CO2-Enhanced Thermolytic H2 Release from Ammonia Borane - The

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CO2-Enhanced Thermolytic H2 Release from Ammonia Borane Junshe Zhang, Yu Zhao, Daniel L. Akins, and Jae W. Lee* Department of Chemical Engineering and Department of Chemistry, The City College of New York, New York, New York 10031, United States

bS Supporting Information ABSTRACT: This work presents the accelerated dehydrogenation of ammonia borane (AB) at 85 °C under carbon dioxide atmosphere. The presence of CO2 significantly enhances the thermolysis kinetics, and the hydrogen release becomes faster as the CO2 pressure increases from 1.70 to 3.67 bar. Around 4 bar of CO2, 1.33 equiv of hydrogen is released from AB in 1 h. The accelerated AB thermolysis is due to CO2 reacting with AB, or intermediates like diammoniate of diborane (DADB), or primary thermolysis products including linear dimers of aminoborane (LDAB), which accelerates AB thermolysis, and this reaction has been confirmed by ATR-FTIR, Raman, and 11B and 13C solid-state MAS NMR measurements.

1. INTRODUCTION Ammonia borane (AB, NH3BH3) is a promising on-board, solid-state hydrogen storage material.1 Desorption of hydrogen from AB is usually achieved via hydrolysis,2,3 methanolysis,4
6 or thermolysis.7
9 The thermolysis of AB involves three sequential steps occurring around 110, 150, and >500 °C, with 6.5 wt % of hydrogen (with respect to the initial mass of AB) liberated in each step.1 A low dehydrogenation rate for temperatures around 85 °C (the working temperature of polymer electrolyte membrane fuel cell) makes pristine AB infeasible as an on-board hydrogen storage medium. Several approaches have been discussed to enhance the thermolysis kinetics of AB: for example, compositing AB with mesoporous materials like SBA-15,10 carbon cryogel (CC),11 carbon frameworks,12 and metal organic frameworks (MOFs);13 dispersing AB in ionic liquids;14,15 catalyzing over transition metals;16
18 and adding chemical promoters like NH4Cl and LiH.19,20 One major drawback associated with the first two methods is the low hydrogen storage capacity, as a result of the mass fraction of mesoporous materials and ionic liquids being usually very high. The latter two approaches have the drawback of requiring expensive materials, except for NH4Cl-doping, which is a less effective option as compared to the others.10,14,20 As a result, the search continues to find cheap and effective promoters for hydrogen release without negatively impacting the storage capacity. In this work, we present the promoting effect of CO2 on the thermal decomposition of solid AB at 85 °C. We have found that CO2 significantly enhances the rate of AB thermolysis and can result in 8.63 wt % of hydrogen (with regard to the initial mass of AB) generated in 1 h. Also, we have observed that the H2 promoting effect is dependent on the pressure of CO2. We interpret our observations based on both macroscopic and microscopic data including in situ investigations of the interaction between CO2 and AB. r 2011 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Materials. Ammonia borane (AB) with a purity of 97% was obtained from Sigma-Aldrich. Ethane with a purity of >99% was supplied by Praxair. CO2 with a purity of >99.8% was purchased from T. W. Smith. Boric acid with a purity of 99.999% (metals basis) was purchased from Sigma-Aldrich. All chemicals were used as received, without further purification. Moisture and some CO2 were removed from compressed air by a moisture trap (LGMT-4-HP, Agilent). Polyaminoborane (PAB) was prepared by heating AB from room temperature up to 115 °C at 1 °C/min under 1 bar of air. 2.2. Thermal Kinetics. Thermolysis was carried out on a highpressure Micro-DSC VII (SETARAM). A detailed description of this equipment has been provided in our prior publication.21 The total volume of the sample cell and pipelines with fittings was 6.38 mL. Most of the pipelines and fittings were exposed to normal ambient conditions, while only the cells were subject to the heating and cooling cycles. Pressure was monitored using a CET9005GY7 pressure transducer (SensorTechnics, 0
5 bar). After 15.5 mg of AB was loaded into the sample cell, the cell was evacuated, purged twice with CO2 or air, and then evacuated again using a 25 mL syringe. This was followed by loading CO2 or air to a preset pressure at room temperature. The sample and reference cells were maintained at 25 °C for 5 min, then they were heated to 85 °C at 1 °C/min, followed by maintaining at this temperature for 3 h. Finally, the cells were cooled to 25 °C at 3 °C/min. 2.3. AB Thermolysis under 21.7 bar of CO2. The thermolysis under high-pressure CO2 was conducted using a 450 mL highpressure reactor that was customized by Parr Instruments. The Received: January 3, 2011 Revised: February 18, 2011 Published: April 01, 2011 8386

