Ceria Promoted Ni/SiO₂ as an Efficient Catalyst for Carbon Dioxide Reforming of Methane

Abstract

The Ni/SiO₂ and the ceria-promoted Ni-CeO₂/SiO₂ were prepared by the impregnation method and co-impregnation method, respectively. The performance of the carbon dioxide reforming of methane (CDR) over Ni/SiO₂ and Ni-CeO₂/SiO₂ was investigated under the conditions of CH₄/CO₂ = 1.0, T = 800 °C, and GHSV = 60,000 mL·g⁻¹·h⁻¹. As a result, a high CDR performance, especially stability, was obtained over Ni-CeO₂/SiO₂, in which the conversion of CH₄ was very similar to that of the thermodynamic equilibrium (88%), and a negligible decrease in CH₄ conversion was observed after 50 h of the CDR reaction. Ni/SiO₂ and Ni-CeO₂/SiO₂ before and after the CDR reaction were subjected to structural characterization by XRD, TEM, TG–DSC, and physical adsorption. It was found that the addition of CeO₂ into Ni/SiO₂ significantly affected its surface area, the size and dispersion of Ni, the reduction behavior, and the coking properties. Moreover, the redox property of Ce³⁺-Ce⁴⁺, which accelerates the gasification of the coke, made Ni-CeO₂/SiO₂ successfully operate for 50 h without observable deactivation. Thus, the developed catalyst is very promising for the CDR.

1. Introduction

The “dual carbon” goal, i.e., peaking carbon emissions by 2030 and achieving carbon neutrality by 2060, is an inherent requirement and inevitable trend to promote high-quality and sustainable social and economic development. Carbon dioxide (CO₂) is one of the main greenhouse gases, but it is also an important C1 resource in nature. The targeted catalytic conversion of CO₂ is an important topic of CO₂ conversion and one of the important ways to achieve the “dual carbon” goal [1,2]. Thus, the CH₄-CO₂ reforming (CDR) technology combined with CH₄ has attracted considerable interest and has been the subject of numerous studies and reports [3,4]. The CDR process makes use of two major greenhouse gases, namely CO₂ and CH₄, and is highly significant for mitigating greenhouse gas emissions. Moreover, the H₂/CO ratio of the synthesized syngas is nearly 1. This syngas can serve as a feedstock for the synthesis of carbonyls and organic oxygen-containing compounds, and it is also suitable for the production of liquid fuels via Fischer–Tropsch (FT) reactions, as described in references [5,6]. What is more crucial is that the CDR reaction enables the direct preparation of syngas from CH₄ and CO₂ present in flue gas, eliminating the need for the pre-separation of CO₂. This feature can substantially cut down industrial costs, as reported in references [7,8]. Therefore, the design of the CDR catalyst with high performance has an important application for CO₂ reduction and utilization.

Generally, precious metals (Rh, Ru, Pt, and Pd) and transition metals (Ni, Co, and Fe) were used as active components for the CDR catalysts. Although precious-metal catalysts have excellent catalytic activity and stability, their high cost limits their application as industrial catalysts [9,10]. Ni-based catalysts are considered to be the most promising CDR catalysts due to their catalytic activity, comparable to precious metals under certain conditions and associated with a low cost [3,4,11]. However, the high-temperature sintering and severe coking of Ni catalysts in CDR reactions lead to rapid catalyst deactivation [12,13]. Therefore, designing and preparing Ni catalysts with higher anti-sintering and anti-carbon deposition performance is also a major challenge faced as a result of their industrialization process.

A thorough review of the relevant literature indicates that the catalytic performance of Ni-based catalysts in CDR reactions is predominantly influenced by several key factors. These include the particle size, distribution, and interaction with the support of Ni, as well as the surface structure of Ni. Among these, the particle size of Ni plays a crucial role in the formation of carbon deposits, as evidenced by references [8,14,15,16,17]. Specifically, smaller Ni particles have been shown to effectively suppress carbon deposition, as reported in references [17,18]. Nevertheless, the high-temperature reduction and reaction conditions inherent to the CDR process can induce aggregation and growth of the active Ni component. This growth leads to the formation of carbon deposits that can cover the reaction’s active sites, thereby decreasing the catalyst’s catalytic activity and increasing its susceptibility to deactivation. Therefore, the key to improving the catalytic performance of Ni-based catalysts in CDR reactions is to enhance the dispersion of Ni, reduce the particle size of Ni, and prevent the aggregation of Ni particles in high-temperature reforming reactions [3,8,19].

