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

Reactions of CO and O-2 on neutral and anionic Cum 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 Cu-20(-) cluster can adsorb CO and O-2 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 O-2 molecule upon adsorption is crucial to determine the rate of CO oxidation. The CO oxidation proceeds efficiently on both Cu-20 and Cu-20(-) clusters, when O-2 is pre-adsorbed dissociatively. The ER mechanism has a lower reaction barrier than the LH mechanism on the neutral Cu-20(-). In general, CO oxidation occurs more readily on the anionic Cu-20(-) (effective reaction barriers 0.1-0.3 eV) than on the neutral Cu-20(-) cluster (0.3-0.5 eV). Moreover, Cu-20(-) 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 Cu-20(-) cluster, which demonstrates that Cu clusters may serve as good catalyst for CO2 chemistry.