Dendrimer oxide supports deposition

Dendrimers can be used as templates for synthesis of monodisperse gold particles, which can then be deposited onto oxide supports metal particles are encapsulated within dendrimers, and the size of the particles can be tuned by varying the metal-to-dendrimer ratio prior to reduction. 

The details of the deposition and dissolution processes of the DAB-dend-(NHCOFc) in tetra n-butyl ammonium perchorate (TBAP)/CH2Cl2 solution were investigated using the electrochemical quartz crystal microbalance (EQCM) technique as well as admittance measurement of the quartz crystal resonator by Takada etal. [77]. It was found that the oxidized form of the dendrimers deposited onto the Pt electrode likely due to the low solubility of the salt composed of the oxidized dendrimer (ferricenium form) and C104 anions. On the other hand, the reduced form of the dendrimers easily redissolved except for the first monolayer, which appeared to be strongly adsorbed. Further, the mass-transfer process, during the redox reaction of the adsorbed dendrimers in an AN solution, was found to be of the anion exchange type. The resistance measurements of the quartz crystal resonator based on the admittance also supported the results obtained by EQCM. 

Figure 4.9b shows that the as-synthesized Pt nanoparticles, without any reduction pretreatment, have very low activity. Upon reduction at 423 K for 1 h, the activity of SBA-15 supported Pt DENs was enhanced sevenfold. If the reduction temperature was too high (573 and 673 K), the Pt DENs lost their activity, possibly due to sintering and carbon deposition, which decreases the surface area of the Pt DENs. The low activity of the as-synthesized Pt DENs could be attributed to their high oxidation state, which is not active for ethylene hydrogenation. From XPS measurement of the sample (Fig. 4.5), only 7 % of the Pt was reduced to the metallic state after treating Pt PAMAM dendrimer complexes with NaBIU for 8 h. This observation agrees with a detailed study performed by Crooks et al. [90]. The high oxidation state could also explain why Amiridis et al. couldn t observe any CO adsorption on Pt DENs without any high-temperature reduction treatment in the gas phase, as shown in Fig. 4.10 [93]. In this study, the Pt PAMAM dendrimer complexes were only reduced using H2 for 1 h at room temperature. By the end of this treatment, the majority of the Pt ions would still remain in their high oxidation state [90] and would not adsorb CO molecules.

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