Core-Shell Structured Ni @ SiO 2 Catalysts Exhibiting Excellent Catalytic Performance for Syngas Methanation Reactions

In this study, we prepared core-shell structured Ni@SiO2 catalysts using chemical precipitation and modified Stöber methods. The obtained Ni@SiO2 samples exhibited excellent catalysis performances, including high CO conversion of 99.0% and CH4 yield of 89.8%. Moreover, Ni@SiO2 exhibited excellent catalytic stability during a 100 h lifetime test, which was superior to that of the Ni/SiO2 catalyst. The prepared samples were characterized using a series of techniques, and the results indicated that the catalytic performance for syngas methanation reaction of the Ni@SiO2 sample was markedly improved owing to its nanoreactor structure. The strong interaction between the Ni core and the SiO2 shell effectively restrained the growth of particles, and the deposition of C species.


Introduction
Today's society faces many energy challenges, including environmental pollution, expensive crude oil prices, and short supply of natural gas resources [1].Natural gas is a promising energy source as it is good quality fuel and a chemical raw material.Over the past decade, the coal gasification method for preparing synthetic gas has been extensively investigated.CO methanation is one of the most important reactions investigated for this process [2][3][4][5][6].
Conversion of coal to methane via gasification and methanation processes can not only utilize the abundant coal resources but also meet the rapidly growing demand for natural gas in coal-rich regions or countries [7].The methanation of syngas has been considered as the most critical step in the processing of coal to produce synthetic natural gas [8][9][10].In recent years, highly efficient and stable catalysts for syngas methanation have been developed.
Noble metals, such as Pd, Rh, and Ru, exhibit high chemical stability and excellent activity for syngas methanation at low temperatures [11][12][13].However, the limited reserves and high prices of noble metals greatly limit their industrial applications.Owing to the excellent activity as comparable to the noble metal catalysts and the low-cost, Ni-based catalysts have been widely used for syngas methantion [14].However, using conventional Ni-based catalysts still has enormous challenges.The methanation of CO is a highly exothermic process; hence, the heat produced on the surface of the catalyst would cause serious problems, such as C deposition and aggregation of Ni nanoparticles.
In recent years, catalysts with a core-shell structure have been widely used in various catalysis fields.The metal component of active nanoparticles is coated with an inert nanoporous oxide shell, and the outer shells effectively protect the active nanoparticle cores from sintering when reactions are performed at high temperature.The unique physical and/or chemical interactions can result in synergic effects.Chao Wang et al. [15] successfully synthesized the Ni@graphene composites comprising a graphene shell and a Ni metal core using an arc-discharge method.The outer carbon sheath prevents the inner Ni nanoparticle from being etched when the sample is exposed to air, H 2 O 2 , or acids.Takenaka et al. [16] reported that a SiO 2 -coated nickel-based catalyst exhibited excellent catalytic performance and good stability in partial oxidation of methane (POM) reaction, as compared with conventional Ni/Al 2 O 3 .Li et al. [17] synthesized Ni@meso-SiO 2 catalysts exhibiting excellent catalytic performance in the POM reaction to synthesize gas.Ni particles encapsulated in core-shell structured catalysts exhibited high catalytic activities and enhanced inhibition toward C deposition [18,19].Li et al. [20] successfully synthesized transition metal (Ni, Cu, Co) coated in hollow zeolite single crystals by recrystallization and impregnated method, in hydrogenation reaction, coated Ni particles showed excellent size-selectivity performance.Based on the above-mentioned studies, it can be inferred that Ni@SiO 2 with active Ni as the core and SiO 2 as the coated shell may exhibit excellent catalytic performance in CO methanation, owing to its unique structure.
Herein, we report the synthesis of Ni@SiO 2 catalysts as well as their catalytic activity and stability for methanation reaction at different temperatures.Characterization methods, such as N 2 physical adsorption, X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H 2 -TPR), thermo-gravimetric analysis (TGA), and transmission electron microscopy (TEM), were employed for understanding the relationship between the catalytic performance and the physical structure of the catalyst.

