J. Phys. Chem. 1996, 100, 3137-3143
3137
Discussion on the Coordination of Ni2+ Ions to Lattice Oxygens in Calcined Faujasite-Type Zeolites Followed by Diffuse Reflectance Spectroscopy Christine Lepetit* and Michel Che Laboratoire de Re´ actiVite´ de Surface, URA 1106 CNRS, UniVersite´ P. et M. Curie, Tour 54-55, 2e` me e´ tage, 4, Place Jussieu, 75252 Paris Cedex 05, France ReceiVed: June 20, 1995; In Final Form: September 13, 1995X
The coordination of Ni2+ ions to lattice oxygens in calcined X and Y zeolites has been extensively studied. A survey of the literature is presented here. From X-ray diffraction, Ni2+ ions are known to occupy preferentially the hexagonal prisms (SI) in octahedral symmetry. The main d-d absorption bands of Ni2+ ions in calcined NiX and NiY were assigned on this basis. The corresponding low value of the crystal field (10Dq ≈ 6400 cm-1) was accounted for by unusual long Ni-O bond lengths at SI sites. We demonstrate here that this assignment is not consistent with recently reported extended X-ray absorption fine structure measurements and that the absorption bands should be reassigned to Ni2+ cations in distorted tetrahedral and trigonal (D3h) coordinations. These cations are located in the supercages and/or sodalite cavities.
Introduction Molecular sieves and particularly transition-metal-exchanged zeolites are key materials for catalysis.1 Faujasite-type zeolites such as X and Y zeolites are among the most studied microporous materials. Their crystal structure consists of a tetrahedral arrangement of sodalite units (three-dimensional array of SiO4 and AlO4 tetrahedra in the form of a truncated octahedron) that are connected by hexagonal prisms. This arrangement induces the presence of large cavities called supercages (Figure 1). The extraframework cations (which balance the negative charges created by the AlO4 tetrahedra of the lattice) may be mainly located in the following positions2 (Figure 1) : SI lies in the center of the hexagonal prism and is surrounded by an octahedron of oxygens; SI′ lies in the sodalite cage near the center of the hexagonal window of the prism and is close to three framework oxygen atoms; SII′ lies in the sodalite cage near the center of the hexagonal window of the sodalite and is close to three framework oxygen atoms; SII in the supercage is the mirror image of SII′ with respect to the hexagonal window. The coordination of Ni in faujasite-type zeolites has been extensively studied using several analyses techniques such as powder X-ray diffraction (XRD),3-5 diffuse reflectance spectroscopy (DRS),6,7 and, more recently, extended X-ray absorption fine structure (EXAFS).8 After calcination of the zeolite samples, the extraframework Ni2+ cations are distributed among the zeolitic sites2 (Figure 1), but as shown by XRD,4,5 they preferentially occupy the hexagonal prisms (SI) with 6-fold coordination. Consequently, octahedral symmetry was suggested to explain the d-d spectra of nickel-exchanged faujasites. The absorption bands found for regular octahedral NiII complexes with oxygen-donor ligands such as water, including hydrated Ni21Y, are shown in Table 1. However, calcined NiY presents DRS absorption bands, as shown in Table 2, that occur out of the range for the regular octahedral complexes of Table 1. Various explanations of this discrepancy have been given : (i) distorted octahedron;9 (ii) elongated Ni-O distances7,10 compared to those in the [Ni(H2O)6]2+ complex; (iii) square planar coordination for Ni2+.11 We suggest here that these * Corresponding author at the address above or by e-mail : cml@ccr. jussieu.fr. X Abstract published in AdVance ACS Abstracts, December 15, 1995.
0022-3654/96/20100-3137$12.00/0
Figure 1. Various cavities of a faujasite-type zeolite and possible positions for extraframework cations.
