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Al catalyzed with TiN, TiMn2 and LaNi5
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 0 e1 0 2 1 4
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogenation of LiH/Al catalyzed with TiN, TiMn2 and LaNi5 Felix F. Azenwi, Henrietta W. Langmi, G. Sean McGrady* Department of Chemistry, University of New Brunswick, PO Box 4400, Fredericton, New Brunswick E3B 5A3, Canada
article info
abstract
Article history:
In order to investigate the catalytic effect of TiN, TiMn2 and LaNi5 on the hydrogen storage
Received 3 January 2012
capacity of LiAlH4, 2 mol% of the catalyst was milled with LiH/Al and then hydrogenated in
Received in revised form
Me2O. Doping with TiN, TiMn2 or LaNi5 led to substantial hydrogenation of LiH/Al in
28 March 2012
accordance with the formation of LiAlH4. In each case the amount of hydrogen absorbed
Accepted 30 March 2012
was dependent on the catalyst and the ball-to-powder ratio used during milling. A high
Available online 4 May 2012
ball-to-powder ratio results in an improvement in the hydrogen storage capacity of LiAlH4. For each ball-to-powder ratio the highest hydrogen storage capacity was recorded for the
Keywords:
TiN-catalyzed sample; hydrogen storage capacity increased from 3.2 to 4.8 to 6.0 wt.% H as
Hydrogen storage material
the ball to-powder ratio increased from 10:1 to 20:1 to 40:1. The high levels of hydroge-
Lithium alanate
nation of LiH/Al catalyzed with TiN, TiMn2 and LaNi5 are remarkable because for the LiAlH4
Catalyst
system only a TiCl3 catalyst has previously been shown to result in rehydrogenation of the
Ball milling
dehydrogenated products to LiAlH4. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fossil fuels are a relatively cheap source of energy, which accounts for their widespread usage, but environmental awareness is paving the way for alternative sources of energy. Hydrogen, amongst others like solar, wind and biomass, is a major contributor in the clean and sustainable energy sector. However, its application is limited and dependent on its physical form. Due to the low density and high flammability of hydrogen, gaseous and liquid hydrogen require large volumes and/or heavy, thick-walled containers for transportation and storage. A possible solution to this would be the use of hydrogen rich materials, such as complex metal hydrides, as hydrogen storage materials, releasing and reabsorbing hydrogen gas at acceptable rates and temperatures. Extensive research has been conducted on these materials, but high desorption temperatures and general irreversibility remain significant stumbling blocks [1,2]. This impasse was broken when Bogdanovic et al. successfully reversed the
dehydrogenation reactions of NaAlH4 by adding small amounts of Ti dopant [3]. Other examples of complex metal hydrides under investigation include LiAlH4, Mg(AlH4)2, Ca(AlH4)2, LiBH4 and NaBH4, with hydrogen contents of 10.5, 9.3, 7.8, 18.4 and 10.6 wt.%, respectively [1,2]. Theoretically, LiAlH4 can release 7.9 wt.% H below 220 C, which makes it one of the best candidate materials for hydrogen storage. Desorption of hydrogen occurs in two stages at 160e180 C and 180e220 C according to reactions (1) and (2), respectively [2]. 3LiAlH4 /Li3 AlH6 þ 2Al þ 3H2
5:3 wt:%H
Li3 AlH6 þ 2Al/3LiH þ 3Al þ 3=2H2
2:6 wt:%H
(1) (2)
The main limitations of LiAlH4 as a hydrogen storage material are the irreversibility of its dehydrogenation reactions under practical conditions and its relatively slow desorption kinetics [1,2,4]. A plethora of studies has demonstrated successful dehydrogenation of LiAlH4 to LiH and Al,
* Corresponding author. E-mail address: [email protected] (G. Sean McGrady). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.160
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 0 e1 0 2 1 4
with an improvement in dehydrogenation kinetics and lowering of desorption temperature when a catalyst is added [5e9]. Nevertheless, failure of subsequent rehydrogenation to LiAlH4 was also highlighted [7e9]. It has been reported that reversal of reaction (1) at room temperature requires more than 103 bar hydrogen pressure [4]. However, partial reversibility to Li3AlH6 occurs when LiAlH4 is doped with various catalysts, including LaCl3, Ce(SO4)2, Ni, Ti and TiC [10e12]. These have often resulted in less than 1 wt.% H reabsorption capacity. The highest amount of reydrogenation (1.9 wt.% H) was achieved at 165 C and 95 bar H2 when 5 mol% TiC was used as a catalyst [12]. In solution, reaction (1) can be more easily reversed; using TiCl3 catalyst complete rehydrogenation of LiAlH4 in THF was achieved [13,14], and in Me2O more than 7 wt.% H was re-absorbed [15]. To the best of our knowledge, amongst all the catalysts reported, TiCl3 is the only one that has afforded very high levels of regenerated LiAlH4 capable of meeting the demands of hydrogen storage applications. In the present work, we report for the first time substantial formation of LiAlH4 from LiH/Al catalyzed with TiN, TiMn2 and LaNi5. Formation of LiAlH4 from catalyzed LiH/Al provides a path to regeneration of LiAlH4 from its dehydrogenated products. Nano-sized TiN has already been shown to enhance the hydrogen storage properties of NaAlH4 [16,17]. Kim et al. [17] reported that TiN nanopowder synthesized by mechanochemical high-energy ball milling of TiCl3 and Li3N shows better catalytic influence on the cyclability and hydrogen capacity of NaAlH4 than TiCl3. The other dopants used in this study, TiMn2 and LaNi5, are AB2 and AB5 alloys, respectively. These alloys on their own are known to store up to 2 wt.% H at near ambient conditions [18,19].
