Impurity Gas Analysis of the Decomposition of ... - ACS Publications

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Impurity Gas Analysis of the Decomposition of Complex Hydrides A. Borgschulte,*,† E. Callini,† B. Probst,† A. Jain,†,‡ S. Kato,† O. Friedrichs,† A. Remhof,† M. Bielmann,† A. J. Ramirez-Cuesta,§ and A. Z€uttel† †

Empa, Swiss Federal Laboratories for Materials Testing and Research, Hydrogen & Energy, CH-8600 D€ubendorf, Switzerland Centre for Non-Conventional Energy Resources, University of Rajasthan, Jaipur 302004, India § ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, United Kingdom ‡

ABSTRACT: This study aims at an investigation of the impurity gases emitted during the decomposition of borohydrides. For this we have set up a quantitative gas analysis based on a combination of FTIR spectroscopy and gravimetry. We show that the emission of various intermediates, in particular diborane, depends sensitively on the reaction conditions, including gas mean free path lengths, hydrogen backpressure, and sample pretreatment. Adductfree Mg(BH4)2 and LiBH4 emit diborane only at the impurity level, while for LiZn2(BH4)5 diborane is the main decomposition product. The decomposition reaction of LiZn2(BH4)5 proceeds via a collision-induced dissociation of Zn(BH4)2 in Ar at ambient pressures. Various additives were tested aiming at catalyzing the decomposition of the desorbed diborane.

I. INTRODUCTION Light-weight metal hydrides such as the borohydrides are promising hydrogen-storage materials.1 These hydrides are difficult to synthesize in high purity and, more importantly, might release other gases than hydrogen upon decomposition.2 In addition to fundamental interest, the proof of the existence of highly poisonous/corrosive gases such as diborane3/HF4 and their suppression is of high technical importance. Mass spectrometry (MS), as frequently used in the community, has difficulties in the quantification of the various gas species. This problem is caused by the high vacuum needed for the MS measurements, at which different intermediates might be released than those formed at technically realistic pressures. If the decomposition of the sample takes place at finite pressures, differential pumping is required, during which the desorbed species may change or condense.5 Infrared spectroscopy is sensitive to vibrations originating from gaseous molecules and can be utilized to analyze the gaseous decomposition products of complex hydrides also at high-pressure environments. We have combined a gravimetric analysis with infrared spectroscopy, enabling the quantitative measurement of the gaseous decomposition products. The results obtained from these measurements are compared to MS measurement on decomposition reactions taking place in ultrahigh vacuum and at finite pressure measured after differential pumping. There have been several reports on diborane emission during the decomposition of borohydrides.3 The relative amount varies with compound and specific experimental conditions. To shed light on this problem, we apply the method first to LiZn2(BH4)5, known as a diborane source.6 We then discuss the archetypical compound LiBH4 and the recently discussed system Mg(BH4)2. r 2011 American Chemical Society

Empirically it was found that borohydrides desorbing at lower temperatures release a considerable amount of diborane, while the borohydrides, which desorb at higher temperature, release mainly hydrogen.8 LiBH4 desorbs at higher temperatures only and emits diborane at the impurity level in contrast to LiZn2(BH4)5, which decomposes below 100 °C. Mg(BH4)2 was chosen as a borderline case between the two extreme cases.

II. EXPERIMENTAL SECTION LiBH4 and ZnCl2 were purchased from SigmaAldrich Fine Chemicals (95%). LiZn2(BH4)5 was synthesized by ball-milling of LiBH4 and ZnCl2 in a Spex 8000 M mixer mill for 90 min using a stainless steel vial and 10 mm stainless steel balls with a sample to ball mass ratio of 1:8. The corresponding metathesis reaction is7,8 5LiBH4 þ 2ZnCl2 f LiZn2 ðBH4 Þ5 þ 4LiCl

