Metallic States at the Edges of MoS 2 Clusters
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Metallic States at the Edges of MoS2 Clusters D. H. Galvan Æ A. Posada Amarillas Æ M. Jose´-Yacama´n
Received: 27 March 2009 / Accepted: 18 August 2009 / Published online: 4 September 2009 Ó Springer Science+Business Media, LLC 2009
Abstract In this work we suggest that certain structures present in real commercial catalysts maybe related to low dimensional structures and present a theoretical study of two MoS2 clusters (one made of 61 atoms and the second one made of 68 atoms). The 61 atoms cluster has 7 sulfur atoms decorating at the top of sulfur (S) layer in order to analyze the effects produced on the electronic properties. The first cluster yielded semiconductor behavior with a forbidden energy gap Eg of the order of 0.11 eV, while the second cluster decorated with seven S atoms yielded metallic behavior. A careful analysis to the partial density of states (PDOS) indicate that the contributions to the total DOS in the vicinity of the Fermi level, from the seven S atoms located at the surface is negligible, they serve only as promoters to the metallic behavior to the system. Keywords Tight-binding calculations Dehydrodesulfurization catalysts
D. H. Galvan (&) Centro de Nanociencias y Nanotecnologı´a, Universidad Nacional Auto´noma de Me´xico, Apartado Postal 2681, C. P. 22800, Ensenada, BC, Mexico e-mail: [email protected] A. P. Amarillas Departamento de Investigacio´n en Fı´sica, Universidad de Sonora, Apartado Postal 5-088, C. P. 83190, Hermosillo, Sonora, Mexico e-mail: [email protected] M. Jose´-Yacama´n Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, TX 78249, USA e-mail: [email protected]
1 Introduction The most commonly used industrial hydro treating catalysts for sulfur removal from heavy oils are based on cobalt or nickel promoted MoS2. A basic understanding of the mechanisms that involve the catalytic reactions are still not well understood .Those catalysts consist of small MoS2 ? x particles of finite size supported on alumina; When observed at high resolution they often show elongated wire like structures, as the ones observed in model systems observed in Fig. 1a. A more deep analysis of these wires show a remarkable structure, which resembles to the theoretical models used by a number of groups [1–7]. Figure 1b shows how a typical catalyst can be formed by these platelets of MoS2 that have an enhanced selectivity for hydrogenation [8]. The hexagonal contrast observed at the tip was measured to correspond to MoS2–2H basal plane thus the structure grows along a direction perpendicular to, either {10.0} or the {11.0}. It has been customary to assume that the platelet-like structure observed in Fig. 1 correspond to transversal sections of large MoS2 crystallites. It has been shown [1–5] by the groups at Haldor Topsoe Labs and at the University of Aarhus the existence of metallic one-dimensional states on MoS2 clusters and established a new view of the catalytic active sites. It is very important to determine the relevance of these findings fo
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