Hydrogen Desorption Reaction between Hydrogen-Containing

Apr 12, 2010 - vacuum condition at 200 °C for 8 h in advance. The annealed. hBN of 300 mg and 20 ZrO2 balls with 8 mm in diameter were put into a mil...
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Hydrogen Desorption Reaction between Hydrogen-Containing Functional Groups and Lithium Hydride Hiroki Miyaoka, Takayuki Ichikawa,* Hironobu Fujii, and Yoshitsugu Kojima Institute for AdVanced Materials Research, Hiroshima UniVersity, 1-3-1 Kagamiyama, Higashi-Hiroshima, 739-8530, Japan ReceiVed: February 20, 2010; ReVised Manuscript ReceiVed: March 18, 2010

Three kinds of hydrogen-containing products were synthesized from hBN by ball-milling under hydrogen (H2), methane (CH4), and ammonia (NH3) atmosphere, in which these products are named BNnanoXHx (X ) C, N). BNnanoHx, BNnanoCHx, and BNnanoNHx desorbed H2, CH4, and NH3, respectively, in the wide temperature range from 100 to 900 °C. From infrared spectroscopy for these products, the existence of some polarized functional groups such as B-H, C-H, or N-H bonds in the products was clarified. Afterward, the products were mixed with lithium hydride (LiH) to synthesize the composite by ball-milling. All the BNnanoXHx and LiH composites can release mainly H2 around 200 °C, indicating that the hydrogenated states as the polarized functional groups in each product would be destabilized by an interaction with LiH. Therefore, the BNnanoXHx and LiH systems are recognized as a hydrogen storage system with H2 release at relatively low temperature due to the characteristic interaction. 1. Introduction High performance hydrogen storage materials are required to establish suitable transportation technologies for a clean and sustainable society. In particular, much attention has been paid to hydrogen storage materials based on light elements as an on-board application because it is possible that high gravimetric and volumetric hydrogen densities are realized.1,2 Since Dillon et al. reported on single-walled carbon nanotubes as a hydrogen storage material in 1997,3 many kinds of materials based on carbon (C) have been investigated all over the world. Among them, we have focused on nanostructural hydrogenated graphite (CnanoHx) and have investigated its hydrogen storage properties.4-10 CnanoHx was synthesized from graphite powder by mechanical ball-milling under a hydrogen atmosphere for 80 h. This product possesses high hydrogen storage capacity, about 4 mass %, because hydrogen atoms are chemisorbed by stable C-H bonding at the edges and defects of graphene sheets induced by mechanical ball-milling.9 Then, graphite crystallite has been destroyed down to nanometer scale. The hydrocarbon groups such as -CH, -CH2, and -CH3 in CnanoHx have been clarified by neutron scattering measurements11,12 and infrared absorption spectroscopy.13 On the other hand, CnanoHx has several essential disadvantages for practical use as a hydrogen storage material. To release H2, a high temperature of more than 700 °C is required. Moreover, some hydrocarbons such as methane (CH4) or ethane (C2H6) are also emitted with H2 desorption. Furthermore, it is difficult to rehydrogenate this product under moderate pressure and temperature for the on-board application. Recently, the hydrogen desorption properties and the reversibility of CnanoHx were improved by synthesizing the nanocomposites with alkali (-earth) metal hydrides (MHs), lithium hydride (LiH), sodium hydride (NaH), magnesium hydride (MgH2), or calcium hydride (CaH2).14-18 Among them, the CnanoHx-LiH composite can store rechargeable hydrogen of 5.0 mass % at 350 °C. Additionally, the amount of hydrocarbon * Corresponding author. Telephone & Fax: +81-82-424-5744. E-mail: [email protected].

