Effect of reduction in liquid phase on the properties and the catalytic activity of Pd/Al2O3 catalysts

Article abstract of DOI:10.1016/j.jcat.2011.11.018

The change in structural and surface properties of Pd/Al₂O ₃ catalysts as the effect of reduction in liquid phase during preparation is investigated by a multi-technical approach. It has been found that, although the treatment leaves an external shell of oxidized Pd because of the exposure to air of final wet catalyst, it affects both the catalyst reducibility and the properties of the supported Pd nanoparticles, in terms of dispersion, particle size, morphology, interaction with the support, structural order, and surface properties. Consequently, unreduced and pre-reduced catalysts show different performances in the debenzylation of 4-benzyloxyphenol reaction.

Full text of DOI:10.1016/j.jcat.2011.11.018

Journal of Catalysis 287 (2012) 44–54
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Effect of reduction in liquid phase on the properties and the catalytic activity
of Pd/Al O catalysts
2
3
Elena Groppo a, , Giovanni Agostini , Andrea Piovano , Naresh B. Muddada a,2, Giuseppe Leofanti b,
⇑
a
a,1
c
d
e
a
Riccardo Pellegrini , Giuseppe Portale , Alessandro Longo , Carlo Lamberti
a
Department of Inorganic, Physical and Materials Chemistry, INSTM Reference Center and NIS Centre of Excellence, Università di Torino, Via P. Giuria 7, I-10125 Torino, Italy
Consultant, Via Firenze 43, 20010 Canegrate (Milano), Italy
Chimet SpA – Catalyst Division, Via di Pescaiola 74, I-52041 Viciomaggio (Arezzo), Italy
Netherlands Organization for Scientific Research at ESRF, BP 220, F-38043 Grenoble Cedex 9, France
Istituto per lo Studio dei Materiali Nanostrutturati del CNR, Via Ugo La Malfa 153, I-90146 Palermo, Italy
b
c
d
e
a r t i c l e i n f o
a b s t r a c t
Article history:
The change in structural and surface properties of Pd/Al O catalysts as the effect of reduction in liquid
2
3
Received 24 August 2011
Revised 11 November 2011
Accepted 30 November 2011
Available online 12 January 2012
phase during preparation is investigated by a multi-technical approach. It has been found that, although
the treatment leaves an external shell of oxidized Pd because of the exposure to air of final wet catalyst, it
affects both the catalyst reducibility and the properties of the supported Pd nanoparticles, in terms of dis-
persion, particle size, morphology, interaction with the support, structural order, and surface properties.
Consequently, unreduced and pre-reduced catalysts show different performances in the debenzylation of
Keywords:
4-benzyloxyphenol reaction.
Pd-based catalysts
Heterogeneous catalysis
Characterization techniques
Temperature-programmed reduction
FT-IR spectroscopy
CO chemisorption
TEM
Ó 2011 Elsevier Inc. All rights reserved.
X-ray absorption spectroscopy
SAXS
1
. Introduction
Pd-supported catalysts are widely used in hydrogenation reac-
process in which they are used, Pd catalysts can be supplied
directly in this oxidized state or can be further reduced by the cat-
alyst manufacturer itself through different reducing agents (among
which sodium formate, sodium borohydride, and hydrogen are the
most common ones).
Although the active Pd phase in reductive conditions typical of
hydrogenation reaction is metal Pd, the oxidation state of the start-
ing catalyst (i.e., unreduced or pre-reduced) influences its catalytic
performances in such a way that sometimes it is more convenient
to use an unreduced catalyst [9–11], and some others a reduced
catalyst shows better performances [12,13]. This suggests that
the physical–chemical properties of the supported Pd particles de-
pend on the reduction state and on the reduction method. How-
ever, a clear relationship among the reduction state of the
catalyst as loaded into the reactor, its physical–chemical proper-
ties, and the catalytic performances, was never demonstrated so
far.
tions for the synthesis of fine chemicals (e.g., active pharmaceutical
ingredients) [1] and bulk chemicals (e.g., terephthalic acid) [2].
Although Pd catalysts find application for reactions in gas phase
[
3], more often they are used for reactions in liquid phase [4–7],
and thus more conveniently supplied in the wet form, i.e., with
pores filled by water. Pd catalysts are usually prepared by support-
ing a Pd precursor on high surface area carriers, among which me-
2 3
tal oxides, such as Al O and carbons are the most used. We have
recently demonstrated that, whichever is the support, following
the deposition–precipitation method, Pd is supported in the form
of a Pd-oxide or hydroxide [8]. Depending on the specific catalytic
⇑
Corresponding author. Fax: +39 011 6707855.
E-mail address: elena.groppo@unito.it (E. Groppo).
Present address: Institut Laue Langevin, 6 Rue Jules Horowitz BP 156, F-38042
We will show that the catalytic performances of alumina-
supported Pd-based catalysts in the debenzylation of 4-benzyloxy-
phenol (chosen as test reaction) are influenced by a pre-reduction
in liquid phase, in terms of induction time, activity, and turnover
1
Grenoble Cedex 9, France.
2
Present address: inGAP Centre of Research-based Innovation, Department of
Chemistry, University of Oslo, Sem Saerlandsvei 26, N-0315 Oslo, Norway.
0
021-9517/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2011.11.018

E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
45
frequency. In an attempt to understand the origin of these changes,
we performed a systematic investigation of the structural and sur-
face properties of Pd/Al catalysts as a function of the reduction
To take into account the possible modification of the
pore size distribution upon Pd precipitation, a fraction of the
Al support was subjected to the deposition–precipitation pro-
2 3
c-Al O
c-
2
O
3
2 3
O
state (unreduced or pre-reduced with different reducing agents).
The oxidation state of the supported Pd phase was evaluated by
means of TPR, whereas the surface oxidation state was determined
by FT-IR spectroscopy of CO adsorbed at 100 K. CO chemisorption,
TEM, and SAXS were successively adopted to determine the disper-
sion and the morphology of the Pd particle. At last, Pd hydride
decomposition in TPR experiments and FT-IR of CO adsorbed at
room temperature were used to investigate the order/disorder
characteristics of bulk and surface Pd particles, as well as the nat-
ure of the exposed surface sites. This multi-technical approach, al-
ready applied by us to investigate various Pd-supported catalysts
cess without adding the Pd precursor. This sample was used to
simulate the contribution of the support in the SAXS experiments.
