# Oxide Supported Cobalt Catalysts for CO 2 Hydrogenation to Hydrocarbons: Recent Progress

Canio Scarfiello, Doan Pham Minh, Katerina Soulantica, Philippe Serp

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Canio Scarfiello, Doan Pham Minh, Katerina Soulantica, Philippe Serp. Oxide Supported Cobalt Catalysts for CO 2 Hydrogenation to Hydrocarbons: Recent Progress. Advanced Materials Interfaces, 2023, 10 (15), pp.2202516. 10.1002/admi.202202516. hal-04078884

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# Oxide Supported Cobalt Catalysts for CO₂ Hydrogenation to Hydrocarbons: Recent Progress

Canio Scarfiello,* Doan Pham Minh, Katerina Soulantica, and Philippe Serp*

Carbon capture and utilization represents a promising strategy to meet the global energy and climate goals. Under specific conditions, CO₂ catalytic hydrogenation with renewable H₂ can transform waste CO₂ into a chemical feedstock for added-value energy carriers and chemicals. CO₂-Fischer-Tropsch synthesis-based-hydrocarbons should contribute to the creation of a circular carbon economy with a significant impact on anthropogenic emission into the atmosphere. This review summarizes the progress achieved toward the single-step hydrogenation of CO₂ to long-chain hydrocarbons over oxide-supported Co-based catalysts. Mechanistic aspects are discussed in relation to thermodynamic and kinetic limitations. The main parameters that must be taken into consideration to increase the activity and the selectivity toward compounds of two or more carbon atoms (C₂+) are discussed in detail: cobalt active phase, support and metal-support interfaces, and promoters. Finally, particular focus is dedicated to the role of reducible oxide supports and their surface defects on the activation of CO₂, as well as on the regulation and evolution of metal-support interactions.

## 1. Introduction

The current concentration of CO₂ in the atmosphere is ≈410 parts per million (ppm) and is expected to reach 950 ppm by the end of the century, triggering significant changes in the earth's atmosphere.[1] Over 40% of world's total CO₂ emissions come from fossil fuels, which combustion accounts for 80–90% of world's power generation.[1] From a rational point of view, “prevent” is better than “cure” and consequently, a transition toward carbon-free renewable sources is on the way.[2] This will require decades, huge investments, and significant political as well as technological efforts. Furthermore, some sectors such as carbon-intensive industries (i.e. cements and chemicals industries), intrinsically produce CO₂.[3] In this context, carbon capture and utilization (CCU) represents a promising strategy to meet the global energy and climate goals. Under specific conditions, CO₂ hydrogenation with renewable H₂ can transform waste CO₂ into a chemical feedstock for added-value energy carriers and chemicals.[2,3] In this optic, catalytic hydrogenation of CO₂ received increasing scientific attention over the last decades. Most researchers focused their attention on the hydrogenation to C₁ products such as methane and methanol, which can nowadays be obtained by established industrial by established industrial-scale technologies.[2,4] Conversely, the production of C₂+ hydrocarbons is more challenging due to the high C–C coupling barrier, and the numerous competing reactions generating C₁ products.[4] However, compounds of two or more carbon atoms (C₂+) possess higher volumetric energy densities, which increase with the chain length, and can be easily transported off-grid.[2–4] Reductive C=O bond cleavage of the CO₂ molecule requires a high energy input of ≈750 kJ mol⁻¹. Should this energy effort be paid, the production of higher-value products in a single stage is more interesting than the partial or total hydrogenation into C₁ products.[2] C₂+ hydrocarbons can be currently produced via the Fischer–Tropsch synthesis (FTS) process using syngas (gas mixture rich in CO and H₂) as feedstock. Compared to petroleum-based products, ultra-clean FTS-based hydrocarbons are free of sulfur, nitrogen, aromatics, and other poisoning species, and can be directly used in subsequent refining processes or immediate commercial applications.[5] CO₂-FTS-based hydrocarbons would allow the creation of a circular carbon economy with a significant impact on anthropogenic emission into the atmosphere. Provided that

C. Scarfiello, D. Pham Minh
Université de Toulouse
IMT Mines Albi
UMR CNRS 5302
Centre RAPSODEE
Campus Jarlard, Albi, Cedex 09 81013, France
E-mail: canio.scarfiello@mines-albi.fr

C. Scarfiello, K. Soulantica
LPCNO
Université de Toulouse
CNRS
INSA
UPS
135 avenue de Rangueil, Toulouse 31077, France
C. Scarfiello, P. Serp
LCC
CNRS-UPR 8241
ENSIACET
Université de Toulouse
4, allée Emile Monso – CS 44362, Toulouse, Cedex 4 31030, France
E-mail: philippe.serp@ensiacet.fr

The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/admi.202202516.

© 2023 The Authors. Advanced Materials Interfaces published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.

DOI: 10.1002/admi.202202516
CO₂-FTS is conducted under the same industrial conditions, with the same catalysts and product distribution as traditional FTS, the process would be extremely competitive. Due to their high C–C coupling activity in the FTS process, cobalt-based catalysts are good candidates for direct CO₂ hydrogenation to C₂⁺ hydrocarbons. Nevertheless, unmodified Co-based catalysts act differently when CO₂ substitutes CO in the feed, producing mainly methane. However, it is known from the 50’s that alkali- lized cobalt catalysts can be active for the catalytic hydrogena- tion of carbon dioxide to higher hydrocarbons.[6] Since then, it was proposed that a careful choice of the metal oxide support, of the alkaline metal promoters, and regulation of the metal- support interface can enhance the intrinsically marginal activity of Co-based catalysts for the reverse water-gas shift reaction (RWGS), and decrease excess methanation. Similarly, despite their higher activity for RWGS, Fe and Fe-Co based catalysts require careful design and promoter addition to improve their activity and selectivity toward C₂⁺ products.[7–10] Moreover, due to their lower hydrogenation ability, such catalysts produce more olefins and oxygenates products.[7–10]

Here we present an overview of the progress achieved toward the single-step hydrogenation of CO₂ to long-chain hydrocarbons over oxide-supported Co-based catalysts. Mecha- nistic aspects are discussed in relation to thermodynamic and kinetic limitations. The main parameters that affect the activity and the selectivity toward C₂⁺ products are discussed in detail: cobalt active phase, support and metal-support interfaces, and promoters. Finally, particular focus is dedicated to the role of reducible oxides as supports and their surface defects on the activation of CO₂, as well as on the regulation and evolution of metal-support interactions.

## 2. Thermodynamic and Kinetic Restrictions Over Metallic Cobalt Catalysts

Metallic cobalt, especially in its hexagonal close-packed (hcp) phase, represents one of the best choices for the conversion of syngas to hydrocarbons via FTS.[5] Due to their superior chain- growth capability, high stability and low activity for the water-gas shift (WGS) reaction, Co-based catalysts are typically employed in the low temperature FTS process (220–250 °C) to produce heavy hydrocarbons with a high carbon efficiency.[4,5] This sur- face polymerization reaction leads to a hydrocarbon product pattern that can be ideally modeled by an Anderson–Schulz– Flory (ASF) distribution (Figure 1), mathematically expressed as Wn = n(1-α)²α n⁻¹,[2] where Wn represents the weight fraction of products with n carbon atoms in their chain, and α is the chain-growth probability. According to the ASF distribution, only CH₄ and C₂₁⁺ hydrocarbons can be obtained with high selectivity at low and high α value, respectively.[5] Conversely, a careful control of α is necessary to directly obtain specific middle-distillate hydrocarbons.[5] Due to such restrictions, Co- based catalysts are traditionally employed in FTS to maximize the production of heavy hydrocarbons (C₂₁⁺, α > 0.9), which, in turn, are converted into the desired middle distillate mix blend via a downstream hydrocracking refining treatment.[5]

The utilization of such a process for the direct synthesis of CO₂-based heavy hydrocarbons would represent an important

