New Analysis of Oxidation State and Coordination


The coordination number of the carbon atom in the adsorbed CO is estimated to be 1.4, ... The Journal of Physical Chemistry Letters 2010, 1 (18) , 264...
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J. Phys. Chem. B 1999, 103, 89-96

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New Analysis of Oxidation State and Coordination Environment of Copper Ion-Exchanged in ZSM-5 Zeolite Ryotaro Kumashiro,† Yasushige Kuroda,‡ and Mahiko Nagao*,† Research Laboratory for Surface Science and Department of Chemistry, Faculty of Science, Okayama UniVersity, Tsushima, Okayama 700-8530, Japan ReceiVed: April 21, 1998; In Final Form: October 6, 1998

The changes of structure and electronic state of copper ion species during the heat treatment of copper ionexchanged ZSM-5 zeolite (CuZSM-5) as well as the interaction with CO molecules have been investigated by using various spectroscopic techniques such as infrared (IR) and emission spectroscopy (ES), electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) consisting of a XANES (X-ray absorption near edge structure) and an EXAFS (extended X-ray absorption fine structure) and through the measurements of heat of adsorption and adsorption isotherm. About 70% of the divalent copper ions (Cu2+) exchanged in CuZSM-5 were found to be reduced to the monovalent copper ions (Cu+) during the heat treatment at 873 K in vacuo, the latter species having a linear or a planar coordination structure with a coordination number of 2 or 3 with respect to the nearest-neighboring oxygen atoms at a distance of 1.98 Å. It was found from the ES data that the Cu+ species strongly interact with CO molecules at room temperature. The ratios of the number of CO species interacting with Cu+ species, the former being obtained from the adsorption isotherm data and the latter being obtained from the XANES data, are estimated to be 0.97 for the irreversible CO adsorption and 1.14 for the reversible CO adsorption, respectively. Furthermore, it was revealed that there exist at least two types of Cu+ species differing in the strength of the interaction with CO; one gives a heat of adsorption of about 120 kJ mol-1 and an IR absorption band at 2159 cm-1 due to the irreversibly adsorbed CO species, and the other exhibits 100 kJ mol-1 and a band at 2151 cm-1, which is also ascribed to a similar species. When CO is adsorbed on the Cu+ species, the coordination structure around the Cu+ species changes and the distance between the Cu+ ion and the nearest-neighboring oxygen atom changes from 1.98 to 2.05 Å, as is evidenced from the EXAFS data. The coordination number of the carbon atom in the adsorbed CO is estimated to be 1.4, and a value of 1.89 Å is obtained as a distance of Cu-C. The coordination structure recovers by the heat treatment at 573 K. This implies that the irreversible adsorption of CO molecules is responsible for the change of coordination structure around the Cu+ species. Combination of IR and XAFS data leads to the interpretation that in the irreversible CO adsorption the Cu+ species on which a CO molecule is strongly adsorbed to give an IR band at 2159 cm-1 is coordinated to two lattice oxygen atoms and that the Cu+ species on which a CO molecule is weakly adsorbed to give a 2151 cm-1-band is coordinated to three lattice oxygen atoms.

Introduction Copper ion-exchanged ZSM-5-type zeolite (CuZSM-5) is known to exhibit high activity in the catalytic decomposition of NOx as well as in the selective and catalytic reduction of NO, and hence, extensive research has continued, directing toward a further application of this material to a cleanup catalyst for NOx.1-9 In the NOx decomposition reaction the participation of copper ion in CuZSM-5 is evident from a number of experimental results. However, the details of the state of the copper ion in such a reaction are still unknown. Indeed, Shelef10,11 reported the importance of Cu2+ species in the NOx decomposition reaction. On the basis of ESR data, Jacomo et al.12 also stated in their recent paper that the majority of copper ion species in the CuZSM-5 sample exists as Cu2+ species. * To whom correspondence should be addressed. E-mail: [email protected] cc.okayama-u.ac.jp. Fax: +81-86-251-7903. † Research Laboratory for Surface Science. ‡ Department of Chemistry.

