arXivDaily arXiv每日学术速递 周一至周五更新
arXiv 2008.08765physics.chem-phcond-mat.mtrl-sci
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催化剂最佳电子构型的动力学表达式:以氨分解为例

Kinetic Expression for Optimal Catalyst Electronic Configuration: The Case of Ammonia Decomposition

Nigora Turaeva, Rebecca Fushimi, Gregory Yablonsky

摘要:

提出了一种新的氨分解稳态动力学模型,并结合金属催化剂的电子性质进行了分析。该模型基于经典的Temkin-Ertl机理,并根据Wolkenstein电子理论进行了修正,通过引入催化剂的自由电子来改变吸附物种的化学性质。沃尔肯斯坦最初的理论只适用于半导体,但通过包括d带模型,电子理论可以扩展到金属。对于简化的和完整的反应机制,包括电子步骤,我们提出了一个稳态速率方程的费米能级的金属上的依赖创建一个火山形的依赖。根据动力学模型,增加催化剂的费米能级,接近与吸附的氮分子的反键状态,将增加中性氮分子的分数,并增强其脱附。同时,氨分子的强化学吸附通过在吸附物键中的额外的自由催化剂电子的参与而容易地进行。结果,反应速率提高并达到其最大值。接近与氨分子的反键状态的催化剂的进一步增加的费米能级将导致带负电荷的氨分子的更小部分和更少的脱氢。同时,中性氮分子的解吸发生而没有损害。结果,反应速率降低。详细的动力学模型进行了比较,最近的实验测量的氨分解铁,钴和CoFe催化剂。

英文摘要:

A new steady-state kinetic model of ammonia decomposition is presented and analyzed regarding the electronic properties of metal catalysts. The model is based on the classical Temkin-Ertl mechanism and modified in accordance with Wolkenstein electronic theory by implementing participation of free electrons of the catalyst to change the chemical nature of adsorbed species. Wolkenstein original theory only applied to semiconductors but by including the d-band model, the electronic theory can be extended to metals. For both simplified and full reaction mechanisms, including electronic steps, we present a steady-state rate equation where the dependence on the Fermi level of the metal creates a volcano-shaped dependence. According to the kinetic model, an increasing Fermi level of the catalyst, that approaching the antibonding state with adsorbed nitrogen molecules, will increase the fraction of neutral nitrogen molecules and enhance their the desorption. Concurrently, strong chemisorption of ammonia molecules proceeds easily through participation of additional free catalyst electrons in the adsorbate bond. As a result, the reaction rate is enhanced and reaches its maximum value. A further increasing Fermi level of the catalyst that approaches the antibonding state with ammonia molecules will result in a smaller fraction of negatively charged ammonia molecules and less dehydrogenation. Concurrently, the desorption of neutral nitrogen molecules occurs without impairment. As a result, the reaction rate decreases. The detailed kinetic model is compared to recent experimental measurements of ammonia decomposition on iron, cobalt and CoFe bimetallic catalysts.

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