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The study of Xe adsorption behavior in meso-size pores of carbon black materials using laser-polarized 129Xe NMR spectroscopy
Magnetic Resonance Imaging 21 (2003) 401– 403
The study of Xe adsorption behavior in meso-size pores of carbon black materials using laser-polarized 129Xe NMR spectroscopy Koji Saitoa,*, Atsuomi Kimurab, Hideaki Fujiwarab a
Nippon Steel Corporation, Advanced Technology Research Laboratory, Osaka University, 20-1 Shintomi, Futtsu City, 293-8511, Japan b Faculty of Medicine, Osaka University, 20-1 Shintomi, Futtsu City, 293-8511, Japan
Abstract The meso size pores of carbon black materials with Pt critically affect catalysts which play an important role for fuel cells of electric vehicles. Time-consuming BET methods are usually used to measure the physisorption enthalpy which determines the characteristics of catalysts. The laser polarized method enhances 129Xe polarization by 4 orders of magnitude, overcoming a low sensitivity, making this measurement technique faster than conventional experiments. In this paper, we first demonstrate Laser-Polarized 129Xe NMR Spectroscopy for studying carbon black materials with Pt of fuel cells of electric vehicles in order to determine the physisorption enthalpy. At the same time, T1 experiments using Laser-Polarized 129Xe will be discussed in order to clarify the surface condition and adsorption behavior. © 2003 Elsevier Inc. All rights reserved. Keywords: Laser-Polarized Xe; Meso-pore; Carbon black; Physisorption enthalpy
1. Introduction
2. Experimental
The meso size pores of carbon black materials with Pt are very critical and sensitive for the workings of catalysis which is an important process for fuel cells of electric vehicles [1]. Time-consuming BET methods are usually used to measure the physisorption enthalpy which determines the characteristics of catalysts. 129Xe NMR spectroscopy has also been adapted to determine the pore size of catalysts [2,3], but also it takes a long time to determine the physisorption enthalpy because of the low sensitivity. The laser polarized method enhances 129Xe polarization by 4 orders of magnitude, overcoming a low sensitivity [4], making this measurement technique faster than conventional experiments. In this paper, we first demonstrate Laser-Polarized 129Xe NMR Spectroscopy, which has been shown to be successful for studying carbon black materials with Pt of the fuel cells of electric vehicles in order to decide the physisorption enthalpy. At the same time, T1 experiments using Laser-Polarized 129Xe are presented in order to clarify the surface condition and adsorption behavior.
An optical pumping cell was placed at the side of a superconducting NMR magnet (vertical type) used for NMR measurements in a fringe field of about 12 mT. A Pyrex cylinder cell with a diameter of 60 mm and a length of 100 mm was used. Rb was sealed into the cell under high vacuum and Xe gas was supplied from a cylinder through a pre-drying vessel containing K-Na alloy. The alloy is also useful for removing oxygen and carbon dioxide, which are possibly included in the Xe gas and react with Rb metal. The metallic cluster was checked for high sensitivity of the alloy when passing the Xe gas over the metal alloy. When a small amount of the polarized Xe gas was drawn from the optical pumping cell to the NMR tube for sample preparation, the gas was automatically refilled from the cylinder. A 50 mL plastic syringe was used to temporarily hold Xe gas from the cylinder. The pressure of Xe gas was 1 atm throughout the present experiment, and no other gases were mixed. This mode of gas treatment contributes to the ease of operation: the polarized Xe gas needs no solidification, since neither N2 nor He is mixed; so separation is not necessary. Also, this mode is safe for handling the glass apparatus. In contrast, high-pressure gas, sometimes as high as 10 atm, is often used to broaden the D1 transition of the Rb vapor and to enhance the polarizability as much as
* Corresponding author. Tel.: ⫹81-439-80-2270; fax: ⫹81-439-802746. E-mail address: [email protected] (K. Saito). 0730-725X/03/$ – see front matter © 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0730-725X(03)00150-4
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K. Saito et al. / Magnetic Resonance Imaging 21 (2003) 401– 403
Fig. 1. Typical results for carbon black with added Pt.
