J. Phys. Chem. 1992, 96, 1875-1880 general conclusions. Under typical CVD conditions, when acetylene is the dominant gaseous reaction product, the growth of diamond should proceed by the addition of acetylene. Yet, such additions cannot take place on atomically flat surfaces and require the existence of atomic-size steps. These steps are most likely to be formed by the addition of methyl radicals to (1 11) radical sites2 or to (100) monohydride dimer radicals (Figure 9). In fact, such a "methyl followed by acetylene" addition reaction appears favorable on both energetic3 and kinetics24 grounds. In environments containing large concentrations of methyl radicals and not much acetylene, the most likely candidates for diamond growth are two reactions: the addition of CH3 to a (100) monohydride dimer radical, suggested in this work (see Figure 9), and the formation of a diradical intermediate on a (1 11) surface, i.e., two adjacent chemisorbed methylene groups as shown in Figure 9 of ref 3 [this reaction referred to as a (1 10)-surface growth reaction by Y a r b r o ~ g h ~ ~ However, ]. due to the less (24) Frenklach, M.; Wang, H. Phys. Reu. E . 1991,43, 1520. Frenklach, M. In Proceedings of the Second International Symposium on Diamond Materials; Purdes, A. J., Angus, J. C., Davis, R.F., Meyerson, B. M., Spear, K. E., Yoder, M., Eds.; The Electrochemical Society: Pennington, NJ, 1991; p 142.
1875
favorable energetics of the former and steric requirements for the latter, the rate of diamond growth via these reactions should be lower than in the previous "methyl followed by acetylene" addition case. If the gas-phase abundance of C atoms becomes comparable to that of CH3 and CZH2,the reaction sequence in Figure 13 should contribute to diamond growth. This C-addition reaction would not only build up the diamond lattice by itself, but would also provide an atomic-size step for a subsequent addition of acetylene.
Acknowledgment. The computations were performed using the facilities of the Pennsylvania State University Center for Academic Computing. The work was supported in part by Innovative Science and Technology Program of the Strategic Defence Initiative Organization (SDIO/IST) via the U S . Office of Naval Research, under Contract N00014-86-K-0443. Registry No. H, 12385-13-6; H2,1333-74-0; C2H2.74-86-2; CO, 630-08-0; C, 7440-44-0; diamond, 7782-40-3. (25) Yarbrough, W. A. In Diamond Optics I K Holly, S.,Feldman, A., Eds.; Proc. SPIE-Int. SOC.Opt. Eng., in press.
Kinetic Studies on the Thermal Cis-Trans Isomerization of an Azo Compound in the Assembled Monolayer Film Zhong-Fan Liu,+ Kenichi Morigaki, Tadashi Enomoto, Kazuhito Hashimoto, and Akira Fujishima* Department of Synthetic Chemistry, Faculty of Engineering, The University of Tokyo, Hongo, Tokyo 113, Japan (Received: August 26, 1991)
The thermal cis-trans isomerization of an amphiphilic azo compound in the assembled monolayer film has been investigated kinetically, with an electrochemicalapproach being employed to follow the reaction process. Isomerization in the rigid monolayer film was found to follow first-order kinetics as in the liquid phase, at least in the earlier stage of reaction. The rate constant can be expressed by k = 10s,5*l.5exp (-(16.5 f 1.0 kcal mol-')/RT) s-l. Interestingly, in spite of the rigid and highly-oriented and entropy (AS') were quite comparable to those in chloroform film structure, activation parameters such as enthalpy ( M ) solution, whereas they were distinctly smaller than those of azobenzene in crystal. The kinetic behavior observed is attributed to the effects of the assembled monolayer film environment employed and implies an inversion mechanism for the thermal isomerization in the monolayer film.
