  The Langmuir-Hinshelwood barrier (ELH) and the pre-exponential factor
($\nu$LH) for CO oxidation have been measured at high temperature on hexagonal
arrays of Pd clusters supported on an ultrathin alumina film on Ni3Al(111). The
Pd clusters have a sharp size distribution and the mean sizes are: 174$\pm$13,
360$\pm$19 and 768$\pm$28 atoms. ELH and $\nu$LH are determined from the
initial reaction rate of a CO molecular beam with a saturation layer of
adsorbed oxygen on the Pd clusters, measured at different temperatures
(493$\le$T (K) $\le$613). The largest particles (3.5 nm) give values of ELH and
$\nu$LH similar to those measured on Pd (111) [2]. However, smaller particles
(2.7 and 2.1 nm) show very different behavior. The origin of this size effect
is discussed in terms of variation of the electronic structure and of the
atomic structure of the Pd clusters.
