J. Phys. Chem. 1994,98, 13017-13021
13017
Electron Spin Resonance Study of Ni(1) Stabilized in Nickel-Substituted and Nickel Ion-Exchanged Synthetic Hydroxyhectorites Hirohisa Yamada,? Naoto Amma,$ and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas 77204-5641 Received: July 7, 1994; In Final Form: October 1, 1994@
Hydroxyhectorites containing Ni(I1) in lattice sites (Mg-NiHect) and in ion-exchange sites (NiMg-Hect) are synthesized and characterized by electron spin resonance (ESR) spectroscopy after y-irradiation at 77 K to generate Ni(1). Two Ni(1) species, species A (gll = 2.51 and g l = 2.09) and species B (gll = 2.24 and g l = 2.1 l), are identified in the ESR spectrum of Mg-NiHect. Only one Ni(1) species ( g l = 2.09) is identified in NiMg-Hect. After annealing at room temperature, the intensity of the ESR spectra of Ni(1) species in NiMg-Hect increased substantially with a concomitant decrease of the ESR intensity of trapped hydrogen atoms. The intensity of Ni(1) species B in Mg-NiHect did not increase after annealing at room temperature, but the intensity of Ni(1) species A increased slightly. The Ni(1) species in NiMg-Hect was not stable after adsorption of D20 or CD30H and annealing at room temperature. But Ni(1) species B in Mg-NiHect was stable after adsorption of DzO or CD30H and annealing at room temperature, while Ni(1) species A was unstable. According to these ESR characteristics and to chemical analyses before and after treatment with ethylenediaminetetraacetic acidsodium acetate, the Ni(1) sites are assigned as follows. The Ni(1) site in NiMg-Hect is assigned to a nonframework ion-exchange site in the interlayer space. Ni(1) species A in Mg-NiHect is assigned to Ni(I) in an edge framework site in the octahedral sheet of a clay layer, and Ni(1) species B in Mg-NiHect is assigned to Ni(1) in an interior octahedral framework site in the octahedral sheet of a clay layer.
Introduction Hydroxyhectoriteis a member of the smectite group of clays. The smectite clay minerals are pseudo-two-dimensional solids and have a layered structure in which each layer is composed of one octahedral sheet sandwiched by two tetrahedral sheets, and the negatively charged layers are held together by electrostatic interactions with exchangeable metal cations in the interlayer region. Because of the relatively high surface areas and abundant surface acid sites, the smectite clay minerals are used as catalysts and catalyst supports. Smectite clay minerals containing transition metal ions can be tailor-made for specific catalytic applications. There are two types of sites for transition metal ions in smectite clay minerals. One is an ion-exchange, nonframework site in the interlayer space, and the other is a lattice framework site within the clay structure. Smectite clay minerals containing transition metal ions in lattice sites are found in nature and have been synthesized.'V2 Substitution of ions of the same valence, that is, Mg(II) by Fe(I1) or Al(1II) by Fe(III), are common in octahedral lattice sites. Ni(II), Co(II), Zn(II), Mn(II), V(III), and Cr(II1) have been found as substituted ions in octahedral lattice sites. Synthetic, Ni-substituted saponite clay has been shown to be an efficient catalyst for the selective dimerization of ethene.3 The catalytic behavior of hectorite-like smectites containing Ni(11) substituted in the lattice was recently investigated for the decomposition of 2-propan01.~Ni-substituted synthetic micemonomorillonite clays have also been shown to be active hydroisomerization t Present address: National Institute for Research in Inorganic Materials, Namiki 1, Tsukuba, Ibaraki 305, Japan. *Present address: Department of Applied Chemistry and Materials Technology,Faculty of Engineering, Shizuoka University, Hamamatsu 432, Japan. Abstract published in Advance ACS Absrrucrs, November 1, 1994. @
0022-365419412098-13017$04.5010
In the present work, we evaluate the incorporation of nickel ion into lattice sites of hydroxyhectorite and show the first generation and characterizationof Ni(1) species in smectite clay minerals by electron spin resonance (ESR) spectroscopy. A critical comparison of the ESR spectroscopic results between Ni(1) in lattice framework sites and in nonframework ionexchange sites of hydroxyhectorite was made.
