Download Promotional Effect of Co or Ni Impurity : An Electronic Structure Study

Promotional Effect of Co or Ni Impurity
in the Catalytic Activity of MoS2:
An Electronic Structure Study
R. GÓMEZ-BALDERAS,1,2 J. M. MARTÍNEZ-MAGADÁN,2
R. SANTAMARIA,2,∗ C. AMADOR1
1
Departamento de Física y Química Teórica, Facultad de Química, UNAM, D.F. 04510, México
Programa de Simulación Molecular, Instituto Mexicano del Petróleo, Eje Central Lázaro Cárdenas 152,
Apdo. Post. 15-805, D.F. 07730, México
2
Received 4 February 2000; revised 13 June 2000; accepted 14 June 2000
ABSTRACT: It has been observed that the catalytic activity of MoS2 crystals is
enhanced when either Co or Ni atoms are added. The presence of these atoms leads to
electronic rearrangements, which are considered the source of catalytic improvement.
However, the relation between the electronic properties and the enhancement of the
catalytic activity is not yet fully understood. In order to get insight into the electronic-level
changes that affect the catalyst performance, a solid-state density functional study has
been carried out for Mo, Co/Mo, and Ni/Mo sulfides, using bulk and surface models. The
MoS2 crystallize in a well-known layered structure, which has been used together with
the supercell model to simulate the (10 1̄ 0) edge surface of MoS2 . The binary sulfides were
obtained substituting Co or Ni by Mo from the original MoS2 bulk model. The electronic
structure in a nonmagnetic state is analyzed and, in particular, the density of states of
metal and sulfur atoms for the surface and bulk are compared. Finally, we discuss the
important role that these properties play in the hydrodesulfurization reaction and
concluded that Mo at the surface remains the relevant reactive atomic center in the
bimetallic systems, whereas Co and Ni are responsible for increasing the Mo reactivity at
c 2000 John Wiley & Sons, Inc. Int J Quantum Chem 80: 406–415, 2000
the surface. Key words: hydrodesulfurization; MoS2 catalytic activity; Co and Ni promotional
effects; LMTO; density functional theory
Correspondence to: R. Gómez-Balderas.
∗ On sabbatical leave from the Instituto de Física, UNAM.
Contract grant sponsor: DGAPA-UNAM.
Contract grant sponsor: Instituto Mexicano del Petróleo.
Contract grant number: FIES-96-15-III.
International Journal of Quantum Chemistry, Vol. 80, 406–415 (2000)
c 2000 John Wiley & Sons, Inc.
CATALYTIC ACTIVITY OF MoS2
Introduction
C
rude petroleum contains a great variety of organic compounds, and many of them contain
sulfur. The catalytic hydrodesulfurization (HDS)
process consists in the transformation of organosulfurs into hydrocarbons plus hydrogen sulfide (H2 S).
Among the transition-metal sulfides (TMS) that catalyze this process, the MoS2 -based catalysts are the
most widely used in the petroleum refinement industry. It is known that addition of different transition metals to MoS2 , such as Co or Ni, leads to a
pronounced enhancement of the MoS2 catalytic activity up to a factor of 10 with respect to that of
the unpromoted MoS2 crystallites [1]; the doping of
MoS2 allows a performance at similar rates as the
Rh2 S3 and RuS2 structures, two of the most active
HDS catalysts. The activity of these bimetallic sulfides is also known to be higher than that of CoS or
NiS.
A large number of investigations have been carried out in order to analyze the structural changes
in the MoS2 crystals (because of the insertion of
Co or Ni atoms) and their corresponding impact
on the catalytic activity [2 – 5]. It has been observed
that the presence of different metals to Mo in the
MoS2 crystal can lead to the formation of different phases [6] due to several possible factors, like
the promoter metal loading, impregnation procedure, calcination, or sulfiding temperatures of the
catalyst [2]. General agreement has been reached
on the formation of Co–Mo–S and Ni–Mo–S catalyst phases, and valuable experimental results [7, 8]
suggest that a Co–Mo–S phase exists with a slab
structure, similar to that of MoS2 .
The metallic sites located on the catalytic surface
represent the major part of active centers in the HDS
process. The metal atoms are exposed, unsaturated,
and available for binding reactant guest molecules.
