Highly Active Low Cobalt Content-Based Bulk MoS2 Hydrodesulfurization Catalysts with a Unique Impact of H2S

Article abstract of DOI:10.1155/2018/8076534

A series of unsupported MoS₂, Co₉S₈, and Co-promoted MoS₂ catalysts have been synthesized by tuned impregnation and successive thermal annealing methods using a continuous flow of a mixture of H₂ and H₂S gases. The resulting catalysts were evaluated in terms of their activity and selectivity for the hydrodesulfurization of dibenzothiophene (DBT) both in the absence and the presence of H₂S. The inclusion of Co onto MoS₂ affected both the hydrogenation and direct desulfurization reactions, with the latter (production of biphenyl) being magnified to a much greater degree than the former. Interestingly, low cobalt/molybdenum ratio of ca. 0.05 of the catalyst exhibited outstanding promotion efficiency in the hydrodesulfurization reaction. However, as cobalt is added, the synergy effect drastically decreased. H₂S in the reaction mixture led to a remarkable step up in the product from the direct desulfurization reaction route with the most notable increases occurring for the product from the hydrogenation reaction pathway. The HDS activity of such catalysts was much higher than that of the commercial CoMo/Al₂O₃. The promotion by H₂S was discussed.

Full text of DOI:10.1155/2018/8076534

Hindawi
Journal of Chemistry
Volume 2018, Article ID 8076534, 9 pages
https://doi.org/10.1155/2018/8076534
Research Article
Highly Active Low Cobalt Content-Based Bulk MoS2
Hydrodesulfurization Catalysts with a Unique Impact of H2S
,
Hamdy Farag ,1 2 Abd-Alrahman Embaby,3 Masahiro Kishida,2
Abdel-Nasser A. El-Hendawy,4 and Mohamed Mahmoud Nasef5
1Chemistry Department, Faculty of Science, Mansoura University, Mansoura 35516, Egypt
2Department of Material Process Engineering, Graduate School of Engineering, Kyushu University, Motooka 744,
Fukuoka 819-0395, Japan
3Geology Department, Faculty of Science, Damietta University, Damietta 34517, Egypt
4Physical Chemistry Department, National Research Center, 12622 Dokki, Cairo, Egypt
5Chemical Engineering Department, Universiti Teknologi Petronas, 32610 Seri Iskandar, Perak, Malaysia
Correspondence should be addressed to Hamdy Farag; hamdy.farag@gmail.com
Received 9 April 2018; Revised 6 July 2018; Accepted 17 July 2018; Published 3 September 2018
Academic Editor: Mohammad A. Al-Ghouti
Copyright © 2018 Hamdy Farag et al. +is 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.
A series of unsupported MoS2, Co9S8, and Co-promoted MoS2 catalysts have been synthesized by tuned impregnation and
successive thermal annealing methods using a continuous flow of a mixture of H2 and H2S gases. +e resulting catalysts were
evaluated in terms of their activity and selectivity for the hydrodesulfurization of dibenzothiophene (DBT) both in the absence and
the presence of H2S. +e inclusion of Co onto MoS2 affected both the hydrogenation and direct desulfurization reactions, with the
latter (production of biphenyl) being magnified to a much greater degree than the former. Interestingly, low cobalt/molybdenum
ratio of ca. 0.05 of the catalyst exhibited outstanding promotion efficiency in the hydrodesulfurization reaction. However, as cobalt
is added, the synergy effect drastically decreased. H2S in the reaction mixture led to a remarkable step up in the product from the
direct desulfurization reaction route with the most notable increases occurring for the product from the hydrogenation reaction
pathway. +e HDS activity of such catalysts was much higher than that of the commercial CoMo/Al2O3. +e promotion by H2S
was discussed.
Mo-based Al2O3 catalysts. Such catalysts are proved to be
highly active for thiophenes and benzothiophenes sulfur
elimination. However, they are not sufficiently active for
desulfurizing compounds such as dibenzothiophene (DBT),
and it is analogous. +is is particularly problematic because
these compounds are the major sulfur-containing species
remaining in the middle distillate fractions and in the at-
mospheric or heavy residues. +e MoS2 phases are the main
catalysts in hydrothermal treatment reactions such as
hydrodesulfurization (HDS). +e addition of Co to sup-
ported MoS2 catalysts has been studied intensively, and its
role as a promoter in HDS reactions is well established.
