Nano- to macro-scale design of metal ammines

5 Nano- to macro-scale design of metal ammines 

The important properties of the metal ammines when used as materials for indirect hydrogen storage can be understood and explained at the atomic scale. In this section, an atomic scale model of the sample system Mg(NH3)6Cl2 based on density functional theory (DFT) calculations shows how atomic structures, energies and dynamics can be connected to macro-scale properties such as shape, uptake and decomposition rates, porosity and reaction enthalpies. The model is based on experimentally observed structures (Olovsson, 1965 Partin and O Keeffe, 1991 Leineweber et al, 1999, 2000 Hummelshpj et al, 2006) and it is expected to be applicable to other metal ammines. A detailed understanding of the systems at the atomic scale is a prerequisite for the design of new and better materials. 

From the atomic structures it is possible to understand why the ah- and desorption processes are fast, why they are easily reversible and how an observed self-porosity may arise. DFT calculations, using the DACAPO planewave basis set implementation (Hammer et al, 1999), confirm the stability of the experimentally proposed structures, as seen in Fig. 19.7. To confirm that a structure is in fact stable, one compares the energy with the energies of the other phases, e.g. the monoamine phase is stable since 

In Fig. 19.7, it is seen that the uptake of ammonia in MgCl2 can happen continuously if the MgCl2 layers (a) are cleaved into chains when the first ammonia molecules are introduced to form Mg(NH3)Cl2 (b). These double octahedral chains are then cleaved once more to form octahedral chains of the Mg(NH3)2Cl2 salt (c). When the last four ammonia molecules are introduced, the chains are stretched and rearranged to produce the K2PtCl6 stmcture of Mg(NH3)6Cl2 (Olovsson, 1965) (d) see also Fig. 19.5. For the desorption process, the direction can simply be reversed. 

The material is known to retain its macroscopic shape during the ab-/ desorption of ammonia (Fig. 19.3), which implies an ab-/desorption mechanism where ammonia diffuses into the bulk and takes the system stepwise through (a) (b) (c) (d) in Fig. 19.7 and vice versa. For this to occur readily. 

---