J. Phys. Chem. 1996, 100, 4213-4218
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Small Silver Clusters in Smectite Clay Interlayers J. Michalik,† H. Yamada,‡ D. R. Brown,§ and L. Kevan* Department of Chemistry, UniVersity of Houston, Houston, Texas 77204-5641 ReceiVed: September 28, 1995; In Final Form: December 8, 1995X
Electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies have been used to characterize small paramagnetic Ag clusters and their precursors, generated in the interlayer regions of montmorillonite, hectorite, and saponite smectite clays. Samples of clays, ion exchanged with Ag+ and solvated with water or methanol, were γ-irradiated at 77 K and monitored as the temperature was increased. No evidence for cluster formation was found in hydrated clays. However, when solvated with methanol, all three clays are able to stabilize Ag32+ and Ag43+ in interlayer sites. Differences in the behavior of the three clays were observed, particularly in their abilities to stabilize Ag0 centers formed at 77 K. Montmorillonite is able to stabilize weakly coordinated Ag0 even at room temperature, whereas Ag0 is unstable above 200 K in the other two clays. Differences are explained in terms of the structures of the clays.
Introduction We have previously studied Ag+-exchanged clays, using the Ag+ ion as a probe for the behavior of monovalent metal cations in ion-exchange sites.1,2 In this earlier work, Ag+ ions were radiolytically reduced at 77 K to paramagnetic Ag0 atoms, which were in turn characterized using electron spin resonance (ESR) techniques. At 77 K the coordination of the parent Ag+ ion does not change significantly on reduction to Ag0, so the ESR spectrum of the reduced species reflects the environment of the parent ion. In some matrices Ag0 atoms, generated in this way at 77 K, migrate to form small, discrete Ag clusters when the temperature is increased.3 These clusters are formed between migrating Ag0 atoms and unreduced Ag+ ions. They typically contain between two and six Ag nuclei and are paramagnetic and amenable to study with ESR. The structures and the stabilities of these clusters depend on the matrix in which they are formed. In the work reported here we have extended our earlier work to study the formation of such Ag clusters in smectite clay matrices for the first time. Small supported Ag clusters of this type are of general catalytic importance as precursors to the formation of supported metallic Ag particles. They have also been shown to be of specific importance in Ag+-exchanged zeolites, which are active catalysts in the photolytic decomposition of water.4-6 In this catalytic process the Ag+ ion is first reduced and then oxidized back to Ag+. The form of the reduced Ag species produced in the first half of the process is important in controlling catalytic activity. Optimum activity is associated with small (20 µm by hydraulic elutriation and used as the starting material. Then 500 mg of glass and 500 µL of distilled water were sealed in a gold tube. This sealed tube was then treated at 100 MPa and 325 °C for 10 days in a rapid quench-type hydrothermal apparatus.15 At the end of the reaction the gold tube was weighed to confirm that it has not ruptured. The XRD pattern of the resulting montmorillonite was similar to that of natural montmorillonite.14 Saponite was prepared under the same temperature and pressure conditions as montmorillonite. A glass with a composition corresponding to a formula Na0.55(Mg2.7Al0.3)(Si3.15Al0.85)O10(OH)2 was prepared from 1.560 g of Na2CO3,
