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Properties of yolk–shell structured Ni@SiO2 nanocatalyst and its catalytic performance in carbon dioxide reforming of methane to syngas
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Properties of yolk–shell structured Ni@SiO2 nanocatalyst and its catalytic performance in carbon dioxide reforming of methane to syngas Weiwei Yang, Huimin Liu, Yuming Li, Juan Zhang, Hao Wu, Dehua He ∗ Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 24 October 2014 Received in revised form 23 March 2015 Accepted 6 April 2015 Available online xxx Keywords: Ni@SiO2 -yolk–shell Carbon dioxide reforming Carbon deposition behavior Stability
a b s t r a c t Ni@SiO2 -yolk–shell catalyst was prepared by the method of etching Ni@SiO2 -core–shell with hydrochloric acid, while the core–shell catalyst was prepared by the stöber method. In comparison, Ni–SiO2 –AE (without yolk–shell structure) was used to denote the catalyst prepared by the similar method as Ni@SiO2 -yolk–shell but without fully encapsulation of Ni particles by SiO2 . The catalytic performances of these three catalysts were investigated in carbon dioxide reforming of methane to syngas (CRM). The reaction was conducted under different conditions by varying GHSVs of reactants and reaction temperatures to further test the stability of the catalysts. The properties of the as-prepared and spent Ni@SiO2 -yolk–shell, Ni@SiO2 -core–shell and Ni–SiO2 –AE catalysts were characterized by TEM/HRTEM, XRD, TG-DSC to measure the morphology, crystallinity and carbon deposition, etc. It was found that compared to Ni@SiO2 -core–shell and Ni–SiO2 –AE catalysts, Ni@SiO2 -yolk–shell catalyst behaved remarkably stable performance at the temperature up to 1073 K, and the specific structure and carbon depositionresisted behavior of the Ni@SiO2 -yolk–shell catalyst may help to maintain the stability during the reaction. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Methane is one of the most important fossil energies, and has been widely used as a chemical raw material recently. The utilization of methane includes direct conversion ways (such as oxidative coupling, selective oxidation, aromatization, etc.) and indirect conversion ways (such as steam reforming and carbon dioxide reforming of methane). Carbon dioxide reforming of methane (CRM) could convert two greenhouse gases (CH4 and CO2 ) into syngas, which could be further employed to synthesize high-value added products. Ni-based catalysts have been widely investigated and considered to be potential for the industrial utilization for CRM reaction. However, the sintering of Ni particles and carbon deposition on Ni-based catalysts limited their applications. Therefore, to develop highly active and stable Ni-based catalysts are of vital importance. Usually, CRM reaction was carried out at high temperatures (700–850 ◦ C) in consideration of the thermodynamics and dynamics factors. The active component Ni is tend to be aggregated when
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the operation temperature is above its Tammann temperature (590 ◦ C) [1] during the CRM reaction, which is driven by the reduction of the total surface energy [2]. Decomposition of methane and disproportionation of CO could occur and lead to carbon deposition (including amorphous carbon, encapsulating carbon and filamentous carbon) during CRM reaction process. For the formation of filamentous carbon, Baker’s research [3] indicated that temperature gradient drove the diffusion of carbon to form whisker-like carbon, while Trimm [4] and Snoeck [5,6] regarded the concentration gradient as the force to drive the diffusion of carbon. But it is clear that the formation of the filamentous carbon would not influence greatly on the activity, while it would destroy the structure of the catalysts. On the other hand, the formation of the encapsulating carbon, which was accumulated on the surface of catalysts because of the much rapider speed of methane decomposition than the carbon elimination, would deactivate the activities of catalysts and block the reactor [4,7,8]. According to the previous studies, increasing the interaction between supports and metal active sites [9–13], selecting better thermal conductive supports or additives [14,15], and improving the dispersion of catalysts [13,16–19] all would help to relieve the sintering of the active sites. Meanwhile, the acidic–basic properties [13,20–22], oxidation–reduction properties [17,23], particle sizes
http://dx.doi.org/10.1016/j.cattod.2015.04.012 0920-5861/© 2015 Elsevier B.V. All rights reserved.
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[24–29] and noble metal modifications [30,31] would also influence the carbon deposition. Kawi et al. [13] reported a La2 O3 doped Ni/SiO2 catalyst, and found that La2 O3 influenced the dispersion of Ni and the interaction between SiO2 and Ni active sites as well as the basicity of the catalyst, which resulted in the excellent activity and stability of the catalyst during CRM reaction. Similarly, Yoon et al. [17] employed Ni–Ce/MgAl2 O4 catalyst to investigate the effect of Ce addition during combined steam and carbon dioxide reforming of methane (CSCRM) reaction by varying the Ce/Ni ratio, and found that Ce addition have improved the metal dispersion, reducibility as well as surface oxygen transfer. However, how to effectively prevent the sintering of Ni particles, and consequently to decrease the carbon deposition during the CRM, are key factors for keeping the stability of Ni catalysts. Ni@SiO2 nanomaterials with yolk–shell or core–shell structures are comprised of nickel cores inside silica shells, and can help to prevent the sintering of Ni particles during high temperature reactions. Due to their structural features, this kind of nanomaterials is called “nanoreactor”, and has been used as the catalysts in the reactions, such as steam reforming [32] and partial oxidation [33–35] as well as CO2 reforming [36] of methane to syngas. Song et al. observed [32] that Ni@SiO2 -yolk–shell nanocatalyst behaved relatively stable at high temperature up to 973 K for 240 min during steam reforming of methane (SRM). Takenaka et al. [33] indicated that the interaction between Ni metal particles and coated silica shell made the Ni@SiO2 -core–shell nanocatalyst show better performance than the impregnated Ni/SiO2 catalyst during the reaction of partial oxidation of methane (POM). Au’s research [34,35] found that the sizes of Ni nanoparticles over Ni@porous silica core–shell nanocatalyst were tunable, and the catalytic performance could be improved by doping with other metals. Recently, Kawi et al. [36] found that the silica shell thickness would also influence the structure of the catalyst and the catalytic performance during the CRM reaction due to the strength of interaction between Ni core and silica shell. Compared with SRM, POM and combination of the three ways for methane reforming, CRM is more difficult to prevent carbon deposition because the carbon elimination property of CO2 is much weaker than that of O2 or H2 O, which would easily lead to severe carbon deposition and even block the reactor within a short time. Yolk–shell structured Ni@SiO2 perhaps is a suitable nanoreactor and could effectively prevent the sintering of Ni particles during the CRM. Up to now, it seems that there have been no reports employing such a Ni@SiO2 yolk–shell catalyst (in consideration of the catalyst structure and the Ni core size) in CRM. In this work, we prepared and characterized Ni@SiO2 nanomaterials, especially Ni@SiO2 -yolk–shell, and applied them in CRM reaction. Quite interestingly, Ni@SiO2 -yolk–shell catalyst exhibited excellent performance in CRM. Therefore, the reasons why it could help to solve the sintering problem and what is the carbon deposition behavior of such kind of catalysts, as well as the long-time stability during the CRM reaction were studied in detail.
