Platinum Nanoparticle Functionalized CNTs as Nanoscaffolds and

Nov 18, 2010 - ... for the implementation of the “hydrogen economy” is the question of how to store ...... Miriam Rueda , Luis Miguel Sanz-Moral ,...
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J. Phys. Chem. C 2010, 114, 21885–21890

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Platinum Nanoparticle Functionalized CNTs as Nanoscaffolds and Catalysts To Enhance the Dehydrogenation of Ammonia-Borane S. F. Li, Y. H Guo, W. W. Sun, D. L. Sun, and X. B. Yu* Department of Material Science, Fudan UniVersity, Shanghai 200433, China ReceiVed: September 23, 2010; ReVised Manuscript ReceiVed: NoVember 5, 2010

In this paper, the dehydrogenation properties of ammonia-borane (AB, NH3BH3) modified with platinum nanoparticle functionalized carbon nanotubes (CNTs) (Pt@CNTs) that were prepared through a new “ammoniadeliquescence” method are reported. It has been demonstrated that the synergetic catalysis of CNTs and platinum nanoparticles, and the nanoconfinement of AB incorporated into the CNTs are two crucial factors in enhancing the dehydrogenation of AB. Both CNTs and platinum nanoparticles showed favorable catalytic activities toward the thermolysis of AB, which not only depressed the emission of the poisonous byproduct borazine, but also prevented severe material foaming and expansion during the decomposition. Meanwhile, the nanoconfinement of AB through the “ammonia-deliquescence” method led to enhanced dehydrogenation kinetics, releasing the first equivalent of hydrogen at 70 °C within 5 h, while no hydrogen was released from pristine AB under the same conditions. By the Arrhenius method, the activation energy of the modified AB was calculated to be 106.2 kJ mol-1, which is reduced considerably compared to the activation energy for the pristine AB (137.8 kJ mol-1). These results indicate that the “ammonia-deliquescence” method combined with the utilization of Pt@CNTs as catalyst is an effective approach in modifying the properties of AB with favorable dehydrogenation. 1. Introduction Nowadays, hydrogen is considered as one of the best candidates to satisfy the increasing demand for an efficient and clean energy carrier because of its abundance, high-energy density, and environmental friendliness. However, one of the principle obstacles for the implementation of the “hydrogen economy” is the question of how to store hydrogen on-board more securely and effectively.1,2 To solve this problem, tremendous efforts have been devoted to research and development on materials that can hold sufficient hydrogen in terms of gravimetric and volumetric densities, and, at the same time, possess suitable thermodynamic and kinetic properties.3 After decades of exploration, it has been demonstrated that using solid media, such as sorbent materials4,5 (activated carbon, carbon nanotubes, metal-organic frameworks, etc.) and hydrides6-8 (metal hydrides, complex hydrides, chemical hydrides, etc.), is the safest and most effective way to store hydrogen. Among these hydrogen storage materials, accordingly, ammonia-borane appears to be a suitable hydrogen source and is attracting more and more interest in the field of solid-state hydrogen storage because of its abnormally high hydrogen content of 19.6 wt.% and well-behaved stability under ambient conditions.9-22 However, despite these favorable properties of ammonia-borane, there are three serious drawbacks which need to be overcome in order to make it suitable for practical on-board application: (1) the relatively high dehydrogenation temperature (>100 °C) and low hydrogen release rate;23,24 (2) the emission of a poisonous byproduct (borazine, ammonia, and diborane) and severe material foaming during the dehydrogenation;9-11 and (3) the difficulty of regeneration. To overcome the above drawbacks and improve the dehydrogenation of AB, a number of approaches have been * To whom correspondence should be addressed. Phone and Fax: +8621-5566 4581. E-mail: [email protected].

