Hydrodesulfurization of dibenzothiophene and 4,6-dimethyl-dibenzothiophene: Gallium effect over NiMo/Al2O3 sulfided catalysts

Article abstract of DOI:10.1016/j.jcat.2005.09.011

The influence of gallium on alumina-supported NiMo catalysts was investigated by correlating their physicochemical properties with the hydrodesulfurization (HDS) activity of model molecules. The Ga-γ-Al ₂O₃ supports were prepared by

Full text of DOI:10.1016/j.jcat.2005.09.011

Journal of Catalysis 235 (2005) 403–412
www.elsevier.com/locate/jcat
Hydrodesulfurization of dibenzothiophene and
4,6-dimethyl-dibenzothiophene: Gallium effect over NiMo/Al2O3
sulfided catalysts
Efraín Altamirano a,b, José Antonio de los Reyes a,∗, Florentino Murrieta c, Michel Vrinat b
a
Universidad A. Metropolitana-Iztapalapa, Área de Ingeniería Química, Av. FFCC R. Atlixco No. 186, Col. Vicentina, Iztapalapa, 09340 México, DF, Mexico
b
Institut de Recherches sur la Catalyse, 2 Av. A. Einstein, 69626 Villeurbanne cedex, France
c
Instituto Mexicano del Petróleo, Eje central Lázaro Cárdenas 152, 07730 México, DF, Mexico
Received 3 June 2005; revised 31 August 2005; accepted 5 September 2005
Abstract
The influence of gallium on alumina-supported NiMo catalysts was investigated by correlating their physicochemical properties with the
hydrodesulfurization (HDS) activity of model molecules. The Ga–γ -Al O supports were prepared by impregnation of Ga (0.6–5.9 wt%) on
2
3
γ -Al O . The NiMo catalysts were prepared following the same method on calcined Ga–Al O . The N -physisorption results showed that Ga
2
3
2
3
2
did not affect the textural properties of both γ -Al O and NiMo/γ -Al O . When the Ga–Al O supports were calcined at 723 K all Ga ions
2
3
2
3
2 3
presented a high dispersion, forming a different structure than that observed in bulk Ga O . The XPS and the TPR-S showed that the Ga addition
2
3
modified the structural properties of NiMo depending on the amount of Ga. In the HDS of DBT and 4,6-DMDBT, the Ga–Al O supported NiMo
2
3
catalysts at loadings of Ga below to 1.2 wt% showed higher activity than that of a NiMo/Al O sample.
2
3
2005 Elsevier Inc. All rights reserved.
Keywords: Ga O ; Ga S ; Al O ; NiMo; Hydrodesulfurization; Dibenzothiophene; 4,6-Dimethyl-dibenzothiophene; XPS; TPR
2
3
2
3
2 3
1. Introduction
thors have suggested that the improvement of HDS catalysts
is closely related to the formation of the so-called “NiMoS”
(CoMoS) phase. Thus the amount of the NiMoS phase should
be increased by reducing the concentration of inactive species
to improve the performance of HDS catalysts. As pointed out by
several authors, the carrier plays a fundamental role, because
strong interactions prevent NiMoS phase formation. Addition
of a third metal as an additive may also influence the specia-
tion of Ni and Mo and, consequently, HDS activity. Along this
line, many papers have reported the influence of additives or
modifiers, such as Mg [5–9], Ca [6,8,9], Ti [8], Fe [8], Mn [9]
and Zn [5,10–13]. Unfortunately, the reports on the effects of
these additives are rather conflicting. Martinez and Mitchell [5]
showed that adding small amounts of Mg to CoMo/Al2O3 cat-
alysts resulted in increased HDS activity, whereas adding large
amounts of Mg resulted in decreased HDS activity. Muralidhar
et al. [6] reported that adding 5% of Ca or Mg before or after
Mo and Co impregnation decreased HDS activity. Hercules et
al. [7] reported that Mg forms spinel-type species (MgAl2O4)
The current environmental regulations that impose very low
sulfur content in diesel cuts have spurred research on hy-
drotreating catalysts. The commercial HDS catalysts consist of
an active phase of molybdenum (Mo) promoted by nickel (Ni)
or cobalt (Co) usually supported on alumina. Nevertheless, it
has been recognized since the very first studies related to these
solids (CoMo or NiMo/Al2O3) that alumina is not an inert car-
rier. On one hand, the promoter ion (Co or Ni) can react with
the support and occupy octahedral or tetrahedral sites in ex-
ternal layers or even form an inactive phase such as CoAl2O4
(NiAl2O4) depending on the preparation conditions [1]. On the
other hand, the molybdates interact strongly with the support
from the first steps of catalyst preparation through Mo–O–Al
anchors [2,3]. Topsøe and Topsøe [4] and many other au-
*
Corresponding author.
E-mail address: jarh@xanum.uam.mx (J.A. de los Reyes).
0021-9517/$ – see front matter
doi:10.1016/j.jcat.2005.09.011
2005 Elsevier Inc. All rights reserved.

404
E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
or free oxides on the alumina surface, promoting Mo or W dis-
persion, and Saini et al. [8] reported that Ca cannot easily form
CaAl2O4 species due to its larger ionic radius compared with
Mg (Mg2+, 0.66; Ca2+, 0.94). Transition metals (Me = Ti, Fe,
or Mn) as additives scarcely form MeAl2O4 species and have
poor dispersion when they are impregnated on alumina and cal-
cined around 773 K; therefore, these metals are not suitable for
use in improving HDS activity. Martinez [5] showed that adding
small amounts of Zn to the CoMo/Al2O3 resulted in increased
HDS activity, depending on the sequence of impregnation and
on the calcination temperatures, in agreement with findings of
other authors [11].
nm2 of initial surface area of alumina. For the molybdenum and
nickel 2.8 and 1.2 at nm−2 were added, respectively, leading to
an atomic Ni r = 0.3, with r = Ni/(Ni + Mo).
