J. Phys. Chem. 1995, 99, 15222-15228
15222
Diffuse Reflectance Spectroscopy of Dehydrated Cobalt-Exchanged Faujasite-Type Zeolites: A New Method for Co2+ Siting An A. Verberckmoes, Bert M. Weckhuysen,* Jozefien Pelgrims, and Robert A. Schoonheydt Centrum voor Oppervlaktechemie en Katalyse, K.U.k u v e n , Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium Received: May 11, 1995; In Final Form: July 22, 1 9 9 9
The diffuse reflectance spectra of cobalt in X- and Y-type zeolites with varying Co2+ contents have been investigated after dehydration at 400 and 500 "C. A novel method for the determination and characterization of Co sites in dehydrated zeolites is developed. The spectra were resolved with a decomposition program in five or six Gaussian bands in the visible region (12 500-22 500 cm-I) and three Gaussian bands and two Lorentz bands in the near infrared region (4000-10 000 cm-I). Interactive self-modeling analysis showed the presence of three independent Co species, called pure components. In the visible region component A appeared at 20 000-17 800 cm-I, component B appeared in the 18 800-15 300 cm-' region with three band maxima, and component C appeared with main purity at 13 700-14 900 cm-I. In the near infrared region component B appeared as a triplet in the 4600-7000 cm-I region together with two components of framework hydroxyls at 4500 and 7600 cm-I. With the aid of additional experiments on dehydrated lanthanum-, cesium-, and ammonium-exchanged Co zeolites, the three cobalt-related components (A, B, and C) were assigned as follows: A, (pseudo)octahedral cobalt in the hexagonal prisms (I); B, (pseud0)tetrahedral cobalt at I' (II',II); and C, trigonally coordinated cobalt at the same sites. These findings are discussed in relation with XRD results of Co siting.
Introduction For all applications of transition metal ion (TMI) exchanged zeolites there is a great interest in locating and characterizing the preferred positions of the TMI in order to understand complexation and catalytic properties. The coordination sites can be obtained with X-ray diffraction (XRD), however, such studies are only possible for high TMI loadings. For a study of the coordination at lower loadings a spectroscopic approach, such as diffuse reflectance spectroscopy (DRS), is recommended. Klier was the first to develop a ligand field model to explain the d-d transitions of the 3d TMI, coordinated to six-rings of oxygens in zeolite A.',2 Similar six-rings occur in zeolites X and Y, together with other coordination sites, all of which can be occupied simultaneously. In this paper we have chosen cobalt as a probe for studying the cation siting in X- and Y-type zeolites. The most important cation sites are indicated with Roman numbers in Figure 1. It has been established that during dehydration the hexaaquo complex C o ( H ~ 0 ) 6 ~(oh + symmetry) in the supercage loses water and migrates toward the sodalite cages where it is coordinatively bound to three oxygen ions of the structure and one extra-lattice OH- or 02-group at sites I' and II'.3-8 The electronic spectrum becomes that of a tetrahedral Co2+ species with two ranges of abs~rption:~ the visible (VIS), 20 000-15 400 cm-I, and the near infrared (NIR), 8300-5900 cm-I. In each range, a 3-fold band splitting is often observed. Above 250 "C, a large part of the tetrahedral species migrates in the hexagonal prisms (octahedral site I), but yet a part stays tetrahedrally coordinated at 11' or I' as evidenced by XRD.Io As a consequence, the DRS spectra of Co2+ in X and Y are a superposition of spectra, due to Co2+ on different sites. Up to now, no method has been developed to decompose these spectra into their individual components.
* To whom correspondence should be addressed. 'Abstract published in Advance ACS Abstracts, September 15, 1995.
Figure 1. Faujasite structure with indication of cation sites. Numbers refer to the four crystallographically different oxygens.
We propose a new approach for the spectroscopic speciation of Co2+ on different sites in dehydrated zeolites X and Y. The method is based on (1) decomposition of spectra of homogeneous series of dehydrated Co zeolites into Gaussian andor Lorentz bands and (2) an interactive self-modeling analysis of these series of spectra."-'5
Experimental Section 1. Sample Preparation and Characterization. I .I. CoNaX-CoNaY. Commercial X- and Y-type zeolites from Ventron were stirred in 0.01 M NaCl, washed C1- free, airdried, and stored over a saturated NH&1 solution prior to use. Cobalt zeolites with variable cobalt content were prepared by ion exchange of the so-obtained NaX or NaY in solution (1 g/L)with the amount of CoC126H20 necessary to obtain the desired exchange level at room temperature. The exchange time was 24 h.
