Research Progress of Carbon Deposition on Ni-Based Catalyst for CO₂-CH₄ Reforming

Abstract

As we all know, the massive emission of carbon dioxide has become a huge ecological and environmental problem. The extensive exploration, exploitation, transportation, storage, and use of natural gas resources will result in the emittance of a large amount of the greenhouse gas CH₄. Therefore, the treatment and utilization of the main greenhouse gases, CO₂ and CH₄, are extremely urgent. The CH₄ + CO₂ reaction is usually called the dry methane reforming reaction (CRM/DRM), which can realize the direct conversion and utilization of CH₄ and CO₂, and it is of great significance for carbon emission reduction and the resource utilization of CO₂-rich natural gas. In order to improve the activity, selectivity, and stability of the CO₂-CH₄ reforming catalyst, the highly active and relatively cheap metal Ni is usually used as the active component of the catalyst. In the CO₂-CH₄ reforming process, the widely studied Ni-based catalysts are prone to inactivation due to carbon deposition, which limits their large-scale industrial application. Due to the limitation of thermodynamic equilibrium, the CRM reaction needs to obtain high conversion and selectivity at a high temperature. Therefore, how to improve the anti-carbon deposition ability of the Ni-based catalyst, how to improve its stability, and how to eliminate carbon deposition are the main difficulties faced at present.

1. Introduction

With the increasing use of fossil resources, such as coal, oil, and natural gas, the global CO₂ emissions will continue to rise. In recent years, some countries have been using renewable energy; however, this trend has not been enough to prevent the climate change, polar ice sheet melting, and hurricane intensification caused by the increase in CO₂ emissions. The massive emission of CO₂ not only accelerates the deterioration of the greenhouse effect but also wastes valuable carbon resources. Therefore, CO₂ reduction and resource utilization have become the most noticeable research issues. According to the proportions of different greenhouse gases in the total greenhouse gas emissions calculated by CO₂ equivalent, CH₄ accounts for 16% and is therefore the second villain of the greenhouse effect. Although the emission of CH₄ is far less than that of CO₂, its potential to produce a greenhouse effect is about 20 times more than that of CO₂. Therefore, how to convert the above two greenhouse gases into useful chemicals or chemical raw materials has attracted the great attention of governments and scientists around the world [1,2,3].

As early as 1928, Fischer and Tropsch discovered the carbon dioxide reforming of methane reaction, that is, CO₂ + CH₄ = 2CO + 2H₂ (carbon dioxide reforming of methane, CRM, ΔH = 247 kJ/mol), also known as methane dry reforming. CRM can produce synthesis gas; it is a strongly endothermic reaction, and the synthesis gas obtained has a low H₂/CO ratio, which is more suitable for the subsequent Fischer–Tropsch synthesis reaction [4]. The ratio of H₂/CO in the CRM product is about 1, and it can also be used in chemical reactions such as carbonyl synthesis and hydrocarbon production and to produce clean liquid fuels and high-value chemicals [5,6]. Hence, this reaction can also utilize CO₂ and CH₄, which are two main greenhouse gases; therefore, it has significant industrial value and ecological and environmental significance. The research on this reaction has been further developed, particularly in the past 30 years [7].

CRM and steam reforming of methane (SRM) are catalytic reactions at a high temperature (about 800 °C). The ΔH of CRM = 247 kJ/mol, which is greater than that of SRM (206 kJ/mol), indicating that both CRM and SRM are strongly endothermic reactions, and the endothermic capacity of CRM is nearly 20% higher than that of SRM. Therefore, the reverse reaction of CRM can theoretically release up to 247 kJ/mol of energy. Therefore, the reaction can be used as a good chemical energy transmission system (CETS) to store energy. On the other hand, CRM can be realized through fossil fuels (such as coal, petroleum, etc.), light energy, or nuclear energy, and the above energy can be stored in the product (synthesis gas); then, the synthesis gas can be transported to the place where it is needed for a reverse reaction to release energy.

Over a long period of time, researchers have conducted many studies on the selection and optimization of CRM catalysts, and have achieved fruitful results, making the research on this reaction increasingly broad and deep. Without loss of generality, the relationship between the chemical reaction itself, the type of active component and carrier, the modification of additives, the carbon deposition, and the catalyst performance have been consistently discussed by many researchers. As far as the CRM process is concerned, the main side reaction is the reverse water gas shift reaction (CO₂ + H₂ = CO + H₂O, reverse water gas shift reaction, RWGS), which consumes the H₂ (CH₄ = C + 2H₂) generated by CH₄ cracking and generates a large amount of CO, which can cause carbon deposition on the catalyst through another side reaction—the CO disproportionation reaction (2CO = C + CO₂) [8,9]. Thus, the proportion of CO₂ can be increased from the perspective of chemical equilibrium to inhibit the formation of carbon deposition (that is, the carbon elimination reaction, C + CO₂ = 2CO). Thermodynamic calculation showed that the RWGS reaction can be inhibited or avoided at temperatures above 820 °C, but this needs a lot of energy [10]. Therefore, how to optimize the catalyst structure, match the process conditions, and selectively control the degree of carbon deposition, the reverse water gas shift reaction, carbon elimination, and other chemical reactions involving carbon is of great significance in improving the activity of the CO₂-CH₄ reforming catalyst, inhibiting catalyst deactivation, and extending the service life of the catalyst.

Pakhare et al. [11] introduced DRM literature on a catalyst based on Rh, Ru, Pt, and Pd metals. This includes the effect of these noble metals on the kinetics, mechanism, and deactivation of these catalysts. The inert support catalysts are more prone to deactivation due to carbon deposition than the acidic or basic supports.

At present, the widely used CO₂-CH₄ reforming catalyst is still dominated bythe non-noble metal catalyst, especially the Ni-based catalyst. Its activity is equivalent to that of the noble metal, but it is very easily inactivated due to carbon deposition. Therefore, the development of the Ni-based catalyst with high carbon deposition resistance is the key to realizing the industrialization of CO₂-CH₄ reforming. A large number of studies have shown that carbon deposition in the hydrocarbon conversion process is mainly affected by such factors as the acid–base property of the carrier, the dispersion of the active component, and the interaction between the carrier and the active metal [12,13,14,15,16]. The acid sites on the catalyst surface are not conducive to the adsorption of CO₂, resulting in the carbon deposition rate on the catalyst surface being much higher than the carbon elimination. Conversely, the surface basic sites can inhibit the carbon deposition caused by CO disproportionation to a certain extent, thereby improving the stability of the catalyst. It was found that when the particle size of the active component was less than 10 nm, the catalyst could present a high anti-coking performance [14]. In addition, strong metal-support interaction is also beneficial in improving the anti-carbon deposition performance of the catalyst [15,16].

In this paper, the thermodynamics, kinetics, and reaction mechanism of the CO₂-CH₄ reforming reaction are reviewed. Because Ni-based catalysts exhibit high activity but have the problem of the easy deactivation of carbon deposition, this paper further summarizes the research situation regarding carbon deposition on Ni-based catalysts, including the types of carbon deposition, the amount of carbon deposition, and the elimination of carbon deposition. As to how to improve the anti-carbon deposition ability of the Ni-based catalyst and how to eliminate carbon deposition, this paper focuses on two aspects: one is the resistance of carbon deposition from the perspective of catalyst optimization; the other is the elimination of carbon deposition from the perspective of process condition matching. Finally, how to improve the carbon deposition resistance of Ni-based CRM catalyst is prospected.

