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Article

Three-Dimensional Mesoporous Ni-CeO2 Catalysts with Ni Embedded in the Pore Walls for CO2 Methanation

by
Luhui Wang
1,*,
Junang Hu
1,2,
Hui Liu
3,
Qinhong Wei
1,
Dandan Gong
1,
Liuye Mo
4,
Hengcong Tao
1 and
Chengyang Zhang
1
1
Department of Chemical Engineering, School of Petrochemical Technology and Energy Engineering, Zhejiang Ocean University, Zhoushan 316022, China
2
School of Port and Transportation Engineering, Zhejiang Ocean University, Zhoushan 316022, China
3
School of Food and Pharmaceutical, Zhejiang Ocean University, Zhoushan 316022, China
4
Institute of Innovation & Application, Zhejiang Ocean University, Zhoushan 316022, China
*
Author to whom correspondence should be addressed.
Catalysts 2020, 10(5), 523; https://doi.org/10.3390/catal10050523
Submission received: 8 April 2020 / Revised: 2 May 2020 / Accepted: 6 May 2020 / Published: 8 May 2020
(This article belongs to the Special Issue Catalysts in Environmental and Climate Protection)
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Abstract

:
Mesoporous Ni-based catalysts with Ni confined in nanochannels are widely used in CO2 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-CeO2-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 diffraction (XRD), transmission electron microscopy (TEM) and H2 temperature programmed reduction (H2-TPR). Among the catalysts with different Ni loadings, the 50% Ni-CeO2-CSC with 50 wt % Ni loading exhibited the best catalytic performance in CO2 methanation. Furthermore, the 50% Ni-CeO2-CSC catalyst was stable for 50 h at 300° and 350 °C in CO2 methanation. The characterization results illustrate that the 50% Ni-CeO2-CSC catalyst has Ni particles smaller than 5 nm embedded in the pore walls, and the Ni particles interact with CeO2. On the contrary, the 50% Ni-CeO2-CP catalyst, prepared using the traditional coprecipitation method, is less active and selective for CO2 methanation due to the larger size of the Ni and CeO2 particles. The special three-dimensional mesoporous embedded structure in the 50% Ni-CeO2-CSC can provide more metal–oxide interface and stabilize small Ni particles in pore walls, which makes the catalyst more active and stable in CO2 methanation.

Graphical Abstract

1. Introduction

In recent years, CO2 conversion has attracted much attention [1,2]. CO2 methanation can convert CO2 and renewable H2 to storable and transportable CH4, which is of great significance for greenhouse gas control and the chemical storage of renewable H2 [3,4]. Although the exothermic methanation is thermodynamically favorable at low temperatures, there are significant kinetic limitations for converting CO2 into methane due to the stability of CO2.
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, three-dimensional 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.

2. Results and Discussion

2.1. 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 (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 m2/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.
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.
The XRD patterns of the reduced 50% Ni-CeO2-CSC and 50% Ni-CeO2-CP are shown in Figure 2b. The reduced 50% Ni-CeO2-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-CeO2-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 sintered in the process of catalyst reduction, thus forming large Ni particles. For the reduced 50% Ni-CeO2-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-CeO2-CSC catalyst is more stable than the 50% Ni-CeO2-CSC catalyst in the reduction process.
The transmission electron microscopy (TEM) images of the colloidal SiO2 and the 50% Ni-CeO2-CSC are shown in Figure 3. As shown in Figure 3a, the spherical colloidal SiO2 is about 22 nm in diameter.
Figure 3b,c present the TEM images of the fresh 50% Ni-CeO2-CSC. For the fresh 50% Ni-CeO2-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 SiO2. The diameter of the mesopore is basically the same as that of the colloidal SiO2. 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 CeO2 particles. As shown in Figure 3d, the particles with a d-spacing of 0.312 and 0.209 nm are associated with CeO2(111) and NiO(200) planes, respectively. These results indicate that the NiO in the mesoporous 50% Ni-CeO2-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-CeO2-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 CeO2, which can provide more Ni–CeO2 interface and is conducive to improving the CO2 methanation activity [32].
Figure 4 shows the H2 programmed temperature reduction (H2-TPR) profiles of the NiO and Ni-CeO2 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-CeO2-CP can be assigned to the reduction in the large NiO particles that interacted weakly with CeO2 [32]. However, the Ni-CeO2-CSC catalysts exhibit a broad peak between 300 and 600 °C, which is attributed to the reduction in the surface CeO2 and the small NiO particles that interacted with CeO2 [36].
Because of the low nickel content, the reduction peak of the 20% Ni-CeO2-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 CeO2 was stronger. Shan et al. reported that the highly dispersed NiO strongly interacted with CeO2 and had a higher reduction temperature [36].
Compared with the 50% Ni-CeO2-CP, the Ni-CeO2-CSC catalysts exhibit an obviously higher reduction temperature, indicating that the Ni–CeO2 interaction in the Ni-CeO2-CSC is stronger. The strong interaction in the Ni-CeO2-CSC catalysts could be due to the small NiO particles embedded in the pore wall and in contact with CeO2.
In previous reports, it has been shown that Ni2+ can be incorporated into the CeO2 lattice and form a Ce1-xNixO2 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-CeO2 catalysts.

