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A comparison of Al2O3 and SiO2 supported Ni-based catalysts in their performance for the dry reforming of methane
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 47, Issue 2, February 2019 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2019, 47(2), 199208
RESEARCH PAPER
A comparison of Al2O3 and SiO2 supported Ni-based catalysts in their performance for the dry reforming of methane XU Yan*, DU Xi-hua, LI Jing, WANG Peng, ZHU Jie, GE Feng-juan, ZHOU Jun, SONG Ming, ZHU Wen-you School of Chemistry and Chemical Engineering, Xuzhou University of Technology, Xuzhou 221018, China
Abstract:
Dry reforming of methane (DRM) with CO2 is of great significance in the environmental protection and the utilization of
natural gas. SiO2 and Al2O3 are two typical catalyst supports used in DRM. To elucidate the effect of these two supports on the catalytic performance, in this work, Ni/SiO2 and Ni/Al2O3 catalysts are prepared by the incipient wetness method and characterized by BET, TEM, H2-TPR, XRD, TG and Raman technologies. The results indicate that the performance of Ni-based catalyst is closely related to the properties of support and the Ni/SiO2 and Ni/Al2O3 catalysts are rather different in their DRM performance. Ni/SiO2 catalyst exhibits higher initial activity but poor stability; its catalytic activity decreases rapidly in 15 h for DRM at 800°C. Because of the weak metal-support interaction, Ni species on the Ni/SiO2 catalyst is present as large Ni particles, which may promote the formation of coke precursors, viz., the multi-carbon Cn species, leading to the fast carbonaceous deposition and catalyst deactivation. In contrast, the Ni/Al2O3 catalyst displays a lower activity but a much higher stability; its activity in DRM keeps stable in 50 h. Although Ni particles in the Ni/Al2O3 catalyst is much smaller, the strong metal-support interaction promotes the formation of NiAlxOy species during the catalyst preparation process, which may lead to a decrease in the content of active Ni species and give the Ni/Al 2O3 catalyst a relatively low catalytic activity in DRM; however, the strong metal-support interaction between Ni and Al2O3 is also of benefit to the formation and stabilization of small Ni particles, which can alleviate the carbanceous deposition and afford the Ni/Al 2O3 catalyst a better stability. Key words:
methane; dry reforming; nickel-based catalyst; structure-activity relationship; support effect; Ni/Al2O3; Ni/SiO2
The availability of natural gas in large reserves makes methane a good candidate for C1 chemistry[1]. At the same time, the combustion of natural gas and other fossil fuels to meet the energy demands causes a massive emission of CO 2, leading to the global warming. Hence, the dry reforming of methane (DRM) with CO2 to syngas contributes not only to the mitigation of the global environmental problem, but also to the supply of a valuable feedstock to synthesize various chemicals and liquid fuels via methanol route and Fischer-Tropsch synthesis (FTS)[2–5]. Compared with the noble metal catalysts such as Ru, Pt, Rh and so on, Ni-based catalyst is believed to be most promising for industrial application in DRM due to its relatively high catalytic activity and low price[6,7]. However, the coke formed during reaction on the catalyst can block the reactant molecules from the active sites, which is the main challenge
for the Ni-based catalyst[8,9]. In recent years, various methods have been attempted to improve the activity and stability of the Ni-based catalysts[10–15]. Huang et al[16,17] prepared the mesoporous bimetallic NiO-Y2O3-Al2O3 and CoO-NiO-Al2O3 catalysts by an evaporation-induced self-assembly method; owing to the good textural properties and the addition of Y2O3 and CoO, these bimetallic catalysts exhibited high thermal stability in DRM. To improve the catalyst stability and inhibit the agglomeration of Ni particles, the core-shell structure with Ni or Ni species as the core has also been developed [18–22]. For example, the nano-capsule Ni@SiO2 catalyst featured with mono-dispersed capsule and anchored Ni nano-particles (NPs) in the inner porous shell exhibited excellent catalytic performance in DRM, as the carbon formation could be sterically hindered due to the sealed space[18].
Received: 27-Sep-2018; Revised: 09-Dec-2018. Foundation items: Supported by the National Natural Science Foundation of China (21703194), the Natural Science Foundation of Jiangsu Province (BK20171168, BK20171169), Natural Science Foundation of Jiangsu Higher Education Institutions of China (17KJB530010, 17KJB150038 and 18KJA430015), Key Research Project of Social Development of Xuzhou (KC17154) and Research Project of Xuzhou University of Technology (XKY2017217). *Corresponding author. E-mail: [email protected]. Copyright 2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 1
Conversions of CH4 and CO2 and molar ratio of H2/CO as a function of time on stream over the Ni/SiO2 and Ni/Al2O3 catalysts in DRM at 800°C, with a GHSV of 48000 mL/(gcath) and a CH4:CO2:N2 ratio of 9:9:2
Furthermore, the sandwiched core-shell structured Ni-SiO2@CeO2 catalyst with Ni nanoparticles encapsulated between silica and ceria could effectively inhibit the coke formation for the reforming of biogas under low temperature (600°C)[20]. Numerous studies have proved that small Ni particles and stronger metal-support interaction are of benefit to the enhancement of the catalytic activity and stability[23–25]. SiO2 and Al2O3 are two typical catalyst supports for DRM because of their high thermal stability and surface area. To obtain a Ni-based catalyst with high activity and stability, usually with small Ni particles, SiO2 and Al2O3 supports with various structures have been developed in recent years. Daoura et al[26] prepared the nickel-containing meso-cellular silica foam (MCF) catalysts, which were provided with highly dispersed Ni nanoparticles having strong interaction with the supports. Kim et al[27] prepared the Ni/Al2O3 nanosheets catalyst by a solvothermal method, which displayed highly stable activity in comparison with Ni/Al2O3 with a random morphology. Because SiO2 and Al2O3 are different in many properties, it is important to understand the effect of these two supports on the catalytic performance to lay a firm foundation for the improvement of the Ni-based catalyst for DRM. Therefore, in this work, Ni/SiO2 and Ni/Al2O3 catalysts are prepared by the incipient wetness method and characterized by BET, TEM, H2-TPR, XRD, TG and Raman technologies; their performance in DRM was compared to elucidate the
relationship between the support (SiO2 and Al2O3) and performance of Ni-based catalyst.