2. Experimental 2.1. Catalyst preparation Ni@SiO2 -yolk–shell nanocatalyst was prepared by three steps, including the preparation of Ni nanoparticles, coating Ni particles with SiO2 shell to synthesize Ni@ SiO2 -core–shell nanoparticles and etching Ni@SiO2 -core–shell with hydrochloric acid, as described below [32]. In step 1, Ni nanoparticles were prepared by a polyol process. 1.05 g Ni(acac)2 and 5.35 g PVP were added into 50 mL 1,5-pentanadiol, then the solution was heated from RT to 473 K
in 20 min and stirred at 473 K for 4 h under N2 atmosphere. The solution was further heated to 513 K in 10 min and stirred at 513 K for 1 h. Formed Ni nanoparticles were collected by dissolving in acetone and washing with ethanol for four times. In step 2, the obtained Ni nanoparticles were dispersed in 40 mL ethanol and the obtained suspension solution was dispersed with ultrasonic assistance for 10 min, and then 2.0 mL aqueous ammonia solution (25–28%, 0.907 g/cm3 ) was added into the suspension solution while stirring. This suspension solution was dispersed with ultrasonic assistance for another 2 min. Then 0.1 mL C18 TMS was added into the suspension solution and ultrasonically dispersed for 10 min. After that, 0.1 mL TMOS was added dropwise, and the resulted suspension solution was stirred at room temperature for 1 h. Finally, Ni@SiO2 -core–shell nanocatalyst was obtained after collecting the nanoparticles by centrifugation and washing with ethanol for four times. In step 3, the Ni@SiO2 -core–shell nanocatalyst obtained from the above steps was dispersed in 40 mL ethanol, and 2.5 mL hydrochloric acid dissolved in 10 mL ethanol was added. The resulted suspension solution was stirred and ultrasonically dispersed simultaneously for 30 min. The product obtained by centrifugation was washed with ethanol, and then Ni@SiO2 yolk–shell nanocatalyst was collected. In comparison, Ni–SiO2 –BE nanocatalyst was prepared by the similar method as Ni@SiO2 -core–shell nanocatalyst but the prepared SiO2 sol and Ni nanoparticles were directly mixed together in advance, as described below. Firstly, TMOS was hydrolyzed for 30 min. Then Ni nanoparticles, which were obtained by the method as mentioned above, were added into hydrolyzed TMOS. The Ni nanoparticles mixed with silica were stirred for another 30 min. Then, the obtained Ni–SiO2 –BE by the centrifugal separation was also etched with hydrochloric acid in the same process as the preparation of Ni@SiO2 -yolk–shell, and finally the collected solid was dried, and then Ni–SiO2 –AE nanocatalyst was obtained.
2.2. Catalyst characterization Transmission electron microscopy (TEM, Hitachi Limited, H7650B) and High Resolution Transmission Electron Microscopy (HRTEM, JEOL Ltd, JEM-2010) were used to observe the morphologies and particle sizes of the catalysts. The sample was ultrasonically dispersed for 30 min in ethanol, and then was dripped to the copper mesh. The particle size distributions of the samples could be derived by measuring the sizes of Ni particles through TEM iamges. The crystalline phases of the catalysts were investigated by Xray diffraction (XRD, Bruker D8 Advance X-Ray Diffractometer). The instrument was powered at 40 kV and 200 mA, using Cu K␣ radiation. The scan angle was from 10◦ to 90◦ with a scan speed of 10◦ /min. The crystal sizes of Ni particles were calculated by Scherrer equation. The loadings of Ni were measured by Inductively Coupled Plasma (ICP, ThermoFisher, IRIS Intrepid II XSP), while the main operating parameters were 1150 W, 26.0 PSI, 1.0 LPM, and 100 r/min. The amounts of carbon deposition on the catalysts were measured by thermal gravity analysis (TG), while the types of carbon deposited could be differentiated by Differential Scanning calorimetry (DSC, on Mettler Toledo TGA/SDTA851e ). The sample was heated from room temperature to 800 ◦ C with a rate of 10 ◦ C/min under flowing air. Raman spectroscopy was conducted at LabRAM HR Evolution with a 514 nm laser to measure the different types of carbon deposited on catalysts. The scan range from 100 to 3000 cm−1 , but the main information located at 1200–1700 cm−1 .
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Table 1 Physicochemical properties of Ni@SiO2 samples.
2.3. Catalyst performance test CRM reaction was conducted in a continuous flow, vertical fixedbed quartz tube reactor (internal diameter 6 mm, external diameter 8 mm) under atmosphere pressure. Before reaction, the catalyst was reduced at 500 ◦ C for 2 h with a flow of 35% H2 /Ar. After reduction, the catalyst bed temperature was raised to the reaction temperature, and then the reactant mixture CH4 and CO2 with the molar ratio of 1:1 was introduced into the reactor. The composition of effluent gas was analyzed by on-line gas chromatographs (GC) with a thermal conductivity detector. The method for calculating the conversions of CH4 and CO2 , as well as the selectivities of CO and H2 , was available elsewhere [37].
Sample
Carbon deposition (%)b
Ni size (nm)
Fresh (TEM) Fresh (XRD) Spent (XRD) Ni@SiO2 65.5 core–shell Ni@SiO2 42.4 yolk–shell Ni–SiO2 –AE 51.6 a b c d e
3. Results and discussion
Ni content (%)a
c
57.6
53.9 ± 5.4
45.7
46.2c
22.0d
46.4 ± 5.7
41.2
26.7d
49.2e
51.8 ± 20.0 48.7
26.4e
Measured by ICP. Calculated by TG. After 5 h reaction. After 100 h reaction. After 1.5 h reaction.
3.1. Characterization of as-prepared catalysts Fig. 1 shows the XRD patterns of the as-prepared, reduced, and spent Ni@SiO2 nanocatalysts. Fig. 1(a)–(d) revealed the crystal structures of Ni nanoparticles, Ni@SiO2 -core–shell, Ni@SiO2 yolk–shell and Ni–SiO2 –AE, respectively. The diffraction peaks of Ni(1 1 1), Ni(2 0 0) and Ni(2 2 0) at 2 = 44.5◦ , 51.8◦ and 76.4◦ could be assigned to cubic-Ni phase (JCPDS no. 65-2865), while other peaks could be ascribed to hexagonal-Ni phase (JCPDS no. 45-1027) except the one located at 2 = 23.5◦ . The weak and broad peak at 2 = 23.5◦ should be assigned to silica. Interestingly, the hexagonal phase Ni all changed into cubic phase Ni after reducing the samples at 500 ◦ C for 2 h, as shown in Fig. S1. As shown in Table 1, the crystal sizes of as prepared Ni NPs, Ni@SiO2 -core–shell, Ni@SiO2 yolk–shell and Ni–SiO2 –AE calculated by Scherrer equation were 44.5, 45.7, 41.2 and 48.7 nm, respectively. Fig. 2 illustrates the TEM and HRTEM images of the fresh Ni nanoparticles (Fig. 2(a)), Ni@SiO2 -core–shell (Fig. 2(b and c)), Ni@SiO2 -yolk–shell (Fig. 2(d and e)), and Ni–SiO2 –AE (Fig. 2(f)). Compared with Fig. 2(c) and (e), it can be seen that the inside Ni cores were close to the outside SiO2 shells for Ni@SiO2 -core–shell, while for Ni@SiO2 -yolk–shell, hollow spaces existed clearly after etching Ni@SiO2 -core–shell with HCl. Based on the results from TEM, the particle size distributions were measured and listed in Table 1. The average sizes were 53.9 nm both for Ni nanoparticles and the Ni core of Ni@SiO2 -core–shell. And the average Ni size dropped to 46.4 nm after etching Ni@SiO2 -core–shell with HCl. For Ni–SiO2 –AE, the average Ni size was 51.8 nm, and some of the
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Fig. 1. XRD patterns of (a) Fresh Ni NPs; (b) Fresh Ni@SiO2 -core–shell nanocatalyst, (c) Fresh Ni@SiO2 -yolk–shell nanocatalyst, (d) Fresh Ni–SiO2 –AE nanocatalyst.