Characterization of the Catalysts
Figure 1 shows the XRD results obtained for NiO@SiO 2 and NiO/SiO 2 , revealing that both the catalysts exhibited similar diffraction peaks.The diffraction peaks observed at 2θ = 37.3 • , 43.2 • , and 63.9 • could be attributed to (111), (200), and (220) of the NiO phase, respectively (JCPDS No. 78-0429).There could still be oxidized nickel species present as amorphous phases or clusters too small to be observed by XRD. Figure 2 shows the XRD pattern of the catalyst obtained after performing in situ reduction.The peaks observed at 44.  and the outer shells effectively protect the active nanoparticle cores from sintering when reactions are performed at high temperature.The unique physical and/or chemical interactions can result in synergic effects.Chao Wang et al. [15] successfully synthesized the Ni@graphene composites comprising a graphene shell and a Ni metal core using an arc-discharge method.The outer carbon sheath prevents the inner Ni nanoparticle from being etched when the sample is exposed to air, H2O2, or acids.Takenaka et al. [16] reported that a SiO2-coated nickel-based catalyst exhibited excellent catalytic performance and good stability in partial oxidation of methane (POM) reaction, as compared with conventional Ni/Al2O3.Li et al. [17] synthesized Ni@meso-SiO2 catalysts exhibiting excellent catalytic performance in the POM reaction to synthesize gas.Ni particles encapsulated in core-shell structured catalysts exhibited high catalytic activities and enhanced inhibition toward C deposition [18,19].Li et al. [20] successfully synthesized transition metal (Ni, Cu, Co) coated in hollow zeolite single crystals by recrystallization and impregnated method, in hydrogenation reaction, coated Ni particles showed excellent size-selectivity performance.Based on the above-mentioned studies, it can be inferred that Ni@SiO2 with active Ni as the core and SiO2 as the coated shell may exhibit excellent catalytic performance in CO methanation, owing to its unique structure.Herein, we report the synthesis of Ni@SiO2 catalysts as well as their catalytic activity and stability for methanation reaction at different temperatures.Characterization methods, such as N2 physical adsorption, X-ray diffraction (XRD), hydrogen temperature-programmed reduction (H2-TPR), thermo-gravimetric analysis (TGA), and transmission electron microscopy (TEM), were employed for understanding the relationship between the catalytic performance and the physical structure of the catalyst.

