Project Summary: CATALYSIS SCIENCE INITIATIVE: Supported Molecular Catalysts: Synthesis, In-Situ Characterization and Performance. Final Report

The major goal of this project was to gain a broad understanding of a typical supported metal catalyst, namely Re/γ-Al2O3, from ab initio theory and simulations of both structure and x-ray absorption spectra (XAS). Such spectra provide a powerful tool to elucidate both the local environment and electronic structure of specific atoms in complex structures. The interpretation of these spectra is enhanced when theoretical methods are used in simulations. Re is an important catalytic species that performs a variety of hydrocarbon conversions. The structures of supported metal catalysts are intended to understand the processes in which they are involved. Although they can be improved by trial and error, a better approach to determine their properties is to use ab initio methods. For this purpose we proposed to integrate multiple scattering methods and density functional theory (DFT) to simulate XAS. This work also developed extensions for catalyst research of the FEFF8 XAS code developed by our group. We previously demonstrated the power of FEFF8 in elucidating the effect of cluster size and geometry on the Pt L-edge XAS of small Pt clusters. In the present project we carried out DFT calculations to determine the preferred interaction sites for isolated Re atoms and dimers on γ-Al2O3. The results were then used to study changes in the XAS of Re/Al2O3. The simulations showed that the strong Re-O binding results in noticeable surface rearrangements. The most stable site corresponds to a Re atom interacting with four O atoms in a surface-exposed Al vacancy, while the remaining sites are less stable and fairly close in binding energy. The DFT calculations provide an estimate of the atomic charge of the absorbing atoms, which is usually associated with the intensity of the white line in XAS. The Re charge follows the number of Re-O bonds. Experimental EXAFS data was obtained at the Re L3-edge of a Re/γ-Al2O3 catalyst that was reduced in situ. When compared with Re metal, Re(IV) oxide and Re(VII), the edge position of the white line of the Re L3-edge XANES of the dry reduced catalyst clearly indicates that the Re is reduced. However, the intensity of the white line is comparable to that of Re(IV), showing the strong oxophilic nature of Re. The Fourier transform (FT) of these spectra show a strong peak at ∼2.5 Angstrom that is likely due to Re-Re neighbors. Models were then constructed based on the theoretical minima, allowing the Re atom to move slightly on the γ-Al2O3 surface. A statistical comparison of the EXAFS models indicates that the triply bound minimum gave the best match to experiment. This project also made two other key developments: First, a new, efficient algorithm for ab initio calculations of EXAFS Debye-Waller (DW) factors. Such DW factors constitute one of the main components in the modern XAFS formula, and are essential for the quantitative treatment of thermal vibrations. Our method is based on ab initio calculations of the dynamical matrix using codes such as ABINIT and GAUSSIAN, together with a Lanczos algorithm for the phonon spectra. The method is general and can be applied to crystalline, amorphous and molecular systems, providing results in very good agreement with experiment. Second, our group has developed methods, which are well suited for the analysis of catalysts that have a large number of unknown parameters. Beginning with structural models, we restrain the parameters using uncertainties determined a priori using Bayesian analysis methods. The advantage of using Bayesian restraints is that the parameter space naturally separates into parameters that are truly relevant to a fit. In conclusion, we successfully carried out studies of supported metal catalysts using DFT and mapped the interaction between a Re atom and the (110) γ-Al2O3 surface, determining several chemically important surface sites based on an analysis of the XAS. We have also introduced new algorithms for calculations of XAS Debye-Waller factors and improved Bayesian analysis procedures, which are especially relevant to catalyst investigations.