- Email: [email protected]
LaNiO3 nanocube embedded in mesoporous silica for dry reforming of methane with enhanced coking resistance
Accepted Manuscript LaNiO3 nanocube embedded in mesoporous silica for dry reforming of methane with enhanced coking resistance Li Zhang, Jie Lian, Le Li, Cheng Peng, Wenming Liu, Xianglan Xu, Xiuzhong Fang, Zheng Wang, Xiang Wang, Hongen Peng PII:
S1387-1811(18)30086-6
DOI:
10.1016/j.micromeso.2018.02.021
Reference:
MICMAT 8784
To appear in:
Microporous and Mesoporous Materials
Received Date: 29 August 2017 Revised Date:
23 December 2017
Accepted Date: 19 February 2018
Please cite this article as: L. Zhang, J. Lian, L. Li, C. Peng, W. Liu, X. Xu, X. Fang, Z. Wang, X. Wang, H. Peng, LaNiO3 nanocube embedded in mesoporous silica for dry reforming of methane with enhanced coking resistance, Microporous and Mesoporous Materials (2018), doi: 10.1016/ j.micromeso.2018.02.021. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Graphical Abstract
Herein, a perfect perovskite LaNiO3 nano-cube was encapsulated in perpendicular silica
shell
to
form
a
novel
core-shell
structured
catalyst
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mesoporous
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EP
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(LaNiO3-cube@meso-SiO2) with enhanced coking resistance for DRM reaction.
ACCEPTED MANUSCRIPT LaNiO3 nanocube embedded in mesoporous silica for dry reforming of methane with enhanced coking resistance Li Zhanga†, Jie Liana†, Le Lia, Cheng Penga, Wenming Liua, Xianglan Xua, Xiuzhong
a
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Fanga, Zheng Wangb, Xiang Wanga, Hongen Penga* Institute of Applied Chemistry, College of Chemistry, Nanchang University, 999
b
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Xuefu Road, Nanchang, Jiangxi 330031, China.
State Key Laboratory of High-efficiency Utilization of Coal & Green Chemical
* Email: [email protected] †
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Engineering, Ningxia University, Yinchuan, 750021, China.
These authors contribute equally to this work.
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Abstract:
Methane dry reforming (DRM) is a fascinating reaction which can effectively utilize two abundant and greenhouse gases (CO2 and CH4) to prepare syngas (CO and H2). The
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design and synthesis of high coking resistance Ni based catalysts are still a challenge.
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Herein, a perfect perovskite LaNiO3 nano-cube was encapsulated in perpendicular mesoporous
silica
shell
(LaNiO3-cube@meso-SiO2)
to
form for
a
DRM
novel
core-shell
reaction.
The
structured
catalyst
morphology
of
LaNiO3-cube@meso-SiO2 retained well even after reduced at high temperature, and the effect of shell thickness on its catalytic performance was also studied in detail. It’s found that the mesoporous silica shell has positively effects to improve the coking resistance of Ni/La2O3 (derived from LaNiO3 nano-cube after reduction). When the 1
ACCEPTED MANUSCRIPT shell thickness increased, the formation of coking deposition is decreased. The reasons for this novel core-shell structured catalyst with superior DRM performance should be attributed to the dual confinement effects. One is the strong metal-support
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interaction (SMSI) between Ni and La2O3 and silica, and the other is derived from the perpendicular mesoporous silica shell. The dual confinement strategy developed in
reaction or other high temperature thermal catalysts.
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this work can be used to design other high coking resistance catalysts for DRM
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Key words: Perovskite; LaNiO3 nanocube; Perpendicular mesoporous silica shell; Core-shell structure; Dry reforming of methane; Coke resistance
1. Introduction
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Dry reforming of methane (DRM) has been intensively studied due to its important application in chemical industry of producing synthesis gas and hydrogen, and most
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importantly, which can effectively utilize two major greenhouse gases (CH4 and CO2)[1-7]. Due to its inherent strong endothermic effect, the DRM reaction has also
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been considered to be an ideal way to store energy[8, 9]. However, dry reforming technique has the disadvantage of rapid catalyst deactivation due to carbon deposition and sintering of both support and the active metal particles at high reaction temperatures[10, 11], which can degrade the activity of the catalyst quickly and also block the reactors[12]. The coke originates from two reactions (methane decomposition and CO disproportionation) [10, 13-16]:
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ACCEPTED MANUSCRIPT CH4→C+H2
△H=75KJ/mol
2CO→C+CO2
△H=-172 KJ/mol
Nickel and noble metals such as Pt, Ru, and Rh have been the most investigated
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catalysts for DRM. Noble metal catalysts show high activity and resistance toward carbon formation, despite their excellent performance[17]. However, the high price and
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limited availability restrict their application in industry. The Ni-based catalysts have much lower cost due to their extensive resources and can offer an alternative.
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Unfortunately, the Ni-based catalysts are sensitive to deactivation as result of sintering and coking under harsh reforming conditions. Hence from an industry application view, it is still necessary to improve Ni-based catalysts with respect to both activity and carbon resistance.
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LaNiO3 with perovskite structure was thought to be an excellent precursor of Ni/La2O3 which can obtain relatively uniform dispersion of Ni thus reducing Ni0
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segregation (strong metal-support interaction between Ni and supports). And the
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generated La2O3 tends to interact with CO2 to form La2O2CO3 which help suppress coke formation in DRM[18-20]. In recent years, core-shell structured catalysts were prepared and intensively investigated due to their advantages including availability of uniform particle distribution, coking and sintering resistance at high reaction temperature[11, 17, 21-30] Kawi et al.[29, 31] prepared silica coated Ni-Mg core-shell catalysts and used in DRM, these results reveal that the core-shell structured catalysts showed high and stable activity attributed to smaller Ni particle
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ACCEPTED MANUSCRIPT size and stronger interaction between metal and support. Thus, the core-shell structured Ni based catalysts with dual confinement effect can effectively inhibited Ni sintering and carbon deposition.
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Herein, a novel core-shell structured Ni based catalyst with perfect perovskite LaNiO3 nano-cube as core and perpendicular mesoporous silica as shell was designed
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and synthesized. The morphology of LaNiO3-cube@meso-SiO2 retained well even after reduced at high temperature, and the effect of shell thickness on its catalytic
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performance was also studied in detail. It’s found that the mesoporous silica shell has positively effect to improve the coking resistance of Ni/La2O3 (derived from LaNiO3 nano-cube after reduction). When the shell thickness increased, the formation of coking deposition is decreased. The reasons for this novel core-shell structured
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catalyst with superior DRM performance should be attributed to the dual confinement effects. One is the strong metal-support interaction (SMSI) between Ni and La2O3 and
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SiO2, and the other is derived from the perpendicular mesoporous silica shell.
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2. Experimental
2.1 Catalyst preparation All the reagents are analytical and used as received without any further purification.
2.1.1 Synthesis of LaNiO3 nanocube LaNiO3 with perfect nano-cube morphology was prepared according to the literature with little modification[32, 33]. In typical synthesis procedure, 3.031 g La(NO3)3·6H2O and 1.525 g Ni(NO3)3·6H2O and 1.970 g glycine were
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ACCEPTED MANUSCRIPT simultaneously dissolved in 375 ml deionized water to form a transparent solution, then concentrated NH3·H2O was added into the solution dropwise under vigorous stirring to maintain the pH value at about 7.7, after 30 min stirring, the solution was
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transferred into a Teflon-lined with 500 ml capacity. Afterward, the Teflon liner was sealed in the stainless steel autoclave and maintained at 180 °C for 24 h. The product
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was washed with deionized water and ethanol for several times. Finally, the achieved solids were dried at 110 °C overnight and calcined at 550 °C for 2 h with a heating
morphology
for
the
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rate of 2 °C/min in air atmosphere to achieve LaNiO3 with perfect nano-cube following
LaNiO3-cube@meso-SiO2.
synthesis
of
core-shell
structured
2.1.2 Synthesis of LaNiO3-cube@meso-SiO2
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LaNiO3 nanocube encapsulated in perpendicular mesoporpus silica was prepared by the modified stöber method using hexadecyl trimethyl ammonium bromide (CTAB)
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as the meso-structured directing agent. Firstly, 0.071 g CTAB was dissolved in 20 g
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deionized water, 12 g alcohol and 0.25 g ammonia under stirring to form a transparent solution, then 0.1 g LaNiO3 was added to the solution by ultrasonic dispersion in aqueous solution for about 30 min. With vigorous stirring at 25 ℃, 0.112 g TEOS was dropped into the above solution slowly, after reaction for 4 h with continuously stirring, the solids were collected by centrifugation at 10000 rpm, then washed several times with distilled water and ethanol until pH reached neutral. Finally, the obtained solid was dried at 100 °C for 5 h and calcined at 550 °C for 5 h in air atmosphere with
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ACCEPTED MANUSCRIPT a heating rate of 2 °C/min. the thickness of the mesoporous silica shell was controlled by weight ratio of TEOS/Core (g/g), the more TEOS was added, the thicker mesoporous silica shell was formed. LaNiO3 nanocube coated by mesoporous SiO2
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shell were noted by LaNiO3-cube@meso-SiO2-x where x stands for the mass ratio of TEOS to LaNiO3.
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For comparison, nickel nanoparticles supported on Al2O3 were also prepared via conventional wet impregnation method.
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2.2 Catalysts characterization
The scanning electron microscope (SEM) images were taken on a Hitachi S-4800 field emission scanning electron microscope. The Transmission Electron Microscope (TEM) images were taken on a TecnaiTM F30 transmission electron microscope.
