J. Phys. Chem. 1993,97, 7054-7060
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Effects of Reduction and Reoxidation on the Infrared Spectra from Cu-Y and Cu-ZSM-5 Zeolites Jozsef Valyont and W. Keith Hall' Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: December 1 , 1992; In Final Form: March 1 1 , 1993
Several zeolite preparations of varying Si/Al ratios were studied with their base-exchange cations in the Cu2+ and in the Cu+ states. Good linear correlations were established between certain lattice vibration frequencies and the lattice aluminum concentration (Al/unit cell) of the Cu-Y preparation. A better fit of the data was obtained with the pore systems filled with H20 than with dry samples. These correlations held for the preparations both before reduction and after reoxidation following reduction with C O or H2, showing that dealumination had not occurred during this cycle. Three frequency ranges were found to be significant, viz., those in the 560-600-cm-I region stemming from vibrations of the double 6-rings, those in the 780-825-cm-I region corresponding to T - 0 vibrations of the external linkages between tetrahedra, and a band at about 905 cm-l (at 935 cm-l for Cu-ZSM-5) which was associated with the extralattice oxygen introduced during base exchange with zeolites charge-balanced with bivalent, but not with monovalent, cations. This latter band disappeared when the oxygen was removed by reduction and reappeared on reoxidation. Four frequencies were observed in the O H stretching region: at around 3745 cm-l (the invariant ubiquitous SiOH vibration), the Brernsted acid bands at 3640 and 3550 cm-1, and a band near 3675 cm-1 assignable to a C u O H stretch. This latter band increased in intensity as the Si/Al ratio increased and the A1 T-sites moved further apart. These same bands decreased in intensity on reduction with CO, but increased when H2 was used. Isotopic substitution of O H by OD effected the expected isotopic shift in the O H bands, but did not affect the 907-cm-l band. Substitution of l8O for l60produced a shift from 909 to 895 cm-l (Av = 14 cm-I). The data suggested that the species responsible for the 3675-cm-I band is convertible into that for the 907-cm-I species. These results support the view that much of the extralattice oxygen is associated with copper-containing species.
Introduction It has been repeatedly demonstrated14 that zeolites chargebalanced with Cu base-exchangecations can be reversibly reduced and reoxidized. In this process the zeolite acts as an oxygen carrier. A one electron redox couple is developed between Cu2+ and Cu+, as confirmed by EPR and gravimetric studies, and when the former is reduced with CO, one molecule of COZis produced for every CO consumed in the stoichiometricreduction of Cu2+ to Cu+.3 The origin of this oxygen has remained illdefined. The present work was undertaken to elucidate this question. The problem can be best understood in the following way: Boudart and c o - w ~ r k e r s ~showed -~ that when Fe2+cations were base-exchanged into a Y zeolite under anaerobic conditions, a stable Fe2+/Fe3+ redox couple was developed with the addition or subtraction of one atom of oxygen for every two iron ions. When Cu2+ was base-exchanged into the same zeolite, a similar circumstance was observed,2JJ but in this instance the couple was Cu+/Cu2+. Seemingly, the catalyst had to first be reduced before it can become an oxygen carrier. Thus, Jacobs and Beyer2 suggested that, on the first reduction with CO, a lattice oxygen is removed to form the stoichiometric amount of C02. They further supposed that the lattice was reconstituted by expelling an alumina fragment which they depicted as A10+. This together with the Cu+ formed on reduction charge-balanced the lattice. After this preliminary reduction, the catalyst could then be reoxidized to form extralattice oxygen (ELO) in stoichiometric relation to the Cu+ oxidized to Cuz+. From here on, the chemistry was thought to follow that established for the Fe2+/Fe3+system. Our earlier work3 was in apparent agreement with this picture, which was adopted in its interpretation. More recently, however,
* To whom all correspondence should be addressed.
t On leave from the Central Institute for Chemistry, Hungarian Academy
of Sciences, H1525 Budapest, Hungary.
