Sulfidation and thiophene hydrodesulfurization activity of nickel tungsten sulfide model catalysts, prepared without and with chelating agents

Article abstract of DOI:10.1006/jcat.2000.3013

The production of low-sulfur diesel fuels is presently an important topic in oil refineries due to more stringent environmental legislation. HDS of refinery streams is performed with catalysts consisting of Mo promoted with Ni or Co, or W promoted with Ni, all in the sulfided state. The sulfidation behavior of NiW/SiO₂ catalysts prepared in the presence and absence of various chelating agents (i.e., 1,2-cyclohexanediamine-N,N,N',N'-tetraacetic acid (CyDTA), ethylenediaminetetraacetic acid, or nitrilotriacetic acid) was studied. The rate of sulfidation with the activity in thiophene HDS was correlated. W/SiO₂ was converted to WS₂ at 150°-350°C. This phase indicated significant activity for the HDS of thiophene at 400°C. Complexing Ni with CyDTA slowed down the sulfidation of Ni where WS₂ had already formed. Nickel sulfidation started when the CyDTA complex decomposed. As a result, nickel atoms released by the chelating agent can move to the reactive edges of the WS₂ to form a finely dispersed sulfide. This provided the highest activity for thiophene HDS.

Full text of DOI:10.1006/jcat.2000.3013

Journal of Catalysis 196, 180–189 (2000)
doi:10.1006/jcat.2000.3013, available online at http://www.idealibrary.com on
Sulfidation and Thiophene Hydrodesulfurization Activity of Nickel
Tungsten Sulfide Model Catalysts, Prepared without
and with Chelating Agents
1
G. Kishan, L. Coulier, V. H. J. de Beer, J. A. R. van Veen, and J. W. Niemantsverdriet
Schuit Institute of Catalysis, Eindhoven University of Technology, 5600 MB Eindhoven, The Netherlands
Received June 2, 2000; revised July 18, 2000; accepted July 18, 2000
While sulfidation of supported oxidic Mo, CoMo, and NiMo
Silica-supported NiWS catalysts with a high activity for catalysts has received considerable attention (2–12), much
thiophene hydrodesulfurization are obtained when chelating less is known about the sulfidation of W-based catalysts.
0
0
agents such as 1,2-cyclohexanediamine-N,N,N ,N -tetraacetic acid
CyDTA), ethylenediaminetetraacetic acid, or nitrilotriacetic acid
As NiW-sulfide catalysts are becoming increasingly impor-
tant for deep HDS, more detailed knowledge about their
preparation chemistry is desirable.
W and Mo exhibit clearly different behavior with respect
to sulfidation. Scheffer et al. (3) and Breysse et al. (12)
showed that NiW/Al2O3 catalysts are much more difficult to
sulfide than their Mo-based counterparts. Reinhoudt et al.
(
are added in the impregnation stage. X-ray photoelectron spec-
troscopy has been used to follow the state of Ni and W during the
temperature programmed sulfidation of Ni–W model catalysts pre-
pared with and without chelating agents on planar SiO2 films on
silicon substrates. Fully sulfided catalysts have been tested in thio-
phene hydrodesulfurization. The activity increases with increasing
Ni content and reaches a plateau at a Ni : W atomic ratio of 0.66.
(
13) reported that complete sulfidation of W resulted in
In NiW catalysts prepared without additives the sulfidation of Ni the highest thiophene HDS activity (tested in gas phase),
precedes that of W. However, Ni sulfide formed at low temperatures while the highest dibenzothiophene HDS activity (tested
changes its structure at high temperatures where WS2 is present, in trickle flow) was found when the sulfidation of W was
as indicated by the Ni XPS binding energy, which we tentatively
attribute to redispersion of sulfidic Ni over WS2. Chelating agents
stabilize Ni against sulfide formation at low temperature, the effect
being strongest when CyDTA is applied. CyDTA retards the sulfi-
dation of Ni to temperatures where all W has already been sulfided.
monly referred to as CoMoS and NiMoS phases (2, 4, 5, 14–
This complete reversal in the order in which the two elements con-
incomplete.
