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  In recent years, anthropogenic carbon dioxide emission has received global concerns^[1].CO₂ reforming of methane can utilize CO₂ as a feedstock. This process converts CH₄ and CO₂ to H₂ and CO, and the ratio of H₂/CO is suitable for Fischer-Tropsch synthesis, carbonylation and hydroformylation^[2-4]. One of the main challenges on CO₂ reforming of methane is to develop stable and coke-resistant catalyst. The noble metal catalysts (Pd, Rh, Pt, Ru, Ir) have high activity and low carbon deposition^[5]. However, high cost has restricted their application. Nickel catalysts have been widely used in carbon dioxide reforming of methane due to low cost and considerable activity compared to the noble catalysts. However, the nickel catalysts are easily sintered and deactivated via carbon deposition^[6]. Much efforts have been devoted to improve the performance of the nickel catalysts, such as using more stable supports, adding promoters (alkaline metal oxide, alkaline earth metal oxide and rare earth metal oxide) to improve the activity of catalysts, altering preparation method to increasing the dispersion of nickel and so on^{[4, 5, 7]}. García-Vargas et al^[7] found Ni-Mg/SiC catalyst had a high catalytic performance and stability (with a lower coke formation) during the methane tri-reforming reaction since Mg provoked a decrease of Ni particle size and an increase of both the interaction between nickel and support.

  SiC, as a catalyst support, has excellent chemical stability, high mechanical strength and good thermal conductivity^{[8, 9]}. Yu et al^[10] prepared Ni/SiC catalyst for methanation of CO, and found that the catalyst exhibited high activity and stability. Zhi et al^[11] found that Ni/SiC was also excellent catalyst for methanation of carbon dioxide. After adding promoter (La₂O₃) to Ni/SiC, the interaction between nickel nanoparticles and SiC was enhanced. Besides, carbon deposition and sintering of nickel nanoparticles were effectively suppressed. In our previous work, Ni-Sm_x/SiC catalysts (Ni: 9%, x=0, 2%, 3%, 4%, 5%, 7%) were prepared by impregnation method^[12]. Ni-Sm₅/SiC catalyst exhibited high activity and low carbon deposition in CO₂ reforming of CH₄ under 10000 mL/(g·h) of GHSV at 800℃. The addition of Sm₂O₃ enhanced the interaction between nickel nanoparticles and SiC support, and Sm₂O₃, as an alkaline rare earth metal oxide can efficiently absorb CO₂ to promote carbon deposition-elimination process, correspondingly increased the stability of catalyst. However, the catalytic performance and carbon deposition of Ni-Sm_x/SiC catalysts prepared by the impregnation method are not yet satisfied because the dispersion of nickel nanoparticles was not homogeneous and the nickel particles size was large.

  The hydrothermal synthesis is an efficient method to increase the dispersion of active component and control the size of metal nanoparticles^{[13, 14]}. Besides, different nickel precursors may have effects on the performance of catalysts for carbon dioxide reforming of methane. In this paper, we prepare Ni-Sm/SiC catalysts (Ni: 9%, Sm: 5%) by a hydrothermal route using nickel nitrate and nickel acetylacetonate as nickel precursors, respectively. The catalysts show high activity and stability in CO₂ reforming of CH₄, and the coke deposition is well controlled.

  1   Experimental

  1.1   Catalyst preparation

  SiC support was prepared by a sol-gel and carbothermal reduction process^[15]. The amounts of nickel and samarium are fixed on 9% and 5% (by weight), respectively. Taking the catalyst with nickel nitrate precursor for example, 0.1479g Sm (NO₃)₃·6H₂O and 0.503g Ni (NO₃)₂·6H₂O were firstly dissolved into 60mL deionized water. Then 1g of SiC, 0.73g of cetyltrimethyl ammonium bromide and 0.6g of urea were added into the solution. After stirring for an hour, the suspension was transferred to a Teflon-lined stainless steel autoclave of 80mL and then heated at 150℃ for 12h. Then cooling to the room temperature, the solid was obtained by centrifugation and then dried at 110℃ for 12h. Finally, the solid was calcined in a muffle furnace at 500℃ for 4h to get the catalyst (denoted as HN). The catalyst with nickel acetylacetonate precursor was synthesized by the same way (denoted as HA). Both catalysts were directly used for carbon dioxide reforming of methane without prereduction.

