- Email: [email protected]
The effect of additives on the hydrogen storage properties of NaAlH4
international journal of hydrogen energy 34 (2009) 2701–2704
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
The effect of additives on the hydrogen storage properties of NaAlH4 Zheng Xueping*, Liu Shenglin, Li Donglin School of Materials Science and Engineering, Chang’an University, 30 Xueyuan Road, Xi’an 710061, China
article info
abstract
Article history:
The effect of Ti, Co, Ni and LaCl3 on hydrogen release of NaAlH4 was studied by pressure-
Received 19 December 2008
content-temperature (PCT) equipment. The result showed that the sample doped with
Received in revised form
3 mol% LaCl3 presented the largest amount of hydrogen release. Increasing the amount of
24 January 2009
LaCl3 from 1 to 6 mol% caused such marked changes in behavior that the amount and rate
Accepted 26 January 2009
of hydrogen release increased first and then decreased. In addition, the study on the
Available online 20 February 2009
rehydrogenation temperature of NaAlH4 doped with 3 mol% LaCl3 showed that the doped sample during the first rehydrogenation cycle carried out in PCT at 110 C under 8 MPa after
Keywords:
being discharged of hydrogen at 270 C presented the largest amount of hydrogen release.
Additives
ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Hydrogen storage properties NaAlH4 Dehydrogenation amount Rehydrogenation temperature
1.
Introduction
Hydrogen storage has been the subject of intensive research in recent years since hydrogen is pollution-free and can readily be produced from renewable energy resources [1,2]. Complex hydrides, such as NaAlH4 and LiAlH4, are attractive as hydrogen storage compounds due to their high hydrogen content and low weight. In 1997, Bogdanovic´ and Schwickardi [3] demonstrated that the addition of Ti compounds and other transition metal compounds enhanced the desorption kinetics of NaAlH4 and, furthermore, that rehydrogenation was possible under moderate conditions. This finding has created an entirely new prospect for complex hydrides based on Al as promising reversible hydrogen storage media. Theoretically, NaAlH4 can reversibly store 5.6 wt% H2. Unlike intermetallic hydrides, these materials release hydrogen through a series of decomposition reactions as depicted in reactions (1) and (2) [3–6].
3NaAlH4 / Na3AlH6 þ 2Al þ 3H2
(1)
2Na3AlH6 / 6NaH þ 2Al þ 3H2
(2)
Unfortunately, however, hydrogen absorption and desorption kinetics of these systems are generally poor [7]. Recently, much work proved that further kinetic enhancement can be achieved by alternate doping methods and catalysts [8–11]. Wang J, et al. [12,13] reported the first systematic study on the dehydrogenation and hydrogenation kinetics of NaAlH4 co-doped with TiCl3, graphite, FeCl3 and ZrCl4. The results showed that the samples of NaAlH4 when co-doped with binary and ternary combinations of Ti, Zr and Fe at 4 mol% total catalyst content exhibited synergistic behavior, with respect to improving the dehydrogenation kinetics of the first decomposition reaction over that of sample of NaAlH4 doped with 4 mol% Ti or Zr as single catalysts. However, the effects of Ti, Zr and Fe as co-dopants on
* Corresponding author. Tel.: þ86 13991873613. E-mail address: [email protected] (Z. Xueping). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.066
2702
international journal of hydrogen energy 34 (2009) 2701–2704
2.
