Three-Dimensional Mesoporous Ni-CeO 2 Catalysts with Ni Embedded in the Pore Walls for CO 2 Methanation

: Mesoporous Ni-based catalysts with Ni conﬁned in nanochannels are widely used in CO 2 methanation. However, when Ni loadings are high, the nanochannels are easily blocked by nickel particles, which reduces the catalytic performance. In this work, three-dimensional mesoporous Ni-CeO 2 -CSC catalysts with high Ni loadings (20 − 80 wt %) were prepared using a colloidal solution combustion method, and characterized by nitrogen adsorption–desorption, X-ray di ﬀ raction (XRD), transmission electron microscopy (TEM) and H 2 temperature programmed reduction (H 2 -TPR). Among the catalysts with di ﬀ erent Ni loadings, the 50% Ni-CeO 2 -CSC with 50 wt % Ni loading exhibited the best catalytic performance in CO 2 methanation. Furthermore, the 50% Ni-CeO 2 -CSC catalyst was stable for 50 h at 300 ◦ and 350 ◦ C in CO 2 methanation. The characterization results illustrate that the 50% Ni-CeO 2 -CSC catalyst has Ni particles smaller than 5 nm embedded in the pore walls, and the Ni particles interact with CeO 2 . On the contrary, the 50% Ni-CeO 2 -CP catalyst, prepared using the traditional coprecipitation method, is less active and selective for CO 2 methanation due to the larger size of the Ni and CeO 2 particles. The special three-dimensional mesoporous embedded structure in the 50% Ni-CeO 2 -CSC can provide more metal–oxide interface and stabilize small Ni particles in pore walls, which makes the catalyst more active and stable in CO 2 methanation.


Introduction
In recent years, CO 2 conversion has attracted much attention [1,2]. CO 2 methanation can convert CO 2 and renewable H 2 to storable and transportable CH 4 , which is of great significance for greenhouse gas control and the chemical storage of renewable H 2 [3,4]. Although the exothermic methanation is thermodynamically favorable at low temperatures, there are significant kinetic limitations for converting CO 2 into methane due to the stability of CO 2 .
A large number of studies have shown that the Rh [5], Ru [6,7], Pd [8] and Ni [9] catalysts are active in CO 2 methanation. Precious metal catalysts have a higher activity than Ni catalysts, but the high cost of precious metal catalysts limits their large-scale application in industry. Ni catalysts have a relatively low cost and have attracted extensive attention in the field of CO 2 methanation. To improve the methanation activity of Ni catalysts, many methods have been proposed to prepare highly dispersed Ni catalysts [10][11][12][13][14]. The general strategy to improve the dispersion of Ni is to load the Ni onto a support with a high specific surface area [15][16][17]. However, small particles of Ni with high dispersion tend to aggregate and grow into large particles in a long-time reaction, leading to catalyst deactivation [11,18]. Embedded structures can improve the thermal stability of Ni-based catalysts. Embedded Ni catalysts, such as Ni@HZSM-5 [19], Ni@MOF [20,21] and Ni@C [22], have been used for carbon dioxide methanation and shown good stability.
Mesoporous materials are widely used to prepare embedded catalysts [23][24][25][26]. Metal particles can be embedded in the pore channels [25] or pore walls [27]. Metal particles embedded in the pore channels are a conventional structure of embedded catalysts, but the high metal load can lead to channel blockage, which is not conducive to mass transfer [26]. On the contrary, the metal particles embedded in the pore wall can not only improve the thermal stability of the catalyst, but also avoid the blockage of the channel, which is conducive to mass transfer. Moreover, the metal particles embedded in the pore wall are in close contact with the support and have abundant metal-support interfaces, which is conducive to improving the methanation activity [28].
As CeO 2 can adsorb and activate CO 2 , Ni-CeO 2 catalysts prepared by different methods have been applied to CO 2 methanation and exhibit excellent catalytic performance [28][29][30][31][32]. Small nickel particles and abundant Ni-CeO 2 interfaces are important for improving the carbon dioxide activity of the catalyst [28,32]. Recently, a facial colloidal solution combustion (CSC) method was reported for the synthesis of three-dimensional mesoporous materials [33][34][35]. In this paper, in order to increase the Ni-CeO 2 interface and improve the thermal stability of a highly dispersed Ni-CeO 2 catalyst, three-dimensional mesoporous Ni-CeO 2 -CSC catalysts with Ni embedded in the pore walls were prepared using a colloidal solution combustion method. The catalysts had abundant metal-support interfaces and small nickel particles, showing good catalytic performance in CO 2 methanation.

