Sintering of supported metal particles: Au on TiO2(110)

COLL 453

Stephen C. Parker,, Department of Physics and Astronomy, Carleton College, Northfield, MN 55057 and Charles T. Campbell,, Department of Chemistry, University of Washington, P. O. Box 351700, Seattle, WA 98195.

Sintering of nanoscale-sized metal particles on oxide supports has been theoretically modeled and experimentally measured for gold supported on titanium dioxide.  Au is vapour-deposited onto a TiO2(110) surface under ultra-high vacuum (UHV) conditions and characterized using X-Ray Photoelectron Spectroscopy (XPS) and Low Energy Ion Scattering (LEIS).  Experimental measurements of the particle sintering are performed using Temperature-Programmed Low Energy Ion Scattering (TP-LEIS), a technique designed to collect information about the topmost layer of the surface during a linear temperature ramp.  This technique works well to investigate the very broad range of temperatures (which is analogous to a broad time scale) over which the particles sinter.  Microcalorimetry measurements have shown that metal particles smaller than ~6nm have much higher surface free energies than large particles.  Incorporating this result into a sintering model (which is based on work done by Wynblatt and Gjostein1 but correcting several limitations of the model that are incorrect for particles of small size) and using a modified bond-additivity (MBA) model to better estimate particle size energetics, the broad range of temperatures over which the catalysts sinter can be theoretically reproduced.  This technique has also been used to model the long time-scale isothermal sintering of a catalyst.  Particle distributions as a function of time using the MBA model are contrasted with models that assume a surface free energy independent of particle size.  

1Wynblatt, P. and Gjostein, N. A., Supported metal crystallites, in Progress in Solid State Chemistry, edited by McCaldin, J. O. and Somorjai, G. A., (Oxford, 1975), Vol. 9, p. 21.