0022-365419512099-15222$09.00/0 0 1995 American Chemical Society
Cobalt-Exchanged Faujasite-Type Zeolites
J. Phys. Chem., Vol. 99, No. 41, 1995 15223
TABLE 1: Exchangeable Cation Content (milliequiv-g-') of the Zeolites sample
co2+
La3+
cs+
COl39Y
0.09 0.17 0.41 0.73 1.26 2.13
COO 5x COO 8x COl7X co3 5x co5 9x COll 5x co187x
0.07 0.12 0.25 0.5 1 0.86 1.69 2.74
C O I5NHsY ~ c03 2NKY Co2LaY
2.07 0.49 0.31 0.26 1.64
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 2.42 4.22
0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0
0
0.39
2.36 4.19
0 0
COO 6 y COl IY co2 6 y c04
1Y
cos 2y
col c s c o j 07Y Lalo 2Y L a d
0 0
1.2. NHd+(H+)-, Csf-, and La3+-Exchanged Zeolites. The same commercial X- and Y-type zeolites were stirred in 0.01 M NaCmaOH (5050). Samples of CoLaX(Y) were then prepared by first exchanging in 0.0021 M (0.0014 M) La[NO&6H20 at room temperature with a solidniquid ratio of 1 g/L. The LaX(Y) were then washed anion free, air-dried, and calcined in a crucible in a muffle furnace at 550 "C for 24 h. The subsequent Co2+ exchange was performed as described above. For the CoNH4Y samples, NH4Y (Zeocat) was stirred in 0.00016 M CoC126H20 solution. CoCsY was obtained by stirring COY in a 0.004 M CsCl solution. Table 1 contains the Co2+, La3+, and Cs+ contents of the samples, determined by atomic absorption spectroscopy (AAS)after HFEI2SO4 dissolution of known quantities of zeolites. 2. Pretreatment and Spectroscopy. The samples were dried and granulated. The size fraction of 0.25-0.40 mm was loaded into a quartz flow cell with a suprasil window for DRS. The samples were calcined at 400 and 500 "C during 24 h in an oxygen stream. The samples were cooled in 0 2 , and the sample cell was flushed with a He flow for 12 h before spectra were taken. Diffuse reflectance spectra were taken on a Varian Cary 5 UV-VIS-NlR spectrophotometer at room temperature. The spectra were recorded against a halon white reflectance standard in the range 200-2500 nm. 3. SPECTRA CALC. The spectra were decomposed with a SPECTRA CALC program of Galactic Ind. C ~ r p . 'For ~ the deconvolution with SPECTRA CALC, we chose to decompose the spectra in Gaussian bands (with the exception of Lorentz bands for the relatively sharp framework hydroxyl bands) because of (1) instrumental band broadening, (2) band broadening due to site inhomogeneity (SUA1 ratio varies from site to site), and (3) unresolved vibronic components within each d-d band. 4. SIMPLISMA. SIMPLISMA (SIMPLe-to-use Interactive Self-Modeling Analysis) is a statistical technique, and a successful application requires therefore a series of minimum size of homogeneous spectra."-15 This means spectra varying in only one chemical parameter, e.g., the Co content of the zeolite. Consequently, the conditions of preparation and spectroscopic measurement must be the same. For each homogeneous series of spectra the starting point and end point of the wavelength interval and the number of data points in that interval were equalized, such that the spectroscopic data series were suitable to load into SIMPLISMA as uniform matrices. The SIM-
PLISMA program calculates (in a MATLAB environment) the spectrum of the mean intensity values of the series and the standard deviation spectrum. When dividing the standard deviation values through their mean and multiplying with a weight factor, one obtains the purity spectrum. When the latter is adjusted to reduce pure variables of low intensity (=noise), it is called the purity-corrected standard deviation spectrum. A pure variable is defined as a variable of which the intensity is due to only one of the compounds in the mixture. The spectrum that corresponds with a pure variable is called a pure spectrum. The self-interactive method of SIMPLISMA exists in the interaction of the user with the proposal, done by the program, for the position of a pure variable. Some general rules for the analysis can be formulated: (1) the proposed variable with maximum intensity in the purity spectrum must also have a significant intensity in the purity-corrected standard deviation spectrum, (2) the pure spectra must be (preponderant) positive, and (3) when the proper number of "pure spectra" has been chosen, the difference between the original and reconstructed data must be noise. In this study, we disposed of four data sets composed of six spectra for COY and seven spectra for COX with varying Co contents and taken both after dehydration at 400 and 500 "C. For each data set the data matrix was built up as follows: an n x m matrix with n the wavelengths and m the Kubelka-Munk intensities of the different spectra ordered with increasing Co content. The inverse of this matrix was taken before loading into SIMPLISMA. The analysis is based on the following calculations: for each wavelength the mean of the Kubelka-Munk intensities for the different spectra of the data set is calculated as well as the standard deviation. The first purity and puritycorrected standard deviation spectra are given as were defined before, and the user accepts a first pure variable. With this variable a pure spectrum and an intensity course with increasing Co content correspond. After subtracting the f i s t pure spectrum a second set of purity and purity-corrected standard deviation spectra is given. At the end only noise should be left. The SIMPLISMA analysis thus results in a number of pure spectra according to different Co species. When multiplying the pure spectra with the intensity contribution of the different Co loadings, one obtains the pure spectra of all the individual spectra of the data series. Summarizing, although SIMPLISMA is a user-friendly tool, data sets are often not very user friendly. Therefore, proper data acquisition may be a time-consuming process.
