J. Phys. Chem. 1995,99, 10621-10628
10621
Specific Feature of Copper Ion-Exchanged Mordenite for Dinitrogen Adsorption at Room Temperature Yasushige Kuroda* and Yuzo Yoshikawa Coordination Chemistry Laboratories, Institute for Molecular Science, Myodaiji-cho, Okazaki 444, Japan
Shin-ichi Konno, Hideaki Hamano, and Hironobu Maeda Department of Chemistry, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700, Japan
Ryotaro Kumashiro and Mahiko Nagao Research Laboratory for Surface Science, Faculty of Science, Okayama University, Tsushima-naka, Okayama 700, Japan Received: March 28, 1 9 9 9
Adsorption properties of copper ion-exchanged mordenite (CUM)for dinitrogen molecules ( N 2 ) were examined at 298 K. The intensive IR absorption band observed at 2299 cm-' was attributed to the NZspecies strongly adsorbed on CUM. The interaction of NZwith CUM is explored using adsorption calorimetry, X-ray absorption fine structure (XAFS), electron spin resonance (ESR), and photoemission spectroscopy. The differential heat and entropy of adsorption for N2 on CUM were 60 kJ mol-' and 60 J K-'mol-' at the initial stage of adsorption, respectively, and those for NZ on NaM (sodium-type mordenite) gave the values of 32 kJ mol-' and 130 J K-' mol-', revealing that the N2 molecules are in the localized state resulting from the strong interaction with CUM. The monolayer capacity is estimated to be 4.12 cm3 g-' for NZ on CUM-150, which gives a value of 0.22 for the Nz/Cu ratio. XAFS and emission data for CUM degassed at 873 K exhibit pair bands at 8.983 and 8.994 keV and 18 700 and 20 800 cm-I, respectively. The former pair band is assigned to the 1s-4p transition, and the latter pair band is assigned to the 3d94s1-3dl0 transition. It is also found that the ESR band intensity for Cu(I1) decreases with increasing pretreatment temperature. These spectral data are reasonably explained by assuming the presence of Cu(1) species in mordenite. It is proved from the emission data that the adsorption site including Cu(1) species easily formed by heat treatment at 873 K in vacuo is effective for N2 adsorption. Such easy conversion of Cu(II) to Cu(1) may be due to the spatial distribution of ion-exchanged sites on mordenite. The appearance of a strong IR band at 2299 cm-' is due to the adsorption of N2 on the Cu(1) species and to the induction of a transition moment by the strong field of this site. Although a rather high value of heat of adsorption might suggest chemisorption, it is made plausible that this type of N2 adsorption is physisorption.
Introduction The synthetic zeolites are extensively used as catalysts or adsorbents in a variety of chemical reactions and separation processes.lsZ In particular, a great deal of attention was given to the zeolite molecular sieves containing transition metal ions. Notably, copper ion-exchanged zeolite (ZSM-5 type) has been reported to be active in the catalytic decomposition of the nitrogen oxides that are considered as one of the causes of air p ~ l l u t i o n . ~ -Especially, ~ zeolite containing copper ions in greater amount than that expected from a stoichiometric consideration, which is referred to as the nonstoichiometrically copper ion-exchanged one, is supposed to be appreciably effective for the decomposition of nitrogen oxides4 Therefore, much research has been carried out with the intention of elucidating the role of copper ion in catalytic activity. It is worthwhile to examine the geometry around the copper ion and its electronic structure in considering the feature of cataly~is.~.' The state of copper ions exchanged in mordenite has already been examined.8 Developing from these considerations, it is interesting to investigate the adsorption properties of copper ionexchanged zeolite with nonstoichiometry and to understand them in relation to the state of exchanged copper ion. @
Abstract published in Advance ACS Absrracts, June 1, 1995.
0022-3654/95/2099-10621$09.00/0
We have been investigating the adsorption properties of zeolites along these lines and found a new feature that the mordenite-type zeolite can strongly adsorb dinitrogen molecules at room temperature (298 K).9 The interaction of such molecules with the solid surface is particularly interesting in the field of catalysis, e.g., N2 f i ~ a t i o n . ' ~ -Indeed, '~ a number of works have been directed to this field.I5-l9 A majority of them, however, are concerned with the works carried out at low temperatures or at high gas pressures. Few works examined a strong adsorption of N2 on oxide samples at room temperature. Therefore, it is very interesting to investigate the surface properties of copper ion-exchanged mordenite for N2 adsorption at room temperature. The purpose of the present study is to elucidate the exchanged state of copper ion in mordenite and to obtain information on the adsorption sites for strong Nz adsorption. Unusual properties of copper ion-exchanged mordenite for N:! adsorption at room temperature are discussed on the basis of the data of adsorption calorimetry, IR, XAFS, and emission spectroscopy.
