The nature of antimony-enriched surface layer of Fe–Sb mixed oxides

Abstract

Antimony segregation is a common feature in Fe–Sb mixed oxides, which have been widely applied as catalysts in selective oxidation and ammoxidation reactions. This paper attempts to shed a light on the cause of such a common feature and on the nature of the antimony-enriched surface layer over FeSbO 4 by means of XPS surface analysis. Single-phase FeSbO 4 samples prepared by different methods were studied, and the antimony in their surface layer is a mixture of both Sb 5+ and Sb 3+ rather than single Sb 5+. Their surface composition is close to FeSb 2O 6, which could be described as (FeSbO 4)(Sb 2O 4) δ , δ = 0.5, and it is not “Fe(II)Sb(V) 2O 6” as suggested in literature. Fe–Sb mixed oxides with Sb/Fe > 1 (mol/mol) are mixtures of FeSbO 4 and Sb 2O 4, and the surface of FeSbO 4 grains would be a layer of (FeSbO 4)(Sb 2O 4) δ , δ ≥ 0.5. Fe–Sb mixed oxides with Sb/Fe < 1 are mixtures of FeSbO 4 and Fe 2O 3, and the surface of FeSbO 4 grains would be a layer of (FeSbO 4)(Sb 2O 4) δ , δ ≤ 0.5, but the remaining Fe 2O 3 would be encapsulated by a layer of FeSbO 4.