Advancements in Cobalt-Based Catalysts for Enhanced CO₂ Hydrogenation: Mechanisms, Applications, and Future Directions: A Short Review

Abstract

In 2020, China put forward the national energy and economic development strategy goal of “carbon peak and carbon neutrality”; in this context, the hydrogenation of carbon dioxide into clean energy and high-value-added chemicals can effectively alleviate the current environmental pressure. This process represents a crucial avenue for the advancement of green energy and the realisation of a sustainable energy development strategy. Among the efficient catalysts designed for CO₂ hydrogenation reactions, transition metal cobalt has garnered extensive attention from researchers due to its relatively abundant reserves and low economic cost. This paper first introduces the thermodynamic process of carbon dioxide hydrogenation and discusses methods to improve the efficiency of the catalytic reaction from a thermodynamic perspective. It then briefly describes the reaction mechanism of cobalt-based catalysts in the carbon dioxide hydrogenation reaction. Based on this understanding, this paper reviews recent research on the application of cobalt-based catalysts in the hydrogenation of carbon dioxide to produce methane, hydrocarbon chemicals, and alcohols. Finally, the methods to improve the catalytic efficiency of these catalysts are discussed, and future research directions are proposed.

1. Introduction

As the world’s population grows, the economy develops, and the standard of living improves, the demand for fossil energy is increasing rapidly. This heightened consumption of fossil energy has led to a significant rise in CO₂ emissions into the atmosphere. The insulating effect of CO₂ contributes to climate issues such as the greenhouse effect, sea level rise, and acid rain, posing threats to the survival of humans, wildlife, and plants [1]. Historically, nature maintained a balance between the production and consumption of atmospheric CO₂ through the absorption and dissolution by land and oceans. However, increased human activities and environmental degradation have disrupted this balance, resulting in a steady rise in atmospheric CO₂ concentrations [2,3,4]. Balancing CO₂ emissions with absorption while meeting energy needs is a critical challenge. In response, scientists have proposed carbon capture, utilisation, and storage (CCUS) technology pathways, as shown in Figure 1.

CCUS, which encompasses the capture of emitted CO₂ from industrial processes or directly from the atmosphere, its subsequent transportation to a suitable storage site, and its permanent underground storage or utilisation for enhanced oil recovery (EOR) or other industrial applications [5,6], represents one of the most effective contemporary strategies for achieving chemical resource utilisation of CO₂. At present, it has been extensively documented as a means of retrofitting thermal power generation, petrochemicals, plastic recycling, blast furnace steelmaking, and numerous other industrial sectors [7,8,9]. At this juncture, the cost of carbon sequestration in China is approximately RMB 500/tonne. The low carbon price is insufficient to offset the expense of capture and sequestration, and the conventional CCS technology necessitates optimal geological conditions, which presents a challenge in terms of economic viability [10,11,12]. It is therefore more economically viable to utilise high-purity carbon captured through carbon capture technology as a raw material for the manufacture of industrial products, including plastics, oil, and natural gas [13]. One such avenue of research is the utilisation of CO₂ hydrogenation for the production of fuel, which offers the dual benefit of carbon recycling and the alleviation of the global fossil energy crunch.

The CO₂ hydrogenation reaction necessitates the availability of a substantial quantity of cost-effective and environmentally benign hydrogen sources. In recent years, the advent of novel energy generation technologies has led to a significant expansion in the utilisation of renewable energy sources, including hydro, geothermal, wind, and solar, for electricity generation. The resulting electricity is employed in the production of hydrogen through the electrolysis of water, thereby facilitating the greening of hydrogen sources. Hydrogen produced from renewable energy sources can be converted via heterogeneous catalysis into high-value-added chemicals (including gasoline, diesel, methane, methanol, ethanol, and other advanced alcohols). This process not only mitigates the ecological and environmental problems caused by excessive CO₂ emissions from human activities but also provides a green liquid fuel. High-purity hydrocarbon fuels, upon combustion, produce CO₂ and H₂O, thus achieving a sustainable carbon cycle of “energy–CO₂–energy”. This approach holds significant practical importance and promising application prospects for “green development”, combining environmental protection and energy savings [14].

To date, a large number of homogeneous and heterogeneous catalysts have been reported for CO₂ hydrogenation [15]. Under relatively mild reaction conditions, homogeneous catalysts show high activity and selectivity in CO₂ hydrogenation, and the reaction mechanism is easy to be studied because of the single active centre; the reaction products are mainly formic acid, methanol, methane, etc. [15,16,17]. However, they also have the disadvantages of difficult separation of catalyst and substrate and short service life. Compared with homogeneous catalysts, heterogeneous catalysts have the advantages of easy recycling, longer service life, and higher stability, which are favourable for industrial applications [15,18]. Therefore, heterogeneous phase catalysis has been more widely studied. Heterogeneous catalytic reactions for CO₂ hydrogenation are mainly divided into electrocatalysis, photocatalysis, and thermal catalysis. The first two methods are challenging to achieve industrial production in the near future due to their low efficiency. Therefore, this article focuses exclusively on the thermal heterogeneous catalytic process for CO₂ hydrogenation. The CO₂ hydrogenation reaction catalysed by multiphase catalysts represents a current research focus, with metals including Cu, Ni, Co, In, and Fe having been widely reported as effective CO₂ hydrogenation catalysts. In recent years, numerous reviews on CO₂ hydrogenation reactions have been published [19,20,21]. Among them, Co has a better CO₂ activation ability at low temperatures, has relatively high stability, and is relatively inexpensive, which has received extensive attention from researchers [22,23,24]. Due to the excessive hydrogenation capacity and lack of RWGS activity, Co catalysts have obvious advantages for methanation on Co catalysts and have better performance at low temperatures compared to Ni and Fe catalysts [23,25,26,27,28]. In recent years, we have found that Co-based catalysts have made a breakthrough in obtaining products such as low-carbon olefins, long-chain alkanes, and high-carbon alcohols through the addition of additives, the establishment of interactions with carriers, or the formation of alloy phases [22,29,30,31]. Meanwhile, the sintering resistance and catalyst lifetime were also improved by forming Co₂C active sites or constructing tandem catalysts [32,33,34].

However, few authors have conducted independent reviews on the performance of Co-based catalysts in CO₂ hydrogenation reactions. Accordingly, this paper presents a review of the research progress on thermally catalysed CO₂ hydrogenation over Co-based multiphase catalysts. Firstly, the CO₂ hydrogenation reaction pathways and potential Co-active sites of the catalysts are discussed. Subsequently, the research progress of cobalt-based catalysts for methane, hydrocarbons, and alcohols is summarised in conjunction with the design ideas of the catalysts in terms of additives addition, morphology modification, and size control, among other factors. Finally, this review addresses the challenges associated with the application of CO₂ hydrogenation reactions using Co-based catalysts, as well as potential strategies for addressing these challenges. This review offers insights that can inform the design and synthesis of new, highly efficient catalysts.

