Dalton
Transactions
PAPER

Cite this: Dalton Trans., 2013, 42, 12714

Cluster-derived Ir–Sn/SiO2 catalysts for the catalytic
dehydrogenation of propane: a spectroscopic study†
A. Gallo,a R. Psaro,b M. Guidotti,*b V. Dal Santo,*b R. Della Pergola,c D. Masihd andY. Izumie
Ir–Sn bimetallic silica-based materials have been prepared via deposition of the molecular organometallic
clusters (NEt4)2[Ir4(CO)10(SnCl3)2] and NEt4[Ir6(CO)15(SnCl3)] or via deposition of Sn organometallic
precursor Sn(n-C4H9)4 onto pre-formed Ir metal particles. These solids possess promising properties, inReceived 2nd May 2013,
Accepted 14th May 2013
DOI: 10.1039/c3dt51144h
www.rsc.org/dalton

1.

terms of selectivity, as catalysts for propane dehydrogenation to propene. Detailed CO-adsorption
DRIFTS, XANES and EXAFS characterization studies have been performed on these systems in order tocompare the structural and electronic evolution of systems in relation to the nature of the Ir–Sn bonds
present in the precursor compounds and to propose a structural model of the Ir–Sn species present at
the silica surface of the ﬁnal catalyst.

Introduction

In recent years, the worldwide demand for propene has rapidly
grown, as it plays a pivotal role as a feedstock for a large
variety of polymers and of intermediates for the chemicalindustry. Typically, propene is obtained from steam cracking
and fluid catalytic cracking of naphtha. Nevertheless, due tothe increasing propene demand and unstable oil prices, there
is a real interest in developing alternative routes to alkene production, in particular, from light alkanes that are frequently
wasted in crude oil refining plants. Such a route can be as wella viable starting point to be combined with the exploitation ofrenewable building blocks.1–3 Dehydrogenation of propane isthus a promising alternative and a couple of propane dehydrogenation processes (Oleflex, Catofin or FBD-4) have alreadybeen set up at the industrial level.4
With this aim, bimetallic catalysts have shown good performances for many important reactions, such as hydrogenation ofalkynes and dienes in olefins feedstock,5 selective hydrogenation of unsaturated aldehydes,6 preferential oxidation of CO,7hydrogen production8 and light alkane dehydrogenation.9–13
However, as shown in earlier studies, it has been a diﬃcultaDepartment of Chemical Engineering, University of California, Santa Barbara, CA
93106-5080, USAb
CNR-Istituto di Scienze e Tecnologie Molecolari, via Golgi 19, 20133 Milano, Italy.
E-mail: m.guidotti@istm.cnr.it, v.dalsanto@istm.cnr.itcDipartimento di Scienze dell’ambiente e del territorio e di Scienze della Terra,
Università di Milano-Bicocca, piazza della Scienza 1, 20126 Milano, Italyd
KAUST Catalysis Center, King Abdullah University of Science and Technology,
Thuwal 23955-6900, Saudi ArabiaeDepartment of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33,
Inage-ku, Chiba 263-8522, Japan† Dedicated to the memory of Dr Laura Sordelli.

12714 | Dalton Trans., 2013, 42, 12714–12724

task to understand the rationale behind a promotion eﬀect of
an inert metal on the catalytic behavior of a noble one.14 For
example, a relevant decrease in hydrogenolysis properties of aGroup 8, 9 or 10 metal occurs when a Group 14 metal is added
as a promoter.15–17 But the reason for such a decrease is still amatter of debate.
So far, this behavior has been principally ascribed to two
diﬀerent eﬀects: the so-called geometric and electronic eﬀects(cf. ref. 8 and references therein). The geometric eﬀect is theresult of the dilution of surface metal atoms by atoms ofanother metal (the promoter) and the surface ensembles ofthe main metal component are thus reduced in size. Such aneﬀect is related to “structure sensitive” and “structure insensitive” reactions. In fact, some reactions, such as hydrogenolysis,18,19 require a defined number of contiguous surface active
metal atoms to proceed, whilst others, like dehydrogenation,
do not. On the other hand, the electronic eﬀect is related to achange in noble metal surface electronic density due to thepresence of inert metal atoms and to the diﬀerence in electronic aﬃnity between the main metal component and thepromoter. As a consequence, a diﬀerent interaction with thereacting molecules (adsorption/desorption of reagents/reaction
intermediates/products) can occur and modify their catalytic
properties. However, the discrimination between electronic
and geometric eﬀects has not yet been adequately understoodfor many catalytic systems. Together with these major structural and/or morphological eﬀects, other important positiveeﬀects can arise from the addition of a metal promoter inthese systems, such as improved coke resistance, enhanced
thermal stability and increased resistance to poisoning.
Pt–Sn systems have been deeply studied by many authors
because of their good catalytic performances in various reactions like reforming20 and dehydrogenation,21 in particular, ofThis journal is © The Royal Society of Chemistry 2013