However, it seems to be accepted by many researchers that the Cu+ species play an important role in the NOx decomposition reaction.1,13-15 We have already investigated the coordination structure and electronic state of copper ion exchanged in the mordenite sample (CuM) and found that the copper ions can be exchanged in an amount greater than that expected from a stoichiometric consideration, where the exchanged copper ions are supposed to exist as Cu2+ dimer species bridged by hydroxyl groups.16-18 Furthermore, it has also been elucidated that these species are reduced to the Cu+ species during the heat treatment at higher temperatures in vacuo and that the Cu+ species thus formed have a serious influence on the adsorptive properties of zeolites.19 A large number of studies where carbon monoxide (CO) is used as a probe molecule have been made since the usefulness of this molecule was recognized in analyzing the state of the Cu+ species in zeolite.20-24 On the other hand, some papers

10.1021/jp981935t CCC: $18.00 © 1999 American Chemical Society Published on Web 12/17/1998

90 J. Phys. Chem. B, Vol. 103, No. 1, 1999 described the interaction between the Cu2+ species and CO molecules.25,26 Incidentally, there are no reports referring to a quantitative analysis of the interaction between the copper ions and CO molecules, and only few presumptions based on the ES and EXAFS data are given.27,28 It is very important and worthwhile, therefore, to reveal the state of copper ions, taking account of the amounts of monovalent and divalent copper ions and of the CO/Cu ratio in the interaction between the copper ions and CO molecules. This information will be useful for revealing the role of copper ion species in the NOx decomposition. In the present study, we intended to elucidate the state of the copper ion in the CuZSM-5-type zeolite by revealing the coordination structure of Cu+ species and the interaction with CO molecules using various spectroscopic techniques such as IR spectroscopy, emission spectroscopy (ES), electron spin resonance (ESR), and X-ray absorption fine structure (XAFS) and through the measurements of adsorption isotherm and differential heat of adsorption of CO molecules. Experimental Section The starting material of a sodium-type ZSM-5 zeolite (NaZSM-5; Si/Al ratio of 11.9) was supplied by Tosoh Co. The copper ion exchange was carried out by stirring the NaZSM-5 sample (ca. 5 g) in a 0.3 M CuCl2 solution (100 cm3) at 363 K for 1 h. This procedure was repeated to obtain the excessively ion-exchanged samples. The extent of ion exchange was determined by the chelatometric method. The copper ionexchanged sample thus obtained is designated as CuZSM-5-X, where X denotes the copper ion-exchanging capacity in percentage. The CuZSM-5 sample was pretreated at 873 K under a reduced pressure of 1.3 mPa for 4 h prior to every measurement throughout this work. Such a pretreatment at high temperature promotes a reduction of copper ion species, which makes it possible to characterize the Cu+ species in more detail and correctly. CO gas (99.95%) purchased from Nilaco Co. was used as a probe substance without further purification. The emission spectra were measured at ambient temperature by using a Hitachi F-2000 fluorescence spectrophotometer. The exciting light (300 nm) was focused on the sample cell in situ, and the emission was observed at a right angle to the incident beam.19 ESR spectra for the sample pretreated at 873 K were recorded at X-band frequency (ca. 9.5 GHz) with a JEOL-FE3XG spectrometer. The intensity of the ESR spectra, relative to the spectrum for the 300 K treated sample, was calculated by a double-integral process of the measured spectra with the aid of commercial software. For the measurement of IR spectra a self-supporting disk of 10 mm in diameter was prepared by compressing the CuZSM-5 sample (ca. 8 mg) under a pressure of 120 kg cm-2, and it was mounted in an in situ cell with KRS-5 windows, which is capable of pretreatment at a higher temperature and of adsorption-desorption operation.29 The spectra were recorded at room temperature on a Mattson 3020 FTIR spectrophotometer with a resolution of 4 cm-1 in transmission mode. XAFS measurement was performed using synchrotron radiation of Photon Factory in the Institute of Materials Structure Science (High Energy Accelerator Organization, KEK, Tsukuba). Cu K-edge XANES and EXAFS spectra for the CuZSM-5 and reference samples were recorded in transmission mode using a beam-line BL-10B of KEK with a channel-cut silicon (311)

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Figure 1. XANES spectra for the CuZSM-5-181 sample evacuated at various temperatures (a) and for the reference samples (b). Spectrum number corresponds to the following evacuation temperatures: (1) 300 K; (2) 373 K; (3) 473 K; (4) 573 K; (5) 673 K; (6) 773 K; (7) 873 K.

