Sonochemical preparation of supported hydrodesulfurization catalysts

Article abstract of DOI:10.1021/ja010516y

Sonochemical preparation of Co and Ni promoted MoS₂ supported on alumina was achieved by high-intensity ultrasonic irradiation of isodurene solutions containing molybdenum carbonyl, dicobalt octacarbonyl, elemental sulfur, and Al₂O₃ or Ni-Al₂O₃ under Ar flow. The sonochemically prepared catalysts were characterized by elemental analysis, XPS, SEM, TEM, and XEDS, and hydrodesulfurization (HDS) activity evaluated for thiophene and dibenzothiophene substrates. The TEM studies on the sonochemically prepared catalysts indicate the formation of layered hexagonal MoS₂ (lattice fringes ～6.2 A) on the alumina support. The sonochemically prepared Co-Mo-S/Al₂O₃, Ni-Mo-S/Al₂O₃, and Co-Ni-Mo-S/Al₂O₃ are extremely active catalysts for the HDS of thiophene and dibenzothiophene, with activities several fold those of comparable commercial catalysts under identical conditions. The layered structure of MoS₂ remained intact after 120 h of HDS, and the catalyst is reusable.

Full text of DOI:10.1021/ja010516y

8310
J. Am. Chem. Soc. 2001, 123, 8310-8316
Sonochemical Preparation of Supported Hydrodesulfurization
Catalysts
N. Arul Dhas, Arash Ekhtiarzadeh, and Kenneth S. Suslick*
Contribution from the School of Chemical Sciences, UniVersity of Illinois at Urbana-Champaign,
6
00 South, Mathews AVenue, Urbana, Illinois 61801
ReceiVed February 26, 2001
Abstract: Sonochemical preparation of Co and Ni promoted MoS2 supported on alumina was achieved by
high-intensity ultrasonic irradiation of isodurene solutions containing molybdenum carbonyl, dicobalt
octacarbonyl, elemental sulfur, and Al2O3 or Ni-Al2O3 under Ar flow. The sonochemically prepared catalysts
were characterized by elemental analysis, XPS, SEM, TEM, and XEDS, and hydrodesulfurization (HDS) activity
evaluated for thiophene and dibenzothiophene substrates. The TEM studies on the sonochemically prepared
catalysts indicate the formation of layered hexagonal MoS2 (lattice fringes ∼6.2 Å) on the alumina support.
The sonochemically prepared Co-Mo-S/Al2O3, Ni-Mo-S/Al2O3, and Co-Ni-Mo-S/Al2O3 are extremely
active catalysts for the HDS of thiophene and dibenzothiophene, with activities severalfold those of comparable
commercial catalysts under identical conditions. The layered structure of MoS2 remained intact after 120 h of
HDS, and the catalyst is reusable.
Introduction
studies suggest that Co in this structure decorates the edges of
MoS2 slabs and is coordinated to five sulfur atoms in a square
pyramidal geometry in the (1 0 -1 0) edge planes of MoS2.6
Formation of this so-called “Co-Mo-S” structure is known
Removing heteroatoms, especially sulfur, from petroleum
constituents has always been an integral part of petroleum
,12
1
2
processing. In a recent EPA regulatory change, a 10-fold
reduction in fuel sulfur emission makes the need for improved
hydrodesulfurization (HDS) catalysts urgent. Alumina-supported
molybdenum sulfide promoted with cobalt or nickel has been
for several decades the catalyst of choice for hydrodesulfuriza-
6
to dominate the catalytic activity of the system. At higher Co
loading (Co/Mo > 0.4), however, Co9S8 domains are formed
that do not contribute significantly to the HDS activity. Ni can
interact with MoS2 in similar fashion, and there is evidence for
1
2
tion of fossil fuels due to its remarkable activity.1
,3-12
The MoS2
a Ni-Mo-S phase.
4
basal plane is largely inert, while the edges of the MoS2 layer
expose Mo atoms for reaction.13 Therefore, the catalytic activity
is strongly dependent on the specific dispersion of the MoS2
crystal (i.e., the ratio of edge sites to basal plane area) and on
the local environment of the active edge sites. Detailed XAS
Chianelli et al. proposed that Co or Ni adatoms increase the
electron density at the Mo sites and that this results in stronger
Mo-S bonds (and thus weaker C-S bonds), which leads to a
5
higher HDS activity. Smit and Johnson, however, have
proposed that although Co and Ni do indeed contribute electron
density to Mo, this transfer of electron density somewhat reduces
the activity of Mo while greatly increasing the activity of the
Co or Ni; in essence, MoS2 can be viewed as a complex support
for the highly active Co-Mo-S or Ni-Mo-S phase. Topsøe
(
1) (a) Furimsky, E. Appl. Catal. A 1998, 171, 177. (b) Grange, P. Catal.
ReV.-Sci. Eng. 1980, 21, 135.
2) (a) Federal Register, 2000, August 4, 65 (151), 48057-48105. (b)
Chem. Eng. News 2000, 78 (1), 927.
3) Prins, R.; de Beer, H. J.; Somorjai, G. A. Catal. ReV.-Sci. Eng. 1989,
1, 1.
