Structure of the molybdenum sulfide phase in carbon-supported Mo and Co-Mo sulfide catalysts as studied by extended X-ray absorption fine structure spectroscopy

The structure and dispersion of the active phase present in carbon-supported Mo and Co-Mo sulfide catalysts were studied by means of Mo K-edge EXAFS. In situ EXAFS measurements were carried out at liquid N2 temperature on freshly sulfided (at 673 K) catalysts. Due to the high signal-to-noise ratio of the experimental data, detailed information about the structural parameters of the active phase could be obtained. The catalysts probably consist of small MoS2-like particles with an auerage local ordered structure of 5-6 Mo atoms (Mo/C) and 7-8 Mo atoms (Co-Mo/C). Comparison with literature data shows that the MoS2 dispersion in these carbon-supported catalysts is probably as good as in their alumina-supported counterparts. A detailed EXAFS data analysis shows the presence of carbon neighbors next to the Mo atoms in both catalysts with a Mc-C coordination distance of 2.1+/-0.1 A. This short distance could imply an intimate interaction between the active phase and the carbon support, which may explain the high active-phase dispersion. From the value of 2.1 8, it can be inferred that the Mo-C coordination is restricted to the exposed Mo atoms and that a support carbon atom takes the place of a sulfur vacancy. The high-quality EXAFS spectra show evidence for the existence of cobalt neighbors next to the Mo atoms in the Co-Mo/C catalyst, with a Mo-Co coordination distance of 2.8+/-0.1 A.