Supported Ir-Pd nanoalloys: Size-composition correlation and consequences on tetralin hydroconversion properties

Article abstract of DOI:10.1016/j.jcat.2012.05.010

A series of Ir-Pd/SiO₂-Al₂O₃ catalysts has been prepared by incipient wetness co-impregnation of acetylacetonate precursors and treated by reductive thermal decomposition. The decomposition has been followed by combined thermogravimetry, differential thermal analysis, and mass spectrometry, showing a marked difference between the bimetallic and monometallic cases. The catalysts characterization by transmission electron microscopy and single-nanoparticle energy dispersive X-ray spectroscopy reveals that the particles are bimetallic and exhibit a size-composition correlation. More generally, the enrichment of larger particles with the less cohesive element (Pd in the present case) via Ostwald ripening is here claimed to be inherent to the thermal activation treatments used in the preparation of bimetallic catalysts. The catalysts have been tested in the hydroconversion of tetralin, which is considered as a model molecule for gas oil upgrading by selective ring opening. At 350 °C, 4 MPa H₂, and in the presence of ppm amounts of H₂S, hydrogenation, dehydrogenation, and ring-opening/contraction products (ROCPs) are formed. While the activity increases with the Pd content, the selectivity to ROCPs reaches a maximum for the Ir55-Pd45 composition and increases with the sulfur concentration. The structural and catalytic results are combined to propose a qualitative model relying on metal-acid bifunctionality and size-composition correlation. While the particle size drives the number of surface sites, the corresponding surface composition governs the hydrogenation efficiency of these sites. Therefore, by controlling the mean metal composition of the catalyst, it is possible to tune the activity-selectivity balance.

Full text of DOI:10.1016/j.jcat.2012.05.010

Journal of Catalysis 292 (2012) 173–180
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Supported Ir–Pd nanoalloys: Size–composition correlation and consequences
on tetralin hydroconversion properties
a
a
b
a
Laurent Piccolo a, , Salim Nassreddine , Mimoun Aouine , Corinne Ulhaq , Christophe Geantet
⇑
a Institut de Recherches sur la Catalyse et l’environnement de Lyon (IRCELYON), UMR 5256 CNRS & Université Lyon 1, 2 avenue Albert Einstein, F-69626 Villeurbanne Cedex, France
b Institut de Physique et Chimie des Matériaux de Strasbourg (IPCMS), UMR 7504 CNRS & Université de Strasbourg, 23 rue du Loess, BP 43, 67034 Strasbourg Cedex 2, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 17 January 2012
Revised 3 May 2012
Accepted 13 May 2012
Available online 20 June 2012
A series of Ir–Pd/SiO2–Al2O3 catalysts has been prepared by incipient wetness co-impregnation of acetyl-
acetonate precursors and treated by reductive thermal decomposition. The decomposition has been fol-
lowed by combined thermogravimetry, differential thermal analysis, and mass spectrometry, showing a
marked difference between the bimetallic and monometallic cases. The catalysts characterization by
transmission electron microscopy and single-nanoparticle energy dispersive X-ray spectroscopy reveals
that the particles are bimetallic and exhibit a size–composition correlation. More generally, the enrich-
ment of larger particles with the less cohesive element (Pd in the present case) via Ostwald ripening is
here claimed to be inherent to the thermal activation treatments used in the preparation of bimetallic
catalysts. The catalysts have been tested in the hydroconversion of tetralin, which is considered as a
model molecule for gas oil upgrading by selective ring opening. At 350 °C, 4 MPa H2, and in the presence
of ppm amounts of H2S, hydrogenation, dehydrogenation, and ring-opening/contraction products
(ROCPs) are formed. While the activity increases with the Pd content, the selectivity to ROCPs reaches
a maximum for the Ir55–Pd45 composition and increases with the sulfur concentration. The structural
and catalytic results are combined to propose a qualitative model relying on metal–acid bifunctionality
and size–composition correlation. While the particle size drives the number of surface sites, the corre-
sponding surface composition governs the hydrogenation efficiency of these sites. Therefore, by control-
ling the mean metal composition of the catalyst, it is possible to tune the activity–selectivity balance.
Ó 2012 Elsevier Inc. All rights reserved.