Experimental Section Materials. The synthetic sodium-type mordenite (NaM) having a SUA1 ratio of 10, which had been supplied by Tohso Co. (TSZ-644), was used as a starting material. This mordenite 0 1995 American Chemical Society
Kuroda et al.
10622 J. Phys. Chem., Vol. 99,No. 26, 1995
was ion-exchanged at 363 K for several times using an aqueous Cu2+ solution (0.3 mol dn~-~) in order to obtain the copper ionexchanged mordenite with different amounts of copper ions as described Copper ion-exchanged mordenites thus obtained were centrifuged and washed with redistilled water to remove the counterions and then dried at room temperature. These samples were stored in the desiccator. The extent of exchange was determined by assuming that one divalent copper ion can be exchanged for two sodium ions; these samples are designated CUM-X,where X denotes the percentage of exchange. Highly pure dinitrogen was supplied by Chugoku-Kasei Sans0 co. Measurements of Heat of Adsorption and Adsorption Isotherm. The measurement of the adsorption isotherm was performed using a conventional volumetric adsorption apparatus. The equilibrium pressure was measured with a MKS Baratron 3 10-BH pressure sensor. The first adsorption measurement was performed at 298 K for the sample which had been pretreated at 873 K at a reduced pressure of 1 mPa. After the first adsorption measurement, the sample was evacuated at 298 K to remove the adsorbed N2, and then the second adsorption process was monitored at 298 K. The heat of adsorption of NZ was determined directly at 301 K by calorimetry.2’,22 IR Measurement. For the measurement of IR spectra, a selfsupporting disk of 1 cm in diameter was prepared by compressing the powder sample (12 mg) under a pressure of 100 kg cm-2. The sample disk set in an in situ cell was degassed at 873 K for 2 h at a reduced pressure of 1 mPa. The IR spectra were recorded at room temperature and at various pressures of N2 by using a Nicolet FTIR-710 spectrometer. XAFS Measurement. The X-ray absorption measurement was carried out using synchrotron radiation of Photon Factory in the National Laboratory of High Energy Physics at Tsukuba. The Cu K-edge XANES and EXAFS spectra for CUM’Sand the reference samples were recorded in the transmission mode using a beam line BL-1OB under the ring operation conditions of 2.5 GeV and 200 mA of maximum current. The energy resolution was 0.5 eV for XANES and 2.0-3.0 eV for EXAFS measurements. The photon energy, E, was calibrated with respect to a copper foil by assigning 8.9788 keV to the preedge peak on the absorption edge. The zeolite sample was pressed into a thin wafer in a pellet die and sealed in an in situ cell with a greaseless stopcock. The sample was evacuated at a definite temperature for 2 h. The XAFS spectra were also recorded under in situ condition^.^^ ESR Measurement. The electron spin resonance measurements were performed on JEOL-FE3XG operating at about 9.5 GHz. The magnetic field was modulated at 100 kHz, and the derivative curve was recorded. The relative spin density was obtained from the computer-aided integration of the data by the use of a commercially available software. The cell used for ESR measurements was made of fused silica equipped with a greaseless stopcock. Each measurement was carried out in situ condition and at room temperature after evacuating at various temperatures under a vacuum of 1 mPa for 2 h. Photoemission Spectra. The sample cell was designed to get meaningful results by in situ observation. This cell was fabricated from quartz connected to the evacuating system through a greaseless joint and a stopcock. The features of the cell are as follows: it makes possible to (i) treat the sample continuously at elevated temperatures (up to 1273 K) in vucuo, (ii) treat the sample quantitatively with various kinds of gases, and (iii) adsorb the gases at room temperature and at elevated temperatures. Emission spectra were taken by exciting with 33 300 cm-’ at 298 K using a Hitachi 650-10M photolumines-
,m1
I
I
3500
4000
3000
2500
2000
Wave number/cm-’
I
I
1 4I 3 7 a
4000
I
2299
3621 I
3500
3000
I
.
-2500
2000
Wave number/cm-’
Figure 1. IR spectra of NaM and CUM-150which were evacuated at 873 K: (a) before and (b) after the adsorption of NZ at 298 K. (A) NaM and (B) CUM-150.
cence spectrophotometer. The exciting light was focused onto the sample cell, and the emission was observed at a right angle to the incident beam.
