Comparison of the Reactivity of Platinum Cations and Clusters Supported on Ceria or Alumina in Carbon Monoxide Oxidation

The mechanism of CO oxidation on platinum species supported on ceria or alumina was studied via periodic density functional calculations. In order to elucidate the nature of the catalytically active species, various reaction paths involving monoatomic species in different oxidation states-Pt0, Pt2+, and Pt4+-as well as platinum clusters were modelled. Since the oxygen centers that participate in the oxidation process may have diverse origins, the interaction of CO with the oxygen from the ceria support, dissociated O2 molecules on the platinum species, and the oxygen molecule healing the surface oxygen vacancy in the support was considered. Ceria was modeled both as a (111) surface and as a nanoparticle, and gamma-alumina was modeled as a (001) surface. The reaction paths via complexes of Pt2+ and Pt4+ cations were found to have the lowest activation barriers, 22-35 kJ/mol. The calculated activation energies on platinum clusters with high CO coverage supported on the ceria surface and the nanoparticle are also low, 40 and 19 kJ/mol, respectively. The barriers are significantly higher, 90-120 kJ/mol, when the reaction occurs on platinum supported on gamma-alumina or when an oxygen preadsorbed on the platinum cluster is involved in the process. The calculations allowed us to discriminate the role of the oxidant in the modeled reaction paths and to conclude that the activation barriers are low when the oxidants are platinum species and notably higher when the oxidants are Ce4+ cations.