Paper SS-TuP14
Reaction Properties of O₃ and CO Over Gold Surface

Tuesday, October 21, 2008, 6:30 pm, Room Hall D

Gold nanoparticles supported on TiO₂ exhibit high catalytic activity for CO oxidation. Although numerous investigations have been carried out to elucidate the source of this enhanced activity, there are still controversies concerning the active sites and the role of support for the Au/TiO₂ catalyst. In addition to the aforementioned studies, reactions of O₂, O₃ and CO on gold surfaces have been investigated by means of surface science techniques. Recently, we found that O₃ dissociation and CO adsorption depend strongly on the gold surface structure. Here, we report the adsorption and desorption properties of atomic oxygen produced from O₃ exposure and CO adsorption properties on gold single crystals as well as gold deposited on TiO₂(110). XPS measurements confirmed that no dissociative adsorption of O₂ occurred on surfaces of Au(111), Au(100) and Au(311). On the other hand, atomic oxygen was observed on Au(111) and Au(311) upon exposure to O₃, but no atomic oxygen was detected on Au(100). The saturation coverage of atomic oxygen on Au(311) was half of that observed on Au(111), where the exposed (111) face on Au(311) was half of that on Au(111). Furthermore, the initial formation rate of atomic oxygen for Au(311) was half of that for Au(111). These results clearly indicate that O₃ dissociation over gold surfaces proceeded selectively on the (111) face. We found that the adsorption behavior of CO also depended on the gold surface structure. PM-IRAS peaks of CO at 2070-2080 cm^-1 were observed for Au(111) and Au(100) at CO pressures above 0.5 Torr; these peaks were assigned to the CO adsorbed on atop sites (atop-CO). In contrast, the peak due to atop-CO adsorbed on step sites was seen at 2117 cm^-1 for Au(311) at 0.01 Torr. It was thus shown that the step sites on the gold surface were effective for CO adsorption under low CO pressure. Next, we investigated the CO adsorption state for the gold nanoparticles on TiO₂(110). PM-IRAS peak of CO adsorbed on atop sites of gold atom was observed at 2120 cm^-1, which was higher than the frequency of the CO adsorbed on Au(111). The CO frequency observed for the Au/TiO₂ model catalyst agreed with that on step sites for Au(311). We thus found that the TiO₂ support influences the electronic state of the supported gold, resulting in the formation of positively charged gold nanoparticles.