2.
Experimental
2.1.
Sample preparation
reaction was left to stir at room temperature for 24 h. Finally, the excess hydrogen and solvent were vented and the dry powder was collected for further characterization.
2.2.
Sample characterization
Powder X-ray diffraction (XRD) patterns were measured using a Rigaku Miniflex diffractometer (CuKa radiation). Samples were mounted on a plastic holder and covered with parafilm to protect them from contact with air during the measurements. The parafilm resulted in additional diffraction peaks at ca. 21.6 and 24 , which were distinct from diffraction peaks of the samples. Thermogravimetric analysis (TGA) was performed using a TA Instruments Q50 TGA. Typically, about 5 mg of sample was loaded into an Al crucible and sealed with an Al lid in a glovebox. A pinhole pierced in the lid allowed the escape of H2 released during the measurements and minimized exposure of the sample to the atmosphere as it was transferred to the TGA instrument. The sample was heated to 300 C at a ramping rate of 2 C/min under a flowing stream of N2 at a rate of 120 mL/min. Differential scanning calorimetry (DSC) experiments were carried out using a TA Instruments Q20P DSC. About 5 mg of material was placed in an Al pan, which was sealed with an AL lid in a glovebox. The pan was placed on the DSC instrument and the sample was heated to 300 C at 2 C/min under a nitrogen flow of 95 mL/min. Desorption plots for the samples were obtained using a Sieverts-type instrument, PCTPro-2000 manufactured by HyEnergy LLC. In a typical experiment, about 250 mg sample was loaded into a sample holder in a glovebox. This was then attached to the PCT instrument without exposure of the sample to air. The vessel was heated at 2 C/min to 250 C, and then maintained at this temperature for 30 min.
3.
LiH (Aldrich, 95%), Al (Alfa Aesar, 99.97%), TiN (Strem Chemicals, 99þ %-Ti), TiMn2 (Aldrich, H2 storage grade) and LaNi5 (Aldrich, H2 storage grade) were used as received. All materials and samples were handled in a nitrogen-filled glovebox. Ball milling of LiH/Al was carried out in a 1:1 molar ratio, with 2 mol% catalyst (TiN, TiMn2 or LaNi5). In a typical experiment 2 g of the combined powders was loaded in a 50 mL tungsten carbide vessel containing tungsten carbide balls. The number of balls differed according to the desired ball-to-powder ratio. Three different ball-to-powder ratios were employed; i.e. 40:1, 20:1 and 10:1. The powders were mechanically milled at room temperature in a nitrogen atmosphere at a rotational speed of 300 rpm for 12 h using a Retsch PM 100 planetary ball mill. After every 15 min of milling there was a 30 s pause and the rotation was automatically reversed. Hydrogenation reactions in Me2O (Air Liquide Canada Inc., Chemically pure) were carried out in a Parr Instruments 500 mL stainless steel stirred reactor. In a typical procedure, 1.79 g of doped LiH/Al was loaded into the reactor in the glovebox. The reactor was sealed, taken out of the glovebox and then connected to the Me2O cylinder. Me2O (55 g) was transferred to the reactor using a solvent pump and 100 bar H2 gas was added. The
10211
Results and discussion
The effect of TiN, TiMn2 and LaNi5 catalysts on the hydrogenation of LiH/Al was studied in detail. The relationship between the amount of hydrogen released from the hydrogenated product and ball-to-powder ratio was also investigated, as ball milling affects the yield of LiAlH4 generated. Previously, we have demonstrated that undoped LiH/Al does not undergo hydrogenation in Me2O and 100 bar H2. Fig. 1 shows the TGA plots for hydrogenated LiH/Al doped with 2 mol% TiN, TiMn2 and LaNi5. In each case, hydrogen release began at ca. 10 C and continued up to ca. 190 C, with the plots showing two obvious regions of hydrogen release corresponding to reactions (1) and (2). Powder XRD confirmed that LiAlH4 was generated upon hydrogenation (Fig. 2). According to the TGA plots, the first step of hydrogen release occurred at 100e145 C. The yields of LiAlH4, and hence the amount of hydrogen desorbed, were dependent on the ballto-powder ratio used during milling. The yield of LiAlH4 decreased with decreasing ball-to-powder ratio. Using TiN as dopant, a total of 6.0, 4.8 and 3.2 wt.% H was released when the ball-to-powder ratios were 40:1, 20:1 and 10:1, respectively. In the case of TiMn2, the hydrogen desorption capacities were 5.8, 4.7 and 1.4 wt.% H for the ball-to-powder ratios 40:1, 20:1 and 10:1, respectively. For LaNi5, the amount of
10212
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 0 e1 0 2 1 4
Fig. 1 e TGA plots for hydrogenated LiH/Al catalyzed with 2 mol % TiN (aec), TiMn2 (def) and LaNi5 (g and h) for different ball-to-powder ratios (BPRs) used during milling. Sample with (a) TiN catalyst, BPR [ 40:1; (b) TiN catalyst, BPR [ 20:1; (c) TiN catalyst, BPR [ 10:1; (d) TiMn2 catalyst, BPR [ 40:1; (e) TiMn2 catalyst, BPR [ 20:1; (f) TiMn2 catalyst, BPR [ 10:1; (g) LaNi5 catalyst, BPR [ 40:1 and (h) LaNi5 catalyst, BPR [ 20:1.
