J . Phys. Chem. 1990, 94, 8203-8207 The errors quoted for the given rate coefficients were used to estimate upper and lower limits for these reactions. Simulations showed that even when the maximum and minimum values of k l / k 3were used there was only an insignificant change (f5%) in the [HCHO]/[CH,OH] value. ( v ) k l a / k l Branching Ratio. The parameter to which the [HCHO]/[CH,OH] ratio was found to be the most sensitive was the k , , / k , branching ratio. In this case, a 10% change in the branching ratio (e.g., from 0.20 to 0.22) resulted in a similar change in the predicted [HCHO]/[CH,OH] ratio. 2. Recommended Branching Ratio between 223 and 573 K . Although the results presented here lie in an atmospherically important temperature range, extrapolation to higher temperatures is desirable for simulation of combustion conditions. Both the Anastasi et a1.6 and especially Lightfoot et aL2 results were obtained at higher temeratures and are therefore incorporated together with this work in Figure 5 , where k l a / k lis plotted against 1000/T. It should be noted here that the points attributed to Anastasi et a1.6are not raw data points but were calculated from equations fitted to the data and presented in ref 6 . The technique employed by Lightfoot et al.* to determine a and p becomes less sensitive at lower temperatures where channel la and subsequent H 0 2 production (via reaction 2) are less important. For this reason. the measurement at 388 K was not fitted with the rest (22) DeMore, W . B.; Molina, M. J.; Sander, S. P.; Golden, D. M.; Hampson, R. F.; Kurylo, M. J.; Howard, C. J.; and Ravishankara, A. R. 'Chemical Kinetics and Photochemical Data for Use in Stratospheric Modeling"; Evaluation No.5, JPL Publication 87-41, 1987. (23) Veyret, B.; Lesciaux, R.; Rayez, M.; Rayez, J.; Cox, R. A,; Moortgat, G . K . J . Phys. Chem. 1989, 93, 2368. (24) Burrows, J. P.; Moortgat, G . K.; Tyndall, G.S.; Cox, R . A,; Jenkin, M. E., Hayman, G.D.; Veyret, B. J . Phys. Chem. 1989, 93, 2375.
8203
of the data. The fit is described by k , , / k , = a = 1/11 [exp(1330 f 12O/T)]/33 f IO}
+
(C)
which yields a value for a at 298 K of 0.28. When the 40% contribution from channel 3b is considered this becomes k l a / k I= a = 1/11 + [exp(1535 f 120/7')]/51 f IO] (D) yielding a value of a = 0.23 at 298 K. Combining our room temperature branching ratio (0.30) with our recent study' of the kinetics of the methylperoxy self-reaction yields a room temperature rate coefficient at 298 K of k , = 3.54 x cm3 molecule-' s-I.
Conclusion The fraction of the methylperoxy self-reaction that proceeds via the nonterminating channel l a ( a )is temperature dependent and is described by the expression a = 1/11
+ [exp(ll31 f 30/T)]/(19
f 5))
(A)
for the temperature range 223-333 K. Extending the temperature range by including previous measurements gives the result CY
= 1/(1
+ [exp(1330 f 120/T)]/(33
f IO)}
(C)
When a 40% contribution by channel 3b to reaction 3 is considered, eqs B and D replace eqs A and C, respectively.
Acknowledgment. We acknowledge the EEC for financial support within the Environmental Program, and T. J. Wallington and P. D. Lightfoot for making results available prior to publication. Registry No. CH,O,, 2143-58-0.
I n Situ FTIR and XPS Studies of the Hexacyanoferrate Redox System Monika Datta* Department of Chemistry, Delhi University, Delhi I IO 007, India
and Arunabha Datta* Alchemie Research Centre, P.O. Box 155, Thane- Belapur Road, Thane 400 601, India (Received: October IO. 1989; In Final Form: May I, 1990)
Evidence from in situ FTIR studies is provided for the presence of irreversibly adsorbed species on the platinum electrode in the hexacyanoferrate redox system in 0.5 M aqueous K,S04 at neutral pH. Both FTIR and XPS data show that the adsorbed species is Prussian blue, which is bound to platinum through the nitrogen of the CN group.
