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Hydrogen adsorption properties of activated carbons with modified surfaces
Journal of Alloys and Compounds 385 (2004) 257–263
Hydrogen adsorption properties of activated carbons with modified surfaces H. Takagi a,∗ , H. Hatori a , Y. Yamada b , S. Matsuo c , M. Shiraishi c a
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan b Faculty of Engineering, Fukui University, 3-9-1 Bunkyou, Fukui 910-8507, Japan c School of High-Technology for Human Welfare, Tokai University, 317 Nishino, Numazu, Shizuoka 410-0395, Japan Received 26 February 2004; accepted 6 March 2004
Abstract The hydrogen adsorption isotherms of activated carbon fibers loaded with platinum (Pt-ACF) and palladium (Pd-ACF) were accurately measured at ambient temperature over the hydrogen pressure range 0–3.5 MPa by using a high-pressure adsorption apparatus. The amounts of hydrogen adsorbed on the Pt-ACF and Pd-ACF samples were larger than the amount adsorbed on the unmodified ACF sample. Detailed measurement of the hydrogen adsorption isotherms of Pt-ACF and Pd-ACF at a hydrogen pressure up to 0.1 MPa indicated that the increase in adsorption was due to chemisorption of hydrogen on the metal. The hydrogen adsorption properties of ACF oxidized with (NH4 )S2 O8 or reduced in a hydrogen flow at 1073 K were evaluated at 77 and 303 K over the hydrogen pressure range 0–3.5 MPa. The amount of adsorbed hydrogen was decreased by oxidation and increased by reduction. This result is attributable to a change in the pore structure: that is, oxygen functional groups introduced on the carbon surface inhibited hydrogen adsorption into the micropore with an optimum pore diameter. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen adsorption property; Activated carbons; Noble metals; Functional groups
1. Introduction Hydrogen is an ideal substitute for fossil fuels, and a number of research groups have recently reported the hydrogen storage capacity of carbon materials. The high hydrogen storage capacities of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been demonstrated experimentally [1–5]. However, several groups have claimed that the storage capacity of CNTs and CNFs at ambient temperature and high pressure (10 MPa) is less than 0.7 wt.%, and is smaller than the capacity of activated carbon (AC) [6–8]. This discrepancy among the reports is considered to be due to the difficulty of evaluating storage capacity. In addition, the mechanism of hydrogen storage, and the interaction between the carbon surface and hydrogen are not adequately understood. To clarify the mechanism of hydrogen storage in carbon materials and to design an appropriate structure as storage materials, we must accurately analyze
∗ Corresponding author. Tel.: +81-29-861-8298; fax: +81-29-861-8408. E-mail address: [email protected] (H. Takagi).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.139
the hydrogen storage properties by using well-characterized samples. The metal catalysts resided in CNTs and CNFs significantly influence hydrogen storage capacities. It was reported that CNT and CNF samples containing metal catalysts exhibited high capacities at ambient temperature, and the capacities decreased upon removal of the metals [3,4,9]. The reports claim that the high hydrogen storage capacity can be explained by the spillover phenomenon of hydrogen atoms dissociated on the metal (metal oxide), that is, the interactions other than physisorption between hydrogen molecules and carbon surface. These reports may provide a clue to the enhancement of the hydrogen storage capacity of carbon materials. However, CNTs and CNFs are unsuitable for adequately clarifying the influence of metals on the hydrogen storage capacity because these materials are prepared with catalysts containing two or three kinds of transition metals, and therefore the influence of the metals on hydrogen adsorption is not easy to determine. However, the metal catalysts cannot be completely removed without changing the nanotube and nanofiber structure. Therefore, well-characterized materials such as AC are more appropriate for studying the influence of metals on the hydrogen adsorption property.
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Oxidation of carbon materials is a fundamental method for changing their adsorption properties. For example, oxidation increases the amount of NH3 and H2 O adsorbed on AC [10,11]. This increase can be explained by the enhancement of the interaction between the AC surface and adsorbates due to the introduction of oxygen-containing functional groups. Oxidation is also reported to enhance the amount of hydrogen adsorbed on the CNT sample [5]. However, oxidation of CNTs gives rise to structural changes such as the removal of the terminal structure, the formation of defective sites, and the introduction of functional groups [12]. Since the structure of oxidized AC samples can be characterized more easily, AC is a more appropriate material for determining the influence of oxidation on the hydrogen adsorption property. In a previous study [13], we accurately measured the hydrogen adsorption and desorption isotherms of several kinds of activated carbon fibers (ACFs), single-walled carbon nanotubes, and zeolites at 303 K over the hydrogen pressure range 0–3.5 MPa by using a high-pressure adsorption apparatus. The amount of adsorbed hydrogen depended on the pore structure of the samples, and an ACF sample with a high surface area, 2250 m2 /g, exhibited the largest capacity, 0.26 wt.% at 303 K and 3.1 MPa. Hydrogen molecules were physisorbed on the micropores in the ACF sample, and the hydrogen adsorption capacity increased with increasing the micropore volume. In this study, we evaluated the hydrogen adsorption properties of ACF loaded with platinum (Pt-ACF) and palladium (Pd-ACF), which are representative metals that interact strongly with hydrogen, at ambient temperature. Furthermore, we prepared an ACF sample oxidized with a solution of (NH4 )2 S2 O8 , which can add oxygen functional groups without significantly modifying the surface area and pore texture [14,15], and discussed the influence of the functional groups on the hydrogen adsorption property at 77 and 303 K.