<figure><img src="image_2.png" /><figcaption>Figure 1. Weight fractions of different hydrocarbons as a function of the chain-growth probability (α) assuming an ideal ASF distribution. Source: C. Scarfiello.</figcaption></figure>

route toward greener high-density chemical energy storage. Cobalt seems to be the obvious choice for such a purpose, and therefore, over the years several attempts were made by simply switching the feed composition from CO to CO₂ with traditional Co-based FTS catalysts.[11–13] Unfortunately, under industrial relevant operation conditions, the progressive substi- tution of CO by CO₂ in the feed results in limited CO₂ conver- sion levels (<20%) and increased methane selectivity (>70%). Moreover, the chain-growth probability for C₂⁺ hydrocarbon precursors significantly decreases, leading to the production of shorter chain products (α < 0.5–0.6).[2,11–14] Such undesired out- comes can be ascribed to thermodynamic limitations that have direct kinetic consequences.[2,14] Indeed, during CO₂ hydrogena- tion to hydrocarbons, the FTS reaction (R2) occurs only in a second time, after the initial transformation of CO₂ into CO via the RWGS (R1):

$$ \begin{array}{l@{\quad}l@{\quad}l} \text{(R1)CO}_2 + \text{H}_2 \rightarrow \text{CO} + \text{H}_2\text{O} & \Delta\text{H}_{298} = 41.2\,\text{kJ}\cdot\text{mol}^{-1} & \text{RWGS} \\ \text{(R2)nCO} + (2n+1)\text{H}_2 \rightarrow \text{C}_n\text{H}_{2n+2} + n\text{H}_2\text{O} & \Delta\text{H}_{298} = -165.5\,\text{kJ}\cdot\text{mol}^{-1} & \text{FTS} \\ \text{(R3)CO}_2 + 4\text{H}_2 \rightarrow \text{CH}_4 + 2\text{H}_2\text{O} & \Delta\text{H}_{298} = -164.8\,\text{kJ}\cdot\text{mol}^{-1} & \text{Sabatier} \\ \text{(R4)CO} + 3\text{H}_2 \rightarrow \text{CH}_4 + \text{H}_2\text{O} & \Delta\text{H}_{298} = -206\,\text{kJ}\cdot\text{mol}^{\text{‡}} & \text{FTS} \end{array} \quad (1) $$

As the RWGS is a slightly endothermic reaction, CO₂ conver- sion to CO is limited at the low temperatures required for the traditional FTS. Indeed, for a H₂/CO₂ ratio of 3 (stoichiometric ratio for CO₂ hydrogenation to –CH₂–), and temperatures between 220 and 300 °C, only 13–23% of CO₂ can be converted to CO.[14] As the exothermic FTS has no thermodynamic con- straint in this temperature range, the consecutive reaction of the CO can lead to higher CO₂ conversion for the overall pro- cess, provided that the reaction rate of FTS is equal to or higher than that of the RWGS.[14] Moreover, the competitive exothermic formation of methane from CO₂ (R3) and CO (R4) is highly favored.[14] Therefore, as traditional Co-based FTS catalysts are not particularly active for the RWGS, they mainly act as metha- nation catalysts under CO₂/H₂ feeds, and important modifica- tions are needed to modulate such a behavior.[2,14] Careful choice
<figure><img src="image_3.png" /><figcaption>**Figure 2.** Contour plot for the hydrocarbon chain-growth probability ($\alpha$) on a cobalt-based FTS catalyst, as a function of the reaction temperature and $H_2/CO$ ratio (or CO partial pressure) at a constant total pressure of 20 bar. The white lines represent the equilibrium compositions for the RWGS reaction starting from feeds with different $H_2/CO_2$ molar ratios. The standard operational window for cobalt-based catalysts employed in syngas ($CO+H_2$) FTS is indicated by the dashed frame. Reproduced with permission.[2] Copyright 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.</figcaption></figure>

of the metal oxide support, alkaline metal promoters and regulation of the metal-support interface can enhance the intrinsically marginal activity of Co-based catalysts for the RWGS, and decrease excess methanation. The thermodynamic constraint of the RWGS also sets important limitations to the chain propagations on cobalt catalysts, leading to the experimentally observed production of shorter chain hydrocarbons. As discussed by Prieto,[2] and summarized in the **Figure 2**, in the most favorable case for RWGS, under $H_2/CO_2$ feed ratio between 1 and 3 and a total pressure of 20 bar, only $H_2/CO$ molar ratios above 10 and, therefore, CO partial pressure ($P_{CO}$) lower than 1.8 bar are possible. Such a low CO partial pressure set by the RWGS equilibrium limits the chain-growth probability to values below 0.5–0.6, that is far from the industrially relevant $\alpha$ values (>0.8) achievable at relatively high $P_{CO}$ (>10 bar) under traditional FTS conditions.[2] The reasons behind the considerable dependence of the chain-growth probability on $P_{CO}$ are not completely clear, but they seem to be related to small variations of the CO coverage ($\theta_{CO}$) on the metallic cobalt surface. Indeed, under sub-atmospheric CO partial pressure, Co-based FTS catalysts mainly produce methane, despite the fact that $\theta_{CO}$, independent of $P_{H_2}$, is close to saturation. Conversely, high chain-growth probabilities are obtained upon marginal increase of $\theta_{CO}$ at industrially relevant conditions (>5 bar).[2,15,16] According to density functional theory (DFT) calculations, for a CO insertion chain-growth mechanism (Pichler–Schultz mechanism), a very high $\theta_{CO}$ on FTS cobalt catalysts plays a pivotal role in the destabilization of *CH$_x$* ($x$ = 1–2) species, thus facilitating the insertion of CO into these surface species, a key step for chain growth.[2,17] Similarly, DFT simulations on Ru-based FTS catalysts reveal that H-assisted *CO* dissociation in proximity of growing *C$_x$H$_y$* chains is an essential step for chain propagation.[2,18] Therefore, quasi-saturation CO coverages can lead to preferential dissociation of CO (monomer) in the proximity of a few growing chains, leading to high effective chain-growth probability.[2,18]

<figure><img src="image_4.png" /><figcaption>**Figure 3.** $C_{2+}$ selectivity as a function of C/H ratio over anatase and rutile Co/TiO$_2$ catalysts. Reproduced with permission.[20] Copyright 2019, American Chemical Society.</figcaption></figure>

Bredy et al. recently confirmed the relationship between $\theta_{CO}$ on metallic Co (supported on TiO$_2$ and Siralox) and selectivity toward higher hydrocarbons via operando diffuse reflectance spectroscopy (DRIFTs).[19] The selectivity to methane increases monotonously with decreasing $\theta_{CO}$ when CO$_2$ is introduced in the feed alongside CO. Moreover, the structure and oxidation state of the surface of metallic cobalt remains the same whether CO$_2$ or CO are co-fed with $H_2$, thus confirming that a high $\theta_{CO}$ is the key feature to achieve high selectivity for hydrocarbons during CO$_2$ hydrogenation. According to Visconti et al.,[13] the different selectivity between CO and CO$_2$ feeds are related to the different adsorption strengths of the two molecules, which lead to different C/H ratios on the catalyst surface. The weaker CO$_2$ adsorption on metallic cobalt results in a higher local hydrogen fractional coverage, which favors chain termination. Li et al. have proved that the increase of C/H ratio by the addition of promoters (K, Zr, Cs) results in higher $C_{2+}$ selectivity over Co/TiO$_2$ catalysts (**Figure 3**).[20]

Hence, thermodynamic restrictions strongly limit the applicability of traditional Co-based catalysts for the direct production of long chain hydrocarbons under CO$_2$/H$_2$ feed. Significant structural modifications are thus needed to furnish the Co catalysts with active species that can promote the RWGS under industrially relevant conditions, to increase their activity and limit methane production.

Nevertheless, even in combination with effective RWGS functionalities, CO fugacity strongly affects chain-growth probability, leading to the production of shorter chain hydrocarbons ($\alpha$ < 0.5–0.6). Therefore, Co-based catalysts might be used for the direct production of synthetic natural gas mixtures rich in light $C_2-C_4$ hydrocarbons, and CO$_2$-based middle distillate mix (i.e., gasoline, jet fuel, and diesel) without the necessity of a downstream hydrocracking refining treatment.