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The Powder X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 Focus diffractometer instrument operating at 40 kV and 30 mA, with Cu target Kα-ray
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irradiation (λ= 1.5405 Å). Scans were taken with a 2 θ range from 10o to 90o and a step
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of 2o/min. To keep the data comparable, all the samples were tested continuously. The mean crystallite sizes of the samples were calculated using Scherrer equation based on the three strongest peaks of metallic Ni. Hydrogen temperature programed reduction (H2-TPR) profiles were carried out
on FINESORB 3010C instrument in a 10 % H2/Ar gas mixture flow with a flow rate of 30 mL/min. The temperature was increased from room temperature to 800 oC with a ramp of 10 oC/min. Generally, 100 mg catalyst was used for the test. A thermal
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ACCEPTED MANUSCRIPT conductivity detector (TCD) was employed to monitor the H2 consumption. For H2 consumption quantification, CuO (99.99%) was used as the calibration standard sample.
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Thermogravimetric analysis (TGA-DSC) was carried out using thermal analyst to monitor the coke deposition amount of the spent catalysts. The experiments were performed with about 10 mg spent catalysts on a TAQ600 instrument with temperatures
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ranging from 30 to 800 oC in the presence of air and heating rate of 10 oC/min
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The specific surface areas of the samples were measured by nitrogen adsorption-desorption at 77 K on ASAP2020 instrument. Specific surface areas were calculated using Brunauer-Emmett-Teller (BET) method in the relative pressure (P/P0) range of 0.05-0.25. The pore size distributions of the samples were calculated with
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Barrett-Joyner-Halenda (BJH) method. And the average pore sizes were obtained from the peak positions of the distribution curves. The total pore volume was accumulated at
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a relative pressure of P/P0 = 0.99.
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2.3 Activity evaluation
The catalytic activity tests were performed in a fixed bed micro-reactor. Generally,
catalyst (100 mg) was first reduced in situ in a 30 ml min-1, 10 %H2/Ar flow at 700 oC for LaNiO3-cube and LaNiO3-cube@meso-SiO2, and at 800 oC or Ni/Al2O3 for 2 h, respectively. The methane dry reforming reaction was performed in a gas flow of 30 ml min-1 gas mixture flow, with a stoichiometric volume radio of CH4 (99.99 %) CO2 (99.99 %)=1:1. The weight hourly space velocity (WHSV) was 18000 ml h-1 gcat.-1. The
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ACCEPTED MANUSCRIPT outlet gas was cooled by ice-water and then analyzed on-line by using a GC9310 gas chromatograph equipped with a TDX-01 column and a TCD detector. To determine the amount of coke formed during the reaction, all the measured catalysts were tested for
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480 min in the reaction feed. 3. Results and discussion
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3.1 Morphology and texture structure of LaNiO3-cube@meso-SiO2
To elucidate the morphology of LaNiO3-cube@meso-SiO2 and related materials,
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we employed transmission electron microscope (TEM) technique to explore the inner core and outer mesoporous surface shell. The TEM images of fresh catalysts with different shell thickness are shown in Fig.1. Fig.1(a) demonstrates that pure perovskite LaNiO3 with uniform perfect nanocube morphology was successfully
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prepared. The surface of LaNiO3 nanocubes are quite smooth, and the particles size is about 350 nm. After coated with mesoporous silica shell, the core-shell structured
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LaNiO3-cube@meso-SiO2 was achieved (Fig.1b-d). It can be clearly seen that the
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mesopores of the silica sell were perpendicular to the core of LaNiO3 nanocube. The mesoporous silica shell was grown around the core with similar outer morphology shape. And most importantly, the original cubic shape of LaNiO3 cores were not significantly changed. The thickness of the mesoporous SiO2 shells vary from 75 nm to 350 nm according to the addition amount of silica source (TEOS). Therefore, we can conclude that the novel core-shell structured materials with perfect LaNiO3 nano-cube as core and ordered mesoporous silica as shell (LaNiO3-cube@meso-SiO2)
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ACCEPTED MANUSCRIPT was successfully fabricated with tunable shell thickness through a facile modified stöber method To elucidate the reasons leading to the difference in reaction performance, the
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texture properties of the catalysts were measured with N2 adsorption-desorption technique, with the results shown in Fig.2 and Table 1. LaNiO3-cube has the lowest
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surface area (7.7 m2/g) among all the other catalysts excluding Ni/Al2O3(102 m2/g). It is clearly observed in Fig.2 (A) that LaNiO3-cube has a very small amount of nitrogen
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adsorption, and the adsorption curve and desorption curve coincide completely which indicates that no pores exist in LaNiO3. In comparison, all the core-shell structured catalysts showed a type IV isotherm with clearly hysteresis loop in the relative pressure of 0.3-0.9 for the formation of mesopores in these materials[19, 20].
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As listed in Table 1, the pore volume and size of LaNiO3-cube is 0.04 cm3/g and 26.42 nm, while the surface areas of the three core-shell structured catalysts are 225,
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257, 535 m2/g, for [email protected], [email protected],
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[email protected], respectively, which are much higher than the pure core of LaNiO3-cube due to the mesoporous SiO2 with high surface areas[3, 21-23]. Generally, catalyst with larger surface area is favorable for the dispersion of active components, thus obtaining with better reaction performance[24]. While for LaNiO3-cube@meso-SiO2, the active site is Ni/La2O3 core derived from the LaNiO3 nano-cube, the silica shell is not active for DRM. Thus, though changing the shell thickness could not improve the dispersion of the Ni species, there has a suitable value
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ACCEPTED MANUSCRIPT for DRM, which can effectively confine the active sites in the core. In this work, the suitable shell thickness was about 120 nm when the weight ratio of TEOS to core is 1.7. Further increasing the shell thickness, its coking resistance was not evidently improved.
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To elucidate the effects between active sites Ni and SiO2 shells on the reduction process, all the fresh calcined catalysts were tested via H2-TPR technique, with the results listed in Fig. 3. The conventional Ni/Al2O3 catalyst showed a reduction peak
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above 700 oC, which is assigned to the reduction of Ni2+ to Ni0 from NiAl2O4 phase.
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Both LaNiO3-cube and core-shell structured LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) catalysts display two reduction peaks in TPR profiles. It should be noted, up to now, the reduction mechanism of LaNiO3 perovskite structure has not yet reached a commonly accepted cognition. As is reported in many papers[23, 25, 27], the
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reduction peaks at 350-500 oC were ascribed to the reduction of Ni3+ to Ni2+, and reduction peaks at 550-700 oC were ascribed to the reduction of Ni2+ to Ni0.
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Compared with the core LaNiO3-cube, the reduction peaks of LaNiO3@SiO2-x (x=1.1,
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1.7, 2.8) become more wider, and shift to higher temperatures according to the shell thickness,
among
the
three
core-shell
structured
catalysts,
[email protected] has both the largest hydrogen consumption and the reduction temperature (642 ℃ ). It strongly testifies that Ni0 reduced from [email protected] has strong interaction with the SiO2 inner shell during the reduction and the high temperature reaction process, thus inhibiting the formation of coke. Besides, as is reported that there is a certain relationship between the SiO2 shell
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ACCEPTED MANUSCRIPT thickness and the diffusion velocity of H2 molecule in the shell[28, 31]. Therefore, to some extent, the abundant mesoporous shell can decrease the content of active sites which make the reduction peak became wider and lower and shift to higher
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temperature. When the shell thickness further increased, the hydrogen consumption became smaller as the theoretical content of Ni further decreases according to ICP
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analysis (Table 2).
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3.2 Catalytic performance of LaNiO3-cube@meso-SiO2 3.2.1 Activity evaluation of the catalysts
The catalytic performance of the above prepared catalysts for dry reforming of
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methane (DRM) with the function of temperatures are shown in Fig.4. All the data displayed herein were collected after stabilization for 30 min at the corresponding temperature. The obtained results showed that conventional nickel catalyst supported
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on Al2O3 (Ni/Al2O3) has highest initial CH4 conversion about 38 % while all the other
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catalysts exhibit similar initial CH4 conversion (at 700 oC), while the CO2 conversion of all the catalysts have similar initial activity. The CH4 and CO2 conversions were increased with increasing temperature due to the inherent endothermicity of DRM reaction. Usually, the more Ni amount means more active sites, and it is positive for improving their catalytic performances, while in this work,
activity of
LaNiO3-cube@meso-SiO2 with different shell thickness was not evidently decreased. To achieve the true value of Ni amount, all the catalysts were tested by ICP 11
ACCEPTED MANUSCRIPT technology and with the results showed in Table 2. For Ni/Al2O3 catalyst, Ni amount was 11.8 wt.% which was very close to the theoretical value 12 wt.%. However, for perovskite like catalysts, all Ni amount decreased in some degree, LaNiO3 nanocube
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get 1/3 percent, and core-shell structured LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) get 1/2 to the initial theory value, on one hand, unlike impregnation method of
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Ni/Al2O3 catalyst, loss damage of Ni active site was not un-avoided because there exists wash step when preparing LaNiO3 and LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7,
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2.8) catalysts. On the other hand, as is reported elsewhere[34-38], the reasonable preparation process can obtain uniform mesoporous SiO2 shell coated onto the shell without forming single SiO2 particles. When adding LaNiO3 into this stöber reaction system, the LaNiO3 nanocubes were all embedded in mesoporous silica shell. What’s
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more, with the increase the amount of TEOS input, the more serious the loss, the content is almost half of the theoretical Ni content. This should be one of the
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retained well.