Characterization of Fresh and Reduced Catalysts
The N 2 adsorption-desorption isotherms and pore size distributions of the fresh Ni-CeO 2 -CSC catalysts are shown in Figure 1. As shown in Figure 1a, all the catalysts display type IV isotherms with a hysteresis loop, indicating the presence of a mesoporous structure. When the relative pressure (P/P 0 ) approaches 1, the adsorption branch of the isotherm rises, which is due to the adsorption of nitrogen in the macropore. This indicates that a certain amount of macropore exists in these catalysts. In Figure 1b, all the catalysts exhibit a probable pore diameter centered at 20-25 nm. The probable pore diameter is similar to that of the colloidal SiO 2 (about 22 nm) used to prepare the catalyst, indicating that the mesoporous structure is mainly produced by etching the SiO 2 particles.
Catalysts 2020, 10, x FOR PEER REVIEW 2 of 12 A large number of studies have shown that the Rh [5], Ru [6,7], Pd [8] and Ni [9] catalysts are active in CO2 methanation. Precious metal catalysts have a higher activity than Ni catalysts, but the high cost of precious metal catalysts limits their large-scale application in industry. Ni catalysts have a relatively low cost and have attracted extensive attention in the field of CO2 methanation. To improve the methanation activity of Ni catalysts, many methods have been proposed to prepare highly dispersed Ni catalysts [10][11][12][13][14]. The general strategy to improve the dispersion of Ni is to load the Ni onto a support with a high specific surface area [15][16][17]. However, small particles of Ni with high dispersion tend to aggregate and grow into large particles in a long-time reaction, leading to catalyst deactivation [11,18]. Embedded structures can improve the thermal stability of Ni-based catalysts. Embedded Ni catalysts, such as Ni@HZSM-5 [19], Ni@MOF [20,21] and Ni@C [22], have been used for carbon dioxide methanation and shown good stability.
Mesoporous materials are widely used to prepare embedded catalysts [23][24][25][26]. Metal particles can be embedded in the pore channels [25] or pore walls [27]. Metal particles embedded in the pore channels are a conventional structure of embedded catalysts, but the high metal load can lead to channel blockage, which is not conducive to mass transfer [26]. On the contrary，the metal particles embedded in the pore wall can not only improve the thermal stability of the catalyst, but also avoid the blockage of the channel, which is conducive to mass transfer. Moreover, the metal particles embedded in the pore wall are in close contact with the support and have abundant metal-support interfaces, which is conducive to improving the methanation activity [28].
As CeO2 can adsorb and activate CO2, Ni-CeO2 catalysts prepared by different methods have been applied to CO2 methanation and exhibit excellent catalytic performance [28][29][30][31][32]. Small nickel particles and abundant Ni-CeO2 interfaces are important for improving the carbon dioxide activity of the catalyst [28,32]. Recently, a facial colloidal solution combustion (CSC) method was reported for the synthesis of three-dimensional mesoporous materials [33][34][35]. In this paper, in order to increase the Ni-CeO2 interface and improve the thermal stability of a highly dispersed Ni-CeO2 catalyst, threedimensional mesoporous Ni-CeO2-CSC catalysts with Ni embedded in the pore walls were prepared using a colloidal solution combustion method. The catalysts had abundant metal-support interfaces and small nickel particles, showing good catalytic performance in CO2 methanation.

