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RESEARCH PAPER
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C. Prestipino,*a L. Capello,wa F. D’Acapitob and C. Lamberti*a
www.rsc.org/pccp
Local structure of [CuI(CO)2]1 adducts hosted inside ZSM-5
zeolite probed by EXAFS, XANES and IR spectroscopies
a
Department of Inorganic, Physical and Materials Chemistry and NIS Centre of Excellence,
University of Turin, Via P. Giuria 7, 10125 Torino, Italy. E-mail: [email protected];
Fax: þ39011 6707855; Tel: þ39011 6707860
b
INFM-OGG, c/o ESRF, GILDA CRG, BP 220, F-38043 Grenoble, France
Received 18th January 2005, Accepted 21st February 2005
First published as an Advance Article on the web 15th March 2005
EXAFS spectroscopy, analysed in the frame of the multiple scattering theory, has been able to determine the
local structure of [Cu(CO)2]1 complexes hosted inside ZSM-5 channels upon contacting the activated zeolite with
CO from the gas phase at room temperature. We found that the number of coordinated CO molecules (1.8 0.3) is in good agreement with the [Cu(CO)2]1 stoichiometry suggested by IR. The Cu–C distance obtained for
the [Cu(CO)2]1 complex is 1.88 0.02 Å, with a C–O distance (1.12 0.03 Å). This work complements a
previous one [C. Lamberti, G. Turnes Palomino, S. Bordiga, G. Berlier, F. D’Acapito and A. Zecchina, Angew.
Chem. Int. Ed., 2000, 39, 2138], performed at liquid nitrogen temperature, where the structure of [Cu(CO)3]1
complexes was identified by combined EXAFS/XANES/IR spectroscopies. An increase of the Cu–C distance of
0.05 Å by moving from [Cu(CO)2]1 to [Cu(CO)3]1 complexes has been observed, which is the local
rearrangement needed to accommodate a third CO ligand in the first coordination shell of copper. EXAFS
determined that the Cu–C–O bond angle is linear within the error bars (170 101), while IR and XANES
indicate that intrazeolitic [Cu(CO)2]1 complexes have C2v symmetry. The experimentally obtained moieties are in
good agreement with the values obtained with advanced quantum mechanical methods.
DOI: 10.1039/b500780a
1. Introduction
Copper is a transition metal element playing a remarkable role
in red-ox catalysis of several selective, tri-dimensionally organized, inorganic, organic and biological systems.1 In all of
these catalysts the structure of the active sites is rather complex, needing often the interplay between two adjacent copper
centres to operate. Copper(I) exchanged zeolites can be considered as the best known type of inorganic tri-dimensionally
organized catalysts, because of their crystalline structure and of
the controlled dispersion of the metal centres which ensure a
good structural definition to the adsorbing sites.2
It is well stated that copper exchanged ZSM-5 zeolites are
active catalysts in the direct decomposition of NO into N2
and O2.3
The study of this catalytic process deserves a great deal of
practical interest, as nitric oxides are known to be a major
cause of air pollution. For this reason, Cu-ZSM-5 and other
copper-exchanged molecular sieves have been the subject of
many papers having appeared in recent years. Most of the data
reported up to now refer to over-exchanged samples4 prepared
via conventional ion exchange with aqueous solutions of cupric
precursors. As a result of this procedure, samples containing
mixtures of copper ions in different aggregation and oxidation
states are usually obtained. In order to avoid the heterogeneity
of copper species present in the sample (which complicates the
characterisation of the active sites), an alternative method of
preparation can been used. This method is based on the
reaction between the zeolite (in the protonic form) and gaseous
CuCl at 573 K.5–10 The material thus obtained can be considered as a model solid, as it only contains well defined
isolated copper species in a single oxidation state.7 In fact,
because of its model character, the system is able to give clear
w Present address: ESRF, BP 220, F-38043 Grenoble, France.
