Reactivity of diatomic molecules on Cu(100)

The reactivity of the diatomic molecules O₂, N₂, CO and NO on Cu(100) has been studied on cluster models using first principles density functional calculations. For each molecule the dissociation was studied at top, bridge and hollow surface sites. The barriers for dissociation on the energetically most favorable reaction pathways were ordered as O₂ < N₂, with the N₂ and CO barriers high enough to preclude dissociation at moderate operating conditions. The recombination barriers were ordered as CO < NO < N₂ < O₂. These trends were found to be in good agreement with the experiment and they can be explained in terms of the bonding characteristics and the underlying electronic structure for each molecule. In the case of O₂ the dissociation was found to be preceded by a weakly adsorbed state where O₂ is lying parallel to the surface. O₂ dissociation was favored over a hollow surface site. NO, CO and N₂ showed a slight preference for dissociation over the bridge site. An alternative mechanism was found for NO dissociation whereby the NO species goes through a bent adsorbed state in a hollow site and then dissociates over the neighboring bridge of metal atoms. For O₂ and NO we calculated for the lowest energy reaction paths the pre-exponential factors for dissociation and recombination using transition state theory. These were 10₁₃ and 5 × 10₁₂, respectively, both in qualitative agreement with the experiment.