Experimental (XAS, STEM, TPR, and XPS) and Theoretical (DFT) Characterization of Supported Rhenium Catalysts

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• 作者：Simon R. Bare ; Shelly D. Kelly ; Fernando D.Vila ; Edwin Boldingh ; E. Karapetrova ; Josh Kas ; George E. Mickelson ; Frank S. Modica ; Ning Yang ; John J. Rehr
• 刊名：Journal of Physical Chemistry C
• 出版年：2011
• 出版时间：April 7, 2011
• 年：2011
• 卷：115
• 期：13
• 页码：5740-5755
• 全文大小：1367K
• 年卷期：v.115,no.13(April 7, 2011)
• ISSN：1932-7455

文摘

A high surface area supported Re-based catalyst, fundamental to heterogeneous catalysis, is studied in the oxidic and reduced states using a combination of experimental (XAFS, STEM, TPR, and XPS) and theoretical (DFT and X-ray spectroscopy simulations) approaches. In the calcined dried catalyst, the Re species is present as an isolated trioxo(oxoaluminate) Re(VII) species. The temperature at which the Re undergoes reduction is a function of the hydrogen partial pressure and temperature ramp rate, but the maximum rate of reduction occurs in the range 300鈭?00 掳C. Following reduction at 500 or 700 掳C in dry hydrogen, the Re is present as a mixture of species: unreduced trioxo(oxoaluminate) Re(VII) species, Re nanoclusters, and isolated Re atoms. By using a multifaceted approach, it is apparent that the majority species is an isolated Re adatom bound to the alumina support. DFT calculations identify several likely adsorption sites for these Re adatoms on the [110] surface of 纬-Al₂O₃. The final extended X-ray absorption fine structure (EXAFS) model taking into account these three species is used to identify the dominant adsorption site for Re on the alumina surface. FEFF8 X-ray absorption near-edge spectroscopy (XANES) calculations of unsupported and alumina-supported Re nanoclusters provide interpretation of the shape and edge position of the Re L₃-edge XANES after reduction. The presence of moisture during reduction strongly affects the mobility of the Re on the alumina leading to agglomeration. Subsequent air exposure of a reduced catalyst readily reoxidizes the reduced Re. The power of using a combination of analysis tools provides insight into the behavior of dispersed Re on supported alumina under oxidizing and reducing conditions relevant to heterogeneous catalysis.