Grant Johnson - Influence of Stoichiometry and Charge State on the Structure and Reactivity of Cobalt Oxide Clusters with CO

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  Publication Details (including relevant citation   information):


  Volume:   112

    Issue: 45

    Pages: 11330-11340


Published: NOV 13 2008


    Cationic and anionic cobalt oxide clusters, generated by laser   vaporization, were studied using guided-ion-beam mass   spectrometry to obtain insight into their structure and   reactivity with carbon monoxide. Anionic clusters having the   stoichiometries Co2O3-, Co2O5-, Co3O5- and Co3O6- were found to   exhibit dominant products corresponding to the transfer of a   single oxygen atom to CO, indicating the formation of CO2.   Cationic clusters, in contrast, displayed products resulting from   the adsorption of CO onto the cluster accompanied by the loss of   either molecular O-2 or cobalt oxide units. In addition,   collision induced dissociation experiments were conducted with   N-2 and inert xenon gas for the anionic clusters, and xenon gas   for the cationic clusters. It was found that cationic clusters   fragment preferentially through the loss of molecular O-2 whereas   anionic clusters tend to lose both atomic oxygen and cobalt oxide   units. To further analyze how stoichiometry and ionic charge   state influence the structure of cobalt oxide clusters and their   reactivity with CO, first principles theoretical electronic   structure studies within the density functional theory framework   were per-formed. The calculations show that the enhanced   reactivity of specific anionic cobalt oxides with CO is due to   their relatively low atomic oxygen dissociation energy which   makes the oxidation of CO energetically favorable. For cationic   cobalt oxide clusters, in contrast, the oxygen dissociation   energies are calculated to be even lower than for the anionic   species. However, in the cationic clusters, oxygen is calculated   to bind preferentially in a less activated molecular O-2 form.   Furthermore, the CO adsorption energy is calculated to be larger   for cationic clusters than for anionic species. Therefore, the   experimentally observed displacement of weakly bound O-2 units   through the exothermic adsorption of CO onto positively charged   cobalt oxides is energetically favorable. Our joint experimental   and theoretical findings indicate that positively charged sites   in bulk-phase cobalt oxides may serve to bind CO to the catalyst   surface and specific negatively charged sites provide the   activated oxygen which leads to the formation of CO2. These   results provide molecular level insight into how size,   stoichiometry, and ionic charge state influence the oxidation of   CO in the presence of cobalt oxides, an important reaction for   environmental pollution abatement.

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