Ni particles were coated together with SiO2 , while some Ni particles were even uncoated, which were quite nonuniform with a deviation of about ±20 nm, as shown in Fig. 2(f). And these results approximately agreed with the XRD results mentioned above. From the ICP results (Table 1), the Ni content for Ni@SiO2 core–shell was 65.5 wt%, while for Ni@SiO2 -yolk–shell, it decreased to 42.4 wt%. For Ni–SiO2 –AE, the Ni content was 51.6 wt%. 3.2. CRM reaction results The results of CRM reaction over those Ni catalysts at 700 ◦ C with a GHSV = 1.20 × 104 mL/(gcat h) are given in Figs. 3 and 4. For Ni@SiO2 -yolk–shell, as shown in Fig. 3(a), the conversion of CH4 was 43.9%, and the conversion of CO2 (57.3%) was higher than that of CH4 because of the reverse water–gas shift (RWGS) reaction (CO2 + H2 → CO + H2 O). The RWGS reaction also made the selectivity of H2 (83.6%) lower than that of CO (to 99.5%). The relatively sufficient amount of H2 O might help to eliminate the carbon deposition during CRM by the reaction (H2 O + C → CO + H2 ), which might be favorable for the improved stability of the catalyst. In comparison, for Ni–SiO2 –AE nanocatalyst, as shown in Fig. 3(b), the initial conversion of CH4 (74.9%) was higher than that of CO2 (72.4%), while H2 /CO ratio was higher than 1, because of the severe decomposition of methane (CH4 → 2H2 + C, H298k = 75 kJ/mol). This would lead to the quick carbon deposition on the Ni–SiO2 –AE catalyst and the increase of reactor pressure drop. On the other hand, Ni@SiO2 -core–shell, as shown in Fig. 3(c), also behaved similarly with Ni–SiO2 –AE, and the catalyst bed was blocked by carbon deposition within 5 h CRM reaction. To further test the stability of Ni@SiO2 -yolk–shell catalyst, Ni@SiO2 -yolk–shell was evaluated in CRM reaction under different conditions by varying the GHSVs of reactants and reaction temperatures. Firstly, the initial reaction temperature was kept at 700 ◦ C for about 260 h, while GHSVs were increased from 1.16 × 104 mL/(gcat h) to 4.62 × 104 mL/(gcat h) step by step, then decreased to 3.53 × 104 mL/(gcat h). Then the reaction temperature was increased to 800 ◦ C and kept at 800 ◦ C for about 280 h, while GHSVs were decreased from 3.53 × 104 mL/(gcat h) to 1.16 × 104 mL/(gcat h) step by step, then again increased from 1.16 × 104 mL/(gcat h) to 3.46 × 104 mL/(gcat h). And finally, the reaction temperature was decreased to 700 ◦ C again and kept for about 50 h under GHSV = 1.16 × 104 mL/(gcat h). The results are given in Fig. 4. It could be seen that the conversions and H2 /CO ratios would increase or decrease with the decreasing or increasing GHSVs when the temperatures were constant, they would increase or decrease with the increasing or decreasing temperatures when the GHSVs were constant, and kept almost constant when switching the conditions to the same GHSVs and temperatures during
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Fig. 2. (a) TEM image of Ni nanoparticles, (b) TEM and (c) HRTEM images of Ni@SiO2 -core–shell nanocatalyst, (d) TEM and (e) HRTEM images of Ni@SiO2 -yolk–shell nanocatalyst, (f) TEM image of Ni-SiO2 –AE nanocatalyst.
the 590 h reaction. These results further revealed that Ni@SiO2 yolk–shell catalyst was remarkably stable in the CRM reaction. 3.3. Characterization of spent catalysts The deposited carbon on the spent catalysts was characterized by XRD, TEM, HRTEM, TG-DSC and Raman spectroscopy. From Fig. 5 (a)–(c), the peak 2 = 26.4◦ attributed to carbon was of high intensity, which meant the severe carbon deposition on the catalysts after CRM reaction. And the average particle sizes of the catalysts after reaction calculated by Scherrer equation are listed in Table 1. The average size of Ni@SiO2 -yolk–shell decreased from 41.2 to 26.7 nm, and the size of Ni–SiO2 –AE decreased from 48.7 to 26.4 nm, but the size of Ni@SiO2 -core–shell kept almost the same (45.7 and 46.2 nm). This phenomenon was quite elusive, and it will be discussed below. The TEM and HRTEM images of the spent Ni@SiO2 -yolk–shell shown in Fig. 6 also demonstrated the existence of carbon on the catalysts after CRM reaction. For Ni@SiO2 -yolk–shell after
100 h reaction (Fig. 6(c)), filamentous carbon (Fig. 6(d)) and layered encapsulating carbon (Fig. 6(e)) could be observed. But after 590 h reaction, mainly filamentous carbon and almost no encapsulating carbon could be seen on Ni@SiO2 -yolk–shell, as shown in Fig. 6(f). And this phenomenon may be because that the filamentous carbon could grow gradually, while the encapsulating carbon would inhibit the further carbon deposition by preventing the contact between Ni active sites and reactant gases [4]. The TEM image of Ni–SiO2 –AE after 1.5 h reaction (Fig. 6(g)) also demonstrated the formation of filamentous carbon and encapsulating carbon, and the catalyst also sintered seriously. But for Ni@SiO2 core–shell after 5 h reaction, almost only layered encapsulating carbons could be observed, according to TEM results (Fig. 6(a)). The HAADF-STEM images of Ni@SiO2 -core–shell after 5 h reaction (Fig. 6(b)) showed that the Ni core was encapsulated by SiO2 shell and deposited carbon simultaneously. This result revealed that the structure of Ni@SiO2 -core–shell was almost no change after 5 h reaction, but the deposited carbon grew along with the pore in SiO2 shell. According to a previous study [38], the formation of
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Time on stream(h) Fig. 4. Stability test of Ni@SiO2 -yolk–shell nanocatalyst in CRM, (a) CH4 and CO2 conversions, (b) H2 /CO Ratio. Conditions: V(CH4 )/V(CO2 ) = 1(v/v), GHSV(mL/(gcat h)) = 1.16 × 104 (A), 2.36 × 104 (B), 3.53 × 104 (C), 4.62 × 104 (D), 3.53 × 104 (C) at 700 ◦ C; 3.53 × 104 (C ), 2.37 × 104 (B ), 1.16 × 104 (A ), 2.37 × 104 (B ), 3.46 × 104 (C ) at 800 ◦ C; and 3.46 × 104 (C) at 700 ◦ C.
