J. Phys. Chem. 1987, 91, 8-1 1
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where R is the rate of hydrogen evolution (mol/s), and AG is the standard free energy change for the decomposition of water to hydrogen and oxygen (J/mol). If a correction is made for the light absorbed by the polysulfide solution, the efficiency was 0.91%. Further improvements can be made by using a better electrode with a lower overpotential for O2 evolution than Pt (e.g., a nickel-based electrode13) and by using smaller bandgap material (1 3) Tidak, B. V.; Lu, W. T.; Coleman, J. E.; Srinivasan, S. In Comprehensiue Treatise of Electrochemistry, Bockris, J. O M . , Ed.; Plenum: New York, 1981; pp 13-33.
either directly or as underlayers for larger bandgap materials in biphotonic tandem panels. l 4 Such improvements, guided by computer modeling for maximizing efficiency, are currently under investigation in these laboratories.
Acknowledgment. The support of this research by the Gas Research Institute (5982-260-0756) is gratefully acknowledged. We are grateful to the NATO Spanish Scientific Committee for the grant received by S.C.M. to carry out this research. (14) Weber, M. F.; Dignam, M.J. J. Electrochem. SOC.1984,131, 1258.
Calorimetrtc Study on the State of Aromatic Molecules Sorbed on Slllcalite H.Thamm Academy of Sciences of the GDR, Central Institute of Physical Chemistry, GDR-1199 Berlin, Rudower Chaussee 5, GDR (Received: July 26, 1986; In Final Form: October 28, 1986)
Differential heats of sorption have been determined calorimetrically for benzene, toluene, ethylbenzene, and p-xylene on silicate as a function of pore filling. In all cases abrupt changes in the state of the sorbed molecules are observed when the unit cell. While in the case of benzene, toluene, and ethylbenzene sorbate-sorbate amount sorbed exceeds 1 molecule per interaction occurs only above 1 molecule per unit cell, the sorbed p-xylene molecules interact with each other below this loading.
Introduction During the past few years several investigations on sorption equilibria of aromatic molecules on silicalite have been published. The experimental methods applied were gas chromatography,' measurements of sorption and measurements of sorption isochores (isosteres).' These investigations, however, were either restricted to relatively narrow ranges of pore filling and temperature or the experimental points were too widely spaced on the sorption or temperature scale, respectively, so as not to reveal possible changes of the derived thermodynamic functions of the sorbate within small intervals of loading and/or temperature. This may explain that, contrary to ref 1-7, preliminary calorimetric results on the sorption of benzene and toluene on silicalite *~~ considerably more comobtained in our l a b ~ r a t o r y indicated plicated dependences of the differential heats of sorption on pore filling. Unfortunately, the silicalite sample used in these studies contained amorphous SiO, and did not allow for the deduction of exact quantitative relations between the heats of sorption and the sorbed amount. Moreover, the investigation of sorption equilibria on silicalite samples received from different laboratories led us to the conclusion that besides experimental care that has to be taken, as mentioned above, special attention must be paid to the chemical purity and the structural homogeneity of the (1) Lechert, H.; Schweitzer, W. Proceedings of the Sixth International Conference on Zeolites, Bisio, A., Olson, D. H., Eds ; Butterworths: London, 1984; p 210. (2) Anderson, J. R.; Foger, K.; Mole, T.; Rajadyaksha, R. A,; Snaders, J. V. J . Catal. 1979, 58, 114. (3) Olson. D. H.:Kokotailo. G. T.: Lawton. S. L.: Meier. W. M. J . Phvs. Chem. 1981, 85, 2238. (4) Jacobs, P. A,; Beyer, H. K.;Valyon, J. Zeolites 1981, I , 161. (5) Wu, P.; Debebe, A.; Ma, Y. H.Zeolites 1983, 3, 118. (6) Lohse, U.; Fahlke, B. Chem. Tech. (Leipzig) 1983, 35, 350. (7) Pope, C. G. J . Phys. Chem. 1986, 90, 835. (8) Thamm, H.; Regent, N. I. Z . Chem. 1982, 22, 232. (9) Stach, H.; Thamm, H.; Janchen, J.; Fiedler, K.; Schirmer, W. Proceedings of the Sixth International Conference on Zeolites, Bisio, A,, Olson, D. H., Eds.; Butterworths: London, 1984; p 225.