explanations should be reconsidered and that the main DRS absorption bands lie in the range expected for distorted tetrahedral Ni2+ complexes, also evidenced in XRD studies. Since it appears from the literature data that the profile of the DR spectra is very similar for a large range of calcined NiX and NiY zeolites, our studies have been limited to NiY samples. Experimental Section Samples and Thermal Treatments. Samples were either provided by Dr. John Couves and Sir John Thomas (Royal Institution, London UK) or prepared by ourselves using an already published procedure.8 Depending on the exchange time, two samples Ni9Na41Al59Si133O384 and Ni21Na17Al59Si133O384, referred to as Ni9Y (4% Ni (w/w)) and Ni21Y (9% Ni (w/w)), respectively, were obtained. They were dried in the oven at 60 °C. These samples will be hereafter referred to as hydrated samples. Portions of these starting materials were then calcined first in a flow of oxygen (heating rate ) 100 °C/h) at various temperatures (typically 400 °C) for 15 h and then evacuated © 1996 American Chemical Society
3138 J. Phys. Chem., Vol. 100, No. 8, 1996
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TABLE 1: Typical DRS Absorption Bands of Ni2+ Cations in Octahedral Symmetry (Oh)a λ (nm) λ1 λ2 λ2′ λ3 λ3′ ref
crystalline CaNiSi2O6 1219 757 409 14
hydrated Ni21Y [Ni(H2O)6]2+/NaY
[Ni(H2O)6]2+
NiII/MgO
1176 725 395
1162 740 671 406
12
10
1200 729 665 393 this workc
nickel fluosilicate
nickel phyllosilicate (Ni-talc 150)b
1150 711 628 391 451 22
1120 705 663 390 13
Absorption bands are given in the order of decreasing λ1 (nm). Phyllosilicates prepared in autoclaves at 150 °C. See ref. 13. DR spectrum not shown. a
b
c
TABLE 2: Typical DRS Absorption Bands of Ni2+ Cations in Distorted Tetrahedral Symmetrya λ (nm)
NiA (50 °C)b
Ni/SiO2 (500 °C)b
λ1
hidden, expected at 2200 952 610 543 471 32
1786
λ2 λ 3′ λ3′′ λ3′′ ref b
952 613 535 435 31
NiY (400 °C)b 1661 997 616 494 467 this work
a Absorption bands are given in the order of decreasing λ (nm). 1 Calcination temperature.
for 2 h at the same temperature. Such pretreated samples will be hereafter referred to as calcined samples. The calcination step in oxidizing conditions is required to avoid the formation of Ni0 that is observed when the pretreatment is performed under vacuum only. The glass container with the pretreated powder was sealed off under vacuum from the pretreatment line before subsequent transfer under vacuum into a DRS quartz cell was performed. Thermal reduction was performed under 300 Torr of hydrogen in the DRS cell either (i) following a procedure elaborated in our laboratorys150 °C for 1 h 30 min in order to obtain a partial reduction of Ni2+ into Ni+ and to avoid the simultaneous formation of Ni0sor (ii) following a procedure found in the literature480 °C for various times in order to obtain selectively Ni0.5 Diffuse Reflectance Spectroscopy. DR spectra were recorded at room temperature with a Beckman 5270 spectrometer in the range 200-2500 nm. The spectrometer is equipped for reflectance studies with an integration sphere coated with BaSO4 taken as a reference. This reference was not kept in a waterfree atmosphere so that on some spectra at about 2000 nm, the lack of compensation leads to some negative bands. However, the absorption bands discussed here lie out of this range. Results and Discussion DRS Study of Hydrated Samples. The main absorption bands of hydrated Ni21Y (green) are reported in Table 1 and are very similar to those of [Ni(H2O)6]2+.12 It is known from the literature9 that in hydrated samples, Ni2+ cations are present as octahedral [Ni(H2O)6]2+ complexes located in the supercages (SII) and in electrostatic interaction with the zeolite framework. More recently, Dooryhee et al.8 reported that the EXAFS spectrum of hydrated Ni21Y can be interpreted in terms of a single shell of six oxygen atoms with a Ni-O distance of 2.06 Å, thus supporting that nickel is present as fully hydrated hexaaqua cations. A simplified energy level diagram for octahedral Ni2+ is provided in Figure 2. The three spin-allowed electronic transitions are referred to as ν1, ν2, and ν3, whereas ν2′ and ν3′ are spin-forbidden transitions. All these d-d bands are expected to be weak since they are forbidden by the Laporte rule for a centrosymmetric complex. This parity selection rule may,
Figure 2. Simplified correlation diagram between the energy levels of the Ni2+ free ion (d8) and those of the same ion subjected to a crystal field of tetrahedral symmetry (center) and octahedral symmetry (right). Solid line arrows indicate spin-allowed electronic transitions and dotted line arrows spin-forbidden ones (from ref 12, p 82).