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advantage for our novel catalyst which decrease the active sites and its activity
3.2.2 The stability test
To investigate the stability of LaNiO3-cube@meso-SiO2 and related catalysts, the
catalytic performance with 100 mg catalyst amount and a feed molar ratio of CH4/CO2 =1:1 at 800 oC and at atmospheric pressure was taken placed at a space velocity of 18000 ml h-1 gcat-1. The conversions of CH4 and CO2 and H2/CO molar ratio are showed in Fig.5 and Fig.6, respectively. The traditional Ni/Al2O3 catalyst starts from initial CH4 conversion of 82 %, which drop to 60% in 480 min of continuous reaction. And the H2/CO of Ni/Al2O3 exhibits similar regular which means that nickel catalysts 12
ACCEPTED MANUSCRIPT supported on Al2O3 agglomerate very fast while all the core-shell structured catalyst remain stable until the end of the 480 min on reforming. For Ni/LaNiO3, derived from LaNiO3 nanocube, the stability test was stopped quickly due the serious coking formation (Figure 7 a-2). Interestingly, for LaNiO3nanocube encapsulated in
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mesoporous silica shell, the conversion of CH4 and CO2 was not significantly decreased and the H2/CO ratio retained at about 1.0. It is well known that the RWGS and methane cracking are two unavoidable side reactions happening with DRM reaction at 800 oC and the H2/CO ratio should be either higher (If methane
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decomposition is dominant) or lower (If RWGS is dominant). However, for DRM reaction, it also existed CO disproportionation reaction, which is also one of the main
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reason for carbon deposition. Thus, the reasons for H2/CO ratio at ~1.0 are very complex. To resolve this problem, the isotopic tracer method should be adopted. The feasible reasons for this phenomenon should be attributed to the confinement effect derived from the mesoporous silica shells around the active the
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LaNiO3 nano-cube, which can efficiently prevent carbon deposition on the metallic nickel active sites, though also has some carbon was detected through TGA analysis.
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3.4 Characterization of the fresh reduced and spent catalysts
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Fig.7 shows the TEM images of freshly reduced (left) and spent (right) catalysts. It can be evidently seen that LaNiO3-cube after reduced in 10 %H2/Ar atmosphere for 2 h at 700 oC, the origin cubic shape of LaNiO3 was not changed but with some cavity appeared indicating that Ni active site was reduced to form Ni/La2O3 active sites and this should be the main reason of its high initial conversion. For core-shell structured LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) catalysts with various shell thickness as shown in Fig.7 (b-1) - (d-1), the perfect morphology of LaNiO3-cube core remained
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ACCEPTED MANUSCRIPT very well and encapsulated in the mesoporous SiO2 shell. What’s more, the shell thickness distributed from 60 nm to 350 nm. Compared with the spent catalysts shown in Fig 7 (2), LaNiO3-cube without embedded in mesoporous silica, its cube structure
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was totally destroyed after stability test for just only 30 min, and a lot of filamentous carbons were clearly existed which almost equal to the traditional Ni/Al2O3 catalyst.
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However, LaNiO3-cube encapsulated in various thickness of mesoporous SiO2 shells, the core shell structure and cubic morphology remained well and less carbon
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deposition was detected indicating that LaNiO3-cube confined in mesoporous silica shells have higher resistance against coke deposition, which is in line with the TGA-DSC results.
To further testify the morphology of spent catalysts and formation of carbon
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deposition, the optimized [email protected] of fresh and spent catalyst were characterized by scanning electron microscope (SEM), with the images shown in
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Fig.8. It’s evidently observed in Fig.8 (c-1) that the conventional Ni/Al2O3 catalyst
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consists of un-uniform and irregular particles while pure LaNiO3 presents a uniform cubic morphology as listed in Fig.8 (a-1). However, after LaNiO3-cube coated with mesoporous SiO2 shell as displayed in Fig.8 (b-1), the [email protected] sample becomes smooth and evasive. This result further confirm that the LaNiO3-cube was totally embedded in mesoporous silica shell. Fig.8 (a-2) and (c-2) display SEM images after stability test for DRM reaction. It can be clearly seen that both the surface of LaNiO3-cube and Ni/Al2O3 are covered by a large amount of long
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ACCEPTED MANUSCRIPT whisker-like
filamentous
carbon
deposits,
in
comparison,
the
surface of
[email protected] sample is almost clean indicating that the severe coking was effectively suppressed by the mesoporous SiO2 shell.
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The XRD patterns of the fresh calcined and spent catalysts are presented in Fig.9. It’s reported[39] that the calcined temperature of metal oxide is a key factor for crystal
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formation and growth, below 550 oC, the main phase of LaNiO3 are La2O3 and NiO. As is exhibited in Fig 9 (A), no any diffraction peaks are observed obviously for
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LaNiO3 (Ni-La2O3) while only a weak peak at 32.78o (2θ) which correspond to the (110) and (200) diffractions of LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) which re-calcined to remove the meso-structure directing agent. What’s more, the diffraction peaks of SiO2 are similar to the LaNiO3 particles, but no any clear peaks can be found
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due to its weak intensity in these core shell structured catalysts[28, 40, 41]. To further investigate the structure properties of these catalyst, XRD patterns of the spent catalysts
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are shown in Fig. 9 (B), Scherrer’s formula was applied to calculate the crystal size of
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Ni, and the results are given in Table.2. In comparison with the results over LaNiO3-cube (22.0 nm), a slightly larger crystal size of Ni was obtained over Ni/Al2O3 (25.4 nm) while evidently smaller crystal size was observed in the case of LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) catalysts after reduction. As declared before, the reduced Ni0 particles highly distributed on the surface of La2O3 and inner face of SiO2 shell which obtained smaller Ni particle size, and the formation of La2O2CO3 further depressed the formation of coke deposition[18, 42]. This was well
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ACCEPTED MANUSCRIPT agreement with their catalytic performance results. Coke resistance is always thought to be a main reason for the deactivation of DRM reaction catalysts[42, 43]. To achieve clearer information, the spent catalysts
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after for 480 min reaction were collected and measured by TGA-DSC to quantify the amount of carbon deposition, with the results show in Fig.10. As shown in Fig.10 (A),
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all the catalysts showed no weight loss before 300 oC for totally dried before characterization. Ni/Al2O3 has a 56 % weight loss stage, which is accompanied by a
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symmetric exothermic peak at 683 oC in the DSC profile while LaNiO3 has a weight loss of 59 % which is even higher than Ni/Al2O3 proving the formation of more serious coke formation.
In contrast, for the novel core-shell structured
LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) with various shell thickness, much less
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carbon deposition of 12 % was detected，and 4 wt.% for the thicker mesoporous SiO2 shells ([email protected] and [email protected]) maintained
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at 4wt.% indicating that severe coking was effectively suppressed by coating a mesoporous SiO2 shell. As is reported, the tendency of Ni0 metal particles to
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aggregate is trivial because the Ni0 particles are homogeneously and densely distributed on the surface of La2O3 and shielded by SiO2 shell.[44] Therefore, carbon deposition is hard to find from the core-shell structured catalysts. For other Ni-based catalysts without protective structure, coke deposition often occurs directly on Ni particles.
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ACCEPTED MANUSCRIPT 4. Conclusions In summary, a novel core-shell structured catalyst using a perfect perovskite LaNiO3 nano-cube
as
core
and
perpendicular
mesoporous
silica
as
shell
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(LaNiO3-cube@meso-SiO2) was successfully fabricated and applied for dry reforming of methane (DRM) to prepare syngas. It is known that though La2O3 is a suitable
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support for loading Ni active sites owing to its alkalinity to facilitate the adsorption of CO2, strong metal support interaction with Ni, and most importantly to form La2O2CO3
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to eliminate the coke deposition, the coking problem is still existed. The dual confinement strategy used in this work (that is the strong metal support interaction between Ni and La2O3 and the mesoporous silica shell) can efficiently improve its coking resistance for DRM. The shell thickness is also a very important factor to affect
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its coking resistance. When the shell thickness increased to above 60 nm, the carbon deposition can decrease to below 4 wt.% after 480 min reaction. Thus, we believe that
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the dual confinement strategy developed in this work can be used to design other high
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coking resistance catalysts for DRM reaction or other high temperature thermal catalysts.
Acknowledgements
This work is supported by the National Key R&D Program of China (2016YFC0205900), the National Natural Science Foundation of China (21503106, 21566022 and 21773106), the Natural Science Foundation of Jiangxi Province (20171BCB23016 and 20171BAB203024) and the Foundation of State Key 17
ACCEPTED MANUSCRIPT Laboratory of Coal Clean Utilization and Ecological Chemical Engineering (Grant No.
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2016-15) which are greatly acknowledged by the authors.
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[2] L. Li, D.H. Anjum, H. Zhu, Y. Saih, P.V. Laveille, L. D'Souza, J.-M. Basset, ChemCatChem 7 (2015) 427-433.
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[3] D. Pakhare, J. Spivey, Chemical Society Reviews 43 (2014) 7813-7837.
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Mesoporous Materials 251 (2017) 9-18.
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N. Zhang, X. Wang, Applied Catalysis B: Environmental 224 (2018) 488-499.
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[8] G.S. Gallego, C. Batiot-Dupeyrat, J. Barrault, E. Florez, F. Mondragón, Applied Catalysis A: General 334 (2008) 251-258. [9] S. Wang, G.Q. Lu, G.J. Millar, Energy & Fuels 10 (1996) 896-904. [10] M.-S. Fan, A.Z. Abdullah, S. Bhatia, international journal of hydrogen energy 36 (2011) 4875-4886. [11] Z. Li, L. Mo, Y. Kathiraser, S. Kawi, ACS Catalysis 4 (2014) 1526-1536. [12] J. Sehested, Catalysis Today 111 (2006) 103-110.
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[19] H. Peng, X. Zhang, L. Zhang, C. Rao, J. Lian, W. Liu, J. Ying, G. Zhang, Z. Wang, N. Zhang, X. Wang, ChemCatChem 9 (2017) 127-136.
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[20] C. Dai, S. Zhang, A. Zhang, C. Song, C. Shi, X. Guo, Journal of Materials
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Chemistry A 3 (2015) 16461-16468. [21] X. Zheng, S. Tan, L. Dong, S. Li, H. Chen, Chemical Engineering Journal 265 (2015) 147-156.
[22] W. Yang, H. Liu, Y. Li, J. Zhang, H. Wu, D. He, Catalysis Today 259 (2016) 438-445. [23] Y. Cao, M. Lu, J. Fang, L. Shi, D. Zhang, Chemical Communications 53 (2017) 7549-7552.