(
(
14
et al. have shown that at the HDS active edges, coordinatively
unsaturated Mo atoms must be formed by sulfur removal before
HDS can take place. Recent STM studies on single-layer MoS2
have shown that the S vacancies for the HDS can be generated
3
(
4) (a) Chianelli, R. R. Catal. ReV.-Sci. Eng. 1984, 26, 361. (b) Harris,
S.; Chianelli, R. R. J. Catal. 1986, 98, 17. (c) Halbert, T. R.; Ho, T. C.;
Stiefel, E. I.; Chianelli, R. R. J. Catal. 1991, 130, 116.
(
(
(
5) Smit, T. S.; Johnson, K. H. Catal. Lett. 1994, 28, 361.
6) Topsøe, H.; Clausen, B. Catal. ReV.-Sci. Eng. 1984, 26, 420.
7) (a) Vasudevan, P. T.; Fierro, J. L. G. Catal. ReV.-Sci. Eng. 1996, 38,
1
5
by exposure to atomic hydrogen.
The traditional synthesis of promoted MoS2/Al2O3 catalyst
is by simultaneous or successive impregnation of molybdenum
and promoter salts onto alumina, followed by calcination
1
61. (b) Ratnasamy, P.; Sivasanker, S. Catal. ReV.-Sci. Eng. 1980, 22, 401.
(8) (a) Dungey, K. E.; Curtis, M. D. J. Am. Chem. Soc. 1997, 119, 842.
(
b) Brito, J. L.; Barbosa, A. L. J. Catal. 1997, 171, 467.
(9) Bonneau, P. R.; Jarvis, R. F.; Kaner, R. B. Nature 1991, 349, 510.
10) (a) Zdrazil, M. Catal. Today 1988, 3, 269. (b) Vicic, D. A.; Jones,
(
producing oxides), and sulfidation under the flow of H2S in
(
1
,3-7
H2 prior to use.
The HDS activity could be modified by
W. D. J. Am. Chem. Soc. 1999, 121, 7606. (c) Afanasiev, P.; Xia, G. F.;
Gilles, B.; Jouguet, B.; Lacroix, M.; Chem. Mater. 1999, 11, 3216.
using different molybdenum/promoter precursors, and also by
changing the sequence of impregnation of molybdenum and
promoter ions on alumina.
(11) (a) Brenner, J.; Marshall, C. L.; Ellis, L.; Tomczyk, N.; Heising, J.;
Kanatzidis, M. Chem. Mater. 1998, 10, 1244. (b) Cid, R.; Atanasova, P.;
Cordero, R. L.; Palacios, J. M.; Agudo, A. L.; Olson, B. A.; J. Catal. 1999,
1
82, 328.
12) (a) Bouwens, S. M. A. M.; Koningsberger, D. C.; de Beer, V. H.
(
(14) (a) Topsøe, H.; Clausen, B. S.; Massoth, F. E. Hydrotreating
Catalysis, Science and Technology; Springer-Verlag: Berlin, 1996. (b)
Norskov, J. K.; Clausen, B. S.; Topsøe, H. Catal. Lett. 1992, 13, 1.
(15) Helveg, S.; Lauritsen, J. V.; Laegsgaard, E.; Stensgaard, I.; Norskov,
J. K.; Clausen, B. S.; Topsøe, H.; Besenbacher, F. Phys. ReV. Lett. 2000,
84, 951.
J.; Louwers, S. P. A.; Prins, R. Catal. Lett. 1990, 5, 273. (b) Ozkan, U. S.;
Zhang, L.; Shuangyao, N.; Moctezuma, E. J. Catal. 1994, 148, 181.
(13) (a) Salmeron, M.; Somorjai, G. A.; Wold, A.; Chianelli, R. R.; Liang,
K. S. Chem. Phys. Lett. 1982, 90, 105. (b) Kushmerick, J. G.; Weiss, P. S.
J. Phys. Chem. B 1998, 102, 10094.
1
0.1021/ja010516y CCC: $20.00 © 2001 American Chemical Society
Published on Web 08/04/2001

Preparation of Supported Hydrodesulfurization Catalysts
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8311
The preparation of Mo-based supported catalyst with higher
concentration of active metals on the exterior surface and high
was used to internally calibrate the binding energies. To avoid exposing
the catalysts to air, a special transfer vessel was used to load the sample
inside the inert atmosphere box and transfer it to the XPS specimen
exchange chamber.
dispersion still represents a synthetic challenge in the field of
advanced catalysis.1-10 Alternative preparation methods involv-
9
Synthesis of Sonochemical Co-Mo-S/Al
2 3
O Catalyst. The syn-
thetic methodology is similar to our previous sonochemical synthesis
ing metathesis reactions and decomposition of metal sulfur
10
compounds have also been explored. These techniques appear
to cause internal diffusion of active Mo into the support and
produce ordered MoS2 crystals with less edge density. While
18
of nanostructured MoS
in the MoS lattice sonochemically. In a typical preparation, sulfur (150
mg) is first dispersed in isodurene (35 mL) by short pulses of ultrasound
followed by addition of 275 mg of Mo(CO) , 75 mg of Co (CO) , and
750 mg of γ-Al O . The vessel was sealed, transferred out of the inert
2
. The Co promoter ions were also incorporated
2
there are numerous studies on Co-Mo-S/Al2O3 and Ni-Mo-
6
2
8
S/Al2O3 as HDS catalysts,1
-10
there are very few studies on
2
3
the effect on HDS performance of double promotion by
atmosphere box, and sonicated under a flow of Ar for 1.5 h at 80 °C
(60 °C bath temperature). The black slurry was then filtered inside the
inert atmosphere box using a fine frit and washed several times with
pentane. The washed powder was then heated to 80 °C under vacuum
to remove any unreacted precursors and adsorbed solvent. The powder
was annealed by heat treatment at 450 °C under a flow of 10% H in
2
He at 30 mL/min for 10 h using a single pass quartz microreactor,
which also removes the excess sulfur. The elemental analysis gives a
16
bimetallic Co-Ni modification of MoS2/Al2O3, and in fact,
previous studies failed to find any significant enhancement
compared to single promotion.