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The Journal of Physical Chemistry C temperature of the reactor was controlled by circulating the heat transfer fluid from an Isotemp 3006P thermostat (Fisher Scientific), having a stability of (0.01 °C, inside the jacket around the reactor. The temperature of the reactor was monitored with a type-T thermocouple (Omega Engineering). The precision of the temperature measurement was (0.5 °C. The pressure of the reactor was measured using a 9001PDM pressure transducer (Ashcroft, 0
34.5 MPa) with an accuracy of (0.3 bar. About 0.5 g of AB was loaded into the reactor, followed by purging the gas phase with CO2 twice, and then charged to a desired pressure with CO2. Once the temperature of the fluids reached 85 °C, it was connected to the jacket. The reactor temperature approached 85 °C, and the pressure reached 21.7 bar in less than 10 min. Subsequently, the reactor was maintained at 85 °C for 1 h and then cooled to room temperature. The reactor was kept under high-pressure CO2 at ambient temperature for 2 days. IR and Raman spectra of solid products were then acquired immediately after the reactor was opened. 2.4. Hydrogen Yield and CO2 Consumed. In the hydrogenreleased and CO2-consumed measurements, 8.6 mg of AB was loaded into the sample cell. It was evacuated, purged twice with a CO2 and C2H6 mixture (PCO2 = 4.07 bar), then vacuumed again by a 25 mL syringe, followed by charging gas mixtures to 4.42 bar at room temperature. The sample and reference cells were maintained at 25 °C for 5 min, and then heated to 85 °C at 1 °C/min, followed by maintaining this temperature for 1 h. Subsequently, the cells were cooled to
40 °C at 3 °C/min, with this temperature maintained for 20 min before withdrawing ca. 10 mL of gaseous products from the sample cell. Note that the reason for maintaining cells at
40 °C was to minimize the effect of borazine on hydrogen measurements, because borazine can react with sulfuric acid to generate extra hydrogen. After the gaseous products were sampled, the cells were heated to 25 °C at 1.2 °C/min. Once the temperature reached the preset value, solid products were collected and stored under ambient conditions. Analysis of solid products was completed within several hours after they were exposed to air. The gaseous products were bubbled through concentrated sulfuric acid in a sealed 30 mL vial that was manually vacuumed using a 10 mL syringe. The gas composition was analyzed using an HP 5890 SII GC equipped with a MolSieve 5A Plot capillary column (30 m  0.53 mm, Sigma-Aldrich). The carrier gas was argon, and its flow rate was set to 3.74 mL/min. The column temperature was maintained at 35 °C for 4 min, then under program control, advanced to 255 °C at 10 °C/min. The uncertainty in moles of hydrogen released is estimated to be within 8% based on triplicate measurements. 2.5. In Situ Investigations of AB Reacting with CO2. The in situ investigations were performed with an HR800 Horiba Jobin Yvon micro-Raman system (the 632.8 nm line of a He
Ne laser as the excitation source) equipped with a 10 objective lens, a CCD detector, and a high-pressure heating-freezing cell (THMS 600-PS, Linkam Scientific). The cell can be operated up to 14 bar at a temperature range of
120 to 500 °C, and its temperature was controlled with an accuracy of (0.1 °C by using a temperature controller (TMS 94, Linkam Scientific). After one glass slide was mounted onto the sample holder of the cell, a few hundred micrograms of AB were loaded, followed by placing another glass slide on it. The cell was first purged with CO2 twice, and then it was pressurized with CO2 to 1.62 bar. After a few minutes of maintaining the cell under this pressure, the Raman spectrum was collected from 300 to 3500 cm
1. Typically, it took around 13 min to acquire a complete spectrum by CREST

