- Email: [email protected]
Enhancement of hydrogen spillover onto carbon nanotubes with defect feature
Available online at www.sciencedirect.com
Microporous and Mesoporous Materials 109 (2008) 549–559 www.elsevier.com/locate/micromeso
Enhancement of hydrogen spillover onto carbon nanotubes with defect feature Chien-Hung Chen a, Chen-Chia Huang a
b,*
Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Douliu, Yunlin 640, Taiwan, ROC b Department of Chemical Engineering, National Yunlin University of Science and Technology, Douliu, Yunlin 640, Taiwan, ROC Received 3 March 2007; received in revised form 21 May 2007; accepted 1 June 2007 Available online 12 June 2007
Abstract Hydrogen spillover is one of potential ways to enhance hydrogen storage in carbon nanotubes (CNTs). In this study, hydrogen spillover effect on hydrogen storage in cobalt (Co) metal loaded CNTs with different structural characteristics was investigated. The surface structure of CNTs was modified by KOH activation at temperature range from 873 K to 1073 K, in order to produce different defects of CNTs. The experimental results revealed that the specific surface area of the CNTs increased along with activation temperature from 467 m2/g (at 873 K) to 859 m2/g (at 1073 K). The higher surface area of CNTs was obtained by activating the higher temperature. Hydrogen storage capacity in the CNTs was determined by a dynamic thermogravimetric method with heating/cooling cycles between 303 K and 673 K under one atmosphere. Measurements showed the weight change during the heating/cooling cycles for the activated CNT samples was larger than the as-prepared ones. The enhancement of hydrogen storage in the activated CNTs was attributed to hydrogen spillover effect. The defective structure of the CNTs after activation is a significant factor to promote hydrogen spillover. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Hydrogen storage; Carbon nanotubes; Activation; Spillover; Cobalt
1. Introduction Hydrogen is a clean, ideal energy source. In the past few years, several different technologies including compression, liquefaction, metal hydride, and adsorbent material have been developed for hydrogen storage. Among these technologies, the application of adsorbent material is safer and more efficient than any other technology in terms of controlled hydrogen storage. Recently, carbon nano-materials (such as nanotubes and nanofibers) have attracted attention as one of potential adsorbents for hydrogen storage. Dillon et al. [1] first found that single walled carbon nanotubes (SWCNTs) possess excellent hydrogen storage properties with gravimetric density of 5–10 wt.%. Cham*
Corresponding author. Tel.: +886 5 534 2601x4616; fax: +886 5 531 2071. E-mail address: [email protected] (C.-C. Huang). 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.06.003
bers et al. [2] claimed that herringbone type graphite nanofibers can adsorb hydrogen up to 67 wt.% at 100 atm and 300 K. Unfortunately, so far these storage capacities have been unable to be validated by other researchers. In recent publications [3–5], almost all experimental results indicated that hydrogen storage by carbon nano-materials was only 0.1–2.0 wt.%. Based on physisorption, Zhou et al. [3,6] even reported that carbon nanotubes (isosteric heat of adsorption 1.7 kJ/mol) were hopeless for hydrogen storage, compared to activated carbon (AX-21, isosteric heat of adsorption 6.4 kJ/mol). However, these dissimilarities may be caused either by the method of experimental analysis or some disparities in the structural characteristics of carbon nano-materials. Hydrogen spillover is an approach to be considered to enhance hydrogen storage in a carbon nano-structure. Lueking and Yang [7–9] presented a series of papers about hydrogen spillover onto metal catalyst (transitional metal such as Pt, Pd and Ni) supported carbon nanotubes
550
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
(CNTs) that enhanced hydrogen storage properties. Hydrogen molecules dissociate to atomic hydrogen on a metal catalyst and subsequently migrate from the metal to the surface of CNTs. Ab initio molecular orbital study [10] has indicated that atomic hydrogen adsorption was exothermic and stable on the graphite basal plane. Kim et al. [11] reported that hydrogen storage capacity of 2.8 wt.% was achieved by using Ni nanoparticle-dispersed multi-walled carbon nanotubes (MWCNTs). The results indicated that the Ni catalyst allowed hydrogen molecules to be dissociated and be adsorbed to form C–Hn bonding on the carbon structure. Hydrogen storage in activated carbon by hydrogen spillover also has been demonstrated by Zielin˜ski et al. [12]. They reported that the hydrogen spillover would be seen as driving force for hydrogen storage in carbonaceous materials. Furthermore, the spilt-over hydrogen onto the carbon was easily recombined at room temperature and pressure, due to the weak adsorption strength with carbon surface. However, the advantage of hydrogen spillover is that it not only improves hydrogen storage but also increases initial hydrogen adsorption kinetics [13]. In fact, the external surface, inner surface and interplanar spacing of CNTs are potential adsorption sites for hydrogen storage. Generally, the tube length of CNTs is several hundred nanometers, even over micrometers. The long path for hydrogen diffusion into interior of CNTs is a challenge, which controls the hydrogen storage capacity of CNTs. Etching the surface of CNT [14,15] may be a way to solve this problem of mass transfer resistance. Defects or cavities on CNTs surfaces not only represent point of the entry for hydrogen but also increase the surface area and pore volume of CNTs. KOH is one of activation reagents for preparation of activated carbons [16]. Recently, carbon nanotubes and nanofibers activated by KOH have also been reported [17–20]. In this study, we used cobalt to modify both the as-prepared CNTs and the activated CNTs, and compared hydrogen adsorption capacity on the same. The activated CNT samples were prepared by KOH activation at different temperatures to create variable degree of defects of the CNTs structure. The objectives of this work were: (1) to investigate characteristics of the defect structure of CNTs modified by KOH etching, and (2) to study hydrogen storage properties on the defect structure of CNTs.
for 1 h, stirred strongly at 373 K to become concentrated and then dried at 403 K for 12 h. Finally, the Co0.05Mg0.95O catalyst powder was obtained.
2. Experiments
The structure of CNTs was characterized by using a high resolution transmission electron microscope (HRTEM, JEOL, JEM-2010 type). The metal species on the CNTs was characterized by an X-ray photoelectron spectroscopy (XPS, Thermo VG, ESCAlab 250) and by an X-ray diffractometer (XRD, RIGAKU, D/MAX2200). The nitrogen adsorption isotherm for CNT samples was measured by a gas adsorption meter (QUANTACHROME, AUTOSORB-1) at 77 K. The specific surface area and pore size distribution were calculated by using the BET and the BJH methods from N2 isotherm adsorp-
2.1. Catalyst preparation In this study, Co/MgO functioned as the catalyst to synthesize the MWCNTs was prepared by using impregnation method. The molar ratio of Co/Mg was 0.05/0.95. Preparation procedure of the catalyst was as the follows: Co(NO3)2 Æ 6H2O was dissolved in a solution that was prepared by mixing the ethanol solution with magnesium oxide (MgO) powder. Next, the Co/MgO solution was sonicated
2.2. Synthesis of MWCNTs MWCNTs were synthesized by catalytic decomposition of acetylene over Co0.05Mg0.95O catalyst. The prepared Co0.05Mg0.95O powders were loaded into the center of quartz reactor tube in a flowing of H2, and then heated up to 973 K for 30 min. On reaching this temperature, the flow of C2H2/H2 (30/70) mixture was introduced into the quartz reactor for 30 min. The total flow rate was 100 ml/min. The raw MWCNT samples were obtained after cooling down to room temperature. The raw samples were immersed in 37% HCl solution to dissolve impurities such as the metal catalyst. After the samples were filtered, they were washed in de-ionized water and dried at 373 K in the air. CNTs with a high purity (>90%) were obtained and referred to as the as-prepared CNTs. 2.3. KOH activation of CNTs In this study, activated CNTs were prepared by KOH activation. The ratio of KOH and CNT was fixed at 4:1 (g/g). The activation was conducted at 873 K, 973 K, or 1073 K for 1 h under nitrogen flow of 500 ml/min. The activated products were washed with diluted HCl solution to remove potassium species. After filtration the samples were dried at 378 K. The CNTs after KOH activation are referred to as the activated CNTs. 2.4. Co loaded CNTs Both the as-prepared CNTs and the activated CNTs were impregnated with cobalt nitrate using a wet impregnation method. The CNT (100 mg) samples were immersed in 100 ml of different concentration (0.025–0.5 M) Co(NO3)2 solutions, and sonicated for 1 h to ensure the CNT powders dispersedly in the solution. They were shook with 100 rpm rotation rate at 303 K for 24 h. Subsequently, the Co loaded CNT samples were obtained after filtering and drying at 378 K for 24 h. 2.5. Material characterization
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
551
tion/desorption curves, respectively. The amount of Co deposited onto the CNT samples was analyzed by using an atomic absorption spectroscopy (AAS) after the impregnation process. The Fourier transform infrared (FTIR) studies of CNTs before and after H2 adsorption were performed by a spectrometer (FTIR-RAS, Jasco FT/IR-4200 Spectrometer) with a liquid nitrogen cooled MCT-M detector at a resolution of 4 cm1. The CNT samples were prepared by spraying CNTs powders with acetone solution onto the silicon wafer. The acetone was evaporated at room temperature in air. The background spectrum was performed by a clean silicon wafer. A minimum of 1000 scans were collected. 2.6. Hydrogen storage measurement Hydrogen adsorption/desorption experiments were performed using a thermogravimetric analyzer (TG, Cahn, TG2121) with a vacuum system. In this study, ‘ultrahigh purity grade’ hydrogen (99.9995%) was used through a moisture trap (packed SICAPENT powder, Merck) to eliminate moisture impurities. Ten milligrams of CNT samples were loaded into a quartz sample holder in a close TG system. Before performing hydrogen uptake measurements, the TG system was evacuated in order to eliminate possible contamination within the system. Then, hydrogen was introduced into the TG system to pump pressure back to ambient condition. A flowing of hydrogen (20 ml/min) was maintained to purge the system for several hours in order to ensure its cleanliness and weight stability in the TG system. For hydrogen adsorption/desorption measurement, we adopted two-step thermal programs and they were carried out as follows: (1) The CNTs samples were initially heated up to 673 K at a rate of 10 K/min, and then held for 1 h in a flowing of H2 at 20 ml/min. Afterwards, it was cooled down to room temperature until constant weight. The cobalt loaded reduction in the CNTs was performed in this pretreatment. (2) The temperature was heated again from 303 K to 673 K at a heating rate of 5 K/min (desorption curve), and then cooled down to 303 K (adsorption curve). In order to obtain accurate values of hydrogen uptake, the buoyancy effect was taken into consideration during heating/cooling. All weight changes with respect to adsorption/desorption data were corrected using a quartz blank calibration [7]. All hydrogen storage experiments were repeated at least once. 3. Results and discussion 3.1. Material characterization 3.1.1. HRTEM observation Fig. 1 shows the HRTEM micrographs of the as-prepared CNT and the activated CNT samples. From HRTEM observation, most of catalyst particles were removed from the CNT after the purification process. The caps of CNTs were opened. However, a small residual
Fig. 1. TEM micrographics of CNT: (a) the as-prepared CNT and (b) the activated-CNT prepared by KOH activation at 973 K.
