Effect of the sequence of impregnation on the activity and sulfur resistance of Pt-Ni/γ-Al2O3 bimetallic catalysts for the selective hydrogenation of styrene

Article abstract of DOI:10.1016/j.apcata.2012.06.001

The influence of the preparation of bimetallic Pt-Ni catalysts on their activity and sulfur resistance during styrene semi-hydrogenation was studied. The preparation variables assessed were the sequence of impregnation and the kind of nickel precursor use

Full text of DOI:10.1016/j.apcata.2012.06.001

Applied Catalysis A: General 435–436 (2012) 181–186
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Effect of the sequence of impregnation on the activity and sulfur resistance of
Pt–Ni/␥-Al O bimetallic catalysts for the selective hydrogenation of styrene
2
3
Carolina Betti , Juan Badano , M. Juliana Maccarrone , Vanina Mazzieri , Carlos Veraa,b,
a
a
a
a
a,b,∗
Mónica Quiroga
a
Instituto de Investigaciones en Catálisis y Petroquímica INCAPE, FIQ, UNL-CONICET, Santiago del Estero 2654, S3000AOJ Santa Fe, Argentina
Química Inorgánica, Departamentos de Química, Facultad de Ingeniería Química, Universidad Nacional del Litoral, Santiago del Estero 2829, S3000AOJ Santa Fe, Argentina
b
a r t i c l e i n f o
a b s t r a c t
Article history:
The influence of the preparation of bimetallic Pt–Ni catalysts on their activity and sulfur resistance dur-
ing styrene semi-hydrogenation was studied. The preparation variables assessed were the sequence of
impregnation and the kind of nickel precursor used. The catalysts were tested with the styrene hydro-
genation reaction and further characterized by ICP, TPR, XPS and pyridine TPD.
Received 14 March 2012
Received in revised form 29 May 2012
Accepted 1 June 2012
Available online 12 June 2012
All catalysts showed high selectivities to ethylbenzene (>98%). The bimetallic catalysts were more active
∼
than monometallic Pt and the following activity order was found: Pt = NiClPt ≤ NiNPt < PtNiCl < PtNiN.
Key words:
After poisoning with 300 ppm of thiophene the following order of sulfur resistance was found:
Selective hydrogenation
Bimetallic catalysts
Sulfur resistance
Platinum
∼
∼
PtNiCl < Pt ꢀ PtNiN = NiClPt = NiNPt.
Differences in activity and sulfur resistance between the catalysts were attributed to electronic effects,
in turn related to the presence of different electron-rich and electron-deficient surface species of Pt and
Ni interacting between themselves and with the support. The bimetallic catalyst with the highest Cl/Pt
atomic ratio on the surface (as obtained by XPS) proved to be the most sulfur resistant; this higher poison
resistance was rationalized in terms of both steric and electronic effects.
Nickel
©
2012 Elsevier B.V. All rights reserved.
1
. Introduction
properties clearly different from those of the monometallic cata-
lysts [9]. They have a higher activity and selectivity and a greater
resistance to the poisoning by sulfur compounds. Many stud-
ies on bimetallic catalysts for selective hydrogenation have been
performed using both surface characterization techniques and cat-
alytic activity/selectivity tests [9–11]. The greater part of these
studies has involved bimetallic catalysts in reactions of selective
hydrogenation of hydrocarbons of low molecular mass.
In recent works our group has studied the effect of the precur-
sor salt and the temperature of reduction on the activity and poison
resistance of monometallic catalysts for the selective hydrogena-
tion of styrene [12,13]. The objective of this work is to extend this
research to the reaction system of the bimetallic Pt–Ni/␥-alumina
catalyst and the model feedstock of styrene contaminated with
thiophene. The preparation parameters varied are the kind of nickel
precursors and the sequence of impregnation of the metal salts.
Properties especially assessed are the activity, selectivity and sulfur
resistance.
The reactions of selective hydrogenation of vinyllic bonds that
keep unaltered the aromatic ring and that proceed through het-
erogeneous catalysis are of great interest and usefulness for the
petrochemical, fine chemistry and specialty chemicals industries.
