Increased conversion and selectivity of 4-nitrostyrene hydrogenation to 4-aminostyrene on Pt nanoparticles supported on titanium-tungsten mixed oxides

Article abstract of DOI:10.1016/j.apcata.2016.03.031

A catalyst series consisting in platinum nanoparticles photodeposited on pure titania and on W/Ti mixed oxides, these latter prepared by the sol-gel method, were tested in the hydrogenation of 4-nitrostyrene. A remarkable increase in the reaction rate occurred when the catalyst support contained tungsten, with a parallel boosting in the selective reduction of the nitro group. With the selective W-containing catalysts, the reaction proceeded at constant rate (zero order rate law), while the tungsten-free catalyst showed a rate-dependence on the 4-nitrostyrene concentration (positive order reaction). The presence of tungsten in the support is beneficial not only because a higher surface area is obtained, thanks to the stabilization of anatase owing to the presence of tungsten, but also because it allows the photodeposition of smaller, better dispersed platinum particles, on which the adsorption of the aromatic part of 4-nitrostyrene is less favored. Tungsten not only substitutes titanium in the titania lattice, as revealed by HAAF-STEM analysis, but it is also present as WO_x species partly covering the Pt nanoparticles photodeposited on the mixed oxide support, as revealed by an in depth distribution XPS analysis. This accounts for the progressively lower performance observed with increasing tungsten content in the catalysts, the highest conversion and selective hydrogenation of the 4-nitrostyrene nitro group having been achieved on the catalyst with a 1% W/Ti molar ratio.

Full text of DOI:10.1016/j.apcata.2016.03.031

Applied Catalysis A: General 519 (2016) 130–138
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Increased conversion and selectivity of 4-nitrostyrene hydrogenation
to 4-aminostyrene on Pt nanoparticles supported on
titanium-tungsten mixed oxides
Marco Carrusa,b, Marzia Fantauzzic, Francesca Ribonia,d, Martin Makoschb,
Antonella Rossic,e,∗, Elena Sellia,∗∗, Jeroen A. van Bokhovenb,f,∗∗
a Dipartimento di Chimica, Università degli Studi di Milano, via Golgi 19, 20133 Milano, Italy
b Institute for Chemical and Bioengineering, ETH Zurich, 8093 Zürich, Switzerland
c Dipartimento di Scienze Chimiche e Geologiche, Università di Cagliari, 09042 Monserrato (Ca), Italy
d Present address: Department of Materials Science WW4-LKO, University of Erlangen-Nuremberg, Martensstrasse 7, 91058 Erlangen, Germany
e Laboratory of Surface Science & Technology (LSST), Department of Materials, ETH-Hönggerberg, 8093 Zürich, Switzerland
f
Swiss Light Source, Paul Scherrer Institute, 5232 Villigen, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
A catalyst series consisting in platinum nanoparticles photodeposited on pure titania and on W/Ti mixed
oxides, these latter prepared by the sol-gel method, were tested in the hydrogenation of 4-nitrostyrene.
A remarkable increase in the reaction rate occurred when the catalyst support contained tungsten, with a
parallel boosting in the selective reduction of the nitro group. With the selective W-containing catalysts,
the reaction proceeded at constant rate (zero order rate law), while the tungsten-free catalyst showed a
rate-dependence on the 4-nitrostyrene concentration (positive order reaction). The presence of tungsten
in the support is beneficial not only because a higher surface area is obtained, thanks to the stabilization
of anatase owing to the presence of tungsten, but also because it allows the photodeposition of smaller,
better dispersed platinum particles, on which the adsorption of the aromatic part of 4-nitrostyrene is less
favored. Tungsten not only substitutes titanium in the titania lattice, as revealed by HAAF-STEM analysis,
but it is also present as WOx species partly covering the Pt nanoparticles photodeposited on the mixed
oxide support, as revealed by an in depth distribution XPS analysis. This accounts for the progressively
lower performance observed with increasing tungsten content in the catalysts, the highest conversion
and selective hydrogenation of the 4-nitrostyrene nitro group having been achieved on the catalyst with
a 1% W/Ti molar ratio.
Received 12 January 2016
Received in revised form 13 March 2016
Accepted 29 March 2016
Available online 30 March 2016
Keywords:
Selective hydrogenation
Reduction of nitro group
Supported Pt
W-Doped TiO2
X-ray photoelectron spectroscopy (XPS)
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
palladium [4], gold [5] and rhenium [3] nanoparticles supported
on titania are excellent catalysts in the hydrogenation of aromatic
The research directed towards the development and improve-
ment of photocatalysts led to the design of new nanomaterials
prepared exclusively for photocatalytic applications. In particular,
a considerable attention has been paid to titania and its increase
in photoactivity as a result of modifying the surface and the bulk
by metal and/or nonmetal doping [1,2]. However, titania is a tra-
ditional support in thermal catalysis. In particular, platinum [3],
nitro compounds to anilines, with the supported metals acting as
catalytic sites. The aminobenzenes obtained by this reaction are
important intermediates in the manufacture of many agrochemi-
cals, pharmaceuticals, dyes, and pigments [6–8].
