将负载的 Ru/γ-Al2O3 催化剂的粒径和形状与 NH3 分解活性相关联,Journal of the American Chemical Society

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将负载的 Ru/γ-Al2O3 催化剂的粒径和形状与 NH3 分解活性相关联
Journal of the American Chemical Society ( IF 14.4 ) Pub Date : 2009-09-02 , DOI: 10.1021/ja902587k
Ayman M. Karim ₁ , Vinay Prasad ₁ , Giannis Mpourmpakis ₁ , William W. Lonergan ₁ , Anatoly I. Frenkel ₁ , Jingguang G. Chen ₁ , Dionisios G. Vlachos ₁

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虽然已知氨在 Ru 上的合成和分解是结构敏感反应，但颗粒形状对控制颗粒尺寸的影响尚不清楚，从而产生最大周转频率 (TOF)。通过控制催化剂预处理条件，我们改变了负载的 Ru/γ-Al(2)O(3) 催化剂的粒度和形状。Ru 粒子形状是通过结合显微镜、化学吸附和扩展 X 射线吸收精细结构 (EXAFS) 技术重建的。我们表明，通过适当的预处理，颗粒形状可以从圆形的较小的颗粒变为细长的扁平的用于较大的颗粒。密度泛函理论计算表明，煅烧最有可能导致平面结构。我们第一次展示活动（此处为 B(5)）站点的数量高度依赖于粒子形状，并且随着扁平纳米粒子的粒径增加至 7 nm。与半球形纳米粒子的大约 1.8-3 nm 相比，最大 TOF（基于暴露的 Ru 原子总数）和活性 (B(5)) 位点的数量发生在大约 7 nm 的细长纳米粒子处。构建了完整的、基于第一性原理的微动力学模型，首次定量描述了不同粒径和形状对Ru活性的影响，并为表征结果提供了进一步的支持。在非常小的纳米粒子中，粒度多分散性（由于存在较大粒子）似乎是观察到的活性的原因。

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Correlating Particle Size and Shape of Supported Ru/γ-Al2O3Catalysts with NH3Decomposition Activity

While ammonia synthesis and decomposition on Ru are known to be structure-sensitive reactions, the effect of particle shape on controlling the particle size giving maximum turnover frequency (TOF) is not understood. By controlling the catalyst pretreatment conditions, we have varied the particle size and shape of supported Ru/gamma-Al(2)O(3) catalysts. The Ru particle shape was reconstructed by combining microscopy, chemisorption, and extended X-ray absorption fine structure (EXAFS) techniques. We show that the particle shape can change from a round one, for smaller particles, to an elongated, flat one, for larger particles, with suitable pretreatment. Density functional theory calculations suggest that the calcination most likely leads to planar structures. We show for the first time that the number of active (here B(5)) sites is highly dependent on particle shape and increases with particle size up to 7 nm for flat nanoparticles. The maximum TOF (based on total exposed Ru atoms) and number of active (B(5)) sites occur at approximately 7 nm for elongated nanoparticles compared to at approximately 1.8-3 nm for hemispherical nanoparticles. A complete, first-principles based microkinetic model is constructed that can quantitatively describe for the first time the effect of varying particle size and shape on Ru activity and provide further support of the characterization results. In very small nanoparticles, particle size polydispersity (due to the presence of larger particles) appears to be responsible for the observed activity.

更新日期：2009-09-02

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