Synthesis, Characterization and Kinetic Evaluation of Dendrimer-Derived Iridium and Iridium-Palladium Catalysts.

Supported catalysts are very important for industrial, automotive and fuel cell applications. Ir and Pd catalysts possess unique properties that can enhance activity and selectivity for a variety of reactions. However, conventional synthetic methods used to produce monometallic and bimetallic catalysts often result in wide particle size distributions and non-uniform materials which can be difficult to characterize on a fundamental level. Poly(amidoamine) (PAMAM) dendrimers are used to form and stabilize cluster and nanoparticles in solution generating dendrimer metal nanocomposites (DMN) precursors. In the present work, the effectiveness of using DMN precursors to prepare gamma-Al 2O3 supported Ir, Pd and Ir-Pd catalysts is reported. The main part of this work consisted on the developing a synthetic protocol where fourth generation hydroxyl-terminated (G4OH) PAMAM dendrimer is used to produce a 1% Ir/gamma-Al2O3. The complexation process between IrCl3 and dendrimer functional groups in solution was followed using ultraviolet-visible (UV-vis) spectroscopy and X-ray absorption fine structure (EXAFS) spectroscopy. Seven days were required to achieve maximum complexation, resulting in a substitution of two Cl- ions from the precursor with two oxygen ions from the dendrimer functional groups. No discernable reduction of Ir3+ to Ir0 clusters or particles was observed after treatment with reducing agents. The resulting DMN precursors were delivered to gamma-Al2O3 support via standard wet impregnation. In order to expose the metal nanoparticles to the surface of the catalysts thermal dendrimer decomposition and CO adsorption were monitored using transmission Fourier transform infrared (FTIR) spectroscopy. Two activation treatments were obtained yielding narrow particle size distributions (1.4-3 nm) and less than 1% carbon in the surface. Pd/gamma-Al2O3 and Irx-Pdy/gamma-Al 2O3 catalysts were prepared utilizing the DMN approach by two different synthetic routes and with three bimetallic compositions. In general, the use of DMN precursors yielded narrower particle size distribution. These catalysts including Ir/gamma-Al2O3 were compared with the conventional counterparts, characterized and evaluated for benzonitrile hydrogenation. Dendrimer-derived Ir catalyst has higher turnover frequency (TOF) than the conventional, however both present similar selectivity (dibenzylamine). In the other hand, Pd catalysts are even more active than Ir catalysts, however they are selective to benzylamine. Non-monotonically behavior is obtained for the initial reaction rate versus catalyst composition with a maximum at the 50:50% Ir: Pd molar ratio catalysts. Overall, DMN-derived bimetallic precursors form different bimetallic structures or distributions of metallic particles relative to those formed by conventional methods of preparation