Introduction The kinetics of the thermal cis-trans isomerization of azo compounds in liquid phase have been intensively studied during the last 50 years.'-' Little work, however, has been conducted on the assembled monolayer films. Studies of this kind are undoubtedly important from both practical and theoretical points of view. In the first place, azo compounds have been very often employed as trigger^^,^ or probes6" in recent molecular device studies owing to the large and reversible change in geometry during the photochemical cis-trans isomerization. In most of such purpose-oriented studies, usually using thin films, the thermal cis-trans isomerization is the weak point, and its kinetic study becomes necessary. In the second place, the mechanism of the thermal cis-trans isomerization of azo species has also been a controversial problem. Two different mechanisms have been proposed: the reaction may proceed either via a rotation about bond*-" or via an inversion of one of the nitrogen the -N=NWhile the effects of substituent,"-'3J8 solvent polarity,"-I4 and p r e ~ s u r e ' ~are - ~ ~mostly investigated for such mechanistic studies, the isomerization behavior in some special To whom correspondence should be addressed. ' h e n t addrcss: Institute for Molecular Science, Myodaiji, Okazaki 444, Japan.
0022-3654192120961875S03.00.,IO , -, -
molecular environments such as highly-oriented monolayer film may also be expected to offer valuable information. (1) Hartley, G. S.J . Chem. Soc. 1938,633-642. (2) Rau, H.Photochromism; Durr, H., Laurent, H. B., Eds.; Elsevier: New York, 1990 pp 165-192, and references cited there. (3) Griffiths, J. Chem. SOC.Reu. 1972, 1,481-493, and references cited
there. (4) Tazuke, S.;Kurihara, S.; Ikeda, T. Chem. Lett. 1987, 91 1-914. ( 5 ) Ichimura, K.; Suzuki, Y.; Seki, T. Longmuir 1988, 4, 1214-1216. (6) Stadler, R.; Weber, M. Polymer 1986, 27, 1254-1260. (7) Kumar, G. S . ; Neckers, D. C. Chem. Rev. 1989,89, 1915-1925. (8) Halicioglu, T.; Sinanoglu, 0. Ann. N.Y. Acad. Sci. 1969, 308-312. (9) Le Fevre, R. J. W.; Northcott, J. J. Chem. SOC.1953, 867-870. (10) Schulte-Frohlinde, D. Justus Liebigs Ann. Chem. 1958,612,138-144. (11) Wildes, P. D.; Pacifici, J. G.; Irick, G., Jr.; Whitten, D. G. J . Am. Chem. SOC.1971, 93, 2004-2008. (12) Nishimura, N.; Sueyoshi, T.; Yamanaka, H.; Imai, E.; Yamamoto, S.; Hasegawa, S.Bull. Chem. SOC.Jpn. 1976, 49, 1381-1387. (13) Marcandalli, B.;Liddo, L. P.; Fede, C. D.; Bellobono, I. R.J. Chem. SOC.,Perkin Trans. 2 1984, 589-593. (14) Haberfield, P.; Block, P. M.; Lux, M. S. J. Am. Chem. SOC.1975, 97, 5804-5806.
(IS) Asano, T.; Okada, T.; Shinkai, S . ; Shigematsu, S.; Kusano, Y.; Manabe, 0. J. Am. Chem. SOC.1981, 103, 5161-5165. (16) Asano, T.; Yano, T.; Okada, T. J. Am. Chem. SOC.1982, 104, 4900-4904. (17) Nishimura, N.; Tanaka, T.; Asano, M.; Sueishi, Y. J. Chem. SOC., Perkin Trans. 2 1986, 1839-1845.
0 1992 American Chemical Society
1876 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 C~H,~-@-N=N-@-
Liu et al.
o - c ~ H COOH ,~
ABD Figure 1. Molecular structure of the azobenzene amphiphile used in the present work.
I.i.til,in (photostationaly state)
Figure 3. Experimental procedure for following the thermal cis-trans isomerization kinetics. N
I -out Figure 2. Cell for photoelectrochemicai measurements: WE, ABD monolayer film-deposited SnO, glass electrode; RE, Ag/AgCI electrode, CE, Pt electrode. The hydrophilic head of the ABD molecule is represented by a circle attached to the SnO, glass surface.
To follow the isomerization kinetics in thin films, especially in an ultrathin monolayer film, an effective quantification method is indispensable. The spectrophotometric method conuentionally employed for liquid-phase studies is not so suitable in such cases since the film absorption is often beyond the detection limit of the spectrophotometer. In this paper, an electrochemical approach is employed to follow the thermal isomerization kinetics of an amphiphilic azo compound in the assembled monolayer film. This novel approach is established upon a route-specific hybrid effect that we found in previous studies:21-23the cis isomer is electrochemically reduced to a hydrazine compound (-NH-NH-) at substantially more anodic potential than the trans isomer, and the hydrazine compound thus produced is exclusively oxidized to the trans isomer. This electrochemical approach has been shown to be particularly effective for quantification of the ultrathin monolayer film as compared to the spectrophotometric method. Based on the kinetic results obtained, effects of the rigid and highly-oriented film structure on the isomerization kinetics have been discussed.
