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RESEARCH PAPER PCCP 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 This journal is & The Owner Societies 2005 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. 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