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Electron spin resonance studies on silver atoms in imogolite fibers
Applied Clay Science 19 Ž2001. 173–178 www.elsevier.nlrlocaterclay
Electron spin resonance studies on silver atoms in imogolite fibers Hirohisa Yamada a,) , Jacek Michalik b, Jaroslaw Sadlo b, Joanna Perlinska b, Satoru Takenouchi a , Shuichi Shimomura a , Yoshisige Uchida a a
AdÕanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1 Tsukuba, Ibaraki 305-0044, Japan b Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland Received 4 May 2000; received in revised form 1 September 2000; accepted 25 September 2000
Abstract The formation and stabilization of reduced silver species in imogolite have been studied by electron spin resonance ŽESR. spectroscopy. Ag-loaded imogolite samples after degassing and dehydration were g-irradiated at 77 K and monitored by ESR as the temperature increased. Some samples were exposed to methanol vapour after dehydration. It was found that imogolite shows exceptional ability to stabilize silver atoms. In dehydrated Ag-imogolite silver atoms generated at low temperature remain stable at room temperature. Silver atoms are also formed in imogolite samples exposed to methanol. However, in contrast to silver agglomeration in molecular sieves and smectites exposed to methanol there is no indication of the formation of cationic silver clusters in Ag-imogolite. It is postulated that there are special trapping sites in imogolite structure which effectively stabilize silver atoms. q 2001 Published by Elsevier Science B.V. Keywords: ESR spectroscopy; Imogolite; Silver atoms; g-Irradiation
1. Introduction Silver is a transition metal that has been proved to be very active catalytically when adsorbed on various oxides. Earlier, we had been studying the mechanism of radiation-induced silver agglomeration in smectites ŽMichalik et al., 1996a. and zeolites ŽMichalik and Kevan, 1986; Michalik, 1996; Michalik et al., 1996b, 1998.. Ag 0 atoms radiolytically
)
Corresponding author. Tel.: q81-298-51-3354; fax: q81-29852-7449. E-mail address: [email protected] ŽH. Yamada..
generated at 77 K migrate to the nearby Agq cations when temperature rises and form small silver clusters. The cluster structure and stability depend on the matrix in which they are formed. In the present work, we focus our attention on the formation of reduced silver species in a different type of aluminosilicate matrix-imogolite ŽWada and Yoshinaga, 1968; Farmer and Russell, 1973; Wada, 1977.. Imogolite has a net composition ŽHO. 3 Al 2O 3 SiOH and its structure consists of hollow tubes with an outer diameter of 2 nm and the length of a few micrometers. The tubes contain curved gibbsite sheets with silicate groups replacing hydroxy groups on the inner surface. AlOH groups are located on the outer surface. The surface properties of imogolite
0169-1317r01r$ - see front matter q 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 1 3 1 7 Ž 0 1 . 0 0 0 5 6 - 4
174
H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
have attracted considerable interests, especially related to cation adsorption and to immobilization of metallic particles ŽLiz-Marzan ´ and Philipse, 1995.. The reduced silver species stabilized in imogolite are compared with those observed earlier in zeolites ŽMichalik and Kevan, 1986; Sadlo et al., 1995; Michalik, 1996; Michalik et al., 1996b, 1998., silicoaluminophosphate molecular sieves ŽMichalik et al., 1995. and smectite clays ŽBrown et al., 1991; Michalik et al., 1996a..
