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

Abstract

The molybdate species and their coordination structures on gamma-alumina are characterized by UV resonance Raman spectroscopy and UV-vis diffuse reflectance spectroscopy. The tetrahedral and octahedral molybdate species can be definitely identified on the basis of the resonance Raman effect. The evolution of species has been studied during the catalyst preparation under various conditions including the pH 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 property of gamma-alumina. When the impregnating solution is at pH = 10.0, only tetrahedral molybdate species are formed on gamma-alumina, while both tetrahedral and octahedral species are formed on gamma-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 (NH4)(6)Mo7O24, some tetrahedral molybdate species polymerize to the octahedral species during calcination even when the loading of molybdate is extremely low. However, the molybdate species hardly aggregate into the octahedral species when the sample prepared by the impregnating solution of Na2MoO4. It turns out that the surface pH value (i.e., the pH of region) is a key factor governing the coordination structure of the molybdate species.