### 3. Cobalt Active Phase

Metallic cobalt can exist in three different crystal phases: $\alpha$-Co (hexagonal-close-packed, hcp), $\beta$-Co (face-centered-cubic, fcc),
and ε-Co (cubic-primitive, cp). [21-25] The latter is a metastable phase and can easily be transformed into hcp-Co and fcc-Co, which are the most common crystal phases for traditional FTS catalysts. [21-26] A phase transition from the low temperature hcp-Co phase to the high temperature fcc-Co occurs at ≈400 °C in bulk cobalt, [21,27] but can take place at lower temperatures (≤300 °C) for metal nanoparticles (NPs). [28] hcp-cobalt NPs present higher FTS activity due to the more favorable direct dissociation of CO, while H-assisted CO dissociation takes place on fcc-cobalt. [21] However, hcp-Co seems to be less resistant to water-induced re-oxidation and cobalt carbide formation. [21] It is generally admitted that for the traditional FTS, the optimum metallic cobalt NP size is between 6 and 10 nm, depending on the catalyst type and reaction conditions. [5] Below this range, Co NPs generally produce more methane and are more prone to re-oxidation. [21] Above this value, turnover frequency (TOF) is generally lower, and C₅⁺ selectivity remains almost unchanged. [5,21,29]

Regarding CO₂-FTS, most of the metallic Co-based catalysts contain fcc-cobalt. To the best of our knowledge, a robust comparison between hcp- and fcc-cobalt NPs supported on a metallic oxide doesn't exist in the current literature. However, such a study has been carried out for unsupported hcp- and fcc-cobalt NPs by Li et al. [30] Under a molar ratio of H₂/CO₂ = 4 and a pressure of 30 bar, hcp-Co shows higher activity than fcc-Co. CO₂ dissociates directly into chemisorbed CO* and O* on both cobalt phases, but the different adsorption strengths of the CO* intermediate lead to different product selectivity. On hcp-Co, strongly adsorbed CO* is hydrogenated to methane, while over fcc-Co, due to a weaker adsorption, it easily desorbs to produce gaseous CO.

The effect of cobalt particle size on CO₂ hydrogenation is still significantly understudied and will likely become an important discussion topic in the future. It has been reported that for CO₂ hydrogenation, 10 nm Co particles supported on mesoporous silica (MCF-17) display higher TOF than 3 and 7 nm ones. [31] On the other hand, no significant differences in terms of product distribution are detected at a pressure of 6 bar and H₂/CO₂ ratio of 3. [31] Similarly, for a promoted Co-Na-Mo based catalyst, NPs smaller than 2 nm supported on MgO show low RWGS conversion and negligible FTS activity. [32] Larger Co NPs (≈15 nm)

supported on SiO₂ and ZSM-5 lead to higher CO₂ conversion and hydrocarbon selectivity. [32] Finally, further increase of Co NPs size to 25–30 nm has a detrimental effect on the global CO₂ conversion. [32]

Metallic cobalt is generally accepted as the active phase during traditional FTS, and is the most studied cobalt phase for CO₂-FTS as well. [5,21] However, metallic cobalt and cobalt oxides often coexist under FTS conditions; [21] and, in 2014, evidences of a highly active cobalt oxide catalyst (CoO/TiO₂) for the FTS and CO₂-FTS reactions were presented by Melaet et al. [33] The higher activity and selectivity of such catalyst were ascribed to the formation of an active and unique interface between CoO and the TiO₂ support. Similar results were obtained in a more recent and complete study on the activity and selectivity of metallic Co and CoO supported on reducible (CeO₂ and TiO₂) and non-reducible (SiO₂ and Al₂O₃) metal oxides. [3] Metallic cobalt is more active than CoO, except when supported on TiO₂ P25 (a mixture of rutile and anatase TiO₂ phases). CoO/TiO₂ produces also more C₂⁺ products compared to its metallic counterpart, with a higher content of olefins, due to its lower hydrogenation ability. Conversely, Co/TiO₂ exclusively produces paraffins. DRIFTs analysis highlights that metallic Co catalysts follow the direct dissociation mechanism, indicated by the presence of adsorbed CO as intermediate. The latter is not present on CoO catalysts, which instead favor a H-assisted mechanism (Figure 4), characterized by formyl, formate and carbonate intermediates. [3] These results are in good agreement with theoretical works, which found that CO adsorption is strong on metallic Co (−1.99 eV = −192 kJ mol⁻¹) and weak on CoO (−0.33 eV = −32 kJ mol⁻¹). [3,34] Kinetic insights reveal that direct dissociation occurs at higher rate than H-assisted pathway. Thus, CoO-based catalysts can benefit from a higher H₂ partial pressure, which is instead detrimental for metallic cobalt ones.

Wang et al. [35] have recently shown that, for cobalt active species, the regulation of their valence state can lead to a different selectivity during CO₂ hydrogenation. According to this study, CoO particles are extremely active for the production of CO via RWGS, while metallic Co species promote methanation. [35] FTIR characterization highlights that, on CoO, adsorbed formates are key intermediates for the formation of CO. Conversely, on Co⁰ sites, the carboxylate intermediate evolves to

<figure><img src="image_5.png" /><figcaption>**Figure 4.** Simplified reaction pathways for CO₂ hydrogenation to hydrocarbons over Co-based catalysts. In the direct dissociation mechanism, the COads intermediate can either desorb or form Cads and then hydrocarbon products. The H-assisted pathway involves surface carbonates, formates, and formyl as intermediates. The latter can either be fully hydrogenated to methane or converted into olefins or paraffins via C—C coupling (FTS). Reproduced under terms of the CC-BY license. [3] Copyright 2022, The Author(s), published by Springer Nature.</figcaption></figure>
adsorbed CO, which in turn is hydrogenated to CH₄.[35] Notably, weak CO adsorption on CoO favors its desorption. Stronger CO adsorption on metallic Co, along with higher surface hydrogenation resulting from enhanced H₂ activation, leads to CH₄ formation.[35]

Indeed, metallic Co is more active than CoO for H₂ dissociation, leading to a high amount of surface-active H species.[36-38] Moreover, the high electron density near the Fermi level provides Co⁰ with excited electrons for the different hydrogenation steps.[36,39] Both DFT and experimental results show better CO₂ activation on Co than on CoO.[3,36,39] Additionally, Coδ⁺ sites at CoOₓ-CeO₂ interface limit formate hydrogenation to methane,[40] while the synergy between Co and Zr in uniform Co-O-Zr sites accelerates the decomposition of formates to CO, leading to superior RWGS performances.[35,41] CoO can be obtained via a controlled reduction,[3] or can be formed in situ via re-oxidation of small metallic cobalt NPs.[21] Finally, CoO can be stabilized via strong metal support interaction (SMSI), as illustrated in the following section of this review.[3,21,33] As CoO seems to be particularly active for RWGS reaction, while metallic Co is known for its optimal CO-FTS selectivity, a close cooperation between these different active sites might lead to superior CO₂-FTS selectivity toward long chain hydrocarbons. However, as summarized by Lin et al.,[5] "synergistic dual site" in close proximity, namely Co⁰-Co₂C,[42-44] Co⁰-Coδ⁺.[45,46] or Co-Co²⁺/Co₂C,[47] can promote the formation of higher alcohols. The metallic cobalt is necessary for the initial CO cleavage and the consecutive formation *CHₓ species.[5] The second site, most likely a carbide, although it has been proposed to work also with Coδ⁺/Co²⁺, is necessary for the non-dissociative adsorption and insertion of CO.[5] However, the fabrication of stable dual site structures remains a challenge.[5]

Besides metallic and oxide cobalt, cobalt carbide can also be formed during traditional CO-FTS.[21] The role of this phase is quite controversial, as some studies correlate its presence to the formation of olefins and oxygenates,[21,43,48-50] while others to catalyst deactivation.[21,51-53] As previously mentioned, cobalt carbide is an active site for CO non-dissociative adsorption and insertion in dual site structures during the formation of higher alcohols.[5,36] According to Yu et al.,[54] a Co₂C/γAl₂O₃ catalyst is very active for CO₂ methanation, with a CH₄ selectivity close to 100%. Additionally, Khangale et al.[55] suggest that the formation of Co₂C in a Co-K/Al₂O₃ catalyst is responsible for catalyst deactivation and increasing CH₄ formation with increasing reaction time. However, recent works report that a simple morphological modulation of Co₂C nanoprisms can lead to excellent low-temperature RWGS or cascade RWGS-FTS reaction activity, leading to the production of olefins and alcohols.[56,57] The role of cobalt carbides in CO₂ hydrogenation is still significantly understudied and will likely become an important discussion topic in the future.

