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Article

The Effect of Si on CO2 Methanation over Ni-xSi/ZrO2 Catalysts at Low Temperature

by
Li Li
1,
Ye Wang
2,
Qing Zhao
3 and
Changwei Hu
1,2,*
1
Key Laboratory of Green Chemistry and Technology, Ministry of Education, College of Chemistry, Sichuan University, Chengdu 610064, China
2
College of Chemical Engineering, Sichuan University, Chengdu 610064, China
3
Analytical & Testing Center, Sichuan University, Chengdu 610064, China
*
Author to whom correspondence should be addressed.
Catalysts 2021, 11(1), 67; https://doi.org/10.3390/catal11010067
Submission received: 19 November 2020 / Revised: 29 December 2020 / Accepted: 3 January 2021 / Published: 5 January 2021
(This article belongs to the Special Issue New Trends in Catalysis for Sustainable CO2 Conversion)
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Abstract

:
A series of Ni-xSi/ZrO2 (x = 0, 0.1, 0.5, 1 wt%, the controlled contents of Si) catalysts with a controlled nickel content of 10 wt% were prepared by the co-impregnation method with ZrO2 as support and Si as a promoter. The effect of different amounts of Si on the catalytic performance was investigated for CO2 methanation with the stoichiometric H2/CO2 molar ratio (4/1). The catalysts were characterized by BET, XRF, H2-TPR, H2-TPD, H2-chemisorption, CO2-TPD, XRD, TEM, XPS, and TG-DSC. It was found that adding the appropriate amount of Si could improve the catalytic performance of Ni/ZrO2 catalyst at a low reaction temperature (250 °C). Among all the catalysts studied, the Ni-0.1Si/ZrO2 catalyst showed the highest catalytic activity, with H2 and CO2 conversion of 73.4% and 72.5%, respectively and the yield of CH4 was 72.2%. Meanwhile, the catalyst showed high stability and no deactivation within a 10 h test. Adding the appropriate amount of Si could enhance the interaction between Ni and ZrO2, and increase the Ni dispersion, the amounts of active sites including surface Ni0, oxygen vacancies, and strong basic sites on the catalyst surface. These might be the reasons for the high activity and selectivity of the Ni-0.1Si/ZrO2 catalyst.

