforming, partial oxidation, and autothermal reforming [22,23,24,25,26,27,28,29].

2. Catalytic Materials for CO₂ Methanation Reaction

This section provides a concise and precise description of the experimental results, their interpretation, and the experimental conclusions that can be drawn.

2.1. Noble Metal Catalysts

2.1.1. Rhodium

Rh is one of the most investigated metals for the CO₂ methanation reaction. Particular emphasis has so far been placed on the hypotheses of a mechanism that allows methane to be formed on the surface of the catalyst, especially in the presence of alumina as a support. The steps leading to methane could be: (i) chemisorption of carbon dioxide; (ii) dissociation of carbon dioxide into CO and O adsorbed on the surface; (iii) reaction of dissociated species with hydrogen [30]. The oxidation state of the metal may also play an important role in the evolution of the reaction, since CO₂ oxidizes the catalyst. Moreover, the production of methane depends upon the temperature, pressure, presence, and absence of promoters. Obviously, when varying the Rh content different metal particle sizes are formed, and at low temperatures (130–150 °C) the activity of larger particle sizes of Rh was found to be higher than that of smaller ones. Furthermore, the addition of Ba and K on the Al₂O₃ support allows significant differences in the catalytic behavior in the temperature range 300–700 °C. CH₄ was preferentially formed below 500 °C on Ba-containing and pure Rh/Al₂O₃ while, at higher temperatures, significant amounts of CO were formed. Only CO was observed with the K-containing catalyst [30,31,32,33].

The presence of O₂ could have a positive effect on the methanation of CO₂ and CO. Belus et al. highlighted that oxygen, in low proportion, improves the catalytic performance of the Rh/γ-Al₂O₃ catalyst, but if the amount of O₂ is too high, a negative effect is observed [31]. In Figure 3 the methane production, over Rh(1%)/γ-Al₂O₃ catalyst, at 125 °C, as a function of CO/O₂ and CO₂/O₂ is reported. DRIFTS experiments explain that the positive effect of oxygen is due to the formation of more active species, such as gem-dicarbonyl. On the contrary, the negative effect could be attributed to the inevitable oxidation of the catalyst [31].

A synergistic effect was recently observed by mechanically mixing Rh/γ–Al₂O₃ and Ni/activated carbon (Ni/AC). It appears that higher production of CH_4, with respect to the single catalysts, is not due to a chemical interaction since no formation of new structures occurs when the catalysts are mixed. Authors hypothesize that Rh/γ-Al₂O₃ is able to efficiently adsorb CO₂, whereas Ni/AC adsorbs a high amount of H₂ but a small amount of CO₂, resulting in high CO₂ conversion and methane formation [34].

TiO₂ has been also extensively investigated as a support for Rh in the CO₂ methanation reaction, especially at low temperatures. Rh/TiO₂ is one of the most active catalysts for the reaction, but the high cost of the metal prevents its widespread utilization at industrial scale. Such a high activity is thought to be attributed to a metal–support interaction that facilitates the breaking of the C=O bond, resulting in an increase of the catalytic activity [35,36,37,38,39,40,41,42]. Recently, the Density Functional Theory method (DFT) has been employed to study the CO₂ methanation reaction on an Rh₁/TiO₂ (101) model [43]. The results show the co-adsorption properties of the Rh₁/TiO₂ catalyst surface. By the proposed mechanism, CO₂ and H₂ are co-adsorbed on the Rh atom and, subsequently, can react with each other to form CO. Further adsorption of H₂ is inhibited on the CO adsorbed on Rh; therefore, the consecutive CO hydrogenation does not occur. The proposed mechanism explains why, experimentally, a high selectivity of Rh₁/TiO₂ to CO is observed, as a consequence of the frontier orbital charge density symmetry matching principle [44].

2.1.2. Ruthenium

Ru is one of the most active methanation catalysts. Its catalytic activity and selectivity to CH₄ are, however, largely dependent on the dispersion of the metallic phase (at high dispersion the apparent activation energy reaches a minimum), on the type of the support, and an addition of modifiers/promoters that can more or less chemically interact with the metal [45,46,47,48,49,50,51,52,53,54]. Ru catalysts have been supported on a number of oxide materials, such as Al₂O₃, TiO₂, SiO₂, MgO, MgAl₂O₄, C, and Ce_0.8Zr_0.2O₂ [55,56]. The activity of pre-reduced 3%Ru/Al₂O₃ catalyst (CO₂ conversion, CH₄ and CO yields), as a function of the reaction temperature, is reported in Figure 4 [53]. The best catalytic performance occurs at 673 K, with the highest CO₂ conversion, CH₄ yield, and limited CO yield.

In particular, in order to lower the reaction temperature in CO₂ methanation reaction, Ru/TiO₂ catalysts have been used since the 1980s. Akamaru et al. [57] reported that they failed to reproduce the results of Gratzel et al. [58,59] using Ru/TiO₂ catalysts prepared under the same experimental conditions. The main reason was the difficulty of controlling the preparation conditions in a wet process. Therefore, a dry technique, called polyhedral barrel sputtering, was developed; this technique, according to [60], is able to disperse metal nanoparticles on the support in clean conditions and control their size and deposition density. Using a Ru/TiO₂ catalyst prepared with this technique, an onset temperature of 60 °C for CH₄ generation was observed that increases upon increasing the size of metal particles. Recently, Xu et al. have demonstrated that the Ru particle size is not the main reason for the different behavior of Ru/rutile(TiO₂)catalyst [61]. They reported that the increasing of hydroxyl groups on the TiO₂ layer strongly increases the CO₂ dissociation. The suggested mechanism assumes that the adsorbed CO₂ reacts with the hydroxyl groups, producing CO via formate species intermediates. So, the pre-treatment temperature of the support should not be higher than 800 °C, because over this temperature value condensation of the hydroxyl groups occurs and the support does not play its important role.

Results of DFT analysis attribute the high activity of the catalyst to the difference between the Ru structure and the Ru surface, and to the weak charge transfer from adsorbed species to Ru atoms [57].

It is recognized that CeO₂ plays an important role in promoting CO₂ methanation and enhancing CH₄ formation. On account of its basicity, CO₂ is adsorbed on CeO₂ and reduced due to the oxygen vacancies on CeO₂, resulting in high CO₂ conversion at T ≤ 350 °C. The addition of 30% CeO₂ to 2 wt % Ru/Al₂O₃ leads to an enlarged specific surface area of the catalyst. Furthermore, the reaction intermediates (formates, carbonates) react with H₂ faster on this catalyst than over Ru/Al₂O₃ [62]. Ceria is also a very good methanation catalyst when doped with 0.05 wt % Ru. Electron microscopy and XRD analyses suggest that Ru is incorporated into the ceria lattice. At T = 450 °C, this catalyst converts 55% CO₂ with 99% selectivity to CH₄. The reaction takes place on the reduced Ce_0.95Ru_0.05O₂; the role of Ru is to lower the reduction temperature with respect to pure ceria [63].

