J. Phys. Chem. 1995,99, 16327-16337
16327
Coordination of Cu Ions in High-Silica Zeolite Matrices. Cu+ Photoluminescence, IR of NO Adsorbed on Cu2+,and Cu2+ESR Study J. DgdeEek, Z. Sobalik, Z. Tvariifkovti, D. Kauckf, and B. Wichterlovti* J. Heyrousk$ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejikoua 3, 182 23 Prague 8, Czech Republic
Received: March 22, 1995; In Final Form: August 1, 199.5@
The Cu ion siting-coordination in MFI, mordenite, erionite, and Y zeolite matrices was investigated employing Cu+ luminescence, IR spectra of NO adsorbed on Cu2+, and ESR spectra of Cu2+ ions. Four discrete Cuf emission bands (with decays) at 450 nm (35ps), 480 nm (55 ps), 510 nm, and 540 nm (120 ps) were identified and ascribed to four individual types of Cu ion sites. The Cu site distribution varied with the Cu loading and SUA1 framework ratio. It has been shown that the Cu siting in high-silica zeolites is controlled by the local Si-A1 sequences, which affects the redox properties of the individual sites. The Cu+ emission bands were in sound agreement with characteristic shifts of the main IR bands of NO adsorbed on Cu2+ ions, found at about 1921, 1912, 1906, and 1895 cm-I. This indicated different charges on the four Cu ion sites. The ESR spectra provide qualitative information on the existence of two square pyramidal and one square planar Cu2+ complexes. It is suggested that the Cu sites balanced by pairs of framework A1 atoms (Cu+ emissions at 450 and 480 nm) exhibit square pyramidal coordination, while those adjacent to a single framework A1 atom are coordinated close to square planar environment. The latter sites possess substantially higher reducibility compared to the other sites and are responsible for the Cu-ZSM-5 unique activity in NO decomposition.
Introduction Cu ions implanted in molecular sieve matrices have been recognized as promising catalysts for redox reactions such as decomposition of NO,'-4 NO selective reduction with ammonia5 and with hydrocarbons,6 and methane traces complete oxidati0n.I Because of the unique activity of Cu ions planted in a ZSM-5 matrix in NO decomposition, considerable attention has been paid to the preparation of the ion-exchanged andor overexchanged Cu-ZSM-5 zeolite^^^^ their redox properties,I0 as well as to Cu-NO interaction c ~ m p l e x e s . ~ ~An ~ *attempt " has also been made to clear up the mechanism of NO decomposition over Cu-ZSM-5 by monitoring in situ IR spectra of adsorbed intermediates during the NO decomposition reaction. A redox Cu2+-Cu+ cycle has been suggested, and Cu(NO)(N02) adsorbed species have been e ~ i d e n c e d . ' ~ ,How'~ ever, the true mechanism is still far from being elucidated. Even though a substantially higher catalytic activity in NO decomposition (in TOF per Cu atom) has been found with CuZSM-5, moreover strongly depending on the Cu concentration, in comparison with Cu-Y and Cu-m~rdenite,~sufficient information on the siting -coordination of the Cu cations is missing, especially in high-silica matrices.I3 Particular interest has been focused on the Cu species occumng in highly Culoaded ZSM-5, exhibiting the highest activity in NO decomposition. It has been assumed that the precursor of the active site is the (Cu2+-OH)+ cation incorporated into the zeolite, which results in the copper overexchange l e ~ e l . ~ Recently .'~ we have shownI6that the active Cu species in the overexchanged Z S M J zeolites are not Cu-0 clusters, as suggested in ref 17, but Cu ions in defined cationic sites of the ZSM-5 structure. Moreover, this Cu ion coordination is found in the ZSM-5 matrix also at low Cu loading (below CdAl = 0.5); cf. ref 18. The most comprehensive insight, based on UV-vis and ESR spectra, into the Cu ion siting in A, X, Y, and mordenite
* To whom correspondence should be addressed. @Abstractpublished in Advance ACS Abstracts, October 1, 1995.
