SURFACE-CHEMISTRY OF MAGNESIUM OXIDE-SUPPORTED MOLYBDENUM OXIDE - AN INSITU RAMAN-SPECTROSCOPIC STUDY

Supported molybdenum oxide on magnesium oxide was investigated by in situ Raman spectroscopy as a function of molybdenum loading and calcination temperature in order to determine the molecular structures of the surface molybdenum oxide species. Molybdenum loadings from 1.0 to 26.7 wt % were studied after calcination at temperatures from 80 to 700-degrees-C. For samples calcined at 600-degrees-C in dry air the observed structure of the molybdenum oxide species was dependent upon weight loading. At low molybdenum loadings (<3.3% Mo), an isolated, highly distorted octahedral molybdena species is present on the MgO surface. At intermediate loadings (6.7-13.3 wt % Mo), an octahedrally coordinated polymolybdenum oxide is identified. At loadings of Mo greater than 20.0 wt %, crystalline magnesium molybdate (MgMoO4) is found to be the dominant structure on the surface. Both the isolated octahedral molybdena and polymolybdena species transform to tetrahedrally coordinated species upon hydration at room temperature. The influence of calcination temperature for fixed loading of molybdenum on MgO was also examined. The structure of the surface molybdate species changes from tetrahedral coordination to octahedral coordination at a temperature around 400-degrees-C. At temperatures above 450-degrees-C, polymolybdena species are formed which are stable up to 700-degrees-C. The observations are discussed in terms of the acid-base chemistry of both molybdenum oxide and magnesium oxide. The influence of the magnesium oxide structure on the coordination of the surface molybdate is also noted.