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), ultraviolet and visible diffuse reflectance spectroscopy (UV-DRS), and temperature-programmed reduction (TPR) are used to characterize a series of WO3/CeO2 samples. The results indicate that the dispersion capacity of tungsten oxide 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. For the calcined samples, two distinctly different tungsten species have been identified by various methods. At low WO3 loading, only the highly dispersed tungsten oxide species are found on the surface possibly formed by the incorporation of the dispersed W6+ ions into the surface vacant sites of CeO2. 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 shown that calcination has a dramatic effect on the structure of the final product, which might mostly eliminate the differences of the precursors and result in final products with almost a same structure. TPR results of WO3/CeO2, CuO/CeO2, and CuO/WO3-CeO2 samples reveal that the reduction behaviors of CuO dispersed on CeO2 and on WO3 premodified CeO2, i.e., WO3-CeO2, are apparently different. The result emphasizes the importance of the surface structure of the support on the properties of the dispersed metal oxide species; the conclusion is also supported by UV-DRS results. The coordination environments of the dispersed tungsten oxide and copper oxide species are discussed on the basis of the incorporation model (Chen, Y.; Zhang, L. Catal. Lett. 1992, 12, 51).