1 1.1
Methods and materials Catalyst preparation
The SiO2 and Al2O3 supported Ni-based catalysts were prepared by the incipient impregnation method with a Ni loading of 10%. An aqueous solution containing the desired amounts of Ni(NO3)2·6H2O was added to the support and then dried slowly in a rotary evaporator under vacuum at 80°C for 2 h, followed by drying at 120°C in an oven overnight and calcining in air at 550°C for 3 h. The calcined catalyst was crushed and sieved to 40–60 mesh size for catalytic tests. 1.2
Catalytic test
The DRM reaction was performed in a fixed bed reactor (a quartz tube reactor with i.d. of 9 mm) at atmospheric pressure. 0.2 g of shaped catalyst (40–60 mesh) was placed at the center of reactor. Before the reaction, the catalyst was reduced at 750°C for 2 h by pure H2 with the flow rate of 60 mL/min. The temperature was then raised to the designed reaction temperature (800°C) and a flow of gas mixture with CH4:CO2:N2 molar ratio of 9:9:2 was fed into the reactor (160 mL/min).
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 2
N2 adsorption and desorption isotherms and size distribution (inset) of the reduced Ni/SiO 2 catalyst (a), reduced Ni/Al2O3 catalyst (b), spent Ni/SiO2 catalyst (c), and spent Ni/Al2O3 catalyst (d) Table 1 2
–1
Textural properties of various catalysts
Catalyst
ABET/(m g )
vpore/(cm3g–1)
Pore diameter d/nm
vpore/ABET (10−9 m)
Reduced Ni/SiO2
162.6
0.99
15.5
6.1
Reduced Ni/Al2O3
159.3
0.34
6.2
2.1
Spent Ni/SiO2
89.9
0.45
17.5
–
Spent Ni/Al2O3
152.1
0.35
7.4
–
Gaseous products were analyzed online by gas chromatography (Agilent GC 7820A with a TCD detector and a Porapak Q columnand a 5A molecular sieve column). The conversions of CH4 (xCH4) and CO2 (xCO2) were calculated with the following formulas: FCH4-in − FCH4 -out xCH4 = ×100% (1) FCH4-in FCO2-in − FCO2 -out xCO2 = ×100% (2) FCO2-in where Fi-in and Fi-out are the gas flow rate of i in the feed and the effluent, respectively. 1.3
Catalyst characterization
The BET surface area and pore size of the reduced and spent catalysts were measured with an ASAP 2020 instrument (Micromeritics, USA). Nitrogen adsorption-desorption isotherms were obtained at liquid nitrogen temperature (−195.6°C). Before the measurement, the samples were degassed under vacuum condition at 200°C for 6 h.
The transmission electron microscopy (TEM) images of the reduced and spent catalysts were obtained on a JEM-2100 transmission electron microscope (JEOL, Japan) at an acceleration voltage of 200 kV. The statistics of Ni particles were obtained by using more than 150 observable Ni particles from the TEM images. H2 temperature programmed reduction (H2-TPR) tests of the fresh catalysts were conducted in an auto-controlled flow reactor system (TP-5076, Tianjin Xianquan Instrument, Co., Ltd., China) equipped with a thermal conductivity detector (TCD). 50 mg of catalyst sample was charged in a quartz-tube reactor and pretreated in a N2 stream at 473 K for 1 h and then cooled down to 303 K. After that, the sample was heated in a 30 mL/min flow of 5%H2/N2 to 950°C at a rate of 10°C/min to obtain the H2-TPR profiles. The powder X-ray diffraction (XRD) patterns of reduced and spent catalysts were recorded with a Bruker AXS D8 Advance diffractometer using Cu K radiation ( = 0.15406 nm) over a 2 range of 5°–90°at a scanning rate of 2(°)/min.
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 3
TEM images and the Ni particle size distribution of the freshly reduced catalysts (a) and (c): Ni/SiO2 catalyst; (b) and (d): Ni/Al2O3 catalyst
Thermogravimetric (TG) analysis of the spent catalysts was carried out on a Mettler-Toledo TGA-1100SF Thermogravimetric analyzer, to estimate the coke amounts in the spent catalysts after reaction for 50 h. Raman analysis of the coke on the spent catalyst was operated on a Leica DMLM Raman instrument (Renishaw Company, UK). The argon ion laser of 514 nm was employed and the spectrum continuous scanning range was 200–2000 cm−1.
2 2.1
Results and discussion Catalytic test results
Figure 1 shows the catalytic performances of the Ni/SiO2 and Ni/Al2O3 catalysts in DRM (CH4 + CO2 → 2CO + 2H2) at 800°C with a GHSV of 48000 mL/(gcath) and a CH4:CO2:N2 ratio of 9:9:2. Over the Ni/SiO2 catalyst, the conversions of CH4 and CO2 exhibit a rapid drop from 78% and 85% to 51% and 65%, respectively, in the early 15 h, and then become relatively stable; after reaction for 50 h, the conversions of CH4 and CO2 decrease to 48% and 63%, respectively. In contrast, the Ni/Al2O3 catalyst displays a stable catalytic performance over the whole reaction period; the conversions of CH4 and CO2 are 60% and 72% at the first hour, and then decrease slightly to 57% and 70%, respectively, after reaction
for 50 h. Moreover, the conversion of CO2 is higher than that of CH4 over both catalysts, due to the reverse water gas shift (RWGS) reaction (CO2 + H2 → H2O + CO)[28–30]. It is worth noting that the gap between the CH4 conversion and CO2 conversion over Ni/SiO2 catalyst increases from 7% to 15% during the reaction, whereas it increases slightly from 12% to 13% over the Ni/Al2O3 catalyst. The increase in the gap between the conversions of CH4 and CO2 indicates the enhancement of the RWGS reaction; in other words, above results suggest that the RWGS reaction is relatively enhanced with the decrease of catalytic activity in the DRM reaction. Besides, the similar tendency is also observed in the H2/CO molar ratio of produced syngas which is lower than 1 over both catalysts. 2.2
Catalyst characterization results
Figure 2 shows the N2 adsorption-desorption isotherms of two catalysts. According to the five kinds of isotherms from BDDT pore model, they belong to the type IV isotherms, corresponding to typical mesoporous materials[31]. The pore structure is related to the shape of hysteresis loop in the adsorption-desorption isotherm. The hysteresis loop of Ni/SiO2 is H3 loop; its adsorption capacity increases monotonously with the increase of pressure.