1. Introduction

With the release of a large amount of CO2 into the atmosphere, the greenhouse effect has increased in recent years [1,2]. Facing this great challenge, there are three main ways to control CO2 emissions: 1. Reducing CO2 emissions, 2. capture and storage of CO2, and 3. chemical conversion and utilization of CO2 [3,4,5]. Converting CO2 into value-added chemicals is by far the most cost-effective method [6,7,8]. Clean and renewable energy resources (wind, solar, and tidal energy) produce discontinuous electricity, which is not capable of being connected to the grid. Whereas, hydrogen can be generated by electrolysis using this discontinuous electricity [9,10,11]. With this H2 supply, the CO2 methanation reaction is attracting more and more interest due to the use of both carbon dioxide and hydrogen obtained from renewable energy. Compared with hydrogen, methane has many advantages, which could be easily liquefied, stored, transported, and used by the natural gas infrastructure [12,13]. In 1902, Sabatier et al. came up with the CO2 methanation reaction (also called the Sabatier reaction) firstly, CO2 + 4 H2 → CH4 + 2 H2O, Δ G = −130.8 KJ·mol−1, Δ H = −165 KJ·mol−1 [14]. It can be seen that the CO2 methanation reaction is highly exothermic, which is thermodynamically favorable but kinetically constrained [15]. Meanwhile, the heat released during the reaction will lead to the sintering of metal particles and deactivation of the catalysts [9,16]. Therefore, designing a high active and stable catalyst operating at low temperature is important [17].
Many studies have shown that VIII metals exhibit a catalytic performance for CO2 methanation, especially Ru [18], Rh [19], Pd [20], and Ni [21]. Although noble metal catalysts exhibit a higher catalytic activity and CH4 selectivity at low temperature, their large-scale industrial applications are limited by the high cost [21,22]. Therefore, Nickel as the most practical active metal has been widely investigated in the CO2 methanation reaction due to its abundance, low cost, and high activity [23,24,25]. Moreover, the properties of support play an important role in catalytic performance including the surface properties, the ability to disperse the active phase, and metal-support interaction. It is crucial to choose an appropriate support in preparing effective catalysts [26,27]. Many different metal oxides, such as CeO2 [28], MgO [29], Al2O3 [30], TiO2 [31,32], SiO2 [9,33], ZrO2 [34,35,36,37,38], and Y2O3 [39], have been used as support to promote CO2 methanation. ZrO2 becomes the most promising support and is getting increased attention since it has a higher concentration of oxygen defects [5]. Xu et al. [35] studied the CO2 methanation mechanism on the Ni/ZrO2 catalyst by in-situ FTIR and DFT methods, and found that c-ZrO2 could improve the electron mobility, the reducibility of Ni, and thus increase the catalytic activity of the catalysts. Martínez et al. [36] found that Ni/ZrO2 had a higher catalytic activity due to the strong interaction between Ni and ZrO2 by comparing the catalytic performance of Ni/ZrO2, Ni/SiO2, and Ni/MgAl2O4 catalysts. Hu et al. [37] also used ZrO2 as support, due to its good synergetic function, to explore the effect of La on the CO2 methanation reaction. Jia et al. [38] found that the structure of the catalyst had a significant influence on the catalytic performance. In addition, 71.9% CO2 conversion and 69.5% CH4 yield at 300 °C could be obtained on the Ni/ZrO2 catalyst prepared by the DBD plasma decomposition of nickel nitrate, while the CO2 conversion and CH4 yield were only 32.9% and 30.3% on the catalyst prepared thermally. Many researchers also reported that Ni/CeZrO2 catalysts exhibited an excellent low temperature catalytic performance for CO2 methanation due to the oxygen vacancies and high oxygen storage capacity of CeZrO2 [40,41,42].
In addition, promoters could further improve the catalytic activity. Therefore, the addition of a second element into nickel-based catalysts is considered as an effective method to improve the catalytic activity and stability at low temperature [14,21,43]. There are many types of promoters, including alkaline earth metals (Mg, Ca) [44,45], noble metals (Pt, Pd, Rh) [43], rare earth metals (La, Ce, Sm) [46], etc. The activation of CO2 can be enhanced by changing the surface basicity of the catalyst and the metal-support interaction [30,44,46]. Guilera et al. [30] studied the metal-oxide promoted Ni/Al2O3 catalyst, which exhibited a higher catalytic performance compared with the Ni/Al2O3 catalyst, due to the increase in basic sites and nickel dispersion. Xu et al. [46] found that the surface basicity and the intensity of CO2 chemisorption on Ni-based catalysts promoted by rare earth metals greatly increased, which could enhance the low-temperature catalytic activity of the catalysts. Wang et al. [47] found that the Ni-Si/ZrO2 catalyst (Si as promoter) had a better catalytic performance on the DRM reaction than the Ni-Zr/SiO2 catalyst (Zr as promoter). The Si on the Ni-Si/ZrO2 catalyst could promote the dispersion of Ni and improve the stability of metal Ni in the DRM reaction process. The highly dispersed Ni species on Ni-Si/ZrO2 increased the activation of CH4 and CO2, thus increasing the catalytic activity.
Since CO2 adsorption and activation were the key steps in both the DRM reaction and CO2 methanation, and CO2, CH4, and H2 co-existed in both the two systems. This study tried to apply the Ni-Si/ZrO2 catalyst to CO2 methanation to improve the activation of CO2 and promote CO2 methanation. Therefore, Ni-xSi/ZrO2 (Ni is 10 wt%, x = 0, 0.1, 0.5, 1 wt%) catalysts with different contents of Si were prepared by the co-impregnation method. The catalysts were characterized by different methods, including BET, H2-TPR, H2-TPD, H2-chemisorption, CO2-TPD, XRD, TEM, XPS, and TG-DSC, to explore the influence of Si.

