The dissociation of O2 on Ni(100) has been studied using a cluster model approach. The three principally different reaction pathways, over an on‐top position, over a bridge position, and over a fourfold hollow position, were considered. The dissociation mechanisms were found to be very similar for these pathways. In the entrance channel a chemisorbed, peroxo‐form, of molecular O2 is first formed, which is strongly bound to the Ni(100) surface by two polar covalent bonds. The binding energy at the fourfold hollow site is found to be 78 kcal/mol, which is about 20 kcal/mol larger than for the other two sites, and much larger than the chemisorption energies for the experimentally observed O2 on Pt(111) and Ag(110). The reason for this difference is discussed. In a simplified valence‐bond picture the wave function of this molecularly bound O2 has a large component of a πu to πg excited state of O2. The dissociation of O2 then proceeds by two stepwise electron transfers from the surface over to the O2u orbital, which completes the breaking of the O–O bond. In this latter process the energy passes over a local barrier, which is still far below the long distance asymptote, however. The local barrier height is much higher for the fourfold hollow dissociation, 35 kcal/mol over the local molecular minimum, than for the other two pathways, where the barrier height is only 6–8 kcal/mol. The 3d orbitals on nickel remain passive for all the three dissociation pathways, which is in line with the fact that also nontransition metals dissociate O2. This behavior is in contrast to the dissociation of H2 on Ni(100), where the 3d orbitals play a key role for the on‐top dissociation.

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