Progress in reaction mechanisms and catalyst development of ceria-based catalysts for low-temperature CO₂ methanation

Abstract

With the development of carbon dioxide (CO₂) capture, storage and utilization (CCSU) technologies, CO₂ has gradually become a desired feedstock for the production of value-added chemicals like methane (CH₄). Ceria (CeO₂)-based catalysts have gained much attention because of their potential to efficiently hydrogenate CO₂ to CH₄ under mild conditions. Here we systematically outline the advances in CeO₂-based catalysts for CO₂ methanation mainly from the perspective of mechanism investigation and catalyst development. Various in situ/operando and ex situ technologies have verified that active metal and oxygen vacancies at the metal/metal oxide–CeO₂ interface act as the prime active sites to promote the formation and hydrogenation of key intermediates during the CO₂ methanation reaction. Kinetic analysis and in situ DRIFT characterization combined with theoretical calculations revealed that the reaction mechanism toward CO₂ methanation is sensitive to active sites, and the formate route versus the carboxyl (CO*) route has been widely detected as the main methanation pathway over CeO₂-based catalysts. Additionally, mainstream strategies to improve CeO₂-based catalysts include optimizing reducibility, adjusting the distribution of basic sites, dispersing active metal supported on CeO₂, and increasing the amount of oxygen vacancies or additional active sites for CO₂ adsorption and selective hydrogenation into CH₄. Finally, perspectives on the deeper understanding of active sites and intermediates’ evolution and the challenges of CeO₂-based catalysts for CO₂ methanation in future investigations are presented.

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1. Introduction

The overutilization of fossil fuels has caused enormous amounts of CO₂ to be emitted into the atmosphere over the past decades, contributing to a substantial waste of carbon feedstock and triggering a series of environmental concerns.^1–4 Besides, the demand for natural gas in China is rising, driven by the coal-to-gas switching process.⁵ Confronting both of the above threats simultaneously has driven a lot of research on CO₂ hydrogenation technologies for clean energy (i.e., methane and methanol) production.^6–10 Concerning the hydrogenation of CO₂ into methanol, the conversion of CO₂ is about 20% even at 300 °C and 5 MPa,^11–13 which does not contribute significantly to CO₂ mitigation. For CO₂ methanation, however, the CO₂ conversion can reach 80% at 300 °C and the CH₄ selectivity is up to 100%.^14–16 Moreover, CH₄ can be efficiently converted into methanol under a mild reaction condition (150 °C and 2 MPa).¹⁷ Therefore, the hydrogenation of CO₂ to produce synthetic natural gas is a promising strategy to make a breakthrough on these above issues.

Methane (CH₄), as a major component of natural gas, is an important feedstock for fuel production.¹⁸ CO₂ methanation, the so-called Sabatier reaction (CO₂ + 4H₂ → CH₄ + 2H₂O, ΔH_298K = −165.0 kJ mol⁻¹) is a highly exothermic process, which is favored at low temperatures.^19–22 In addition, carrying out this reaction at a low temperature inhibits the endothermic reverse water–gas shift reaction (CO₂ + H₂ → CO + H₂O, ΔH_298K = 41.2 kJ mol⁻¹),²³ which effectively reduces the generation of by-product CO. The CO₂ hydrogenation to CH₄via heterogeneous catalysis has many advantages: (1) the CO₂ methanation process can be integrated into industrial areas that produce substantial CO₂ gas (e.g., coking and chemical plant, steel factory, cement factory, waste incineration plant, heavy crude refinery, etc.), due to the mature CO₂ capture and storage (CCS) technologies; (2) the produced CH₄ can be injected into the existing natural gas pipeline network for storage and transportation, which prevents CH₄ from escaping into the atmosphere;²⁴ (3) the required H₂ can be generated by surplus renewable electricity (wind, solar, and hydro) via water electrolysis;^25,26 (4) CO₂ methanation can be performed at moderate temperatures and atmospheric pressure, which significantly reduces operating costs. In principle, the CO₂ methanation technique is a promising power-to-gas concept for a sustainable future.

However, CO₂ methanation is kinetically limited due to the chemical stability and highly C=O bond energy (806 kJ mol⁻¹) of CO₂ molecules.²⁷ Hence, the current urgent demand is for the development of catalysts with superior activity for CO₂ activation at low temperatures. Almost all the CO₂ hydrogenation catalysts are developed as supported catalysts.²⁸ Generally, active metal is the main active component, as it provides active sites for C=O bond activation and dissociation of the H₂ molecule. Existing studies have reported that noble metals (e.g., Rh, Pd, Ir, and Ru) or non-noble metals (e.g., Ni, Co, and Fe) loaded on various metal oxides (e.g., Al₂O₃, SiO₂, ZrO₂, TiO₂, Y₂O₃, and CeO₂, etc.) have been evaluated to be active for methanation.^15,29–43 Among them, Ni-based catalysts have gained extensive attention because of their relatively high methane yields and cheaper prices than the noble ones.⁴⁴ However, they are prone to deactivation due to coke formation and sintering at high temperatures.⁴⁵ Ru-based catalysts are also a promising candidate for methanation applications, as they are more durable than Ni and less expensive than Rh, Pd, and Ir.^46–48 Furthermore, it has been reported that the support may affect the reaction mechanism by adjusting surface properties and promoting metal dispersion, and thus govern catalytic performance.⁴⁹ Hence, it is instructive to systematically understand the function of the support, specifically to account for the complex metal–support interaction (MSI).

As an easily accessible rare-earth material, ceria (CeO₂) exhibits significant superiority as a support for methanation catalysts, because of its highly tunable nature involving MSI and oxygen vacancy distribution.^50–58 In addition, CeO₂ can modify the reducibility and surface properties of catalysts due to its ability to interact with other components.^45,59–64 We used the keywords “CeO₂” and “methanation” to search citation reports in Web of Science. As depicted in Fig. 1a, both the number of publications and citations have increased between 2016 and 2021, suggesting that CeO₂-based catalysts are an ongoing research hotspot in the methanation field. Of note, Ru/CeO₂ and Ni/CeO₂ catalysts have been widely reported as promising candidates for industrial purposes due to their outstanding low-temperature methanation activity.^{19,32,65–70} However, the essential understanding of the reaction mechanism and structure–activity relationship between Ru/Ni and CeO₂ remains limited, which impedes the development of new catalysts.

Fig. 1 (a) The citation report from 2016 to 2021 in Web of Science for CeO₂-based catalysts in methanation application. (b) Schematic illustration of active sites, reaction mechanism, and catalyst development for CO₂ methanation.

Several excellent reviews have been reported on CO₂ methanation, covering methanation catalytic technology (e.g., thermocatalysis, photocatalysis, and plasma catalysis),^71–73 catalyst development (including monometallic and bimetallic catalysts, alloys, and functionalized support),^9,17,74,75 reaction mechanisms,⁷⁶ product selectivity,^77–79 and reactor technologies.⁷³ Nevertheless, the role played by CeO₂ in CO₂ methanation catalysts has rarely been reviewed in detail. A comprehensive understanding is essential, considering that many valuable research articles on CeO₂-based catalysts for CO₂ methanation have been reported in the past few years.^28,67,80

The specific objective of this review is to determine how the types of active sites, surface properties, and MSI of the Ru/CeO₂ and Ni/CeO₂ catalysts impact the reaction routes, and thus the catalytic performance. We discuss up-to-date findings about the promotion effects of CeO₂ for low-temperature methanation and insight into the structure–activity relationship and reaction mechanism. This review is organized following the interaction between CeO₂ and H₂/CO₂, the identification of active sites, the influence of MSI, methanation mechanisms involving adsorption and hydrogenation of the reactants, and the strategies for CeO₂-based catalyst modification (Fig. 1b). Subsequently, the perspective and suggestions for further understanding the CO₂ methanation mechanism and catalyst development of CeO₂-based materials are discussed.

2. Understanding ceria-based catalysts for CO₂ methanation

2.1. Interaction of CeO₂ with H₂ and CO₂

The redox process with oxygen storage and release over CeO₂ is one of the most interesting behaviors in the CO₂ hydrogenation reaction. Schweke et al.⁸¹ performed a complementary temperature programmed reduction/desorption (TPR/TPD) measurement to identify the interaction of CeO₂ with H₂. They observed two peaks of H₂ consumption assigned to the generation of surface and bulk oxygen vacancies at 350 °C and 700 °C, respectively, and found that the H₂ consumption occurred concomitantly with the H₂O desorption. Generally, the stoichiometric CeO₂ with a face-centered cubic (FCC) structure (Fig. 2Ia) is unstable in an H₂-rich environment. The dislodgment of an O atom under an H₂ atmosphere would leave two electrons to reduce the adjacent two Ce⁴⁺ to Ce³⁺ (Fig. 2Ib).⁸² This process triggers the generation of oxygen vacancies due to the electrostatic balance mechanism.⁸³ It is generally accepted that the oxygen vacancy plays a critical role in the adsorption and activation of the oxygen-containing molecule, i.e., CO₂.⁸⁴

Fig. 2 (I) Ideal face-centered cubic structure of CeO₂ (a) and crystal structure of CeO₂ with one oxygen vacancy (b). Reproduced from ref. 82 with permission from Elsevier, copyright 2018. (II) Stable configurations of CO₂ molecule adsorbed on a CeO₂(111) facet (a–c cross-sectional and d–f top view). Reproduced from ref. 86 with permission from American Chemical Society, copyright 2013. (III) Adsorption and photoreduction of CO₂ at oxygen vacancy and OH site. Reproduced from ref. 89 with permission from American Chemical Society, copyright 2020.

Understanding the interaction of CO₂ with CeO₂ is of high importance for CO₂ hydrogenation. In 2014, Yoshikawa et al.⁸⁵ performed CO₂ pulse injection measurements and found that the CeO₂-based adsorbents showed the strongest CO₂ adsorption capacity among various single metal oxides (Al₂O₃, SiO₂, and ZrO₂). In situ FT-IR spectroscopy observed the bonds assigned to monodentate carbonate, bidentate carbonate, polydentate carbonate, and hydrogen carbonate on the tested CeO₂ surface. Detailed adsorption mechanisms of CO₂ on CeO₂ were studied by Hahn et al.⁸⁶ using hybrid functional (PBE0) and density functional theory (DFT)+U calculations. This indicated that the isolated CO₂ molecules interact with CeO₂(111) more favorably to form two stable carbonate configurations, monodentate and bidentate carbonate, rather than the linear adsorption configuration (Fig. 2IIa–f). For these two carbonate species, the anti-bonding orbitals of the C atom could interact with the O atom on the CeO₂ surface and subsequently form a stable chemical bond. However, the role of oxygen vacancy was not discussed in these studies.

Recently, Ruhaimi et al.⁸⁷ confirmed that oxygen vacancies on the CeO₂ surface could serve as CO₂ adsorption sites. Typically, the electronegative O atom of CO₂ could provide electrons to oxygen vacancies on CeO₂, lead to the polarization of adsorbed CO₂ molecules and reduce the activation energy of CO₂. Additional evidence has been provided by Zhu et al.⁸⁸ Compared with the ideal CeO₂(110) facet, the CO₂ molecule on CeO₂(110) with one oxygen vacancy exhibited more pronounced geometrical changes (more tortuous O–C–O angle and stretched C=O bond lengths), and resulted in a lower CO₂ adsorption energy (−1.93 eV vs. −1.44 eV). Besides oxygen vacancy, Zhu et al.⁸⁹ demonstrated that the hydroxyl (OH) group on the CeO₂ surface could act as an adsorption site for CO₂. Generally, the O atom of OH groups on CeO₂ tends to act as Lewis basicity to interact with the C atom of CO₂ due to its electronegativity, thus promoting the adsorption and reduction of CO₂ (Fig. 2III). In summary, the above-mentioned experimental and theoretical studies indicate that the interactions between CeO₂ and methanation reactants (CO₂ and H₂) may play a positive role in CO₂ methanation.

