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Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 5 ) 1 e1 2
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Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon W. Djeridi a,b,*, N. Ben Mansour c, A. Ouederni a, P.L. Llewellyn b, L. El Mir c,d a
Research Laboratory, Engineering Process and Industrial Systems, National School of Engineers of Gabes, University of Gabes, St Omar Ibn Elkhattab, 6029 Gabes, Tunisia b Chimistry Laboratory of Provence, University Aix-Marseille I, II, III- CNRS, UMR 6264, Centre de Saint Jerome, 13397 Marseille, France c Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Gabes University, Faculty of Sciences in Gabes, Gabes, Tunisia d Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Department of Physics, 11623 Riyadh, Saudi Arabia
article info
abstract
Article history:
Three different sets of microporous carbon were produced at various activation tempera-
Received 31 January 2015
tures: the first set was prepared from lignocellulosic material, the second set was prepared
Received in revised form
by mixing pyrogallol with formaldehyde (PF), and the third set was prepared by incorpo-
19 April 2015
ration of nickel oxide in PF (PF/Ni). The objective of this work is to compare pure CH4
Accepted 1 May 2015
sorption isotherms for well-characterized the three sets of porous carbon prepared by
Available online xxx
different methods to determine the most suitable porous carbon for CH4 storage. Methane adsorption on several microporous carbon has been investigated using a manometric
Keywords:
adsorption method. The experiments were carried out using CH4 pure gas at 298 K from 1 to
Microporous carbon
30 bar. The effect of the preparatory condition on the properties, textures, and adsorption
Microstructure
gas of the prepared samples has been analyzed. The samples were characterized by
Activation temperature
adsorption of N2 (77 K), scanning electron microscope (SEM) and transmission electron
Nickel oxide
microscope (TEM). It was found that both the type of raw material and the conditions of
CH4 storage
activation (activation temperatures), had a huge influence on the microporosity of the resultants samples and their CH4 capture capacities. The sample that presented the maximum CH4 capture capacities at 298 K, was prepared from PF and was activated at 1000 C. This sample showed the highest narrow microporosity volume (0.46 cm3/g) and greater affinity for methane, thus confirming that only pores of the less than 1 nm are effective for CH4 adsorption. The result shows that the addition of nickel oxide is not improved the adsorbed methane amount. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Research Laboratory, Engineering Process and Industrial Systems, National school of Engineers of Gabes, University of Gabes, St Omar Ibn Elkhattab, 6029 Gabes, Tunisia. Tel.: þ216 75392053; fax: þ216 75392100. E-mail address: [email protected] (W. Djeridi). http://dx.doi.org/10.1016/j.ijhydene.2015.05.010 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Introduction Microporous carbons, prepared from lignocellulosics materials and obtained after pyrolysis of some organic compounds, have been extensively studied for the past 20 years [1e9]. Due to their large surface area and porosity, activated carbon plays an important role in the technology of adsorbing gas. The production of an economical carbon adsorbent for gas storage has been actively pursued in recent years. In physical adsorption, the size and volume of the pores are important, thus activated carbons are used for the sorption/separation of light gases [10,11]. Although the activated carbons structural properties, including surface area and pore size distribution are effective in regards to the adsorptive capacity of gas, when specific interactions between gas and adsorbent act in the adsorption process, other features such as surface chemistry should also be taken into consideration. Due to the increasing demand of natural gas (NG) throughout the world, the adsorbed natural gas (ANG) storage system has become popular in the field of developing efficient gas storage and transportation technologies. Natural gas (NG) has emerged as an alternative energy source in transportation sector as it provides clean combustion and hence lowers exhaust pollution. It is mostly composed of methane (CH4) which has the highest heating value per unit mass of hydrocarbon fuels [12]. Moreover, its competitive price and copious availability makes NG a tangible fuel in energy sectors. Capture, separation and storage of this gas is present social issues essential both in environmentally and economically. Numerous theoretical and experimental studies conclude that adsorption capacity of methane is closely related to the textural characteristics of the activated carbons [13e17]. Therefore, the preparation conditions and the activation processes are necessary to be tuned to characterize a good carbon adsorbent for natural gas storage [18]. For all these reasons, optimizations of the adsorption properties of these porous solids by working on the structure, nature and the pore size are always a topical subject. Several studies have been carried out on methane storage in carbon materials and it has been concluded that highly microporous materials with a narrow micropore size distribution centered at a pore size of around 0.8 nm are the most suitable for this purpose [19,20]. Recently, some works have shown that the presence of certain metals (Ni, Co, Pt) improves the gas storage capacity of carbon materials [21e27]. In fact metal-doped materials are attractive for several applications: catalyst [28,29], double-layer capacitors or electrochemical applications [30,31], magnetic systems [32], hydrogen storage [21,33]. Much effort has been performed to relate the physical and chemical properties of carbon materials with their behavior as support and their further performance in diverse applications. In this work, three different series of activated carbon were prepared under various conditions in order to study the effect of the different conditions, raw material and adding of nickel oxide on their final textural and properties and on their subsequent methane storage capacity. The first series was prepared from lignocellulosic materials (olives stones were chosen as raw material, since they are extensively generated in Tunisia, olives stones were obtained from local olive manufactures) at different pyrolysis temperature. The second
series was obtained after pyrolysis of some organic compounds, based upon the gelation Pyrogallol-Formaldhyde (PF) xerogel using picric acid as catalyst. The third series was obtained after nickel incorporated in nano-carbon structures based on Pyrogallol-Formaldhyde (PF/Ni) at different pyrolysis temperatures.
Experimental methods Sample preparation The first series of activated carbon pellets was prepared in our laboratory using olive stones in two steps process: chemical activation and carbonization. In the chemical process, some amount of granular olive stones was mixed with aqueous solution containing 50% H3PO4 (w/w) at the weight ratio of 1/3. The suspension of the olive stones in chemical impregnation solution was mixed at 110 C for 9 h. Impregnate olive stones powder was used as raw material; with a size of about 80 mm. A mass of 0.6 g of the raw material was placed between the pistons in the dice and the mechanical pressure, P, was loaded on them for 1 min by an oil hydraulic machine. After the compression, the impregnate olive stones ejected from the dice were columnar pellets. These pellets were thermally activated in a fixed bed vertical reactor tubular furnace under nitrogen continuous flow for 3 h at different carbonization temperatures in the range from 350 C to 1000 C. After cooling, activated carbon pellet was washed for several times with hot water until the pH of washing solution became neutral. The samples were dried at 110 C to get the final product. The nomenclature of each sample includes the pyrolysis temperature and the abbreviation of activated carbon pellets (ACP): for example the activated carbon pellet carbonized at 410 C will be denoted ACP410. The preparation of the first series was described in detail in our previous work [34]. The synthesis of the second series of porous carbon has been done in three steps according to El Mir et al. protocol [35]. In this series three pyrolysis temperatures were considered (150 C for sample PF-150, 650 C for sample PF-650 and 1000 C for sample PF-1000). The synthesis of third porous carbon series reached by 5% mass ratio of NiO nanoparticles has been done in three steps as described elsewhere [36]. In this case the thermal treatment was carried out in a tubular furnace under nitrogen atmosphere at 150 C (PF/Ni-150), 650 C (PF/Ni-650) and 1000 C (PF/Ni-1000).