Recently, many new strategies were tried to improve the anti-sintering and anti-carbon deposition performance of Ni-based catalysts. Firstly, the presence of Ni in the support of hydrotalcite or perovskite structures can significantly increase the dispersibility of Ni [13,16]. However, due to the tendency of catalysts with such structures to collapse at higher reaction temperatures (600–900 °C), their application in CDR reactions with high-temperature reaction characteristics is limited [20,21,22]. Moreover, highly dispersed Ni-based catalysts were prepared by the confinement effect of the support, such as MCM-41 and SBA-15, which were the ordered mesoporous materials with a high specific surface area [11,23,24]. Unfortunately, Ni particles more easily sinter during high-temperature reduction and CDR processes because of the weak interaction between Ni and the support. Secondly, the preparation conditions and techniques exert a substantial influence on the particle size and dispersion of the active components in the catalyst. In an attempt to decrease the particle size of Ni and enhance its dispersibility, several novel catalyst preparation methods have been explored. These include low-temperature combustion synthesis, evaporation-induced self-assembly (EISA), and plasma-assisted treatment, as reported in references [4,8,25,26,27,28]. While these methods are effective in improving the dispersion of Ni, they involve more complicated operations and come with higher costs. Thirdly, rare-earth materials such as CeO₂, ZrO₂, La₂O₃, and Y₂O₃ [29,30,31,32] and non-precious metals (Co, Cu, Sn, and Fe) were used as additives to inhibit the aggregation and growth of Ni particles during the CDR at high-temperature [32,33,34]. Recently, the preparation of Ni-based catalysts was prepared by adding β-cyclodextrin and glucose, which significantly improved the dispersibility of Ni [24], and the additives were removed by the calcination process, resulting in highly dispersed Ni-based catalysts. CeO₂ has been proven to be an effective promoter and/or supporter of Ni-based catalysts towards the CDR, which exhibited a higher oxygen storage capacity (OSC) and thermal stability than the pure CeO₂. Moreover, a high OSC can enhance the gasification of the deposited coke over the catalyst by storing and delivering active oxygen species [33,35,36,37]. However, due to the small surface area and low thermal stability of the pure CeO₂, mixed oxides by doping CeO₂ into Al₂O₃, SiO₂ compounds such as SBA-15, ZrO₂, etc., are commonly investigated [36,38,39,40].

In this study, considering the high OSC property of the CeO₂, Ni-CeO₂/SiO₂ was prepared with the co-impregnation method, and was evaluated for the CDR. A smaller crystal size and higher dispersion of Ni was shown with the Ni/SiO₂ over the Ni-CeO₂/SiO₂. As a result of the high activity stability being obtained over the Ni-CeO₂/SiO₂, in particular, no observable deactivation was detected after 50 h of testing for the CDR. These results can be explained by the smaller Ni particle size and high OSC of CeO₂ by the redox properties of Ce⁴⁺-Ce³⁺, which effectively inhibits the sintering of Ni particles and the formation of carbon deposits, leading to a high-performance catalyst for the CDR.

2. Results and Discussion

2.1. Structural and Textural Properties of Ni-Based Catalysts

2.1.1. Structural Properties

The XRD patterns of the Ni/SiO₂ and Ni-CeO₂/SiO₂ are shown in Figure 1. They showed very similar XRD patterns and the broad peak at 2θ of 15–35° was easily assigned to the amorphous silica according to reference [21]. The well-separated diffractions at 2θ of about 37, 43, 63, and 75° were also observed, and these can be attributed to the (111), (200), (220), and (311) lattice planes of the cubic NiO [21,38]. However, for Ni-CeO₂/SiO₂, there was no observable diffraction assigned to cerium species. This situation can be attributed to two possible reasons. Firstly, these oxides might have formed with a small crystal size. Secondly, their content could have been so low that XRD was unable to detect them. Additionally, the prominent diffraction peak corresponding to the (200) plane was employed to calculate the crystal size of NiO using Scherrer’s formula. The calculated results are presented in

Table 1. Summary of the textural and crystal properties of Ni/SiO₂ and Ni-CeO₂/SiO₂.

