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Article

CO2 Methanation over Rare Earth Doped Ni-Based Mesoporous Ce0.8Zr0.2O2 with Enhanced Low-Temperature Activity

by
Zhenglong Yang
†,
Yan Cui
†,
Pengxiang Ge
,
Mindong Chen
* and
Leilei Xu
*
Collaborative Innovation Center of Atmospheric Environment and Equipment Technology, Jiangsu Key Laboratory of Atmospheric Environment Monitoring and Pollution Control, School of Environmental Science and Engineering, Nanjing University of Information Science & Technology, Nanjing 210044, China
*
Authors to whom correspondence should be addressed.
†
These authors equally contribute to this manuscript.
Catalysts 2021, 11(4), 463; https://0-doi-org.brum.beds.ac.uk/10.3390/catal11040463
Submission received: 16 March 2021 / Revised: 30 March 2021 / Accepted: 30 March 2021 / Published: 1 April 2021
(This article belongs to the Special Issue Metal Nanoparticle Catalysis)
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Abstract

:
The Ni-based catalysts have a wide range of industrial applications due to its low cost, but its activity of CO2 methanation is not comparable to that of precious metal catalysts. In order to solve this problem, Ni-based mesoporous Ce0.8Zr0.2O2 solid solution catalysts doped with rare earth were prepared by the incipient impregnation method and directly used as catalysts for the methanation of CO2. The catalysts were characterized systematically by X-ray powder diffraction (XRD), N2 physisorption, transmission electron microscopy (TEM), energy-dispersed spectroscopy (EDS) mapping, X-ray photoelectron spectroscopy (XPS), H2 temperature programmed reduction (H2-TPR), CO2 temperature programmed desorption (CO2-TPD), and so on. The results show that Ni is highly dispersed in the mesoporous skeleton, forming a strong metal-skeleton interaction. Therefore, under the condition of CO2 methanation, the hot sintering of metallic Ni nanoparticles can be effectively inhibited so that these mesoporous catalysts have good stability without obvious deactivation. The rare earth doping can significantly increase the surface alkalinity of catalyst and enhance the chemisorption of CO2. In addition, the rare earth elements also act as electron modifiers to help activate CO2 molecules. Therefore, the rare earth doped Ni-based mesoporous Ce0.8Zr0.2O2 solid solution catalysts are expected to be an efficient catalyst for the methanation of CO2 at low-temperature.

