J. Phys. Chem. 1993,97,6943-6944
6943
129XeNMR Study of Small Sodium Particles in NaY Zeolite E. Trescos,+L. C. de MBnorval,'**and F. Rachdit Groupe de Dynamique des Phases Condenskes URA-233 CNRS, USTL Place E. Bataillon, 34095 Montpellier Ckdex 5, France, and Luboratoire de Chimie Organique Physique et Cinktique Chimique Appliqukes, URA-418 CNRS, ENSCM 8 rue de 1'Ecole Normale, 34053 Montpellier Ckdex 1, France Received: February 18, 1993: In Final Form: May 10,1993
The distribution of sodium metal particles inside the cavities of a NaY zeolite has been investigated using 129Xe NMR spectroscopy. For a sample prepared by vapor-phase deposition at 520 K, the 129XeN M R spectrum shows three lines which are interpreted in terms of domains of a nonuniformly distributed metal particles, oxidized particles, and empty cavities. After annealing the sample at 670 K, the lz9Xespectrum collapses to one single line, characteristic of a narrow particle size distribution. Oxidation of the sample by oxygen decreases the chemical shift of the 129XeN M R line, which is mainly due to a loss of paramagnetism and a volume contraction of the small sodium metal particles.
Introduction A zeolite is a useful matrix for producing quantum size sodium particles' due to its well-defined crystalline structure with an internal porous open framework of molecular dimensions. Martens et al.z have shown that these metal-loaded systems act as basic catalysts. Two different methods have been developed to obtain these systems: the vapor-phase deposition3and the thermal decomposition of the sodium azide.4 Using ESR measurements, Xu and KevanSshowed that there are no significant differences in the results obtained following the formation of the sodium particles by the two methods. Of particular interest is to obtain information about the size and the distribution of sodium particles inside the cavities of the zeolite. lz9Xe NMR is a powerful technique for the characterization of metal particles supported on zeolites."* The aim of this work is to apply this technique to gain insight into the size distribution of the particles and the homogeneity of the system.
Experimental Section NaY zeolite has been synthesized at the School of Chemistry in Montpellier using a standard proced~re.~ The zeolite was dehydrated in a gradient furnace at 750 K under vacuum ( Torr) overnight. The temperature was then lowered to 520 K, and the zeolite was exposed to sodiumvapor. After exposure, the color of the sample changed to bright red. The product was transferred to a 10" NMR tube equipped with a glass vessel stopcock and exposed to a given xenon pressure. lz9XeNMR spectra wereobtainedat room temperature with a AC250L Bruker spectrometeroperating at a frequencyof 69.19 MHz. Typically, 2000 signal acquisitions were accumulated for each spectrum with a recycle delay of 0.5 s between a / 2 pulses. Chemical shift measurements are precise to within 1 ppm and are expressed relative to xenon gas at zero pressure.1° They are considered as positive in this paper.
Results and Discussion Since the early work of Rabo et a1.,3 it is well known that two distinct species are formed in a NaY zeolite by exposure to sodium vapor: ionic clusters Na43+ in the sodalite cages and small metal particles in thesupercagecavities.' lZ9XeNMRisonlyaneffective probe for investigating the occupancy of the supercage cavities, since a xenon atom (atomic diameter = 0.44 nm) cannot enter
* To whom correspondence should he addressed.
t URA-233 CNRS,USTL.
XURA-418 CNRS, ENSCM.
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(PPm) Figure 1. lZ9XeNMR spectra obtained at room temperature and 400 Torr of xenon pressure on (a) a NaY zeolite loaded with sodium metal, (b) after annealing at 670 K,and (c, d, and e) during successiveadsorption of oxygen gas.
the sodalite cage (free aperture of -0.25 nm); hence, the ionic clusters will be indetectable. Figure l a displays the lz9XeNMR spectrum of xenon adsorbed at 400 Torr in the supercagecavities of the loaded sodium zeolite sample. This spectrum is composed of three resonance lines assigned todifferent domains in the zeolite sample. By domain, we understand a crystallite or a collection of crystallites joined together in a given part of the sample." The broader line (AH= 24 ppm) centered at 134 ppm is assigned to xenon atoms in domains of the zeolite sample loaded with
0022-3654/93/2097-6943S04.00/0 0 1993 American Chemical Society
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6944 The Journal of Physical Chemistry, Vol. 97, No. 27, 1993 160
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Xenon pressure (Torr) Figure 2. Variation in '29Xe chemical shift with the xenon pressure ( T = 293 K) in a N a Y zeolite loaded with sodium metal: ( 0 )domains where the supercage cavities are occupied by sodium metal particles; (A) domains of oxidized sodium metal particles; (X) domains without metal particles; (0) dehydrated N a Y zeolite considered as a reference for the chemical shift.
