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ZrO2@SiO2 catalyst for the partial oxidation of methane to synthesis gas
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Coking resistant Ni/ZrO2@SiO2 catalyst for the partial oxidation of methane to synthesis gas Chuanmin Ding a, Ganggang Ai a, Kan Zhang b, Qinbo Yuan c, Yulin Han d, Xishun Ma d, Junwen Wang a,*, Shibin Liu a,* a
College of Chemistry & Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China c Shanxi Academy of Environmental Research, Taiyuan 030021, PR China d Shanxi Fenxi Mining (Group) Co., Ltd, Jiexiu 032000, PR China b
article info
abstract
Article history:
Carbon deposition, which may reduce the number of active sites or remove metal particles
Received 31 January 2015
from the catalyst surface, is an urgent issue for the partial oxidation of methane (POM) to
Received in revised form 18 March 2015
synthesis gas. To solve this problem, Ni/ZrO2@SiO2 catalysts were prepared by a modified € ber method. The investigation was focused mainly on the role of ZrO2 addition and Sto
Accepted 19 March 2015
mesopore silica shell in preventing carbon deposition. The structural properties and car-
Available online 23 April 2015
bon deposition of catalysts were characterized by XRD, TEM, N2 adsorption and TG techniques. The oxygen transfer capacity and reducibility of catalysts were evaluated by
Keywords:
oxygen storage capacity (OSC) and temperature programmed reduction (TPR). Inspiringly,
Partial oxidation of methane
the Ni/ZrO2@SiO2 catalyst was proved to be more active and possessed less carbon depo-
Carbon deposition
sition due to the higher reducibility and oxygen storage/release capacity. Importantly,
Silica shell
compared with the support catalysts, the catalysts coated by mesopore silica shell showed
Synthesis gas
exceptional resistance to coking, because the edge and corner atoms favor to carbon deposition were selectively blocked by silica shell, in addition, the size of the pore channel prevented growth up of carbon filament. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has been identified as a promising candidate for future energy systems [1]. Hydrogen production from natural gas has been spurred by exploration and production of methane from shale basins in the hydrogen energy field recently [2,3]. Partial oxidation of methane (POM), which provides the suitable H2/CO ratio for methanol and alkane synthesis by the FischereTropsch process, has attracted much attention. In addition, the POM is a mild exothermic reaction
and requires less energy compared with the conventional endothermic steam methane reforming (SMR) and carbon dioxide reforming of methane (CRM) process. In spite of many advances in this reaction, catalyst deactivation caused by coke deposition and sintering is still a serious issue [4,5]. Due to high cost of noble metals, it is more practical to develop nickel-based catalyst. Regardless of highly catalytic activity, supported nickel catalysts tend to deactivate due to sintering and coking. The phenomenon of sintering has been relieved by ingenious design of coreeshell catalysts [6,7].
* Corresponding authors. Tel./fax: þ86 0351 6014 498. E-mail addresses: [email protected] (J. Wang), [email protected] (S. Liu). http://dx.doi.org/10.1016/j.ijhydene.2015.03.094 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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However, carbon deposition on nickel-based catalysts still is obstacle to industrial application [8e10]. As a result, very intensive research efforts focus on minimizing coke deposition over nickel-based catalysts for POM reaction. Among several hypotheses on the mechanism of coke formation on nickel catalysts, two reactions stand out as being probably the most important: methane cracking and the Boudouard reaction: 1
CH4 /C þ 2H2
DH ¼ 74:8 kJ mol
2CO/C þ CO2
DH ¼ 172:8 kJ mol
ðmethane crackingÞ 1
ðBoudouard reactionÞ
The former reaction is endothermic and favored at higher temperature, while the latter is exothermic and favored at lower temperature. It is thought that the methane cracking reaction is the main reason of carbon formation on nickel catalyst under high temperature condition [11,12]. Since carbon formation of nickel catalysts is structuresensitive, modifications to reduce the crystallite sizes or increase interaction with support are typically used, which includes addition of alkaline metal oxide or the use of rare earth metal oxide as support [13,14]. Especially, NiOeMgO solid solution was proved to be resistant to coking due to high stability € zdemir et al. [17,18] as well as the weak basicity of MgO [15,16]. o found that carbon deposition was greatly affected by support basicity and decreased with increasing Lewis basicity/acidity on Ni/MgAl2O4 catalysts. In addition, the lattice oxygen contributed to eliminating carbon deposition [19]. Oxygen species adsorbed on the surface of the catalyst may react with the coke to form CO. However, at surface of nickel-based catalyst, not all the coke was removed, perhaps due to lack of sufficient activated O2 molecules or too high content of carbon [20]. Some studies indicated that the ZrO2 as supporters can enhance the performance of the catalyst due to redox property and high thermal stability [21]. The addition of ZrO2, which possesses catalytic property of the carbon gasification, could be good alternative in Ni-catalyzed POM reaction. Lu et al. [22] prepared the palladium catalysts via atomic layer deposition (ALD) process. The metal nanoparticles (NPs) were coated by alumina layer (thickness: ~8 nm, pore diameter: ~6 nm). They found the coreeshell catalyst showed effective resistance to carbon deposition and the product yield was improved on all Pd@Al2O3 catalysts. Taking into account acidity of alumina, metal NPs coated by silica shell may be a better choice. In this context, research focused on the effect of ZrO2 on improving the activation of reactant molecules and suppressing the build-up of carbon deposition at the catalyst surface. Combined with the formation mechanism of coke on nickel catalyst, the role of mesopore silica shell in inhibiting carbon deposition will be illuminated.
Experimental Materials The Ni/ZrO2@SiO2 catalysts were synthesized according to the literature with a little modification [7]. Firstly, 300 mg of
polyethylene glycol (average MW ¼ 20 000, Aldrich) and 1.0 g NaOH were dissolved in 40 mL deionized water as the stock solution (A). Meanwhile, another solution (B) was prepared by dissolving Ni(NO3)2$6H2O and Zr(NO3)4$5H2O with different stoichiometric ratio in 40 mL deionized water. Following that, the solution (B) was added dropwise into the solution (A), and the resulted solution was stirred for 1 h at 25 C. The precipitates were collected by centrifuging at 6000 rpm for 10 min and washed several times with deionized water and ethanol. Finally, the sample was dried at 50 C for 16 h and calcined in air at 500 C for 2 h. The atomic ratios of Ni/Zr were 1:1, 1:0.5, 1:1.5, respectively, and the counterpart NiO/ZrO2 samples were referred to as 1NiO0.5ZrO2, 1NiO1ZrO2 and 1NiO1.5ZrO2, respectively. The coreeshell structure NiO/ZrO2@SiO2 catalysts were € ber method. 0.2 g of as-synthesized prepared by a modified Sto NiO/ZrO2 NPs was dispersed in 100 mL of poly-(vinylpyrrolidone) (K30, 1.0 g) ethanol solution and stirred for 12 h, then 10 mL of NH3$H2O (25 wt%) was added. The resulted suspension was sonicated in an ultrasound cleaner (SK1200H, 40 KHz, 100 W) for 30 min, 5 mL of tetraethyl orthosilicate (TEOS, 0.1 mL, 99%, Aldrich) ethanol solution was added to the suspension. One hour later, the sample was collected by centrifugation and washed with deionized water and ethanol. The coreeshell products were dried at 80 C in air for 6 h and calcined in air at 500 C for 2 h, which were marked as 1NiO1ZrO2@SiO2, 1NiO1.5ZrO2@SiO2, 1NiO0.5ZrO2@SiO2, respectively. Prior to the reaction, the catalysts were separately reduced to 1Ni0.5ZrO2@SiO2, 1Ni1ZrO2@SiO2, 1Ni1.5ZrO2@SiO2 in flowing of mixture gas (20% H2 balanced with N2) at 750 C for 4 h. By comparison, the Ni/ZrO2 catalyst was prepared by conventional impregnation method and the Ni@SiO2 catalyst was prepared by parallel method without Zr(NO3)4$5H2O.