2. Thermodynamic Considerations in CO₂ Hydrogenation

As the highest oxidation state of carbon, CO₂ has a high enthalpy of formation (~394 kJ·mol⁻¹), making it thermodynamically unfavourable for chemical transformations. Therefore, H₂, with a high Gibbs free energy of formation, is usually added to CO₂ transformations to supply the necessary energy for the reaction and facilitate the process [35,36,37]. For instance, the enthalpy change for the reaction of CO₂ to CO is significantly reduced when H₂ is added, as shown in the following equations (Equations (1) and (2)):

CO₂ → CO + 1/2O₂
(ΔH^°_298K = 293.0 kJ/mol  ΔG^°_298K = 257.2 kJ/mol)

(1)

CO₂ + H₂ → CO + H₂O
(ΔH^°_298K = 41.2 kJ/mol  ΔG^°_298K =28.6 kJ/mol)

(2)

CO₂ hydrogenation was first proposed by Charles Ross Prichard and Paul Sabatier in the early 20th century. They identified that the high activation energy barrier of the carbon–oxygen double bond in CO₂ was the main obstacle to the hydrogenation reaction. The researchers used catalyst design ideas such as optimising metal–carrier interaction (MSI), constructing symmetry-breaking active centres, and regulating the number of oxygen vacancies on the surface, which lowered the thermodynamic energy barriers to carbon dioxide activation, facilitated the carbon dioxide hydrogenation reaction, and regulated the distribution of reaction products [38,39,40]. Over the past century, researchers have explored and developed various catalyst systems to achieve high selectivity for a wide range of products such as methane, methanol, long-chain hydrocarbons, and higher alcohols [22,41], while also investigating possible reaction pathways.

CO₂ emitted from industrial activities and H₂ produced from clean energy sources follow different reaction pathways on various catalysts, resulting in distinct product compositions (Figure 2). Methane is the most readily obtained product of the reaction. Among the many methanation catalysts, Ni-based catalysts have been extensively studied in industry due to their low cost and availability [42]. However, at high reaction temperatures, Ni particles are prone to sintering or carbon deposition, leading to deactivation.

In the realm of methanol production, copper is the most extensively studied metal. Cu/ZnO/Al₂O₃ catalysts have been widely implemented in industrial production, yet they also suffer from issues such as sintering or carbon deposits, which shorten the catalysts’ lifespan [43,44]. Consequently, researchers have investigated alternative transition metal and noble metal catalysts for the efficient and stable production of C₁ products. Among these, Co catalysts have emerged as a promising option due to their low cost, compatibility with mild reaction conditions, ability to catalyse reactions at low temperatures [23,45], and resistance to high-temperature sintering [26]. Additionally, they offer a longer catalytic lifetime, enhancing their potential for practical applications [33].

The significant potential of Co-based catalysts lies in their ability to promote carbon chain growth. This is particularly relevant in the synthesis of long-chain compounds, which often require the use of methanol or CO as platform molecules for conversion to higher carbon products via Fischer-Tropsch Synthesis (FTS) and methanol-to-hydrocarbon (MTH) processes [38,46]. It is possible to modify FTS catalysts (e.g., Co-based, Fe-based catalysts) or MTH catalysts to combine the reverse water–gas shift (RWGS) reaction with FTS and high-temperature methanol synthesis with MTH, thereby producing C₂₊ hydrocarbons through the direct hydrogenation of CO₂.

The synthesis of high-carbon alcohols is more challenging than that of C₂₊ hydrocarbons. Since the 1980s, researchers have systematically investigated the catalysts used in the hydrogenation of CO₂ to high-carbon alcohols, focusing on four principal metals: Rh, Cu, Mo, and Co [47].

On the technical side, scholars generally agree that the activation (hydrogenation) of carbon dioxide is more susceptible to kinetics rather than thermodynamics [48]. For example, thermodynamically, the number of molecules in a reverse water–gas reaction (RWGS) does not change during the course of the reaction, so increasing the pressure has no effect on the CO yield (Equation (2)).

In contrast, the hydrogenation of CO₂ to methanol is a thermodynamically exothermic process (Equation (3)) and is thermodynamically favourable. Furthermore, since the reaction of CO₂ hydrogenation to methanol involves a reduction in the number of molecules, high pressure favours the conversion of CO₂ and the high selectivity of methanol. Considering the Gibbs free energy, the synthesis of ethanol from CO₂ is thermodynamically more favourable, with a free energy change of less than zero (Equation (4)). This suggests that the reaction is more likely to proceed in a forward direction under identical conditions.

Additionally, studies have indicated that the ratio of CO₂ to H₂ in the feedstock gas significantly influences the reaction efficiency. For CO₂ hydrogenation to C₁ products, a higher H₂ content in the feedstock gas correlates with a higher CO₂ conversion rate and product selectivity [38].

CO₂ + 3H₂ → CH₃OH + H₂O
(ΔH^°_298K = −49.5 kJ/mol  ΔG^°_298K = 3.5 kJ/mol)

(3)

CO₂ + 3H₂ → 1/2 CH₃OH + 3/2 H₂O
(ΔH^°_298K = −86.7 kJ/mol  ΔG^°_298K = −32.4 kJ/mol)

(4)

In conclusion, the incorporation of catalysts or alterations to thermodynamic parameters can enhance the efficiency of CO₂ hydrogenation. This paper presents a review of Co-based catalysts capable of efficiently converting carbon sources.

3. Mechanism of Reaction Co-Based Catalysts

3.1. Detailed Pathways of CO₂ Hydrogenation and Catalyst Behaviour

The researchers have investigated the reaction mechanism based on the characterisation results obtained by in situ XRD, in situ DRIFTS, in situ FT–IR spectroscopy, and other techniques, combined with simulation calculations. They have proposed several potential reaction pathways for CO₂ hydrogenation over Co-based catalysts. This paper outlines the most prevalent reaction pathways.

For C₁ products such as methane and methanol, three principal pathways are observed [36,49,50,51], as illustrated in Figure 3. One pathway is the formate pathway, where CO₂ hydrogenation produces *HCOO intermediates. These intermediates undergo further cleavage of the C–O and C–H bonds to produce *CH_xO, which is subsequently hydrogenated to produce CH₃OH. Alternatively, the hydrogenation of the C–O bond after it is broken again can produce *CH_x, leading to the formation of CH₄. Zhang et al. employed a phosphorylation strategy to modify the surface of a structurally stable layered double-hydroxide-derived Co-Al catalyst (CoAlLDH) for the hydrogenation of CO₂ to generate methanol [32]. The results of in situ DRIFTS and DFT calculations (Figure 4) demonstrated that the reaction proceeded via the formic acid pathway. The phosphorylated surface oxygen vacancies underwent significant electron transfer, facilitating the direct hydrogenation of the key intermediate *H₃CO to methanol by inhibiting the cleavage of the C–O bond in *H₃CO.

The second pathway is the RWGS + CO-hydro pathway, where CO₂ is first hydrogenated to produce *HOCO intermediates. The C–O bond in the intermediates is then broken to produce CO, which can either be directly desorbed as CO or further hydrogenated to form *CH_xO intermediates. These intermediates continue to be hydrogenated, resulting in the formation of C₁ products. Lu et al. investigated the kinetic and thermodynamic behaviour of Co/MoS₂ single-atom catalysts in the CO₂ hydrogenation reaction [52]. Theoretical studies were conducted to elucidate the kinetic and thermodynamic behaviour of CO₂ reduction reaction on Co/MoS₂ through the utilisation of DFT simulations, transition state calculations, and reaction rate constants for a multitude of potential pathways. The findings indicated that the optimal pathway for the reaction of CO₂ with hydrogen is the combination of the RWGS and CO hydrotransformation. The entire reaction pathway can be summarised as *CO₂ → *CO → *CHO → *CH₂O → *CH₂OH and *CH₃O → CH₃OH, as shown in Figure 5. The third pathway is the direct hydrogenation pathway. In this pathway, a C–O bond in CO₂ breaks to form *CO, which either dissociates to CO or undergoes further hydrogenation to form methane.