X
X Parafilm
X
Intensity (a.u.)
(b)
(a) 20
30
40
50
60
70
80
90
2 Theta (degrees) Fig. 2 e Powder XRD plot for (a) LiAlH4 from ICDD and (b) hydrogenated LiH/Al catalyzed with 2 mol% TiN for a ballto-powder ratio of 40:1 used during milling.
desorbed hydrogen recorded was even lower, reaching only 3.7 and 1.0 wt.% H for ball-to-powder ratios of 40:1 and 20:1, respectively. The hydrogen desorption capacities for the doped LiAlH4 are less than the theoretical value of 7.9 wt.% H indicating incomplete hydrogenation. This difference can also be accounted for, at least in part, by the additional weight of the catalyst, since the capacities are expressed as a percentage of the entire sample. Another reason for the disparity may be the fact that the LiH starting material had a quoted purity of 95%. The general trend of increasing yield with increasing ballto-powder ratio can be explained by the decrease in particle size caused by milling. Ball milling is a high-energy technique that results in the grinding and intimate mixing of particles. High ball-to-powder ratios will produce finer particles and increase the surface area for catalytic activity, accounting for the increase in hydrogen release in the ball-to-powder ratio order 10:1 < 20:1 < 40:1. The greatest effect was noticed for the sample containing TiMn2 catalyst, which showed an increase in hydrogen uptake of 3.3 wt.% H from 10:1 to 20:1 ball-topowder ratio. The role of both TiMn2 and LaNi5 in achieving hydrogenation of LiH/Al can be attributed to the fact that these catalysts are both hydrogen storage materials themselves at ambient temperatures [18,19]. Their presence in minute quantities in the LiH/Al milled powder was sufficient to trigger hydrogenation of the mixture at room temperature in Me2O. The significant difference in the desorbed hydrogen content between TiMn2 and LaNi5-catalyzed samples with corresponding ball-to-powder ratios may be associated with the difference in the weight of the two alloys. The weight of LaNi5 is more than double that of TiMn2; moreover, LaNi5 has a lower hydrogen storage capacity (ca. 1.5 wt.%) than TiMn2 (ca. 1.9 wt.%). For each ball-to-powder ratio studied the best results were obtained for the samples catalyzed with TiN, with an impressively high value of 6.0 wt.% H being obtained for a ball-to-powder... ratio of 40:1. Interestingly, this value exceeds that previously reported for rehydrogenated LiAlH4 doped with 2 mol% of the established TiCl3 catalyst [15,20]. This result is significant because TiN has previously been shown to enhance the hydrogen storage properties of the widely studied NaAlH4 [16,17]. Since TiN and TiCl3 each contains Ti in an oxidation state of þ3, it is tempting to infer that this valency may play a role in the hydrogenation of LiH/ Al. However, it has been reported that Ti(III) is reduced to Ti(0) in the TiCl3-doped NaAlH4 system [21], so such speculation may be dangerous. Further studies are underway to investigate in detail the part played by these catalysts amongst others. Hydrogenated LiH/Al doped with 2 mol% TiN was further examined using the PCT instrument. Hydrogen desorption curves were recorded as the samples were heated to 250 C (Fig. 3). The amount of desorbed hydrogen increased with increasing ball-to-powder ratio used during milling, in accordance with the TGA results. However, the plots do not show the two-step desorption observed in the TGA plots because of the difference in measurement technique. The hydrogen storage capacity measured for the 40:1 ball-to-powder ratio sample was 5.7 wt.% H. This sample had the fastest desorption kinetics, and it clearly started releasing hydrogen at least 20 C lower than the 20:1 and 10:1 ball-to-powder ratio samples.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 0 e1 0 2 1 4
300
6
4.
5
250
(b)
4 3
200
(a)
150
o
2
Temperature ( C)
Desorbed Hydrogen (wt.%)
(c)
100
1 50
0 0
1
2
3
Time (h)
Fig. 3 e Desorption plots for hydrogenated LiH/Al catalyzed with 2 mol% TiN for different ball-to-powder ratios (BPRs) used during milling. Sample with (a) BPR [ 10:1; (b) BPR [ 20:1 and (c) BPR [ 40:1.