Finally, unsupported PdO sample was prepared as a reference for
TPR experiments following a procedure similar to that adopted
for the catalysts preparation, but omitting the support. The final
unsupported samples were dried overnight in air at 393 K.
2.2. Catalysts activity in debenzylation reaction
2 3
The catalytic performance of the Pd/Al O samples was tested in
the debenzylation of 4-benzyloxyphenol to hydroquinone and tol-
3
[
8,14–17], is fundamental to understand the role of pre-reduction
uene. The reaction was carried out in a 300 cm glass reactor
in determining the reducibility of supported Pd-oxide or hydroxide
and the properties of the final Pd nanoparticles.
equipped with a double mantle for water circulation and a gas
injection stirrer (Premex, br1 series). Water at the required tem-
perature was circulated inside the reactor mantle by means of a
thermostatic bath. In a typical experiment, the reactor was charged
with 500 mg of dry catalyst and then a solution of 10 g of 4-benzyl-
oxyphenol in 140 ml of ethanol was poured into the reactor. The
reactor was purged first with nitrogen and then with hydrogen.
Hydrogenation was performed at atmospheric pressure, at a tem-
perature of 308 K, and with a stirring speed of 2000 rpm.
2
. Experimental
2.1. Catalysts preparation and nomenclature
Supported Pd samples were prepared in the Chimet Laborato-
2
ꢀ1
ries on
0
c
-Al
2
O
3
(surface area = 121 m g
;
pore volume =
By plotting the hydrogen consumption versus time, a straight
line is obtained, demonstrating that the reaction has a zero-order
with respect to the substrate. The slope of this line corresponds
3
ꢀ1
.43 cm g ) following the deposition–precipitation method, with
Na PdCl as palladium precursor and Na CO as basic agent [8].
2
4
2
3
3
ꢀ1
Three Pd loadings were investigated: 5.0 wt.%, which is relevant
for industrial applications, and 2.0 and 3.5 wt.% for comparison.
to the activity (hereafter r), expressed in cm STP min , and the
intercept with the abscissa has been taken as the measure of the
induction time. The mean activity of surface Pd atoms can be de-
fined in terms of turnover frequency (TOF), which is the number
ꢀ
All catalysts were carefully washed until complete Cl removal
was achieved and were kept in their wet state (ca. 50% of water)
until measurement. The same procedure was followed to prepare
supported Pd samples on a wood-based activated carbon (surface
2
of H molecules consumed per second per number of surface atoms
determined by an independent technique [21], in this work by CO
chemisorption.
To prove that the reaction proceeds without mass transfer con-
straints in the conditions adopted herein, several tests with differ-
ent catalyst amounts (i.e., 300, 500, and 700 mg), different catalyst
2
ꢀ1
3
ꢀ1
area = 980 m g ; pore volume = 0.62 cm g ), which were used
for the evaluation of mass transfer phenomena in the catalytic
tests. For the 5.0 wt.% catalyst, after the deposition of palladium
as hydroxide, reduction was carried out in liquid phase by means
of three different reducing agents, i.e., HCOONa, NaBH
4
, or hydro-
particle size (i.e., 5 and 23 lm), and at different temperatures (i.e.,
gen at 338 K. For reduction with hydrogen, the treatment was per-
formed inside a stainless steel autoclave, under 10 bar of pressure.
As for unreduced catalysts, the reduced catalysts were successively
water-washed until the residual Cl was completely removed and
stored in their wet state.
293, 308, and 323 K) were performed [22–24]. The effect of particle
size on catalytic activity was evaluated on PdCw(F), having a par-
ticle size of 23
Mastesizer 2000 instrument). The catalyst particle size was de-
creased to 5 m by grinding. CO chemisorption data demonstrated
lm (measured by LALLS technique with Malvern
ꢀ
l
Hereafter, the nomenclature PdAl and PdCw are used to indicate
the unreduced catalysts prepared on alumina (Al) and wood car-
bon (Cw), respectively. The Pd loading is indicated before the PdAl
label; thus, 5PdAl refers to the 5.0 wt.% loading. When a reduction
in the liquid phase is performed, a label indicating the reducing
agent is added. Thus, 5PdAl(F), 5PdAl(B), and 5PdAl(H) refer to alu-
mina-supported palladium catalysts (5.0 wt.% loading) reduced
with formate, borohydride, and hydrogen, respectively, whereas
PdCw(F) refers to carbon-supported palladium catalyst reduced
with formate. All the Pd-supported samples were stored in their
wet state. TPR data provided evidence that the drying atmosphere
greatly affects the reduction temperature of the catalysts: a sample
dried in inert atmosphere starts to be reduced at lower tempera-
ture than the same sample dried in air. The reason is attributed
to the auto-reduction of a very small fraction of defective sites,
as demonstrated also by FT-IR spectroscopy of CO adsorbed at
that the grinding procedure does not affect the palladium disper-
sion. By using a simplified approach [22], the reciprocal plot 1/r
versus 1/m (Fig. 1a) allowed us to calculate the mass transfer coef-
ficient for the transfer of the gas to liquid phase (k ) and the pseu-
m
do-kinetic constant, which includes the mass transport to the
solid–liquid interface, as well as internally to the porous solid
3
(k
evidence that the transfer of the gas (i.e., H
the reaction rate; and this happens at almost the same extent
regardless of the catalyst particle size (5 or 23 m).
). The ratio k /k
r m r
resulted in a value higher than 10 , providing
2
) is much quicker than
l
The Arrhenius plot shown in Fig. 1b further confirms the above-
mentioned results, giving an activation energy value equal to
52.3 kJ/mol (12.5 kcal/mol). According to Roberts [24], when a
reaction is controlled by either gas/liquid or liquid/solid mass
transport, the observed activation energy should be less than
5 kcal/mol, whereas activation energy values greater than
10 kcal/mol (as in our case) are an indication that the reaction pro-
ceeds without mass transfer constraints, either with or without
pore diffusion. Moreover, the fact that the reciprocal plot of
Fig. 1a is the same for both catalysts having a different particle size
1
2
00 K (vide infra) and as already suggested in the literature [18–
0]. This observation is relevant in order to avoid misleading com-
parisons with similar data in literature. However, it was found that
all the properties of Pd-supported particles before and after reduc-
2
+
tion (such as the Pd /Pdtot ratio and the Pd dispersion) are not af-
fected by the drying procedure. Therefore, the drying step was
always conducted in air (unless specified).
