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Modification of the H2 Desorption Properties of LiAlH4 through Doping with Ti Henrietta W. Langmi,† G. Sean McGrady,*,† Xiangfeng Liu,† and Craig M. Jensen‡ Department of Chemistry, UniVersity of New Brunswick, PO Box 4400, Fredericton, NB E3B 5A3, Canada, and Department of Chemistry, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822-2275 ReceiVed: March 24, 2010; ReVised Manuscript ReceiVed: May 4, 2010
Undoped LiAlH4 displays complex thermal behavior in which melting at ∼170 °C is accompanied by spontaneous decomposition to produce Al, H2, and Li3AlH6: this latter species then decomposes further to LiH, H2, and Al at ∼225 °C. For undoped LiAlH4, melting corresponds to initial absorption, then release, of the latent heat of fusion as LiAlH4 decomposes to LiH, Li3AlH6, Al, and H2, and the first three species spontaneously solidify. Doping LiAlH4 with low levels of TiCl3 advances its thermal decomposition by 60-75 °C, bringing this event below the melting point of LiAlH4 and uncoupling it from the composite melting-decomposition-solidification event that appears at ca. 170 °C for the undoped material. As a result, the desorption of H2 from both LiAlH4 and Li3AlH6 are each revealed to be intrinsically endothermic events, in contrast with previous reports that described the decomposition of LiAlH4 to produce H2 as an exothermic process. Accordingly, the release of H2 by Ti-doped LiAlH4 may be varied and shut off by close control of the sample temperature, in contrast with the undoped material, whose H2 release characteristics are compromised by its melting, with immediate spontaneous and uncontrolled decomposition. 1. Introduction There has been intensive research on hydrogen storage materials by various research groups globally. High capacity hydrogen storage systems such as aluminum hydride, AlH3, and its complexes, LiAlH4 and NaAlH4, have been investigated as candidate materials,1-6 possessing theoretical hydrogen wt % values of 10.0, 7.9, and 5.6, respectively. When doped with a Ti catalyst, these materials exhibit attractively low hydrogen desorption temperatures. We recently demonstrated for the first time that Ti-doped LiAlH4 can operate as a reversible hydrogen storage material that can release g7 wt % hydrogen commencing at temperatures below 100 °C.7 During the course of our investigations of this doped material, it became clear to us that the thermochemistry of LiAlH4 was poorly documented, with inconsistent reports in the literature dating back some four decades. Accordingly, we report here an investigation of the properties of both as-received and Ti-doped LiAlH4, and the resolution of some long-standing questions surrounding the thermochemistry that characterizes its decomposition and release of H2. There have been numerous studies of the thermal decomposition of LiAlH4 over the past several decades.3,8-16Whereas there are some commonalities in these reports,8-10 there are also conflicting interpretations of the results.9,11,12 In the earliest study, Block and Gray observed three endotherms and two exotherms in a DSC study of LiAlH4.8 The first exothermic event at 153 °C was ascribed to the presence of an impurity, whereas the first endothermic peak at 170 °C was assigned to a reversible phase change. Subsequent thermal events occurring after this phase transition were reported to be associated with the evolution of hydrogen according to eqs 1-3 * Corresponding author. † University of New Brunswick. ‡ University of Hawaii at Manoa.
