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The dehydrogenation performance and reaction mechanisms of Li3AlH6 with TiF3 additive
international journal of hydrogen energy 35 (2010) 4554–4561
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The dehydrogenation performance and reaction mechanisms of Li3AlH6 with TiF3 additive Shu-Sheng Liu a,b, Yao Zhang a, Li-Xian Sun a,*, Jian Zhang a, Jun-Ning Zhao a, Fen Xu c,*, Feng-Lei Huang d a
Materials and Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Graduate School of Chinese Academy of Sciences, Beijing 100049, China c Faculty of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, China d State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China b
article info
abstract
Article history:
For Li3AlH6 prepared by mechanical milling method, the dissociation reaction enthalpy and
Received 2 September 2009
activation energy are calculated to be 22.1 kJ mol1 H2 and 133.7 2.7 kJ mol1, respectively.
Received in revised form
The dehydrogenation performance of Li3AlH6 is greatly enhanced by TiF3 additive, espe-
10 December 2009
cially in the kinetic behaviors. For the Li3AlH6 þ 10 mol% TiF3 sample, the starting
Accepted 17 December 2009
temperature of dehydrogenation is obviously decreased by 60 C from that of pure Li3AlH6
Available online 28 March 2010
(190 C), and 3.0 wt.% H2 may be released within 1000 s at 120 C under an initial vacuum.
Keywords:
improves due to the decrease in the activation energy. The X-ray diffraction (XRD) together
Dehydrogenation
with thermogravimetric analysis (TGA) results show that there are three mechanochemical
Titanium fluoride
reactions involved during milling: i) Li3AlH6 þ TiF3 / 3 LiF þ Al þ Ti þ 3H2, ii) Ti þ H2 /
With the amount of TiF3 increasing, the starting temperature decreases and the kinetics
Alanate
TiH2, iii) 3 Al þ Ti / Al3Ti. The in-situ formed Ti species (TiH2 and Al3Ti) co-catalyze the
Kinetic
thermal dehydrogenation of Li3AlH6.
Mechanism
1.
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Introduction
Hydrogen storage is a key technical challenge for onboard application of hydrogen. High-pressure and cryogenic hydrogen storage systems are impractical for vehicular applications due to safety concerns and volumetric constraints. Attention has been focused on solid-state hydrogen storage materials due to the significant advantages related to safety, energy efficiency and operational convenience [1,2]. Metal hydrides [3], complex hydrides [4], carbon materials [5], and MOFs [6] are all investigated as hydrogen carriers. Among these materials, light metal complex hydrides have attracted intensive research interest because
of their high hydrogen storage capacities and moderate working temperatures. Lithium alanate (LiAlH4) is one of the promising hydrogen storage materials and has been widely investigated in recent years. It can decompose in three steps (R1, R2 and R3) with a hydrogen liberation amounting to 10.6 wt.%. [7,8] 150w175 C
3 LiAlH4 ! Li3 AlH6 þ 2Al þ 3H2 ; 180w220 C
Li3 AlH6 ! 3 LiH þ Al þ 3=2 H2 ; >400 C
ð5:3 wt:%H2 Þ ð2:6 wt:% H2 Þ
3 LiH þ 3 Al ! 3 LiAl þ 3=2 H2 ð2:6 wt:%H2 Þ
* Corresponding authors. Tel./fax: þ86 411 84379213. E-mail addresses: [email protected] (L.-X. Sun), [email protected] (F. Xu). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.108
R1 R2 R3
international journal of hydrogen energy 35 (2010) 4554–4561
The first two reactions, R1 and R2, attract much attention at present, since their reaction temperatures are close to the working temperature of fuel cell system. Various catalysts have been added to LiAlH4 to improve its hydrogen storage properties, including such effective metal halides as TiCl3$1/ 3AlCl3 [9,10], VCl3 [10], NiCl2 [11] and TiF3 [12,13]. Under the catalysis of these metal halides, LiAlH4 decomposes partly below 100 C mainly corresponding to R1 stage. However, the equilibrium pressure for R1 is estimated to be above 100 MPa at 25 C [14]. On the other hand, the equilibrium pressure for R2 is much lower. By ab initio calculations, Ke et al. [15] predicted the equilibrium pressure for R2 to be around 10 MPa at 0 C. It means that Li3AlH6 is a thermodynamically attractive complex for hydrogen storage with a capacity of 5.6 wt.% H2. The method for the preparation of Li3AlH6 via the mechanochemical reaction between LiAlH4 and LiH (R4) has been widely used in the past decade [8,16–18]. The dehydrogenation temperature of Li3AlH6 has been reduced by about 30 C and the kinetics has been significantly improved by Ti5Si3 [19] or Al3Ti [20] additives. TiCl3 [18–20] shows more pronounced catalytic effect on R2, since it reacts with Li3AlH6 to form Al3Ti, which may act as a dehydrogenation catalyst. Ti(OPr)4 is also a remarkable catalyst [17], which was co-milled with LiAlH4 and LiH to produce nanocrystalline Ti-doped Li3AlH6 with a starting dehydrogenation temperature of 100 C induced by the Ti0/Ti3þ/Ti4þ defect sites. ball milling
LiAlH4 þ 2LiH ! Li3 AlH6
R4
In the past few years, TiF3 has attracted broad scientific interest due to its excellent catalytic effect on many metal hydrides, such as MgH2 [21], Mg2NiH4 [21], Na2LiAlH6 [22]. Wang et al. reported that TiF3 is superior to TiCl3 as a dopant precursor in preparation of catalytically enhanced NaAlH4 [23] and Na3AlH6 [24] systems. They believed that the F anion is substantially incorporated into the doped structure, and in some way contributes to the catalytic enhancement. Our previous studies have demonstrated the significant catalytic effect of TiF3 on both LiBH4 [25] and LiAlH4 [13]. In the case of LiAlH4, adding 4 mol% TiF3 makes LiAlH4 start to decompose at 80 C with 6.3 wt.% H2 release up to 200 C. TiF3 probably reacts with LiAlH4 to form the catalyst, Al3Ti, which catalyzes the mechanochemical reactions during ball milling and the thermochemical reactions under increased temperature. However, the catalytic effect of TiF3 on Li3AlH6 is still unclear. We are curious if the same pathway occurs in the reactions. In order to find the answer to this question we further explore the dehydrogenation performance and reaction mechanisms of Li3AlH6 with TiF3 additive. On the other hand, the enthalpy change of reaction (DH ) and apparent activation energy (Ea) are two important thermal parameters to evaluate the thermodynamic and kinetic properties of a material or reaction [26]. Although numerous results related to these two parameters of Li3AlH6 (R2) have been reported, they are all based on the products of LiAlH4 decomposition (R1). The data related to pure Li3AlH6 prepared by ball milling have not been previously reported to the best of our knowledge. In this work, we prepared pristine Li3AlH6 and Li3AlH6 with TiF3 additive through mechanochemical milling. Both the thermal properties and the dehydrogenation performance of
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as-prepared Li3AlH6 with TiF3 additive were investigated. The reaction mechanisms of Li3AlH6 with TiF3 additive were also discussed.