however, be relaxed (i) if any distortion occurs so that the effective group to which the molecule belongs is therefore no longer Oh or (ii) by the mechanism of vibronic coupling,15 which is in practice most important. The presence of spin-forbidden transitions, such as ν2′, has been ascribed to the influence of the spin-orbit coupling in “mixing” a spin singlet (1Eg) with the 3T1g(F) spin triplet, thereby allowing the spin-forbidden transition to gain intensity from the spin-allowed transition.16 From the Tanabe-Sugano diagram for d8 ions in an octahedral field,12 it is apparent that the energy of 1Eg is insensitive to the crystal field (10Dq), whereas the energy of 3T1g(F) increases with the ratio 10Dq/B, where B is the Racah parameter. In the range of the crossing point of the 1E and 3T (F) terms, they become close in energy and the g 1g “mixing” of both terms is therefore greatly enhanced. Thus, the resulting spin-forbidden transition ν2′ is more intense. The crystal field (10Dq) is given by the first transition ν1, i.e., by the relation
10Dq (cm-1) ) 1/λ1 where λ1 is expressed in cm. DRS Study of Calcined Samples. Ni9Y and Ni21Y samples calcined at 400 °C are pink and their DR spectra are compared in Figure 3. For both samples, the main absorption bands are located at 1661, 997, 616, 494, 467, 312, and 270 nm (Table
Coordination of Ni2+ Ions in Faujasite Zeolites
J. Phys. Chem., Vol. 100, No. 8, 1996 3139 18). They confirmed the multiple site interpretation of BriendFaure et al.7 Nevertheless, they noticed that the d-d bands of octahedral Ni2+ are expected to be weak since they are forbidden by the Laporte rule. Weak bands occurring in all the samples were therefore assigned to Ni2+ ions in SI sites: λ1 ) 1563 nm, λ2 ) 955 nm, and λ3 ) 513 nm. The low corresponding crystal field value (10Dq ) 6398 cm-1) was also ascribed to a long Ni-O bond length in the hexagonal prism. However, recent EXAFS studies of calcined Ni21Y (300 °C) revealed an average Ni-O distance of only 2.02 Å.8 In these samples 3/4 of the SI sites are occupied by a nickel cation, since Gallezot and Imelik3 showed that the number of occupied SI sites is limited to 12 over 16 per unit cell, and this whatever the dehydration temperature between 300 and 600 °C. By contrast, the simultaneous XRD measurements indicated a Ni-O distance of 2.28 Å for nickel cations sitting in a SI site.8 This apparent discrepancy was already explained by Olson.19 Relative to their positions in natural faujasite where all SI cavities are empty, the six oxygens of the hexagonal prism have all shifted about 0.5 Å toward the nickel ion located at the center of the hexagonal prism. The aluminosilicate framework is able to adjust to changes in cation occupancy. The XRD-measured Ni-O bond length is therefore only an average value that includes oxygen positions for occupied (3/4, d(Ni-O)true) and empty (1/4, d(center of SI site-O) ) 2.60 Å20) hexagonal prisms.8 The XRD average bond length is therefore given by8
2.28 ) (1/4)2.60 + (3/4)d(Ni-O)true
Figure 3. DRS spectra of samples calcined at 400 °C: (a) Ni9Y; (b) Ni21Y. Note that the shift between the spectra is real and was not arbitraryly introduced.
2). The band located at 312 nm is assigned to a O2- f T(Si4+, Al3+) charge-transfer inside the zeolite structure since the same band is observed for the starting NaY without nickel. Let us first look at the assignments of the other bands, generally reported in the literature. Regular Octahedron. Ni31NaX compounds were studied by Briend-Faure et al.7 after calcination at temperatures ranging from 300 to 600 °C. The reflectance spectra were explained in terms of three coordinations for nickel cations: octahedral, tetrahedral, and trigonal. The following bands were attributed to Ni2+ located in SI sites with octahedral coordination. For spinallowed transitions, λ1 ) 2222 nm (hidden by surface hydroxyl groups vibrations), λ2 ) 1041 nm, and λ3 ) 444 nm. For spinforbidden transitions, λ2′ ) 909 nm, and λ3′ ) 625 nm. The corresponding crystal field value (10Dq ) 4500 cm-1) is lower than that for analogous regular octahedral species such as [Ni(H2O)6]2+ or Ni2+-doped MgO10,17 (Table 1). This low value of the crystal field was ascribed to a long Ni-O bond length (2.29-2.37Å in the SI site, depending on the nickel content, as measured by XRD3) compared to the Ni-OH2 bond length (2.06 Å).8 Yet, the average Ni-O distance R may be correlated to the crystal field by Jørgensen’s formula: Dq ) KR-5, where the constant of proportionality K depends on the effective charge of the oxygen ligands and on the d electron core distance.18 When nickel-doped MgO (10Dq ) 8600 cm-1 and RNi/MgO ) 2.10 Å) is taken as the reference compound, then
Dq ) DqNi/MgO(RNi/MgO/R)5 Schoonheydt et al.10 studied calcined NinCaX and NipCaY zeolites of various nickel content (1 e n e 30 and 1 e p e