Ru supported on TiO₂–Al₂O₃ exhibits a much higher (3.1 times) activity than that on supported Al₂O₃. This result was attributed to the smaller averaged particle size of Ru supported on TiO₂–Al₂O₃ (2.8 nm) versus the 4.3 nm measured on Al₂O₃, resulting from the interaction between the metal and the rutile TiO₂, which hinders the aggregation of Ru particles [64].

Very recently, trimetallic catalysts drew attention to the reaction of the Baker group [65,66,67], which highlights how the adsorption strength of CO₂ is controlled by the Lewis basicity of the catalyst, the d-band center of the metal surface, and the charge transfer from the metal surface to the chemisorbed CO₂. Pd, Rh, and Ru supported on Mn/Cu–Ce–Al₂O₃ catalysts were also investigated and Ru/Mn/Cu–Al₂O₃ was found to be the most promising catalyst (10.9 wt % Ru loading, calcination temperature 1035 °C). Baker et al. claim the suitability of their catalysts for industrial application.

2.1.3. Palladium

A good catalytic performance has also been observed with Pd-based catalysts. Pd is able to dissociate molecular hydrogen [68,69] and makes available hydrogen atoms for the subsequent transfer and reaction with activated surface carbonate species formed by the reaction of CO₂ on a Mg-containing oxide [70,71,72],with the aim of providing a pathway to minimize CO formation by using metal oxides that inhibit CO desorption. Intermixed Pd and Mg sites are obtained by using the reverse microemulsion synthesis route; 95% selectivity to CH₄ and 59% CO₂ conversion have been measured at 450 °C [73].

More recently, shape-controlled Pd nanoparticles embedded in mesoporous silica have been tested in the reaction; their performance has been compared with a Pd/SiO₂ catalyst prepared by wet impregnation. The encapsulation was demonstrated to have a better stability towards sintering. Moreover, different activities and selectivities for the CO₂ methanation were demonstrated by the different exposed facets (100, 111) of the metal [74].

2.2. Nickel-Based Catalysts

Supported Ni catalysts are the most widely investigated materials for CO₂ methanation due to their high efficiency in CH₄ production and low cost [75,76,77,78,79,80,81,82,83,84,85,86,87,88,89]. A lot of supports have been investigated for Ni catalysts since, as is well known, the catalytic performance strongly depends upon the nature and properties of the support. Its influence can be generally linked to physico-chemical peculiarities: (i) varying the dispersion of the active phase; (ii) modifying the reducibility of the oxide precursors by tuning the interaction between the active phase and the support.

In this part of the review, we will examine the role of the most investigated supports for the hydrogen reaction with CO₂ to CH₄ (Al₂O₃, zeolites, SiO₂, CeO₂, ZrO₂, and Ce–ZrO₂) with particular emphasis on the role of promoters/modifiers in their catalytic performance in the reaction. Other materials were recently investigated as nickel supports, such as hydrotalcite, carbon nanotubes, and W–Mg oxides. Section 2.2.4 and Section 2.2.5 will be dedicated to the main outcomes reached using these unconventional supports.

2.2.1. Alumina-Supported Nickel

The Ni/Al₂O₃ catalyst shows a high catalytic activity, although it suffers from severe carbon deposition or poor stability due to the high reaction temperature used [88,90]. Therefore, the aim throughout the years was to develop catalysts able to show both high activity and resistance to carbonaceous deposits in the reaction. So, since the pioneering work of Trimm et al. [91], a lot of papers have been published on the argument. We will examine here only the most recent ones, since the number of studies covering the topic is huge, and other previous reviews can easily be found in the literature. Rahamani et al. [92] have prepared, by impregnation, a series of Ni catalysts supported on mesoporous nanocrystalline γ-Al₂O₃, having high surface area and different Ni contents. The influence of calcination temperature was also investigated on CO₂ conversion and CH₄ selectivity. The catalyst with 20 wt % Ni shows higher activity and stability between 200 °C and 350 °C. According to He et al. [89], the combination of highly dispersed Ni particles with a strong basic support is thought to be responsible for the high performance of the Ni–Al hydrotalcite-derived catalyst. Such basic sites are originated from the formation of the Ni–O–Al structure; 100% CH₄ selectivity and 82% CO₂ conversion are reached at 350 °C.

In [93] a study on the methane yield as a function of different Ni-based compositions and reaction temperatures (250–500 °C) is reported. The data suggest that good performance is obtained at the expense of a CO intermediate on the corners of nanoparticles interacting with alumina, likely via an oxygenate mechanism.

In the light of a fluctuating supply of renewable hydrogen, various IR studies, under both stationary and transient conditions, have been done in order to get insight into the reaction mechanism. Since Ni is very prone to oxidation, studies have been carried out under “operando” conditions [94,95,96,97,98].

The results reported in [94] (81% CO₂ conversion, 80% CH₄ yield at 400 °C with a 23 wt % of Ni/CaO/Al₂O₃ catalyst), however, strongly suggest the importance of conducting experiments under realistic (i.e., transient) conditions. To ameliorate the performance of Ni/Al₂O₃ catalysts via substantial modifications of some specific (structural, electronic) properties, the addition of several promoters has been also attempted. Among these, CeO₂ has often been employed on account of some positive effects, such as: (i) improvement of the thermal stability of Al₂O₃; (ii) promotion of the dispersion of the metal onto the support; (iii) change of the properties of the metal due to strong metal–support interaction (SMSI) [41,89,99,100,101].

Moreover, CeO₂ is a well-known oxygen storage material, able to store and release in a reversible manner large amounts of oxygen depending on the experimental conditions adopted [102]. The results show that the presence of CeO₂ has significant influence on CO₂ conversion (for Ni/Al₂O₃, from 350 °C to 400 °C, 45% vs. 71% with a CeO₂ content of 2 wt %). CH₄ selectivity, independently from the CeO₂ content, is higher than 99%. Stability was measured for a reaction time up to 120 h [103]. In [104], the effect of different promoters (CeO₂, MnO₂, IrO₂, La₂O₃) on the catalytic performance of Ni supported on Al₂O₃ mesoporous nanoparticles was investigated. Both CO₂ conversion and CH₄ selectivity were demonstrated to be affected by the different promoters. The catalyst promoted by ceria exhibits the highest activity; also in this case, the catalyst with 2 wt % ceria was the most active and selective to CH₄. The best results were obtained at 350 °C.