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sample used. The presence even of minor quantities of framework Al,exchangeable cations, or structural OH groups has a significant influence on the sorption equilibria on silicalite or the isostructural ZSM-5 molecular sieves, respectively.10 Therefore it is important to note that the following results refer to a 100% crystalline silicalite sample of high chemical purity that corresponds to the end member of the ZSM-5 series with Si/A1 approaching infinity. To study systematically the energies of interaction of aromatic molecules on silicalite we investigated the sorption of benzene, toluene, ethylbenzene, and p-xylene. For comparison we also measured the differential heats of sorption of n-hexane on the same silicalite sample.
Experimental Section The differential heats of sorption were measured by a Calvet-type microcalorimeter (Setaram) at 301 K. Both the thermokinetic curve of the calorimeter and the pressure in the gas phase were continuously recorded to ensure the attaining of equilibrium data. Silicalite was kindly supplied by Dr. B. Fahlke, Central Institute of Inorganic Chemistry, Academy of Sciences of the GDR. The wet chemical analysis gave no indication of the presence of aluminum or sodium in the sorbent. The crystallinity of the silicalite sample was checked by X-ray diffractometry, sorption capacity measurements, IR spectrometry, and SEM. Prior to application the sorbent was activated in high vacuum ( < l W 3 Pa) for 24 h at 673 K. The hydrocarbons sorbed were gas chromatographically pure. Results and Discussion In Figure 1 the calorimetrically determined differential molar heats of sorption for benzene on silicalite are given together with sorption enthalpies derived from isochoric measurements by Pope.' Both heat curves have two common features: the constancy of the heats of sorption up to a 1 molecule per 'I4 unit cell (uc)
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(10) Thamm, H. Zeolites, in press.
0 1987 American Chemical Society
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and a maximum of unusual height above a = 1 molecule per 1 / 4 uc. The calorimetric heat curve, however, obviously reflects more details about the state of benzene on silicalite than the isochoric curve. Contrary to the isochoric results, the calorimetrically determined heat curve shows very high Q values at a < 0.1 molecules per uc and two maxima as well as two minima at high sorptions. (The systematic difference in the absolute magnitude of the sorption heats between both of these heat curves may have numerous reasons. However, since it has no influence on the following discussion it will be considered elsewhere.) Enhanced heats of sorption for molecules with electric dipole or large quadrupole moments such as benzene at very low sorbed amounts are found for nearly all solids. They are due to the chemical or structural nonideality of the sorbents. Measurements of sorption isotherms or isochores usually do not include this sorption interval because of the difficulties in measuring correct equilibrium pressures in the low-pressure region. The constancy of the heats of sorption up to a 1 molecule per uc and their decline with further sorption suggest that there are energetically favored sorption sites for benzene in silicalite (one site in each 1/4 uc) which are sufficiently separated from each other to prevent sorbate-sorbate interactions. Since all experimental results given in this work represent equilibrium states, the minimum in the heat curve suggests that the molecules sorbed uc occupy energetically less favorable above a = 1 molecule per sorption sites. The fact that the minimum is relatively small indicates that the filling of these sites is superimposed with the interactions of the sorbed molecules with each other. The subsequent stepwise increase in the heats of sorption a t moderate sorbed amounts seems to be associated with a cooperative redistribution and/or reorientation of the sorbed molecules (e.g., formation of benzene dimers). A second change of the state of the sorbed phase (e.g., formation of larger clusters) is indicated 2 molecules per 1 / 4 uc. by the second maximum at a To confirm the complicated course of the heat of sorption curve of benzene on silicalite by an experimentally independent method
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c .s Figure 1. Differential molar heats of sorption of benzene on silicalite: 0,first run; 0,second run; dashed curve, ref 7. J
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Figure 2. Differential molar heats of sorption of toluene on silicalite: 0, first run; 0,second run; dashed curve, ref 7 .
