Promotion of Hydrogen Release from Ammonia Borane with

Dec 29, 2008 - E-mail: [email protected] (S.M.K.); [email protected] (T.A.)., † ... Young Joon Choi , Yimin Xu , Wendy J. Shaw , and Ewa C. ...
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J. Phys. Chem. C 2009, 113, 1098–1103

Promotion of Hydrogen Release from Ammonia Borane with Mechanically Activated Hexagonal Boron Nitride Doinita Neiner,† Abhijeet Karkamkar,‡ John C. Linehan,‡ Bruce Arey,‡ Tom Autrey,*,‡ and Susan M. Kauzlarich*,† Department of Chemistry, One Shields AVenue, UniVersity of California, DaVis, California 95616, and Pacific Northwest National Laboratory, Richland, Washington 99352 ReceiVed: October 2, 2008; ReVised Manuscript ReceiVed: NoVember 16, 2008

Nanoscale hexagonal BN additive for ammonia borane, AB, is shown to decrease the onset temperature for hydrogen release. Both the nano-BN and the AB:nano-BN samples are prepared by ball milling. The materials are characterized by X-ray powder diffraction, 11B muclear magnetic resonance, thermogravimetric analysis, differential scanning calorimetry, and mass spectrometry, and the hydrogen release is measured by a volumetric gas burette system. Several effects of the mixtures of AB:nano-BN are shown to be beneficial in comparison with neat AB. These are the decrease of the dehydrogenation temperature, the decrease in NH3 formation, as well as the decrease of the exothermicity of hydrogen release with increasing the nano-BN concentration. Introduction An energy economy based on hydrogen may prove to be one answer to growing concerns of increasing energy independence and decreasing air pollution and greenhouse gas emissions. Hydrogen storage is a key enabling technology for the advancement of hydrogen and fuel cell power technologies in transportation, stationary, and portable applications.1 Ammonia borane (NH3BH3, AB), 19.6 wt % of hydrogen, has received increased attention because of its potential use as a hydrogen storage media.2 There are two distinct approaches to releasing hydrogen from AB: (1) solvolysis,3-5 which leads to borate esters, and (2) thermolysis,6-10 which leads to polyborazylene. While solvolysis takes place at room temperature in the presence of a catalyst or an acid, thermolysis of neat AB requires higher temperatures, above 85 °C, as well as the presence of additives to increase the rate of hydrogen release and decrease the dehydrogenation temperature.7,8,11,12 Neat AB decomposes upon melting at 114 °C, vigorously evolving hydrogen gas. Dehydrogenation from AB can occur at lower temperatures, below 100 °C, although at slower rates. Mild conditions have been reported that use silica scaffolds,13 metal catalysts,4,14 and ionic liquids.7 The dehydrogenation of AB in ionic liquids occurs at 95 °C with the release of 1.6 equiv of hydrogen in 22 h.7 The use of a 1:1 mixture of AB and mesoporous silica SBA-15, as a scaffold, enabled the release of 1 equiv of hydrogen at temperatures as low as 50 °C in 85 min.13 Recently it has been reported that addition of trace quantities of diamoniate of diborane ([NH3BH2NH3][BH4] DADB), a product of AB isomerization, to neat AB significantly reduces the induction time or onset temperature at which hydrogen is released.15 Nanophase boron nitride (nano-BN) additives to AB could provide a similar role as DADB and serve as a scaffold: both of which have been reported to decrease the onset temperature of H2 release. Furthermore, BN does not introduce any additional elements that are not already present in the composition of * To whom correspondence should be addressed. E-mail: smkauzlarich@ ucdavis.edu (S.M.K.); [email protected] (T.A.). † University of California, Davis. ‡ Pacific Northwest National Laboratory.