2. Thermodynamics of CO₂-CH₄ Reforming

CO₂-CH₄ reforming mainly includes the following reactions:

CH₄ + CO₂ = 2H₂ + 2CO, ΔH (298 K) = 247 kJ/mol,

(1)

H₂ + CO₂ = H₂O + CO, ΔH (298 K) = 41 kJ/mol,

(2)

2CO = CO₂ + C, ΔH (298 K) = −172 kJ/mol,

(3)

CH₄ = C + 2H₂, ΔH (298 K) = 75 kJ/mol,

(4)

CO + H₂ = C + H₂O, ΔH (298 K) = −131 kJ/mol,

(5)

Zhang et al. [17] obtained the thermodynamic equilibrium constants of the above reactions as a function of temperature through thermodynamic calculations, as shown in Figure 1.

As both methane and carbon dioxide are very stable, CRM is an extremely strong endothermic reaction. Meanwhile, the RWGS reaction is an important side reaction in the CRM process which reduces the H₂/CO ratio in the product [18,19]. In the CRM reaction process, high temperature (>1000 K) and low pressure (~1 atm) are usually required to obtain the efficient conversion of methane and carbon dioxide to syngas. At a higher pressure, it can promote the RWGS reaction and reduce the H₂/CO ratio. Therefore, the ratio of carbon dioxide and methane in the feed gas has a great influence on the H₂/CO ratio. Many experiments showed that the ideal H₂/CO ratio of 1 can be achieved when the CO₂/CH₄ ratio is about 1. With the increase in the CO₂/CH₄ ratio, the H₂/CO ratio decreases. This trend is more obvious at higher pressures (10 atm).

In addition to RWGS, there are two other side reactions, methane decomposition and CO disproportionation, which also occur in the CRM process [11]. These two reactions lead to the formation of carbon deposition, which leads to the deactivation of the catalyst. In order to study the formation of the carbon deposition, Lu et al. [19] calculated the limit temperatures of the two side reactions (where the Gibbs free energy change is zero). The CO₂-CH₄ reaction can be accompanied by methane cracking at above 640 °C, while the reverse water gas shift reaction starts at above 820 °C, without CO disproportionation (2CO = C + CO₂, the Boudouard reaction). In the range of 557–700 °C, carbon is mainly formed by methane cracking or the Boudouard reaction. Under the pressure of 0.01–0.1 atm, the feed ratio of CO₂/CH₄ = 1 can reach the equilibrium conversion rate. At a fixed temperature, the rate at a low pressure is always higher than that at a high pressure. Under the pressure of 0.01 atm, the rate reached 90% at 550 °C, and did not reach 90% until 700 °C at 0.1 atm. There is an upper temperature limit for carbon deposition, and the temperature increases with the increase in reaction pressure and the decrease in the CO₂/CH₄ ratio. Therefore, the formation of carbon deposition at a certain temperature can be inhibited by reducing the reaction pressure and increasing the proportion of carbon dioxide in the feed gas.

The main reactions of carbon deposition are methane decomposition and the carbon monoxide disproportionation reaction. Severe carbon deposition will lead to blockage or even deactivation of the catalyst bed. According to thermodynamic analysis, the carbon monoxide disproportionation reaction is a strongly exothermic reaction, which mainly occurs in a relatively low temperature range (<650 °C), and methane cracking reaction is a strongly endothermic reaction, which mainly occurs in a relatively high temperature range. Therefore, low temperature and high pressure are beneficial to carbon monoxide disproportionation, and high temperature and low pressure are beneficial to methane cracking, and the main reaction temperature is in the range of 557–700 °C. When the temperature is higher than 600 °C, the amount of carbon deposition will increase rapidly. However, with the increase in reaction temperature, the disproportionation reaction of carbon monoxide will be inhibited, and methane cracking will become the main reaction of the carbon deposition [11,20,21]. However, a high reaction temperature often leads to both the sintering of the active metal and carbon deposition on the catalyst. Sometimes, the fibrous carbon formed during the reaction process has a high mechanical strength, which will damage the catalyst and lead to rapid deactivation of the catalyst [18,19,22].

Adding O₂ to CRM system can remove the carbon deposition formed to promote the regeneration of the catalyst, and the heat released from the reaction can also accelerate the decomposition of the methane. For periodic operation, the addition of oxygen (CO₂/O₂ ratio of 7/3) during the regeneration process at 750 °C significantly improved the stability and activity of the catalyst. During the stability experiment, the catalytic performance of the Ni/SiO₂·MgO catalyst for CRM in the presence of O₂ increased with the increase in O₂ content and reaction temperature [23]. In addition, the introduction of another auxiliary means, such as light and plasma treatment, can break the energy barrier of the reaction and improve the conversion rate of the reactants. Some researchers used Au as the plasma promoter for the first time to improve the reforming performance of noble metal-based catalysts. The results showed that visible light irradiation could significantly improve the reforming activity of the Rh-Au/SBA-15 catalyst. The maximum CO₂ conversion rate under light conditions is 1.7 times higher than that under dark conditions. The test at 400 °C showed that the CO₂ conversion rate under light conditions was 2.4 times higher than that under dark conditions. Kinetic measurement showed that the activation energy was reduced by 30% under light conditions [24].

3. Kinetics of CO₂-CH₄ Reforming (Reaction Mechanism)

With the development of research technology at the micro-level, the perspective of the theoretical research has gradually shifted from thermodynamics to kinetics. The view on the adsorption and dissociation of reactants and products provides an ideal explanation for the kinetics of the CRM reaction.

Presently, the focus of the research on CO₂-CH₄ reforming has two aspects. On the one hand, it is necessary to find new catalysts and additives to improve the catalyst activity and carbon deposition resistance. On the other hand, it is necessary to study the reaction mechanism in detail by the kinetic method, with the aim of deeply understanding the reforming reaction and designing a new catalyst according to the mechanism. Kinetic study is one of the best methods to reveal the intrinsic activity of the used catalyst. Due to the high activity and low price of Ni in CO₂-CH₄ reforming, the study of Ni kinetics has attracted more and more attention. Most studies have focused on exploring the rate-determining step in the CO₂-CH₄ reforming process. Discussing the rate-determining step of the reaction can lead to further understanding of the catalytic reaction mechanism and the design and improvement of the catalyst and reaction conditions. It was found that the kinetic process of CO₂-CH₄ reforming mainly includes the adsorption and dissociation of the reactants, CH₄ and CO₂, as well as the formation and desorption of the products, H₂ and CO. In order to correctly understand the kinetics and reaction mechanism of the reaction, we should understand the adsorption, desorption, and reaction properties of the reactants and products on the catalyst.

CRM reaction is a complex process. The mechanism of CRM varies greatly according to the difficulty in forming reaction intermediates in different catalyst systems and reaction conditions [25]. Many studies have shown that the key step of CRM is the adsorption and dissociation of CH₄ on the catalyst surface.

The reaction mechanism of CO₂-CH₄ reforming is closely related to the type and composition of the catalyst. At present, there is no uniform conclusion on the reaction mechanism of CO₂-CH₄ dioxide reforming. Many researchers have explored the mechanisms of different catalysts.

Bodrov et al. [26] first proposed the CO₂-CH₄ reforming reaction principle on a Ni-based catalyst, including the following basic steps:

CH₄+*⟶ CH₂* + H₂,

(6)

CO₂+* ⟷ CO + O*,

(7)

O* + H₂ ⟷ H₂O+*,

(8)

CH₂* + H₂O ⟷ CO* + 2H₂,

(9)

CO* ⟷ CO+*,

(10)

In the above formula, “*” represents the active site, (6) represents an irreversible slow reaction, and the other steps are reversible reactions.

