Research Progress on Stability Control on Ni-Based Catalysts for Methane Dry Reforming

: CO 2 reforming of CH 4 (DRM) utilizes the greenhouse gases of CH 4 and CO 2 to obtain the synthesis gas, benefiting the achievement of carbon neutrality. However, the deactivation of Ni-based catalysts caused by sintering and carbon deposition limits the industrial application. Focusing on stability improvement, this review first summarizes the reaction mechanism and deactivation mechanism in DRM and then discusses the impact of catalyst active components, supports, and interfacial structure. Finally, we propose the design direction of stable Ni-based catalysts towards DRM, providing guidance for the future development of catalysts suitable for industrial production.


Introduction
Methane (CH 4 ) and carbon dioxide (CO 2 ) are two major greenhouse gases but also important carbon resources [1][2][3][4].Catalytic conversion and utilization of these two greenhouse gases through chemical pathways is of positive significance for mitigating the greenhouse effect and closing the carbon cycle [5].Additionally, CH 4 and CO 2 are important components of biogas, which come from a wide range of sources [6].Methane dry reforming (DRM) can convert CH 4 and CO 2 into carbon-rich synthesis gas, which is particularly suitable for hydroformylation synthesis to fulfill the high-value utilization of biogas.DRM, which is strongly endothermic thermodynamically, needs to be performed under high temperature (700-1000 • C) conditions.On the one hand, this leads to high energy consumption, and on the other hand, there is catalyst deactivation caused by high-temperature sintering and carbon deposition.Therefore, how to improve the stability and low-temperature activity of the DRM catalyst has become the key to industrial application [7].Among various metal-based DRM catalysts, the inexpensive earth-abundant Ni-based catalysts stand out because they show high catalytic activity, comparable to noble metals such as Ru and Rh [8].However, Ni-based catalysts also deliver excellent C-H bond cleavage ability.The formed carbon species deposit on the Ni surface and form stable carbon structures, leading to the block of metal-active sites and inducing deactivation [9][10][11][12].
To understand and seek possible solutions to catalyst deactivation issues, this review first reveals the reaction mechanism and deactivation mechanism of DRM and then discusses the influence of catalyst active components, supports, and microenvironment.Finally, the development direction of stable DRM catalysts is proposed, which provides guidance for the design of catalysts suitable for industrial applications (Figure 1).

DRM Reaction Mechanism and Deactivation Mechanism
The reaction network of DRM is complex.The reaction (Entry 1), Table 1 is accompanied by side reactions such as the reverse water gas shift (RWGS) reaction (Entry 2) and carbon deposition reaction, including CH4 dissociation (Entry 3), CO disproportionation, namely, the Boudouard reaction (Entry 4) and CO redox (Entry 5).
As can be seen from the above reaction equation, DRM is strongly endothermic, and a high temperature is required to promote the reaction.Due to the presence of the side reaction of (Entry 2), the CO:H2 mole ratio is less than 1.Since the DRM reaction is carried out in the thermodynamic carbon deposition zone (Entry 3), it is impossible to improve the carbon deposition of the DRM reaction by changing the thermodynamic conditions of the reaction.Therefore, this review focuses on possible kinetic ways to increase the reaction rate of DRM while decreasing the reaction rates of side reactions by the adjustment of influencing factors.

Reaction Mechanism
The DRM mechanism has been extensively studied [13,14].It is generally believed that DRM reactions on Ni-based catalysts follow these steps: (1) Activation of CH4.It generally occurs on the active site of the metal and at the interface between the metal and the support.CH4 adsorbs on these active metal surfaces and gradually dissociates to C* or CH* [15][16][17].
(2) Activation of CO2.The activation of CO2 usually occurs on the support or at the interface between the support or the support rather than on metal.The obtained H from the CH4 dissociation on Ni may assist CO2 activation via COOH* or HCOO* species [18].The rate-limiting step varies depending on the system.The dissociation of CH4 on the catalyst is generally considered to be a rate control step for DRM [18].M-O* species formed on metal surfaces after preferential activation of CO2 can promote methane activation [19].However, some studies have shown that CO2 dissociation at high temperatures may be a rate-limiting step [20].For bifunctional catalysts, such as Pd/ZrO2, it is a multi-step-controlled reaction [21].

DRM Reaction Mechanism and Deactivation Mechanism
The reaction network of DRM is complex.The reaction (Entry 1), Table 1 is accompanied by side reactions such as the reverse water gas shift (RWGS) reaction (Entry 2) and carbon deposition reaction, including CH 4 dissociation (Entry 3), CO disproportionation, namely, the Boudouard reaction (Entry 4) and CO redox (Entry 5).As can be seen from the above reaction equation, DRM is strongly endothermic, and a high temperature is required to promote the reaction.Due to the presence of the side reaction of (Entry 2), the CO:H 2 mole ratio is less than 1.Since the DRM reaction is carried out in the thermodynamic carbon deposition zone (Entry 3), it is impossible to improve the carbon deposition of the DRM reaction by changing the thermodynamic conditions of the reaction.Therefore, this review focuses on possible kinetic ways to increase the reaction rate of DRM while decreasing the reaction rates of side reactions by the adjustment of influencing factors.

Reaction Mechanism
The DRM mechanism has been extensively studied [13,14].It is generally believed that DRM reactions on Ni-based catalysts follow these steps: (1) Activation of CH 4 .It generally occurs on the active site of the metal and at the interface between the metal and the support.CH 4 adsorbs on these active metal surfaces and gradually dissociates to C* or CH* [15][16][17].
(2) Activation of CO 2 .The activation of CO 2 usually occurs on the support or at the interface between the support or the support rather than on metal.The obtained H from the CH 4 dissociation on Ni may assist CO 2 activation via COOH* or HCOO* species [18].The rate-limiting step varies depending on the system.The dissociation of CH 4 on the catalyst is generally considered to be a rate control step for DRM [18].M-O* species formed on metal surfaces after preferential activation of CO 2 can promote methane activation [19].However, some studies have shown that CO 2 dissociation at high temperatures may be a rate-limiting step [20].For bifunctional catalysts, such as Pd/ZrO 2 , it is a multi-stepcontrolled reaction [21].
(3) Oxidation and decomposition of intermediates.C* or CH*, which is from the CH 4 dissociation, is oxidized by O* to produce CO* or CHO*, and then CHO* is decomposed into CO* and H* [21].The origin of O* is diverse, which could come from the decomposition of CO 2 or from the oxide support [22][23][24].For those supports with high oxygen release capacity and lattice oxygen mobility, such as La 2 O 3 [23] and CeO 2 [23], direct oxidation of CH x species by the lattice oxygen (O lattice ) via a Mars−van Krevelen-type redox mechanism is proposed.In this mechanism, the active carbon intermediates from methane dissociation are oxidized directly by the lattice oxygen of the support to form CO and oxygen vacancies, avoiding the formation of carbon species on the Ni surface [23].The oxygen vacancies then serve as the activation sites for CO 2 dissociation into CO and O [23].
(4) Formation of hydroxyl groups.H* generated by CH 4 decomposition could diffuse from the surface of active metal particles to the support under the DRM reaction condition and react with the O* species generated by CO 2 decomposition to form OH species [23].
(5) Syngas formation.In the final step, syngas is formed as a result of a radical recombination.