Characterization of Fresh and Reduced Catalysts
The N2 adsorption-desorption isotherms and pore size distributions of the fresh Ni-CeO2-CSC catalysts are shown in Figure 1. As shown in Figure 1a, all the catalysts display type IV isotherms with a hysteresis loop, indicating the presence of a mesoporous structure. When the relative pressure (P/P0) approaches 1, the adsorption branch of the isotherm rises, which is due to the adsorption of nitrogen in the macropore. This indicates that a certain amount of macropore exists in these catalysts. In Figure 1b, all the catalysts exhibit a probable pore diameter centered at 20-25 nm. The probable pore diameter is similar to that of the colloidal SiO2 (about 22 nm) used to prepare the catalyst, indicating that the mesoporous structure is mainly produced by etching the SiO2 particles.  Table 1 lists the Brunauer-Emmett-Teller (BET) surface areas (S BET ) of the fresh Ni-CeO 2 catalysts prepared by the CSC and coprecipitation (CP) methods. For the mesoporous Ni-CeO 2 -CSC catalysts, the S BET of the catalyst decreases with the increase in nickel content. The S BET of the 50% Ni-CeO 2 -CSC is 121.5 m 2 /g, which is more than three times that of the 50% Ni-CeO 2 -CP catalyst prepared by the coprecipitation method. The results show that the mesoporous Ni-CeO 2 -CSC catalysts have larger S BETs than the 50% Ni-CeO 2 -CP catalyst. The X-ray diffraction patterns of the fresh Ni-CeO 2 are shown in Figure 2a. The fresh 20% Ni-CeO 2 -CSC exhibits the characteristic diffraction peaks of CeO 2 , and no peaks of NiO are observed. The broad diffraction peaks and low intensity could indicate small crystallites or solids with low crystallinity, such as amorphous phase. From the following HRTEM image, we found small crystallites, and no amorphous phase was observed. Therefore, the broad diffraction peaks and low intensity are due to the small crystallites. The XRD result indicates that NiO is highly dispersed in the 20% Ni-CeO 2 -CSC catalyst. When the Ni loading increased to 50%, the weak and broad diffraction peaks of CeO 2 and NiO were observed, indicating that NiO and CeO 2 particle sizes are small in the catalyst. The 80% Ni-CeO 2 -CSC catalyst exhibits an obvious NiO diffraction peak, and no peaks of CeO 2 are observed in the catalyst. The 50% Ni-CeO 2 -CP catalyst shows sharp diffraction peaks of NiO and CeO 2 , indicating that the 50% Ni-CeO 2 -CP catalyst has large NiO and CeO 2 crystal particles. Table 1 presents the crystal sizes of NiO and CeO 2 calculated by the Scherrer equation. The sizes in the Ni-CeO 2 -CSC catalyst are less than 5 nm. When the Ni loading is less than or equal to 50%, the diffraction peak of the NiO in the Ni-CeO 2 -CSC is too weak to calculate the crystal size. Even when the nickel content reaches 80%, the average NiO size is only 3.7 nm. In contrast, the NiO and CeO 2 size in the 50% Ni-CeO 2 -CP catalyst are 8.2 and 8.9 nm, respectively. The results indicate that the colloidal solution combustion is an effective method for preparing highly dispersed Ni-CeO 2 catalysts with NiO and CeO 2 particles smaller than 5 nm.  Table 1 lists the Brunauer-Emmett-Teller (BET) surface areas (SBET) of the fresh Ni-CeO2 catalysts prepared by the CSC and coprecipitation (CP) methods. For the mesoporous Ni-CeO2-CSC catalysts, the SBET of the catalyst decreases with the increase in nickel content. The SBET of the 50% Ni-CeO2-CSC is 121.5 m 2 /g, which is more than three times that of the 50% Ni-CeO2-CP catalyst prepared by the coprecipitation method. The results show that the mesoporous Ni-CeO2-CSC catalysts have larger SBETs than the 50% Ni-CeO2-CP catalyst.
a Crystal size was calculated using the Scherrer equation according to the XRD result. b The diffraction peak is too weak.
The X-ray diffraction patterns of the fresh Ni-CeO2 are shown in Figure 2a. The fresh 20% Ni-CeO2-CSC exhibits the characteristic diffraction peaks of CeO2, and no peaks of NiO are observed. The broad diffraction peaks and low intensity could indicate small crystallites or solids with low crystallinity, such as amorphous phase. From the following HRTEM image, we found small crystallites, and no amorphous phase was observed. Therefore, the broad diffraction peaks and low intensity are due to the small crystallites. The XRD result indicates that NiO is highly dispersed in the 20% Ni-CeO2-CSC catalyst. When the Ni loading increased to 50%, the weak and broad diffraction peaks of CeO2 and NiO were observed, indicating that NiO and CeO2 particle sizes are small in the catalyst. The 80% Ni-CeO2-CSC catalyst exhibits an obvious NiO diffraction peak, and no peaks of CeO2 are