2.
Experimental
2.1.
Sample preparation
The starting materials, LiH (98%), LiAlH4 (97%) and TiF3 (99%) were all purchased from Alfa Aesar and used without further purification. All material handlings were performed in an MBraun Unilab glove box filled with high purity Ar (99.999%), with oxygen and water vapor contents both less than 0.1 ppm. Li3AlH6 was synthesized through the mechanochemical reaction between LiH and LiAlH4 similar to the method reported in the literature [17–20]. LiH and LiAlH4 with a mole ratio of 2:1 were loaded into a 100 mL stainless milling pot with 24 steel balls (10 mm in diameter). Ball milling was performed using a QM-1SP2 planetary ball mill under 0.1 MPa argon atmosphere at a rotation speed of 541 rpm. We obtained pure Li3AlH6 after 8 h milling. Different amounts (2, 10, 20, 25 and 50 mol%) of TiF3 were added to Li3AlH6, respectively. All the mixtures were milled for 0.5 h under the same conditions as those used above.
2.2.
Sample characterization
XRD analysis was carried out using a PANalytical X-ray Diffractometer (X’Pert MPD PRO, Cu Ka, 40 kV, 40 mA). The samples were shielded by Mylar films (diffraction peak w 26 2q) from the atmospheric oxygen and moisture. Thermogravimetric analysis (TGA) was carried out in Cahn Thermax 500 with a heating rate of 10 C min1 in a flow of high purity Ar. Differential scanning calorimetry (DSC) was conducted using a TA Q1000 at different heating rates (2.5, 5, 10 and 20 C min1) under a flow (50 ml min1) of high purity Ar. The isothermal dehydrogenation was performed using a Sieverts-type apparatus (PCT, Advanced Materials Corporation, USA) under an initial pressure of 105 MPa H2. The surface morphology of the composites was observed using scanning electron microscopy (SEM, JSM6360LV).
3.
Results and discussion
3.1. XRD characterization and thermal properties of as-prepared Li3AlH6 The XRD pattern of as-prepared Li3AlH6 is shown in Fig. 1. All the reflections belong to Li3AlH6, and are in good agreement with the literature [16–20]. This means that Li3AlH6 could be prepared by the reaction between LiAlH4 and LiH (R4) within 8 h. By means of the Scherrer equation [b ¼ l/(B cos q), where b is the crystallite size, l is the X-ray wavelength and B is the full width at half maximum (FWHM)], the average crystallite size of Li3AlH6 could be determined as 22 nm. Fig. 2 shows the DSC curves of Li3AlH6 at different heating rates: 2.5, 5, 10 and 20 C min1. Only one endothermic peak,
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international journal of hydrogen energy 35 (2010) 4554–4561
Fig. 1 – XRD pattern of as-prepared Li3AlH6.
Fig. 2 – DSC curves of Li3AlH6 at different heating rates: 2.5, 5, 10 and 20 8C minL1.
corresponding to Li3AlH6 decomposition, can be seen in each curve. At a heating rate of 10 C min1, the endothermic peak starts at 195 C and ends at 245 C. As known, DSC measurement is applied as a suitable and quick method for determination of dissociation reaction enthalpy by evaluating the peak area [27]. We used all four curves in Fig. 2 to calculate an average reaction enthalpy of R2 (22.1 kJ mol1 H2). It is between 13.3 [28] and 29.0 [29] kJ mol1 H2 both obtained by thermal analyses. The apparent activation energy, Ea, can be obtained using Kissinger equation [30]:
Isothermal dehydrogenations of as-prepared Li3AlH6 and Li3AlH6 with TiF3 additive at 120 C are shown in Fig. 5. Without TiF3, only about 0.35 wt.% H2 is released from asprepared Li3AlH6 even after 36000 s. After TiF3 is added, the kinetics of dehydrogenation is greatly improved, and the dehydrogenation goes on more rapidly with the increase of TiF3. With 2 mol% TiF3, Li3AlH6 can release 3.9 wt.% H2 in 10000 s, and the remaining hydrogen can be released when the dehydrogenation time is prolonged to 36000 s. As shown in the inset plot of Fig. 5, about 3.0 and 2.4 wt.% H2 may be released within 1000 s from Li3AlH6 þ 10 mol% TiF3 and Li3AlH6 þ 20 mol% TiF3 mixtures, respectively, and the latter performs more rapidly and releases almost its maximum dehydrogenation amount (2.6 wt.%, 6000 s) at 120 C. These results are attractive considering the U. S. Department of Energy target for the operation temperature.
d ln Tb2 p E ¼ a R d T1p where b, Tp and R are the heating rate, the peak temperature of the endothermic peak and the gas constant, respectively. As shown in Fig. 3, the apparent activation energy for the decomposition of as-prepared Li3AlH6 is 133.7 2.7 kJ mol1, which is close to the previously reported value of 130 15 kJ mol1 [28]. The differences between our results and those reported in the literature in both DH and Ea may be because the material Li3AlH6 tested in the present study is synthesized by the mechanochemical reaction instead of the thermal decomposition of LiAlH4.
3.3.
Reaction mechanisms of Li3AlH6 with TiF3 additive
Fig. 6 shows the SEM images of as-prepared Li3AlH6 and Li3AlH6 with TiF3 additive after milling. From the inset image,
3.2. Dehydrogenation performance of Li3AlH6 with TiF3 additive Non-isothermal dehydrogenation performance of samples with TiF3 additive was achieved by TGA measurements, shown in Fig. 4. The starting temperature of Li3AlH6 þ 2 mol% TiF3 mixture (Fig. 4b) is about 150 C, which is 40 C lower than that of as-prepared Li3AlH6 (Fig. 4a). With the additive amount increasing, the starting temperatures decrease from 130 C for Li3AlH6 þ 10 mol% TiF3 mixture to 105 C for Li3AlH6 þ 20 mol% TiF3 mixture. However, the amount of released hydrogen is decreasing and only 2.6 wt.% H2 is released from Li3AlH6 þ 20 mol% TiF3 mixture.
