Structural effects of supported Pd particles in D-2 and CO oxidation experiments

In the temperature range 100-300 degrees C, isothermic D-2 and CO titration experiments have been performed on oxygen pre-exposed 1.5, 4, 20 and 200 n m thick Pd films, epitaxially cube-on-cube grown on MgO(100). The oxygen pr e-exposures varied from 0.2 to 150 L. The three thinnest films consisted of faceted particles, mainly bounded by Pd(111) and Pd(100) surfaces, while t he 200 nm sample was a continuous, flat Pd(100) film with atomic steps. Ato mic force microscopy demonstrated that the titration experiment did not aff ect the structure of the (large) Pd particles in the 20 nm film. The reacti on behavior of the 1.5 nm film can be understood by assuming that the film consists of(lll)bounded Pd particles, where O-2 and D-2 dissociate and spil l over to the oxide support, whereas CO adsorbs and reacts both on the Pd a nd on the oxide. The CO oxidation thus occurs in parallel on Pd and on MgO, while the D-2 oxidation is sequential, first occurring on the Pd and then on the MgO. To explain the D2O desorption curves for the 4 and 20 nm films, spillover to the MgO support as well as reactions on the Pd(lll) and Pd(10 0) facets have to be taken into account. The greater the oxygen pre-exposur e, the higher the probability that the D-2 molecules dissociate primarily o n the Pd(lll) surfaces. (C) 1999 Elsevier Science B.V. All rights reserved.