A DFT and HRTEM Study of MoS2/Co: Locating Promoters in Catalytic Nanostructures

Journal: 2010 MRS Fall Meeting

Manuscript ID: 913573

Manuscript Type: Symposium EE

Date Submitted by the Author:

18-Nov-2010

Complete List of Authors: Ramos, Manuel; Universidad Autonoma de Cuidad Juarez, Fisica y Matematicas Berhault, Gilles; IRCELYON, CNRS-Université de Lyon Farias, Rurik; Universidad Autonoma de Cuidad Juarez, Fisica y Matematicas Elizalde, Jose; Universidad Autonoma de Cuidad Juarez, Fisica y Matematicas Ferrer, Domingo; University of Texas at Austin, Microelectronics Research Center Torres, Brenda; UTEP, MRTI Chianelli, Russell; UTEP, MRTI

Keywords: catalytic, nanostructure, transmission electron microscopy (TEM)

A DFT and HRTEM Study of MoS2/Co: Locating Promoters in Catalytic Nanostructures Manuel Ramos1), 2), Gilles Berhault3), Jose Rurik Farias1), Jose Trinidad Elizalde1), Domingo Ferrer4), Brenda Torres2) and R. R. Chianelli2) 1 Departamento de Física y Matemáticas, Instituto de Ingeniería y Tecnología-UACJ, Avenida del Charro #610, Cuidad Juárez, MX. CP.32310 2 University of Texas at El Paso, Materials Research and Technology Institute, 303 Burges Hall, El Paso, Texas 79902, U.S.A 3 Institut de Recherches sur la Catalyse et l’Environnement, IRCELYON, CNRS –Université de Lyon, Villeurbanne, 69100, France 4 Microelectronics Research Center, University of Texas at Austin, Austin, TX. 78751, U.S.A. ABSTRACT Locating cobalt promoters on catalytically active MoS2 structures is a challenging task to achieve due to microscopic scale size on the MoS2 nanostructures. Previous reports in the literature indicate that the specific location for Co on MoS2 slabs are the (1010)-plane creating either a sulfur-Co or Molybdenum-Co edge termination. We present here results obtained from a DFT (Density Functional Theory) study done on the interface between MoS2 and Co9S8 crystal structures; the interface show an interesting thiocubane core which is responsible for Mo-S-Co bonding. In order to understand electronic properties of thiocubane interface, Density of States, Band Structure and Mulliken Population Analysis calculations were implemented using Cambridge Serial Total Energy Package (CASTEP). Results indicate a strong electron donation from Co to Mo through intermediate sulfur atom bonded to both metals while an enhanced metallic character is also found. INTRODUCTION Because of its uses during hydrodesulphurization (HDS) reaction MoS2 (Molybdenum Sulfide) is one of the best candidates to act as catalysts for sulfur removal from crude oil. Previous studies [1] indicate that when it is promoted with Ni or Co a significant increase in catalytic activity is observed. This fact caused MoS2 to be studied extensively in the past two decades [2]. The location of promoters (Nickel, Cobalt) has been done by others by conventional characterization techniques X-ray diffraction [3], Field Emission Electron Microscope (TEM and SEM) [4], Mössbauer spectroscopy [5] and lastly with the aid of quantum computational methods [6]. Even though these techniques were implemented to understand the relationship between Mo, S and Co (Ni) atoms, they were unsuccessful to achieved the nature of chemical bonding, that could happen because of scale size, which in most of the cases reports found in the literature reveals solely physical properties like d-spacing, folding, stacking size, surface contact and dislocations. Few are the reports published in the literature as compared with hundreds on conventional techniques that provide information about density of states, band structure and Mulliken population analysis [7, 8] for MoS2/Co9S8 structure. The objective of the present study

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was to address the chemical nature of MoS2 when is promoted with Cobalt, on doing that we focus more on a thiocubane-Co8S9/Mo core [9] and treated as an individual entity using density functional theory methods.

