New B,N-hydrides: Characterization and Chemistry
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New B,N-hydrides: Characterization and Chemistry Paul A. Anderson1, Philip A. Chater1, William I. F. David2, Ian C. Evans1 and Alexandra L. Kersting1 1 School of Chemistry, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK. 2 ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon., OX11 0QX, UK. ABSTRACT Three different classes of boron and nitrogen containing light metal complex hydrides have been investigated, resulting from the reactions of LiNH2 with LiBH4, NaNH2 with NaBH4 and MHx (where M = Li, Na and Ca) with NH3BH3. A rich variety of new phases has been identified, which exhibit modified decomposition pathways and onset temperatures of hydrogen desorption as low as 40°C. In each case the composition of phases formed has been examined in detail and the products of thermal decomposition—both solid and gaseous—have been determined. INTRODUCTION Light metal complex hydrides containing both boron and nitrogen, have been shown to release relatively large amounts of hydrogen (up to 13.5% by weight) at moderate temperatures (as low as 100°C) [1–10]. In spite of these desirable features, the potential usefulness of this class of materials is currently compromised by a number of less favourable aspects, such as a tendency for hydrogen desorption to be accompanied by the release of products such as NH3 or borazine that would poison fuel cells. Here we present results from three promising systems in an attempt to provide a fuller understanding of the complex and subtle chemistry governing their thermal decomposition properties. EXPERIMENTAL DETAILS Reactions of nMNH2 + (1−n)MBH4, where n = 1, 0.916& , 0.833& , 0.75, 0.666& , 0.583& , 0.5, 0.416& , 0.333& , 0.25, 0.166& , 0.083& and 0, were investigated for lithium and sodium. Samples were ground intimately by hand in an argon filled glove box (< 2 ppm O2, < 1 ppm H2O); these will be referred to as ground samples. For lithium samples, a portion of each ground sample was heated at 190°C for 12 hours in a tube furnace under argon gas at 1 bar; the products will be referred to as heated samples. A portion of each heated sample was subsequently reground in an argon filled glove box and annealed at 90°C for 12 hours; these will be referred to as annealed samples. Three extra samples close to the composition needed to form Li4BH4(NH2)3 (n = 0.775, 0.742 and 0.725) were also prepared. For sodium samples, a portion of each ground sample was evacuated and sealed inside a quartz tube, which was heated at 180°C for 12 hours and then annealed at 90°C for 12 hours; the products will be referred to as annealed samples. For MHx + NH3BH3 samples, the metal hydride and ammonia borane were ground intimately by hand in the
desired ratio an argon filled glove box. The samples were then heated at the desired temperature for 12 hours in a tube furnace under argon gas at 1 bar. Powder synchrotron X-ray diffraction data were collected on the ID31 diffractometer at the ESRF, Grenoble, at a wavelength of 0.80157 Å for the lithium heated and annealed samples
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