(c)
♦ SiO2 ♥ Carbon • Ni(cubic)
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• Fig. 3. CRM reaction results (CH4 and CO2 Conversion, and H2 /CO Ratio) over (a) Ni@SiO2 -yolk–shell nanocatalyst; (b) Ni/SiO2 -AE nanocatalyst; (c) Ni@SiO2 -core–shell nanocatalyst, Conditions: 700 ◦ C, V(CH4 )/V(CO2 ) = 1(v/v), GHSV = 1.20 × 104 mL/(gcat h).
♦ 20
filamentous carbon involved the reshaping of the Ni nanoclusters and the transport of carbon atoms. The different carbon species on Ni@SiO2 -yolk–shell and Ni@SiO2 -core–shell might be due to the different structures of the two catalysts. Core–shell structured nanocatalyst may have the property to prevent the formation of filamentous carbon, because the SiO2 shell might affect the reshaping of inside Ni core for Ni@SiO2 -core–shell nanocatalyst. On the other hand, a hollow space between the inside Ni cores and outside SiO2 shells for Ni@SiO2 -yolk–shell and Ni–SiO2 –AE nanocatalysts would not affect the reshaping process. The particle size characterization results of the catalysts after reaction might support this inference. Then the encapsulating carbon deposited on Ni@SiO2 -yolk–shell might be due to the inhomogeneous etching.
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Fig. 5. XRD patterns of (a) Ni@SiO2 -core–shell nanocatalyst after 5 h reaction; (b) Ni@SiO2 -yolk–shell nanocatalyst after 100 h reaction; (c) Ni–SiO2 –AE nanocatalyst after 1.5 h reaction.
The weight loss of the spent Ni@SiO2 -yolk–shell (100 h on stream) was 22.03% by TG analysis, according to the results shown in Fig. 7 (a) and Table 1. On the other hand, the amount of carbon deposition was high up to 49.22% for the spent Ni–SiO2 –AE (only 1.5 h on stream) as shown in Fig. 7(b), and the amount was 57.62% on the spent Ni@SiO2 -core–shell (5 h on stream) as shown in Fig. 7(c). It was obviously that the rate of carbon deposition over Ni–SiO2 –AE was higher than that of Ni@SiO2 -core–shell, and both
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Fig. 6. (a) TEM, and (b) EDX mapping images of Ni@SiO2 -core–shell nanocatalyst after 5 h reaction, (c) TEM, and (d, e) HRTEM images of Ni@SiO2 -yolk–shell nanocatalyst after 100 h reaction, (f) TEM image of Ni@SiO2 -yolk–shell nanocatalyst after 590 h reaction, (g) TEM image of Ni–SiO2 –AE nanocatalyst after 1.5 h reaction. Inset in (a) shows the corresponding high-magnification HAADF-STEM images.
of them were much higher than that of Ni@SiO2 -yolk–shell. From the DSC results (Fig. 7), two oxygen consumption peaks existed due to the complete oxidation of deposited carbon on the catalysts, which could be denoted as LT (low temperature) and HT (low temperature) carbons, respectively. For Ni@SiO2 -core–shell after reaction, only layered encapsulating carbon was observed, and only HT oxygen consumption peak could be seen. Therefore, LT and HT peaks might be ascribed to filamentous (LT) and layered
encapsulating (HT) carbons, respectively. After quantitative analysis, the percentages of LT and HT carbons deposited on Ni–SiO2 –AE were 73% and 27% respectively, while the amounts of deposited LT and HT carbon on Ni@SiO2 -yolk–shell were 43% and 57%. Meanwhile, the deposited HT carbon on Ni@SiO2 -core–shell was 100% after 5 h reaction. The percentage of layered encapsulating carbon of these three catalysts fell into the order of Ni@SiO2 -core–shell (100%)> Ni@SiO2 -yolk–shell (57%) > Ni–SiO2 –AE (27%).
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had the highest structured level or graphitization level. And this order was consistent with the order of layered encapsulating carbon percentage mentioned above, which may reflect that layered encapsulating carbon had much higher structured level or graphitization level than filamentous carbon.
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Fig. 8. Raman spectroscopy results of spent nanocatalysts: (a) Ni@SiO2 -yolk–shell after 100 h reaction; (b) Ni–SiO2 –AE after 1.5 h reaction; (c) Ni@SiO2 -core–shell after 5 h reaction.
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600
700
A yolk–shell structured Ni@SiO2 -nanocatalyst (Ni@SiO2 yolk–shell) with a core size of 46.4 ± 5.7 nm had been synthesized and used in the CRM reaction. It behaved remarkably stable at high temperature up to 1073 K, and the carbon deposited on this catalyst was mainly filamentous carbon and a small amount of encapsulating carbon. This yolk–shell structured Ni@SiO2 nanocatalyst could obviously reduce the sintering of Ni particles and carbon deposition. Ni@SiO2 -core–shell showed less stable in CRM under the same conditions as Ni@SiO2 -yolk–shell, due to the deposition of layered encapsulating carbon. On the other hand, as a comparison, Ni–SiO2 –AE sintered seriously, and carbon deposition was also quite serious. The amount of carbon deposition on Ni–SiO2 –AE was high up to 49.22 wt% after only 1.5 h reaction, among which 57% was encapsulating carbon and 43% was filamentous carbon. Acknowledgements
800
o
Temperature( C) Fig. 7. TG-DSC results of spent nanocatalysts: (a) Ni@SiO2 -yolk–shell after 100 h reaction; (b) Ni–SiO2 –AE after 1.5 h reaction; (c) Ni@SiO2 -core–shell after 5 h reaction.
Raman spectroscopy is one of the most effective and nondestructive characterization methods to investigate the surface properties of deposited carbon. D band, at about 1350 cm−1 , is originated from defects or disorder of carbon materials; while G band, at about 1580 cm−1 , is associated with the symmetry and order degree of carbon materials [39–42]. The ratio of intensity (IG /ID ) reflects the structured level or graphitization level of carbon materials. From the Raman results (Fig. 8), the IG /ID values of the carbon deposited on these catalysts showed an order of Ni@SiO2 core–shell (2.10) > Ni@SiO2 -yolk–shell (1.51) > Ni–SiO2 –AE (1.24), which meant that the carbon deposited on Ni@SiO2 -core–shell
We gracefully acknowledge the financial support from the National Basic Research Program of China (973 Program, 2011CB201405), National Natural Science Foundation of China (grant no. 20673064) and Specialized Research Fund for the Doctoral Program of Higher Education of Ministry of Education of China (20131018984). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.04. 012 References [1] D.L. Trimm, Catal. Today 49 (1999) 3–10. [2] J. Sehested, Catal. Today 111 (2006) 103–110.