1. Introduction

In the past hundred years, with the growth of the world population and the development of the global economy, CO2 emissions from the burning of fossil fuels have been on the rise [1,2]. Therefore, the massive emissions of CO2 have also led to many environmental problems such as global warming, sea level rise, polar ice melting and so on [3]. It is reported that CO2 in the atmosphere is threatening the Earth’s atmospheric system [4,5]. However, CO2 is also considered to be one of the largest and cheapest carbon sources in the world, and the treatment and disposal technology of CO2 has attracted worldwide attention [6]. Paui Sabatier first proposed the methanation of the CO2 reaction (CO2 (g) + 4H2 (g) → CH4 (g) + 2H2O (g), ∆H298K = −165.4 kJ/mol, ∆G298K = −130.8 kJ/mol) in 1897 [5]. This reaction can not only realize the utilization of CO2 resources, but also has potential economic and environmental value, which is a widely studied topic [7]. Hashimoto et al. [4] also proposed a global CO2 cycle strategy to solve the problem of global CO2 emissions. Due to the exothermic properties of carbon dioxide, methanation of CO2 can be carried out at low temperature. However, this process is a kinetic-controlled reaction due to the high activation energy of the stable CO2, which will cause a slow reaction rate and poor catalytic activity in low-temperature regions [8]. Therefore, in order to achieve high activity of CO2 methanation at low temperature, it is very important to use a highly efficient catalyst. In addition, due to the strong exothermic properties of the CO2 methanation catalyst bed, hot spots often exist in the catalyst bed during the reaction process, which leads to the hot sintering of the metal active center [9,10].
In the catalysts for CO2 methanation, the supported metals of Group VIII have been proved to be active. The Ru, Rh, and Ni are the most studied active metals due to their excellent catalytic activity [11,12]. The Ru and Rh-based catalysts’ activity at low temperature is far better than that of Ni-based catalysts, but the preparation cost is higher, thus limiting its large-scale application in industry. Thus, Ni-based catalysts are extensively studied and recognized as the most active non-noble metal catalysts because they usually perform comparable activities to the noble metal catalysts at high temperature [13,14]. However, the active site of Ni metal is prone to hot sintering and can quickly deactivate the catalyst [15,16]. In addition, the low temperature activity of Ni-based catalysts is not as good as noble metal catalysts because of distinguished d and s outer layer orbits. Therefore, it is of great significance to study and develop Ni-based CO2 methanation catalysts with high activity and high stability [17,18].
Enhancing the activity of carbon dioxide at low temperatures and constructing strong metal-carrier interaction (SMSI) are potential solutions [19,20]. Therefore, the activation of CO2 can be enhanced by changing the surface alkalinity and metal-carrier interactions by doping rare earth elements (La, Pr, Yb, Sm, etc.) [21,22,23] as catalytic agents. Compared with other elements, rare earth elements, due to their unique d-orbital electronic structure, can not only be used as a basic modifier for the metal active site, but can also regulate the electronic properties of the metal active site [24]. Zhi et al. [25] successfully prepared Ni/SiC and La2O3 modified Ni/SiC by the impregnation method, and used for the CO2 methanation reaction. The results showed that La2O3 modified Ni/SiC had better catalytic activity and stability than pure Ni/SiC. They found that La2O3 could effectively inhibit the growth of nano-sized NiO, improve the dispersion of NiO, and enhance the interaction between NiO and SiC. The La2O3 could also change the electronic environment around Ni atoms, so that CO2, the reactant on Ni atoms, was easier to activate. Takano et al. [26] found that compared with the Ni–Zr catalyst, the Ni–Zr–Sm catalyst showed higher CO2 methanation activity. The main reason is that the Zr4+ ion is replaced by the Sm3+ ion to form tetragonal ZrO2, and the oxygen vacancy may interact strongly with the oxygen atom in the CO2 molecule, thus weakening the strength of the C=O bond, leading to the enhancement of the hydrogenation of CO2 to form CH4 and H2O. Therefore, rare earth doped Ni based catalysts are promising catalysts toward CO2 methanation.
It is well known that the performance of the catalyst also depends on the characteristics of the carrier to a large extent as the catalytic carrier will also significantly affect the various properties of the catalyst such as metal-support interaction [27], acid-base performance [28], redox performance [29], etc. Oxides such as SiO2 [30], Al2O3 [31], ZrO2 [32], and CeO2 [33] have been widely used as carriers of Ni-based CO2 methanation catalysts. Among them, CeO2 shows a unique redox performance and high sample capacity [34,35,36], which is of great significance for the activation of carbon dioxide in the process of catalytic methane. However, due to the poor stability of pure CeO2, its redox effect will be lost with the increase in temperature. It has been reported that Zr atoms can greatly stabilize the CeO2 matrix by combining the CeO2 lattice with isomorphic substitution [37,38]. In addition, due to the difference in ionic radii between Ce4+ (0.97 A) and Zr4+ (0.84 A), the addition of ZrO2 will cause serious deformation of the crystal structure and form defects in the crystal structure. Therefore, this can promote the activation of CO2 in the methanation of CO2.
Herein, the rare earth (La, Sm, Pr, Yb) doped Ni-based bimetal catalysts supported on the mesoporous Ce0.8Zr0.2O2 solid solution (Ce/Zr = 80/20 molar ratio) were synthesized and utilized as the catalysts for CO2 methanation to further improve the low-temperature catalytic performance. It can be found that the Ni species are highly dispersed in mesoporous skeleton and forming strong metal-skeleton interaction. Therefore, under the condition of CO2 methanation, the hot sintering of metallic Ni nanoparticles can be effectively inhibited so that these mesoporous catalysts have good stability without obvious deactivation. Rare earth doping enhances the chemisorption of CO2 by increasing the surface alkalinity of the catalyst. In addition, the addition of rare earth elements can help activate carbon dioxide molecules. A series of characterization methods (XRD, TEM, N2 physical adsorption, XPS, CO2-TPD, H2-TPR) were used to characterize the catalyst in detail, which will be discussed in this paper.

2. Results and Discussion

2.1. Characterizations of the Catalysts

2.1.1. X-ray Diffraction (XRD) Analysis

The XRD analyses were carried out to determine the crystalline phases of the catalysts. The XRD patterns of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts are shown in Figure 1. It was noteworthy that all the catalysts displayed six different diffraction peaks at 28.5°, 33.0°, 47.7°, 56.6°, 69.8°, and 77.6°, which were ascribed to (1 1 1), (2 0 0), (2 2 0), (3 1 1), (4 0 0), and (3 3 1) crystalline planes, respectively. This indicated that M-Ce80Zr20 support (Ce0.8Zr0.2O2) was a cubic crystalline structure (PDF-#-34-0394). Moreover, the absence of the ZrO2 diffraction peak indicated that ZrO2 was successfully integrated into the CeO2 lattice to form a solid solution while maintaining the crystal structure of fluorite [39]. As for the NiO (PDF-#-78-0429), it was interesting to observe that the NiO peak strength of the 15Ni2R/M-Ce80Zr20 catalysts were much weaker than that of 15Ni/M-Ce80Zr20, demonstrating better NiO dispersion over the 15Ni2R/M-Ce80Zr20 catalysts. Among them, the NiO diffraction peak of the 15Ni2La/M-Ce80Zr20 catalyst was the weakest, which also indicated that the doping of rare earth La was most conducive to the dispersion of NiO. Therefore, the characteristic peaks of the rare earth were not observed over these doped 15Ni2R/M-Ce80Zr20 catalysts because of the high dispersion and low loading amount.