Catalyst Deactivation Mechanism
In the DRM reaction, the deactivation of the catalyst is mainly caused by carbon deposition and sintering of the active components.
Carbon deposition is the process of carbon species covering the active sites, reducing the contact between reactants and active metals.Carbon species can be classified into different types based on morphology, properties, and relative deactivation rate.It includes the adsorbed atomic carbon C α , polymeric carbon filaments C β , graphitic carbon C c , and carbon whiskers C v (Figure 2).C α is formed from the dissociation of reactants and products of DRM, CO 2 , CH 4 , and CO on the catalyst.When the temperature is above 250 • C, the generation rate of C α will exceed its gasification rate and accumulate on the surface.As it accumulates, they react together to convert into C β .As the temperature continues to rise, C α and C β will convert into carbon whiskers C v and graphite carbon C c .It is noteworthy that not every kind of carbon species reduces the activity of the catalyst.For example, C α and C β can be eliminated by oxidants formed during the reaction with little impact on the catalytic activity.However, at 500-900 • C, the carbon whiskers C v exhibit thermodynamic stability with a thickness of over 80 atomic diameters, making it difficult to oxidize and remove.In addition, when generating a large amount of C v , it will cause the active metal components to move and aggregate.Graphite carbon C c is also difficult to eliminate through oxidation reactions [25].It will cover the surface of the catalyst, even completely envelop the active sites, block pores, isolate internal active components, and prevent them from contacting reactants, thus deactivating the catalysts.In addition, C c can also block the reactor, increase the load pressure of the catalytic bed, and damage the structure of the catalyst.In summary, at the typical DRM temperature, 500-900 • C, the main carbon species are carbon whiskers C v and graphite carbon C c [25].Therefore, for Ni-based catalysts in the DRM process, it is crucial to solve the problem of carbon deposition to maintain the activity and stability simultaneously.
Methane 2024, 3, FOR PEER REVIEW (3) Oxidation and decomposition of intermediates.C* or CH*, which is from the dissociation, is oxidized by O* to produce CO* or CHO*, and then CHO* is decompo into CO* and H* [21].The origin of O* is diverse, which could come from the decomp tion of CO2 or from the oxide support [22][23][24].For those supports with high oxygen rele capacity and lattice oxygen mobility, such as La2O3 [23] and CeO2 [23], direct oxidatio CHx species by the lattice oxygen (Olattice) via a Mars−van Krevelen-type redox mechan is proposed.In this mechanism, the active carbon intermediates from methane disso tion are oxidized directly by the lattice oxygen of the support to form CO and oxy vacancies, avoiding the formation of carbon species on the Ni surface [23].The oxy vacancies then serve as the activation sites for CO2 dissociation into CO and O [23].
(4) Formation of hydroxyl groups.H* generated by CH4 decomposition could diff from the surface of active metal particles to the support under the DRM reaction condi and react with the O* species generated by CO2 decomposition to form OH species [2 (5) Syngas formation.In the final step, syngas is formed as a result of a radical rec bination.

Catalyst Deactivation Mechanism
In the DRM reaction, the deactivation of the catalyst is mainly caused by carbon d osition and sintering of the active components.
Carbon deposition is the process of carbon species covering the active sites, reduc the contact between reactants and active metals.Carbon species can be classified into ferent types based on morphology, properties, and relative deactivation rate.It inclu the adsorbed atomic carbon Cα, polymeric carbon filaments Cβ, graphitic carbon Cc, carbon whiskers Cv (Figure 2).Cα is formed from the dissociation of reactants and pr ucts of DRM, CO2, CH4, and CO on the catalyst.When the temperature is above 250 the generation rate of Cα will exceed its gasification rate and accumulate on the surf As it accumulates, they react together to convert into Cβ.As the temperature continue rise, Cα and Cβ will convert into carbon whiskers Cv and graphite carbon Cc.It is notew thy that not every kind of carbon species reduces the activity of the catalyst.For exam Cα and Cβ can be eliminated by oxidants formed during the reaction with little impac the catalytic activity.However, at 500-900 °C, the carbon whiskers Cv exhibit thermo namic stability with a thickness of over 80 atomic diameters, making it difficult to oxid and remove.In addition, when generating a large amount of Cv, it will cause the ac metal components to move and aggregate.Graphite carbon Cc is also difficult to elimin through oxidation reactions [25].It will cover the surface of the catalyst, even comple envelop the active sites, block pores, isolate internal active components, and prevent th from contacting reactants, thus deactivating the catalysts.In addition, Cc can also bl the reactor, increase the load pressure of the catalytic bed, and damage the structur the catalyst.In summary, at the typical DRM temperature, 500-900 °C, the main car species are carbon whiskers Cv and graphite carbon Cc [25].Therefore, for Ni-based c lysts in the DRM process, it is crucial to solve the problem of carbon deposition to main the activity and stability simultaneously.Sintering is the migration and agglomeration of the active metal at high temperatures, which reduces the number of active sites, resulting in a decrease in the activity of the catalyst [26].The oxidation of active metals during the reaction and its reaction with the support or promoters, as well as the collapse of the pore structure of the catalyst support at high temperatures, will also cause catalyst deactivation.Although it is difficult to avoid carbon deposition and sintering of the active components, the above problems can be effectively mitigated by the design and modulation of the catalyst.
Additionally, the possible presence of temperature hysteresis phenomenon in the typical endothermic reaction of DRM could be one of the reasons for deactivation.However, the mechanism of its occurrence is still unclear.Wang et al. [27] studied the hysteresis phenomenon of the Ni/SBA-15 catalyst in the DRM reaction and deeply analyzed the reasons and mechanisms behind this phenomenon (Figure 3).The ignition temperature (T ign ) is 425 • C, and the text temperature is 295 • C. At temperatures below T ign , the surface Ni 0 is oxidized to NiO by CO 2 or O in the support, making the reaction difficult to proceed.At T ign , NiO reduction and methane cracking occur simultaneously, generating carbon nanotubes that inhibit the oxidation of Ni 0 .When the reaction temperature is higher than T ign , carbon nanotubes generated by methane cracking form a unique catalyst structure with surface Ni, which promotes the catalyst to still have catalytic activity at temperatures lower than T ign .When it is below Text (the activity decreases to 0% at Text), CH 4 cannot continue to reduce NiO, the cycle between the NiO and Ni is broken, and the catalyst is deactivated [27] (Figure 3).
Methane 2024, 3, FOR PEER REVIEW 4 Sintering is the migration and agglomeration of the active metal at high temperatures, which reduces the number of active sites, resulting in a decrease in the activity of the catalyst [26].The oxidation of active metals during the reaction and its reaction with the support or promoters, as well as the collapse of the pore structure of the catalyst support at high temperatures, will also cause catalyst deactivation.Although it is difficult to avoid carbon deposition and sintering of the active components, the above problems can be effectively mitigated by the design and modulation of the catalyst.
Additionally, the possible presence of temperature hysteresis phenomenon in the typical endothermic reaction of DRM could be one of the reasons for deactivation.However, the mechanism of its occurrence is still unclear.Wang et al. [27] studied the hysteresis phenomenon of the Ni/SBA-15 catalyst in the DRM reaction and deeply analyzed the reasons and mechanisms behind this phenomenon (Figure 3).The ignition temperature (Tign) is 425 °C, and the text temperature is 295 °C.At temperatures below Tign, the surface Ni 0 is oxidized to NiO by CO2 or O in the support, making the reaction difficult to proceed.At Tign, NiO reduction and methane cracking occur simultaneously, generating carbon nanotubes that inhibit the oxidation of Ni 0 .When the reaction temperature is higher than Tign, carbon nanotubes generated by methane cracking form a unique catalyst structure with surface Ni, which promotes the catalyst to still have catalytic activity at temperatures lower than Tign.When it is below Text (the activity decreases to 0% at Text), CH4 cannot continue to reduce NiO, the cycle between the NiO and Ni is broken, and the catalyst is deactivated [27] (Figure 3).