2. Experimental 2.1. Sample preparation Two kinds of ACF samples (ACF-1, ACF-2), derived from phenolic resin, were used in this study. The Pt-ACF and Pd-ACF samples were prepared by the impregnation of ACF-1 with aqueous solutions of H2 PtCl6 and PdCl2 , respectively. After the impregnation at ambient temperature for 48 h, the supernatant water was removed, and the samples were then dried at 383 K for 2 h under an inert atmosphere. The reduction treatment was carried out in a hydrogen flow at 473 or 673 K for 1 h. After reduction, the samples were washed with pure water to remove the retained chloride and were then reduced again under the same conditions for 1 h. The Pt-ACF (Pd-ACF) samples reduced at 473 and 673 K are designated Pt-ACF-473 (Pd-ACF-473) and Pt-ACF-673 (Pd-ACF-673), respectively.
ACF-1 and ACF-2 were oxidized with 1 M solution of (NH4 )2 S2 O8 at ambient temperature for 48 h [15] and then washed with pure water to afford the oxidized samples, Ox-ACF-1 and Ox-ACF-2. ACF-1 was reduced in a hydrogen flow at 1073 K for 1 h. The reduced ACF-1 sample is designated H-ACF-1. 2.2. Characterization of samples The pore structures of the ACF samples were estimated from the nitrogen adsorption isotherms at 77 K by using the αs -plot. The nitrogen adsorption isotherm of nonporous carbon black was adopted as the standard isotherm for the construction of the αs -plot. The amounts of the functional groups in the Ox-ACF and H-ACF samples were estimated from the desorbed quantities of CO and CO2 by means of temperature-programmed desorption (TPD). The samples were heated to 1223 K in an argon flow at a rate of 5 K/min, and the amounts of CO and CO2 were recorded with a quadrupole mass spectrometer as a function of temperature. Prior to the TPD measurements, the samples were heated at 423 K in an argon flow for 2 h and then subjected to the measurement without exposure to air. The metal contents in the Pt-ACF and Pd-ACF samples were determined by X-ray fluorescence analysis. The average diameters of the metal particles in the Pt-ACF and Pd-ACF samples were estimated from the 111 peak of the X-ray diffraction (XRD) profile by means of Scherrer’s equation. 2.3. Measurement of hydrogen adsorption isotherms Hydrogen adsorption isotherms of the ACF samples at 77 and 303 K up to a hydrogen pressure of 0.1 MPa were measured by using a volumetric apparatus (BELSORP 28SA, BEL, Japan) after pretreatment at 393 K and 1 Pa for 12 h. High-purity hydrogen (99.99999%) was used in this study. Hydrogen adsorption isotherms at 303 K over the 0–3.5 MPa range were measured by using a manually controlled apparatus for high-pressure adsorption (Fig. 1). The following procedures ensured the accuracy of the experimental results. (1) The principal parts of the apparatus were held in an air thermostat to keep its temperature at 303 ± 0.1 K. (2) We determined that no hydrogen adsorbed onto the sample cell walls and that the apparatus was leak-free for at least 2 h at each step of equilibration. (3) Approximately 1.0 g of sample was placed in the sample cell. Prior to measurement, the cell was evacuated at 393 K for 24 h. In the case of the Pt-ACF and Pd-ACF samples, after the cell was evacuated at 393 K for 12 h, hydrogen gas was introduced, and a pressure of 1 MPa was then maintained at 393 K for 1 h in order to reduce the metal in the Pt-ACF and Pd-ACF samples. Finally, the cell was evacuated at 393 K for 12 h again, and the hydrogen adsorption isotherms were then measured. (4) The time adopted for equilibration was
H. Takagi et al. / Journal of Alloys and Compounds 385 (2004) 257–263
259
Fig. 1. A schematic diagram of the volumetric apparatus for high-pressure adsorption.
30 min at each step. (5) The amount of hydrogen sorption for LaNi5 that was measured with this apparatus agreed with that previously reported [8]. (6) The desorption isotherm was measured to confirm the accuracy of the results.
3. Results and discussion 3.1. Hydrogen adsorption properties of Pt-ACF and Pd-ACF Table 1 shows the porosity parameters, metal contents and average diameters of metal particles in the Pt-ACF and Pd-ACF samples. For all the samples, the specific surface area of micropore (Smicro ) was much larger than the specific external surface area (Sext ). The average micropore width (Wave ) was estimated from Smicro and the micropore volume (Vmicro ) by assuming a slit-shaped pore. The values of Smicro and Vmicro were slightly decreased by the metal loading. The Wave values of the Pt-ACF and Pd-ACF samples were much smaller than the average diameters of the metal particles, as determined by XRD (DXRD ), which indicates that most of the metal particles were deposited on the surface of the
activated carbon, not inside the micropores. The DXRD values of both Pt-ACF and Pd-ACF samples after the reduction at 673 K were larger than those after reduction at 473 K. Fig. 2 shows the hydrogen adsorption and desorption isotherms of ACF-1, Pt-ACF, and Pd-ACF at 303 K over the hydrogen pressure range 0–3.5 MPa. The amount of adsorbed hydrogen is expressed on the basis of the ACF sample weight, exclusive of metal, in order to facilitate our discussion of the effect of metal loading on the hydrogen adsorption property. The agreement between the adsorption and desorption isotherms indicates that the data are sufficiently accurate and that the adsorption process is completely reversible. The amounts of hydrogen adsorbed on Pt-ACF-473 and Pt-ACF-673 at 3.1 MPa were 0.257 and 0.251 wt.%, respectively, and both amounts were larger than the amount adsorbed on ACF, 0.236 wt.%. In the case of Pd-ACF, the amount of adsorbed hydrogen was also increased by metal loading, and the values were 0.285 wt.% for Pd-ACF-473 and 0.282 wt.% for Pd-ACF-673. The slopes of the Pt-ACF and Pd-ACF isotherms were equal to the slope of the ACF isotherm. Fig. 3 shows the detailed hydrogen adsorption isotherms of Pt-ACF and Pd-ACF at 303 K up to a hydrogen pressure
Table 1 Characteristics of Pt-ACF and Pd-ACF samples Sample
Reduction temperature (K)
Smicro a (m2 /g)
Sext b (m2 /g)
Vmicro c (cm3 /g)
Wave d (nm)
Metal content (wt.%)
DXRD e (nm)
ACF-1 Pt-ACF-473 Pt-ACF-673 Pd-ACF-473 Pd-ACF-673
– 473 673 473 673
1640 1600 1590 1600 1610
40 30 50 30 40
0.62 0.60 0.60 0.61 0.61
0.75 0.75 0.75 0.76 0.76
– 9.5 10.6 6.9 7.4
– 3.1 5.7 5.8 9.2
a b c d e
Specific surface area of micropore. Specific external surface area. Micropore volume. Average micropore width. Average diameter of metal particles determined by XRD.