Introduction The hexacyanoferrate redox couple has long been considered to be a model system for the study of heterogeneous electrontransfer reactions with the oxidation/reduction in this system believed to be taking place through an outer-sphere electrontransfer mechanism. However, it has now been shown that the reaction is much more complex and that the electron-transfer rate is dependent on the nature and concentration of the supporting cation owing to the influence of cation adsorption,, ion pairing in the solution and in the double layer,* retardation of diffusion ( 1 ) Frumkin, A. M.; Petry, 0. A.; Nikoaleva-Fedorovich, N . V . Electro-
chim. Acra 1963. 8. 177.
by ion pairing3 and the formation of an activated complex involving the cation and the complex anion already paired with at least one other ~ a t i o n . It ~ has also been suggesteds-' that electrode-adsorbed dimeric species consisting of the oxidized and the reduced forms of the anion, coupled through a bridging cation, (2) Gierst, L.; Vandenbergen, L.; Nicholas, E.; Fraboui, A. J . Electrochem. Soc. 1966, 113, 1025. (3) Dieman, D. J.; Fawcett, W. R. J . Electroanal. Chem. 1972, 34, 27. (4) Peter, L. M.; Durr, W.; Bindra, P.; Gerischer, H. J . Electroanal. Chem. 1976, 71, 31. ( 5 ) Schleinitz, K. D.; Landsberg R.; Lowis of Menav, G. V. J . Electroanal. Chem. 1970, 28, 279, 287. ( 6 ) Sohr, R.; Muller, L.; Landsberg, R. J . Electroanol. Chem. 1974, 50, 55. (7) Sohr, R.; Muller, L. Electrochim. Acta 1975, 20, 451
0022-3654/90/2094-8203%02.50/0 0 1990 American Chemical Society
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The Journal of Physical Chemistry, Vol. 94, No. 21, I990
are involved in the redox reaction. In addition, evidence for the presence of other adsorbed species such as hexacyanoferrate and cyanide ions8 and Fe(CN)] species, resulting from the partial decomposition of Fe(CN)63-I" during heterogeneous charge t r a n ~ f e r . ~have . ' ~ also been provided from radiotracer, voltammetric, and in situ FTIR techniques. Subsequently, we presented preliminary evidence for the presence of adsorbed Prussian blue (PB) on the electrode surface in the hexacyanoferrate system," and a similar suggestion has also been made by Niwa and Doblhofer.12 The present work therefore describes the more elaborate attempts that have been made using a combination of in situ FTIR and ex situ XPS techniques to detect and characterize the adsorbed species on a platinum electrode in the [Fe(CN)6]3-'4- system in aqueous K2S04solution.
Experimental Section The electrochemical instrumentation consisted of a PAR Model 173 potentiostat and a PAR Model 175 function generator. The working electrode was a 1-cm-diameter flat disk of polycrystalline platinum sealed at the end of a stainless steel rod. The whole rod was press-fitted in a Teflon tube so that only the front surface of platinum was in contact with the solution. The electrode surface was polished with 0.05-llm y-alumina and cleaned ultrasonically and then electrochemically." The secondary electrode was a platinum loop arranged symmetrically around the working electrode, and a saturated calomel (IBM) electrode was used as the reference. Details of the design and construction of the electrochemical cell used have been described earlier.'l All measurements were made through a thin-layer solution (ca. 1 pm) of 10 m M K4Fe(CN), in 0.5 M aqueous K2S04solution at neutral pH. All glass parts were cleaned by boiling in H N 0 3 / H 2 S 0 4solution followed by steam-cleaning with triply distilled water. All salts were of AR grade and were recrystallized before use. FTIR spectra were recorded on a Nicolet 7199 FTIR spectrometer with a retroreflectance attachment." A wire grid polarizer was used to prevent the s-polarized radiation from reaching the detector. Data collection was done by low-frequency potential modulation of the electrode surface.II Six co-added interferograms were collected after the steady-state condition had been reached and stored at each of the two set potential limits (E,f and Esample) separately. The cycle was repeated n times until two sets of 6n co-added interferograms of the desired signal-to-noise ratio were obtained. In order to eliminate the interference of solution species, a time interval was introduced between each cycle. Typically, and six interferograms the electrode was pulsed for 6 s at Esample were collected, and then with the potential at Eref.an interval of 10 s