In a typical preparation, first a solution of AHM was pre-
pared and the pH was adjusted to 8 by adding a basic so-
lution (NH4OH, Baker). Then, the support was impregnated
and after 12 h, the solids were dried under a stream of air
(5.16 × 10−3 molmin−1) at 393 K for 12 h, with a heating rate
of 3 K min−1. The impregnation of nickel was performed with
a solution of nickel nitrate at constant pH of 5.4. After 12 h, the
obtained solids were dried at 393 K for 720 min and calcined
at 673 K for 4 h under a stream of air (5.16 × 10−3 molmin−1
)
Gallium has been scarcely studied as an additive of HDT
catalysts. However, some papers have been devoted to the
characterization of Ga as an additive in alumina-supported
CoMo/Al2O3 and NiMo/Al2O3 catalysts. In early works,
Cimino et al. [10] and Lo Jacono et al. [14] observed a high
affinity of Ga3+ to the tetrahedral sites of alumina, modifying
with a heating rate of 1 K min−1
.
The oxide phases (mass of sample ca. 1 g) were sulfided
at 673 K for 2 h with a mixture of H2S/H2 (15% H2S) at a
total flow rate of 2.97 × 10−3 molmin−1 and a heating rate of
5 K min−1. After this step, the catalysts were cooled to room
temperature and flushed with N2 for about 30 min and kept in
sealed bottles under argon.
2+
the ratio of tetrahedral/octahedral species of Ni (Ni2te+t /Ni
)
oct
in the Ni/Al2O3 solid. A similar effect was observed when
gallium was added to the CoMo catalyst, and these authors
2.2. Characterization techniques
2+
observed that the Co2te+t /Co ratio changed as a function of
oct
the metal loading (Ga, Co, and Mo) and of the impregnation
sequence. As far as we know, these catalysts were not tested
in HDS reactions; therefore, in this context, our experimental
goals are aimed at investigating the additive effect of gallium.
Indeed, recent results reported a positive effect of Ga as a
second promoter to a NiMo catalyst in the pyridine hydroden-
itrogenation (HDN) [15], and mixed gallium/aluminum oxides
have been used as support for CoMo catalysts in the HDS of
thiophene [16]. In this paper we report the study of a series
of NiMo/Ga–Al2O3 catalysts with a gallium content ranging
from 0 to 5.9 wt%. These catalysts were tested in the HDS
of dibenzothiophene and 4,6-dimethyl-dibenzothiophene, and
a discussion of these results considering the characterization
in the sulfided state by X-ray diffraction (XRD), X-ray pho-
toelectron spectroscopy (XPS), and temperature-programmed
reduction on the sulfided samples (TPR-S) is presented.
After the final calcination, the metal contents were deter-
mined by atomic absorption after appropriate dissolution of the
solids samples; the results are presented in Table 1. This ta-
ble also gives the nomenclature and specification of all of the
catalysts. Elemental sulfur analyses were performed after com-
bustion at 1623 K in a CS-mat 5500 instrument (Ströhlein).
The emitted amounts of SO2 were analyzed by infrared spec-
troscopy.
Surface area, pore volume, and pore size distribution were
obtained from N2 adsorption and desorption isotherms using
the conventional BET and BJH methods. The samples were first
dried at 393 K for 2 h and subsequently outgassed at 573 K
under vacuum. Then N2 adsorption measurements were carried
out at 73 K.
The XRD patterns of the calcined and sulfided catalysts were
recorded on a Bruker D5005 diffractometer using Cu-Kα radi-
ation (0.154184 nm) at 3–80◦ with a 0.02◦ step size and 1 s at
every step.
2. Experimental
XPS measurements were performed on a VG Instrument
type ESCALAB 200R spectrometer equipped with an Al-Kα
source (hν = 1486.6 eV). The shifts of the peak core line due
to the charge of the sample were corrected by taking the Al-
2p line of the catalyst support, γ -Al2O3 (Al 2p, 74.0 eV) as
a reference. For the XPS measurements, the presulfided pow-
der sample was introduced into an argon-filled glove box and
pressed on indium foil fixed on the sample holder under the
protection of inert Ar atmosphere. The sample holder was then
transferred into the preparation chamber of the XPS equipment
and passed into the analysis chamber after evacuation overnight
(10−9 Pa). The XPS spectrum corresponds to the plot of the
variation in the numbers of emitted electrons versus their ki-
netic energy values, that is, their binding energy values.
In the TPR-S experiments, an appropriate amount of the sul-
fided sample (50 mg) was maintained between two layers of
quartz wool in a U-shaped quartz tube reactor with an outside
2.1. Catalyst preparation
The supports were prepared by pore-filling impregnation
of a commercial γ -Al2O3 (high purity; BET surface area,
240 m2 g−1; pore volume (Vp), 0.63 cm3 g−1; particle size, 80–
100 µm) with an aqueous solution of Ga(NO3)3 ·xH2O (Aldrich
Chemical) at constant pH of 5.4 to obtain supports with 0, 0.6,
1.2, 1.8, 2.9, and 5.9 wt% of metal. After 12 h, the obtained
solids were dried at 393 K for 12 h and calcined at 723 K for
4 h under a flow of air (5.16 × 10−3 molmin−1) with a heating
rate of 1 K min−1
.
NiMo catalysts were prepared by the pore-filling method
on calcined Ga2O3/γ -Al2O3 samples (Ga–Al2O3) with aque-
ous solutions of ammonium heptamolybdate [AHM, (NH4)6
Mo7O24 · 4H2O; Merck] and nickel nitrate [Ni(NO3)2 · 6H2O;
Aldrich Chemical]. The loading of the metals was estimated per

E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
405
Table 1
Textural properties, chemical analysis and nomenclature of prepared catalysts.
