Influence of ceria on Pd activity for the CO+O-2 reaction

The behavior of a series of palladium catalysts supported on alumina, ceria , and ceria/alumina for the CO + O-2 reaction has been analyzed by a combin ation of electron transmission microscopy, infrared and electron paramagnet ic resonance spectroscopies, and catalytic test studies. Ceria is shown to decrease the onset of the reaction by ca. 130 K due to an enhanced activati on of both reactant molecules. Even at room temperature, ceria facilitates activation of CO by promoting the formation of metallic palladium and that of oxygen by the presence of reactive vacancies at the Pd-Ce interface. The optimum ceria promoting effect in the CO conversion is observed for pallad ium particles in contact with 3-dimensional ceria supported particles, whic h seem to be oriented by their interaction with alumina, and ascribed to th e specific characteristics of the anionic vacancies located at the correspo nding Pd-Ce interface. The bulk ceria support, however, induces an oxidatio n-deactivation process of palladium at medium and high reaction temperature s which hinders reaction between Pd-bonded CO and Ce-bonded oxygen at the P d-Ce interface. (C) 1999 Academic Press.