NATURE OF ADSORPTION SITES ON SULFIDED MO CATALYSTS AND THEIR SELECTIVITY IN CHEMISORPTION OF PROBE MOLECULES

Oxygen chemisorption has been studied on sulfided Mo catalysts support ed on SiO2 with differing dispersions as a function of pretreatment. I t was found that presulfided samples purged in He at 350 degrees C che misorb about half as much O-2 as samples reduced first in H-2 at 350 d egrees C, followed by He purge at the same temperature. From this stud y in combination with other studies of chemisorption of probe molecule s such as H2S, CO, O-2, and NO on sulfided Mo catalysts, a two-site ch emisorption model is proposed. The adsorption sites are intrinsically related to the geometric and electronic character of the MoS2 two-dime nsional layer structure. Frontier orbital concepts and Hoffmann's theo ry of bonding on surfaces are employed to postulate the selectivity in chemisorption of probe molecules on the two adsorption sites and to c orrelate quantitatively the chemisorption amount of various probe mole cules. The model suggests that Mo-S sites with unsaturated coordinatio n located in S-Mo-S layers are adsorption sites for probe molecules H2 S, CO, O-2, and NO, while S sites located in S-S layers only adsorb O- 2 and NO.