Keywords:
Nanoalloys
Iridium–Palladium
Amorphous silica–alumina
Ostwald ripening
Tetralin
Selective ring opening
1. Introduction
sired role of the acid function is to favor contraction of C6 rings via
isomerization on Brönsted acid sites, prior to ring opening. Never-
Going further than aromatics saturation, the selective ring
opening of polyaromatic hydrocarbons is a promising route to im-
prove the combustion properties of diesel fuels and consequently
reduce undesirable emissions [1]. In order to increase significantly
the cetane number, unselective cracking and branching isomeriza-
tion must be avoided [2]. Ideally, linear paraffins could be formed
by the selective opening of all rings. In addition, due to the pres-
ence of residual sulfur in pre-hydrotreated fuels, the catalysts must
be thioresistant.
Most of researches on selective ring opening have used
metal–acid bifunctional catalysts. The role of the metal function
is to activate hydrogen for hydrogenation–dehydrogenation steps
and possibly favor selective hydrogenolysis of the rings. Hydrogen-
olysis is known to be catalyzed by noble metals such as Ru, Rh, and
Ir. The pioneering work of McVicker et al. has shown the ability of
Ir to promote hydrogenolysis of C6 rings [3]. However, even on Ir,
C6 ring opening is much slower than C5 ring opening. Thus, the de-
theless, the catalyst acidity should be mild enough to prevent
cracking and secondary isomerization. Moreover, through electron
transfer, the support acidity provides thioresistance to the metal
nanoparticles [1].
The current strategies in selective ring opening consist in the
use of microporous or mesoporous acidic materials loaded with
noble metals. However, as a conclusion of several studies using
decalin, tetralin, or naphthalene as model molecules [3–17], it re-
mains challenging to obtain both a resistance to poisoning by sul-
fur, and a high selectivity ratio between C10 ring-opening products
and C<10 cracking products. Moreover, along with ring-opening
products, a large proportion of decalin skeletal isomers (or ‘‘ring-
contraction products’’) is generally formed. Although less desirable
than ring-opening products in terms of cetane number, such com-
pounds could be important intermediates to ring-opening steps in
real feedstocks. Indeed, using bifunctional catalysts, several works
have obtained significant cetane number enhancements for pre-
hydrotreated light cycle oil feedstocks [10,13,18].
We have recently shown that Ir supported on amorphous silica–
alumina (ASA) is thioresistant and active in tetralin hydrogenation,
⇑
Corresponding author.
E-mail address: laurent.piccolo@ircelyon.univ-lyon1.fr (L. Piccolo).
0021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.05.010

174
L. Piccolo et al. / Journal of Catalysis 292 (2012) 173–180
Table 1
ring contraction, and ring opening [16,19]. The selectivity to ring-
opening/contraction products (ROCPs) is roughly proportional to
the acid/metal site number ratio. Thus, a low-loaded (<1 wt.%) Ir
catalyst supported on an acidic ASA (40 wt.% silica in our case)
was found to be an optimal combination in terms of selectivity
to ROCPs (>50%) and cracking products (<10%). However, the selec-
tivity to ring-opening products remained low (<10%) [16].
Palladium exhibits excellent properties in terms of hydrogen
activation and thioresistance [20]. In an attempt to further improve
the catalytic performances of our ASA-supported system, we have
investigated the structural and catalytic properties of Ir–Pd/ASA
bimetallic catalysts obtained by co-impregnation of ASA with acet-
ylacetonates. Transition-metal acetylacetonates have proven to be
efficient precursors to prepare contaminant-free supported cata-
lysts with a high degree of metal dispersion and thermal stability
[21]. However, unlike for other Pd-based bimetallic systems [22]
such as Cu–Pd [23,24], Mn–Pd [25], Pt–Pd [26], Ni–Pd [27,28],
and Co–Pd[29], the synthesis of Ir–Pd catalysts by combining acet-
ylacetonates has never been attempted to our knowledge. In the
bulk phase, similar to that of Ir–Pt, the phase diagram of Ir–Pd pre-
sents a miscibility gap below 1500 °C at 50 at.% composition and
ranging from ca. 11 to 98 at.% Pd at 500 °C [30]. This may increase
the difficulty to obtain Ir–Pd nanoalloys with respect to the above-
mentioned Pd-based ones.
Metal composition of the samples, as determined by ICP-OES.
