Size-Dependence of Adsorption Properties of Metal Nanoparticles: A Density Functional Study on Palladium Nanoclusters

Interatomic distances in metal nanoparticles are reduced from their values in the bulk. We studied computationally how this size-dependent geometry change (from the bulk) relates to the size-dependence of other properties of large metal clusters, including their reactivity. For this purpose, using an all-electron scalar-relativistic density-functional approach, we calculated structures and binding energies for the example of CO adsorption on 3-fold hollow sites at the center of (111) facets of cuboctahedral nanoscale clusters Pd n (n = 55−260). The average nearest-neighbor Pd−Pd distance of optimized structures is 4−7 pm (2−3%) shorter than the extrapolated limit of the lateral distance within an infinite (111) surface. In consequence, the energy of CO adsorption on a cluster of ∼100 atoms is ∼15 kJ mol−1 smaller than the extrapolated limit. On the basis of these results, we suggest a strategy for modeling particles of larger size, e.g. of 1000 atoms and more, with the help of smaller model particles of up to ∼300 atoms where one keeps the core of a model cluster fixed at the bulk structure and restricts the structure optimization to the outermost shell of cluster atoms.