Study of the interactions between MoO3 and alpha-Fe2O3

alpha-Fe3O3-supported molybdenum catalysts were prepared by heating a mixture of MoO3 and alpha-Fe2O3. XRD, XPS, LRS, FT-IR, and Mossbauer spectroscopy were used to study the interactions between MoO3 and alpha-F2O3. At the temperature of 693 K, the dispersion capacity of MoO3 on alpha-Fe2O3 determined by XRD and XPS is 0.80 mmol MoO3/100 m(2) alpha-Fe2O3, i.e., 4.8 Mo6+/nm(2). LRS and FT-IR results show that at low MoO3 loading (1.8 Mo6+/nm(2)), Mo6+ cations are located in the tetrahedral sites of the alpha-Fe2O3 surface. The occcupation of octahedral surface vacant sites increases with the MoO3 loading. Considering the fact that each Mo6+ is accompanied by 3O(2-) anions and that alpha-Fe2O3 has a hexagonal structure, almost all the incorporated Mo6+ on the surface are in octahedral coordination environment. Based on the assumptions that the (001) plane of alpha-Fe2O3 is preferentially exposed on the surface and that all the usable surface vacant sites have been occupied, the formation of a close-packed layer on the alpha-Fe2O3 surface by the O2- anions linked with the incorporated Mo6+ can be expected, which is in good agreement with the result predicted by the incorporation model proposed previously. A relationship between the residual bulk MoO3 and the calcination time shows that Mo6+ ions occupy the surface vacant sites of alpha-Fe2O3 in two stages. The first stage may correspond to the migration of Mo6+ cations from the bulk MoO3 to the tetrahedral surface vacant sites on the surface. The second stage may correspond to the migration of Mo6+ from the bulk MoO3 into the octahedral unoccupied vacant sites. Mossbauer spectroscopy and XRD results indicate that a new phase, Fe-2(MoO4)(3), is formed when the sample containing 10.0Mo(6+)/nm(2) alpha-Fe2O3 was calcined at 743 K, suggesting that the calcination temperature is important to the interaction extent between MoO3 and alpha-F2O3. (C) 1997 Academic Press.