\bibitem{sheintuch89}   M.~Sheintuch, S.~Brandon,  \hskip 2mm
                        {\it Deterministic Approaches to Problems of Diffusion, Reaction and Adsorption in a Fractal Porous Catalyst},
                        Chem. Engng. Sci. {\bf 44} (1), 69-79, 1989.