MoS2-based catalysts with no promoter have very low ac-
tivities towards HDS reactions. +e significant enhancement
in the HDS performance of these catalysts following the
1. Introduction
Significant research efforts have been directed towards the
development of catalysts for better reducing the sulfur
contents of petroleum fuel fractions [1–4]. Dwindling oil
supplies, especially those extracted under vacuum distilla-
tion, and recent stringent environmental regulations aimed
at limiting the sulfur contents of transportation fuels have
resulted in a strong demand for improved hydrothermal
treatment techniques. Clean fuels, such as sulfur- and
nitrogen-free fuels, not only have the advantage of being
environmentally benign transportation fuels but can also be
used in several other emerging energy-related fields, in-
cluding fuel cells [5]. +e main catalysts used in hydro-
thermal treatment processes are Ni- and/or Co-containing

2
Journal of Chemistry
incorporation of Co has been attributed to the synergy
between the Co and Mo phases [6–9]. +e details of this
synergy have been investigated extensively in previous oc-
casions [10–12]. However, the influence of the incorporation
of Co on the selectivity of such MoS2-based catalysts towards
the HDS reactions is far from being fully understood [13–
17]. +e nature of the interactions between the Mo support
and the substrate can have a significant impact on the ef-
ficiency of the synergy caused by Co and/or Ni promoters.
On the contrary, the function of the promoter in the bulk
MoS2 in the HDS reaction remains beyond a complete
coverage. Catalysts of this type have high activities in HDS
reactions [18]. Although a large number of reports are
available for CoMo-based catalysts, relatively very little
information has been published pertaining to the un-
supported CoMo catalysts. A better understanding of how
Co or Mo modifies the activity of bulk MoS2 or Co9S8
catalysts could provide valuable information for the de-
velopment of new Mo-based catalysts for hydrothermal
treatment processes. In this study, the DBT HDS reactions
using bulk MoS2, Mo-Co9S8, and Co-MoS2 catalysts con-
taining various amounts of Co, especially in low cobalt
concentration range, were investigated. +e potential of Co
as a promoter of the bulk MoS2 catalyst is dealt with. +e
effects of the hydrogen sulfide matrix on the catalytic
performance of the HDS reaction of DBT were also
evaluated.
heating in a vacuum oven at 120°C overnight. All catalysts
were thereafter subjected to sulfidation with a stream of
a 1 : 9 (v/v) mixture of H2S/H2 gases (5 ml/min) at 400°C.
+e resulting samples were denoted as C-AHSG- and
C-ATS-, I to III based on their Co loading. Another sample
was synthesized in which the prepared Co9S8 phase was
impregnated with a certain amount of an alcoholic solution
of molybdenum acetylacetonate following the typical
procedure mentioned before until obtaining the sulfide
form of the catalyst. +is sample (denoted as MPC) rep-
resents the Mo-promoted cobalt sulfide catalyst of ca. 0.4 of
the Co/(Co + Mo) atomic ratio. +e commercial CoMo/Al2O3
catalyst was also studied for comparison. All Mo-sulfide
samples exhibited a hexagonal molybdenite-2H structure
(JCPDS# 65-0160) according to the XRD patterns of AHS,
AHS-G, and ATS catalysts shown in Figure 1, which is in
agreement with literature [20].
No phases other than MoS2 were detected. On the
contrary, the diffraction patterns of the synthesized Co-
sulfide phase matched well with the JCPDS# 73-1442 of
Co9S8. +e AHS-G and ATS catalysts had MoS2 crystallite
sizes of approximately ≈5 nm. Transmission electron mi-
croscopy images (obtained from TEM; JEOL-2000EX) for
C-ATS-I were depicted in Figure 2. Five to 10 layers of MoS2
is obviously noted. Interlayer spacing of ca. 0.62 nm was
determined from the (002) XRD peak at 2θ of 14.2.
2.2. Activity Measurements. +e catalysts were investigated
for the DBT HDS reaction. DBT was selected as a model
compound for these reactions because it is a representative
of some of the sulfur-containing refractory compounds
found in middle distillates and heavy residue. All tests were
implemented under a 3 MPa of H2 pressure and at 340°C.
