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 spectrosc opy. Analysis of the oxidic catalysts showed that on both Mo/Al2O3 and Mo/TiO2 the molybdenum oxide particles possess a highly distorted oct ahedral 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 s mall. Upon addition of Co the particle size is increased to approximat ely four Mo atoms per particle. EXAFS data analysis showed the presenc e of second shell Al and Ti neighbours indicating a linkage of the mol ybdenum oxide particles to the support via Mo-O-X (X = Al or Ti) bonds . Upon sulfidation of the oxidic catalysts small MoS2 particles are fo rmed 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 coordin ation number of this Mo-O contribution correlated with the MoS2 partic le 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 pr events them from sintering. It is proposed that the activity of these catalysts can be controlled by optimising the molybdenum sulfide-suppo rt interaction, which consists of the observed Mo-O-support bonds. (C) 1997 Academic Press