Alon Seri-Levy

Fractal analysis of surface geometry effects on catalytic reactions

Abstract We study the effects of the surface morphology of catalysts on their performance. The surface geometry parameter is varied by taking surfaces of different fractal dimensions. The reaction mechanisms studied are the Eley-Rideal process (diffusion controled) and some modifications of it (chemically controled and side-reaction of the reactant). Theory predicts B ( t ) = K t 3− D /2 were B ( t ) is the amount reacted after time t , D is the surface fractal dimension, and K is a collection of constants. In a detailed simulation study we verified the theoretical prediction and were able to retrieve not only D , but, for the first time also K . Conditions under which an increase in surface roughness can either enhance or quench the efficiency of the catalytic reaction are identified and explained.

Photoprocesses on Fractal Surfaces

Publisher Summary This chapter discusses photoprocesses on fractal surfaces. The initial interest in fractal geometry stemmed mainly from the striking similarity between computer-generated objects, as formed by applying the rules of this geometry, and “real” objects as found in many natural or man-made objects. The algorithms used for creating the fractal objects are usually quite simple and involve an iterative construction procedure. Because of this iterative procedure, the objects thus obtained have the property of being self-similar—that is, the various magnifications of the object look similar. The chapter describes the use of molecules as yardsticks. The first yardstick for the resolution analysis discussed, is the molecule interacting with the surface. For an irregular surface, the smaller the molecule is, the finer are the surface details it can probe and the larger will be the apparent surface area, A, as determined from monolayer coverage. Rather than measuring area with a set of molecules, one can perform an equivalent resolution analysis—namely, to take only one yardstick and to measure the area of the object as a function of its size.

Surface Geometry Effects on Adsorption-Desorption Hysteresis, Latent Adsorption and Adsorption-Probability Plots

Abstract We perform molecular-level two-dimensional Monte-Carlo simulations of adsorption/desorption processes in order to investigate the effects that surface geometric irregularity has on these processes. The degree of surface irregularity is gradually changed by taking various profiles, starting from a straight line and ending in a microporous line. We correlate geometry to the appearance of adsorption-desorption hysteresis and to the evolution of latent adsorption (trapped, unadsorbed molecules).

An Integrated Fractal Analysis of Silica: Electronic Energy Transfer, Small-Angle X-ray-Scattering and Adsorption

D. Rojanski-Pines (a), D. Huppert (a), H.B. Bale (b), The fractal dimension,D, of mesoporous silica gel was determined by four independent approaches: Analysis of one step dipolar energy transfer between adsorbed rhodamime B and malachite green; Analysis of small-angle X-ray scattering at the Porod regime; tiling the surface with molecules of different cross sectional area by adsorption; Analysing the change in surface area as a function of particle size. All four techniques agree in pointing to extreme surface wiggliness with D -.3. Besides confirmation of earlier indications to this high value, our mUltiple approach contributes to the mutual corroboration of the various theories involved. For comparison, flat-surfaced non-porous silica (Aerosil) was studied. All methods give D = 2.0.