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 4
H2-TPR profiles of the Ni/SiO2 and Ni/Al2O3 catalysts
In contrast, the hysteresis loop of Ni/Al2O3 is H2 loop, which is a characteristic of porous materials. Besides, the BJH pore size distribution curves suggest that the most probable pore diameters of the Ni/SiO2 and Ni/Al2O3 catalysts are 19 and 6 nm, respectively. Table 1 gives the textural properties of the reduced and spent catalysts. The reduced Ni/SiO2 and Ni/Al2O3 catalysts have a similar BET surface area (ABET, about 160 m²/g), though Ni/SiO2 has a larger total pore volume (vpore) and pore diameter than Ni/Al2O3. However, for the spent catalyst, the surface areas (ABET) of the Ni/SiO2 and Ni/Al2O3 catalysts are decreased to 89.9 and 152 m²/g, respectively. Moreover, the total pore volume of the spent Ni/SiO2 catalyst is decreased by 50%, whereas that of the spent Ni/Al2O3 catalyst is almost unchanged. The decrease in the surface area and the total pore volume of the spent Ni/SiO2 catalyst may be ascribed to the formation of carbon deposits during the reaction for 50 h, which can cover the active sites and fill in the pore of SiO 2 support, leading to a decrease in the catalytic activity. The slight increase in the pore diameter of both catalysts may be due to the stacking of Ni particles, carbon deposits and partial dilapidation of support. Although there is no direct relationship between the pore volume and diameter and the catalytic property, Saha et al[32] pointed out that a high pore volume/surface area (vpore/ABET) ratio contributed to high catalytic performance, because high porosity could improve the metal dispersion. In this work, the pore volume/surface area ratios of the Ni/SiO2 and Ni/Al2O3 catalysts are 6.1×10−9 and 2.1×10−9 m, respectively; the Ni/SiO2 catalyst does exhibit better initial activity than the Ni/Al2O3 catalyst, but with poor stability. The TEM images of the reduced catalysts are depicted in Figure 3. It was reported that a higher vpore/ABET ratio was beneficial for a smaller crystal metal size and higher metal dispersion[33]. However, the Ni/Al2O3 catalyst with a smaller vpore/ABET ratio displays smaller and uniform Ni particles. The Ni particle size is just 4.1±2.2 nm over the Ni/Al 2O3 catalyst,
whereas it is 31.3±13.5 nm over the Ni/SiO2 catalyst. Moreover, the Ni/Al2O3 catalyst with smaller Ni particles does not exhibit better catalytic activity, which conflicts with the reported results[23–25]. This paradox should be ascribed to the different properties between SiO2 and Al2O3 supports. Besides the physical structure of the support, the interaction between Ni and support also plays an very important role in the microstructures of the resultant catalysts. Figure 4 shows the reduction behavior of the fresh catalysts to clarify the interaction between Ni and support. The Ni/SiO 2 catalyst shows a sharp reduction peak at 390°C and two shoulders at 350 and 480°C, whereas the Ni/Al2O3 catalyst displays three less distinct and broad peaks at 480, 640 and 760°C. Generally, the peaks at low temperature (400–500°C) are assigned to the reduction of NiO species which have weak interaction with support, whereas the peaks at intermediate temperature (500–600°C) are assigned to the reduction of NiO species having medium strength interaction with support. In contrast, the peaks in the high temperature zone (> 600°C) are attributed to the reduction of NiO species with strong chemical interaction with support[34]. Obviously, the interaction between Ni and SiO2 support is weak and the weakly bound NiO species generally exist in big particle size and can easily migrate and aggregate during the reduction and reaction process[35]. For the Ni/Al2O3 catalyst, the higher reduction temperature indicates a stronger interaction between Ni and Al2O3 support; the broad reduction peak at 550–750°C is assigned to the reduction of NiAlxOy phase[36]. Most of Ni supported on Al2O3 has a strong chemical interaction with support, whereas there is hardly any chemical interaction between Ni and SiO2 support, as the formation of nickel silicate needs an alkaline environment[37,38]. It was reported that the NiAlxOy species can be easily formed but cannot be readily reduced[39]. Numaguchi et al[40] found that NiAl2O4 could be sufficiently reduced with H2/N2 (30/70, by volume) mixture at 1023 K for 20 h.