+e experiments were conducted using a stainless steel batch
microautoclave reactor (100 mL) equipped with a magnetic
stirrer and stainless steel filter, which allowed convenient
withdrawal of small samples from the reaction mixtures at
regular time intervals. A decane solution of DBT (1 wt.%)
was used as the reaction feedstock. Some reaction runs were
conducted in the presence of Cu powder (ca. 0.7 g), which
was used as a scrubber for the H2S produced as a by-product
during the HDS reaction. A blank run was also conducted
with Cu in the absence of a catalyst, which confirmed that Cu
did not exhibit any activity towards the DBT HDS reaction.
+e effect of the reaction matrix was evaluated by in-
vestigating the reaction over the present catalysts in both
with and without the existence of H2S in the feedstocks.
Immediately before the reaction test, the catalyst was once
more subjected to a sulfidation step with a mixture of
H2S/H2 gases. +e catalyst and the reaction mixture (a
2. Experimental
2.1. Materials and Methods. Cobalt acetate tetrahydrate
[Co(C2H3O2)2·4H2O], ammonium heptamolybdate tetrahy-
drate [(NH4)6Mo7O24·4H2O] (AH), ammonium tetrathio-
molybdate [NH4]2[MoS4] (AT), molybdenum acetylacetonate
[C10H14MoO6], DBT, and decane were supplied from Wako
Chemical Industries, Ltd. All chemicals were exploited as
received. +e bulk MoS2 catalysts, AHS and ATS, were
prepared by heat annealing the corresponding Mo precursors
(AH and AT) in the presence of a concurrent flow of a 1 : 9
(v/v) mixture of H2S/H2 gas at 830°C and 400°C, respectively.
Co9S8 (CS) was also synthesized from the heat annealing of
Co-acetate tetrahydrate at 400°C following the same pro-
cedure as before. +e details of such processes have been
described elsewhere [19]. A sample of the AHS material
(obtained by sulfiding the ammonium heptamolybdate tet-
rahydrate precursor) was comminuted using a mill with an
inner volume of 100 mL equipped with a media of zirconia
beads, which was purged with He prior to being used. +e
resulting material was denoted as AHS-G. +e BET surface
areas of AHS, AHS-G, and ATS, which were measured using
automatic Micromeritics ASAP 2010 instrument by N2
adsorption-desorption technique at −196°C, were approxi-
mately >10, 115, and 65 m2/g, respectively. +e AHS-G and
ATS sulfide samples were used as preliminary supports for the
addition of the Co precursor. +e Mo-sulfide phase was
impregnated with a 1 :1 (v/v) mixture of water/alcohol
containing a specific amount of cobalt acetate tetrahydrate
at an ambient temperature. +e solution was then subjected to
sonication for 3 h, followed by drying in air and thereafter
typical of 15 mL of 1 wt.% DBT in decane) were loaded in
situ into the reactor, which was subsequently pressurized
with H2 and heated to 340°C under continuous stirring at
1000 rpm. Small samples of the reaction mixture (0.1–
0.2 mL) were withdrawn from the reactor periodically for
analysis to determine the rate of conversion. Gas chroma-
tography (Agilent HP 6890) and GC-mass spectrometry
(GC-MS) equipped with an Agilent HP 5970 MS were used

Journal of Chemistry
3
Co8S9
C-ATS-I
ATS
AHS-G
AHS
0
30
60
2θ
Figure 1: XRD patterns of the AHS, AHS-G, ATS, C-ATS-I, and Co9S8 catalysts.
(a)
(b)
Figure 2: TEM images of the C-ATS-I catalyst.
to analyze the reactions. All GC analyses were conducted on
involve the initial formation of H4-DBT and its isomers by
partial hydrogenation of DBT, followed by further reduction
of these intermediates to give CB and (2) direct de-
sulfurization (DDS), which would produce BP directly by
the C-S bond scission. ꢀese data were treated kinetically
and evaluated according to the model that was recently
developed for consecutive-parallel reactions [21]. Based on
this approach, we used the following di‡erential equations to
calculate the catalytic constants:
a methylsiloxane capillary column (0.32 mm × 50 m).