Michalik et al. 3.137 g of Al2O3, 5.823 g of MgO, and 10.127 g of SiO2.17 The hydrothermal product was purified by hydraulic elutriation to remove the small amount of quartz impurities. Synthetic saponite exhibited a similar XRD pattern to natural saponite.14 ESR Sample Preparation. Samples of the synthetic clays were exchanged with Ag+ using a 1% suspension of clay in 1.0 mol dm3 AgNO3 solution overnight. The clays were filtered, dialyzed, and dried at 40 °C. Samples of powdered clay were sealed in 2 mm i.d. by 3 mm i.d. Suprasil quartz tubes and were dehydrated on a vacuum line under flowing O2 overnight at appropriate temperatures. Samples were exposed to methanol at room temperature while connected to the vacuum line. Deuterated methanol (CH3OD, 9.5 atom %) was supplied by Aldrich. ESR and ESEM Measurements. All samples were irradiated at 77 K in a Gammacell 220 60Co γ source with a dose rate of 5.3 kGy h-1. ESR spectra were recorded on a Bruker ESP 300 X-band spectrometer in the temperature range 120290 K using a variable-temperature Bruker unit. ESEM signals were measured on a Bruker ESP 380 pulsed ESR spectrometer at temperatures in the range 4.5-5.4 K using a helium flow cryostat. A three-pulse sequence 90°-τ-90°-T-90° was used and T swept. The first interpulse time τ was set at 0.28 µm to suppress modulation from 27Al. 2D modulation was analyzed using an HCP 386 IBM PC compatible computer. Simulations of the experimental modulation envelopes were performed using the analytical expressions developed by Dikanov et al.18 Best fits were obtained by varying the number of interacting 2D nuclei N, the interaction distance between the electron spin and the nearest 2D nuclei R, and the isotropic hyperfine interaction Aiso. Powder X-ray Diffraction. X-ray diffraction data were collected on a Philips Model 1840 X-ray diffractometer using Cu KR radiation. Clay samples were deposited on glass slides from aqueous suspensions and then activated under O2 in a furnace. Where appropriate, samples were equilibrated with solvent vapor for 48 h prior to recording diffraction patterns. Samples were transferred to the diffractometer from a desiccator as quickly as possible to avoid significant rehydration. Results Ag+-Exchanged Montmorillonite. Dehydrated at 250 °C under Flowing O2. The X-ray diffraction pattern of Ag+ montmorillonite dehydrated at 250 °C showed a d001 spacing of 0.98 nm, verifying that the clay lattice layers are completely collapsed, with no solvent in the interlayer regions. The ESR spectra recorded after irradiation at 77 K and subsequent annealing appear in Figure 2. The first spectrum shown, recorded at 100 K, shows a pair of doublets characteristic of Ag0 centers (species A). The large hyperfine splitting of 628 G for 109Ag0 is very close to the free atom value19 and suggests that the parent Ag+ ion is relatively weakly coordinated. As the temperature is increased this Ag0 signal decreases in intensity, and a second Ag0 signal (species B), which is probably underlying the first at the lower temperatures, remains. This exhibits broader lines and has a lower hyperfine splitting of 549 G, suggesting that it is derived from Ag+ ions more strongly coordinated than the first. The thermal stability of this Ag0 center is remarkable; the signal is still strongly detected after annealing the sample to room temperature for more than an hour (Figure 1c). Further well-resolved features, showing fine structure, appear close to the center of the spectrum recorded at 300 K. These features are certainly associated with a paramagnetic Ag center since they are not seen in unexchanged samples. They could possibly be the central two features in a quartet due to a
Silver Clusters in Smectite Clay Interlayers
J. Phys. Chem., Vol. 100, No. 10, 1996 4215
Figure 4. ESR spectra of Ag-montmorillonite/CH3OD γ-irradiated at 77 K and annealed to 170 and 290 K and recorded at those temperatures. Figure 2. Temperature evolution of ESR spectra at the temperatures indicated of Ag-montmorillonite dehydrated at 250 °C under flowing O2 and γ-irradiated at 77 K.
Figure 3. Experimental and simulated ESR spectra at 160 K of Agmontmorillonite/CH3OD γ-irradiated at 77 K and annealed to 160 K. The simulation is a sum of the three simulated spectra shown below it for Ag32+, Ag-CH2OH+ and Ag0.