There is also experimental evidence [9] indicating
that cobalt and nickel promoters can be found in the
(10 1̄ 0) edge surfaces, located at the same planes as
molybdenum atoms. Still, important questions remain about the coordination of Co and Ni, whether
it is tetrahedral, octahedral, or some other one.
The induced rearrangement in the electronic
structure of sulfides caused by the substitution of Co
or Ni for Mo has been also the subject of considerable research efforts. There mainly exist two points
of view: (1) the Co and Ni may act as charge donators on Mo [1, 10]. (2) The promoter atom itself may
become the actual catalytic site [11]. In connection
with the former point, it is believed that the two
main factors responsible for the enhanced activity
of Co(Ni)/Mo sulfide are: (a) the covalent contribution to the metal–sulfur bond strength and (b) the
increase of d electrons associated with Mo. This second factor is considered as the dominant effect of
the promoter. Although this conclusion was reached
through a study with a low-level computational
technique, the SCF-Xα scattered wave method, it
may be expected that a more rigorous approach
does not produce significant changes on the interpretation.
It was also found that varying the transition
metal, from Ti to Ni, next to a Mo atom results
in downward shifts of the 3d-orbital energies, by
an approximate amount of 1.5 eV, thus getting
them closer to the 3p-sulfur orbital energies. In the
presence of Co or Ni, Mo is reduced relative to
Mo in MoS2 and, on the other hand, Mo is oxidized in presence of Cu relative to Mo in MoS2 [1].
Also, the donation/back-donation mechanism has
been proposed for the HDS reaction. Raybaud et
al. found [12] that charge is donated from the sulfur atom toward the nonbonding d-surface states of
the metal atoms, and charge is back-donated from
the bonding d-surface states to antibonding C–S–π
states, for tiophene over MoS2 .
Recently, Toulhoat et al. [13] proposed another
explanation of the promotional effect. They introduce a definition of the metal–sulfur bond strength,
which considers the cohesive energy of the bulk per
metal–sulfur bond. High values of this parameter,
as in MoS2 , imply a low HDS activity since the residence time of the guest sulfur would be greater
over the metallic center. Low values of the metal–
sulfur bond strength, as in Co or Ni sulfides, imply
also a low HDS activity because the residence time
would be now too small. Then, Toulhoat et al. [13]
expect intermediate bond strengths for (Co, Ni)Mo
bimetallic sulfurs between the two previous limits.
As a consequence, the catalytic activity predicted
for these mixed sulfurs should be in the range of
that exhibited by RuS2 or OsS2 . The metal–sulfur
bond strengths would explain the promoting effect
of Co(Ni) impurities, allowing an optimal residence
time of the guest sulfur over the active site in the
HDS process.
The catalytic phenomenon, which involves the
breaking and formation of new chemical bonds, is
considered to take place on the active surface of
bimetallic TMS catalyst. Surface science can help
us to understand the catalytic process by analyzing
electronic rearrangements and their role in the sul-
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407
GÓMEZ-BALDERAS ET AL.
fur affinity for metals. In the present contribution,
we consider the interaction between the Co and Ni
promoters with Mo and investigate, with the use of
solid-state density functional theory, the electronic
structure modifications of Co/MoS2 and Ni/MoS2
edge surfaces. We assume a slab chemistry model
with Co/Ni atoms positioned at the (10 1̄ 0) edge
plane, which is expected to give a reasonable active
surface model.
It is well established that pure transition-metal
sulfides such as CoS, NiS, and MoS2 have a lower
catalytic activity than mixed crystals such as Co/
MoS2 and Ni/MoS2 . In order to get insight into the
causes that lead to catalytic differences, we analyze
the electronic structure of different systems through
the study of their density of states (DOS). We will
focus our attention on metallic atoms at the surface,
which are the active sites responsible for catalytic
transformations in the HDS process of organosulfur
compounds.
The empty intense states of unsaturated Mo
atoms, located at the edge surface of MoS2 , present
just above of the Fermi level, are responsible for
the charge acceptor properties of the surfaces, although a detailed study of the bonding with the
adsorbed molecule would be necessary in order to
quantify the acceptor properties. Our analysis of
the enhanced catalytic activity, induced by promotional effects of inserted Co/Ni metals in MoS2 , will
be made on that basis, namely, the intensity of the
states near the Fermi level, which are understood to
indicate reactivity indexes.