leading to d(Ni-O)true ) 2.16 Å, closer to the 2.02 Å obtained from EXAFS analysis. These results have been rationalized by computer modeling.21 The calculations show that there is a pronounced inward relaxation (0.4-0.5 Å) of the surrounding oxygen ions to accommodate nickel cations at SI sites. The Ni cation sitting in a SI site is therefore surrounded by six oxygen atoms located at about 2.10 Å.8 Previous DRS assignments of Schoonheydt et al.10 and Briend-Faure et al.7 for octahedral Ni2+ cations located in SI sites, based on an apparently long Ni-O bond length, are no longer in line with these recent findings. Distorted Octahedron. Another interpretation was given by Sendoda et al.9 working on Ni13X and Ni41X samples evacuated at 500 °C. These authors assigned the broad d-d bands observed at 901, 625, and 455 nm to Ni2+ ions located in SI sites with pseudooctahedral symmetry. This interpretation may be questioned since from XRD studies, SI sites are always described as near-perfect octahedra.3,8,19 Let us, however, examine whether this is consistent with the literature data concerning minerals where Ni2+ lies in a distorted octahedral environment of six oxygens. A slight distortion of the octahedron results in an nonGaussian shape of the bands,22 which eventually broaden as observed for nickel hydroxide where there is a weak trigonal distortion at the nickel center. Similarly, during the preparation of Ni/SiO2 by use of cationic exchange starting from Ni2+(NH3)6 in electrostatic interaction with silica at a suitable pH, the substitution of ammonia by two water and two silica surface ligands (SiO-) results in the [Ni(tSiO)2(H2O)2(NH3)2] complex of distorted octahedral symmetry and in a bathochromic shift and broadening of the corresponding DRS bands.23 However, in both cases, the DR spectrum is very similar to the one for the regular octahedral analog (number of bands, position, intensity). A significant distortion of the octahedron modifies the symmetry around the nickel center from Oh to D4h (tetragonal
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TABLE 3: Examples of Absorption Bands of Ni2+ Cations in Distorted Octahedral Symmetrya clinoenstatite
olivine
λ1
1450
1538-1330
λ2 λ 2′ b
787
781 707
λ 3′ b λ3 10Dq(cm-1) R (Å)c symmetry ref
456 408 6900 2.18 C2V 25
490 422 7368 2.15 C2V 24
nepouite λ1 λ2 λ3 λ4 λ4′ b λ5 λ6 10Dq R (Å)c symmetry ref
1187 1010 731 625 668 420 375 9410 2.06 C3V 26
a In nm. b Spin-forbidden transition. c Average Ni-O distance given by Jørgensen’s formula of ref 18.
distortion), C2V or C3V (trigonal distortion). The orbital triplets (in Oh) may show partial or complete resolution into their singly or doubly degenerate components.12 From the correlation diagram corresponding to D4h and C2V symmetry,12,24 six spinallowed transitions are expected depending on the resolution conditions. In olivines24 and clinoenstatite,25 only three spinallowed transitions (Table 3) are observed for nickel ions in the distorted octahedral environment of C2V symmetry, the latter being deduced from XRD measurements. In these compound, the Ni-O distance estimated from Jørgensen’s formula18 (Table 3) is longer than the one of Ni/MgO and yields to a bathochromic shift of the three bands associated with the regular octahedron (Table 1). In the case of nepouite, where the symmetry of the distorted octahedron is C3V, the DR spectrum may be decomposed into seven Gaussian-shaped bands (Table 3), in agreement with the corresponding correlation diagram.26 The allowed transitions are split into two components, whereas the spin-forbidden ones are not affected. The value 10Dq ) 9410 cm-1 is in agreement with the short average Ni-O distance of 2.06 Å deduced from XRD measurements. The crystal field strength resulting from the interpretation of Sendoda (10Dq ) 11 100 cm-1) is found to be larger than that for [Ni(H2O)6]2+ (10Dq ) 8500 cm-1). This finding disagrees with the recently established ligand spectrochemical series where lattice oxygens were found to be σ-donors, π-donor ligands, and weaker than water.27 Moreover, it is associated with a short average Ni-O distance (i.e., 2.02 Å), which is not consistent with the geometry of the SI site as described above (average Ni-O distance of 2.10 Å8). The DRS bands observed in this work at 1661, 997, 616, 494, and 467 nm cannot be assigned to a distorted octahedron since they lie out of the range of the ones reported in Table 3. Moreover, the average Ni-O distance deduced from Jørgensen’s formula18 and λ1 ) 1661 nm happens to be 2.26 Å, which, as we mentioned above, is not consistent with the recent combined EXAFS and XRD studies. This is particularly clear in Figure 4 where logn(λ1) varies linearly with logn(Ni-O distance) in regular or distorted Ni(O)6 species taken from Tables 1 and 3 except for the calcined NiY sample discussed in this work. The regular or distorted octahedron assignment may therefore be ruled out. It appears that the DRS study of the reduction of NiY zeolites may give us a clue to understand the DR spectra. DRS Study of Reduced Samples. The DR spectra of the Ni21Y sample after calcination at 600 °C followed by thermal reduction for 1 or 16 h in molecular hydrogen are compared in Figure 5. As the reduction proceeds, the d-d absorption bands described above (Figure 5a) decrease in intensity (Figure 5b) and disappear after 16 h of thermal reduction (Figure 5c),
Figure 4. Variation of logn(λ1) vs logn(average Ni-O distance) for Ni(O)6 species reported in Tables 1 and 3 (b) and for calcined NiY (this work) (+). The slope of the straight line is 4.8 (close to 5). Numbers in parentheses refer to the literature source.
