Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder

Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder

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Significantly improved dehydrogenation of LiAlH4 catalysed with TiO2 nanopowder M. Ismail a,b, Y. Zhao a,c,*, X.B. Yu a,d, I.P. Nevirkovets a, S.X. Dou a a

Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2519, Australia Department of Physical Sciences, Faculty of Science and Technology, Universiti Malaysia Terengganu, 21030 Kuala Terengganu, Malaysia c School of Mechanical, Materials and Mechatronics Engineering, University of Wollongong, NSW 2522, Australia d Department of Materials Science, Fudan University, Shanghai 200433, China b

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abstract

Article history:

The effects of TiO2 nanopowder addition on the dehydrogenation behaviour of LiAlH4 have

Received 17 December 2010

been studied. The 5 wt.% TiO2-added LiAlH4 sample showed a significant improvement in

Received in revised form

dehydrogenation rate compared to that of undoped LiAlH4, with the dehydrogenation

28 March 2011

temperature reduced from 150  C to 60  C. Kinetic desorption results show that the added

Accepted 8 April 2011

LiAlH4 released about 5.2 wt% hydrogen within 30 min at 100  C, while the as-received

Available online 17 May 2011

LiAlH4 just released below 0.2 wt.% hydrogen within same time at 120  C. From the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is

Keywords:

114 kJ/mol for pure LiAlH4 and 49 kJ/mol for the 5 wt.% TiO2 added LiAlH4, indicating that

LiAlH4

TiO2 nanopowder adding significantly decreased the activation energy for hydrogen

TiO2 nanopowder

desorption of LiAlH4. X-ray diffraction and Fourier transform infrared analysis show that

Desorption temperature

there is no phase change in the cell volume or on the AleH bonds of the LiAlH4 due to

Kinetics desorption

admixture of TiO2 after milling. X-ray photoelectron spectroscopy results show no changes in the Ti 2p spectra for TiO2 after milling and after dehydrogenation. The improved dehydrogenation behaviour of LiAlH4 in the presence of TiO2 is believed to be due to the high defect density introduced at the surfaces of the TiO2 particles during the milling process. Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.

1.

Introduction

Catalyst investigation plays a vital role in the development of hydrogen storage materials, especially for the high-capacity complex hydrides such as LiBH4, NaAlH4, and LiAlH4. The main purposes in catalyst investigation of hydrogen storage materials are to reduce the decomposition temperature, enhance the desorption kinetics, and improve the reversibility. Growing interest has been attracted by LiAlH4 of the alanate family. LiAlH4 theoretically contains 10.5 wt.% H2, which

makes it a preferable hydrogen storage medium to NaAlH4 in terms of gravimetric capacity for hydrogen vehicles. Many studies have been done to improve the dehydrogenation behaviour of LiAlH4 through using various dopant, which include (1) pure metals such as V, Ti, Fe, Ni [1] and nanometric Ni [2,3] (2) alloys such as Ti3Al and TiAl3 [4] (3) metal/transition metal halides such as TiCl3, AlCl3, FeCl3 [1], NiCl2 [5], NbF5 [6], LaCl3 [7] and MnCl2 [8] (4) carbon materials such as carbon black [1] and carbon nanofibres (CNFs) [9] (5) metal hydrides such as TiH2 [1] and (6) composites such as SWCNT-metallic

* Corresponding author. Institute for Superconducting and Electronic Materials, University of Wollongong, Wollongong, NSW 2519, Australia. Tel./fax: þ61 2 4221 5731. E-mail address: [email protected] (Y. Zhao). 0360-3199/$ e see front matter Copyright ª 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2011.04.074