Results 1. Qualitative Spectroscopy. Figure 2 gives the overall DRS spectra of c05.9xand COg.2Y dehydrated at 500 "C. The intensities after dehydration at 500 "C are lower than those after dehydration at 400 "C. In the VIS region a triplet is more or less observed. In the NIR, the triplet is asymmetric and superposed on two sharp bands at 7250 and 4550 cm-I, assigned to respectively 2v and v 6 hydroxyl vibration~.~~-I* The DRS spectra in the VIS region of a series of COX zeolites after dehydration at 500 "C are shown in Figure 3. At low loadings a broad band is visible with at least two band maxima. As the loading increases, four bands become apparent and, possibly, a shoulder above 20 OOO cm-'. Above 1.7 Co/uc the most intense band shifts from 17 000 to 17 200 cm-'. The band at 15 750 cm-l shifts to 15 300 cm-l for loadings above 3.5 Co/uc. These observations are also made for the spectra of COX dehydrated at 400 "C. Figure 4 gives the spectra of COY dehydrated at 400 "C in the VIS region. Already at the lowest loadings four bands are apparent. At higher loadings a shoulder above 20 000
+
Verberckmoes et al.
15224 J. Phys. Chem., Vol. 99, No. 41, 1995 45
l h
-
32.5
0
= f r
1
cox
X
(5.9 Co/UC)
80
z
P
x
7.5
-5
1:
10
10375
NM
Figure 2. DRS spectra of Co5.9X and Cos.2Y dehydrated at 500 "C.
I
I
6250
6125
4000
Wavelength (cm- 1 )
2000
1000
Figure 5. Near infrared region of the diffuse reflectance spectra of dehydrated COY zeolites after dehydration at 500 "C. The number of
Co2+per unit cell is given with each spectrum. 100
1
x7 0
r
a 5 100 P
I
X
50
X
f
Y
X
s
Y
0 0
Wavelength (em-1)
20000
Figure 3. Visible region of the diffuse reflectance spectra of COX zeolites dehydrated at 500 "C. The numbers are the amount of cobalt
15000
Wavelength (em- 1 )
ions per unit cell.
Figure 6. Visible region of the diffuse reflectance spectra of Co2LaY and ColsLaX dehydrated at 500 "C.
-I
the Co content. A weak broad band can also be observed around 11 000 cm-' . The NIR region of COXis similar to that of COY in Figure 5 . Figure 6 shows the spectra of Co2LaY and Col.sLaX dehydrated at 400 and 500 "C. In the latter case, the intensities after dehydration at 500 "C are higher than those after dehydration at 400 OC, contrary to the observations on the COX and COY spectra. For co],!&ax the spectra have similar characteristics independent of the calcination temperature. There is a clearly resolved band at 14 350 cm-I and a poorly resolved band system in the region 16 000-18 700 cm-I. C02Lay, calcined at 400 "C, is characterized by a triplet at 15 550, 17 200, and 18 900 cm-' with an ill-defined shoulder around 13 950 cm-I. Upon calcination at 500 "C this shoulder gains intensity at the expense of the 15 550 cm-' band. The spectra of dehydrated CSCOIO.~Y are similar to the spectra of Co14.1Y of Figure 4. The spectra of CoN&Y were also comparable to those of COYwith the same Co content, dehydrated at the same temperature. 2. Spectral Decomposition. Our goal is 2-fold: (1) to decompose the spectra into a consistent set of Gaussian bands with the deconvolution program SPECTRA CALC, based on the method of least-squares and (2) to get more insight into the number and nature of the pure species making up the spectra, with the aid of the simple-to-use interactive self-modeling analysis SIMPLISMA. 2.1. Decomposition of Spectra (SPECTRA CALC). The VIS region of the spectra, obtained on samples with varying Co
100
I
0
r
50
f i
P
X
0
I
I 20600
15000
Wavelength (em-1)
Figure 4. Visible region of the diffuse reflectance spectra of COY zeolites dehydrated at 400 "C. The number of Co2+per unit cell is given with each spectrum.
cm-' becomes apparent. For COY calcined at 400 "C a shift of the band at 15 300 cm-' to 15 600 cm-' is observed when going from 8.2 to 14.1 Cohc. This shift is not apparent in the COY spectra after dehydration at 500 "C. The shift is opposite to that for X-type Co zeolites. In Figure 5 the NIR DRS spectra of COY dehydrated at 500 "C are given. These spectra have the same features as the NIR spectra of COY dehydrated at 400 "C. Three broad bands can be observed. The relative intensities of the bands around 5500 and 6300 cm-' vary as a function of
Cobalt-Exchanged Faujasite-Type Zeolites
J. Phys. Chem., Vol. 99, No. 41, 1995 15225
..