Even when the definition of local quantities in
extended systems may be arguable, they are useful
for the interpretation of our results. Therefore, we
compute the charge projection onto atomic sites for
valence states in the following way:
Z EF
Nlν (ε) dε,
Qlν =
−∞
where l is an angular momentum index, ν indicates
the atomic site, and Nlν is the density of states projected onto the atomic site and angular momentum.
Hereafter we will refer to Qlν as the atomic orbital
population.
Methodology
We employ first-principles density functional
theory (DFT) to compute the electronic structure
of sulfide crystals assuming a nonmagnetic state.
We resort in particular to the Kohn–Sham version
408
of DFT, where monoelectronic Schrödinger-type
equations for periodic systems are solved selfconsistently. The electronic exchange and correlation energy terms are treated in the local density
approximation, using a parameterized form of the
exchange and correlation potentials of the homogenous electron gas, initially proposed by Hedin and
Lundqvist [14] and von Barth and Hedin [15].
We employed the linear muffin-tin orbitals
(LMTO) method in the atomic sphere approximation (ASA) [16] in our ab initio calculations, as implemented in the ESOCS program [17], version 4.0.0.
In the solution of the Schrödinger equation, ESOCS
adopts atom-centered potentials of the muffin-tin
type, with spherical symmetry, and a constant potential in the interstitial region. The solutions are
given as linear combinations of muffin-tin orbitals
and obey the Bloch theorem to produce a periodic wave function suitable for extended systems
like crystals and surfaces. Once the solutions are
obtained, the density of states can be calculated
and projected onto the different atomic sites. The
analysis of results is focused on the behavior of the
density of states, but special attention is given to
those states around the Fermi level.
We note that usually valence electrons for sulfur include 3s and 3p orbitals, nevertheless, our
preliminary calculations showed that the band associated with the 3s orbital is far away from the Fermi
level (−13 eV) and, therefore, we do not take it into
consideration in the foregoing discussion. The 3s orbital is contemplated as a core state and the 4s, 3p,
and 3d orbitals constitute the sulfur valence states.
For the case of transition-metals cobalt and nickel,
we employ 3d, 4s, and 4p valence orbitals, and for
the case of molybdenum the 4d, 5s, and 5p orbitals
are used as valence states. Our preliminary calculations also showed that spin-polarized results differ
slightly from the spin-compensated results, practically giving no variation in the density of states, and
maintaining our conclusions essentially unchanged.
Modeling
We have analyzed bulk and surface models of
the MoS2 , Co/MoS2 , and Ni/MoS2 structures. In the
pure MoS2 crystal (space group P63 /mmc) the Mo
atom is located at the center of a trigonal bipyramid
formed by S atoms. The bulk structure is composed
of two layers of molybdenum at z = 14 and z = 34
positions in the cell, and each Mo layer is sandwiched between sulfur layers. The S–Mo–S sheets
VOL. 80, NO. 3
CATALYTIC ACTIVITY OF MoS2
FIGURE 1. Representation of the bulk model, a = 9.468 Å, b = 9.468 Å, and c = 12.275 Å. Selected atoms for DOS
analysis are labeled and displayed as balls.
are stacked in the (001) direction in the sequence
. . . BCBCBC. . . (layers of Mo in bold) and are held
together by weak van der Waals interactions between S atoms. Our bulk model for the pure MoS2
crystal is shown in Figure 1. Three rows of trigonal prisms in the y direction and three rows in the
x direction form the unit cell, which has 54 atoms
in the composition Mo18 S36 , and is periodically repeated in the x, y, and z directions. The unit cell of
the Co(Ni)/MoS2 bulk is obtained by replacing a
Co(Ni) by a Mo atom. The lattice parameters that
were used to build the unit cell were a = 9.468 Å,
b = 9.468 Å, and c = 12.275 Å, which correspond
to experimental parameters of the MoS2 crystal [18].
