Influence of Charge State on Catalytic Oxidation Reactions at Metal Oxide Clusters Containing Radical Oxygen Centers

Evidence obtained by guided-ion-beam mass spectrometry experiments and density functional theory calculations indicates that by adding one oxygen atom with a full octet of valence electrons (O²⁻) to stoichiometric cationic zirconium oxide clusters (ZrO₂)_x⁺ (x = 1−4), a series of anionic clusters (Zr_xO_2x+1)⁻ (x = 1−4) are formed which contain radical oxygen centers with elongated (elongation ≈ 0.24 ± 0.02 Å) metal−oxygen bonds. These anionic clusters oxidize carbon monoxide, strongly associate acetylene, and weakly associate ethylene, in contrast to the cationic species which were found previously to be highly active toward the oxidation of all three molecules. Theoretical investigations indicate that a critical hydrogen transfer step necessary for the oxidation of ethylene and acetylene at metal oxide clusters containing radical oxygen centers is energetically favorable for cationic clusters but unfavorable for the corresponding anionic species. The calculated electrostatic potential of the cluster reveals that in the case of cations, a favorable interaction with nucleophilic molecules takes place over the whole surface of the (ZrO₂)_x⁺ (x = 1−4) clusters, compared to a restricted interaction of ethylene and acetylene with the less coordinated zirconium atom in the case of the anionic (Zr_xO_2x+1)⁻ (x = 1−4) species. Therefore, in spite of the common presence of a radical oxygen center in specific anionic and cationic stoichiometries, the extent to which various classes of reactions are promoted is influenced by charge state. Moreover, the (Zr_xO_2x+1)⁻ (x = 1−4) series of anionic clusters may be regenerated by reacting oxygen deficient clusters with a strong oxidizer. This indicates that not only cationic species, as shown previously, but also anionic clusters may promote multiple cycles of carbon monoxide oxidation.