Characterization of the Catalysts
Figure 1 shows the XRD results obtained for NiO@SiO2 and NiO/SiO2, revealing that both the catalysts exhibited similar diffraction peaks.The diffraction peaks observed at 2θ = 37.3°, 43.2°, and 63.9° could be attributed to (111), (200), and (220) of the NiO phase, respectively (JCPDS No. 78-0429).There could still be oxidized nickel species present as amorphous phases or clusters too small to be observed by XRD. Figure 2 shows the XRD pattern of the catalyst obtained after performing in situ reduction.The peaks observed at 44.3°, 52.5°, and 76.8°correspond to the typical crystalline planes of Ni, indicating that NiO was completely reduced to Ni metal.XRD results show that the NiO and Ni nanoparticles in the core-shell catalyst before and after reduction exhibit a smaller grain size.The surface area, pore volume, and average pore size of NiO@SiO2 and NiO/SiO2 are shown in Table 1.The core-shell structured NiO@SiO2 exhibits a larger surface area and pore size.This could be because the NiO particles filled and blocked some of the silica pores in the NiO/SiO2 samples.The core-shell structured NiO@SiO2 catalysts exhibited smaller NiO crystallite sizes, as well as a modified SiO2 texture.The porous SiO2 shell allows the free access and transfer of gaseous reactant and product molecules.Thus, core-shell structures comprising metal particles on high surface area supports are known to be the most effective heterogeneous catalysts [21].
Table1.Characteristics of NiO@SiO2 and NiO/SiO2 samples determined by N2 sorption and XRD. Figure 3a,b illustrates the Transmission Electron Microscopy (TEM) image and the morphological features of the NiO@SiO2 and NiO/SiO2 samples.By examining 50-100 NiO particles, the diameter of the NiO cores in NiO@SiO2 catalyst is calculated to be ~11.2nm, which is much smaller than that of NiO/SiO2 (28.7 nm).HAADF-STEM measurements confirmed the existence of the core-shell structure.From Figure 3c,d, the distribution of the catalyst supports and the active components in the two samples seems to be significantly different.In case of the NiO@SiO2 catalyst, NiO core is evenly distributed on the SiO2 shell, whereas NiO nano-particles are distributed on the silica spheres in the case of the NiO/SiO2 sample.Figure 3e,f shows the TEM images of NiO@SiO2 and NiO/SiO2 both after hydrogen treatment, the core-shell structure of NiO@SiO2 sample can be well maintained after treated at 550 °C, 20% H2/Ar for 3 h, and the NiO nanoparticles are uniformly dispersed without obvious agglomeration.For NiO/SiO2, the aggregation of NiO nanoparticles appeared.The surface area, pore volume, and average pore size of NiO@SiO 2 and NiO/SiO 2 are shown in Table 1.The core-shell structured NiO@SiO 2 exhibits a larger surface area and pore size.This could be because the NiO particles filled and blocked some of the silica pores in the NiO/SiO 2 samples.The core-shell structured NiO@SiO 2 catalysts exhibited smaller NiO crystallite sizes, as well as a modified SiO 2 texture.The porous SiO 2 shell allows the free access and transfer of gaseous reactant and product molecules.Thus, core-shell structures comprising metal particles on high surface area supports are known to be the most effective heterogeneous catalysts [21].Figure 3a,b illustrates the Transmission Electron Microscopy (TEM) image and the morphological features of the NiO@SiO 2 and NiO/SiO 2 samples.By examining 50-100 NiO particles, the diameter of the NiO cores in NiO@SiO 2 catalyst is calculated to be ~11.2nm, which is much smaller than that of NiO/SiO 2 (28.7 nm).HAADF-STEM measurements confirmed the existence of the core-shell structure.From Figure 3c,d, the distribution of the catalyst supports and the active components in the two samples seems to be significantly different.In case of the NiO@SiO 2 catalyst, NiO core is evenly distributed on the SiO 2 shell, whereas NiO nano-particles are distributed on the silica spheres in the case of the NiO/SiO 2 sample.Figure 3e,f shows the TEM images of NiO@SiO 2 and NiO/SiO 2 both after hydrogen treatment, the core-shell structure of NiO@SiO 2 sample can be well maintained after treated at 550 • C, 20% H 2 /Ar for 3 h, and the NiO nanoparticles are uniformly dispersed without obvious agglomeration.For NiO/SiO 2 , the aggregation of NiO nanoparticles appeared.H 2 Temperature Programmed Reduction (H 2 -TPR) technique was employed in order to study the reduction behavior of supported oxide catalysts, and determine the effect of the core-shell structure on NiO encapsulation in the SiO 2 support, as well as the catalytic performance of the samples.In the case of NiO/SiO 2 , the NiO reduction peak is observed at a lower temperature (200-400 • C), while the H 2 reduction peak obtained for the NiO@SiO 2 catalyst can be divided into two peaks (Figure 4) the first peak observed in the range of 200-320 • C is associated with the NiO species and pure NiO [4].In this case, the interaction between NiO and SiO 2 is relatively weak [22], the second peak observed in the temperature range of 350-600 • C is associated with the NiO species forcefully interacted with the SiO 2 support [16].These results demonstrate that the interaction between the NiO nano-particles and the SiO 2 support becomes stronger after the encapsulation, which consistently matches the pioneer's results [19,23].The formation of this NiO species, which strongly interacted with coated SiO 2 , would enhance the catalytic performance and life for the syngas methanation reaction.