(i.e., 5 and 23 lm) provides evidence that the reaction rate depends
only on the chemical reaction taking place on active Pd sites, with-
out the influence of internal pore diffusion [23].

4
6
E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
In a typical experiment, the catalyst (200 mg) was reduced in situ
at 393 K according to the following procedure: (i) the catalyst is
loaded inside the U-tube, (ii) it is heated in He up to 393 K (ramp
ꢀ1
2
rate of 10 K min ), (iii) H is fed for 30 min, and (iv) the sample
ꢀ1
is cooled down to 323 K in He (ramp rate of 10 K min ). A CO/
Pd average stoichiometry = 1 was assumed to calculate the Pd dis-
persion. This assumption was verified by measurements performed
simultaneously in a static volumetric apparatus on three samples
reduced at two different temperatures (393 and 673 K) using both,
2 2
H and O as probe molecules; these measurements gave a O/Pd
average stoichiometry close to 1 [14,26]. Comparing the results ob-
tained according to the two methods, a CO/O ratio in the 0.94–1.13
range was obtained, which is a strong support for a CO/Pd average
stoichiometry = 1.
2.3.2. Transmission Electron Microscopy (TEM)
Transmission electron micrographs were obtained using a JEOL
3
2
010-UHR instrument operating at 300 kV, equipped with a
k ꢁ 2 k pixels Gatan US1000 CCD camera. Samples were depos-
ited on a copper grid covered with a lacey carbon film.
2.3.3. Small-Angle X-ray Scattering (SAXS)
SAXS measurements were taken at the synchrotron DUBBLE
beamline BM26B [27] at the European Synchrotron Radiation Facil-
ities (ESRF, Grenoble, F). Experiments were carried out using a
monochromatic 12 keV X-ray beam. The H-hutch of BM26B is
equipped to perform SAXS/WAXS experiments and was designed
with a maximum of flexibility that allows one to study several
problems, particularly in the field of nanoscience [27]. In our
experiments, the small-angle scattering images were recorded by
placing a two-dimensional PilatusPilatus 1 M detector (by Dectris)
at 6 m. Thus, a large scattering vector range was covered,
ꢀ
1
ꢀ1
0
.1 < q < 3.5 nm (being q [nm ] = 10 ꢁ 1.014E [19] sinh the scat-
Fig. 1. Mass transfer evaluation in the debenzylation of 4-benzyloxyphenol. Part
a): reciprocal plot of activity (r) versus the catalyst amount (m). Reaction
(
tering vector, where h is the scattering angle and E is the energy of
conditions: PdCw(F) catalyst, atmospheric pressure, 308 K, 2000 rpm stirring speed,
catalyst loading: 300, 500, and 700 mg. Part (b): Arrhenius plot. PdCw(F) catalyst,
atmospheric pressure, catalyst loading 500 mg, 2000 rpm stirring speed, temper-
ature: 293, 308, and 323 K.
the X-ray beam) and the maximum d-spacing resolution was
6
2.8 nm. Silver Behenate was used as standard to determine the
centre of the beam and the scattering vector scale. Then, the isotro-
pic SAXS intensity profiles were obtained by the integration of the
two-dimensional images radially averaged around the centre of the
primary beam by using FIT2D software [28–30]. The analysis de-
tails will be described in a forthcoming paper.
2
2
.3. Catalysts characterization
.3.1. Temperature-Programmed Reduction (TPR) and CO
Chemisorption
2.3.4. FT-IR spectroscopy of adsorbed CO
A Micromeritics Autochem 2910 instrument was used for both,
TPR and CO chemisorption measurements. For the TPR measure-
ments, the instrument was equipped with a CryoCooler device
allowing to perform the measurements starting from 153 K. The
water formed during the reduction process was trapped inside a
molecular sieve getter located between the sample holder and
the detector. A typical procedure was as follows: 100 mg of sample
The FT-IR spectra of CO adsorbed on the catalysts were recorded
ꢀ1
at a resolution of 2 cm on a Bruker Vertex 70 instrument, in
transmission mode. In situ experiments were carried out in a
homemade quartz cell equipped with KBr windows allowing to
perform sample activation and successive measurements in the
ꢀ4
100–823 K temperature range, at pressures from 10 to 760 Torr.
The catalysts were pressed into self-supporting pellets and acti-
vated in the same cells used for the measurement. Both activation
and measurements were performed using grease-free cells and
vacuum lines.
(
either wet or dry) is introduced in the sample holder and, when
necessary, heated in situ by a nitrogen flow at 393 K for 2 h
[
(
(
8,15]. Then, the sample is cooled down to 153 K under Ar flow
3
ꢀ1
50 cm min ). Successively, the flow is switched to 5% H
2
in Ar
A first set of experiments was aimed to determine the surface
oxidation state of the Pd phase on the fresh samples. For this rea-
son, the samples were heated at 393 K in air, followed by a rapid
outgassing to remove the air from the cell; the outgassing time
was kept as short as possible, in order to avoid any possible Pd2+
auto-reduction. In order to minimize changes on the catalyst sur-
face, due to the reaction of the CO probe with the supported Pd-
hydroxide phase, FT-IR experiments were performed by dosing
CO (equilibrium pressure PCO = 5 Torr) on samples pre-cooled
down to 100 K. For comparison, the same experiment was per-
formed also on samples heated at 393 K in vacuum and on 5PdAl
3
ꢀ1
50 cm min ) and maintained throughout the analysis. Once a
baseline is established, the temperature ramp is started at a rate
ꢀ1
of 5 K min up to 623 K. Tmax and Tmin indicate the temperatures
of the maximum rate of H consumption or release, respectively.
start is the temperature of the beginning of H consumption, de-
fined as the temperature where the first derivative of the TCD sig-
2
T
2
ꢀ
4
nal versus temperature exceeds the arbitrary value of 5 ꢁ 10
.
Note that all the TPR curves shown hereafter present a minimum
around 230 K, which is an instrumental artefact due to a small
change in the heating rate.