Figure 5. DRS spectra (a) Ni21Y calcined at 600 °C; (b) Ni21Y calcined at 600 °C and then reduced for 1 h (480 °C, H2); (c) Ni21Y calcined at 600 °C and then reduced for 16 h (480 °C, H2). Note that the shift between the spectra is real and was not arbitraryly introduced.
whereas a continuous background related to the formation of Ni0 particles develops. Subsequent adsorption of oxygen at room temperature left the DR spectrum unchanged. Oxygen is
Coordination of Ni2+ Ions in Faujasite Zeolites
J. Phys. Chem., Vol. 100, No. 8, 1996 3141
TABLE 4: Location of Ni2+ Cations after Different Sample Treatments (from Reference 5)a Number of ions per unit cell site
Ni20Yb
thermally reduced (H2, 480 °C, 15 h) Ni20Yb
SII Na+ + Ni2+ S I′ SI
22.9(9) 17 + 5.9 4.4(4) 11.7(3)
17.0 (1) 17 + 0 1.3(3) 11.4(3)
a Figures in parentheses correspond to the standard error in the last significant digit. b After evacuation at 500 °C for 15 h. From ref. 5.
TABLE 5: Typical DRS Absorption Bands of Ni2+ Cations in Tetrahedral Symmetrya
a
λ (nm)
NiLaX
Ni/ZnO
Ni/β-Al2O3
λ1 λ2 λ3 λ3′ ref
2660 1220 621 571 10
2173 1199 655 618 29
2000 1180 670 560 30
Absorption bands are given in the order of decreasing λ1 (nm).
not able to decrease the metallic character of nickel as it was observed in the case of palladium.28 It is known that in the presence of metal, the specular reflectance is predominant and decreases drastically the sensitivity of detection of other species.28 However, the comparison of spectra 5b and 5c clearly shows the disappearance of the d-d bands discussed above, while the Ni0 background and the SiO-H vibrations located in the near-IR at 2230 and 1375 nm remain unchanged. The absorption bands of calcined NiY are therefore related to NiII species that disappear upon thermal reduction under hydrogen. Yet, Briend-Faure et al.5 reported that in these experimental conditions, nickel cations located in SI sites are not affected by thermal reduction, whereas Ni cations located in SII or SI′ sites are almost completely reduced to Ni0 (Table 4). The same conclusions were drawn from recent EXAFS studies.8 Since NiII ions at SI sites are not affected by thermal reduction, the absorption bands of calcined NiY have to be assigned to nickel cations that are located in the sodalite or supercage cavities and that can be reduced to Ni0 atoms. Reassignment of the DR Spectra. As discussed above, the main absorption bands of the DR spectrum (Table 2) cannot be assigned to octahedral or distorted octahedral symmetry. They also lie out of the range of five-coordinated nickel species.12 In the square pyramidal (SP) symmetry, five DRS bands are expected as for KNiPO4: 2080, 1200, 930, 520, and 450 nm.30 Similarly, in a trigonal bipyramid geometry, five DRS bands are expected as for Ni(dien)Cl2, 2000, 1040, 930, 530, and 460 nm,30 but the intensity of the band located at 530 nm (spin-forbidden transition) is low. Finally, these symmetries, especially the SP one, are not expected in the NiY structure after calcination at 400 °C. As a result, tetrahedral and trigonal coordinations appear to be the most likely. The main absorption bands of calcined Ni21Y (Table 2 and Figure 3) are quite different from those of known tetrahedral complexes (Table 5). Yet, they lie in the range of the ones assigned to distorted tetrahedral complexes (Table 2). The correlation diagram and the expected DRS transitions for regular tetrahedral complexes are given in Figure 2. The distortion of the tetrahedron induces the splitting of ν3 into three components as observed for NiII ions on silica13,31 or in A zeolite.32 Indeed, when a ligand with a donor atom of strength comparable to that of skeletal oxygens (O) is adsorbed onto a trigonal ion Ni(O)3, the spin triplets T1 and T2 of the tetrahedral