structures was given by Schoonheydt.14 In Y zeolites, the existence of Cu2+ ions in SI1 (large cavities) and in SII" and SI" sites (small cavities) has been well proven. However, the coordination of Cu in high-silica mordenites, in MFI structure, and in erionite is poorly defined. The only information is based on the ESR spectra of Cu2+ ions in mordenite and MFI mat rice^.'^^^^ The spectra exhibited axially symmetrical ESR signals of Cu2+ with four hyperfine lines in both parallel and perpendicular components. The larger A values (170-180 G ) in the parallel component of the axially symmetrical ESR signal were suggested to reflect a square planar Cu ion environment, while the lower ones (140-150 G) were connected with the square pyramidal symmetry of the ligand field of the Cu ion. The Cu2+ ions in square planar coordination were assumed to be formed from the (Cu2+-OH)+ precursor balanced by a single framework A1 atom and responsible for Cu-overexchange in ZSM-5 zeolites, 6,1 This paper is a continuation of the preceding study'* investigating siting and redox properties of the Cu ions planted in ZSM-5 structure by photoluminescence of Cu+ ions obtained after reduction of Cu2+-ZSM-5 to Cu+-ZSM-5. It has been suggested that, depending on the SUA1 ratio and Cu ion concentration, the originally exchanged divalent Cu ions are located in the vicinity of two (Cu+ emission at 480 nm) or a single (Cu+ emission at 540 nm) skeletal aluminum atom. The Cu ions adjacent to the single skeletal A1 atom have appeared to be those responsible for the high decomposition activity of the exchanged and overexchanged Cu-ZSM-5 zeo1ites.l6 Attention is paid here to a detailed investigation of the coordination-siting of the Cu ions in mordenite MFI, erionite, and Y structures by photoluminescence spectra of Cu+, by a shift in vibration of NO adsorbed on the Cu2+ ions using IR spectroscopy, and by Cu2+ ESR spectra. UV-vis and ESR spectroscopies, owing to poorly resolved spectra at low Cu loading and spin-spin interactions occurring possibly at higher Cu concentrations, respectively, cannot be applied in all cases. The highly sensitive Cu+ emission spectra, together with the I5s1
0022-3654/95/2099-16327$09.00/0 0 1995 American Chemical Society
16328 J. Phys. Chem., Vol. 99, No. 44, 1995
DEdeEek et al.
TABLE 1: Composition of Original Zeolites
H-ZSM-5 Na-ZSM-5 H-ZSM-5 Na-ZSM-5 H-ZSM-5 Na-(Fe)ZSM-5 HNa-M HNa-E HNa-Y
93.80 91.79 94.90 94.12 99.79 97.14 89.45 77.15 69.18
4.60 5.63 3.60 3.32 0.13 0.22 8.79 18.93 23.28
0.01 2.22 0.09 2.54 0.06 1.51 0.28 0.74 4.50
0.02 C0.01 0.01 0.5 (see Figure 3), is connected with a further increase only in the intensity of the Cui emission at 540 nm and not that at 480 nm. Moreover, the intensity at 480 nm does not decrease and no new bands are detected compared to Cu-ZSM-5 with CdA1 well below 0.5. This indicates that the Cu sites responsible for the Cu overexchange are those adjacent to single framework A1 atoms and are occupied also well below complete exchange. However, such Cu coordination prevails especially in ZSM-5 approaching or exceeding a CdA1 value of 0.5. Therefore, it is worthwhile to point out that no new defined Cu species were detected, which might be connected exclusively with the Cu over-exchanged ZSM-5 zeolites. A significant migration of the Cu ions under zeolite heat treatment in various atmospheres (CO, oxygen, in vacuo) followed by a standard reduction in hydrogen to reach monovalent Cu was not observed until the Cu ions were not reduced into the zerovalent state (details in ref 18). In addition to the high-intensity emission bands discussed above, the intensity maxima in the spectra were deconvoluted as individual emissions at 450 and 605 nm. The very low intensity band at 450 nm (decay time of 35 ps), present in the spectra of all the Cu zeolites, and clearly seen in the low Culoaded ZSM-5, is connected with a structurally defined site, discussed below. This Cu site saturates at a very low Cu content (Figure 3) and is hardly detectable in high-silica ZSM-5. The 605 nm band exhibits very low and similar intensity for all the zeolites studied, regardless of the structural type and content of the framework aluminum. However, when the Cu-silicalite was not washed with distilled water after the Cu ion exchange, the spectrum intensity at 605 nm considerably increased. Therefore, this band does not reflect a distinct structural site, but corresponds to Cu ions bonded via terminal Si-OH groups, which understandably can be easily removed down to traces
16330 J. Phys. Chem., Vol. 99, No. 44, 1995
DgdeEek et al.