Experimental Section The synthesis of Ni(I1)-substituted hydroxyhectorite (NaNiHect) was performed by hydrothermal reaction using a wet uniform gel with a composition corresponding to N%.s(Mg2,45Ni0.05Li0.5)Si4011by a method similar to that of Urabe et al.,3 ~ synthesis was as Luca et al.,8 and Toni and I ~ a s a k i . The follows: Tetraethyl orthosilicate (1 1.467 g, Aldrich) was added to 25 mL of ethanol and stirred at about 60 "C for 3 h. To this solution was added MgC1206H20 (6.575 g, Aldrich) and NiC126H20 (0.164 g, Aldrich) dissolved in 50 mL of deionized water. This ethanollwater solution was stirred more than 4 h to obtain a homogeneous solution, and then 15 wt % of NaOH (Aldrich) solution was added dropwise over a few minutes until a final pH of 11 was obtained. After the solution was stirred overnight, the precipitate containing Mg, Ni, and Si was filtered and washed several times with deionized water. The solids were redispersed in deionized water, and 0.289 g of Li OHnH20 (Aldrich) and 0.275 g of NaOH (Aldrich) were added. The suspension was aged with stimng at room temperature overnight to form a uniform gel. The reaction mixture was placed in a Teflon cup fitted into a stainless steel pressure vessel and heated in an oven at 200 "C and autogeneous pressure for 7 days. The synthesized Na-NiHect was dispersed in a 1 M MgC126HzO solution. The suspension was filtered, and the clay was again stirred with a 1 M MgC126H20 solution. This treatment was 0 1994 American Chemical Society
Yamada et al.
13018 J. Phys. Chem., Vol. 98, No. 49, 1994 twice repeated to fully exchange Mg(I1) for Na(1) to give MgNiHect, which was finally filtered and washed to remove excess cations. Partially Ni(II)-exchanged hydroxyhectorite was prepared by solution ion exchange of sodium hydrohectorite (Na-Hect). Na-Hect was prepared in a similar manner as for Na-NiHect except that the nickel salt was deleted. Na-Hect was synthesized at 200 "C and autogeneous pressure for 7 days, using a wet uniform gel with a composition of Nao,5(Mg2.5Lio.s)Si4011. The following chemicals were used to prepare the gel: 11.445 g of tetraethyl orthosilicate (Aldrich), 6.981 g of MgC126H20 (Aldrich), 0.289 g of LiOHoH20 (Aldrich), and 0.275 g of NaOH (Aldrich). The synthesized Na-Hect was converted to the Mg(11)-exchanged form (Mg-Hect) by thrice exchanging with 1 M MgC126H20 solution followed by fitration. Mg(II) was used as the major exchangeable cation for Ni(II) because of the similarity in size of these two ions. Replacement of part of the exchangeable Mg(I1) by Ni(I1) was achieved by dispersing 1 g of Mg-Hect in 100 mL of 0.05 M NiC126H20. The suspension was stirred overnight at room temperature and then filtered and washed. The partially Ni(I1)-exchanged hydroxyhectorite is denoted as NiMg-Hect. Na-NiHect and Na-Hect were air-dried and saturated with ethylene glycol before examination by powder X-ray diffraction (XRD) with a Philips PW1840 diffractometer to identify phase purity. The ethylene glycol treatment causes intercalation of ethylene glycol into the interlayer space of the hectorite and expands the basal spacing to 17 A.lo This spacing is characteristic of swelled smectites in general. The chemical composition of the samples (Mg-NiHect, NiMg-Hect, and completely Ni(I1)-exchanged hydroxyhectorite, Ni-Hect) was measured by electron probe microanalysis before and their treatment with an ethylenediaminetetraacetic acid (EDTA)/sodium acetate pH = 5.5 buffer. Ni-Hect was obtained by repeated exchange of Na-Hect by 1 M NiC126H20. The treatment with EDTA buffer solution was used to remove exchangeable Ni(II) and surface-bound Ni(II) from the hectorite. This was done by stirring 1 g of the hectorite with 100 mL of 10 mM EDTA buffer solution overnight and then repeating the treatment. The hectorite suspension was then filtered and washed. Air-dried samples were loaded into 2 mm i.d. x 3 mm 0.d. Suprasil