CO chemisorption measurements were performed at 323 K by a
2
reduced in H gas at 393 K. A second set of experiments was aimed
2
dynamic pulse method on samples pre-reduced in H at 393 K [25].
to characterize the surface sites on the reduced Pd particles by FT-

E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
47
IR of CO adsorbed at room temperature, following the approach al-
ready discussed in our previous works [14,15,17,31]. In that case,
more evident effect is the disappearance of any induction time. Sec-
ondly, pre-reduced 5PdAl catalysts show a greater activity than the
unreduced one and, consequently, a greater TOF.
the Pd phase was pre-reduced in H
process consisted in two subsequent H
2
gas at 373 K. The reduction
dosages (equilibrium pres-
2
The data summarized in Table 1 unequivocally indicate that the
pre-reduction procedure affects the catalyst performance. How-
ever, its effects on the oxidation degree and reducibility of sup-
ported Pd phase, as well as the morphology, the structural order
and the surface properties of final Pd nanoparticles were never
investigated in detail. This would be of a great interest in view of
a rational choice of the best preparation procedure. The following
sections will deal with the investigation of each one of the
above-mentioned catalyst properties. In most cases, the measure-
ments were taken on 5PdAl and 5PdAl(F), the latter chosen as a
representative for the pre-reduced catalysts.
sure PH2 = 120 Torr, contact time = 10 min), followed by outgassing
at 373 K; then, the samples were cooled down to room tempera-
ture in a dynamic vacuum. All the outgassed samples were con-
tacted in situ with CO (PCO = 50 Torr); PCO was then gradually
ꢀ
4
decreased stepwise from 50 to 10 Torr, and the corresponding
FT-IR spectra were recorded. The final spectrum corresponds to
CO species irreversibly adsorbed at 300 K.
The intensities of all the FT-IR spectra reported in this work
were normalized to the pellet weight in order to take into account
the different optical thickness of the samples, i.e., the quantity of
Pd crossed by the IR beam. Consequently, as all samples contain
the same amount of Pd, the reported intensity of a given IR absorp-
tion band along the whole set of samples is directly proportional to
the amount of carbonyl species responsible of that band, so that all
the spectra are directly comparable each other.
3.2. Oxidation state of the supported Pd phase
3.2.1. Oxidation degree: TPR results
The oxidation state of the supported Pd phase in both the unre-
duced and pre-reduced catalyst was evaluated at first by means of
TPR, which is one of the most common techniques used to investi-
gate the reducibility of supported metal species [35–37]. However,
the experiments on palladium are complicated by the fact that
3
. Results and discussion
3.1. Catalyst performances in the debenzylation of 4-benzyloxyphenol
2
+
0
reduction of Pd precursors to Pd starts below room temperature,
thus requiring the use of an apparatus capable to operate also at a
low temperature (in this work from 153 K). This is the reason why
very few works in literature show TPR data on the reduction of
The catalytic performances of unreduced and pre-reduced
samples were tested in a debenzylation reaction, a widely used
reaction in fine chemicals and pharmaceutical industry where
functional groups protection or de-protection is required [32–
2
+
supported Pd compounds [8,15,38–40].
The TPR profiles of 5PdAl and 5PdAl(F) are shown in Fig. 2. The
two curves are characterized by a reduction peak starting at tem-
3
4]. Herein, 4-benzyloxyphenol was selected as a model substrate.
Upon hydrogenation, the C–O bond is cleaved, with the attendant
production of toluene and hydroquinone, according to Scheme 1.
Table 1 summarizes the catalytic performances of all the inves-
tigated catalysts in terms of (i) induction time, (ii) activity and (iii)
turnover frequency (TOF), calculated on the basis of the number of
surface Pd atoms as obtained by CO chemisorptions measurements
perature lower than 275 K due to the Pd ? Pd0 reduction, fol-
lowed by a smaller negative peak centred around 330 K due to
the decomposition of the Pd hydride formed at lower temperature.
The integrated area below the positive peak (after correction for
both bulk and surface hydride formation) gives the overall hydro-
gen consumption in the reduction process and allows to calculate
2+
(
vide infra). As far as unreduced catalysts are concerned, the activity
2
+
the Pd fraction present in each sample. The so obtained data
are summarized in the last column of Table 2 and indicate that:
linearly increases with Pd loading, being the TOF almost constant;
the induction time decreases upon increasing the Pd concentration.
Coming to the pre-reduced samples, all the recorded parameters
are greatly affected by the pre-reduction treatment. The first and
(
1) In 5PdAl (sample 2 in Table 2), all the supported Pd is in the
2
+
2+
Pd form within the accuracy of the technique (the Pd /Pdtot
ratio is determined with an incertitude of ±0.03); therefore,
0
no reduction to Pd occurs during the impregnation step.
CH3
OH
O
H
2
+
cat
OH
OH
Scheme 1. Debenzylation of 4-benzyloxyphenol to form hydroquinone and
toluene.
Table 1
Catalyst activity (in cm3 min ) and turnover frequency (TOF, s , defined as the
ꢀ1
ꢀ1
number of
2
H molecules consumed per second per number of surface atoms
determined by CO chemisorption as an independent technique) of unreduced PdAl
catalysts as a function of the Pd loading and of pre-reduced 5PdAl catalysts.
Activity (cm3 minꢀ1
)
TOF (s
ꢀ1
)
Sample
Induction time (min)
2
3
5
5
5
5
PdAl
.5PdAl
PdAl
PdAl(F)
PdAl(B)
PdAl(H)
27
20
11
0
0
0
5.0
6.2
11.3
12.9
13.7
14.4
0.098
0.071
0.093
0.156
0.176
0.170
Fig. 2. TPR curves of 5PdAl (black) and 5PdAl(F) (red) samples dried in air at 393 K.
The labels on the TPR curves refer to the sample code of Table 2. (For interpretation
of the references to colour in this figure legend, the reader is referred to the web
version of this article.)

4
8
E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
Table 2
Summary of the TPR data obtained for 5PdAl and 5PdAl(F) samples subjected to
different thermal treatments. Also, the data obtained for a bulk, unsupported, PdO
sample are reported for comparison (samples
0 and 1). Tstart and Tmax refer,
respectively, to the starting and maximum temperature values of the hydrogen
consumption peak due to the Pd ? Pd0 reduction; Pd /Pdtot is the fraction of Pd
2
+
2+
2+
present in the catalyst (see Section 3.2).
Code
Sample
Treatment
Air 393 K
T
start (K)
T
max (K)
Pd2+/Pdtot
0
1
2
PdO
PdO
5PdAl
5PdAl
5PdAl
5PdAl
5PdAl(F)
5PdAl(F)
5PdAl(F)
5PdAl(F)
5PdAl(F)
260
173
265
247
–
175
213
163
–
298
198
308
271
–
256
315
223
–
1.00
0.16
0.97
1.02
0.00
0.35
0.57
0.58
0.00
0.25
0.52
H
2
393 K + air 393 K
Air 393 K
0
2
N
2
H
2
H
2
393 K
393 K
393 K + air 300 K
3
4
5
Air 393 K
0
5
N
2
H
2
H
2
H
2
393 K
393 K
393 K + air 300 K
393 K + air 393 K
6
7
8
172
201
n.d.