sorption isosteres were also measured in our laboratory.” Although the crystallinity of the silicalite batch applied in ref l l was worse than that of the silicalite sample used in the current work, the comparison of the results of both these works clearly demonstrates that a satisfactory agreement Between the isosteric and the calorimetric heat of sorption curves of benzene on silicalite is reached when the number of isosteres investigated is sufficiently high. Moreover, a strong temperature dependence of the heat of sorption curve of benzene on silicalite at a > 1 molecule per uc is reported in ref 11. Further experimental work, however, is necessary (e.g.$ direct heat of sorption or heat capacity measurements over broad intervals of temperature) to study this effect in detail. In Figure 2 the calorimetric heat of sorption curve for toluene on silicalite obtained in this work is represented together with the isosteric heat of sorption curve from ref 7. As in the case of benzene, the calorimetrically determined differential heats of sorption of toluene on silicalite are constant up to a 1 molecule per 1/4 uc and pass through a minimum when the sorbed amount is further increased. Contrary to benzene, however, the minimum in the heat curve for toluene is followed up by a second plateau which extends from moderate pore fillings up to saturation capacity. While the first plateau in the heat of sorption curve of toluene indicates that the sorbate-sorbate interaction is neglibible, the second plateau can be accounted for by assuming constant sorbatesorbate interaction energies within the corresponding sorption range. This means that ordered sorbate structures are likely to occur before as well as after the change of the state of the sorbed molecules at a = 1 molecule per 1 / 4 uc, the main difference between these two sorbate structures being the lack of sorbatesorbate interactions in the first state. Presumably, below the “critical” sorbed amount of 1 molecule per uc the toluene molecules are sorbed as monomers while above this loading the
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(1 1) Wendt, R.; Thamm, H.; Fiedler, K.; Stach, H. Z Phys. Chew (Leipzig) 1985, 266, 289.
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10 The Journal of Physical Chemistry, Vol. 91, No. 1, 1987 TABLE I: Saturation Capacities 8 , and Total Sorbate Length I,” on Silicalite at 301 K molecule length, nm 0.74 0.86 0.98 0.98 1.03
sorbate benzene toluene ethylbenzene p-xylene n-hexane
a,, theor,b m o i e c ~ l e s / ’ /uc ~ 3.0 2.5 2.2 2.2 2.1
a-,sxptl
cm3/g 0.134 0.129 0.125 0.168 0.178
molecules/’/4 uc 2.1 1.7 1.5 1.9 1.9
I,, nm/uc
a,,cxptl/a..,tbcor 0.7 1 0.68 0.66 0.88 0.94
6.2 5.8 5.9 7.5 7.8
“See text for definition of I,. bRefered to a theoretical pore volume of 0.19 cm3 per g.‘*
In contrast to this, n-hexane fills 94% of the pore volume and its total sorbate length is considerably greater than the pore length. This means that at saturation capacity the sorbed n-hexane molecules overlap or coil at the intersections of the straight and sinusoidal channels, while the benzene, toluene, and ethylbenzene molecules, at least principally, may arrange in an “end-to-end” configuration. In other words, the term “end-to-end” configuration often referred to in the literature (for example, see ref 4) to describe the sorbate structures of n-paraffins in silicalite (possibly) more correctly characterizes the arrangement of the abovementioned aromatic molecules. To illustrate the pecularities of the sorption of the aromatics studied the heat of sorption curve for n-hexane on silicalite is also presented in Figure 3. As was found for other n-alkanes and n-alkenes on ~ i l i c a l i t e , ~the , ’ ~ differential heats of sorption for n-hexane continuously increase with pore filling and neither in the heat curve nor in the sorption isotherm are discontinuities or hysteresis phenomena observed. The sorption isotherms for benzene, toluene, and ethylbenzene, contrary to n-hexane, exhibit steps corresponding to the abrupt 1 molecule per ‘I4 increase in the heat of sorption curves at a uc as well as more or less pronounced hysteresis loops. (The graphs of the isotherms which are similar to that of the type IV of Brunauer’s classificationI5 are not shown for brevity of presentation.) These observations may be understood in terms of a phase transition or a change in the state of the sorbed molecules, respectively, as discussed earlier. Both the constancy of the dif1 molecule per uc and ferential heats of sorption up to a their subsequent decline, however, are observed below the stepwise increase in the sorption isotherms. Consequently, they are not associated with two-phase phenomena in the sorbed state but reflect energetic heterogeneity of the pore volume in silicalite as already assumed in the discussion of the course of the heat curve for benzene. Evidently, benzene, toluene, and ethylbenzene, on one hand, and n-hexane, on the other, represent two types of sorption on silicalite, the former being characterized by