LiAlH4 f LiAlH2 + H2 (187-218°C)
(1)
LiAlH2 f LiH + Al + 1/2H2 (228-282°C)
(2)
LiH f Li + 1/2H2 (370-483°C)
(3)
Although these equations describe the overall stoichiometry of the reactions, they do not imply the existence of independent species such as LiAlH2. Equation 1 was reported to be exothermic, whereas eqs 2 and 3 were reported to be endothermic. Subsequent work by Mikheeva and Arkipov9 confirmed the results of Block and Gray8 but suggested that the first endothermic effect at 170-173 °C is due to polymorphism and that the release of hydrogen associated with the second exothermic effect at 190 °C is accompanied by the formation of Li3AlH6 and Al. Therefore, eqs 1 and 2 can be recast as eqs 4 and 5
LiAlH4 f 1/3Li3AlH6 + 2/3Al + H2
(4)
Li3AlH6 f 3 LiH + Al + 1/3H2
(5)
The final stage of decomposition was reported to involve a reaction between LiH and Al to form LiAl,11 eq 6
LiH + Al f LiAl + 1/2H2
(6)
Whereas later studies10 gave results that are in accord with those of Mikheeva and Arkipov,9 Dymova et al.11 reported that the first endothermic effect is not due to a polymorphic transformation of LiAlH4 but corresponds to melting of the material. They observed a decrease in hydrogen content and a darkening of
10.1021/jp102641p 2010 American Chemical Society Published on Web 05/19/2010
H2 Desorption Properties of LiAlH4 LiAlH4 upon prolonged storage. The enthalpies of eqs 4 and 5 at room temperature were calculated to be -18.5 and 14.46 kJ mol-1; respectively. More recently, solid-state DFT calculations have predicted endothermic reaction enthalpies of 9.79 and 15.72 kJ mol-1; respectively at room temperature.13 The hydrogen content of LiAlH4 has been shown to be dependent on the carbon content and duration of mechanical activation of the sample.14 With increasing milling time, a decrease in the intensity of the exothermic peak associated with the transformation of LiAlH4 to Li3AlH6 and an increase in the intensity of the endothermic peak corresponding to the decomposition of Li3AlH6 were reported.3 Further work by Balema et al.15 demonstrated that when LiAlH4 is doped with Al3Ti and milled, only two thermal events are observed in the DTA trace between room temperature and 300 °C. An exothermic event at 122-150 °C was assigned to the decomposition of LiAlH4 to Li3AlH6, and an endothermic event at 178-242 °C was attributed to the decomposition of Li3AlH6. No melting of LiAlH4 was observed, and the first decomposition reaction occurred at ∼50 °C lower than for undoped LiAlH4. Chen et al.12 studied a 2:1 mixture of LiH/LiAlH4 and reported two endotherms and one exotherm in the DTA plot. Meanwhile, two endothermic events were observed in the DSC curve. In both cases, the endotherms were associated with the decomposition of LiAlH4 into Li3AlH6, Al, and H2, and subsequent decomposition of Li3AlH6. This report did not attribute the first endothermic peak to the melting transition, as assigned by other studies,11 but rather to decomposition of LiAlH4, although the subsequent exothermic event was reported to be associated with a sudden loss of hydrogen. By doping LiAlH4 with TiCl3 · 1 /3AlCl3, Chen et al.16 showed that the decomposition of LiAlH4 commences 60 °C lower than that of undoped LiAlH4. Andreasen17 confirmed that such doping leads to a lowering of the decomposition temperature of LiAlH4 below its melting point, such that two endotherms corresponding to the decomposition reactions, eqs 4 and 5, were observed. However, some unreacted LiAlH4 remained that resulted in a melting transition in the DSC trace. We previously reported that Ti-doped LiAlH4 can be prepared intact and in quantitative yield by the direct hydrogenation of LiH/Al in Me2O,7 corresponding to the reversal of eqs 4 and 5. Here we describe a detailed study of doped LiAlH4 produced in this manner, and we report remarkable modification of the thermal decomposition of LiAlH4 following Ti doping in this manner.
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Figure 1. DSC plots for undoped and doped LiAlH4: (a) as-received (undoped) LiAlH4; (b) Ti-doped LiAlH4 [(1) 0.05 mol % TiCl3; (2) -0.2 mol %; (3) 0.5 mol %].
