Decomposition Pathway of Ammonia Borane on the Surface of Nano-BN

Jul 26, 2010 - Materials Science, One Shields AVenue, UniVersity of California, .... Current address: Doinita Neiner, Pacific Northwest National Labor...
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J. Phys. Chem. C 2010, 114, 13935–13941

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Decomposition Pathway of Ammonia Borane on the Surface of Nano-BN Doinita Neiner,*,‡,†,# Avery Luedtke,‡ Abhijeet Karkamkar,‡ Wendy Shaw,‡ Jialing Wang,† Nigel D. Browning,†,§,| Tom Autrey,*,‡ and Susan M. Kauzlarich*,† Department of Chemistry, One Shields AVenue, UniVersity of California, DaVis, California 95616, Pacific Northwest National Laboratory, Richland, Washington 99352, Department of Chemical Engineering and Materials Science, One Shields AVenue, UniVersity of California, DaVis, California 95616, and Condensed Matter and Materials DiVision, Physical and Life Sciences Directorate, Lawrence LiVermore National Laboratory, LiVermore, California 94550 ReceiVed: May 10, 2010; ReVised Manuscript ReceiVed: July 2, 2010

Ammonia borane (AB) is under significant investigation as a possible hydrogen storage material. While chemical additives have been shown to lower the temperature for hydrogen release from ammonia borane, many provide additional complications in the regeneration cycle. Mechanically alloyed hexagonal boron nitride (nano-BN) has been shown to facilitate the release of hydrogen from AB at lower temperature, with minimal induction time and less exothermicity, and inert nano-BN may be easily removed during any regeneration of the spent AB. The samples were prepared by mechanically alloying AB with nano-BN. Raman spectroscopy indicates that the AB/nano-BN samples are physical mixtures of AB and h-BN. The release of hydrogen from AB/ nano-BN mixtures as well as the decomposition products was characterized by 11B magic angle spinning (MAS) solid state NMR spectroscopy, TGA/DSC/MS with 15N-labeled AB, and solution 11B NMR spectroscopy. The 11B MAS solid state NMR spectrum shows that diammoniate of diborane (DADB) is present in the mechanically alloyed mixture, which drastically shortens the induction period for hydrogen release from AB. Analysis of the TGA/DSC/MS spectra with 15N-labeled AB shows that all the borazine (BZ) produced in the reaction comes from AB and that increasing nano-BN surface area results in increased amounts of BZ. However, under high temperature, 150 °C, isothermal conditions, the amount of BZ released significantly decreases. High resolution transmission electron microscopy (HRTEM), selected area diffraction (SAD), and electron energy loss spectroscopy (EELS) of the initial and final nano-BN additive provide evidence for crystallinity loss but not significant chemical changes. The higher concentration of BZ observed for lowtemperature dehydrogenation of AB/nano-BN mixtures versus neat AB is attributed to a surface interaction that favors the formation of precursors which ultimately result in BZ. This pathway can be avoided through isothermal heating at temperatures lower than 150 °C. Introduction A number of materials have been developed over the past few years that are promising as hydrogen storage materials: these include metal-organic frameworks (MOFs) and other framework structures,1-10 carbon based materials,11-15 water clathrates,16-19 metal hydrides,20-25 and aminoboranes.26-28 One attractive chemical hydride is ammonia borane (AB) because of its high hydrogen content (19.6%) and mild hydrogen release conditions.29-32 The use of AB in fuel cells has also been demonstrated.33,34 AB releases hydrogen in solution at 25 °C in the presence of a catalyst or an acid,34-43 or in the solid state above 85 °C.29,44,45 The conditions for hydrogen release in solid AB can be modified by the presence of an additive such as silica,46,47 ionic liquids,48 nano-BN,49 catalysts,50-53 ammonium chloride,47 transition metal chlorides (Co, Ni),54 and carbon cryogels,55 to name a few. Each * To whom correspondence should be addressed. E-mail: tom.autrey@ pnl.gov (T.A.) and [email protected] (S.M.K.). # Current address: Doinita Neiner, Pacific Northwest National Laboratory, Radiomaterials Chemistry, PO Box 999, MSIN: P7-25, Richland, WA 99354, USA, E-mail: [email protected]. † Department of Chemistry, One Shields Avenue, University of California. ‡ Pacific Northwest National Laboratory. § Department of Chemical Engineering and Materials Science, One Shields Avenue, University of California. | Lawrence Livermore National Laboratory.

