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H 2 adsorption on multiwalled carbon nanotubes at low temperatures and low pressures

H 2 adsorption on multiwalled carbon nanotubes at low temperatures and low pressures ` Dipartimento di Fisica, Universita della Calabria and INFN, Gruppo Collegato di Cosenza, 87036 Arcavacata di Rende, Italy 2 ` Dipartimento di Metodologie Fisiche e Chimiche, Universita di Catania, 95131 Catania, Italy (Received 16 July 2008; published 18 November 2008) We present an experimental study on H2 adsorption on multiwalled carbon nanotubes (MWCNTs) at low temperatures (12–30 K) and low pressures (2 Â 10À5 Torr) using the temperature programmed desorption technique. Our results show that the molecular hydrogen uptake increases nearly exponentially from 6 Â 10À9 wt: % at 24.5 K to 2 Â 10À7 wt: % at 12.5 K and that the desorption kinetics is of the first order. Comparative measurements indicate that MWCNTs have an adsorption capacity about two orders higher than that of activated carbon (charcoal) making them a possible candidate as hydrogen cryosorber for eventual applications in accelerators and synchrotrons. DOI: 10.1103/PhysRevSTAB.11.113201 PACS numbers: 07.20.Mc, 07.30.Cy, 29.20.Àc I. INTRODUCTION Pumping of molecular hydrogen in the Large Hadron Collider is designed by installation of cryosorbers on cryogenic elements operating at very low temperatures [1]. Among many specific requirements for the best choice of cryosorbent materials, the adsorption capacity is certainly a discriminating factor. Though many types of cryosorbers have been tested in the past [2,3], active porous carbon is still the far most important cryosorbent material due to its high specific surface area and large pore volume [4–6]. Usually, three different techniques are widely applied to study the H2 adsorption rate, capacity, sticking and capture probability, and storage in solids: the volumetric method, which measures the pressure drop owing to hydrogen absorption after loading the specimen contained in a constant volume [7]; the gravimetric method, which measures the sample weight changes due to absorption or desorption [8,9]; and the temperature programmed desorption (TPD) technique [10]. This latest measures the hydrogen signal during desorption in high vacuum using mass spectrometry; it is highly sensitive allowing one to study samples with masses even below 1 mg and is especially suited for loading of small quantities at low pressures. In an earliest work, Dillon et al. estimated the hydrogen uptake by single walled carbon nanotubes to be 5–10 wt. % for adsorption conditions of about 0.5 bar and 273 K, followed by cooling and pumping to 133 K and 10À6 Torr [10]. The heat of adsorption determined from the TPD measurements was close to 20 kJ=mol. Wilson et al. measured isotherms of H2 and D2 deposited on single walled, closed end carbon nanotube bundles and observed two well distinct steps corresponding to adsorption on grooves/interstitials and the graphene surface [11]. In this work we present a TPD study on H2 adsorption on multiwalled carbon nanotubes (MWCNTs) and on charcoal at very low pressure and temperatures as a function of gas dose. We show that the molecular hydrogen uptake 1098-4402=08=11(11)=113201(4) decreases with the adsorption temperature and is in the order of 10À7 wt: %, that the desorption kinetics is of the first order, and that the desorption energy is 2:82 Æ 0:16 KJ=mol. Comparative measurements indicate that MWCNTs have an adsorption capacity about two orders higher than that of activated carbon (charcoal) making them a possible candidate as hydrogen cryosorber for eventual applications in accelerators and synchrotrons. II. EXPERIMENTAL MWCNTs were synthesized by chemical vapor deposition using C2 H4 and subsequently purified. As illustrated in Fig. 1, transmission electron microscopy (TEM) shows that FIG. 1. Transmission electron micrograph of multiwalled carbon nanotube sample used in this study. 113201-1 Ó 2008 The American Physical Society F. XU et al. our MWCNT sample was made of individual nanotubes with an outer (inner) diameter of 10–15 (3–6) nm and that many of them were open ended. Microporosity measurements yielded a specific surface of 174 m2 =g. 10 mg of MWCNTs were dispersed in orthodichlorobenzene and sonicated in a glass container. The sample then was deposited directly onto the copper head (6 cm2 ) of a cold finger with closed He cycling and dried. A quite uniform thin film was obtained and its adhesion to the copper surface remained stable during all the measurements. The experiments were conducted in a UHV chamber (base pressure in the low 10À9 Torr range) evacuated with a dry turbo pump. H2 gas (purity of 