For CoMo or NiMo supported on Al2O3 or SiO2 a fairly
complete picture of the active phases exists. These consist
of Co or Ni atoms on the edges of MoS2 slabs and are com-
2
0). An extended X-ray absorption fine structure (EXAFS)
vert to sulfides is seen as the key step in preparing highly active
NiWS HDS catalysts.
study by Louwers and Prins (21) on carbon-supported NiW
catalysts provides evidence that a NiWS phase analogous
to the CoMoS phase exists.
�c 2000 Academic Press
Key Words: model catalyst; sulfidation; XPS; nickel; tungsten;
silica; hydrodesulfurization; chelating ligands; thiophene.
Addition of chelating agents such as nitrilotriacetic acid
(
NTA), ethylenediaminetetraacetic acid (EDTA), and 1,2-
0
0
INTRODUCTION
cyclohexanediamine-N,N,N ,N -tetraacetic acid (CyDTA)
has a beneficial effect on the catalytic activity of CoMo,
The production of low-sulfur diesel fuels is currently an NiMo, and NiW catalysts, irrespective of the support (14–
important topic in oil refineries due to more stringent en- 18, 22, 23). Work in the laboratories of Prins (15–17) and
vironmental legislation (1). Hydrodesulfurization (HDS) our own group (18–20) has identified retardation of Ni and
of refinery streams is carried out with catalysts consisting Co sulfide formation as the key step in enabling the forma-
of Mo promoted with Ni or Co, or W promoted with Ni, tion of the active NiMoS and CoMoS phases (18–20). Re-
all in the sulfided state (2–5). In the preparation, sulfida- cently Shimizu et al. (22) also showed that chelating agents
tion of the oxidic precursors in a mixture of H2S/H2 or in improve the activity in HDS of benzothiophene and in hy-
the sulfur-containing hydrocarbon feed is an essential step. drogenation of o-xylene over NiW/Al O catalysts.
2
3
Model catalysts consisting of a flat conducting substrate
covered with a thin SiO2 or Al2O3 layer on top of which
the active phase is deposited have been proven very use-
ful in catalyst preparation studies (24). The advantage of
1
To whom correspondence should be addressed at P.O. Box 513,
600 MB Eindhoven, The Netherlands. Fax: 0031 40 245 5054. E-mail:
J.W.Niemantsverdriet@tue.nl.
5
180
0021-9517/00 $35.00
Copyright �c 2000 by Academic Press
All rights of reproduction in any form reserved.

SULFIDATION AND THIOPHENE HYDRODESULFURIZATION ACTIVITY OF NiWS CATALYSTS
181
�
these model systems, having a conducting substrate, is that CyDTA) were heated at a rate of 2 C/min (other catalysts
�
sample charging is largely eliminated, resulting in much bet- at 5 C/min) to the desired temperature and kept there for
ter resolution of X-ray photoelectron spectra (XPS) with 30 min. After sulfidation, the reactor was cooled to room
respect to high-surface-area catalysts. To keep the impreg- temperature under helium gas flow and then brought to
nation chemistry identical to that of industrial catalysts, we a glove-box, where the model catalyst was mounted in a
prepare our model catalysts by spin coating from aqueous transfer vessel for transport to XPS under N2 atmosphere.
solutions on the flat, thin SiO2 or Al2O3 layers, which mim-
XPS spectra were obtained by using a VG Escalab
ics the conventional pore- volume impregnation used for 200MK spectrometer equipped with a dual Al/Mg K�
high-surface-area catalysts (25, 26). These models of hy- X-ray source and a hemispherical analyzer with a five-
drotreating catalysts exhibit “realistic” activities and prod- channel detector. Measurements were recorded with con-
uct distributions in thiophene HDS (18–20).
stant pass energy of 20 eV. Binding energies were corrected
In this article, we investigate the sulfidation behavior for charging by using Si 2p peak of SiO2 at 103.3 eV as a
of NiW/SiO2 catalysts prepared without and with various reference. Binding energy values thus determined are esti-
chelating agents and correlate the rate of sulfidation with mated to possess an accuracy of � 0.2 eV.
the activity in thiophene hydrodesulfurization. A prelimi-
Model catalysts were tested in thiophene HDS (gas
nary report on the effect of adding CyDTA in the prepa- phase) to determine the activity and product distribution.
ration of NiW/SiO2 model catalysts has been published Measurements were carried out in a microflow reactor un-
�
elsewhere (27).
der standard conditions (1.5 bar, 400 C, 4% thiophene/H2).
