Binding sites, geometry, and energetics of propene at nanoparticulate Au/TiO₂

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).