99.999%) was introduced in the chamber through a precision leak valve while keeping the sample at a desired adsorption temperature Tad and a fixed pressure of 2 Â 10À5 Torr (measured at room temperature and corrected for the ion gauge sensitivity factor ) for a chosen time interval, then the gas was pumped out. The sample temperature was monitored with a Au=0:7%Fe-chromel thermocouple. Thermal desorption was achieved by heating the head of the cold finger with a linear ramp and the desorbed H2 was measured with a Phys. Rev. ST Accel. Beams 11, 113201 (2008) quadruple mass analyzer. No appreciable contamination gas signals were detected neither in the adsorption nor in the desorption processes. Complete reversibility of charging and discharging cycles was routinely verified and a total reproducibility was observed. The vacuum system pumping speed was calibrated by admitting a known quantity of hydrogen into the chamber at desired quadruple reading pressures and monitoring the time needed for evacuation. A linear dependence on the pressure was observed. The desorption rate was obtained by multiplying the detected hydrogen partial pressure (the integral intensity of a quadruple mass spectrum centered at m ¼ 2 a:u: composed of some 50 data points) by the system pumping speed and the total amount of desorbed hydrogen then could be evaluated from the integration of the desorption rate-time curves with an estimated error of about 20%. The density of MWCNTs should be close to 2 g cmÀ2 once deposited on the cold finger in a compact form (not measured). For small quantities of our samples (up to FIG. 2. (Color) Temperature programmed desorption traces of molecular hydrogen adsorbed at Tad ¼ 15 K and Pad ¼ 2 Â 10À5 Torr. The dose is expressed in Langmuir (1 L ¼ 106 Torr sec ). FIG. 3. (Color) Molecular hydrogen adsorbed on MWCNTs as a function of H2 dose for adsorption temperature of Tad ¼ 15 K. Also shown are the results for a charcoal sample. The active surfaces are 174 m2 =g and 23 m2 =g for MWCNTs and charcoal, respectively. 113201-2 H2 ADSORPTION ON MULTIWALLED CARBON . . . 10 mg on a surface of 6 cm2 ), we found the sorption capacity is proportional to the sample mass. III. RESULTS AND DISCUSSIONS In Fig. 2 we illustrate some representative H2 TPD traces taken at a constant adsorption temperature of Tad ¼ 15 K and varying exposure time. The H2 dose È was expressed in units of Langmuir (1 L ¼ 10À6 Torr sec ) and the sample heating rate was fixed at ¼ 5 K= min . These spectra are substantially narrower than those previously reported in literature for H2 dosing at higher temperatures and pressures [10,12]. These TPD traces are quite similar to each other and the spectrum maxima remain unchanged as the hydrogen adsorption increases clearly indicating a first order desorption mechanism. The total molecular hydrogen content, as determined from the integral of the desorbed H2 TPD trace curves, is plotted in Fig. 3 as a function of È for Tad ¼ 15 K. It is interesting to note that it increases linearly with È at low doses and saturates at about 8 Â 10À8 wt:% for È ! 104 L. The saturation at a very low value and the relatively narrow line shape would suggest that hydrogen adsorbs only on a limited number of sites. Phys. Rev. ST Accel. Beams 11, 113201 (2008) To gain some information on the hydrogen binding energy we varied the sample heating rate for constant 2 È ¼ 2100 L and Tad ¼ 19 K. In Fig. 4 we plot lnðTmax = Þ À1 , where T versus Tmax max is the temperature corresponding to the desorption rate maximum. The slope of the linear fit yields a desorption energy of Edes ¼ 340 Æ 20 K (or Edes ¼ 2:82 Æ 0:16 KJ=mol). It is interesting to note that, though there is a large body of experimental studies on hydrogen adsorption on carbon nanotubes because of its potential application as a storage material, very few measurements were undertaken below 77 K [11,13]. To the best of our knowledge, no experimental data are available in literature for adsorption at very low pressures. The binding energy value found in this study is lower than that for H2 on graphite (480 K) [14] in accord with the isothermal adsorption measurements reported by Wilson et al. (3:3 kJ=mol) [11]. This suggests that hydrogen molecules mostly adsorb on the outer surfaces and the reduction in binding energy value is generally attributed to the surface curvature [11,15–17]. This site assignment is in accord with the TEM observation that our sample was essentially made of individual nanotubes (Fig. 1). On the other hand, adsorption on inner walls would imply a much stronger bonding [11,18] and 2 FIG. 4. (Color) Plot of lnðTmax = Þ versus Tmax À1 for a series of TPD spectra taken at fixed Tad ¼ 19 K, È ¼ 2160 L, and varying heating rate . Tmax is the temperature corresponding to the desorption rate maximum. FIG. 5. Saturated molecular hydrogen content adsorbed on MWCNTs as a function of adsorption temperature of Tad . 