2
About 5 cm of model catalyst was placed inside a glass re-
actor. First, the model catalyst was sulfided at 400 C for
0 min as described above. Then a mixture of 4%
�
EXPERIMENTAL
3
thiophene/H2 was passed through the reactor at a rate of 50
ml/min and at 400 C. After 5 min the reactor was closed and
Silica model supports were prepared by oxidizing a Si
100) wafer in air at 750 C for 24 h. The thickness of the
�
�
(
operated as a batch reactor. Although this procedure has
the risk that the reaction mixture is insufficiently homoge-
neous due to lack of mixing, the activities on a per tungsten
basis are similar to those measured in flow mode with a
high-surface-area catalyst. After the desired reaction time,
a sample was taken from the reactor with a calibrated gas
tight syringe for gas chromatograph analysis of the reaction
products. Then the reactor was flushed with thiophene/H2
for 5 min and closed again for the next reaction. Blank runs
of the empty reactor and bare model support were also per-
formed. The activity of the model catalysts is expressed as
the percentage of thiophene converted into products per 5
amorphous SiO2 overlayer is estimated as 90 nm and the
roughness of the SiO2 layer is below 1 nm. After calcination
the wafers were cleaned in a mixture of H2O2 and NH4OH
�
(
3/2 v/v) at 65 C. Subsequently, the surface was hydrox-
ylated by boiling the model supports in water for 30 min,
which leads to a hydroxyl population of 4–5 OH groups/nm2
(
28). Next the wafer was covered with a solution of the cata-
lyst precursors and spin-coated under N2 atmosphere at
2
800 rpm (26).
The catalyst precursor compounds were ammonium
metatungstate, nickel nitrate (Ni(NO3)2�6H2O), NTA,
EDTA, and CyDTA, all obtained from Merck. The con-
centration of W and Ni in the precursor solutions was ad-
justed to result in a loading of 6 W atoms/nm and 2 Ni
atoms/nm after spin coating for samples that were used in
2
cm of catalyst and has been corrected for conversion of the
empty SiO2/Si(100) model support (19).
2
2
the XPS studies, whereas for HDS activity studies the Ni
loadingwasvaried from 1to 6atoms/nm . The samplescont-
2
RESULTS
aining NTA, EDTA, and CyDTA were prepared by spin
coating with ammonical solutions, which contained the pre-
cursors of W, Ni, and NTA (or EDTA or CyDTA) with an
atomic ratio of 3 :1 :1, such that the amount of chelating
agent was equivalent to that of Ni.
First, we will discuss the state of Ni and W during sulfi-
dation, as determined by XPS. Next we present the results
of thiophene HDS and correlate the catalytic activity with
the relative sulfidation rates of Ni and W.
After being spin-coated, the catalysts were dried and
calcined in a glass reactor under a 20% O2/Ar gas flow
Sulfidation of Tungsten in W/SiO2 and NiW/SiO2 Catalysts
�
at 1.5 bar. The catalysts were heated to 500 C at a rate
Figure 1A shows W 4f XPS spectra of the uncalcined
�
of 5 C/min and were kept at the desired temperature for NiW/SiO2 model catalyst after sulfidation in a H2S/H2 mix-
3
0min. Catalystscontainingchelatingagents(NTA, EDTA, ture at different temperatures. It appeared that the sul-
and CyDTA) were used without calcination. fidation of W proceeded similarly in W/SiO2 and in the
The model catalysts were sulfided in a glass tube reactor NiW/SiO2 catalysts prepared with or without chelating
with a mixture of 10% H2S/H2 at a flow rate of 60 ml/min. agents. We therefore only discuss the series of W 4f spectra
The catalysts with chelating agents (NTA, EDTA, and in Fig. 1A.

1
82
KISHAN ET AL.
FIG. 1. W 4f (A) and Ni 2p (B) XPS spectra of uncalcined NiW/SiO2 model catalyst after stepwise sulfidation at various temperatures.