ammonia borane and, therefore, may facilitate the off-board regeneration. Specifically, hexagonal boron nitride (h-BN) may provide a solid support that promotes hydrogen release by chemically interacting with AB to lower the hydrogen release temperature and reducing induction time and has the advantage that it would not require any additional concerns for the regeneration of AB currently under investigation.16-18 Herein, we present the activation of hydrogen release from AB by the presence of mechanically activated h-BN. Hydrogen is released at lower temperatures, and the dehydrogenation is less exothermic than with neat AB. This approach provides all the benefits of a lightweight scaffold without additional foreign contamination that might complicate AB regeneration. Experimental Section 1. Synthesis. Microcrystalline h-BN 99% from Aldrich was used as received. All of the manipulations, except ball milling, were performed in a nitrogen-filled drybox. The ball milling was performed in a 3/4 in. diameter and 21/8 in. long tungsten carbide lined vial with a methacrylate center cylinder and two tungsten carbide 5/16 in. balls. The h-BN was ball milled separately for 1 h in a Spex 8000 ball mill and characterized by powder X-ray diffraction. This material will be referred to in this paper as nano-BN. Three samples were prepared with the weight percentage of AB in the mixture of AB and nanoBN of 80 wt % (referred to as 4:1), 50 wt % (referred to as 1:1), and 20 wt % (referred to as 1:4). Typically, 880 mg of AB and 220 mg of nano-BN powder for the 4:1 sample, 220 mg AB and 220 mg nano-BN for the 1:1, and 98 mg AB and 406 mg nano-BN for the 1:4 were weighed and loaded in the ball mill vial in the drybox. The ball mill vial was covered with Parafilm prior to removing it from the drybox. The ball milling was then performed in air for 30 min. These three samples were reproduced three times with similar results. Two more samples were prepared by adding AB to nano-BN in THF and then removing the solvent under dynamic vacuum. The wet chemistry did not lead to a 1:1 AB:nano-BN but to ratios of 2:1 and 4:1 AB:BN. Regardless of amounts of starting AB, higher loadings

10.1021/jp8087385 CCC: $40.75  2009 American Chemical Society Published on Web 12/29/2008

Promotion of Hydrogen Release from AB of BN were not accomplished. The results for these samples were similar to those with the same ratio obtained by ball milling. 2. Characterization. X-ray powder diffraction studies were carried out on a Philips X’Pert System (Cu KR radiation λ ) 1.5418 Å) equipped with a graphite monochromator. Data were collected in a continuous scan mode between 20 and 60° and step size 0.02 2θ. The size of the nano-BN nanocrystalline domains for all of the samples was determined after separation from AB in order to remove the contribution from overlapping peaks. Instrument parameters were determined with h-BN. Domain size was calculated from the diffraction pattern using the profile fit incorporated in the Jade MDI program package. Adsorption isotherms of nitrogen at 77 K were measured using a QuantaChrome Autosorb-6 surface area and porosity analyzer. Before each gas adsorption measurement, the samples were degassed at 100 °C overnight in a QuantaChrome Autosorb degasser. The surface area measurement was performed only on the 1:1 sample after heating with 1 K/min to 200 °C (after dehydrogenation) and on the nano-BN after ball milling. A gas burette system based on a standard hydrides test glassware kit (Sigma-Aldrich, model Z516198-1KT-A) equipped with automated data acquisition module was used to measure the volume of hydrogen gas released from the AB:BN mixtures. The flask containing the mixture was immersed in a mineral oil bath heated at 90 and 150 °C, respectively. The 250-mL graduated burette for the gas displacement measurements contains a light mineral oil. The absolute volume of gas was determined from the fluid displacement.19 Thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and mass spectrometry (MS) were performed on a TGA/DSC STA 449 Jupiter Netzsch instrument equipped with an Aelos QMS 403C MS by heating the samples under flowing argon. The MS uses a standard electron impact ionization detector. Typically, the powders were loaded in alumina crucibles in the drybox. The samples were brought under inert atmosphere and loaded in the TGA/DSC/MS instrument. The data were obtained by heating the samples under Ar gas from room temperature to 200 °C at 1 K/min. Bloch decay solid state 11B{1H} magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectra were collected on a Varian VXR-300 operating at 96 MHz with a 3 µs 90° and a 5 s recycle time. The samples were packed in 5-mm rotors under an inert atmosphere for use in the Varian MAS probe with a spinning speed of 8.5 KHz. An external chemical shift standard of BF3-etherate, 0 ppm, was used. The 11B{1H} MAS NMR spectra for the nano-BN and 1:1 sample are provided in Supporting Information. Results and Discussion Boron nitride in the form of nanocrystalline domains (nanoBN) was prepared by ball milling commercial h-BN powder for 1 h. Figure 1 compares the X-ray powder diffraction data and the SEM data for nano-BN with the 1:1 mixture of nanoBN ball milled in the presence of AB. The broad peaks in the diffraction patterns are consistent with h-BN that has nanosized grains, and the 1:1 AB:nano-BN can be fully indexed as a mixture of pure phase AB and nano-BN. The asymmetric broadening of the Bragg peaks in the X-ray powder pattern is caused by lattice defects such as shearing of lattice planes, Frank dislocations, and stacking faults. These defects are introduced by ball milling BN for times approaching 1 h. Longer ball milling times (180 h) are reported to produce amorphous BN.20 All the mixtures of AB:nano-BN showed similar diffraction