Characterization The synthesized and prepared products were characterized using a JEOL JSM-6300 scanning electron microscope (SEM) and a JEM-200CX transmission electron microscope (TEM). The surface area and the porous texture of the carbons were characterized using nitrogen sorptiometry at 196 C on the automatic Quantachrome Autosorb-1 apparatus in the range of relative pressure from 106 to 1. High purity nitrogen was used up to 6.0 (99.9999%) purity, before the measurement; all samples were outgassed at 250 C for 24 h. The nitrogen adsorptionedesorption isotherms were used to determine the following parameters: specific surface area SBET, total micropore volume Vmicro, and mean pore diameter dp.
Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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CH4 sorption experiment Methane adsorption was carried out at 298 K up to 30 bar in a high pressure volumetric system on the porous carbon previously outgassed at 150 C under vacuum. A diagram of the high-throughput adsorption apparatus is presented in our previous work [34], it was used to determine adsorption gas capacities with a good uncertainties of about 3% [37].
Results ACP methane storage Scanning electron micrographs observation of activated carbon pellets (ACP410, ACP500, and ACP1000) are shown in Fig. 1. The examination of the structure of samples shows the presence of macropores of various sizes and geometries at the surface. Micro-graphs show some regular structured shape: plane and thin stacked forms of a few mm and with large interparticle voids. Meanwhile, Fig. 1A displays the SEM micrograph of activated carbon pellets structure obtained at 410 C pyrolysis temperature; the sample exhibits significant microporous texture in which pore sizes are about a few mm in diameter. Carbon microstructures appear between these pores. In addition, the SEM micrographs of Fig. 1B and C also show that the increment of pyrolysis temperature from 500 C to 1000 C had increased the porosity of the surface area. Fig. 2
3
shows the N2 sorption isotherms of the activated carbon pellets samples at 77 K. All the samples present type I isotherms according to the UIPAC classification [38], indicating that they are microporous materials. However, this shape suggests a strong development of microporosity. It is characteristic of the presence of microporous structure with rather narrow pore size distribution. Activation and pyrolysis of the raw olives stones by inert gas (N2) allow an increase in the porosity. This rise is probably due to the elimination of volatile matters produced during pyrolysis and release of the surface to create new pores in the carbon structure. From the analysis of the isotherms of Fig. 2, the main differences between the samples are their micropore volumes and their pore size distribution as it can be deduced from the sharp knees of their isotherms. Table 1 shows the textural properties of the samples obtained, and it can be seen an increases in specific surface area from 532 to 1014 m2/g and of the micropore volume from 0.241 to 0.459 cm3/g were measured in the activated carbon pellets samples as the pyrolysis temperature was increased from 350 C to 1000 C. Fig. 3 shows the CH4 adsorption capacity corresponding to all samples at room temperature and in pressure range up 25 bar. It can be seen that in general, CH4 adsorption capacity increases with the micropore volume with the existence of some exceptions. The highest CH4 adsorption capacity corresponds to the sample with the highest micropore volume (ACP410) (see Table 1). Comparing the shape of CH4 adsorption isotherms with that of N2 adsorption isotherms (Fig. 2), it can
Fig. 1 e Scanning electron micrographs of (A) ACP410, (B) ACP500 and (C) ACP1000 samples. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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300
ACP350 ACP410 ACP470 ACP500 ACP600 ACP800 ACP1000
a
3
V /cm .g
-1
250
200
150
0,0
0,2
0,4
0,6
0,8
1,0
P/P0
Fig. 2 e Adsorption isotherm of N2 on activated carbon pellet. Va is the volume of liquid nitrogen adsorbed per gram of sample.
be said that they follow similar trends, which indicates that CH4 adsorption mainly occurs in the microporosity. Fig. 4 represents BET surface area determined from N2 adsorption data versus the CH4 adsorption capacity at 25 bar. A linear relation is found between CH4 storage capacity and BET surface area. We see that there are samples with micropore volume higher than the above values with deviate upward with respect to this line. This deviation happens for all those samples which have a considerable contribution of narrow porosity (pore size lower than 0.8 nm) which is not accessible to N2 at 77 K but which can be reached by CH4 at room temperature. These results show the importance of both micropore volume and micropore size distribution on CH4 uptake.
PF methane storage Fig. 5 displays for SEM micrographs of synthetic porous carbon obtained at different pyrolysis temperatures, which exhibit significant differences in agglomeration particles. For the PF150 a large macropores appear between these particles, micropores and perhaps some mesopores exist in these micrometer-size polymeric particles. Whilst, for the PF-650
Table 1 e Pore properties of activated carbon pellets from olive stones and Methane adsorption capacity at 25 bar and room temperature. Sample BET (m2/ Vmicro (cm3/ dp g) g) (nm) ACP350 ACP410 ACP470 ACP500 ACP600 ACP800 ACP1000
826 1014 765 700 632 532 915
0.381 0.459 0.349 0.329 0.279 0.241 0.404
1.91 1.85 1.86 1.95 1.83 1.87 1.79
CH4 uptake (mmol/g) 4.09 4.69 3.57 3.21 4.07 3.94 4.63
and PF-1000 carbon samples, the carbon microparticles aggregate and macropores volume decreases with pyrolysis temperature; which means that heat treatment improves the network structure density. For PF-1000 carbon sample, no macropores were seen from the SEM micrograph, since particles with 1e5 mm diameters appear to coagulate together leaving little space between them. The surface area and pore volume of this carbon indicate that these particles are essentially microporous. These results are consistent with porosity measurements. Images of transmission electron microscopy (TEM) were performed on PF composites treated under different pyrolysis temperatures. These images show in Fig. 6 that our composites are characterized by nanopores. The average pore size is less than 2 nm, whereas the PF composite are microporous. The diameter of the pores increases with increasing pyrolysis temperature. Indeed, the PF-1000 (Fig. 6c) has pores larger than those of the other samples. The TEM data confirm that the interconnected solid nanoparticles comprise an opencelled network with continuous nanodimension porosity. These observations are also consistent with porosity measurement. The N2 adsorptionedesorption isotherms at 77 K of synthetic porous carbon polymerized by picric acid are displayed in Fig. 7. These isotherms are generally of type I in the Brunauer, Emmet and Teller (BET) classification [38], and indicating the synthetic porous carbon are dominated by the microporosity, whatever the pyrolysis temperature used in the preparation. The order of N2 adsorption capacity from the lowest to highest is: PF-150 < PF-650 < PF-1000. This N2 capacity corresponds directly to the microporosity volume of synthetic porous carbon. The shape of PF-650 and PF-1000 are characteristic of the presence of microporous structure with a rather narrow pore size distribution. Following with the analysis of the isotherms of Fig. 7, the main differences between the samples are their micropore volumes and their pore
Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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mmol CH4 /g
4
3
2
ACP350 ACP410 ACP470 ACP500 ACP600 ACP800 ACP1000
1
0 0
5
10
15
20
25
Pressure (bar) Fig. 3 e CH4 adsorption isotherm at room temperature.