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Fig. 2. Hydrogen adsorption isotherms at 303 K over the hydrogen pressure range 0–3.5 MPa: (a) Pt-ACF; (b) Pd-ACF. Filled symbols, adsorption; open symbols; desorption.
Fig. 3. Hydrogen adsorption isotherms of Pt-ACF and Pd-ACF samples at 303 K up to a hydrogen pressure of 0.1 MPa.
of 0.1 MPa. The largest increase in the amount of hydrogen adsorbed on Pt-ACF and Pd-ACF was obtained over the hydrogen pressure range 0–0.01 MPa. Pt-ACF-473 adsorbed more hydrogen than Pt-ACF-673 and less than Pd-ACF. The slopes of the Pt-ACF and Pd-ACF adsorption isotherms in the 0.01–0.1 MPa range were equal to the slope of the ACF isotherm. By assuming that the difference of the amounts of adsorbed hydrogen between the Pt-ACF (Pd-ACF) and ACF samples corresponded to chemisorption of hydrogen on the metal, we estimated the H/metal molar ratio and the surface area of the metal particles (Table 2). The surface area of the Pt particles estimated from the amount of adsorbed hydrogen Table 2 Characteristics of metals on Pt-ACF and Pd-ACF samples Sample
H/metal
S.A.ads a (m2 /g)
S.A.XRD b (m2 /g)
Pt-ACF-473 Pt-ACF-673 Pd-ACF-473 Pd-ACF-673
0.32 0.18 0.67 0.61
87 51 – –
90 49 83 53
a Surface area of metal estimated from the amount of adsorbed hydrogen. b Surface area of metal estimated from the average diameter determined by XRD.
(S.A.ads ) of both Pt-ACF-473 and Pt-ACF-673 was equal to the surface area calculated from the average diameter determined by XRD (S.A.XRD ). In the case of Pt, the surface of the particles is the active site for hydrogen chemisorption [16]. Thus, the agreement of S.A.ads and S.A.XRD indicates that the increase in the amount of hydrogen adsorbed on the ACF sample due to Pt loading results from chemisorption on the surface of Pt particles. Since the average diameter of the Pt particles in Pt-ACF-473 was smaller than that in Pt-ACF-673, the H/metal ratio and the surface area of the Pt particles in Pt-ACF-473 were larger than those in Pt-ACF-673. The H/metal ratios for Pd-ACF were higher than those for Pt-ACF. This result can be explained by the fact that Pd forms metal hydride. The H/metal ratio for Pd-ACF was approximately equal to that of the Pd particle [17]. Therefore, the increase in the amount of hydrogen adsorbed on the ACF sample due to Pd loading can be attributed to the formation of metal hydride. Our results led us to conclude that the increase in the amount of the adsorbed hydrogen due to Pt and Pd loading corresponded to the hydrogen chemisorption on the Pt particles and the formation of Pd hydride, respectively. That is, the increase in the amount of hydrogen adsorbed on the ACF sample due to the spillover phenomenon was not observed, and may require additional conditions, higher adsorption temperatures and/or the presence of the sites to accept the spiltover hydrogen atoms on the carbon surface [18,19]. 3.2. Hydrogen adsorption properties of oxidized and reduced ACF samples Table 3 lists the porosity parameters for the oxidized and reduced ACF samples. The Smicro and Vmicro values for the ACF sample were decreased by oxidation, whereas Wave was increased. Reduction increased Smicro and Vmicro and decreased Wave . The decrease in Smicro and Vmicro as the result of oxidation can be attributed to the presence of the oxygen functional groups introduced on the inside wall or at the entrance of the micropore [20]. In addition, the increase in the sample weight caused by the introduction of oxygen reduces
H. Takagi et al. / Journal of Alloys and Compounds 385 (2004) 257–263 Table 3 Porosity parameters for ACF, Ox-ACF, and H-ACF-1 samples
Table 4 Amounts of CO and CO2 desorbed up to 1223 K
Sample
Smicro a (m2 /g)
Sext b (m2 /g)
Vmicro c (cm3 /g)
Wave d (nm)
ACF-1 Ox-ACF-1 H-ACF-1 ACF-2 Ox-ACF-2
1640 1340 (1550)e 1730 (1680)e 1090 840 (960)e
40 40 30 30 20
0.62 0.52 (0.60)c 0.63 (0.61)e 0.35 0.28 (0.33)
0.75 0.78 0.73 0.65 0.68
a b c d e
261
Specific surface area of micropore. Specific external surface area. Micropore volume. Average micropore width. Value on the basis of the weight of unmodified ACF.
the values of Smicro and Vmicro per sample. Therefore, we carried out TPD measurements on the Ox-ACF and H-ACF samples to estimate the amount of oxygen introduced on the surface of the ACF samples in the forms of functional groups. Fig. 4 shows the TPD profiles of the ACF, Ox-ACF, and H-ACF-1 samples up to 1223 K. The desorption of CO in the 600–1100 K range is derived from the decomposition of phenol, carbonyl, acid anhydride, and quinone groups. The desorption of CO2 in the 450–900 K range is derived from the decomposition of carboxyl, acid anhydride, and lactone groups. The amounts of desorbed CO and CO2 from the Ox-ACF samples were much larger than those from the unmodified ACF samples. This result indicates the introduction of the functional groups on the surface of ACF samples by oxidation with (NH4 )2 S2 O8 . The decrease in the amounts of CO and CO2 due to reduction indicates the removal of the functional groups. Table 4 shows the amounts of CO and CO2 desorbed from the ACF samples up to 1223 K. The oxygen contents in the ACF samples were calculated from the amounts of desorbed CO and CO2 . The oxygen content of ACF-1 increased from 4.0 to 17.0 wt.% as the result of the oxidation. The oxygen content of Ox-ACF-2 was also much larger than that of ACF-2 but was smaller than that of Ox-ACF-1, owing to the lower surface area of ACF-2 relative to that of ACF-1 [15]. By contrast, reduction of ACF-1 decreased the oxygen content from 4.0 to 0.2 wt.%.