Benzene, toluene and xylenes, commonly known as the BTX
fraction, are useful solvents and intermediate. One important
source of BTX in the cracking of different petroleum cuts [1–3].
These streams can have up to 1000 ppm of sulfur compounds, thio-
phene being among the most common ones [1,2]. These sulfur
compounds are the main cause of the loss of activity and selectivity
of metal catalysts in several refinery processes [4,5].
In the petrochemical industry gasoline and BTX streams com-
ing from the cracking of petroleum cuts must be purified in
order to minimize the concentration of olefins and diolefins. The
widespread method of purification is the selective hydrogena-
tion of vinyllic compounds, keeping the aromatic rings unaltered
[
6–8]. The employed catalysts must have a great resistance to
different sulfur compounds. A particular type of supported cata-
lysts is that of the highly dispersed bimetallic catalysts, that have
2. Experimental
2.1. Catalysts preparation
∗ Corresponding author. Fax: +54 342 4531068.
E-mail address: mquiroga@fiq.unl.edu.ar (M. Quiroga).
Bimetallic catalysts were prepared by means of succes-
sive impregnations over a previously calcined ␥-Al O3 support
2
0
926-860X/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2012.06.001

1
82
C. Betti et al. / Applied Catalysis A: General 435–436 (2012) 181–186
2
−1
(
Ketjen CK 300, SBET = 224 m g ). For the incorporation of the met-
pyridine. 200 mg of the previously reduced catalyst samples were
used. They were first impregnated with an excess of pyridine at
room temperature. The excess of pyridine was then evaporated
under a fume hood at room temperature until a dry powder was
obtained. Then the samples were put in a fixed bed reactor under
als to the support a technique of incipient wetness impregnation
was used. The point of incipient wetness was detected at the value
−
1
of 0.7 mL g . The impregnating solutions were H PtCl ·H O (Strem
2
6
2
Chemicals, Cat. No. 78-0200, purity > 99.9%) and NiCl (Merck, CAS:
2
−
1
7
718-54-9, purity > 98%). These solutions were acidified to pH = 1
a constant flow of nitrogen (40 mL min ). A first stage of desorp-
tion of weakly adsorbed pyridine and stabilization of the sample
was performed by heating the sample at 383 K for 1 h. Then the
temperature was increased from 383 to 823 K at a heating rate
with dropwise addition of HCl. The solutions were used to obtain
the monometallic catalysts of Pt and NiCl. A solution acidified to
pH = 1 with HNO3 containing dissolved Ni(NO ) ·6H O (Fluka, Cat.
3
2
2
−
1
No. 72253, purity > 98.5%) was used to obtain the monometallic NiN
catalyst. The concentrations of the solutions were adjusted in order
to get loads of ca. 1 wt% of Pt and 2 wt% of Ni on the final catalysts.
The monometallic catalysts were dried 24 h in a stove at 383 K, and
then they were calcined in dry air for 3 h at 823 K. Finally they were
of 10 K min . The gases issued by the reactor were directly sent
to a flame ionization detector (FID) and the signal of the detector
was recorded continuously together with the temperature of the
sample.
X-ray diffraction (XRD) spectra of the powdered samples were
obtained in a Shimadzu XD-1 instrument using CuK␣ radiation
−
1
reduced for 1 h at 673 K in a hydrogen stream (110 mL min flow
rate).
◦
◦
(ꢀ = 1.5405
A˚ ) filtered with Ni, in the 15 < 2ꢁ < 85 range and at a
◦
−1
The impregnation of the second metal was performed using a
similar procedure and taking the monometallic catalyst as sup-
port. The acidic solution of H PtCl was added to the monometallic
scan speed of 1 min . The samples were powdered and reduced
ex situ under a hydrogen flow. Then they were cooled down to
room temperature in nitrogen flow and put into the chamber of
the equipment to record the spectrum.