observed in the catalyst. The 50% Ni-CeO2-CP catalyst shows sharp diffraction peaks of NiO and CeO2, indicating that the 50% Ni-CeO2-CP catalyst has large NiO and CeO2 crystal particles. Table 1 presents the crystal sizes of NiO and CeO2 calculated by the Scherrer equation. The sizes in the Ni-CeO2-CSC catalyst are less than 5 nm. When the Ni loading is less than or equal to 50%, the diffraction peak of the NiO in the Ni-CeO2-CSC is too weak to calculate the crystal size. Even when the nickel content reaches 80%, the average NiO size is only 3.7 nm. In contrast, the NiO and CeO2 size in the 50% Ni-CeO2-CP catalyst are 8.2 and 8.9 nm, respectively. The results indicate that the colloidal solution combustion is an effective method for preparing highly dispersed Ni-CeO2 catalysts with NiO and CeO2 particles smaller than 5 nm. 20 30  The XRD patterns of the reduced 50% Ni-CeO 2 -CSC and 50% Ni-CeO 2 -CP are shown in Figure 2b. The reduced 50% Ni-CeO 2 -CP presents sharp Ni peaks, indicating that the crystal size of the Ni in it was large. Table 2 shows that the Ni size of the reduced 50% Ni-CeO 2 -CP was 24.8 nm, which is 2.8 times the NiO size of the fresh catalyst. This indicates that the Ni particles were unstable and Catalysts 2020, 10, 523 4 of 12 sintered in the process of catalyst reduction, thus forming large Ni particles. For the reduced 50% Ni-CeO 2 -CSC catalyst, the crystal size of the Ni cannot be calculated using the Scherrer equation because the diffraction peaks of the Ni are too weak, suggesting that the particle size of the Ni is small. The results indicate that the 50% Ni-CeO 2 -CSC catalyst is more stable than the 50% Ni-CeO 2 -CSC catalyst in the reduction process. The transmission electron microscopy (TEM) images of the colloidal SiO 2 and the 50% Ni-CeO 2 -CSC are shown in Figure 3. As shown in Figure 3a, the spherical colloidal SiO 2 is about 22 nm in diameter. Figure 3b,c present the TEM images of the fresh 50% Ni-CeO 2 -CSC. For the fresh 50% Ni-CeO 2 -CSC catalyst, an ordered mesopore of about 20 nm in diameter is observed in Figure 3b. During the catalyst preparation, the mesoporous structure was formed after etching the SiO 2 . The diameter of the mesopore is basically the same as that of the colloidal SiO 2 . Figure 3c shows that the pore wall thickness is about 5 nm, and the pore wall is composed of nanoparticles smaller than 5 nm. This is consistent with the XRD results. Figure 3d reveals that the pore wall contains small NiO and CeO 2 particles. As shown in Figure 3d, the particles with a d-spacing of 0.312 and 0.209 nm are associated with CeO 2 (111) and NiO(200) planes, respectively. These results indicate that the NiO in the mesoporous 50% Ni-CeO 2 -CSC catalyst is embedded in the pore wall rather than filled in the pore channel.
Figure 3e,f present the TEM images of the reduced 50% Ni-CeO 2 -CSC catalyst. Figure 3e shows the mesoporous structure of the reduced catalyst, indicating that the mesoporous structure is stable during the reduction process. As shown in Figure 3f, small particles of Ni embedded in the pore wall are in close contact with CeO 2 , which can provide more Ni-CeO 2 interface and is conducive to improving the CO 2 methanation activity [32]. Figure 4 shows the H 2 programmed temperature reduction (H 2 -TPR) profiles of the NiO and Ni-CeO 2 catalysts. NiO was prepared by calcining nickel nitrate hexahydrate at 450 • C for 4 h. For pure NiO, only one hydrogen consumption peak located at 355 • C was observed. Combined with the XRD characterization results, the reduction peak at 403 • C of the 50% Ni-CeO 2 -CP can be assigned to the reduction in the large NiO particles that interacted weakly with CeO 2 [32]. However, the Ni-CeO 2 -CSC catalysts exhibit a broad peak between 300 and 600 • C, which is attributed to the reduction in the surface CeO 2 and the small NiO particles that interacted with CeO 2 [36].
Because of the low nickel content, the reduction peak of the 20% Ni-CeO 2 -CSC catalyst is small. In addition, the reduction temperature of the catalyst is higher, which may be because the nickel particles in the catalyst were smaller and the interaction with CeO 2 was stronger. Shan et al. reported that the highly dispersed NiO strongly interacted with CeO 2 and had a higher reduction temperature [36].
Compared with the 50% Ni-CeO 2 -CP, the Ni-CeO 2 -CSC catalysts exhibit an obviously higher reduction temperature, indicating that the Ni-CeO 2 interaction in the Ni-CeO 2 -CSC is stronger. The strong interaction in the Ni-CeO 2 -CSC catalysts could be due to the small NiO particles embedded in the pore wall and in contact with CeO 2 .