TABLE 4: Characterization of the Cut Luminescence Bands" decav (m)
A,,,,,(nm) 410 420 440 450 470 475 505 515 540
70
120
560 615 450 480 5 10 540 605
-
.-V)
35 55 110
matrix
Cu+ complex structure
alumina ZSM-5 alumina ZSM-5 Y ZSM-5 Y alumina Y ZSM-5 ZSM-5 alumina X I 4A Y 4A M, E, ZSM-5 M, E, ZSM-5, Y. B M, E, Y, B ZSM-5, E, M, B, (Fe)ZSM-S ZSM-5, Y, M, (Fe)ZSM-S
Cu+ monomer, C?, AIOCuf Cu+ monomer, CZ,, Cu+ monomer 11', C?, Cu+ monomer Cut-Cut. 3.2 A I', c3, cu+-cu+ Cu+ monomer Cu+-Cut. 2.6 A
002-
0
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a, 001-
000500
600
700
860
.-
wavelength (nm)
Figure 4. Cu+ emission spectrum of Cu-ferrisilicalite.
(Fitted
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by careful washing. The same conclusion was drawn by Beer et al.23for Cu ions supported on Cu-A zeolite and Spoto et al.24for Cu-ZSM-5 zeolites. The Cu2+ ions were also exchanged in the ferrisilicate with MFI structure (Si/Fe 115), which did not contain any aluminum in the framework. The spectrum of Cu+-(Fe)ZSM-5 (Figure 4) exhibits a dominant band at 540 nm together with a band of a lower intensity at 605 nm. From analogy with the alumosilicate matrix, the Cu2+ions were assumed to be predominantly exchanged in the sites adjacent to single framework Fe atoms. Low intensity at 480 nm indicates that the ferrisilicate contained a very low number of (-FeOSiOFe-) pairing sites. The presence and intensity of the Cu' emission band at 605 nm correspond to the Cu bonding to Si-OH groups, as described for alumosilicates. It should also be noted that the above results indicate that the Cu+ !uminescence bands (dominant at 540 nm) are not sensitive to differences in the basicity of the alumosilicate and ferrisilicate framework oxygens. Luminescence spectra of Cu+Na-Y have already been investigated by Strome and Klier;25.26 see Table 4. They found that the dehydrated CuNa-Y zeolite exhibited a band at 540 nm, which was reversibly (slowly in days) transformed into the 480 nm band under the zeolite exposure to CO atmosphere. That led to a conclusion on the migration of Cu ions under CO atmosphere from sodalite to large cavities. Figure 5 depicts the luminescence spectra of CuNa-Y with different Cu loadings. It is clearly seen that the spectra features depend considerably on the Cu concentration; at low Cu contents the band at 480 nm prevails and a low intensity at 450 nm can also be found after the spectrum deconvolution. At a higher Cu loading, the intensity at 480 and 450 nm is preserved and bands
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Cu Ions in High-Silica Zeolite Matrices I
J. Phys. Chem., Vol. 99, No. 44, 1995 16331
1-
CulAl 0.01 SilAl 3.6
8
i
6-
c .-8
5-
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4-
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0400
500
450
600
550
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Figure 6. Cu+ emission spectra of Cu-erionite with different Cu loading. (Fitted spectra as full line; experimental spectra as dotted line.) - 1
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Figure 7. Cu+ emission spectra of Cu-mordenite.
