Surface interactions of MoO3/ α-Fe2O3 system

Abstract

α-Fe2O3-supported molybdena catalysts have been prepared by heating a mixture of MoO3 and α-Fe2O3. XRD, XPS, LRS, TG-DTA and Mossbauer spectroscopy were used to characterize the interactions between MoO3 and α-Fe2O3. The dispersion capacity of MoO3 on the surface of α-Fe2O3 determined by XRD and XPS was 0.8 mmol/100m2 α-Fe2O3 in the samples calcined at 420°C. For the sample with low MoO3 loading, LRS and FT-IR results showed that Mo6+ ions were located in the tetrahedral vacant sites on the surface of α-Fe2O3, signed as Mo-I. The amount of Mo-II species, formed by Mo6+ ions incorporated into the octahedral vacant sites, increased with the MoO3 loading. Based on the assumption that the (001) plane of α-Fe2O3 is preferentially exposed, almost all the Mo6+ ions of the dispersed molybdena species existed at the surface octahedral sites for the sample with MoO3 loading close or beyond the dispersion capacity, and formed the Mo-II species. In this case, the capping O2− ions linking with the incorporated Mo6+ ions formed a surface epitaxial structure, which was in good agreement with the results predicted by the incorporation model proposed previously. XRD and Mossbauer spectroscopy of the MoO3 / α-Fe2O3 samples calcined at different temperatures showed that the calcination temperature could strongly influence the interaction extent: (i) at 420°C, MoO3 dispersed on the surface of α-Fe2O3 and formed surface Mo species; (ii) at 500°C, MoO3 reacted with the bulk of α-Fe2O3 and formed Fe2(MoO4)3 compound.