Elucidation of Active Sites for CH4 Catalytic Oxidation over Pd/CeO2 Via Tailoring Metal–Support Interactions,ACS Catalysis

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Elucidation of Active Sites for CH4 Catalytic Oxidation over Pd/CeO2 Via Tailoring Metal–Support Interactions
ACS Catalysis ( IF 11.3 ) Pub Date : 2021-04-26 , DOI: 10.1021/acscatal.1c00839
Shiyuan Chen ₁ , Songda Li ₁ , Ruiyang You ₁ , Ziyi Guo ₂ , Fei Wang ₁ , Guanxing Li ₁ , Wentao Yuan ₁ , Beien Zhu _{3,

4} , Yi Gao _{3,

4} , Ze Zhang ₁ , Hangsheng Yang ₁ , Yong Wang ₁

Affiliation

Pd/CeO₂ has attracted great attention owing to its unique activity for methane catalytic oxidation; however, the active sites for CH₄ catalytic oxidation still remain elusive, which affects the comprehensive understanding of the catalytic mechanism. In this work, the structures of PdO_x nanoparticles (NPs) loaded on octahedrons, cubes, and rods of nanocrystal CeO₂ supports were systematically studied by Cs-corrected HRTEM/STEM, XPS, and Raman spectroscopy. Our results indicate that the Pd species on CeO₂ supports are morphology-dependent: PdO NPs (Pd²⁺) on octahedrons, PdO_x (x = 1–2) clusters (1–2 nm) on cubes, and dispersed Pd⁴⁺ ions on the CeO₂ rods. Additionally, the chemical states of Pd can be tuned in oxidizing/reducing atmospheres via interactions between Pd and CeO₂. Detailed studies reveal that the Pd²⁺ species are the active centers for the catalytic oxidation of methane. The activity of Pd⁰ could be ascribed to Pd²⁺ produced through the gradual oxidation of Pd⁰ during the CH₄ oxidation. Further, Pd⁴⁺ in the CeO₂ lattice is inactive for CH₄ oxidation. In situ Fourier transform infrared spectroscopy results suggest that the mechanism of CH₄ oxidation reaction on PdO_x/CeO₂ follows the Mars–van Krevelen mechanism, and adsorbed CO can be produced in CH₄ decomposition over Pd²⁺ in the absence of gas-phase oxygen. As revealed by density functional theory calculations, the incomplete coordination of Pd²⁺ ions and adjacent oxygen atoms has excellent activity in cracking the C–H bond of CH₄, which leads to high methane oxidation ability.

中文翻译：

通过定制金属-载体相互作用阐明Pd / CeO _2上CH ₄催化氧化的活性位点

Pd / CeO ₂由于具有独特的甲烷催化氧化活性而备受关注。但是，CH ₄催化氧化的活性位仍然难以捉摸，这影响了对催化机理的全面理解。在这项工作中，通过Cs校正的HRTEM / STEM，XPS和拉曼光谱系统地研究了负载在八面体，立方体和纳米晶CeO ₂载体棒上的PdO _x纳米颗粒（NP）的结构。我们的结果表明，CeO ₂载体上的Pd种类取决于形态：八面体上的PdO NP（Pd ²⁺），立方体上的PdO _x（x = 1-2）簇（1-2 nm）和分散的PdCeO ₂棒上的⁴⁺离子。另外，可以通过Pd和CeO ₂之间的相互作用在氧化/还原气氛中调节Pd的化学状态。详细的研究表明，Pd ²⁺物种是甲烷催化氧化的活性中心。Pd ⁰的活性可以归因于在CH ₄氧化过程中Pd ⁰逐渐氧化产生的Pd ²⁺。另外，CeO ₂晶格中的Pd ⁴⁺对于CH ₄氧化是惰性的。原位傅里叶变换红外光谱结果表明CH ₄的机理PdO _x / CeO ₂上的氧化反应遵循Mars–van Krevelen机理，在不存在气相氧的情况下，在Pd ^2+上CH ₄分解时可生成吸附的CO 。正如密度泛函理论计算所揭示的那样，Pd ²⁺离子与邻近的氧原子的不完全配位在裂解CH ₄的C–H键方面具有出色的活性，从而具有很高的甲烷氧化能力。

更新日期：2021-05-07

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