emission from this composite is strongly suppressed compared with emission from CnanoHx itself,15 and as the dehydrogenated state, we have detected the Li2C2 phase by means of neutron scattering measurements.17 Because Li2C2 should be more stable than the separated Li and C phases, the expected enthalpy change corresponding to hydrogen desorption from the composite was expected to be much lower than for each component. These results suggest that the polarized C-H groups in CnanoHx and LiH are able to react with each other, resulting in both of them being destabilized. Therefore, the chemisorbed hydrogen is released at a lower temperature compared with the decomposition temperature of each component,19 which is in fact more than 650 °C. It is well-known that hexagonal boron nitride (hBN) possesses a layered structure, which is the same structure as graphite. So far, it is clarified that the hydrogenation of hBN proceeds during ball-milling under a H2 atmosphere, as reported by Kojima et al.22 and Wang et al.20,21 As a result, about 2 mass % of hydrogen is stably absorbed in the ball-milled hBN. This product desorbs only H2 with an increase in a temperature without NH3 or boron hydrides (BHx) emissions. This behavior indicates that the layered structure of hBN is more stable than that of graphite, because the ball-milled graphite desorbs not only hydrogen but also a large amount of hydrocarbons, in other words, the carbon atoms in the hexatomic ring of graphite structure are released with the hydrogen. In addition, hBN is known as a dehydrogenated state of ammonia borane (NH3BH3), which is one of the promising hydrogen storage materials at present because it can release H2 around 100 °C.23-25 With the H2 desorption reactions of NH3BH3, various kinds of intermediate phases and byproduct, diborane (B2H6), aminoborane (NH2BH2), and borazine (B3N3H6), are formed, and hBN is a final dehydrogenated product. Moreover, metal amidoboranes such as LiNH2BH3 and NaNH2BH3 were also reported, and they desorb H2 at a lower temperature than NH3BH3.26-28 Xiong et al. reported that the reaction product of LiNH3BH3 was borazine or a borazine-like compound, composed of a hexatomic ring like benzene. On the other hand, the H2 desorption reaction of NH3BH3 or metal

10.1021/jp101533z  2010 American Chemical Society Published on Web 04/12/2010

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Figure 1. Thermal gas desorption profiles of BNnanoXHx (upper) synthesized by the milling for 80 h and TG profiles (lower) of BNnanoXHx synthesized by the milling for 8, 32, and 80 h corresponding to weight loss with an increase in a temperature to 900 °C: (a) BNnanoHx; (b) BNnanoCHx; (c) BNnanoNHx. S16/S2 inserted in Figure 1b is the peak area ratio of CH4 to H2.

amidoboranes is an exothermic reaction, indicating that the reaction is not reversible thermodynamically. On the basis of the above reports, we have expected that some hBN based hydrogen storage materials can be synthesized by ball-milling under different kinds of hydrogen-containing atmosphere, e.g., CH4 or NH3, to absorb much more hydrogen in the ball-milled hBN. In this work, three kinds of hBN based products were synthesized by ball-milling under hydrogen (H2), methane (CH4), and ammonia (NH3) atmosphere, in which these products were named BNnanoXHx (X ) C, N). Furthermore, the composites of BNnanoXHx and LiH were synthesized by the milling method to destabilize the hydrogenated states, expecting the reaction between polarized functional groups in BNnanoXHx and LiH. All the products and composites were characterized by a thermal desorption analysis and structure investigations. On the basis of the results, their hydrogen desorption reactions were discussed to understand how hydrogen is desorbed. The results obtained by these studies may provide some useful information for the regeneration of NH3BH3 or metal amidoboranes. 2. Experimental Procedures 2.1. Sample Preparation. BNnanoHx, BNnanoCHx, and BNnanoNHx were synthesized from hBN powder (99%, Aldrich) by ball-milling using a rocking (vibrating) ball-mill apparatus (RM-10, Seiwa, Giken Co. Ltd.) under H2 (1.0 MPa), CH4 (1.0 MPa), and NH3 (about 0.9 MPa), respectively. To remove adsorbed water, the as-received hBN was annealed under a vacuum condition at 200 °C for 8 h in advance. The annealed hBN of 300 mg and 20 ZrO2 balls with 8 mm in diameter were put into a milling vessel made of Cr steel (SKD-11, Umetoku Co. Ltd.) with the inner volume of ∼30 cm3. And then, the hBN was milled under each atmosphere at room temperature, where the milling time was chosen to be 8, 32, and 80 h. After that, each product was milled for 80 h, that is, BNnanoXHx, was mechanically mixed with LiH using a planetary (rotating) ballmill apparatus (P7, Fritsch) for 2 h under 1.0 MPa argon (Ar) atmosphere to obtain a close contact between both the components, where totally 300 mg of BNnanoXHx and a LiH mixture with 20 ZrO2 balls was put into the Cr steel vessel. The molar ratio of BN and LiH was chosen to be 1:1, assuming that the molar weight of BNnanoXHx is the same as that of BN itself because the accurate amount of the carbon and nitrogen included in the products was not able to be estimated at present, as mentioned in the following part. The synthesized composites were named BNnanoXHx-LiH. As a reference, hydrogenated nanostructural graphite (CnanoHx) was synthesized from graphite powder (Strem Chemicals, 99.999%) by ball-milling under H2