Samples	SBET (m2·g−1) a	Vp (cm3·g−1) b	Dp (nm) c	dNiO (nm) d	dNi (nm) e	Dispersion (D%) f
SiO2	572	0.45	3.0
Ni/SiO2	352	0.36	3.5	13.4	13.7	7.1
Ni-CeO2/SiO2	382	0.38	3.5	12.8	10.3	9.4

^a: Specific surface area measured by the BET method. ^b: Total pore volumes derived using the BJH method, which is based on the analysis of adsorption curves. ^c: Average pore diameter obtained through the application of the BJH method. ^d: Calculation performed with Scherrer’s formula, utilizing the (200) diffraction peak of the fresh catalyst. ^e: Calculation performed with Scherrer’s formula, utilizing the (111) diffraction peak of the reduced catalyst. ^f: Determination according to the formula of D% = 97/d_Ni [42].

. Compared with Ni/SiO₂, which showed a slightly larger NiO crystal size of 13.4 nm, a much smaller NiO crystal size of 12.8 nm was given over Ni-CeO₂/SiO₂. These results indicate that the NiO crystal size was decreased by the addition of the mixed oxide of CeO₂, which is agreeable with the reference results [41].

Metallic Ni plays a crucial role as an active component in the CDR reaction. XRD characterization was performed on the reduced Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts to determine the crystal size of metallic Ni, and the results are presented in Figure 2. When compared with the XRD results in Figure 1, the reduced Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts still exhibited very similar XRD patterns, with an amorphous silica peak at 2θ values ranging from 15° to 35°. This indicates that the silica support remained unreduced under the applied reduction conditions. Notably, distinct diffraction peaks appeared at 2θ values of 45°, 52°, and 77°, which corresponded to the (111), (200), and (220) diffractions of metallic Ni, as reported in reference [38]. This suggests that nickel oxides could be readily reduced to metallic Ni under the reduction conditions employed. The well-resolved and intense diffraction peak at 2θ = 45° was utilized to calculate the crystal size of metallic Ni using Scherrer’s formula, and these calculated values are listed in Table 1. Additionally, the dispersion of Ni on the Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts was calculated according to the method described in reference [42], and the results are also tabulated in Table 1. A comparison of the crystal sizes revealed that the crystal size of Ni in the reduced Ni/SiO₂ catalyst was larger than that of NiO (Table 1). This phenomenon can be attributed to the aggregation of metallic Ni during high-temperature reduction, which aligns with the results of our previous research [43]. Interestingly, in the case of the reduced Ni-CeO₂/SiO₂ catalyst, the crystal size of Ni was smaller than that of NiO (Table 1), suggesting that CeO₂ effectively inhibited the aggregation of metallic Ni during high-temperature reduction [33,37]. To further investigate the size distribution of Ni particles, TEM analysis was conducted on the reduced Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts, and the results are illustrated in Figure 3. As is depicted in Figure 3, the Ni-CeO₂/SiO₂ catalyst exhibited relatively smaller Ni particle sizes and a more homogeneous particle distribution compared to the Ni/SiO₂ catalyst. These TEM observations are in good agreement with the XRD results presented in Figure 2.

2.1.2. Textural Properties of Ni-Based Catalysts

Table 1 also presents a summary of the textural properties of the Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts, which were determined through the analysis of N₂ adsorption–desorption isotherms. For the SiO₂, the BET surface area, pore volume, and pore diameter were 572 m²·g⁻¹, 0.45 cm³·g⁻¹, and 3.0 nm, respectively. After impregnation, the BET surface area and pore volume over Ni/SiO₂ and Ni-CeO₂/SiO₂ were decreased to 352–382 m²·g⁻¹ and 0.36–0.38 cm³·g⁻¹, while the pore diameter was slightly increase to 3.5 nm. Those results can be explained by the impregnation process. When the textural properties of Ni/SiO₂ and Ni-CeO₂/SiO₂ were carefully compared, the BET surface area and pore volume of Ni-CeO₂/SiO₂ was notably larger than the Ni/SiO₂, i.e., 382 m²·g⁻¹ and 0.38 cm³·g⁻¹ for Ni-CeO₂/SiO₂, and 352 m²·g⁻¹ and 0.36 cm³·g⁻¹ for Ni/SiO₂, respectively. This can be easily understandable together with the much smaller porosity of CeO₂, as reported in references [41,44].