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ACCEPTED MANUSCRIPT [24] X. Zhao, M. Lu, H. Li, J. Fang, L. Shi, D. Zhang, New Journal of Chemistry 41 (2017) 4869-4878. [25] X. Zhao, H. Li, J. Zhang, L. Shi, D. Zhang, International Journal of Hydrogen
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Energy 41 (2016) 2447-2456. [26] T. Xie, X. Zhao, J. Zhang, L. Shi, D. Zhang, International Journal of Hydrogen
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Energy 40 (2015) 9685-9695.
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7250-7253.
[28] X. Du, D. Zhang, R. Gao, L. Huang, L. Shi, J. Zhang, Chemical Communications 49 (2013) 6770-6772.
3556-3575.
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[29] S. Kawi, Y. Kathiraser, J. Ni, U. Oemar, Z. Li, E.T. Saw, ChemSusChem 8 (2015)
[30] Z. Li, Y. Kathiraser, J. Ashok, U. Oemar, S. Kawi, Langmuir 30 (2014)
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14694-14705.
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[31] Z. Bian, S. Kawi, ChemCatChem, 2018, in press, DOI: 10.1002/cctc.201701024 [32] J. Zhang, Y. Zhao, X. Zhao, Z. Liu, W. Chen, Scientific Reports 4 (2014) 6005. [33] S. Singh, D. Zubenko, B.A. Rosen, Acs Catalysis 6 (2016). [34] M.M.Y. Chen, A. Katz, Langmuir 18 (2002) 8566-8572. [35] F. Grasset, R. Marchand, A.M. Marie, D. Fauchadour, F. Fajardie, J Colloid Interface Sci 299 (2006) 726-732. [36] K.T. Li, M.H. Hsu, I. Wang, Catalysis Communications 9 (2008) 2257-2260.
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ACCEPTED MANUSCRIPT [37] J.N. Park, A.J. Forman, W. Tang, J. Cheng, Y.S. Hu, H. Lin, E.W. Mcfarland, Small 4 (2008) 1694-1697. [38] H.P. Peng, R.P. Liang, Z. Li, J.D. Qiu, Electrochimica Acta 56 (2011) 4231-4236.
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[39] X. Zheng, S. Tan, L. Dong, S. Li, H. Chen, International Journal of Hydrogen Energy 39 (2014) 11360-11367.
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[40] K.M. Kang, H.W. Kim, I.W. Shim, H.Y. Kwak, Fuel & Energy Abstracts 92 (2011) 1236-1243.
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[41] H.W. Kim, K.M. Kang, H.Y. Kwak, International Journal of Hydrogen Energy 34 (2009) 3351-3359.
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[43] E.-h. Yang, Y.-s. Noh, S. Ramesh, S.S. Lim, D.J. Moon, Fuel Processing Technology 134 (2015) 404-413.
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[44] X. Zheng, S. Tan, L. Dong, S. Li, H. Chen, International Journal of Hydrogen
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Energy 39 (2014) 11360-11367.
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ACCEPTED MANUSCRIPT Figure Captions Fig.1. TEM images of fresh LaNiO3 nanocube and embedded in mesoporous silica with various thickness: (a) LaNiO3 nanocube; (b) [email protected]; (c)
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[email protected]; (d) [email protected].
Fig.2 (A) N2 adsorption-desorption isotherms and (B) pore size distribution profiles of
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LaNiO3-cube@meso-SiO2 and related materials. Fig.3 H2-TPR profiles of the freshly calcined catalysts.
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Fig.4 Catalytic performance of LaNiO3-cube@meso-SiO2 and related materials with the function of temperature for dry reforming of methane: (A)CH4 conversion, (B)CO2 conversion. Reaction conditions: Pressure = 0.1MPa, CO2: CH4 = 1:1, weight hourly space velocity (WHSV) = 18000 ml h-1 g cat-1.
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Fig.5 Stability tests of LaNiO3-cube@meso-SiO2 and related materials for methane dry reforming: (A) CH4 conversion, (B) CO2 conversion; Reaction conditions:
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Pressure=0.1MPa, CO2: CH4=1:1, WHSV = 18000 ml h-1 g cat-1.
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Fig.6 H2/CO ratio of LaNiO3-cube@meso-SiO2 with various shell thickness: (A) [email protected];
(B)
[email protected];
(C)
[email protected]; (D) Ni/Al2O3. Reaction conditions: Pressure=0.1MPa, CO2: CH4=1:1, WHSV=18000 ml h-1 g cat-1. Fig.7 TEM images of the freshly reduced and spent catalysts: (a) LaNiO3-cube; (b) [email protected];
(c)
[email protected];
[email protected]; (e) Ni/Al2O3; (1) freshly reduced; (2) spent.
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(d)
ACCEPTED MANUSCRIPT Fig.8 SEM images of catalysts: (a) LaNiO3-cube; (b) [email protected]; (c) Ni/Al2O3. (1):fresh reduced and (2): spent. Fig.9 XRD patterns of the (A) freshly calcined and (B) used catalysts: (a)
(d) [email protected]; (e) Ni/Al2O3.
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LaNiO3-cube; (b) [email protected]; (c) [email protected];
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Fig.10 TGA-DSC profiles of the spent catalysts after 8 h stability tests.
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Table 1 The physicochemical properties of LaNiO3-cube@meso-SiO2 and related catalysts
(m2/g)
LaNiO3 nanocube
7.7
[email protected]
225
[email protected]
257
[email protected]
535
Ni/Al2O3
102
Pore
volume
diametre
(cm3/g) 0.04
Ni crystal size[b]
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Samples
Pore
(nm)
26.42
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SBET[a]
(nm)
22
0.26
5.13
--
0.33
4.91
--
0.32
2.63
--
--
--
25.4
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[a] Calculated by N2-adsorption and desorption by BJH method.
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[b] Crystallite size calculated using the Debye-Scherrer equation.
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Table 2 The physiochemical properties of catalysts. Ni content[b](%)
thickness[a](nm)
True
Theory
LaNiO3 cube
--
20.4
23.9
[email protected]
75
[email protected]
130
[email protected] Ni/Al2O3
[a] Mean size of different state.
18.1
8.1
16.1
350
6.0
13.2
--
11.8
12.0
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[b] Calculated using ICP.
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Samples
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Shell
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Fig.1 TEM images of fresh calcined LaNiO3 nanocube and embedded in mesoporous with
various
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silica
(c)
(a)
LaNiO3
nanocube;
[email protected];
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[email protected];
thickness:
[email protected].
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(b) (d)
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LaNiO3-cube
Adsorption Volume (cm /g)
3
[email protected] [email protected]
0.45 0.40 0.35
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1.0
15nm
(B)
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2nm
0.25
4nm
0.20
[email protected]
0.10
[email protected]
0.05
0.00 0
LaNiO3-cube [email protected]
0.15
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3
0.30
0.4 0.6 0.8 Relative Pressure (P/P0)
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0.50
0.2
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12Ni/Al2O3
0.0
dV/dD (cm /g)
(A)
[email protected]
12Ni/Al2O3
5
10
15
20
25
30
35
40
45
50
55
60
Pore size (nm)
Fig.2 (A) N2 adsorption-desorption isotherms and (B) pore size distribution profiles of LaNiO3-cube@meso-SiO2 and related materials.
28
O
O
LaNiO3-cube
611 C
O
420 C
O
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625 C O
422 C
[email protected]
O
642 C
[email protected]
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H2 Consumption (a.u.)
428 C
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O
406 C
O
563 C
O
365 C
[email protected] Ni/Al2O3
100
200
300
400
500
600
Temperature ( C)
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O
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Fig.3 H2-TPR profiles of the freshly calcined catalysts.
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700
800
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100 (A)
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CH4 Conversion (%)
80
60
40
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LaNiO3-cube [email protected] [email protected]
20
[email protected]
0
600
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Ni/Al2O3
650
700
750
800
O
Temperature ( C) 100 (B)
60
40
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CO2 Conversion (%)
80
EP
20
600
AC C
0
LaNiO3-cube [email protected] [email protected] [email protected] Ni/Al2O3
650
700
O
750
800
Temperature ( C)
Fig.4 Catalytic performance of LaNiO3-cube@meso-SiO2 and related materials with the function of temperature for dry reforming of methane: (A)CH4 conversion, (B)CO2 conversion. Reaction conditions: Pressure = 0.1MPa, CO2: CH4 = 1:1, weight hourly space velocity (WHSV) = 18000 ml h-1 g cat-1.
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100
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(A)
LaNiO3-cube
40
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60
[email protected] [email protected] [email protected]
20
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CH4 Conversion (%)
80
Ni/Al2O3
0
0
CO2 Conversion (%)
80
60
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20
0
(B)
LaNiO3-cube
EP
40
8
TE D
100
2 4 6 Time on stream (h)
[email protected] [email protected] [email protected] Ni/Al2O3
0
2 4 6 Time on stream (h)
8
Fig.5 Stability tests of LaNiO3-cube@meso-SiO2 and related materials for methane dry reforming: (A) CH4 conversion, (B) CO2 conversion; Reaction conditions: Pressure=0.1MPa, CO2: CH4=1:1, WHSV = 18000 ml h-1 g cat-1.
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2.0
(A) 1.6
[email protected]
1.6
H2/CO
1.2
8
0.0
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2 4 6 Time on stream (h)
H2/CO
0.4
H2/CO
0.8
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0.8
0
2 4 6 Time on stream (h)
H2/CO
[email protected] 1.2
0
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2.0
(B)
0.4
8
0.0
2.0
2.0
(D)
(C) [email protected]
1.6
1.6
1.2
H2/CO
1.2
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0.8
H2/CO
2
4 6 Time on stream (h)
0.8
0.4
8
0.0
H2/CO
0
2 4 6 Time on stream (h)
8
0.4
0.0
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0
H2/CO
Ni/Al2O3
Fig.6 H2/CO ratio of LaNiO3-cube@meso-SiO2 with various shell thickness: (A)
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[email protected];
(B)
[email protected];
(C)
[email protected]; (D) Ni/Al2O3. Reaction conditions: Pressure=0.1MPa, CO2: CH4=1:1, WHSV=18000 ml h-1 g cat-1.