3. Results and Discussion
3.1.CatalystPerformances. ꢀe main HDS reaction products
obtained under the operating terms described above over the
present catalysts were cyclohexylbenzene (CB), biphenyl
(BP), 1,2,3,4-tetrahydrodibenzothiophene (H4-DBT), and
H2S. Trace quantities of the di‡erent isomers of partially
hydrogenated DBT were also detected. Based on these
products, we have proposed two reaction pathways for DBT
HDS, which are depicted in Scheme 1.
−k0t
0
CA ꢀ CA exp
,
0
−k03t
−k04t
0
CA k1
−k0t
−k0t
CB ꢀ
exp
− exp
,
,
ꢀ
ꢀ
ꢁ
ꢁ
k03 − k0
(1)
ꢀe rate constants for these transformations were esti-
mated by ˆtting the experimental data using a nonlinear
least square analysis on the supposition that the HDS re-
action behaves kinetically as pseudo-ˆrst-order [3, 4, 8, 9].
Figure 3 shows the ˆt curves for the DBT HDS over the
C-ATS-I catalyst under di‡erent levels of H2S.
CA0 k02
CC ꢀ
exp
− exp
k04 − k0
where k0 ꢀ k1K1 + k2K2 and k1 and k2 denote the DDS and
the HYD intrinsic kinetic rate constants, respectively. K1
and K2 point to the constants of DBT adsorption at equi-
librium onto the DDS and HYD reaction sites, respectively.
ꢀe reaction system was classiˆed into two reaction
routes, including (1) hydrogenation (HYD), which would

4
Journal of Chemistry
DBT
S
BP
CoMo/Al2O3 catalysts. However, DBT HDS proceeded to
a lower activity over the C-AHS-G catalysts than it did over
the bare commuted MoS2 catalyst. ꢀe di‡erence in the
activity of these two systems could be attributed to the
delicate structure of the commuted MoS2 as a primary
support, which led to di‡erences in the speciˆc surface areas
of these catalysts (the surface area (BET) of the AHS-G
catalyst was 110 m2/g, and therefore, it is much higher than
that of the C-AHS-G-II catalyst, which was less than 5 m2/g).
ꢀus, commuted MoS2 may not be a recommended pre-
liminary support for Co promotion. Taken together, these
results indicate that the selectivity proˆles of the catalysts
need to be analyzed in a greater detail to develop a better
view of the changes in the activity following the impreg-
nation of the catalysts with Co. In this study, the selectivity
was estimated according to the determined ratio of the HYD
to the DDS rate constant, that is, kHYD/kDDS. One should be
careful when considering the catalytic selectivity according
to the product distributions because the selectivity in this
case underwent signiˆcant mobile changes depending on the
level of conversion (Figure 4). When the bare MoS2 catalysts
were used, the HDS of DBT proceeded preferentially by the
HYD route, as shown in Table 1. ꢀe relatively high se-
lectivity observed in this case for the products resulting from
the HYD route is contrary to the common view that DBT
HDS proceeds mostly by a DDS route. ꢀese results
therefore highlight the Ÿexibility of the HDS reaction, in the
sense that it can proceed by these routes with no path
limitation to either of them.
H2S
(A)
(B)
k1
k3
k2
H2S
(D)
(C)
k4
S
H4-DBT
CHB
Scheme 1: Reaction scheme for HDS of dibenzothiophene.
1
0.5
ꢀe impregnation of Co onto the ATS catalyst (C-ATS
series of catalysts) led to a signiˆcant development in the
overall HDS activity. One may notice the obvious pioneer Co
promotion in the HDS of these catalysts, especially for the
C-ATS-I catalyst. ꢀis catalyst showed a remarkable ten-
dency towards the DDS pathway (ca. 1∼2-fold higher activity
towards the DDS route over the HYD route (Table 1)) for the
HDS of DBT, whilst the bare MoS2 catalysts exhibited
a preference for the HYD pathway. ꢀe change in the activity
of the Co-containing catalyst was accompanied by an ob-
vious shift in its selectivity towards the BP product. ꢀis
tendency towards the DDS pathway has been reported
previously for the conventional MoS2-based catalysts as
a result of the impregnation of a Co promoter [5]. ꢀese
results therefore demonstrate that the HYD and DDS re-
action pathways occur in parallel and can be independently
manipulated. Notably, all Co catalysts prepared in this study
underwent the HDS reaction preferentially by the DDS
pathway, and a clear correlation between the selectivity
and activity of the HDS reactions could be established
(Figure 6(a)). It is noteworthy mentioning that the trend in
the selectivity of these reactions was in accordance with
similar reports from the literature for related systems [3].