and simple spectroscopic, energetic and structural data that
can be used for comparison with computational calculations
obtained by advanced quantum chemical methods.11 The high
reactivity of such Cu1 cations, that are able to fix N2 molecules
even at room temperature,7,12 has been explained in terms of a
remarkably high coordinative unsaturation. Very recently, it
has been observed that cuprous ions hosted in ZSM-5 zeolite
are even able to bind H2 at room temperature and relatively
low equilibrium pressures.13–15 Cuprous ions are in fact able to
form, depending on equilibrium pressure and temperature,
Cu1(NO), Cu1(NO)2, Cu1(CO), Cu1(CO)2 and Cu1(CO)3
complexes of high structural and spectroscopic quality to be
compared with the analogous complexes typical of homogeneous chemistry.16
2. Experimental
Cu1-ZSM-5 samples have been prepared via the gas phase
exchanged and gaseous CuCl at 573 K as anticipated in the
Introduction and as described in detail elsewhere.7
EXAFS and high resolution XANES spectra were collected
at the ESRF (GILDA BM8). The monochromator was
equipped with two Si(311) crystals, while harmonic rejection
was achieved using Pd mirrors having a cut-off energy of
21 keV. To ensure high quality XANES spectra, the geometry
of the beamline was optimised to improve the energy resolution: vertical slits, located at 23 m from the source, were set to
0.4 mm, ensuring at 9 keV an actual energy resolution better
than 0.3 eV. A direct energy/angle calibration for each spectrum has been performed by measuring the absorption spectrum of a Cu-metal foil located after the second ionisation
chamber, as described elsewhere.17 This experimental set-up
avoids any problems related to small energy shifts. The first
maximum of the XANES derivative spectrum of the Cu metal
foil (corresponding to the 1s - 4p electronic transition of Cu0
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Phys. Chem. Chem. Phys., 2005, 7, 1743–1746
1743
has been defined as 8979.0 eV. The EXAFS spectra, collected
three times and averaged before the data analysis (before and
after CO dosage), have been sampled up to 9800 eV with a
variable step in energy resulting in a maximum step of 0.05 Å1
in k space. A description of the cell used to measure an
activated sample under controlled temperature and atmosphere can be found elsewhere.18
For IR spectra a Brucker IFS 66 FTIR spectrometer
equipped with an HgCdTe cryodetector has been used. For
all spectra a resolution of 2 cm1 has been adopted. Samples
have been studied in transmission mode on thin self-supported
wafers. CO has been dosed in the gas phase through a vacuum
manifold directly connected with the measure cell on containing the activated zeolite sample, as described in more detail
elsewhere.7
3. Results and discussion
IR spectroscopy has been very informative on the structure of
intrazeolitic [CuI(CO)n]1 (n ¼ 1, 2, 3) species and the main
results can be summarized as follows.6–9 (i) At low CO
equilibrium pressures (pCO) linear CuI(CO) complexes are
formed, which are characterized by n~(CO) ¼ 2157 cm1. (ii)
By increasing pCO the formation of C2v [CuI(CO)2]1 adducts is
observed, as testified by the doublet at(~
n sym(CO) ¼ 2178 cm1
1
and n~asym(CO) ¼ 2151 cm , due to the symmetric and
antisymmetric stretching modes of the di-carbonyl complex,
see Fig. 1a. (iii) By decreasing the temperature to about 80 K
CuI(CO)3 complexes in C3v symmetry are obtained characterized by appearance of a new IR doublet at n~(CO) ¼ 2167 and
2192 cm1. It is worth noting that homogeneous counterparts
form linear [CuI(CO)2]1 complexes and planar [CuI(CO)3]1
adducts of DNh and D3h symmetry, respectively.19 The distortion from the ideal linear and planar symmetry, observed for
di- and tri-carbonyl species, is associated with the interaction
with the zeolite walls acting as a polydentate ligand. Following
ref. 20, this distortion can be considered as an external parameter which is reflecting the interaction with the negatively
charged framework (that is the heterogeneous counterpart of
the [AsF6] anion in the homogeneous complexes synthesized
by Rack et al.19). Successively, again the group of Strauss
reported copper(I) di-carbonyls, stabilized by anions of larger
size, [CuI(CO)2]1[N(SO2CF3)2] and [CuI(CO)2]1[(1-BnCB11F11)] that have bent C2v CuI(CO)2 moieties21 like those
observed in ZSM-5. The formation at RT of di-carbonyls
indicates that there is a strong similarity between the chemistry
towards CO of CuI in superacidic media (where they are in
contact with extremely weak basic anions like AsF6) and the
1
Fig. 1 (a) IR spectra of CO dosed at room temperature on Cu -ZSM5 zeolite. (b) Room temperature XANES spectra of Cu1-ZSM-5
zeolite before and after contact with CO, dotted and full lines,
respectively (pCO ¼ 40 Torr).
