Characterization and hydrodesulfurization activity of W-based catalysts supported on Al2O3-TiO2 mixed oxides

Article abstract of DOI:10.1006/jcat.1997.1713

Characterization and catalytic activity of tungsten-based hydrotreating catalysts supported on Al₂O₃(1 - x)-TiO₂(x) mixed oxides (x=0.0, 0.1, 0.5, 0.7, 0.95, and 1.0) have been obtained. The supports were characterized by

Full text of DOI:10.1006/jcat.1997.1713

JOURNAL OF CATALYSIS 170, 108–122 (1997)
ARTICLE NO. CA971713
Characterization and Hydrodesulfurization Activity of W-Based Catalysts
–
Supported on Al2O3 TiO2 Mixed Oxides
J. Ramı´rez and A. Gutie´rrez-Alejandre
UNICAT, Departamento de Ingenier´ıa Qu´ımica, Facultad de Qu´ımica, UNAM, Cd. Universitaria, Me´xico D.F. 04510, Me´xico
Received September 5, 1996; revised April 1, 1997; accepted April 2, 1997
In the case of the tungsten-based catalysts, the effect of the
support is of great interest since it has been clearly estab-
lished (2–6) that in the preparation of these catalysts using
the conventional alumina support, during the calcination
step, a very strong interaction between the tungsten-oxided
species and the alumina is developed. This strong interac-
tion precludes the adequate sulfidation of the catalyst (7, 8)
leading therefore to poor activity. It has been demonstrated
that treatment of these catalysts at 673 K for 4 h in a sul-
fiding mixture of H2/H2S (15% v/v) achieves only about
70% sulfidation, whereas complete sulfidation is obtained
with molybdenum-based catalysts under the same condi-
tions (8).
In the attempts to overcome this difficulty researchers
in the field have tried, either the modification of the alu-
mina support or the use of other less interacting supports
(1, 9–11). Among the supports studied, titania seems an
interesting option in view of the high activity results ob-
tained previously on Mo/TiO2 systems (1). However, the
titania supports present the disadvantage of a low surface
area (Sg ꢂ 50 m2/g) and poor thermal stability compared to
their alumina counterparts (Sg ꢂ 200 m2/g).
Characterization and catalytic activity of tungsten-based hy-
–
drotreating catalysts supported on Al2O3(1 ꢀ x) TiO2(x) mixed ox-
ides (x = 0.0, 0.1, 0.5, 0.7, 0.95, and 1.0) have been obtained. The
supportswerecharacterized byX-raydiffraction (XRD), X-raypho-
toelectron spectroscopy, and FT-Raman spectroscopy. The catalysts
in their oxided state were characterized by surface area, XRD, UV–
Vis diffuse reflectance, FT-Raman spectroscopy, and temperature-
programmed reduction (TPR). The catalysts in their sulfided form
were characterized by FTIR spectroscopy and high-resolution elec-
tron microscopy. The catalytic activity of the different catalysts was
evaluated in the thiophene hydrodesulfurization reaction at atmo-
spheric pressure. The results indicate that the support composition
and properties determine the type of tungsten species present on
the surface. The tetrahedral tungsten species present on the alu-
mina support change gradually to different octahedral species as
the contents of titania in the support is increased. The presence
of titania in the support leads to oxotungstate species with greater
lateral interaction than in the case of the pure alumina-supported
catalyst. These changes in the type of tungsten species are accompa-
nied by changes in the reducibility and number of species detected
by TPR. From the characterization results a picture of the struc-
tural changes in the tungsten species as the titania content in the
support is increased is proposed. The changes in the catalytic ac-
tivity trend with titania content can be rationalized in terms of the
changes in the dispersion and morphology of the WS2 crystallites,
which are in turn related to the type of tungsten species in the oxidic
precursors, and also, in the rich titania catalysts (x = 0.95, 1.0), to
the effect that a sulfided and/or reduced layer of titania appears to
have on the reaction mechanism leading to much higher activities
than those expected from purely structural changes of the tungsten
c
The use of alumina–titania-mixed oxides supports seems
a good alternative to overcome the problems of the poor
sulfidation shown by the alumina-supported catalysts and
the low surface area of those supported on titania (11).
As for the characterization of the WO3/Al2O3 catalysts,
not as many studies as for the Mo system have been made.
However, tungsten-based alumina-supported catalysts, al-
though less studied than their molybdenum counterparts,
have been characterized by means of different techniques
such as Raman spectroscopy, UV–Vis diffuse reflectance
spectroscopy (DRS), X-ray photoelectron spectroscopy
(XPS), X-ray diffraction, and others (4, 6, 12–21). In spite
of these efforts, there is still some controversy on whether
the tungsten is in tetra or octahedral coordination on the
surface of the alumina (4, 12). The reports from the litera-
ture indicate that the structure and dispersion of the oxidic
tungsten phases present in the alumina supported catalysts,
depend on the tungsten load and on the calcination temper-
ature (4, 5, 12, 13, 22–27). Also, it has been reported that on
species.
ꢁ 1997 Academic Press
INTRODUCTION
Over the past years, Mo- and W-based catalysts sup-
ported on alumina have been widely used in hydrotreat-
ment processes. Of these two catalytic systems, the one
based on tungsten has been the least studied. Also, for
these catalytic systems it is now well known that the type
of support plays an important role and that their activity
may be improved by the use of an adequate support (1).
108
0021-9517/97 $25.00
c
Copyright ꢁ 1997 by Academic Press
All rights of reproduction in any form reserved.