ð1Þ

To study the impact of additives on the decomposition of LiZn2(BH4)5, 2 mol % of Ru (Merck, 99.9%), Fe2O3 (Fluka, >97%), Ni nanosize activated powder (Sigma-Aldrich, 99.9%), MgO nanopowder (Sigma-Aldrich, 99%), Mo powder (Fluka, 99.7%), and Cr2O3 (Sigma-Aldrich, 99.9%), respectively, were ballmilled with LiZn2(BH4)5 + 4LiCl (Spex 8000 M mixer mill for 90 min using stainless vial and balls with a sample to ball mass ratio of 1:8). All values of concentrations (e.g., mass losses, Received: June 14, 2011 Revised: July 14, 2011 Published: July 21, 2011 17220

dx.doi.org/10.1021/jp205566q | J. Phys. Chem. C 2011, 115, 17220–17226

The Journal of Physical Chemistry C

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Figure 1. Sketch of experimental setups for (a) mass spectrometry in UHV and (b) combined gravimetric and IR analysis.

additives) are expressed with respect to the initial composite 5LiBH4 + 2ZnCl2. Mg(BH4)2 (95%) was purchased from Sigma-Aldrich. The as-delivered compound was Mg(BH4)2 3 2S(CH3)2, as confirmed by XRD. Heating in vacuum at 150 °C revealed the R-phase. Hydrogen desorption was studied either by monitoring the emitted species from a heated sample in UHV environment by mass spectrometry (base pressure of the system below 109 mbar, used MS: Pfeiffer 402, see Figure 1a) or by following the weight change using a magnetic suspension balance (Rubotherm, Bochum) in combination with infrared gas analysis (Bruker Alpha spectrometer equipped with a 8 cm gas cell at a resolution of 0.9 cm1; see Figure 1b). In the latter case, the desorption takes place in a 250 mL/min flow of hydrogen at 1 bar, guaranteeing defined thermodynamic conditions. That means that the desorption temperature Tdes is related to the stability ΔH and entropy ΔS assuming thermodynamic equilibrium Tdes ¼

ΔH ΔS

ð2Þ

If the only gaseous decomposition product is hydrogen, the entropy change upon desorption is expected to be of the same order for most hydrides (ΔS = 130 J K1 mol1 for a hydrogen pressure of 1 bar), the decomposition temperature being a measure of the stability. Under UHV condition as used for MS measurements, the decomposition is hindered by kinetic constraints only. In addition to the infrared gas analysis, a mass spectrometer (residual gas analyzer SRS) was attached to the balance system via differential pumping. Heating rates in all thermal decomposition experiments were 1 K/min. The vibrational properties of B2H6 were calculated by densityfunctional theory and the plane-wave pseudopotential method as implemented in the CASTEP code.9,10 Pseudopotentials were of the optimized norm-conserving variety11 with a plane-wave cutoff of 450 eV. Calculations were performed under the PBE approximation to exchange-correlation.12 Brillouin-zone integration was performed according to the MonkhorstPack scheme with a 2 3 2 mesh of k-points, which gave convergence of all modes to a precision of better than 3 cm1. Pseudopotential errors in the frequencies were estimated at no more than 1% from a comparison of alternative pseudopotentials.

III. RESULTS A. Measurement Principle. The infrared transmission T is a function of the concentration of the infrared absorbing gas

Figure 2. (Top) Mass spectrometry measurements of the decomposition of LiZn2(BH4)5 in UHV. (Bottom) Mass spectrometry measurements of the decomposition of LiZn2(BH4)5 measured via differential pumping. The inset is a comparison of a measured spectrum with a reference spectrum of B2H6 (red bars, NIST17).

species c and its infrared molar decadic coefficient k (Lambert Beer law13,14): T ¼ I=I0 ¼ 10kcl ¼ 10A