Samples
Catalysts 2017, 7, 21 5 of 12 H2 Temperature Programmed Reduction (H2-TPR) technique was employed in order to study the reduction behavior of supported oxide catalysts, and determine the effect of the core-shell structure on NiO encapsulation in the SiO2 support, as well as the catalytic performance of the samples.In the case of NiO/SiO2, the NiO reduction peak is observed at a lower temperature (200-400 °C), while the H2 reduction peak obtained for the NiO@SiO2 catalyst can be divided into two peaks (Figure 4) the first peak observed in the range of 200-320°C is associated with the NiO species and pure NiO [4].In this case, the interaction between NiO and SiO2 is relatively weak [22], the second peak observed in the temperature range of 350-600 °C is associated with the NiO species forcefully interacted with the SiO2 support [16].These results demonstrate that the interaction between the NiO nano-particles and the SiO2 support becomes stronger after the encapsulation, which consistently matches the pioneer's results [19,23].The formation of this NiO species, which strongly interacted with coated SiO2, would enhance the catalytic performance and life for the syngas methanation reaction.

Catalytic Performances of the Catalysts
The catalytic performance of the samples was tested under 1.5 MPa in the test temperature range of 250-550 °C. Figure 5A shows the results obtained for the Ni@SiO2 catalyst in the temperature range of 250-500 °C, demonstrating the maximum CO conversion, close to 100%.In the case of the Ni/SiO2 catalyst, the best activity was achieved in a narrow temperature range of 400-450 °C which is slightly lower than that of the Ni@SiO2 catalyst.Due to the influence of kinetic factors, the CO conversion increased with an increase in the temperature at the low temperature range.However, due to the limitation of thermodynamic equilibrium, the CO conversion decreased at the high-temperature range [24].Figure 5B illustrates the CH4 yield comparison of different samples.For both of the catalysts, the yield of CH4 increased with an increase in the reaction temperature before reducing the samples, which could be attributed to the thermodynamic limitation.As the two samples prepared using different methods exhibited different structures, we speculate that the catalytic performance could be greatly influenced by the structure of the catalyst.When the same amount of Ni was used (10 wt %), the catalyst with the core-shell structure exhibited a higher CO conversion and CH4 yield.

Catalytic Performances of the Catalysts
The catalytic performance of the samples was tested under 1.5 MPa in the test temperature range of 250-550 • C. Figure 5A shows the results obtained for the Ni@SiO 2 catalyst in the temperature range of 250-500 • C, demonstrating the maximum CO conversion, close to 100%.In the case of the Ni/SiO 2 catalyst, the best activity was achieved in a narrow temperature range of 400-450 • C which is slightly lower than that of the Ni@SiO 2 catalyst.Due to the influence of kinetic factors, the CO conversion increased with an increase in the temperature at the low temperature range.However, due to the limitation of thermodynamic equilibrium, the CO conversion decreased at the high-temperature range [24].Figure 5B illustrates the CH 4 yield comparison of different samples.For both of the catalysts, the yield of CH 4 increased with an increase in the reaction temperature before reducing the samples, which could be attributed to the thermodynamic limitation.As the two samples prepared using different methods exhibited different structures, we speculate that the catalytic performance could be greatly influenced by the structure of the catalyst.When the same amount of Ni was used (10 wt %), the catalyst with the core-shell structure exhibited a higher CO conversion and CH 4 yield.On examining the results obtained with the catalyst performance test and the XRD, TEM, and TPR measurements, samples containing small NiO particles are found to exhibit an excellent catalyst performance.The sintering of the Ni core can be effectively reduced owing to the narrow space between the core and the shell as a strong interaction might cause an increase in the reduction temperature of catalyst, TPR analysis results support our speculation [23].Amphipathic and nonionic PVP polymers can be adsorbed on colloidal particles, and PVP have been commonly employed in the preparation of mesoporous samples.On the other hand, the good dispersity of the active component could be helpful in the formation of uniform pores.The uniform pores are also conducive to the entering of reactant and product [17,23].

