Hydrodesulfurization of thiophene over few-layer MoS2 covered with cobalt and nickel nanoparticles

Article abstract of DOI:10.1002/cplu.201300012

The hydrodesulfurization reaction of thiophene has been investigated over graphene-like few-layer MoS₂ as well as over Co- and Ni-nanoparticle-covered few-layer MoS₂. Conversion of thiophene to n-butane over few-layer MoS₂ is superior to that over bulk MoS ₂; the percent conversion is approximately 64 % at 450 °C. However, over few-layer MoS₂ covered with Co or Ni nanoparticles the conversion increases to approximately 98 % at around 375 °C as a result of the smaller nanoparticles being more effective. Less is more: Hydrodesulfurization of thiophene has been carried out using graphene-like few-layer MoS₂ with conversion of approximately 64 %. Further conversion was enhanced to 98 % when Co or Ni nanoparticles covered the surface of MoS₂ (see figure). Copyright

Full text of DOI:10.1002/cplu.201300012

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DOI: 10.1002/cplu.201300012
Hydrodesulfurization of Thiophene over Few-Layer MoS2
Covered with Cobalt and Nickel Nanoparticles
Bolla Govinda Rao, H. S. S. Ramakrishna Matte, Piyush Chaturbedy, and C. N. R. Rao*[a]
The hydrodesulfurization reaction of thiophene has been inves-
tigated over graphene-like few-layer MoS2 as well as over Co-
and Ni-nanoparticle-covered few-layer MoS2. Conversion of
thiophene to n-butane over few-layer MoS2 is superior to that
over bulk MoS2; the percent conversion is approximately 64%
at 4508C. However, over few-layer MoS2 covered with Co or Ni
nanoparticles the conversion increases to approximately 98%
at around 3758C as a result of the smaller nanoparticles being
more effective.
Introduction
Hydrotreatment of petroleum feed stock is an important indus-
trial process with direct implications for our environment.[1] Of
the different hydrotreatment processes, hydrodesulfurization
(HDS) is of concern to us in this study. The HDS reaction is
generally carried out at around 3008C to remove sulfur from
thiophenes by treatment with hydrogen over a heterogeneous
catalyst. Transition-metal oxides,[2] phosphides,[3] nitrides, and
carbides[4,5] as well as metal nanoparticles supported appropri-
ately have been employed as catalysts for the HDS reaction.
CoMo (CoO+MoO3) on various supports such as AlMCM-41
(4008C), TiA (3508C), and AlMCM-48 (4008C) have also been
used for the HDS of thiophene, which provided conversions of
approximately 54, approximately 90, and approximately 85%,
respectively.[6–8] Tungsten phosphide exhibits 67% conversion
of dibenzothiophene at 4008C. Molybdenum nitride supported
on alumina has been used for HDS of thiophene with a catalyt-
ic conversion of 79% at 3608C. HDS of thiophene on molybde-
num carbide at 4208C was shown to give approximately 23%
conversion. HDS of thiophene over Pt, Pt–Pd, Pt–Rh, and Pt–Ru
nanoparticles supported on FSM-16 at 4008C gave conversions
of approximately 95, 73, 55, and 50%, respectively.[9] The other
family of catalysts for HDS of thiophene are MoS2 and cobalt-
or nickel-covered MoS2. HDS with a MoS2 crystal gave a catalyt-
ic conversion of approximately 11% at 3508C.[10] Co- and Ni-in-
corporated MoS2 prepared sonochemically and supported on
alumina show a conversion of approximately 40% at 3508C for
this reaction.[11] Crystalline MoS2 impregnated with cobalt gave
a conversion of 28.8% at 4008C.[12] Tenne et al.[13] reported that
WS2 nanotubes covered with cobalt nanoparticles exhibit a cat-
alytic conversion of approximately 12% for the HDS of thio-
phene at 6008C. Cheng et al.[14] reported good catalytic effi-
ciency for the HDS of thiophene and its derivatives over MoS2
nanotubes decorated with Ni.
Over the last three years, there has been considerable re-
search into graphene-like inorganic materials such as MoS2 and
MoSe2. Single- as well as few-layer MoS2 have been prepared
by different methods.[15,16] We considered it important to inves-
tigate the HDS reaction of thiophene on graphene-like MoS2
nanostructures (Scheme 1).