Dalton Transactions
propane22 and isobutane.23,24 Tin aﬀects deeply Pt catalytic
properties. In particular, Sn prevents Pt-catalyzed hydrogenolysis and also particles sintering and can reduce the rate ofcoke formation.25–27 However, characterization studies contradict each other about the role of tin as a promoter. Someauthors, on the basis of temperature programmed desorption(TPD) of adsorbed CO monitored by infrared spectroscopy andof calorimetric measurements of H2 and CO adsorption,21,25,28
observed a decrease in the number of surface contiguous Pt
atoms due to the presence of tin that acts as a diluting component, thus attributing the Sn promotion mainly to the geometric eﬀect. On the other hand, from XPS and H2 or CO
adsorption studies, a change in binding energy (BE) that ismore consistent with the electronic eﬀect was detected.29 The
lowering of the BE for platinum was, in fact, interpreted as anelectron transfer from tin to platinum.
Another bimetallic system similar to Pt–Sn is the Ir–Sn one.
Yet, characterization of them is still rare. Iridium is a noble
metal exhibiting strong hydrogenolysis properties that can be
deeply inhibited by the presence of Sn. In fact, Ir–Sn catalysts,
supported on Na-Y zeolite or silica, showed good performancesin propane dehydrogenation.30 Moreover, this behavior
depends basically on the synthesis method of catalysts and on
the nature of the support. For silica-supported samples, only
bimetallic cluster-derived catalysts showed propene selectivity
up to 95%. In contrast, for Na–Y-supported samples, both bimetallic cluster-derived catalysts and monometallic precursorderived ones were very selective for dehydrogenation.
Some years ago, some of us showed excellent performancesof bimetallic catalysts obtained from Ir–Sn carbonyl cluster
precursors or prepared by deposition of an organometallic Sn
precursor onto pre-formed Ir nanoparticles in the gas-phasedehydrogenation of propane at 773 K.31 In particular, an
optimal selectivity to propene and a high catalyst stability wererecorded. In order to study the generation and structuralfeatures of bimetallic supported Ir–Sn nanoparticles, we deposited two clusters with pre-formed Ir–Sn bonds onto orderedhigh surface area silica supports. In this work, we show theresults of the characterization of such bimetallic clusterderived and monometallic precursor-derived silica supported
Ir–Sn catalysts. We studied both iridium and tin sites bymeans of DRIFTS-monitored CO chemisorption, as well as ofIr L3-edge and Sn K-edge X-ray absorption spectroscopy (XAS)
in order to understand the promotion eﬀect of tin. Indeed, theaim is to understand how tin aﬀects iridium. The correlationof the results of previous catalytic tests with characterization
studies will allow us to better understand the chemicalsurrounding of the active metal sites on the support.

2.

Experimental

2.1.

Catalysts preparation

Two mesoporous silica materials were used as supports: acommercially available non-ordered silica purchased fromGrace Davison (denoted as SiO2; mean pore diameter 20.4 nm,

This journal is © The Royal Society of Chemistry 2013

Paper

Scheme 1 The solid state structures of the clusters [Ir4(CO)10(SnCl3)(μ-SnCl3)]2−
and [Ir6(CO)15(μ3-SnCl3)]−. Blue: iridium; light grey: tin; green: chlorine (carbonylomitted).

total pore volume 1.48 cm3 g−1 and specific surface area
290 m2 g−1) and an ordered purely siliceous MCM-41 (meanpore diameter 2.8 nm, total pore volume 1.36 cm3 g−1 andspecific surface area 1024 m2 g−1). SiO2 was stirred for 5 h in
1 M HNO3 and then washed before use with ultrapure deionised water (MilliQ; 18 MΩ cm−1) until neutrality was reached.
MCM-41 was synthesized with a pore wall thickness of about
2 nm according to the procedure described in the literature.32
Two main synthetic ways were followed to obtain the catalysts: a “single source” method and a “dual source” one.
Single source precursors were (NEt4)2[Ir4(CO)10(SnCl3)2] andNEt4[Ir6(CO)15(SnCl3)] organometallic clusters, prepared as
described elsewhere33 (Scheme 1).
Before impregnation, the non-ordered silica was treated at
773 K in air for 5 h and in vacuo overnight, stirred in MilliQ
water for 4 h, dried and then treated at 573 K in air for 5 h andin vacuo overnight. Similarly, MCM-41 was treated at 573 K in
air and then left in vacuo overnight. Impregnation occurred via
an anhydrous CH2Cl2 (Fluka) solution of the cluster precursor.
The suspension was stirred for 12 h under Ar. The solvent wasthen removed under vacuum in the case of SiO2, whereas byfiltration for MCM-41.
Monometallic and dual source bimetallic catalysts were alsosynthesised using monometallic precursors. First, monometallic Ir samples were prepared by impregnating the supports
with a solution of IrCl3·3H2O (Engelhardt) in a hydrochloric
acid aqueous solution (typically 1.5 mL of 36% HCl in 60 mLof MilliQ water). After drying, the iridium-containing samples
were calcined for 1 h under oxygen flow (100 mL min−1; 573 K)
and then reduced for 1 h under hydrogen flow (100 mL min−1;
643 K). Then, bimetallic samples were synthesised by evaporation of a solution of Sn(n-C4H9)4 (Aldrich, 93%) in anhydrous
toluene (Riedel de Haen).
All operations were conducted under Ar inert atmosphere,
unless otherwise specified, and all catalysts were stored under
Ar atmosphere prior to use. The metal contents of the prepared catalysts, and the nature of the precursors they are
obtained from, are summarised in Table 1.
In summary, Ir5.9 + Sn/SiO2 and Ir1.8 + Sn/SiO2 were obtained
via the dual source method, whereas [Ir4Sn2]/SiO2, [Ir4Sn2]/
MCM-41 and [Ir6Sn]/MCM-41 via the single source one.

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Table 1

Catalysts composition and metal particle dispersion

Catalyst

Precursor

Ir
contenta(wt%)

Sn
contenta
(wt%)

NS/
NTb
(%)

Ir/SiO2
Ir5.9 + Sn/SiO2
Ir1.8 + Sn/SiO2
Ir/MCM-41[Ir4Sn2]/SiO2[Ir4Sn2]/
MCM-41[Ir6Sn]/MCM-41

IrCl3
IrCl3 + Sn(n-Bu)4
IrCl3 + Sn(n-Bu)4
IrCl3(NEt4)2[Ir4(CO)10(SnCl3)2](NEt4)2[Ir4(CO)10(SnCl3)2]

0.86
0.78
0.78
1.28
1.02
0.68

—
0.08
0.27
—
0.29
0.22

58
54
65
70
70
73

(NEt4)[Ir6(CO)15(SnCl3)]

0.71

0.11

71

a

As obtained from the ICP-AES analysis. b Dispersions calculated as
the surface atoms to total atoms ratio from average diameters basedon HR-TEM observations assuming Ir fcc packing in a truncated cubooctahedron geometry for reduced samples only.

2.2.