2.2. Identification of active sites over ceria-based catalysts for methanation of CeO₂ with H₂ and CO₂

The interaction of CeO₂ with CO₂ methanation reactants is a prerequisite for the application of CeO₂ in this reaction. Unfortunately, the single CeO₂ catalyst is extremely inefficient for CO₂ activation. Varvoutis et al.⁹⁰ recently evaluated the CO₂ methanation performance over hydrothermally synthesized CeO₂-nanorods and commercial CeO₂ (Sigma Aldrich, >99.5%) samples and found that only little CO was formed via reverse water–gas shift (RWGS) reaction, indicating that the CH₄ yield over bare CeO₂ is negligible. Similarly, Li et al.³² found no apparent CO₂ conversion over the physical mixture of Ni particles and CeO₂ compared with a high-performance Ni/CeO₂ catalyst prepared by a wet-impregnation method. Therefore, it is essential to systematically identify the catalytic sites for CO₂ methanation over CeO₂-based catalysts.

Zhang et al.⁹¹ investigated the CO₂ methanation reaction over Ni supported on an ideal CeO₂(111) surface by first principles calculations based on DFT. They concluded that the CeO₂ support promoted the dispersion of Ni particles and the electronic interaction of charge transfer between Ni and CeO₂. As shown in Fig. 3I, the top site of the Ni particle exhibited a lower CO₂ adsorption energy (−158.89 kJ mol⁻¹) than that of Ni particle side (−85.58 kJ mol⁻¹) and metal–support interface (−136.53 kJ mol⁻¹), indicating that the top of Ni is the predominant catalytic site for CO₂ methanation. In contrast, Li et al.³² argued that CO₂ prefers to adsorb and activate at the interface of the Ni/CeO₂ catalyst, while the Ni particle is the site of H₂ dissociation to afford active H atoms for the subsequent hydrogenation reactions. This view was approved by Cárdenas-Arenas et al.,⁹² who suggested that the reduced NiO–CeO₂ interface is the chemisorption region of CO₂, and the adsorbed CO₂ species could react with H atoms to produce CH₄ (Fig. 3II).

Fig. 3 (I) Adsorption energies of CO₂ adsorbed at top, side sites of Ni particle and Ni–CeO₂ interface. Reproduced from ref. 91 with permission from Elsevier, copyright 2021. (II) Reduction of NiO–CeO₂ interface and hydrogenation of CO₂. Reproduced from ref. 92 with permission from Elsevier, copyright 2020. (III) Operando visible Raman spectra of Ru/CeO₂ in the pre-reduction process (A), the CO₂ methanation process (B), and the corresponding I_D/I_F2g value (C). Generation of oxygen vacancy, OH group, and Ce³⁺ in the reduction process of Ru/CeO₂ catalyst (D). Reproduced from ref. 84 with permission from American Chemical Society, copyright 2016. (IV) Schematic diagram of reactor cell for in situ Raman. (V) In situ Raman spectra in the reduction process (a), methanation reaction process (b), and the corresponding intensity ratio of I_D/I_F2g over Ni/CeO₂ (c). Reproduced from ref. 99 with permission from American Chemical Society, copyright 2022.

Recent findings further identified the catalytic position of Ni/CeO₂ catalysts. Lin et al.²⁴ performed a series of reduction–oxidation treatments to tune the interaction of Ni particles and CeO₂ support. They found that strong Ni–CeO₂ interaction was detected after a high-temperature reduction of Ni/CeO₂, which resulted in the partial encapsulation of Ni particles as well as subsequent decreased ability to activate H₂ molecules. This further limited the hydrogenation of CO₂. However, Varvoutis et al.⁹⁰ claimed both Ni sites and the Ni–CeO₂ interface cannot be regarded as methanation performance descriptors of Ni/CeO₂ catalysts. They synthesized a series of CeO₂ nanorods loaded with Ni of different particle sizes, and found that the optimum methanation activity could be obtained only by coordinating the competitive relationship between Ni–CeO₂ perimeter and large Ni particles. This boosted activity could be mostly correlated to the under-coordinated step/edge and kink/corner sites on the Ni surface. It can be seen that detailed characterizations and meticulous catalyst synthesis technologies combined with theoretical models are required to determine the exact location of the active sites over Ni/CeO₂ catalysts.

In addition to discerning where the active sites are, it is important to understand what the active sites are. It was reported that the basic and oxygen vacancies sites on CeO₂ make it promising support for CO₂ adsorption and conversion at low temperatures.⁸⁸ Muroyama et al.¹⁵ prepared a series of Ni loaded on various metal oxides (Y₂O₃, Sm₂O₃, ZrO₂, CeO₂, Al₂O₃, and La₂O₃) catalysts and combined CO₂-TPD techniques to explore the distribution of basic sites over these samples. For Ni/CeO₂, the main CO₂ desorption was detected below 180 °C, denoting the existence of a high concentration of weak basic sites on the Ni/CeO₂ surface. Subsequently, Yan et al.¹⁴ claimed that the adsorbed species of CO₂ at OH sites with weak basicity can react with H atoms to produce CH₄. It should be noted that the CO₂ methanation activity is particularly relevant to the number of weak (OH groups) and medium (metal–oxygen pairs) basic sites.^49,93

Furthermore, Hao et al.⁹⁴ attributed the high performance of Ni/CeO₂ mainly to the oxygen vacancies with the interaction of Ni–CeO₂. They observed that Ni/CeO₂ exhibited hundreds of times higher CO₂ conversion rates than that of Ni/SiO₂ without oxygen vacancy in a temperature range from 200 to 300 °C. Subsequently, their work confirmed that the turnover frequency (TOF) of CO₂ increased with the decrease of Ni sizes (8–21 nm) over Ni/CeO₂ catalysts. A similar experiment has also been performed by Marconi et al.;⁹⁵ however, a positive correlation was observed between the TOF for CO₂ conversion and Ni particle size, and CH₄ prefers to be produced on large Ni particles (8 nm). Moreover, Lin et al.⁹⁶ suggested that the larger Ni particles (8 nm) on the CeO₂ surface accelerated the hydrogenation of key intermediates for CO₂ methanation. In this light, Varvoutis et al.⁹⁰ believed that the differences in the extent of H₂ spillover and the dispersion of Ni particles led to these contradicting reports.

López-Rodríguez et al.⁹⁷ monitored the active sites on Ni/CeO₂ catalyst for CO₂ methanation by in situ near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS). The Ni0 sites and oxygen vacancies located at the Ni–CeO₂ interface were confirmed to be the active sites for H₂ dissociation and CO₂ dissociation, respectively, while a small amount of NiO and Ni-carbonates hydroxyls acted as spectators in methanation. Besides, they found that the reduction rates of Ni²⁺ and Ce⁴⁺ gradually became faster than their CO₂ oxidation rates as the reaction temperature increased. Therefore, an appropriate CO₂ methanation temperature may be beneficial for maintaining the chemical state of the active sites on Ni/CeO₂ catalysts.

In terms of Ru/CeO₂ catalysts, Guo et al.²⁸ reported that the Ce³⁺–OH and Ru sites on Ru/CeO₂ nanowires were recognized as the catalytic sites for CO₂ dissociation and carbonyl hydrogenation, respectively. The TOF over as-prepared Ru/CeO₂ catalysts followed the order of Ru nanoclusters/CeO₂ > Ru single atoms/CeO₂ > Ru nanoparticles/CeO₂. It should be noted that the CeO₂-supported Ru nanoclusters catalyst with the lowest oxygen vacancy concentration exhibited the highest CO₂ methanation performance, suggesting that the rate-determining step occurred on Ru species. However, the oxygen vacancy content on the reacted Ru/CeO₂ catalysts showed a great decrease after the methanation process, indicating that the oxygen vacancies indeed contributed to CO₂ methanation. Subsequently, these authors examined the impact of Ru particle size on H₂ dissociation capacity by H₂-TPR and H₂-TPD experiments. The H₂ spillover intensity detected on CeO₂-supported Ru nanoparticles (ca. 4 nm) was much higher than for Ru nanoclusters (ca. 1.2 nm) and Ru single-atom samples. This may be due to the low coverage of H₂ on these small metal particles and single-atom sites. Moreover, the in situ DRIFTS results confirmed that the Ce³⁺–OH structure greatly facilitated the transformation of CO₂ to formate species. Overall, it is well established that active Ru sites, oxygen vacancies, and OH groups play important roles in the hydrogenation of CO₂ into CH₄ over metal nanoparticles-supported CeO₂ catalysts. Also, the structural sensitivity induced by metal size effects is prevalent in CO₂ methanation.

Nevertheless, based on the results of kinetic analysis, Wang et al.⁹⁸ presented the opposite view. They deposited Ru particles on CeO₂ supports with different shapes. Although these Ru/CeO₂ catalysts displayed different CO₂ conversion, the similar apparent activation energies (75 kJ mol⁻¹) indicated the same active site on as-prepared Ru/CeO₂ catalysts. It is worth mentioning that the TOF values with respect to the surface oxygen vacancy were similar (2.7 × 10⁻⁴ s⁻¹), while the values with respect to exposed Ru atom were markedly different for all Ru/CeO₂ catalysts (6.61 × 10⁻⁵–6.06 × 10⁻⁴ s⁻¹). Therefore, the oxygen vacancy, not the Ru site, was the major active site for the rate-determining step in CO₂ methanation over Ru/CeO₂ catalysts.

Oxygen vacancy, one of the vital defects on CeO₂-based catalysts, plays an important role in CO₂ adsorption and hydrogenation. Detecting the evolution of oxygen vacancy during the process of pre-reduction and CO₂ methanation reaction by in situ/operando Raman characterization is beneficial for revealing the role of oxygen vacancy in methanation. Generally, the ratio of peak intensities at ∼570 cm⁻¹ (defect-induced (D) mode) and ∼460 cm⁻¹ (first-order F_2g mode) is a quantitative measurement of oxygen vacancy concentration.⁸⁴ Wang et al.⁸⁴ found that the oxygen vacancy concentration increased as the reduction temperature was raised from room temperature to 400 °C (Fig. 3IIIA and C). In the methanation process, the oxygen vacancy concentration decreased sharply from room temperature to 100 °C and remained at a low level in a temperature range of 200–400 °C (Fig. 3IIIB and C). These results indicated that oxygen vacancies were gradually generated on the Ru/CeO₂ surface in the reduction process, and the oxygen vacancies were indeed actively involved in the methanation reaction. Combining operando X-ray absorption near edge structure (XANES) and operando infrared (IR) results, they deemed that oxygen vacancies, OH groups, and Ce³⁺ sites were generated almost synchronously during the reduction process of Ru species. As shown in Fig. 3IIID, metallic Ru with the ability to split H₂ molecules was first produced in the reduction process. Afterward, the Ce–O bond on the CeO₂ surface was attacked by active H atoms due to the hydrogen-spillover effect, which accelerated the generation of oxygen vacancies, OH groups, and Ce³⁺. The detailed evolution of these active species in CO₂ methanation will be presented later in this review.

