Modern Research in Catalysis, 2013, 2, 164-171 Published Online October 2013 (
Experimental and Theoretical Properties of
MoS2+x Nanoplatelets
D. H. Galvan1,2, A. Posada Amarillas3, N. Elizondo2, M. José-Yacamán4
1On Sabbatical from Centro de Nanociencias y Nanotecnología,
Universidad Nacional Autónoma de México, Ensenada, México
2Department of Chemical Engineering and International Center for Nanotechnology & Advanced Materials,
University of Texas at Austin, Austin, USA
3Departamento de Investigación en Física, Universidad de Sonora, Hermosillo, México
4Department of Physics and Astronomy, University of Texas at San Antonio, One UTSA Circle, San Antonio, USA
Received April 6, 2013; revised July 5, 2013; accepted August 18, 2013
Copyright © 2013 D. H. Galvan et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
The synthesis and the catalysis in the HDS of DBT reaction of nanostructured self-supported catalyst containing MoS2+x
nanoplatelets have been investigated. Enhancement of higher activity observed in sulfide catalyst sample (d) with re-
spect to the ex situ and in situ references is more closely related to the morphology change of particles (nanoplatelets).
In this work, we suggest that certain structures present in model catalysts maybe related to low dimensional structures
and present a theoretical study of two MoS2 clusters (one made of 34 atoms/cluster and the second one made of 41 at-
oms/cluster), to these clusters seven sulfur atoms were randomly located at the surface of the sulfur layer, in order to
simulate certain structures resembling arrow shaped nanoplatelets that were found in a High Resolution TEM analysis
performed in some MoS2 samples. Additionally, one of the goals is to enquire about the electronic properties presented
in such structures when the clusters terminated as Mo- or S-edge and if it could be correlated to the catalyst behavior of
these compounds. To the 34 atoms/cluster Mo-edge yielded metallic behavior while the second cluster the 41 atoms/
cluster S-edge yielded a semiconductor behavior with a forbidden energy gap Eg of the order of 3.6 eV between the
Valence and Conductions bands respectively. Moreover, to the same clusters enunciated formerly, when the sulfur at-
oms were located at the surface of the S-layer, for the first cluster (34 atoms/cluster) yielded a more metallic behavior,
while the second one (41 atoms/cluster) yielded an isolator behavior. Our results agree with the experimental and theo-
retical results presented by several groups in different laboratories arriving to the conclusion that the S-Mo-S Mo-edge
arrow heads structures could be responsible to the enhancement of the catalytic activity on the MoS2 studied samples.
Keywords: Hydrodesulfurization; Tight-Binding; Catalyst; Tem; Clusters
1. Introduction
The most commonly used industrial hydro treating cata-
lysts for sulfur removal from heavy oils are based on
cobalt or nickel promoted MoS2 [1].
Atom-resolved scanning tunneling microscopy (STM)
studies of catalyst model systems have recently given the
first direct insight into the atomic-scale structure of mo-
del MoS2 nanoclusters and the promoted CoMoS struc-
tures [2,3]. These model systems consisted of a few na-
nometer wide, gold-supported MoS2 and CoMoS nano-
clusters, and with the STM, it was possible to characterize
their geometrical and electronic structure in great detail.
On this basis, new important insight was obtained on the
nanocluster morphology, on the atomic-scale structure of
the catalytically important edges, and on the formation of
sulfur vacancies. It was also shown that the nanocluster
morphology and exact edge structures were sensitive to
reaction conditions [4-8].
Those catalysts consist of small MoS2+x particles of fi-
nite size supported on alumina. A more deep analysis of
these wires shows a remarkable structure which resembles
to the theoretical models used by several groups [1-11].