Support
Catalyst
N
adsorption measurements
Metal content (wt%)
2
A
BET
V
d
p
Ga
Ni
Mo
p
γ -Al O
0.6Ga–Al O
1.2Ga–Al O
1.8Ga–Al O
2.9Ga–Al O
5.9Ga–Al O
NiMo
213
218
220
219
215
196
0.55
0.49
0.50
0.52
0.43
0.44
100
102
100
100
98
0.00
0.61
1.20
1.74
2.86
5.91
2.43
2.32
2.34
2.35
2.30
2.34
8.64
8.47
8.46
8.41
8.41
8.34
2
3
0.6NiMo
1.2NiMo
1.8NiMo
2.9NiMo
5.9NiMo
2
3
3
3
3
3
2
2
2
96
2
2
−1 −1
, surface area (m g ); V , pore volume (ml g ); and d , pore diameter (Å).
p p
A
BET
diameter of 6 mm and inside diameter of 4 mm. The quantity of
H2S was measured by a photoionization detector (PID) (type PI
52-02A; HNU Systems). The energy of the lamp was 10.2 eV,
approaching the amount needed for the ionization of H2S. Be-
fore the reduction step, the detector was calibrated with a stan-
dardized gas mixture of 500 ppm H2S/Ar (the exact content of
H2S with a relative precision of 3%). When the PID signal was
stabilized, the reactor was switched to purified H2 (flow rate,
50 ml min−1) to first purge the rest of H2S for 10 min; then
the sample was reduced at a temperature that was increased lin-
early up to 1323 K at a rate of 4 K min−1). The composition
of the outlet gas mixture was analyzed every minute with an
automated sampling valve. The amount of H2S was calculated
according to previous studies [17,18].
a hydrogen atmosphere of 5 MPa for 8 h, using 500 mg of sul-
fided catalyst and 1.41×10−3 mol of 4,6-DMDBT dissolved in
150 ml of dodecane (Aldrich Chemical), with 1.4 × 10−3 mol
of hexadecane (Aldrich Chemical) used as an internal standard
(IS) reference for quantitative analysis. The reactor was flushed
with nitrogen and heated under stirring to reach the reaction
temperature; then hydrogen was introduced (ptot = 5 MPa).
The reaction time was determined from this moment. The to-
tal pressure was controlled constantly during the course of the
reaction by adding hydrogen to compensate for its consump-
tion. Samples were periodically collected and analyzed quanti-
tatively by GC. The reactant conversion and the products yields
were determined relative to the IS.
3. Results
2.3. Catalytic activity tests
3.1. N2 Adsorption measurements
The HDS of DBT (Aldrich; 99.9%) was carried out in a
continuous-flow microreactor in the vapor phase working with
The catalysts with different Ga content demonstrated almost
the same values for the surface area (Table 1). However, a slight
decrease (ca. 11%) was observed for the sample containing
5.9 wt% Ga. The pore volume (V p) and the pore diameter (dp)
did not vary significantly for the whole series.
a total flow of 6 l h−1, pH = 4 MPa, pDBT = 2.4 kPa, and
2
pH S = 5.2 kPa, at 513–533 K. The catalyst mass was approx-
2
imately 50 mg. The products were analyzed by gas chromatog-
raphy (GC).
The HDS of 4,6-DMDBT was carried out in a three-phase
continuous-flow microreactor with dodecane as the solvent,
3.2. XRD
working with a total mass liquid flow of 3.8 g h−1, pH
=
2
3 MPa, and H2 flow = 0.026 l min−1, at three different temper-
atures (553, 573, and 593 K). The catalyst mass was approxi-
mately 50 mg. The products were analyzed by GC.
The specific reaction rates were calculated according to the
following expression:
In all Ga–Al2O3 supports, the diffraction patterns were prac-
tically the same as that of alumina, suggesting that gallium is
highly dispersed with a particle size <4 nm, in agreement with
transmission electron microscopy results published by Nishi et
al. [19]. The presence of gallium produced no significant mod-
ification of the XRD patterns of all NiMo-containing samples
after calcination or sulfidation (patterns not shown here).
F
r = (ConvDBT),
(1)
m
where r is the specific rate of disappearance of DBT or 4,6-
DMDBT (mol g−1 s−1), F is the molar flow rate of the reactant
(mol s−1), ConvDBT is the conversion of DBT or 4,6-DMDBT,
and m is the catalyst mass (g). All rates were estimated at low
conversion (<15%).
Additional experiments were performed in batch mode to as-
sess the evolution with time of the selectivities in the HDS of
4,6-DMDBT. This reaction was performed in a 300-ml batch
reactor, magnetically stirred (1000 rpm) (Parr Instruments),
equipped with four baffles on the wall to prevent vortex for-
mation. The test conditions were a temperature of 593 K under
3.3. TPR-S analyses
3.3.1. TPR-S of α-Ga2S3 and sulfided γ -alumina
The TPR-S pattern of the bulk commercial α-Ga2S3 is
shown in Fig. 1a. Three large peaks were observed between 800
and 1350 K, indicating a three-step reduction. The quantitative
analysis indicated that the α-Ga2S3 was reduced in about 74%.
Due to the experimental conditions, we could not continue the
reduction above 1350 K, although we did plot a confidential
interval to complete the reduction pattern (Fig. 1b). Hence we
suggest, taking into account literature results [20], proceeding

406
E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
Table 2
H S production in the TPR-S of sulfided supports (Ga–Al O ) and catalysts
2
2 3
(0–5.9NiMo)
a
a
Support
Zone I
Zone II
Solid
Total
−1
−1
−1
(µmol g )
cat
(µmol g
)
(µmol g
)
cat
cat
γ -Al O
0.6Ga–Al O
1.2Ga–Al O
1.8Ga–Al O
0
0
NiMo
1736.58
1748.30
1717.89
1727.26
1692.25
1696.11
532.30
2
3
19.0
38.2
41.1
60.4
130
319
0.50
3.00
24.3
55.0
137
0.6NiMo
1.2NiMo
1.8NiMo
2.9NiMo
2
3
3
3
3
3
2
2
2.9Ga–Al O
2
5.9Ga–Al O
2
5.9NiMo
c
b
5.9Ga–Al O
2
355
α-Ga S
2 3
3
a
Data corrected by the extraction of H S produced in the alumina TPR-S.
2
Support sulfided at 873 K.
The mass analyzed correspond to the loading of gallium present in the sup-
b
c
Fig. 1. TPR-S patterns: (a) experimental reduction of commercial sample
port 5.9Ga–Al O .
α-Ga S and (b) complete predicted reduction pattern of α-Ga S .
