[论文解读] Coking-Resistant Sub-Nano Dehydrogenation Catalysts: Pt$_n$Sn$_x$/SiO$_2$ (n = 4, 7)

Timothy J. Gorey, Borna Zandkarimi+4 more|arXiv (Cornell University)|Mar 10, 2020

Catalysis and Oxidation Reactions被引用 4

一句话总结

本研究提出了一种PtₙSnₓ/SiO₂（n = 4, 7）亚纳米团簇催化剂，通过选择性H₂/SnCl₄沉积法将Pt团簇与Sn合金化，有效抑制焦化和烧结现象，适用于烃类脱氢反应。该催化剂通过抑制双σ-乙烯吸附态及脱氢路径，降低乙烷转化为乙烯的能垒，同时消除碳沉积与团簇聚集。

ABSTRACT

We present a combined experimental/theoretical study of Pt<sub><i>n</i></sub>/SiO<sub>2</sub> and Pt<sub><i>n</i></sub>Sn<sub><i>x</i></sub>/SiO<sub>2</sub> (<i>n</i> = 4, 7) model catalysts for the endothermic dehydrogenation of hydrocarbons, using the ethylene intermediate as a model reactant. Mass-selected Pt<sub><i>n</i></sub> clusters were deposited onto amorphous SiO<sub>2</sub>/Si­(100) to make the Pt<sub><i>n</i></sub>SiO<sub>2</sub> model catalysts. To produce Pt<sub><i>n</i></sub>Sn<sub><i>x</i></sub> clusters, size-selected Pt<sub><i>n</i></sub> was used to seed selective deposition of Sn on Pt via a self-limiting H<sub>2</sub>/SnCl<sub>4</sub>/H<sub>2</sub> reaction sequence. Model catalysts were analyzed using C<sub>2</sub>D<sub>4</sub> and CO temperature-programmed desorption (TPD), low-energy ion scattering (ISS), X-ray photoelectron spectroscopy (XPS), plane wave density functional theory (DFT) global optimization combined with a statistical mechanical description of the catalytic interface, and a DFT mechanistic study. Supported pure Pt<sub><i>n</i></sub> clusters are found to be highly active toward dehydrogenation of C<sub>2</sub>D<sub>4</sub>, quickly deactivating due to a combination of carbon deposition and sintering, resulting in loss of accessible Pt sites. Addition of Sn to Pt<sub><i>n</i></sub> clusters results in the complete suppression of C<sub>2</sub>D<sub>4</sub> dehydrogenation and carbon deposition and also stabilizes the clusters against thermal sintering. Theory shows that both systems have thermal access to a multitude of cluster structures and adsorbate configurations that form a statistical ensemble. While Pt<sub>4</sub>/SiO<sub>2</sub> clusters bind ethylene in both di-σ- and π-bonded configurations, Pt<sub>4</sub>Sn<sub>3</sub>/SiO<sub>2</sub> binds C<sub>2</sub>H<sub>4</sub> only in the π mode, with di-σ bonding suppressed by a combination of electronic and geometric features of the PtSn clusters. Dehydrogenation reaction profiles on the accessible cluster isomers were calculated using the climbing image nudged elastic band (CI-NEB) method. Dehydrogenation of di-σ-bound ethylene is computed to dominate and is suppressed by Sn addition, in agreement with the experiments. DFT indicates that, after Sn alloying, the barrier for ethane-to-ethylene conversion is lower than that for unwanted ethylene dehydrogenation.

研究动机与目标

• 开发抗焦化亚纳米Pt基催化剂，用于吸热烃类脱氢反应。
• 阐明Sn合金化如何改变Pt团簇上乙烯的吸附几何构型与反应性。
• 在反应条件下稳定Pt团簇，防止热烧结与碳沉积。
• 阐明Pt-Sn团簇中催化选择性与稳定性增强的电子与几何起源。
• 将实验观测结果与团簇动态行为及反应路径的DFT模拟相衔接。

提出的方法

• 通过质量选择法将Pt₄与Pt₇团簇沉积于非晶态SiO₂/Si(100)基底上，构建模型催化剂。
• 采用自限制性H₂/SnCl₄反应序列选择性地将Sn沉积于预形成的Pt团簇上，形成PtₙSnₓ团簇。
• 程序升温脱附（TPD）结合C₂D₄与CO，探测表面反应性与吸附态。
• 低能离子散射（ISS）与X射线光电子能谱（XPS）表征表面组成与氧化态。
• 采用平面波DFT全局优化方法识别稳定团簇异构体与吸附物种构型。
• 利用爬坡图像弹性带法（CI-NEB）计算乙烯脱氢路径的反应能垒。

实验结果

研究问题

• RQ1 Sn合金化如何改变亚纳米Pt团簇上乙烯的吸附几何构型？
• RQ2 Sn在脱氢过程中如何抑制碳沉积与团簇烧结？
• RQ3 PtₙSnₓ团簇中的电子与几何效应如何影响乙烷转化为乙烯与乙烯脱氢路径的相对能垒？
• RQ4 团簇动态行为与多种异构体结构在多大程度上影响催化性能？
• RQ5 DFT模拟能否准确预测Pt-Sn体系中双σ键合与焦化路径的抑制？

主要发现

• 纯Pt₄/SiO₂与Pt₇/SiO₂团簇在C₂D₄脱氢过程中因碳沉积与烧结而迅速失活。
• Pt₄Sn₃/SiO₂团簇完全抑制了乙烯脱氢与碳沉积，未检测到任何焦化现象。
• Sn合金化有效稳定团簇，防止热烧结，维持活性位点的分散性。
• DFT计算表明，Pt₄Sn₃/SiO₂仅以π-模式吸附乙烯，双σ键合被电子与几何效应所抑制。
• 在Pt₄Sn₃/SiO₂上，乙烷转化为乙烯的反应能垒低于乙烯脱氢路径，有利于目标产物的生成。
• 热力学可及的多种团簇异构体与吸附物种构型形成统计系综，Sn使主导反应路径远离焦化路径。