Silica-Gel Mixtures and the Effect of

LiBH4 that has been ball-milled with different additives and subsequently mixed with mesoporous ... have little influence on the amount of released di...
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J. Phys. Chem. C 2007, 111, 14026-14029

Diborane Release from LiBH4/Silica-Gel Mixtures and the Effect of Additives J. Kostka, W. Lohstroh,* M. Fichtner, and H. Hahn Institute of Nanotechnology, Forschungszentrum Karlsruhe GmbH, Postfach 3640, 76021 Karlsruhe, Germany ReceiVed: May 16, 2007; In Final Form: July 3, 2007

LiBH4 that has been ball-milled with different additives and subsequently mixed with mesoporous silica-gel has been investigated using thermal desorption spectroscopy and thermogravimetry. Mixtures of LiBH4 and mesoporous silica-gel (composition 50:50 wt %) release hydrogen at ∼260 °C. Samples, where LiBH4 was ball-milled prior to mixing with silica-gel with 5 mol % PdCl2, NiCl2, LaCl3 or TiCl3, have similar decomposition temperatures. Mass spectrometry data of the emitted gas streams show that almost all hydrogen desorption events are accompanied by the evolution of gaseous diborane, B2H6. The relative diborane concentration above 350 °C is considerably lowered by LaCl3 and TiCl3, whereas the other additives investigated have little influence on the amount of released diborane.

I. Introduction In the search for high-capacity hydrogen storage materials with potential to fulfill the requirements for mobile applications, the interest has recently turned toward borohydrides,1-4 such as LiBH4. This material has a high gravimetric and volumetric hydrogen storage density of 18.5 wt % and 121 kg/m3. This warrants intensive research on its storage properties; however, up to now, the conditions for reversible hydrogen uptake and release have been harsh. The majority of the bound hydrogen is only released from the melt (Tm ) 275 °C, ref 5) at temperatures between 480 - 490 °C. Rehydrogenation can be achieved only at a pressure of 15 MPa H2 and a temperature of 650 °C.3,6 The overall reaction was proposed to be (eq 1),

LiBH4 f LiH + B + 3/2 H2

(1)

including several intermediate steps (i.e., a state with a composition ‘LiBH2’ as intermediate species).1 Zu¨ttel et al.1 reported a lowering of the decomposition temperature for LiBH4 mixed with 75 wt % SiO2. Likewise, additives based on oxides or chlorides, such as TiCl3, TiO2, SnO2, V2O3, or ZrO2, promote hydrogen desorption at lower temperatures. Moreover, the rehydrogenation conditions of these mixtures are milder as compared to pure LiBH4; hydrogen uptake occurs at 600 °C and 7 MPa H2 pressure.7 The appearance of TiB2 during hydrogen cycling fed speculations of a beneficial effect of TiB2 on the rehydrogenation properties. Vajo et al.8 used mixtures of LiBH4 + 1/2MgH2 to stabilize the decomposition products in the form of LiH + MgB2 + 3/2H2. The reaction enthalpy of this alternative reaction path is reduced by 25 kJ/mol H2 as compared to the decomposition of eq 1. In addition to the problems associated with high temperatures and pressures required for the reversible reactions, a major limitation for applications could be the production of BxHy species during decomposition.7 Diborane (B2H6, which is gaseous at standard conditions), is not only poisonous, but boron is lost also from the system with detrimental effects for the capacity. * To whom correspondence [email protected].