The catalytic hydrogenation of simple aromatic nitro com-
pounds poses only a few problems and is, therefore, implemented
on a large scale. However, it is difficult to selectively reduce the
nitro group in a catalytic process when other reducible groups
are present in the aromatic molecule, especially when hydrogen
is employed as reducing agent. Significant effort has, thus, been
made to develop more selective catalysts [9], although an increase
in selectivity was often accompanied by a decrease in activity. For
example, Corma’s group [3,10] showed that gold particles, sup-
ported on titania or iron oxide, catalyze the reduction of various
∗
Corresponding author at: Dipartimento di Scienze Chimiche e Geologiche, Uni-
versità di Cagliari, 09042 Monserrato (Ca), Italy.
∗∗
Corresponding authors.
E-mail addresses:
rossi@unica.it, antonella.rossi@mat.ethz.ch (A. Rossi), elena.selli@unimi.it (E. Selli),
jeroen.vanbokhoven@chem.ethz.ch (J.A. van Bokhoven).
http://dx.doi.org/10.1016/j.apcata.2016.03.031
0926-860X/© 2016 Elsevier B.V. All rights reserved.

M. Carrus et al. / Applied Catalysis A: General 519 (2016) 130–138
131
functionalized aromatic nitro compounds with high chemoselec-
2.3. Catalysts characterization
tivity and without the unwanted accumulation of explosive phenyl
hydroxylamine byproducts. Selectivity was successfully addressed
in another way, i.e. by modifying the platinum deposited on tita-
nia with organic thiols to affect the adsorption mode and control
selectivity [11], or by the use of soluble nanoparticles poisoned
with phosphine ligands or self-assembled monolayers [12]. A
recent review paper addressed how chemoselectivity to nitro group
reduction can be induced in traditionally unselective metals by
controlling the architecture of the metal catalysts, avoiding the
addition of toxic substances [13].
We have taken a different approach in the present work, the goal
of which is to ascertain whether the composition of the support
affects the the conversion and selectivity of the reaction. The selec-
tive catalytic reduction of the nitro group of 4-nitrostyrene was,
thus, investigated over platinum nanoparticles, photodeposited
on titanium-tungsten mixed oxides with W/Ti molar ratios in
the 0–0.05 range, prepared by the sol-gel method. There was an
increase in the reaction rate and in selectivity to 4-aminostyrene
when doping with tungsten was optimal. A detailed investigation
of the materials by different techniques made it possible to cor-
relate the activity and selectivity to the composition of the active
platinum nanoparticles, the structure of which is affected by the
presence of tungsten.
X-ray powder diffraction (XRPD) measurements were per-
formed with a Philips PW3020 powder diffractometer, operating at
40 kV and 40 mA, and Cu K␣ radiation (␭ = 1.54056 Å) as the X-ray
source. The diffractograms were recorded by continuous scanning
between 20◦ and 80◦ 2␽ angles with a 0.05◦ step. Quantitative
phase analysis was carried out according to the Rietveld refine-
ment method with the GSAS software, as described elsewhere [16].
The average crystallite size was obtained by applying the Scherrer
equation. The detection limit of the technique is estimated to be
equal to 1%.
X-ray photoelectron spectroscopy (XPS) experiments were per-
formed by means of a PHI Quantera SXM spectrometer (ULVAC
PHI, Chanhassen, MN, USA) and an Al K␣ radiation (␭ = 1486.6 eV)
monochromatic source with a beam diameter ranging from 9 to
200 ␮m. Photoelectron spectra were recorded in fixed analyzer
transmission mode. The binding energies were referred to the C
1s peak at 285.0 eV. XPS analysis was performed on pure PtO2,
WO3 (both purchased from Aldrich) and TiO2 (the TW0 sample),
and on PtTW0, PtTW1, PtTW3 and PtTW5. A detailed description
of the instrument and of the calibration procedure can be found
elsewhere [17].
Energy dispersive X-ray (EDX) spectroscopy experiments, car-
ried out using a Zeiss Sigma Scanning Electron Microscope,
equipped with a Bruker Quantax 400 EDS detector (30 mm2 X Flash
silicon drift detector), provided information about the surface com-
position of the catalysts.