301
2
Fig. 3. TPR signals for 5PdAl(F) reduced in H at 393 K (grey) and of the same
(
2) In 5PdAl(F) (sample 5 in Table 2), more than half of the Pd
phase is still oxidized. The TPR results are confirmed by
XANES and EXAFS data (not shown). This observation is
quite unexpected since the catalyst was reduced in Na-
sample successively treated in air at 300 K (blue) and 393 K (green), compared to
the TPR signal of 5PdAl(F) dried in air at 393 K (red). The labels on the TPR curves
refer to the sample code of Table 2. (For interpretation of the references to colour in
this figure legend, the reader is referred to the web version of this article.)
2
+
formate. It is worth underlining that the fraction of Pd
0.57–0.58) is much larger than that expected for a surface
passivation, i.e., formation of a surface monolayer of oxygen
(
+
[
26]; in such a case, the Pd2 fraction value would be of 0.24,
as calculated on account of the dispersion values obtained
from CO chemisorption data (vide infra). The same extent
of subsurface oxidation is observed also when PdAl(F) is
0
dried in inert atmosphere (sample 5 in Table 2), demon-
strating that Pd is partially oxidized already in the wet sam-
ple and that drying in air did not alter the Pd2 fraction.
3) Although it is still highly oxidized, 5PdAl(F) starts to be
reduced at lower temperature than PdAl.
+
(
In principle, the presence of about 50% of oxidized Pd in the
PdAl(F) sample (see Table 2) can be explained by two main
5
hypothesis: (i) incomplete reduction of the supported Pd-hydrox-
0
ide by Na-formate; (ii) re-oxidation of fully reduced Pd , due to la-
ter exposure of the wet catalyst to air. The Pd surface would be
oxidized in any case, since it is well known that O chemisorbs dis-
2
sociatively on metal Pd [41–43]. However, in the case (i), both the
core and the surface of the Pd particles should be oxidized,
whereas in the case (ii), the oxidation process should involve only
the surface and the subsurface layers. In order to get more insight
into the nature of the partially oxidized Pd particles in the PdAl(F)
sample, a sequence of TPR measurements was taken on a PdAl
sample subjected to ad hoc conceived thermal treatments. The
TPR curves are shown in Fig. 3, whereas the corresponding Tstart
and Tmax values, as well as the Pd2 fraction are summarized in
Table 2.
Fig. 4. FT-IR spectra of CO adsorbed at 100 K on 5PdAl and 5PdAl(F) samples dried
either in air (black and green spectra) or in vacuum (blue and red spectra) at 393 K,
compared to the spectrum obtained in the same conditions on 5PdAl reduced in H
2
at 393 K (grey). The numeric labels refer to the sample code of Table 2. (For
interpretation of the references to colour in this figure legend, the reader is referred
to the web version of this article.)
+
ity. The shape as well as Tstart, Tmax, and the integrated area of the
TPR reduction peak of sample 5PdAl(F) dried in air (sample 5 in
Fig. 3 and Table 2) are close to that of sample 5PdAl(F) reduced
The TPR profile of 5PdAl(F) reduced in H
and Table 2) showed no consumption of H
decomposition peak at 328 K demonstrating that thermal treat-
2
gas (sample 6 in Fig. 3
2
but only the hydride
2
in H gas and treated in air at 393 K (sample 8 in Fig. 3 and Table 2).
gas causes a complete Pd2 ? Pd reduction (in agree-
+
0
ment in H
2
Because the latter is covered by an external PdO layer formed dur-
ing re-oxidation of the fully reduced Pd, the result supports the
hypothesis (ii) formulated above: Na-formate fully reduces the
ment with XAS results, not shown). When the sample is exposed to
0
air at room temperature (sample 7 in Fig. 3 and Table 2), Pd is par-
tially re-oxidized, as testified by the appearance of the Pd2 ? Pd0
reduction peak. However, the re-oxidized Pd is only a fraction of
the total Pd (about 25%) and likely corresponds to a surface mono-
+
0
supported Pd-hydroxide phase; then, the resulting Pd is partially
re-oxidized by air on both, surface and subsurface.
0
layer only (dispersion = 24%, vide infra). A larger fraction of Pd
3.2.2. Surface oxidation state: FT-IR of CO adsorbed at 100 K
(
52%) is re-oxidized when the 5PdAl(F) sample reduced in H
2
gas
In order to definitely demonstrate that the Pd phase in 5PdAl(F)
has a metal core-oxide layer structure, we have performed a FT-IR
experiment of CO adsorbed at 100 K. The low temperature is nec-
essary to avoid reaction of CO with the supported Pd2+ phase
is treated in air at 393 K (sample 8 in Fig. 3 and Table 2). In addi-
tion, the thinner the re-oxidized Pd2 layer, the easier its reduction
+
(i.e., lower Tstart and Tmax), probably as a result of surface defectiv-

E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
49
[
20,44–46]. Fig. 4 shows the FT-IR spectra of CO adsorbed at low
temperature on the 5PdAl and 5PdAl(F) samples dried in air at
93 K (samples 2 and 5, respectively). Both spectra are dominated
Fig. 5 are a selection of those reported in Table 2 and include sam-
ples at different degrees of oxidation: (i) the two fully oxidized
samples 5PdAl (sample 2) and bulk PdO (sample 0), (ii) the par-
tially oxidized sample 5PdAl(F) (sample 5), and (iii) the partially
oxidized samples obtained from 5PdAl, 5PdAl(F), and PdO reduced
3
ꢀ1
by an IR absorption band at around 2148 cm , which is due to CO
physisorbed on the support [47], whereas no IR absorption bands
at frequencies lower than 2100 cm are observed for both sam-
ples, demonstrating that no accessible metal Pd is present in those
cases. Apparently, no IR absorption bands ascribable to carbonyl
species formed on Pd species are observed. However, linear car-
bonyls on Pd sites have been reported to give absorption in the
2
quency range dominated by the IR absorption band due to physi-
sorbed CO.
The same experiment was repeated for 5PdAl and 5PdAl(F) dried
in vacuum, with the aim to exclude that the absence of accessible
Pd sites on 5PdAl(F) was induced by the drying procedure in air.