EXPERIMENT Based on High Resolution Transmission Electron Microscope images for MoS2/Co9S8 before HDS reaction (figure 1) an interface computational model was constructed using Cerius2, later one portion (“seed”) of the interface was selected for further investigations using Materials Studio. The seed was placed in a supercell (space group P1, non-symmetry) with a = 16.0 Å, b = 17.0 Å, and c = 15.0 Å and α = β = γ = 90o and subjected to band structure and density of states calculations using Cambridge Serial Total Energy Package (CASTEP) with a revised Perdew-Burke-Ernzerhof functional (general gradient approximation), with a cutoff energy of 340 eV in the reciprocal space for gamma point only and a self-consistent field (SCF) convergence threshold of 1x10-6 and ultra-soft pseudo potential of 290 eV for Mo and 220 eV for S and 300 eV for Co. This had found that best matching planar contact between both structures were (111) and (010) edge plane for Co9S8 and MoS2 respectively as presented in figure 2, further observation of the interface pay our attention to an interesting thiocubane three fold coordinated Mo-S-Co and Co-Mo bonding, the core was separated from the interface model and subjected to Mulliken analysis [10] in order to understand the electronic nature primarily from Co-Mo metallic bonding. DISCUSSION

Figure 1 (left) Presents a 5nm resolution image that shows the interaction between MoS2 and Co9S8 structures, the image was obtained experimentally using a FEI Tecnai TF20 (200kV) equipped with a STEM unit, high-angle annular dark-field (HAADF) detector and X-Twin lenses. Dispersion of (101) edge MoS2 slabs onto (111) Co9S8 was observed, the “fringes” characteristic of slabs shown along their edges are more commonly detected. Molybdenum disulfide is a layered compound formed of more and less stacked slabs maintained by van der Waals forces; figure 1 (right) presents a computational model and a TEM simulation created using a full dynamical calculation by multi-slice method using projected potential ∑

=

−=n

i

Ub

iieaUf

1

)( 2

)( , where ai and bi are coefficients to be determined. The variable U = (u,

v, w) is used to represent coordinates in reciprocal space (Fourier space) quantities (spatial frequencies) and f(U) the atomic dispersion factor [11]. Aim on explaining TEM micrographs and the interaction between MoS2 and Co9S8 were presented before by others, their explanation lead to two important models 1) the Intercalation Model [12] and 2) the Rim-Edge model [13], both models agreed that planar contact has to be on sulfur either molybdenum edge termination and cobalt sulfide, due to energy required for permutation between S, Mo and Co to exist. Those “rim” sites are locations able to perform hydrogenation and carbon-sulfur (C-S) rupture steps during hydrodesulphurization reaction of dibenzothiophene, while exterior layers, “edge” sites able only to break C-S bonds are on interior layers. Catalytic functionality depends solely on the

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edge according to rim-edge model which are sites of location on hydrogenation of DBT molecules on HDS reaction when processing crude oil.

Figure 1: A) left: A 5 nm resolution HRTEM image of MoS2 slabs making contact with Co9S8. B) Right: Bright field and Dark field HRTEM simulation of MoS2/Co9S8 interface model.

Figure 2: Left: Crystal structure models of MoS2, Center Co9S8 structure both used for Band Structure and DOS calculations corresponding results are presented in figure 3. Right:

Thiocubane-Co8S9/Mo core (blue: cobalt, green: molybdenum, yellow: sulfur) Band structure and density of states calculations for MoS2, Co9S8 and the thiocubane core were carried out using CASTEP [14] which is a based DFT computational algorithm, results of total energy are presented in table 1 and DOS plots in figure 3 where one can observed a pure metallic character of Co9S8 [15] and semiconductor behavior on MoS2 because of the gap at Fermi level [16]. Further investigations of population analysis for thiocubane-Co8S9/Mo were carried using Mulliken analysis, which provides partial estimation of the atomic charges based on linear combination of molecular and atomic orbital results are presenting on table 2. The density matrix can be written as ∑=

iiiCCD νµµν 2 for all close shell systems where each molecular orbital is

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doubly occupied, the population matrix will have terms µνµν )(DSP = where Pµν is the overlap

matrix of the basis functions. Finally the gross atom population will be defined as:

AAA GAPZQ −= , which QA represents atomic charge, atomic number is ZA and gross atom population is GAPA [7].