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(Continuous Rapid Extended Scanning Technology) and high resolution scanning with 4 times subpixel. Once the acquisition was completed, the cell was pressurized to a higher value and maintained under this pressure for a few minutes before acquiring another spectrum. This procedure was repeated twice until the pressure reached 5.9 bar at ambient temperature. When the recording under the above pressure was finished, the cell was heated to 95 °C at 10 °C/min. The time when the temperature reached the target value is referred to as the 0 min CO2 exposure. Subsequently, the Raman spectra were acquired by CREST scanning over a 20 min interval. 2.6. Characterization of Solid Products. The solid products of AB thermolysis were characterized by attenuated total reflection ATR-FTIR, micro-Raman, 11B and 13C solid-state MAS NMR, and thermal analysis. ATR-FTIR analyses were recorded on a Varian 7000 FTIR using MIRacle ATR accessory, the internal reflection element of which is a Ge single reflection plate. FTIR spectra were recorded at 2 cm
1 resolution from 850 to 4000 cm
1. Raman spectra were obtained by using the microRaman described above, which is equipped with a 100 objective lens. Samples were placed on a cover glass and excited with 632.8 nm HeNe laser radiation. The full spectrum was obtained by multiple-windows scanning between 500 and 3500 cm
1. 11 B and 13C solid-state MAS NMR spectra were recorded at 150.74 and 192.34 MHz, respectively, on a Varian VNMRS system equipped with BioMAS probe (3.2 mm), operating in a 14.1 T magnet. Samples were spun at 10 kHz for 11B and 9 kHz for 13C with the sample temperature regulated at 25 °C.22 The 13C solid-state MAS NMR spectra were referenced to the methylene carbon of adamantane at 38.48 ppm.23 The chemical shifts of 11B were referenced to 0 ppm based on a calculation from frequency ratios of 11B and 13C, as recommended by IUPAC guidelines.24 A combined TGA-DSC (Q600) technique from TA Instruments was employed to simultaneously detect the weight loss and heat flow. For this latter study, after loading 4.69 mg of solid products into the sample cup (alumina crucible), the sample and reference cups were heated from room temperature to 50 °C at 2 °C/min, and were then kept at this temperature for 15 min. Finally, they were heated to 600 °C at 5 °C/min. Also, in another experiment, 1.86 mg of AB was loaded into the sample cup, and then the sample and reference cups were subjected to the above heating procedure. All experiments were performed under a nitrogen flow of 100 mL/min.

3. RESULTS AND DISCUSSION The kinetics and thermodynamics of AB thermolysis under different atmospheres were investigated using a high-pressure differential scanning calorimeter (HP-DSC), which allows the pressure and heat flow into/out of a closed cell to be measured simultaneously. As shown in Figure 1, the pressure profiles display sigmoidal kinetics behaviors under 3.32 bar of air and three pressures of pure CO2. Moreover, the pressure increase in the first 60 min, during which the temperature is linearly increased from 25 to 85 °C, is very small under 3.32 bar of air and 1.7, 2.61, and 3.67 bar of CO2. Unless noted elsewhere, the pressure is referenced to the initial value at room temperature. A notable observation is that the thermolysis in the isothermal period, that is, 3 h at 85 °C, is much faster under CO2 than under air. Specifically, the pressure increase during the first hour of the isothermal period is 0.33 and 1.83 bar for 3.32 bar of air and 2.61 bar of CO2, respectively. Further examination of Figure 1 reveals 8387

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Figure 1. Pressure profiles of AB thermolysis for 3.32 bar of air and CO2 pressures of 1.70, 2.61, and 3.67 bar.

Figure 2. Heat flow profiles of AB thermolysis for 3.32 bar of air and CO2 pressures of 1.70, 2.61, and 3.67 bar.