2
3
2
3
2 3
Fig. 2a shows the TPR-S pattern of the support γ -Al2O3 sul-
fided at 673 K. Production of H2S was observed between 293
and 400 K, representing the desorbed H2S. Another zone of
H2S production occurred between 600 and 900 K, correspond-
ing to the reduction of some impurities [21,22] or to the reduc-
tion of some elemental sulfur produced by H2S decomposition
during the sulfidation. It has been reported that the reduction of
this elemental sulfur can be found only over the support and not
over Ni/Al2O3, Mo/Al2O3, or NiMo/Al2O3 solids [23].
3.3.2. TPR-S of Ga–Al2O3 supports
The TPR-S patterns of Ga–Al2O3 supports sulfided at 673 K
and with various loadings are shown in Fig. 2b–f. The profiles
changed as the amount of gallium was increased. There are two
important zones of H2S production: zone I, in the range of 450–
950 K, in which a broad peak increases with the loading of Ga,
and zone II (950–1350 K), in which the intensity becomes more
significant than that of zone I at a loading over 2.9% of gallium.
The total amount of H2S and the evolution of H2S in each zone
are reported in Table 2. The H2S desorbed below 450 K was not
considered, because this peak was present in all samples.
In Fig. 1, the TPR-S of α-Ga2S3 shows a completely dif-
ferent reduction pattern than that of the Ga–Al2O3 series
(Figs. 2b–f), suggesting that the gallium species on the alumina
surface is different than that in the bulk gallium sulfide.
Fig. 2. TPR-S patterns of (a) γ -Al O , (b) 0.6Ga–Al O , (c) 1.2Ga–Al O ,
2
3
2
3
2 3
(d) 1.8Ga–Al O , (e) 2.9Ga–Al O , and (f) 5.9Ga–Al O . All samples were
2
3
2
3
2 3
sulfided at 673 K.
with the reduction of α-Ga2S3 as follows:
Ga2S3 + H2800–1100 K2GaS + H2S,
→
(1)
(2)
2Ga + H2960–1290 KGa2S + H2S,
→
Ga2S + H21100–1600 K
→
Ga0 + H2S.
(3)
3.3.3. TPR-S of NiMo catalysts
In the experimental TPR-S pattern (Fig. 1a), the peak between
800 and 1100 K [step (1)] corresponds to 42% of the total H2S
produced (area under the curve), the peak between 960 and
1290 K [step (2)] represents 42% of the total H2S, and the peak
between 1100 and 1350 K [step (3)] corresponds to 16% of the
total H2S. The temperature difference (ꢀT ) between each ad-
jacent peak maximum is ca. 160 K.
The quantitative analyses in TPR-S (considering a 10% er-
ror) and the elemental sulfur chemical analysis indicate that the
H2S produced in each step is consistent with the stoichiometry
of the species involved in the reduction steps. These results led
us to consider that the pattern suggested in Fig. 1b (where the
H2S in each step represents 33% of the total H2S produced) is a
helpful approximation to determine the complete Ga2S3 reduc-
tion.
Due to the fact that the reduction of gallium sulfide could
be superposed to the reduction of the NiMo sulfide phase, we
determined, for the sake of simplicity, that all NiMo catalysts
could be deconvoluted in the same way. Interpretation of the
patterns is based on work by Arnoldy et al. and Mangnus et al.
[22,24,25], who classified the H2S produced during the TPR-S
in four types, as described below.
The deconvolution treatment was adjusted to four peaks that
correspond to the four types of H2S sources. For the TPR-S of
NiMo (Fig. 3), the first peak appeared between 350 and 500 K
with a maximum at 393 K (peak 1). The H2S produced in this
region is assigned mainly to the desorbed H2S (type 1), and
some part could be produced by the hydrogenation of nonstoi-
chiometric sulfur (Sx). Peak 2, between 300 and 900 K with a

E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
407
The catalyst with the highest amount of Ga (5.9NiMo)
showed important differences for the four peaks of NiMo (see
Fig. 3d). The highest temperature of peak 1 increased by 10 K
compared with peak 1 for the other catalysts, and the area of
this peak was slightly higher. Peak 2 showed a significant de-
crease in area (28%) and a downshift of 20 K, suggesting poor
production of H2S type 2. For peak 3, the highest temperature
continued moving down ca. 804 K, and its area was ca. 15%
greater than that of NiMo. This modification could be due to
the existence of higher amounts of nickel species in the sulfided
state over the Ga–Al2O3 surface. Peak 4 showed the highest
intensity in the patterns of all catalysts, but the maximum tem-
perature also occurred at 1150 K. This increase could be caused
by the H2S produced by the noticeable reduction of gallium due
to its high loading in this sample.
Fig. 3. TPR-S patterns of (a) NiMo, (b) 0.6NiMo, (c) 1.8NiMo and (d) 5.9NiMo
catalysts. All catalysts were sulfided at 673 K. Dashed area is assigned to nickel
species.
3.4. XPS analysis
3.4.1. XPS of Ga–Al2O3 support
maximum at 651 K, could be assigned to H2S produced by a re-
combination of S–H groups, a reaction of H2 and S–H (type 2),
and a reaction that may involve a surface nonstoichiometric sul-
fur (Sx) anion and H2, forming H2S and anion vacancies on the
catalyst (type 3) [21,26],
To investigate the effect of the sulfiding process over gal-
lium and the interaction of this metal with the support, the
Ga-containing samples were analyzed by XPS. The samples
were β-Ga2O3, α-Ga2S3 (both commercially supplied), 5.9Ga–
Al2O3 after calcination, and 5.9GaS–Al2O3 sulfided under typ-
ical conditions. In the first two cases, the internal reference was
the C 1s with binding energy (BE) of 284.5 eV [29] arising from
carbon contamination. The BE of Al 2p (74 eV) was used for
the other two samples, as explained in the Experimental section.
Regarding the BE for Ga2O3, the Ga 3d signal is normally
used as a reference because it is the most intense signal. How-
ever, a partial overlapping of the Ga 3d signal by the O 2s peak
occurs when Ga is supported on alumina, introducing an error
in the peak position measurements. Therefore, all of our analy-
ses for gallium-containing samples were performed using the
Ga 2p3/2 level. Table 3 summarizes the BE values for both sig-
nals for comparison purposes.
SSSS
S2SS
+ H2 ꢀ
+ H2S.
| | | |
| | | |
Sulfur anion vacancies have been suggested as catalytic sites
for hydrotreatment reactions [26], and it is quite possible that
in TPR-S the presence of H2 in the range of 400–700 K could
create such catalytic sites.