(1.0 MPa) for 80 h. All the samples were handled in a glovebox (MP-P60W, Miwa MFG) filled with purified Ar gas (>99.9999%) to avoid an oxidation. 2.2. Experimental Technique. Thermal gas desorption properties of the as-prepared products and composites were examined by thermal desorption mass spectroscopy (TDMS) (M-QA200TS, Anelva) connected to thermogravimetry (TG) and differential thermal analysis (DTA) (TG8120, Rigaku), where this equipment is installed inside the glovebox to minimize the influence of exposing the samples to air. In the thermal analysis, high purity helium (He) gas (>99.9999%) flowed as a carrier gas, and the heating rates were fixed at 10 °C/min for BNnanoXHx and 5 °C/min for the BNnanoXHx-LiH composites, respectively. Some kinds of fragments of possible desorption gases were also monitored in TDMS measurements for all the samples to assign the desorption gases; e.g., for NH3 (molecular weight is 17), m/z ) 14, 15, and 16 as well as 17 were also measured. Structural properties were investigated by X-ray diffraction (XRD) measurements (RINT-2100, Rigaku, Cu KR radiation), where all the samples were covered by a polyimide sheet (Kapton, Du Pont-Toray Co. Ltd.) in the glovebox to avoid oxidation during the XRD measurement. Fourier-transform infrared (FT-IR) spectroscopy (Spectrum One, Perkin-Elmer) was performed by using a diffuse reflection cell to examine IR active stretching modes. For the measurements, all the samples were diluted by potassium bromide (KBr) down to 10 mass %. The FT-IR equipment is installed inside a homemade glovebox filled with purified argon gas. 3. Results and Discussion 3.1. Characterization of BNnanoXHx (X ) C, N). Figure 1 shows the TDMS profiles of (a) BNnanoHx, (b) BNnanoCHx, and (c) BNnanoNHx synthesized by ball-milling for 80 h. The TG profiles of BNnanoHx milled for 80 h and of BNnanoCHx and BNnanoNHx milled for 8, 32, and 80 h are shown in Figure 1 as well. As shown in the TDMS profiles of Figure 1a, BNnanoHx synthesized by ball-milling under H2 atmosphere desorbed only H2, which is indicated by mass number 2, in the wide temperature range from 100 to 900 °C without other gas emissions such as H2O, NH3 or boron hydrides such as BH3 and B2H6, and this H2 desorption peak was located around 650 °C. Due to the desorption of H2 by heating to 900 °C, about 1.0 mass % weight loss was revealed, as shown in the TG profile of Figure 1a. BNnanoCHx was synthesized from the hBN by ball-milling under CH4 atmosphere to form the different kinds of polarized

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Figure 2. XRD patterns of the annealed hBN, the as-synthesized BNnanoXHx by different milling times, and the 80 h milled BNnanoXHx after heating to 900 °C. As a reference, hBN in database (PDF #34-0421) are also shown.