quartz tubes, evacuated for 17 h (< Torr) at room temperature, and then y-irradiated at 77 K to a total dose of 0.6 Mrad to reduce Ni(I1) to paramagnetic Ni(1). y-Radiolysis was carried out using a % ' o source with a dose rate of 0.3 Mrad h-l. In order to examine the interaction between Ni(1) in the hydroxyhectorites with adsorbates, the y-irradiated samples were exposed to D20 (24 Torr) or CD30H (18 Torr) at their room temperature vapor pressures as indicated for 30 s. The deuterated compounds were from Aldrich Chemical Co. These samples were then frozen in liquid nitrogen, sealed, and stored in liquid nitrogen at 77 K. Electron spin resonance spectra were recorded at 77 K on a modified Varian E-4 spectrometer interfaced to a Tracor Northem TN-1710 signal averager. Each spectrum was obtained by multiple scans to achieve satisfactory signal-to-noise. The magnetic field was calibrated with a Varian E-500 gaussmeter. The microwave frequency was measured by a Hewlett-Packard HP 5342A frequency counter.
Results Powder X-ray Diffraction. The powder XRD patterns of Na-Hect (Figure la) and Na-NiHect (Figure lb) are similar
2.60
0
20
40
11.52
60
CuKa,20 Figure 1. X-ray powder diffraction patterns of (a) synthetic Ni-free (b) synthetic Nihydroxyhectorite (Na-Hect), Nao.~(Mgz,5Lio,5)S4011, substituted hydroxyhectorite (Na-NiHect), Nao.s(Mg~.4~Nio.osLi.5)S 4 0 1 1 ,and (c) commercial synthetic Laponite RD. The numbers indicate the corresponding d spacing in angstroms. to that of natural hectorite'Jl and to the commercially available synthetic fluorohectorite called Laponite RD (Figure IC). They show no evidence of impurity phases. Good agreement was obtained with the d spacings of reflections from natural hectorite. Both Na-Hect and Na-NiHect could be expanded from dm1 = 12.3 dm1 = 17.2 A on treatment with ethylene glycol. The value of d m = 1.52 A confirmed its identification as a trioctahedral smectite.1-2J1 Electron Probe Microanalysis. Electron probe microanalysis on Mg-NiHect after EDTA treatment indicated that the EDTA treatment removed only a few percent of the Ni(II) originally present. This indicates that most of the Ni(I1) indeed occupies sites in the hydroxyhectorite lattice since EDTA complexes Ni(II) very strongly and is expected to remove it easily from surface sites and ion-exchange sites. On the other hand, the treatment of NiMg-Hect with the EDTA buffer solution successfully removed more than 98% of the Ni(II), which is known to be in ion-exchange sites. EDTA also extracted all the Ni(I1) from Ni-Hect, the completely Ni(I1)exchanged hectorite. Electron Spin Resonance. Ni(1) ions formed by reduction of Ni(II) ions can be stabilized in si1icoaluminophosphates,l2 aluminosilicates zeolite^),^^-^^ and silicas.16 Two different methods have been used to reduce Ni(I1) in these materials: thermal reduction at 473-573 K under 100-500 Torr hydrogen pressure and photoreduction by ultraviolet irradiation at 77 K under 100 Torr of hydrogen. However, these methods did not reduce Ni(I1) to stabilized Ni(1) in hydroxyhectorites based on no ESR signal of Ni(1) being observed. Thus irradiation at 77 K was tried, which did generate stable Ni(1) hydroxyhectorites. For a 0.6 Mrad dose less than 1% of the total Ni(II) was reduced and stabilized as Ni(I). ( i ) Comparison of Mg-Hect, NiMg-Hect and Mg-NiHect. The ESR spectra of Mg-Hect, NiMg-Hect, and Mg-NiHect are shown in Figure 2, and the parameters are given in Table 1 with those of Ni-containing silicoaluminophosphates, aluminosilicates, and silicas. No ESR signal of Ni(1) was seen before y-irradiation. There are obvious differences among the ESR
J. Phys. Chem., Vol. 98, No. 49, 1994 13019
Ni(1) Stabilized in Synthetic Hydroxyhectorites
H
g = 2.00
H
200 G
-
glA = 2.09
c--t-,
Figure 2. ESR spectra at 77 K of (a) Mg-Hect, (b) NiMg-Hect, and (c) Mg-NiHect after evacuation at room temperature for 17 h and y-irradiation at 77 K.