2. Results and Discussion

2.1. The Catalytic Performance of Ni-xSi/ZrO2 Catalysts

The catalytic performance of Ni-xSi/ZrO2 catalysts was investigated over the temperature range from 200 to 400 °C under atmospheric pressure. The results of the catalytic test were presented in Figure 1, including the conversion of H2 and CO2, and the selectivity and yield of CH4. The catalytic activity increased as the temperature increased until it was thermodynamically limited by the equilibrium. Among all catalysts, the Ni-0.1Si/ZrO2 catalyst was the most active over the whole temperature range, followed by the Ni/ZrO2 catalyst, which exhibited an excellent catalytic activity at 250 °C. On the unpromoted Ni/ZrO2 catalyst, the CO2 conversion and CH4 yield were 66.6% and 66.2%, respectively with the 99.4% CH4 selectivity. Si-promoted Ni/ZrO2 catalysts exhibited a different catalytic activity, which changed with the amount of Si. The Ni-0.1Si/ZrO2 catalyst exhibited the highest CO2 conversion of 72.5% and H2 conversion of 73.4% among the studied catalysts. Simultaneously, it showed the highest CH4 selectivity of 99.6% and the highest CH4 yield of 72.2%, whose catalytic activity was about 6% higher than the Ni/ZrO2 catalyst. However, the Ni-0.5Si/ZrO2 catalyst showed a rather low activity, with 10% H2 conversion and 9.8% CO2 conversion. The lowest catalytic activity was obtained on the Ni-1Si/ZrO2 catalyst, with only 1% CH4 yield. In general, adding the appropriate Si was beneficial to CO2 methanation.
Then, the catalytic stability of Ni-xSi/ZrO2 catalysts was investigated at 250 °C where the catalyst exhibited a high activity. The results of the catalytic test were presented in Figure 2. All the catalysts exhibited the high stability with 10 h on stream. TG-DSC analysis results showed that the weight of all spent catalysts did not decrease during the heating process (Figure S1), which illustrated that there was no carbon deposition on the catalysts after reaction.

2.2. The Textural Properties of Ni-xSi/ZrO2 Catalysts

The physical properties of Ni-xSi/ZrO2 catalysts were characterized by N2 adsorption-desorption experiments. Figure 3 showed the N2 adsorption-desorption isotherms, pore volume, and size distribution of the catalysts. All isotherms of the catalysts were assigned to the type IV isotherm and the P/P0 for the hysteresis loop was 0.7~0.9, which indicated that all the catalysts were with the mesoporous structure, and that also could be proved by the pore size distribution in Figure 2B [48,49,50]. The textural properties of catalysts were summarized in Table 1. The specific surface area was calculated by the Brunauer-Emmett-Teller (BET) method and the pore size and volume were calculated by the Barrett-Joyner-Halenda (BJH) method. Obviously, it could be seen that the BET results of catalysts with a different Si content had no significant differences, which suggested that the different loading of Si did not damage the pore structure of the Ni/ZrO2 catalyst.
The actual loadings of Ni and Si were determined by the X-ray fluorescence (XRF) test. The content of Ni on Ni/ZrO2, Ni-0.1Si/ZrO2, Ni-0.5Si/ZrO2, and Ni-1Si/ZrO2 were 6.74%, 6.66%, 6.51%, and 5.79%, respectively. The actual contents of Ni on these catalysts were very close, indicating the successful loading of Ni onto ZrO2. The content of Si on Ni-0.1Si/ZrO2, Ni-0.5Si/ZrO2, and Ni-1Si/ZrO2 were 0.11%, 0.34%, and 0.58%.

2.3. The Reducibility of Ni-xSi/ZrO2 Catalysts

The TPR profiles of calcined Ni-xSi/ZrO2 catalysts and support ZrO2 were presented in Figure 4. There was a broad peak at around 500 °C on the ZrO2. Three main different reduction peaks (α, β, and γ) could be observed on the Ni-xSi/ZrO2 catalysts, which suggested that there were three kinds of nickel oxide species. The α peak (317–327 °C) was attributed to the reduction of the bulk NiO species on the surface of the catalysts. The β peak (340–366 °C) and γ peak (419–428 °C) were assigned to the reduction of NiO species that interacted weakly and strongly with support, respectively. The amount of each NiO species was summarized in Table 2. The minimum amount of bulk NiO species could be observed, indicating that a little bulk NiO species existed on all catalysts. The amount of β peak was over 60% on both Ni-0.5Si/ZrO2 and Ni-1Si/ZrO2 catalysts. Associated with the catalytic activity, the NiO species that interacted with ZrO2 weakly was not good for the CO2 methanation reaction after reduction, resulting in a lower catalytic activity. While Ni-0.1Si/ZrO2 and Ni/ZrO2 catalysts showed the higher amount of the γ peak contributing to a higher catalytic activity, suggesting that the strong interaction between the nickel and the support could promote the dispersion of Ni on the support [51]. Furthermore, the β and γ peaks of promoted Ni/ZrO2 catalysts shifted to a lower temperature compared with that of the Ni/ZrO2 catalyst, suggesting that the appropriate Si (0.1 wt%) promoted the dispersion of Ni [50,51]. The peaks of the Ni-0.1Si/ZrO2 catalyst shifted to the lowest temperature (the β at 340 °C and the γ at 419 °C), indicating the highest Ni dispersion, which could increase the catalytic activity of catalysts. Therefore, the strong nickel-support interaction promoted the dispersion of Ni on the support, which was beneficial to the catalytic performance [1,50].