negligible sorbateuc and the abrupt sorbate interaction below a = 1 molecule per set-in of interactions between the sorbed molecules above this loading. The second type is characterized by sorbate-sorbate interactions beginning at low sorbed amounts and progressively increasing with pore filling as is usually the case for nonpolar sorbates and microporous sorbents (isotherm type I according to BrunauerIs). The state of p-xylene on silicalite must be regarded as intermediate between these two cases. While in accordance with ref 3 a n d 6 the sorption isotherm of p-xylene exhibits a pronounced step at a 1 molecule per uc as well as a hysteresis loop, the differential heats of sorption of p-xylene, like that of n-hexane, continuously increase beginning at low sorbed amounts (Figure 3), i.e. the sorbed p-xylene molecules interact with each other already below a = 1 molecule per uc. In contrast to n-hexane,
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sorbed toluene molecules exist as dimers or larger clusters which increase in number or size with further pore filling. Very surprising is the difference between the heat of sorption curves of ethylbenzene and p-xylene (Figure 3). From the lengths of these molecules which are comparable to the lengths of the uc one would expect the sorbed channel sections within ‘I4 molecules to interact with each other already at a < 1 molecule per uc. While Q = f ( a ) for p-xylene is in agreement with this assumption, the heat curve for ethylbenzene, similar to benzene and toluene, exhibits a plateau up to a 1 molecule per uc followed by a minimum and a steep increase again. This means that, as in the case of benzene and toluene, one ethylbenzene uc without interacting molecule may be accommodated per ‘I4 with its neighbors. One reason for this may be the lower symmetry of the ethylbenzene molecule in conjunction with its electric dipole moment, rendering sorbatesorbate interactions for ethylbenzene at low sorptions in silicalite more difficult than for p-xylene. In Table I are given the saturation capacities for the sorption systems studied expressed as cm3 of the liquid sorbate per g, as molecules per uc, and as total sorbate length assuming an “end-to-end” configuration of the sorbed molecules, respectively. The values listed in Table I show that benzene, toluene, and ethylbenzene occupy only about 2/3 of the available pore volume in silicalite and that the total sorbate length per unit cell for these molecules is less than the available pore length which corresponds to 6.62 nm per uc.I3
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(12) Flanigen, E. M.; Bennet, J. M.; Grose, R. W.; Cohen, J. P.; Patton, R. L.; Kirchner, R. M.; Smith, J. V. Nature (London) 1978, 271, 512. (13) The available pore length is estimated from structure data given in ref 12. Length of the straight channels: 3.96 nm per unit cell. Length of the sinusoidal channels without intersections: 2.66 nm per unit cell. From this follows an available pore length of 6.62 nm per unit cell, in contrast to ref 4, where the intersections of the two channel types were counted twice. (14) Thamm, H.; Stach, H.; Fiebig, W. Zeolites 1983, 3, 95. (1 5 ) Brunauer, S. The Adsorption of Gases and Vapors; Oxford University Press: London, 1944; p 150.
J. Phys. Chem. 1987, 91, 11-14 the heat curve for p-xylene exhibits a weak maximum at a = 1 molecule per I/., uc and its last part resembles that of the heat curve for toluene. This together with the observed step in the sorption isotherm suggests that at a = 1 molecule per uc the arrangement of the p-xylene molecules within the silicalite pores becomes more ordered. The 88% pore filling as well as the total sorbate length of 7.5 nm per uc found at saturation capacity (Table
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I) suggest a configuration of the sorbedp-xylene molecules which allows them to interact with each other by their CH3 groups at the intersections of straight and sinusoidal channels. Acknowledgment. The author gratefully acknowledges the assistance of Mr. Dieter Schielinski in the calorimetric experiments.
Vibrational Frequency Shifts of Adsorbed Pyridazlne on a Silver Electrode Studied by SERS Machiko Takahashi,* Motomi Niwa, and Masatoki Ito Department of Chemistry, Faculty of Science and Technology, Keio University, Hiyoshi 3- 14- 1, Kohoku-ku, Yokohama 223, Japan (Received: August 18, 1986)
The surface-enhanced Raman spectrum of pyridazine adsorbed on a silver electrode has been studied in order to elucidate the origin of the adsorption-induced vibrational frequency shifts. The vibrations that experience relatively large frequency shifts are found to have large amounts of the N-N internal coordinate in their potential energy distribution and are influenced by an increase in the N-N stretching force constant. We conclude that pyridazine is adsorbed through the donation of the lone pair electrons from the nl-nz orbital, and the change of electron density on the N-N bond correlates with the vibrational frequency shifts.