Scheme 1. The hydrodesulfurization of thiophene on graphene-like MoS2
nanostructures.
The HDS reaction over few-layer MoS2 and also over few-
layer MoS2 covered with cobalt and nickel nanoparticles was
carried out. The effect of the size as well as loading of these
metal nanoparticles on the reaction were examined. The re-
sults have been most encouraging for the HDS reaction of
thiophene over few-layer MoS2 covered with small Co or Ni
nanoparticles, with the conversion reaching 98% at 3758C.
Results and Discussion
In Figure 1a, we show the XRD pattern of few-layer MoS2 pre-
pared by us. The absence of the (002) reflection and the pres-
ence of (100) and (110) reflections suggest the few-layer char-
acter of MoS2. A typical TEM image of the few-layer MoS2 is
presented in Figure 2a and a high-resolution image in Fig-
ure 2b. An AFM image and the height profile are shown in Fig-
ure 2c to confirm the presence of four layers of MoS2 in the
sample (height of 2.6 nm). The optimum conditions for the
HDS reaction were standarized with respect to the various pa-
rameters such as weight of the catalyst and flow rate and the
[a] B. G. Rao, Dr. H. S. S. R. Matte, P. Chaturbedy, Prof. C. N. R. Rao
Chemistry and Physics of Materials Unit
International Centre for Material Research
and CSIR Centre of Excellence in Chemistry
Jawaharlal Nehru Centre for Advanced Scientific Research
Jakkur P.O., Bangalore 560064 (India)
E-mail: cnrrao@jncasr.ac.in
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Figure 1. XRD patterns of (a) few-layer MoS2, (b) cobalt-nanoparticle-covered
few-layer MoS2, and (c) nickel-nanoparticle-covered few-layer MoS2. Triangles,
few-layer MoS2; asterisks, metal nanoparticles.
Figure 3. TEM images of (a) 10 and (b) 15 nm cobalt-nanoparticle-covered
few-layer MoS2. Histograms of the particle-size distributions of metal nano-
particles are shown as insets. 10 nm cobalt-nanoparticle-covered few-layer
MoS2 with (c) low and (d) high loading.
The HDS reaction of thiophene was carried out on Co-cov-
ered few-layer MoS2. Figure 4a shows the temperature varia-
tion of the conversion on these surfaces. With 15 nm-sized Co-
nanoparticle-covered MoS2, the conversion is 83% at 4258C.
For MoS2 covered with 10 nm-sized Co nanoparticles, the con-
version efficiency is 97.8% at 3758C. These results clearly
Figure 2. a) TEM image of few-layer MoS2, (b) HREM image of few-layer
MoS2, (c) AFM image of few-layer MoS2, and d) temperature variation of the
conversion of thiophene over few-layer MoS2.
reaction was carried out only with few-layer MoS2. In Figure 2d,
we show the temperature variation of the conversion of thio-
phene over few-layer MoS2. Thiophene starts to convert to n-
butane at 2508C. The activity increases with increase in tem-
perature and reaches a maximum conversion of 63.9% at
4508C. The conversion observed by us is superior to that re-
ported in the literature for bulk MoS2 and MoS2 nanotubes.[14]
It appears that the two-dimensional morphology of MoS2 with
many edges (Figure 2b) favors the HDS reaction of thiophene.
To increase the catalytic activity, cobalt or nickel nanoparti-
cles were deposited on the surface of few-layer MoS2 by in situ
thermal decomposition of the metal precursors. This method
allows a good control over the size and loading of the nano-
particles. The XRD pattern of the composite of few-layer MoS2
with cobalt nanoparticles shows the characteristic reflections
of MoS2 and cobalt (Figure 1b). TEM images of the Co–MoS2
composites with 10 and 15 nm-sized nanoparticles are shown
in Figure 3a and b, respectively. The histograms of the particle
distribution of the nanoparticles are inset in these figures.
Figure 4. a) Temperature variation of the conversion of thiophene over few-
layer MoS2 covered with 10 and 15 nm cobalt nanoparticles. b) Temperature
variation of the conversion of thiophene at different loadings of 10 nm
cobalt nanoparticles.
reveal that the catalytic activity increases with the decrease in
the size of the Co nanoparticles. Thiophene starts to convert
to n-butane even at 2758C over Co-covered few-layer MoS2.