Catalyst characterization N2 adsorption measurement was performed on a QUADRSORB SI material physical structure determinator. Before measurement, the sample was degassed at 300 C for 3 h. The BET surface area was calculated from a multipoint BET analysis of the N2 adsorption isotherms. X-ray powder diffraction (XRD) data were obtained on a SHIMADZU-6000 automated powder diffractometer, using Cu Ka radiation. The TEM images were taken over a JEOL JEM-2100F instrument operated at 100 kV. The test of carbon deposition was carried out by the same reactor under the same reaction condition. The coke quantity was measured by the TA Q600 thermogravimetric analyzer. OSC measurements were carried out in a microreactor coupled to a quadrupole mass spectrometer. The samples were reduced in flowing H2 and heated to 800 C. Then the samples were cooled to 450 C in flowing He stream and a 5% O2/He mixture was passed through the catalyst until the oxygen uptake was finished. The samples were cooled to room temperature and heated to 800 C at rate of 10 C/min. The amount of O2 consumed on the catalysts was recorded taking into account a previous calibration of the mass spectrometer. H2 TPR was performed in a conventional setup equipped with a thermal conductive detector. 30 mg of catalysts were heated to 110 C at rate of 10 C/min under Ar flow and kept at
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this temperature for 1 h to remove adsorbed water. After cooled down to room temperature, the sample was switched to a 25% H2/N2 (V/V, 60 mL/min) mixture. The sample temperature was programmed to 750 C at rate of 10 C/min.
Catalyst evaluation Catalytic reactions were conducted in a fixed-bed tubular quartz reactor (i.d. 10 nm), which was heated by electric power. The catalyst diluted with quartz chips (60-80 Mesh) was packed and sandwiched by two quartz wool beds. A thermocouple was placed at the middle part of the catalyst bed to monitor and control temperature. Control experiments using a reactor containing only quartz chips without catalysts demonstrated that conversion of CH4 at 750 C was <1%. The reactant feed composed of CH4 and O2 (V/V ¼ 2/1) was introduced into the reactor at rate of 420 mL/min (gas hour space velocity (GHSV) of 5 104 mL h1 g1) at atmospheric pressure. The product mixtures and unconverted reactants were analyzed by a gas chromatograph (Haixin GC 920) equipped with TCD and FID detector. The products at the reactor outlet were divided into two streams which were analyzed differently. H2, O2 and CH4 were separated by stainless steel column containing 5A molecular sieve and analyzed by TCD detector; while CO, CH4 and CO2 were separated by a stainless steel column containing carbon molecular sieve and analyzed by FID detector. CO and CO2 were converted to CH4 through a reformer before they reached the detector. All components occurred on two detectors were related by CH4.
Results and discussion Characterization Phase composition X-ray powder diffraction (XRD) was tested to obtain the diffraction patterns of the reduced and used Ni/ZrO2@SiO2 catalysts. Fig. 1A showed the typical peaks at 2q ¼ 24.2 , 28.2 , 31.4 and 34.3 , corresponding to monoclinic ZrO2 crystal phase (JCPDS 37-1484) and the characteristic peak at 2q ¼ 30.3 attributed to tetragonal ZrO2 crystal phase (JCPDS 501089) (curve ‘b’,‘c’,‘d’). It was worth noting that the reflections of monoclinic ZrO2 crystal phase became weaker after reaction, while reflections of tetragonal phase the increased (Fig. 1B). It is universally accepted that the monoclinic phase of ZrO2 started to transform to tetragonal phase when the calcinations temperature was higher than 1000 C. Considering the reaction temperature of experiment was only 750 C, it is reasonably deduced that combustion reforming reaction may occur on catalysts. The high temperature may accelerate simultaneously methane cracking reaction, which increases the rate of carbon deposition. After reduction, the NiO species were completely transformed into metallic state Ni0, as revealed by the reflections of the (111), (200) and (220) planes at 2q ¼ 44 , 52 , 76 (JCPDS 040850) (Fig. 1A). Furthermore, compared with the reduced Ni@SiO2 catalyst, the Ni reflections of Ni/ZrO2@SiO2 catalysts were weaker, indicating the ZrO2 prevents growing up of Ni crystal. It is generally acknowledged that Ni NPs with small
Fig. 1 e XRD patterns of catalysts: (A) catalysts reduced at 750 C for 4 h, (B) used catalysts. a-Ni@SiO2, b1Ni1ZrO2@SiO2, c-1Ni0.5ZrO2@SiO2, d-1Ni1.5ZrO2@SiO2.
size can effectively suppress sintering and coking [23]. Therefore, the addition of ZrO2 was favorable to improve the structural stability. Even more important, only the used 1Ni1ZrO2@SiO2 catalyst showed the absence of C and NiO species (curve‘c’in Fig. 1B). The reflections of C and NiO on used Ni@SiO2 catalyst was stronger than the Ni/ZrO2@SiO2 catalysts (curve‘a’ in Fig. 1B). The oxidation of Ni metal to NiO resulted in deactivation of the catalysts for the reaction [24]. There are some results indicating the formation of oxidation state species was related with the carbon deposition and the amount of carbon deposition depends on that of oxidation state species [25], which can explain the occurrence of NiO species.
TEM There is a popular belief that coreeshell structure possesses ability of preventing sintering of active metal NPs. As we can see from the Fig. 2A, The NiO/ZrO2 NPs prepared by coprecipitation method aggregated easily due to the magnetism and high surface energy. After coated by silica shell, the NiO/ZrO2 NPs were isolated from each other and the coreeshell NiO/ ZrO2@SiO2 catalysts were obtained (Fig. 2B). The coreeshell structure was stable even though suffering from the harsh reaction (Fig. 2C). By comparison, the spherical NiO/ZrO2 NPs were obtained by impregnation method (Fig. 2D). After reaction, the nanoclusters tended to obtain a highly elongated shape, and tubular carbon structure formed around the NiO/ ZrO2 NPs (Fig. 2E). The result was in accord with the report of the Nørskov [26]. Fig. 2E1 showed the shell-like carbon was also observed on used Ni/ZrO2 catalyst. Compared with the used Ni/ZrO2@SiO2 coreeshell catalysts, the carbon deposition on used Ni/ZrO2 support catalyst was severe. The tubular or
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Fig. 2 e TEM images of samples: (A) NiO/ZrO2 nanoparticles prepared by coprecipitation method, (B) fresh NiO/ZrO2@SiO2 catalysts, (C) used Ni/ZrO2@SiO2 catalysts, (D) fresh NiO/ZrO2 catalyst prepared by impregnation method, (E) TEM image showing a tubular carbon nanofibre structure on used Ni/ZrO2 catalyst, (E1) High-resolution micrograph of shell-like carbon deposition obtained on used Ni/ZrO2 catalyst.
shell-like carbon simultaneously occurred, which may decrease the mass transfer rate or reduce active sites on the surface of catalysts, further causing the catalyst deactivation. On the contrary, the fibrous carbons did not have sufficient room to grow in Ni/ZrO2@SiO2 catalysts, because the metal particles were uniformly covered with silica [24].
larger surface areas than Ni@SiO2 catalysts indicating ZrO2 promoted surface area of Ni-based catalysts [27]. However, the surface areas and pore volumes of Ni/ZrO2@SiO2 catalysts with different Ni/Zr ratio were close. It is worth noting that the surface area and pore volume of 1Ni/1ZrO2 are much less than those of 1Ni1ZrO2@SiO2, due to abundance of pores in the silica shell.
