Abstract
Alumina supported V 2O 5, MoO 3, Fe 2O 3, Fe 2O 3V 2O 5, and MoO 3V 2O 5 catalysts were investigated by Raman spectroscopy as a function of calcination temperature under ambient conditions where the surfaces are hydrated. The single metal oxide systems (V 2O 5, MoO 3, or Fe 2O 3) and the mixed metal oxide systems (Fe 2O 3V 2O 5 and MoO 3V 2O 5) are present as surface metal oxide species on the alumina support at intermediate calcination temperatures. For the single metal oxide species, the surface vanadium oxide species consists of metavanadate species ([VO 3] x− n ), the surface molybdenum oxide species consists of heptamolybdate (Mo 7O 6− 24) and octamolybdate (Mo 8O 4− 26) species, and the surface iron oxide species is primarily present as a surface spinel containing Fe 3+ ions in the surface region of the alumina support. Addition of iron oxide and molybdenum oxide to the V 2O 5/Al 2O 3 transforms the surface metavanadate species to surface pyrovanadate species (V 2O 2− 7) and surface decavanadate species (V 10O 6− 28), respectively. The thermal stability of the Supported metal oxide on the alumina support depends on the specific nature of the surface metal oxide species. The supported iron oxide on alumina has a high thermal stability, similar to the unpromoted alumina, which is probably related to Fe 3+ absorption into the surface region of the alumina lattice. In contrast, the supported molybdenum oxide on alumina system is not stable above 800°C due to significant loss in the alumina surface area and volatilization of molybdenum oxide. The surface vanadium oxide species on alumina is stable up to a calcination temperature of 800°C and transforms to crystalline V 2O 5 at higher calcination temperatures due to significant loss in surface area of the alumina support. The mixed metal oxide species (Fe 2O 3V 2O 5 and MoO 3V 2O 5) on alumina could not stabilize the surface vanadium oxide on alumina above 800°, and accelerated the rate of surface area loss and phase transformations. No FeVO 4, AlVO 4, or VMoO compounds were formed in these mixed metal oxide species on alumina at elevated temperatures.
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