Article pubs.acs.org/JPCC
Dehydrogenation of Ammonia Borane Confined by Low-Density Porous Aromatic Framework Ye Peng,†,‡ Teng Ben,† Yi Jia,§ Dongjiang Yang,‡,⊥ Huijun Zhao,⊥ Shilun Qiu,*,† and Xiangdong Yao*,‡,§ †
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, 130012 Changchun, P. R. China Queensland Micro- and Nanotechnology Centre, Griffith University, Nathan, Queensland 4111, Australia § ARC Centre of Excellence for Functional Nanomaterials, The University of Queensland, St. Lucia, Queensland 4072, Australia ⊥ Centre for Clean Energy and Environment, Griffith University, Gold Coast Campus, Queensland 4222, Australia ‡
ABSTRACT: Ammonia borane (AB) has been considered as an outstanding candidate material for on-board hydrogen storage due to its high stoichiometric hydrogen content (19.6 wt %) and moderate dehydrogenation temperature. However, slow dehydrogenation kinetics below 100 °C and release of volatile byproducts (ammonia, borazine, and diborane) limited its practical applications. In this work, low-density and highly porous aromatic framework (PAF-1; BET, 4657 cm2 g−1; pore volume, 2.55 cm3 g−1) was utilized as a template for the first time to nanoconfine AB molecules. The dehydrogenation behavior of the confined AB was studied by temperatureprogrammed desorption mass spectrometry (TPD-MS) and pressure−composition−temperature (PCT) analyses. It was found that the AB molecules can be fully confined within the nanopores when the weight ratio of AB/PAF-1 is around 1:1. More importantly, AB started to dehydrogenate at very low temperature (around 50 °C) with the peak of 77 °C in the absence of any volatile byproducts such as ammonia, borazine, or diborane. Furthermore, about 4 wt % of hydrogen was evolved in the first 25 min at 75 °C which is 27 times higher than the pristine AB, displaying higher kinetics at low temperatures. Compared with other porous supports such as MOFs, the PAF-1 has a very low framework density because it is built up only by light C and H elements. This could significantly improve the hydrogen systemic gravimetric capacity of the AB-confined system and thus increase feasibility in practical applications. complex hydrides (e.g., NaAlH4, H3NBH3), etc.7 However, each of the methods has its own disadvantages; thus, it is still a challenge to put them into practical applications. For instance, Cheng et al. confirmed that the hydrogen storage capacity of the CNTs is less than 1.7 wt % under a pressure of 12 MPa and at room temperature,8 which is far below the 2015 targets (5.5 wt % of mass and 45 g L−1 of volume for the system) set by the U.S. Department of Energy (DOE).9 Yan et al. reported the most promising hydrogen storage results on MOF structure of NOTT-112,10 which achieved 10 wt % hydrogen storage capacity under 77 bar and 77 K. Unfortunately, the best hydrogen storage results on MOFs were only achieved at low temperatures owing to the weak isosteric heats of adsorption involved, which is obviously not in the operational temperature range of PEMFCs.11,12 Metal hydrides, in particular magnesium hydride with theoretical gravimetric storage capacity of 7.6 wt %, are limited for practical application mainly because of the slow hydrogen sorption kinetics and the high thermodynamic stability (required very high temperature to release hydro-
1. INTRODUCTION Development of economy and society needs renewable, versatile, clean, and efficient energy to substitute the conventional fossil fuels for automobiles. Following the use of the proton exchange membrane fuel cells (PEMFCs), highperformance hydrogen vehicles have attracted tremendous interest due to high-energy density, environmental friendly byproducts, and recycling properties of hydrogen. The main concern is how to store and transport this highly flammable gas in safe and efficient ways. There are several basic principles for practical on-board hydrogen storage: (i) high volumetric/ gravimetric storage capacity; (ii) moderate operating temperature with fast kinetics; (iii) low cost and/or reversibility.1 The traditional liquid hydrogen storage facilities require the additional energy and heavy container to maintain a cryogenic state (boiling point: −253 °C). High-pressure storage of hydrogen gas is limited by the weight of the storage canisters and also has the safety problem for on-board transport applications. In the last years, several solid-state methods have been investigated on hydrogen storage,2 such as carbon nanotubes (CNTs),3 porous materials adsorption (e.g., metal− organic frameworks (MOFs), zeolitic imidazolate frameworks (ZIFs)),4,5 metal hydrides (e.g., MgH2),6 imides (e.g., LiNH2), © 2012 American Chemical Society
Received: August 13, 2012 Revised: October 27, 2012 Published: November 15, 2012 25694
dx.doi.org/10.1021/jp308066e | J. Phys. Chem. C 2012, 116, 25694−25700
The Journal of Physical Chemistry C
Article
Scheme 1. (a) Diamond Topology; (b) Framework Structure of PAF-1 with the Topology of Diamond (Blue for Carbon Atoms and Grey for Hydrogen Atoms); (c) Confinement of AB within the Pores of PAF-1