2
6
catalysts of NiN and NiCl to obtain the catalysts NiNPt and NiClPt,
respectively. Similarly, acidic solutions of NiCl and Ni(NO ) were
2
3 2
added to the monometallic Pt catalyst to obtain the catalysts PtNiCl
and PtNiN, respectively. The bimetallic catalysts were then dried for
2.3. Catalytic tests
2
4 h in a stove at 383 K, calcined for 3 h at 823 K and finally reduced
The reaction test for assessing the activity, selectivity and sulfur
−1
for 1 h at 673 K in H2 (110 mL min ). It was found that the spe-
cific surface area of the support was practically not modified by
the addition of the different metal precursors or by the subsequent
thermal treatment.
resistance of the prepared catalysts was the selective hydrogena-
tion of styrene. The reaction was performed in a stainless steel,
PTFE coated, stirred tank reactor, operated in batch mode, at 353 K,
MPa hydrogen pressure and 1200 rpm stirring rate. In each test
.3 g of the catalyst and 200 mL of a solution of 5% (vol/vol) of
2
0
2.2. Characterization of the catalysts
styrene (Aldrich, Cat. No. S497-2, purity > 99%) in toluene (Merck,
Cat. No. TX0735-44, purity > 99%) were used. n-Decane (Fluka, Cat.
No. 30550, purity > 98%) was added as an internal chromatographic
standard. In the case of the sulfur resistance tests, 300 ppm of thio-
phene were also added to the reaction mixture. The PTFE lining
of the reactor ensured that no chemical contamination from steel
occurred [14]. Reactants and products were analyzed in a gas chro-
matograph, using a 30 m, J&W INNOWax 19091N-213 capillary
column. The runs were carried out in triplicates with a recorded
average experimental error of 3%.
The Pt and Ni content of the catalysts were determined by Induc-
tively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES)
using a Perkin Elmer ICP OPTIMA 2100 instrument. The samples
were immersed in a dilute solution of sulfuric acid at 363 K before
each analysis.
The electronic state of the surface species of Pt and Ni and the
atomic ratios of the different elements was determined by X-ray
photoelectron spectroscopy (XPS). The analysis was performed in
a Multitech UniSpecs equipment with a dual XR-50 Mg/Al X-ray
source and a Specs Phoibos 150 hemispherical analyzer working in
transmission, fixed analysis (FAT) mode. All the samples were dried
at 353 K and they were examined in potassium bromide disks in a
concentration ranging 0.5–1% to assure non saturated spectra. The
samples were reduced ex situ 1 h at 673 K and were then heated in
3. Results and discussion
3.1. Catalysts characterization
a flow of H /Ar (5%, vol/vol) at 673 K for 10 min in the instrument
Table 1 contains the catalyst notation used. The values of mass
concentration determined by the ICP technique were 1 wt% of Pt
and 2.1 wt% of Ni, with an atomic ratio of Ni/Pt = 7.
Figs. 1 and 2 contain the XPS spectra of the Pt 4d5/2 and Ni
2p3/2 species on the catalysts. The binding energies (BE) were deter-
mined by the fitting of the curves. The Pt/Al and Cl/Pt atomic ratios
were calculated from the ratio of the corresponding peak areas (see
Table 1).
2
chamber before recording the spectrum. The spectra were obtained
with an energy step of 30 eV, using a Mg anode operated at 200 W,
−8
and at a total pressure lower than 2 × 10 mbar. A careful decon-
volution of the spectra was made and the areas under the peaks
were estimated by calculating the integral of each peak after sub-
tracting a Shirley background and fitting the experimental peaks to
a combination of Lorentzian/Gaussian lines of 30–70% proportions.
The binding energy used as a reference was the C 1s 284.6 eV sig-
nal. Since there is an interference between the Al 2p and Pt 4f7/2
As it can be seen in Fig. 1 all catalysts have the Pt 4d5/2 peak
at low values of binding energy, in the 312.3–313.5 eV range. From
0
binding energies, the peak studied was the Pt 4d5/2
.
literature reports this peak could be attributed to Pt [15,16]. When
The study of the reducibility of the surface species was per-
formed by temperature programmed reduction (TPR) in an Ohkura
TP2002 apparatus equipped with a thermal conductivity detector.
