CeO2- and CaO-Promoted Precipitation Method for One-Step Preparation of Vermiculite-Based Multilayer Mesoporous Ni-Based Catalysts for Dry Reforming of Methane.

In this paper, a molecular sieve (VSiO2) prepared from modified vermiculite is used as a support, and a multilayer mesoporous catalyst, Ni-VSiO2, is prepared while the active components are loaded in one step by the precipitation method. The catalyst is further modified by adding additives Ca and Ce to prepare the catalyst Ni-5x-VSiO2 (x = Ce, Ca) and is used for the dry reforming of methane reaction. The catalyst is characterized by X-ray fluorescence, Brunauer-Emmett-Teller analysis, scanning electron microscopy, hydrogen temperature-programmed reduction test, transmission electron microscopy, thermogravimetric analysis, and other technical means. The result shows that under a normal pressure of 750 °C, the catalyst Ni-Ca-VSiO2 has good stability. The catalyst Ni-Ce-VSiO2 has good activity, stability and carbon deposition resistance, and the conversion rates of CO2 and CH4 are 88% and 78%, respectively. This is because the mesoporous structure allows Ni nanoparticles to enter the pores of the catalyst support, thereby inhibiting the aggregation of the active component Ni and improving its sintering resistance. CeO2 additives provide more oxygen vacancies to inhibit the formation of carbon deposits. At the same time, the strong interaction between the active component Ni and the additive CeO2 is also beneficial to improve its sintering resistance.

Introduction

The main gases of the greenhouse effect are methane and carbon dioxide, and the greenhouse effect seriously threatens human health and the environment.[1−3] The use of fossil fuels in modern society has increased the emissions of the greenhouse gases such as methane and carbon dioxide.[4] Fortunately, the dry reforming of methane (DRM) process can use these two greenhouse gases to prepare CO and H2 with an industrial application value. The product has a H2/CO ratio of about 1, and it can be directly used as a raw material for the Fischer–Tropsch synthesis.[5−7] One of its products, hydrogen, is a clean and sustainable energy source, which plays an important role in solving the problem of energy sustainability.

At present, a large number of studies have shown that precious metals (Pt, Ru, Pb, Ir, Rh, etc.) as basic catalysts have good catalytic activity and carbon resistance in DRM reactions.[8,9] However, due to the high price of precious metals, their industrial value in DRM reactions is greatly limited. Hence, a large number of researchers began to pay attention to nonprecious metal catalysts, and in the existing research results, Ni-based catalysts have the best catalytic effect from the point of view of reaction activity, and they are also the most likely nonprecious metal catalysts to replace precious metal catalysts[10,11] because Ni nanoparticles have stronger carbon–hydrogen bond breaking ability than other transition-metal nanoparticles.[12,13] According to the research results of some scholars, the size of Ni nanoparticles is closely related to their activity and resistance to carbon deposition.[14,15] Therefore, the use of smaller Ni particle sizes is one of the effective methods to obtain a more stable catalyst.[16] For DRM catalysts, the preparation conditions, the nature of the support, and the types of additives all have an impact on the catalytic performance.[17−19] Therefore, it is a very effective way to improve the performance of the catalyst to select a suitable catalyst support and auxiliary agent. Studies have shown that alkaline earth metals can neutralize the acidity of the catalyst surface.[20] Alkaline earth metals can not only reduce the methane cracking and dehydrogenation capacity[21] but also increase the amount of CO2 adsorbed on the catalyst surface to improve the carbon removal effect of CO2. The addition of rare earth metals can greatly increase the adsorption and dissociation capacity of oxygen[22] and promote the adsorption and activation of CO2 molecules by the catalyst,[23] and it can also enhance the interaction between the support and the metal active component, thereby inhibiting the growth of Ni particles.[24] Oxygen vacancies in some rare earth metals make the catalysts have good resistance to carbon deposition.[25]