Stability of the Catalysts
The 100 h lifetime test was conducted at 500 °C under a pressure of 1.5 MPa, and Gas Hour Space Velocity (GHSV) = 36,000 h −1 and the test results are displayed in Figure 6 For Ni@SiO2, no obvious decrease in the CO conversion is observed with the in 100 h stability test.For the Ni/SiO2 catalyst, the CO conversion is significantly decreased after 12 h.Thus, Ni@SiO2 exhibited excellent stability under selected reaction conditions.On examining the results obtained with the catalyst performance test and the XRD, TEM, and TPR measurements, samples containing small NiO particles are found to exhibit an excellent catalyst performance.The sintering of the Ni core can be effectively reduced owing to the narrow space between the core and the shell as a strong interaction might cause an increase in the reduction temperature of catalyst, TPR analysis results support our speculation [23].Amphipathic and non-ionic PVP polymers can be adsorbed on colloidal particles, and PVP have been commonly employed in the preparation of mesoporous samples.On the other hand, the good dispersity of the active component could be helpful in the formation of uniform pores.The uniform pores are also conducive to the entering of reactant and product [17,23].

Stability of the Catalysts
The 100 h lifetime test was conducted at 500 • C under a pressure of 1.5 MPa, and Gas Hour Space Velocity (GHSV) = 36,000 h −1 and the test results are displayed in Figure 6 For Ni@SiO 2 , no obvious decrease in the CO conversion is observed with the in 100 h stability test.For the Ni/SiO 2 catalyst, the CO conversion is significantly decreased after 12 h.Thus, Ni@SiO 2 exhibited excellent stability under selected reaction conditions.Figure 6.Stability of the Ni@SiO2 catalyst for CO methanation at 500 °C, 1.5 MPa, 36,000 mL/g/h.

Characterization of the Used Catalysts
Figure 7a,b demonstrate the TEM image of NiO@SiO2 and Ni/SiO2catalysts obtained after performing the activity test.The average particle size of the NiO encapsulated in SiO2 sample is around 27.1 nm, and the diameter of NiO on the surface of the silica particles is around 45.3 nm.Since it is difficult to achieve complete individual particle encapsulation, there is a degree of aggregation in the NiO@SiO2 catalyst after the reaction, this result is consistent with the literature [16,23].The NiO@SiO2 catalyst exhibits a smaller particle size and a higher uniform dispersion than those of NiO/SiO2, indicating that the SiO2 shell effectively protects the NiO cores, in the calcination process performed at high temperatures.Exposed metal nanoparticles are commonly air-sensitive [16]; however, after coating metal nanoparticles with SiO2, their thermal stability improves [25].From the TPR results, owing to the stronger interaction between the NiO core and the SiO2 shell, the inhibition of the active component of Ni aggregationin the reaction process.The TGA curves of the Ni-based catalysts obtained after performing the lifetime test are shown in Figure 8. TGA curves indicate the weight loss of the samples at two temperature ranges.The first weight loss was observed below 400 °C due to the removal of organic components and physically adsorbed water molecules.The second weight loss observed could be associated to the oxidation of graphitized C species occurred in the 400-600 °C range [26,27].In the TGA test temperature range, the weight of the Ni@SiO2 catalyst sample reduces approximately by 8.94%, which could be attributed

Characterization of the Used Catalysts
Figure 7a,b demonstrate the TEM image of NiO@SiO 2 and Ni/SiO 2 catalysts obtained after performing the activity test.The average particle size of the NiO encapsulated in SiO 2 sample is around 27.1 nm, and the diameter of NiO on the surface of the silica particles is around 45.3 nm.Since it is difficult to achieve complete individual particle encapsulation, there is a degree of aggregation in the NiO@SiO 2 catalyst after the reaction, this result is consistent with the literature [16,23].The NiO@SiO 2 catalyst exhibits a smaller particle size and a higher uniform dispersion than those of NiO/SiO 2 , indicating that the SiO 2 shell effectively protects the NiO cores, in the calcination process performed at high temperatures.Exposed metal nanoparticles are commonly air-sensitive [16]; however, after coating metal nanoparticles with SiO 2 , their thermal stability improves [25].From the TPR results, owing to the stronger interaction between the NiO core and the SiO 2 shell, the inhibition of the active component of Ni aggregationin the reaction process.