that the thermolysis becomes faster as the CO2 pressure varies from 1.7 to 3.67 bar. However, the pressure increase at the end of thermolysis is essentially independent of CO2 pressure. Additionally, the final pressure attained under CO2 is greater than that under air. In our calorimetric investigations of AB thermolysis under different atmospheres (Figure 2), only one principal exothermic peak is detected on the DSC curves for the isothermal period under air. However, two exothermic peaks are observed under 1.7 and 2.61 bar of CO2, but only one intense exothermic peak with an unresolved shoulder is evidenced under 3.67 bar of CO2. Interestingly, as the CO2 pressure increases from 1.7 to 3.67 bar, one observes that the onset of second exothermic peak occurs earlier and the area of first exothermic peak increases. We interpret the first exothermic event as being related to the reaction of CO2 with AB, or intermediates like diammoniate of diborane (DADB, [(NH3)2BH2]þ[BH4]
),25 or primary thermolysis products including linear dimers of aminoborane (LDAB, NH3BH2NH2BH3),21,26 because the exothermic heat is strongly CO2 pressure dependent. It should be noted that the heat release could lead to the sample temperature increase and, consequently, the acceleration of thermolysis rate. However, the fast thermolysis kinetics under CO2 cannot be attributed mainly to this exothermic heat, because the exothermic heat under 1.7

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Figure 3. Pressure and heat flow profiles of AB thermolysis for 4.42 bar of a mixture of CO2 and C2H6 (PCO2 = 4.07 bar).

bar of CO2 is relatively small (see Figure 2), but the pressure increase is dramatically enhanced for the same CO2 pressure (see Figure 1). To obtain more insight into CO2 on the thermolysis of AB, we utilized a mixture of CO2 and C2H6 (CO2 partial pressure, PCO2 = 4.07 bar) and determined the amount of CO2 consumed and that of hydrogen released in 1 h at 85 °C, for a total pressure 4.42 bar (see Figure 3). It is important to note that ethane, as a reference in our chromatographic analysis, is inert with regards to AB thermolysis (see the Supporting Information). Under the above condition, 0.0620 mmol of CO2 was consumed and 0.371 mmol of hydrogen was produced for 0.278 mmol of AB, corresponding to a hydrogen yield of 8.63 wt % with respect to the initial mass of AB. Thus, not only the rate of hydrogen release, but also the yield of hydrogen in the CO2 atmosphere is greater than for NH4Cl-doping and thermal treatment (7.2
7.4 wt % in 3 h at 85 °C).21 The assumption of CO2 reacting with AB is consistent with a pressure decrease immediately preceding the fast pressure increase (see Figure 3). The same behavior is observed for CO2 pressures of 2.61 and 3.67 bar (see Figure 1). Additionally, we observed that the mole fraction of hydrogen in the gaseous product is higher than 0.9 according to GC analysis and the ideal gas law. Also, no other carbon-containing species except CO2 are detected by GC analysis for the H2SO4-scrubbed gas mixtures. Moreover, we observe that the two exothermic peaks merge into one as the pressure of CO2 increases from 2.67 to 4.07 bar (see Figures 2 and 3). These observations lead us to conclude that CO2 reacts with AB, or intermediates like DADB, or primary thermolysis products including LDAB, to produce new compounds that initiate or catalyze the dehydrogenation at moderate temperatures. This reaction has been confirmed, in large measure, through ATR-FTIR, micro-Raman, and 11B and 13C MAS NMR measurements of solid thermolysis products. Specifically, IR spectra of solid products for a total pressure of 4.42 bar, of a mixture of CO2 and C2H6 (PCO2 = 4.07 bar) after 1 h at 85 °C, contain spectral bands of both AB and polyaminoborane (Figure 4 and Table 1). Polyaminoborane (PAB) is the solid product of the first dehydrogenation of pristine AB. However, the relative amount of AB in the solid product is extremely small, because no decomposition peak attributable to the first step of AB dehydrogenation is detected in either DSC or differential thermogravimetric (DTG) curves (Figure S9 in the Supporting Information). When compared to AB and its solid thermolysis products (PAB) in the absence of CO2, a new band at 8388

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Figure 4. IR spectra of AB, PAB, and solid thermolysis products for 4.42 bar of a mixture of CO2 and C2H6 (PCO2 = 4.07 bar) and 21.7 bar (at 85 °C) of CO2.

Table 1. Observed Infrared Frequencies of AB, PAB, and Solid Thermolysis Products for 4.07 bar and 21.7 bar (at 85 °C) of CO2 from 3500 to 850 cm
1

Figure 5. Raman spectra of AB, PAB, and solid thermolysis products for 21.7 bar (at 85 °C) of CO2.