Experimental
NaAlH4 (S93 wt.% pure) was purchased from J&K Chemical Ltd. Ti, Co, Ni and LaCl3$7H2O were obtained from the Central Iron and Steel Research Institute. In this study, NaAlH4, Ti, Co and Ni were used as received with no additional purification in order to obtain a simple and economic process. Due to a mass of crystal water in LaCl3$7H2O, dehydration was carried out before NaAlH4 was mixed with LaCl3 in order to prevent the effect of the crystal water on NaAlH4. LaCl3$7H2O was heated to 150 C and then kept at the temperature of 3 h under vacuum condition. All operations on the samples were done under dry argon atmosphere in a glove box to prevent reaction with moisture and oxygen in the air. NaAlH4, usually 1 g, was mixed with Ti, Co, Ni and LaCl3 by ball-milling for 10 min at a gyration rate of 200 rpm using a Spex mill. Four hardened steel vials sealed under argon with fourteen steel balls (1 g each) were used. Air-cooling of the vials were employed to prevent the heating during the ball-milling process. The ballmilled samples were then transferred to 3-ml glass bottles in a glove box under dry argon atmosphere. Hydrogen desorption experiments were carried out in pressure-content-temperature (PCT) apparatus. This can be operated up to 10 Mpa and at 600 C. The pressure of hydrogen released in relation to volume was displayed by a pressure transducer. The experimental studies were done by a reactor. This consisted of two parts: heater and sample vessel. The former was used to connect with the pressure transducer and thermocouple. It had a 2.2 cm outside diameter (OD), 0.5 cm wall and 20 cm internal length. It was loaded with the sample vessel (1 cm OD, 0.1 cm wall and 5 cm internal length). The sample vessel was loaded with about 0.1 g of NaAlH4. The reactor was heated with an air furnace. During heating, the hydrogen released overflowed from the sample vessel firstly into the heater and then into the transit pressure transducer. The value of hydrogen pressure can be clearly read. According to the computing formula, the weight percentage of release hydrogen is obtained. However, it is necessary to be pointed out that the calculation about the weight percentage of release hydrogen includes the heavy additives. Rehydrogenation studies were carried out with NaAlH4 doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3. After the first complete dehydrogenation (first two reactions), the samples were kept at different temperatures under w8 MPa hydrogen pressure for 3 h, and then carried out complete dehydrogenation. The phase structure of LaCl3 was determined by a MXP21VAHF X-ray diffractometer (XRD with Cu Ka radiation). X-ray diffraction was done at a tube voltage of 40 kV and
a tube current of 200 mA. The X-ray intensity was measured over a diffraction angle from 10 to 90 with a velocity of 0.02 per step.
3.
Results and discussion
The effects of various catalysts on the hydrogen release characteristics of NaAlH4 were firstly studied. Fig. 1 shows thermal desorption of NaAlH4 samples doped with 3 mol% Ti, Co, Ni and LaCl3 under 150 C. It is very marked that the dehydrogenation amount of the sample doped with 3 mol% LaCl3 is significantly higher than that of the samples doped with 3 mol% Ti, Co and Ni. This indicates that doping with LaCl3 is more advantage to enhance hydrogen storage performance of NaAlH4 than doping with Ti, Co and Ni. In order to further analyze the effect of LaCl3 on dehydrogenation of NaAlH4, thermal desorption of NaAlH4 doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3 was carried out. Because the LaCl3 concentration in the samples is high, especially, in the sample doped with 6 mol% LaCl3, it is important to consider hydrogen absorption of LaCl3 itself. Fig. 2 gives the rehydrogenation curve of LaCl3 at 150 C under 8 MPa hydrogen pressure. It can be clearly seen that hydrogen pressure has no significant change after the LaCl3 sample was laid for 3 h under hydrogen condition. XRD measurements were carried out in order to study the structural changes of the phases. The results are presented in Fig. 3. It can be seen that the major phases presented in the XRD pattern from LaCl3 have no obvious changes before and after hydrogen absorption except for the weakening of the intensity of some diffraction peaks. These proved that LaCl3 has no hydrogen adsorption capability. This result is in accord with the research on La2O3 reported by Gil-Jae Lee [14]. It is found that NaAlH4 with lanthanide oxides reversibly stores hydrogen and these oxides remain stable in NaAlH4. However, the study on La2O3 and LaCl3 by T Sun et al. [7] indicated that La was first hydrogenised as LaH2, and forms same kinds of La–Al alloys, e.g. La3Al11, when the temperature
Amount of hydrogen desorption/wt%
the second decomposition reaction were not as pronounced as their effects on the first reaction. Furthermore, the effect of the rare earth elements on the hydrogenation/dehydrogenation behavior has been studied by D Pukazhselvan, et al. recently [10]. Compared to catalyst Ti, mischmetal was a more effective catalyst for enhancing the desorption kinetics and rehydrogenation of NaAlH4. Here, we chose Ti, Co, Ni and LaCl3 as catalysts and analyzed the effect of the catalysts on the hydrogen storage properties of NaAlH4.
3.5 3.0 2.5 2.0
3mol% Ti 3mol% LaCl3
1.5
3mol% Co 3mol% Ni
1.0 0.5 0.0 0
50
100
150
200
250
300
350
Time/min Fig. 1 – Thermal desorption of doped NaAlH4 samples under 150 8C.