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Fig.7 TEM images of the freshly reduced and spent catalysts: (a) LaNiO3-cube; (b) [email protected];
(c)
[email protected];
[email protected]; (e) Ni/Al2O3; (1) freshly reduced; (2) spent. 33
(d)
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Fig.8 SEM images of catalysts: (a) LaNiO3-cube; (b) [email protected];
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(c) Ni/Al2O3. (1):fresh reduced and (2): spent.
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⊗-NiAl2O4 •−Ni ♣-LaNiO3
(A)
⊗
⊗
⊗
⊗
⊗
•
(e)
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⊗
Intensity (a.u.)
♣
(d)
(c)
(b)
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(a)
LaNiO3(PDF#33-0710)
(B)
40
60
2θ (°)
Π
Π−C •− −Ni ∀−La2O2CO3
∀
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Intensity (a.u.)
•
∀
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20
80
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∀
∀
40
∀
(e) (d) (c)
(b) •
∀
2θ (°)
•
60
(a)
80
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Fig.9 XRD patterns of the (A) freshly calcined and (B) used catalysts: (a) LaNiO3-cube; (b) [email protected]; (c) [email protected]; (d) [email protected]; (e) Ni/Al2O3.
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80 59% 56%
60
LaNiO3-cube
[email protected]
40
[email protected]
20 100
200
300
(B)
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12%
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Weight loss (%)
4%
(A)
100
400
500
Temperature (°C)
600
700
800
683
LaNiO3-cube
583
648
[email protected]
[email protected]
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Ni/Al2O3
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Heat Flow (mW)
[email protected]
200
300
400
500
600
Temperature (°C)
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100
508
700
800
Fig.10 TGA-DSC profiles of the spent catalysts after 8 h stability tests.
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ACCEPTED MANUSCRIPT Research Highlights
A perfect core-shell structured methane dry reforming catalyst with perovskite LaNiO3 nano-cube as core and perpendicular mesoporous silica as shell was
The shell thickness has positive effect on coking resistance.
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successfully synthesized.
LaNiO3-cube@meso-SiO2 has the enhanced coking resistance due to the dual
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confinement effect.
S1387-1811(18)30086-6
DOI:
10.1016/j.micromeso.2018.02.021
Reference:
MICMAT 8784
To appear in:
Microporous and Mesoporous Materials
Received Date: 29 August 2017 Revised Date:
23 December 2017
Accepted Date: 19 February 2018
Please cite this article as: L. Zhang, J. Lian, L. Li, C. Peng, W. Liu, X. Xu, X. Fang, Z. Wang, X. Wang, H. Peng, LaNiO3 nanocube embedded in mesoporous silica for dry reforming of methane with enhanced coking resistance, Microporous and Mesoporous Materials (2018), doi: 10.1016/ j.micromeso.2018.02.021. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Graphical Abstract
Herein, a perfect perovskite LaNiO3 nano-cube was encapsulated in perpendicular silica
shell
to
form
a
novel
core-shell
structured
catalyst
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mesoporous
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EP
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(LaNiO3-cube@meso-SiO2) with enhanced coking resistance for DRM reaction.
ACCEPTED MANUSCRIPT LaNiO3 nanocube embedded in mesoporous silica for dry reforming of methane with enhanced coking resistance Li Zhanga†, Jie Liana†, Le Lia, Cheng Penga, Wenming Liua, Xianglan Xua, Xiuzhong
a
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Fanga, Zheng Wangb, Xiang Wanga, Hongen Penga* Institute of Applied Chemistry, College of Chemistry, Nanchang University, 999
b
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Xuefu Road, Nanchang, Jiangxi 330031, China.
State Key Laboratory of High-efficiency Utilization of Coal & Green Chemical
* Email: [email protected] †
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Engineering, Ningxia University, Yinchuan, 750021, China.
These authors contribute equally to this work.
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Abstract:
Methane dry reforming (DRM) is a fascinating reaction which can effectively utilize two abundant and greenhouse gases (CO2 and CH4) to prepare syngas (CO and H2). The
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design and synthesis of high coking resistance Ni based catalysts are still a challenge.
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Herein, a perfect perovskite LaNiO3 nano-cube was encapsulated in perpendicular mesoporous
silica
shell
(LaNiO3-cube@meso-SiO2)
to
form for
a
DRM
novel
core-shell
reaction.
The
structured
catalyst
morphology
of
LaNiO3-cube@meso-SiO2 retained well even after reduced at high temperature, and the effect of shell thickness on its catalytic performance was also studied in detail. It’s found that the mesoporous silica shell has positively effects to improve the coking resistance of Ni/La2O3 (derived from LaNiO3 nano-cube after reduction). When the 1
ACCEPTED MANUSCRIPT shell thickness increased, the formation of coking deposition is decreased. The reasons for this novel core-shell structured catalyst with superior DRM performance should be attributed to the dual confinement effects. One is the strong metal-support
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interaction (SMSI) between Ni and La2O3 and silica, and the other is derived from the perpendicular mesoporous silica shell. The dual confinement strategy developed in
reaction or other high temperature thermal catalysts.
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this work can be used to design other high coking resistance catalysts for DRM
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Key words: Perovskite; LaNiO3 nanocube; Perpendicular mesoporous silica shell; Core-shell structure; Dry reforming of methane; Coke resistance
1. Introduction
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Dry reforming of methane (DRM) has been intensively studied due to its important application in chemical industry of producing synthesis gas and hydrogen, and most
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importantly, which can effectively utilize two major greenhouse gases (CH4 and CO2)[1-7]. Due to its inherent strong endothermic effect, the DRM reaction has also
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been considered to be an ideal way to store energy[8, 9]. However, dry reforming technique has the disadvantage of rapid catalyst deactivation due to carbon deposition and sintering of both support and the active metal particles at high reaction temperatures[10, 11], which can degrade the activity of the catalyst quickly and also block the reactors[12]. The coke originates from two reactions (methane decomposition and CO disproportionation) [10, 13-16]:
2
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△H=75KJ/mol
2CO→C+CO2
△H=-172 KJ/mol
Nickel and noble metals such as Pt, Ru, and Rh have been the most investigated
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catalysts for DRM. Noble metal catalysts show high activity and resistance toward carbon formation, despite their excellent performance[17]. However, the high price and
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limited availability restrict their application in industry. The Ni-based catalysts have much lower cost due to their extensive resources and can offer an alternative.
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Unfortunately, the Ni-based catalysts are sensitive to deactivation as result of sintering and coking under harsh reforming conditions. Hence from an industry application view, it is still necessary to improve Ni-based catalysts with respect to both activity and carbon resistance.
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LaNiO3 with perovskite structure was thought to be an excellent precursor of Ni/La2O3 which can obtain relatively uniform dispersion of Ni thus reducing Ni0
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segregation (strong metal-support interaction between Ni and supports). And the
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generated La2O3 tends to interact with CO2 to form La2O2CO3 which help suppress coke formation in DRM[18-20]. In recent years, core-shell structured catalysts were prepared and intensively investigated due to their advantages including availability of uniform particle distribution, coking and sintering resistance at high reaction temperature[11, 17, 21-30] Kawi et al.[29, 31] prepared silica coated Ni-Mg core-shell catalysts and used in DRM, these results reveal that the core-shell structured catalysts showed high and stable activity attributed to smaller Ni particle
3
ACCEPTED MANUSCRIPT size and stronger interaction between metal and support. Thus, the core-shell structured Ni based catalysts with dual confinement effect can effectively inhibited Ni sintering and carbon deposition.
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Herein, a novel core-shell structured Ni based catalyst with perfect perovskite LaNiO3 nano-cube as core and perpendicular mesoporous silica as shell was designed
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and synthesized. The morphology of LaNiO3-cube@meso-SiO2 retained well even after reduced at high temperature, and the effect of shell thickness on its catalytic
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performance was also studied in detail. It’s found that the mesoporous silica shell has positively effect to improve the coking resistance of Ni/La2O3 (derived from LaNiO3 nano-cube after reduction). When the shell thickness increased, the formation of coking deposition is decreased. The reasons for this novel core-shell structured
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catalyst with superior DRM performance should be attributed to the dual confinement effects. One is the strong metal-support interaction (SMSI) between Ni and La2O3 and
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SiO2, and the other is derived from the perpendicular mesoporous silica shell.
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2. Experimental
2.1 Catalyst preparation All the reagents are analytical and used as received without any further purification.
2.1.1 Synthesis of LaNiO3 nanocube LaNiO3 with perfect nano-cube morphology was prepared according to the literature with little modification[32, 33]. In typical synthesis procedure, 3.031 g La(NO3)3·6H2O and 1.525 g Ni(NO3)3·6H2O and 1.970 g glycine were
4
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transferred into a Teflon-lined with 500 ml capacity. Afterward, the Teflon liner was sealed in the stainless steel autoclave and maintained at 180 °C for 24 h. The product
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was washed with deionized water and ethanol for several times. Finally, the achieved solids were dried at 110 °C overnight and calcined at 550 °C for 2 h with a heating
morphology
for
the
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rate of 2 °C/min in air atmosphere to achieve LaNiO3 with perfect nano-cube following
LaNiO3-cube@meso-SiO2.
synthesis
of
core-shell
structured
2.1.2 Synthesis of LaNiO3-cube@meso-SiO2
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LaNiO3 nanocube encapsulated in perpendicular mesoporpus silica was prepared by the modified stöber method using hexadecyl trimethyl ammonium bromide (CTAB)
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as the meso-structured directing agent. Firstly, 0.071 g CTAB was dissolved in 20 g
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deionized water, 12 g alcohol and 0.25 g ammonia under stirring to form a transparent solution, then 0.1 g LaNiO3 was added to the solution by ultrasonic dispersion in aqueous solution for about 30 min. With vigorous stirring at 25 ℃, 0.112 g TEOS was dropped into the above solution slowly, after reaction for 4 h with continuously stirring, the solids were collected by centrifugation at 10000 rpm, then washed several times with distilled water and ethanol until pH reached neutral. Finally, the obtained solid was dried at 100 °C for 5 h and calcined at 550 °C for 5 h in air atmosphere with
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shell were noted by LaNiO3-cube@meso-SiO2-x where x stands for the mass ratio of TEOS to LaNiO3.