Due to their excellent catalytic performance, two-dimensional materials have attracted the attention of many scholars.[26] As a two-dimensional layered aluminosilicate with water molecules between layers,[27] vermiculite has good cationicity, adsorption, and swelling properties.[28] However, the interlayer spacing of vermiculite is relatively close, which is not conducive to the gas phase spreading between them. Some scholars use the microwave expansion of vermiculite for catalyst preparation.[29] Compared with the traditional thermal expansion method, the microwave-assisted preparation not only shortens the catalyst preparation time but also saves energy.[30] However, due to the low specific surface area of vermiculite (0.5 m2/g) and the weak interaction between the support and the active components, it has many limitations when it is used in the DRM reaction.[31] Therefore, the modification technology of vermiculite has gradually entered everyone’s field of vision. Some scholars have used acid treatment to prepare two-dimensional porous silica nanonetworks, which have excellent properties such as large specific surface area and porosity.[32] It is precisely because the expanded-acidified vermiculite-based SiO2 molecular sieve (VSiO2) has a high specific surface area, layered porous structure, and high mechanical strength, and it also has the potential as a support for DRM.

Although it has been reported many times that alkali metals and alkaline earth metals are supported on supports for DRM reactions, there is no literature report on their use in vermiculite-based molecular sieve (VSiO2) supported Ni-based catalysts for the DRM reaction.

In this paper, a molecular sieve (VSiO2) prepared from modified vermiculite is used as a support, and a multilayer mesoporous catalyst, Ni-VSiO2, is prepared while supporting the active components by a one-step precipitation method. The catalyst is further modified by adding additives Ca and Ce to prepare a catalyst Ni-5x-VSiO2 (x = Ce, Ca) and use it for the DRM reaction. The dual effects of the multilayer mesoporous structure and CeO2 and CaO additives make the catalyst have higher activity, stability, and carbon deposition resistance.

Results and Discussion

X-ray Fluorescence Analysis

The chemical composition analysis results of original vermiculite (VMT), expanded vermiculite (EVMT), and expanded and modified vermiculite (VSiO2) are shown in Table . From the table, the raw vermiculite (VMT) mainly contains metal and nonmetal oxides such as SiO2, MgO, Al2O3, Fe2O3, K2O, TiO2, CaO, and Na2O. The chemical composition of the EVMT processed by microwave is basically unchanged. It can be concluded that there is not much change in the chemical composition of the vermiculite and vermiculite ore after microwave expansion, indicating that the microwave expansion does not change its chemical composition too much. However, EVMT becomes vermiculite-based molecular sieve (VSiO2) after acid treatment, and its chemical composition and structure have undergone some changes. Most of the metal oxides in vermiculite are dissolved by acid, especially the SiO2 in its chemical composition has increased significantly to 95.34%, forming a layered porous molecular sieve with a silicon–oxygen tetrahedron as the framework structure.[33] To a certain extent, compared with unacidified vermiculite, acidified vermiculite has more pores, larger specific surface area, and better material properties.

Table 1

Chemical Composition (wt %) Analysis of VMT, EXVMT, and VSiO2

	chemical composition (%)
sample	SiO2	MgO	Al2O3	Fe2O3	K2O	TiO2	CaO	Na2O
VMT	41.94	28.05	13.5	6.004	6.00	1.49	1.45	1.60
EVMT	43.15	28.74	13.7	5.080	5.683	1.27	0.967	1.43
VSiO2	95.34	1.85	0.894	0.464	0.543	0.621	0.290

Scanning Electron Microscopy Analysis

It can be clearly seen from Figure a that the raw vermiculite has a layered structure. Compared with the raw vermiculite (VMT) in Figure a, the interlayer spacing of EVMT in Figure b is larger. We have successfully expanded the interlayer spacing of vermiculite, namely, EVMT.[29] From the vermiculite-based nanomolecular sieve shown in Figure c, the vermiculite-based molecular sieve is successfully synthesized by EVMT using acid solution, and we can clearly observe the layer of the VSiO2 nanomolecular sieve. From the N2 adsorption–desorption curve shown in Figure , which is a type IV isotherm, it can be seen that the sample has a mesoporous structure, and we have successfully prepared a vermiculite-based molecular sieve (VSiO2).[34]

Figure 1

Scanning electron microscopy (SEM) images: (a) VMT, (b) EVMT, and (c) VSiO2.

