(766c) Synthesis and Characterization of Subnanometer Sized Rh Particles on ZrO2

Since conventional preparation techniques often provide limited control over the structure and composition of resulting catalytic materials, alternative synthetic routes have been proposed based on the use of colloidal nanoparticles preformed in solutions as precursors. The preparation of colloidal metal suspensions requires the chemical reduction of transition metal salts in an aqueous or organic medium in the presence of surfactants that act as protective agents to preserve the colloidal state of metal particles. Several organic molecules have been used in this role among which the poly (amidoamine) (PAMAM) dendrimers have attracted growing attention. PAMAM dendrimers not only have various functional groups that are capable of interacting with cations of various metals, but they also have relatively open interior pockets which can be used for trapping metal nanoparticles, and therefore, protecting them from aggregation until they are delivered to a support surface. It has been shown that a subsequent grafting of metal-dendrimer nanocomposites onto various metal oxide supports leads to narrower particle size distributions as compared to those observed with conventional impregnated catalysts. However, many aspects of this new preparation route still remain unclear.

Our goal was to develop a better molecular-level understanding of the processes taking place during the synthesis of (Rh)-PAMAM nanocomposites in solutions and how these materials can be used as precursors for the preparation of supported catalysts. STEM and EXAFS were used for the characterization of such materials during the various preparation steps and the data indicate that that highly dispersed Rh/ZrO₂ catalysts with extremely narrow distributions of Rh particles can be successfully prepared via the dendrimer synthetic root. In addition, various samples were characterized by XPS and FTIR in an effort to probe the oxidation state of the Rh species formed. Finally, the catalytic activity of various Rh/ZrO₂ catalysts has been evaluated for CO and C₃H₆ oxidation and ethane hydrogenolysis reactions.