Thermal Dehydration and Vibrational Spectra of Hydrated Sodium Metaborates

Sodium metaborate hydrates are a class of compounds represented by the stoichiometry NaBO2 center dot xH(2)O. Recently, sodium metaborate has received attention as the byproduct of sodium borohydride hydrolysis, a reaction that is under consideration for hydrogen storage applications. The aim of this work was to understand the disposition of water in the crystal structure of hydrated sodium metaborates and to characterize the thermal stability and dehydration of the various hydrated species to optimize hydrogen storage efficiency as well as recyclability of the borate. Observations from a suite of analytical techniques including thermal analyses (thermogravimetric analysis/differential scanning calorimetry), X-ray diffraction, and Raman spectroscopy were correlated to characterize the dehydration mechanism of commercially available sodium metaborates, with an emphasis on the dihydrate (x = 2). A transformation from tetrahedrally coordinated boron to trigonal boron occurs when NaB(OH)(4) (x = 2) is heated between 25 and 400 degrees C. The first dehydration to N(a)3[B3O5(OH)(2)] (x = 1/3) releases 5 mol of water between 83 and 155 degrees C. The final mole of water is released between 249 and 280 degrees C, and Na3B3O6 (x = 0) is formed. Raman spectra are reported for x = 2 and 1/3 for the first time. First-principles density functional theory was used to compute Raman spectra of the x = 1/3 and 2 material in order to assign the modes. We found reasonably good agreement between the experimentally measured and calculated vibrational frequencies.