In previous reports, it has been shown that Ni 2+ can be incorporated into the CeO 2 lattice and form a Ce 1-x Ni x O 2 solid solution, which leads to the formation of an oxygen vacancy [36]. The oxygen adsorbed on the oxygen vacancy in the solid solution can be reduced below 300 • C [32,36,37]. However, in the TPR patterns shown in Figure 4, there is no obvious reduction peak below 300 • C, which indicates that it is difficult to generate a solid solution in our Ni-CeO 2 catalysts.    Figure 4. H2 programmed temperature reduction profiles of the NiO, the 50% Ni-CeO2-CP and the Ni-CeO2-CSC. Figure 5 shows the catalytic performance of the Ni-CeO2-CSC catalysts. At low temperatures, the CO2 conversion and CH4 selectivity of the 20% Ni-CeO2-CSC catalyst were low. This could be due to most of the Ni surface being covered by CeO2 in the 20% Ni-CeO2-CSC catalyst. When the nickel content increased from 20% to 50%, the CO2 conversion and CH4 selectivity of the Ni-CeO2-CSC increased significantly. When the nickel content further increased to 80%, the conversion and selectivity of the catalyst decreased. The Ni-CSC catalyst with 100% Ni content showed low CO2 conversion and CH4 selectivity, especially at low temperatures. The 50% Ni-CeO2-CSC catalyst had the best catalytic performance among the Ni-CeO2-CSC catalysts. At temperatures higher than 400 °C, the conversion and selectivity of the catalyst decreased slightly, due to the limitation of the thermodynamic equilibrium of CO2 methanation and the formation of CO in the reverse water-gas shift reaction [38,39]. The comparison of the catalytic activities of the 50% Ni-CeO2-CP and 50% Ni-CeO2-CSC catalysts is shown in Figure 6. Compared with the 50% Ni-CeO2-CP, 50% Ni-CeO2-CSC had a higher CO2 conversion and CH4 selectivity in the temperature range of 250 to 450 °C. At 250 °C, the CO2 conversions of the 50% Ni-CeO2-CSC and the 50% Ni-CeO2-CP were 33% and 4%, respectively. The former is about eight times the latter, indicating that the 50% Ni-CeO2-CSC catalyst has better lowtemperature activity.  Figure 5 shows the catalytic performance of the Ni-CeO 2 -CSC catalysts. At low temperatures, the CO 2 conversion and CH 4 selectivity of the 20% Ni-CeO 2 -CSC catalyst were low. This could be due to most of the Ni surface being covered by CeO 2 in the 20% Ni-CeO 2 -CSC catalyst. When the nickel content increased from 20% to 50%, the CO 2 conversion and CH 4 selectivity of the Ni-CeO 2 -CSC increased significantly. When the nickel content further increased to 80%, the conversion and selectivity of the catalyst decreased. The Ni-CSC catalyst with 100% Ni content showed low CO 2 conversion and CH 4 selectivity, especially at low temperatures. The 50% Ni-CeO 2 -CSC catalyst had the best catalytic performance among the Ni-CeO 2 -CSC catalysts. At temperatures higher than 400 • C, the conversion and selectivity of the catalyst decreased slightly, due to the limitation of the thermodynamic equilibrium of CO 2 methanation and the formation of CO in the reverse water-gas shift reaction [38,39].  Figure 5 shows the catalytic performance of the Ni-CeO2-CSC catalysts. At low temperatures, the CO2 conversion and CH4 selectivity of the 20% Ni-CeO2-CSC catalyst were low. This could be due to most of the Ni surface being covered by CeO2 in the 20% Ni-CeO2-CSC catalyst. When the nickel content increased from 20% to 50%, the CO2 conversion and CH4 selectivity of the Ni-CeO2-CSC increased significantly. When the nickel content further increased to 80%, the conversion and selectivity of the catalyst decreased. The Ni-CSC catalyst with 100% Ni content showed low CO2 conversion and CH4 selectivity, especially at low temperatures. The 50% Ni-CeO2-CSC catalyst had the best catalytic performance among the Ni-CeO2-CSC catalysts. At temperatures higher than 400 °C, the conversion and selectivity of the catalyst decreased slightly, due to the limitation of the thermodynamic equilibrium of CO2 methanation and the formation of CO in the reverse water-gas shift reaction [38,39]. The comparison of the catalytic activities of the 50% Ni-CeO2-CP and 50% Ni-CeO2-CSC catalysts is shown in Figure 6. Compared with the 50% Ni-CeO2-CP, 50% Ni-CeO2-CSC had a higher CO2 conversion and CH4 selectivity in the temperature range of 250 to 450 °C. At 250 °C, the CO2 conversions of the 50% Ni-CeO2-CSC and the 50% Ni-CeO2-CP were 33% and 4%, respectively. The former is about eight times the latter, indicating that the 50% Ni-CeO2-CSC catalyst has better lowtemperature activity. The comparison of the catalytic activities of the 50% Ni-CeO 2 -CP and 50% Ni-CeO 2 -CSC catalysts is shown in Figure 6. Compared with the 50% Ni-CeO 2 -CP, 50% Ni-CeO 2 -CSC had a higher CO 2 conversion and CH 4 selectivity in the temperature range of 250 to 450 • C. At 250 • C, the CO 2 conversions of the 50% Ni-CeO 2 -CSC and the 50% Ni-CeO 2 -CP were 33% and 4%, respectively. The former is about eight times the latter, indicating that the 50% Ni-CeO 2 -CSC catalyst has better low-temperature activity. Small Ni particles can absorb and dissociate H2, and CeO2 can absorb and activate CO2. The Ni-CeO2 interface facilitates further reactions between the two adsorbed species to form methane [32]. The three-dimensional mesoporous catalyst prepared by the CSC method can keep the particle size of the Ni and CeO2 less than 5 nm in the catalyst with a high Ni content, and at the same time the Ni embedded in the pore wall provides an abundant Ni-CeO2 interface, meaning that the 50% Ni-CeO2-CSC catalyst has excellent activity and selectivity for CO2 methanation. On the contrary, due to the larger size of the Ni and CeO2 particles, the 50% Ni-CeO2-CP catalyst is less active and selective for CO2 methanation. The excellent catalytic performance of the 50% Ni-CeO2-CSC catalyst is related to the large number of small Ni particles and abundant Ni-CeO2 interfaces in the three-dimensional mesoporous structure.