The W 4f spectrum of the unsulfided W/SiO2 and Sulfidation of Nickel in Ni/SiO2 and NiW/SiO2 Catalysts
NiW/SiO2 catalysts exhibits a doublet with the 4f7/2 peak
at 36.0 � 0.1 eV, which is characteristic of W-oxide with an
Ni/SiO2. Figure 2A shows the Ni 2p XPS spectra of a
oxidation state of 6 + (29). The small peak at 41.9 eV with calcined Ni/SiO2 model catalyst recorded after sulfidation
FWHM of 1.9 eV is assigned to the W 5p3/2 state. In case at various temperatures. The Ni 2p spectrum of the unsul-
chelating agents such as NTA, EDTA, and CyDTA were fided catalyst exhibits a Ni 2p doublet at 856.8 eV and a
applied in the preparation, the 4f7/2 binding energy was shake-up feature at higher binding energy, which is a char-
about 0.4 eV lower, indicating interaction of W with the acteristic pattern for nickel oxide with an oxidation state of
complexing agent.
3+ (29). A shoulder at 854.9 eV in the spectrum of fresh
�
Sulfidation starts to affect the W at about 150 C, as ev- Ni/SiO2 points to the presence of hydrated oxide (29), pos-
idenced by the appearance of a shoulder at lower binding sibly formed when the calcined catalyst was exposed to the
energy. Separate studies indicate that this is due to reduc- ambient atmosphere. The Ni 2p XPS spectrum obtained for
tion of the W-oxide, without incorporation of sulfur into the model catalyst, upon sulfidation at room temperature,
the structure (30). W-sulfide formation is characterized by exhibited an additional doublet at lower binding energy
the appearance of the W 4f7/2 peak at 32.6 eV and is com- (� 854.0 eV). With increasing temperature, the intensity of
�
pleted at temperatures between 300 and 350 C. The spec- this additional doublet was found to increase at the expense
trum shows a single W 4f doublet at 32.6 eV, characteristic of the doublet at 856.8 eV, which disappeared completely
�
of WS2 (29). The S 2p spectra of the W/SiO2 catalyst (not at � 100 C. The Ni 2p doublet with a binding energy of
�
shown) confirm that the sulfidation starts at around 150 C 854.0 eV measured after sulfidation at higher temperatures
and that the area of S 2p increases with increasing sulfida- corresponds to that of bulk Ni sulfide, Ni3S2 (20, 29). The
tion temperature. The S 2p spectra can be fitted with a single Ni 2p doublet of Ni sulfide also shows shake-up features at
doublet with S 2p3/2 binding energy of 161.8 eV, consistent higher binding energy, although the intensity of the peaks
2�
with the S -type ligands present in WS2.
is less than in the case of oxidic Ni.
Neither calcination of the catalysts, nor variation of the
The S 2p spectra (not shown) confirm that the sulfidation
W loading, nor addition of chelating agents affected the starts already at room temperature. From the Ni 2p/S 2p
sulfidation rate of tungsten significantly.
intensity ratio it was concluded that the sulfidation of Ni is

SULFIDATION AND THIOPHENE HYDRODESULFURIZATION ACTIVITY OF NiWS CATALYSTS
183
FIG. 2. Ni 2p XPS spectra of (A) Ni/SiO2, (B) calcined NiW/SiO2, (C) NiW–NTA/SiO2, and (D) NiW–EDTA/SiO2 model catalysts after sulfidation
at various temperatures. The spectra show that the presence of complexing agents such as NTA and EDTA retards the sulfidation of Ni at higher
temperatures.

1
84
KISHAN ET AL.
TABLE 1
fidation begins varies with the ligands in the order NTA <
EDTA < CyDTA. However, in all cases, the binding energy
of the final Ni sulfide is the same (853.9 � 0.1 eV).
Binding Energies and FWHM of Ni 2p and W 4f XPS Peaks
of Fully Oxidic and Sulfidic NiW Model Catalysts
NiW/SiO2. Figure 1 shows the Ni 2p XPS spectra to-
Niox 2p3/2
NiSulf 2p3/2
Wox 4f7/2
Wsulf 4f7/2
gether with those of W 4f of the uncalcined silica-supported
Catalyst
Ni/SiO2
Ni-EDTA/SiO2
W/SiO2
(eV)
(eV)
(eV)
(eV)
2
NiW catalyst with a loading of 6 W atoms/nm and Ni 2
2
856.8 (3.0) 854.0 (2.4)
856.1 (2.5) 853.9 (2.5)
—
—
—
—
atoms/nm after sulfidation at various temperatures. Peak
positions of the oxidic and sulfidic contribution of both the
Ni 2p3/2 and the W 4f7/2 photoemission lines and degree
of sulfidation of both elements upon the various sulfida-
tion temperatures are presented in Table 2. As Fig. 1 and
—
—
35.9 (1.9) 32.6 (1.6)
857.1 (2.6) 854.4 (2.6) 36.1 (1.6) 32.6 (1.7)
54.9 (2.1)
NiW/SiO2
8
NiW/SiO2 calc.
