Varying oxygen coverage on Cu55 and its effect on CO oxidation.

Adsorption of molecular oxygen on a Cu55 cluster and the resulting oxidation effects have been investigated by spin-polarized density functional theory (DFT). The optimal structure for each Cu55O2N (N = 1-20) complex has been obtained via a sequential addition of O2 and systematic screening of the preferable adsorption sites. Upon structural optimization, several O2 molecules dissociate readily on Cu55 at different oxygen coverages, and further DFT molecular dynamics simulations at 300 K confirm the instability (small dissociation barrier) of the remaining O2 and a spontaneous movement of some oxygen atoms from the surface sites towards the cluster interior. The Cu55 cluster and its oxidized derivatives have been placed on a γ-Al2O3(100) surface to study the cluster-support interaction, and furthermore, CO oxidation reactions on both Cu55(O)2N and Cu55(O)2N/γ-Al2O3(100) have been studied as a function of oxygen coverage. The CO oxidation reaction barrier is rather insensitive to the oxygen coverage regardless of the support, indicating a small increase in activity with the number of surface oxygen atoms.