crystal monochromator under ring-operating conditions of 2.5 GeV and 300 mA of maximum current. The energy resolution was 0.5 and 2.0-5.0 eV for the XANES and EXAFS measurements, respectively. About 50 mg of sample was compressed into a disk of 10 mm in diameter under a pressure of 50 kg cm-2, and it was settled in the in situ cell. The pretreatment condition of the sample and the adsorption procedure were the same as for the case of IR measurement. The coordination number of the oxygen atom bringing about a backscattering and the interatomic distance between copper and oxygen atoms were estimated from the EXAFS data. Using Maeda’s procedure,30 we employed the least-squares minimization technique to curvefit the Fourier filtered data with a multiple-term semiempirical expression of the EXAFS formula. Simultaneous measurements of the adsorption heat and the adsorption isotherm of CO were performed at 301 K using an adiabatic-type adsorption calorimeter.31,32 The first measurement (first adsorption) was carried out for the sample pretreated at 873 K. After this sample was evacuated at 301 K for 4 h, the second measurement (second adsorption) was performed again at 301 K. The amount of desorbed CO gas was determined by a successive ignition-loss method for the CuZSM-5 sample, which had been pretreated at 873 K under a reduced pressure of 1.3 mPa and succeedingly exposed to CO gas at a pressure of 13.3 kPa for 12 h. Results and Discussion Change of the State of Copper Ions by Heat Treatment. Figure 1 shows the XANES spectra for the CuZSM-5-181 sample evacuated at various temperatures between 300 and 873 K. The Cu K-edge XANES spectra for the reference samples of Cu2O, Cu(OH)2, CuO, and metal copper are also shown in Figure 1. For the sample treated at 300 K a weak band is observed at a photon energy of 8.978 keV. Since the same band is observed for the reference samples of Cu(OH)2 and CuO, this band can be assigned to the 1s-3d transition,33,34 being indicative of the presence of divalent copper ions. As the pretreatment temperature rises, especially after heat treatment at 573 K, the spectral pattern changes and the characteristic bands centered at 8.983 and 8.993 keV become distinct. Further

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Figure 3. Comparison of XANES spectra obtained experimentally with those obtained by calculation for the CuZSM-5-181 sample. The thick solid line represents a calculated spectrum, and each symbol shows spectra obtained experimentally for the samples evacuated at 300 K (O) and 873 K (4) and for the reference [Cu(NH3)2]+ sample (b). Figure 2. Emission spectra for the CuZSM-5-147 sample evacuated at the respective temperatures: (1) 473 K; (2) 573 K; (3) 673 K; (4) 773 K; (5) 873 K.

increase in the treatment temperature enhances the intensities of these bands. In particular, the appearance of the former strong band is characteristic of this system. The fact that similar bands are observed for the reference sample (Cu2O) indicates that these bands are due to the 1s-4pz and 1s-4px,y transitions, respectively, being characteristic of a Cu+ species.35 On the other hand, also for the metal copper sample a distinct band is observed at 8.983 keV, though the feature of the band pattern above 8.993 keV is different from that for the CuZSM-5 sample, which excludes the possibility of the formation of metal copper during the heat treatment. It is quite evident from these facts that the Cu+ species are formed in the CuZSM-5 sample by the heat treatment at 873 K in vacuo. Furthermore, the appearance of two bands due to the 1s-4p transition implies that the coordination environment of Cu+ ions takes a linear or a planar configuration. Recently, on the basis of the measurements of ESR and magnetic susceptibility, Jacomo et al.12 have reported that the Cu2+ species with a low symmetry are formed during the heat treatment and that no evidence for the formation of Cu+ species can be obtained. In contrast with their results, our present results confirm the formation of the Cu+ species. In addition we also measured ESR spectra and observed an axially resolved spectrum due to the Cu2+ species even for the sample treated at 873 K. From these experimental facts, it is obvious that the Cu2+ species remain in the 873 K treated sample. These species may be recognized as CuO-like species, as proposed by Jacomo et al.12 Emission spectra for the CuZSM-5-147 sample evacuated at various temperatures are shown in Figure 2. No emission bands can be observed for the sample evacuated at 473 K, but after the heat treatment at temperatures above 573 K the emission bands become observable. The intensity of these bands increases with increasing evacuation temperature, and after the sample was treated at 873 K two bands centered at 480 and 540 nm are enhanced. The temperature at which the emission bands appear corresponds well to the temperature at which the 1s4p band in the XANES spectra appears. Therefore, these emission bands are obviously ascribed to the emission from the Cu+ species. By reference to the emission data obtained so far,36,37 these emission bands are due to the 3d94s1-3d10 transition of the Cu+ species.