N2 sorption Plenty of nanometer pores could form in Ni/ZrO2@SiO2 coreeshell catalysts after calcination, which provide the diffusion channel for the POM reaction. The N2 sorption was performed to investigate the porosity of silica shell. The adsorptionedesorption isotherm of 1Ni1ZrO2@SiO2 catalyst showed type IV capillary condensation indicating the mesopore structure in the SiO2 shell (Fig. 3a). Fig. 3b showed a typical curve with average pore diameter of 5 nm, which form in the shell due to the template of PVP. Table 1 summarizes BET surface areas and pore volumes of all synthesized samples. The Ni/ZrO2@SiO2 catalysts show
H2-TPR, TG and oxygen storage capacity The TPR results of NiO/ZrO2@SiO2, NiO@SiO2 and NiO/ZrO2 at the same calcination temperature were shown in Fig. 4. The broad hydrogen consumption peaks in the range 300e500 C can be assigned to the reduction of NiO species in NiO@SiO2 catalyst [28]. However, all the Ni/ZrO2@SiO2 catalysts showed two distinct reduction peaks at low temperature (400 C) and high temperature (520 C), indicating that there existed two different types of NiO species. Compared with the reduction peak of NiO@SiO2 catalyst, the low temperature peaks of Ni/ ZrO2@SiO2 catalyst were assigned to reduction of NiO species
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Fig. 4 e H2 TPR profiles of NiO/ZrO2, NiO@SiO2 and NiO/ ZrO2@SiO2 catalysts.
Fig. 3 e (a) Adsorptionedesorption isotherms and (b) pore size distribution of Ni/ZrO2@SiO2 catalyst after calcination in air at 700 C for 3 h.
interacted with SiO2 shell. While the peaks at the high temperature were attributed to NiO species interacted with ZrO2. Obviously, the more amount of NiO species strongly interacted with ZrO2 in Ni/ZrO2@SiO2 catalyst. Meanwhile, the starting reduction temperature of Ni/ZrO2@SiO2 catalysts (~350 C) is higher than that of NiO@SiO2 catalyst (~300 C). It was evident that the addition of ZrO2 increases the reducibility of catalysts. The two similar reduction peaks were observed on NiO/ZrO2 catalyst: the low temperature peak (350 C) was assigned to the reduction of bulk NiO species and the high temperature peak (450 C) was attributed to reduction of NiO interacted with ZrO2. Comparing the reduction temperature of NiO/ZrO2@SiO2 and NiO/ZrO2, we found the silica shell made both of NiO species more difficult to be reduced. The reduction temperature of NiO species (420 C) in NiO@SiO2 was higher than that of bulk NiO species in NiO/ZrO2 catalyst (350 C) due to stronger coreeshell interaction [24]. Therefore, the addition of ZrO2 and silica shell increased the reducibility
of catalysts, which may be a fatal factor to the inhibition of coking [29]. Further, the carbon deposition was taken into consideration as a factor in catalyst deactivation and the amount of carbon deposited on catalysts was evaluated by TG technique. TG results confirmed the fact that 1Ni/1ZrO2@SiO2 catalyst had minimum carbon deposition among those prepared catalysts. As seen from Table 1, Ni/ZrO2@SiO2 catalyst showed less coking than Ni@SiO2 catalyst indicating ZrO2 can prevent the carbon deposition. The result was in accord with the XRD analysis. What is noteworthy is that the carbon deposition of supported NiO/ZrO2 catalyst was most severe comparing with the coreeshell catalysts, which also was verified by TEM analysis (Fig. 2). The result indicates silica shell is of great importance to inhibition of carbon deposition. Table 1 also presents the oxygen uptakes of the catalysts. The obtained O2 consumption of Ni@SiO2 catalyst was obviously less than those of Ni/ZrO2@SiO2 catalysts, indicating the addition of ZrO2 increased the storage capacity and mobility of oxygen. Moreover, the amounts of O2 uptake increased with increasing amount of ZrO2 in Ni/ZrO2@SiO2 catalysts. The O2 uptakes of 1Ni1ZrO2@SiO2 and 1Ni1.5ZrO2@SiO2 catalysts were more than those of 1Ni0.5ZrO2@SiO2 and Ni@SiO2 catalysts, meanwhile the result of TG showed the carbon depositions of 1Ni/0.5ZrO2@SiO2 and Ni@SiO2 catalysts were more severe, indicating the high storage capacity and mobility of oxygen is favor to reduce the carbon deposition. The Ni
Table 1 e Characteristics of catalysts determined by N2 sorption, OSC and TG. Samples 1Ni/0.5ZrO2@SiO2 1Ni/1ZrO2@SiO2 1Ni/1.5ZrO2@SiO2 Ni@SiO2 1Ni/1ZrO2 a b c d e
Surface area (m2/g)a
Pore volume (cm3/g)a
O2 uptake (mmol/g catalyst)b
Ni loading (wt%)c
Ni crystal size (nm)d
Carbon deposition 1 e rate ðgc g1 cat h Þ
45.17 45.65 42.32 40.75 18.51
0.63 0.58 0.70 0.72 0.055
38.29 42.93 46.75 10.70 43.05
31.2 24.8 18.5 46.1 30.7
13.25 10.07 9.87 19.72 6.61
0.017 0.0052 0.0089 0.024 0.14
N2 sorption, after calcined at 700 C for 3 h. Calculated from OSC experiments. Measured by XRF. Calculated from the full width at half maximum (FWHM) of the reflection of Ni (111) plane in the XRD using the Scherrer equation. Calculated from thermogravimetry.
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loadings of Ni/ZrO2@SiO2 catalysts decreased with decreasing Ni/Zr ratio (Table 1). Ni@SiO2 catalyst possessed the most loading amount of Ni among all the catalysts. The catalysts were mixed with quartz chips to attain the same loading amount of Ni during the activity test.
POM catalytic activity and stability The catalytic performances of Ni/ZrO2@SiO2 catalysts with different ratio of Ni/Zr were shown in Fig. 5AeC. As we can see, all of the Ni/ZrO2@SiO2 catalysts exhibited an induction period of catalytic activity due to the metal particles coated by the silica shell. Fascinatingly, 1Ni1ZrO2@SiO2 catalysts owned higher initial methane conversion and CO and H2 selectivity than other catalysts. The catalytic activity decreased in order of 1Ni1ZrO2@SiO2 > 1Ni1.5ZrO2@SiO2 > 1Ni0.5ZrO2@SiO2 catalysts, which can be explained by amounts of carbon deposition on the catalysts (Table 1). Particularly, the least amount of carbon deposition on 1Ni1ZrO2@SiO2 catalyst affords the highest CO selectivity. Further, the stability tests of the Ni/ZrO2, Ni@SiO2 and Ni/ ZrO2@SiO2 catalysts were carried out at 750 C (Fig. 5D). The catalyst Ni/ZrO2@SiO2 had a prolonged lifetime in the stability test. The methane conversion of Ni/ZrO2@SiO2 catalyst increased with the reaction time and stabilized at 95%, moreover, the CO selectivity of Ni/ZrO2@SiO2 catalysts was stable even suffering from 160 h. It is worth noting that the CO selectivity of Ni/ZrO2@SiO2 catalyst was superior to other
catalysts, which was attributed to less carbon deposition (Table 1). The catalyst deactivation from coke formation was a strong function of particle size. The Ni crystal sizes of reduced samples were listed in Table 1. Compared with the Ni@SiO2 and Ni/ZrO2@SiO2 catalysts, the Ni crystal size in Ni/ZrO2 catalyst was smallest. It was accepted that the turnover frequency increased with decreasing particle size, which explained the high methane conversion (Fig. 5D). However, the decreasing size also attributed to the increasing fraction of edge and corner surface sites [30]. As the start sites of accumulation of carbon atom from methane cracking, more edge and corner atoms accelerate the carbon deposition [22], which was also verified by TEM images (Fig. 2E). Even though the methane conversion of Ni/ZrO2 catalyst was higher, the CO selectivity declined rapidly with the extended reaction time compared with the coreeshell catalysts. Combined with the TEM images (Fig. 2E), it was deduced that plenty of methane was transformed to the carbon deposition on catalysts, which in turn caused the decline of catalytic activity. By contrary, the silica shell exerts a significant effect on holding back the formation of carbon deposition and the coreeshell structure affords the stable catalytic activity. It was evident that catalytic performance of Ni/ZrO2@SiO2 catalyst was superior to the Ni@SiO2 catalyst (Fig. 5D). The result may be attributed to high reducibility and oxygen transfer ability, which reduces carbon deposition or promotes the carbon removal from the metal surface.
