Tous droits de propriété htellecluelle reserves. Reproduction et
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`
`E L S E V I E R M i c r o p o r o u s a n d M e s o p o r o u s M a t e r i a l s
`
`3 2 ( 1 9 9 9 ) 6 3 - 7 4
`
`- n w i s a s s
`www.elsevicr.nl/locate/micmat
`
`Siting of the Cu^ ions in dehydrated ion exchanged synthetic
`and natural chabasites: a Cu^ photoluminescence study
`
`J. Dëdeéek*, B. Wichterlová, P. Kubát
`J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejikova
`CZ-I8223 Prague
`8. Czech Republic
`
`3,
`
`Received 18 M a y 1998; received in revised f o r m 19 April 1999; accepted f o r publication 26 A p r i l 1999
`
`Abstract
`
`C u ^ emission spectra o f Cu^* ion exchanged and reduced natural a n d synthetic CuNa-, CuCa-, CuCs- and CuBa-
`chabasites were used to identify cationic sites o f the Cu"*^ luminescence centres in this zeolite. T w o different C u ^
`emission bands with maxima at 500 and 540 nm were observed; the emission at
`540 nm prevailed at higher Cu
`loadings. The concentration dependence o f the intensity o f 500 a n d 540 n m emission bands indicated that both bands
`correspond to the single C u ^ ions. T h e effect o f the presence o f co-cations in Cu-chabasites on the luminescence
`spectra w a s employed f o r the identification o f the cationic sites corresponding to the individual emission bands.
`Cu"^ emission a t 500 n m reflects the Cu"^ ion located at the cationic site in the eight-membered ring. C u ^ emission a t
`5 4 0 n m is attributed to the C u ^ ion coordinated to three oxygen atoms o f the regular six-membered ring. T h e Cu"*"
`ion in this site is suggested t o b e located a b o v e the six-ring plane, similar to the siting o f C u ^ ions in
`Y zeolite.
`© 1999 Elsevier Science B.V. A l l rights reserved.
`
`Keywords; C a l i o n siting; C u - c h a b a s i t e ; C u * emission; L u m i n e s c e n c e
`
`1. Introduction
`
`Zeolites containing Cu ions attract attention
`owing to their high catalytic activity in N O [1-5]
`and N j O decomposition [6] and selective catalytic
`reduction (SCR) of N O with ammonia [7-9] and
`hydrocarbons [10-12]. The Cu"*" ions were sug­
`gested [13] t o be catalytic centres in N O and
`N2O decompositions. The essential question is the
`structure of the reaction centres. Recently, a cat­
`ionic site of the Cu"^ ions active in the N O
`decomposition over C u ~ Z S M - 5 was proposed
`
`• C o r r e s p o n d i n g a u t h o r .
`E-mail address:
`dedecek@jh-inst.cas.cz ( J . D ¿ d e ¿ e k )
`
`[14,15]. But there is stUi a lack of general informa­
`tion on the siting and coordination of the C u ^
`ions in zeolites.
`With aluminium-rich zeolites, such as chabasite,
`faujasite and A-type zeolites, the cationic sites of
`the Cu^"*" ions are known [16-19]. This knowledge
`is based on the results of X-ray diffraction ( X R D )
`experiments, b u t this technique does not, in prin­
`ciple, allow
`the determination of the oxidation
`state of the cations (since they have similar electron
`density functions), thus bringing difficulties in the
`identification of the Cu^ ion siting. For zeolites
`with a low aluminium content, such as ZSM-5,
`ferrierite and beta, which are not suitable for X R D
`investigations, there is a complete lack of informa­
`tion on the cation siting in these structures.
`
`1 3 8 7 - 1 8 1 1 / 9 9 / $- s e e f r o n t m a t t e r © 1999 Elsevier Science B.V. A l l rights reserved.
`Pll-. 8 1 3 8 7 - 1 8 1 1 ( 9 9 ) 0 0 0 9 0 - 6
`
`LRD4948088088
`
`Umicore AG & Co. KG
`Exhibit 1107
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`6 4
`
`J. Dédeiek et ai
`
`/ Microporous and Mesoporous
`
`Maleriah
`
`32 ( ¡999) 63-74
`
`Emission spectroscopy has been found to be a
`powerful tool for characterisation of the C u ^ ions
`incorporated in zeolites
`[20-23], but the assign­
`ment of the observed Cu"*" emission bands to
`defined C u ^ species is not yet resolved. The main
`interest was focused o n the C u ions exhibiting a n
`emission band at 540 nm, which was suggested t o
`be a catalytic centre in N O decomposition in high-
`silica zeolites [14,23]. Recently, we reported that
`this 540 n m emission band can represent two
`different centres with different types of lumines­
`cence kinetics; one o f them exists in high-silica
`zeolites, and the other is connected with alumin­
`ium-rich zeolites [14,23]. Strome and Klier [21,24]
`attributed the Cu"*" emission a t 540 n m in faujasite
`t o the single Cu"*" ion located in the regular six-
`ring of site r . C u ^ ions located in this site exhibit
`^3v symmetry of the
`ligand field of framework
`oxygen atoms. This suggestion was based on the
`X R D data of the siting in
`Y zeolite [17j.