Few works have been published on Ni supported on zeolites for CO₂ methanation [49,105,106,107]. Recently, Graca et al. [108] prepared, by ion exchange and impregnation, Ni–Ce catalysts supported on a partially exchanged HNaUSY zeolite.

Since temperatures >800 °C are necessary to reduce the exchanged Ni species, those present on the catalyst prepared by Graca et al. [108] are not in the metal form during the reaction; the activity observed was attributed to the zeolite support. Moreover, addition of ceria causes an improvement in both CO₂ conversion and CH₄ selectivity [41,108,109,110]. Thus, the catalyst properties result from the synergistic effect between the metal active sites and the promoters. The “operando” IR spectroscopy technique was also used to get a better insight into the mechanism of the reaction. According to [111], dissociated hydrogen reacts with carbonates and/or physisorbed CO₂, giving rise firstly to formation of monodentate formates, then carbonyls, and finally to CH₄.

Recently, Rivero-Mendoza et al. reported La–Ni/γ-Al₂O₃ catalytic activity relative to the Sabatier reaction [112], where catalytic studies coupled with in situ DRIFTS analyses provide insights on the CO₂ methanation mechanism. The activation process of the catalyst promotes formation of oxidized Ni particles (Ni²⁺) decorated with LaO_x moieties. During the reaction, in situ reduction of Ni occurs and LaO_x species return to their initial oxidation state (La₂O₃). Since the in situ DRIFTS analyses results show the presence of adsorbed formates and CO species, it is reasonable to think that the Sabatier reaction, catalyzed by the La–Ni/γ-Al₂O₃ catalyst, advances by the CO-based mechanism.

2.2.2. Silica-Supported Nickel

Metal–support interactions are generally present, although at a variable extent in t heterogeneous catalytic systems, implying that different combinations result in different performances in terms of activity and selectivity for a given process [113,114,115]. Among the numerous SiO₂-based materials used as supports for metal catalysts, mesostructured silica nanoparticles (MSN) have recently found wide application in several processes and reactions, such as in drug delivery [116,117], biomedical imaging [118,119], and catalysis [120,121]. Their unique features include (i) nanosized dimension; (ii) ordered structures; (iii) very high surface area; (iv) large pore volume; (v) tunable pore size from 1.5 to 10 nm; (vi) tailorable pore diameters to host particles of different dimensions [122].

Recently, Aziz et al. have studied such materials, promoted by Ni, for CO₂ methanation [123,124,125]. In [126] MSN (mesostructured silica nanoparticles) and Ni/MSN were prepared by sol-gel and impregnation methods. The reaction was carried out at temperatures between 150 °C and 450 °C. Ni/MSN was compared with other types of support; the activity of the reaction follows the order Ni/MSN > Ni/MCM-41 > Ni/HY > Ni/SiO₂ > Ni/γ-Al₂O₃. IR characterization supports the hypothesis that the high activity of Ni/MSN is due to the high concentration of basic sites on account of the presence of both intra- and inter-particle porosity. The carbon species detected on the surface was attributed to the presence of defect sites and/or oxygen vacancies in MSN. The role of Ni sites is that of dissociating H₂ to form atomic hydrogen, which, by reacting with the surface carbon species, forms CH₄. A 200-h endurance test shows no deactivation for the Ni/MSN catalyst.

In [124], various metals were loaded on MSNs (Rh, Ru, Ni, Fe, Ir, Cu, Zn, V, Cr, Mn, Al, and Zr). The catalytic performance measurements clearly indicate that surface centers containing metallic and associated basic and/or oxygen vacancy sites are responsible for the catalytic activity. This latter, at 350 °C, follows the order Rh/MSN > Ru/MSN > Ni/MSN > Ir/MSN > Fe/MSN > Cu/MSN. However, on an areal basis, Ni/MSN exhibits the best performance. In the light of the catalytic activity tests and physico-chemical characterization results, a new, plausible pathway in the mechanism of CO₂ methanation on metal based MSNs is formulated [12,124]. Firstly, CO₂ and H₂ are adsorbed on metal sites, and subsequently dissociate to form CO, O, and H atoms and migrate on the MSN surface. CO interacts with the oxides of MSN and forms bridged and linear carbonyls. Bidentate formate is also formed by interaction with atomic hydrogen. At the same time, the O atom splits onto the MSN surface and is stabilized in the oxygen vacancy near the metal site. Hydroxyls are formed on the MSN surface by interaction of adsorbed oxygen with atomic hydrogen, and, by reaction with another hydrogen atom, water is formed [12,124]. Then, the carbon species is hydrogenated to CH₄. According to the Aziz et al., linear and bridged carbonyl, as well as bidentate formate, could be intermediates during the reaction course, as previously suggested [127,128,129,130,131].

In short, the role of MSN is the seat of sites for carbonyl species that are precursors for methane formation. In [125] the effect of Ni loading and water vapor on the performance and properties of Ni/MSN for CO₂ methanation was investigated. It is well known that metal loading on a supported catalyst significantly affects its dispersion and interaction with the support, in turn affecting the catalytic behavior. In general, the lower the metal loading, the higher its dispersion, and vice versa. In this study, results show that after increasing Ni loading from 1 to 10 wt %, a decrease in crystallinity, surface area, and basicity of the catalysts is observed. On the other hand, the catalytic activity follows the order 10%Ni/MSN ≈ 5Ni/MSN > 3Ni/MSN > 1Ni/MSN at 400 °C with 100% CO₂ conversion. It is then necessary to find a proper balance between Ni loading and basic sites concentration to achieve a high catalytic activity. As for the influence of water vapor on the feed stream, its presence was found to decrease the carbonyl species concentration on the surface of Ni/MSN, thus negatively affecting the catalytic activity of the reaction. It was suggested that such a negative effect could be attributed to the formation of CO₂ through the water gas shift (WGS) reaction between intermediate CO and excess water. Moreover, Ni sintering is favored by the presence of water vapor [132]. Finally, the optimal conditions for the reaction over Ni/MSN were singled out by response surface methodology (RSM). Accordingly, 85% CO₂ conversion could be achieved at 341 °C, 69.105 mL_cat·g⁻¹·h⁻¹ gas hourly space velocity (GHSV), and 3.68 H₂/CO₂ ratio.

Amorphous pure silica materials, well known as mesoporous materials, such as highly ordered hexagonal structure MCM-41, or large pore size and high specific surface area SBA-15 material, are typical supports largely studied by the scientific community for different catalytic applications.

However, few scientific papers have been published on the performance of Ni-incorporated MCM-41 catalysts in the methanation reaction.