Introduction Great advances have recently been made in two vibrational spectroscopic methods, i.e. surface-enhanced Raman spectroscopy and modulated IR spectroscopy. In situ information about the adsorbed species at an electrode surface can be acquired with these methods. It is possible to deduce the orientation of the adsorbed species on the electrode from the infrared results, comparing the intensity and the frequency with those of the free molecule. However, the IR signal from sub-monolayer species is usually too weak to detect. SERS spectroscopy, on the other hand, is able to obtain information about the submonolayer adsorbates because of its high sensitivity. The main origin of SERS is thought to be a combination of an increase in the surface eiectromagnetic field and a charge-transfer resonance, but its mechanism has not yet been completely elucidated. At the present stage, however, we focus our attention on finding how the vibrational frequencies of adsorbed species shift from those in bulk phase. A SER spectrum is found to be very informative because of its high resolution and sensitivity. The frequency shift of adsorbates has been discussed mainly with respect to small chemical species such as C O and CN-.I In the case of pyridine adsorbed on the surfaces of silica or alumina,* the vibrational frequencies at the ring breathing and C H stretching regions are known to be shifted to higher frequency. The frequency shift has been explained by adsorption through the lone pair of the nitrogen atom on the Lewis acid or Bransted acid adsorption sites, or by hydrogen bonding with a surface hydroxyl group. Weaver et al.3 reported the SERS of benzene, ethylene, and their derivatives on a gold electrode and concluded that the coordination through the unsaturated groups shifts the frequencies of the respective vibrations, due to electron donation and backdonation between the adsorbate and the metal. In general, however, the amount of shift of the vibrational frequency varies with each vibration, and, until now, no substantial attention has (1) Ishi, S.;Ohno, Y.; Viswanathan, B. Surf: Sci. 1985, 161, 349. Billman, J.; Otto, A. Surf: Sci. 1984, 138, 1, and references therein. (2) Yamada, H.; Yamamoto, Y. J. Chem. Soc., Faraday Trans. I 1979, 75, 1215. ( 3 ) Gao, P.; Weaver, M. J. J. Phys. Chem. 1985,89, 5040. Patterson, M. L.;Weaver, M. J. J. Phys. Chem. 1985, 89, 5046.
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been paid to the difference in the behavior between each vibration. In this paper, we report the SER spectrum of pyridazine on a silver electrode. Pyridazine has two nitrogen atoms in adjacent positions and is found to be a suitable example to investigate the frequency shift induced by adsorption. With the information about the electronic structure and the potential energy distribution of each normal vibration, the adsorption-induced frequency shift is interpreted as being due to the donation of the nonbonding electrons of nitrogen atoms to the metal.
Experimental Section Commercially available pyridazine was purified by distillation. A polycrystalline silver electrode was mechanically polished with mol/L pyridazine and 1 X 10-l alumina and immersed in 5 x mol/L KC1 aqueous solution. All potentials were measured against a Ag/AgCl (4 mol/L) electrode. As for the oxidation-reduction cycle, the electrode potential was swept between a respective voltage and +0.25 V at a rate of 50 mV/s. All the apparatus for Raman spectral measurement were the same as described in a previous paper! Raman spectra were excited with 514.5-nm light from an Ar ion laser. Results and Discussion Figure 1 shows the Raman spectra of pyridazine under four different conditions: In (a) and (b), SER spectra observed at -0.05 and -0.55 V are given, respectively. The normal Raman spectra of pyridazine in an aqueous solution and the liquid phase are also given in (c) and (d), respectively. In SER spectra, almost all the bands could be assigned to the corresponding vibrations in bulk phase without ambiguity. SERS of pyridazine decreased in intensity when the applied potential was more positive than -0.05 V or more negative than -0.55 V. Overall spectral features changed only slightly with applied potential. Therefore, it is clear that the adsorbed form of pyridazine at the Ag surface does not change significantly under the present experimental conditions. The observed vibrational frequencies are given in Table I, together with the values and the assignment reported by Ozono et ale5 It is apparent from Table I that, when pyridazine is (4) Takahashi, M.; Fujita, M.; Ito, M. Chem. Phys. Lett. 1984, 109, 112.
0 1987 American Chemical Society