Later, Hansen et al. [27] improved the above mechanism and proposed that the dissociation of CH₄ on Ni/MgO can be divided into two steps:

CH₄+*→CH₃* + H*,

(11)

CH₃* + (3 − x)*→CH_x* + (3 − x)H*,

(12)

Osaki et al. [28] analyzed the surface pulse reaction rate of a Ni/MgO catalyst and found that the adsorption and dissociation of CH₄ can directly generate gaseous H₂, namely

CH₄+*→CH_x* + (4 − x)/2H₂*,

(13)

It was experimentally concluded that the dissociation of CH₄ was the rate-determining step of the CO₂-CH₄ reforming reaction.

For Ni and Pt-based catalysts, Bradford et al. [29] believed that CH₄ first dissociated into CH_x* and H₂, while CO₂ dissociated under the action of H* to form CO* and OH*; then, CH_x* reacted with OH* to form CH_xO*, and finally, CH_xO* decomposed into CO and H₂.

Wei et al. [30] studied the kinetics of the CO₂-CH₄ reaction on a Ni/MgO catalyst by using carbon dioxide and methane isotopes. They observed an obvious isotope effect of CH₄ dissociation and speculated that CH₄ dissociation was the rate-determining step of the reforming reaction. In the field of theoretical research, Wang et al. [31] used the DFT (density functional theory) method to study the principle of the CO₂-CH₄ reaction on the surface of perfect Ni(111). The calculation results showed that the principle of CO₂-CH₄ reforming is:

• CO₂ decomposes to generate O and CO, while CH₄ gradually cleaves H on the surface to generate CH and H₂;
• CH is oxidized to obtain CHO;
• The main product CO is obtained by the dissociation reaction of CHO;
• H₂ and CO are desorbed from Ni(111) to form free H₂ and CO.

Although the activation mechanisms of different catalytic systems are quite different, the existing research results show that the dehydrogenation cracking of CH₄ on the metal surface is a common and critical process. That is, CH₄ is decomposed into surface CH_x (x = 1–3) and H (CH₄→CH₃→CH₂→CH). On the other hand, the adsorption and activation mechanism of CO₂ is very important for the decarbonization process, because it not only generates the key product CO, but also provides a surface oxygen species for CH₄ reforming, which is the core intermediate for the elimination of carbon deposition. CO₂ activation consists of two steps: the first step is CO₂ chemical adsorption and the formation of an anionic CO₂^δ- precursor on the surface [32,33]; in the second step, the CO₂^δ- precursor is dissociated into surface adsorbed CO and O species. Therefore, CO₂ is the only source of the oxygen atom in the reaction gas and is the supplier of active oxygen species on the catalyst surface [34,35,36,37,38]. In addition, the activation path of CO₂ varies with the acidity and alkalinity of the support, which has a certain effect on the anti-carbon deposition performance of a Ni-based catalyst. Generally, at the interface between the active metal and the support, the acidic support can promote the dissociation of CH₄, but the stronger the acidity, the easier it is to produce carbon deposition [39].

It is known that increasing the adsorbed oxygen species over the catalyst surface is really effective in promoting the catalytic activity and restraining the side reaction (RWGS). Simultaneously, the adsorbed oxygen species are effective in suppressing/removing the deposited carbon, thereby alleviating catalyst deactivation [40].

The higher CO₂ activity enhanced the oxidation rate of the surface carbon generated from the side reactions, thereby resulting in a higher reforming rate and in the inhibition of the coke formation, especially the detrimental graphitic encapsulating carbon on an active nickel surface [41].

Based on the combined results of catalytic testing and characterization, Ni/Ce_0.9Eu_0.1O_1.95-HT can accelerate the rate of CO₂ activation and promote the conversion of CH₄ into CO instead of into coke deposition, leading to a relatively good performance for the DRM reaction [42].

Burghaus [43] clarified the correlation between CO₂ adsorption kinetics and the surface structure characteristics of various metals and oxides, including metals (Cu, Cr), metal oxides (ZnO, TiO₂, CaO), model catalysts (Cu/ZnO, Zn/Cu), and nano-catalysts. The binding energy of CO₂ and metal oxides is generally greater than that of metals. When CO₂ chemisorption occurs on CaO, the C atom combines with the O site of CaO, and the surface carbonate formed is very stable. Its decomposition and desorption temperature is as high as 1100 K.

Pan et al. [44] used DFT calculation and found that the adsorption and activation of a 3D transition metal dimer (M₂/γ-Al₂O₃, M = Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Cu) supported by γ-Al₂O₃ were consistent with the experimental results reported in many of the studies. CO₂ adsorbed on M₂/γ-Al₂O₃, a negatively charged species is formed, forming a metal dimer; γ-Al₂O₃ supports could provide electrons to the adsorbed CO₂ to activate it, and the most favorable adsorption position was at the interface between the metal dimer and the support; so, the highly dispersed metal particles showed good activity. In addition, the hydroxyl group on the surface of the carrier reduces the amount of charge transferred from the metal dimer to the CO₂ and weakened the chemical adsorption of CO₂.

The addition of La₂O₃ to the Ni/γ-Al₂O₃ catalyst could inhibit the carbon deposition in CO₂-CH₄ reforming. Some researchers have found that in the CO₂-CH₄ reaction, La₂O₃ interacts with CO₂ on the Ni/La₂O₃ catalyst to generate La₂O₂CO₃ [45], and La₂O₂CO₃ decomposes CO and provides oxygen species, and the oxygen species can react with carbon species accumulated after the dissociation of CH₄ on Ni grains to generate CO, thus achieving the effect of inhibiting carbon deposition.

4. Carbon Deposition and Elimination on Ni-Based Catalyst

The Ni-based catalyst is widely used in industrial processes because of its high activity, good stability, and low cost, but the biggest problem is that the catalysts are easily inactivated. There are three main ways that deactivation occurs: carbon deposition, sintering, and poisoning. Among them, the most important factor causing catalyst inactivation in the carbon-related reaction process was carbon deposition. Hence, the reaction mechanism of carbon deposition and the inhibition of carbon deposition need to be further studied. The several existing inhibition methods can be divided into two types: one involves the resistance of carbon deposition from the perspective of catalyst optimization, and the other involves the elimination of carbon deposition from the perspective of process condition matching.

4.1. Formation and Type of Carbon Deposition

The deactivation of the Ni-based catalyst is mainly due to carbon deposition on the catalyst surface. The raw materials for the CO₂-CH₄ reforming reaction are all carbon-containing gases. Methane cracking (CH₄ = C + 2H₂) and carbon monoxide disproportionation (2CO = C + CO₂) will inevitably form carbon on the catalyst surface [11].

According to existing studies, the types of carbon deposition can be divided into amorphous carbon, polymerized carbon, carbon nanotubes, graphitized carbon, and filamentous carbon [46]. Amorphous carbon is composed of carbon atoms adsorbed on the metal active center; these atoms have high reactivity and can be removed by an oxidation reaction (C + O₂ = CO₂) at about 200 °C. Polymerized carbon composed of partially hydrogenated carbon-carbon chains has low reactivity, but it is still a kind of carbon species that can be oxidized and eliminated under mild conditions or eliminated under appropriate process conditions (such as excessive CO₂). Graphitized carbon is a ring structure composed of six carbon atoms and needs higher a reaction temperature to be oxidized and eliminated. It belongs to inert carbon deposition. Filamentous carbon and carbon nanotubes can block the pores of the catalyst and gradually reduce its activity until it is completely deactivated [7,47,48].