Experimental Section Reagents. The amphiphilic azobenzene derivative, 4-octyl4’- [ (5-~arboxypentamethylene)oxy]azobenzene (ABD; whose molecular structure is shown in Figure l), was purchased from Dojindo Laboratory (Kumamoto, Japan). All the chemicals were of reagent grade and used without further purification. Fabrication of Monolayer Films The ABD monolayer film was deposited onto a transparent SnOz glass substrate by the conventional Langmuir-Blodgett method using a commercial instrument (Kyowa, HBM-AP). A 0.2 mM CdC12 aqueous solution was used as the subphase, and no special pH adjustment was made. Chloroform was used as the spreading solvent with the ABD concentration being 1.5-2.5 mM in the solution. The SnOz glass with a lateral resistance of 35 s2 was purchased from Asahi Glass Co. (Tokyo, Japan). The surface of the Sn02glass was always hydrophilically treated by immersing it into hot sulfuric acid (50% by volume) for 10 min before use. The monolayer film was fabricated by dipping the substrate into the aqueous subphase and (18) Brown, E. V.; Granneman, G. R. J . Am. Chem. SOC. 1975, 97, 621-627. (19) Otruba, J . P., 111; Weiss, R. G. J . Org. Chem. 1983, 48, 3448-3453. (20) Wolf, E.; Cammenga, H . K. Z . Phys. Chem. Neue Folge, Bd. 1977, 107, S.21-S.38. (21) Liu, Z. F.; Hashimoto, K.; Fujishima, A. Nature 1990,347,658-660. (22) Liu, Z. F.; Loo,B. H.; Hashimoto, K.; Fujishima, A. J . Elecrroanal. Chem. 1991, 297, 133-144. (23) Liu, Z . F.! Hashimoto, K.; Fujishima, A. J . Elecfroanal. Chem., submitted for publication.
-0.4 -0.2 0 0.2 0.4 E ( V vs AgIAgCL) Figure 4. Cyclic voltammograms of trans- (dashed line) and cis- (solid
line) ABD monolayer films on a SnO, glass electrode. The shadowed part was used to calculate the cis amount in the film. The sweep rate was 20 mV s d . raisiig it at a rate of 10 mm min-I. Because the SnOz glass surface was hydrophilic, only one monolayer of ABD with the hydrophobic alkyl group exposed to the air was formed on the Sn02glass during the dipping and raising process. All of the monolayer films were made at a constant surface pressure of 25 mN m-l, and the subphase temperature was controlled at 20 O C by a thermostat (Tokyo Rikakikai, UC-55). Photoelectrochemical Measurements. The experimental setup for photoelectrochemical measurements is shown in Figure 2. The ABD monolayer film-modified SnO, glass was used as a working electrode (WE), and a Pt wire as a counter electrode (CE). The potential of the working electrode was controlled versus a Ag/AgCl (saturated KCl) reference electrode (RE) by a potentiostat (Toho, 2040). A 0.1 M aqueous sodium perchlorate solution, buffered to pH 7.0 as with the Britton-Robinson method,% was employed as the electrolyte, which was thermostat4 through a waterjacketed cell by the same apparatus used for film fabrication. Before each experiment, the electrolyte was deaerated with high-purity argon gas for 30 min. A 500-W xenon lamp (Ushio Electric, UI-501C) was used for inducing a trans to cis isomerization of ABD in the monolayer film. The excitation wavelengths were isolated by a glass filter centered a t 356 nm with a bandpass of 60 nm (Kenko, U-360). Procedure for Determination of Isomerization Kinetics. (1) Electrochemical Method. The experimental procedure for following the thermal cis-trans isomerization kinetics is shown in Figure 3. The reaction was always started from a photostationary state, which was produced by UV-irradiating the original trans-monolayer film for 2 min. For convenience, the cis amount in the photostationary state (n(O),in moles) was taken as a ref(24) Perrin, D. D.;Dempsey, B. Buffers for p H und Metal Ion Control; Chapman and Hall: London, 1974; p 154.