Sample
name
Pd content/metal content
(at.%)
Metal weight/catalyst weight
(wt.%)
Ir/ASA
0
11
45
89
100
0.96
1.04
0.91
1.02
0.85
Ir89–Pd11/ASA
Ir55–Pd45/ASA
Ir11–P89/ASA
Pd/ASA
by incipient wetness (co-)impregnation of the supports with Ir ace-
tylacetonate, Ir(acac)3, and/or Pd acetylacetonate, Pd(acac)2. These
precursors (Sigma–Aldrich, purity 97%) were dissolved in toluene,
using the concentration needed to obtain a metal loading of
1.0 wt.%. After maturation during 2 h at room temperature, the sam-
ples were dried at 120 °C overnight and reduced in an H2 flow at
350–500 °C for 6 h. In the case of Ir/ASA, we have shown that the cat-
alysts must be treated by direct H2 reduction of acac-impregnated
ASA, without pre-calcination, to avoid particle agglomeration [42].
The compositions of the so-called Ir(100-x)–Pd(x)/ASA sample
series (x in at.%), as determined from inductively coupled plas-
ma–optical emission spectrometry (ICP-OES, Activa – Horiba Jobin
Yvon), are reported in Table 1. Prior to each activity measurement,
the samples were further exposed in situ to an H2 flow for 2 h at
350 °C.
Besides, few catalytic studies have been devoted to the Ir–Pd
system. Recently, Persson et al. synthesized a number of Pd-based
bimetallic catalysts for methane combustion. Ir–Pd/c-Al2O3 was
prepared by impregnation using IrCl3 and Pd(NO3)2 precursors,
2.2. Materials characterization techniques
but separate particles were obtained [31]. López-De Jesús et al.
prepared Ir–Pd/c-Al2O3 catalysts from IrCl3 and PdCl2 for liquid-
The TG–DTA–MS (thermogravimetry–differential thermal anal-
ysis–mass spectrometry) system is described in the electronic Sup-
plementary material file.
phase hydrogenation of benzonitrile and evidenced a nanoparticle
size–composition correlation [32]. Using a colloidal method, Shen
et al. synthesized Ir–Pd/C catalysts and observed a positive effect
of Ir addition to Pd for ethanol oxidation in alkaline media [33].
Similarly to our study, Rocha et al. investigated tetralin hydrocon-
version over a dealuminated HY zeolite loaded with Ir–Pd particles,
which were obtained from (NH4)3IrCl6 and Pd(NH3)4Cl2 precursors
[34]. Nevertheless, no ROCPs were formed on this catalyst, and the
hydrogenation activity was found comparable to that of a reference
Pd–Pt catalyst. For the selective opening of one-ring or two-ring
hydrocarbons, several other combinations of noble metals have
been reported: Ir–Pt [6,10,11,35], Pd–Pt [5,14], Os–Ru [5,9] Pd–
Rh [13], and Pt–Rh [36]. However, a frequent limitation of the
works dealing with bimetallic catalysts is the poor direct investiga-
tion of the nanoparticles structure, allowing determining whether
they are themselves bimetallic or whether monometallic particles
coexist [37–39].
The carbon-replicated samples were observed by high-resolu-
tion transmission electron microscopy (HRTEM). The microscopes
used were Jeol 2010 (200 kV, LaB6 filament, 0.19 nm resolution),
2010F (200 kV, FEG, 0.19 nm resolution), and 2100F (200 kV, FEG,
Cs-corrected probe, 0.11 nm STEM resolution). They were all
equipped with energy dispersive X-ray spectroscopy (EDS). Scan-
ning TEM (STEM) was performed with the 2100F. Atomic-level
high-resolution high-angle annular dark field (HAADF, Z-contrast)
and bright field images were recorded.
Size histograms were obtained from the statistical treatment of
the micrographs, by analyzing more than 300 particles. The metal
particle size was averaged over the metal surface distribution nid2,
i
which is more relevant to catalysis than the number distribution ni
(ni is the number of particles in the diameter range di). The ‘‘sur-
face-weighted mean size’’ hdisurf is given by
R
nid3=
R
nid2.
i
i
In this article, we will show how the size and composition of
ASA-supported Ir–Pd nanoparticles affect the activity–selectivity
balance in high-pressure tetralin hydroconversion.