TABLE 1: ESR P Values of NYI) in Various Matrices PII PI ref NiMg-Hect U 2.09 this work this work 2.5 1 NiHect 2.09 this work 2.24 2.11 NiH-SAPO-11 12 2.460 2.099 12 NiAPSO-11 2.463 2.099 13 NiCa-Y zeolite 2.151 2.061 14 U NiCa-X zeolite 2.090 15 U NiCa-X zeolite 2.096 2.71 16 Ni-silica 2.07 ~
200 G
Figure 3. ESR spectra at 77 K of (a) NiMg-Hect after y-irradiation at 77 K, (b) NiMg-Hect after annealing (a) at room temperature for 1 min, () NiMg-Hect after annealing (a) at room temperatur for 3 min, and (d) NiMg-Hect after annealing (a) at room temperature for 60 min. H
The gll peak is too weak to be observed. spectra. The spectrum of Mg-Hect (Figure 2a) consists of several intense lines around g = 2.00 due to defect centers at least partially associated with hole centers on oxygen generated in the silicate clay lattice during y-irradiati011.l~ The sharp intense line labeled H is the low-field line of a characteristic -500 G doublet from hydrogen atoms also generated and trapped during irradiation at 77 K. The high-field line of hydrogen atoms is beyond the field range shown in these figures. The defect centers and hydrogen atoms are also observed in the ESR spectra of NiMg-Hect and Mg-NiHect. The ESR spectrum of NiMg-Hect (Figure 2b) indicates only one Ni(1) species based on a weak, barely resolved signal at g l = 2.09. The gll component is apparently too weak to observe. Two signals are evident in the spectrum of Mg-NiHect (Figure 2c): one at gll = 2.51 is designated as species A, an the other at gll = 2.24 is designated as species B. In the perpendicular region of the spectrum of Figure 2c, two g l signals are evident. These two signals are assigned as glA = 2.09 and g l B = 2.1 1. The justification for this assignment is supported by the results of annealing at room temperature described below. (ii) Annealing Behavior at Room Temperature. Figure 3 shows the changes in the ESR spectra at 77 K of NiMg-Hect after annealing at room temperature for different times. After annealing at room temperature for 1 min, the intensity of g l of Ni(1) increased slightly. After annealing for 3 min, the intensity of g l increased greatly with a concomital decrease of the H atom intensity. After annealing for 60 min, the signal of g l
gA l = 2.09
A
Figure 4. ESR spectra at 77 K of (a) Mg-NiHect after y-irradiation at 77 K, (b) Mg-NiHect after annealing (a) at room temperature for 1 min, and (c) Mg-NiHect after annealing (a) at room temperature for 60 min. did not change further from that after 3 min, but the H atom signal completely disappeared. The ESR spectra at 77 K of Mg-NiHect after annealing at room temperature are shown in Figure 4. After annealing at room temperature for 1 min, the intensity of the signals at gll = 2.24 and g l = 2.1 1 increased slightly with the disappearance of the H atom signal, but the intensity of the gll = 2.51 and g l = 2.09 features did not change. Annealing of the sample at room temperature for 60 min caused a substantial decrease in the intensity of the signals at gll = 2.51 and g l = 2.09, but the signals at gll= 2.24 and g i 2.1 1 still did not change. Therefore, the signals at gll= 2.51 and g l = 2.09 are designated as species
Yamada et al.
