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Effects of TiCl3-decorated MWCNTs addition on the dehydrogenation behavior and stability of LiAlH4
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Effects of TiCl3-decorated MWCNTs addition on the dehydrogenation behavior and stability of LiAlH4 Chia-Yen Tan a, Wen-Ta Tsai a,b,* a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan
article info
abstract
Article history:
A complex catalyst for enhancing the dehydrogenation kinetics of LiAlH4 was developed by
Received 22 August 2014
using an impregnation process to decorate TiCl3 on multiwall carbon nanotubes
Received in revised form
(MWCNTs). The effects of these composite catalysts on the dehydrogenation behavior were
29 September 2014
investigated by using thermal gravimetric analysis (TGA) and in-situ synchrotron X-ray
Accepted 6 October 2014
diffraction (XRD) technique. The experimental results showed that the initial dehydroge-
Available online 29 October 2014
nation temperature could be lowered by adding the appropriate amount of TiCl3eMWCNTs
Keywords:
20 wt% TiCl3eMWCNTs was added.
LiAlH4
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
composite. LiAlH4 became unstable and decomposed even at room temperature when
reserved.
MWCNTs TiCl3 Dehydrogenation In-situ synchrotron X-ray diffraction
3LiH / 3Li þ 3/2H2 (Above 400 C, 2.6 wt% H2)
Introduction Complex metal hydrides are promising materials for hydrogen storage applications, with good safety, high hydrogen storage capacity and relatively low dehydrogenation temperature [1e3]. Among all the complex metal hydrides, LiAlH4 is known for its theoretically high gravimetric hydrogen storage density of 10.5 wt%. The typical dehydrogenation process of LiAlH4 contains the following consecutive reactions [4e6]: 3LiAlH4 / Li3AlH6 þ 2Al þ 3H2 (160e180 C, 5.3 wt% H2)
(1)
Li3AlH6 / 3LiH þ Al þ 3/2H2 (180e220 C, 2.6 wt% H2)
(2)
(3)
The decomposition of LiH in the third reaction requires a temperature of up to 400 C, which is impractical for normal usage. Therefore, most of hydrogen released from LiAlH4 comes from the first two reactions, which is theoretically 7.9 wt%. Nevertheless, the dehydrogenation temperature of the first two reactions is still too high to meet the criteria set by the US Department of Energy (DOE) for light-duty vehicle applications [7]. A number of researchers have thus examined the use of metal catalysts [8e10] and Ti based chemical compounds [11e24] in order to further reduce the dehydrogenation temperature of LiAlH4. Among all the Ti-based catalysts, titanium chlorides (including TiCl3 and TiCl4) have shown to have the
* Corresponding author. 1, Ta Hsueh Road, Tainan 701, Taiwan. Tel.: þ886 62757575x62927; fax: þ886 62754395. E-mail address: [email protected] (W.-T. Tsai). http://dx.doi.org/10.1016/j.ijhydene.2014.10.036 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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most profound effects on lowering the dehydrogenation temperature of LiAlH4 [11,18,19]. Easton et al. [11] reported that doping of TiCl3 by 2 mol% (7.6 wt%) could reduce the onset dehydrogenation temperature to below 70 C. They also reported room temperature decomposition of LiAlH4 with the addition of TiCl3. Fu et al. [18,19] later reported similar observations on the effects of TiCl3 and TiCl4 doping in lowering the dehydrogenation temperature of LiAlH4. In an attempt to explain the physical mechanism by density functional theory calculations, Wohlwend et al. [21] indicated that the presence of Ti-containing additives in LiAlH4 might alter the charge density in the surrounding AlH 4 tetrahedra, affecting the H binding energy and leading to prompt desorption of H2 [21]. Besides the Ti based catalysts, nano-sized carbon materials such as carbon nano-fibers (CNFs) [25,26], single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) [27,28], were also reported to exhibit catalytic effects in modifying and improving the dehydrogenation kinetics of complex metal hydrides [28]. A few studies have attempted to explore the roles of carbon additives in catalyzing the dehydrogenation reactions of various complex metal hydrides. Kumar et al. [25] indicated that CNFs could provide additional transition sites for hydrogen transfer during dehydrogenation of LiAlH4. Berseth et al. [29] suggested that the electron affinity of the carbon additives might change the H-removal energy of the interacting complex metal hydride, and assist its dissociation reaction. The aim of this study was to develop a novel complex catalyst consisting of MWCNTs and TiCl3 catalysts. The TiCl3 decorated MWCNTs were prepared by an impregnation process. The effects of TiCl3 decorated MWCNTs on the dehydrogenation behavior of LiAlH4 were explored with high pressure thermo-gravimetric analysis (HPTGA) and in-situ XRD analysis.
Experimental procedure Sample preparation TiCl3 particles were decorated on MWCNTs (purity > 95 %, outer diameter 20e30 nm; length 5e15 mm, UniRegion, BioTech) by impregnation. To prepare the TiCl3eMWCNTs catalyst, 0.3 g of as-received MWCNTs was added to 0.3 g of TiCl3 solution (Acros, Titanium(III) chloride 20 % w/w in 2 N hydrochloride solution). The solution was subsequently treated with ultrasonic vibration for 30 min, followed by vacuum drying for 12 h to obtain the TiCl3eMWCNTs mixed powders. The concentration of TiCl3 was 16.6 wt% in the final impregnated powders. The vacuum dried TiCl3eMWCNTs powders with specific contents (0e20 wt%) were then mixed with lithium aluminum hydride (LiAlH4, Chemetall, 97% purity) and stored in a 75 ml cylindrical stainless steel vessel. Sample loading was performed in a N2-purified glove box. The vessel also contained stainless steel balls with an average diameter of 4.8 mm. The ball-to-powder weight ratio was maintained at 10:1. Before being installed to a ball-milling machine (SPEX 800), the sample-loaded stainless steel vessel was cooled in liquid N2 to prevent overheating in the course of ball milling at
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1700 rpm for 10 min. The process was repeated three times to ensure complete mixing of the powders.