ð3Þ

where l is the optical path length in the medium. The dedadic absorbance A is measured for most common systems (see, e.g. ref 17). With knowing the total flow of the gas, the rate of (infrared active) desorbed gas species can be quantified. The flow is adapted to the measurement system: the dead volume of the system V0 (balance heating cell plus infrared gas cell) is around 100 mL. To guarantee a response time τ of less than 1 min, the flux has to be higher than f > V0/τ = 100 mL/min. On the other hand, a high flow dilutes the desorbed gas species and thus lowers the detection limit. The amount of gas species is related to the mass of the sample, which is also defined by the precision of the balance. With eq 3, the molar weight of the gas species (M), the total weight of the sample (m0), reference absorbance (Aref), and the corresponding conversion factors (cell lengths lref and lmeas and reference concentration cref), one obtains the rate of mass change measured by IR: m_ Ameas lref M ¼ cref f ð4Þ m0 IR m0 Aref lmeas 17221

dx.doi.org/10.1021/jp205566q |J. Phys. Chem. C 2011, 115, 17220–17226


Figure 3. Infrared transmission spectra of gaseous decomposition products from LiZn2(BH4)5, reference spectrum of diborane (NIST, from ref 17), and an IR spectrum from electronic structure calculations (DFT). The inset is an enlargement showing rovibrational fine structures. For comparison, the corresponding intensities have been converted (eq 7); see text for details.

As shown later, the measurement is semiquantitative even without a calibration factor over nearly 4 orders of magnitude. However, deviations, which are partly corrected by applying a calibration factor, might be due the following sources of errors: • uncertainty of the total sample mass and uncertainty of the weight change measurements; • uncertainty of the flow, particular at high desorption rates; [The flow is determined before the heating cell (see Figure 1), i.e., the total flow is that of the carrier gas (H2 or Ar) plus emitted gases. The latter is usually neglected.] • uncertainty of the reference absorbance; (The measured absorbance depends on parameters such as resolution and might also differ from spectrometer to spectrometer.) • deviation from the LambertBeer law (eq 3) at high concentrations, i.e., high optical densities. The total error may be obtained by comparing the total gravimetrically measured mass loss with the integrated IR rate, if the weight change originates from species measured by IR only. The ratio may be used as a calibration factor. With this, the systematic error of the measurements is significantly reduced. The statistical error from the above listed error sources sums up to around 1020%. B. Decomposition of LiZn2(BH4)5. LiZn2(BH4)5 decomposes under emission of diborane and hydrogen.6,15 The exact reaction path is under discussion due to difficulties in quantification of the desorption products. Figure 2 shows mass spectrometry measurements of the decomposition of LiZn2(BH4)5 in UHV. A multitude of peaks is found, which may be assigned as follows: • m/z = 10, 11, 12, 13 and 20, 21, ... 27 are fragments of diborane (i.e., 11BHn, n = 0, 1, 2; 11B2Hn, n = 0, ..., 717) and corresponding 10B isotopomers. • m/z = 64, 66, (67), 68, (70) are Zn isotopes. • m/z = 30 + n, 40 + n, 70 + n, and 80 + n might be assigned to higher boranes.16 However, decomposition of, for example, decaborane should lead to fragments resulting in m/z = 50 + n, 60 + n as well. With the appearance of Zn, an

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Figure 4. Decomposition of LiZn2(BH4)5 in 1 bar of hydrogen as measured gravimetrically (full squares) and optically (hollow circles). The inset shows the absolute weight change during heating up to 300 °C in vacuum.

assignment to fragments of Zn(BH4)2, i.e., Zn(BHx) and Zn(BHx)2, is likely. In addition to the UHV measurements, MS measurements have been applied to the decomposition of LiZn2(BH4)5 in rough vacuum (=1 mbar) via a differential pumping. In this case, only signals originating from diborane and impurity gases such as water and CO2 are observed (see Figure 2, bottom). To quantify the decomposition products, we performed combined gravimetric and IR measurements. A typical FTIR spectrum is shown in Figure 3. All structures in the measured spectrum are due to vibrations from diborane. Water and CO2 lines are present at the impurity level only mainly originating from the differential pumping system. The IR spectrum of diborane consists of various vibrational and rovibrational transitions.18,19 The ground-state IR transitions by electronic structure calculations are in good agreement with the experiment. However, the overtones (such as transition ν13) and the rotational fine structure are not included in the calculations. Furthermore, the experimental resolution is too low to resolve the detailed rovibrational spectrum of diborane (see, e.g. ref 19). Using the main IR transition of diborane (ν17) at 1602 cm1, the decomposition is followed quantitatively by IR, as shown in Figure 4. The perfect agreement with the mass signal of the balance over 4 orders of magnitude is reflected by comparing the total mass loss derived by the two measurements methods: the optically measured one corresponding to B2H6 is 10.65 mass %, giving a total mass loss of 11.4 mass % assuming the desorption of one H2 per B2H6. The gravimetrically measured value up to 120 °C is 15.2 mass %. At higher temperatures an additional weight loss is observed (see Figure 4), while signals for the diborane or other IR-active species remain at the impurity level. The possibility of quantifying the amount of diborane allows a systematic investigation of additives, possibly changing the reaction mechanism, eventually reducing the amount of emitted diborane. Figure 5 compares the effect of additives on the total mass loss and emitted diborane. The diborane signal was calibrated to the corresponding mass signal of the pure LiZn2(BH4)5 measured gravimetrically. Although the 17222