additive has its own advantages and disadvantages: some reduce the dehydrogenation temperature (such as silica or nano-BN), some improve the kinetics of hydrogen release (such as ionic liquids), and some reduce the impurities such as ammonia and BZ (such as silica or CoCl2). For instance, encapsulating AB in silica, SBA-15, has been shown to significantly reduce the dehydrogenation temperature, reduce the impurities from AB (BZ and NH3), and enhance the kinetics of hydrogen release from AB.46 In a previous report, nano-BN was used as a solid support for AB, and it was shown to decrease both the dehydrogenation temperature and exothermicity of hydrogen release.49 For a material to be considered viable for hydrogen storage, regeneration should also be demonstrated and hence the possibility of regenerating AB from spent fuel (NBHx) is an active area of investigation.56,57 One of the advantages that nano-BN offers is chemical stability, without additional elemental contamination, that may enable chemical processing during regeneration of AB from the spent fuel, NBHx. However, while AB/nano-BN mixtures give less ammonia than AB, the BZ yields are higher.49 This result suggests that either a different mechanism of dehydrogenation of AB/nano-BN as compared with SBA-15 is involved or new competing reaction pathways are important. It has been shown that the AB dehydrogenation pathway strongly depends upon the solvent and the solid support.39,43,45,47,49,52,53,55,58-60

10.1021/jp1042602  2010 American Chemical Society Published on Web 07/26/2010

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The dehydrogenation of AB mechanically alloyed with nanoBN in the ratios 1:1 and 1:4 AB/nano-BN was investigated with Raman spectroscopy, MAS and solution 11B NMR spectroscopy, TGA/DSC/MS, BZ quantification, HRTEM, SAD, and EELS in order to provide insight into the mechanism of hydrogen release. Since nano-BN provides all the advantages of an excellent additive to amenable recycling, gaining insight into the mechanism of release may allow for optimization of the process and ultimately lead to a hydrogen storage material based on AB without BZ as a side product and with a simple regeneration procedure. Experimental Section 1. Synthesis. Microcrystalline h-BN (99%) was obtained from Sigma-Aldrich and was used as received. Unless otherwise noted, all manipulations were performed under inert atmosphere in a nitrogen filled drybox. Mechanically alloying or ball milling was performed outside the drybox in a Spex 8000 ball mill with a 3/4 in. diameter and 1 7/8 in. long tungsten carbide lined vial with a methacrylate center cylinder and two tungsten carbide 5/16 in. balls. The ball milling vessel was loaded in a drybox and wrapped in Parafilm for the milling process. The h-BN was ball milled for 1 h (referred to as nano-BN) and characterized by powder X-ray diffraction. Samples were prepared with a weight ratio AB/nano-BN of 1:1 (220 mg:220 mg) and 1:4 (98 mg:406 mg) and milled for 30 min. Samples containing 15Nlabeled AB/nano-BN were prepared in 1:1 and 1:4 ratios, and were analyzed by TG/MS spectroscopy to detect BZ. 2. Characterization. Raman Spectroscopy. Raman data were collected at room temperature for nano-BN, AB, 1:1 AB/nanoBN, and 1:4 AB/nano-BN. The spectra were collected on a Renishaw MicroRaman 1000 spectrometer, equipped with a 514 nm Ar+ laser of 25 mW, a 20× objective, and a charge-coupled device camera. The spectral resolution was ∼4 cm-1 and the peak accuracy was 0.10-0.15 cm-1. Raman spectra were referenced to the Si single crystal peak at 520 cm-1 for all samples. Solid State Magic Angle Spinning (MAS) 11B NMR. In a procedure similar to previously published NMR experiments for AB dehydrogenation,59 samples were packed in boron-free silicon nitride rotors with a pinhole in their vessel cap for hydrogen release. The samples were packed in air, but were run under nitrogen gas. The 11B NMR spectra were referenced to NaBH4 (-41 ppm). Experiments were performed on a Varian Unity spectrometer at 18.8 T (800 MHz 1H frequency) and used a 4 mm Doty 3 channel MAS variable-temperature probe. The proton decoupled experiments were run with a 4 µs 90° pulse, a 10 s pulse delay, and 14 kHz sample spinning. Unless otherwise noted, the first NMR spectrum was recorded at room temperature, and the 1:1 AB/nano-BN and the 1:4 AB/nanoBN samples were heated in the magnet at 80 and 60 °C, respectively. The temperature was calibrated by using the chemical shift of 207Pb in Pb(NO3)2 as a function of temperature. The set point was reached within 4 min and stable (2 °C over the duration of the NMR experiment. ThermograWimetric Analysis (TGA) and Mass Spectrometry (MS). TGA/MS experiments were performed on an STA 449 Jupiter Netzsch instrument equipped with an Aelos QMS 403C MS by heating the samples under flowing argon. The MS instrument employs a standard electron impact ionization detector. The samples were loaded in alumina crucibles in the drybox and were transferred under inert atmosphere and loaded in the TGA/MS instrument. The data were obtained by heating the samples under Ar gas, 25 mL/min, from room temperature to 200 °C at 1 deg/min.