TABLE I sample
AH(ppm)
N a Y dehydrated Na-Nay" Na-Nayb Na-NayC Na-NaY annealed Na-NaY oxidized
1.5 f 0.3 24* 1
slope 6xCvs Pxe 6xCa t PxC = 0 (*l ppm) (ppm/Torr) k0.005 61
103 66 61
0.067 0.068 0.070 0.068
10 k 1 2.7 f 0.3
(1 For domains with sodium metal particles. With oxidized sodium metal particles. For empty cavities.
metal sodium particles. The narrow and least shifted line at -88 ppm is due to xenon adsorbed in unloaded sodium metal particles domains. We assign the intermediate line at -94 ppm to domains containing oxidized sodium metal particles, since the xenon gas used in our experiment was contaminated with a small amount of oxygen. This could be observed during the first adsorption of xenon gas by a decoloration in the top part of the sample. In Figure 2, we have plotted the lz9Xechemical shift of these three NMR lines as a function of xenon pressure. We also present in the same figure the pressure dependence of the chemical shift of Iz9Xeadsorbed on NaY zeolite, dehydrated at 750 K, for reference purposes. All chemical shifts plotted are linear functions with respect to the xenon pressure, with a slope of approximately 0.067 ppm per Torr (Table I). We can clearly observe t h a t the chemical shift of Iz9Xeof the least shifted line in Figure l a is equivalent to the reference chemical shift. No change in the slope for the broader line indicates that the diffusion of xenon atoms in the domain containing sodium particles is not affected by these particles.12 For the extrapolated chemical shift values for zero xenon pressure for the two lines correspondingto domains of loaded sodium particles and empty cavities, we observe a huge difference of 42 ppm. This difference is the fingerprint for the presence of small sodium particles in the supercage cavities of the zeolite and is due to the supplementary interaction of the xenon atoms with metal particles. Figure l b shows the 129XeNMR spectrum of the sample after annealing it at 670 K for 1 h and cooling to room temperature for several hours. We observe the disappearance of the lines attributed to domains of empty cavities and cavities containingoxidized particles. The resonance assigned
to domains of sodium metal particles becomes narrower with a line width of AH = 10 ppm, and its position is shifted upfield by about 15 ppm at a xenon pressure of 400 Torr. The line width of the broad line before annealing was mainly due to both a heterogeneous distribution of the sodium particles in the supercage cavities and a distribution of the local magnetic fields created by the unpaired electrons. These local magnetic fields are also responsible for the paramagnetic contribution to the chemical shift for the Iz9XeNMR line, which is averaged by the motion of the xenon atoms. After the annealing, the decrease observed in the chemical shift and the narrowing of the line can be understood by considering two mechanisms leading to a decrease of the average local magnetic field experienced by the xenon atoms and a homogenization of the particles to a smaller size. These are due to the occupancy of the empty cavities and the diffusion of a small quantity of sodium atoms out of the zeolite seen by the formation of a very thin mirror of sodium metal on the glass vessel. Following adsorptionof smallquantities of oxygen (Figure lc-e), we observe the appearance of a line at 97 ppm which increases strongly, while the intensity of the other line decreases. We also observed that the color at the top of the sample evolves to white as more and more oxygen was added. For a given quantity of oxygen, the line at 117 ppm completely disappeared and led to a maximum in the intensity of the 97 ppm line (Figure le). We assign this line to the domains of the fully oxidized particles in the zeolite cavities. The decrease in the chemical shift observed in Figure lc-e can be explained by the disappearance of the paramagnetism and a volume contraction of the oxidized particles compared to the volume of the same sodium metal particles. The narrowing of this line (AH= 2.7 ppm), which is approximately twice the line width observed for the xenon atoms adsorbed in the unloaded NaY zeolite, is also due to the disappearance of the paramagnetism and indicates that the oxidized particles are still homogeneouslydistributed in the cavities. Conclusion The results obtained in this work clearly indicate that Iz9Xe NMR of adsorbed xenon is a very sensitive useful probe for the investigation of the size distribution of quantum size sodium particles supported on supercage cavities of NaY zeolite. The precise knowledge of these parameters is of fundamental interest in the study of the quantum size effects in sodium particles by ESR and NMR studies under way in our laboratory. A complement to this work would be the extension to samples of different sodium loading. Acknowledgment. The authors are grateful to Dr. F. Fajula for providing the NaY zeolite and for many helpful discussions and G. Masson for glass vessels preparation.
References and Notes (1) Harrison, M. R.; Edwards, P. P.; Winowski, J.; Thomas, J. M.; Johnson, D. C.; Page, C. J. J. Solid State Chem. 1984, 54, 330. (2) Martens, L. R.; Vermeiren, W. J.; Huybreechts, D. R.; Grobet, P. J.; Jacobs, P. A. In Proceedings of the 9th International Congress on Catalysis, Calgary, 1988; p 420. (3) Rabo, J. A.; Angel, C. L.; Kasai, P. H.; Schomaker, V. Discuss. Faraday SOC.1968, 41, 328. (4) Martens, L. R.; Grobet, P. J.; Jacobs, P. A. Nature 1985,315,568. (5) Xu,B.; Kevan, L.J. Chem. SOC.,Faraday. Tram. 1991.87, 2843. ( 6 ) de Mbnorval, L. C.; Ito, T.; Fraissard, J. J . Chem. Soc., Faraday Trans. 1 1982, 78, 403. (7) Schoemaker, R.; Apple, T. J. Phys. Chem. 1987,91,4024. (8) Cho, S. J.; Jung, S. M.;Shul, Y.G.; Ryoo, R. J. Phys. Chem. 1992, 96, 9922.
(9) Barrer, R. M. In Hydrothermal Chemistry of Zeolites; Academic Press: London, 1982. (10) Jameson, A. K.; Jameson, C. J.; Gutowsky, H. S. J. Chem. Phys. 1970,53, 2310.
(1 1) Chmelka, B. F.; Pearson, J. G.; Liu, S.B.; Ryoo, R.; de MCnorval, L. C.; Pines, A. J. Phys. Chem. 1991, 95, 303. (12) de Mbnorval, L. C.; Raftery, D.; Liu, S. B.;Takegoshi, K.; Ryoo, R.; Pines, A. J. Phys. Chem. 1990, 94, 27.
