HETEROGENEITY OF SURFACES AND MATERIALS, AS REFLECTED IN MULTIFRACTALANALYSIS

A large variety of heterogeneous processes take place at solid interfa ces. The heterogeneity of surfaces and materials plays a key role in m ost of those processes. Spatial variations in surface and material pro perties can result from complex geometries and chemical heterogeneitie s. To treat quantitatively the effects of complex geometries on specif ic processes, it is therefore important to characterize the geometrica l and topological features associated with most porous solids. Many te chniques have been developed for this purpose. Among them the simple g as adsorption technique gives the most information, From the standard nitrogen adsorption isotherms, values such as total pore volume, BET s urface area and pore size distribution (PSD) can be obtained, However, more information about the complex structure of surfaces and material s could be obtained by adopting some new approaches or theories. Recen t developments in percolation and fractal analysis of adsorption data will be reviewed in this article, The emphasis is placed on the practi cal application of these theories to obtain measures of the porous str ucture, and to interpret such results. The difficulties of applying th e fractal concepts are also presented. In addition to percolation and fractal analysis, it has been recently realized that in many cases a m ore detailed characterization of disordered structures can be obtained by applying multifractal scaling analysis (MSA), Some discussion abou t its development and application to the geometrical characterization of the microstructure of heterogeneous materials is also given in this article. For chemical heterogeneities, the heterogeneous spatial dist ribution of components or elements in multicomponent materials as well as site residence or adsorption probabilities in catalysts and adsorb ents can be described in terms of a fractal measure or multifractal di stribution. This will also be reviewed in this article, On the other h and, the realization that many of these sophisticated geometrical stru ctures are fractal-like has provided an appropriate approach to the st udy of chemical and physical processes taking place in such environmen ts. The applicability of fractal and multifractal analysis to the char acterization of heterogeneous processes, in particular, to illustrate the influence of the surface geometry on specific processes, will be g iven with a focus on heterogeneous catalysis reactions.