functional groups from BNnanoHx. Regarding the hydrogenated state of this product, it is expected that hydrogen atoms are absorbed as the C-H groups as well as B-H and N-H groups in hBN, where H and CHx should be formed due to dissociation of CH4 during ball-milling. The TDMS profile of BNnanoCHx is shown in Figure 1b, where mass numbers 2, 16, and 28, respectively, correspond to H2, CH4, and C2H6. TDMS results of BNnanoCHx with other milling times were omitted because they revealed almost the same profiles. S16/S2 inserted in Figure 1b is the peak area ratio of CH4 to H2. Not only hydrogen but also some hydrocarbons were desorbed with the temperature increasing to 900 °C without any other gas emissions. As shown in the TG profiles of Figure 1b, BNnanoCHx synthesized by various milling times revealed the 5.0 mass % weight loss even for the 8 h milled product. Furthermore, the value of weight loss increased to 9.5 mass % with the gain in milling time to 80 h. BNnanoNHx was synthesized from the hBN by ball-milling under NH3 atmosphere to form some N-H groups in hBN. The TDMS profiles of the 80 h milled BNnanoNHx are shown in Figure 1c, where there was not much difference among the results of all BNnanoNHx at the different milling times. The mass numbers 2, 17, and 28 were assigned to H2, NH3, and nitrogen (N2). With heating to 900 °C, NH3 was gradually desorbed from 100 °C. Simultaneous desorption of H2 and N2 above 500 °C indicates the decomposition of the NH3 molecule because the intensity gain of the gas synchronized with a decrease of NH3 intensity. These behaviors indicate that the chemisorbed hydrogen atoms are desorbed as only NH3 from the BNnanoNHx product. The TG profiles of Figure 1c show the weight loss due to NH3 desorption of each BNnanoNHx synthesized by different milling times, indicating a tendency to increase NH3 desorption by longer milling. The XRD patterns of the annealed hBN, the as-synthesized BNnanoXHx by different milling times, and the 80 h milled BNnanoXHx after heating to 900 °C are shown in Figure 2. In the XRD patterns of Figure 2a in the case of BNnanoHx, the diffraction peaks corresponding to hBN disappeared after 80 h of milling, indicating that the layered structure of hBN had been destroyed down to nanometer scale during the ballmilling process. As shown in Figure 2b, in the case of BNnanoCHx, the diffraction peaks originating in the hBN structure were not found after ball-milling even for 8 h. This behavior is similar to the case of BNnanoHx.20 On the other hand, BNnanoNHx showed a different structural change with the milling. The peaks assigned to hBN were weakened and broadened with an increase in the milling time from 8 to 80 h; however, the hBN structure clearly remained even after 80 h of milling, as shown in Figure 2c. With respect to this behavior, it seems likely that a destruction of hBN structure is prevented by the NH3 atmosphere. Actually, the same phenomenon has been confirmed in the case of graphite milled under an NH3 atmosphere for 80 h

Figure 3. FT-IR spectra of (a) the annealed hBN, (b) 80 h milled BNnanoHx, (c) BNnanoCHx, (d) BNnanoNHx, and (e) CnanoHx as a reference.

(see Supporting Information). Therefore, it is expected that the adsorptive NH3 molecule or dissociated molecules such as NHx immediately terminated the active edges and defects that were induced by ball-milling, with the result that the milling effect is elastically weakened by the stabilization of the hBN structure. For all the products, the diffraction patterns of 80 h milled samples were not changed before and after heating to 900 °C, suggesting that the hBN structure was not crystallized with the gas desorption by heating. In Figure 3, the FT-IR spectra of BNnanoXHx synthesized by 80 h milling and the host hBN annealed at 200 °C are shown. The spectrum of hBN showed characteristic peaks around 3020, 2800, 2530, 2330, and 780 cm-1, as shown by arrows in Figure 3a. It seems likely that the broad peak at 3420 cm-1 is assigned to the O-H stretching mode, which might indicate the existence of a small amount of a remaining water after annealing. Among them, the peaks at 3020, 2800, and 2330 cm-1 completely disappeared after milling under a H2 atmosphere, as shown in Figure 3b, indicating that these peaks would originate in chemical bonding states of the host hBN. This result was consistent with the XRD measurements. However, the peak at 780 cm-1 remained after milling, which was assigned to the absorption of out-of-plane B-N-B on the B3N3 ring structure, as reported before,22,23 indicating that the hexatomic ring structure composed of B and N is preserved although the hBN structure is broken down to nanometer size by ball-milling. On the other hand, two new peaks were observed at 3400 and 2500 cm-1. Even though the peak at 3400 cm-1 was located at almost the same position as the peak of the O-H stretching mode, the peak shape was clearly sharper and larger than that shown in the spectrum of host hBN. Thus, the two peaks originate in the hydrogenated state of BNnanoHx, which was previously examined.22 In that paper, the peaks corresponding to the N-H and B-H stretching mode had been observed at 3440 and 2520 cm-1, respectively, where these peak positions were quite consistent with the results in this work. Therefore, the peaks around 3400 and 2500 cm-1 were assigned to the N-H and

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Figure 4. (a) Thermal gas desorption and TG-DTA profiles, (b) XRD patterns before/after heating to 500 °C, and (c) FT-IR spectra before/after heating at different temperatures of the BNnanoHx-LiH composite.