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Properties of yolk–shell structured Ni@SiO2 nanocatalyst and its catalytic performance in carbon dioxide reforming of methane to syngas Weiwei Yang, Huimin Liu, Yuming Li, Juan Zhang, Hao Wu, Dehua He ∗ Innovative Catalysis Program, Key Lab of Organic Optoelectronics & Molecular Engineering of Ministry of Education, Department of Chemistry, Tsinghua University, Beijing 100084, China
a r t i c l e
i n f o
Article history: Received 24 October 2014 Received in revised form 23 March 2015 Accepted 6 April 2015 Available online xxx Keywords: Ni@SiO2 -yolk–shell Carbon dioxide reforming Carbon deposition behavior Stability
a b s t r a c t Ni@SiO2 -yolk–shell catalyst was prepared by the method of etching Ni@SiO2 -core–shell with hydrochloric acid, while the core–shell catalyst was prepared by the stöber method. In comparison, Ni–SiO2 –AE (without yolk–shell structure) was used to denote the catalyst prepared by the similar method as Ni@SiO2 -yolk–shell but without fully encapsulation of Ni particles by SiO2 . The catalytic performances of these three catalysts were investigated in carbon dioxide reforming of methane to syngas (CRM). The reaction was conducted under different conditions by varying GHSVs of reactants and reaction temperatures to further test the stability of the catalysts. The properties of the as-prepared and spent Ni@SiO2 -yolk–shell, Ni@SiO2 -core–shell and Ni–SiO2 –AE catalysts were characterized by TEM/HRTEM, XRD, TG-DSC to measure the morphology, crystallinity and carbon deposition, etc. It was found that compared to Ni@SiO2 -core–shell and Ni–SiO2 –AE catalysts, Ni@SiO2 -yolk–shell catalyst behaved remarkably stable performance at the temperature up to 1073 K, and the specific structure and carbon depositionresisted behavior of the Ni@SiO2 -yolk–shell catalyst may help to maintain the stability during the reaction. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Methane is one of the most important fossil energies, and has been widely used as a chemical raw material recently. The utilization of methane includes direct conversion ways (such as oxidative coupling, selective oxidation, aromatization, etc.) and indirect conversion ways (such as steam reforming and carbon dioxide reforming of methane). Carbon dioxide reforming of methane (CRM) could convert two greenhouse gases (CH4 and CO2 ) into syngas, which could be further employed to synthesize high-value added products. Ni-based catalysts have been widely investigated and considered to be potential for the industrial utilization for CRM reaction. However, the sintering of Ni particles and carbon deposition on Ni-based catalysts limited their applications. Therefore, to develop highly active and stable Ni-based catalysts are of vital importance. Usually, CRM reaction was carried out at high temperatures (700–850 ◦ C) in consideration of the thermodynamics and dynamics factors. The active component Ni is tend to be aggregated when
∗ Corresponding author. Tel.: +86 10 62773346; fax: +86 10 62773346. E-mail address: [email protected] (D. He).
the operation temperature is above its Tammann temperature (590 ◦ C) [1] during the CRM reaction, which is driven by the reduction of the total surface energy [2]. Decomposition of methane and disproportionation of CO could occur and lead to carbon deposition (including amorphous carbon, encapsulating carbon and filamentous carbon) during CRM reaction process. For the formation of filamentous carbon, Baker’s research [3] indicated that temperature gradient drove the diffusion of carbon to form whisker-like carbon, while Trimm [4] and Snoeck [5,6] regarded the concentration gradient as the force to drive the diffusion of carbon. But it is clear that the formation of the filamentous carbon would not influence greatly on the activity, while it would destroy the structure of the catalysts. On the other hand, the formation of the encapsulating carbon, which was accumulated on the surface of catalysts because of the much rapider speed of methane decomposition than the carbon elimination, would deactivate the activities of catalysts and block the reactor [4,7,8]. According to the previous studies, increasing the interaction between supports and metal active sites [9–13], selecting better thermal conductive supports or additives [14,15], and improving the dispersion of catalysts [13,16–19] all would help to relieve the sintering of the active sites. Meanwhile, the acidic–basic properties [13,20–22], oxidation–reduction properties [17,23], particle sizes
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[24–29] and noble metal modifications [30,31] would also influence the carbon deposition. Kawi et al. [13] reported a La2 O3 doped Ni/SiO2 catalyst, and found that La2 O3 influenced the dispersion of Ni and the interaction between SiO2 and Ni active sites as well as the basicity of the catalyst, which resulted in the excellent activity and stability of the catalyst during CRM reaction. Similarly, Yoon et al. [17] employed Ni–Ce/MgAl2 O4 catalyst to investigate the effect of Ce addition during combined steam and carbon dioxide reforming of methane (CSCRM) reaction by varying the Ce/Ni ratio, and found that Ce addition have improved the metal dispersion, reducibility as well as surface oxygen transfer. However, how to effectively prevent the sintering of Ni particles, and consequently to decrease the carbon deposition during the CRM, are key factors for keeping the stability of Ni catalysts. Ni@SiO2 nanomaterials with yolk–shell or core–shell structures are comprised of nickel cores inside silica shells, and can help to prevent the sintering of Ni particles during high temperature reactions. Due to their structural features, this kind of nanomaterials is called “nanoreactor”, and has been used as the catalysts in the reactions, such as steam reforming [32] and partial oxidation [33–35] as well as CO2 reforming [36] of methane to syngas. Song et al. observed [32] that Ni@SiO2 -yolk–shell nanocatalyst behaved relatively stable at high temperature up to 973 K for 240 min during steam reforming of methane (SRM). Takenaka et al. [33] indicated that the interaction between Ni metal particles and coated silica shell made the Ni@SiO2 -core–shell nanocatalyst show better performance than the impregnated Ni/SiO2 catalyst during the reaction of partial oxidation of methane (POM). Au’s research [34,35] found that the sizes of Ni nanoparticles over Ni@porous silica core–shell nanocatalyst were tunable, and the catalytic performance could be improved by doping with other metals. Recently, Kawi et al. [36] found that the silica shell thickness would also influence the structure of the catalyst and the catalytic performance during the CRM reaction due to the strength of interaction between Ni core and silica shell. Compared with SRM, POM and combination of the three ways for methane reforming, CRM is more difficult to prevent carbon deposition because the carbon elimination property of CO2 is much weaker than that of O2 or H2 O, which would easily lead to severe carbon deposition and even block the reactor within a short time. Yolk–shell structured Ni@SiO2 perhaps is a suitable nanoreactor and could effectively prevent the sintering of Ni particles during the CRM. Up to now, it seems that there have been no reports employing such a Ni@SiO2 yolk–shell catalyst (in consideration of the catalyst structure and the Ni core size) in CRM. In this work, we prepared and characterized Ni@SiO2 nanomaterials, especially Ni@SiO2 -yolk–shell, and applied them in CRM reaction. Quite interestingly, Ni@SiO2 -yolk–shell catalyst exhibited excellent performance in CRM. Therefore, the reasons why it could help to solve the sintering problem and what is the carbon deposition behavior of such kind of catalysts, as well as the long-time stability during the CRM reaction were studied in detail.
