UV Resonance Raman Spectroscopic Studies on the Genesis of Highly Dispersed Surface Molybdate Species on γ-Alumina

The surface molybdate species and their coordination structures on γ- alumina are characterized by UV resonance Raman spectroscopy and UV−vis diffuse reflectance spectroscopy. The surface tetrahedral and octahedral molybdate species can be definitely identified on the basis of the resonance Raman effect. The evolution of surface species has been studied during the catalyst preparation under various conditions including the pH value of the impregnating solution and the pretreatment temperature. It is shown that the coordination structures of the supported molybdate in the wet state are dependent on the structures of molybdate in the impregnation solution as well as the surface property of γ-alumina. When the impregnating solution is at pH = 10.0, only tetrahedral molybdate species are formed on γ-alumina, while both tetrahedral and octahedral species are formed on γ-alumina at pH = 3.0, although the molybdate is in only the octahedral form in the impregnation solution at pH = 3.0. For the samples prepared by the impregnating solution of (NH 4 ) 6 Mo 7 O 24, some surface tetrahedral molybdate species polymerize to the octahedral species during calcination even when the loading of molybdate is extremely low. However, the surface molybdate species hardly aggregate into the octahedral species when the sample prepared by the impregnating solution of Na 2 MoO 4 . It turns out that the “surface pH value” (i.e., the pH value of surface region) is a key factor governing the coordination structure of the surface molybdate species.