2. Experimental
Scanning transmission electron microscopy (STEM) measure-
ments were performed on an aberration-corrected HD-2700CS
Hitachi STEM microscope operated at an acceleration potential of
200 kV (electron gun: cold-field emitter) with an annular dark field
detector (HAADF). Analytical investigations of selected spots and
areas were performed in the normal mode with an energy dis-
persive X-ray spectrometer (EDX, Genesis Spectrum version 6.2
(EDAX)) attached to the microscope. A few drops of the material,
suspended in ethanol, were deposited onto a perforated carbon foil
supported on a copper grid. After drying, the grid was mounted on
the single tilt holder of the microscope.
2.1. Synthesis of the supporting materials
A pure TiO2 sample, labeled as TW0, was synthesized by a sol-
gel method, starting from titanium(IV) isopropoxide (TTIP Aldrich,
purity 97%) as the titanium precursor, according to a procedure sim-
ilar to that described elsewhere [14]. An anhydrous ethanol solution
(100 cm3, purity > 99.8%) containing 10 cm3 of dissolved TTIP was
heated at 30 ◦C under vigorous stirring. Then 34 cm3 of water were
added dropwise in order to obtain a molar ratio of Ti/H2O = 1/58.
Tungsten-doped TiO2 samples were obtained by adding the
required amount of tungsten(VI) hexa-ethoxide (W(OC2H5)6, Alfa-
Aesar 99.8%, 5 wt.% in ethanol) to the TTIP ethanol solution to obtain
nominal W/Ti molar ratios of 1.0, 3.0 and 5.0%. These catalysts were
labeled TWx, with x referring to the W/Ti percent molar ratio.
After stirring and refluxing for 1 h, the suspensions were con-
centrated under reduced pressure at 35 ◦C. The resulting white
slurries were kept in an oven at 70 ◦C overnight to eliminate organic
compounds and then calcined at 500 ◦C in a 100-cm3 min−1 air-
flow for 4 h. All solutions were prepared with ultra-pure water
(18.2 Mꢀ cm), supplied by a Millipore Direct-Q 3 water purification
system.
The specific surface area of the materials was determined by
N2 adsorption in a Micromeritics Tristar II 3020 V1.03 apparatus
according to the Brunauer, Emmett and Teller (BET) method. The
samples were pre-treated at 300 ◦C for 4 h in vacuo.
2.4. 4-Nitrostyrene hydrogenation
All hydrogenation runs were performed in 50-cm3 Premex
stainless steel autoclaves [12] with polyetheretherketone inlets.
The typical reaction mixture consisted of solvent (20 g, toluene,
puriss > 99%, Fluka Analytical), an internal standard (1 mmol
mesitylene > 99%, Sigma Aldrich), PtTWx (x = 0, 1, 3, 5) catalyst
(50 mg) and substrate (0.67 mmol 4-nitrostyrene > 95%, TCI). An
autoclave was filled with this mixture, sealed and purged three
times with H2 under stirring. The autoclave was then pressurized
to 10 bar by using H2 and heated to 80 ◦C under constant stirring
to start the reaction. Samples were taken at fixed time intervals
by means of a sample tube. The samples were filtered and ana-
lyzed in an Agilent 7820 gas chromatograph equipped with an
apolar 30 m HP 5 MS column. The temperature program consisted
of 20 ◦C min−1 heating steps from 80 to 300 ◦C at a 60/l split ratio.
Activity results are expressed in terms of 4-nitrostyrene conversion
(moles reacted over the initial number of moles) and selectivity
to 4-aminostyrene (moles of 4-aminostyrene produced over the
moles of reacted 4-nitrostyrene).
2.2. Platinum deposition
Platinum nanoparticles were photo-deposited on the oxide
powders according to the procedure described elsewhere [15],
starting from 6 vol.% of methanol/water suspensions containing
3 g dm−3 of oxide powder and the amount of H2PtCl6 (Aldrich) to
obtain a 0.5 wt.% nominal metal loading. PtIV photo-reduction was
achieved by irradiating the suspensions for 2 h in a 15 cm3 min−1
nitrogen flow by means of an immersion fluorescent, low-pressure
mercury arc lamp. Platinum-modified powders (PtTWx) were
recovered after at least three cycles of centrifugation, each followed
by washing with water, up to the complete removal of residual ions
and organic precursors. The samples were dried at 70 ◦C for one day
and stored in the absence of light and humidity.

