| AVS 64th International Symposium & Exhibition | |
| Fundamental Discoveries in Heterogeneous Catalysis Focus Topic | Thursday Sessions |
| Session HC-ThP |
| Session: | Fundamental Discoveries in Heterogeneous Catalysis Poster Session |
| Presenter: | Yoshihide Watanabe, Toyota Central R&D Labs. Inc., Japan |
| Authors: | Y. Watanabe, Toyota Central R&D Labs. Inc., Japan A. Beniya, Toyota Central R&D Labs. Inc., Japan |
| Correspondent: | Click to Email |
Size-selected metal clusters on oxide surfaces are a subject of considerable interest because of their distinctive size-dependent catalytic properties. The most precise approach to prepare a model catalyst is deposition of size-selected clusters using a mass-filtered cluster ion beam. It is important to prepare a uniformly deposited surface and avoid cluster-aggregations for investigation of size-dependent catalytic activity.
Pt clusters produced using a DC magnetron-sputtering source were mass-selected using a quadrupole mass filter. The parallel plate deflectors were placed to scan the cluster ion beam. When two sine voltage waveforms are applied to the orthogonal deflectors, the ion trajectory on the surface produces a Lissajous pattern. The ion trajectory fills the sample surface uniformly with an irrational frequency ratio. The advantages of this method are simplicity and low cost of setup compared with raster scanning method.
In this study, size-selected Pt clusters were deposited uniformly on surfaces by scanning the cluster ions in the form of Lissajous pattern. We confirmed that size-selected clusters can be deposited uniformly on a surface by Pt 4f intensity mapping. A uniform cluster distribution was also confirmed using a scanning tunneling microscope.
In high-density condition, the Pt clusters deposited without Lissajous scan could aggregate and form different size clusters. Cluster aggregation probability depends on the deposited density. The isolated Ptn clusters and their aggregates were estimated to be distributed with position-dependence. At 0.1 ML of total coverage, 33% of the clusters coalesce to form larger clusters. This result indicates that the coverage needs to be low enough to neglect the cluster aggregation effect.
The adsorption states of CO molecules on size-selected Ptn (n = 7, 15, 20) clusters deposited on the Al2O3/NiAl(110) surface were also investigated with and without a Lissajous scan. Two peaks at 2020 and 2040 cm−1 are observed in infrared reflection absorption spectroscopy (IRAS) spectra of adsorbed 13CO at saturation coverage. These peaks are assigned to adsorbed CO at the on-top site of slightly cationic Pt atoms interacting with substrate oxygen atoms (2040 cm−1), and of neutral Pt atoms bound by the Pt–Pt bond (2020 cm−1). Temperature programed desorption (TPD) spectra of saturated 13CO were also investigated. Combining the IRAS and TPD results, we determined that CO molecules bind to slightly cationic Pt atoms with an adsorption energy of 0.7–1.0 eV, and bind to neutral Pt atoms with an adsorption energy of 1.4 eV.