Influence of Surface Reactions on Complex Hydride Reversibility

Abstract

Alkali metal hexahydride alanates, M 3 AlH 6 , are known to exist for M = Li, Na, and K. All three release hydrogen, forming MH and Al as the solid phase products. The reverse of this reaction, 6MH + 2Al + 3H 2 → 2M 3 AlH 6 , occurs without a catalyst for M = K, occurs only with a catalyst for M = Na, and has not been observed, even with a catalyst, for M = Li. Differences in the reactivities of the LiH, NaH, and KH surfaces may contribute to the observed differences in rehydriding. We have examined the reactivities of the low-energy MH(100) surfaces with respect to gas phase H 2 , H, O, O 2 , and H 2 O in order to test this hypothesis. We have found that H 2 weakly physisorbs and that H is unbound to all three MH surfaces, relative to gas phase H 2 . Atomic oxygen is very strongly bound to all three surfaces and O 2 dissociates without a barrier at low coverage on the LiH and NaH surfaces. The KH surface is more resistant to O 2 dissociation, but molecular O 2 strongly binds to the surface. We have identified dissociation pathways for H 2 O on all three MH surfaces, which results in the formation of surface metal hydroxide and gas phase H 2 . The zero-point energy corrected dissociation activation energies for H 2 O are 23.0, 13.8, and 18.4 kJ/mol for LiH, NaH, and KH, respectively. We have performed kinetic modeling of the H 2 O dissociation process resulting from MH surfaces exposed to vapor phase water at room temperature and partial pressures of 0.03 bar (100% relative humidity) and 10 -6 bar (1 ppm). In both cases, our modeling predicts that all three MH surfaces will be essentially completely covered with a monolayer of OH groups in less than 10 ms. We therefore conclude that there are no substantial differences in the reactivities of the MH surfaces that can account for the observed differences in their abilities to form the hexahydride alanate phase.