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1877
Thermal Cis-Trans Isomerization in Monolayer
0.8 1
100
i
90 0.6
0
-
6oti, 40
0.4
\
0.2
, 0
0
10
20
30
40
10
0
50
20
Time (min.)
Figure 5. Change of isomeric composition with time for ABD monolayer films at different temperatures.
50
40
Figure 6. First-order plots for the therAal cis-trans isomerization of ABD in the assembled monolayer films.
erence for calculating the percentage of cis isomer in the monolayer film. Hence the cis percentage (a,) at time t (including t = 0) was calculated by a, = n(t)/n(O) X 100
30
Time (min.)
-7
(1)
where n(t) is the amount of cis isomer at time t , which was evaluated from the cyclic voltammogram of the corresponding monolayer film by integrating the reduction peak of the cis isomer. The integration was performed by carefully weighing the peak area using a microbalance (Mettler, AE166). This is possible because of the substantially different electrochemical reductivity between the trans and cis isomer^^^-^^ as shown in Figure 4, where the shadowed part corresponds to the cis reduction. Since the original trans-monolayer film was always regenerated after electrochemical o x i d a t i ~ n , * the ~ - ~time ~ course of the thermal reaction was achieved simply by repeating the above operations. (2) Spectrophotometric Method. For the sake of comparison, the conventional spectrophotometric approach was also tried. In this case, the experiments were conducted without use of an aqueous electrolyte, Le., in a dry state. The temperatures of the monolayer films were controlled by a cryostat (Oxford Instruments Ltd., ITC4). Before each experiment, the sample chamber was evacuated to ca. Torr by a vacuum pump (Sinku Kiko, VPC-050) to protect the sample surface from moisture. The cis isomer was produced by UV-irradiating the trans-monolayer film for 2 min. The thermal isomerization was then followed by monitoring the absorbance change a t 325 nm, the absorption maximum of the trans isomer in the assembled monolayer film (see Figure 8a), using a UV-visible spectrophotometer (Shimadzu, UVIDEC-650). For experimental convenience, the absorption at t = 0 was taken as a reference and was subtracted from all the absolute absorbance data. Furthermore, to increase the absorbance value, two pieces of monolayer film-deposited SnOzglass substrate, spaced by 75 pm of Teflon sheet, were used for each spectrophotometric measurement. In addition, the spectrophotometric method was also employed to follow the thermal isomerization kinetics of the cis isomer in chloroform solution. In this case, the temperatures were controlled through a water-jacketed cell holder (Shimadzu). The cis isomer was created by UV-irradiating the chloroform solution of trans isomer (38 NM) for ca. 1 min. The reaction was then followed by monitoring the absorbance change at 352 nm, the absorption maximum of the trans isomer in chloroform solution (see Figure 8c).
Results (1) Electrochemical Results. The experimental results obtained from the electrochemical quantification method are expressed as the cis percentage, a,, as a function of time for each temperature, as shown in Figure 5. It can be seen that the cis percentage in the monolayer film decreased monotonically as the reaction
-8
-C
-9 -10 -1 1 3.1
3.2
3.3
3.4 103
3.5
3.6
3.7
(im
Figure 7. Arrhenius plot of the thermal cis-trans isomerization rate constants of ABD in the assembled monolayer film. TABLE I: Temperature Effect on the Thermal Isomerization Rate Constant of cis-ABD in Different Reaction Environments i04k,s-] (at T,"C) wet film dry film in CHC13
3 0.31
10 0.58 0.03
20 1.38 0.07 0.05
25
0.09
30 3.95 0.30 0.17
35
0.30
41 8.94 0.73 0.51
proceeded throughout the temperature range studied. Since the film molecules peeled off slowly into the aqueous electrolyte solution at higher temperatures, which was evidenced by the decrease of the cis amount in the photostationary state, Le., the reference data of cis isomer at t = 0, all of the kinetic studies were confined to a relatively earlier stage of reaction in order to prevent a large experimental error. For the same reason, the reference data of the cis isomer at t = 0 were always measured freshly for each run. A small variance in the reference data, Le., the initial concentration of cis isomer, may not affect the acquired kinetic parameters because the first-order rate constant is independent of the initial concentration of reactant (vide infra). The thermal cis-trans isomerization of azo species is known to follow first-order reaction kinetics in a wide variety of media and phases.*-I9 For the present system, such a first-order relation can be expressed by In (100/a,) = kt (2) where a,, k, and t are cis percentage, rate constant, and reaction time, respectively. Figure 6 shows the first-order plots of cis-ABD in the assembled monolayer film for several temperatures. Fairly good linearities were observed for all the temperatures studied. From the least-squares slope of the first-order plot, the rate constant of the thermal isomerization a t each temperature was
1878 The Journal of Physical Chemistry, Vol. 96, No. 4, 1992
Liu et al.