This can be ascribed to the finer, more homogeneous particles obtained for the high 40:1... ball-to-powder ratio sample. Fig. 4 shows DSC results for hydrogenated samples (ball-topowder ratio ¼ 40:1). The two endothermic peaks characteristic of TiCl3-doped LiAlH4 were also observed here [22]. As was the case with the TiCl3-doped samples, the first peak with a maximum around 130e145 C is assigned to the release of hydrogen, as described by reaction (1); i.e. the conversion of LiAlH4 to Li3AlH6. Accordingly, the second peak with maximum at 165e180 C corresponds to the decomposition of Li3AlH6 to LiH and Al. These two endothermic events in the DSC plot correlate exactly with the two mass-loss steps observed in the TGA plot (Fig. 1). It can also be seen from the DSC curves that the amount of heat flow for the sample containing LaNi5 catalyst is relatively small compared to the samples containing TiMn2 and TiN. This is in agreement with the lower hydrogen desorption capacity observed for the LaNi5-catalyzed sample.
Fig. 4 e DSC plots for hydrogenated LiH/Al catalyzed with 2 mol % TiN, TiMn2 and LaNi5 for a ball-to-powder ratio of 40:1 used during milling. Sample with (a) LaNi5 catalyst; (b) TiMn2 catalyst and (c) TiN catalyst.
10213
Conclusions
This work has demonstrated that doping LiH/Al with TiN, TiMn2 and LaNi5 followed by hydrogenation in Me2O leads to the generation of high levels of LiAlH4, as evident from hydrogen desorption capacities. Prior to this study, only TiCl3 had been reported to catalyze the regeneration of LiAlH4 to any significant extent. In this study, the yield of LiAlH4 was found to depend on the catalyst and the ball-to-powder ratio used in the activation of the LiH/Al/catalyst substrate. The TiN-catalyzed sample milled at a ball-to-powder ratio of 40:1 gave the best hydrogen storage capacity of 6.0 wt.% H. Meanwhile, the lowest capacity was observed for the LaNi5catalyzed sample milled at ball-to-powder ratio of 20:1. Although TiMn2 and LaNi5 are hydrogen storage alloys in their own right, their performance in enhancing the hydrogenation of LiH/Al was inferior to that of TiN, as the corresponding hydrogen storage capacities of the TiMn2- and LaNi5-doped samples were less than those of TiN-doped samples. However, the levels of hydrogenation for the samples containing TiMn2 and LaNi5 were still significant.
Acknowledgments We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Foundation for Innovation (CFI), for financial support of this work.
references
[1] Orimo S, Nakamori Y, Eliseo JR, Zu¨ttel A, Jensen CM. Complex hydrides for hydrogen storage. Chem Rev 2007;107: 4111e32. [2] Jain IP, Jain P, Jain A. Novel hydrogen materials: a review of lightweight complex hydrides. J Alloys Compd 2010;503: 303e39. B, Schwickardi M. Ti-doped alkali metal [3] Bogdanovic aluminium hydrides as potential novel reversible hydrogen storage materials. J Alloys Compd 1997;253:1e9. [4] Jang J-W, Shim J-H, Cho YW, Lee B- J. Thermodynamic calculation of LiH4Li3AlH64LiAlH4 reactions. J Alloys Compd 2006;420:286e90. [5] Sun T, Huang CK, Wang H, Sun LX, Zhu M. The effect of doping NiCl2 on the dehydrogenation properties of LiAlH4. Int J Hydrogen Energy 2008;33:6216e21. [6] Ismail M, Zhao Y, Yu XB, Dou SX. Effects of NbF5 addition on the hydrogen storage properties of LiAlH4. Int. J. Hydrogen Energy 2010;35:2361e7. [7] Suttisawat Y, Rangsunvigit P, Kitiyanan B, Muangsin N, Kulprathipanja S. Catalytic effect of Zr and Hf on hydrogen desorption/absorption of NaAlH4 and LiAlH4. Int J Hydrogen Energy 2007;32:1277e85. [8] Liu S-S, Sun L-X, Zhang Y, Xu F, Zhang J, Chu H-L, et al. Effect of ball milling time on the hydrogen storage properties of TiF3-doped LiAlH4. Int J Hydrogen Energy 2009;34:8079e85. [9] Varin RA, Zbroniec L. The effects of nanometric nickel (n-Ni) catalyst on the dehydrogenation and rehydrogenation