To summarize, it seems that depending on the support, both metallic cobalt and and cobalt oxide can be active phases during the CO₂-FTS reaction to long chain hydrocarbons. Metallic cobalt is more active on most supports, while CoO is a better choice for TiO₂ support. Such a behavior stems from the formation of a unique interface between the cobalt oxide and the reducible oxide, which will be discussed in more details in the following sections of this review. Moreover, CoO shows interesting activity

toward the RWGS reaction. Nonetheless, further investigations are definitively needed to clarify the role of CoO and Co⁰-CoO interfaces during CO₂-FTS, as well as the possible formation of cobalt carbide and its consequences on CO₂ hydrogenation.

## 4. Effect of Support and Metal-Support Interfaces

Due to its utilization in industry, Al₂O₃ is the most extensively studied support for traditional Co-based FTS catalysts.[58] SiO₂, TiO₂, and ZrO₂ have been also largely employed in the preparation of Co-based FTS catalysts.[58] Metal-support interactions (MSI) play a pivotal role on the dispersion, reduction, and activation behavior of the active metal phase.[5] SMSI can lead, mainly through electronic and geometric effects, to different situations. On one hand, we can have the formation of undesired refractory compounds (e.g., CoAl and CoSi) or the complete encapsulation of the active metallic phase.[5,58] On the other hand, it has been shown that charge transfer, metal surface coverage by a thin layer of reducible oxide, and formation of special metal-oxide interfaces can result in highly efficient FTS catalysts.[5,59-62] The latter are common features of reducible metal oxides, which are further characterized by the formation of oxygen vacancies (Ovac) on their surface. Oxygen vacancies can be extremely useful for FTS, as they can promote CO₂[63] as well as CO dissociation.[59]

The effect of support and metal-support interfaces is even more important during CO₂ hydrogenation. Recently, ten Have et al.[3] have studied the CO₂ hydrogenation over metallic Co and CoO (≈10 wt%), supported on reducible (CeO₂ and TiO₂) and non-reducible (SiO₂ and Al₂O₃) oxides under CO₂-FTS relevant conditions (T = 250 °C, P = 20 bar, H₂/CO₂ = 3). Cobalt particles size above 10 nm on all the supports (14–17 nm for SiO₂, Al₂O₃ and TiO₂; and 37 nm for CeO₂) avoided interfering size effects on the activity.[3] As already mentioned in the previous section, CoO/TiO₂ is the most active and selective system. However, among the investigated supports, TiO₂ leads to the higher activity even with metallic cobalt NPs (Figure 5). The reason lies in the optimum reducibility of the TiO₂ support, which allows the weakening of the CO bond. Indeed, for the reduced Co/TiO₂, a red shift of the COads peak (1994, 1992, 1987, and 1980 cm⁻¹ for Co/SiO₂, Co/Al₂O₃, Co/CeO₂, and Co/TiO₂, respectively), indicates a weaker CO bond on reducible oxides, as observed by DRIFTS.[3] Moreover, evident and broad signals of surface (bi)carbonates and formates were detected for the catalysts prepared on reducible supports, and not on non-reducible oxides.[3] This is due to the different basicity of the supports (TiO₂ > CeO₂ > Al₂O₃ > SiO₂).[3] CO₂ can interact directly with O²⁻ surface ions and -OH surface groups, leading to the formation of carbonate and bicarbonate species, respectively.[3,64] Formates originate from the interaction of CO₂ with surface Ovac, which are easily formed on reducible oxides such as TiO₂ and CeO₂.[3,65] Besides, hydrogen spillover is known to occur ten orders of magnitude faster on TiO₂ than on Al₂O₃.[66] Finally, a peak splitting was observed for carbonates on Co/TiO₂. The latter can be ascribed to different types of coordination and/or different adsorption centers, suggesting the formation of a new interface between Co and TiO₂ with different adsorption features.[3,67-69]
<figure><img src="image_6.png" /><figcaption>**Figure 5.** Catalytic activity (cobalt-time-yield (CTY)) (a) and selectivity (b) of Co-based catalysts as CoO (suffix: -ox) and metallic Co (suffix: -red) supported on reducible (CeO₂ and TiO₂) and non-reducible (SiO₂ and Al₂O₃) oxides. Reaction conditions: T = 250 °C, P = 20 bar, H₂/CO₂ = 3. Time-on-stream = 10 h. Reproduced under terms of the CC-BY license.[3] Copyright 2022, The Author(s), published by Springer Nature.</figcaption></figure>

In addition, metal oxide supports with the same chemical composition but different crystal phases can strongly affect the catalytic performance of the final catalysts,[20,70] both in terms of activity and selectivity. The support allowing the higher activity and selectivity of the previous study[3] is a commercial TiO₂-P25, which contains a mixture of anatase (80%) and rutile (20%). Cobalt catalysts with the same metal loading (≈10 wt%) and particle size (20 nm) were investigated on the two pure TiO₂ crystal forms by Li et al.[20] Co/rutile-TiO₂ catalyst shows higher activity and selectively for CO₂ hydrogenation to CH₄. Conversely, Co/anatase-TiO₂ catalyst has a lower CO₂ conversion, and produces mainly CO. By simply increasing the calcination temperature of the anatase-TiO₂ at 800 °C, the product selectivity completely changes from CO to CH₄, and the CO₂ conversion increases to the same values of the catalyst prepared on the rutile-TiO₂ support. Such a change is due to the surface phase transition of anatase to the rutile phase, thus confirming the pivotal role of the support crystal phase.[20] DRIFTs and temperature programed desorption (TPD) characterizations suggest that the different activity and selectivity can mainly be attributed to the different ability of anatase and rutile to adsorb CO₂, CO, and H₂. Indeed, the weak bond of *CO intermediate over Co/anatase-TiO₂ leads to its immediate desorption as gas-phase CO, and negligible CO₂ conversion. Conversely, stronger adsorption on Co/rutile-TiO₂ enables the formation of the key intermediate formate species, which are further converted to CH₄, along with higher CO₂ conversion.[20] The modification of such catalysts by addition of different promoters (K, Zr, and Cs) leads to stronger adsorptions, allowing increasing surface C/H ratio, and consequently the C₂+ selectivity (Figure 3). For the rutile-based catalysts, the C₂+ selectivity increases slowly at low C/H ratio < 0.5, and quickly at C/H > 0.5. Distinctly, on anatase-

based catalysts, higher increase occurs at C/H < 0.5 and lower at C/H > 0.5.[20] Notably, anatase-supported catalysts always have higher C₂+ selectivity than the rutile-based ones, even at the same C/H ratio. The higher C₂+ selectivity of anatase supported catalysts is attributed to specific metal-support interactions. Additionally, for both catalysts Co NPs are covered by a thin TiO₂ overlayer, the thickness of which can vary from 2 to 4 nm depending on the calcination treatment. X-Ray photoelectron spectroscopy (XPS) also reveals the presence of anion vacancies and defects that can adsorb and activate CO₂ and CO.[20]

As mentioned in the previous section, in 2014, Melaet et al.[33] reported a particularly active CoO/TiO₂ catalyst. In their work, 10 nm Co NPs are prepared via colloidal route and then dispersed on macroporous TiO₂ and mesoporous SiO₂ (MCF-17). Over the SiO₂ support, metallic cobalt shows higher TOF than CoO during both CO₂ (H₂/CO₂ = 3) and CO (H₂/CO = 1) hydrogenation at 250 °C and 5 atm of pressure. Conversely, for the TiO₂ support, CoO performs better than its metallic counterpart, despite a higher olefin production due to the lower hydrogenation ability.[33] To obtain CoO and metallic Co, the samples are treated under H₂ at 250 and 450 °C, respectively. XPS characterizations demonstrate that after the treatment at 250 °C in H₂, the content of Co on the surface is equal to 29 atomic %, and drops to 20 atomic % at 450 °C. Additionally, at 250 °C, Ti on the surface is only partially reduced, whereas full reduction to Ti³⁺ is achieved at 450 °C. Therefore, the high temperature reduction leads to an encapsulation of the metallic Co active phase, while CoO wetting of the support takes place at 250 °C (Figure 6).

