Hydrogenation of α, β unsaturated aldehydes over polycrystalline, (111) and (100) preferentially oriented Pt nanoparticles supported on carbon

Article abstract of DOI:10.1016/j.jcat.2007.10.010

The influence of the shape/surface structure of Pt nanoparticles on the selective hydrogenation of crotonaldehyde and cinnamaldehyde has been studied. (111) and (100) preferentially oriented Pt nanoparticles (10 nm) as well as polyoriented Pt nanoparticles (3 nm) were synthesized, characterized (by TEM, cyclic voltammetry and adsorption microcalorimetry) and their catalytic properties evaluated. TEM analysis provided information about the size and shape of the Pt nanoparticles, whereas cyclic voltammetry allowed gaining qualitative and quantitative information about their surface structure. Thus, small Pt nanoparticles (≈ 3 nm) were revealed to have a polyoriented surface, containing high ratio of corner and edges atoms to terrace atoms, whereas large Pt nanoparticles (≈ 10 nm) were shown to have larger Pt domains with (100) and (111) surface structures. Microcalorimetric results for CO adsorption showed higher values of initial heat for polyoriented Pt/C compared to preferentially oriented samples, thus accounting for a higher amount of highly unsaturated surface platinum atoms for Pt/C, in agreement with cyclic voltammetry. The catalytic performances of the samples showed a strong structure-sensitive character for both reactions, with TOF values following the trend Pt(100)/C > Pt(111)/C > Pt/C. Moreover, Pt(111)/C showed higher selectivities to unsaturated alcohol than Pt(100)/C and Pt/C samples.

Full text of DOI:10.1016/j.jcat.2007.10.010

Journal of Catalysis 253 (2008) 159–166
www.elsevier.com/locate/jcat
Hydrogenation of α, β unsaturated aldehydes over polycrystalline,
(111) and (100) preferentially oriented Pt nanoparticles supported on carbon
J.C. Serrano-Ruiz a, A. López-Cudero b, J. Solla-Gullón b, A. Sepúlveda-Escribano a,∗, A. Aldaz b,
F. Rodríguez-Reinoso a
a
Departamento de Química Inorgánica, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain
b
Instituto Universitario de Electroquímica, Universidad de Alicante, Apartado 99, 03080 Alicante, Spain
Received 20 June 2007; revised 21 September 2007; accepted 12 October 2007
Available online 26 November 2007
Abstract
The influence of the shape/surface structure of Pt nanoparticles on the selective hydrogenation of crotonaldehyde and cinnamaldehyde has
been studied. (111) and (100) preferentially oriented Pt nanoparticles (10 nm) as well as polyoriented Pt nanoparticles (3 nm) were synthesized,
characterized (by TEM, cyclic voltammetry and adsorption microcalorimetry) and their catalytic properties evaluated. TEM analysis provided
information about the size and shape of the Pt nanoparticles, whereas cyclic voltammetry allowed gaining qualitative and quantitative information
about their surface structure. Thus, small Pt nanoparticles (≈3 nm) were revealed to have a polyoriented surface, containing high ratio of corner
and edges atoms to terrace atoms, whereas large Pt nanoparticles (≈10 nm) were shown to have larger Pt domains with (100) and (111) surface
structures. Microcalorimetric results for CO adsorption showed higher values of initial heat for polyoriented Pt/C compared to preferentially ori-
ented samples, thus accounting for a higher amount of highly unsaturated surface platinum atoms for Pt/C, in agreement with cyclic voltammetry.
The catalytic performances of the samples showed a strong structure-sensitive character for both reactions, with TOF values following the trend
Pt(100)/C > Pt(111)/C > Pt/C. Moreover, Pt(111)/C showed higher selectivities to unsaturated alcohol than Pt(100)/C and Pt/C samples.
© 2007 Elsevier Inc. All rights reserved.
Keywords: Nanoparticles; Platinum; Unsaturated aldehydes; Preferential surface orientation
1. Introduction
affected by the local surface structure (relative concentration
of steps, terraces, etc.) [5]. Nevertheless, the effect of metal
nanoparticle size on catalytic or electrocatalytic activities is bet-
ter understood at present than the influence of the shape/surface
structure, since size control of the nanoparticles has gener-
ally been easier than shape/surface structure control. In spite
of that, some papers have demonstrated that it is possible to
synthesize quasi-monodispersed Pt nanoparticles of controlled
particle shape and that the catalytic or electrocatalytic prop-
erties of these nanoparticles are under shape/surface structure
control. Thus, El-Sayed et al. have reported about the effect
of the shape of Pt nanoparticles on their catalytic properties
[6–11]. Reactions such as propylene hydrogenation, electron-
transfer and Suzuki reactions have revealed the importance of
controlling the shape of the nanoparticles. Somorjai et al. have
also shown the importance of the shape/surface structure using
ethylene hydrogenation and carbon monoxide adsorption and
oxidation as surface reaction probes [12,13]. Aldaz et al. have
The influence of the surface structure of a metal particle on
its catalytic and electrocatalytic properties has been extensively
reported [1–3]. Nevertheless, in the case of nanoparticles, the
effect of the surface structure has been generally related to the
so-called “size effect.” Thus, some reactions have been shown
to be strongly dependent on the particle size, whereas others
are independent of the particle size. Boudart, in 1969, classi-
fied the first type of reactions as structure sensitive, and the
second type as structure insensitive [4]. Later studies related
the structure sensitive character of a given reaction to the struc-
ture sensitive character of the chemical bond formed between
the reactant and the catalyst surface, the strength of which is
*
Corresponding author. Fax: +34 965 90 34 54.