gen).13 The imides also suffer from the very high temperature required to release the hydrogen for practical applications.14 Ammonia borane (H3NBH3, AB) is considered as an outstanding candidate material for on-board hydrogen storage due to its moderate dehydrogenation temperature (∼100 °C), high stability, nonflammability, and very high stoichiometric hydrogen content (19.6 wt %), assuming to meet the ultimate targets of the DOE (7.5 wt %). However, the practical application of pristine AB is restricted by its slow dehydrogenation kinetics below 100 °C and the release of the harmful byproducts (ammonia, borazine, and diborane) in the dehydrogenation process. Many methods have been reported to increase the rate of H2 release at relatively low temperatures, such as addition of metallic or metal complex catalytic,15 formation of alkali-, alkaline-earth metal hydrides,16 etc. However, the hydrogen release rate is still too slow (takes a few hours) at low temperatures to be practically used in real hydrogen storage systems although the kinetics has been significantly improved. Recently, a new method, confined AB inside nanopores of various porous materials, has been applied, and it is confirmed that this method is able to dramatically increase the hydrogen release at a low temperature below 100 °C.17−21 For example, Gutowska et al. loaded AB on the mesoporous silica SBA-15 to enhance greatly the hydrogen release rate and modify the enthalpy of AB decomposition.17 Li et al. reported that AB confined by Li-doped CMK-3 can completely eliminate the release of harmful volatile byproducts including ammonia and also significantly enhance the hydrogen release at low temperatures.21 More recently, Li et al.18 confined AB in a metal−organic framework JUC-32 and realized the hydrogen release of 10.2 wt % H2 at 95 °C in a few minutes, without any impurities. Accordingly, it is concluded that AB confined in porous materials can improve dramatically the hydrogen release rate at a low temperature below 100 °C, e.g., overcome the barrier of kinetics for hydrogen release for on-board applications. However, the AB-confined system suffers significantly from the decrease of hydrogen storage capacity as the template materials such as SBA-15, CMK-3, and JUC-32 have no contribution to the hydrogen capacity of the confined system. To minimize this negative effect of the template materials on the hydrogen capacity, a critical issue is to find a high surface area porous material with a very low density and high pore volume that can increase the AB loading rate as much as possible. Fortunately, novel porous aromatic frameworks (PAFs) have successfully been synthesized with very high thermal stability, larger specific surface area and pore volume, and very low density.22−25 For instance, PAF-1, built up only by
C and H light elements with diamond topology and narrow pore distribution (∼1.4 nm), has a BET surface area of 4657 cm2 g−1, its pore volume is as large as 2.55 cm3 g−1, and its thermal decomposition temperature in air is as high as 500 °C. More importantly, its density is as low as 0.3 g cm−3. Accordingly, it is hypothesized that the PAF-1 is an ideal template material to confine AB and potentially achieves fast hydrogen release kinetics and high hydrogen storage capacity of the confined AB system simultaneously. In the present work, AB confined in PAF-1 was synthesized (see Scheme 1), and its thermal dehydrogenation process was investigated by temperature-programmed desorption mass spectrometry (TPD-MS) and pressure−composition−temperature (PCT). It was found that the small AB molecules (∼0.45 nm) have been nanoconfined into the micropores of PAF-1. The loaded AB started to dehydrogenate at ∼50 °C, which is a big shift to low temperature compared with the unconfined AB (∼110 °C). Over 8 wt % hydrogen of AB was released in 50 min at 95 °C, which is equal to ∼4 wt % of the system. This hydrogen capacity is much higher than that of previous AB-confined systems.17−21
2. EXPERIMENTAL SECTION Synthesis of PAF-1. PAF-1 was synthesized according to the previous literature.23 Briefly, 1.05 mL of 1,5-cyclooctadiene was added to 120 mL of anhydrous DMF solution of 2.25 g of bis(1,5-cyclooctadiene)nickel(0) [Ni(cod)2] and 1.28 g of 2,2′bipyridyl. Tetrakis(4-bromophenyl)methane (1.00 g) was added to the purple solution after aging for 1 h at 80 °C. The mixture was stirred for 3 days. Concentrated HCl was added to the mixture in an ice bath. After filtration, the residue was washed with H2O, THF, and CHCl3, respectively, and dried at 200 °C under vacuum for 10 h to obtain ∼500 mg offwhite powder. The final product was kept in a glovebox. Synthesis of AB-PAF-1. AB was loaded into PAF-1 by solution infusion process.18 Typically, AB (purchased from Sigma-Aldrich, 97% with no further purification and kept in argon-filled glovebox) was resolved in anhydrous methanol (purchased from Sigma-Aldrich) in a glovebox at room temperature (0.5 M). The weight ratios of AB and PAF-1 were 3:1, 2:1, 1:1, 1:2 (as xAB-PAF1, x = 3, 2, 1, 0.5), respectively. A required amount of PAF-1 was added to the solution and stirred for 4 h. The mixture was dried at room temperature under high vacuum overnight to remove methanol and then stored in a glovebox. 25695
dx.doi.org/10.1021/jp308066e | J. Phys. Chem. C 2012, 116, 25694−25700
The Journal of Physical Chemistry C
Article
Figure 1. (A) TPD-MS spectra of AB (black ash line) and AB-PAF-1 (red line). (B) Time dependences of hydrogen release from AB-PAF-1 at 95 °C (blue), 85 °C (black), 75 °C (red), and pristine AB at 85 °C (orange). (C) Arrhenius treatment of PCT rate data yields a straight line with a gradient that is proportional to the apparent activation energy of AB-PAF-1 (Ea ≈ 116 kJ mol−1).