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Figure 1. X-ray powder diffraction data with SEM images (insets 1 and 2) for (a) BN ball milled for 1 h, nano-BN and (b) 1:1 AB:nanoBN sample. In the X-ray powder diffraction data nano-BN is an asterisk and AB is a triangle.

patterns with no new diffraction peaks. The size of the nanoBN grains after ball milling was calculated from the X-ray powder diffraction data to be 10.8(1) nm. The size of the nanoBN grains after ball milling with AB was calculated to be 10.7(1) nm for the 4:1 sample, 9.5(3) nm for the 1:1 sample, and 9.8(1) nm for the 1:4 sample. Consistent with this grain size, the surface area of the nano-BN was measured by the Brunauer-Emmett-Teller method to be 251 m2/g. The surface area of 99% h-BN is reported to be 12 m2/g,21 and hence there is an approximately 20-fold increase in the surface area that occurs by ball milling. The insets in Figure 1 show the scanning electron microscopy (SEM) data corresponding to nano-BN and the 1:1 AB:nano-BN sample. The SEM images for the ball milled nano-BN are consistent with the X-ray data and show that there are well-defined grains of the order of approximately 10 nm as well as of nanodomains and agglomerates of larger size, indicating that ball milling induces a defective, nanocrystalline state.20 By comparison of part a with part b of Figure 1, one can see that AB fully covers the surface of nano-BN so that the image (Figure 1b) shows larger features. This observation is consistent with the surface area measurement of the 1:1 AB:nano-BN sample, which is reduced to 23.6 m2/g compared with the nano-BN. The size of the BN grains after heating the 1:1 sample at 200 °C is slightly smaller, 8.7(1) nm, indicating that there is no sintering of the sample with heating. Table 1 presents a summary of the ball milling time, nano-BN grain size, TG weight loss, onset temperatures, and heat released from all of the samples used in this study. In Table 1, the TGA and DSC data has been normalized to the amount of AB in the sample by multiplying the raw weight loss by 1.2 for the 4:1, 2 for the 1:1, and 5 for the 1:4 sample. The TGA/DSC results for the AB:nano-BN samples presented in Table 1 are an average of three TGA/DSC measurements performed on three different batches of composition 4:1, 1:1, and 1:4 AB:nano-BN. The dehydrogenation of AB occurs in three steps that are generally represented by the transformation of AB to polyaminoborane (PAB, (H2NBH2)x) and H2, according to eq 1, PAB to polyiminoborane (PIB, (HNBH)x) and H2, represented by eq 2, and PIB to BN and H2, according to eq 310,16,22

H3NBH3 f (H2NBH2)x+H2 (H2NBH2)x f (HNBH)x+xH2 (HNBH)x f BN + xH2

90-120°C

(1)

120-160°C

(2)

well above 500°C

(3)

Figure 2 presents the TGA/DSC/MS (hydrogen and borazine only) data for the neat AB and for the 4:1, 1:1, and 1:4 AB:

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TABLE 1: Sample Preparation and Characterization Data for the Materials Used in this Studya sample AB wt % BN wt % diameter of nano BN (nm) SA (m2/g) milling time (min) onset (1) °C ∆H 1 (kJ/molAB) onset (2)°C ∆H 2 (kJ/molAB) normalized weight loss

neat AB 100

114 21.6 151 33.3 16

4:1 80 20 10.7(1) 30 99.5 ( 3.2 16.1 ( 4.4 137.5 ( 8.8 4.4 ( 2.9 37.9 ( 15.8

1:1 50 50 9.5(3) 23 30 89.6 ( 3.3 15.1 ( 0.7 134.4 ( 8.7 1.4 ( 0.4 45 ( 1.7

1:4 20 80 9.8(1) 30 70.6 ( 5.5 5.7 ( 5.6 120.5 ( 8.7 2.2 ( 0.6 30.3 ( 5.5

nano-BN

10.8(1) 251 60

a SA ) surface area; onset (n) ) temperature at which the nth equivalent of H2 is released; ∆H n ) dehydrogenation for the nth equivalent of H2 released.

nano-BN samples. The TGA data presented in Figure 2a show that the weight loss (e.g., hydrogen release) starts at lower temperatures in the AB:nano-BN samples as compared to neat AB. The normalized weight loss, presented in Table 1, increases with the addition of nano-BN to AB with the initial value of 16% for neat AB increasing to 37.9 ( 15.8% for the 4:1 AB: nano-BN to 45 ( 1.7% for 1:1 AB:nano-BN and 30.3 ( 5.5% for 1:4 AB:nano-BN. The large standard deviation for the 4:1 sample may be a result of inhomogeneities of that sample loading, presumably due to insufficient nano-BN interface for the amount of AB. Figure 2b presents the normalized DSC data for neat AB and the 4:1, 1:1 and 1:4 AB:nano-BN samples. The onset for the release of the first equivalent of H2, AB to PAB (eq 1), decreases with increasing nano-BN content (i.e., it is 114 °C for neat AB, 99.5 ( 3.2 °C for 4:1, 89.6 ( 3.3 °C for 1:1, and 70.6 ( 5.5 °C for the 1:4 sample). Also, the normalized heat release associated with the hydrogen release, eq 1, appears to decrease with increasing nano-BN content. The integrated area under the first DSC peak is 21.6 kJ/mol for neat AB, 16.1 ( 4.4 kJ/mol for the 4:1, 15.1 ( 0.7 kJ/mol for the 1:1 AB: nano-BN sample, and 5.7 ( 5.6 kJ/mol for the 1:4 AB:nanoBN sample. The second exotherm, corresponding to the second dehydrogenation step (i.e., PAB to PIB, eq 2) is also shifted to lower temperatures for the ball milled samples of AB:nanoBN as compared to neat AB. The effect of ball milling AB with the nano-BN on this second exotherm is illustrated in the inset of Figure 2b. This second exotherm onset is at 151 °C in neat AB and shifts to 137.5 ( 8.8 °C in the 4:1, to 134.4 ( 8.7 °C in the 1:1 AB:nano-BN sample, and 120.5 ( 8.7 °C in the 1:4 AB:nano-BN sample. The heat release calculated from the integrated area under the second DSC peak is 33.3 kJ/mol for the neat AB, 4.4 ( 2.9 kJ/mol for the 4:1, 1.4 ( 0.4 kJ/mol for the 1:1, and 2.2 ( 0.6 kJ/mol for the 1:4 AB:nano-BN samples. Parts c and d of Figure 2 present the normalized MS signals for hydrogen and borazine. The MS signals have been normalized to the amount of AB in the neat AB for each of the 4:1, 1:1, and 1:4 AB:nano-BN samples by multiplying the MS signal times [mneat AB (mg)/mAB in the AB:nano-BN sample (mg)]. In addition, the MS signal for both hydrogen and borazine was divided by 10-10. As one can see from Figure 2c, the first equivalent of hydrogen is released at lower temperatures as compared to neat AB, with increasing the nano-BN concentration. Figure 2d presents the MS normalized signal for borazine. The addition of nano-BN eliminates the first release of borazine and shifts the second one to lower temperatures concurrent with release of the second equivalent of hydrogen (e.g., PAB to PIB (eq 2)). However, the borazine signal appears to increase with increasing the nano-BN concentration consistent with the greater observed mass loss when the second equivalent of hydrogen is desorbed. While a reaction of BN with AB to form borazine is