2. Experimental 2.1. Catalyst preparation Ni@SiO2 -yolk–shell nanocatalyst was prepared by three steps, including the preparation of Ni nanoparticles, coating Ni particles with SiO2 shell to synthesize Ni@ SiO2 -core–shell nanoparticles and etching Ni@SiO2 -core–shell with hydrochloric acid, as described below [32]. In step 1, Ni nanoparticles were prepared by a polyol process. 1.05 g Ni(acac)2 and 5.35 g PVP were added into 50 mL 1,5-pentanadiol, then the solution was heated from RT to 473 K
in 20 min and stirred at 473 K for 4 h under N2 atmosphere. The solution was further heated to 513 K in 10 min and stirred at 513 K for 1 h. Formed Ni nanoparticles were collected by dissolving in acetone and washing with ethanol for four times. In step 2, the obtained Ni nanoparticles were dispersed in 40 mL ethanol and the obtained suspension solution was dispersed with ultrasonic assistance for 10 min, and then 2.0 mL aqueous ammonia solution (25–28%, 0.907 g/cm3 ) was added into the suspension solution while stirring. This suspension solution was dispersed with ultrasonic assistance for another 2 min. Then 0.1 mL C18 TMS was added into the suspension solution and ultrasonically dispersed for 10 min. After that, 0.1 mL TMOS was added dropwise, and the resulted suspension solution was stirred at room temperature for 1 h. Finally, Ni@SiO2 -core–shell nanocatalyst was obtained after collecting the nanoparticles by centrifugation and washing with ethanol for four times. In step 3, the Ni@SiO2 -core–shell nanocatalyst obtained from the above steps was dispersed in 40 mL ethanol, and 2.5 mL hydrochloric acid dissolved in 10 mL ethanol was added. The resulted suspension solution was stirred and ultrasonically dispersed simultaneously for 30 min. The product obtained by centrifugation was washed with ethanol, and then Ni@SiO2 yolk–shell nanocatalyst was collected. In comparison, Ni–SiO2 –BE nanocatalyst was prepared by the similar method as Ni@SiO2 -core–shell nanocatalyst but the prepared SiO2 sol and Ni nanoparticles were directly mixed together in advance, as described below. Firstly, TMOS was hydrolyzed for 30 min. Then Ni nanoparticles, which were obtained by the method as mentioned above, were added into hydrolyzed TMOS. The Ni nanoparticles mixed with silica were stirred for another 30 min. Then, the obtained Ni–SiO2 –BE by the centrifugal separation was also etched with hydrochloric acid in the same process as the preparation of Ni@SiO2 -yolk–shell, and finally the collected solid was dried, and then Ni–SiO2 –AE nanocatalyst was obtained.
2.2. Catalyst characterization Transmission electron microscopy (TEM, Hitachi Limited, H7650B) and High Resolution Transmission Electron Microscopy (HRTEM, JEOL Ltd, JEM-2010) were used to observe the morphologies and particle sizes of the catalysts. The sample was ultrasonically dispersed for 30 min in ethanol, and then was dripped to the copper mesh. The particle size distributions of the samples could be derived by measuring the sizes of Ni particles through TEM iamges. The crystalline phases of the catalysts were investigated by Xray diffraction (XRD, Bruker D8 Advance X-Ray Diffractometer). The instrument was powered at 40 kV and 200 mA, using Cu K␣ radiation. The scan angle was from 10◦ to 90◦ with a scan speed of 10◦ /min. The crystal sizes of Ni particles were calculated by Scherrer equation. The loadings of Ni were measured by Inductively Coupled Plasma (ICP, ThermoFisher, IRIS Intrepid II XSP), while the main operating parameters were 1150 W, 26.0 PSI, 1.0 LPM, and 100 r/min. The amounts of carbon deposition on the catalysts were measured by thermal gravity analysis (TG), while the types of carbon deposited could be differentiated by Differential Scanning calorimetry (DSC, on Mettler Toledo TGA/SDTA851e ). The sample was heated from room temperature to 800 ◦ C with a rate of 10 ◦ C/min under flowing air. Raman spectroscopy was conducted at LabRAM HR Evolution with a 514 nm laser to measure the different types of carbon deposited on catalysts. The scan range from 100 to 3000 cm−1 , but the main information located at 1200–1700 cm−1 .
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Table 1 Physicochemical properties of Ni@SiO2 samples.
2.3. Catalyst performance test CRM reaction was conducted in a continuous flow, vertical fixedbed quartz tube reactor (internal diameter 6 mm, external diameter 8 mm) under atmosphere pressure. Before reaction, the catalyst was reduced at 500 ◦ C for 2 h with a flow of 35% H2 /Ar. After reduction, the catalyst bed temperature was raised to the reaction temperature, and then the reactant mixture CH4 and CO2 with the molar ratio of 1:1 was introduced into the reactor. The composition of effluent gas was analyzed by on-line gas chromatographs (GC) with a thermal conductivity detector. The method for calculating the conversions of CH4 and CO2 , as well as the selectivities of CO and H2 , was available elsewhere [37].
Sample
Carbon deposition (%)b
Ni size (nm)
Fresh (TEM) Fresh (XRD) Spent (XRD) Ni@SiO2 65.5 core–shell Ni@SiO2 42.4 yolk–shell Ni–SiO2 –AE 51.6 a b c d e
3. Results and discussion
Ni content (%)a
c
57.6
53.9 ± 5.4
45.7
46.2c
22.0d
46.4 ± 5.7
41.2
26.7d
49.2e
51.8 ± 20.0 48.7
26.4e
Measured by ICP. Calculated by TG. After 5 h reaction. After 100 h reaction. After 1.5 h reaction.