Neiner et al. BZ Trapping Experiments. The samples were loaded in the drybox and transferred to the experimental setup under inert atmosphere. In a typical experiment, the solid samples (AB or AB/nano-BN) were heated in a stainless steel reaction vessel fitted with a gas inlet and outlet. Argon was used as the inert carrier gas at a 20-30 mL/min flow rate to carry volatile gases from the reaction vessel. The vessel containing between 80 and 100 mg of AB was heated at a controlled rate or isothermally with a programmable tube furnace. The carrier gas was passed through a glass frit immersed in 1,2-dimethoxyethane (glyme) or tetrahydrofuran (THF) (10 mL) solution cooled at -78 °C in a dry ice bath. Two sets of heating schemes were followed: ramped and isothermal. In the ramped scheme, the samples were heated at 0.5 deg/min, 1 deg/min to 200 °C. In the isothermal profile, the furnace was heated at 150 °C until the temperature stabilized, and the cell containing the sample was inserted. Volatile products including BZ were collected for 2 h. The isothermal experiments were performed under argon. The glyme or THF solution was analyzed by solution 11B NMR spectroscopy after heating. Solution 11B NMR. 11B NMR spectroscopy was used to quantify the amount of BZ present in the glyme or THF solution. 11 B NMR spectroscopic experiments were performed on a Varian spectrometer (500 MHz for 1H). In all cases, the spectra were recorded both with and without decoupling. The 11B chemical shifts were referenced to BF3 OEt2 ([δ] ) 0 ppm) at 20 °C. In all cases 1 mL of glyme solution was used. AB was used as an internal standard for quantification of BZ. A standard solution of 2.33 mg/mL of AB in glyme or THF was prepared, and this standard solution (2 mL) and BZ containing sample (2 mL) were mixed in a vial, and then 1 mL of the resulting solution was used for the analysis by 11B NMR spectroscopy. Quantitative yields of BZ were obtained by integration of the 11 B NMR signals obtained in the purge-trap solution. HRTEM, SAD, and EELS. HRTEM images and SAD patterns were recorded with a field emission gun JEOL JEM 2500SE electron microscope operated at 200 kV. EEL spectra were measured on a postcolumn Gatan imaging filter. The probe size was 2 mm, energy dispersion was 0.20 eV/channel, and total acquisition time was 20 s. Hexagonal and cubic BN powders from Sigma-Aldrich and Alfa Aesar, respectively, were used as standards for comparison. All samples were dispersed in ethanol and dropped on copper grids coated with an ultrathin carbon film with a lacey carbon film as support. Results and Discussion The preparation of the AB:nano/BN samples has been previously reported along with data showing that both the hydrogen release temperature and exothermicity of H2 release from AB are lowered in the presence of nano-BN.49 In addition, it was noted that H2 release from AB/nano-BN resulted in less NH3 and more BZ relative to neat AB.49 These results are surprisingly different from AB combined with other solid additives such as SBA-15 and mesoporous carbon.46,55,61 Since nano-BN provides all the advantages of an excellent additive amenable to recycling, gaining insight into the mechanism of release may allow for optimization of the process and ultimately hydrogen release without BZ as a side product with facile regeneration. The Raman spectra of nano-BN, neat AB, and the mixtures of AB/nano-BN (Figure 1) show that BN has only one signal at 1380 cm-1 attributed to B-N stretch, and AB has signals for the B-N stretch (785 cm-1), the B-H stretches (2280 and 2330 cm-1), and the N-H stretches (3316 and 3250 cm-1).62,63