we have found4 that the lattice is not dealuminated as pictured.2 In this work measurements of lattice parameter by X-ray diffraction (XRD)and the lattice aluminum concentration by 29Si and 27Al MAS NMR were used to establish that significant dealumination did not occur during the reduction with CO. This led us to suggest that the E L 0 must be incorporated during the preparation of the material as part of the base-exchange chemistry. Further insight into the nature of the E L 0 was sought in the present work. Both MijssbauerSand IR7studies of F e Y zeolites led Boudart and co-workers to locate the E L 0 atom bridged between two Fe3+ ions inside the sodalite unit. Whereas this assignment was in apparent agreement with the physical data, it poses a difficult when one considers these sites as intermediates in the catalytic mechanisms of reactions such as CO + * / 2 0 2 = COz or CO NO = COz 1/2N2, which we Rave studied.8 Yet, the kinetics of these reactions suggest that the E L 0 plays this role even in silica-rich preparations where even second nearest neighbor A1 T-sites are rare. Boudart and co-workers used H2 for the reduction step where the E L 0 atom was removed as HzO, and 0 2 as the oxidizing agent where the oxygen atoms were deposited in the catalyst. Unless two such atoms can be deposited within a single sodalite cage, at least one atom of the 02 molecule must be able to diffuse over a relatively long distance to find a second site. Similarly, whereas H2 can enter a sodalite unit fairly easily, it would seem unlikely that molecules such as CO, C02, and N20 would find it easy to penetrate the 6-rings with the rapidity necessary to maintain the observed catalytic rates in the temperature range between 523 and 773 K. It is even more puzzling that such species can exist in high-silica materials such as Cu-ZSM-5, where Li and Armor9 have shown that NO is decomposed with the release of N2at room temperature leaving two atomsof oxygen behind. Although the conclusion reached in our earlier study: that the
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Reduction and Reoxidation from Cu Zeolites
The Journal of Physical Chemistry, Vol. 97,No. 27, 1993 7055
zeolite was not dealuminated to a significant extent by reduction, seemed inescapable, two serious difficulties were encountered. Because of the presence of paramagnetic Cu2+ centers, a direct determination of the 29Si resonance could not be made. It was necessary, therefore, to remove these centers by back-exchange of the Cu2+ ions with NH4+. Secondly, the method gave no information concerning the center on which the E L 0 was held. Consequently, another approach was sought. The lattice vibration region of the I R spectrum has been used previouslyl0-l4 for determination of lattice aluminum concentration. Flanigen et a1.10 were first to exploit this method. Through extensive studies of a large number of zeolites, they were able to divide the spectroscopic assignments into those corresponding to vibrations of the internal tetrahedra and those stemming from the external linkages between tetrahedra. Thus, with the Y zeolites they assigned the symmetric stretching mode, ~ 2in, the 750-820-cm-l region, and the vibrations of the double 6-rings in the 500-650-cm-l region, and correlated these vibrational frequencies inversely with the A1concentration in the zeolite. This work was extended by others.11-13 Thus, Sohn et al.13 used 29SiMAS N M R to determine the lattice aluminum concentration and were able to correlate this variable with the I R spectra with an accuracy of f l Al/unit cell. In our previous work? using XRD and MAS N M R techniques, the accuracy was adequate for the Y zeolite system, but not for high silica materials such as HZSM-5. We thought, therefore, that IR might be found useful for the ZSM-5 zeolites which contain -5 Al/unit cell. The notion that the E L 0 is introduced during base exchange of bivalent ions into the zeolite is supported by a considerable body of data. IR s t u d i e ~ ~suggest ~ - ' ~ that partially neutralized divalent cations such as CuOH+ or [Cu-O-Cuj2+ may be deposited in the zeolite depending upon the conditions of dehydration. These findings have been supported by the single crystal X-ray study of Olsonlgand by the IR studies of the oxidized form of Fe-Y of Dalla Betta et al.' In a following X-ray diffraction study, however, no oxide bridging complex of the [Fe-O-FeI4+ type could be detected.20 In the Cu-Y system evidence of the analogous bridging oxygen has been forthcoming from XRD,21 electron paramagnetic resonance (EPR),22 and TPD23.24studies of spontaneous desorption of 0 2 . Recently, data suggesting the presence of such bridging oxygen in high silica material such as Cu-mordenite and Cu-ZSM-5 have been p r e ~ e n t e d . ~Al~-~~ though the interpretation of the data suggesting such bridges seems to be quite straightforward, it is difficult to visualize how such bi-ion complexes exist in these high-silica materials where the A1 T-sites are remote from each other. Moreover, no mechanism has been suggested for the recombination of these remote E L 0 atoms to desorb 0 2 . The high-silica Cu-ZSM-5 zeolites, unlike t h e y zeolitesystem, are typically over-exchanged with copper ions. It has been argued4 that this may occur because the Cu2+ are exchanged for Na+ as singly charged species such as CuOH+. Can all or part of the E L 0 be this species? Can infrared studies produce further evidence concerning this catalytically important species? These questions were the motivation for the present investigation.