In 2007, Du et al. employed C16Ni/MCM-41 catalysts (with C16 indicating a 16 carbon alkyl template producing MCM-41 pores with 2.9 nm diameter) with different amounts of incorporated Ni to be tested for CO₂ methanation [126]. A 3 wt % Ni catalyst at T = 300 °C shows 100% methane selectivity, although a very low CO₂ conversion (about 10% max) is observed. In our opinion, the main drawback of this type of support is due to its limited stability in the presence of steam. Since water is one of the products of methanation reactions, these supports should be useful only for catalyst activity at very low temperatures.

Very recently, Xin et al., from the Chemical Engineering School of Shanghai, studied mesoporous materials as a support for nickel catalysts and their application in CO methanation [7,133]. Both catalysts tested by Xin and co-workers—Ni-SBA-15, prepared by a traditional impregnation method [7], and Ni-MCM-41, synthesized by a hydrothermal synthesis method [133]—show promising behavior in CO methanation.

Methanation of producer gas (CO, H₂, CO₂, and other trace species [134,135] was also recently investigated on NiO/SBA-15 [136,137,138,139]. Firstly, CO₂ conversion increases on increasing the H₂/CO₂ ratio, up to a value of 6, and the maximum CO₂ conversion increases on increasing the NiO loading regardless of the preparation method (heat treatment (HT) or solvent impregnation (SI)) [140]. However, on using the HT method the NiO particles are preferentially dispersed outside the SBA-15 pores, whereas by the SI method they are included into the SBA-15 pores. As expected, the maximum CO₂ conversion and CH₄ yield, regardless of the preparation method, are obtained on increasing the NiO loading.

2.2.3. Zirconia- and Ceria-Supported Catalysts

The interest in ZrO₂ as a support for catalysts to be used in CO₂ methanation is mainly due to its acidic/basic characteristics and CO/CO₂ adsorption capability [141,142,143]. ZrO₂ exists in three polymorphic structures (monoclinic (m), tetragonal (t), and cubic (c)). The monoclinic phase is thermodynamically the most stable below 1000 °C; the other phases are stabilized at such lower temperatures by doping the solid phase with low valent cations. Polymorphs of ZrO₂ greatly influence their catalytic properties. For CO₂ methanation, Ni/t-ZrO₂ prepared from amorphous Ni–Zr alloys exhibits very high activity and almost 100% selectivity to methane at 200–300 °C [144,145]. It has been demonstrated that the catalytic activity increases with the amount of t-ZrO₂ [144,146], achieving the maximum methanation rate at 300 °C with a catalyst prepared from amorphous alloy precursors. The catalytic activity can be significantly enhanced on Ni/ZrO₂ catalysts prepared by Ni/Zr/earth elements alloys [147,148,149,150]. In the Ni/Zr/5at%Sm, samarium stabilizes tetragonal zirconia and increases the number of surface nickel atoms [146].

More recently, a highly efficient Ni/ZrO₂ catalyst doped with Yb₂O₃ for co-methanation of CO and CO₂ was developed. Over Ni6Zr2Yb at 300 °C, total conversion of CO and CO₂ was observed. More interestingly, a heat resistance test shows that, after 24 h of reaction at 800 °C, followed by going down to 300 °C, the conversion does not change the 89% value. Characterization results demonstrate that both activity and thermal stability are due to the formation of a (Zr-Yb) oxide having the cubic ZrO₂ structure. The promoting action of Yb₂O₃ was associated with the high solubility of Yb₂O₃ in the ZrO₂ lattice (the ionic radius of Yb⁺³, 0.087 nm, is close to that of Zr⁺⁴, 0.079 nm) [149].

Moreover, a 50% at%Ni–50%at(Zr-Sm oxide) with Zr/Sm = 5 calcined at 650 °C or 800 °C shows high activity for CO₂ methanation. The increase in Sm⁺³ ion content in the ZrO₂ lattice on increasing the calcination temperature leads to an increase in oxygen vacancies. According to the study of CO and CO₂ adsorption on tetragonal and monoclinic ZrO₂, the CO₂ adsorption capacity of m-ZrO₂ is more than one order of magnitude higher than that of t-ZrO₂, and the CO adsorption capacity of m-ZrO₂ is from 5 to 10 times higher than of t-ZrO₂ depending on the adsorption temperature [142]. As a consequence, the rate-determining step for the reaction cannot be the adsorption of CO₂ and CO. In this study, the high activity of Ni/t-ZrO₂ derives from the presence of oxygen vacancies, which allow a strong interaction with oxygen and weaken the C–O bond strength, leading to the enhancement of hydrogenation of CO₂ to CH₄ [150].

The most investigated rare-earth oxide as promoter of Ni–ZrO₂ catalysts for CO₂ methanation was ceria. Ceria was also studied as a single support for efficient Ni catalysts in CO₂ methanation [81,151,152,153,154], although its synergic effect with zirconia seems to increase the nickel performance.

The main role of ceria is attributed to its high oxygen storage capacity and ability to highly disperse nickel [155,156,157,158,159]. The Ce/Zr composition is a factor that has a strong influence on the catalytic performance of ceria-zirconia (CZ) in several types of reactions such as the reduction in three-way catalysts [160], the methane partial oxidation, and ethanol/methane reforming [161,162,163].

In [159] the C/Z composition of the mixed oxides fixes the ratio between the Ni²⁺ incorporated into the CZ fluorite structure and the surface Ni⁰. Ni²⁺ enhances the catalyst specific activity; thus, a compromise between the two nickel species has to be found in order to optimize the catalytic system. Accordingly, a C/Z mass ratio of 60/40 leads to the best Ni²⁺/Ni⁰ ratio, resulting in the best catalytic activity. The mechanism of the reaction was also studied by Ocampo et al. on Ni/CZ under stoichiometric conditions [155,164]. The CO₂ conversion was observed to increase from 250 °C to 400 °C towards the thermodynamic value. Mono- and bidentate carbonates are present already at 150 °C, as detected by FTIR analysis. Thus, from the evidence shown in the work, the main reaction pathway does not involve CO as an intermediate. CO₂ adsorbs on the sites of mild basicity to form covalent carbonates, hydrogen carbonates, and then bidentate carbonates. These species are reduced and hydrogenated by H atoms formed by the dissociation of hydrogen on the surface of Ni⁰ particles to form formates and finally release CH₄. However, a parallel pathway is hypothesized involving CO formed by a redox cycle on reduced ceria.

