WOx effect on the catalytic properties of Pt particles on Al2O3

Article abstract of DOI:10.1016/j.jallcom.2008.08.143

The effect of WOx on the microstructure, Lewis acidity and catalytic conversion of Pt/Al₂O₃ catalysts was studied. High surface areas were obtained at low W concentration and the WOx species decreased the Al₂O₃

Full text of DOI:10.1016/j.jallcom.2008.08.143

Journal of Alloys and Compounds 483 (2009) 450–452
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
WOx effect on the catalytic properties of Pt particles on Al2O3
J.L. Contrerasa,b,∗, G.A. Fuentesb, L.A. Garcíad, J. Salmonesc, B. Zeifertc
a Universidad Autónoma Metropolitana-Azcapotzalco, Sn. Pablo 180, 02200 México, D.F., Mexico
b Universidad Autónoma Metropolitana-Iztapalapa, Dpto. de IPH, C.P. 09340, México, D.F., Mexico
c Instituto Politécnico Nacional, ESIQIE Edificio 7 UPALM Zacatenco, 07738 México, D.F., Mexico
d Instituto Politécnico Nacional, ESIT, UPALM Zacatenco, 07738 México, D.F., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
The effect of WOx on the microstructure, Lewis acidity and catalytic conversion of Pt/Al2O3 catalysts was
studied. High surface areas were obtained at low W concentration and the WOx species decreased the
Al2O3 surface area and Lewis acid sites. WOx species in tetrahedral coordination were found. Multiple
layers of close-packed spheres forming pores were seen on the Al2O3. The addition of WOx produced
semi-cubic particles and the spheres disappeared. At low W concentrations (0.5–2 wt%W) dehydroci-
clization and isomerization selectivities were higher than the hydrocracking selectivity. On the contrary
high hydrocracking selectivity was found for samples with high W concentration (4–22 wt%W).
© 2008 Elsevier B.V. All rights reserved.
Received 30 August 2007
Received in revised form 31 July 2008
Accepted 9 August 2008
Available online 18 November 2008
Keywords:
Surfaces and interfaces
Sol–gel synthesis
Scanning and transmission electron
microscopy
Nanostructures
1. Introduction
ratio (W/Pt < 7) dehydrocyclization was high whereas a maximum
of isomerization was found at 24 wt%W, however the conversion
The Pt sintering on the Al2O3 surface for many industrial cata-
lysts is an important cause of catalyst deactivation and has evident
economic implications [1]. As a result, stabilization of the catalyti-
cally active Pt during harsh operating conditions is very important.
Among different options, the sub-monolayer range of WOx on
Al2O3 appears interesting for high temperature applications [2]
because under those conditions W6+ stabilizes the transitional
Al2O3 phases while itself becoming refractory to reduction [3–6]
and against high temperature steam treatments [7]. This stabi-
lization appears to be caused by the formation of surface mixed
oxides of W6+ strongly bonded with Al3+ anchoring the Al2O3 sur-
face against structure rearrangements produced by operation at
high temperatures. There are precedents for this positive effect on
SiO2 [8] and Al2O3 based catalysts [9]. Pt supported on WOx/Al2O3
catalyst has been suggested for the industrial nafta reforming pro-
cess [10]. These solids are bifunctional catalysts having the metallic
(Pt) and acid functions (Lewis acid sites) for the reactions. For
the Pt-WOx on commercial Al2O3 catalysts the hydrocracking, iso-
merization and dehydrocyclization selectivities of n-heptane have
been affected by the W/Pt atomic ratio [2]. At low W/Pt atomic
decreased as the W/Pt ratio increased. In this study we investigated
the WOx effect on the superficial Lewis acidity of Al2O3 sol–gel,
surface area and catalytic hydroconversion of n-heptane.
2. Experimental
Al2O3 sol–gel was prepared from aluminum trisecbutoxide (Aldrich) and ethylic
alcohol (Tecsiquim) using a molar ratio of 1/50 at 60 ◦C. These components were
stirred during 1 h and the gel produced was dried at 110 ◦C. (NH4)12 W12 O41 ·5H2O,
(Aldrich) was impregnated onto the gel which was dried at 110 ◦C and calcined
in air at 600 ◦C for 12 h. The W concentration was varied from 0.5 to 22 wt% (or
0.278–15.5 mole%W) (Table 1). In all the samples an average of 0.37 wt% of Pt was
supported on Al2O3 sol–gel using H2PtCl6 (Aldrich). Surface area measurements
(Table 1) using BET method of adsorption of N2, were carried out in the ASAP-2000
Micromeritics digisorb. The acid sites distribution was determined from infrared
spectroscopy of pyridine adsorbed previously in a small catalyst leaf of 0.02 g using a
Nicolet 71 infrared spectrometer and the desorption temperature was changed from
25 to 400 ◦C with temperature steps of 100 ◦C. The Raman spectra were obtained
fromaNicoletAlmegaspectrometer(Neodymiumlaserat532 nm)at25 ◦C. Scanning
electron microscopy was applied at the samples prepared, using a Jeol JSM-5910LV
microscope (Au atoms were deposited to enhance the observation). The catalysts
were evaluated (0.1 g) in a conventional flow microreactor with on-line GC-analysis
(Gow-Mac 550). We used n-heptane hydroconversion at 0.8 atm, using H2 as a carrier
gas (1 ml/s). The glass saturator with n-heptane was kept at 25 ◦C. The chromato-
graph column of SE-30 (10 m) and a FID detector was used.
3. Results and discussion
∗
Corresponding autor at: Universidad Autónoma Metropolitana-Azcapotzalco,
Sn. Pablo 180, México, D.F. 02200, Mexico. Tel.: +52 55 53189065x116;
fax: +52 55 53947378.
The samples showed high surface areas (298 m2/g) and the
addition of WOx to the Al2O3 decreased the area and Lewis acid
E-mail address: jlcl@correo.azc.uam.mx (J.L. Contreras).
0925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2008.08.143