was allowed before the six interferograms were collected. This allows time for any species electrogenerated at E,,, le to diffuse away from the electrode surface into the bulk. T i e co-added interferograms collected at Erefand Esample were Fourier-transrespectively. The formed into single-beam spectra Rnf and Rsample, final results were displayed as A R / R where A R / R = Rref Rumple/Rrcp This is the normalized difference spectrum and represents only the changes between the two states. In the present case, therefore, all peaks extending to positive values of A R / R will signify decreased absorption at that wavelength at sample potential relative to the reference potential, and vice versa. If there is no change in the infrared spectrum due to change in potential, the difference spectrum will be a straight line at zero value of AR/ R. For XPS studies, the electrode was emersed at +0.4 V, washed repeatedly with triply distilled water, and then immediately transferred to the spectrometer. The spectra were recorded on a VG ESCA-3 MK I 1 spectrometer using AI Ka X-rays (hv = 1486.6 eV) as the exciting source. All core electron binding energies were referred to the Fermi level and the 3d5,, peak (368.0 (8) Wieckowski, A.; Szklarczyk, M. J . Electrmnal. Chem. 1982, 142, 157. (9) Kawiak. J.; Jedral, T.; Galus, 2.J . Elecrrmnal. Chem. 1983, 145, 163. (IO) Pons, S.; Datta, M.; McAleer, J. F.; Hinman, A. S . J . Elecrroonol. Chem. 1984. 160. 369. ( I 1 ) Datta M.; Datta, A. Spectrosc. Lett. 1986, 19, 993. ( I 2) Niwa. K.; Doblhofer. K. Electrochim. Acto 1986, 31. 439.
1
2400
2100
'
zdoo
Id00
WAVENUMBERS
Figure 1. Normalized difference spectra between -0.2 and +0.4 V of aqueous ferrocyanide solution: (A) 6 X 20 scans, (B) 6 X 100 scans.
eV) of metallic silver. For computing the N:Fe ratio of the adsorbed species, the ratio of the areas of the nitrogen and iron peaks of PB was taken to be 3.0, and the multiplicative factor that emerged was used in the calculation. The PB used was prepared by a published method.I3
Results and Discussion In Situ FTIR Studies. The spectra of the species present at the electrode/electrolyte interface obtained by the potential modulation technique (as described in the Experimental Section) is shown in Figure 1. The band at 2040 cm-l is due to the depletion of ferrocyanide at positive potentials whereas the band at 2 1 14 cm-I represents the formation of ferricyanide. These bands correspond to the F,, mode of the C=N stretching vibration of the ferro- and ferricyanide molecules, respectively. In addition, however, there is a band at 2098 cm-I which disappears when the number of scans is increased from 20 to 100. The presence of this band is indicative of irreversible adsorption on the electrode surface, and its disappearance on longer signal averaging probably indicates the formation of a limiting thickness of the adsorbed material. This is because during the growth of the adsorbed layer there would always be a difference in the intensity of the band at the reference and sample potentials, but after the limiting thickness is reached the intensity of the band would be the same at both potentials and would therefore disappear in the difference spectrum. The difference spectrum recorded as a function of anodic potential (Figure 2) shows that the intensity of the bands at 2040 and 21 14 cm-l increases up to 0.2 V and then decreases. These observations are indicative of the depletion of solution species and therefore provide indirect evidence for electrochemical irreversibility in the system. There is further evidence of irreversible adsorption from the single-beam spectra (Figure 3a,b). At the reference potential of -0.2 V there is a band at 2040 cm-l due to the vCN of ferrocyanide and a broad band at 2100 cm-' which is a combination band of water. On stepping up the potential to 0.4 V (Figure 3a), bands at 2098 and 21 14 cm-I appear, and on reversing the potential to -0.2 V, the 2040-cm-l band reappears with reduced intensity while the 2098-cm-l band remains essentially unchanged. This indicates that, at 0.4 V, the ferrocyanide is oxidized to ferricyanide and some other new species with a band at 2098 cm-I is formed. On potential reversal, the ferricyanide is reduced back to ferrocyanide, but a part of the original ferrocyanide is lost in forming the irreversibly adsorbed species. On the other hand, when (13) Wilde, R . E.: Ghosh S. N.; Marshall, B. J. Inorg. Cfiem. 1970, 9, 2512.