Figure 2

N2 adsorption–desorption curve.

Brunauer–Emmett–Teller Analysis

It can be seen from Table that the specific surface area of VSiO2 is 621 m2/g, indicating that we have successfully increased the specific surface area of vermiculite from 0.5 m2/g to a very high level. In all samples, the adsorption–desorption curves have typical type IV isotherms with H2 hysteresis loops, indicating that all materials are ordered mesoporous materials. The specific surface area of the Ni/VSiO2 catalyst prepared by the impregnation method is 431 m2/g, while the catalyst prepared by the precipitation method has a specific surface area of 239 m2/g, and the pore volume becomes larger; this indicates that the active component Ni in the catalyst prepared by the precipitation one-step method is more evenly distributed on the support. Due to the etching effect of the alkaline precipitation agent, the etching effect makes the surface of the catalyst more porous. Some Ni nanoparticles will be attached to the catalyst pores and interlayer spacing, which will increase the number of active sites and increase the interaction force between the support and the active components. It can be seen from Table and the N2 adsorption–desorption graph that the specific surface area of the catalyst Ni-5x-VSiO2 (x = Ce, Ca) added with the auxiliary agent is slightly larger than that of Ni-VSiO2, which indicates that the addition of the auxiliary agent is helpful for the dispersion of active components, and the addition of additives does not have much effect on the structure of Ni-VSiO2.

Table 2

Structural Properties of the Support, Ni/VSiO2, Ni-VSiO2, and Ni-5x-VSiO2 (x = Ce, Ca)

samples	specific surface area (m2/g)	pore volume (cm3·g–1)	average pore diameter (nm)	NiM sizea reduced (nm)
VMT	0.5	0.003
VSiO2	621	0.221	2.855
Ni/VSiO2	431	0.281	2.609	14
Ni-VSiO2	239	0.599	9.997	13
Ni-Ce-VSiO2	279	0.517	7.404	8
Ni-Ca-VSiO2	267	0.418	6.260	11

Calculated by the Scherrer formula.

X-ray Diffraction Analysis

Figure shows the X-ray diffraction (XRD) patterns of the catalyst and VSiO2 samples. It can be seen from Figure a that all samples have a peak at 2θ of about 26.7, which is a characteristic peak of VSiO2, which is a vermiculite-based molecular sieve. The above four catalysts have three characteristic peaks at 37.5, 43.5, and 62.9, which are characteristic peaks of NiO, indicating that Ni(NO3)2 thermally decomposes during the calcination process to form NiO nanoparticles, indicating that this series of catalysts have been successfully prepared. However, the characteristic peaks of Ni-VSiO2, Ni-Ce-VSiO2, and Ni-Ca-VSiO2 are not obvious because the catalyst prepared by the precipitation method has better dispersibility, resulting in weaker characteristic peaks. In Figure a, the peaks of diffracted substances of different metals can be seen, indicating that the promoter exists in the fresh catalyst in the form of oxides. From Figure a, the peak intensity of the catalyst prepared by the impregnation method at NiO is higher than that of other catalysts. It can be calculated from the Debye–Scherrer formula that the catalyst prepared by the impregnation method is compared with the catalyst prepared by the precipitation method. In comparison, the catalyst particles prepared by the impregnation method are larger and can be obtained from the Brunauer–Emmett–Teller (BET) data, and the catalyst prepared by the precipitation method has better dispersibility. It can be concluded that the catalysts prepared by different methods have an effect on the dispersibility and particle size of the active component Ni of the catalyst. In the catalyst with two additives, the peaks of different promoter oxides can be seen, and the intensity is very weak, indicating that different metal oxides and Ni are highly dispersed, and the addition of promoters has an effect on the structure of the catalyst. There is not much impact. As shown in Figure b, we can see that the characteristic peak of VSiO2 still exists. There are three significant diffraction peaks at 2θ values of 44.5, 52, and 76.5, which belong to the crystal plane diffraction peaks of the elemental metal Ni, indicating that Ni has been successfully reduced. As shown in Table , the Ni particle sizes of the catalysts Ni/VSiO2, Ni-VSiO2, Ni-Ce-VSiO2, and Ni-Ca-VSiO2 are estimated using Scherrer’s formula, which are 14, 13, 8, and 11 nm, respectively, The Ni-Ce-VSiO2 catalyst has the smallest Ni particle size. Studies have shown that within a certain range, the smaller the Ni particle size, the better its stability.[35]