Mo et al. [49] studied the effect of reaction time on carbon deposition on a Ni-Al₂O₃ catalyst. Figure 2 shows the TPH spectra of samples after a reforming reaction at different times. According to the report [50], 200–350 °C is the first type of hydrogenation peak, which belongs to the amorphous α type of carbon species, which is the active intermediate of the decarbonization reaction (CO₂ + C = 2CO) and is also the desired type of carbon deposition for a carbon-related reaction. It is easy for this type of carbon to be converted into a slightly less active carbon at a high-temperature β type of carbon species; 350–500 °C is the second type of hydrogenation peak, which belongs to the β₁ type carbon species, which do not form a strong interaction with the carrier and are easy to deposit in the catalyst pore, or they enter the catalyst lattice to form carbon nanotubes or filamentous carbon; they have decarbonization reaction activity at higher temperatures but easily become inert when aggregated for a long time at high-temperature γ carbon species; 500–700 °C is the third type of hydrogenation peak, belonging to the γ type of carbon species, such as graphite carbon, whose activity is lower than α carbon and β carbon and is an important reason for the irreversible deactivation of the catalyst due to carbon deposition. The literature also showed that [51], after the CRM reaction occurred on the surface of the Ni-CaO-ZrO₂ catalyst for 1 h, the temperature-programmed hydrogenation reaction characterization (TPH) found that the coking hydrogenation peak at about 800 °C was attributable to β₂ types of carbon species, namely the fourth type of hydrogenation peak. It can be seen from Figure 2 that in the process of the CO₂-CH₄ reforming reaction on the Ni-Al₂O₃ catalyst, the surface hydrogenation peaks of the carbon species generated are all at 200–500 °C, that is, the amorphous carbon C_α and filamentous carbon C_β1 type exists [52,53]. However, no obvious hydrogenation peak was found at about 600 and 800 °C. Therefore, it can be considered that in the range of 10 h, the CO₂-CH₄ reforming reaction almost did not form the C_γ and C_β2 types of carbon species.

According to the degree of difficulty of carbon elimination under the conditions of the reforming reaction, the above carbon deposition can be classified as active carbon deposition (amorphous carbon), transitional carbon deposition (polymeric carbon, carbon nanotubes, and filamentous carbon), and inactive carbon deposition (graphitized carbon).

Mo et al. [49] observed the carbon deposition on the catalyst surface at different reaction times and found that the fibrous carbon deposition began to accumulate after 2 h of reaction, and after 5 h, it was observed that there were many fibrous or rod-shaped morphologies with larger diameters. With the extension of reaction time, the amount of fibrous carbon deposition increased significantly, while the diameter of the carbon fibers decreased significantly, which is probably because of the occurrence of a carbon elimination reaction (C + CO₂ = 2CO) in CO₂-CH₄ reforming (Figure 3). The results also showed that each sample had two hydrogenation peaks, one low-temperature hydrogenation peak and one high-temperature hydrogenation peak, corresponding to two types of carbon deposition species [49]. With the increase in reaction temperature, the high-temperature hydrogenation peak of the carbon deposition moves to a high temperature. The reaction temperature increased from 650 °C to 850 °C, and the peak temperature of the high-temperature hydrogenation peak increased from 425 °C to 455 °C, which may be due to the fact that filamentous carbon is easily converted into graphite carbon at high temperatures, and the hydrogenation reaction needs to be carried out at a higher temperature [54].

Figure 4 showed the morphological characteristics of carbon on the catalyst surface after the reforming reaction. It can be seen from the figure that a large amount of filamentous carbon is formed on the catalyst surface and is even covering the catalyst surface in a large range. It can be speculated that if the carbon deposition continues, it will completely cover the catalyst surface and cause catalyst deactivation [46].

4.2. Resistance and Elimination of Carbon Deposition

In CO₂-CH₄ reforming, the carbon deposition rate usually depends on its formation rate and elimination rate. When the elimination rate of carbon deposition is higher than the formation, the carbon deposition can be inhibited [55]. According to the cause of carbon deposition in the CO₂-CH₄ reaction, the carbon deposition can be suppressed by two aspects: catalyst modification and process conditions optimization.

In the process of CO₂-CH₄ reforming, carbon deposition mainly comes from methane cracking and carbon monoxide disproportionation. The dehydrogenation of CH₄ on the metal surface generates carbon species CH_x (x = 0–3, CH₄→C + 2H₂), which do not react with the surface oxygen species generated by the timely adsorption and dissociation of CO₂ to generate carbon species of CO (C + CO₂→2CO), and the carbon species may accumulate on the metal surface, causing carbon deposition. As the carbon dioxide content in the reactant gas increases, the adsorption rate of methane and its subsequent dissociation rate (i.e., the cracking of methane) decrease. At the same time, the oxidation rate of carbon species on the catalyst surface increases. Therefore, the amount of carbon deposited on the active site is reduced, which can significantly improve the stability of the Ni-based catalyst. In an atmosphere with sufficient CO₂, during a long catalytic process, carbon will migrate and accumulate in the center of the Ni crystal, forming a hollow fiber type structure from bottom to top. The process of carbon deposition and carbon elimination on a Ni/CeO₂ catalyst is shown in Figure 5 [56].

Kuijpers [57] found the size sensitivity of CH₄ activation in the study of Ni-based catalysts, that is, CH₄ preferentially dissociates on smaller Ni grains. Osaki et al. [28,58] reported the x value of CH_x species on different catalysts: Ni/MgO = 2.7, Ni/ZnO = 2.5, Ni/Al₂O₃ = 2.4, Ni/TiO₂ = 1.9, Ni/SiO₂ = 1.0, and Co/Al₂O₃ = 0.75. It can be seen that for the same metal, the x value of basic carrier is higher, that is, the degree of CH₄ dissociation increases with the increase in carrier acidity. On the other hand, the activation of the C-H bond requires electrons from the surface of the Ni; so, the electronic environment around the Ni is also extremely important [59]. For example, the strong metal-support interaction will significantly affect the activity of Ni to dissociate CH₄ [60,61].

Horiuchi et al. [7] believe that alkaline metal oxides can enhance the adsorption capacity of CO₂ and generate more active oxygen atoms (O_ad); O_ad can effectively prevent the adsorption of CH_x,ad on the active center of Ni metal through the reaction of CH_x,ad + O_ad→CO + H₂, thus avoiding the surface carbon deposition caused by the cracking of CH_x,ad. Therefore, adding additives such as alkali or alkaline oxide can enhance the basicity of the carrier surface and the ability to absorb CO₂, change the electronic density of the metal active center, effectively improve the catalytic activity of the catalyst, and inhibit the carbon deposition on the catalyst surface [62].

4.2.1. Resistance of Carbon Deposition from the Perspective of Catalyst

Effect of Ni Grain Size on the Deposition of Carbon

Studies have shown that high dispersion of active metal on the surface of the support can reduce the agglomeration size and effectively inhibit carbon deposition [11,63,64]. It has also been shown that only when the size of the active component is larger than a certain critical size (for example, ≥9 nm) can lead the carbon simple substance to form nuclei [7]. The small size and high dispersion of the active component can effectively inhibit the nucleation and growth of carbon whiskers. Therefore, by selecting the appropriate support, additive, and preparation method, the metal dispersion and particle size can be adjusted to effectively inhibit the occurrence of carbon deposition and improve the anti-carbon deposition performance [65]. In addition, the addition of an appropriate additive can also improve the surface alkalinity of the catalyst, strengthen the adsorption of CO₂, promote the elimination of deposited carbon species, and enhance the anti-carbon deposition performance of the catalyst.