113201-3 F. XU et al. diffusion into those open ended nanotubes. Quasielastic neutron scattering measurements performed by Narehood et al. also indicated the absence of H2 mobility in SWNTs at temperatures below 30 K [19]. For comparison, in Fig. 3 we also present the total H2 adsorption amount of 10 mg activated charcoal powders (specific surface area 23 m2 =g) under exactly the same experimental conditions. It exhibits very similar characteristics as that of MWCNTs but is about two orders smaller. Finally, in Fig. 5 we illustrate the saturated total amount of adsorbed hydrogen as a function of adsorption temperature Tad . As expected, the total hydrogen adsorption capacity decreases with the cryosorber temperature. For Tad ! 27 K, no H2 adsorption is observed. Our results contrast sharply with those reported for hydrogen storage measurements at higher temperatures and pressures and strongly indicate that under high vacuum conditions, hydrogen does not adsorb on the nanotube surface beyond the sublimation temperature and the diffusion is essentially inhibited, as already confirmed by previous works. We mention that the testing measurements on carbon nanofibers actually used in the LHC were also taken at higher pressures and the so determined sorption capacity can be much higher than the real values in UHV conditions [5,6]. More quantitative works are currently being undertaken to determine the realistic possibility of using nanotubes as cryosorbers in UHV systems. IV. CONCLUSIONS In conclusion, we presented first experimental measurements of molecular hydrogen adsorption on MWCNTs at low pressures (10À5 Torr) and low temperatures (12– 30 K). We showed that the adsorption capacity decreases sharply with temperature and is in the order of 10À7 wt: % at 12 K. This figure is about two orders higher than that obtained for commercial active carbon, making MWCNTs a potential candidate as cryosorbent materials for accelerators. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Physical Review Special Topics - Accelerators and Beams American Physical Society (APS)

H 2 adsorption on multiwalled carbon nanotubes at low temperatures and low pressures

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Publisher
American Physical Society (APS)
Copyright
Copyright © 2008 The American Physical Society
ISSN
1098-4402
DOI
10.1103/PhysRevSTAB.11.113201
Publisher site
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Abstract

` Dipartimento di Fisica, Universita della Calabria and INFN, Gruppo Collegato di Cosenza, 87036 Arcavacata di Rende, Italy 2 ` Dipartimento di Metodologie Fisiche e Chimiche, Universita di Catania, 95131 Catania, Italy (Received 16 July 2008; published 18 November 2008) We present an experimental study on H2 adsorption on multiwalled carbon nanotubes (MWCNTs) at low temperatures (12–30 K) and low pressures (2 Â 10À5 Torr) using the temperature programmed desorption technique. Our results show that the molecular hydrogen uptake increases nearly exponentially from 6 Â 10À9 wt: % at 24.5 K to 2 Â 10À7 wt: % at 12.5 K and that the desorption kinetics is of the first order. Comparative measurements indicate that MWCNTs have an adsorption capacity about two orders higher than that of activated carbon (charcoal) making them a possible candidate as hydrogen cryosorber for eventual applications in accelerators and synchrotrons. DOI: 10.1103/PhysRevSTAB.11.113201 PACS numbers: 07.20.Mc, 07.30.Cy, 29.20.Àc I. INTRODUCTION Pumping of molecular hydrogen in the Large Hadron Collider is designed by installation of cryosorbers on cryogenic elements operating at very low temperatures [1]. Among many specific requirements for the best choice of cryosorbent materials, the adsorption capacity is certainly a discriminating factor. Though many types of cryosorbers have been tested in the past [2,3], active porous carbon is still the far most important cryosorbent material due to its high specific surface area and large pore volume [4–6]. Usually, three different techniques are widely applied to study the H2 adsorption rate, capacity, sticking and capture probability, and storage in solids: the volumetric method, which measures the pressure drop owing to hydrogen absorption after loading the specimen contained in a constant volume [7]; the gravimetric method, which measures the sample weight changes due to absorption or desorption [8,9]; and the temperature programmed desorption (TPD) technique [10]. This latest measures the hydrogen signal during desorption in high vacuum using mass