NiW-NTA/SiO2
NiW-EDTA/SiO2
856.9 (3.2) 854.4 (2.4) 36.0 (1.8) 32.6 (1.7)
856.0 (3.1) 854.4 (2.7) 35.6 (2.2) 32.5 (1.8)
856.0 (2.7) 854.5 (2.4) 35.7 (2.2) 32.6 (1.6)
�
Table 2 show, the sulfidation of W starts at around 150 C
�
and is completed at 350 C, whereas that of Ni starts at room
temperature and is finished at 150 C. Hence, sulfidation
NiW-CyDTA/SiO2 855.4 (2.4) 854.5 (2.3) 35.6 (2.0) 32.6 (1.6)
�
of Ni 2p and that of W 4f occur in separate temperature
regimes.
Calcination of the NiW/SiO2 catalyst retards the sulfida-
tion of Ni somewhat. The Ni 2p XPS spectra of this catalyst
�
complete at temperatures around 100 C. The binding en-
ergy of Ni sulfide does not change in the entire sulfidation
temperature range (25 to 400 C).
�
(
Fig. 2B) indicate that Ni sulfidation starts at room tempera-
Addition of EDTA to the impregnating solution from
which the Ni/SiO2 is prepared leads to a slower sulfidation
of the Ni (not shown). The Ni 2p spectrum of the unsul-
fided Ni–EDTA catalyst exhibited a pattern characteristic
of Ni2 with the Ni 2p doublet at 856.1 eV accompanied
by shake-up features at higher binding energy. Hence, the
binding energy for Ni–EDTA (856.1 eV) is lower than that
observed for the catalysts without EDTA (856.8 eV; see
Table 1), providing evidence for complexation of the Ni by
the EDTA. Sulfidation of Ni–EDTA/SiO2 started at tem-
ture but converts more slowly to the sulfided state than the
uncalcined NiW catalyst does, as is evident if the Ni spectra
after reaction at 50 C are compared.
�
Interestingly, the binding energy of Ni shifts gradually
+
�
from 854.0 eV after sulfidation at 150 C (characteristic of
Ni3S2) to 854.4 eV at higher temperatures as W converts
to the sulfidic state. Such a shift has not been observed for
Ni/SiO2 with or without chelating agents. We discuss the
indication of this shift under Discussion.
�
peratures above 200 C, i.e., significantly later than mea-
sured for the standard Ni/SiO2 catalyst discussed above.
Sulfidation of Nickel in NiW/SiO2 Catalysts Prepared with
Chelating Agents
After completion of the sulfidation at temperatures around
�
2
50 C, the Ni 2p3/2 binding energy of 853.9 eV indicated for-
Figures2C and 2D illustrate the effect ofNTA and EDTA
mation of bulk Ni3S2. Similar experiments were performed on the sulfidation of Ni in NiW/SiO2 catalysts. Clearly,
with Ni–NTA/SiO2 and Ni–CyDTA/SiO2 catalysts and gave the effect is to stabilize the Ni such that sulfidation oc-
very similar results although the temperature at which sul- curs at higher temperatures than observed for either Ni
TABLE 2
The Binding Energy and FWHM of the Oxidic and Sulfidic Contribution of the Ni 2p3/2 and W 4f7/2 Emission
Lines and Degree of Sulfidation of Both Elements upon the Various Sulfidation Temperature for Uncalcined
NiW/SiO2 Model Catalysts
Ni 2p3/2 (eV)
W 4f7/2 (eV)
Sulfidation
temperature
Oxide
Sulfide
% Nisulfide
Oxide
Sulfide
% Wsulfide
Unsulfided
857.1 (2.6)
—
—
36.1 (1.6)
—
—
8
54.9 (2.1)
�
2
5
5 C
856.9 (3.1)
856.6 (3.3)
856.4 (2.9)
854.0 (2.4)
853.9 (2.2)
853.9 (2.2)
854.0 (2.5)
854.0 (2.3)
854.2 (2.6)
854.3 (2.6)
854.4 (2.6)
46
55
80
100
100
100
100
100
36.0 (1.8)
36.0 (1.7)
36.0 (1.7)
35.9 (1.7)
35.7 (2.3)
36.0 (2.0)
—
—
—
—
—
—
—
6
54
91
100
100
�
0 C
�
1
1
2
3
3
4
00 C
�
50 C
—
—
—
—
—
33.0 (1.8)
33.0 (2.0)
32.5 (1.7)
32.6 (1.6)
32.6 (1.7)
�
00 C
�
00 C
�
50 C
�
00 C
—

SULFIDATION AND THIOPHENE HYDRODESULFURIZATION ACTIVITY OF NiWS CATALYSTS
185
FIG. 3. (A) W 4f, (B) Ni 2p, (C) S 2p, and (D) N 1s XPS spectra of NiW–CyDTA/SiO2 model catalyst as a function of sulfidation temperature,
showing that CyDTA retards the sulfidation of Ni at temperatures where WS2 has been formed. Ni sulfide formation parallels the disappearance of the
N 1s signal, indicating that the decomposition of CyDTA determines the rate of Ni sulfidation.