developed recently, including hydrolysis,12,13 transition metal14 or acid catalysts,15 and nanoconfinement by nanoscaffolds,9,11,16 as well as chemical modification of AB through replacing one of its H with an alkali or alkaline earth element to form metal amido-borane,17-22 and so forth. Among these explorations, nanoconfinement by nanoscaffolds has been demonstrated to be an effective route to improve the dehydrogenation properties of AB. For example, after an internal coating of AB in nanoscaffolds (SBA-15), using methanol as solvent, Tom Autrey9 and his co-workers found that the kinetics and thermodynamics of AB in H2 release were improved. Meanwhile, Cao et al.11 lowered the activation energy of AB and suppressed the emission of harmful byproduct by dissolving AB in tert-butyl alcohol and spreading it into carbon cryogels and BN-modified carbon cryogels. However, the emission of byproduct (borazine) from the above two systems was only partly restrained, and the issue of elimination of the solvents (i.e., methanol and tert-butyl alcohol) was brought into the system as another new problem. Therefore, exploring new approaches that not only proceed with simple synthesis routes but also can combine the strong points of both nanoconfinement and catalysis to jointly enhance the dehydrogenation of AB is still a challenging issue. In this paper, we introduce a new route, the “ammoniadeliquescence” method, to synthesize nanosized AB incorporated into platinum nanoparticle-functionalized carbon nanotubes (Pt@CNTs) template, which combines the synergetic catalysis of CNTs and Pt nanoparticles, as well as the nanoconfinement of AB, resulting in a significant improvement in the dehydrogenation of AB. 2. Experimental Section NH3BH3 (90% Sigma-Aldrich) and H2PtCl6 · 6H2O (99.9% Alfa-Aesar) were used as received. Raw CNTs (95%) were

10.1021/jp1091152  2010 American Chemical Society Published on Web 11/18/2010

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purchased from Applied Sciences Inc. The purity of the ammonia used in the experiment is approximately 99%. Preparation of Pt@CNTs. The Pt@CNTs were synthesized according to a previous report in the literature.25 Raw multiwalled carbon nanotubes (MWCNTs) were refluxed in HNO3 (68 wt %) for 14 h at 135 °C in an oil bath to remove the amorphous carbon and open the ends of the CNTs. Then, the liquid was filtered and washed with deionized water, followed by drying at 100 °C for 12 h. One gram of the obtained CNTs was immersed in 6 mL of acetone solution containing 300 mg of H2PtCl6 · 6H2O. Through ultrasonic treatment and stirring, CNTs were dispersed homogeneously in the solution. At the same time, the acetone was slowly vaporized to form an inklike solution under a carefully controlled process. The Pt precursor was then loaded onto the surface of the CNTs. After the vaporization of the solvent, the mixture was dried in an oven at 80 °C. Then, the dried mixture was heated in H2 flow at 300 °C for 3 h. Subsequently, the Pt nanoparticle-functionalized CNTs were outgassed for 2 h at 300 °C for desorption of H2 from the resultant Pt clusters. Preparation of AB Loading on Pt@CNTs under Ammonia Atmosphere. AB and the as-prepared Pt@CNTs were directly mixed together with a mass ratio of 1:1 in a glovebox filled with argon. AB and the mixture was ball milled on a QM-3SP2 planetary ball mill at 470 rpm under argon for 0.5 h. The mass ratio of ball-to-powder was 20:1. The milled AB/Pt@CNTs mixture was then put into an ice water bath under NH3 flow. A visible deliquescence of AB was observed under these circumstances, resulting from the formation of liquid AB · xNH3 complex26 (Supporting Information Figures S1 and S2). By ultrasonic treatment, the liquid AB · xNH3 was first coated on the surface of the Pt@CNTs, and then part of the liquid AB · xNH3 was impregnated into the CNT-channels by capillary forces. Finally, this sample was exposed to vacuum at room temperature for 3 h to eliminate the ammonia, leading to the formation of AB/Pt@CNTs. Characterizations. Powder X-ray diffraction (XRD, Rigaku D/max 2400) measurements were conducted to confirm the phase structure. Powders were spread on a Si single crystal before measurements were taken. Amorphous polymer tape was used to cover the surface of the powder to avoid oxidation during the XRD measurement. Morphologies of samples were observed on a scanning electron microscope (SEM) (FE-SEM S-4800). Thermogravimetry-mass spectroscopy (TG-MS, Netzsch STA 409C combined system) was conducted under 1 bar argon from room temperature to 250 °C with a heating rate of 5 °C min-1 for the analysis of the evolved gas during decomposition of samples. Volumetric release for quantitative measurements of hydrogen desorption from samples was carried out on a Sievert’s type apparatus under 1 bar argon. Approximately 0.1 g of the sample was loaded and heated with a heating rate of 5 °C min-1 from room temperature to 250 °C. The pressure data (P) and the temperature data (T) were recorded automatically at every other 6 s. Finally, according to the equation: PV ) nRT, where R is the gas constant, the mol (n) of the gas released from the sample could be calculated. The pore structure of the samples wasanalyzedbymeansofnitrogensorption(Brunauer-Emmett-Teller (BET) technique) at -196 °C by using a Quantachrome NOVA 4200e instrument. For the density measurements on the AB, Pt@CNT, and AB/Pt@CNTs (1:1 by weight), samples were submerged in hexyl hydride (n-hexane) at 20 °C. According to Archimedes’ principle, the volume of a sample is equal to that of the n-hexane spilled over. Then, according to the known density (0.662 g cm-3) of n-hexane at 20 °C, the specific density