E-mail address: asepul@ua.es (A. Sepúlveda-Escribano).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.10.010

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also reported the influence of the particle shape/surface struc-
ture on the electrocatalytic properties of Pt nanoparticles using
ammonia oxidation and carbon monoxide adsorption–oxidation
as model reactions [14,15].
2.5 × 10−4 M tri-sodium citrate was prepared. Then, the de-
sired amount of ice cold 0.1 M NaBH4 solution was added all
at once with continuous stirring. After 30 min, an appropriate
amount of carbon black (Vulcan XC-72) was added to the so-
lution and stirred for 2 h. Finally, two or three NaOH pellets
were added to produce the precipitation of the nanoparticles on
the carbon support. After complete precipitation, the supported
nanoparticles were washed 3–4 times with ultra-pure water.
On the other hand, preferentially oriented nanoparticles
were prepared by using the so-called colloidal method [16,17].
Briefly, a variable amount of sodium polyacrylate (PA) aque-
From the point of view of the synthesis, and since the pi-
oneer work of El-Sayed et al. [16], some contributions have
appeared in relation to the preparation of nanoparticles with
controlled shape [17–19]. These new methodologies have been
mainly focused on the synthesis of Au and Pt nanoparticles with
preferential shape/surface structure, due to the great number of
reactions where this kind of materials can be employed.
An interesting model reaction is the hydrogenation of
α, β-unsaturated aldehydes (acrolein, crotonaldehyde, cin-
namaldehyde, etc.). Here, the selective hydrogenation of the
carbonyl group, which is preferred because the unsaturated
alcohols are valuable intermediates for the production of per-
fumes and pharmaceuticals, is very complicate, and with usual
hydrogenation catalysts complete hydrogenation (both C=C
and C=O groups) to yield the primary alcohol or selective hy-
drogenation of the olefin bond to give the saturated aldehyde are
often obtained. Low selectivity toward crotyl alcohol is gener-
ally obtained with Pt supported catalysts, although it can be
increased by using different approaches, such as an increment
of the platinum particle size and by the use as support of a re-
ducible oxide such as TiO2 [20–22]. An increase in selectivity
toward the unsaturated alcohol in the liquid phase hydrogena-
tion of crotonaldehyde over Pt nanoparticles supported on SiO2
and Al2O3 (from 9 to 35%) has been obtained by increasing
their particle sizes (from 2 to 6 nm) [21]. It has been shown
that the selectivity to the primary products, butyraldehyde and
crotyl alcohol, critically depends on the Pt particle size [20].
The presence of a high fraction of Pt(111) surfaces in large Pt
nanoparticles favors the adsorption of crotonaldehyde via the
carbonyl bond, this leading to its preferential hydrogenation to
yield the unsaturated alcohol, the product of interest. On the
other hand, both double bonds can be adsorbed without any re-
striction on Pt(100) and on edges and corners [20]. Under these
conditions, the saturated aldehyde is preferentially produced,
as the hydrogenation of the C=C bond is favored by both ther-
modynamic and kinetics reasons. In this way, the synthesis of
well-defined shape Pt nanoparticles represents a key point for
the design of more active and selective catalysts for this kind of
reactions.
ous solution was added to a 100 ml of an aged 1 × 10−4
M
solution containing the desired Pt precursor. Thus, K2PtCl4
was employed for the synthesis of the “(100)-Pt” nanoparticles
whereas H2PtCl6 was used for the “(111)-Pt” nanoparticles. Af-
ter 20 min of Ar bubbling, the Pt ions were reduced by bubbling
H2 for 5 min. The reaction vessel was then sealed and the so-
lution was left overnight. After complete reduction (12–24 h),
an appropriate amount of carbon (Vulcan XC-72) was added
and stirred for two hours. After that, two or three NaOH pel-
lets were added to produce the precipitation of the supported
nanoparticles. After complete precipitation, the nanoparticles
were washed 3–4 times with ultra-pure water. All samples were
prepared to obtain a platinum loading of 5 wt%. The particle
size and morphology of all the supported nanoparticle sam-
ples were characterized by transmission electron microscopy
(TEM).