Peak 3, between 600 and 1250 K with a maximum at 903 K,
is assigned to H2S (type 4) produced by the reduction of dis-
persed NiS species and by the reduction of Ni3S2 produced by
the sintering of the NiMoS phase (NiMoS → Ni3S2 + MoS2),
which is not directly observable under TPR-S conditions [24].
Peak 4, between 700 and 1350 K with a maximum at 1150 K,
is ascribed to H2S (type 4) produced by the reduction of well-
dispersed molybdenum species [23,28].
The TPR-S pattern for 0.6NiMo (Fig. 3b) exhibited a simi-
lar shape as that observed for NiMo (Fig. 3a). Peaks 1 and 2 did
not show any modification compared with the same peaks for
the NiMo catalyst. In contrast, peak 3 showed an increased area
but a lower maximum reduction temperature (863 K) compared
with the equivalent peak for NiMo. These differences suggest
an easier reduction, probably due to better dispersion of the
nickel species caused by the presence of highly dispersed Ga
and to better sulfidation of nickel. The intensity of peak 4 was
higher than that measured for the NiMo sample.
The TPR-S pattern for 1.8NiMo (Fig. 3c) did not reveal any
important changes over peaks 1 and 2, but the area of the lat-
ter was lower by about 7% than that reported for the NiMo
solid. Peak 3 exhibited significant differences compared with
the NiMo catalyst; a diminution of 80 K over its maximum
and an increase in area of ca. 8% was observed. No signifi-
cant changes were observed for peak 4, except a slight increase
of intensity.
The Ga 2p3/2 BE (1118.4
0.1 eV) for the bulk-sulfided
sample, α-Ga2S3, showed a difference of −0.3 eV ( 0.2) with
respect to that of the oxide sample, β-Ga2O3. It is generally ac-
cepted that sulfidation of metals produces a slight BE reduction
at the same oxidation state. The BE of Ga 2p3/2 for the cal-
cined 5.9Ga–Al2O3 sample showed a difference of about 1.0 eV
( 0.2) with respect to that of the sulfided 5.9Ga–Al2O3 sample.
The BE difference between the oxide- and sulfide-supported
samples suggests that at least a fraction of gallium supported
on alumina was sulfided, which is consistent with findings of
the TPR-S analyses (Table 2). The differences in BE between
the bulk and the supported samples suggest that the gallium
species on the alumina surface are different from that of the
bulk. Therefore, the BE at 1119.5 eV ( 0.1) of Ga 2p3/2 is
assigned to the supported Ga3+δ+ species on the calcined ma-
terial, whereas the value of 1118.7 eV ( 0.1) is assigned to the
Ga3+ species in bulk Ga2O3.
3.4.2. XPS of NiMo catalysts
For the NiMo sulfided catalysts with or without Ga, the Mo
3d band is characterized by a doublet with BE (Mo 3d5/2
)

408
E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
Table 3
Binding energies of catalysts in oxide or sulfide state referenced to Al 2p peak position (74.0 eV). Ga 3d and Ga 3p
referenced to carbon peak position (284.5 eV). See comments in the text
of commercial supplied powders are
3/2
Sample code
Treatment
T (K)
Binding energies (eV)
Ga 3d
Ga 2p
Ni 2p
Mo 3d
Ni 2p /Al 2p
3/2
3/2
5/2
3/2
NiMo
0.6NiMo
5.9NiMo
Sulfidation
Sulfidation
Sulfidation
Calcination
673
673
673
732
673
–
–
–
–
–
853.9
854.1
853.6
228.7
228.6
228.3
0.039
0.067
0.045
20.0
19.9
21.1
20.3
20.5
20.2
1118.3
1118.0
1119.5
1118.5
1118.7
1118.4
5.9Ga–Al O
2
3
Sulfidation
–
–
–
–
–
–
–
–
–
–
–
–
Ga O
–
–
–
–
–
2
3
3
Ga S
2
3
a
Ga O
2
20.5
a
Ga O
19.0
2
a
Ga
a
18.7
From Ref. [29].
Fig. 5. Comparison of XPS spectra of Ni 2p
0.6NiMo and (c) 5.9NiMo, including curve fitting results.
region for (a) NiMo, (b)
Fig. 4. Comparison of XPS spectra of the Mo 3d and S 2s regions for (a) NiMo,
(b) 0.6NiMo, and (c) 5.9NiMo, including curve fitting results.
3/2
5.9NiMo with respect to the Ni 2p BE for the reference NiMo
catalyst. The deconvolution of the Ni 2p region (Fig. 5) revealed
a small peak with BE = 859 eV ( 0.1) in all cases. This peak
could be assigned to nonsulfided or reduced NiO that could be
in the form of NiAl2O4 [30–32], and the size of this peak de-
creased when the Ga loading increased. The BE shift of Ni 2p
and the size reduction of the peak at 859 eV ( 0.1) suggest that
some alumina sites that have chemical interactions with nickel,
forming NiAl2O4, could be occupied by gallium species, con-
ducting to a higher concentration of free Ni species that could
be more readily sulfided, as discussed earlier for the TPR-S re-
sults. The Ni 2p3/2/Al 2p ratio increased considerably in the
presence of Ga (cf. Table 3). Besides, it should be considered
that the reduction in BE of the Ni 2p peak could result from a
lower proportion of surface Ni species in the “NiMoS” phase.
This hypothesis is based on the idea that the electron density
equals 228.7 eV ( 0.1) (Fig. 4) regardless of the Ga presence.
The S 2p level for all NiMo catalysts is characterized by a nar-
row peak with BE 161.6 eV ( 0.1), and its position does not
change with the amount of Ga. A partial overlapping between
S 2p and Ga 3d signals was observed for the sulfided samples,
which makes the assessment of the peak position less accurate.
The XPS atomic ratios S 2s/Mo 3d were calculated after appro-
priate deconvolution for all of the NiMo sulfided catalysts, and
it was noted that the catalysts with gallium (0.6 and 5.9%) ex-
hibited a significantly higher S 2s/Mo 3d ratio than the NiMo
catalyst (results not shown). This excess could be ascribed to
the sulfur of gallium sulfide species and to a better sulfidation
of the Mo and Ni species.
