Electronic, structural and chemical properties of gold clusters on ultra-thin oxide films
Julkaistu sarjassaResearch report / Department of Physics, University of Jyväskylä
Two decades ago, it was found that gold is catalytically active as clusters although it is known to be inert as bulk material. In the work presented in this thesis, density functional theory calculations have been applied to show that the properties of gold clusters and small molecules, namely O2 and NO2, are di erent on oxide thin lms supported by metal carriers than on single crystal oxide surfaces. Collectively, these di erences are called thin- lm e ects. The e ects and the mechanisms behind them have been studied with atomistic calculations and rationalized with simple physical models. The signi cance of the thin- lm e ects for heterogeneous catalysis is discussed. It has been found that on MgO/Mo thin lm, the adsorption of gold clusters is strong, whereas on single crystal MgO, the adsorption is weak. On MgO/Mo and MgO/Ag, the clusters are charged and tend to maximize their surface contact whereas on single crystal MgO, they are essentially neutral and the smallest clusters have only few atoms in contact with the surface. As the clusters are charged, their adsorption energy correlates to the electron a nity of gas-phase clusters. The tiniest Au2−6 favor chain-like structures on MgO/Mo. The larger clusters of around 7-20 atoms are two dimensional and planar on MgO/Ag. These structures are not favored by the clusters adsorbed on single crystal MgO. Furthermore, also O2 and NO2 become charged on the studied thin- lms. In addition to electron a nity of the adsorbate, the strength of the thin- lm e ects seem to depend on oxide lm thickness and the work function of the substrate. The long-ranged character of the e ect can be addressed to image charge and polarization interaction. It has been predicted that the thin- lm e ects are signi cant when the lm thickness is less than 1-2 nm. The electronic states of the chain and planar clusters can be rationalized in the light of one or two dimensional harmonic oscillator model depending on the cluster size. E ectively, a cluster can be seen as a harmonic potential to which the 6s valence electrons of gold atoms and the charge transferred from the substrate are con ned. By applying these ideas, it has been possible to identify experimentally observed clusters in a combined scanning tunneling microscopy and density functional theory study. This is an important step in characterizing thin- lm based cluster catalytic model systems. Calculations of O2 adsorption on Au1−6/MgO/Mo show that O2 tends to stick at the ends of the chains. However, adsorption to bare MgO/Mo terrace is slightly more favorable. Based on these results, one can suggest a CO oxidation scheme in which Au traps CO molecules whereas O2 sticks on the terraces. As the surface di usion barrier of O2 is low, the molecules occasionally reach Au clusters with CO and the CO2 is released on the cluster boundary. It should be noted, however, that also other reaction pathways can be possible and they may depend on the carrier metal and the cluster size.
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