Recently, sonochemical reactions of volatile organometallics
in an alkane medium have proven to be a unique technique to
prepare high-surface-area nanophase metals, alloys, carbides,
17,18
sulfides, nitrides, and supported catalysts.
In this study, we
5
.5% Mo and ∼4.0% Co loading on alumina support. Similarly,
report a simple sonochemical synthesis, characterization, and
kinetic studies of HDS activity of Co-Mo-S/Al2O3, Ni-Mo-
S/Al2O3, and Co-Ni-Mo-S/Al2O3. The catalytic results are
compared with the standard commercial catalysts.
sonochemical preparations were made of unpromoted MoS /Al O and
2
2
3
unsupported Co-Mo-S for comparison.
Synthesis of Sonochemical Ni-Mo-S/Al
2
O
3
Catalyst. We have
/Al because
) are exceptionally toxic and
dangerous to use.19 Nickel coated on Al
(Ni-Al ) has been used
both as the Al support and as the source for Ni in the sonochemical
preparation of Ni-Mo-S/Al . To prepare Ni-Al , γ-Al is first
soaked in an aqueous solution of Ni(N )(N COO) ‚H O complex
for 1 h, dried in air, and heat treated under 10% H in He at 300 °C for
2 h. Ni(N )(N COO) ‚H O was synthesized and characterized
according to methods outlined in the literature. The sonochemical
Ni-Mo-S/Al catalyst is then made by the same method as Co-
Mo-S/Al by using Ni-Al (750 mg), Mo(CO) (275 mg), and
sulfur (150 mg). The initial sonochemical Ni-Mo-S/Al was
annealed by heat treatment at 450 °C under a flow of 10% H in He
designed a different route to incorporate Ni in MoS
volatile Ni precursors (e.g., Ni(CO)
2
2 3
O
4
O
2 3
2 3
O
Experimental Section
2 3
O
Materials and Equipment. Unless otherwise noted, all synthesis,
preparation, and handling of the materials were done in an Ar
O
2 3
2
O
3
2 3
O
2
H
5
2
H
3
3
2
atmosphere inert atmosphere box with O
2
< 0.5 ppm. Pentane was
2
dried and distilled over Na/benzophenone and degassed prior to use.
Isodurene (1,2,3,5-tetramethyl benzene, Aldrich) was dried and distilled
2
H
5
H
2 3
3
2
2
0
over Na and degassed. Sulfur (99%), Mo(CO)
6
(98%), and Co
2
(CO)
8
2 3
O
(stabilized with 1-5% hexanes) were purchased from Strem and used
O
2 3
O
2 3
6
without further purification. Alumina (neutral 150 mesh, Aldrich) was
heat treated at 400 °C under vacuum for 24 h to remove any adsorbed
water. Thiophene (99% Aldrich) and dibenzothiophene (99%, Acros)
were used without further purification. Industrial catalysts were obtained
from Crosfield/ICI Catalysts. Crosfield 465 1/20 (Co/Mo oxides on
2 3
O
2
(30 mL/min) for 10 h using a single pass quartz microreactor. The
sonochemically prepared catalyst typically has a 5.5% Mo and ∼2.8%
Ni loading.
γ-Al
on γ-Al
2
O
3
: 10.6% Mo, 2.9%Co) and Crosfield 565 1/20 (Ni/Mo oxides
: 20.0% Mo, 3.7% Ni) were sulfided with 1:10 H S/H
Synthesis of Sonochemical CoNi-Mo-S/Al
preparation of CoNi-Mo-S/Al , we use 3.5% Ni-Al
Co (CO) , and sulfur. The sonochemical Co-Ni-Mo-S/Al
is then made by the same method as Co-Mo-S/Al by using Ni-
Al (750 mg), Mo(CO) (275 mg), Co (CO) (40 mg), and sulfur
(150 mg). The initial sonochemical Ni-Mo-S/Al was annealed
by heat treatment at 450 °C for 10 h under 10% H in He flow (30
2
O
3
Catalyst. For the
, Mo(CO)
catalyst
O
2 3
2
2
O
2 3
2
O
3
6
,
flowing at 45 mL/min at 450 °C for 2 h prior to use. All sonications
2
8
2 3
O
were done at 20 kHz using a Sonics & Materials VCX600 with a 1
2 3
O
2
-2
cm titanium horn and an acoustic intensity of ∼50 W cm in a glass
2
O
3
6
2
8
vessel under an argon atmosphere.
2
O
3
Reactions were analyzed with a Hewlett-Packard 6890 GC system
equipped with an HP-5973 Mass Selective Detector fitted with a HP-
2
mL/min). The catalyst has a 5.5% Mo, 1.5% Co, and ∼2.5% Ni loading.
