Rippled and Helical MoS 2 Nanowire Catalysts: An Aberration Corrected STEM Study
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Rippled and Helical MoS2 Nanowire Catalysts: An Aberration Corrected STEM Study Francis Leonard Deepak • Rodrigo Esparza Belsay Borges • Xo´chitl Lo´pez-Lozano • Miguel Jose-Yacaman
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Received: 8 October 2010 / Accepted: 9 January 2011 / Published online: 26 January 2011 Ó Springer Science+Business Media, LLC 2011
Abstract Aberration corrected (Cs) scanning transmission electron microscopy (STEM) has been used for the first time to characterize MoS2 catalysts (supported on Al2O3 substrates) to provide detailed information of its shape and structure. The high-resolution imaging reveals unprecedented morphologies present in the MoS2 catalyst that have never been observed before with other experimental techniques because of the insufficient image contrast and/or resolution. High angle annular dark field (HAADF)-STEM images shows very clearly that the catalyst is formed by elongated chains with a twisted and helical structure. Based on the HAADF-STEM images, we built three atomic models to illustrate the different morphologies found in the MoS2 catalyst. The existence of these nanostructures opens the posibility for novel catalyticaly active edge morphologies in MoS2-based nanocatalysts. Keywords MoS2 Hydrodesulphurization catalysts HAADF–STEM
F. L. Deepak R. Esparza B. Borges X. Lo´pez-Lozano M. Jose-Yacaman (&) Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio 78249, TX, USA e-mail: [email protected] F. L. Deepak International Iberian Nanotechnology Laboratory, Avda Mestre Jose Veiga, 4715 Braga, Portugal B. Borges Departamento de Quı´mica, Universidad Simo´n Bolı´var, Apartado 89000, Caracas 1080-A, Venezuela
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1 Introduction Environmental regulations regarding the content of sulfur in fuels have become more stringent in many countries during the past few years. Hence there is a considerable effort to improve the efficiency of catalysts used for the hydrodesulfurization (HDS) process in the oil industry [1]. HDS are catalytic processes that involve Mo or W-based catalysts, often doped with other transition metals. The presence of sulfur compounds in fuels is undesirable because their combustion results in formation of sulfur oxides which, after being released into the atmosphere cause acid rain [2]. Thus the use of cobalt or nickel doped molybdenum sulfide catalysts supported over alumina help yield sulfur free hydrocarbons and H2S [3, 4]. The removal of sulfur heterocyclic compounds such as dibenzothiophene (DBT) and its more hindered analogues, 4-methyldibenzothiophene (4-MeDBT) and 4,6-dimethyldibenzothiophene (4,6-Me2DBT), from oil (the latter being the major causes for the high sulfur levels in it) pose important challenges to the oil industry given their resistance to HDS [5–7]. MoS2 is a layered material consisting of weakly bounded S-Mo-S layers. Different nanomaterials of MoS2 have been prepared and used to study the hydrodesulphurization catalytic processes. These include MoS2–Ni nanocomposites which were used as catalysts for th
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