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In this paper, we report on the thermal decomposition of LiBH4 that has been ball-milled with different additives and mixed with mesoporous silica-gel. The hydrogen release temperatures and the composition of the emitted gases were investigated using thermal desorption spectroscopy under highvacuum conditions. The capacity was monitored with thermogravimetry under He-flow conditions. II. Experimental Mixtures of as received LiBH4 (Aldrich, 95%) and 5 mol % of the respective additives (i.e., TiCl3, LaCl3, PdCl2, and NiCl2) were ball-milled for 2 h in a planetary mill (600 rpm, Si4N3 vial and balls, ball-to-powder ratio of 20:1). All handling of the powders took place in a glove box in a purified argon atmosphere. X-ray diffraction (XRD) data were obtained using a Philips X’pert system (λ ) 0.15418 nm) in Bragg-Brentano geometry. A Netzsch STA 409C thermogravimetric (TG) apparatus coupled to a Balzers mass spectrometer QS 422 (MS) was used for the thermal investigations and the analysis of the emitted gas stream. Both the sample chamber and the mass spectrometer were operated under a high vacuum (3‚10-6 mbar), and the distance between sample and entrance to the mass spectometer amounted to only a few cm. The temperature ranged from 25 to 550 °C with a constant heating rate (β) of 3 K/min. For the TG-MS experiments, the ball-milled nanocomposites were mixed in a mortar with nanoporous silica-gel (Merck, 3 nm median pore size, ratio 50:50 wt %) because experiments on pure LiBH4 (i.e., without mesoporous silica-gel) yielded irregular bursts of H2 in the MS data due to gas bubbles leaving the molten material. In contrast, desorption from LiBH4 mixed with mesoporous silica-gel shows a regular behavior. Special measures were taken to minimize oxygen and water contamination during transfer of the mixed powders into the TG-MS apparatus. To access the maximum amount of the released gases, open Al2O3 crucibles were used for the measurements. To determine the amount of BxHy species desorbed from the sample, the mass spectroscopy signals were calibrated beforehand using a gas mixture of H2/B2H6 with a composition of 99:1 vol %. The hydrogen storage capacity of the samples was verified in thermogravimetric measurements under He-flow conditions because the results obtained in a high vacuum showed large

10.1021/jp073783k CCC: $37.00 © 2007 American Chemical Society Published on Web 08/29/2007

Diborane Release from LiBH4/Silica-Gel Mixtures

Figure 1. Thermogravimetric data of LiBH4 ball-milled with various additives and subsequently mixed by hand with silica-gel. The heating rate was 5 K/min and the mass is normalized to the amount of LiBH4. For comparison, panel a shows the TG data of pure LiBH4 (ball-milled for 2 h) as well. The line indicates the melting temperature of LiBH4.

variations, presumably due to solid or liquid material escaping the sample holder. III. Results After milling LiBH4 with the chloride additives TiCl3, NiCl2, or PdCl2, the XRD pattern shows reflections stemming from LiBH4 and LiCl. During mechanical milling the metal ions of the saline additives are thus reduced, most likely to the zerovalent state, although the corresponding metal peaks could not be identified in the patterns. Presumably, the metal clusters are too small to be detected. LaCl3 is an exception from this general behavior, that is, the starting materials LiBH4 and LaCl3 and a new unidentified peak were observed in the XRD data. Figure 1 shows the thermogravimetric data under He-flow conditions (β ) 5 K/min) of the various samples mixed with silica-gel. For comparison, the TG data of pure LiBH4 (ballmilled for 2 h) are also shown. Pure LiBH4 starts to decompose around its melting temperature of 275 °C; however, the majority of hydrogen is released only above 400 °C. Mixing LiBH4 with silica-gel dramatically reduces the desorption temperature as reported by Zu¨ttel et al.1 However, in comparison to the data published in refs 1 and 2, the decomposition in the present samples starts at even lower temperatures. With He-flow conditions and a heating rate of 5 K/min, a mass loss of 5 wt % was observed at 300 °C, whereas, for similar heating rates, Zu¨ttel et al. found a comparable mass loss at 400 °C. One possible explanation for the lower decomposition temperatures observed in this study might be the use of silica-gel powder with a high specific surface area. In comparison to mixing with nanoporous silica-gel, the effect of the other additives introduced during ball milling is small, amounting to ∼10 °C at the maximum. For all samples (except for the LaCl3 additive), the total weight loss at 550 °C is ≈1213 wt % (normalized to the amount of LiBH4). This figure agrees well with a decomposition into LiH + B + 3/2H2 as described by eq 1. Samples with LaCl3 additives show an additional decomposition step at 185 °C, where ∼2 wt % are lost, and the total mass loss at 550 °C amounts to 16 wt %. The composition of the emitted gas stream was investigated by means of thermal desorption spectroscopy in a high vacuum. The results for hydrogen (m/z ) 2) are shown in Figure 2. The data obtained for pure LiBH4 (ball-milled for 2 h) are also included. Around the melting temperature, a strong H2 desorption peak is observed, followed by a second one around 450 °C, in agreement with the TG measurements. Note, that for pure LiBH4 the MS signal strength is only a qualitative measure