The yield of butane is much higher than that over clean MoS2
alone. The effects of nanoparticle loading on the catalytic effi-
ciency were studied. For this purpose, 10 nm-sized cobalt
nanoparticles were deposited on few-layer MoS2 to different
extents (23 and 35.5 wt%) as shown in the TEM images in Fig-
ure 3c and d. As revealed in Figure 4b, we do not see much
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difference in the conversion with different nanoparticle load-
ings. A conversion of around 98% at 3758C was obtained re-
gardless of the loading.
Conclusion
In conclusion, few-layer MoS2 is superior to bulk MoS2 as a cata-
lyst for the HDS of thiophene. The conversion of thiophene to
n-butane is further improved on few-layer MoS2 surfaces cov-
ered with Co or Ni nanoparticles, with a conversion of nearly
98% at 3758C. Smaller Co or Ni nanoparticles further favor the
HDS reaction of thiophene.
The XRD pattern of nickel-nanoparticle-covered few-layer
MoS2 in Figure 1c shows reflections corresponding to nickel
and MoS2. The TEM image in Figure 5a shows nickel nanoparti-
cles covered on the surface of MoS2 sheets. The average diam-
eter of the nickel nanoparticles is approximately 20 nm as can
Experimental Section
Synthesis of various MoS2 layers
Few-layer MoS2 was prepared by reacting molybdic acid with an
excess amount of thiourea (molybdic acid/thiourea 1:48) at 5008C
for 3 h under a nitrogen atmosphere. After the sample had been
cooled to room temperature, the black product obtained was used
for further analysis.[15]
Composites of few-layer MoS2 with cobalt nanoparticles were pre-
pared by means of the thermal decomposition method. In a typical
reaction, few-layer MoS2 (170 mg) was dissolved in o-dichloroben-
zene (4 mL) and sonicated for 30 min. To this solution, oleic acid
(0.1 mL) and trioctylphosphine oxide (TOPO; 50 mg) were added at
1828C, followed by injection of cobalt carbonyl (0.27 g) dissolved
in o-dichlorobenzene (3 mL). This mixture was heated at reflux for
10 and 30 min to obtain 10 and 15 nm-sized particles decorated
on the few-layer MoS2, respectively.[20] The suspension was centri-
fuged and the solid product was kept in a vacuum oven and dried
at 408C for 24 h. The extent of loading of the 10 nm metal nano-
particles was varied by varying the amount of cobalt coverage
(0.139 g for low loading and 0.549 g for high loading), by the addi-
tion of oleic acid (50 mL/0.2 mL) and TOPO (25/100 mg) at 1828C
followed by the injection of cobalt carbonyl dissolved in o-dichlor-
obenzene (3 mL) and heated at reflux for 10 min. The suspension
was centrifuged and the solid product collected was allowed to
dry in a vacuum oven at 408C for 24 h. The weight percentages of
the 10 nm-sized particles were approximately 23, approximately
27.5, and approximately 35.5 wt% and for the 15 nm-sized particle
were approximately 39 wt%.
Figure 5. a) TEM image of nickel-nanoparticle-covered few-layer MoS2. A his-
togram of the particle-size distribution of metal nanoparticles is shown as
an inset. b) Temperature-dependent conversion of thiophene over few-layer
MoS2 covered with 25 and 20 nm nickel nanoparticles.
be seen from the inset in the figure. A composite of few-layer
MoS2 with nickel nanoparticles of approximately 25 nm diame-
ter was also synthesized. The weight percentages of the nickel
nanoparticles on few-layer MoS2 were 29 and 41.5 wt% for the
20 and 25 nm nanoparticles, respectively. The HDS of thio-
phene over few-layer MoS2 covered with both 20 and 25 nm-
sized nickel nanoparticles was carried out and the results are
presented in Figure 5b. The MoS2–Ni composite with 25 nm-
sized particles shows a conversion of 82.7% at 4258C. A con-
version of 94% at 3758C was obtained with the 20 nm-sized
nickel nanoparticles. Clearly, the smaller Ni nanoparticles favor
higher conversion. The conversion of thiophene to butane in
the present study involves the minimum amount of Ni and the
lowest temperature compared with reports in the literature.