Sample
CO (mmol/g)
CO2 (mmol/g)
Oa (wt.%)
ACF-1 Ox-ACF-1 H-ACF-1 ACF-2 Ox-ACF-2
1.54 5.65 0.08 1.17 5.10
0.21 2.50 0.03 0.28 2.31
4.0 17.0 0.2 2.8 15.6
a
Calculated from the amounts of CO and CO2 .
Therefore, the values of Smicro and Vmicro on the basis of the weight of the unmodified ACF sample were estimated from Eq. (1) and were shown in Table 3: Smicro modified Smicro = (1a) 1 − {OOx-ACF(H-ACF) − OACF } modified Vmicro =
Vmicro 1 − {OOx-ACF(H-ACF) − OACF }
(1b)
where OOx-ACF(H-ACF) and OACF are the oxygen contents of the Ox-ACF (H-ACF) and ACF samples, respectively. Fig. 5 shows the hydrogen adsorption and desorption isotherms of ACF, Ox-ACF, and H-ACF-1 samples at 77 and 303 K over the hydrogen pressure range 0–3.5 MPa. The adsorbed amounts are expressed as percentages of the sample weights. The amounts of hydrogen adsorbed on the ACF samples at both 77 and 303 K were remarkably decreased by oxidation and slightly increased by reduction. The amounts of hydrogen adsorbed at 77 K and 0.1 MPa and at 303 K and 3.1 MPa on the basis of the unmodified ACF sample weight were estimated from Eq. (2): modified amount of hydrogen amount of hydrogen = 1 − {OOx-ACF(H-ACF) − OACF }
(2)
The relationship between the amount of adsorbed hydrogen and Vmicro is shown in Fig. 6. (The Vmicro values are also expressed on the basis of the unmodified ACF sample weight.) The decrease in the amount of adsorbed hydrogen at both 77 and 303 K due to oxidation was greater than the decrease in the Vmicro value; that is, the oxygen functional groups
Fig. 4. TPD profiles of ACF, Ox-ACF, and H-ACF-1 samples: (a) CO; (b) CO2 .
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Fig. 5. Hydrogen adsorption isotherms of ACF, Ox-ACF, and H-ACF-1 samples: (a) 77 K up to 0.1 MPa; (b) 303 K up to 3.5 MPa. Filled symbols, adsorption; open symbols; desorption.
Fig. 6. Relationship between the amount of adsorbed hydrogen and the micropore volume: (a) 77 K and 0.1 MPa; (b) 303 K and 3.1 MPa.
introduced on the inside wall or at the entrance of the micropore inhibited physisorption of hydrogen molecules more than nitrogen molecules. As shown in Table 3, the Wave values of the ACF samples were increased by oxidation. This result suggests that the functional groups inhibited adsorption of nitrogen molecules on the micropore with the pore size smaller than Wave , 0.75 nm for ACF-1 and 0.65 nm for ACF-2. The optimum pore diameter for hydrogen adsorption has been reported to be 0.56 nm theoretically and 0.66 nm experimentally [21,22]. Thus, the decrease in the hydrogen adsorption capacity due to oxidation can be attributed to the change in the pore structure, that is, to the inhibition of hydrogen adsorption on the micropore with the optimum pore diameter by the oxygen-containing functional groups on the inside wall or at the entrance. On the other hand, the increase in the amount of hydrogen adsorbed on the H-ACF-1 sample can be attributed to the removal of the functional groups. This explanation is supported by the decrease in the Wave value after reduction. 4. Conclusions The hydrogen adsorption and desorption isotherms of Pt-ACF, Pd-ACF, Ox-ACF, and H-ACF at 77 and 303 K over
the hydrogen pressure range 0–3.5 MPa were measured, and the influence of metal loading and oxidation and reduction on the hydrogen adsorption property was discussed. The obtained conclusions are as follows: 1. The amounts of hydrogen adsorbed on the Pt-ACF and Pd-ACF samples at 303 K were larger than the amount adsorbed on the unmodified ACF sample. Detailed measurement of the hydrogen adsorption isotherms of the Pt-ACF and Pd-ACF samples up to a hydrogen pressure of 0.1 MPa indicated that the increase in the amount of the adsorbed hydrogen due to metal loading corresponded to hydrogen chemisorption on the Pt particles and the formation of Pd hydride. 2. The amounts of hydrogen adsorbed on the ACF samples at both 77 and 303 K were remarkably decreased by oxidation and slightly increased by reduction. The relationship between the Vmicro values and the amounts of adsorbed hydrogen based on the weight of the unmodified ACF sample indicated that the changes in hydrogen adsorption capacity with oxidation and reduction could be attributed to changes in the pore structure, that is, to the inhibition of hydrogen adsorption on the micropore with the optimum pore diameter by the oxygen-containing functional groups.