2.2. Catalytic Performance

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 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.
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.
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 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.

2.3. 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 three-dimensional 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.

3. Experimental

3.1. 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 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-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.

3.2. 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 distribution was calculated from the quenched solid density functional theory (QSDFT) method using the calculation model of N2 adsorbed on carbon (slit/cylindrical/spherical pores, adsorption branch).
The X-ray diffractometer (XRD) experiments were conducted on a powder X-ray diffractometer (DX-2700, Haoyuan Corporation, Dandong, China) with a Cu Kα anode. The transmission electron microscopy (TEM) experiments were performed on a Tecnai G2 F20 microscope (FEI Company, Hillsboro, OR, USA) at 200 kV. The H2 temperature programmed reduction was performed on a TP-5080 apparatus (Xianquan, Tianjin, China) using 5% H2/Ar (30 mL min−1).

3.3. Catalytic Performance

The catalytic test was performed in a fixed-bed quartz reactor (8 mm i.d.) at 0.1 MPa. Before the reaction, a 50 mg sample mixed with 200 mg inert silica was reduced at 450 °C for 40 min with 20% H2/Ar (50 mL min−1). The reaction feed was a mixed gas (CO2/ H2/ Ar = 1/4/5, 100 mL min−1). The weight hourly space velocity (WHSV) was 120,000 mL gcat−1 h−1. The Ar was used as the internal standard gas for calculating the CO2 conversion. After the removal of the water by a cold trap, at each rection temperature, the exit gases were sampled and analyzed four times by gas chromatography (Techcomp GC-7900) with an error of less than 2%.

4. Conclusions

In this study, three-dimensional mesoporous Ni-CeO2-CSC catalysts with different Ni contents, prepared using the CSC method, were used for CO2 methanation. The Ni-CeO2-CSC catalysts had high SBETs. In the Ni-CeO2-CSC catalysts, small Ni particles were embedded in the pore walls which interacted with CeO2. This special embedded structure enables the catalyst to have more Ni–CeO2 interface, while maintaining a particle size of less than 5 nm, even when the nickel content reaches 50%. Among the Ni-CeO2-CSC catalysts with different nickel contents, the 50% Ni-CeO2-CSC catalyst showed the best catalytic performance. Compared to the 50% Ni-CeO2-CP catalyst, the 50% Ni-CeO2-CSC catalyst had a higher CO2 conversion and CH4 selectivity due to its smaller Ni particle size and more Ni–CeO2 interface. In addition, the 50% Ni-CeO2-CSC catalyst exhibited excellent stability due to the confinement effect of the embedded structure.

Author Contributions

Conceptualization, L.W.; data curation, J.H.; formal analysis, L.W., J.H., H.L., Q.W., D.G., L.M., H.T. and C.Z.; funding acquisition, L.W.; investigation, L.W. and J.H.; writing—original draft, L.W., J.H. and H.L.; writing—review and editing, H.L., Q.W. and D.G. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the Fundamental Research Funds for Zhejiang Provincial Universities and Research Institutes (No. 2019JZ00003) and the Science and Technology Foundation of Zhoushan (No. 2018C21013).

Conflicts of Interest

The authors declare no conflict of interest.

