Reactions of O₂ with the Fe₁₉, Fe₉Pt₁₀ and Pt₁₉ clusters: a computational study

Interactions of O₂ with the Fe₁₉, Fe₉Pt₁₀ and Pt₁₉ clusters were studied by density functional theory within the generalized gradient approximation. To arrive at a medium-sized FePt cluster with the Fe : Pt ratio near 1 : 1, we performed a thorough search for the lowest energy states in three series Fe_n-1Pt_n, Fe_nPt_n and Fe_nPt_n-1, 2 ≤ n ≤ 8, by adding one atom at a time. The lowest energy isomers (FePt)₈ were used for constructing an Fe₉Pt₁₀ cluster with a square bipyramidal geometry. Next, we simulated reactions of O₂ dissociation on this cluster as well as on the Fe₁₉ and Pt₁₉ clusters. The dioxygen dissociation patterns are substantially different in all three cases. Although the pathways of O₂ dissociation on the Fe₁₉ and Fe₉Pt₁₀ clusters contain a single transition state, there are two branches of states created by spin relaxation of the initial states in the Fe₁₉ case, whereas there is no change in the total spin in the Fe₉Pt₁₀ case. Compared to Fe₁₉ and Fe₉Pt₁₀, O₂ dissociation on Pt₁₉ follows a more complex, four-step energy pathway. In addition, we have optimized the clusters Fe₁₉O_n, Fe₉Pt₁₀O_n and Pt₁₉O_n, n = 1-2, to evaluate binding energies of O atoms in both series and desorption energies of O₂ from the clusters with n = 2. The energy of O₂ desorption from the Fe₉Pt₁₀O₂ cluster is somewhat larger than the half-sum of the desorption energies from the Fe₁₉O₂ and Pt₁₉O₂ clusters (4.27 eV), which indicates some bias toward the iron cluster.