Regulation of Carbon Deposition on Ni-Based DRM Catalysts
Many strategies, such as structural and electronic engineering, are employed to adjust the properties of the catalysts.For the control of carbon deposition on Ni-based DRM catalysts, the design includes the following aspects.

Ni Configuration
The configuration of Ni matters during the catalytic reaction [28,29].The zero-valenced Ni centers, namely metallic Ni, are required for DRM, while the limited amount of metallic Ni minimizes carbon deposition [30][31][32].Wang et al. [15] found that for the Ni-Zr/SiO2 catalyst, the Ni 0 species was oxidized to a NiO species under the reaction conditions and could not be restored, leading to its deactivation.

Regulation of Carbon Deposition on Ni-Based DRM Catalysts
Many strategies, such as structural and electronic engineering, are employed to adjust the properties of the catalysts.For the control of carbon deposition on Ni-based DRM catalysts, the design includes the following aspects.

Ni Configuration
The configuration of Ni matters during the catalytic reaction [28,29].The zero-valenced Ni centers, namely metallic Ni, are required for DRM, while the limited amount of metallic Ni minimizes carbon deposition [30][31][32].Wang et al. [15] found that for the Ni-Zr/SiO 2 catalyst, the Ni 0 species was oxidized to a NiO species under the reaction conditions and could not be restored, leading to its deactivation.
The carbon deposition causes the deformation and recombination of the active metal particles.Due to the friction exerted by carbon deposits, the Ni particles in the Ni-based catalyst may become pear-shaped and transfer to elongated conical at high temperatures.In addition, since high Ni dispersion reduces metal sintering and hinders C v formation, the stability of the catalyst can be enhanced by increasing the dispersion of Ni and adopting the appropriate operating temperature for the Ni-based catalyst.Note that agglomeration and sintering can again increase above an optimal limit of metal dispersion.

Ni Particle Size
The size of Ni particles is another important factor controlling the stability of DRM catalysts.Recently, in order to fully utilize the metal center [33], a series of single-atom catalysts have been developed for DRM applications.Mohcin Akri, et al. [34] designed and prepared a stable Ni single-atom catalyst through interaction with Ce-doped hydroxyapatite (HAP), showing a high activity of DRM and excellent carbon resistance ability.Experimental and computational studies have shown that isolated nickel atoms are inherently carbon-resistant because they have the unique property of activating only the first carbon-hydrogen bond in CH 4 , thus avoiding the deep decomposition of methane into deposited carbon species.It was found that the cerium doping of HAP support stabilized the atomically dispersed Ni, which significantly improved the anti-sintering performance of the 0.5Ni 1 /HAP-Ce catalyst.After 65 h reactions, the conversion of CO 2 and CH 4 retains 90% of the initial with a small decrease.The HAP-Ce support has almost no activity towards DRM, indicating that its activity mainly comes from atomically dispersed Ni.The main role of Ce is to stabilize atomically dispersed Ni rather than to inhibit carbon deposition.This observation is consistent with results from recently reported Fe single-atom catalysts catalyzing non-oxidized coupling reactions of CH 4 with ethylene and aromatics [35], in which no carbon deposition was observed.Theoretical calculations also show that the oxide-supported Pt single-atom catalyst can only efficiently activate the first C-H bond in CH 4 , matching well with the experimental results [36].

Second Active Metal
As a non-precious metal element, Ni shows high catalytic activity but low antideposition ability towards the DRM reaction process.While precious metals (Ru, Rh, Ir, Pd, Pt, etc.) own high activity and good carbon-resistant properties, they are expensive.Therefore, adding a small number of precious metals as co-active components [37] could achieve excellent activity and stability simultaneously with high cost-effectiveness [38].In general, a large number of active Ni components in Ni-based catalysts migrate to the outside of the pore, causing Ni to sinter and forming large-sized particles.The addition of a second component, such as Pt, can greatly avoid the diffusion of active metals to the outside, decrease the size of metal particles, and reduce the sintering of metal particles in the pore.Recently, Li et al. [39] designed and constructed a highly dispersed Ni-Ir/MgAl 2 O 4 alloy catalyst for the DRM reaction.Based on the synergistic effect between Ni, Ir, and MgAl 2 O 4 , high catalytic activity and stability were achieved concurrently.Ni is the main activation center of CH 4 , while the Ir can effectively utilize the surface carbonates formed by the adsorption of CO 2 on the MgAl 2 O 4 support to generate active oxygen species, which can further be used for the elimination of the carbon species generated after the activation of CH 4 (Figure 4).Based on the equilibrium mechanism between the active site and carbon deposition, the dynamic balance between carbon deposition and elimination can be achieved by regulating the active site so that the carbon deposition in this system will not continue to increase with the increased reaction time.The optimal Ni 3 Ir 6 /MgAl 2 O 4 catalyst achieved near-zero carbon deposition during a 600 h high-temperature DRM reaction [39].
effective dispersion of Ni and the avoidance of active components sintering can prolong the service life of catalysts.