2. Experimental Natural imogolite from a gel-like film in weathered pumice ŽKitakami, Iwate Prefecture, Japan. was used for our study ŽMiyauchi and Aomine, 1966; Wada and Yoshinaga, 1968; Henmi and Wada, 1976.. The gel-like film, which is translucent and slightly contaminated by iron oxide, fills up the interspaces among weathered pumice grains. The pieces of the film were collected by a sieve, washed by distilled water, and subsequently small pumice fragments were removed by a pincette. The collected materials were
treated with H 2 O 2 , and then by the Na-citrate–dithionite–bicarbonate method for removing organic matters and extractable oxides. Transmission electron microscopy of purified sample showed a spider’s web-like network structure ŽFig. 1.. In the holes of this structure, individual fibers were seen at various places. This morphology is very typical for imogolite. The AlrSi ratio was determined to be 1.80 by ICP method. Silver cations were loaded to imogolite by stirring with an aqueous solution of silver nitrate overnight at room temperature. Then the imogolite sample was filtered and washed with distilled water several times and dried at room temperature. The silver content was determined by ICP method to be 5.8 wt.%. Samples of powdered imogolite were placed into 2 mm i.d. by 3 mm o.d. Suprasil quartz tubes, evacuated at room temperature and then dehydrated under vacuum with gradually increasing temperature till 2008C. Sample was exposed to methanol under its vapour pressure at room temperature while connected to the vacuum line. All samples were irradiated at 77 K in a 60 Co source with a dose of 4 kGy. The ESR spectra were recorded with Bruker ESP-300e spectrometer in the
Fig. 1. Transmission electron micrograph of imogolite.
H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
temperature range 110–310 K using Bruker variable temperature unit.
3. Result The ESR spectra of dehydrated Ag-imogolite irradiated at 77 K and annealed at different temperatures are presented in Fig. 2. The spectra consist of strong singlet at g s 2 region Žwith intensity not fully shown. associated with paramagnetic defects in imogolite framework and isotropic doublet with ESR parameters: A iso s 57 mT and g iso s 1.992, which are characteristic for Ag 0 atoms ŽBrown et al., 1976; Brown and Kevan, 1986; Michalik, 1996.. The low intensity doublets labeled H observed at 110 K, represents hydrogen atoms generated radiolytically in the quartz tubicngs. The intensity of Ag 0 doublet decreases during the annealing in the temperature range of 110–310 K but in contrast to molecular sieves Ag 0 decay does not result in the formation of cationic silver clusters. About 25% of silver atoms is immobilized so strongly in imogolite matrix that they are observed at room temperature for days as
175
far as sample remains degassed. After admission of air Ag 0 signal disappears completely after 30 min. In hydrated Ag-imogolite matrices which were degassed at room temperature Ag 0 atoms decay so fast that characteristic doublet is not observed at all at 110 K. To check how other adsorbates affect Ag 0 stabilization the dehydrated imogolite sample was exposed to the methanol vapour at room temperature. The ESR spectra of Ag-imogoliterCH 3 OH sample irradiated at 77 K and recorded at increasing temperatures are shown in Fig. 3. At 110 K, the spectrum consists of intense triplet B: A iso s 2.4 mT of PCH 2 OH radical and the doublet of Ag 0 atoms with line intensity much lower than in dehydrated samples. Upon annealing at 170 K, a new doublet A with A iso s 9.9 mT appears but the intensity of Ag 0 lines is nearly the same as at 110 K. The ESR doublets with similar hyperfine splittings were earlier recorded in g-irradiated molecular sieves and clays loaded with Agq cations and were assigned to silver hydroxymethyl radicals Ag P CH 2 OHq which are formed by the attack of PCH 2 OH radicals on Agq cations ŽWasowicz et al., 1992; Michalik et al., 1995, 1996a.. Ag 0 spectrum starts decaying at 170 K and is barely seen above 230 K ŽFig. 3.. In imogolite
Fig. 2. ESR spectra of dehydrated Ag-imogolite g-irradiated at 77 K and annealed at 110 K Ža., 310 K Žb. and open to air at room temperature Žc..
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H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
Fig. 3. ESR spectra of Ag-imogoliterCH 3 OH g-irradiated at 77 K and annealed at different temperature.
samples exposed to methanol, the spectra of cationic silver clusters are not recorded during thermal annealing in contrast to smectite clays.