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For comparison, nickel nanoparticles supported on Al2O3 were also prepared via conventional wet impregnation method.
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2.2 Catalysts characterization
The scanning electron microscope (SEM) images were taken on a Hitachi S-4800 field emission scanning electron microscope. The Transmission Electron Microscope (TEM) images were taken on a TecnaiTM F30 transmission electron microscope.
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The Powder X-ray diffraction (XRD) patterns were recorded on a Bruker AXS D8 Focus diffractometer instrument operating at 40 kV and 30 mA, with Cu target Kα-ray
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irradiation (λ= 1.5405 Å). Scans were taken with a 2 θ range from 10o to 90o and a step
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of 2o/min. To keep the data comparable, all the samples were tested continuously. The mean crystallite sizes of the samples were calculated using Scherrer equation based on the three strongest peaks of metallic Ni. Hydrogen temperature programed reduction (H2-TPR) profiles were carried out
on FINESORB 3010C instrument in a 10 % H2/Ar gas mixture flow with a flow rate of 30 mL/min. The temperature was increased from room temperature to 800 oC with a ramp of 10 oC/min. Generally, 100 mg catalyst was used for the test. A thermal
6
ACCEPTED MANUSCRIPT conductivity detector (TCD) was employed to monitor the H2 consumption. For H2 consumption quantification, CuO (99.99%) was used as the calibration standard sample.
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Thermogravimetric analysis (TGA-DSC) was carried out using thermal analyst to monitor the coke deposition amount of the spent catalysts. The experiments were performed with about 10 mg spent catalysts on a TAQ600 instrument with temperatures
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ranging from 30 to 800 oC in the presence of air and heating rate of 10 oC/min
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The specific surface areas of the samples were measured by nitrogen adsorption-desorption at 77 K on ASAP2020 instrument. Specific surface areas were calculated using Brunauer-Emmett-Teller (BET) method in the relative pressure (P/P0) range of 0.05-0.25. The pore size distributions of the samples were calculated with
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Barrett-Joyner-Halenda (BJH) method. And the average pore sizes were obtained from the peak positions of the distribution curves. The total pore volume was accumulated at
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a relative pressure of P/P0 = 0.99.
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2.3 Activity evaluation
The catalytic activity tests were performed in a fixed bed micro-reactor. Generally,
catalyst (100 mg) was first reduced in situ in a 30 ml min-1, 10 %H2/Ar flow at 700 oC for LaNiO3-cube and LaNiO3-cube@meso-SiO2, and at 800 oC or Ni/Al2O3 for 2 h, respectively. The methane dry reforming reaction was performed in a gas flow of 30 ml min-1 gas mixture flow, with a stoichiometric volume radio of CH4 (99.99 %) CO2 (99.99 %)=1:1. The weight hourly space velocity (WHSV) was 18000 ml h-1 gcat.-1. The
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ACCEPTED MANUSCRIPT outlet gas was cooled by ice-water and then analyzed on-line by using a GC9310 gas chromatograph equipped with a TDX-01 column and a TCD detector. To determine the amount of coke formed during the reaction, all the measured catalysts were tested for
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480 min in the reaction feed. 3. Results and discussion
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3.1 Morphology and texture structure of LaNiO3-cube@meso-SiO2
To elucidate the morphology of LaNiO3-cube@meso-SiO2 and related materials,
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we employed transmission electron microscope (TEM) technique to explore the inner core and outer mesoporous surface shell. The TEM images of fresh catalysts with different shell thickness are shown in Fig.1. Fig.1(a) demonstrates that pure perovskite LaNiO3 with uniform perfect nanocube morphology was successfully
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prepared. The surface of LaNiO3 nanocubes are quite smooth, and the particles size is about 350 nm. After coated with mesoporous silica shell, the core-shell structured
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LaNiO3-cube@meso-SiO2 was achieved (Fig.1b-d). It can be clearly seen that the
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mesopores of the silica sell were perpendicular to the core of LaNiO3 nanocube. The mesoporous silica shell was grown around the core with similar outer morphology shape. And most importantly, the original cubic shape of LaNiO3 cores were not significantly changed. The thickness of the mesoporous SiO2 shells vary from 75 nm to 350 nm according to the addition amount of silica source (TEOS). Therefore, we can conclude that the novel core-shell structured materials with perfect LaNiO3 nano-cube as core and ordered mesoporous silica as shell (LaNiO3-cube@meso-SiO2)
8
ACCEPTED MANUSCRIPT was successfully fabricated with tunable shell thickness through a facile modified stöber method To elucidate the reasons leading to the difference in reaction performance, the
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texture properties of the catalysts were measured with N2 adsorption-desorption technique, with the results shown in Fig.2 and Table 1. LaNiO3-cube has the lowest
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surface area (7.7 m2/g) among all the other catalysts excluding Ni/Al2O3(102 m2/g). It is clearly observed in Fig.2 (A) that LaNiO3-cube has a very small amount of nitrogen
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adsorption, and the adsorption curve and desorption curve coincide completely which indicates that no pores exist in LaNiO3. In comparison, all the core-shell structured catalysts showed a type IV isotherm with clearly hysteresis loop in the relative pressure of 0.3-0.9 for the formation of mesopores in these materials[19, 20].
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As listed in Table 1, the pore volume and size of LaNiO3-cube is 0.04 cm3/g and 26.42 nm, while the surface areas of the three core-shell structured catalysts are 225,
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257, 535 m2/g, for [email protected], [email protected],
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[email protected], respectively, which are much higher than the pure core of LaNiO3-cube due to the mesoporous SiO2 with high surface areas[3, 21-23]. Generally, catalyst with larger surface area is favorable for the dispersion of active components, thus obtaining with better reaction performance[24]. While for LaNiO3-cube@meso-SiO2, the active site is Ni/La2O3 core derived from the LaNiO3 nano-cube, the silica shell is not active for DRM. Thus, though changing the shell thickness could not improve the dispersion of the Ni species, there has a suitable value
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ACCEPTED MANUSCRIPT for DRM, which can effectively confine the active sites in the core. In this work, the suitable shell thickness was about 120 nm when the weight ratio of TEOS to core is 1.7. Further increasing the shell thickness, its coking resistance was not evidently improved.
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To elucidate the effects between active sites Ni and SiO2 shells on the reduction process, all the fresh calcined catalysts were tested via H2-TPR technique, with the results listed in Fig. 3. The conventional Ni/Al2O3 catalyst showed a reduction peak
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above 700 oC, which is assigned to the reduction of Ni2+ to Ni0 from NiAl2O4 phase.
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Both LaNiO3-cube and core-shell structured LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) catalysts display two reduction peaks in TPR profiles. It should be noted, up to now, the reduction mechanism of LaNiO3 perovskite structure has not yet reached a commonly accepted cognition. As is reported in many papers[23, 25, 27], the
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reduction peaks at 350-500 oC were ascribed to the reduction of Ni3+ to Ni2+, and reduction peaks at 550-700 oC were ascribed to the reduction of Ni2+ to Ni0.
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Compared with the core LaNiO3-cube, the reduction peaks of LaNiO3@SiO2-x (x=1.1,
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1.7, 2.8) become more wider, and shift to higher temperatures according to the shell thickness,
among
the
three
core-shell
structured
catalysts,
[email protected] has both the largest hydrogen consumption and the reduction temperature (642 ℃ ). It strongly testifies that Ni0 reduced from [email protected] has strong interaction with the SiO2 inner shell during the reduction and the high temperature reaction process, thus inhibiting the formation of coke. Besides, as is reported that there is a certain relationship between the SiO2 shell
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ACCEPTED MANUSCRIPT thickness and the diffusion velocity of H2 molecule in the shell[28, 31]. Therefore, to some extent, the abundant mesoporous shell can decrease the content of active sites which make the reduction peak became wider and lower and shift to higher
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temperature. When the shell thickness further increased, the hydrogen consumption became smaller as the theoretical content of Ni further decreases according to ICP
SC
analysis (Table 2).
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3.2 Catalytic performance of LaNiO3-cube@meso-SiO2 3.2.1 Activity evaluation of the catalysts
The catalytic performance of the above prepared catalysts for dry reforming of
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methane (DRM) with the function of temperatures are shown in Fig.4. All the data displayed herein were collected after stabilization for 30 min at the corresponding temperature. The obtained results showed that conventional nickel catalyst supported
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on Al2O3 (Ni/Al2O3) has highest initial CH4 conversion about 38 % while all the other
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catalysts exhibit similar initial CH4 conversion (at 700 oC), while the CO2 conversion of all the catalysts have similar initial activity. The CH4 and CO2 conversions were increased with increasing temperature due to the inherent endothermicity of DRM reaction. Usually, the more Ni amount means more active sites, and it is positive for improving their catalytic performances, while in this work,
activity of
LaNiO3-cube@meso-SiO2 with different shell thickness was not evidently decreased. To achieve the true value of Ni amount, all the catalysts were tested by ICP 11
ACCEPTED MANUSCRIPT technology and with the results showed in Table 2. For Ni/Al2O3 catalyst, Ni amount was 11.8 wt.% which was very close to the theoretical value 12 wt.%. However, for perovskite like catalysts, all Ni amount decreased in some degree, LaNiO3 nanocube
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get 1/3 percent, and core-shell structured LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) get 1/2 to the initial theory value, on one hand, unlike impregnation method of
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Ni/Al2O3 catalyst, loss damage of Ni active site was not un-avoided because there exists wash step when preparing LaNiO3 and LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7,
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2.8) catalysts. On the other hand, as is reported elsewhere[34-38], the reasonable preparation process can obtain uniform mesoporous SiO2 shell coated onto the shell without forming single SiO2 particles. When adding LaNiO3 into this stöber reaction system, the LaNiO3 nanocubes were all embedded in mesoporous silica shell. What’s
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more, with the increase the amount of TEOS input, the more serious the loss, the content is almost half of the theoretical Ni content. This should be one of the
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retained well.