The atomic sphere radius of sulfur is rS = 1.5261 Å,
of molybdenum is rMo = 1.4677 Å, and of cobalt and
nickel are rCo = rNi = 1.2711 Å. In order to satisfy
the ASA condition, empty spheres are added in such
a way that the final packing of atoms plus empty
spheres is a hexagonal closed packed structure. The
radii of the empty spheres are adjusted for both bulk
and surface models, accordingly with the atomic
sphere radii. The maximum allowed sphere overlap is 15% of the total cell volume. For instance, the
bulk empty sphere radius is 1.3061 Å and the surface empty sphere radius is 1.4376 Å, for the MoS2
system.
The (10 1̄ 0) edge surface is modeled using supercells built from two S–Mo–S sheets stacked in
the z direction. Each sheet consists of trigonal prism
rows (two in the y direction, plus terminal molybdenum atoms, and three in the x direction). In the
y direction the slabs are separated by 10.52 Å, and
empty spheres fill this space. The entire supercell
structure, including the empty spheres, is hexagonal
closed packed and has the following dimensions:
a = 9.468 Å, b = 18.936 Å, and c = 12.275 Å.
Figure 2 shows the main features of our surface
FIGURE 2. Representation of the (10 1̄ 0) surface, a = 9.468 Å, b = 18.936 Å, and c = 12.275 Å. Side view shows
layers 1, 2, and 3 as L1, L2, and L3. Atoms for which DOS is analyzed are labeled and displayed as balls.
INTERNATIONAL JOURNAL OF QUANTUM CHEMISTRY
409
GÓMEZ-BALDERAS ET AL.
model. In the bimetallic surfaces the promoter atom
is located in a molybdenum position. The surfaces
studied expose unsaturated metal atoms as well
as sulfur atoms. Toulhoat et al. claim that reconstruction of the surface is expected to be moderate
for transition-metal sulfides with respect to that
observed in the transition metal [13]. Using ab initio molecular dynamics simulations, Raybaud et al.
have demonstrated that the (10 1̄ 0) edge surface
suffers small relaxations and remains stable at the
usual temperatures of HDS reactions [19]. Hence, in
the present approach, atoms at the surface are fixed
in the bulk-terminated geometry.
Results and Discussion
MoS2
The MoS2 compound is an important catalyst
component used in the refining industry, and previous studies exist on its crystal structure and electronic features. For instance, Raybaud et al. [19]
have performed band structure calculations of the
MoS2 bulk, by working within the generalized gradient approximation of DFT and using a plane wave
basis set. They have shown the mixture of the 4d
bands of molybdenum with the 3p bands of sulfur
and identified the MoS2 bulk as a semiconductor
with a gap of Eg = 0.89 eV. We have performed similar calculations for the MoS2 bulk, and our results
are in good agreement with the density of states obtained by Raybaud et al. [19]. The fact that ESOCS
reproduces previous results that were obtained with
a different approach gives us confidence to describe with similar accuracy the electronic structure
of bimetallic compounds. Figure 3 shows the MoS2
bulk total density of states projected onto the Mo
and S atoms, as well as the global DOS. This figure
also gives the Mo 4d and S 3p states (represented
by dotted lines in the graph), which appear at the
same energy interval, and essentially determine the
band structure. A gap of 1.3 eV between occupied
and unoccupied states is shown at the Fermi level,
the experimental value for this gap being 1.3 eV [19].
This fortuitous match, possibly due to error cancellation, should not be regarded too seriously given
the well-known usual underestimation of band gaps
by local density methods. Nevertheless, the existence of this gap characterizes the MoS2 crystal as
a semiconductor, if we consider that most semiconductors such as germanium, silicon, or GaAs have
energy gaps lower than 2 eV [20]. Above the Fermi
410
FIGURE 3. Atomic site projected DOS for pure MoS2 in
the bulk model.
level, molybdenum exhibits unoccupied states with
higher intensities than sulfur, consequently, Mo has
the highest probability to accept electrons or, equivalently, Mo is more electrophilic than S.
In regard to the MoS2 (10 1̄ 0) crystal surface, Raybaud et al. [19] used a supercell model consisting of
a slab of 72 atoms (in a Mo24 S48 composition). Their
layer separation was 12.8 Å and they only allowed
the first MoS2 layer to relax to simulate a stable surface structure. They found that the surface remains
metastable in the bulk-terminated geometry, except
for small relaxations that have little influence on
the electronic DOS. Even when we use a smaller
model without surface relaxation, a three-layer slab
supercell model with 54 atoms (in a Mo18 S36 composition) with a layer separation of 10.52 Å, the
relevant features of the system are reproduced when
we compare with the result of Raybaud.