Catalytic Performance
A comparison of catalyst activity with other Ni-based catalysts found in the literature is listed in Table 3. Compared with the Ni-based catalysts reported in the literature, the 50% Ni-CeO2-CSC catalyst exhibits an excellent CO2 conversion rate in low-temperature CO2 methanation.  [11] The stability test of the 50% Ni-CeO2-CSC catalyst was conducted at 300 and 350 °C, and the results are shown in Figure 7. The catalytic performance was stable for 50 h, and the CO2 conversions at 300 and 350 °C remained around 68% and 82%, respectively. The CH4 selectivity was higher than Small Ni particles can absorb and dissociate H 2 , and CeO 2 can absorb and activate CO 2 . The Ni-CeO 2 interface facilitates further reactions between the two adsorbed species to form methane [32]. The three-dimensional mesoporous catalyst prepared by the CSC method can keep the particle size of the Ni and CeO 2 less than 5 nm in the catalyst with a high Ni content, and at the same time the Ni embedded in the pore wall provides an abundant Ni-CeO 2 interface, meaning that the 50% Ni-CeO 2 -CSC catalyst has excellent activity and selectivity for CO 2 methanation. On the contrary, due to the larger size of the Ni and CeO 2 particles, the 50% Ni-CeO 2 -CP catalyst is less active and selective for CO 2 methanation. The excellent catalytic performance of the 50% Ni-CeO 2 -CSC catalyst is related to the large number of small Ni particles and abundant Ni-CeO 2 interfaces in the three-dimensional mesoporous structure.
A comparison of catalyst activity with other Ni-based catalysts found in the literature is listed in Table 3. Compared with the Ni-based catalysts reported in the literature, the 50% Ni-CeO 2 -CSC catalyst exhibits an excellent CO 2 conversion rate in low-temperature CO 2 methanation. The stability test of the 50% Ni-CeO 2 -CSC catalyst was conducted at 300 and 350 • C, and the results are shown in Figure 7. The catalytic performance was stable for 50 h, and the CO 2 conversions at 300 and 350 • C remained around 68% and 82%, respectively. The CH 4 selectivity was higher than 99%. The results show that the 50% Ni-CeO 2 -CSC catalyst has good stability. The embedded structure of the 50% Ni-CeO 2 -CSC catalyst can prevent the sintering or aggregation of the highly dispersed Ni during the methanation reaction, thus improving the stability of the catalyst. The excellent stability of the catalyst could be due to the pore wall embedded structure of the catalyst. 99%. The results show that the 50% Ni-CeO2-CSC catalyst has good stability. The embedded structure of the 50% Ni-CeO2-CSC catalyst can prevent the sintering or aggregation of the highly dispersed Ni during the methanation reaction, thus improving the stability of the catalyst. The excellent stability of the catalyst could be due to the pore wall embedded structure of the catalyst.

Characterization of the Used Catalyst
The 50% Ni-CeO2-CSC catalyst after the stability test at 300 °C was characterized by TEM and XRD. The three-dimensional mesoporous structure with the spherical pores of the catalyst is clearly visible in Figure 8a. As can be seen from the high-resolution TEM image (Figure 8b), Ni and CeO2 particles less than 5 nm constituted the pore wall, and the metal Ni particles were embedded in the pore wall and in close contact with CeO2. The TEM results show that the mesoporous structure and particle size did not change significantly, which further confirms the stability of the threedimensional mesoporous structure. The XRD pattern of the used 50% Ni-CeO2-CSC catalyst is shown in Figure 8c: the Ni and CeO2 peaks are broad and very weak, indicating that the Ni and CeO2 particles were small and stable in the 50% Ni-CeO2-CSC catalyst during the methanation reaction. This result further confirms that the three-dimensional embedded structure can prevent the small nickel particles from sintering or aggregating, thus making the catalyst stable in the CO2 methanation reaction.