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M. Carrus et al. / Applied Catalysis A: General 519 (2016) 130–138
Table 1
The HAAF-STEM images of the investigated titanium-tungsten
mixed oxide materials (Fig. 2) show the nanoparticles before and
after the photo-deposition of platinum nanoparticles. On average,
the oxide particles size appears to be less than half that of pure
TW0 and PtTW0 (see for example Figs. 2 b vs 1), confirming the dA
values reported in Table 1. Furthermore, all images, in particular
those obtained from platinum-free samples, prove the presence of
atomically dispersed tungsten, revealed as bright spots in the tita-
nia bulk. With increasing tungsten amount, the number of bright
spots increases (Fig. 2a to c). The bright spots (W) are on the
bright lines, thus showing that W is substituting Ti (Fig. 2c). The
homogeneity of the observed W dispersion leads to the conclu-
sion that W probably resides both in the TiO2 lattice and on the
oxide surface, where it can interact with platinum and/or influ-
ence the 4-nitrostyrene hydrogenation reaction. Larger (1.5–2 nm)
bright spots appear in the images obtained for the same samples
after platinum photo-deposition (Fig. 2d–f), confirming the pres-
ence of nanometer-sized platinum particles on the surface of the
PtTWx catalysts. Notably, the platinum nanoparticles deposited on
the mixed oxides are smaller in size (1–2 nm, with the exception
of PtW5, the nanoparticles of which are 1–3 nm) than those which
were photo-deposited on PtTW0 (2–5 nm) (Figs. 1 and 2).
A detailed EDX analysis, performed on a slightly larger (about
3 nm in diameter) metal nanoparticle on the surface of the PtTW5
sample (Fig. 3, upper panel) demonstrates the presence of both plat-
inum and tungsten in this nanoparticle and support the existence
of direct interactions between platinum and tungsten. However,
this was not proven in the case of the PtTW1 and PtTW3 sam-
ples, probably because of their rather low tungsten content, below
the sensitivity of the EDX instrument. The very high dispersion of
tungsten visible in the STEM monographs of Fig. 2 also points to
platinum and tungsten to be in close proximity.
BET specific surface area (SSA), anatase crystallite dimensions (dA), percent amount
of platinum atoms (at.Pt) and W/Ti percent molar ratio in the investigated catalysts.
Sample
SSA (m2/g)
dA (nm)a
atPt (%)b,c
(W/Ti)×100b,c
PtTW0
PtTW1
PtTW3
PtTW5
40
13
6
5
0.39 (0.04)
0.20 (0.04)
0.3 (0.1)
0
206
202
198
1.0 (0.1)
3.3 (0.2)
5.3 (0.3)
6
0.20 (0.04)
a
obtained from XRD analysis.
obtained from XPS analysis.
standard deviation (in parentheses) calculated over three independent XPS mea-
b
c
surements.
3. Results and discussion
3.1. Catalysts characterization
Table 1 gives the results of BET, XRD and XPS analyses of the
investigated catalysts. XRD patterns (Fig. S1 in the Supplemen-
tary material) demonstrate that the photo-deposition of platinum
nanoparticles did not modify the phase composition of the sup-
port, which consisted of pure anatase in both pure titania and
tungsten-containing titania samples. The crystallite dimensions,
calculated from XRD data by means of the Scherrer equation, indi-
cate a remarkable decrease in the particle size of anatase in samples
containing tungsten, with respect to pure titania (Table 1). At the
same time, the tungsten-containing titania samples had a much
larger BET surface area. The W/Ti percentage molar ratios, obtained
by XPS analysis, were in excellent agreement with the expected
values within the range of experimental uncertainty.
There was no evidence of a crystalline phase corresponding
to the formation of pure tungsten trioxide in the XRD spectra
of tungsten-containing samples, indicating that tungsten trioxide
may be amorphous or may be present in a concentration below the
detection limit of XRD (LOD = 1%). Moreover, tungsten ions may be
easily incorporated into the titania lattice, either by substituting
titanium ions to form W-O-Ti bonds or by being located at inter-
stitial sites. W6+ can substitute Ti4+ in the titania lattice, the ionic
radius of W6+ being 0.060 nm and that of Ti4+ 0.0605 nm [18].
The TEM micrographs of the TW0 and PtTW0 samples (Fig. 1)
confirm that pure titania crystallites are larger than 10 nm, in agree-
ment with the XRD results (Table 1). Nano-sized platinum particles
(2–5 nm) are visible after platinum photo-deposition (Fig. 1b).
EDX maps were acquired for the PtTW0 sample (Fig. S2 in the
Supplementary material) and confirm the presence of platinum
nanoparticles (Fig. S2c).
XPS characterization was then carried out to determine the
composition of all PtTW samples, and, in particular, to check the
presence of tungsten in PtTW1 and PtTW3 as well. This technique
detects low concentrations of tungsten and gives insight into the
chemical composition and oxidation state of surface elements.