size distribution. Thus, almost PF-1000 is with homogeneous pore size distribution and nanoporosity as it can be deduced from the sharp knees of their isotherms and TEM micrographs (Fig. 6). The microporous specific surface areas determined by the conventional BET method varies from 560 to 920 m2/g, with micropore volumes between 0.268 and 0.460 cm3/g (Table 2). The mean micropore size determined from BET surface area and the pore volume in the approximation of cylindrical is close to 2 nm. Fig. 8 presents the methane adsorption isotherms on a volumetric basis at room temperature on synthetic porous
carbon samples. For PF-1000 and PF-650 samples, isotherm behavior is of type I (as defined by the UIPAC classification). Reflecting that those materials are essentially microporous (monolayer adsorption). It can be seen that CH4 adsorption capacity increases with the micropore volume. The highest methane adsorption capacity corresponds to the sample with the highest micropore volume (PF-1000) (see Table 2). Comparing the shape of CH4 adsorption isotherms with that of N2 adsorption isotherms (Fig. 7), it can be said that they follow similar trends which indicates that methane adsorption is also affected by pore size distribution of the samples. It can be
1200 ACP410 ACP800
2
BET surface area (m /g)
1000
800
ACP500
ACP1000
ACP470
600
ACP600 ACP800
400
200
0 3,0
3,5
4,0
4,5
mmol CH4/g Fig. 4 e Relationship between the CH4 adsorption capacity at 25 bar and room temperature with the BET surface area calculated from N2 adsorption isotherms. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Fig. 5 e SEM images of (a): PF-150, (b): PF-650 and (c): PF-1000.
observed that the methane increases with the increases of pressure. The results also show that the amount of methane adsorbed in the PF-1000 and PF-650 is larger than in the PF-150, where progressive thermal treatment improve their adsorption capacities, i.e., PF-150 < PF-650 < PF-1000. This is in agreement with the higher development of porosity obtained
at higher pyrolysis temperature. Moreover, the heat treatment improves the network structure density, besides for the PF650 and PF-1000 carbon samples, the carbon microparticles aggregate and micropores volume decreases with pyrolysis temperature which promotes the methane adsorption. The existence of a reliable correlation between methane
Fig. 6 e TEM images of (a): PF-150, (b): PF-650 and (c) PF-1000. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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to the micropore volume and that possible deviations from this behavior can be ascribed to either the presence of microporosity not accessible to N2 at 77 K, or to the existence of supermicroporosity where methane could adsorbed with a different density. To maximize methane storage capacity of synthetic porous carbon, there are several options: maximize micropore volume while minimizing mesopore and macropore volume, control pore size distribution so that the majority of micropores are y1.14 nm, the modeled optimum pore size for methane storage [19] and minimize interparticle volume by grinding and pelletizing the activity carbon products.
Influence of NiO doping PF microporous carbon on methane storage Fig. 7 e Adsorption-desorption isotherm of nitrogen composites; (a): PF-150, (b): PF-650 and (c): PF-1000.
adsorption and the porous texture of activated carbons is of interest as it would allow us to check the suitability of a given material if high pressure equipment is not available. Several researchers have discussed these correlation [39,40]. Their main conclusion is that methane adsorption has to be related
The SEM images for the studied PF/Ni nanocomposites are illustrated in Fig. 9. According to these images, the increase of the pyrolysis temperature contributed to the agglomeration of the nanoparticles. TEM images performed on the NiO nanoparticles and the PF/Ni-650 samples are shown in Fig. 10. Fig. 10a shows NiO nanoparticles, of about 30 nm in size with cubic shape, before their incorporation. The TEM image of PF/Ni-650 (Fig. 10b) indicates that material was composed by metallic nickel nanoparticles dispersed in carbon matrix. It is worthwhile to notice that nickel oxide was
Table 2 e Pore properties of nanoporous PF carbon and Methane adsorption capacity at 25 bar and room temperature. Sample code
Temperature of thermal treatment, Tp ( C)
BET (m2/g)
Vmicro (cm3/g)
dp (nm)
CH4 uptake (mmol/g)
150 650 1000
561.9 720.4 920.6
0.268 0.334 0.460
1.9 1.9 2.1
0.25 4.29 5.50
PF-150 PF-650 PF-1000
6
a
mmol CH4 /g
5
b
4
3
2
1
c 0
0
5
10
15
20
25
30
Pressure (bar) Fig. 8 e Methane adsorption isotherms at room temperature; (a): PF-1000, (b): PF-650 and (c): PF-150. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Fig. 9 e SEM images of (a): PF/Ni-650 and (b): PF/Ni-1000.