The corresponding FT-IR spectra (samples 2 and 5 , respectively)
ꢀ1
2
at 393 K in H and re-oxidized in air at RT and 393 K. All the data lie
on a straight line, meaning that more oxidized are the Pd particles,
less easy is their reduction. It is well known that re-oxidation of
metal Pd causes an increase in surface irregularity, up to a surface
fragmentation [62–64]. This is a consequence of both the larger
volume of PdO with respect to that of metal Pd and its lower sur-
face tension. However, by increasing the oxidation degree, the oxi-
dized layer becomes progressively more regular approaching the
order of true oxide, such as in samples 5PdAl and bulk PdO. The
easier reducibility of 5PdAl(F) with respect to 5PdAl would be
the consequence of a greater disorder of the few external PdO lay-
ers when compared to the bulk-like PdO phase in 5PdAl.
2
+
2
+
ꢀ1
160–2140 cm range [20,44–46,48–51], that is precisely the fre-
0
0
0
show, in addition to the IR absorption band due to physisorbed
ꢀ
1
CO, very weak IR absorption bands around 2100 and 1950 cm
3.4. Properties of reduced Pd
characteristic of carbonyls formed on Pd0 [14,17,20,31,52–61].
However, these bands are much less intense than those obtained
The data shown in the previous sections definitely demonstrate
that, in spite of a large amount of oxidized Pd phase still present,
the pre-reduced 5PdAl(F) catalyst is reduced more easily than the
unreduced one. However, once the catalysts are in the reducing
reaction environment, it could be that the starting differences van-
ish. Therefore, it is important to know whether the different reduc-
ibility observed on the fresh catalysts corresponds to different
properties, such as particle size, morphology, and defectivity, of
the Pd nanoparticles in the reducing environment (i.e., during the
catalytic act), thus explaining the different observed catalytic per-
formances (see Table 1). A direct investigation on both 5PdAl and
2
by dosing CO on 5PdAl sample reduced in H at 393 K (sample 3,
grey spectrum in Fig. 4) and are assigned to CO adsorbed on the
small fraction of defective sites that have been auto-reduced during
the sample activation (see Section 2). The FT-IR spectra shown in
Fig. 4 validate the hypothesis formulated above on the basis of
TPR results: the surface oxidation state of the supported Pd phase
is +2 for both, 5PdAl and 5PdAl(F) samples.
3.3. Reducibility of oxidized samples
5
PdAl(F) samples after reduction in reaction conditions would be
In the previous paragraphs, we have demonstrated that the sur-
complicated by several factors, including the possible presence of
compounds from reaction medium; therefore, the samples have
been treated in H
face of Pd particles is fully oxidized in both unreduced and pre-re-
duced catalysts. Thus, the oxidation state of Pd surface sites cannot
be accounted for the different catalytic performances induced by
pre-reduction. An alternative explanation comes from the observa-
tion that 5PdAl(F) is reduced at temperature lower than 5PdAl, as
already shown in Fig. 2 and summarized in Table 2. This result sug-
gests that the two oxidized phases in 5PdAl and 5PdAl(F) have dif-
ferent properties.
2
gas at 393 K in order to completely reduce the
Pd phase.
3.4.1. Pd dispersion and interaction of Pd particles with the support
(
CO chemisorption, TEM, and SAXS)
CO chemisorption was already demonstrated to be an efficient
technique for investigating the dispersion of Pd nanoparticles
i.e., the ratio between surface and total metal atoms) when poi-
soning phenomena of the exposed metal surface are negligible
8,14]. From the dispersion values summarized in Table 3, it is evi-
The easier reducibility of 5PdAl(F) can be explained by analyz-
ing the results of TPR experiments summarized in Fig. 5, where
the starting reduction temperature Tstart, which is the most signif-
icant parameter indicating the easiness of catalyst reduction, is re-
(
[
dent that Pd dispersion (Dchemi) slightly decreases with increasing
Pd concentration, as already reported in literature. Moreover, the
pre-reduced catalysts show a lower dispersion than the unreduced
one, independently of the reducing agent.
2
+
ported versus the Pd fraction in the samples. The data shown in
A first hypothesis to explain the loss of Pd dispersion after pre-
reduction in liquid phase is the occurrence of a relevant sintering
of the Pd particles; in this case, TEM investigation should reveal
an increase in the average particle diameter going from unreduced
Table 3
Pd dispersion as obtained by CO chemisorption data (Dchemi) and, when available, by
SAXS data (DSAXS), minimum of the negative peak due to the decomposition of the
formed Pd hydride (Tmin) and corresponding H/Pd in Pd hydride, as obtained by TPR
on the whole set of samples investigated in this work.
Sample
PdO
D
chemi (%)
D
SAXS (%)
T
min (K)
H/Pd in Pd hydride
0.2
38
37
36
24
23
25
–
339
321
320
322
328
329
327
0.50
0.17
0.17
0.19
0.27
0.29
0.28
2
3
5
5
5
5
PdAl
.5PdAl
PdAl
PdAl(F)
PdAl(B)
PdAl(H)
n.a.
n.a.
38
31
31
Fig. 5. Starting reduction temperature (Tstart) as a function of the Pd2+ fraction in
the catalysts as determined by TPR experiments. The labels on the scattered points
refer to the sample code of Table 2.
32

5
0
E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
to pre-reduced catalysts. For this reason, a series of TEM images
were collected on both 5PdAl and 5PdAl(F); three representative
images are shown in Fig. 6a–c. In all cases, Pd particles having a
fcc structure (as evidenced by electron diffraction, see insets in
Fig. 6a–c) are homogeneously distributed on the support in an iso-
lated form, whereas no aggregates are observed (Figure not
shown). A careful analysis of several TEM images revealed that
the average Pd particle size increases moderately after the reduc-
tion process, but it cannot account for the whole loss of Pd disper-
sion as found by CO chemisorption. Unfortunately, it is not possible
to quantitatively estimate the change in particle size as already
done in previous cases [14,15], mainly because the cubo-
octahedral morphology of Pd particles exhibits (111) and (100)
faces in different ratio (see arrows and labels in Fig. 6) that makes
difficult the identification of the particle diameter. As an example,
Fig. 6b shows a Pd particle terminating mainly with (111) faces,
whereas in the Pd particle shown in Fig. 6a and c, the extension
of (100) faces is larger. In all the cases, the particle diameter would
result in a different value according to the direction in which it is
evaluated. As TEM provides only the projection of the particle on
the plane perpendicular to the electron beam axis, and not a tri-
dimensional reconstruction of the particles, a reliable particle size
distribution cannot be obtained in the present conditions,
contrarily to what done in the past on samples characterized by
particles with symmetrical shape [14,15].
a scale factor for the support contribution (accounting for a slightly
different packing of the two capillaries) and the Pd particle size dis-
tribution in terms of a mean value and a
r for a log-normal distri-
bution [65,66]. Fig. 7b displays the particle size distribution
obtained by analyzing the SAXS data of 5PdAl and of the pre-re-
duced samples. For 5PdAl, the Pd particle size is centred around
1.5 nm, corresponding to a dispersion DSAXS = 38% (Table 3) [14].