Figure 6(b) displays the association between the reaction
activity of the C-ATS catalysts and the Co/(Co + Mo) ratio.
It is obvious that the C-ATS-I catalyst, which had 0.05
Co/(Co + Mo) ratio, exhibited the highest HDS activity of all
of the catalysts belonging to this series. Interestingly, such
ratio is far lower than those commonly reported for con-
ventional Co-Mo-based catalysts, where the average is
0
0
100
200
Time (min)
300
400
Theoretical with H2S
Experimental with H2S
Theoretical no H2S
Experimental no H2S
Figure 3: Pseudo-ˆrst-order plots for the HDS reactions of DBT
over the C-ATS-I catalyst in the absence and presence of H2S.
ꢀe kinetic parameters were adjusted for each compound
according to the following formula: k0 ꢀ knKn, where Kn
and kn are the equilibrium adsorption nand intrinsic kinetic
rate constants of each compound, respectively. ꢀis treat-
ment provided a better estimation of the individual con-
tributions of the hydrogenation and direct desulfurization
reaction pathways to the HDS reaction. Furthermore, this
process allowed more reliable quantiˆcation of the indi-
vidual reaction rates according to the contribution of each
reaction pathway. ꢀe experimental data were solved and
ˆtted to the model using the Mathcad program. ꢀe esti-
mated activity data are listed in Table 1.
Figures 4 and 5 show the nonlinear curves that were
ˆtted with the product yields from the HDS reaction over the
C-ATS-1 catalyst in the presence and in the absence of H2S,
respectively, using the model described above. All catalysts
tested in this study provided a reasonable ˆt to the data
exposed from the HDS reaction. ꢀe C-ATS-I catalyst
displayed much higher level of reaction activity than
the ATS, AHS-G, C-AHS-G, MPC, and commercial

Journal of Chemistry
5
Table 1: Activities of the catalysts towards the HDS of dibenzothiophene.
Individual activity
Products at 50%
conversion level
(sec−1 g·cat. ·10
)
Co/(Co + Mo)
HDS activity
−1
−4
Catalyst
Level of H2Sa
−1
−4
atomic ratio
(sec–1 g·cat. ·10
)
k1b
k2c
k2/k1
BP
23
8
23
8
46
43
29
43
28
37
27
43
37
24
CB
H4-DBT
L
H
L
H
L
H
H
L
H
L
H
L
H
H
24
77
10
15
3.9
6.7
191
112
21
14
20
127
194
1.6
1.8
0.8
14
62
1.4
4.0
1.2
4.7
0.1
0.14
0.8
0.2
0.7
0.4
0.8
0.12
0.4
1
25
29
24
28
3
2
13
3
AHS-G
ATS
—
—
9
4.8
31.4
20
38
14
1
211d
128d
38
e
CoMo/Al2O3
C-AHSG-I
C-AHSG-II
0.27
0.02
0.23
16
6
1
17
16
6
17
12
18
6
5
17
33
184
352
1.8f
2.6f
1.6f
3
1
13
5
57
1
C-ATS-I
0.05
158
0.2
0.8
0.8
5
0.40
—
—
1
MPC
CS
12
22
1
4
aLevel of H2S: low (L) and high (H) denote pressures of ca. 3 and 20 kPa, respectively. bApparent rate constant for the direct desulfurization route, 1st order.
cApparent rate constant for the hydrogenation route, 1st order. dNormalized to the metal sulˆde content. eCommercial catalyst Co, 3.2%, and Mo, 13.7%, in
terms of the weight percent. Pseudo-zero-order rate constant, × 1017 molecule/(g·S). ꢀe uncertainty was within 5–10%.
f
0.8
0.6
0.4
0.2
0
0.6
0.4
0.2
0
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Fraction converted
1
0
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9
Fraction converted
1
BP theoretical
BP experimental
CB theoretical
CB experimental
H4-DBT theoretical
H4-DBT experimental
BP theoretical
BP experimental
CB theoretical
CB experimental
H4-DBT theoretical
H4-DBT experimental
Figure 4: Transformation of dibenzothiophene over the C-ATS-I
catalyst in the presence of H2S.