not expected, there is some precedent from theory that suggests that (BN)12 clusters react with hydrogen to form B12N12H24, which has the same stoichiometry as borazine (B3N3H6).23 The higher concentration of borazine is in direct contrast to AB embedded in mesoporous silica or carbon aerogels where the borazinesignaldecreasesrelativetotheneatABdecomposition.13,24,25 To better understand these results, volumetric gas burette measurements at 90 and 150 °C along with subsequent 11B{1H} MAS NMR were performed, and the results are presented in Figure 3. In addition, for comparison purposes, a neat AB sample has been also measured under the same conditions (e.g., 90 and 150 °C), and the data are presented in Figure 3. As one can see from Figure 3a, approximately 0.96 equivalents of hydrogen evolve at 90 °C from the 1:1 sample, as compared to 1 mol equiv. when neat AB decomposes at 90 °C.15 In addition to the amount of hydrogen released, the concentration of ammonia produced in the decomposition of 1:1 AB:nano-BN at 90 °C was measured with a Draiger tube to be about 20 ppm. The inset presents the 11B{1H} MAS before and after this measurement. The peak at -36 ppm is consistent with DADBlike products previously observed in the decomposition of solid and solution neat AB.7,10,26 The broad feature around -24 ppm can be attributed to BH3 structures, such as unreacted AB and terminal BH3 species in the PAB oligomers. The peaks centered around -16 ppm can be assigned to BH2 species in the PAB oligomers. The other downfield peaks may result from the corresponding BH species in the PAB oligomers and further downfield to sp2 boron species. Figure 3b shows the volumetric burette measurement at 150 °C for both neat AB and a 1:1 AB:nano-BN sample. The inset presents the 11B{1H} MAS before and after the measurement. As shown in Figure 3b, ca. 1.9 equiv. of hydrogen desorb from the 1:1 AB:nano-BN sample at 150 °C; this is comparable to the quantity of hydrogen observed from neat AB after the first 15 min at the same temperature. At 150 °C and longer reaction times, the neat AB desorbs 2.6 equiv of hydrogen, while the 1:1 sample remains constant at 1.9 equiv. The 11B{1H} MAS NMR spectrum after heating at 150 °C (Figure 3b inset) shows the presence of BN and a very broad feature at around -20 ppm. The concentration of ammonia produced in the decomposition of 1:1 AB:nano-BN at 150 °C is cca. 20 ppm as measured with a Draiger tube, in contrast to ca. 200 ppm ammonia from neat AB, which releases 2.6 equiv of hydrogen at 150 °C after 900 min. Thus the addition of nano-BN to AB reduces the amount of ammonia released in the decomposition of AB. Figure 3b shows that neat AB yields ∼2.6 equiv of hydrogen, which corresponds to ∼16% weight loss observed for the neat AB in the TGA experiment. In contrast, the 1:1 AB:nano-BN, also presented in Figure 3b, shows that this sample yields ∼1.9 equiv of hydrogen, which correspond to

Promotion of Hydrogen Release from AB

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Figure 3. Hydrogen release from AB and AB:nano-BN 1:1 as measured by a gas burette system at (a) 90 and (b) 150 °C. The insets are the 11B MAS NMR before and after the burette measurement at that temperature.

Figure 2. (a) TGA, (b) DSC (inset shows an expanded view), (c) MS hydrogen signal, and (d) MS borazine signal for neat AB (black), 4:1 AB:nano-BN (red), 1:1 AB:nano-BN (blue), 1:4 AB:nano-BN (green). DSC and MS traces have been normalized for the amount of AB.