Thermal dehydrogenation analysis The dehydrogenation behavior of the LiAlH4 e admixed with TiCl3eMWCNTs was evaluated by thermogravimetric analysis (TGA) with a high pressure microbalance (Cahn D-110). The sample with an initial weight of ca. 200 mg was loaded in a quartz crucible and transferred into a high pressure chamber. The chamber was evacuated to 1 104 torr, followed by charging H2 gas (99.99% purity) to ambient pressure. Once the microbalance system stabilized, heating was applied. Two heating processes were adopted in this study, as follows: (1) continuous heating from room temperature to 300 C at a heating rate of 5 C min1, and (2) isothermal heating at 90 C, 100 C, and 110 C for 2 h, respectively. The weight change of the sample was continuously measured during each heating process. The weight percentage of hydrogen released from each sample at the specific temperature was calculated.
Material characterization The morphology of the TiCl3eMWCNTs catalyst was examined by a transmission electron microscope (TEM). The chemical composition of the TiCl3eMWCNTs catalyst was analyzed by a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS). The crystal structure transition of LiAlH4 during the dehydrogenation reaction was identified by in-situ synchrotron X-ray diffraction (in-situ XRD) employing beamline 17A, at the National Synchrontron Radiation Research Center in Hsinchu, Taiwan. To perform insitu XRD analysis, the sample was first loaded in a 0.5 mm diameter glass capillary tube, and then mounted to the specimen holder. N2 gas was introduced at one end of the capillary tube to form a protective atmosphere. The sample was heated from room temperature to 350 C at a heating rate of 5 C min1 by a hot air blower. During heating the sample was exposed to the synchrotron X-ray with a wavelength of ˚ for 132 s, and this was repeated at intervals of 240 s. 1.033105 A The 2-D diffraction patterns were collected continuously and converted to 1-D patterns by Fit2D software. The transition of the crystal structure during the heating process was thus analyzed.
Results and discussion Material characterization TiCl3eMWCNTs catalyst Fig. 1 shows the TEM images of MWCNTs decorated with TiCl3 (hereafter referred as TiCl3eMWCNTs). The diameter of the MWCNTs was around 20e30 nm. As shown in this figure, the MWCNTs tangled together after impregnation and vacuum drying. The TiCl3 particles were deposited on the walls of MWCNTs with a size around 10e20 nm. The distribution of TiCl3 particles was not homogeneous, and some of the TiCl3 particles became aggregated, as seen in Fig. 1. The microstructure of the TiCl3-impreganated MWCNTs was also examined by SEM. Fig. 2(a) shows the typical SEM image, while
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Thermogravimetric analysis
Fig. 1 e TEM bright field image of TiCl3eMWCNTs catalysts.
Fig. 2(b) demonstrates the color mappings of Ti and Cl, revealing the distribution of TiCl3 particles in the MWCNT scaffolds. The EDS results shown in Fig. 2(c) also confirm the successful impregnation of TiCl3 into the MWCNTs scaffolds. Fig. 2(d) shows the morphology of the LiAlH4 powder admixed with 20 wt% TiCl3eMWCNTs catalysts, showing the scattering of MWCNTs over the milled LiAlH4 powders.
The dehydrogenation behaviors of LiAlH4 samples with and without additives were evaluated quantitatively by thermogravimetric analysis (TGA). The weight loss of each sample was measured continuously during heating, from room temperature to 300 C at a rate of 5 C min1. As shown in Fig. 3, a twostep dehydrogenation process was found for all the samples studied. Based on the TGA results, the temperature at which the sample had 0.25 wt% weight loss was determined as the initial dehydrogenation temperature. For the as-milled LiAlH4, the initial hydrogen desorption temperature was 170 C, in agreement with that reported in the literature [28]. A second dehydrogenation temperature was observed as the sample was continuously heated to 220 C. The total amount of hydrogen released was as high as 6.5 wt% when the temperature was raised to 260 C, and thereafter remained almost unchanged. The TGA results for LiAlH4 with various catalyst additions are also shown in Fig. 3, and the percentage weight change for each of these was measured on the basis of total weight, including that of the catalyst. By adding 20 wt% of MWCNTs to LiAlH4, the onset of the hydrogen desorption temperature decreased to 140 C, while the second stage dehydrogenation commenced at 190 C. The catalytic effect of MWCNTs in assisting the dehydrogenation of LiAlH4 was thus confirmed. However, only 4.6 wt% of hydrogen was released when the MWCNT-added LiAlH4 was heated to 300 C, much less than that without MWCNT addition. When MWCNTs were decorated with TiCl3, a further lowering of the dehydrogenation temperature of LiAlH4 was observed with the addition of the mixed catalyst, in both the first and second stages. The role of
Fig. 2 e (a) SEM image of vacuum dried TiCl3eMWCNTs catalysts, (b) EDS color mapping of Ti and Cl signals, (c) EDS analysis spectrum of the TiCl3eMWCNTs catalysts, and (d) SEM image of LiAlH4-admixed with 20 wt% TiCl3eMWCNTs catalyst.
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Fig. 3 e TGA results of ball-milled LiAlH4, LiAlH4-admixed with 20 wt% MWCNT and LiAlH4-admixed with 5 wt%, 10 wt%, and 20 wt% TiCl3eMWCNTs, heated from room temperature to 300 C at a rate of 5 C min¡1.