DRIFT characterization

IR spectra were collected fusing a home-made DRIFTS cell,which allows in situ treatment of the samples,34 inserted in the
beam path of a DigilabFTS-60A spectrometer. The catalysts
were loaded under inert atmosphere in the reaction chamber.
Helium was fluxed for a few minutes (at the rate of 25 mL
min−1). After He purging, H2 was admitted (25 mL min−1) atroom temperature for 15 min. Activation conditions weresimilar to the ones of catalytic tests:31 the catalysts were heated
to 773 K under hydrogen (25 ml min−1) at the ramping rate of
10 K min−1 for 1 h. After cooling down to room temperature inflowing He (25 mL min−1), CO was fluxed in the cell (25 mL
min−1). When the adsorption band intensity reached constantvalues, He was introduced to remove gaseous and weakly
bound CO.
2.3.

EXAFS–XANES spectroscopy

EXAFS (extended X-ray absorption fine structure) and XANES(X-ray absorption near edge spectroscopy) measurements at
the Ir L3-edge and Sn K-edge have been performed on the following samples: (a) pure cluster complexes (NEt4)2[Ir4(CO)10(SnCl3)2] and (NEt4)[Ir6(CO)15(SnCl3)]; (b) silica supported(NEt4)2[Ir4(CO)10(SnCl3)2] and (NEt4)[Ir6CO)15(SnCl3)] samples
after reduction (Table 1), respectively, denoted as [Ir4Sn2]/SiO2,[Ir4Sn2]/MCM-41 and [Ir6Sn]/MCM-41; (c) samples (b) at theend of a catalytic run; (d) the dual step sample Ir1.8 + Sn/SiO2
after reduction; and (e) previous samples, as in (d), at the endof a catalytic run.
The experiments on samples at point (a) have been carriedout at the Sn K-edge and Ir L3-edge at the SAMBA beamline of
the SOLEIL synchrotron in Saclay (Paris, France), using adouble-crystal Si(220) monochromator (resolution of 1.6 eV atthe Sn edge and 0.6 eV at the Ir edge) from the ring operated at
2.75 GeV, and at the KEK-PF-AR synchrotron facility at thebeamline NW10A (Tsukuba, Japan) using a double-crystal
Si(311) monochromator from the ring operated at 6.5 GeV,
respectively.

12716 | Dalton Trans., 2013, 42, 12714–12724

The spectra have been recorded in transmission mode on
samples sealed inside sample-holders loaded under argon
atmosphere and measured at room temperature over a rangeof 1000 eV above the edge, with a scanning step of 0.05 eV and
2–4 seconds integration per point. Each spectrum measurement has been repeated three or four times for the signal-tonoise ratio optimization and for the error bars evaluation.
The Ir L3-edge experiments on samples from points (b) to(e) have been carried out at the XAFS beamline of the Elettra
synchrotron in Trieste (Italy), with a double-crystal Si(311)monochromator and the ring operated at 2.0 GeV. The spectra
have been recorded in transmission mode on samples sealed
inside sample-holders loaded under a nitrogen atmosphereand measured at room temperature over a range of 1000 eV
above the edge, with a sampling step of 2 eV and 2–4 seconds
integration per data point. Each spectrum measurement was
repeated three or four times for the signal-to-noise ratio optimization and for the error bars evaluation.
Data analysis has been performed with the FEFF8 software
package.35 Experimental EXAFS χ(k) functions have been
extracted from the absorption spectra with a standard
procedure and Fourier transformed over a k-range of
25–130 nm−1. The main peaks of the Fourier transformed
modulus have been filtered and analyzed with a nonlinear
least-squares fit program which provides the atomic species,
the coordination number (N) of nearest neighbours, their distance R from the absorber atom and the disorder σDW factor
for each shell.
Sn K-edge XANES spectra on a part of the samples at points(a)–(e) were measured at 290 K on beamline 10B at KEK influorescence detection mode using a Lytle detector. Sn-edge
EXAFS data on a part of the samples at points (a)–(e) wereobtained in transmission mode at KEK-PF-AR. The curve fit
parameters were derived from EXAFS spectra for SnO powder
and (NEt4)2[Ir4(CO)10(SnCl3)2] for Sn–O, Sn–Cl, and Sn–Ir
bonds, respectively. As references standards for the evaluation,
theoretical phase and amplitude functions generated fromFEFF8.235 for Sn–O, Sn–Cl, Sn–Sn, Ir–O and Ir–Ir bonds, the
EXAFS spectra of SnO, SnCl2 and Sn foil, IrO2 and Ir foil,
respectively, have been also employed. The curve fit parameters are first evaluated from the interatomic distances andthe coordination number in crystal data of pure clusters[Ir4(CO)12],36 [Ir6(CO)16],37 (NEt4)2[Ir4(CO)10(SnCl3)2] and (NEt4)[Ir6(CO)15(SnCl3)].38

3.

Results

3.1.

DRIFT characterization

Carbon monoxide is a suitable probe to investigate the chemical nature and the electronic properties of Ir species. Since atroom temperature CO chemisorbs only on Ir sites, it can beused as a selective probe also in the presence of Sn. As acommon behaviour to all samples, CO stretching bands weredetected only above 1900 cm−1, indicating the absence ofbridged CO species.39 In particular, after CO exposure one IRThis journal is © The Royal Society of Chemistry 2013