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Figure 2. Hydrogen desorption spectra of LiBH4 ball-milled with 5 mol % of saline additive and subsequently mixed with silica-gel. Solid lines show the MS signal of m/z ) 2. Measurements were taken at a pressure of 3 × 10-6 mbar and a heating rate of 3 K/min. The vertical line indicates the reported melting temperature of LiBH4. The arrows point to maxima of the relative diborane concentration in the emitted gas stream (see Figure 4 and corresponding text). For comparison, the data obtained for pure LiBH4 (ball-milled for 2 h) are included, although the obtained data are of qualitative nature (see text).

because, at the melting temperature, material is thrown out of the crucible (with a corresponding weight loss larger than 40 wt. %); hence, the signal for the first and second H2 desorption events originate from different amounts of sample. For LiBH4 mixed with silica-gel this problem is far less severe, and Figure 2, panels b-f, shows hydrogen desorption data for these samples. The beneficial effect of silica-gel could be due to the very efficient wetting of molten LiBH4 (or its reaction products) with oxide surfaces such as SiO2 or Al2O3, which was observed in the experiments. The lower surface tension could allow easier hydrogen release from the melt, and, at the same time, it moderates the desorption in a way that less molten material is thrown up with gas bubbles, enabling measurements in a high vacuum. However, it should be noted that in the mixtures a considerable amount of hydrogen is released below the melting temperature. Without further additive (Figure 2b), the experiment shows two distinct desorption events in which the majority of the hydrogen is set free. The first desorption starts around 170 °C, reaching a maximum H2 release around T ) 260 °C and continuing until ∼340 °C. From the MS data it is obvious that decomposition occurs via several intermediate steps. Above 350 °C, a second, weaker hydrogen desorption event is observed, ending around 500 °C. Moreover, a small amount of H2 is released around the polymorphic phase transition at 110 °C. Compared to He-flow conditions, the desorption temperatures obtained in a high vacuum are shifted to lower temperatures due to different background pressures (in addition to the smaller heating rate employed). Nevertheless, the features remain similar. For all additives investigated, the majority of the H2 release occurs between 170 and 340 °C (Figure 2, panels c-f). TiCl3, NiCl2, and PdCl2 have a small beneficial effect on the desorption temperature, with the main H2 release shifted ∼10 °C lower. Samples with LaCl3 additions show a distinct additional desorption at 170 °C, as already seen in the TG data under Heflow conditions. During the thermal analysis, the characteristic intensity pattern of diborane9 was observed. Almost all hydrogen release is linked to diborane emission, as exemplarily shown in Figure 3 for LiBH4 (without additive) mixed with silica-gel; each intermediate H2 desorption peak is reflected in the characteristic B2H6 mass fragments, whereas other masses, measured simulta-

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Kostka et al.

Figure 5. Decomposition reaction scheme of possible parallel and consecutive reactions that lead to the observed products in the gas phase.

The extra feature observed in samples with LaCl3 additions at T ) 170 °C is also reflected in the B2H6 emission pattern; the low-temperature desorption is accompanied by a RC ratio of 13%. Nevertheless, the shoulder of the peak, stretching to temperatures above 200 °C, indicates that the onset of the main H2 desorption shows a similar relative diborane concentration to that observed in the other samples. Figure 3. Thermal desorption behavior of LiBH4 (ball-milled for 2 h) mixed with silica-gel (50:50 wt %). (a) MS signal for m/z ) 2 and (b) the characteristic mass fragments of B2H6. The legend gives the mass signals sorted according to their measured intensity (with the strongest signal m/z ) 26).

