Deliquescence of NaBH4 from Density Functional Theory and Experiments

We report a theoretical investigation of H2O adsorption on the NaBH4(100) surface based on first principles density functional theory with inclusion of dispersion corrections in order to explore the initial stages of deliquescence at the molecular level. In the zero coverage limit, H2O is found to bind strongly to sodium sites on NaBH4(100) through O center dot center dot center dot Na and O-H center dot center dot center dot H-B attractions. As the coverage increases H2O molecules adsorb on boron sites. H atoms in the adsorbed H2O monomer adopt tilted down (15 degrees-20 degrees) configurations with respect to the NaBH4(100) surface, which undergoes reconstruction in response to adsorbed H2O by rotations of BH4- groups of up to 90 degrees and slight distortions of the positions of Na+ and BH4-. The adsorption energy per H2O is roughly independent of water coverage up to at least a coverage of four monolayers, suggesting that it is energetically feasible for water to condense on the surface, in agreement with experiments. We have experimentally studied the deliquescence of a mixture of NaBH4 with 10 wt % CoCl2. We found that CoCl2 lowers the deliquescence temperature compared to that for pure NaBH4 at a given vapor phase mole fraction of water; i.e., the deliquescence relative humidity is increased because of addition of CoCl2. Thus, while CoCl2 is a catalyst for aqueous phase hydrolysis of NaBH4, it actually inhibits deliquescence and hence delays the onset of steam hydrolysis.