Surface adsorption and activation of propene, to catalytically produce propene oxide, is the first step in the industrially important gas-phase epoxidation reaction. Motivated by the significant practical importance of this reaction, site-specific adsorption of propene on nanoparticulate titania-supported Au (Au/TiO2) has been systematically characterized through Fourier transform infrared (FTIR) spectroscopy and density functional theoretical (DFT) calculations. The infrared spectra, recorded during propene uptake at low surface temperatures, and DFT calculations identified two distinct propene-surface binding motifs. Propene was found to bind to the surface through a π-interaction at TiO2 sites remote from the Au particles and through a (πσ)-interaction at a single atomic Au site, distinguishable through the stretching frequency of the propene double bond. Temperature-programmed desorption and calculated binding enthalpies for the minimum-energy configurations revealed the propene-Au interaction to have a stronger binding energy relative to the propene-TiO2 interaction. Upon coordination to Au, the double bond of propene was found to weaken and elongate, a possible first step in the activation and epoxidation to form propene oxide. (Chemical Equation Presented).
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This material is based upon work supported by the U. S. Army Research Laboratory and the U. S. Army Research Office under grant number W911NF-14-1-0159. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the ARO, or the U.S. Government. The U.S. Government is authorized to reproduce and distribute reprints for Governmental purposes notwithstanding any copyright annotation thereon. We also gratefully thank the XSEDE computing resources from Texas Advanced Computing Center and San Diego Supercomputer Center for all of the DFT calculations. This research was supported in part by an appointment to the Postgraduate Research Participation Program at the U.S. Edgewood Chemical Biological Center administered by the Oak Ridge Institute for Science and Education through an interagency agreement between the U.S. Department of Energy and USAECBC.
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