Dalton Transactions
band appeared in the region between 1900 and 2100 cm−1.
Such evidence, reported also by other authors, is consistent
with the presence of CO linearly chemisorbed on dispersed Ir
metallic particles.40
The attribution of IR absorption bands due to CO adsorbed
on Ir sites is not a trivial task, since a commonly acceptedinterpretation is lacking in the available literature. The particle-size eﬀect was studied by many authors and they generally
agree that an increase in particles size leads to IR bands at
higher frequency.41 However, the same authors have studied
low and highly dispersed Ir particles on alumina and they haveassigned the two bands located at 2050 and at 2075 cm−1
respectively to CO adsorbed on coordinatively-unsaturated(at edges, corners, etc.) sites and on coordinatively-saturated(on planes) sites.
Alternative assignments of high, medium and low frequency bands were proposed by Solymosi et al.42 Three IR CO
absorption bands were reported on reduced 5% Ir/Al2O3 catalysts located respectively at 2102, 2081 and 2040 cm−1. The
2012 and 2040 cm−1 frequency bands were attributed to symmetric and antisymmetric stretchings of Ir dicarbonyl species,
while the one of 2081 cm−1 was attributed to CO linearlybonded to Ir clusters. Notably on silica supported 1% Irsample, the authors found only one band located at
2070 cm−1.
Comparable results were found by Tanaka et al. in aluminaand silica-supported Ir4(CO)12 catalysts.43 The presence of tin
in bimetallic Ir–Sn catalysts caused a diﬀerent IR adsorption
pattern and a general decrease in intensity was observed. For
all bimetallic samples, obtained via either the single or dual
source approach, the low wavenumber components of theadsorption bands increased in the IR pattern.
An alternative attribution of IR bands was proposed by
McVicker et al.44 The high frequency band (2060 cm−1) was
attributed to Ir(CO)2-dicarbonyl species with two non-equivalent CO, thus showing only one CO stretching, the low frequency band (located at 2020 cm−1) was attributed to Ir(CO)monocarbonyl species. Since smaller particles present ahigher fraction of edge and corner atoms able to chemisorb up
to two CO per Ir atom, whereas Ir atoms in larger particles canchemisorb only one CO per metal atom, high frequency bands
were attributed to highly dispersed Ir catalysts, and the low frequency one to samples with lower dispersion. A similar band
located at 2070 cm−1 was observed by Erdöhely et al. for highly
dispersed Ir on silica, magnesia and titania.40
Our results were in good agreement with those of McVicker
and Erdöhely. The most dispersed samples are the ones supported over high-specific surface area MCM-41, showing linear
adsorbed CO absorption falling at higher wavenumbers withrespect to non-ordered SiO2 supported catalysts that, in turn,show bands located at lower wavenumbers. A strong dependence of band position on metal dispersion was found formonometallic Ir catalysts supported on silica materials(namely Ir/SiO2 and Ir/MCM-41) (Fig. 1a,b). Highly dispersed
metallic particles caused a blue shift of CO IR absorption
bands. In detail, in highly dispersed Ir/MCM-41 samples anThis journal is © The Royal Society of Chemistry 2013

Paper

Fig. 1 DRIFT spectra for CO absorption on (a) Ir/MCM-41; (b) Ir/SiO2; (c) Ir1.8 +Sn/SiO2; (d) [Ir4Sn2]/MCM-41; (e) [Ir6Sn1]/MCM-41; (f ) [Ir4Sn2]/SiO2.

asymmetric CO chemisorption band at ca. 2070 cm−1 wasdetected (Fig. 1a), whereas a decrease in intensity and a red
shift to 2040 cm−1 occurred for less dispersed Ir/SiO2 samples(Fig. 1b). The asymmetric adsorption revealed two main
components: the main one located at 2039 cm−1 and a weakshoulder at 2003 cm−1 (a very weak shoulder at about
2070 cm−1 is also present). After deposition of Sn and activation, the IR pattern in the carbonyl region changed further(Fig. 1c). The presence of Sn, from Sn(n-Bu)4, mainly aﬀected
the relative intensity of the component of the CO linearlyadsorbed on the nanoparticles at 2039 cm−1. In particular, forthe sample Ir1.8 + Sn/SiO2 the band intensity at 2039 cm−1
decreased. Then, the peak maximum shifted to 2030 cm−1 andthe shoulders at 2005 cm−1 and at 2070 cm−1 became moreevident.
MCM-41 supported samples revealed similar features witha diﬀerent relative intensity. Monometallic Ir catalyst showed abroad band with a maximum at 2067 cm−1, typical of highly
dispersed metallic particles; two shoulders located respectively
at 2034 and 2000 cm−1 were also observed.
In conclusion, for dual source samples three bands ascribable to adsorbed CO were revealed. As a general consideration
the relative intensity between these bands is strongly influenced by two main factors: the particles dimension for monometallic Ir samples and the presence of Sn for bimetallic ones.
The addition of tin decreased the overall CO chemisorption,
but influenced strongly the adsorption band at higher wavenumbers (2070 and 2030 cm−1); thus, the relative intensity ofthe band at 2000 cm−1 was higher for bimetallic catalysts thanthe monometallic ones.
Single source catalysts showed some diﬀerences in IR
absorption patterns, when compared with dual source derived
samples (Fig. 1d–f ). In fact, [Ir4Sn2]/MCM-41 revealed a COadsorption band with a maximum at 2020 cm−1 and two
shoulders, one located at 2070 cm−1 and a more relevant one
at 2000 cm−1. In the same way, the non-ordered silica supported cluster, [Ir4Sn2]/SiO2, showed the same band after activation, but with a lower contribution of the two shoulders. In
contrast, three similar components were detected for [Ir6Sn1]/

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MCM-41, although their relative intensity was completely
diﬀerent (Fig. 1e). Indeed, also for this sample the band
maximum was centred on the middle wavenumbers position,
but with a more relevant contribution of a shoulder at higher
wavenumbers. In conclusion, we found a general shift of theabsorption band maxima towards lower wavenumbers when
we compare cluster-derived samples to dual source-derived
ones. This suggests a better interaction between Ir and Sn whena bimetallic precursor is used for the synthesis. In addition,
the relative intensity of lower components was more relevant.
In order to study the samples surface stability under reaction conditions, CO chemisorption studies were also performed on the Ir/MCM-41 and [Ir4Sn2]/MCM-41 systems after areaction time of 24 h. Before chemisorption experiments the
samples were treated again under H2 at 773 K for 1 h. During
such a treatment no CH4 evolution was detected by means of
on-line QMS analysis on the outlet stream. Nevertheless, somecoke residues were detected on the catalysts after 24 h on
stream (1.06 and 2.42 wt% in carbon content for Ir/MCM-41
and [Ir4Sn2]/MCM-41, respectively, as from elemental C, H, N
analysis). It is thus likely that these carbonaceous deposits,
which are stable under such conditions, are graphitic innature (the so-called “hard coke”).45
After catalysis, Ir/MCM-41 samples showed a decrease in IR
absorption and a shift of absorption maximum of the chemisorbed CO band to 2050 cm−1. As a result of the reaction, thehigher component at 2070 cm−1 was not present any more.
Also the IR pattern for [Ir4Sn2]/MCM-41 changed after catalysis(Fig. 2). The intensity ratio of the 2020 to 1995 cm−1 band wasreverted, now the 1995 cm−1 band being more intense. Thus,
as a general behaviour, after catalysis there was a shift of the
CO adsorption bands towards lower wavenumbers. Furthermore, this shift was more evident for the monometallic
Ir sample than for the cluster-derived bimetallic one.
3.2.

