Structure and bonding of Pd clusters supported on alpha-Al2O3(0001)
F. Illas1, J. R. B. Gomes1
and Z. Lodziana2
1) Departament de Química
Física i Centre de Recerca en Química Teòrica,
Universitat de Barcelona i
Parc Científic de Barcelona,
C/ Martí i Franquès 1,
E-08028 Barcelona, Spain
2) Center for Atomic Scale
Materials Physics, DTU, Building 307,
Aluminum oxides have been successfully employed as supports
in heterogeneous catalysis and in the research of semiconductor devices. The
simplest aluminum oxide structure is a-Al2O3
although the corresponding surfaces are not so simple [1]. Likewise, the size
of metal particles supported on a-Al2O3
has a crucial role on the properties of the supported catalyst [2]. Because of
its potential use as “three-way” catalysts, supported palladium clusters have
been subject of a large number of studies although, unfortunately, most of them
focus on Pd particles with diameters of ~ 30-40 Å and almost no information
exists concerning the first steps of Pd growth on the oxide substrate.
The present work reports results from theoretical methods
used to study the adsorption of Pd atoms and of Pd3 and Pd4
particles on the Al-terminated a-Al2O3(0001)
surface. Both, embedded clusters and periodic
approaches have been used coupled to first principles (hybrid and/or
gradient-corrected) density functional theory. Results obtained from both
approaches are in agreement and predict that Pd atoms induce large surface
relaxation [3,4]. The
structure of supported Pd3 is largely distorted from the gas phase
equilibrium geometry whereas the structure of supported Pd4 is less
distorted and reminiscent of the most stable gas phase isomer. The results obtained
for the different structures of supported Pd4 suggest that there is
a competition between three- and two-dimensional growth of the supported
crystallites [5].
1.- J. R. B. Gomes, I. de P. R. Moreira, P. Reinhardt, A.
Wander, B. G. Searle, N. M. Harrison, F. Illas, Chem.
Phys. Lett., 341 (2001) 412 and references therein.
2.- T. Dellwig, J. Hartmann, J. Libuda, I. Meusel, G. Rupprechter, H. Unterhalt, H. –J. Freund, J. Mol. Catal. A: Chemical, 162 (2000) 51.
3.- J.R.B. Gomes, F. Illas, N. Cruz Hernández, A. Márquez, J. F. Sanz, Phys. Rev. B, 65 (2002) 124414
4.- J.R.B.
Gomes, F. Illas, N. Cruz Hernández, J. F. Sanz, A. Wander, N. M. Harrison, J. Chem. Phys., 116 (2002) 1684
5.- J. R. B. Gomes, Z. Lodziana, F. Illas, J. Phys. Chem. B, submitted for publication.