ESR on silver clusters in zeolite a

893

Surface Science 156 (1985) 893-898 North-Holland, Amsterdam

ESR ON SILVER CLUSTERS IN ZEOLITE A P.J. GROBET

* and R.A. SCHOONHEYDT

Katholieke Universiteit Leuven, Laboratorium B - 3030 Leuven (Heverlee), Belgium Received

*

voor Opperulaktechemie,

Kardinanl

Mercierlaan

92,

26 July 1984

ESR results on silver cluster formation in zeolite A with different loadings are presented. Depending on the reduction degree hyperfine spectra with seven and nine lines and two types of conduction electron spin resonance lines are detected. These signals could be assigned to the evolution in silver cluster formation in and outside the zeolite structure. Zeolites are good supports to prove the experimental evidence of the quantum size effect in small metal particles.

1. Introduction Zeolites loaded with metals, especially noble metals, are known as important catalysts for industrial processes [l]. By their high cation exchange capacity it is indeed possible to incorporate various metals in zeolite structure dispersed as single ions which can be reduced, by molecular hydrogen for example, to highly dispersed metal particles. In order to make very efficient use of these metal-loaded zeolites it is necessary to have an idea about (i) the dimension of the metal clusters, (ii) the location of the metal clusters inside the zeolite structure, and (iii) the reduction mechanism and metal cluster mobility. The study of the metal particle nature in zeolites has been done by X-ray [2], optical diffuse reflectance spectroscopy [3], fluorescence emission excitation [4] or temperature programmed ad- and de-sorption [5]. In this paper we wish to present ESR measurements on silver particles in zeolite A. There are several reasons to choose this silver support system. Starting from NaA zeolite it is very easy to exchange the sodium ion by silver in any stoichiometric amount [6]. The reduction of the silver ions is simply performed by reaction with molecular hydrogen; doing the reduction at room temperature the formation of silver particles inside the zeolite is expected [7]. Hermerschmidt and Haul [8] confirmed unambiguously the presence of Ag,“+ clusters and submicroscopic silver crystallites by preliminary in-situ ESR

* Senior Research

Associate

of the National

Fund for Scientific

0039-6028/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

Research

B.V.

of Belgium.

894

P..J. Grobei, R.A. Schoonheydt / Ag clusters in reolite A

measurements on AgA and AgX zeolites during the reduction process. The cage structure of zeolite A [6] consists of truncated octahedra (P-cages) of SiO, or AIO, tetrahedra with a free diameter of 0.66 nm which enclose cavities (a-cages) with a free diameter of 1.14 nm. The octahedra are also linked to each other by smaller cubic cavities (double four-rings). The silver ions are located in the CX-and P-cages and after reduction silver clusters are expected inside these cavities depending on the accessibility of molecular hydrogen into the cages. By a moderate reduction and before further sintering the size of these clusters is restricted by the dimensions of the free space inside the cages. The rather well-defined shaping of metal clusters by the zeolite structure is also of fundamental interest for the study of the physical and chemical properties of small metal particles. Some theoretical predictions about the quantum size [9,10] for instance can be verified in much better experimental conditions.

2. Experimental The anhydrous unit-cell composition Linde Division) samples are: NaIOAgz(AQ

)rZ(SiOZ II2

of our NaAg,A

or

NaAg,A,

Na,Ag,(AlG,),,(SiG,),,

or

NaAg,A,

AgI~(AIG~)I,(SiG~),,

or

AgA.

zeolite (Union

Carbide,

The sodium-silver ratios are estimated from synthesis which was performed by aqueous ion exchange followed by washing and drying in the dark. The dehydration of the samples is done by heating in vacuum or in a helium gas flow. The reduction of the samples is performed by a pulse method or a flow method. The pulse reduction method consists of the admission of molecular hydrogen at a certain pressure (3 mbar-1 bar) in the sample holder during a restricted time interval; after this activation, the hydrogen gas is pumped off and replaced by 1 bar helium. When severe reduction of the samples was needed a flow of hydrogen gas (1 cm3/s) was used in the sample holder. The ESR measurements are performed in a Bruker spectrometer (ER 200 D-SRC) at 9.5 GHz and room temperature. The measurements and sample manipulations are done in complete darkness.

