April, 1963
FoILs
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POLYSTYRENESULFONIC ACIDAND ITS SALTS
ent work, the results obtained are in qualitative agreement with those of Benesi, apart from the high acid strength observed with the alumina. By means of the indicator method, Ballou, et aL,* also have found that alumina has high acid strength only when the adsorbed water vapor is completely eliminated. Such a high acid strength of alumina would be expected from the results of the desorption method described above, but there seems to be some disagreement between the conclusions obtained from the (desorption and indicator methods. From the result in Table I, it is concluded that almost all of the acid sites on silica-alumina are very strong, since essentially the same acidity value has been obtained at different pKa values. On the other hand, the desorption method indicates the presence of weak as (8) E. V. Ballou, R. T. Elarth, and R. 4.Flinn, J . Phys. Chem., 66, 1639 (1961).
771
well as strong acid sites on silica-alumina as seen in Fig. 1. The reason for such discrepancy is not apparent. It is a well known fact that the Benesi method is based upon the following assumption: the acid strength on solid catalyst can be discussed in terms of the H o function or pK, value of indicators, both referring to homogeneous media, in spite of the fact that the acidity is determined from the color change of the indicator adsorbed on solid catalysts. Further work is necessary to check whether such an assumption is indeed true. Acknowledgments.-It is a pleasure for the author to acknowledge Professor P. H. Emmett of Johns Hopkins University and Professor 0. Toyama of University of Osaka Prefecture, to whom he is indebted for valuable discussion and encouragement in this work. The author also wishes to thank Mr. K. Togano and Mr. S. Kobayashi for their assistance.
FOIILS OF POLYSTYRENESULFONIC ACID AND ITS SALTS. VIII. LOIT-TEMPERATURE INVESTIGATION OF THE INFRARED CONTINUOUS ABSORPTION SPECTRUM OF AQUEOUS ACID SOLUTIONS BY G. ZUNDELAND G.-M. SCHWAB Institute for Physical Chemistry, the LTniversity, Munich, Germany Received August 37, 1962 I n studying aqueous acid solutions, a continuous absorption spectrum is observed in the infrared. The present work discuse,esthe question of whether this continuous absorption is connected with thermal vibrations in the hydrate complexes, or whether the high exchange probability of the excess proton in these hydrate complexes is a necessary condition for the existence of the absorption continuum. To decide this, we investigated polystyrenesulfonic acid a t 9O’K. by infrared spectroscopy. It was found that the continuous absorption is a t least as strong a t 90°K. as at 298°K. From this it follows that the continuous absorption has no connection with the thermal vibrations in the complexes. Finally, the high exchange probability of the excess proton is discussed as a cause for the existence of the continuum.
Continuous absorption has been observed in the infrared spectra of aqueous acid solutions, for the first time in the higher frequency regions as early as 1933 by Suhrmann and Breyerl and later by Meerlender2; in the lower frequency regions it was observed by Falk and Gigu&re3and by Ackermann4and later discussed by Wicke, Eigen and A ~ k e r m a n n Eigen ,~ and de Maeyer6 and A ~ k e r m a n n . ~ We have investigated this continuum more closely in polystyrenesulfortic acid foils7 and have shown the following: The polystyrenesulfonic acid foils contain according to the degree of hydration -SO,-H30+, respectively more strongly hydrated complexes as, for instance, -S03-H70:,+. In these complexes, the proton is largely free movable. Therefore, it is best described in proton boundary structures as done in Fig. 1 for the --S03-H703+ complex. The present study clarifies the temperature dependence of the absorption continuum. I n order to make understandable the question answered by the present (1) R. Suhrmenn and F. Breyer, 2. physik. Chem., B23, 193 (1933). (2) G.Meerlender, Dissertation (R.Suhrmann), Braunschweig, 1959. (3) M. Falk a n d P. A. GiiruBre, Can. J . Chem., 36, 1195 (1957). (4) Th. Aokermann, 2. physik. Chem. (Frankfurt), 27, 253 (1961). ( 5 ) E. Wicke, M. Eigen, and Th. Ackermann, 2. physik. C h e n . (Frankfurt), 1,340 (1954). (6) M. Eigen and L. de Maeyer, Proc. Roy. Soc. (London), 8247, 505 (1958). (7) G. Zundel, H. Woller, and G.-M. Schwab, 2. Elektrochem., 66, 129 (1962).
