Particle size effect on the Langmuir-Hinshelwood barrier for CO oxidation on regular arrays of Pd clusters supported on ultrathin alumina films.

The Langmuir-Hinshelwood barrier (E) and the pre-exponential factor (ν) for CO oxidation have been measured at high temperatures on hexagonal arrays of Pd clusters supported on an ultrathin alumina film on NiAl (111). The Pd clusters have a sharp size distribution, and the mean sizes are 174 ± 13, 360 ± 19, and 768 ± 28 atoms. E and ν 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 ≤ T(K) ≤ 613]. The largest particles (3.5 nm) give values of E and ν similar to those measured on Pd (111) [T. Engel and G. Ertl, J. Chem. Phys. 69, 1267 (1978)]. However, smaller particles (2.7 and 2.1 nm) show very different behaviors. 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.