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[11] [12]
[13] [14]
[15]
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 0 e1 0 2 1 4
behaviour of ball milled lithium alanate (LiAlH4). J Alloys Compd 2010;506:928e39. Xueping Z, Ping L, Humail IS, Fuqiang A, Guoqing W, Xuanhui Q. Effects of catalyst LaCl3 on hydrogen storage properties of lithium alanate (LiAlH4). Int J Hydrogen Energy 2007;32:4957e60. Xueping Z, Shenglin L. Study on hydrogen storage properties of LiAlH4. J Alloys Compd 2009;481:761e3. Rafi-ud-din, Zhang L, Ping L, Xuanhui Q. Catalytic effects of nano-sized TiC additions on the hydrogen storage properties of LiAlH4. J Alloys Compd 2010;508:119e28. Graetz J, Wegrzyn J, Reilly JJ. Regeneration of lithium aluminum hydride. J Am Chem Soc 2008;130:17790e4. Wang J, Ebner AD, Ritter JA. Physiochemical pathway for cyclic dehydrogenation and rehydrogenation of LiAlH4. J Am Chem Soc 2006;128:5949e54. Liu X, McGrady GS, Langmi HW, Jensen CM. Facile cycling of Ti-doped LiAlH4 for high performance hydrogen storage. J Am Chem Soc 2009;131:5032e3. B, Felderhoff M, Kaskel S, Pommerin A, Bogdanovic Schlichte K, Schu¨th F. Improved hydrogen storage properties
[17]
[18] [19] [20]
[21]
[22]
of Ti-doped sodium alanate using titanium nanoparticles as doping agents. Adv Mater 2003;15:1012e5. Kim J-W, Shim J-H, Kim SC, Remhof A, Borgschulte A, Friedrichs O, et al. Catalytic effect of titanium nitride nanopowder on hydrogen desorption properties of NaAlH4 and its stability in NaAlH4. J Power Sources 2009;192:582e7. Zu¨ttel A. Hydrogen storage methods. Naturwissenschaften 2004;91:157e72. Schlapbach L, Zu¨ttel A. Hydrogen-storage materials for mobile applications. Nature 2001;414:353e8. Liu X, Langmi HW, Beattie SD, Azenwi FF, McGrady GS, Jensen CM. Ti-doped LiAlH4 for hydrogen storage: synthesis, catalyst loading and cycling performance. J Am Chem Soc 2011;133:15593e7. Le´on A, Kircher O, Rothe J, Fichtner M. Chemical state and local structure around titanium atoms in NaAlH4 doped with TiCl3 using X-ray absorption spectroscopy. J Phys Chem B 2004;108:16372e6. Langmi HW, McGrady GS, Liu X, Jensen CM. Modification of the H2 desorption properties of LiAlH4 through doping with Ti. J Phys Chem C 2010;114:10666e9.
Available online at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/he
Hydrogenation of LiH/Al catalyzed with TiN, TiMn2 and LaNi5 Felix F. Azenwi, Henrietta W. Langmi, G. Sean McGrady* Department of Chemistry, University of New Brunswick, PO Box 4400, Fredericton, New Brunswick E3B 5A3, Canada
article info
abstract
Article history:
In order to investigate the catalytic effect of TiN, TiMn2 and LaNi5 on the hydrogen storage
Received 3 January 2012
capacity of LiAlH4, 2 mol% of the catalyst was milled with LiH/Al and then hydrogenated in
Received in revised form
Me2O. Doping with TiN, TiMn2 or LaNi5 led to substantial hydrogenation of LiH/Al in
28 March 2012
accordance with the formation of LiAlH4. In each case the amount of hydrogen absorbed
Accepted 30 March 2012
was dependent on the catalyst and the ball-to-powder ratio used during milling. A high
Available online 4 May 2012
ball-to-powder ratio results in an improvement in the hydrogen storage capacity of LiAlH4. For each ball-to-powder ratio the highest hydrogen storage capacity was recorded for the
Keywords:
TiN-catalyzed sample; hydrogen storage capacity increased from 3.2 to 4.8 to 6.0 wt.% H as
Hydrogen storage material
the ball to-powder ratio increased from 10:1 to 20:1 to 40:1. The high levels of hydroge-
Lithium alanate
nation of LiH/Al catalyzed with TiN, TiMn2 and LaNi5 are remarkable because for the LiAlH4
Catalyst
system only a TiCl3 catalyst has previously been shown to result in rehydrogenation of the
Ball milling
dehydrogenated products to LiAlH4. Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
1.