The samples were pretreated in situ for 30 min in an air stream at
comparing the BE of the bimetallic catalysts with those of the
monometallic ones, a shift can be seen between 0.8 and 1.4 eV
ı−
toward lower BE values, indicating the presence of Pt with a
0
higher availability of electrons than Pt . This could be due to the
6
73 K for 30 min. Then they were cooled down to room tempera-
formation of metallic bonds or alloying, occurring at low tempera-
tures [17,18]. On the other side a second peak with BE between
314.8 and 316.7 eV could be attributed to the presence of com-
ture in an Ar stream. Finally they were reduced with a 5% (vol/vol)
H in Ar stream while heating with a 10 K min rate to a final value
of 1173 K.
−1
2
ı+
plex electrodeficient Pt species, with 0 < ı < 2, stabilized by the
The amount and strength of the acid sites of the catalysts were
measured by means of temperature programmed desorption of
presence of remaining chloride ions [15]. The third peak with BE
of 317.6 ± 0.4 eV would indicate the presence of another complex

C. Betti et al. / Applied Catalysis A: General 435–436 (2012) 181–186
183
Table 1
Catalysts naming convention. Pt and Ni binding energies and atomic superficial ratios as determined by XPS.
Catalyst
1st impa
2nd impa
Pt 4d5/2 BE (eV)
Ni 2p3/2 BE (eV)
Pt/Al (at/at)
Cl/Pt (at/at)
Pt
Pt
Ni
Ni
Pt
Pt
–
313.5 (54%)
312.6 (21%)
312.4 (35%)
312.3 (35%)
312.7 (47%)
316.7 (46%)
314.9 (57%)
314.8 (43%)
314.9 (47%)
315.3 (53%)
0.0035
0.0038
0.0035
0.0036
0.0039
0.96
2.49
2.07
1.33
1.13
NiClPt
NiNPt
PtNiCl
PtNiN
Pt
Pt
Ni
Ni
317.7 (22%)
317.2 (22%)
318.0 (18%)
852.5 (3%)
852.7 (4%)
852.7 (28%)
852.9 (20%)
856.2 (97%)
856.3 (96%)
856.1 (72%)
856.1 (80%)
NiN: Ni(NO3)2 precursor; NiCl: NiCl2 precursor.
a
imp = impregnation step.
alumina [15,19,20]. Other authors suggest the presence of species
a)
b)
NiO·x(Al O ) (0 ≤ x ≤ 1) [21] that would be formed during the ther-
2
3
mal treatments. In the case of the bimetallic NiNPt and NiClPt
catalysts the signal assigned to Ni0 is smaller than the signal of
the bimetallic catalysts prepared with an inverse order of impreg-
nation, PtNiN and PtNiCl. These would have 20 and 28% completely
reduced Ni. This could be due to the way of preparation of these
catalysts. The bimetallic catalysts firstly impregnated with Ni were
calcined and reduced twice and would have a stronger interaction
with the support, thus making the reduction of the Ni species diffi-
cult. The bimetallic catalysts have slightly higher Ni 2p3/2 BE values
in comparison to the Ni monometallic catalyst (852.3 eV [19]), pos-
sibly because Pt is bonded to Ni through a metal–metal or alloy
bond. Since Pt is more electronegative it would have a higher elec-
c)
d)
e)
ı−
tron density (Pt ) and Ni would be electrodeficient.
The XPS spectra of the catalysts had a peak at about 198.5 eV
that would correspond to Cl 2p3/2, and that could be associated
to chloride species that were not eliminated during the thermal
treatment stages [15]. With respect to the atomic ratios obtained
by XPS (Table 1), a similar Pt/Al ratio for all catalysts was found.
The catalysts with a higher Cl/Pt ratio are the bimetallic catalysts
in which Pt was impregnated the last.