HDS of Thiophene. Catalytic studies of HDS of thiophene were
performed at atmospheric pressure in a single-pass microreactor. The
thiophene vapors were carried at a constant partial pressure of 75 mbar
5
MS column. All SEM micrographs were obtained using a Hitachi
S-4700 field emission SEM. All TEM micrographs were obtained using
either a Phillips CM-12 microscope operating at 120 kV or a Vacuum
Generators VG HB-501 STEM using a 6 Å probe and operating at
3
by a flow of hydrogen [flow rate ) 23 cm (STP)/min]. The reaction
1
00 kV. XRD data were collected on a Rigaku D-max diffractometer
products were analyzed with an on-line HP GC-MS and a 5730A HP
GC fitted with a 9-ft n-octane/Porasil C column and a flame ionization
detector.
using Cu KR radiation. The XPS analysis was performed using a
Physical Electronics Φ-5400 with a nonmonochromatized Mg KR
source (1253.6 eV), operating at 15 kV. The C(1s) peak at 284.6 eV
HDS of Dibenzothiophene. DBT HDS was accomplished using a
home-built stainless steel pressure vessel and sample withdrawal system.
Catalyst (30-50 mg) was dispersed in 50 mL of decane containing
(16) (a) Guemez, M. B.; Cambra, J. F.; Arias, P. L.; Egia, B.; Uria, P.
M.; Fierro, J. L. G. Inf. Technol. 1995, 6, 15. (b) Kameoka T.; Sato, T.;
Yoshimura, Y.; Shimada, H.; Matsubashi, N.; Nishijima, A. Sekiyu
Gakkaishi 1994, 37, 497.
1
% w:w dibenzothiophene (40 mM, 370 mg). The vessel was flushed
with H and pressurized to 3.4 MPa (500 psig) and heated to 375 °C
during 1 h. For consistency, t was denoted as the time when the target
temperature was reached (at the end of ramping). In all cases, reactions
reached equilibrium within 15 min after t . Samples were withdrawn
in duplicate at 30, 45, 60, 75, and 90 min after t . The sampling system
2
(17) (a) Suslick, K. S. Annu. ReV. Mater. Sci. 1999, 29, 295. (b) Suslick,
0
K. S. In Handbook of Heterogeneous Catalysis; Ertl, G., Knozinger, H.,
Weitkamp, J., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 3, Chapter 8.6,
pp 1350-1357. (c) Suslick, K. S.; Choe, S.-B.; Cichowlas, A. A.; Grinstaff,
M. W. Nature 1991, 353, 414. (d) Grinstaff, M. W.; Salamon, M. B.;
Suslick, K. S. Phys. ReV. B 1993, 48, 269. (e) Hyeon, T.; Fang, M.; Suslick,
K. S. J. Am. Chem. Soc. 1996, 118, 5492. (f) Suslick. K. S.; Fang, M.;
Hyeon, T. J. Am. Chem. Soc. 1996, 118, 11960. (g) Dhas, N. A.; Raj, C.
P.; Gedanken, A. Chem. Mater. 1998, 10, 1446. (h) Dhas, N. A.; Gedanken,
A. Appl. Phys. Lett. 1998, 72, 2514. (i) Dhas, N. A.; Gedanken, A. J. Phys.
Chem. B 1997, 101, 3159. (j) Ashokkumar, M.; Grieser, F. ReV. Chem.
Eng. 1999, 15, 41.
0
0
was equipped with a 2 µm stainless steel filter to prevent the removal
of catalyst from the system. Aliquot size was about 0.25 mL. The
samples were then centrifuged to remove any solid particulate, diluted
1:10 in pentane, and analyzed with GC-MS.
(19) Brauer, G. Handbook of preparatiVe inorganic chemistry; Academic
Press: New York, 1965; Vol. 2.
(
18) Mdeleni, M. M.; Hyeon, T.; Suslick, K. S. J. Am. Chem. Soc. 1998,
(20) (a) Ravindranathan, P.; Patil, K. C. Ceram. Bull. 1987, 22, 3261.
(b) Dhas N. A.; Patil K. C. J. Solid State Chem. 1993, 102, 440.
1
20, 6190.

8312 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Dhas et al.
Results and Discussion
Sonochemistry introduces a fundamentally different way of
making materials by taking advantage of the intense energy
within a collapsing bubble in a sound field.16,21 A bubble
expands and contracts in response to the negative and positive
pressure induced by a sound field. The bubble grows larger
during each cycle due to rectified diffusion and inertial growth.
Once the bubble reaches a critical size, the next compression
wave can create a violent collapse, the last stage of which is a
near-adiabatic process. The temperatures during this stage reach
2
2
upward of 5000 K with pressures above 300 atm. In our
experimental conditions, Mo(CO)6 vapor in the bubble decom-
poses to give Mo atoms due to the local hot spots formed after
bubble collapse. The hot Mo atoms coming out of the bubble
react with the sulfur in the medium to form nanostructured MoS2
1
7
(
size ∼3 nm) and deposit on the alumina support as small
clusters. The added Co2(CO)8 decomposes in a fashion similar
to give Co atoms along with Mo atoms during the bubble
collapse.
Nickel hydrazine carboxylate complex, Ni(N2H5)(N2H3-
COO)3‚H2O, has been used as precursor for Ni for the
preparation of sonochemical Ni-Mo-S/Al2O3. We were at-
tracted by the low-temperature autocatalytic decomposition of
the complex to yield pure nanocrystalline Ni (∼3 nm) particles.
