Nanoparticles and 3D Supported Nanomaterials

Standing is still under development, although clearly increased stability for clusters with an even number of electrons is observed for Cu, Ag, Au, V and Cr [9]. 

For metal hydride clusters, much less is known about electronic and geometrical size effects, except for very small clusters that can be relatively easily traced with computational methods. Results on other ionic compounds, such as ZnS clusters, indicate less well defined size effects, and only for small cluster sizes [11]. As the size dependence will be clearly different than for metal clusters, potentially there is a large influence of size on the stability differences between small hydride and the corresponding metal clusters. However, the practical impact of these effects is probably limited, as in the bulk experimental preparation of light metal (hydride) clusters, generally, polydisperse samples are obtained, and hence pronounced effects of 

A different approach to the stability of hydride nanoparticles is bottom-up, using computational techniques to construct equilibrium (lowest energy) configurations for the clusters, nanoparticles or thin films of the metal and corresponding hydride, and evaluate the difference in stability between the two as a function of particle size. Although the possibilities are rapidly increasing, these calculations are still limited to small cluster sizes due to computational restraints. As a relevant example we vhll only briefiy discuss various types of calculations for the case of the ionic hydrides MgH2 and NaH. 

Not only thermodynamic factors, but also the slow kinetics ofhydrogen sorption are a barrier to practical application for many materials. In this section we will briefly discuss the expected impact of the particle size on kinetics. First, it is important to realize which step in the hydrogen sorption process is rate limiting, which depends not only on the type of material, but also on the specific experimental conditions. Taking the absorption ofhydrogen (which is usually slower than the desorption at a given temperature) as an example, the following steps can be discerned  

4) diffusion through the hydride layer to the interface with the metallic phase 

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