Experimental Section Catalysts. The zeolites investigated are listed in Table I. They are identified by giving the cationic form, the zeolite type, and the Si/Al ratio. Preparations made from these parent materials are further identified by giving the percent of exchange (or overexchange) as the final number. The Na-Y and the NH4-Y zeolites were obtained from Union Carbide Co. and are identified in the first column, using their nomenclature. The Na-ZSM-5 samples were furnished by the Air Products and Chemicals Co.; they were template free preparations synthesized in their laboratories. The Cu- and Mgexchanged preparations were made using conventional ion
TABLE I: Identification of the Zeolites Studied starting material Union Carbide's our preparations studied identification identification' (after base exchange) Y-52 Na-Y -2.5 Mg-Y-2.5 and Cu-Y-2.8-80 NH4-Y-2.5 Mg-Y-2.5 and Cu-Y-2.5-82 Y-62 LZ-210 NHd-Y-3.2 Mg-Y-2.5 and Cu-Y-3.2-85 LZ-210 (9.0) NH4-Y-4.5 Mg-Y-2.5 and Cu-Y-4.5-93 LZ-210 (12.0) NH4-Y-6.2 Mg-Y-2.5 and Cu-Y-6.2-91 Na-ZSM-5-14 Cu-ZSM-5-14-114 Na-ZSM-5-26 Cu-ZSM-5-26- 166 a The physicochemical properties of these samples were determined and are published e l s e ~ h e r e . ~ - * ~ ~ ~ ~ exchange procedures. Further physicochemical data on these samples are supplied else~here.~,29JO IR Measurements. A Cygnus Model 1OOFT-IR spectrometer was used to collect the infrared spectra. Self-supporting, pressed zeolite wafers were made and mounted on a movable rack and studied in the cell used by Segawa et a1.8 This permitted pretreatment in flowing gas or under vacuum at any desired temperature, but the spectra were all recorded at room temperature. KBr windows were used. For spectra in the 400-1000and 3400-3800-~m-~ regions thesample thickness was maintained at 3-4mg/cm2. Wafers of about 10 mg/cm2 were used to examine the same regions of the oxidized and CO reduced catalysts. The cell construction permitted the sample to be raised by action of a magnet into a heated part of the cell where treatments in flowing gas or under vacuum were carried out. Wafers were then lowered into slots between the optical windows for spectroscopic measurement a t room temperature. Spectra were obtained by accumulating a t least 100 scans at a resolution of about 8 cm-1. Peak maxima were determined automatically by the built-in computer of the spectrometer. Gases. H2, He, and 0 2 were ultrahigh purity grade products furnished by Linde. These were further dried and purified by passing them through traps filled with zeolite a t dry ice or liquid nitrogen temperatures. A 4% CO in He mixture was prepared from ultrahigh purity gases provided by Matheson Gas Products co.