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catalyst [10] and [email protected] [9]. To the best of our knowledge, no studies have been reported on LiAlH4 added with metal oxides, which are popular as catalysts for hydrogen storage materials [11e13]. Among the metal oxides, TiO2 shows potential as a catalyst for improving the hydrogen sorption properties of hydrogen storage materials, especially for NaAlH4 and MgH2. A study by Suttisawat et al. [14] shows that TiO2 doped NaAlH4 lowered the temperature of the first desorption to150  C, a decrease of 35  C compared to undoped one. They also revealed that NaAlH4 catalysed with TiO2 desorbs about 5.0 wt.% H2 during the first cycle with enhanced kinetics. In addition, Lee et al. [15] studied the catalytic effects of nanosized TiO2 powder on the desorption kinetics of NaAlH4. Their results showed that TiO2 nanopowder improves the desorption kinetics and cyclic property of NaAlH4. Both of these studies claimed that the enhancement in hydrogenation properties of NaAlH4 may be attributed to the formation of amorphous TieAl as a reaction product between TiO2 and NaAlH4, and that this amorphous TieAl acts as the real catalyst in the NaAlH4eTiO2 system. According to a study by Jung et al. [16], the use of nanosized rutile TiO2 in MgH2 creates a greater reactive surface with hydrogen, thus affecting the hydrogen absorption kinetics. The capacity was increased by forming an ultra-fine nanocomposite of MgH2- rutile TiO2. Recently, Croston et al. [17] found that reduction of the Ti4þ ions in TiO2 to metallic Ti0 appears to result in the formation of active species responsible for catalysing the MgH2 dehydrogenation reaction. They also claimed that the surface area of oxide also plays a critical role in de/rehydrogenation effects. It would be interesting to see the role of TiO2 in LiAlH4. However, TiO2 adding effects in LiAlH4 have not been reported so far, to the best of the authors’ knowledge. In this study, we present the effects of TiO2 nanopowder on the dehydrogenation behaviour of LiAlH4 and attempt to understand the mechanism of how TiO2 nanopowder acts as a catalyst in LiAlH4.

2.

Experimental

LiAlH4 (95% pure, Sigma Aldrich) was mixed with TiO2 anatase (nanopowder, <25 nm particle size, 99.7% trace metal basis, Sigma Aldrich) under Ar atmosphere in an MBraun Unilab glove box. This mixture was loaded together with hardened stainless steel balls into a sealed stainless steel vial and then milled in a planetary ball mill (QM-3SP2) for 1 h, by first milling for 0.5 h, resting for 6 min, and then milling for another 0.5 h in a different direction at the rate of 400 rpm. The ratio of the weight of the balls to the weight of the powder was 40:1. X-ray diffraction (XRD) analysis was performed using a GBC MMA X-ray diffractometer with Cu Ka radiation. qe2q scans were carried out over diffraction angles from 20 to 80 with a speed of 2.00 /min. Before the measurement, a small amount of sample was spread uniformly on the sample holder, which was wrapped with plastic wrap to prevent oxidation. Fourier transformation infrared (FTIR) spectroscopy analyses were carried out using a Shimadzu IRPrestige-21 model. Samples were analysed in attenuated total reflectance mode (ATR) using the Pike MIRacle accessory equipped with a Ge crystal (Pike Technology). 40 scans were carried out between

800 and 2000 cm1 with a spectral resolution of 4 cm1. The morphology of the samples was investigated using a JEOL JSM 6460A scanning electron microscope (SEM) with an energy dispersive X-ray spectroscopy (EDS) detector of X-ray mapping capability. The dehydrogenation experiments were performed in a Sieverts-type pressure-composition-temperature (PCT) apparatus (Advanced Materials Corporation). About 200 mg of sample was loaded into a sample vessel in the glove box. For desorption purposes, all the samples were heated in a vacuum chamber, and the amount of desorbed hydrogen was measured to determine the lowest decomposition temperature. The heating rate for the desorption experiment was 5  C/min, and samples were heated from room temperature to 260  C. Differential scanning calorimetry (DSC) analysis of the dehydrogenation process was carried out on a Mettler Toledo TGA/DSC 1. About 5e8 mg of sample was loaded into an alumina crucible in the glove box. The crucible was then placed in a sealed glass bottle in order to prevent oxidation during transportation from the glove box to the DSC apparatus. An empty alumina crucible was used for reference. The samples were heated from room temperature to 300  C under 1 atm flowing argon atmosphere, and the heating rate was 20  C/min. X-ray photoelectron spectroscopy (XPS) of the pure TiO2 and TiO2-doped LiAlH4 before and after desorption was conducted using a SPECS PHOIBOS 100 Analyser installed in a high-vacuum chamber with the base pressure below 108 mbar; X-ray excitation was provided by Al Ka radiation with photon energy hn ¼ 1486.6 eV at the high voltage of 12 kV and power of 120 W. The XPS binding energy spectra were recorded at the pass energy of 20 eV in the fixed analyser transmission mode, and the XPS spectra of the doped sample were collected after bombardment of the sample using an Ar ion source with ion energy of 5 keV. Samples were prepared inside an Ar glove box, by dusting powders onto an adhesive carbon tape. The samples were then placed in a sealed container in order to reduce oxidation during transportation from the glove box to the XPS apparatus. Analysis of the XPS data was carried out using the commercial CasaXPS2.3.15 software package. The background was corrected using the linear approximation. Raman spectroscopy was carried out to investigate the defect site in the crystal structure of TiO2 surface using JOBIN YVON HR800 Confocal Raman system with 632.8 nm diode laser excitation on a 300 lines/mm grating at room temperature.

