ARTICLE pubs.acs.org/JPCC
First-Principles Characterization of Potassium Intercalation in Hexagonal 2H-MoS2 Amity Andersen,*,† Shawn M. Kathmann,‡ Michael A. Lilga,§ Karl O. Albrecht,§ Richard T. Hallen,§ and Donghai Mei*,‡ †
Environmental Molecular Sciences Laboratory, Pacific Northwest National Laboratory, Richland, Washington 99352, United States Fundamental and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States § Chemical & Biological Process Development, Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, United States ‡
ABSTRACT: Periodic density functional theory calculations were performed to study the structural and electronic properties of potassium intercalated into hexagonal MoS2 (2H-MoS2). Metallic potassium (K) atoms are incrementally loaded in the hexagonal sites of the interstitial spaces between MoS2 layers of the 2H-MoS2 bulk structure generating KxMoS2 (0.125 e x e 1.0) structures. To accommodate the potassium atoms, the interstitial spacing c parameter in the 2H-MoS2 bulk expands to 15.871 Å in K0.125MoS2. The second lowest potassium loading concentration (K0.25MoS2) results in the largest interstitial spacing expansion (to c = 16.617 Å). Our calculations show that there is a small gradual contraction of the interstitial spacing as the potassium loading increases with c = 14.785 Å for KMoS2. This interstitial contraction is correlated with an in-plane expansion of the MoS2 layers, which is in good agreement with experimental X-ray diffraction (XRD) measurements. The electronic analysis shows that potassium readily donates its 4s electron to the conduction band of the MoS2, and is largely ionic in character. As a result of the electron donation, the KxMoS2 system changes from a semiconductor to a more metallic system with increasing potassium intercalation. For loadings 0.25 e x e 0.625, triangular Mo Mo Mo moieties are prominent and tend to form interlayer rhombitrihexagonal tessellated patterns. Intercalation of H2O molecules that solvate the K cations is likely to occur in catalytic conditions. The inclusion of two H2O molecules per K atom in the K0.25MoS2 structure shows good agreement with XRD measurements.
1. INTRODUCTION MoS2-based catalysts have been widely used in industrial hydrodesulfurization (HDS) processes.1 When promoted with a heavy alkali metal (K, Cs, or Rb), MoS2-based catalysts are promising catalysts used for synthesis of mixed higher alcohols from biomass derived syngas.2 By doping heavy alkali metals, syngas hydrogenation leads to the formation of mixed higher alcohols instead of hydrocarbons, primarily methane, when unpromoted pure MoS2-based catalysts were used.2 5 Although many experimental studies address the important role the doped alkali metal promoters play in the selectivity toward alcohols,3,6 12 there is very little understanding of where these alkali metals reside in MoS2 catalysts and how they affect the selectivity toward higher alcohols.3,9 Lee et al. reported that alkali promoted MoS2 catalysts with substantial alkali ion migration into the subsurface and bulk show high selectivity for linear hydrocarbon chain growth, which can contribute to higher (C2+) alcohols.3,5,9 In the previous study, we have addressed the adsorption of K on the surface of the catalytically active MoS2(100) edge surface, and its importance in the alcohol selectivity of this catalyst.13 Herein, we r 2012 American Chemical Society
will address the structural and electronic effects on the bulk 2H-MoS2 material upon intercalation of potassium. Potassium is one of the most common promoters used in MoS2 catalysts. Potassium is typically added as a salt in which the K+ is readily separated from the anion such as K2CO3, KOH, and K2S, which have positive pKa values.3 The anions of these salts can readily undergo chemical transformation to form neutral stable molecules (as in the case of K2CO3 and KOH) and/or readily integrate into the MoS2 catalyst, donating electrons to the MoS2 conduction band. Under syngas conversion conditions with H2S present, Lee et al. found, using energy dispersive X-ray spectroscopy (EDS), that, with potassium salts having anions that readily convert to neutral molecules and surface species that donate electron charge to the MoS2 substrate during an induction period, the remaining free K cations readily spread over the surface of the MoS2 catalyst.3 When the catalyst is oxidized over a period of time or promoted with a salt where the K+ cannot be Received: July 11, 2011 Revised: December 9, 2011 Published: January 05, 2012 1826