Results and Discussion The infrared spectra of Cu zeolites in the 400-1 100-cm-1 region contain bands which are characteristic of the particular zeolite structure.1s14 As shown in Figure 1, the spectra are all similar, but the peak frequencies vary depending upon whether the pore systems are filled with H2O or have been degassed at an elevated temperature. Also the peak frequencies are shifted with the structure of thezeolite and with their Si/Al ratios. Thecharacter of the base-exchange cation also may exert a minor effect. The extent of these variations may be seen in Table 11, where the frequencies are listed for a series of catalysts of interest herein. In the dehydrated zeolites the corresponding bands appeared at a higher frequency than with the hydrated samples. For example, the 788-cm-1 band assigned to the symmetrical stretching vibration, YZ, of the external T-0 linkages10 shifted to 791 cm-1 upon dehydration. Similarly the band at 570 cm-1 assigned to vibrations of the double 6-rings (D6R) shifted to 596 cm-1 upon removal of H20.Note that the frequency variations from sample to sample in the hydrated state were generally smaller than when the samples were dehydrated. This was not unexpected since in the former case the cations are dissociated from the lattice into the liquid phase in the supercages. This may be taken as another indication that the cations have a significant effect on the framework vibrations when they are deposited on the H2O-free lattice. The position of these two bands may also be correlated with the lattice aluminum concentration of the samples. These
7056 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993
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Wavenumber (cm-') Figure 1. IR spectra of Cu-Y and Cu-ZSM-5 zeolites of different Si/A1 ratiosin hydrated (w) anddehydrated (d) (wet or dry) states. Dehydrated samples were obtained by treating the wafers in flowing 02 at 773 K for 3 h followed by 20-min evacuation at 473 K (a, c, e, g). The remaining spectra (b, d, f, and h) were recorded with pore volumes filled with H20. The preparations were Cu-Y-2-5-82 (a, b), Cu-Y-3.2-85 (c, d), CuZSM-5-14-114 (e, f), and Cu-ZSM-5-26-166 (g, h).
TABLE II: Effects of the Base-Exchange Cation and Hydration on the Position of the Bands Assigned to the Symmetrical Stretching Mode, YZ, of the External T-0 Linkages (750-820 cm-1) and Double 6-Ring Vibrations (500-650 cm-'). frequency, cm-l hydrated dehydratedc y2
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cationic formb (sym T-O stetch) D6R (sym T-O stretch) D6R H-Yd 789 575 803 570 NH4-Y-2.5 784 574 e e Na-Y -2.5 788 576 786 582 Mg-Y-2.5 786 572 797 e CU-Y-2.5-80 788 570 79 1 596 Assignmentswere taken from ref 10. Cationic forms were prepared by ionexchange fromNa-Y-2.5 sample. Sampleswere treatedin flowing 0 2 at 773 K overnight and evacuated at 473 K for 30 min. Treatment c converted NH4-Y to H-Y. This dehydrated sample was equilibrated with the ambient atmosphere at room temperature to get the hydrated H-Y. e Ill-defined peak. relationships are shown in Figure 2 for the hydrated (w) and dehydrated (d) samples. Clearly in both cases the dried samples had higher frequencies than the wet ones. In all cases, however, linear relationships were obtained from which the accuracy for the determination of the lattice aluminum concentration can be judged, Le., the steeper the slope, the higher the accuracy. These slopes are listed in Table I11 for the data available. Less variation from these curves occurred with the wet samples than with the dry ones. Lunsford and co-workersl3 claimed for their samples an accuracy of A1 Al/unit cell. The present results suggest that, in general, this was optimistic because of thevarious factors which can lead to deviations from these lines. Nevertheless, these changes were very significant for the Y zeolites but were too small to be useful with the two Cu-ZSM-5 compositions studied herein. This unfortunate circumstance is not too surprising when one realizes that it is not possible to have large changes in the lattice aluminum concentration in samples having high Si/Al
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A I / U. C. Figure 2. Correlation between the positions of the symmetricstretch, Q, and the double 6-ring (D6R) bands and the aluminum concentration in Cu-Y zeolites in wet (w) and dried (d) samples. The zeolite wafers were treated in situ in flowing 0 2 at 773 K overnight and evacuated at 473 K for 30 min before spectra were recorded and band positions were determined (0). Spectra from the hydrated samples (w) were recorded before above treatment (X) and after equilibrating the dry samples with the ambient atmosphere (A). Cu-Y samples were prepared from NHdt forms of different B/A1 ratios by exchanging with Cu2+ions. These are labeled as Cu-Y-2.5-82, Cu-Y-3.2-85, Cu-Y-4.5-93, and Cu-Y-6.291. Catalyst Cu-Y-2.5-80 was prepared by exchange of the Na form; these data are given by the filled symbols for the hydrated ( 0 )and the dehydrated (-+) samples, respectively. The maximum variation of the nine points from line was about 6 Al/unit cell or 4-5 cm-I, Le., small compared with range of the variables shown. The effect of dehydration was more pronounced, especially for the D6R.