Fig. 5
XRD patterns of the reduced and spent Ni/SiO2 and Ni/Al2O3 catalysts
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 6
TGA profiles (a) and Raman spectra (b) of the spent catalysts after 50 h stability test for DRM
Fig. 7
TEM images of the spent catalysts after 50 h stability test (a) and (b): Ni/Al2O3 catalyst; (c) and (d): Ni/SiO2 catalyst
The size of Ni particles of Ni/Al2O3 catalyst depends strongly on the reducibility of the Ni precursor; lower reducibility leads to smaller nickel particles[41]. Besides, as metallic Ni is the active component for the DRM reaction, the lower reducibility may also reduce the amount of active sites over Ni/Al2O3 catalyst, leading to a lower catalytic activity. On the other hand, as the catalyst is prepared by the incipient wetness impregnation method, most of the Ni particles should be restrained in the pore space of the support. The larger pore volume and pore diameter of SiO2 support can offer the space for the formation of large Ni particles, exactly as shown by the TEM results in Figure 3. The XRD patters of the reduced and spent catalyst are shown in Figure 5. Compared with the Ni/Al2O3 catalyst, the Ni/SiO2 catalyst shows two obvious sharp peaks at 44.5°and 52.2°, corresponding to metallic Ni (JCPDS 1-1260). The
stronger intensity of the Ni diffraction peaks over Ni/SiO 2 indicates better crystallinity and larger particles, which is in line with the TEM results. In contrast, the Ni diffraction peaks of the Ni/Al2O3 catalyst are low and broad, suggesting small Ni particle size. Moreover, it is worth noting that the Ni diffraction peaks are still rather low for the spent Ni/Al2O3 catalyst after 50 h reaction, as the strong interaction between Ni and Al2O3 can effectively inhibit the migration and agglomeration of Ni particles during the reaction process. For the spent Ni/SiO2 catalyst, however, because of the weak interaction between Ni and SiO2, the Ni particles are easy to grow up according to the Ostwald ripening theory, leading to much sharper diffraction peaks of Ni. Besides, there is an obvious sharp peak at 26°on the spent Ni/SiO2 catalyst, which corresponding to the carbon formed during the reaction.
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 8
Scheme of the preparation processes for the Ni/SiO2 and Ni/Al2O3 catalyst
To characterize the coke formed on the different catalysts, TGA and Raman spectroscopy are used, as shown in Figure 6. There is a weight loss between 500 and 700°C for the spent catalysts, as shown in Figure 6(a). The weight losses from the elimination of coke deposited on the spent Ni/SiO2 and Ni/Al2O3 catalysts are 51.4% and 9.6%, respectively; apparently, the coke deposited on Ni/SiO2 is five times more than that on Ni/Al2O3. Figure 6(b) displays the graphite degree of the coke which is related to the reactivity of coke. For the spent catalyst, there are two strong peaks in 1000–2000 cm−1, located at 1350 cm−1 (D band) and 1580 cm−1 (G band), corresponding to the reactive coke with a defect structure and the inertia graphite with an ordered structure, respectively. The ID/IG ratio calculated using a Gaussian curve fitting algorithm is usually used to indicate the degree of graphite with a non-ordered state[42]. The ID/IG values for the Ni/SiO2 and Ni/Al2O3 catalysts are 1.7 and 2.6, respectively; that is, there is more reactive coke on the Ni/Al2O3 catalyst, which should benefit from the small Ni particles. As reported by Aleksandrov et al[43], multicarbon Cn species were preferably formed on large rather than on small Ni particles, which were the potential precursors of carbon deposits such as graphene or coke, leading to the catalyst deactivation. The morphology of the coke formed on spent catalysts was characterized by TEM and shown in Figure 7. Obviously, compared with the encapsulating carbon, there are more whiskers carbon on the catalyst surface, which is in accordance with the Raman results. As a result of the larger Ni particles, there are more carbon deposits with larger diameters on the spent Ni/SiO2 catalyst. Although the amount of carbon on the spent Ni/SiO2 catalyst is over 50%, the catalyst is still active in DRM, which can be explained by the fact that the active sites could be “re-located” on the tips of the whisker carbon and sustain its accessibility to the reactant gases [13,44]. In the present work, it has been demonstrated that the Ni/Al2O3 catalyst exhibits higher stability in DRM, with less amount but more reactive coke deposits after 50 h reaction, which should be ascribed to the small Ni particles and strong metal-support interaction, as illustrated by the scheme in Figure 8. The randomly connected large pore channels which can provide the space for the growth of particles and the weak
metal-support interaction which cannot hinder the migration and agglomeration of Ni particles are the two reasons for the Ni/SiO2 catalyst with large Ni particles; the large Ni particles may further grow up during the reaction and suffers from sever coke deposits, resulting in the decrease in catalytic performance during reaction. In contrast, the Ni/Al2O3 catalyst has small pore channels, which may contribute to the formation of small Ni particles; the interaction between Ni and Al2O3 can be strengthened not only by the small Ni particles, but also by the formation of NiAlxOy species. Generally, the NiAlxOy spinels can be easily formed but cannot be readily reduced; the formed spinels can reduce the amount of active Ni species, leading to a low activity but better coke resistance for the reforming reaction[39]. In other words, the formed NiAlxOy (invalid Ni species, which cannot be reduced under current conditions) with extra strong interaction should be responsible for the lower catalytic activity but better stability of the Ni/Al2O3 catalyst.
3
Conclusions
The Ni/SiO2 and Ni/Al2O3 catalysts are prepared by the incipient wetness method and there catalytic performance in the dry reforming of methane (DRM) with CO2 was compared to elucidate a relation between of the support properties and the catalytic performance. The results indicate that the performance of Ni-based catalyst in DRM is closely related to the properties of support and the Ni/SiO2 and Ni/Al2O3 catalysts are rather different in their DRM performance. Ni/SiO2 catalyst exhibits higher initial activity but poor stability; its catalytic activity decreases rapidly in 15 h for DRM at 800°C. Because of the weak metal-support interaction, Ni species in the Ni/SiO2 catalyst is present as large Ni particles, which may promote the formation of coke precursors, leading to the fast carbonaceous deposition and catalyst deactivation. In contrast, the Ni/Al2O3 catalyst displays a lower activity but a much higher stability; its activity in DRM keeps stable in 50 h. Although Ni particles in the Ni/Al2O3 catalyst is much smaller, the strong metal-support interaction promotes to the formation of NiAlxOy species during the catalyst preparation
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
process, which may lead to a decrease in the content of active Ni spieces and give the Ni/Al2O3 catalyst a relatively low catalytic activity in DRM; however, the strong metal-support interaction between Ni and Al2O3 is also of benefit to the formation and stabilization of small Ni particles, which can alleviate the carbanceous deposition and afford the Ni/Al 2O3 catalyst a better stability. In a word, the appropriate metal-support interaction is crucial to get a nickel-based catalyst with higher activity and stability, as well as the anti-coking ability.
