Oxidation of Magnesia-Supported Pd30 Nanoclusters and Catalyzed CO Combustion: Size-Selected Experiments and First-Principles Theory

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• 作者：Bokwon Yoon ; Uzi Landman ; Vahideh Habibpour ; Chris Harding ; Sebastian Kunz ; Ueli Heiz ; Michael Moseler ; Michael Walter
• 刊名：The Journal of Physical Chemistry C
• 出版年：2012
• 出版时间：May 3, 2012
• 年：2012
• 卷：116
• 期：17
• 页码：9594-9607
• 全文大小：855K
• 年卷期：v.116,no.17(May 3, 2012)
• ISSN：1932-7455

文摘

Experimental and theoretical investigations of the oxidation reaction of CO to form carbon dioxide, catalyzed by size-selected Pd₃₀ clusters soft-landed on MgO(100), are described. The consequences of pretreatment of the deposited clusters with molecular oxygen, ¹⁶O₂, at a temperature of 370 K followed by annealing at around 450 K are explored. Subsequent to the above pretreatment stage, the system was cooled to 120 K, and after exposure to ¹⁸O₂ and ¹³C¹⁶O the temperature was ramped and a temperature-programmed reaction (TPR) spectrum recorded. The onset of catalyzed combustion of CO starts at a temperature of 180 K, and the TPR spectrum shows oxidation to occur over a broad temperature range, up to 550 K. Using first-principles density-functional theory, the optimal adsorption geometry of the Pd₃₀ cluster on the MgO(100) surface is found to be a square-base pyramidal structure, with an excess electronic charge of 1.25e, originating from the underlying magnesia support, found to be localized near the interfacial region of the cluster with the supporting surface. Structural and energetic properties of a variety of oxygen adsorption sites on the supported palladium cluster and effects due to multiple adsorbed O₂ molecules were explored. It is found that the barriers for dissociation of the adsorbed molecules depend strongly on the locations of the adsorption sites, with very small (<0.1 eV) dissociation energy barriers found for adsorption sites on the Pd₃₀ cluster that are closer to its interface with the Mg(100) surface. This correlates with our finding that adsorption at these interfacial sites is accompanied by excess charge accumulation on the adsorbed molecule through excess partial (0.25e) occupation of the molecular antibonding 2蟺* orbital, resulting in activation of the molecule to a peroxo-like state. This activation mechanism depletes the excess charge on the cluster, resulting in a self-limiting partial oxidation of the cluster. The information obtained through isotope labeling in the TPR experiments is explored through first-principles quantum simulations of various reaction pathways, with a focus on the multiple coadsorption system Pd₃₀O₁₀(CO)₁₃/MgO. These theoretical calculations allow us to correlate the measured isotope and temperature-dependent TPR features, with operative Langmuire鈥揌inshelwood, LH, and Mars鈥搗an Krevelen-type (MvK) reaction mechanisms, catalyzed by the partially oxidized cluster. The LH mechanism was found to contribute to the reaction at lower temperatures, while the MvK dominates for higher temperatures.