Phase cooperation in oxidation catalysis. Structural studies of the iron antimonate–antimony oxide system

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Abstract
The compositions FeSb2O6 and FeSb5O12 of the two-phase FeSbO4–α-Sb2O4 system, an active and selective catalyst for the oxidation and ammoxidation of propylene, have been structurally characterized by Rietveld analysis of powder neutron-diffraction data. Results of the analysis indicate that the presence of Sb2O4 has no effect on the bulk structural parameters of FeSbO4. Specifically (a) the unit cell of FeSbO4 does not depend upon the presence of Sb2O4 or calcination temperature, (b) antimony atoms are not found in the intersticies of the coexisting iron antimonate and (c) the apparent Sb/Fe ratio is 1 in iron antimonate. Additionally, the Sb/Fe occupancy in the rutile FeSbO4 structure is random as no supercell reflections were observed. Results of scanning electron microscope and X-ray photoelectron spectroscopy experiments have been interpreted to show that Sb enrichment occurs on coprepared samples of the two-phase mixture. Based on this evidence and the lack of alteration of the bulk structures of both phases it is suggested that surface alteration in this two-phase system is the key to enhanced selective catalytic oxidation activity.

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