cobalt catalyst on the CNT samples was confirmed by EDS analysis (data not shown). The diameters of tubes ranged from 5 nm to 15 nm, and length was in a range of several hundred nanometers. Further, the structure of the as-prepared CNTs was observed with no significant defect, as shown in Fig. 1a. In this study, the activated CNTs with defect structure were prepared by KOH activation under various temperature conditions (873–1073 K). As being activated at 973 K, massive defects produced on the surface of the CNTs, as presented in Fig. 1b. More details about the CNTs activation were discussed in the next section.
552
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
For the Co loaded as-prepared CNT, Co metal particles size of 10 nm distributed over the external surface of CNT samples were observed in Fig. 2. 3.1.2. X-ray photoelectron spectroscopy (XPS) The Co 2p XPS spectra of Co loaded CNTs before and after H2 reduction are presented in Fig. 3. For both Co
Fig. 2. TEM micrographics of the Co loaded as-prepared CNT with 14.62 wt.% Co.
loaded the as-prepared and the activated CNTs before H2 reduction, the XPS Co 2p3/2 peak with subsidiary peak was observed and the position of Co2p3/2 peak shifted to the region of higher binding energy. The spectra indicated that some concentration of isolated Co2+ species distributed onto the Co loaded CNTs [21,22] after wet impregnation. After H2 reduction at 673 K, the individual peak of XPS Co 2P3/2 at 780.6–780.9 ev of the binding energy was found. Further the difference in binding energy (DE) between peaks of Co 2P3/2 and Co 2P1/2 was calculated as 15 ev. The results indicated that Co3O4 is dominant cobalt phase on the CNTs [23]. Khodakov et al. [21] reported that Co3O4 transited to CoO at a range of 500–600 K, and CoO phase reduced to metallic Co at a range of 600–700 K, demonstrated by the weight loss of TGA. Schanke et al. [24] also identified the reduction of Co3O4 by TPR studies. The result indicated that the two-step reduction of Co3O4 (via CoO) to metallic Co was found at 592 K and 640 K. The similar studies about Co species reduction have been reported in previous reports [25,26]. In this study, the presence of Co3O4 and metallic Co phases onto the CNTs after H2 reduction at 673 K were found by XRD analysis (data no shown), similar results reported by Gandia et al. [27]. And, the XPS spectra exhibited that Co species onto the CNTs was Co3O4 phase after H2 reduction. The presence of Co3O4 phase was attributed to the metallic Co re-oxidized by passivation at room temperature under air atmosphere [27]. 3.1.3. Thermogravimetric analysis (TGA) Fig. 4 shows the differential thermogravimetry (DTG) curves of bulk cobalt nitrate, the Co loaded as-prepared CNT and the Co loaded activated CNTs. As shown in Fig. 4a, the significant weight loss peak was obtained at 502 K. Comparison of the weight loss ratio, the ratio of Wfinal (the final weight after reduction) to Winitial (the initial weight before reduction) was 0.204, which was similar to the molecular weight ratio of 0.203 between Co and Co(NO3)2 Æ 6H2O. The result implied the cobalt nitrate completed reduction to metallic Co at 502 K in H2. Furthermore, two major peaks were found for each Co loaded CNT samples. Weight loss of left peaks was usually larger than that of right peak. According to the DTG curves, XRD analysis and previous reports [21,22,24–26], it is suggested that cobalt nitrate and small amount of Co3O4 deposited onto the surface of CNTs after the impregnation process. The weight loss of left peak was attributed as cobalt nitrate ! metallic Co, and the weight loss of right peak was as Co3O4 (via CoO) ! Co. 3.2. CNTs activation
Fig. 3. XPS Co 2p spectra of Co loaded CNTs before and after H2 reduction at 673 K for 1 h. (a) Co loaded as-prepared CNTs; (b) Co loaded activated-CNTs.
The comparison of activation temperature effect on specific surface area and yield for CNT samples is presented in Fig. 5. The specific surface areas of CNT at 873, 973 and 1073 K activation were 467, 764 and 859 m2/g, respectively, higher than that of the as-prepared CNT (453 m2/g). The
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
Fig. 4. DTG curves of (a) bulk cobalt nitrate, (b) Co loaded as-prepared CNTs and (c) Co loaded activated-CNTs. Operation conditions: heated up to 673 K at a rate of 10 K/min in H2, and then held at 673 K for l h.
553
after KOH activation implied that defects on the graphite structure were formed by carbonaceous gasification. Additionally, excessive defect sites on the graphite walls of the CNTs caused structure collapse and decreased yield after activation process. Some previous papers [16,28,29] have reported that K2CO3, C–O–K metal salts and metallic K were considered as products after the activation of KOH in a N2 or Ar atmosphere. Both CO and CO2 were detected in the effluent during the activation process. Fig. 6 shows the pore distribution of the as-prepared CNTs and the activated CNTs prepared at different temperatures. It is found that pore volume of the CNT after KOH activation increased remarkably, particular for pore ˚ . In KOH activation at 873 K and diameters less than 100 A 973 K, the main pore volume increment was in the range of ˚ . When the CNTs were activated at 1073 K, the 20–30 A ˚ . The ranges of pore enhancement shifted to 40–100 A increase of pore volume was ascribed to defect or cavity sites formed onto the external graphite walls. At higher activation temperature, the defects or cavities on the CNTs became deeper and wider from the exterior to the interior of graphite layers. The collapse of two or more small defect sites to form a large defect size was possible in excessive activation, such as the CNTs activated at 1073 K. Parts of carbon molecules of CNTs were even decomposed (burnt off) by KOH attack. Raman spectroscopy is an important tool to provide information about the crystal structure and the presence of disorder of the CNTs. In the Raman spectra, two main peaks were observed in the high frequency region, at 1580 cm1 (G band) and 1350 cm1 (D band). The G band indicates an original graphitic structure, and the D band reflects the presence of either disordering feature or amorphous carbon [30–32]. The intensity ratio of D band to G band was employed to characterize the feature of
Fig. 5. Effect of activation temperature on specific surface area and yield of the activated CNT.
specific surface area of the activated CNT obviously increased with higher activation temperature. Obviously, the yield of the CNT decreased remarkably at higher activation temperature. The incremental surface area of CNTs
Fig. 6. Pore size distribution of the as-prepared CNT and the activated CNTs under different activation temperatures.
554
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
CNTs. In this study, we adopted the intensity ratio of D band to G band (ID/IG) to analyze the feature of structure change of CNTs after KOH activation with various temperatures (873–1073 K). Fig. 7 reveals the ID/IG ratio of the as-prepared CNTs and the activated CNTs at variable temperature. Overall, the ratio of ID/IG of the activated CNTs was higher than that of the as-prepared CNTs.
Fig. 7. The ID/IG ratio of the as-prepared and the activated CNTs with different activation temperature.
The result indicated that the graphitic structure of CNTs certainly became disordered after KOH activation. It is interesting that the ratio of ID/IG of the activated CNTs at 873 K activation was larger than that of other activation condition (973 K and 1073 K). The cause was attributed that the disordered structure of the external surface of CNTs decomposed at the high temperature. On the other hands, the defect structure of CNTs was produced after KOH activation, since the disordered carbon material from graphite sheets transforming was decomposed. This suggestion can explain that the pore volume and the specific surface area of activated CNTs enhanced with increasing the activation temperature. Based on the above discussion, we proposed a possible activation mechanism of CNTs by KOH activation, as shown in Fig. 8. The produced defects on the external surface of CNTs were developed by two steps: disordering and destroying. When CNTs activated at low temperature, the graphite sheets of CNTs became disordered structure by KOH attacking gradually. No significant defects produced on the external surface of CNTs, because the disordered carbons have only a little amount of decomposition. As the activation condition at high temperature, more disordered carbons formed and graphite sheet decomposed simultaneously. Then, significant defect sites produced on the external of graphite layers of CNTs. Moreover, if the excessive activation performed, the CNTs defect either became larger or structure destroyed completely.
Fig. 8. Schematic representation of CNTs activation mechanism.