TABLE 1: Proportion of Cu+ Species in CuZSM-5-181 Sample Treated at Various Temperatures treat. temp (K) proportion of Cu+ species (%)

300 0

373 6

473 14

573 35

673 46

773 56

873 70

The relative amount of Cu+ species in the copper ionexchanged zeolite sample can be estimated from the XANES data.38,39 Here, we assume that all the copper ions in the CuZSM-5-181 sample evacuated at 300 K are Cu2+ ions. These Cu2+ ions remain in the sample even after treatment at 873 K, as described above. To estimate the amounts of Cu+ species in the sample, it is necessary to use a reference sample that contains only monovalent copper ions. Moen et al.40 examined the XANES spectra of Cu2O and [Cu(NH3)2]+, both of which contain only monovalent copper ions, and reported that the ratio of the intensity of the 1s-4pz transition band (at 8.983 keV) to that of the 1s-4px,y transition band (at 8.993 keV) differs remarkably between these two samples; the ratio is about 0.6 for the former sample if it is assumed to be unity for the latter sample. They ascribed this difference to the differences in the formation probability of the exciton and its lifetime. The present XANES data also show that the band intensity is relatively smaller for Cu2O (Figure 1b) than for the zeolite sample (e.g., spectrum 7 in Figure 1). Therefore, Cu2O is not an adequate reference sample for estimating the amount of Cu+ species. Since the copper ions in a high-silica type of zeolite like ZSM-5 can be regarded as being dispersed, [Cu(NH3)2]+ may be useful as a reference sample. We can reproduce the experimental spectrum by combining the spectrum for the sample evacuated at 300 K and that for [Cu(NH3)2]+ in an appropriate ratio. The resulting data are shown in Figure 3, together with the experimental spectrum for the 873 K treated CuZSM-5-181 sample. It can be seen from this figure that the calculated data, which are obtained by assuming that the amount of Cu+ species is about 70% of total copper ions, are in fair agreement with the experimental data for the 873 K treated sample. This result implies that about 70% of the copper ions in the CuZSM-5 sample is reduced to the monovalent species. The proportion of the Cu+ species that is produced by the reduction of Cu2+ species (as in the exchanged state) can be estimated by the same method, and the values obtained for the samples evacuated at various temperatures are listed in Table 1. This is the first case where the amount of Cu+ species could be estimated from the XANES data in the course of heat treatment. The proportion of