2.2. Reduction Behavior of Ni-Based Catalysts

Figure 4 displays the H₂-TPR profiles of the Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts. For both the Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts, two distinct reduction peaks were observed: one at around 328 °C and another large, broad peak at approximately 550 °C. Previous research has established that NiO undergoes direct reduction to metallic Ni without any intermediate stages. Consequently, the reduction peaks in varying temperature ranges can be plausibly associated with the reduction of different Ni species, as reported in references [3,23,38]. Typically, the peak at lower temperatures (below 400 °C) is attributed to the reduction of larger NiO particles, as indicated in references [23,38]. Conversely, the peak at higher temperatures is considered to result from the reduction of relatively smaller NiO particles or NiO that has a strong interaction with the support, as documented in references [24,38,43]. According to the XRD results shown in Figure 1, only NiO species were identified in the Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts. Therefore, it is reasonable to conclude that the peak at the lower temperature of 328 °C corresponds to the reduction of larger NiO particles, while the primary reduction peaks at around 550 °C can be attributed to the reduction of smaller NiO particles and NiO with stronger interactions with the SiO₂ support.

When the H₂-TPR curves of the Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts were carefully compared, a reduction peak of about 550 °C was observed for Ni-CeO₂/SiO₂, which was higher than that of Ni/SiO₂ (524 °C). This phenomenon can be accounted for by the reduction of NiO with a smaller particle size, and this explanation aligns well with the findings from both XRD and TEM analyses. Moreover, a bigger reduction in the peak area, especially the reduction in the peak area at higher temperatures, was also observed over Ni-CeO₂/SiO₂. It is well known that CeO₂ can be reducible due to the reduction of Ce⁴⁺ to Ce³⁺.

2.3. Catalytic Performance

The CDR performance of Ni/SiO₂ and Ni-CeO₂/SiO₂ catalysts was investigated under the conditions of a CH₄/CO₂ ratio of 1:0, a temperature of 800 °C, a gas hourly space velocity (GHSV) of 60,000 mL·g⁻¹·h⁻¹, and a pressure of 1 atm. The time-on-stream (TOS) results are illustrated in Figure 5. Initially, all of the Ni-based catalysts exhibited a relatively high CH₄ conversion rate of approximately 88%, a value that was close to the equilibrium CH₄ conversion level. However, an obviously different stability was obtained over that of Ni/SiO₂ and Ni-CeO₂/SiO₂. Although the conversion of CH₄ was kept at 87% within a TOS of 10 h, an observable decrease in CH₄ conversion was shown over the Ni/SiO₂, and the CH₄ conversion was decreased to 70% after 50 h of the CDR, while a negligible decrease in CH₄ conversion was shown over Ni-CeO₂/SiO₂, indicating its high stability. Thus, the addition of the mixed oxide of CeO₂ into Ni/SiO₂ led to a significant increase in the CDR stability, which can be explained by the smaller Ni particle sizes and higher anti-sintering of Ni.

2.4. Characterization of Used Catalysts

It is commonly known that coking plays an important role in determining the stability of the CDR catalyst. Thus, the used Ni/SiO₂ and Ni-CeO₂/SiO₂ were subjected to TG–DSC characterization, and the results are shown in Figure 6. Both Ni/SiO₂ and Ni-CeO₂/SiO₂ show a weight increase from 200 to 500 °C, which was explained by oxidization of the metallic Ni in air under the TG condition [3,43]. Moreover, a 4.2% weight decrease was observed over used Ni/SiO₂, while a 2.1% weight decrease was observed over used Ni-CeO₂/SiO₂. These results indicate that coking can be gasified by the OSC of CeO₂, which can release free O by Ce⁴⁺ to Ce³⁺. As was expected from the CDR results, the addition of CeO₂ into Ni/SiO₂ alleviates the coking of the catalyst, leading to the much higher stability of Ni-CeO₂/SiO₂.