group (Figure 2) may show complete resolution into their triply degenerate components.33 In this work, as in the ones related to the silica support,13,31 the splitting is observed only on ν3 (Table 2). The main absorption bands of calcined NiY (Table 2) are therefore assigned to four-coordinated Ni2+ species (referred to as Ni4c2+) in distorted tetrahedral symmetry. These cations may be located in sodalite cavities (SI′) and/or in supercages (SII). Bands of lower intensity located at 1600, 903, and 270 nm (Table 6) are attributed to three-coordinated cations (referred to as Ni3c2+) since they lie in ranges similar to those observed by Klier and Ralek32 and Cornet et al.34 for Ni3c2+ ions in D3h and C3V symmetry, respectively (supports of A zeolite and alumina, respectively). The largest visible band is located at 270 nm. Both the latter and the band located at 1600 nm increase in intensity with nickel content (Figure 3) and with calcination temperature (spectra a and b of Figures 6). Since the amount of occupied SI sites is limited to 12 per unit cell,3 upon an increase in the nickel content, the amount of nickel cations located in other sites (i.e., Ni4c2+ and Ni3c2+ in SI′ and SII sites) also increases. As the calcination temperature increases above 300 °C, SI sites are saturated3 and one can expect more Ni4c2+ to be transformed into Ni3c2+. The reduction conditions of Ni21Y were adapted in order to avoid the formation of metallic nickel and to obtain selectively Ni+. On the DR spectra of the samples reduced in these mild conditions, the above bands (1600 and 270 nm) decrease in intensity (spectra b and c of Figure 6). Under these experimental conditions, (H2)Ni+(O)3 complexes located in SI′35 were characterized by electron paramagnetic resonance. The Ni+ species seem, therefore, to come from the reduction of the Ni3c2+ species. Most of the latter are therefore more likely to sit in sodalite cavities. Polak et al.36 gave the theoretical interpretation of the DRS of Ni3c2+ (Figure 7) and showed that the transition energies of both symmetries (D3h and C3V) are expected to be similar. Three bands located at 765, 735, and 702 nm are observed on silica,31,37 whereas they are not observed on alumina,34 in the Y zeolite (this work), and in the A zeolite.32 This suggests a C3V symmetry for alumina and a D3h symmetry for the others. The weakest bands of the DR spectrum located at 1600 and 903 nm together with the band located at 270 nm (Table 6) may therefore be assigned to three-coordinated Ni3c2+ cations in D3h symmetry and located in the sodalite cavities. The final assignment of the DR spectra of calcined NiY that we therefore suggest from this work is summarized in Table 7. Ni Cations Located in SI Sites. Two possibilities may be considered: (i)The DRS absorption bands of nickel cations located in SI are hidden by the more intense bands arising from low-symmetry Ni2+ species; (ii) Ni2+ cations located in SI sites are not DRS sensitive. It is not possible to choose between these possibilities at this time. However, some other examples, for which sites of high symmetry or in octahedral symmetry are also DRS-silent, are reported hereafter. After calcination at 400 °C of nickel-exchanged L zeolites,38 the d-d bands of nickel are almost completely removed. Nickel cations, previously present in distorted octahedral coordination, are assumed to migrate to a highly symmetrical site E where they become bound to eight oxygens. After calcination at 700 °C, Ni/SiO2 samples have been shown by EXAFS to be made of a mixture of supported, isolated Ni4c2+ and Ni6c2+ engaged in an unknown phase.39 The relative proportions of both components depend on the nickel deposition
3142 J. Phys. Chem., Vol. 100, No. 8, 1996
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TABLE 6: Typical DRS Absorption Bands of Known Trigonal Ni2+ Cationsa a
assignment from Figure 6 3A′
2(F)
3A′
2(F)
f 3A′′1(F), 3A′′2(F) f 3E′(F), 3E′′(F)c 3A′ (F) f 1E′(G), 1A′ (D), 2 1 1 E′′(D) 3A′ (F) f 3A′ (P) 3A′ (F) f 2 2 2 1E′(D) 3 A′2(F) f 3E′′(P) ref
NiA (D3h) (330 °C) λ (nm)
Ni3c2+/SiO2 (C3V) (700 °C) λ (nm)
Ni3c2+/SiO2 (1000 °C) λ (nm)
Ni3c2+/Al2O3 (500 °C) λ (nm)
Ni21Y (400 °C) λ (nm)
hidden 1100 781, 690, 621 (all weak) 439
1612 hidden 765, 735, 702 (all intense) 397
1666 hidden 666, 700 intense
1715 hidden not observed
1600 (sh)b 903 (sh)b not observed
390
425
hidden
278 32
300 31
broad 37
not specified 34
270 this work
a Assignments for spin-allowed transitions are in bold characters, whereas for spin-forbidden ones, they are in standard characters. b sh refers to shoulder. c These energy levels are too close to lead to separate bands, in contrast to the expectation from Figure 7.