Fig. 10 e TEM micrograph of (a): NiO nanoparticles and (b): PF/Ni-650.
reduced to metallic nickel which leads to the decreasing of the nanoparticles size. However, the carbon matrix stills in the amorphous phase. The TEM images of PF/Ni-1000 sample presented in Fig. 11 exhibit the existence of carbon nanotubes. The nickel nanoparticles in the carbon matrix played an important role for the appearance of nanotubes (Fig. 11a). The nanotubes germination may be originated from the difference of the surface tension between the nickel nanoparticles, which is very high due to its nanometric size and the graphite material
[41]. Fig. 11b depicts the TEM image of the multiwall carbon nanotubes (MWNT) with the inter-plane distance of 0.34 nm which is equivalent approximately to the distance between the graphene layers in a graphite crystal [42]. The surface area available for adsorption is a key property of porous solids and it often used to compare different materials. The most widely used procedure for evaluating the surface area of finely divided and porous materials is the Brunauer-Emett-Teller or BET method [38]. Fig. 12 shows the N2 sorptionedesorption isotherms of Ni-doped carbon
Fig. 11 e TEM images of PF/Ni-1000. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Fig. 12 e Adsorption isotherm of N2 on porous carbon. Va is the volume of liquid nitrogen adsorbed per gram of sample.
composites with 5 wt.% of NiO at 77 K shapes of the PF/Ni-650, show that this isotherm are type IV in the Brunauer Emett and Teller (BET) classification. This is characteristic of mesoporous solid and the hysterisis is usually associated with the filling and emptying of the mesopores by capillary condensation. The shape of the PF/Ni-1000 isotherm is type V in the Brunauer Emett and Teller (BET) classification. This is typical for nonwetting mesoporous materials, where the convex isotherm at low P/P0 indicates weak host-gas interactions while the hysterisis loop is associated with the mechanism of pore filling and emptying by capillary condensation. The nitrogen sorption results and the adsorption equilibrium capacities of CH4 on all three adsorbents at 298 K and elevated pressures (25 bar) are summarized in Table 3. The order of N2 adsorption capacity from the lowest to highest is: PF/Ni-150 < PF/Ni650 < PF/Ni-1000. This N2 capacity corresponds directly to the microporosity volume of synthetic porous carbon. Adsorption isotherms for CH4 are presented in Fig. 13. The difference between the PF/Ni-150 and PF/Ni-1000 samples is related to a small difference in the synthesis conditions (pyrolysis temperature). This explains the systematically lower amounts adsorbed. Looking at the PF/Ni-650, it is noticeable that at high pressures the solid have the largest micropore volume (PF/Ni-650, 0.222 cm3 g1) also has the highest uptakes while the solid with the smallest pore volume (PF/Ni-150, 0.020 cm3 g1) adsorbs the least.
Comparison of CH4 sorption To verify the effect of adding nickel oxide and the choice of raw material for activated carbon preparation in the methane
adsorption capabilities, 298 K isothermal curves at pressure of 30 bar for the methane gas were obtained. Fig. 14 shows the methane adsorption of the prepared samples (ACP410, PF-650, PF-1000, PF/Ni-650 and PF/Ni-1000). It was found that the PF1000 showed an adsorption capacity of 5.5 mmol/g while that of the ACP410 showed 4.69 mmol/g. This result indicates that porous organic samples prepared by mixing formaldhyde with pyragallol (PF) has a tendency to adsorb methane higher than microporous carbons, prepared from lignocellulosic materials. In fact PF-1000 carbon sample possesses a very wide micropore volume; the carbon microparticles aggregate and macropores volume decreases. The average pore size of PF-1000 is less than 1 nm (Fig. 6) which optimizes the density of the adsorbed phase. Also, the methane capacity in the PF650 and PF/Ni-650 were approximately 4.29 mmol/g and 4.54 mmol/g respectively. PF/Ni-650 sample showed a higher adsorption capacity than that non reached by NiO (PF-650). Indeed comparing PF-650 and PF/Ni-650, adding nickel oxide increases the methane adsorbed amount although the specific surface area decreased from 720 m2/g to 460 m2/g, this indicates that the surface properties enhanced the methane adsorption. Meanwhile the TEM image of PF/Ni-650 (Fig. 10b) indicates that material was composed by metallic nickel nanoparticles. It is worthwhile to notice that nickel oxide was reduced to metallic nickel. Adding nickel dispersed in carbon matrix enhanced the methane adsorption, besides the transition metals have acid features (electron acceptors) and methane gas is a Lewis basic in that it done electron from Lewis acid (electron acceptors). Therefore, the basic methane can attract forces with transition metal. This explains the increase in the amount of adsorbed methane despite the
Table 3 e Properties of synthetic porous carbon PF/Ni and methane adsorption capacity at 25 bar and room temperature. Sample code PF/Ni-150 PF/Ni-650 PF/Ni-1000
Temperature of thermal treatment, Tp ( C)
BET (m2/g)
Vmicro (cm3/g)
dp (nm)
CH4 uptake (mmol/g)
150 650 1000
10 < 461 163
0 0.222 0.133
e 2.62 4.81
0.74 4.54 1.52
Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Pressure (bar) Fig. 13 e CH4 adsorption isotherm at room temperature of (a) PF/Ni-150, (b) PF/Ni-650 and (c) PF/Ni-1000. decrease in the specific surface area. It can be stated that the presence of nickel particles in the carbon surfaces enhances the adsorption of methane due to the acidebase properties. Also, the methane capacity in the PF-1000 and PF/Ni-1000 were approximately 5.5 mmol/g and 1.5 mmol/g respectively. PF-1000 sample showed a higher adsorption capacity than that PF/Ni-1000. Indeed when the incorporated nanocomposites are treated with high pyrolysis temperature (more than 650 C), involve the creation of carbon nanotubes that is not the case in PF sample, this prevents the gas from
Conclusion This study provides experimental data for methane adsorption on three sets samples from different raw material.
ACP410 PF1000 PF/Ni650 PF650 PF/Ni1000
6
5
mmol CH4/g
penetrating in to the pores. This explains the systematically lower amounts of adsorbed CH4 by PF/Ni-1000. The porosity of this sample was totally blocked, leading to a decrease in BET surface area.
4
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0 0
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25
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Pressure (bar) Fig. 14 e CH4 adsorption isotherm at room temperature. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Adsorption experiments were conducted using a volumetric method. Our experimental isotherms have an evident character of type I (Langmuir type), reflecting adsorption in a microporous adsorbent with a good affinity with CH4. So, the adsorption could be considered as reversible, local and limited to a monolayer. We have shown that maximum adsorption capacities for CH4 are proportional to the microporous volume and also to the narrow pore size distribution. It was found that different pyrolysis temperature influence methane storage capacity. It was demonstrated that the methane adsorption capacity not only depends on the micropore volume but also strongly depends on the narrow pore size distribution. Microporous carbon synthesized from pyrogallolformaldhyde (PF) has a great affinity to adsorb methane, due to the essentially micropores character of the synthetic porous carbon, micropore size distribution and its low mesopore concentration. Such material seem to be particularly adapted for methane storage. Adding nickel oxide increases the amount adsorbed for some sample and decreases for other. Consequently, the adsorbed methane amount depends on the both microporosity and acidebase properties.