possible. In the present study a polarizability of about 3% was attained, as judged from the enhancement in S/N ratio of the 129Xe NMR signal. This value is better than a reported value of 1% for 800-Torr Xe gas in an uncoated Pyrex cell. Silicone coating will surely increase the Xe polarization: probably it will increase the polarization by an order of magnitude. But it was not attempted in the present study, since the signal enhancement was enough, even without such coating. A laser diode was used with 15 W output power to excite the Rb vapor. The polarizing cell was situated in an oven made by a polypropylene sheet to which a constant-temperature air blower, Leister Hot Window S, was attached to maintain the oven temperature at 120°langfenp1041 C. The polarizing system is basically made using Pyrex glass, with an optically ground plate for the window of the polarizing cell and Pyrex stop cocks with highvacuum O-rings from Young Co. are used to manipulate the Xe gas flow. NMR measurements were made on a Varian INOVA 400 WB spectrometer operating at 9.4 T and ambient temperature. A 10 mm size of probe was used for 129 Xe (110.5 MHz) so that a wider range of sample size could be dealt with. The glass or gelatin bulbs were first put into a 10 mm NMR tube, to be filled with polarized Xe gas. The gas inlet specially made on the bulb surface was sealed by adhesive tape after the bulb was filled with polarized Xe gas. The bulbs were then put into another 10 mm NMR tube and the 129Xe NMR spectrum was measured.
3. Results and discussion The T1 was measured by repetitively applying small flip-angle pulses and by analyzing the intensity data according to the equation ln(Sn) ⫽ 共ln(cos ␣) ⫺ TR/T1兲*共n ⫺ 1兲 ⫹ ln(S1)
(1)
where S1 and Sn are the signal intensities for the 1st and nth pulses, respectively, ␣ is the flip angle, and TR is the pulse repetition time. This equation means that plots of ln(Sn) against n-l will give a straight line with a slope of ln(cos ␣ - TR/T1. Exact determination of the flip angle ␣ needs special consideration in laser-polarized experiments, because the ordinary determination method does not apply in this case. That is, the laser-polarized magnetization is not recovered once it is tilted away from z-axis to monitor the pulse angle in the trial and error method. In such a case, repetition of the measurement using a 2 ␣-degree pulse immediately after the ␣-degree pulse can give an estimate of the pulse angle.The difference in the slopes of the plots of Eq. [1] for the ␣- and 2 ␣-degree pulses becomes slope共2 ␣ 兲 ⫺ slope共 ␣ 兲 ⫽ ln(cos共2 ␣ 兲) ⫺ cos共2 ␣ 兲 ⫽ ln共2cos2 ␣ ⫺ 1兲/cos共 ␣ 兲
Fig. 2. Continuous measurements of carbon black materials which are treated with acid and 800 °C by every on 1 sec.
(2)
K. Saito et al. / Magnetic Resonance Imaging 21 (2003) 401– 403
403
fenp1041 1 sec, for both acid and heat treated samples (Fig. 2). Because these treatments are very effective in generating new surface conditions (roughness and new adsorption site etc.), this technique can get information about the surface condition of these samples. Then the relationship between 1/T1 relaxation time and chemical shift drift rate (ppm/sec.) is found (Fig. 3). Therefore, this phenomenon is explained by Xe movement from a semi-stable adsorption site at the surface to a more stable adsorption site. Finally, information obtained by the Laser-Polarized method is useful in order to design high activity catalysts, determined by surface conditions, for fuel cells of electric vehicles. Fig. 3. The relationship between Chemical shift drift rate and 1/T1.
