A Mo-K edge XAFS study of the metal sulfide-support interaction in (Co)Mo supported alumina and titania catalysts

The metal-support interaction in oxidic and sulfided (Co)Mo/Al2O3 and (Co)Mo/TiO2 catalysts has been studied with X-ray absorption spectroscopy. Analysis of the oxidic catalysts showed that on both Mo/Al2O3 and Mo/TiO2 the molybdenum oxide particles possess a highly distorted octahedral structure with Mo-O distances ranging from 1.71 Angstrom to 1.94 Angstrom. The second shell Mo-Mo coordination number of less than 1.0 indicated that the size of the molybdenum oxide particles is very small. Upon addition of Co the particle size is increased to approximately four Mo atoms per particle. EXAFS data analysis showed the presence of second shell Al and Ti neighbours indicating a linkage of the molybdenum oxide particles to the support via Mo-O-X (X = Al or Ti) bonds. Upon sulfidation of the oxidic catalysts small MoS2 particles are formed with second shell Mo-Mo coordination numbers ranging from 0.8 to 3.9. Angstrom Mo-O contribution at 2.0 Angstrom was found in the first coordination shell of Mo in the sulfided catalysts. Since the coordination number of this Mo-O contribution correlated with the MoS2 particle size, deduced from the second shell Mo-Mo coordination number, this Mo-O contribution was assigned as an interfacial MO-O-support linkage. This Mo-O-support linkage stabilises the small MoS2 particles and prevents them from sintering. It is proposed that the activity of these catalysts can be controlled by optimising the molybdenum sulfide-support interaction, which consists of the observed Mo-O-support bonds. (C) 1997 Academic Press