experiments we shall discuss first two alternative hypot heses. The Origin of the Absorption Continuum.-It is possible to imagine two ways in which the continuum could be produced. First Possibility.-Between the oxygen atom of “HzO” and that of “H3O+” in the hydrate complex is a potential with a small barrier. This potential barrier has to be small for the reason that t.he proton frequently changes from one position to the other. We may now suppose that the height and breadth of such a small potential barrier are particularly strongly affected by the thermal vibrational and rotary motions of the molecules in these complexes. The changes of the barrier effected by thermal motions could then be large in comparison with the height and breadth of the barrier a t low temperatures. A model which makes this conceivable as a cause for the continuum was discussed by us in ref. 7. Second Possibility.-The absorption continuum is caused by the high exchange probability of the excess proton within the complexes (see below). The low-temperature test permits of a decision between these two possibilities. The Experiments and their Results We investigatedaa two polystyrenesulfonic acid foils,8b one 5% cross-linkedEc (Fig. 2) and one 10%
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cross-linked (Fig. 3 ) , each sulfonated for 5 days. The 5% cross-linked one mas naturally much more strongly sulfonated within the 5 days than the 10% crosslinked one. The 5% cross-linked one was t'ested with relative humidities of the air in contact with the sample of 33, 22.5, 11, 7, and approximately 0.1% (Fig. 2 ) . The 10% cross-linked foil was tested with relat,ive humidities of 71, 33, and 11%, as well as after three days' drying over Pz06(Fig. 3). All measurements in each of these series were made on one and the same sa,mple. The spectrum drawn with a broken line is in each case that at 298"K., that with a continuous line is that after instantaneous cooling of the sample to 90OK. The spectra as shown in Fig. 2 and 3 will supply the following findings. The continuous absorption is a t least as strong after cooling the specimen from 298" to 90OK. (comparison of the broken and continuous curves in each case). This holds for the 5% cross-linked, strongly sulfonated sample (Fig. 2 ) as well as for the 10% cross-linked one. Further, this result is independent of the moisture content of the specimen. (8) (a) All measurements were made with the IR-Spectrophotometer, model 221 with grating, of the firm Bodenseexr-erke Perkin-Elmer G.m.b.H., Uberlingen, Germany. It is intended t o publish details of the method, in particular the container used, in the "Zeitschrift f u r Chemie-lngenieurTechnik." (b) For methods of preparation see: G. Zundel, H. Koller, and G.-M. Schwab, 2. Natwfoorschung, 16b, 716 (1961). (c) By per cent-crosslinking is understood the weight-percentage of divinylbenzene added in polymerization.
The possibility is not ruled out that the intensity of the continuous absorption is even slightly increased by cooling (see in particular Fig. 2 ) . With the 10% cross-linked, weakly sulfonated sample, it is nevertheless observed that the absorption 011 the sides of the OH-bands decreases as a result of cooling. This presents the illusory appearance of a decrease of the continuous absorption here on cooling. It is nevertheless observed that the maximum extinction of the OH-bands increases considerably on cooling. This shows that this decrease of extinction on the sides of the bands is caused by a sharpening of the bands with decrease of temperature. This is naturally due to increasing restriction of thermal vibrations in these complexes on cooling the sample. The same change with temperature decrease can also be observed occasionally on other bands, however not so distinctly. Discussion of Results This experimental result is decisive : the thermal vibrational and rotary motions are not effective as a cause of continuous absorption. In this case the production of continuous absorption by the large exchange probability of the protons in the complexes must be discussed. If the lifetime of a state decreases, the associated band becomes broader. hIeerleizder2 has discussed his results from this standpoint. I n our work, however, we do not observe broadened bands, but an absorption continuum as shown by the experimental results. We consider it probable that, besides the fast proton transfer, coupling of the individual complexes with one another is necessary for the production of continuous absorption. The transfer of a proton within a complex presumably leads to variations of the field in the im-
THERMAL DIsSOCI.~TIOSO F ETHYL hLCOHOL
April, 1963
mediate neighborhood of this complex. A coupliiig of the individual complexes could be effected by these fields. This has to be clarified later on. Nevertheless, the present work has shown clearly that the fast proton transfers within the hydrate cornplexes are a necessary condition for the existence of absorption continuurn. Thus, vice-versa, an absorption continuum with corresponding properties7 can be regarded as a criterium for such fast proton transfers. Acknowledgment.-We thank Dr. E. Weidemanii of
773
the Institute for Theoretical Physics of the University of Munich for many long discussions and valuable suggestions as well as for carrying out interesting calculations. We are indebted to Dr. Th. Ackermann of the Institute for Physical Chemistry of the University of Muenster, Restphalia, and Dr. 31. Eigeii of the Max Planck Institute for Physical Chemistry, Goettiiigen, for helpful discussions. We are further particularly indebted to the Ileutsche Forschungsg~m~iiiscliaftfor supporting this work.