XANES

The Ir L3-edge XANES analysis (Fig. 3) evidenced that the twopure clusters (NEt4)2[Ir4(CO)10(SnCl3)2] and NEt4[Ir6(CO)15(SnCl3)],

Fig. 2 DRIFT spectra for CO absorption on (a) Ir/MCM-41 before catalysis; (b)Ir/MCM-41 after 24 h catalysis; (c) Ir4Sn2/MCM-4 before catalysis; (d) Ir4Sn2/
MCM-41 after 24 h catalysis.

12718 | Dalton Trans., 2013, 42, 12714–12724

Dalton Transactions

Fig. 3 Ir L3-edge XANES spectra for Ir metal (g), Ir/SiO2 (f ), Ir1.8 + Sn/SiO2 (e),(NEt4)2[Ir4(CO)10(SnCl3)2] (d), [Ir4Sn2]/SiO2 (c), NEt4[Ir6(CO)15(SnCl3)] (b), and[Ir6Sn]/MCM-41 (a).

exhibit an Ir edge energy position and white line structure
corresponding to the Ir0 state due to the predominant
metal–metal contribution from the core of the metal clusters (Scheme 1). This trend is also in agreement with the CO
IR band position which corresponds to those of the complex[Ir4(CO)12]0,46 while an absorption edge shift of 2 eV towards
lower energy with respect to the reference metal foil wasobserved in the data for all impregnated-reduced catalyst
systems, demonstrating an average electron-rich character of
Ir interacting with the support and/or the Sn atoms in contact
with the catalyst metal particles. Any electron density variation
on Ir among the diﬀerent catalyst systems, due to particle size
eﬀect or interaction with the Sn, was below the energy resolution attained, under these conditions, in the spectra at theIr L3 edge.
The XANES analysis at the tin K-edge showed the following
results: in the pure bimetallic cluster (NEt4)2[Ir4(CO)10(SnCl3)2]
dispersed in boron nitride (BN), the energy values of the Sn
absorption edge and the white line peak top were close to
those for the SnO reference compound (Fig. 4b,d), so the Sn
species was essentially maintaining the native divalent state.
Due to diﬀerent coordination environments for the bimetallic
cluster, the post-edge peak features were diminished, and thewhite line peak top intensity to post-edge depth ratio was low
as compared to SnO. The catalyst prepared from bimetallic
cluster NEt4[Ir6(CO)10(SnCl3)] impregnation and following
reduction in H2 at 773 K exhibited higher edge energy, close to
the Sn4+ state (Fig. 4c,g). The [Ir4Sn2]/SiO2 catalyst preparedfrom the bimetallic [NEt4]2[Ir4(CO)10(SnCl3)2] cluster after

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Dalton Transactions

Paper
energy shift is slightly smaller (0.2–0.3 eV) for the supported
bimetallic cluster on MCM-41 compared for that for Irx + Sn/SiO2.
This may be due to the diﬀerence of average metal particle
size: 1.5 nm for supported bimetallic cluster samples and1.9 nm for Irx + Sn/SiO2 samples.
3.3.

Fig. 4 Sn K-edge XANES spectra for Sn metal (a), SnIIO (b), SnIVO2 (c),(NEt4)2[Ir4(CO)10(SnCl3)2] (d), Ir1.8 + Sn/SiO2 (e), Ir5.9 + Sn/SiO2 (f ), [Ir6Sn]/MCM-41(g), and [Ir4Sn2]/SiO2 (h).

treatment at 773 K in H2 also showed a tetravalent state of Sn
species even though starting Sn was divalent (Fig. 4c,h). Conversely, the reduction of the samples synthesized by depositionof Sn(n-Bu)4 led to a Sn edge position comparable to that of
SnO2. In the latter case the Sn absorption edge and white line
peak top energy values were the highest for the catalyst withthe lowest Sn contents (Ir/Sn = 6) (Fig. 4e,f ). Such a behaviour
can be correlated to the EXAFS edge shift, due to the diminution in electronic density (see section 3.3). Also the white line
peak top to post-edge depth ratio decreased with an increaseof Sn amount loaded in the catalysts (Ir/Sn = 2).
The spectra (e) and (f ) for Irx + Sn/SiO2 samples (Fig. 4)
were basically similar to the one for SnO2, but at a closer look,a weak shoulder feature was present in such spectra at 29 201
eV that was absent in spectrum (c) for SnO2 (Fig. 4; referenceline position). This shoulder feature suggests also the presenceof metallic Sn sites in Ir nanoparticles in addition to tetravalent Sn sites.
In summary, Sn K-edge XANES patterns for the Ir–Sn
species supported on SiO2 and MCM-41 after reduction weresimilar to SnO2, but the white line peak top to post-edge depth
ratio was the highest for [Ir4Sn2]/SiO2.
In particular, the trend in Fig. 4 for the Irx + Sn/SiO2
samples evidences how the absorption edge progressively
shifts towards the lower energy side, as the Sn amount in the
catalyst increases. This trend implies that Sn acts as aneﬀective electron donor to Ir and the donation extent per Sn
atom is greater as the Sn amount becomes relatively smaller.
In fact, the higher the total Sn amount, the higher is theoverall electron-rich character of Ir. Moreover, the oxidation
state for Sn of (IV) is reached when the Ir/Sn ratio is 6 (Fig. 4c,f ). The XANES spectra for [Ir4Sn2]/MCM-41 and [Ir6Sn]/
MCM-41 are basically similar to those for Ir + Sn/SiO2 (Ir/Sn =2 and 6), respectively (spectra not shown). The extent of edge