paramagnetic Ag3 center such as Ag32+, and the hyperfine structure could be due to the expected second-order splitting on these middle two features. This assignment is very speculative, however, since the remaining outer two features cannot be clearly identified. Dehydrated at 250 °C and Exposure to Methanol Vapor at 25 °C. Exposure to methanol vapor increased d001 to 1.53 nm, corresponding to an interlayer spacing of 0.55 nm, sufficient to accommodate a single molecular layer of methanol. The ESR spectrum of this sample following irradiation at 77 K and recorded at 160 K is shown in Figure 3. This spectrum is simulated in the same figure, in terms of a Ag32+ cluster giving a quartet (fine structure on the outer features is due to the
presence of two Ag isotopes, and fine structure on the inner features is a consequence of second-order splitting), a broad doublet due to a Ag0 center (similar to species B described above), and a second doublet which is assigned to an Ag+ ion coupled to a CH2OH radical formed on irradiation of the methanol. This relatively intense doublet has been observed previously on irradiation of frozen methanolic Ag+ solutions, and its assignment to a coupled Ag-CH2OH+ species has become its accepted interpretation.11,20 The hyperfine splittings and g parameters used for the spectral simulations in Figure 3 are as follows: A(Ag0) ) 549 G, g(Ag0) ) 1.969; A(Ag‚CH2OH+) ) 159 G, g(Ag‚CH2OH+) ) 2.0045; 107Ag32+: A⊥ ) 192 G, A| ) 187 G, g⊥ ) 1.992, g| ) 1.962; 109Ag32+: A⊥ ) 222 G, A| ) 216 G, g⊥ ) 1.992, g| ) 1.962. Note that the Ag isotopes are not resolved in the Ag0 and Ag‚CH2OH+ component spectra. The simulation in Figure 3 clearly supports the formation of a Ag32+ cluster in montmorillonite for the first time. When the temperature is increased (Figure 4a), additional features appear which make up an evenly spaced quintet; and it seems that the Ag32+ quartet is replaced by an Ag43+ quintet, but at 170 K quartet T is still seen. Characteristic anisotropic spectral features due to the Ag electron loss center Ag2+ are also clear in the spectrum recorded at 170 K, although it is possible that these are also present at the lower temperature and simply sharpen on annealing. Note that because of the overlap of spectral features from Ag0 species with the outer lines of Ag32+ and Ag43+, the relative intensities of the outer lines of the cluster species are not characteristic of the cluster species alone. The identification of the silver ionic cluster species is unambiguous in these complex multispecies systems because the magnetic resonance parameters are approximately known from previous work in zeolites where the spectra are quite clear.21,22 The Ag0 spectrum (species B) remains intense at 170 K. On further annealing to 290 K (Figure 4b), the cluster spectrum and the Ag2+ spectrum decay, but the Ag0 center remains, suggesting again that this is a very stable center. Its ESR parameters change a little during annealing, with the isotropic splitting constant increasing from 549 G at 110 K to 615 G at 290 K. This is not surprising and suggests that the coordination around the atom, which is dictated by the coordination of the parent Ag+ ion at low temperature, relaxes as the sample is warmed to room temperature.
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Michalik et al.
Figure 5. Experimental (s) and simulated (- - -) three-pulse ESEM spectra of Ag32+ and Ag43+ in Ag-montmorillonite exposed to CH3OD.
Three-pulse ESEM decays recorded at field positions corresponding to Ag32+ (110 K spectrum) and Ag43+ (170 K spectrum) are shown in Figure 5. In both cases deuterated methanol (CH3OD) is used and deuterium modulation monitored. Modulation patterns are simulated in terms of the number of deuterium nuclei (N) interacting with the unpaired electron spin and their distance (R) from the spin. For Ag32+ N ) 3 and R ) 0.29 nm so this cluster is directly coordinated to three methanol molecules, and for Ag43+ N ) 3 and R ) 0.30 nm so this cluster also is directly coordinated to three methanol molecules. Ag+-Exchanged Hectorite. Dehydrated