Fig. 4 presents the X-ray photoelectron survey spectra. Signals
from all the elements in the catalysts (titanium, oxygen, plat-
inum and tungsten) were detected, together with small amounts
of carbon due to organic contamination. Table S1 (Supplementary
material) gives the surface composition of the materials. The W/Ti
ratios (Table 1) are in good agreement with the expected values
(
10% accuracy). Reproducible XPS analyses of the samples were
repeated 18 months after the first acquisition.
Fig. 1. TEM images of (a) TW0 and (b) PtTW0.

M. Carrus et al. / Applied Catalysis A: General 519 (2016) 130–138
133
Fig. 2. HAAF-STEM images of (a) TW1,(b) TW3, (c) TW5, (d) PtTW1, (e) PtTW3, (f) PtTW5.
Table 2
Binding energy (eV) of the main photoelectron peak determined by curve fitting after iterative Shirley-Sherwood background subtraction.a
Ti 2p3/2
Ti 3p
W 4f7/2
–
Pt 4f7/2
–
O 1sb
TW0
459.0 (0.1)
37.5 (0.1)
O
2− = 530.3 (0.1) − 79%
OH− = 531.5 (0.1) − 13%
H2Oads = 532.7 (0.1) − 8%
PtTW0
PtTW1
PtTW3
459.0 (0.1)
459.2 (0.2)
459.2 (0.2)
459.1 (0.1)
37.5 (0.1)
37.6 (0.2)
37.7 (0.2)
37.6 (0.1)
–
Pt0 = 70.7 (0.1)
PtIV = 73.9 (0.1)
O
2− = 529.7 (0.1) − 88%
OH− = 530.7 (0.1) − 7%
H2Oads = 531.7 (0.1) − 5%
36.2 (0.2)
36.1 (0.2)
36.1 (0.1)
Pt0 = 71.0 (0.2)
PtIV = 74.2 (0.1)
O (1) = 530.3 (0.1) − 81%
O (2) = 531.0 (0.1) − 12%
O (3) = 532.2 (0.1) − 7%
Pt0 = 71.0 (0.2)
PtIV = 74.4 (0.2)
O (1) = 530.3 (0.1) − 79%
O (2) = 531.0 (0.1) − 14%
O (3) = 532.3 (0.1) − 7%
PtTW5
Pt0 = 71.0 (0.1)
PtIV = 74.3 (0.2)
O (1) = 530.3 (0.1) − 78%
O (2) = 531.0 (0.1) − 14%
O (3) = 532.2 (0.1) − 8%
a
Binding energies are given as the mean of three or more independent measurements with the corresponding standard deviations (in parentheses).
The percentage O 1s photoelectron peaks is also reported.
b
Furthermore, high-resolution XPS spectra were acquired to
due to hydroxyl species [21] at 531.5 eV and a third peak at 532.7 eV
exploit the unique capability of XPS to provide information about
the chemical state of the elements. The spectra of Ti 2p, W 4d and of
W 4f, Pt 4f and O 1s were curve-fitted by means of model functions
with the same parameters as the pure oxides. Curve fitting param-
eters, such as the Gaussian-Lorentzian ratio of the product function
and the full width at half maximum of the peak height, were kept
constant during data processing, while the binding energy and the
area of the peaks were not. Table 2 gives the binding energy (BE)
of the main photoelectron signals together with the assignment to
the respective chemical states.
Ti 2p3/2 peaks (Fig. 4) of samples without W consist of a single
component at 459.0 eV. The single component is assigned to TiO2,
in agreement with literature data [19,20]. The BE shift in tungsten-
containing samples is within the experimental error.
Fig. 5 reports the O 1s and Pt 4f high-resolution XPS spectra. O 1s
signals in pure TiO2 have three components. The most intense one,
at 530.3 eV, is ascribed to O in TiO2, which also has a component
due to absorbed water (see TW0 in Table 2).
Upon photo-deposition of platinum nanoparticles, all O 1s com-
ponents undergo a shift to lower binding energy (sample PtTW0 vs
sample TW0), with the component at 531.5 eV shifting to 530.7 eV
(ꢁ = −0.8 eV) in PtTW0.
In tungsten-containing samples, the most intense O 1s compo-
nent is at 530.3 eV and is due to TiO2 together with a component
at 531.0 eV, which is ascribed to WO3, and a small shoulder at
532.2 eV. The intensity of the component at 531.0 eV increases, as
expected, with increasing tungsten content in the oxide support.
Pt 4f peaks (Fig. 5, right panels) consist of two doublets (Pt 4f7/2
and Pt 4f5/2), separated by 3.35 eV, with an intensity ratio of 4:3.