`On the other hand, Anpo and coworkers [25,26]
`attributed the C u ^ ion emission at 510 n m to the
`single C u ^ ion and the emission band at
`540 n m
`was ascribed t o bridging C u ^ " C u ^ species. Their
`suggestion was based o n the EXAFS study of
`Cu+—Y zeolites.
`Also, factors controlling the emission wave­
`length of the Cu"^ ions located in zeolite matrices
`are not well established. The model proposed by
`Texter et al. [27] explains the different wavelengths
`emitted by the C u * ion implanted in various
`matrices by the repulsion of the Cu* excited state
`by the surrounding lattice. But, with regard t o the
`zeolite structural and compositional features, this
`model does not allow identification of the mecha­
`nism controlling the repulsion of the C u * ions in
`zeolites. Both the changes of the symmetry and
`strength of the
`ligand field ( L F ) of the single
`C u * ion or induced changes of the L F by the
`formation of the Cu'^'-'-Cu^ species affect the
`repulsion of the C u * excited state, i.e. the C u *
`emission wavelength.
`In the present paper, the structure of chabasite
`is used
`t o elucidate the origin of the emission
`bands at 500 and 540 nm. Both the C u * emission
`bands are attributed t o the single C u * ions, the
`band a t 500 n m t o the C u * located in the eight-
`
`membered ring, and the band at 540 n m t o the C u
`* ion located in the regular six-membered ring.
`This assignment was based on a comparison of
`the observed dependence of the intensity of the
`individual emission bands on the C u concentration
`and o n the effect of the presence of non-transition
`metal co-cations with b l o w n siting on the C u *
`emission spectra.
`
`2. Experimental
`
`2.1. Zeolite preparation
`
`The synthesis of chabasite was performed
`according t o the procedure of Gaffney [28]. Zeolite
`Y (Si/Al = 2.7) in ammonium form was used as a
`source material and was mixed with a solution of
`potassium hydroxide. The batch composition was
`0.17Na20;2.0K20:Al203:5.4Si02:224H20. The
`synthesis took place in a polypropylene bottle with
`a screw-top lid at a temperature of 368 K for 96 h
`without agitation. After synthesis, the product was
`recovered by filtration, washed repeatedly with
`deionised water and dried at ambient temperature.
`The crystallinity and phase purity were determined
`using a Siemens D5005 X-ray powder diffracto-
`meter with C u K a radiation and N i filter in the
`range of 20 of 5-50°. The diffraction pattern is
`depicted in Fig. 1 (a).
`Synthetic chabasite and natural sedimentary
`chabasite from North Korea — chemical composi­
`tion (weight percent) 63.89% SÍO2, 17.48% AljOj,
`8.37% FeîOa, 5.15% K j O . 3.10% CaO, 1.21%
`MgO, 0.40% TÍO2 and 0.39% Na^O [the X R D
`pattern is given in Fig. 1(b)] — were equilibrated
`four times with 0.5 M NaCl (20 ml of solution per
`1 g of zeolite) for 12 h . After the ion exchange, the
`Na-chabasites were washed with distilled water
`and dried at room temperature.
`Ca-, Ba- and
`Cs-chabasites were prepared by the equilibration
`of Na-chabasite with 0.1 M Ca(N03)2, Ba(N03)2
`and CsCI solutions respectively. These zeolites
`were washed with distilled water and dried at
`ambient temperature. The conditions of Ca^*,
`Ba^* and Cs* ion exchanges are given in Table 1.
`Cu^*-chabasite samples with C u concentrations
`varying from 0.20 t o 7.60 wt% were prepared by
`
`Umicore AG & Co. KG
`Exhibit 1107
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`

`

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`
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`
`J. Dédeâek el ai f Microporous andMesoporous
`
`Malerials 32 (1999)63-74 6 5
`
`a
`
`rations and chemical compositions of
`Cu-chabasites are given in Tables 2 and 3.
`
`J Jjil jjLL
`
`b
`
`\ A illi
`
`i 1
`"1 '
`10 2 0 3 0 4 0 6 0
`
` '
`
`2 6
`
`1 '
`
`r
`
`F i g . 1. X R D p a t t e r n s o f ( a ) s y n t h e t i c ( b ) n a t u r a l se<ii}nentary
`chabasites.