Introduction
Fossil fuels are a relatively cheap source of energy, which accounts for their widespread usage, but environmental awareness is paving the way for alternative sources of energy. Hydrogen, amongst others like solar, wind and biomass, is a major contributor in the clean and sustainable energy sector. However, its application is limited and dependent on its physical form. Due to the low density and high flammability of hydrogen, gaseous and liquid hydrogen require large volumes and/or heavy, thick-walled containers for transportation and storage. A possible solution to this would be the use of hydrogen rich materials, such as complex metal hydrides, as hydrogen storage materials, releasing and reabsorbing hydrogen gas at acceptable rates and temperatures. Extensive research has been conducted on these materials, but high desorption temperatures and general irreversibility remain significant stumbling blocks [1,2]. This impasse was broken when Bogdanovic et al. successfully reversed the
dehydrogenation reactions of NaAlH4 by adding small amounts of Ti dopant [3]. Other examples of complex metal hydrides under investigation include LiAlH4, Mg(AlH4)2, Ca(AlH4)2, LiBH4 and NaBH4, with hydrogen contents of 10.5, 9.3, 7.8, 18.4 and 10.6 wt.%, respectively [1,2]. Theoretically, LiAlH4 can release 7.9 wt.% H below 220 C, which makes it one of the best candidate materials for hydrogen storage. Desorption of hydrogen occurs in two stages at 160e180 C and 180e220 C according to reactions (1) and (2), respectively [2]. 3LiAlH4 /Li3 AlH6 þ 2Al þ 3H2
5:3 wt:%H
Li3 AlH6 þ 2Al/3LiH þ 3Al þ 3=2H2
2:6 wt:%H
(1) (2)
The main limitations of LiAlH4 as a hydrogen storage material are the irreversibility of its dehydrogenation reactions under practical conditions and its relatively slow desorption kinetics [1,2,4]. A plethora of studies has demonstrated successful dehydrogenation of LiAlH4 to LiH and Al,
* Corresponding author. E-mail address: [email protected] (G. Sean McGrady). 0360-3199/$ e see front matter Copyright ª 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2012.03.160
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 0 e1 0 2 1 4
with an improvement in dehydrogenation kinetics and lowering of desorption temperature when a catalyst is added [5e9]. Nevertheless, failure of subsequent rehydrogenation to LiAlH4 was also highlighted [7e9]. It has been reported that reversal of reaction (1) at room temperature requires more than 103 bar hydrogen pressure [4]. However, partial reversibility to Li3AlH6 occurs when LiAlH4 is doped with various catalysts, including LaCl3, Ce(SO4)2, Ni, Ti and TiC [10e12]. These have often resulted in less than 1 wt.% H reabsorption capacity. The highest amount of reydrogenation (1.9 wt.% H) was achieved at 165 C and 95 bar H2 when 5 mol% TiC was used as a catalyst [12]. In solution, reaction (1) can be more easily reversed; using TiCl3 catalyst complete rehydrogenation of LiAlH4 in THF was achieved [13,14], and in Me2O more than 7 wt.% H was re-absorbed [15]. To the best of our knowledge, amongst all the catalysts reported, TiCl3 is the only one that has afforded very high levels of regenerated LiAlH4 capable of meeting the demands of hydrogen storage applications. In the present work, we report for the first time substantial formation of LiAlH4 from LiH/Al catalyzed with TiN, TiMn2 and LaNi5. Formation of LiAlH4 from catalyzed LiH/Al provides a path to regeneration of LiAlH4 from its dehydrogenated products. Nano-sized TiN has already been shown to enhance the hydrogen storage properties of NaAlH4 [16,17]. Kim et al. [17] reported that TiN nanopowder synthesized by mechanochemical high-energy ball milling of TiCl3 and Li3N shows better catalytic influence on the cyclability and hydrogen capacity of NaAlH4 than TiCl3. The other dopants used in this study, TiMn2 and LaNi5, are AB2 and AB5 alloys, respectively. These alloys on their own are known to store up to 2 wt.% H at near ambient conditions [18,19].
2.
Experimental
2.1.
Sample preparation
reaction was left to stir at room temperature for 24 h. Finally, the excess hydrogen and solvent were vented and the dry powder was collected for further characterization.
2.2.
Sample characterization
Powder X-ray diffraction (XRD) patterns were measured using a Rigaku Miniflex diffractometer (CuKa radiation). Samples were mounted on a plastic holder and covered with parafilm to protect them from contact with air during the measurements. The parafilm resulted in additional diffraction peaks at ca. 21.6 and 24 , which were distinct from diffraction peaks of the samples. Thermogravimetric analysis (TGA) was performed using a TA Instruments Q50 TGA. Typically, about 5 mg of sample was loaded into an Al crucible and sealed with an Al lid in a glovebox. A pinhole pierced in the lid allowed the escape of H2 released during the measurements and minimized exposure of the sample to the atmosphere as it was transferred to the TGA instrument. The sample was heated to 300 C at a ramping rate of 2 C/min under a flowing stream of N2 at a rate of 120 mL/min. Differential scanning calorimetry (DSC) experiments were carried out using a TA Instruments Q20P DSC. About 5 mg of material was placed in an Al pan, which was sealed with an AL lid in a glovebox. The pan was placed on the DSC instrument and the sample was heated to 300 C at 2 C/min under a nitrogen flow of 95 mL/min. Desorption plots for the samples were obtained using a Sieverts-type instrument, PCTPro-2000 manufactured by HyEnergy LLC. In a typical experiment, about 250 mg sample was loaded into a sample holder in a glovebox. This was then attached to the PCT instrument without exposure of the sample to air. The vessel was heated at 2 C/min to 250 C, and then maintained at this temperature for 30 min.
3.