2.3. Catalytic testing and product identification
The experiments were carried out in a gas-phase flow fixed bed
catalytic microreactor described in details elsewhere [16]. The
standard reaction conditions for tetralin hydroconversion were
H2 4 MPa, tetralin 6 kPa, and 350 °C. The H2S concentration was
varied between 0 and 200 ppm. The catalyst weight and the mass
flow rate were adjusted for conversion control (contact time range
2–20 s).
2. Experimental
2.1. Materials and catalyst preparation
Amorphous silica–alumina (ASA, commercial name SIRAL) was
supplied by Sasol (Germany), following a patented preparation pro-
cedure [40,41]. SIRAL-40, containing 40 wt.% silica, was chosen as a
support for its optimal performances, in correlation with its Brön-
sted acidity [19]. The powder, received in hydrated form, was acti-
vated by heating at 550 °C in air for 3 h. It resulted in dehydration
and transformation of the alumina part from AlO(OH) (boehmite)
For the catalytic tests at fixed tetralin conversion (48–56%), the
catalyst weight was varied (50, 40, 20, 10, and 7 mg for increasing
Pd contents), while the total flow rate was kept in the range 160–
200 mL minꢀ1, and the H2S concentration was 100 ppm.
The gas composition at the reactor outlet was determined by on-
line gas sampling and gas chromatography (GC). The GC-FID was a
HP 5890 II equipped with a HP-1 column (25 m ꢁ 0.2 mm ꢁ
to
area, pore volume, and pore diameter were 50
0.90 mL gꢀ1, and 6.4 nm, respectively. The catalysts were prepared
c-Al2O3. For SIRAL-40, the average oxide grain size, BET surface
l
m, 500 m2 gꢀ1
,
0.5
lm). In a typical catalytic test, at least 50 isomers of C10 aromatic
and saturated compounds are formed and can hardly be separated

L. Piccolo et al. / Journal of Catalysis 292 (2012) 173–180
175
and identified, except by (post-reaction) comprehensive two-
dimensional gas chromatography (GC ꢁ GC) [16,43,44]. The corre-
sponding instrument is described in the Supplementary file.
In order to determine the individual particle composition, we
have performed EDS with various probe diameters (2–100 nm) on
the Ir–Pd samples. Bimetallic particles have been hardly detected
in the Ir89–Pd11 and Ir11–P89 samples, which is not surprising
with regard to their composition. However, the nanoparticles in
the Ir55–Pd45 sample are bimetallic, as shown by Fig. 3. Interest-
ingly, the Pd content of the nanoparticles increases roughly linearly
with their size. Consistently with the size histogram of Fig. 2c, the
nanoparticle size ranges between 1 and 5 nm. According to Fig. 3,
these sizes correspond to Pd contents of ca. 0 and 100 at.%, respec-
tively. Only the particles with intermediate size (3.0 0.2 nm) have
the composition of the overall sample (45 at.% Pd from ICP) within
5% in standard deviation. Accurate measurements on two Ir55–
Pd45 samples using, for each one, two different spectrometers,
were all consistent with this behavior.
3. Results
The reductive thermal decomposition of the metal precursors
has been studied by TG–DTA–MS. The results are reported in the
Supplementary file. To summarize the most significant features
(Fig. S1), Ir(acac)3 and Pd(acac)2 decompose essentially in to light
alkanes (methane and C2+ alkanes) at ca. 350 °C with low exother-
micity, while the mixture decomposes exothermically to methane
at 475 °C. Thus, mixing the two precursors somewhat hinders the
decomposition process, and the bimetallic case is far from being
a superposition of the single-metallic ones. Instead, the results sug-
gest a strong interaction between Ir and Pd species.