J.L. Contreras et al. / Journal of Alloys and Compounds 483 (2009) 450–452
451
Table 1
Surface area, Lewis acidity, conversion and selectivity of Pt/WOx/Al2O3 catalysts at 350 ◦C.
Catalyst
W (wt%)
Area (BET) (m2/g)
Lewis aciditya (␮mol/g)
Xb (%)
Shc (%)
Si (%)
Sc (%)
P
0
0.5
1
2
4
8
16
22
299
298
238
230
245
246
235
229
210
194
180
197
190
160
120
103
11
15
13
11
9
6
9
9
39
23
21
28
42
57
49
53
28
37
39
40
27
30
30
26
33
40
40
32
31
13
21
21
PW0.5
PW1
PW2
PW4
PW8
PW16
PW22
a
Evaluated at 100 ◦C.
b
c
X is conversion.
Selectivities of hydrocracking (Sh), isomerization (Si) and dehydrocyclization (Sc) reactions.
sites which were partially covered by the W-oxides (Table 1). The
decrease of area in samples from 0.5 wt% to 22 wt%W was 23% and
thenasmallmaximumisshownat8 wt%W, thismaximumhasbeen
found in Ref. [11]. A single main band was found by Raman spectra
(Fig. 1) located at 980 cm−1. These spectra are similar to the spectra
ofaqueous solutions ofoxoanions ofW6+ and thisfact has suggested
that W6+ species are very well dispersed and isolated onto Al2O3
[12]. As the W concentration increased from PW1 to PW16 samples,
the Raman intensity increased such as Iannibello et al. [12] found
in their samples. We did not find crystalline octahedral species of
WO3 having Raman vibration bands at 807, 712 and 263 cm−1 in
the sample PW16. The band between 965 and 990 cm−1 (Fig. 1) has
been related with oxo-tungsten well dispersed tetrahedral species
[5,12–13]. The Raman tetrahedral bands are the result of the W=O
symmetric (931 cm−1) and antisymmetric (830 cm−1) stretching
12−
modes. In aqueous solution the W
O
species have showed the
12
42
major Raman bands located at 977 and 963 cm−1 [14]. The role of
these tungstates (or oxo-tungstates) was to decrease the strong
Lewis acid sites of the Al2O3 and to stabilize this support under
high reduction temperatures [9]. Multiple layers of close-packed
spheres (average diameter of 350 nm) forming pores were seen by
SEM in the Al2O3 sol–gel (Fig. 2). The addition of WOx produced
semi-cubic large particles (average size 10 ␮m) and the spheres dis-
appeared (Fig. 3). The presence of WOx species strong bonded with
Al2O3 [5,13] produced changes in the geometrical shape of particles.
As W concentration increased from 1 to 22 wt% the pore diameter
decreased21%. Simultaneouslythediameterofsemi-cubicparticles
increased. These changes suggested that Knudsen diffusion of n-
Fig. 2. SEM of Al2O3 sol–gel.
heptane molecules could decrease. Therefore the reactants could be
with residence times high and the hydrocracking selectivity could
increase.
The conversion of h-heptane as a function of reaction temper-
ature (Fig. 4) showed that W (at less than 2 wt%W) promoted this
reaction between 350 and 450 ◦C. However, at 500 ◦C, W acted as
inhibitor. The Pt/Al2O3 catalyst (P) was used as reference. Due to
isomerization reaction is thermodynamically favorable at low tem-
peratures we studied the reaction selectivities at 350 ◦C (Table 1).
Fig. 1. Raman spectra of the Pt/WOx-Al2O3 catalysts.
Fig. 3. SEM of Pt/WOx-Al2O3 sol–gel (1 wt%W).