400
2703
international journal of hydrogen energy 34 (2009) 2701–2704
3.5
Amount of hydrogen desorption/wt%
Hydrogen pressure/MPa
8.10
8.08
8.06
8.04
8.02
8.00
0
20
40
60
80
100
120
140
160
180
200
3.0 2.5 2.0 1.5
1 mol% 2 mol%
1.0
3 mol% 4 mol%
0.5
5 mol% 6 mol%
0.0 -50
0
50
100
Time/min
150
200
250
300
350
400
450
Time/min
Fig. 2 – Rehydrogenation curve of LaCl3 under 8 MPa hydrogen pressure.
Fig. 4 – Thermal desorption of NaAlH4 doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3 under 150 8C.
was raised to a certain level. Therefore, it is very important to study further the state of the La additives in NaAlH4 and the reaction principle of the La additives with NaAlH4. Fig. 4 presents the curves of hydrogen release of the samples doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3 under 150 C. The result shows that the samples doped with 3 and 1 mol% LaCl3 present the largest and least amount of hydrogen release, respectively. In addition, it is not difficult to be seen from the slope of curves that the dehydrogenation rate of the sample doped with 3 mol% is the fastest. The rate of hydrogen release shows a gradual increase with increase of doping concentration from 1 to 3 mol%. However, when doping concentration is higher than 3 mol%, the rate of hydrogen release shows a gradual decrease (Fig. 5). Fig. 6 shows the dehydrogenation amount of NaAlH4 as a function of the LaCl3 concentration. It is obvious that increasing the amount of LaCl3 from 1 to 6 mol% caused such
marked changes in behavior that the amount of hydrogen release increased first and then decreased. In the next study, the main aim is to find out and optimize the rehydrogenation temperature of the sample doped with 3 mol% LaCl3. After being degassed under vacuum at room temperature for 1 h, the doped samples could be dehydrogenated at 270 C, and then the samples were subjected for rehydrogenation in reactor at 100, 110, 130, 150 and 180 C under 8 MPa hydrogen pressure for 4 h. After that sample was cooled down to room temperature to stabilize the hydride, it can be used for dehydrogenation. Fig. 7 compares the thermal desorption process of the doped samples carried out in PCT under vacuum condition. It is very easy to be seen that the sample which absorbed hydrogen under 110 C shows the largest dehydrogenation amount. Dehydrogenation amount of rehydrogenation samples drops gradually with increase of rehydrogenation temperature. In addition, dehydrogenation amount of rehydrogenation samples also drops obviously when the rehydrogenation temperature is lower than 110 C.
2.0
Dehydrogenation rate/wt%/min
Intensity/counts
- LaCl3 - LaOCl - La(OH)3 or LaCl3•7H2O - LaCl3•7H2O
(b)
(a)
0
20
40
60
80
2Θ Fig. 3 – XRD data for LaCl3 before (a) and after hydrogen adsorption (b).
100
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
1
2
3
4
5
6
Contentration/mol% Fig. 5 – Dehydrogenation rate for NaAlH4 as a function of added LaCl3.
2704
international journal of hydrogen energy 34 (2009) 2701–2704
Amount of hydrogen desorption/wt%
3.5
temperature of NaAlH4 doped with 3 mol% LaCl3 shows that the doped sample during the first rehydrogenation cycle carried out in PCT at 110 C under 8 MPa after being discharge hydrogen at 270 C presents the largest amount of hydrogen release. The dehydrogenation amount of rehydrogenation samples drops gradually with increase of rehydrogenation temperature.
3.0 2.5 2.0 1.5 1.0
Acknowledgement
0.5 0.0
1
2
3
4
5
This work is supported by National Natural Science Foundation funded projects (50806007).
6
Contentration/mol% Fig. 6 – Dehydrogenation for NaAlH4 as a function of added LaCl3.
Therefore, the optimal rehydrogenation temperature is at 110 C.
4.