LiH (Aldrich, 95%), Al (Alfa Aesar, 99.97%), TiN (Strem Chemicals, 99þ %-Ti), TiMn2 (Aldrich, H2 storage grade) and LaNi5 (Aldrich, H2 storage grade) were used as received. All materials and samples were handled in a nitrogen-filled glovebox. Ball milling of LiH/Al was carried out in a 1:1 molar ratio, with 2 mol% catalyst (TiN, TiMn2 or LaNi5). In a typical experiment 2 g of the combined powders was loaded in a 50 mL tungsten carbide vessel containing tungsten carbide balls. The number of balls differed according to the desired ball-to-powder ratio. Three different ball-to-powder ratios were employed; i.e. 40:1, 20:1 and 10:1. The powders were mechanically milled at room temperature in a nitrogen atmosphere at a rotational speed of 300 rpm for 12 h using a Retsch PM 100 planetary ball mill. After every 15 min of milling there was a 30 s pause and the rotation was automatically reversed. Hydrogenation reactions in Me2O (Air Liquide Canada Inc., Chemically pure) were carried out in a Parr Instruments 500 mL stainless steel stirred reactor. In a typical procedure, 1.79 g of doped LiH/Al was loaded into the reactor in the glovebox. The reactor was sealed, taken out of the glovebox and then connected to the Me2O cylinder. Me2O (55 g) was transferred to the reactor using a solvent pump and 100 bar H2 gas was added. The
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Results and discussion
The effect of TiN, TiMn2 and LaNi5 catalysts on the hydrogenation of LiH/Al was studied in detail. The relationship between the amount of hydrogen released from the hydrogenated product and ball-to-powder ratio was also investigated, as ball milling affects the yield of LiAlH4 generated. Previously, we have demonstrated that undoped LiH/Al does not undergo hydrogenation in Me2O and 100 bar H2. Fig. 1 shows the TGA plots for hydrogenated LiH/Al doped with 2 mol% TiN, TiMn2 and LaNi5. In each case, hydrogen release began at ca. 10 C and continued up to ca. 190 C, with the plots showing two obvious regions of hydrogen release corresponding to reactions (1) and (2). Powder XRD confirmed that LiAlH4 was generated upon hydrogenation (Fig. 2). According to the TGA plots, the first step of hydrogen release occurred at 100e145 C. The yields of LiAlH4, and hence the amount of hydrogen desorbed, were dependent on the ballto-powder ratio used during milling. The yield of LiAlH4 decreased with decreasing ball-to-powder ratio. Using TiN as dopant, a total of 6.0, 4.8 and 3.2 wt.% H was released when the ball-to-powder ratios were 40:1, 20:1 and 10:1, respectively. In the case of TiMn2, the hydrogen desorption capacities were 5.8, 4.7 and 1.4 wt.% H for the ball-to-powder ratios 40:1, 20:1 and 10:1, respectively. For LaNi5, the amount of
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i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 0 e1 0 2 1 4
Fig. 1 e TGA plots for hydrogenated LiH/Al catalyzed with 2 mol % TiN (aec), TiMn2 (def) and LaNi5 (g and h) for different ball-to-powder ratios (BPRs) used during milling. Sample with (a) TiN catalyst, BPR [ 40:1; (b) TiN catalyst, BPR [ 20:1; (c) TiN catalyst, BPR [ 10:1; (d) TiMn2 catalyst, BPR [ 40:1; (e) TiMn2 catalyst, BPR [ 20:1; (f) TiMn2 catalyst, BPR [ 10:1; (g) LaNi5 catalyst, BPR [ 40:1 and (h) LaNi5 catalyst, BPR [ 20:1.
X
X Parafilm
X
Intensity (a.u.)
(b)
(a) 20
30
40
50
60
70
80
90
2 Theta (degrees) Fig. 2 e Powder XRD plot for (a) LiAlH4 from ICDD and (b) hydrogenated LiH/Al catalyzed with 2 mol% TiN for a ballto-powder ratio of 40:1 used during milling.