92 J. Phys. Chem. B, Vol. 103, No. 1, 1999 the Cu+ species obtained for the present system corresponds well to the variation of emission spectra with heat treatment and also to the results of Anpo et al.27 In our previous study of the CuM system41 and the CuZSM-5 system,42 the characterization of the Cu+ species was performed by using CO as a probe molecule. In these systems, the sample was pretreated at 723 K and the reduction ratio was estimated to be about 50%. On the other hand, in the present study the sample is heat-treated at 873 K, which raises the reduction ratio up to 70%, as shown in Table 1. The higher reduction ratio makes it possible to characterize the Cu+ species in more detail and correctly. It is evident from the arguments mentioned above that the copper ion exchanged in the ZSM-5 zeolite is reduced from a divalent state to a monovalent one by the heat treatment. We have already proposed a reduction mechanism for the copper ion-exchanged mordenite (CuM) having the same Si/Al ratio as in the present zeolite sample.19 Taking into consideration that ZSM-5 is a high-silica type of zeolite, we may assume the same reduction mechanism as in the case of CuM. Interaction of Cu+ Species with CO Molecules: Emission Spectra and Heat of Adsorption. Figure 4 represents a variation of emission spectra in the course of adsorption and desorption of CO for the 873 K treated CuZSM-5-147 sample. The emission bands reduce their intensities appreciably in the initial adsorption stage and then eventually vanish at the completion of the chemisorbed layer (spectrum 7 in Figure 4a). This is confirmed by the fact that no emission band can be observed for the sample evacuated at 300 K after CO adsorption (spectrum 14). Taking into consideration that the emission band is due to the Cu+ species, it is obvious that the CO molecules interact strongly with the Cu+ species. The emission bands are observed again when the treatment temperature is raised (Figure 4b). The temperature at which the emission bands reappear is consistent with that at which the CO desorption starts. This fact also indicates a strong interaction between the Cu+ species and CO molecules. To get the information on the states of effective sites for CO adsorption, the measurements of the heat of adsorption and adsorption isotherm of CO were performed on the 873 K treated CuZSM-5-147 sample at 301 K. (These data are shown in Figure S1 in Supporting Information.) The monolayer capacities were estimated to be 27.3 and 14.8 cm3 (STP) g-1 for the first and the second adsorption isotherms, respectively. The difference between them, 12.5 cm3 g-1, can be assumed to be the chemisorbed amount. Therefore, the ratio CO/Cu is evaluated to be 0.68 and 0.80 for the chemisorption and the physisorption, respectively, by considering both values of the ion-exchanging capacity and the adsorbed amount. In addition, taking into account that about 70% of the total copper ions are monovalent copper ions as mentioned above, it is obvious that the interaction between the Cu+ ions and CO molecules takes place in ratios of 1:0.97 and 1:1.14 with respect to the irreversible and the reversible adsorption, respectively. It is apparent that in the presence of CO gas the Cu+ species further interacts with a CO molecule to form [O(L)nCu(CO)2]+ (here, O(L) means lattice oxygen). These results give quantitative support to the data obtained by Lamberti et al.28 However, at least in the temperature range examined here, we could not confirm the presence of [O(L)nCu(CO)3]+ species as proposed by them. The differential heat of adsorption of CO on CuZSM-5-147 (the first adsorption) gave 120 kJ mol-1 in the initial stage of irreversible adsorption, and then it decreased to 100 kJ mol-1, giving rise to a plateau on the heat curve (Figure S1 in Supporting Information). Beyond this region, the heat of

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Figure 4. Emission spectra for the CuZSM-5-147 sample during CO adsorption (a) and desorption (b). The sample was treated in the following manner: (1) evacuated at 873 K; (2-7) equilibrated with CO gas of increasing but nearly zero pressure at 300 K; successively equilibrated with CO gas under a pressures of (8) 13 Pa, (9) 40 Pa, (10) 106 Pa, (11) 840 Pa, (12) 3.46 kPa, and (13) 13.0 kPa; successively reevacuated at (14) 300 K, (15) 373 K, (16) 473 K, (17) 573 K, and (18) 873 K.

adsorption drastically decreased to 45 kJ mol-1 in the reversible adsorption region and then exhibited a gradual decrease. The feature of the heat curve for the second adsorption resembles that for the first adsorption in the reversible region (>12 cm3 g-1). It is quite obvious that such variation of the heat of adsorption with the adsorbed amount is characteristic of the interaction between the copper ion species in CuZSM-5 and the CO molecules. The adsorption range in the first adsorption, where the heats of adsorption are relatively larger than those in the second adsorption, corresponds well to the amount of irreversibly adsorbed CO estimated above, which supports the conception of strong bonding (chemisorption) between the Cu+ species and CO molecules. Moreover, the existence of three adsorption ranges in the chemisorption region, respectively, corresponding to the heats of adsorption of 120 (plateau), 100 (plateau), and 100-60 kJ mol-1 (decrease), suggests the

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Figure 6. EXAFS spectra for the CuZSM-5-181 sample (a) and for the reference samples (b). The sample was treated in the following manner: (1) evacuated at 873 K; (2) equilibrated with CO gas under a pressure of 13.3 kPa at 300 K; successively reevacuated at (3) 300 K, (4) 373 K, (5) 473 K, and (6) 573 K.