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W-BASED CATALYSTS SUPPORTED ON Al2O3 TiO2 MIXED OXIDES
109
alumina supported tungsten catalysts the reduction of the thiophene hydrodesulfurization, which was used as a model
W6+ species takes place in a single step to W0 (17, 28).
Regarding the studies of tungsten supported on titania,
only a few studies have been reported (17, 26–29, 30, 31).
Temperature-programmed reduction studies (17, 28) show
that in these catalysts, in contrast with their alumina sup-
ported counterparts, the reduction of the WO3 species oc-
curs in two steps (W6+ → W4+ → W0). During this process,
the titania anatase phase is transformed to rutile although it
has been found that this transition is inhibited by the pres-
ence of the tungsten species (29). It has been also found that
at low tungsten loading (ꢃ6 wt% WO3), wolframyl species
in interaction with the support are present (29).
In a work by Hilbrig et al. (27), on the characterization of
WO3/TiO2 catalystsbyEXAFSand XANES, theyproposed
a structural model in which islands of surface tungstate
species are formed by branched chains of WO5 units with
terminal WO4 units anchored onto the titania surface via
reaction.
EXPERIMENTAL
Supports and Catalysts Preparation
A series of tungsten-based catalysts supported on
–
Al2O3 TiO2 mixed oxides with variable amounts of TiO2,
expressed as the molar ratio x = TiO2/(TiO2 + Al2O3) (x =
0, 0.1, 0.5, 0.7, 0.95, 1.0), have been prepared. The supports
were prepared by the coprecipitation of titanium and alu-
minum isopropoxides following the same method described
previously (11). Aluminum isopropoxide was dissolved in
n-propanol (150 ml per gram of isopropoxide) under vigor-
ous stirring. The required amount of titanium isopropoxide
was added to this solution and the coprecipitation of the
corresponding hydroxides was achieved by slowly adding
to the above solution an amount of demineralized water
equivalent to 33 times the required stoichiometric amount.
The precipitate was left under slow agitation for 24 h and
then was filtered and washed five times with water in an
amount of 250 ml per gram of product each time. Later on,
the solid was dried at 373 K for 24 h and calcined at 773 K
– –
W O Ti bonds. In this structure, the adsorption of one or
two water molecules increases the oxygen coordination to
six for all the tungsten surface sites. They also concluded
that the length of these chains, and thus the WO4/WO5 ra-
tio, is determined by the degree of tungsten oxide surface
coverage. In a recent work by Kim et al. (32) a series of sup-
ported tungsten oxide catalysts on different supports were
structurally characterized by in situ Raman spectroscopy.
In this work they proposed that the surface tungsten oxide
species on Al2O3 and TiO2 at high surface coverage possess
a highly distorted, octahedrally coordinated surface tung-
–
for 24 h. The solids prepared were named Al Ti(x), where
x is the molar ratio TiO2/(TiO2 + Al2O3).
A series of W catalysts containing 2.8 atoms of W per
square nanometer of support surface were prepared by
impregnating the above supports (pore volume method),
using the appropriate amounts of an aqueous solution
of ammonium metatungstate ((NH4)6H2W12O40). The im-
pregnated samples were dried at 373 K for 12 h and later
calcined at 773 K for 3 h; hereafter the catalysts will be
==
sten oxide structure with one short W O bond (mono-oxo
species), while at low surface coverage an isolated, tetrahe-
dral coordinated surface tungsten oxide species is present.
In another work by Wachs (33) under ambient conditions,
and where the surface was hydrated, their results showed
that the molecular structures of the surface metal oxide
species were determined by the net pH at the PZC (point
of zero charge) of the supported metal oxide system.
To date there have been no reports on the characteri-
zation and HDS activity of tungsten-based catalysts sup-
–
referred as W/Al Ti(x).
Supports and Catalyst Characterization
The specific surface areas of the samples were obtained
by nitrogen physisorption in an automated BET appara-
tus. X-ray diffraction of the supports and catalysts were
obtained in a Siemens D-500 diffractometer using the
Cu Kꢀ radiation (ꢁ = 1.5418). XPS measurements were per-
formed in a Perkin–Elmer spectrometer using monochro-
matic Al Kꢀ radiation. The C1s binding energy of spurious
carbon (BE = 284.5 eV) was taken as reference. The Raman
spectra were recorded on a Nicolet 950 FT-Raman spec-
trometer equipped with a InGaAs detector and a Nd : YAG
laser source, in the frequency zone 50–3500 cmꢀ1. The spec-
tral slit width was typically 4 cmꢀ1. In all the spectra shown
here, the contribution of the support has been subtracted
in order to minimize the fluorescence effect present in the
alumina support. UV–Vis diffuse reflectance spectra in the
wavelength region 250–2500 nm were obtained in a Cary
5 E UV–Vis–NIR spectrometer using BaSO4 as reference
for the supports and the support itself as reference for
the catalysts. Temperature-programmed reduction (TPR)
–
ported on Al2O3 TiO2 mixed oxides. It seems interesting
then to study the changes that occur in the tungsten oxidic
precursors as the nature of the support changes from alu-
–
mina to titania passing through Al2O3 TiO2 mixed-oxide
supports, with variable amounts of titania, and the rela-
tionship between the type of tungsten surface species with
the catalytic activity in HDS reaction. It is therefore, the
object of the present work to characterize the oxidic precur-
sors by different techniques (nitrogen physisorption, XRD,
TPR, XPS, Raman spectroscopy, and DRS), and the sul-
fided catalysts (HREM and FTIR) obtained with a series
–
of tungsten catalysts suppported on mixed Al2O3 TiO2 sup-
ports with variable amounts of TiO2 going from pure Al2O3
to pure TiO2 and relate the results from these characteri-
zations to the changes in catalytic activity observed in the

´ ´
RAMIREZ AND GUTIERREZ-ALEJANDRE
110
measurements were performed in a flow apparatus with an
argon–hydrogen (70% Ar v/v) mixture as reducing gas and
a thermal conductivity detector. The gas mixture flow was
25 ml/min and the heating rate of the samples was 10 K/min.
Samples of 0.25 g were used in each measurement. Before
TPR, the samples were pretreated in air at 773 K for 2 h.
Hydrogen consumption was evaluated using as a standard
reference the reduction of V2O5, as previously reported
(34). The infrared spectra of the oxided state of the sam-
ples were recorded in the 4000–200 cmꢀ1 range on a Nicolet
510 FT-IR spectrometer working at a resolution of 4 cmꢀ1
and with 400 scans. These samples were recorded using KBr
wafers. Compensation of support absorption was used by
subtraction of a spectrum of an equivalent amount of sup-
port from the spectra of the samples.
TABLE 1
2
–
Surface Area SBET (m /g) and Catalytic Activities of the W/Al2O3
TiO2(x) Catalysts in the Thiophene HDS Reaction
x = TiO2/(Al2O3 + TiO2)
0.0
0.1
0.5
0.7
0.95
1.0
SBET
R1
R2
205
2.53
0.52
196
4.81
0.94
188
5.85
1.10
162
7.20
1.17
68
7.62
0.56
10
29.7
0.30
Note. R1 = (mol/s/m2) ꢄ 1010; R2 = (mol/s/g) ꢄ 107.
RESULTS
Surface Area
The results from the surface area measurements are
shown in Table 1 for each catalyst composition. It may be
observed from this table that the surface area of the cata-
lysts presents only a slight drop between the samples with
x = 0 (Sg = 205 m2/g) and x = 0.5 (Sg = 188 m2/g), to later
descend in a more pronounced way between x = 0.7 (Sg =
162 m2/g) and x = 1.0 (Sg = 10 m2/g). All the catalysts
showed a monomodal pore size distribution curve, which
gave an indication of the absence of important segregation
of the different oxide supports in the catalysts formulations.
Catalytic Activity
Before the activity tests, the catalysts were sulfided under
a H2/H2S 15% (v/v) gas mixture for 4 h at 673 K. After that,
measurement of catalytic activity was conducted in a typical
microreactor apparatus. Hydrogen was bubbled through a
saturator filled with thiophene, maintained at 273 K. The
gas mixture was passed through a heated Pyrex reactor with
temperature regulation operating at atmospheric pressure
which contained 0.1 g of catalyst. Reactants and products
analysis were performed by on-line gas chromatography.
All the catalytic activities reported here are steady-state
values reached after the catalyst had been stabilized 12 h.
Activity measurements were obtained at 603 K. The exper-
iments were performed at low conversions, below 15% and
rates of reaction were calculated as R = (Q/W)X, where R
is the specific rate (moles of thiophene transformed per sec-
ond per gram of catalyst, Q is the molar flow of reactant per
second, W is the mass of catalyst in grams, and X is the frac-
tional thiophene conversion, defined as moles of thiophene
converted per mole of thiophene in the feed.
X-Ray Diffraction (XRD)
The diffractograms of the catalysts taken in the range
2ꢂ = 19–90 are shown in Fig. 1. No diffraction lines of any
tungsten phase were detected in the different samples and
the only peaks present in the diffractograms corresponded
–
to the ꢃ -Al2O3 phase (W/Al Ti(x = 0.0, 0.1) catalysts) or
–
the anatase phase (W/Al Ti(x = 0.95, 1.0) catalysts). In the
–
W/Al Ti(1.0) sample some traces of poorly crystallized ru-
tile were also observed. With respect to the samples with
x = 0.5 and x = 0.7, the diffractograms show a completely
–
FIG. 1. X-ray diffractograms of the W/Al Ti(x) catalysts.