13020 J. Phys. Chem., Vol. 98, No. 49, 1994 H
(4
"
,
200 G
Figure 5. ESR spectra at 77 K of (a) NiMg-Hect after y-irradiation at 77 K and adsorption of methanol, (b) NiMg-Hect after annealing (a) at room temperature for 1 min, and (c) NiMg-Hect after annealing (a) at room temperature for 10 min. A, and those at gll = 2.24 and g l = 2.11 as species B, as assigned in the previous section. (iii) Interaction of Ni(I) in NiMg-Hect and Mg-NiHect with Methanol and Water. Changes in the ESR signals of Ni(1) in NiMg-Hect after adsorption of methanol and annealing at room temperature are shown in Figure 5 . Just after adsorption of methanol (Figure 5a), the signal at g l = 2.09 and the sharp signal of H atoms are observed. After annealing for 1 min (Figure 5b), a slight increase in the intensity of the signal at g l = 2.09 occurs with the disappearance of the H atom signal. The signal at g l = 2.09 disappears after annealing for 10 min (Figure 5c). Similar phenomena are observed for NiMg-Hect after adsorption of water and annealing at room temperature (spectra not shown). Immediately after water adsorption, the signals at the gll = 2.24 and g l = 2.09 are observed together with the sharp H atom signal. After annealing for 1 min, a slight increase of the intensity of the signals at gll= 2.24 and g l = 2.09 occurs with the disappearance of the H atom signal. The signals at gll = 2.24 and g l = 2.09 disappear after annealing at room temperature for 60 min. The annealing behavior at room temperature of Mg-NiHect after adsorption of methanol is shown in Figure 6. Immediately after adsorption of methanol, both species A (gll= 2.51 and g l = 2.09) and species B (gll= 2.24 and g l = 2.11) are still seen (Figure 6a). After annealing at room temperature for 1 min, the intensity of both signals slightly decreases (Figure 6b). After annealing for 60 min, only species B remains, and species A has disappeared (Figure 6c). The ESR spectral changes for Mg-NiHect after adsorption of water and annealing at room temperature are the same as for adsorption of methanol. Directly after adsorption of water, both species A and B are still observed. But the intensity of species A decreases greatly after annealing for 2 min and totally disappears after 60 min.