3.2. Catalyst performances in tetralin hydroconversion
3.1. Structural characterization of the metal nanoparticles by
TEM–EDS
The three bimetallic catalysts and the two monometallic ones
have been assessed in tetralin hydroconversion at 350 °C with up
to 200 ppm H2S in the feed. Fig. S2 in the Supplementary file shows
GC ꢁ GC chromatograms acquired in the same conditions for Ir/
ASA and Pd/ASA. In addition to tetralin (reactant), naphthalene
(dehydrogenation product), and cis and trans decalins (hydrogena-
tion products), five categories of C10 products appear on both
images: methyl-indans (‘‘aromatic ring-contraction products,’’ the
four position isomers), alkyl-benzenes (‘‘aromatic one-ring-open-
ing products,’’ mostly n-butyl-benzene), decalin skeletal isomers
(‘‘saturated ring-contraction products,’’ the largest family, includ-
ing dimethyl-bicyclo-octanes and methyl-bicyclo-nonanes), alkyl-
cycloalkanes (‘‘saturated one-ring-opening products,’’ mostly
n-butyl-cyclohexane and n-pentyl-cyclopentane), and C10 paraffins
(‘‘saturated two-ring-opening products,’’ mostly n-decane). Paraf-
fins are visible only in the case of low-metal-loaded catalysts
[46]. A deeper analysis of the products distribution was performed
in Refs. [16,44,46]. In the present paper, in order to facilitate the
presentation of the catalysis results, the products issuing from ring
opening and ring contraction have been gathered together in a
same family, so-called ROCP. Note that the fraction of ring-opening
products is less than ca. 10% of the ROCPs at half-conversion [16].
The amounts of hydrocarbons with more (alkylation products) or
less (cracking products) than 10 carbon atoms are negligible for
the 1 wt.% metal-loaded catalysts considered herein, even at
100% conversion (Fig. S3).
Fig. 1 shows STEM images of the Ir55–Pd45 sample. The nano-
particles are well-dispersed and essentially exhibit a truncated
octahedral shape, as expected for thermally equilibrated fcc parti-
cles in the considered size range (see also Ref. [16] for Ir). However,
some particles exhibit rough facets and, besides, small clusters and
even isolated Ir atoms have been evidenced by STEM–HAADF.
Fig. 2a–e shows the particle size distributions corresponding to
the series of Ir(100-x)–Pd(x)/ASA catalysts (x = 0, 10, 50, 90, and
100) derived from conventional TEM image analysis. For x = 50,
the histogram for the sample reduced at 550 °C (instead of
350 °C for the other samples) after impregnation is also reported
(Fig. 3c, R550). This shows that the additional 200 °C do not affect
significantly the characteristics of the final catalyst, contrary to
what TG–DTA–MS would have suggested (Fig. S1). This is ex-
plained by the isothermal reduction of the solids during 8 h prior
to the catalytic tests, at variance from the TG conditions.
Strikingly, the distribution gradually shifts to larger sizes and
becomes wider as x increases. These trends are summarized in
Fig. 2f. For monometallic Ir and Pd catalysts, the mean particle
sizes are 1.5 nm (full width at half-maximum 0.6 nm) and 4.5 nm
(FWHM 2.5 nm), respectively. As TG–DTA–MS ones, these TEM
results suggest an interaction between Ir and Pd species during
the preparation process, and the formation of bimetallic particles.
The results in terms of selectivity to ROCPs and total activity, for
fixed sulfur content (100 ppm H2S) and isoconversion (ca. 50%), are
presented in Fig. 4. As shown by Fig. 4a, the activity increases with
the Pd content. The corresponding turnover frequency (TOF, i.e.,
tetralin consumption activity normalized by the number of surface
metal atoms calculated from the size distribution, Fig. 4b) increases
from 7.6 ꢁ 10ꢀ2 sꢀ1 (Ir catalyst) to 1.1 sꢀ1 (Pd catalyst). Fig. 4b re-
veals that the selectivities to ROCPs of the bimetallic catalysts are
higher than those of the monometallic ones (14% for Ir and 16%
for Pd). The maximum, 22%, is obtained for the Ir55–Pd45 sample.
Note that the selectivity of such bifunctional catalysts can be im-
proved by increasing the acid/metal site number ratio, but this
comes along with cracking and alkylation [46].
The results for variable sulfur contents are reported in the
Supplementary file (Figs. S3 and S4). Briefly, the Pd content has no
significant effect on the catalysts thioresistance, that is, the deacti-
vation kinetic order with respect to H2S is similar for all the catalysts
(tetralin conversion rate proportional to CꢀH20S:7ꢂ0:2). However, the
selectivity of Pd-rich catalysts is strongly affected by the sulfur con-
tent of the feed. Going from Ir/ASA to Pd/ASA, it is observed that the
dehydrogenation of tetralin vanishes and the selectivity to ROCPs
becomes dependent on the sulfur content. When the H2S concentra-
Fig. 1. STEM images of the C-replicated Ir55–Pd45 sample.
