Mechanism of hydroxyl radical generation from a silica surface: Molecular orbital calculations
Mechanism of hydroxyl radical generation from a silica surface: Molecular orbital calculations
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DOI：
10.1021/jp0543025
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发表时间：
2005-11-24
影响因子：
3.3
通讯作者：
Kubicki, JD
中科院分区：
文献类型：
--
作者：
Narayanasamy, J;Kubicki, JD
通过量子力学计算研究了 H2O 分子与破裂二氧化硅表面簇模型的相互作用。对代表二氧化硅表面均裂（相当于 Si 中心点和相当于 SiO 中心点）的两个簇和代表二氧化硅表面异裂（相当于 Si+ 和相当于 Si-O-）的两个簇进行了建模。计算了这些二氧化硅表面与水反应的反应物和产物的振动频率，并与实验结果进行了比较。对模型离子态和自由基态的吉布斯自由能和势能进行了比较，并且根据计算方法，预计自由基位点对将更加稳定约-70至-85 kJ/mol。这些计算表明，当二氧化硅在真空中断裂时，有利于均裂。研究了这四个模型表面位点与 H2O 相互作用的反应途径。 H2O 与 SiO 中心点的反应预计会生成 OH 中心点。还计算了这些反应的速率常数，并预测了相当于 SiO 中心点 + H2O -> 相当于 SiOH + OH 中心点的反应的快速平衡。还预测了上述与 H2O 反应中平衡时相当于 SiO 中心点位点的有限数量的稳定性，这意味着如果断裂键的位点不重新聚合形成硅氧烷基团，则二氧化硅表面产生 OH 中心点自由基的长期能力。
The interaction of an H2O molecule with cluster models of fractured silica surfaces was studied by means of quantum mechanical calculations. Two clusters representing homolytic cleavage (equivalent to Si-center dot and equivalent to SiO center dot) and two representing heterolytic cleavage (equivalent to Si+ and equivalent to Si-O-) of silica surfaces were modeled. Vibrational frequencies of the reactants and products of these silica surfaces reacting with H2O have been calculated and compare favorably with experiment. Comparisons of the Gibbs free and potential energies for the model ionic and radical states were made, and the radical pair of sites was predicted to be more stable by approximately -70 to -85 kJ/mol, depending on the computational methodology. These calculations suggest that when silica is fractured in a vacuum homolytic cleavage is favored. Reaction pathways were investigated for these four model surface sites interacting with H2O. The reaction of H2O with equivalent to SiO center dot was predicted to generate OH center dot. Rate constants for these reactions were also calculated and predict a rapid equibrium for the reaction equivalent to SiO center dot + H2O -> equivalent to SiOH + OH center dot.. Stability of a finite number of equivalent to SiO center dot sites at equilibrium in the above reaction with H2O was also predicted, which implies a long-term ability of silica surfaces to produce OH center dot radicals if the sites of the broken bonds do not repolymerize to form siloxane groups.