"
I
>
I 5L11)C
/0111>
Wavelength (Cm- 1 )
Figure 7. Decomposed visible region spectrum of at 400 "C.
c03.5xdehydrated
30
I 0
8
4
10
12
20
# Coluc -t
-0-
400 C
.+
-Q-
500 C
400
c
500 C
Figure 9. Intensities of the sum of bands c, d, and e in the near infrared and visible regions as a function of Co content for COY and COX dehydrated at 400 and 500 "C.
u0 P
3 Y
I 0
8
4
12
20
16
# Coluc - A -
-t-
400OC
a 60O0C
-0- f
400°C
+
I 6OOaC
Figure 8. Intensities of bands a and f as a function of the Co content for COY and COX dehydrated at 400 and 500 "C. Bands a and f a r e defined around 13 800 and 20 000 cm-', respectively.
contents at a specific dehydration temperature, has been decomposed in five or six bands, of which the band positions were estimated by eye. The bands have for each zeolite type the same position. The spectra are baseline corrected, which means that a straight line between the beginning and end point of the spectrum is drawn, as shown in Figure 7. This figure shows, as an example, the decomposition of the 12 500-22 500 cm-' region for dehydrated Co3.5X at 400 "C. Six bands have to be invoked. The bands are called a, b, c, d, e, f, respectively at 13 700-13 800,14 600,15 450 f 200,17 100 f 100,18 700 f 100, and 20 000 f 100 cm-'. Bands c, d, and e are typical tetrahedral triplet bands, for which we arbitrarily have taken the same width for each band at 1650 f 50 cm-'. For COY the same bands are needed except band b. Figure 8 gives the intensities of bands a and f for COX and COY after dehydration at 400 and 500 "C as a function of Co content. The following observations are made: for COX the intensities of a and f increase concomittantly until 4 Co/uc for both calcination temperatures, then the intensities level off until 12 Co/uc (except for band f a t 500 "C) and increase further at higher Co loading. For COY the intensity of band f increases with Co content for
both calcination temperatures, while the intensity of band a grows to a maximum at 1 Co/uc and remains constant (500 "C) and decreases slightly (400 "C) with increasing Co loading. In Figure 9 the sum of the triplet bands c, d, and e is given as a function of Co content. For COX this sum is independent of the calcination temperature whatever the Co content. For COY, on the contrary, there is a significant difference between the two temperatures; the spectra obtained after calcination at 400 "C are more intense than those obtained after calcination at 500 "C. Both for X and Y zeolites, the intensity increases rapidly up to 4 Co2+/uc, levels off more or less, and increases again with Co content above 6 Co2+/uc. The decomposition reveals a shift of band c between low and high loading, as was already observed by eye in the spectra of Figures 3 and 4. For COY (400 "C) this band is positioned around 15 200 cm-' at low Co loading and shifts to 15 500 cm-' for 14.1 Co/uc. For COX band c is at 15 700 cm-' for low Co-loading and shifts to 15 300 cm-' for higher loadings. Application of SPECTRA CALC in the NIR results in three bands around 5350,6300, and 7600 cm-' for COY and around 5400,6300, and 7750 cm-' for COX. The sum of the intensities of the three bands in the NIR as a function of Co content resembles the intensity course of the sum of bands c, d, and e of the concomitant zeolite type in the VIS. This is given in Figure 9. Decomposition of the spectra of the NK+-, Cs+-, and La3+exchanged zeolites was performed in the VIS region. For the decomposition of the NK+-and Cs+-exchanged zeolites the same bands were used as mentioned before. The band positions of the La3+-exchangedzeolites are given in Table 2. Five bands are resolved for CoLaY at similar positions as were resolved for COY by SPECTRA CALC and SIMPLISMA. Only four bands were resolved for CoLaX. 2.2. Interactive Self-Modeling Analysis (SIMPLISMA). The SIMPLISMA analysis of the VIS region resulted for all the data series of COY and COX in three pure components. Figure 10 shows examples of the pure spectra. The low-frequency (LF) component consists of a single asymmetric band at 14 000 f
15226 J. Phys. Chem., Vol. 99, No. 41, 1995
Verberckmoes et al.