Hexacyanoferrate Redox System
The Journal oJPhy.rica1 Chemistry, Vol. 94, No. 21, 1990 8205 I
l a
u
I0.1V
200
)O
WAVENUMBERS 00 2-2
WAVENUMBERS
Figure 2. Normalized difference spectra of aqueous ferrocyanide solution as a function of anodic potential. The reference potential was - 0 . 2 V.
the anodic potential is stepped up to 1.0 V (Figure 3b), the 2098-cm-' band, due to the adsorbed species, is the predominant one with the formation of a small amount of ferricyanide being indicated by the broadening and asymmetry of the 2098-cm-] band and the pronounced reduction in the intensity of the ferrocyanide band at 2040 cm-' on potential reversal. Thus, at both 0.4 and 1.O V, there is clear evidence of irreversible adsorption. In fact, the 2098-cm-' band was found to persist even after flushing the electrode/electrolyte interface with fresh solution. To provide more conclusive proof of adsorption, therefore, a different method of data collection was used in which 100 scans were taken at the reference potential and each of the sample potentials. The normalized difference spectra obtained this way (Figure 4) clearly reveal the presence of the 2098-cm-' band which grows in intensity with increasing positive potential. Correspondingly, the intensities of the 2040- and 2114-cm-' bands increase initially up to 0.25 V and then start decreasing, which parallels the appearance at 0.25 V of the 2098-cm-] band and its subsequent increase in intensity. It is obvious therefore that there is irreversible adsorption in the hexacyanoferrate system, and the 2098-cm-' band can be assigned to the ucNof the adsorbed species. The position of this band suggests that this species could be either adsorbed ferrocyanide, ferricyanide, decomposition products of these complex ions, such as free cyanide and Fe(CN),, or PB. The possibility of ferrocyanide being the adsorbed species could be ruled out because adsorption was found to take place only at positive potential of 0.25 V and above, where ferrocyanide would be oxidized. As regards the adsorption of ferricyanide, it was found that when the experiment was done using K,[Fe(CN),] instead of K,[Fe(CN),] as the substrate in the same supporting electrolyte solution (0.5 M K2S04),neither the 2098-cm-I band nor bands due to any other adsorbed species were observed (Figure 5). It is very unlikely, therefore, that the adsorbed species is ferricyanide or any of its degradation products as suggested by some authors.s-iO As far as the adsorption of free cyanide is concerned, it was observed that, in a system containing cyanide along with sulfate, the latter was preferentially adsorbed on the electrode surface with the sulfate acting as a scavenger for the cyanide. The vCN of PB on the other hand occurs around 2080 cm-I,l4 and it does seem very likely that the adsorbed species is PB with the positive shift (14) Ghosh
S.N . J . Inorg. Nucl. Chem. 1974, 36. 2465.
0
2f50
21'00
20'50
21 0 0
WAVENUMBERS
Figure 3. Potential dependence of the single-beam spectra of aqueous ferrocyanide solution. TABLE I: Comparison between the Binding Energies (eV) of Prussian Blue and the Species Adsorbed on the Electrode element
adsorbed species
Prussian blue
Fe2p1/,(Fe3+) Fe2p1/2We2+) Fe2p3i2(Fe3+) FeZp3/2We2+) N 15
723.9 721.6 712.0 708.7 400.2 398.2 377.7 295.7 293.1 284.8 77.8 74.3
723.8 721.4 711.9 708.6
K3s '3Pb/2 K3P3/2
c,,
Pt4fSi2 Pt4f712
397.9 377.9 295.8 293.1 284.7
of the uCN to 2098 cm-', indicating that its interaction with the metal surface takes place through end-on nitrogen interaction. However, since the adsorbed species was found (from ex situ FTIR spectra) to be stable even on emersion of the electrode, a more definitive characterization of the adsorbed species was done using X-ray photoelectron spectroscopy (XPS).
