J. Phys. Chem. 1992, 96, 5059-5063 mechanism also prevailed for DPBF degradation on the airequilibrated surfaces of ZnO and Ti02. However, in degassed samples, semiconductor supports such as TiOz and ZnO promote the photodegradation by directly participating in the chargetransfer process. The intrinsic properties of the support material are important in guiding the course of a photochemical reaction. The present study also highlights the importance of photochemical studies on solid surfaces for degrading environmentally hazardous organic contaminants.
Acknowledgment. The work described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. This is Contribution No. NDRL-3402 from the Notre Dame Radiation Laboratory. We would like to thank Mr. Ulick Stafford for his help in measuring the surface area of ZnO powder. Registry No. DPBF, 5471-63-6; AZO,,1344-28-1; Ti02, 13463-67-7; ZnO, 1314-13-2; 02,7782-44-7.
References and Notes (1) Nicholson. W. J.: Moore. J. A. Ann. N.Y. Acad. Sci. 1979. 320. 1. (2) Moore, J. A.; Mkonnel1,’E. E.; Dalgard, D. W.; Harris, M. W. Ann. N.Y. Acad. Sci. 1979, 320, 151. (3) Poland, A,; Greenlee, W. F.; Kende, S. A. S. Ann. N.Y. Acad. Sci. 1979.320,214. (4) Poland, A.; Glover, E.; Kende, A. S . J. Biol. Chem. 1976,251,4936. ( 5 ) Czuczwa, J.; Niessen, F.; Hites, R. A. Chemosphere 1985, 14, 1175. (6) Hutzinger, 0.;Clumich, M. J.; Berg, M. V. D.; Olie, K. Chemosphere 1984, 16, 581. (7) Rappe, C.; Marklund, S.; Buser, H. R.; Bosshardt, M. Chemosphere 1978, 7 , 269. (8) (a) Hygran, N.; Rappe, C.; Lindstrom, G.;Hansson, M.; Bergquist, P. A,; Marklund, S.;Donnellof, L.; Hardell, L.; Olsen, M. In Chlorinated Dioxin and Dibenzofurans in the Total Environment;Rappe, C., Chowdhary, G.,Keith, L., Eds.;Lewis Publishers: Chelsea, MI, 1986; Vol. 111, pp 17-34. (b) Hites, R. A. Acc. Chem. Res. 1990, 23, 194.
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(9) Tieman, T. 0.;Wagel, D. J.; Vanness, G. F.; Garrett, J. H.; Solch, J. G.; Rogers, C. Chemosphere 1989, 19, 573. (IO) (a) Zepp, R. G.;Schlotzhauer, P. F.; Sink, R. M. Environ. Sci. Technol. 1985,19,74. (b) Zepp, R. G. In Humic Substances and Their Role in the Emiromtent; Frimmel, F. H., Christman, R. F., Eds.; John Wilcy and Sons: New York, 1988; pp 193-214. (1 1) (a) Pelizetti, E., Schiavello, M., Eds.Photochemical Conversionand Storage of Solar Energy; Kluwer Academic Publishers: Dordrecht, Netherlands, 1991. (b) Ollis, D. F.; Turchi, C. Environ. Prog. 1990, 9, 229. (12) Kormann, C.; Bahnemann, D. W.; Hoffman, M. R. Environ. Sci. Technol. 1991, 25, 494. (13) Mathews, R. W. J. Coral. 1988, 3, 264. (14) (a) AI-Ekabi, H.; Serpone, N. J. Phys. Chem. 1988, 92, 5726. (b) Serpone, N.; Borgarello, E.; Harris, R.; Cahill, P.; Borgarello, M. Solar Energy Mater. 1986, 14, 121. (15) Menassa, P. E.; Mak, M. K. S.; Langford, C. H. Environ. Technol.
Lett. 1988, 9, 825. (16) Gopidas, K. R.; Kamat, P. V. J. Phys. Chem. 1989,93,6428. (17) Lala, D.; Rabek, J. R.; Ranby, B. Eur. Polym. J. 1980. 16, 735. (18) Stevens, B.; On,J. A.; Christy, C. N. J. Phys. Chem. 1981,85,210. (19) Stevens, B.; Small, R. D., Jr. Chem. Phys. Lett. 1979, 61, 233. (20) Vinodgopal, K.; Kamat, P. V. J . Photochem. Photobiol., A 1992,63, 119. (21) Oelkrug, D.; Hemming, W.; Fiillerman, R.; Giintha, R.; Honner, W.; Krabichler, G.;Schafer, M.; Uhl, S. Pure Appl. Chem. 1986, 58, 1207. (22) Kessler, R. W.; Krabichler, S.;Uhl, S.;Oelkrug, D.; Hagan, W. D.; Hyslop, J.; Wilkinson, F. Opt. Acta 1983, 30, 1099. (23) Stone, F. S. In Surface Properties and Catalysis by Nonmetals; Bonelle, J. P., Ed.; Reidel: Dordrecht, 1983; pp 237-272. (24) Ford, W. E.; Hiratsuka, H.; Kamat, P. V. J. Phys. Chem. 1989,93, 6692. (25) Gorman, A. A,; Rodgers, M. A. J. Chem. Soc. Reu. 1981,10,205. (26) Kamat, P. V.; Gopidas, K.R.; Weir, D. Chem. Phys. Lett. 1988,149, 491. (27) Gopidas, K. R.; Kamat, P. V.; George, M. V. Mol. Cryst. Liq. Cryst. 1990,183,403. (28) Rothenberger, G.;Moser, J.; Grltzel, M.; Serpone, N.; Sharma, D. K. J. Am. Chem. Soc. 1985, 107, 8054. (29) Gopidas, K. R.; Bohorquez, M.; Kamat, P. V. J. Phys. Chem. 1990, 94. 6435.