Pan et al. hold the presence of medium basic sites responsible for the high activity of Ni/CZ with respect to Ni/SiO₂ [165]. The equilibrium CO₂ conversion is reached at 340 °C. CO₂–TPD profiles of Ni/CZ show weak and medium sites, different from Ni/Al₂O₃, which shows weak and strong basic sites; CO₂ adsorbed on strong basic sites cannot desorb from Ni/Al₂O₃ surface until 700 °C. Consequently, the strongly adsorbed CO₂ on the surface does not participate in the reaction carried out from 220 °C to 400 °C. According to [166], CO₂ reacts with surface hydroxyls and surface oxygen to form hydrogen and monodentate carbonate, respectively [164,166,167,168,169,170,171,172]. Formate species could be also important intermediates during the reaction [63,173,174]. Combining CO₂–TPD and FTIR results, medium basic sites promote formation of monodentate formate and its hydrogenation during the CO₂ methanation, in accordance with [165,175]. The tetragonal phases, thought to be responsible for the activity in CZ-supported Ni catalysts, are distinguished into t, t’, and t”, according to the type of tetragonal distortion and its nature. The t phase is a stable one and is formed through a diffusional phase decomposition; t’ is obtained through a diffusionless transition and is metastable. The t” form is intermediate between t’ and cubic [176,177,178,179].

The H₂–TPR profiles reported in [180] show peaks in the temperature range from 400 °C to 600 °C for CZ, attributed to a reduction of surface oxygen and bulk oxygen in Ce_0.5Zr_0.5O₂, respectively [42,181,182], while the peaks around 360 °C are assigned to the reduction of surface oxygen in CZ and of the bulk NiO, having weak interaction with the support. Peaks at 400–600 °C are related to bulk oxygen in CZ and NiO having strong interaction with the support, or Ni ions incorporated into CZ [183]. In Figure 5 the qualitative behavior of TPR profiles (the data from our research are not yet published) of nickel catalysts supported on doped ceria with increasing nickel content is reported.

Takano et al. recently investigated a 50 wt % Ni/(Zr + Ca) catalyst (molar ratios 0.125–0.333) affording 100% CH₄ selectivity with a CO₂ conversion of 90% at T = 400 °C. The comparison with 50 wt % Ni/(Zr + Sm) catalyst highlighted the higher conversion under the same flow rate conditions. The Ca content dependence of the catalytic activity has been related, according to [184], to the number of oxygen vacancies in t-ZrO₂ since calcium is a more effective additive than samarium. Hydrogen is atomically absorbed on Ni, and is able to reduce carbonates to formate, formaldehyde, methoxy species, and methane.

Other relevant papers on supported ZrO₂ and ceria Ni catalysts that must be cited for their significant results are [185,186,187]; moreover, an interesting paper regarding unconventional supports for nickel catalysts has been published recently by Abate et al. [188]. This is a pioneering research work in which quaternary system catalysts have been prepared. In particular, the authors prepared a Ni-based catalyst by wet impregnation technique applied to γ-Al₂O₃ powder as a host for ZrO₂, TiO₂, and CeO₂ oxides. The best catalyst, named Ni/C15, is composed of a quaternary system with 55% γ-Al₂O₃ and 15% of each of the other oxides. The catalytic performance of the Ni/C15 catalyst in CO₂ methanation is superior to that of similar catalysts prepared with lower amounts of quaternary oxides (Ni/C5 and Ni/C10) and to that of nickel supported on commercial γ-Al₂O₃. A larger amount of all four oxides seems to favor the formation of large-sized nickel particles that are more active at lower temperatures. At 300 °C the CO₂ conversion to CH₄ is higher than 80%.

2.2.4. Hydrotalcite-Supported Nickel

With respect to the aforementioned conventional supports, other materials are emerging as potential, and sometime promising, supports for nickel catalyst in CO₂ methanation. Among these, the hydrotalcite-like compounds are noteworthy.

Hydrotalcite-like compounds, also known as layered double hydroxides, are layered structure materials composed of a mixed hydroxide of divalent/trivalent metals. The general representative chemical formula is [M_1−x²⁺M_x³⁺(OH)₂](Aⁿ⁻)_x/n•y H₂O, where M²⁺ and M³⁺ are any divalent and trivalent cations, Aⁿ⁻ is an exchangeable interlayer anion (organic or inorganic), and the water molecules are either crystalline in the lattice or physisorbed on the surface [189].

Several authors have studied these materials as supports for Ni catalysts (Ni–Al/hydrotalcite), comparing their catalytic activity in CO₂ methanation with commercial Ni/γ-Al₂O₃ catalysts. Taking together the Abate et al. [190] and He et al. [191] results, it is possible to display the activity of Ni–Al/hydrotalcite catalysts prepared at three different pH values (12, 10, and 8.7). Both works report a high Ni dispersion on the hydrotalcite support with respect to the conventional catalyst (Ni/γ-Al₂O₃), indicating that this new support is a promising material for preparing stable and well-dispersed Ni catalysts. Even if the amount of loaded nickel on the support ranges between 75 and 80 wt %, the authors demonstrated that the catalysts possess a narrow nickel particle size distribution in the range of 3–9 nm. This means that more reducible nickel species can be obtained. The catalytic performance of Ni–Al–Hydrotalcite catalysts shows a maximum activity at 300 °C, with similar CO₂ conversion and CH₄ yield (85% for catalysts prepared by Abate et al. and 80%–82% for the catalyst prepared by He et al.).

Aluminum is the main metal present in this type of material; therefore, many works on the argument report the effect of the Ni/Al ratio on the catalyst performance. Gabrovska et al. [192] studied the variation of Ni²⁺/Al³⁺ molar ratio effect on the catalytic activity of Ni-Al-Hydrotalcite. The effect on the catalyst performance cannot be defined directly but has to be correlated to other fundamental parameters of the CO₂ methanation process, such as reaction temperature, space velocity, and reduction temperature. The catalyst with the highest amount of Ni (Ni²⁺/Al³⁺ = 3) shows a talovite-like phase and higher conversion after reduction at 400–450 °C, under all reaction temperatures and space velocities tested.

The catalyst with the highest amount of Al (Ni²⁺/Al³⁺ = 0.5), and, of course, the lowest amount of nickel, shows high methanation activity, at 260 °C only after reduction at a high temperature (higher than 500 °C), in order to reduce the NiAl₂O₄ species.

The hydrotalcite material seems to be a promising support for Ni catalysts due to the possibility of hosting a large amount of catalyst (metal) without losing the important peculiarity of high particle size dispersion. The great quantity of aluminum present in these materials must be taken into account because it affects the reducibility of species that are formed with nickel, retarding the nickel metal sintering.