Table 1: Obtained Total Energy for: MoS2, Co9S8 and MoS2/ Co9S8. All calculations were done

using CASTEP with a revised Perdew-Burke-Ernzerhof functional. Figure 3: CASTEP total density of states for: MoS2 and Co9S8, the calculations were done using

a revised Perdew-Burke-Ernzerhof functional. Figure 4 (Left) presents charge density on thiocubane (Co-Mo-S) found from a combination of MoS2 and Co9S8 structures, (Right) partial projected DOS which indicates an strong metallic character which seems to be important from a catalytic stand point, since it would not only hinder the hydrogenating character of “edge” sites, also the electronic donation occurred from Cobalt to Molybdenum indicated the nature of active sites on the edge sulfur termination and are in agreement with the Rim-Edge model. Mulliken analysis shows a set of highly coordinated Mo atoms, the Mulliken calculations lead to charges of + 0.49 - +0.53 for the Mo atoms. In bulk MoS2, the Mo charge is +0.61. The direct bonding between Co and Mo therefore results in a limited charge donation from cobalt to molybdenum. Inversely, cobalt atoms of the cubic array that are directly connected to Mo show much more negative (or less positive) charges (-0.07 and +0.03) than those expected for similar Co atoms in bulk Co9S8 (+0.16). Table 2 presents charge values for each specimen contained in thiocubane-Co8S9/Mo structure.

Type of Structure Total Energy (eV/atom)

MoS2 312.31 Co9S8

Thiocubane-Co8S9/Mo core 685.48

849.93

CASTEP Density of States

0

20

40

60

80

100

120

140

160

180

-70 -60 -50 -40 -30 -20 -10 0 10

Energy (eV)

DO

S (

ele

ctr

on

/ato

m)

MoS2

Co9S8

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Projected Partial Density of States for Thiocubane

0

5

10

15

20

25

30

35

-70 -60 -50 -40 -30 -20 -10 0 10

Energy (eV)

De

ns

ity

of

Sta

tes

(e

lec

tro

n/e

V)

s-Orb (eV)

p-Orbital(eV)

d-orbital(eV)

Sum Orbitals (eV)

Figure 4. Partial projected density of states results for thiocubane-Co8S9/Mo obtained on CASTEP, Inset: Charge density plot for thiocubane-Co8S9/Mo at two different views.

Bond Population Bond Length (Å) Bond Population Bond Length (Å) Co1-Mo1 -0.43 1.908 Co1-Mo1 0.37 2.238 S2-Co1 0.33 2.068 Co1-Co7 0.01 2.343 S2-Co1 0.34 2.068 Co2-Co8 -0.25 2.343 S1-Co3 0.35 2.068 Co3-Co8 -0.40 2.343 S1-Co6 0.22 2.068 Co1-Co5 -0.25 2.343 S3-Co4 0.33 2.068 Co3-Co6 -0.20 2.343 S3-Co5 0.33 2.068 Co4-Co5 -0.19 2.343 S3-Co1 0.32 2.068 Co4-Co6 0.11 2.343 S5-Co5 0.31 2.068 Co3-Co5 -0.15 2.343 S4-Co2 0.28 2.068 Co2-Co7 0.36 2.343 S2-Co3 0.34 2.068 Co1-Co8 -0.34 2.343 S1-Mo1 0.51 2.411 S4-Mo2 0.15 2.593 S1-Mo2 0.37 2.413 S1-Co8 0.12 2.764

Co8-Mo1 -0.07 2.546 Co2-Mo2 0.16 2.938 S2-Co5 0.34 2.068 Co4-Co7 0.12 2.343

Table 2: Summary of charge values from Mulliken population analysis on thiocubane-Co8S9/Mo

obtained during CASTEP total energy calculations.

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CONCLUSIONS

HRTEM techniques was implemented as a direct tool for observation and understanding on the interaction between MoS2 and Co9S8 crystal structures, later the implementation of quantum computational molecular modeling in order to reproduced HREM results lead us on a new path to follow which was the understanding of chemical nature on thiocubane core, that particular thiocubane-Co8S9/Mo-S core was treated as an individual entity subjected to total energy calculations for density of states and Mulliken population analysis, results from both have concluded a strong metallic character, created specially from an electron donation of cobalt to molybdenum, which represents the nature of active sites in promoted MoS2 catalyst.

ACKNOWLEDGMENTS The authors want to thank Consejo Nacional de Ciencia y Tecnlogía, Mexico, Instituto de

Ingeniería y Tecnología de Universidad Autónoma de Cuidad Juárez, the Materials Research and Technology Institute of UTEP and Microelectronic research center of UT-Austin. REFERENCES 1. Chianelli, R. R.; Berhault, G.; Raybaud, P.; Kasztelan, S.; Hafner, J. and Toulhoat, H., Applied

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