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Density functional theory investigation of Mo3sx nanoclusters : a theoretical study of the hydrodesulfurization pathway

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Item Summary

Title: Density functional theory investigation of Mo3sx nanoclusters : a theoretical study of the hydrodesulfurization pathway
Authors: Gold, Keegan Laurel
Keywords: Molybdenum disulfide
oil industry
Issue Date: May 2014
Publisher: [Honolulu] : [University of Hawaii at Manoa], [May 2014]
Abstract: Molybdenum disulfide (MoS2) has long been used as a hydrodesulfurization (HDS) catalyst in the oil industry. The size-dependent efficacy of MoS2 clusters has only recently been established revealing a need for studies of the smaller nanocatalysts. As the demand for ultra-low sulfur petroleum products continues to grow, crude oil quality is concurrently declining. Understanding these catalysts and their mechanisms is now more imperative than ever to the oil industry. Therefore, this thesis presents a B3LYP density functional theory (DFT) investigation of Mo3S5-9 nanoclusters The first part of this thesis explores potential structures of Mo3S5-9 nanoclusters. Analysis of these bare clusters reveals that the S coordination about the Mo directly affects the energetics of the cluster and find that triangular Mo centers are more stable when the Mo has 4 > 3 > 2 > 5 S coordination. We then investigate how H2 and 2H2 interact with these low energy bare Mo3S6-8 clusters. This is a novel idea and, to our best knowledge, neither the energetics nor the mechanisms of this addition have been explored until now. We discover that these are highly heterogeneous systems and that hydrogen interacts not only with S but with Mo as well. It is also discovered that H2 can form as a low-energy binding complex on the Mo of the cluster. Combinations of low energy Mo3SxH2 structures often resulted in low energy Mo3SxH4 structures found which acts as a confirmation of established bonding patterns.
The formation and desorption of H2S is explored as H2S is a known product in the hydrogenation phase of the HDS process. It is seen that an increase in S number on the clusters leads to an increase in the formation of H2S and a decrease in H2S desorption energy. This pattern is attributed to the lack of sulfur vacancy sites on higher sulfided complexes. It is then determined that S vacancies are located on the less sulfided Mo3Sx complexes, particularly Mo3S6.
The last part of the thesis explores several mechanisms to mimic the HDS process using methanethiol (CH3SH) as an organo-thiol molecule to be desulfurized. First, CH3SH is added to several previously-determined low energy structures and its adsorption energy is calculated. It is discovered that the best adsorbing structure is a Mo3S6H2 structure. To make this cluster, a reaction pathway is found such that the low energy Mo3S7 cluster undergoes the addition of H2 to produce H2S and a sulfur-vacant Mo3S6 cluster. Next, H2 is added to the cluster to produce the Mo3S6H2 cluster. After the addition of CH3SH to this cluster as a reactant, a single mechanism is discovered which results in Mo3S7H2 and CH4 as the products. The activation barrier of this desulfurizing reaction is then compared to that of Mo3S6 and the decomposition of isolated CH3SH. This comparison reveals that the addition of H2 to the cluster makes the reaction more energetically favorable and that the nanocluster is indeed acting as a catalyst by lowering the barrier.
Description: M.S. University of Hawaii at Manoa 2014.
Includes bibliographical references.
Rights: All UHM dissertations and theses are protected by copyright. They may be viewed from this source for any purpose, but reproduction or distribution in any format is prohibited without written permission from the copyright owner.
Appears in Collections:M.S. - Chemistry

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