3.

Result and discussion

Fig. 1 displays the XRD patterns of the as-received LiAlH4, the as-milled LiAlH4, and the LiAlH4 þ 5 wt.% TiO2. For the asreceived LiAlH4, except for the plastic wrap peak, the spectra show that LiAlH4 crystallises in the monoclinic space group P21/c, which in good agreement with the crystal structure of LiAlH4 reported by Hauback et al. [18]. The ball-milled LiAlH4 showed slightly weaker intensity compared to the as-received sample. After milling with 5 wt.% TiO2, the diffraction peaks of LiAlH4 become weaker than those of the as-milled LiAlH4,

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Fig. 1 e X-ray diffraction patterns of as-received LiAlH4, asmilled LiAlH4, and LiAlH4 D 5 wt.% TiO2. Filled circle corresponds to the plastic wrap peaks.

and no additional peaks were observed. No peak of TiO2 nanopowder was observed after adding. The TiO2 nanopowders have intrinsically weak X-ray signal comparing to the LiAlH4 powders, owing to their small particle sizes. And the low proportion TiO2 addition in the LiAlH4 samples makes it even difficult to be distinguished from the noisy background. The FTIR spectra of the as-received LiAlH4, the as-milled LiAlH4, and the LiAlH4 þ 5 wt.% TiO2 are shown in Fig. 2. For LiAlH4, there are two regions of active infrared vibrations of the AleH bonds [19]: two AleH stretching modes between 1600 and 1800 cm1, and two LieAleH bending modes between 800 and 900 cm1. From the spectra shown in Fig. 2, all the stretching and bending modes occur for all samples, and they are in good agreement with results previously reported [19]. After detailed FTIR analysis, it has been found that there is no phase change due to admixture of TiO2 after milling, which is in good agreement with the XRD measurements. Fig. 3 shows the FESEM images of the as-milled LiAlH4 and the LiAlH4 þ 5 wt.% TiO2. As it can be seen, the FESEM images do not show significant difference in microstructure of pure and added sample after ball milling. Both of the samples show sub-micrometre particle features but the particle sizes are not

evenly distributed. Our added TiO2 particles are of 25 nm original size and it is difficult to observe the embedded TiO2 in the LiAlH4 matrix from the FESEM images. Fig. 4 shows the dehydrogenation results for the as-received LiAlH4, the as-milled LiAlH4, and the LiAlH4 added with 5 wt.% TiO2. The as-received LiAlH4 starts to release hydrogen at about 150  C for the first step, R1 (3LiAlH4 / Li3AlH6 þ 2Al þ 3H2, where 5.3 wt.% H2 is theoretically released), and desorbs about 4.99 wt% hydrogen. The second step reaction, R2 (Li3AlH6 / 3LiH þ Al þ 3/2H2, where 2.6 wt.% H2 is theoretically released), starts to release hydrogen at 189  C and desorbs about 2.53 wt.% hydrogen. After adding with 5 wt% TiO2, it is clear that the onset dehydrogenation temperature of the LiAlH4 was dramatically decreased to 60  C and the full dehydrogenation was completed below 200  C, which is 90  C lower than for the as-received LiAlH4. The total amount of hydrogen release is retained at about 7.5 wt.%. Fig. 5 presents the DSC curves of the as-received LiAlH4, asmilled LiAlH4, and the LiAlH4 þ 5 wt.% TiO2. For the asreceived LiAlH4, there are two exothermic processes and two endothermic processes. The first exothermic peak (170  C) is assigned to the interaction of LiAlH4 with surface hydroxyl impurities [20], and the second exothermic peak (220  C) corresponds to the decomposition of liquid LiAlH4 (R1) [21]. For the endothermic process, the first peak (187  C) corresponds to the melting of LiAlH4 and the second peak (271  C) corresponds to the decomposition of Li3AlH6 (R2). It is worth noting that due to the different heating rates and the different atmospheres in the DSC and PCT, the decomposition temperature by DSC is slightly higher. After adding with 5 wt.% TiO2, the number of thermal events of LiAlH4 is reduced from four to only two: one broad exothermic process, with a peak at 151  C, and one broad endothermic process, with a peak at 221  C. The exothermic process corresponds to the decomposition of solid LiAlH4 (R1), and the endothermic process corresponds to the decomposition of Li3AlH4 (R2). Clearly, the melting peak of LiAlH4 disappears after adding with TiO2. The disappearance of the melting peak from the DSC trace should be attributed to the fact that the decomposition temperature of the first stage is lower than the melting temperature of LiAlH4. Fig. 6(a) and (b) shows the isothermal dehydriding kinetics measurements at different temperatures for the LiAlH4 þ 5 wt.% TiO2 and for the as-received LiAlH4. From the results, it can be seen that adding with TiO2 caused LiAlH4 to release as much as 5.2 wt.% hydrogen within 30 min at 100  C (Fig. 6(a)). The dehydrogenation rate of the TiO2-added sample at 100  C is much faster than that of the as-received LiAlH4 at 150  C (Fig. 6(b)). This kinetic enhancement is related to the energy barrier for H2 release from LiAlH4. The activation energy for decomposition of the LiAlH4 has been reduced by adding with TiO2. To calculate the activation energy of the as-received LiAlH4 and TiO2-added LiAlH4, the Arrhenius equation was used. From the Arrhenius equation, k ¼ k0 expð  EA =RTÞ