Figure 5: Transformation of dibenzothiophene over the C-ATS-I
catalyst in the absence of H2S.
generally around 0.3. However, the present results are
generally in a trend with other studies where low Co con-
centration catalysts were reported to be active for hydro-
genation and hydrogenolysis reactions [22, 23]. ꢀe
selectivity for the HYD reaction was clearly e‡ected by the
amount of Co incorporated into the existing catalyst. ꢀe
results in Figure 6(b) show that the tendency towards the
HYD reaction decreased with the increase in Co content.
ꢀese results therefore may indicate that only a limited
number of active Mo and S sites can be further activated by
the inclusion of Co. ꢀese sites were assumed to be in the
type II CoMoS phase.
ꢀe active sites of HDS catalysts are coordinately un-
saturated sites. ꢀe incorporation of promoters such as Co
and Ni can lead to improvements in the supply of spillover
hydrogen. ꢀis may lead to an increase in the DDS and HYD
activities [9, 12, 15, 16, 21]. It is therefore envisaged that
these catalysts must contain di‡erent types of active sites to
allow them to catalyze multiple reactions in parallel. ꢀe Co
atoms incorporated into the MoS2 catalysts could occupy the
active sites on their surface. ꢀe enlargement in the catalytic
activity of the Co-containing MoS2 catalysts (i.e., C-ATS
catalysts) compared with the bare MoS2 catalyst could be
attributed to the occurrence of a large synergistic e‡ect
(Table 2) between Co and Mo atoms. A synergistic e‡ect of
this type could potentially result in the formation of two
di‡erent Co-based active sites for the DDS and HYD re-
actions. ꢀe Co atoms could therefore occupy di‡erent
positions on the surface (i.e., edges and corners) of the
unsaturated MoS2 sites, resulting in new sites. However, not
all of these sites would be accessible for the HDS reaction of
the substrate. ꢀe results therefore imply that the higher
activity of these compounds could be attributed to an in-
crease in the e‡ectiveness of the active sites. ꢀe corner-edge

6
Journal of Chemistry
400
300
200
100
0
400
300
200
100
0
C-ATS-I
C-ATS-II
C-ATS-III
0.30
ATS
0.00
0.05
0.10
0
0.2
0.4
0.6
0.8
1
Co/(Co + Mo), atomic ratio
Selectivity, HYD/DDS
HYD
DDS
(a)
(b)
Figure 6: E‡ect of the Co content on the HDS activity (a) and the relationship between the activity and the selectivity (b).
Table 2: Changes in the selectivity of the HDS reaction for the MoS2-based catalysts caused by Co and H2S.
Comparison of the average fold change (increase or decreases) in the activity of the ATS catalyst due to the inclusion of Co
Reaction condition
Catalyst
Absence of H2S
Presence of H2S
C-ATS-I
+33
+12
C-ATS-I
+29
DDS
HYD
DBT
+5
Comparison of the average fold change (increase or decrease) in the activity caused by H2S
Catalyst
C-AHSG-II
+1.4
AHS-G
+1.5
C-ATS-I
+1.5
ATS
+1.7
+6.5
CoMo/Al2O3
−1.7
DDS
HYD
DBT
+4.3
+4.4
+2.8
−1.3
+, increase; −, decrease.
model [24–26] can be used to explain the di‡erences in the
selectivity proˆles of promoted and unpromoted MoS2
catalysts for the DDS route. ꢀe Mo atoms at the corner sites
of the stacked cluster would probably be unsaturated be-
cause of the steric conˆguration of the hexagonal crystals in
the catalyst. ꢀere could also be a high proportion of co-
ordinately unsaturated sites in this case. It can be concluded
that the selectivity of the catalysts for the DDS reaction
correlates highly with the number of MoS2 layers. Nikulshin
et al. [27, 28] in studies for the HDS over CoMo-based
catalysts showed that the (Co/Mo) edge ratio is directly
proportional with the catalyst HDS activity. ꢀey further
revealed that, with increasing the cobalt content, the HDS
selectivity turns slightly towards HYD but still the pre-
dominant route is the DDS. To date, our preliminary results
have shown that the low content of the Co-promoted MoS2
catalyst, especially the C-ATS catalysts, and led to an in-
crease in the overall activity. On the contrary, MPC catalyst
exhibited low HDS performance. Furthermore, the reaction
followed pseudo-zero-order kinetics (Figures S1–S3).
treatments of these experimental results are presented in
Table 1. Several clear and interesting trends can be observed.