–
W-BASED CATALYSTS SUPPORTED ON Al2O3 TiO2 MIXED OXIDES
111
amorphous system. The above observations indicate that
TABLE 2
at this measuring scale, the tungsten phases were well dis-
persed on the different supports forming either an amor-
phous phase or microcrystallites of less than 4.0 nm in size.
–
XPS Results for Al Ti(x) Supports
Binding
energies (eV)a
UV–Vis (DRS)
x
(Ti/Al)XPS
(Ti/Al)theoretical
Ti2p
Al2p
O1s
According to the literature (35), the bands corresponding
to octahedral (Oh) and tetrahedral (Td) tungsten geometry
appear in the spectral region of 250–280 nm. The maxima 0.5
0.0
0.1
0
0
—
74.1
73.7
74.2
74.2
74.3
—
530.1
530.1
530.1
529.9
529.8
529.9
0.050
0.413
0.840
3.647
—
0.055
0.500
1.166
9.500
—
457.8
458.3
458.3
458.6
458.7
0.7
0.95
1.0
of the bands have been reported in 250 nm for octahedral
tungsten, in 340 nm for WO3 which also contains tungsten
in octahedral coordination, and in 275 nm for tetrahedral
tungsten species in Al2(WO4)3.
a The C1s binding energy (BE = 284.5 eV) was taken as reference.
–
The electronic spectra of the W/Al Ti(x) catalysts, using
nation since normally the octahedral bands are much less
intense than tetrahedral ones.
the corresponding support as reference, are shown in Fig. 2.
–
As can be observed in Fig. 2, for the W/Al Ti(0.0) catalyst
As interesting fact that should be noted here is that hav-
ing the W6+ a d0 electronic configuration it should not show
any d–d transition bands. The bands previously observed in
the literature and in the present work most probablyare due
to charge transfers which induce a slightly reduced state in
the tungsten.
only an intense single band is present in the region of the
spectrum where the contributions from tungsten W6+ in
tetrahedral coordination appear. As the content of TiO2 in
the support is increased, the intensity of the bands decrease
and the maximum shifts to higher wavelength.
The maximum in the most intense absorption band of
–
It appears then from the above observations that the in-
corporation of TiO2 to the catalyst support leads to a higher
population of octahedral tungsten species and in the tita-
nium rich samples mainly tungsten in octahedral coordina-
tion seems to be present.
the W/Al Ti(0.1) catalyst appears in 306 nm and does not
correspond to either tetrahedral (ꢁ = 275 nm) or octahedral
(ꢁ = 340 nm) well-defined tungsten species. A shoulder in
this band is also present in the spectrum of this catalysts
in the region 350–390 nm which has been assigned to the
presence of octahedral tungsten in WO3 (30).
As the contents of TiO2 is increased further, only a single
band in the region of octahedral tungsten species (ꢁ = 350–
390 nm) was evident. The decrease in the relative intensity
of this band as the titanium content is increased is probably
due to the drop in specific area of the titanium rich catalysts,
which is accompanied by the corresponding drop in the
amount of tungsten per gram of catalyst.
X-Ray Photoelectron Spectroscopy
Table 2 shows the experimental (XPS) and theoretical
–
results of the Ti/Al atomic surface ratio for the Al Ti(x)
support samples. In Table 2, it can be observed that the
–
Ti/Al ratio increases almost linearly from the Al Ti(0.0)
–
to the Al Ti(0.5) support sample, to later increase more
slowly in the titanium rich samples (x = 0.7 and 0.95). This
behavior could be due to an agglomeration of TiO2 on the
surface leading therefore to a depart from linearity in the
XPS Ti signal. With respect to the binding energies (BE) of
the aluminum and titanium elements, also given in Table 2,
they are reported in the literature (36) at 74.10 and 458.5 eV,
respectively. These values are in good agreement with the
ones obtained in this work for the pure oxide samples (x = 0
and 1.0). In case of mixed oxide supports, a small shift in the
The change observed in the intensity of the tungsten
band (ꢁ = 250–400 nm) for the samples with x = 0 to x = 0.5,
which have approximately the same specific surface area,
also indicates a shift from tetrahedral to octahedral coordi-
–
BE of the Ti2p electrons from the Al Ti(0.1) (457.8 eV), to
–
the Al Ti(1.0) support (458.7 eV) was observed, this shift
toward higher BE indicates a change in the Ti4+ ions sur-
roundings.
FT-Raman Spectroscopy
The FT-Raman experiments are useful to inquire on the
structural characteristics of supports and catalysts.
Supports. The titanium oxide presentstwo main distinct
crystallographicstructures:anatase and rutile. Accordingto
–
FIG. 2. Diffuse reflectance electronic spectra of the W/Al Ti(x) cata-
lysts.