Direct Observation of Intracrystalline Transport Diffusion in Zeolites by Xenon-I 29 NMR Jorg Kiirger,* Harry Heifer, Thomas Wutscherk, Fachbereich Physik der Universitat Leipzig, Linnbtrasse 5, D- 0-7010 Leipzig, Federal Republic of Germany
Stefan Emst, Jens Weitkamp, Imtitut f i r Technische Chemie der Universitat Stuttgart, Pfaffenwaldring 55, D- W-7000 Stuttgart, Federal Republic of Germany
and Jacques Fraissard Laboratoire de Chimie des Surfaces, AssociC au CNRS, URA 1428, UniversitC Pierre et Marie Curie, 4, Place Jussieu. F- 75230 Paris, France (Received: December 18. 1991)
Xenon-129 NMR is used to determine intracrystalline concentration profiles during sorption experiments. In this way, a direct discrimination between the limiting cases of barrier-controlled and diffusion-controlled uptake becomes possible. For molecular uptake is found to be controlled by intracrystalline diffusion. The determined value benzene in zeolite ZSM-5, m2 s-* at room temperature is in satisfactory agreement with the results of conventional uptake experiments. of D, = 1.3 X
Introduction The differences between the data for intracrystalline molecular diffusion in zeolites, as resulting from different experimental techniques have attracted the interest of researchers of various branches of science.l.* Generally, methods applied under equilibrium conditions (pulsed field gradient (PFG) NMR s ~ e c t r ~ p y l quasielastic 3-~ neutron scattering,6q7and molecular dynamics simulations-10)have led to coinciding results. There is little doubt, therefore, that in this way the correct coefficient of intracrystalline self-diffusion, D, is measured. This quantity 0022-3654/92/2096-5059$O3
may be introduced either by Einstein’s relation ( $ ( t ) ) = 6Dt (1) as a factor of proportionality between the mean square displacement and the observation time, or by Fick’s first law j * = -D grad c* (2)
as a factor of proportionality between the flux density of labeled molecules and their concentration gradient, where the mobilities of the labeled and unlabeled molecules are assumed to be equal,
.OO/O 0 1992 American Chemical Society
5060 The Journal of Physical Chemistry, Vol. 96, No. 12, 1992 and where the total concentration, Le., the sum of the concentrations of the labeled and unlabeled molecules, is constant. The diffusivity under nonequilibrium conditions, Le., the coefficient of transport diffusion D,,is defined by the nonequilibrium version of Fick’s first law j = -D,grad c
(3)
where now, in contrast to eq 2, the total flux density and the gradient of the total concentration are considered. Due to the different physical situation during the measurement, the coefficients of self-diffusion (D)and transport diffusion (0,) cannot be expected to be identical. However, being based on the same elementary processes of molecular motion, transport and selfdiffusion should at least be correlated to each other, being identical in the limit of small concentrations. It turns out, however, that in many cases the transport diffusivities are much smaller than In contrast expected on the basis of the self-diffusion to the above-mentioned methods to study molecular motion under equilibrium conditions, the techniques that have been applied up till now to measure transport diffusivities (adsorption/desorption,’,” permeation,12 and frequency response’’ measurements) are macroscopic; i.e., the directly measured quantities are the total concentrations of the molecules in the adsorbed and/or gaseous phases, and the time dependence of these concentrations, rather than the intracrystalline fluxes and concentration profiles. The determination of intracrvstalline transmrt diffusivities must therefore be based on mod& of molecula; mass transfer between the adsorbent and the gas phase, and it cannot be excluded that differences between the results of equilibrium and nonequilibrium measurements are S i p l y caused by the fact that the overall uptake process is controlled by influences different from intracrystalline diffusion. The direct measurement of intracrystalline concentration profiles and mass transfer under nonequilibrium conditions could therefore substantially contribute to a correct correlation between equilibrium and nonequilibrium measurements of molecular diffusion in microporous adsorbents. As a sensitive tool for studying adsorbate-adsorbent interact i o n ~ , ’ ~xenon-1 * ’ ~ 29 NMR spectroscopy has been used to characterize the local structure and properties of zeolites16and of other microporous materials.” Since the chemical shift of xenon- 129 NMR depends also on the nature and the concentration of molecules adsorbed in addition to xenon within the zeolite structure,’6,18most recently xenon-129 NMR has also been applied to map the sorbate concentration within macroscopic NMR samples.1F21 Considering the adsorption/desorption process of benzene in zeolite ZSM-5, in the present communication the prospects and limitations of xenon-1 29 N M R spectroscopy for a direct measurement of intracrystalline transport diffusivities will be discussed.