Results and Discussion Adsorption of Dinitrogen at 298 K. The IR spectra for
NaM and CUM-150 samples taken before and after the introductions of dinitrogen (N2) at 298 K are shown in Figure 1. Two absorption bands are observed in the OH stretching region. A sharp band at 3745 cm-I (in the case of CUM-150, 3748 cm-I) is assignable to the invariant ubiquitous SiOH v i b r a t i ~ n , ~ ~ - ~ ~ and a broad band centered at 3648 cm-’ (in the case of CuM150, 3621 cm-I) is due to the acidic OH groups (Bronsted acid).24-26It is clear from these figures that the absorption band assigned to the adsorbed dinitrogen molecules cannot be observed for NaM even after the addition of N2, while that an additional strong band appears at 2299 cm-I for CUM-150. This band seems to be characteristic of a CUM sample. Since a homonuclear diatomic molecule, just like an N2 molecule, is intrinsically IR-inactive, the question can be raised as to what kind of species is responsible for this strong band. Experimentally, in every case, N2 adsorption was reversible, and the intensity of the 2299 cm-I band was reduced as the gas pressure decreased and vanished immediately upon evacuation at 298 K. From these facts, it is reasonable to confirm that the intense band at 2299 cm-’ is attributed to the NEN stretching vibration of N2 species physisorbed strongly on CUM-150, though its frequency is obviously lowered compared with the 2331 cm-’ for the free N2 molecule.27 The appearance of a strong absorption band due to the adsorbed N2 in the present system
J. Phys. Chem., Vol. 99, No. 26, 1995 10623
Copper Ion-Exchanged Mordenite
L.JI
b
""I
A
I
w 2.0
p 1.0
't
e0
-
5 0.5
0
1.0 2.0 3.0 4.0 51) 6.0 7.0 Volume adsorbed/cdMJ?h-'
'" '" '~ 0
>
0
e
-p 0'
, -
e al
I 10
I
5
5
15
Pressure/k Pa
Figure 3. Integrated absorbance at 2299 cm-' for N2 adsorbed on CUM-150,which was evacuated at 873 K, at 298 K as a function of
equilibrium pressure. Inset shows the relation between the integrated absorbance of N2 and the volume adsorbed. The amount of Nz was obtained with the aid of the adsorption isotherm data. 70 I
I
I 10
5
10
Pressure/ kPa
1
15
Pressure/kPa
Figure 2. Adsorption isotherm of N2 at 298 K on NaM (A) and CuM150 (B) after evacuation at 873 K in vacuo. Open and filled circles
represent the first and second adsorptions, respectively. may be exceptional. Strong interaction between the adsorbed N2 molecule and copper ion in mordenite is likely to take place through the formation of Cu-Nz b ~ n d i n g . ~ The adsorption isotherms of N2 at 298 K for the samples of NaM and CUM-150degassed at 873 K are shown in Figure 2. The amount of N2 adsorbed on NaM increases almost proportionally with increasing pressure, or as it is said, the adsorption obeys Henry's law. The adsorbed amount is about 1.7 cm3 g-I at 10 H a , being similar to that obtained by Furuyama and Morimoto.28 The agreement between the first and the second adsorption isotherms suggests a reversible adsorption for this system. On the other hand, the adsorption isotherm for CuM150 is of Langmuir type, quite different from that for NaM. The adsorbed amount of N2 is much larger for CUM-150 than for NaM, being indicative of strong interaction on the former surface. The monolayer capacities (V,) for CUM-150 estimated by Langmuir plots are 4.50 cm3 g-' (0.24 N2 molecule per copper ion) and 4.12 cm3 g-' (0.22 N2 molecule per copper ion) for the first adsorption and second one, respectively. The difference in V, values between the first and second adsorptions corresponds to the amount of N2 strongly adsorbed on mordenite. In order to confirm this irreversible adsorption, the gas evolved on heating the sample at 473 K was analyzed by mass spectrometry, and it was identified as N2. However, there was no evidence for such adsorbed species in the IR spectra taken after evacuating the sample at 298 K. This may be due to either their lower concentration or overlapping with the IR bands for the bulk material. We have only limited information on these adsorbed species, and hence, we are incompetent to discuss them furthermore at present. Figure 3 shows the pressure dependence of the integrated absorbance of the v(NEN) band at 2299 cm-I for N2 adsorbed
0
1.0 2.0 Volume adsorbed/cm3(STP)g-'
Figure 4. Differential heats of adsorption and adsorption isotherms of N2 at 301 K on NaM and CUM-1 14 samples pretreated at 723 K. Open and filled circles represent the adsorption heats and isotherms on NaM and CUM-114, respectively.
