Early Stage of the Dehydrogenation of NaAlH4 by Ab Initio Rare Event Simulations

We investigated the chemical species formed in the early stage of the first dehydrogenation reaction of (undoped) NaAlH4 (NaAlH4 reversible arrow 1/3Na(3)AlH(6) + 2/3Al + H-2(up arrow)). We found that the experimental barrier to dehydrogenation (120 kj/mol) is compatible with the Al-H bond breaking of an AlH4 unit. We observed the formation of AlH3, AlH5, NaH, and NaH2. We computed the free energy profiles for the process of formation of the two most frequent species, AlH5 and NaH. While the free energy barriers for creating the species are comparable, the sample containing NaH is thermodynamically much more stable than the one containing AlH5. We did not observe the formation of H-2 nor of the other products of the complete reaction. We attribute the lack of formation of H-2 at this early stage to the fact that the hydrogen released in the sample is negatively charged and cannot quickly oxidate in the absence of chemical species that can efficiently be reduced. This finding suggests a possible mechanism for the catalytic action of Ti in Ti-doped samples. Finally, we studied the concentration of the various AlHi species as a function of the distance from the surface and found that species with higher negative charge are formed far from the surface, while neutral species are formed preferably doser to the surface, in particular in the top layer.