/
0 0
300
425
550
Wavelength (nm) Figure 8. Absorption spectra of ABD molecules: (a) trans-monolayer film; (b) trans-dominated trans/& mixed-monolayer film, which was obtained by UV-irradiating the trans-monolayer film for 2 min; (c) trans isomer in chloroform solution. 0.008
vj
0.006
10
I
I
I
I
20
30
40
50
60
Time (min.)
Figure 10. Comparison of the first-order plots obtained from electrochemical (solid line) and spectrophotometric (dashed line) methods (20 OC). The vertical lines indicate maximum uncertainties.
monolayers, considering the double sides of the glass substrate), which were used to increase the absorption. Assuming that Beer's law still holds true for monolayer absorption, the fmt-order relation is then given by
In (A" - A o ) / @ " - A ) = kt
(3)
where A , Ao, and A" are absorbances at 325 nm at times t , zero, and infinity, respectively, and k is the rate constant. In Figure .-! 0.004 10, the typical first-order plot thus obtained a t 20 OC (dashed -al line) is shown. For comparison, the first-order plot obtained from K the electrochemical method at the same temperature is replotted 0.002 in Figure 10 (solid line). The error range for each datum is also marked in both cases. Obviously the error ranges of data in the 0.000 spectrophotometric measurement are distinctly larger than those 0 120 240 360 480 600 in the electrochemical measurement. Similar results were obtained for all the other temperatures studied. The rate constants at Time (min.) different temperatures obtained from the least-squares slopes of Figure 9. Absorbance change at 325 nm of the ABD monolayer film with the first-order plots are also summarized in Table I. time at 20 OC. The absorption at t = 0 was subtracted from all the The large errors observed in the spectrophotometric meaabsolute absorbance data. surements are ascribed to the extremely small absorption of ABD molecules in the ultrathin monolayer film. The error range of evaluated, and all of them are listed in Table I. In Figure 7,the the rate constant in this case was estimated to be more than &a%. Arrhenius plot of the rate constants is given, which also shows On the other hand, the data errors in the electrochemical meaa fairly good linearity. surements mainly arise from the integration step of the cis re(2) SpectrophotometricResults. The spectrophotometric study duction peak for calculating the cis amount in the monolayer film, was performed for two purposes: (1) to understand the advantages which is because of the existence of a non-faradaic current comof the electrochemical approach and (2) to investigate the effect ponent.22-27 As one of the important features of the electroof the aqueous electrolyte. To ensure the kinetic investigations chemical techniques, a high accuracy of measurement can be being done in a truly dry condition, all of the monolayer film achieved by using a suitable sweep rate of pote~~tial.~' In the samples were kept in a silica gel desiccator for at least 24 h before present experimental conditions, the error range of the rate the experiments. Figure 8 shows the absorption spectra of the constant was estimated to be within *lo%. trans-ABD and the trans-dominated trans/cis ABD mixture in homehationin chloroform Solution. The thermal (3) the assembled monolayer film, in which the trans/& ABD mixture isomerization of cis-ABD molecules was also studied in chloroform was obtained by UV-irradiating the rrans-ABD for 2 min. For solution. In this case, the first-order plots exhibit excellent lincomparison, the absorption spectrum of the trans-ABD in chloroform solution is also given in the same figure. The intense 1 ~ * earities for all the temperatures studied, indicating that the reaction solution follows first-order kinetics, similar to the transition band centered at 352 nm in chloroform s o l ~ t i o n ~ . ~ . ~i n~chloroform *~~ behavior of the other azo compounds. The rate constants are was considerably blue-shifted to 325 nm in the monolayer film. summarized in Table I. This implies that a strong coupling between the -N=Nchromophores occurs because of the highly-ordered film strucDiscussion ture.z5,26 The peak absorption of the T-T* band was chosen to Activation Parameters. The activation energy (E,) and the monitor the change of cis-ABD in the monolayer film during the preexponential factor ( A ) of the thermal cis-trans isomerization thermal isomerization. Figure 9 shows the typical absorbance can be obtained from the Arrhenius plot of rate constants. The change at 325 nm taken at 20 OC. As anticipated, considerably values of enthalpy (AH')and entropy (AS') of activation are then large fluctuations of absorbance were observed though the two calculated through the relationship AH*= E, - RT and AS*= pieces of monolayer film-deposited Sn02 glass (actually four R[ln A -In (kT/h)- 1.00].*8*28In Table 11, the related activation
2
-
( 2 5 ) Liu, Z. F.; Loo, B. H.; Baba, R.; Fujishima, A. Chem. Lett. 1990, 1023-1026. ( 2 6 ) Nakahara, H.; Fukuda, K.: Simomura, S.; Kunitake, T. Nippon Kagaku Kuishi 1988, 1001-1010.