Fig. 3 – Kissinger plot for dehydrogenation of as-prepared Li3AlH6.
international journal of hydrogen energy 35 (2010) 4554–4561
Fig. 4 – Non-isothermal (10 8C minL1) dehydrogenations of as-prepared Li3AlH6 (a) and Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d).
we can see that the grains of as-prepared Li3AlH6 are already very small (10–50 nm), in agreement with the XRD result. But these small grains tend to agglomerate. When TiF3 is added, there is not obvious change in the grain size of Li3AlH6. So the size effect of Li3AlH6 on the dehydrogenation is negligible. Fig. 7 displays the XRD patterns of Li3AlH6 with TiF3 additive after milling. It is found that the reflections of Li3AlH6 become weaker with the additive amount increasing. When 50 mol% TiF3 is added, Li3AlH6 phase vanishes completely. Such intermediate phases as TiH2 (35.1, 40.5 2q) [18] and Al/LiF can be identified. We also observed a shoulder on the high angle side
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Fig. 5 – Isothermal (120 8C) dehydrogenations of as-prepared Li3AlH6 (a) and Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d).
of Al (111) (39 2q) in the XRD pattern, which might be assigned to a metastable phase of Al–Ti alloy, L12–Al3Ti [31]. It was also formed during ball milling in both TiCl3-doped NaAlH4 [32] and TiCl3-doped Li3AlH6 systems [18]. The lattice constant of this ˚ using the MDI Jade 5.0 new phase is evaluated to be 3.991 A software. This is very close to the lattice constant of L12–Al3Ti ˚ determined by Hong et al. [33] and 3.9856 A ˚ of 3.98 0.01 A determined by Zhang et al. [34]. This implies that the newly formed fcc phase in the present study is also L12–Al3Ti. From these observations, we assume that there may be three reactions (R5, R6 and R7) happening in Li3AlH6 with TiF3 additive
Fig. 6 – SEM images of as-prepared Li3AlH6 (a) and Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d). The inset images are the higher magnification micrographs.
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ball milling
Ti þ H2 ! TiH2
R6
ball milling
Ti þ 3Al ! Al3 Ti
Fig. 7 – XRD patterns of Li3AlH6 with different amounts of TiF3 additive: 2 mol% (a), 10 mol% (b), 25 mol% (c) and 50 mol% (d) after 0.5 h ball milling.
during ball milling. First, Li3AlH6 reacts with TiF3 by R5 to produce active metals: Al and Ti. According to R5, the H2 pressure will rise to above 0.4 MPa in the milling pot if 50 mol% TiF3 is added. R6 is a self-propagating reaction with a very large heat of formation and it has been reported to take place in 0.1 MPa H2 by mechanical milling [35,36]. The Ti particles with new surfaces and defects are easy to react with H2 under the impact of high-energy milling. The formed TiH2 has an average crystalline size of about 5 nm in Li3AlH6 þ 50 mol% TiF3 mixture, and the fine grains are easy to disperse in the mixture. At the same time, a competitive reaction R7 may happen because Al3Ti is a much more thermodynamically stable phase (DG+f ¼136 kJ mol1) than TiH2 (DG+f ¼86 kJ mol1) and AlTi (DG+f ¼72 kJ mol1) [37]. Initial milling results in a lamellar microstructure of the as-formed Al particles through deformation, welding and fracture. Further milling makes the Ti atoms dissolve into the Al matrix as a result of an interdiffusion reaction across the increasing interphase layer. The diffractions of Al3Ti are weak compared to other phases in the mixtures due to its high dispersion, large peak widths, small amount or some amorphous state [38].
ball milling
Li3 AlH6 þ TiF3 ! 3 LiF þ Al þ Ti þ 3 H2
R5
R7
To validate our hypothesis, we calculated the theoretical amounts of dehydrogenation during TGA analyses excluding the H2 loss accompanying the milling process. As shown in Table 1, the dehydrogenation amounts obtained by experiment are in accordance with the theoretical data for all the samples. It should be noted that TiH2 does not decompose to release H2 below 250 C [35]. Therefore, the above three mechanochemical reactions do occur in Li3AlH6 with TiF3 additive during ball milling, and produce the active species like TiH2 and Al3Ti. Fig. 8 shows the phase changes of Li3AlH6 þ 20 mol% TiF3 and Li3AlH6 þ 50 mol% TiF3 mixtures after isothermal dehydrogenation at different temperatures. For Li3AlH6 þ 20 mol% TiF3, the reflections of Li3AlH6 have already disappeared after dehydrogenation at 100 C for 4 h (Fig. 8a), which indicates R2 is almost completed at this condition. TiH2 seems to be unchangeable in all the patterns because 250 C is far below its decomposition temperature [35]. The reflections (39.2 (Fig. 8 inset), 45.5, 66.2 and 79.3 2q) of L12–Al3Ti become more obvious as temperature goes up, which corresponds to the growth of particles. The similar result was obtained when TiCl3-doped LiAlH4 was annealed to 150 C [39]. So, there is only one reaction (R2) involved during heating Li3AlH6 with TiF3 additive below 250 C. It is concluded that the amount of TiF3 is rapidly reduced by Li3AlH6 giving rise to nanocrystalline Ti and Al with H2 release during the ball milling process. The milling energy is significant enough to lead to the formation of TiH2 and Al3Ti. In the following heating process the intermetallic compound, Al3Ti becomes ordered and grows while TiH2 does not decompose within the testing temperature range.