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Figure 1. XRD patterns for the pristine AB, CNTs, Pt@CNTs, ballmilled AB/CNTs, ball-milled AB/Pt@CNTs, and the loaded AB/ Pt@CNTs. The origin of several diffraction peaks below 20° is assigned to the polymer tape used to cover the surface of the powder to avoid oxidation during the XRD measurement.

of AB, Pt@CNTs, and AB/Pt@CNTs could be calculated. Differential scanning calorimetry (DSC, Netzsch 200) was conducted under 1 bar argon from room temperature to 250 °C at a heating rate of 5 °C min-1 to verify the thermal change. The DSC data was differentiated in the OriginPro 8 to obtain the onset and terminal temperature of each exothermic and endothermic peak. Then the area (n mJ/mg) of every peak under the DSC curve between the onset and terminal temperature could be approximately calculated by integrating in the OriginPro 8. On the basis of the area of the peaks, the reaction enthalpy data could be calculated. 3. Results and Discussion Figure 1 shows the XRD patterns for the loaded AB/ Pt@CNTs compared with the ball-milled AB/CNTs and AB/ Pt@CNTs. After loading Pt on CNTs, the peaks ascribed to Pt appear, indicative of the formation of the Pt@CNT hybrid. The ball milling of AB with CNTs or Pt@CNTs results in additional broad peaks with 2θ at 22.5-25° and 33-35°, which are assigned to AB, implying that the AB did not decompose or react with the CNTs or Pt during the ball milling. Note that the peaks of AB in the ball-milled samples are much broader than in the pristine sample, indicating that ball milling caused a reduction in the crystal grain size of AB. Furthermore, compared with the ball-milled samples, the peaks of AB in the loaded AB/Pt@CNTs are almost no difference, which indicates that the “ammonia-deliquescence” method also resulted in a physical mixing of AB with Pt@CNTs, and only part of the AB was confined into the channels of Pt@CNTs template and the rest formed layers on the outside (see later discussion). Figure 2 shows SEM images of the loaded AB/Pt@CNTs compared with the ball-milled AB/Pt@CNTs and AB/CNTs. The CNTs before and after treatment are also shown as references. It can be observed that after treatment by nitric acid the ends of the raw CNTs were opened (Figure 2b, open ends marked with arrows). After loading Pt on the treated CNTs, the Pt nanoparticles were uniformly supported on the surface of the open-ended CNTs, with an average particle diameter of 3-8 nm (Figure 2c). Energy dispersive spectroscopy (EDS) revealed that the content of Pt in the Pt@CNTs was about 10 wt % (Supporting Information Figure S3). In the images of the ball-milled AB/CNTs and AB/Pt@CNTs samples (Figure 2d,e), bulk AB powders with a particle size of a few micrometers are

