EXAFS characterization of dendrimer-Pt nanocomposites used for the preparation of Pt/γ-Al2O3 catalysts

Pt/gamma-Al2O3 catalysts were prepared using hydroxyl-terminated generation four (G4OH) PAMAM dendrimers as the templating agents and the various steps of the preparation process were monitored by extended X-ray absorption fine structure (EXAFS) spectroscopy. The EXAFS results indicate that, upon hydrolysis, chlorine ligands in the H2PtCl6 and K2PtCl4 precursors were partially replaced by aquo ligands to form [PtCl3(H2O)(3)](+) and [PtCl2(H2O)(2)] species, respectively. The results further suggest that, after interaction of such species with the dendrimer molecules, chlorine ligands from the first coordination shell of Pt were replaced by nitrogen atoms from the dendrimer interior, indicating that complexation took place. This process was accompanied by a substantial transfer of electron density from the dendrimer to platinum, indicating that the dendrimer plays the role of a ligand. Following treatment of the H2PtCl6/G(4)OH and K2PtCl4/G(4)OH complexes with NaBH4, no substantial changes were observed in the electronic or coordination environment of platinum, indicating that metal nanoparticles were not formed during this step under our experimental conditions. However, when the reduction treatment was performed with H-2, the formation of extremely small platinum clusters, incorporating no more than four Pt atoms was observed. The nuclearity of these clusters depends on the length of the hydrogen treatment. These Pt species remained strongly bonded to the dendrimer. Formation of larger platinum nanoparticles, with an average diameter of approximately 10 angstrom, was finally observed after the deposition and drying of the H2PtCl6/G(4)OH nanocomposites on a gamma-Al2O3 surface, suggesting that the formation of such nanoparticles may be related to the collapse of the dendrimer structure. The platinum nanoparticles formed appear to have high mobility because subsequent thermal treatment in O-2/H-2, used to remove the dendrimer component, led to further sintering.