Finally, Wierzbicki et al. modified the redox properties of a hydrotalcite-based catalyst by lanthanum incorporation [193]. The presence of La increases the basicity of the support and, simultaneously weakens the interactions between Ni and the Al–Mg/Hydrotalcite matrix, producing more reducible nickel species.

2.2.5. Other Supported-Nickel Catalysts

Carbon nanotubes were also investigated as nickel supports for CO₂ methanation [194]. However, they do not show innovative properties as supports for Ni catalyst because the catalytic performance of Ni–CNT is high only in the initial stage of reaction but decreases over time. As occurs for other supports, on introducing ceria on the CNTs surface the catalyst activity and stability increase, due to the promoting effect of ceria on nickel species dispersion and their reduction [195].

Recently, a synergic effect between Ni and a W–Mg mixed oxide has been reported by Yan et al. [195]. The researchers studied a series of W-doped Ni–Mg mixed oxide catalysts in CO₂ methanation, demonstrating their excellent activity, CH₄ selectivity, and stability up to 450 °C. Particularly, the maximum CO₂ conversion is attained when the W:Ni molar ratio in the final catalyst is 1:1.

W, in fact, plays an important role in the catalyst performance: its presence promotes the formation of CO₂ adsorption sites, enhancing the catalyst activity and simultaneously favoring the Ni–Mg interactions, producing very sintering-resistant Ni species, and leading to a very stable NiWMgO_x catalyst.

2.2.6. Nickel–Iron-Based Catalysts

The last part of this review is entirely dedicated to catalysts active in the CO₂ methanation reaction based on Ni–Fe systems, either as alloys or ferrites. The first papers on the application of these classes of catalysts in the reaction date back to the early 1990s, and recently they have found renewed interest due to the possible ways of exploiting CO₂ for obtaining methane as a promising energy source for methane production and/or in Fischer–Tropsch Synthesis (FTS).

Recently, Merkache et al. published a scientific work in which the activity of iron catalyst in the CO₂ hydrogenation was reported [196]. They tested a mesoporous-supported iron catalyst, Fe–KIT-6, synthesized for the first time with high iron loading. The high density of the iron sites strongly improves the CO₂ methanation selectivity of the catalyst at high temperatures [196]. However, it is well known that the activity and selectivity of iron catalysts are a function not only of the iron loading but also of the oxidation state of iron atoms.

Magnetite and M(II)-bearing ferrites form a family of oxygen-deficient spinel ferrites represented by the general formula M_xFe_3−xO_4−δ, where δ represents the reduction degree of the ferrite. The oxygen deficient ferrite (ODF) is formed by hydrogen reduction at 300 °C [197]:

M_xFe_3−xO₄ + δH₂ = M_xFe_3−xO_4−δ + δH₂O.

ODF decomposes CO₂ to carbon and oxygen. Carbon deposits on the ferrite surface, while oxygen penetrates the lattice of the ODF; therefore, carbon reacts with H₂ to form CH₄ at 200–300 °C [198,199,200,201,202]. Several ferrites (M_xFe_3−xO₄ where M = Fe, Ni, Co, Cu, Zn, Mg, Mn) have been investigated The most active results are for the Ni_xFe_3−xO₄, with over 86% CH₄ yield at 200–300 °C [199,202,203].

In [197] ultrafine Ni(II) ferrite (UNF) with 36% Ni²⁺ substitution is synthesized and tested at 300 °C in CO₂ methanation. The yield of CH₄ obtained on the UNF catalyst was 1.5–6.0 times (depending of GHSV) larger than that of the nickel ferrite (NF) catalyst. Furthermore, investigations on UNFs have mainly regarded methanation by using waste heat in stationary sources such as power plants and carbon recycling systems in flue gas in LNG power plants [204,205].

From 2004 to 2007 an interesting series of papers appeared in the literature [206,207,208,209] that, starting from fundamental concepts of heterogeneous catalysis, were applied by the authors to the methanation reaction. According to [210], one of the fundamental results is the well-known volcano curve. This is obtained when the activity of catalysts, for a given reaction, is plotted as a function of a parameter related to the ability of the surface to form chemical bonds with reactants, intermediates, or products [211,212].

Starting from the point of view that several elementary reactions at metal surfaces show a linear Bronsted–Evans–Polanyi (BEP) relation [213] between the activation energy and the reaction energy, and reactions belonging to the same class follow the same relation, Bligaard et al. [206] investigated the implications of this finding on the kinetics of surface catalyzed chemical processes.

For the scope of the present review, the analysis presented in [206] can be used to understand the volcano curve for the methanation reaction, although related only to the CO methanation (Figure 6). Secondly, such analysis deals only with the catalytic activity; for reactions where selectivity plays a role, it cannot be used.

DFT (Density Functional Theory) is used to describe semi-quantitatively the kinetics of catalytic reactions on the basis of thermochemistry and activation energies [214,215,216].

Following the previous study [206], in [207] a screening of several intermetallic alloys to be used for CO methanation has been reported including experimental results. According to the authors, the two important characteristics of a catalyst surface in determining the rate of the reaction are the barrier for CO dissociation and the stability of the atomic C and O intermediates. The BEP relation reported in [207] shows the volcano dependence on the rate of E_diss [206]. For weak adsorption at the right in the volcano curve the barrier for dissociation is high, thus limiting the reaction rate; on the other branch (left) a strong adsorption leads to a low rate of removal of adsorbed C and O to from the reaction products. The interpolation model used in the study suggests a potential candidate for improving the methanation reaction use of alloy catalysts formed by combining the elements on the left and right branches of the curve. Moreover, a multi-objective optimization target is included in the investigation; in the economics literature, a typical solution is represented by the Pareto-optimal set [217,218].

In [207] it is clearly shown that, taking into consideration all the above reported considerations, Ni–Fe alloys have to be considered the best candidates for an “all-inclusive” material of choice for the reaction. The experimental results fully confirm the predictions from the computational screening. On the other hand, Ni–Fe alloys, also tested for CO₂ methanation, have not shown results concordant with those here discussed [219,220]. This, in our opinion, could be due to the different operational conditions adopted in the investigations.

The same approach (computational screening/experimental verifications) was followed in [208] for both CO₂ hydrogenation and simultaneous CO and CO₂ hydrogenation. The former reaction was carried out at 250 °C with a gas mixture containing 9% CO₂ in 91% H₂. The latter was conducted with a gas containing 2% CO, 2% CO₂, and 96% H₂ at 220–230 °C. It has been shown that the conversion of CO₂ to methane is significantly higher over the Ni–Fe alloy catalyst in comparison to pure Ni. For the simultaneous CO–CO₂ hydrogenation, the best catalysts have a Ni/Fe ratio >1. However, the pure Fe catalyst shows a lower activity than pure Ni.