at 250 °C in Flowing O2. The measured d001 spacing of the clay of 0.98 nm corresponds again to complete collapse of the interlayer regions. The ESR spectra recorded after 77 K irradiation are broadly similar to those for dehydrated Ag+-exchanged montmorillonite. A similar Ag0 species dominates at low temperature, and as the sample is annealed this signal disappears to reveal a second more stable Ag0 species, like the Ag0 (species B) detected in Ag+-exchanged montmorillonite. However, in contrast to species B in montmorillonite, this Ag0 center does not resist thermal annealing above 200 K and disappears completely when the sample is annealed to room temperature. A further difference between Ag+-exchanged hectorite and montmorillonite is that the two lines with well-resolved fine structure which appear in montmorillonite on annealing to 300 K, and which have been tentatively assigned to the inner two features of a quartet of a Ag32+ cluster (Figure 2c), are not detected in hectorite. This may indicate that cluster formation in dehydrated hectorite is less facile than in dehydrated montmorillonite. In contrast, however, dehydrated hectorite does appear to stabilize the electron loss center Ag2+ more successfully than montmorillonite, and the Ag2+ parallel and perpendicular features are quite intense even up to 260 K. Dehydrated at 250 °C and Exposure to Methanol Vapor at 25 °C. The layer spacing in the Ag+-exchanged hectorite exposed to methanol was found to be 1.55 nm, similar to that in montmorillonite, and corresponding again to only a single molecular layer of methanol in the interlayer region. The ESR spectra of methanol-treated Ag+-exchanged hectorite following irradiation appear in Figure 6. They are similar to those recorded for the equivalent montmorillonite sample in that a quartet due to Ag32+ appears to be present even at low temperature together with a doublet due to Ag-CH2OH+. On annealing, a Ag43+ quintet replaces the quartet (Figure 6b) in the same way as in montmorillonite. However, the very stable Ag0 center (species B), which persists to room temperature
Figure 6. ESR spectra of Ag-hectorite/CH3OD γ-irradiated at 77 K and annealed to 115 and 180 K and recorded at those temperatures.
Figure 7. ESR spectra of Ag-saponite/CH3OD γ-irradiated at 77 K and annealed to 120 and 230 K and recorded at those temperatures.
in montmorillonite, is not seen in hectorite, and all Ag0 signals decay by 180 K. Ag+-Exchanged Saponite. Dehydrated at 250 °C in Flowing O2. The results for dehydrated Ag+-exchanged saponite are very similar to those for Ag+-exchanged hectorite. X-ray diffraction confirmed that the interlayer spacing is completely collapsed. The ESR spectrum of the sample following 77 K irradiation shows Ag0 and Ag2+ formation but, as with Ag+-exchanged hectorite, the Ag0 spectra disappear on annealing above 200 K, and there is no evidence for Ag cluster formation. Dehydrated at 250 °C and Exposure to Methanol Vapor at 25 °C. The interlayer spacing from X-ray diffraction is 0.55 nm, similar to that for montmorillonite and hectorite under these conditions. The ESR spectra following irradiation (Figure 7) are similar to those for Ag+-exchanged hectorite. At low temperature Ag0, Ag32+, and Ag-CH2OH+ are seen, and on annealing a quintet assigned to Ag43+ replaces the Ag32+ quartet.
Silver Clusters in Smectite Clay Interlayers The Ag-CH2OH+ spectrum disappears above 200 K. Significantly, and again in contrast to the behavior of Ag+-exchanged montmorillonite, the Ag0 centers formed on irradiation are not stable, and their spectra disappear on heating to 200 K. Discussion The results demonstrate that small Ag clusters can be formed and stabilized in clay interlayers in the presence of methanol. This contrasts with the behavior of clays with water in the interlayer regions which, on the basis of our previous work, are unable to stabilize identifiable Ag clusters.1,2 It is becoming increasingly clear that migration of Ag0 and cluster formation is more facile in frozen methanolic environments than in frozen aqueous environments. No evidence