Oxidation State and Symmetry of Magnesia-Supported Pd₁₃O_<i>x</i> 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 Pd₁₃ clusters deposited on MgO surfaces grown on Mo(100) is explored. It is shown that molecular oxygen dissociates easily on the supported Pd₁₃ cluster, leading to facile partial oxidation to form Pd₁₃O₄ clusters with <i>C</i>_4<i>v</i> symmetry. Increasing the oxidation temperature to 370 K results in nonsymmetric Pd₁₃O₆ 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 Pd₁₃O₄ cluster. Subsequent reactions on the resulting lower-symmetry Pd₁₃O_<i>x</i> (<i>x</i> < 4) clusters entail lower activation energies. The nonsymmetric Pd₁₃O₆ clusters show lower temperature-catalyzed CO combustion, already starting at cryogenic temperature