Chemistry Department Seminar

"Thermal Stability, Electronic, and Catalytic Properties of Supported Metal Clusters: Size, Interparticle Interactions, Oxidation and Support Effects"

Presented by Professor Beatriz Roldan-Cuenya, University of Central Florida

Thursday, January 31, 2008, 11:00 am — Hamilton Seminar Room, Bldg. 555

The next generation of nanocatalysts requires detailed knowledge of the correlation between their structure (geometrical and electronic) and their activity. Size- and shape-selected Au and AuFe nanoclusters with well defined intercluster distances were synthesized by diblock copolymer encapsulation. Significant changes in the electronic local density of states (LDOS) of TiC-supported Au clusters, in particular, the onset of non-metallic behavior, were observed by scanning tunneling spectroscopy with decreasing cluster size. In addition, evidence for substrate-induced modifications in the LDOS of interfacial gold atoms was found [1]. When the Au clusters are deposited on reduced TiO2, the LDOS distribution within the clusters is inhomogeneous and appears to be enhanced at low-coordinated Au atoms. The possibility of electron transfer from the reduced TiO2 support to the nanoclusters will be discussed. The enhancement of electron density inside and around metal clusters is of scientific importance, since it has been predicted to facilitate the adsorption and dissociation of reactants with low activation energies. Our temperature programmed desorption measurements (TPD) indicate a size-dependency in the catalytic activity of Au/TiC for low temperature CO oxidation [2]. Furthermore, interparticle interactions were found to affect the activity and life-time of our catalysts [3].
The thermal and chemical stability of oxidized gold species formed upon cluster exposure to atomic oxygen was investigated using a combination of temperature-, time- and CO dosing-dependent X-ray photoelectron spectroscopy (XPS) and TPD [4]. Our work demonstrates that low temperature exposure to atomic oxygen leads to the formation of surface, as well as sub-surface gold oxide on Au nanoparticles. Interestingly, the presence of a reducible TiO2 substrate leads to a lower gold oxide stability compared to that on SiO2, possibly due to a TiO2 oxygen vacancy-mediated decomposition process.

Hosted by: Jose Rodriguez

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