The Journal of
Physical Chemistry
0 Copyright, 1984, by the American Chemical Society
VOLUME 88, NUMBER 19
SEPTEMBER 13,1984
LETTERS Partially Zinc Ion-Exchanged K-A Zeolites as Molecular Sieves Distinguishing Oxygen from Nitrogen Masakazu Iwamoto,* Ken-ichiro Yamaguchi, Yoshiomi Akutagawa, and Shuichi Kagawa Department of Industrial Chemistry, Faculty of Engineering, Nagasaki University, Nagasaki 852, Japan (Received: March 27, 1984)
Adsorption behaviors of O2 and N, on partially zinc ion-exchanged K-A zeolites have been studied in the range 195-323 K and 0-101.3 Wa. When the degree of exchange of zinc ions was less than ca. 50%, the amounts of 0, adsorbed on ZnK-A zeolites were greater than those of N2 and both amounts increased with increasing adsorption temperature. Opposite adsorption behaviors were observed on ZnK-A with exchange levels above 70%, which were similar to those on Ca,-A. The ZnK-A having 50-60% exchange level showed intermediate properties between the above two groups of ZnK-A zeolites. The results have been interpreted by the change of the window size. With the exchange level below ca. 50%, it was presumably altered to the intermediate size between the molecular diameters of O2 and N2 due to the change of the electrostatic field in the zeolite through the exchange of potassium ions with zinc ions.
Introduction Although the molecular sieving character of A-type zeolites has been well-known, the window size can be changed only stepwise through exchange of cations blocking the windows, which (causeslimitations of the utilization of molecular sieving functions. For example, K12-A (potassium ion-exchanged A-type zeolite having a window size of approximately 3 A) and Na,,-A ( 4 A) (cannot separate oxygen from nitrogen because of their close :molecular sizes (kinetic diameters are on average 3.54 8,for 0, ,and 3.75 A for N2).'s2 That is, neither O2 nor N2 can pass through the windows of KI2-A, while the passage of both O2and N, through those of Na12-A is free. We wish here to report that ]partially zinc ion-exchanged K-A zeolites (ZnK-A) could adsorb (1) Koresh, J.; Soffer, A. J . Chem. Soc., Faraday Trans. 1 1980, 76,2457, 2472, 2507. (2) Breck, D. W. "Zeolite Molecular Sieves''; Wiley-Interscience: New York, 1974.
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a greater amount of oxygen than nitrogen and the dependencies of the adsorbed amounts on the adsorption temperature are quite different from those of NaI2-A and Ca6-A. The present results suggest that the window size of K-A is fine controlled upon exchange of potassium and zinc ions and is close to the molecular sizes of 0, and N2.
Experimental Section The mother zeolite of KI2-A powder was obtained from Union Carbide Co. (Lot. No. 340911 131) and treated with an aqueous KOH solution to ensure complete Kf exchange. Ion exchange was carried out at 353 K with a solution of ZnC12 by the method of Seff et aL3 and the sample was washed extensively at ambient temperature. The composition of the zeolite was determined by (3) Raghavan, N. V.;Seff, K. J . Phys. Chem. 1976,80,2133. Kim, Y . ; Seff, K. J . Phys. Chem. 1980,84, 2823. McCusker, L. B.; Seff, K. J . Phys. Chem. 1981, 85, 405.
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The Journal of Physical Chemistry, Vol. 88, No. 19, 1984
Letters
1-17 J2o0 N&-A I
L
SODALlTE
N2
02 02
/
ZnK-A-41
...._
,
1 2 3 “ 7 ADSORPTION TIME ( h )
8
A
_
-
-
N2
Figure 1. Adsorption of oxygen (a,c,e) and nitrogen (b,d,f) on various A-type zeolites at 273 K. The initial pressure of oxygen or nitrogen for adsorption were 57.3 kPa for ZnK-A-41 (a,b), 13.3 kPa for ZnK-A-58 (c,d), and 30.4 kPa for Na12-A (e$).
atomic adsorption spectrometry. Here a Zn2,46K7,08-A zeolite has been written as ZnK-A-41 where 41 indicates the exchange level of zinc ions. An uptake of sample gas could be followed by measuring the pressure drop in a static system. After being mounted in a sample tube, each zeolite sample (1 .O g) was slowly heated to 673 K. Subsequently, it was degassed at 673 K for 30 min, exposed to oxygen (1 3.3 P a ) for 30 min, and evacuated again for 30 min. Following this pretreatment, the sample was subjected to an adsorption experiment.
