Studies of size selected palladium and iridium model catalysts

This dissertation describes size-dependent investigations of supported palladium and iridium clusters, used as model catalysts for carbon monoxide oxidation and hydrazine decomposition reactions. Chapter 1 provides an introduction into the role of size-dependent studies within the greater scope of catalysis research, with a primary focus on the changing nature of metal particles with size at the small scale (<25 atoms per particle), and the important role of support interactions in determining particle chemistry.;An investigation of CO oxidation over titania (110) supported Pd(n) particles (n = 1, 2, 4, 7, 10, 16, 20, 25) via stepwise dosing and reacting protocols is given in Chapter 2, with experimental results showing a correlation between nonmonotonically varying cluster activities and their accompanying electronic properties as a function of size. In Chapter 3, the same catalyst-reaction system is studied as a function of surface temperature and total oxygen exposure during the oxygen dose, and is found to be limited by oxygen binding under the conditions investigated within the previous chapter.;Chapter 4 makes use of He flux-dependent ion scattering experiments to examine the thermal stability of silica supported Ir(n) (n = 1, 2, 10) as a function of cluster impact energy. The results of this work show that Ir agglomerates into larger structures when excited to temperatures expected during industrial breakdown of hydrazine, and cannot be stabilized by embedding the clusters within the support. Using the techniques developed in the previous chapter, Chapter 5 provides an in depth characterization of the strong metal-support interaction (SMSI), which leads to deactivation of titania supported Pd(n) towards CO-oxidation.;In Chapter 6 we build upon the insights obtained within Chapters 3 and 5 to create a fully developed picture of those changes to the physical properties of the catalysts that exert the greatest influence over the carbon dioxide formation mechanism during the course of preparation and reaction. From this, it is proposed that overall CO-oxidation activity for titania supported Pd(n) (n<26), as observed in Chapter 2, is governed by a complex combination of oxygen activation efficiency and the degree to which SMSI and O adsorption act to attenuate subsequent CO binding over the most stable sites on top of the clusters.