3
05
310
315
320
325
Binding Energy (eV)
Fig. 1. XPS profiles of Pt 4d5/2: (a) Pt; (b) NiClPt; (c) NiNPt; (d) PtNiCl and (e) PtNiN.
electrodeficient Ptꢂ+ species, Ptꢂ+ClxOy, with 2 < ꢂ < 4 [15], formed
during the calcination or the subsequent reduction process [16]. In
Table 1 it can be seen that the PtNiCl is the catalyst that has species
with the highest value of electrodeficiency, 318 eV.
Fig. 3 shows the TPR traces of the calcined Pt mono and bimetal-
lic catalysts. Lieske et al. [22] reported that Pt/Al O prepared from
2
3
chlorinated precursors should have two peaks in the TPR trace
because of two complex surface species that contain chlorides. The
IV
peak at 533 K would be related to [Pt (OH)xCly] and the one at
When analyzing the BE region corresponding to the 2p3/2
spectrum of nickel, it can be seen (Fig. 2) that the bimetallic
catalysts have two peaks, the first one at about 852.7 ± 0.2 eV
IV
5
63 K to [Pt OxCly]. The formation of these species mainly depends
on the temperature of calcination of the catalyst [23]. Reyes et al.
[24] have attributed the peak between 420–460 K to the reduction
of PtO2 and the peak at about 600 K to the reduction of PtO Cl .
x y
0
that could be assigned to Ni [15] and another at 856.2 ± 0.1 eV
2+
attributed to electrodeficient species of Ni . According to litera-
ture reports the latter could be Ni species strongly interacting with
Finally Navarro et al. [25] reported that silica–alumina supported
Pt catalysts prepared from H PtCl6 had a broad reduction peak at
2
503
b )
a)
b)
5
04.7
989
8
93
c )
d )
971
5
02
8
24
c)
d)
5
27
603
8
46
1058
e )
489
730
9
83
e)
8
50
855
860
4
00
500
600
700
800
900
1000
1100
Binding Energy (eV)
Temperature (K)
Fig. 2. XPS profiles of Ni 2p3/2: (b) NiClPt; (c) NiNPt; (d) PtNiCl and (e) PtNiN.
Fig. 3. TPR traces of: (a) Pt; (b) NiClPt; (c) NiNPt; (d) PtNiCl and (e) PtNiN catalysts.

1
84
C. Betti et al. / Applied Catalysis A: General 435–436 (2012) 181–186
Table 2
Pyridine TPD results.
3
00–700 K. They suggested the presence of oxychlorinated species
such as Pt(OH)xCly, PtOxCly and PtO2.
With respect to the Ni catalysts supported over alumina, some
authors define three different reduction temperature ranges: (I)
up to 600 K only the Ni oxides with weak or null interaction with
the support (bulk NiO) are reduced; (II) between 600 and 1000 K
the reduction of Ni oxides strongly interacting with the support
occurs; (III) between 1000 and 1273 K nickel aluminates (NiAl O )
Catalyst
Distribution of acid
strength (a. u.)
Total acid
concentration (a.u.)
9
73–873 K
873–773 K
(medium)
373–773 K
(low)
(high)
Pt
–
1.80
3.20
0.23
5.80
0.84
2.50
4.50
0.89
2.10
0.71
27.00
18.00
1.1
9.7
4.7
29.5
22.5
2
4
PtNiCl
PtNiN
NiClPt
NiNPt
are reduced [26–29].
According to the surveyed literature the broad peak between
0.37
3
23–673 K in Fig. 3(a)–(c) (Pt, NiClPt and NiNPt TPR plots), could
ı+
be due to the reduction of oxychlorinated Pt OxCly species. In the
case of the bimetallic catalysts in which Pt was impregnated first
and Ni second (Fig. 3(d) and (e)) the first peak is broader than the
same peak for monometallic Pt. Hence it could correspond to the
simultaneous reduction of both Pt and bulk NiO [26,27,29–31]. In
the case of PtNiN the first peak can be found at lower temperatures
0
Pt particles close to the surface and have low amounts of Ni . The
values shown in Table 2 suggest the following decreasing order
of total acidity: NiClPt > NiNPt ꢁ PtNiCl > PtNiN > Pt. This is also the
order of the superficial Cl/Pt atomic ratio as measured by XPS
(
shown in Table 1). Carvalho et al. [33] reported that a chlorinated
(
489 K) indicating that in that catalyst the Pt species are more eas-
support had 2.9 times more acidity than pure alumina and that
the replacement of different kinds of OH groups by Cl produced
acid sites of different strength. Chlorine was also thought to dis-
place water molecules coordinated to surface Al atoms of different
chemical nature.
