Heterogeneous Catalysis DOI: 10.1002/ange.201300538 Synergy between XANES Spectroscopy and DFT to Elucidate the Amorphous Structure of Heterogeneous Catalysts: TiO 2 -Supported Molybdenum Oxide Catalysts** Asma Tougerti, Elise Berrier, Anne-Sophie Mamede, Camille LaFontaine, ValØrie Briois, Yves Joly, Edmond Payen, Jean-FranÅois Paul, and Sylvain Cristol* The properties of heterogeneous catalysts are directly corre- lated to the molecular structure of the active sites which often consists of nanometer-scale particles of transition metals (in a metallic, oxide or sulfide form) dispersed on an oxide support. [1] The complexity of physico-chemical phenomena occurring during the catalyst synthesis/activation stages often leads to the formation of unknown supported phases featur- ing new ill-defined sites. [2] Their identification requires an in- depth characterization at the molecular scale of the catalyst with the use of various spectroscopic tools. Despite the valuable information so-obtained, these techniques some- times are not able to provide the overall structure of the active species responsible for the catalytic activity. The use of theoretical tools to establish direct correlation between spectroscopic fingerprints and structural/electronic properties of catalysts is a powerful and quite new approach for unraveling the structure of catalysts. Thanks to its chemical and electronic (orbital) selectivity, X-ray absorption near-edge structure (XANES) spectroscopy is the technique of choice for molecular-scale characterization of catalysts. Furthermore, in the XANES spectra, the long mean free path of the photoelectron, induced by its small kinetic energy (E c < 50–100 eV), provides a high contribution of multiple scattering events of the photoelectron allowing to probe the three-dimensional structure (3D) around the absorbing atom. [3] XANES spectroscopy is however highly dependent on electronic parameters that make its fine interpretation difficult. This often leads to a restricted use of XANES spectra for the determination of the oxidation state of an absorber atom or/and basic consideration on its local symmetry. It is however possible to obtain a finer interpretation of spectra by calculation of XANES transi- tions. Indeed, the multiple scattering (MS) theory is well adapted to reproduce the XANES transitions observed at K edges of elements heavier than Li and at L 2,3 edges of elements heavier than Cd. [3] We propose herein to use MS simulations based on structural models predicted by DFT calculations for inter- preting the Mo K edge fingerprints observed for TiO 2 -sup- ported molybdenum oxide catalysts which are widely used for olefin metathesis, [4] selective oxidation reactions, [5] and hydro- treatment. [6] The structure of the oxomolybdate species formed during the catalyst activation is still unknown even though it has been extensively characterized by various spectroscopies. [7] A three-step methodology was followed to unravel the catalyst structure: first simulations of XANES spectra of Mo reference compounds (ammonium heptamo- lybdate (hereafter noted AHM), (NH 4 ) 3 [Al(OH) 6 Mo 6 O 18 ] (noted AlMo 6 ), a-(NH 4 ) 4 [Mo 8 O 26 ] (noted Mo 8 O 26 )) were performed to obtain direct correlation between spectroscopic fingerprints and structural properties of known structures. Then, these fingerprints are identified in the spectrum of the supported catalyst to generate catalyst structures for DFT optimization. Finally, the optimized geometry is used for modeling XANES spectra using MS theory. Figure 1 A presents Mo K edge experimental spectra of the activated catalyst (350 8C in oxygen), labeled hereafter 7.5 MoTi, together with four reference compounds in which Figure 1. A) Mo K edge XANES spectra of model compounds and the 7.5 MoTi catalyst (Norm. m(E) = normalized absorption). B) Zoom on Mo K edge XANES spectra for octahedral model compounds and the 7.5 MoTi catalyst at 75 eV beyond the pre-edge. [*] Dr. A. Tougerti, Dr. E. Berrier, Dr. A.-S. Mamede, Prof. E. Payen, Prof. J.-F. Paul, Prof. S. Cristol UniversitØ Lille 1 UnitØ de Catalyse et de Chimie du Solide, UMR CNRS 8181 59655 Villeneuve d’Ascq Cedex (France) E-mail: sylvain.cristol@univ-lille1.fr Dr. C. LaFontaine, Dr. V. Briois Synchrotron Soleil Gif-sur-Yvette (France) Dr. Y. Joly Institut NØel, CNRS-UniversitØ J. Fourier Grenoble (France) [**] We wish to acknowledge the Agence Nationale de la Recherche for funding under the contract number ANR-07-BLAN-0265-01 (Spec- troscopie d’Absorption X Operando). XANES = X-ray absorption near-edge structure. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/anie.201300538. . Angewandte Zuschriften 6568  2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Angew. Chem. 2013, 125, 6568 –6572