Results and Discussion Typical adsorption behaviors of 0, and N 2 on selected ZnK-A and Na,,-A zeolites at 273 K are shown in Figure 1. It has been clarified in the ZnK-A-28, -41, and -58 systems that adsorption of O2 and N z reached equilibrium for adsorption times greater than about 7 h and the adsorption was diffusion-limited. Figure 1 shows that the amount and rate of 0, adsorption over ZnK-A-41 were greater than those of N2 adsorption, whereas the amount of O2 adsorbed on ZnK-A-58 at equilibrium was smaller than that of N, though the adsorption rate of 0, was greater than that of N,. When the degree of exchange of zinc ion was greater than ca. 70%, both the amount and the rate of O2 adsorption were smaller than those of N2. The adsorption behaviors on Na12-A are also depicted in the figure for comparison. It was confirmed in separate experiments that the adsorption-desorption cycles of 0, and N 2 were fully reversible on all the zeolites used here. The amounts of 0, and N 2 adsorbed on the zeolites at equilibrium increased monotonicly with increasing adsorption pressure. For example, the amounts of 0, adsorbed on ZnK-A-41 at 273 K were 11.6, 24.4, and 40.8 pmol/g of zeolite at 23.6, 36.7, and 49.2 kPa of equilibrium pressure, and those of N 2 were 1.8, 4.6, and 6.1, pmol/g of zeolite at 23.7, 35.5, and 43.2 kPa. On the other hand, the dependence of their amounts on the adsorption temperature were characteristically changed with the exchange level of zinc ions. At an equilibrium pressure of 40.5 kPa, the amounts of 0, and N, adsorbed (in pmol/g of zeolite) on ZnKA-41 were 8.1 and 3.8 at 195 K, 28.9 and 5.7 at 273 K, 44.1 and 28.7 at 300 K, and 50.4 and 37.5 at 323 K, respectively. The ZnK-A-28 showed results similar to those of ZnK-A-41 except that the amounts of gases adsorbed were smaller than those on
Figure 2. Model for the effective change of window size in K-A zeolites upon exchange with ZnZt. A and B represent positions of the cations before and after the ion exchange, respectively.
the latter; 11.7 pmol of 0, and 1.8 pmol of N2 were adsorbed on 1.0 g of the former at 273 K and 40.5 kPa. In contrast, on ZnK-A-51 the repective amounts (in prnollg of zeolite) were 53.5 and 31.0 at 273 K, 63.0 and 50.0 a t 300 K, and 49.6 and 48.8 at 323 K. On Znk-A-58 they were 950 and 691 at 195 K, and 225 and 440 at 273 K. On ZnK-A-81 they were 869 and 1624 at 195 K, 125 and 290 at 273 K, and 80.2 and 165 at 300 K. The following characteristics can be pointed out from the above observations. With exchange levels of 28 and 41%, the amounts of O2 and N2 adsorbed both increased with the adsorption temperature, and that of O2 was always greater than that of N,, in the range 195-323 K. In the case of ZnK-A-81, the amount adsorbed decreased with temperature, and O2 adsorption was always less than N2. ZnK-A41 and -58 showed intermediate behavior. For comparison, it should be noted that K12-A adsorbs neither O2 nor N 2 owing to its narrow windows, while on Na12and Ca6-A zeolites, where both O2 and N 2 can freely pass the windows, more N, is adsorbed than O2in the range 195-323 K. The amounts of O2 and N2 adsorbed both decrease with increasing adsorption temperatures2 It is obvious that the adsorption behaviors on ZnK-A-28 and -41 are entirely different from those on Na12- and Ca6-A, but that on ZnK-A-81 is similar to those. Therefore, the preferential adsorption of O2 over N2 is not due to the chemical adsorption of O2 onto zinc ions exchanged into the zeolite lattice. This has been further supported by the observation that the adsorption