The evolution of the adsorption energy of carbon monoxide (CO) molecules on palladium (Pd) clusters as function of Pd particle size from the molecular regime (less than ~100 atoms per particle) to the bulk regime has been revealed. This adsorption energy is retrieved from the residence time of CO molecules on the Pd clusters, measured by a pulsed molecular beam technique, versus temperature. Unprecedented accuracy on the determination of the particle size has been achieved here by using a regular array of metal clusters exhibiting a size dispersion down to the ultimate limit of a Poisson distribution. This allows getting rid of the convolution effects that generally occur when considering particles grown through other techniques.

Hexagonal arrays of Pd and PdAu clusters are produced by condensation of metal vapors under UHV on a nanostructured alumina ultrathin film at 320 K. The alumina presents a hexagonal network of point defects that are nucleation centers for Pd. The growth of the Pd clusters is uniform leading to a very narrow size distribution in the range of size of few atoms to about 400 atoms. The number density of clusters is fixed by the density of nodes of the nanostructure of the alumina film (6.5 × 10(12) cm(-2)). The PdAu bimetallic particles grow in a two-steps process. First Pd is deposited then Au is deposited and grows exclusively on the preformed Pd clusters. The size and the composition of the clusters are independently controlled by the amounts of the deposited metals. The shape, size, density and organization of the clusters are studied in situ by STM. The long range order is studied in situ by Grazing Incidence Small Angle X-ray Scattering (GISAXS). The organization of the arrays of clusters is stable up to 600 K. These arrays of Pd and PdAu clusters on alumina are well suited model catalysts to study the effects of size, shape and composition on their reactivity.