4. Conclusion Therefore, cos ␣ can be determined from the measurement using a pulse angle (and hence a pulse duration time) twice as large as the initial one. In the present study ␣ thus determined was typically 16.5° [5] It is well-known that the chemical shift parameter ␦0, obtained by extrapolation of chemical shift to zero xenon pressure, may be considered characteristic of the pore structure, and the relationship between ␦0 and the dimensions of the pores and channels are very useful equations. Additionally, the temperature variation of the Xe chemical shift defines an average heat of the Xe physisorption enthalpy within porous Vycor, in calorimetric measurements [6]. As we have checked that these relationships are available in the case of carbon materials with Pt, from the viewpoint of measurement time, the Laser-Polarized 129Xe NMR Spectroscopy method (only 1 h to get both pore size information and physisorption enthalpy) has a very big advantage over the conventional BET method (at least 5 days). The results for 20 kinds of carbon materials with Pt using this technique lead to the relationship between the average heat of the Xe physisorption enthalpy and the activity of catalysis for the fuel cells of electric vehicles shown in Fig. 1. Because in the case of 33% Pt, the highest activity of catalysis is performed, the average heat of the Xe physisorption enthalpy is an important and useful factor to design fuel cells of electric vehicles. Next, we found an interesting phenomenon, that ␦0 moved to low-fields and then levels off to a constant value when the measurements were performed every lang-
We first demonstrated Laser-Polarized 129Xe NMR Spectroscopy which has been shown to be successful for studying carbon black materials with Pt of the fuel cells of electric vehicles in order to decide the physisorption enthalpy. At the same time, T1 experiments using LaserPolarized 129Xe are presented in order to clarify the surface condition and adsorption behavior. Finally, many industrial applications are waiting to apply the Laser-Polarized 129Xe NMR method.
References [1] Uchida H, Watanabe M Reformate Fuel Cells for Electric Vehicles. Feramu 2001;6:971– 6. [2] Simonov PA, Filimonova SV, Kryukova GN, Moroz EM, Likholobov VA, Kuretzky T, Boehm HP. 129Xe NMR study of carbonaceous materials: effects of surface chemistry and nanotexture. Carbon 1999; 37:591– 600. [3] McGrath KJ. 129Xe Nuclear Magnetic Resonance investigation of carbon black aggregate morphology. Carbon 1999;37:1443– 48. [4] Goodson BM. Nuclear Magnetic Resonance of Laser-Polarized Noble Gases in Molecules Materials,and Organisms. J Magn Reson 2002; 155:157–216. [5] Fujiwara H, Kimura A, Yanagawa Y, Kamiya T, Hattori M, Hiraga T. Relaxation behavior of Laser-Polarized 129Xe Gas: Size dependency and wall effect of the T1 relaxation time in glass and gelatin Bulbs. J Magn Reson 2001;150:56 – 60. [6] Pasquier V, Levitz P, Tinet D, Delville A. 129Xe NMR as Probe of the Dynamics of Gas Confined in Porous Vycor. Magn Reson Imaging 1996;14:971–3.
The study of Xe adsorption behavior in meso-size pores of carbon black materials using laser-polarized 129Xe NMR spectroscopy Koji Saitoa,*, Atsuomi Kimurab, Hideaki Fujiwarab a
Nippon Steel Corporation, Advanced Technology Research Laboratory, Osaka University, 20-1 Shintomi, Futtsu City, 293-8511, Japan b Faculty of Medicine, Osaka University, 20-1 Shintomi, Futtsu City, 293-8511, Japan
Abstract The meso size pores of carbon black materials with Pt critically affect catalysts which play an important role for fuel cells of electric vehicles. Time-consuming BET methods are usually used to measure the physisorption enthalpy which determines the characteristics of catalysts. The laser polarized method enhances 129Xe polarization by 4 orders of magnitude, overcoming a low sensitivity, making this measurement technique faster than conventional experiments. In this paper, we first demonstrate Laser-Polarized 129Xe NMR Spectroscopy for studying carbon black materials with Pt of fuel cells of electric vehicles in order to determine the physisorption enthalpy. At the same time, T1 experiments using Laser-Polarized 129Xe will be discussed in order to clarify the surface condition and adsorption behavior. © 2003 Elsevier Inc. All rights reserved. Keywords: Laser-Polarized Xe; Meso-pore; Carbon black; Physisorption enthalpy
1. Introduction
2. Experimental
The meso size pores of carbon black materials with Pt are very critical and sensitive for the workings of catalysis which is an important process for fuel cells of electric vehicles [1]. Time-consuming BET methods are usually used to measure the physisorption enthalpy which determines the characteristics of catalysts. 129Xe NMR spectroscopy has also been adapted to determine the pore size of catalysts [2,3], but also it takes a long time to determine the physisorption enthalpy because of the low sensitivity. The laser polarized method enhances 129Xe polarization by 4 orders of magnitude, overcoming a low sensitivity [4], making this measurement technique faster than conventional experiments. In this paper, we first demonstrate Laser-Polarized 129Xe NMR Spectroscopy, which has been shown to be successful for studying carbon black materials with Pt of the fuel cells of electric vehicles in order to decide the physisorption enthalpy. At the same time, T1 experiments using Laser-Polarized 129Xe are presented in order to clarify the surface condition and adsorption behavior.