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Figure 3. 1H MAS NMR spectra for 1:1 AB/nano-BN at (a) room temperature for 0 min, and (b-f) at 80 °C for 4, 6, 11, 18, and 28 min, respectively. The MAS 11B and 1H NMR spectra were collected from the same sample by nonconcurrently sampling the 1H and 11B channels. Figure 1. Raman spectra for (a) nano-BN, (b) AB/nano-BN 1:1, (c) AB/nano-BN 1:4, and (d) AB.

BH2 groups from polyaminoboranes (PAB, (H2NBH2)x) begin to grow. PAB forms during the first step of dehydrogenation of neat AB according to the equation32,49,66,67

H3NBH3 f (H2NBH2)x + H2

Figure 2. 11B MAS NMR (18.1 T) spectra for 1:1 AB/nano-BN at (a) room temperature at 0 min, and (b-f) at 80 °C for 2, 5, 8, 19, and 30 min, respectively. The MAS 11B and 1H NMR spectra were collected from the same sample by nonconcurrently sampling the 1H and 11B channels.

Also, 1:1 AB/nano-BN shows a combination of peaks for nanoBN and AB without new stretches or significant changes in frequency, and 1:4 AB/nano-BN shows signals for nano-BN and very weak signals for AB. The 1:4 AB/nano-BN sample shows the stretch for B-N at 1380 cm-1 and very broad peaks associated with AB. These spectra are consistent with previously reported X-ray diffraction data of AB/nano-BN, which showed these samples to be physical mixtures of the two components.49 The 11B MAS NMR spectrum for an 1:1 AB/nano-BN heated in situ at 80 °C is shown in Figure 2. The 11B MAS NMR spectrum of 1:4 AB/nano-BN heated in situ at 60 °C is very similar and is available in the Supporting Information. In both data sets, the initial spectrum was taken at room temperature, and the decomposition was monitored as a function of time at 80 °C for the 1:1 AB/nano-BN and at 60 °C for the 1:4 AB/ nano-BN. At room temperature, there are signals for hexagonal BN (30 ppm),64 AB (-22 ppm), and the diammoniate of diborane [DADB, (NH3BH2NH3)(BH4)] (-36 and -13 ppm).59 The BH2 moiety of DADB at -13 ppm is very weak.59 The NMR spectrum obtained after 2 min at 80 °C show that the amount of DADB increases and the signal for AB sharpens indicating a more rapid rotation of the AB molecules.65 After 5-8 min, DADB and the AB peaks are the major peaks, which is similar to the dehydrogenation of neat AB reported previously.59,65 Also new peaks between -5 and -15 ppm corresponding to BH and

(90 - 120 °C)

For the subsequent 13-30 min, the resulting NMR spectra show that the intensity of peaks associated with DADB and AB decreases, and the PAB resonances broaden. The NMR resonance for nano-BN at 30 ppm remains unchanged during the dehydrogenation of AB. The dehydrogenation of AB involves three steps, induction, nucleation, and growth, as previously reported.59 In the nucleation period the dihydrogen bonding is disrupted and a mobile AB phase is formed.59 This mobile form of AB forms DADB in the nucleation step. Then in the growth step, the mobile AB and DADB react to produce the observed polyamino and polyimino boranes. If a small amount of DADB (5%) is added to AB, the induction period of dehydrogenation of AB at 88 °C is greatly reduced.47,59 The presence of DADB in the 1:1 AB/nano-BN sample after ball milling and before heating may be a result of self-heating from mechanically alloying AB with nano-BN or an added stabilizing environment provided by nanoBN, similar to ionic liquids. As DADB is present in the 1:1 AB/nano-BN sample before heating it is not surprising that the dehydrogenation is observed with nearly no induction time as shown previously from volumetric analysis of the H2 released from AB/DADB mixtures.47 However, the presence of DADB does not reduce the dehydrogenation temperature or the exothermicity of hydrogen release from AB. The 1H MAS NMR spectra evolution from 1:1 AB/nano-BN monitored as a function of time at 80 °C (Figure 3) shows that upon heating the two broad peaks for BH3 (0.6 ppm) and NH3 (3.5 ppm) groups in AB sharpen after 4-7 min, and a new resonance at -1.1 ppm grows. The signal at -1.1 ppm is consistent with the [BH4]- moiety of DADB. After 11-18 min, new peaks arise in the 1H NMR as the dehydrogenation proceeds that may be related to the newly formed polyaminoboranes (PAB). These peaks become broader as the heating continues for 24-28 min due to an inhibition of the rotational mobility of the molecules. The species present in 1H MAS NMR spectra of AB/nano-BN are similar to the ones observed upon dehydrogenation of AB at 90 °C. The presence of DADB in the spectrum at room temperature and the lower temperature

