Heating Rate-Dependent Dehydrogenation in the Thermal

Article pubs.acs.org/JPCC

Heating Rate-Dependent Dehydrogenation in the Thermal Decomposition Process of Mg(BH4)2·6NH3 Yanjing Yang, Yongfeng Liu,* You Li, Mingxia Gao, and Hongge Pan State Key Laboratory of Silicon Materials, Key Laboratory of Advanced Materials and Applications for Batteries of Zhejiang Province, and Department of Materials Science and Engineering, Zhejiang University, Hangzhou 310027, China S Supporting Information *

ABSTRACT: The detailed mechanism of thermal decomposition of Mg(BH4)2·6NH3 synthesized via a mechanochemical reaction between Mg(BH4)2 and NH3 at room temperature was investigated for the first time. A six-step decomposition process, which involves several parallel and interrelated reactions, was elucidated through a series of structural examinations and property evaluations. First, the thermal decomposition of Mg(BH4)2·6NH3 evolves 3 equiv of NH3 and forms Mg(BH4)2·3NH3. Subsequently, Mg(BH4)2·3NH3 decomposes to release an additional 1 equiv of NH3 and 3 equiv of H2 to produce the [MgNBHNH3][BH4] polymer. And then, [MgNBHNH3][BH4] further desorbs 3 equiv of H2 through a three-step reaction to give rise to the formation of the polymer intermediates of [MgNBHNH2][BH4], MgNBHNH2BH2, and MgNBNHBH, respectively. Finally, an additional 1 equiv of H2 is liberated from MgNBNHBH to yield Mg and BN as the resultant solid products. In total, about 7 equiv of H2 and 4 equiv of NH3 are released together from Mg(BH4)2·6NH3 upon heating. Moreover, there is a strong dependence of the gas compositions released from Mg(BH4)2·6NH3 on the heating rate because the decomposition reaction of Mg(BH4)2·3NH3 is sensitive to the heating rate, as the faster heating rate induces a lower ammonia evolution. The finding in this work provides us with insights into the dehydrogenation mechanisms of the metal borohydride ammoniates as hydrogen storage media.

1. INTRODUCTION

Recently, a new strategy has been proposed to reduce the operating temperature of hydrogen desorption from metal borohydrides by coordinating with NH3 to form ammoniates.16 A series of metal borohydride ammoniates, including LiBH 4 ·NH 3 , Mg(BH 4 ) 2 ·NH 3 , Mg(BH 4 ) 2 ·2NH 3 , Mg(BH 4 ) 2 ·3NH 3 , Ca(BH 4 ) 2 ·2NH 3 , Al(BH 4 ) 2 ·6NH 3 , Zn(BH4)2·2NH3, LiMg(BH4)3·2NH3, and Li2Al(BH4)5·6NH3 have been synthesized and investigated as hydrogen storage media.16−28 Such metal borohydride ammoniates achieve a local combination of the N−Hδ+···Hδ−−B dihydrogen bonds upon heating, consequently resulting in tunable thermodynamic stabilities and increased hydrogen densities.24,25 It was reported that only a trace amount of hydrogen was released from LiBH4·NH3 and Ca(BH4)2·2NH3 at temperatures below 300 °C under dynamic conditions.17,18,20 However, in a closed vessel, ∼17.8 wt % of hydrogen release was observed for the Co-catalyzed LiBH4·(NH3)4/3 in the temperature range of 135−250 °C.26 More interestingly, hydrogen is the major product at the decomposition of Mg(BH4)2·NH3, Mg(BH4)2·2NH3, Mg(BH4)2·3NH3, Al(BH4)3·6NH3, and Li2Al(BH4)5·6NH3 upon heating.19,21−23,27,28 Mg(BH4)2·2NH3 decomposes to liberate ∼16 wt % of hydrogen with an onset