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Catalysts 10 00523 g001 550
Figure 1. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of the fresh Ni-CeO2-CSC catalysts.
Figure 1. (a) N2 adsorption–desorption isotherms and (b) pore size distributions of the fresh Ni-CeO2-CSC catalysts.
Catalysts 10 00523 g001
Catalysts 10 00523 g002 550
Figure 2. X-ray diffraction (XRD) patterns of Ni-CeO2: (a) fresh catalysts, (b) reduced catalysts.
Figure 2. X-ray diffraction (XRD) patterns of Ni-CeO2: (a) fresh catalysts, (b) reduced catalysts.
Catalysts 10 00523 g002
Catalysts 10 00523 g003 550
Figure 3. Transmission electron microscopy images of (a) the colloidal SiO2, (b–d) the fresh 50% Ni-CeO2-CSC and (e,f) the reduced 50% Ni-CeO2-CSC.
Figure 3. Transmission electron microscopy images of (a) the colloidal SiO2, (b–d) the fresh 50% Ni-CeO2-CSC and (e,f) the reduced 50% Ni-CeO2-CSC.
Catalysts 10 00523 g003
Catalysts 10 00523 g004 550
Figure 4. H2 programmed temperature reduction profiles of the NiO, the 50% Ni-CeO2-CP and the Ni-CeO2-CSC.
Figure 4. H2 programmed temperature reduction profiles of the NiO, the 50% Ni-CeO2-CP and the Ni-CeO2-CSC.
Catalysts 10 00523 g004
Catalysts 10 00523 g005 550
Figure 5. (a) CO2 conversion and (b) CH4 selectivity of the Ni-CeO2-CSC catalysts in CO2 methanation.
Figure 5. (a) CO2 conversion and (b) CH4 selectivity of the Ni-CeO2-CSC catalysts in CO2 methanation.
Catalysts 10 00523 g005
Catalysts 10 00523 g006 550
Figure 6. (a) CO2 conversion and (b) CH4 selectivity over the 50% Ni-CeO2-CSC and 50% Ni-CeO2-CP catalysts in CO2 methanation.
Figure 6. (a) CO2 conversion and (b) CH4 selectivity over the 50% Ni-CeO2-CSC and 50% Ni-CeO2-CP catalysts in CO2 methanation.
Catalysts 10 00523 g006
Catalysts 10 00523 g007 550
Figure 7. Stability of the 50%Ni-CeO2-CSC in CO2 methanation: (a) stability at 300 °C, (b) stability at 350 °C.
Figure 7. Stability of the 50%Ni-CeO2-CSC in CO2 methanation: (a) stability at 300 °C, (b) stability at 350 °C.
Catalysts 10 00523 g007
Catalysts 10 00523 g008a 550Catalysts 10 00523 g008b 550
Figure 8. (a,b) Transmission electron microscopy images and (c) X-ray diffraction pattern of the used 50% Ni-CeO2-CSC catalyst.
Figure 8. (a,b) Transmission electron microscopy images and (c) X-ray diffraction pattern of the used 50% Ni-CeO2-CSC catalyst.
Catalysts 10 00523 g008aCatalysts 10 00523 g008b
Catalysts 10 00523 sch001 550
Scheme 1. Synthesis route of the Ni-CeO2-CSC catalysts.
Scheme 1. Synthesis route of the Ni-CeO2-CSC catalysts.
Catalysts 10 00523 sch001
Table
Table 1. Physicochemical properties of the fresh Ni-CeO2.
Table 1. Physicochemical properties of the fresh Ni-CeO2.
SamplesSBET (m2/g)CeO2 (nm) aNiO (nm) a
20% Ni-CeO2-CSC134.03.9/ b
50% Ni-CeO2-CSC121.53.1/ b
80% Ni-CeO2-CSC94.9/ b3.7
50% Ni-CeO2-CP38.78.28.9
a Crystal size was calculated using the Scherrer equation according to the XRD result. b The diffraction peak is too weak.
Table
Table 2. Cristal size of the reduced 50% Ni-CeO2 catalysts.
Table 2. Cristal size of the reduced 50% Ni-CeO2 catalysts.
CatalystsCeO2 (nm) aNi (nm) a
Reduced 50% Ni-CeO2-CSC3.2/ b
Reduced 50% Ni-CeO2-CP11.824.8
a Crystal size was calculated using the Scherrer equation according to the XRD result. b The diffraction peak is too weak.
Table
Table 3. CO2 conversion rate and CH4 selectivity of the 50% Ni-CeO2-CSC catalyst and recently reported low-temperature CO2 methanation catalysts in the literature at 0.1 MPa.
Table 3. CO2 conversion rate and CH4 selectivity of the 50% Ni-CeO2-CSC catalyst and recently reported low-temperature CO2 methanation catalysts in the literature at 0.1 MPa.
CatalystWHSV
(mL.gcat−1.h−1)		Reaction Temperature (°C)XCO2 (%)CO2 Conversion Rate (×10−5 molCO2/gcat/s)Ref.
50% Ni-CeO2-CSC120,000250334.91This Work
3007010.42This Work
20% Ni-Ce/RGO36,000250201.79[40]
300807.14[40]
25% Ni/Al2O39,00025070.17[41]
300501.24[41]
15% Ni/ZrO248,000250151.61[42]
300606.43[42]
10% Ni/CeO2-ZrO220,000250460.91[43]
300551.09[43]
6% Ni/ZrO215,000250842.50[11]
6% Ni-MgO/ZrO215,000250902.68[11]

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MDPI and ACS Style

Wang, L.; Hu, J.; Liu, H.; Wei, Q.; Gong, D.; Mo, L.; Tao, H.; Zhang, C. Three-Dimensional Mesoporous Ni-CeO2 Catalysts with Ni Embedded in the Pore Walls for CO2 Methanation. Catalysts 2020, 10, 523. https://doi.org/10.3390/catal10050523

AMA Style

Wang L, Hu J, Liu H, Wei Q, Gong D, Mo L, Tao H, Zhang C. Three-Dimensional Mesoporous Ni-CeO2 Catalysts with Ni Embedded in the Pore Walls for CO2 Methanation. Catalysts. 2020; 10(5):523. https://doi.org/10.3390/catal10050523

Chicago/Turabian Style

Wang, Luhui, Junang Hu, Hui Liu, Qinhong Wei, Dandan Gong, Liuye Mo, Hengcong Tao, and Chengyang Zhang. 2020. "Three-Dimensional Mesoporous Ni-CeO2 Catalysts with Ni Embedded in the Pore Walls for CO2 Methanation" Catalysts 10, no. 5: 523. https://doi.org/10.3390/catal10050523

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