Supported Gold and Platinum Clusters: Stability under Vacuum and Hydrogen at Elevated Temperatures; Optical Properties*

The Achilles heal of supported clusters remains their low stability at elevated temperatures or when exposed to reactive gases. In this paper, the stability of Au$_{n}$ and Pt$_{n}$ clusters (n=6-10) supported on SiO$_{2}$, Al$_{2}$O$_{3}$ {\&} TiO$_{2}$ films is addressed. The clusters were heated in vacuum and in H$_{2}$ atmosphere, their stability monitored by synchrotron grazing incidence small angle X-ray scattering. Pt clusters supported on Al$_{2}$O$_{3}$ did not undergo sintering in vacuum and when exposed to hydrogen during a lengthy heat treatment reaching 400C; Au clusters on SiO$_{2}$ remained stable up to 350C. These temperatures are considerably higher than those characteristic for the onset of the catalytic activity of these clusters. Results on heat-induced structural isomerization of clusters will be shown. Single-particle UV-VIS spectra of Au-particles obtained by dark-field microscopy will be presented as well.