Figure 3

(a) XRD patterns of the fresh catalyst. (b) XRD patterns of reduced catalysts.

H2-TPR Analysis

Hydrogen-temperature-programmed reduction (H2-TPR) test technology is an effective method to study the reduction behavior of catalysts. In Figure , the peak on the left is the low-temperature reduction peak, which is mainly due to the reduction of small NiO particles and the weak interaction between the active component and the support; the peak on the right is the high-temperature reduction peak, which is mainly due to the reduction of the layered structure in the VSiO2 and the larger NiO particles in the surface pores and the catalyst surface, and the strong interaction between the active components and the support. The catalyst prepared by the impregnation method has a low-temperature reduction peak at ∼350 °C, and the peak at ∼550 °C is a high-temperature reduction peak. The Ni-VSiO2 catalyst prepared by the precipitation method is mainly at ∼400 and ∼500 °C, indicating that Ni-VSiO2 has a more dispersed distribution of active component Ni and stronger interaction force than Ni/VSiO2. Compared with the catalyst without the promoter, the reduction peaks of the two catalysts Ni-5x-VSiO2 (x = Ce, Ca) added with the promoter have the first and second peaks shifted to a higher temperature and lower intensity. This means that CeO2 and CaO have a strong interaction between the support and the active phase, and according to the literature, a stronger metal–support interaction can prevent nickel from sintering and agglomeration, thereby losing active sites and leading to catalyst deactivation.[24]

Figure 4

H2-TPR curves of fresh catalysts: Ni/VSiO2, Ni-VSiO2, and Ni-5x-VSiO2 (x = Ce, Ca).

Catalytic Performance

The results of the 8 h stability test of the catalyst at a temperature of 750 °C and a space velocity of 18 000 mL/(g·h) are shown in Figure . It can be clearly seen from Figure that all of the catalysts have a good initial activity and the conversion rate of CO2 is always higher than that of CH4, which is related to the reverse water gas shift reaction and reverse Boudouard reaction. Further observation showed that the catalyst Ni/VSiO2 not only has a poor initial conversion rate but also has a poor stability. This may be due to the large Ni particles of the catalyst Ni/VSiO2 prepared by the impregnation method, poor dispersion, and easy sintering and carbon deposition.

Figure 5

(a) CH4 conversion and (b) CO2 conversion. Reaction conditions: P = atmospheric pressure, T = 750 °C, CH4/CO2 = 1, GHSV = 18 000 mL/(g h).