Hydrogen holds great potential as an environmentally benign, reliable, and affordable energy carrier, especially for vehicular applications. However, lack of safe, efficient, and economical hydrogen storage technologies hinders the implementation of a hydrogen-based economy.1 In the past decades, great efforts have been devoted to exploring and developing solid-state hydrogen storage materials, which are often preferred over pressurized gas and liquefied hydrogen storage because of their high gravimetric and volumetric storage capacities and safe operating pressures.2−5 Recently, metal alanates, borohydrides, and amides/imides have been extensively investigated and developed for their hydrogen storage properties.6−9 Of them, metal borohydrides, including LiBH4, Mg(BH4)2, and Ca(BH4)2, are regarded as promising hydrogen storage materials because they can offer high gravimetric and volumetric hydrogen densities.9−11 Magnesium borohydride, for example, possesses a gravimetric hydrogen density of 14.9 wt % and a volumetric hydrogen density of 112 g/L that can meet the system targets set by the U.S. Department of Energy.10,12 In particular, a favorable desorption enthalpy makes Mg(BH4)2 a more attractive candidate in comparison with other borohydrides. However, the majority of its dehydrogenation occurs still above 300 °C, which is too high for practical applications.13−15 © 2013 American Chemical Society

Received: May 4, 2013 Revised: July 17, 2013 Published: August 6, 2013 16326

dx.doi.org/10.1021/jp404424m | J. Phys. Chem. C 2013, 117, 16326−16335

The Journal of Physical Chemistry C

Article

dehydrogenation temperature of 120 °C,19 which makes it competitive with ammonia borane as a hydrogen storage candidate. The difference in the coordination strength of NH3 to the cation (i.e., Mg2+ or Li+) is the most important reason for the different thermal decomposition behaviors of Mg(BH4)2·xNH3 (x = 1−3) and LiBH4·NH3.17−19,27,28 Since Mg2+ has a more positive charge with respect to Li+, it is possible to form a stronger coordination bond with NH3. More importantly, thermal decomposition of Mg(BH4)2·2NH3 is endothermic in nature,19 which induces a controllable hydrogen-release process and more potential reversibility, consequently attracting increasing attention. Four ammoniates have been reported for Mg(BH4)2, including Mg(BH4)2·NH3, Mg(BH4)2·2NH3, Mg(BH4)2·3NH3, and Mg(BH4)2·6NH3.19,27,28 Mg(BH4)2·6NH3 is of special interest, since it possesses the highest theoretical hydrogen contents (16.8 wt %). As a saturated ammoniate of Mg(BH4)2, Mg(BH4)2·6NH3 was first synthesized by electrolysis of alkalimetal borohydrides with a magnesium anode in liquid ammonia 50 years ago.29 After that, a variety of approaches have been developed for the preparation of Mg(BH4)2·6NH3.30−33 By reacting ammonia with the Mg(BH4)2 etherate solution in ether at low temperature, Mg(BH4)2·6NH3 was obtained in high purity.30,31 Recently, Soloveichik et al. solved the structure of Mg(BH4)2·6NH3, which crystallizes in the cubic space group Fm3̅m with unit cell parameter a = 10.82(1) Å and is isostructural to Mg(NH3)6Cl2.19 Each Mg2+ atom is octahedrally coordinated by six NH3 groups, while the BH4− units occupy the (1/4, 1/4, 1/4) tetrahedral sites. Heating Mg(BH4)2·6NH3 at 118 °C in a dynamic vacuum for 4 h yielded the resultant product of Mg(BH4)2·2NH3 after liberating 4 mol of ammonia.19 Furthermore, hydrogen was detected as the dominant gaseous product upon further heating of Mg(BH4) 2·2NH3, which leads to the formation of magnesium boronitrides, BN, and amorphous boron as the resultant products.19 However, the solid-state intermediates during thermal decomposition of Mg(BH4)2·6NH3 have not been investigated and identified, and the detailed decomposition process of Mg(BH4 ) 2 ·6NH 3 as well as Mg(BH4)2·2NH3 is still unclear so far. It is believed to be very important for understanding the hydrogen generation mechanism of metal borohydride ammoniates. In addition, the purity of hydrogen release is strongly desired to be significantly improved by utilizing the intermediate compounds due to their tunable compositions, which also is particularly meaningful for practical applications. It is therefore of both fundamental interest and practical importance to elaborate the details of the thermal decomposition of Mg(BH4)2·6NH3. In this work, the high-purity Mg(BH4 ) 2 ·6NH 3 was synthesized via a facile solvent-free approach based on the solid−gas reaction between Mg(BH4)2 and ammonia at ambient temperature. The thermal decomposition behaviors of the as-prepared Mg(BH4)2·6NH3 were systematically investigated by a series of structural analyses and property evaluations. A six-step decomposition process was proposed for Mg(BH4)2·6NH3 under dynamic heating conditions. In particular, a strong dependence of the gas constituent on the heating rate for the decomposition of Mg(BH4)2·6NH3 was found, as the faster heating rate leads to lower ammonia evolution. The reason behind this phenomenon is discussed and elucidated.