The latter enables the formation of an extended unique interface between CoO and TiO₂ with enhanced activity for CO and CO₂ hydrogenation.[33]

<figure><img src="image_7.png" /><figcaption>**Figure 6.** Metallic Co encapsulation and CoO wetting over TiO₂ after reduction at 450 and 250 °C, respectively. Reproduced with permission.[33] Copyright 2014, American Chemical Society.</figcaption></figure>
As illustrated by Khangale et al.,[71] an unpromoted 15 wt% Co/ZrO₂ catalyst produces mainly methane with a selectivity of 99.4%, which decreases upon K promotion. For promoted Co-Na-Mo catalysts, the systems involving CeO₂ and TiO₂ supports provide higher α values than the ones prepared on SiO₂, Al₂O₃, MgO, ZrO₂, and ZSM-5.[32]

Therefore, the effect of the support is extremely important during CO₂ hydrogenation to hydrocarbons. Independently from the cobalt active phase, catalysts prepared on reducible oxides, and especially TiO₂ are more active than those prepared on irreducible oxides.

On one hand, the higher activity stems from the ability of reducible oxides to create specific metal-support interfaces, originating from SMSI, which can strongly benefit CO₂ activation to different products, from C₂⁺ to CH₄ and CO. For instance, Co/CeO₂ catalysts show superior performance for CO₂ hydrogenation to CH₄ due the formation of O_vac.[72] and higher reducibility linked to Co-CeO₂ interactions.[73] Additionally, the selectivity of Ir/TiO₂ catalysts can be completely changed from CH₄ to CO thanks to the formation of a reduced TiOₓ overlayer around Ir NPs.[74] To benefit from SMSI, it is necessary to find the optimum interaction strength that allows the formation of highly active interfaces, without inducing complete encapsulation that can be extremely detrimental.[75] A more detailed analysis of SMSI and its evolution for the most active system (Co/TiO₂) is given in the section 4.3.

On the other hand, reducible oxides are rich of surface defects that can be exploited for the direct activation of CO₂, CO, and H₂. Moreover, defects can play important roles during the preparation of Co-based catalysts, from direct reduction of the different cobalt precursor to the promotion of SMSI. A more detail analysis of these effects is available in the sections 4.1 and 4.2.

## 4.1. CO₂ Activation Over Surface Defects

CO₂ is a linear non-polar molecule with two equivalent C=O double bonds and a high oxidation state of carbon (+4) that makes it thermodynamically very stable. Therefore, reducible C=O bond cleavage requires a high energy input (≈750 kJ·mol⁻¹), which can be reduced via a proper activation.[76] CO₂ activation generally involves altering the molecular properties, such as the C=O bond length or O=C=O angle, and can occur both nucleophilically and/or electrophilically through the carbon or oxygen atom, respectively.[76] Generally, the activation of CO₂ molecule over heterogeneous catalysts involves its adsorption, followed by an electron transfer from the catalyst to the molecule. In such a route, metal NPs can serve as active sites for full or partial transfer, leading to the formation of CO₂⁻ or CO₂δ⁻, respectively. The interaction of CO₂ with single metals is generally weak, but can be improved by the addition of promoters (e.g., alkali[76-78]), or the formation of alloys.[76] However, some metals (Fe, Ni, and Co) can activate CO₂ more strongly as single metals than as constituents of an alloy.[76,79] On cobalt, DFT calculations demonstrated that CO₂ activation depends on particle size: Co₅₅ nanoclusters show higher CO₂ dissociation activity than Co₁₃ and Co₃₈.[80] Additionally, CO₂ dissociation becomes easier for all metallic clusters in the presence of H₂.[81]

On the surface of stoichiometric metal oxides, CO₂ activation can occur over both metal (M⁺⁺) and oxygen (O²⁻) ions. It can take place via coordination of CO₂ terminal oxygen atoms to one or two adjacent metal ions, while the carbon atom of CO₂ can interact with surface oxygen sites. These interactions result in monodentate or bidentate carbonate species. CO₂ activation can also occur via the σ-bond or π-bond activation on metal and oxygen ions, respectively.[76,82] On defect-rich non-stoichiometric metal oxides (e.g., TiO₂₋ₓ, CeO₂₋ₓ, etc.), O_vac can interact directly with carbon and oxygen atoms of CO₂, leading to enhanced CO₂ adsorption. According to DFT calculations, CO₂ adsorption on reduced ceria (110) is thermodynamically favored compared to adsorption on the stoichiometric ceria (110) surface.[76,83] Similarly, CO₂ is preferentially adsorbed at the O_vac defects of the TiO₂ (110) surface.[76,84] On the stoichiometric TiO₂ (001) surface, CO₂ dissociation is not observed, and only monodentate carbonate species can be obtained via DFT.[63,76] The introduction of O_vac defects generates new adsorption configuration with the formation of a CO molecule, which can easily desorb.[63,76] CO₂ chemisorption was also studied at room temperature using in situ DRIFTs. Besides the carbonate and bicarbonate species resulting from the interaction with the oxygen sites, CO₂ chemisorption is also observed at Ce³⁺, Ce⁴⁺, Ti³⁺, and Ti⁴⁺ sites.[76,85] Therefore, O_vac can enhance CO₂ adsorption and dissociation via the creation of a high number of stronger binding sites.[86] Additionally, O_vac can promote specific reaction pathways, via stabilization of key intermediates. Bobadilla et al.[87] investigated the RWGS reaction on Au/Al₂O₃ and Au/TiO₂ catalysts. On both catalysts, CO₂ initial activation occurs on the supports, as Au NPs are not able to promote direct dissociation to CO and O.[87] In the case of Au/Al₂O₃, CO₂ initially adsorbs on the hydroxyls of the Al₂O₃ to generate bicarbonate species. Then, H atoms activated on gold spill over to the support to react with the bicarbonates, leading to the formation of “fast formates,” which can finally decompose to CO.[87] Conversely, on the reducible TiO₂ support, the reaction proceeds at lower temperatures via a redox mechanism involving the direct participation of Ti³⁺, surface hydroxyl and O_vac to form hydroxycarbonyl intermediates, which further decompose to CO and water (Figure 7).[87]

A direct correlation between the relative concentration of O_vac on the TiO₂ support and the RWGS reaction rate was evidenced. Moreover, a low level of CO production was observed during a reference catalytic test using bare TiO₂, highlighting that CO₂ activation can takes place also in absence of Au species.[87] Thus, the main role of the metal NPs is the H₂ activation, and its subsequent spillover to the TiO₂ surface to increase the number of O_vac for CO₂ activation.[87] In this respect, the formation of O_vac is highly favored at the metal-oxide interface. This is not only due to the increased presence of highly reducing H species,[87] but also to the higher reactivity of the O atoms at the boundary region.[88] The latter can be more easily removed in comparison to the other O atoms of the surface. Indeed, the electron density of the reduced system tends to be localized on undercoordinated cations (e.g., Ce³⁺), which are largely present at the nanoscale.[88] Moreover, nanostructures are in general more flexible than extended surfaces or bulk materials: atomic relaxation around the O_vac occurs at lower costs, thus stabilizing the defect.[88] Finally, the proximal metal
<figure><img src="image_8.png" /><figcaption>**Figure 7.** Suggested mechanism for RWGS reaction over Au/TiO₂ catalysts. Reproduced with permission.[87] Copyright 2018, American Chemical Society.</figcaption></figure>

NPs play a pivotal role in delocalizing the excess of electrons, resulting from the generation of a neutral vacancy.[88]

Hence, providing Co-based catalysts supported on reducible oxides with large number of Ovac can be a useful strategy to increase the activity for CO₂ hydrogenation, and tune the selectivity toward the desired products. A comprehensive description of the different techniques for creating Ovac is beyond the scope of this review. However, the reader can find several examples in the literature concerning Ovac formation and characterization over CeO₂[1,89] and TiO₂.[90–93]

