A study on the surface properties of ceria-supported tungsten and copper oxides

Laser Raman spectroscopy (LRS), Fourier transform infrared spectroscopy (FT -IR), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), ultr aviolet and visible diffuse reflectance spectroscopy (UV-DRS), and temperat ure-programmed reduction (TPR) are used to characterize a series of WO3/CeO 2 samples. The results indicate that the dispersion capacity of tungsten ox ide is about 4.8W(6+) ions nm(-2)(CeO2) and the structure of the supported tungsten oxide species is closely related to its loading amount on ceria. F or the calcined samples, two distinctly different tungsten species have bee n identified by various methods. At low WO3 loading, only the highly disper sed tungsten oxide species are found on the surface possibly formed by the incorporation of the dispersed W6+ ions into the surface vacant sites of Ce O2. increasing the loading amount of tungsten oxide to a value above 4.8W(6 +) ions nm(-2)(CeO2) leads to the formation of crystalline WO3. LRS and IR results of WO3/CeO2 samples prepared by using different precursors have sho wn that calcination has a dramatic effect on the structure of the final pro duct, which might mostly eliminate the differences of the precursors and re sult in final products with almost a same structure. TPR results of WO3/CeO 2, CuO/CeO2, and CuO/WO3-CeO2 samples reveal that the reduction behaviors o f CuO dispersed on CeO2 and on WO3 premodified CeO2, i.e., WO3-CeO2, are ap parently different. The result emphasizes the importance of the surface str ucture of the support on the properties of the dispersed metal oxide specie s; the conclusion is also supported by UV-DRS results. The coordination env ironments of the dispersed tungsten oxide and copper oxide species are disc ussed on the basis of the incorporation model (Chen, Y.; Zhang, L. Catal. L ett. 1992, 12, 51).