  1.2   Catalyst characterization

  BET surface area was characterized by Micromeritics Tristar 3000 at nitrogen temperature at 77K. The amount of Ni and Sm was measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Thermo iCAP 6300). X-ray diffraction (XRD) analysis was conducted on MiniFlexII using Cu Kα radiation at 30kV and 15mA. Temperature-programmed reduction (TPR) experiments were conducted in a Micromeritics Xianquan TP 5080 with a thermal conductivity detector under a flow of 10% H₂/N₂.Thermogravimetry and differential thermal analysis (TG-DTA) were performed on Rigaku analyzer with a temperature range from room temperature to 900℃ with a heating rate of 5℃/min in air atmosphere. The morphology of catalyst was analyzed by JEOL-2100 transmission electron microscopy (TEM).

  1.3   Catalytic activity test

  Carbon dioxide reforming of methane reaction was conducted in a fixed-bed quartz reactor with an interior diameter 10mm at atmospheric pressure. 200mg catalyst was put into the reactor and fixed by quartz cotton on both ends. The reforming reaction was performed for 120h under 10000mL/(g·h) of GHSV (molar ratio of CH₄/CO₂ was 1) at 800℃. Gas effluents were analyzed with a gas chromatograph (Techcomp GC-7900).

  2   Results and discussion

  2.1   BET surface area and ICP

  The surface area and metal loadings of catalysts are listed in Table 1. Pure SiC has a surface area of 36.8m²/g. The surface areas of HN and HA catalysts are 82.2 and 74.7m²/g, respectively. These values are obviously higher than those of pure SiC and the Ni-Sm₅/SiC catalyst prepared by impregnation method (with a BET surface area of 25.3m²/g)^[12]. This could result from that the nanoparticles of active components formed by the hydrothermal route are well dispersed and with smaller size. This is further confirmed by the XRD and TEM results. The analysis of ICP demonstrates that Ni loading and Sm loading of both catalysts are around 9% and 5% which are close to theoretical values, respectively.

  Table 1.  BET and ICP results of the Ni-Sm/SiC catalysts

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  SamplePure SiCHN catalystHA catalystSurface area
  A/(m²·g^-1)36.882.274.7Sm loading w/% -4.914.57

  Ni loading w/%	-	8.62	8.52

  Table 1.  BET and ICP results of the Ni-Sm/SiC catalysts

  2.2   XRD analysis

  The XRD patterns of HN and HA catalysts are shown in Figure 1. For the fresh catalysts (Figure 1(a)), the diffraction peaks centered at 35.7°, 41.5°, 60.1°, 71.9° and 75.7°correspond to (111), (200), (220), (311) and (222) planes of SiC^[15]. The diffraction peak at 33.6° is corresponding to the stacking fault of SiC^[15]. The diffraction peaks of NiO usually appear at 37.3°, 43.3° and 62.9^{[11, 16]}. However, these peaks are not observed in the XRD patterns, indicating that NiO nanoparticles are well dispersed and the size of the NiO nanoparticles is too small to be detected. There is a small diffraction peak of Sm₂O₃ at 28.3° for the both catalysts^[17]. After 120h reaction, there is no change in the SiC diffraction peaks, suggesting that SiC has excellent stability and chemical inertness as the support. The new diffraction peaks centered at 44.5°, 51.8° and 76.4° correspond to (111), (200) and (220) planes of metallic Ni for the both catalysts (Figure 1(b))^[11]. The crystalline sizes of metallic Ni for HN and HA catalysts are 14.5 and 20.1nm calculated by the XRD measurements, respectively. This demonstrates that NiO is reduced to Ni⁰ during the reaction. Besides, we observed the obvious diffraction peaks of Sm₂O₃ at 28.3°，47.0°and 55.7°for the HN catalyst after the reaction^[18]. This indicates that the size of Sm₂O₃ particles in HN catalysts increases during the reaction. The difference in the size of Sm₂O₃ particles between HN and HA catalysts may be caused by different nickel precursors.

  Figure 1.  XRD patterns of the fresh (a) and used (b) Ni-Sm/SiC catalysts

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  2.3   H₂-TPR analysis

  H₂-TPR analysis is employed to characterize the interaction between support and active components. The HN and HA catalysts have similar H₂-TPR profiles (Figure 2). The tiny peak at around 350℃ is ascribed to the reduction of pure NiO, suggesting that there is only small amount of large and bare NiO particles on the SiC surface^{[9, 12]}. The main H₂ consumption peak between 450 and 750℃ can be ascribed to the reduction of NiO species with strong interaction with the support. These indicate that nickel nanoparticles are homogeneously dispersed. It is in agreement with the XRD and TEM results. The interaction between support and nickel nanoparticles is enhanced, and thus the stability of catalysts is improved. For Ni-Sm/SiC catalyst prepared by impregnation method, there is a sharp reduction peak at 430℃^[12]. H₂-TPR profile of 5% Sm/SiC suggests that Sm³⁺ can not be reduced even at 900℃ (shown in Figure 2).