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
3.3. Effect of residual metal and structure of CNTs In order to clear the effect of residual metal (Co) and structure of CNT on hydrogen storage capacity, concentrate HNO3 solution was used to treat CNT samples (including the as-prepared and the activated CNTs) to further eliminate residual Co catalyst. The amounts of residual Co onto the as-prepared and the activated CNT samples are collected in Table 1. The weight changes of the as-prepared and the activated CNTs before and after 5 N HNO3 washing upon the heating/cooling cycles, are shown in Fig. 9. The amount of residual Co onto the asprepared CNTs was larger than that onto the activated CNT samples. However, the weight change of the as-prepared CNTs was only 0.22 ± 0.07 wt.%. The weight change of the activated CNTs increased along with activation temperature. Even without Co residues (after HNO3 wash), the high temperature (973 K or 1073 K) activated CNTs still possessed more than 1.0 wt.% of weight change. The causes of hydrogen storage improvement into the activated CNT samples are attributed to the following. (1) The edge carbon was created around defect sites of graphite walls on CNT, produced during activation treatment. Consequently, hydrogen was possibly chemisorbed with edge carbon [10]. (2) The high surface area of the activated CNT implied the presence of defects or cavities on the graphite walls, as shown in Fig. 1b. The defect sites
Table 1 Amount of residual Co onto the CNT samples Samples
Residual Co (wt.%)
As-prepared 873 K activation 973 K activation 1073 K activation
1.02 0.16 0.09 0.15
Fig. 9. Weight changes of the activated CNT samples before and after 5 N HNO3 washing upon the heating/cooling cycles.
555
on the structure provided the entry of hydrogen spreading into the interplanar spacing, which enhanced hydrogen storage into CNTs [33,34]. 3.4. Effect of Co loaded onto CNTs The hydrogen adsorption/desorption curves of the Co loaded CNTs are shown in Fig. 10. All adsorption/desorption curves were corrected by the blank experiment under the same thermal programs. The weigh change of CNTs was determined by the weight differential upon the heating-cooling cycles. The Co loaded-activated CNTs possessed the highest weight change compared to other samples. Additionally, both the Co loaded as-prepared CNTs and the Co loaded-activated CNTs had similar adsorption/desorption behavior. It is observed that weight increased sharply below 400 K in the adsorption curve. Two successive hydrogen adsorption/desorption curves on the Co loaded CNT samples are presented in Fig. 11. As shown in Fig. 11, the adsorption/desorption experiment is well reproducible. The results implied that hydrogen storage on the Co loaded CNTs is reversible. The weight changes of the Co loaded CNT samples upon the heating/cooling cycles are presented in Fig. 12 and Table 2. The weight change increased along with the amount of Co loaded onto the CNTs. Obviously, the different structural characteristics of CNTs possessed remarkably different capabilities of hydrogen uptake. Under similar Co loaded content, the weight change of the Co loaded as-prepared CNT was significantly smaller than that of the Co loaded-activated CNT. The maximum weight changes of the Co loaded-activated CNT (activated at 973 K) and of the Co loaded as-prepared CNT were 3.41 ± 0.05 wt.% and 1.36 ± 0.03 wt.%, respectively.
Fig. 10. TG profiles of the adsorption/desorption cycle. (a) The asprepared CNT, (b) the Co loaded as-prepared CNT with 5.67 wt.% Co, (c) the Co loaded-activated CNT with 4.41 wt.% Co.
556
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
Fig. 11. TG profiles of two successive adsorption/desorption cycles of the Co loaded activated CNTs.
Fig. 12. Weight changes of the different Co content loaded CNT samples upon the heating/cooling cycles.
However, the weight change of the CNT without Co impregnation was only 0.22 ± 0.07 wt.%. Obviously, Co at active site played an important role in promoting hydrogen adsorption. As a catalyst, the loaded Co (metallic state) dissociated hydrogen molecules [25,35]. The hydrogen atoms then diffused to the surface of CNTs and adsorbed onto the specific sites of CNTs. This phenomenon is called ‘‘hydrogen spillover’’ by previous literatures [7,9,11,35,36]. Mu et al. [15] developed the hydrogen storage capacity of 4.5 wt.% using Pd decorated CNTs with defect structure under 10.7 MPa at room temperature. Yoo et al. [14] reported that the hydrogen capacity of 1.5 wt.% was
received by defective CNT with Pd under 1 atm and at 573 K. In present work, the largest hydrogen capacity of 3.41 ± 0.05 wt.% for Co loaded activated CNTs was obtained under 1 atm and at moderate temperature, that revealed similar results by previous literature [14,15]. The presence of defect structure onto CNT was demonstrated to be a remarkable factor for enhancing hydrogen storage by hydrogen spillover. The external surface, inner surface and interplanar spacing of CNTs are available for hydrogen storage. The advantage of hydrogen spillover applied in hydrogen storage of CNTs as follows: the dissociated hydrogen (the radius of hydrogen atom with the radius of van der Waals ˚ ) can be easily diffused into the interplanar is about 0.46 A ˚ space (3.4 A) of CNTs, compared with hydrogen molecule ˚ ). Therefore, hydrogen spillover (a kinetic diameter 2.89 A behavior was considered as a possible way to improve hydrogen storage in the CNTs. In this study, it is interesting that the hydrogen spillover phenomenon on the Co loaded as-prepared CNTs was less significant than that on the Co loaded-activated CNTs. Owing to no significant defect over the structure, it was difficult for atomic hydrogen to migrate into the interplanar space of the as-prepared CNTs. The dissociated hydrogen only migrated from Co metal to the external surface of the CNTs. Conversely, hydrogen atoms easily diffused and migrated from external surface into interplanar spacing via the defect sites on the surface of the activated CNTs, causing hydrogen spillover enhancement, as illustrated in Fig. 13. Moreover, the interface between Co metal and CNT surface was considered to be an energy barrier that significantly affected atomic hydrogen diffusion from a metal site to other adsorption sites on CNT. Lachawiec et al. [37] reported that physical ‘‘bridges’’ were employed to improve the contact between two components, which enhanced hydrogen storage by hydrogen spillover. In this study, the Co loaded CNTs were prepared by using a wet impregnation method. Under TEM observation (see Fig. 2), it is clearly seen that Co metal particles contacted proximally on the surface of the CNTs. Accordingly, the problem of barrier between Co and CNT in this study could be reduced. 3.5. Hydrogen spillover properties An atom ratio of H to Co metal is a common method to describe the behavior of hydrogen spillover onto supports, is defined as H HT H0T ¼ M M
ð1Þ
where HT denotes the total H atoms uptake by CNTs with a loaded catalyst; H’T denotes the capacity of H atoms uptake by CNTs without catalyst; M denotes the amount of metal (such as Co) loaded onto the CNTs. The atom ratio between H and metal is employed not only to appear the behavior of hydrogen spillover but also to obtain the rela-
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
557
Table 2 Compassion of impregnated Co amount, hydrogen storage capacity, a ratio of H/Co and enhancement factor Sample
Co content (wt.%)
H2 capacity (wt.%)
A ratio of H/Co
Enhancement factor (g)
As-prepared CNT impregnation condition 0.025 M 2.02 ± 0.05 0.05 M 2.31 ± 0.20 0.1 M 5.23 ± 0.08 0.2 M 5.67 ± 0.02 0.5 M 14.62 ± 0.01
0.51 ± 0.08 0.51 ± 0.10 0.73 ± 0.07 1.11 ± 0.14 1.36 ± 0.03
0.69 4.84 3.91 4.62 2.58
0.97 1.42 1.76 2.14 2.54
Activated CNT-973 K impregnation condition 0.025 M 1.27 ± 0.03 0.05 M 1.64 ± 0.02 0.1 M 2.95 ± 0.17 0.2 M 4.41 ± 0.01 0.5 M 9.61 ± 0.15
1.84 ± 0.02 1.63 ± 0.09 1.94 ± 0.04 2.90 ± 0.02 3.41 ± 0.05
37.88 23.54 16.85 19.97 9.20
1.87 1.71 1.95 2.84 3.25
Activated CNT-873 K impregnation condition 0.2 M 4.63 ± 0.01
2.08 ± 0.17
–
–
Activated CNT-1073 K impregnation condition 0.2 M 5.30 ± 0.31
3.06 ± 0.11
–
–
Fig. 13. Schematic representation of hydrogen spillover onto the different structural characteristics of CNTs. (a) Hydrogen spillover over the asprepared CNTs; (b) hydrogen spillover over the activated CNTs.
tionship between the structural characteristics of CNTs and hydrogen spillover. If the ratio of H to Co metal is quantitatively greater than one, this implies that hydrogen spillover occurred onto the surface of the CNT. In this study, it was assumed that the weight change of heating/cooling cycles was attributed to hydrogen adsorption/desorption in the CNTs. Table 2 lists the atom ratio of two structural characteristics CNTs (as-prepared and
activated) along with different Co loaded content. Obviously, the as-prepared CNTs and the activated CNTs with varied Co contents revealed different atom ratio trends. The atom ratio of H to Co for the Co loaded as-prepared CNTs was effectively independent of the amount of Co loaded. However, it is also clearly found that the atom ratio ranged from 9 to 37 for the Co loaded-activated CNT, and decayed with increasing amount of Co loaded onto the CNTs. Indeed, hydrogen spillover took place over the Co loaded CNTs, since the value of atom ratio was larger than one. The dissimilarity of hydrogen spillover on the as-prepared CNTs and the activated CNTs was attributed to different structural characteristics. The defect structure of the activated CNTs obtained an excellent hydrogen spillover property, compared to the as-prepared CNTs with no significant defect structure. We believe that the presence of defects or cavities on the surface of CNT might be a path to improve hydrogen spillover into the interior of CNTs. The presence of defects or cavities on the surface of CNTs created and shortened the path of dissociating hydrogen atom diffusion. Consequently, hydrogen atoms more easily diffused from Co metal to interplanar space via defect sites. The capacity of hydrogen adsorption on the Co loadedactivated CNTs was strongly enhanced since the interplanar space stored hydrogen atoms effectively by hydrogen spillover. For the Co loaded-activated CNT, it was comprehended that the atom ratio decreased remarkably with increasing amount of Co deposited onto the CNTs. The saturation capacity of CNTs was assumed to be a constant in the equilibrium state. Each hydrogen source as Co metal site plays a role that dissociated hydrogen molecule to atom state, and hydrogen atoms migrated to other adsorption sites such as the surface of CNT. When suitable number of sources developed on the surface of CNTs, the produce rate of hydrogen atoms was improved, that enhanced atoms diffusion and adsorption onto the CNTs effectively.
