Nanoporous Inorganic Solids
A porous inorganic material is one which does not have pores and voids through and through. The voids show a translational repetition in 3-D space, while no regularity is necessary for a material to be termed “porous.” A typical and relatively simple porous system is one type of dispersion classically described in colloid science, namely foam or, better, solid foam (M. Jaroniec, 2002). In correlation with this, the most common way to think about a porous material is as a material with gas-solid interfaces as the most dominant characteristic. This already indicates that classical colloid and interface science as the creation of interfaces due to nucleation phenomena.
Decreasing interface energy, and stabilization of interfaces is of elemental importance in the formation process of nanoporous materials. These factors are often omitted because the final products are stable. Indeed, they are just metastable. This metastability is due to the rigid character of the void-surrounding network, which is covalently cross-linked in most cases. However, it should be noticed that most of the porous materials reported in the literature or those that are of high technical relevance are not stable by thermodynamic means (F. Rodríguez-Reinoso, 2002). As soon as kinetic energy boundaries are overcome, materials start to break down. One example should elucidate this. Porous silica, for instance, is just metastable.
As soon as the temperature is raised and the melting point is reached, primary particles in the network begin to fuse and it comes to phase separation into a nonporous silica phase. Finally, at very high temperatures, the thermodynamic stable phase of SiO2 quartz emerges. Control over interface energy and metastabilization of nanodimensional holes becomes of special importance when the task is to produce nanoporous materials (F. Rodríguez-Reinoso, 2002). For this review, the whole range of pores from 1 nm to 1000 nm s interpreted as nanoporous materials.
Additionally, we will skip the strict limitation of just regarding 3-D distributed solid-gas interfaces as porous materials. Instead, when the pores happen to be led with something the materials will also be judged to be porous as long as it is possible to remove this filling. The pores might be filled by any matter that is different from the surrounding network. However, it should be noted that nanoporous materials filled with pores might be more correctly described as nanostructured materials (T.J. Pinnavaia, 2000).
Accordingly, porous materials might be classified by the size of pores or may be distinguished by different network materials. Before nanoporous materials are considered in further detail it might be good to give a brief overview of the areas in which nanoporous materials are of interest (M. Jaroniec, 2002). One can find a whole variety of nanoporous materials in nature executing many different functions. The most common task for...