In recent years, PAMAM dendrimers have been used as templates for the preparation of supported metal catalysts. This approach takes advantage of several unique characteristics of these dendrimers such as the ability to complex metal cations in solution and to protect them from aggregation until they are impregnated onto a support. After the impregnation step, the active metal is encapsulated by the dendrimer and, therefore, the dendrimer component must be removed to render the metal sites accessible. The first literature examples in this area have illustrated that the use of metal-dendrimer nanocomposites as precursors leads to a greater control of metal nanoparticle sizes and better distribution of the active metal phase on the support surface. However, little is known regarding the extent to which noble metals are complexed with the dendrimers in solution, and what factors can affect the complexation process. Furthermore, it is apparent that high temperature thermal treatments used to remove the dendrimer component frequently lead to sintering of the supported metal nanoparticles.
Our goal is to develop a better molecular-level understanding of the processes taking place during the synthesis of (Rh, Pt)-PAMAM nanocomposites in solution and the subsequent impregnation and dendrimer removal steps. The extent and strength of complexation of Rh and Pt with G4OH PAMAM dendrimer was quantitatively evaluated. Results of AA, UV-Vis, FTIR, XPS, EXAFS, and STEM measurements illustrate how the metal/dendrimer ratio, the dialysis, the solution pH, and the choice of the dendrimer removal procedure can influence the extent of metal-to-dendrimer binding and the dispersion of supported metal species. More specifically, the size of supported Rh nanoparticles can be precisely controlled by varying the Rh/dendrimer ratio, only when the pH of the complexation solution is adjusted to neutral conditions and the solution is dialyzed after the complexation to remove free metal species and impurities. This work also shows that O2 plasma can be used effectively at room temperature for the removal of the dendrimer component and the activation of the metal sites. Such treatment does not significantly affect the structure of the metal particles and the support, thus, minimizing the metal particle sintering observed when thermal activation protocols are used.
|Advisor:||Amiridis, Michael D.|
|Commitee:||Alexeev, Oleg S., Ploehn, Harry J., Vogt, Thomas|
|School:||University of South Carolina|
|School Location:||United States -- South Carolina|
|Source:||DAI-B 72/11, Dissertation Abstracts International|
|Subjects:||Inorganic chemistry, Chemical engineering|
|Keywords:||Catalysts, Nanoscale dendrimer, Noble metals, Plasma|
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