3. Results The results are qualitatively the same for the different silver-loaded samples. In all samples two ESR Fe 3+ lines are present with g factors 4.3 and 2.2 (fig.

P.J. Grobet, R.A. Schoonheydt

/ Ag clusters in zeolite A

895

Fig. 1. The ESR spectrum of Fe 3+ in unreduced AgA zeolite; g,, is a free radical signal.

1) which can be attributed to framework iron sites and precipitated iron impurities [ll]; a g= 2.1 exchangeable Fe3+ line is also present but is sometimes hidden by other ESR lines. After thermal dehydration in vacuum or in a helium flow (till 700 K) some samples show also a hyperfine splitted Mn2+ spectrum; all samples show after this treatment a strong free radical signal which can be burned out by an oxygen flow treatment for several hours at about 700 K. During the dehydration process the samples change their colour from white to yellow (500 K) and finally to brick red at 700 K. We detected

Fig. 2. The spectra of Api+ reduction.

clusters

in reduced

NaAg,A;

(a) mild reduction,

(b) stronger

896

P.J. Grobet, R.A. Schoonheydt

/ Ag clusters in reolite A

weak ESR signals of reduced silver ions but the observation is hindered by the presence of the abovementioned impurity signals. By gradually reducing the samples at room temperature four kinds of ESR silver cluster lines are successively seen. In the first stage we observe (fig. 2a) a growing seven-line spectrum for all samples (g = 2.025 f 0.003). The observed hyperfine splitting (a = 6.76 & 0.07 mT) can be assigned to the interaction of an unpaired electron with six equivalent silver nuclei (Z = l/2). The isotope effect is not resolved due to the small coupling constant and the large linewidth. This spectrum indicates the presence of charged Aggf clusters; no information on the charge of the clusters is obtained from these ESR measurements (x = 5, 3 or 1). This result is consistent with the findings of Hermerschmidt et al. [S] although some differences in g factor and hyperfine coupling constant a are found. By further reducing the samples the hyperfine splitted spectrum disappeared and a second type of silver cluster signal is observed (fig. 3a). This single line (g = 2.011 f 0.003) has an asymmetric shape and is assigned to conduction electron spin resonance (CESR) in silver crystallines. This CESR signal grows first and then decreases. The g factor remains constant but the asymmetry of the lineshape changes somewhat. Superimposed on this signal a weak nine-line hyperfine spectrum is seen in a further reduction stage (g = 2.025 + 0.005; a = 5.2 f 0.4 mT; fig. 3b). The deformed base line in fig. 3b is due to a broad signal which grows by still further reduction at the expense of the other signals. It results into a signal seen in fig. 4; this second CESR line has a linewidth of 10 mT and the g factor equals 1.981 + 0.003. It vanishes when the reduction goes on. This line was also seen by Hermerschmidt et al. but the CESR line of , g= 2.011 to.003

1

1OmT

b

9=2.025'0.005 a=5.2?0.4mT

Fig. 3. Conduction electron spin resonance signals of silver crystallites and a signal of A&+ clusters in different stages of reduced NaAg,A.

silver

897

P.J. Grobet, R.A. Schoonheydt / Ag clusters in reolite A

g= 1.§81+0.003

1

J

1OmT

Fig. 4. The conduction

electron

spin resonance

in severely reduced

AgA (1 cm3/s

H,

flow, 72 h).

g = 2.011 and the nine-line spectrum was not observed by these authors. It is important to notice that the four signals due to different reduction stages of the silver ions in zeolite A could be stabilized by pumping off the samples and keeping them under helium atmosphere. In contrast to this, He~ersch~dt et al. detected their signals as a function of time while their samples were kept under a constant hydrogen pressure.

