Surface interactions of MoO3/α-Fe2O3 system

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α-Fe2O3-supported molybdena catalysts have been prepared by heating a mixture of MoO3 and α-Fe2O3. XRD, XPS, LRS, TG-DTA and Mssbauer 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℃. 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 Mssbauer 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℃, MoO3 dispersed on the surface of α-Fe2O3 and formed surface Mo species;(ⅱ) at 500℃, MoO3 reacted with the bulk of α-Fe2O3 and formed Fe2(MoO4)3 compound.
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