In comparison, the Ni-VSiO2 catalyst prepared in one step by the precipitation method has smaller particles, better dispersibility, and better initial activity. This can be obtained by XRD, but there is still room for improvement in stability. It can be seen from Figure that compared with Ni/VSiO2, the carbon deposit of Ni-VSiO2 has been greatly reduced. This is because the catalyst is prepared in one step, which makes the pore size of VSiO2 larger again, and the layered mesoporous structure of the catalyst allows a part of Ni nanoparticles to enter the pores of the catalyst support, thereby inhibiting the aggregation of the active component Ni and improving the sintering resistance. After adding the alkaline earth metal oxide CaO promoter, the stability of the catalyst is improved, and the initial turnaround rate is also improved. However, CeO2 shows the best performance with rare earth metal oxides. Its stability is the best among the four catalysts, and there is basically no decrease in the stability test of 8 h. It is mainly due to the strong metal–support interaction (as shown by H2-TPR) and the strong adsorption and dissociation of Ce elements, which promote the adsorption and activation of CO2 molecules by the catalyst.[23] Because of the existence of CeO2, its ability to resist carbon deposition will be greatly enhanced (as shown by thermogravimetric (TG) analysis). As can be seen in Table , the performance of the catalyst in this work is comparable to that reported in the literature.

Figure 7

TG curve after the catalyst stability test.

Table 3

Comparison of Dry Reforming Conversion Rate of Different Samples under Similar Conditions

catalyst	temperature reflex	CH4 conversion rate	CO2 conversion rate	references
Ni@SiO2	750	60	71	(36)
5Ni/La2O3-LOC	700	70	75	(12)
Ni/MgO	750	46.1	51.4	(37)
La-Ni/SBA-15	750	67	80	(38)
NixMgxO	700	70	82.5	(39)
Ni-Ce-VSiO2	750	78.6	88	this work

Transmission Electron Microscopy Analysis

It can be seen from Figure a that Ni particles are mainly distributed on the surface of the catalyst support and partly enter the pores of the support, and a small amount is distributed on the edge of the support. This is because the edges and corners of the two-dimensional material have low-coordination active sites, which can stabilize the Ni nanoparticles on the edge of the support, and these nanoparticles are easier to be reduced in the reaction atmosphere than the Ni particles on the surface of the support. The particle size distribution bar in Figure c shows that the Ni particles in the Ni-Ce-VSiO2 catalyst are highly dispersed on the VSiO2 support and the particle size distribution is mainly concentrated in the range of 4–10 nm. After the 8 h stability test, the Ni particles are slightly agglomerated The main reason is that the active component Ni migrates and aggregates on the surface of the support to reduce its own surface energy under high-temperature conditions. It can be seen from the transmission electron microscopy (TEM) image (Figure b) that the Ni-Ce-VSiO2 catalyst after 8 h of reaction is mainly concentrated in the range of 7–19 nm, and the particle size distribution is shown in Figure d. Compared with the prereaction catalyst, the particle size has increased, which is due to the partial sintering of reactive components at high temperature for a long time. In addition, the the Ni-Ce-VSiO2 catalyst that has reacted for 8 h did not produce obvious carbon deposits. This is due to the elimination of carbon deposits by the lattice oxygen–oxygen vacancies of CeO2 in the reaction process and the strong adsorption of Ce elements. The dissociation effect,[40,41] to a certain extent, improves the antioxidant capacity of Ni. From the mapping diagram, it can be seen that both Ni and Ce are well dispersed on the support. This is mainly due to the good interaction between the active component Ni and the auxiliary Ce, which is beneficial for maintaining the good stability of the catalyst, which agrees with the H2-TPR characterization results.

Figure 6

(a) TEM image and (c) particle size distribution image of reduced Ni-Ce-VSiO2. (b) TEM image and (d) particle size distribution image of the spent catalyst Ni-Ce-VSiO2. TEM mapping of Ni-Ce-VSiO2 after reduction.