In Figure 4, we give the Mo and S density of
states for each layer of sheet B for the slab model,
together with the DOS of the bulk. If we compare
layer 1 molybdenum atoms at the surface (referred
hereafter as Mo1) with molybdenum in the bulk,
the bulk shows a gap of 1.3 eV, contrary to the surface, which hardly shows any gap. The lack of a
gap at the surface is associated to the shift of the
Mo1 empty states toward the Fermi level, due to
VOL. 80, NO. 3
CATALYTIC ACTIVITY OF MoS2
tron donor/acceptor character. This feature is related with the donation/back-donation mechanism,
which is due to the surface.
Although, it would not be determinant to discuss
chemical trends when charge differences are of the
order of 0.01, the population analysis seems to be
consistent with this electron donor/acceptor character provided by the DOS results. From Table I, Mo2
in the bulk has 4.18, 0.58, and 0.37 electrons in the d,
p, and s shells, and Mo1 at the surface has 4.23, 0.40,
and 0.33 electrons in the d, p, and s shells, respectively. Consequently, the surface raises the electron
donor/acceptor character, diminishing the s and p
populations (also participating in donation) and increasing the d population (back-donation).
We also stressed the considerable amount of
charge allocated on the empty spheres sites, whose
radius is 1.3061 Å. In the bulk, as shown in Table I, the total charge on these sites is 0.37 electrons;
this charge counterbalance the atomic sites charge
ensuring the electroneutrality of the system. In the
case of interstitial spheres in the surface model,
whose radius is now 1.4376 Å, they have 0.56 electrons. In connection with the empty spheres located
between slabs, we observe for the surface model a
variation from 0.27 electrons just above the surface
to 4.5 × 10−5 electrons a half distance between slabs.
This fact validates our model since the electronic
density is mainly spread in the slab region.
FIGURE 4. Atomic site projected DOS for (10 1̄ 0)
surface of pure MoS2 . The Mo and S densities of states
of MoS2 bulk are shown for comparison.
the lower coordination of Mo at the surface. The
increased amount of occupied and unoccupied surface states around the Fermi level associated to
Mo1 suggests that this type of atom has a elecTABLE I
Projected charges by angular momentum and total charges by atomic sites.a
MoS2
Mo1
Bulk
l
0
1
2
3
Total
Surface
l
0
1
2
3
Total
a Total
0.33
0.40
4.23
0.10
5.06
Mo2
S1
0.37
0.58
4.18
0.14
5.27
0.03
3.74
0.31
0.10
4.18
0.42
0.64
4.23
0.15
5.44
0.04
3.57
0.36
0.11
4.08
Co/MoS2
Sphere
Mo1
0.37
0.34
0.39
4.22
0.10
5.05
Mo2
S1
Co
0.38
0.57
4.20
0.15
5.30
0.04
3.70
0.28
0.09
4.11
0.38
0.44
7.46
0.03
8.31
0.42
0.64
4.25
0.15
5.46
0.04
3.59
0.32
0.10
4.05
0.31
0.29
7.56
0.02
8.18
Ni/MoS2
Sphere
Mo1
0.39
0.34
0.39
4.21
0.10
5.04
Mo2
S1
Ni
Sphere
0.38
0.57
4.20
0.15
5.30
0.04
3.71
0.28
0.09
4.12
0.44
0.48
8.36
0.03
9.31
0.39
0.42
0.64
4.25
0.15
5.46
0.04
3.61
0.32
0.09
4.06
0.38
0.34
8.40
0.02
9.14
empty sphere charges are also included in the case of bulk models.
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411
GÓMEZ-BALDERAS ET AL.