TABLE 2: Comparison of the Frequencies (cm-') of the Bands Obtained with SPECTRA CALC (SC) and the Pure Components Obtained with SIMPLISMA (S) and Frequencies of the Bands of La3+-ExchangedZeolites. The Symbols T, LF, and HF Represent Respectively the Triplet Component, Low-Frequency Component and High-Frequency Component COY
S
cox
Component 1: LF or Band a 14 900 13 700
sc
14 000 13 700
S
Component 2: T or Bands c, d, and e 18 700-17 150-15 650 18 750-17 000-15 950 18 700-17 100-15 300 18 800-17 100-15 550
S
Component 3: HF or Band f 20 000-17 850 19 600-17 850 20000 20 000
sc sc
CoLa
Y
+
SC 400 "C 20 000 18 800-17 100-15 450 14 200 500 "C 19 900 18 600-17 100-15 700 14 400
+
X
+
19 000-18 000-16 500 14 600
+
19 200-18 000-16 500 14 500
+ +
100 cm-'. The triplet (T) component has three maxima at 15 650, 17 150, and 18 700 cm-'. The high-frequency (HF) component has two main bands at 20 000 and 17 850 cm-'. The separation into pure components is not perfect. One can see in Figure 10 that for COY dehydrated at 400 "C the LF component is contaminated with the T component and that the HF component is contaminated by the LF component. A similar contamination of the components can be observed in Figure 10 for the COX zeolites calcined at 400 "C, although the SIMPLISMA analysis of the COYzeolites calcined at 500 "C (Figure lob) resulted in a much better resolution of the pure components. Figure 11 gives the intensity course as a function of cobalt content for COY calcined at 400 "C. The HF component increases almost linearly with the Co2+ content up to 8 Co2+/ uc and levels off. The LF component attains its maximum intensity at 1 Co2+/uc, and the triplet rapidly increases in intensity up to 2 Co2+/uc,levels off between 2 and 8 Co2+/uc, and increases for 14 Co2+/uc. This latter evolution is in qualitative agreement with the intensity evolution of the cde triplet obtained with SPECTRA CALC for COYcalcined at 400 "C given in Figure 9. The intensity courses of, respectively, the LF and HF components, obtained with SIMPLISMA (Figure l l ) , resemble those of the a and f bands, obtained with SPECTRA CALC (Figure 8, COY). A good agreement between SPECTRA CALC and SIMPLISMA results was also observed for COY calcined at 500 "C and COX calcined at 400 "C. In Table 2 the frequencies of the decomposed bands (SPECTRA C A L C ) and pure components (SIMPLISMA) are summarized. This table also gives the frequencies of the spectra of the Laexchanged zeolites. One can see that upon a qualitative agreement in intensity course, there is also a frequency agreement between band a and the LF component, band f and the HF component, and bands c, d, and e with the T component. SIMPLISMA was also applied to the NIR region for COY after dehydration at 400 "C. Three components were resolved. Two for the hydroxyls at 4500 and 7600 cm-I and one component that covers the three decomposed Gaussian bands (SPECTRA CALC). When investigating the intensity course of the three components as a function of the Co content, the following observations were made: (1) there is a correspondence between the NIR triplet and the VIS triplet, ( 2 ) the intensity of
D
Wavelength (nm)
Figure 10. The global spectrum and resolved pure components of the data series of (a) COY dehydrated at 400 "C, (b) COY dehydrated at 500 "C, and (c) COX dehydrated at 400 "C. 14
-
I
I
I ,'+-'
I
1
[,$ox'-+-+ 0
-+-
2
LF
6
4
- A
8
# COlUC T
12
IO
-0-
14
HF
Figure 11. Intensity course of the pure components in the visible region for COY dehydrated at 400 "C.