Thermogravimetric Analysis

Figure shows the TG of each catalyst after 8 h of reaction at 750 °C. In this study, the amount of carbon deposition and its performance were measured by TG scanning calorimetry in an air atmosphere. As shown in the Figure , there is a small amount of weight loss in the range of 50–200 °C, mainly to remove the water vapor adsorbed on the catalyst surface and a small amount of gas impurities. There is a certain amount of weight gain in the range of 200–500 °C, which is due to the oxidation of Ni particles on the catalyst surface to NiO, which increases the quality of the catalyst; There is one significant weight loss in the range of 500–900 °C. The main reason is that the removal of carbon deposits in the catalyst reduces the quality of the catalyst. Among several catalysts, the thermal weight loss of Ni-Ce-VSiO2 is the smallest, which is 4.42%, which shows that the catalyst Ni-Ce-VSiO2 has good carbon resistance performance. This is mainly because the catalyst Ni-Ce-SiO2 has a stronger interaction force and CeO2 contains a large number of oxygen vacancy sites (Ce2O3), and these oxygen vacancy sites can react with NiO to form CeO2 and Ni (Ce2O3 + NiO → 2CeO2 + Ni). At the same time, a portion of the carbon deposits in the reaction can react with CeO2 at high temperatures to generate new oxygen holes (C + 4CeO2 → 2Ce2O3 + CO2), causing some amounts of the carbon deposits to be consumed. It is further proved that the addition of the additive CeO2 significantly improves the antisintering and antioxidation properties of Ni in the catalyst[13,42] and inhibits the formation of carbon deposits in the catalyst, and the catalyst exhibits excellent stability.

Conclusions

In summary, by using expanded acidified vermiculite, a layered porous vermiculite-based molecular sieve (VSiO2) with a large specific surface area was prepared. The mesoporous structure is etched out by using the precipitation method to load the active components in one step to prepare the vermiculite-based multilayer mesoporous catalyst Ni-VSiO2, and then certain amounts of additives are added to modify it to prepare the catalyst Ni-5x-VSiO2 (x = Ce, Ca); the prepared catalyst is used in the DRM reaction.

Due to the etching effect of the precipitant, the pore size of the vermiculite-based molecular sieve becomes larger again, and its layered mesoporous structure allows a part of the Ni nanoparticles to enter the pores of the catalyst support, thereby inhibiting the aggregation of the active component Ni and improving the sintering resistance. Compared with the traditional Ni/VSiO2 impregnation method, the one-step preparation of the vermiculite-based multilayer mesoporous Ni-based catalyst (Ni-VSiO2) improves the stability and conversion rate of the DRM reaction.

CeO2 and CaO were added as additives to the Ni-VSiO2 catalyst prepared by the precipitation method. It can be seen from H2-TPR and TG that the addition of CeO2 additives can strengthen the interaction between the active component Ni and the additive Ce. The oxygen holes of CeO2 can eliminate carbon deposits during the reaction, and the strong adsorption and dissociation of Ce elements. These all further improve the stability and conversion rate of the catalyst in the DRM reaction and the carbon deposition resistance.

Experimental Section

Materials

Preparation of Catalyst Support (VSiO2)

The VMT (Yuli County, Xinjiang) is added into a gold pan and washed, and then the impurities are removed by magnetic separation, and the vermiculite is dried. Then, the dried vermiculite is added into hydrogen peroxide (25%) at a ratio to 1:10 The solid–liquid mixture is heated in a water bath at 75 °C for 60 min. After a simple filtration treatment, it is placed in a microwave oven (700 w) and dried to obtain EVMT, which is processed and passed through a 100 mesh sieve. A certain amount of EVMT is added into 2 M hydrochloric acid at a solid–liquid ratio of 1:20. The mixture is stirred at a constant temperature at 75 °C for 180 min, filtered, and washed with deionized water to neutrality, and dried at 100 °C for 12 h to obtain VSiO2.