`
`Table 1
`P r e p a r a t i o n c o n d i t i o n s f o r Cs*-, C a ^ * - a n d Ba^*-chabasites
`
`2 ^ o l i t e ' Solution/zeolite ( m l / g ) E x c h a n g e t i m e ( h )
`
`C s N a - C H A B / N
`
`B a N a - C H A B / N
`
`C a N a - C H A B / N
`
`C a N a - C H A B / S
`
`4 0
`6 0
`4 0
`6 0
`4 0
`8 0
`8 0
`8 0
`
`6
`12
`6
`12
`6
`10
`3
`12
`
`* N ; n a t u r a l s e d i m e n t a r y c h a b a s i t e ; S: synthetic chabasite.
`
`Na-, Ca-, Cs- and
`the ion exchange of
`Ba-chabasites with aqueous solutions of Cu ace­
`tate. The p H of Cu-acetate-chabasite solutions
`varied during the Ion exchange procedure from
`5.1 t o 5.6. Samples were carefully washed with
`distilled water, dried at ambient temperature and
`grained. Detailed conditions of the sample prepa­
`
`2.2. Cu* emission
`
`photolumin­
`Prior t o monitoring of the Cu*
`escence spectra, the Cu^"*" zeolites were calcined in
`an oxygen stream at 350®C for 3 h t o remove all
`traces of organic compounds from the zeolite, well
`known as C u * luminescence quenchers [21,24,28].
`Then, they were dehydrated at 450''C with subse­
`quent reduction in hydrogen of 4 x 10^ P a for 20 s
`u p t o 64 min at 450°C o r in carbon monoxide of
`5.3 X 10^ Pa for 40 min at 450''C. The intensity of
`the C u * emission depended not only on the Cu/Al
`zeolite composition, but also on the time of the
`reduction, as is described in Section 3. For determi­
`nation of the distribution of the C u * ions in
`chabasite, the samples were reduced in hydrogen
`at conditions yielding maximum concentrations of
`the monovalent copper. Under the chosen condi­
`tions, 90% of the copper in the zeolite was in a
`monovalent state and n o significant amounts were
`reduced t o the zero-valent copper, as was proven
`by D R UV~VIS spectra (not shown here). After
`the reduction of Cu^* zeolites, the samples were
`evacuated at the temperature of reduction for
`15 min, transferred under vacuum t o the silica cell
`and sealed. The details of the reduction procedure
`of divalent t o monovalent copper, which have been
`described in detail elsewhere
`[23,29-31], are dis­
`cussed below.
`C u * emission spectra were recorded using a
`nanosecond laser kinetic spectrometer (Applied
`Photophysics). Cu*-zeolites were excited by the
`laser beam of the XeCl excimer laser (Lambda
`Physik 205, emission wavelength 308 nm, pulse
`width 28 ns, pulse energy
`100 mJ). The 320 n m
`filter was situated between a
`2 m m thick silica
`cell and the monochromator. The emission signal
`was detected with a n R
`928 photomultiplier
`(Hamamatsu), recorded with a P M 3325 oscillo­
`scope and processed by computer. All the lumines­
`cence measurements were carried out at room
`temperature. The C u * emission spectra were con­
`structed from the values of luminescence intensity
`at the individual wavelengths of emission at
`selected times after excitation (2, 5,10,20, 50, 100
`
`Umicore AG & Co. KG
`Exhibit 1107
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`

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`
`¡nleUeciuelle).
`
`66 J. Dédelek et al.
`
`/ Microporous and Mesoporous
`
`Malerials 32 (1999) 63-74
`
`T a b l e 2
`P r e p a r a t i o n c o n d i t i o n s f o r Cu^"^-chabasi$es
`
`Zeolite*
`
`C u / A I
`
`C u c o n c e n t r a t i o n in s o l u t i o n ( M )
`
`Solution/zeolite ( m i / g )
`
`E x c h a n g e t i m e ( h )
`
`C u N a - C H A B / N
`C u N a - C H A B / N
`C u N a - C H A B / N
`C u N a - C H A D / N
`C u N a - C H A B / N * -
`C u N a - C H A B / N * »
`C u N a - C H A B / S
`C u N a - C H A B / S
`C u N a - C H A B / S
`C u N a - C H A B / S
`C u N a - C H A B / S "
`C u C s - C H A B / N
`C u C a - C H A B / N
`C u B a - C H A B / N
`C u C a - C H A B / S
`
`O.OJ
`O . l l
`0 . 1 7
`0 . 2 8
`0 . 3 4
`0 . 3 8
`O.OJ
`0 . 0 8
`0 . 1 5
`0 . 2 2
`0 . 3 2
`0 . 0 5
`0 . 1 0
`0 . 0 9
`0 . 0 3
`
`O.OOJ
`0 . 0 0 5
`0.01
`0.01
`0.01
`0.1
`0 . 0 0 1
`0 . 0 0 5
`0.01
`O.OI
`0.01
`0.01
`0.01
`0.01
`0.01
`
`* N ; n a t u r a l s e d i m e n t a r y c b a b a s i t e ; S: s y n t h e t i c chabasite.
`'• TwO'Step i o n exchange.