CNTs Enhance the Dehydrogenation of Ammonia-Borane

Figure 2. SEM images of (a) raw CNTs, (b) CNTs with open-ends, (c) Pt@CNTs, (d) ball-milled AB/CNTs, (e) ball-milled AB/Pt@CNTs, and (f) loaded AB/Pt@CNTs.

observed. However, no bulk AB powders are present in the loaded AB/Pt@CNTs sample (Figure 2f) prepared by the “ammonia-deliquescence” method, suggesting that part of the AB was probably encapsulated into the CNT-channels and the rest was uniformly coated on the surface of the Pt@CNTs. To verify the incorporation of AB into the CNT-channels, BET and density measurements of the loaded AB/Pt@CNT sample compared with the Pt@CNT were carried out. The BET specific surface area of the loaded AB/Pt@CNTs (40.6 m2 g-1), as calculated from the Barrett-Joyner-Halenda (BJH) model, is much lower than that for the Pt@CNTs sample (168.5 m2 g-1), along with a significantly reduced intensity in the pore size distribution (Supporting Information Figure S4), indicating the possibility of (1) successful incorporation of a large amount of AB into the CNT-channels or (2) that the open-ended CNTs were blocked by the AB molecules. To clarify this phenomenon, further density measurements were conducted. According to Archimedes’ principle, the specific densities of AB, Pt@CNTs, and AB/Pt@CNTs were calculated to be 0.785, 1.665, and 1.035 g cm-3, respectively. Theoretically, the specific density of AB/ Pt@CNTs can be calculated by the equation

DAB/Pt@CNTs )

WAB + WPt@CNTs WAB WPt@CNTs + + VB 0.785 1.665

where DAB/Pt@CNTs is the theoretical specific density of AB/ Pt@CNTs, WAB and WPt@CNTs are the respective weights of AB and Pt@CNTs in the sample, and VB is the pore volume of Pt@CNTs blocked by AB. According to the equation, while the open ends of CNTs are completely blocked by AB, the minimum value of DAB/Pt@CNTs is 0.835 g cm-3, and the