Catalyst Preparation

A total of 0.495 g of Ni(NO3)2·6H2O is taken and deionized water is added to obtain a solution. An appropriate amount of VSiO2 support is weighed and added to the prepared aqueous solution; the mixture is heated to 65 °C. About 7.5 mL of 0.5 M NaOH solution is slowly added dropwise to the above solid–liquid mixture to maintain a certain alkaline condition, and the mixture is stirred for 8 h, filtered, washed to neutrality, and dried in a constant temperature oven for 12 h. Then, it was calcined at 550 °C for 3 h to obtain the catalyst precursor. Before the catalytic reaction, it was reduced for 90 min in an atmosphere containing H2 to obtain a fresh Ni-VSiO2 catalyst.

An appropriate amount of VSiO2 support was weighed and added to the prepared Ce(NO3)3 and Ca(NO3)2 aqueous solution. The mass of the auxiliary metal oxide accounts for 5 wt % of the support mass. The mixture was dried at 100 °C for 12 h, and then it was calcined at 550 °C for 3 h to obtain the VSiO2 support (CeO2, CaO)-VSiO2 supported by the metal oxide promoter. Then, 0.495 g of Ni(NO3)2·6H2O was added to deionized water, and the previous (CeO2, CaO)-VSiO2 was added to it. Once the temperature was increased to 65 °C, about 7.5 mL of 0.5 M NaOH solution was slowly added dropwise to the above solid–liquid mixture to maintain a certain alkaline condition. The mixture was stirred for 8 h, filtered, washed to neutrality, and then dried in a constant temperature oven for 12 h. Then, it was calcined at 550 °C for 3 h to obtain the catalyst precursor. Before the catalytic reaction, it was reduced for 90 min in an atmosphere containing H2 to obtain a fresh Ni-5x-VSiO2 (x = Ce, Ca) catalyst.

As a control, an impregnation method was used to prepare Ni/VSiO2 with the same Ni loading.

Catalyst Characterization

The chemical composition of the original and modified vermiculite samples was analyzed with an X-ray fluorescence (XRF) analyzer (Bruker, TIGERS8, Germany).

The morphology of the vermiculite (VMT) before and after the expansion acidification modification (SEM, S4800, Hitachi, Japan) was observed with a scanning electron microscope under an accelerating voltage of 15 kV.

The pore structure characteristics (specific surface area, pore volume, and pore size distribution) of the vermiculite and catalyst were measured at 196 °C using a N2 adsorption–desorption analyzer (BET, ASAP2460, Micromeritics, USA). The sample was vacuum degassed at 200 °C for 8 h in advance.

The crystal phase structure identification of all samples was determined using an X-ray diffractometer (XRD, D8-Advance, Bruker, Germany). According to the Debye–Scherrer formula, the average crystal size of the nickel species on the reduced catalyst was calculated using the half-width at half-height of the strongest peak of the diffraction pattern.

The data of temperature programmed reduction was obtained using an automatic chemical adsorption analyzer (H2-TPR, AUTOChem2920, Micromeritics, USA).

The morphology observation and particle size of the samples were characterized by a TEM (TALOS F200, FEI, USA).

A synchronous thermogravimetric analyzer (TG, STA449F5, NEZSCH, Germany) was used for in situ thermogravimetric analysis.

Catalytic Activity and Stability Tests

The stability test of the catalyst was performed using DRM under normal pressure to test the stability of the catalyst. The instrument is a fixed bed reactor. A total of 0.1 g of the catalyst was taken and placed it into the middle of the stainless steel tube. The inner diameter of the stainless steel tube is 10 mm, and the length is 460 mm. Under a N2 atmosphere, the temperature is increased from room temperature at 10 °C/min to 700 °C, and the mixture was in situ reduced with 50 mL/min 10% H2−Ar gas for 90 min, and then the temperature was increased to 750 °C under the protection of a N2 atmosphere. Finally, the dried mixed gas of methane and carbon dioxide is introduced for the reaction, and after the gas reaction is stable for 30 min, it is passed into the gas chromatograph (Fuli 9790) for analysis. The experimental conditions are T = 750 °C, CH4/CO2/N2 = 13:13:10 mL/min, and the space velocity is 18 000 mL/(g h).