2. EXPERIMENTAL SECTION All the handling of samples was performed in either a Schlenk apparatus or a glovebox (MBRAUN 200B) equipped with a circulation purifier which can keep the concentration of O2 and H2O below 0.1 ppm during operation. 2.1. Material Preparation. Sodium borohydride (NaBH4, 98%) and magnesium chloride (MgCl2, 99%) were purchased from Alfa Aesar and used as received. Anhydrous diethyl ether was delivered from Sinopharm Chemical and further purified with calcium hydride (CaH2), and anhydrous ammonia gas was used as received. Mg(BH4)2 was prepared through the metathesis reaction between NaBH4 and MgCl2 in diethyl ether as described elsewhere.34 Mg(BH4)2·6NH3 was synthesized by ball milling Mg(BH4)2 under 6 atm of ammonia atmosphere on a planetary ball mill for 24 h, rotating at 300 rpm. 2.2. Structural Characterization. The powder X-ray diffraction (XRD) data were recorded on a PANalytical X’Pert Pro X-ray diffractometer with Cu Kα radiation at 40 kV and 40 mA from 3° to 90° (2θ). A specially designed sample container was used to protect samples from oxygen and moisture contaminations during transfer and testing. Vibrational characteristics of B−H and N−H bonds were identified by Bruker Tensor 27 Fourier transform infrared (FTIR) spectroscopy. The spectra of all the samples (as KBr pellets with a KBr-to-sample weight ratio of about 100:1) were acquired in the range of 4000−400 cm−1, and the transmission mode was adopted with a resolution of 4 cm−1. For each spectrum, 16 scans were run and accumulated. Solid-state 11B MAS NMR experiments were performed on a Bruker Avance II 300 MHz spectrometer at room temperature operating at 96.216 MHz. Solid powder samples were packed into 7 mm ZrO2 rotors with a Kel-F cap in the glovebox. All spectra were plotted after 1024 scans with a rotor-spinning rate of 5 kHz. Chemical shifts (δ) were reported in parts per million (ppm) referenced to solid NaBH4 (−41 ppm). 2.3. Property Evaluation. A home-made temperatureprogrammed-desorption (TPD) system equipped with online mass spectroscopy (MS, Hiden QIC-20) was used to measure the thermal decomposition behavior as a function of temperature. Approximately 40 mg of sample was loaded into a tube reactor, and pure argon was used as purge gas at a flow rate of 20 mL/min. The thermal decomposition behaviors of Mg(BH4)2·6NH3 were quantitatively evaluated with a home-made Sieverts-type apparatus. Typically, about 80 mg of sample was loaded into a stainless steel reactor in the glovebox and then connected to the Sieverts-type apparatus. After evacuating the system to 10−3 atm, the sample was heated to a desired temperature at variable heating rates of 2−10 °C/min. The pressure and temperature data were recorded automatically. The quantity of gas released during the experiments was calculated from the pressure change according to the equation of state. Simultaneous thermogravimetric analysis and differential scanning calorimetry examination (TG-DSC) were conducted on a Netzsch STA449F3 thermal analyzer to detect the weight loss and heat flow during the decomposition of Mg(BH4)2·6NH3. About 2 mg of sample was loaded into an Al crucible, and the sample was gradually heated from room temperature to 600 °C at 2−10 °C/min. 16327