2.1.2. N2 Physisorption Analysis

The structural properties of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts were investigated by N2 physisorption. The N2 adsorption-desorption isotherms and pore size distributions of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts are displayed in Figure 2. As can be observed in Figure 2a, all the catalysts displayed the type IV H2-shaped isotherms based on the IUPAC classification [40]. Therefore, the catalysts still maintained the mesoporous structure after loading the metal and calcination. Furthermore, the mesoporous catalysts in Figure 2b showed the pore diameter distribution with the peaks in the 9.9 nm range. Compared with the 15Ni/M-Ce80Zr20 catalyst, the 15Ni2R/M-Ce80Zr20 catalysts displayed slightly bigger pore diameters. This indicated that the shrinkage of the mesoporous framework was effectively inhibited after the preparation and calcination of the catalysts [40].
Furthermore, the values of the specific surface areas, pore volumes, and average pore diameters of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts are summed up in Table 1. It was noted that the specific surface area of the 15Ni2R/M-Ce80Zr20 catalyst was larger than that of the 15Ni/M-Ce80Zr20 catalyst. This suggested that the loading of the Ni and the rare earth caused the partial blockage of the mesoporous channels of the catalysts. However, the average pore diameters of these catalysts did not suffer any decrease. This demonstrated that the catalyst successfully retained the mesoporous skeleton and provided sufficient thermal stability.

2.1.3. TEM, STEM and EDS-Mapping Analyses

The TEM characterization of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts were performed to determine the dispersions of surface NiO. The TEM images are displayed in Figure 3. As can be observed in Figure 3, the NiO nanoparticles of around 10.0 nm were highly dispersed among the M-Ce80Zr20 with wormlike mesoporous channels. As for the high-resolution TEM images of these catalysts, the lattice fringe spacing around 0.24 nm, 0.21 nm, and 0.15 nm could be observed over all the catalyst surfaces, which were ascribed to the (1 1 1), (2 0 0), and (2 2 0) planes of NiO, respectively.
The spatial distribution of elements in the mesoporous structure of the catalyst was studied by using scanning transmission electron microscopy (STEM) and EDS-mapping spectra. Figure 4 presents the STEM-EDS element mapping images. It could be seen that Ce, Ni, Zr, and the rare earths (La, Sm, Pr, Yb) were evenly dispersed in the catalyst, which indicated that the Ni and rare earth doping had been evenly dispersed on the surface of M-Ce80Zr20.

2.1.4. H2-TPR Analysis

The H2-TPR analyses were conducted to investigate the metal-support interactions of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts by comparing the reduction peak positions and the shapes of the profiles in Figure 5. As can see from the figure, the main reduction peak appeared in the range of 447.0 °C to 480.0 °C, which should be attributed to NiO and rare earth with strong metal-support interaction. There was only one reduction peak in the H2-TPR profile, indicating the homogeneity of Ni species in the entire mesoporous skeleton. In addition, compared with the original 15Ni/M-Ce80Zr20, the reduction peak of the rare earth doped 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts moved to the low temperature region, suggesting that the existence of rare earth enhanced the reducibility of Ni2+ in 15Ni2R/M-Ce80Zr20.

2.1.5. CO2-TPD Analysis

The CO2-TPD analyses of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts were conducted to characterize the surface alkalinity of the catalysts. It is universally accepted that the CO2 chemisorbed over the strong alkaline sites could be desorbed at high temperature because the strength of the catalyst basicity could be closely related to the desorption temperature [41]. As can be seen from Figure 6, the CO2-TPD profile shapes of all these catalysts were similar. Specifically, they found two groups of CO2 desorption peaks around 155.0 °C and 350.0 °C, suggesting that both weak (155 °C) and strong (350.0 °C) alkaline sites with different intensities existed in these catalysts [42,43]. In addition, compared with 15Ni/M-Ce80Zr20, the rare earth doped 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts of the CO2 desorption peak was stronger, demonstrating that the rare earth greatly increased the number of surface basic sites. This proved that rare earth (La, Sm, Pr, Yb) could promote the improvement of surface basic sites [44,45,46,47,48]. The surface basic sites enhanced by rare earth contributed to the low temperature activation of CO2 methanation.