spectrometry; it is highly sensitive allowing one to study samples with masses even below 1 mg and is especially suited for loading of small quantities at low pressures. In an earliest work, Dillon et al. estimated the hydrogen uptake by single walled carbon nanotubes to be 5–10 wt. % for adsorption conditions of about 0.5 bar and 273 K, followed by cooling and pumping to 133 K and 10À6 Torr [10]. The heat of adsorption determined from the TPD measurements was close to 20 kJ=mol. Wilson et al. measured isotherms of H2 and D2 deposited on single walled, closed end carbon nanotube bundles and observed two well distinct steps corresponding to adsorption on grooves/interstitials and the graphene surface [11]. In this work we present a TPD study on H2 adsorption on multiwalled carbon nanotubes (MWCNTs) and on charcoal at very low pressure and temperatures as a function of gas dose. We show that the molecular hydrogen uptake 1098-4402=08=11(11)=113201(4) decreases with the adsorption temperature and is in the order of 10À7 wt: %, that the desorption kinetics is of the first order, and that the desorption energy is 2:82 Æ 0:16 KJ=mol. Comparative measurements indicate that MWCNTs have an adsorption capacity about two orders higher than that of activated carbon (charcoal) making them a possible candidate as hydrogen cryosorber for eventual applications in accelerators and synchrotrons. II. EXPERIMENTAL MWCNTs were synthesized by chemical vapor deposition using C2 H4 and subsequently purified. As illustrated in Fig. 1, transmission electron microscopy (TEM) shows that FIG. 1. Transmission electron micrograph of multiwalled carbon nanotube sample used in this study. 113201-1 Ó 2008 The American Physical Society F. XU et al. our MWCNT sample was made of individual nanotubes with an outer (inner) diameter of 10–15 (3–6) nm and that many of them were open ended. Microporosity measurements yielded a specific surface of 174 m2 =g. 10 mg of MWCNTs were dispersed in orthodichlorobenzene and sonicated in a glass container. The sample then was deposited directly onto the copper head (6 cm2 ) of a cold finger with closed He cycling and dried. A quite uniform thin film was obtained and its adhesion to the copper surface remained stable during all the measurements. The experiments were conducted in a UHV chamber (base pressure in the low 10À9 Torr range) evacuated with a dry turbo pump. H2 gas (purity of 99.999%) was introduced in the chamber through a precision leak valve while keeping the sample at a desired adsorption temperature Tad and a fixed pressure of 2 Â 10À5 Torr (measured at room temperature and corrected for the ion gauge sensitivity factor ) for a chosen time interval, then the gas was pumped out. The sample temperature was monitored with a Au=0:7%Fe-chromel thermocouple. Thermal desorption was achieved by heating the head of the cold finger with a linear ramp and the desorbed H2 was measured with a Phys. Rev. ST Accel. Beams 11, 113201 (2008) quadruple mass analyzer. No appreciable contamination gas signals were detected neither in the adsorption nor in the desorption processes. Complete reversibility of charging and discharging cycles was routinely verified and a total reproducibility was observed. The vacuum system pumping speed was calibrated by admitting a known quantity of hydrogen into the chamber at desired quadruple reading pressures and monitoring the time needed for evacuation. A linear dependence on the pressure was observed. The desorption rate was obtained by multiplying the detected hydrogen partial pressure (the integral intensity of a quadruple mass spectrum centered at m ¼ 2 a:u: composed of some 50 data points) by the system pumping speed and the total amount of desorbed hydrogen then could be evaluated from the integration of the desorption rate-time curves with an estimated error of about 20%. The density of MWCNTs should be close to 2 g cmÀ2 once deposited on the cold finger in a compact form (not measured). For small quantities of our samples (up to FIG. 2. (Color) Temperature programmed desorption traces of molecular hydrogen adsorbed at Tad ¼ 15 K and Pad ¼ 2 Â 10À5 Torr. The dose is expressed in Langmuir (1 L ¼ 106 Torr sec ). FIG. 3. (Color) Molecular hydrogen adsorbed on MWCNTs as a function of H2 dose for adsorption temperature of Tad ¼ 15 K. Also shown are the results for a charcoal sample. The active surfaces are 174 m2 =g and 23 m2 =g for MWCNTs and charcoal, respectively. 113201-2 H2 ADSORPTION ON MULTIWALLED CARBON . . . 