1744
Phys. Chem. Chem. Phys., 2005, 7, 1743–1746
chemistry of CuI in CuI-ZSM-5 (where the role of counteranion is assumed by zeolate anion). This result is in agreement
with the fact that the conjugated acid H-ZSM-5 is very strong,
like the triflic acid.22 Further and more direct information
about the internal and external parameters can be obtained by
X-ray absorption spectroscopy (XAS) studies.
Before entering in the details of the XANES spectrum in CO
atmosphere, let us briefly comment on the characteristics of the
spectrum of the Cu1-ZSM-5 sample under vacuum (dotted line
in Fig. 1b). It presents a very intense pre-edge peak at 8983.5
eV, accompanied by a less intense but still well resolved
component at 8986.6 eV, that have recently been attributed
to the 1s - 4pxy and 1s - 4pz electronic transitions, respectively.5 The splitting of these two transitions (3.1 eV) indicates
that bare Cu1 cations are in an axial (cylindrical) symmetry in
the channels of the ZSM-5 zeolite. Please note that this is
the same local symmetry probed by Cu21 hosted in the
same matrix as observed by EPR, showing g1 g//.a g2 ¼
g3 g>.5,7
Once CO has been dosed (pCO ¼ 40 Torr) at room temperature on Cu1-ZSM-5 (full line spectrum in Fig. 1b). As testified
by IR spectroscopy, see Fig. 1a, in such conditions CO forms
Cu1(CO)2 complexes inside the zeolite channels.7–10 Upon CO
adsorption, a strong decrease of the intensity of the 1s-4pxy
peak is observed, together with a small red shift (0.5 eV). Upon
CO adsorption, the second pre-edge XANES feature at 8986.6
eV, is no more visible.
In agreement with previous studies,6,7,9 the EXAFS data
analysis of Cu1-ZSM-5 in vacuo (before CO dosage) resulted in
2.5 0.3 framework oxygen atoms located at 2.00 0.02 Å.
As for the sample contacted with 40 Torr of CO at room
temperature, the EXAFS data were analysed in the framework
of the multiple scattering theory by using the GNXAS code,23
as described in ref. 6 where the structure of the [Cu(CO)3]1
complex formed at liquid nitrogen temperature has been
treated in detail. In that work, it has been shown that the
[Cu(CO)3]1 complex is in a C3v-like geometry with three
equivalent CO molecules characterized by a Cu–C distance of
1.93 0.02 Å, a C–O distance of 1.12 0.03 Å and a Cu–C–O
bond angle linear within the error bars (180 101). In that
case, the experimental spectrum was successfully simulated
using scattering contributions [Cu(CO)3]1 complex only, that
is without any contribution of the zeolitic framework. This fact
has been interpreted in terms of the extraction of the Cu(I)
cations by three CO ligands into a more central position inside
the channels.6 Parallel IR and synchrotron radiation XRD
Fig. 2 k-Weighted w(k) functions of, from top to bottom: Cu–C single
scattering (SS) contribution, Cu–O SS contribution, Cu–C–O MS
contribution, the sum of the three previous theoretical contributions
(FIT) and the experimental curve (collected at room temperature with
pCO ¼ 40 Torr), superimposed for comparison, and the corresponding
residual function.