This journal is © The Royal Society of Chemistry 2013

EXAFS

The fit results for the Ir L3-edge of [Ir4Sn2] and [Ir6Sn] on
MCM-41 samples showed (Fig. 5 and Table 4) the formation ofvery small metal particles with a coordination number for the
Ir–Ir scattering path of 3.4 and 3.1, respectively. Namely, for
the Ir–Ir shell, an N of 4 corresponds to 0.8–0.9 nm average
diameter of the Ir particle. A strong interaction with surface
oxygen atoms was evidenced by the short distance Ir–O scattering path resulting from the fit. The metal dispersion of thetwo cluster-derived catalysts over MCM-41 is higher than thatin any other reduced sample (see Table 4), in agreement withthe average metal particle sizes detected by HR-TEM. Notably,
even with a molar ratio of Ir/Sn of 1.8, no Ir–Sn scattering path
was detected. This suggests a partial segregation of Sn species
at the interface between the Ir nanoparticle and the silica
support.
After reduction the structural evolution of the two clusterderived catalysts was dramatically diﬀerent on non-orderedsilica and on MCM-41. In fact, on silica, larger metal particles
were produced (an NIr–Ir of 6.5 corresponding to 1.4 nm
average diameter) with respect to the 0.9 nm average particle
size on MCM-41, with a parallel decrease of the support
surface interaction contribution. The reduction of the silicasupported system obtained via the dual source indeed produces larger particles (Table 4). It is however worth mentioning that the catalytic properties of these samples arecomparable. This excludes, therefore, a primary eﬀect on thedehydrogenation activity of the dispersion or a relevant interaction with the support oxygen of the metal particles.
The Sn K-edge data of [Ir4Sn2]/MCM-41 and [Ir4Sn2]/SiO2
catalysts suggest the coexistence of Sn interacting with oxygenFig. 5 Fourier transform of the Ir L3-edge EXAFS spectra ( phase shift uncorrected) of [Ir4Sn2]/MCM-41 (red), [Ir4Sn2]/SiO2 (black) and Ir1.8 + Sn/SiO2(blue) samples.

Dalton Trans., 2013, 42, 12714–12724 | 12719

PaperTable 2

Dalton Transactions
EXAFS data ﬁt at the Sn K-edge for pure compounds and silica-supported catalysts

Shell Sn–O

Shell Sn–O

Shell Sn–Cl

Shell Sn–Ir and Sn–Sn

Shell Sn–Ir and Sn–Sn(NEt4)2[Ir4(CO)10(SnCl3)2]——[Ir4Sn2]/MCM-41

N = 4.6
σ = 0.063
R = 2.040
ΔE = 4.4
N = 3.3
σ = 0.063
R = 2.037
ΔE = 1.2
N = 2.0
σ = 0.075
R = 1.997
ΔE = −21.5—

N=3
σ = 0.056
R = 2.426—

N=2
σ = 0.05
R = 2.584———

N = 4.5
σ = 0.050
R = 2.106
ΔE = −1.0—

N = 0.8
σ = 0.069
R = 2.502
ΔE = 6.6—

N=1
σ = 0.05
R = 2.718
N = 4.6
σ = 0.089
R = 2.865
ΔE = −21.1
N = 1.2
σ = 0.069
R = 2.830
ΔE = −33.6
N = 0.6
σ = 0.035
R = 2.887
ΔE = −41.3[Ir4Sn2]/SiO2

Ir1.8 + Sn/SiO2

The distance is expressed in angstroms, the DW factor in angstroms, and the energy shift in eV.

Table 3

EXAFS data ﬁt at the Ir L3-edge for pure compounds

Ir4(CO)12Ir6(CO)16(NEt4)2[Ir4(CO)10(SnCl3)2]

Shell Ir–C

Shell Ir–Sn

Shell Ir–Sn2

Shell Ir–Ir

Shell Ir–Ir

Shell Ir–O

N=3
σ = 0.085
R = 1.86
ΔE = 11.8
N=2
σ = 0.085
R = 1.86
ΔE = 11.8
N = 2.5
σ = 0.085
R = 1.86
ΔE = 11.8

—

—

—

—

—

N = 1.5
σ = 0.079
R = 2.580
ΔE = 8.9

—
—
—
—

N=3
σ = 0.019
R = 2.67
ΔE = 0
N=4
σ = 0.07
R = 2.77
ΔE = 0N = 1.5
σ = 0.019
R = 2.67
ΔE = 0

N=3
σ = 0.079
R = 3.24
ΔE = 0
N=2
σ = 0.079
R = 3.05
ΔE = 0
N=2
σ = 0.079
R = 3.24
ΔE = 0—N = 1.5
σ = 0.009
R = 2.80
ΔE = 0The distance is expressed in angstroms, the DW factor in angstroms, and the energy shift in eV.

and metallic Sn–Ir alloy (Fig. 6), as evidenced by an Sn–O scattering path at around 0.2 nm and two Sn–Ir ones at 0.25 nm,
detected only for the SiO2 supported catalyst, and at 0.28 nm.
Due to the featureless pattern of Sn K-edge XANES for supported Ir–Sn samples, the shoulder peak at 29 201 eV due toSn(0) sites was almost buried beneath the pattern of Sn(IV)sites. In addition, the energy resolution at the Sn K-edge wasgreater (3–5 eV) compared to that at the Ir L3-edge and the contribution from the minor component was diﬃcult to detect in
the broader spectra. In this sense, Sn K-edge EXAFS was moresensitive in detecting metallic Sn–Ir alloy as the interatomic
pair peak in the Fourier transform (Fig. 6), rather than theoverlapping, broad pattern in the Sn K-edge XANES (Fig. 4).
The population of alloy seems to be greater in the caseof MCM-41. The absence of a long distance Sn–O isconsistent with the absence of SnO2 nanoparticles.47 No Cl
surrounding the Sn, which was present in the precursor
organometallic cluster, was evidenced for either sample. Insummary, after reduction, the Cl contribution was completely
removed by O neighbours and some of the Sn was alloyedwith Ir in well-dispersed metal particles. No bulk tin oxide
was formed.

