Anomalous scaling of diffusion and reaction processes on fractal catalysts

R. Gutfraind*, M. Sheintuch

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

13 Scopus citations


The geometry of a heterogeneous catalyst is a crucial parameter in determining its performance. Many catalytic systems can be described well in terms of fractal geometry. Here we study the rate dependence of various operating conditions in diffusion-reaction processes occuring on fractal rough surfaces, such as catalyst supports, and fractal subsets such as steps and kinks that constitute the active sites in metal crystallites. In a process of diffusion and reaction in a catalyst with external fractal surface, exposed to a fixed reactant concentration, the catalyst exhibits an increasingly larger area for faster reactions; the overall rate scales as k(Ds/k)2-Df/2, where k is the first-order reaction rate constant and Ds is the diffusivity in the solid. A subfractal, like a Cantor Set, to which molecules diffuse through a stagnant fluid layer of thickness δ and react instantaneously with the catalytic set, exhibits an overall rate that scales like δ-Df; i.e., this subset is less sensitive to mass transfer resistance than a smooth surface. With finite k, the inverse overall rate can be approximated as the sum of the inverse rates at the two asymptotes of very slow and very fast reactions. A fractal line exposed to a fixed reactant concentration at its enveloping boundary, represent a pore network which is an optimal configuration since porosity is higher at the larger pores.

Original languageEnglish
Pages (from-to)2787-2792
Number of pages6
JournalChemical Engineering Science
Issue number9-11
StatePublished - 8 Jun 1992
Externally publishedYes


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