`
`T a b l e 3
`C h e m i c a l c o m p o s i t i o n s o f Cu^'^^-chabasites
`
`Zeolite* C u / A I M e / A ! " C a / A l N a / A l K / A I M g / A l F e / A l T i / A l
`
`2 5
`2 5
`5 0
`100
`8 0 + 8 0
`1 2 + 2 0
`2 5
`6 5
`6 0
`110
`U O + I I O
`4 0
`4 0
`4 0
`4 5
`
`3
`3
`9
`17
`3 + 1 2
`3 + 1 4
`1
`1
`3
`12
`3 + 1 5
`1
`1
`Í
`1.5
`
`N a - C H A B / N
`N a - C H A B / S
`C u N a - C H A B / N
`C u N a - C H A B / N
`C u N a - C H A B / N
`C u N a - C H A B / N
`C u N a - C H A S / N
`C u N a - C H A B / N
`C u N a - C H A B / S
`C u N a - C H A B / S
`C u N a C H A B / S
`C u N a ^ H A B / S
`C u N a - C H A B / S
`C u O - C H A B / N
`C u B a - C H A B / N
`C u C a - C H A B / N
`C u C a - C H A B / S
`
`-
`0.01
`o.n
`0 . 1 7
`0 , 2 8
`0 . 3 4
`0 . 3 8
`0.01
`0 . 0 8
`0 . 1 5
`0 , 2 2
`0 . 3 2
`0 . 0 5
`0 . 0 9
`0 . 1 0
`0 . 0 3
`
`_
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`
`-
`0 . 4 2
`0 . 2 5
`
`-
`
`-
`
`O.JI
`0.01
`0.11
`0 . 0 8
`0 . 0 9
`0 . 0 7
`0 . 0 5
`0.07
`
`-
`
`-
`
`-
`
`-
`
`-
`0
`0 . 0 4
`0 . 1 9
`0.41
`
`0 . 4 3
`0.94
`0 . 4 0
`0 . 2 5
`0 . 2 2
`0 . 1 7
`0 . 1 2
`0 . 0 8
`0 . 9 9
`0.84
`0 . 6 8
`0.51
`0 . 2 6
`0 . 0 3
`0.03
`0 . 0 8
`0 . 0 6
`
`OAS
`
`-
`0 . 1 5
`0 . J 3
`0 . 1 3
`0 . 1 3
`0 . 1 4
`0 . 1 4
`
`-
`
`-
`
`-
`
`-
`
`-
`0.11
`0 . 1 1
`0 . 1 1
`
`-
`
`0 . 0 6
`
`-
`0.06
`0 . 0 3
`0
`0
`0
`0
`
`-
`
`-
`
`-
`
`-
`
`-
`0 . 0 8
`0 . 0 2
`0
`
`-
`
`0 . 3 0
`0
`0 . 3 0
`0.30
`0 . 3 3
`0 . 3 3
`0 . 3 5
`0.31
`
`-
`
`-
`
`-
`
`-
`
`-
`0 . 2 9
`0 . 3 3
`0 . 3 0
`
`-
`
`0 . 0 2
`
`-
`0 . 0 2
`0.01
`0.01
`0 . 0 3
`0 . 0 2
`0 . 0 2
`
`-
`
`-
`
`-
`
`-
`
`-
`
`0 . 0 2
`0 . 0 2
`0 . 0 2
`
`-
`
`* N : n a t u r a l s e d i m e n t a r y c h a b a s i t e ; S : s y n t h e t i c chabasite.
`** Me/AJ is C s / A l f o r C u C s - c h a b a s i t e a n d B a / A I f o r C u B a - c h a b a s i t e .
`
`and 200 us) using the Applied Photophysics
`Kinetic Spectrometer software. Decomposition of
`the luminescence spectra to the Gaussian curves
`and final data processing was carried out using the
`Microcal Origin 4.1 software (Microcal Software).
`
`3. Results
`
`Typical emission spectra of the
`2. Both the
`Cu'^Na-chabasite are shown in Fig.
`asymmetry of the emission spectrum and its shift
`
`Umicore AG & Co. KG
`Exhibit 1107
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`
`representation interdites sans autorisation (code de la propriété intellectuelle).
`
`J. Dèdeiek et al.
`
`/ Microporous and Mesoporous Materials
`
`32 ( ¡999) 63-74 6 7
`
`vt c
`0>
`c
`c
`o
`
`e
`
`1 ff
`
`wavelength (nm)
`
`F i g . 3 . E>ecomposition o f t h e emission s p e c t r u m o f
`C u * - c h a b a s i t c ( C u / A i = 0 . 3 8 ) t o G a u s s i a n b a n d s : Cu"^ emission
`s p e c t r u m ( O ) ; resulting
`ñ t t o t h e G a u s s i a n c u r v e s
`( — ) ;
`G a u s s i a n c u r v e s c o m p o s i n g s p e c t r u m ( - - ) . Cu^'^-chabasite w a s
`r e d u c e d in h y d r o g e n o f 4 x 10^ P a f o r 2 m i n a t 450®C.