## 4.2. Defect Mediated Reduction/Growth of Metal Nanoparticles

Besides their ability to directly activate CO₂, surface defects on reducible oxides can also play an important role during the preparation of supported catalysts. Indeed, the electrons located on the Ovac can directly interact with the ionic metal precursors via an in situ redox reaction, leading to the spontaneous formation of metallic NPs. As such process does not require any foreign reducing agents or stabilizing molecules, and it takes place in a single step, it can be exploited for the preparation of several metal/semiconductor composites, with superior performance during both photo[94–96] and thermal catalysis.[97] Moreover, the metal particle size can be controlled by different parameters, from the amount of metal precursors[94] to the amount of surface defects,[97] as both these parameters can affect the nucleation and growth kinetics of metal NPs. Certainly, the possibility of controlling the metal particle size by tuning the amount of surface defects is the most interesting option. A negative correlation between Pd dispersion and surface defects concentration was highlighted by Cao et al.[97] Indeed, on a low-defect CeO₂ support, the strong electrostatic interaction with the metal precursor (with the consequent formation of many Pd nuclei), and the weak reducing capacity of the support lead to the

formation of smaller particles. Conversely, the fewer Pd nuclei and the faster growth on the defect rich CeO₂ lead to the formation of larger Pd NPs. The latter show also higher electron density than smaller NPs. Such electron enrichment, due to SMSI, favors the H₂ activation and consequently the spillover, which in turn contributes to the in situ formation of Ovac for CO₂ activation. Further evidence of the SMSI formation during the defect mediated reduction of metal precursor resides in the presence of gold NPs partially embedded in the surface of TiO₂, as demonstrated by Pan et al.[95] The SMSI is known to play a crucial role in regulating the catalytic activity, the selectivity and the stability of metal NPs supported on reducible oxides. It is also known that the existence of Ovac on the TiO₂ surface, either from reduction or doping, can largely favor decoration and encapsulation of Pd clusters.[98,99] The SMSI was initially thought to be an exclusive feature of group VIII metals, characterized by high work function (φ) and surface energy. However, it has been recently shown not only that SMSI is also possible for metals with a lower work function or surface energy (γ) such as gold, but in addition, that for the Au/TiO₂ system the SMSI is more likely to occur on large NPs (≈9 and 13 nm) than on small ones (≈3 and 7 nm).[100]

It must be noted that all the above-mentioned examples of catalyst preparation via defect mediated reduction of metal precursors involve noble metals. This is likely due to two main characteristics of noble metals: i) their high reducibility; and ii) the low metal loading that is usually employed for the preparation of such catalysts. Conversely, for a non-noble metal such as Co, which has a negative reduction potential (Co²⁺ + e⁻ → Co(s) (E° = -0.282 V)) and is usually employed at high loading (>10 wt%), the role of the defect mediated reduction of ionic precursors during catalyst preparation has not been clarified yet. However, Qiu et al.[101] have recently shown that Ovac on TiO₂ can readily reduce pre-synthesized individual Co₃O₄ NPs directly into CoO/Co⁰. It is possible to rationalize the impact
<figure><img src="image_9.png" /><figcaption>**Figure 8.** Co₃O₄ NPs reduction into CoO/Co⁰ by O_vac on rutile substrate and subsequent reductions by H₂ reduction (350 °C) and syngas adsorption (220 °C). Adapted under terms of the CC-BY license.[101] Copyright 2022, The Authors. Published by American Chemical Society.</figcaption></figure>

of O_vac on the Co NPs on the base of the standard potentials: Ti⁴⁺ + e⁻ → Ti³⁺ (–0.56 V); Co²⁺ + 2e⁻ → Co (–0.28 V); and Co³⁺ + e⁻ → Co²⁺ (1.82 V). The potential for the reduction/oxidation of Co/Ti is positive: Co³⁺ + Ti³⁺ → Co²⁺ + Ti⁴⁺ (2.38 V), and Co²⁺ + 2Ti³⁺ → Co + 2Ti⁴⁺ (0.84 V). Thus, the reduction of Co³⁺ to Co²⁺ and Co²⁺ to Co⁰ by surface O_vac/Ti³⁺ is spontaneous if these species are present in sufficient quantities, and does not need any additional reducing agent.[101] Notably, the extent of the reduction is dependent on the NP size, with smaller particles (<8 nm) being more reduced than the larger ones. Indeed, O_vac are particularly good at reducing the edges of larger particles, while the core remains partially oxidized (Figure 8).

The latter can be further reduced after H₂ and syngas treatments, accompanied by the consumption of O_vac after H₂ exposure. Conversely, an increase in the amount of O_vac is observed after the treatment in syngas. Finally, O_vac prevents the complete reoxidation of small Co NPs (>8 nm) during syngas exposure.[101]

Therefore, introduction of O_vac on reducible supports is a promising and straightforward method to develop new catalytic materials with higher reducibility and stability. Moreover, the combination of such synthetic strategy with careful post-synthesis treatments may promote the formation of specific metal-support interfaces with superior performance for CO₂ hydrogenation.

### 4.3. SMSI Evolution for Co/TiO₂ Catalysts

Since the first report of SMSI in 1978 by Tauster et al.,[102] great interest toward this effect arose in the catalysis community. In the earlier studies, SMSI was characterized by the inhibition of CO and H₂ chemisorption on group VIII metals supported

on TiO₂ after a high temperature reduction.[98] The SMSI effect was explained as an electron transfer between the support and the metal. Nowadays, it is clear that SMSI can promote three different effects: i) electronic; ii) geometric; and iii) bifunctional.[98] The electronic effect consists in the charge redistribution that can occur at the interface between the metal and the support. The degree of electron transfer depends on different factors, spanning from the surface defects on the oxide to the size of the metal clusters.[98] The geometric effect involves a partial (decoration) or total (encapsulation) covering of the metal clusters surface by a TiOₓ layer, usually after high temperature reduction (450–500 °C).[98] The commonly accepted two-step encapsulation mechanism involves first the mass transport of interstitial Ti cations (Tiⁿ⁺, n = 3,4) near the surface region, promoted by the high diffusivity of Ti in TiO₂ at high temperatures. To fulfill such step, the work function of the metal must be higher than that of TiO₂ (φₜiO₂(110) ≈ 5.2 eV).[99] The second step involves the mass transport of TiOₓ (x < 2) onto the surface of the metallic cluster. Metals with higher surface energy (γₘ) than the one of the oxides are required (γₘ > γₜiOₓ).[98,99] Therefore, reduced or n-type doped oxides with small surface energies favor encapsulation.[99] The geometric effect can also induce morphological changes in the metals. Indeed, metal particles can be flattened and stabilized on the partially reduced surface of the oxide support.[98,103] The bifunctional effect involves the creation of new reaction sites at the boundary between the metal and the support. These new sites show completely different properties in terms of lattice constant, electron density and composition, which can significantly modify the catalytic activity and the selectivity. The bifunctional effect includes the possibility for the reactive species to migrate or spillover either from the metal or the support to the boundary or edge where the chemical reactions takes place.[98]
<figure><img src="image_10.png" /><figcaption>**Figure 9.** a) HRTEM images of a Co/TiO₂ catalyst depicting the decoration of Co metal NPs by an amorphous TiO₂ layer. Adapted with permission.[104] Copyright 2011, Royal Society of Chemistry. STEM images of b) Co/TiO₂ reduced at 600 °C (layer size: 2.8 nm) c) Co/TiO₂ calcined and reduced at 600 °C (layer size: 4.0 nm). Adapted with permission.[105] Copyright 2015, Elsevier.</figcaption></figure>

However, our comprehension of the SMSI is continuously evolving over the years. For instance, transition metals with small work function or low surface energies such as Cu, Ag, Au, and Co were initially thought to withstand encapsulation.[99] while recent studies have shown that decoration and encapsulation are possible also for these and other metals. Direct evidence of the SMSI encapsulation effect in a 10 wt% Co/TiO₂ (pure anatase) catalyst was highlighted by TEM imaging by De la Peña et al.[104] After high temperature reduction (500 °C, 2 h), two types of cobalt NPs were identified on this sample: i) partially encapsulated Co⁰ NPs; and ii) Co⁰ NPs covered by a TiOₓ amorphous overlayer with a thickness of a few angstroms. Some of these encapsulated Co NPs show some fringes due to the non-epitaxial growth between the metal and the support (Figure 9a). Moreover, the formation of Co—O—Ti bonds and the suppression of CO hydrogenation activity during FTS catalytic tests confirm the presence of SMSI. Similar results were obtained also by Lee et al.[105] on a catalyst containing ≈5 wt% of Co supported on commercial TiO₂-P25. A TiOₓ layer showing striations and a thickness between 2.8 and 4.0 nm is formed after high temperature calcination and reduction (Figure 9b,c). Characterizations unveiled that the formation of the TiOₓ layer occurs already on the Co₃O₄ particles during the calcination step (300–400 °C). The reduction to metallic Co can be complete or only superficial, depending on the reduction temperature, and the final thickness of the TiOₓ layer is largely influenced by the treatment conditions.