2.4. The H2-TPD and H2-Chemisorption of Ni-xSi/ZrO2 Catalysts

The amounts of active sites of reduced Ni-xSi/ZrO2 catalysts were measured by H2-TPD and H2-chemisorption. Two peaks could be clearly observed in the patterns of reduced catalysts depicted in Figure 5. The first peak appeared at around 92–111 °C, which was assigned to the weak active site, and a broad peak around at 500 °C could be found over the catalysts, corresponding to the strong active site with the strong H2 adsorption [51,52]. The temperature of two H2 desorption peaks was different with different contents of Si. The highest temperature with 111 and 517 °C of H2 desorption peaks on the Ni-0.1Si/ZrO2 catalyst suggested the strong H2 adsorption to the active sites, which could be assigned to the high dispersion of Ni and the increasing number of active sites [51,52,53]. Considering the possible spill over in TPD over the catalysts, the pulse chemisorption of H2 was also carried out. The dispersion of Ni obtained by H2-chemisorption was shown in Table 3. The Ni dispersion of Ni/ZrO2, Ni-0.1Si/ZrO2, Ni-0.5Si/ZrO2, and Ni-1Si/ZrO2 were 0.65%, 0.67%, 0.34%, and 0.28%, respectively. The Ni-0.1Si/ZrO2 catalyst showed the highest dispersion of Ni, which was slightly higher than the Ni/ZrO2 catalyst. The amounts of adsorbed H2 decreased significantly on Ni-0.5Si/ZrO2 and Ni-1Si/ZrO2 catalysts, suggesting that the amounts of Ni decreased, which corresponded to their lower catalytic activity.

2.5. The Results of CO2-TPD on Ni-xSi/ZrO2 Catalysts

The CO2 desorption capability was explored by the CO2-TPD measurement and the profiles were shown in Figure 6, which was used to describe the basicity of catalysts usually. Two main CO2 desorption peaks could be observed over each catalyst, which were classified as the adsorbed CO2 at weak basic sites (361–410 °C) and strong basic sites (577–604 °C), respectively. Based on the pioneer studies, the CO2 adsorbed on the weak basic sites could be desorbed at a low temperature and that absorbed on the strong basic sites could be desorbed at a high temperature [44]. The contents of weak and strong basic sites were calculated by the area of the peaks (Table 4). It could be seen that the content of strong basic sites on Ni-0.1Si/ZrO2 was the highest (71%) among all the catalysts studied, which could inhabit carbon formation effectively [54,55]. Le et al. [56] found that the stronger CO2 desorption peak around 500 °C on the Ni-CeO2 catalyst suggested the strong CO2 adsorption ability, which had a positive effect on the catalytic performance. In addition, the CO2 adsorption peak of strong basic sites slightly shifted to a higher temperature with the increasing of Si, suggesting that the addition of Si promoted the adsorption of CO2. The intensified surface basicity of the Ni-0.1Si/ZrO2 catalyst could promote the adsorption of CO2 thus promoting the CO2 methanation reaction [42,44,46].

2.6. Crystallite Structure of Ni-xSi/ZrO2 Catalysts

The XRD patterns of catalysts could be seen in Figure 7. Before reduction, the diffraction peaks at 37.2, 43.3, and 62.9° were attributed to NiO (PDF no. 47-1049), as shown in Figure 7a [1,26]. The diffraction peaks at 44.5, 51.8, and 76.4° corresponded to the Ni metal (PDF no. 04-0850) over reduced catalysts, as shown in Figure 7b, and there was no diffraction peak of NiO [57]. In addition, no obvious diffraction peak of Si species was detected on Ni-xSi/ZrO2 catalysts, suggesting that Si was in a high dispersion or amorphous state [51]. The crystallite sizes of NiO and Ni metal were calculated by the Scherrer equation at 2 θ = 43.3° and 2 θ = 44.5°, respectively, and the results were listed in Table 5. It could be seen that the particle sizes of NiO had no significant changes and the crystallite sizes of NiO were all around 25 nm on Ni-xSi/ZrO2 catalysts before reduction. The grain sizes of the Ni metal were around 22 nm on reduced Ni-xSi/ZrO2 catalysts, which also showed no obvious variation.

2.7. The TEM Images of Ni-xSi/ZrO2 Catalysts

The TEM images of reduced Ni-xSi/ZrO2 catalysts were presented in Figure 8. It can be seen that all the catalysts exhibited a similar morphology, and the particle sizes were distributed between 10 and 30 nm. From Figure 8B, Ni was more evenly distributed with particle sizes of 18–20 nm on the Ni-0.1Si/ZrO2 catalyst. However, on the Ni-1Si/ZrO2 catalyst, the size distribution of nickel particles was uneven, and there were big nickel particles. From Figure 8D, bulk particles were found on the Ni-1Si/ZrO2 catalyst, which might be bulk Ni particles or the mixture of Ni and Si.