`
`The decay of the C u ^ emission intensity at
`500 n m is documented in Fig. 4, where the normal­
`ised luminescence decay is shown. The dependence
`of the luminescence intensity on time followed a
`double-exponential dependence, described by
`
`I = f o [ a i e x p ( - / / T j ) + a 2 e x p í - í / i a ) ] ,
`
`( 1 )
`
`where I is the luminescence intensity at time t(t~
`0 for time of excitation), 4 is the initial lumines­
`cence intensity, Ti and r j are luminescence decay
`times and Oj and ű2
`pre-exponential factors.
`The emission centre with an emission at
`500 nm,
`both for the natural and synthetic chabasite, was
`characterised by
`the decay times
`10+5 and
`80±10fts; the lower one represents the life time
`of the lowest excited state and is used as a charac­
`teristic of the C u ^ ion located in a zeolite (see
`Refs. [22,23,30]). Owing to the strong overlap of
`the emission band at
`540 n m by the band at
`500 nm, its decay time could not be estimated.
`However, the 540 nm luminescence prevails in the
`spectra recorded at a longer time after the excita­
`2 ^is; the
`tion compared with that recorded at
`decay time of the 540 nm emission is substantially
`longer than that of the 500 nm emission.
`In order t o determine the distribution of the
`two C u ^ species in chabasite, it was necessary t o
`
`350 4 0 0 4 5 0 6 0 0 5 5 0 600 6 5 0 7 0 0
`wavelength (nm)
`
`F i g . 2. (a) N o r m a l i s e d intensity o f t h e C u ' emission o f
`Cu'^-chabasitc ( C u / A l = 0 . 3 8 ) r e c o r d e d a t
`2 ( — ) a n d 100 p s
`( - - ) a f t e r excitation. C u ^ * - c h a b a s i l e w a s reduced i n h y d r o g e n
`o f 4 X 1 0 ' P a f o r 2 m i n a t 4 5 0 ° C . ( b ) Effect o f reduction t i m e
`o f t h e C u * - c h a b a s i t e { C u / A I = 0 . 3 8 ) o n t h e normalised C u *
`emission intensity. C u ^ ^ - c h a b a s i t e w a s r e d u c e d in h y d r o g e n o f
`4 X 1 0 ' P a f o r I ( — ) a n d 6 4 m i n ( - - ) a t 4 5 0 ° C .
`
`to a longer wavelength with increasing time after
`excitation indicated the presence of at least two
`emission bands with different luminescence decays
`(Fig. 2(a)J. This was also indicated by the maxi­
`mum shifts to shorter wavelengths with increasing
`reduction time [Fig. 2(b)]. These shifts reflected
`the presence of different Cu"*" luminescence centres
`exhibiting different reducibilities in the
`Cu^"*'-»Cu'*"-*Cu° process. Decomposition of the
`emission spectra of CuNa-chabasites t o the
`Gaussian bands and second derivative mode analy­
`sis (not shown in Fig. 2 ) showed the presence of
`two emission bands with maxima at 500 ± 5 and
`5 4 0 ± 5 nm, as illustrated in Fig. 3. The decomposi­
`tion of the C a * luminescence spectra to the
`Gaussian bands was discussed in detail elsewhere
`[22,23].
`
`Umicore AG & Co. KG
`Exhibit 1107
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`

`Tous droits de prophete inteilecłuelle reserves. Reproduction et representation interdites sans autorisation (code de la propriété intellectuelle).
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`6 8 J. Dédeiek ei al.
`
`Materials 32 (¡999) 63-74
`
`,0
`
`Ë 0,5-
`
`0,0-
`
`0 200 400
`
`t i m e ( ^ s )
`
`a l 5 0 0 nm o f
`decay of t h e e m i s s i o n
`F i g . 4.
`i^ormaUsed
`C u ' ^ - c h a b a s i t e ( C u / A l « 0 . 0 1 ) : e x p e r i m e n t a l d a l a ( — ) ; d o u b l e -
`e x p o n e n t i a l fit
`(—^). C u ' ^ - c h a b a s i t e w a s r e d u c c d in h y d r o g e n
`o f 4 x l O ' P a f o r 2 n i i n a t 4 5 0 »C.
`
`use conditions of the Cu^^-chabasite reduction,
`denoted here as optimum conditions, which en­
`abled us
`to achieve simultaneously a maximum
`concentration of monovalent Cu"*" ions in the
`zeolite. The dependence of the integrated intensity
`of the emission bands a t 500 and 540 n m on the
`reduction time for natural Cu-chabasite is shown
`in Fig. 5, and n o difference was found using syn­
`thetic Cu-chabasite. The reduction time at which
`the maximum in normalised intensities overlapped
`was taken as the optimum reduction time. As with
`the dependence of the emission intensity on the
`reduction time, the C\i* ions corresponding to the
`emission band at 540 n m are more easily reduced
`fromCu^^ to Cu^ and from C u ^ t o Cu® compared
`with those corresponding t o the 500 n m emission.