The most intense doublet at 71.0 0.2 eV, in agreement with val-
ues reported in the literature, is assigned to nano-sized platinum
supported on oxides such as SiO2 [22], TiO2 and WO3. Strong metal
support interactions occurred in such systems [23]. When platinum
nanoparticles smaller than 5 nm are not supported on oxides, there
is a positive shift [24] compared to platinum powders [25,26]. The

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M. Carrus et al. / Applied Catalysis A: General 519 (2016) 130–138
Fig. 3. EDX spectrum (lower panel), recorded by focusing on the red cross in the upper panel of the HAAF-STEM image of PtTW5. The copper signal originates from the copper
grid. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
doublet at about 74.4 eV is assigned to small amounts of PtO2 (PtIV
)
of the run. The production rate of 4-ethylnitrobenzene was always
higher than that of 4-ethylaniline. After a longer reaction time (not
shown), all 4-ethylnitrobenzene was converted into fully hydro-
genated 4-ethylaniline, the final reduction product. The carbon
balance was always close to 100%, indicating that almost no side
reactions occurred. Finally, only more or less negligible amounts
of 4-aminostyrene were detected during the runs. The exponential
decay and the initial high selectivity to 4-ethylnitrobenzene agree
with previous studies with a Pt/TiO2 catalyst [11], even though that
catalyst had smaller platinum particles (1-1.5 nm).
In contrast, there was high selectivity to 4-aminostyrene with
all W-containing PtTWx (x > 0) catalysts, together with a four-
fold higher conversion rate of 4-nitrostyrene (Table 3); significant
production of 4-ethylaniline was observed in parallel (Fig. 6b).
The concentration of 4-nitrostyrene decreased linearly, not expo-
nentially, which suggests that, with this catalyst, the surface
concentration of the reactant is much higher, resulting in zero-
order behavior. The best performance was achieved with PtTW1,
with 66% selectivity to 4-aminostyrene at 80% conversion, more
than six times higher than that obtained with PtTW0. When the
on the surface of the samples [24,27]. The PtIV/Pt0 ratio in PtTW0 is
0.14 and is affected by the presence of tungsten in the oxide sup-
port; the ratios in PtTW1, PtTW3 and PtTW5 are 0.22, 0.07 and 0.06,
respectively. A third component is visible in the Pt 4f region, labeled
Ti 3s SAT in Fig. 5. It falls at 76 eV and is a charge transfer shake-up
satellite accompanying the photoionization from the Ti 3s orbital
[28].
The most intense W peak, W 4f, is found at 36.1 eV; overlap with
the Ti 3p peak (BE = 37.6 eV) is shown in Fig. 4. The binding energy
of W 4f is in agreement with that for WO3 [29]. No shifts dependent
on the Ti/W ratio were observed.
3.2. Catalytic activity in 4-nitrostyrene hydrogenation
With the Pt/TiO2 (PtTW0) catalyst, the concentration of
4-nitrostyrene decreased exponentially over time, with no 4-
nitrostyrene left after about 60 min (Fig. 6a). The main product
was 4-ethylnitrobenzene and smaller amounts of 4-ethylaniline
were detected in the reaction system about 10 min after the start

M. Carrus et al. / Applied Catalysis A: General 519 (2016) 130–138
135
Fig. 4. XPS survey and high-resolution spectra of the investigated catalysts, in the Ti 2p, Pt 4f, Ti 3p and W 4f binding energy regions. X-ray Source: monochromatic Al K␣.
Table 3
Results of 4-nitrostyrene (S) hydrogenation at 80% conversion in terms of selectivity
ꢁ(-d[S]/dt)(PtTW1–PtTW0) = 0.0616 mmol g−1 s−1
to 4-aminostyrene (NH), formation rate of the latter (d[NH]/dt) and 4-nitrostyrene
conversion rate (-d[S]/dt).a
This indicates that the higher rate of 4-aminostyrene produc-
Sample Selectivity to NH d[NH]/dt (mmol g−1 s−1
)
−d[S]/dt (mmol g−1 s−1
)
tion (ꢁ(d[NH]/dt)) is close to the higher overall rate of substrate
conversion (ꢁ(-d[S]/dt)). Thus, not only the selectivity in nitro group
reduction is promoted by the presence of tungsten in the support,
but also the observed increase in the conversion rate of the sub-
strate (-d[S]/dt) is mainly due to the higher rate of 4-aminostyrene
production (d[NH]/dt).
After longer reaction times (not shown) with the PtTWx oxides,
4-aminostyrene was fully converted into the completely hydro-
genated 4-ethylaniline, with no phenyl-hydroxylamine derivatives
present throughout the reaction.