4. Discussion The ESR results clearly prove that dehydrated imogolite fibers are very effective stabilizers of silver atoms. Such stabilizing effect was earlier found in smectite clays ŽMichalik et al., 1996a., but is very rare in molecular sieves. This effect is rather unexpected because in smectite clays exchangeable cations located in the interlayer space usually show higher mobility than exchangeable cations in zeolites. In Ag-montmorillonite matrix silver atoms produced
radiolytically at 77 K are still observed at room temperature just as Ag 0 in imogolite. To explain such unusual stability of Ag 0 atoms, it was postulated ŽMichalik et al., 1996a. that on dehydration at 2508C some of Agq cations became trapped in the so-called hexagonal cavities in the clay surface. The six-membered rings of silicon atoms with bridging oxygens in tetrahedral layers in clay lattice can strongly chelate cation of appropriate size as Agq cations. If a trapped Agq cation captures an electron as a consequence of irradiation, the resultant Ag 0 atoms, which is larger than the parent ion, would remain trapped in the cavity. The other Ag 0 atoms easily migrate through interlayer to the surface where they form metallic particles. In dehydrated montmorillonite, no ESR evidence was found for the forma-
H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
tion of cationic silver clusters, which usually are detected in zeolites. However, when solvated with methanol, montmorillonite is able to stabilize Ag 2q 3 and Ag 3q clusters in interlayer sites. The role of 4 methanol molecules in the silver agglomeration process in porous materials is at least twofold. First, by scavenging holes they prevent Ag 2q formation. Owing to that, the concentration of both Ag 0 and Agq, the species active in agglomeration, is higher. Second, by blocking clay interlayers methanol can decrease long-distance mobility of silver atoms and clusters to reduce the formation of larger metallic particles. The concept of Ag 0 stabilization in hexagonal cavities in clay surface ŽMichalik et al., 1996a. cannot be adopted for Ag 0 atoms trapped in imogolite for structural reasons because there are not hexagonal cavities in imogolite lattice. Besides, smectite clays show cation exchange capacity associated with lattice negative charges. Imogolite lattice is neutral so cations can only be sorbed physically on imogolite surface. One can distinguish three types of porosity in imogolite structure: Ži. intra-tube pores of about 1 nm, Žii. inter-tube spaces between tubes in parallel arrays which vary with hydration state, and Žiii. irregular pores between bundles of tubes in a cross-linked network of fiber bundles. Some studies suggest that the sites of salt adsorption are inter-tube ones ŽFarmer et al., 1983.. It was also shown that small platinum metal particles are adsorbed at outer surface of imogolite fibers ŽLiz-Marzan ´ and Philipse, 1995.. So, it seems reasonable to assume that Agq cations and Ag 0 atoms produced radiolytically at low temperature are located on the outer surface of imogolite fibers. In hydrated samples Agq cations are solvated by H 2 O molecules. Thus, silver atoms generated by irradiation are able to react with H 2 O molecules even at low temperature. This explains why Ag 0 doublet was not recorded in hydrated imogolite sample. On dehydration to 2008C inter-tube pores collapse and the cross-linked network of fiber bundles are denser. In general, the free space channels becomes narrower which makes Ag 0 migration more difficult. Thus, the ESR doublet of Ag 0 atoms is easily observed in temperature range 110–250 K. However, the stability of Ag 0 atoms at room temperature is very unique and to explain this effect we postulate that some Agq cations upon dehydration
177