A large amount of work has been carried out on decomposition
of metal hydrazine carboxylate complexes under aerobic condi-
tions for the synthesis of ultrafine particles of pervoskites and
spinel metal oxides.20 The fine particle nature of the oxide
residue is due to the evolution of a large amount of gases during
the decomposition of hydrazine carboxylate complexes. We have
exploited this feature to prepare metallic clusters by decompos-
ing the hydrazine carboxylate complex under a 1:10 H2/He flow.
Characterization. The powder XRD of the sonochemical
sample primarily showed the reflections of γ-Al2O3 with small
broad peaks correspond to MoS2. The nanostructured nature of
the sonochemical MoS2 particles causes the XRD line broaden-
ing and does not yield any information about the crystal
structures of active components. SEM micrographs of sonochem-
ically prepared catalysts show the MoS2 particles made up of
small spherical clusters as observed earlier for bulk sonochemi-
1
7
cal MoS2. Each spherical cluster is made up of finer particles
of ∼3 nm diameter, and the cluster rests atop a porous platelet
of the alumina support. Representative micrographs of the
sonochemical and commercial Ni-Mo-S/Al2O3 are shown in
Figure 1. The SEM-XEDS analysis on the small spherical
clusters showed increased signal intensity of the Mo compared
to Al and O. This confirms our conclusion that in the
sonochemically prepared catalysts, the small spherical clusters
on the alumina support are indeed mostly MoS2. The SEM
morphologies of sonochemically prepared Co-Mo-S/Al2O3
and Co-Ni-Mo-S/Al2O3 are identical to the Ni-Mo-S/
Al2O3. The elemental analysis and composition have also been
examined using XEDS analysis coupled to SEM. The SEM-
XEDS analysis profiles of sonochemically prepared catalyst are
shown in Figure 2. The presence of Co and Ni has been
confirmed by the characteristic signals of K lines at ∼7 and
Figure 1. SEM micrograph of sonochemically prepared (A and C)
and commercial Ni-Mo-S/Al O (B and D). XEDS confirms that the
2 3
numerous small clusters in the sonochemically prepared catalyst are
(21) (a) Ultrasound: Its Chemical, Physical, and Biological Effects;
Suslick, K. S., Ed.; VCH Press: New York, 1988. (b) Sonochemistry and
Sonoluminescence; Crum, L. A., Mason, T. J., Reisse, J. L., Suslick, K. S.,
Eds.; Kluwer Academic Publishers: Dordrecht, The Netherlands, 1999.
Ni-Mo-S, whereas the occasional small clusters in the commercial
catalyst (Crosfield) are Al O .
2 3
(
22) (a) Flint, E. B.; Suslick, K. S. Science 1991, 253, 1325. (b)
McNamara, W. B., III; Didenko, Y.; Suslick, K. S. Nature 1999, 401, 772.
c) McNamara, W. B., III; Didenko, Y.; Suslick, K. S. J. Am. Chem. Soc.
000, 122, 8563. (d) McNamara, W. B., III; Didenko, Y.; Suslick, K. S.
Phys. ReV. Lett. 2000, 84, 777.
∼7.5 keV, respectively. The L lines of Co and Ni can also be
seen at ∼1 keV. Unambiguous determination of sulfur and
molybdenum were not possible on the SEM due to the overlap
of the S K and Mo L lines at ∼2.5 keV. The Mo K lines,
(
2

Preparation of Supported Hydrodesulfurization Catalysts
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8313
Figure 2. SEM-XEDS profiles of sonochemically prepared catalyst:
(
Al
a) Co-Mo-S/Al
2
2 3 2 3
O , (b) Ni-Mo-S/Al O , and (c) Co-Ni-Mo-S/
3
O .
however, could be resolved at around 17.5 keV using high-
resolution STEM-XEDS analysis (discussed below). The XEDS
peak intensities were calibrated using a Cu grid for the
calculation of elemental composition. The percentage of Mo is
enhanced by 3-fold compared with the bulk elemental analysis
values which indicate the surface enhancement of Mo (penetra-
tion depth under these conditions for XEDS ∼4.5 µm).
Therefore, the SEM-XEDS studies on sonochemically prepared
catalysts confirm that the active component is indeed mostly
on the exterior surface of the support.
The TEM of sonochemically prepared catalyst shows the
lattice fringes of MoS2 layers with interlayer spacing of ∼6.2
Å, the same as those for bulk MoS2.23 The TEM shown in Figure
3
shows the microstructure of sonochemically prepared Co-
Figure 3. TEM images: (A) sonochemical Co-Mo-S/Al
2 3
O and (B)
Mo-S/Al2O3 and commercial Co-Mo-S/Al2O3 catalysts. The
lattice fringes of MoS2 are clearly seen on the surface of the
support. Comparing TEM of the industrial and sonochemically
prepared catalysts reveals an important difference: the industrial
catalyst is mostly composed of multi-stacked and ordered layers
with few wrinkled fringes, while the sonochemical Co-Mo-S
catalyst shows short lengths of wrinkled layers that are highly
disordered, and have only minimal layer stacking. The disor-
dered, fractured, and defective nature of sonochemically pre-
pared MoS2 layers are inherent in having clusters made up of
nanometer sized, highly dispersed MoS2 particles. The sonochem-
ically prepared MoS2 has increased edge density and higher
dispersion, both of which are desirable for HDS activity.
Similarly layered MoS2 structures are also observed for the
sonochemically prepared Ni-Mo-S/Al2O3 and Co-Ni-Mo-
S/Al2O3.
