C-X Bond Activation and Reactivity On Doped Metal Oxides

Using a combination of theory and experiment, we have investigated a strategy for the introduction of atomic dopants into otherwise inactive host materials to produce active catalysts. The strategy is demonstrated for Ti and Al dopants into the inert ZnO host to activate two reaction pathways for CO oxidation which are both different from the traditional Mars-van-Krevelen (MVK) mechanism and for methane activation on ceria. In the MVK mechanism the metal oxide surface supplies oxygen atoms to the reductant through formation of a surface oxygen vacancy. In contrast, doped ZnO promotes the adsorption and activation of dioxygen on the dopant sites for reaction with CO. Further, surface dopant sites that do not coordinate gas phase dioxygen may themselves give rise to a dopant induced oxygen shuttle by which a surface oxygen vacancy is created from next-nearest neighbor sites through the energetically favorable O transfer to the dopant. Pt atoms doped into ceria are shown to direct the reaction pathway of methane with oxygen to partial oxidation producing synthesis gas from complete combustion. The proposed mechanisms are supported by density functional theory calculations and by experiments involving isotopically labeled molecular oxygen.