For the synthesis of C₂₊ products, a large number of studies have found RWGS to be a prerequisite step in the reaction [53,54,55,56]. The key to the formation of C₂₊ products is the insertion of *CH_x into other intermediates [50,57]. The intermediate *CH_x is formed mainly by the conversion of *CO and *HCOO, and the insertion of one or more *CH_x into another intermediate results in the formation of a C–C bond, which lengthens the carbon chain [58,59]. Co-based catalysts are excellent FTS catalysts, capable of promoting the growth of the product carbon chain in FTS, but have difficulty in promoting the RWGS reaction. This provides ideas for the design of efficient catalysts and it is common practice to develop tandem catalysts with the aim of producing C₂₊ hydrocarbons via the RWGS + FTS pathway and high-carbon alcohols via the RWGS + HAS pathway. Canio and colleagues prepared Pd-modified Co/TiO₂ catalysts, observing that Pd positively affects RWGS activity. It also enhances the reducibility of cobalt through the spillover effect, thus promoting Co-based CO-FTS catalysts [53].

3.2. Synergistic Effects and Active Sites in CO₂ Hydrogenation

In the case of homogeneous catalysts, all hydrogenation reactions are understood to follow a deprotonation mechanism. This involves the active centre of the metal catalyst undergoing redox cycling during the reaction. The reported mechanisms for Co-based homogeneous catalysts include the Co(-I)/Co(I) catalytic cycle, the Co(I)/Co(III) catalytic cycle, and the Co(-I)/Co(0)/Co(+I) catalytic cycle [56,60,61,62]. In contrast, in the case of heterogeneous catalysts, the reaction typically occurs without any alteration to the state of the active centre. The catalytic active sites on Co-based heterogeneous catalysts are complex, with Co⁰, Co²⁺, Co^δ⁺, and Co₂C, among others, all considered active sites for CO₂ hydrogenation [53,63]. Different intermediate species form at different active sites, and these intermediates determine the reaction pathways that influence the efficiency and products of the CO₂ hydrogenation reaction. Have and colleagues investigated the CO₂ hydrogenation reaction mechanism of various Co species. The results of the Modulated Excitation (ME) Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) are presented in Figure 6. The CoO active phase follows the hydrogen-assisted pathway, with major intermediates including *CO₃, *HCOO, *CHO, and others [64]. In contrast, the metallic Co⁰ active phase follows a direct dissociation pathway, with *CO as the major intermediate. Kinetically, the direct dissociation pathway is faster, resulting in the production of C₁ products, while the hydrogen-assisted pathway is more favourable for the production of C₂₊ hydrocarbons.

In conventional Fischer-Tropsch Synthesis (FTS) reactions, the formation of Co₂C is typically regarded as a significant contributor to catalyst deactivation. However, recent studies have shown that Co₂C can facilitate non-dissociative adsorption and CO insertion, thereby enhancing the selectivity of oxygenated compounds in the product [65]. Davis et al. prepared Na-promoted cobalt oxide catalysts (1%Na-20%Co/SiO₂) using SiO₂ as the carrier and investigated the impact of distinct pretreatment conditions on the Co-active species [66]. When hydrogen was used as the reducing gas fraction, cobalt (III) oxide was fully reduced to metallic cobalt following reduction at 350 °C, with methane identified as the primary product. When the temperature was reduced to 250 °C, Co₃O₄ was reduced to CoO, leading to a reduction in the hydrogenation ability of the catalyst and a corresponding decrease in methane selectivity. Following the reduction to CO, the active phases of CoO and Co₂C were obtained, resulting in a further reduction in CH₄ selectivity to 15.3%, while alcohol selectivity increased to 73.2%. Zhang et al. employed the in situ DRIFTS technique to analyse NaCo-Si₃N₄ catalysts hydrogenated in incoming CO₂ and observed that the intermediates of the RWGS pathway, *CO and *CH_x, and the in situ formation of the Co₂C site promoted the coupling of *CO and *CH_x, which improved ethanol selectivity [57]. It is therefore proposed that Co⁰ is more capable of hydrogenation than Co²⁺ active sites and that the Co₂C active phase is key for inhibiting methane formation and enhancing alcohol selectivity.

Synergistic interactions between multiple active sites or alloy phases formed with other metals can significantly affect the efficiency and products of the reaction. Fu et al. explored the effect of metal–oxide interactions on the CO₂ hydrogenation reaction by calcining mixed CoAl hydroxides at different temperatures [67]. Their findings indicated that selecting moderate calcination temperatures could effectively balance the Co–metal oxide interactions, thus enabling the optimal production of Co⁰-Co²⁺ synergism. This moderate Co⁰-Co²⁺ synergism was found to promote CO₂ hydrogenation via the HCOO* pathway, with methane being the main product (with a selectivity of over 98%). The coexistence of Co⁰ and CoO_x is now widely acknowledged as a pivotal characteristic of Co-based catalysts for ethanol generation [59]. Ding et al. conducted density functional theory (DFT) simulations on Co⁰(111) and CoO(200) surfaces with the objective of deepening their understanding of the mechanism underlying ethanol production over CoO_x catalysts. The initial CO₂* hydrogenation process on Co(111) was found to require a lower activation potential barrier and to be more active for CO₂ conversion than CoO(200). The strong hydrogenation of the Co(0) site promoted the formation of CH_x* species more readily, while the CoO site had a more favourable formation of HCOO* species. These species could be coupled with the HCOO* species on the CoO_x site to inhibit methane formation and improve ethanol selectivity [68]. Wang and his team employed in situ infrared (in situ FT-IR) and ¹H nuclear magnetic resonance (¹H NMR) characterisation techniques to examine the CoAlO_x catalysts [45]. Their findings indicated that the optimised Co-CoO phase enhanced the efficiency of the insertion of *CH_x onto *HCOO intermediates, facilitating the formation of acetate intermediates for ethanol production. Further, it was demonstrated that the CoNi alloy phase on the CoNiAlO_x catalyst facilitated the generation of *CHx intermediates. The hydrogenation of *CH_x to ethanol was achieved by linking it to *HCOO on the CoO surface [58]. The following year, a team used in situ DRIFTS spectra and XPS to characterise the Co@Si_0.95 catalyst. They showed that *CH₃O on Co@Si is an intermediate in CH₃OH formation. The irreducible oxygen on the surface of CoO prevents the breaking of the C–O bond in *CH₃O, allowing for the hydrogenation of more *CH₃O to form CH₃OH rather than CH₄ [69].

Given that Co⁰ plays a role in the dissociative activation of CO₂ and H₂, and Co^δ⁺ facilitates the non-dissociative activation of C–O, the simultaneous presence of Co⁰ and Coδ⁺-active sites is considered a beneficial approach for inhibiting methanation and promoting carbon chain growth. This approach aims to improve the performance of catalytic synthesis of higher alcohol [70]. An et al. investigated the catalytic properties of CoGaxAl_2−xO₄/SiO₂ (x = 0, 0.5, 1.0, 1.5, 2.0) catalysts for CO₂ hydrogenation to ethanol [71]. Their results demonstrated that the formation of Co⁰-Co^δ+-active sites on the catalyst surface, due to the strong interaction between Ga₂O₃ and Co, facilitated ethanol synthesis by coupling *CH_x and CO intermediates. This led to an ethanol selectivity of 20.1% over the optimal catalyst (270 °C, 3.0 MPa).

In conclusion, the active sites of Co exhibit varying degrees of activation capacity for H₂ and CO₂, and are capable of forming distinct stable intermediates. The synergistic effect between different sites allows for the regulation of CO₂ hydrogenation activity and product selectivity by constructing multi-site catalysts [72].

4. Optimisation of Co-Based Catalysts

Co is one of the most reactive Group VIII metals for hydrogenation, exhibiting excellent low-temperature hydrogenation capabilities. It has been extensively studied for applications in Fischer-Tropsch synthesis [46]. Co-based catalysts also have a strong adsorption activation capacity for CO₂, with complex active centres that facilitate hydrogenation on Co⁰, CoO, Co₂C, and Co₂N species [64,73,74]. The component ratios and structures of the different Co species determine the CO₂ conversion rate and the types of products formed [22].

The strong hydrogenation capacity of Co catalysts typically results in methane as the dominant product of CO₂ hydrogenation. To achieve the selective formation of long-chain hydrocarbons or oxygenated products [75], it is necessary to weaken this hydrogenation capacity, control the degree of CO₂ reduction, and promote C–C bond coupling [38]. According to the literature, Co catalysts can aid in carbon chain growth by forming alloys with other metals or interacting with supports [53,76,77]. Additionally, the incorporation of additives can alter the surface acidity, alkalinity, strength, and hardness of the catalysts, as well as the surface and electronic structures, thereby affecting the CO₂ hydrogenation reaction rate and product distribution by adjusting the reaction pathways [22].

For example, in a K-Co⁰ catalyst, the addition of the alkali metal K as an additive increased the electron density around Co⁰ and promoted CO₂ adsorption, significantly enhancing the CO₂ conversion rate [71]. Using in situ characterisation techniques to analyse the microstructure and surface active species of the catalysts in their working state, it was found that CO₂ on pure-phase Co catalysts was adsorbed and dissociated to CO*, which then either desorbed directly as CO or was hydrogenated to CH₄. Conversely, some CO₂ adsorbed on the K-Co⁰ surface was hydrogenated to form HCOO* intermediates, which were further reduced through a C–C coupling step to produce C₂₊ products.

Developments in materials chemistry have led to an interest in new catalysts such as metal–organic frameworks (MOFs) and bifunctional catalysts [78,79,80]. Among these, bifunctional catalysts, which possess two different kinds of active centres, can facilitate a variety of synergistic reactions simultaneously. They exhibit high stability and the ability to precisely regulate product selectivity [81,82], making them a focus of research and expanding new avenues in the study of CO₂ hydrogenation catalysts.

4.1. Detailed Pathways of CO₂ Hydrogenation and Catalyst Behaviour

Methane is the main component of natural gas, with a hydrogen content of up to 25%, making it an advanced gaseous fuel and an important raw material for the production of chemical products such as hydrogen energy, carbon black, carbon monoxide, acetylene, hydrocyanic acid, and formaldehyde. In addition, the Gibbs free energy of the carbon dioxide methanation reaction is less than zero (Equation (5)), so methane at room temperature is relatively stable. It can be used as a medium for storing hydrogen to effectively solve the problem of hydrogen being difficult to store, and can then transport it to the industrial development pain point [83,84]. The methanation reaction, also known as the Sabatier reaction [51,84], requires CO₂ activation at high temperatures due to the structural stability of the linear, non-polar molecule. However, thermodynamically, methanation is a highly exothermic reaction with a decreasing gas volume (Equation (5)), which can occur spontaneously at room temperature [79,84,85].

CO₂ + 3H₂ → CH₄ + H₂O(g)
(ΔH^°_298K = −165 kJ/mol  ΔG^°_298K = −113.6 kJ/mol)

(5)

High temperatures promote the endothermic reverse water–gas shift (RWGS) reaction, producing CO as a by-product. Therefore, the reaction should preferably occur at low temperatures [25]. Among Group VIII metals, Co is the most active for hydrogenation and exhibits low-temperature activity [84], making it promising for methanation reactions. Tu et al. reported that Co-Zr_0.1-B-O amorphous catalysts, with Zr added as an additive, could carry out CO₂ methanation reactions as early as 140 °C. The CO₂ conversion at 180 °C could reach 10.7 mmol CO₂ g_cat⁻¹ h⁻¹, with a methane selectivity of 97.8%, comparable to noble metal catalysts under similar conditions [23].

The addition of carriers is a common means of catalyst optimisation, and metal–carrier interactions can effectively tune the performance of multiphase catalysts. Liang et al. loaded Co and Ni onto Al₂O₃ and found that Co/Al₂O₃ catalysts possessed better activity and stability, with a lower reaction onset temperature compared to Ni/Al₂O₃ (Figure 7a,b) [26]. The coordination effect between the metal Co and the support Al₂O₃ promotes the formation of reaction intermediates (bicarbonate, carbonate, and formate), resulting in higher methanation catalyst activity. After the reaction, analysis using TG, TPO-MS, and XRD (Figure 7e–h) revealed that less coke formed on the surface of Co/Al₂O₃ catalysts due to the stronger interaction between Co and Al₂O₃ compared to Ni. This strong interaction improves resistance to coking and reduces catalyst sintering, leading to better stability. Bogdan et al. compared the performance of Ni/CNT and Co/CNT catalysts for CO₂ hydro-methanation under supercritical conditions and showed that Co/CNT catalysts were more active than Ni/CNT catalysts in low H₂/CO₂ feed gas ratios, with almost 100% methane selectivity and no by-product CO [27]. According to these studies, Co demonstrates better hydrogenation capacity and greater stability than Ni. CeO₂ and ZrO₂ are considered important Co-based catalyst carriers. Díez-Ramírez and his team investigated the CO₂ methanation performance of cobalt-based catalysts with different metal carriers added (CeO₂, ZrO₂, Gd₂O₃, ZnO₂) at low temperatures (200–300 °C) and atmospheric pressure [28]. Among them, the Co–Ce interaction on the Co/CeO₂ catalyst enhanced the hydrogenation performance of the catalyst, and the methanation activity was superior to that of the other catalysts. The better the dispersion of metal nanoparticles on the carrier, the stronger the metal–carrier interaction. Li et al. found that the hydroxyl and carboxyl groups on citric acid, a low-cost complexing agent, favoured the dispersion of the metal on the support. The dispersion of Co on the surface of 2%Co-ZrO₂ catalysts prepared with citric acid was better, increasing the strength of the Co–ZrO₂ interaction [86]. This strong interaction between metal Co and ZrO₂ supports contributed to forming more reduction-active sites and oxygen vacancies, promoting CO₂ adsorption and significantly improving CO₂ conversion (85%) and CH₄ selectivity (99%). Suitable carrier size facilitates the dispersion of metal nanoparticles. Alexander et al. controlled the metal–carrier interaction strength on Co/CeZrO₄ catalysts by varying the size of the carriers [87]. It was found that cobalt nanoparticles on CZ carriers in the range of 20–30 nm showed better dispersion and stronger metal–carrier interactions, and the cobalt nanoparticles were more stable during the reduction step and therefore had higher CO₂ methanation activity compared to CZ carriers with smaller particle sizes. Meanwhile, the characterisation results demonstrated that the easy formation of oxygen vacancies and oxygen overflow in cerium dioxide is the key to high CO₂ methanation activity. In addition, the use of novel MOF materials with large surface areas as carriers can achieve high dispersion of active sites and improve the catalytic performance [88]. In a study by Ampaiah et al., using cerium-zirconium oxide (CZ) as a carrier in the MOF stencilling method resulted in enhanced catalytic activity compared to the conventional co-precipitation method [70,89]. Incorporating CZ–MOF carriers increased the specific surface area and pore volume, facilitating the dispersion of Co₃O₄. This elevated the oxygen vacancies on the catalyst surface, enhancing CO₂ adsorption capacity.