558
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
Conversely, excessive sources cover the surface of the CNTs and block the adsorption sites, thus decreasing hydrogen storage in the CNTs. Lueking and Yang [9] proposed an enhancement factor (g) to describe the degree of hydrogen spillover onto carbon materials, which is defined as Q g ¼ c0 Qc
ð2Þ
where Qc denotes the capacity of H2 spillover and adsorption onto CNTs; Qc’ denotes the H2 capacity of CNTs without catalyst (such as Co). Since Qc cannot be obtained directly by experimental measurement, it was expressed: Qc ¼ QT QM
ð3Þ
where QT denotes the total H2 capacity of CNTs with a supported catalyst; QM denotes the capacity of H2 uptake by catalyst, assuming that each metal atom adsorbed one hydrogen atom (dissociation adsorption). The enhancement factor should be greater than one if hydrogen spillover took place onto the surface of material. In this study, the enhancement factor (g) was calculated for the as-prepared and the activated CNT samples with different amount of Co loaded, as collected in Table 2. The value of enhancement factor increased with increasing the amount of Co loaded onto the CNT samples. The enhancement factor of the activated CNTs was higher than that of the as-prepared CNT. For high Co loaded content (>5 wt.%), the enhancement factor of the activated CNTs was even larger than 3. The trend of enhancement factor for both CNT samples was similar, indicating that hydrogen spillover enhancement strongly depended on catalyst metal content. Robell et al. [38] reported that dissociated hydrogen diffused from catalyst into carbon surface, which is a slow process. This result indicated the surface diffusion is reaction control step about the spillover process. The center number of catalyst decided the number of spilt-over hydrogen on the surface of carbon. On the other hand, more catalyst centers enhanced hydrogen atoms concentration on the CNTs, caused by the capacity of atomic hydrogen into the CNTs was improved. It is an important problem how the atomic hydrogen was attracted by CNTs, physisorption or chemisorption. Kim et al. [11] reported that spiltover H atoms from Ni catalyst would be chemical bonding with the surfaces of MWNTs under 4 MPa at 300 K, and the C–Hn stretching vibration in the range of 2850–3300 cm1 was observed by FTIR studies of Ni dispersed MWNTs after hydrogenation. Roland et al. [39] developed that two possible states of dissociated hydrogen, ionized state and neutral state. Concerning hydrogen chemisorption, two possible ways were generally considered: strong chemisorption by ionized hydrogen and week chemisorption by neutral hydrogen. In our case, from the FTIR studies (data not shown), the absorbance between 2850 and 3300 cm1 with the C–Hn stretching vibration changed insignificantly before and after H2 adsorption for the Co loaded CNTs. However,
the presence of defects onto the CNTs was positively to improve the performance of hydrogen spillover and storage in the CNTs. More detailed studies needed to demonstrate in future works. 4. Conclusions In this study, the Co loaded CNT samples with the different structures were investigated to evaluate their hydrogen storage properties. All weight changes of CNTs during hydrogen adsorption/desorption were determined by a TGA heating/cooling cycles (303–673 K) in H2. The surface characteristics of CNTs might significantly control hydrogen spillover enhancement. From our results, we concluded several points as follow: 1. Both the specific surface area and pore volume of CNTs were increased by KOH activation, due to the defect structure formation. 2. Hydrogen spillover was available to prove hydrogen storage enhancement in CNTs. 3. The defect or cavity sites on the external surface of CNTs played an important role in promoting hydrogen atom migration from the external surface into the interplanar spacing between graphite layers. References [1] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [2] A. Chambers, C. Park, R.T.K. Baker, N.M. Rodriguez, J. Phys. Chem. B 102 (1998) 4253. [3] L. Zhou, Y. Zhou, Y. Sun, Int. J. Hydrogen Energ. 29 (2004) 475. [4] H. Takagi, H. Hatori, Y. Soneda, N. Yoshizawa, Y. Yamada, Mater. Sci. Eng. B 108 (2004) 143. [5] C. Zhang, X.S. Lu, A.Z. Gu, Int. J. Hydrogen Energ. 29 (2004) 1271. [6] L. Zhou, Y. Zhou, Y. Sun, Int. J. Hydrogen Energ. 31 (2006) 259. [7] A. Lueking, R.T. Yang, J. Catal. 206 (2002) 165. [8] A.D. Lueking, R.T. Yang, AIChE J. 49 (2003) 1556. [9] A.D. Lueking, R.T. Yang, Appl. Catal. A 265 (2004) 259. [10] F.H. Yang, R.T. Yang, Carbon 40 (2002) 437. [11] H.S. Kim, H. Lee, K.S. Han, J.H. Kim, M.S. Song, M.S. Park, J.Y. Lee, J.K. Kang, J. Phys. Chem. B 109 (2005) 8983. [12] A. Zielin˜ski, R. Wojcieszak, S. Monteverdi, M. Mercy, M.M. Bettahar, Catal. Commun. 6 (2005) 777. [13] R. Zacharia, K.Y. Kim, A.K.M.F. Kibria, K.S. Nahm, Chem. Phys. Lett. 412 (2005) 369. [14] E. Yoo, T. Habe, J. Nakamura, Sci. Technol. Adv. Mater. 6 (2005) 615. [15] S.C. Mu, H.L. Tang, S.H. Qian, M. Pan, R.Z. Yuan, Carbon 44 (2006) 762. [16] M.A. Lillo-Ro´denas, D. Cazorla-Amoro´s, A. Linares-Solano, Carbon 41 (2003) 267. [17] Q. Jiang, M.Z. Qu, B.L. Zhang, Z.L. Yu, Carbon 40 (2002) 2743. [18] E. Raymundo-Pin˜ero, D. Cazorla-Amoro´s, A. Linares-Solano, S. Delpeux, E. Frackowiak, K. Szostak, F. Be´guin, Carbon 40 (2002) 1614. [19] S.H. Yoon, S. Lim, Y. Song, Y. Ota, W. Qiao, A. Tanaka, I. Mochida, Carbon 42 (2004) 1723. [20] C.-H. Chen, C.-C. Huang, Int. J. Hydrogen Energ 32 (2007) 237. [21] A.Y. Khodakov, A. Griboval-Constant, R. Bechara, V.L. Zholobenko, J. Catal. 206 (2002) 230.
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559 [22] A.Y. Khodakov, R. Bechara, A. Griboval-Constant, Appl. Catal. A 254 (2003) 273. [23] R. Riva, H. Miessner, R. Vitali, G.D. Piero, Appl. Catal. A 196 (2000) 111. [24] D. Schanke, S. Vada, E.A. Blekkan, A.M. Hilmen, A. Hoff, A. Holmen, J. Catal. 156 (1995) 85. [25] A.Y. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier, B. Moisson, P. Chaumette, J. Catal. 168 (1997) 16. [26] S. Sun, N. Tsubaki, K. Fujimoto, Appl. Catal. A 202 (2000) 121. [27] L.M. Gandia, M. Montes, J. Catal. 145 (1994) 276. [28] N. Yoshizawa, K. Maruyama, Y. Yamada, E. Ishikawa, M. Kobayashi, Y. Toda, M. Shiraishi, Fuel 81 (2002) 1717. [29] T. Yang, A.C. Lua, Micropor. Mesopor. Mater. 63 (2003) 113. [30] Y.M. Shyu, F.C.N. Hong, Mater. Chem. Phys. 72 (2001) 223.
559
[31] Y.J. Yoon, J.C. Bae, H.K. Baik, S.J. Cho, S.J. Lee, K.M. Song, N.S. Myung, Chem. Phys. Lett. 366 (2002) 109. [32] C.J. Lee, J. Park, J.A. Yu, Chem. Phys. Lett. 360 (2002) 250. [33] P.X. Hou, S.T. Xu, Z. Ying, Q.H. Yang, C. Liu, H.M. Cheng, Carbon 41 (2003) 2471. [34] X. Li, H. Zhu, L. Ci, C. Xu, Z. Mao, B. Wei, J. Liang, D. Wu, Carbon 39 (2001) 2077. [35] W.C. Conner, J.L. Falconer, Chem. Rev. 95 (1995) 759. [36] J.T. Miller, B.L. Meyers, F.S. Modica, G.S. Lane, M. Vaarkamp, D.C. Koningsberger, J. Catal. 143 (1993) 395. [37] A.J. Lachawiec, G. Qi, R.T. Yang, Langmuir 21 (2005) 11418. [38] A.J. Robell, E.V. Ballou, M. Boudart, J. Phys. Chem. 68 (1964) 2748. [39] U. Roland, T. Braunschweig, F. Roessner, J. Mol. Catal. A: Chem. 127 (1997) 61.