TiCl3 in catalyzing the dehydrogenation reactions was also concentration dependent. The respective dehydrogenation temperatures of LiAlH4, with or without catalyst addition, are summarized in Fig. 4 and Table 1. As revealed in Table 1, the first stage dehydrogenation temperature could be lowered to 80 C. The synergistic effect of the complex TiCl3eMWCNTs in affecting desorption of hydrogen from LiAlH4 is clearly demonstrated. It is worth noting that the total amount of hydrogen released from LiAlH4 with the addition of 5 wt% TiCl3eMWCNTs could reach 6.2 wt%, close to that from the plain LiAlH4. However, the total amount of desorbed hydrogen decreased with increasing TiCl3eMWCNTs content, beyond 5 wt%, even though the dehydrogenation temperatures were substantially lower. The amounts of total hydrogen released when LiAlH4 was heated both with and without the addition of a catalyst are also shown in Table 1. In Fig. 4 and Table 1, the
Fig. 4 e Variations of onset dehydrogenation temperature and total amount of hydrogen released from ball-milled LiAlH4, LiAlH4-admixed with 20 wt% MWCNTs and LiAlH4admixed with 5 wt%, 10 wt%, and 20 wt% TiCl3eMWCNTs, respectively.
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percentages of weight change calculated by excluding the amount of catalyst added are also presented to reveal the intrinsic amount of hydrogen released solely from LiAlH4. The results demonstrate that MWCNTs with and without TiCl3 decoration could lower the dehydrogenation temperature, but the total amounts of hydrogen released from LiAlH4 were all higher than 83% of the theoretical capacity with the addition of catalysts. Fig. 5 shows the isothermal dehydrogenation curves of LiAlH4 e admixed with 20 wt% TiCl3eMWCNTs at 90 C, 100 C and 110 C for 2 h. The noticeable amount of weight loss detected at each temperature indicates the occurrence of a dehydrogenation reaction. As shown in this figure, the magnitude of the initial slope of each curve increased with increasing temperature, showing the temperature-dependent kinetic behavior. Specifically, the dehydrogenation reaction rates at 90 C, 100 C and 110 C were 0.2 mg min1, 0.36 mg min1 and 0.66 mg min1, respectively. The amounts of hydrogen released were 2.6 wt%, 3.1 wt%, and 3.6 wt% at the three different isothermal temperatures, which were less than the theoretical hydrogen capacity, indicating the incompleteness of the first stage dehydrogenation reaction of the LiAlH4-admixed with 20 wt% TiCl3eMWCNTs at the temperatures studied.
In-situ synchrotron X-ray diffraction analysis The dehydrogenation reaction of LiAlH4 e admixed with 20 wt % TiCl3eMWCNTs was closely examined by in-situ synchrotron XRD analysis. The sample was cooled by liquid N2 and then ball-milled for 30 min and held at room temperature for 24 h before in-situ XRD analysis. During in-situ synchrotron X-ray diffraction analysis, the sample was heated at a rate of 5 C min1 from room temperature to 350 C to induce thermal dehydrogenation reactions. The diffraction patterns were collected every 20 C, as shown in Fig. 6(a). The XRD patterns obtained at room temperature show that the sample consists of the LiAlH4, Li3AlH6 and Al phases. As mentioned above, Li3AlH6 is the reaction product of the first stage dehydrogenation of LiAlH4. The presence of the diffraction peaks of Li3AlH6 indicated that TiCl3eMWCNTs could strongly catalyze the decomposition of LiAlH4, even at room temperature. In our previous investigation, the first stage dehydrogenation temperature of LiAlH4 could be lowered from 170 C to 140 C with the addition of plain MWCNTs [28]. The in-situ synchrotron Xray diffraction results obtained in this study clearly indicate that the catalyzing power of MWCNTs could be substantially promoted with TiCl3 decoration. However, as found in the TGA analysis, and shown in Table 1, the first stage dehydrogenation temperature was 75 C if TiCl3eMWCNTs was added to LiAlH4. In that case, the sample was analyzed immediately after ballmilling, without prolonged temporary storage. The sample used for the in-situ synchrotron X-ray diffraction analysis, however, was stored for 24 h after ball-milling. The results indicated the strong synergistic effect of TiCl3 and MWCNTs in catalyzing the decomposition of LiAlH4 at room temperature. The continued dehydrogenation of LiAlH4 with increasing temperature caused a gradual disappearance of the peaks corresponding to this phase. When the sample was heated to nearly 90 C, the peaks of LiAlH4 phase disappeared while
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Table 1 e List of the first and the second stage dehydrogenation temperatures and amount of hydrogen released from ball milled LiAlH4, LiAlH4-admixed with 20 wt% MWCNTs and LiAlH4-admixed with 5 wt%, 10 wt%, and 20 wt% TiCl3eMWCNTs. Composition LiAlH4 LiAlH4 LiAlH4 LiAlH4 LiAlH4
e e e e
20 wt% MWCNTs 5 wt% TiCl3eMWCNTs 10 wt% TiCl3eMWCNTs 20 wt% TiCl3eMWCNTs
T1 ( C)
T2 ( C)
Hydrogen released (wt%, including MWCNTs)
Hydrogen released (wt%, excluding MWCNTs)
Theoretical hydrogen capacity
171 148 102 78 75
206 190 140 123 113
6.21 4.61 6.20 5.30 4.68
6.21 5.76 6.52 5.89 5.85
7.9 6.3 7.5 7.1 6.3
those of Li3AlH6 still remained. These results suggest the solid to liquid transformation of LiAlH4 occurred at 90 C. The presence of the diffraction peaks of Li3AlH6 at temperatures beyond 90 C and until 150 C suggest that the LiAlH4 liquid phase dehydrogenation reaction could take place with the addition of TiCl3eMWCNTs, similar to the results found with plain LiAlH4 [30,31]. In Fig. 6, the peaks corresponding to Li3AlH6 existed until the sample was continuously heated to 130 C, where their intensity started to fade out, indicating the occurrence of the second stage dehydrogenation reaction similar to reaction (2), but without catalyst addition. The second stage dehydrogenation reaction was completed at 150 C, as revealed in Fig. 6, followed by the existence of the LiH phase. The temperature where the second stage dehydrogenation reaction commenced was lowered from 206 C for plain Li3AlH6 to 190 C and 113 C for those with 20 wt% MWCNTs and 20 wt% TiCl3eMWCNTs addition, respectively. The reinforced role of TiCl3 in catalyzing the dehydrogenation of LiAlH4 and Li3AlH6 was thus quite obvious. The temperature-dependent X-ray diffraction peak intensities for various phases are show in Fig. 6(a) and summarized in Fig. 6(b). The co-existence of the LiAlH4 and Li3AlH6 phases in the sample at room temperature indicates that the LiAlH4 might have decomposed to Li3AlH6 before the in-situ synchrotron XRD analysis, as mentioned above. Since the samples had been stored in a N2-purified glove box at room temperature for 24 h before performing in-situ synchrotron XRD analysis, the early decomposition phenomenon of LiAlH4
Fig. 5 e Isothermal dehydrogenation curves of the LiAlH4admixed with 20 wt% TiCl3eMWCNTs catalyst at 90 C, 100 C and 110 C.