RADIATION EFFECTS O S p - AKD n-TYPE CATALYSTS KSED I N THE THERMAL DISSOCIATION OF ETHYL ALCOHOL BY ~ ~ A T T EDOKATO O Mines Branch, Department of Mines and Technical Surveys, Ottawa, Ont., Canada Receaved August 68, 1.968 The dissociation of anhydrous ethyl alcohol has been studied over the temperature range 330-430’ with the employment (of catalysts that had been exposed to a range of estimated “integrated” neutron doses of up to 96.6 X 10‘8 n./cm.*. The catalysts are %no, an n-type semiconductor, prepared by decomposition of the carbonate, and Crz03, a p-type semiconductor, prepared by dehydration of the hydroxide. Measurable changes in yield and decomposition mechanism have been observed a8 compared with the unirradiated catalysts. The variation in the behavior of the Catalysts has been related to the various neutron doses received and the results are discussed in the framework of the electronic theory of catalysis on semiconductors.
Introduction I n recent years, there has been an increasing interest in the effect of nuclear radiation on solid inorganic catalysts. This is due to the fact that catalytic behavior is strongly dependent on the structure and energy conditions of the catalyst and its structure may be modified under nuclear irradiati~n.l-~The object of the present paper is to discuss the behavior, as catalysts, of ZnO, an n-type semiconductor, and Crz08,a p-type semiconductor, before and after irradiation in the Chalk River reactor. Studies have been made of the xell-known thermal (dissociation reaction of ethyl alcohol that proceeds along tmo paths
+ Hz L--+CzH4 + H20
r--+RCHO R-CH20H
hydrogenation dehydration
The particular reaction path taken is determined at the first stage of reaction, the adsorption of the alcohol molecule, and depends on the particular bond broken in the molecule, the OH or C-OH bond; this in turn depends on the nature of the catalyst. With ZnO and Cr203 catalysts both reactions occur. It is well known that the reactor irradiation of a solid semiconductor can generally produce the following effects: (a) direct ionization; (b) change in the degree of disorder in the lattice of the solid irradiated; and (c) nuclear reaction in the solid. Each of these effects produces a variation in the number of free carriers in the solid. The first effect is only a temporary effect. The second one can be removed by heating the sample at a high temperature. The third one, which leads to the (1) I. I. Barry, “Report to the International Congress on Catalysis,” Paris, July 1960. (2) T. Brown and I. Maxim, X a t u r e , 192, 598 (1961). (3) R. H. Bragg, F L. Morrits, R. Holtzman, P. Y. Feng, and F. Pizzarello, Wright Air Development Center, Technical Report 59, 986 (1959). (4) H. W. Kohn, G. E. Moore, and E. H. Taylor, Oak Ridge Kational Laboratory, Progress Report 2589, 17 (1958).
production of impurities of different chemical character in the lattice of the solid, is a permanent effect and irreversible. I n the last twenty years many authors, such as H a ~ f f e ,Schmab,6 ~ Garner,? Wolkenstein,8 and others have developed an electronic theory of catalysis on semiconductors. According to this theory the nature of the substrate bonding determines the transfer of electrons from the adsorbed molecule to the unfilled electronic levels in the solid catalyst or from a solid with an excess of electrons to the adsorbed molecule. The well-known catalytic activity of the transition metals is thus correlated with their unfilled d-bands, and the different activities of n- and p-type semiconductors with their capability to donate or accept electrons, respectivelyP lo The relative activity of the catalyst, in this case for dehydration or dehydrogenation, depends on the position of the Fermi level. The lowering of the Fermi level retards dehydrogenation and accelerates dehydration In this framework, the gamma and neutron irradiation of a solid catalyst would be expected to alter its catalytic activity because the products of irradiation include trapped electrons and holes (in non-metals), displaced atoms, and impurities of a different chemical nature, due to the neutron-induced transmutations, all of which influence the electronic structure of the solid catalyst and hence its ability to interchange electrons with the adsorbed molecules on the surface. Apparatus and Procedure.-The reaction, the decomposition of the ethyl alcohol, was observed at atmospheric pressure and a t ( 5 ) K. Hauffe, “Semiconductor Surface Physics,” University of Pennsylvania Press, 1957. (6) G. 111. Schnab, ref. 5 . (7) W. E. Garner, “Chemistry of the Solid State,” Butterworths, London, 195.5. (8) F. F. Wolkenstein, Advances zn Catalysis. 12, 121 (1960). (9) S. Z. Roginsky, 0 . V. Krylov, and E. A. Fokina, Bull, 4cad. Sc7. USSR, Dzv. Chem. Sa., 442 (1952). (IO) W. E. Garner, Rdnanresin Catalyszs, 9 , 169 (19571,