Table 2. Band Position (cm
1) and Assignments of Raman Spectra of AB, PAB, and Solid Thermolysis Products for 21.7 bar (at 85 °C) of CO2 AB

3251 .s 3315 .s 3248 .w

PAB 3289 .br,m 3247 .br,m

4.07 bar 3291 .br,m 3247 .br,m 3124 .sh,w

21.7 bar 3379 .br,m 3232 .br,m 3079 .br,m 2800 s. h,w

2361 .w 2324 .s 2280 .w 1602 .m 1375 .s

1156 .s 1060 .s

2380 .m 2348 .m 2305 .m 1559 .m 1359 .m

2363 .m 2330 .m 2280 .sh,w

assignment

assignment asym. N
H stretching

3245 .s

3173 .w

3215 .sh,vw

sym. N
H stretching

3113 .w

ov

N
H stretching

2897 .sh,vw

ov

N
H stretching

2803 s.

C
H stretching

N
H stretching

2717 .s

C
H stretching asym. B
H stretching

C
H stretching

2375 .m

B
H stretching

2331 .w

B
H stretching B
H stretching

2280 .s 1597 .w

2346 .s,br

asym. B
H stretech sym. B
H stretching 1693 .w

CdO stretching asym. NH3 deformation

1564 .m

∼1640 .sh,w

1686 .s

CdO stretching

1567 .m

1577 .s

NH3 deformation

1446 .vw

1465 .vw

ov

NH3 deformation

1373 .m

1373 .vs

sym. NH3 deformation

1380 .sh,w 1350 .m

B
O stretching asym. B(3)
O stretching

1387 .s

∼1384 .sh,w

1336 .m

1341 .s

B
O stretching

1240 .sh,w

1241 .m

C
O stretching

1177 .w

C
O stretching

1190 .m

BH3 deformation BH3 deformation

1160 .m

1186 .s,br 1150 .s,br

1162.s

1049 .w

1052 .s

1064 .vw

NBH rocking

asym. BH3 deformation

1192 .s,br

sym. BH3 deformation 1038 .vw

1058 .w

878 .m

878 .s

NBH rock sym. B(3)
O stretching

1092 .s

C
N stretching

798 .m

B
N stretching

1024 s.

C
O stretching

783 .vs

B
N stretching

728 .w

919 s. 891 .br,w 860 .w

thermolysis products

3315 .w

thermolysis products AB

PAB

880 w .

732 .sh,br

NBH rocking

B
O stretching B
N stretching

1336 cm
1, assigned to an asymmetric B
O stretching mode of BO3 groups,27
30 appears when CO2 is present. Also, two discernible shoulders band exist, around 1240 and 1640 cm
1, which are likely attributable, respectively, to C
O and CdO stretching vibrations.27
30 To further test for the existence of B
O and CdO bonds, we acquired IR and Raman spectra of the solid thermolysis products, resulting from keeping AB under CO2 with an initial pressure of 21.7 bar at 85 °C for 1 h in a high-pressure reactor. We find, in the

IR spectra, that the band at 1336 cm
1 becomes very strong, and what was originally a shoulder band at 1640 cm
1 for lower CO2 pressures becomes a strong resolved band. There are another two notable effects resulting from application of the elevated pressure: first, bands attributable to BH3 group vanish; second, IR frequencies associated with C
H stretching mode appear. The putative disappearance of BH3 and the creation of C
H functionalities are also supported by Raman spectral measurements (Figure 5 and Table 2). Additionally, a strong band at 878 cm
1, assignable to a symmetrical B
O stretching mode of BO3 groups, is observed in the Raman spectrum. 8389

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Figure 6.

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B (a) and 13C (b) MAS NMR of solid thermolysis products for 21.7 bar (at 85 °C) of CO2.

11

Figure 7. Raman spectra of AB under four CO2 pressures at ambient temperature.

