Applications and mechanistic understanding of rare earth oxides in catalysis

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Rare earth oxides (REOs) have attracted considerable attention due to their favorable properties for several catalytic applications, in particular complete and partial oxidation reactions. In spite of this potential, however, only ceria has been studied in great detail so far. We propose a collaborative project to broaden the scope and follow a twofold approach by preparing selected REOs as both well-defined thin films for studies under UHV conditions and nanostructured materials for catalytic studies under practical reaction conditions. As reducible oxides, REOs exhibit variable valence states and thus stoichiometries combined with high oxygen mobility within the oxide lattice. These properties enable them to actively participate in surface redox reactions through the so-called Mars van Krevelen mechanism whereby lattice oxygen is exchanged with adsorbed reactants. As examples with a largely different chemistry, Tb (oxidation states: +3, +4) and Sm (oxidation states: +2, +3) will be chosen. Taking the oxidative coupling of methane (OCM) as a test reaction, we are aiming at a basic understanding of the selectivity of REOs toward partial vs. complete oxidation reactions, and an exploration of methods for tuning the catalytic selectivity of REOs through alkali-doping and nanoscale size effects.