references
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Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon W. Djeridi a,b,*, N. Ben Mansour c, A. Ouederni a, P.L. Llewellyn b, L. El Mir c,d a
Research Laboratory, Engineering Process and Industrial Systems, National School of Engineers of Gabes, University of Gabes, St Omar Ibn Elkhattab, 6029 Gabes, Tunisia b Chimistry Laboratory of Provence, University Aix-Marseille I, II, III- CNRS, UMR 6264, Centre de Saint Jerome, 13397 Marseille, France c Laboratory of Physics of Materials and Nanomaterials Applied at Environment (LaPhyMNE), Gabes University, Faculty of Sciences in Gabes, Gabes, Tunisia d Al Imam Mohammad Ibn Saud Islamic University (IMSIU), College of Sciences, Department of Physics, 11623 Riyadh, Saudi Arabia
article info
abstract
Article history:
Three different sets of microporous carbon were produced at various activation tempera-
Received 31 January 2015
tures: the first set was prepared from lignocellulosic material, the second set was prepared
Received in revised form
by mixing pyrogallol with formaldehyde (PF), and the third set was prepared by incorpo-
19 April 2015
ration of nickel oxide in PF (PF/Ni). The objective of this work is to compare pure CH4
Accepted 1 May 2015
sorption isotherms for well-characterized the three sets of porous carbon prepared by
Available online xxx
different methods to determine the most suitable porous carbon for CH4 storage. Methane adsorption on several microporous carbon has been investigated using a manometric
Keywords:
adsorption method. The experiments were carried out using CH4 pure gas at 298 K from 1 to
Microporous carbon
30 bar. The effect of the preparatory condition on the properties, textures, and adsorption
Microstructure
gas of the prepared samples has been analyzed. The samples were characterized by
Activation temperature
adsorption of N2 (77 K), scanning electron microscope (SEM) and transmission electron
Nickel oxide
microscope (TEM). It was found that both the type of raw material and the conditions of
CH4 storage
activation (activation temperatures), had a huge influence on the microporosity of the resultants samples and their CH4 capture capacities. The sample that presented the maximum CH4 capture capacities at 298 K, was prepared from PF and was activated at 1000 C. This sample showed the highest narrow microporosity volume (0.46 cm3/g) and greater affinity for methane, thus confirming that only pores of the less than 1 nm are effective for CH4 adsorption. The result shows that the addition of nickel oxide is not improved the adsorbed methane amount. Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
* Corresponding author. Research Laboratory, Engineering Process and Industrial Systems, National school of Engineers of Gabes, University of Gabes, St Omar Ibn Elkhattab, 6029 Gabes, Tunisia. Tel.: þ216 75392053; fax: þ216 75392100. E-mail address: [email protected] (W. Djeridi). http://dx.doi.org/10.1016/j.ijhydene.2015.05.010 0360-3199/Copyright © 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Introduction Microporous carbons, prepared from lignocellulosics materials and obtained after pyrolysis of some organic compounds, have been extensively studied for the past 20 years [1e9]. Due to their large surface area and porosity, activated carbon plays an important role in the technology of adsorbing gas. The production of an economical carbon adsorbent for gas storage has been actively pursued in recent years. In physical adsorption, the size and volume of the pores are important, thus activated carbons are used for the sorption/separation of light gases [10,11]. Although the activated carbons structural properties, including surface area and pore size distribution are effective in regards to the adsorptive capacity of gas, when specific interactions between gas and adsorbent act in the adsorption process, other features such as surface chemistry should also be taken into consideration. Due to the increasing demand of natural gas (NG) throughout the world, the adsorbed natural gas (ANG) storage system has become popular in the field of developing efficient gas storage and transportation technologies. Natural gas (NG) has emerged as an alternative energy source in transportation sector as it provides clean combustion and hence lowers exhaust pollution. It is mostly composed of methane (CH4) which has the highest heating value per unit mass of hydrocarbon fuels [12]. Moreover, its competitive price and copious availability makes NG a tangible fuel in energy sectors. Capture, separation and storage of this gas is present social issues essential both in environmentally and economically. Numerous theoretical and experimental studies conclude that adsorption capacity of methane is closely related to the textural characteristics of the activated carbons [13e17]. Therefore, the preparation conditions and the activation processes are necessary to be tuned to characterize a good carbon adsorbent for natural gas storage [18]. For all these reasons, optimizations of the adsorption properties of these porous solids by working on the structure, nature and the pore size are always a topical subject. Several studies have been carried out on methane storage in carbon materials and it has been concluded that highly microporous materials with a narrow micropore size distribution centered at a pore size of around 0.8 nm are the most suitable for this purpose [19,20]. Recently, some works have shown that the presence of certain metals (Ni, Co, Pt) improves the gas storage capacity of carbon materials [21e27]. In fact metal-doped materials are attractive for several applications: catalyst [28,29], double-layer capacitors or electrochemical applications [30,31], magnetic systems [32], hydrogen storage [21,33]. Much effort has been performed to relate the physical and chemical properties of carbon materials with their behavior as support and their further performance in diverse applications. In this work, three different series of activated carbon were prepared under various conditions in order to study the effect of the different conditions, raw material and adding of nickel oxide on their final textural and properties and on their subsequent methane storage capacity. The first series was prepared from lignocellulosic materials (olives stones were chosen as raw material, since they are extensively generated in Tunisia, olives stones were obtained from local olive manufactures) at different pyrolysis temperature. The second
series was obtained after pyrolysis of some organic compounds, based upon the gelation Pyrogallol-Formaldhyde (PF) xerogel using picric acid as catalyst. The third series was obtained after nickel incorporated in nano-carbon structures based on Pyrogallol-Formaldhyde (PF/Ni) at different pyrolysis temperatures.
Experimental methods Sample preparation The first series of activated carbon pellets was prepared in our laboratory using olive stones in two steps process: chemical activation and carbonization. In the chemical process, some amount of granular olive stones was mixed with aqueous solution containing 50% H3PO4 (w/w) at the weight ratio of 1/3. The suspension of the olive stones in chemical impregnation solution was mixed at 110 C for 9 h. Impregnate olive stones powder was used as raw material; with a size of about 80 mm. A mass of 0.6 g of the raw material was placed between the pistons in the dice and the mechanical pressure, P, was loaded on them for 1 min by an oil hydraulic machine. After the compression, the impregnate olive stones ejected from the dice were columnar pellets. These pellets were thermally activated in a fixed bed vertical reactor tubular furnace under nitrogen continuous flow for 3 h at different carbonization temperatures in the range from 350 C to 1000 C. After cooling, activated carbon pellet was washed for several times with hot water until the pH of washing solution became neutral. The samples were dried at 110 C to get the final product. The nomenclature of each sample includes the pyrolysis temperature and the abbreviation of activated carbon pellets (ACP): for example the activated carbon pellet carbonized at 410 C will be denoted ACP410. The preparation of the first series was described in detail in our previous work [34]. The synthesis of the second series of porous carbon has been done in three steps according to El Mir et al. protocol [35]. In this series three pyrolysis temperatures were considered (150 C for sample PF-150, 650 C for sample PF-650 and 1000 C for sample PF-1000). The synthesis of third porous carbon series reached by 5% mass ratio of NiO nanoparticles has been done in three steps as described elsewhere [36]. In this case the thermal treatment was carried out in a tubular furnace under nitrogen atmosphere at 150 C (PF/Ni-150), 650 C (PF/Ni-650) and 1000 C (PF/Ni-1000).
Characterization The synthesized and prepared products were characterized using a JEOL JSM-6300 scanning electron microscope (SEM) and a JEM-200CX transmission electron microscope (TEM). The surface area and the porous texture of the carbons were characterized using nitrogen sorptiometry at 196 C on the automatic Quantachrome Autosorb-1 apparatus in the range of relative pressure from 106 to 1. High purity nitrogen was used up to 6.0 (99.9999%) purity, before the measurement; all samples were outgassed at 250 C for 24 h. The nitrogen adsorptionedesorption isotherms were used to determine the following parameters: specific surface area SBET, total micropore volume Vmicro, and mean pore diameter dp.
Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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CH4 sorption experiment Methane adsorption was carried out at 298 K up to 30 bar in a high pressure volumetric system on the porous carbon previously outgassed at 150 C under vacuum. A diagram of the high-throughput adsorption apparatus is presented in our previous work [34], it was used to determine adsorption gas capacities with a good uncertainties of about 3% [37].
Results ACP methane storage Scanning electron micrographs observation of activated carbon pellets (ACP410, ACP500, and ACP1000) are shown in Fig. 1. The examination of the structure of samples shows the presence of macropores of various sizes and geometries at the surface. Micro-graphs show some regular structured shape: plane and thin stacked forms of a few mm and with large interparticle voids. Meanwhile, Fig. 1A displays the SEM micrograph of activated carbon pellets structure obtained at 410 C pyrolysis temperature; the sample exhibits significant microporous texture in which pore sizes are about a few mm in diameter. Carbon microstructures appear between these pores. In addition, the SEM micrographs of Fig. 1B and C also show that the increment of pyrolysis temperature from 500 C to 1000 C had increased the porosity of the surface area. Fig. 2
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shows the N2 sorption isotherms of the activated carbon pellets samples at 77 K. All the samples present type I isotherms according to the UIPAC classification [38], indicating that they are microporous materials. However, this shape suggests a strong development of microporosity. It is characteristic of the presence of microporous structure with rather narrow pore size distribution. Activation and pyrolysis of the raw olives stones by inert gas (N2) allow an increase in the porosity. This rise is probably due to the elimination of volatile matters produced during pyrolysis and release of the surface to create new pores in the carbon structure. From the analysis of the isotherms of Fig. 2, the main differences between the samples are their micropore volumes and their pore size distribution as it can be deduced from the sharp knees of their isotherms. Table 1 shows the textural properties of the samples obtained, and it can be seen an increases in specific surface area from 532 to 1014 m2/g and of the micropore volume from 0.241 to 0.459 cm3/g were measured in the activated carbon pellets samples as the pyrolysis temperature was increased from 350 C to 1000 C. Fig. 3 shows the CH4 adsorption capacity corresponding to all samples at room temperature and in pressure range up 25 bar. It can be seen that in general, CH4 adsorption capacity increases with the micropore volume with the existence of some exceptions. The highest CH4 adsorption capacity corresponds to the sample with the highest micropore volume (ACP410) (see Table 1). Comparing the shape of CH4 adsorption isotherms with that of N2 adsorption isotherms (Fig. 2), it can
Fig. 1 e Scanning electron micrographs of (A) ACP410, (B) ACP500 and (C) ACP1000 samples. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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300
ACP350 ACP410 ACP470 ACP500 ACP600 ACP800 ACP1000
a
3
V /cm .g
-1
250
200
150
0,0
0,2
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P/P0
Fig. 2 e Adsorption isotherm of N2 on activated carbon pellet. Va is the volume of liquid nitrogen adsorbed per gram of sample.
be said that they follow similar trends, which indicates that CH4 adsorption mainly occurs in the microporosity. Fig. 4 represents BET surface area determined from N2 adsorption data versus the CH4 adsorption capacity at 25 bar. A linear relation is found between CH4 storage capacity and BET surface area. We see that there are samples with micropore volume higher than the above values with deviate upward with respect to this line. This deviation happens for all those samples which have a considerable contribution of narrow porosity (pore size lower than 0.8 nm) which is not accessible to N2 at 77 K but which can be reached by CH4 at room temperature. These results show the importance of both micropore volume and micropore size distribution on CH4 uptake.
PF methane storage Fig. 5 displays for SEM micrographs of synthetic porous carbon obtained at different pyrolysis temperatures, which exhibit significant differences in agglomeration particles. For the PF150 a large macropores appear between these particles, micropores and perhaps some mesopores exist in these micrometer-size polymeric particles. Whilst, for the PF-650
Table 1 e Pore properties of activated carbon pellets from olive stones and Methane adsorption capacity at 25 bar and room temperature. Sample BET (m2/ Vmicro (cm3/ dp g) g) (nm) ACP350 ACP410 ACP470 ACP500 ACP600 ACP800 ACP1000
826 1014 765 700 632 532 915
0.381 0.459 0.349 0.329 0.279 0.241 0.404
1.91 1.85 1.86 1.95 1.83 1.87 1.79
CH4 uptake (mmol/g) 4.09 4.69 3.57 3.21 4.07 3.94 4.63
and PF-1000 carbon samples, the carbon microparticles aggregate and macropores volume decreases with pyrolysis temperature; which means that heat treatment improves the network structure density. For PF-1000 carbon sample, no macropores were seen from the SEM micrograph, since particles with 1e5 mm diameters appear to coagulate together leaving little space between them. The surface area and pore volume of this carbon indicate that these particles are essentially microporous. These results are consistent with porosity measurements. Images of transmission electron microscopy (TEM) were performed on PF composites treated under different pyrolysis temperatures. These images show in Fig. 6 that our composites are characterized by nanopores. The average pore size is less than 2 nm, whereas the PF composite are microporous. The diameter of the pores increases with increasing pyrolysis temperature. Indeed, the PF-1000 (Fig. 6c) has pores larger than those of the other samples. The TEM data confirm that the interconnected solid nanoparticles comprise an opencelled network with continuous nanodimension porosity. These observations are also consistent with porosity measurement. The N2 adsorptionedesorption isotherms at 77 K of synthetic porous carbon polymerized by picric acid are displayed in Fig. 7. These isotherms are generally of type I in the Brunauer, Emmet and Teller (BET) classification [38], and indicating the synthetic porous carbon are dominated by the microporosity, whatever the pyrolysis temperature used in the preparation. The order of N2 adsorption capacity from the lowest to highest is: PF-150 < PF-650 < PF-1000. This N2 capacity corresponds directly to the microporosity volume of synthetic porous carbon. The shape of PF-650 and PF-1000 are characteristic of the presence of microporous structure with a rather narrow pore size distribution. Following with the analysis of the isotherms of Fig. 7, the main differences between the samples are their micropore volumes and their pore
Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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mmol CH4 /g
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ACP350 ACP410 ACP470 ACP500 ACP600 ACP800 ACP1000
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0 0
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Pressure (bar) Fig. 3 e CH4 adsorption isotherm at room temperature.