ily reduced than those found on the monometallic catalyst [18]. In
the case of PtNiCl the position of the first peak is shifted to higher
temperatures (527 K) suggesting that the Pt particles are harder
to reduce (in comparison to the monometallic catalyst). Reduction
peaks can also be found at 600–1000 K in the case of the bimetallic
catalysts. These peaks could be related to the reduction of NiO in
strong interaction with the support [24,29,31,32]. In the case of the
bimetallic NiNPt and NiClPt catalysts the position of the peaks is
slightly shifted to higher temperatures, indicating that Ni species
on these catalysts have a lower reducibility. This could be due to
a greater interaction of NiO with other surface species or with the
support. It must be recalled that Ni on the NiNPt and NiClPt catalysts
suffered a double calcination and reduction treatment.
The X-ray diffractograms of all catalysts had only three peaks
at 2ꢁ = 37.7 , 46.0 and 67.0 , corresponding to the structure of
◦
◦
◦
◦
␥
-Al O [34]. No peak at 39.9 related to the ꢂ1 1 1ꢃ reflections of
2
3
0
Pt could be found, neither the peaks corresponding to the ꢂ2 0 0ꢃ
0
planes of PtO or Pt [35–37], or the peaks due to bulk NiO. This was
2
attributed to the small particle size and the relatively high limit of
detection of the XRD technique [38]. Heracleous et al. [39] have
reported that in catalysts of Ni supported over alumina a minimum
Ni mass content of 15% is needed in order to detect the NiO (bulk)
diffraction lines at 2ꢁ = 43.3 , 63.0 , 75.5 and 79.5 . Salagre et al.
21] have reported that the low intensity diffraction lines of NiO
particles of Ni/Al O catalysts would only be detected at Ni contents
According to the literature and the temperature of reduction
used (673 K), for all the prepared catalysts the broad peak between
◦
◦
◦
◦
[
3
00 and 673 K should be related to the reduction of PtO and the
2
2
3
ı+
IV
oxychlorinated species Pt OxCly or [Pt (OH)xCly]. For the PtNiN
and PtNiCl bimetallic catalysts it would also be related to the reduc-
tion of Ni in weak interaction with the support. The TPR results
agree with the XPS results since in all the samples there were Pt ,
Pt and Pt oxychlorinated species, while on the surface of the
higher than 26.6%. Due to this low sensitivity of the XRD method
the Pt and NiO phases cannot be detected in our case.
0
3
.2. Catalytic tests
ı+
ꢂ+
0
bimetallic PtNiN and PtNiCl catalysts more Ni was found. Also the
The selectivity found in all the tests performed was higher than
8% (styrene to ethylbenzene). Also no products of hydrogenolysis
fact that the PtNiN has a first peak at low temperatures is in agree-
9
ı−
ment with the higher amount of reduced platinum (Pt ) found in
of thiophene could be detected. Fig. 4 shows the results of total
conversion of styrene as a function of time-on-stream for the mono
and bimetallic catalysts, in the absence of poisons. It can be seen
that all bimetallic catalysts display a higher total conversion than
the monometallic catalyst. The most active catalyst is PtNiN and
the XPS spectra of the bimetallic catalysts. The lower reducibility of
the Ni species on the NiNPt and NiClPt catalysts (shown in Fig. 3(b)
0
and (c)) also agrees well with the lower amount of Ni species found
by XPS.
