Cerium Oxide Nanostructures on Titania: Effect of the Structure and Stoichiometry on the Reactivity Toward Ethanol Oxidation

Nanocatalysis is a multidisciplinary field in which catalytic reactions are explored, understood, and improved by means of surface science methods. Model systems are a good playground to develop new interfaces where uncommon geometries and special active sites are stabilized. Cerium oxide deposited on titanium dioxide represents a good example of this. Thanks to a charge transfer from the titania support, Ce³⁺ sites grow at the interface. Thick cerium oxide layers show both Ce³⁺ and Ce⁴⁺ oxidation states, whereas only Ce³⁺ is detected in the monolayer. This demonstrates that the charge transfer is limited to the interface. In the present study, we grew and characterized two cerium oxide layers having different thicknesses, namely, 2 and 6 monolayer equivalents, on rutile (110), and investigated their reactivity toward the partial oxidation of ethanol. Our results show that the selectivity and the reaction mechanism are affected by the thickness of the cerium oxide. In particular, a stoichiometric reaction involving surface oxygen sites takes place on the thinner reduced film, whereas at higher ceria coverage, the presence of the Ce³⁺/Ce⁴⁺ redox couple favors a catalytic reaction in which gas-phase oxygen is activated on ceria and then reacts with adsorbed ethanol.