3.4. Kinetic analysis and catalytic mechanism of Li3AlH6 with TiF3 additive Fig. 9 exhibits the DSC curves of Li3AlH6 with different amounts of TiF3 additive with only one endothermic peak on each curve. The reaction enthalpies are 21.9, 22.6 and 21.8 kJ mol1 H2, respectively, for Li3AlH6 þ 2 mol% TiF3, Li3AlH6 þ 10 mol% TiF3 and Li3AlH6 þ 20 mol% TiF3 mixtures. They are all close to those of as-prepared Li3AlH6. It means that the influence of TiF3 on the thermodynamic properties of
Table 1 – Thermodynamic/kinetic data and dehydrogenation amounts of as-prepared Li3AlH6 and Li3AlH6 with TiF3 additive. Amount of TiF3 (mol%) 0 2 10 20 25 50
Actual amount of H2 released during TGA (wt.%)
Theoretical amount of H2 released considering R5 (wt.%)
DH (kJ mol1 H2)
5.3 4.9 3.8 2.6 1.9 0
5.6 5.0 4.0 2.7 2.1 0
22.1 21.9 22.6 21.8 – –
Ea (kJ mol1) 133.7 2.7 125.3 1.6 104.4 3.3 85.8 0.2 – –
international journal of hydrogen energy 35 (2010) 4554–4561
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Fig. 9 – DSC (10 8C minL1) curves of as-prepared Li3AlH6 (a) and Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d).
Fig. 8 – XRD patterns of Li3AlH6 with TiF3 additive after isothermal dehydrogenations at different temperatures: 100 8C (a), 150 8C (b) and 250 8C (c) for Li3AlH6 with 20 mol% TiF3; 150 8C (d) and 250 8C (e) for Li3AlH6 with 50 mol% TiF3.
Li3AlH6 could be negligible. Even though, the peak temperatures for the endothermic reaction of the samples are gradually reduced with the augmentation of TiF3 in the mixtures. It is likely due to the enhancement in the dehydrogenation kinetics. The apparent activation energies of Li3AlH6 þ 2 mol% TiF3, Li3AlH6 þ 10 mol% TiF3 and Li3AlH6 þ 20 mol% TiF3 mixtures were evaluated using Kissinger analyses. As shown in Fig. 10, Ea decreases dramatically with addtive amount increasing. When 20 mol% TiF3 is added, Ea can be drastically decreased to 85.8 0.2 kJ mol1 from 133.7 2.7 kJ mol1 for as-prepared Li3AlH6 (Table 1). We conclude that adding TiF3 greatly improves the kinetics of Li3AlH6 dehydrogenation, and it is a plausible explanation to the above dehydrogenation behaviors. Al3Ti was reported to form in-situ and serve as an excellent catalyst in TiCl3-doped NaAlH4 [32], LiAlH4 [39] and Li3AlH6 [18] systems. TiH2 catalyst was proved to form in Ti-doped NaAlH4 during milling [40], and theoretical calculation also pointed out the catalytic mechanism of TiH2 [41]. Both Al3Ti and TiH2
are formed in-situ in Li3AlH6 with TiF3 additive in the present study. Available information provides an evidence that the activity of in-situ prepared nano/micro-crystalline heterogeneous catalyst with an extremely large active surface area significantly exceeds that of a doped bulk catalyst. This can justify the superior dehydrogenation kinetics of Li3AlH6 with TiF3 additive over Ti5Si3 [19] or Al3Ti [20] additives. Ti is a transition metal with a number of d-orbital vacancies, which facilitate the interaction between transition metal and hydrogen atoms of Li3AlH6 [39]. The in-situ formed Al3Ti and TiH2 disperse widely on the surfaces of much larger Li3AlH6 globelets, acting as the catalytic active sites for the dehydriding process. Also, TiH2 may destabilize the Al–H bonds [40]. When Li3AlH6 and TiH2 are close enough, local electrostatic attraction may cause the breaking of Al–H bands if combined with an increased atomic vibration at elevated temperature. As a result, Li3AlH6 decomposes and two H atoms combine to form H2. Therefore, it is Al3Ti and TiH2
Fig. 10 – Kissinger plots for dehydrogenations of Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d).
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that make the energy barrier of H dissociation (Ea) in Li3AlH6 decrease. The kinetic enhancement with the augmentation of TiF3 could be caused by the increase of the concentration of these active species.
4.
Conclusions
Li3AlH6 was successfully prepared by ball milling the mixture of LiAlH4 and LiH with a mole ratio of 1:2. It starts to decompose around 190 C. The reaction enthalpy and apparent activation energy of Li3AlH6 dehydrogenation are 22.1 kJ mol1 H2 and 133.7 2.7 kJ mol1, respectively. When Li3AlH6 is mixed with TiF3, it begins to release H2 at much lower temperature and the dehydrogenation speed is increased remarkably. For Li3AlH6 þ 10 mol% TiF3 mixture, 3.0 wt.% H2 can be released within 1000 s at 120 C. This is close to the need of practical application. Kinetic analyses indicate that the apparent activation energy of Li3AlH6 dehydrogenation has been greatly reduced by 29 kJ mol1 in Li3AlH6 þ 10 mol% TiF3 mixture. This enhancement attributes to the in-situ formed active catalysts, Al3Ti and TiH2, during the ball milling process. Therefore, TiF3 can be considered as an excellent additive on the dehydrogenation of Li3AlH6.
Acknowledgements The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (No. 20833009, 20873148, 20903095, 50671098, 50901070 and U0734005), 863 projects (2007AA05Z115 and 2007AA05Z102), the National Basic Research Program (973 program) of China (2010CB631303), Beijing Institute of Technology (Grant No. KFJJ10-1Z) and IUPAC (Project No. 2008-0063-100). The authors also would like to express their appreciation to Dr. Michael Frenkel of the U. S. National Institute of Standards and Technology (Boulder, Colorado) for his valuable advice and suggestions.
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[31] Moon KI, Lee KS. Development of nanocrystalline Al–Ti alloy powders by reactive ball milling. J Alloys Compd 1998;264:258–66. [32] Majzoub EH, Gross KJ. Titanium–halide catalyst-precursors in sodium aluminum hydrides. J Alloys Compd 2003;356–357: 363–7. [33] Hong QZ, Lilienfeld DA, Mayer JW. Thermal and ion-induced, metastable-cublic Al3M phases in Al-Ti and Al-Hf thin films. J Appl Phys 1988;64:4478–83. [34] Zhang F, Lu L, Lai MO. Study of thermal stability of mechanically alloyed Ti–75% Al powders. J Alloys Compd 2000;297:211–8. [35] Zhang H, Kisi EH. Formation of titanium hydride at room temperature by ball milling. J Phys Condens Matter 1997;9: L185–90. [36] Dunlap RA, Small DA, MacKay GR, O’Brien JW, Dahn JR, Cheng ZH. Materials preparation by ball milling. Can J Phys 2000;78:211–29.
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[37] Kang XD, Wang P, Song XP, Yao XD, Lu GQ, Cheng HM. Catalytic effect of Al3Ti on the reversible dehydrogenation of NaAlH4. J Alloys Compd 2006;424:365–9. [38] Kang XD, Wang P, Cheng HM. In situ formation of Ti hydride and its catalytic effect in doped NaAlH4 prepared by milling NaH/Al with metallic Ti powder. Int J Hydrogen Energy 2007; 32:2943–8. [39] Kojima Y, Kawai Y, Matsumoto M, Haga T. Hydrogen release of catalyzed lithium aluminum hydride by a mechanochemical reaction. J Alloys Compd 2008;462: 275–8. [40] Wang P, Kang XD, Cheng HM. Exploration of the nature of active Ti species in metallic Ti-doped NaAlH4. J Phys Chem B 2005;109:20131–6. [41] Marashdeh A, Olsen RA, Løvvik OM, Kroes GJ. Density functional theory study of the TiH2 interaction with a NaAlH4 cluster. J Phys Chem C 2008;112:15759–64.
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The dehydrogenation performance and reaction mechanisms of Li3AlH6 with TiF3 additive Shu-Sheng Liu a,b, Yao Zhang a, Li-Xian Sun a,*, Jian Zhang a, Jun-Ning Zhao a, Fen Xu c,*, Feng-Lei Huang d a
Materials and Thermochemistry Laboratory, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian 116023, China Graduate School of Chinese Academy of Sciences, Beijing 100049, China c Faculty of Chemistry and Chemical Engineering, Liaoning Normal University, Dalian 116029, China d State Key Laboratory of Explosion Science and Technology, Beijing Institute of Technology, Beijing 100081, China b
article info
abstract
Article history:
For Li3AlH6 prepared by mechanical milling method, the dissociation reaction enthalpy and
Received 2 September 2009
activation energy are calculated to be 22.1 kJ mol1 H2 and 133.7 2.7 kJ mol1, respectively.
Received in revised form
The dehydrogenation performance of Li3AlH6 is greatly enhanced by TiF3 additive, espe-
10 December 2009
cially in the kinetic behaviors. For the Li3AlH6 þ 10 mol% TiF3 sample, the starting
Accepted 17 December 2009
temperature of dehydrogenation is obviously decreased by 60 C from that of pure Li3AlH6
Available online 28 March 2010
(190 C), and 3.0 wt.% H2 may be released within 1000 s at 120 C under an initial vacuum.
Keywords:
improves due to the decrease in the activation energy. The X-ray diffraction (XRD) together
Dehydrogenation
with thermogravimetric analysis (TGA) results show that there are three mechanochemical
Titanium fluoride
reactions involved during milling: i) Li3AlH6 þ TiF3 / 3 LiF þ Al þ Ti þ 3H2, ii) Ti þ H2 /
With the amount of TiF3 increasing, the starting temperature decreases and the kinetics
Alanate
TiH2, iii) 3 Al þ Ti / Al3Ti. The in-situ formed Ti species (TiH2 and Al3Ti) co-catalyze the
Kinetic
thermal dehydrogenation of Li3AlH6.
Mechanism
1.
ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved.
Introduction
Hydrogen storage is a key technical challenge for onboard application of hydrogen. High-pressure and cryogenic hydrogen storage systems are impractical for vehicular applications due to safety concerns and volumetric constraints. Attention has been focused on solid-state hydrogen storage materials due to the significant advantages related to safety, energy efficiency and operational convenience [1,2]. Metal hydrides [3], complex hydrides [4], carbon materials [5], and MOFs [6] are all investigated as hydrogen carriers. Among these materials, light metal complex hydrides have attracted intensive research interest because
of their high hydrogen storage capacities and moderate working temperatures. Lithium alanate (LiAlH4) is one of the promising hydrogen storage materials and has been widely investigated in recent years. It can decompose in three steps (R1, R2 and R3) with a hydrogen liberation amounting to 10.6 wt.%. [7,8] 150w175 C
3 LiAlH4 ! Li3 AlH6 þ 2Al þ 3H2 ; 180w220 C
Li3 AlH6 ! 3 LiH þ Al þ 3=2 H2 ; >400 C
ð5:3 wt:%H2 Þ ð2:6 wt:% H2 Þ
3 LiH þ 3 Al ! 3 LiAl þ 3=2 H2 ð2:6 wt:%H2 Þ
* Corresponding authors. Tel./fax: þ86 411 84379213. E-mail addresses: [email protected] (L.-X. Sun), [email protected] (F. Xu). 0360-3199/$ – see front matter ª 2009 Professor T. Nejat Veziroglu. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2009.12.108
R1 R2 R3
international journal of hydrogen energy 35 (2010) 4554–4561
The first two reactions, R1 and R2, attract much attention at present, since their reaction temperatures are close to the working temperature of fuel cell system. Various catalysts have been added to LiAlH4 to improve its hydrogen storage properties, including such effective metal halides as TiCl3$1/ 3AlCl3 [9,10], VCl3 [10], NiCl2 [11] and TiF3 [12,13]. Under the catalysis of these metal halides, LiAlH4 decomposes partly below 100 C mainly corresponding to R1 stage. However, the equilibrium pressure for R1 is estimated to be above 100 MPa at 25 C [14]. On the other hand, the equilibrium pressure for R2 is much lower. By ab initio calculations, Ke et al. [15] predicted the equilibrium pressure for R2 to be around 10 MPa at 0 C. It means that Li3AlH6 is a thermodynamically attractive complex for hydrogen storage with a capacity of 5.6 wt.% H2. The method for the preparation of Li3AlH6 via the mechanochemical reaction between LiAlH4 and LiH (R4) has been widely used in the past decade [8,16–18]. The dehydrogenation temperature of Li3AlH6 has been reduced by about 30 C and the kinetics has been significantly improved by Ti5Si3 [19] or Al3Ti [20] additives. TiCl3 [18–20] shows more pronounced catalytic effect on R2, since it reacts with Li3AlH6 to form Al3Ti, which may act as a dehydrogenation catalyst. Ti(OPr)4 is also a remarkable catalyst [17], which was co-milled with LiAlH4 and LiH to produce nanocrystalline Ti-doped Li3AlH6 with a starting dehydrogenation temperature of 100 C induced by the Ti0/Ti3þ/Ti4þ defect sites. ball milling
LiAlH4 þ 2LiH ! Li3 AlH6
R4
In the past few years, TiF3 has attracted broad scientific interest due to its excellent catalytic effect on many metal hydrides, such as MgH2 [21], Mg2NiH4 [21], Na2LiAlH6 [22]. Wang et al. reported that TiF3 is superior to TiCl3 as a dopant precursor in preparation of catalytically enhanced NaAlH4 [23] and Na3AlH6 [24] systems. They believed that the F anion is substantially incorporated into the doped structure, and in some way contributes to the catalytic enhancement. Our previous studies have demonstrated the significant catalytic effect of TiF3 on both LiBH4 [25] and LiAlH4 [13]. In the case of LiAlH4, adding 4 mol% TiF3 makes LiAlH4 start to decompose at 80 C with 6.3 wt.% H2 release up to 200 C. TiF3 probably reacts with LiAlH4 to form the catalyst, Al3Ti, which catalyzes the mechanochemical reactions during ball milling and the thermochemical reactions under increased temperature. However, the catalytic effect of TiF3 on Li3AlH6 is still unclear. We are curious if the same pathway occurs in the reactions. In order to find the answer to this question we further explore the dehydrogenation performance and reaction mechanisms of Li3AlH6 with TiF3 additive. On the other hand, the enthalpy change of reaction (DH ) and apparent activation energy (Ea) are two important thermal parameters to evaluate the thermodynamic and kinetic properties of a material or reaction [26]. Although numerous results related to these two parameters of Li3AlH6 (R2) have been reported, they are all based on the products of LiAlH4 decomposition (R1). The data related to pure Li3AlH6 prepared by ball milling have not been previously reported to the best of our knowledge. In this work, we prepared pristine Li3AlH6 and Li3AlH6 with TiF3 additive through mechanochemical milling. Both the thermal properties and the dehydrogenation performance of
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as-prepared Li3AlH6 with TiF3 additive were investigated. The reaction mechanisms of Li3AlH6 with TiF3 additive were also discussed.
