Ab initio DFT study of hydrogen dissociation on MoS2, NiMoS, and CoMoS: mechanism, kinetics, and vibrational frequencies
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 HD 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 SH 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 Mo-edge and the 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 S-edge can be fully covered by sulfur dimers with adsorbed hydrogen [17]. For nickel-promoted MoS2 catalysts, nickel prefers the metal edge [5], [6], and a fully nickel-promoted metal edge (termed Ni-edge) is completely uncovered by sulfur atoms [4], [5], [6], [21]. For cobalt-promoted catalysts, cobalt prefers the S-edge, and the fully cobalt-promoted 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 metal edge surfaces of NiMoS [22]. In the present work, the mechanism and kinetics of hydrogen dissociation on unpromoted and promoted Mo-edge and 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 () Mo-edge
Table 1 summarizes the possible structures and relative energies of hydrogen species on the stable 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 Mo-edge and Ni-promoted metal edge are similar (0.91 and 0.87, respectively). The hydrogen species on the Mo-edge and the Ni-promoted 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 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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