might be trapped inside small isolated cavities, which are collapsed inter-tube pores completely surrounded by imogolite fiber bundles. These trapping sites should be similarly effective for Ag 0 stabilization as hexagonal cavities in montmorillonite clay. According to this mechanism, silver atoms in the presence of methanol molecules should be unstable as in hydrated samples. Experimental results do not prove such a conclusion. In imogolite exposed to methanol, Ag 0 atoms are not as stable as in dehydrated samples but Ag 0 doublet is still seen at 230 K. It should be stressed however, that before exposure to methanol imogolite sample was dehydrated at 2008C. Upon methanol adsorption, the inter-tube pores are probably not rebuilt completely and some Agq might be located in isolated cavities which keep them immobile till 230 K. In conclusion, this work has shown the remarkable ability of imogolite fibers to stabilize silver atoms at room temperature. It was postulated that the most stable Ag 0 atoms are produced radiolytically from Agq cations trapped in cavities surrounded by crossed bundles of imogolite fibers. Acknowledgements The authors are grateful to Dr. Shin-ichiro Wada, Kyushu University, for supplying imogolite sample. References Brown, D.R., Kevan, L., 1986. Comparative electron spin resonance and optical absorption studies of silver-exchanged sodium Y zeolites: silver centers formed on dehydration, oxidation, and subsequent g-irradiation. J. Phys. Chem. 90, 1129–1133. Brown, D.R., Findlay, T.J.V., Symons, M.C.R., 1976. Radiation mechanisms: Part 12. E.s.r. studies of electron capture by silverŽI. ions, nitrate ions and their ion pairs and clusters in methyl cyanide. J. Chem. Soc., Faraday Trans. 72, 1792–1798. Brown, D.R., Luca, V., Kevan, L., 1991. Electron paramagnetic resonance and electron spin echo modulation analysis of silver atom environment in g-irradiated silver-exchanged sodium montmorillonite and its Al 13 pillared derivative. J. Chem. Soc., Faraday Trans. 87, 2749–2754. Farmer, V.C., Russell, J.D., 1973. The structure and genesis of allophane and imogolite; their distribution in non-volcanic soils. In: De Boodt, M.F., Hayes, M.H.B., Herbillon, A. ŽEds.., Soil Colloids and their Association in Aggregate. Plenum, New York, pp. 165–178.
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Farmer, V.C., Adams, M.J., Fraser, A.R., Palmieri, F., 1983. Synthetic imogolite: properties, synthesis, and possible applications. Clay Miner. 18, 459–472. Henmi, T., Wada, K., 1976. Morphology and composition of allophane. Am. Mineral. 61, 379–390. Liz-Marzan, ´ L.M., Philipse, A.P., 1995. Stable hydrosols of metallic and bimetallic nanoparticles immobilized on imogolite fibers. J. Phys. Chem. 99, 15120–15128. Michalik, J., 1996. Silver atoms and clusters in molecular sieves and clays. Appl. Magn. Reson. 10, 507–537. Michalik, J., Kevan, L., 1986. Paramagnetic silver clusters in Ag–NaA zeolite: electron spin resonance and diffuse reflectance spectroscopic studies. J. Am. Chem. Soc. 108, 4247–4253. Michalik, J., Azuma, N., Sadlo, J., Kevan, L., 1995. Silver agglomeration in SAPO-5 and SAPO-11 molecular sieves. J. Phys. Chem. 99, 4679–4686. Michalik, J., Yamada, H., Brown, D.R., Kevan, L., 1996a. Small silver clusters in smectite clay interlayers. J. Phys. Chem. 100, 4213–4218. Michalik, J., Sadlo, J., Yu, J.S., Kevan, L., 1996b. Tetrameric
silver clusters in rho zeolite stable above room temperature— ESR studies. Colloids Surf. A 115, 239–247. Michalik, J., Sadlo, J., Kodaira, T., Shimomura, S., Yamada, H., 1998. ESR and optical studies of cationic silver clusters in zeolite rho. J. Radioanal. Nucl. Chem. 232, 135–137. Miyauchi, N., Aomine, S., 1966. Mineralogy of gel-like substance in the pumice bed in Kanuma and Kitakami districts. Soil Sci. Plant Nutr. ŽTokyo. 12, 19–22. Sadlo, J., Wasowicz, T., Michalik, J., 1995. Radiation-induced silver agglomeration in molecular sieves: a comparison between A and X zeolites. Radiat. Phys. Chem. 45, 909–915. Wada, K., 1977. Allophane and imogolite. In: Dixon, J.B., Weeds, S.B. ŽEds.., Minerals in Solid Environments. Soil Science Society America, Madison, WI, pp. 603–638. Wada, K., Yoshinaga, N., 1968. The structure of AimogoliteB. Am. Mineral. 54, 50–71. Wasowicz, T., Mikosz, J., Sadlo, J., Michalik, J., 1992. Organosilver radicals in gamma-irradiated Ag–NaA zeolites with methanol adsorbate. J. Chem. Soc., Perkin Trans. 2, 1487– 1491.