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tion, the spectrum is evidently shifted to lower frequencies, clearly indicating a substantial increase in the intensity at 450 nm. This is in contrast to the other zeolite structural types, where the Cu sites with emission at 450 nm are readily saturated at relatively very low Cu loadings. Thus, the Cu ions embedded in the mordenite matrix demonstrated that the emission at 450 nm reflects a defined structural site and not some defects. Simultaneously with the increase in intensity at 450 nm, a comparable intensity increase in the band at 510 nm is found, analogous to the erionite and Y structures. It has been shown that the Cu+ emission spectra revealed four different defined Cu sitings-coordinations reflected in the bands at 450,480,510, and 540 nm, common to MFI, erionite, mordenite, and faujasite zeolite structural types. The population
of these sites differs depending on the Cu loading as well as SUA1 framework ratio and zeolite structural type. The Cu+ emission reflects the energy difference between the lowest d9s' and the d'O levels, which is controlled by the strength and symmetry of the Cu+ ion ligand field. As all the Cu+ ions are located in an environment of framework oxygen atoms, the strength of the ligand field can be assumed to be dependent only on its symmetry. Thus, the individual wavelengths (with defined lifetimes) in the Cu+ emission spectra can be assigned to the individual Cu coordinations. It has been verified with the Cu' emission at 480 nm, for which the excitation spectrum is equal for Cu+ in different matrices2' Table 4 summarizes the emission bands correspondingto Cu+ ions loaded idon various matrices presented in the literature. Barrie et al.27 investigated Cu oxidic species supported on alumina, a material which might contain several, possibly illdefined, Cu coordinations. They observed individual Cu+ emission bands at 410, 440, 515, and 540 nm, which were ascribed to Cu+ monomeric complexes with G, and C2h ligand field symmetry and to Cu+-Cu+ dimer species with Cu+-Cu+ bond lengths of 3.2 and 2.6 A, respectively. They assumed that the shorter the distance between the Cu+ ions in the dimer, the higher the wavelength of the Cu+ emission should be. With the zeolitic matrices, Anpo et alS2*observed Cu+ emissions at 450 and 540 nm for Cu-ZSM-5 and assumed that the former band belongs to monomeric and the latter to Cu+-Cu+ dimers. Spoto et al.24 also detected Cu+ luminescence at 480 and 540 nm for Cu+-ZSM-5 prepared by the ion exchange of gaseous CuCl with the m - Z S M - 5 zeolite; however, they did not make a definite conclusion about the Cu+ ion coordination. According to the literature, the Cu+ emission at 540 nm can be related to Cu+-Cu+ dimers suggested on alumina27or to a single Cu+ ion coordination in a planar environment slightly withdrawn from the plane of the skeletal oxygen^.^^ However, the existence of the dimeric Cu+ species in ZSM-5 matrix is doubtful. Single framework A1 atoms are far apart, and thus formation of long-range Cu-O-Cu bridging moieties bearing an extraframework oxygen is hardly possible. Moreover, random distances between framework A1 atoms, which should control distances in Cu+ dimers, are not in line with the discrete wavelength of the luminescence band (540 nm) for zeolites of various SUA1 ratios. On the other hand, the monomeric Cu+ ion seems to be more realistic (cf. ref 29). However, the structure of the Cu2+ ion bearing one extralattice oxygen after the zeolite dehydration is not exactly specified. Larsen et al.I5 suggested formation of monovalent Cu2+O- species obtained after dehydration of two (Cu2+-OH)+ ions. In this paper easy reduction of the Cu ions adjacent to a single Al atom, leading to Cut ions, is presented (see below). In general, two approaches in the explanation of the Cu ion siting in zeolites can be employed: (i) Metal ions occupy well-defined "rigid" extraframework sites, as generally accepted in faujasites (SI, SI', SII, SII') and mordenites (SI, SII, SIV, SVI), cf. refs 14 and 30, but up to now not defined in zeolites of MFI and other high-silica structures. The metal ion coordination in these defined sites is assumed to be controlled by the geometry of the oxygen rings and cation nature, regardless of the framework c o m p o ~ i t i o n . ~ ~ (ii) Metal ions are stabilized in the vicinity of a discrete, defined local A1 arrangement in the framework, which predominantly controls their siting. This approach appears to be important for high-silica materials, where a low number of A1 atoms is present in the f r a m e ~ o r k . ~ ' In principle the (-AlO(SiO),Al-) framework arrangements with n = 1--m can occur in all the zeolite structure regardless