Figure 5. XANES spectra for the CuZSM-5-181 sample. (a) The sample was treated in the following manner: (1) evacuated at 873 K; (2) equilibrated with CO gas under a pressure of 13.3 kPa at 300 K; successively reevacuated at (3) 300 K, (4) 373 K, (5) 473 K, and (6) 573 K. (b) Thick solid line indicates a difference spectrum obtained by subtracting spectrum 2 (O) from spectrum 1 (b).

presence of at least three kinds of Cu+ species interacting with CO molecules differently in the interaction energy. These adsorption ranges also correspond to the adsorption amounts of 4.5, 6.0, and 1.5 cm3 (STP) g-1 for the respective adsorption heats. The desorption behavior of CO from the CuZSM-5-147 sample on which CO molecules had been preadsorbed was examined by the successive ignition-loss method (Figure S2 in Supporting Information). The desorbed CO amounted to about 12 cm3 g-1 in total, corresponding well to the chemisorbed amount (12.5 cm3 g-1). From the analysis of this desorption curve, it was found that there exist at least three kinds of adsorption sites in a ratio of 2:3:1. This ratio also corresponds to the heat of adsorption data, namely, the ratio of three adsorption ranges described above. All these facts indicate the presence of three kinds of chemisorption sites in the CuZSM-5 sample. State of the Cu+ Species in CuZSM-5 and Interaction with CO Molecules: XANES Spectra. Cu K-edge XANES spectra for the CuZSM-5-181 sample were taken under various conditions, and they are shown in Figure 5a. For the sample evacuated at 873 K two bands are observed at 8.983 and 8.993 keV

(spectrum 1), which are due to the 1s-4p transition characteristic of the Cu+ species. Such splitting of the 1s-4p transition band suggests a linear or a planar coordination structure of the Cu+ species. By dosing of the CO gas (at 13.3 kPa), the band at 8.983 keV reduces remarkably its intensity and a new band is observed at 8.981 keV (spectrum 2). These observations indicate a change of coordination structure around the Cu+ ion by CO adsorption. Kau et al.35 studied the metal complex of copper to elucidate the relationship between the XANES spectra and the coordination structure. According to their data, in the case of a planar coordination of ligand around the Cu+ ion the band due to the 1s-4p transition had split into two bands, while in the case of a tetrahedral coordination such splitting did not take place. Here, it is worthwhile to examine a variation of XANES spectra with CO adsorption. To show this variation more clearly, the difference spectrum was obtained by subtracting the spectrum for the sample with adsorbed CO molecules from that for the sample evacuated at 873 K, the result being illustrated in Figure 5b. By adsorption of CO, the intensity of the band at 8.983 keV decreases accompanied by the appearance of a band at 8.981 keV that may be caused by a shift of the former band. The presence of two bands (at 8.981 and 8.993 keV) even after CO adsorption suggests the existence of a species having a planar coordination structure. However, the decreased intensity of the 8.981 keV band predicts the possibility of inclusion of the tetrahedral coordination structure. By desorption of CO at increasing temperature, the spectral pattern gradually resembles that for the 873 K treated sample before CO adsorption, and eventually it recovers completely after evacuation at 573 K (spectrum 6 in Figure 5a). By employment of the XANES data for both CO-adsorbed and CO-desorbed samples, a synthetic spectrum can be obtained for each sample evacuated at 373 and 473 K. The amount of remaining CO can also be estimated to be 70 and 28% for the sample evacuated at 373 and 473 K, respectively, by the same analytical method as used in Figure 3. Figure 6 shows the EXAFS spectra for the CuZSM-5-181 sample treated under various conditions. In the radial distribution function for the sample evacuated at 873 K (spectrum 1), the