The XPS spectra of Ni 2p changed its position slightly in
the presence of Ga (Fig. 5). The Ni 2p BE shifted upward by
0.2 eV ( 0.2) for 0.6NiMo and downward by 0.3 eV ( 0.2) for

E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
409
Table 4
Table 5
Transformation of DBT in a continuous flow reactor, and the deactivation factor
observed in the HDS of DBT and 4,6-DMDBT
Activities and selectivities for the HDS of 4,6-DMDBT in a batch reactor at
593 K
1
T
2
α
Catalyst
r
i
r
r
S
S
S /S
DDS HYD
Catalyst
r
α
HYD
HYD
DDS
DDS
DBT
T
NiMo
6.7
8.8
7.4
6.4
4.4
3.2
23.6
9.0
7.8
4.3
1.7
0.5
16.4
11.4
9.2
2.3
2.9
NiMo
15
19
17
12
12
9
4.4
6.1
5.1
4.3
4.4
3.6
10.6
12.9
11.9
7.7
7.6
6.7
29
32
30
36
37
40
71
68
70
64
63
60
0.40
0.47
0.42
0.56
0.58
0.66
0.6NiMo
1.2NiMo
1.8NiMo
2.9NiMo
5.9NiMo
0.6NiMo
1.2NiMo
1.8NiMo
2.9NiMo
5.9NiMo
2.2
−8
−1 −1
g ). Conversion under 15%.
r
, transformation rate of DBT (10 mol s
DBT
−8
−1 −1
s
r , initial transformation rate of 4,6-DMDBT (10 mol g
).
1
−2
−1
i
α , deactivation factor corresponding to the HDS of DBT (10
(
% h );
T
r
, transformation rate of 4,6-DMDBT through the direct desulfurization
DDS
0.02).
−8
−1 −1
s ).
pathway (10 mol g
, transformation rate of 4,6-DMDBT through the hydrogenation pathway
2
T
α , deactivation factor corresponding to the HDS of 4,6-DMDBT
r
HYD
−2
−1
−8
−1 −1
(10 % h ); ( 0.02).
(10 mol g
and S
s
).
are, respectively, the initial selectivities of direct desulfuriza-
S
HYD
DDS
tion and hydrogenation pathways ( 2), which were estimated by multiplying
the initial rate transformation by the corresponding fraction of each pathway.
continuous-flow tests of the NiMo catalysts with different
amounts of gallium. For all three temperatures, the highest ac-
tivities were obtained for the 0.6NiMo catalyst and a decrease
of activity was observed for higher Ga loadings, following al-
most the same tendency of the foregoing results exposed for
the HDS of DBT. Moreover, the stabilities of the NiMo cata-
lysts with gallium were quite similar to those observed in the
HDS of DBT in the continuous flow reactor. The catalysts with
the higher gallium amount had the better stability (Table 4).
The transformation of 4,6-DMDBT occurs through two par-
allel routes. The direct desulfurization pathway (DDS) yields
mainly 3,3-dimethylbiphenyl, whereas the hydrogenation route
(HYD) involves preliminary hydrogenation of one aromatic
ring, giving 4,6-dimethyl-tetrahydro- and 4,6-dimethyl-hexa-
hydrodibenzothiophene or analogues. These intermediates can
be desulfurized to 3,3ꢀ-dimethylcyclohexyltoluene and 3,3ꢀ-di-
methylbicyclohexyl [35–38].
Table 5 lists the catalytic activities determined for the NiMo
catalysts with different loadings of gallium after additional ex-
periments with 4,6-DMDBT in a batch reactor. The catalyst
0.6NiMo was 27% more active than the reference catalyst, and
the 1.2NiMo showed an increase of ca. 14%. This enhancing
effect disappeared when the amount of gallium was >1.2 wt%;
that is, the activity for the (1.8–2.9)NiMo catalysts was 20%
lower than that of the NiMo catalyst.
Fig. 6. Transformation of 4,6-DMDBT in a continuous flow reactor at different
temperatures.
transfer from Ni to Mo leads to a higher BE of Ni 2p, suggest-
ing a high formal positive charge on Ni atoms [33,34].
3.5. Catalytic tests
3.5.1. HDS of dibenzothiophene
Table 4 gives the results of the DBT transformation rates
and reports a deactivation factor to obtain information about
the decreased reaction rate with time on stream and thus gain
insight into the stability of the samples under these conditions.
The deactivation factor (αT) is expressed in percent per hour
and is obtained from the slope of the conversion data plotted
versus time when the pseudostationary phase was reached af-
ter ca. 5 h on stream. Deactivation was measured after 50 h of
reaction. The DBT transformation rates over the 0.6NiMo and
1.2NiMo catalysts were respectively 31 and 10% higher than
that of the NiMo sample. However, higher Ga loadings pro-
voked lower catalytic activities than that of the reference solid.
In addition, all of the catalysts with gallium exhibited a better
stability than the NiMo reference catalyst.
For the most active gallium-containing NiMo catalyst, the
increased catalytic activity was reflected to a slightly higher ex-
tent over the DDS pathway rate (rDDS) than over the HYD rate
(rHYD). The transformation of 4,6-DMDBT through the DDS
route (rDDS) was 35 and 16% higher over the 0.6NiMo and
1.2NiMo catalysts, respectively, than over NiMo (Table 5). The
transformation of 4,6-DMDBT through the HYD route (rHYD
)
was 23% higher over the 0.6NiMo and 12% over the 1.2NiMo
catalyst. The 1.8NiMo and the 2.9NiMo catalysts had the same
rDDS as the NiMo catalyst, whereas the rDDS of the 5.9NiMo
catalyst was reduced by 18%. Furthermore, rHYD was more
greatly affected by the larger amounts of gallium, with this
pathway decreasing by 27% for the 1.8NiMo catalyst, by 28%
for the 2.9NiMo catalyst, and by 47% for the 5.9NiMo cata-
lyst. Table 5 summarizes the SDDS/SHYD ratios for all of the
3.5.2. HDS of 4,6-dimethyl-dibenzothiophene
Fig. 6 presents the reaction rates for the HDS of 4,6-
DMDBT determined at three different temperatures in the

410
E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
catalysts. From these data, it is difficult to establish a direct cor-
relation between this value and the Ga content in the catalysts.