Datta and Datta
8206 The Journal of Physical Chemistry, Vol. 94, No. 21, 1990
____
r
1
2040
I
82
I
78
I
74
I
70
I
66
BINDING ENERGY ( e V ) 2200
2150
2100
2050
2000
1950
1900
WAVENUMBERS
Figure 4. Normalized difference spectra (without potential modulation) of aqueous ferrocyanide solution as a function of anodic potential. The reference potential was -0.2 V .
r
8 00
I
2150
21'00
2d50
2 00
WAVENUMBERS
Figure 5. Normalized difference spectra of 10 mM aqueous ferricyanide in 0.5 M K2S0, at various potentials.
XPS Studies. It is evident (Figure 6a, Table I) that the binding energies (BE'S) of nitrogen and iron of the adsorbed species
Figure 6. (a, top) Comparison of the XPS (Fez,, and Nls) of the species adsorbed on the platinum electrode (A) and Prussian blue (B): (+ + +) experimental spectra, (-) fitted spectra. (b, bottom) Platinum peaks in the XPS of the species adsorbed on the platinum electrode: (+ + +) experimental spectra, (-) fitted spectra.
correspond very closely with those of PB. In addition, the N:Fe ratio for the adsorbed material is 3.1: 1, indicating further that it is indeed Prussion blue and not ferro- or ferricyanide. It is knownI5 that PB can exist in two different forms, namely, the "insoluble" Fe,)"[Fe11(CN)6]3 and the "soluble" KFe111Fe11(CN)6. Actually both forms are highly insoluble (Ksp= lo*) in water, and the distinction refers to the ease with which the potassium compound can be peptized. However, in our case since photoelectron peaks due to potassium were observed, the adsorbed PB must correspond to the "soluble" form. The presence of an additional nitrogen peak on the higher BE side (Table I) in the case of the adsorbed species indicates that the PB interacts with the electrode surface through the nitrogen of the C N group. At the same time since only one carbon peak was observed, it would appear that the carbon of the CN group is not bonded to the surface. It may be mentioned here that shifts of about 2.0 eV in the BE'S of both carbon and nitrogen have been reportedi6 in the case of the adsorption through both carbon and nitrogen atoms of acetonitrile on nickel and palladium. Hence, the adsorption of PB on platinum can be assumed to be end-on through the nitrogen. This also fits in with the infrared data since the very observance of the vCN of the adsorbed Prussian blue indicates that the C=N bond should have its dipole oriented perpendicular to the electrode surface in accordance with surface selection rules. The shift of 2.0 eV on adsorption indicates a strong interaction of the PB with the electrode surface. In fact, an empirical relation between the binding energy and the effective charge on the nitrogen has been suggested" from Mulliken population data based on a b initio calculations of a series of nitrogen-containing comM.;Neff, V. D. J . Phys. Chem. 1981, 85, 1225. (16) Kishi, K.; Ikeda, S.SurJ Sci. 1981, 107, 405. ( 1 7) Sundberg P.; Larsson, R.;Folkesson, B. J . Electron Spectrosc. Relar. Phenom. 1988, 46, 19. ( 1 5 ) Ellis, D.; Eckhott,
J. Phys. Chem. 1990, 94, 8207-8212 pounds and well-calibrated XPS data. By use of this relation, an effective charge of -0.46 is indicated on the uncoordinated nitrogen of PB whereas the corresponding charge on the nitrogen presumably bound to the platinum is -0.17. In addition to the photoelectron peaks of the adsorbed species, peaks due to the platinum substrate are also observed (Figure 6b). This indicates that both the F H R and XPS data are representative of the adsorbed layer bound to the electrode surface rather than that of the first few monolayers of a thick adsorbed layer. At the same time the area of the platinum 4f7/2 peak is about 16 times that of the iron 2p312.peak of adsorbed PB, although the photoionization cross section of the 4f7/2 subshell of platinum is only slightly lower than that of the iron 2p3,2 subshell.ls Typically, the sampling depth of the spectrometer is about 0-20 A with the escape depth of photoelectrons from the platinum 4f levels being higher than those from the iron 2p levels because of their higher kinetic energy. Nevertheless, the higher intensity of the platinum peaks suggests that more photoelectrons from the substrate rather than from the adsorbed species were detected. Consequently, the thickness of the adsorbed layer should be much less than 20 A and would presumably be in the monolayer range. Conclusion It is quite evident therefore that irreversible adsorption takes place on the platinum electrode in the hexacyanoferrate redox system in 0.5 M K2SO4 at neutral pH. The adsorbed species is the "soluble" form of PB, which is bound to the electrode surface through the nitrogen of the C N group as evident from both the (18) Scotfield J. H. J . Elecfron Spectrosc. Relat. Phenom. 1976, 8,129.
