Published: March 08, 2011 r2011 American Chemical Society 5740 dx.doi.org/10.1021/jp1105218 | J. Phys. Chem. C 2011, 115, 5740–5755 ARTICLE pubs.acs.org/JPCC Experimental (XAS, STEM, TPR, and XPS) and Theoretical (DFT) Characterization of Supported Rhenium Catalysts Simon R. Bare,* ,† Shelly D. Kelly, § Fernando D.Vila, || Edwin Boldingh, † E. Karapetrova, ‡ Josh Kas, || George E. Mickelson, † Frank S. Modica, † Ning Yang, ‡,^ and John J. Rehr || † UOP LLC, A Honeywell Company, Des Plaines, Illinois 60016, United States ‡ Argonne National Laboratory, Argonne, Illinois 60439, United States § EXAFS Analysis, Bolingbrook, Illinois 60440, United States ) Department of Physics, University of Washington, Seattle, Washington 98195, United States b S Supporting Information ’ INTRODUCTION Supported rhenium catalysts are of great technological im- portance in heterogeneous catalysis. In the oxidic form, rhenium oxide dispersed on high surface area alumina has been shown to have high activity and high selectivity in olefin metathesis. 1 Supported Re catalysts also show high activity for selective catalytic oxidation of simple alcohols 2,3 and exhibit activity in hydrodesulfurization and hydrodenitrogenation. 4 When rhe- nium forms a bimetallic cluster with platinum, the subsequent Pt-Re clusters are used in petroleum-reforming catalysts. 5,6 Given this ubiquitous nature of rhenium-based catalysts, it is not surprising that they have been the focus of several character- ization efforts which are briefly summarized below. Rhenium is a particularly interesting metal as it is strongly oxophilic and its chemistry is therefore expected to be dominated by its propensity for forming strong metal-oxygen bonds, quite different from the more traditionally studied group VIII metal-based catalysts. In reviewing the prior characterization work, it is clear that the nature of the supported rhenium species for catalysts prepared by standard methods using, for example, incipient wetness impregnation of soluble rhenium compounds is complex and warrants further detailed studies under controlled conditions. For example, the reducibility of oxidic rhenium on alumina has been a topic of discussion for decades, and the issue has still not been fully resolved. Going back to the 1970s there are numerous papers discussing the reducibility of rhenium on alumina. 7-9 The debate centered on how much of the rhenium is reduced. This debate continued into the 1980s 10,11 with similar discrepancies as to the temperature of maximum reduction and the amount of reduction. In the current decade the issue of the reducibility of rhenium continues to be a topic of discussion. 12 From these and other studies it can be concluded that the rhenium loading, calcination temperature, and effect of adsorbed moisture all play a role in determining the reducibility of rhenium. XAFS has previously been used to probe the structure of supported rhenium catalysts. Ellison et al. 13 concluded that after Received: November 3, 2010 Revised: January 19, 2011 ABSTRACT: 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 300-400 °C. Following reduction at 500 or 700 °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 γ-Al 2 O 3 . 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 L 3 -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.