Microporous and Mesoporous Materials 109 (2008) 549–559 www.elsevier.com/locate/micromeso
Enhancement of hydrogen spillover onto carbon nanotubes with defect feature Chien-Hung Chen a, Chen-Chia Huang a
b,*
Graduate School of Engineering Science and Technology, National Yunlin University of Science and Technology, Douliu, Yunlin 640, Taiwan, ROC b Department of Chemical Engineering, National Yunlin University of Science and Technology, Douliu, Yunlin 640, Taiwan, ROC Received 3 March 2007; received in revised form 21 May 2007; accepted 1 June 2007 Available online 12 June 2007
Abstract Hydrogen spillover is one of potential ways to enhance hydrogen storage in carbon nanotubes (CNTs). In this study, hydrogen spillover effect on hydrogen storage in cobalt (Co) metal loaded CNTs with different structural characteristics was investigated. The surface structure of CNTs was modified by KOH activation at temperature range from 873 K to 1073 K, in order to produce different defects of CNTs. The experimental results revealed that the specific surface area of the CNTs increased along with activation temperature from 467 m2/g (at 873 K) to 859 m2/g (at 1073 K). The higher surface area of CNTs was obtained by activating the higher temperature. Hydrogen storage capacity in the CNTs was determined by a dynamic thermogravimetric method with heating/cooling cycles between 303 K and 673 K under one atmosphere. Measurements showed the weight change during the heating/cooling cycles for the activated CNT samples was larger than the as-prepared ones. The enhancement of hydrogen storage in the activated CNTs was attributed to hydrogen spillover effect. The defective structure of the CNTs after activation is a significant factor to promote hydrogen spillover. Ó 2007 Elsevier Inc. All rights reserved. Keywords: Hydrogen storage; Carbon nanotubes; Activation; Spillover; Cobalt
1. Introduction Hydrogen is a clean, ideal energy source. In the past few years, several different technologies including compression, liquefaction, metal hydride, and adsorbent material have been developed for hydrogen storage. Among these technologies, the application of adsorbent material is safer and more efficient than any other technology in terms of controlled hydrogen storage. Recently, carbon nano-materials (such as nanotubes and nanofibers) have attracted attention as one of potential adsorbents for hydrogen storage. Dillon et al. [1] first found that single walled carbon nanotubes (SWCNTs) possess excellent hydrogen storage properties with gravimetric density of 5–10 wt.%. Cham*
Corresponding author. Tel.: +886 5 534 2601x4616; fax: +886 5 531 2071. E-mail address: [email protected] (C.-C. Huang). 1387-1811/$ - see front matter Ó 2007 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2007.06.003
bers et al. [2] claimed that herringbone type graphite nanofibers can adsorb hydrogen up to 67 wt.% at 100 atm and 300 K. Unfortunately, so far these storage capacities have been unable to be validated by other researchers. In recent publications [3–5], almost all experimental results indicated that hydrogen storage by carbon nano-materials was only 0.1–2.0 wt.%. Based on physisorption, Zhou et al. [3,6] even reported that carbon nanotubes (isosteric heat of adsorption 1.7 kJ/mol) were hopeless for hydrogen storage, compared to activated carbon (AX-21, isosteric heat of adsorption 6.4 kJ/mol). However, these dissimilarities may be caused either by the method of experimental analysis or some disparities in the structural characteristics of carbon nano-materials. Hydrogen spillover is an approach to be considered to enhance hydrogen storage in a carbon nano-structure. Lueking and Yang [7–9] presented a series of papers about hydrogen spillover onto metal catalyst (transitional metal such as Pt, Pd and Ni) supported carbon nanotubes
550
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
(CNTs) that enhanced hydrogen storage properties. Hydrogen molecules dissociate to atomic hydrogen on a metal catalyst and subsequently migrate from the metal to the surface of CNTs. Ab initio molecular orbital study [10] has indicated that atomic hydrogen adsorption was exothermic and stable on the graphite basal plane. Kim et al. [11] reported that hydrogen storage capacity of 2.8 wt.% was achieved by using Ni nanoparticle-dispersed multi-walled carbon nanotubes (MWCNTs). The results indicated that the Ni catalyst allowed hydrogen molecules to be dissociated and be adsorbed to form C–Hn bonding on the carbon structure. Hydrogen storage in activated carbon by hydrogen spillover also has been demonstrated by Zielin˜ski et al. [12]. They reported that the hydrogen spillover would be seen as driving force for hydrogen storage in carbonaceous materials. Furthermore, the spilt-over hydrogen onto the carbon was easily recombined at room temperature and pressure, due to the weak adsorption strength with carbon surface. However, the advantage of hydrogen spillover is that it not only improves hydrogen storage but also increases initial hydrogen adsorption kinetics [13]. In fact, the external surface, inner surface and interplanar spacing of CNTs are potential adsorption sites for hydrogen storage. Generally, the tube length of CNTs is several hundred nanometers, even over micrometers. The long path for hydrogen diffusion into interior of CNTs is a challenge, which controls the hydrogen storage capacity of CNTs. Etching the surface of CNT [14,15] may be a way to solve this problem of mass transfer resistance. Defects or cavities on CNTs surfaces not only represent point of the entry for hydrogen but also increase the surface area and pore volume of CNTs. KOH is one of activation reagents for preparation of activated carbons [16]. Recently, carbon nanotubes and nanofibers activated by KOH have also been reported [17–20]. In this study, we used cobalt to modify both the as-prepared CNTs and the activated CNTs, and compared hydrogen adsorption capacity on the same. The activated CNT samples were prepared by KOH activation at different temperatures to create variable degree of defects of the CNTs structure. The objectives of this work were: (1) to investigate characteristics of the defect structure of CNTs modified by KOH etching, and (2) to study hydrogen storage properties on the defect structure of CNTs.
for 1 h, stirred strongly at 373 K to become concentrated and then dried at 403 K for 12 h. Finally, the Co0.05Mg0.95O catalyst powder was obtained.
2. Experiments
The structure of CNTs was characterized by using a high resolution transmission electron microscope (HRTEM, JEOL, JEM-2010 type). The metal species on the CNTs was characterized by an X-ray photoelectron spectroscopy (XPS, Thermo VG, ESCAlab 250) and by an X-ray diffractometer (XRD, RIGAKU, D/MAX2200). The nitrogen adsorption isotherm for CNT samples was measured by a gas adsorption meter (QUANTACHROME, AUTOSORB-1) at 77 K. The specific surface area and pore size distribution were calculated by using the BET and the BJH methods from N2 isotherm adsorp-
2.1. Catalyst preparation In this study, Co/MgO functioned as the catalyst to synthesize the MWCNTs was prepared by using impregnation method. The molar ratio of Co/Mg was 0.05/0.95. Preparation procedure of the catalyst was as the follows: Co(NO3)2 Æ 6H2O was dissolved in a solution that was prepared by mixing the ethanol solution with magnesium oxide (MgO) powder. Next, the Co/MgO solution was sonicated
2.2. Synthesis of MWCNTs MWCNTs were synthesized by catalytic decomposition of acetylene over Co0.05Mg0.95O catalyst. The prepared Co0.05Mg0.95O powders were loaded into the center of quartz reactor tube in a flowing of H2, and then heated up to 973 K for 30 min. On reaching this temperature, the flow of C2H2/H2 (30/70) mixture was introduced into the quartz reactor for 30 min. The total flow rate was 100 ml/min. The raw MWCNT samples were obtained after cooling down to room temperature. The raw samples were immersed in 37% HCl solution to dissolve impurities such as the metal catalyst. After the samples were filtered, they were washed in de-ionized water and dried at 373 K in the air. CNTs with a high purity (>90%) were obtained and referred to as the as-prepared CNTs. 2.3. KOH activation of CNTs In this study, activated CNTs were prepared by KOH activation. The ratio of KOH and CNT was fixed at 4:1 (g/g). The activation was conducted at 873 K, 973 K, or 1073 K for 1 h under nitrogen flow of 500 ml/min. The activated products were washed with diluted HCl solution to remove potassium species. After filtration the samples were dried at 378 K. The CNTs after KOH activation are referred to as the activated CNTs. 2.4. Co loaded CNTs Both the as-prepared CNTs and the activated CNTs were impregnated with cobalt nitrate using a wet impregnation method. The CNT (100 mg) samples were immersed in 100 ml of different concentration (0.025–0.5 M) Co(NO3)2 solutions, and sonicated for 1 h to ensure the CNT powders dispersedly in the solution. They were shook with 100 rpm rotation rate at 303 K for 24 h. Subsequently, the Co loaded CNT samples were obtained after filtering and drying at 378 K for 24 h. 2.5. Material characterization
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
551
tion/desorption curves, respectively. The amount of Co deposited onto the CNT samples was analyzed by using an atomic absorption spectroscopy (AAS) after the impregnation process. The Fourier transform infrared (FTIR) studies of CNTs before and after H2 adsorption were performed by a spectrometer (FTIR-RAS, Jasco FT/IR-4200 Spectrometer) with a liquid nitrogen cooled MCT-M detector at a resolution of 4 cm1. The CNT samples were prepared by spraying CNTs powders with acetone solution onto the silicon wafer. The acetone was evaporated at room temperature in air. The background spectrum was performed by a clean silicon wafer. A minimum of 1000 scans were collected. 2.6. Hydrogen storage measurement Hydrogen adsorption/desorption experiments were performed using a thermogravimetric analyzer (TG, Cahn, TG2121) with a vacuum system. In this study, ‘ultrahigh purity grade’ hydrogen (99.9995%) was used through a moisture trap (packed SICAPENT powder, Merck) to eliminate moisture impurities. Ten milligrams of CNT samples were loaded into a quartz sample holder in a close TG system. Before performing hydrogen uptake measurements, the TG system was evacuated in order to eliminate possible contamination within the system. Then, hydrogen was introduced into the TG system to pump pressure back to ambient condition. A flowing of hydrogen (20 ml/min) was maintained to purge the system for several hours in order to ensure its cleanliness and weight stability in the TG system. For hydrogen adsorption/desorption measurement, we adopted two-step thermal programs and they were carried out as follows: (1) The CNTs samples were initially heated up to 673 K at a rate of 10 K/min, and then held for 1 h in a flowing of H2 at 20 ml/min. Afterwards, it was cooled down to room temperature until constant weight. The cobalt loaded reduction in the CNTs was performed in this pretreatment. (2) The temperature was heated again from 303 K to 673 K at a heating rate of 5 K/min (desorption curve), and then cooled down to 303 K (adsorption curve). In order to obtain accurate values of hydrogen uptake, the buoyancy effect was taken into consideration during heating/cooling. All weight changes with respect to adsorption/desorption data were corrected using a quartz blank calibration [7]. All hydrogen storage experiments were repeated at least once. 3. Results and discussion 3.1. Material characterization 3.1.1. HRTEM observation Fig. 1 shows the HRTEM micrographs of the as-prepared CNT and the activated CNT samples. From HRTEM observation, most of catalyst particles were removed from the CNT after the purification process. The caps of CNTs were opened. However, a small residual
Fig. 1. TEM micrographics of CNT: (a) the as-prepared CNT and (b) the activated-CNT prepared by KOH activation at 973 K.
