The structural evolution of small copper clusters of up to 15 atoms and the dissociative chemisorption of H2 on the minimum energy clusters are studied systematically using density functional theory. The preferred copper sites for chemisorption are identified and the transition state structures and activation barriers for clusters four to nine atoms are determined and found to be inconsistent with the empirical Bronsted-Evans-Polanyi relationship. The physicochemical properties of the clusters are computed and compared with the bulk and surface values. The results indicate that a phase transition must occur in the going from cluster to bulk.

The sequential growth of small copper clusters up to 15 atoms and the dissociative chemisorption of H 2 on the minimum energy clusters are studied systematically using density functional theory under the generalized gradient approximation. We found that small Cu n clusters grow by adopting a triangular growth pathway. The pentagon bipyramid structural arrangements are strongly favored energetically in the growth and the new addition in the cluster occurs preferably at a site where the atom is capable of interacting with more adjacent atoms. To understand the evolution of small copper clusters, we also performed calculations on selected icosahedral clusters ͑for n = 13, 19, 25, 55͒ and fcc-like clusters ͑n = 14, 23, 32, 41͒. By extrapolating/ interpolating the binding energies of triangular clusters, icosahedral clusters, and bulk-like clusters, we found that structural transitions from the triangular growth clusters to the icosahedral and fcc-like clusters occur at approximately n = 16 and n = 32, respectively. Subsequently, we performed extensive calculations on the dissociative chemisorption of H 2 on the minimum energy clusters. The chemisorption likely occurs near the most acute metal site with the two H atoms residing on the edges, which differs significantly from the chemisorption on Cu surfaces that usually takes place at the hollow sites.

Transition state theory is used to estimate rate constants for dissociative chemisorption of H 2 on copper clusters. Activation energies and transition state partition functions are obtained from density functional theory for small clusters of less than 10 atoms. The violation of the Bronsted-Evans-Polanyi relation, which was previously observed for these clusters, is explained in terms of structural relaxation due to the chemisorption process. For large clusters, the impact of chemisorption on the global structure of the clusters is reduced. This restores the validity of the Bronsted-Evans-Polanyi relation and allows an extrapolation scheme for nano-size clusters to be developed.

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