For example, the presence of H2S resulted in a slight rise in
the formation of BP (DDS route product). However, the
inclusion of H2S also drove to a remarkable positive change
in the tendency of the reaction towards the HYD route and
the production of CB. Table 2 shows that the presence of H2S
led to a rise in the activity of the present catalysts by in-
creasing the rates of the HYD and DDS reactions by ap-
proximately 2.5- to 4-fold and 1.4- to 1.7-fold, respectively.
ꢀe promotional trends of H2S towards the DDS reaction
were very similar for all of the prepared catalysts. For in-
stance, the enhancement in the activity resulting from the
inclusion of H2S was much more pronounced for the HYD
reaction route, especially in case of the bare MoS2 catalyst.
ꢀe HYD pathway contributed to more than 80% of DBT
HDS over AHS-G and/or ATS catalysts. H2S inŸuence on
the HDS reaction has been discussed extensively in various
studies [29–35], which consensually suggested that H2S
suppresses the DDS route, whilst having very little impact on
the HYD route. In contrast, several research groups, in-
cluding our own, have reported that some transition metal
sulˆde catalysts can e‡ectively promote the HYD reaction
when they are carried out in the presence of H2S [36–38].
Guernalec et al. [38] correlated this positive behavior to the
increase of the -SH concentration on the catalyst surface.
However, our obtained results represent a unique catalytic
3.2. Impact of H2S. ꢀe H2S matrix could also have a sig-
niˆcant impact on the behaviors of the catalysts developed in
this study for the HDS reaction. ꢀe activities and selec-
tivities of the current catalysts towards DBT HDS varied
considerably depending on the existence of H2S in the re-
action medium. ꢀe parameters derived from the kinetic

Journal of Chemistry
7
trend in which the use of H2S may promote the HDS re-
action over unsupported Co-promoted MoS2 catalysts. ꢀe
increase observed in the catalytic activity in presence of H2S
reported here is contrary to the well-known observation that
H2S severely inhibits the HDS reactions that are conducted
over conventional catalysts. ꢀis is evident from the data
highlighting the inhibition of the commercial CoMo/Al2O3
catalyst by H2S presented in Tables 1 and 2. Interestingly,
this reaction (impacted by H2S) was found to be reversible
indicating that H2S may have caused no permanent changes
to the structure of the catalyst. Taken together, these results
demonstrate that our collective understanding of H2S in-
Ÿuence on the selectivity of the HDS reaction is rather in-
complete. Crystallite sizes in the extent of 4-5 nm for the MoS2
crystallites appeared to exert a speciˆc catalytic performance,
which deˆnitely depends on how such catalyst is synthesized.
ꢀe nature of substrate, catalyst, and reaction conditions
therefore appear to be of considerable importance for
quantitative evaluation of this phenomenon.
9
6
3
0
0
0.1
0.2
0.3
0.4
Co/(Co + Mo)
Figure 7: Cobalt content as a function of synergy promotion.
