The synthesis of Ir/γ-Al 2 O 3 using the dendrimer metal nanocomposites (DMN) approach is reported. Fourth generation hydroxyl-terminated polyamidoamine dendrimer was complexed with Ir 3+ in aqueous solution and the process monitored using ultraviolet-visible and X-ray absorption spectroscopy. No discernible reduction of Ir 3+ to form zerovalent nanoparticles was observed after bubbling hydrogen or adding NaBH 4 into the complex solution. Standard wet impregnation of the DMN precursors were used to prepare Ir/γ-Al 2 O 3 , 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. IntroductionTransition metal-based heterogeneous catalysts are frequently employed in numerous industrial reactions, as well as environmental applications such as automotive catalytic converters and fuel cell technologies. Among transition metals, noble metals are widely used since they possess unique properties that enhance activity and selectivity in chemical reactions. Supported iridium catalysts are outstanding candidates for a variety of catalytic reactions due to their stability, activity and selectivity under reaction conditions. 1-6 For example, Ir catalysts possess unique characteristics for effective and selective synthesis of substituted N-aryl-hydroxylamines, chloro-substituted anilines and both symmetric and asymmetric azoxybenzenes. 6 Furthermore, supported Ir has been successfully employed for hydrogenation of R, -unsaturated aldehydes to selectively produce unsaturated alcohols. 4,5,7-11 Supported Ir catalysts are commonly prepared via wet impregnation 4,5,12-16 and incipient wetness impregnation. 11,[17][18][19][20] In supported metal catalysts, it is often desired to minimize the amount of noble metal needed (i.e., cost) by maximizing the metal surface area per unit volume. Consequently, the ability to produce small and highly dispersed metal nanoparticles is of great interest. However, conventional synthetic techniques (e.g., impregnation, ion-exchange) followed by calcination/reduction steps often result in wide particle size distributions. In addition to less than optimal performance, such nonuniform materials can also be difficult to characterize. 2...

The kinetics of liquid-phase hydrogenation of benzonitrile have been examined alumina supported Ir, Pd and Ir-Pd catalysts. Benzonitrile disappearance TOF is 10-fold higher for Pd catalyst than it is for Ir catalyst. Benzylamine is produced preferentially over Pd, whereas dibenzylamine is produced principally over Ir. Non-monotonic activity and selectivity relationships with bimetallic composition are obtained.

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