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advantage for our novel catalyst which decrease the active sites and its activity
3.2.2 The stability test
To investigate the stability of LaNiO3-cube@meso-SiO2 and related catalysts, the
catalytic performance with 100 mg catalyst amount and a feed molar ratio of CH4/CO2 =1:1 at 800 oC and at atmospheric pressure was taken placed at a space velocity of 18000 ml h-1 gcat-1. The conversions of CH4 and CO2 and H2/CO molar ratio are showed in Fig.5 and Fig.6, respectively. The traditional Ni/Al2O3 catalyst starts from initial CH4 conversion of 82 %, which drop to 60% in 480 min of continuous reaction. And the H2/CO of Ni/Al2O3 exhibits similar regular which means that nickel catalysts 12
ACCEPTED MANUSCRIPT supported on Al2O3 agglomerate very fast while all the core-shell structured catalyst remain stable until the end of the 480 min on reforming. For Ni/LaNiO3, derived from LaNiO3 nanocube, the stability test was stopped quickly due the serious coking formation (Figure 7 a-2). Interestingly, for LaNiO3nanocube encapsulated in
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mesoporous silica shell, the conversion of CH4 and CO2 was not significantly decreased and the H2/CO ratio retained at about 1.0. It is well known that the RWGS and methane cracking are two unavoidable side reactions happening with DRM reaction at 800 oC and the H2/CO ratio should be either higher (If methane
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decomposition is dominant) or lower (If RWGS is dominant). However, for DRM reaction, it also existed CO disproportionation reaction, which is also one of the main
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reason for carbon deposition. Thus, the reasons for H2/CO ratio at ~1.0 are very complex. To resolve this problem, the isotopic tracer method should be adopted. The feasible reasons for this phenomenon should be attributed to the confinement effect derived from the mesoporous silica shells around the active the
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LaNiO3 nano-cube, which can efficiently prevent carbon deposition on the metallic nickel active sites, though also has some carbon was detected through TGA analysis.
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3.4 Characterization of the fresh reduced and spent catalysts
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Fig.7 shows the TEM images of freshly reduced (left) and spent (right) catalysts. It can be evidently seen that LaNiO3-cube after reduced in 10 %H2/Ar atmosphere for 2 h at 700 oC, the origin cubic shape of LaNiO3 was not changed but with some cavity appeared indicating that Ni active site was reduced to form Ni/La2O3 active sites and this should be the main reason of its high initial conversion. For core-shell structured LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) catalysts with various shell thickness as shown in Fig.7 (b-1) - (d-1), the perfect morphology of LaNiO3-cube core remained
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ACCEPTED MANUSCRIPT very well and encapsulated in the mesoporous SiO2 shell. What’s more, the shell thickness distributed from 60 nm to 350 nm. Compared with the spent catalysts shown in Fig 7 (2), LaNiO3-cube without embedded in mesoporous silica, its cube structure
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was totally destroyed after stability test for just only 30 min, and a lot of filamentous carbons were clearly existed which almost equal to the traditional Ni/Al2O3 catalyst.
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However, LaNiO3-cube encapsulated in various thickness of mesoporous SiO2 shells, the core shell structure and cubic morphology remained well and less carbon
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deposition was detected indicating that LaNiO3-cube confined in mesoporous silica shells have higher resistance against coke deposition, which is in line with the TGA-DSC results.
To further testify the morphology of spent catalysts and formation of carbon
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deposition, the optimized [email protected] of fresh and spent catalyst were characterized by scanning electron microscope (SEM), with the images shown in
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Fig.8. It’s evidently observed in Fig.8 (c-1) that the conventional Ni/Al2O3 catalyst
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consists of un-uniform and irregular particles while pure LaNiO3 presents a uniform cubic morphology as listed in Fig.8 (a-1). However, after LaNiO3-cube coated with mesoporous SiO2 shell as displayed in Fig.8 (b-1), the [email protected] sample becomes smooth and evasive. This result further confirm that the LaNiO3-cube was totally embedded in mesoporous silica shell. Fig.8 (a-2) and (c-2) display SEM images after stability test for DRM reaction. It can be clearly seen that both the surface of LaNiO3-cube and Ni/Al2O3 are covered by a large amount of long
14
ACCEPTED MANUSCRIPT whisker-like
filamentous
carbon
deposits,
in
comparison,
the
surface of
[email protected] sample is almost clean indicating that the severe coking was effectively suppressed by the mesoporous SiO2 shell.
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The XRD patterns of the fresh calcined and spent catalysts are presented in Fig.9. It’s reported[39] that the calcined temperature of metal oxide is a key factor for crystal
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formation and growth, below 550 oC, the main phase of LaNiO3 are La2O3 and NiO. As is exhibited in Fig 9 (A), no any diffraction peaks are observed obviously for
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LaNiO3 (Ni-La2O3) while only a weak peak at 32.78o (2θ) which correspond to the (110) and (200) diffractions of LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) which re-calcined to remove the meso-structure directing agent. What’s more, the diffraction peaks of SiO2 are similar to the LaNiO3 particles, but no any clear peaks can be found
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due to its weak intensity in these core shell structured catalysts[28, 40, 41]. To further investigate the structure properties of these catalyst, XRD patterns of the spent catalysts
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are shown in Fig. 9 (B), Scherrer’s formula was applied to calculate the crystal size of
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Ni, and the results are given in Table.2. In comparison with the results over LaNiO3-cube (22.0 nm), a slightly larger crystal size of Ni was obtained over Ni/Al2O3 (25.4 nm) while evidently smaller crystal size was observed in the case of LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) catalysts after reduction. As declared before, the reduced Ni0 particles highly distributed on the surface of La2O3 and inner face of SiO2 shell which obtained smaller Ni particle size, and the formation of La2O2CO3 further depressed the formation of coke deposition[18, 42]. This was well
15
ACCEPTED MANUSCRIPT agreement with their catalytic performance results. Coke resistance is always thought to be a main reason for the deactivation of DRM reaction catalysts[42, 43]. To achieve clearer information, the spent catalysts
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after for 480 min reaction were collected and measured by TGA-DSC to quantify the amount of carbon deposition, with the results show in Fig.10. As shown in Fig.10 (A),
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all the catalysts showed no weight loss before 300 oC for totally dried before characterization. Ni/Al2O3 has a 56 % weight loss stage, which is accompanied by a
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symmetric exothermic peak at 683 oC in the DSC profile while LaNiO3 has a weight loss of 59 % which is even higher than Ni/Al2O3 proving the formation of more serious coke formation.
In contrast, for the novel core-shell structured
LaNiO3-cube@meso-SiO2-x (x=1.1, 1.7, 2.8) with various shell thickness, much less
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carbon deposition of 12 % was detected，and 4 wt.% for the thicker mesoporous SiO2 shells ([email protected] and [email protected]) maintained
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at 4wt.% indicating that severe coking was effectively suppressed by coating a mesoporous SiO2 shell. As is reported, the tendency of Ni0 metal particles to
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aggregate is trivial because the Ni0 particles are homogeneously and densely distributed on the surface of La2O3 and shielded by SiO2 shell.[44] Therefore, carbon deposition is hard to find from the core-shell structured catalysts. For other Ni-based catalysts without protective structure, coke deposition often occurs directly on Ni particles.
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ACCEPTED MANUSCRIPT 4. Conclusions In summary, a novel core-shell structured catalyst using a perfect perovskite LaNiO3 nano-cube
as
core
and
perpendicular
mesoporous
silica
as
shell
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(LaNiO3-cube@meso-SiO2) was successfully fabricated and applied for dry reforming of methane (DRM) to prepare syngas. It is known that though La2O3 is a suitable
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support for loading Ni active sites owing to its alkalinity to facilitate the adsorption of CO2, strong metal support interaction with Ni, and most importantly to form La2O2CO3
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to eliminate the coke deposition, the coking problem is still existed. The dual confinement strategy used in this work (that is the strong metal support interaction between Ni and La2O3 and the mesoporous silica shell) can efficiently improve its coking resistance for DRM. The shell thickness is also a very important factor to affect
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its coking resistance. When the shell thickness increased to above 60 nm, the carbon deposition can decrease to below 4 wt.% after 480 min reaction. Thus, we believe that
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the dual confinement strategy developed in this work can be used to design other high
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coking resistance catalysts for DRM reaction or other high temperature thermal catalysts.
Acknowledgements
This work is supported by the National Key R&D Program of China (2016YFC0205900), the National Natural Science Foundation of China (21503106, 21566022 and 21773106), the Natural Science Foundation of Jiangxi Province (20171BCB23016 and 20171BAB203024) and the Foundation of State Key 17
ACCEPTED MANUSCRIPT Laboratory of Coal Clean Utilization and Ecological Chemical Engineering (Grant No.
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2016-15) which are greatly acknowledged by the authors.
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Energy 41 (2016) 2447-2456. [26] T. Xie, X. Zhao, J. Zhang, L. Shi, D. Zhang, International Journal of Hydrogen
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ACCEPTED MANUSCRIPT Figure Captions Fig.1. TEM images of fresh LaNiO3 nanocube and embedded in mesoporous silica with various thickness: (a) LaNiO3 nanocube; (b) [email protected]; (c)
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[email protected]; (d) [email protected].
Fig.2 (A) N2 adsorption-desorption isotherms and (B) pore size distribution profiles of
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LaNiO3-cube@meso-SiO2 and related materials. Fig.3 H2-TPR profiles of the freshly calcined catalysts.