´ ´
RAMIREZ AND GUTIERREZ-ALEJANDRE
112
–
FIG. 3. (a) FT-Raman spectra of the pure alumina (x = 0) and pure titania (x = 1.0) supports. (b) FT-Raman spectra of Al Ti(x) mixed-oxide
supports.
literature reports (26, 37–39) Raman active bands for the 371 cmꢀ1. However, the precise identification of all the dif-
anatase appear at 144, 199, 399, 520, 643, and 798 cmꢀ1, and ferent modes of bond vibration of the tungsten species is
for rutile at 144, 448, and 611 cmꢀ1. The ꢃ -alumina support, obscured by the presence of TiO2 strong bands which ap-
on the contrary, has been reported to have no Raman active pear below 700 cmꢀ1
.
–
modes (5).
The Raman spectra of the W/Al Ti(x) catalysts taken at
–
The Raman spectra of the Al Ti(x) supports are pre- ambient conditions, after subtraction of the support spec-
sented in Figs. 3a and 3b. In Fig. 3a, where the spectra cor- tra, are shown in Fig. 4. According to the literature (26), the
responding to the pure oxides are presented, it can be ob-
served that the alumina support presents the phenomenon
of fluorescence which has been previously reported by oth-
ers (5). For the pure titania support Raman bands with max-
ima in 144, 196, 396, 520, 636, and 798 cmꢀ1, corresponding
to the different vibrational Raman active modes of anatase
are observed. Also, a small shoulder present at 441 cmꢀ1
,
and which maybe assigned to the presence ofsmallamounts
of rutile (37) is present in this sample. In Fig. 3b where the
Raman spectra ofthe mixed oxidesispresented it ispossible
to see that the fluorescence effect due to alumina diminishes
with the titania loading. It is evident that only at the high
titania loading the fluorescence effect disappears indicating
a full coverage of the alumina surface by the titania.
Catalysts. A further insight about the structuralchanges
of the different tungsten phases present in the catalysts,
as the titanium loading was varied, was obtained by FT-
Raman. The results from this technique point out the exis-
tence of various tungsten species in the catalyst oxidic pre-
cursors. According to the literature, characteristic bands
of tungsten oxidic phases appear at ꢃ973 cmꢀ1 and in
ꢃ853 cmꢀ1 for the symmetric and antisymmetric vibrational
==
modes of terminal W O bonds in two-dimensional tung-
sten oxide species in interaction with the support. The char-
acteristic vibrational modes of WO3 have been reported
at 805, 706, and 273 cmꢀ1 (16). The bands correspond-
ing to Al2(WO4)3 typically appear at 1046–1057, 386, and
–
FIG. 4. FT-Raman spectra of W/Al Ti(x) catalysts.

–
W-BASED CATALYSTS SUPPORTED ON Al2O3 TiO2 MIXED OXIDES
113
band with a maximum in 967 cmꢀ1, and which is present
in all the catalyst formulations, has been assigned to the
==
symmetric stretching vibration mode of W O bonding in
two-dimensional oxotungstate species in interaction with
the support, which may be in tetrahedral or octahedral co-
ordination depending on the surface tungsten loading and
also on the level of hydration. As the content of titanium
in the support is increased this band (ꢃ967 cmꢀ1) shifts
toward higher frequencies, indicating different degrees of
distortion. A similar shift has been reported, for catalysts
supported on alumina and on titania (26, 37), as due to dif-
ferences in the degree of hydration of the tungsten species
and also as a consequence of a greater lateral interaction
which affects the hydration and distortion of the oxotung-
sten species. In our samples, since the Raman spectra were
recorded under ambient conditions a certain degree of hy-
dration is expected.
–
FIG. 5. TPR thermograms of Al Ti(x) supports.
The spectra also show the appearance of a band in
803 cmꢀ1 which is more evident in the catalysts with high
titania content (x = 0.7, 0.95, and 1.0). This band has been
reported in previous works as due to crystalline WO3 (5).
However, since the 803 cmꢀ1 band in our spectra was rather
weak, one should be careful not to confuse it with an also
Tmax = 1073 K shifted to lower temperatures and appeared
–
at Tmax = 1008 K in the Al Ti(0.7) sample. In this sample
there was also evident the reduction of a high-temperature
species with Tmax > 1173 K. In the samples with the high-
weak band due to the presence of anatase at (ꢃ798 cmꢀ1
)
–
est titanium content (Al Ti(0.95, 1.0)), the detection of the
corresponding to an overtone of the anatase band at
396 cmꢀ1. This question will be addressed later on in the
discussion of results.
reduction peaks was more difficult due to the low tung-
sten loading per gram of catalyst which was a consequence
of their lower surface area. Nevertheless, a low reduction
temperature peak (Tmax ꢃ 837 K), and the beginning of the
high-reduction temperature peak (Tmax > 1173 K) were evi-
dent in both supports. The peak which reduction took place
at intermediate temperatures and which Tmax of reduction
shifted from 1073 to 1008 K seems to be due to the reduc-
tion of titanium species with different degrees of interaction
with the alumina.
The small band appearing in the region of 840–880 cmꢀ1
– –
has been assigned to the asymmetric mode of a W O W
linkage which has been mentioned to correspond to a sur-
face polytungsten oxide species on the surface of the sup-
port oxide (30).
Temperature-Programmed Reduction
The resultsfrom the titania pure sample indicate in agree-
ment with previous reports (17), that the low reduction
temperature peak (Tmax = 837 K) is due to the reduction
In order to inquire about the number, nature, and re-
ducibility of the tungsten species present in the catalyst
oxidic precursors, temperature-programmed reduction ex-
periments were performed with the supports and catalysts.
of anatase. The high reduction temperature peak (Tmax
>
1173 K) is most probably due to the reduction of the ru-
tile phase formed from anatase during the heating process
since the transition temperature for the anatase to rutile
transformation is about 1023 K.
The above results indicate that at low titanium loading a
strong interaction exists between the titanium oxide species
and the alumina matrix, whereas at higher titanium loading
(x > 0.5), some segregation of anatase occurs, causing the
appearance of low-temperature reduction peaks.
Supports. Figure 5 presents the TPR thermograms of
the different supports. In Fig. 5, one observes, in agree-
ment with previous reports (17), that no reduction takes
–
place in the pure alumina sample. Also, in the Al Ti(0.1)
sample, no clear peak of reduction was evident although a
small hydrogen consumption was detected at the high tem-
–
peratures. On the other hand, in the Al Ti(0.5) sample an
intense broad peak of reduction with a maximum at 1073 K
was clearly evident. This peak presented a shoulder around
–
Catalysts. The W/Al Ti(x) catalysts TPR profiles are
853 K and in the high-temperature range (T > 1173 K). It shown in Fig. 6. Clearly, the analysis of Fig. 6 reveals that
–
was also clear in this sample the beginning of the reduction the W/Al Ti(x) system is quite complex, however, taking
of another species with Tmax > 1273 K. into account the previous reports for the W/Al2O3 and the
–
For the Al Ti(0.7) support, the shoulder present in the W/TiO2 (17, 28) systems, and the results outlined above for
previous sample at about 853 K developed into a clear peak the supports used here, the interpretation of the TPR pro-
–
–
and the peak which in the Al Ti(0.5) sample appeared at files of the W/Al Ti(x) system appears possible.