4. Discussion These ESR experiments prove that during the reduction of silver-loaded zeolite A certain stages of cluster formation can be followed. The presence of Ag;+ and Ag, J+ clusters which fit in the sodalite cage are detected. During the reduction process the charge of these clusters (dimension around 0.7 nm) decreases and the hyperfine spectra vanish when Ag,” or Agz clusters are formed since only intermediates with an odd number of electrons contribute to the ESR signal. Once the clusters are no longer charged the silver atoms become mobile and leave the sodalite cage to form probably bigger clusters in the a-cage. The narrow CESR signal is assigned to this kind of clusters which exhibit a wider cluster size range (dimensions around 1 nm) giving rise to an asymmetric lineshape. The broad CESR signal is coming from still larger crystallites probably located at the boundaries of the zeolite particles. From the presence of this CESR signal at room temperature Hermerschmidt et al. estimated a cluster size of about 5 nm [S]. Metal clusters with a size of 10 nm or more give rise to very broad ESR signals which cannot be observed at room temperature. The appearance of these CESR signals at room temperature is explained by the quantum size effect; the influence of this effect on the ESR parameters is described by Kawabata [9] and Buttet et al. [lo]. As to the linewidth, Kawabata predicts a quadratic dependence on the size of the metal cluster; this can explain the broader linewidth of the second CESR line (10 mT) with

898

P.J. Grobet, R.A. Schoonheydr / Ag clusters in zeoiite A

respect to the first narrow CESR line (2 mT). Our results show a larger g value as the particle size decreases which is in contradiction with the Kawabata theory (predicting lower g values) (91. The g value of the broad CESR line (fig. 4) is exactly the bulk silver value of 1.983 [12,13]; the g value of the narrow signal (fig. 3) is in accord with that of Bore1 [14] on silver particles of 2 nm in solid C,H, and CO,. Furter studies on the quantum size effect of small metal particles are needed to obtain a better insight into the behaviour of the linewidth and g value of the ESR signals; zeolites are certainly good supports to give valuable experimental results for the verification of the quantum size effect.

References [l] J.B. Uytterhoeven, Acta Phys. Chem. 24 (1978) 53. [2] L.R. Gellens, W.J. Mortier, R.A. Schoonheydt and J.B. Uytterhoeven, J. Phys. Chem. 85 (1981) 2783. [3] R. Kellerman and J. Texter, J. Chem. Phys. 70 (1979) 1562. [4] G.A. Ozin et al., in: Intrazeohte Chemistry, Eds. G.D. Stucky and F.G. Dweyer, ACS Symp. Series 218 (1983) p. 409. [S] P.A. Jacobs, J.B. Uytterhoeven and H.K. Beyer, J. Chem. Sot. Faraday Trans. I. 75 (1979) 56. [6] D.W. Breck, in: Zeolite Molecular Sieves (Wiley, New York. 1973). [7] P.A. Jacobs, in: Metal Microstructures in Zeohtes, Eds. P.A. Jacobs, N.I. Jaeger, P. Jiru and G. Schulz-Ekloff (Elsevier, Amsterdam, 1982) p. 71. [S] D. Hermerschmidt and R. Haul, Ber. Bunsenges. Physik. Chem. 84 (1980) 902. [9] A. Kawabata, J. Phys. Sot. Japan 29 (1970) 902. [lo] J. Butte& R. Car and C.W. Myles, Phys. Rev. B26 (1982) 2414. [II] E.G. Derouane, M. Mestdagh and L. Vielvoye, J. Catalysis 33 (1974) 169. [12] S. Schuitz, M.R. Shanabarger and P.M. Platzman, Phys. Rev. Letters 19 (1967) 749. [13] F. Beuneu and P. Monod, Phys. Rev. B13 (1976) 3424. 1141 J.P. Bore], Phys. Letters A57 (1976) 253.