Recently, our group performed in situ Raman spectra (Fig. 3IV) to systematically monitor the changes in oxygen vacancy density over Ni/CeO₂ catalyst during the reduction and subsequent methanation reaction processes.⁹⁹ In the reduction process (Fig. 3Va and c), the oxygen vacancy concentration over Ni/CeO₂ surged with the increment of temperature from 50 to 250 °C, and stayed relatively stable from 250 to 450 °C. This confirmed the progressive generation of oxygen vacancies during the reduction treatment over Ni/CeO₂. Subsequently, when the inlet gas was switched from 25% H₂/75% N₂ to 5% CO₂/20% H₂/75% N₂ (Fig. 3Vb and c), the evolution of oxygen vacancy concentration exhibited a concave-like shape, which decreased from 50 to 250 °C and then increased from 250 to 350 °C. This suggested that CO₂ could dramatically occupy the oxygen vacancies and participate in the subsequent hydrogenation process.

2.3. Effect of metal–support interaction for catalytic performance

In the field of heterogeneous catalysis, the MSI effect plays an extremely important role in dispersing metal particles, tuning redox properties, and resisting catalyst sintering and coke.¹⁰⁰

This significant effect may alter the microstructure of the catalyst due to geometric and/or electronic effects, and thus the catalytic performance.²⁴ MSI is a well-recognized concept that mainly relies on strong metal–support interaction (SMSI) and electronic metal–support interactions (EMSIs), in which the SMSI effect was studied more than 40 years ago by Tauster et al.¹⁰¹ Generally, the SMSI effect acts solely on the catalysts supported on a reducible support (such as CeO₂, TiO₂, Fe₃O₄, and ZnO),^102–105 and is highly dependent on the reaction conditions. The major characteristic of the typical SMSI effect is the migration and encapsulation of partially reduced oxides to metal particles during the reaction process.¹⁰⁶

Matte et al.¹⁰⁷ discovered a CeO_2−x encapsulation overlayer on Ni nanoparticles after an H₂ reduction treatment at 500 °C. A similar phenomenon on a Ni/CeO₂ sample was observed by Li et al.³² They deemed that the coverage of Ni particles by reduced CeO₂ decreased the exposed surface of active Ni, which accordingly impaired the capacity for CO₂ and/or H₂ activation. Specifically, the CO₂ consumption rate over Ni/CeO₂ catalysts at 523 K significantly decreased (from ∼32 to ∼7 h⁻¹) due to SMSI induced by the increase of pre-reduction temperature (from 723 K to 973 K) (Fig. 4I). Ho et al.¹⁰⁸ prepared a series of CeO₂ supported by Ni or Ru with different loadings by a direct coating process. They found the low metal content enhanced the MSI effect, but these samples exhibited a poor catalytic performance (CO₂ conversion below 10% and CH₄ selectivity below 70%) at 350 °C, while the catalyst with high Ni loading (45%) achieved ∼65% CO₂ conversion and ∼92% CH₄ selectivity. This could be attributed to SMSI and unbalanced sites for H₂ dissociation and CO₂ activation. In this respect, Lin et al.²⁴ found that the oxidative treatment alleviated the encapsulation of the CeO_2−x layer on the Ni surface, but this did not fully recover the CO₂ methanation activity over Ni/CeO₂. Recently, Pu et al.¹⁰⁹ reported that SMSI on Ni/CeO₂ catalysts was greatly influenced by CeO₂ shapes. They found that the extent of encapsulation of Ni particles induced by SMSI followed the sequence of Ni/CeO₂-nanooctahedra > Ni/CeO₂-nanocube > Ni/CeO₂-nanorod (Fig. 4II). Therefore, SMSI between Ni and CeO₂ is not only affected by Ni content, but is also affected by CeO₂ properties. The geometric surface restructuring process induced by the SMSI effect may significantly affect the CO₂ methanation reaction, due to the metal nanoparticles undertaking the role of CO₂ and H₂ activation.

Fig. 4 (I) HRTEM images of Ni/CeO₂ after a reduction treatment at (A) 773 K, (B) 973 K, and (C) effect of strong metal–support interaction induced by reduction temperature on CO₂ consumption rate over Ni/CeO₂ catalysts. Methanation condition: GHSV = 6.6 × 10⁴ h⁻¹; T_react = 523 K. Reproduced from ref. 32 with permission from Elsevier, copyright 2018. (II) HR-TEM images and schematic illustration of SMSI influence on Ni particles supported on CeO₂ with different shapes. Reproduced from ref. 109 with permission from Elsevier, copyright 2022. (III) HRTEM images for reduced Ni/CeO₂ samples with different Ni content (A) 1%, (B) 2.5%, and (C) 5%. Reproduced from ref. 94 with permission from Elsevier, copyright 2021.

In addition, the modification of methanation catalysts by the appropriate degree of MSI effect has been extensively established. On the one hand, Hao et al.⁹⁴ observed that spherical Ni particles were moderately embedded in the nanocrystalline CeO₂ due to the MSI effect, which is beneficial for maintaining the stability of Ni particles at low Ni content (Fig. 4IIIA–C). On the other hand, the redox properties of Ni/CeO₂ and Ni/γ-Al₂O₃ catalysts were compared by Cárdenas-Arenas et al.⁹² They found that the reduction of NiO on CeO₂ occurred at a much lower temperature than that on γ-Al₂O₃ support. This implied that the moderate interaction between NiO and CeO₂ support improved the reduction of NiO species. Thus, Ni/CeO₂ with more metallic Ni sites achieved ∼60% CO₂ conversion at 300 °C, much higher than that of Ni/γ-Al₂O₃ (∼2%).

Rui et al.⁶⁶ investigated the MSI effect toward Ni/CeO₂ catalyst prepared by plasma decomposition (Ni/CeO₂-P) and conventional thermal calcination (Ni/CeO₂-C) strategies. They suggested that the stronger MSI induced by plasma decomposition treatment prompted the generation of Ni–O–Ce coordination. This not only inhibited the aggregation of NiO particles by an anchoring effect, but also enhanced the activity of O atoms at Ni–CeO₂ interfacial regions. As expected, Ni/CeO₂-P with better reducibility and reactivity exhibited a high CO₂ conversion of 84.2% at 275 °C, while it is only 34.7% over Ni/CeO₂-C. In this regard, Ruiz Puigdollers et al.¹¹⁰ believed that the O atoms in the periphery between metal and support are more reactive than those in other regions, and accordingly, generation of oxygen vacancies is easier at the metal–support interface sites.

In the case of Ru/CeO₂ catalysts, a recent report explored the effect of Ru content (1–5 wt%) on Ru/CeO₂ catalysts for CO₂ methanation. López-Rodríguez et al.¹¹¹ found that the optimum catalyst with 2.5 wt% Ru detected the highest proportion of Ruⁿ⁺ species with strong interaction with the CeO₂ support. This MSI effect improved the reducibility of Ru cations at low temperatures, and thus achieved a high CO₂ conversion of ∼80% with 100% CH₄ selectivity at 290 °C.

In the CO₂ activation situation, the EMSI effect can induce partial charge transfer from the support to raise the electron density at metal sites, which facilitated the active intermediate conversion.⁷³ Tada et al.¹¹² reported that the activation from carbonyl species into CH₄ on metal can be promoted due to the EMSI effect. This is attributed to the ability of the electron-rich metal to donate electrons to the antibonding π-orbital of the adsorbed carbonyl species, and thus the C–O bond is easily activated. However, Wang et al.⁹⁸ found a different phenomenon about the EMSI effect. They employed different CeO₂ shapes (nanorods, nanopolyhedrons, and nanocubes, denoted as NRs, NPs, and NCs) as the support for Ru particles. Subsequently, Ruⁿ⁺ species at the interface region attributed to the electron transfer from Ru to CeO₂ support were observed in Ru/CeO₂-NRs and Ru/CeO₂-NPs samples. This electronic interaction impacted the concentration of oxygen vacancies on Ru/CeO₂ catalysts. According to Raman spectra analysis, the oxygen vacancy concentration on the three bare CeO₂ supports is decreased in the following order: CeO₂-NRs > CeO₂-NCs > CeO₂-NPs. As for Ru/CeO₂ samples, it should be noted that the above order was changed to Ru/CeO₂-NCs > Ru/CeO₂-NRs > Ru/CeO₂-NPs. This may be due to the electronic interaction in Ru/CeO₂-NRs and Ru/CeO₂-NPs leading to the decrease of Ru⁰ proportion, thus weakening the H₂ activation ability. Finally, the Ru/CeO₂-NCs exhibited an outstanding catalytic rate of 4.85 × 10⁻⁸ mol g_cat⁻¹ s⁻¹ even at a low temperature of 150 °C.

Likewise, Guo et al.²⁸ prepared CeO₂-supported Ru single atoms (SA), nanoclusters (NC), and nanoparticles (NP) by adjusting the deposition amount of RuCl₃ to investigate the effect of electronic interaction strength in Ru/CeO₂ catalysts for CO₂ methanation. The X-ray photoelectron spectroscopy (XPS) results indicated that the strength of electronic interaction is decreased in the sequence Ru(SA)/CeO₂ > Ru(NC)/CeO₂ > Ru(NP)/CeO₂. However, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) combined with the DFT calculations further demonstrated that the strong charge transfer induced by the EMSI effect is disadvantageous for the activation of carbonyl species. The TOF_CO2 of as-prepared Ru/CeO₂ samples at 190 °C ranked in the following order: Ru(NC)/CeO₂ (7.41 × 10⁻³ s⁻¹) > Ru(SA)/CeO₂ (4.59 × 10⁻³ s⁻¹) > Ru(NP)/CeO₂ (5.30 × 10⁻⁴ s⁻¹). It is important to note that the methanation activity is not only affected by electronic interaction but also by the H-spillover effect, hence it is difficult to obtain a direct relationship between EMSI strength and CO₂ methanation performance.

Collectively, the SMSI effect is widely recognized to be unfavorable for CO₂ methanation as it may lead to the encapsulation of active metal sites. However, based on the existing literature, the impacts of the EMSI effect on CO₂ methanation are still unclear, and more in-depth experimental and theoretical studies are required to fully understand this interaction.

3. CO₂ methanation mechanism over ceria-based catalysts

Fundamentally understanding the mechanism of heterogeneous catalytic reaction is a pivotal premise for designing efficient catalysts.^113,114 Although much effort has been made to reveal the reaction mechanism of CO₂ methanation by experimental or/and theoretical methods, a consensus has not yet been reached. As shown in Fig. 5, the C–O bond cleavage mechanism and the formate mechanism are widely recognized. The former involves the direct dissociation or hydrogen-assisted dissociation of CO₂ on the active metal surface to CO* (* represents an adsorbed species). Subsequently, the CO* species react with H*, leading to CH₄ production. The latter involves the CO₂ adsorbed on the support or metal–support interface hydrogenation to formate (HCOO*) intermediates, which are subsequently hydrogenated to CH₄.

Fig. 5 Possible CO₂ methanation pathways.

3.1. CO₂ methanation mechanism over Ni/CeO₂ catalysts

CO₂ methanation pathways over Ni/CeO₂ catalysts have been examined by many researchers because of the potential practical application of inexpensive Ni-based catalysts.^91,115–117 On the basis of DFT calculations, in 2021, Zhang et al.⁹¹ reported that the RWGS + CO-hydro route over the Ni/CeO₂(111) surface is more favorable than the direct CO₂ dissociation route and formate route. Ni nanocluster acted as the major activation site and facilitated the following pathway: CO₂* → HOCO* → CO* → HCO* → H₂CO* → CH₂* → CH₃* → CH₄*. Among them, the C–O bond cleavage of the H₂CO* intermediate is the rate-limiting step of this route. Interestingly, in 2015, Ren et al.¹¹⁸ suggested that the direct dissociation of CO₂ is the optimal methanation pathway over the Ni(111) surface, and that CO* → C* + O* is the rate-limiting step. Hence, Zhang et al.⁹¹ argued that the CeO₂ support played a key role in changing the optimal route and the corresponding rate-determining step of CO₂ methanation over Ni-based catalysts.