For the electrochemical experiments, and due to the low
Pt metal loading (5 wt%), unsupported Pt nanoparticles were
employed. The procedure used for the electrochemical char-
acterization of the nanoparticles has been previously reported
[24–26]. The electrochemical measurements were performed in
a 0.5 M H2SO4 solution at room temperature. Fresh electrolyte
solutions were prepared every time an experiment was carried
out from Milli-Q water and Merck sulphuric acid. A three-
electrode electrochemical cell was used. The electrode potential
was controlled using a PGSTAT30 AUTOLAB system. The
counter electrode was a gold wire. Potentials were measured
against a reversible hydrogen electrode (RHE) connected to the
cell through a Luggin capillary. Potentials are quoted versus this
reference electrode. Before each experiment, the gold collec-
tor was mechanically polished with alumina and rinsed with
ultra-pure water to eliminate the nanoparticles from previous
experiments.
Differential heats of CO adsorption were measured at 300 K
in a Setaram BT2.15D heat-flux microcalorimeter using the
method described in detail elsewhere [27]. The microcalorime-
ter was connected to a high vacuum (base pressure <10−5 Torr)
volumetric system employing Baratron capacitance manome-
ters for precision pressure measurement (0.5 × 10−4 Torr). The
maximum apparent leak rate of the volumetric system (includ-
ing the calorimetric cells) was 10−5 Torr min−1 in a system vol-
ume of approximately 80 cm3 (i.e., 10−5 µmol min−1). The pro-
cedure for microcalorimetric measurements used in this study
is described below: each sample (about 0.25 g) was treated ex
situ in ultrapure hydrogen (99.999% with further purification,
Air Liquide) for 1 h (2 K min−1 ramp, 200 ml min−1) at 473 K
Thus, the aim of this work has been to study the hydrogena-
tion of crotonaldehyde and cinnamaldehyde over polyoriented
and (100) and (111) preferentially oriented carbon-supported Pt
nanoparticles, in order to assess the structure sensitive character
of these reactions on platinum.
2. Experimental
Two different protocols have been used to prepare Pt
nanoparticles. Polyoriented Pt nanoparticles were synthesized
using sodium borohydride as reducing agent in the presence
of citrate and using a methodology similar to that previously
reported for the preparation of gold seed nanoparticles [23].
An aqueous solution containing 2.5 × 10−4 M H2PtCl6 and

J.C. Serrano-Ruiz et al. / Journal of Catalysis 253 (2008) 159–166
161
and then the sample was purged for 1 h at the same temperature
in ultrahigh purity helium (99.999% with further purification,
Air Liquide) in order to remove adsorbed hydrogen. Then, it
was sealed in a Pyrex NMR tube capsule and broken in a spe-
cial calorimetric cell [28] after the sample had attained thermal
equilibrium with the calorimeter. After the capsule was broken,
the calorimetric data were collected by sequentially introducing
small doses (1–10 µmol) of CO (99.997% with further purifica-
tion, Air Liquide) onto the sample until it became saturated.
The resulting heat response for each dose was recorded as a
function of time and integrated to determine the energy released
(mJ). The amount of gas adsorbed (µmol) was determined volu-
metrically from the dose, equilibrium pressures, and the system
volume and temperature. The time required for the pressure to
equilibrate in the microcalorimeter after each dose was approx-
imately 1–15 min, and the heat response was monitored for
20–30 min after each dose to ensure that all heat was detected
and to allow the heat response to return to the baseline value.
The differential heat (kJ mol−1), defined as the negative of the
enthalpy change of adsorption per mole of gas adsorbed, was
then calculated for each dose by dividing the heat released by
the amount adsorbed.
lyzed by gas chromatography using a flame ionization detector
and a Carbowax 20 M 58/90 semicapillary column.
3. Results and discussion
3.1. Transmission electron microscopy (TEM)
Fig. 1 shows some typical TEM images of the supported
and unsupported Pt nanoparticles prepared in this work. Plat-
inum nanoparticles prepared in the presence of citrate, A1 (sup-
ported) and A2 (unsupported), show a quasi-spherical shape
and an average particle size of 2.5 0.5 nm. In addition, a ho-
mogeneous distribution of the Pt nanoparticles on the carbon
support can be clearly observed, whereas some agglomeration
was detected for the unsupported nanoparticles.
On the other hand, preferentially oriented Pt nanoparticles
(images B and C) show well-defined shapes, which can be bet-
ter observed in the unsupported samples (B2 and C2), as in this
condition the influence of the support not only in the contrast
of the image but also in the dispersion of the Pt nanoparticles,
can be avoided. Thus, the presence of cubic-shaped nanopar-
ticles in clearly shown in image B2, which corresponds to the
(100)-Pt nanoparticles. In this sample, particle sizes are around
The catalytic behavior of the samples in the vapor-phase
hydrogenation of crotonaldehyde (2-butenal [CROALD]) was
tested in a microflow reactor at atmospheric pressure under dif-
ferential conditions. Before the catalytic experiments, the cata-
lysts (around 50 mg) were reduced in situ at 473 K (2 K min−1
ramp) under flowing hydrogen (50 ml min−1) for 1 h, and then
cooled under hydrogen to the reaction temperature (333 K).