Fig. 5 e (A)CH4 conversion, (B) CO selectivity, (C) H2 selectivity with time on stream in POM over Ni/ZrO2@SiO2 catalysts with different Ni/Zr ratio at GHSV ¼ 5 £ 104 mL h¡1 g¡1, at 750 C. (D) Activity comparison of Ni/ZrO2, Ni@SiO2 and Ni/ZrO2@SiO2 catalysts for 160 h at GHSV ¼ 2.5 £ 105 mL h¡1 g¡1, at 750 C. CH4/O2 ¼ 2:1 (V/V), atmospheric pressure.
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Reaction routes of carbon formation Carbon deposition, which may reduce the number of active sites through the formation of whiskers or the removal of metal particles from the surface, is a severe issue for the metal catalysts. The factor leading to carbon deposition is manifold, containing metal type, metal crystallite size, metal structure, and reaction conditions and so on. Compared with the noble metal, carbon deposition rate of nickel catalysts is rapid, because mobility and solubility of carbon in the noble metals were reduced [31]. The POM is structure sensitive reaction. Generally, catalyst structure, such as porous structure, also affects coke formation, which decreases with decreasing pore size [32]. Meanwhile, the rate and extent of coke formation increase with increasing acid strength and concentration of support [18]. In addition, reaction temperature is of crucial importance. When reaction temperature is above 600 C, carbon species on nickel catalyst mainly is pyrolytic carbon obtained by thermal cracking of hydrocarbon and deposition of C precursors on catalyst. Whisker carbon with Ni crystal at top develops by diffusion of C through Ni-crystal and nucleation at >450 C [33]. The formation mechanisms of coke on supported catalyst are listed in Fig. 6. Three kinds of carbon deposition over the used nickel catalysts, including filaments, tubes and shell, had been studied by electron micrograph [34]. As low-coordinated surface sites, the edge and corner atoms play a central role in both sintering and coking [22]. Nørskov et al. [26] observed that carbon nanofibers developed initially at step edges of nickel surfaces companied with restructuring of atomic step edges involving surface diffusion of both carbon and nickel atoms. Hydrocarbons dissociate on the nickel surface to produce
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highly reactive carbon species. Most of the carbon species are gasified but some are converted to less active carbon, which may deposited on edge and corner surface sites and dissolve in the nickel crystallite. The dissolved carbon diffuses through the nickel crystal to nucleate and precipitate. The formation of filaments or tubes carbon was determined by interaction between the active metal particle and support. If the binding force between metal and support is strong, deposited carbon grow outward from surface to form fibrous carbon (Fig. 6A). Otherwise, carbon nucleates and grows at the rear of the crystallite, lifting the nickel crystallite from the catalyst surface, which explains the formation of tubes carbon (Fig. 6B). However, the metal crystallite lifted by carbon tube may drop, which results in loss of active metal. Similarly, the weak interaction may accelerate formation of carbon deposition around the metal particles, which then are coated by carbon shell. The shell carbon prevents the reaction gas contacting the active sites, which results in the catalyst deactivation (Fig. 6C).
The role of silica shell to resist carbon deposition Traditional strategies to resist or remove carbon deposition includes: controlling of metal crystallite size as well as increasing steam/carbon ratios in the feed [35]. Bearing in mind the formation mechanism of carbon deposition, it can be concluded that the edge and corner atoms are the key of carbon deposition. The edge and corner atoms as well as platform atoms are active for POM reaction. However, the edge and corner atoms are considered as the active sites for carbon deposition. The low coordinated surface sites are favor to CeC bond scission and hydrogen stripping to produce C1
Fig. 6 e Formation mechanism of carbon deposition (A, B, C) and coke inhibition of silica shell (D).
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fragments, which results in carbon deposition [3]. The severe carbon deposition on Ni/ZrO2 catalyst confirms the claim that more edge and corner atoms are favor to carbon deposition. While the carbon deposition can be efficiently suppressed by blocking the edge and corner atoms using silica shell (Fig. 6D). Moreover, the results of N2 adsorption suggest the size of the pore channels in the silica shell is ~5 nm (Fig. 3b), which inhibits carbon filament formation, because the typical filament diameter is much larger (~17 nm). In addition, the pore channel hinders the radical chains reactions that are necessary to form coke. The TEM showed metal particles were uniformly covered with silica, which resulted in fibrous carbons did not have sufficient room to grow [24]. Nielsen [36] suggested that the ensembles size control played a critical role in minimizing formation of coke in steam reforming. Coke formation would require more ensembles of surface sites (6e7 nickel atoms) than those required for steam reforming (3e4 nickel atoms). Guo et al. [37] obtained single iron sites by spreading out the iron atoms in a silica matrix for nonoxidative conversion of methane. They found the absence of adjacent iron sites prevented catalytic CeC coupling, further carbon deposition. Therefore, it is reasoned that the decreasing ensemble size by segmentation of silica shell is favor to minimization of carbon deposition.
Conclusions In this work, the Ni/ZrO2@SiO2, Ni@SiO2 coreeshell catalysts and Ni/ZrO2 support catalyst were synthesized and investigated for the catalytic partial oxidation of methane to synthesis gas. The effects of the silica shell and ZrO2 on reducibility, structural properties, and prevention of carbon deposition were characterized by XRD, TEM, N2 sorption, TG, H2-TPR and OSC techniques. The lifetime test of 160 h showed the Ni/ ZrO2@SiO2 catalyst owned more stable and higher catalytic activity due to reduced carbon deposition during POM reaction, differing in sharp reduction of CO selectivity (from 88% to 75%) on Ni/ZrO2 catalyst and low methane conversion (84%) on Ni@SiO2 catalyst. Among three catalysts with different Ni/Zr ratio, the Ni/ZrO2@SiO2 catalyst (Ni/Zr ¼ 1) was proved to be more active with higher methane conversion (99%) and product selectivity (94% for CO and 85% for H2). The results showed silica shell played critical role in resistance of carbon deposition, by blocking the active edge and corner atoms and preventing the growth up of carbon fiber. The addition of ZrO2 effectively improved reducibility and oxygen storage capacity of catalysts, which was favor to reduce carbon deposition.
