负载型纳米金催化剂的表面重排和蒸发动力学
Surface Rearrangement and Evaporation Kinetics of Supported Gold Nanoparticle Catalysts
摘要：
由负载型金属纳米颗粒组成的非均相催化剂通常从其大比例的欠配位表面位点获得优异的催化活性，所述表面位点促进反应物分子的吸附。同时，这些高能表面构型是不稳定的，导致纳米颗粒生长或降解，并最终失去催化活性。催化纳米颗粒的表面形态对催化活性、选择性以及降解速率至关重要，然而，众所周知，苛刻的反应条件会导致表面结构发生变化。尽管如此，有限的研究集中在理解纳米颗粒表面刻面和降解速率或机制之间的联系。在这里，我们研究了一个模型Au负载的催化剂系统在一定的温度范围内使用的组合\textit{in situ}透射电子显微镜，动力学蒙特卡罗模拟，和密度泛函理论计算，以建立一个原子的图片如何在表面结构和原子配位环境的变化导致转变的演变机制作为温度的函数。通过结合实验结果，产生直接观察的动态形状变化和粒子蒸发速率，与计算技术，这使得了解纳米粒子演变的基本热力学和动力学，我们说明了一个两步的演变机制，其中移动吸附原子形成通过解吸从低配位方面，随后蒸发掉的粒子表面。通过了解温度在表面扩散和蒸发之间竞争中的作用，我们能够展示单个原子运动如何导致颗粒尺度的形态变化，并合理解释为什么在几乎相同的纳米颗粒系统中颗粒之间的蒸发速率会有所不同。
英文摘要：
Heterogeneous catalysts consisting of supported metallic nanoparticles typically derive exceptional catalytic activity from their large proportion of under-coordinated surface sites which promote adsorption of reactant molecules. Simultaneously, these high energy surface configurations are unstable, leading to nanoparticle growth or degradation, and eventually a loss of catalytic activity. Surface morphology of catalytic nanoparticles is paramount to catalytic activity, selectivity, as well as degradation rates, however, it is well-known that harsh reaction conditions can cause the surface structure to change. Still, limited research has focused on understanding the link between nanoparticle surface facets and degradation rates or mechanisms. Here, we study a model Au supported catalyst system over a range of temperatures using a combination of \textit{in situ} Transmission Electron Microscopy, kinetic Monte Carlo simulations, and density functional theory calculations to establish an atomistic picture of how variations in surface structures and atomic coordination environments lead to shifting evolution mechanisms as a function of temperature. By combining experimental results, which yield direct observation of dynamic shape changes and particle evaporation rates, with computational techniques, which enable understanding the fundamental thermodynamics and kinetics of nanoparticle evolution, we illustrate a two-step evolution mechanism in which mobile adatoms form through desorption from low-coordination facets and subsequently evaporate off the particle surface. By understanding the role of temperature in the competition between surface diffusion and evaporation, we are able to show how individual atomic movements lead to particle-scale morphological changes, and rationalize why evaporation rates vary between particles in a system of nearly identical nanoparticles.
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