the 4500 cm-I OH band remains almost constant whatever the Co content, and (3) the intensity of the 7600 cm-I OH band increases with increasing Co content. It is also a relatively broad band. This may be due to contamination with neighboring bands or with an underlying unresolved band, e.g., of octahedral co2+.I9
J. Phys. Chem., Vol. 99, No. 41, 1995 15227
Cobalt-Exchanged Faujasite-Type Zeolites
Discussion
1. Spectral Decomposition and Siting. The DRS spectra of the d-d transitions of Co2+ in dehydrated COX and COY zeolites are complex. Any interpretation in terms of cation siting must be based on decomposition of the spectra in terms of pure components, i.e., spectra characteristic for Co2+ on a specific site. The spectral characteristics and the evolution of the intensities with the Co2+ content of the zeolites are then used for the siting of Co2+. On the basis of the known cation sites (Figure 1) and XRD results one expects (pseudo) octahedral coordination (site I) and (pseud0)tetrahedral coordination (sites I' and II'AI), while sites 111and 111' are unlikely to be occupied. Octahedral Co2+ has only weak symmetry-forbidden d-d transitions. Therefore any intensity change of the overall spectrum can be interpreted qualitatively in terms of increasing or decreasing occupation of site I with respect to 1', 11', and 11. The following qualitative features emerge, when a comparison is made of the spectra of all the samples at the two calcination temperatures: (1) For all the COXand COY zeolites the overall intensity is lower after calcination at 500 than at 400 "C. This suggests a supplementary migration of Co2+ to the hexagonal prisms when the temperature is increased from 400 to 500 "C. (2) Cs+ and NI&+(H+) do not significantly influence the Co2+ spectra. This means that these ions do not influence the Co2+ distribution. As Cs+ and H+ are mainly in the supercage,20it follows that the majority of the Co2+ ions is located in the hexagonal prisms and in the cubooctahedra. (3) La3+, on the other hand, strongly influences the Co2+ spectra, thus competing with Co2+for the same sites in the hexagonal prisms and in the cubooctahedra.2' This competition is temperature dependent for Y-type zeolites, because the spectrum after calcination at 500 "C is different from that obtained after calcination at 400 "C. The spectra of CoLaX are much more intense than those of CoLaY, suggesting preferential occupation of site I' in the former case or that La3+ more effectively blocks sites I in X than in Y. This is in agreement with literature data.22,23 2. Methodical Approach. The main features of the spectra were analyzed (1) by eye, (2) after decomposition (SPECTRA CALC), and (3) after component analysis (SIMPLISMA) and thus with increasing accuracy. The spectral regions of interest were the NIR and VIS regions. By eye, four to five bands in the VIS region were noticed and five bands in the NIR. These bands were also obtained with SPECTRA CALC. Sometimes a sixth band had to be invoked for deconvolution of the COX spectra in the VIS. This is band b around 14 600 cm-'. We can not give it a physical interpretation at this moment. SIMPLISMA determined three pure components in the VIS region and three in the NIR region. In the NIR only one component is due to Co2+; the other two can be ascribed to framework hydroxyls. Both the resolved bands in the VIS and the NIR regions (SPECTRA CALC) could be grouped according the pure components. In the visible region bands a, cde, and f correspond respectively to components 1, 2, and 3 (Table 2 ) . This resulted from a correspondence in intensity course with Co content and in absorption between the resolved bands and the determined components. The three Gaussian bands in the NIR correspond to component 2. This resulted from a correspondence of the intensity courses of the VIS and NIR triplet bands. The band decomposition by SPECTRA CALC was thus confirmed with SIMPLISMA. 3. Component Assignment to Coordination Site. 3.1. (Pseudo)tetrahedral Co2+. Both in the VIS and NIR regions SPECTRA CALC resolves three bands which are recognized as only one component with SIMPLISMA, corresponding to only one absorbing species. Such triplets are typical for
tetrahedral ~ y m m e t r y . ~The ~ . ~band ~ separation within each triplet is almost symmetrical. Therefore we take the central points of each triplet as the tetrahedral v2 and v3 transitions:
= -
v2 = 4A2(F)
4T1(F)
6450 cm-' (400 "C), 6400 cm-' (500 "C)
v3 = 4 ~ 2 ( ~ )
4Tl(P) = 17 000 cm-'(400 "C), 17 100 cm-' (500 "C) v3 is relatively high when a comparison is made with reference
compounds.26 In view of (1) the preference of Co2+for sites I' and I,27 (2) the absence of significant effects of Cs+, and (3) the effect of La3+,we propose I' as the most likely coordination site. Indeed, for X-type zeolites with a blocked site I, the intensities of the spectra have increased due to a forced increase of tetrahedral coordination. For Y-type zeolites, site I' is blocked and less Co2+ has tetrahedral symmetry, expressed in the lower intensities of the CoLaY spectra. Because of a coordination of Co to three oxygens of the zeolite windows and to one extra-lattice oxygen, there are two types of oxygens. As a consequence, there is a lowering of the ideal tetrahedral symmetry and we prefer the term pseudotetrahedral symmetry. 3.2. (Pseudo)octahedral Co2+. Band f at 20 000 cm-' can be assigned to (pseudo)octahedral coordination (v3) of Co2+ in the hexagonal prisms. A broad band in the region 10 20012 500 cm-' (Figure 2) can be ascribed to the v2 transition ( V I is probably masked by the triplet bands in the NIR). Moreover, this broad band is apparent in all the spectra of calcined COX and -Y zeolites, Cs- and N&+-exchanged COY-zeolites, and CoLaY and not in the spectra of calcined CoLaX and CoA.'s2 In the first zeolites Co2+ can enter the hexagonal prisms. In the latter two not, because the hexagonal prisms are blocked (LaX) or not present (zeolite A). Thus, we believe this component to be a pseudooctahedral species in the hexagonal prism. 3.3. Trigonal Co2+. A third component is the low-frequency (LF) component in the VIS region, corresponding to the 13 800 cm-' band. It is a characteristic band of dehydrated COX and COY zeolites.8,28This band has been interpreted by Hutta and Lunsford as characteristic for Co2+ in the distorted octahedral site We rather ascribe the 13 800 cm-' band to trigonal planar coordinated Co2+ at the hexagonal oxygen window, because a similar band is present in the spectra of anhydrous CoAe2 In addition, this 13 800 cm-' band disappears in dehydrated CoA when adsorbing ethylene and thus going from a trigonal to a more tetrahedral coordination. Although the LF component has its main purity around 13 800 cm-I, we suppose the trigonally coordinated Co species also to have bands in the 18 800- 15 300 cm-' region, as can be deduced from the spectra of anhydrous C O A ' . ~and from the contamination of the LF component with the triplet component (Figure 10). At this moment, we do not known how well the pseudotetrahedral and trigonal components are separated by SIMPLISMA. But the present purpose was to show that the methodology works, rather than obtaining the exact spectra for each type of Co2+. Additional work on CoA is in progress to elucidate this point. 4. Cation Distribution. XRD measurements are useful to locate cations, but they are only possible for high TMI loadings. We suggest the use of quantitative information of decomposed diffuse reflectance spectra to predict the preferred positions of TMI at lower loadings. From the literature we dispose of XRD results of dehydrated C O ~ ~ Yof, which ' ~ , ~ the ~ pretreatment and loading is comparable
Verberckmoes et al.
