Exafs Characterization of Pt/Dendrimer Nanocomposites Used for the Preparation of Pt/Al2o3 Catalysts
Attilio Siani, Oleg S. Alexeev, Christopher Williams, Harry J. Ploehn, and Michael D. Amiridis. Chemical Engineering, University of South Carolina, 301 Main St., Columbia, SC 29208
Conventional methods used for the preparation of supported metal catalysts employ the deposition of the metal precursors from solutions and provide limited control over the structure of the resulting materials. Consequently, alternative synthetic routes have been developed based on the ability of templating materials to stabilize metal nanoparticles in solution. Tighter control of particle size can be achieved in these cases. Among such templating materials, poly(amidoamine) dendrimers have attracted growing attention. The preparation of metal-dendrimer nanocomposites involves the complexation of metal cations with the dendrimer's interior amine/amide groups followed by a reduction step to form nanoparticles encapsulated within the dendrimer structure. EXAFS spectroscopy was used to characterize the various steps of the preparation of supported Pt catalysts using G4OH dendrimers as templates. The results indicate that upon hydrolysis chlorine ligands of the precursors are partially replaced by aquo ligands to form partially hydrated species. The subsequent interaction of these species with G4OH leads to partial replacement of chlorine ligands by nitrogen from the dendrimer, indicating that complexation takes place. The process was accompanied by a transfer of electron density from the dendrimer to the metal cations, indicating that the dendrimer plays the role of a ligand. Treatment of such nanocomposites nanocomposites with H2 or NaBH4 does not change substantially the electronic or coordination environment of the metal cations, and no metal nanoparticles are formed during this step. Instead, the formation of nanoparticles with an average diameter of approximately 10 Å was observed after the deposition and drying of the metal/dendrimer nanocomposites on the surface of the oxidic supports, implying that their formation may be related to the collapse of the dendrimer and the removal of aquo ligands. The dendrimer-covered nanoparticles appear to have high mobility and subsequent thermal treatments in O2/H2 can lead to further sintering.