desorbed hydrogen recorded was even lower, reaching only 3.7 and 1.0 wt.% H for ball-to-powder ratios of 40:1 and 20:1, respectively. The hydrogen desorption capacities for the doped LiAlH4 are less than the theoretical value of 7.9 wt.% H indicating incomplete hydrogenation. This difference can also be accounted for, at least in part, by the additional weight of the catalyst, since the capacities are expressed as a percentage of the entire sample. Another reason for the disparity may be the fact that the LiH starting material had a quoted purity of 95%. The general trend of increasing yield with increasing ballto-powder ratio can be explained by the decrease in particle size caused by milling. Ball milling is a high-energy technique that results in the grinding and intimate mixing of particles. High ball-to-powder ratios will produce finer particles and increase the surface area for catalytic activity, accounting for the increase in hydrogen release in the ball-to-powder ratio order 10:1 < 20:1 < 40:1. The greatest effect was noticed for the sample containing TiMn2 catalyst, which showed an increase in hydrogen uptake of 3.3 wt.% H from 10:1 to 20:1 ball-topowder ratio. The role of both TiMn2 and LaNi5 in achieving hydrogenation of LiH/Al can be attributed to the fact that these catalysts are both hydrogen storage materials themselves at ambient temperatures [18,19]. Their presence in minute quantities in the LiH/Al milled powder was sufficient to trigger hydrogenation of the mixture at room temperature in Me2O. The significant difference in the desorbed hydrogen content between TiMn2 and LaNi5-catalyzed samples with corresponding ball-to-powder ratios may be associated with the difference in the weight of the two alloys. The weight of LaNi5 is more than double that of TiMn2; moreover, LaNi5 has a lower hydrogen storage capacity (ca. 1.5 wt.%) than TiMn2 (ca. 1.9 wt.%). For each ball-to-powder ratio studied the best results were obtained for the samples catalyzed with TiN, with an impressively high value of 6.0 wt.% H being obtained for a ball-to-powder... ratio of 40:1. Interestingly, this value exceeds that previously reported for rehydrogenated LiAlH4 doped with 2 mol% of the established TiCl3 catalyst [15,20]. This result is significant because TiN has previously been shown to enhance the hydrogen storage properties of the widely studied NaAlH4 [16,17]. Since TiN and TiCl3 each contains Ti in an oxidation state of þ3, it is tempting to infer that this valency may play a role in the hydrogenation of LiH/ Al. However, it has been reported that Ti(III) is reduced to Ti(0) in the TiCl3-doped NaAlH4 system [21], so such speculation may be dangerous. Further studies are underway to investigate in detail the part played by these catalysts amongst others. Hydrogenated LiH/Al doped with 2 mol% TiN was further examined using the PCT instrument. Hydrogen desorption curves were recorded as the samples were heated to 250 C (Fig. 3). The amount of desorbed hydrogen increased with increasing ball-to-powder ratio used during milling, in accordance with the TGA results. However, the plots do not show the two-step desorption observed in the TGA plots because of the difference in measurement technique. The hydrogen storage capacity measured for the 40:1 ball-to-powder ratio sample was 5.7 wt.% H. This sample had the fastest desorption kinetics, and it clearly started releasing hydrogen at least 20 C lower than the 20:1 and 10:1 ball-to-powder ratio samples.
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y 3 7 ( 2 0 1 2 ) 1 0 2 1 0 e1 0 2 1 4
300
6
4.
5
250
(b)
4 3
200
(a)
150
o
2
Temperature ( C)
Desorbed Hydrogen (wt.%)
(c)
100
1 50
0 0
1
2
3
Time (h)
Fig. 3 e Desorption plots for hydrogenated LiH/Al catalyzed with 2 mol% TiN for different ball-to-powder ratios (BPRs) used during milling. Sample with (a) BPR [ 10:1; (b) BPR [ 20:1 and (c) BPR [ 40:1.
This can be ascribed to the finer, more homogeneous particles obtained for the high 40:1... ball-to-powder ratio sample. Fig. 4 shows DSC results for hydrogenated samples (ball-topowder ratio ¼ 40:1). The two endothermic peaks characteristic of TiCl3-doped LiAlH4 were also observed here [22]. As was the case with the TiCl3-doped samples, the first peak with a maximum around 130e145 C is assigned to the release of hydrogen, as described by reaction (1); i.e. the conversion of LiAlH4 to Li3AlH6. Accordingly, the second peak with maximum at 165e180 C corresponds to the decomposition of Li3AlH6 to LiH and Al. These two endothermic events in the DSC plot correlate exactly with the two mass-loss steps observed in the TGA plot (Fig. 1). It can also be seen from the DSC curves that the amount of heat flow for the sample containing LaNi5 catalyst is relatively small compared to the samples containing TiMn2 and TiN. This is in agreement with the lower hydrogen desorption capacity observed for the LaNi5-catalyzed sample.
Fig. 4 e DSC plots for hydrogenated LiH/Al catalyzed with 2 mol % TiN, TiMn2 and LaNi5 for a ball-to-powder ratio of 40:1 used during milling. Sample with (a) LaNi5 catalyst; (b) TiMn2 catalyst and (c) TiN catalyst.
10213
Conclusions
This work has demonstrated that doping LiH/Al with TiN, TiMn2 and LaNi5 followed by hydrogenation in Me2O leads to the generation of high levels of LiAlH4, as evident from hydrogen desorption capacities. Prior to this study, only TiCl3 had been reported to catalyze the regeneration of LiAlH4 to any significant extent. In this study, the yield of LiAlH4 was found to depend on the catalyst and the ball-to-powder ratio used in the activation of the LiH/Al/catalyst substrate. The TiN-catalyzed sample milled at a ball-to-powder ratio of 40:1 gave the best hydrogen storage capacity of 6.0 wt.% H. Meanwhile, the lowest capacity was observed for the LaNi5catalyzed sample milled at ball-to-powder ratio of 20:1. Although TiMn2 and LaNi5 are hydrogen storage alloys in their own right, their performance in enhancing the hydrogenation of LiH/Al was inferior to that of TiN, as the corresponding hydrogen storage capacities of the TiMn2- and LaNi5-doped samples were less than those of TiN-doped samples. However, the levels of hydrogenation for the samples containing TiMn2 and LaNi5 were still significant.
Acknowledgments We are grateful to the Natural Sciences and Engineering Research Council of Canada (NSERC), and the Canadian Foundation for Innovation (CFI), for financial support of this work.
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