WHY EXAFS UNDERESTIMATED THE SIZE OF SMALL SUPPORTED MOS2 PARTICLES

EXAFS spectra of distorted and disordered small MoS2 particles were si mulated by means of the FEFF7 program. The simulated EXAFS-spectra of several models were compared with observed EXAFS spectra of small MoS2 particles which had been characterized by TEM (Calais et al. J. Catal . 1998, 174, 130). The EXAFS spectra could be reproduced well by a mod el in which Mo-Mo distances at the edges of the MoS2 particles are 0.1 6 Angstrom longer than in bulk MoS2 and the Mo atom positions are stat istically disordered. The simulation demonstrated that the diameter of the MoS2 slabs and the degree of distortion at the MoS2 edges cannot be determined simultaneously and that the assumption that Mo atoms in the particles occupy regular positions causes a substantial underestim ation of the MoS2 diameter. A novel function relating the coordination number of the nearest Mo-Mo contribution to the MoS2 slab diameter is proposed. With this function, the diameter of the MoS2 slabs, estimat ed from EXAFS data, agrees with that of TEM data.