Figures 1(a) and (b) show how a typical catalyst can
be formed by these nanoplatelets of MoS2 that also show
an enhanced selectivity toward hydrogenation [12]. Two
types of structures can be seen: triangular platelets and
cone shaped platelets. These structures contain metallic
edge sites (brims).
opyright © 2013 SciRes. MRC
Figure 1. Shows that the model catalyst of MoS2 presents
he contrast observed at the tip was measured and
n assumed that the platelet-like structure ob-
2+x (x 0.5) catalysts has been
2. Experimental and Calculations
d by a hydro-
crystalline structures with an arrow shaped nanoplatelets
by High Resolution Electron Microscopy (HREM).
rresponds to 2H-MoS2 basal plane, thus the structure
grows along a direction perpendicular to either {10.0} or
to {11.0}.
It has bee
rved in Figure 1(b) corresponds to a transversal sec-
tion of large MoS2 crystallites. It has been shown by the
group of Haldor Topsøe Laboratory and the University of
Aarhus the existence of metallic one dimensional state on
MoS2 clusters and establishes a new view of the catalytic
active sites. Traditionally, it was believed that the hydro-
desulfurization (HDS) reactions occur at sulfur vacancies
and the metallic edges (the so-called brim sites) on the
MoS2, but in situ STM observations of the interaction
between MoS2 clusters and thiophene indicate activity at
the metallic edges, which seem to be terminated by sulfur
dimers. The catalytic behavior changes strongly depend
on the type of surfaces present in the material [13]. It is
extremely important to determine the relevance of these
findings for real systems.
Recently, a model MoS
ported [13]; this material showed an increased activity
for HDS and selectivity toward hydrogenation, contained
one-dimensional structures like the one shown in Figure
1. This behavior may be attributed to metallic states at
the edges of the nano wires. In the present paper, the syn-
thesis and the catalysis in the HDS of DBT reaction of
nanostructured self-supported catalyst containing MoS2+x
nanoplatelets have been investigated. Moreover, we dem-
onstrate a new way to simulate the platelet arrow head
structures that appear in some MoS2 samples and fur-
thermore to investigate if the platelet-like structures show
a special type of behavior (metallic, semiconductor or
insulator) depending on how the cluster terminates Mo-
or S-edge.
A film of α-MoO3 nanoribbons was prepare
thermal process. The procedure consisted of adding drop
wise a 4 M solution of HCl to a saturated solution of so-
dium molybdate. The mixture is placed in a Teflon-lined
autoclave and left at 423 K for 8 h (precursor of sample
d). Once the reaction time was completed, the product
was filtered and dried. Sections of these films were re-
acted with a stream of H2S gas mixed with 90% inert gas
N2 and 10% H2 in a 9/1 volume ratio at 723 K for 1.5 h,
in order to produce the corresponding sulfide of molyb-
denum (catalyst d). The unreacted excess of H2S was
neutralized with a saturated solution of NaOH.
Specific surface areas of catalysts were det
ith a Nova 1000 series from Quantachrome by nitrogen
adsorption at 77 K, with the BET method. Samples were
degassed under vacuum at 523 K before nitrogen adsorp-
tion. Reference catalysts were prepared by decomposi-
tion of ammonium thiomolybdate (ATM). One of them
was sulfided ex situ in a tubular reactor with 15% vol-
ume H2S/H2 flow at 673 K for 4 h (heating rate 4 K/min)
before catalytic testing; the other one was left to be sul-
fided in situ during the reaction. The decomposition of
ATM precursor is a well-known reaction that occurs very
fast, generating MoS2, NH3, and H2S [14].
The hydrodesulfurization (HDS) of dibe
BT) has been studied as a model reaction of HDS of
Copyright © 2013 SciRes. MRC
petroleum feedstock. For this work the HDS was carried
out in a Parr Model 4522 high-pressure batch reactor.
The catalysts were placed in the reactor with a gram of
each one respectively (ex situ catalysts (1 g) or in situ
catalysts, the appropriate amount of ATM to yield 1 g of
MoS2) along with the reaction mixture (5% weight of
DBT in decalin), then pressurized with hydrogen and
heated to 623 K at a rate of 10 K/min under a constant
agitation of 600 rpm.