XU et al. [66] prepared Ni/La₂O₃/γ-Al₂O₃ and Ni/La₂O₃/α-Al₂O₃ catalysts and found that when the size of the Ni particles was less than 15 nm, the carbon deposition was significantly reduced (Figure 6), which made the catalyst have higher catalytic activity and stability. CRNIVEC et al. [67] found that the catalyst surface with metal active particles less than 6 nm has excellent resistance to carbon deposition. LIU et al. [68] found that when nickel particles were less than 5 nm, they had an obvious inhibition effect on carbon deposition.

Li et al. [69] analyzed the surface carbon on a Ni/MgO catalyst based on density functional theory and found that the size of the active component Ni had a great influence on the anti-carbon deposition performance of the catalyst. The three different sizes of active metals, Ni4, Ni8, and Ni12 were loaded on the surface of MgO, showing significant differences in catalytic activity and stability. Small size Ni4 can reduce the activation energy of the CH₄ dissociation adsorption, the CH dissociation, and the Coxidation, thus improving the CO₂-CH₄ reforming performance.

In their study, Mo et al. [70,71] found that after the reduction in the Ni-based catalyst based on NiAl₂O₄ spinel, the size of the Ni crystal was small and could effectively prevent the high-temperature sintering of the active component. By increasing the calcination temperature, the proportion of NiAl₂O₄ spinel (the active component precursor) in the catalyst was increased, and the size of the active component was effectively reduced; the stability of the catalyst was improved, and the amount of carbon deposited on the catalyst was obviously reduced. It was also found that there were two Ni precursors, crystalline NiO (calcinated at lower temperature (≤600 °C) and spinel NiAl₂O₄ (calcinated at higher temperature (≥700 °C), indicating that the calcination temperature significantly affected the interaction force between the metal and the support. The higher the calcination temperature, the stronger the force, and the more the spinel phase Ni species that formed. This type of Ni species gave a small size of active component after reduction, which could obtain higher CO₂ and CH₄ conversion and H₂ selectivity [72].

Inhibition of Carbon Deposition from the Stability of Ni Component

In general, researchers add some additives to the Ni-based catalyst to improve the dispersion and stability of the nickel, promoting the reforming reaction and inhibiting the formation of carbon deposition, which is a research hotspot in this reaction. The additives commonly used are mainly divided into two categories. The first type of additive is alkaline oxides, including MgO, CaO, K₂O, etc. [73,74]. It was found that the addition of alkaline oxides to a Ni-based catalyst can promote dispersion of the Ni and inhibit carbon deposition. On the other hand, the alkaline medium can improve the adsorption performance of CO₂ (weak acid gas). Some researchers [74] believed that the addition of alkali metals can inhibit carbon deposition and improve the activity and stability of the catalyst. Mo et al. [75] studied the effect of CaO on the structure, reforming performance, and carbon deposition of the Ni-Al₂O₃ catalyst. The results showed that the activity of Ni-Ca-4 was higher, with the conversion rate of CH₄ and CO₂ of 52.0% and 96.7%, respectively. The amount of carbon deposited on the catalyst was lower, and the type of the carbon was attributed to an amorphous one, presenting a good anti-carbon deposition performance.

Another kind of additive is rare earth oxides [73], such as CeO₂ and La₂O₃, which can achieve both high activity and stability for CO₂-CH₄ reforming. By adding rare earth oxide, the crystal phase, pore structure, and mechanical strength of the catalyst could be significantly changed, thereby improving the activity, stability, and selectivity of the catalyst. For example, the addition of La₂O₃ to the Ni/γ-Al₂O₃ catalyst could inhibit the carbon deposition in CO₂-CH₄ reforming. Some researchers found that in the CO₂-CH₄ reaction, La₂O₃ interacts with CO₂ on the Ni/La₂O₃ catalyst to generate La₂O₂CO₃ [52]; La₂O₂CO₃ decomposes CO and provides oxygen species, and the oxygen species can react with the carbon species accumulated after the dissociation of CH₄ on Ni grains to generate CO, thus achieving the effect of inhibiting carbon deposition. Other additives, such as metal Cr [76] and mixed oxide CeO₂-ZrO₂ [77,78], can also improve the activity and stability of the catalyst.

Mo et al. [70] prepared a series of La₂O₃-NiO-Al₂O₃ catalysts with different La loading to improve the performance of the Ni-based catalyst for CO₂-CH₄ reforming. The results showed that the precursor of the active component mainly exists in the form of NiAl₂O₄ spinel. The “confinement effect” of La₂O₃ on Ni grains can inhibit the sintering of the active component, prevent carbon deposition, and improve the reforming performance. Mo et al. [79] also prepared a Ni-Al₂O₃ catalyst with Ca, Co, and Ce as additives by the combustion method. The results showed that the activity order of the catalysts was followed by Co-Ni-Al₂O₃>Ca-Ni-Al₂O₃>Ni-Al₂O₃>Ce-Ni-Al₂O₃. Carbon deposition analysis showed that Ca-Ni-Al₂O₃ presented poor carbon deposition resistance, and a certain amount of graphitic carbon was generated on the catalyst. The dry reforming performance of Ni catalysts supported by different supports is shown in

Table 1. Catalytic performance of Ni-based catalysts with different supports.

Active Component	Support	Mass Fraction of Active Component /%	Temperature /°C	Time /h	SV /(mL·g−1·h−1)	CH4 Conversion Rate /%	CO2 Conversion Rate /%	Carbon Deposition /%	Reference
LaNiO3	SBA-15	10	700	60	36,000	78	73	4.47	[80]
LaNiO3	MCM-41	10	700	60	36,000	75	71	4.83	[80]
LaNiO3	SiO2	10	700	60	36,000	68	64	5.67	[80]
Ni	MgO	20	750	2.3	168,000	46.13→34.3	51.4→37.6	2.648	[81]
Ni	Al2O3-T	10	700	5	24,000	80	90	——	[82]
Ni	Al2O3-S	10	700	5	24,000	68	79	——	[82]
Ni	Al2O3	10	700	5	24,000	72	75	——	[82]
Ni	γ-Al2O3-S	10	700	5	48,000	56.0→52.2	——	——	[83]
Ni	γ-Al2O3-P	10	700	5	48,000	52.2→39.3	——	——	[83]
Co-Ni	CeO2	——	600	10	12,000	77	80	——	[84]
Ni	MgO-ZrO2	10	700	60	16,000	84.7	86.5	20	[85]
Ni	ZrO2-RC-100	5	700	7	42,000	67.4→46.5	68.4→58.2	66.3	[86]
Ni	ZrO2-ELTN	5	700	7	42,000	42.3→31.9	52.3→43.9	25.2	[86]
Ni	ZrO2-Z-3215	5	700	7	42,000	62.2→45.3	69.5→58.4	38.3	[86]
Ni	ZrO2(MK)	5	700	7	42,000	51.2→36.3	56.7→46.4	46.9	[86]
Ni	ZrO2-O2	10	750	10	24,000	78→64	86→73	——	[87]
Ni	ZrO2	5	750	36	24,000	83→78	——	——	[87]

Application of High-Activity Bimetallic Catalysts

The introduction of a second metal to obtain a bimetallic Ni-based catalyst is also considered to be an effective and practical strategy to improve the performance of the CRM catalyst. The synergistic effect between Ni and the second metal can significantly improve the activity and carbon deposition resistance of the Ni-based catalyst [17,88,89,90,91].