94 J. Phys. Chem. B, Vol. 103, No. 1, 1999 band due to backscattering is observed at a distance of 1.98 Å (phase-shift corrected). Since such a band is observed for all the reference samples, Cu2O, CuO, and Cu(OH)2, and by taking account of the ionic radii of copper and oxygen ions, it can be ascribed to the backscattering from the nearest-neighboring oxygen atom. In this case, the coordination number of oxygen is estimated to be 2.5, being in good agreement with the value obtained for CuZSM-5 by Lamberti et al.28 and also with that for CuM by us.41 This value means that two- and threecoordinated oxygen atoms are involved. When CO gas (13.3 kPa) is introduced, three bands appear at distances of 1.89, 2.05, and 2.91 Å (spectrum 2). Taking into consideration that the Cu+ species interact strongly with CO and that the bond length of C-O in a CO gas molecule is 1.13 Å, we can assign these bands to (Cu)-C(CO), (Cu)-O(lattice), and (Cu)-O(CO), respectively. In these estimations [KCu(CN)2] was used as a reference material to evaluate the ionic distance of Cu-C.44 The value of 1.89 Å for the distance of Cu-C is very close to the value of 1.856 Å for the Cu-C(CO) distance in Cu(CO)Cl having a tetrahedral coordination structure.45 Subsequent evacuation at increasing temperatures brings about a gradual recovery of the spectral pattern, and finally, a complete recovery is observed after evacuation at 573 K, in accord with the variations in XANES spectra. From the optimal parameters obtained by the fitting of two shells, (Cu)-C(CO) and (Cu)-O(lattice), the coordination number of the carbon atom is estimated to be 1.4 when the coordination number of oxygen is taken as 2.5 as described above. This value of 1.4 is found to be in good agreement with the value of 1.48 for the CO/Cu ratio; here, the number of CO molecules refers to the total amount of CO adsorption including both irreversible and reversible adsorption, and the number of Cu is expressed as the total number of monovalent and divalent copper ions. The fact that the distance between the copper ion and the nearest-neighboring lattice oxygen changed from 1.98 Å (before CO adsorption) to 2.05 Å (after CO adsorption) is explained as follows. By strong interaction between the Cu+ species and CO molecules, the former species is attracted to the latter molecule situated in a position distant from the lattice oxygen, resulting in a stable configuration. This concept is supported by the fact that through the desorption of CO at increasing temperatures the band due to the backscattering by the nearest-neighboring oxygen reverts to the original position, as is also supported by the XANES data. Thus, the variation of EXAFS spectra, the bond length, and the coordination number can be reasonably explained by taking account of the CO adsorption. This is the first case where the coordination structure around the Cu+ species is revealed when CO is adsorbed on the Cu+ species in zeolite. Infrared Spectra. The IR spectra for the CuZSM-5-147 sample were obtained under various conditions (Figure S3 in Supporting Information). In accordance with the previous results,41 a broad band appeared at around 2155 cm-1 with increasing CO dosing. Fitting analysis of this broad band yielded two bands at 2151 and 2159 cm-1, and the progressive adsorption enhanced the former band appreciably. When the adsorbed phase was in equilibrium with CO gas in the reversible adsorption region, the intensity of the 2151 cm-1 band increased remarkably, accompanied by an appearance of a new band at 2175 cm-1 by further dosing of CO gas. The addition of CO eventually caused the disappearance of the 2159 cm-1 band. By reference to the assignments made by Spoto et al.46 and by Iwamoto and Hoshino,47 the bands at 2175 and 2151 cm-1 observed in the present system can also be assigned to the

Kumashiro et al.

Figure 7. CO content remaining on the sample after evacuation at various temperatures. These data were obtained from the CO desorption curve (b), IR band area (O), and XANES data ([).

