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2008, 112, 3164-3169 Published on Web 02/13/2008
NMR Confirmation for Formation of [B12H12]2- Complexes during Hydrogen Desorption from Metal Borohydrides Son-Jong Hwang,*,† Robert C. Bowman, Jr.‡ Joseph W. Reiter,‡ Job Rijssenbeek,§ Grigorii L. Soloveichik,§ Ji-Cheng Zhao,§ Houria Kabbour,| and Channing C. Ahn| DiVision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California 91109, GE Global Research, Niskayuna, New York 12309, and DiVision of Engineering and Applied Science, California Institute of Technology, Pasadena, California 91125 ReceiVed: NoVember 14, 2007; In Final Form: January 22, 2008
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B NMR spectroscopy has been employed to identify the reaction intermediates and products formed in the amorphous phase during the thermal hydrogen desorption of metal tetrahydroborates (borohydrides) LiBH4, Mg(BH4)2, LiSc(BH4)4, and the mixed Ca(AlH4)2-LiBH4 system. The 11B magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) spectral features of the amorphous intermediate species closely coincide with those of a model compound, closo-borane K2B12H12 that contains the [B12H12]2- anion. The presence of [B12H12]2- in the partially decomposed borohydrides was further confirmed by high-resolution solution 11B and 1H NMR spectra after dissolution of the intermediate desorption powders in water. The formation of the closo-borane structure is observed as a major intermediate species in all of the metal borohydride systems we have examined.
Introduction A critical challenge to the implementation of hydrogenpowered vehicles is the development of suitable on-board hydrogen storage systems and materials that can satisfy the performance targets proposed by the U.S. Department of Energy.1,2 Among the hydrogen storage options, solid-state systems offer several advantages with respect to volume and safety albeit with significant weight penalties for nearly all metal hydrides.2 Metal borohydrides M(BH4)n have among the highest theoretical hydrogen storage capacities when compared to most traditional metal hydrides and other complex hydrides such as alanates or amides. Consequently, the synthesis, properties, and hydrogen storage behavior of the lighter element borohydrides (i.e., LiBH4 and Mg(BH4)2) are being investigated extensively.3-27 The generic maximum release of hydrogen from M(BH4)n during thermal desorption is thought typically to follow the reaction paths4-8
M(BH4)n f M + nB + (2n)H2
(1)
M(BH4)n f MHx + nB + [2n-x/2]H2
(2)
depending on whether a stable binary hydride (i.e., MHx) or elemental metal (M) is the final product. In either case, B is presumed to be in an amorphous state since it is not detected * To whom correspondence should be addressed. † Division of Chemistry and Chemical Engineering, California Institute of Technology. ‡ Jet Propulsion Laboratory, California Institute of Technology. § GE Global Research. | Division of Engineering and Applied Science, California Institute of Technology.
10.1021/jp710894t CCC: $40.75
by X-ray diffraction.3,11,13,14,24,27 Unfortunately, there are several issues with reactions 1 and 2 that include: (i) the hydrogen pressures are much too low at the temperatures available from proton exchange membrane (PEM) fuel cells, (ii) slow kinetics, (iii) concurrent evolution6,15,25 of toxic diborane (B2H6) during desorption contaminating PEM fuel cells and decreasing cycle life, and (iv) limited regeneration of the borohydride following desorption if it occurs at all. The desorption pressures of some borohydrides have been increased by modifying their thermodynamic properties using variants of the “destabilization” approach,10,26,28,29 forming ternary borohydrides,7,14,20 or reacting them with halides or oxides.3,11 Some additives (e.g., TiCl3 and MgB2) have been observed10,18,20 to enhance the reaction rates for LiBH4 and other borohydrides, and sometimes reduce or eliminate the formation of borane gas.14,25 However, none of these materials currently satisfy the DOE targets. The regeneration of borohydrides following desorption remains a challenge. For example, reformation of LiBH4 required heating to ∼873 K with ∼35 MPa pressure of hydrogen.8 Only the LiBH4/MgH2 mixtures exhibit relatively easy reformation of LiBH4 below 700 K, and its reformation is incomplete if the desorption pressures are below ∼0.3 MPa.10,24,27 Several groups have found that LiBH4 usually decomposes in two or more stages rather than the single transitions suggested by reactions 1 or 2. Sequential formation of several hypothetical phases “LiBH3”, “LiBH2”, and “LiBH” have been suggested to occur but with little or no experimental verification.3,9,23 Since identification of these substoichiometric borohydrides is severely hampered by the absence of clearly discernible X-ray or neutron diffraction peaks from reacted (or dehydrogenated) samples, these phases were presumed to be © 2008 American Chemical Society
Letters
J. Phys. Chem. C, Vol. 112, No. 9, 2008 3165 Experimental Section Commercial LiBH4 powder purchased from Sigma-Aldrich was desorbed without further purification. Preparation of the Mg(BH4)2 phases was performed by the desolvation process reported by Her, et al.19 The synthesis of scandium borohydride was achieved using ball-milling following a metathesis reaction similar to one described by Nakamori, et al.:14
ScCl3 + 3 LiBH4 f Sc(BH4)3 + 3 LiCl
Figure 1. Structure of the B12H122- anion.