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3. RESULTS AND DISCUSSION 3.1. Characterization of As-Prepared Mg(BH4)2·6NH3. The hexaammoniate of magnesium borohydride, Mg(BH4)2·6NH3, was prepared by ball milling Mg(BH4)2 under 6 atm of NH3 atmosphere for 24 h at room temperature. After milling for 12 h, the milling vessel was evacuated once and refilled with fresh ammonia gas. A temperature rise was detected as the ammonia gas was filled in the vessel, indicating an exothermic nature for the chemical reaction between Mg(BH4)2 and NH3. Further prolonging the milling to 24 h, a white loose solid-state powder was finally obtained as the resultant product. Figure 1a shows the XRD patterns of

Figure 2. (a, b) TPD-MS and (c) DSC curves of the as-prepared Mg(BH4)2·6NH3 at different heating rates.

decomposition of Mg(BH4)2·6NH3 (Figure 2c). Three thermal events were observed one after the other while the temperature was elevated from room temperature to 375 °C, as a strong endothermic peak is followed by an exothermic peak and a weak endothermic peak. The first strong endothermic peak can be attributed to the deammoniation of Mg(BH4)2·6NH3, since the ammoniation of Mg(BH4)2 is a strong exothermic reaction as mentioned above. The slight difference in the operating temperature detected by DSC and TPD-MS is possibly due to the different positions of the thermocouple. In the DSC experiments, the thermocouple is placed below the bottom of the Al crucible while it is inserted into the samples for TPD-MS measurements. The second exothermic peak originates from the mixed hydrogen and ammonia desorption, as also observed in other B−N−H systems.35,36 The third weak endothermic peak corresponds to the release of pure hydrogen. Here, it is noteworthy that the integral area of the NH3 signal (corrected by the relative sensitivity of the MS spectroscopy to the corresponding species) was distinctly decreased with increasing the heating rate as shown in Figure 2b, implying a declined amount of NH3 evolution. To quantify the weight loss and the amount of gas release with temperatures, TG and volumetric release measurements were further performed, and the results are presented in Figure 3 and Table 1. It is found that there is a strong dependence of gas constituent on the heating rate, as the faster heating rate results in a distinct decline in the ammonia evolution. At the heating rate of 2 °C/min, ∼3.96 equiv of NH3 is evolved while it is 3.30 equiv of NH3 at 10 °C/min as shown in Table 1. Correspondingly, the amount of hydrogen desorption is increased from 6.95 equiv of H2 (2 °C/min) to 8.09 equiv of H2 (10 °C/min). Specifically, the thermal decomposition of Mg(BH4)2·6NH3 is highly sensitive to the heating rate. A similar phenomenon was also reported in ammonia borane, sodium amidoborane, and LiBH4·NH3 systems.16,35,36 This suggests that several parallel reactions with different kinetics possibly proceed simultaneously during the thermal decomposition of Mg(BH4)2·6NH3. 3.3. Thermal Decomposition Mechanism of Mg(BH4)2·6NH3. As discussed above, the thermal decomposition behavior of Mg(BH4)2·6NH3 is rather complicated, as it possibly contains several parallel reactions. To get further

Figure 1. (a) XRD patterns and (b) FTIR spectra of Mg(BH4)2 and as-prepared Mg(BH4)2·6NH3.

