ARTICLE pubs.acs.org/JPCC
Synthesis Mechanism of Alkali Borohydrides by Heterolytic Diborane Splitting Robin Gremaud,* Andreas Borgschulte, Oliver Friedrichs, and Andreas Z€uttel
:: Empa, Swiss Federal Laboratories for Materials Testing and Research, Hydrogen & Energy, Uberlandstrasse 129, CH-8600 D€ubendorf, Switzerland
bS Supporting Information ABSTRACT: Similar to alane in alanates, borane species are assumed to be the mass transport intermediate in the hydrogen storage reaction MH þ B þ 3/2H2 S MBH4 with M = Li and Na. One possible substep of this reaction is the interaction of diborane with the alkali hydride. In this paper, we unravel the synthesis mechanism of alkali borohydrides by solid-gas reaction of alkali hydrides and diborane gas by H/D isotope labeling of the reaction educts (e.g., LiD þ B2H6). The labeling enables us to trace the hydrogen/deuterium atoms in the borohydride product by Raman scattering and in the gas by infrared spectrometry measurements. We conclude that, during the LiBH4 synthesis from LiH, the entire BH4 unit is transferred from the diborane to the Liþ cation. This provides clear evidence for the heterolytic splitting of diborane on alkali hydrides and implies exchange of BH4 with H ions of the underlying hydride. The detection of Li-H bonds at the surface of newly formed LiBH4 confirms the importance of H- defects for the synthesis of borohydrides.
’ INTRODUCTION Hydrogenation reactions of solids, as an example of a gassolid interaction, are of fundamental interest and have a wide application spectrum, such as hydrogen storage in metal hydrides.1 The most important elementary step during the course of reaction is the hydrogen splitting on the surface (dissociation). Next steps are diffusion of atomic hydrogen and the formation of a hydride. Because hydrogen is a nonpolar covalently bond molecule, it has to be polarized prior splitting. In transition metal hydrides, the polarization originates from the interaction of s electrons of hydrogen with the metal d bands.2 After splitting, the atomic hydrogen easily diffuses by hopping from interstitial to interstitial site.3 Thus, transition metal hydrides such as LaNi5Hx satisfy all requirements to be used as hydrogen storage materials (fast kinetics, appropriate stability), except the gravimetric hydrogen storage density. Light-weight complex hydrides such as borohydrides4 can store hydrogen up to 20 mass % H2. However, hydrogen uptake by a composite of, e.g., an alkali hydride and boron, as well as subsequent hydrogen release MH þ B þ 3=2H2 hintermediateshMBH4 ðM ¼ Li, Na, :::Þ
ð1Þ
occurs at elevated pressures and temperatures only,5-8 and no suitable catalyst has been found sofar for the formation/decomposition of r 2010 American Chemical Society
borohydrides. Because of the missing of d states in boron, the abovediscussed H-interstitial model fails in describing the reaction mechanism. Particularly, gas-solid interaction and diffusion, which have been found as most important kinetic constraints in d metal hydrides,9 are little understood in borohydrides. In this case, not only hydrogen but also borane gases are involved in the reaction.10-12 Recently, we showed that the synthesis of LiBH4 is possible under mild conditions (ambient pressure at 390 K) if starting from LiH under gaseous diborane atmosphere,13 opening the way for a solvent-free synthesis route of borohydrides14 MH þ 1=2B2 H6 f MBH4 ðM ¼ Li, Na, :::Þ
ð2Þ
This facilitates the synthesis of various borohydrides that are not easily separated from their solvent when using a wet chemistry synthesis route.15 This is also strong hint that the mass transport intermediate in the total hydrogen storage reaction 1 is a borane species, similar to what was found in NaAlH4.16,17 However, it has been demonstrated that diborane exposure only is not sufficient to obtain a satisfactory reaction yield, owing to the formation of a LiH-core/LiBH4-shell structure that prevents further reaction.18 It was shown that this kinetic constraint Received: August 2, 2010 Revised: November 19, 2010 Published: December 22, 2010 2489
dx.doi.org/10.1021/jp107266d | J. Phys. Chem. C 2011, 115, 2489–2496
The Journal of Physical Chemistry C
ARTICLE
can be overcome by ball-milling under diborane atmosphere providing fresh reactive surfaces and enhance interdiffusion.18 Additionally, the decomposition of many borohydrides results in the emission of significant amount of diborane gas (and possibly higher boranes) at temperatures below 300 °C.10-12 These facts raise new questions: what are the surface processes and how are the products transported into the underlying bulk. In particular, we want to shed light on the gas-solid interaction of diborane with alkali hydrides (homolytic/heterolytic splitting) and on the subsequent diffusion of the formed BH4 anions. In this paper, we study the reaction 2 by hydrogen-deuterium labeling of the reaction educts. We then use Raman spectroscopy for characterization of the products, as the isotope shift is easily detected by vibrational spectroscopy (Raman or infrared)19-21 and provides a structural probe for individual B-H bonds in borohydrides.22,23 The labeling of involved compounds allows us to discriminate between hydrogen originating from the alkali hydride or from the diborane and to conclude on the reaction mechanism. We provide clear evidence of the heterolytic splitting of diborane on alkali hydrides. The resulting BH4 anion is subsequently exchanged with the H- ions of the underlying hydride. The paper is organized as follows: First we expose LiH to fully deuterated diborane (B2D6), forming a layer of borohydride on the surface of LiH (core-shell case). To avoid isotope scrambling as much as possible, we apply the lowest possible reaction temperature (100 °C). The borohydride layer is then analyzed by Raman spectroscopy to determine what kind of B(H4-nDn)- is present on the surface and conclude on the reaction mechanism. We focus on the The B-H and B-D stretching spectral region that delivers all the necessary information to discriminate between various B(H4-nDn)- units. For cross-checking, the reverse reaction (B2H6 on LiD) is also considered. Furthermore, the Raman spectrum of a fully reacted sample, i.e., LiD ball-milled at room-temperature in fully protonated diborane- is measured and compared with the core-shell case. To conclude on the reaction mechanism, we then study the kinetics of the borohydride formation is studied by gravimetry and infrared spectrometry of the gaseous products. We finally show that NaBH4 can also be synthesized in a similar way from NaH, confirming the generality of the reaction mechanism found in LiBH4. Reference Raman spectra are provided by isotope scrambling experiments on the alkali borohydrides, i.e., exposure of the solid alkali borohydrides to deuterium gas.
’ METHODS Sample Preparation. LiH, NaH, LiBH4, NaBH4, and ZnCl2 are purchased from Sigma-Aldrich Fine Chemicals, Switzerland, and LiBD4 and NaBD4 from CatChem, Czech Republic (98% D). The samples were handled solely in argon glove boxes for preparation and measured without contact to air. The source for B2H6 and B2D6 was prepared by the metathesis reaction of LiBH4 and LiBD4, respectively, with ZnCl2 in a Spex 8000 M mixer mill for 90 min according to the following reaction14,24
5LiBH4 þ 2ZnCl2 f LiZn2 ðBH4 Þ5 þ 4LiCl
ð3Þ
Figure 1 shows the Raman spectra of the LiZn2 borohydride and deuteride used as B2H6 and B2D6 sources. The peaks corresponding to the various types of vibrations are clearly separated, with the “external” lattice vibrations for Raman shifts