When the working temperature was reached, sampling
mercial spent catalysts were pulver-
late the arrow like structures from
r chromatographic analysis was performed to deter-
mine conversion versus time dependence; the reaction
was run for 5 h. Reaction products were analyzed with an
AutoSystem XL gas chromatograph (Perkin Elmer In-
struments) with a 9-ft, 1/8-inch-diameter packed column
containing OV-17, on Chromosorb WAW 80/100 as the
separating phase.
Samples of com
ed and dispersed in isopropanol. A droplet of this sus-
pension was deposited on lacey carbon copper grids for
transmission electron analysis (TEM). The catalyst sam-
ples were analyzed with the aid of a JEOL 2010 F mi-
croscope equipped with a Schottky-type field emission
gun, ultra-high resolution pole piece, and a Scanning-
Transmission (STEM) unit with a high angle annular
dark field detector (HAADF) operating at 200 kV, a Ga-
tan CCD camera was used for image acquisition.
The calculations reported in this work, have been
d out by means of the tight-binding method [15] with-
in the Extended Huckel [16] framework using YAeH-
MOP (Yet Another extended Huckel Molecular Orbital
Program) computer package with f-orbitals [17,18]. The
Extended Huckel method is a semi empirical approach
for solving Schrödinger equation for a system of elec-
trons, based on the variational theorem. In this approach,
explicit electron correlation is not considered except for
the intrinsic contributions included in the parameter set.
More details about the mathematical formulation of this
method have been described elsewhere [19] and will be
omitted here. The calculations were performed in super-
computer Berenice 32 (32 parallel processors) Silicon
Graphics Origin 2000 using the input file accordingly to
each specific case.
In order to simu
gures 1(a) and (b) we constructed the appropriate Mo-
or S-MoS2 systems starting from an infinite hexagonal
array like the one depicted in Figure 2. Notice Figure 2
indicates that it has been constructed from Mo dark
spheres and S white spheres respectively, then construct
a finite x-y and an infinite z-structure in order to simu-
late the appropriate cluster under consideration. From the
selected cluster we simulate a three dimensional S-Mo-S
arrangement just considering that the selected cluster
ought to terminate as Mo or S-edge respectively, as de-
picted in Figures 2(a) and (b). On the same figure, on
Figure 2. (a) Inifinite hexagonal array of Mo and S atoms;
e right hand top side it is possible to find a small
S atoms used through-
(b) Three layers of S-Mo-S to create the appropriate struc-
hatched rectangle terminating on Mo atoms all the way
around, consequently it is called Mo-edge, while on the
same figure we can locate similar hatched rectangle on
the lower left corner terminating of S atoms, hence is
called S-edge. For the Mo-edge MoS2 cluster it was nec-
essary to use 16 Mo plus 18 S in order to construct a 34
atoms/cluster as the one depicted in Figure 2 right hand
side of the figure. Alternatively in order to construct the
S-edge MoS2 left hand side of Figure 2, it was necessary
to use 9 Mo plus 32 S atoms in order to build 41/atoms/
cluster respectively. Moreover, in order to perform a
more realistic experimental image, seven sulfur atoms
were randomly located at the top the sulfur layer, hence
for the S-Mo-S Mo-edge plus seven S-atoms, a new clus-
ter made of 41 atoms/cluster were selected, mean while,
for the S-Mo-S S-edge a 48 atom cluster were selected. It
is good to point out that in order to generate the appro-
priate clusters they aroused from crystalline MoS2 using
the primitive vectors: a = 3.1604 Å, c = 12.295 Å and
space group P63/mmc (194) [20].
Atomic parameters for Mo and
t the calculations were obtained from Alvarez et al.
and provided in Table 1 [21]. It is necessary to stress that
experimental lattice parameters instead of optimized val-
ues were used searching for a best match between our
results with the available experimental information pro-
vided in the literature.
Copyright © 2013 SciRes. MRC
3. Results and Discussion
f α-MoO3 nanoribbons,
t at 90/10 and the inert
e phases, molybdenum dioxide
ursor. It
r hydrogen-
e surface of
those used in indus-
rough the so-called direct desulfurization pathway
We sulfidized sections of a film o
keeping the HS/H ratio constan
2 2
carrier gas was N2. The sulfidation process produces me-
tallic grey powders whose morphology differs from that
one of the parent oxide.