Fig. 2 e FTIR spectra of as-received LiAlH4, as-milled LiAlH4, and LiAlH4 D 5 wt.% TiO2.

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(1)

where k is the rate of dehydrogenation, k0 is a temperatureindependent coefficient, EA is the apparent activation energy for hydride decomposition, R is the gas constant, and T is the

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Fig. 3 e FESEM image of (a) the as-milled LiAlH4 and (b) the LiAlH4 added with 5 wt.% TiO2 by ball milling.

absolute temperature. As shown in Fig. 7, by plotting ln(k) vs. 1/T, the apparent activation energy, EA, for H2 release from as-received LiAlH4 and TiO2-added LiAlH4 can be estimated. From the calculation, the apparent activation energy of dehydrogenation, EA, for the as-received LiAlH4 is 114 kJ/mol. This value is lowered by 65 kJ/mol after adding with 5 wt.% TiO2 (EA ¼ 49 kJ/mol for the TiO2-added LiAlH4). Owing to this lowering of the activation energy, the decomposition of the LiAlH4 was improved significantly. The apparent activation energy of TiO2-added LiAlH4 obtained in this work is similar to that for LiAlH4 doped with 2 mol% TiCl3$1/3AlCl3 (42.6 kJ/mol) [22]. XPS spectroscopy of the TiO2-added LiAlH4 was carried out to investigate the nature of the Ti species after milling and to investigate whether there was any change in these species after dehydrogenation. In this investigation, a sample with 20 wt.% TiO2 was used. For the sample with 5 wt.% TiO2, the Ti 2p transition was hard to detect, probably due to the concentration of Ti at the surface being too low. For comparison, pure TiO2 also was investigated. Fig. 7 shows the variation in the Ti 2p transition for these samples. The pure TiO2 sample showed the characteristic peak of Ti4þ, situated at 464.6 eV for 2p1/2 and 458.7 eV for 2p3/2, as shown in Fig. 8(a), which is in accordance with the literature [23].

The Ti 2p transition was less pronounced in the TiO2-added LiAlH4 samples than in the pure TiO2, similar to what was reported by Croston et al. [17] for TiO2-doped MgH2. In order to better identify the response from Ti in the spectra recorded for the TiO2-added LiAlH4 sample after ball milling and after desorption, in situ ion milling (for 10 min) using an Ar ion source with ion energy of 5 keV was employed. This resulted in a better signal-to-noise value for the Ti 2p peak as compared with that recorded for the same sample before the ion milling, although the noise level was still comparable with the signal. We performed deconvolution of the peak structure using the CasaXPS2.3.15 software package for fitting the peak structure obtained for the 20 wt.% TiO2-added LiAlH4 samples; Fig. 8(b) and (c) shows the fitting results for the samples before and after desorption. For the sample before desorption, the position of the Ti 2p3/2 peak and of the Ti 2p1/2 peak is 458.8 eV and 464.7 eV, respectively. After desorption, there was no significant change for the Ti 2p transition as observed in our XPS measurements. (The position of the Ti 2p3/2 and the Ti 2p1/2 peaks was 458.8 eV and 464.8 eV, respectively.) In order to examine the defect site in the crystal structure of TiO2 surface during the milling process, Raman spectroscopy was used. In this investigation four samples were measured;

Fig. 4 e Thermal desorption of as-received LiAlH4, asmilled LiAlH4, and LiAlH4 D 5 wt.% TiO2.