Then, the catalysts were contacted with a reaction mixture (to-
tal flow: 50 cm3 min−1; H2/CROALD molar ratio of 26) con-
taining purified hydrogen and crotonaldehyde (Fluka, >99.5%)
prepared by passing a hydrogen flow through a thermosta-
bilized saturator (at 293 K) containing the unsaturated alde-
hyde. The selectivity to the desired product was calculated as
Si = (moles of producti/moles of products)×100. The concen-
trations of the reactants and the products at the reactor outlet
were determined by on-line gas chromatography with a Car-
bowax 20 M 58/90 semicapillary column.
9
3 nm. The presence of these Pt nanoparticles with a cu-
bic shape is a first indication of the higher presence of (100) Pt
atoms on the surface. Images C correspond to (111)-Pt nanopar-
ticles. In the unsupported samples (C2) some tetrahedral and
The liquid phase hydrogenation of cinnamaldehyde was car-
ried out in a 300 ml stirred autoclave (Autoclave Engineers, fit-
ted with a system for liquid sampling) at 383 K and at constant
pressure of 5 MPa of hydrogen. Before the catalytic experi-
ments, the catalysts (100 mg) were reduced at 473 K (2 K min−1
ramp) under flowing hydrogen (50 ml min−1, 99.999%) for 1 h,
and suspended in 60 ml of 2-propanol (Riedel-de Haën, 99.9%)
previously bubbled with helium for 20 min in order to remove
the solved oxygen. After three pressure/vent cycles with helium
and three more with hydrogen, the temperature was raised to
383 K under 2 MPa of hydrogen and remained at this tempera-
ture for 2 h. Then a mixture of substrate (0.5 ml of cinnamalde-
hyde, Aldrich 99%) and isopropanol (40 ml) was loaded into
the autoclave through an external cylinder (previously flushed
with helium and hydrogen) under 8 MPa hydrogen pressure.
Zero time was taken at this moment and stirring was switched
on (800 rpm). Reaction progress with time was followed taking
periodically liquid samples of the reactor. The liquid was ana-
Fig. 1. TEM pictures corresponding to supported (1) and unsupported (2) Pt
nanoparticles: polyoriented (A1, A2), preferentially oriented (100) (B1, B2),
and preferentially oriented (111) (C1, C2) surface orientation.

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J.C. Serrano-Ruiz et al. / Journal of Catalysis 253 (2008) 159–166
hexagonal Pt nanoparticles can be observed, with particle sizes
around 10 2 nm. The existence of tetrahedral and hexagonal
Pt nanoparticles is again a first evidence of a higher presence
of Pt surface atoms with a (111) ordering. It is necessary to re-
mark that all samples were reduced in situ, at 473 K under a
hydrogen flow for 1 h before microcalorimetric and catalytic
experiments. It was assessed by TEM that no relevant size and
shape modification was produced during this pre-treatment.
tion and the symmetry of the adsorption states are indicative
of the surface cleanliness. Fig. 2a is characteristic of the small
semi-spherical Pt nanoparticles prepared in presence of citrate.
The voltammogram looks similar to that reported for polycrys-
talline platinum electrodes. Thus, the voltammogram shows the
presence of adsorption states associated to (110) and (100) sites
at 0.12 and 0.27 V, respectively. Moreover, a shoulder around
0.35 V is apparent, being characteristic of (100) terraces. In ad-
dition, the unusual adsorption state around 0.5 V, characteristic
of small (111) ordered surface domains, can be observed.
On the other hand, Figs. 2b and 2c show some characteristic
voltammetric profiles of “(100)-Pt” and “(111)-Pt” nanoparti-
cles prepared in colloidal solution. For “(100)-Pt” nanoparticles
(Fig. 2b) a very sharp peak at 0.27 V is observed. In addi-
tion, the well-marked state at 0.37 V, typical of (100) terrace
sites, is clearly defined. These results point out that these Pt
nanoparticles have a (100) preferential surface structure. More-
over, it is important to note that part of these Pt(100) sites are
present at the surface as wide (100) terraces as suggested by
the peak at 0.37 V. These electrochemical observations are in
agreement with previous results obtained by Ahmadi et al., who
pointed out that the nanoparticles obtained are preferentially cu-
bic [16,17].
