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Synthesis and Characterization of Dendrimer-Derived Supported Iridium Catalysts

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journal contribution
posted on 11.09.2008, 00:00 by Yaritza M. López-De Jesús, Aurélie Vicente, Gwendoline Lafaye, Patrice Marécot, Christopher T. Williams
The synthesis of Ir/γ-Al2O3 using the dendrimer metal nanocomposites (DMN) approach is reported. Fourth generation hydroxyl-terminated polyamidoamine dendrimer was complexed with Ir3+ in aqueous solution and the process monitored using ultraviolet−visible and X-ray absorption spectroscopy. No discernible reduction of Ir3+ to form zerovalent nanoparticles was observed after bubbling hydrogen or adding NaBH4 into the complex solution. Standard wet impregnation of the DMN precursors were used to prepare Ir/γ-Al2O3, which were compared with conventionally prepared samples. In situ transmission Fourier transform infrared spectroscopy during dendrimer thermal decomposition in different atmospheres and CO adsorption allowed for identification of catalyst activation treatments that expose the maximum metal surface area. The particle size distributions of these catalysts were investigated using high resolution transmission electron microscopy, revealing that all of the catalysts have small particle sizes (0.4−3 nm) with narrow distributions. An optimized oxidation/reduction treatment produced a DMN-derived supported catalyst with higher metallic dispersion. DMN-derived catalysts were tested for liquid-phase hydrogenation of benzonitrile, and show an increase in TOF with increasing dispersion. The selectivity toward dibenzylamine is affected by the catalyst preparation method, with the oxidation/reduction treatment resulting in lower selectivity.