To study the influence of different support phases on the extension of SMSI, Bertella et al. prepared Ru-promoted Co catalysts (0.5 wt% Ru, 10 wt% Co) on both anatase and rutile TiO₂ for CO-FTS.[106] According to this study, the extent of SMSI decoration is more significant for Co supported on anatase. Moreover, on the anatase based catalyst, the SMSI is partially reversible during the FTS reaction (220 °C, 20 bar, H₂/CO = 2). Most of the previously mentioned studies are performed on traditional Co/TiO₂ catalysts, in form of powder or pellets, with particles in close proximity resulting from the high metal loading (up to 20 wt%). Consequently, the characterization of the SMSI and its evolution during the different synthesis steps and post-synthesis treatments is extremely difficult. To have a better understanding of the SMSI evolution in cobalt-based catalysts, Qiu et al. prepared two well-defined model cobalt samples using flat single crystal SiOₓ-Si (110) and rutile-TiO₂ (110) supports, covered by a monolayer of highly monodispersed

Co NPs with a large inter-particle distance (>100 nm).[107] A combination of surface sensitive spectroscopic and microscopic methods was employed to characterize the evolution of MSI during reduction-oxidation-reduction (ROR) treatments. Such treatments are commonly used industrially to regenerate or enhance the catalytic activity by improving metal dispersion, reducibility and MSI.[62,107–110] Weak interactions on SiOₓ-Si allow the complete reduction of Co NPs, although they migrate and agglomerate during ROR (reduction at 350 or 500 °C, oxidation at 300 °C). In contrast, stronger MSI on TiO₂ leads to only partial reduction of surface exposed cobalt. Moreover, SMSI over TiO₂ avoids the agglomeration of Co NPs, which can however spread on the support and eventually assume a fried-egg-like shape (Figure 10).[107]

Such a spreading increases the exposed surface area of Co NPs and their overall electronic state, both of which may affect catalytic activity and selectivity. Nonetheless, if the spreading becomes extensive, this can lead to non-reducible CoTiO₃ species, which may be detrimental to reactions involving metallic cobalt as an active phase.[107]

## 5. Effect of Promoters

Promoters are crucial for FTS catalysts, as they can enhance activity, stability and selectivity.[5] Co-based FTS catalysts usually contain noble metal promoters,[5,111] which can promote the reduction of metal oxides into active metal particles, thus lowering the temperature during the activation and regeneration procedures, and limiting oxidation during the FTS.[111] Moreover, noble metal promotion can also affect the catalytic properties under relevant FTS conditions. The activity usually benefits from noble metal addition, while the effects on C₅⁺ selectivity can be highly dependent on the promoter used.[112] Alkaline promoters can decrease the selectivity to methane, by favoring the formation of higher hydrocarbons, along with an increase of olefin concentration in the gasoline product fraction.[113] The addition of alkaline promoters is even more important during CO₂-FTS. Indeed, such promoters, having high basicity, can enhance CO₂ adsorption, thus limiting the formation of CH₄ and increasing C₅⁺ selectivity.[114,115] According to Li et al.,[30] the addition of K decreases the difference in catalytic performances between unsupported hcp- and fcc-Co phases. CO₂ conversion increases over both K-hcp-Co and K-fcc-Co, reaching similar
<figure><img src="image_11.png" /><figcaption>**Figure 10.** Outline of the Co NP evolution on TiO₂(110) and SiOₓSi(100) supports after ROR. The spreading of Co NPs onto the surface of TiO₂ forms a fried-egg shape resulting in strong interaction with the support to produce CoTiO₃, while Co NPs on SiOₓSi tend to move and agglomerate into bigger particles. Reproduced under terms of CC-BY license.[107] Copyright 2020, The Authors. Published by Royal Society of Chemistry.</figcaption></figure>

values, CO becomes the dominant product and C₂+ start to be formed, with a selectivity that increases to ≈25% at 400 °C. Potassium addition increases the electron density around Co NPs, strengthening CO₂ adsorption and leading to a different reaction pathway.[30] Similarly, the addition of K to a 15 wt% Co/Al₂O₃ catalyst decreases CH₄ selectivity and increases C₂+ production, with an optimum K loading of 6 wt%.[116] Promotion by Zr, K, and Cs improves CO₂, CO and H₂ adsorption over anatase- and rutile-TiO₂ supported cobalt catalysts.[20] Zr addition modifies the reaction pathway over anatase-supported catalysts toward formate intermediate species, enabling the subsequent hydrogenation of CO to CH₄ and C₂+ species. The surface C/H ratio benefits from promoter addition (unpromoted < Zr-promoted < K-Zr-promoted ≈Cs-Zr-promoted) resulting in a higher C₂+ selectivity (Figure 3).[20]

Alkaline promoters play a pivotal role also for cobalt-based bimetallic catalysts. Indeed, the introduction of metals that are more active for RWGS reaction (e.g., Fe, Cu) can slightly improve the selectivity toward C₂⁺ products. Such improvement becomes noticeable when alkaline promoters are combined to the bimetallic catalysts. Shi et al.[114] investigated the CO₂ hydrogenation to long-chain hydrocarbons over a series of K-promoted (0–3.5 wt%) Co-Cu/TiO₂ catalysts. The addition of suitable amounts of K (2.5 wt%) suppresses the CH₄ formation and increases the C₅⁺ selectivity. This trend is related to the enhanced CO₂ chemisorption and the reduced H₂ adsorption detected upon K promotion.[114] Similar results were obtained in a follow-up study,[115] where the authors examined the promotion by various alkali metals (Li, Na, K, Rb, and Cs). Among the different catalysts, the Na-promoted Co-Cu/TiO₂, because of its stronger basicity, shows the highest C₅⁺ selectivity.[115] Likewise, alkali-promoted Fe-Co catalysts show superior selectivity toward C₂⁺ products and negligible CH₄ production.[9,77,117,118] However, such catalysts usually produce more light olefins and present lower overall activity than monometallic Co-based catalysts.[9,10]

The improved selectivity originates from the ability of alkali-promoted Fe oxides and carbides to enhance RWGS and CO hydrogenation to C₂⁺, with similar behavior and product distribution than traditional FTS Fe-based catalysts.[9,77,117,118] Indeed, on Fe-based catalysts, Fe₃O₄ is normally responsible for the RWGS reaction, while iron carbides account for C—C coupling via traditional FTS.[8,119] However, during CO₂-FTS, high CH₄ selectivity is obtained for almost all unpromoted iron-based catalysts.[8] Alkali promoters can significantly enhance the formation of longer chain hydrocarbons and olefins in several ways. They can i) promote the carburization of iron species; ii) enhance CO₂ and CO adsorption; iii) suppress H₂ adsorption on the catalyst surface; and/or iv) suppress re-adsorption and re-hydrogenation of olefins.[8,120–124] Therefore, the higher CO/H₂ and CO₂/H₂ ratios promote CO₂ conversion and olefin selectivity.[8] According to Jiang et al.[125] the addition of a small amount of Co to a K-promoted iron-based catalysts can increase both CO₂ conversion and selectivity toward C₂⁺ hydrocarbons. Indeed, in such bimetallic Fe-Co catalysts, cobalt can contribute to the increase of the CO conversion via traditional FTS.[125] The intimate contact between the two metals facilitates the spillover of the CO intermediate from the Fe₃O₄ where it is produced via RWGS, to the cobalt sites. Therefore, CO conversion can proceed on both Co and Fe₅C₂ sites.[8,125] However, bimetallic Fe-Co catalysts usually contain larger amount of Fe compared to the one of Co, which acts as a promoter.[117,126] Therefore, a detailed description of such bimetallic catalysts is beyond the scope of this review. The reader can find several detailed reviews concerning CO₂ hydrogenation on Fe-based, as well as comparisons between Co and Fe-based catalysts in the literature.[4,8,127,128]