In order to discuss the synergistic effect and the basic principle for improving the performance of the used catalyst, the researchers prepared a series of bimetallic Ni-based CRM catalysts. The results showed that Ni-Pt [92], Ni-Co [93], and Ni-Cu [94] showed better activity and carbon deposition resistance. In general, bimetallic Ni-based CRM catalysts include Ni-noble metals (Pt, Ru, etc.) and Ni-transition metals (Co, Fe, and Cu) [95,96]. Ni-noble metal bimetallic catalysts have three advantages: the promotion of reduction, surface modification, and surface reconstruction. Noble metals usually contribute to the reduction in NiO crystal, thereby increasing the number of active sites [97,98,99,100]. In terms of surface modification, the surface properties of Ni can be changed by adding a trace noble metal. In addition, the surface reconstruction of the bimetallic particles can be caused by temperature or adsorbate [101,102]. GARCIÁ-DIÉGUEZG et al. [103] prepared a Ni-Pt bimetallic catalyst for a CRM reaction. Compared with the Ni catalyst, the Ni-Pt bimetallic catalyst formed a Ni-Pt alloy with higher activity and lower carbon deposition. Although only a small amount of precious metals was added to the Ni-based catalyst, the production cost of the catalyst still increased. Therefore, some researchers doped transition metals such as Co, Fe, and Cu into a Ni-based catalyst to construct a CRM bimetallic catalyst to reduce the industrial production cost [104]. Co, Fe, and Cu have a strong synergistic effect in the bimetallic system. Of course, the specific effects of the three metals are different [105,106,107,108,109]. Some researchers discussed the effect of Ni-Co, Ni-Fe, and Ni-Cu bimetallic catalysts. The introduction of the second active component, Co or Cu, into the Ni-based catalyst helped to improve the catalytic activity and carbon deposition resistance [110].

The Ni-Co bimetallic catalyst shows a stronger synergistic effect [111,112,113]. Additionally, the Ni/Co ratio, which can adjust the surface composition of Ni-Co clusters, plays a crucial role in the Ni-Co bimetallic system [114,115,116,117]. Generally, a small amount of Co can optimize the adjustment process, while excessive Co will cause the catalyst to be oxidized. The promotive effect of Co is mainly due to its strong affinity for oxygen species, enhancing the ability to eliminate carbon deposition on the catalyst [111,118,119]. The Ni-Co/Al₂O₃ bimetallic catalyst showed high thermal stability at 800 °C and effectively inhibited the side reaction of RWGS [120]. Turap et al. [84] prepared a Ni-Co/CeO₂ bimetallic catalyst for CRM reaction and found that the strong oxygen affinity of Co and the strong oxygen storage capacity of CeO₂ were helpful in eliminating carbon deposition. As the Co/Ni ratio was up to 0.8, the catalyst presented better activity and stability. Li et al. [121] studied the catalytic performance of a bimetallic Ni-Co/Al₂O₃ catalyst for CRM and found that the addition of metal Co can form a Ni-Co alloy, increasing the activation energy of CH₄ dissociation, thus inhibiting the CH₄ cracking activity. At the same time, the addition of Co could improve the oxygen affinity of the catalyst and remove carbon deposition. Liang et al. [122] used a one-pot method to synthesize an attapulgite-derived MFI (ADM) zeolite-coated Ni-Co alloy. The results showed that the Ni-Co alloy existed stably in the CRM process, which was conducive to the formation of electron-rich Ni metal and significantly improved the fracture ability of the C-H bond. At the same time, ADM not only firmly anchors metal sites through pore structure or layered system, but also provides rich CO₂ adsorption/activation centers, realizing high CRM reaction activity and improving the anti-carbon deposition performance.

Cu can partly replace Ni to improve catalyst activity and carbon deposition resistance [110]. Song et al. [123] constructed a Ni-Cu bimetallic catalyst. The catalyst with a 0.25–0.50 Cu/Ni ratio showed good activity, stability, and carbon deposition resistance, while the catalyst with higher and lower Cu/Ni ratios would be deactivated due to serious carbon deposition. The excellent performance of the optimized Ni-Cu/Mg(Al)O catalyst was related to the synergistic effect between Ni and Cu. On the one hand, the alloying of Ni and Cu inhibited the deep dissociation of methane, and the carbon species obtained were more easily gasified (carbon eliminated). On the other hand, Cu provided active sites for the dissociation of CO₂, leading to the formation of active oxygen species. The alloying of Ni and Cu reduced the decomposition rate of CH₄, promoted the dissociation of CO₂, and effectively inhibited carbon deposition. Other studies showed that [124], during the CRM reaction, the addition of Cu had a significant effect on the activity and anti-carbon deposition performance of the Ni/CeO₂ catalyst, and the formation of a Ni-O-Ce solid solution generated more oxygen vacancies, improving catalytic activity.

Fe has always played a certain role in promoting the CRM reaction. Both Fe and Ni are iron elements with similar element properties, and the two metals can be alloyed in a certain proportion to make a catalyst with good catalytic performance [93,125]. The research from Kim et al. [126] showed that the catalysts supported solely with Ni or Fe presented the problems of fast deactivation and a low conversion rate, respectively, while the bimetallic Ni-Fe catalyst showed good activity and stability in the CRM reaction. By further analysis, it was found that the promotion of Fe in a Ni-Fe alloy was due to the cracking of CH₄ on the active metal Ni to produce H₂ and carbon. A part of Fe reacts with CO₂ to generate FeO, which falls off from the alloy. Additionally, the carbon can react with FeO and be oxidized to generate CO. Then, FeO is reduced to Fe, which is the original Ni-Fe alloy. This decarburization reaction cycle is conducive to the reducing of the surface carbon on the catalyst. The anti-carbon deposition performance of different bimetallic catalysts is shown in

Table 2. Carbon deposition resistance of bimetallic catalysts in DRM.

Catalysts	SV /(mL·g−1·h−1)	Feed Ratio	Temperature /K	CH4 Conversion Rate /%	CO2 Conversion Rate /%	Carbon Deposition /%
Ru-Ni/Al2O3 [127]	60,000		1023	94.00	97.00	0.32
Co-Ni/CeO2 [84]	30,000	CH4:CO2 = 1:1	1073	80.10	82.20	10.00
NiFe/Al2O3 [128]	12,000	CH4:CO2 = 1:1	823	26.60	37.80	2.30
NiCu/Al2O3 [129]	18,000	CH4:CO2:He = 1:1:8	923	65.00	64.34	6.40
Ni-Co/Al2O3 [130]	54,000	CH4:CO2:N2 = 2:2:1	1023	96.10	92.20	1.00
NiPt/Al2O3 [100]		CH4:CO2:Ar = 45:45:10	1023	86.00	87.00	7.00

.

Selection of Support

The support is a very important part of a catalyst. In a CO₂-CH₄ reforming reaction, the commonly used supports are Al₂O₃, MgO, CeO₂, TiO₂, SiO₂, etc. Although the support itself has no activity in the reaction, it can change the overall performance of the catalyst. The physical and chemical properties of the support, such as surface morphology, pore structure, interaction with active component, and the resulting differences due to the support, such as surface–interface structure, surface composition, grain size, and dispersion of the active component, can affect the existence form of the active component precursor and the catalyst activity, selectivity, stability, and carbon deposition resistance. Many studies have pointed out that strong interaction between the support and the active component is conducive to improving the dispersion and sintering resistance of the active metal, resulting in a high carbon resistance performance.