Mg(BH4)2 before and after ammoniation. It is clear that the characteristic diffraction peaks of Mg(BH4)2 disappeared completely after full ammoniation, and a set of new XRD diffractions peaks was identified, which is assigned to the singlephase Mg(BH4)2·6NH3 with a face centered-cubic structure, as proposed by Soloveichik et al.19 The quite sharp reflections indicate a good crystallinity for the as-synthesized Mg(BH4)2·6NH3. Further FTIR examination confirmed the formation of the pure Mg(BH4)2·6NH3. As shown in Figure 1b, two absorption bands at 3360 and 3266 cm−1 were observed after the ammoniation of Mg(BH4)2, which originate from the symmetric and asymmetric N−H stretching modes, respectively. Interestingly, it is found that there is a splitting and red shift for the B−H stretching modes and BH2 deformation modes, respectively, in the FTIR spectrum with respect to Mg(BH4)2, which matches well with the previous report.19 3.2. Thermal Decomposition Behavior of As-Prepared Mg(BH4)2·6NH3. Figure 2 shows the TPD-MS and DSC curves of the as-prepared Mg(BH4)2·6NH3 at various heating rates (2−10 °C/min). As expected, the thermal decomposition curves shift to higher temperatures with increasing heating rate (Figure 2a and b). At a heating rate of 2 °C/min, three distinct peaks were detected for the H2 signal at 182, 215, and 238 °C, respectively. With rising heating rate, the first peak is gradually weakened and even disappears at 10 °C/min. In the meanwhile, the NH3 signal is also declined dramatically (Figure 2b). DSC measurements show a complicated heat flow behavior for the 16328



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Figure 3. (a) TG and (b) volumetric release curves of the as-prepared Mg(BH4)2·6NH3 at different heating rates.

Table 1. Composition of Gas Released during the Decomposition of Mg(BH4)2·6NH3 at Different Heating Rates heating rates (°C/min) mass loss (wt %) amt of gas released (equiv) composition of gases (for one Mg(BH4)2·6NH3 unit)

2 51.99 10.91 3.96 equiv of NH3 and 6.95 equiv of H2


7.5 47.44 11.18 3.44 equiv of NH3 and 7.74 equiv of H2


∼0.97 equiv of NH3 was evolved. Moreover, the TPD curve of the corresponding residue remains almost unchanged except for the slightly declined first decomposition peak and the hightemperature shift of the third peak. As decomposed at 150 °C, simultaneous NH3 and H2 release was observed, and the corresponding amount was determined to be ∼3.37 equiv of NH3 and 2.47 equiv of H2. XRD results present a new set of diffraction peaks in addition to the characteristic reflections of Mg(BH4)2·6NH3, as shown in Figure 5a. These new peaks are attributed to magnesium borohydride triammoniate, Mg(BH4)2·3NH3, which was first synthesized by Wagner et al. 25 years ago.27,28 Further FTIR examination indicates that the typical absorbances of Mg(BH4)2·6NH3 at 3360, 1087, and 637 cm−1 are distinctly weakened, suggesting its consumption at such an operating temperature. Moreover, an absorption peak at 1118 cm−1 assignable to Mg(BH4)2·3NH3 is also discernible. TPD measurement on the solid residue after decomposition at 150 °C reveals that the first decomposition peak almost disappears (Figure 6a). We therefore believe that Mg(BH4)2·6NH3 first decomposes to evolve NH3 and form a lower-coordinated ammoniate of Mg(BH4)2·3NH3 as described below:

insight into the mechanism of thermal decomposition, Mg(BH4)2·6NH3 was first heated to different desired temperatures at 2 °C/min and then dwelled for enough time until the relative reactions were completed. And then, the solid residues were collected for XRD, FTIR, and TPD measurements to identify the chemical species involved in the decomposition reactions. Figure 4 illustrates the volumetric release curves of

Mg(BH4)2 · 6NH3 → Mg(BH4)2 · 3NH3 + 3NH3

(1)

Moreover, it should be pointed out that there are three new infrared absorption peaks at 3418, 1485, and 770 cm−1 with considerable intensities in the FTIR spectrum after decomposition at 150 °C, as shown in Figure 5b. It is known that the absorption peaks at 1485 and 770 cm−1 locate in the typical B− N vibration range, and the absorbance at 3418 cm−1 originates from the N−H stretching vibrations.37,38 The presence of the B−N vibration indicates that there is a strong chemical interaction between NH3 groups and [BH4]− anions, which is reasonably responsible for the release of hydrogen at 150 °C as stated above.

