Oxygen Exchange Rates At Aluminum Polynuclear Complexes: Models For Mineral Surfaces

凯西和菲利普斯-0101246本项目的主要目标是确定大型粘土状铝复合物和水的结构位点之间的单个氧交换速率定律。 铝纳米团簇被选择为具有与土壤矿物表面共同的结构元素和类似的质子表面电荷密度。 与大量的矿物与流体反应的研究不同，我们的实验产生的信息交换个别氧与散装溶液的规模，是适合于计算机模拟的速率定律。 这种分子尺度的研究需要重新考虑目前对水中矿物质反应机制的许多想法。 例如：1）矿物溶解的重要氧显然是分子深处高度配位的氧。 2)质子转移到桥接羟基，这之前交换的氧，通常发生在内部，而不是从溶液中的质子吸收。 3)表面结合水分子的解离控制着许多吸附反应的速率，但在个别位点的速率在很大程度上是未知的。 我们发现，这些大的polyoxocations的溶剂分解率落入相同的范围内溶解的铝单体，这表明粘土和质子化氢氧化铝矿物的边缘的速率是相似的。 4)氧交换速率对金属取代高度敏感，其方式无法从批量研究或通过X射线晶体学确定的键长变化中预期。

Casey and PhillipsEAR-0101246The main goal of this project is to determine rate laws for exchange of individual oxygens between structural sites in large clay-like aluminum complexes and water. The aluminum nanoclusters are chosen to have structural elements in common with soil mineral surfaces and a similar surface charge density of protons. Unlike bulk studies of mineral reactions with fluid, our experiments yield information about rate laws for exchange of individual oxygens with the bulk solution at a scale that is suitable for computer simulation. This molecular-scale research requires reconsideration of much current thinking about mechanisms for mineral reaction in water. For example: 1) The important oxygens for mineral dissolution are apparently highly coordinated oxygens deeper in the molecule. 2) Proton transfer to the bridging hydroxyls, which precedes exchange of the oxygens, commonly takes place internally and not by proton uptake from the solution. 3) Dissociation of a surface-bound water molecule controls the rates of many adsorption reactions, yet the rates at individual sites are largely unknown. We find that these solvolysis rates for large polyoxocations fall into the same range as dissolved aluminum monomers, suggesting that the rates for edges of clays and protonated aluminum hydroxide minerals are similar. 4) The rates of oxygen exchange are highly sensitive to metal substitutions, in ways that could not have been anticipated from bulk studies or by changes in from bond lengths determined by X-ray crystallography.