Crystallite Size Determination of Highly Dispersed Unsupported MoS2Catalysts

Abstract

Crystallite size determination of MoS2particles is still a controversial problem fuelling the debate on the morphology of hydrotreating catalysts. The size of MoS2crystallites determined from extended X-ray absorption fine structures (EXAFS) is close to 1 nm, whereas TEM picture analysis give an average size of 3 to 4 nm. It is obvious that these techniques have intrinsic strengths and weaknesses and that the results may not be comparable because of these differences. However, it is our intention to obtain a better understanding of these crystallite sizes by studying the dispersed unsupported solids. Various techniques were used: nitrogen adsorption–desorption, X-ray diffraction (XRD), XAFS, TEM, and temperature-programmed reduction (TPR) and the results were compared and discussed. The samples were prepared by thiosalt decomposition or molten salt synthesis. These solids presented different morphologies, the latter method providing higher surface area samples. The specific surface areas varied from 4 to 125 m2/g and, depending on the method of preparation, microporous and mesoporous solids were obtained. 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 periphery of the crystallites.