`The effect of C u loading on the normalised
`Cu"*^ emission
`spectrum of CuNa-chabasite
`reduced at optimum conditions is shown in Fig. 6.
`With increasing concentration of the C u ^ ions in
`zeolites, the emission spectrum was shifted t o
`longer wavelengths. A larger shift was observed
`with natural chabasite [shift of the maximum of
`the emission band from
`500 t o 515 nm, see
`Fig. 6(a)J than with the synthetic Cu-chabasite of
`similar C u loading [shift from 515 t o 525 nm, see
`Fig. 6(b)j. It showed an increasing relative popula­
`tion of the Cu * ions emitting a t 540 n m compared
`
`optimum reduction time
`
`1 — . — I — . — J — I — 1 — I — I — I — J — I — [ —
`0 10 20 30 40 60 60
`time (min)
`
`F i g . 5 . T h e effect o f r é d u c t i o n t i m e o f C u * - c h a b a s i t e ( C u / A I =
`0 . 3 8 ) o n t h e n o r m a l i s e d integrated intensity o f t h e emission
`b a n d s . C u * e m i s s i o n b a n d a t
`5 0 0 ( Ü ) a n d 5 4 0 n m ( O ) ,
`C u ^ * - c h a b a s i t e w a s r e d u c c d i n h y d r o g e n o f 4 x 10^ P a f o r 2 m i n
`a t 4 5 0 ' C .
`
`with those emitting at 500 nm. This is reflected in
`the dependence of the ratio of the integrated
`emission of the 540 and 500 n m bands on the Cu
`loading, which is given in Fig.
`7. This ratio
`increased with increasing Cu loadings with both
`the natural and synthetic Cu-chabasites. On the
`other band, the population of the centre emitting
`at 540 n m was significantly higher in synthetic
`chabasite than that in the natural chabasite with
`a comparable C u loading. A t low Cu loadings,
`only the emission band at 500 n m was present in
`the spectrum of the natural Cu-chabasite. The
`dependence of the integrated intensity of the indivi­
`dual luminescence bands on the Cu concentration
`is given in Fig. 8. The intensity of the 540 n m band
`increased with increasing Cu loadings in the whole
`C u concentration range for both natural and syn­
`thetic Cu«chabasites, whereas the intensity of the
`500 n m emission was saturated at high Cu
`loadings.
`The effect of the presence of co-cations on the
`emission spectrum of the Cu"^ ions in chabasite is
`documented in Fig. 9. When Ca^^ ions are present
`in Cu-chabasite, the emission spectrum is shifted
`to a shorter wavelength, compared with the emis­
`sion spectrum of CuNa-chabasite with a similar
`Cu loading. On the contrary, the presence of Cs"^
`
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`
`/ Microporous and Mesoporous
`
`Malerials 32 ( ¡999) 63-74 6 9
`
`M
`
`c I
`
`c
`o
`
`eOO
`350 4 0 0 4 5 0 500 5 5 0
`wavelength (nm)
`
`6 5 0 7 0 0
`
`- « — r
`
`C u / A l
`
`F i g . 6 . T h e effect o f C u l o a d i n g o n t h e n o r m a l i s e d emission o f
`a ) n a t u r a l C u * - c h a b a s i t c , C u / A I 0 . 0 1 ( — ) , O . i l ( - - ) , 0 . I 7 ( — )
`a n d 0 . 3 8 ( ) ;
`( b ) Synthetic C u - c h a b a s i t e , C u / A I 0.08 (-•• ) ,
`0 . 1 5 ( - - ) a n d 0 . 3 2 ( — ) . C u ^ * - c h a b a s i t e s w e r e r e d u c e d a t o p t i ­
`m u m c o n d i t i o n s (see F i g . 5 ) .
`
`F i g . 8 . T h e effect o f t h e C u l o a d i n g o n t h e integrated emission
`intensity o f n a t u r a l ( a ) a n d synthetic ( b ) C u - c h a b a s i t e . Cu*^
`emission b a n d s a t 5 0 0 ( • ) a n d 5 4 0 n m ( O ) . Cu^'^-chabasites
`w e r e r e d u c e d a t o p t i m u m c o n d i t i o n s (see F i g . 5 ) .
`
`and co-cations is accompanied by a spectra!
`shift t o longer wavelengths. Decomposition of the
`the Gaussian bands and the second
`spectra to
`derivative mode analysis did not indicate any new
`band in the emission spectrum, and only bands
`with maxima at 500 and 540 n m were found. Thus,
`the presence of Ca^^ ions increases the relative
`intensity of the 500 n m band in the spectrum. On
`the other hand, Ba^"*" and Cs^ co-cations increase
`the relative intensity of the 540 n m band.
