Recent Advancement in Understanding the Structure of DENs

Since the development of the dendrimer-templated synthesis of metal nanoparticles, there have been many fnndamental questions raised about the synthesis processes and final structures of DENs. Many efforts have been devoted to answering these questions, which are the fonndation of these dendrimer-based synthesis techniques. The answers to these questions could also explain the observed catalytic properties of DENs. In this section, we discuss several recent reports that provide deeper understanding of the synthesis and structure of DENs. 

Rh ions behave similarly to PF+ ions, but the resonance Raman peaks at 1,560 and 1,440 cm are much weaker, as shown by spectrum 3 in Fig. 4.4a, because the LMCT band for RhG40H (Rh + mixed with fourth-generation PAMAM dendrimers) has much less overlap with the 244 nm Raman excitation (Fig. 4.4b). 

During the synthesis of Pt DENs with PAMAM dendrimers, NaBH4 is the most frequently used reducing agent. In a typical synthesis, 10- to 100-fold of NaBH4 is used to completely reduce the metal precursors bonded in PAMAM dendrimers 

What is the real oxidation state of Pt in DENs To answer this question, Somorjai et al. measured the XPS spectrum of Pt2o DENs, as shown in Fig. 4.5, which shows 

Combined with other techniques, such as transmission electron microscopy (TEM), EXAFS, and high-energy X-ray diffraction (XRD), a bimodal distribution was proposed, as shown in Fig. 4.7 [90]. The proposed model suggests that each PE /G60H complex can only be fully reduced or fuUy intact after reduction with NaBFLt. The same work also suggests an autocatalytic mechanism in which Pt seeds form very slowly inside PAMAM dendrimers and a fast catalytic reduction of Pt  

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Another very important category of dendrimer catalysis is based on nanoparticles synthesized directly inside the dendrimers being the active component for catalytic reactions. The current review will focus on Polyamidoamine (PAMAM) dendrimer-encapsulated nanoparticles (DENs). This topic has been reviewed several times [22-24,26,42]. In this review, the techniques developed for the synthesis of mono- and bi-metallic DENs will only be briefly overviewed. The focus of this review is on the most recent advances in understanding the structure of DENs and newly developed apphcations of DENs in catalysis. 

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