TABLE III: Negative Slopes of the Correlation between the Band Position and the Aluminum Content for Y Zeolites AwlAAllunit cell (cm-l/(Al/unit cell)) hydrated dehydrated v2
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cationic form' (sym T-O stretch) D6R (sym T-O stretch) D6R NH4+ 0.99 0.59 ? ? H+ 0.99 0.59 0.64 0.42 cu2+ 0.66 0.69 1.11 0.12 a The H-Y and the Cu-Y samples were prepared from the commercially available NH4-Y samples. They were pretreated in flowing 02 at 773 K, followed by 20-min evacuated at 473 K the NH4-Y was converted intotheH-form. HydratedH-Y wasobtained by exposingthedehydrated samples to the ambient atmosphere. Cu-Y samples were prepared by ion exchange. ratios and the scatter of our data was -5 Al/unit cell, i.e., about the same as the A1 concentration in these preparations. The results obtained from spectra taken at various steps in CO/Oz redox cycles at 773 and 873 K are listed in Table IV. The frequencies of the maxima for the oxidized (dried) catalysts did not vary significantly throughout these experiments. Moreover, the u2 frequency for the rehydrated sample at the end of these experiments was within the experimental error identical to that of the (wet) starting material. The same may be said about the D6R vibrations. These varied little throughout the experiments except for the effects of hydration. The u2 vibrations were very noticeably affected by reduction but were restored to their initial positions on reoxidation. These data show that no irreversible structural changes occurred during these redox cycles as would be expected on the basis of the model of Jacobs and Beyer.2 Thus, these data confirm our earlier finding^.^ The spectra of Cu-Y-2.5-80 following dehydration at low temperatures are shown in Figure 3. In the OH region (A)
Reduction and Reoxidation from Cu Zeolites
TABLE I V IR Spectroscopic Data of Cu-Y-2.5-80Catalyst after Different Treatments frequency, cm-1 pretreatment vz (sym T-O stretch) D6R 1. without treatment 789 570 79 1 2. oxidized,' 773 K 596 3. reduced! 773 K 743 596 4. reoxidized,O773 K 792 598 5 . reoxidized,@873 K 793 599 C 6. reduced! 873 K 598 792 7. reoxidized,' 873 K 599 C 8. reduced,* 873 K 597 194 9. reoxidized,O873 K 599 790 10. rehydratedd 573 a Treatment in flowing 02 overnight at the indicated temperature was followed by 30-min evacuation at 473 K. Wafer was treated in a flowing mixture of 4% CO in He for 1 h and was evacuated for 30 min at the indicated temperature. Flat plateau without characteristic maximum in the 770-800-~m-~ region. Following step 9 the zeolite was contacted with the atmosphere overnight.