∼12% mass loss. However, the TGA data (Figure 2a) show a 24% weight loss, which corresponds to a normalized to AB weight loss of 48% (the sample is 1:1 AB:BN). The only other significant volatile product observed in the MS is borazine, which occurs at the second hydrogen release. Thus, if one

assumes that the mass difference results from borazine formation, the yield in borazine is ca. 33%. It has been reported that ball milling titanium metal powder with h-BN stimulates hydrogen adsorption and also reduces the adsorption temperature. It was proposed that the milling process promotes the interaction between the hydride forming metal, TiHx, and hydrogen by making an oxide-free high surface area material. In addition, ball milling titanium with h-BN significantly increases the hydrogen adsorption rate and greatly reduces the adsorption temperature from 471 to 323 °C.21 This effect was attributed to the presence of new types of occupation sites in the Ti lattice described as interface-located active sites with incorporated nitrogen. A similar type of activation also may be hypothesized for AB:nano-BN ball milled mixtures: as the nanoBN content in the mixture is increased, there will be a greater proportion of AB at the nano-BN interface. Given that there is a steady decrease in hydrogen release temperature with increasing nano-BN content, it may suggest that the interface between nucleation sites on the nano-BN and AB are important to lowering the temperature for hydrogen release. It is possible that nano-BN surface disrupts the dihydrogen bonding, B-H · · · H-N, present in neat AB and, therefore, lowers the induction period for AB dehydrogenation. Alternatively, the “heating” of the samples prepared under the ball milling conditions (e.g., 30 min in a high energy ball mill) could result in the introduction of nucleation sites (i.e., similar to the formation of DADB in the neat AB that reduces the induction period).10,15 It is possible that DADB formation is induced by ball milling but cannot be discerned by low-field 11B{1H} MAS NMR. In previous work, it was shown that disruption of the

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SCHEME 1: Enchanced Cyclization Promoted on the Nano-BN Surface

dihydrogen bonding facilitates isomerization of AB to DADB. Once the DADB is formed in the presence of AB, the release of H2 is initiated.15 The apparently higher borazine yields can be explained by proposing three possible mechanisms: (1) surface-enhanced cyclization, (2) base-initiated cyclization, and (3) acid-initiated cyclization. There are at least two competing pathways: (a) ionic polymerization and (b) cyclization. It may be that the surface of nano-BN facilitates the cyclization pathway, as shown in Scheme 1. It has recently been reported that ball milling can induce dangling bonds in BN. It has been shown that such dangling bonds trap ammonia and hydrogen to form BN-NHx and BN-Hx.27 If that is the case than one can imagine that those BN-NHx could act as basic sites, which can react with DADB to produce cycloborazanes that are the precursors for borazine.28 Alternatively, the B site in nano-BN can act as a Lewis acid.29 This site can catalyze the dehydrogenation of AB leading to cyclic products.30 Cyclotriborazene (CTB), indicated in Scheme 1, is not stable at these temperatures and forms borazine (BZ), which is observed by MS. The combination of the disruption in bonding and addition of nucleation sites may provide an overall effect of lowering the dehydrogenation temperature but would be expected to have little effect on the exothermicity of hydrogen release. For the first hydrogen release, PAB oligomers should be produced, and if PAB oligomers are destabilized at the interface relative to the stability of AB, then the formation of PAB would be expected to be less exothermic. Recent theoretical calculations show that coiling of the oligomers increase the stability of the PAB products.31 If there is less coiling of PAB on the nanoBN support, there may be less heat released in the formation of PAB. It is also interesting to contrast the slightly lower yield of hydrogen desorbed from the AB:nano-BN mixtures compared to the neat AB, specifically at longer reaction times where there may be more cross-linking between PAB oligomers. Consequently, if there is less coiling in the PAB products formed at the interface of nano-BN then there may be lower probability of self-cross-linking reactions and fewer cross-linking reactions that would lower the yields of hydrogen release, consistent with what is observed in the AB:nano-BN mixtures. The use of nano-BN as a scaffold for AB may have advantages over the use of mesoporous silica scaffolds. We have shown that the spent AB products can be washed away from the nano-BN preserving the high surface area of the nano-BN

