REMARKABLE SPREADING BEHAVIOR OF MOLYBDENA ON SILICA CATALYSTS - AN INSITU EXAFS-RAMAN STUDY

In contrast to the frequently reported lack of interaction between hexavalent molybdenum and SiO2 and the tendency of silica-supported MoO3 to coalescence, it has been found that on dehydration small molybdenum oxide clusters spread over a silica support. A combined Raman spectroscopy-X-ray absorption study shows a significantly altered structure of the molybdenum oxide phase after dehydration. In EXAFS the total Mo-Mo coordination number drops from 3.27 to 0.20 after an in situ thermal treatment at 673 K. The increase of the peak in the XANES region (1s --> 4d) indicates that the coordination sphere of the molybdenum atoms strongly alters after dehydration. The Raman spectra reflect the change of the structure through a shift of the position of the terminal Mo = O bond from 944 to 986 cm-1 and the disappearance of the bridged Mo-O-Mo vibration at 880 cm-1. It is concluded that dehydration produces almost isolated molybdenum sites in this highly dispersed sample. Water ligands stabilize the oligomeric clusters under ambient conditions; the removal of water causes spreading of these clusters.