cobalt catalyst on the CNT samples was confirmed by EDS analysis (data not shown). The diameters of tubes ranged from 5 nm to 15 nm, and length was in a range of several hundred nanometers. Further, the structure of the as-prepared CNTs was observed with no significant defect, as shown in Fig. 1a. In this study, the activated CNTs with defect structure were prepared by KOH activation under various temperature conditions (873–1073 K). As being activated at 973 K, massive defects produced on the surface of the CNTs, as presented in Fig. 1b. More details about the CNTs activation were discussed in the next section.
552
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
For the Co loaded as-prepared CNT, Co metal particles size of 10 nm distributed over the external surface of CNT samples were observed in Fig. 2. 3.1.2. X-ray photoelectron spectroscopy (XPS) The Co 2p XPS spectra of Co loaded CNTs before and after H2 reduction are presented in Fig. 3. For both Co
Fig. 2. TEM micrographics of the Co loaded as-prepared CNT with 14.62 wt.% Co.
loaded the as-prepared and the activated CNTs before H2 reduction, the XPS Co 2p3/2 peak with subsidiary peak was observed and the position of Co2p3/2 peak shifted to the region of higher binding energy. The spectra indicated that some concentration of isolated Co2+ species distributed onto the Co loaded CNTs [21,22] after wet impregnation. After H2 reduction at 673 K, the individual peak of XPS Co 2P3/2 at 780.6–780.9 ev of the binding energy was found. Further the difference in binding energy (DE) between peaks of Co 2P3/2 and Co 2P1/2 was calculated as 15 ev. The results indicated that Co3O4 is dominant cobalt phase on the CNTs [23]. Khodakov et al. [21] reported that Co3O4 transited to CoO at a range of 500–600 K, and CoO phase reduced to metallic Co at a range of 600–700 K, demonstrated by the weight loss of TGA. Schanke et al. [24] also identified the reduction of Co3O4 by TPR studies. The result indicated that the two-step reduction of Co3O4 (via CoO) to metallic Co was found at 592 K and 640 K. The similar studies about Co species reduction have been reported in previous reports [25,26]. In this study, the presence of Co3O4 and metallic Co phases onto the CNTs after H2 reduction at 673 K were found by XRD analysis (data no shown), similar results reported by Gandia et al. [27]. And, the XPS spectra exhibited that Co species onto the CNTs was Co3O4 phase after H2 reduction. The presence of Co3O4 phase was attributed to the metallic Co re-oxidized by passivation at room temperature under air atmosphere [27]. 3.1.3. Thermogravimetric analysis (TGA) Fig. 4 shows the differential thermogravimetry (DTG) curves of bulk cobalt nitrate, the Co loaded as-prepared CNT and the Co loaded activated CNTs. As shown in Fig. 4a, the significant weight loss peak was obtained at 502 K. Comparison of the weight loss ratio, the ratio of Wfinal (the final weight after reduction) to Winitial (the initial weight before reduction) was 0.204, which was similar to the molecular weight ratio of 0.203 between Co and Co(NO3)2 Æ 6H2O. The result implied the cobalt nitrate completed reduction to metallic Co at 502 K in H2. Furthermore, two major peaks were found for each Co loaded CNT samples. Weight loss of left peaks was usually larger than that of right peak. According to the DTG curves, XRD analysis and previous reports [21,22,24–26], it is suggested that cobalt nitrate and small amount of Co3O4 deposited onto the surface of CNTs after the impregnation process. The weight loss of left peak was attributed as cobalt nitrate ! metallic Co, and the weight loss of right peak was as Co3O4 (via CoO) ! Co. 3.2. CNTs activation
Fig. 3. XPS Co 2p spectra of Co loaded CNTs before and after H2 reduction at 673 K for 1 h. (a) Co loaded as-prepared CNTs; (b) Co loaded activated-CNTs.
The comparison of activation temperature effect on specific surface area and yield for CNT samples is presented in Fig. 5. The specific surface areas of CNT at 873, 973 and 1073 K activation were 467, 764 and 859 m2/g, respectively, higher than that of the as-prepared CNT (453 m2/g). The
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
Fig. 4. DTG curves of (a) bulk cobalt nitrate, (b) Co loaded as-prepared CNTs and (c) Co loaded activated-CNTs. Operation conditions: heated up to 673 K at a rate of 10 K/min in H2, and then held at 673 K for l h.
553
after KOH activation implied that defects on the graphite structure were formed by carbonaceous gasification. Additionally, excessive defect sites on the graphite walls of the CNTs caused structure collapse and decreased yield after activation process. Some previous papers [16,28,29] have reported that K2CO3, C–O–K metal salts and metallic K were considered as products after the activation of KOH in a N2 or Ar atmosphere. Both CO and CO2 were detected in the effluent during the activation process. Fig. 6 shows the pore distribution of the as-prepared CNTs and the activated CNTs prepared at different temperatures. It is found that pore volume of the CNT after KOH activation increased remarkably, particular for pore ˚ . In KOH activation at 873 K and diameters less than 100 A 973 K, the main pore volume increment was in the range of ˚ . When the CNTs were activated at 1073 K, the 20–30 A ˚ . The ranges of pore enhancement shifted to 40–100 A increase of pore volume was ascribed to defect or cavity sites formed onto the external graphite walls. At higher activation temperature, the defects or cavities on the CNTs became deeper and wider from the exterior to the interior of graphite layers. The collapse of two or more small defect sites to form a large defect size was possible in excessive activation, such as the CNTs activated at 1073 K. Parts of carbon molecules of CNTs were even decomposed (burnt off) by KOH attack. Raman spectroscopy is an important tool to provide information about the crystal structure and the presence of disorder of the CNTs. In the Raman spectra, two main peaks were observed in the high frequency region, at 1580 cm1 (G band) and 1350 cm1 (D band). The G band indicates an original graphitic structure, and the D band reflects the presence of either disordering feature or amorphous carbon [30–32]. The intensity ratio of D band to G band was employed to characterize the feature of
Fig. 5. Effect of activation temperature on specific surface area and yield of the activated CNT.
specific surface area of the activated CNT obviously increased with higher activation temperature. Obviously, the yield of the CNT decreased remarkably at higher activation temperature. The incremental surface area of CNTs
Fig. 6. Pore size distribution of the as-prepared CNT and the activated CNTs under different activation temperatures.
554
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
CNTs. In this study, we adopted the intensity ratio of D band to G band (ID/IG) to analyze the feature of structure change of CNTs after KOH activation with various temperatures (873–1073 K). Fig. 7 reveals the ID/IG ratio of the as-prepared CNTs and the activated CNTs at variable temperature. Overall, the ratio of ID/IG of the activated CNTs was higher than that of the as-prepared CNTs.
Fig. 7. The ID/IG ratio of the as-prepared and the activated CNTs with different activation temperature.
The result indicated that the graphitic structure of CNTs certainly became disordered after KOH activation. It is interesting that the ratio of ID/IG of the activated CNTs at 873 K activation was larger than that of other activation condition (973 K and 1073 K). The cause was attributed that the disordered structure of the external surface of CNTs decomposed at the high temperature. On the other hands, the defect structure of CNTs was produced after KOH activation, since the disordered carbon material from graphite sheets transforming was decomposed. This suggestion can explain that the pore volume and the specific surface area of activated CNTs enhanced with increasing the activation temperature. Based on the above discussion, we proposed a possible activation mechanism of CNTs by KOH activation, as shown in Fig. 8. The produced defects on the external surface of CNTs were developed by two steps: disordering and destroying. When CNTs activated at low temperature, the graphite sheets of CNTs became disordered structure by KOH attacking gradually. No significant defects produced on the external surface of CNTs, because the disordered carbons have only a little amount of decomposition. As the activation condition at high temperature, more disordered carbons formed and graphite sheet decomposed simultaneously. Then, significant defect sites produced on the external of graphite layers of CNTs. Moreover, if the excessive activation performed, the CNTs defect either became larger or structure destroyed completely.
Fig. 8. Schematic representation of CNTs activation mechanism.