In case of catalyst d, X-ray diffraction showed the pre-
sence of two crystallin
ugarinovite, JCPDS 78-1073) and molybdenum disul-
fide (Molybdenite-2H, JCPDS 65-0160) (Figure 3). We
also perform the quantitative phase analysis by X-ray
powder diffraction dates by the internal-standard method
[22]. The amounts of the MoO2 phase and the MoS2
phase are 82% and 18% in weight respectively.
The catalyst d contains MoS2 nanoplatelets and it was
prepared from α-MoO3 nanoribbons like prec
acted with H2S/H2 under reaction conditions. MoS2
nanoplatelets and the MoO2 were obtained.
The catalytic activity of catalyst d for HDS was of
12.8 × 107 mol·s1·g1 and the selectivity fo
tion was of 1.7. The d catalyst is more efficient in ac-
tivity and selectivity than both ex situ and than in situ
references has can be seen from Table 2.
The material tested for catalytic activity presented a
surface area of 14 m2/g. Compared with th
ference catalysts, sample d presents a typical value for
this kind of catalyst, intermediate between the in situ and
ex situ references (see Table 2).
The HDS of DBT was studied under conditions of 623
K and 3.4 MPa, which are close to
al applications. DBT was chosen because it is consid-
ered an appropriate compound for the investigation of
activity and reaction mechanisms of proposed HDS cata-
The HDS of DBT yields two main products: biphenyl
(BP) th
DS) and cyclohexilbenzene (CHB) and tetrahydrodi-
020406080 100
Intensity (a.u.)
2 (degrees)
* MoS2.
. MoO2
Figure 3. X-ray diffraction pattern of the sample d tested
for the catalytic properties.
(eV) and (Valence orbital
Table 1. Atomic parameters used in the Extended Huckel
tight-binding calculations, Hii
ionization potential and exponent of Slater type orbitals).
The d-orbitals for Mo are given as a linear combination of
two Slater type orbitals. Each exponent is followed by a
weighting coefficient in parentheses. A modified Wolfsberg-
Helmholtz formula was used to calculate Hij [28].
Atom OrbitalHii i1 C1 i2
5s 8.34 1.96
5p 5.241.90
Mo (0.6397) 1.90 (0.6097)
4d 10.50 4.54
3s 20.00 2.12
3p 11.00 1.83
Table 2. Sue acuby the BET equation from
itrogen absorption at 77 K, rate constants (k), and HYD/
rfacrea callated
DDS ratios for HDS of DBT (3.4 MPa of hydrogen, 623 K).
Sample Surface area
before reaction
Rate constant
k (×107)
(±0.5) m/g
2(m 11) ol·s ·g
D 14 12.8 1.
Ex situ
57 7. 1.
9 1.8 0.5
In situ 0 4
phene (TH-DBT) throh the hydrogenative b
enzothio ug
athway (HYD) as can be seen in Figure 4(a). Since
these two pathways are parallel and competitive, the se-
lectivity (HYD/DDS) is determined by [23]:
The experimental constant rate (pseudo-zero-o
cause t
rder be-
he DBT concentration decreased linearly with
time) is given in moles of DBT transformed by second in
1 g of catalyst, and it was calculated from the experi-
mental slope of the plots of DBT concentration versus
time according to the equation:
00 s
1mol1000 mmol34 mmol1 g
Where 34 mmol is the initial concentration of DBT.
Slope1 HrHr36
The pseudo-zero-order rate constant values (k) were
lculated from the slope of the experimental data.
Figure 4 (b) shows the catalytic activity and the distri-
bution of the reaction products of HDS of DBT of cata-
lyst d. The final hydrogenation (HYD) products were
cyclohexylbenzene (CHB) and tetrahydrodibenzothio-
phene (TH-DBT), and biphenyl (BP) of direct desulfuri-
Copyright © 2013 SciRes. MRC
Figure 4. (a) Scheme of the HDS of DBT pathways; (b) Ca-
talytic activity plots for the sample d, con-
The molar selectivity HYD/DDS for hydrogenation
um of 1.7 in sulfide catalyst (for catalyst
MoS clusters
HDS of DBT for
sidering pure MoS2 like the active phase; (c) Molar selectiv-
ity HYD/DDS plots of sample d.
zation (DDS).
reached a maxim
has can be seen from Figure 4(c).