2.8. Chemical State of the Elements on Ni-xSi/ZrO2 Catalysts

The surface element composition and chemical state of reduced Ni-xSi/ZrO2 catalysts were obtained by the XPS experiment, which were shown in Figure 9. There were four main peaks in the Ni 2p spectra. The peak at around 852 eV was assigned to the characteristic peak of Ni0 [7]. There were two characteristic peaks of Ni2+, P1 (about 854 eV) was the low energy peak and belonged to the peak of NiO, while P2 (about 856 eV) corresponded to Ni (OH)2 [25]. The peak at around 860 eV was the companioning peak of Ni2+, produced by the orbital spin splitting [50]. The percentages of different elements on the surface of catalysts were summarized in Table 6. The surface content of Ni0 was the highest on the Ni-0.1Si/ZrO2 catalyst, which was 2.15%. More Ni0 could provide more active sites and facilitate CO2 methanation [58]. It could be found that the surface content of Si was higher than its actual loading, suggesting that Si enriched on the catalysts surface and part of Ni might be covered by Si, especially on Ni-0.5Si/ZrO2 and Ni-1Si/ZrO2 catalysts. The O 1s spectrum exhibited two types of oxygen species, as shown in Figure 9B. The peak at 530–530.46 eV was attributed to the lattice oxygen (Oα) and the peak at 528.9–529.1 eV belonged to the surface oxygen (Oβ) [28,59]. Based on the areas of Oα and Oβ, the ratios of the oxygen vacancies could be obtained by calculating the ratio of Oβ to OT (OT = Oα + Oβ) [5,22,38]. In Table 6, the reduced Ni-0.1Si/ZrO2 catalyst exhibited the highest ratio (0.589) of Oβ to OT compared with other catalysts, suggesting the highest amount of oxygen vacancies. The presence of oxygen vacancies was beneficial to the adsorption of CO2, which could promote CO2 activation [23,58]. Jiang et al. [17] found that the content of surface oxygen on the Mn promoted Ni/bentonite catalyst was 83.55%, which was higher than that of the unpromoted Ni/bentonite catalyst (74.85%), representing the higher amount of oxygen vacancies. The increased oxygen vacancies were helpful to the adsorption and dissociation of CO2 on the catalyst. To sum up, the Ni-0.1Si/ZrO2 catalyst exhibited the highest catalytic activity and stability due to the highest amount of Ni0 and oxygen vacancies on the surface.

3. Materials and Methods

3.1. Catalysts Preparation

The ZrO2 support was prepared by the precipitation method. A certain amount of Zr (NO3)4∙5H2O (ChengduKelong, China) was dissolved in deionized water with continuous stirring until dissolved completely. Then, NH3∙H2O was added to the above solution to achieve a pH value of 9. After stirring for 2 h, the mixture was aged for 24 h at room temperature. After that, the mixture was filtered and washed three times with deionized water. The obtained sample was dried at 110 °C for 4 h and then calcined at 500 °C for 5 h (with the rate of 2 °C /min) to obtain the ZrO2 support.
The impregnation method was used to synthesize the Ni-xSi/ZrO2 catalysts (ZrO2 was used as the support and Si was used as the promoter with the load of 0, 0.1, 0.5, and 1 wt%). The designed amount of Ni (NO3)2∙6H2O (ChengduKelong, Chengdu, China) and different amounts of (C2H5O)4Si (Alfa Aesar Chemicals, Shanghai, China) were dissolved in a certain amount of absolute ethanol (Chengdu Chron Chemical, Chengdu, China). The ZrO2 support was impregnated with the aforesaid ethanol solution for 24 h at room temperature. Then, these samples were dried at 80 °C for 2 h and 110 °C for 4 h. Finally, the above mixtures were heated to 500 °C (with the rate of 2 °C/min) and calcined at 500 °C for 5 h under air flow, then, the Ni-xSi/ZrO2 catalysts were obtained (x = 0, 0.1, 0.5, 1 wt%).