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RAMIREZ AND GUTIERREZ-ALEJANDRE
114
surface areas of the rich titanium samples (x = 0.95 and 1.0)
are quite low compared to the other catalyst samples with
less titanium. Also, in the study of TPR profiles of different
catalysts it is not advisable to change the size of the sample
since, as has been demonstrated before (40), the TPR re-
sults from samples of different size may not be comparable.
–
However, in this sample (W/Al Ti(1.0)), a small hydrogen
consumption due to the reduction of the anatase present
in the support was detected at about 893 K. Another even
smaller hydrogen consumption peak, most probably due to
the reduction of W6+ species appeared at about 1053 K and
the beginning of the high reduction temperature peak, con-
sistent with the reduction of rutile started to appear above
1173 K.
The above results confirm that the incorporation of tita-
nium oxide to the catalyst support induces a change in the
type of tungsten oxide species which is reflected in an eas-
ier reduction than in the pure alumina supported catalyst.
It appears that as the titanium loading is increased, a higher
population of easily reducible octahedral tungsten species
is present in the catalysts.
High-Resolution Electron Microscopy (HREM)
After sulfidation, the catalysts were characterized by
HREM. Electron microscopy is a helpful technique for
obtaining evidence of the changes in the morphology, dis-
persion, and homogeneity of the supported metal sulfides.
However, in the interpretation of the results, one must be
aware of the limitations of the technique. Clearly, one of the
–
FIG. 6. TPR thermograms of W/Al Ti(x) catalysts.
It has been reported previously using tungsten loading limits of the limits of the technique comes from the difficulty
similar to that used in this study that the reduction of the in observing very small WS2 particles of less than 1 nm in
tungsten oxide species supported on alumina presents only length. Also, duringthe observations, onlythe WS2 platelets
one high-temperature peak due to the reduction of tetra- which have the c axis perpendicular to the incident beam
hedral tungsten species in strong interaction with the alu- will be observed. In our case, the fact that the same tungsten
–
mina support. The TPR profile of the W/Al Ti(0) catalyst surface concentration was used in the preparation of all the
is clearly in agreement with those previous reports. As the catalysts diminished the effects induced by the changes in
content of titania is increased in the catalysts, the Tmax of texture of the different catalyst supports. Nevertheless, it
reduction of this peak shifts to lower temperatures and may be possible that some of the changes observed in the
the appearance of another reduction peak, which is only tungsten sulfide crystallites may not only be due to changes
–
a small shoulder in the W/Al Ti(0.1) sample, but that de- in the chemical nature of the support but also to changes in
velops into a clear peak with Tmax = 1185 and 1173 K in the texture of the support. With these precautions in mind
–
–
the W/Al Ti(0.5) and W/Al Ti(0.7) catalyst samples is ob- we can then go through the analysis of the micrographs of
served.
In the W/Al Ti(0.95) catalyst sample, only a broad re-
the sulfided catalysts.
The micrographs of the catalysts (Figs. 7a and 7b) show
–
duction peak with Tmax = 1154 K is clearly evident. In this the typical lattice fringes representing the basal plane of
˚
sample it is observed also the beginning of the reduction of WS2 structures with 6.18 A interplanar distances (8, 41).
a high reduction temperature species which, from the re- These structures appear well dispersed in all the samples;
sults obtained for the pure supports, seems to be due to the however, changes in the stacking, length, and population
beginning of the reduction of rutile.
were observed depending on the amount of titania in the
The results from the W/Al Ti(1.0) sample did not al- catalyst support.
–
lowed a clear interpretation due to the low intensity of the
Measurement of length and number of layers of 300–400
signals which were a result of the relatively low amount of WS2 crystallites allowed us to estimate the overall changes
tungsten in thissample. It must be remembered that allcata- in the relative dispersion in the different catalyst samples.
lysts had a tungsten content of 2.8 W atoms/nm2 and that the Figures 8 and 9 show the percentage distribution plots for

–
–
FIG. 7. (a) Typical micrograph of the W/Al Ti(0) catalyst showing WS2 crystallites. (b) Typical micrograph of the W/Al Ti(0.95) catalyst showing
WS2 crystallites.

´ ´
RAMIREZ AND GUTIERREZ-ALEJANDRE
116
–
FIG. 8. WS2 crystallite size distribution plot in the W/Al Ti(x) sulfided catalysts.
the length (L) and number of layers (N), respectively, of tanium loading. However, since partially sulfided tungsten
–
the WS2 structures found in the W/Al Ti(x) catalyst series. is not clearly observed by this technique and it is well known
–
In Fig. 8 it is possible to see that for the W/Al Ti(0.0) that the tungsten oxide precursor is difficult to sulfide when
catalyst, the majority of the WS2 crystallites have a length supported on alumina, only about 40% at the sulfiding
˚
between 20 and 30 A, whereas for the catalysts supported conditions of this work, the above results on the growth
˚
in the mixed oxides the main length is between 30 and 40 A. of the WS2 crystallites with titania loading may be due to ei-
–
Regarding the W/Al Ti(1.0) catalyst, the maximum num- ther a better sulfidation of the catalyst or a lateral growth of
˚
ber of crystallites lies in 50–60 A length. It appears then the crystallites resulting from the migration of the tungsten-
that the length of the WS2 crystallites increases with the ti- sulfided specieswhich becomesmore important asthe inter-
action with the support is diminished by the incorporation
of titania into the catalyst support. As for the number of
layers observed in the WS2 crystallites, the results in Fig. 9
indicate that the presence of titania in the support induces
a low stacking of the WS2 crystallites. It should be consid-
ered that it may be possible that under reaction conditions
the sulfided phase can sinter or be further sulfided. There-
fore, the size and stacking of the WS2 crystallites measured
before reaction should be taken with due care. In our exper-
iments we expect, due to the conditions of sulfidation, an
incomplete sulfidation of the tungsten phase, especially in
the catalysts supported on alumina-rich supports. Because
of this, an attempt to correlate catalytic activity with the
number of edge sites present on the catalyst was not made.
Such an exercise is more suitable for Mo-based catalysts or
for Ni-promoted tungsten catalysts where higher degrees
of sulfidation are observed, as one of us has reported pre-
viously (42, 43).
Catalytic Activity
The catalytic activity of the sulfided form of the
–
FIG. 9. Distribution plot of the number of layers in WS2 crystallites.
W/Al Ti(x) catalysts was tested in the hydrodesulfurization