is found for dimeric Ag clusters in this work, and the trimeric Ag32+ cluster appears to form at relatively low temperatures. This suggests that migration of Ag0 through the frozen methanolic interlayer is relatively easy. The average separation between Ag+ ion-exchange sites on the surface of the clay is 1-2 nm so migration must occur over as much as 4 nm to form trimeric clusters at temperatures close to 77 K. In most other inorganic host matrices the mobility of Ag0 is evidently lower, and much higher temperatures are required to generate clusters of this size. The ESEM results on Ag32+ and Ag43+ show them both to be solvated by only three methanol molecules. These observations are consistent with the trimeric cluster being a trigonalplanar species lying in the plane of the clay. With only one molecular layer of solvent in the interlayer, this would constrain solvation to three methanol molecules coordinating the three Ag atoms/ions, with solvent molecules and cluster in the same plane. The tetrameric cluster is expected to be close to tetrahedral. Coordination by three methanol molecules could be explained if three of the four Ag atoms/ions lie in the plane of the clay and are each solvated by single methanols, and the fourth Ag is out of this plane and more closely associated with the clay surface and therefore less accessible to solvent molecules. Although such a structure would not give a binomial quintet in the ESR spectrum, the experimentally observed line widths are such that some silver nucleus inequivalency could be present and still be consistent with the ESR spectrum. Trimeric and tetrameric Ag clusters with similar ESR spectra are found in several other matrices,3,8-12 and as the number of matrices for cluster formation increases, so these clusters are emerging as those most commonly formed. Only when the matrix offers specific sites which can stabilize other size clusters, such as the Ag6 species in zeolite A,23 have such been observed. Possibly the most significant result of this work is the observation that, in Ag+-exchanged montmorillonite, a Ag0 species is generated at 77 K which is still detectable after annealing to room temperature. This is remarkably stable for radiolytically produced Ag0 which, in most matrices, is unstable at 200 K and above. The hyperfine splitting suggests that at low temperatures, where it presumably retains much of the coordination which existed for the parent Ag+ ion, there is a small amount of spin delocalization onto the ligands, but at higher temperatures the splitting increases to close to the free atom value so that there is little spin delocalization onto the ligands and the original coordination relaxes. Evidently, however, it does not relax sufficiently for the Ag0 to migrate from its original site and form clusters. Equally remarkable is the observation that this very stable Ag0 center is formed in montmorillonite but not in saponite or
J. Phys. Chem., Vol. 100, No. 10, 1996 4217 hectorite. Its formation also seems to depend on the clay being dehydrated at high temperature, since we failed to detect the center, even in montmorillonite, in samples which had not been subjected to vigorous dehydration.1,2 A possible explanation is that on dehydration at 250 °C some of the Ag+ becomes trapped in the so-called hexagonal cavities in the clay surface (Figure 1). The six-membered rings of silicon atoms with bridging oxygens in the tetrahedral layers of the clay lattice can strongly chelate cations of appropriate size. The potassium ion, for example, which has a similar ionic radius to the Ag+ ion, is known to become irreversibly trapped in these sites when K+-exchanged clays are dehydrated at high temperatures.24 Any Ag+ ions which become trapped in these sites would be retained when the clay is resolvated. If a trapped Ag+ then captures an electron as a consequence of irradiation, the resultant Ag0 atom, which is larger than the parent ion, would remain trapped in the cavity. This uniquely stable and inaccessible site for Ag+/Ag0 could account for the high thermal stability of the Ag0 center. It