When a base is adsorbed over an acid surface, strong bonds
appear and high temperatures are needed to desorb it. In the
case of pyridine the molecules adsorbed over weak acid sites are
desorbed at relatively low temperatures and those adsorbed over
strong acid sites are desorbed at much higher temperatures. The
amount of base desorbed in different temperature ranges is a mea-
sure of the distribution of the acid strength. The pyridine TPD
traces were thus integrated to give the total amount of weak acid
sites (room temperature to 773 K), mild acid sites (773–873 K) and
strong acid sites (temperatures higher than 873 K). These acidity
values are included in Table 2. A marked difference can be seen
between the amount and main kind of acidity for the different cat-
alysts. For example the total acidity of the bimetallic catalysts is
∼
the order of activity found is: PtNiN > PtNiCl > NiNPt ≥ NiClPt = Pt.
1
00
90
8
7
6
0
0
0
50
4
3
2
0
0
0
4
–27 times the acidity of the monometallic catalyst. In the case of
the bimetallic catalysts there is also a great effect of the kind of
metal precursor and the order of impregnation on the total acid-
ity and the acid strength distribution. These results agree with
those of the distribution of the Pt and Ni electrodeficient species
as measured by XPS, since both species would provide Lewis acid
sites. It can be seen that catalysts with high total acidity have
10
0
0
20
40
60
80
100
120
140
160
180
200
Time (min)
Fig. 4. Styrene total conversion as a function of time in the absence of poison. (᭹)
Pt, (ꢀ) NiClPt, (ꢁ) NiNPt, (ꢂ) PtNiCl and (+) PtNiN.

C. Betti et al. / Applied Catalysis A: General 435–436 (2012) 181–186
185
Table 3
Values of the initial rate of hydrogenation of styrene hydrogenation rate.
Catalyst
Sulfur-free condition
Sulfur-poisoned condition
˛
0
sf
−1
−1
R2
0
−1
−1
)
R2
r
(mol L min
)
r
(mol L min
sp
Pt
0.27 ± 0.01
0.27 ± 0.02
0.28 ± 0.02
0.32 ± 0.02
0.34 ± 0.01
0.995
0.988
0.990
0.996
0.998
0.14 ± 0.01
0.18 ± 0.03
0.19 ± 0.02
0.13 ± 0.01
0.18 ± 0.01
0.997
0.970
0.981
0.998
0.994
0.49
0.33
0.31
0.59
0.45
NiClPt
NiNPt
PtNiCl
PtNiN
˛
: fraction of poisoned sites
Fig. 5 shows the results of total conversion of styrene in the
presence ot thiophene as a function of time-on-stream. A decrease
in total conversion can be seen as a result of the loss of active sites.
electrons in the 5d Pt orbital in comparison to monometallic Pt, the
cleavage of the H H bonds would be favored on these catalysts. This
would explain the higher activity of the bimetallic Pt–Ni catalysts.
It has also been well established that the mechanism of poison-
ing of Group VIII metals by sulfur compounds is due to an effect
of donation of electrons from the metal to sulfur. Particularly thio-
phene interacts with the surface of metals in a planar way through
the ␲ electrons of the aromatic nucleus (weak ␩5 bond) [13,41,42].
During the thiophene poisoning tests some of the platinum elec-
trons of the d orbital are shared with the S atom and for this reason
the metal would have a lower amount of electrons available for
promoting the cleavage of the hydrogen bond. The catalytic activ-
ity is thus reduced for all the catalysts after poisoning occurs. In
this sense the differences in activity and sulfur resistance between
the catalysts could partly be attributed to electronic effects.
The NiNPt and NiClPt bimetallic catalysts have the highest val-
ues of the superficial Cl/Pt atomic ratio (XPS data of Table 1) and are
also the most resistant to sulfur poisoning. It is possible that Pt oxy-
chlorinated species prevent the adsorption of thiophene both by a
steric hindrance effect (big size of oxychlorinated species) and an
electronic one (high electronegativity of chlorine). Bimetallic PtNiCl
catalysts turned out to be the least sulfur resistant. This is likely due
to its high amount of exposed Ni0 and the presence of Pt species
with the highest availability of electrons (with BE of 312.3 eV). Both
effects would promote a strong adsorption of thiophene and thus
an enhanced blocking of active sites.
∼
∼
The activity order is now: NiNPt = NiClPt = PtNiN ꢁ Pt > PtNiCl.