H. Takagi et al. / Journal of Alloys and Compounds 385 (2004) 257–263
Acknowledgements This work was supported by a grant (Support of Young Researchers with a Term) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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Hydrogen adsorption properties of activated carbons with modified surfaces H. Takagi a,∗ , H. Hatori a , Y. Yamada b , S. Matsuo c , M. Shiraishi c a
National Institute of Advanced Industrial Science and Technology (AIST), AIST Tsukuba West, 16-1 Onogawa, Tsukuba, Ibaraki 305-8569, Japan b Faculty of Engineering, Fukui University, 3-9-1 Bunkyou, Fukui 910-8507, Japan c School of High-Technology for Human Welfare, Tokai University, 317 Nishino, Numazu, Shizuoka 410-0395, Japan Received 26 February 2004; accepted 6 March 2004
Abstract The hydrogen adsorption isotherms of activated carbon fibers loaded with platinum (Pt-ACF) and palladium (Pd-ACF) were accurately measured at ambient temperature over the hydrogen pressure range 0–3.5 MPa by using a high-pressure adsorption apparatus. The amounts of hydrogen adsorbed on the Pt-ACF and Pd-ACF samples were larger than the amount adsorbed on the unmodified ACF sample. Detailed measurement of the hydrogen adsorption isotherms of Pt-ACF and Pd-ACF at a hydrogen pressure up to 0.1 MPa indicated that the increase in adsorption was due to chemisorption of hydrogen on the metal. The hydrogen adsorption properties of ACF oxidized with (NH4 )S2 O8 or reduced in a hydrogen flow at 1073 K were evaluated at 77 and 303 K over the hydrogen pressure range 0–3.5 MPa. The amount of adsorbed hydrogen was decreased by oxidation and increased by reduction. This result is attributable to a change in the pore structure: that is, oxygen functional groups introduced on the carbon surface inhibited hydrogen adsorption into the micropore with an optimum pore diameter. © 2004 Elsevier B.V. All rights reserved. Keywords: Hydrogen adsorption property; Activated carbons; Noble metals; Functional groups
1. Introduction Hydrogen is an ideal substitute for fossil fuels, and a number of research groups have recently reported the hydrogen storage capacity of carbon materials. The high hydrogen storage capacities of carbon nanotubes (CNTs) and carbon nanofibers (CNFs) have been demonstrated experimentally [1–5]. However, several groups have claimed that the storage capacity of CNTs and CNFs at ambient temperature and high pressure (10 MPa) is less than 0.7 wt.%, and is smaller than the capacity of activated carbon (AC) [6–8]. This discrepancy among the reports is considered to be due to the difficulty of evaluating storage capacity. In addition, the mechanism of hydrogen storage, and the interaction between the carbon surface and hydrogen are not adequately understood. To clarify the mechanism of hydrogen storage in carbon materials and to design an appropriate structure as storage materials, we must accurately analyze
∗ Corresponding author. Tel.: +81-29-861-8298; fax: +81-29-861-8408. E-mail address: [email protected] (H. Takagi).
0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.03.139
the hydrogen storage properties by using well-characterized samples. The metal catalysts resided in CNTs and CNFs significantly influence hydrogen storage capacities. It was reported that CNT and CNF samples containing metal catalysts exhibited high capacities at ambient temperature, and the capacities decreased upon removal of the metals [3,4,9]. The reports claim that the high hydrogen storage capacity can be explained by the spillover phenomenon of hydrogen atoms dissociated on the metal (metal oxide), that is, the interactions other than physisorption between hydrogen molecules and carbon surface. These reports may provide a clue to the enhancement of the hydrogen storage capacity of carbon materials. However, CNTs and CNFs are unsuitable for adequately clarifying the influence of metals on the hydrogen storage capacity because these materials are prepared with catalysts containing two or three kinds of transition metals, and therefore the influence of the metals on hydrogen adsorption is not easy to determine. However, the metal catalysts cannot be completely removed without changing the nanotube and nanofiber structure. Therefore, well-characterized materials such as AC are more appropriate for studying the influence of metals on the hydrogen adsorption property.
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Oxidation of carbon materials is a fundamental method for changing their adsorption properties. For example, oxidation increases the amount of NH3 and H2 O adsorbed on AC [10,11]. This increase can be explained by the enhancement of the interaction between the AC surface and adsorbates due to the introduction of oxygen-containing functional groups. Oxidation is also reported to enhance the amount of hydrogen adsorbed on the CNT sample [5]. However, oxidation of CNTs gives rise to structural changes such as the removal of the terminal structure, the formation of defective sites, and the introduction of functional groups [12]. Since the structure of oxidized AC samples can be characterized more easily, AC is a more appropriate material for determining the influence of oxidation on the hydrogen adsorption property. In a previous study [13], we accurately measured the hydrogen adsorption and desorption isotherms of several kinds of activated carbon fibers (ACFs), single-walled carbon nanotubes, and zeolites at 303 K over the hydrogen pressure range 0–3.5 MPa by using a high-pressure adsorption apparatus. The amount of adsorbed hydrogen depended on the pore structure of the samples, and an ACF sample with a high surface area, 2250 m2 /g, exhibited the largest capacity, 0.26 wt.% at 303 K and 3.1 MPa. Hydrogen molecules were physisorbed on the micropores in the ACF sample, and the hydrogen adsorption capacity increased with increasing the micropore volume. In this study, we evaluated the hydrogen adsorption properties of ACF loaded with platinum (Pt-ACF) and palladium (Pd-ACF), which are representative metals that interact strongly with hydrogen, at ambient temperature. Furthermore, we prepared an ACF sample oxidized with a solution of (NH4 )2 S2 O8 , which can add oxygen functional groups without significantly modifying the surface area and pore texture [14,15], and discussed the influence of the functional groups on the hydrogen adsorption property at 77 and 303 K.