Figure 7. Simplified correlation diagram between the energy levels of the Ni2+ free ion (d8) and those of the same ion subjected to a crystal field of trigonal symmetry (D3h). Solid line arrows indicate spin-allowed electronic transitions and dotted line arrows spin-forbidden ones (from ref. 36.)
TABLE 7: Proposed Reassignment of the DRS Absorption Bands of Calcined Nickel-Exchanged Faujasite-Type Zeolites
Figure 6. DRS spectra: (a) Ni21Y calcined at 400 °C; (b) Ni21Y calcined at 500 °C; (c) Ni21Y calcined at 500 °C and then reduced for 1 h 30 min (150 °C, H2). Note that the shift between the spectra is real and was not arbitraryly introduced.
method onto the support. Whatever the relative proportions, six-coordinated sites are never observed by DRS. In hydrothermally synthesized CoAPO-5 ((CoxAlyPz)O2, ntemplate, mH2O), with cobalt content x < 2.5 × 10-4, Co2+ ions are found to be in a tetrahedral environment of oxygen atoms,40 i.e., in isomorphous substitution for Al3+ in the lattice. For higher cobalt loadings, most of the Co2+ ions are in extralattice positions in octahedral symmetry. However, the latter Co2+ ions are never observed by DRS, which shows lattice Co2+ ions in tetrahedral symmetry only. Conclusions In this work, we have shown that for calcined NiX or NiY zeolites, the DRS absorption bands previously assigned to nickel
absorption bands (nm)
assignment from this work
see the following
312
O2- f T (Si4+, Al3+) charge-transfer
Figure 3
1661 997 616 494 467
Ni4c2+ distorted Td in SI′ and/or SII sites
Table 2 Figure 3
1600 903 270
Ni3c2+ D3h in SI′ sites
Table 6 Figures 3 and 6a,b
cations sitting in SI sites lie out of the range of octahedral or distorted octahedral nickel species with an average Ni-O distance of about 2.10 Å. This distance was recently obtained for SI sites from combined XRD and EXAFS studies.8 The DRS absorption bands disappear upon reduction of Ni2+ ions into Ni0 although it is known that under these conditions, Ni2+ cations sitting in SI sites are not reduced. The main DRS absorption bands are, therefore, reassigned to distorted tetrahedral and trigonal nickel species. The four-coordinated cations Ni4c2+, in distorted tetrahedral symmetry, are located in sodalite cavities (SI′ sites) and/or in supercages (SII sites), whereas the three-coordinated Ni3c2+ cations, in trigonal D3h symmetry, are more likely to sit in sodalite cavities (SI′ sites).
Coordination of Ni2+ Ions in Faujasite Zeolites This work illustrates the limitations of the DRS analysis technique, which appears to be more sensitive to low-symmetry species. Therefore, high-symmetry species may be hidden even if they are present in significant amounts in the sample. This study suggests once again that it is always necessary to use a combination of analysis techniques to characterize a catalyst, each technique giving only an incomplete picture of the whole system. Acknowledgment. The authors thank Dr. J. Couves and Sir J. M. Thomas for providing some of the NiY samples and Professor C. Bennett, Professor F. Bozon-Verduraz, Professor G. Calas, and Dr. J. Y. Carriat for very fruitful discussions. References and Notes (1) (a) Ben Taarit, Y.; Che, M. Catalysis by Zeolites; Imelik, B., et al., Eds.; Elsevier: Amsterdam, 1980; p 167. (b)Sachtler, W. M. H.; Zhang, Z. AdV. Catal. 1993, 39, 129. (2) Breck, D. W. J. Chem. Educ. 1964, 41, 678. (3) Gallezot, P.; Imelik, B. J. Phys. Chem. 1973, 77, 652. (4) Couves, J. W.; Jones, R. H.; Thomas, J. M.; Smith B. J. AdV. Mater. 1990, 2, 181. (5) Briend-Faure, M.; Jeanjean, J.; Kermarec, M.; Delafosse, D. J. Chem. Soc., Faraday Trans. 1 1978, 74, 1538. (6) Garbowski, E.; Kodratoff, Y.; Mathieu, M. V.; Imelik, B. J. Chim. Phys. Phys.-Chim. Biol. 1972, 69, 1386. (7) Briend-Faure, M.; Jeanjean, J.; Spector, G.; Delafosse, D.; BozonVerduraz, F. J. Chim. Phys. Phys.-Chim. Biol. 1982, 79, 489. (8) Dooryhee, E.; Catlow, C. R. A.; Couves, J. W.; Maddox, P. J.; Thomas, J. M.; Greaves, G. N.; Steel, A. T.; Townsend, R. P. J. Phys. Chem. 1991, 95, 4514. (9) Sendoda, Y.; Ono, Y.; Keii, T. J. Catal. 1975, 39, 357. (10) Schoonheydt, R. A.; Roodhooft, D.; Leeman, H. Zeolites 1987, 7, 412. (11) Hass, E. C.; Plath, P. J. J. Mol. Catal. 1982, 14, 35. (12) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Studies in Physical Theoretical Chemistry, Vol. 33; Elsevier: Amsterdam, 1984, 33. (13) Carriat, J. Y. The`se de Doctorat, Universite´ P. et M. Curie, 1994; p 37. (14) Galoisy, L.; Calas, G. Am. Mineral. 1991, 76, 1777.