2.1.6. X-ray Photoelectron Spectroscopy (XPS) Analysis

The XPS measurement of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts were applied to obtain the detailed coordination states and more detailed information about the surface elements. The Ce 3d, Zr 3d, Ni 2p, and O 1s XPS profiles of 15Ni2R/M-Ce80Zr20 are shown in Figure 7. The Ce 3d spectrum in Figure 7a was divided into eight overlapping peaks. According to previous reports, these peaks could be divided into Ce 3d5/2 (V, V′, V″, V‴) and Ce 3d3/2 (U, U′, U″, U‴) [49]. The U (901.0 eV), U″ (908.0 eV), U‴ (917.0 eV) and V (882.0 eV), V″ (889.0 eV), V‴ (901.0 eV) peaks were derived from the Ce4+ cation in +4 valence. These were caused by electrons moving from a complete O 2p orbital to an empty Ce 4f orbital. The presence of U′ and V′ in the Ce 3d spectrum indicated that Ce3+ exists in the +3 valence state [50,51,52]. Therefore, it could be concluded that there were +4 and +3 cerium surface types of the 15Ni2R/M-Ce80Zr20 catalysts studied, and Ce4+ was the predominant species. The Zr 3d spectra of catalysts are displayed in Figure 7b. As could be observed, almost all catalysts displayed the Zr 3d spectra with Zr 3d5/2 peaks centered around 182.0 eV, which corresponded to the Zr4+ in the +4 oxidation state [53]. From the O 1s spectra of these catalysts in Figure 7c, it was noteworthy that almost all catalysts had main peaks and shoulder peaks at 530.0 eV and 533.0 eV, respectively, indicating the presence of O2−. The appearance of acromion was related to the nonequivalence of the oxidation environment. It has been reported that the acromial peak near 533.0 eV corresponds to O2− bound with Ce3+. As a result, the oxygen vacancy was formed around the Ce3+ species to maintain the charge neutrality [49,54]. Thus, the O 1s shoulder peak at higher binding energy could be generated. The Ni 2p spectra of these catalysts are shown in Figure 7d. It can be clearly seen that each catalyst had a Ni 2p3/2 main peak at 854.0 eV and a Ni 2p1/2 main peak at 872.0 eV. Compared with pure NiO (Ni 2p3/2, 853.6eV), the 15Ni2R/M-Ce80Zr20 catalysts had a higher binding energy [55,56]. Therefore, the above H2-TPR analysis confirmed that the oxidation state of Ni species in these catalysts mainly existed in the form of Ni2+, and there was a strong metal–support interaction.
Furthermore, the XPS spectra of the Pr 3d, Sm 3d, Yb 4d, and La 3d of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts are shown in Figure 8. As showen in Figure 8a, the Pr 3d spectra of the 15Ni2Pr/M-Ce80Zr20 catalyst displayed a Pr 3d5/2 peak at 933.0 eV. This was different from the Pr 3d5/2 peaks of PrO2 (935.0 eV) and Pr2O3 (932.9 eV), and might be a mixture of PrO2 and Pr2O3 [57]. During the methanation of CO2, the change in valence state would produce an oxygen vacancy and activate the C=O bond of CO2. The Sm 3d spectrum of 15Ni2Sm/M-Ce80Zr20 (Figure 8b) showed the Sm 3d5/2 peak near 1082.0 eV and the Sm 3d3/2 peak near 1110.0 eV. This indicated that the Sm element existed in the form of Sm3+ in the 15Ni2Sm/M-Ce80Zr20 catalyst [57]. The Yb 4d spectrum in Figure 8c of 15Ni2Yb/M-Ce80Zr20 displayed the Yb 4d5/2 centered at 183.0 eV, suggesting the presence of Yb3+ cation species in the +3 valence [58]. As can be seen in Figure 8d, the peak of La 3d5/2 was located at about 834.0 eV, and the peak of the satellite was located at about 838.0 eV. The ΔE between the two was about 4.0 eV, which was the characteristic feature of the La3+ in La2(CO3)3. The surface La2O3 and CO2 interacted to form La2(CO3)3, when the 15Ni2La/M-Ce80Zr20 catalyst was exposed to the atmosphere [59].
In addition, the binding energies of the surface elements of the catalysts based on XPS in Table 2. The results showed that almost all the catalysts had the same Ni 2p3/2, O 1s, Ce 3d5/2, and Zr 3d5/2 binding energies of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts. As shown in Table 3, it could be seen that rare earth doped Ni-based mesoporous Ce0.8Zr0.2O2 catalysts had different O 1s shoulder peak area ratios, which further affects the catalytic performance of CO2 methanation. Among them, the O 1s shoulder peak area of the 15Ni2La/M-Ce80Zr20 catalyst was larger than that of the other catalysts, which indicated that the surface redox performance of the 15Ni2La/M-Ce80Zr20 catalyst was obviously better than that of other catalysts in terms of the number of oxygen vacancies on the surface.