–
W-BASED CATALYSTS SUPPORTED ON Al2O3 TiO2 MIXED OXIDES
117
of thiophene at atmospheric pressure. However, since the the support practically consists of titanium oxide (anatase)
surface area of the different catalysts formulations varies since at this titanium concentration practically no fluores-
between a wide range of values, comparison of the catalytic cence originated by the alumina appears and only the char-
activities will be best made in terms of the reaction rates acteristic anatase Raman bands at 144, 196, 396, 520, 636,
expressed as thiophene moles transformed per second and and 798 cmꢀ1 are clearly evident. This result is consistent
per square meter. Table 1 shows the activity of the cata- with the XRD observations. The quantitative determina-
lysts at 603 K as the content of titania is varied. It can be tion of the relative amounts of Ti and Al on the surface is
observed in Table 1 that the maximum activity per gram beyond the scope of the present work.
–
of catalysts is observed for the W/Al Ti(0.7) catalyst. How-
In line with the above findings the distribution of tita-
ever, this result can be easily explained since all the catalysts nium on the surface of the supports measured by XPS also
were prepared with the same tungsten surface concentra- showed that the Ti/Al atomic ratio increased linearly up
tion (2.8 W atoms/nm2) and the specific surface area of the to titania contents of x = 0.5 and then dropped slightly at
supports reached a maximum at x = 0.7. However, if one x = 0.7 and more at x = 0.95. It appears that after the titania
looks at the catalytic activities expressed per square me- loading of x = 0.5, which also corresponds to the theoretical
ter, it is observed that the activity of the catalysts increases amount for the formation of aluminum titanate (Al2TiO5),
slightly and linearly with titanium loading from the catalyst some segregation of titania on the surface of the support
supported on pure alumina (x = 0.0) to the catalysts with takes place. The changes observed in the binding energy
x = 0.95. The catalyst supported on pure titania, however, of titanium which changed from 458.7 eV in the pure tita-
–
shows a sharp increase in catalytic activity. These differ- nia sample to 457.8 eV in the Al Ti(0.1) support, although
– –
ences in activity could be due to changes induced by the small, indicate the possibility of the formation of Ti O Al
support in the dispersion of the tungsten-sulfided phase, bridges, since the influence of the Al3+ ions would cause
to the level of sulfidation of the tungsten phases or to an a shift to lower binding energies in the titanium electrons.
–
effect caused during reaction by the presence of a partly re- For the Al Ti(0.95) rich titanium sample, the binding en-
duced and/or sulfided titanium phase which could act as a ergy is the same as in the pure titania sample, indicating
promoter. Furthermore, these changes in catalytic activity that important amounts of TiO2 are present on the surface.
must be also related to the structural changes of the tung-
sten phases, induced by the incorporation of titanium to the Catalysts
catalysts, in the oxided state catalyst precursors.
The tungsten addition to the supports produces only a
small decrease in surface area indicating a uniform distri-
bution of the tungsten oxide on the surface of the supports.
DISCUSSION
Since the X-ray diffraction patterns of the different cata-
lysts formulations did not show any diffraction lines differ-
Supports
–
–
One of the interesting features of using the Al Ti(x) ent from those due to the Al Ti(x) supports, the tungsten
mixed oxides as catalysts supports is that higher surface oxidic phase at this scale of observation seems to be well
areas than those obtained in the pure oxides are obtained, dispersed on the surface of the different supports.
even at high titanium loading. The results from the pore size
With respect to the changes induced in the structure of
distribution curves obtained for all the supports showed a the oxidic tungsten phases by the incorporation of titania
homogeneous unimodal distribution of pore sizes indicat- in the support, the shifts observed in the DRS absorption
ing mainly the existence of a true microcomposite structure bands, once the spectra of the support was subtracted from
rather than a mixture of the individual materials.
each sample (Fig. 2), indicate, at the level of tungsten load-
The X-ray diffraction patterns of the supports indicate ing used in this work, the preference of the W6+ ions for
that, at this scale of observation, in the support samples a tetrahedral coordination when supported on alumina.
with x = 0 to x = 0.7 the titanium is well dispersed. It is However, as the titanium loading increases, the shift in the
only in the titanium-rich samples (x = 0.95, 1.0) that bigger absorption edge toward higher wavelengths indicates a re-
anatase crystals are detected. Also, a slight segregation of duction in the tetrahedral species and an increase in the
poorly crystallized titania in the form of rutile was detected octahedral ones which is more notorious at the high titania
in the samples with x = 0.95 and x = 1.0. Another interest- loading. It appears then that in the pure alumina support
ing feature present in the support diffractograms is that the only tetrahedral species are present, at intermediate tita-
incorporation of titania appears to reduce the crystallinity nia contents (x = 0.1–0.5) both tetrahedral and octahedral
of the ꢃ -alumina phase present with respect to that in the or highly distorted species are present, and at high titania
pure alumina support. In fact the supports with x = 0.5 and contents mostly octahedral tungsten species are present.
x = 0.7 show a completely amorphous pattern.
The decrease in the intensity of the DRS absorption bands
The evolution of the FT-Raman spectra of the supports with the titanium loading is also in line with the fact that
with titanium loading show that at x = 0.95 the surface of the intensity of the octahedral bands is smaller than for

´ ´
RAMIREZ AND GUTIERREZ-ALEJANDRE
118
tetrahedral ones. Furthermore, the absence of octahedral
TABLE 3
–
bands in the W/Al Ti(0.0) catalysts is in line with the TPR
Total Hydrogen Consumption (HC) and Degree of Reducibilitya
results of this sample in which, also in agreement with the
literature (17, 28), only the existence of a single reduc-
tion peak at 1343 K which has been assigned to tungsten
species in tetrahedral coordination in strong interaction
with the alumina support is observed (Fig. 6). When a small
amount of titanium is incorporated to the alumina support
–
for W/Al Ti(x) Catalysts
x
0.0 0.10 0.50 0.70 0.95
1.00
HC (mmol of H2/m2) ꢄ 104 9.93 17.01 19.45 17.34 30.47 56.30
Reducibility (% )
33.2 60.0 68.5 66.6 94.8 164.0
a Calculated assuming that only W is reduced (W6+ → W0).
–
(W/Al Ti(0.1) sample), the observed shift in the Tmax of re-
duction from 1343 to 1236 K indicates the presence of tung-
sten specieswith lessinteraction with the support than those
present in the pure alumina supported sample. This effect is
this reason that the degree of reducibility estimated under
the above assumption leads to values higher than 100% in
the pure titania supported sample. Although it is not pos-
sible to separate in an accurate way the contributions of
tungsten and titanium reduction to the total hydrogen con-
sumption, it is clear that the incorporation of small amounts
of titanium lead to a sharp increase in the consumption of
hydrogen and therefore in the degree of reducibility. Since
this trend does not continue from x = 0.1 to x = 0.7 one
may reasonable assume that great part of the increase in
the hydrogen consumption with titanium loading in these
samples (x = 0.1, 0.5, 0.7) is due to the increased reduction
of the tungsten species rather than to titanium reduction.
It is interesting to note that the trend of the degree of re-
ducibility with the titanium loading follows the same shape
asthe catalytic intrinsic activityversustitanium content (see
Fig. 10 and Table 1). Clearly, the variations in the degree of
reducibility of the catalysts and their catalytic activity will
be related to the type of tungsten species present in the
oxided catalyst precursors.
–
more clearly observed in the W/Al Ti(x = 0.5, 0.7) samples
where a clear reduction peak develops at Tmax = 1185 K
–
in the W/Al Ti(0.5) sample and at Tmax = 1173 K in the
–
W/Al Ti(0.7) catalyst. In these two samples, in addition to
the high-temperature reduction peak, we also observe the
–
appearance of a peak with Tmax = 1073 K (W/Al Ti(0.5)
–
sample) and with Tmax = 1006 K (W/Al Ti(0.7) sample),
which are due to the reduction of the titania in the support
(see Fig. 6), and also, possibly, to the reduction of some
tungsten species in octahedral coordination. The reduction
of W6+ species has been reported (17) to occur in tungsten
catalysts supported on pure titania, in two stages, the first
W
6+ → W4+ at Tmax = 1017 K and the second, W4+ → W0,
at Tmax = 1248 K. The appearance of this second reduction
stage is due to the stabilization of the W4+ oxidation state
by the titania.
As mentioned previously, the titanium rich samples
–
(W/Al Ti(x = 0.95, 1.0)) due to their the small surface area
and therefore low amount of tungsten per gram of catalyst,
do not lend themselves to a clear interpretation. Never-
theless, in these samples in which the support surface is
practically titania, the presence of tetrahedral tungsten, if
it existed, would be only minor and could be confused with
the start of the reduction of the rutile species formed during
the TPR processes.
The results from the Raman analysis of the oxidic catalyst
samples confirm the existence of various tungsten species in
the catalyst precursors. The intense 957–977 cmꢀ1 band ap-
–
pearing in all the W/Al Ti(x) catalysts (Fig. 4) has been
–
The reduction peak that in the W/Al Ti(0.95) sample
which appears at Tmax = 1154 K has been assigned to tung-
sten species, possibly WO3. Salvati et al. (4) have also re-
ported the reduction of tungsten species at Tmax = 1150 K
in NiW/Al2O3 catalysts; however, they do not state clearly
the nature of this tungsten species. The broad small reduc-
–
tion peak observed in the W/Al Ti(1.0) sample cannot be
clearly interpreted.
According to the above results, the increase of titanium
in the catalyst support produces a change in the distribution
of tungsten species, leading to species with less interaction
with the support which are more easily reduced. This last
statement is confirmed by the total hydrogen consumption
results and by the degree of reducibility estimated assum-
ing that only the tungsten is reduced during the process
(see Table 3). This assumption is clearly not accurate since
–
FIG. 10. Degree of reducibility of the W/Al Ti(x) oxidic catalyst pre-
part of the titania in the support is also reduced. It is for cursors.