In the last study of the series [209], some mono- and bi-metallic Ni–Fe catalysts supported on MgAl₂O₄ and Al₂O₃ were tested in CO methanation (reaction conditions: 2% vol. CO in H₂, 25,000 h⁻¹ GHSV, reduction at 500 °C in CO/H₂ mixture for 4 h, reaction temperature in the range 200–300 °C).

Bimetallic catalysts with compositions 25Fe75Ni and 50Fe50Ni show better activity compared with monometallic Ni and Fe catalysts. The selectivity to CH₄ was found to increase at higher CO conversions and higher Ni loading in the catalysts. The maximum catalytic activity and the highest selectivity to methane are observed for a 20 wt % total metal loading on MgAl₂O₄.

DFT was recently applied to investigate the three different mechanisms of CO₂ methanation on Ni(111) surfaces with and without formation of CO as an intermediate [221]. The three mechanisms fall into two categories [222,223,224]: conversion of CO₂ to CO prior to methanation involving a CO intermediate [225,226], and direct hydrogenation of CO₂ to methane without the formation of CO as an intermediate [63,127,227,228].

On the basis of the calculated energy barrier values for each of the three paths and the correlated determination of the rate determining step (r.d.s.), path 2 (the decomposition of CO₂ into CO and O, further decomposition of CO into C and O (r.d.s), and hydrogenation of C to CH₄)) appears to be the most accredited mechanism.

Other recent relevant studies devoted to Ni–Fe-based catalysts for methanation reactions regarded the effect of the Fe content on Al₂O₃-supported catalysts in the co-methanation of CO and CO₂ [229]; the influence of the second metal for CO hydrogenation [230]; the effect of the preparation method [231]; and the effect of the addition of a noble metal (Ru) [232] on the activity and selectivity of the reactions investigated.

In [229] the maximum carbon conversion and CH₄ selectivity on Ni_xFe_1−x/Al₂O₃ (20 wt % metal content on the support) was observed for the Ni_0.7Fe_0.3/Al₂O₃ catalyst. Higher Fe content causes a decrease in C conversion and CH₄ productivity because of the occurrence of WGS reaction and C₂–C₅ hydrocarbon formation.

Alumina xerogel (AX)—used as a support of Ni catalysts with the addition of a second metal M (M = Fe, Ni, Co, Ce, La)—was tested in CO hydrogenation (feed composition CO:H₂:N₂ = 1:3:1.7) at 230 °C and 10 bar. Both CO conversion and CH₄ yield decrease in the order 30Ni10FeAX > 30Ni10NiAX > 30Ni10CoAX > 30Ni10CeAX > 30Ni10LaAX. The low performance exhibited by both Ce and La as a second metal appears to denote that transition metals (Fe, Co, Ni) are more effective in promoting the reaction than lanthanides (Ce, La). Moreover, on the basis of temperature-programmed surface reaction (TPSR) experiments, the higher performance of 30Ni10FeAX was attributed to the optimal CO dissociation energy and the largest H₂ adsorption [230]. Following the latter two studies discussed above, Hwang et al. investigated, on the same catalytic system (AX-supported Ni-Fe), the effect of the precipitation agent, on the catalysts prepared by coprecipitation, on methane production from CO₂ and H₂. The precipitation agent used ((NH₄)₂CO₃, Na₂CO₃, NH₄OH, or NaOH) has a decisive role on the catalytic performance since it influences the extent of the active surface area. Therefore, among the catalysts tested, NiFeAl-(NH₄)₂CO₃, affording the smallest particle size, shows the best results in terms of CO₂ conversion and CH₄ yield [231].

The alumina xerogel support was again used to investigate the effect of the addition of different ruthenium contents (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) on the activity of the same reaction; in operative conditions the feed composition is CO₂:H₂:N₂ = 1:4:1.7, P = 10 bar, T = 220 °C. A close correlation between the metal surface area and the amount of desorbed CO₂ (measured by CO₂–TPD) was found. Both CO₂ conversion and CH₄ yield show a well-defined volcano-shaped trend with respect to Ru content. Furthermore, the CH₄ yield increases on increasing the metal surface area and the amount of desorbed CO₂ [232].

Bimetallic Ni–Fe catalysts were tested in the CO methanation reaction for the production of substitute natural gas (SNG) under industrial (0.1–0.3 MPa) total methanation conditions to be adopted in processes related to the Fischer–Tropsch synthesis [232,233,234,235,236]. Thus, in [5] the activity of bimetallic Ni₃Fe/γ-Al₂O₃ was first compared with that of a reference Ni/γ-Al₂O₃ under conditions of 3.0 MPa and H₂/CO = 3.1. Ni₃Fe/γ-Al₂O₃ achieves the 100% CO conversion at 225 °C, whereas Ni/γ-Al₂O₃ reaches total CO conversion at 275 °C. Also, in the high temperature range of 500–600 °C, the CO conversion of the bimetallic catalyst is higher than that of the monometallic catalyst, with 99% selectivity to CH₄ from 300 °C to 450 °C. The influence of the Ni/Fe ratio on the catalytic activity of a number of bimetallic Ni–Fe catalysts in CO methanation at 0.1 MPa was also investigated. Ni₃Fe_1.8/γ-Al₂O₃ shows the lowest CO conversion and CH₄ selectivity, even lower than that of Ni/γ-Al₂O₃. Clearly, the higher Fe content is responsible for the poor catalyst performance. Comparing all the results obtained in high-pressure methanation test with the low-pressure experiments reported in [5], Tian et al. conclude that the performance of Ni–Fe catalysts is not determined by the content of individual Ni or Fe, but by the synergistic effect of the two metals in the catalyst. Another process able to produce CH₄ has been reported to be the decomposition of CO₂ to carbon and oxygen [237,238,239,240]. In this regard, ferrite catalysts have been previously used for this reaction and being non-stoichiometric or oxygen-deficient materials are able to decompose water to hydrogen and CO₂ to carbon [241,242,243,244,245].

In [246] the decomposition of CO₂ was carried out on the nanophase NiFe₂O₄ (NFN, Ni_xFe_3−xO_4−δ, 0 < δ < 1) at 300 °C to obtain CO, CO₂, or CH₄. After the CO₂ decomposition, the reactor was flushed with hydrogen to carry out the methanation process. The two reaction routes for decomposing CO₂ by oxygen-deficient ferrites are: (i) decomposition to carbon and oxygen; (ii) decomposition into carbon monoxide and oxygen [247,248,249,250,251].