Conclusions
The effect of doping with Ti, Co, Ni and LaCl3 on the hydrogen release capacity of NaAlH4 was studied by PCT experiment. The dehydrogenation amount of the NaAlH4 doped with LaCl3 is markedly higher than that of the samples doped with Ti, Co and Ni. The study on hydrogen release of NaAlH4 doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3 under 150 C shows that the sample doped with 3 mol% LaCl3 presents the largest amount and rate of hydrogen release. Increasing the amount of LaCl3 from 1 to 6 mol% caused such marked changes in behavior that amount and rate of hydrogen release increased first and then decreased. In addition, the study on the rehydrogenation
Amount of hydrogen desorption/wt%
3.5 110°C
3.0
130°C
2.5
100°C
2.0
150°C 180°C
1.5 1.0 0.5 0.0 80
100
120
140
160
180
200
220
240
260
280
Temperature/°C Fig. 7 – Dehydrogenation curves of the samples doped with 3mol% LaCl3 after rehydrogenation under different temperatures.
references
[1] Berry Gene D, Salvador M. Aceves. Onboard storage alternatives for hydrogen vehicles. Energy Fuels 1998;12(1): 49–55. [2] Andrer CM, Walmsley J, Blanchard D, Brinks HW, Holmestad R, Hauback BC. Electron microscopy studies of lithium aluminium hydrides. J Alloys Compd 2005;395:307–12. [3] Bogdanovic´ B, Schwickardi M. Ti-doped alkali metal aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloys Compd 1997;253–254:1–9. [4] Easton DS, Schneibel JH, Speakman SA. Factors affecting hydrogen release form lithium alanate (LiAlH4). J Alloys Compd 2005;398:245–8. [5] Brinks HW, Hauback BC, Norby P, Fjellva˚g H. The decomposition of LiAlD4 studied by in-situ X-ray and neutron diffraction. J Alloys Compd 2003;351:222–7. [6] Løvvik OM. Crystal structure and thermodynamic stability of the lithium alanates LiAlH4 and Li3AlH6. Phys Rev B 2004;69: 134117–9. [7] Sun T, Zhou B, Wang H, Zhu M. The effect of doping rareearth chloride dopant on the dehydrogenation properties of NaAlH4 and its catalytic mechanism. Int J Hydrogen Energy 2008;33:2260–7. [8] Scndrock G, Gross K, Thomas G. Effect of Ti-catalyst content on the reversible hydrogen storage properties of the sodium alanates. J Alloys Compd 2002;339:299–308. [9] Lee Gil-Jae, Kim Ti Woo, Shim Jae-Hyeok, Whan Cho Young, Lee Kyung Sub. Synthesis of ultrafine titanium aluminide powders and their catalytic enhancement in dehydrogenation kinetics of NaAlH4. Scripta Materialia 2007; 56:125–8. [10] Pukazhselvan D, Sterlin Leo Hudson M, Kumar Gupta Bipin, Shaz MA, Srivastava ON. Investigations on the desorption kinetics of Mm-doped NaAlH4. J Alloys Compd 2007;439:243–8. [11] Wang T, Wang J, Ebner AD, Ritter JA. Reversible hydrogen storage properties of NaAlH4 catalyzed with scandium. J Alloys Compd 2008;450:293–300. [12] Wang J, Ebner AD, Zidan R, Ritter JA. Synergistic effects of codopants on the dehydrogenation kinetics of sodium aluminium hydride. J Alloys Compd 2005;391:245–55. [13] Wang J, Ebner AD, Prozorov T, Zidan R, Ritter JA. Effect of graphite as a co-dopant on the dehydrogenation and hydrogenation kinetics of Ti-doped sodium aluminium hydride. J Alloys Compd 2005;395:252–62. [14] Lee Gil-Jae, Shim Jae-Hyeok, Whan Cho Young, Lee Kyung Sub. Reversible hydrogen storage in NaAlH4 catalyzed with lanthanide oxides. Int J Hydrogen Energy 2007;32:1911–5.
Available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
The effect of additives on the hydrogen storage properties of NaAlH4 Zheng Xueping*, Liu Shenglin, Li Donglin School of Materials Science and Engineering, Chang’an University, 30 Xueyuan Road, Xi’an 710061, China
article info
abstract
Article history:
The effect of Ti, Co, Ni and LaCl3 on hydrogen release of NaAlH4 was studied by pressure-
Received 19 December 2008
content-temperature (PCT) equipment. The result showed that the sample doped with
Received in revised form
3 mol% LaCl3 presented the largest amount of hydrogen release. Increasing the amount of
24 January 2009
LaCl3 from 1 to 6 mol% caused such marked changes in behavior that the amount and rate
Accepted 26 January 2009
of hydrogen release increased first and then decreased. In addition, the study on the
Available online 20 February 2009
rehydrogenation temperature of NaAlH4 doped with 3 mol% LaCl3 showed that the doped sample during the first rehydrogenation cycle carried out in PCT at 110 C under 8 MPa after
Keywords:
being discharged of hydrogen at 270 C presented the largest amount of hydrogen release.
