Reactivity and Properties of Au/TiO2 Nanocatalysts

Gold particles with diameters in the nanometer range in contact with reducible oxide supports exhibit catalytic activity at temperatures well below 273 K. Many studies have suggested that the catalytic activity occurs at interface sites between the metal and the support. Here, using infrared (IR) spectroscopy, I show the oxidation of CO, ethylene, and carboxylic acids (acetic, propionic and butyric acid) on a Au/TiO2 catalyst with ~3 nm diameter Au nanoparticles in order to further explore the role of the Au/TiO2 interface. At the interface, the first step in all the oxidation processes is the dissociation of O2 at the Au-Ti4+ dual site, where one dioxygen O atom is bonded to the Au and the other O is bonded to the Ti4+ site of the metal oxide support, to form activated O adatoms on the surface. Then for ethylene and the carboxylic acids, in sequential steps, acidic C-H bonds are activated, followed by C-O and C-C (for acid oxidation) bond scission yielding adsorbed ketenylidene, Au2C=C=O, an exotic species formed just before full oxidation. The kinetics and full mechanisms are discussed.

Additionally, I explore the effect of the metal oxide support, TiO2, on ~3 nm Au nanoparticles by adsorbing donor molecules (CH4, C2H6 and C3H8) and separately an acceptor molecule (SF6) on only the TiO2 support. In order to observe the effect these molecules have on the Au nanoparticles, CO is initially adsorbed on only the Au sites. As donor or acceptor molecules adsorb on the TiO2, IR spectroscopy is used to observe the frequency change in the Au-CO IR absorbance bands. For donor/TiO2 adsorption, electrons transfer from the TiO2 to the Au causing a redshift; whereas, for acceptor/TiO2 adsorption, electrons transfer from the Au nanoparticles to the TiO2 support causing a blueshift in the IR frequency. In comparison, electron transfer was not observed on a Au/SiO2 catalyst. Additional methods confirm these results.