EXAFS Characterization of Dendrimer ‐ Derived Pt / γ ‐ Al 2 O 3 A

The various steps involved in the preparation of a Pt/y-Al203 material using hydroxyl-terminated generation four (G40H) PAMAM dendrimers as templates were monitored by EXAFS. The results indicate that CI ligands in the Pt precursors (H2PtCl6 and K2PtCl4) were partially replaced by aquo ligands upon hydrolysis to form [PtCl3(H20)3] + and [PtCl2(H20)2] species. After interaction of such species with G40H, Cl ligands from the first coordination shell of Pt were further replaced by nitrogen atoms from the dendrimer interior, indicating the complexation of Pt with the dendrimer. This process was accompanied by a transfer of the electron density from the dendrimer to Pt, indicating that the former plays the role of a ligand. Following treatment of the H2PtCl6/G40H and K2PtCl4/G40H composites with NaBH4, no substantial changes were detecteded in the electronic or coordination environment of Pt, and no formation of metal nanoparticles was observed. However, when the reduction treatment was performed with H2, the formation of extremely small Pt clusters incorporating no more than 4 Pt atoms was observed. These Pt species remained strongly bonded to the dendrimer and their nuclearity depends on the length of the H2 treatment. Formation of Pt nanoparticles with an average diameter of approximately 10 A was finally observed after the deposition of H2PtCl6/G40H on y-Al203 and drying, suggesting that their formation may be related to the collapse of the dendrimer structure. The Pt nanoparticles formed appear to have high mobility, since subsequent thermal treatment in 02/H2 led to further sintering.