The molybdenum and sulfur bands of layer 2 (as
seen in Fig. 4) start to transform onto the DOS of the
bulk, but it should take deeper layers in a thicker
slab model to resemble that of the bulk. Layer 3 is
equivalent by symmetry to layer 1, thus when analyzing layer 3 sheet B, we will have the complete
electronic structure picture of layer 1 with surface
and subsurface Mo and S sites. In contrast with
layer 1, the particular importance of layer 3 sheet
B is that its main contribution comes from sulfur at
the surface, which shows a relatively large intensity
of states at the Fermi level. This region of the (10 1̄ 0)
surface is supposed to be not active in the HDS reactions.
Surface generation produces a decrease in the
charge populations of surface and subsurface
atoms, but an increase in the bulklike atoms of this
model. From Table I, the total charge of Mo1 at the
surface diminishes 0.21 valence electrons, and the
subsurface S1 charge decreases 0.10 electrons with
respect to the corresponding atoms in the bulk. In
contrast, Mo2 has 0.23 valence electrons more than
in the bulk.
Co/MoS2
The main results for Co embedded in the MoS2
bulk are presented in Figure 5. The density of states
for the S1 sulfur (first neighbor) and the Mo2 molybdenum (second neighbor) are also depicted there.
These atoms are the closest neighbors to Co and
are expected to show relevant changes in their electronic structure. If we compare the graphics of MoS2
bulk (Fig. 3) with those of MoS2 bulk doped with
Co, the presence of new intense states are evident
around the Fermi level, which partially fill the former gap that characterized the MoS2 bulk.
As we approach the Fermi level from the occupied states region, Co depletes the density of states
of Mo2 and S1, producing a gap just below the Fermi
level. The largest peaks belong to Co compared with
those of Mo2 and S1.
FIGURE 5. Atomic site projected DOS for Co/MoS2 and Ni/MoS2 bulk models.
412
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CATALYTIC ACTIVITY OF MoS2
FIGURE 6. Atomic site projected DOS for Co/MoS2 and Ni/MoS2 (10 1̄ 0) surfaces.
In regard to the effects produced by Co inserted
at the MoS2 surface (Fig. 6), we have analyzed the
DOS of Co, Mo1, and S1 in layer 1 and Mo2 in
layer 2. They all are localized in the B sheet (see
Fig. 2). Energy gaps are not observed for any atom
in Figure 6. In the occupied DOS portion, Co shows
the largest intensity values in comparison with the
other atoms, but Mo1 has the most intense peak
in the unoccupied portion, certainly an effect produced by the presence of Co. The largest Mo1 DOS
in this region gives a charge-acceptor character to
this atom, favoring the interaction with sulfur. On
the other hand, Mo2 has a charge-donor character
and seems to be less affected than Mo1 because its
spectrum looks similar to that of Mo2 in the pure
MoS2 crystal (Fig. 4). If we compare the DOS of
Co in the bulk with Co at the surface, the gap that
appears before the Fermi level in the bulk (Fig. 5)
disappear when Co is at the surface (Fig. 6). This
feature is also observed for Mo1 and S1 and may be
taken as a clear fingerprint to discriminate between
the surface and bulk for this doped crystal.
In analyzing the charge distribution, Table I
shows that Mo2 in the Co/MoS2 bulk system has
a total valence charge of 5.30 electrons. By comparing this value with molybdenum in the pure MoS2
(5.27 electrons), Mo2 is chemically reduced because
of the Co impurity. The chemical reduction of Mo
in Co/MoS2 and in Ni/MoS2 relative to Mo in pure
MoS2 has been also previously observed [1, 10]. On
the other hand, the total valence charge of S1 is
4.11 electrons, 0.07 less than in the pure MoS2 . Now,
when the surface is present, as we observe in the undoped MoS2 , the total valence charge also decreases
in the surface and subsurface atoms of the Co/MoS2
system, from a value of 5.30, 4.11, and 8.31 electrons
to 5.05, 4.05, and 8.18 electrons for Mo1, S1, and Co,
respectively. In analyzing the populations by angular momentum, Mo2 in the bulk has 4.20, 0.57, and
0.38 electrons for the d, p, and s shells, and Mo1 at
the surface gives 4.22, 0.39, and 0.34 electrons for the
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413
GÓMEZ-BALDERAS ET AL.
d, p, and s shells, respectively. Correspondingly, Co
in the bulk gets 7.46, 0.44, and 0.38 valence electrons
for the d, p, and s shells, while Co at the surface has
7.56, 0.29, and 0.31 electrons for the d, p, and s shells,
respectively. Again, the surface raises the electron
donor/acceptor character, diminishing s, and p populations (involved in the donation) and increasing
the d population (back-donation) in the Mo and Co
metallic atoms at the surface.