The stronger the interaction between the support and the metal, the less likely the catalyst will be reduced. If it can be reduced under certain conditions, then the smaller the metal particles, the better the dispersion. The excessive surface acidity of the support leads to catalyst deactivation through methane decomposition. Similarly, excessive surface basicity leads to catalyst deactivation through the Boudouard reaction as well as through the formation of metal oxides [39]. Hao et al. [131] reported that a close combination of Ni and carrier caused by a strong metal-support interaction promoted the transfer of transition species at the interface and the transfer of electrons, leading to the transformation of non-inert carbon species in the reaction process and avoiding the forming of inert carbon deposition. At the same time, strong metal-support interaction can effectively inhibit the sintering and growth of Ni particles under the reaction conditions and can have a certain stabilizing effect on Ni particles, thereby improving the performance of the catalyst. Liu et al. [60] found through their research that Ni/CeO₂ was very active in the CRM reaction and that strong metal-support interaction enhanced the dissociation reaction activity of Ni to CH₄ and inhibited the formation of carbon deposition. Ruckenstein et al. [132] prepared a Ni/TiO₂ catalyst and found that there was a strong interaction between Ni and TiO₂, which led to the reduction in the free energy of the system. TiO_x can promote the elimination of carbon to a certain extent, but TiO_x molecules migrate on the surface during the reduction process, covering the active sites of Ni. A large amount of filamentous carbon was formed on Ni supported on CeO₂ and on CeO₂ doped with iso-valent Zr, while a negligible amount was formed on Ni supported on CeO₂ doped with aliovalent Sm or La (Figure 7). The ceria dopants can change the interaction of Ni with the support [133].

The pore structure of the support has a great influence on the performance of the catalyst and has a limited domain effect on the active component. It has been found that micropores (<2nm) are not conducive to the dispersion of metal particles; mesopores (2–50 nm) can make the catalyst have a large specific surface area; macropores (>50nm) can promote the diffusion of reactant and product molecule, make gas molecules fully contact the catalyst, and increase the number of exposed Ni active sites [134]. Due to the limitation of the mesoporous structure of the support, Ni particles exist in the pores on the catalyst as much as possible, with high dispersion, which is conducive to strong metal-support interaction, thus reducing the formation of carbon deposition [135]. The catalyst with multistage pore structure has higher carbon capacity and a lower carbon deposition deactivation rate due to the addition of different levels of pores [134,136]. Du et al. [137] reported a CRM catalyst of HT-NiMgAl with a multistage pore structure. The multistage pore structure of this catalyst effectively increased the specific surface area of the catalyst, improving the dispersion of Ni particles; it could effectively inhibit carbon deposition due to its role in limiting the region of the active component.

After the reduction in the catalyst, the Ni atoms are easily sintered at high temperature, which leads to the reduction in the dispersion of Ni atoms on the surface and the increase in the concentration difference between the bulk Ni atoms and the surface Ni atoms so that the Ni atoms dissolved in the support will migrate to the surface under the promotion of the concentration gradient, supplementing the dispersion of the surface Ni atoms. Studies showed that the Co/MgO catalyst provides a strong Lewis alkaline environment due to the formation of solid solution CoMgO_x, which effectively stabilizes the Co nanoparticles on the surface of the support. Due to the alternating polar nanolayer structure of O^2- and Mg²⁺ and the existence of a large number of O^2- Lewis alkaline sites on the surface of MgO(111), the anti-sintering ability and anti-carbon deposition performance of the catalyst have been improved [138,139]. In addition, the support has a Lewis base, which can increase the alkalinity of the catalyst, promote the adsorption and dissociation of CO₂, and eliminate carbon deposition in the reaction. At the same time, the alkalinity of the support can inhibit the growth of Ni metal particles at high temperatures, thus improving the activity, stability, and carbon deposition resistance of the catalyst [140]. Jafarbegloo et al. [141] prepared a NiO-MgO catalyst and found that the strong Lewis base of MgO absorbed a large amount of carbon dioxide, improved the conversion rate of carbon dioxide, and eliminated carbon deposition on the catalyst surface.

Li et al. [142] prepared an iron-rich biomass-derived carbon for the CO₂-CH₄ reforming and found that it had higher activity than non-iron-rich carbon. Before 800 °C, the order of the iron-rich carbon promoting the reforming reaction was followed by Fe-C₂ (10% Fe content) > Fe-C₃ (20% Fe content) > Fe-C₁ (5% Fe content). After 800 °C, Fe-C₂ can still achieve the maximum CH₄ conversion rate. In addition, the catalytic activity of Fe-C₂ to CH₄ at 800 °C was better than that of other catalysts at higher temperature. By further measuring the carbon catalyst used, it was found that the weights of iron-rich carbon and non-iron-rich carbon increased by 0.2% and 0.9%, respectively. Therefore, it can be proved that the carbon deposition on the carbon catalyst is less, which effectively eliminates the carbon deposition. After the test, the iron-rich carbon had less carbon deposition, mainly in the form of filamentous carbon, which was more easily removed by carbon removal reaction.

Most biomass carbons are alkaline, and their ash contains a large amount of alkali metals (K, Na) and alkaline earth metals (Ca, Mg), which can promote the formation of alkaline sites, facilitate the adsorption and dissociation of CO₂, and inhibit the formation of carbon deposition. It has been reported that alkali/alkaline earth metals are one of the main reasons for biomass carbon to promote CO₂-CH₄ reforming [142]. Zhang et al. [143] studied the role of alkali/alkaline earth metals in tar reforming. The results showed that alkali/alkaline earth metals promoted the interaction between the active metal Ni and the carrier and inhibited the sintering of Ni. Alkali/alkaline earth metals cause more oxygen to be adsorbed on the surface of the catalyst, which has strong oxidizability. It can react with reaction intermediates or C, avoid the deposition of C on the catalyst, and inhibit carbon deposition [142,143,144]. San et al. [145] studied the role of alkali metal K and speculated that K can promote C gasification reaction and cover some active sites to inhibit CH₄ decomposition and reduce carbon deposition, but the coverage of active sites will also have a certain negative impact on the reforming reaction. Wu et al. [146] used CaO as the carrier to theoretically calculate that the presence of CaO adsorbed more CO₂ to the participate in the CO₂-CH₄ reforming and that CO₂ dissociated at the interface between Ni and CaO; in addition, the oxygen species produced by dissociation and carbon deposition on the surface of the catalyst generated CO, which extended the service life of the catalyst.

Application of Confined Catalyst

A confined catalyst can effectively confine the active center on the catalyst in different ways, which mainly include lattice limit, pore limit, core-shell limit, surface space limit, and multiple limits.

Lattice confinement can effectively anchor precious metal or non-precious metal on the regularly arranged spatial skeleton and can improve the dispersion of active centers. Ruitenbeek et al. [147] used a catalyst composed of a single iron atom in the lattice confined region; it had high activity and selectivity in the reaction and almost no carbon deposition. The surface confined catalyst had a high specific surface area, highly ordered pore structure, and narrow pore size distribution. Wang et al. [148] used dendritic mesoporous SiO₂ (DMS) as a carrier to prepare an alkali metal oxide modified low-temperature carbon deposition-resistant Ni-based catalyst and applied it to a reforming reaction, which showed excellent low-temperature carbon deposition resistance.