HAADF image of a ca. 3-nm-sized particle is shown in insert.
A high-resolution

176
L. Piccolo et al. / Journal of Catalysis 292 (2012) 173–180
Fig. 2. (a–e) Surface-weighted particle size histograms for the Ir–Pd/ASA series. In the case of the Ir55–Pd45 sample, the reductions at 350 °C (standard treatment) and 550 °C
are compared. (f) Center and full width at half-maximum (FWHM) of the size distributions vs. Pd content.
tion increases from 0 to 200 ppm, the naphthalene/decalins selec-
tivity ratio increases for Ir, while the ROCPs/decalins one increases
for Pd. The behavior of Ir–Pd is different from those of both Ir and
Pd: Dehydrogenation is significant, and the selectivity to ROCPs is
positively sulfur-dependent (see the Supplementary file for details).
reduction [32]. As in our case, the authors have observed an increase
in the particle size with the Pd content. They have ascribed this cor-
relationto a greater mobilityof Pd, withrespect to Ir, over alumina. Ir
would prevent the sintering or agglomeration of Pd during the prep-
aration. A size–Pd concentration correlation has also been observed
for Au–Pd/C catalysts prepared by incipient wetness impregnation
with chlorinated precursors followed by calcination [47].
4. Discussion
We suggest that our results originate from the combination of
two facts. First, there is a strong interaction between Ir and Pd acac
species. This is demonstrated by (i) TG–DTA–MS experiments
(Fig. S1), which show a significant difference between the bimetal-
lic and monometallic cases; (ii) EDS analyses (Fig. 3) directly prov-
ing the bimetallic character of the nanoparticles; and (iii) TEM
observations (Fig. 2a–f) showing a gradual upshift and spreading
4.1. Metal-dependent particle size and size-dependent particle
composition
Recently, López-De Jesús et al. have synthesized Ir–Pd/
catalysts by impregnation of chlorinated precursors and calcination-
c-Al2O3

L. Piccolo et al. / Journal of Catalysis 292 (2012) 173–180
177
of the size distribution when the Pd concentration increases. The
interaction between Ir and Pd species during the preparation is
confirmed by the abrupt variations of the particle size vs. sample
composition curve at the extreme compositions (Fig. 2f). Note that
a previous work by Renouprez et al. has evidenced the possibility
to form mixed Cu–Pd bis-acetylacetonate crystals, showing the
significant interaction between the acac precursors before ligand
removal [24].
Second, Ir and Pd species interact differently with the ASA sup-
port. Indeed, while the heating of Ir(acac)3/ASA in H2 yields
1.5 0.3 nm Ir particles, that of Pd(acac)2/ASA gives rise to
4.5 1.3 nm Pd particles. This suggests a faster diffusion of Pd spe-
cies on the support, as already proposed by López-De Jesús et al. In
regard to the similarities of our results with those for Ir–Pd/c-Al2O3
synthesized from chlorinated precursors by these authors [32], the
difference of interaction with the ASA surface, which is alumina-
rich [41]. In summary, appears to arise from the metal atoms
themselves, that is, Ir interacts more strongly than Pd with the
support.
Fig. 3. Nanoparticle composition (determined by EDS) vs. nanoparticle size (TEM)
for Ir55–Pd45/ASA. 2–5 nm- and 10–100 nm-sized EDS probes were used for
individual particles (open symbols) and groups of particles (filled symbols),
respectively. Typically, 20 particles were simultaneously analyzed when using a
25-nm-diameter probe. Two samples, reduced at 350 °C (squares) or 550 °C
From the above two arguments, our results can be qualitatively
explained by considering the nanoparticles growth process
through ‘‘Ostwald ripening’’ during the thermal ramp used for
the catalysts preparation. It consists of the thermally activated
detachment of metal atoms from small clusters and their surface
diffusion on the support toward bigger particles [48]. In other
words, the small particles, which have the higher evaporation
rates, get smaller or even disappear at the benefit of the large
particles, which get larger. For nanoalloys containing species with
different mobilities, this phenomenon is expected to cause size-
dependent particle composition. This has been recently observed
for Au–Pd nanoparticles exposed to hydrogen (large particles are
Pd-enriched) [49], and Pt–Co particles annealed under ultrahigh
vacuum (large particles are Co-enriched) [50]. Our Ir–Pd system
is believed to be formally equivalent to the Au–Pd and Pt–Co ones,
with a faster detachment or surface diffusion of Pd atoms as com-
pared to Ir atoms during annealing in H2. In particular, the surface
(circles), were analyzed. The red lines are
a linear fit of the data and the
corresponding standard deviation limits. (For interpretation of the references
to color in this figure legend, the reader is referred to the web version of this
article.)