2.
Experimental
2.1.
Sample preparation
The starting materials, LiH (98%), LiAlH4 (97%) and TiF3 (99%) were all purchased from Alfa Aesar and used without further purification. All material handlings were performed in an MBraun Unilab glove box filled with high purity Ar (99.999%), with oxygen and water vapor contents both less than 0.1 ppm. Li3AlH6 was synthesized through the mechanochemical reaction between LiH and LiAlH4 similar to the method reported in the literature [17–20]. LiH and LiAlH4 with a mole ratio of 2:1 were loaded into a 100 mL stainless milling pot with 24 steel balls (10 mm in diameter). Ball milling was performed using a QM-1SP2 planetary ball mill under 0.1 MPa argon atmosphere at a rotation speed of 541 rpm. We obtained pure Li3AlH6 after 8 h milling. Different amounts (2, 10, 20, 25 and 50 mol%) of TiF3 were added to Li3AlH6, respectively. All the mixtures were milled for 0.5 h under the same conditions as those used above.
2.2.
Sample characterization
XRD analysis was carried out using a PANalytical X-ray Diffractometer (X’Pert MPD PRO, Cu Ka, 40 kV, 40 mA). The samples were shielded by Mylar films (diffraction peak w 26 2q) from the atmospheric oxygen and moisture. Thermogravimetric analysis (TGA) was carried out in Cahn Thermax 500 with a heating rate of 10 C min1 in a flow of high purity Ar. Differential scanning calorimetry (DSC) was conducted using a TA Q1000 at different heating rates (2.5, 5, 10 and 20 C min1) under a flow (50 ml min1) of high purity Ar. The isothermal dehydrogenation was performed using a Sieverts-type apparatus (PCT, Advanced Materials Corporation, USA) under an initial pressure of 105 MPa H2. The surface morphology of the composites was observed using scanning electron microscopy (SEM, JSM6360LV).
3.
Results and discussion
3.1. XRD characterization and thermal properties of as-prepared Li3AlH6 The XRD pattern of as-prepared Li3AlH6 is shown in Fig. 1. All the reflections belong to Li3AlH6, and are in good agreement with the literature [16–20]. This means that Li3AlH6 could be prepared by the reaction between LiAlH4 and LiH (R4) within 8 h. By means of the Scherrer equation [b ¼ l/(B cos q), where b is the crystallite size, l is the X-ray wavelength and B is the full width at half maximum (FWHM)], the average crystallite size of Li3AlH6 could be determined as 22 nm. Fig. 2 shows the DSC curves of Li3AlH6 at different heating rates: 2.5, 5, 10 and 20 C min1. Only one endothermic peak,
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Fig. 1 – XRD pattern of as-prepared Li3AlH6.
Fig. 2 – DSC curves of Li3AlH6 at different heating rates: 2.5, 5, 10 and 20 8C minL1.
corresponding to Li3AlH6 decomposition, can be seen in each curve. At a heating rate of 10 C min1, the endothermic peak starts at 195 C and ends at 245 C. As known, DSC measurement is applied as a suitable and quick method for determination of dissociation reaction enthalpy by evaluating the peak area [27]. We used all four curves in Fig. 2 to calculate an average reaction enthalpy of R2 (22.1 kJ mol1 H2). It is between 13.3 [28] and 29.0 [29] kJ mol1 H2 both obtained by thermal analyses. The apparent activation energy, Ea, can be obtained using Kissinger equation [30]:
Isothermal dehydrogenations of as-prepared Li3AlH6 and Li3AlH6 with TiF3 additive at 120 C are shown in Fig. 5. Without TiF3, only about 0.35 wt.% H2 is released from asprepared Li3AlH6 even after 36000 s. After TiF3 is added, the kinetics of dehydrogenation is greatly improved, and the dehydrogenation goes on more rapidly with the increase of TiF3. With 2 mol% TiF3, Li3AlH6 can release 3.9 wt.% H2 in 10000 s, and the remaining hydrogen can be released when the dehydrogenation time is prolonged to 36000 s. As shown in the inset plot of Fig. 5, about 3.0 and 2.4 wt.% H2 may be released within 1000 s from Li3AlH6 þ 10 mol% TiF3 and Li3AlH6 þ 20 mol% TiF3 mixtures, respectively, and the latter performs more rapidly and releases almost its maximum dehydrogenation amount (2.6 wt.%, 6000 s) at 120 C. These results are attractive considering the U. S. Department of Energy target for the operation temperature.
d ln Tb2 p E ¼ a R d T1p where b, Tp and R are the heating rate, the peak temperature of the endothermic peak and the gas constant, respectively. As shown in Fig. 3, the apparent activation energy for the decomposition of as-prepared Li3AlH6 is 133.7 2.7 kJ mol1, which is close to the previously reported value of 130 15 kJ mol1 [28]. The differences between our results and those reported in the literature in both DH and Ea may be because the material Li3AlH6 tested in the present study is synthesized by the mechanochemical reaction instead of the thermal decomposition of LiAlH4.