Band structure calculations for the 34 atoms/cluster
Mo-edge and the 41 atoms/cluster S-edge2
ith and without seven S-atoms at the top the S-layer
were calculated using 51 k-points for each case, and sam-
pling the First Brillouin zone (FBZ) as depicted in Fig-
ures 5 and 6(a) and (b), respectively.
K k
Figure 5. (a) and (b) Band structure calculations for S-Mo-
S Mo-edge and S-Mo-S S-edith no sulfur atoms in the
ts the Wigner-Seitz cell for
a hexagonal configuration used in the calculations. The
M (1/2 0 0) of /a.
ge w
surface of S-layer. The inset depicts the Wigner-Seitz cell
for a hexagonal configuration.
The inset in Figure 5 depic
rmi level is indicated by a horizontal dotted line sepa-
rating the valence band (VB) from the conduction band
(CB) respectively. Energy in eV vs k-values were plotted
for each case, ranging from (0 0 0) to K (1/3 1/3 0) to
Copyright © 2013 SciRes. MRC
Figure 6. (a) and (b) Energy bands for S-Mo-S Mo-edge and
S-Mo-S S-edge with seve n sulfur atoms decorated randoml
at the surface of S-layer.
parated by a forbidden gap ran-
gi from 1.0 to 1.9 eV [24,25]. It is mandatory to men-
ecorated at the surface of the S-layer are
the S layer. Notice that Ef level is located
s were randomly decorated on the sur-
p due to the seven sulfur atoms
It is accepted that 2H-MoS2 crystalline bands are split
into three sub bands se
n that the cluster behavior could be expected to be
different than the crystalline MoS2 but the cluster should
preserve some of the properties inherited from the crystal
where it came from. Our calculations yielded indication
of this event.
Energy bands for the 34 atoms/cluster Mo-edge and
for the 41 atoms/cluster S-edge without and with seven
sulfur atoms d
picted in Figures 5(a) and (b) and Figures 6(a) and (b)
Figure 5(a) yields information regarding S-Mo-S Mo-
edge (34 atoms/cluster) without the seven sulfur atoms at
the surface of
ithin two multiple degenerate bands, providing indica-
tion that the system is semi metallic. Whereas, Figure
5(b) yields information about S-Mo-S S-edge (41/atoms/
cluster) without S-atoms located randomly at the surface
of the S-layer. Opposite to the case of Figure 5(a), the Ef
is located at the top of a multiple degenerate band yield-
ing a forbidden gap Eg ~ 3.6 eV yielding information of a
In addition, Figures 6(a) and (b) yielded information
regarding S-Mo-S Mo-edge and S-Mo-S S-edge when
seven sulfur atom
ce of the S-layer to form 41 atoms/cluster and 48 at-
oms/cluster respectively.
Figure 6(a) yielded information regarding S-Mo-S
Mo-edge with seven S-atoms at the surface of the S layer.
The Ef has been shifted u
d it is now located in between three or more multiple
degenerate bands, providing information that the new
system has become more metallic than the case of Figure
5(a). On the other hand, Figire 6(b) yielded information
regarding S-Mo-S S-edge plus seven sulfur atoms deco-
rating the sulfur layer. The Ef has been shifted up pro-
moting a large gap and the system becomes an insulator.
Bollinger et al. [1] reported that one-dimensional me-
tallic states formed at the edges of a single layer of MoS2
clusters, showing that if MoS2 gets confined to a size
here only edges are present, both the geometric and the
electronic configurations will be different from the bulk.
Hence, it is expected that the nanofibers, undergoes a
structural rearrangement preserving a resemblance to the
crystal with a subsequent formation of metallic states.
