Piras Alessandro, Colussi Sara, Trovarelli Alessandro, Sergo Valter, Llorca Jordi, Psaro Rinaldo, Sordelli Laura
Dipartimento di Scienze e Tecnologie Chimiche, via del Cotonificio 108, Università di Udine, 33100 Udine, Italy.
J Phys Chem B. 2005 Jun 9;109(22):11110-8. doi: 10.1021/jp0440737.
A series of CeO(2)/Al(2)O(3) samples with different ceria loadings in the range 0-25 wt % (0, 2, 5, 7.5, 15, and 25%) were prepared by incipient wetness and studied using several complementary techniques such as Brunauer-Emmett-Teller (BET), X-ray diffraction (XRD), temperature-programmed reduction (TPR), Raman, high-resolution transmission electron microscopy (HRTEM), and extended X-ray absorption fine structure (EXAFS). The aim of the investigation was to understand the behavior of ceria when deposited on alumina and treated under oxidizing and reducing conditions at high temperature (T >/= 1273 K). It is shown that ceria can partially stabilize alumina toward the formation of low-surface-area phases up to 1373 K under oxidizing conditions, while enhanced stabilization is observed under reducing conditions, being effective up to 1473 K. A detailed quantitative temperature-programmed reduction (TPR) analysis made at different loadings and calcination temperatures allowed us to identify three characteristic regions where the reduction of small and large ceria crystallites occurs with the formation of CeAlO(3) crystallites at high temperature. These are likely responsible for surface-area stabilization. For dispersed ceria samples, reduction takes place almost exclusively at low temperature (<700 K), while a shift to higher temperatures is observed upon increasing the ceria particle size. A fraction of Ce, in samples at low loadings, is stable in the lower oxidation state, even if subjected to strongly oxidizing conditions.
通过初湿浸渍法制备了一系列二氧化铈负载量在0 - 25 wt%（0、2、5、7.5、15和25%）范围内的CeO₂/Al₂O₃样品，并使用多种互补技术进行研究，如布鲁诺尔-埃米特-泰勒（BET）法、X射线衍射（XRD）、程序升温还原（TPR）、拉曼光谱、高分辨率透射电子显微镜（HRTEM）和扩展X射线吸收精细结构（EXAFS）。研究目的是了解二氧化铈沉积在氧化铝上并在高温（T≥1273 K）的氧化和还原条件下处理时的行为。结果表明，在氧化条件下，二氧化铈可在高达1373 K的温度下部分稳定氧化铝，防止其形成低比表面积相；而在还原条件下观察到增强的稳定性，在高达1473 K时仍有效。在不同负载量和煅烧温度下进行的详细定量程序升温还原（TPR）分析使我们能够确定三个特征区域，在这些区域中，大小不同的二氧化铈微晶发生还原，并在高温下形成CeAlO₃微晶。这些可能是比表面积稳定的原因。对于分散的二氧化铈样品，还原几乎只在低温（<700 K）下发生，而随着二氧化铈粒径的增加，还原温度会向更高温度移动。即使在强氧化条件下，低负载量样品中的一部分铈也能以较低氧化态稳定存在。