3.2. Catalytic Activity Test

The catalytic activity test was carried out in a fixed-bed continuous flow micro-quartz-tube reactor with 10 mm in diameter at atmospheric pressure. There was a thermocouple near the reactor close to the fixed-bed, which was used to follow the temperature of the catalysts during the test. Before the activity test, 0.50 g of the catalyst was heated up to 450 °C (10 °C/min) and then reduced at a constant temperature of 450 °C for 1 h in H2/Ar (F(H2) = F(Ar) = 30 mL/min) mixture gas. After that, the catalyst was cooled down to the reaction temperature before the introduction of the mixture of reactants (H2/CO2 = 4, F = 150 mL/min) for the CO2 methanation reaction. The effluent from the reactor passed through a condensing device and was analyzed online by a gas chromatograph (plot-C2000 capillary column) per hour.
X C O 2 =   n C O 2 , i n − n C O 2 , o u t n C O 2 , i n × 100 % ,
X H 2 =   n H 2 , i n − n H 2 , o u t n H 2 , i n × 100 % ,
S C H 4 =   n C H 4 , o u r n C H 4 , o u r + n C O , o u r × 100 % ,
Y C H 4 =   X H 2 × S C H 4
where XCO2 and XH2 were the conversion of CO2 and H2, SCH4 was the selectivity of CH4, and YCH4 was the yield of CH4.

3.3. Catalysts Characterization

The physical property test was conducted in the Micromeritics Tristar II 3020 instrument by using the N2 adsorption-desorption method. Before the measurements, about 0.1 g samples were outgassed at 150 °C for 2 h and then at 300 °C for 2 h under a vacuum.
The actual Ni and Si loadings of the fresh catalysts were determined by the X-ray fluorescence (XRF) test. Ni and Si were detected by the Ni Kα line and Si Kα line, respectively.
The hydrogen temperature-programmed reduction (H2-TPR) measurement was carried out with a Micromeritics AutoChem II Chemisorption Analyzer. At first, about 100 mg of the catalyst was pretreated with Ar flow at 150 °C for 30 min. Then, the TPR experiment was performed from 50 to 800 °C in the H2/Ar (10/90 vol%) flow with the heating rate of 8 °C/min. The TCD detector was used to monitor the H2 consumption.
The temperature-programmed desorption of H2 (H2-TPD) was performed on the same equipment for H2-TPR. About 100 mg of the reduced catalyst was pretreated at 500 °C for 1 h in Ar flow. Next, H2 was absorbed at 50 °C for 1 h in 10% H2/Ar. After cleaning the excess unabsorbed H2, the catalyst was heated to 800 °C with a heating rate of 10 °C /min under Ar flow. The results were detected by a TCD detector. The H2 pulse chemisorption was also processed on the equipment. About 100 mg of the reduced catalyst was pretreated at 450 °C for 1 h in Ar flow and cooled down to 50 °C at the same atmosphere for beginning the H2 adsorption. A gas mixture of 10% H2 balance Ar was pulsed over the catalyst for chemisorption measurements.
Before the temperature-programmed desorption of CO2 (CO2-TPD), about 100 mg of the reduced catalyst (reduced at 450 °C) was pretreated at 500 °C in He flow for 1 h to remove surface impurities. Then, CO2 was absorbed at 50 °C for 1 h in 10% CO2/He. After cleaning the excess unabsorbed CO2, the catalyst was heated to 900 °C with a heating rate of 10 °C/min in He flow. The observed curves were fitted into two Gaussian peaks.
The X-ray diffraction (XRD) was performed on a DX-1000 CSC diffractometer instrument, operating at 40 kV and 25 mA with a Cu Kα radiation source for the calcined and reduced catalysts. The data was recorded over the scattering angle range of 2θ from 10 to 80°, with a scan step with of 0.03°.
Transmission electron microscopy (TEM) was used to characterize the reduced catalysts on the Tecnai G2 F20 machine. The twin instrument with the 0.20 nm resolution was used, and the acceleration voltage was 200 Kv.
The analysis of X-ray photoelectron spectroscopy (XPS) was carried out on a KRATOS spectrometer with an AXIS Ultra DLD. The Al Kα monochromatized line was operated at the accelerating power of 25 W. In addition, the binding energy was calibrated with C 1 s 284.6 eV.
Thermogravimetric (TG) and differential scanning calorimetry analysis (DSC) was used to characterize the deposited carbon of the spent Ni-xSi/ZrO2 catalysts, using the NETZSCH TG209F1 instrument. Before the test, the sample was placed until a better gas equilibrium. Then, the temperature was increased from 30 to 800 °C with a 5 °C·min−1 heating rate in air flow with a rate of 60 mL·min−1.