Characterization of the Used Catalyst
The 50% Ni-CeO 2 -CSC catalyst after the stability test at 300 • C was characterized by TEM and XRD. The three-dimensional mesoporous structure with the spherical pores of the catalyst is clearly visible in Figure 8a. As can be seen from the high-resolution TEM image (Figure 8b), Ni and CeO 2 particles less than 5 nm constituted the pore wall, and the metal Ni particles were embedded in the pore wall and in close contact with CeO 2 . The TEM results show that the mesoporous structure and particle size did not change significantly, which further confirms the stability of the three-dimensional mesoporous structure. The XRD pattern of the used 50% Ni-CeO 2 -CSC catalyst is shown in Figure 8c: the Ni and CeO 2 peaks are broad and very weak, indicating that the Ni and CeO 2 particles were small and stable in the 50% Ni-CeO 2 -CSC catalyst during the methanation reaction. This result further confirms that the three-dimensional embedded structure can prevent the small nickel particles from sintering or aggregating, thus making the catalyst stable in the CO 2 methanation reaction.
99%. The results show that the 50% Ni-CeO2-CSC catalyst has good stability. The embedded structure of the 50% Ni-CeO2-CSC catalyst can prevent the sintering or aggregation of the highly dispersed Ni during the methanation reaction, thus improving the stability of the catalyst. The excellent stability of the catalyst could be due to the pore wall embedded structure of the catalyst.

Characterization of the Used Catalyst
The 50% Ni-CeO2-CSC catalyst after the stability test at 300 °C was characterized by TEM and XRD. The three-dimensional mesoporous structure with the spherical pores of the catalyst is clearly visible in Figure 8a. As can be seen from the high-resolution TEM image (Figure 8b), Ni and CeO2 particles less than 5 nm constituted the pore wall, and the metal Ni particles were embedded in the pore wall and in close contact with CeO2. The TEM results show that the mesoporous structure and particle size did not change significantly, which further confirms the stability of the threedimensional mesoporous structure. The XRD pattern of the used 50% Ni-CeO2-CSC catalyst is shown in Figure 8c: the Ni and CeO2 peaks are broad and very weak, indicating that the Ni and CeO2 particles were small and stable in the 50% Ni-CeO2-CSC catalyst during the methanation reaction. This result further confirms that the three-dimensional embedded structure can prevent the small nickel particles from sintering or aggregating, thus making the catalyst stable in the CO2 methanation reaction.

Synthesis of Catalysts
A series of Ni-CeO2-CSC catalysts were prepared with the CSC method, as shown in Scheme 1. In the solution combustion reaction, metal nitrates were used as the oxidizers, and glycine as the fuel. When preparing 1 g Ni-CeO2-CSC catalyst, 0.6 g glycine and an amount of Ce(NO3)2·6H2O, Ni(NO3)2·6H2O were dissolved in 6.3 mL deionized water, and the solution was ultrasonic for 20 min. A 1.26 mL colloidal SiO2 LUDOX TMA (Sigma-Aldrich, Saint Louis, USA; 34 wt %, diameter of 22 nm) was added to the solution and continued to be ultrasonic for 20 min. Then, the solution was heated over a hot plate at 210 °C. After a few minutes of evaporation, a combustion reaction occurred, releasing large amounts of gas and forming a solid powder. The powder was calcined at 450 °C in air for 4 hours, and was then treated with a 2 M NaOH solution at 80 °C for 4 h. After washing with ethanol and water 3 times and drying for 12 h at 80 °C, the catalyst was obtained and named as xNi-CeO2-CSC, where x represents the weight percentage of the nickel. Three catalysts with nickel content of 20%, 50% and 80% were prepared. A pure NiO catalyst, denoted as Ni-CSC, was prepared with the same method. The 50% Ni-CeO2-CP catalyst was prepared with a coprecipitation method. An NaOH solution was fed dropwise into an aqueous mixture of Ce(NO3)2 and Ni(NO3)2 until the pH of the mixture was 10. The mixture was aged at room temperature for 24 h. The obtained precipitate was filtered and washed with water, and then dried at 80 °C for 12 h. Finally, the 50% Ni-CeO2-CP was obtained by calcining the precipitate at 450 °C for 4 h.