Constructing systematic synergies between metals by adding metal additives can affect the catalytic performance, and the researchers designed Ni-Co bimetallic catalysts to improve the methanation performance. Li et al. found that in the Ni-Co-MgO bimetallic catalyst, a strong synergistic interaction between Co and Ni facilitated the CO₂ methanation reaction. In this process, the active site Co^δ⁺ activated CO₂ to form a monodentate carbonate intermediate, which then reacted with H* dissociated at Ni⁰ to form methane (Figure 8) [90]. The redox nature of Co in the Ni-Co/CeO₂-ZrO₂ catalysts reduced carbon deposition and sintering, to which Ni catalysts are prone, thus improving reaction efficiency and stability [91]. It was also observed that when CH₄ was present in the feedstock, the methanation reaction was carried out simultaneously with the low-temperature reforming reaction, enhancing catalytic activity.

Mesoporous oxides, with their large specific surface area and sufficient pore space, have great application value as catalyst carriers. Zhou et al. developed Co-based catalysts for CO₂ hydrotreating methanation using novel mesoporous supports. Both Co/KIT-6 (KIT-6 is a porous silica molecular sieve) and Co/meso-SiO₂ catalysts were prepared with well-defined mesoporous structures and highly dispersed surface Co particles. The Co/KIT-6 catalysts, with a specific surface area of 368.9 m²/g and a highly ordered bicontinuous mesoporous structure, exhibited better CO₂ hydrogenation activity and methane selectivity than Co/meso-SiO₂ catalysts at high reaction temperatures [92].

Co carbides and nitrides are also very reactive. Yu et al. developed a Co₂C/γ-Al₂O₃ catalyst for CO₂ hydrogenation at 300 °C and 3 bar, demonstrating an 89% conversion and 99% selectivity to CH₄ [73]. This performance indicated higher CO₂ conversion and methane selectivity compared to CoO_x/γ-Al₂O₃ catalysts. Azzaz and colleagues prepared Co₄N/γ-Al₂O₃ catalysts for CO₂ methanation using γ-Al₂O₃ as a carrier. Their results showed that Co₄N exhibited higher CO₂ methanation activity than Co. Co₄N/γ-Al₂O₃ demonstrated reduced susceptibility to coke deposition and sintering of metal particles and good anti-deactivation performance. This was attributed to the introduction of nitride, which resulted in nitrogen vacancies, increasing the catalyst surface’s alkalinity and enhancing CO₂ and H₂ adsorption [93].

Table 1. CO₂ hydrogenation to methane over Co-based catalysts.

Entry	Catalyst	T (°C)	P (MPa)	GHSV b	WHSV c	SV b	CO2 Conv. (%)	CH4 Sel. (%)	H2/CO2	Ref.
1	Co-Zr0.1-B-O a	180	8.0	-	-	-	-	97.8	1.0	[23]
2	15 wt% Co/Al2O3	400	1 atm	16,000	-	-	82	80	4.0	[26]
3	25 wt% Ni/Al2O3	400	1 atm	16,000	-	-	75	69	4.0	[25]
4	Co(10)/CNT	250	8.0	-	-	-	24	100	1.0	[27]
5	Ni(10)/CNT	250	8.0	-	-	-	4	100	1.0	[27]
6	Ni-Co-MgO	290	-	20,000	-	-	80.5	99.6	4.0	[90]
7	Ni-Co/CeO2-ZrO2	350	-	-	12,000	-	61	97	4.0	[91]
8	2%Co-ZrO2	400	3.0	-	-	7200	85	99	4.0	[86]
9	Co/KIT-6	280	-	22,000	-	-	48.9	100	4.6	[92]
10	Co/meso-SiO2	300	-	22,000	-	-	28	68.1	4.6	[89]
11	20 wt%Co/CZ-MOF	320	1.5	-	-	1500	81.2	100	3.0	[73]
12	Co2C/γ-Al2O3	300	0.3	60,000	-	-	89	99	4.0	[93]
13	20Co4N/γ-Al2O3	300	1.5	5000	-	-	98	98	4.0	[93]
14	Co/CeO2	300	0.1	-	-	-	97.0	≈100	9.0	[28]
15	10CoCZ700	225	0.1	-	-	-	50	≈46	4.0	[87]

^a. Kettle reactor ^b. The unit is mL·g⁻¹·h^{−1 c}. The unit is mL·g⁻¹·hcat⁻¹.

summarises the catalytic performance of recently reported Co-based catalysts for CO₂ methanation.

In summary, compared with Ni and noble metal catalysts, Co-based catalysts show advantages of low reaction temperature, relatively good stability, and low cost in CO₂ hydrogenation to methane, holding good prospects for industrialisation. Researchers optimised the Co-active site by adjusting the chemical microenvironment to promote C–O bond cleavage and H₂ dissociation, thereby improving methanation efficiency.

4.2. CO₂ Hydrogenation to Hydrocarbon Chemicals and Fuels

CO₂ hydrogenation can produce low-carbon hydrocarbons and liquid fuels, with low-carbon olefins (ethylene, propylene, etc.) being the most basic raw materials in the chemical industry. Liquid fuels, including gasoline (C₅–C₁₁), aviation fuel (C₈–C₁₆), and diesel fuel (C₁₀–C₂₀), are important energy sources today, derived from petroleum processing [84,94]. Given the declining availability of oil and the global pursuit of sustainable energy sources, converting carbon dioxide into hydrogen via non-oil pathways offers dual benefits: mitigating carbon emissions and reducing reliance on oil resources [75,95].

The Fischer-Tropsch Synthesis (FTS) process, which uses syngas as a feedstock to synthesise C₂₊ hydrocarbons, has a long history. Numerous highly effective FTS catalysts have also shown high activity in CO₂ hydrogenation reactions [96]. Co-based catalysts are high-performing FTS catalysts with substantial carbon chain growth capacity. However, due to their low reverse water–gas shift (RWGS) activity, the primary CO₂ hydrogenation product is CH₄ [29,97]. To enhance the selectivity for C₂₊ hydrocarbons in CO₂ hydrogenation products, researchers typically incorporate components with RWGS activity into Co catalysts [38,98].