In the case of the “(111)-Pt” nanoparticles (Fig. 2c), some
important features must be noted. First at all, the adsorption
state around 0.5 V, characteristic of small (111) ordered sur-
face domains, is much more clearly marked than in the previous
cases. This feature is directly related to the presence of bidimen-
sionally ordered (111) domains. On the other hand, the sharp
peaks at 0.12 and 0.26 V are also present in the voltammetric
profile. These sharp peaks are observed on stepped surfaces vic-
inal to Pt(111) with monoatomic steps with (110) or (100) sites,
respectively. Finally, a well-marked adsorption state at 0.37 V
points out that these nanoparticles have also large (100) do-
mains. Similar voltammetric profiles were reported by Attard
et al. [31] after a sintering process of the Pt/graphite samples.
The surface structure of similar Pt nanoparticles, both pref-
erentially and poly oriented have been previously electrochem-
ical characterized using the irreversibly adsorptions of Bi and
Ge, where Bi is able to identify the presence of Pt(111) sur-
face sites whereas Ge is sensitive to the presence of Pt(100)
surface sites [32–35]. In these references, quantitative informa-
tion about the presence of some characteristic types of surface
sites (Pt(111) and Pt(100)) on the surface of Pt nanoparticles
was gained. In addition, according to the surface stoichiom-
etry, where each Bi blocks three Pt(111) sites whereas each
Ge blocks four Pt(100) sites, information about the percentage
of small Pt surface domains as well as the presence of edges
and corner surface sites which can not be detected by Bi and
Ge and where low coordination Pt atoms are presented, could
also be estimated. Thus, in the case of the small polyoriented
Pt nanoparticles only a 30–35% of the surface atoms could
be detected, whereas for large and preferentially oriented Pt
nanoparticles around a 70–90% of the total surface atoms could
be identified. These results indicate that, for the small polyori-
ented Pt nanoparticles, the amount of small surface domains
and low coordination surface atoms is significantly higher than
3.2. Electrochemical characterization
A very good characterization of the structure of a single
nanoparticle can be obtained by ex situ HRTEM after simu-
lation of the images [29]. However, the analysis of a statisti-
cally significant number of nanoparticles with such a technique
seems, currently, unrealistic. Moreover only partial information
can be obtained for a given sample of nanoparticles and the real
global distribution of surface sites in the nanoparticles could not
coincide with the partial distribution found using TEM.
In order to characterize in situ the shape/surface structure of
the Pt nanoparticles, a surface sensitive technique or process
has to be used. Electrochemistry provides some surface sen-
sitive reactions that can be used as tool to characterize the
surface structure. In the case of platinum, it is known that the
so-called hydrogen adsorption/desorption process is very sen-
sitive to the Pt surface structure [30] and this fact is put in
evidence in the voltammetric profiles shown in Fig. 2. These
profiles correspond to the so-called reversible hydrogen ad-
sorption/desorption process on nanoparticles synthesized using
different methods. The voltammetric profile works as a finger-
print for any Pt surface. However, in a general way, the direct
estimation of the amount of different sites present at the surface
is not feasible, since it would require a too complex deconvo-
lution of the voltammograms. Nevertheless with the help of Pt
model surfaces it is possible to qualitatively characterize each
feature of the voltammetric profile.
Fig. 2 shows the characteristic voltammetric profiles of the
different Pt nanoparticles in 0.5 M sulfuric acid. The defini-
Fig. 2. Voltammograms corresponding to (a) poly, (b) (100), and (c) (111) Pt
−1
nanoparticles. Test solution: 0.5 M H SO , sweep rate 50 mV s
.
2
4

J.C. Serrano-Ruiz et al. / Journal of Catalysis 253 (2008) 159–166
163
in the case of the preferentially oriented samples. With regard
to preferentially oriented samples, irreversible adsorptions of Bi
and Ge allowed to detect only the 70% of surface platinum sites
in the case of Pt(111), whereas this value was increased until a
level of 90% for Pt(100). This finding reveals the presence of
a higher amount of small domains and low coordination sur-
face platinum atoms in the case of Pt(111)/C catalyst, which
seems to be in agreement with the voltammetric results dis-
cussed above.
described in the TEM section. Although the CO adsorption
capacity was similar for both samples, important differences
were found in initial adsorption heats. Thus, Pt(111)/C showed
a unusual high initial heat (107 kJ mol−1), close to the typi-
cal values for polycrystalline Pt/C catalysts [36,39]. This result
can be tentatively explained on the basis of a higher number
of highly unsaturated platinum atoms present in this sample. In
fact, the results of irreversible adsorption of Bi and Ge revealed
the presence of a higher amount of low coordination surface
atoms in Pt(111)/C as compared to Pt(100)/C, although this fact
does not seem to be important enough to explain by itself the
difference in initial heats (107 vs 90 kJ mol−1). Furthermore,
Pt(111)/C showed higher adsorption heats than Pt(100)/C in
all range of CO coverages studied, not only in the initial re-
gion. The explanation for this behavior can be found taking into
account the TEM and voltammetric results. Thus, Pt(100) par-
ticles showed a well defined cubic shape (Fig. 1B) with wide
domains of (100) terraces, whereas Pt(111) nanoparticles pre-
sented a more faceted shape (Fig. 1C) with small (111) ordered
surface domains and a noticeable amount of stepped surfaces
and monoatomic steps connecting the (111) facets (prominent
peak at 0.12 V in Fig. 2c). Thus, the presence of a larger
amount of these linking sites with lower coordination number in
Pt(111)/C could account for the higher adsorption heats showed
by this sample.