2. Experimental 2.1. Sample preparation Two kinds of ACF samples (ACF-1, ACF-2), derived from phenolic resin, were used in this study. The Pt-ACF and Pd-ACF samples were prepared by the impregnation of ACF-1 with aqueous solutions of H2 PtCl6 and PdCl2 , respectively. After the impregnation at ambient temperature for 48 h, the supernatant water was removed, and the samples were then dried at 383 K for 2 h under an inert atmosphere. The reduction treatment was carried out in a hydrogen flow at 473 or 673 K for 1 h. After reduction, the samples were washed with pure water to remove the retained chloride and were then reduced again under the same conditions for 1 h. The Pt-ACF (Pd-ACF) samples reduced at 473 and 673 K are designated Pt-ACF-473 (Pd-ACF-473) and Pt-ACF-673 (Pd-ACF-673), respectively.
ACF-1 and ACF-2 were oxidized with 1 M solution of (NH4 )2 S2 O8 at ambient temperature for 48 h [15] and then washed with pure water to afford the oxidized samples, Ox-ACF-1 and Ox-ACF-2. ACF-1 was reduced in a hydrogen flow at 1073 K for 1 h. The reduced ACF-1 sample is designated H-ACF-1. 2.2. Characterization of samples The pore structures of the ACF samples were estimated from the nitrogen adsorption isotherms at 77 K by using the αs -plot. The nitrogen adsorption isotherm of nonporous carbon black was adopted as the standard isotherm for the construction of the αs -plot. The amounts of the functional groups in the Ox-ACF and H-ACF samples were estimated from the desorbed quantities of CO and CO2 by means of temperature-programmed desorption (TPD). The samples were heated to 1223 K in an argon flow at a rate of 5 K/min, and the amounts of CO and CO2 were recorded with a quadrupole mass spectrometer as a function of temperature. Prior to the TPD measurements, the samples were heated at 423 K in an argon flow for 2 h and then subjected to the measurement without exposure to air. The metal contents in the Pt-ACF and Pd-ACF samples were determined by X-ray fluorescence analysis. The average diameters of the metal particles in the Pt-ACF and Pd-ACF samples were estimated from the 111 peak of the X-ray diffraction (XRD) profile by means of Scherrer’s equation. 2.3. Measurement of hydrogen adsorption isotherms Hydrogen adsorption isotherms of the ACF samples at 77 and 303 K up to a hydrogen pressure of 0.1 MPa were measured by using a volumetric apparatus (BELSORP 28SA, BEL, Japan) after pretreatment at 393 K and 1 Pa for 12 h. High-purity hydrogen (99.99999%) was used in this study. Hydrogen adsorption isotherms at 303 K over the 0–3.5 MPa range were measured by using a manually controlled apparatus for high-pressure adsorption (Fig. 1). The following procedures ensured the accuracy of the experimental results. (1) The principal parts of the apparatus were held in an air thermostat to keep its temperature at 303 ± 0.1 K. (2) We determined that no hydrogen adsorbed onto the sample cell walls and that the apparatus was leak-free for at least 2 h at each step of equilibration. (3) Approximately 1.0 g of sample was placed in the sample cell. Prior to measurement, the cell was evacuated at 393 K for 24 h. In the case of the Pt-ACF and Pd-ACF samples, after the cell was evacuated at 393 K for 12 h, hydrogen gas was introduced, and a pressure of 1 MPa was then maintained at 393 K for 1 h in order to reduce the metal in the Pt-ACF and Pd-ACF samples. Finally, the cell was evacuated at 393 K for 12 h again, and the hydrogen adsorption isotherms were then measured. (4) The time adopted for equilibration was
H. Takagi et al. / Journal of Alloys and Compounds 385 (2004) 257–263
259
Fig. 1. A schematic diagram of the volumetric apparatus for high-pressure adsorption.
30 min at each step. (5) The amount of hydrogen sorption for LaNi5 that was measured with this apparatus agreed with that previously reported [8]. (6) The desorption isotherm was measured to confirm the accuracy of the results.
3. Results and discussion 3.1. Hydrogen adsorption properties of Pt-ACF and Pd-ACF Table 1 shows the porosity parameters, metal contents and average diameters of metal particles in the Pt-ACF and Pd-ACF samples. For all the samples, the specific surface area of micropore (Smicro ) was much larger than the specific external surface area (Sext ). The average micropore width (Wave ) was estimated from Smicro and the micropore volume (Vmicro ) by assuming a slit-shaped pore. The values of Smicro and Vmicro were slightly decreased by the metal loading. The Wave values of the Pt-ACF and Pd-ACF samples were much smaller than the average diameters of the metal particles, as determined by XRD (DXRD ), which indicates that most of the metal particles were deposited on the surface of the
activated carbon, not inside the micropores. The DXRD values of both Pt-ACF and Pd-ACF samples after the reduction at 673 K were larger than those after reduction at 473 K. Fig. 2 shows the hydrogen adsorption and desorption isotherms of ACF-1, Pt-ACF, and Pd-ACF at 303 K over the hydrogen pressure range 0–3.5 MPa. The amount of adsorbed hydrogen is expressed on the basis of the ACF sample weight, exclusive of metal, in order to facilitate our discussion of the effect of metal loading on the hydrogen adsorption property. The agreement between the adsorption and desorption isotherms indicates that the data are sufficiently accurate and that the adsorption process is completely reversible. The amounts of hydrogen adsorbed on Pt-ACF-473 and Pt-ACF-673 at 3.1 MPa were 0.257 and 0.251 wt.%, respectively, and both amounts were larger than the amount adsorbed on ACF, 0.236 wt.%. In the case of Pd-ACF, the amount of adsorbed hydrogen was also increased by metal loading, and the values were 0.285 wt.% for Pd-ACF-473 and 0.282 wt.% for Pd-ACF-673. The slopes of the Pt-ACF and Pd-ACF isotherms were equal to the slope of the ACF isotherm. Fig. 3 shows the detailed hydrogen adsorption isotherms of Pt-ACF and Pd-ACF at 303 K up to a hydrogen pressure
Table 1 Characteristics of Pt-ACF and Pd-ACF samples Sample
Reduction temperature (K)
Smicro a (m2 /g)
Sext b (m2 /g)
Vmicro c (cm3 /g)
Wave d (nm)
Metal content (wt.%)
DXRD e (nm)
ACF-1 Pt-ACF-473 Pt-ACF-673 Pd-ACF-473 Pd-ACF-673
– 473 673 473 673
1640 1600 1590 1600 1610
40 30 50 30 40
0.62 0.60 0.60 0.61 0.61
0.75 0.75 0.75 0.76 0.76
– 9.5 10.6 6.9 7.4
– 3.1 5.7 5.8 9.2
a b c d e
Specific surface area of micropore. Specific external surface area. Micropore volume. Average micropore width. Average diameter of metal particles determined by XRD.