Cu catalysts, extensively researched for WGS, are also employed in RWGS reactions. Consequently, Co-Cu bimetallic catalysts serve as highly effective catalysts for hydrogenating CO₂ to long-chain hydrocarbons [24]. In 1950, Russell and Miller found that adding Cu to Co-based catalysts could lower the catalytic reaction temperature, reduce CH₄ generation, and promote liquid hydrocarbon production [99]. Alkali metals (K, Na, Cs), characterised by high electron density, facilitate CO₂ adsorption, inhibit methane production, and promote carbon chain growth in the product [54,100]. Shi et al. reported the synthesis of alkali metal-modified CoCu/TiO₂ catalysts. Doping with alkali metals altered the structural properties and crystal structure of the catalysts, enhanced CO₂ chemisorption, reduced H₂ adsorption, effectively inhibited CH₄ formation, and improved the selectivity of C₅₊ hydrocarbons, achieving a selectivity of up to 42.1% for C₅₊ hydrocarbons at 250 °C and 5.0 MPa [30,101].

Fe is an excellent catalyst for the FTS process, exhibiting high activity in the RWGS reaction and playing a pivotal role in CO₂ hydrogenation to long-chain hydrocarbons [20,102]. Recent studies on Co-Fe bimetallic catalysts show that Co can advance the chain extension reaction and inhibit carbon deposit generation, thereby improving catalyst activity [53,103]. Wang et al. designed catalysts using carbon shells to achieve a controlled distribution of Co and Fe. The isolated active site of Fe-Co catalysts, separated by the inside and outside of the carbon shells, effectively promoted CO₂ hydrogenation efficiency, suppressed methanogenesis, and improved the selectivity of C₅₊ hydrocarbons, increasing it from 19.8% to 39.7% [55], as Figure 9 shows. Kim et al. prepared Na-modified CoFe₂O₄/CNT catalysts for the CO₂ hydrogenation reaction. Catalysts with 0.32 wt% Na exhibited a selectivity of 38.8% for C_2–4 olefins and up to 40.9% for C₅+ hydrocarbons [104]. Xu et al. synthesised a series of homogeneous K-containing spinel-type ZnCoFe_2−xO₄ nanoparticle catalysts via ZnCoFe-LDHs precursors, which exhibited excellent properties for the hydrogenation of CO₂ to light olefins [105]. A combination of structural characterisation and reaction results demonstrated that an increase in the Co content facilitated the reduction of Co and Fe species in hydrogen, thereby promoting the formation of more CoFe alloy phases. The electron-rich Fe0 atoms in the CoFe alloys enhanced the dissociation of CO intermediates and significantly contributed to the formation of iron and cobalt carbons, thus improving the reactivity of the catalysts in the production of hydrocarbons and simultaneously suppressing the CO₂ methanation and the secondary hydrogenation reaction of olefins. Hwang et al. induced the formation of FeCo alloys using N-coordinated Co atomic structures (Co-NC) as carriers. DFT calculations show that the FeCo alloy structure promotes FTS while inhibiting CH₄ formation [31].

The introduction of other transition metals, such as Mo, Mn, Pd, etc., is also favourable for the formation and stabilisation of the active phase and improves the selectivity of long-chain hydrocarbons in the product. Owen et al. reported that a Na-Co-Mo/SiO₂-TiO₂ catalyst exhibited a 13.5% CO₂ conversion, 37.3% selectivity for C_2–4 olefins, and 23.1% selectivity for CH₄ at 200 °C and 0.1 MPa. They determined that the surface area of the carriers and the metal–oxide carrier interactions influence Co particle size, which subsequently affects the activity and selectivity of the catalysts [106]. He et al. discovered that Co₆/MnO_x nanocatalysts could effectively catalyse CO₂ hydrogenation, with a selectivity of up to 53.2% for liquid hydrocarbons (C_5–26) in the total product and a very low CO selectivity of 0.4%. TPD data indicated that incorporating the Mn promoter enhanced CO₂ adsorption, inhibited H₂ adsorption over the Co catalysts, and drove the generation of long chains [30]. Canio et al. used Pd as an additive to enhance the CO₂ hydrogenation performance of Co/TiO₂ catalysts. Their findings revealed that the interactions between the two metals were not more pronounced or favourable, and that the mixed monometallic catalysts exhibited higher fuel selectivity than the bimetallic catalysts [107].

The spatial arrangement of catalysts synthesised by methods such as co-precipitation, impregnation, and physical mixing is not subject to precise control [31,108]. The development of well-designed and structurally well-defined bifunctional catalysts not only contributes to the creation of new catalysts but also elucidates reaction mechanisms. Xie et al. designed a bifunctional CeO₂-Pt@mSiO₂-Co core–shell catalyst for synthesising hydrocarbons with the target product C_2–4 using two metal–oxide interfaces to catalyse the sequential reaction [49]. In this case, the higher RWGS activity of CeO₂/Pt favoured CO formation, which underwent an F-T reaction to produce C_2–4 hydrocarbons at the adjacent Co/mSiO₂ interface (Figure 10). Heuntae et al. reported a Mn-promoted Co@CoO_x/Co₂C core–shell catalyst, where oxygen vacancies on the CoO_x in the shell layer activated the RWGS reaction. The resulting CO was transported to the Co₂C phase near the nucleus and the metal Co phase to continue the reaction, resulting in the production of long-chain hydrocarbons via the FTS reaction, with a selectivity of up to 14.9% of the C₂₁₊ hydrocarbons under optimal reaction conditions [109].

In conclusion, the researchers employed a strategy of modulating the interactions between the active sites of Co and other metals. This involved the introduction of alkali metals, transition metals, or the construction of structurally precise bifunctional catalysts. The objective was to inhibit methane generation, promote C–C chain growth, and improve C₂₊ hydrocarbon selectivity.

Table 2. CO₂ Hydrogenation to hydrocarbon chemicals and fuels over Co-based catalysts.

Entry	Catalyst	T (°C)	P (MPa)	CO2 Conv. (%)	CO Sel. (%)	Product Selectivity (%)				H2/ CO2	Ref.
						CH4	C=2–4	C2–4	C5+
1	C/Co3@C/Fe	320	2.0	33.4	23.8	14.4	36.8	9.1	39.7	3.0	[55]
2	Na-CoCu/TiO2	250	5.0	18.4	30.2	26.1	31.8		42.1	3.0	[101]
3	K-CoCu/TiO2	250	5.0	13.0	35.1	34.1	30.8		35.1	3.0	[110]
4	Na-Co-Mo/SiO2-TiO2	200	0.1	13.5	66.7	23.1	37.3	15.4	24.2	3.0	[106]
5	Na-CoFe2O4/CNT	340	1.0	34.4	18.6	14.8	5.5	38.8	40.9	3.0	[104]
6	ZnCo0.5Fe1.5O4	320	2.5	49.6	5.8	18.9	6.2	36.1	38.7	3.0	[105]
7	FeK/Co-NC	300	2.5	51.7	2.8	22.2	34.2		43.6	3.0	[31]
8	Co6/MnOx a	200	8.0	15.3	0.4	46.6			53.4	1.0	[30]
9	10Co/TiO2* + 1Pd/TiO2*	220	2.0	19.4	0.8	66.4	12.7		20.1	2.0	[107]
10	CeO2-Pt@mSiO2-Co	250	0.62	24	74	60	40		0	3.0	[49]
11	Mn-Co@CoOx/Co2C	270	4.0	64.7	0.2	44.2	2.0	17.8	36.0	3.0	[109]

^a. Kettle reactor.

summarises the catalytic performance of recently reported Co-based catalysts for CO₂ to hydrocarbon chemicals and fuels.