10 mg on a surface of 6 cm2 ), we found the sorption capacity is proportional to the sample mass. III. RESULTS AND DISCUSSIONS In Fig. 2 we illustrate some representative H2 TPD traces taken at a constant adsorption temperature of Tad ¼ 15 K and varying exposure time. The H2 dose È was expressed in units of Langmuir (1 L ¼ 10À6 Torr sec ) and the sample heating rate was fixed at ¼ 5 K= min . These spectra are substantially narrower than those previously reported in literature for H2 dosing at higher temperatures and pressures [10,12]. These TPD traces are quite similar to each other and the spectrum maxima remain unchanged as the hydrogen adsorption increases clearly indicating a first order desorption mechanism. The total molecular hydrogen content, as determined from the integral of the desorbed H2 TPD trace curves, is plotted in Fig. 3 as a function of È for Tad ¼ 15 K. It is interesting to note that it increases linearly with È at low doses and saturates at about 8 Â 10À8 wt:% for È ! 104 L. The saturation at a very low value and the relatively narrow line shape would suggest that hydrogen adsorbs only on a limited number of sites. Phys. Rev. ST Accel. Beams 11, 113201 (2008) To gain some information on the hydrogen binding energy we varied the sample heating rate for constant 2 È ¼ 2100 L and Tad ¼ 19 K. In Fig. 4 we plot lnðTmax = Þ À1 , where T versus Tmax max is the temperature corresponding to the desorption rate maximum. The slope of the linear fit yields a desorption energy of Edes ¼ 340 Æ 20 K (or Edes ¼ 2:82 Æ 0:16 KJ=mol). It is interesting to note that, though there is a large body of experimental studies on hydrogen adsorption on carbon nanotubes because of its potential application as a storage material, very few measurements were undertaken below 77 K [11,13]. To the best of our knowledge, no experimental data are available in literature for adsorption at very low pressures. The binding energy value found in this study is lower than that for H2 on graphite (480 K) [14] in accord with the isothermal adsorption measurements reported by Wilson et al. (3:3 kJ=mol) [11]. This suggests that hydrogen molecules mostly adsorb on the outer surfaces and the reduction in binding energy value is generally attributed to the surface curvature [11,15–17]. This site assignment is in accord with the TEM observation that our sample was essentially made of individual nanotubes (Fig. 1). On the other hand, adsorption on inner walls would imply a much stronger bonding [11,18] and 2 FIG. 4. (Color) Plot of lnðTmax = Þ versus Tmax À1 for a series of TPD spectra taken at fixed Tad ¼ 19 K, È ¼ 2160 L, and varying heating rate . Tmax is the temperature corresponding to the desorption rate maximum. FIG. 5. Saturated molecular hydrogen content adsorbed on MWCNTs as a function of adsorption temperature of Tad . 113201-3 F. XU et al. diffusion into those open ended nanotubes. Quasielastic neutron scattering measurements performed by Narehood et al. also indicated the absence of H2 mobility in SWNTs at temperatures below 30 K [19]. For comparison, in Fig. 3 we also present the total H2 adsorption amount of 10 mg activated charcoal powders (specific surface area 23 m2 =g) under exactly the same experimental conditions. It exhibits very similar characteristics as that of MWCNTs but is about two orders smaller. Finally, in Fig. 5 we illustrate the saturated total amount of adsorbed hydrogen as a function of adsorption temperature Tad . As expected, the total hydrogen adsorption capacity decreases with the cryosorber temperature. For Tad ! 27 K, no H2 adsorption is observed. Our results contrast sharply with those reported for hydrogen storage measurements at higher temperatures and pressures and strongly indicate that under high vacuum conditions, hydrogen does not adsorb on the nanotube surface beyond the sublimation temperature and the diffusion is essentially inhibited, as already confirmed by previous works. We mention that the testing measurements on carbon nanofibers actually used in the LHC were also taken at higher pressures and the so determined sorption capacity can be much higher than the real values in UHV conditions [5,6]. More quantitative works are currently being undertaken to determine the realistic possibility of using nanotubes as cryosorbers in UHV systems. IV. CONCLUSIONS In conclusion, we presented first experimental measurements of molecular hydrogen adsorption on MWCNTs at low pressures (10À5 Torr) and low temperatures (12– 30 K). We showed that the adsorption capacity decreases sharply with temperature and is in the order of 10À7 wt: % at 12 K. This figure is about two orders higher than that obtained for commercial active carbon, making MWCNTs a potential candidate as cryosorbent materials for accelerators.

Journal

Physical Review Special Topics - Accelerators and BeamsAmerican Physical Society (APS)

Published: Nov 3, 2008

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