Effect of Platinum Nanocluster Size and Titania Surface Structure upon CO Sturface Chemistry on Platinum-Supported TiO_2 (110)

摘要

The adsorption chemistry of CO on clean and Pt-supported TiO_2 (110) was investigated. It was found that surface structure of TiO_2 plays an important role inthe chemistry thattakes place at the surface. On the reduced (1 x 2)-reconstructed surface, CO desorbed at 140 and 170 K, while only desorption at 140 K was observed on the stoichiometric (1 x 1) surface. Additionally, CO dissociation, possibly due to the reduction by Ti~(3+), was observed on the Pt-supported (1 x 2) surface. On the Pt-covered surfaces, the chemistry of CO adsorption and desorption strongly depends onthe size of Pt nanoclusters. With a decrease in cluster size, CO was found to desorb at higher temperatures. This unusual desorpion chemistry is likely related to quantum size effects of Pt nanoclusters. Scanning tunneling spectra revealed that clusters below 20 A in diameter exhibited nonemtallic behavior, while those above 40 A were metallic. This transition of the properties of Pt nanoclusters from metallic to nonmetallic as the cluster size decreases correlates with stronger interaction of CO with Pt observed in temperature-programmed desorption spectra.