Electron spin resonance studies on silver atoms in imogolite fibers Hirohisa Yamada a,) , Jacek Michalik b, Jaroslaw Sadlo b, Joanna Perlinska b, Satoru Takenouchi a , Shuichi Shimomura a , Yoshisige Uchida a a
AdÕanced Materials Laboratory, National Institute for Materials Science, Namiki 1-1 Tsukuba, Ibaraki 305-0044, Japan b Institute of Nuclear Chemistry and Technology, Dorodna 16, 03-195 Warsaw, Poland Received 4 May 2000; received in revised form 1 September 2000; accepted 25 September 2000
Abstract The formation and stabilization of reduced silver species in imogolite have been studied by electron spin resonance ŽESR. spectroscopy. Ag-loaded imogolite samples after degassing and dehydration were g-irradiated at 77 K and monitored by ESR as the temperature increased. Some samples were exposed to methanol vapour after dehydration. It was found that imogolite shows exceptional ability to stabilize silver atoms. In dehydrated Ag-imogolite silver atoms generated at low temperature remain stable at room temperature. Silver atoms are also formed in imogolite samples exposed to methanol. However, in contrast to silver agglomeration in molecular sieves and smectites exposed to methanol there is no indication of the formation of cationic silver clusters in Ag-imogolite. It is postulated that there are special trapping sites in imogolite structure which effectively stabilize silver atoms. q 2001 Published by Elsevier Science B.V. Keywords: ESR spectroscopy; Imogolite; Silver atoms; g-Irradiation
1. Introduction Silver is a transition metal that has been proved to be very active catalytically when adsorbed on various oxides. Earlier, we had been studying the mechanism of radiation-induced silver agglomeration in smectites ŽMichalik et al., 1996a. and zeolites ŽMichalik and Kevan, 1986; Michalik, 1996; Michalik et al., 1996b, 1998.. Ag 0 atoms radiolytically
)
Corresponding author. Tel.: q81-298-51-3354; fax: q81-29852-7449. E-mail address: [email protected] ŽH. Yamada..
generated at 77 K migrate to the nearby Agq cations when temperature rises and form small silver clusters. The cluster structure and stability depend on the matrix in which they are formed. In the present work, we focus our attention on the formation of reduced silver species in a different type of aluminosilicate matrix-imogolite ŽWada and Yoshinaga, 1968; Farmer and Russell, 1973; Wada, 1977.. Imogolite has a net composition ŽHO. 3 Al 2O 3 SiOH and its structure consists of hollow tubes with an outer diameter of 2 nm and the length of a few micrometers. The tubes contain curved gibbsite sheets with silicate groups replacing hydroxy groups on the inner surface. AlOH groups are located on the outer surface. The surface properties of imogolite
0169-1317r01r$ - see front matter q 2001 Published by Elsevier Science B.V. PII: S 0 1 6 9 - 1 3 1 7 Ž 0 1 . 0 0 0 5 6 - 4
174
H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
have attracted considerable interests, especially related to cation adsorption and to immobilization of metallic particles ŽLiz-Marzan ´ and Philipse, 1995.. The reduced silver species stabilized in imogolite are compared with those observed earlier in zeolites ŽMichalik and Kevan, 1986; Sadlo et al., 1995; Michalik, 1996; Michalik et al., 1996b, 1998., silicoaluminophosphate molecular sieves ŽMichalik et al., 1995. and smectite clays ŽBrown et al., 1991; Michalik et al., 1996a..