Figure 4. Isothermal desorption curves of the as-prepared Mg(BH4)2·6NH3 at different temperatures.

Mg(BH4)2·6NH3 at different temperatures. The corresponding amounts of ammonia and hydrogen released were calculated by considering simultaneously the weight loss (not shown here). Figures 5a, b and 6a show XRD patterns, FTIR spectra, and TPD curves of the solid residues of Mg(BH 4 ) 2 ·6NH 3 decomposed at different temperatures, respectively. It is seen that the structural feature of the decomposition product at 140 °C is identical to the as-prepared Mg(BH4)2·6NH3 although 16329



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Interestingly, the FTIR spectrum of the decomposition product remains almost unchanged, whereas no diffraction peak was observed in the XRD profile. This fact suggests that the first step of decomposition of Mg(BH4)2·6NH3 has completed, and the solid residues are the noncrystalline species after decomposition at 160 °C. According to the evolved gas constituent and the corresponding amount, the average composition of the decomposition product is written as MgB2N2H8. Further increasing the decomposition temperature induces only hydrogen release with the absence of NH3. At 180 °C, an additional 1.1 equiv of H2 was released in comparison with the decomposition at 160 °C, which yields a solid residue with the nominal composition of MgB2N2H6. At 200 and 350 °C, hydrogen release amounts to 5.18 and 6.07 equiv of H2, respectively, in addition to the liberation of 4 equiv of NH3. Thus, the nominal compositions of the decomposition products at 200 and 350 °C were determined to be MgB2N2H4 and MgB2N2H2, respectively. While elevating the decomposition temperature to 600 °C, the total amount of gas evolution is 10.91 mol and the final solid products are composed of Mg and BN, as shown in Figures 3−5. Thus, the decomposition reactions occurring at 150−600 °C can be roughly expressed as follows: Mg(BH4)2 ·3NH3 → MgB2N2H8 + NH3 + 3H 2

(2)

MgB2N2H8 → MgB2N2H6 + H 2

(3)

MgB2N2H6 → MgB2N2H4 + H 2

(4)

MgB2N2H4 → MgB2N2H 2 + H 2

(5)

MgB2N2H 2 → Mg + 2BN + H 2

(6)

Similar to the sample decomposed at 160 °C, XRD examinations show that there still is no specific diffraction peak for the decomposition products at 180−350 °C, indicating their noncrystalline nature. Fortunately, the signature vibrations of N−H, B−H, and B−N bonds can be identified in these samples by FTIR. As shown in Figure 5b, the B−N absorbances at 1485 and 770 cm−1 are discernible after decomposition at 180 °C. In the meanwhile, an N−H vibration at 3418 cm−1 and three B−H vibrations at 2386, 2291, and 2225 cm−1 were also detected with considerably decreased intensities. However, the asymmetric stretching mode of the N−H bond in NH3 groups at 3360 cm−1 disappears, suggesting the consumption of NH3 groups. These results indicate that decomposing at 180 °C possibly gives rise to the generation of a new Mg−B−N−Hbased compound. The existence of the B−H vibrations at 2386, 2291, and 2225 cm−1 demonstrates the integrity of the [BH4]− group, which is very similar to the feature of the B−H vibration in Li4BH4(NH2)3.39 Further solid-state 11B NMR provided another valuable evidence in confirming the persistence of [BH4]− groups, as the tetracoordinated boron was detected at the chemical shift of −40.7 ppm (Figure 7).40 In comparison with the as-prepared Mg(BH4)2·6NH3 (−37.0 ppm), the 11B resonance in the decomposition product at 180 °C shifts subtly to upfield, implying a more shielded local environment of the B nucleus due to the uncoordinated BH4 units.40 According to the above discussion, the decomposition product at 180 °C with the nominal composition MgB2N2H6 can be expressed as [MgNBNH2][BH4]. Identically, the decomposition product at 160 °C with the above-mentioned average composition of MgB2N2H8 can also be denoted as [MgNBHNH3][BH4], since

Figure 5. (a) XRD patterns and (b) FTIR spectra of the decomposed Mg(BH4)2·6NH3 samples at different temperatures.

