A Mo K Edge XAFS Study of the Metal Sulfide-Support Interaction in (Co)Mo Supported Alumina and Titania Catalysts.

Abstract

The metal-support interaction in oxidic and sulfided (Co)Mo/Al{2}O{3}and (Co)Mo/TiO{2}catalysts has been studied with X-ray absorption spectroscopy. Analysis of the oxidic catalysts showed that on both Mo/Al{2}O{3}and Mo/TiO{2}the molybdenum oxide particles possess a highly distorted octahedral structure with Mo-O distances ranging from 1.71 A to 1.94 A. 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 MoS{2}particles are formed with second shell Mo-Mo coordination numbers ranging from 0.8 to 3.9. A Mo-O contribution at 2.0 A 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 MoS{2}particle size, deduced from the second shell Mo-Mo coordination number, this Mo-O contribution was assigned as an interfacial Mo-O{s}{u}{p}{p}{o}{r}{t}linkage. This Mo-O{s}{u}{p}{p}{o}{r}{t}linkage stabilises the small MoS{2}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{s}{u}{p}{p}{o}{r}{t}bonds.