`Identical results t o those reported in the previ­
`ous paragraph for Cu'^-chabasite reduced in
`hydrogen at optimum conditions were also
`observed with Cu'^'-chabasite reduced in carbon
`10, where a
`monoxide. This is illustrated in Fig.
`comparison of the C u ^ emission spectra of
`Cu-chabasite reduced in hydrogen at optimum
`conditions and in carbon monoxide for 40 min is
`given. As no difference in the normalised emission
`spectra of Cu-chabasite reduced in hydrogen and
`
`o
`
`y
`
`
`
`o\
`
`\o
`
`•
`
`\ • \
`
`•
`
`1.6-
`
`1.2-
`
`0,8'
`
`0,4-
`
`0.0-
`
`o '
`
`o
`
`0,0 0,1
`
`0.2
`Cu/AI
`
`0.3 0,4
`
`F i g . 7 . T h e effect o f C u l o a d i n g o n t h e r a t i o o f integrated inten­
`sity o f t h e C u * emission b a n d s a t 5 4 0 a n d 5 0 0 n m o f synthetic
`{ • ) a n d n a t u r a l ( O ) C u - c h a b a s i t e . C u ^ * - c h a b a s i l e s w e r e
`r e d u c e d a t o p t i m u m c o n d i t i o n s (see Fig. 5 ) .
`
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`70
`
`J. Dëdeiek et al.
`
`/ Microporous and Mesoporous
`
`Materials 32 ( 1999) 63-74
`
`«)
`e
`£
`c
`c
`o
`M
`ü
`E
`c
`•o «
`«o
`
`3S0
`
`4 0 0
`
`4 5 0 5 0 0
`wavelength ( n m )
`
`7 0 0
`
`F i g . 9 . T h e effect o f t h e presence o f co-cacions o n t h e n o r m a h s e d
`emission s p e c t r u m o f C u ' ^ - c h a b a s i t e : ( a ) synthetic C u N a - {—)
`a n d C u C a - c h a b a s i t e ( • • • ) ; ( b ) n a t u r a l C u N a - ( — ) , C u B a -
`( — ) , C u C a - ( — ) a n d C u C s - c b a b a s i l e
`( ) .
`C u ^ ^ -
`c h a b a s i t e s w e r e r e d u c e d a t o p t i m u m c o n d i t i o n s (see F i g . 5 ) .
`
`400 450 600 650 600 650
`wavelength (nm)
`
`F i g . 10. T h e effect o f t h e r e d u c t i o n a t m o s p h e r e o n t h e n o r ­
`malised emission spectra o f Cu"^-chabasite ( C u / A l =
`0 . 3 4 ) . R e d u c t i o n i n h y d r o g e n o f
`4 x 1 0 ^ P a f o r 2 m i n a l
`450®C ( — ) , a n d in c a r b o n m o n o x i d e o f 5 . 3 x 1 0 ' P a f o r 4 0 m i n
`a t 4 5 0 ' C ( - - ) .
`
`carbon monoxide was observed, we conclude that
`the population of both C u ^ emission centres a t
`500 and 540 n m depends only on the C u loading
`in the zeolite a n d n o t o n the nature of the reduction
`atmosphere. T h e intensities of the Cu"^ emission
`spectra of the Cu^^-chabasites reduced in carbon
`monoxide were slightly higher ( 2 - 5 % of integrated
`emission intensity) than those of Cu^'^-chabasites
`reduced in hydrogen. This agrees with the reported
`quantitative reduction of Cu^"^ t o C u ^ in zeolites
`by carbon monoxide reported elsewhere [22].
`
`4. Discussion
`
`N o difference was found between the Cu"^ emis­
`sion centres and their behaviour in synthetic and
`natural Cu-chabasites. All the C u ions were
`reflected in the emission spectra of synthetic and
`natural Cu-chabasite, as follows f r o m the quantita­
`tive reduction o f the Cu^^ ions evidenced by the
`V I S - N I R D R spectra. This indicates, that C u ions
`were exchanged only t o the chabasite structure
`and not incorporated into the non-zeolitic concom­
`itants present in the sedimentary natural chabasite,
`evidenced b y the disproportion in the analysis of
`natural chabasite and by the X R D diffraction [see
`Fig. 1(b)]. I t follows that the Cu"*" emission of
`natural chabasite represents only the C u ^ ions a t
`cationic sites of the chabasite structure.
`T h e Cu"*" luminescence bands a t 500 and 540 n m
`can correspond t o two different emission centres,
`formed by (i) single Cu"^ ions in two different
`cationic sites
`o r (ii) single C u ^ ions a n d
`Cu"^ •••Cu'^ pairs, exhibiting different dependences
`on the C u concentration.