(27) Bard A. J.; Faulkner, L. R. Electrochemical MethodsFundamentals and Applications; John Wiley & Sons: New York,1980 pp 21 3-242.
The Journal of Physical Chemistry, Vol. 96, No. 4, 1992 1879
Thermal Cis-Trans Isomerization in Monolayer TABLE II: Activation Parameters for the Thermal Isomerization of cis-ABD in Different Reaction Environments
E.,
kcal mol-' wet film dry film in CHCll crystal'
16.5 f 1.0 20.4 f 4.0 21.2 f 0.5 53.3 f 4.5
f
rotation
AH* I
A, s-l kcal mol-l AS', eu 108,5*1.S15.9 f 1.0 -21.7 f 6.8 10'0*3,0 19.8 f 4.0 -14.3 f 13.7 1O1Of0.* 20.6 f 0.5 -12.4 f 3.6 1031*3,052.7 4.5 81.5 f 13.7
*
For azobenzene, ref 20.
parameters calculated at 293 K are summarized. For comparison, similar parameters of azobenzene in crystal a t the same temperatureZ0are also listed in Table 11. From Tables I and 11, we noted that the kinetic results obtained from the electrochemical and the spectrophotometric methods show a small difference. Taking into account the considerably larger errors of the spectrophotometric data, such a difference may not have an exact meaning. In other words, the wet electrochemical measurement may not give any additional effect on the isomerization kinetics as compared with the dry spectrophotometric measurement. It is clear from Table I1 that the activation parameters of cis-ABD in the assembled monolayer film are distinctly smaller than those of azobenzene in crystal, whereas they are quite comparable with those of cis-ABD in chloroform solution. In the first place, for the crystalline-phase isomerization, the apparent activation enthalpy of azobenzene is more than 3 times the value of the cis-ABD isomer in the monolayer film. This large difference is believed to arise from the specific reaction environment rather than from the small change in molecular structure, considering the substituent effect on the activation enthalpy. For instance, the AH*change induced by similar substitutions on the 4 and 4' positions of the phenyl rings were less than 5 kcal mol-I for benzene and n-butyl stearate solutions.19 The difference in kinetic behaviors in the crystal and in the assembled monolayer film is also reflected by the following facts. In the crystal case, a topochemical behavior is r e p ~ r t e d : * ~the * ~reaction ~ proceeds via induction, acceleration, and decay periods, leading to a sigmoid behavior of the cis percentage versus time.zo*29 However, such behavior was not observed in the monolayer film, as is clearly seen in Figure 5 , where the cis percentage decreased monotonically as a function of time, as it does in chloroform solution. In the second place, the enthalpy and entropy of activation of azo compounds in the liquid phase are known to be affected by solvent polarity.'I-l4 In general, the higher the polarity of the solvent, the larger both AH* and AS*,but the Gibbs free energy of activation, AC* (AG*= AH*- TAS*),varies little because the change of AlP is compensated by that of TAS*, which is called the compenratjon effect of A P and AS*.1213,M The typical values of AH* and AS*of azobenzene-typezazo compounds in the liquid phase are 20-25 kcal mol-' and 0 to -15 eu, respectively. Obviously both AlP and AS*of cis-ABD in chloroform solution are in good agreement with such typical values. Here it may not be important to argue how similar or how different are the activation parameters (AH* or AS') between the monolayer film and the chloroform solution. The attention should be paid to the distinctly small AH* and AS* values in the rigid and highly-oriented film structure as compared with those in the crystal. Effects of the Assembled Monolayer Film Structure. The distinctly small activation enthalpy of cis-ABD in the assembled monolayer film is believed to be attributed to the specific film structure. On the one hand, it would be reasonable to predict a relatively smaller activation enthalpy for the monolayer film as compared to the crystal. The large activation enthalpy required for crystalline-phase isomerization is ascribed to the existence of strain energy at the interface of cis and trans lattices.29 Obviously that kind of strain energy may not exist in the highly-ordered
*
inversion
Figure 11. Rotation and inversion mechanisms for the thermal cis-trans isomerization of azo compounds.