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Figure 4. TG/MS data obtained by heating the samples under Ar flow at 1 deg/min to 200 °C for (a) 15N-labeled AB, (b) 15N-labeled 1:1 AB:nano-BN, and (c) 15N-labeled 1:4 AB/nano-BN. TG axis is on the left and MS axis is on the right.

TABLE 1:

15

N-Labeled BZ Results from TG/MS

sample 15

N AB N AB/nano-BN 1:1 15 N AB/nano-BN 1:4 15

sample integrated intensity integrated intensity mass (mg) M ) 80 (×10-9) M ) 83 (×10-9) ratio 3.7 23 21

24.35 80.45 253.78

86.61 256.48 889.98

0.28 0.31 0.29

dehydrogenation compared to neat AB is consistent with previously reported results.49 The previous report on the dehydrogenation of AB/nano-BN suggests that the amount of BZ produced is greater than that of neat AB.49 Although the reason for the larger quantity of BZ could not be determined, some possibilities include the following: (i) a decomposition reaction of AB facilitated by the presence of nano-BN, (ii) hexagonal nano-BN acting as a template to enhance cyclization of the AB decomposition products, or (iii) heat transfer to AB mediated by the interaction with nano-BN. To investigate the reason for larger BZ yields for AB/nano-BN, isotopically enriched AB (15NH3BH3) was synthesized and ball milled with natural abundance nano-BN in a ratio of 1:1 and 1:4 15NH3BH3/nano-BN and compared with neat 15NH3BH3. The TGA traces of 15NH3BH3, 1:1 15NH3BH3/ nano-BN, and 1:4 15NH3BH3/nano-BN from ambient temperature to 200 °C with the MS channel of BZ are shown in Figure 4. The m/e range 80-83 in the MS corresponds to labeled BZ B315N3H6 ) 83, unlabeled BZ ) 80. These data are presented in the Supporting Information. Table 1 summarizes the results obtained for the integrated area under the MS peaks for m/e 80 (unlabeled BZ) and 83 (labeled BZ) and their ratios. Two observations are notable: (i) the isotope ratio of m/e, A[B3N3H6]/