size distribution. Thus, almost PF-1000 is with homogeneous pore size distribution and nanoporosity as it can be deduced from the sharp knees of their isotherms and TEM micrographs (Fig. 6). The microporous specific surface areas determined by the conventional BET method varies from 560 to 920 m2/g, with micropore volumes between 0.268 and 0.460 cm3/g (Table 2). The mean micropore size determined from BET surface area and the pore volume in the approximation of cylindrical is close to 2 nm. Fig. 8 presents the methane adsorption isotherms on a volumetric basis at room temperature on synthetic porous
carbon samples. For PF-1000 and PF-650 samples, isotherm behavior is of type I (as defined by the UIPAC classification). Reflecting that those materials are essentially microporous (monolayer adsorption). It can be seen that CH4 adsorption capacity increases with the micropore volume. The highest methane adsorption capacity corresponds to the sample with the highest micropore volume (PF-1000) (see Table 2). Comparing the shape of CH4 adsorption isotherms with that of N2 adsorption isotherms (Fig. 7), it can be said that they follow similar trends which indicates that methane adsorption is also affected by pore size distribution of the samples. It can be
1200 ACP410 ACP800
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BET surface area (m /g)
1000
800
ACP500
ACP1000
ACP470
600
ACP600 ACP800
400
200
0 3,0
3,5
4,0
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mmol CH4/g Fig. 4 e Relationship between the CH4 adsorption capacity at 25 bar and room temperature with the BET surface area calculated from N2 adsorption isotherms. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Fig. 5 e SEM images of (a): PF-150, (b): PF-650 and (c): PF-1000.
observed that the methane increases with the increases of pressure. The results also show that the amount of methane adsorbed in the PF-1000 and PF-650 is larger than in the PF-150, where progressive thermal treatment improve their adsorption capacities, i.e., PF-150 < PF-650 < PF-1000. This is in agreement with the higher development of porosity obtained
at higher pyrolysis temperature. Moreover, the heat treatment improves the network structure density, besides for the PF650 and PF-1000 carbon samples, the carbon microparticles aggregate and micropores volume decreases with pyrolysis temperature which promotes the methane adsorption. The existence of a reliable correlation between methane
Fig. 6 e TEM images of (a): PF-150, (b): PF-650 and (c) PF-1000. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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to the micropore volume and that possible deviations from this behavior can be ascribed to either the presence of microporosity not accessible to N2 at 77 K, or to the existence of supermicroporosity where methane could adsorbed with a different density. To maximize methane storage capacity of synthetic porous carbon, there are several options: maximize micropore volume while minimizing mesopore and macropore volume, control pore size distribution so that the majority of micropores are y1.14 nm, the modeled optimum pore size for methane storage [19] and minimize interparticle volume by grinding and pelletizing the activity carbon products.
Influence of NiO doping PF microporous carbon on methane storage Fig. 7 e Adsorption-desorption isotherm of nitrogen composites; (a): PF-150, (b): PF-650 and (c): PF-1000.
adsorption and the porous texture of activated carbons is of interest as it would allow us to check the suitability of a given material if high pressure equipment is not available. Several researchers have discussed these correlation [39,40]. Their main conclusion is that methane adsorption has to be related
The SEM images for the studied PF/Ni nanocomposites are illustrated in Fig. 9. According to these images, the increase of the pyrolysis temperature contributed to the agglomeration of the nanoparticles. TEM images performed on the NiO nanoparticles and the PF/Ni-650 samples are shown in Fig. 10. Fig. 10a shows NiO nanoparticles, of about 30 nm in size with cubic shape, before their incorporation. The TEM image of PF/Ni-650 (Fig. 10b) indicates that material was composed by metallic nickel nanoparticles dispersed in carbon matrix. It is worthwhile to notice that nickel oxide was
Table 2 e Pore properties of nanoporous PF carbon and Methane adsorption capacity at 25 bar and room temperature. Sample code
Temperature of thermal treatment, Tp ( C)
BET (m2/g)
Vmicro (cm3/g)
dp (nm)
CH4 uptake (mmol/g)
150 650 1000
561.9 720.4 920.6
0.268 0.334 0.460
1.9 1.9 2.1
0.25 4.29 5.50
PF-150 PF-650 PF-1000
6
a
mmol CH4 /g
5
b
4
3
2
1
c 0
0
5
10
15
20
25
30
Pressure (bar) Fig. 8 e Methane adsorption isotherms at room temperature; (a): PF-1000, (b): PF-650 and (c): PF-150. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Fig. 9 e SEM images of (a): PF/Ni-650 and (b): PF/Ni-1000.
Fig. 10 e TEM micrograph of (a): NiO nanoparticles and (b): PF/Ni-650.
reduced to metallic nickel which leads to the decreasing of the nanoparticles size. However, the carbon matrix stills in the amorphous phase. The TEM images of PF/Ni-1000 sample presented in Fig. 11 exhibit the existence of carbon nanotubes. The nickel nanoparticles in the carbon matrix played an important role for the appearance of nanotubes (Fig. 11a). The nanotubes germination may be originated from the difference of the surface tension between the nickel nanoparticles, which is very high due to its nanometric size and the graphite material
[41]. Fig. 11b depicts the TEM image of the multiwall carbon nanotubes (MWNT) with the inter-plane distance of 0.34 nm which is equivalent approximately to the distance between the graphene layers in a graphite crystal [42]. The surface area available for adsorption is a key property of porous solids and it often used to compare different materials. The most widely used procedure for evaluating the surface area of finely divided and porous materials is the Brunauer-Emett-Teller or BET method [38]. Fig. 12 shows the N2 sorptionedesorption isotherms of Ni-doped carbon
Fig. 11 e TEM images of PF/Ni-1000. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Fig. 12 e Adsorption isotherm of N2 on porous carbon. Va is the volume of liquid nitrogen adsorbed per gram of sample.