monolayer film structure. On the other hand, the ABD molecules in the present system were densely assembled due to the high film-fabrication pressure. The dense film structure was evidenced by its inhibition of the photochemical trans to cis isomerization, which is accompanied with an increase of molecular occupying volume.25 Because of the lack of free space in the rigid monolayer film, only about 20% of the trans-ABD was converted to the cis-ABD by the UV-irradiation preceding thermal isomerization. As a comparison, more than 90%of the rrans-ABD was photoisomerized to the cis-ABD in chloroform solution under the same experimental c ~ n d i t i o n . ~In~ addition, as mentioned above, a strong coupling between the -N=Nchromophores of ABD molecules was observed in the monolayer film. This may also be evidence for the dense film structure. Apparently the volumeincreasing cis-ABD formation in the dense monolayer film would induce a lateral tension between ABD molecules, which leads to the cis-ABD being at a relatively unfavorable state. The above specific structural features of the monolayer film are considered to result in the significantly small activation enthalpy. The aqueous electrolyte employed for the electrochemical measurements was also expected to affect the isomerization kinetics somehow, e.g., by a partial solvation or a proton catalysis.18 However, such effects could not be evidenced experimentally since no significant difference was observed between the wet electrochemical and the dry spectrophotometric results, as is clearly shown in Table 11. Isomerization Mechanism. As mentioned in the Introduction, both rotation and inversion have been proposed as possible mechanisms for the thermal cis-trans isomerization of azo species,&zocf. Figure 11. The present study was aimed primarily a t determination of the activation parameters in the assembled monolayer film. It would be difficult to conclude unequivocally whether the thermal isomerization occurs via a rotation or an inversion mechanism, based on the results obtained. However, the inversion mechanism may be more favorable to elucidate the small activation enthalpy and the strongly negative activation In the first place, theoretical calculations based on the CND0/2 method predict activation enthalpy for the rotation mechanism to be more than 50 kcal moP,l8J2which is significantly higher than the experimental value obtained in the present study. Hence the inversion mechanism is energetically favorable. In the second place, the strongly negative AS*indicates a loss of much molecular freedom in going to the transition stateaZ0This would not apply to the rotation mechanism because of the Ir-band rupture in the transition state.12-18*20On the contrary, the inversion mechanism may work well since the formation of a linear transition
~~
(28) Petersen, R. C. J. Org. Chem. 1964, 29, 3133-3135. (29) Tsuda, M.; Kuratani, K. Bull. Chem. Soc. Jpn. 1964,37,1284-1288. ( 3 0 ) Leffler, J. E. J . Org. Chem. 1955, 20, 1202-1231.
(31) Liu, Z. F. Ph.D. Thesis; The University of Tokyo, 1990; Chapter 11. (32) Ljunggren, S.;Wettermark, G. Acra Chem. Scand. 1971, 25, 1599-1 603.
