Size dependent reaction kinetics on supported model catalysts: A molecular beam/IRAS study of the CO oxidation on alumina-supported Pd particles

We have employed molecular beam techniques combined with in situ IR reflection absorption spectroscopy to study the CO oxidation kinetics on oxide-supported model catalysts. As model systems we utilize Pd particles of different size and morphology grown under ultrahigh vacuum conditions on a well-ordered alumina film on NiAl(110). Previously, these systems have been characterized in detail with respect to their geometric and electronic structure. Using a combination of two beam sources crossed on the sample surface, the transient behavior and the steady-state reaction rate are systematically probed over a wide range of CO and oxygen fluxes and at different sample temperatures. Comparing different particle sizes and structures, characteristic differences are observed with respect to both the steady-state reaction rate and the transient behavior upon switching off the CO beam source. The origin of these effects is discussed with respect to the different CO and oxygen adsorption properties of small Pd particles. To directly monitor changes in the CO adsorption during the reaction, we have performed in situ IR absorption spectroscopy under steady-state conditions. Whereas strong rearrangements of the CO adsorbate layer are found for large and well-ordered particles as a function of the applied CO and oxygen fluxes, no similar effects are observed on small Pd aggregates.