The particle size distribution for the three pre-reduced catalysts
is centred around the same average value as 5PdAl, but presents
a tail toward greater values, resulting into slightly lower DSAXS val-
ues with respect to 5PdAl (Table 3). These results provide a clear
and quantitative evidence that the pre-reduction causes an in-
crease in the Pd particle size. However, the decrease in DSAXS by
going from unreduced to pre-reduced samples is smaller than
the decrease in Dchemi. This means that the increase in the Pd par-
ticles size cannot account for all the loss of dispersion measured by
CO chemisorption data, suggesting that in the pre-reduced sam-
ples, not all the surface of the Pd particles is accessible to the CO
probe.
As a matter of fact, in addition to the moderate increase in Pd
particle size by going from 5PdAl to 5PdAl(F), TEM images revealed
the occurrence of an embedding of the Pd particles into the Al O
2 3
support, a phenomenon that undoubtedly contributes to the loss
of Pd dispersion after reduction in the liquid phase. In fact,
although already a fraction of Pd particles were partially buried
On the contrary, quantitative information on the particle size
distribution was obtained by SAXS measurements. With respect
to TEM, SAXS technique has the advantage to get information on
the whole particles population. Fig. 7a shows, as an example, the
experimental SAXS curve for 5PdAl (red circles) compared to that
2 3
into Al O in 5PdAl sample, an important fraction was protruding
out of the surface (an example is shown in Fig. 6c). In contrast,
nearly all the Pd particles are partially embedded into the support
in 5PdAl(F) sample (Fig. 6b). A strong metal–support interaction is
typically observed in the presence of highly reducible supports
of the bare
c
-Al
2
O
3
support (grey full line). The experimental SAXS
2 2 2
(such as TiO and CeO ), and in those cases, H reduction at high
profiles of the pre-reduced sample (not reported) have a similar
signal-to-noise ratio. Overlapped to the experimental data in
Fig. 7a, also the best fit (black full line) and the theoretical signal
obtained by considering a distribution of spherical Pd particles
temperature (P773 K) brings about a relevant decrease in the
chemisorption ability of the metal. Although the final effect is sim-
ilar, the phenomenon discussed herein has a different origin and
reflects a high mobility of the Al
2 3
O support, even in the absence
(
dotted line) are reported. The fit has been performed optimizing
of H at high temperature. Recently, we have shown that relevant
2
Fig. 6. Selected TEM micrographs of a few Pd particles on both 5PdAl (parts a and c) and 5PdAl(F) (part b) samples (top part) and schematic representation of the exposed
faces (bottom part). The insets show the electron diffraction of each particle, demonstrating the fcc structure.

E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
51
Fig. 7. Part (a): Experimental SAXS curve of 5PdAl (dots) compared to the signal of the bare Al support (grey). Also, the theoretical signal obtained by considering spherical Pd
particles (dotted line) and the best fit (black line) is reported. Part (b): particle size distribution for the unreduced and pre-reduced samples obtained by analyzing the SAXS
data.
3.4.2. Pd disorder (TPR and FT-IR of adsorbed CO)
Additional information on the properties of the supported Pd
particles can be obtained by TPR data in the region of the negative
peak due to hydride decomposition. A magnification of TPR curves
of 5PdAl and pre-reduced samples in the Pd hydride decomposition
region is shown in Fig. 8. All the samples were subjected to the
same treatment performed during CO chemisorption measure-
2
ments, i.e., drying in air at 393 K followed by reduction in H at
the same temperature. The curves of unreduced and pre-reduced
samples differ in both integrated area of the negative peak and
peak position. The integrated area allows the calculation of the
H/Pd ratio in Pd hydride, as summarized in Table 3. The tempera-
ture of hydride decomposition appears to be strictly correlated to
H/Pd ratio suggesting that both of them are dependent on the same
property. As already pointed out, the reduction in liquid medium
confers to Pd particles specific characteristics, independently on
reductant.
It is well known that the H/Pd value decreases upon increasing
the Pd dispersion; the same trend is shown by the temperature of
the hydride decomposition [39,67–74]. However, both quantities
depend also on the internal disorder of the Pd particles, due to both
framework defects, impurities or decorations: the lower the H/Pd
ratio, the more disordered the Pd particles [68–70,72,74–76]. The
two factors affecting the H/Pd ratio and the decomposition temper-
ature of the hydride are often correlated each other because the
smallest particles tend to be more disordered than the bigger ones
[75]. Thus, it is difficult to decide the importance of the relative
contribution of the two characteristics, i.e., particle size and
internal disorder, to H/Pd ratio and decomposition temperature.
According to TEM and SAXS evaluations, that point out only a mod-
est difference of Pd particle size between 5PdAl and 5PdAl(F), we
must conclude that the Pd particles of the unreduced catalyst are
more disordered than those of pre-reduced ones, probably because
of the milder conditions and slower rate of the reduction in liquid
Fig. 8. TPR signals of PdO bulk, unreduced, and pre-reduced catalysts dried in air at
93 K and then reduced in H at 393 K, in the region of the negative peak due to
hydride decomposition.
3
2
changes in the morphology of some supports (e.g.,
SiO –Al ) can occur during catalyst preparation [16]. In particu-
lar, an increase in surface area and decrease in the pore volume
were observed on -Al after the Pd precipitation. These
morphological changes were attributed to the mobility of Al , in-
2 3
c-Al O and
2
2 3
O
c
2 3
O
2 3
O
duced by the basic medium used during Pd deposition–precipita-
tion. The presence of a fraction of Pd particles embedded in the
support already in 5PdAl (see Fig. 6a) is in agreement with the
results discussed above. The phenomenon becomes much more
evident in 5PdAl(F) because the catalyst remains for longer time
in an alkaline solution (Na-formate used for reduction), thus
2 3
favouring a further mobility of the Al O support. Summarizing,
reduction in the liquid phase brings about a decrease in Pd disper-
sion that is partially explained by a sintering of Pd particles (quan-
titatively determined by SAXS measurements) and partially by
2
medium with respect to those of reduction with H in gas phase.