energy of Pd (2.1 J mꢀ2) is much lower than that of Ir (3.0 J mꢀ2
)
[51], suggesting a lower activation energy for Pd detachment.
Moreover, as mentioned above, Pd atoms diffuse faster than Ir
atoms on the support.
The above-mentioned studies of Au–Pd and Pt–Co used physical
deposition on amorphous carbon as synthesis method. However,
the previously cited studies on Au–Pd [47] and Ir–Pd [32] used
chemical preparation methods, as well as the present work. This
shows that the previous concept is extendable to practical bimetal-
lic catalysts. Obviously, the situation is more complex for chemical
impregnation than for physical deposition. Indeed, in the former
case, the nucleation and growth of the nanoparticles is simulta-
neous to the precursors decomposition during the thermal ramp.
The size–composition correlation has important consequences
on the surface composition of the metal nanoparticles. Using the
actual metal loadings (Table 1) and particle size distributions
(Fig. 2), and assuming that the particles are spherical (diameter
di), we have estimated the numbers of surface metal sites for the
Ir, Pd, and Ir55–Pd45 catalysts (Fig. 5a). For the latter, we have
approximated the linear correlation of Fig. 3 by (di ꢀ 1)/4 (di in
nm). In addition, we have assumed that the surface composition
of the particles is identical to that of the bulk, which is evidently
an ideal case since surface segregation may occur. Thus, the overall
metal dispersions have been calculated according to:
X
nid2
i
Fig. 4. Activity in tetralin hydroconversion expressed as rate and turnover
frequency (a), and selectivity to ring-opening/contraction products (b) vs. Pd
concentration for the Ir–Pd/ASA series. The concentration of H2S in the feed was
100 ppm. The conversion was kept constant and equal to ca. 50%.
NS
N
nS
nV
6
nS
i
X
ðIr or PdÞ ¼ 6
¼
ð1Þ
nid3
< d>surf nV
i
i

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L. Piccolo et al. / Journal of Catalysis 292 (2012) 173–180
intermediates, which in turn may be hydrogenated to decalins.
These metallic steps are in competition with acidic ones, consisting
in the protonation by Brönsted acid sites of octalins having diffused
to the support. Subsequently, the corresponding carbocations may
undergo a number of transformations including skeletal isomeriza-
tion, b-scission, hydride transfer, and deprotonation, leading to the
ROCPs [7,45,52]. Thus, Scheme 1 reveals a competition between
octalins hydrogenation on the metal and protonation on the sup-
port. This competition is a key factor in controlling the decalins/
ROCPs selectivity ratio. Indeed, in the case of Ir/ASA, by increasing
the particle size or decreasing the metal loading, we have shown
that the selectivity to ROCPs increased with the acid/metal site
number ratio [16,46].
In the case of Ir–Pd/ASA catalysts, a satisfactory interpretation
of the results requires to account for the structural parameters
established in Section 3.1,that is, the increase in the particle size
with the Pd content, both at the global (whole catalyst) and local
(nanoparticle) scales. The increase of the selectivity with Pd con-
centration in the Ir-rich composition domain (Fig. 4b) is ascribed
to a decrease in the number of metallic sites, as in the Ir/ASA case.
In this domain, due to the fact that the larger particles contain the
larger amounts of Pd, the overall amount of surface Pd remains low
(Fig. 5) and the acid/Ir site number ratio criterion prevails.
However, not only the number, but also the nature of the metal
sites plays a role, as shown here by the similar selectivities to ROC-
Ps found for Ir/ASA (14%) and Pd/ASA (16%), in spite of the different
numbers of exposed metal sites. Indeed, taking the mean particles
sizes, the (similar) metal loadings, and the molar weights into ac-
count, the Ir catalyst exhibits almost twice the number of surface
metal sites with respect to the Pd catalyst, for the same amount
of acid sites (Fig. 5a). These results suggest that, although the addi-
tion of Pd does not modify the reaction mechanism, it induces a
change in the kinetic regime. This is confirmed by the qualitatively
similar but quantitatively different products distributions mea-
sured by GC ꢁ GC for Ir/ASA and Pd/ASA (Fig. S2).