3.3. CO adsorption microcalorimetry
Fig. 3 plots the evolution of the differential adsorption heats
with CO coverage for catalysts previously reduced at 473 K. It
can be observed in all cases that the heat of adsorption decreases
as the CO coverage increases, this being due to adsorption on
weaker sites and/or to interactions between adsorbed species.
The drastic drop in the differential heat at higher coverage is
indicative of saturation of the accessible platinum metallic sur-
face sites. The polycrystalline Pt/C catalyst showed an initial
adsorption heat of 115 kJ mol−1, which is in good agreement
with the value reported by Silvestre-Albero et al. [36] for a
0.2 wt% Pt/C catalyst prepared by a conventional impregna-
tion method, and using H2PtCl6 as platinum precursor. Adsorp-
tion heats hardly decreased until a coverage of 12 µmol g−1
,
and then they suddenly dropped to the weakly adsorbed CO
level, taken as 40 kJ mol−1 [37]. Consequently, the saturation
coverage for this sample (amount of CO necessary to totally
saturate the platinum surface) was around 20 µmol g−1. It is in-
teresting to observe that polycrystalline Pt/C catalyst showed a
higher initial heat than preferentially oriented samples. It is well
known that the heat of adsorption increases with decreasing av-
erage coordination number of the surface metal atoms, due to
preferential adsorption on low coordination sites like edges and
corners [38]. Thus, this finding would indicate that the amount
of highly unsaturated platinum atoms and open faces in this cat-
alyst is higher than in the preferentially oriented samples, which
is compatible with the voltammetric and irreversible Bi and Ge
adsorptions results described in the previous section.
The heat-coverage profiles were different for preferentially
oriented Pt(100)/C and Pt(111)/C catalysts. The differential
heats strongly decreased with coverage and, in both cases, the
saturation coverage was reached at about 5 µmol g−1. This re-
sult is coherent with the higher platinum particle size obtained
for these samples (≈10 nm) as compared to Pt/C (≈3 nm),
3.4. Catalytic behavior
3.4.1. Crotonaldehyde hydrogenation
The performance of the catalysts (previously reduced at
473 K) was tested in the vapor phase hydrogenation of croton-
aldehyde at 333 K. The evolution of catalytic activity (TOF,
calculated in basis of CO uptakes at 300 K) with time on
stream for the three catalysts studied is showed in Fig. 4. It
can be seen that, after a period of 20 min with a strong de-
activation, the activity remained quite stable with time. This
fast deactivation of the catalysts is typical of this reaction [40]
and has been previously related to the decarbonylation of the
reactant molecule yielding carbon monoxide, which is irrever-
sively adsorbed on platinum at the reaction conditions [20]. It
can be clearly appreciated that the specific activity strongly de-
pends on the platinum face exposed, which is a clear indicative
of the structure-sensitive character of this reaction [41]. Fur-
Fig. 3. Differential heats of CO adsorption at 300 K vs CO coverage for Pt/C,
Pt(111)/C, and Pt(100)/C catalysts, previously reduced at 473 K.
Fig. 4. TOF in crotonaldehyde hydrogenation at 333 K as a function of time on
stream for Pt/C, Pt(111)/C, and Pt(100)/C catalysts.

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thermore, it is interesting to observe that polycrystalline Pt/C
showed lower TOF values than preferentially oriented cata-
lysts. Englisch et al. [20], in a very complete work on structure
sensitivity of crotonaldehyde hydrogenation over Pt/SiO2, also
found lower activity for catalysts with smaller metal particles,
and this behavior was attributed to a faster deactivation caused
by strong adsorption of crotonaldehyde on low coordination
sites like edges and corners. However, this fact has not been
detected in our case, and deactivation profiles were very simi-
lar for the three catalysts under study. In the same way, Singh
et al. [42] found a 25-fold increase in TOF for citral hydro-
genation as the average platinum crystallite size increases from
1 to 5 nm. Again, the higher participation of low coordination
number surface atoms as the metal particle size decreases was
invoked to explain this behavior. Note that these highly exposed
atoms are assumed to be the most abundant on the surface of
Pt/C as indicated by voltammetric and microcalorimetric results
(see Figs. 2a and 3).
Fig. 5. Reaction scheme for cinnamaldehyde hydrogenation.