15228 J. Phys. Chem., Vol. 99, No. 41, 1995
TABLE 3: Co2+-Distributionalong Pseudo-octahedral and Pseudo-tetrahedral/TrigonalSites at Low Loading, Calculated with SDectra Calc Areas ~~
calculated Co2+ distribution
adjusted Kubelka-Munk band areas dehydrated zeolite
+
f
(cde a)/ 26.8
I
I'
% Co2f at site I
12425.1 14969.7
2788.4 2575.4
0.41 0.43
0.09 0.07
81.6 86
12836.1 13379.4
3496.8 3385
0.86 0.88
0.24 0.22
78 80
14311.5 22978.2
6729.2 5200.9
1.77 2.12
0.83 0.48
68 81.5
32786.4 3 1144.9
8196.6 5447.8
3.76 4
0.94 0.7
80 85
57095.9 4021 1
963 1 7963.5
7 6.8
1.2 1.4
85 83
COO 5y
400 "C 500 "C C O l IY 400 "C 500 "C COZ 6y 400 "C 500 OC co4 7y
400 "C 500 "C
cos zy 400 "C 500 "C
to our highest loaded COY sample dehydrated at 500 "C. A distribution of 11.3 Co at site I and 2.3 Co at I' was determined with XRD. This can be used to obtain the ratio of the molar extinction coefficient of Co2+ at I'D. Pseudooctahedral Co2+ can (approximately) be quantified by the area of the decomposed band f at 20 000 cm-]. The Co species at mainly I' can be tetrahedral or trigonal, as stated above, and is quantified by the sum of the areas of bands cde and a. When dividing the area of pseudooctahedral coordination (band b) by 11.3 and the area of tetrahedravtrigonal coordination (bands a and cde) by 2.3, the extinction coefficient of tetrahedrdtrigonal symmetry is found to be 26.8 times that of octahedral symmetry, or the extinction ratio of Co2+at I'D is 26.8. When applying this ratio for lower loadings, we can calculate the cation distribution from the areas of the decomposed bands of the spectra of lower Co loaded Y-zeolites. This is given in Table 3. We find that (1) for all loadings around 80% of Co2+ is in site I, (2) site I has a slightly higher occupancy at 500 OC (Table 3), except for co8.2Y. This is in agreement with the experimental finding that the spectra of Co(X/Y) zeolites calcined at 500 OC are of lower intensity, thus indicating an increased population of c o in oh with a forbidden, low-intensity d-d transition. Intuitively, one should have guessed that the percentage of Co2+ in site I increases with decreasing loading, because of the well-known preference of divalent cations for site I. The reason for this difference with expectation is probably the extra-lattice oxygen in the pseudotetrahedral coordination, which prevents Co2+ migration to site I. A similar situation was also observed in an earlier study of Ni-loaded faujasites X and Y.29
Conclusions In this paper a novel approach is presented to decompose diffuse reflectance spectra of Co-loaded faujasite-type zeolites into pure components in order to obtain the different absorbing species. The method is a self-modeling mixture analysis called SIMPLISMA, and its reliability has been proven by the correspondence with a spectral decomposition program (SPECTRA CALC). Three independent components of the coordination of Co in zeolites are identified: component A, a high-frequency component with two band maxima around 20 OOO-19 600 and 17 850 cm-I; component B, a triplet component, occurring in the 18 800-15 300 and 4600-7000 cm-I regions and component C, a low-frequency component with main purity at 14 900-
13 700 cm-I, The following spectroscopic assignments were put forward: component A is due to Coz+ located in the hexagonal prism, component B to pseudotetrahedral cobalt at site I' (II'AI); while component C belongs to Co2+ in trigonal symmetry. The ratio of the extinction coefficient of Co2+ at sites I' and I was established to be 26.8. With this value the site occupancies of Co2+ can be calculated for all loadings.