amorphous or nanocrystalline. By combining first principles calculations of phase stability with Raman spectroscopy measurements, Orimo, et al.5,12,13 concluded that decomposition of LiBH4 occurs via formation of one or more polyhedral borane phases (i.e., Li2B12H12 and perhaps similar “BnHn” containing compounds) prior to forming elemental boron and LiH as the final products. However, no further characterization of the intermediates was reported to support these claims. Whether or not such compounds form as intermediates during decomposition of other M(BH4)n systems has not been effectively studied up to now. In the present work, solid-state NMR methods were employed to characterize/identify the amorphous intermediates formed during decomposition reactions of various metal borohydride M(BH4)n systems. The most probable intermediate candidates contain the [B12H12]2- anion30,31 which is a hydrolytically stable polyhedral borane complex with a closo structure.31 Its structure is a regular icosahedron of boron atoms with terminal B-H bonds as shown in Figure 1. Considering the fact that some of elemental boron polymorphs (which should be the final product of the decomposition reaction from reactions 1 and 2) contain icosahedral boron frameworks,31 the formation of [B12H12]2anions could be a natural choice as an intermediate during the decomposition to elemental boron. In the literature,32,33 among the intensive researches in borane chemistry, thermal conversion of some boranes (i.e., Et4N-BH4) to polyboranes (BnHm) including [B12H12]2- has been reported. Pyrolysis is known to rearrange polyboranes to more stable [B12H12]2- in the absence of oxygen.34 While MBH4 (NaBH4 or KBH4) mixed with trimthyl- and triethylamine borane was reported34,35 to form [B12H12]2-, solid-state conversion of MBH4 to MB12H12 has not been investigated. Due to the fact that the borohydride decomposition results in formation of amorphous phases, NMR becomes the most unambiguous spectroscopic tool. NMR is an ideal technique to study local structural environments and has been well suited for elucidating structures of amorphous or glassy materials.36 In this study, high-resolution magic angle spinning (MAS) NMR methods have provided clear evidence that a [B12H12]2- species is a major intermediate when M(BH4)n is dehydrogenated. The stable nature of the closo-borane compound (BnHn) in an aqueous environment allowed us to further confirm its structural identity by use of solution-phase NMR. Formation of the same intermediate was observed for several borohydride phases based on Li, Mg, and Sc, highlighting the universality of reaction pathway regardless of the accompanying metal element. These results shed new insight into the hydrogen storage behavior and reversibility potential for borohydrides.
(3)
Anhydrous ScCl3 (Alfa Aesar, 99.9%) and LiBH4 powders were mixed together in an argon gas filled glove box then placed into an 80 mL neoprene gasket sealed steel vessel along with five steel balls. The mixture was next milled in a Fritschpulverizette 6 planetary mill at 500 rpm for 3 h. Combining the MAS and CPMAS spectra of 6Li, 11B, and 45Sc nuclei, we have determined that the resulting product was the ionic LiSc(BH4)4 phase instead of Sc(BH4)3 as assumed in reaction 3. These results will be reported in more detail elsewhere. Finally, a physical mixture of LiBH4 with Ca(AlH4)2 with a 6:1 molar ratio was prepared using Ca(AlH4)2 previously made by ball milling CaH2 (Alfa Aesar, 95%) and AlH3 (Dow Chemical) as described Kabbour et al.37 Due to the high reactivity of these compounds with moisture and oxygen, all handling and filling processes of these powders were performed in purified argon atmosphere glove boxes. Decompositions of the various borohydride-based systems were performed under a range of conditions. The thermal decomposition treatments for LiBH4, LiSc(BH4)4, and the Ca(AlH4)2/6LiBH4 mixture were typically performed by stepwise heating of 400-600 mg of the material up to 400-500 °C in a stainless steel Sieverts type apparatus with evolution of released hydrogen gas into previously evacuated calibrated volumes. The Mg(BH4)2 sample desorbed ∼11.5 wt % hydrogen content during a 1 °C/min ramp to a maximum temperature of 450 °C38 at which point magnesium metal and some MgO impurity were identified by powder X-ray diffraction (XRD). Samples of Mg(BH4)2 heated to 550 °C released an additional 1.7 wt % and XRD of the products indicated only MgB2. In Table 1, we have included desorption conditions and the measured amount of hydrogen desorbed from the samples examined during these NMR studies. Solid-state MAS NMR spectra were measured using a Bruker Avance 500 MHz spectrometer with a wide bore 11.7 T magnet and employing a boron-free Bruker 4 mm CPMAS probe. The spectral frequencies were 500.23 MHz for the 1H nucleus and 160.50 MHz for the 11B nucleus. NMR shifts are reported in parts per million (ppm) when externally referenced to tetramethylsilane (TMS) and BF3‚O(CH2CH3)2 at 0 ppm for 1H and 11B nuclei, respectively. The powder samples collected after decomposition reactions were packed into a 4 mm ZrO2 rotor and were sealed with a kel-F cap. Sample spinning was performed under dry nitrogen gas. The one-dimensional (1D) 11B MAS NMR spectra were acquired after a 0.5 µs single pulse (
