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ZB 19 - Characterization of processes in first atomic layers of a solid surface (A.Jablonski)


Metal oxide catalyst doped with nanoparticles. A. Jablonski, A. Bilinski, O. Chenyayeva, A. Kosinski, M. Krawczyk, W. Lisowski, K. Nikiforov, J. W. Sobczak

The subject of our studies are metal oxides doped with nanoparticles of other metals, which are used as supports for catalysts. The changes in the electron state of oxide-support  and doped metal-interactive chemical species induced by various preparation method, thermal processing and the impact of the selected reactive gases are examined using XPS, AES, SPM.
Ceria oxide doped by nanosize particles of gold and one of the metals: Co, Sn, Mn, Fe, is a model system of investigations. The method of preparation of such catalysts significantly affect the electron state of both oxide support and doped metals. An important prerequisite is to obtain highly dispersed nanoparticles of metals whose interaction with the  oxide support leads to the formation of catalytically active systems. Particularly important is Au doping. Unlike traditional metal supported catalysts, metallic gold nanoparticles are not active in the catalytic reaction and for the catalytic activity are responsible the nonmetallic, positively charged gold nanoparticles (described as Au+, which are strongly bound with CeO2. The presence of such nanoparticles on the catalyst surface shows the XPS spectrum of Au 4f , recorded on PHI 5000 VersaProbe spectrometer for Au/CeO2 sample doped with Mn.
Methods of preparation significantly affect the electron state of gold species supported on CeO2 support;  in addition to the desired states of Au+, also the negatively charged states of gold species Au- can be generated as a result of electron transfer from the ceria support, or as a result of the agglomeration the metallic Au crystallites may be formed, which do not generate the catalytic activity. In the model system Au-CeO2, the active site location of Au nanoparticles in ceria support is formed by oxygen vacancies, lattice sites with a deficit of oxygen, in which the cerium atoms are at a lower oxidation state (Ce+3) compared to the basic structure of the crystal lattice of CeO2 (Ce+4).
Model systems are also studied to determine the selected parameters of electron transport, particularly the mean free path by EPES. This parameter is very important in the practical quantitative analysis in XPS and AES spectroscopy. These studies are important for understanding the course of basic catalytic processes, selection of the optimal reaction conditions and the selection of the optimal catalyst support for the chosen reaction. Systems of metal nanoparticles on oxide supports are known as catalyst systems, with an exceptionally wide range of application in environmental processes: low-temperature gas combustion (removal of hydrocarbons, oxides of nitrogen) and low temperature water gas conversion reaction as a source of pure hydrogen for energy (fuel cells, clean motor fuel) and also photocatalytic removal of pollution from surface waters.