left to stir at room temperature for 24 h, after which the pressure was vented and the product (Ti-doped LiAlH4) was collected and analyzed. The powder X-ray diffraction pattern of the fine, dry product attested to the formation of LiAlH4 in quantitative yield, with no evidence of residual LiH or Al, nor of any LiAlH4.nMe2O adduct. Investigation of the thermal decomposition of both undoped and Ti-doped LiAlH4 was carried out using differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). For the DSC experiments, 7 mg of material was placed in an Al pan, which was crimped with an Al cover in a glovebox. The pan was mounted on the DSC instrument (TA Instruments Q50P DSC), and the sample was heated to 300 at 2 °C min-1 under a nitrogen flow of 95 mL min-1. TGA was performed on a TA Instruments Q50 TGA. About 8 mg of sample was placed in an Al pan, which was crimped with an Al cover in a glovebox. A pinhole pierced in the cover enabled the escape of H2 evolved during the measurements while minimizing exposure of the sample to the atmosphere as it was transferred to the TGA instrument. The sample was heated to 300 °C at 2 °C min-1 in a flowing stream of nitrogen (120 mL min-1). 3. Results and Discussion
2. Experimental Section The starting materials LiH (Aldrich, 95%), Al (Alfa Aesar, 99.97%), TiCl3 (Aldrich, 99.999%), and LiAlH4 (Aldrich, 95%) were used without further purification. All material and sample handling was carried out in a nitrogen-filled glovebox. A 1:1 molar ratio of LiH/Al powder was doped with 0.05, 0.2, and 0.5 mol % TiCl3. Typically, a 5 g sample consisting of LiH, Al, and TiCl3 was loaded in a 250 mL stainless steel vessel containing five stainless steel balls of diameter 20 mm. The ball-to-powder mass ratio was ∼32:1. The sample was ballmilled at 300 rpm for 12 h using a Retsch PM 100 planetary ball mill. The milling direction was automatically reversed every 15 min following a 30 s pause. The doped sample was hydrogenated using a previously established procedure.7 About 650 mg doped material was loaded in a 450 mL stainless steel reaction vessel in a glovebox. Next, 55 g Me2O (Air Liquide, chemically pure) was transferred to the vessel, and H2 gas (100 bar) was added. The vessel was sealed, and the reaction was
Hydrogenation in Me2O solution of commercial LiH/Al powders doped with TiCl3 produces Ti-doped LiAlH4, as does the regeneration of the latter from deliberately decomposed Tidoped LiAlH4.7 The thermal decomposition behaviors of asreceived (commercial) and Ti-doped, milled, and Ti-doped LiAlH4 samples investigated by DSC are presented in Figure 1. The plot for as-received LiAlH4 shows four characteristic peaks: a weak exotherm at 136 °C, an endotherm around 169 °C, a strong exothermic peak at 175 °C, and a broad endothermic peak around 212 °C. Observation of these four events is in agreement with previous studies.8,17 These features are assigned to the presence of impurities, melting of LiAlH4, decomposition of molten LiAlH4 to Li3AlH6, and decomposition of Li3AlH6 to LiH and Al, respectively. On the basis of the DSC measurements, the melting enthalpy of as-received LiAlH4 is calculated to be 7.41 kJ mol-1. The enthalpies of the subsequent exothermic event and the second dehydrogenation reaction are -6.53 and 6.90 kJ mol-1; respectively.
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In contrast with the DSC plot for as-received LiAlH4 (Figure 1a), those for Ti-doped LiAlH4 samples (Figure, 1b) are strikingly different, displaying two endothermic features. The first of these occurs at 112, 102, or 98 °C for LiAlH4 containing 0.05, 0.2, and 0.5 mol % TiCl3; respectively. This endothermic event is attributed to the direct decomposition of solid LiAlH4 to Li3AlH6 (eq 4). The second endothermic event appears at 167, 162, or 156 °C for LiAlH4 containing 0.05, 0.2, and 0.5 mol % TiCl3, respectively. This feature is ascribed to the decomposition of Li3AlH6 to LiH and Al (eq 5). These results demonstrate that the decomposition temperature