A few comments on the catalytic mechanism of the HDS re-
action involving MoS2 and Co/Ni-covered MoS2 are in order.
The catalytic activity of MoS2 sheets is considered to arise from
the edge structure.[17] There will be more edges in few-layer
MoS2 than in the bulk sample. The presence of edges in few-
layer MoS2 has indeed been demonstrated recently.[15,18] The re-
sults obtained by us clearly point to the role of the Co/Ni
nanoparticles as well. The nanoparticles would be associated
with a lower enthalpy of chemisorption of hydrogen, which
would favor dissociation.[14] The Co/Ni atoms in the nanoparti-
cles would also be more effective in removing sulfur from thio-
phene.[19]
Few-layer MoS2 covered with nickel nanoparticles were prepared
as follows. Few-layer MoS2 (170 mg) and nickel acetylacetonate
(0.256 g/0.4 g) was dissolved in oleylamine (3.68 mL) in a 25 mL
round-bottomed flask and heated to 1008C under a nitrogen at-
mosphere followed by addition of trioctylphosphane (TOP; 2.5 mL).
At 2208C the mixture was heated at reflux for 30 min and 2 h to
obtain 20 and 25 nm-sized nickel nanoparticle-decorated few-layer
MoS2, respectively.[21] The suspension so obtained was centrifuged,
washed several times with ethanol, and the obtained black prod-
uct was allowed to dry in a vacuum oven at 408C for 24 h. The
weight percentage of the 20 and 25 nm-sized particles was ap-
proximately 29 and approximately 41.5, respectively.
Characterization
XRD patterns were recorded with CuKa radiation using a Rich-Sie-
fert XRD 300-TT diffractometer. TEM images were viewed with
a JEOL TEM 3010 instrument fitted with a Gatan CCD camera oper-
ating at an accelerating voltage of 300 kV. The gas products were
analyzed by using an Agilent GC 6890N instrument. Inductively
coupled plasma optical emission spectroscopy (ICP-OES) was used
to determine the weight percentage of the metal nanoparticles.
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[3] P. Clark, W. Li, S. T. Oyama, J. Catal. 2001, 200, 140.
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Hydrodesulfurization of thiophene was carried out in a stainless-
steel microreactor, 0.6 cm in diameter and 30 cm in length. The
catalyst (100 mg) was packed at the middle of the reactor tube by
using glass wool. Thiophene was placed in a quartz trap, one end
of which was connected to a H2 flow reactor and the other end to
the microreactor. The vapors of thiophene were passed over the
catalyst along with H2 at a flow rate of 6.3 mLminÀ1. The outlet of
the sample reactor was connected to a gas chromatograph. The re-
action was carried out at different temperatures and the reaction
temperatures were monitored using a thermocouple attached to
the furnace. The reaction products were analyzed using a flame
ionization detector in an Agilent gas chromatograph equipped
with a polyethyleneglycol capillary column (HP-INNOWAX, 30 mꢁ
0.25 mmꢁ0.25 mm).
[16] H. S. S. R. Matte, B. Plowman, R. Datta, C. N. R. Rao, Dalton Trans. 2011,
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Acknowledgements
[18] C. N. R. Rao, H. S. S. R. Matte, K. S. Subrahmanyam, U. Maitra, Chem. Sci.
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We thank Professor M. Eswaramoorthy for use of laboratory fa-
cilities.
[20] Y. Bao, W. An, C. H. Turner, K. M. Krishnan, Langmuir 2010, 26, 478–483.
[21] K. P. Donegan, J. F. Godsell, D. J. Otway, M. A. Morris, S. Roy, J. D.
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Keywords: desulfurization
nanoparticles · thiophene
·
graphene
·
molybdenum
·
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Received: January 11, 2013
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Less is more: Hydrodesulfurization of
thiophene has been carried out using
graphene-like few-layer MoS2 with con-
version of approximately 64%. Further
conversion was enhanced to 98% when
Co or Ni nanoparticles covered the sur-
face of MoS2 (see figure).
B. G. Rao, H. S. S. R. Matte, P. Chaturbedy,
C. N. R. Rao*
&& – &&
Hydrodesulfurization of Thiophene
over Few-Layer MoS2 Covered with
Cobalt and Nickel Nanoparticles
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