With regards to selectivity, the main product formed in all
cases was the saturated aldehyde (butanal). Thus, the selectivity
to the unsaturated alcohol was almost negligible for polycrys-
talline Pt/C and Pt(100)/C catalysts while Pt(111)/C showed
certain selectivity to crotyl alcohol (4% molar fraction). It is
well known that there is an effect of the metal morfology on
the adsorption geometry of unsaturated aldehydes, leading to
different types of reaction pathways and finally, different selec-
tivities [43]. The theoretical calculations of Delbecq et al. [44]
suggest that, in the case of (100) and open faces or steps, croton-
aldehyde adsorption involves both double bonds, in a so-called
η4 planar coordination, this geometry leading to the preferen-
tial hydrogenation of the C=C bond for kinetic reasons. This
fact would explain the low selectivity toward unsaturated alco-
hol found for Pt(100)/C and Pt/C catalysts, where the fraction
of (100) facets and low coordinated atoms, respectively, is very
high. On the other hand, the dense (111) metal faces are not
favorable for the C=C coordination, and crotonaldehyde ad-
sorption via carbonyl bond (di-σC=O) is energetically favored.
Thus, a greater participation of these faces in the catalysts sur-
face would improve the selectivity to unsaturated alcohol. The
low value of selectivity obtained for Pt(111)/C in our study
could be due to the presence of a noticeable fraction of stepped
surfaces vicinal to (111) small domains (see prominent peaks
at 0.12 and 0.26 V in Fig. 2c) since, in these surfaces, the
crotonaldehyde coordination is shifted to a πC=C mode [43].
Furthermore, voltammetric results showed that the surface of
Pt(111)/C is not exclusively participated by (111) facets and
the presence of large (100) domains (peak at 0.37 V in Fig. 2c)
could also be responsible of the low value of selectivity to un-
saturated alcohol.
Fig. 6. Conversion of cinnamaldehyde as a function of time for Pt/C, Pt(111)/C,
and Pt(100)/C catalysts.
Table 1
Rate constant, TOF and selectivity to the saturated aldehyde and the unsatu-
rated alcohol at 10% conversion in cinnamaldehyde hydrogenation for Pt/C,
Pt(111)/C, and Pt(100)/C catalysts
−1
−1
a
Catalyst
Rate constant (min
)
TOF (s
)
Selectivity (%)
HALD
CALC
Pt/C
Pt(100)/C
Pt(111)/C
0.016
0.008
0.005
0.003
0.040
0.020
82
61
49
16
36
48
a
Values at 10% conversion.
cinnamaldehyde as a function of reaction time for the three cat-
alysts under study. Initial activities were expressed in term of
rate constant (min−1), which were obtained from the slope of
the cinnamaldehyde concentration-time plot and considering a
first rate law equation. The activity per active site (TOF) was
calculated using the CO uptakes to determinate the number of
surface metal sites and the results are summarized in Table 1.
As it can be seen, both the rate constant (Table 1) and the con-
version of cinnamaldehyde reached for similar reaction times
(Fig. 6), followed the trend: Pt/C > Pt(100)/C > Pt(111)/C.
However, when the rate is normalized by the number of acces-
sible platinum atoms (TOF), the trend changes to: Pt(100)/C >
Pt(111)/C > Pt/C which is consistent with that obtained in cro-
tonaldehyde hydrogenation (Fig. 4).
3.4.2. Cinnamaldehyde hydrogenation
Fig. 5 shows the main reaction pathways that can occur
during cinnamaldehyde hydrogenation. Under the experimen-
tal conditions used in the present work only these four products
were detected. Activity and selectivity of the catalysts in the
hydrogenation of cinnamaldehyde were evaluated in the liq-
uid phase at 383 K and 5 MPa. Fig. 6 plots the conversion of
Regarding the products distribution, again the main reaction
product was the saturated aldehyde (HALD) although, con-
versely to crotonaldehyde hydrogenation, the three catalysts

J.C. Serrano-Ruiz et al. / Journal of Catalysis 253 (2008) 159–166
165
under study yielded noticeable amounts of cinnamyl alcohol
(CALC). This fact is not strange since it is well known that the
selective hydrogenation of the carbonyl bond is more difficult
to achieve when lower aldehydes are involved and the reaction
takes place in gas phase [45]. Table 1 also shows the selec-
tivity to cinnamyl alcohol (CALC) and hydrocinnamaldehyde
(HALD) at equal conversion level (10%) for the three catalysts
under study. It is interesting to observe that polycrystalline Pt/C
showed lower selectivity to CALC than preferentially oriented
samples. This fact can be explained in terms of particle size. It
is well known that metal particle size has a strong influence in
the selectivity to cinnamyl alcohol in the case of monometallic
platinum catalysts [46]. Thus, the selectivity for C=O hydro-
genation increased with increasing crystallite size. Note that
Pt/C showed an average platinum particle size of 3 nm while
in preferred oriented samples, Pt(100)/C and Pt(111)/C, plat-
inum is in form of large particles of around 10 nm size (see
Fig. 1). Repulsive interactions between the phenyl ring and the
large metal crystallites, which hinders the interaction via the
C=C bond and enhances interaction via C=O bond, is pro-
posed to explain this behavior. This effect is absent in the case
of small metal particles because both double bonds, C=C and
C=O, could approach the surface with less repulsive interac-
tions between the phenyl ring and the metal surface [47].