Besides the increased CO₂ adsorption stemming from the enhanced basicity of the promoted catalytic surface, alkali and alkaline earth promotion can also favor the generation of oxygen vacancies on reducible supports, and improve the final metal dispersion.[129,130] According to Liu et al.,[129] the modification of
a CeO₂ support by the addition of different alkaline earth metal oxides with a M/Ce (M = Mg, Ca, Sr, Ba) molar ratio of 1/9 via sol-gel method leads to the formation of more Ovac on the final Ni/M₀.₁CeOₓ catalyst. Moreover, such modification increases both strength and number of the moderate alkaline sites and the Ni dispersion.[129]

Finally, noble metal promotion does not improve the selectivity for higher hydrocarbons.[131] The higher surface hydrogenation achievable by the addition of different noble metals can benefit the overall activity and suppress olefin production, however increasing CH₄ selectivity.[131] The dual promotion with transition and alkali metals was proved to be more successful than noble-alkali metal combination: Co-Na-Mo/SiO₂ has similar C₅⁺ selectivity to the one of Co-K-Pt/SiO₂ catalysts, albeit with higher conversion.[131]

Hence, the addition of alkali promoters can increase CO₂ adsorption and the intrinsically marginal activity of Co-based catalysts for RWGS, helping to mitigate excess methanation and increase overall C₂⁺ selectivity. However, chain-growth probability remains modest even in presence of alkaline promoters, varying from 0.55 to 0.65 in all cases.[2,32]

## 6. Conclusions

CO₂-FTS-based hydrocarbons could allow the creation of a circular carbon economy with a significant impact on anthropogenic emissions into the atmosphere. Due to their high C—C coupling activity in the conventional FTS process, cobalt-based catalysts are good candidates for direct CO₂ hydrogenation to C₂⁺ hydrocarbons. Unfortunately, Co-based catalysts act differently when CO₂ substitutes CO in the feed, producing mainly methane.

This review summarizes the progress achieved toward the single-step hydrogenation of CO₂ to long-chain hydrocarbons over oxide-supported Co-based catalysts under traditional FTS conditions. The main conclusions and perspectives are listed below:

i. Due to weak CO₂ adsorption and RWGS thermodynamic constraints, C/H surface ratio and CO coverage are generally low over Co-based catalysts, leading to the preferential production of CH₄ and short-chain hydrocarbons. Methane formation can be decreased and C₂⁺ selectivity can be increased by the careful choice of cobalt active phase, metal oxide support, regulation of the metal-support interfaces and addition of alkaline metal promoters.

ii. Both metallic cobalt and cobalt oxide (CoO) can be the active phase during CO₂-FTS reaction, depending on the support used. Metallic cobalt is more active on most supports, while CoO appears to be a better choice for TiO₂ supports. Such a behavior stems from the formation of a unique interface between the cobalt active phase and the TiO₂ support. Due to its lower hydrogenation ability, CoO/TiO₂ produces less CH₄ and more C₂⁺ products compared to its metallic counterpart, with a higher content of olefins. Moreover, CoO shows interesting activity toward the RWGS reaction. Nonetheless, further investigations are needed to clarify the role of CoO and Co⁰-CoO interfaces during CO₂-FTS, as well as the possible formation of cobalt carbide and its consequences on CO₂ hydrogenation.

iii. Independently from the cobalt active phase, reducible oxides and especially TiO₂ are better adapted for this reaction than irreducible oxides. The higher activity obtained on TiO₂-based catalysts stems from the ability of reducible oxides to create specific metal-support interfaces, originating from SMSI, which can strongly benefit the CO₂ activation to different products, from C₂⁺ to CH₄ and CO. Moreover, reducible oxides and metal-oxide interfaces are rich in surface defects (Ovac, Ti³⁺) that can be exploited for direct activation of CO₂. Providing Co-based catalysts supported on reducible oxides with large number of Ovac can be an efficient strategy to increase the activity during CO₂ hydrogenation and tune the selectivity toward the desired products.

iv. The oxygen vacancies can directly interact with metal precursors and affect the evolution of SMSI. The introduction of Ovac on reducible supports is a promising and straightforward method to develop new catalytic materials with higher reducibility and stability. Moreover, the combination of such synthetic strategies with careful post-synthesis treatments might promote the formation of specific metal-support interfaces with superior performance for CO₂ hydrogenation.

v. The addition of alkali promoters can increase CO₂ adsorption, C/H surface ratio and the intrinsically marginal activity of Co-based catalysts for RWGS, helping to mitigate excess methanation and to increase overall C₂⁺ selectivity. The addition of other transition metals, which favor RWGS (e.g., Cu and Fe) in combination with alkaline promoters can as well improve the overall activity and selectivity. Finally, the modification of reducible supports by alkaline earth metals can promote the formation of Ovac.

vi. Despite the application of the above-mentioned strategies, chain growth probability (α) remains limited by the low CO fugacity and consequent surface coverage (θCO). Therefore, Co-based catalysts might be used, under pure CO₂/H₂ feeds, for the direct production of synthetic natural gas mixture rich in light C₂–C₄ hydrocarbons and CO₂-based middle distillate mix (i.e., gasoline, jet fuel, diesel), without the necessity of a downstream hydrocracking refining treatment. Further improvements of the product distribution (higher α) can be obtained via utilization of mixed CO/CO₂/H₂ feeds (e.g., the ones resulting from biomass gasification), which can ensure higher θCO.

vii. Further implementation of Co-based catalysts for CO₂ hydrogenation should be focused, first of all, on the clarification of cobalt particle size effect. Then, the activity of hcp-Co supported on reducible TiO₂ should be investigated. Finally, due to the pivotal role played by the support on the activation of CO₂, large efforts should be devoted to the development of an extremely active TiO₂ support, rich in Ovac and alkaline promoters, to ensure proper CO₂ activation and metal-interface formation.

## Acknowledgements

This work was supported by the Agence Nationale de la Recherche (project ANR-19-CE07-0030), which is gratefully acknowledged.

## Conflict of Interest

The authors declare no conflict of interest.
Keywords

CO₂ hydrogenation, cobalt catalysts, liquid fuels, metal oxides, metal-support interaction, oxygen vacancy, promoters

Received: January 3, 2023

Revised: March 7, 2023

Published online:

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<figure><img src="image_12.png" /></figure>

**Canio Scarfiello** obtained his bachelor's and master's degree in industrial chemistry from the Sapienza University of Rome. He is currently developing his Ph.D. research on carbon dioxide hydrogenation to liquid fuels at the RAPSODEE, LPCNO and LCC laboratories.

<figure><img src="image_13.png" /></figure>

**Doan Pham Minh** received his Ph.D. degree in chemistry at the University Lyon 1, France in 2006. He is currently full professor and deputy director of the RAPSODEE research center, UMR CNRS 5302 at IMT Mines Albi. His research is focused on the design of new performing catalysts for applications in the production of green hydrogen, biomethane, and biofuels, as well as on the development of refractory ceramics and processes for high temperature sensible heat storage.

<figure><img src="image_14.png" /></figure>

**Katerina Soulantica** received her Ph.D. in chemistry from the University of Athens (Greece) in 1995. From 1995 to 1998 she was a post-doctoral fellow in Spain (Prof. P. Espinet, Valladolid). In 1999, she joined the group of Dr. B. Chaudret at the LCC (Toulouse). In 2005, she joined the LPCNO laboratory as a research associate and since 2008 she is a CNRS researcher in the nanostructures and organometallic chemistry (NOC) group of LPCNO. Her research domain is the synthesis of metal nanoparticles and their applications in microelectronics, biodetection, and catalysis.

<figure><img src="image_15.png" /></figure>

**Philippe Serp** is a professor of inorganic chemistry in Toulouse University. He received his Ph.D. degree from Toulouse University in 1994 and engaged in postdoctoral research at the University of Porto and Louvain-la-Neuve from 1995 to 1997. His research interests in the laboratory of coordination chemistry include the preparation of nanostructured catalytic materials, and molecular approaches to understand supported catalysis; fields in which with co-workers he has published over 200 papers among them twenty European and U.S. patents. His research has been recognized by the Catalysis (2004) and Industrial Chemistry (2012) Division Awards of the French Chemical Society.