might occur at room temperature and at less than 24 h. The room temperature stability of LiAlH4 with the addition of TiCl3eMWCNTs is discussed in the following section by performing ex-situ XRD analysis under different storage times.
Fig. 6 e (a) In-situ synchrotron XRD patterns of LiAlH4admixed with 20 wt% TiCl3eMWCNTs heated from room temperature to 350 C; (b) variations of the highest peak intensity with temperature for the major species appearing during heating.
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Identification of stability of LiAlH4 with TiCl3eMWCNTs addition In order to better understand the stability of LiAlH4 e admixed with 20 wt% TiCl3eMWCNTs stored at room temperature, the crystal structures of the immediately ball-milled sample and those stored in a N2-filled glove box at room temperature for several intervals of 12 h were analyzed by X-ray diffraction. The time-dependent XRD patterns are shown in Fig. 7. For the admixed with 20 wt% fresh ball-milled LiAlH4 TiCl3eMWCNTs, the ex-situ XRD patterns reveal only the peaks of LiAlH4. As shown in Fig. 7, the peaks corresponding to Li3AlH6 and Al, which were the first stage dehydrogenation products, started to appear after 12 h storage. As the storage time increased, the peak intensities for these two species also rose, accompanied by decreasing peak intensities of LiAlH4, indicating the continuing decomposition of LiAlH4, even when stored at room temperature. By storing at room temperature longer than 36 h, the diffraction peaks of LiAlH4 disappeared, indicating complete decomposition of this phase to from Li3AlH6 and Al. The results of ex-situ XRD analysis further confirm the strong catalyzing power of TiCl3eMWCNTs for the dehydrogenation reaction of LiAlH4. The weight losses of various as-milled LiAlH4, with and without catalyst loading, were measured in a N2-filled glove box at room temperature at 4-h intervals. As shown in Fig. 8, the weight of pure LiAlH4 remained constant throughout the whole measurement. An initial slight weight loss of about 0.5 wt% was observed when 20 wt% MWCNTs was added to LiAlH4, which then remained almost unchanged. A significant and noticeable weight loss, however, was found when LiAlH4 was admixed with 20 wt% TiCl3eMWCNTs. The amount of weight loss associated with hydrogen released from the mixture increased with increasing holding time, indicating the occurrence of a continuous dehydrogenation reaction of LiAlH4. A weight loss of 3.2 wt% (4.0 wt% excluding the weight of catalyst) was measured when the holding time reached 40 h and beyond. Since the theoretical amount of hydrogen that could be released from the first stage dehydrogenation
Fig. 8 e Time-dependent weight loss of ball milled LiAlH4, LiAlH4-admixed with 20 wt% MWCNT and LiAlH4-admixed with 20 wt% TiCl3eMWCNTs, measured every 4 h after ball milling.
reaction of LiAlH4 was 5.3 wt%, about 75% of hydrogen could be released at room temperature with the addition of 20 wt% TiCl3eMWCNTs. These results again demonstrate the important catalytic role of TiCl3 in the dehydrogenation reaction. The destabilizing behavior of the milled LiAlH4 was reported by Easton et al. [11], who suggested that the addition of 2 mol% of TiCl3 might lead to its early decomposition. The present study found direct evidence of the catalytic role of TiCl3 combined with MWCNTs in destabilizing LiAlH4 at room temperature. The incorporation of MWCNTs provided the skeleton for uniform dispersion of TiCl3, and consequently enhanced the decomposition rate of LiAlH4.
Conclusion This study demonstrates the catalytic effect of TiCl3 decorated MWCNTs on the dehydrogenation behavior and stability of LiAlH4. MWCNTs can be decorated with TiCl3 particles using an impregnation process. The TGA results show that MWCNTs can cause a lowering of the first stage dehydrogenation temperature of LiAlH4. A more profound catalytic effect can be obtained if MWCNTs are decorated with TiCl3, which is concentration dependent. Both in-situ and ex-situ XRD analyses indicate that room temperature decomposition of LiAlH4 can occur with the addition of 20 wt% TiCl3eMWCNTs.
Acknowledgments
Fig. 7 e Time-dependent XRD patterns of the LiAlH4admixed with 20 wt% TiCl3eMWCNTs, collected every 12 h after ball milling.
The authors gratefully acknowledge the financial support received for this study from the Ministry of Science and Technology (Republic of China, Taiwan), under grant 1022221-E-006-159-MY2, and the Research Center for Energy Technology and Strategy, National Cheng Kung University, under grant D100-23003.