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RESEARCH PAPER
A comparison of Al2O3 and SiO2 supported Ni-based catalysts in their performance for the dry reforming of methane XU Yan*, DU Xi-hua, LI Jing, WANG Peng, ZHU Jie, GE Feng-juan, ZHOU Jun, SONG Ming, ZHU Wen-you School of Chemistry and Chemical Engineering, Xuzhou University of Technology, Xuzhou 221018, China
Abstract:
Dry reforming of methane (DRM) with CO2 is of great significance in the environmental protection and the utilization of
natural gas. SiO2 and Al2O3 are two typical catalyst supports used in DRM. To elucidate the effect of these two supports on the catalytic performance, in this work, Ni/SiO2 and Ni/Al2O3 catalysts are prepared by the incipient wetness method and characterized by BET, TEM, H2-TPR, XRD, TG and Raman technologies. The results indicate that the performance of Ni-based catalyst is closely related to the properties of support and the Ni/SiO2 and Ni/Al2O3 catalysts are rather different in their DRM performance. Ni/SiO2 catalyst exhibits higher initial activity but poor stability; its catalytic activity decreases rapidly in 15 h for DRM at 800°C. Because of the weak metal-support interaction, Ni species on the Ni/SiO2 catalyst is present as large Ni particles, which may promote the formation of coke precursors, viz., the multi-carbon Cn species, leading to the fast carbonaceous deposition and catalyst deactivation. In contrast, the Ni/Al2O3 catalyst displays a lower activity but a much higher stability; its activity in DRM keeps stable in 50 h. Although Ni particles in the Ni/Al2O3 catalyst is much smaller, the strong metal-support interaction promotes the formation of NiAlxOy species during the catalyst preparation process, which may lead to a decrease in the content of active Ni species and give the Ni/Al 2O3 catalyst a relatively low catalytic activity in DRM; however, the strong metal-support interaction between Ni and Al2O3 is also of benefit to the formation and stabilization of small Ni particles, which can alleviate the carbanceous deposition and afford the Ni/Al 2O3 catalyst a better stability. Key words:
methane; dry reforming; nickel-based catalyst; structure-activity relationship; support effect; Ni/Al2O3; Ni/SiO2
The availability of natural gas in large reserves makes methane a good candidate for C1 chemistry[1]. At the same time, the combustion of natural gas and other fossil fuels to meet the energy demands causes a massive emission of CO 2, leading to the global warming. Hence, the dry reforming of methane (DRM) with CO2 to syngas contributes not only to the mitigation of the global environmental problem, but also to the supply of a valuable feedstock to synthesize various chemicals and liquid fuels via methanol route and Fischer-Tropsch synthesis (FTS)[2–5]. Compared with the noble metal catalysts such as Ru, Pt, Rh and so on, Ni-based catalyst is believed to be most promising for industrial application in DRM due to its relatively high catalytic activity and low price[6,7]. However, the coke formed during reaction on the catalyst can block the reactant molecules from the active sites, which is the main challenge
for the Ni-based catalyst[8,9]. In recent years, various methods have been attempted to improve the activity and stability of the Ni-based catalysts[10–15]. Huang et al[16,17] prepared the mesoporous bimetallic NiO-Y2O3-Al2O3 and CoO-NiO-Al2O3 catalysts by an evaporation-induced self-assembly method; owing to the good textural properties and the addition of Y2O3 and CoO, these bimetallic catalysts exhibited high thermal stability in DRM. To improve the catalyst stability and inhibit the agglomeration of Ni particles, the core-shell structure with Ni or Ni species as the core has also been developed [18–22]. For example, the nano-capsule Ni@SiO2 catalyst featured with mono-dispersed capsule and anchored Ni nano-particles (NPs) in the inner porous shell exhibited excellent catalytic performance in DRM, as the carbon formation could be sterically hindered due to the sealed space[18].
Received: 27-Sep-2018; Revised: 09-Dec-2018. Foundation items: Supported by the National Natural Science Foundation of China (21703194), the Natural Science Foundation of Jiangsu Province (BK20171168, BK20171169), Natural Science Foundation of Jiangsu Higher Education Institutions of China (17KJB530010, 17KJB150038 and 18KJA430015), Key Research Project of Social Development of Xuzhou (KC17154) and Research Project of Xuzhou University of Technology (XKY2017217). *Corresponding author. E-mail: [email protected]. Copyright 2019, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 1
Conversions of CH4 and CO2 and molar ratio of H2/CO as a function of time on stream over the Ni/SiO2 and Ni/Al2O3 catalysts in DRM at 800°C, with a GHSV of 48000 mL/(gcath) and a CH4:CO2:N2 ratio of 9:9:2
Furthermore, the sandwiched core-shell structured Ni-SiO2@CeO2 catalyst with Ni nanoparticles encapsulated between silica and ceria could effectively inhibit the coke formation for the reforming of biogas under low temperature (600°C)[20]. Numerous studies have proved that small Ni particles and stronger metal-support interaction are of benefit to the enhancement of the catalytic activity and stability[23–25]. SiO2 and Al2O3 are two typical catalyst supports for DRM because of their high thermal stability and surface area. To obtain a Ni-based catalyst with high activity and stability, usually with small Ni particles, SiO2 and Al2O3 supports with various structures have been developed in recent years. Daoura et al[26] prepared the nickel-containing meso-cellular silica foam (MCF) catalysts, which were provided with highly dispersed Ni nanoparticles having strong interaction with the supports. Kim et al[27] prepared the Ni/Al2O3 nanosheets catalyst by a solvothermal method, which displayed highly stable activity in comparison with Ni/Al2O3 with a random morphology. Because SiO2 and Al2O3 are different in many properties, it is important to understand the effect of these two supports on the catalytic performance to lay a firm foundation for the improvement of the Ni-based catalyst for DRM. Therefore, in this work, Ni/SiO2 and Ni/Al2O3 catalysts are prepared by the incipient wetness method and characterized by BET, TEM, H2-TPR, XRD, TG and Raman technologies; their performance in DRM was compared to elucidate the
relationship between the support (SiO2 and Al2O3) and performance of Ni-based catalyst.
