Elsevier

Journal of Catalysis

Volume 233, Issue 2, 25 July 2005, Pages 411-421
Journal of Catalysis

Ab initio DFT study of hydrogen dissociation on MoS2, NiMoS, and CoMoS: mechanism, kinetics, and vibrational frequencies

https://doi.org/10.1016/j.jcat.2005.05.009Get rights and content

Abstract

The present study provides detailed discussions about the structures, relative stabilities, and vibrational frequencies of hydrogen species on MoS2, NiMoS, and CoMoS catalyst edge surfaces. The transition states and activation energies for molecular hydrogen dissociation and surface migration of atomic hydrogen on catalyst edge surfaces have been calculated by complete linear synchronous transit (LST) and quadratic synchronous transit (QST) search methods. It has been found that the heterolytic dissociation of molecular hydrogen at a pair of sulfur–metal sites to form an single bondSH group and a metal hydride is energetically preferred. The dissociation of molecular hydrogen on the Ni-promoted (101¯0) metal edge of NiMoS requires slightly lower activation energy than that on the unpromoted (101¯0) Mo-edge of MoS2 (0.87 and 0.91 eV, respectively). The dissociation of molecular hydrogen on the unpromoted (1¯010) S-edge requires a large activation energy (about 1.0 eV), and the addition of cobalt to the (1¯010) S-edge significantly lowers the dissociation energy to approximately 0.6 eV. The atomic hydrogen species on the (1¯010) S-edge and the Co-promoted (1¯010) S-edge are less mobile than on the (101¯0) Mo-edge of MoS2 or the Ni-promoted (101¯0) metal edge of NiMoS. The calculated vibrational frequencies of different surface hydrogen species agree well with reported experimental observations and have provided references for further spectroscopic experiments.

Introduction

Hydrotreating processes play an essential role in producing clean transportation fuels to meet increasingly stringent environmental regulations. Molybdenum sulfides promoted by nickel (or cobalt) have been widely used as hydrotreating catalysts for the removal of sulfur, nitrogen, and other impurities from oil fractions. Many aspects of the structures and properties of the catalytically active phase have been well defined by extensive experimental and theoretical studies [1], [2], [3], [4], [5], [6]. Whereas the reaction networks of hydrodesulfurization (HDS) and hydrodenitrogenation (HDN) for many model compounds have been well documented [1], [2], [7], the detailed mechanisms and energetics for elementary reactions on hydrotreating catalyst surfaces are poorly understood.

Hydrotreating reactions require the activation of hydrogen on the catalyst surface and the subsequent reactions between adsorbed hydrogen species and organic molecules. Experimental studies have shown that hydrogen and hydrogen sulfide can be adsorbed and dissociated on promoted and unpromoted molybdenum sulfide catalysts ([8], and references therein). It has previously been shown that the amount of hydrogen adsorbed at temperatures below 373 K is very low and increases with temperature, indicating that the adsorption of hydrogen on MoS2 is an activated process [9]. Furthermore, deuterium tracer studies have shown Hsingle bondD isotopic exchange between H2 and D2 in the presence of molybdenum sulfide catalysts [10], [11], [12]. These results indicate that hydrogen dissociates and adsorbs on MoS2 as atomic hydrogen species, which are mobile and can recombine to form molecular hydrogen. Possible structures of surface hydrogen species, including surface metal hydrides and single bondSH species, have been proposed based on experimental observations [8], [9]. The relative stabilities of different hydrogen species [4], [13], [14], [15], [16], [17], [18] and the kinetics of hydrogen dissociation have been investigated with the use of theoretical calculations [15], [19], and some of the most relevant work has been performed by Travert and co-workers [15].

The active sites of molybdenum sulfide catalysts are located at edge surfaces of the MoS2 hcp layered structure [1], [2], [3]. For the (101¯0) Mo-edge and the (1¯010) S-edge of unpromoted MoS2, molybdenum atoms on the edge surfaces are covered by bridge sulfur atoms under reaction conditions [4], [5], [6], [13], [15], [17], [20]. At high H2S concentrations, the (1¯010) S-edge can be fully covered by sulfur dimers with adsorbed hydrogen [17]. For nickel-promoted MoS2 catalysts, nickel prefers the (101¯0) metal edge [5], [6], and a fully nickel-promoted (101¯0) metal edge (termed Ni-edge) is completely uncovered by sulfur atoms [4], [5], [6], [21]. For cobalt-promoted catalysts, cobalt prefers the (1¯010) S-edge, and the fully cobalt-promoted (1¯010) S-edge (termed Co–S-edge) is covered by bridge sulfur atoms under reaction conditions [5], [6]. Previously we studied the relative stabilities of different hydrogen species on MoS2 and NiMoS edge surfaces [18], and the hydrogenation of pyridine and pyrrole by surface hydrogen species on Ni-promoted (101¯0) metal edge surfaces of NiMoS [22]. In the present work, the mechanism and kinetics of hydrogen dissociation on unpromoted and promoted (101¯0) Mo-edge and (1¯010) S-edge surfaces are studied. In addition, the vibrational frequencies of hydrogen species on various edge surfaces are calculated and compared with experimental observations to validate the DFT calculations. The present results are compared with previous studies, and the implications of these results for hydrotreating reactions are discussed.

Section snippets

Methods

The calculations are based on density-functional theory (DFT) and were performed with Materials Studio DMol3 from Accelrys (version 2.2) [23]. The double-numerical plus polarization functions (DNP) and Becke exchange [24] plus Perdew–Wang approximation [25] nonlocal functionals (GGA-PW91) are used in all calculations. The real space cutoff radius is 4.5 Å. All electron basis sets are used for light elements, such as hydrogen and sulfur. Effective core potentials [26], [27] are used to treat

On the unpromoted MoS2 (101¯0) Mo-edge

Table 1 summarizes the possible structures and relative energies of hydrogen species on the stable (101¯0) Mo-edge of unpromoted MoS2, with the clean surface and gas-phase molecular hydrogen as energetic references. The data included in brackets were obtained with the larger 6-Mo supercell, which includes six MoS2 units along the edge surface [18]. Similar relative energies for the same structure of hydrogen species on the surface were obtained with the 2-Mo and 6-Mo models. Homolytic hydrogen

Conclusions

The activation energies for hydrogen dissociation on the unpromoted (101¯0) Mo-edge and Ni-promoted (101¯0) metal edge are similar (0.91 and 0.87, respectively). The hydrogen species on the (101¯0) Mo-edge and the Ni-promoted (101¯0) metal edge are different in chemical properties, which results in different reactivities toward further surface reactions (hydrogenation). The activation energy for hydrogen dissociation on the Co-promoted (1¯010) S-edge is very low (0.58 eV), and dissociated

Acknowledgements

This work is supported by Syncrude Canada Ltd. and the Natural Sciences and Engineering Research Council (NSERC) under grant no. CRDPJ 261129.

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