Acknowledgments The authors gratefully acknowledge the financial support from the Project of “Utilization of Low Rank Coal” Strategic Leading Special Fund, Chinese Academy of Sciences (XDA07070800, XDA-07070400), the Program of Overseas Science and Technology Activity (2012-35), Shanxi Province and the cooperation program from Shanxi Fenxi mining (Group) Co., Ltd.
references
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Coking resistant Ni/ZrO2@SiO2 catalyst for the partial oxidation of methane to synthesis gas Chuanmin Ding a, Ganggang Ai a, Kan Zhang b, Qinbo Yuan c, Yulin Han d, Xishun Ma d, Junwen Wang a,*, Shibin Liu a,* a
College of Chemistry & Chemical Engineering, Taiyuan University of Technology, Taiyuan 030024, PR China Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China c Shanxi Academy of Environmental Research, Taiyuan 030021, PR China d Shanxi Fenxi Mining (Group) Co., Ltd, Jiexiu 032000, PR China b
article info
abstract
Article history:
Carbon deposition, which may reduce the number of active sites or remove metal particles
Received 31 January 2015
from the catalyst surface, is an urgent issue for the partial oxidation of methane (POM) to
Received in revised form 18 March 2015
synthesis gas. To solve this problem, Ni/ZrO2@SiO2 catalysts were prepared by a modified € ber method. The investigation was focused mainly on the role of ZrO2 addition and Sto
Accepted 19 March 2015
mesopore silica shell in preventing carbon deposition. The structural properties and car-
Available online 23 April 2015
bon deposition of catalysts were characterized by XRD, TEM, N2 adsorption and TG techniques. The oxygen transfer capacity and reducibility of catalysts were evaluated by
Keywords:
oxygen storage capacity (OSC) and temperature programmed reduction (TPR). Inspiringly,
Partial oxidation of methane
the Ni/ZrO2@SiO2 catalyst was proved to be more active and possessed less carbon depo-
Carbon deposition
sition due to the higher reducibility and oxygen storage/release capacity. Importantly,
Silica shell
compared with the support catalysts, the catalysts coated by mesopore silica shell showed
Synthesis gas
exceptional resistance to coking, because the edge and corner atoms favor to carbon deposition were selectively blocked by silica shell, in addition, the size of the pore channel prevented growth up of carbon filament. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen has been identified as a promising candidate for future energy systems [1]. Hydrogen production from natural gas has been spurred by exploration and production of methane from shale basins in the hydrogen energy field recently [2,3]. Partial oxidation of methane (POM), which provides the suitable H2/CO ratio for methanol and alkane synthesis by the FischereTropsch process, has attracted much attention. In addition, the POM is a mild exothermic reaction
and requires less energy compared with the conventional endothermic steam methane reforming (SMR) and carbon dioxide reforming of methane (CRM) process. In spite of many advances in this reaction, catalyst deactivation caused by coke deposition and sintering is still a serious issue [4,5]. Due to high cost of noble metals, it is more practical to develop nickel-based catalyst. Regardless of highly catalytic activity, supported nickel catalysts tend to deactivate due to sintering and coking. The phenomenon of sintering has been relieved by ingenious design of coreeshell catalysts [6,7].
* Corresponding authors. Tel./fax: þ86 0351 6014 498. E-mail addresses: [email protected] (J. Wang), [email protected] (S. Liu). http://dx.doi.org/10.1016/j.ijhydene.2015.03.094 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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However, carbon deposition on nickel-based catalysts still is obstacle to industrial application [8e10]. As a result, very intensive research efforts focus on minimizing coke deposition over nickel-based catalysts for POM reaction. Among several hypotheses on the mechanism of coke formation on nickel catalysts, two reactions stand out as being probably the most important: methane cracking and the Boudouard reaction: 1
CH4 /C þ 2H2
DH ¼ 74:8 kJ mol
2CO/C þ CO2
DH ¼ 172:8 kJ mol
ðmethane crackingÞ 1
ðBoudouard reactionÞ
The former reaction is endothermic and favored at higher temperature, while the latter is exothermic and favored at lower temperature. It is thought that the methane cracking reaction is the main reason of carbon formation on nickel catalyst under high temperature condition [11,12]. Since carbon formation of nickel catalysts is structuresensitive, modifications to reduce the crystallite sizes or increase interaction with support are typically used, which includes addition of alkaline metal oxide or the use of rare earth metal oxide as support [13,14]. Especially, NiOeMgO solid solution was proved to be resistant to coking due to high stability € zdemir et al. [17,18] as well as the weak basicity of MgO [15,16]. o found that carbon deposition was greatly affected by support basicity and decreased with increasing Lewis basicity/acidity on Ni/MgAl2O4 catalysts. In addition, the lattice oxygen contributed to eliminating carbon deposition [19]. Oxygen species adsorbed on the surface of the catalyst may react with the coke to form CO. However, at surface of nickel-based catalyst, not all the coke was removed, perhaps due to lack of sufficient activated O2 molecules or too high content of carbon [20]. Some studies indicated that the ZrO2 as supporters can enhance the performance of the catalyst due to redox property and high thermal stability [21]. The addition of ZrO2, which possesses catalytic property of the carbon gasification, could be good alternative in Ni-catalyzed POM reaction. Lu et al. [22] prepared the palladium catalysts via atomic layer deposition (ALD) process. The metal nanoparticles (NPs) were coated by alumina layer (thickness: ~8 nm, pore diameter: ~6 nm). They found the coreeshell catalyst showed effective resistance to carbon deposition and the product yield was improved on all Pd@Al2O3 catalysts. Taking into account acidity of alumina, metal NPs coated by silica shell may be a better choice. In this context, research focused on the effect of ZrO2 on improving the activation of reactant molecules and suppressing the build-up of carbon deposition at the catalyst surface. Combined with the formation mechanism of coke on nickel catalyst, the role of mesopore silica shell in inhibiting carbon deposition will be illuminated.
Experimental Materials The Ni/ZrO2@SiO2 catalysts were synthesized according to the literature with a little modification [7]. Firstly, 300 mg of
polyethylene glycol (average MW ¼ 20 000, Aldrich) and 1.0 g NaOH were dissolved in 40 mL deionized water as the stock solution (A). Meanwhile, another solution (B) was prepared by dissolving Ni(NO3)2$6H2O and Zr(NO3)4$5H2O with different stoichiometric ratio in 40 mL deionized water. Following that, the solution (B) was added dropwise into the solution (A), and the resulted solution was stirred for 1 h at 25 C. The precipitates were collected by centrifuging at 6000 rpm for 10 min and washed several times with deionized water and ethanol. Finally, the sample was dried at 50 C for 16 h and calcined in air at 500 C for 2 h. The atomic ratios of Ni/Zr were 1:1, 1:0.5, 1:1.5, respectively, and the counterpart NiO/ZrO2 samples were referred to as 1NiO0.5ZrO2, 1NiO1ZrO2 and 1NiO1.5ZrO2, respectively. The coreeshell structure NiO/ZrO2@SiO2 catalysts were € ber method. 0.2 g of as-synthesized prepared by a modified Sto NiO/ZrO2 NPs was dispersed in 100 mL of poly-(vinylpyrrolidone) (K30, 1.0 g) ethanol solution and stirred for 12 h, then 10 mL of NH3$H2O (25 wt%) was added. The resulted suspension was sonicated in an ultrasound cleaner (SK1200H, 40 KHz, 100 W) for 30 min, 5 mL of tetraethyl orthosilicate (TEOS, 0.1 mL, 99%, Aldrich) ethanol solution was added to the suspension. One hour later, the sample was collected by centrifugation and washed with deionized water and ethanol. The coreeshell products were dried at 80 C in air for 6 h and calcined in air at 500 C for 2 h, which were marked as 1NiO1ZrO2@SiO2, 1NiO1.5ZrO2@SiO2, 1NiO0.5ZrO2@SiO2, respectively. Prior to the reaction, the catalysts were separately reduced to 1Ni0.5ZrO2@SiO2, 1Ni1ZrO2@SiO2, 1Ni1.5ZrO2@SiO2 in flowing of mixture gas (20% H2 balanced with N2) at 750 C for 4 h. By comparison, the Ni/ZrO2 catalyst was prepared by conventional impregnation method and the Ni@SiO2 catalyst was prepared by parallel method without Zr(NO3)4$5H2O.