As has been reported in many studies [30,41], the OSC of CeO₂ can enhance the gasification of the deposited coke via the Ce⁴⁺-Ce³⁺ redox, leading to a decreased amount of coke deposited over the catalyst. This can be the main reason for alleviated coking over Ni-CeO₂/SiO₂. To further confirm this, the fresh and reduced Ni-CeO₂/SiO₂ catalysts were characterized by XPS, and the Ce3d XPS results are shown in Figure 7. As is revealed in Figure 7, the binding energies of Ce 3d5/2 and Ce 3d3/2 over the reduced Ni-CeO₂/SiO₂ were lower and broader than those of the fresh Ni-CeO₂/SiO₂. Thus, the amount of Ce³⁺ over the reduced catalyst was higher than that of the fresh catalyst, indicating that the Ce⁴⁺-Ce³⁺ redox easily occurred under the pretreatment conditions. This is consistent with the H₂-TPR results. Thus, as a result of the high OSC induced by the added CeO₂, the removal of the coke deposited over the Ni-CeO₂/SiO₂ is enhanced, leading to its significantly improved stability for the CDR.

For the CDR, besides the coke deposition, the sintering of Ni is also an important factor in determining the Ni-based catalytic stability. To reveal the impact of the sintering on the stability of the catalyst, the Ni/SiO₂ and Ni-CeO₂/SiO₂ were subjected to TEM after conducting the CDR test for 20 h, and the results are shown in Figure 8. Compared with Figure 3, obvious Ni particle sintering was detected over the used Ni/SiO₂ and Ni-CeO₂/SiO₂. However, after carefully comparing the size and distribution of Ni particles, it was observed that used Ni/SiO₂ showed the bigger particle size and widest distribution of Ni while Ni-CeO₂/SiO₂ exhibited a smaller particle size and narrower distribution of Ni. These results are exactly consistent with the CDR results. Moreover, Ni-CeO₂/SiO₂ and Ni/SiO₂ after conducting the CDR test for 50 h were in-situ regenerated with 3% O₂/Ar at 700 °C for 1 h. As is shown in Figure 9, after catalyst regeneration, the initial activity of the CDR over Ni-CeO₂/SiO₂ was fully recovered, while an obvious decrease in CH₄ conversion after regeneration was obtained over Ni/SiO₂, and with the increase in time on stream, a rapid decrease in CH₄ conversion was also shown during the 20 h CDR testing. However, the regenerated Ni-CeO₂/SiO₂ also showed high stability for 20 h. It is generally known that the small nickel particle shows super activity and coke resistance [28,45], which can be applied to explain our results. Thus, coking is the key factor in determining the stability of Ni-CeO₂/SiO₂.

3. Experimental

3.1. Catalyst Preparation

The Ni-CeO₂/SiO₂ catalyst was fabricated via the co-impregnation technique. In the Ni-based catalysts, the nickel content was fixed at 10 wt.%, and the molar ratio of Ce to Ni was set at 0.1. Prior to catalyst preparation, commercial SiO₂ (Q-15, purchased (Fujisilicia Chemical Ltd., Kyoto, Japan) underwent calcination at 500 °C for 4 h to eliminate adsorbed moisture and gases. Ni(NO₃)₂·6H₂O and Ce(NO₃)₃·6H₂O were employed as the precursors for Ni and CeO₂, respectively. Subsequently, the predetermined amounts of Ni(NO₃)₂·6H₂O and Ce(NO₃)₃·6H₂O were dissolved in distilled water. The calcined SiO₂ was then added to this solution, which was vigorously shaken for 30 min and subsequently left to stand overnight. The sample was dried at 80 °C for 12 h and then calcined at 500 °C for 4 h. Similarly, the Ni/SiO₂ catalyst was prepared using the impregnation method. The nickel content in the Ni/SiO₂ catalyst was also maintained at 10 wt.%, and the preparation process was identical, except that no ceria was included.

3.2. Catalyst Characterization

X-ray diffraction (XRD) testing was operated using a X-ray diffractor (D8 advanced, Bruker, Germany) with monochromatized Cu/Kα radiation at 40 kV and 40 mA.

The BET test was carried out on a 2460 adsorption apparatus (Micromeritics, Norcross, GA, USA). The sample was de-gassed at 300 °C for 10 h and then tested under liquid nitrogen (−196 °C) to obtain the textural properties.

