Oxygen adsorption on well-defined gold particles on TiO2(110)

Very tiny An particles on TiO2 show excellent activity and selectivity in a number of oxidation reactions. We have studied the vapor deposition of:Au onto a TiO2(110) surface using x-ray photoelectron spectroscopy, low energy ion scattering, low; energy electron diffraction, and temperature programmed description (TPD) and found that we can prepare Au islands with controlled thicknesses from one to several monolayers. In order to understand at the atomic level the unusual catalytic activity in oxidation reactions of this system, we have studied oxygen adsorption on Au/TiO2(110) as a function of Au island thickness. A hot filament was used to dose gaseous oxygen atoms, since O-2 gas does not readily dissociatively adsorb on Au. The saturation coverage of oxygen adatoms on Au particles approaches 1.0 ML, similar to that found on bulk Au(110) and (111). TPD results show higher desorption temperatures (645 K) for ultrathin gold particles on TiO2(110) than thicker particles (520-545 K). This implies that O-a bonds much more strongly to ultrathin islands of Au. Thus from Bronsted relations, ultrathin gold particles should be able to dissociatively adsorb O-2 more readily than thick gold particles. We postulate that the catalytic activity in oxidation reactions demonstrated by thin (small) Au particles on TiO2 is related to the positive influence this stronger bonding to oxygen has on the dissociative adsorption rate of O-2. (C) 1999 American Vacuum Society. [S0734-2101(99)02704-9].