The presence of B
O, CdO, and C
H bonds is also verified by 11B and 13C solid-state MAS NMR spectra. In the 11B solidstate MAS NMR spectrum (Figure 6a), a sharp peak at 1.7 ppm and a broad peak at 15.7 ppm are observed, corresponding to tetrahedral BO4 groups and trigonal BO3 groups, respectively. The 13C solid MAS NMR spectrum displays three principal peaks at 25.5, 50.9, and 166.5 ppm besides CO2 resonances (Figure 6b), which are assigned to aliphatic, OCH3, and COO
groups, respectively. This assignment is supported by IR and Raman spectra (Figures 4 and 5, Tables 1 and 2). Upon combining macroscopic investigations and microscopic results, we conclude that CO2 reacts with AB, or intermediates, or dehydrogenation products at 85 °C or lower. This reaction either enhances the concentration of other active species like boronium cations ([NH3BH2NH3]þ) or produces catalytic compounds containing the
COOH group for the thermolysis of AB as previously showed that some solid acids such as polyphosphoric acid can initiate AB dehydrogenation.31 Additionally, the interaction between CO2 and AB possibly disrupts the hydrogen-bonding network, resulting in defect formation in AB crystals and consequently promoting thermolytic hydrogen release from AB. Further insight into the enhanced

Figure 8. Raman spectra of AB under 5.9 bar of CO2 at 95 °C for 4 h.

thermolysis kinetics and the reaction of CO2 with AB was obtained from in situ investigations using micro-Raman. We found through in situ investigations that three new bands appear at 1286, 1341, and 1388 cm
1 after AB is exposed to CO2 at ambient temperature (Figure 7). As shown in Figure 7, the intensity of these bands becomes stronger, while the bands related to AB continue to exist as CO2 pressure increases from 1.62 to 5.9 bar. Concurrently, the intensity of the symmetric NH3 deformation mode at 1373 cm
1 becomes weaker. The bands at 1286 and 1388 cm
1 are assigned to C
N and B
O stretching modes, respectively, and the band at 1341 cm
1 is tentatively assigned to OdCdO stretching vibration. It is evident, therefore, that the crystal surface of AB chemically adsorbs CO2 at ambient temperature with nitrogen being bound to carbon and boron to one of oxygen atoms. We believe the chemical adsorption of CO2 could disrupt the hydrogen-bonding network of AB, promoting thermolytic hydrogen release. To further our understanding of the reaction of CO2 with AB, we collected Raman spectra within 240 min exposure to 5.9 bar of CO2 at 95 °C (Figure 8). As the temperature increases from ambient temperature to 95 °C, the bands related to the OdCdO stretching mode vanish, suggesting that OdCdO is an intermediate species. It should be noted here that the time shown in Figure 8 is the start 8390

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The Journal of Physical Chemistry C of acquiring the spectrum. After 40 min exposure, the spectrum exhibits C
H stretching modes at ca. 2780 and 2890 cm
1 and N
H stretching vibrations. Also, we found that the B
H bonds vanish after the C
H bonds form (inset in Figure 8). As Figure 8 shows, several new bands between 1000 and 1750 cm
1 appear and then vanish as the CO2 exposure time increases from 100 to 160 min. Our current available evidence does not permit us to assign these bands. Another notable finding is that both OBO bending (at 516 cm
1) and CdO stretching (at 1690 cm
1) modes are observed after 220 min exposure. It appears that methoxy groups evolve via OdCdO f —O
CdO f — OCH3. Nevertheless, the details of the formation of methoxy, carboxyl, and aliphatic groups and the role of the reaction of CO2 with AB in accelerating thermolytic H2 release from AB will be further clarified in future works.

4. CONCLUSIONS We have found that 1.33 equiv of hydrogen is released from AB in 1 h at 85 °C when the thermolysis is performed under a CO2 pressure around 4 bar. We also ascertain that the kinetics of AB thermolysis at 85 °C is enhanced at increased CO2 pressures, and an important initial step involves CO2 reacting with AB, or intermediates, or primary thermolysis products. To the best of our knowledge, this is the first reported study dealing with acceleration of dehydrogenation rate of solid AB at temperatures less than 85 °C through the use of CO2, a very cheap, abundant, and naturally occurring material. ’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information of thermolysis of AB under ethane, argon, and air, IR spectra of solid thermolysis products under different atmospheres, and thermographs and DTG data for AB and solid thermolysis products for 4.42 bar of a mixture of CO2 and C2H6. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (212) 650-6688. Fax: (212) 650-6660. E-mail: lee@ che.ccny.cuny.edu.