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Fig. 2. Hydrogen adsorption isotherms at 303 K over the hydrogen pressure range 0–3.5 MPa: (a) Pt-ACF; (b) Pd-ACF. Filled symbols, adsorption; open symbols; desorption.
Fig. 3. Hydrogen adsorption isotherms of Pt-ACF and Pd-ACF samples at 303 K up to a hydrogen pressure of 0.1 MPa.
of 0.1 MPa. The largest increase in the amount of hydrogen adsorbed on Pt-ACF and Pd-ACF was obtained over the hydrogen pressure range 0–0.01 MPa. Pt-ACF-473 adsorbed more hydrogen than Pt-ACF-673 and less than Pd-ACF. The slopes of the Pt-ACF and Pd-ACF adsorption isotherms in the 0.01–0.1 MPa range were equal to the slope of the ACF isotherm. By assuming that the difference of the amounts of adsorbed hydrogen between the Pt-ACF (Pd-ACF) and ACF samples corresponded to chemisorption of hydrogen on the metal, we estimated the H/metal molar ratio and the surface area of the metal particles (Table 2). The surface area of the Pt particles estimated from the amount of adsorbed hydrogen Table 2 Characteristics of metals on Pt-ACF and Pd-ACF samples Sample
H/metal
S.A.ads a (m2 /g)
S.A.XRD b (m2 /g)
Pt-ACF-473 Pt-ACF-673 Pd-ACF-473 Pd-ACF-673
0.32 0.18 0.67 0.61
87 51 – –
90 49 83 53
a Surface area of metal estimated from the amount of adsorbed hydrogen. b Surface area of metal estimated from the average diameter determined by XRD.
(S.A.ads ) of both Pt-ACF-473 and Pt-ACF-673 was equal to the surface area calculated from the average diameter determined by XRD (S.A.XRD ). In the case of Pt, the surface of the particles is the active site for hydrogen chemisorption [16]. Thus, the agreement of S.A.ads and S.A.XRD indicates that the increase in the amount of hydrogen adsorbed on the ACF sample due to Pt loading results from chemisorption on the surface of Pt particles. Since the average diameter of the Pt particles in Pt-ACF-473 was smaller than that in Pt-ACF-673, the H/metal ratio and the surface area of the Pt particles in Pt-ACF-473 were larger than those in Pt-ACF-673. The H/metal ratios for Pd-ACF were higher than those for Pt-ACF. This result can be explained by the fact that Pd forms metal hydride. The H/metal ratio for Pd-ACF was approximately equal to that of the Pd particle [17]. Therefore, the increase in the amount of hydrogen adsorbed on the ACF sample due to Pd loading can be attributed to the formation of metal hydride. Our results led us to conclude that the increase in the amount of the adsorbed hydrogen due to Pt and Pd loading corresponded to the hydrogen chemisorption on the Pt particles and the formation of Pd hydride, respectively. That is, the increase in the amount of hydrogen adsorbed on the ACF sample due to the spillover phenomenon was not observed, and may require additional conditions, higher adsorption temperatures and/or the presence of the sites to accept the spiltover hydrogen atoms on the carbon surface [18,19]. 3.2. Hydrogen adsorption properties of oxidized and reduced ACF samples Table 3 lists the porosity parameters for the oxidized and reduced ACF samples. The Smicro and Vmicro values for the ACF sample were decreased by oxidation, whereas Wave was increased. Reduction increased Smicro and Vmicro and decreased Wave . The decrease in Smicro and Vmicro as the result of oxidation can be attributed to the presence of the oxygen functional groups introduced on the inside wall or at the entrance of the micropore [20]. In addition, the increase in the sample weight caused by the introduction of oxygen reduces
H. Takagi et al. / Journal of Alloys and Compounds 385 (2004) 257–263 Table 3 Porosity parameters for ACF, Ox-ACF, and H-ACF-1 samples
Table 4 Amounts of CO and CO2 desorbed up to 1223 K
Sample
Smicro a (m2 /g)
Sext b (m2 /g)
Vmicro c (cm3 /g)
Wave d (nm)
ACF-1 Ox-ACF-1 H-ACF-1 ACF-2 Ox-ACF-2
1640 1340 (1550)e 1730 (1680)e 1090 840 (960)e
40 40 30 30 20
0.62 0.52 (0.60)c 0.63 (0.61)e 0.35 0.28 (0.33)
0.75 0.78 0.73 0.65 0.68
a b c d e
261
Specific surface area of micropore. Specific external surface area. Micropore volume. Average micropore width. Value on the basis of the weight of unmodified ACF.
the values of Smicro and Vmicro per sample. Therefore, we carried out TPD measurements on the Ox-ACF and H-ACF samples to estimate the amount of oxygen introduced on the surface of the ACF samples in the forms of functional groups. Fig. 4 shows the TPD profiles of the ACF, Ox-ACF, and H-ACF-1 samples up to 1223 K. The desorption of CO in the 600–1100 K range is derived from the decomposition of phenol, carbonyl, acid anhydride, and quinone groups. The desorption of CO2 in the 450–900 K range is derived from the decomposition of carboxyl, acid anhydride, and lactone groups. The amounts of desorbed CO and CO2 from the Ox-ACF samples were much larger than those from the unmodified ACF samples. This result indicates the introduction of the functional groups on the surface of ACF samples by oxidation with (NH4 )2 S2 O8 . The decrease in the amounts of CO and CO2 due to reduction indicates the removal of the functional groups. Table 4 shows the amounts of CO and CO2 desorbed from the ACF samples up to 1223 K. The oxygen contents in the ACF samples were calculated from the amounts of desorbed CO and CO2 . The oxygen content of ACF-1 increased from 4.0 to 17.0 wt.% as the result of the oxidation. The oxygen content of Ox-ACF-2 was also much larger than that of ACF-2 but was smaller than that of Ox-ACF-1, owing to the lower surface area of ACF-2 relative to that of ACF-1 [15]. By contrast, reduction of ACF-1 decreased the oxygen content from 4.0 to 0.2 wt.%.