3. CHARACTERIZATION Powder X-ray diffraction (PXRD) was performed on a Rigaku MIniflex diffractometer using monochromatic Co Kα radiation. The measuring 2θ range was from 4° to 60° at the rate of 1.2°/ min. The voltage and the current were set to be 40 kV and 40 mA, respectively. N2 sorption isotherms measurements were performed on a Micro Meritics Tristar II Surface Area and Pore Size Analyzer at 77 K. The Fourier transform infrared spectroscopy (FTIR) spectra were collected (4000−400 cm−1 region) on a Nicolet 6700 FTIR spectrometer. Temperature-programmed desorption mass spectrometry (TPD-MS): MS data were collected by Brooks 5850E mass flow S2 controllers attached with a glass reactor in a tube furnace. The temperature was raised from 25 to 200 °C at a rate of 1 °C/min with helium flow at a rate of 50 mL/min. Hydrogen storage properties were examined by pressure− composition−temperature (PCT) analyses on an automated Sieverts’ apparatus (Suzuki Shokan PCT H2 Absorption Rig), and the desorption measurements were performed at various temperatures under 1 kPa.
borazine, ammonia, and diborane, respectively (Figure 1A-b, c, d, black dot lines). This limits the practical application of pristine AB because even a trace amount of ammonia will poison the catalyst of fuel cells. In the AB-PAF-1 system, the decomposition of nanophase AB in the pores starts at about 50 °C, and the decomposition peak shifts from 105 °C of pure AB to 77 °C, which is reduced by 28 °C. The temperature reduced is more significant than that of AB in SBA-15 and CMK-3 systems (about 10 °C), and similar to that of AB in the JUC-32 system (30 °C). More importantly, the second decomposition peak is not observed, which suggests there are no volatile byproducts released in the decomposition process confirmed by MS measurement (Figure 1A-b, c, d, red lines). Time dependences of hydrogen release at different temperatures for AB and AB-PAF-1 were studied by PCT analysis (Figure 1B). Pristine AB releases less than 1 wt % H2 at 85 °C within 1 h at very slow kinetics. However, the release rate of AB loaded in PAF-1 is obviously much faster at the same temperature. For instance, 5.3 wt % of H2 releases from ABPAF-1 sample within 25 min, and the value reaches 7.0 wt % within 180 min. Even at lower temperatures, the rate and the total amount of H2 release are significant. It releases 3.5 and 5.5 wt % at 75 °C within 25 and 180 min, respectively. It is also observed that the AB confined in PAF-1 can release 7.2 wt % H2 at 95 °C within 25 min and 10.8 wt % in 180 min. TPD-MS results show no volatile byproducts of ammonia, borazine, or diborane signs during the thermal decomposition process up to 200 °C. Activation energy for hydrogen release from AB-PAF-1 could be calculated by the Arrhenius equation (Figure 1C). The value is Ea ≈ 116 kJ mol−1, which is lower than Ea ≈ 184 kJ mol−1 of pristine AB21 and thus provides direct evidence for the possibility of nanoconfinement effect as the dehydrogenationenhanced role in the AB-PAF-1 system.
4. RESULTS AND DISCUSSION Thermal decomposition properties were studied by the TPDMS (Figure 1A) for AB and AB-PAF-1 (weight ratio of AB and PAF-1 is 1:1). Apparently, the pristine AB has two-step decomposition (