Discussion A difference in the location of Ni(II) between Mg-NiHect and NiMg-Hect is c o n f i i e d by electron probe microanalysis before and after EDTA treatment. The EDTA treatment
gllB= 2.24
Figure 6. ESR spectra at 77 K of (a) Mg-NiHect after y-irradiation at 77 K and adsorption of methanol, (b) Mg-NiHect after annealing (a) at room temperature for 2 min, and (c) Mg-NiHect after annealing (a) at room temperature for 60 min. removes only a small amount of Ni(I1) from Mg-Ni.Hect in which Ni(II) was added to the synthesis mixture. In contrast, almost all of the Ni(I1) is removed by EDTA treatment from synthetic hydroxyhectorite in which Ni(I1) was introduced after synthesis by ion exchange. These results indicate that the Ni(11) ion in Mg-NiHect is incorporated into a framework site. Similar results were previously obtained for Cu(I1)-substituted synthetic fluorohectorite vLrsus Cu(I1)-exchanged fluorohectorite.8 The substitution of Mg(I1) by Ni(I1) in the octahedral sheet during synthesis seems chemically logical due to the similarity of their ionic radii and valences. Newman and Brown18 showed by chemical analysis that in natural smectites the most common octahedral cations are Al(III), Fe(III), Fe(II), and Mg(I1) but that other cations such as Ni(II), Co(II), and Cu(II) can also occur. The synthesis of Ni-substituted saponite which contains Ni(II) ions in place of Mg(II) has also been r e p ~ r t e d .Saponite ~ is a trioctahedral smectite similar to hectorite except that it contains no Li(1) ions, and the layer charge is created by substitution of A1 for Si in the tetrahedral sheet. Inorganic interlayer complexes containing Ni(II) in the interlayer space of hectorite have also been synthesi~ed.'~There is abundant optical evidence that Ni ions in phyllosilicates are divalent and octahedrally coordinated, rather than tetrahedrally coordinated.3*20-23 The ESR spectra show that Ni(1) can be generated by y-irradiation and stabilized in Mg-NiHect and NiMg-Hect. After evacuation at room temperature and y-irradiation at 77 K, one Ni(1) species was observed for NiMg-Hect, but two Ni(1) species were observed for Mg-NiHect. It is reasonable that only one Ni(1) species is obtained in NiMg-Hect, because the Ni(1) is located in a nonframework, cation-exchangeable site in the interlayer space. A Ni(1) species in an ionexchangeable site in the interlayer space is expected to interact easily with adsorbates. This is consistent with the reactivity of Ni(1) in NiMg-Hect with methanol and water. By analogy to the reactivity of Ni(1) in silicoaluminophosphate-11 (SAPO-11) to cause water decomposition and form Ni(II)-02- and Ni(I)-(02)n,12 we suggest the same reactions here. The characteristic ESR spectrum of Ni(II)-02- cannot be resolved from the silicate defect centers produced by y-radiolysis. In SAPO-11, Ni(1) coordinates methanol but does not decompose it, and the ESR spectrum of Ni(1) only changes
Ni(1) Stabilized in Synthetic Hydroxyhectorites slightly. However, in hectorite, the Ni(1) ESR spectrum disappears, indicating reaction with methanol possibly causing its decomposition. Possible paramagnetic intermediates could not be observed due to overlap with the silicate defect centers. In Ni(II)-substituted hectorite, Mg-NiHect, Ni(II) is expected to occupy a site either neighboring or not neighboring an octahedron containing Li(1). Recent characterization of Cu(11)-substituted synthetic f l u o r o h e ~ t o r i t eand ~ ~ ~hydroxyhec~ t ~ r i t with e ~ ~ESR and electron spin echo modulation (ESEM) spectroscopies indicated that there are three types of Cu(II) sites in the octahedral sheet: (1) Cu(I1) in an octahedral site with Li(1) in a neighboring octahedral site, (2) Cu(II) in an octahedral site with no Li(1) in a neighboring octahedral site, and (3) Cu(11)in an edge octahedral site. But in Mg-NiHect, only two Ni(1) species are observed. Perhaps two Ni(1) sites are not resolved by ESR. It has not been possible to observe ESEM signals from Ni(1) to probe whether Li(1) is adjacent to it or not. However, the results with adsorbates indicate that Ni(1) species A is more reactive and hence suggests that Ni(1) species A is an edge site in an octahedral sheet which is more accessible to adsorbates. Ni(1) species B is then assigned to an interior octahedral site in an octahedral sheet that does not border the interlayer space and is not readily accessible to adsorbates.