composites with 5 wt.% of NiO at 77 K shapes of the PF/Ni-650, show that this isotherm are type IV in the Brunauer Emett and Teller (BET) classification. This is characteristic of mesoporous solid and the hysterisis is usually associated with the filling and emptying of the mesopores by capillary condensation. The shape of the PF/Ni-1000 isotherm is type V in the Brunauer Emett and Teller (BET) classification. This is typical for nonwetting mesoporous materials, where the convex isotherm at low P/P0 indicates weak host-gas interactions while the hysterisis loop is associated with the mechanism of pore filling and emptying by capillary condensation. The nitrogen sorption results and the adsorption equilibrium capacities of CH4 on all three adsorbents at 298 K and elevated pressures (25 bar) are summarized in Table 3. The order of N2 adsorption capacity from the lowest to highest is: PF/Ni-150 < PF/Ni650 < PF/Ni-1000. This N2 capacity corresponds directly to the microporosity volume of synthetic porous carbon. Adsorption isotherms for CH4 are presented in Fig. 13. The difference between the PF/Ni-150 and PF/Ni-1000 samples is related to a small difference in the synthesis conditions (pyrolysis temperature). This explains the systematically lower amounts adsorbed. Looking at the PF/Ni-650, it is noticeable that at high pressures the solid have the largest micropore volume (PF/Ni-650, 0.222 cm3 g1) also has the highest uptakes while the solid with the smallest pore volume (PF/Ni-150, 0.020 cm3 g1) adsorbs the least.
Comparison of CH4 sorption To verify the effect of adding nickel oxide and the choice of raw material for activated carbon preparation in the methane
adsorption capabilities, 298 K isothermal curves at pressure of 30 bar for the methane gas were obtained. Fig. 14 shows the methane adsorption of the prepared samples (ACP410, PF-650, PF-1000, PF/Ni-650 and PF/Ni-1000). It was found that the PF1000 showed an adsorption capacity of 5.5 mmol/g while that of the ACP410 showed 4.69 mmol/g. This result indicates that porous organic samples prepared by mixing formaldhyde with pyragallol (PF) has a tendency to adsorb methane higher than microporous carbons, prepared from lignocellulosic materials. In fact PF-1000 carbon sample possesses a very wide micropore volume; the carbon microparticles aggregate and macropores volume decreases. The average pore size of PF-1000 is less than 1 nm (Fig. 6) which optimizes the density of the adsorbed phase. Also, the methane capacity in the PF650 and PF/Ni-650 were approximately 4.29 mmol/g and 4.54 mmol/g respectively. PF/Ni-650 sample showed a higher adsorption capacity than that non reached by NiO (PF-650). Indeed comparing PF-650 and PF/Ni-650, adding nickel oxide increases the methane adsorbed amount although the specific surface area decreased from 720 m2/g to 460 m2/g, this indicates that the surface properties enhanced the methane adsorption. Meanwhile the TEM image of PF/Ni-650 (Fig. 10b) indicates that material was composed by metallic nickel nanoparticles. It is worthwhile to notice that nickel oxide was reduced to metallic nickel. Adding nickel dispersed in carbon matrix enhanced the methane adsorption, besides the transition metals have acid features (electron acceptors) and methane gas is a Lewis basic in that it done electron from Lewis acid (electron acceptors). Therefore, the basic methane can attract forces with transition metal. This explains the increase in the amount of adsorbed methane despite the
Table 3 e Properties of synthetic porous carbon PF/Ni and methane adsorption capacity at 25 bar and room temperature. Sample code PF/Ni-150 PF/Ni-650 PF/Ni-1000
Temperature of thermal treatment, Tp ( C)
BET (m2/g)
Vmicro (cm3/g)
dp (nm)
CH4 uptake (mmol/g)
150 650 1000
10 < 461 163
0 0.222 0.133
e 2.62 4.81
0.74 4.54 1.52
Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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5
b
mmol CH4/g
4
3
2
c 1
a 0 0
5
10
15
20
25
Pressure (bar) Fig. 13 e CH4 adsorption isotherm at room temperature of (a) PF/Ni-150, (b) PF/Ni-650 and (c) PF/Ni-1000. decrease in the specific surface area. It can be stated that the presence of nickel particles in the carbon surfaces enhances the adsorption of methane due to the acidebase properties. Also, the methane capacity in the PF-1000 and PF/Ni-1000 were approximately 5.5 mmol/g and 1.5 mmol/g respectively. PF-1000 sample showed a higher adsorption capacity than that PF/Ni-1000. Indeed when the incorporated nanocomposites are treated with high pyrolysis temperature (more than 650 C), involve the creation of carbon nanotubes that is not the case in PF sample, this prevents the gas from
Conclusion This study provides experimental data for methane adsorption on three sets samples from different raw material.
ACP410 PF1000 PF/Ni650 PF650 PF/Ni1000
6
5
mmol CH4/g
penetrating in to the pores. This explains the systematically lower amounts of adsorbed CH4 by PF/Ni-1000. The porosity of this sample was totally blocked, leading to a decrease in BET surface area.
4
3
2
1
0 0
5
10
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20
25
30
Pressure (bar) Fig. 14 e CH4 adsorption isotherm at room temperature. Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010
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Adsorption experiments were conducted using a volumetric method. Our experimental isotherms have an evident character of type I (Langmuir type), reflecting adsorption in a microporous adsorbent with a good affinity with CH4. So, the adsorption could be considered as reversible, local and limited to a monolayer. We have shown that maximum adsorption capacities for CH4 are proportional to the microporous volume and also to the narrow pore size distribution. It was found that different pyrolysis temperature influence methane storage capacity. It was demonstrated that the methane adsorption capacity not only depends on the micropore volume but also strongly depends on the narrow pore size distribution. Microporous carbon synthesized from pyrogallolformaldhyde (PF) has a great affinity to adsorb methane, due to the essentially micropores character of the synthetic porous carbon, micropore size distribution and its low mesopore concentration. Such material seem to be particularly adapted for methane storage. Adding nickel oxide increases the amount adsorbed for some sample and decreases for other. Consequently, the adsorbed methane amount depends on the both microporosity and acidebase properties.
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Please cite this article in press as: Djeridi W, et al., Influence of the raw material and nickel oxide on the CH4 capture capacity behaviors of microporous carbon, International Journal of Hydrogen Energy (2015), http://dx.doi.org/10.1016/ j.ijhydene.2015.05.010