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Fig.4 Catalytic performance of LaNiO3-cube@meso-SiO2 and related materials with the function of temperature for dry reforming of methane: (A)CH4 conversion, (B)CO2 conversion. Reaction conditions: Pressure = 0.1MPa, CO2: CH4 = 1:1, weight hourly space velocity (WHSV) = 18000 ml h-1 g cat-1.
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Fig.5 Stability tests of LaNiO3-cube@meso-SiO2 and related materials for methane dry reforming: (A) CH4 conversion, (B) CO2 conversion; Reaction conditions:
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Pressure=0.1MPa, CO2: CH4=1:1, WHSV = 18000 ml h-1 g cat-1.
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Fig.6 H2/CO ratio of LaNiO3-cube@meso-SiO2 with various shell thickness: (A) [email protected];
(B)
[email protected];
(C)
[email protected]; (D) Ni/Al2O3. Reaction conditions: Pressure=0.1MPa, CO2: CH4=1:1, WHSV=18000 ml h-1 g cat-1. Fig.7 TEM images of the freshly reduced and spent catalysts: (a) LaNiO3-cube; (b) [email protected];
(c)
[email protected];
[email protected]; (e) Ni/Al2O3; (1) freshly reduced; (2) spent.
23
(d)
ACCEPTED MANUSCRIPT Fig.8 SEM images of catalysts: (a) LaNiO3-cube; (b) [email protected]; (c) Ni/Al2O3. (1):fresh reduced and (2): spent. Fig.9 XRD patterns of the (A) freshly calcined and (B) used catalysts: (a)
(d) [email protected]; (e) Ni/Al2O3.
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LaNiO3-cube; (b) [email protected]; (c) [email protected];
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Fig.10 TGA-DSC profiles of the spent catalysts after 8 h stability tests.
24
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Table 1 The physicochemical properties of LaNiO3-cube@meso-SiO2 and related catalysts
(m2/g)
LaNiO3 nanocube
7.7
[email protected]
225
[email protected]
257
[email protected]
535
Ni/Al2O3
102
Pore
volume
diametre
(cm3/g) 0.04
Ni crystal size[b]
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Samples
Pore
(nm)
26.42
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SBET[a]
(nm)
22
0.26
5.13
--
0.33
4.91
--
0.32
2.63
--
--
--
25.4
TE D
[a] Calculated by N2-adsorption and desorption by BJH method.
AC C
EP
[b] Crystallite size calculated using the Debye-Scherrer equation.
25
ACCEPTED MANUSCRIPT
RI PT
Table 2 The physiochemical properties of catalysts. Ni content[b](%)
thickness[a](nm)
True
Theory
LaNiO3 cube
--
20.4
23.9
[email protected]
75
[email protected]
130
[email protected] Ni/Al2O3
[a] Mean size of different state.
18.1
8.1
16.1
350
6.0
13.2
--
11.8
12.0
AC C
EP
TE D
[b] Calculated using ICP.
12.2
M AN U
Samples
SC
Shell
26
TE D
M AN U
SC
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ACCEPTED MANUSCRIPT
Fig.1 TEM images of fresh calcined LaNiO3 nanocube and embedded in mesoporous with
various
EP
silica
(c)
(a)
LaNiO3
nanocube;
[email protected];
AC C
[email protected];
thickness:
[email protected].
27
(b) (d)
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ACCEPTED MANUSCRIPT
LaNiO3-cube
Adsorption Volume (cm /g)
3
[email protected] [email protected]
0.45 0.40 0.35
M AN U
1.0
15nm
(B)
EP
2nm
0.25
4nm
0.20
[email protected]
0.10
[email protected]
0.05
0.00 0
LaNiO3-cube [email protected]
0.15
AC C
3
0.30
0.4 0.6 0.8 Relative Pressure (P/P0)
TE D
0.50
0.2
SC
12Ni/Al2O3
0.0
dV/dD (cm /g)
(A)
[email protected]
12Ni/Al2O3
5
10
15
20
25
30
35
40
45
50
55
60
Pore size (nm)
Fig.2 (A) N2 adsorption-desorption isotherms and (B) pore size distribution profiles of LaNiO3-cube@meso-SiO2 and related materials.
28
O
O
LaNiO3-cube
611 C
O
420 C
O
SC
625 C O
422 C
[email protected]
O
642 C
[email protected]
M AN U
H2 Consumption (a.u.)
428 C
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ACCEPTED MANUSCRIPT
O
406 C
O
563 C
O
365 C
[email protected] Ni/Al2O3
100
200
300
400
500
600
Temperature ( C)
TE D
O
AC C
EP
Fig.3 H2-TPR profiles of the freshly calcined catalysts.
29
700
800
ACCEPTED MANUSCRIPT
100 (A)
RI PT
CH4 Conversion (%)
80
60
40
SC
LaNiO3-cube [email protected] [email protected]
20
[email protected]
0
600
M AN U
Ni/Al2O3
650
700
750
800
O
Temperature ( C) 100 (B)
60
40
TE D
CO2 Conversion (%)
80
EP
20
600
AC C
0
LaNiO3-cube [email protected] [email protected] [email protected] Ni/Al2O3
650
700
O
750
800
Temperature ( C)
Fig.4 Catalytic performance of LaNiO3-cube@meso-SiO2 and related materials with the function of temperature for dry reforming of methane: (A)CH4 conversion, (B)CO2 conversion. Reaction conditions: Pressure = 0.1MPa, CO2: CH4 = 1:1, weight hourly space velocity (WHSV) = 18000 ml h-1 g cat-1.
30
ACCEPTED MANUSCRIPT
100
RI PT
(A)
LaNiO3-cube
40
SC
60
[email protected] [email protected] [email protected]
20
M AN U
CH4 Conversion (%)
80
Ni/Al2O3
0
0
CO2 Conversion (%)
80
60
AC C
20
0
(B)
LaNiO3-cube
EP
40
8
TE D
100
2 4 6 Time on stream (h)
[email protected] [email protected] [email protected] Ni/Al2O3
0
2 4 6 Time on stream (h)
8
Fig.5 Stability tests of LaNiO3-cube@meso-SiO2 and related materials for methane dry reforming: (A) CH4 conversion, (B) CO2 conversion; Reaction conditions: Pressure=0.1MPa, CO2: CH4=1:1, WHSV = 18000 ml h-1 g cat-1.
31
ACCEPTED MANUSCRIPT
2.0
(A) 1.6
[email protected]
1.6
H2/CO
1.2
8
0.0
M AN U
2 4 6 Time on stream (h)
H2/CO
0.4
H2/CO
0.8
SC
0.8
0
2 4 6 Time on stream (h)
H2/CO
[email protected] 1.2
0
RI PT
2.0
(B)
0.4
8
0.0
2.0
2.0
(D)
(C) [email protected]
1.6
1.6
1.2
H2/CO
1.2
TE D
0.8
H2/CO
2
4 6 Time on stream (h)
0.8
0.4
8
0.0
H2/CO
0
2 4 6 Time on stream (h)
8
0.4
0.0
EP
0
H2/CO
Ni/Al2O3
Fig.6 H2/CO ratio of LaNiO3-cube@meso-SiO2 with various shell thickness: (A)
AC C
[email protected];
(B)
[email protected];
(C)
[email protected]; (D) Ni/Al2O3. Reaction conditions: Pressure=0.1MPa, CO2: CH4=1:1, WHSV=18000 ml h-1 g cat-1.
32
AC C
EP
TE D
M AN U
SC
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ACCEPTED MANUSCRIPT
Fig.7 TEM images of the freshly reduced and spent catalysts: (a) LaNiO3-cube; (b) [email protected];
(c)
[email protected];
[email protected]; (e) Ni/Al2O3; (1) freshly reduced; (2) spent. 33
(d)
TE D
M AN U
SC
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ACCEPTED MANUSCRIPT
EP
Fig.8 SEM images of catalysts: (a) LaNiO3-cube; (b) [email protected];
AC C
(c) Ni/Al2O3. (1):fresh reduced and (2): spent.
34
ACCEPTED MANUSCRIPT
⊗-NiAl2O4 •−Ni ♣-LaNiO3
(A)
⊗
⊗
⊗
⊗
⊗
•
(e)
RI PT
⊗
Intensity (a.u.)
♣
(d)
(c)
(b)
SC
(a)
LaNiO3(PDF#33-0710)
(B)
40
60
2θ (°)
Π
Π−C •− −Ni ∀−La2O2CO3
∀
TE D
Intensity (a.u.)
•
∀
EP
20
80
M AN U
20
∀
∀
40
∀
(e) (d) (c)
(b) •
∀
2θ (°)
•
60
(a)
80
AC C
Fig.9 XRD patterns of the (A) freshly calcined and (B) used catalysts: (a) LaNiO3-cube; (b) [email protected]; (c) [email protected]; (d) [email protected]; (e) Ni/Al2O3.
35
ACCEPTED MANUSCRIPT
RI PT
80 59% 56%
60
LaNiO3-cube
[email protected]
40
[email protected]
20 100
200
300
(B)
M AN U
[email protected] Ni/Al2O3
12%
SC
Weight loss (%)
4%
(A)
100
400
500
Temperature (°C)
600
700
800
683
LaNiO3-cube
583
648
[email protected]
[email protected]
TE D
Ni/Al2O3
EP
Heat Flow (mW)
[email protected]
200
300
400
500
600
Temperature (°C)
AC C
100
508
700
800
Fig.10 TGA-DSC profiles of the spent catalysts after 8 h stability tests.
36
ACCEPTED MANUSCRIPT Research Highlights
A perfect core-shell structured methane dry reforming catalyst with perovskite LaNiO3 nano-cube as core and perpendicular mesoporous silica as shell was
The shell thickness has positive effect on coking resistance.
RI PT
successfully synthesized.
LaNiO3-cube@meso-SiO2 has the enhanced coking resistance due to the dual
AC C
EP
TE D
M AN U
SC
confinement effect.