碳六十在熱水狀態下的反應動力學研究和其地質意涵

Abstract

C60最初是由Kroto（1985）等人為了要模擬富碳紅巨星周圍環境而使用之雷射燒融裝置中所發現的，因此人們推測在自然界中也很有可能產生C60；而現今自然界中發現富勒烯（典型為C60和C70）的地點不勝枚舉，一般認為這些地方富勒烯的成因皆是因為高能量而造成碳被熔融成氣相而重新凝聚成的，然而一些地方發現的富勒烯卻推翻這種說法，即富勒烯也有可能是當地有機物的變質產物之一，也就是有可能在低能量狀態下生成。本研究希望藉由C60在熱水狀態(hydro- thermal condition)下的反應動力學及熱穩定性的研究來推估自然界富勒烯在溫度和時間上可存在的條件，以及討論一些自然界富勒烯現今為何稀少的原因。 實驗方法是將C60粉末和蒸餾水以9:1的比例裝填密封在金管中，並使用高溫高壓儀器做定溫定壓的長期控制。實驗溫度分別為525、550、575、600和625℃，儀器壓力則皆定在5kb，而經校正後則約為3~4kb間，反應時間為30分鐘到140小時。實驗產物使用拉曼光譜儀(Raman spectrum)和X-光粉末繞射儀(XRD)來判定C60和未定形碳(amorphous carbon)兩者間相對強度比例上的變化，並以X-光粉末繞射儀的分析結果做分解比例的定量。此外，為進一步了解實驗產物實際結構上的變化，故取數個不同分解比例的實驗產物和真空狀態下所做的碳六十全分解產物做TEM的觀察。 初步結果顯示C60在熱水狀態下零級和一級的反應活化能分別為271和278 kJ/mol，由於前人使用的動力學模型為一級反應，因此本研究沿用前人模式以一級反應結果來外推低溫下自然界富勒烯的生存條件。外推結果顯示C60在290℃以下的溫度可生存至約10萬年，而若要生存超過10億年，則需在220℃以下的溫度；而將本研究結果與無水狀態下C60熱解反應動力學機制相比，熱水狀態下的C60會比無水狀態下的熱解反應快約106倍，顯示水會明顯地加速C60熱解反應；因此，在自然界裡的環境中，若水的活性較低的話，則這些富勒烯可以存在於較高的溫度環境下，或能存活較長的時間。另外，以拉曼光譜儀和TEM的分析比對XRD的結果顯示，當C60面心立方結晶分解時，C60分子本身也會逐漸轉變成未定形碳，也表示使用XRD定量的結果是可信的。

Fullerenes (typically C60) are carbon cage molecules initially synthesized during laser ablation of carbon by Kroto et al. in 1985. The occurrence of nature fullerene has been reported from meteorites, sediments from the K-T boundary and P-T boundary, carbon-rich rocks (organic origin), pillow lavas cutting black shales, and others such as fulgurite, ink sticks, dinosaur eggs and tree char. Some of these, like graphite and diamond, may probably be the metamorphic products of local organic matter. The objective of this study is to examine the kinetics that C60 transforms into amorphous carbon and estimate the temperature and time that fullerenes can survive in the earth, especially under hydrothermal condition in the crust. C60 powder and distiller water were sealed into gold capsules and reacted at the temperature, 525, 550, 575, 600, 625°C, and the pressure at 3~4 kbar for periods between 30 min and 168 h. The run products were characterized using the Raman spectrum and TEM while the transformation ratio of C60 to amorphous carbon is determined by x-ray diffraction. The preliminary results show that the decomposition rate of C60 can be described by a first-order kinetic model with an activation energy of 278 kJmol-1. The extrapolation to geological hydrothermal condition reveals that C60 will survive about 0.1 million years at 290°C and 1 billion years at 220°C, which represents the time-temperature limit that C60 can survive under hydrothermal condition. Compared with the non-hydrothermal fullerene decomposition kinetics, it reveals that water strongly accelerates transformation of fullerenes. This implies that fullerene may survive at longer time or higher temperature if the water activity of geological environment is lower than hydrothermal environment. In addition, the Raman spectrum and TEM results confirm that the fcc crystal structure of C60 revealed by XRD has been thermally decomposed along with the disintegration of the C60 molecule revealed by Raman spectroscopy.