OXYGEN-CHEMISORPTION AND LASER RAMAN-SPECTROSCOPY OF UNSUPPORTED AND SILICA-SUPPORTED MOLYBDENUM OXIDE

Samples of unsupported and silica-supported molybdenum oxide were characterized by oxygen chemisorption, X-ray diffraction, and laser Raman spectroscopy. In the supported materials, the interaction between molybdenum oxide and silica was weak, and for high-concentration samples, the molybdenum oxide was mostly in the form of small crystallites of bulk structure. At very low concentrations, however, laser Raman spectroscopy indicated the presence of a highly dispersed, monomeric species. The structure of this surface molybdate is probably that of a distorted tetrahedron with two short M = O bonds and two long M-O bonds. Oxygen chemisorption at 630 K gave correct values of dispersion in both the low and high ranges of molybdenum concentration. It indicated close to 100% dispersion for the surface species and a rapid dropoff in dispersion in the higher concentration samples, consistent with X-ray diffraction results. Thus, the chemisorption method developed here can be used to titrate surface molybdenum centers in unsupported and silica-supported MoO3.