Instead, Hongmanorom et al.¹¹⁹ prepared an ordered mesoporous CeO₂ support (mpCeO₂) to experimentally explore the difference in methanation pathway between Ni/mpCeO₂ and conventional Ni/CeO₂. As shown in Fig. 6I, CH₄ was produced over conventional Ni/CeO₂ mainly via an associative mechanism (*CO₂ → *CO₃/*HCO₃ → *HCOO → CH₄). In situ DRIFTS results indicated that the carbonates and bicarbonates were converted into formate species that would further hydrogenate to CH₄ by dissociated H atoms, and no bands for *CO species were detected on the conventional Ni/CeO₂ surface. However, Ni/mpCeO₂ with rich oxygen vacancies, abundant Ni–O–Ce interfacial sites, and highly dispersed Ni particles exhibited a simultaneous dissociative (*CO₂ → *CO → *HCO → CH₄) and associative mechanism.

Fig. 6 (I) Proposed CO₂ methanation mechanism over Ni/mpCeO₂ and Ni/CeO₂ catalysts. Reproduced from ref. 119 with permission from Elsevier, copyright 2021. (II) Proposed reaction pathway over Ni/CeO_2−x: CO₂ adsorption at 180 °C (a), CO₂ methanation at 180 °C (b), and CO₂ methanation over 260 °C (c). Reproduced from ref. 120 with permission from American Chemical Society, copyright 2019. (III) Proposed CO₂ methanation mechanism over Ni–CeO₂. Reproduced from ref. 121 with permission from Elsevier, copyright 2018. (IV) CO₂ possible methanation route over Ni/CeO₂-SGM. Reproduced from ref. 80 with permission from Elsevier, copyright 2020. (V) Proposed CO₂ methanation mechanism and catalytic performance over Ni/CeO₂. Reproduced from ref. 99 with permission from American Chemical Society, copyright 2022.

Lee et al.¹²⁰ suggested that different reaction pathways dominated at different temperatures. In the low-temperature range of 180–240 °C (Fig. 6IIa and b), Ce³⁺ sites with oxygen vacancies over Ni/CeO₂ catalyst activated CO₂ to form carbonate species and then hydrogenated to formate and finally CH₄. When the temperature exceeded 260 °C (Fig. 6IIc), the CO₂ could be dissociated into CO* on the Ni surface due to the high activation energy. Then, the CO* species were hydrogenated until the CH₄ generation. However, Yu et al.¹²¹ argued CO* species over Ni–CeO₂ can be attributed to the formate decomposition at the Ce³⁺ site. Moreover, they found that the CO* species was not an active intermediate and existed as a by-product (Fig. 6III).

In this light, Ye et al.⁸⁰ believed that the hydrogenation of CO* species was a highly dissociated H demands process. They prepared a Ni/CeO₂ catalyst with 16.75% Ni loading to provide enough sites for H₂ dissociation, thus the DRIFTS results indicated that some CO* presented at Ni sites can react with H* to form formyl groups that can produce CH₄ by CO* hydrogenation. In addition, they traced the evolution of formate species by injecting formic acid into the reaction system. Stable formate bands on the Ni/CeO₂ surface were observed at 300 °C. Subsequently the band associated with CH₄ appeared when H₂ was introduced into the system. Therefore, the formate route played an important role in CO₂ methanation over Ni/CeO₂ catalyst (Fig. 6IV). Recently, Onrubia-Calvo et al.¹²² performed 124 kinetic experiments to establish a kinetic model that reflects the intrinsic CO₂ methanation activity over Ni/CeO₂ catalyst. They found the formate Langmuir–Hinshelwood–Hougen–Watson mechanistic model, which considered the hydrogenation of bicarbonate to formate as rate-determining step, in good line with the kinetic data and the results of in situ DRIFTS.

Of note, although the formate route has been considered to be the dominant CO₂ methanation pathway over Ni/CeO₂ catalysts, the contribution of the CO* route cannot be ignored. It is a pity that the exact origin of CO* is still controversial because of the structure-sensitivity of the CO₂ methanation reaction. Typically, formate species can be converted to CO* species or directly hydrogenated to CH₄.^123,124 However, Pan et al.¹²⁵ argued that CO* can be obtained by direct dissociation of CO₂. The dynamic evolutions of CO* species on Ni–CeO₂–Y₂O₃ catalyst have been explored by Zhu et al.⁸⁸ They observed that the consumption rate of CO* was faster than that of formate species after the reaction gas was switched from CO₂/H₂ to H₂. This confirmed that the CO* was generated by the direct CO₂ dissociation, not by formate. However, no CO* was detected on the Ni–CeO₂ surface due to the lower oxygen vacancy concentration than Ni–CeO₂–Y₂O₃. A similar finding was reported by Zhang et al.¹²⁶ They found that the high population of oxygen vacancies and medium-strength basic sites may be promoted by both the formate and CO* pathways.

Recently, our group found that the frustrated Lewis pairs constructed by oxygen vacancies and OH groups could promote CO₂ dissociation at a low temperature of 225 °C, and the CO* route as well as formate pathway together enhanced the low-temperature methanation activity over Ni/CeO₂ catalyst (Fig. 6V).⁹⁹ Unfortunately, it remains unclear what role oxygen vacancies play in the dissociation of carbon dioxide.

3.2. CO₂ methanation mechanism over Ru/CeO₂ catalysts

Ni-based catalysts may be deactivated because of the loss of active metal induced by the formation of mobile nickel subcarbonyls at low temperatures.² Although Ru is more expensive than Ni, the CO₂ methanation mechanism over Ru/CeO₂ catalysts still commands great attention due to the considerable methanation activity and stability of Ru-based catalysts.

Ma et al.¹²⁷ investigated the methanation mechanism and site requirements on the ideal CeO₂(110)-supported Ru_n (n represents the number of atoms) surface by a first-principles approach (Fig. 7Ia). The reaction pathway was observed to be sensitive to the number of Ru atoms. On the Ru₁/CeO₂ surface, the single Ru atom and nearby lattice O atoms were recognized as active sites, and CH₄ was generated by a carboxylate (COOH*) route. By comparison, the Ru nanocluster was identified as the major catalytic site on the Ru₄/CeO₂ and Ru₈/CeO₂ surface, and the CO* became the dominant route. Compared with the CO mechanism, which exhibited a significant ensemble effect and required 3–4 Ru atoms, the COOH mechanism was less structure sensitive and only required 1–2 Ru atoms. Of note, the elementary step of the accepted formate route (CO₂* + H* → HCOO*) on all Ru/CeO₂ surfaces needs to overcome a higher barrier than that of CO* and COOH* routes. Moreover, Ru₄/CeO₂ showed a lower apparent activation energy of 0.91 eV than that of Ru₁/CeO₂ (1.65 eV) and Ru₈/CeO₂ (1.41 eV), indicating that the CO₂ molecule could be easily activated over Ru₄/CeO₂ (Fig. 7Ib). The first four influential steps are depicted in Fig. 7Ic. It appeared that the rate-determining step switched from CO* + H* → HCO* over Ru₁/CeO₂ and Ru₄/CeO₂ to CH₂* + H* → CH₃* over Ru₈/CeO₂ with the increasing of Ru atom numbers. Meanwhile, the second most influential step switched from CO₂* + H* → COOH* (Ru₁/CeO₂) to CO₂* + * → CO* + O* (Ru₄/CeO₂ and Ru₈/CeO₂), indicating that CO₂ dissociation required more Ru atoms.

Fig. 7 (I) CO₂ methanation over CeO₂(110)-supported Ru_n (n represents the number of atoms) (a), Arrhenius plot (b), and the influence of the elementary steps on the TOF of CH₄ (c). Reproduced from ref. 127 with permission from American Chemical Society, copyright 2021. (II) In situ DRIFTS spectra recorded at 220 °C on reduced (a and d) Ru(SA)/CeO₂, (b and e) Ru(NC)/CeO₂, and (c and f) Ru(NP)/CeO₂. The results were recorded under atmosphere switched from 1% CO₂/4% H₂/He to 5% H₂/He after stabilization for 60 min for Ru(SA)/CeO₂ and Ru(NP)/CeO₂, and 30 min for Ru(NC)/CeO₂. (g) The CO₂ methanation reaction pathways over Ru/CeO₂ catalysts based on the results of in situ DRIFTS. Reproduced from ref. 28 with permission from American Chemical Society, copyright 2018. (III) (A) Formate route for CO₂ methanation over Ru/CeO₂, and (B) CO₂ conversion over Ru/CeO₂ (a) and Ru/α-Al₂O₃ (b). Reproduced from ref. 84 with permission from American Chemical Society, copyright 2016.

A finding raised questions about this interpretation. Gou et al.²⁸ argued that the rate-determining step of methanation occurred at Ru sites on CeO₂-supported Ru singles atom (SA), nanoclusters (NC), and nanoparticles (NP) catalysts. According to the results of in situ DRIFTS (Fig. 7IIa–f), the CO route was confirmed as the dominant CO₂ methanation pathway over all Ru/CeO₂ samples, and the bicarbonate (HCO₃* at 3623, 1396, and 1048 cm⁻¹), carboxylate (O₂C at 1559, 1508, and 1305 cm⁻¹), and bridged formate (HCOO* at 3017, 2836, 1580, and 1339 cm⁻¹) were the active intermediates. Additionally, they found that different reaction pathways took place on CeO₂ supported by different Ru structures. The reaction pathway is summarized in Fig. 7IIg. Specifically, the CO₂-derived HCO₃* was converted to HCOO* (path 1), bidentate CO₃* (path 2) or bridged CO₃* (path 3) species and then to form Ru–CO*. Subsequently, the CO* species react with the active H* atoms to produce CH₄ or direct desorption. In parallel, the adsorbed CO₂ could convert to O₂C* (path 4) and then form Ru–CO*.

Sharma et al.¹²⁸ reported a different methanation pathway involving methoxy species over a Ru-substituted CeO₂ sample (Ce_0.95Ru_0.05O₂). Based on TPR, DRIFTS and DFT results, the CO* species was first hydrogenated by dissociated H and underwent CO₂* → CO* → OCH₂* → OCH₃* → CH₄. In sharp contrast, for the conventional supported metal/oxide sample, the CO* was evolved to formate followed by hydrogenation to form CH₄. According to their earlier study,¹²⁹ Ru was substituted in a +4 oxidation state in the CeO₂ lattice and thus formed Ce_0.95Ru_0.05O₂. These works indicated that the CO₂ methanation pathway over Ru/CeO₂ catalysts was significantly impacted by the chemical environment of the Ru species.

