On the Stability of Cu₅ Catalysts in Air Using Multireference Perturbation Theory

An ab initio study of the interaction of O₂, the most abundant radical and oxidant species in the atmosphere, with a Cu₅ cluster, a new generation atomic metal catalyst, is presented. The open-shell nature of the reactant species is properly accounted for by using the multireference perturbation theory, allowing the experimentally confirmed resistivity of Cu₅ clusters toward oxidation to be investigated. Approximate reaction pathways for the transition from physisorption to chemisorption are calculated for the interaction of O₂ with quasi-iso-energetic trapezoidal planar and trigonal bipyramidal structures. Within the multireference approach, the transition barrier for O₂ activation can be interpreted as an avoided crossing between adiabatic states (neutral and ionic), which provides new insights into the charge-transfer process and gives better estimates for this hard to localize and therefore often neglected first intermediate state. For Cu₅ arranged in a bipyramidal structure, the O-O bond cleavage is confirmed as the rate-determining step. However, for planar Cu₅, the high energy barrier for O₂ activation, related to a very pronounced avoided crossing when going from physisorption to chemisorption, determines the reactivity in this case.