An optical pumping cell was placed at the side of a superconducting NMR magnet (vertical type) used for NMR measurements in a fringe field of about 12 mT. A Pyrex cylinder cell with a diameter of 60 mm and a length of 100 mm was used. Rb was sealed into the cell under high vacuum and Xe gas was supplied from a cylinder through a pre-drying vessel containing K-Na alloy. The alloy is also useful for removing oxygen and carbon dioxide, which are possibly included in the Xe gas and react with Rb metal. The metallic cluster was checked for high sensitivity of the alloy when passing the Xe gas over the metal alloy. When a small amount of the polarized Xe gas was drawn from the optical pumping cell to the NMR tube for sample preparation, the gas was automatically refilled from the cylinder. A 50 mL plastic syringe was used to temporarily hold Xe gas from the cylinder. The pressure of Xe gas was 1 atm throughout the present experiment, and no other gases were mixed. This mode of gas treatment contributes to the ease of operation: the polarized Xe gas needs no solidification, since neither N2 nor He is mixed; so separation is not necessary. Also, this mode is safe for handling the glass apparatus. In contrast, high-pressure gas, sometimes as high as 10 atm, is often used to broaden the D1 transition of the Rb vapor and to enhance the polarizability as much as
* Corresponding author. Tel.: ⫹81-439-80-2270; fax: ⫹81-439-802746. E-mail address: [email protected] (K. Saito). 0730-725X/03/$ – see front matter © 2003 Elsevier Inc. All rights reserved. doi:10.1016/S0730-725X(03)00150-4
402
K. Saito et al. / Magnetic Resonance Imaging 21 (2003) 401– 403
Fig. 1. Typical results for carbon black with added Pt.
possible. In the present study a polarizability of about 3% was attained, as judged from the enhancement in S/N ratio of the 129Xe NMR signal. This value is better than a reported value of 1% for 800-Torr Xe gas in an uncoated Pyrex cell. Silicone coating will surely increase the Xe polarization: probably it will increase the polarization by an order of magnitude. But it was not attempted in the present study, since the signal enhancement was enough, even without such coating. A laser diode was used with 15 W output power to excite the Rb vapor. The polarizing cell was situated in an oven made by a polypropylene sheet to which a constant-temperature air blower, Leister Hot Window S, was attached to maintain the oven temperature at 120°langfenp1041 C. The polarizing system is basically made using Pyrex glass, with an optically ground plate for the window of the polarizing cell and Pyrex stop cocks with highvacuum O-rings from Young Co. are used to manipulate the Xe gas flow. NMR measurements were made on a Varian INOVA 400 WB spectrometer operating at 9.4 T and ambient temperature. A 10 mm size of probe was used for 129 Xe (110.5 MHz) so that a wider range of sample size could be dealt with. The glass or gelatin bulbs were first put into a 10 mm NMR tube, to be filled with polarized Xe gas. The gas inlet specially made on the bulb surface was sealed by adhesive tape after the bulb was filled with polarized Xe gas. The bulbs were then put into another 10 mm NMR tube and the 129Xe NMR spectrum was measured.
3. Results and discussion The T1 was measured by repetitively applying small flip-angle pulses and by analyzing the intensity data according to the equation ln(Sn) ⫽ 共ln(cos ␣) ⫺ TR/T1兲*共n ⫺ 1兲 ⫹ ln(S1)
(1)
where S1 and Sn are the signal intensities for the 1st and nth pulses, respectively, ␣ is the flip angle, and TR is the pulse repetition time. This equation means that plots of ln(Sn) against n-l will give a straight line with a slope of ln(cos ␣ - TR/T1. Exact determination of the flip angle ␣ needs special consideration in laser-polarized experiments, because the ordinary determination method does not apply in this case. That is, the laser-polarized magnetization is not recovered once it is tilted away from z-axis to monitor the pulse angle in the trial and error method. In such a case, repetition of the measurement using a 2 ␣-degree pulse immediately after the ␣-degree pulse can give an estimate of the pulse angle.The difference in the slopes of the plots of Eq. [1] for the ␣- and 2 ␣-degree pulses becomes slope共2 ␣ 兲 ⫺ slope共 ␣ 兲 ⫽ ln(cos共2 ␣ 兲) ⫺ cos共2 ␣ 兲 ⫽ ln共2cos2 ␣ ⫺ 1兲/cos共 ␣ 兲
Fig. 2. Continuous measurements of carbon black materials which are treated with acid and 800 °C by every on 1 sec.