’ ACKNOWLEDGMENT We are grateful for the support of the National Science Foundation for this work (under Grant No. HRD-0833180). We also would like to thank Prof. Morris for providing a high
pressure cell and Prof. Stark and her student Xundong Guan for the assistance with solid-state MAS NMR measurements. ’ REFERENCES (1) Staubitz, A.; Robertson, A. P. M.; Manners, I. Ammonia-Borane and Related Compounds as Dihydrogen Sources. Chem. Rev. 2010, 110, 4079–4124. (2) Chandra, M.; Xu, Q. A High-Performance Hydrogen Generation System: Transition Metal-catalyzed Dissociation and Hydrolysis of Ammonia
Borane. J. Power Sources 2006, 156, 190–194. (3) Xu, Q.; Chandra, M. Catalytic Activities of Non-noble Metals for Hydrogen Generation from Aqueous Ammonia-Borane at Room Temperature. J. Power Sources 2006, 163, 364–370.

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(4) Ramachandran, P. V.; Gagare, P. D. Preparation of Ammonia Borane in High Yield and Purity, Methanolysis, and Regeneration. Inorg. Chem. 2007, 46, 7810–7817. (5) Kalidindi, S. B.; Sanyal, U.; Jagirdar, B, R. Nanostructured Cu and Cu@Cu2O Core Shell Catalysts for Hydrogen Generation from Ammonia-Borane. Phys. Chem. Chem. Phys. 2008, 10, 5870– 5874. (6) Caliskan, S.; Zahmakiran, M.; Ozkar, S. Zeolite Confined Rhodium(0) Nanoclusters as Highly Active, Reusable, and Long-Lived Catalyst in the Methanolysis of Ammonia-Borane. Appl. Catal., B 2010, 93, 387–394. (7) Hu, M. G.; Geanangel, A.; Wendlandt, W. W. The Thermal Decomposition of Ammonia Borane. Thermochim. Acta 1978, 23, 249–255. (8) Wolf, G.; Baumann, J.; Baitalow, F.; Hoffman, P. Calorimetric Process Monitoring of Thermal Decomposition of B
N
H Compounds. Thermochim. Acta 2000, 343, 19–25. (9) Baitalow, F.; Baumann, J.; Wolf, G.; Jaenicke-R€oβler, K.; Leitner, G. Thermal Decomposition of B
N
H Compounds Investigated by Using Combined Thermoanalytical Methods. Thermochim. Acta 2002, 391, 159–168. (10) Gutowska, A.; Li, L.; Shin, Y.; Wang, C. M.; Li, X. S.; Linehan, J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw, W.; Gutowski, M.; Autrey, T. Nanoscaffold Mediates Hydrogen Release and the Reactivity of Ammonia Borane. Angew. Chem., Int. Ed. 2005, 44, 3578–3582. (11) Feaver, A.; Sepehri, S.; Shamberger, P.; Stowe, A.; Autrey, T.; Cao, G. Z. Coherent Carbon Cryogel-Ammonia Borane Nanocomposites for H2 Storage. J. Phys. Chem. B 2007, 111, 7469–7472. (12) Li, L.; Yao, X.; Sun, C.; Du, A.; Cheng, L.; Zhu, Z.; Yu, C.; Zou, J.; Smith, S. C.; Wang, P.; Cheng, H.-M.; Frost, R. L.; Lu, G. Q. LithiumCatalyzed Dehydrogenation of Ammonia Borane within Mesoporous Carbon Framework for Chemical Hydrogen Storage. Adv. Funct. Mater. 2009, 19, 265–271. (13) Li, Z.; Zhu, G.; Lu, G.; Qiu, S.; Yao, X. Ammina Borane Confined by a Metal-Organic Framework for Chemical Hydrogen Storage: Enhancing Kinetics and Eliminating Ammonia. J. Am. Chem. Soc. 2010, 132, 1490–1491. (14) Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon, L. G. Amineborane-Based Chemical Hydrogen Storage:Enhanced Ammonia Borane Dehydrogenation in Ionic Liquids. J. Am. Chem. Soc. 2006, 128, 7748–7749. (15) Himmelberger, D. W.; Alden, L. R.; Bluhm, M. E.; Sneddon, L. G. Ammonia Borane Hydrogen Release in Ionic Liquids. Inorg. Chem. 2009, 48, 9883–9889. (16) Denny, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. Efficient Catalysis of Ammonia Borane Dehydrogenation. J. Am. Chem. Soc. 2006, 128, 12048–12049. (17) Keaton, R. J.; Blacquiere, J. M.; Baker, R. T. Base Metal Catalyzed Dehydrogenation of Ammonia-Borane for Chemical Hydrogen Storage. J. Am. Chem. Soc. 2007, 129, 1844–1845. (18) Dietrich, B. L.; Goldberg, K. I.; Heinekey, D. M.; Autrey, T.; Linehan, J. C. Iridium-Catalyzed Dehydrogenation of Substituted Amine Boranes: Kinetics, Thermodynamics, and Implications for Hydrogen Storage. Inorg. Chem. 2008, 47, 8585–8585. (19) Heldebrant, D. J.; Karkamkar, A.; Hess, N. J.; Bowden, M.; Rassat, S.; Zheng, F.; Rappe, K.; Autrey, T. The Effects of Chemical Additives on the Induction Phase in Solid-State Thermal Decomposition of Ammonia Borane. Chem. Mater. 2008, 20, 5332–5336. (20) Bluhm, M. E.; Bradely, M. G.; Sneddon, L. G. Promoted Hydrogen Release from Ammonia Borane. Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2006, 51, 571–572. (21) Zhang, J. S.; Zhao, Y.; Akins, D. L.; Lee, J. W. Thermal Decomposition and Spectroscopic Studies of Preheated Ammonia Borane. J. Phys. Chem. C 2010, 114, 19529–19534. (22) Guan, X.; Stark, R. E. A General Protocol for Temperature Calibration of MAS NMR Probes at Arbitrary Spinning Speeds. Solid State Nucl. Magn. Reson. 2010, 38, 74–76. 8391

