Hydriding and Dehydriding Kinetics of Sodium Alanate at Constant Pressure Thermodynamic Driving Forces

A study was done to compare the hydriding and dehydriding kinetics of the first two decomposition steps in NaAlH4. In the first step, NaAlH4 decomposes forming Na3AlH6. In the second step, Na3AlH6 decomposes forming NaH. This comparison was made using a novel procedure in which the ratio of the equilibrium plateau pressure (P-m) to the opposing pressure (P-op), or the N-value, was the same. This represents the first time that such a comparison has been made in a complex hydride displaying two decomposition steps. Since the Gibbs free energy change is proportional to In(P-m/P-op), it was concluded that these experiments were carried out under constant thermodynamic driving forces. It was found that, under these conditions, the first decomposition step occurs about an order of magnitude faster than the second decomposition step. Experiments were also done to compare absorption and desorption rates for the second decomposition step. It was found that, using the same N-value, the absorption rates were about 20 times faster than desorption. Modeling studies showed that the absorption kinetics are most likely controlled by reaction at a moving boundary. Desorption reactions displayed a more complex behavior in which no single model described the entire process. Indications are that nucleation and growth control the reaction rate in the early stages but that diffusion through a product layer may control the rate in latter stages.