Recently, more details of methanation behavior and mechanism over Ru/CeO₂ have been provided by López-Rodríguez et al.¹³⁰ The (NAP-XPS) results indicated the reduced Ru/CeO₂ can be slightly oxidized below 300 °C under reaction conditions of CO₂ methanation (CO₂ + H₂). Combined with the in situ DRIFTS results, it can be seen that the chemisorbed CO₂ on Ru sites was dissociated into Ru–CO* and O atoms, which could potentially oxidize the surface of Ru⁰ and CeO₂. Then, DFT calculations demonstrated that the hydrogenation of Ru–CO* is the rate-determining step in this reaction system. Besides, CO₂ can be adsorbed at the surface oxygen site and oxygen vacancy site of CeO₂ to form carbonate and carboxylate species, respectively, while carboxylates can evolve into formate species. These formate species which followed decomposed to CO on CeO₂ above 200 °C and served as an additional reservoir of Ru–CO*.

To systematically understand the catalytic behavior and the relationship between the reaction pathway and active site over Ru/CeO₂, three kinds of operando characterizations (XANES, Raman, and IR) were used by Wang et al.⁸⁴ They found that the concentrations of Ce³⁺, OH groups, and oxygen vacancies decreased at different temperatures, which indicated these active sites function at different steps of this reaction system. A complete formate route is displayed in Fig. 7IIIA on the basis of the steady-state isotope transient kinetic analysis (SSITKA) type operando DRIFTS results. First, Ce³⁺ was responsible for the activation of CO₂ into carboxylate at 25 °C, then the carboxylate species was hydrogenated to formate by the surface OH group at the same temperature. As the temperature increased to 150 °C, the oxygen vacancy was activated and thus promoted the conversion of formate to methanol (detected as the rate-determining step). Finally, the methanol was easily hydrogenated to form CH₄. In contrast, the CO* route activated by metallic Ru was not observed over the Ru/α-Al₂O₃ sample until 250 °C, resulting in a low CO₂ conversion (Fig. 7IIIB).

In summary, the CO₂ methanation mechanism over CeO₂-based catalysts has been disputed and no consensus has yet been reached (Table 1). Based on the above findings, it can be concluded that mechanistic studies are largely limited by the characterization techniques employed by researchers. For example, Zhang et al.⁹¹ and Ma et al.¹²⁷ used DFT calculations to explore the methanation pathway over CeO₂-based catalysts. However, the theoretical results were inconsistent with the experimental results, because the influence of the abundant OH groups and oxygen vacancies over the CeO₂ surface for the reaction pathway was not taken into account. Additionally, although in situ DRIFTS is a reliable characterization technique for the detection of surface-adsorbed species on the tested catalyst, it is difficult to specifically probe the evolution of active species during the reaction. Perhaps pulse technology combined with spectrokinetics studies could make breakthroughs on this issue.

Table 1 Summary of dominant reaction pathway over various CeO₂-based catalysts for CO₂ methanation

Sample	Method	Dominant reaction pathway	Ref.
Ni/CeO2	DFT calculations	CO* route	91
Ni/CeO2	In situ DRIFTS	Formate route	119
Ni/mpCeO2		Formate route
Ni/CeO2−x	In situ DRIFTS	Formate route (180–240 °C) CO* route (>260 °C)	120
Ni/CeO2-SGM	In situ DRIFTS	Formate route	80
Ni/CeO2	In situ DRIFTS	Formate route	99
Ru1/CeO2	DFT calculations	Carboxylate route	127
Ru4/CeO2		CO* route
Ru8/CeO2		CO* route
Ru(SA)/CeO2	In situ DRIFTS + DFT calculations	CO* route	28
Ru(NC)/CeO2		CO* route
Ru(NP)/CeO2		CO* route
Ce0.95Ru0.05O2	In situ DRIFTS + DFT calculations	CO* route	128
Ru/CeO2		Formate route
Ru/CeO2	In situ DRIFTS + DFT calculations	CO* route	130
Ru/CeO2	SSITKA type Operando DRIFTS	Formate route	84

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4. Strategies to raise the catalytic performance over ceria-based catalysts

From the above discussion, it is clear that the CO₂ methanation undergoes CO₂ adsorption, generation of intermediates via interaction between the active site and CO₂ molecule, H₂ dissociation, hydrogenation of active intermediates, and CH₄ production. So, the enhanced catalytic performance could be established on CeO₂-based catalysts by increasing the number of sites for CO₂ adsorption/activation and accelerating the dissociation and spillover of the H₂ molecule. Several strategies for modifying the existing CeO₂-catalysts and corresponding CO₂ methanation activity are summarized in Table 2.

Table 2 Summary of various CeO₂-based catalysts for CO₂ methanation

Strategy	Sample	Reaction gases	GHSV (mL gcat^−1 h^−1)	Temperature (°C)	CO2 conv. (%)	CH4 select. (%)	Ref.
a The unit is h^−1. — not shown in the literature.
Constructing efficient structure	Ni/CeO2-SGM	20% CO2/80% H2	40 000	250	82.5	94.8	80
	NiCe(35)_HT	11% CO2/48% H2/41% N2	72 000	300	76	99	131
	NiCe(15)_IWI				66	98
	Ni/CeO2-600 °C	10% CO2/40% H2/50% N2	45 000	300	∼56	∼99	132
	10Ni/CeO2-M-1	20% CO2/80% H2	45 000	325	∼79	∼100	133
	10Ni/CeO2-M				∼69	∼100
	10Ni/CeO2-C				∼30	∼100
	Pellet Ni3Ce1	17% CO2/67% H2/17% N2	38 200^a	300	∼62	∼96	108
	Foam Ni3Ce1				∼22	∼95
	Ni/CeO2–NCT	10% CO2/40% H/50% Ar	45 000	340	91.1	100	134
	NiO–CeO2 (np)	16% CO2/64% H2/20% N2	60 000	300	∼55	∼95	135
	NiO–CeO2 (3DOM)				∼3	∼50
	Ni–CeO2-CSC	10% CO2/40% H/50% Ar	120 000	300	70	∼100	136
	Porous NC3	10% CO2/40% H/50% Ar	60 000	300	85.6	100	137
	NiNPs@CeO2NF	20% CO2/80% H2	36 000	300	82.3	∼99	138
	NiNPs/CeO2				∼68	∼98
	NiNPs/CeO2NF				∼70	∼98
Synthesizing special morphology	Ni/CeO2-NR	10% CO2/40% H2/50% He	24 000	250	∼23	∼98	139
	Ni/CeO2-NC				∼10	∼96
	Ni/CeO2-PH	10% CO2/40% H2/50% He	21 000	250	41	∼99	140
	Ni/CeO2-NR				21.7	∼97
	Ni/CeO2-NP				15.7	∼98
	Ni/CeO2-NC				11	∼95
	Ni/CeO2-P	19% CO2/76% H2/5% N2	6000	260	73	100	141
	Ru/CeO2/r	10% CO2/40% H2/50% N2	72 000	300	∼72	∼99	67
	Ru/CeO2/o				∼56	∼99
	Ru/CeO2/c				∼14	∼99
Metal/oxide loading	Na/Ni/CeO2	1% CO2/50% H2/49% He	60 000	250	∼97	∼	142
	Ni/Ca0.1Ce0.9Ox	20% CO2/80% H2	36 000	290	75	99	143
	Ni/Sr0.1Ce0.9Ox				∼66	∼99
	Ni/Mg0.1Ce0.9Ox				∼58	∼99
	Ni/Ba0.1Ce0.9Ox				∼53	∼99
	12Ni3Co/M–Ce80Zr20	20% CO2/80% H2	12 000	300	∼72	∼99	144
	12Ni3Fe/M–Ce80Zr20				∼68	∼98
	12Ni3Mn/M–Ce80Zr20				∼62	∼98
	12Ni3Cu/M–Ce80Zr20				∼8	—
	1Co15Ni/CeO2	15% CO2/60% H/25% Ar	52 000^a	250	74	∼100	145
	RuNi/CZ	20% CO2/80% H2	24 000	350	53	93	146
	Ni/Pr–Ce	10% CO2/40% H/50% Ar	25 000	350	54.5	100	147
	Ni/Sm–Ce				44.9	100
	Ni/Mg–Ce				43.2	100
	Ni/Pr10Ce	20% CO2/80% H2	25 000	300	46	98	148
	5Ni/CeO2–Y2.0%	15% CO2/60% H/25% Ar	60 000	300	∼74	∼100	149
	NiCeO2–La-600	18% CO2/72% H/10% N2	30 000	300	∼88	∼100	126
	Ru/Ce0.9Cr0.1Ox	20% CO2/80% H2	36 000	250	∼70	∼100	19
	Ru/Ce3PrOx	10% CO2/40% H2/50% He	9000^a	270	∼55	100	150
Activation treatment	Ni/CeO2–H2	20% CO2/80% H2	36 000	300	70.9	99.7	24
	Ni/CeO2–N2				67.7	99.8
	Ni/CeO2–air				64.4	99.8
	Ni/CeO2-250	3.5% CO2/10% H2/87.5% N2	20 000	300	0	0	151
	Ni/CeO2-350				74.7	99.1
	Ni/CeO2-450				53.1	99
	Ni/CeO2-650				16.7	91.1

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4.1. Constructing efficient structured CeO₂-based catalysts

The structure of the support not only impacts the dispersion of active metal, but also greatly changes the reducibility and basicity of the catalyst.^49,152 Ye et al.⁸⁰ synthesized a nanostructured Ni/CeO₂-SGM that has a higher specific surface area than a conventional Ni/CeO₂-IM catalyst by a sol–gel method (Fig. 8I). The finding showed that the modified structure offered an array of advantages in Ni dispersion, reducibility, and CO₂ adsorption. Accordingly, even at a low temperature of 250 °C, the Ni/CeO₂-SGM presented a high CO₂ conversion of 80.5% and maintained it for more than 100 h. Therefore, modification of the CeO₂-based catalyst structure is a direct means to improve the catalytic activity.

Fig. 8 Synthesis route and corresponding catalytic performance of (I) Ni/CeO₂-SGM (reaction conditions: GHSV = 10 000 mL g⁻¹ h⁻¹, n(H₂) : n(CO₂) = 4 : 1). Reproduced from ref. 80 with permission from Elsevier, copyright 2020, (II) Ni/CeO₂-MOF (reaction conditions: GHSV = 45 000 mL g⁻¹ h⁻¹, n(H₂) : n(CO₂) : n(N₂) = 4 : 1 : 5). Reproduced from ref. 132 with permission from Elsevier, copyright 2021, (III) Ni–CeO₂-CSC (reaction conditions: GHSV = 120 000 mL g⁻¹ h⁻¹, n(H₂) : n(CO₂) : n(Ar) = 4 : 1 : 5). Reproduced from ref. 136 with permission from MDPI, copyright 2020, and (IV) NiNPs@CeO₂NF (reaction conditions: GHSV = 36 000 mL g⁻¹ h⁻¹, n(H₂) : n(CO₂) = 4 : 1). Reproduced from ref. 138 with permission from Elsevier, copyright 2022.

To avoid the sintering of Ni particles on conventional metal/support structure during the preparation and reaction process, Atzori et al.¹³¹ prepared a series of NiO–CeO₂ mixed oxides (NiCe(x)_HT) with different Ni loadings (5–35 wt%) using SBA-15 as the template. A counterpart pure CeO₂ sample was prepared by the same hard template method, and then Ni particles were deposited by incipient wetness impregnation (NiCe(x)_IWI). For the NiCe(x)_HT catalysts, the MSI effectively prevents the sintering and aggregation of active Ni particles even at a high Ni content (35 wt%). Accordingly, CO₂ conversion reached 76% at 300 °C over NiCe(35)_HT, which is higher than that over NiCe(35)_IWI (57%). On the contrary, NiCe(15)_IWI exhibited the highest CO₂ conversion of 66% among the as-prepared NiCe(x)_IWI catalysts. Based on the TEM results of NiCe(x)_IWI samples, an obvious Ni aggregation was observed with the increase in Ni content, leading to a decrease in methanation activity.