12720 | Dalton Trans., 2013, 42, 12714–12724

For the dual source-derived catalyst, the fit results of the
EXAFS data of the Ir L3 edge showed that by increasing
the Ir/Sn atomic ratio from 2 to 6, no relevant variation in theIr surrounding, i.e. the NIr–M coordination number, was detectable (Tables 3 and 4).
Interesting is the presence of a second Sn–O scattering path
at 0.28 Å for the fit of the EXAFS Sn K-edge data of Ir1.8 + Sn/
SiO2, which may be consistent with the presence of bulk
SnO2,47 and the lower coordination number for the path Sn–Ir.
The fit result is consistent with a lower amount of Sn in the Irparticles and the presence of some bulk SnO2.
After a catalysis run of 24 h, the average size of Ir nanoparticles over Ir/MCM-41 catalyst (1.8 nm) was about twice as large
as on the fresh reduced catalyst (0.7 nm), while the [Ir4Sn2]/
MCM-41 metal particles do not increase in size (0.9–1.1 nm;
Fig. 7). For both samples after catalysis, the peak corresponding to the O surface atoms (0.18 nm, phase shift uncorrected) was significantly reduced, even in the case where
the metal phase remained well dispersed. This fact strongly
suggests that the Ir atoms previously in intimate contact withthe support were covered by carbon residues due to thedehydrogenation of propane.

This journal is © The Royal Society of Chemistry 2013

Dalton Transactions

Paper

Table 4 EXAFS data ﬁt at the Ir L3-edge, for silica- and MCM-41-supported
catalysts

Shell Ir–O
[Ir4Sn2]/SiO2—[Ir4Sn2]/MCM-41

N = 3.5
σ = 0.076
R = 2.000
ΔE0 = 14N = 4.9
σ = 0.10
R = 1.993
ΔE0 = 12.6—

[Ir6Sn]/MCM-41

Ir1.8 + Sn/SiO2

Ir5.9 + Sn/SiO2

Ir/MCM-41

Ir/MCM-41
after 24 hcatalysis run[Ir4Sn2]/MCM-41
after 24 hcatalysis run

N = 1.8
σ = 0.078
R = 1.992
ΔE0 = 12.8
N = 4.9
σ = 0.10
R = 1.990
ΔE0 = 13.9
N = 1.8
σ = 0.10
R = 1.973
ΔE0 = 11.7
N = 2.8
σ = 0.10
R = 1.961
ΔE0 = 11.4

Shell
Ir–Ir andIr–Sn
N = 6.5
σ = 0.079
R = 2.692
ΔE0 = −2.8
N = 3.4
σ = 0.085
R = 2.597
ΔE0 = −10
N = 3.1
σ = 0.081
R = 2.666
ΔE0 = 0.0
N = 6.9
σ = 0.074
R = 2.702
ΔE0 = 0.0
N = 6.9
σ = 0.078
R = 2.691
ΔE0 = 0.0
N = 2.8
σ = 0.071
R = 2.691
ΔE0 = 0.0
N = 7.6
σ = 0.085
R = 2.680
ΔE0 = 0.3
N = 4.7
σ = 0.091
R = 2.663
ΔE0 = 0.2

Shell
Ir–Sn

DEXAFS(Å)

DHR-TEM

—

14

11.8

—

9

10.9

—

8

11.5

—

15

13.6

—

15

17.9

—

7

11.9

—

18

—

11

Fig. 6 Sn K-edge FFT EXAFS spectra of [Ir4Sn2]/MCM-41, [Ir4Sn2]/SiO2 andIr1.8 + Sn/SiO2 after reduction under H2 ﬂow.

10.8

The distance is expressed in angstroms, the DW factor in angstroms,
and the energy shift in eV.

4.

Discussion

In a previous paper31 mainly focused on the catalytic behaviorof Ir–Sn bimetallic catalysts, the catalytic performances wereascribed to geometrical eﬀects, inhibiting the formation of
adjacent-atom aggregates (responsible for hydrogenolysis), as
well as to electronic eﬀects, reducing the electronic density of
the Ir sites, exerted by Sn atoms. Ir–Sn systems, obtained via
the single source method, showed very high selectivity (95%) topropene at interesting values of propane conversion (18%) for
the dehydrogenation of propane at 773 K and a high stability
during the reaction time (negligible loss of activity and no
agglomeration of the metallic particles).31 However, neither
the exact localization of Sn with respect to Ir sites nor the electronic eﬀect of the Sn sites on the Ir ones (by evaluation of theoxidation states of Sn species) could be determined in thatwork. Such parameters are of great importance for making
hypotheses on structure–activity relationships and, thus, fora rational design of nanostructures and, in particular, ofeﬀective and selective catalytic sites.
Upon impregnation and interaction with the support
surface, the (NEt4)2[Ir4(CO)12(SnCl3)2] cluster deposited onto

This journal is © The Royal Society of Chemistry 2013

Fig. 7 Ir L3-edge FFT EXAFS spectra (phase shift uncorrected) of [Ir4Sn2]/MCM-41
before (red) and after a 24 h catalysis run (green).

MCM-41 did not change the metal frame nuclearity (similar
coordination numbers at EXAFS Ir-edge and Sn-edge). The two
Sn–Ir distances characteristic of the cluster cage structure are
only slightly changed into one at longer R (2.847 Å). After
reduction, the Cl contribution was completely removed by
O neighbours and the Sn is alloyed with Ir in small metal
particles. No bulk tin oxide was formed. The same happens
also when starting from the Ir6Sn cluster.
The Sn K-edge XANES data show that as the Sn amount in
the cluster is decreased, the absorption edge shifts toward thehigher energy side. This trend suggests that Sn acts as aneﬀective electron donor to Ir and the donation extent per Sn
atom is greater as the Sn amount became relatively smaller.
The higher the total Sn amount, the higher is the overall electron-rich character of the Ir.