Conclusions Comparative ESR studies between Ni(II)-substituted hydroxyhectorite (Mg-NiHect) and Ni(II)-exchanged hydroxyhectorite (NiMg-Hect), after radiolytic reduction to Ni(I), have accomplished differentiation of the Ni(1) location in these two hectorites. One Ni(1) species in a nonframework ion-exchange site is generated in NiMg-Hect and is reactive to water and methanol. In contrast, two Ni(1) species in framework sites are generated in Mg-NiHect; one is reactive and the other is unreactive toward water and methanol. The reactive framework site is assigned to an edge site that is part of an octahedral sheet, and the nonreactive framework site is assigned to an interior octahedral site in hectorite. The differentiation between nonframework and framework sites is confirmed by the effect of EDTA on removing Ni(II) from ion-exchanged NiMg-Hect but not from synthesized Mg-NiHect.
J. Phys. Chem., Vol. 98, No. 49, 1994 13021
Acknowledgment. This work was supported by the Robert A. Welch Foundation and the National Science Foundation. H.Y is grateful to M. Koga and K. Torii for information on the synthesis of hectorite. References and Notes (1) Brindley, G. W. In Crystal Structures of Clay Minerals and their X-ray IdentiFcation; Brindley, G. W., Brown, G., Eds.; Mineralogical Society: London, 1984; Chapter 2. (2) Giiven, N. In Hydrous Phyllosilicates (exclusive of micas); Bailey, S . E., Ed.; Reviews in Mineralogy Volume 19; Mineralogical Society of America: Washington, DC, 1988; Chapter 13. (3) Urabe, K.; Koga, M.; Izumi, Y. J . Chem. Soc., Chem. Commun. 1989, 807. (4) Nishiyama, Y.; Arai, M.; Guo, S.; Sonehara, N.; Naito, T.; Torii, K. Appl. Catal. 1993, 95, 171. (5) Swift, H. E.; Black, E. R. Id.Eng. Chem. Prod. Res. Dev. 1974, 13, 106. (6) van Santen, R. A. Reel. Trav. Chim. Pays-Bas 1982, 101, 157. (7) Gaff, J.; van Santen, R. A.; Knoester, R.; Wingerden, B. J . Chem. SOC., Chem. Commun. 1982, 655. (8) Luca, V.; Chen, X.; Kevan, L. Chem. Mater. 1991, 3, 1073. (9) Torii, K.; Iwasaki, T. Chem. Lett. 1986, 2021. (10) MacEvan, D. M. C.; Wilson M. J. In Crystal Structures of Clay Minerals and their X-ray IdentiFcation; Brindley, G. W., Brown, G., Eds.; Mineralogical Society: London, 1984; Chapter 3. (1 1) Brown, G.; Brindley, G. W. In Crystal Structures of Clay Minerals and their X-ray Identification; Brindley, G. W., Brown, G., Eds.; Mineralogical Society: London, 1984; Chapter 5. (12) Azuma, N.; Lee, C. W.; Kevan, L. J . Phys. Chem. 1994,98, 1217. (13) Ghosh, A. K.; Kevan, L. J . Phys. Chem. 1990, 94, 3117. (14) Kermarec, M.; Olivier, D.; Richard, M. J . Phys. Chem. 1982, 86, 2818. (15) Michalik, J.; Narayana, M.; Kevan, L. J. Phys. Chem. 1984, 88, 5236. (16) Bogus, W.; Kevan, L. J. Phys. Chem. 1989, 93, 3223. (17) Luca, V.; Brown, D. R.; Kevan, L. J . Phys. Chem. 1991,95, 10065. (18) Newman, A. C. D.; Brown, G. In Chemistry of Clays and Clay Minerals; Newman, A. C. D., Ed.; Mineralogical Society: London, 1987; pp 1-128. (19) Torii, K.; Iwasaki, T. Chem. Lett. 1988, 2045. (20) Faye, G. H. Can. Mineral. 1974, 12, 389. (21) Cervelle, B. D.; Maquet, M. Clay Miner. 1982, 17, 377. (22) Manceau, A.; Calas, G.; Decarreau, A. Clay Miner. 1985,20,367. (23) Manceau, A.; Calas, G. Clay Miner. 1987, 22, 357. (24) Luca, V.; Kevan, L. J . Phys. Chem. 1992, 96, 3391.