X-ray photoelectron spectrometry (XPS) was performed using an X-ray photoelectron spectrometer (Kratos Analytical Ltd., Manchester, UK), which was equipped with an Al monochromatic X-ray source.

Transmission electron microscopy (TEM) images were acquired using a transmission electron microscope (JEM-2100, JEOL, Tokyo, Japan). The samples were placed on carbon-enhanced copper grids for observation.

Hydrogen temperature-programmed reduction (H₂-TPR) experiments were conducted using a chemisorption apparatus (Micromeritics Autochem 2920, USA). Initially, the sample was pre-heated in an argon (Ar) atmosphere at 300 °C for 30 min. Subsequently, the H₂-TPR process was initiated, with the temperature increasing from 50 °C to 1000 °C at a heating rate of 10 °C per minute. This was carried out under a 10% H₂/Ar gas mixture. The rate of hydrogen consumption was monitored by a thermal conductivity detector (TCD).

Thermogravimetric analysis–differential scanning calorimetry (TG–DSC) was performed on a thermoanalyzer system (TA Instruments, Q1000DSC + LNCS+FACSQ600SDT, New Castle, DE, USA). The sample was heated from room temperature to 1000 °C at a rate of 10 °C per minute, and the entire process was carried out in an air atmosphere.

3.3. Activity Evaluation of the Catalysts

The CDR catalytic performance experiments were carried out using a fixed-bed reactor (quartz tube, i.d. = 8 mm). Typically, 0.10 g of catalyst (40–60 mesh) was diluted with 0.90 g of quartz sands (40–60 mesh), which was loaded in the quartz tube by two quartz layers. Before the CDR testing, the catalyst was reduced with 50 mL/min 20% H₂/N₂ at 700 °C for 150 min, then it was purged with N₂ and heated to 800 °C. A gas mixture of CO₂ and CH₄ with a volume ratio of VCH₄/VCO₂ = 1:0 was introduced into the reactor. The CO₂ dry reforming (CDR) reaction was evaluated at 800 °C, 1 atmosphere, and a gas hourly space velocity (GHSV) of 60,000 mL·g⁻¹·h⁻¹. After condensing the water vapor in the effluent gas, it was analyzed in real time using a gas chromatograph (GC9790Ⅱ, manufactured by Zhejiang Fuli Chromatographic Analysis Co., Ltd., Zhejiang, China). The composition of the exhaust gas was determined by a thermal conductivity detector (TCD) equipped with Molecular Sieve 5A and Porapak Q capillary columns, with argon (Ar) serving as the carrier gas. F represents the total gas flow rate. The conversion of CH₄, denoted as XCH₄, was calculated according to the following formula:

$$\mathrm{X}{\mathrm{C}\mathrm{H}}_4\left(\mathrm{\%}\right)={\displaystyle \frac{\mathrm{F}{\mathrm{C}\mathrm{H}}_{4,\mathrm{i}\mathrm{n}}-\mathrm{F}{\mathrm{C}\mathrm{H}}_{4,\mathrm{o}\mathrm{u}\mathrm{t}}}{\mathrm{F}{\mathrm{C}\mathrm{H}}_{4,\mathrm{i}\mathrm{n}}}}\times 100\%$$

4. Conclusions

The effect of CeO₂ on the stability of the Ni/SiO₂ for CDR was investigated. In comparison with Ni/SiO₂, the Ni-CeO₂/SiO₂ showed a lower surface area, total pore volume, and average pore diameter. Moreover, the addition of CeO₂ into Ni/SiO₂ also decreased the crystal size of the metal Ni, while the dispersion of Ni was increased. The approximate equilibrium CH₄ conversion and a negligible deactivation were obtained over the Ni-CeO₂/SiO₂ during a CDR test of 50 h, but a clear deactivation was found in the case of the Ni/SiO₂. Importantly, the high stability of the Ni-CeO₂/SiO₂ for the CDR can be effectively attributed to the alleviated coking, where the main contributors are the redox properties of Ce⁴⁺-Ce³⁺ over the catalysts. Based on the characterization results and the CDR results over the regenerated catalyst, the excellent resistance to the sintering of Ni and significantly alleviated coking over Ni-CeO₂/SiO₂ were responsible for its high activity and stability towards the CDR.