A[15N3B3H6], was the same for both neat AB and the AB/nano-BN samples and (ii) the MS signal for BZ decreases as 1:4 AB/ nano-BN > 1:1 AB/nano-BN > AB, which is consistent with our initial observations. The ratios of the integrated intensities in the MS for the most intense peaks of B3N3H6 (m/e 80) and 15 N3B3H6 (m/e 83) for (15NH3BH3), 1:1 (15NH3BH3):nano-BN, and 1:4 (15NH3BH3):nano-BN are A[B3N3H6]/A[15N3B3H6] ≈ 0.3 in all cases. This means that the BZ released by AB/nano-BN mixtures has no nitrogen that came from BN, but only from AB, as an increase in 14N containing BZ would have been expected from the AB/nano-BN had nano-BN been involved. Since the compounds observed by MAS 11B NMR spectroscopy in the decomposition of the AB or AB/nano-BN are similar, the mechanism for dehydrogenation of AB on the surface on nano-BN is likely similar to that of AB. Therefore, the greater yield of BZ observed for 1:4 AB/nano-BN as compared to neat AB may be due to a surface interaction that increases the rate of BZ formation. To quantify the yield of BZ produced from neat AB or AB/ nano-BN, these samples were heated under an Ar gas purged system and any volatile material was trapped by bubbling through an ether (glyme or tetrahydrofuran). An inert gas flow (3-30 mL/min) was passed through the AB samples loaded into a stainless steel vessel fitted with gas inlet and outlet. The outlet was connected to a flask with a frit that contains an ether (glyme, tetrahydrofuran) cooled with dry ice. The vessel containing AB was heated in a tube furnace at various ramp rates (Table 2) or inserted into a preheated furnace at 150 °C. The hydrogen decoupled 11B NMR spectra obtained from the BZ trapping experiments for the samples AB, 1:1 AB/nanoBN, and 1:4 AB/nano-BN (Figure 5 and the Supporting Information) show as major signals BZ (30 ppm) and AB (-23 ppm). The 11B NMR spectra in Figure 5 have been normalized to the amount of AB present in each sample and show that the amount of BZ increases as the amount of nano-BN increases: 1:4 AB/nano-BN shows more BZ than 1:1 AB/nano-BN. The AB present in the 11B NMR spectra sublimes from the heated sample vessel and dissolves in the trap containing the glyme solution. The 1H coupled 11B NMR spectrum shows the peak for BZ as a doublet (JBH ) 135 Hz) and AB as a quartet (JBH ) 95 Hz) and is provided as Supporting Information. The yields of BZ as a function of reaction conditions are summarized in Table 2. The amount of BZ formed upon heating was calculated as follows: mol % BZ ) (mol BZ)/(mol AB starting material) or mass % BZ ) (mass BZ)/(mass AB starting material). It is interesting to note that when heating AB under isothermal conditions at 150 °C little BZ is observed, 0.5 mol % (1.2 mass %). On the other hand, the yield of BZ increases in the presence of nano-BN for identical reaction conditions. Heating AB/nanoBN (1:1) at ramp rates between 0.5 and 1 deg/ min gives BZ in yields of 2.4-3 mol % (6.5-7.5 mass %) and AB/nanoBN (1:4) at ramp rates from 1 deg/min gives a BZ yield of 4.8 mol % (12.7 mass %). Under isothermal heating at 150 °C the yield of BZ is 1.6 mol % (4.1 mass %). To probe if hexagonal nano-BN is acting as a template to favor cyclization of the AB decomposition products, a 1:4 AB: nanographite sample was prepared by ball milling graphite for 1 h and the resulting powder was ball milled with AB for an additional 30 min, similar to the preparation of AB/nano-BN. Upon heating from 25 to 200 °C at 1 deg/min ramp rate, the amount of BZ trapped from 1:4 AB/nanographite was 0.4 mass %. Under the same conditions, neat AB evolves 5 mass % and AB/nano-BN 1:4 13 mass %. Although the hexagonal sheets of graphite are similar to that of h-BN, nano-BN promotes the

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TABLE 2: BZ Trapping Experiment Resultsa no.

a

sample

1 2 3

AB AB AB

1

AB

1 2 3 4 5

AB:BN 1:1 AB/nano-BN 1:1 AB/nano-BN 1:4 AB/nano-BN 1:1 AB:graphite 1:4

heating scheme

Ar flow (mL/min)

mole % BZ

AB neat, ramped experiments (variables: heating rate and flow rate) 0.5 deg/min to 200 °C 30 1.1 ( 0.5 1 deg/min to 200 °C 30 1.8 ( 0.7 1 deg/min to 200 °C 3 2.2 AB neat, isothermal experiment 150 °C isothermal 30

0.5 ( 0.26

AB/nano, BN samples (variables: nano-BN concentration, heating scheme) 0.5 deg/min to 200 °C 30 2.9 ( 0.7 1 deg/min to 200 °C 30 2.4 ( 0.02 1 deg/min to 200 °C 30 4.8 ( 0.5 150 °C isothermal 30 1.6 ( 0.4 1 deg/min to 200 °C 30 0.2

wt % BZ 2.9 ( 1.4 4.7 ( 1.9 5.8 1.2 ( 0.7 7.5 ( 1.8 6.4 ( 0.07 12.7 ( 1.2 4.1 ( 1.1 0.4

The solvent in the trap is glyme or THF.