Fig. 5 e DSC traces of as-received LiAlH4, as-milled LiAlH4 and LiAlH4 D 5 wt.% TiO2.

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associated to the defect site in the crystal structure of TiO2 surface [24]. From the results, we assume that the same phenomenon was occurred for TiO2 surfaces in TiO2-added LiAlH4 sample. It would be more evidential to observe the peak shift in the TiO2 added samples, but the signal of TiO2 is too weak to be detected by Raman spectroscopy due to the small concentration of TiO2 in LiAlH4. Fig. 10 illustrates the comparison of the dehydrogenation profile of 5 wt.% and 20 wt.% TiO2 added LiAlH4 samples. From the results, we can see that the 20 wt.% TiO2 doped LiAlH4 sample shows an increase of desorption rate compare than that of 5 wt.% TiO2 doped LiAlH4 sample. Heated up to 100  C,

Fig. 6 e Isothermal dehydrogenation kinetics of (a) LiAlH4 added with 5 wt.% TiO2 and (b) as-received LiAlH4 at different temperatures.

pure TiO2, as-milled TiO2 (1 h ball mill), 20 wt.% TiO2-added LiAlH4 before and after desorption. Fig. 9 shows the Raman spectra for pure TiO2 and as-milled TiO2. For the pure TiO2, the spectrum situated at 143, 196, 396, 517 and 636 cm1 are corresponds to anatase TiO2, which is in accordance with the literature [24,25]. After 1 h ball milled, Raman spectrums of anatase located at 143, 196 and 517 cm1 apparently more weaken and the spectrums located at 396 and 636 cm1 shift to 420 and 620 cm1. This shift may be due to transformation from anatase TiO2 to srilankite or rutile TiO2 as reported by Pan et al. [25]. A weaken and shift spectrum after milling are

Fig. 7 e The plot of ln(k) vs. 1/T for (a) as-received LiAlH4 and (b) LiAlH4 D 5 wt.% TiO2.

Fig. 8 e XPS Ti 2p spectra for (a) pure TiO2, (b) LiAlH4 D 20 wt.% TiO2 before desorption, and (c) LiAlH4 D 20 wt.% TiO2 after desorption.

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Fig. 9 e Raman spectra for pure TiO2 and as-milled TiO2.

Fig. 11 e X-ray diffraction patterns of LiAlH4 D 20 wt.% TiO2. Filled diamonds and filled circle correspond to the Al and plastic wrap peaks, respectively.

the 20 wt.% TiO2 doped LiAlH4 sample desorbed about 4.4 wt.% hydrogen meanwhile the 5 wt.% TiO2 doped LiAlH4 sample just desorbed about 1.8 wt.% hydrogen. A stoichiometric reaction between LiAlH4 and 20 wt.% TiO2 may occur during ball milling process. In order to examine the phase transfer after ball milling for 20 wt.% TiO2 added LiAlH4 sample, XRD was used. Fig. 11 displays the XRD pattern of the 20 wt.% TiO2 added LiAlH4 after 1 h of milling. From the XRD result, the post-milled 20 wt.% doped LiAlH4 sample showed the weak additional peaks of Al, indicating that doping with 20 wt.% TiO2 induced the decomposition of LiAlH4 that occurred during ball milling. Besides the LiAlH4 and Al peaks, we couldn’t detect the peaks of TiAl3, Al2O3 and Al(-Ti) that are presumed to be reaction product between LiAlH4 and TiO2. We believe that there is no reaction occurs between LiAlH4 and TiO2 during ball milling process. This assumption is supported by XPS results in which there is no change in the Ti species after ball milling. According to [26], the bonding strength of TieO is so strong (bonding enthalpy of 672 kJ/mol) that TiO2 cannot be easily reduced by Al in LiAlH4 at low temperature. The decomposition of 20 wt.% TiO2 added LiAlH4 during the ball milling process may due to the catalytic effect of TiO2 surfaces.

