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New Insights into the Mechanism of Hydrolysis of Sodium Borohydride

Amy M. Beaird, Eyma Y. Marrero-Alfonso, Thomas A. Davis, and Michael A. Matthews. University of South Carolina, Department of Chemical Engineering, Columbia, SC 29208

Chemical hydrides have promise as a compact medium for hydrogen storage at low temperatures and pressures. Hydrogen is produced when a chemical hydride, such as NaBH4, exothermically reacts with water.

NaBH4 + (2+x) H2O → 4H2 + NaBO2 xH2O + heat

The traditional hydrolysis of sodium borohydride with liquid water has been extensively studied and the reaction and its kinetic limitations are well documented in literature. Although thermodynamically favored, the aqueous hydrolysis does not proceed to completion even in excess water because of the formation of basic byproducts that inhibit the reaction. Therefore an acid catalyst is required to adjust the pH and increase the hydrogen yield. In addition, the hydride must be stored as a caustic solution to prevent premature reaction.

Alternatively, our approach is to expose solid chemical hydrides to steam. We have demonstrated essentially 100% yield without the need for a catalyst and without the need for storage in caustic solution. Clearly, the reaction mechanism and the pathway for steam hydrolysis differ from traditional liquid hydrolysis.

The purpose of this study is to elucidate the mechanism and physical phenomena associated with steam hydrolysis of sodium borohydride an alternative pathway that may significantly improve hydrogen storage by chemical hydrides. In-situ visual observation of the reaction and its phase behavior was accomplished with a glass reactor and borescope camera. Reaction products were analyzed by 11B NMR from both the steam and aqueous hydrolysis reactions for comparison. The reaction rate and conversion appear to rely on the ability of the hydride to absorb water vapor and then dissolve in it, a phenomenon called deliquescence. This process depends on the reaction temperature, pressure and mole fraction of water. Based on this study, deliquescence is necessary for reaction, but only occurs above 30% humidity at 110oC and requires higher humidity as reaction temperature is increased. Analysis of the reaction products indicates that a BH3 containing intermediate found in the aqueous reaction is not present in the steam pathway, indicating a different reaction mechanism. Comparison of liquid and vapor phase hydrolysis provides valuable insight into the reaction mechanisms.