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Figure 5. Comparison of various additives intended for reducing the amount of emitted diborane during the decomposition of LiZn2(BH4)5. (Top panel) peak temperature at maximum mass loss; (bottom panel) mass loss due to decomposition. Lines: total (blue line) and diborane (red line) related mass loss expected from reaction 5; the dashed lines represent the corresponding mass loss from reaction 6. Circles: measured total (full blue circles) and diborane (derived from IR, hollow red circles) mass loss.

used additives differ significantly, no marked effect was found. The choice of additives was made from literature reports on the effective catalytic effect on the B2H6 decomposition.20 Ru and Ni are also widely used as hydrogenation catalysts, MgO is a frequent impurity in reactive hydride composites, and Fe2O3 and Cr2O3 may represent the class of 3d oxides. Over Mo(100) surfaces, complete decomposition of diborane occurs already below room temperature.21 Indeed, this additive was the only one showing a small deviation from the pure system. Furthermore, the peak temperatures being sensitive to changes of the reaction kinetics do not differ significantly, implying that the additives do not alter the decomposition mechanism. This suggests that the additives work—if at all—only as a surface catalyst for diborane decomposition. The residence time and the amount of the catalysts seem to be too small to markedly increase the decomposition rate of diborane. In all cases including the pure system, the total mass change is around 20% less than expected from a decomposition into only diborane and hydrogen: LiZn2 ðBH4 Þ5 f 5=2B2 H6 þ 2H2 þ 2Zn þ LiH Δm=m ¼ 19:4%

ð5Þ

An additional decomposition step without the release of diborane is observed at higher temperatures (see Figure 4). Furthermore, LiH is not observed as a solid decomposition product.7 We thus suggest the following decomposition reaction: LiZn2 ðBH4 Þ5 f

2ZnðBH4 Þ2 þ LiBH4

! f 2B2 H6 þ 2H2 þ 2Zn þ LiBH4 ðStep 1Þ

f2B2 H6 þ 7=2H2 þ 2Zn þ LiH þ B

ðStep 2Þ

Δm=m ¼ 15:4% ðStep 1Þ, 0:8% ðStep 2Þ

ð6Þ

A comparison of experimental and theoretical values is plotted in Figure 5, corroborating the two-step decomposition pathway. The first step of the reaction pathway was also suggested by Cerny et al.22 Furthermore, UHV-MS measurements imply that

Figure 6. Selected FTIR spectra of the gas emitted during decomposition of LiBH4 in 1 bar of hydrogen. Temperature range is 25450 °C and heating ramp 1 °C/min.

also step 1 is a two-step process, in which LiZn2(BH4)5 forms first LiBH4 and unstable and volatile Zn(BH4)2, which immediately decomposes into Zn and diborane. This conclusion is corroborated by recent density functional calculations,23 predicting that LiZn2(BH4)5 is unstable with respect to decomposition into LiZn(BH4)3 + Zn(BH4)2. With using the decomposition reaction 6, the infrared data is recalibrated with the mass signal. For comparison with theory, the calculated infrared intensity, being the integrated molar absorbance AI, is converted into the experimentally infrared absorption coefficient assuming a bandwidth ν1/2 of 3000 m1:24 " # 2 D 1 k ½m2 mol1 ¼ AI 42:2561 Å amu