PtTW0 11%
PtTW1 66%
PtTW3 53%
PtTW5 45%
0.0016
0.057
0.039
0.027
0.017
0.078
0.070
0.050
a
Reaction conditions: 80 ◦C, 10 bar of hydrogen. 0.67 mmol of 4-nitrostyrene in
20 g of toluene, with 50 mg of catalyst, in a 50 cm3 autoclave.
content of tungsten is above 1%, the selectivity to 4-aminostyrene is
slightly lower, i.e. 53 and 45% with PtTW3 and PtTW5, respectively,
and the 4-nitrostyrene conversion rate is lower (see Table 3).
The higher selectivity to 4-aminostyrene with the tungsten-
containing catalysts was consistent not only with an increased rate
of 4-aminostyrene formation (see d[NH]/dt values in Table 3), but
also with an increase in the overall rate of substrate conversion (-
d[S]/dt). A comparison of the results with PtTW0 and PtTW1 gave:
3.3. Effects of the support oxide composition on the reaction rate
and selectivity
With all tungsten-containing catalysts the formation of 4-
ethylnitrobenzene, the main product obtained with PtTW0, took
place at a much lower rate than with PtTW0, i.e. the selectivities to
4-aminostyrene and to 4-ethylnitrobenzene were reversed (Fig. 6b
vs a).
ꢁ(d[NH]/dt)(PtTW1–PtTW0) = 0.0554 mmol g−1 s−1

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M. Carrus et al. / Applied Catalysis A: General 519 (2016) 130–138
Fig. 5. O 1s and Pt 4f high-resolution X-ray photoelectron spectra.
Furthermore, with PtTW1 full conversion of the 4-nitrostyrene
concentration with time (Fig. 6b), i.e. according to the zero order
rate law, not with a positive order as with the tungsten-free cata-
lyst (Fig. 6a). According to the Langmuir-Hinshelwood model, this
substrate was achieved within the first 15 min, i.e. in about a quarter
of the time required when the reaction was carried out with PtTW0.
Of particular importance is the linear decrease of 4-nitrostyrene

M. Carrus et al. / Applied Catalysis A: General 519 (2016) 130–138
137
Fig. 6. Evolution of the 4-nitrostyrene substrate (᭹), of the intermediates and product, 4-ethylnitrobenzene (ꢀ), 4-aminostyrene (ᮀ) and 4-ethylaniline (ꢁ), and carbon
balance (᭿) in the liquid phase hydrogenation of 4-nitrostyrene on (a) PtTW0 and (b) PtTW1. Reaction conditions: 80 ◦C, 10 bar of hydrogen. 0.67 mmol of 4-nitrostyrene in
20 g of toluene, with 50 mg of catalyst, in a 50 cm3 autoclave.
implies that there is greater coverage of the substrate on the active
sites of the catalyst in the tungsten-doped titania.
TW0 to 530.7 eV (−0.8 eV) in the presence of platinum nanoparti-
cles (PtTW0) and then further shifts back to 531.0 eV (+0.3 eV with
respect to PtTW0) in tungsten-containing PtTWx (x > 0) oxides.
Thus, oxygen atoms appear to play a role in platinum-tungsten
interactions, probably acting as W-O-Pt bridges between the two
metals.
The higher activity of tungsten-containing catalysts originates,
at least in part, from the greater specific surface area of the
tungsten-containing materials. This effect is common to titania
materials doped with different elements that stabilize the anatase
phase [2]. The larger surface allows a greater dispersion of smaller
platinum nanoparticles (Table 1). This is in line with the lower
Pt/Ti ratios for the tungsten-containing catalysts determined by
XPS analysis (Table S1) and is confirmed by the images shown in
Figs. 1 and 2.
As demonstrated by Corma et al. [3] by carbon monoxide
adsorption followed by IR spectroscopy, such smaller platinum
nanoparticles have a smaller fraction of metal atoms in exposed
faces with respect to atoms in the corner of crystallites. The same
authors demonstrated that the rate of nitrobenzene hydrogenation
on Pt/Al2O3 is slightly affected by the amount (and consequently
the size) of platinum particles on the alumina surface, while the rate
of styrene hydrogenation increases with the amount of platinum on
alumina, i.e. with increasing the fraction of facets on the platinum
nanoparticles, where preferential adsorption through the aromatic
and C C moieties may occur. Thus in substituted nitroaromatics
containing olefinic groups, such as 4-nitrostyrene, the chemose-
lectivity towards the reduction of the nitro group increases when
the size of the platinum particles decreases. This contributes to
the higher selectivity to 4-aminostyrene of the tungsten-containing
catalysts, where smaller-sized platinum nanoparticles were photo-
deposited.
