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.