The electronic states of metals and sulfur in the catalyst were
probed by X-ray photoelectron spectroscopy (XPS).2 XPS of
the sonochemically prepared catalyst showed that the Mo is
concentrated at the surface (X-ray penetration depth ∼50 Å).
The XPS signal intensities were corrected with sensitivity factors
for the calculations of atomic concentration. The atomic
concentration calculations indicated that there is a 5-fold
enhancement of surface Mo versus the bulk material for the
sonochemically prepared catalyst. On the other hand, the
commercial catalyst did not show Mo surface enhancement in
XPS.
commercial (Crosfield) Co-Mo-S/Al
2
3
O .
differences ∆E1 and ∆E2 (which are obtained by subtracting
the BE of M(2p3/2) (M ) Co, Ni) and Mo(3d5/2) from the
S(2p3/2), respectively). The XPS BE, ∆E1, and ∆E2 values of
unsupported sonochemical Co-Mo-S and commercial sup-
ported samples are also included in Table 1 for comparison.
It is known that Co interacts with the Al2O3 support due to
the facile formation of spinel CoAl2O4 oxide phase; the spinel
6
phase actually decreases HDS activity. The BE of sonochemical
unsupported Co-Mo-S and supported Co-Mo-S/Al2O3 is
identical, indicating a negligible interaction between the support
and cobalt under the sonochemical samples. This is further
supported by BE values of a commercial carbon supported
catalyst (Co-Mo-S/C), which are identical to those of sup-
ported and unsupported sonochemically prepared catalysts. The
peak positions of M, Mo, and S in the sonochemically prepared
catalyst are in good agreement with the conventional samples.25
We also performed STEM-XEDS spot analysis on the
sonochemical samples to determine if the promoter-metal and
Mo atoms coexist spatially or if they phase segregate. Figure
4
4
a shows a STEM micrograph of sonochemical Ni-Mo-S/
Al2O3 catalyst. The circles represent the locations and the areas
of analysis. Analysis was done both on and off MoS2 fringes
(“on-fringe” and “off-fringe”, respectively), and the intensities
(24) Moulder J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook
of X-ray Photoelectron Spectroscopy; Chastain J., Ed.; Perkin-Elmer: Eden
Prairie, MN, 1992.
Table 1 summarizes the XPS binding energies (BE) of Mo,
Co, Ni, and S of sonochemical Co-Mo-S/Al2O3, Ni-Mo-
S/Al2O3, and Co-Ni-Mo-S/Al2O3, as well as binding energy
(25) (a) Alstrup, I.; Chorkendorff, I.; Candia, R.; Clausen, B. S.; Topsøe,
H. J. Catal. 1982, 77, 397. (b) Bouwens, S. M. A. M.; van Zon, F. B. M.;
van Dijk, M. P.; van der Kraan, A. M.; de Beer, V. H. J.; van Veen, J. A.
R.; Koningsberger, D. C. J. Catal. 1994, 146, 375.
(23) Inamura, K.; Prins, R. J. Catal. 1994, 147, 515.

8314 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Dhas et al.
Table 1. XPS Binding Energy Values for Sonochemically Prepared Catalysts
a
a
Co-Mo-S /Cb
Co-Mo-S unsupported
Co-Mo-S /Al
O
2 3
Ni-Mo-S /Al
O
2 3
2 3
Co-Ni-Mo-S /Al O
M (Co, Ni) 2p3/2
S 2p3/2
778.6
162.4
616.2
229.1
66.7
778.6 (778.6)
162.5 (161.2)
616.1 (617.4)
229.0 (228.1)
66.5 (66.9)
853.6 (852.8)
162.6 (162.1)
691.0 (690.7)
229.2 (228.8)
66.6 (66.7)
778.6, 853.5
162.5
616.1, 691.0
229.2
778.6
162.5
616.1
229.1
66.7
∆
E
1
(M 2p3/2 - S 2p/32
)
Mo 3d5/2
∆
E
2
(Mo 3d5/2 - S 2p/32
)
66.7
a
Values determined for the equivalent conventional catalyst (ref 25) are given in parentheses. b Reference 25.
Figure 5. Longevity of catalytic activity for hydrodesulfurization of
thiophene at 400 °C. From left to right at each time: sonochemically
prepared CoNi-Mo-S/Al
Al ; Ni-Mo-S/Al , treated thermally with H
Crosfield) Co-Mo-S/Al ; and commercial (Crosfield) Co-Mo-
S/Al
O
2 3
; sonochemically prepared Co-Mo-S/
O
2 3
O
2 3
2
in He; commercial
(
2 3
O
2 3
O .
surface area of the support. Given the different degree of
dispersion for sonochemical and industrial catalysts and the fact
that the active sites are located at the edges of MoS2 layers,
2
6,27
activity is normalized to the mass of metal,
rather than to
the surface area. The stability of the sonochemical and com-
mercial catalysts was measured over the course of 5 days by
continuous HDS of thiophene using a single-pass microreactor
at 400 °C. Figure 5 displays the turnover frequencies as function
of catalytic use over time. The stabilities of the promoted
sonochemically prepared catalysts were calculated using a first-
order exponential fit after reaching a steady state conversion
(60 h). The half-lives for the sonochemically prepared catalysts
are around 25 days, comparable to the commercial catalyst. In
addition, the sonochemically prepared catalysts can be easily
regenerated at 450 °C for 10 h under 10% H2 in He flow and
return to nearly their initial activity.