Characterization of the Used Catalysts
Figure 7a,b demonstrate the TEM image of NiO@SiO2 and Ni/SiO2catalysts obtained after performing the activity test.The average particle size of the NiO encapsulated in SiO2 sample is around 27.1 nm, and the diameter of NiO on the surface of the silica particles is around 45.3 nm.Since it is difficult to achieve complete individual particle encapsulation, there is a degree of aggregation in the NiO@SiO2 catalyst after the reaction, this result is consistent with the literature [16,23].The NiO@SiO2 catalyst exhibits a smaller particle size and a higher uniform dispersion than those of NiO/SiO2, indicating that the SiO2 shell effectively protects the NiO cores, in the calcination process performed at high temperatures.Exposed metal nanoparticles are commonly air-sensitive [16]; however, after coating metal nanoparticles with SiO2, their thermal stability improves [25].From the TPR results, owing to the stronger interaction between the NiO core and the SiO2 shell, the inhibition of the active component of Ni aggregationin the reaction process.The TGA curves of the Ni-based catalysts obtained after performing the lifetime test are shown in Figure 8. TGA curves indicate the weight loss of the samples at two temperature ranges.The first weight loss was observed below 400 °C due to the removal of organic components and physically adsorbed water molecules.The second weight loss observed could be associated to the oxidation of graphitized C species occurred in the 400-600 °C range [26,27].In the TGA test temperature range, the weight of the Ni@SiO2 catalyst sample reduces approximately by 8.94%, which could be attributed The TGA curves of the Ni-based catalysts obtained after performing the lifetime test are shown in Figure 8. TGA curves indicate the weight loss of the samples at two temperature ranges.The first weight loss was observed below 400 • C due to the removal of organic components and physically adsorbed water molecules.The second weight loss observed could be associated to the oxidation of graphitized C species occurred in the 400-600 • C range [26,27].In the TGA test temperature range, the weight of the Ni@SiO 2 catalyst sample reduces approximately by 8.94%, which could be attributed to the production of C that probably causes reduction in catalyst activity as obtained in the performance test.However, the weight of the Ni/SiO 2 catalyst sample is reduced by 56.73%, demonstrating that the C deposition occurs easily at the Ni/SiO 2 catalyst surface by encapsulating Ni nanoparticles in the SiO 2 support, the C deposition behavior is clearly changed.The strong interactionbetween the Ni core and the SiO 2 shell effectively inhibits the formation of C deposits.TGA analysis provides evidence for the excellent CO conversion for Ni@SiO 2 obtained in the 100-h lifetime activity test.
Catalysts 2017, 7, 21 8 of 12 to the production of C that probably causes reduction in catalyst activity as obtained in the performance test.However, the weight of the Ni/SiO2 catalyst sample is reduced by 56.73%, demonstrating that the C deposition occurs easily at the Ni/SiO2 catalyst surface by encapsulating Ni nanoparticles in the SiO2 support, the C deposition behavior is clearly changed.The strong interactionbetween the Ni core and the SiO2 shell effectively inhibits the formation of C deposits.TGA analysis provides evidence for the excellent CO conversion for Ni@SiO2 obtained in the 100-h lifetime activity test.

Catalyst Preparation
Nickel nitrate (Ni (NO3)2•6H2O) was acquired from Aladdin (Aladdin Industrial Corporation, Los Angeles, Southern CA, USA), and polyethylene glycol (PEG) (average MW = 20,000) and NH3•H2O (25 wt %) were obtained from Chengdu Kelong Chemical Reagent Factory.Tetraethylorthosilicate (TEOS, SiO2, 28%) was purchased from Tianjin Da mao Chemical Reagent Factory, Tianjin, China.Sodium hydroxide (NaOH) was obtained from Tianjin Yong Sheng Fine Chemical Co., Ltd., Tianjin, China.Absolute ethyl alcohol was acquired from Tianjin Fu Yu Fine Chemical Co., Ltd., Tianjin, China.All of the materials used in this study were of AR grade and were directly used without any further purification.