4. Conclusions

Adding the appropriate amount of Si could increase the catalytic activity of Ni/ZrO2 catalyst, and the Ni-0.1Si/ZrO2 catalyst showed the highest catalytic activity and stability. The strong interaction between Ni and ZrO2 could promote the dispersion of Ni on the support, and the strong basic sites on the catalyst were beneficial to the absorption of CO2, thus to the CO2 methanation reaction on the Ni-0.1Si/ZrO2 catalyst. In addition, the higher amount of surface Ni0 could provide more active sites, and the more oxygen vacancies were beneficial to the absorption and activation of CO2 on the 0.1Si/ZrO2 catalyst.

Supplementary Materials

The following are available online at https://www.mdpi.com/2073-4344/11/1/67/s1. Figure S1. The TG-DSC profile of the spent Ni-xSi/ZrO2 catalysts (A) Ni /ZrO2 catalyst, (B) Ni-0.1Si/ZrO2 catalyst, (C)Ni-0.5Si/ZrO2 catalyst, and (D) Ni-1Si/ZrO2 catalyst.

Author Contributions

Conceptualization, methodology, L.L., Y.W. and C.H.; software, validation, formal analysis, investigation, resources, data curation, writing—original draft preparation, L.L. and Q.Z.; writing—review and editing, visualization, supervision, Y.W. and C.H.; project administration, funding acquisition, C.H. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Key R and D Program of China (2018YFB1501404), the 111 program (B17030), and Fundamental Research Funds for the Central Universities.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