For the same purpose, Feng et al.¹³² designed a Ni/CeO₂ catalyst with high Ni dispersion and good durability using a Ce-based metal–organic framework (MOF) (Fig. 8II). At 350 °C, the Ni/CeO₂ sample with MOF structure (Ni/CeO₂-600 °C) yielded a higher CO₂ conversion of 71.0% than the conventional catalyst (∼55%) prepared by co-precipitation method (Ni/CeO₂-p). Also, benefiting from the confinement effect of the MOF structure, only slight sintering of Ni nanoparticles on the Ni/CeO₂ sample was observed (from 2.3 to ∼3 nm) after a 30 h durability test at 350 °C. Tang et al.¹³³ reported the difference in methanation performance between Ni deposited on Ce-MOF (Ni/CeO₂-M) and Ni/CeO₂-MOF (Ni/CeO₂-M-1) prepared by a one-pot method to further explore the effect of catalyst structure on CO₂ methanation. Although both the Ni/CeO₂ catalysts contained the same component and content, Ni/CeO₂-M-1 possessed a higher methanation activity of ∼79% at 325 °C compared with Ni/CeO₂-M (∼69%). This can be attributed to the stronger MSI over Ni/CeO₂-M-1, and further induced higher NiO dispersion and oxygen vacancy concentration.

The synthesis method largely determines the structural characteristics of the Ni/CeO₂ catalysts. Generally, CeO₂-based materials are less porous than conventional Al₂O₃, which may inhibit the dispersion of active metals. Fortunately, the utilization of templates in the synthesis process can optimize the pore structure of the CeO₂-based catalyst, and thus promote catalytic activity. Zhou et al.¹³⁴ prepared a series of CeO₂, prepared by hard-template (CT), soft-template (CS), and precipitation (CP) methods, that served as support of Ni particles. They found the Ni crystal particle size, specific surface area, and pore structure were governed by the CeO₂ support with different structures. Among the Ni/CeO₂ samples, the NCT catalyst with the largest specific surface area and the highest Ni dispersion had the strongest capacity to hydrogenate CO₂, and thus achieved a high CO₂ conversion of 91.1% at 340 °C. Similarly, Cárdenas-Arenas et al.¹³⁵ synthesized NiO–CeO₂ nanoparticles (NiO–CeO₂ (np)) prepared by a reversed microemulsion method, and the resulting catalyst exhibited CO₂ conversion of ∼55% at 300 °C. However, the NiO–CeO₂ sample (NiO–CeO₂ (3DOM)) with a three-dimensionally ordered macroporous (3DOM) structure, by hard-template method, showed a poor CO₂ conversion of ∼5%. The difference in specific surface area and reducibility was the primary reason for the substantial difference in methanation.

Wang et al.¹³⁶ synthesized a three-dimensional mesoporous Ni–CeO₂ sample (Ni–CeO₂-CSC) containing highly dispersed Ni particles (Fig. 8III). This special structure supplied an efficient Ni–CeO₂ interface for CO₂ activation, and thus enhanced the methanation activity over Ni–CeO₂-CSC. Also, they observed that Ni particles smaller than 5 nm can be embedded in the pore walls, which prevented the Ni sintering during the methanation reaction. Consequently, the CO₂ conversion of ∼70% can be maintained at 300 °C under a high GHSV of 120 000 mL g⁻¹ h⁻¹ for 50 h over Ni–CeO₂-CSC. To further optimize the pore structure of the Ni/CeO₂ catalyst, Zhou et al.¹³⁷ prepared a series of NiCe composite samples with different n_Ni/n_Ce ratios by a soft template method. The physicochemical properties of as-prepared NiCe catalysts were highly correlated with the n_Ni/n_Ce ratio. They found that the NC3 (n_Ni/n_Ce ratio = 3) with porous structure and high specific surface area showed high performance for CO₂ hydrogenation to CH₄ at 300 °C, with CO₂ conversion and CH₄ selectivity of 85.6% and 100%, respectively. Meanwhile, this nano-porous structure of NC3 was beneficial for the maintenance of methanation activity.

Recently, Hu et al.¹³⁸ reported Ni nanoparticles encapsulated in highly mesoporous CeO₂ nanofiber (NiNPs@CeO₂NF) fabricated by the co-electrospinning method (Fig. 8IV). They found that the structure of CeO₂ nanofibers could promote the production of oxygen vacancies, and those confined NiNPs could further enhance the methanation activity. Compared with the samples (NiNPs/CeO₂ and NiNPs/CeO₂NF) prepared by traditional impregnation, NiNPs@CeO₂NF possessed the highest oxygen vacancy concentration and thus exhibited a superior CO₂ conversion of 82.3% at 300 °C, which was higher than that of NiNPs/CeO₂ (∼68%) and NiNPs/CeO₂NF (∼70%). In addition, attributed to the confined effect of NiNPs@CeO₂NF, no significant deactivation was observed on NiNPs@CeO₂NF during a 60 h durability testing at 400 °C.

In summary, these studies confirmed that the catalytic activity over Ni/CeO₂ catalysts in CO₂ methanation can be enhanced by optimization of the structure. An efficient structured CeO₂-based catalyst not only decelerated the sintering of active metal, but also accelerated the generation of oxygen vacancy, and consequently, the CO₂ hydrogenation processes.

4.2. Synthesizing specific morphology

In addition to optimizing the structure of CeO₂-based catalysts, synthesizing the specific morphology is another approach to improve the catalyst's nature and activity. The effect of different morphology of CeO₂ support on the activity of CeO₂-based catalysts for CO₂ hydrogenation has been studied extensively due to the highly tunable crystal texture.^153–156 For CO₂ methanation, the activity over the CeO₂-based catalysts is strongly affected by the morphology of CeO₂. Compared with a commercial nano-CeO₂ (Aladdin, 20 nm, 99.5% metals basis)-supported Ni catalyst (∼60%), Ni/CeO₂-nanoplates (Ni/CeO₂-P) exhibited a higher CO₂ conversion of 73% at 260 °C.¹⁴¹ Besides, the TOF of Ni/CeO₂-P was up to 1.7 fold higher than Ni/CeO₂ sample at 200 °C. It should be noted that the H₂-TPD results indicated a similar Ni particle size on these Ni/CeO₂ catalysts. Thus, Du et al.¹⁴¹ suggested that the difference in distinct catalytic activity over Ni/CeO₂ and Ni/CeO₂-P was attributed to the CeO₂ supports with different shapes.

Generally, CeO₂ with different shapes have been synthesized by hydrothermal process under different hydrothermal conditions.^157,158 Bian et al.¹³⁹ deposited Ni particles on two CeO₂ supports with different shapes (nanorod and nanocube, denoted as NR and NC), and evaluated the low-temperature methanation activity over these samples. At a temperature range of 200–250 °C, the CO₂ conversion over Ni/CeO₂-NR was consistently higher than that over Ni/CeO₂-NC. They proposed that the high activity was related to a large number of oxygen vacancies resulting from the CeO₂-NR structure. This apparent discrepancy in oxygen vacancy concentration has a direct influence on the generation of a key intermediate (formate).

Subsequently, Jomjaree et al.¹⁴⁰ prepared additional CeO₂ supports including nanopolyhedron (PH) and nanoparticle (NP) to further complement this study. The results of a series of characterizations indicated the significant difference in textural property, reducibility, and oxygen vacancy content, which further impacted the catalytic activity over these Ni/CeO₂ catalysts. As a result, the CO₂ conversion ranked in the following order: Ni/CeO₂-PH > Ni/CeO₂-NR > Ni/CeO₂-NP > Ni/CeO₂-NC. Interestingly, a lower CO₂ conversion of 21.7% than that over Ni/CeO₂-PH (41%) was observed over the Ni/CeO₂-NR at 250 °C, even though it contained the highest surface area and reducibility. The authors suggested that the negative SMSI effect between Ni and CeO₂-NR was responsible for the worse low-temperature CO₂ methanation performance over the Ni/CeO₂-NR.

Furthermore, the preferentially exposed facet of CeO₂ is highly correlated with its morphology.^{153,159–161} This characteristic offered an appropriate model for exploring the impact of the CeO₂ crystal facet on CO₂ methanation. Generally, the behaviors of oxygen vacancy generation and lattice oxygen migration on the CeO₂ surface were highly correlated with its crystal structure.^27,162 The DFT calculations indicated that the stabilities of CeO₂ crystal facets decreased in the order of (111) > (110) > (100), while the oxygen vacancy formation energy decreased in the following order: (111) > (100) > (110).¹⁶³ Hence, the CeO₂(110) facet has the potential to produce oxygen vacancies and maintain the stability of these vacancy sites, which is favorable for the hydrogenation of CO₂ into CH₄.

Wang et al.⁹⁸ linked the oxygen vacancy content to CeO₂ exposed facets, and further revealed the structure–activity relationship over Ru/CeO₂-nanorods (NRs), Ru/CeO₂-nanopolyhedrons (NPs), and Ru/CeO₂-nanocubes (NCs) catalysts. The HR-TEM images displayed the primary CeO₂ lattice fringes of (110), (100), and (111) facets observed on Ru/CeO₂-NRs, Ru/CeO₂-NCs, and Ru/CeO₂-NPs, respectively. The subsequent catalytic activity experiments indicated that the reaction rates at 150 °C decreased in the order: Ru/CeO₂-NCs > Ru/CeO₂-NPs > Ru/CeO₂-NRs. The Raman spectra certified that Ru/CeO₂-NCs contained the highest oxygen vacancy concentration among these Ru/CeO₂ samples, which greatly benefited the activation of CO₂. However, the difference in Ru dispersion on these catalysts may impact the CO₂ conversion. Hence, the exclusion of the influence of particle size is warranted for further exploration of the crystal facet effect.

Sakpal et al.⁶⁷ prepared three CeO₂ supports with different shapes by a hydrothermal method, and supported Ru particles with similar size (∼2 nm). SEM and TEM images (Fig. 9a–l) showed that CeO₂(111) and (100) lattice fringes were observed on CeO₂ rods (CeO₂/r) and CeO₂ cubes (CeO₂/c), respectively, while CeO₂ octahedra (CeO₂/o) exposed (110) and (111) facets. Raman spectra combined with the results of TPR and XPS demonstrated that the addition of Ru particles enhanced the reducibility of CeO₂, and further increased oxygen vacancy concentration. Ru/CeO₂/r exhibited a higher CO₂ conversion of ∼72% than Ru/CeO₂/o (∼56%) and Ru/CeO₂/c (∼14%) at 300 °C (Fig. 9m), and no significant deactivation was observed over these catalysts within 24 h testing (Fig. 9n). This trend was consistent with the trend in oxygen vacancy content on as-prepared catalysts.

Fig. 9 SEM and TEM images of CeO₂/r (a and b), CeO₂/o (e and f), and CeO₂/c (i and j). TEM images of Ru/CeO₂/r (fresh, c and spent, d), Ru/CeO₂/o (fresh, g and spent, h), and Ru/CeO₂/c (fresh, k and spent, l). CO₂ conversion over Ru/CeO₂ catalysts (m), and CO₂ conversion at 350 °C against time on stream (n) (reaction conditions: GHSV = 72 000 mL g⁻¹ h⁻¹, n(H₂) : n(CO₂) : n(N₂) = 4 : 1 : 5). Reproduced from ref. 67 with permission from Elsevier, copyright 2018.