All HDS tests were performed at different reaction condi-
tions. Based on these results, the enhancing catalytic effect of
gallium at low loading over NiMo catalysts activity was con-
firmed.
4. Discussion
4.1. Effect of gallium over alumina
The textural properties of the unloaded support did not
change significantly when gallium was present, probably due
to the high dispersion of this metal. XRD did not show any
peaks that could suggest the presence of crystals of Ga2O3, in
agreement with the literature results [19,39]. The XPS analy-
ses showed that the BE values of Ga 2p3/2 and Ga 3d that
we observed and that are described in the literature for bulk
compounds (Table 3) are systematically lower than those for
the supported gallium species. Although a size effect in XPS
has not been reported for metal oxide particles, it has been
shown that the development of a TiO2–SiO2 interface produced
an upshift in the Ti 2p XPS peak and a downshift in the Ti
Auger parameter [40]. The values of these shifts in supported
gallium oxide indicate changes in the extra-atomic relaxation
energy rather than a variation of the initial state [41,42]. In
principle, these differences in relaxation could be related to
crystal size and/or support effects. As discussed earlier, the
gallium is highly dispersed on alumina and this leads to a
strong Ga–support interaction. Shimizu et al. [19,43–45] pre-
pared Ga2O3/Al2O3 and Ga2O3/SiO2 by impregnation with an
aqueous solution of Ga(NO3)3 followed by evaporation and cal-
cination at 873 and 823 K. The X-ray absorption near edge
structure (XANES) spectrum showed that the local structure of
Ga2O3 on SiO2 was similar to that of octahedral (labeled Gao–
O) α-Ga2O3. The XANES spectrum of Ga2O3/Al2O3 did not
resemble the individual spectra of β- or α-Ga2O3, indicating
that the Ga ions have a specific local structure as a conse-
quence of the strong interaction with the alumina support show-
ing a tetrahedral coordination (labeled Gat–O) [19]. Extended
X-ray absorption fine structure (EXAFS) analysis showed that
the bond distances of Gat–O and Gao–O were almost constant
for the sample <20 wt% Ga (Rt = 1.70 Å and Ro = 1.95 Å).
These distances are shorter than the bond distances of β-Ga2O3
(Rt = 1.86 Å and Ro = 2.02 Å) and rather close to those of γ -
Al2O3 (Rt = 1.72 Å and Ro = 1.95 Å) [44]. These findings and
our results suggest that the gallium ions are incorporated in the
alumina surface below the monolayer charge (Ga < 20 wt%).
This would be quite similar to the model of a surface spinel pro-
posed by Chin and Hercules [46] for Ni/Al2O3 and Co/Al2O3
systems, in which the surface spinel is formed by diffusion of
cations during calcination into the surface cation site of the
alumina. In that way, the XPS results are consistent with the
literature and indicate a strong interaction between the Ga and
the support.
Fig. 7. Evolution of the rate transformation of 4,6-DMDBT as a function of
the gallium species. Ga , tetrahedrally coordinated; Ga , gallium octahedrally
coordinated.
t
o
This is in agreement with the XPS results showing a down-
shift in the BE for Ga 2p3/2 from 1119.5 eV (oxidic state)
to 1118.5 eV. The quantitative TPR-S analyses and the ele-
mental sulfur analyses revealed that the Ga sulfidation was
incomplete at 673 K, considering that the stoichiometric com-
putations showed a 65% deficit of the expected gallium/sulfur
atomic ratio for the five sulfided supports. To verify this finding,
we sulfided the 5.9Ga–Al2O3 sample at 873 K and found that
the quantitative analyses and the sulfur chemical analyses had
greater sulfur production than that for the sample 5.9Ga–Al2O3
sulfided at 673 K (Table 2). Therefore, under typical sulfiding
conditions, some part of Ga remained in the oxidic state.
In the TPR-S patterns of the supports (Ga–Al2O3), H2S pro-
duction occurred in two different zones (Fig. 2). The difference
of temperature (ꢀT ) between the two more intense peaks in
each zone was ca. 600 K. This is three times larger that the ꢀT
between the peaks observed in the reduction of Ga2S3 (bulk),
where the ꢀT between the adjacent peaks was about 160 K (see
Fig. 1). Hence, for the supported gallium samples, neither the
H2S produced in zone I nor the H2S produced in zone II can
be assigned to a consecutive reduction similar to that occur-
ring in the unsupported samples. It can be suggested that each
reduction zone corresponds to a different species of gallium
supported on alumina. Hence, in the TPR-S, the production of
H2S at low Ga content (zone I) can be assigned to the tetrahe-
dral species (Gat) and zone II, which started to be more intense
at high Ga content, can be assigned to the octahedral species
(Gao) (Fig. 7). Therefore, the ratio of Gat/Gao decreased as the
Ga loading increased. This is in agreement with the results of
Shimizu et al. [43–45].
4.2. Effect of gallium on the NiMo/γ -Al2O3 catalysts
The more remarkable features of the NiMo TPR-S are as
follows:
1. The type 2 and type 3 H2S, produced between 300 and
900 K (peak 2), presented the same intensity and area un-
der the curve for the catalysts with low Ga content; whereas
In the TPR-S analyses, H2S production in all samples sug-
gests that gallium is sulfided under typical sulfiding conditions.

E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
411
this production was reduced for the catalysts with a higher
Ga content.
shift in the BE (0.4 0.2 eV) was observed in the Mo 3d level.
This suggests that Ga at low loadings increases the sulfidation
of MoO3, although high Ga loadings could negatively affect the
dispersion of MoO3. Clearly, the increased amount of sulfidable
Ni species cannot compensate for the decrease of MoO3 disper-
sion. No clear relationship can be established between our XPS
results and the dispersion of Mo, because of the interference
of gallium (mainly at high loadings) over the atomic ratio of
Mo 3d/Al 2p.
