Thermal decomposition of generation-4 polyamidoamine dendrimer films: decomposition catalyzed by dendrimer-encapsulated Pt particles

The thermal decomposition of hydroxyl-terminated generation-4 polyamidoamine dendrimer (G4OH) films deposited on Au surfaces has been compared with decomposition of the same dendrimer encapsulating an approximately 40-atom Pt particle (Pt-G4OH). Infrared absorption reflection spectroscopy studies showed that, when the films were heated in air to various temperatures up to 275 degrees C, the disappearance of the amide vibrational modes occurred at lower temperature for the Pt-G4OH film. Dendrimer decomposition was also investigated by thermogravimetric analysis (TGA) in both air and argon atmospheres. For the G4OH dendrimer, complete decomposition was achieved in air at 500 degrees C, while decomposition of the Pt-G4OH dendrimer was completed at 400 degrees C, leaving only platinum metal behind. In a nonoxidizing argon atmosphere, a greater fraction of the G4OH decomposed below 300 degrees C, but all of the dendrimer fragments were not removed until heating above 550 degrees C. In contrast, Pt-G4OH decomposition in argon was similar to that in air, except that decomposition occurred at temperatures approximately 15 degrees C higher. Thermal decomposition of the dendrimer films on Au surfaces was also studied by temperature programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) under ultrahigh vacuum conditions. Heating the G4OH films to 250 degrees C during the TPD experiment induced the desorption of large dendrimer fragments at 55, 72, 84, 97, 127, 146, and 261 amu. For the Pt-G4OH films, mass fragments above 98 amu were not observed at any temperature, but much greater intensities for H(2) desorption were detected compared to that of the G4OH film. XPS studies of the G4OH films demonstrated that significant bond breaking in the dendrimer did not occur until temperatures above 250 degrees C and heating to 450 degrees C caused dissociation of C=O, C-O, and C-N bonds. For the Pt-G4OH dendrimer films, carbon-oxygen and carbon-nitrogen bond scission was observed at room temperature, and further decomposition to atomic species occurred after heating to 450 degrees C. All of these results are consistent with the fact that the Pt particles inside the G4OH dendrimer catalyze thermal decomposition, allowing dendrimer decomposition to occur at lower temperatures. However, the Pt particles also catalyze bond scission within the dendrimer fragments so that decomposition of the dendrimer to gaseous hydrogen is the dominant reaction pathway compared to desorption of the larger dendrimer fragments observed in the absence of Pt particles.