J. Phys. Chem. C, Vol. 114, No. 49, 2010 21887 maximum value of DAB/Pt@CNTs, which assumes a full incorporation of AB into the pores of Pt@CNTs, is 1.067 g cm-3. By a combination of the BET result and the density of the AB/ Pt@CNTs tested by Archimedes’ principle (1.035 g cm-3), it can be safely concluded that the majority of the pores of the Pt@CNTs have been impregnated by AB. The filling rate is as high as 88.9%, calculated by (0.52 - VB)/0.52. (The cumulative pore volume of Pt@CNTs tested by BET was approximately 0.52 cm3 g-1.) This is also in good agreement with the BET result. The above results strongly confirm that the “ammoniadeliquescence” method is an effective approach to confine AB in mesoporous materials, along with the following advantages: (1) no solvent is involved during preparation; (2) the loaded AB content is controllable; and (3) the AB is formed uniformly. However, it is noting that based on the cumulative pore volume of Pt@CNTs (0.52 cm3 g-1) and the specific densities of AB (0.785 g cm-3), the maximum content of AB confined into the channels of 1 g Pt@CNTs is calculated to be 0.4082 g (0.52 × 0.785). Therefore, the exact content of AB confined into the channels of Pt@CNTs is 0.3629 g (0.4082 × 88.9%), resulting in about 0.6371 g AB formed layers on the outside of the Pt@CNTs, which can explain the phenomenon of visible AB peaks presented in the loaded AB/Pt@CNTs sample as did in the ball milled AB/Pt@CNTs (Figure 1). The TG-MS results for all the studied samples are shown in Figure 3. Similar to previous reports in the literature, the pristine AB released the first equivalent of H2 at temperatures above 120 °C and the second equivalent of H2 at ∼160 °C, accompanied by a large evolution of borazine at 150-200 °C.9,10,23 After ball milling with CNTs, apparent depression of borazine release was achieved. Given ball milling exhibited little improvement on depressing the emission of borazine from AB (Supporting Information Figure S5), the above result indicates that CNTs are effective in improving the dehydrogenation of AB. In the case of the ball-milled AB/Pt@CNTs sample, further restraint of the evolution of borazine is observed, and the dehydrogenation temperature is slightly shifted to lower temperature. The improvement due to Pt of the dehydrogenation of AB was further confirmed by an AB/PtCl2 mixture (Supporting Information Figure S6). These results clearly demonstrate that Pt has a synergetic catalytic effect toward improving the dehydrogenation of AB. As for the loaded AB/Pt@CNTs sample, even further improvement, particularly in the dehydrogenation kinetics, can be obtained. This sample showed first and second desorption peaks at about 108 and 150 °C, respectively, which are 12 and 10 °C lower than for the pristine AB. The improvement in the kinetics can be ascribed to the nanoconfinement of AB through the “ammonia-deliquescence” method, with uniform dispersion on the surface and in the interior of the open-ended CNTs, as described above. The TG results in Figure 3b indicate that the weight loss of the modified AB is decreased compared to the pristine AB. For the loaded AB/Pt@CNTs sample in particular, the total weight loss by 225 °C is 10.8 wt %, which is comparable with its theoretical hydrogen capacity (9.8 wt %, calculated from half of the theoretical hydrogen capacity of AB (19.6 wt %), owe to the addition of the 50% Pt@CNTs in the composite), suggesting a significant decrease in the emission of byproduct. From combination of the volumetric (Supporting Information Figure S7) and TG results (Figure 3b), the proportion of gas evolution has been calculated and is shown in Figure 3c. The figure makes it clear that the emission of byproduct is decreased gradually in the order of AB > ball-milled AB/CNTs > ball-milled AB/ Pt@CNTs > loaded AB/Pt@CNTs, in which the hydrogen purity

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Figure 4. Photographs of samples (a) before dehydrogenation and (b) after dehydrogenation. (Left vial: pristine AB, 50 mg. Middle vial: ballmilled AB/Pt@CNTs, 100 mg. Right vial: loaded AB/Pt@CNTs, 100 mg).

Figure 3. (a) MS spectra and (b) TG curves for the pristine AB, ballmilled AB/CNTs, ball-milled AB/Pt@CNTs, and loaded AB/Pt@CNTs. (c) Content of borazine released from all the samples. The mass ratio of AB to templates for all the composites is 1:1.

in the loaded AB/Pt@CNTs is as high as 98.9 mol %. Besides the depression of the evolution of borazine and the improvement of the dehydrogenation kinetics, another important observation for the loaded AB/Pt@CNTs is that no material foaming and expansion occurred during the decomposition, as shown in Figure 4. After dehydrogenation, the volume of the AB is >20 times larger than that of the pristine AB, indicating serious material foaming during hydrogen release, which is another fatal drawback for practical application as a hydrogen storage material, due to the dramatic decrease in the volumetric hydrogen density. However, only a small volume increase occurs for the ball-milled AB/Pt@CNTs, and no change can be observed for the loaded AB/Pt@CNTs. The complete prevention of material foaming and expansion in the loaded AB/Pt@CNTs may be attributed to its decreased dehydrogenation temperature, now centered at 108 °C below its melting point (i.e., 114 °C), where reacting species set up linkages with each other to form polyaminoborane (PAB), whose melting point is higher than that of the pristine AB.10