Figure 6. (a) TPD-MS curves (H2) of the decomposed Mg(BH4)2·6NH3 samples at different temperatures. (b) TPD-MS curves (H2) of Mg(BH4)2·6NH3 decomposed at 160 °C for different durations.

As decomposition proceeds at 160 °C, the TG and volumetric release results indicate that ∼4 equiv of NH3 and 2.86 equiv of H2 were simultaneously liberated (Figure 4). 16330



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the absorbance of the B−N vibration at 1670−930 cm−1 was developed to a relative symmetric broad peak with significantly increased intensity. At the same time, there is a further blue shift to 787 cm−1 for the out-of-plane B−N−B vibration, suggesting a stronger bonding of B−N, which is similar to the vibration characteristics of NHBH.43 As shown in Figures 4 and 5a, only one mole of hydrogen was released at 200−350 °C and the solid residue remains the polymer form. We therefore believe that the decomposition product with the nominal composition of MgB2N2H2 can be expressed as MgNBNHBH. Hence, reactions 5 and 6 can be rearranged as follows: MgNBNH2BH 2 → MgNBNHBH + H 2 (10) MgNBNHBH → Mg + 2BN + H 2

(11)

According to the above discussion, a six-step reaction process was proposed for the thermal decomposition of Mg(BH4)2·6NH3 as shown in Figure 8. At the initial heating

Figure 7. Solid-state 11 B NMR spectra of Mg(BH 4 ) 2 ·6NH 3 decomposed at different temperatures.

a similar FTIR spectrum was identified to some extent. Thus, the decomposition reactions (reactions 2 and 3) occurring at 150−180 °C can be rewritten as follows: Mg(BH4)2 · 3NH3 → [MgNBHNH3][BH4] + NH3 + 3H 2 (7)

[MgNBHNH3][BH4] → [MgNBNH2][BH4] + H 2

(8)

Moreover, it is noteworthy that an absorbance at 770 cm−1 was observed at 150−180 °C, which is assigned to the out-ofplane B−N−B bonds in the B3N3 ring structure. This fact indicates that the decomposition intermediates, MgB2N2H8 and MgB2N2H6, at 160 and 180 °C should have a ring structure similar to that of polyborazine.41,42 In other words, the Bcontaining groups and the N-containing groups in the neighboring Mg−B−N-H compound molecules interact with each other to create the Mg−B−N-H clusters or chains, i.e., the Mg−B−N-H polymer. After decomposition at 200 °C, the N−H vibration at 3418 cm−1 is still discernible in the solid residue, indicating the persistence of the -NH2 groups. However, only a broad absorption peak was detected at 2257 cm−1 for the B−H vibration with the distinctly decreased intensity and the slight red-shift. This result implies the consumption of the BH4 groups and the weakening of the B−H bonding. More interestingly, the absorbances at 1670−930 cm−1 assignable to the B−N vibration are incorporated into an asymmetric broad peak at 1454 cm−1. Furthermore, a blue shift was observed from 770 to 781 cm−1 for the out-of-plane B−N−B vibration. These changes make the features of the B−N vibration more close to those in polyborazine as reported previously.41,42 We therefore believe that a B3N3 ring structure possibly persists after decomposition at 200 °C, since the decomposition product still presents the polymer form with the release of an additional 1 mol of hydrogen. Thus, the decomposition product with the nominal composition of MgB2N2H4 can be represented as MgNBNH2BH2 and reaction 4 can be re-expressed as follows: [MgNBNH2][BH4] → MgNBNH2BH 2 + H 2

Figure 8. Proposed decomposition mechanism of Mg(BH4)2·6NH3.

stage (

Heating Rate-Dependent Dehydrogenation in the Thermal

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