B-H bonding, respectively. In addition, the IR spectrum of this product was similar to that of poly(aminoborane) ({NH2BH2}n) formed by heat treatment of ammoniaborane (NH3BH3), as reported by Kim et al.23 These results indicate that hydrogen atoms are chemisorbed as the B-H and N-H groups at the edges and defects in the nanostructural hBN generated by ballmilling. The above results are almost comparable to the reports by Kojima et al.22 and Wang et al.20,21 In the FT-IR spectrum of BNnanoCHx shown in Figure 3c, the peaks assigned to hBN around 3020, 2800, 2530, and 2330 cm-1 disappeared. At 3400, 3000-2840, and 2500 cm-1, the new IR absorption peaks were clearly observed. The spectrum of CnanoHx shown in Figure 3e revealed the peaks corresponding to the C-H stretching modes of -CH2 and -CH3 groups in the region from 3000 to 2840 cm-1, which were reported before by Ogita et al.13 Therefore, the peaks observed in the region of 3000-2840 cm-1 were ascribed to such C-H groups. Two other peaks were located in the same positions as the peaks observed in the spectrum of BNnanoHx, indicating that the hydrogen atoms were chemisorbed as the B-H and N-H bonds as well as C-H bonds. The peak corresponding to the out-of-plane B-N-B absorption was observed at 780 cm-1, suggesting that the hexatomic structure was preserved at the nanometer scale like BNnanoHx. From the above results, the hydrogen atoms (H) and hydrocarbon groups (-CH2, -CH3) are chemisorbed at the active sites such as the edges of the BN layers and defects, resulting in the B-H, C-H, and N-H bonds being formed. The FT-IR spectrum of BNnanoNHx is shown in Figure 3d. By comparing with the spectrum of the host hBN, we clarified that the peaks around 2800, 2530, and 2330 cm-1 originated in hBN clearly remained even after 80 h of milling. This result suggested that the destruction of hBN was suppressed by the effect of the NH3 atmosphere, where it was consistent with the results of XRD measurements. Moreover, the IR spectrum showed the peak at 780 cm-1, indicating that the N3B3 ring structure still remained. On the other hand, two kinds of peaks appeared at 3400 and 3500 cm-1. It was expected that these peaks were assigned to the different types of N-H stretching mode, e.g., -NH and -NH2 groups. Although it was expected that N-H as well as B-H bonding should be formed, the existence of the B-H bonding was unable to be detected. From the obtained experimental facts, it has been clarified that BNnanoXHx (X ) C, N) possesses the characteristic

hydrogenated state with the B-H, C-H, or N-H groups, depending on the milling atmosphere, H2, CH4, or NH3, at the active sites in the BN layers formed by the milling. These functional groups possess a polarization because they are detectable by IR spectroscopy. Such bonding states would possess wide thermodynamic stabilities because the thermal gas desorption profiles of all the products and IR absorption peaks corresponding to their bonds are quite broad. Generally, the ballmilling mechanically induces structural defects and distortion in hBN, with the result that the functional groups formed at the above sites would have different stabilities. In addition to the above results, no clear endothermic or exothermic peak was observed in the DTA of all the products, suggesting that the enthalpy change with the gradual gas desorption due to the dispersive bonding energy of functional group was not detectable by a resolution of our apparatus. Therefore, the above hydrogen absorption and desorption are recognized as characteristic properties of BNnanoXHx synthesized by ball-milling. 3.2. Hydrogen Desorption Properties of BNnanoXHx (X ) C, N)-LiH Composites. 3.2.1. BNnanoHx-LiH Composite. Hydrogen desorption from the BNnanoHx-LiH composite started at 100 °C and made several broad peaks, which are located around 200, 350 and more than 500 °C, as shown in Figure 4a, suggesting that hydrogen atoms in this composite were desorbed with several step reactions. The first hydrogen desorption temperature was much lower than those of BNnanoHx and LiH themselves. The TG profile indicated that this composite can desorb about 2.0 mass % H2 with heating to 500 °C. No clear peaks were observed around 200 °C in DTA. The profile above 300 °C showed gradual endothermic reactions, which were synchronized with the H2 desorption profile in TDMS. Figure 4b shows the results of XRD measurements of the BNnanoHx-LiH composite before and after the TG-DTA-TDMS measurement. In the XRD profile of the as-synthesized composite, the peaks corresponding to LiH and no other peaks were found. The LiH phase disappeared after hydrogen desorption by heating to 500 °C, indicating that at least LiH in this composite desorbed hydrogen even though LiH itself needs more than 650 °C to release hydrogen. In addition to the above results, a broad peak was observed around 25° in the sample after hydrogen desorption. Its position was close to that of the main diffraction peak of hBN. Here, this structural change was not found during the thermal decomposition process of BNnanoHx

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Figure 5. (a) Thermal gas desorption and TG-DTA profiles, (b) XRD patterns before/after heating to 500 °C, and (c) FT-IR spectra before/after heating at different temperatures of the BNnanoCHx-LiH composite.