Regulation of Supports
The support is generally inert in the DRM reaction.However, it can affect the stability and mechanical strength of the catalyst via the interaction between the active component and the support.Moreover, the surface area, pore structure, thermal stability, oxidationreducibility, and surface alkalinity of the support can affect the carbon deposition behavior of the Ni-based catalyst [40].

Support Structure
As described above, the support can affect the particle size and dispersion of the active metal in the catalyst and directly affect the activity and stability of the DRM catalysts.For the Ni-based catalysts, the use of a support with a high specific surface area and a finely controlled pore structure can obtain small Ni nanoparticles, improve the dispersion of the active component, prevent the sintering of metal particles, and reduce carbon deposition [40].Al2O3-supported Ni-based catalysts, one of the widely employed DRM catalysts, exhibit a high catalytic activity because of their good structural characteristics, strong metal-support interaction (SMSI) effect, and high dispersion of Ni particles.The porous 3D structure enables the Ni nanoparticles to remain uniformly dispersed in the catalyst.In addition, the formation of the spinel structure NiAl2O4 during DRM makes the catalyst highly stable.However, the generation of the NiAl2O4 phase also limits the metal utilization of the active Ni site, thereby reducing the DRM reactivity.The formed NiAl2O4 In sum, based on the investigation of the influence of the Ni configuration, the size of the Ni particles, and doped metal components on catalyst stability, it is found that the effective dispersion of Ni and the avoidance of active components sintering can prolong the service life of catalysts.

Regulation of Supports
The support is generally inert in the DRM reaction.However, it can affect the stability and mechanical strength of the catalyst via the interaction between the active component and the support.Moreover, the surface area, pore structure, thermal stability, oxidationreducibility, and surface alkalinity of the support can affect the carbon deposition behavior of the Ni-based catalyst [40].

Support Structure
As described above, the support can affect the particle size and dispersion of the active metal in the catalyst and directly affect the activity and stability of the DRM catalysts.For the Ni-based catalysts, the use of a support with a high specific surface area and a finely controlled pore structure can obtain small Ni nanoparticles, improve the dispersion of the active component, prevent the sintering of metal particles, and reduce carbon deposition [40].Al 2 O 3 -supported Ni-based catalysts, one of the widely employed DRM catalysts, exhibit a high catalytic activity because of their good structural characteristics, strong metal-support interaction (SMSI) effect, and high dispersion of Ni particles.The porous 3D structure enables the Ni nanoparticles to remain uniformly dispersed in the catalyst.In addition, the formation of the spinel structure NiAl 2 O 4 during DRM makes the catalyst highly stable.However, the generation of the NiAl 2 O 4 phase also limits the metal utilization of the active Ni site, thereby reducing the DRM reactivity.The formed NiAl 2 O 4 spinel is unstable and can be reduced to Ni during the DRM reaction by a reductive gas mixture of CO and H 2 .After adding an appropriate amount of MgO to the Ni@Al 2 O 3 catalyst, the spinel structure MgAl 2 O 4 generated during the reaction also contributes to the resistance to carbon deposition [49].
ZrO 2 is another commonly used support due to its high oxygen storage capacity and high thermodynamic stability [50].The crystal structure of ZrO 2 is closely related to the catalytic activity and stability of the catalysts [21].Naeem et al. [51] prepared Ni@Al 2 O 3 , Ni@CeO 2 , and Ni@ZrO 2 using polyol and surfactant-assisted methods.Among these catalysts, Ni@ZrO 2 prepared using the polyol method delivers the highest reactivity, while the Ni@ZrO 2 obtained using the surfactant-assisted method shows the lowest reactivity.This difference in catalytic performance is due to the phase state of ZrO 2 .The ZrO 2 of the monoclinic crystal system was mainly prepared using the polyol method, while the ZrO 2 of the cubic crystal system was mainly prepared using the surfactant-assisted method [51].
The former shows better redox properties than the latter and favors the adsorption and activation of CO 2 better, leading to higher reactivity [51].
The distribution of active metal components at different locations of the support also affects carbon deposition.For example, Yan et al. [52] [52].At 700 • C, the CO 2 and CH 4 conversion rates of the former (87.3% and 78.5%) were higher than those of the latter (80.5% and 67.8%).Ni/CeO 2 -SiO 2 -P was stable in long-term tests, while Ni/CeO 2 -SiO 2 -C showed poor stability, and the activity was significantly reduced within 10 h.The improved performance and stability of the Ni/CeO 2 -SiO 2 -P resulted from rich reactive oxygen species and more accessible formate species located at the metal-support interface [52] (Figure 5).
Methane 2024, 3, FOR PEER REVIEW 7 spinel is unstable and can be reduced to Ni during the DRM reaction by a reductive gas mixture of CO and H2.After adding an appropriate amount of MgO to the Ni@Al2O3 catalyst, the spinel structure MgAl2O4 generated during the reaction also contributes to the resistance to carbon deposition [49].
ZrO2 is another commonly used support due to its high oxygen storage capacity and high thermodynamic stability [50].The crystal structure of ZrO2 is closely related to the catalytic activity and stability of the catalysts [21].Naeem et al. [51] prepared Ni@Al2O3, Ni@CeO2, and Ni@ZrO2 using polyol and surfactant-assisted methods.Among these catalysts, Ni@ZrO2 prepared using the polyol method delivers the highest reactivity, while the Ni@ZrO2 obtained using the surfactant-assisted method shows the lowest reactivity.This difference in catalytic performance is due to the phase state of ZrO2.The ZrO2 of the monoclinic crystal system was mainly prepared using the polyol method, while the ZrO2 of the cubic crystal system was mainly prepared using the surfactant-assisted method [51].The former shows better redox properties than the latter and favors the adsorption and activation of CO2 better, leading to higher reactivity [51].
The distribution of active metal components at different locations of the support also affects carbon deposition.For example, Yan et al. [52] prepared and designed two types of Ni/CeO2-SiO2 catalysts, including CeO2 (Ni/CeO2-SiO2-P) in close contact with Ni nanoparticles or CeO2 (Ni/CeO2-SiO2-C) away from the Ni nanoparticles for DRM.Compared with Ni/CeO2-SiO2-C, Ni/CeO2-SiO2-P exhibits superior low-temperature activity and H2/CO ratio [52].At 700 °C, the CO2 and CH4 conversion rates of the former (87.3% and 78.5%) were higher than those of the latter (80.5% and 67.8%).Ni/CeO2-SiO2-P was stable in long-term tests, while Ni/CeO2-SiO2-C showed poor stability, and the activity was significantly reduced within 10 h.The improved performance and stability of the Ni/CeO2-SiO2-P resulted from rich reactive oxygen species and more accessible formate species located at the metal-support interface [52] (Figure 5).

