Ozturk O, Black T J, Perrine K, Pizzolato K, Williams C T, Parsons F W, Ratliff J S, Gao J, Murphy C J, Xie H, Ploehn H J, Chen D A
Department of Chemistry and Biochemistry, University of South Carolina, Columbia, SC 29208, USA.
Langmuir. 2005 Apr 26;21(9):3998-4006. doi: 10.1021/la047242n.
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.
已将沉积在金表面的端羟基四代聚酰胺-胺树枝状大分子（G4OH）薄膜的热分解情况，与包裹着约40个原子的铂颗粒的相同树枝状大分子（Pt-G4OH）的分解情况进行了比较。红外吸收反射光谱研究表明，当薄膜在空气中加热到高达275℃的不同温度时，Pt-G4OH薄膜中酰胺振动模式在较低温度下消失。还通过热重分析（TGA）在空气和氩气气氛中研究了树枝状大分子的分解。对于G4OH树枝状大分子，在空气中500℃时实现了完全分解，而Pt-G4OH树枝状大分子在400℃时分解完成，仅留下铂金属。在非氧化性氩气气氛中，G4OH的较大部分在300℃以下分解，但直到加热到550℃以上所有树枝状大分子碎片才被去除。相比之下，Pt-G4OH在氩气中的分解与在空气中相似，只是分解发生的温度大约高15℃。还在超高真空条件下通过程序升温脱附（TPD）和X射线光电子能谱（XPS）研究了金表面树枝状大分子薄膜的热分解。在TPD实验中将G4OH薄膜加热到250℃会在55、72、84、97、127、146和261原子质量单位处诱导大的树枝状大分子碎片脱附。对于Pt-G4OH薄膜，在任何温度下都未观察到质量大于98原子质量单位的碎片，但与G4OH薄膜相比，检测到的H₂脱附强度要大得多。G4OH薄膜的XPS研究表明，直到温度高于250℃树枝状大分子中才发生显著的键断裂，加热到450℃会导致C=O、C-O和C-N键解离。对于Pt-G4OH树枝状大分子薄膜，在室温下观察到碳-氧和碳-氮键断裂，加热到450℃后进一步分解为原子物种。所有这些结果都与以下事实一致，即G4OH树枝状大分子内部的铂颗粒催化热分解，使树枝状大分子在较低温度下发生分解。然而，铂颗粒也催化树枝状大分子碎片内的键断裂，因此与在没有铂颗粒时观察到的较大树枝状大分子碎片脱附相比，树枝状大分子分解为气态氢是主要的反应途径。