3.3.
Reaction mechanisms of Li3AlH6 with TiF3 additive
Fig. 6 shows the SEM images of as-prepared Li3AlH6 and Li3AlH6 with TiF3 additive after milling. From the inset image,
3.2. Dehydrogenation performance of Li3AlH6 with TiF3 additive Non-isothermal dehydrogenation performance of samples with TiF3 additive was achieved by TGA measurements, shown in Fig. 4. The starting temperature of Li3AlH6 þ 2 mol% TiF3 mixture (Fig. 4b) is about 150 C, which is 40 C lower than that of as-prepared Li3AlH6 (Fig. 4a). With the additive amount increasing, the starting temperatures decrease from 130 C for Li3AlH6 þ 10 mol% TiF3 mixture to 105 C for Li3AlH6 þ 20 mol% TiF3 mixture. However, the amount of released hydrogen is decreasing and only 2.6 wt.% H2 is released from Li3AlH6 þ 20 mol% TiF3 mixture.
Fig. 3 – Kissinger plot for dehydrogenation of as-prepared Li3AlH6.
international journal of hydrogen energy 35 (2010) 4554–4561
Fig. 4 – Non-isothermal (10 8C minL1) dehydrogenations of as-prepared Li3AlH6 (a) and Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d).
we can see that the grains of as-prepared Li3AlH6 are already very small (10–50 nm), in agreement with the XRD result. But these small grains tend to agglomerate. When TiF3 is added, there is not obvious change in the grain size of Li3AlH6. So the size effect of Li3AlH6 on the dehydrogenation is negligible. Fig. 7 displays the XRD patterns of Li3AlH6 with TiF3 additive after milling. It is found that the reflections of Li3AlH6 become weaker with the additive amount increasing. When 50 mol% TiF3 is added, Li3AlH6 phase vanishes completely. Such intermediate phases as TiH2 (35.1, 40.5 2q) [18] and Al/LiF can be identified. We also observed a shoulder on the high angle side
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Fig. 5 – Isothermal (120 8C) dehydrogenations of as-prepared Li3AlH6 (a) and Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d).
of Al (111) (39 2q) in the XRD pattern, which might be assigned to a metastable phase of Al–Ti alloy, L12–Al3Ti [31]. It was also formed during ball milling in both TiCl3-doped NaAlH4 [32] and TiCl3-doped Li3AlH6 systems [18]. The lattice constant of this ˚ using the MDI Jade 5.0 new phase is evaluated to be 3.991 A software. This is very close to the lattice constant of L12–Al3Ti ˚ determined by Hong et al. [33] and 3.9856 A ˚ of 3.98 0.01 A determined by Zhang et al. [34]. This implies that the newly formed fcc phase in the present study is also L12–Al3Ti. From these observations, we assume that there may be three reactions (R5, R6 and R7) happening in Li3AlH6 with TiF3 additive
Fig. 6 – SEM images of as-prepared Li3AlH6 (a) and Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d). The inset images are the higher magnification micrographs.
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ball milling
Ti þ H2 ! TiH2
R6
ball milling
Ti þ 3Al ! Al3 Ti
Fig. 7 – XRD patterns of Li3AlH6 with different amounts of TiF3 additive: 2 mol% (a), 10 mol% (b), 25 mol% (c) and 50 mol% (d) after 0.5 h ball milling.
during ball milling. First, Li3AlH6 reacts with TiF3 by R5 to produce active metals: Al and Ti. According to R5, the H2 pressure will rise to above 0.4 MPa in the milling pot if 50 mol% TiF3 is added. R6 is a self-propagating reaction with a very large heat of formation and it has been reported to take place in 0.1 MPa H2 by mechanical milling [35,36]. The Ti particles with new surfaces and defects are easy to react with H2 under the impact of high-energy milling. The formed TiH2 has an average crystalline size of about 5 nm in Li3AlH6 þ 50 mol% TiF3 mixture, and the fine grains are easy to disperse in the mixture. At the same time, a competitive reaction R7 may happen because Al3Ti is a much more thermodynamically stable phase (DG+f ¼136 kJ mol1) than TiH2 (DG+f ¼86 kJ mol1) and AlTi (DG+f ¼72 kJ mol1) [37]. Initial milling results in a lamellar microstructure of the as-formed Al particles through deformation, welding and fracture. Further milling makes the Ti atoms dissolve into the Al matrix as a result of an interdiffusion reaction across the increasing interphase layer. The diffractions of Al3Ti are weak compared to other phases in the mixtures due to its high dispersion, large peak widths, small amount or some amorphous state [38].
ball milling
Li3 AlH6 þ TiF3 ! 3 LiF þ Al þ Ti þ 3 H2
R5
R7
To validate our hypothesis, we calculated the theoretical amounts of dehydrogenation during TGA analyses excluding the H2 loss accompanying the milling process. As shown in Table 1, the dehydrogenation amounts obtained by experiment are in accordance with the theoretical data for all the samples. It should be noted that TiH2 does not decompose to release H2 below 250 C [35]. Therefore, the above three mechanochemical reactions do occur in Li3AlH6 with TiF3 additive during ball milling, and produce the active species like TiH2 and Al3Ti. Fig. 8 shows the phase changes of Li3AlH6 þ 20 mol% TiF3 and Li3AlH6 þ 50 mol% TiF3 mixtures after isothermal dehydrogenation at different temperatures. For Li3AlH6 þ 20 mol% TiF3, the reflections of Li3AlH6 have already disappeared after dehydrogenation at 100 C for 4 h (Fig. 8a), which indicates R2 is almost completed at this condition. TiH2 seems to be unchangeable in all the patterns because 250 C is far below its decomposition temperature [35]. The reflections (39.2 (Fig. 8 inset), 45.5, 66.2 and 79.3 2q) of L12–Al3Ti become more obvious as temperature goes up, which corresponds to the growth of particles. The similar result was obtained when TiCl3-doped LiAlH4 was annealed to 150 C [39]. So, there is only one reaction (R2) involved during heating Li3AlH6 with TiF3 additive below 250 C. It is concluded that the amount of TiF3 is rapidly reduced by Li3AlH6 giving rise to nanocrystalline Ti and Al with H2 release during the ball milling process. The milling energy is significant enough to lead to the formation of TiH2 and Al3Ti. In the following heating process the intermetallic compound, Al3Ti becomes ordered and grows while TiH2 does not decompose within the testing temperature range.