1
86
KISHAN ET AL.
or NiW/SiO2 catalysts. The stabilizing effect of EDTA is
stronger than that of NTA. We will discuss the effect of the
even stronger chelating agent CyDTA in more detail.
Figure 3 shows the evolution of W, Ni, S, and N in the
NiW–CyDTA/SiO2 catalyst during sulfidation in H2S/H2.
The W 4f spectrum of the unsulfided catalysts in Fig. 3A
shows a doublet at 35.6 eV with full width at half-maximum
(
FWHM) of 2.0, which is characteristic of oxidic W in an
oxidation state of 6 + (29). The Ni 2p spectrum of the unsul-
fided catalyst shows the characteristic pattern of oxidic Ni,
however, with an unusually low binding energy of 855.4 eV,
which we attribute to complexation with CyDTA. The N 1s
peak around 400 eV is characteristic of the CyDTA ligand.
The S 2p region shows no emission as expected.
Upon sulfidation, W is the first element to be affected,
�
which occurs around 150 C. The S 2p region shows the ap-
pearance of sulfur. Ni, however, is not affected until tem-
FIG. 4. Degree of sulfidation of Ni and W in various NiW catalysts as a
�
peratures of 250 C are reached. Sulfidation of Ni is com- function of sulfidation temperature. Note that complexing agents stabilize
�
plete at 300 C. Close inspection of N 1s spectra reveals that Ni against sulfide formation.
the intensity of the nitrogen signal disappears at tempera-
�
tures above 250 C. Hence, we conclude that sulfidation of
�
below 250 C. However, all NiW catalysts sulfided at tem-
Ni is retarded until the Ni–CyDTA complex decomposes. A
similar situation occurred in high-surface-area NiMo/SiO2
catalysts prepared with NTA as the chelating complex, as
reported in an EXAFS study by Medici and Prins (16).
The conversion from oxidic to sulfidic phases is reflected
by the S 2p spectra (Fig. 3C). In the spectra obtained after
sulfidation at 150 and 200 C, the signal corresponds to the
sulfidic part of W only, whereas from 250 C onward the
S 2p peaks have the contribution of both Ni and W sulfide.
�
peratures of 300 C and higher exhibit a Ni 2p3/2 binding
energy that is significantly, i.e., 0.35–0.50 eV, higher than
that of bulk Ni3S2 and indicative of a different structure.
As this shift in binding energy occurs at the temperatures
where WS2 forms, we believe that WS2 accommodates the
change in Ni sulfide structure.
�
�
Thiophene Hydrodesulfurization
The S 2p spectra can be fitted with a doublet of S 2p3/2
All catalysts were tested on their performance in thio-
binding energy of 161.8 eV, consistent with sulfur with a phene HDS. As an example, Fig. 6 shows the yield of the
main products and the total conversion of thiophene of the
formal charge of � 2 (8, 9).
The results in Fig. 3 clearly show that addition of CyDTA NiW–EDTA/SiO catalyst versus reaction time in a batch
2
in the preparation completely reverses the order in which
Ni and W convert to sulfides (W first, Ni second) compared
to the standard NiW/SiO2 catalyst, where Ni sulfidation pre-
cedes that of W.
