Bridging oxygen as a site for proton adsorption on the vitreous silica surface.
Bridging oxygen as a site for proton adsorption on the vitreous silica surface.
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桥接氧作为质子吸附在玻璃二氧化硅表面上的位点。
DOI：
10.1063/1.3205946
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发表时间：
2009
期刊：
影响因子：
--
通讯作者：
S. Garofalini
中科院分区：
文献类型：
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作者：
Glenn K. Lockwood;S. Garofalini
分子动力学计算机模拟被用来研究质子化的桥接氧（Si-O-Si）的网站上存在的玻璃状二氧化硅表面上接触水使用解离水电位。与基于无约束分子类似物（如H（7）Si（2）O（7）（+）分子）的第一性原理计算相反，当嵌入固体表面时，相邻SiO（4）四面体的非常有限的柔性意味着对质子吸附的几何响应相对较小，需要易于吸附的位点。模拟结果表明，质子化的桥接氧发生在预置网站的桥接角在125度-135度的范围内，远低于散装二氧化硅平均约150度，符合各种从头计算，和这样的网站的一小部分是存在于所有的环尺寸。在模拟中观察到的各种角度的干桥和质子化桥之间的能量差与在玻璃质二氧化硅表面中遇到的整个桥接角度范围内的量子计算完全一致。桥连角接近130度的那些位点更稳定地支持吸附的质子，导致质子保持吸附更长的时间。玻璃质二氧化硅在所有环尺寸上具有必要的角应变分布，以允许质子吸附到表面处的桥接氧上，形成酸性表面基团，其充当质子在表面附近转移的理想中间步骤。除了水合氢离子的形成和水辅助的质子在液体中的转移，质子可以快速移动通过应变桥，倾向于瞬态质子吸附的水-二氧化硅界面。因此，在二氧化硅表面上的任何给定位置处的过量质子可以根据局部环境通过水辅助或应变桥辅助扩散来移动。这样做的结果将是净移民比只可能有一种机制时更快。这些模拟结果表明，进行大尺寸和时间尺度的模拟的结构异质性玻璃质二氧化硅暴露于水来描述质子在水和二氧化硅表面之间的界面处的传输的重要性。
Molecular dynamics computer simulations were used to study the protonation of bridging oxygen (Si-O-Si) sites present on the vitreous silica surface in contact with water using a dissociative water potential. In contrast to first-principles calculations based on unconstrained molecular analogs, such as H(7)Si(2)O(7)(+) molecules, the very limited flexibility of neighboring SiO(4) tetrahedra when embedded in a solid surface means that there is a relatively minor geometric response to proton adsorption, requiring sites predisposed to adsorption. Simulation results indicate that protonation of bridging oxygen occurs at predisposed sites with bridging angles in the 125 degrees-135 degrees range, well below the bulk silica mean of approximately 150 degrees, consistent with various ab initio calculations, and that a small fraction of such sites are present in all ring sizes. The energy differences between dry and protonated bridges at various angles observed in the simulations coincide completely with quantum calculations over the entire range of bridging angles encountered in the vitreous silica surface. Those sites with bridging angles near 130 degrees support adsorbed protons more stably, resulting in the proton remaining adsorbed for longer periods of time. Vitreous silica has the necessary distribution of angular strain over all ring sizes to allow protons to adsorb onto bridging oxygen at the surface, forming acidic surface groups that serve as ideal intermediate steps in proton transfer near the surface. In addition to hydronium formation and water-assisted proton transfer in the liquid, protons can rapidly move across the water-silica interface via strained bridges that are predisposed to transient proton adsorption. Thus, an excess proton at any given location on a silica surface can move by either water-assisted or strained bridge-assisted diffusion depending on the local environment. The result of this would be net migration that is faster than it would be if only one mechanism is possible. These simulation results indicate the importance of performing large size and time scale simulations of the structurally heterogeneous vitreous silica exposed to water to describe proton transport at the interface between water and the silica surface.