absence of H2S. ꢀe major enhancement was in the HYD
reactions of DBT. ꢀe MoS2 and Co-containing MoS2
catalysts contain acidic sites on their surfaces of two di‡erent
types, including (i) Lewis acidic sites, that is, the sulfur
vacancies on the Mo and Co atoms, and (ii) Brønsted acidic
sites. ꢀe latter of these two sites would contain the -SH and
-SH2 groups. ꢀe -SH sites may act to eliminate sulfur from
DBT and the HYD reactions (under certain circumstances),
whilst the -SH2 sites would favorably be involved in hy-
drogenation reactions. ꢀe involvement of these active sites
in the HDS reaction would be dependent on their availability
with the appropriate geometry to interact with the substrate
[8, 35]. Given that the Co-S bond is relatively weaker than
the Mo-S bond, the Co sites would be more acidic than the
Mo sites [39]. ꢀis diversity in the acidity of the two sites
could explain the higher DDS activity of the Co-containing
MoS2 catalysts compared with the unpromoted MoS2 cat-
alyst. ꢀe cleavage of the S-C bonds is most likely to occur
with greater ease over the Co-promoted catalyst. ꢀis
suggestion is also consistent with the results obtained for the
commercial CoMo/Al2O3 catalyst (Table 1). It has therefore
been suggested that the inclusion of H2S may result in new
active sites through interactions with vacant sulfur sites on
the MoS2 and Co-containing MoS2 catalysts. ꢀe quality of
these sites for promoting or suppressing the HYD and
hydrogenolysis reactions would probably be dependent not
only on their conˆguration and concentration but also on
the structure of the substrate layers. ꢀe data shown in
Table 2 provide a summary of the e‡ects of Co and H2S on
the HDS reaction. ꢀe main points that could be drawn from
the data are as follows:
3.3. SynergyEꢀect. ꢀe results of this study clearly show that
the incorporation of Co had a decisive inŸuence on the
catalytic performance of the unsupported MoS2 catalysts
(series C-ATS). It is noteworthy mentioning that the results
are consistent with the well-known promoting e‡ects of
conventional supported MoS2 catalysts. Taken together,
these results suggest that the HDS reaction resulted from
divergent active sites. ꢀe synergy between the Co and Mo
phases appeared to be on line with that claimed for the well-
known Co-Mo-S type II materials, such as the supported Co-
Mo catalysts [10], which are assumed to be responsible for
catalyzing HDS reactions. All C-ATS catalysts prepared in
the present study exhibited some degree of synergy between
the Co and Mo atoms. ꢀis synergistic interaction enhanced
the HDS reaction by promoting the DDS route to a higher
extent than that of the HYD route. ꢀis result indicated that
the Co-based sites were more e‡ective for the C-S bond
scission. ꢀus, it can be stated that the incorporation of Co
not only improved the activity of the catalyst but also
modiˆed the selectivity along the two pathways. Figure 7
shows the relation between the synergy factor (SF, activity
ratio of (Co-MoS2/MoS2)) and the Co-added ratio. Calcu-
lations based on the density functional theory (DFT) for
such system have predicted that the sulfur and the metal
edges of the catalyst may conduct the DDS reactions [34, 35].
ꢀese DFTstudies have also shown that the Co atoms can be
accommodated on the rims of the MoS2 crystallites espe-
cially on the rims of the sulfur atoms.
(1) ꢀe incorporation of Co (low concentration range)
into the unsupported MoS2 catalyst led to a signiˆ-
cant activity boost in the DBT HDS reaction, as well
as resistance to the inhibition phenomena caused by
H2S.
(2) ꢀe Co atoms preferentially may occupy the edge
sites of the MoS2 catalyst [34, 35].
(3) H2S inclusion in the HDS reaction feedstock led to
an enhancement in the hydrogenation activities of all
the studied catalysts.
It has been suggested that the HYD reactions most likely
occur on the brim sites and the metal edges. In the present
study, CB was isolated as the major product from the re-
actions catalyzed by the bare MoS2 catalyst, which indicated
that the HYD pathway was favored under these conditions.
However, Table 1 shows that the share from the DDS route
was higher than that of the HYD route when the HDS re-
action was conducted over the C-ATS-I catalyst. ꢀe overall
HDS activities of the ATS and C-ATS catalysts in the
presence of H2S were much higher than those in case of the

8
Journal of Chemistry
(4) +e incorporation of Co into the MoS2 catalyst
resulted in a drastic selectivity change towards the
DDS reaction, which increased by an order of
magnitude over the HYD reaction.
S3: product distribution curves of DBT HDS over the MPC
catalyst in the absence of H2S at 340°C and 3 MPa H2.
(Supplementary Materials)
(5) +e presumable Co-SH2 active sites may be much
more effective for the HDS reaction than the Mo-SH2
active sites.
(6) Mo may, however slightly, promote the Co9S8 cat-
alyst in the HDS reactions.
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Conflicts of Interest
+e authors declare that they have no conflicts of interest.
Acknowledgments
+is work was supported by Kyushu University, Japan, and
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Supplementary Materials
Figure S1: pseudo-zero-order kinetics of HDS of DBT over
the MPC catalyst in the presence or absence of H2S. Figure
S2: product distribution curves of DBT HDS over the MPC
catalyst in the presence of H2S at 340°C and 3 MPa H2. Figure

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