First-principle density functional theory (DFT) calculations are used to calculate the most stable structures and heats of adsorption of trimethyl and triethyl phosphites on an Fe₃O₄ surface in order to understand their behavior as lubricant additives. Previous surface science studies of phosphate and phosphite esters on iron oxide surface have shown that they react by sequentially desorbing the corresponding alcohol and aldehyde in equimolar amounts. This implies that the reactions are limited by the rate of alkoxide removal, followed by a rapid reaction to form the alcohol and aldehyde. It is found that the trialkyl phosphites, dialkyl phosphite, and monoalkyl phosphite all adsorb with the phosphorus atoms bound to surface Fe³⁺ ions, with the alkoxy groups close to parallel to the surface. The heat of adsorption increases as the alkoxide groups are removed. The postulate that the rate-limiting step in the decomposition of the phosphate esters is the sequential removal of the alkoxide group is tested by plotting the experimental activation energy obtained from temperature-programed desorption experiments versus the corresponding heats of adsorption calculated by DFT. A linear dependence shows that the reaction obeys the so-called Evans–Polanyi relation, thereby confirming the above postulate. The slope of the plot is close to unity and thus implies that the structure of the transition state of the reaction resembles the product.

Graphic Abstract

中文翻译：

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密度泛函理论研究亚磷酸三甲酯和亚磷酸三乙酯在Fe 3 O 4上的吸附和反应

第一原理密度泛函理论（DFT）计算用于计算最稳定的结构和亚磷酸三甲酯和亚磷酸三乙酯在Fe ₃ O ₄表面上的吸附热，以了解它们作为润滑剂添加剂的行为。先前对氧化铁表面上的磷酸酯和亚磷酸酯的表面科学研究表明，它们通过依次解吸等摩尔量的相应醇和醛来反应。这意味着该反应受到醇盐去除速率的限制，随后迅速反应形成醇和醛。发现亚磷酸三烷基酯，亚磷酸二烷基酯和亚磷酸单烷基酯均吸附有结合在表面Fe ³⁺上的磷原子。离子，其烷氧基接近平行于表面。吸附热随着醇盐基团的去除而增加。通过绘制从程序升温脱附实验获得的实验活化能与DFT计算得出的相应吸附热的关系图，测试了磷酸酯分解中的限速步骤是依次除去烷氧基的假设。线性相关性表明该反应符合所谓的Evans-Polanyi关系，从而证实了上述假设。曲线的斜率接近于1，因此暗示反应的过渡态的结构类似于产物。

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