dx.doi.org/10.1021/jp200049y |J. Phys. Chem. C 2011, 115, 8386–8392

The Journal of Physical Chemistry C

ARTICLE

(23) Morcombe, C. R.; Zilm, K. W. Chemical Shift Referencing in MAS Solid State NMR. J. Magn. Reson. 2003, 162, 479–486. (24) Harris, R. K.; Becker, E. D.; Cabral de Menezes, S. M.; Goodfellow, R.; Granger, P. NMR Nomenclature: Nuclear Spin Properties and Conventions for Chemical Shifts: IUPAC Recommendations 2001. Solid State Nucl. Magn. Reson. 2002, 22, 458–483. (25) Stowe, A. C.; Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T. In Situ Solid State 11B MAS-NMR Studies of the Thermal Decomposition of Ammonia Borane: Mechanistic Studies of the Hydrogen Release Pathways from a Solid State Hydrogen Storage Material. Phys. Chem. Chem. Phys. 2007, 9, 1831–1836. (26) Shaw, W. J.; Bowden, M.; Karkamkar, A.; Howar, C. J.; Heldebrant, D. J.; Hess, N. J.; Linehan, J. C.; Autrey, T. Characterization of a New Phase of Ammonia Borane. Energy Environ. Sci. 2010, 3, 796–804. (27) Servoss, R. R.; Clark, H. M. Vibrational Spectra of Normal and B10-Labeled Methyl Borate. J. Chem. Phys. 1957, 26, 1179–1184. (28) Miller, N. M. Cyclic Borane Derivatives of Amino Acids. Inorg. Chem. 1974, 13, 1459–1467. (29) Carter, J. C.; Luther, G. W., III; Moye, A. L. Infrared Spectra of Trimethylamine Carbon Monoxide-Borane and Its Deuterated Derivatives. Appl. Spectrosc. 1974, 28, 427–430. (30) Liu, J.; Xia., S.; Gao, S. FT-IR and Raman Spectroscopic Study of Hydrated Borates. Spectrochim. Acta 1995, 51A, 519–532. (31) Stephen, F. H.; Baker, R. T.; Matus, M. H.; Grant, D. J.; Dixon, D. A. Acid Initiation of Ammonia-Borane Dehydrogenation for Hydrogen Storage. Angew. Chem., Int. Ed. 2007, 46, 746–749.

8392

dx.doi.org/10.1021/jp200049y |J. Phys. Chem. C 2011, 115, 8386–8392