2.2. Catalytic Performance toward CO2 Methanation

2.2.1. The Promoting Effect of Rare Earth Doping on Catalytic Activity

The catalytic activity of 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts at different reaction temperatures was investigated under the specified conditions (H2/CO2 = 4, GHSV = 12,000 mL/(g∙h), 1 atm), and their catalytic performances are shown in Figure 9. As can be seen from Figure 9a, the rare earth doped 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts had the higher CO2 conversion rate than that of the original 15Ni/M-Ce80Zr20catalyst. Specifically, 15Ni2La/M-Ce80Zr20 had a much higher CO2 conversion rate than the other catalysts, especially in the low temperature range. In addition, it can be found that the trend of the CO2 conversion curve of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts were completely different from that of the equilibrium CO2 conversion curve. This was mainly due to the contradiction between kinetics and thermodynamics. In particular, according to previous literature [60], CO2 methanation is controlled by the kinetics at low temperatures due to the inert property of the stable CO2 molecule, which is then controlled by thermodynamics at high temperature. Although the lower reaction temperature was theoretically conducive to achieving a higher equilibrium CO2 conversion rate, the large kinetic barrier limited the methanation of CO2 activity at low-temperature. Therefore, the doping of rare earth can greatly improve the CO2 methanation activity of Ni-based catalysts, and La was considered to be the best rare earth modifier among these rare earths. The CH4 selectivity of 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts at different temperatures is shown in Figure 9b. It can be seen that with the increase in reaction temperature, the equilibrium selectivity of CH4 gradually decreased, which was due to the generation of CO gas at the water gas shift (RWGS) side of the reaction, which reduces the selectivity of CH4 [9,61]. In addition, the CH4 selectivity of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts were higher than that of the 15Ni/M-Ce80Zr20 catalyst. This indicates that rare earth elements also promoted the selectivity of CH4.
In general, these rare earth doped 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts have higher catalytic activity at low temperature than the original 15Ni/M-Ce80Zr20 catalyst. The results showed that the rare earth dopants (La) played an important role in improving the catalytic activity of the catalysts at low temperature.

2.2.2. Catalytic Stability Tests

An important problem for CO2 methanation is to conduct long-term stability experiments on catalysts because the exothermic property, the hot sintering of metal Ni active site, led to rapid subsequent deactivation. The 40 h catalytic stability tests at 400 °C were also conducted over 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts under the specified conditions (H2/CO2 = 4, GHSV = 12,000 mL/(g∙h), 1 atm) and their profiles are exhibited in Figure 10. As shown in Figure 10a, the CO2 conversions over the 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts did not decrease after 40 h stability tests. Furthermore, the 15Ni2La/M-Ce80Zr20 catalyst doped with rare earth also displayed higher activity than the 15Ni/M-Ce80Zr20 single metal counterparts. Its excellent performance should be attributed to the limiting effect of mesoporous skeleton on the stabilization of metal Ni active sites, forming a strong metal-skeleton interaction in the mesoporous skeleton. Similarly, it was noticeable in Figure 10b that the CH4 selectivity over the 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts were also stable during the stability tests, suggesting that severe hot sintering of the metal Ni active center had been effectively avoided. The stable catalytic activities and selectivity over the 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts could be attributed to the outstanding sintering-proof properties. In general, the 15Ni2La/M-Ce80Zr20 catalyst was provided with not only advanced low-temperature activity, but also outstanding catalytic stability. Figure 11 shows the XRD analysis of the spent 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts after 40 h long-term stability tests. The results showed that the catalyst exhibited the characteristic peak of Ni (PDF-#-45-1027) after the stability test. Moreover, the metal Ni peak of 15Ni2La/M-Ce80Zr20 was weaker than that of 15Ni/M-Ce80Zr20, which indicated that 15Ni2La/M-Ce80Zr20 had better dispersion of metal Ni. The FWHM reflected the diameter of metal Ni in the catalysts. The smaller the diameter, the larger the FWHM that can be obtained. It could be obtained that the FWHM of the metal Ni diffraction peak of 15Ni2La/M-Ce80Zr20 was higher than that of 15Ni/M-Ce80Zr20, which indicated that the metal Ni in 15Ni2La/M-Ce80Zr20 was smaller and could be uniformly distributed in the catalysts. Therefore, due to the mesoporous structure of M-Ce80Zr20, 15Ni2La/M-Ce80Zr20 showed excellent thermal sintering resistance.

