Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al2O3-Supported Rh Nanoparticles,The Journal of Physical Chemistry C

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Theoretical and Experimental Characterization of Adsorbed CO and NO on γ-Al2O3-Supported Rh Nanoparticles
The Journal of Physical Chemistry C ( IF 3.3 ) Pub Date : 2021-09-02 , DOI: 10.1021/acs.jpcc.1c05160
Alexander J. Hoffman ₁ , Chithra Asokan ₂ , Nicholas Gadinas ₂ , Pavlo Kravchenko ₁ , Andrew “Bean” Getsoian ₃ , Phillip Christopher ₂ , David Hibbitts ₁

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Rh active sites are critical for NO_x reduction 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 the in situ formation 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 Rh₂₀₁ near 1 ML. Similarly, NO* binds to threefold sites and saturates the Rh(111) surface at 0.67 ML but saturates the Rh₂₀₁ particle surface at 1.38 ML, indicating that more NO* binds than there are Rh_surf atoms. Moreover, the adlayers on the Rh₂₀₁ particle 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-Al₂O₃ show 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 Rh₂₀₁ particles 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.

中文翻译：

γ-Al2O3 负载的 Rh 纳米颗粒上吸附 CO 和 NO 的理论和实验表征

Rh 活性位点对于汽车三元催化剂中的NO _x还原至关重要。工业催化剂中使用的低 Rh 负载导致小纳米粒子和单原子 Rh 物质的混合物。这种活性位点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，缺乏氧化物负载的 Rh 纳米颗粒表面的曲率，这引发了关于周期性表面模型系统有效性的问题。在这里，我们将 DFT 与探针分子傅里叶变换红外 (FTIR) 光谱和合成的负载 Rh 催化剂的高分辨率扫描透射电子显微镜相结合，以确保免受单原子 Rh 物质的原位形成，以比较周期性和纳米粒子 DFT 模型，以描述 CO 和 NO 与支持的 Rh 纳米粒子的相互作用。我们专注于比较模型系统——Rh (111) 和 201 原子立方八面体 Rh 纳米颗粒（Rh ₂₀₁；直径约 1.7 nm）的行为——以解释 CO 和 NO 结合到具有平均颗粒的 Rh 纳米颗粒的行为直径约 2.6 nm。我们的 DFT 计算表明，CO* 在 Rh(111) 上占据三重和顶部模式的混合物，在 0.56 ML CO* (473 K，1 bar) 时饱和，而 CO*在接近 1 ML 时使 Rh ₂₀₁饱和。类似地，NO* 结合三重位点并使 Rh(111) 表面饱和，浓度为 0.67 ML，但使 Rh ₂₀₁饱和1.38 ML 的颗粒表面，表明结合的 NO* 比 Rh_冲浪原子多。此外，Rh ₂₀₁颗粒上的吸附层主要包含顶部结合的 CO*，颗粒边缘可能存在桥接 CO*，主要包含三重 NO*，桥接和顶部结合的 NO* 结合到边缘和角落。这些结合模式和更高的覆盖率是通过这些纳米粒子的曲率和表面金属 - 金属键的扩展（这两种键都不会发生在 Rh（111）上）实现的，它们共同允许吸附层横向松弛，减少内部应变。CO* 在 10 wt % Rh/γ-Al ₂ O ₃上的 FTIR 数据显示主要是顶部结合模式 (2067 cm ^–1 )，在桥附近有小宽峰 (1955 cm^–1 ) 和三重 (1865 cm ^–1 ) 区域。同时，NO* FTIR 光谱还显示顶部 (1820 cm ^–1 ) 和三重 (1685 cm ^–1 ) NO* 特征的混合物，在反应条件下观察到类似特征（5 mbar NO, 1 mbar CO, 478 K），表明 NO* 在催化过程中支配着 Rh 表面。Rh ₂₀₁吸附层的频率计算粒子产生的主频率更接近 FTIR 光谱中观察到的频率，并展示了覆盖率和偶极-偶极耦合如何影响振动频率和表面曲率。总之，这些结果表明，Rh 表面曲率改变了约 2.6 nm 直径的 Rh 纳米粒子的 NO* 和 CO* 的结构和光谱特性，这必须在 DFT 模型中准确反映。

更新日期：2021-09-16

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