is lowered as the Ti content increases, and the first dehydrogenation step (eq 4) occurs before the melting point of LiAlH4 is reached. This reaction is clearly endothermic and occurs approximately 60-75 °C lower than the corresponding and apparently exothermic feature for undoped LiAlH4. Furthermore, from these DSC data, the enthalpy of the first dehydrogenation of LiAlH4 doped with 0.05% TiCl3 (eq 4) is calculated to be 0.86 kJ mol-1. This value is in excellent agreement with the enthalpy difference (0.88 kJ mol-1) between the melting transition and the exothermic event immediately following it that we observed for as-received LiAlH4. Hence, we conclude that the composite endothermicexothermic feature at 169-175 °C in Figure 1a corresponds to the absorption of latent heat of melting by LiAlH4, followed by immediate decomposition of the molten salt and evolution of H2, and subsequent release of the latent heat of fusion of Li3AlH6 and Al once the H2 has been released. Therefore, the first dehydrogenation step in the thermal decomposition of LiAlH4 (eq 4) is intrinsically endothermic. Doping LiAlH4 with Ti activates the material toward dehydrogenation, advancing the onset of eq 4 and separating H2 evolution from the melting of the salt. The endothermic nature of eq 4 may be taken to imply that this can be reversed in a direct solid-gas reaction. However, the very low positive enthalpy for this reaction makes such an outcome unlikely at workable temperatures and hydrogen pressures. The enthalpy of the second dehydrogenation reaction (eq 5) of Ti-doped LiAlH4 is measured to be 6.94 kJ mol-1, in good agreement with the value for the as-received undoped material (6.90 kJ mol-1). This reaction also occurs 45-55 °C lower in the Ti-doped material. The endothermic nature of both dehydrogenation reactions is consistent with the values reported from solid-state DFT calculations,13 although the calculated values are higher than those obtained experimentally in this study. Other studies have reported doping of LiAlH4 with various catalysts and have studied its effects on the thermal properties of the material. In contrast with the results, we present here for TiCl3-doped LiAlH4, it has been reported that when LiAlH4 is doped with 3 mol % Al3Ti through ball milling the first dehydrogenation reaction is exothermic, whereas the second is endothermic.15 However, it has also been reported that when LiAlH4 is doped with 2 mol % TiCl3 · 1/3AlCl3 via ball milling, the first and second dehydrogenation reactions are both endothermic,16,17 in agreement with our results. The current study differs from these previous studies in several ways. First, the previous studies doped commercial LiAlH4 through milling.16,17 In our work, freshly synthesized Ti-doped LiAlH4 was generated by hydrogenation of a premilled LiH/Al/TiCl3 substrate. Second, we employed much lower dopant concentrations than those employed in the previous studies.16,17 When 2 mol % TiCl3 · 1/3AlCl3 was used, the dehydrogenation temperature of LiAlH4 was lowered,17 but not to the same extent as for LiAlH4 doped with as little as 0.05 mol % TiCl3 reported here. In addition, the DSC plots presented in this work show no melting transition, whereas
Langmi et al.
Figure 2. DSC and TGA plots for undoped and doped LiAlH4: (a) TGA plot for as-received (undoped) LiAlH4; (b) DSC plot for as-received LiAlH4; (c) TGA plot for LiAlH4 doped with 0.05 mol % TiCl3 and (d) DSC plot for LiAlH4 doped with 0.05 mol % TiCl3.
