Shamil Shaikhutdinov, Randall Meyer, Hans-Joachim Freund,
In the last decade, gold nanoparticles have begun to garner attention for unique catalytic properties such as low temperature CO oxidation. In order to elucidate the reaction mechanism and determine structure-activity relationships we developed well-defined model systems involving gold particles vapor deposited on thin oxide films. Of particular interest are the role of the support and the origin of any differences in catalytic activity between gold catalysts on varying supports. In this work, we report on the preparation, characterization and CO adsorption on gold supported on Al2O3, FeO, Fe3O4, and Fe2O3 films.
The experiments were performed in ultra-high vacuum. Thin alumina films were grown by oxidation of a NiAl(110) single crystal. Iron oxide films were prepared by iron deposition-oxidation on a Pt(111) substrate. The quality of the films was judged by characteristic diffraction patterns and scanning tunneling microscopy (STM). The morphology of the Au deposits was studied by STM, and CO adsorption was examined by temperature programmed desorption (TPD).
Gold deposition leads to formation of three-dimensional particles for all the substrates studied. However, the nucleation density, size distribution and structure depend on the support as revealed by STM. TPD experiments show that CO desorption temperature is sensitive to average Au particle size. It gradually shifts from ~ 250 to 170 K as size increases. Therefore the results point to a particle size effect with respect to CO adsorption: small particles adsorb CO more strongly. Interestingly, the desorption temperature for the smallest particles may extend to 300 K which is in the temperature regime of real gold catalysts for low temperature CO oxidation. Note also, that such a "high temperature" state has never been observed on gold single crystals.
CO TPD spectra were also shown to depend on the substrate temperature of the gold deposition (77 K vs. 300 K). The latter fact is understood in terms of temperature dependent metal nucleation and particle growth on oxide supports.
For the particles annealed to 400 K, we have revealed three distinct phenomena. First, annealing results in a remarkable drop of the integral CO desorption intensity. Second, the desorption peak shifts to lower temperatures, with the shift being larger at a low Au loading. Third, for the annealed samples, the desorption temperature is nearly independent of the deposited amounts of gold above some value depending upon the substrate.
These effects can be explained in terms of sintering of the gold deposits upon heating, thus forming larger particles and hence decreasing total surface area. Particle restructuring seems to occur simultaneously to annealing, resulting in a loss of the low coordinated Au atoms, on which CO may adsorb according to theoretical calculations. The sintering of Au particles depends on the support: it proceeds more easily on FeO than on Fe3O4 or Al2O3 possibly due to the different defect structures. Our results also indicate the CO adsorption strength dependence on particle size may disappear as particles exceed some critical size, which is about 4 nm based on STM images.
In order to examine the stability of the gold particles in a reactive atmosphere, we employed in situ STM experiments. First results showed that the Fe3O4 supported gold particles were remarkably stable, showing little or no change even when heated to 130 ºC in 10 mbar O2. Similar experiments for other oxide films are currently in progress.