Catalyst characterization N2 adsorption measurement was performed on a QUADRSORB SI material physical structure determinator. Before measurement, the sample was degassed at 300 C for 3 h. The BET surface area was calculated from a multipoint BET analysis of the N2 adsorption isotherms. X-ray powder diffraction (XRD) data were obtained on a SHIMADZU-6000 automated powder diffractometer, using Cu Ka radiation. The TEM images were taken over a JEOL JEM-2100F instrument operated at 100 kV. The test of carbon deposition was carried out by the same reactor under the same reaction condition. The coke quantity was measured by the TA Q600 thermogravimetric analyzer. OSC measurements were carried out in a microreactor coupled to a quadrupole mass spectrometer. The samples were reduced in flowing H2 and heated to 800 C. Then the samples were cooled to 450 C in flowing He stream and a 5% O2/He mixture was passed through the catalyst until the oxygen uptake was finished. The samples were cooled to room temperature and heated to 800 C at rate of 10 C/min. The amount of O2 consumed on the catalysts was recorded taking into account a previous calibration of the mass spectrometer. H2 TPR was performed in a conventional setup equipped with a thermal conductive detector. 30 mg of catalysts were heated to 110 C at rate of 10 C/min under Ar flow and kept at
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this temperature for 1 h to remove adsorbed water. After cooled down to room temperature, the sample was switched to a 25% H2/N2 (V/V, 60 mL/min) mixture. The sample temperature was programmed to 750 C at rate of 10 C/min.
Catalyst evaluation Catalytic reactions were conducted in a fixed-bed tubular quartz reactor (i.d. 10 nm), which was heated by electric power. The catalyst diluted with quartz chips (60-80 Mesh) was packed and sandwiched by two quartz wool beds. A thermocouple was placed at the middle part of the catalyst bed to monitor and control temperature. Control experiments using a reactor containing only quartz chips without catalysts demonstrated that conversion of CH4 at 750 C was <1%. The reactant feed composed of CH4 and O2 (V/V ¼ 2/1) was introduced into the reactor at rate of 420 mL/min (gas hour space velocity (GHSV) of 5 104 mL h1 g1) at atmospheric pressure. The product mixtures and unconverted reactants were analyzed by a gas chromatograph (Haixin GC 920) equipped with TCD and FID detector. The products at the reactor outlet were divided into two streams which were analyzed differently. H2, O2 and CH4 were separated by stainless steel column containing 5A molecular sieve and analyzed by TCD detector; while CO, CH4 and CO2 were separated by a stainless steel column containing carbon molecular sieve and analyzed by FID detector. CO and CO2 were converted to CH4 through a reformer before they reached the detector. All components occurred on two detectors were related by CH4.
Results and discussion Characterization Phase composition X-ray powder diffraction (XRD) was tested to obtain the diffraction patterns of the reduced and used Ni/ZrO2@SiO2 catalysts. Fig. 1A showed the typical peaks at 2q ¼ 24.2 , 28.2 , 31.4 and 34.3 , corresponding to monoclinic ZrO2 crystal phase (JCPDS 37-1484) and the characteristic peak at 2q ¼ 30.3 attributed to tetragonal ZrO2 crystal phase (JCPDS 501089) (curve ‘b’,‘c’,‘d’). It was worth noting that the reflections of monoclinic ZrO2 crystal phase became weaker after reaction, while reflections of tetragonal phase the increased (Fig. 1B). It is universally accepted that the monoclinic phase of ZrO2 started to transform to tetragonal phase when the calcinations temperature was higher than 1000 C. Considering the reaction temperature of experiment was only 750 C, it is reasonably deduced that combustion reforming reaction may occur on catalysts. The high temperature may accelerate simultaneously methane cracking reaction, which increases the rate of carbon deposition. After reduction, the NiO species were completely transformed into metallic state Ni0, as revealed by the reflections of the (111), (200) and (220) planes at 2q ¼ 44 , 52 , 76 (JCPDS 040850) (Fig. 1A). Furthermore, compared with the reduced Ni@SiO2 catalyst, the Ni reflections of Ni/ZrO2@SiO2 catalysts were weaker, indicating the ZrO2 prevents growing up of Ni crystal. It is generally acknowledged that Ni NPs with small
Fig. 1 e XRD patterns of catalysts: (A) catalysts reduced at 750 C for 4 h, (B) used catalysts. a-Ni@SiO2, b1Ni1ZrO2@SiO2, c-1Ni0.5ZrO2@SiO2, d-1Ni1.5ZrO2@SiO2.
size can effectively suppress sintering and coking [23]. Therefore, the addition of ZrO2 was favorable to improve the structural stability. Even more important, only the used 1Ni1ZrO2@SiO2 catalyst showed the absence of C and NiO species (curve‘c’in Fig. 1B). The reflections of C and NiO on used Ni@SiO2 catalyst was stronger than the Ni/ZrO2@SiO2 catalysts (curve‘a’ in Fig. 1B). The oxidation of Ni metal to NiO resulted in deactivation of the catalysts for the reaction [24]. There are some results indicating the formation of oxidation state species was related with the carbon deposition and the amount of carbon deposition depends on that of oxidation state species [25], which can explain the occurrence of NiO species.
TEM There is a popular belief that coreeshell structure possesses ability of preventing sintering of active metal NPs. As we can see from the Fig. 2A, The NiO/ZrO2 NPs prepared by coprecipitation method aggregated easily due to the magnetism and high surface energy. After coated by silica shell, the NiO/ZrO2 NPs were isolated from each other and the coreeshell NiO/ ZrO2@SiO2 catalysts were obtained (Fig. 2B). The coreeshell structure was stable even though suffering from the harsh reaction (Fig. 2C). By comparison, the spherical NiO/ZrO2 NPs were obtained by impregnation method (Fig. 2D). After reaction, the nanoclusters tended to obtain a highly elongated shape, and tubular carbon structure formed around the NiO/ ZrO2 NPs (Fig. 2E). The result was in accord with the report of the Nørskov [26]. Fig. 2E1 showed the shell-like carbon was also observed on used Ni/ZrO2 catalyst. Compared with the used Ni/ZrO2@SiO2 coreeshell catalysts, the carbon deposition on used Ni/ZrO2 support catalyst was severe. The tubular or
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Fig. 2 e TEM images of samples: (A) NiO/ZrO2 nanoparticles prepared by coprecipitation method, (B) fresh NiO/ZrO2@SiO2 catalysts, (C) used Ni/ZrO2@SiO2 catalysts, (D) fresh NiO/ZrO2 catalyst prepared by impregnation method, (E) TEM image showing a tubular carbon nanofibre structure on used Ni/ZrO2 catalyst, (E1) High-resolution micrograph of shell-like carbon deposition obtained on used Ni/ZrO2 catalyst.
shell-like carbon simultaneously occurred, which may decrease the mass transfer rate or reduce active sites on the surface of catalysts, further causing the catalyst deactivation. On the contrary, the fibrous carbons did not have sufficient room to grow in Ni/ZrO2@SiO2 catalysts, because the metal particles were uniformly covered with silica [24].
larger surface areas than Ni@SiO2 catalysts indicating ZrO2 promoted surface area of Ni-based catalysts [27]. However, the surface areas and pore volumes of Ni/ZrO2@SiO2 catalysts with different Ni/Zr ratio were close. It is worth noting that the surface area and pore volume of 1Ni/1ZrO2 are much less than those of 1Ni1ZrO2@SiO2, due to abundance of pores in the silica shell.