Comparison Sulfidation of NiW Catalysts
Figure 4 shows the degree of sulfidation of W and Ni for
all the catalysts as a function of sulfidation temperature,
as derived from the XPS spectra. Sulfidation of W in all
catalysts is similar and is indicated by the heavy line in Fig. 4.
One clearly observes the retarding effect of calcination and
of the chelating agents NTA, EDTA, and CyDTA on the
sulfidation of Ni.
Figure 5 shows the Ni 2p3/2 binding energy of the Ni-
sulfide phase versus sulfidation temperature. A nickel-only
(
Ni/SiO2) catalyst, which already forms sulfide at room tem-
perature, contains Ni with a binding energy of 854.0 eV,
similar to that of bulk Ni3S2 irrespective of sulfidation tem-
perature. Also the XPS spectrum of the standard NiW/SiO2
FIG. 5. Ni 2p3/2 binding energy of Ni sulfide in various catalysts as a
catalyst exhibits this phase for sulfidation temperatures function of sulfidation temperature.

SULFIDATION AND THIOPHENE HYDRODESULFURIZATION ACTIVITY OF NiWS CATALYSTS
187
FIG. 8. Thiophene hydrodesulfurization over NiW model catalysts
supported on silica prepared in different ways, showing the product dis-
tribution after 1 h of batch reaction at 400 C.
FIG. 6. Thiophene hydrodesulfurization experiment on NiW–
�
EDTA/SiO2 model catalyst. The loading of Ni and W is 2 and 6
2
atoms/nm , respectively. The figure shows the product yield in a batch
�
reaction at 400 C as a function of reaction time.
dominant product. As the reaction progresses, the conver-
sion increases and the relative amount of the secondary
products, trans-2-butene and cis-2-butene, increases. At
high conversion after a longer reaction time than indicated
in Fig. 6, trans-2-butene even becomes the main product.
Figure 7 illustrates the promoting effect of Ni on the HDS
activity of tungsten in NiW–CyDTA/SiO2 model catalysts.
The activity increases with Ni loading until a Ni-W ratio of
2
experiment. The yields are based on a 5-cm surface area
of catalyst and have been corrected for conversion of an
empty SiO2 support in a blank experiment. These blanks
showed some conversion of thiophene (0.03% ) to mainly
methane, ethane, and propane, thought to be due to ther-
mal decomposition of thiophene, which may be assisted by
the reactor wall. The product distribution in Fig. 6 shows
the most important products, 1-butene, trans-2-butene, and
cis-2-butene. We also observed minor quantities of C1–C3
cracking products (not shown). Butane was not observed.
The yield of products increases roughly linearly with reac-
tion time, indicating that no significant deactivation occurs.
At low conversions the primary product, 1-butene, is the
0
.66 is reached, after which the activity levels off. Interest-
ingly, the optimum Ni : W atomic ratio of 0.66 is equal to
that in commercial NiW/Al2O3 catalysts (2).
Figure 8 compares the catalytic performance of all
the NiW/SiO2 catalysts with optimum loading of 6 W
2
2
atoms/nm and 4 Ni atoms/nm . The beneficial effect of Ni
on W and of the chelating agents on the activity of the
NiW/SiO2 catalysts is evident.
DISCUSSION
Promotion of W by Ni in sulfidic catalysts leads to a sig-
nificant increase in catalytic activity for the hydrodesulfur-
ization of thiophene to 1- and 2-butene, and the promoting
effect becomes significantly higher if chelating agents such
as NTA, EDTA, and CyDTA are added in the impregnation
stage. The activity increases with increasing Ni : W atomic
ratio and reaches a plateau at a ratio of 0.66.
An important conclusion from this work is that a clear
correlation exists between the thiophene HDS activity of
NiW/SiO2 catalysts and the order in which the initially ox-
idic Ni and W convert to the sulfidic state during presul-
fidation in H2S/H2. This becomes immediately apparent if
one compares Fig. 8, where the catalytic performance of
different NiW/SiO2 is shown, with Fig. 4, showing the de-
gree of sulfidation of Ni and W versus temperature. There is
a one-to-one correspondence between the extent to which
�
FIG. 7. Product yield of thiophene hydrodesulfurization at 400 C for
1
h in a batch reaction over NiW–CyDTA/SiO2 model catalysts, as a func-
tion of Ni loading, showing that the optimum composition corresponds to
a Ni : W ratio of 0.66.