on CUM-150. This integrated absorbance increases remarkably at the initial stage of adsorption and reaches a saturated value near 5 kPa of the NZ pressure; the IR absorption isotherm of N2 on CUM-150 is concave to the pressure axis. Such a tendency, being expressed by Langmuir equation, is also observed in the adsorption isotherm for the same system (Figure 2). This fact implies that the appearance of the 2299 cm-' band is due to the existence of the stronger adsorption sites on CuM150. Using adsorption isotherm data, the integrated absorbance of the 2299 cm-' band is replotted against the adsorbed amount in the inset of Figure 3. The intensity increases linearly with increasing amount of adsorbed Nz, and then it is depressed at 4.3 cm3 g-' , being consistent with the monolayer capacity described above. From these facts, it is sure that N2 molecules in the first physisorbed layer are responsible for the 2299 cm-' band. The heats of adsorption of N2 on NaM and CUM-114 pretreated at 723 K are determined at 301 K by a direct calorimetric method, the results of which are represented in Figure 4 as a function of the adsorbed amount. This is the first case of the direct measurement of the heat of adsorption of N2
10624 J. Phys. Chem., Vol. 99, No. 26, 1995
Kuroda et al.
state. This result also leads to the conclusion that the adsorbed N2 interacts strongly with CUM. 1401 Analysis of the Adsorption Sites for Dinitrogen. It is found that the strong adsorption of dinitrogen (N2) on copper ionexchanged mordenite occurs at 298 K. A remaining problem is to clarify the adsorption sites where such strong interaction takes place. XAFS observations were performed in order to confirm the change in the exchanged state of copper ions in mordenite when the sample is subjected to the heat treatment in vacuo. The Fourier transforms (FT) of the EXAFS function for CUM-215 evacuated at various temperatures are illustrated in Figure 6, together with the data for reference materials: Cu(OH)2, CuO, and Cu20. The radial structural function of CuML I I 1 I 215 evacuated at 298 K shows two strong backscatterings at 0 05 1.o 15 2.0 1.95 and 3.05 8, (phase-shift corrected). From the detailed Volume adsorbed/cd(S.T.!?)g-l comparison of this EXAFS function for CUM-215 with those Figure 5. Differential entropy of adsorbed Nz on NaM and CUM-114 for possible oxides and hydroxide, the exchanged species bears samples evacuated at 723 K. SL and SS mean the entropies of liquid closer resemblance to Cu(OH)2 than to CuO and Cu20; N2 at 77.32 K and solid N2 at 63.14 K, respectively. Open and filled accordingly, the first band is attributed to the backscattering circles represent the adsorption entropies on NaM and CUM-114, respectively. from the nearest 0 atoms and the second band mainly to that from the second-nearest copper ions.8 The estimated values near room temperature, and the values obtained by this method for the first shell by curve fitting are shown in Table 1. With are considered to be more reliable than those by other methods increasing evacuation temperature of the sample, the intensity such as IR spectroscopy and the isotherm m e t h ~ d . ' ~ * The ~ * - ~ ~ of the first band decreases and the second band splits into two interaction energy between NaM and N2 is evaluated to be about bands at 473 K. The spectrum for CUM-215 pretreated at 573 30 kJ mol-' in the whole range of adsorption, which is indicative K is characterized by a strong band at 1.95 8, and weak bands of a weak interaction in this system. This result is consistent at 2.85 and 3.35 8, (phase-shift corrected), showing close with our previous result9 and with that obtained by Furuyama resemblance to that for CuO. When the sample is evacuated at and Morimoto.28 On the other hand, the heat of adsorption of 773 K, the peaks other than the 1.95 8, peak disappear virtually. N2 on CUM-1 14 gives a high value of 60 kJ mol-' at the initial It can be deduced from these observations that the hydroxylstage of adsorption, and it decreases with increasing coverage bridged dimer, Cu(OH)~-likespecies, is formed in mordenite to reach the same value as in the case of NaM beyond the at 298 K,8 which transforms to a CuO cluster at about 473 K monolayer coverage. As can be seen from Figure 4, there is a and finally to a Cu species bonded to the lattice oxygens. distinct difference