Reddy Benjaram M, Lakshmanan Pandian, Khan Ataullah, López-Cartes Carlos, Rojas Teresa C, Fernandez Asunción
Inorganic and Physical Chemistry Division, Indian Institute of Chemical Technology, Hyderabad-500007, India.
J Phys Chem B. 2005 Feb 10;109(5):1781-7. doi: 10.1021/jp045723+.
Structural characteristics of CeO(2)-ZrO(2)/TiO(2) (CZ/T) and V(2)O(5)/CeO(2)-ZrO(2)/TiO(2) (V/CZ/T) mixed oxide catalysts have been investigated using X-ray diffraction (XRD), BET surface area, Raman spectroscopy (RS), and high-resolution transmission electron microscopy (HREM) techniques. The CeO(2)-ZrO(2) (1:1 mole ratio) solid solution was deposited over a finely powdered TiO(2) support by a deposition precipitation method. A nominal 5 wt % V(2)O(5) was impregnated over the calcined (773 K) CZ/T mixed oxide carrier by a wet impregnation technique. The obtained CZ/T and V/CZ/T samples were further subjected to thermal treatments from 773 to 1073 K to understand the dispersion and temperature stability of these materials. In the case of CZ/T samples, the XRD results suggest the formation of different cubic and tetragonal Ce-Zr-oxide phases, Ce(0.75)Zr(0.25)O(2), Ce(0.6)Zr(0.4)O(2), Ce(0.5)Zr(0.5)O(2), and Ce(0.16)Zr(0.84)O(2) in varying proportions depending on the treatment temperature. With increasing calcination temperature from 773 to 1073 K, the intensity of the lines pertaining to cubic Ce(0.6)Zr(0.4)O(2) and Ce(0.5)Zr(0.5)O(2) phases increased at the expense of cubic Ce(0.75)Zr(0.25)O(2), indicating more incorporation of zirconia into the ceria lattice. The TiO(2) was mainly in the anatase form whose crystallite size also increased with increasing treatment temperature. A better crystallization and more incorporation of zirconia into the ceria lattice was noted when CZ/T was impregnated with V(2)O(5). However, no crystalline V(2)O(5) could be seen from both XRD and RS measurements. In particular, a preferential formation of CeVO(4) compound and an intense tetragonal Ce(0.16)Zr(0.84)O(2) phase were noted beyond 873 K. The HREM results indicate, in the case of CZ/T samples, a well-dispersed Ce-Zr-oxide of the size approximately 5 nm over the bigger crystals ( approximately 40 nm) of TiO(2) when treated at 873 K. The exact structural features of these crystals as determined by digital diffraction analysis of experimental images reveal that the Ce-Zr-oxides are mainly in the cubic fluorite geometry and the TiO(2) is in anatase form. A better crystallization of Ce-Zr-oxides ( approximately 8 nm) over the surface of bigger crystals of TiO(2) was noted at 1073 K. A further enhancement in the crystallite size and zirconia-rich tetragonal phase was noted in the case of V/CZ/T samples. Further, the structure of CeVO(4) formed was also clearly identified in conformity with XRD and RS results.
采用X射线衍射（XRD）、BET比表面积、拉曼光谱（RS）和高分辨率透射电子显微镜（HREM）技术研究了CeO(2)-ZrO(2)/TiO(2)（CZ/T）和V(2)O(5)/CeO(2)-ZrO(2)/TiO(2)（V/CZ/T）混合氧化物催化剂的结构特征。通过沉积沉淀法将CeO(2)-ZrO(2)（1:1摩尔比）固溶体负载在细粉状TiO(2)载体上。采用湿浸渍技术将标称5 wt%的V(2)O(5)浸渍在煅烧（773 K）的CZ/T混合氧化物载体上。对获得的CZ/T和V/CZ/T样品进一步进行773至1073 K的热处理，以了解这些材料的分散性和温度稳定性。对于CZ/T样品，XRD结果表明，根据处理温度的不同，会形成不同比例的立方和四方Ce-Zr-氧化物相，即Ce(0.75)Zr(0.25)O(2)、Ce(0.6)Zr(0.4)O(2)、Ce(0.5)Zr(0.5)O(2)和Ce(0.16)Zr(0.84)O(2)。随着煅烧温度从773 K升高到1073 K，与立方Ce(0.6)Zr(0.4)O(2)和Ce(0.5)Zr(0.5)O(2)相相关的谱线强度增加，而立方Ce(0.75)Zr(0.25)O(2)的强度降低，这表明更多的氧化锆掺入氧化铈晶格中。TiO(2)主要为锐钛矿形式，其晶粒尺寸也随处理温度的升高而增大。当CZ/T浸渍V(2)O(5)时，氧化锆在氧化铈晶格中的结晶更好，掺入量更多。然而，XRD和RS测量均未观察到结晶的V(2)O(5)。特别是，在873 K以上，观察到CeVO(4)化合物的优先形成和强烈的四方Ce(0.16)Zr(0.84)O(2)相。HREM结果表明，对于CZ/T样品，在873 K处理时，尺寸约为5 nm的Ce-Zr-氧化物在较大的TiO(2)晶体（约40 nm）上分散良好。通过对实验图像的数字衍射分析确定的这些晶体的确切结构特征表明，Ce-Zr-氧化物主要为立方萤石结构，TiO(2)为锐钛矿形式。在1073 K时，观察到Ce-Zr-氧化物（约8 nm）在较大TiO(2)晶体表面的结晶更好。在V/CZ/T样品中，晶粒尺寸进一步增大，富氧化锆的四方相更加明显。此外，所形成的CeVO(4)的结构也与XRD和RS结果一致，得到了明确鉴定。