–
W-BASED CATALYSTS SUPPORTED ON Al2O3 TiO2 MIXED OXIDES
119
assigned to the symmetric stretching vibration mode of
==
W
O bonds in two-dimensional tungsten species in inter-
action with the support. The shift observed in this band
when the titanium loading is increased may be due to dif-
ferent degrees of hydration which will also be related to the
level of lateral interaction and distortion of the oxotungsten
species. Apparently, the interaction of the water molecules
with the oxidictungsten surface speciesincreasesthe degree
of disorder or changes the symmetry of the bonds affect-
ing, therefore, the symmetric stretching vibrational mode
==
in the W O terminal bonds. A similar effect has been re-
ported for alumina-supported tungsten catalysts when the
tungsten loading is increased (44). In fact, the degree of
hydration of the tungsten oxidic species and their tungsten
loading are related since an increase in the concentration of
tungsten species on the surface promotes increased lateral
interactions which impede the hydration of the tungsten
species. The results from our laboratory indicate that the
coverage used in this work are below the monolayer and,
FIG. 11. FT-Raman spectra of tungsten catalysts supported on com-
mercial TiO2 (DT51 Rhone Poulanc, SBET = 90 m2/g), at different tungsten
loading. (a) 1.4, (b) 2.8, and (c) 4.2 W atoms per square nanometer.
therefore, according to the literature (32, 33), on pure alu- supported catalysts, since previous works (32) have stated
mina, we should expect in the dehydrated state only the the monolayer at the point where the appearance of WO3
presence of tetrahedral tungsten species, while under am- is clearly detected by Raman spectroscopy.
bient conditions, where some hydration occurs, a shift in
The Raman spectra of the W/TiO2 catalysts with differ-
the Raman peaks would be observed due to the degree of ent tungsten loading (Fig. 11) shows that the 803 cmꢀ1 band
hydration which also changes the degree of tungsten coor- only increases significantly when the tungsten loading ex-
dination. Indeed, we observe on the alumina support under ceeds 2.8 W atoms/nm2 indicating that the band observed
TPR conditions, which can be considered asdehydrated, the at 1.4 and 2.8 W atoms/nm2 loadings was mainly due to
presence of only one type of tetrahedral tungsten species, anatase. This result also indicates that for this support, the
while in the Raman characterization which was taken un- monolayer coverage for W/TiO2 catalysts is between 2.8
der ambient conditions, a shift in the Raman peak corre- and 3.5 W atoms/nm2, which in our case are equivalent to
==
sponding to W O vibrations of tungsten surface species in 8.84 and 10.8 wt% WO3. This result is fair agreement with
interaction with the support is observed for the different the one reported by Vuurman et al. in Ref. (30) which cor-
samples.
responds to 3.7 W atoms/nm2. A value of 4.2 W atoms per
The existence of a highly dispersed crystalline WO3 phase square nanometer has also been reported recently (33). Ac-
–
in some of the samples seems also possible due to the
presence of the weak band in 803 cmꢀ1 characteristic of
this species. However, as was mentioned before, the titania
cording to these results, in our W/Al Ti(x) catalysts with a
tungsten loading of 2.8 W atoms/nm2, the band appearing
at 803 cmꢀ1, and which increases with the titania loading,
may be assigned to an anatase contribution rather than to
the presence of WO3 crystallites.
(anatase phase) presents also a weak band in ꢃ798 cmꢀ1
,
and therefore, the small increase observed in the 803 cmꢀ1
band as the titanium content was raised may be also the re-
sult of an increased contribution of the ꢃ798 cmꢀ1 anatase
band.
Regarding the behavior of the band appearing at about
ꢃ967 cmꢀ1, it is observed that in the W(y)/TiO2 catalysts
this band shifts from 971 to 985 cmꢀ1 when going from the
low (y = 1.4) to the high (y = 4.2) tungsten loading. This
result indicates that the shift in this band, observed in the
In order to resolve the question if crystalline WO3 was
formed in our catalysts, and also if the shift observed in the
ꢃ967 cmꢀ1 band was due to an increased lateral interac-
tion of the tungsten species, Raman analysis was made on
an additional series of titania supported tungsten catalysts
(W(y)/TiO2, where y = 1.4, 2.8, 3.5, and 4.2 W atoms/nm2)
prepared using a commercial high-surface TiO2 support
(TiO2 DT51 Rhone Poulenc, Sg = 90 m2/g). The reason for
using a TiO2 support with higher surface area is to amplify
the tungsten signal per gram of catalyst. These additional
experiments will also allow us to determine at which load-
ing the tungsten oxide monolayer is exceeded in WO3/TiO2-
–
W/Al Ti(x) catalysts when the titania content was varied
from pure alumina to pure titania may well be due to an
increased lateral interaction of the tungsten species. This
increased lateral interaction will also be promoted by the
diminished interaction of the tungsten oxidic phases with
the support as the titanium loading is increased.
In line with the above results, the HREM observations in
the sulfided samples indicate that the WS2 crystallites in the
W/Al Ti(1.0) sample tend to growth laterally and to wrap
–
the TiO2 particles in contrast to the randomly distributed