Acidity and Alkalinity of the Support Surface
In the high-temperature DRM process, the support affects the acidity and alkalinity of the catalyst, and the change in acidity and alkalinity further affects the reaction mechanism and carbon deposition.For Ni-based catalysts, the support properties affect the adsorption and dissociation of CO 2 and CH 4 on the catalyst and determine the reaction pathway of DRM [53].If the inert support (SiO 2 ) is used as the catalyst support, the reaction follows a single-function pathway, that is, CH 4 and CO 2 are adsorbed and activated on the surface of the active metal, and the inert support is only used as the skeleton of the catalyst, providing large pore volume, good thermal stability, and good dispersibility for the active metal [42,53].CO 2 is an acid gas, and using an alkaline support may lead to an increase in the adsorption capacity of CO 2 .Introducing the Lewis alkali support reduces the formation of deposited carbon and improves stability [54].As illustrated in Reference [51], the Ni@ZrO 2 prepared using the surfactant-assisted method with high basicity shows better coke-resistant ability but lower reactivity compared with the one prepared by the surfactant-assisted method with low basicity.However, the basic support decreases the adsorption of CH 4 and may reduce the DRM reaction activity.The acidic support reduces the adsorption of CO 2 but promotes the CH 4 adsorption, which may favor the generation of side products to result in increased carbon deposits.When the weakly acidic or weakly basic (CeO 2 , La 2 O 3 ) support is employed as the support of the Ni-based catalyst, DRM takes place via a bifunctional reaction pathway, that is, CH 4 is activated on the active metal, and CO 2 is activated on the support or interface [55][56][57].Alkaline additives, such as alkali metals, are used to increase the surface alkalinity of the catalyst, thereby accelerating the activation of CO 2 and the gasification of carbon [58].

Support Defects
Surface defects of metal oxides provide a way to stabilize thermodynamically unstable metal atoms and as active sites to promote adsorption and chemical transformation of reactants [21].
For non-redox support Al 2 O 3 , the surface defects are also used in regulating the catalytic reactivity of reactions.In general, the so-called defect sites on Al 2 O 3 are mainly related to the coordination of the unsaturated Al site and the adjacent lattice O atoms.The under-coordinately tricoordinated Al 3+ sites are closely related to surface dehydroxylation.Zhang et al. [59] proposed a new comprehension of the special role of defect sites on γ-Al 2 O 3 .Ni/CeO 2 -SiO 2 catalyst was prepared using the plasma decomposition method, which is characterized by close contact between the CeO 2 and Ni nanoparticles.In previous work, plasma-decomposed nickel catalysts showed smaller nickel particle sizes, fewer defect sites, and stronger nickel-support interactions compared to thermal decomposed nickel catalysts.The development of the acid etching strategy has been applied to the directional removal of OH groups on the surface of the γ-Al Redox oxide supports, such as CeO 2 and TiO 2 , have a high concentration of highly reactive oxygen species and unique redox properties (M 4+ ↔M 3+ ) [60][61][62][63][64][65][66].The surface defects anchor Ni, forming a strong interaction between the metal and the support, preventing the metal from sintering.Reactive oxygen species can act as local sources or reservoirs of oxygen species, which are involved in reactions that occur on oxide surfaces [23].The effective barrier for methane activation decreases to 0.15 eV on Ni/CeO 2−x (111) compared with 0.9 eV on Ni(111) [67].Recent studies show that Co-substituted CeO 2 is more stable than Ni-substituted CeO 2 under the DRM reaction conditions at 800 • C. Surface lattice oxygen reactivity plays a vital role in catalytic stability.The vacancy formation is favored in Co-substituted CeO 2 as compared to Ni-substituted CeO 2 , leading to a higher availability of surface oxygen in Co-substituted catalyst than Ni-substituted CeO 2 for the reaction [68].
Interfacial metal-O lattice favors the O-assisted CH 4 dissociation, forms CH x O intermediate, and finally yields CO.The reacted O lattice can be re-supplied by CO 2 dissociation at the metal-CeO 2 interface [69].
It is worth pointing out that recent works mostly use combined strategies to regulate catalyst activity and stability jointly.For example, Li et al. [70] claim that the high Ni dispersion, intensified Ni-support interaction, the enlarged oxygen vacancies, the increased t-ZrO 2 content, and enhanced reducibility of NiO led by oxygen vacancies co-boost the superior catalytic activity and stability of the developed LA-Ni/ZrO 2 catalyst.Although a larger amount of coke is deposited on the spent LA-Ni/ZrO 2 catalyst in comparison with that on the spent Ni/ZrO 2 , the developed LA-Ni/ZrO 2 illustrates much higher catalytic stability than Ni/ZrO 2 , ascribed to the superior thermal sintering resistance of Ni nanoparticles and the different coke morphologies induced by enforced interaction of Ni and the ZrO 2 support [70].Similarly, among a series of Ni-Ce x Zr 1−x O 2 and Ni-Ce x Ti 1−x O 2 catalysts with varying Ce/Zr and Ce/Ti molar ratios (x = 0, 0.25, 0.5, 0.75, and 1), Ni-Ce 0.5 Zr 0.5 O 2 and Ni-Ce 0.5 Ti 0.5 O 2 delivered the highest apparent activity and stability for combined steam and CO 2 reforming of CH 4 in each series due to small Ni particles, strong metal-support interaction, and high oxygen vacancy concentration [71].Ding et al. [72] also find that doping the optimized amount of cerium (Ce/Mg = 0.12) into Ni/MgO catalyst can improve activity and stability, which are attributed to the easy reducibility of NiO x species, small Ni particle size, and abundant oxygen vacancies resulting from the doping of cerium.