This journal is & The Owner Societies 2005
Fig. 3 Pictorial representation of intrazeolitic [Cu(CO)2]1 complexes
formed at RT inside the ZSM-5 channels. The zeolitic framework has
been represented with sticks while the sphere representation has been
adopted for both Cu1 cations and CO molecules. This model is in
agreement with the experimental IR, XANES and EXAFS results
summarized in this work.
data collected on different Cu1-exchanged zeolites supported
this thesis.8,24
The same experimental set-up has been used here but the
EXAFS spectra have been collected at room temperature.
Under these conditions, from IR and microcalorimetric data
the formation of [Cu(CO)2]1 complexes is inferred. The results
are illustrated in Fig. 2 where the calculated and experimental
EXAFS signal together with the partial contributions of the
different 2 body (g2 single scattering) and 3 body (g3 multiple
scattering) configurations are compared. Being the solvation
power of two CO molecules less important than that of three,
the presence of a contribution from the zeolitic framework has
been necessary to reproduce the experimental spectrum (top
curve in Fig. 2), consisting in 2.3 0.3 oxygen atoms located at
2.11 0.03 Å. The number of framework oxygen neighbours
is in good agreement with that found before the CO dosage
(2.5 0.3, see above) while the Cu(I)–OF distance has been
considerably stretched (þ0.11 0.03 Å) with respect to the
zeolite in vacuo. Concerning the scattering due to the carbonyl
ligands, our EXAFS study results in a number of coordinated
CO molecules of 1.8 0.3, being so in agreement with the
[Cu(CO)2]1 stoichiometry suggested by IR, see Fig. 1a and
refs. 6–10 and microcalorimetry.9,10 The Cu–C distance obtained for the [Cu(CO)2]1 complex is 1.88 0.02 Å, the C–O
distance (1.12 0.03 Å) is in good agreement with the gasphase value (1.128 Å) and the Cu–C–O bond angle is linear
within the error bars (170 101), in agreement with indirect IR
evidences (vide supra).
Parallel IR and XANES experiments (Fig. 1) indicate that
intrazeolitic [Cu(CO)2]1 complexes are in C2v symmetry. A
pictorial representation of intrazeolitic [Cu(CO)2]1 complexes
formed at RT inside the ZSM-5 channels is reported in Fig. 3.
An Increase of the Cu–C distance of 0.05 Å by moving from
[Cu(CO)2]1 (this work) to [Cu(CO)3]1 (see ref. 6) complexes is
expected to accommodate a third CO ligand in the first coordination shell of copper. The experimentally obtained Cu–C
with advanced quantum mechanical methods:25 Lupinetti
et al.25a (1.891 Å), Ramprasad et al.25b (1.900 Å) and Sodupe
et al.25c (1.969 Å). The loss of framework oxygen coordination
by Cu(I) upon CO coordination has recently been observed in
the ab initio study of the group of Nachtigall.11e,26 on several
cationic sites in both MFI and FER frameworks, where also
an almost linear geometry has been found: 1711 r Cu–C–O
r 1791.
that the phenomenon can actually be followed in situ by
EXAFS and XANES spectroscopies when the appropriate
experimental set-up is available.18
The number of coordinated CO molecules obtained by EXAFS data analysis (1.8 0.3) is in good agreement with the
[Cu(CO)2]1 stoichiometry suggested by IR and testified by
microcalorimetric experiments.9,10 The Cu–C distance obtained
for the [Cu(CO)2]1 complex is 1.88 0.02 Å, with a C–O
distance (1.12 0.03 Å). The increase of the Cu–C distance of
0.05 Å observed by moving from [Cu(CO)2]1 to [Cu(CO)3]1
complexes is a consequence of the local rearrangement needed
to accommodate a third CO ligand in the first coordination
shell of copper. EXAFS determined that the Cu–C–O bond
angle is linear within the error bars (170 101), while IR and
XANES indicate that intrazeolitic [Cu(CO)2]1 complexes are
in C2v symmetry. The experimental values reported here are in
good agreement with the values obtained with advanced
quantum mechanical methods.
