The dispersion of molybdena on ceria

Abstract

The dispersion and surface structure of molybdena supported on CeO₂ have been studied by using XRD, XPS, LRS and FTIR spectroscopies. The samples were prepared by mixing MoO₃ and CeO₂ and by impregnating CeO₂ with an aqueous solution of ammonium polymolybdate. The results indicate that the dispersion capacity of MoO₃ is 4.8 Mo⁶⁺ nm^–2(CeO₂) and the structure of molybdena depends on the loading of molybdenum on ceria. At low loadings, only a surface species was formed, possibly resulting from the incorporation of Mo⁶⁺ into the surface vacant sites of CeO₂. Beyond the dispersion capacity, the excess bulk MoO₃ remained in the samples. The Raman spectra of MoO₃/CeO₂ samples prepared with different precursors show that calcination dramatically affects the state of molybdenum and results in the formation of the same surface species, indicating that the structure of the surface species depends on the surface structure of the support. The comparison of the XRD results of MoO₃/CeO₂ and MoO₃/ZnO-modified CeO₂ leads to the conclusion that the surface vacancies play the key role in determining the dispersion of MoO₃ on CeO₂. In addition, the coordination environment of the surface molybdenum species is discussed according to the incorporation model.