To gain the distinctly enhanced kinetics, the time dependence of the first step H2 release from the pristine AB and loaded AB/Pt@CNTs at different temperatures was measured, as shown in Figure 5. It can be seen that the loaded AB/Pt@CNTs sample releases the majority of the first equivalent of hydrogen at 70 °C within 5 h without any observable induction period (Figure 5b), while no hydrogen release is observed for the pristine AB under the same conditions and an induction period of about 2 h is required, even at 80 °C (Figure 5a), clearly indicating that the loaded AB/Pt@CNTs sample has an enhanced dehydrogenation rate compared to the pristine AB. The activation energies can be calculated from the various isothermal hydrogen desorption curves through the Arrhenius equation as shown below

( )

K ) A exp

-Ea RT

where K is the rate constant; A is the pre-exponential factor; R is the gas constant; T is absolute temperature; and Ea is the activation energy. The plot of ln K versus 1/T is linear, and the slope of the resulting line corresponds to the value of the activation energy. The activation energy for releasing the first equivalent of hydrogen from the pristine AB is calculated to be 137.8 kJ mol-1, which is comparable to the value reported in the literature.10 As for the loaded AB/Pt@CNTs sample, the activation energy is decreased to ∼106.2 kJ mol-1, indicating a considerable enhancement of the dehydrogenation kinetics of AB. On the basis of the above results, it is evident that the loaded AB/Pt@CNTs produced by the “ammonia-deliquescence” method exhibit improved dehydrogenation capabilities in contrast to the

CNTs Enhance the Dehydrogenation of Ammonia-Borane

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Figure 6. DSC profiles for the dehydrogenation of pristine AB, ballmilled AB/CNTs, ball-milled AB/Pt@CNTs, and loaded AB/Pt@CNTs. The heating rate is 5 °C min-1.

Figure 5. Time-dependent isothermal volumetric release, allowing quantitative measurements of the first step hydrogen release from (a) pristine AB and (b) loaded AB/Pt@CNTs at various temperatures. (c) Arrhenius treatment of the temperature-dependent data, which give the activation energy of pristine AB and the loaded AB/Pt@CNTs.

pristine AB. To understand the role of the CNTs and Pt, as well as the effect of nanoconfinement on the improvement in the thermodynamics and kinetics of AB, DSC measurements were carried out on different samples, as shown in Figure 6. The thermal decomposition of AB shows an endothermic dip at approximately 110 °C (assigned to the melting of AB) and two exothermic peaks, one at approximately 120 °C and a smaller one at 160 °C, which are associated with the release of the first and second equivalent of hydrogen, respectively.11,23 However, the endothermic nature of AB melting (∼110 °C) is hardly detectable for the loaded AB/Pt@CNTs relative to the

