\bibitem{Sapoval01}     B.Sapoval, J.S.Andrade Jr. and M.Filoche  \hskip 2mm 
                        {\it Catalytic Effectiveness of Irregular Interfaces and 
                        Rough Pores: the Land Surveyor Approximation}, Chem.Eng.Science 
                        {\bf 56}, 5011, 2001.