Size, Stoichiometry, Dimensionality, and Ca Doping of Manganese Oxide-Based Water Oxidation Clusters: An Oxyl/Hydroxy...

Gas-phase ion-trap reactivity experiments and density functional simulations reveal that water oxidation to H₂O₂ mediated by (calcium) manganese oxide clusters proceeds via formation of a terminal oxyl radical followed by oxyl/hydroxy O-O coupling. This mechanism is predicted to be energetically feasible for Mn₂O⁺ (y = 2-4) and the binary CaMn₃O₄⁺, in agreement with the experimental observations. In contrast, the reaction does not proceed for the tetramanganese oxides Mn₄O⁺ (y = 4-6) under these experimental conditions. This is attributed to the high fluxionality of the tetramanganese clusters, resulting in the instability of the terminal oxyl radical as well as an energetically unfavorable change of the spin state required for H₂O₂ formation. Ca doping, yielding a symmetry-broken lower-symmetry three-dimensional (3D) CaMn₃O₄⁺ cluster, results in structural stabilization of the oxyl radical configuration, accompanied by a favorable coupling between potential energy surfaces with different spin states, thus enabling the cluster-mediated water oxidation reaction and H₂O₂ formation.