Acknowledgements
Luciana Capello thanks the INFM grant for her stage at the
ESRF during her Thesis degree in Materials Science. The cell
used for performing in situ XANES/EXAFS measurements has
been realized in collaboration with the GILDA beamline and
INFM OGG in Grenoble (F. Danca, F. La Manna and R.
Felici) and supported by INFM PURS project. The scientists
and technicians of BM8 (GILDA) of the ESRF are gratefully
acknowledged for their fundamental support during data
acquisition. We are indebted to Profs. G. Spoto, S. Boridga
and A. Zecchina (University of Torino) and with Prof. V. Bolis
(University of Piemonte Orientale) for fruitful discussions.
References
1
2
3
4
5
6
7
8
9
10
11
4. Conclusions
In this work we complete the X-ray absorption study of
[Cu(CO)n]1 complexes formed at RT inside ZSM-5 channels
by reporting the structure of the [Cu(CO)2]1 adducts. It
represents the natural complement of the investigation of the
[Cu(CO)3]1 complexes, performed at liquid nitrogen temperature and reported elsewhere.6 The picture emerging from the
two combined works is that the temperature and pCO can be
readily used as thermodynamic parameters to tune the nuclearity of [Cu(CO)n]1 adducts hosted inside ZSM-5 channels and
12
13
This journal is & The Owner Societies 2005
(a) E. I. Solomon, U. M. Sundaram and T. E. Machonkin, Chem.
Rev., 1996, 96, 2563; (b) R. H. Holm, P. Kennepohl and E. I.
Solomon, Chem. Rev., 1996, 96, 2239; (c) S. Ferguson-Miller and
G. T. Babcock, Chem. Rev., 1996, 96, 2889.
K. Klier, Langmuir, 1988, 4, 13.
M. Iwamoto and H. Hamada, Catal. Today, 1991, 10, 57.
Following the convention introduced by Iwamoto, a 100% exchange is reached when a Cu21 ion is introduced for each two
monovalent counterions, i.e., for each two Al31 ions of the
framework.
C. Prestipino, G. Berlier, F. X. Llabrés i Xamena, G. Spoto,
S. Bordiga, A. Zecchina, G. Turnes Palomino, T. Yamamoto and
C. Lamberti, Chem. Phys. Lett., 2002, 363, 389.
C. Lamberti, G. Turnes Palomino, S. Bordiga, G. Berlier, F.
D’Acapito and A. Zecchina, Angew. Chem. Int. Ed., 2000, 39,
2138.
C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina,
F. Geobaldo, G. Vlaic and M. Bellatreccia, J. Phys. Chem. B,
1997, 101, 344, and refs. therein.
A. Zecchina, S. Bordiga, G. Turnes Palomino, D. Scarano, C.
Lamberti and M. Salvalaggio, J. Phys. Chem. B, 1999, 103, 3833.
V. Bolis, S. Maggiorini, L. Meda, F. D’Acapito, G. Turnes
Palomino, S. Bordiga and C. Lamberti, J. Chem. Phys., 2000,
113, 9248.
V. Bolis, A. Barbaglia, S. Bordiga, C. Lamberti and A. Zecchina,
J. Phys. Chem. B, 2004, 108, 9970.
(a) L. Rodriguez-Santiago, M. Sierka, V. Branchadell, M. Sodupe
and J. Sauer, J. Am. Chem. Soc., 1998, 120, 1545; (b) D. Nachtigallova, P. Nachtigall, M. Sierka and J. Sauer, Phys. Chem. Chem.
Phys., 1999, 1, 2019; (c) D. Nachtigallova, P. Nachtigall and J.