support and simple filtration provides the spent fuel for regeneration. BN does not contaminate the sample with additional elements not present in AB, such as Li or Si compounds. This provides advantages for regeneration by eliminating additional separation steps (steps that will cost energy and affect the overall efficiency) that would become imminent if one has in mind regeneration of the spent fuel. At the same time, the fact that BN maintains its nanodomains after heating at 200 °C (does not sinter) suggests that it may be reused. Moreover, the fact that BN is chemically stable suggests that it could resist the digestion and regeneration processes virtually intact, and therefore it would only need to be added once, e.g., at the initial fill. The fact that the hydrogen release is less exothermic for the 4:1, 1:1, and 1:4 samples as compared to neat AB (see Table 1) may provide a lower energy pathway to regenerate the AB, assuming that the mechanism can be deduced. In addition, the use of ball milling over wet chemistry as in the case of silica scaffolds has the advantage of using dry powders, thus eliminating possible solvent residues. Further work using high-field NMR should provide more insight into the changes in chemical and physical properties of hydrogen release from AB on nanoBN supports. Conclusions Ball milled mixtures of AB:nano-BN modify the properties of hydrogen release from AB. In addition to decreasing the reaction enthalpy for H2 release from AB, both the onset temperature for H2 release and the concentration of the byproduct ammonia are significantly decreased relative to neat AB. Increased amounts of borazine generation relative to other additives are observed and a mechanism for the formation is proposed. Several beneficial effects that pertain to the dehydrogenation properties of the mixtures of AB:nano-BN are notable, such as the decrease of the dehydrogenation temperature, the decrease in NH3 formation, as well as the decrease of the exothermicity of hydrogen release with increasing the nano-BN concentration. The lower exothermicity of hydrogen release from AB in the nano-BN may result from physical and chemical interactions between the PAB oligomers and the interface of the BN support. Acknowledgment. The authors thank the U.S. Department of Energy′s Center of Excellence for Chemical Hydrogen

Promotion of Hydrogen Release from AB Storage for funding. A portion of the research described in this paper was performed in the W.R. Wiley Environmental Molecular Sciences Laboratory, a national scientific user facility sponsored by the Department of Energy′s Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Pacific Northwest National Laboratory is operated for the DOE by Battelle. Supporting Information Available: MAS Spectra of the MAS 11B{1H} NMR for the nano-BN after ball mill and AB: nano-BN 1:1 sample and X-ray powder diffraction for all samples are available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) G. C. M. Dresselhaus; Buchanan, M. National Research Council and National Academy of Engineering, committee on AlternatiVes and Strategies for Future Hydrogen Production and Use; The Hydrogen Economy: Opportunities, Costs, Barriers and R&D Needs, 2004. (2) Marder, T. B. Angew. Chem., Int. Ed. 2007, 46, 8116. (3) Chandra, M.; Xu, Q. J. Power Sources 2006, 156, 190. (4) Cheng, F. Y.; Ma, H.; Li, Y. M.; Chen, J. Inorg. Chem. 2007, 46, 788. (5) Chandra, M.; Xu, Q. J. Power Sources 2007, 168, 135. (6) Bowden, M.; Autrey, T.; Brown, I.; Ryan, M. Curr. Appl. Phys. 2008, 8, 498. (7) Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon, L. G. J. Am. Chem. Soc. 2006, 128, 7748. (8) Hu, M. G.; Geanangel, R. A.; Wendlandt, W. W. Thermochim. Acta 1978, 23, 249. (9) Baitalow, F.; Baumann, J.; Wolf, G.; Jaenicke-Rossler, K.; Leitner, G. Thermochim. Acta 2002, 391, 159. (10) Stowe, A. C.; Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T. Phys. Chem. Chem. Phys. 2007, 9, 1831. (11) Hu, M. G.; Vanpaasschen, J. M.; Geanangel, R. A. J. Inor. Nucl. Chem. 1977, 39, 2147.

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