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
3.3. Effect of residual metal and structure of CNTs In order to clear the effect of residual metal (Co) and structure of CNT on hydrogen storage capacity, concentrate HNO3 solution was used to treat CNT samples (including the as-prepared and the activated CNTs) to further eliminate residual Co catalyst. The amounts of residual Co onto the as-prepared and the activated CNT samples are collected in Table 1. The weight changes of the as-prepared and the activated CNTs before and after 5 N HNO3 washing upon the heating/cooling cycles, are shown in Fig. 9. The amount of residual Co onto the asprepared CNTs was larger than that onto the activated CNT samples. However, the weight change of the as-prepared CNTs was only 0.22 ± 0.07 wt.%. The weight change of the activated CNTs increased along with activation temperature. Even without Co residues (after HNO3 wash), the high temperature (973 K or 1073 K) activated CNTs still possessed more than 1.0 wt.% of weight change. The causes of hydrogen storage improvement into the activated CNT samples are attributed to the following. (1) The edge carbon was created around defect sites of graphite walls on CNT, produced during activation treatment. Consequently, hydrogen was possibly chemisorbed with edge carbon [10]. (2) The high surface area of the activated CNT implied the presence of defects or cavities on the graphite walls, as shown in Fig. 1b. The defect sites
Table 1 Amount of residual Co onto the CNT samples Samples
Residual Co (wt.%)
As-prepared 873 K activation 973 K activation 1073 K activation
1.02 0.16 0.09 0.15
Fig. 9. Weight changes of the activated CNT samples before and after 5 N HNO3 washing upon the heating/cooling cycles.
555
on the structure provided the entry of hydrogen spreading into the interplanar spacing, which enhanced hydrogen storage into CNTs [33,34]. 3.4. Effect of Co loaded onto CNTs The hydrogen adsorption/desorption curves of the Co loaded CNTs are shown in Fig. 10. All adsorption/desorption curves were corrected by the blank experiment under the same thermal programs. The weigh change of CNTs was determined by the weight differential upon the heating-cooling cycles. The Co loaded-activated CNTs possessed the highest weight change compared to other samples. Additionally, both the Co loaded as-prepared CNTs and the Co loaded-activated CNTs had similar adsorption/desorption behavior. It is observed that weight increased sharply below 400 K in the adsorption curve. Two successive hydrogen adsorption/desorption curves on the Co loaded CNT samples are presented in Fig. 11. As shown in Fig. 11, the adsorption/desorption experiment is well reproducible. The results implied that hydrogen storage on the Co loaded CNTs is reversible. The weight changes of the Co loaded CNT samples upon the heating/cooling cycles are presented in Fig. 12 and Table 2. The weight change increased along with the amount of Co loaded onto the CNTs. Obviously, the different structural characteristics of CNTs possessed remarkably different capabilities of hydrogen uptake. Under similar Co loaded content, the weight change of the Co loaded as-prepared CNT was significantly smaller than that of the Co loaded-activated CNT. The maximum weight changes of the Co loaded-activated CNT (activated at 973 K) and of the Co loaded as-prepared CNT were 3.41 ± 0.05 wt.% and 1.36 ± 0.03 wt.%, respectively.
Fig. 10. TG profiles of the adsorption/desorption cycle. (a) The asprepared CNT, (b) the Co loaded as-prepared CNT with 5.67 wt.% Co, (c) the Co loaded-activated CNT with 4.41 wt.% Co.
556
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
Fig. 11. TG profiles of two successive adsorption/desorption cycles of the Co loaded activated CNTs.
Fig. 12. Weight changes of the different Co content loaded CNT samples upon the heating/cooling cycles.
However, the weight change of the CNT without Co impregnation was only 0.22 ± 0.07 wt.%. Obviously, Co at active site played an important role in promoting hydrogen adsorption. As a catalyst, the loaded Co (metallic state) dissociated hydrogen molecules [25,35]. The hydrogen atoms then diffused to the surface of CNTs and adsorbed onto the specific sites of CNTs. This phenomenon is called ‘‘hydrogen spillover’’ by previous literatures [7,9,11,35,36]. Mu et al. [15] developed the hydrogen storage capacity of 4.5 wt.% using Pd decorated CNTs with defect structure under 10.7 MPa at room temperature. Yoo et al. [14] reported that the hydrogen capacity of 1.5 wt.% was
received by defective CNT with Pd under 1 atm and at 573 K. In present work, the largest hydrogen capacity of 3.41 ± 0.05 wt.% for Co loaded activated CNTs was obtained under 1 atm and at moderate temperature, that revealed similar results by previous literature [14,15]. The presence of defect structure onto CNT was demonstrated to be a remarkable factor for enhancing hydrogen storage by hydrogen spillover. The external surface, inner surface and interplanar spacing of CNTs are available for hydrogen storage. The advantage of hydrogen spillover applied in hydrogen storage of CNTs as follows: the dissociated hydrogen (the radius of hydrogen atom with the radius of van der Waals ˚ ) can be easily diffused into the interplanar is about 0.46 A ˚ space (3.4 A) of CNTs, compared with hydrogen molecule ˚ ). Therefore, hydrogen spillover (a kinetic diameter 2.89 A behavior was considered as a possible way to improve hydrogen storage in the CNTs. In this study, it is interesting that the hydrogen spillover phenomenon on the Co loaded as-prepared CNTs was less significant than that on the Co loaded-activated CNTs. Owing to no significant defect over the structure, it was difficult for atomic hydrogen to migrate into the interplanar space of the as-prepared CNTs. The dissociated hydrogen only migrated from Co metal to the external surface of the CNTs. Conversely, hydrogen atoms easily diffused and migrated from external surface into interplanar spacing via the defect sites on the surface of the activated CNTs, causing hydrogen spillover enhancement, as illustrated in Fig. 13. Moreover, the interface between Co metal and CNT surface was considered to be an energy barrier that significantly affected atomic hydrogen diffusion from a metal site to other adsorption sites on CNT. Lachawiec et al. [37] reported that physical ‘‘bridges’’ were employed to improve the contact between two components, which enhanced hydrogen storage by hydrogen spillover. In this study, the Co loaded CNTs were prepared by using a wet impregnation method. Under TEM observation (see Fig. 2), it is clearly seen that Co metal particles contacted proximally on the surface of the CNTs. Accordingly, the problem of barrier between Co and CNT in this study could be reduced. 3.5. Hydrogen spillover properties An atom ratio of H to Co metal is a common method to describe the behavior of hydrogen spillover onto supports, is defined as H HT H0T ¼ M M
ð1Þ
where HT denotes the total H atoms uptake by CNTs with a loaded catalyst; H’T denotes the capacity of H atoms uptake by CNTs without catalyst; M denotes the amount of metal (such as Co) loaded onto the CNTs. The atom ratio between H and metal is employed not only to appear the behavior of hydrogen spillover but also to obtain the rela-
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
557
Table 2 Compassion of impregnated Co amount, hydrogen storage capacity, a ratio of H/Co and enhancement factor Sample
Co content (wt.%)
H2 capacity (wt.%)
A ratio of H/Co
Enhancement factor (g)
As-prepared CNT impregnation condition 0.025 M 2.02 ± 0.05 0.05 M 2.31 ± 0.20 0.1 M 5.23 ± 0.08 0.2 M 5.67 ± 0.02 0.5 M 14.62 ± 0.01
0.51 ± 0.08 0.51 ± 0.10 0.73 ± 0.07 1.11 ± 0.14 1.36 ± 0.03
0.69 4.84 3.91 4.62 2.58
0.97 1.42 1.76 2.14 2.54
Activated CNT-973 K impregnation condition 0.025 M 1.27 ± 0.03 0.05 M 1.64 ± 0.02 0.1 M 2.95 ± 0.17 0.2 M 4.41 ± 0.01 0.5 M 9.61 ± 0.15
1.84 ± 0.02 1.63 ± 0.09 1.94 ± 0.04 2.90 ± 0.02 3.41 ± 0.05
37.88 23.54 16.85 19.97 9.20
1.87 1.71 1.95 2.84 3.25
Activated CNT-873 K impregnation condition 0.2 M 4.63 ± 0.01
2.08 ± 0.17
–
–
Activated CNT-1073 K impregnation condition 0.2 M 5.30 ± 0.31
3.06 ± 0.11
–
–
Fig. 13. Schematic representation of hydrogen spillover onto the different structural characteristics of CNTs. (a) Hydrogen spillover over the asprepared CNTs; (b) hydrogen spillover over the activated CNTs.
tionship between the structural characteristics of CNTs and hydrogen spillover. If the ratio of H to Co metal is quantitatively greater than one, this implies that hydrogen spillover occurred onto the surface of the CNT. In this study, it was assumed that the weight change of heating/cooling cycles was attributed to hydrogen adsorption/desorption in the CNTs. Table 2 lists the atom ratio of two structural characteristics CNTs (as-prepared and
activated) along with different Co loaded content. Obviously, the as-prepared CNTs and the activated CNTs with varied Co contents revealed different atom ratio trends. The atom ratio of H to Co for the Co loaded as-prepared CNTs was effectively independent of the amount of Co loaded. However, it is also clearly found that the atom ratio ranged from 9 to 37 for the Co loaded-activated CNT, and decayed with increasing amount of Co loaded onto the CNTs. Indeed, hydrogen spillover took place over the Co loaded CNTs, since the value of atom ratio was larger than one. The dissimilarity of hydrogen spillover on the as-prepared CNTs and the activated CNTs was attributed to different structural characteristics. The defect structure of the activated CNTs obtained an excellent hydrogen spillover property, compared to the as-prepared CNTs with no significant defect structure. We believe that the presence of defects or cavities on the surface of CNT might be a path to improve hydrogen spillover into the interior of CNTs. The presence of defects or cavities on the surface of CNTs created and shortened the path of dissociating hydrogen atom diffusion. Consequently, hydrogen atoms more easily diffused from Co metal to interplanar space via defect sites. The capacity of hydrogen adsorption on the Co loadedactivated CNTs was strongly enhanced since the interplanar space stored hydrogen atoms effectively by hydrogen spillover. For the Co loaded-activated CNT, it was comprehended that the atom ratio decreased remarkably with increasing amount of Co deposited onto the CNTs. The saturation capacity of CNTs was assumed to be a constant in the equilibrium state. Each hydrogen source as Co metal site plays a role that dissociated hydrogen molecule to atom state, and hydrogen atoms migrated to other adsorption sites such as the surface of CNT. When suitable number of sources developed on the surface of CNTs, the produce rate of hydrogen atoms was improved, that enhanced atoms diffusion and adsorption onto the CNTs effectively.