1 1.1
Methods and materials Catalyst preparation
The SiO2 and Al2O3 supported Ni-based catalysts were prepared by the incipient impregnation method with a Ni loading of 10%. An aqueous solution containing the desired amounts of Ni(NO3)2·6H2O was added to the support and then dried slowly in a rotary evaporator under vacuum at 80°C for 2 h, followed by drying at 120°C in an oven overnight and calcining in air at 550°C for 3 h. The calcined catalyst was crushed and sieved to 40–60 mesh size for catalytic tests. 1.2
Catalytic test
The DRM reaction was performed in a fixed bed reactor (a quartz tube reactor with i.d. of 9 mm) at atmospheric pressure. 0.2 g of shaped catalyst (40–60 mesh) was placed at the center of reactor. Before the reaction, the catalyst was reduced at 750°C for 2 h by pure H2 with the flow rate of 60 mL/min. The temperature was then raised to the designed reaction temperature (800°C) and a flow of gas mixture with CH4:CO2:N2 molar ratio of 9:9:2 was fed into the reactor (160 mL/min).
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 2
N2 adsorption and desorption isotherms and size distribution (inset) of the reduced Ni/SiO 2 catalyst (a), reduced Ni/Al2O3 catalyst (b), spent Ni/SiO2 catalyst (c), and spent Ni/Al2O3 catalyst (d) Table 1 2
–1
Textural properties of various catalysts
Catalyst
ABET/(m g )
vpore/(cm3g–1)
Pore diameter d/nm
vpore/ABET (10−9 m)
Reduced Ni/SiO2
162.6
0.99
15.5
6.1
Reduced Ni/Al2O3
159.3
0.34
6.2
2.1
Spent Ni/SiO2
89.9
0.45
17.5
–
Spent Ni/Al2O3
152.1
0.35
7.4
–
Gaseous products were analyzed online by gas chromatography (Agilent GC 7820A with a TCD detector and a Porapak Q columnand a 5A molecular sieve column). The conversions of CH4 (xCH4) and CO2 (xCO2) were calculated with the following formulas: FCH4-in − FCH4 -out xCH4 = ×100% (1) FCH4-in FCO2-in − FCO2 -out xCO2 = ×100% (2) FCO2-in where Fi-in and Fi-out are the gas flow rate of i in the feed and the effluent, respectively. 1.3
Catalyst characterization
The BET surface area and pore size of the reduced and spent catalysts were measured with an ASAP 2020 instrument (Micromeritics, USA). Nitrogen adsorption-desorption isotherms were obtained at liquid nitrogen temperature (−195.6°C). Before the measurement, the samples were degassed under vacuum condition at 200°C for 6 h.
The transmission electron microscopy (TEM) images of the reduced and spent catalysts were obtained on a JEM-2100 transmission electron microscope (JEOL, Japan) at an acceleration voltage of 200 kV. The statistics of Ni particles were obtained by using more than 150 observable Ni particles from the TEM images. H2 temperature programmed reduction (H2-TPR) tests of the fresh catalysts were conducted in an auto-controlled flow reactor system (TP-5076, Tianjin Xianquan Instrument, Co., Ltd., China) equipped with a thermal conductivity detector (TCD). 50 mg of catalyst sample was charged in a quartz-tube reactor and pretreated in a N2 stream at 473 K for 1 h and then cooled down to 303 K. After that, the sample was heated in a 30 mL/min flow of 5%H2/N2 to 950°C at a rate of 10°C/min to obtain the H2-TPR profiles. The powder X-ray diffraction (XRD) patterns of reduced and spent catalysts were recorded with a Bruker AXS D8 Advance diffractometer using Cu K radiation ( = 0.15406 nm) over a 2 range of 5°–90°at a scanning rate of 2(°)/min.
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 3
TEM images and the Ni particle size distribution of the freshly reduced catalysts (a) and (c): Ni/SiO2 catalyst; (b) and (d): Ni/Al2O3 catalyst
Thermogravimetric (TG) analysis of the spent catalysts was carried out on a Mettler-Toledo TGA-1100SF Thermogravimetric analyzer, to estimate the coke amounts in the spent catalysts after reaction for 50 h. Raman analysis of the coke on the spent catalyst was operated on a Leica DMLM Raman instrument (Renishaw Company, UK). The argon ion laser of 514 nm was employed and the spectrum continuous scanning range was 200–2000 cm−1.
2 2.1
Results and discussion Catalytic test results
Figure 1 shows the catalytic performances of the Ni/SiO2 and Ni/Al2O3 catalysts in DRM (CH4 + CO2 → 2CO + 2H2) at 800°C with a GHSV of 48000 mL/(gcath) and a CH4:CO2:N2 ratio of 9:9:2. Over the Ni/SiO2 catalyst, the conversions of CH4 and CO2 exhibit a rapid drop from 78% and 85% to 51% and 65%, respectively, in the early 15 h, and then become relatively stable; after reaction for 50 h, the conversions of CH4 and CO2 decrease to 48% and 63%, respectively. In contrast, the Ni/Al2O3 catalyst displays a stable catalytic performance over the whole reaction period; the conversions of CH4 and CO2 are 60% and 72% at the first hour, and then decrease slightly to 57% and 70%, respectively, after reaction
for 50 h. Moreover, the conversion of CO2 is higher than that of CH4 over both catalysts, due to the reverse water gas shift (RWGS) reaction (CO2 + H2 → H2O + CO)[28–30]. It is worth noting that the gap between the CH4 conversion and CO2 conversion over Ni/SiO2 catalyst increases from 7% to 15% during the reaction, whereas it increases slightly from 12% to 13% over the Ni/Al2O3 catalyst. The increase in the gap between the conversions of CH4 and CO2 indicates the enhancement of the RWGS reaction; in other words, above results suggest that the RWGS reaction is relatively enhanced with the decrease of catalytic activity in the DRM reaction. Besides, the similar tendency is also observed in the H2/CO molar ratio of produced syngas which is lower than 1 over both catalysts. 2.2
Catalyst characterization results
Figure 2 shows the N2 adsorption-desorption isotherms of two catalysts. According to the five kinds of isotherms from BDDT pore model, they belong to the type IV isotherms, corresponding to typical mesoporous materials[31]. The pore structure is related to the shape of hysteresis loop in the adsorption-desorption isotherm. The hysteresis loop of Ni/SiO2 is H3 loop; its adsorption capacity increases monotonously with the increase of pressure.