Acknowledgment. A.A.V. and B.M.W. acknowledge a research grant of the I.W.T. (Belgium) and a research grant as assistant of the National Fund for Scientific Research of Belgium (N.F.W.O.), respectively. The authors thank D. L. Massart, L. Ceusta Shchez, and B. Walczak (VUB, Belgium) for use of SIMPLISMA and for help with data handling. We gratefully acknowledge W. Windig (Kodak) who developed the SIMPLISMA program. We thank also J. B. Uytterhoeven for the use of the SPECTRA CALC Program. This work was financially supported by the Geconcerteerde Onderzoeksactie (GOA) of the Flemish Government and by the FKFO. References and Notes (1) Klier, K. Adu. Chem. Ser. 1971, 101, 480. (2) Klier, K.; Kellerman, R.; Hutta, P. J. J. Chem. Phys. 1974, 61, 4224. (3) Vansant, E. F.; Lunsford, J. H. Adu. Chem. Ser. 1973, 121, 441. (4) Klier, K. J. Am. Chem. Soc. 1969, 91, 5392. (5) Egerton, T. A.; Hagan, A.; Stone, F. S.; Vickerman, J. C. J . Chem. SOC., Faraday Trans. 1 1972, 68, 723. (6) Hoser, H.; Krzyzanowski, S.; Trifiro, F. J . Chem. Soc., Faraday Trans. I 1975, 71, 665. (7) Heilbron, M. A.; Vickerman, J. C. J. Catal. 1974, 33, 434. (8) Hutta, P. J.; Lunsford, J. H. J. Chem. Phys. 1977, 66, 4716. (9) Praliaud, H.; Coudurier, G. J . Chem. SOC.,Faraday Trans. 1 1979, 75, 2601. (10) Gallezot, P.; Imelik, B. J . Chim. Phys. 1974, 71, 155. (11) Windig, W.; Guilment, J. Anal. Chem. 1991, 63, 1425. (12) Windig, W.; Heckler, C. E.; Agblevor, F. A,; Evans, R. J. Chemom. Intell. Lab. Syst. 1992, 14, 195. (13) Windig, W.; Stephenson, D. A. Anal. Chem. 1992, 64, 2735. (14) Windig, W.; Markel, S. J . Mol. Struct. 1993, 292, 161. (15) Cuesta SBnchez, F.; Massart, D. L. Anal. Chim. Acta 1994, 298, 331. (16) Weckhuysen, B. M.; De Ridder, L. M.; Schoonheydt, R. A. J . Phys. Chem. 1993,97,4756. Weckhuysen, B. M.; Verberckmoes, A. A.; Buttiens, A. L.; Schoonheydt, R. A. J . Phys. Chem. 1994, 98, 579. (17) Schoonheydt, R. A.; Pelgrims, J. J. Chem.SOC., Dalton Trans. 1981, 914. (18) Tanabe, K., Hattori, H., Yamaguchi, T., Tanoka, T., Eds. In Proceedings of the International Symposium on Acid-base Catalysis; Kodansha, Tokyo, 1989; p 147. (19) Lever, A. B. P. Inorganic Electron Spectroscopy, 2nd ed.; Elsevier: Amsterdam, 1984. (20) Lai, P. P.; Rees, L. V. C. J. Chem. SOC., Faraday Trans. 1 1976, 72, 1809. (21) Lai, P. P.; Rees, L. V. C. J. Chem. Soc., Faraday Trans 1 1976, 72, 1827.
(22) Costenoble, M. L.; Mortier, W. J.; Uytterhoeven, J. B. J. Chem. Soc., Faraday Trans. 1 1978, 74, 466. (23) Smith, J. V.; Bennett, J. M.; Flanigen, E. M. Nature 1967, 213, 241. (24) Weakliem, H. A. J . Chem. Phys. 1962, 36, 2117. (25) Urbina, de Navarro, C.; Machado, F.; L6pez. M.; Maspero, D.; Perez-Pariente, J. Zeolites 1995, 15, 157. (26) Konig, E. S t c c t . Bonding (Berlin) 1971, 9, 175. (27) Mortier, W. J. Compilation of Extra-Framework Sites in Zeolites; Buttenvorths: Guildford, 1982. (28) Schoonheydt, R. A,; Van Wouwe, D.; Vanhove, M. J . Colloid Interface Sei. 1981, 83, 279. (29) Schoonheydt, R. A.; Roodhooft, D.; Leeman, H. Zeolites 1987, 7 , 412. JP95 1309L
