8.1 Introduction

Nanostructured materials exhibit a host of interesting new phenomena directly related to their reduced dimensionality. Not only the electronic, magnetic, and optical properties but also chemical, electrochemical, and catalytic properties of nanostructured materials are very different from those of the bulk form and depend sensitively on size, shape, and composition (Kirk and Reed, 1992; Berry, et al., 1996). The large surface-to-volume ratio and the variations in geometry and electronic structure have a dramatic effect on transport and catalytic properties. For example, the reactivity of small clusters has been found to vary by orders of magnitude when the cluster size is changed by only a few atoms (Whetten, et al., 1985). Another example is hydrogen storage in metals. It is well known that most metals do not absorb hydrogen, and even among those that do, hydrogen is typically adsorbed dissociatively on surfaces with a hydrogen-to-metal ratio of 1. This limit is not applicable small clusters. It is shown that small positively charged clusters of Ni, Pd, and Pt generated in molecular beams and containing between 2 and 60 atoms can absorb up to 8 hydrogen atoms per metal atom (Cox, et al., 1990). The number of absorbed hydrogen atoms decreases with increasing cluster size and approaches 1 for clusters having more than 60 atoms, simplying that small clusters may be very useful for hydrogen storage. Yet another case of the effect of size on properties is observed in nanophase powders of lithium magnesium oxide, which...