1
88
KISHAN ET AL.
sulfidation of nickel is retarded and the activity of the sured by XPS and compared with the activity in thiophene
NiWS/SiO2 catalyst in thiophene HDS.
HDS.
These findings are readily rationalized if one assumes
that the optimum structure of the active phase in a sul-
fidic NiW/SiO2 catalyst is the analogue of the well-known
CoMoS phase (2, 4, 5, 14, 16, 18, 19, 21), in which sulfidic Ni
decorates the edges of WS2 particles. Retarding the sulfi-
dation of Ni by coordinating it to a sufficiently stable agent
such as CyDTA is favorable, because the Ni is released
at temperatures where WS2 has already been formed. Of
course, we cannot rule out the possibility that the chelat-
ing agents play a beneficial role with respect to the size of
the WS2 particles as well, but we believe that the stabilizing
effect on the Ni is the main contributor. The detailed XPS
measurements of the NiW–CyDTA/SiO2 system in Fig. 3
clearly illustrate this point: Ni starts to form sulfides at the
temperatures where the complex decomposes, as revealed
by the intensity decrease of the N 1s spectra characteristic
of the CyDTA complex.
�
Ni/SiO2 catalyst forms bulk nickel sulfide in H2S/H2 at
relatively low temperatures; this catalyst is largely inactive
in thiophene HDS.
�
W/SiO2 converts to WS2 at significantly higher temper-
�
atures (150–350 C); this phase shows significant activity for
�
the HDS of thiophene at 400 C.
�
In NiW/SiO2 catalysts prepared by conventional im-
pregnation, Ni converts more rapidly to the sulfidic state
before the W starts. However, the Ni3S2 phase formed at
low temperatures restructures at temperatures where WS2
has formed; the resulting NiWS phase is about four times
more active for thiophene HDS than WS2.
�
Optimum promotion of WS2 by Ni is observed for
Ni : W atomic ratios of 0.66. Higher Ni content does not
lead to higher activity.
�
Complexing to Ni with chelating agents like NTA or
EDTA retards the sulfidation of Ni to higher temperatures,
such that both Ni and W form sulfides in the same tempera-
ture range. This leads to higher HDS activity than measured
from the standard NiW/SiO2 catalyst.
Note that NTA was sufficiently stable to achieve the de-
sired retardation of Co in CoMo (16, 18, 19) and Ni in NiMo
(
15, 20) catalysts, because Mo forms sulfides at lower tem-
peratures. As W is significantly more difficult to sulfide than
Mo, chelating agents more stable than NTA such as EDTA
and CyDTA are necessary to achieve the desired effect in
NiWS catalysts.
Nonetheless, one cannot state that retardation of Ni sul-
fidation is the sole determining factor in reaching a high
level of activity, as sulfidation of a standard NiW catalyst,
in which Ni sulfidation entirely precedes that of W, shows
significant HDS activity as well and significantly higher than
that of a single WS2/SiO2 catalyst. The Ni 2p XPS spectra
of this catalyst give a hint why this is so: As Fig. 5 shows,
�
Complexing Ni with CyDTA retards the sulfidation
of Ni where WS2 has already formed. Nickel sulfidation
starts when the CyDTA complex decomposes. As a result,
nickel atoms released by the chelating agent can move to
the reactive edges of the WS2 to form a finely dispersed
sulfide. This gives the highest activity for thiophene HDS.
ACKNOWLEDGMENTS
This work has been performed under the auspices of NIOK, The
Netherlands Institute for Catalysis Research, with financial support from
the binding energy of Ni in partially sulfided NiW/SiO2 in- the Chemical Science Division of The Netherlands Organization for Scien-
creases from 854.0 eV, the value of Ni3S2, to 854.4 eV, the tific Research (NWO-CW) and The Netherlands Technology Foundation
(
STW).
value corresponding to the active state of the catalyst, at
the temperatures where WS2 formation is complete. This
strongly suggests that the Ni3S2 formed at low temperatures
rearranges at higher temperatures. Interpreted in terms of
the CoMoS structure, it is likely that the nickel redisperses
over the edges of the WS2 crystallites, as proposed earlier
by Reinhoudt et al. (11).
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