  Figure 2.  H₂-TPR profiles of the Ni-Sm/SiC catalysts

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  2.4   TG-DTA analysis

  The amount of carbon deposition is an important factor which influences the performance of catalysts for carbon dioxide reforming of methane. TG-DTA results of HN and HA catalysts are shown in Figure 3.

  Figure 3.  TG (a) and DTA (b) profiles of the used catalysts

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  After 120h reforming reaction，the amounts of carbon deposition on HN and HA catalyst are 4.5% and 6.4%, respectively (Figure 3 (a)). They are much lower than the carbon deposition of Ni/SiC (13.5%) and Ni-Sm₅/SiC (7.2%) catalysts prepared by impregnation method^[12]. It suggests that the carbon deposition is suppressed due to the homogeneous dispersion of Ni nanoparticles. Besides, there is a tiny peak ranging from 300 to 400℃ for the both catalysts which is a result of Ni⁰ oxidation^[19]. Figure 3 (b) shows the DTA profiles of both catalysts. The exothermic peaks caused by carbon combustion of HN and HA catalysts are around 570 and 600℃, respectively. The different exothermic peak positions are due to the different structure and morphology of coke. The TG-DTA analysis demonstrates that the catalysts prepared by hydrothermal method have good resistance to carbon deposition.

  2.5   Catalytic performance

  The catalytic performance of HN and HA catalysts for carbon dioxide reforming of methane are investigated. They both exhibit excellent activity and stability as shown in Figure 4.

  Figure 4.  Evolution of conversion with time on stream over different catalysts

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  For the HN catalyst (Figure 4(a)), the conversions of CH₄ and CO₂ are above 96% and 92%, respectively. During the 120 h reaction, there is no loss of activity. The HA catalyst exhibits the similar activity as HN catalyst (Figure 4(b)). The conversions of CH₄ and CO₂ of HA catalyst are around 95% and 90%, respectively. However, there is an obvious fluctuation of CO₂ conversion in the first few hours of the reaction which means the existence of carbon deposition-elimination process (CH₄=C+2H₂, CO₂+C=2CO)^[20]. The conversions of CH₄ and CO₂ of the Ni-Sm₅/SiC catalyst prepared by impregnation method are around 95% and 91%, respectively^[12]. The TEM images of fresh and used HN catalysts are compared as shown in Figure 5.

  Figure 5.  TEM images of the (a) fresh and (b) used HN catalysts

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  There were only some silk-like structures detected for fresh HN catalyst (Figure 5(a)). It indicated the active component was well-distributed over the surface of SiC support and the active metal particles can not be seen by HRTEM analysis. Combined with BET and XRD results, it further confirms the active metal nanoparticles formed by hydrothermal synthesis have ultra small size and high dispersion^[21]. After the reaction, the metal nanoparticles can be obviously observed with an average size of 11.5nm which is close to the size (14.5nm) calculated by XRD measurements. Besides, some crystalline carbon nanotubes can be found on the catalyst (Figure 5(b)).

  3   Conclusions

  In this paper, the Ni-Sm/SiC (Ni: 9%, Sm: 5%) catalysts prepared by a hydrothermal route show excellent activity and stability for CO₂ reforming of CH₄. Different nickel precursors nearly have no effect on the properties of the Ni-Sm/SiC catalysts. Nickel nanoparticles are uniformly dispersed and have strong interaction with SiC support by the hydrothermal method. Besides, the carbon deposition is well controlled.

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  Figure 1  XRD patterns of the fresh (a) and used (b) Ni-Sm/SiC catalysts

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  Figure 2  H₂-TPR profiles of the Ni-Sm/SiC catalysts

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  Figure 3  TG (a) and DTA (b) profiles of the used catalysts

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  Figure 4  Evolution of conversion with time on stream over different catalysts

  (a): HN; (b): HA reaction conditions: temperature=800℃，GHSV=10000 mL/(g·h), CH₄/CO₂(molar ratio)=1.0

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  Figure 5  TEM images of the (a) fresh and (b) used HN catalysts

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  Table 1.  BET and ICP results of the Ni-Sm/SiC catalysts

  SamplePure SiCHN catalystHA catalystSurface area
  A/(m²·g^-1)36.882.274.7Sm loading w/% -4.914.57

  Ni loading w/%	-	8.62	8.52

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