Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al 2 O 3 -Supported Rh Nanoparticles
Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al 2 O 3 -Supported Rh Nanoparticles
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γ-Al 2 O 3 负载的 Rh 纳米粒子吸附 CO 和 NO 的理论和实验表征
DOI：
10.1021/acs.jpcc.1c05160
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发表时间：
2021
期刊：
影响因子：
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通讯作者：
Hibbitts, David
中科院分区：
文献类型：
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作者：
Hoffman, Alexander J.;Asokan, Chithra;Gadinas, Nicholas;Kravchenko, Pavlo;Getsoian, Andrew “Bean”;Christopher, Phillip;Hibbitts, David
Rh活性中心是汽车三效催化剂中NOx还原的关键。工业催化剂中使用的低Rh负载导致小纳米颗粒和单原子Rh物质的混合物。这种活性位点的异质性使表征和反应性的解释复杂化，使得结构-功能关系的发展具有挑战性。Rh催化剂的密度泛函理论（DFT）研究通常采用平坦的周期性表面，其缺乏氧化物支撑的Rh纳米颗粒表面的曲率，从而引起关于周期性表面模型系统的有效性的问题。在这里，我们结合联合收割机DFT与探针分子傅里叶变换红外（FTIR）光谱和高分辨率扫描透射电子显微镜的负载铑催化剂合成，以确保对thesituformation的单原子铑物种比较周期性和纳米颗粒DFT模型描述CO和NO与负载铑纳米颗粒的相互作用。我们着重比较模型系统Rh（111）和201个原子的立方八面体Rh纳米颗粒（Rh 201;直径约1.7 nm）的行为，以解释CO和NO与平均粒径约2.6 nm的Rh纳米颗粒结合的行为。我们的DFT计算表明，CO* 占据Rh（111）上的三重和顶部模式的混合物，饱和在0.56 ML CO*（473 K，1巴），而CO* 饱和Rh 201接近1 ML。类似地，NO* 结合到三重位点，并在0.67 ML处饱和Rh（111）表面，但在1.38 ML处饱和Rh 201颗粒表面，表明结合的NO* 多于Rh表面活性剂。此外，Rh 201颗粒上的吸附层主要含有atop结合的CO*，其中桥CO* 可能在颗粒边缘上，并且主要含有三重NO*，其中桥和atop结合的NO* 结合到边缘和角落。这些结合模式和更高的覆盖率是由这些纳米颗粒的曲率和表面金属-金属键的膨胀（两者都不能发生在Rh（111）上）共同允许吸附层横向松弛，减少内部应变。CO* 在10wt%Rh/γ-Al_2O_3上的红外光谱数据显示，CO* 主要以顶部结合模式（2067 cm ~（-1））为主，在桥（1955 cm ~（-1））和三重（1865 cm ~（-1））区域附近有小而宽的峰。同时，NO* FTIR光谱还显示了顶部（1820 cm-1）和三重（1685 cm-1）NO* 特征的混合物，在反应条件下（5 mbar NO，1 mbar CO，478 K）观察到类似的特征，表明NO* 在催化过程中占主导地位的Rh表面。频率计算这些adlayers的Rh 201粒子产生的主频率，更接近于在FTIR光谱中观察到的，并演示了如何覆盖和偶极偶极耦合影响振动频率与表面曲率。综上所述，这些结果表明，Rh表面曲率改变的结构和光谱特性的NO* 和CO* 的Rh纳米粒子的直径为2.6 nm，这必须准确地反映在DFT模型。
Rh active sites are critical for NOxreduction in automotive three-way catalysts. Low Rh loadings used in industrial catalysts lead to a mixture of small nanoparticles and single-atom Rh species. This active-site heterogeneity complicates the interpretation of characterization and reactivity, making the development of structure–function relationships challenging. Density functional theory (DFT) investigations of Rh catalysts often employ flat, periodic surfaces, which lack the curvature of oxide-supported Rh nanoparticle surfaces, raising questions about the validity of periodic surface model systems. Here, we combine DFT with probe molecule Fourier transform infrared (FTIR) spectroscopy and high-resolution scanning transmission electron microscopy of supported Rh catalysts synthesized to insure against thein situformation of single-atom Rh species to compare periodic and nanoparticle DFT models for describing the interaction of CO and NO with supported Rh nanoparticles. We focus on comparing the behavior of model systems—Rh(111) and a 201-atom cubo-octahedral Rh nanoparticle (Rh201; ∼1.7 nm diameter)—to explain the behavior of CO and NO bound to Rh nanoparticles with an average particle diameter of ∼2.6 nm. Our DFT calculations indicate that CO* occupies a mixture of threefold and atop modes on Rh(111), saturating at 0.56 ML CO* (473 K, 1 bar), while CO* saturates Rh201near 1 ML. Similarly, NO* binds to threefold sites and saturates the Rh(111) surface at 0.67 ML but saturates the Rh201particle surface at 1.38 ML, indicating that more NO* binds than there are Rhsurfatoms. Moreover, the adlayers on the Rh201particle contain predominantly atop-bound CO*, with bridge CO* possible on particle edges and predominantly threefold NO* with bridge- and atop-bound NO* bound to edges and corners. These binding modes and higher coverages are made possible by the curvature of these nanoparticles and by the expansion of surface metal–metal bonds—neither of which can occur on Rh(111)—which together permit the adlayer to laterally relax, reducing internal strain. FTIR data for CO* on 10 wt % Rh/γ-Al2O3show predominantly atop binding modes (2067 cm–1) with small broad peaks near bridge (1955 cm–1) and threefold (1865 cm–1) regions. Meanwhile, NO* FTIR spectroscopy also shows a mixture of atop (1820 cm–1) and threefold (1685 cm–1) NO* features, with similar features observed at reaction conditions (5 mbar NO, 1 mbar CO, 478 K), indicating that NO* dominates Rh surfaces during catalysis. Frequency calculations on these adlayers of Rh201particles yield dominant frequencies that more closely resemble those observed in FTIR spectra and demonstrate how coverage and dipole–dipole coupling affect vibrational frequencies with surface curvature. Taken together, these results indicate that the Rh surface curvature alters the structure and spectral characteristics of NO* and CO* for Rh nanoparticles of ∼2.6 nm diameter, which must be accurately reflected in DFT models.