dx.doi.org/10.1021/jp206555b | J. Phys. Chem. C 2012, 116, 1826–1832
The Journal of Physical Chemistry C readily separated from the salt anion over MoS2 substrate (e.g., K2SO4), sulfates are found on the surface, and the K+ tends to migrate into the subsurface and bulk. The migration mechanism for how these sulfate surface species allow substantial migration of K+ into the subsurface is not understood.3,9 According to Lee et al., K+ surface species drive alcohol selectivity (possibly via direct adsorption of CO and blocking of sites responsible for CO dissociation), whereas K+ subsurface/bulk species drive chain growth for higher linear alcohols possibly because of the supply of free K 4s electrons in the conduction band of the MoS2 material.3 Experimental studies on alkali-MoS2 materials were also carried out where metallic K (or other alkali metals) is added either in the vapor phase or as a solute in ammonia. Intercalation of K leads to interesting electronic properties such as superconductivity (up to ∼7 K).14 16 Moreover, these materials are highly hygroscopic and readily intercalate solvent molecules such as water with the alkali metal.14 18 Wypych and co-workers have found, that intercalation of K in MoS √ 2 tends to form 2a 2a superstructures.19,20 A wavy a a 3a surface superstructure with sulfur chains in an aqueous K-intercalated system was observed using scanning tunneling microscopy (STM).19,20 The intercalation of water with the potassium can affect the activity of the water-gas-shift reaction occurring over alkalipromoted MoS2(100) catalysts.11 With no moisture present, Somoano et al. reported XRD measurements with a c parameter expansion of 35% (16.5804 Å).16 Zak et al. measured a 50% increase (18.42 Å) in the c lattice parameter and suggested that one to two water molecules per K atom were possibly intercalated with the K atoms in 2H-MoS2.18 To the best of our knowledge, a theoretical investigation of the intercalation of potassium alone or with H2O molecules has not been reported. This paper is arranged as follows. In section 2, we describe the details of our computational approach. In section 3, we then present and discuss our results regarding the effects of multiple K interstitial loadings on the atomistic and electronic structure of the bulk 2H-MoS2 system. Also in section 3, we discuss the atomistic and electronic structure changes that 2H-MoS2 undergoes with the insertion of K atoms and H2O molecules into the interstitial spaces. In section 4, we present our conclusions.
2. COMPUTATIONAL DETAILS All spin-polarized calculations were carried out using the Quantum-Espresso plane-wave density functional theory (DFT) package.21 Vanderbilt ultrasoft pseudopotentials22 24 were used. Because of the fact that the K K distances in the KxMoS2 (x > 0.25) were expected to be much shorter than that in the bcc K metal bulk, and the K S distances were expected to be relatively short (∼3.2 Å) based on our previous study,13 a semicore pseudopotential was chosen for K (3s23p64s1). A similar potential was employed for simulations of high pressure K metal and alloy phases in the literature where the inclusion of the semicore pseudowave functions were required to obtain close agreement with all-electron methods such as the full potential linearized augmented planewave (FP-LAPW) method.25 28 A semicore pseudopotential was chosen for Mo (3s23p64s13d5), and a valence-only pseudopotential was chosen for S (3s23p4) in this work. Scalar relativistic corrections29 are included for all pseudopotentials employed. The bulk lattice parameters: a = b=3.160 Å, c = 12.294 Å, α = β = 90° and γ = 120° for the
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Figure 1. Optimized bulk 2H-MoS2 structure: the side view (left) and the basal view (right). Mo atoms are in blue, and S atoms are in yellow.
hexagonal trigonal prismatic 2H-MoS2 crystal (space group P63/mmc) were taken from experimental crystallographic data in the literature (see Figure 1).30 A full optimization of lattice parameters and atomic positions for the 2H-MoS2 bulk model was performed using the generalized gradient approximation (GGA) exchange-correlation functional of Perdew, Burke, and Enzerhof (PBE),31 and a Monkhorst-Pack32 k-point grid of 9 9 2 for the sampling of the Brillouin zone was applied. The energy cutoff for the plane-wave basis set expansion was set to 30 Ry, and the charge density cutoff was set to 300 Ry. For all calculations, the energy convergence tolerance for self-consistent field (SCF) convergence was set to