8207
blue shift of the uCN of adsorbed PB in the infrared spectrum and the presence of a higher binding energy peak due to the nitrogen of adsorbed PB in the XPS. The chemical reactions by which adherent PB films are formed on metal surfaces is still not clearly understood. In our case, however, it is observed that PB is formed on the electrode surface only when ferrocyanide and not ferricyanide is used as the substrate. A possible mechanism therefore for the formation of PB could be that initially, at low positive potential (below 0.1 V), ferrocyanide is adsorbed on the electrode surface. At increasing positive potential, however, the ferrocyanide in solution is oxidized whereas the adsorbed ferrocyanide being more stable is not. The ferricyanide thus generated in solution, close to the electrode surface, then reacts with the adsorbed ferrocyanide to form PB. In constrast, when ferricyanide is used as the substrate, it is unlikely to form a stable adsorbed layer because the electrode surface would be expected to catalyze its reduction or perhaps decomposition.I2Js The formation of PB via an adsorbed layer of ferrocyanide would also explain why a limiting thickness of PB is formed. The lack of infrared evidence however for the proposed adsorption of ferrocyanide is probably due to the fact that the adsorption is nonspecific (electrostatic) in nature and is far too small to be detectable.12 It may be pointed out that the layer adsorbed on the electrode surface would, as reported,lsJ9 be porous enough to allow electron transfer to take place.
Acknowledgment. A.D. is thankful to IC1 India Limited for financial support of this work. (19) Itaya, K.; Ataka, T.; Toshnima, S. J . Am. Chem. Soc. 1982, 104, 4167.
Molecular Assemblies of ((Dodecyloxy)methyl)-l8-crown-6 in Water Sumio Ozeki,* Department of Chemistry, Faculty of Science,'Chiba University, 1 - 33 Yayoi-cho, Chiba 260, Japan
Akira Kojima, Laboratory of Chemistry, Tokyo Dental College, Chiba 260, Japan
Shigeharu Harada, Hamamatsu College of Shizuoka Prefectural University, Hamamatsu 432, Japan
Seiichi Inokuma, Hideo Takahashi, Tsunehiko Kuwamura,* Department of Synthetic Chemistry, Faculty of Engineering, Gunma University, 1-5- 1 Tenjin-cho, Kiryu 376, Japan
Hirotaka Uchiyama, Masahiko Abe, and Keizo Ogino Faculty of Science and Technology, Science University of Tokyo, Noda, Chiba 278, Japan (Received: July 27, 1989; In Final Form: June 6 , 1990) Molecular assemblies of ((dodecyloxy)methyl)-IS-crown-6(C,2-OM-crown) in water have been studied by dynamic light scattering, ultracentrifugation, and viscosity. The micelles of C12-OM-crowngrow with increasing surfactant concentration after aggregation of premicelles around the cmc. The hydrodynamic radius of the micelles increases from 6 to 20 nm, through the coexistence region of two kinds of micelles. The rodlike model for the large micelles, whose aggregation number is 2300 f 400 beyond 0.5 g/dL, seems to be consistent with the intrinsic viscosity values and geometrical consideration. The micellar growth is associated with dehydration from crown head groups. The cyclization effects of the head group on the micelle formation are discussed.
Introduction Crown ethers have an excellent selectivity and high chelating ability for cations. The transportation of ions from water to oil 'To whom correspondence should be addressed.
0022-3654/90/2094-8207$02.50/0
is not easy for crown compounds, although they have been used as ion carriers across bilayer or liquid membrane and phasetransfer catalysts.'V2 Recently, several reports3" showed that new ( I ) Mclaughlin, S.G . ;Szabo, G.; Ciani, S.;Eisenman, G. J . Membr. Biol. 1972, 9, 3.
0 1990 American Chemical Society
