CRYSTALLITE SIZE DETERMINATION OF HIGHLY DISPERSED UNSUPPORTED MOS2 CATALYSTS

Crystallite size determination of MoS2 particles is still a controvers ial problem fuelling the debate on the morphology of hydro-treating ca talysts, The size of MoS2 crystallites determined from extended X-ray absorption fine structures (EXAFS) is close to 1 nm, whereas TEM pictu re analysis give an average size of 3 to 3 mo, It is obvious that thes e techniques have intrinsic strengths and weaknesses and that the resu lts may not be comparable because of these differences, However, it is our intention to obtain a better understanding of these crystallite s izes by studying the dispersed unsupported solids, Various techniques were used: nitrogen adsorption-desorption, X-hay diffraction (XRD), XA FS, TEM, and temperature-programmed reduction (TPR) and the results we re compared and discussed, The samples were prepared by thiosalt decom position or molten salt synthesis, These solids presented different mo rphologies, the latter method providing higher surface area samples. T he specific surface areas varied from 4 to 125 m(2)/g and, depending o n the method of preparation, microporous and mesoporous solids were ob tained. XRD, TEM, and TPR were found to be complementary and coherent methods for determining the size parameter, EXAFS interpretation does not give consistent results due to local reorganization at the periphe ry of the crystallites, (C) 1998 Academic Press.