In order to compare the sulfur resistance of the catalysts the
initial rates of reaction of styrene were determined for the tests
0
0
with sulfur-free (r ) and sulfur-poisoned (r ) feed. A kinetic rate
sf
sp
model was used that assumed a zero order with respect to styrene
that seemed in accord with the conversion patterns found. Table 3
shows the values of the initial reaction rates, as obtained by least
squares fitting with the MicroMath Scientist program. As indicated
in the experimental section all tests were performed in triplicates.
0
0
In this sense the error associated to (r ) and (r ) in Table 3 corre-
sf
sp
0
sponds to the standard deviation of r as obtained by propagation of
the error of the ethylbenzene yield (2–4% error). The values of the
coefficient of regression R2 indicate that the fitting of the model
is good in all cases. From the regressed values of the two kinetic
constants, the fraction of poisoned sites (˛) was determined as:
0
0
˛
= 1 − (r /r ).
sf sp
It can be seen (Table 3) that the NiNPt and NiClPt bimetallic cat-
alysts in which Pt was impregnated last, are those with the lowest
amount of poisoned sites. According to the other characterization
0
ı−
results these catalysts have a low amount of Ni (by XPS and TPR),
a high value of the Cl/Pt ratio and the highest amount of total acid-
ity. With a related but opposite trend, the PtNiCl catalyst which has
0
ı−
the highest amount of Ni and Pt species with the highest elec-
tronic availability, shows the highest amount of poisoned active
sites.
The XPS results indicate that all catalysts have different kinds
of platinum species and that those on the surface of the bimetallic
catalysts have a higher electronic availability (species with lower
BE). During the reaction of hydrogenation the cleavage of the H–H
bond is favored by the interaction of the electron-rich d orbitals of
the metal with the molecular antibonding orbitals of hydrogen [40].
Since the bimetallic catalysts have a higher amount of available
4
. Conclusions
The effect of the sequence of impregnation and of the kind of
nickel precursor used was evaluated for a series of bimetallic Pt–Ni
catalysts. The properties assessed were the catalytic activity and the
sulfur resistance, during the selective hydrogenation of styrene. All
the bimetallic catalysts were found to be more active than the Pt
monometallic catalyst.
1
00
0
ı−
The XPS results point to the presence of different Pt , Pt and
Ptı+OxCly species. The presence of Pt species would indicate the
formation of a metallic bond or a Pt–Ni alloy. Species of Pt with
high availability of electrons were found on the bimetallic catalysts
suggesting that an electronic effect would be partly responsible
for their higher conversion of styrene, either in the presence or
the absence of thiophene. The order of total conversion of styrene
ı−
9
0
0
0
0
0
0
0
0
0
8
7
6
5
4
3
2
1
∼
in the absence of sulfur was: Pt = NiClPt ≤ NiNPt < PtNiCl < PtNiN.
After the poisoning with 300 ppm thiophene the order changed to:
∼
∼
PtNiCl < Pt ꢀ PtNiN = NiClPt = NiNPt. The bimetallic catalysts pre-
pared by successive impregnation of Ni and Pt had a high surface
content of chlorine, a low amount of Ni0 and a high total acidity.
These catalysts were the most sulfur resistant because of steric and
electronic effects.
0
0
20
40
60
80
100
120
140
160
180
200
The great poisoning of the PtNiCl bimetallic catalyst would be
due to the easy adsorption of the thiophene sulfur atoms over the
Time (min)
relatively abundant Ni0 and Pt species with high availability of
ı−
Fig. 5. Styrene total conversion as a function of time in the presence of poison. (ꢄ)
Pt, (♦) NiClPt, (ꢃ) NiNPt, (ꢅ) PtNiCl and (+) PtNiN.
electrons.

1
86
C. Betti et al. / Applied Catalysis A: General 435–436 (2012) 181–186
Acknowledgements
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978.
1
The authors are gratefully indebted to CONICET, Universidad
Nacional del Litoral and the ANPCyT for their funding of this work.
The technical assistance of Carlos Mazzaro and Daniel Coria is also
greatly thanked.
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