Hydrocarbon reactions on MoS₂ revisited, I: Activation of MoS₂ and interaction with hydrogen studied by transient kinetic experiments

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• 作者：Mykola Polyakov ; Maurits W.E. van den Berg ; Thomas Hanft ; Martha Poisot ; Wolfgang Bensch ; Martin Muhler ; Wolfgang Grü ; nert
• 关键词：Molybdenum sulfide ; Active sites ; Chemisorption ; Hydrogen activation ; XPS ; XAFS
• 刊名：Journal of Catalysis
• 出版年：2008
• 出版时间：15 May 2008
• 年：2008
• 卷：256
• 期：1
• 页码：126-136
• 全文大小：694 K

文摘

MoS₂ was prepared by thermal decomposition of ammonium tetrathiomolybdate in inert gas at 723–773 K and activated by procedures involving evacuation at 723 K and/or reduction at different temperatures, predominately 573 K. Bulk and surface properties of these samples were studied by X-ray diffraction (XRD), extended X-ray absorption fine structure (EXAFS), X-ray photoelectron spectroscopy (XPS), nitrogen physisorption, oxygen chemisorption (273 K, pulse mode), and isotope exchange with D₂ to determine the quantity of exchangeable hydrogen. In addition, the quantity of hydrogenation-active surface hydrogen was determined by hydrogenation of ethylene in absence of gas-phase hydrogen. No obvious relationship was found between the S/Mo stoichiometry and the degree of Mo exposure; depending on the pretreatment, exposed Mo ions can be detected by oxygen chemisorption at widely varying quantities while the S/Mo ratio remains 2. At more severe reduction treatments to remove additional sulfur, the oxygen chemisorption capacity decreases, likely due to migration of defects into the bulk. The reactivity of MoS₂, as expressed by the temperatures required to expose Mo ions and the quantity of exchangeable hydrogen, varies significantly between batches prepared by similar routes. Therefore, thorough surface characterization by a set of techniques including reactivity studies is an indispensable prerequisite for any meaningful study investigating the relationship between catalytic properties with preparation procedures. After activation, MoS₂ holds hydrogen, which is able to hydrogenate ethylene in absence of gas-phase H₂; the quantity of this hydrogenation-active surface hydrogen is not correlated with the hydrogenation activity, however.