Characterization of Catalysts
The N2 isotherms of the calcined catalyst were performed at −196 °C on a gas sorption instrument (Quantachrome, Autosorb-iQ). Before the test, the sample was degassed at 300 °C for 8 h. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area. The pore size

Synthesis of Catalysts
A series of Ni-CeO 2 -CSC catalysts were prepared with the CSC method, as shown in Scheme 1. In the solution combustion reaction, metal nitrates were used as the oxidizers, and glycine as the fuel. When preparing 1 g Ni-CeO 2 -CSC catalyst, 0.6 g glycine and an amount of Ce(NO 3 ) 2 ·6H 2 O, Ni(NO 3 ) 2 ·6H 2 O were dissolved in 6.3 mL deionized water, and the solution was ultrasonic for 20 min. A 1.26 mL colloidal SiO 2 LUDOX TMA (Sigma-Aldrich, Saint Louis, USA; 34 wt %, diameter of 22 nm) was added to the solution and continued to be ultrasonic for 20 min. Then, the solution was heated over a hot plate at 210 • C. After a few minutes of evaporation, a combustion reaction occurred, releasing large amounts of gas and forming a solid powder. The powder was calcined at 450 • C in air for 4 h, and was then treated with a 2 M NaOH solution at 80 • C for 4 h. After washing with ethanol and water 3 times and drying for 12 h at 80 • C, the catalyst was obtained and named as xNi-CeO 2 -CSC, where x represents the weight percentage of the nickel. Three catalysts with nickel content of 20%, 50% and 80% were prepared. A pure NiO catalyst, denoted as Ni-CSC, was prepared with the same method.

Synthesis of Catalysts
A series of Ni-CeO2-CSC catalysts were prepared with the CSC method, as shown in Scheme 1. In the solution combustion reaction, metal nitrates were used as the oxidizers, and glycine as the fuel. When preparing 1 g Ni-CeO2-CSC catalyst, 0.6 g glycine and an amount of Ce(NO3)2·6H2O, Ni(NO3)2·6H2O were dissolved in 6.3 mL deionized water, and the solution was ultrasonic for 20 min. A 1.26 mL colloidal SiO2 LUDOX TMA (Sigma-Aldrich, Saint Louis, USA; 34 wt %, diameter of 22 nm) was added to the solution and continued to be ultrasonic for 20 min. Then, the solution was heated over a hot plate at 210 °C. After a few minutes of evaporation, a combustion reaction occurred, releasing large amounts of gas and forming a solid powder. The powder was calcined at 450 °C in air for 4 hours, and was then treated with a 2 M NaOH solution at 80 °C for 4 h. After washing with ethanol and water 3 times and drying for 12 h at 80 °C, the catalyst was obtained and named as xNi-CeO2-CSC, where x represents the weight percentage of the nickel. Three catalysts with nickel content of 20%, 50% and 80% were prepared. A pure NiO catalyst, denoted as Ni-CSC, was prepared with the same method. The 50% Ni-CeO2-CP catalyst was prepared with a coprecipitation method. An NaOH solution was fed dropwise into an aqueous mixture of Ce(NO3)2 and Ni(NO3)2 until the pH of the mixture was 10. The mixture was aged at room temperature for 24 h. The obtained precipitate was filtered and washed with water, and then dried at 80 °C for 12 h. Finally, the 50% Ni-CeO2-CP was obtained by calcining the precipitate at 450 °C for 4 h.

Characterization of Catalysts
The N2 isotherms of the calcined catalyst were performed at −196 °C on a gas sorption instrument (Quantachrome, Autosorb-iQ). Before the test, the sample was degassed at 300 °C for 8 h. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area. The pore size The 50% Ni-CeO 2 -CP catalyst was prepared with a coprecipitation method. An NaOH solution was fed dropwise into an aqueous mixture of Ce(NO 3 ) 2 and Ni(NO 3 ) 2 until the pH of the mixture was 10. The mixture was aged at room temperature for 24 h. The obtained precipitate was filtered and washed with water, and then dried at 80 • C for 12 h. Finally, the 50% Ni-CeO 2 -CP was obtained by calcining the precipitate at 450 • C for 4 h.

Characterization of Catalysts
The N 2 isotherms of the calcined catalyst were performed at −196 • C on a gas sorption instrument (Quantachrome, Autosorb-iQ). Before the test, the sample was degassed at 300 • C for 8 h. The Brunauer-Emmett-Teller (BET) method was used to calculate the specific surface area. The pore size