Preparation of Ni@SiO2 Catalyst
The catalyst was prepared in two steps.In the first step, NiO nanoparticles were prepared using a chemical precipitation method.The preparation procedure can be described as follows: 330 mg PEG (average MW = 20,000) and 1.0 g NaOH was dissolved in 100 mL deionized water.Subsequently, 2.9 g nickel nitrate was added to 40 mL deionized water, and the obtained solution was added dropwise to the NaOH solution.The obtained mixture was stirred for 2 h.The solid product obtained was collected by suction filtration and washed with absolute ethyl alcohol and deionized water several times.The product was dried at 50 °C for 24 h followed by subjecting to calcinations at 450 °C for 2 h.
In the second step, NiO@SiO2 was prepared using a modified Stöber method [28,29].In a typical reaction, 0.2 g of the prepared NiO nanoparticles, 1.0 g PVP (K30), and 100 mL anhydrous ethanol were mixed and stirred for 12 h.10 mL of aqueous solution of ammonia (28%) was added to this solution, followed by sonicating the suspension for 0.5 h.Subsequently, TEOS (SiO2, 28%) was dissolved in 5 mL anhydrous ethanol in desired amounts at room temperature.This solution was then injected into the suspension.After 1 h, the product was isolated by centrifugation and washed

Catalyst Preparation
Nickel nitrate (Ni (NO 3 ) 2 •6H 2 O) was acquired from Aladdin (Aladdin Industrial Corporation, Los Angeles, Southern CA, USA), and polyethylene glycol (PEG) (average MW = 20,000) and NH 3 •H 2 O (25 wt %) were obtained from Chengdu Kelong Chemical Reagent Factory.Tetraethylorthosilicate (TEOS, SiO 2 , 28%) was purchased from Tianjin Da mao Chemical Reagent Factory, Tianjin, China.Sodium hydroxide (NaOH) was obtained from Tianjin Yong Sheng Fine Chemical Co., Ltd., Tianjin, China.Absolute ethyl alcohol was acquired from Tianjin Fu Yu Fine Chemical Co., Ltd., Tianjin, China.All of the materials used in this study were of AR grade and were directly used without any further purification.

Preparation of Ni@SiO 2 Catalyst
The catalyst was prepared in two steps.In the first step, NiO nanoparticles were prepared using a chemical precipitation method.The preparation procedure can be described as follows: 330 mg PEG (average MW = 20,000) and 1.0 g NaOH was dissolved in 100 mL deionized water.Subsequently, 2.9 g nickel nitrate was added to 40 mL deionized water, and the obtained solution was added dropwise to the NaOH solution.The obtained mixture was stirred for 2 h.The solid product obtained was collected by suction filtration and washed with absolute ethyl alcohol and deionized water several times.The product was dried at 50 • C for 24 h followed by subjecting to calcinations at 450 • C for 2 h.
In the second step, NiO@SiO 2 was prepared using a modified Stöber method [28,29].In a typical reaction, 0.2 g of the prepared NiO nanoparticles, 1.0 g PVP (K30), and 100 mL anhydrous ethanol were mixed and stirred for 12 h.10 mL of aqueous solution of ammonia (28%) was added to this solution, followed by sonicating the suspension for 0.5 h.Subsequently, TEOS (SiO 2 , 28%) was dissolved in 5 mL anhydrous ethanol in desired amounts at room temperature.This solution was then injected into the suspension.After 1 h, the product was isolated by centrifugation and washed with ethanol, followed by drying in air at 80 • C for 6 h.The sample was than calcined at 550 • C for 3 h in air.The coated sample thus obtained is named as NiO@SiO 2 .

Preparation of Ni/SiO 2 Catalyst
For comparison purposes, SiO 2 support was prepared by the modified Stöber method.The same procedure as the second step was used.The Ni/SiO 2 was prepared using a conventional impregnation method as reported earlier [30].The loading of nickel in Ni@SiO 2 and Ni/SiO 2 catalysts is the same, both are 10 wt %.