Sauer, Phys. Chem. Chem. Phys., 2001, 3, 1552; (d) P. Nachtigall,
M. Davidova and D. Nachtigallova, J. Phys. Chem. B, 2001, 105,
3510; (e) M. Davidova, D. Nachtigallova, R. Bulanek and
P. Nachtigall, J. Phys. Chem. B, 2003, 107, 2327.
Y. Kuroda, T. Okamoto, R. Kumashiro, Y. Yoshikawa and
M. Nagao, Chem. Commun., 2002, 1758.
X. Solans-Monfort, V. Branchadell, M. Sodupe, C. M. ZicovichWilson, E. Gribov, G. Spoto, C. Busco and P. Ugliengo, J. Phys.
Chem. B, 2004, 108, 8278.
Phys. Chem. Chem. Phys., 2005, 7, 1743–1746
1745
14 A. I. Serykh and V. B. Kazansky, Phys. Chem. Chem. Phys., 2004,
6, 5250.
15 G. Spoto, E. Gribov, S. Bordiga, C. Lamberti, G. Ricchiardi, D.
Scarano and A. Zecchina, Chem. Commun., 2004, 2768.
16 (a) A. J. Lupinetti, S. H. Strauss and G. Frenking, Prog. Inorg.
Chem., 2001, 49, 1; (b) S. H. Strauss, J. Chem. Soc., Dalton Trans.,
2000, 1; (c) Q. Xu, Coord. Chem. Rev., 2002, 231, 83, and
references therein.
17 C. Lamberti, S. Bordiga, F. Bonino, C. Prestipino, G. Berlier, L.
Capello, F. D’Acapito, F. X. Llabrés i Xamena and A. Zecchina,
Phys. Chem. Chem. Phys., 2003, 5, 4502.
18 C. Lamberti, C. Prestipino, S. Bordiga, G. Berlier, G. Spoto, A.
Zecchina, F. La Manna, F. Danca R. Felici and P. Roy, Nucl.
Instrum. Methods Phys. Res., Sect. B, 2003, 200, 196.
19 J. J. Rack, J. D. Webb and S. H. Strauss, Inorg. Chem., 1996, 35,
277.
20 H. Willner and F. Aubke, Angew. Chem., Int. Ed. Engl., 1997, 36,
2402.
1746
Phys. Chem. Chem. Phys., 2005, 7, 1743–1746
21 (a) O. G. Polyakov, S. M. Ivanova, C. M. Gaudinski, S. M. Miller,
O. P. Anderson and S. H. Strauss, Organometallics, 1999, 13, 3769;
(b) S. M. Ivanova, S. V. Ivanov, S. M. Miller, O. P. Anderson,
K. A. Solntsev and S. H. Strauss, Inorg. Chem., 1999, 38, 3756.
22 C. Pazé, S. Bordiga, C. Lamberti, M. Salvalaggio, A. Zecchina
and G. Bellussi, J. Phys. Chem. B, 1997, 101, 4740.
23 (a) A. Filipponi, A. Di Cicco and C. R. Natoli, Phys. Rev. B, 1995,
52, 15122; (b) A. Filippini and A. Di Cicco, Phys. Rev. B, 1995, 52,
15135.
24 G. Turnes Palomino, S. Bordiga, A. Zecchina, G. L. Marra and C.
Lamberti, J. Phys. Chem. B, 2000, 104, 8641.
25 (a) A. J. Lupinetti, S. Fau, G. Frenking and S. H. Strauss, J. Phys.
Chem. A, 1997, 101, 9551; (b) R. Ramprasad, W. F. Schneider, K.
C. Hass and J. B. Adams, J. Phys. Chem. B, 1997, 101, 1940; (c)
M. Sodupe, V. Branchadell, M. Rosi and C. W. Bauschlicher Jr,
J. Phys. Chem. A, 1997, 101, 7854.
26 O. Bludský, M. Šilhan, D. Nachtigallová and P. Nachtigall,
J. Phys. Chem. A, 2003, 107, 10381.
This journal is & The Owner Societies 2005