in the heat of adsorption of N2 between CUM Taking into account the lack of the second peak in the EXAFS and NaM. Consistent with the IR findings, it is clearly radial distribution functions for the 773 and 873 K treated evidenced from the calorimetric results that the copper ions in samples, the species formed upon evacuation at 773 or 873 K mordenite have stronger interaction with N2 compared with may have a large static or dynamic disorder. sodium ions in mordenite. The initial heat of adsorption, 60 From the XANES study, we can obtain the detailed informakJ mol-', is considerably higher than those for other systems tion on the chemical state of the copper ions in mordenite. Figure such as N2WAl203 (9.2 kJ mol-'),29 N 2 M i 0 2 (42.7 kI 7 shows Cu K-edge X-ray absorption and their differential mol-'),30 N2EnO (19.3 kJ mol-'),31 N2/Ti02 (1 1 kJ mol-'),'7 spectra collected for the CUM-215 sample, together with the and NdLiZSM-5 (36.5 kJ mol-')32 and is comparable to the reference samples, Cu(OH)2, CuO, and Cu20. The spectra for Nz/Ni/Si02 system (50.2 kJ mol-').33 It can be said with CUM-215evacuated at 298 K, Cu(OH)2, and CuO give a weak certainty that the strong interaction is characteristicof the present band at 8.978 keV and strong bands around 8.988 and 8.998 N2-CUM system. keV. On the other hand, CUZOexhibits strong bands at 8.983 Entropy considerations enable us to elucidate the adsorption and around 8.997 keV.37,38The band at 8.978 keV is assigned phenomena in more detail. The differential entropy, SA, of N2 to 1s-3d transition, and its appearance indicates the presence physisorbed on CUMwas calculated by the following equation33 of Cu(I1) species. The CUM-215 sample evacuated at 298 K also gives the same band, which shows the existence of the SA = -qdiff/T R ln@O/p) -tS , -R (1) divalent copper ions in this sample. As for the geometric factor, it is expected that with increasing tetrahedral distortion the intensity of the 8.978 keV band increases due to Cu 4p-3d where p o is the standard pressure (1 atm), p the equilibrium mixing. On the basis of the data of XANES and the coordinapressure, and SG the standard entropy of Nz gas (191.5 J K-' tion number obtained from the EXAFS data (shown in Table mol-' at 298 K and 1 atm).35 The SA values thus calculated l), it can be assumed that the Cu species in mordenite have a are plotted against the surface coverage, 8, in Figure 5. Here, tetragonal symmetry rather than a tetrahedral one. For the SL (79.81 J K-' mol-') and SS (56.95 J K-' mol-') are the tetragonal Cu(I1) sites, there may be an additional transition entropies of liquid N2 at 77.32 K and solid N2 at 63.14 K, resulting from the 1s-4p transition; the intense bands at around re~pectively.~~ The entropy of adsorbed species on CUM-1 14 8.988 and 8.998 keV for the Cu2+ species are ascribed to this is 60 J K-' mol-' at the initial stage and increases to reach 120 transition whose energy will depend on the electronic and J K-' mol-' at the adsorbed amount of 1.5 cm3 g-I, while that geometric structure around Cu(I1) ions. The tetragonal symon NaM decreases from 140 to 110 J K-' mol-' with increasing volume adsorbed. From these facts, it may be said that the metry which has a short equatorial 0-Cu-0 bond increases state of adsorbed N2 on CUM is analogous to that of the solid the ligand field along x , y axes and shifts a l s - 4 ~ , , ~transition N2 (localized state) at the initial stage of adsorption, in contrast to the higher energy side (8.998 keV) than the 4p, component to the case of NaM where the adsorbed N2 is rather in a mobile (8.988 keV). In addition, for the d9 ion, the signals due to the
+
Copper Ion-Exchanged Mordenite
J. Phys. Chem., Vol. 99, No. 26, 1995 10625
A
2
0 2 J
d
LO a
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0,
3
EO z$ 2 0 2
0 0 9 3 0 Distance/ A
4
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B 12-
'
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1
2 3 4 Distance/8,
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6
Figure 6. (A) Fourier transform of the EXAFS function for CuM215 obtained at 300 K after evacuation at the respective temperatures: (a) 300, (b) 373, (c) 473, (d) 573, (e) 673, (f) 773, and (8) 873 K. (B) Fourier transform of the EXAFS function for reference samples obtained at 300 K: (a) Cu(OH)z, (b) CuO, and (c) CUZO.