Additives
ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved.
Hydrogen storage properties NaAlH4 Dehydrogenation amount Rehydrogenation temperature
1.
Introduction
Hydrogen storage has been the subject of intensive research in recent years since hydrogen is pollution-free and can readily be produced from renewable energy resources [1,2]. Complex hydrides, such as NaAlH4 and LiAlH4, are attractive as hydrogen storage compounds due to their high hydrogen content and low weight. In 1997, Bogdanovic´ and Schwickardi [3] demonstrated that the addition of Ti compounds and other transition metal compounds enhanced the desorption kinetics of NaAlH4 and, furthermore, that rehydrogenation was possible under moderate conditions. This finding has created an entirely new prospect for complex hydrides based on Al as promising reversible hydrogen storage media. Theoretically, NaAlH4 can reversibly store 5.6 wt% H2. Unlike intermetallic hydrides, these materials release hydrogen through a series of decomposition reactions as depicted in reactions (1) and (2) [3–6].
3NaAlH4 / Na3AlH6 þ 2Al þ 3H2
(1)
2Na3AlH6 / 6NaH þ 2Al þ 3H2
(2)
Unfortunately, however, hydrogen absorption and desorption kinetics of these systems are generally poor [7]. Recently, much work proved that further kinetic enhancement can be achieved by alternate doping methods and catalysts [8–11]. Wang J, et al. [12,13] reported the first systematic study on the dehydrogenation and hydrogenation kinetics of NaAlH4 co-doped with TiCl3, graphite, FeCl3 and ZrCl4. The results showed that the samples of NaAlH4 when co-doped with binary and ternary combinations of Ti, Zr and Fe at 4 mol% total catalyst content exhibited synergistic behavior, with respect to improving the dehydrogenation kinetics of the first decomposition reaction over that of sample of NaAlH4 doped with 4 mol% Ti or Zr as single catalysts. However, the effects of Ti, Zr and Fe as co-dopants on
* Corresponding author. Tel.: þ86 13991873613. E-mail address: [email protected] (Z. Xueping). 0360-3199/$ – see front matter ª 2009 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.01.066
2702
international journal of hydrogen energy 34 (2009) 2701–2704
2.
Experimental
NaAlH4 (S93 wt.% pure) was purchased from J&K Chemical Ltd. Ti, Co, Ni and LaCl3$7H2O were obtained from the Central Iron and Steel Research Institute. In this study, NaAlH4, Ti, Co and Ni were used as received with no additional purification in order to obtain a simple and economic process. Due to a mass of crystal water in LaCl3$7H2O, dehydration was carried out before NaAlH4 was mixed with LaCl3 in order to prevent the effect of the crystal water on NaAlH4. LaCl3$7H2O was heated to 150 C and then kept at the temperature of 3 h under vacuum condition. All operations on the samples were done under dry argon atmosphere in a glove box to prevent reaction with moisture and oxygen in the air. NaAlH4, usually 1 g, was mixed with Ti, Co, Ni and LaCl3 by ball-milling for 10 min at a gyration rate of 200 rpm using a Spex mill. Four hardened steel vials sealed under argon with fourteen steel balls (1 g each) were used. Air-cooling of the vials were employed to prevent the heating during the ball-milling process. The ballmilled samples were then transferred to 3-ml glass bottles in a glove box under dry argon atmosphere. Hydrogen desorption experiments were carried out in pressure-content-temperature (PCT) apparatus. This can be operated up to 10 Mpa and at 600 C. The pressure of hydrogen released in relation to volume was displayed by a pressure transducer. The experimental studies were done by a reactor. This consisted of two parts: heater and sample vessel. The former was used to connect with the pressure transducer and thermocouple. It had a 2.2 cm outside diameter (OD), 0.5 cm wall and 20 cm internal length. It was loaded with the sample vessel (1 cm OD, 0.1 cm wall and 5 cm internal length). The sample vessel was loaded with about 0.1 g of NaAlH4. The reactor was heated with an air furnace. During heating, the hydrogen released overflowed from the sample vessel firstly into the heater and then into the transit pressure transducer. The value of hydrogen pressure can be clearly read. According to the computing formula, the weight percentage of release hydrogen is obtained. However, it is necessary to be pointed out that the calculation about the weight percentage of release hydrogen includes the heavy additives. Rehydrogenation studies were carried out with NaAlH4 doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3. After the first complete dehydrogenation (first two reactions), the samples were kept at different temperatures under w8 MPa hydrogen pressure for 3 h, and then carried out complete dehydrogenation. The phase structure of LaCl3 was determined by a MXP21VAHF X-ray diffractometer (XRD with Cu Ka radiation). X-ray diffraction was done at a tube voltage of 40 kV and
a tube current of 200 mA. The X-ray intensity was measured over a diffraction angle from 10 to 90 with a velocity of 0.02 per step.