New Catalyst Support
Employing novel materials, such as MXene, as the support of Ni-based catalysts has also received much attention.For example, MXene is a class of layered two-dimensional (2D) transition metal carbides [73].Due to its large specific surface area, high conductivity and strong redox properties, and the presence of hydroxyl groups or terminal oxygen on the surface, the multilayer structure of V 2 CT x MXene maintains a certain degree of extension in the environment of inert gas (N 2 ) and reducing gas (H 2 ).And in the presence of CO 2 and air, V 2 CT x MXene can be completely oxidized (V 2 CT x is converted to V 2 O 5 ) [74].After 100 h of reaction, its stability is still very high; no carbon deposition has been found, showing excellent resistance towards carbon deposition.It has the potential to be used as a precursor, support, or catalyst for the DRM reaction [74].Another 2D support is nitride, such as BN.Because BN has the advantages of high thermal stability, strong chemical inertness, good thermal conductivity, and corrosion resistance, it shows greater application potential than traditional oxides under harsh reaction environments.As expected, BNsupported Ni-based catalysts show good resistance to carbon deposition during DRM [19].However, due to the inert surface of BN, Ni particles are prone to sintering on the surface of the BN, which leads to an unstable Ni-BN interface.Based on this, a research group first reduced the particle size of Ni particles by reducing the Ni loading (0.3 wt%) to increase the proportion of Ni-BN interfaces and then confined the Ni particles to the edge of BN by functionalizing amorphous SiO 2 generated by functional group decomposition on boron nitride (BN f ), thereby constructing a stable Ni-BN interface.The results show that Ni/BN f shows excellent resistance to carbon deposition during DRM, and Ni remains in a metallic state after reaction for 10 h.For comparison, under the same DRM conditions, no carbon deposition is observed on the surface of the Ni/BN f catalyst, while a few deposited carbons are produced on the surface of the Ni/α-Al 2 O 3 catalyst.Additionally, under the condition of the CH 4 pyrolysis experiment alone, no obvious carbon deposition is observed on the Ni/BN f catalyst (Figure 6), while carbon deposition on the Ni/α-Al 2 O 3 catalyst surface continues to occur, and deposited carbon covered all surfaces of the catalyst.In situ tests and theoretical calculations show that the catalyst promotes the first C-H cleavage and inhibits the last C-H bond cleavage of CH 4 .When CH 4 and CO 2 are injected at the same time, CO 2 preferentially activates on the Ni surfaces.Altogether, it makes the catalyst present excellent resistance to carbon deposition [19].
injected at the same time, CO2 preferentially activates on the Ni surfaces.Altogether, it makes the catalyst present excellent resistance to carbon deposition [19].In sum, the carbon deposition behavior of the Ni-based catalyst can be adjusted by the pore structure, oxidation-reducibility, and surface alkalinity of the support.

Interface Structure Control
The interfacial structure is widely confirmed to influence the properties of the composites [75], such as the resistance towards carbon deposition during Ni-based catalysts catalyzing DRM [76].Recently, studies have shown that adjusting the interface and strengthening the interaction between Ni and the support play a vital role in the synthesis of efficient and stable Ni-based DRM catalysts.Agglomeration of Ni nanoparticles can be avoided by regulating the SMSI between the metal (Ni) and the support.Compared to bare Ni NPs, the greater the contribution of the Ni-support interface, the stronger the adsorption or activation of the reactants [77].Stronger Ni-support interactions yield more uniform distribution and smaller Ni nanoparticles, which is conducive to high activity and stability.Jeon et al. [76] propose the strategy of suppressing Ni nanoparticle aggregation during the reduction process via in situ reduction using dry methane (CH4/CO2) as the reducing gas.It increases the exposed interface between the Ni and CeO2 and results in large amounts of the Ni 3+ phase at the catalyst surface, of which SMSI is dramatically enhanced (Figure 7).SMSI originates from Ni-doped sites on the CeO2 (denoted as the Ni 3+ phase).The more Ni 3+ on the surface, the more SMSI is formed near the exposed Ni nanoparticles, the smaller the size of Ni nanoparticles prepared, and the better the durability and activity.The developed catalyst achieved a methane conversion of 60% for 550 h at a low DRM operating temperature of 600 °C [76].In sum, the carbon deposition behavior of the Ni-based catalyst can be adjusted by the pore structure, oxidation-reducibility, and surface alkalinity of the support.

Interface Structure Control
The interfacial structure is widely confirmed to influence the properties of the composites [75], such as the resistance towards carbon deposition during Ni-based catalysts catalyzing DRM [76].Recently, studies have shown that adjusting the interface and strengthening the interaction between Ni and the support play a vital role in the synthesis of efficient and stable Ni-based DRM catalysts.Agglomeration of Ni nanoparticles can be avoided by regulating the SMSI between the metal (Ni) and the support.Compared to bare Ni NPs, the greater the contribution of the Ni-support interface, the stronger the adsorption or activation of the reactants [77].Stronger Ni-support interactions yield more uniform distribution and smaller Ni nanoparticles, which is conducive to high activity and stability.Jeon et al. [76] propose the strategy of suppressing Ni nanoparticle aggregation during the reduction process via in situ reduction using dry methane (CH 4 /CO 2 ) as the reducing gas.It increases the exposed interface between the Ni and CeO 2 and results in large amounts of the Ni 3+ phase at the catalyst surface, of which SMSI is dramatically enhanced (Figure 7).SMSI originates from Ni-doped sites on the CeO 2 (denoted as the Ni 3+ phase).The more Ni 3+ on the surface, the more SMSI is formed near the exposed Ni nanoparticles, the smaller the size of Ni nanoparticles prepared, and the better the durability and activity.The developed catalyst achieved a methane conversion of 60% for 550 h at a low DRM operating temperature of 600 • C [76].CH4 and CO2 conversion at 600 °C with dry methane (CH4/CO2/N2 = 1:1:2) and GHSV of 10,000 of the fabricated catalyst after CH4/CO2 reduction.Insert: Scheme of the concentration gradient the Ni 3+ and Ce 3+ fraction at the bulk and surface of the nanoparticles.Reprinted with permiss from [76].Copyright {2022} American Chemical Society.

Special Structure Design
The configuration of the Ni centers, size of Ni particles, and exposed crystal surfa are the key factors affecting its catalytic performance.Because Ni particles are on the su face of the catalyst and are exposed to the reaction stream directly, the supported Ni nan catalysts prepared by conventional impregnation or precipitation migrate and aggreg during the DRM.Therefore, the design of special structures [78][79][80][81][82], e.g., core-shell [2 to protect metal active centers is the key to improving the stability of the catalysts.E ploying this strategy, researchers improve the resistance ability of Ni-based DRM cataly towards sintering and carbon deposition.

Reverse Metal-on-Oxide
Recently, some research teams innovatively constructed nanostructured oxides the surface of metals to form reverse oxide-on-metal catalysts [44,64].Due to the inter tion between the oxide and the metal, the oxide formed on the metal surface is a sing layer dispersed, metastable structure, and metal active centers with coordination unsa rated are formed at the oxide-metal boundary, which can efficiently activate O2, H2O, a other molecules, and can be successfully used in low-temperature catalytic oxidation, e Another group has successfully grown nano-ZrO2 films on Pt, Cu, Pd, and Ni, denoted ZrO2@M (M = Ni, Pt, Cu, Pd), using chemical vapor deposition and applying them to t DRM reaction.This new type of reverse catalyst shows excellent resistance to carbon de osition.It was found that the activity and stability of DRM are closely related to Zr 0 .G phitic deposits are anti-segregated into Ni 0 nanoparticles to provide restored CH4 adsor tion sites and near-surface/dissolved C atoms, which migrate toward the Ni 0 /ZrO2 int face and induce local ZrxCy formation.The resulting oxygen-deficient carbidic pha boundary sites assist in kinetically enhanced CO2 activation toward CO [44,64].