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 4
H2-TPR profiles of the Ni/SiO2 and Ni/Al2O3 catalysts
In contrast, the hysteresis loop of Ni/Al2O3 is H2 loop, which is a characteristic of porous materials. Besides, the BJH pore size distribution curves suggest that the most probable pore diameters of the Ni/SiO2 and Ni/Al2O3 catalysts are 19 and 6 nm, respectively. Table 1 gives the textural properties of the reduced and spent catalysts. The reduced Ni/SiO2 and Ni/Al2O3 catalysts have a similar BET surface area (ABET, about 160 m²/g), though Ni/SiO2 has a larger total pore volume (vpore) and pore diameter than Ni/Al2O3. However, for the spent catalyst, the surface areas (ABET) of the Ni/SiO2 and Ni/Al2O3 catalysts are decreased to 89.9 and 152 m²/g, respectively. Moreover, the total pore volume of the spent Ni/SiO2 catalyst is decreased by 50%, whereas that of the spent Ni/Al2O3 catalyst is almost unchanged. The decrease in the surface area and the total pore volume of the spent Ni/SiO2 catalyst may be ascribed to the formation of carbon deposits during the reaction for 50 h, which can cover the active sites and fill in the pore of SiO 2 support, leading to a decrease in the catalytic activity. The slight increase in the pore diameter of both catalysts may be due to the stacking of Ni particles, carbon deposits and partial dilapidation of support. Although there is no direct relationship between the pore volume and diameter and the catalytic property, Saha et al[32] pointed out that a high pore volume/surface area (vpore/ABET) ratio contributed to high catalytic performance, because high porosity could improve the metal dispersion. In this work, the pore volume/surface area ratios of the Ni/SiO2 and Ni/Al2O3 catalysts are 6.1×10−9 and 2.1×10−9 m, respectively; the Ni/SiO2 catalyst does exhibit better initial activity than the Ni/Al2O3 catalyst, but with poor stability. The TEM images of the reduced catalysts are depicted in Figure 3. It was reported that a higher vpore/ABET ratio was beneficial for a smaller crystal metal size and higher metal dispersion[33]. However, the Ni/Al2O3 catalyst with a smaller vpore/ABET ratio displays smaller and uniform Ni particles. The Ni particle size is just 4.1±2.2 nm over the Ni/Al 2O3 catalyst,
whereas it is 31.3±13.5 nm over the Ni/SiO2 catalyst. Moreover, the Ni/Al2O3 catalyst with smaller Ni particles does not exhibit better catalytic activity, which conflicts with the reported results[23–25]. This paradox should be ascribed to the different properties between SiO2 and Al2O3 supports. Besides the physical structure of the support, the interaction between Ni and support also plays an very important role in the microstructures of the resultant catalysts. Figure 4 shows the reduction behavior of the fresh catalysts to clarify the interaction between Ni and support. The Ni/SiO 2 catalyst shows a sharp reduction peak at 390°C and two shoulders at 350 and 480°C, whereas the Ni/Al2O3 catalyst displays three less distinct and broad peaks at 480, 640 and 760°C. Generally, the peaks at low temperature (400–500°C) are assigned to the reduction of NiO species which have weak interaction with support, whereas the peaks at intermediate temperature (500–600°C) are assigned to the reduction of NiO species having medium strength interaction with support. In contrast, the peaks in the high temperature zone (> 600°C) are attributed to the reduction of NiO species with strong chemical interaction with support[34]. Obviously, the interaction between Ni and SiO2 support is weak and the weakly bound NiO species generally exist in big particle size and can easily migrate and aggregate during the reduction and reaction process[35]. For the Ni/Al2O3 catalyst, the higher reduction temperature indicates a stronger interaction between Ni and Al2O3 support; the broad reduction peak at 550–750°C is assigned to the reduction of NiAlxOy phase[36]. Most of Ni supported on Al2O3 has a strong chemical interaction with support, whereas there is hardly any chemical interaction between Ni and SiO2 support, as the formation of nickel silicate needs an alkaline environment[37,38]. It was reported that the NiAlxOy species can be easily formed but cannot be readily reduced[39]. Numaguchi et al[40] found that NiAl2O4 could be sufficiently reduced with H2/N2 (30/70, by volume) mixture at 1023 K for 20 h.
Fig. 5
XRD patterns of the reduced and spent Ni/SiO2 and Ni/Al2O3 catalysts
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 6
TGA profiles (a) and Raman spectra (b) of the spent catalysts after 50 h stability test for DRM
Fig. 7
TEM images of the spent catalysts after 50 h stability test (a) and (b): Ni/Al2O3 catalyst; (c) and (d): Ni/SiO2 catalyst
The size of Ni particles of Ni/Al2O3 catalyst depends strongly on the reducibility of the Ni precursor; lower reducibility leads to smaller nickel particles[41]. Besides, as metallic Ni is the active component for the DRM reaction, the lower reducibility may also reduce the amount of active sites over Ni/Al2O3 catalyst, leading to a lower catalytic activity. On the other hand, as the catalyst is prepared by the incipient wetness impregnation method, most of the Ni particles should be restrained in the pore space of the support. The larger pore volume and pore diameter of SiO2 support can offer the space for the formation of large Ni particles, exactly as shown by the TEM results in Figure 3. The XRD patters of the reduced and spent catalyst are shown in Figure 5. Compared with the Ni/Al2O3 catalyst, the Ni/SiO2 catalyst shows two obvious sharp peaks at 44.5°and 52.2°, corresponding to metallic Ni (JCPDS 1-1260). The
stronger intensity of the Ni diffraction peaks over Ni/SiO 2 indicates better crystallinity and larger particles, which is in line with the TEM results. In contrast, the Ni diffraction peaks of the Ni/Al2O3 catalyst are low and broad, suggesting small Ni particle size. Moreover, it is worth noting that the Ni diffraction peaks are still rather low for the spent Ni/Al2O3 catalyst after 50 h reaction, as the strong interaction between Ni and Al2O3 can effectively inhibit the migration and agglomeration of Ni particles during the reaction process. For the spent Ni/SiO2 catalyst, however, because of the weak interaction between Ni and SiO2, the Ni particles are easy to grow up according to the Ostwald ripening theory, leading to much sharper diffraction peaks of Ni. Besides, there is an obvious sharp peak at 26°on the spent Ni/SiO2 catalyst, which corresponding to the carbon formed during the reaction.