TABLE 1: Parameters for CUM-215 (First Shell) pretreatment condition (K) coord no. bond length (A) 300 373 473 573 673 773 873
4.2 4.3 4.0 3.8 3.1 2.8 2.5
1.95 1.94 1.95 1.95 1.98 2.01 2.02
uz 0.008 0.008 0.009 0.010 0.010 0.010 0.010
transitions of l s - 4 ~ and ~ l s - 4 ~ ~split , ~ into t w o bands, respectively, arising from the existence of well-screened and
10626 J. Phys. Chem., Vol. 99, No. 26, I995
Kuroda et al.
poorly-screened core-hole final states.39 The features of these splittings are seen in Figure 7. Information on the geometry of the Cu(I1) site and the electronic state of Cu(II) in mordenite can be obtained by comparing the K-edge spectra of Cu(I1) in mordenite with those of Cu(OH)2 and CuO. It can be easily seen that the band position and the spectral structure for Cu in mordenite is close to that for Cu(OH)2. In conclusion, it can be said that the copper ions in CUM evacuated at 298 K exist in such a hydroxyl-bridged form as Cu(OH)2. When the sample is heated at 473 or 573 K, these spectral structures resemble that for CuO. Further treatment at higher temperatures than 773 K gives strong bands at 8.983 and 8.994 keV. The lack of a band at 8.978 keV is strong evidence that the reduction of Cu(I1) species to Cu(1) species proceeds by the heat treatment in vucuo. The spectrum for the 873 K treated sample agrees with that for Cu20. The peaks at 8.983 and 8.997 keV observed for Cu20 are due to the Cu 1s-4p transition whose energy depends on the geometry of the ligand field. Cu2O has a linear ligand field at the copper ion which raises the 4p, orbital energy (8.997 keV) due to the repulsive interaction along the ligand220 240 260 280 300 320 340 360 380 metal bonds. Thus, the l s - 4 ~ ~transition ,~ is in lower energy H/mT region to which the peak at 8.983 keV can be assigned. Alternatively, in a tetrahedral ligand field all 4p orbitals experience a repulsive interaction and are raised in energy; it is expected, therefore, that there is no low-energy peak (8.983 keV) in the spectrum for the tetrahedral Cu(1) model c ~ m p l e x e s . ~ ~ ~ ~ ~ As shown in Figure 7, the Cu(1) edge at 8.983 keV is very sharp and intensive, and hence the Cu(1) sites in mordenite appear to exist as a planar or linear structure having three or two coordinations, respectively. As a result, EXAFS and XANES findings would be reasonably explained by the presence of such polynuclear species as [CU~(OH)~](~-Y)+ in mordenite at 298 K. This species can be converted to CuO species and finally to Cu(1) species by the heat treatment. It is worth noting that the energy of the l s - 4 ~transition ~ for CUM evacuated at 873 K is changed to lower energy than that for Cu20, because the ligand field of oxygen of Cu-0 in Cu20 is stronger than that in mordenite. This consideration seems to be consistent with that based on the bond length of Cu-0 in mordenite (2.02 A) Temperature/K and in Cu20 (1.88 A). Figure 8. (A) ESR spectra for CUM-215 sample measured at 298 K: ESR spectra for the CUM-215 sample treated at various evacuated at (a) 298, (b) 373, (c) 473, (d) 573, (e) 673, (0773, and ( g ) temperatures in vacuo are shown in Figure 8A. The spectral 873 K. (B) Variation of the peak area of ESR spectra with pretreatment pattern for the sample treated at 298 K is analogous to that for temperatures. ESR intensities were referenced to the 298 K evacuated hydrated copper ion measured at low temperature; the spectrum state. contains a weak band with four hyperfine lines on the lowfield side and a strong band unable to separate on the highheat treatment, as evidenced by the decrease in signal intensity. field side. The gl and gll values equal 2.06 and 2.34, Therefore, ratios of ESR signal intensity at different evacuation respectively, with a hyperfhe constant of about 10.6 mT for temperatures are shown in Figure 8B. In this figure, ESR the component of low field. However, spectra due to the intensities were referenced to the 298 K evacuated state. These exchange-coupled pairs observed by Chao and Lunsford4 and data have also provided quantitative information on the reduction Schlick et ~ 1 . ~were ' not found in our system, even for process that occurs when the sample is pretreated by heating at nonstoichiometrically copper ion-exchanged samples, Le., CUMhigher temperatures under vacuum, suggesting that cupric ions 215, as in similar results obtained by Larsen et Increasing are autoreduced to cuprous ions. While cuprous ions