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Effects of TiCl3-decorated MWCNTs addition on the dehydrogenation behavior and stability of LiAlH4 Chia-Yen Tan a, Wen-Ta Tsai a,b,* a b
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 701, Taiwan Research Center for Energy Technology and Strategy, National Cheng Kung University, Tainan 701, Taiwan
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abstract
Article history:
A complex catalyst for enhancing the dehydrogenation kinetics of LiAlH4 was developed by
Received 22 August 2014
using an impregnation process to decorate TiCl3 on multiwall carbon nanotubes
Received in revised form
(MWCNTs). The effects of these composite catalysts on the dehydrogenation behavior were
29 September 2014
investigated by using thermal gravimetric analysis (TGA) and in-situ synchrotron X-ray
Accepted 6 October 2014
diffraction (XRD) technique. The experimental results showed that the initial dehydroge-
Available online 29 October 2014
nation temperature could be lowered by adding the appropriate amount of TiCl3eMWCNTs
Keywords:
20 wt% TiCl3eMWCNTs was added.
LiAlH4
Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights
composite. LiAlH4 became unstable and decomposed even at room temperature when
reserved.
MWCNTs TiCl3 Dehydrogenation In-situ synchrotron X-ray diffraction
3LiH / 3Li þ 3/2H2 (Above 400 C, 2.6 wt% H2)
Introduction Complex metal hydrides are promising materials for hydrogen storage applications, with good safety, high hydrogen storage capacity and relatively low dehydrogenation temperature [1e3]. Among all the complex metal hydrides, LiAlH4 is known for its theoretically high gravimetric hydrogen storage density of 10.5 wt%. The typical dehydrogenation process of LiAlH4 contains the following consecutive reactions [4e6]: 3LiAlH4 / Li3AlH6 þ 2Al þ 3H2 (160e180 C, 5.3 wt% H2)
(1)
Li3AlH6 / 3LiH þ Al þ 3/2H2 (180e220 C, 2.6 wt% H2)
(2)
(3)
The decomposition of LiH in the third reaction requires a temperature of up to 400 C, which is impractical for normal usage. Therefore, most of hydrogen released from LiAlH4 comes from the first two reactions, which is theoretically 7.9 wt%. Nevertheless, the dehydrogenation temperature of the first two reactions is still too high to meet the criteria set by the US Department of Energy (DOE) for light-duty vehicle applications [7]. A number of researchers have thus examined the use of metal catalysts [8e10] and Ti based chemical compounds [11e24] in order to further reduce the dehydrogenation temperature of LiAlH4. Among all the Ti-based catalysts, titanium chlorides (including TiCl3 and TiCl4) have shown to have the
* Corresponding author. 1, Ta Hsueh Road, Tainan 701, Taiwan. Tel.: þ886 62757575x62927; fax: þ886 62754395. E-mail address: [email protected] (W.-T. Tsai). http://dx.doi.org/10.1016/j.ijhydene.2014.10.036 0360-3199/Copyright © 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
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most profound effects on lowering the dehydrogenation temperature of LiAlH4 [11,18,19]. Easton et al. [11] reported that doping of TiCl3 by 2 mol% (7.6 wt%) could reduce the onset dehydrogenation temperature to below 70 C. They also reported room temperature decomposition of LiAlH4 with the addition of TiCl3. Fu et al. [18,19] later reported similar observations on the effects of TiCl3 and TiCl4 doping in lowering the dehydrogenation temperature of LiAlH4. In an attempt to explain the physical mechanism by density functional theory calculations, Wohlwend et al. [21] indicated that the presence of Ti-containing additives in LiAlH4 might alter the charge density in the surrounding AlH 4 tetrahedra, affecting the H binding energy and leading to prompt desorption of H2 [21]. Besides the Ti based catalysts, nano-sized carbon materials such as carbon nano-fibers (CNFs) [25,26], single-walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs) [27,28], were also reported to exhibit catalytic effects in modifying and improving the dehydrogenation kinetics of complex metal hydrides [28]. A few studies have attempted to explore the roles of carbon additives in catalyzing the dehydrogenation reactions of various complex metal hydrides. Kumar et al. [25] indicated that CNFs could provide additional transition sites for hydrogen transfer during dehydrogenation of LiAlH4. Berseth et al. [29] suggested that the electron affinity of the carbon additives might change the H-removal energy of the interacting complex metal hydride, and assist its dissociation reaction. The aim of this study was to develop a novel complex catalyst consisting of MWCNTs and TiCl3 catalysts. The TiCl3 decorated MWCNTs were prepared by an impregnation process. The effects of TiCl3 decorated MWCNTs on the dehydrogenation behavior of LiAlH4 were explored with high pressure thermo-gravimetric analysis (HPTGA) and in-situ XRD analysis.
Experimental procedure Sample preparation TiCl3 particles were decorated on MWCNTs (purity > 95 %, outer diameter 20e30 nm; length 5e15 mm, UniRegion, BioTech) by impregnation. To prepare the TiCl3eMWCNTs catalyst, 0.3 g of as-received MWCNTs was added to 0.3 g of TiCl3 solution (Acros, Titanium(III) chloride 20 % w/w in 2 N hydrochloride solution). The solution was subsequently treated with ultrasonic vibration for 30 min, followed by vacuum drying for 12 h to obtain the TiCl3eMWCNTs mixed powders. The concentration of TiCl3 was 16.6 wt% in the final impregnated powders. The vacuum dried TiCl3eMWCNTs powders with specific contents (0e20 wt%) were then mixed with lithium aluminum hydride (LiAlH4, Chemetall, 97% purity) and stored in a 75 ml cylindrical stainless steel vessel. Sample loading was performed in a N2-purified glove box. The vessel also contained stainless steel balls with an average diameter of 4.8 mm. The ball-to-powder weight ratio was maintained at 10:1. Before being installed to a ball-milling machine (SPEX 800), the sample-loaded stainless steel vessel was cooled in liquid N2 to prevent overheating in the course of ball milling at
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1700 rpm for 10 min. The process was repeated three times to ensure complete mixing of the powders.