behavior of a zinc ion-exchanged Na-A zeolite was essentially equal to that of Na,,-A. On the basis of the experimental and theoretical ~ o r k ,most ~,~ of divalent cations in a A-type zeolite dehydrated at 673-873 K occupy site I (six-membered oxygen ring sites). In contrast, K+ ions preferentially occupy site I1 (eight-membered oxygen ring sites, window). When the exchange level of the K+ ions by divalent ions exceeds 8/12 (67%), therefore, window with no blocking cations begin to appear2 and the zeolite shows adsorption behavior similar to that of Ca6-A. This is the reason for the adsorption properties of ZnK-A-81. We wish here to suggest the following model to explain the behaviors of ZnK-A-41 and -28. As shown in Figure 2, the K+ ion at site I1 blocking the 8-ring window moves slightly from the original position because of electrostatic repulsion between the K+ and Zn2+ions and electrostatic attraction between the K+ ion and the vacant site (having an anionic character) resulting from the exchange of one divalent ion for two monovalent ions. As a result, the effective window size is presumably increased to values close to the molecular diameters of 0, and N2. The model in Figure 2 is supported by X-ray diffraction analyses of Seff and
(4) For example, Seff, K. Acc. Chem. Res. 1974, 9, 121. Takaishi, T.; Yatsurugi, Y.; Usa, Y.; Kuratomi, T. J . Chem. SOC., Faraday Trans. 1 1975, 122, 1700. McCusker, L. B.; Seff, K. J . Am. Chem. SOC.1978, 100, 3091. Subramanian, V.; Seff, K. J . Phys. Chem. 1980, 84, 2928. Rees, L V. C.; Berry, T. A. “Molecular Sieves”; Society of Chemical Industry: London, 1968; p 149. Ogawa, K.; Natta, M.; Aomura, K. J . Phys. Chem. 1978, 82, 1655; 1979, 83, 1235; 1980, 84, 1061.
J . Phys. Chem. 1984, 88, 4197-4199 c o - ~ o r k e r s .It~ is clear that the increase in the amounts of O2 and N 2 adsorbed on ZnK-A-28 and -41 at higher adsorption temperatures is due to the thermal vibration of the windowblocking cation^.^-^
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The present results have great potential utility with regard to separations in the 02,N 2 system. They also indicate that the window size may be finely adjusted for other separations by selecting an appropriate pair of monovalent and divalent cations and determining the most effective level of exchange. I
(5) For example, Fraenkel, D.; Shabtai, J. J. Am. Chem. SOC.1977, 99, 7074. Fraenkel, D. J. Chem. SOC.,Faraday Trans. I 1981, 77,2029,2041; Chem. Tech. 1981,61. Itabashi, K.;Takaishi, T.; Ohgushi, T. Bull, Chem. SOC. Jpn. 1981,54,1943. Takaishi, T.; Kamei, Y.; Yusa, A.; Ohgushi, T. Bull. Chem. SOC.Jpn. 1981, 54, 48.
Acknowledgment. This work was supported in part by a Grant-in-Aid from the Ministry of Education, Science, and Culture of Japan and from the Mitsubishi Heavy Industry Ltd.
Evidence for Inversion Layer Formation at the p-GaAs/Persulfate Solution Interface under Strong Cathodic Bias: Electroluminescence Caused by Carrier Injection Kohei Uosaki* and Hideaki Kita Department of Chemistry, Faculty of Science, Hokkaido University, Sapporo 060, Japarl (Received: May 31, 1984)
Electroluminescence (EL) due to a band-to-band transition is observed at negatively biased p-GaAs in solutions containing the persulfate ion (S,Ogz-). The intensity of EL is stronger at p-GaAs electrodes with higher carrier concentrations. These results prove inversion layer formation. Under inversion conditions the electrons in the conduction band recombine radiatively with the holes injected into the valence band by SzOg2-and/or SO4-. in the solution.