2303 1:57ν1=2 ½m1

ð7Þ

Figure 3 demonstrates the quantitative agreement between calculations and experiment. This opens the possibility of the investigation of compounds without necessarily having reference standards. C. Decomposition of LiBH4. The gas purity of hydrogen released from LiBH4 has been the subject of numerous investigations using mainly mass spectrometry. In UHV, a variety of fractions possibly from gaseous LiBH4,25 diborane, and other boranes has been observed. However, it is estimated from a careful MS study that only 0.2 mass % diborane relative to the amount of emitted hydrogen is released.26 The amount of diborane from LiBH4 is further reduced when the desorption takes place in a hydrogen atmosphere. Figure 6 shows FTIR spectra of the gas emitted during decomposition of LiBH4 in 1 bar of H2. The spectra show only little structures above the noise level. Clear spectral signatures are from water, tetrahydrofuran (THF), and CH4. Main diborane peaks are expected to appear at the indicated positions. Although no clear peak is visible, the intensity at 1602 cm1 changes slightly. If this change is due to diborane, the thus derived concentration of diborane may be taken as its upper limit. The corresponding mass losses are plotted in Figure 7. Relevant 17223



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Figure 7. Decomposition of LiBH4 in 1 bar of H2 as measured gravimetrically (dots) and optically (lines). The green line indicates the sensitivity limit of diborane as estimated from the signal-to-noise ratio.

mass loss is observed for THF and water only. THF and water are residues of the synthesis purification process during manufacture of the compound. The impurity CH4 is probably from the reduction of THF, an ether, by the decomposing LiBH4 (so-called ether splitting). The detected species are added to a total IR-mass loss and compared to the total mass loss measured gravimetrically. Integration gives 13.8 mass% and 0.8 mass % for total and IR- mass loss, respectively. This corresponds to around 5 mass% of impurities, in good agreement with the company’s specification of LiBH4 of 95% purity. D. Decomposition of Mg(BH4)2. Mg(BH4)2 is a promising hydrogen-storage material with respect to its high hydrogen content of 14.9% and its rather low desorption temperature of below 300 °C.27 Partial reversibility has been shown.28,29 A desorption temperature of 300 °C was proposed to be the minimum temperature required for decomposition without diborane emission.8 We thus employed the combined gravimetryIR study on the decomposition of Mg(BH4)2 to test this hypothesis. Figure 8 shows the rate of decomposition of Mg(BH4)2 3 2S(CH3)2. Interestingly, the material emits a significant amount of diborane (ΔmB2H6/ΔmH2 = 1 mass %) around 90 °C. Annealing at 150 °C transforms the material into pure Mg(BH4)2. Figure 8 shows the corresponding X-ray patterns of the as-received and annealed phase (compare to ref 29). Apparently, no further diborane emission is observed from the pure Mg(BH4)2 phase (Figure 8), although negligible diborane emission is only reached after long annealing steps (intermediate steps are not shown). A further piece of information is given by the infrared spectra of the emitted gas. They display the characteristics of diborane and impurities of water and CH4. Most likely, the dimethyl sulfide [S(CH3)2] is reduced similar to the observed ether splitting in LiBH4 (see above). It is noted that, in 1 bar of hydrogen, the maximum temperature of the used system is not high enough to enable full decomposition. Indeed, the exact decomposition pathway depends on the applied hydrogen pressure.30 In Figure 8, the decomposition of Mg(BH4)2 in 1 bar of Ar is added. The measured profile changes drastically when compared to decomposition in hydrogen. Decomposition in Ar comes close to decomposition in vacuum, and indeed the

Figure 8. Decomposition of Mg(BH4)2 3 2S(CH3)2 (top) and Mg(BH4)2 (bottom) in 1 bar of H2 as measured gravimetrically (dots) and optically (lines, only dibroane). The decomposition of Mg(BH4)2 in 1 bar of Ar is also shown (bottom, hollow spheres, only gravimetry). The inset shows X-ray patterns of Mg(BH4)2 3 2S(CH3)2 and Mg(BH4)2.

measurements in Ar are practically identical to published results on thermodesorption into vacuum. Furthermore, the total mass loss is around 14.6 mass %, close to the theoretical value of complete decomposition of Mg(BH4)2 into Mg, B (or MgB2), and hydrogen of 14.9 mass %. The small difference might be due to the partial decomposition during removal of the adduct.