3000 1000 800 600 400 Wavenumber (cm-' ) Figure 3. IR spectra of Cu-Y-2.5-80 (a, band c) and Mg-Y (d) in the hydroxyl (A) and latticevibrational (B) regions. A spectrum was recorded without any sample treatment (a), after purging the cell with dry He at room temperature overnight (b), after treatment in flowing 0 2 at 373 K for 1 h followed by a 20-min evacuation at 373 K (c) and after treatment of Mg-Y in flowing 0 2 at 673 K for 1 h followed by 3 0 4 1 1evacuation at 473 K (d). The Cu-Y treated in this way was similar to curved except for the frequency of the bands in the 3680- and 904-cm-I regions. The scale expansions were X = 1.0 for curves a and b and X = 0.2 for curves c and d. 3800
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hydrogen bonding dominated the spectra. Most of the bulk water was removed by purging overnight at room temperature with dry gas, and most of the loosely held H 2 0was removed by evacuation at 373 K. Weak bands at about 3680 cm-1 appeared in these spectra and may be assigned to CuOH+. Also a very weak band appeared at around 3622 cm-l, suggesting the formation of a small amount of Brernsted hydroxyl. All of these bands were eliminated during an 0 2 treatment at 773 K followed by an evacuation a t 473 K, but see Figure 4. The spectrum for an MgY (d) is shown for comparison. The results in Figure 3A may be compared with those obtained simultaneously in the lattice vibration region shown in Figure 3B. Interestingly, a new band appeared on dehydration of the Cu-Y near 905 cm-1 and at about 935 cm-1 with the Mg-Y samples. Very similar spectra have been published7 for F e Y where this band appeared at 895 cm-1. The position of these bands was independent of the aluminum concentration. Moreover, they could be reversibly removed and regenerated on hydration and dehydration, respectively (Figure 1). Note the diminishing intensity of the band around 720 cm-1 as HzO was removed. The 19vibration a t 793 cm-1 and the D6R vibration a t around 590 cm-1 remained relatively constant. These data are shown as curves
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Wavenumber (cm-' ) Figure 4. IR spectra of Cu-Y zeolites of different Si/AI ratios in the hydroxyl region. Sampleswerepretreatedinflowing02at 773 Kovemight and evacuated at 473 K for 30 min. The spectra were recorded at room temperature with (a) Cu-Y-6.2-91, (b) Cu-Y-45-93, (c) Cu-Y-3.285, and (d) Cu-Y-2.5-82. These spectra were obtained at X = 0.1 after computer averaging of over 100 scans. a - c and may be compared with those obtained from the same zeolite exchanged with Mgz+ (curve d). Although the same two bands appeared in the OH region, their frequencies were higher (3696 vs 3680 cm-l). Otherwise the spectra were very similar. Also the spectrum recorded after heating the sample used for Figure 3c a t 773 K (not shown) contained no discernible bands in the OH region with X = 1 but was almost identical in the lattice vibration region. Significantly, the band at 905 cm-1 did not appear in Na-Y. The OH region of the spectra taken from a series of Cu-Y zeolites of increasing lattice aluminum concentration is shown in Figure 4 with an expanded absorbance scale. The striking feature is the increasing intensity of the CuOH band near 3678 cm-1 as the lattice aluminum concentration decreased while the remaining bands remained relatively constant. As the aluminum ion concentration decreased, the charge-compensating Cu2+ ions moved farther apart and it became more difficult to convert CuOH groups into Cu-0-Cu bridged species, vide infra. In all cases Brernsted bands were present, as was the ubiquitous silanol near 3744 cm-1. The appearance of the Brernsted bands is under~ t a n d a b l e .A~ part of the negative charge on the lattice is chargecompensated by protons, rather than by Cu The strong band near 905 cm-l which developed on dehydration (Figure 3B) was quite stable with increasing temperature up to at least 773 K (Figure 5a). However, it was eliminated by reduction with C O (Figure Sb), or with H2 (Figure Sc), but it could be regenerated on reoxidation (Figure 5d). Similar results have been reported for The corresponding data for the OH region are shown in Figure 6A,B for samples reduced in C O or in H2, respectively, and reoxidized. The spectrum obtained for the freshly prepared sample after treatment in flowing 0 2 overnight at 673 K is presented in Figure 6Aa. It matches most closely the spectrum shown in Figure 4d, and all of the remaining spectra may be referenced to it. Reduction with CO tended to further decrease thevery low intensity of these OH bands. Perhaps this was simply due to further dehydroxylation on extended treatment at 673 K. Reduction with H2, on the other hand, produced relatively strong bands in the region of the Brernsted hydroxyl groups (Figure 6B). These bands increased in intensity as the reduction temperature was raised from 473 to 573 K and to 673 K. As demonstrated previously by EPR and stoichiometry,4.30 at the lower temperature the Cu2+ was reduced to Cu+ but when the temperature was
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Wavenumber (cm-l) Figure 5. IR spectra of Cu-Y-2.5-80 after treatment in flowing 02 overnight at 773 K (a) and after reduction in CO at 773 K (b) or in H2 at 673 K (c). Spectrum d was obtained after reoxidation of the CO reduced sample. Note the removal of the 909-912-cm-Lband on reduction and its reappearance on treatment with 02.