Introduction When a p-type semiconductor in solution is cathodically biased very strongly, either a deep depletion or an inversion layer is formed within the semiconductor.' Inversion layer formation has a significant effect on the electron transfer kinetics at the semiconductor/solution interface2 and it is very important to probe the inversion layer formation. Several methods have been applied for this p ~ r p o s e . ~ Electroluminescence (EL) at the semiconductor/electrolyte interface is proved to be useful in understanding the electrochemical reaction mechanism at semiconductor electrodes4 and is usually caused by minority carrier injection: Le., hole injection to the valence band of n-type semiconductors and electron injection to the conduction band of p-type semiconductors, although no report on electroluminescence of this kind at ptype semiconductors in solution is available.6 If the inversion layer is formed at a ptype semiconductor surface, the EL caused by the hole injection should be observed. In this paper, we report EL due to a band-to-band transition at the p-GaAs/persulfate solution interface for the first time at any p-type semiconductor/electrolyte interface, proving (1) S. R. Morrison, "Electrochemistry at Semiconductor and Oxidized Metal Electrodes", Plenum Press, New York, 1980. (2) J. A. Turner, J. Manassen, and A. J. Nozik, ACSSymp. Ser., No. 146, 253 (1981). (3) (a) D. J. Benard and P. Handler, Surf. Sci., 40, 141 (1973). (b) W. Kautek and H. Gerischer, Eer. Eunsenges. Phys. Chem., 84,645 (1980). (c) C. D. Jaeger, H. Gerischer, and W. Kautek, ibid., 86, 20 (1982). (4) (a) H. H. Streckert, B. R. Karas, D. J. Morano, and A. B. Ellis, J. Phys. Chem., 84, 3232 (1980). (b) H. H. Streckert, J. Tong, and A. B. Ellis, J. Am. Chem. SOC.,104, 581 (1982). (c) H. Gobrecht, M. Schaldach, F. Hein, and W. Paatsch, Electrochim. Acta, 13, 1279 (1968). (d) R. Memming and G. Schwandt, Electrochim. Acta, 13, 1299 (1968). (e) K. H. Beckmann and R. Memming, J. Electrochem. SOC., 116, 368 (1968). ( f )B. Pettinger, H.-R. Schoppel, T. Yokoyama, and H. Gerischer, Eer. Eunsenges. Phys. Chem., 78, 1024 (1974). (8) T. Yamase and H. Gerischer, ibid., 87, 349 (1983). (h) R. W. Noufi, P. A. Kohl, S. N. Frank, and A. J. Bard, J. Electrochem. SOC., 125, 246 (1978). (j)F.-R. Fan, P. Leempoel, and A. J. Bard, J. Electrochem. SOC.,130, 1866 (1983). ( 5 ) H. Gerischer, J. Elecrrochem. Soc., 125, 218C (1978). (6) E. Aharon-Shalom and A. Heller, J. Phys. Chem., 87, 4913 (1983).
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TABLE I: Properties of p-GaAs mobility/ cm2.V-l.
sample
dopant
1
Zn Zn
2 3
Cd
acceptor
specific
density/cm-)
resistance/Q.cm
S-1
0.0051 1 0.0236 0.176
64.3 112.7
1.09
x ioi9
2.35 X lo1* 2.3 X 10"
149
the formation of the inversion layer. Experimental Section Materials. p-GaAs single crystal wafers were obtained from Morgan Semiconductor, Inc., and their properties are summarized in Table I. Ohmic contacts to the crystals were made by using an In-Zn alloy. The procedure for mounting electrodes was similar to that reported before.' The electrode surface was etched in HNO,-HCl (1:l) before each experiment. Reagent grade NaOH and K2S20sobtained from Wako Pure Chemicals Co. Ltd. and water purified by the Milli-Q water purification system (Millipore Corp. Ltd.) were used to prepare electrolyte solutions. Electrochemical and Luminescent Measurements. An ordinary two-compartment cell with a flat window was used for all the measurements. The potential of the electrode was controlled by a potentiostat (Hokuto Denko, HA-301) with respect to a Ag/ AgCl reference electrode. A large Pt foil which surrounded a GaAs working electrode was used as a counterelectrode. The EL intensity was monitored with a photomultiplier (Hamamatsu Photonics Co. Ltd., R406), having a response similar to S-1. Either a programable function generator or a 12-bit D/A converter controlled by a personal computer (Nippon Electric Co. Ltd., PC-8801) was used to provide external potential to the potentiostat. The current and EL response were recorded on an X-Y-t recorder (Rika Denki Co. Ltd., RW-11T). A monochromator (Ritsu Oyo Kogaku Co. Ltd., MC-20N), having a grating with 1200 grooves/nm blazed at 0.75 km, which was controlled by the personal computer via P I 0 interface, was used to obtain emission (7) K. Uosaki and H. Kita, J. Electrochem. Soc., 128, 2153 (1981)
0 1984 American Chemical Society