As is seen in Figure 5 (and discussed below), the sonochemi-
cal Co-Ni-Mo-S/Al2O3 catalyst shows higher activity for
HDS of thiophene than the Co or Ni promoted sonochemical
and commercial catalysts. The variation of catalytic activity as
a function of Co wt % in Co-Ni-Mo-S/Al2O3 is shown in
Figure 6. The activity values plotted here were measured after
2 3
Figure 4. (A) STEM of sonochemically prepared Ni-Mo-S/Al O
showing MoS
2
lattice fringes and displaying the areas of analysis. (B)
scaled to
Spot XEDS analysis on and off fringes for Ni-Mo-S/Al
2
O
3
Mo K intensity. The inset shows an enlarged view of Ni K/Cu K and
Mo K regions.
of Mo KR (17.5 eV), Ni KR (7.5 eV), and Al KR (1.5 eV)
were determined. For comparison purposes, the intensities of
the spectra were scaled based on the Mo KR peak intensity.
Within the scope of this analysis, there were no significant
changes in the intensity of Mo KR versus the Ni KR peak.
XEDS overlay spectra (Figure 4b) shows the presence of Mo,
Ni, and S in the on-fringe spot. The only change observed that
depended on location was the intensity of the Al KR peak, which
is very weak on-fringe. We have obtained similar results in
sonochemical Co-Mo-S/Al2O3 and Co-Ni-Mo-S/Al2O3.
The XEDS spot analyses of the sonochemical samples on-fringe
and off-fringe indicate that the Co or Ni promoters and Mo are
uniformly distributed on the alumina sample. The XEDS results
were consistent with the STEM electron probe mapping analysis
of the sonochemically prepared catalysts.
6
0 h of use. As the concentration of Co promoter increases in
Co-Ni-MoS/Al2O3, the catalytic activity increases to an
optimum composition of 2% Co and 2.8% Ni.
It is worth mentioning that the HDS activity of Ni and Co
promoted sonochemically prepared catalyst is higher after
thermal treatment with 10% H2 in He than if treated only with
He. This is most likely because H2 removes sulfur atoms from
the edges of the Co-Mo-S or Ni-Mo-S structures, resulting
in coordinatively unsaturated Mo and promoter atoms that are
28
more active catalytic sites for HDS.
(26) Pecoraro, T. A.; Chianelli, R. R. J. Catal. 1981, 67, 430.
(27) Ramanathan, S.; Oyama, S. T. J. Phys. Chem. 1995, 99, 16365.
(28) Byskov, L. S.; Norskov, J. K.; Clausen, B. S.; Topsøe, H. C. Prepr.
HDS of Thiophene. The specific surface areas of the
pretreated sonochemical and commercial catalysts were ∼170
2
and ∼220 m /g, respectively, but this simply reflects the total
Am. Chem. Soc., DiV. Pet. Chem. 1998, 43, 12.

Preparation of Supported Hydrodesulfurization Catalysts
J. Am. Chem. Soc., Vol. 123, No. 34, 2001 8315
Figure 6. Variation of catalytic activity for thiophene hydrodesulfu-
rization as a function of Co amount in sonochemically prepared Co-
Ni-Mo-S/Al O .
2 3
Figure 8. Specific activity for dibenzothiophene hydrodesulfurization
by sonochemical and commercial (Crosfield) catalysts.
linear decrease in the concentration of dibenzothiophene (DBT)
over time. This indicates zero order kinetics, observed when
the substrate concentration is high and turnover is determined
by the availability of active sites on the catalyst. Figure 8 shows
specific activity of each catalyst measured as molecules of DBT
consumed per gram of metal per minute.
Comparing sonochemically prepared catalysts to industrial
ones, it is clear that sonochemically prepared catalysts are
superior for HDS of DBT. The sonochemical Co-Mo-S/Al2O3
is more active than the sonochemical Ni-Mo-S/Al2O3, and
they are about two and four times more active than their
commercial counterparts, respectively. The enhancements in
activity of the sonochemically prepared catalyst could be due
to improved mass transport since the active species are
predominantly on the outside surface of the alumina support.
The beneficial effect on the HDS performance of Co-Ni-Mo-
S/Al2O3 is probably due to the different stages and nature of
incorporation of Co and Ni to MoS2.
Figure 7. Product distribution of thiophene hydrodesulfurization.
Scheme 1
Product distribution afforded by an HDS catalyst is also of
importance. Numerous studies have been carried out on
coordination of DBT and other thiophenic species on model
3
2-36
catalysts.
undergo either hydrodesulfurization or hydrogenation. Scheme
shows the general pathways for DBT reactions. The two major
Dibenzothiophene adsorbed on the catalyst can
2
products of hydrodesulfurization of DBT are biphenyl (BP) and
cyclohexylbenzene (CHB). BP is the direct HDS product and
is desired due to its commercial application and smaller
hydrogen consumption, while CHB is a product of HDS of
tetrahydrodibenzothiophene (TH-DBT), which itself is an
incomplete hydrogenation product of DBT. Classic experimental
and theoretical work by Chianelli, Daage, and Murray have
shown that HDS can happen on the rim and edge sites of a
multilayer MoS2 catalyst, while hydrogenation probably only
The principal products detected by GC-MS were the C4
hydrocarbons: butane, 1-butene, trans-2-butene, and cis-2-
butene (Figure 7). We do not see butadiene due to the high
reactivity of the catalyst (40% conversion); the amount of
tetrahyrothiophene in the product is <0.2%. The promoted
catalysts show high selectivity for butenes compared with the
unpromoted MoS2, which gives more butane. In the absence of
promoters the butadiene or thiophene strongly adsorbs on MoS2
and undergoes extensive hydrogenation.29 On the basis of the
observed products, a plausible mechanism for thiophene HDS
would involve initial homolytic cleavage of the thiophene C-S
bond and hydrogenolysis to give H2S and butadiene. Butadiene
undergoes rapid hydrogenation to 1-butene, which is subse-
quently hydrogenated slowly to butane or isomerized to a
thermodynamic mixture of cis- and trans-2-butenes (Scheme
3
2,33
happens on the rim sites.