3. Experimental and Methods

3.1. Catalyst Preparation

3.1.1. The Fabrication of the Mesoporous Ce–Zr Solid Solution Support

The mesoporous Ce–Zr solid solution material (Ce0.8Zr0.2O2) was synthesized by the EISA strategy reported elsewhere [38,62]. In the preparation process, 1.0 g of P123 (Molecular Weight (MW) = 5800, EO20PO70EO20, Aladdin, Shanghai, China) was first dissolved in 20.0 mL of anhydrous ethanol (C2H5OH) and the P123 was completely dissolved by strong magnetic stirring. Then, 8 mmol of Ce(NO3)3·6H2Oand 2 mmol of ZrOCl2·8H2O were added to the above solution and magnetic agitation was performed with strong force until the solution was transparent. The transparent solution was stirred into a culture dish and covered with a PE membrane with small holes. The final solution was moved to a culture dish and placed in a 100 °C convection oven with relative humidity less than 50% for solvent volatilization and template assembly. After experiencing the 48 h EISA process, the golden yellow dried gel in the culture dish was transferred to the crucible and calcined at 500 °C for 5 h. The final mesoporous Ce–Zr solid solution with the 80/20 Ce/Zr molar ratio was abbreviated as M-Ce80Zr20.

3.1.2. The Preparation of the Catalysts

The Ni(NO3)2·6H2O, La(NO3)3·6H2O, Sm(NO3)3·6H2O, Pr(NO3)3·6H2O and Yb(NO3)3·5H2O were selected as the metal precursors and all catalysts were prepared by the incipient impregnation method using the M-Ce80Zr20 as the support. The weight percentages of the Ni and rare earth (R = La, Sm, Pr, Yb) were controlled at 15.0 wt.% and 2.0 wt.%, respectively. The catalyst precursors were dried at 100 °C in a convection oven overnight and then calcined at 500 °C for 5 h. The final bimetal catalysts with 15.0 wt.% Ni and 2.0 wt.% rare earth were denoted as 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb).

3.2. Catalyst Characterizations

The XRD patterns of the catalysts were recorded on a XRD-6100 diffractometer (Shimadzu, Kyoto, Japan) with Cu Kα radiation and scanning rate of 3°/min in the range of 20–80°.
The N2 physisorption analysis was carried out on an Autosorb-iQ-AG-MP instrument (Quantachrome, Boynton Beach, US). The Brunauer–Emmett–Teller (BET) specific surface area, Barret–Joyner–Halenda (BJH) pore volume, and pore size distribution of the sample were calculated based on the N2 adsorption branch of the isotherm.
The TEM images, STEM images, and EDS mapping were taken on a high-resolution transmission electron microscopy (FEI TECNAI G2 F20, Hillsboro, OR, USA). The sample was uniformly dispersed in absolute ethanol with the assistance of ultrasonic and then dropped on the copper grid sample holder coated with a carbon layer.
The valence state and composition of the surface elements in the catalysts were characterized by XPS. The binding energy of the catalyst was calibrated by using C1s (284.5 eV) as the internal standard.
The H2-TPR analyses of the catalysts were carried out on a micro fixed reactor with a LC-D200 mass spectrometer (TILON, US) as the detector. In a specific test, 200 mg of sample was loaded in a quartz tube (i.d. = 5.0 mm). The sample was first purged with Ar (20.0 mL/min) at 300 °C for 30 min before the regular analysis. Then, the 5 vol.% H2-95 vol.% Ar mixture stream (50.0 mL/min) was introduced. After the baseline of the H2 mass signal (m/z = 2) was stable, the temperature was programmatically increased from 25 °C to 800 °C with the rate of 20 °C /min and the H2 mass signal was recorded with the LC-D200 mass spectrometer during the whole H2-TPR process.
The CO2-TPD analyses of the samples were analyzed on the same apparatus of the H2-TPR described above. Specifically, 100 mg of catalyst was loaded and absorbed CO2 in a CO2 stream (10.0 mL/min) at 25 °C for 30 min. Then, the sample was purged by the Ar stream until the CO2 baseline (m/z = 44) was stable. Finally, the sample was programmatically heated from 25 °C to 800 °C with a ramp of 20 °C/min under Ar stream (40.0 mL/min) and the CO2 signal during the CO2-TPD was recorded by the mass spectrometer.

3.3. Catalyst Evaluation

Catalytic activity tests were conducted in a micro fixed bed reactor equipped with a quartz tube (i.d. = 10 mm) and the reaction quartz tube was placed vertically in a tube furnace. The temperature of the reaction was detected and controlled by the thermocouple located in the center of the catalyst bed. The gas flows of the feed gases were controlled by the mass flow controllers (MFC, Brooks Instrument, Hatfield, UK) and the ratio of the reactants (H2/CO2) was controlled at 4/1 without any dilution. The reaction pressure is controlled at 1.0 atm and the reaction temperature was investigated in the range of 200–450 °C. For each test, 100 mg of catalyst was loaded and the space velocity was set as 12,000 mL/(g∙h). The 40 h stability tests were performed at 400 °C with the GHSV of 12,000 mL/(g·h). Before the catalytic reactions, the catalyst in the reactor was first pre-reduced with H2/N2 = (10.0/10.0 mL/min) at 500 °C for 3 h. A Perkin Elmer GC-680 gas chromatograph equipped with a thermal conductivity detector (TCD) and a hydrogen flame detector (FID) was used for online analysis of the products.