Kong et al. [149] prepared microporous molecular sieve S-1 with rich pores and high specific surface area, and effectively embedded the active component Ni in the pores and applied it to CO₂-CH₄ reforming. The results showed that the catalyst had excellent activity and stability at 650 °C and 0.5 MPa for 100 h. The thermogravimetric test of the catalyst after reaction did not find weight loss and indicated that the S-1-encapsulated Ni-based catalyst had excellent carbon deposition resistance. The main reason was that the catalyst channel effectively restricted the aggregation of Ni particles, which made the Ni disperse uniformly and reduced the size of the Ni.

Core-shell catalysts mainly include two types: one is the close contact type; the other is the eggshell type (the active component is separated from the shell). Zhang et al. [150] wrapped the perovskite LaNiO₃ nano-cube in the mesoporous silica shell to form a new core-shell structure catalyst, which was used in the CO₂-CH₄ reforming and showed excellent carbon deposition resistance. Compared with the eggshell catalyst, the core-shell catalyst had a contact interface between the core and shell, which resulted in enhanced interaction, inhibition of the movement of the active center, and reduction in the particle size. Liu et al. [151] designed a high-performance In-Ni@SiO₂ close-contact nanocore-shell catalyst. The In-Ni@SiO₂ catalyst had higher activity compared to the Ni@SiO₂ catalyst. CO₂ and CH₄ reacted at 800 °C for 430 h and still maintained 90% conversion. After reaction, compared with the other supported catalyst, they had less carbon deposition, better stability, and anti-carbon deposition performance.

Multiple restriction can limit the active center, reduce its exposure, and improve carbon deposition resistance on the catalyst. Wang et al. [152] prepared a Ni@La₂O₃/SiO₂ catalyst, and the results showed that an amorphous La₂O₃ layer was coated on the SiO₂, while small Ni nanoparticles were encapsulated in the La₂O₃ layer. As Ni nanoparticles were encapsulated in the La₂O₃ amorphous layer, it could effectively inhibit the formation of carbon deposition in CO₂-CH₄ reforming.

4.2.2. Eliminate Carbon Deposition from Process Condition Matching

The conversion rate of carbon dioxide and methane varies with the ratio of reaction gas, space velocity, reactor size, and catalyst dosage.

Selection of Operating Conditions (Temperature, Pressure, etc.)

Nematollahi et al. [153] conducted the same thermodynamic simulation under different pressures and found that the conversion rate of CH₄ and CO₂ and the amount ratio of the H₂/CO substances decreased significantly with the increase in operating pressure. This is due to the fact that the CRM reforming is a reaction with an increase in volume. The lower the pressure, the better the reaction. The high-pressure environment inhibits the conversion of the reactants. Some researchers conducted thermodynamic simulation on the influence of temperature, CH₄/CO₂ ratio, reaction pressure, and other oxidants on the formation of carbon deposition and proposed that high conversion and less carbon deposition could be obtained by operating at a high temperature, low pressure, and high CH₄/CO₂ ratio above 850 °C [17,154,155]. Bao et al. [156] prepared a NiCeMgAl double-porous (mesoporous–mesoporous) catalyst. When the space velocity was lower than 96,000 h⁻¹, the larger mesopores provides a fast transport channel for the reactants and product molecules. At a higher space velocity (such as 120,000 h⁻¹), the conversion rate was reduced because the reactants could not fully diffuse to the active center in the catalyst. The thermogravimetric analysis results showed that with the (Ni₁₅CeMgAl) the total weight loss of the dual porous catalyst after reaction was 16.8%, of which amorphous carbon accounted for 2.5%, carbon nanotubes accounted for 9%, graphite-like carbon accounted for 1%, and the others comprised the desorption of adsorbed small molecules. It was further found that carbon nanotubes could act as a carrier to continue the reaction and prolong the service life of the catalyst.

Adjustment and Matching of Reaction Gases

Carbon species deposited on the catalyst surface can usually be eliminated by the oxidation of CO₂ through the carbon elimination reaction (CO₂ + C→2CO). Therefore, the total amount of carbon deposition on the catalyst depends on the balance between methane cracking, carbon monoxide disproportionation, and the decarburization reaction [63], which can be considered from the two aspects of the inhibition of the carbon deposition reaction and the promotion of the decarburization reaction to improve the anti-carbon deposition performance of the catalyst. Of course, increasing the proportion of CO₂ can inhibit the formation of carbon deposition but increasing the proportion of CO₂ will promote the occurrence of side reactions and lead to increased separation costs in the later period. Therefore, the determination of the CO₂/CH₄ ratio should be combined with various factors. Mo et al. [49] found that with the increase in the CO₂/CH₄ ratio, the amount of carbon deposition on the catalyst surface gradually decreased, and the area and intensity of the high-temperature hydrogenation peak gradually weakened, indicating that low activity β carbon was significantly reduced due to the increase in the proportion of CO₂. The results also showed that the addition of CO₂ played an important role in preventing the transformation from active carbon to inactive carbon.

Adding steam or oxygen to the reaction for mixed reforming can also reduce carbon deposition on the catalyst. Li et al. [157] prepared a Ni/CeO₂-ZrO₂-Al₂O₃ catalyst, carried out a CO₂-CH₄ reforming reaction with and without steam, and measured the amount of carbon deposition. The results showed that the addition of steam to the reaction gas could significantly reduce the carbon deposition, improving the stability of the catalytic reaction. O’Connor et al. [158] found that the Ni/Al₂O₃ catalyst had high activity at 550–800 °C under the conditions of CO₂ reforming and the partial oxidation of methane. With the increase in the O₂ addition, almost no surface carbon deposition was found, but the activity of the catalyst decreased with time.

LI et al. [159] conducted a study employing the action of microwave-irradiated biological semi-coke; the experimental study of CO₂/steam-combined CH₄ reforming was carried out. The characteristics of the combined reforming reaction were examined, and the effects of the combined reforming reaction on the quality of the syngas, the loss of biochar, the surface characteristics, and the functional groups were discussed. The results showed that the combined reforming reaction could promote the conversion of the reaction gas, causing the the average value of the volume ratio of H₂/CO in the syngas to be within 90, the min reaction time to rise to 0.923, and the H₂/CO gas volume ratio to be closer to 1.

5. Conclusions and Prospect

CRM reforming not only promotes the utilization of CH₄ and CO₂ but also plays an important role in mitigating the greenhouse effect and reducing carbon emissions. It is an effective means of achieving carbon peaking and carbon neutralization and has good industrial value and application prospects. The key to the stable operation of the reaction is the construction of the catalyst, and the easy sintering of the active component and carbon deposition on the catalyst in the reaction is the core problem that needs to be solved urgently. As the preferred catalyst for this reaction, the Ni-based catalyst also faces the above problems. This paper briefly introduces the thermodynamics, kinetics, and reaction mechanism of the CRM reaction and focuses on the research progress of carbon deposition and carbon elimination on the used catalysts. The following prospects are put forward in terms of inhibiting carbon deposition in order to improve the activity and stability of the CRM catalyst:

• More advanced characterization methods should be used to explore the reaction mechanism and carbon deposition mechanism of the CRM reaction on a Ni-based catalyst, and the reaction mechanism and anti-carbon deposition mechanism of the Ni-based catalyst should be further clarified.
• By introducing different types of additives to regulate the number of alkaline sites on the surface of the catalyst, the adsorption performance of CO₂ may be enhanced, and more adsorbed oxygen may be generated; the gasification process of the carbon deposition may also be promoted.
• By DFT or other calculations, the formation and elimination mechanism of carbon deposition can be discussed in depth, and the catalyst design scheme can correspondingly be optimized to inhibit carbon deposition.