J. Phys. Chem., Vol. 100, No. 8, 1996 3143 (15) Reference 12, p 167. (16) Greenwood, N. N.; Earnshaw, A. Chemistry of the Elements; Pergamon Press: Oxford, 1984, 1345. (17) Bragg, L.; Caringbull, C. F. Crystal Structure of Minerals; Bell & Sons Ltd: London, 1965. (18) Jørgensen, C. K. Inorganic Complexes; Academic Press: New York, 1963. (19) Olson, D. H. J. Phys. Chem. 1968, 72, 4366. (20) Baur, W. H. Am. Mineral. 1964, 49, 697. (21) George, A. R.; Catlow, C. R. A.; Thomas, J. M. Catal. Lett. 1991, 8, 193. (22) Manceau, A.; Calas, G.; Decarreau, A. Clay Miner. 1985, 20, 367. (23) Bonneviot, L.; Legendre, O.; Kermarec, M.; Olivier, D.; Che, M. J. Colloid Interface Sci. 1990, 134, 534. (24) Hu, X.; Langer, K.; Bostro¨m, D. Eur. J. Mineral. 1990, 2, 29. (25) White, W. B.; Mc Carthy, G. J.; Scheetz B. E. Am. Mineral 1971, 56, 72. (26) Cervelle, B. D.; Maquet, A. Clay Miner. 1982, 17, 377. (27) Che, M. Proc. Int. Congr. Catal., 10th 1992 ; 1993, A, 31. (28) Rakai, A.; Tessier, D.; Bozon-Verduraz, F. New J. Chem. 1992, 16, 869. (29) Weakliem, H. A. J. Chem. Phys. 1962, 36, 2117. (30) Galoisy, L.; Calas, G. Geochim. Cosmochim. Acta 1993, 57, 3613. (31) Olivier, D.; Bonneviot, L.; Cai, F. X.; Che, M.; Gihr, P.; Kermarec, M.; Lepetit-Pourcelot, C.; Morin, B. Bull. Soc. Chim. Fr. 1985, 3, 370. (32) Klier, K.; Ralek, M. J. Phys. Chem. Solids 1968, 29, 951. (33) Klier, K.; Hutta, P. J.; Kellerman, R. ACS Symp. Ser. 1977, 40, 108. (34) Cornet, D.; Hemidy, J. F.; Mariette, C. NouV. J. Chim. 1984, 8, 159. (35) (a) Olivier, D.; Richard, M.; Che, M.; Bozon-Verduraz, F.; Clarkson, R. B. J. Phys. Chem. 1980, 84, 423. (b)Kermarec, M.; Olivier, D.; Richard, M.; Che, M.; Bozon-Verduraz, F.; Clarkson, R. B. J. Phys. Chem. 1982, 86, 2818. (c) Schoonheydt, R. A.; Roodhooft, D. J. Phys. Chem. 1986, 90, 6319. (36) Polak, R.; Cerny, V. J. Phys. Chem. Solids 1968, 29, 945. (37) Rebenstorf, B. Acta Chem. Scand. Ser. A 1978, 32, 195. (38) Coughlan, B.; Mc Cann, W. A.; Carroll, W. M. J. Colloid Interface Sci. 1977, 62, 229. (39) Reference 13, p 165. (40) Schoonheydt, R. A.; De Vos, R.; Pelgrims, J.; Leeman H. Zeolites: Facts, Figures, Future; Jacobs, P. A., Van Santen, R. A., Eds.; Elsevier: Amsterdam, 1989; p 559.
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