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

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      Publication Details (including relevant citation   information):


      Volume: 131

      Issue: 15

      Pages: 5460-5470


    Published: APR 22 2009


        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   (ZrxO2x+1)(-) = 1-4) are formed which contain radical oxygen   centers with elongated (elongation approximate to 0.24 +/- 0.02   angstrom) 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 (ZrxO2x+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 (ZrxO2x+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.

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