N2 sorption Plenty of nanometer pores could form in Ni/ZrO2@SiO2 coreeshell catalysts after calcination, which provide the diffusion channel for the POM reaction. The N2 sorption was performed to investigate the porosity of silica shell. The adsorptionedesorption isotherm of 1Ni1ZrO2@SiO2 catalyst showed type IV capillary condensation indicating the mesopore structure in the SiO2 shell (Fig. 3a). Fig. 3b showed a typical curve with average pore diameter of 5 nm, which form in the shell due to the template of PVP. Table 1 summarizes BET surface areas and pore volumes of all synthesized samples. The Ni/ZrO2@SiO2 catalysts show
H2-TPR, TG and oxygen storage capacity The TPR results of NiO/ZrO2@SiO2, NiO@SiO2 and NiO/ZrO2 at the same calcination temperature were shown in Fig. 4. The broad hydrogen consumption peaks in the range 300e500 C can be assigned to the reduction of NiO species in NiO@SiO2 catalyst [28]. However, all the Ni/ZrO2@SiO2 catalysts showed two distinct reduction peaks at low temperature (400 C) and high temperature (520 C), indicating that there existed two different types of NiO species. Compared with the reduction peak of NiO@SiO2 catalyst, the low temperature peaks of Ni/ ZrO2@SiO2 catalyst were assigned to reduction of NiO species
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Fig. 4 e H2 TPR profiles of NiO/ZrO2, NiO@SiO2 and NiO/ ZrO2@SiO2 catalysts.
Fig. 3 e (a) Adsorptionedesorption isotherms and (b) pore size distribution of Ni/ZrO2@SiO2 catalyst after calcination in air at 700 C for 3 h.
interacted with SiO2 shell. While the peaks at the high temperature were attributed to NiO species interacted with ZrO2. Obviously, the more amount of NiO species strongly interacted with ZrO2 in Ni/ZrO2@SiO2 catalyst. Meanwhile, the starting reduction temperature of Ni/ZrO2@SiO2 catalysts (~350 C) is higher than that of NiO@SiO2 catalyst (~300 C). It was evident that the addition of ZrO2 increases the reducibility of catalysts. The two similar reduction peaks were observed on NiO/ZrO2 catalyst: the low temperature peak (350 C) was assigned to the reduction of bulk NiO species and the high temperature peak (450 C) was attributed to reduction of NiO interacted with ZrO2. Comparing the reduction temperature of NiO/ZrO2@SiO2 and NiO/ZrO2, we found the silica shell made both of NiO species more difficult to be reduced. The reduction temperature of NiO species (420 C) in NiO@SiO2 was higher than that of bulk NiO species in NiO/ZrO2 catalyst (350 C) due to stronger coreeshell interaction [24]. Therefore, the addition of ZrO2 and silica shell increased the reducibility
of catalysts, which may be a fatal factor to the inhibition of coking [29]. Further, the carbon deposition was taken into consideration as a factor in catalyst deactivation and the amount of carbon deposited on catalysts was evaluated by TG technique. TG results confirmed the fact that 1Ni/1ZrO2@SiO2 catalyst had minimum carbon deposition among those prepared catalysts. As seen from Table 1, Ni/ZrO2@SiO2 catalyst showed less coking than Ni@SiO2 catalyst indicating ZrO2 can prevent the carbon deposition. The result was in accord with the XRD analysis. What is noteworthy is that the carbon deposition of supported NiO/ZrO2 catalyst was most severe comparing with the coreeshell catalysts, which also was verified by TEM analysis (Fig. 2). The result indicates silica shell is of great importance to inhibition of carbon deposition. Table 1 also presents the oxygen uptakes of the catalysts. The obtained O2 consumption of Ni@SiO2 catalyst was obviously less than those of Ni/ZrO2@SiO2 catalysts, indicating the addition of ZrO2 increased the storage capacity and mobility of oxygen. Moreover, the amounts of O2 uptake increased with increasing amount of ZrO2 in Ni/ZrO2@SiO2 catalysts. The O2 uptakes of 1Ni1ZrO2@SiO2 and 1Ni1.5ZrO2@SiO2 catalysts were more than those of 1Ni0.5ZrO2@SiO2 and Ni@SiO2 catalysts, meanwhile the result of TG showed the carbon depositions of 1Ni/0.5ZrO2@SiO2 and Ni@SiO2 catalysts were more severe, indicating the high storage capacity and mobility of oxygen is favor to reduce the carbon deposition. The Ni
Table 1 e Characteristics of catalysts determined by N2 sorption, OSC and TG. Samples 1Ni/0.5ZrO2@SiO2 1Ni/1ZrO2@SiO2 1Ni/1.5ZrO2@SiO2 Ni@SiO2 1Ni/1ZrO2 a b c d e
Surface area (m2/g)a
Pore volume (cm3/g)a
O2 uptake (mmol/g catalyst)b
Ni loading (wt%)c
Ni crystal size (nm)d
Carbon deposition 1 e rate ðgc g1 cat h Þ
45.17 45.65 42.32 40.75 18.51
0.63 0.58 0.70 0.72 0.055
38.29 42.93 46.75 10.70 43.05
31.2 24.8 18.5 46.1 30.7
13.25 10.07 9.87 19.72 6.61
0.017 0.0052 0.0089 0.024 0.14
N2 sorption, after calcined at 700 C for 3 h. Calculated from OSC experiments. Measured by XRF. Calculated from the full width at half maximum (FWHM) of the reflection of Ni (111) plane in the XRD using the Scherrer equation. Calculated from thermogravimetry.
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loadings of Ni/ZrO2@SiO2 catalysts decreased with decreasing Ni/Zr ratio (Table 1). Ni@SiO2 catalyst possessed the most loading amount of Ni among all the catalysts. The catalysts were mixed with quartz chips to attain the same loading amount of Ni during the activity test.
POM catalytic activity and stability The catalytic performances of Ni/ZrO2@SiO2 catalysts with different ratio of Ni/Zr were shown in Fig. 5AeC. As we can see, all of the Ni/ZrO2@SiO2 catalysts exhibited an induction period of catalytic activity due to the metal particles coated by the silica shell. Fascinatingly, 1Ni1ZrO2@SiO2 catalysts owned higher initial methane conversion and CO and H2 selectivity than other catalysts. The catalytic activity decreased in order of 1Ni1ZrO2@SiO2 > 1Ni1.5ZrO2@SiO2 > 1Ni0.5ZrO2@SiO2 catalysts, which can be explained by amounts of carbon deposition on the catalysts (Table 1). Particularly, the least amount of carbon deposition on 1Ni1ZrO2@SiO2 catalyst affords the highest CO selectivity. Further, the stability tests of the Ni/ZrO2, Ni@SiO2 and Ni/ ZrO2@SiO2 catalysts were carried out at 750 C (Fig. 5D). The catalyst Ni/ZrO2@SiO2 had a prolonged lifetime in the stability test. The methane conversion of Ni/ZrO2@SiO2 catalyst increased with the reaction time and stabilized at 95%, moreover, the CO selectivity of Ni/ZrO2@SiO2 catalysts was stable even suffering from 160 h. It is worth noting that the CO selectivity of Ni/ZrO2@SiO2 catalyst was superior to other
catalysts, which was attributed to less carbon deposition (Table 1). The catalyst deactivation from coke formation was a strong function of particle size. The Ni crystal sizes of reduced samples were listed in Table 1. Compared with the Ni@SiO2 and Ni/ZrO2@SiO2 catalysts, the Ni crystal size in Ni/ZrO2 catalyst was smallest. It was accepted that the turnover frequency increased with decreasing particle size, which explained the high methane conversion (Fig. 5D). However, the decreasing size also attributed to the increasing fraction of edge and corner surface sites [30]. As the start sites of accumulation of carbon atom from methane cracking, more edge and corner atoms accelerate the carbon deposition [22], which was also verified by TEM images (Fig. 2E). Even though the methane conversion of Ni/ZrO2 catalyst was higher, the CO selectivity declined rapidly with the extended reaction time compared with the coreeshell catalysts. Combined with the TEM images (Fig. 2E), it was deduced that plenty of methane was transformed to the carbon deposition on catalysts, which in turn caused the decline of catalytic activity. By contrary, the silica shell exerts a significant effect on holding back the formation of carbon deposition and the coreeshell structure affords the stable catalytic activity. It was evident that catalytic performance of Ni/ZrO2@SiO2 catalyst was superior to the Ni@SiO2 catalyst (Fig. 5D). The result may be attributed to high reducibility and oxygen transfer ability, which reduces carbon deposition or promotes the carbon removal from the metal surface.