Thermal dehydrogenation analysis The dehydrogenation behavior of the LiAlH4 e admixed with TiCl3eMWCNTs was evaluated by thermogravimetric analysis (TGA) with a high pressure microbalance (Cahn D-110). The sample with an initial weight of ca. 200 mg was loaded in a quartz crucible and transferred into a high pressure chamber. The chamber was evacuated to 1 104 torr, followed by charging H2 gas (99.99% purity) to ambient pressure. Once the microbalance system stabilized, heating was applied. Two heating processes were adopted in this study, as follows: (1) continuous heating from room temperature to 300 C at a heating rate of 5 C min1, and (2) isothermal heating at 90 C, 100 C, and 110 C for 2 h, respectively. The weight change of the sample was continuously measured during each heating process. The weight percentage of hydrogen released from each sample at the specific temperature was calculated.
Material characterization The morphology of the TiCl3eMWCNTs catalyst was examined by a transmission electron microscope (TEM). The chemical composition of the TiCl3eMWCNTs catalyst was analyzed by a scanning electron microscope (SEM) with an energy dispersive spectrometer (EDS). The crystal structure transition of LiAlH4 during the dehydrogenation reaction was identified by in-situ synchrotron X-ray diffraction (in-situ XRD) employing beamline 17A, at the National Synchrontron Radiation Research Center in Hsinchu, Taiwan. To perform insitu XRD analysis, the sample was first loaded in a 0.5 mm diameter glass capillary tube, and then mounted to the specimen holder. N2 gas was introduced at one end of the capillary tube to form a protective atmosphere. The sample was heated from room temperature to 350 C at a heating rate of 5 C min1 by a hot air blower. During heating the sample was exposed to the synchrotron X-ray with a wavelength of ˚ for 132 s, and this was repeated at intervals of 240 s. 1.033105 A The 2-D diffraction patterns were collected continuously and converted to 1-D patterns by Fit2D software. The transition of the crystal structure during the heating process was thus analyzed.
Results and discussion Material characterization TiCl3eMWCNTs catalyst Fig. 1 shows the TEM images of MWCNTs decorated with TiCl3 (hereafter referred as TiCl3eMWCNTs). The diameter of the MWCNTs was around 20e30 nm. As shown in this figure, the MWCNTs tangled together after impregnation and vacuum drying. The TiCl3 particles were deposited on the walls of MWCNTs with a size around 10e20 nm. The distribution of TiCl3 particles was not homogeneous, and some of the TiCl3 particles became aggregated, as seen in Fig. 1. The microstructure of the TiCl3-impreganated MWCNTs was also examined by SEM. Fig. 2(a) shows the typical SEM image, while
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Thermogravimetric analysis
Fig. 1 e TEM bright field image of TiCl3eMWCNTs catalysts.
Fig. 2(b) demonstrates the color mappings of Ti and Cl, revealing the distribution of TiCl3 particles in the MWCNT scaffolds. The EDS results shown in Fig. 2(c) also confirm the successful impregnation of TiCl3 into the MWCNTs scaffolds. Fig. 2(d) shows the morphology of the LiAlH4 powder admixed with 20 wt% TiCl3eMWCNTs catalysts, showing the scattering of MWCNTs over the milled LiAlH4 powders.
The dehydrogenation behaviors of LiAlH4 samples with and without additives were evaluated quantitatively by thermogravimetric analysis (TGA). The weight loss of each sample was measured continuously during heating, from room temperature to 300 C at a rate of 5 C min1. As shown in Fig. 3, a twostep dehydrogenation process was found for all the samples studied. Based on the TGA results, the temperature at which the sample had 0.25 wt% weight loss was determined as the initial dehydrogenation temperature. For the as-milled LiAlH4, the initial hydrogen desorption temperature was 170 C, in agreement with that reported in the literature [28]. A second dehydrogenation temperature was observed as the sample was continuously heated to 220 C. The total amount of hydrogen released was as high as 6.5 wt% when the temperature was raised to 260 C, and thereafter remained almost unchanged. The TGA results for LiAlH4 with various catalyst additions are also shown in Fig. 3, and the percentage weight change for each of these was measured on the basis of total weight, including that of the catalyst. By adding 20 wt% of MWCNTs to LiAlH4, the onset of the hydrogen desorption temperature decreased to 140 C, while the second stage dehydrogenation commenced at 190 C. The catalytic effect of MWCNTs in assisting the dehydrogenation of LiAlH4 was thus confirmed. However, only 4.6 wt% of hydrogen was released when the MWCNT-added LiAlH4 was heated to 300 C, much less than that without MWCNT addition. When MWCNTs were decorated with TiCl3, a further lowering of the dehydrogenation temperature of LiAlH4 was observed with the addition of the mixed catalyst, in both the first and second stages. The role of
Fig. 2 e (a) SEM image of vacuum dried TiCl3eMWCNTs catalysts, (b) EDS color mapping of Ti and Cl signals, (c) EDS analysis spectrum of the TiCl3eMWCNTs catalysts, and (d) SEM image of LiAlH4-admixed with 20 wt% TiCl3eMWCNTs catalyst.
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Fig. 3 e TGA results of ball-milled LiAlH4, LiAlH4-admixed with 20 wt% MWCNT and LiAlH4-admixed with 5 wt%, 10 wt%, and 20 wt% TiCl3eMWCNTs, heated from room temperature to 300 C at a rate of 5 C min¡1.