IV. DISCUSSION AND CONCLUSIONS This study aims at an investigation of the impurity gases emitted during the decomposition of borohydrides. For this we have set up a quantitative gas analysis based on a combination of FTIR spectroscopy and gravimetry. We would like to emphasis that this combination allows for a semiquantitative analysis without extra calibration as needed for, e.g., mass spectrometry. This is particularly important, because the handling of toxic gases such as diborane is dangerous and difficult. Furthermore, calibration references may not always be available. The gas analysis during decomposition of the borohydrides takes place under realistic conditions, in particular at finite hydrogen backpressure. The emitted gases are investigated by mass spectrometry in UHV environment and after differential pumping and by combined gravimetryIR spectroscopy. The basic differences between the three experimental setups are • thermodynamics [Decomposition in vacuum is kinetically driven, and at finite hydrogen backpressure an additional thermodynamic constraint is added (compare also eq 2). In the case of LiBH4, the differences between Ar and 1 bar of hydrogen is negligible, because the decomposition takes place at temperatures corresponding to an equilibrium pressure of p g 1 bar.31,32 In the case of Mg(BH4)2, the 17224


The Journal of Physical Chemistry C hydrogen environment may change the whole decomposition pathway.27,30] • the UHV setup [which allows for a detection of unstable intermediates during decomposition. Although the experiments using differential pumping also take place in vacuum (g1 mbar), the emitted molecules may interact with the gas and walls and may decompose further or condense. A quantitative number to distinguish between the two scenarios is the ratio of the mean free path to the experiment’s dimension, e.g. the distance between sample and spectrometer. At pressures of 109106 mbar, as used in the UHV experiments, the mean free path is longer than 1 m. In this case, the mass spectrometer measures all emitted molecules. In the other case, the mean free path is on the order of micrometers to be compared to a sample spectrometer distance of around 1 m. Thus, the molecules are multiply scattered and can react with the wall.] The importance of the setup is demonstrated by the measurement of decomposition of LiZn2(BH4)5. UHV mass spectrometry detects the emission of Zn(BH4)2, which is not observed by spectrometry using differential pumping and FTIR. Apparently, LiZn2(BH4)5 decomposes into highly unstable Zn(BH4)2 and LiBH4. Zn(BH4)2 decomposes subsequently into diborane, Zn, and H2 according to reaction 6. Although FTIR spectra do not detect the intermediates, the gravimetric and spectroscopic mass ratios support this reaction pathway. At higher temperatures an additional decomposition step is observed, which might be related to the decomposition of LiBH4. The presence of the hypothetical LiBH4 was not confirmed by X-ray diffraction most likely due to its noncrystalline structure. However, NaBH4 was detected7 during decomposition of the sister compound NaZn2(BH4)5 supporting also the (intermediate) presence of LiBH4 in LiZn2(BH4)5. Density functional calculations23 predict that LiZn2(BH4)5 is unstable with respect to decomposition into LiZn(BH4)3 + Zn(BH4)2. The eventual formation of LiBH4 might thus again be a multistep reaction. In surface chemistry, differences in surface reactions at high and low pressures had been observed and circumscribed as collision-induced processes.35 In these processes, a gas-phase molecule collides with an adsorbate and may trigger the adsorbate to dissociate on the surface [e.g., dissociation of O2 chemisorbed on Ag(001)36] or to desorb from the surface [e.g., collision induced desorption of CH4 on Ni(111)37]. Our results suggest a similar behavior, i.e., a collision-induced dissociation of Zn(BH4)2 in Ar at ambient pressures. Various additives were tested aiming at catalyzing the decomposition of the desorbed diborane. It was found that the additives work only as a surface catalyst for diborane decomposition, and the residence time and the amount of the catalysts seem to be too small to markedly increase the decomposition rate of diborane. In the case of LiBH4, only traces of diborane were detected by FTIR, in contrast to UHV-MS results. These results may be explained in three ways: (i) Diborane is emitted. However, at high temperatures diborane decomposes on the way to the detector far away from the educt, e.g., at the walls of the reactor. Alternatively, diborane is not emitted, because either it is never formed (ii) or it is only present as an intermediate at the reactants’ surfaces or interfaces (iii). The observation of diborane by UHV-MS is in agreement with hypotheses i and iii. As described above, unstable short-lived species may be detectable in UHV due to the lack of interactions. Scenario iii is similar to