2. The H2S assigned to the reduction of NiS and Ni3S2 (pro-
duced by the sintering of the NiMoS phase) showed a
downshift in its maximum temperature and an increase in
its area in the presence of gallium. These differences sug-
gest an easier reduction of the nickel species.
Regarding the XPS results, the main effect of Ga was on the
BE of Ni 2p. The BE of Ni 2p was 853.9 eV for the NiMo cata-
lyst supported over alumina and 854.1 eV for the 0.6NiMo cat-
alyst; the ꢀBE between the NiMo reference and the 5.9NiMo
catalyst was 0.3 eV. For both catalysts, it has been proposed
that Co (Ni) ions supported on alumina could be present as di-
luted NiAl2O4 species in tetrahedral coordination [46] and on
the surface of the support as a NiAl2O4-like phase in mainly
octahedral coordination [27,47]. It has been suggested that the
subsurface NiAl2O4 species are not sulfidable, whereas the sur-
face NiAl2O4-like species are partially sulfidable at 673 K [24].
Therefore, a fraction of Ni was not sulfided, and, consequently,
these nickel species do not participate in the HDS activity.
Bearing in mind the TPR-S results, the XPS results could
have two explanations:
4.3. Gallium effect on the NiMo catalyst activity
As noted earlier, small amounts of gallium enhanced the ac-
tivity of the NiMo catalyst in the HDS of DBT and 4,6-DMDBT
by about 30% for the 0.6NiMo and 15% for the 1.2NiMo. This
enhancement is most likely due to the occupation of the tetra-
hedral sites by gallium, resulting in an increase in the amount
of nickel species that can participate in the MoS2 decoration.
A similar effect has been observed from adding small amounts
of Zn [6,11–13] or Mg [6–9] to the alumina before the impreg-
nation of Mo and Ni (Co). These metals (Me) can form species
like MeAl2O4, which decrease the inactive NiAl2O4 species
and consequently increase the HDS activity. Moreover, the re-
sults shown in Fig. 7 correlate the HDS activity with the gal-
lium species present in the catalytic surface detected by TPR-S.
The maximum activity is reached when the gallium species in
the catalysts are mainly tetrahedral (Gat). When the octahedral
gallium species (Gao) start to increase, the activity of the cata-
lysts decreases in almost a parallel manner with the decreasing
Gat/Gao ratio, implying that the gallium effect is no longer pos-
itive at relatively high amounts (ꢁ1.8 wt%).
For the 0.6NiMo catalyst, the transformation of the 4,6-
DMDBT through the DDS was 35% higher than that over the
reference catalyst, and the rHYD was 23% higher than that of
the NiMo catalyst. These results indicate an increase in MoS2
slab promotion, in agreement with the XPS results for the
0.6NiMo catalyst. Furthermore, some results in the literature
suggest that better promotion is reflected mainly in the trans-
formation of 4,6-DMDBT through the DDS pathway [49]. For
the 1.2NiMo catalysts, the rDDS increases about 16% compared
with the NiMo catalysts, whereas the rHYD was 12% higher.
This implies less MoS2 promotion than in the 0.6NiMo cata-
lyst but more MoS2 promotion than in the NiMo catalyst. In
contrast, when the amount of gallium was increased, both path-
ways started to decrease, although the rHYD was affected more
significantly than the rDDS. Considering the XPS results for the
5.9NiMo catalyst, we suggest that, in contrast to the NiMo,
0.6NiMo, and 1.2NiMo catalysts, the NiMoS phase is poorly
formulated, leading to a change in selectivity approaching that
of the MoS2/Al2O3 catalysts, as reported previously [38].
Therefore, we can propose that the promoter (Ni) is better as-
sociated with the molybdenum (MoS2), creating more NiMoS
phase, whereas at the same time that the S anions are slightly
more basic, provoking a better C–S bond cleavage when the
amount of Ga is <1.2 wt%.
1. The addition of small amounts of gallium could result in
an interaction with tetrahedral sites (as discussed earlier),
leading to an increase in the free nickel species. This sug-
gests that more Ni participates in the NiMoS phase and in
free Ni as NiS, although these two phases could have oppo-
site effects on the BE of Ni 2p. In other words, if the amount
of NiS species increased on the surface of the alumina, then
a downshift in BE would be observed, as has been shown
previously [48], but an increase in the Ni species that par-
ticipate in the NiMoS phase could result in an upshift in BE
of the Ni 2p, due to the charge transfer of Ni to the MoS2
slabs [26]. The catalytic results support this latter idea, as
discussed later in the paper.
2. The addition of relatively high amounts of Ga could result
in the interaction of gallium with both tetrahedral and oc-
tahedral sites, leading to a dramatic increase in the octahe-
drally surrounded Ni ions. At the same time, the promoter
(Ni) may not be well associated with the molybdenum
(MoS2) due to a negative effect of the octahedral gallium
on the dispersion of the MoS2. When the amount of Ga
increases, the tetrahedral Al3+ neighboring sites starts to
be covered by gallium, and, consequently, Ga particle size
increases, leading to the formation of octahedral gallium
species.
As discussed earlier, Ga interacts mainly with the tetrahe-
dral sites of alumina at low loadings. MoO3 also interacts with
these sites [48]. In this work, XPS analysis reveals a diminu-
tion of the Mo5+ species (Fig. 4) between the reference and
the 0.6NiMo catalyst, suggesting a better sulfidation of MoO3.
Therefore, the presence of Ga at low loadings seems to be pos-
itive for the HDS properties, considering the high activity for
the NiMo catalysts. In the case of high Ga loading, a down-

412
E. Altamirano et al. / Journal of Catalysis 235 (2005) 403–412
5. Conclusions
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1.2 wt%) had a positive effect and induced increased activ-
ity of a typical NiMo supported over gamma alumina without
modifying its initial textural properties. XPS and TPR-S analy-
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Ni species participating in the decoration of the NiMoS phase,
leading to increased HDS activity. This effect might be caused
by the existence of gallium in tetrahedral coordination over the
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This research was performed under the auspices of the In-
stitut de Recherches sur la Catalyse, Instituto Mexicano del
Petróleo, and CONACYT (Project 42204-Y). E. Altamirano
is grateful to the Postgraduate Cooperation Program (PCP
France–Mexico) and to CONACyT for a graduate scholarship.
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