4.3. Innovations in CO₂-to-Methanol Catalysis

George A. Olah, the 2006 Nobel Prize winner in chemistry, first introduced the concept of the “methanol economy” in his book “Beyond the Oil and Gas Era—The Methanol Economy” (Figure 11). This model represents a green cycle of C1 chemistry and offers a potential solution to the problem of fossil fuel resource scarcity [111]. Alcohols possess several advantageous properties, including high energy density, low vapor pressure, and affinity for water. These characteristics render them excellent fuels, solvents, and chemical raw materials, with a vast and growing market demand [112,113].

The conversion of carbon dioxide (CO₂) into methanol is a thermodynamically exothermic process (Equation (3)). Furthermore, it is a reaction process in which the number of molecules is reduced. Consequently, low temperatures and high pressures are conducive to CO₂ conversion and high methanol selectivity. To promote the reaction, researchers added metal additives with low-temperature hydrogenation activity, as reported by Zeng et al. His team prepared a Pt₃Co catalyst utilising the noble metal Pt as an additive [114]. FT-IR results confirmed that the higher negative charge density of the Pt atoms at the apex of the Pt₃Co octopod promotes the activation of CO₂, thereby improving catalytic activity and increasing the yield of methanol from CO₂ hydrogenation.

The use of oxide carriers allows for modification of the size, shape, and chemical state of the catalyst, which can significantly affect the reaction and interfacial activity [115]. In a recent study, Li et al. reported a MnO_x/m-Co₃O₄ catalyst with a MnO_x/CoO interface that exhibited enhanced catalytic activity and methanol selectivity compared to MnO nanoparticles and mesoporous Co₃O₄ carriers alone [116]. Wang et al. used a method of loading Co nanoparticles onto amorphous SiO₂, resulting in the formation of Co–O–SiOₙ bonds on the Co@Si catalyst, which inhibited the formation of CO and CH₄. Additionally, this catalyst formed a stable intermediate, CH₃O*, improving methanol selectivity. At 320 °C, the catalyst demonstrated a selectivity of 70.5% for methanol and an CO₂ conversion of 8.6% [69].

The synthesis of catalysts with the aid of stencil agents can create more active sites and reaction spaces, thereby improving reaction efficiency. Alexey et al. employed gradual pyrolytic oxidative decomposition of indium-impregnated ZIF-67(Co) MOFs to form In₂O₃@Co₃O₄ reticulated shell composites [117]. This process yielded a maximum methanol yield of 0.65 g_MeOH g_cat⁻¹ h⁻¹ over 100 h, with methanol selectivity as high as 87%, superior to that of commercial Cu/ZnO/Al₂O₃ catalysts. Screening tests revealed that lower temperatures effectively inhibited side reactions (methanation and RWGS), reducing CO and CH₄ selectivities to 11% and 2%, respectively. Lian et al. synthesised Co/C-N materials using ZIF-67 calcined under a N₂ atmosphere. They then prepared Co@Co₃O₄/C-N catalysts by partially oxidising Co/C-N in air under different conditions [118]. It was observed that hydrogen dissociated from the surface of the metal cobalt, and partial oxidation of Co/C-N reduced the cobalt content and hydrogen dissociation ability, decreasing CO₂ conversion while increasing methanol selectivity. The catalyst exhibited the highest methanol yield of 2.0 mmol g cat⁻¹ h⁻¹ at 220 °C.

Co-based catalysts demonstrate efficacy in CO₂ hydrogenation to methanol and show considerable potential in synthesising higher-carbon alcohols [22]. The addition of metal additives or oxide supports modulates the Co-active sites on the catalyst surface, altering its ability to dissociate H₂ and break C–O bonds [53,76]. Wang et al. reported a CoAlO_x catalyst for selective hydrogenation of CO₂ to synthesise ethanol, adjusting the catalyst’s activity composition by using different pre-reduction temperatures [45]. The catalyst was optimised for the surface Co-CoO phase following reduction at 600 °C, enhancing acetate intermediate generation and ethanol selectivity in the product, with ethanol selectivity reaching up to 92.1% (Figure 12). It is crucial to adjust the oxidation state of Co in order to form the Co-Co^δ⁺-active phase, with the aim of improving alcohol selectivity for Co-based catalysts [65]. Zheng et al. prepared a series of LaCo₁₋_xGa_xO₃ chalcogenide catalysts for ethanol production using the citric acid complexation method [119]. The interaction between Co and Ga resulted in the presence of Co^δ⁺ at the reduced interface. The synergistic effect of Co⁰ and Co^δ⁺ moderately weakened the hydrogenation capacity of Co⁰, inhibited CO₂ methanation, and promoted ethanol formation. The conversion over LaCo_0.7Ga₀.₃O₃ of CO₂ was 9.8%, with 74.7% selectivity to alcohols and 88.1% ethanol content in the alcohol mixture.

The structural morphology of catalysts significantly impacts the conversion and selectivity of reactions. As shown in Figure 13, Ouyang et al. reported using Pt/Co₃O₄-r (nanorods) and Pt/Co₃O₄-p (nanoplates) catalysts with Pt additives for the hydrogenation of CO₂ to generate C₂₊ alcohols [120]. The combined effect of Pt/Co on the catalysts after reduction at 200 °C and the partially reduced Co₃O_4−x surface oxygen vacancies enhanced the adsorption of CO₂ and H₂, facilitating the reaction. The selectivity of the Pt/Co₃O₄-r catalyst for C₂₊ alcohols was found to be 14.8%, while the C₂₊ alcohol selectivity of the Pt/Co₃O₄-p catalyst was determined to be 19.2%. The team then synthesised Co₃O₄-m (mesoporous) using mesoporous silica (KIT-6) as a template [121]. The ordered mesoporous structure on Co₃O₄-m promoted carbon chain growth and reduced methane selectivity. To further reduce methane content as a by-product, platinum, a noble metal with strong RWGS capability, was introduced into the Pt/Co₃O₄-m catalyst. This demonstrated a decrease in methane selectivity and an increase in CO and alcohol selectivity.

Cu is the most widely studied catalyst for CO₂ hydrogenation to alcohols. The introduction of Cu as an additive can promote the formation of high-carbon alcohols. Muhammad et al. prepared bimetallic Cu-Co tandem catalysts by decorating Co nanoparticles on a dendritic Cu substrate [33]. The Cu-Co interface facilitated the generation of acetaldehyde intermediates for the direct and selective production of n-butanol-rich C₃₊ alcohols. Additionally, the Cu-Co catalysts demonstrated remarkable stability over 1000 h, effectively inhibiting re-oxidation and carbon deposition. Incorporating non-metallic elements, such as carbon (C) and phosphorus (P), into the catalyst structure can inhibit CO* intermediate dissociation and promote alcohol synthesis [32,34]. Zhang et al. constructed a Co-Co₂C catalyst with dual active sites, which exhibited an electron-loading effect on carbon species [34]. This increased the CO₂ conversion rate and facilitated non-dissociation of CO_x intermediates, resulting in increased methanol selectivity while maintaining high ethanol selectivity. Introducing carbon elements effectively inhibited re-oxidation and carbon deposition, stabilising the Cu-Co catalyst. Furthermore, carbon elements stabilised the microstructure of the active components, providing the catalyst with good anti-sintering properties and improved catalytic stability.

