CO oxidation catalyzed by neutral and anionic Cu20 clusters: Relationship between charge and activity

Reactions of CO and O₂ on neutral and anionic Cu₂₀ clusters have been investigated by spin-polarized density functional theory. Three reaction mechanisms of CO oxidation are explored: reactions with atomic oxygen (dissociated O₂) as well as reactions with molecular oxygen, including Langmuir-Hinshelwood (LH) and Eley-Rideal (ER) mechanisms. The adsorption energies, reaction pathways, and reaction barriers for CO oxidation are calculated systematically. The anionic Cu₂₀ ^- cluster can adsorb CO and O₂ more strongly than the neutral counterpart due to the superatomic shell closing of 20 valence electrons which leaves one electron above the band gap. The activation of O₂ molecule upon adsorption is crucial to determine the rate of CO oxidation. The CO oxidation proceeds efficiently on both Cu₂₀ and Cu₂₀ ^- clusters, when O₂ is pre-adsorbed dissociatively. The ER mechanism has a lower reaction barrier than the LH mechanism on the neutral Cu₂₀. In general, CO oxidation occurs more readily on the anionic Cu₂₀ ^- (effective reaction barriers 0.1-0.3 eV) than on the neutral Cu₂₀ cluster (0.3-0.5 eV). Moreover, Cu₂₀ ^- exhibits enhanced binding for CO₂. From the analysis of the reverse direction of CO oxidation, it is observed that the transition of CO₂ to CO + O can occur on the Cu₂₀ ^- cluster, which demonstrates that Cu clusters may serve as good catalyst for CO₂ chemistry.