Catalyst Characterization
Powder XRD measurements were carried out in order to examine the crystal structure of the Ni-based catalyst.With a Bruker Advanced D8 X-ray diffractometer using Cu-Kα irradiation (λ = 1.5406Å) as the X-ray source at 40 kV and 40 mA.An angular range of 10 • -90 • was employed with a step size of 0.02.The grain size was calculated using Scherrer's formula.
Brunauer-Emmett-Teller (BET) surface area analysis was employed to reveal the porous structure and the nitrogen adsorption isotherm of the samples with a Micromeritics ASAP 2020 system (Micromeritics Instrument Ltd., Norcross, GA, USA).All the samples were degassed under vacuum at 300 • C for 6 h.The average pore size and the pore volume of the samples were calculated using the Barrett-Joyner-Halenda method.The BET surface area was determined using the BET method.
The morphological features of the sample were analyzed using a Tecnai G2 F20 TEM (FEI, Hillsboro, OR, USA) operating at 200 kV.The distribution of Ni metal and SiO 2 support on the catalyst was examined using high-angle annular dark field scanning TEM (HAADF-STEM) (FEI, Hillsboro, OR, USA).Before performing the TEM measurements, the catalyst powder was dispersed in ethanol using ultrasonic cleaners at room temperature for 30 min.
H 2 -TPR experiment was conducted in a quartz tube reactor using 0.1 g of the catalyst samples.Before performing the analysis, all samples were degassed at 110 • C for 4 h.TPR measurements were performed in 10% H 2 /Ar mixture, with a gas flow of 40 mL/min, heating rate of 10 • C/min, and reduction temperature ranging from room temperature to 900 • C.
TGA of the samples was performed with a TGA/DTA system (SDT Q600, TA Instruments, new castle, DE, USA) by heating the samples from room temperature to 900 • C in O 2 flow of 10 mL/min.

Evaluation of the Catalyst Performance
The catalyst activity of Ni@SiO 2 and Ni/SiO 2 for CO methanation was evaluated.Syngas methanation was carried out in a stainless steel tube having an internal diameter of 10 mm in a fixed-bed reactor at 1.5 MPa pressure.The catalytic performance test was performed at the temperature of range of 250 • C-550 • C with an increase of 50 • C in each step.
Initially, a certain amount of Ni@SiO 2 catalyst (0.14 g, 100 mesh) was placed in the upper filling medium quartz sand (30 mesh), in a stainless steel tubular reactor (length: 40 cm, inner diameter: 10 mm).The Ni@SiO 2 catalyst was purged with N 2 30 mL/min.Before performing the test, the catalyst was treated with online reduction, at 500 • C, (reduction of pure hydrogen gas, flow rate of 50 mL/min, for 2 h).After the reduction, when the temperature was decreased to 250 • C, the feed syngas (H 2 :N 2 :CO molar ratio of 3:1:1) was introduced, In order to analyze the gaseous products obtained; the products were analyzed using gas chromatography techniques.
In order to evaluate the CO conversion and the CH 4 selectivity, at each temperature, the outlet gases were collected and analyzed after 30 min of steady-state operation.The CO conversion (X CO ) and CH 4 selectivity (S CH 4 ) are defined as follows: n CO(in) × 100% (3)

Conclusions
In summary, core-shell structured Ni@SiO 2 catalysts were prepared using the modified Stöber method.The obtained Ni@SiO 2 catalyst particles exhibited higher catalytic activity than that of the Ni/SiO 2 catalyst prepared by the conventional impregnation method.Under the operation conditions employed (1.5 MPa, H 2 /CO/N 2 = 3:1:1), nearly 100% CO conversion was achieved over the Ni@SiO 2 catalyst sample along with a CH 4 yield of ~90% (400-500 • C).In the 100-h stability test, the core-shell structure of Ni@SiO 2 shows excellent stability; CO conversion is always greater than 95%.The enhanced performance obtained for Ni@SiO 2 could be mainly attributed to the stronger interaction of the Ni@SiO 2 catalyst.Particles having highly uniform-sized Ni particles Furthermore, TGA results demonstrated that the core-shell structure might cause a reduction in the production of C deposits on the surface of the Ni active component.
•, 52.5 • , and 76.8 • correspond to the typical crystalline planes of Ni, indicating that NiO was completely reduced to Ni metal.XRD results show that the NiO and Ni nanoparticles in the core-shell catalyst before and after reduction exhibit a smaller grain size.

Table 1 .
Characteristics of NiO@SiO 2 and NiO/SiO 2 samples determined by N 2 sorption and XRD.