4. Conclusions

The rare earth (R = La, Sm, Pr, Yb) doped Ni-based mesoporous Ce0.8Zr0.2O2 were prepared via the incipient co-impregnation method for CO2 methanation. Compared with the Ni-based single catalyst, the rare earth doped catalysts showed much higher low-temperature activities. The addition of rare earth greatly improved the surface basicity and electronic properties of the catalysts, and promoted the chemisorption and activation of CO2. Thus, the results showed that the 15Ni2La/M-Ce80Zr20 catalyst had the optimum catalytic performance. In addition, by limiting the Ni species in the mesoporous carrier, a strong interaction between the active site of Ni and the mesoporous skeleton is formed, which can effectively stabilize the metallic nickel nanoparticles through the restriction effect. In addition, the 15Ni2La/M-Ce80Zr20 and 15Ni/M-Ce80Zr20 catalyst had excellent stability without any deactivation after 40 h tests. In summary, a series of the rare earth doped Ni-based mesoporous Ce0.8Zr0.2O2 catalysts prepared in this study can be used as CO2 methanation catalysts with strong low temperature activity.

Author Contributions

Conceptualization, Z.Y.; Data curation, Z.Y. and Y.C.; Formal analysis, P.G.; Funding acquisition, M.C.; Investigation, Z.Y. and Y.C.; Methodology, Z.Y. and Y.C.; Project administration, M.C.; Resources, M.C.; Supervision, M.C.; Validation, L.X.; Writing—original draft, Z.Y.; Writing—review and editing, Y.C. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by the National Natural Science Foundation of China (grant number 21976094); and the National Key Research and Development Project (grant number 2018YFC0213802).

Data Availability Statement

The data supporting the findings of this study are available by reasonable request to [email protected].