2. Experimental Natural imogolite from a gel-like film in weathered pumice ŽKitakami, Iwate Prefecture, Japan. was used for our study ŽMiyauchi and Aomine, 1966; Wada and Yoshinaga, 1968; Henmi and Wada, 1976.. The gel-like film, which is translucent and slightly contaminated by iron oxide, fills up the interspaces among weathered pumice grains. The pieces of the film were collected by a sieve, washed by distilled water, and subsequently small pumice fragments were removed by a pincette. The collected materials were
treated with H 2 O 2 , and then by the Na-citrate–dithionite–bicarbonate method for removing organic matters and extractable oxides. Transmission electron microscopy of purified sample showed a spider’s web-like network structure ŽFig. 1.. In the holes of this structure, individual fibers were seen at various places. This morphology is very typical for imogolite. The AlrSi ratio was determined to be 1.80 by ICP method. Silver cations were loaded to imogolite by stirring with an aqueous solution of silver nitrate overnight at room temperature. Then the imogolite sample was filtered and washed with distilled water several times and dried at room temperature. The silver content was determined by ICP method to be 5.8 wt.%. Samples of powdered imogolite were placed into 2 mm i.d. by 3 mm o.d. Suprasil quartz tubes, evacuated at room temperature and then dehydrated under vacuum with gradually increasing temperature till 2008C. Sample was exposed to methanol under its vapour pressure at room temperature while connected to the vacuum line. All samples were irradiated at 77 K in a 60 Co source with a dose of 4 kGy. The ESR spectra were recorded with Bruker ESP-300e spectrometer in the
Fig. 1. Transmission electron micrograph of imogolite.
H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
temperature range 110–310 K using Bruker variable temperature unit.
3. Result The ESR spectra of dehydrated Ag-imogolite irradiated at 77 K and annealed at different temperatures are presented in Fig. 2. The spectra consist of strong singlet at g s 2 region Žwith intensity not fully shown. associated with paramagnetic defects in imogolite framework and isotropic doublet with ESR parameters: A iso s 57 mT and g iso s 1.992, which are characteristic for Ag 0 atoms ŽBrown et al., 1976; Brown and Kevan, 1986; Michalik, 1996.. The low intensity doublets labeled H observed at 110 K, represents hydrogen atoms generated radiolytically in the quartz tubicngs. The intensity of Ag 0 doublet decreases during the annealing in the temperature range of 110–310 K but in contrast to molecular sieves Ag 0 decay does not result in the formation of cationic silver clusters. About 25% of silver atoms is immobilized so strongly in imogolite matrix that they are observed at room temperature for days as
175
far as sample remains degassed. After admission of air Ag 0 signal disappears completely after 30 min. In hydrated Ag-imogolite matrices which were degassed at room temperature Ag 0 atoms decay so fast that characteristic doublet is not observed at all at 110 K. To check how other adsorbates affect Ag 0 stabilization the dehydrated imogolite sample was exposed to the methanol vapour at room temperature. The ESR spectra of Ag-imogoliterCH 3 OH sample irradiated at 77 K and recorded at increasing temperatures are shown in Fig. 3. At 110 K, the spectrum consists of intense triplet B: A iso s 2.4 mT of PCH 2 OH radical and the doublet of Ag 0 atoms with line intensity much lower than in dehydrated samples. Upon annealing at 170 K, a new doublet A with A iso s 9.9 mT appears but the intensity of Ag 0 lines is nearly the same as at 110 K. The ESR doublets with similar hyperfine splittings were earlier recorded in g-irradiated molecular sieves and clays loaded with Agq cations and were assigned to silver hydroxymethyl radicals Ag P CH 2 OHq which are formed by the attack of PCH 2 OH radicals on Agq cations ŽWasowicz et al., 1992; Michalik et al., 1995, 1996a.. Ag 0 spectrum starts decaying at 170 K and is barely seen above 230 K ŽFig. 3.. In imogolite
Fig. 2. ESR spectra of dehydrated Ag-imogolite g-irradiated at 77 K and annealed at 110 K Ža., 310 K Žb. and open to air at room temperature Žc..
