Adsorbate-induced restructuring and pressure-dependent adsorption on metal nanoparticles studied by electron microscopy and sum frequency generation spectroscopy

Several examples are presented that illustrate how the internal and surface structure of oxide supported Pt nanoparticles can change under reaction conditions. High temperatures and the presence of adsorbates may lead to different effects: (1) changes in the particle size distribution (sintering or redispersion) that may affect the crystallographic and electronic structure of the nanoparticles, (2) restructuring of the particle surface facets, and (3) crystallization of large polycrystalline aggregates. Three different model systems are described that are well-suited for high resolution transmission electron microscopy (HRTEM) and/or surface science techniques. To examine the influence of high gas pressure on the structure of adsorbates, infrared-visible sum frequency generation (SFG) surface vibrational spectroscopy is applied to monitor CO adsorption on supported Pd nanoparticles from 10-7 to 200 mbar. The adsorption site occupancy of CO, in particular the on-top population, strongly depends on pressure and temperature preventing a simple extrapolation of low pressure results to reaction conditions