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 (1 11) surface. In consequence, the energy of CO adsorption on a cluster of similar to 100 atoms is similar to 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 similar 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.