Formation of Formic Acid, Formaldehyde, and Carbon Dioxide by Electron-Induced Chemistry in Ices of Water and Carbon Monoxide
Formation of Formic Acid, Formaldehyde, and Carbon Dioxide by Electron-Induced Chemistry in Ices of Water and Carbon Monoxide
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DOI：
10.1021/acsearthspacechem.9b00168
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发表时间：
2019-09-19
影响因子：
3.4
通讯作者：
Bredehoeft, Jan H.
中科院分区：
文献类型：
--
作者：
Schmidt, Fabian;Swiderek, Petra;Bredehoeft, Jan H.
通过不同类型的辐射处理由水和一氧化碳组成的低温冰，已知会导致二氧化碳、甲酸、甲醛和甲醇的形成。在这项研究中，我们研究了能量在2到20 eV之间的电子辐照下的这些反应，这个能量范围通常在次级电子中发现。这是第一次用足够细的步宽能量来监测反应，以解决和确定导致特定产物的初级电子-分子相互作用。这使我们能够通过将这些初级电子-分子相互作用与最终产物联系起来来阐明反应机制。在这些反应中，HCO中心点和HOCO中心点自由基是关键中间体。结果表明，HCO中心点的中间体主要生成甲醛，而HOCO中心点的中间体主要生成甲酸。值得注意的是，共振电子附着过程在特征能量范围内增强了甲醛和甲酸的形成。相反，导致二氧化碳的反应没有共振能量依赖，但可以追溯到非共振中性解离过程。这表明二氧化碳与这两种中间体都没有联系。这与之前的实验研究相反，之前的实验研究提出二氧化碳是由HOCO中心点失去h中心点自由基形成的。然而，我们的结果证实了理论研究的预测，即HOCO中心点形成二氧化碳的效率不是很高，因为HOCO中心点呈现出一个能量阱，并迅速失去它在冰基质中可能拥有的任何多余能量。相反，我们提供的证据表明，在水和一氧化碳的低温冰中，导致二氧化碳形成的主要电子-分子相互作用是水中性解离成O原子和H-2。形成的O原子直接与一氧化碳反应生成二氧化碳。
The processing of cryogenic ices consisting of water and carbon monoxide by different types of radiation is known to lead to the formation of carbon dioxide, formic acid, formaldehyde, and also methanol. In this study, we have investigated these reactions upon electron irradiation with energies between 2 and 20 eV, an energy range typically found in secondary electrons. This is the first time that the reactions have been monitored with a sufficiently fine step width in energy to resolve and identify the primary electron- molecule interactions leading to the specific products. This enables us to elucidate reaction mechanisms by linking these primary electron-molecule interactions to final products. In these reactions, HCO center dot and HOCO center dot radicals are key intermediates. Our results show that the HCO center dot intermediate predominantly leads to formaldehyde, while HOCO center dot is intermediate to the formation of formic acid. Noticeably, the formation of both formaldehyde and formic acid is enhanced within characteristic energy ranges by resonant electron attachment processes. In contrast, the reactions leading to carbon dioxide show no resonant energy dependence but can be traced back to nonresonant neutral dissociation processes. This reveals that carbon dioxide is linked to neither of these two intermediates. This is in contrast to prior experimental studies, which have proposed that carbon dioxide is formed by loss of a H-center dot radical from HOCO center dot. However, our results confirm theoretical studies that have predicted that carbon dioxide formation from HOCO center dot is not very efficient, because HOCO center dot presents an energetic well and quickly loses any excess energy it might have in an ice matrix. Instead, we provide evidence that the primary electron-molecule interaction leading to the formation of carbon dioxide in cryogenic ices of water and carbon monoxide is the neutral dissociation of water into O atoms and H-2. The so-formed O atoms then react directly with carbon monoxide to yield carbon dioxide.