(2)
K. Saito et al. / Magnetic Resonance Imaging 21 (2003) 401– 403
403
fenp1041 1 sec, for both acid and heat treated samples (Fig. 2). Because these treatments are very effective in generating new surface conditions (roughness and new adsorption site etc.), this technique can get information about the surface condition of these samples. Then the relationship between 1/T1 relaxation time and chemical shift drift rate (ppm/sec.) is found (Fig. 3). Therefore, this phenomenon is explained by Xe movement from a semi-stable adsorption site at the surface to a more stable adsorption site. Finally, information obtained by the Laser-Polarized method is useful in order to design high activity catalysts, determined by surface conditions, for fuel cells of electric vehicles. Fig. 3. The relationship between Chemical shift drift rate and 1/T1.
4. Conclusion Therefore, cos ␣ can be determined from the measurement using a pulse angle (and hence a pulse duration time) twice as large as the initial one. In the present study ␣ thus determined was typically 16.5° [5] It is well-known that the chemical shift parameter ␦0, obtained by extrapolation of chemical shift to zero xenon pressure, may be considered characteristic of the pore structure, and the relationship between ␦0 and the dimensions of the pores and channels are very useful equations. Additionally, the temperature variation of the Xe chemical shift defines an average heat of the Xe physisorption enthalpy within porous Vycor, in calorimetric measurements [6]. As we have checked that these relationships are available in the case of carbon materials with Pt, from the viewpoint of measurement time, the Laser-Polarized 129Xe NMR Spectroscopy method (only 1 h to get both pore size information and physisorption enthalpy) has a very big advantage over the conventional BET method (at least 5 days). The results for 20 kinds of carbon materials with Pt using this technique lead to the relationship between the average heat of the Xe physisorption enthalpy and the activity of catalysis for the fuel cells of electric vehicles shown in Fig. 1. Because in the case of 33% Pt, the highest activity of catalysis is performed, the average heat of the Xe physisorption enthalpy is an important and useful factor to design fuel cells of electric vehicles. Next, we found an interesting phenomenon, that ␦0 moved to low-fields and then levels off to a constant value when the measurements were performed every lang-
We first demonstrated Laser-Polarized 129Xe NMR Spectroscopy which has been shown to be successful for studying carbon black materials with Pt of the fuel cells of electric vehicles in order to decide the physisorption enthalpy. At the same time, T1 experiments using LaserPolarized 129Xe are presented in order to clarify the surface condition and adsorption behavior. Finally, many industrial applications are waiting to apply the Laser-Polarized 129Xe NMR method.
References [1] Uchida H, Watanabe M Reformate Fuel Cells for Electric Vehicles. Feramu 2001;6:971– 6. [2] Simonov PA, Filimonova SV, Kryukova GN, Moroz EM, Likholobov VA, Kuretzky T, Boehm HP. 129Xe NMR study of carbonaceous materials: effects of surface chemistry and nanotexture. Carbon 1999; 37:591– 600. [3] McGrath KJ. 129Xe Nuclear Magnetic Resonance investigation of carbon black aggregate morphology. Carbon 1999;37:1443– 48. [4] Goodson BM. Nuclear Magnetic Resonance of Laser-Polarized Noble Gases in Molecules Materials,and Organisms. J Magn Reson 2002; 155:157–216. [5] Fujiwara H, Kimura A, Yanagawa Y, Kamiya T, Hattori M, Hiraga T. Relaxation behavior of Laser-Polarized 129Xe Gas: Size dependency and wall effect of the T1 relaxation time in glass and gelatin Bulbs. J Magn Reson 2001;150:56 – 60. [6] Pasquier V, Levitz P, Tinet D, Delville A. 129Xe NMR as Probe of the Dynamics of Gas Confined in Porous Vycor. Magn Reson Imaging 1996;14:971–3.