remains to be explained why Ag0 generated in hectorite and saponite cannot be stabilized in this way. The major difference between montmorillonite and these other two clays is that montmorillonite is a dioctahedral clay, in which only two-thirds of the lattice octahedral sites are occupied, while hectorite and saponite are both trioctahedral clays, in which all the octahedral sites are occupied. We can only propose that the difference in behavior toward trapping Ag+ and stabilizing Ag0 might be related to the composition of the octahedral lattice layer. Possibly the larger Mg(II) ion in the trioctahedral clays changes the dimensions of the hexagonal cavities in the trioctahedral clays so that chelation of Ag+ is less effective. Alternatively, some electrostatic phenomena associated with the vacant octahedral sites in the dioctahedral clays may facilitate coordination in these clays. In conclusion, this work has shown that small Ag clusters can be formed and stabilized in the interlayers of solventexpandable (smectite) layered clays. The important role of the interlayer solvent has also been demonstrated. Relatively small differences between nominally similar clays have been identified which seem to have an important effect on the ability of the clays to trap Ag+, exemplifying the subtle factors which control reduction mechanisms in this type of inorganic matrix for Ag+ ion catalysts. Acknowledgment. This research was supported by NATO Grant HTECH.CRG 930778, the U.S. National Science Foundation, and the R. A. Welch Foundation. References and Notes (1) Luca, V.; Brown, D. R.; Kevan, L. J. Phys. Chem. 1991, 95, 10065. (2) Brown, D. R.; Luca, V.; Kevan, L. J. Chem. Soc., Faraday Trans. 1991, 87, 2749. (3) (a) Morton, J. R.; Preston, K. F. In Electron Magnetic Resonance of the Solid State; Weil, J. A., Bowman, M. K., Morton, J. R., Preston, K. F., Eds.; Canadian Society of Chemistry: Ottawa, 1987; pp 295-308. (b) Forbes, C. E.; Symons, M. C. R. Mol. Phys. 1974, 27, 467. (4) Meyer, T. J. Acc. Chem. Res. 1989, 22, 163. (5) Photochemical ConVersion and Storage of Solar Energy; Pelizzetti, E.; Schiavello, M., Eds.; Kluwer: Dordrecht, 1991. (6) Calzaferri, G.; Li, J.; Waldeck, B. Coord. Chem. ReV. 1991, 111, 193. (7) Calzaferri, G.; Hug, S.; Hugentobler, T.; Sulzberger, B. J. Photochem. 1984, 28, 109. (8) Michalik, J.; Kevan, L. J. Am. Chem. Soc. 1986, 108, 4247. (9) Michalik, J.; Wasowicz, T. In Zeolites for the Nineties; Janson, J. C., Moseov, L., Post, M. F. M., Eds.; Elsevier: Amsterdam, 1989; p 359. (10) Sadlo, J.; Wasowicz, T.; Michalik, J. Radiat. Phys. Chem. 1995, 45, 909.
4218 J. Phys. Chem., Vol. 100, No. 10, 1996 (11) Michalik, J.; Azuma, N.; Sadlo, J.; Kevan, L. J. Phys. Chem. 1995, 99, 4679. (12) Michalik, J.; Zamadics, M.; Sadlo, J.; Kevan, L. J. Phys. Chem. 1993, 97, 10440. (13) Yamada, H.; Azuma, N.; Kevan, L. J. Phys. Chem. 1994, 98, 13017. (14) Brindley, G. W. In Crystal Structure of Clay Minerals and Their X-Ray Identification; Brindley, G. W., Brown, G., Eds.; Mineralogical Society: London, 1984; Chapter 2. (15) Yamada, H.; Fujita, T.; Nakazawa, H. J. Ceram. Soc. Jpn. 1988, 96, 1041. (16) Yamada, H.; Nakazawa, H.; Yoshiaka, K.; Fujita, T. Clay Miner. 1991, 26, 359. (17) Iiyama, J. T.; Roy, R. Clay Miner. Bull. 1963, 5, 161. (18) Dikanov, S. A.; Shubin, A. A.; Parmon, V. N. J. Magn. Reson. 1981, 42, 474.
Michalik et al. (19) Brown, D. R.; Findlay, T. J. V.; Symons, M. C. R. J. Phys. Chem. 1976, 72, 1792. (20) (a) Wasowicz, T.; Mikosz, J.; Sadlo, J.; Michalik, J. J. Chem. Soc., Perkin Trans. 2 1992, 1487. (b) Janes, R.; Stevens, A. D.; Symons, M. C. R. J. Chem. Soc., Faraday Trans. 1 1989, 85, 3973. (21) van der Pol, A.; Reijerse, E. J.; de Boer, E.; Wasowicz, T.; Michalik, J. Mol. Phys. 1992, 75, 37. (22) Xu, B.; Kevan, L. J. Phys. Chem. 1991, 95, 1147. (23) Hermerschmidt, D.; Haul, R. Ber. Bunsen-Ges. Phys. Chem. 1980, 84, 902. (24) (a) Van Olphen, H. An Introduction to Clay Colloid Chemistry; Wiley: New York, 1977; p 68. (b) Van Olphen, H. Clays Clay Miner. 1966, 14, 393.
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