176
H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
Fig. 3. ESR spectra of Ag-imogoliterCH 3 OH g-irradiated at 77 K and annealed at different temperature.
samples exposed to methanol, the spectra of cationic silver clusters are not recorded during thermal annealing in contrast to smectite clays.
4. Discussion The ESR results clearly prove that dehydrated imogolite fibers are very effective stabilizers of silver atoms. Such stabilizing effect was earlier found in smectite clays ŽMichalik et al., 1996a., but is very rare in molecular sieves. This effect is rather unexpected because in smectite clays exchangeable cations located in the interlayer space usually show higher mobility than exchangeable cations in zeolites. In Ag-montmorillonite matrix silver atoms produced
radiolytically at 77 K are still observed at room temperature just as Ag 0 in imogolite. To explain such unusual stability of Ag 0 atoms, it was postulated ŽMichalik et al., 1996a. that on dehydration at 2508C some of Agq cations became trapped in the so-called hexagonal cavities in the clay surface. The six-membered rings of silicon atoms with bridging oxygens in tetrahedral layers in clay lattice can strongly chelate cation of appropriate size as Agq cations. If a trapped Agq cation captures an electron as a consequence of irradiation, the resultant Ag 0 atoms, which is larger than the parent ion, would remain trapped in the cavity. The other Ag 0 atoms easily migrate through interlayer to the surface where they form metallic particles. In dehydrated montmorillonite, no ESR evidence was found for the forma-
H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
tion of cationic silver clusters, which usually are detected in zeolites. However, when solvated with methanol, montmorillonite is able to stabilize Ag 2q 3 and Ag 3q clusters in interlayer sites. The role of 4 methanol molecules in the silver agglomeration process in porous materials is at least twofold. First, by scavenging holes they prevent Ag 2q formation. Owing to that, the concentration of both Ag 0 and Agq, the species active in agglomeration, is higher. Second, by blocking clay interlayers methanol can decrease long-distance mobility of silver atoms and clusters to reduce the formation of larger metallic particles. The concept of Ag 0 stabilization in hexagonal cavities in clay surface ŽMichalik et al., 1996a. cannot be adopted for Ag 0 atoms trapped in imogolite for structural reasons because there are not hexagonal cavities in imogolite lattice. Besides, smectite clays show cation exchange capacity associated with lattice negative charges. Imogolite lattice is neutral so cations can only be sorbed physically on imogolite surface. One can distinguish three types of porosity in imogolite structure: Ži. intra-tube pores of about 1 nm, Žii. inter-tube spaces between tubes in parallel arrays which vary with hydration state, and Žiii. irregular pores between bundles of tubes in a cross-linked network of fiber bundles. Some studies suggest that the sites of salt adsorption are inter-tube ones ŽFarmer et al., 1983.. It was also shown that small platinum metal particles are adsorbed at outer surface of imogolite fibers ŽLiz-Marzan ´ and Philipse, 1995.. So, it seems reasonable to assume that Agq cations and Ag 0 atoms produced radiolytically at low temperature are located on the outer surface of imogolite fibers. In hydrated samples Agq cations are solvated by H 2 O molecules. Thus, silver atoms generated by irradiation are able to react with H 2 O molecules even at low temperature. This explains why Ag 0 doublet was not recorded in hydrated imogolite sample. On dehydration to 2008C inter-tube pores collapse and the cross-linked network of fiber bundles are denser. In general, the free space channels becomes narrower which makes Ag 0 migration more difficult. Thus, the ESR doublet of Ag 0 atoms is easily observed in temperature range 110–250 K. However, the stability of Ag 0 atoms at room temperature is very unique and to explain this effect we postulate that some Agq cations upon dehydration
