PAMAM Dendrimer-Derived Ir/Al 2 O 3 Catalysts: An EXAFS Characterization
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PAMAM Dendrimer-Derived Ir/Al2O3 Catalysts: An EXAFS Characterization Yaritza M. Lo´pez-De Jesu´s Æ Christopher T. Williams
Received: 20 July 2009 / Accepted: 25 August 2009 / Published online: 11 September 2009 Ó Springer Science+Business Media, LLC 2009
Abstract Extended X-ray absorption fine structure (EXAFS) spectroscopy was used to characterize several synthesis stages of hydroxyl-terminated generation four (G4OH) PAMAM dendrimer-derived Ir/c-Al2O3 catalyst. The EXAFS results indicate that Ir3? forms complexes with dendrimer functional groups through displacement of two Cl- ion ligands. These complexes are very stable in solution, and no reduction of Ir3? to Ir nanoparticles or clusters is observed after introduction of reducing agents such as NaBH4 or H2. These Ir3?-dendrimer complexes remain essentially intact after support impregnation. The formation of 1–2 nm particles occurs when the catalysts are treated with O2/H2 or H2, and the dendrimer-derived catalysts exhibit a lower degree of metal support interaction. Keywords Iridium Nanoparticles EXAFS Alumina Dendrimer
1 Introduction Over the past several decades, catalysis researchers have been committed to the development and improvement of transition metal-based heterogeneous catalysts. These types of catalyst are utilized extensively for industrial reactions, energy and automotive applications. In supported metal catalysts, it is often desired to minimize the amount of noble metal needed (and thus the catalyst cost) by maximizing the surface area per unit volume. New synthetic techniques have been developed that offer improved design Y. M. Lo´pez-De Jesu´s C. T. Williams (&) Department of Chemical Engineering, Swearingen Engineering Center, University of South Carolina, Columbia, SC 29208, USA e-mail: [email protected]
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and control of supported metal catalyst structure. These new approaches offer control of structure at the atomic level through tailoring the composition and (in some cases) the environment of the active sites. Among these innovative techniques, poly(amidoamine) (PAMAM) dendrimers have been used as templating and stabilizing agents to synthesize several highly dispersed monometallic and bimetallic heterogeneous catalysts [1–10]. PAMAM dendrimers are monodisperse, hyperbranched polymers that emanate from a central core with repetitive branching units, having specific molecular structure and controllable size. These dendrimers contain interior void spaces composed of tertiary amines and secondary amides that can be used to form dendrimer metal nanocomposites (DMN) [2, 11, 12]. The formation of DMN is expected to occur by complexing metal ions with the interior functional groups. It has been reported that these metal cations can be reduced with sodium boron hydride (NaBH4) to a zero valence state once they are complexed inside the dendrimer, resulting in the formation and stabilization of metal nanoparticles or clusters [13, 14]. Particularly, hydroxyl-terminated poly(amidoamine) (PAMAM-OH) dendrimers have been used as a chel
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