A Method To Study Intracrystalline Transport Diffusion by Xenon-129 NMR One of the main problems of the measurement of transport diffusion in traditional sorption experiments is that the uptake may be controlled by a variety of processes different from intracrystalline diffusion, as e.g., the penetration of the adsorbate into the adsorption vessel (“valve” the permeation of the adsorbate through the bed of ~rystallites,’~~ and the dissipation of spectroscopic of the heat of a d ~ ~ r p t i o n .It~is~an ~ ~advantage -~ techniques like PFG NMR,3-5 quasielastic neutron s~attering,~.’ and IR spectroscopy26that they are able to record directly the intrinsic processes within the sample. In spectroscopic experiments it is therefore possible to circumvent the above-mentioned complications by simultaneously observing adsorption and desorption within the same bed of crystallites. For this purpose we have applied the following procedure: The zeolite material is prepared in two fractions of identical quantity: one portion (fraction A) as a loose assemblage within the sealed NMR sample tube, loaded with benzene and xenon, and a second portion (fraction B) of unloaded zeolite contained in a glass vial, which is also located in the sealed NMR sample tube, together with a striker. The adsorption/desorption process is initiated by
Karger et al. crushing the glass vial and vigorously shaking the sample in order to obtain an intimate mixture of the loaded and unloaded crystallites. It is assumed that both the mixing of the crystallites and the distribution of the xenon over the whole sample are accomplished within a time interval sufficiently short in comparison to adsorption and desorption on the individual crystallites. For demonstrating the principle of the method, in the following it is further assumed for simplicity that (i) neither the transport nor the adsorption properties of the adsorbate molecules under study (benzene) are affected by the probe (xenon), (ii) the adsorbate concentration in the gas phase is negligibly small in comparison with the adsorbed phase, and (iii) the adsorbate mobility is independent of concentration. Under these conditions and assuming diffusion-controlled adsorption/desorption with the transport diffusivity D,,the concentration profile within the individual crystallites is given by the relation2’
where the plus and minus signs refer to the case of adsorption and desorption, respectively, and where the crystallites are assumed to be of spherical shape with radius R. In the other limiting case of bamer-controlled sorption, i.e., for intracrystalline diffusivities high enough to guarantee uniform concentration over the individual crystals, one has c ( r , t ) / c , = c ( t ) / c , = 1 ‘F exp(-(at/3R))
(5)
where 1 / a denotes the strength of the surface barrier (surface film resistance’Vz8). Figure 1 shows the time dependence of the concentration profiles within a pair of crystallites as given by eqs 4 and 5. Under the assumption that xenon-129 NMR is able to probe the real local concentration c, the time dependence of the xenon-129 NMR spectrum results as the weighted mean
I(w)= JZ(W,C)
p(ctr) dc
(6)
over the individual NMR signals Z(w,c) corresponding to a concentration c of the adsorbate. p(c,t) dc denotes the fraction of the zeolite bulk phase loaded at time t with the adsorbate of concentration c to c + dc. The quantity p(c,t) is related to the intracrystalline concentration profile function c(r,t) by the expression p(c,t) = ~ ~ ( c , t ) / f R 3
(7)
where r(c,t) is the inverse function of c(r,t). Figure 2 shows the time dependence of the xenon- 129 NMR spectra calculated for the two limiting cases of diffusion- and barrier-controlled sorption (eqs 4 and 5), assuming that the xenon concentration is unaffected by the adsorbate and that Z(w,c) is given by a simple Lorentzian with a chemical shift proportional to the sorbate concentration and a line width independent of concentration. The line width has been set equal to one-fifth of the initial line separation. It turns out that the nature of the controlling mechanism during molecular uptake is distinctly reflected in the evolution of the xenon-129 N M R spectra: Corresponding to the fact that for barrier-controlled adsorption/desorption the intracrystalline concentration may assume only two values (Figure 1b), the distinction between the two lines is preserved over nearly the whole process (Figure 2b), while for diffusion-controlled adsorption/ desorption the wide range of intracrystalline concentrations (Figure la) leads to a rapid coalescence of the two lines (Figure 2a).
The Study of Transport Diffusion of Benzene in Zeolite ZSM-5 The zeolite crystallites of type ZSM-5 were synthesized by using TPA as a template amrding to the procedure described by Miiller and Unger29 and had a size of about 30 X 30 X 100 pm. To remove the template, they were calcined over 24 h at a temperature of 600 OC. Before the measurements, the samples were activated over 12 h at 400 O C at a pressure of less than 1C2Pa. Afterwards,
The Journal of Physical Chemistry, Vol. 96, NO. 12, 1992 5061
Intracrystalline Transport in Zeolites
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