TiCl3 in catalyzing the dehydrogenation reactions was also concentration dependent. The respective dehydrogenation temperatures of LiAlH4, with or without catalyst addition, are summarized in Fig. 4 and Table 1. As revealed in Table 1, the first stage dehydrogenation temperature could be lowered to 80 C. The synergistic effect of the complex TiCl3eMWCNTs in affecting desorption of hydrogen from LiAlH4 is clearly demonstrated. It is worth noting that the total amount of hydrogen released from LiAlH4 with the addition of 5 wt% TiCl3eMWCNTs could reach 6.2 wt%, close to that from the plain LiAlH4. However, the total amount of desorbed hydrogen decreased with increasing TiCl3eMWCNTs content, beyond 5 wt%, even though the dehydrogenation temperatures were substantially lower. The amounts of total hydrogen released when LiAlH4 was heated both with and without the addition of a catalyst are also shown in Table 1. In Fig. 4 and Table 1, the
Fig. 4 e Variations of onset dehydrogenation temperature and total amount of hydrogen released from ball-milled LiAlH4, LiAlH4-admixed with 20 wt% MWCNTs and LiAlH4admixed with 5 wt%, 10 wt%, and 20 wt% TiCl3eMWCNTs, respectively.
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percentages of weight change calculated by excluding the amount of catalyst added are also presented to reveal the intrinsic amount of hydrogen released solely from LiAlH4. The results demonstrate that MWCNTs with and without TiCl3 decoration could lower the dehydrogenation temperature, but the total amounts of hydrogen released from LiAlH4 were all higher than 83% of the theoretical capacity with the addition of catalysts. Fig. 5 shows the isothermal dehydrogenation curves of LiAlH4 e admixed with 20 wt% TiCl3eMWCNTs at 90 C, 100 C and 110 C for 2 h. The noticeable amount of weight loss detected at each temperature indicates the occurrence of a dehydrogenation reaction. As shown in this figure, the magnitude of the initial slope of each curve increased with increasing temperature, showing the temperature-dependent kinetic behavior. Specifically, the dehydrogenation reaction rates at 90 C, 100 C and 110 C were 0.2 mg min1, 0.36 mg min1 and 0.66 mg min1, respectively. The amounts of hydrogen released were 2.6 wt%, 3.1 wt%, and 3.6 wt% at the three different isothermal temperatures, which were less than the theoretical hydrogen capacity, indicating the incompleteness of the first stage dehydrogenation reaction of the LiAlH4-admixed with 20 wt% TiCl3eMWCNTs at the temperatures studied.
In-situ synchrotron X-ray diffraction analysis The dehydrogenation reaction of LiAlH4 e admixed with 20 wt % TiCl3eMWCNTs was closely examined by in-situ synchrotron XRD analysis. The sample was cooled by liquid N2 and then ball-milled for 30 min and held at room temperature for 24 h before in-situ XRD analysis. During in-situ synchrotron X-ray diffraction analysis, the sample was heated at a rate of 5 C min1 from room temperature to 350 C to induce thermal dehydrogenation reactions. The diffraction patterns were collected every 20 C, as shown in Fig. 6(a). The XRD patterns obtained at room temperature show that the sample consists of the LiAlH4, Li3AlH6 and Al phases. As mentioned above, Li3AlH6 is the reaction product of the first stage dehydrogenation of LiAlH4. The presence of the diffraction peaks of Li3AlH6 indicated that TiCl3eMWCNTs could strongly catalyze the decomposition of LiAlH4, even at room temperature. In our previous investigation, the first stage dehydrogenation temperature of LiAlH4 could be lowered from 170 C to 140 C with the addition of plain MWCNTs [28]. The in-situ synchrotron Xray diffraction results obtained in this study clearly indicate that the catalyzing power of MWCNTs could be substantially promoted with TiCl3 decoration. However, as found in the TGA analysis, and shown in Table 1, the first stage dehydrogenation temperature was 75 C if TiCl3eMWCNTs was added to LiAlH4. In that case, the sample was analyzed immediately after ballmilling, without prolonged temporary storage. The sample used for the in-situ synchrotron X-ray diffraction analysis, however, was stored for 24 h after ball-milling. The results indicated the strong synergistic effect of TiCl3 and MWCNTs in catalyzing the decomposition of LiAlH4 at room temperature. The continued dehydrogenation of LiAlH4 with increasing temperature caused a gradual disappearance of the peaks corresponding to this phase. When the sample was heated to nearly 90 C, the peaks of LiAlH4 phase disappeared while
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Table 1 e List of the first and the second stage dehydrogenation temperatures and amount of hydrogen released from ball milled LiAlH4, LiAlH4-admixed with 20 wt% MWCNTs and LiAlH4-admixed with 5 wt%, 10 wt%, and 20 wt% TiCl3eMWCNTs. Composition LiAlH4 LiAlH4 LiAlH4 LiAlH4 LiAlH4
e e e e
20 wt% MWCNTs 5 wt% TiCl3eMWCNTs 10 wt% TiCl3eMWCNTs 20 wt% TiCl3eMWCNTs
T1 ( C)
T2 ( C)
Hydrogen released (wt%, including MWCNTs)
Hydrogen released (wt%, excluding MWCNTs)
Theoretical hydrogen capacity
171 148 102 78 75
206 190 140 123 113
6.21 4.61 6.20 5.30 4.68
6.21 5.76 6.52 5.89 5.85
7.9 6.3 7.5 7.1 6.3
those of Li3AlH6 still remained. These results suggest the solid to liquid transformation of LiAlH4 occurred at 90 C. The presence of the diffraction peaks of Li3AlH6 at temperatures beyond 90 C and until 150 C suggest that the LiAlH4 liquid phase dehydrogenation reaction could take place with the addition of TiCl3eMWCNTs, similar to the results found with plain LiAlH4 [30,31]. In Fig. 6, the peaks corresponding to Li3AlH6 existed until the sample was continuously heated to 130 C, where their intensity started to fade out, indicating the occurrence of the second stage dehydrogenation reaction similar to reaction (2), but without catalyst addition. The second stage dehydrogenation reaction was completed at 150 C, as revealed in Fig. 6, followed by the existence of the LiH phase. The temperature where the second stage dehydrogenation reaction commenced was lowered from 206 C for plain Li3AlH6 to 190 C and 113 C for those with 20 wt% MWCNTs and 20 wt% TiCl3eMWCNTs addition, respectively. The reinforced role of TiCl3 in catalyzing the dehydrogenation of LiAlH4 and Li3AlH6 was thus quite obvious. The temperature-dependent X-ray diffraction peak intensities for various phases are show in Fig. 6(a) and summarized in Fig. 6(b). The co-existence of the LiAlH4 and Li3AlH6 phases in the sample at room temperature indicates that the LiAlH4 might have decomposed to Li3AlH6 before the in-situ synchrotron XRD analysis, as mentioned above. Since the samples had been stored in a N2-purified glove box at room temperature for 24 h before performing in-situ synchrotron XRD analysis, the early decomposition phenomenon of LiAlH4
Fig. 5 e Isothermal dehydrogenation curves of the LiAlH4admixed with 20 wt% TiCl3eMWCNTs catalyst at 90 C, 100 C and 110 C.