Ni/MoS2
The results for Ni embedded in the MoS2 bulk are
presented in Figure 5, as well as for Mo2 molybdenum and the S1 sulfur. Although the total density of
states of Ni in the bulk differs in quantitative form
from that of Co, the curves of Ni follow a similar
pattern to the Co curves, as it would be expected
from a rigid band model. The Mo2 and S1 total density of states also resemble that of Co/MoS2 .
In regard to the Ni/MoS2 surface, Ni does not
show relevant peaks in or near the Fermi level, and
we can hardly associate a charge acceptor or donor
character to the Ni atom. In general the DOS curves
of Mo1, Mo2, and S1 resemble the corresponding
curves of the Co/MoS2 surface. Due to the fact that
the Ni/MoS2 and Co/MoS2 surface DOS are similar, it is likely that these systems show an equivalent
chemical performance, except at the specific sites
where the Ni or Co atoms are located. A charge
analysis also reveals a similar behavior of Co/MoS2.
From Table I it is observed that total charge decreases in the Mo1 and S1 surface atoms and increases in Mo2 subsurface atoms when bulk and
surface models are compared. A projected angular
moment population also gives the same trends as in
the case of Co/MoS2; that is, the s and p populations
decrease in the metal atoms, but the d population is
increased.
Promoter atoms in the bulk produce a slight increase of 0.02 electrons in the Mo 4d population
for both bimetallic systems, as compared with that
of the pure MoS2 . However, we observe a small
decrease in the 4d population of Mo1 if we compare the pure MoS2 surface with bimetallic surfaces.
Hence, the back-donation character of Mo1 at the
bimetallic surfaces would be reduced by the presence of impurity atoms. In contrast, Co and Ni 3d
populations are increased 0.10 and 0.04 electrons or,
equivalently, the back-donation character of Co and
Ni is increased at the surface.
414
Conclusions
The MoS2 crystal structure is an effective catalyst component commonly used in the oil-refining
industry. The insertion of Co or Ni into the MoS2
structure is known to increase the catalytic activity.
In order to understand the improved catalytic performance of the doped MoS2 crystal, we carried out
electronic structure calculations for MoS2 bulk and
surface models. Our results confirm the picture in
which Co and Ni atoms transfer charge to Mo, increasing its 4d electron density. A relative chemical
reduction of Mo subsurface atoms of the (10 1̄ 0)
surface was also shown in the presence of Co and
Ni atoms. As long as the density of states is a relevant parameter for describing reactivity, our results
indicate that the electronic structure of Mo at the
surface is very similar in both bimetallic sulfides
Co/MoS2 and Ni/MoS2 , which have a similar catalytic activity. This fact suggests that molybdenum
at the surface is the relevant atomic center for the
reactivity in the hydrodesulfurization process. On
the other hand, Co and Ni exhibit different electronic structure at the (10 1̄ 0) surface; however, they
are responsible for increasing the reactivity of the
MoS2 surface. Then, they can not be considered as
the central atomic sites in the hydrodesulfurization
process.
The catalytic reaction mechanism involves charge
redistribution induced by the adsorption of the
guest sulfide molecule onto the MoS2 surface. In
fact, charge flows from molecular orbitals of the
donor to the molecular orbitals of surface atoms.
Such a step is favored by the insertion of promoters that produce a shift of unoccupied states from
higher energies to the Fermi level. On the other
hand, it is established that back-donation, from
metal to antibonding molecular states, facilitates the
weakening of a carbon–sulfur bond. Our results
suggest that the back-donation process is not favored by molybdenum 4d orbitals but favored by
promoter 3d orbitals at the surface.
ACKNOWLEDGMENTS
One of us (R.G.B.) acknowledges support from
DGAPA-UNAM. Financial support from Instituto
Mexicano del Petróleo (Ref. num. FIES-96-15-III) is
also acknowledged.
VOL. 80, NO. 3
CATALYTIC ACTIVITY OF MoS2
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