are not the evacuation temperature of the CUM-215samples causes the observable by ESR measurement, the information on these ions ESR signals of Cu2+ to change to well-resolved, structured has been observed on photoemission data. signals, suggesting the ligand field around the Cu2+site changes The emission spectra of the samples pretreated at various from H20 to OH groups and then partly to lattice 02ions of temperatures in vacuo are shown in Figure 9. These spectra the zeolite. For example, a rather well-resolved higher-field were taken at room temperature by means of an excitation at spectrum for 673 K treated sample represents the superposition 33 300 cm-I. The samples treated at 298, 373, 473, and 573 of two main signals for gll; one gives the value of 2.287 (All = K are emission-silent under the present experimental conditions. 14.5 mT) and the other 2.228 (All = 17.0 mT), indicating at On the other hand, the samples treated at 673, 773, and 873 K least two exchangeable sites and the increase in the ligand field exhibit emissions centered at 20 800 and 18 700 cm-I. The around the copper ion in mordenite of higher temperature treated intensities of these luminescence bands increase with increasing samples compared to the 298 K treated sample. Another striking temperature of the treatment. The photoemission spectra of Cu+ feature is that the concentration of Cu(I1) also changes during in alkali halide hosts have been extensively examined.43 For
J. Phys. Chem., Vol. 99, No. 26, 1995 10627
Copper Ion-Exchanged Mordenite
25
20
25
Wave number/ 10~cm-l
Figure 9. Emission spectra for CUM-150at 298 K evacuated at (a) 298, (b) 473, (c) 673, and (d) 873 K. The excitation wavenumber is 33 000 cm-I for all spectra.
these hosts, the observed emission is assigned to the 3d94sl3dI0 transition. Furthermore, Cu+-doped /?-alumina shows an emission centered at 22 700 cm-I when it was excited with ultraviolet rays at room temperature. The assignment of this transition to the 3d94s'-3dI0 transition has also been given on the basis of emission energy and lifetime.44 Recently, Dedecek and W i c h t e r l ~ v aand ~ ~Anpo et ~ 2 1 reported . ~ ~ that the reduced Cu+/ZSM-5 samples exhibit photoemission at around 25 00016 500 cm-l, attributed to radiative decay from Cu+ in ZSM5. Therefore, the emission from CUM can be reasonably assigned to the Cu+ species; it may be due to the spin-forbidden 3Eg-'Al, transition as reported by Texter et aL4' This conclusion is strongly supported by the fact that the emission occurs on the sample pretreated at the same temperature as in the case of XANES bands assigned to Cu(1). For the Cu+-doped /?-alumina crystal, another emission was observed at about 19 000 cm-I, and it was assigned to a dimer species in a variety of sites in the conduction plane.44948 Therefore, the appearance of the emission band at 18 700 cm-' suggests the presence of the dimeric species of copper ions on CUM evacuated at 873 K. However, this result seems to conflict with the present EXAFS data; no second-nearest peak was observed. This discrepancy can be explained on the basis of static or dynamic disorder resulting from the distribution of CuCu distance, as described in the preceding section of this paper. The changes in photoemission spectra taken under various equilibrium pressures of N2 are depicted in Figure 10. As mentioned above, the emission bands appear at 20800 and 18 700 cm-l by evacuation of CUM at 873 K, indicating the existence of monomeric and dimeric Cu(1) species. The intensities of these emission bands successively decrease with increasing N2 pressure. The emission from the CUM sample finally ceases at 13.8 kPa of NZ pressure. This emission spectrum was restored by evacuation of the sample at 298 K. Moreover, the rate of decrease in emission with respect to the pressure of N2 seems to correspond well to the increasing rate of the IR absorption band for N2. From these facts, it may be concluded that the active sites for N2 adsorption at room temperature are the Cu+ species which were created by evacuating the sample at higher temperatures. This emission loss upon adsorption of N2 may be caused by electron exchange
20 Wave number/lO%m-'
Figure 10. Emission spectra for CUM-150at 298 K after evacuation of the sample at 873 K in vacuo with the different equilibrium pressures of N2, (a) 0, (b) 0.7, (c) 2.8, (d) 6.9, and (e) 13.8 kPa, and (f) reevacuation at 298 K after adsorption of N2 at 13.8 P a .