The preceding work indicates that Co-based catalysts are not readily capable of forming stable oxygenated intermediates due to their pronounced hydrogenation activity. The principal product is methane, and the selectivity for alcohols is typically poor. Researchers have constructed synergistic active sites, such as Co⁰-Co²⁺, Co⁰-Co^δ⁺, Co-Co₂C, etc., to inhibit the methanation reaction and improve alcohol selectivity by adding additives, carriers, or adjusting the catalyst structure.

Table 3. CO₂ Hydrogenation to hydrocarbon chemicals and fuels over Co-based catalysts.

Entry	Catalyst	T (°C)	P (MPa)	CO2 Conv. (%)	Product Selectivity (%)				H2/CO2	Ref.
					CO	CH4	CH3OH	C2+OH
1	Pt3Co a	150	3.2	-	Produced methanol = 17.3mmol(5h)				3.0	[114]
2	MnOx/m-Co3O4	250	0.4	-	SMethanol = 30%, SEthylene = 10%, SDimeyhyi = 5%				3.2	[116]
3	Co@Si0.95	320	2.0	8.6	8	21.5	70.5	-	3.0	[69]
4	In2O3@Co3O4	250	5.0	-	11	2.0	87	-	4.0	[117]
5	Co@Co3O4/C-N a	220	2.0	18.6	0	79.5	18.3	1.2	3.0	[118]
6	CoAlOx-600 a	140	4.0	-	-	-	-	>92.1	3.0	[45]
7	Pt/Co3O4-r	200	2.0	27.8	0	60.0	14.8		3.0	[120]
8	Pt/Co3O4-p	200	2.0	22.4	0	77.8	19.2		3.0	[120]
9	Pt/Co3O4-m	200	2.0	10.7	28.3	14.9	47.2		3.0	[120]
10	LaCo0.7Ga0.3O3	240	3.0	9.8	0	23.1	74.7		3.0	[119]
11	Na-CuCo-9	330	4.0	22.1	20.5	18.0	4.1	SC3+OH = 27.4%	1.0	[121]
12	600-CDM (CoGa1.0Al1.0O4/SiO2)	270	3.0	3.7	-	-	29.5	16.9 C-mol%	-	[34]

^a. Kettle reactor.

summarises the catalytic performance of recently reported Co-based catalysts for CO₂ to hydrocarbon chemicals and fuels.

5. Limitations and Future Perspectives

5.1. Limitations

Co-based catalysts play a pivotal role in the hydrogenation of CO₂ to methane, hydrocarbon chemicals, and alcohols. This is due to the strong hydrogen activation ability of Co, which increases the likelihood of CH₄ formation. This paper discusses the reaction mechanisms of CO₂ hydrogenation with Co-based catalysts, introduces optimised Co-based catalysts for CO₂ methanation, and explores the generation of hydrocarbon chemicals and alcohols through the addition of auxiliaries and carrier modifications. Despite the development of various strategies to optimise the nature and structure of Co-based catalysts, the results remain unsatisfactory. Firstly, the potential of Co-based catalysts lies more in the synthesis of long-chain carbon hydrocarbons than in CO₂ methanation. However, current CO₂ conversion and selectivity for long-chain products do not meet industrial standards, preventing commercial applications. Secondly, achieving precise selectivity for higher alcohols in CO₂ hydrogenation with Co-based catalysts remains a significant challenge due to numerous by-products, including CO, methane, and other C₂₊ hydrocarbons. Thirdly, the understanding of the reaction mechanism remains incomplete. Although numerous researchers have discussed various active sites on Co catalysts and proposed potential reaction pathways, current characterisation methods do not allow for sufficiently clear and direct observation of complex molecular and atomic motions during the reaction. Furthermore, the complex synergistic effects between multiple active sites hinder a deeper exploration of the reaction mechanism. The lack of clarity in the reaction mechanism constrains researchers’ ability to design high-performance catalysts. Finally, the issue of a sustainable source of hydrogen in the feedstock gas is one of the limiting conditions for the industrialisation of CO₂ hydrogenation, and if the source of hydrogen cannot be guaranteed to be clean, the idea of CO₂ hydrogenation reactions to produce green fuels will be dashed. Currently, the main technologies for hydrogen production from renewable sources are hydrolysis, biomass thermochemical conversion, and biomass bioconversion [122,123]. Most of these technologies are still in the laboratory stage, and electrolysis of water, which has been used for industrial production, suffers from high costs [123]. How to obtain affordable and sustainable hydrogen is still a direction that researchers have to strive to explore.

5.2. Future Perspectives

In light of these considerations, the following strategies are proposed to address these issues:

(1) Design of Catalysts for Synergistic Interaction of Multiple Active Sites: Based on the current understanding of single active-site catalysts, constructing catalysts that achieve synthetic goals through synergistic interactions is promising; for example, enhancing the efficacy of Co-based catalysts that follow the RGWS–FTS pathway by introducing a low-temperature, high-activity RWGS active site. Alternatively, we can learn from syngas reactions to design HAS multifunctional catalysts that utilise the limiting or moderating effect of zeolites on reaction centres to enhance selectivity for higher carbon alcohols in CO₂ hydrogenation reactions.

(2) Development and Popularisation of Advanced Characterisation Techniques: Developing novel techniques based on existing in situ characterisation methods, such as in situ XRD, DRIFTS, and FT-IR, to enhance instrument sensitivity and refine the experimental design. Introducing cutting-edge technologies, such as attosecond lasers, can record instantaneous electronic motion processes, providing insights into molecular and atomistic-scale interactions and dynamics. This enables precise identification and continuous monitoring of active components and key intermediates during CO₂ hydrogenation on catalysts.

(3) Introduction of Novel Computational Techniques: Theoretical computation is a valuable tool for elucidating reaction pathways. Recent novel computational techniques, including quantum computing and reaction modelling, integrated with machine learning algorithms, can aid in theoretical interpretation and prediction of catalysts. Purposefully modelling the microenvironment surrounding different Co active sites and analysing free energy barriers to identify the decisive step in the CO₂ hydrogenation reaction rate can provide theoretical assistance in constructing catalysts with high selectivity and stability for CO₂ hydrogenation.

(4) Development of Reactors to Enhance Catalytic Efficiency: Beyond investigating novel and more efficient catalysts, efforts should be directed towards the design and optimisation of reactors. For instance, membrane reactors, which combine reaction and separation steps, can circumvent equilibrium limitations in converting CO₂ to liquid fuels by continuously and selectively removing water as a by-product. This results in a notable enhancement in CO₂ conversion and liquid fuel yield. Membrane reactors are currently used in CO₂ hydrogenation to methanol [124,125,126].

These strategies aim to address the current challenges in optimising Co-based catalysts for CO₂ hydrogenation, paving the way for improved catalytic performance and potential commercial applications.