16332 J. Phys. Chem., Vol. 99, No. 44, 1995 of the framework SUA1 ratio. For high-silica ZSM-5 zeolite Derouane and Fripiat,2' employing the quantum chemical ab initio (LCAO-SCF-MO) calculations of the AI siting in MFI structure, modeled by the pentameric ring clusters, suggested its siting in T2 and TI' framework positions, thus forming (-AIOSiOAl-) pairs. With the mordenite structure the calculated discrete energy minima for the divalent Ni cation in the vicinity of (-AlO(SiO),AI-) sequences with n = 1 or 2 were found.3' Expectably, the Ni?' cation preferred location adjacent to the (-AlOSiOAl-) site, and the effect of the structure was manifested in the finding that location of Ni2+ adjacent to (-AIO(SiO)zAl-) in side pockets tumed out to be the most stable configuration. No energy minima were found for Ni?+ adjacent to A1 arrangements with n 2 3. This is in agreement with 29SiNMR studies suggesting that the A1 atoms in mordenite framework are in a paired arrar~gement.~? Employing the approach (ii) in evaluation of the Cu+ emission spectra of Cu zeolites with consideration of the published energy minima for various arrangements of A1 in the framework, the following conclusions can be made. The Cu+ emissions at 450 and 480 nm for Cu-ZSM-5 should correspond to the Cu+ ions originated from the Cu2+ balanced by two framework A1 atoms in (-AlO(SiO),Al-) arrangements with n = 1 or 2. The existence of A1 pairs in ZSM-5 framework was indicated by Fyfe et al.,33 employing 29SiNMR, who revealed besides Si(OAl) environment also Si( 1Al) and Si(2A1) surroundings. The character of the Cu sites reflected in the 510 nm emission band remains questionable; these sites are probably absent in the ZSM-5 zeolites and occur only in mordenite, erionite, and Y zeolites with a higher Cu loading. This indicates that this site is not energetically very favorable. The Cu+ emission at 540 nm is ascribed to the Cu site adjacent to a single framework A1 atom. Its relative population is highest in the ZSM-5 zeolites, especially in those with a high Si/AI ratio and high Cu loading. The Cu ion is expected to be introduced into this site as monovalent (Cu2+--0H)+ (see refs 4,12, and 15) charge balanced by a single AI atom. Under the zeolite dehydration the divalent cation is transformed readily into Cu' (ref 15 and cf. IR spectra of adsorbed NO presented here). IR Spectra of Adsorbed NO on Cuz' Ions. IR spectra of NO (42 Torr) adsorbed on Cu2+-ZSM-5 differing in both Cu/ A1 and SUA1 ratios are exemplified in Figure 8. It has been found that using a pressure over 30 Torr of NO brings about full saturation of the Cu centers accessible to NO adsorption. In the spectral region of nitrosyl species the Cu2+ zeolites yield a complex band with a maximum at about 1910 cm-'. Deconvolution and curve-fitting of this complex maximum at 1910 cm-' yield two prominent individual bands at 1895 and 1912 cm-'. In addition, a band at 1921 cm-' of much lower intensity was indicated in the second-derivation mode of the spectra, but was not considered in the deconvolution procedure. In some cases, mostly for high Cu loading, the spectrum of the Cu2+-N0 in ZSM-5 exhibits a very low intensity band at 1906 cm-' (Figure 8). It is worthwhile to point out that these bands were found for all the investigated Cu-ZSM-5 zeolites, and no additional band was found with the Cu overexchanged zeolites. When part of the Cu ions in the zeolite are in the monovalent state, additional bands in the region ranging from 1700 to 1850 cm-' are observed (Figure 9); the band at 1812 cm-' is assigned to a Cu' mononitrosyl complex, and the pair of bands at 1735 and 1826 cm-' reflect formation of copper dinitrosyl species (see refs 4, 11, 13, 28). The appearance of the Cu' ions in oxidized zeolites is probably due to autoreduction during evacuation of the sample prior to NO admission, as
DEdeEek et al.
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