Size and Site Dependence of the Catalytic Activity of Iridium Clusters toward Ethane Dehydrogenation

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• 作者：Yingbin Ge ; Hao Jiang ; Russell Kato ; Prasuna Gummagatta
• 刊名：Journal of Physical Chemistry A
• 出版年：2016
• 出版时间：December 1, 2016
• 年：2016
• 卷：120
• 期：47
• 页码：9500-9508
• 全文大小：654K
• ISSN：1520-5215

文摘

This research focuses on optimizing transition metal nanocatalyst immobilization and activity to enhance ethane dehydrogenation. Ethane dehydrogenation, catalyzed by thermally stable Ir_n (n = 8, 12, 18) atomic clusters that exhibit a cuboid structure, was studied using the B3LYP method with triple-ζ basis sets. Relativistic effects and dispersion corrections were included in the calculations. In the dehydrogenation reaction Ir_n + C₂H₆ → H–Ir_n–C₂H₅ → (H)₂–Ir_n–C₂H₄, the first H-elimination is the rate-limiting step, primarily because the reaction releases sufficient heat to facilitate the second H-elimination. The catalytic activity of the Ir clusters strongly depends on the Ir cluster size and the specific catalytic site. Cubic Ir₈ is the least reactive toward H-elimination in ethane: Ir₈ + C₂H₆ → H–Ir₈–C₂H₅ has a large (65 kJ/mol) energy barrier, whereas Ir₁₂ (3 × 2 × 2 cuboid) and Ir₁₈ (3 × 3 × 2 cuboid) lower this energy barrier to 22 and 3 kJ/mol, respectively. The site dependence is as prominent as the size effect. For example, the energy barrier for the Ir₁₈ + C₂H₆ → H–Ir₁₈–C₂H₅ reaction is 3, 48, and 71 kJ/mol at the corner, edge, or face-center sites of the Ir₁₈ cuboid, respectively. Energy release due to Ir cluster insertion into an ethane C–H bond facilitates hydrogen migration on the Ir cluster surface, and the second H-elimination of ethane. In an oxygen-rich environment, oxygen molecules may be absorbed on the Ir cluster surface. The oxygen atoms bonded to the Ir cluster surface may slightly increase the energy barrier for H-elimination in ethane. However, the adsorption of oxygen and its reaction with H atoms on the Ir cluster releases sufficient heat to yield an overall thermodynamically favored reaction: Ir_n + C₂H₆ + ¹/₂O₂ → Ir_n + C₂H₄ + H₂O. These results will be useful toward reducing the energy cost of ethane dehydrogenation in industry.