3.4. Kinetic analysis and catalytic mechanism of Li3AlH6 with TiF3 additive Fig. 9 exhibits the DSC curves of Li3AlH6 with different amounts of TiF3 additive with only one endothermic peak on each curve. The reaction enthalpies are 21.9, 22.6 and 21.8 kJ mol1 H2, respectively, for Li3AlH6 þ 2 mol% TiF3, Li3AlH6 þ 10 mol% TiF3 and Li3AlH6 þ 20 mol% TiF3 mixtures. They are all close to those of as-prepared Li3AlH6. It means that the influence of TiF3 on the thermodynamic properties of
Table 1 – Thermodynamic/kinetic data and dehydrogenation amounts of as-prepared Li3AlH6 and Li3AlH6 with TiF3 additive. Amount of TiF3 (mol%) 0 2 10 20 25 50
Actual amount of H2 released during TGA (wt.%)
Theoretical amount of H2 released considering R5 (wt.%)
DH (kJ mol1 H2)
5.3 4.9 3.8 2.6 1.9 0
5.6 5.0 4.0 2.7 2.1 0
22.1 21.9 22.6 21.8 – –
Ea (kJ mol1) 133.7 2.7 125.3 1.6 104.4 3.3 85.8 0.2 – –
international journal of hydrogen energy 35 (2010) 4554–4561
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Fig. 9 – DSC (10 8C minL1) curves of as-prepared Li3AlH6 (a) and Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d).
Fig. 8 – XRD patterns of Li3AlH6 with TiF3 additive after isothermal dehydrogenations at different temperatures: 100 8C (a), 150 8C (b) and 250 8C (c) for Li3AlH6 with 20 mol% TiF3; 150 8C (d) and 250 8C (e) for Li3AlH6 with 50 mol% TiF3.
Li3AlH6 could be negligible. Even though, the peak temperatures for the endothermic reaction of the samples are gradually reduced with the augmentation of TiF3 in the mixtures. It is likely due to the enhancement in the dehydrogenation kinetics. The apparent activation energies of Li3AlH6 þ 2 mol% TiF3, Li3AlH6 þ 10 mol% TiF3 and Li3AlH6 þ 20 mol% TiF3 mixtures were evaluated using Kissinger analyses. As shown in Fig. 10, Ea decreases dramatically with addtive amount increasing. When 20 mol% TiF3 is added, Ea can be drastically decreased to 85.8 0.2 kJ mol1 from 133.7 2.7 kJ mol1 for as-prepared Li3AlH6 (Table 1). We conclude that adding TiF3 greatly improves the kinetics of Li3AlH6 dehydrogenation, and it is a plausible explanation to the above dehydrogenation behaviors. Al3Ti was reported to form in-situ and serve as an excellent catalyst in TiCl3-doped NaAlH4 [32], LiAlH4 [39] and Li3AlH6 [18] systems. TiH2 catalyst was proved to form in Ti-doped NaAlH4 during milling [40], and theoretical calculation also pointed out the catalytic mechanism of TiH2 [41]. Both Al3Ti and TiH2
are formed in-situ in Li3AlH6 with TiF3 additive in the present study. Available information provides an evidence that the activity of in-situ prepared nano/micro-crystalline heterogeneous catalyst with an extremely large active surface area significantly exceeds that of a doped bulk catalyst. This can justify the superior dehydrogenation kinetics of Li3AlH6 with TiF3 additive over Ti5Si3 [19] or Al3Ti [20] additives. Ti is a transition metal with a number of d-orbital vacancies, which facilitate the interaction between transition metal and hydrogen atoms of Li3AlH6 [39]. The in-situ formed Al3Ti and TiH2 disperse widely on the surfaces of much larger Li3AlH6 globelets, acting as the catalytic active sites for the dehydriding process. Also, TiH2 may destabilize the Al–H bonds [40]. When Li3AlH6 and TiH2 are close enough, local electrostatic attraction may cause the breaking of Al–H bands if combined with an increased atomic vibration at elevated temperature. As a result, Li3AlH6 decomposes and two H atoms combine to form H2. Therefore, it is Al3Ti and TiH2
Fig. 10 – Kissinger plots for dehydrogenations of Li3AlH6 with different amounts of TiF3 additive: 2 mol% (b), 10 mol% (c) and 20 mol% (d).
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that make the energy barrier of H dissociation (Ea) in Li3AlH6 decrease. The kinetic enhancement with the augmentation of TiF3 could be caused by the increase of the concentration of these active species.
4.
Conclusions
Li3AlH6 was successfully prepared by ball milling the mixture of LiAlH4 and LiH with a mole ratio of 1:2. It starts to decompose around 190 C. The reaction enthalpy and apparent activation energy of Li3AlH6 dehydrogenation are 22.1 kJ mol1 H2 and 133.7 2.7 kJ mol1, respectively. When Li3AlH6 is mixed with TiF3, it begins to release H2 at much lower temperature and the dehydrogenation speed is increased remarkably. For Li3AlH6 þ 10 mol% TiF3 mixture, 3.0 wt.% H2 can be released within 1000 s at 120 C. This is close to the need of practical application. Kinetic analyses indicate that the apparent activation energy of Li3AlH6 dehydrogenation has been greatly reduced by 29 kJ mol1 in Li3AlH6 þ 10 mol% TiF3 mixture. This enhancement attributes to the in-situ formed active catalysts, Al3Ti and TiH2, during the ball milling process. Therefore, TiF3 can be considered as an excellent additive on the dehydrogenation of Li3AlH6.
Acknowledgements The authors gratefully acknowledge the financial support for this work from the National Natural Science Foundation of China (No. 20833009, 20873148, 20903095, 50671098, 50901070 and U0734005), 863 projects (2007AA05Z115 and 2007AA05Z102), the National Basic Research Program (973 program) of China (2010CB631303), Beijing Institute of Technology (Grant No. KFJJ10-1Z) and IUPAC (Project No. 2008-0063-100). The authors also would like to express their appreciation to Dr. Michael Frenkel of the U. S. National Institute of Standards and Technology (Boulder, Colorado) for his valuable advice and suggestions.
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