FT-IR spectroscopy of adsorbed CO has been used to obtain
information on the surface regularity of the final Pd particles; these
data provide information complementary to those obtained by
TPR, which gave indication on the Pd disorder in the bulk. Indeed,
FT-IR spectroscopy of adsorbed CO is a well-established technique
to obtain information on the exposed faces of supported metal
2 3
their embedding into Al O (which is observed by TEM and
explains the discrepancy between Dchemi and DSAXS for the pre-
reduced catalysts), due to the high mobility of the support in the
alkaline environment characterizing the catalyst preparation.

5
2
E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
2
Fig. 9. FT-IR spectra of CO adsorbed at room temperature on unreduced and pre-reduced samples, reduced in H at the 393 K. The sequences of FT-IR spectra show the effect
ꢀ
4
of decreasing PCO, from 50 Torr (blue spectrum) to 10 Torr (red). (For interpretation of the references to color in this figure legend, the reader is referred to the web version
of this article.)
nanoparticles and on their defectivity [14,17,20,31,52–61]. The
FT-IR spectra of CO adsorbed at room temperature on unreduced
and pre-reduced samples subjected to the same treatment per-
formed during CO chemisorption measurements (i.e., dried in air
at 393 K and successively reduced in H at the same temperature)
2
are shown in Fig. 9 as a function of CO pressure. In our experimen-
due to linear carbonyls has a lower intensity; (ii) the IR absorption
ꢀ1
band at 1985 cm , assigned to bridged carbonyls on (100) faces,
is scarcely visible in 5PdAl, suggesting that (100) faces are less ex-
tended than (111) faces. The same IR absorption band is more
prominent in the IR spectra of CO adsorbed on the three pre-
reduced samples. A correlation between the Pd particle size and
the relative proportion of linear and bridged carbonyls in the IR
spectra of adsorbed CO has been already established [92,93]. On
small and amorphous (i.e., highly defective) metal particles, CO is
adsorbed prevalently in the linear (terminal) mode, whereas an in-
crease in the particle size (and thus in the order) results in the for-
mation of a greater number of bridged carbonyls [17,55,60,92–95].
On these bases, the overall sequence of FT-IR spectra shown in
Fig. 9 points out that Pd particles on 5PdAl display a more defective
surface than those on the pre-reduced catalysts. Therefore, pre-
reduction in liquid phase would be responsible for the formation
of bigger Pd particles more ordered, both internally (as obtained
by TPR) and on the surface (as shown by FT-IR spectroscopy of ad-
sorbed CO).
tal conditions, hmax (blue curve) corresponds to T = 300 K and
P
CO = 50 Torr, whereas hmin (red curve) is obtained by evacuation
ꢀ4
at 300 K down to 10 Torr. In all cases, the IR spectrum at the
maximum CO coverage (blue) shows three main IR absorption
bands at around 2090, 1940, and 1985 cm , which are the charac-
teristic of linear and bridged carbonyls on (111) faces and of
bridged carbonyls on (100) faces, respectively [17,20,55,58,77–
ꢀ1
9
1]. Therefore, FT-IR spectra are in good agreement with the
morphology determined by TEM images (Fig. 6), which showed
Pd particles terminating with (111) and (100) faces in different
proportion. All of the above-mentioned IR absorption bands evolve
upon decreasing CO pressure, as a consequence of the decreasing
CO population, as it is well known in literature [17,20,55,58,77–
9
1]. In particular, linear carbonyls are the most reversible adsorbed
species and the corresponding IR absorption band downward shifts
ꢀ
1
of about 15 cm , because of the removal of coupling between
adjacent CO oscillators. At lower CO coverage, an additional IR
4. Conclusions
ꢀ1
absorption band around 2050 cm emerges, which is assigned to
linear carbonyls on defective sites; these species are irreversible
upon outgassing at room temperature.
The present work investigates the relationship between the
reduction state of Pd-based alumina-supported catalysts as loaded
into the reactor, and their properties and catalytic performances in
the debenzylation of 4-benzyloxyphenol. It was found that, after
pre-reduction in the liquid phase, about 50% of the Pd in the cata-
lyst is still oxidized. The reason is attributed to a partial re-oxidiza-
Two main differences are observed between the FT-IR spectra of
CO adsorbed on 5PdAl and those of CO adsorbed on the pre-re-
duced samples. Although the types of surface Pd carbonyls are
the same (same frequency position of all the IR absorption bands),
their relative amount is different (different relative intensity of the
three IR absorption bands). In particular, by comparing the IR spec-
tra at the highest CO coverage, the following differences can be no-
ticed: (i) the IR absorption bands characteristic of linear and
bridged carbonyls have an almost equal intensity in 5PdAl,
whereas for the three pre-reduced samples, the IR absorption band
0
tion of the Pd particles formed during the reduction step, because
of the contact of wet catalyst with the air. The re-oxidation in-
volves both, surface and subsurface. However, the external Pd2+
phase is easier to be reduced than bulk PdO; therefore, the pre-re-
duced catalyst starts faster in reaction conditions, as testified by
the absence of an induction time in the adopted experimental con-
ditions. The easier reducibility of the pre-reduced catalyst has been

E. Groppo et al. / Journal of Catalysis 287 (2012) 44–54
53
explained as due to the greater structural disorder of a few external
PdO layers when compared to a bulk PdO.
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sorption data, Table 3) mainly as a consequence of: (i) a greater
interaction of the Pd particles with the support (embedding), as ob-
served by TEM (Fig. 6), and (ii) a particle sintering (as suggested
qualitatively by TEM and demonstrated quantitatively by SAXS).
Moreover, the pre-reduction in liquid phase brings to an increase
in the order (both internal and superficial) of the Pd particles
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Luca Diano is kindly acknowledged for performing some of the
FT-IR experiments. We are indebted with the whole staff of DUB-
BLE BM26 beamline at the ESRF (particularly to Daniel Hermida
Merino, S. Nikitenko, and W. Brass) where both Pd K-edge EXAFS
[
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