3.1. Characterization of as-prepared catalysts Fig. 1 shows the XRD patterns of the as-prepared, reduced, and spent Ni@SiO2 nanocatalysts. Fig. 1(a)–(d) revealed the crystal structures of Ni nanoparticles, Ni@SiO2 -core–shell, Ni@SiO2 yolk–shell and Ni–SiO2 –AE, respectively. The diffraction peaks of Ni(1 1 1), Ni(2 0 0) and Ni(2 2 0) at 2 = 44.5◦ , 51.8◦ and 76.4◦ could be assigned to cubic-Ni phase (JCPDS no. 65-2865), while other peaks could be ascribed to hexagonal-Ni phase (JCPDS no. 45-1027) except the one located at 2 = 23.5◦ . The weak and broad peak at 2 = 23.5◦ should be assigned to silica. Interestingly, the hexagonal phase Ni all changed into cubic phase Ni after reducing the samples at 500 ◦ C for 2 h, as shown in Fig. S1. As shown in Table 1, the crystal sizes of as prepared Ni NPs, Ni@SiO2 -core–shell, Ni@SiO2 yolk–shell and Ni–SiO2 –AE calculated by Scherrer equation were 44.5, 45.7, 41.2 and 48.7 nm, respectively. Fig. 2 illustrates the TEM and HRTEM images of the fresh Ni nanoparticles (Fig. 2(a)), Ni@SiO2 -core–shell (Fig. 2(b and c)), Ni@SiO2 -yolk–shell (Fig. 2(d and e)), and Ni–SiO2 –AE (Fig. 2(f)). Compared with Fig. 2(c) and (e), it can be seen that the inside Ni cores were close to the outside SiO2 shells for Ni@SiO2 -core–shell, while for Ni@SiO2 -yolk–shell, hollow spaces existed clearly after etching Ni@SiO2 -core–shell with HCl. Based on the results from TEM, the particle size distributions were measured and listed in Table 1. The average sizes were 53.9 nm both for Ni nanoparticles and the Ni core of Ni@SiO2 -core–shell. And the average Ni size dropped to 46.4 nm after etching Ni@SiO2 -core–shell with HCl. For Ni–SiO2 –AE, the average Ni size was 51.8 nm, and some of the
(d)
♦ SiO2 • Ni(cubic) ♣ Ni(Hexagonal)
Intensity
(c)
(b) ♦
♣ •
(a) •
♣♣ 20
40
o
2θ( )
♣ 60
♣
•♣
♣♣
80
Fig. 1. XRD patterns of (a) Fresh Ni NPs; (b) Fresh Ni@SiO2 -core–shell nanocatalyst, (c) Fresh Ni@SiO2 -yolk–shell nanocatalyst, (d) Fresh Ni–SiO2 –AE nanocatalyst.
Ni particles were coated together with SiO2 , while some Ni particles were even uncoated, which were quite nonuniform with a deviation of about ±20 nm, as shown in Fig. 2(f). And these results approximately agreed with the XRD results mentioned above. From the ICP results (Table 1), the Ni content for Ni@SiO2 core–shell was 65.5 wt%, while for Ni@SiO2 -yolk–shell, it decreased to 42.4 wt%. For Ni–SiO2 –AE, the Ni content was 51.6 wt%. 3.2. CRM reaction results The results of CRM reaction over those Ni catalysts at 700 ◦ C with a GHSV = 1.20 × 104 mL/(gcat h) are given in Figs. 3 and 4. For Ni@SiO2 -yolk–shell, as shown in Fig. 3(a), the conversion of CH4 was 43.9%, and the conversion of CO2 (57.3%) was higher than that of CH4 because of the reverse water–gas shift (RWGS) reaction (CO2 + H2 → CO + H2 O). The RWGS reaction also made the selectivity of H2 (83.6%) lower than that of CO (to 99.5%). The relatively sufficient amount of H2 O might help to eliminate the carbon deposition during CRM by the reaction (H2 O + C → CO + H2 ), which might be favorable for the improved stability of the catalyst. In comparison, for Ni–SiO2 –AE nanocatalyst, as shown in Fig. 3(b), the initial conversion of CH4 (74.9%) was higher than that of CO2 (72.4%), while H2 /CO ratio was higher than 1, because of the severe decomposition of methane (CH4 → 2H2 + C, H298k = 75 kJ/mol). This would lead to the quick carbon deposition on the Ni–SiO2 –AE catalyst and the increase of reactor pressure drop. On the other hand, Ni@SiO2 -core–shell, as shown in Fig. 3(c), also behaved similarly with Ni–SiO2 –AE, and the catalyst bed was blocked by carbon deposition within 5 h CRM reaction. To further test the stability of Ni@SiO2 -yolk–shell catalyst, Ni@SiO2 -yolk–shell was evaluated in CRM reaction under different conditions by varying the GHSVs of reactants and reaction temperatures. Firstly, the initial reaction temperature was kept at 700 ◦ C for about 260 h, while GHSVs were increased from 1.16 × 104 mL/(gcat h) to 4.62 × 104 mL/(gcat h) step by step, then decreased to 3.53 × 104 mL/(gcat h). Then the reaction temperature was increased to 800 ◦ C and kept at 800 ◦ C for about 280 h, while GHSVs were decreased from 3.53 × 104 mL/(gcat h) to 1.16 × 104 mL/(gcat h) step by step, then again increased from 1.16 × 104 mL/(gcat h) to 3.46 × 104 mL/(gcat h). And finally, the reaction temperature was decreased to 700 ◦ C again and kept for about 50 h under GHSV = 1.16 × 104 mL/(gcat h). The results are given in Fig. 4. It could be seen that the conversions and H2 /CO ratios would increase or decrease with the decreasing or increasing GHSVs when the temperatures were constant, they would increase or decrease with the increasing or decreasing temperatures when the GHSVs were constant, and kept almost constant when switching the conditions to the same GHSVs and temperatures during
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Fig. 2. (a) TEM image of Ni nanoparticles, (b) TEM and (c) HRTEM images of Ni@SiO2 -core–shell nanocatalyst, (d) TEM and (e) HRTEM images of Ni@SiO2 -yolk–shell nanocatalyst, (f) TEM image of Ni-SiO2 –AE nanocatalyst.
the 590 h reaction. These results further revealed that Ni@SiO2 yolk–shell catalyst was remarkably stable in the CRM reaction. 3.3. Characterization of spent catalysts The deposited carbon on the spent catalysts was characterized by XRD, TEM, HRTEM, TG-DSC and Raman spectroscopy. From Fig. 5 (a)–(c), the peak 2 = 26.4◦ attributed to carbon was of high intensity, which meant the severe carbon deposition on the catalysts after CRM reaction. And the average particle sizes of the catalysts after reaction calculated by Scherrer equation are listed in Table 1. The average size of Ni@SiO2 -yolk–shell decreased from 41.2 to 26.7 nm, and the size of Ni–SiO2 –AE decreased from 48.7 to 26.4 nm, but the size of Ni@SiO2 -core–shell kept almost the same (45.7 and 46.2 nm). This phenomenon was quite elusive, and it will be discussed below. The TEM and HRTEM images of the spent Ni@SiO2 -yolk–shell shown in Fig. 6 also demonstrated the existence of carbon on the catalysts after CRM reaction. For Ni@SiO2 -yolk–shell after
100 h reaction (Fig. 6(c)), filamentous carbon (Fig. 6(d)) and layered encapsulating carbon (Fig. 6(e)) could be observed. But after 590 h reaction, mainly filamentous carbon and almost no encapsulating carbon could be seen on Ni@SiO2 -yolk–shell, as shown in Fig. 6(f). And this phenomenon may be because that the filamentous carbon could grow gradually, while the encapsulating carbon would inhibit the further carbon deposition by preventing the contact between Ni active sites and reactant gases [4]. The TEM image of Ni–SiO2 –AE after 1.5 h reaction (Fig. 6(g)) also demonstrated the formation of filamentous carbon and encapsulating carbon, and the catalyst also sintered seriously. But for Ni@SiO2 core–shell after 5 h reaction, almost only layered encapsulating carbons could be observed, according to TEM results (Fig. 6(a)). The HAADF-STEM images of Ni@SiO2 -core–shell after 5 h reaction (Fig. 6(b)) showed that the Ni core was encapsulated by SiO2 shell and deposited carbon simultaneously. This result revealed that the structure of Ni@SiO2 -core–shell was almost no change after 5 h reaction, but the deposited carbon grew along with the pore in SiO2 shell. According to a previous study [38], the formation of
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Time on stream(h) Fig. 4. Stability test of Ni@SiO2 -yolk–shell nanocatalyst in CRM, (a) CH4 and CO2 conversions, (b) H2 /CO Ratio. Conditions: V(CH4 )/V(CO2 ) = 1(v/v), GHSV(mL/(gcat h)) = 1.16 × 104 (A), 2.36 × 104 (B), 3.53 × 104 (C), 4.62 × 104 (D), 3.53 × 104 (C) at 700 ◦ C; 3.53 × 104 (C ), 2.37 × 104 (B ), 1.16 × 104 (A ), 2.37 × 104 (B ), 3.46 × 104 (C ) at 800 ◦ C; and 3.46 × 104 (C) at 700 ◦ C.