3.
Results and discussion
The effects of various catalysts on the hydrogen release characteristics of NaAlH4 were firstly studied. Fig. 1 shows thermal desorption of NaAlH4 samples doped with 3 mol% Ti, Co, Ni and LaCl3 under 150 C. It is very marked that the dehydrogenation amount of the sample doped with 3 mol% LaCl3 is significantly higher than that of the samples doped with 3 mol% Ti, Co and Ni. This indicates that doping with LaCl3 is more advantage to enhance hydrogen storage performance of NaAlH4 than doping with Ti, Co and Ni. In order to further analyze the effect of LaCl3 on dehydrogenation of NaAlH4, thermal desorption of NaAlH4 doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3 was carried out. Because the LaCl3 concentration in the samples is high, especially, in the sample doped with 6 mol% LaCl3, it is important to consider hydrogen absorption of LaCl3 itself. Fig. 2 gives the rehydrogenation curve of LaCl3 at 150 C under 8 MPa hydrogen pressure. It can be clearly seen that hydrogen pressure has no significant change after the LaCl3 sample was laid for 3 h under hydrogen condition. XRD measurements were carried out in order to study the structural changes of the phases. The results are presented in Fig. 3. It can be seen that the major phases presented in the XRD pattern from LaCl3 have no obvious changes before and after hydrogen absorption except for the weakening of the intensity of some diffraction peaks. These proved that LaCl3 has no hydrogen adsorption capability. This result is in accord with the research on La2O3 reported by Gil-Jae Lee [14]. It is found that NaAlH4 with lanthanide oxides reversibly stores hydrogen and these oxides remain stable in NaAlH4. However, the study on La2O3 and LaCl3 by T Sun et al. [7] indicated that La was first hydrogenised as LaH2, and forms same kinds of La–Al alloys, e.g. La3Al11, when the temperature
Amount of hydrogen desorption/wt%
the second decomposition reaction were not as pronounced as their effects on the first reaction. Furthermore, the effect of the rare earth elements on the hydrogenation/dehydrogenation behavior has been studied by D Pukazhselvan, et al. recently [10]. Compared to catalyst Ti, mischmetal was a more effective catalyst for enhancing the desorption kinetics and rehydrogenation of NaAlH4. Here, we chose Ti, Co, Ni and LaCl3 as catalysts and analyzed the effect of the catalysts on the hydrogen storage properties of NaAlH4.
3.5 3.0 2.5 2.0
3mol% Ti 3mol% LaCl3
1.5
3mol% Co 3mol% Ni
1.0 0.5 0.0 0
50
100
150
200
250
300
350
Time/min Fig. 1 – Thermal desorption of doped NaAlH4 samples under 150 8C.
400
2703
international journal of hydrogen energy 34 (2009) 2701–2704
3.5
Amount of hydrogen desorption/wt%
Hydrogen pressure/MPa
8.10
8.08
8.06
8.04
8.02
8.00
0
20
40
60
80
100
120
140
160
180
200
3.0 2.5 2.0 1.5
1 mol% 2 mol%
1.0
3 mol% 4 mol%
0.5
5 mol% 6 mol%
0.0 -50
0
50
100
Time/min
150
200
250
300
350
400
450
Time/min
Fig. 2 – Rehydrogenation curve of LaCl3 under 8 MPa hydrogen pressure.