Fig. 5 e (A)CH4 conversion, (B) CO selectivity, (C) H2 selectivity with time on stream in POM over Ni/ZrO2@SiO2 catalysts with different Ni/Zr ratio at GHSV ¼ 5 £ 104 mL h¡1 g¡1, at 750 C. (D) Activity comparison of Ni/ZrO2, Ni@SiO2 and Ni/ZrO2@SiO2 catalysts for 160 h at GHSV ¼ 2.5 £ 105 mL h¡1 g¡1, at 750 C. CH4/O2 ¼ 2:1 (V/V), atmospheric pressure.
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Reaction routes of carbon formation Carbon deposition, which may reduce the number of active sites through the formation of whiskers or the removal of metal particles from the surface, is a severe issue for the metal catalysts. The factor leading to carbon deposition is manifold, containing metal type, metal crystallite size, metal structure, and reaction conditions and so on. Compared with the noble metal, carbon deposition rate of nickel catalysts is rapid, because mobility and solubility of carbon in the noble metals were reduced [31]. The POM is structure sensitive reaction. Generally, catalyst structure, such as porous structure, also affects coke formation, which decreases with decreasing pore size [32]. Meanwhile, the rate and extent of coke formation increase with increasing acid strength and concentration of support [18]. In addition, reaction temperature is of crucial importance. When reaction temperature is above 600 C, carbon species on nickel catalyst mainly is pyrolytic carbon obtained by thermal cracking of hydrocarbon and deposition of C precursors on catalyst. Whisker carbon with Ni crystal at top develops by diffusion of C through Ni-crystal and nucleation at >450 C [33]. The formation mechanisms of coke on supported catalyst are listed in Fig. 6. Three kinds of carbon deposition over the used nickel catalysts, including filaments, tubes and shell, had been studied by electron micrograph [34]. As low-coordinated surface sites, the edge and corner atoms play a central role in both sintering and coking [22]. Nørskov et al. [26] observed that carbon nanofibers developed initially at step edges of nickel surfaces companied with restructuring of atomic step edges involving surface diffusion of both carbon and nickel atoms. Hydrocarbons dissociate on the nickel surface to produce
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highly reactive carbon species. Most of the carbon species are gasified but some are converted to less active carbon, which may deposited on edge and corner surface sites and dissolve in the nickel crystallite. The dissolved carbon diffuses through the nickel crystal to nucleate and precipitate. The formation of filaments or tubes carbon was determined by interaction between the active metal particle and support. If the binding force between metal and support is strong, deposited carbon grow outward from surface to form fibrous carbon (Fig. 6A). Otherwise, carbon nucleates and grows at the rear of the crystallite, lifting the nickel crystallite from the catalyst surface, which explains the formation of tubes carbon (Fig. 6B). However, the metal crystallite lifted by carbon tube may drop, which results in loss of active metal. Similarly, the weak interaction may accelerate formation of carbon deposition around the metal particles, which then are coated by carbon shell. The shell carbon prevents the reaction gas contacting the active sites, which results in the catalyst deactivation (Fig. 6C).
The role of silica shell to resist carbon deposition Traditional strategies to resist or remove carbon deposition includes: controlling of metal crystallite size as well as increasing steam/carbon ratios in the feed [35]. Bearing in mind the formation mechanism of carbon deposition, it can be concluded that the edge and corner atoms are the key of carbon deposition. The edge and corner atoms as well as platform atoms are active for POM reaction. However, the edge and corner atoms are considered as the active sites for carbon deposition. The low coordinated surface sites are favor to CeC bond scission and hydrogen stripping to produce C1
Fig. 6 e Formation mechanism of carbon deposition (A, B, C) and coke inhibition of silica shell (D).
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fragments, which results in carbon deposition [3]. The severe carbon deposition on Ni/ZrO2 catalyst confirms the claim that more edge and corner atoms are favor to carbon deposition. While the carbon deposition can be efficiently suppressed by blocking the edge and corner atoms using silica shell (Fig. 6D). Moreover, the results of N2 adsorption suggest the size of the pore channels in the silica shell is ~5 nm (Fig. 3b), which inhibits carbon filament formation, because the typical filament diameter is much larger (~17 nm). In addition, the pore channel hinders the radical chains reactions that are necessary to form coke. The TEM showed metal particles were uniformly covered with silica, which resulted in fibrous carbons did not have sufficient room to grow [24]. Nielsen [36] suggested that the ensembles size control played a critical role in minimizing formation of coke in steam reforming. Coke formation would require more ensembles of surface sites (6e7 nickel atoms) than those required for steam reforming (3e4 nickel atoms). Guo et al. [37] obtained single iron sites by spreading out the iron atoms in a silica matrix for nonoxidative conversion of methane. They found the absence of adjacent iron sites prevented catalytic CeC coupling, further carbon deposition. Therefore, it is reasoned that the decreasing ensemble size by segmentation of silica shell is favor to minimization of carbon deposition.
Conclusions In this work, the Ni/ZrO2@SiO2, Ni@SiO2 coreeshell catalysts and Ni/ZrO2 support catalyst were synthesized and investigated for the catalytic partial oxidation of methane to synthesis gas. The effects of the silica shell and ZrO2 on reducibility, structural properties, and prevention of carbon deposition were characterized by XRD, TEM, N2 sorption, TG, H2-TPR and OSC techniques. The lifetime test of 160 h showed the Ni/ ZrO2@SiO2 catalyst owned more stable and higher catalytic activity due to reduced carbon deposition during POM reaction, differing in sharp reduction of CO selectivity (from 88% to 75%) on Ni/ZrO2 catalyst and low methane conversion (84%) on Ni@SiO2 catalyst. Among three catalysts with different Ni/Zr ratio, the Ni/ZrO2@SiO2 catalyst (Ni/Zr ¼ 1) was proved to be more active with higher methane conversion (99%) and product selectivity (94% for CO and 85% for H2). The results showed silica shell played critical role in resistance of carbon deposition, by blocking the active edge and corner atoms and preventing the growth up of carbon fiber. The addition of ZrO2 effectively improved reducibility and oxygen storage capacity of catalysts, which was favor to reduce carbon deposition.
Acknowledgments The authors gratefully acknowledge the financial support from the Project of “Utilization of Low Rank Coal” Strategic Leading Special Fund, Chinese Academy of Sciences (XDA07070800, XDA-07070400), the Program of Overseas Science and Technology Activity (2012-35), Shanxi Province and the cooperation program from Shanxi Fenxi mining (Group) Co., Ltd.
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