Electron Spin Resonance Studies on Silver Atoms in Imogolite Fibers

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Ž .Applied Clay Science 19 2001 173–178

www.elsevier.nlrlocaterclay

Electron spin resonance studies on silver atoms inimogolite fibers

Hirohisa Yamada a,), Jacek Michalik b, Jaroslaw Sadlo b, Joanna Perlinska b,Satoru Takenouchi a, Shuichi Shimomura a, Yoshisige Uchida a

a Ad Õanced 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 resonanceŽ .ESR 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. q2001 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 vari-

ous oxides. Earlier, we had been studying the mecha-

nism of radiation-induced silver agglomeration inŽ .smectites Michalik et al., 1996a and zeolites

ŽMichalik and Kevan, 1986; Michalik, 1996; Micha-. 0lik et al., 1996b, 1998 . Ag atoms radiolytically

)

Corresponding author. Tel.: q81-298-51-3354; fax:q81-298-

52-7449.

E-mail address: [email protected]Ž .H. Yamada .

generated at 77 K migrate to the nearby Agq cations

when temperature rises and form small silver clus-

ters. 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 differentŽtype of aluminosilicate matrix-imogolite Wada and

Yoshinaga, 1968; Farmer and Russell, 1973; Wada,. Ž .1977 . Imogolite has a net composition HO Al -3 2

O SiOH and its structure consists of hollow tubes3

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.Ž .P I I : S 0 1 6 9 - 1 3 1 7 0 1 0 0 0 5 6 - 4



( ) H. Yamada et al.r Applied Clay Science 19 2001 173–178174

have attracted considerable interests, especially re-

lated to cation adsorption and to immobilization of Ž .metallic particles Liz-Marzan and Philipse, 1995 .´

The reduced silver species stabilized in imogolite

are compared with those observed earlier in zeolitesŽMichalik and Kevan, 1986; Sadlo et al., 1995;

.Michalik, 1996; Michalik et al., 1996b, 1998 , sili-Žcoaluminophosphate molecular sieves Michalik et

. Žal., 1995 and smectite clays Brown et al., 1991;.Michalik et al., 1996a .

2. Experimental

Natural imogolite from a gel-like film in weath-Ž .ered pumice Kitakami, Iwate Prefecture, Japan was

Žused for our study Miyauchi 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 O , and then by the Na-citrate–di-2 2

thionite–bicarbonate method for removing organic

matters and extractable oxides. Transmission elec-

tron microscopy of purified sample showed a spider’sŽ .web-like network structure Fig. 1 . In the holes of

this structure, individual fibers were seen at various

places. This morphology is very typical for imogo-

lite. 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, evacu-

ated at room temperature and then dehydrated under

vacuum with gradually increasing temperature till2008C. Sample was exposed to methanol under its

vapour pressure at room temperature while con-

nected to the vacuum line.

All samples were irradiated at 77 K in a60

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 173–178 175

temperature range 110–310 K using Bruker variable

temperature unit.

3. Result

The ESR spectra of dehydrated Ag-imogolite irra-

diated at 77 K and annealed at different temperatures

are presented in Fig. 2. The spectra consist of strongŽsinglet at gs2 region with intensity not fully

.shown associated with paramagnetic defects in

imogolite framework and isotropic doublet with ESR

parameters: A s57 mT and g s1.992, whichiso iso0 Žare characteristic for Ag atoms Brown 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 doubletdecreases 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

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 OH sample3

irradiated at 77 K and recorded at increasing temper-

atures are shown in Fig. 3. At 110 K, the spectrum

consists of intense triplet B: A s2.4 mT of iso

PCH OH radical and the doublet of Ag0 atoms with2

line intensity much lower than in dehydrated sam-

ples. Upon annealing at 170 K, a new doublet A with

A s9.9 mT appears but the intensity of Ag 0 linesiso

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 loadedwith Agq cations and were assigned to silver hy-

droxymethyl radicals Ag P CH OHq which are2

formed by the attack of PCH OH radicals on Agq2

Žcations Wasowicz et al., 1992; Michalik et al.,. 01995, 1996a . Ag spectrum starts decaying at 170 K

Ž .and is barely seen above 230 K Fig. 3 . In imogolite

Ž . Ž .Fig. 2. ESR spectra of dehydrated Ag-imogolite g-irradiated at 77 K and annealed at 110 K a , 310 K b and open to air at roomŽ .temperature c .




Fig. 3. ESR spectra of Ag-imogoliterCH OH g-irradiated at 77 K and annealed at different temperature.3

samples exposed to methanol, the spectra of cationic

silver clusters are not recorded during thermal an-

nealing in contrast to smectite clays.

4. Discussion

The ESR results clearly prove that dehydrated

imogolite fibers are very effective stabilizers of sil-

ver atoms. Such stabilizing effect was earlier foundŽ .in smectite clays Michalik et al., 1996a , but is very

rare in molecular sieves. This effect is rather unex-

pected 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 Ag0 in imogolite. To explain

such unusual stability of Ag 0 atoms, it was postu-Ž .lated Michalik 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 Ag0

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 montmo-

rillonite, no ESR evidence was found for the forma-



( ) H. Yamada et al.r Applied Clay Science 19 2001 173–178 177

tion of cationic silver clusters, which usually are

detected in zeolites. However, when solvated with

methanol, montmorillonite is able to stabilize Ag 2q3

and Ag3q clusters in interlayer sites. The role of 4

methanol molecules in the silver agglomeration pro-

cess in porous materials is at least twofold. First, by

scavenging holes they prevent Ag 2q formation. Ow-

ing to that, the concentration of both Ag 0 and Agq,

the species active in agglomeration, is higher. Sec-

ond, by blocking clay interlayers methanol can de-

crease long-distance mobility of silver atoms and

clusters to reduce the formation of larger metallic

particles.

The concept of Ag0 stabilization in hexagonalŽ .cavities in clay surface Michalik et al., 1996a

cannot be adopted for Ag 0 atoms trapped in imogo-

lite for structural reasons because there are not

hexagonal cavities in imogolite lattice. Besides,

smectite clays show cation exchange capacity associ-ated with lattice negative charges. Imogolite lattice is

neutral so cations can only be sorbed physically on

imogolite surface. One can distinguish three types of Ž .porosity in imogolite structure: i intra-tube pores of

Ž .about 1 nm, ii inter-tube spaces between tubes in

parallel arrays which vary with hydration state, andŽ .iii 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-tubeŽ .ones Farmer et al., 1983 . It was also shown that

small platinum metal particles are adsorbed at outerŽsurface of imogolite fibers Liz-Marzan and Philipse,´

. q1995 . So, it seems reasonable to assume that Ag

cations and Ag0 atoms produced radiolytically at

low temperature are located on the outer surface of

imogolite fibers. In hydrated samples Agq cations

are solvated by H O molecules. Thus, silver atoms2

generated by irradiation are able to react with H O2

molecules even at low temperature. This explains

why Ag0 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 chan-

nels 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 temper-

ature is very unique and to explain this effect we

postulate that some Agq cations upon dehydration

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 dehy-

drated samples but Ag 0 doublet is still seen at 230

K. It should be stressed however, that before expo-

sure 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 remark-

able ability of imogolite fibers to stabilize silveratoms 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.

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Electron Spin Resonance Studies on Silver Atoms in Imogolite Fibers