Overall, the CeO₂ morphology indeed impacted the CO₂ methanation reaction over the CeO₂-based catalyst. Generally, there were substantial differences in specific surface area and pore structure for CeO₂-based catalysts with different shapes. Therefore, because of these significant differences in textural properties of CeO₂-based catalysts, the influence of the CeO₂ crystal facet involved in CO₂ methanation has not been clearly understood. Unfortunately, the calcination temperature and Ce precursors can alter the crystal facets on the CeO₂ sample,¹⁶³ which further increases the difficulty in linking methanation activity with the exposed crystal facets on CeO₂.

4.3. Loading metal or oxide

The incorporation of metal or oxide is a simple, effective, and economical strategy to raise the methanation activity over CeO₂-based catalysts.^164–167 In general, CO₂ is an acidic gas, which can be adsorbed at alkaline sites on the catalyst surface. Thus, the addition of alkalis on the CeO₂-based catalyst would benefit the CO₂ methanation. Sodium (Na), a readily available alkali metal, has been considered for the modification of the Ni/CeO₂ catalyst. Le et al.¹⁴² prepared Ni/SiO₂ and Ni/CeO₂ samples with 0.1 and 1 wt% Na to understand the effects of Na content for methanation. Based on the results of H₂ chemisorption, the Ni dispersion on Ni/SiO₂ and Ni/CeO₂ obviously decreased with the increasing of Na content, and further impaired the ability to activate H₂. In respect of CO₂ adsorption, the adsorption sites on the Ni/SiO₂ surface increased with the Na ratio. The loss of H₂ dissociation sites and the gain of CO₂ adsorption sites reached a balance on 0.1% Na/Ni/SiO₂ catalyst, and thus a higher CO₂ conversion than Ni/SiO₂ and 1% Na/Ni/SiO₂ samples. On the contrary, a negative influence both on CO₂ adsorption and conversion was observed over Ni/CeO₂ catalysts even with a Na content as low as 0.1 wt%. According to the XPS analysis, Na tended to interact with CeO₂, which may lead to coverage of active sites on the CeO₂ surface.

To explore the influence of adding alkaline earth metal oxides on CO₂ methanation over Ni/CeO₂ catalyst, Liu et al.¹⁴³ prepared a series of NiM_0.1Ce_0.9O_x (M = Ca, Sr, Mg, and Ba) samples. They found that the distributions of these alkaline additives were different. CaO and MgO tended to dissolve into the CeO₂ lattice to form a solid solution, while BaO and SrO tended to exist on the surface of CeO₂. Overall, the addition of alkaline earth metal oxides increased Ni dispersion, the number of CO₂ adsorption sites and oxygen vacancies, and further enhanced the intrinsic catalytic activity over as-prepared catalysts. These positive effects were most pronounced on Ni/Ca_0.1Ce_0.9O_x, thus it showed the highest CO₂ conversion of 75% at 290 °C among all the catalysts. It should be noted that a Ni/Ca_0.1Ce_0.9O_x simply synthesized by a traditional impregnation method possessed a lower methanation activity, which indicated that the Ca species dissolved into the CeO₂ lattice were more effective than that on the CeO₂ surface.

Moreover, it was reported that bimetallic CeO₂-based catalysts can provide additional active sites for the activation of CO₂ and H₂.^168–172 Xu et al.¹⁴⁴ found that the loading of transition metal on Ni monometallic catalyst to build bimetallic catalyst could improve methanation activity over a mesoporous Ce_0.8Zr_0.2O₂-supported Ni particles catalyst (12Ni/M–Ce80Zr20) (Fig. 10I). The influences of adding different 3 wt% transition metals on CO₂ methanation over 12Ni/M–Ce80Zr20 samples were different. Specifically, Co and Fe enhanced the methanation activity, and the contribution of Mn could be neglected, while the negative effect of Cu on CO₂ conversion was observed. Among the as-prepared bimetallic catalysts, 12Ni3Co/M–Ce80Zr20 achieved the highest CO₂ conversion of ∼72% and CH₄ selectivity of ∼99% at 300 °C. In situ DRIFTS and temperature-programmed surface reaction (TPSR) results indicated that the incorporation of Co accelerated the conversion of intermediates and decreased the activation temperature of CO₂, due to the synergistic effect of Ni and Co on the 12Ni3Co/M–Ce80Zr20 catalyst. Recently, Hasrack et al.¹⁴⁵ found that CO₂ methanation performance could be improved over 15Ni/CeO₂ catalysts via adding 1 and 5 wt% of Co. On the basis of activity evaluation and CO₂-TPD results, the relationship between catalytic activity and the number of medium basic sites could be well established. Although the addition of Co decreased the total basicity, the amount of medium basic sites greatly increased, which was widely considered a key factor in the methanation reaction.

Fig. 10 (I) CO₂ methanation over Ni–T bimetal catalysts. Reproduced from ref. 144 with permission from American Chemical Society, copyright 2021. (II) Influence of Pr³⁺ on CO₂ conversion over Ni/CeO₂ catalyst. Reproduced from ref. 148 with permission from Elsevier, copyright 2022. (III) Promotion of Cr³⁺ on Ru/CeO₂ sample for CO₂ methanation. Reproduced from ref. 19 with permission from American Chemical Society, copyright 2021.

Saché et al.¹⁴⁶ prepared a series of supported metal catalysts on CeZr (ZC) solid solution, including Ni/CZ, FeNi/CZ, RuNi/CZ, and RuFeNi/CZ. They found that the incorporation of small amounts of Ru (1 wt%) could greatly enhance CO₂ methanation performance, and achieved a high CO₂ conversion of 53% at 350 °C. However, the addition of Fe inhibited CO₂ conversion over FeNi/CZ catalyst. In the terms of the trimetallic RuFeNi/CZ, the improvement effect of the promoter was not significant. The benefits of adding Ru were mainly reflected in Ni dispersion and electronic properties. Besides, Ru provided additional active sites on the RuNi/CZ catalyst, which accelerated the H₂ dissociation and CO₂ hydrogenation. Recently, Elia et al.¹⁷¹ found a similar trend in their study of monometallic and bimetallic CeO₂-based methanation catalysts. They suggested that the addition of Ru does indeed enhance the methanation activity by improving the dispersion of the Ni phase. It is noteworthy that no deactivation was observed on the Ru–Ni/CeO₂ catalyst during 75 h-on-stream at 350 °C.

The incorporation of oxides is a commonly used method to improve the surface properties of catalysts. Siakavelas et al.¹⁴⁷ reported the CO₂ methanation over these Ni/CeO₂ (Ni/Ce) catalysts modified by Sm₂O₃ (Sm), Pr₂O₃ (Pr), and MgO (Mg). They suggested that the additional Pr³⁺ and Sm³⁺ cations could not only dissolve into the CeO₂ lattice to promote the generation of oxygen vacancies, but also inhibited the agglomeration of Ni particles. It was noted that the Pr³⁺ tended to form Ce⁴⁺–O_V–Pr³⁺ species to produce more oxygen vacancies than the Ni/Sm–Ce sample. As for Ni/Mg–Ce, the MSI effect induced by the addition of Mg²⁺ effectively improved the stability at the CO₂ methanation reaction. Moreover, these additional cations optimized the distribution of basic sites and increased the number of moderate adsorption sites, thus enhancing the catalytic activity over the as-prepared Ni/Ce catalysts. As a result, Ni/Pr–Ce exhibited higher CO₂ conversion of 54.5% than Ni/Sm–Ce (44.9%), Ni/Mg–Ce (43.2%), and Ni/Ce (39.4%) at 350 °C.

Recently, Tsiotsias et al.¹⁴⁸ found that an appropriate amount of Pr³⁺ cations tended to substitute Ce⁴⁺ ions in the CeO₂ lattice and further induced the generation of oxygen vacancies. On the contrary, a high Pr loading (>20%) decreased the concentration of oxygen vacancies. Thus, Ni/Pr–CeO₂ catalysts with low Pr content (Ni/Pr10Ce) showed a higher CO₂ conversion of 46% than Ni/Ce (35%) and Ni/Pr50Ce (21%) at 300 °C (Fig. 10II). In addition, López-Rodríguez et al.¹⁵⁰ found the dual effect of Pr on CO₂ methanation over Ru/CeO₂ catalysts. Specifically, a Ru/CeO₂ sample with low Pr content (∼3 wt%) exhibited a relatively high CO₂ conversion of ∼55% at 270 °C due to a positive effect in oxygen mobility induced by Pr. However, a high Pr addition (∼25 wt%) hindered the dissociation of CO₂ at interface sites, and thus showed a low CO₂ conversion of ∼20%.

Sun et al.¹⁴⁹ found that Y played an important role in modifying Ni/CeO₂ catalyst for CO₂ methanation. To systematically understand the promotion effect of Y, they prepared 5Ni/CeO₂, 5Ni/Y₂O₃, and 5Ni/CeO₂–Y samples with different Y content. Subsequently, it was found that the 5Ni/CeO₂–Y catalyst with appropriate Y content possessed a higher CO₂ methanation activity than 5Ni/CeO₂ and 5Ni/Y₂O₃. The results of CO₂-TPD and H₂-TPR indicated that the basic distribution and reducibility of 5Ni/CeO₂–Y catalysts were largely influenced by Y content. The activity over these 5Ni/CeO₂–Y samples followed the order: 5Ni/CeO₂–Y2.0% > 5Ni/CeO₂–Y1.0% > 5Ni/CeO₂–Y0.5% > 5Ni/CeO₂–Y5.0%. This agreed well with the trend of the number of weak and medium basic sites, oxygen vacancy content, and reduction temperature of NiO.

In terms of Ru/CeO₂ catalysts, Xu et al.¹⁹ reported a high-performance CeO₂ support doped with Cr cation (Cr/Ce = 1 : 9) using a precipitation method (Fig. 10III). Then, 3 wt% Ru was loaded on the Ce_0.9Cr_0.1O_x support to hydrogenate CO₂ to CH₄. They found that the Cr³⁺ could incorporate into the CeO₂ lattice, and thus promoted the formation of surface oxygen vacancies on Ru/Ce_0.9Cr_0.1O_x. Meanwhile, XPS results indicated that the Ru/Ce_0.9Cr_0.1O_x catalyst contained more OH groups than Ru/CeO₂ after a reduction treatment. Similar apparent activation energies were calculated on Ru/Ce_0.9Cr_0.1O_x (74.9 kJ mol⁻¹) and Ru/CeO₂ (79.2 kJ mol⁻¹), indicating that the same methanation mechanism proceeds on these two samples. However, Ru/Ce_0.9Cr_0.1O_x showed a considerable TOF_CO2 (25.1 × 10⁻⁴ s⁻¹) at a low temperature of 150 °C, which was 5.3 times higher than that of Ru/CeO₂. These illustrated that the addition of Cr could indeed improve the intrinsic activity of the Ru/CeO₂ catalyst. In situ FTIR spectroscopy demonstrated the CO* and formate pathways that occurred on Ru/Ce_0.9Cr_0.1O_x and Ru/CeO₂. Moreover, the number of bicarbonates and formates increased markedly as the oxygen vacancies and OH group increased, which greatly enhanced the catalytic performance of Ru/Ce_0.9Cr_0.1O_x for low-temperature CO₂ methanation.

In summary, for CO₂ methanation over CeO₂-based catalysts, the addition of metal or oxide often causes certain positive or negative effects. This depends not only on the nature of the additive, but also on the interaction between CeO₂ and the additive. Generally, increasing the amounts of oxygen vacancies and basic sites, and improving the metal dispersion and reducibility on CeO₂-based catalysts by the addition of promotors can effectively improve the activity for CO₂ methanation.

