Oxidation state and symmetry of magnesia-supported Pd13Ox nanocatalysts influence activation barriers of CO oxidation
Combining temperature-programmed reaction measurements, isotopic labeling experiments, and first-principles spin density functional theory, the dependence of the reaction temperature of catalyzed carbon monoxide oxidation on the oxidation state of Pd 13 clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd 13 cluster, leading to facile partial oxidation to form Pd 13O 4 clusters with C 4v symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd 13O 6 clusters. The higher symmetry, partially oxidized cluster is characterized by a relatively high activation energy for catalyzed combustion of the first CO molecule via a reaction of an adsorbed CO molecule with one of the oxygen atoms of the Pd 13O 4 cluster. Subsequent reactions on the resulting lower-symmetry Pd 13O x (x < 4) clusters entail lower activation energies. The nonsymmetric Pd 13O 6 clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature.