Yaritza M. Lopez

Synthesis and Characterization of Dendrimer-Derived Ir/g-Al₂O₃ Catalysts

MELANIE SCHAAL

YARITZA LOPEZ

          Transition metal-based heterogeneous catalysts are frequently employed in numerous industrial and pharmaceuticals reactions. I addition, they are central for the development of fuel cell technology. Among transition metals, noble metals are widely used since they possess unique properties that enhance activity and selectivity in chemical reactions. In supported metal catalysts, it is often desired to minimize the amount of noble metal needed and thus catalyst cost by maximizing the surface area per unit volume. Consequently, small particles and highly dispersed metal nanoparticles are critical properties of noble metals catalysts that need to be controlled. However, conventional synthetic techniques followed by calcinations/reduction typically result in a wide particle size distribution and non-uniform materials.  Non-uniformity, wide size distributions and unknown compositions are considered drawbacks to studying and understanding reaction mechanisms on supported catalysts.

 Dendrimer-metal nanocomposites (DMN) offer a novel synthetic approach that has been used over the last decade to produce homogeneous and heterogeneous catalysts. Poly(amidoamine) (PAMAM) dendrimers are monodisperse, hyperbranched polymers that emanate from a central core with repetitive branching units, having specific molecular structure and controllable size. They contain hollow pockets that can be used for stabilization and formation of metal nanoparticles. Hydroxyl-terminated poly(amidoamine) (PAMAM-OH) dendrimers can be use as chelating agent of metal ions since they have tertiary amines and secondary amides in their interior. Using DMN approach as shown in figure 1, narrow size distribution can be obtained, since a given generation of PAMAM-OH dendrimers has a known number of chelating sites.

Figure 1. Supported transition metal catalysts synthesis using dendrimer-metal nanocomposites (DMN) approach.

 Supported iridium catalysts are outstanding candidates for a variety of catalytic reactions due to their stability, activity and selectivity under reaction conditions.  However, there is as of yet no information about iridium supported catalyst synthesis using the DMN approach. My research work is based on the synthesis and characterization of dendrimer-derive iridium catalysts. Several characterization techniques have been employed to understand all synthesis stages. UV-Vis spectroscopy is used to investigate the iridium-dendrimer complexation. To expose iridium nanoparticles to the surface of the catalyst thermal removal of the dendrimer dendrimer is performed. This dendrimer decomposition has been studied using transmission FTIR spectroscopy. An optimum catalyst activation treatment was found using FTIR, HRTEM and EXAFS. In addition, iridium-palladium bimetallic dendrimer derive catalysts are going to be synthesized, characterized and investigated for use in liquid-phase hydrogenation reactions.