Fig. 4 – Thermal desorption of NaAlH4 doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3 under 150 8C.
was raised to a certain level. Therefore, it is very important to study further the state of the La additives in NaAlH4 and the reaction principle of the La additives with NaAlH4. Fig. 4 presents the curves of hydrogen release of the samples doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3 under 150 C. The result shows that the samples doped with 3 and 1 mol% LaCl3 present the largest and least amount of hydrogen release, respectively. In addition, it is not difficult to be seen from the slope of curves that the dehydrogenation rate of the sample doped with 3 mol% is the fastest. The rate of hydrogen release shows a gradual increase with increase of doping concentration from 1 to 3 mol%. However, when doping concentration is higher than 3 mol%, the rate of hydrogen release shows a gradual decrease (Fig. 5). Fig. 6 shows the dehydrogenation amount of NaAlH4 as a function of the LaCl3 concentration. It is obvious that increasing the amount of LaCl3 from 1 to 6 mol% caused such
marked changes in behavior that the amount of hydrogen release increased first and then decreased. In the next study, the main aim is to find out and optimize the rehydrogenation temperature of the sample doped with 3 mol% LaCl3. After being degassed under vacuum at room temperature for 1 h, the doped samples could be dehydrogenated at 270 C, and then the samples were subjected for rehydrogenation in reactor at 100, 110, 130, 150 and 180 C under 8 MPa hydrogen pressure for 4 h. After that sample was cooled down to room temperature to stabilize the hydride, it can be used for dehydrogenation. Fig. 7 compares the thermal desorption process of the doped samples carried out in PCT under vacuum condition. It is very easy to be seen that the sample which absorbed hydrogen under 110 C shows the largest dehydrogenation amount. Dehydrogenation amount of rehydrogenation samples drops gradually with increase of rehydrogenation temperature. In addition, dehydrogenation amount of rehydrogenation samples also drops obviously when the rehydrogenation temperature is lower than 110 C.
2.0
Dehydrogenation rate/wt%/min
Intensity/counts
- LaCl3 - LaOCl - La(OH)3 or LaCl3•7H2O - LaCl3•7H2O
(b)
(a)
0
20
40
60
80
2Θ Fig. 3 – XRD data for LaCl3 before (a) and after hydrogen adsorption (b).
100
1.8 1.6 1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.0
1
2
3
4
5
6
Contentration/mol% Fig. 5 – Dehydrogenation rate for NaAlH4 as a function of added LaCl3.
2704
international journal of hydrogen energy 34 (2009) 2701–2704
Amount of hydrogen desorption/wt%
3.5
temperature of NaAlH4 doped with 3 mol% LaCl3 shows that the doped sample during the first rehydrogenation cycle carried out in PCT at 110 C under 8 MPa after being discharge hydrogen at 270 C presents the largest amount of hydrogen release. The dehydrogenation amount of rehydrogenation samples drops gradually with increase of rehydrogenation temperature.
3.0 2.5 2.0 1.5 1.0
Acknowledgement
0.5 0.0
1
2
3
4
5
This work is supported by National Natural Science Foundation funded projects (50806007).
6
Contentration/mol% Fig. 6 – Dehydrogenation for NaAlH4 as a function of added LaCl3.
Therefore, the optimal rehydrogenation temperature is at 110 C.
4.
Conclusions
The effect of doping with Ti, Co, Ni and LaCl3 on the hydrogen release capacity of NaAlH4 was studied by PCT experiment. The dehydrogenation amount of the NaAlH4 doped with LaCl3 is markedly higher than that of the samples doped with Ti, Co and Ni. The study on hydrogen release of NaAlH4 doped with 1, 2, 3, 4, 5 and 6 mol% LaCl3 under 150 C shows that the sample doped with 3 mol% LaCl3 presents the largest amount and rate of hydrogen release. Increasing the amount of LaCl3 from 1 to 6 mol% caused such marked changes in behavior that amount and rate of hydrogen release increased first and then decreased. In addition, the study on the rehydrogenation
Amount of hydrogen desorption/wt%
3.5 110°C
3.0
130°C
2.5
100°C
2.0
150°C 180°C
1.5 1.0 0.5 0.0 80
100
120
140
160
180
200
220
240
260
280
Temperature/°C Fig. 7 – Dehydrogenation curves of the samples doped with 3mol% LaCl3 after rehydrogenation under different temperatures.
references
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