In order to investigate the chemical environments of Al and Ti after dehydrogenation, XRD measurement was performed on the 5 wt.% and 20 wt.% TiO2 added LiAlH4 samples after dehydrogenation at 260  C. As shown in Fig. 12, the spectra show that the sample dehydrogenated at 260  C consists of Al and LiH as the dehydrogenation products, with no phases of LiAlH4 and Li3AlH6, indicating that the first step and second step reactions were completed for LiAlH4. As after milling, no TiO2 or secondary TiO-containing phases were detected from the dehydrogenation spectra, again attributable to the week X-ray diffraction by the nanopowders of TiO2. A possible mechanism for the TiO2 effect in LiAlH4 is that during the milling or the dehydrogenation process, TiO2 reacts with LiAlH4, as reported in the literature for MgH2 [17] and NaAlH4 [14,15]. However, from our XRD analysis, for the 5 wt.% TiO2 doped LiAlH4 sample, we did not detect any phase change due to admixture of TiO2 after milling, signifying that no reaction occurred between TiO2 and LiAlH4 during ball milling. The FTIR result also shows no phase change for the TiO2-added sample after milling. Both results are supported by the XPS results: specifically, the XPS measurements revealed no change in the Ti species after ball milling and after the

Fig. 10 e Thermal desorption of LiAlH4 D 5 wt.% TiO2 and LiAlH4 D 20 wt.% TiO2.

Fig. 12 e X-ray diffraction patterns for dehydrogenation samples of LiAlH4 doped with 5 wt.% and 20 wt.% TiO2. Filled diamonds and filled circle correspond to the Al/LiH and plastic wrap peaks, respectively.

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dehydrogenation process. Therefore, this possibility is in disagreement with the experimental results. Therefore, the surface properties of TiO2 may play a crucial role in improving the dehydrogenation behaviour of LiAlH4. This can be associated with the role of the catalyst as intermediate in the dissociation process of hydrogen molecules. Before the dissociation of hydrogen molecules from LiAlH4 can take place, hydrogen has to be adsorbed at the surface of the TiO2. According to Henrich [27], most nearly perfect TiO2-singlecrystal surfaces are essentially inert to H2, but H2 is absorbed by TiO2-surfaces that contain a higher density of defects in the crystal structure. A weaken and shift spectrum as evidenced in the Raman spectra of the TiO2 milled sample is consistent with the possibility that defects are introduced into the crystal structure of TiO2 surfaces in TiO2 doped sample. So, the significant improvement of the dehydrogenation behaviour of LiAlH4 after adding with 5 wt.% TiO2 nanopowder may be attributed to a high density of defects, which are introduced at the surfaces of the TiO2 particles during the ball-milling process. However, further work is necessary to clarify the exact role of TiO2 addition in LiAlH4 by observation methods such as transmission electron microscopy.

4.

Conclusion

In summary, we have demonstrated that adding with a small amount of TiO2 nanopowder by dry ball milling significantly reduced the decomposition temperature and enhanced the desorption kinetics of LiAlH4. Adding with 5 wt.% TiO2 results in a reduction in the decomposition temperature of LiAlH4 by 90  C compared to the as-received LiAlH4. The added material starts to release hydrogen at 60  C, and dehydrogenation is completed below 200  C, with about 7.50 wt.% H2 desorbed. Furthermore, the kinetic desorption results show that the added LiAlH4 released about 5.2 wt% hydrogen within 30 min at 100  C, while the as-received LiAlH4 just released 0.2 wt.% hydrogen within the same time at 120  C. This indicates that the TiO2-added sample shows a significant improvement in dehydrogenation rate compared to that of the as-received LiAlH4. From the Arrhenius plot of the hydrogen desorption kinetics, the apparent activation energy is 49 kJ/mol for TiO2added LiAlH4, compared to 114 kJ/mol for LiAlH4. Adding with 5 wt.% TiO2 reduced the activation energy of LiAlH4, thus promoting the decomposition at lower temperature. It is believed that the significant effect of TiO2 nanopowder addition on the dehydrogenation behaviour of LiAlH4 is attributable to the catalytic activity of the TiO2 surfaces. From the experimental results, it can be concluded that TiO2 nanopowder is an excellent additive for the dehydrogenation of LiAlH4.

Acknowledgements The authors thank the University of Wollongong for financial support of this research. M. Ismail acknowledges the Ministry of Higher Education Malaysia for a PhD scholarship. Many

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thanks also go to Dr. T. Silver for critical reading of the manuscript.

references

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