Furthermore, it can be seen in Table 1 that Pt(111)/C showed
higher selectivity to CALC than Pt(100)/C (48 vs 36%). In this
case no size effect can be invoked since both samples showed
similar average particle size (10 nm). However, as pointed be-
fore, both catalysts differ in the facets distribution of the metal
particles, with a higher ratio of (111) domains in the case of
Pt(111)/C. Thus, these results clearly indicate that, besides a
size effect, the amount of (111) facets composing the metal
particle play a crucial role in increasing the selectivity to un-
saturated alcohol in these kind of reactions. In this way, previ-
ous studies on prenal hydrogenation on well-defined (111) [48]
and (110) [49] platinum surfaces, found that the main prod-
uct (also at 10% conversion) was the unsaturated alcohol and
the saturated aldehyde, respectively. Interestingly, the same au-
thors also found that prenal hydrogenation on Pt(553) (which
can be considered as a surface with (111) terraces and steps)
resulted in a lower selectivity to the unsaturated alcohol com-
pared to (111) flat surface [50]. Again, the simultaneous pres-
ence of (111) terraces (where the di-σC=O adsorption mode is
preferred) and steps (πC=C adsorption mode preferred) is in-
voked to explain the lower selectivity to the unsaturated alcohol
on this stepped surface. As already seen, it has been proved
that the platinum particles composing Pt(111)/C have a notice-
able amount of stepped surfaces vicinal to (111) small domains
(Fig. 2c), which would explain the noticeable yield of satu-
rated aldehyde (49% HALD, Table 1) obtained for this sample.
It is necessary to bear in mind that substituents at the C=C
bond have a strong influence on the selectivity to the unsat-
urated alcohol and thus, can influence the intrinsic selectivity
determined by the platinum surface. Thus, is not strange that
some works, based on the hydrogenation of the simplest unsat-
urated aldehyde (acrolein), introduced some new points to the
factors controlling the selectivity in this kind of reactions. In
a recent theoretical work, Loffreda et al. [51] concluded that
the key point which determines the selectivity to allyl alcohol
on Pt(111) is not the adsorption mode of the aldehyde, but the
competitive desorption process of the half hydrogenated prod-
ucts (unsaturated alcohol vs saturated aldehyde). Furthermore,
in contradiction with earlier assumptions, Mohr et al. [52] claim
that the surface sites responsible of the C=O hydrogenation
are the highly unsaturated surface atoms situated at corners and
edges, although these conclusions are obtained for a very spe-
cial catalytic system (Au/ZnO), and the same authors indicated
that these conclusions could not be transferred to other catalytic
systems.
4. Conclusions
The effect of the surface structure/shape of Pt nanoparticles
on the crotonaldehyde and cinnamaldehyde hydrogenation re-
actions has been evaluated. Polyoriented and preferentially ori-
ented (100) and (111) Pt nanoparticles have been synthesized
and characterized by electrochemical and microcalorimetric
techniques. TEM images of the corresponding Pt nanoparticles
show semi-spherical, cubic and tetrahedral/hexagonal preferen-
tial shapes, respectively. Cyclic voltammetry results confirm the
existence of a preferentially (100) and (111) surface orienta-
tion. In the case of the smallest Pt nanoparticles (3 nm), the
presence of a polyoriented surface orientation is also confirmed
by cyclic voltammetry. Furthermore, irreversible adsorption Bi
and Ge, as well as CO adsorption microcalorimetry, suggested
that the surface of polyoriented Pt/C is essentially constituted
of highly unsaturated platinum atoms, situated at corners and
edges. Catalytic activity in crotonaldehyde and cinnamaldehyde
hydrogenations, expressed as TOF, showed a strong depen-
dence with the face exposed, which is indicative of the struc-
ture sensitive character of these reactions. Moreover, Pt(111)/C
showed higher selectivities to unsaturated alcohol than Pt/C and
Pt(100)/C. A greater ratio of (111) facets in the surface of this
catalyst is proposed to explain the observed results.
Acknowledgments
Financial support from the Ministerio de Educación y
Ciencia (Spain) (Projects MAT2004-03480-C02-02, CTQ2006-
04071 and NAN2004-09267-C05-05), the Network of Exce-
lence InsidePores (European Commission Contract No. NMP3-
CT2004-500895) and Generalitat Valenciana (acomp07/052) is
gratefully acknowledged.
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