Similarly, Sun et al. [26] performed an investigation
on the details of the edge surfaces of unprompted and
Ni(Co)-promoted WS2 catalysts using density functional
FT) under generalized gradient approximation (GGA)
considering the effect of reaction conditions. Nickel
tends to substitute the tungsten on the W-edge in the Ni-
Figure 7. Total and projected DOS for S-Mo-S Mo-edge
with seven sulfur atoms located randomly at the surface of
the S-layer.
Copyright © 2013 SciRes. MRC
promoted catalyst, while cobalt rather takes the position
of tungsten at the S-edge in the Co-promoted catalyst.
The total and projected density of states (PDOS) for
the S-Mo-S Mo-edge is depicted in Figure 7. Solid line
represents total DOS, while the hatched dotted lines are
the selected projected DOS for each orbital of the selec-
ted atoms. Horizontal dotted line indicates the Fermi
level. We have shown only the S-Mo-S Mo-edge, due
that catalytic enhancement toward hydrogenation is spe-
culated, to be responsible off. Furthermore, we concen-
trate on those orbitals from the atoms which contributes
most to the total DOS in the vicinity of Ef because they
manifest their metallic behavior.
The overlap (hybridization) is formed by Mo d- and p-
with S p-orbitals of the same symmetry. This hybridiza-
tion could be considered to be responsible of the metallic
states presented at the Mo-edge. Moreover, it is neces-
sary to point out that the contributions from the seven
sulfur atoms randomly located at the sulfur layer, con-
tribute almost nothing to the total DOS. Instead, they par-
ticipate as bridges through out the metallic states are
promoted. Similar behavior has been reported by Reshak
et al. [27] in 2H-MoS2 intercalated with lithium where
they reported that Li has a weak hybridization with Mo
and S-states.
4. Conclusions
We have shown the synthesis and the catalysis in the
on of a nanostructured self-supported
ated at the top of
p the
ges of the MoS clusters decorated with S
e to acknowledge E. Aparicio, M.
chnical support.
[1] M. V. Bollinger, J. V. Lauritsen, K. W. Jacobsen, J. K.
Nørstov, S. Hher, “One-Dimen-
sional MetallicPhysical. Review
HDS of DBT reacti
catalyst containing MoS2+x nanoplatelets. The enhance-
ment of higher activity observed in sulfide catalyst d with
respect to the ex situ and in situ references is more close-
ly related to the morphology change of particles (nano-
Cluster calculations made on the S-Mo-S Mo-edge (34
atoms/cluster) cluster and on the S-Mo-S S-edge (41 at-
oms/cluster) without and with seven sulfur atoms ran-
domly located at the surface the sulfur layer yield the
following conclusions.
Our results yielded indication that the fact to form the
S-Mo-S Mo-edge, S-Mo-S S-edge without and with
seven sulfur atoms on the surface the S-layer, their prop-
erties depart from the crystalline MoS2 but not to a great
extent as to preserve resemblance of the microscopic
From the energy band analysis, the following conclu-
sions were obtained:
1) For the cluster S-Mo-S Mo-edge without the S-at-
oms located at the surface the S-layer yielded metallic
2) For the cluster S-Mo-S S-edge without seven sulfur
atoms at the surface of the S-layer yielded semi conduc-
tor behavior with a reported Eg ~ 3.6 eV between the
lence band and the conduction band.
3) For case a) with seven sulfur atoms located at the
top of S-layer, the system becomes more metallic. While
for case b) with seven sulfur atoms loc
layer, the system indicates an insulator behavior.
Total and projected DOS provide information that
there exists hybridization in the vicinity of the Ef for the
S-Mo-S Mo-edge with seven sulfur atoms at the to
layer. The contributions were from Mo d- and p- with
S p-orbitals. The S p-orbital contributions from the seven
sulfur atoms were negligible, which make us believe that
they serve only as a bridge throughout the metallic states
Our results corroborate experimental findings that the
catalytic activity could be correlated to the existence of
metallic ed2
oms at the top the sulfur layer and suggest a direct way
of tailor catalysts.
5. Acknowledgements
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