Figure 4. Relative diborane concentration (gray lines) of various LiBH4/additive/silica-gel mixtures. Black lines show the total amount of released gases (i.e., CB2H6 + CH2). These values are dominated by the hydrogen signals. For reference, the desorption data obtained for pure LiBH4 (ball-milled for 2 h) are included, the same limitations as in Figure 2 apply.

neously, keep a low profile. In Figure 4, the relative B2H6 concentration RC ) CB2H6/(CB2H6 + CH2) (gray line) is shown as a function of temperature, the black lines depict the total amount of released diborane and hydrogen gas. The calibration factors correlating the mass spectrometer signals with the relative concentration of H2 and B2H6 were obtained from a measurement of a standard gas mixture. The majority of hydrogen is released around T ) 260 °C. The relative diborane concentration is below 2% in this temperature range. However, increased values of the ratio RC are observed at temperatures below and above the main hydrogen desorption. For LiBH4, and LiBH4 with additions of TiCl3, NiCl2, and PdCl2, the relative diborane concentration amounts to 4-7% at temperatures around 170 °C. This value coincides with the onset temperature of hydrogen desorption, as seen in Figure 2. For samples with NiCl2 and PdCl2, as well as pure LiBH4, a second RC maxima appears at high temperatures above 350 °C. Remarkably, TiCl3 and LaCl3 additives seem to suppress diborane emission in this temperature range.

IV. Discussion The results presented above underline a strong interlinkage of diborane release and hydrogen desorption from LiBH4, independent of the used additives. The characteristic fragmentation pattern of B2H6 is observed throughout the entire course of hydrogen desorption (see Figure 3). Remarkably, in all cases the very first stages of the main hydrogen desorption are accompanied by enhanced diborane emissions. From the integrated signals of CH2 and CB2H6, shown in Figure 3, the total amount of liberated diborane can be roughly estimated. For LiBH4 mixed with silica-gel, the gas desorbed under highvacuum conditions contains approximately 3 mol % diborane. However, for more realistic operating conditions (e.g., He-flow, or hydrogen back-pressure) the amount of released diborane is lower (see discussion below). Under He-flow conditions, the observed mass loss is consistent with decomposition of LiBH4 into LiH + B + 3/2H2. The only exceptions are samples where LiBH4 was ball-milled with LaCl3. Here, a two-step behavior is observed: (1) 2 wt.% are lost around 170 °C, and (2) another 14 wt % is lost until 550 °C. One explanation could be the formation of a La(BH4)x species. Similar reactions of nLiBH4 with MCln to synthesize M(BH4)2 + nLiCl (M ) Mg, Zn, Sc, Zr, or Hf) have been reported previously,4 although not yet for LaCl3. Hence, we propose a La-boranate phase formation during ball-milling that decomposes into hydrogen and diborane around 170 °C. Apart from this feature using LaCl3, the TG data indicate that diborane emissions are considerably suppressed at normal pressure conditions, otherwise a higher mass loss would be observed (assuming the gas stream comprises 3 mol % diborane as observed in a high-vacuum, the total weight loss would amount to ∼16 wt.%). Nevertheless, we still detect traces of BxHy species in the MS signals, albeit at a much lower intensity.10 Similarly, Au et al.7 reported diborane release from mixtures of LiBH4 + TiO2. The question of why gaseous diborane is emitted at a certain stage of the decomposition is important because B2H6 is toxic to both the natural environment and the catalyst of the fuel cell. Hence, one of the aims in the development of viable hydrogen storage systems based on boranates is the suppression of diborane emission. As yet, little is known about the details of the thermal decomposition process of boranates, and it may be helpful to start with a more general description of the processes when the material is decomposing. Figure 5 depicts a simplified scheme with potential reaction pathways for thermal decomposition of a boranate. The figure shows a series of possible

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consecutive and parallel reactions that lead to the formation of either pure hydrogen plus boron or to the formation of diborane. Each of the reactions is characterized by the respective educt and product(s) and the reaction constant ki. It is obvious that the nature of the different ki will determine which way the system goes depending on the reaction temperature and pressure. An assumption that was made for this scheme is that the formation of diborane occurs predominantly by dimerization of previously produced BH3, probably in the gas phase. Moreover, it cannot be excluded that one of the reaction steps consists of several other steps with intermediate products. One of the first decomposition steps that is likely and that was also proposed earlier in literature11 is the unimolecular decomposition of an (earth alkaline) boranate, according to eq 2.