J . Phys. Chem. 1992, 96, 1880-1888
1880 WE
0
.ABD
Distance
-
Figure 12. (top) Schematic diagram of the ordered ABD monolayer film
structure on a SnO, glass electrode. (bottom) Potential profile through the electrolyte solution in an electrochemical system. state with an sp-hybridized nitrogen atom would be accompanied by a loss of molecular freedom.12-ls~20 In addition, it seems worthwhile to study the effect of an electrochemically-produced electric field on the isomerization kinetics. Obviously interactions between electric field and molecular dipoles would occur in the present system. Since the transition states derived by rotation and inversion mechanisms have distinctly different dipole moments,'4J9reaction kinetics based on the two schemes would be affected differently in response to the change of electric field. Such an approach is particularly
possible using the present experimental arrangement as schematically demonstrated in Figure 12, where the electric field is generated electrochemically between working and counter electrodes. Since the potential gradient mainly occurs in the double-layer vicinity of the working electrode?' the necessarily strong electric field can be easily achieved using the ordinary potential bias. And further, the highly-ordered arrangement of ABD molecules in the electric field may prevent molecular dipoles from compensation with each other and thus enhance the effect of electric field. Work is in progress on this theme.
Conclusion The kinetic parameters for the thermal isomerization of cisABD molecules in the assembled monolayer film are precisely determined using the electrochemical method for the first time. The activation enthalpy in such a reaction environment is found to be distinctly smaller than that of azobenzene in crystal, but quite comparable with that of cis-ABD in chloroform solution. The kinetic behavior observed is attributed to the specific isomerization environment effect created by the assembled monolayer film. The present results suggest the inversion mechanism for the thermal isomerization and also imply that studies on the electrochemically-generated electric field effect may offer more useful information on the reaction mechanism. In addition, from a direct comparison with the conventional spectrophotometric method, the electrochemical approach employed in this study is evidenced to be considerably effective for quantification of the ultrathin monolayer film. Acknowledgment. We thank Drs. S. Suzuki, K. Imura, K. Yamamoto, and K. Hyodo of Mitsubishi Paper Mills for their helpful discussions and gratefully acknowledge the financial support from Mitsubishi Paper Mills. Registry No. ABD, 112360-09-5;SnO,, 18282-10-5.
Microkinetic Analysis of Diverse Experimental Data for Ethylene Hydrogenation on Platinum James E. Rekoske, Randy D. Cortright, Scott A. Goddard, Sanjay B. Sharma, and J. A. Dumesic* Department of Chemical Engineering, University of Wisconsin, Madison, Wisconsin 53706 (Received: September 9, 1991)
Kinetic analysis employing a mechanism that captures the essential surface chemistry of the reaction allows quantitative interpretation of diverse experimental data. This approach is used with a Horiuti-Polanyi mechanism, modified by hydrogen activation steps, to describe the surface chemistry for ethylene hydrogenation over platinum catalysts. In this investigation, kinetic analysis provides a quantitative means of comparing, contrasting, and consolidating results from steady-state kinetic studies, deuterium tracing measurements,vibrational spectroscopy,and temperature programmed desorption. A noncompetitive pathway is dominant at low temperatures, involving sites for hydrogen adsorption that are not blocked by carbonaceous species. At higher temperatures and lower ethylene pressures, more surface sites become available for hydrogen adsorption, and the reaction shifts to a pathway involving competitive hydrogen and ethylene adsorption.
Introduction The quantification of chemical kinetic phenomena on heterogeneous catalysts is an important aspect of assessing the performance of these materials and for understanding the essential surface chemistry that controls catalyst behavior. For example, it is common to describe steady-state reaction kinetics data in terms of rate expressions based on various catalytic reaction mechanisms (e&, Boudartl). Quantitative studies of this type are vital for the comparison of different catalytic materials studied over a range of reaction conditions and for elucidation of relationships between catalyst performance and catalyst chemical properties.
* To whom correspondence should be addressed. 0022-3654/92/2096-1880%03.00/0
A key problem faced in the analysis of kinetic data is that studies of catalytic and surface chemistry are generally conducted over a variety of reaction conditions, as dictated by the limitations of the various experimental techniques used in these studies. Therefore, it is difficult to bring this information together for the purpose of extracting quantitative kinetic information. Accordingly, an important problem in chemical kinetics studies of heterogeneous catalysts is to interpret, coordinate, and generalize results from diverse experimental studies to create a basis for quantitative description of kinetic phenomena. We attempt in ( 1 ) Boudart, M. The Kinetics of Chemical Processes; Butterworth-Heinemann: Boston. 1991.
0 1992 American Chemical Society