Encapsulated Structure
Encapsulating active metal nanoparticles with a protecting shell, such as a carb layer or zeolite [83], is an effective way to increase their stability.For Ni-based catalys using the zeolite with a special pore structure as the protector is widely employed so th the migration of Ni is inhibited effectively, thereby improving the resistance to carb deposition.For example, based on this strategy, researchers encapsulate active Ni nan particles by the silicon-aluminum MFI zeolite (Ni@HZSM-5) [83].They further regul the pore environment of zeolite to enhance the hydrogen spillover so as to keep enou active hydrogen species available around Ni nanoparticles [83].This facilitates the redu tion of CO2 to CO and suppresses the side reaction of RWGS.In a CO2-rich atmosphe

Special Structure Design
The configuration of the Ni centers, size of Ni particles, and exposed crystal surface are the key factors affecting its catalytic performance.Because Ni particles are on the surface of the catalyst and are exposed to the reaction stream directly, the supported Ni nanocatalysts prepared by conventional impregnation or precipitation migrate and aggregate during the DRM.Therefore, the design of special structures [78][79][80][81][82], e.g., core-shell [24], to protect metal active centers is the key to improving the stability of the catalysts.Employing this strategy, researchers improve the resistance ability of Ni-based DRM catalysts towards sintering and carbon deposition.

Reverse Metal-on-Oxide
Recently, some research teams innovatively constructed nanostructured oxides on the surface of metals to form reverse oxide-on-metal catalysts [44,64].Due to the interaction between the oxide and the metal, the oxide formed on the metal surface is a single-layer dispersed, metastable structure, and metal active centers with coordination unsaturated are formed at the oxide-metal boundary, which can efficiently activate O 2 , H 2 O, and other molecules, and can be successfully used in low-temperature catalytic oxidation, etc.Another group has successfully grown nano-ZrO 2 films on Pt, Cu, Pd, and Ni, denoted as ZrO 2 @M (M = Ni, Pt, Cu, Pd), using chemical vapor deposition and applying them to the DRM reaction.This new type of reverse catalyst shows excellent resistance to carbon deposition.It was found that the activity and stability of DRM are closely related to Zr 0 .Graphitic deposits are anti-segregated into Ni 0 nanoparticles to provide restored CH 4 adsorption sites and near-surface/dissolved C atoms, which migrate toward the Ni 0 /ZrO 2 interface and induce local Zr x C y formation.The resulting oxygen-deficient carbidic phase boundary sites assist in kinetically enhanced CO 2 activation toward CO [44,64].

Encapsulated Structure
Encapsulating active metal nanoparticles with a protecting shell, such as a carbon layer or zeolite [83], is an effective way to increase their stability.For Ni-based catalysts, using the zeolite with a special pore structure as the protector is widely employed so that the migration of Ni is inhibited effectively, thereby improving the resistance to carbon deposition.For example, based on this strategy, researchers encapsulate active Ni nanoparticles by the silicon-aluminum MFI zeolite (Ni@HZSM-5) [83].They further regulate the pore environment of zeolite to enhance the hydrogen spillover so as to keep enough active hydrogen species available around Ni nanoparticles [83].This facilitates the reduction of CO 2 to CO and suppresses the side reaction of RWGS.In a CO 2 -rich atmosphere, the methane on the catalytic material can reduce CO 2 by 2.9 (1 mol of methane reduces 2.9 mol CO 2 ), which is better than the most advanced super-DRM process reported so far.The CO Methane 2024, 3 97 yield of 3.9 mol CO mol CH4 −1 on this catalytic material can be carried out for a long time.It is expected to realize the direct conversion and utilization of CO 2 -rich natural gas [83].
Similarly, Wang et al.
[84] prepared small-sized bimetallic NiCo nanoparticles encapsulated in silicalite-2 (S-2) using the in situ growth method.Elemental segregation is observed during the activation process, in which Ni and Co atoms migrate to the outer surface and center of the NiCo alloy, respectively [84].The optimal Ni 0.2 Co 0.3 @S-2 catalyst shows stable and high CH 4 and CO 2 conversion within 100 h without carbon deposition [84].This localized effect, combined with the precise dynamic balance of carbon and oxygen content on the catalyst surface, makes it have high catalytic performance [84].
In another example, a catalyst of attapulgite-derived MFI (ADM) zeolite-encapsulated Ni-Co alloys was fabricated using a one-pot method.Co in Ni-Co alloys transfers electrons to Ni, forming electron-rich Ni metal sites, which significantly improves the ability of C-H bond cleavage [47].Meanwhile, the ADM zeolite not only stabilized metallic sites by pore or layer confinement but also offered massive CO 2 adsorption/activation centers.These improve the ability of the anti-sintering and anti-coking formation as well as the superior DRM activity of the optimal 10Ni 1 Co@ADM-0.1 catalyst [47].
Active metal nanoparticles can also be encapsulated using surface coating oxides (e.g., SiO 2 , >10 nm).He et al. [85] grew Ni nanoparticles in situ from the MgO-Al 2 O 3 matrix and surrounded by a thin layer of 1-2 nm multielement oxide (Ni@NiSiAlMgO x ), and synchronously controlled the size, exposed crystal planes, surface acidity and alkalinity, and redox state of the Ni nanoparticles, thereby improving the stability of Ni catalysts in DRM reactions (Figure 8).The oxide thin layer efficiently isolated and confined the Ni nanoparticles, avoiding fast C-C bond formation on an open facet and protecting the Ni nanoparticles from sintering [85].The appropriately sized Ni particles of around 15 nm balanced the carbon decomposition and elimination rates.The nearby basic sites assist CO 2 adsorption and accelerate carbon elimination.The cooperative metallic Ni and the surficial MEO thin layer containing Ni 2+ enhanced the anti-sintering of Ni@NiSiAlMgO x in a redox atmosphere of MDR [85].
Methane 2024, 3, FOR PEER REVIEW the methane on the catalytic material can reduce CO2 by 2.9 (1 mol of methane reduces mol CO2), which is better than the most advanced super-DRM process reported so far.T CO yield of 3.9 molCO molCH4 −1 on this catalytic material can be carried out for a long tim It is expected to realize the direct conversion and utilization of CO2-rich natural gas [83 Similarly, Wang et al.
[84] prepared small-sized bimetallic NiCo nanoparticles enca sulated in silicalite-2 (S-2) using the in situ growth method.Elemental segregation is o served during the activation process, in which Ni and Co atoms migrate to the outer su face and center of the NiCo alloy, respectively [84].The optimal Ni0.2Co0.3@S-2cataly shows stable and high CH4 and CO2 conversion within 100 h without carbon depositi [84].This localized effect, combined with the precise dynamic balance of carbon and ox gen content on the catalyst surface, makes it have high catalytic performance [84].
In another example, a catalyst of attapulgite-derived MFI (ADM) zeolite-encaps lated Ni-Co alloys was fabricated using a one-pot method.Co in Ni-Co alloys transfe electrons to Ni, forming electron-rich Ni metal sites, which significantly improves the ab ity of C-H bond cleavage [47].Meanwhile, the ADM zeolite not only stabilized metal sites by pore or layer confinement but also offered massive CO2 adsorption/activation ce ters.These improve the ability of the anti-sintering and anti-coking formation as well the superior DRM activity of the optimal 10Ni1Co@ADM-0.1 catalyst [47].
Active metal nanoparticles can also be encapsulated using surface coating oxides g., SiO2, >10 nm).He et al. [85] grew Ni nanoparticles in situ from the MgO-Al2O3 mat and surrounded by a thin layer of 1-2nm multielement oxide (Ni@NiSiAlMgOx), and sy chronously controlled the size, exposed crystal planes, surface acidity and alkalinity, a redox state of the Ni nanoparticles, thereby improving the stability of Ni catalysts in DR reactions (Figure 8).2, the CO:H 2 mole ratio is less than 1 due to the presence of side reaction, the conversion rate of CO 2 is usually higher than that of CH 4 , and the stability can reach 550 h.In addition to the above regulation strategies, the stability of the catalyst can also be improved using other strategies, which are not discussed in this review in detail.