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Wavenumber (cm-') Figure 7. IR spectra of Cu-Y-2.5-80 (a, b) and Cu-ZSM-5-14-114 (c, d) treated in l 6 0 2 (a, c) and l802 (b, d). Spectra a and c were rccorded following treatment in flowing 0 2 at 773 K overnight and evacuation at 473 K for 30 min. The IR cell was then attached to a BET system for exchange with I8O2.Approximately 0.5 mmol I8O2(180/Cu > 200) was circulated through the cell at 773 K for 1 h.
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Figure 6. (A) Spectral changes in the hydroxyl region in oxidation-CO reduction cycles. Cu-Y-2.5-80 was treated in flowing 0 2 at 673 K overnight and evacuated at 473 K for 30 min (a); it was then reduced in 4% CO/He mixture at 673 K for 1 h and evacuated at 673 K for 30 min (b); it was reoxidized in flowing 02 at 673 K for 1 h and evacuated at 473 K for 30 min (c) and finally (d) reduced in 4% CO/He mixture as in step b. (B) The OH-bands of the Cu-Y-2.5-80 sample reduced in flowing Hz at 473 K (a), 573 K (b), and 673 K (c) for 1 h followed by a 30-min evacuation at the reduction temperature. The wafers for each experiment were pretreated in flowing 02 at 773 K overnight. Spectrum d was recorded after the same oxygen treatment using a NH4-Y-2.5 sample yielding H-Y.
raised to 673 K, a substantial fraction of the Cu2+was reduced to CuO. Therefore, the spectrum obtained (Figure 6c) matched almost exactly that of the H-Y preparation made from the same zeolite (Figure 6d). Moreover, the spectra obtained in the lattice vibration region were almost identical for these two preparations (not shown). Thesedata are in agreement with earlier reports31332 that as the Cu2+ ions are reduced with H2, the lattice becomes charge-compensated with protons. Several experiments were made obtaining spectra of the lattice vibrational region following exchange with 1 8 0 2 . Spectra obtained before and after exchange are shown for Cu-Y-2.5-80 and CuZSM-5-14-114 in Figure 7. All of these spectra were quite similar, but note the shift of about 15 cm-l in the 909-cm-l band oftheY zeolite(Figure7a,b) and the937-cm-1 bandoftheZSM-5 sample (Figure 7c,d). A similar result was reported for Fe-Y by Dalla Betta et ale,' where the band at 895 cm-I shifted to 885
cm-1 on treatment with 1 8 0 2 . Moreover, this band could be removed by reduction but was unaffected by deuteration or contact with HzO or D20. These authors assigned this band to a Fe0-Fe antisymmetric stretching mode and cited as supporting evidence bands in the 800-900-~m-~region from several organometallic compounds as well as consistency with their MBssbauer data. The two sets of experimental data (for Fe-Y and Cu-Y zeolites holding ELO) are in excellent agreement with each other and are in accord with the data collected for Mg-Y and CuZSM-5 zeolites. Moreover, on substitution of ' 8 0 for 1 6 0 , the frequency is shifted in the expected direction but is much too small to be attributed to any simple harmonic oscillator. Dalla Betta et al.' calculated a shift of 10 cm-l for the antisymmetric stretching mode, v3, of the three center oscillator, Fe-O-Fe, with an apex angle of 150' based on the "well-known relation" cited by Linevsky et al.3z Unfortunately they seem to have miscalculated. The correst result of this calculation is about 45 cm-l. Moreover, to match our experimental result (15 cm-1) with this model an apex angle of
