Structural effects of supported Pd particles in D2 and CO oxidation experiments

• In the temperature range 100–300°C, isothermic D2 and CO titration experiments have been performed on oxygen pre-exposed 1.5, 4, 20 and 200 nm thick Pd films, epitaxially cube-on-cube grown on MgO(100). The oxygen pre-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 the 200 nm sample was a continuous, flat Pd(100) film with atomic steps. Atomic force microscopy demonstrated that the titration experiment did not affect the structure of the (large) Pd particles in the 20 nm film. The reaction behavior of the 1.5 nm film can be understood by assuming that the film consists of (111)-bounded Pd particles, where O2 and D2 dissociate and spill over to the oxide support, whereas CO adsorbs and reacts both on the Pd and on the oxide. The CO oxidation thus occurs in parallel on Pd and on MgO, while the D2 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(111) and Pd(100) facets have to be taken into account. The greater the oxygen pre-exposure, the higher the probability that the D2 molecules dissociate primarily on the Pd(111) surfaces.

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