pristine AB and the ball-milled samples at the same heating rate (5 °C min-1), indicating that AB in the loaded sample releases the majority of the first equivalent of H2 to form polymeric amino-borane (BH2NH2)x below its melting point.10 This corresponds to the observation of no material foaming for the loaded sample. On the other hand, the decomposition reaction enthalpy for the milled and loaded AB/Pt@CNTs samples (∆H ) -8.1, -7.5, and -2.6 kJ mol-1 for the ballmilled AB/CNTs and AB/Pt@CNTs, and the loaded AB/ Pt@CNTs, respectively) are significantly less than for the pristine AB (∆H ) -23.3 kJ mol-1), indicating that the synergetic effects of CNTs and platinum nanoparticles, as well as the nanoconfinement of AB through the CNTs, lead to a change in the thermodynamics of AB during decomposition. This phenomenon could be explained as follows: if the PAB and polyiminoborane (PIB) polymeric decomposition products, formed in the Pt@CNTs scaffold, do not form the more stabilized cyclization products (borazine), there must be a measurable difference between the reaction enthalpies for H2 loss from the AB/Pt@CNTs and the pristine AB.9 Thus, the decrease in the enthalpy between the milled and loaded AB/ Pt@CNTs is also responsible for the suppression of the volatile side-product in the MS results. As indicated by the XRD and SEM results, ball milling just results in a simple physical mixing, and the change in the thermodynamic properties of the AB in the ball-milled samples can be mainly attributed to the synergetic catalysis of CNTs and platinum nanoparticles. However, for the loaded AB/Pt@CNTs sample, the nanoconfinement of AB may also play a crucial role in improving both the thermodynamic and the kinetic properties, as demonstrated by the further decrease in the decomposition reaction enthalpy, as well as the lowered dehydrogenation temperature. According to the XRD, SEM, BET, and density measurement results, it is concluded that through the “ammonia-deliquescence” method AB was successfully confined inside the CNTchannels or formed uniform layers on the surface of the Pt@CNTs without any reaction or decomposition, as illustrated in Scheme 1. The lower dehydrogenation temperatures and activation energies of the loaded AB/Pt@CNTs can be attributed to the size-dependent surface energy of AB confined inside the channels or formed into layers on the surface. Because the CNTs can reduce the distance for diffusion of hydrogen, they increase the frequency of the reaction, which effectively accelerates the dehydrogenation process and serves as an efficient pathway for heat transfer.11 Reduction in the kinetic barrier was also observed for AB spread on other nanoscaffolds, such as N-B modified

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SCHEME 1: Schematic Model of “AmmoniaDeliquescence” Method for Synthesis of the Loaded AB/Pt@CNTs

Li et al. Acknowledgment. This work was financially supported by the National Natural Science Foundation of China (Grant 51071047), the Program for New Century Excellent Talents in Universities (NCET-08-0135) and the Shanghai Leading Academic Discipline Project (B113). Supporting Information Available: Figures S1-S10. This material is available free of charge via the Internet at http:// pubs.acs.org. References and Notes

carbon cryogel,11 SBA-15,9 or mesoporous carbon frameworks.16 To further indentify the effects of nanoconfinement of AB on its thermodynamic and kinetic properties, various contents of AB were loaded on the Pt@CNTs. The decomposition properties indicate that both the dehydrogenation temperature and the evolution of byproduct were increased with the increase of the increasing AB content in the AB/Pt@CNTs composite (Supporting Information Figures S8 and S9). SEM images reveal that a high content loading of AB fails to form uniform AB instead of apparent agglomerations of AB on a large scale (Supporting Information Figure S10). These results, again, support the important role of the nanoconfinement of AB in improving its dehydrogenation properties. All in all, although the detailed decomposition mechanism in the AB/Pt@CNTs is still not clear, it is concluded that there is a synergetic catalysis of CNTs and Pt nanoparticles, as well as beneficial effects from the nanoconfinement of AB, which are jointly responsible for changing the mechanisms involved in the thermal dehydrogenation of AB. 4. Conclusion An “ammonia-deliquescence” method was successfully developed for loading AB into Pt@CNTs nanoscaffolds. Our experimental results show that this method not only combines the synergetic catalysis of CNTs and Pt nanoparticles, but also encapsulates AB into CNT-channels and coats the CNTs surface with uniform layers. The samples prepared by this method exhibit excellent dehydrogenation properties in contrast to pristine AB: (i) lower hydrogen release temperatures (70-190 °C) without an induction period; (ii) significant depression of the volatile byproduct (borazine); (iii) no foaming and expansion during the dehydrogenation; and (iv) much less exothermic (-2.6 kJ mol-1) reaction than for the pristine AB (-23.3 kJ mol-1) during the decomposition, which is promising for the development of convenient regeneration routes with molecular hydrogen from AB.27 These advantages give the modified AB strong potential as a hydrogen storage material for on-board application.

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