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J.L. Contreras et al. / Journal of Alloys and Compounds 483 (2009) 450–452
4. Conclusions
The addition of WOx to the Pt/Al2O3 catalyst decreased the BET
area and the Al2O3 Lewis acid sites which were partially covered
by the W-oxides and tungstate monomeric species in tetrahedral
coordination and semi-cubic large particles were seen. Pt/Al2O3
catalysts with low WOx concentration (0.5–2 wt%W) produced
isomerization and dehydrociclization selectivity higher than the
corresponding selectivities of the Pt/Al2O3 catalyst and also pro-
duced low hydrocracking selectivity. For high W concentration
(8–22 wt%W) only acid sites remained and Pt sites were absent
producing high hydrocracking selectivity.
References
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Fig. 4. Conversion of n-heptane as a function of temperature of the Pt/WOx-Al2O3
catalysts.
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PW0.5, PW1 and PW2 catalysts (at low WOx concentration) pro-
duced isomerization (Si) and dehycrociclization (Sc) selectivities
higher than the corresponding selectivities of the Pt/Al2O3 catalyst,
also they produced low hydrocracking selectivity. Some studies for
these catalysts, have shown [1–2,9] that WOx species decreased
partially both the metallic Pt sites and the strong Al2O3 Lewis
acid sites. These acid sites were able to produce hydrocracking
products [6]. At low W concentrations (0.5–2 wt%W) the Pt disper-
sions were high [2,9] and the dehydrocyclization and isomerization
reactions were favorable because the acid function (Lewis acidity)
decreased the hydrocracking selectivity. Furthermore in other stud-
ies WOx species stabilized the Pt sites at high temperatures (800 ◦C)
[15]. At high W concentrations (8, 16 and 22 wt%W) the Pt sites
were deactivated, then only weak Lewis acid sites remained on the
Al2O3 surface which produced the high hydrocracking selectivity.
Pt metallic sites are required to complete the isomerization and
dehydrocyclization reactions.