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. X-ray diffraction (XRD) patterns of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
Figure 1. X-ray diffraction (XRD) patterns of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
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Catalysts 11 00463 g002 550
Figure 2. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
Figure 2. N2 adsorption-desorption isotherms (a) and pore size distributions (b) of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
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Catalysts 11 00463 g003 550
Figure 3. Transmission electron microscopy (TEM) images of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts: (a,b) 15Ni2La/M-Ce80Zr20, (c,d) 15Ni2Sm/M-Ce80Zr20, (e,f) 15Ni2Yb/M-Ce80Zr20, (g,h) 15Ni2Pr/M-Ce80Zr20.
Figure 3. Transmission electron microscopy (TEM) images of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts: (a,b) 15Ni2La/M-Ce80Zr20, (c,d) 15Ni2Sm/M-Ce80Zr20, (e,f) 15Ni2Yb/M-Ce80Zr20, (g,h) 15Ni2Pr/M-Ce80Zr20.
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Catalysts 11 00463 g004 550
Figure 4. STEM-EDS element mapping images showing the spatial distribution of Ce, Zr, Ni, and R (R = La, Sm, Pr, Yb) elements: (a) 15Ni2Pr/M-Ce80Zr20, (b) 15Ni2Yb/M-Ce80Zr20, (c) 15Ni2Sm/M-Ce80Zr20, (d) 15Ni2La/M-Ce80Zr20.
Figure 4. STEM-EDS element mapping images showing the spatial distribution of Ce, Zr, Ni, and R (R = La, Sm, Pr, Yb) elements: (a) 15Ni2Pr/M-Ce80Zr20, (b) 15Ni2Yb/M-Ce80Zr20, (c) 15Ni2Sm/M-Ce80Zr20, (d) 15Ni2La/M-Ce80Zr20.
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Catalysts 11 00463 g005 550
Figure 5. H2-TPR profiles of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
Figure 5. H2-TPR profiles of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
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Catalysts 11 00463 g006 550
Figure 6. CO2-TPD profiles of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
Figure 6. CO2-TPD profiles of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
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Catalysts 11 00463 g007 550
Figure 7. X-ray photoelectron spectroscopy (XPS) spectra of (a) Ce 3d, (b) Zr 3d, (c) O 1s, and (d) Ni 2p of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
Figure 7. X-ray photoelectron spectroscopy (XPS) spectra of (a) Ce 3d, (b) Zr 3d, (c) O 1s, and (d) Ni 2p of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
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Catalysts 11 00463 g008 550
Figure 8. XPS spectra of (a) Pr 3d, (b) Sm 3d, (c) Yb 4d and (d) La 3d of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
Figure 8. XPS spectra of (a) Pr 3d, (b) Sm 3d, (c) Yb 4d and (d) La 3d of the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts.
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Catalysts 11 00463 g009 550
Figure 9. The curves of the (a) CO2 conversion and (b) CH4 selectivity versus reaction temperature over the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts; reaction condition: H2/CO2 = 4, GHSV = 12,000 mL/(g∙h), 1 atm.
Figure 9. The curves of the (a) CO2 conversion and (b) CH4 selectivity versus reaction temperature over the 15Ni2R/M-Ce80Zr20 (R = La, Sm, Pr, Yb) catalysts; reaction condition: H2/CO2 = 4, GHSV = 12,000 mL/(g∙h), 1 atm.
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Catalysts 11 00463 g010 550
Figure 10. Forty hour long-term stability tests over the 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts: (a) CO2 conversion, (b) CH4 selectivity; reaction conditions: H2/CO2 = 4, GHSV = 12,000 mL/(g∙h), 400 °C, 1 atm.
Figure 10. Forty hour long-term stability tests over the 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts: (a) CO2 conversion, (b) CH4 selectivity; reaction conditions: H2/CO2 = 4, GHSV = 12,000 mL/(g∙h), 400 °C, 1 atm.
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Figure 11. XRD patterns of the spent 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts after 40 h long-term stability tests.
Figure 11. XRD patterns of the spent 15Ni/M-Ce80Zr20 and 15Ni2La/M-Ce80Zr20 catalysts after 40 h long-term stability tests.
Catalysts 11 00463 g011
Table
Table 1. Structural properties of the catalysts based on the N2 physisorption.
Table 1. Structural properties of the catalysts based on the N2 physisorption.
SamplesSpecific Surface Area (m2/g) Pore Volume (cm3/g)Average Pore Diameter (nm) Isotherm Type
15Ni2La/M-Ce80Zr2051.910.139.93IV H2
15Ni2Pr/M-Ce80Zr2045.540.139.99IV H2
15Ni2Sm/M-Ce80Zr2050.460.169.98IV H2
15Ni2Yb/M-Ce80Zr2055.040.159.97IV H2
15Ni/M-Ce80Zr2045.270.129.26IV H2
Table
Table 2. Binding energies (eV) of the surface elements of the catalysts based on X-ray photoelectron spectroscopy (XPS).
Table 2. Binding energies (eV) of the surface elements of the catalysts based on X-ray photoelectron spectroscopy (XPS).
SamplesNi 2p3/2O 1sCe 3d5/2Zr 3d5/2
15Ni2La/M-Ce80Zr20853.40529.16882.20182.10
15Ni2Pr/M-Ce80Zr20853.35529.14882.22182.05
15Ni2Sm/M-Ce80Zr20853.30529.16882.24182.10
15Ni2Yb/M-Ce80Zr20853.45529.10882.35182.10
15Ni/M-Ce80Zr20853.55529.20882.10182.23
Table
Table 3. O 1s peak areas of the catalysts based on XPS.
Table 3. O 1s peak areas of the catalysts based on XPS.
SamplesO 1s Main Peak AreaO 1s Shoulder Peak AreaO 1s Shoulder Peak Area Ratio (%)
15Ni2La/M-Ce80Zr2062695.8963689.850.39
15Ni2Pr/M-Ce80Zr2080514.5167103.9945.46
15Ni2Sm/M-Ce80Zr2056684.1850302.0747.02
15Ni2Yb/M-Ce80Zr2078341.2466003.9945.72
15Ni/M-Ce80Zr2031806.18128467.1747.23
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Yang, Z.; Cui, Y.; Ge, P.; Chen, M.; Xu, L. CO2 Methanation over Rare Earth Doped Ni-Based Mesoporous Ce0.8Zr0.2O2 with Enhanced Low-Temperature Activity. Catalysts 2021, 11, 463. https://0-doi-org.brum.beds.ac.uk/10.3390/catal11040463

AMA Style

Yang Z, Cui Y, Ge P, Chen M, Xu L. CO2 Methanation over Rare Earth Doped Ni-Based Mesoporous Ce0.8Zr0.2O2 with Enhanced Low-Temperature Activity. Catalysts. 2021; 11(4):463. https://0-doi-org.brum.beds.ac.uk/10.3390/catal11040463

Chicago/Turabian Style

Yang, Zhenglong, Yan Cui, Pengxiang Ge, Mindong Chen, and Leilei Xu. 2021. "CO2 Methanation over Rare Earth Doped Ni-Based Mesoporous Ce0.8Zr0.2O2 with Enhanced Low-Temperature Activity" Catalysts 11, no. 4: 463. https://0-doi-org.brum.beds.ac.uk/10.3390/catal11040463

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