might occur at room temperature and at less than 24 h. The room temperature stability of LiAlH4 with the addition of TiCl3eMWCNTs is discussed in the following section by performing ex-situ XRD analysis under different storage times.
Fig. 6 e (a) In-situ synchrotron XRD patterns of LiAlH4admixed with 20 wt% TiCl3eMWCNTs heated from room temperature to 350 C; (b) variations of the highest peak intensity with temperature for the major species appearing during heating.
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Identification of stability of LiAlH4 with TiCl3eMWCNTs addition In order to better understand the stability of LiAlH4 e admixed with 20 wt% TiCl3eMWCNTs stored at room temperature, the crystal structures of the immediately ball-milled sample and those stored in a N2-filled glove box at room temperature for several intervals of 12 h were analyzed by X-ray diffraction. The time-dependent XRD patterns are shown in Fig. 7. For the admixed with 20 wt% fresh ball-milled LiAlH4 TiCl3eMWCNTs, the ex-situ XRD patterns reveal only the peaks of LiAlH4. As shown in Fig. 7, the peaks corresponding to Li3AlH6 and Al, which were the first stage dehydrogenation products, started to appear after 12 h storage. As the storage time increased, the peak intensities for these two species also rose, accompanied by decreasing peak intensities of LiAlH4, indicating the continuing decomposition of LiAlH4, even when stored at room temperature. By storing at room temperature longer than 36 h, the diffraction peaks of LiAlH4 disappeared, indicating complete decomposition of this phase to from Li3AlH6 and Al. The results of ex-situ XRD analysis further confirm the strong catalyzing power of TiCl3eMWCNTs for the dehydrogenation reaction of LiAlH4. The weight losses of various as-milled LiAlH4, with and without catalyst loading, were measured in a N2-filled glove box at room temperature at 4-h intervals. As shown in Fig. 8, the weight of pure LiAlH4 remained constant throughout the whole measurement. An initial slight weight loss of about 0.5 wt% was observed when 20 wt% MWCNTs was added to LiAlH4, which then remained almost unchanged. A significant and noticeable weight loss, however, was found when LiAlH4 was admixed with 20 wt% TiCl3eMWCNTs. The amount of weight loss associated with hydrogen released from the mixture increased with increasing holding time, indicating the occurrence of a continuous dehydrogenation reaction of LiAlH4. A weight loss of 3.2 wt% (4.0 wt% excluding the weight of catalyst) was measured when the holding time reached 40 h and beyond. Since the theoretical amount of hydrogen that could be released from the first stage dehydrogenation
Fig. 8 e Time-dependent weight loss of ball milled LiAlH4, LiAlH4-admixed with 20 wt% MWCNT and LiAlH4-admixed with 20 wt% TiCl3eMWCNTs, measured every 4 h after ball milling.
reaction of LiAlH4 was 5.3 wt%, about 75% of hydrogen could be released at room temperature with the addition of 20 wt% TiCl3eMWCNTs. These results again demonstrate the important catalytic role of TiCl3 in the dehydrogenation reaction. The destabilizing behavior of the milled LiAlH4 was reported by Easton et al. [11], who suggested that the addition of 2 mol% of TiCl3 might lead to its early decomposition. The present study found direct evidence of the catalytic role of TiCl3 combined with MWCNTs in destabilizing LiAlH4 at room temperature. The incorporation of MWCNTs provided the skeleton for uniform dispersion of TiCl3, and consequently enhanced the decomposition rate of LiAlH4.
Conclusion This study demonstrates the catalytic effect of TiCl3 decorated MWCNTs on the dehydrogenation behavior and stability of LiAlH4. MWCNTs can be decorated with TiCl3 particles using an impregnation process. The TGA results show that MWCNTs can cause a lowering of the first stage dehydrogenation temperature of LiAlH4. A more profound catalytic effect can be obtained if MWCNTs are decorated with TiCl3, which is concentration dependent. Both in-situ and ex-situ XRD analyses indicate that room temperature decomposition of LiAlH4 can occur with the addition of 20 wt% TiCl3eMWCNTs.
Acknowledgments
Fig. 7 e Time-dependent XRD patterns of the LiAlH4admixed with 20 wt% TiCl3eMWCNTs, collected every 12 h after ball milling.
The authors gratefully acknowledge the financial support received for this study from the Ministry of Science and Technology (Republic of China, Taiwan), under grant 1022221-E-006-159-MY2, and the Research Center for Energy Technology and Strategy, National Cheng Kung University, under grant D100-23003.
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