We would like to thank the Analytical and Testing center of Sichuan University for the characterization and we are grateful to Yunfei Tian for his help in the XPS experiments.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. The conversion of H2 (A) and CO2 (B), the selectivity of CH4 (C), and the yield of CH4 (D) on Ni-xSi/ZrO2 catalysts over the temperature range from 200 to 400 °C.
Figure 1. The conversion of H2 (A) and CO2 (B), the selectivity of CH4 (C), and the yield of CH4 (D) on Ni-xSi/ZrO2 catalysts over the temperature range from 200 to 400 °C.
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Catalysts 11 00067 g002 550
Figure 2. The conversion of H2 (A) and CO2 (B), the selectivity of CH4 (C), and the yield of CH4 (D) on Ni-xSi/ZrO2 catalysts at 250 °C with a time on stream (H2/CO2 = 4/1, F = 150 mL/min).
Figure 2. The conversion of H2 (A) and CO2 (B), the selectivity of CH4 (C), and the yield of CH4 (D) on Ni-xSi/ZrO2 catalysts at 250 °C with a time on stream (H2/CO2 = 4/1, F = 150 mL/min).
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Catalysts 11 00067 g003 550
Figure 3. N2 adsorption-desorption isotherms (A) and pore volume and size distribution (B) of Ni-xSi/ZrO2 catalysts.
Figure 3. N2 adsorption-desorption isotherms (A) and pore volume and size distribution (B) of Ni-xSi/ZrO2 catalysts.
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Catalysts 11 00067 g004 550
Figure 4. H2-TPR profiles of Ni-xSi/ZrO2 catalysts and ZrO2.
Figure 4. H2-TPR profiles of Ni-xSi/ZrO2 catalysts and ZrO2.
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Catalysts 11 00067 g005 550
Figure 5. H2-TPD patterns of Ni-xSi/ZrO2 catalysts.
Figure 5. H2-TPD patterns of Ni-xSi/ZrO2 catalysts.
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Figure 6. CO2-TPD profiles of Ni-xSi/ZrO2 catalysts.
Figure 6. CO2-TPD profiles of Ni-xSi/ZrO2 catalysts.
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Figure 7. XRD patterns of Ni-xSi/ZrO2 catalysts (a) before reduction, (b) after reduction.
Figure 7. XRD patterns of Ni-xSi/ZrO2 catalysts (a) before reduction, (b) after reduction.
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Catalysts 11 00067 g008a 550Catalysts 11 00067 g008b 550
Figure 8. TEM images of Ni-xSi/ZrO2 catalysts (A) Ni/ZrO2 catalyst, (B) Ni-0.1Si/ZrO2 catalyst, (C) Ni-0.5Si/ZrO2 catalyst, and (D) Ni-1Si/ZrO2 catalyst.
Figure 8. TEM images of Ni-xSi/ZrO2 catalysts (A) Ni/ZrO2 catalyst, (B) Ni-0.1Si/ZrO2 catalyst, (C) Ni-0.5Si/ZrO2 catalyst, and (D) Ni-1Si/ZrO2 catalyst.
Catalysts 11 00067 g008aCatalysts 11 00067 g008b
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Figure 9. XPS spectra of reduced Ni-xSi/ZrO2 catalysts (A) Ni 2p and (B) O 1s.
Figure 9. XPS spectra of reduced Ni-xSi/ZrO2 catalysts (A) Ni 2p and (B) O 1s.
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Table
Table 1. Textural properties and element contents of Ni-xSi/ZrO2 catalysts.
Table 1. Textural properties and element contents of Ni-xSi/ZrO2 catalysts.
CatalystSBET aVBJH bDp cNi (%) dSi (%) d
(m2·g−1)(m3·g−1)(nm)
Ni/ZrO242.50.1513.56.74-
Ni-0.1Si/ZrO246.00.1612.86.660.11
Ni-0.5Si/ZrO244.60.1513.26.510.34
Ni-1Si/ZrO244.40.1513.25.790.58
a: Surface area (SBET) determined by the BET method; b: BJH adsorption cumulative volume of pores; c: BJH adsorption average pore diameter; d: Obtained by XRF.
Table
Table 2. The amount of each nickel oxide species on the Ni-xSi/ZrO2 catalysts.
Table 2. The amount of each nickel oxide species on the Ni-xSi/ZrO2 catalysts.
Catalystαβγ
Position
(°C)		Content
(%)		Position
(°C)		Content
(%)		Position
(°C)		Content
(%)
Ni/ZrO232113.936620.542865.6
Ni-0.1Si/ZrO23175.034017.541977.5
Ni-0.5Si/ZrO231914.236560.142625.7
Ni-1Si/ZrO23277.836662.142230.1
Table
Table 3. The H2 uptake and Ni dispersion of Ni-xSi/ZrO2 catalysts.
Table 3. The H2 uptake and Ni dispersion of Ni-xSi/ZrO2 catalysts.
CatalystPeak 1Peak 2Total H2 Uptake
(µmol/g) a		Ni Dispersion
(%) b
T
(°C)		H2 Uptake
(µmol/g)		T
(°C)		H2 Uptake
(µmol/g)
Ni/ZrO21024.645125.029.660.65
Ni-0.1Si/ZrO21114.645174.959.590.67
Ni-0.5Si/ZrO21034.285052.596.870.34
Ni-1Si/ZrO2924.065032.516.570.28
a: Obtained by H2-TPD; b: Obtained by H2 chemisorption.
Table
Table 4. Peak position and basic sites of Ni-xSi/ZrO2 catalysts obtained by CO2-TPD.
Table 4. Peak position and basic sites of Ni-xSi/ZrO2 catalysts obtained by CO2-TPD.
CatalystPeak 1Peak 2Total Basicity
(µmol/g)
Position
(°C)		Content
(%)		Position
(°C)		Content
(%)
Ni/ZrO23724457756123
Ni-0.1Si/ZrO24022958871124
Ni-0.5Si/ZrO23614959551127
Ni-1Si/ZrO24104660454124
Table
Table 5. The crystallite sizes of NiO and Ni metal on Ni-xSi/ZrO2 catalysts.
Table 5. The crystallite sizes of NiO and Ni metal on Ni-xSi/ZrO2 catalysts.
CatalystBefore ReductionAfter Reduction
NiONi Metal
Ni/ZrO224 nm22 nm
Ni-0.1Si/ZrO226 nm23 nm
Ni-0.5Si/ZrO226 nm23 nm
Ni-1Si/ZrO225 nm21 nm
Table
Table 6. Surface contents on reduced Ni-xSi/ZrO2 catalysts.
Table 6. Surface contents on reduced Ni-xSi/ZrO2 catalysts.
CatalystNi0 (%)Si (%)Oβ/OT
Ni/ZrO21.8800.56
Ni-0.1Si/ZrO22.150.450.59
Ni-0.5Si/ZrO21.781.910.51
Ni-1Si/ZrO21.273.740.50
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Li, L.; Wang, Y.; Zhao, Q.; Hu, C. The Effect of Si on CO2 Methanation over Ni-xSi/ZrO2 Catalysts at Low Temperature. Catalysts 2021, 11, 67. https://doi.org/10.3390/catal11010067

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Li L, Wang Y, Zhao Q, Hu C. The Effect of Si on CO2 Methanation over Ni-xSi/ZrO2 Catalysts at Low Temperature. Catalysts. 2021; 11(1):67. https://doi.org/10.3390/catal11010067

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Li, Li, Ye Wang, Qing Zhao, and Changwei Hu. 2021. "The Effect of Si on CO2 Methanation over Ni-xSi/ZrO2 Catalysts at Low Temperature" Catalysts 11, no. 1: 67. https://doi.org/10.3390/catal11010067

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