4.4. Exposing more active sites by pre-treatment

The purpose of pre-treatment is to expose more active sites before the methanation reaction is started. Generally, a reduction treatment is needed to generate active metal sites on the catalyst surface for CO₂ and H₂ activation. Lin et al.²⁴ suggested that the activation treatment could determine the strength of MSI, and further impact the methanation activity over Ni/CeO₂ catalysts (Fig. 11Ia). They found that the thermal treatment of Ni/CeO₂ catalysts under N₂ or air atmosphere decreased Ni dispersion and the number of oxygen vacancies, while annealed Ni/CeO₂ under H₂ atmosphere was distinctly beneficial for CO₂ methanation due to an appropriate MSI effect. Subsequently, a series of Ni/CeO₂ catalysts were pre-treated under an H₂ atmosphere with different temperatures (400–550 °C) to verify the interaction between H₂ and the catalyst. The conversion rate of CO₂ over these catalysts followed the order: Ni/CeO₂-450 °C > Ni/CeO₂-400 °C > Ni/CeO₂-500 °C > Ni/CeO₂-550 °C (Fig. 11Ib). The results of H₂ chemisorption indicated the exposed metallic Ni decreased with the increase in reduction temperature. This is attributed to the encapsulation of the Ni surface induced by the SMSI effect. A similar finding was reported by Li et al.;³² the appropriate reduction temperature for Ni/CeO₂ catalyst prepared by wet-impregnation was 400–450 °C.

Fig. 11 (I) Effect of heat treatment atmosphere (a) and reduction temperature (b) on CO₂ conversion and CH₄ selectivity over Ni/CeO₂. Reproduced from ref. 24 with permission from Elsevier, copyright 2021. (II) CO₂ methanation performance over the Ni/CeO₂ samples reduced at different temperatures. Reproduced from ref. 151 with permission from American Chemical Society, copyright 2021.

Huang et al.¹⁵¹ further investigated the effect of reduction temperature for CO₂ methanation over Ni/CeO₂ catalysts synthesized by a wet mixing method. Before the activity evaluation experiment, Ni/CeO₂ was reduced under H₂ flow at a set temperature (300–650 °C). The experimental results indicated the reduction temperature substantially affected CO₂ conversion and CH₄ selectivity over Ni/CeO₂ catalysts. As shown in Fig. 11II, the highest methanation catalytic activity was observed over Ni/CeO₂ sample reduced at 350 °C, and then declined rapidly as the reduction temperature raised to 650 °C. Unexpectedly, the high reduction temperature had a marginal impact on the Ni dispersion and Ni surface area of the Ni/CeO₂ samples. However, many surface oxygen vacancies may be consumed at a high reduction temperature (>450 °C), causing poor catalytic performance.

In summary, an appropriate reduction temperature can indeed enhance the activity toward CO₂ methanation. Unfortunately, little attention has been focused on the effect of pre-treatment conditions for CO₂ methanation. Moreover, the previous views on the poor activity over Ni/CeO₂ catalyst reduced at high temperatures remain controversial. This may be attributed to the different synthetic methods and the content of the metal. The reduction pre-treatment is almost necessary to expose more active sites for CO₂ methanation catalysts. Thus, it is worthwhile to further investigate the interaction between CeO₂-based catalysts and H₂.

4.5. Tolerance of CeO₂-based catalysts

Benefiting from the above-mentioned strategies, the low-temperature CO₂ methanation activity over CeO₂-based catalysts under laboratory-scale experiments has essentially met the requirements. Additionally, carrying out a methanation reaction at low temperature is effective for avoiding catalyst deactivation induced by sintering and carbon deposition.^49,99 However, extra poisoning components such as chlorine, CO, and especially sulfur normally exist in industrial conditions.^173–176 Generally, the poisoning effects are mainly caused by the occupation of active sites and the strong chemisorption of species.⁷⁶

Konishcheva et al.¹⁷³ deemed that chlorine could interact with Ce³⁺ to form CeOCl-like species, which occupied active sites on the surface of CeO₂ and inhibited CO₂ hydrogenation. Afterward, they found that the existence of CO also inhibited the CO₂ methanation reaction due to the active metal sites being clogged by strongly adsorbed CO species.¹⁷⁴ Ahn et al.¹⁷⁵ investigated the impact of H₂S on Ni–Ce–Zr catalyst for CO₂ methanation. They observed that the CO₂ conversion decreased dramatically from ∼70% to ∼10% after the injection of 20–100 ppm H₂S at 220 °C. H₂S preferred to interact with CeO₂ to form Ce₂O₂S species rather than Ni.¹⁷⁷ This may be due to the fact that the formation of Ce₂O₂S is thermodynamically more favorable than that of NiS. In contrast to Ni/CeO₂ and Ru/CeO₂ catalysts, Mo-based methanation catalysts may be promising candidates due to their good sulfur-resistant properties.^178–180 However, the current CO₂ conversion and CH₄ selectivity over Mo-based catalysts cannot meet the demand for efficient CO₂ mitigation and fuel production.

Although the poisoning phenomenon and mechanism of methanation catalysts have been widely reported,^181–184 few studies have focused on the strategies to improve the tolerance of CeO₂-based methanation catalysts. Fortunately, many reports focusing on the preparation of anti-poisoning catalysts can provide constructive information for the perspectives of CeO₂-based catalysts in CO₂ methanation.^60,185–191 Jeong et al.¹⁹² proposed that the hydrothermal treatment of Pd/CeO₂ catalyst promoted the redispersion of Pd species and the formation of surface OH groups. This further increased the CO oxidation activity over Pd/CeO₂ and the resistance of the sample to surface poisoning compounds (Fig. 12I). Using the citric acid-assisted hydrothermal method, Zheng et al.¹⁹³ synthesized a porous Fe-doped CeO₂ catalyst with a flower-like shape to achieve efficient H₂S-selective oxidation activity. Meanwhile, the addition of Fe cations could alleviate the deactivation of CeO₂ induced by sulfation (Fig. 12II). In addition to the modification of the CeO₂ support, Xie et al.¹⁹⁴ reported that the well-dispersed electron-deficient Rh particles may be beneficial for the enhancement of sulfur-tolerant performance.

Fig. 12 (I) Promoting effects of hydrothermal treatment on sulfur resistance of Pd/CeO₂ catalysts. Reproduced from ref. 192 with permission from American Chemical Society, copyright 2017. (II) H₂S oxidation over the Fe-doped CeO₂ nanoflowers. Reproduced from ref. 193 with permission from American Chemical Society, copyright 2020. (III) Enhanced sulfur resistance of octahedral nano-ceria. Reproduced from ref. 195 with permission from American Chemical Society, copyright 2021.

Kim et al.¹⁹⁵ prepared a Pt catalyst supported on CeO₂–Al₂O₃ with the octahedral morphology of CeO₂ for CO oxidation in the presence of SO₂. They found that this support with few Ce³⁺ defects exhibited better sulfur-resistant properties than that of the defective one (Fig. 12III). Recently, Hu et al.¹⁹⁶ studied the effects of the Ce³⁺/Ce⁴⁺ ratio of CeO₂/MoO₃ catalyst on the selective catalytic reduction (SCR) of NO_x with NH₃. Contrary to the report of Kim et al.,¹⁹⁵ Hu et al.¹⁹⁶ suggested that a high Ce³⁺/Ce⁴⁺ ratio (≥0.5) could induce the appearance of the non-bulk electronic states for the CeO₂ species, which led to the enhanced sulfur resistance of CeO₂-based catalyst. Therefore, the applicability of these strategies for enhancing catalyst resistance to CeO₂-based methanation catalysts requires further experimental verification.

5. Summaries and future prospects

This paper thoroughly reviewed the application of CeO₂-based catalysts for CO₂ hydrogenation to CH₄. The existing literature has been summarized to understand the interaction between CO₂/H₂ and CeO₂, to identify active sites, and to reveal the role of metal–support interaction for CO₂ methanation. It was demonstrated that the reaction mechanism was highly correlated with the properties of the active site. Oxygen vacancy, as a key active site on CeO₂, can be created by interacting with H₂, and further promoting CO₂ activation and conversion, while the metallic sites dissociated H₂ to accelerate the hydrogenation of key intermediates. The formate route (CO₂* → CO₃*/HCO₃* → HCOO* → CH₄) was observed to be dominant on most of the CeO₂-based catalysts with abundant oxygen vacancies, although the CO* route (CO₂* → CO* → OCH₂* → OCH₃* → CH₄) was possible.

Based on the above discussion, CeO₂-based catalysts could be optimized through the following ideas: (1) weakening the Ce–O bond to improve the reducibility, (2) adjusting the distribution of basic sites to boost the CO₂ adsorption, (3) dispersing active metal supported on CeO₂ to assist the dissociation of H₂ and CO₂, and (4) increasing the number of oxygen vacancies and additional active sites (such as active metal and OH groups) for CO₂ hydrogenation. Popular strategies such as optimizing the structure of the catalysts, synthesizing specific morphology, and doping additional metal or oxide could be used to effectively achieve these purposes. Also, the pre-treatment was found to impact metal–support interaction, and thus the catalytic activity. The highly tunable nature of CeO₂ makes the CeO₂-based catalyst a promising material for application in CO₂ methanation.

Despite the significant advances that have been made in the understanding and development of CeO₂-based catalysts for CO₂ methanation, several issues remain to be elucidated and solved. First, a current urgent demand is to clearly identify the evolution of active sites during the CO₂ methanation reaction process by multiple in situ/operando characterizations combined with temperature-programmed surface reaction experiments, and further precisely establish the structure–performance relationship. Second, the evolution of active sites under reaction conditions should be linked to the transformation of active intermediates. Toward this goal, spectrokinetics studies and a more realistic model for DFT calculations is needed. Third, exploring a new way to precisely tailor the extent of MSI between active metal and CeO₂ is necessary to understand the roles of MSI in CO₂ methanation. Such a fundamental understanding of the methanation mechanism and catalytic sites at the atomic level is a pivotal premise for designing a better CeO₂-based catalyst.

The CO₂ methanation activity over CeO₂-based catalysts under ideal reaction conditions has shown relatively satisfactory methanation performance. However, from a practical industrial point of view, challenges remain in developing a durable and low-cost CeO₂-based methanation catalyst with considerable low-temperature activity. From a long-term perspective, one major challenge to ensuring a large-scale application of CO₂ methanation is to obtain H₂ economically and sustainably. Water electrolysis driven by renewable energy is an economic and environment-friendly technology for H₂ generation. Besides, other methods of H₂ production such as electrocatalysis, photocatalysis, thermal splitting, and high-temperature chemical cycles have been developed considerably in the last decades. Hence, the recycling of atmospheric CO₂ into fuels or chemicals using cost-effective hydrogen will contribute to a sustainable society. To this end, an in-depth understanding of the reaction mechanism and catalyst development is the direction of future efforts.

Author contributions

Yu Xie: conceptualization, data curation, investigation, and writing – original draft; Junjie Wen: data curation and investigation; Zonglin Li: data curation and investigation; Jianjun Chen: writing – review & editing; Qiulin Zhang: conceptualization, funding acquisition, writing – review & editing; Ping Ning: supervision; Yaoqiang Chen: supervision; Jiming Hao: conceptualization and supervision.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Key R&D Program of China (no. 2019YFC0214403) and Yunnan Fundamental Research Projects (no. 202201BE070001-025).

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