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首页> 外文学位 >Experimental and theoretical studies of the adsorption and photochemistry of dibromodifluoromethane (halon-1202) on a model carbonaceous aerosol surface.
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Experimental and theoretical studies of the adsorption and photochemistry of dibromodifluoromethane (halon-1202) on a model carbonaceous aerosol surface.

机译：在模型碳质气溶胶表面上吸附二溴二氟甲烷（哈龙-1202）和光化学的实验和理论研究。

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The adsorption and photochemistry of dibromodifluoromethane, CF2Br2, on highly ordered pyrolytic graphite (HOPG) was studied by theoretical and experimental methods as a model for heterogeneous photochemical reactions occurring on carbonaceous aerosols in the upper troposphere and lower stratosphere. The monolayer-covered HOPG surface was characterized using temperature-programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and electron energy loss spectroscopy (EELS) both before and after broadband ultraviolet (UV) irradiation from ∼225–350 nm. In the absence of UV radiation, molecular adsorption and desorption of CF2Br 2 was observed (∼140 K) while irradiation of the monolayer resulted in photodissociation of the adsorbate to yield CF2Br and Br atoms. The most prevalent reaction observed in these experiments was the recombination of a large fraction of the photolyzed molecules (∼70%) to reform CF 2Br2 during TPD measurements. Minor channels revealing the formation of Br2 (∼6%) and C2F4Br 2 (∼1.4%) were present as well. Comparison of the estimated integrated photodissociation cross section (1.9 × 10−19 cm 2) to the cross section calculated from the TPD data (4.5 × 10−19 cm2) suggested that photodissociation of CF2Br2 occurred by direct photoabsorption by the adsorbate and not by dissociative electron attachment mediated by the surface. Differences in the distributions of Br2 and C2F 4Br2 when compared to results of gas phase and matrix isolation experiments were attributed to possible structural constraints or reduced surface diffusion for one or more of the adsorbates due to the high density of photogenerated species trapped in the surface layer. Theoretical studies of the preferred adsorption sites and orientations of CF2Br 2 and CF2Br on the HOPG surface were examined using a cluster model approach to gain additional insight into the experimental data. Second-order Møller-Plesset perturbation theory (MP2) calculations were performed using 6-31G* and 6-31G(2d) basis sets and a single layer slab approximation consisting of either a four (pyrene) or seven (coronene) ring polycyclic aromatic hydrocarbon to model the HOPG surface. The preferred adsorption site for CF 2Br2 was found to be located at the hollow site of a six-membered ring for each combination of surfaces and basis sets examined. The interaction between the molecule and the surface was dipolar in nature with a small amount of charge transfer occurring from the surface to the adsorbate as determined from a Mulliken population analysis. For the CF2Br radical, different preferred adsorption sites were found for the four and seven ring substrates independent of the basis set used. The atop site was the preferred location on the four ring system while the bridge site was preferred on the seven ring system. The radical favored these sites because it contains a singly occupied molecular orbital to which electron density can be added stabilizing it on the surface. Rotation of both species on the seven ring surface revealed binding energies similar to those obtained for the starting geometries suggesting that free rotation occurs at each adsorption site. These results were used to rationalize the product distributions obtained from the TPD experiments and the decrease in background intensity of the EELS data after UV irradiation.
机译：通过理论和实验方法研究了二溴二氟甲烷CF 2 Br 2 在高度有序热解石墨（HOPG）上的吸附和光化学反应，作为发生在碳纳米管上的异质光化学反应的模型。对流层和平流层下部的碳质气溶胶。在约225–350 nm的宽波段紫外线（UV）照射之前和之后，使用程序升温脱附（TPD），X射线光电子能谱（XPS）和电子能量损失能谱（EELS）来表征单层覆盖的HOPG表面。 。在没有紫外线辐射的情况下，观察到CF 2 Br 2 的分子吸附和解吸（〜140 K），而单层的照射导致被吸附物发生光解离，从而产生CF 2 Br和Br原子。在这些实验中观察到的最普遍的反应是在TPD测量期间，大部分光解分子（〜70％）重组以重整CF 2 Br 2 。小通道揭示了Br 2 （〜6％）和C 2 F 4 Br 2 （〜 1.4％）也存在。估计的积分光解离截面（1.9×10 −19 cm 2 ）与根据TPD数据计算得到的截面（4.5×10 −19 < / super> cm 2 ）表明CF 2 Br 2 的光解离是由被吸附物直接光吸收引起的，而不是由表面。与气相色谱法和气相色谱法的结果相比，Br 2 和C 2 F 4 Br 2 的分布差异基质分离实验归因于一种或多种吸附物的可能的结构限制或减少的表面扩散，这是由于捕获在表层的光生物质的密度很高。使用聚类模型方法研究了CFPG 2 Br 2 和CF 2 Br在HOPG表面的优选吸附位和取向的理论研究。以获得对实验数据的更多了解。二阶Møller-Plesset微扰理论（MP2）的计算是使用6-31G *和6-31G（2d）基组以及由四个（py）或七个（）环多环芳烃组成的单层平板近似进行的碳氢化合物模拟HOPG表面。发现CF 2 Br 2 的首选吸附位点位于所考察的表面和基集的每种组合的六元环的中空位点。分子与表面之间的相互作用本质上是偶极的，从表面到被吸附物的电荷转移量很小，这是根据穆里肯种群分析确定的。对于CF 2 Br基团，独立于所使用的基础，对于四个和七个环底物发现了不同的优选吸附位点。在四环系统中，首选站点是首选位置，而在七环系统中，首选站点是首选站点。自由基偏爱这些位点，因为它包含一个单独占据的分子轨道，可以向其中添加电子密度以使其稳定在表面上。两种物质在七个环表面上的旋转均显示出与初始几何结构所获得的结合能相似的结合能，这表明自由旋转发生在每个吸附位点。这些结果用于合理化从TPD实验获得的产物分布以及紫外线照射后EELS数据背景强度的降低。

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