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filamentous carbon involved the reshaping of the Ni nanoclusters and the transport of carbon atoms. The different carbon species on Ni@SiO2 -yolk–shell and Ni@SiO2 -core–shell might be due to the different structures of the two catalysts. Core–shell structured nanocatalyst may have the property to prevent the formation of filamentous carbon, because the SiO2 shell might affect the reshaping of inside Ni core for Ni@SiO2 -core–shell nanocatalyst. On the other hand, a hollow space between the inside Ni cores and outside SiO2 shells for Ni@SiO2 -yolk–shell and Ni–SiO2 –AE nanocatalysts would not affect the reshaping process. The particle size characterization results of the catalysts after reaction might support this inference. Then the encapsulating carbon deposited on Ni@SiO2 -yolk–shell might be due to the inhomogeneous etching.
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The weight loss of the spent Ni@SiO2 -yolk–shell (100 h on stream) was 22.03% by TG analysis, according to the results shown in Fig. 7 (a) and Table 1. On the other hand, the amount of carbon deposition was high up to 49.22% for the spent Ni–SiO2 –AE (only 1.5 h on stream) as shown in Fig. 7(b), and the amount was 57.62% on the spent Ni@SiO2 -core–shell (5 h on stream) as shown in Fig. 7(c). It was obviously that the rate of carbon deposition over Ni–SiO2 –AE was higher than that of Ni@SiO2 -core–shell, and both
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Fig. 6. (a) TEM, and (b) EDX mapping images of Ni@SiO2 -core–shell nanocatalyst after 5 h reaction, (c) TEM, and (d, e) HRTEM images of Ni@SiO2 -yolk–shell nanocatalyst after 100 h reaction, (f) TEM image of Ni@SiO2 -yolk–shell nanocatalyst after 590 h reaction, (g) TEM image of Ni–SiO2 –AE nanocatalyst after 1.5 h reaction. Inset in (a) shows the corresponding high-magnification HAADF-STEM images.
of them were much higher than that of Ni@SiO2 -yolk–shell. From the DSC results (Fig. 7), two oxygen consumption peaks existed due to the complete oxidation of deposited carbon on the catalysts, which could be denoted as LT (low temperature) and HT (low temperature) carbons, respectively. For Ni@SiO2 -core–shell after reaction, only layered encapsulating carbon was observed, and only HT oxygen consumption peak could be seen. Therefore, LT and HT peaks might be ascribed to filamentous (LT) and layered
encapsulating (HT) carbons, respectively. After quantitative analysis, the percentages of LT and HT carbons deposited on Ni–SiO2 –AE were 73% and 27% respectively, while the amounts of deposited LT and HT carbon on Ni@SiO2 -yolk–shell were 43% and 57%. Meanwhile, the deposited HT carbon on Ni@SiO2 -core–shell was 100% after 5 h reaction. The percentage of layered encapsulating carbon of these three catalysts fell into the order of Ni@SiO2 -core–shell (100%)> Ni@SiO2 -yolk–shell (57%) > Ni–SiO2 –AE (27%).
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had the highest structured level or graphitization level. And this order was consistent with the order of layered encapsulating carbon percentage mentioned above, which may reflect that layered encapsulating carbon had much higher structured level or graphitization level than filamentous carbon.
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A yolk–shell structured Ni@SiO2 -nanocatalyst (Ni@SiO2 yolk–shell) with a core size of 46.4 ± 5.7 nm had been synthesized and used in the CRM reaction. It behaved remarkably stable at high temperature up to 1073 K, and the carbon deposited on this catalyst was mainly filamentous carbon and a small amount of encapsulating carbon. This yolk–shell structured Ni@SiO2 nanocatalyst could obviously reduce the sintering of Ni particles and carbon deposition. Ni@SiO2 -core–shell showed less stable in CRM under the same conditions as Ni@SiO2 -yolk–shell, due to the deposition of layered encapsulating carbon. On the other hand, as a comparison, Ni–SiO2 –AE sintered seriously, and carbon deposition was also quite serious. The amount of carbon deposition on Ni–SiO2 –AE was high up to 49.22 wt% after only 1.5 h reaction, among which 57% was encapsulating carbon and 43% was filamentous carbon. Acknowledgements
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Temperature( C) Fig. 7. TG-DSC results of spent nanocatalysts: (a) Ni@SiO2 -yolk–shell after 100 h reaction; (b) Ni–SiO2 –AE after 1.5 h reaction; (c) Ni@SiO2 -core–shell after 5 h reaction.
Raman spectroscopy is one of the most effective and nondestructive characterization methods to investigate the surface properties of deposited carbon. D band, at about 1350 cm−1 , is originated from defects or disorder of carbon materials; while G band, at about 1580 cm−1 , is associated with the symmetry and order degree of carbon materials [39–42]. The ratio of intensity (IG /ID ) reflects the structured level or graphitization level of carbon materials. From the Raman results (Fig. 8), the IG /ID values of the carbon deposited on these catalysts showed an order of Ni@SiO2 core–shell (2.10) > Ni@SiO2 -yolk–shell (1.51) > Ni–SiO2 –AE (1.24), which meant that the carbon deposited on Ni@SiO2 -core–shell
We gracefully acknowledge the financial support from the National Basic Research Program of China (973 Program, 2011CB201405), National Natural Science Foundation of China (grant no. 20673064) and Specialized Research Fund for the Doctoral Program of Higher Education of Ministry of Education of China (20131018984). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.cattod.2015.04. 012 References [1] D.L. Trimm, Catal. Today 49 (1999) 3–10. [2] J. Sehested, Catal. Today 111 (2006) 103–110.
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Please cite this article in press as: W. Yang, et al., Properties of yolk–shell structured Ni@SiO2 nanocatalyst and its catalytic performance in carbon dioxide reforming of methane to syngas, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2015.04.012