symmetric and asymmetric stretching vibrations of C-O in the [O(L)nCu(CO)2]+ species, respectively. Such an assignment is clearly evidenced by the quantitative results of XAFS and CO adsorption measurements that two CO molecules are adsorbed on one Cu+ species in the reversible adsorption region. The IR spectra for the CuZSM-5-147 sample evacuated at each temperature after CO adsorption showed that the sample evacuated at 300 K gave a broad band centered at 2155 cm-1 (Figure S4 in Supporting Information). This band could be separated into four bands at 2159, 2151, 2135, and 2112 cm-1. By reference to the assignment made by many researchers20-24 and to the present results of XANES and ES measurements, the bands at 2159, 2151, and 2135 cm-1 are assigned to the C-O stretching vibration in the CO species adsorbed irreversibly on the Cu+ species formed by reduction during the heat treatment in vacuo. The 2135 cm-1 band disappeared preferentially by evacuation at 373 K, and the 2151 cm-1 band almost vanished after the heat treatment at 573 K, and eventually the 2159 cm-1 band disappeared by evacuation at 673 K. Taking into consideration that the ZSM-5 sample as used here is a high-silica-type zeolite and that the copper-supported silica sample gives a band at 2127 cm-1 for CO adsorption,20 the 2135 cm-1 band may be assigned to the C-O stretching vibration in the CO species adsorbed irreversibly on the Cu+ species deposited on the CuZSM-5 sample having a silica-like character. As for the bands at 2159 and 2151 cm-1, we have already observed similar bands in the CuM system and they were assigned to the C-O stretching vibration in the CO species adsorbed irreversibly on the different Cu+ species on the ion-exchanging sites.41 The same assignment as in the case of CuM is valid for the present system of CuZSM5-CO; details of this assignment will be given later. The difference in the temperature at which the band disappears can be regarded as a difference in the strength of interaction between the Cu+ species and CO molecules. From these results it is evident that in the CuZSM-5 sample there exist at least three kinds of Cu+ species differing in the interaction energy with CO molecules; two of them are Cu+ species on the ionexchanging sites and the remaining one is on the silica-like surface. These interpretations are well consistent with the results of heat of adsorption and CO desorption measurements. The amounts of CO estimated from the desorption curve and from the XANES data mentioned above are shown in Figure 7. This figure shows the relationship between the chemisorbed amount, in other words, the amount of remaining CO, and the heat-treatment temperature. Here, we also tried to estimate the band area for each IR spectrum, from which a total band area was obtained. The total band area for each spectrum was first

Cu+ in ZSM-5 Zeolite normalized by the band area of the spectrum for the sample evacuated at room temperature and then converted into the amount of chemisorbed CO molecules by using the chemisorbed amount obtained above. Fairly good agreement can be seen in these three kinds of data, which proves the validity of the present estimation by such methods. From the data of CO desorption as well as the arguments on the coordination number and electronic state based on the XAFS data, we can presume a difference in the wavenumber for these two bands at 2159 and 2151 cm-1. We assumed that when CO molecules are adsorbed on the O(L)2Cu+ and O(L)3Cu+ species, a different manner of charge compensation brings about a different vibration energy to give a different wavenumber; the former species gives the 2159 cm-1 band and the latter one the 2151 cm-1 band. Brand et al.48 estimated the wavenumber of the C-O stretching vibration for the [(H2O)2CuCO]+ and [(H2O)3CuCO]+ species and obtained values of 2140 and 2116 cm-1, respectively. The present results are consistent with theirs if a parallel translation is performed between these results. Eventually, these considerations lead us to propose the following models, type I and type II, for CO adsorption on the Cu+ species.

In conclusion it can be described that as adsorption sites for CO molecules there are at least two kinds of sites in the copper ion-exchanged ZSM-5 zeolite (CuZSM-5). Taking account of the chemisorption model of CO, the difference in the adsorption energy is easily evident; the site of type I exhibits higher adsorption energy than the type II site does. Recently, Lamberti et al.28 and Teraishi et al.49 have proposed a model of the ionexchanging site in the ZSM-5 zeolite. Moreover, two types of ion-exchanging sites have been found in NaZSM-5.50 At all events, the present results revealing the existence of at least two types of ion-exchanging sites are consistent with the results obtained so far. For the excessively copper-ion-exchanged zeolite, the exact assignment of the ion-exchanging site is difficult at present and further study of the ion-exchanging and adsorption sites will be needed. Acknowledgment. This work was supported in part by a grant from the Ministry of Education, Science, Sports, and Culture of Japan (Nos. 06640748, 08640750, and 09218242). A part of this work has been performed under the proposal (No. 94G207) of the Photon Factory Program Advisory Committee. Our special thanks are due to Drs. M. Nomura, A. Koyama, and N. Usami of the Photon Factory (KEK) in Tsukuba for their kind assistance in measuring the XAFS spectra. We also thank Mr. T. Ozaki of the glassblowing workshop of Hiroshima University for technical assistance in making the in situ glass cell. Supporting Information Available: Figures of differential heats and isotherms for the adsorption of CO on CuZSM-5147 (Figure S1), of desorption curve of CO from the CuZSM5-147 sample covered with CO (Figure S2), and of IR spectra of the processes of CO adsorption (Figure S3) and CO desorption (Figure S4) for the 873 K treated CuZSM-5-147 sample. This material is available free of charge via the Internet at http://pubs.acs.org.

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