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

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.