O
0 1 2 3 4 5 Vol u me adsorbed/c m?SI i?)gl
Figure 11. Relative area of emission bands at 18 700 cm-' (open circle) and at 20 800 cm-' (filled circle) to the corresponding one at 20800 cm-l for the 873 K evacuated sample as a function of the adsorbed amount of Nz. Squared marks indicate the values which correspond to the bands for the sample reevacuated at 298 K after adsorption of N2 at 13.8 kPa.
and by charge transfer through the bond formation between Cu+ in mordenite and adsorbed N2. Another important feature is seen from Figure 10; the emission at 18 700 cm-' decreased preferentially compared to that at 20 800 cm-' at the initial stage of N2 adsorption. Deconvolution of the spectra by assuming a Gaussian function will make this point clearer. The results are shown in Figure 1 1 . The emission band at 18 700 cm-I decreases linearly with increasing coverage, and that at 20 800 cm-' decreases more slowly than the former. Therefore, it becomes apparent that the dimeric Cu(1) species is more active than the monomeric ones as an adsorption site for N2, which results in a strong N2 adsorption at 298 K. It seems reasonable to speculate that the state of copper species ion-exchanged in mordenite changes with pretreatment conditions as shown in Scheme 1. At fist, aqua complexes of copper ion formed in the solution are ion-exchanged and deprotonated, and the polynuclear species [A], which are hydroxyl-bridged species (e.g., a dimer), are formed in zeolite. Then, they liberate water and turn to the OH-bridged species
10628 J. Phys. Chem., Vol. 99, No. 26, 1995
SCHEME 1
[AI
iB1
[Cl
[B] which are also coordinated by skeletal oxygens (shown as
OL)when heated in vacuo at about 373 K. These species [B] turn to CuO analogues by the heat treatment at 473 K, as seen from XAFS data, and they transfer to Cu(1) species [C] with evolution of oxygen gas after the evacuation at 673 K. Such a mechanism may be based on the distribution of the sites exchangeable to copper ions. If the ion-exchangeable sites are very far apart, that is, only a small number of aluminum ions are present in the skelton of the zeolite, such a way that two sites having a univalent charge can be replaced by one site having a divalent charge is energetically unstable in view of the concept of charge compensation. In this case, copper ions exchanged may be easily deprotonated and reduced by evacuation of the sample. The Cu(1) ion in the species [C] gives XAFS peaks at 8.983 and 8.994 keV and the emission bands at 18 700 and 20 800 cm-'; it adsorbs N2 molecules strongly, giving rise to the IR absorption band at 2299 cm-'. In IR spectra for adsorbed Nz, the observed band shift to the lower energy side compared to gaseous NZas well as the large increase in absorbance is particularly interesting. These may be due to the induction of a transition moment by the strong field of the special sites. Such a situation can be established by a synergetic effect of the electron donation from the NZ3 0 to the metal d a orbitals and the back-donation from the metal dn to the NZ2n orbitals.
Acknowledgment, We thank Professors S. Motomizu and N. Yamashita of Okayama University for according facility in measuring photoemission spectra and for valuable advice in making the photoemission studies. We also thank Drs. M. Nomura and A. Koyama of the Photon Factory (KEK) for their kind assistance and hospitality in measuring the X A F S spectroscopy. A part of this work has been performed under the proposal of the Photon Factory Program Advisory Committee. The writing of this paper was made possible largely through the grants from the Ministry of Education, Science and Culture of Japan (Nos. 05403009 and 06640748) and from Iketani Science and Technology Foundation, and we acknowledge the generosity of these organizations. References and Notes (1) Katzer, J. R. Molecular Sieves 1I; American Chemical Society: Washington, DC, 1977. (2) Barrer, R. M. Zeolites and Clay Minerals as Sorbents and Molecular Sieves: Academic Press: London, 1978. (3) Iwamoto, M.; Yokoo, S.; Sakai, K.; Kagawa, S. J . Chem. SOC., Faraday Trans. 1 1981, 77, 1629. (4) Iwamoto, M.; Yahiro, H.; Mine, Y.; Kagawa, S. Chem. Lett. 1989, 213. ( 5 ) Valyon, J.; Hall, W. K. J . Phys. Chem. 1993, 97, 1204. (6) Valyon, J.; Hall, W. K. J . Phys. Chem. 1993, 97, 7054. (7) Kevan, L. Acc. Chem. Res. 1987, 20, 1. (8) Kuroda, Y.; Kotani, A.; Maeda, H.; Moriwaki, H.; Morimoto, T.; Nagao, M. J . Chem. SOC.,Faraday Trans. 1992, 88, 1583.
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