itself. These results indicate that hBN-like material composed of Li and nanostructural BN may be formed as the dehydrogenated state. The variation of the chemical bonding states of H atoms in the composite with an increase in temperature is shown in Figure 4c. It was confirmed from a comparison with the FT-IR spectrum in Figure 3b that the B-H and N-H bonding states in BNnanoHx were not changed after synthesizing the composite with LiH. Furthermore, new broad peaks appeared around 2300 cm-1. To understand the chemical bonding state at each reaction step, heat treatments were performed for 8 h at 200 and 350 °C, which corresponded to hydrogen desorption peak temperatures. The B-H stretching mode at 2500 cm-1 was strongly reduced by heat treatment at 200 °C and almost disappeared at 350 °C. On the other hand, the absorption peak of the N-H stretching mode at 3400 cm-1 became gradually weak by heating. However, it still remained after heating to 500 °C, indicating that some parts of N-H bonding states in BNnanoHx could be thermodynamically stable. This peak would disappear by heating above 500 °C because H2 was continuously desorbed at the temperature, as shown in Figure 4a. These results indicate that the B-H bond reacts with LiH at a lower temperature than that of the N-H bond. With regard to the peak at 2300 cm-1, it was slightly grown during heating to 200 °C and almost disappeared at 350 °C. The peak may originate in a new phase, which could be formed from a part of two hydrides during the milling, although it was not identified in this work. From the above results, it is summarized that the hydrogen desorption of the BNnanoHx-LiH composite is induced by the interaction between the polarized functional groups and LiH. Due to this interaction, the hydrogen in both components is desorbed at lower temperature compared with the decomposition temperature of BNnanoHx and LiH themselves. Additionally, it seems likely that a nanocomposite of BN and Li might be formed after dehydrogenation, where it cannot be confirmed in this work. 3.2.2. BNnanoCHx-LiH Composite. As shown in Figure 5a, the hydrogen desorption profile of the BNnanoCHx-LiH composite revealed three broad peaks, which were located around 200, 350, and 500 °C. Among them, the first hydrogen desorption around 200 °C had much larger intensity than the others. Furthermore, CH4 and C2H6 desorption corresponding to mass number 16 and 28, in which the intensities are enlarged 10 times, were also observed in the temperature range from 150

to 400 °C. As shown in the thermal gas desorption profiles, the H2 and hydrocarbons desorption peak temperature, 200 °C, was much lower than those of BNnanoCHx. The relative peak area of CH4 to H2 was S16/S2 ∼ 0.029, and it was 10 times smaller than S16/S2 ∼ 0.250 of BNnanoCHx, indicating that the H2 was really desorbed instead of CH4 and C2H6 at lower temperature even though the hydrocarbons desorbed from BNnanoCHx itself above 400 °C, as shown in Figure 1b. The weight loss with gas desorption obtained by TG was about 6.0 mass %, which included the contribution of a small amount of hydrocarbon desorption. In addition, no endothermic or exothermic peaks were observed in the DTA shown inthe lower part of Figure 5a. XRD patterns of the as-synthesized and dehydrogenated BNnanoCHx-LiH composite are shown in Figure 5b. In the case of the as-synthesized composite, the peaks corresponding to the LiH phase were observed. These peaks disappeared after heating to 500 °C, indicating that hydrogen in LiH was desorbed by the interaction with BNnanoCHx. Small peaks around 26, 31, and 32° in the XRD pattern of the hydrogen desorbed sample in Figure 5b might be assigned to some impurity phases like oxides. After the H2 desorption, hBN-like material may be formed from Li, C, and nanostructural BN because it was confirmed that the broad peak around 25° was grown like the BNnanoHx-LiH composite. Figure 5c shows the results of FT-IR measurements at different temperatures for the BNnanoCHx-LiH composite. From the FT-IR spectrum of the as-synthesized composite, no change of the hydrogenated states, the B-H, C-H, and N-H bonds, were clarified even after making the composite with LiH. Moreover, a new broad peak was observed around 2300 cm-1, where it may originate in the same phase as the new material found in the case of the BNnanoHx-LiH composite. The peak intensity of B-H bonding at 2500 cm-1 was decreased by heat treatment at 200 °C for 8 h as well as the BNnanoHx-LiH composite. On the other hand, the peaks corresponding to the C-H stretching modes in the region from 3000 to 2840 cm-1 remained after heating at 200 °C although its shape was slightly changed. The peak intensity of C-H bonding drastically decreased after heat treatment at 350 °C for 8 h, suggesting that the release of the hydrogen atoms absorbed as the C-H bonds needs the higher temperature. Additionally, a peak appeared around 3230 cm-1 at 350 °C and it was unable to be