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the formation of alane on aluminum surfaces, which is observed by thermodesorption only in UHV environments33 or by surfacesensitive methods.34 The combined FTIRgravimetry measurements do not support scenario i, because the total mass loss is too small to include the desorption of diborane, which later on decomposes at the walls of the reactor. Thus, the measurements corroborate hypothesis iii of diborane being an intermediate of the formation and decomposition of borohydrides in line with previous publications (Friedrichs et al.8,38). Mg(BH4)2 was chosen as a borderline case of compounds expected to not emit diborane.8 The results at hand demonstrate that the emission of diborane depends significantly on the preparation method. The as-received Mg(BH4)2 adduct emits diborane, annealed R-Mg(BH4)2 does not. It is not clear whether the emitted diborane is a residue of the preparation method via BH3 3 S(CH3)2 or the decomposition reaction of Mg(BH4)2 3 S(CH3)2 into Mg(BH4)2 with partial decomposition of the borohydride. These results may be summarized as follows: adduct-free Mg(BH4)2 and LiBH4 emit diborane only at the impurity level, while for LiZn2(BH4)5 diborane is the main decomposition product. These results are in good agreement with recent DFT calculations by Kim et al.,39 proposing a small amount of boranes as equilibrium decomposition products of LiBH4 and Mg(BH4)2. However, the authors point out that a thermodynamic approach does not provide a definitive description of the decomposition mechanism. In this paper, we have shown experimentally that the occurrence of various intermediates depends sensitively on the reaction conditions, including gas mean free path lengths, hydrogen backpressure, and sample pretreatment. In other words: the reaction mechanism is predominantly driven by kinetic constraints. Although a screening for catalysts catalyzing the decomposition of diborane did not show the desired outcome, the results generally raise hope of improving hydrogen sorption kinetics by identification and suppression of the corresponding kinetic constraints.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was financially supported by the Swiss Federal Office of Energy, the Swiss National Science Foundation (SNFProject 200021-129603/1), and the Indo Swiss Joint Research Programme (ISJRP), Grant No. RF14. ’ REFERENCES (1) Z€uttel, A.; Borgschulte, A.; Schlapbach, L. (Eds.) Hydrogen as a Future Energy Carrier; Wiley-VCH: Weinheim, 2008. Eberle, U.; Felderhof, M.; Sch€uth, F. Angew. Chem., Int. Ed. 2009, 48, 6608. Li, H.-W.; Yan, Y.; Orimo, S.-i.; Z€uttel, A.; Jensen, C. M. Energies 2011, 4, 185–214. (2) Lodziana, Z. Phys. Rev. B 2010, 81, 144108. (3) Kostka, J.; Lohstroh, W.; Fichtner, M.; Hahn, H. J. Phys. Chem. C. 2007, 111, 14026. (4) Gosalawit-Utke, R.; Bellosta von Colbe, J. M.; Dornheim, M.; Jensen, T. R.; Cerenius, Y.; Bonatto Minella, C.; Peschke, M.; Bormann, R. Phys. Chem. C 2010, 114, 10291. (5) de Hoffman, E.; Stroobant, V. Mass Spectrometry: Principles and Applications, 2nd ed.; John Wiley and Sons: New York, 2001. 17225



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