M(BH4)2 f BH3 + HMBH4 f 2BH3 + MH2 f H2 + 2BH3 + M (2) In the case of LiBH4, this corresponds to the abstraction of a hydride anion from the [BH4]- complex, to the formation of the binary metal hydride LiH, and to BH3 as indicated by the reaction constant k2. It is possible that there is also another concurrent reaction (k1) where the product is unknown at the moment (“X”). However, in this case the subsequent formation of BH3 as decomposition product of X should be possible. This means that it is likely that there is a reaction network where the first step is either abstraction of a hydride anion and formation of BH3 or the formation of a different, unknown intermediate state that can decompose into the elements. When BH3 is formed it may dimerize and form diborane, or it decomposes into the elements, according to the following reactions (thermodynamic data taken from the NIST data bank12 and from Holleman13).

2BH3 f B2H6 ∆H°r ) -82 kJ/mol BH3

(3)

BH3 f B + 3/2 H2 ∆H°r ) -106.7 kJ/mol BH3

(4)

As we find diborane emitted by the sample, it is likely that k2 and/or k3 are sufficiently large to play a role in the reaction network. A strategy to minimize or prevent the co-emission of diborane should therefore focus on reactions where borane, diborane, and the elements are involved. If BH3 is indeed an intermediate that is either bound to the tetrahydroborate complex,14 as [H3B•BH4]-, or present as a volatile species, then

the strategy should include maximization of k4, k6 and k7 (e.g. by adding an appropriate catalyst). V. Conclusions The thermal desorption of LiBH4, ball-milled with various saline additives and subsequently mixed by hand with mesoporous silica-gel was investigated using TG and MS. Mixing pure LiBH4 with silica-gel shifts the main hydrogen release temperature close to the melting point. In comparison, the additives PdCl2, NiCl2, TiCl3, and LaCl3 yield only small additional variations of the hydrogen release temperature. The composition of the gas stream shows a strong correlation of hydrogen release and diborane liberation. For applications, even trace amounts of the later species has to be avoided, independent of whether diborane is an intermediate of the hydrogen release reaction or is produced in a concurrent reaction. The efficiency of the various tested additives to minimize diborane emissions depends on the nature of the additive; LaCl3 and TiCl3 additions show a significant reduction of B2H6 species at temperatures above 350 °C, whereas other additives have only minor influence. References and Notes (1) Zu¨ttel, A.; Wenger, P.; Sudan, P.; Mauron, P.; Emmenegger, C. J. Power Sources 2003, 118, 1. (2) Zu¨ttel, A.; Rentsch, S.; Fischer, P.; Wenger, P.; Sudan, P.; Mauron, P.; Emmenegger, C. J. Alloys Comp. 2003, 356, 515. (3) Orimo, S.; Nakamori, Y.; Kitahara, G.; Ohba, N.; Towata, S.; Zu¨ttel, A. J. Alloys Comp. 2005, 404, 427. (4) Nakamori, Y.; Miwa, K.; Ninomiya, A.; Ohba, N.; Towata, S.; Zu¨ttel, A.; Orimo, S. Phys. ReV. B 2006, 74, 045126. (5) Fedneva, E. M.; v. Alpatova, L.; Mikheeva, V. I. Russ. J. Inorg. Chem. 1964, 9, 826. (6) Muller, A.; Methey, F.; Bensoam, J. Production of Hydrogen US Patent 4,193,978, 1980. (7) Au, M.; Jurgensen, A. J. Phys. Chem. B 2006, 110, 7062. (8) Vajo, J. J.; Skeith, S. L.; Mertens, F. J. Phys. Chem. B 2005, 109, 3719. (9) Stein, S. E. In NIST Chemistry WebBook, NIST Standard Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology, Gaithersburg MD 20899, U.S.A. 2005; Chapter Mass Spectra. (10) At He-flow conditions, the mass spectrometer was coupled to the sample chamber via a skimmer with an opening of ∼2 mircometer. (11) Bonaccorsi, R.; Charkin, O. P.; Tomasi, J. Inorg. Chem. 1991, 30, 2964. (12) NIST Chemistry WebBook, NIST Standard Reference Database No. 69; Linstrom, P. J., Mallard, W. G., Eds.; National Institute of Standards and Technology: Gaithersburg, Maryland, 2005. (13) Holleman, A. F.; Wiberg, E. Lehrbuch der Anorganischen Chemie, 91st ed.; Walter de Gryter: Berlin 1985; in German. (14) James, B. D.; Wallbridge, M. G. H. Progr. Inorg. Chem. 1970, 11, 99.