Furthermore, selectivity [CHB]/
[
BP] is greatly dependent on partial pressure of H2 and
3
7
temperature. High temperatures and H2 pressures yield more
CHB. The product distribution of sonochemically prepared
(30) Benson, J. W.; Schrader, G. L.; Angelici, R. J. J. Mol. Catal. A:
Chemical 1995, 96, 283.
(
31) Luo, S.; Rauchfuss, T. B.; Gan, Z. J. Am. Chem. Soc. 1993, 115,
943.
(32) Daage, M.; Murray, H. H. Prepr. Am. Chem. Soc., DiV. Pet. Chem.
993, 38, 660.
4
3
0,31
1
).
Tetrahydrothiophene can also undergo HDS to give
1
butane.
(
(
33) Daage, M.; Chianelli, R. R. J. Catal. 1994, 149, 414.
34) Rauchfuss, T. B. Prog. Inorg. Chem. 1991, 30, 259.
HDS of Dibenzothiophene. Analysis of the aliquots with-
drawn from the pressure vessel during the reaction shows a
(35) Tarbuck, T. L.; McCrea, K. R.; Logan, J. W.; Heiser, J. L.; Bussell,
M. E. J. Phys. Chem. B 1998, 102, 7845.
(29) Jayamurthy, M.; Vasudevan, S. J. Phys. Chem. 1994, 98, 6777.
(36) Diemann, E.; Weber, T.; M u¨ ller, A. J. Catal. 1994, 148, 288.

8316 J. Am. Chem. Soc., Vol. 123, No. 34, 2001
Dhas et al.
was ∼1.9%. In all cases, complete hydrogenation products such
as bicyclohexane accounted for less than 0.3% of products.
Conclusion
Co, Ni, and Co-Ni promoted molybdenum sulfide on
alumina catalysts have been generated ultrasonically using
molybdenum carbonyl, elemental sulfur, dicobalt octacarbonyl,
and Al2O3 (or nickel on Al2O3). TEM studies demonstrated that
the sonochemical technique enhances the dispersion of MoS2
layers on the alumina support compared to the conventional,
commercial catalyst (Crosfield). SEM-XEDS and XPS studies
have shown that the Mo is mostly concentrated on the surface
of alumina in the case of the sonochemically prepared catalysts,
but not for conventionally prepared catalysts. STEM-XEDS spot
analysis showed that Mo is not phase segregated from the
promoter metals (Co, Ni, and Co-Ni) on the scale of ∼3 nm,
indicating the formation of binary metal sulfide M-Mo-S
structures. The HDS activity of sonochemical Co promoted
MoS2/Al2O3 is slightly higher than that of the Ni counterpart,
and they are 2-fold times and 3-fold times more active than
their respective commercial counterparts. The bimetallic Co-
Ni promoted MoS2/Al2O3 has HDS activity 3-fold and 7-fold
times better than the commercial Co-Mo-S/Al2O3 and Ni-
Mo-S/Al2O3 catalysts, respectively. The superior HDS activity
of sonochemical MoS2 catalysts is likely to arise from the higher
dispersion of the MoS2 on the support and the consequent higher
concentration of the catalytically active edge sites.
Figure 9. Product distribution of dibenzothiophene hydrodesulfuriza-
tion: CHB, cyclohexylbenzene; BP, biphenyl; and TH-DBT, tetrahy-
drodibenzothiophene.
Scheme 2
Acknowledgment. These studies were supported by the NSF
(CHE-0079124). Surface and electron microscopic analyses
were carried out in the UIUC Center for Microanalysis of
Materials, which is supported by the U.S. Department of Energy,
Division of Materials Sciences under Award No. DEFG02-
ER9645439, through the Frederick Seitz Materials Research
Laboratory at the University of Illinois at Urbana-Champaign.
The assistance of Drs. Ray Twesten and Rick Haasch of the
Center for Microanalysis of Materials is appreciated. The authors
also wish to thank Dr. Robert Quigley of Crosfield/ICI for the
generous donation of the Crosfield catalysts.
catalyst was nearly identical or slightly better than that of the
industrial catalyst (Figure 9). All of the product distribution
measurements were obtained at 74% DBT conversion. For all
catalysts studied, BP was the major product, although the Co-
promoted catalysts were more selective than the Ni-promoted
ones. The amount of TH-DBT also reflected catalyst selectivity.
In the case of Co catalysts, TH-DBT only constituted 1.0-1.3%
of the total product, while for the Ni catalyst, the amount
(37) Singhal, G. H.; Espino, R. L.; Sobel, J. E.; Huff, G. A. J. Catal.
1
981, 67, 457.
JA010516Y