`Considering point ( i ) first, the intensity of two
`emission bands corresponding t o the C u ^ ions in
`two cationic sites in a zeolite should increase o r
`should be saturated. This concentration depen­
`dence o f the luminescence intensity is schematically
`depicted in Fig. 11(a).
`N o w considering point (ii), the intensity of the
`band corresponding t o the single ion should
`increase with the C u concentration, reach a maxi­
`m u m and, a t high C u loadings, when C u ^ " C u ^
`pairs are formed by localisation of the second
`C u ^ ion in the vicinity of the single Cu"*" ion, it
`
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`
`/ Microporous and Mesoporous Materials
`
`32 ( ¡999) 63-74
`
`71
`
`loading (see Fig. 8). This indicates that both bands
`correspond t o single Cu"*^ ions located in two
`different cationic sites. This assignment of the
`Cu^ emission bands t o the single Cu"^ ions is also
`supported by the effect of Ca^^, Cs^ and Ba^^
`co-cations on
`the emission spectra, as is dis­
`cussed below.
`The cationic sites in dehydrated chabasite are
`well known [19,32-36] and are shown schemati­
`cally in Fig. 12. Cations in site I (notation accord­
`ing t o Refs. [32-36]) are located in the chabasite
`cavity and coordinated t o three framework oxygen
`atoms of the regular six-membered ring above the
`six-membered ring plane. Their coordination
`exhibits C^y symmetry. Site II of the chabasite
`cavity is occupied only in partially hydrated cha-
`basites and is not given in Fig. 12. Site UI is inside
`the hexagonal prism, and metal ions are supposed
`to be coordinated to six framework oxygen atoms
`with pseudooctahedral symmetry (OiJ. Site IV
`corresponds t o the cations located in the eight-
`membered ring connecting the chabasite cavities.
`The cation position in the eight-membered ring
`and the number of the coordinated framework
`oxygen atoms depend on the nature of the cation.
`Large cations (Cs"^) are located in the ring centre
`and coordinated t o four oxygen atoms. Smaller
`cations (Ag"^, Mn^^) are assumed t o be coordi­
`nated to four or three oxygen atoms and located
`off the centre of the eight-membered ring (the
`position suggested for these small cations is indi­
`cated in Fig. 12).
`
`m
`
`F i g . 12. C a t i o n i c sites in d e h y d r a t e d c h a b a s i t e a n d t h e i r n o t a ­
`tion a c c o r d i n g t o R e f s . ( 3 2 - 3 6 j .
`
`0 50 100
`
`degree of Cu* exchange (%)
`
`F i g . 11. S c h e m a t i c d e p e n d e n c e o f t h e intensity o f C u emission
`o n t h e C u loading, ( a ) T w o emission c e n t r e s c o r r e s p o n d i n g t o
`t w o différent c a t í o n i c sites ( a . b ) w i t h different p r e f e r e n c e in
`site o c c u p a t i o n a n d n u m b e r o f individual sites
`A^bt A / , + A ' b = A ^ ) . TTie emission intensity o f preferentially
`f o r m e d emission c e n t r e s follows t h e e q u a t i o n
`\ —expi—p^n)\, t h e emission intensity o f t h e emission
`/ ,
`c e n t r e f o r m e d b y a l o w p r e f e r e n w follows t h e e q u a t i o n
`/»i«)]}» w h e r e t h e p a r a m e t e r a , i n c l u d e s
`4 = « i > { « — ^ » [ 1 —
`t h e q u a n t u m yield o f t h e radiative t r a n s i t i o n , p r o b a b i l i t y o f t h e
`a b s o r p t i o n a n d t h e t i m e d e p e n d e n c e o f t h e emission, a n d n is
`t h e n u m b e r o f t h e C u ' ^ i o n s in t h e zeolite, ( b ) T w o emission
`c e n t r e s c o r r e s p o n d i n g t o t h e single C u * Ion a n d Cu"^ p a i r . T h e
`e m i s s i o n intensity o f t h e single Cu*^ i o n s follows t h e e q u a t i o n
`/ , = « , ! A/{1—íA:p[-{l - } ^ flj)));
`t h e e m i s s i o n intensity c o r ­
`r e s p o n d i n g t o C u * p a i r s f o l l o w s t h e e q u a t i o n
`4
`i ^
`{
`1
`(
`1 — Ij ^—"!))}) w h e n n o difference
`i s s u p p o s e d f o r o c c u p a t i o n o f t h e site b y a single C u * i o n a n d
`f o r siting o f t h e C u * i o n s i n t h e vicinity o f a n o t h e r o n e .
`
`should decrease t o a level close t o zero. O n the
`other hand, the intensity of the band corresponding
`t o the Cu"^---Cu'^ pairs should increase in the
`whole concentration range. This model is
`illustrated in Fig. 11 (b).
`However, neither of the intensities of the emis­
`sion bands at 500 or 540 n m decrease with C u
`
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