The experimental results reported in [246] clearly show that the prepared NFNs were effective at decomposing CO₂ into carbon and oxygen after reduction by hydrogen. In conclusion, the overall mechanism of CO₂ decomposition over NiFe₂O₄ nanoparticles can be summarized as follows: (i) H₂ gas decomposes and produces the water molecule, making the NiFe₂O₄ oxygen-deficient; (ii) the oxygen-deficient NiFe₂O₄ adsorbs oxygen atoms from CO₂ and produces carbon on the surface or CO when NiFe₂O₄ is less active; (iii) the produced carbon reacts with H₂ and produces CH₄ by methanation.

A new interesting iron source for supported bimetallic Ni–Fe catalysts preparation has recently been proposed by Wang et al. [252], and consists of using calcined olivine as a promising Ni support for CO₂ methanation. Before this study, the activity of calcined olivine supported nickel has been demonstrated in gasification [253] and steam reforming [254] of biomass. Olivine is a magnesium iron silicate with the general formula (Mg_xFe_1−x)₂SiO₄ and is a common Earth mineral. The peculiarity of the support consists in the possibility of allowing interaction between NiO and olivine structure to lead to a better dispersion of Ni species. This effect is promoted by the simultaneous presence of iron and MgO. First of all, Wang et al. describe the effect of the support calcination on iron species. They observed that iron was extracted by the olivine structure during the calcination process so that more reducible iron species can be formed (analyzed by usual TPR experiments). The amount of reducible iron species increases on increasing the temperature to 800 °C. Form 800 °C to 1200 °C a reversal trend was registered, probably due to the reintegration of extracted iron into the olivine structure. Over 1200 °C the reducible iron amount increases again and MgSiO₃ species are formed, as confirmed by the XRD spectrum. TPR profiles of Ni/olivine catalysts obtained by calcining the support at different temperatures show that for the catalyst calcined at lower temperature nickel is just deposited on the olivine structure with a weak interaction as highly dispersed particles. On the contrary, for catalysts calcined at temperatures >900 °C, a NiO–MgO solid solution phase and grafted NiO species were formed. The best catalyst, obtained by calcination of the support at 350 °C and reduction at 500 °C, is able to reach, under tested conditions, a CO₂ conversion of 98.4% and a CH₄ selectivity of 99.9%.

Important and very recent results that confirm the role of iron on the dispersion of NiO nanoparticles, increasing the quantity of the reduced active nickel species, were reported by Lu et al. regarding zirconia-supported doped nickel catalysts [255] and Pandey et al., regarding five series of Ni–Fe bimetallic supported catalysts [256]. The morphology of Ni–ZrO₂ and Ni–Fe/ZrO₂ catalysts has been compared. Using TEM images, it is possible to notice that the average size of NiO particles on the Ni–ZrO₂ catalyst is higher than the average size of the NiO particles dispersed on Ni–Fe/ZrO₂ catalyst [255].

3. Industrial Applications and Concluding Remarks

The use of carbon dioxide as a feedstock for fuels, and for energy in general, is a promising route that permits adherence to main international protocols on environmental protection. Hydrogenation is an appropriate way to reduce CO₂ and provide a range of substances compatible with existent technologies. The methane produced in this way could be directly injected into already existent pipeline networks or storage infrastructures. Obviously, the main challenge that ensures an environmentally friendly process is obtaining hydrogen from renewable sources and energy. Water electrolysis promoted by renewable energy (wind, solar, etc.) is an economic and clean method of H₂ production. Hydrogen can be directly injected into a natural gas grid or combined with CO₂ by a methanation process.

Energy production by these ways is the basis of the Power-to-Gas concept (P2G). However, currently, this is only a model: there are demo-plants and industrial-scale plants that demonstrate the possibility of producing energy by gas. Germany seems to be at the cutting edge in the P2G technology that provides SNG (Substitute or Synthetic Natural Gas) as a product, since 2009.

In Stuttgard there is a demonstration Power-to-methane plant (250 kW power input as capacity) founded by ZSW (the Center for Solar Energy and Hydrogen Research) by ETOGAS GmbH. ETOGAS, in Wertle, Germany, is an industrial-scale Power-to-methane plant (6300 kW power input as capacity) that produces methane for Audi. The produced methane, named Audi-e-gas, comes from the CO₂ obtained by a waste-biogas plant.

More recently, the Karlsruhe Institute of Technology (KIT) coordinated the innovative HELMETH project (Integrated High-Temperature Electrolysis and Methanation for Effective Power to Gas Conversion) [257]. The project, co-financed by the EU’s Seventh Framework Program, has the objective of proving the highly efficient Power-to-Gas (P2G) technology by thermally integrating high-temperature electrolysis (SOEC Technology) with methanation. The expected efficiency should be higher than 85%, superior to the efficiency for hydrogen generation via conventional water electrolysis. In practice, high-temperature electrolysis (endothermic) and methanation (exothermic) can be coupled and thermally integrated with increasing globally conversion efficiency.

At the same time, the Electrochaea Company has developed a commercial power-to gas system solution using a microorganism (methanogenic archaea) as biocatalyst, patented as BioCat, for a methanation technology to operate at lower costs and with higher flexibility than conventional thermochemical methanation processes [258]. Archaea BioCat exhibits several properties including high mass conversion efficiency and high tolerance to many contaminants typically present in industrial CO₂ sources, such as O₂, H₂S and particulates that can poison common catalysts. Moreover, archaea are highly selective towards methane formation, so minimal post-reaction gas treatment is needed before injecting the produced gas into the gas grid. The power-to-gas technology of the Electrochaea system uses an electrolyzer to produce hydrogen, which is fed into a bioreactor containing the archaea along with carbon dioxide from a biogenic (anaerobic digesters, fermentation plants) or industrial source (fuel gas from combustion processes). In February 2014, Electrochaea began its first commercial-scale demonstration project BioCat: a 1 MW power-to-gas plant based on biological methanation.

High flexibility, reduced technological cost, and high process efficiency make biomethanation technology an attractive source of renewable energy. The global evaluation of biomethanation assets should also include the Life Cycle Sustainability Assessment (LSCA) methodology. Kristjanpoller et al. have published an interesting work on the biomethanation plant assessment [259]. The work pinpoints two important concepts that will guide research and generate proposals for industrial plant improvement: effectiveness and efficiency. The first is related to the extent to which the objectives of the process are met; the second is the measure of the economy in which the resources of the company are used, including the cost of the catalyst.

In any case, even if the mechanism that currently promotes methanation on nanosized metal catalysts, as well as the best-performing catalyst system and the effectiveness of industrial power-to-gas plants are under scientific debate, we can say without a doubt that the commercialization of CO₂ methanation processes plays a pivotal role in the future of environmentally friendly energy systems.