558
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559
Conversely, excessive sources cover the surface of the CNTs and block the adsorption sites, thus decreasing hydrogen storage in the CNTs. Lueking and Yang [9] proposed an enhancement factor (g) to describe the degree of hydrogen spillover onto carbon materials, which is defined as Q g ¼ c0 Qc
ð2Þ
where Qc denotes the capacity of H2 spillover and adsorption onto CNTs; Qc’ denotes the H2 capacity of CNTs without catalyst (such as Co). Since Qc cannot be obtained directly by experimental measurement, it was expressed: Qc ¼ QT QM
ð3Þ
where QT denotes the total H2 capacity of CNTs with a supported catalyst; QM denotes the capacity of H2 uptake by catalyst, assuming that each metal atom adsorbed one hydrogen atom (dissociation adsorption). The enhancement factor should be greater than one if hydrogen spillover took place onto the surface of material. In this study, the enhancement factor (g) was calculated for the as-prepared and the activated CNT samples with different amount of Co loaded, as collected in Table 2. The value of enhancement factor increased with increasing the amount of Co loaded onto the CNT samples. The enhancement factor of the activated CNTs was higher than that of the as-prepared CNT. For high Co loaded content (>5 wt.%), the enhancement factor of the activated CNTs was even larger than 3. The trend of enhancement factor for both CNT samples was similar, indicating that hydrogen spillover enhancement strongly depended on catalyst metal content. Robell et al. [38] reported that dissociated hydrogen diffused from catalyst into carbon surface, which is a slow process. This result indicated the surface diffusion is reaction control step about the spillover process. The center number of catalyst decided the number of spilt-over hydrogen on the surface of carbon. On the other hand, more catalyst centers enhanced hydrogen atoms concentration on the CNTs, caused by the capacity of atomic hydrogen into the CNTs was improved. It is an important problem how the atomic hydrogen was attracted by CNTs, physisorption or chemisorption. Kim et al. [11] reported that spiltover H atoms from Ni catalyst would be chemical bonding with the surfaces of MWNTs under 4 MPa at 300 K, and the C–Hn stretching vibration in the range of 2850–3300 cm1 was observed by FTIR studies of Ni dispersed MWNTs after hydrogenation. Roland et al. [39] developed that two possible states of dissociated hydrogen, ionized state and neutral state. Concerning hydrogen chemisorption, two possible ways were generally considered: strong chemisorption by ionized hydrogen and week chemisorption by neutral hydrogen. In our case, from the FTIR studies (data not shown), the absorbance between 2850 and 3300 cm1 with the C–Hn stretching vibration changed insignificantly before and after H2 adsorption for the Co loaded CNTs. However,
the presence of defects onto the CNTs was positively to improve the performance of hydrogen spillover and storage in the CNTs. More detailed studies needed to demonstrate in future works. 4. Conclusions In this study, the Co loaded CNT samples with the different structures were investigated to evaluate their hydrogen storage properties. All weight changes of CNTs during hydrogen adsorption/desorption were determined by a TGA heating/cooling cycles (303–673 K) in H2. The surface characteristics of CNTs might significantly control hydrogen spillover enhancement. From our results, we concluded several points as follow: 1. Both the specific surface area and pore volume of CNTs were increased by KOH activation, due to the defect structure formation. 2. Hydrogen spillover was available to prove hydrogen storage enhancement in CNTs. 3. The defect or cavity sites on the external surface of CNTs played an important role in promoting hydrogen atom migration from the external surface into the interplanar spacing between graphite layers. References [1] A.C. Dillon, K.M. Jones, T.A. Bekkedahl, C.H. Kiang, D.S. Bethune, M.J. Heben, Nature 386 (1997) 377. [2] A. Chambers, C. Park, R.T.K. Baker, N.M. Rodriguez, J. Phys. Chem. B 102 (1998) 4253. [3] L. Zhou, Y. Zhou, Y. Sun, Int. J. Hydrogen Energ. 29 (2004) 475. [4] H. Takagi, H. Hatori, Y. Soneda, N. Yoshizawa, Y. Yamada, Mater. Sci. Eng. B 108 (2004) 143. [5] C. Zhang, X.S. Lu, A.Z. Gu, Int. J. Hydrogen Energ. 29 (2004) 1271. [6] L. Zhou, Y. Zhou, Y. Sun, Int. J. Hydrogen Energ. 31 (2006) 259. [7] A. Lueking, R.T. Yang, J. Catal. 206 (2002) 165. [8] A.D. Lueking, R.T. Yang, AIChE J. 49 (2003) 1556. [9] A.D. Lueking, R.T. Yang, Appl. Catal. A 265 (2004) 259. [10] F.H. Yang, R.T. Yang, Carbon 40 (2002) 437. [11] H.S. Kim, H. Lee, K.S. Han, J.H. Kim, M.S. Song, M.S. Park, J.Y. Lee, J.K. Kang, J. Phys. Chem. B 109 (2005) 8983. [12] A. Zielin˜ski, R. Wojcieszak, S. Monteverdi, M. Mercy, M.M. Bettahar, Catal. Commun. 6 (2005) 777. [13] R. Zacharia, K.Y. Kim, A.K.M.F. Kibria, K.S. Nahm, Chem. Phys. Lett. 412 (2005) 369. [14] E. Yoo, T. Habe, J. Nakamura, Sci. Technol. Adv. Mater. 6 (2005) 615. [15] S.C. Mu, H.L. Tang, S.H. Qian, M. Pan, R.Z. Yuan, Carbon 44 (2006) 762. [16] M.A. Lillo-Ro´denas, D. Cazorla-Amoro´s, A. Linares-Solano, Carbon 41 (2003) 267. [17] Q. Jiang, M.Z. Qu, B.L. Zhang, Z.L. Yu, Carbon 40 (2002) 2743. [18] E. Raymundo-Pin˜ero, D. Cazorla-Amoro´s, A. Linares-Solano, S. Delpeux, E. Frackowiak, K. Szostak, F. Be´guin, Carbon 40 (2002) 1614. [19] S.H. Yoon, S. Lim, Y. Song, Y. Ota, W. Qiao, A. Tanaka, I. Mochida, Carbon 42 (2004) 1723. [20] C.-H. Chen, C.-C. Huang, Int. J. Hydrogen Energ 32 (2007) 237. [21] A.Y. Khodakov, A. Griboval-Constant, R. Bechara, V.L. Zholobenko, J. Catal. 206 (2002) 230.
C.-H. Chen, C.-C. Huang / Microporous and Mesoporous Materials 109 (2008) 549–559 [22] A.Y. Khodakov, R. Bechara, A. Griboval-Constant, Appl. Catal. A 254 (2003) 273. [23] R. Riva, H. Miessner, R. Vitali, G.D. Piero, Appl. Catal. A 196 (2000) 111. [24] D. Schanke, S. Vada, E.A. Blekkan, A.M. Hilmen, A. Hoff, A. Holmen, J. Catal. 156 (1995) 85. [25] A.Y. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier, B. Moisson, P. Chaumette, J. Catal. 168 (1997) 16. [26] S. Sun, N. Tsubaki, K. Fujimoto, Appl. Catal. A 202 (2000) 121. [27] L.M. Gandia, M. Montes, J. Catal. 145 (1994) 276. [28] N. Yoshizawa, K. Maruyama, Y. Yamada, E. Ishikawa, M. Kobayashi, Y. Toda, M. Shiraishi, Fuel 81 (2002) 1717. [29] T. Yang, A.C. Lua, Micropor. Mesopor. Mater. 63 (2003) 113. [30] Y.M. Shyu, F.C.N. Hong, Mater. Chem. Phys. 72 (2001) 223.
559
[31] Y.J. Yoon, J.C. Bae, H.K. Baik, S.J. Cho, S.J. Lee, K.M. Song, N.S. Myung, Chem. Phys. Lett. 366 (2002) 109. [32] C.J. Lee, J. Park, J.A. Yu, Chem. Phys. Lett. 360 (2002) 250. [33] P.X. Hou, S.T. Xu, Z. Ying, Q.H. Yang, C. Liu, H.M. Cheng, Carbon 41 (2003) 2471. [34] X. Li, H. Zhu, L. Ci, C. Xu, Z. Mao, B. Wei, J. Liang, D. Wu, Carbon 39 (2001) 2077. [35] W.C. Conner, J.L. Falconer, Chem. Rev. 95 (1995) 759. [36] J.T. Miller, B.L. Meyers, F.S. Modica, G.S. Lane, M. Vaarkamp, D.C. Koningsberger, J. Catal. 143 (1993) 395. [37] A.J. Lachawiec, G. Qi, R.T. Yang, Langmuir 21 (2005) 11418. [38] A.J. Robell, E.V. Ballou, M. Boudart, J. Phys. Chem. 68 (1964) 2748. [39] U. Roland, T. Braunschweig, F. Roessner, J. Mol. Catal. A: Chem. 127 (1997) 61.