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
Fig. 8
Scheme of the preparation processes for the Ni/SiO2 and Ni/Al2O3 catalyst
To characterize the coke formed on the different catalysts, TGA and Raman spectroscopy are used, as shown in Figure 6. There is a weight loss between 500 and 700°C for the spent catalysts, as shown in Figure 6(a). The weight losses from the elimination of coke deposited on the spent Ni/SiO2 and Ni/Al2O3 catalysts are 51.4% and 9.6%, respectively; apparently, the coke deposited on Ni/SiO2 is five times more than that on Ni/Al2O3. Figure 6(b) displays the graphite degree of the coke which is related to the reactivity of coke. For the spent catalyst, there are two strong peaks in 1000–2000 cm−1, located at 1350 cm−1 (D band) and 1580 cm−1 (G band), corresponding to the reactive coke with a defect structure and the inertia graphite with an ordered structure, respectively. The ID/IG ratio calculated using a Gaussian curve fitting algorithm is usually used to indicate the degree of graphite with a non-ordered state[42]. The ID/IG values for the Ni/SiO2 and Ni/Al2O3 catalysts are 1.7 and 2.6, respectively; that is, there is more reactive coke on the Ni/Al2O3 catalyst, which should benefit from the small Ni particles. As reported by Aleksandrov et al[43], multicarbon Cn species were preferably formed on large rather than on small Ni particles, which were the potential precursors of carbon deposits such as graphene or coke, leading to the catalyst deactivation. The morphology of the coke formed on spent catalysts was characterized by TEM and shown in Figure 7. Obviously, compared with the encapsulating carbon, there are more whiskers carbon on the catalyst surface, which is in accordance with the Raman results. As a result of the larger Ni particles, there are more carbon deposits with larger diameters on the spent Ni/SiO2 catalyst. Although the amount of carbon on the spent Ni/SiO2 catalyst is over 50%, the catalyst is still active in DRM, which can be explained by the fact that the active sites could be “re-located” on the tips of the whisker carbon and sustain its accessibility to the reactant gases [13,44]. In the present work, it has been demonstrated that the Ni/Al2O3 catalyst exhibits higher stability in DRM, with less amount but more reactive coke deposits after 50 h reaction, which should be ascribed to the small Ni particles and strong metal-support interaction, as illustrated by the scheme in Figure 8. The randomly connected large pore channels which can provide the space for the growth of particles and the weak
metal-support interaction which cannot hinder the migration and agglomeration of Ni particles are the two reasons for the Ni/SiO2 catalyst with large Ni particles; the large Ni particles may further grow up during the reaction and suffers from sever coke deposits, resulting in the decrease in catalytic performance during reaction. In contrast, the Ni/Al2O3 catalyst has small pore channels, which may contribute to the formation of small Ni particles; the interaction between Ni and Al2O3 can be strengthened not only by the small Ni particles, but also by the formation of NiAlxOy species. Generally, the NiAlxOy spinels can be easily formed but cannot be readily reduced; the formed spinels can reduce the amount of active Ni species, leading to a low activity but better coke resistance for the reforming reaction[39]. In other words, the formed NiAlxOy (invalid Ni species, which cannot be reduced under current conditions) with extra strong interaction should be responsible for the lower catalytic activity but better stability of the Ni/Al2O3 catalyst.
3
Conclusions
The Ni/SiO2 and Ni/Al2O3 catalysts are prepared by the incipient wetness method and there catalytic performance in the dry reforming of methane (DRM) with CO2 was compared to elucidate a relation between of the support properties and the catalytic performance. The results indicate that the performance of Ni-based catalyst in DRM is closely related to the properties of support and the Ni/SiO2 and Ni/Al2O3 catalysts are rather different in their DRM performance. Ni/SiO2 catalyst exhibits higher initial activity but poor stability; its catalytic activity decreases rapidly in 15 h for DRM at 800°C. Because of the weak metal-support interaction, Ni species in the Ni/SiO2 catalyst is present as large Ni particles, which may promote the formation of coke precursors, leading to the fast carbonaceous deposition and catalyst deactivation. In contrast, the Ni/Al2O3 catalyst displays a lower activity but a much higher stability; its activity in DRM keeps stable in 50 h. Although Ni particles in the Ni/Al2O3 catalyst is much smaller, the strong metal-support interaction promotes to the formation of NiAlxOy species during the catalyst preparation
XU Yan et al / Journal of Fuel Chemistry and Technology, 2019, 47(2): 199208
process, which may lead to a decrease in the content of active Ni spieces and give the Ni/Al2O3 catalyst a relatively low catalytic activity in DRM; however, the strong metal-support interaction between Ni and Al2O3 is also of benefit to the formation and stabilization of small Ni particles, which can alleviate the carbanceous deposition and afford the Ni/Al 2O3 catalyst a better stability. In a word, the appropriate metal-support interaction is crucial to get a nickel-based catalyst with higher activity and stability, as well as the anti-coking ability.
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