177
might be trapped inside small isolated cavities, which are collapsed inter-tube pores completely surrounded by imogolite fiber bundles. These trapping sites should be similarly effective for Ag 0 stabilization as hexagonal cavities in montmorillonite clay. According to this mechanism, silver atoms in the presence of methanol molecules should be unstable as in hydrated samples. Experimental results do not prove such a conclusion. In imogolite exposed to methanol, Ag 0 atoms are not as stable as in dehydrated samples but Ag 0 doublet is still seen at 230 K. It should be stressed however, that before exposure to methanol imogolite sample was dehydrated at 2008C. Upon methanol adsorption, the inter-tube pores are probably not rebuilt completely and some Agq might be located in isolated cavities which keep them immobile till 230 K. In conclusion, this work has shown the remarkable ability of imogolite fibers to stabilize silver atoms at room temperature. It was postulated that the most stable Ag 0 atoms are produced radiolytically from Agq cations trapped in cavities surrounded by crossed bundles of imogolite fibers. Acknowledgements The authors are grateful to Dr. Shin-ichiro Wada, Kyushu University, for supplying imogolite sample. References Brown, D.R., Kevan, L., 1986. Comparative electron spin resonance and optical absorption studies of silver-exchanged sodium Y zeolites: silver centers formed on dehydration, oxidation, and subsequent g-irradiation. J. Phys. Chem. 90, 1129–1133. Brown, D.R., Findlay, T.J.V., Symons, M.C.R., 1976. Radiation mechanisms: Part 12. E.s.r. studies of electron capture by silverŽI. ions, nitrate ions and their ion pairs and clusters in methyl cyanide. J. Chem. Soc., Faraday Trans. 72, 1792–1798. Brown, D.R., Luca, V., Kevan, L., 1991. Electron paramagnetic resonance and electron spin echo modulation analysis of silver atom environment in g-irradiated silver-exchanged sodium montmorillonite and its Al 13 pillared derivative. J. Chem. Soc., Faraday Trans. 87, 2749–2754. Farmer, V.C., Russell, J.D., 1973. The structure and genesis of allophane and imogolite; their distribution in non-volcanic soils. In: De Boodt, M.F., Hayes, M.H.B., Herbillon, A. ŽEds.., Soil Colloids and their Association in Aggregate. Plenum, New York, pp. 165–178.
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H. Yamada et al.r Applied Clay Science 19 (2001) 173–178
Farmer, V.C., Adams, M.J., Fraser, A.R., Palmieri, F., 1983. Synthetic imogolite: properties, synthesis, and possible applications. Clay Miner. 18, 459–472. Henmi, T., Wada, K., 1976. Morphology and composition of allophane. Am. Mineral. 61, 379–390. Liz-Marzan, ´ L.M., Philipse, A.P., 1995. Stable hydrosols of metallic and bimetallic nanoparticles immobilized on imogolite fibers. J. Phys. Chem. 99, 15120–15128. Michalik, J., 1996. Silver atoms and clusters in molecular sieves and clays. Appl. Magn. Reson. 10, 507–537. Michalik, J., Kevan, L., 1986. Paramagnetic silver clusters in Ag–NaA zeolite: electron spin resonance and diffuse reflectance spectroscopic studies. J. Am. Chem. Soc. 108, 4247–4253. Michalik, J., Azuma, N., Sadlo, J., Kevan, L., 1995. Silver agglomeration in SAPO-5 and SAPO-11 molecular sieves. J. Phys. Chem. 99, 4679–4686. Michalik, J., Yamada, H., Brown, D.R., Kevan, L., 1996a. Small silver clusters in smectite clay interlayers. J. Phys. Chem. 100, 4213–4218. Michalik, J., Sadlo, J., Yu, J.S., Kevan, L., 1996b. Tetrameric
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