´ ´
RAMIREZ AND GUTIERREZ-ALEJANDRE
120
–
and smaller crystallites observed in the W/Al Ti(0.0)
catalyst.
The above findings are also in fair agreement with the
idea proposed previously (33) that the net pH at ZPC of
the support influences the structure of the hydrated surface
species. In our case, the ZPC for alumina and titania are
8.5–9 and 6.0, respectively. Therefore, preferentially tetra-
hedral tungsten species will be expected to be present on
pure alumina and octahedral tungsten species will be pre-
dominant on pure titania. The ZPC of the mixed oxides
would be in an intermediate value and therefore a less
clear definition in the coordination of the tungsten surface
species would be expected. In line with this, our results from
DRS and TPR clearly indicate a shift from tetrahedral to
octahedral tungsten species when the support changes from
pure alumina to pure titania. Furthermore, the appearance
ofDRSbandsin the range (ꢁ = 290–330nm) for the samples
with low titania contents (x = 0.1, 0.5, 0.7) at wavelengths
which do not correspond either to a tetrahedral (ꢃ275 nm)
coordination nor to an octahedral one, seems to point to
the possible existence of pentahedral coordinated tungsten
species. To this respect it has been proposed previously (27)
in a XANES study of tungsten catalysts supported on tita-
nia, that the supported tungsten oxide consists of branched
chains of pentahedral WO5 units and terminal tetrahedral
– –
WO4 units anchored onto the titania surface via W O Ti
bonds. The length of these WO5 units chains and thus the
WO4/WO5 ratio being determined by the degree of tung-
sten oxide coverage and in our case also by the level of
interaction of the tungsten species with the support. It was
proposed by the same authors (27) that on adsorption of
water, the oxygen coordination increases to six for all the
surface tungsten sites. Furthermore, Kim et al. (32) has also
suggested a different degree of polymerization of the tung-
sten oxide species on different supports.
FIG. 12. Schematic model of the possible oxotungsten surface species
–
present in the different W/Al Ti(x) catalyst formulations.
affect the number of tungsten coordinatively unsaturated
sitespresent in the sulfided catalystssince the levelofthe so-
called spill over of hydrogen (45), which has been proposed
to have a bearing in the HDS reaction mechanism, will be
affected by the presence of a sulfided titanium phase. The
significant difference in activity between the pure titania-
supported catalysts and all the other samples may well be
related to thisfact. Resultsfrom our laboratoryindicate that
the presence of the sulfided surface layer of titania plays a
role in the reaction mechanism and induces higher activity,
and that some vacancies created in this layer are capable of
adsorbing NO (46). It should be interesting to undertake
a detailed bulk and surface characterization study of rich
The IR spectra of the oxidic samples also shows a shift
==
in the band position of the W O terminal bonds toward
higher wavelengths with the titania loading. Thus, this
band which in the alumina supported catalysts appears at
963 cmꢀ1, in those supported on titania appears at 993 cmꢀ1
in agreement with the Raman results.
The results from this study allow us to propose a model
for the different surface tungsten oxidic species when the
formulation of the support is changed from pure alumina
to pure titania going through a series of mixed oxides.
A schematic representation of this model is presented in
Fig. 12.
Clearly, apart from the influence that the different tung-
sten precursor species will have on the catalytic activity,
this will be influenced by the nature of the support since
in this case the alumina support does not undergo reduc-
tion under reaction or sulfidation conditions, whereas the
titania support does. Under reaction conditions these dif-
ferences in the state of the supports will be expected to
–
titania Al2O3 TiO2 samples to inquire more on the causes
for the drastic changes in hydrodesulfurization activity ob-
served in the rich titanium side of the support formulation.
From all the above results one may say that in the series of
catalysts studied here the activity trend with titania loading
is the result of the contribution of several effects which are
interrelated. First, the change in the nature of the mixed ox-
–
ide Al2O3 TiO2 support with the titania loading promotes a
change in the oxidic tungsten precursor species from tetra-
hedral to octahedral coordination. This change seems to be

–
W-BASED CATALYSTS SUPPORTED ON Al2O3 TiO2 MIXED OXIDES
121
accompanied by the formation of oxopolytungstates. Sec-
REFERENCES
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results in a change in the level of interaction between the
support and the tungsten phases in both the oxidic and the
sulfided state, which leads to more reducible and quite pos-
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rich catalysts seem to lead to well dispersed oxidic tung-
sten species which promote slightly large WS2 crystallites
than for alumina but with a low level of stacking.
Finally, it is also quite possible that the drastic increase
in activity observed for the pure titania supported catalyst
may be also due to the role that the sulfided titania surface
plays in the reaction mechanism in which quite possibly acts
as a promoter.
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(1981).
–
1. In the Al Ti(x) supports, it is only at the high titania
loading above x = 0.7 that anatase segregates forming crys-
tallites detectable by XRD.
2. The support composition and physicochemical proper-
ties, determine the type of oxotungsten species present on
the surface, leading to a gradual change from tetrahedral
species present on alumina to octahedral species present on
titania.
3. The degree of reducibility of the oxotungsten species
seems to be a consequence of the level of interaction with
the support and also, possibly, to the role of the reduced or
sulfided layer of titanium present on the surface.
4. The presence of titania in the support seems to lead to
oxotungstate species with greater lateral interaction which
affect the size and the level of stacking of the WS2 crystal-
lites in the rich titania catalysts.
5. The observed changes in catalytic activity seem to be
the result of changes in the interaction between the surface
tungsten species and the support which leads to different
surface species on the oxidic precursors and these in turn
lead to a different morphology and quite possibly, different
levels of sulfidation in the final sulfided tungsten species.
It is quite possible that the enhanced activity of the tita-
nia supported catalyst is due to the role that the sulfided
layer of titania plays in the mechanism of the HDS reac-
tion, since no drastic change in the nature of the tungsten
surface species either in the oxidic or sulfided state can ac-
count for the observed sharp increase in activity present in
the pure titania supported sample.
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ACKNOWLEDGMENTS
34. Arnoldy, P., Jonge, J. C., and Moulijn, J. A., J. Phys. Chem. 89, 4517
(1985).
The financial support from PEMEX-Refinacio´n and DGAPA-UNAM
is gratefully acknowledged.
35. Scheffer, B., Heijeinga, J. J., and Moulijn, J. A., J. Phys. Chem. 91, 4752
(1987).

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RAMIREZ AND GUTIERREZ-ALEJANDRE
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