By analogy with acidity, the difference between Ir and Pd can be
discussed in terms of metal sites ‘‘strength’’ (i.e., hydrogenation
efficiency) in the framework of a bifunctional mechanism. The de-
crease in the selectivity to ROCPs at the benefit of the hydrogena-
tion products in the Pd-rich composition domain (Fig. 4b and
Fig. S3) is governed by strength. Indeed, Pd is expected to be much
more active (i.e., ‘‘stronger’’) than Ir for hydrogen activation and the
hydrogenation steps, as confirmed by the higher activity of Pd for
tetralin conversion (Fig. 4a), keeping in mind that hydrogenation
is the main pathway under our conditions. As suggested above,
in the Ir-rich domain, the competition between carbocation and
decalins formations (Scheme 1) is more and more favorable to
the former when the number of Ir sites decreases. Conversely, in
the Pd-rich domain, the competition becomes even more favorable
to decalins formation when the number of Pd sites increases. The
nonlinear shape of the activity vs. composition plot (Fig. 4a) may
Fig. 5. Number of surface metal atoms (a) and surface metal composition (b) as a
function of the total metal composition of Ir(100-x)–Pd(x) catalysts. The filled and
open symbols correspond to the cases of size-dependent particle composition and
homogeneous particle composition, respectively (see text). The lines are guides to
eye.
X
niðdi ꢀ 1Þd2
i
NPSd
NPd
nS
nV
i
X
ðIr55 ꢀ Pd45Þ ¼ 6
ð2Þ
niðdi ꢀ 1Þd3
i
i
where nS is the surface atomic density (nS = 15.7 at. nmꢀ2 for
Ir(111); nS = 15.3 at. nmꢀ2 for Pd(111)) and nV is the bulk atomic
density (nV = 70.7 at. nmꢀ3 for Ir; nV = 67.9 at. nmꢀ3 for Pd).
Fig. 5 allows us to compare this situation with the case of size-
independent particle composition. It appears that with the above
correlation, the surface composition is Ir62Pd37, instead of Ir55Pd45
for homogeneous composition. The decrease of the Pd concentra-
tion may have significant effects on the catalysts properties, as
we discuss below.
4.2. Dependence of the catalytic selectivity on the metal composition
Although the reaction mechanism is discussed in details in a sep-
arate paper [46], a simplified mechanistic reaction scheme will
facilitate the interpretation of the results. Scheme 1 is proposed
on the basis of our previous works on Ir/ASA [16,19,46], as well as
on the present work on Ir–Pd/ASA. It shows that, on the
metal, adsorbed tetralin can be hydrogenated to octalin alkenic
Scheme 1. Simplified reaction scheme showing the most important steps for
tetralin hydroconversion on metal/ASA bifunctional catalysts.

L. Piccolo et al. / Journal of Catalysis 292 (2012) 173–180
179
be due to this change in the reaction regime, which is itself affected
by the surface vs. total metal composition profile (Fig. 5).
In summary, at high Ir concentration, the catalysts contain
‘‘weak’’ but numerous metal sites, whereas at high Pd concentra-
tion, they contain fewer but much stronger metal sites. As previ-
ously mentioned, both situations are unfavorable to the ROCPs
selectivity. Conversely, at intermediate composition, the metallic
function efficiency for hydrogen activation and hydrogenation
reaches a minimum. Overall, this leads to the volcano-shaped
selectivity curve of Fig. 4b. The actual situation might obviously
be more complex. In particular, we cannot rule out a synergistic
electronic effect between Ir and Pd.
and Pascale Mascunan (ICP) for their support in sample character-
ization. We acknowledge the METSA network and Dr. Christine
Goyhenex for allowing us to perform experiments with the 2100F
microscope in Strasbourg. We also thank the French ANR (CatAlk-
Thio project) for supporting the GC ꢁ GC analytical development.
Appendix A. Supplementary material
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.jcat.2012.05.010.
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