Sample
CO (mmol/g)
CO2 (mmol/g)
Oa (wt.%)
ACF-1 Ox-ACF-1 H-ACF-1 ACF-2 Ox-ACF-2
1.54 5.65 0.08 1.17 5.10
0.21 2.50 0.03 0.28 2.31
4.0 17.0 0.2 2.8 15.6
a
Calculated from the amounts of CO and CO2 .
Therefore, the values of Smicro and Vmicro on the basis of the weight of the unmodified ACF sample were estimated from Eq. (1) and were shown in Table 3: Smicro modified Smicro = (1a) 1 − {OOx-ACF(H-ACF) − OACF } modified Vmicro =
Vmicro 1 − {OOx-ACF(H-ACF) − OACF }
(1b)
where OOx-ACF(H-ACF) and OACF are the oxygen contents of the Ox-ACF (H-ACF) and ACF samples, respectively. Fig. 5 shows the hydrogen adsorption and desorption isotherms of ACF, Ox-ACF, and H-ACF-1 samples at 77 and 303 K over the hydrogen pressure range 0–3.5 MPa. The adsorbed amounts are expressed as percentages of the sample weights. The amounts of hydrogen adsorbed on the ACF samples at both 77 and 303 K were remarkably decreased by oxidation and slightly increased by reduction. The amounts of hydrogen adsorbed at 77 K and 0.1 MPa and at 303 K and 3.1 MPa on the basis of the unmodified ACF sample weight were estimated from Eq. (2): modified amount of hydrogen amount of hydrogen = 1 − {OOx-ACF(H-ACF) − OACF }
(2)
The relationship between the amount of adsorbed hydrogen and Vmicro is shown in Fig. 6. (The Vmicro values are also expressed on the basis of the unmodified ACF sample weight.) The decrease in the amount of adsorbed hydrogen at both 77 and 303 K due to oxidation was greater than the decrease in the Vmicro value; that is, the oxygen functional groups
Fig. 4. TPD profiles of ACF, Ox-ACF, and H-ACF-1 samples: (a) CO; (b) CO2 .
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Fig. 5. Hydrogen adsorption isotherms of ACF, Ox-ACF, and H-ACF-1 samples: (a) 77 K up to 0.1 MPa; (b) 303 K up to 3.5 MPa. Filled symbols, adsorption; open symbols; desorption.
Fig. 6. Relationship between the amount of adsorbed hydrogen and the micropore volume: (a) 77 K and 0.1 MPa; (b) 303 K and 3.1 MPa.
introduced on the inside wall or at the entrance of the micropore inhibited physisorption of hydrogen molecules more than nitrogen molecules. As shown in Table 3, the Wave values of the ACF samples were increased by oxidation. This result suggests that the functional groups inhibited adsorption of nitrogen molecules on the micropore with the pore size smaller than Wave , 0.75 nm for ACF-1 and 0.65 nm for ACF-2. The optimum pore diameter for hydrogen adsorption has been reported to be 0.56 nm theoretically and 0.66 nm experimentally [21,22]. Thus, the decrease in the hydrogen adsorption capacity due to oxidation can be attributed to the change in the pore structure, that is, to the inhibition of hydrogen adsorption on the micropore with the optimum pore diameter by the oxygen-containing functional groups on the inside wall or at the entrance. On the other hand, the increase in the amount of hydrogen adsorbed on the H-ACF-1 sample can be attributed to the removal of the functional groups. This explanation is supported by the decrease in the Wave value after reduction. 4. Conclusions The hydrogen adsorption and desorption isotherms of Pt-ACF, Pd-ACF, Ox-ACF, and H-ACF at 77 and 303 K over
the hydrogen pressure range 0–3.5 MPa were measured, and the influence of metal loading and oxidation and reduction on the hydrogen adsorption property was discussed. The obtained conclusions are as follows: 1. The amounts of hydrogen adsorbed on the Pt-ACF and Pd-ACF samples at 303 K were larger than the amount adsorbed on the unmodified ACF sample. Detailed measurement of the hydrogen adsorption isotherms of the Pt-ACF and Pd-ACF samples up to a hydrogen pressure of 0.1 MPa indicated that the increase in the amount of the adsorbed hydrogen due to metal loading corresponded to hydrogen chemisorption on the Pt particles and the formation of Pd hydride. 2. The amounts of hydrogen adsorbed on the ACF samples at both 77 and 303 K were remarkably decreased by oxidation and slightly increased by reduction. The relationship between the Vmicro values and the amounts of adsorbed hydrogen based on the weight of the unmodified ACF sample indicated that the changes in hydrogen adsorption capacity with oxidation and reduction could be attributed to changes in the pore structure, that is, to the inhibition of hydrogen adsorption on the micropore with the optimum pore diameter by the oxygen-containing functional groups.
H. Takagi et al. / Journal of Alloys and Compounds 385 (2004) 257–263
Acknowledgements This work was supported by a grant (Support of Young Researchers with a Term) from the Ministry of Education, Culture, Sports, Science and Technology of Japan.
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