Structural properties of Sb–Sn—O mixed oxide catalysts

Abstract

Mixed oxides of tin and antimony were investigated by X-ray diffraction and Mössbauer spectrometry as a function of chemical composition and of firing temperature. At low calcination temperatures, antimony is present as Sb⁵⁺ dissolved in the SnO₂ lattice at 5 atom % Sb and a mixture Sb⁵⁺+ Sb³⁺ at higher concentrations.

Upon calcination a demixing of the solid solution occurs and an antimony oxide phase is formed. From X-ray diffraction, this phase is identified as a bidimensional Sb₂O₄ layer. The balance of electrical charges in the lattice is achieved by two different mechanisms as a function of the Sb content: delocalization of electrons in the conductivity band at low antimony content and reduction of Sb⁵⁺ to Sb³⁺ at higher antimony percentage. Good selectivities are obtained for propene oxidation when the system is biphasic; it is postulated that the actual catalyst consists of an oriented film of Sb₂O₄ supported by the Sb⁵⁺–SnO₂ solid solution.