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Figure 6. (a) Thermal gas desorption and TG-DTA profiles, (b) XRD patterns before/after heating to 500 °C, and (c) FT-IR spectra before/after heating at different temperatures of the BNnanoNHx-LiH composite.

assigned currently. This peak might originate in an intermediate phase composed of B, N, and C atoms because it can be found only in the BNnanoCHx-LiH composite. On the other hand, the peak of the N-H bonding existed at 500 °C. However, the intensity of this peak was slightly weakened, indicating that the hydrogen atoms chemisorbed as the N-H bond were gradually released at higher temperatures compared with other bonds. The spectral change by the interaction between the functional groups and LiH with heat treatments revealed a systematic tendency. Strictly speaking, the suitable temperature for the interaction with LiH is related to the bond energies of the B-H, C-H, and N-H groups. The bove results indicate that the BNnanoCHx-LiH composite desorbs H2 as the main desorption gas from both hydrides by the interaction between the polarized functional groups and LiH. Noteworthy, the hydrogen atoms chemisorbed as the -CH2 and -CH3 groups are desorbed as H2 from 200 to 350 °C, different from the case of BNnanoCHx itself. 3.2.3. BNnanoNHx-LiH Composite. As shown in Figure 6a, the BNnanoNHx-LiH composite desorbed hydrogen around 200 °C without a detection of any NH3 gas emission corresponding to mass number 17, although only NH3 was released from BNnanoNHx itself. The TG profile revealed 2.0 mass % with H2 desorption by heating to 500 °C. This value directly corresponds with the H2 capacity of this composite. In DTA, a small endothermic peak was observed around 200 °C. Therefore, it was expected that this composite can be rehydrogenated thermodynamically. As shown in upper XRD pattern of Figure 6b, the peaks assigned to the LiH phase were found before H2 desorption. Although the diffraction peaks of hBN were observed for BNnanoNHx itself as shown in Figure 2c, it was noticed that these peaks were obviously weakened after being milled with LiH. This result indicates that the remaining hBN structure after synthesizing BNnanoNHx would be broken down to nanometer size by ball-milling under Ar gas for 2 h to make the composite. After the dehydrogenating treatment by heating to 500 °C, the peak intensity of LiH was suppressed and a broad peak around 25° appeared. This result suggests that this composite forms BNnanoN and Li compound after dehydrogenation as well as other composites. Quite small and relatively sharp peaks observed around 31, 32, and 42° might be caused by impurities.

The FT-IR spectrum of the as-synthesized BNnanoNHx-LiH composite in Figure 6c was almost the same as the spectrum of BNnanoNHx itself, indicating that the characteristic peaks corresponding to hBN were not changed after synthesizing this composite, contrary to the structural change observed by the XRD measurements, strictly speaking, the hBN structure still remained even though the destruction of hBN proceeded by the ball-milling to synthesize the composite. In the case of this composite, the peak around 2300 cm-1 assigned to a new phase was not observed, suggesting that the formation of the new phase may be caused by nanostructure. After the dehydrogenation under vacuum conditions at 200 °C, which was the main hydrogen desorption temperature, the peak at 3500 cm-1, which may originate in NH2-like bonding states, disappeared. And, then, the intensity of the peak located at 3400 cm-1 was obviously weakened during heating to 500 °C. Thus, the hydrogen desorption reaction would be continued above 500 °C, actually the hydrogen desorption profile obtained by TDMS measurement (Figure 6a) was in agreement with the result. From the above results, it is summarized that the hydrogen atoms chemisorbed as the N-H bonding in BNnanoNHx are desorbed as H2 at 200 °C without NH3 emission by the interaction between the polarized N-H groups and LiH even though BNnanoNHx itself desorbs only NH3. 4. Conclusions In this work, BNnanoHx, BNnanoCHx, and BNnanoNHx were synthesized from the hBN powder by ball-milling under a H2, CH4, and NH3 atmosphere, respectively. In each BNnanoXHx (X ) C, N) product, hydrogen atoms were chemisorbed as some kind of polarized functional group such as the B-H, C-H, or N-H bond at the edges and defects of B3N3 ring structure in hBN induced by ball-milling, which are characterized by FTIR measurements. BNnanoHx, BNnanoCHx, and BNnanoNHx, respectively desorbed H2, CH4, and NH3 in the wide temperature range from 100 to 900 °C. Therefore, it is concluded that the hydrogenated states and gas desorption properties strongly depend on the milling atmosphere. All the BNnanoXHx-LiH composites released H2 as a main desorbing gas around 200 °C. As we expected in the Introduction, the H2 desorption temperature was much lower than the