that for LiAlH4 doped with 2 mol % TiCl3 · 1/3AlCl3 was reported to exhibit a melting peak corresponding to unreacted LiAlH4. The Ti-doped LiAlH4 samples prepared and characterized in this study display much lower decomposition temperatures and no melting transition as a result of the homogeneous dispersion and more intimate interaction of the dopant with LiAlH4. The mass loss corresponding to H2 desorption for each of the samples reported here was investigated by TGA analysis. The results for undoped and doped (0.05 mol % TiCl3) LiAlH4 samples are plotted alongside corresponding DSC data in Figure 2. In each case, the two-step dehydrogenation of LiAlH4 is clearly seen. For as-received LiAlH4, the onset temperature for dehydrogenation is ca. 160 °C, and approximately 4.6 and 1.9 wt % hydrogen is released in the first and second dehydrogenation reactions (eqs 4 and 5). These events are in good accord with the features shown in the overlain DSC plot. For LiAlH4 doped with 0.05 mol % TiCl3, the first dehydrogenation step (eq 4) releases ∼5.0 wt % hydrogen commencing around 75 °C, with the second reaction (eq 5) giving a further 2.0 wt %. Once again, these features accord well with the observation of two distinct endothermic peaks in the DSC plot. 4. Conclusions Ti-doped LiAlH4 was prepared by direct hydrogenation in Me2O solution of commercial LiH, Al, and TiCl3. Both as-received (undoped) and Ti-doped LiAlH4 displayed a twostage thermal decomposition between 50 and 300 °C, according to eqs 4 and 5. For undoped LiAlH4, the first stage is obscured by two larger antagonistic and overlapping features corresponding to initial absorption, then release, of latent heat of fusion as LiAlH4 melts and decomposes to Li3AlH6 and Al, and these latter two subsequently resolidify: all three events occur within a ∼5 °C temperature range. Doping with TiCl3 lowers the onset temperature of eq 4 by 60-75 °C, bringing it well below the melting point of LiAlH4 and uncoupling it from the meltingdecomposition-solidification nexus that appears around 170 °C
H2 Desorption Properties of LiAlH4 for the undoped material. As a result, eq 4 is clearly revealed as endothermic, in contrast with several previous reports. By preparing a high purity sample of LiAlH4 with low levels of homogeneously dispersed Ti catalyst, we have been able to characterize unambiguously the thermochemistry corresponding to the release of H2 by this attractive, high capacity hydrogen storage material. For comparison, the widely studied congener Ti-doped NaAlH4 is reported to have endothermic enthalpies of 37 and 47 kJ mol-1, respectively, for its first and second dehydrogenation reactions (i.e., the reactions corresponding to eqs 4 and 5) and begins to release hydrogen at ∼80 °C18 with a total hydrogen capacity around two-thirds that of the Ti-doped LiAlH4 discussed here. References and Notes (1) Ritter, J. A.; Ebner, A. D.; Wang, J.; Zidan, R. Mater. Today 2003, 9, 18. (2) Schu¨th, F.; Bogdanovic´, B.; Felderhoff, M. Chem. Commun. 2003, 20, 2249. (3) Balema, V.; Pecharsky, V.; Dennis, K. J. Alloys Compd. 2000, 313, 69. (4) Sandrock, G.; Reilly, J.; Graetz, J.; Zhou, W.; Johnson, J.; Wegrzyn, J. J. Alloys Compd. 2006, 421, 185.
J. Phys. Chem. C, Vol. 114, No. 23, 2010 10669 (5) Kato, T.; Nakamori, Y.; Orimo, S.; Brown, C.; Jensen, C. M. J. Alloys Compd. 2007, 446, 276. (6) Orimo, S.; Nakamori, Y.; Eliseo, J. R.; Zu¨ttel, A.; Jensen, C. M. Chem. ReV. 2007, 107, 4111. (7) Liu, X.; McGrady, G. S.; Langmi, H. W.; Jensen, C. M. J. Am. Chem. Soc. 2009, 131, 5032. (8) Block, J.; Gray, A. P. Inorg. Chem. 1965, 4, 304. (9) Mikheeva, V. I.; Arkipov, S. M. Russ. J. Inorg. Chem. 1967, 12, 1066. (10) Dilts, J. A.; Ashby, E. C. Inorg. Chem. 1972, 11, 1230. (11) Dymova, T. N.; Aleksandrov, D. P.; Konoplev, V. N.; Salina, T. A.; Sizaereva, A. S. Russ. J. Coord. Chem. 1994, 20, 263. (12) Chen, J.; Kuriyama, N.; Takeshita, H. T.; Sakai, T. AdV. Eng. Mater. 2001, 3, 695. (13) Løvvik, O. M.; Opalka, S. M.; Brinks, H. W.; Hauback, B. C. Phys. ReV. B 2004, 69, 134117-1. (14) Mal’tseva, N. N.; Golovanova, A. I. Russ. J. Appl. Chem. 2000, 73, 747. (15) Balema, V. P.; Wiench, J. W.; Dennis, K. W.; Pruski, M.; Pecharsky, V. K. J. Alloys Compd. 2001, 329, 108. (16) Chen, J.; Kuriyama, N.; Xu, Q.; Takeshita, H. T.; Sakai, T. J. Phys. Chem. B 2001, 105, 11214. (17) Andreasen, A. J. Alloys Compd. 2006, 419, 40. (18) Bogdanovic´, B.; Brand, R. A.; Marjanovic, A.; Schwickardi, M.; Tolle, J. J. Alloys Compd. 2000, 302, 36.
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