Adsorption of Small Hydrocarbons on the Three-Fold PdGa Surfaces: The Road to Selective Hydrogenation

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• 作者：Jan Prinz ; Carlo A. Pignedoli ; Quirin S. St枚ckl ; Marc Armbr眉ster ; Harald Brune ; Oliver Gr枚ning ; Roland Widmer ; Daniele Passerone
• 刊名：Journal of the American Chemical Society
• 出版年：2014
• 出版时间：August 20, 2014
• 年：2014
• 卷：136
• 期：33
• 页码：11792-11798
• 全文大小：393K
• ISSN：1520-5126

文摘

Intermetallic compounds are a promising class of materials as stable and selective heterogeneous catalysts. Here, the (111) and (鈭?鈥?鈥?) single crystal surfaces of the PdGa intermetallic compound were studied as model catalysts with regard to the selective hydrogenation of acetylene (C₂H₂) to ethylene (C₂H₄). The distinct atomic surface structures exhibit isolated active centers of single atomic and three atomic Pd ensembles, respectively. For the two prototypal model catalyst surfaces, the adsorption sites and configurations for hydrogen (H₂), acetylene, and ethylene were investigated by combining scanning tunneling microscopy, temperature-programmed desorption, and ab initio modeling. The topmost Pd surface atoms provide the preferred adsorption sites for all studied molecules. The structural difference of the Pd ensembles has a significant influence on the adsorption energy and configuration of C₂H₂, while the influence of the ensemble structure is weak for C₂H₄ and H₂ adsorption. To approach the question of catalytic performance, we simulated the reaction pathways for the heterogeneous catalytic hydrogenation of acetylene on the two surfaces by means of density functional theory. Due to the geometrical separation of the Pd sites on the surfaces, the steric approach of the reactants (H and C₂H_x) was found to be of importance to the energetics of the reaction. The presented study gives a direct comparison of binding properties of catalytic Pd on-top sites vs three-fold Pd hollow sites and is therefore of major relevance to the knowledge-based design of highly selective hydrogenation catalysts.