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decomposition temperatures of BNnanoXHx or LiH. From XRD and FT-IR measurements, it was clarified that the polarized functional groups and LiH were consumed by the H2 desorption, indicating that H2 came from both the hydrogenated states due to the mutual destabilization of them. All the composites revealed several step reactions to release H2 with an increase in temperature. These reactions originated in the interaction between the polarized functional groups with different bonding energies and LiH. After the H2 desorption reaction, BNnanoX and Li would form a hBN-like compound as the dehydrogenated state because the appearance of a broad peak was confirmed at almost the same position of the main diffraction peak of hBN in XRD patterns. Regarding the products, the formation of LiBNH or Li3BN2 would be thought of as possible compounds. LiBNH was reported as a dehydrogenated state of lithium amidoborane LiNH2BH3 by Xiong et al.26 On the other hand, Li3BN2 was reported as a dehydrogenated state of Li3BN2H8 by Aoki et al.30,31,33 and Pinkerton et al.29,32 To identify the dehydrogenated states, further research by other useful analyses, e.g., nuclear magnetic resonance (NMR), is required. Additionally, the exothermic or endothermic peak with the H2 desorption reaction was not revealed in DTA for almost all the composites, and only the BNnanoNHx-LiH composite showed small endothermic peak synchronized with the H2 desorption. The small enthalpy change with release of H2 is recognized as the characteristic feature on the composites. From the experimental facts in this work, it is concluded that the BNnanoXHx-LiH composite should be categorized into a hydrogen storage system characterized by the reaction between the hydrogen-containing functional groups and LiH. Acknowledgment. This work was supported by the project “Advanced Fundamental Research Project on Hydrogen Storage Materials” of the New Energy and Industrial Technology Development Organization (NEDO) and Research Fellowships of the Japan Society for the Promotion of Science for young Scientists (JSPS). We gratefully acknowledge Dr. Shigehito Isobe, Dr. Satoshi Hino, and Dr. Biswajit Paik for their help in this work. Supporting Information Available: XRD patterns. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes (1) Schlapbach, L.; Zu¨ttel, A. Hydrogen-storage materials for mobile applications. Nature 2001, 414, 353–358. (2) Grochala, W.; Edwards, P. P. Thermal decomposition of the noninterstitial hydrides for the storage and production of hydrogen. Chem. ReV. 2004, 104, 1283–1315. (3) Dillon, A. C.; Jones, K. M.; Bekkedahl, T. A.; Kiang, C. H.; Bethune, D. S.; Heben, M. J. Storage of hydrogen in single-walled carbon nanotubes. Nature 1997, 386, 377–379. (4) Orimo, S.; Majer, G.; Fukunaga, T.; Zu¨ttel, A.; Schlapbach, L.; Fujii, H. Hydrogen in the mechanically prepared nanostructured graphite. Appl. Phys. Lett. 1999, 75, 3093–3095. (5) Orimo, S.; Matsushima, T.; Fujii, H.; Fukunaga, T.; Majer, G. Hydrogen desorption property of mechanically prepared nanostructured graphite. J. Appl. Phys. 2001, 90, 1545–1549. (6) Chen, D. M.; Ichikawa, T.; Fujii, H.; Ogita, N.; Udagawa, M.; Kitano, Y.; Tanabe, E. Unusual hydrogen absorption properties in graphite mechanically milled under various hydrogen pressures up to 6 MPa. J. Alloys Compd. 2003, 354, L5–L9. (7) Isobe, S.; Ichikawa, T.; Gottwald, J. I.; Gomibuchi, E.; Fujii, H. Catalytic effect of 3d transition metals on hydrogen storage properties in mechanically milled graphite. J. Phys. Chem. Solids 2004, 65, 535–539. (8) Ichikawa, T.; Chen, D. M.; Isobe, S.; Gomibuchi, E.; Fujii, H. Hydrogen storage properties on mechanically milled graphite. Mater. Sci. Eng. B: Solid State Mater. AdV. Technol. 2004, 108, 138–142.

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