Dalton Trans., 2013, 42, 12714–12724 | 12721

Paper
The morphology of the samples evolved in a diﬀerent way
upon reduction, depending on the sample preparation
approach. In fact, by reducing the samples prepared depositing bimetallic clusters on silica (single source), small alloy
metal particles with Sn interacting with surface O atoms areformed with an increase of the average metal cage size withrespect to the starting cluster (metal coordination around Ir
6.5 vs. 3.5 corresponding to very small particles of about
30 atoms, consistent with an average particle diameter of
1.2 nm, as from HR-TEM data). Furthermore, the total Ir
coordination around Sn decreases from 4 to 2, suggesting that
only 50% of the Sn atoms are still alloyed with Ir, the remaining being atomically dispersed on the support surface (eventhough no evident presence of bulk tin oxide aggregates was
detected by EDX analysis in the range of 5 nm scale31).
In contrast, by reduction of the samples prepared via thedual source approach supported on non-ordered silica, some
SnOx species were produced together with small alloyed Ir–Sn
metal particles and a further increase of the average
metal cage size was revealed based on the Sn and Ir EXAFS
analyses.
The dual step system Ir1.8 + Sn/SiO2 after reduction shows
metal particles with an average size of 1.4 nm (as from
HR-TEM, and N = 6.9 at Ir-edge EXAFS), with an even lower
fraction of tin alloyed with Ir (N = 0.6 at Sn-edge EXAFS) andthe formation of some bulk tin oxide particles.
By varying the molar ratio of Ir/Sn, the metal particle size
after reduction does not significantly change, as Ir-edge EXAFS
and HR-TEM provide average diameters of 1.7–1.8 nm for theIr5.9 + Sn catalysts.
The Sn–Ir alloy seems to be the active phase for the selective
dehydrogenation, most typically observed for [Ir4Sn2]/MCM-41(Table 2; N(Sn–Ir) = 4.6 in total). The dominant oxidic Sn
species, most typically observed for Ir1.8 + Sn/SiO2 (NSn–O =6.5–6.6 in total), plays virtually no role in catalysis. The catalytic results of Ir1.8 + Sn/SiO2 were indeed similar to those of
Ir/SiO2. The curve fit results for [Ir4Sn2]/SiO2 showed an intermediate situation between the two samples: a mixture of Ir–Sn
alloy and Sn in the higher oxidation state. The extent of Ir–Sn
alloy formation was correlated with the selective dehydrogenation catalysis.
In addition, the eﬀect of Sn–Ir interaction on the MCM-41supported catalysts structure is clearly visible in the following
behaviour. After 24 hours of catalysis, the Ir catalyst underwent
a consistent sintering eﬀect (the final metal particle average
size increased from 0.7 to 1.8 nm, which is more than twice aslarge as in the freshly reduced sample), with a corresponding
decrease of the total amount of chemisorbed CO. In contrast,
the [Ir4Sn2]-derived catalyst preserved intact its starting size
(1.1 nm) and its linear CO components ratio and position: theCO chemisorption decrease was due to a partial metal surface
coverage by carbonaceous residues, as suggested also by the Iredge EXAFS data and by C, H, N elemental analysis, showing a
total carbon content of 2.42 wt%.
A structural model can be proposed (Scheme 2), where tin
is present in two states: (i) as metallic tin, Sn(0), in bimetallic

12722 | Dalton Trans., 2013, 42, 12714–12724

Dalton Transactions

Scheme 2

Ir–Sn nanoparticles; (ii) as oxidized phase, SnOx, where x =1–2, at the interface between the bimetallic Ir–Sn nanoparticles and the silica supports. The relative distribution of tin in
the two phases (and oxidation states) strongly depends on themetal precursor, on the tin loading, and to a minor extent on
the silica support.
The presence of a layer of SnOx on the surface of metallic
nanoparticles cannot be excluded, as it is reported for other bimetallic systems, like Ru–Sn.13 At high Sn loadings, Sn could
be also present as segregated SnOx nanoparticles dispersed on
the support.
On both samples, after catalysis, the peak corresponding to
the O surface atoms largely decreased, even in the absence of aremarkable sintering of metal particles (NIr–Ir increased from
3.4 to 4.7 and particle mean diameter from 0.9 to 1.1 nm),which strongly suggests that some of the Ir atoms previously instrong contact with the support are now covered by carbon
residues, as suggested also by the strong decrease of theoverall integrated area of DRIFT spectra CO bands and confirmed by values of C, H, N elemental analysis spanning therange of 0.9–2.9 wt% of carbon.

5.

Conclusions

Ir–Sn bimetallic particles, supported on silica with diﬀerent
morphologies, showed remarkable catalytic properties in theselective dehydrogenation of light alkanes. The surface Ir–Sn
can be obtained via deposition of the molecular organometallic Ir–Sn clusters or via deposition of an Sn organometallic precursor onto pre-formed Ir metal particles.
From our data, a neat ensemble eﬀect in the catalyst could
not be established, since Sn–Ir and Ir–Ir interactions were notclearly distinguishable. However, a noteworthy Sn → Ir
donation was shown by XANES measurements, suggesting atleast a relevant electronic eﬀect in the metallic particles. It is
likely that a concurrent counterbalance of the two eﬀects leads
to the peculiar catalytic properties of these solids in thedehydrogenation reaction.
On the basis of the analytical techniques employed, someappreciable diﬀerences could be pointed out, depending on
the nature of the precursors. The particles prepared from themolecular bimetallic clusters are more stable, both in dimensions and in composition: chlorides are immediately lost upon
reduction, the Ir–Sn ratio showed small variations, and theThis journal is © The Royal Society of Chemistry 2013

Dalton Transactions
metal particle dimensions were relatively constant, even if
much larger than the molecular dimensions of the starting
clusters. In contrast, the particles prepared via the dual source
approach showed the formation of tin oxide, corresponding toan increase of the Ir/Sn ratio in the alloy and a larger size.
Thus, the presence of SnOx species, decorating the Ir particles,
does not hamper sintering of the noble metal. Finally, theSn–Ir ratio in the precursor seems to be of little influence forthe properties of the final supported metal particles.

Acknowledgements
The SOLEIL synchrotron light laboratory in Saclay (Paris,
France) and the staﬀ of the SAMBA beamline are gratefully
acknowledged for the support and technical assistance. The Xray absorption experiments were performed with the approvalof the Photon Factory Proposal Review Committee (No.
2005G221) and the SPring-8 Program Review Committee(2004A0122-NXa300-np and 2003B0386-NXa-np). R.P., M.G.
and V.D.S. thank the Italian Ministry of Education, University
and Research through the Project “ItalNanoNet” (Rete Nazionale
di Ricerca sulle Nanoscienze; Prot. No. RBPR05JH2P).

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