Reduction of PtIV and WVI may occur under reaction conditions.
It is worth underlining, however, that the best performing PtTW1
catalyst (Table 3) in terms of both 4-nitrostyrene conversion and
selectivity to 4-aminostyrene exhibits the highest PtIV/Pt0 ratio.
A possible explanation of the maximum activity and selectivity
attained with this catalyst is provided by the in-depth distribution
analysis of platinum, which can be estimated non-destructively by
XPS, by calculating the Pt 4f to Pt 4d area ratios, i.e. with an analysis
based on two different core lines of the same element. The Pt 4d
peaks are found at a kinetic energy of 1173.6 eV (BE 313 eV), the
Pt 4f peaks at 1412.6 eV (BE 74 eV). Thus, the Pt 4f signals provide
information to a greater depth than those of Pt 4d.
The area ratios of the two XPS signals were found to be 0.8 for
sample PtTW0, in agreement with the theoretical value, 0.6 for sam-
ple PtTW1, 0.5 for PtTW3 and 0.4 for PtTW5. This finding seems to
support the fact that the higher the tungsten content, the thicker
the layer through which the Pt 4f photoelectrons travel until they
leave the sample. These results lead to the conclusion that WOx
is not only dispersed on the support, but it also covers the plat-
inum nanoparticles. Assuming a core-shell model (about 2 nm Pt
nanoparticles diameter), the thickness of the shell is estimated to
range from 1.8 nm for PtTW1 to 3.0 nm for PtTW5. Thus the high-
est activity and selectivity obtained with the PtTW1 sample result
from the preferential adsorption of 4-nitrostyrene through the nitro
group on small platinum particles, which are only slightly covered
by the WOx surface layer. With increasing the tungsten content of
the catalysts, the WOx layer on platinum nanoparticles active sites
would increase, and both activity and selectivity are expected to
consequently decrease.
Moreover, apart from size effects of platinum nanoparticles,
the presence of tungsten in such catalysts also leads to a lower
hydrophilicity of the oxide surfaces, as demonstrated by the dis-
appearance of most of the OH surface groups, replaced by oxygen
linked to tungsten (see Table 2). This indicates that in tungsten-
containing oxides, tungsten not only substitutes titanium in the
anatase lattice (Fig. 2), but is also present as WOx species on the
mixed oxide surface, which possibly affect the size of the plat-
inum particles photo-deposited on the mixed oxide support and
the 4-nitrostyrene adsorption mode.
4. Conclusions
XPS analysis also indicate that PtIV is stable in the metal
nanoparticles when tungsten is present in the oxide support. In
the XPS spectra measured after exposure to air, the signal of PtIV
shifts to positive binding energy (+0.4 eV) when tungsten is present
in the support oxide (Table 2). Furthermore, whereas the BE of the
W 4f signal appears to be unaffected by the smaller amounts of
tungsten in the oxide support (Table 2), the O 1s signal, assigned
to the hydroxyl groups bound to titania, shifts from 531.5 eV in
Selective hydrogenation of 4-nitrostyrene to 4-aminostyrene is
achieved by using platinum supported on tungsten oxide-doped
titania. The presence of tungsten enhances the rate of reduc-
tion of the nitro group and suppresses that of the olefinic group,
thereby changing the reaction from positive to zero order in 4-
nitrostyrene. Tungsten stabilizes the smaller particle size of both
titania and platinum, enhancing surface activity and the formation
of Pt nanoparticles with a high number of metal sites in corners

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M. Carrus et al. / Applied Catalysis A: General 519 (2016) 130–138
and axes, on which 4-nitrobenzene preferentially adsorbs through
the nitro group. The best performance, in both reaction rate and
selectivity to 4-aminostyrene, of the PtTW1 mixed oxide support
catalyst with a 1% W/Ti molar ratio results from an optimal com-
bination of high surface area, small-sized platinum nanoparticles
allowing the preferential adsorption of 4-nitrostyrene through the
nitro group and low coverage of amorphous WOx species on the
platinum active sites.
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This work originates from the stage that MC made at the ICB of
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Department of Chemistry of the University of Milan. The authors are
grateful to Prof. Nicholas Spencer (Laboratory for Surface Science
and Technology, ETH Zurich) for access to the Surface Analytical
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spectrometers. Our thanks also go to the Electron Microscopy Cen-
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The Sardinia Regional Government is gratefully acknowledged
for financial support (P.O.R. Sardegna F.S.E. Operational Program
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2007–2013-Axis IV Human Resources, Objective 1.3, Line of Activ-
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Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.03.
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