Figure 5. 11B solution NMR data obtained on the glyme solution after the trapping borazine experiments for (a) neat AB, (b) 1:1 AB/nanoBN, and (c) 1:4 AB/nano-BN. Proton decoupled spectra are shown. The 1H coupled spectra for BH (borazine) and BH3 (AB) are shown as an inset.

formation of BZ from AB/nano-BN whereas graphite inhibits BZ formation. BZ is formed at lower temperatures from AB/ nano-BN than in neat AB as observed by TGA/DSC/MS (Figure 4). These observations are consistent with the relative yields of BZ in the isothermal experiment at 150 °C, in which AB releases 0.5 mol % BZ and AB/nano-BN 1:1 releases 1.6 mol % BZ as measured by 11B NMR spectroscopy. Figure. 6 shows HRTEM and SAD images for nano-BN and 1:1 AB/nano-BN sample after dehydrogenation. The nanosize grains with turbostratic stacking caused by high energy ball milling are observed in the HRTEM images of nano-BN and 1:1 AB/nano-BN. The corresponding B-K edge and N-K edge core-loss EELS of the nano-BN, AB/nano-BN, and standards hexagonal BN and cubic BN are presented in Figure. 7. Both AB and AB/nano-BN show the characteristic π* peaks of hexagonal BN in B-K and N-K edges corresponding to the transition of the 1s electrons to the empty π* orbitals in sp2hybridized materials. The different intensity ratio of π* and σ* is due to the loss of crystallinity, crystal orientation, and the number of dangling bonds. A meaningful mathematical treatment would require a large number of sample points for analysis.68,69 The amorphous C-K edge at 290-300 eV shown in the spectrum of dehydrogenated 1:1 AB/nano-BN is not always present, and can be attributed to the carbon contamination from the pump during the EELS study. Summary A detailed characterization of the solid and volatile dehydrogenation products of mechanically alloyed AB/nano-BN

Figure 6. HRTEM and SAD (inset) for (a) nano-BN and (b) dehydrogenated 1:1 AB:nano-BN samples.

mixtures has been presented. Dehydrogenation of AB/nano-BN produces similar compounds as that reported for neat AB. Ball milling AB with nano-BN forms small amounts of DADB and

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B NMR data showing the coupled and decoupled spectra obtained from the glyme solutions used to trap borazine, and AB/nano-BN 1:4 at 60 °C, and a picture of the experimental setup used to trap borazine. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes

Figure 7. (a) B-K edge and (b) N-K edge EELS of hexagonal BN, cubic BN, nano-BN and 1:1 AB:nano-BN.

this has been reported to reduce the induction period for hydrogen release. BZ released from AB and AB/nano-BN mixtures has been quantified by 11B NMR spectroscopy by trapping the volatiles in ether solutions. The quantification confirmed that the amount of BZ is slightly higher for the 1:1 AB/nano-BN as compared to AB while the 1:4 AB/nano-BN gives double the BZ compared to AB. The ramp rate has little effect on the amount of BZ produced from neat AB and the 1:1 AB/nano-BN sample. The amount of BZ produced from all of the samples under isothermal conditions at 150 °C was found to be lower. The TG/MS experiments on 15N-labeled AB indicate that BZ is produced from AB and not by reaction with nano-BN. These results suggest that the mechanism of hydrogen release and BZ production is affected by interaction of AB with the nano-BN surface. It is possible that the ring structure of BZ is favored by slow heating rates and interaction with the nanoBN surface. These interactions lead to lower temperatures and exothermicity of hydrogen release suggesting that a high surface area material like nano-BN may provide an optimal support for AB dehydrogenation. Acknowledgment. This research was funded by the U.S. Department of Energy Office of Energy Efficiency and Renewable Energy as part of the Chemical Hydrogen Storage CoE. The microscopy research was supported by DOE DE-FG0203ER46057. MAS NMR studies were performed using EMSL, a national scientific user facility sponsored by the Department of Energy’s Office of Biological and Environmental Research located at Pacific Northwest National Laboratory (PNNL). PNNL is operated for the U.S. DOE by Battelle. Supporting Information Available: Spectra of MS data for labeled 15N neat AB, labeled AB/nano-BN 1:1 and 1:4 samples,

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