Conclusions and Outlook
The rapid carbon deposition and catalyst sintering induced by high reaction temperature limit the industrial applications of Ni-based catalysts toward DRM reaction.In this review, the mechanisms of reaction and deactivation during DRM reaction are analyzed, and the influencing factors and possible solutions regarding the stability of Ni-based catalysts are summarized.The catalytic activity and stability are co-affected by the active metal, support, interface, etc.The stability of the catalyst can be improved by multi-scale regulation.The metal dispersion is related to stability, which has an optimal limit.Increasing the surface basicity of the support could favor the activation of CO 2 and consequently inhibit carbon deposition on the catalyst.The protection of active centers by the construction of special structures, e.g., encapsulating Ni by oxides, can also improve the stability of catalysts.
Future research can focus on the following aspects: (1) Develop new preparation methods of catalysts to increase the dispersion of Ni particles on the support and adjust the interaction between the metal and the support; (2) increase the number of adsorbed oxygen species on the catalyst surface by the release of partial lattice oxygen, and promote the combination of adsorbed oxygen with adsorbed carbon to form CO, eliminating carbon deposition on the catalyst surface; (3) couple to the reaction of CO hydrogenation, modifying the reaction thermodynamically.
This review provides guidance for the rational design of stable Ni-based catalysts in the industrial application of DRM.

Figure 2 .
Figure 2. Carbon formation, deposition, and transformation pathways on the Ni-based catalyst Reprinted with permission from Elsevier, copyright 2019.

Figure 3 .
Figure 3. Schematic diagram of the variation in catalysts in the DRM hysteresis reaction under different conditions [27].Reprinted with permission from Elsevier, copyright 2023.

Figure 3 .
Figure 3. Schematic diagram of the variation in catalysts in the DRM hysteresis reaction under different conditions [27].Reprinted with permission from Elsevier, copyright 2023.
prepared and designed two types of Ni/CeO 2 -SiO 2 catalysts, including CeO 2 (Ni/CeO 2 -SiO 2 -P) in close contact with Ni nanoparticles or CeO 2 (Ni/CeO 2 -SiO 2 -C) away from the Ni nanoparticles for DRM.Compared with Ni/CeO 2 -SiO 2 -C, Ni/CeO 2 -SiO 2 -P exhibits superior low-temperature activity and H 2 /CO ratio
2 O 3 , thereby generating the necessary vacancy defects to anchor the active Ni site, and the moderate interaction of Ni with the modified γ-Al 2 O 3 support has improved the utilization of Ni.These designed defects promote the adsorption and activation of CO 2 and the activation of CH 4 , thus improving the stability and activity of traditional Ni/Al 2 O 3 catalysts [59].

Figure 6 .
Figure 6.(A,B) TEM images of the Ni/BNf and XAS images.(C) Activity and (D) stability of catalyst [19].The figures are published on CCS chemistry and are available online [19].

Figure 6 .
Figure 6.(A,B) TEM images of the Ni/BN f and XAS images.(C) Activity and (D) stability of catalyst[19].The figures are published on CCS chemistry and are available online[19].

Figure 7 .
Figure 7. (A) Illustration of the formation and role SMSI in anti-coking formation during DRM.CH4 and CO2 conversion at 600 °C with dry methane (CH4/CO2/N2 = 1:1:2) and GHSV of 10,000 of the fabricated catalyst after CH4/CO2 reduction.Insert: Scheme of the concentration gradient the Ni 3+ and Ce 3+ fraction at the bulk and surface of the nanoparticles.Reprinted with permiss from[76].Copyright {2022} American Chemical Society.

Figure 7 .
Figure 7. (A) Illustration of the formation and role SMSI in anti-coking formation during DRM.(B) CH 4 and CO 2 conversion at 600 • C with dry methane (CH 4 /CO 2 /N 2 = 1:1:2) and GHSV of 10,000 h −1 of the fabricated catalyst after CH 4 /CO 2 reduction.Insert: Scheme of the concentration gradient of the Ni 3+ and Ce 3+ fraction at the bulk and surface of the nanoparticles.Reprinted with permission from [76].Copyright {2022} American Chemical Society.
The oxide thin layer efficiently isolated and confined the Ni nanop ticles, avoiding fast C-C bond formation on an open facet and protecting the Ni nanop ticles from sintering [85].The appropriately sized Ni particles of around 15 nm balanc the carbon decomposition and elimination rates.The nearby basic sites assist CO2 adsor tion and accelerate carbon elimination.The cooperative metallic Ni and the surficial ME thin layer containing Ni 2+ enhanced the anti-sintering of Ni@NiSiAlMgOx in a redox mosphere of MDR [85].

Figure 8 ., 3 98
Figure 8. (A) High-resolution STEM image; (B) long-term stability tests of Ni@NiSiAlMgO x at 673 and 873 K; (C) schematic representation of the MEO thin layer-confined and -stabilized Ni catalyst and the reference sample.Reprinted with permission from [85].Copyright {2021} American Chemical Society.

Table 1 .
Standard reaction enthalpy of reactions involved in DRM.

Table 1 .
Standard reaction enthalpy of reactions involved in DRM.

Table 2 .
Performances of the Ni-based catalysts toward DRM.