Langmuir 1986, 2, 824-828
824
Adsorption Sites for Water on Graphite. 3. Effect of Oxidation Treatment of Sample Kazuhisa Miurat and Tetsuo Morimoto*f Department of General Education, T s u y a m a National College of Technology, Tsuyama 708, J a p a n , and Department of Chemistry, Faculty of Science, Okayama University, Okayama 700, J a p a n Received J u n e 23, 1986. I n Final Form: September 4, 1986 T h e adsorption isotherms of H 2 0 were measured on graphite samples which were partially burnt in a n HzO-O2 mixture and then outgassed at higher temperatures. On these samples, the gas content and acidic surface functional groups were determined. The isosteric heat of adsorption of H20,qst,calculated from the second adsorption isotherms, decreases more steeply when the pretreatment temperature of graphite is raised. The qst curve on the 1000 "C treated sample reveals a distinct minimum, a t which the qst value is very small compared with the heat of liquefaction of H20. I t has been found that these phenomena depend on the variety and quantity of surface oxides on graphite and that the qst minimum appears when each amount of the HzO-, COP-,and CO-desorbing oxides is extremely small.
Introduction It has been known that the prism surface of graphite has various kinds of functional groups which serve the adsorption sites for polar molecules.'-5 In the previous pap e r ~ , ~the , ' adsorption sites for HzO on graphite were investigated b y measuring the HzO adsorption isotherm on differently pretreated samples and b y calculating t h e isosteric heat of adsorption of HzO, qat. It was found that the number of the HzO adsorption sites decreases with rising temperature of pretreatment of the sample, and at the same time the qat value also decreases. In the extreme case of the 1000 "C treated graphite, a distinct minimum appears on the qat curve, the minimum value being less than the heat of liquefaction of HzO, HL. The pair sites of the HzO- and C0,-desorbing oxides were found t o a c t more strongly as HzO adsorption sites. CO-desorbing oxides d i d not reduce t h e qst value below the HL value. The autoclave treatment of graphite at 300 "C decreases the amount of surface oxides.' Leine et al. have reported that the O2 treatment of Graphon gives rise to a surface which is a b u n d a n t i n the CO-desorbing oxides.s In the present investigation, we have studied t h e pretreatment of a natural graphite in a HzO-O2 mixture and subsequent HzO adsorption measurements.
Experimental Section Materials. The original natural graphite was supplied from Nippon Kokuen Co., which was mined in Sri Lanka. The sample is 99.5% in purity and contains 0.5% ash. The original sample was treated a t 600 "C in a flowing gas mixture of O2 and H 2 0in a quartz tube shown in Figure 1. The gas mixture was 760 torr (1torr = 133.3 Pa) total pressure, containing 24 torr of H20,and the flow rate was 16.0 cm3/min. The quartz tube was rotated in a tubular furnace for 7 h, which resulted in a 20% loss of the sample weight. The sample thus formed was outgassed in a vacuum of torr at room temperature and then heated in vacuo for 5 h at 25,400,700, and loo0 "C respectively for OG-25,OG-400, OG-700, and OG-1000. The specific surface area of the samples, measured by the N2adsorption, was found to be the same for every sample, 7.36 f 0.32 m2/g, regardless of different heating temperatures. The SEM observation was made through an electron microscope, JEOL JSM-35. Measurement of the Water Adsorption Isotherm. Just after the heat treatment of the sample at a given temperature, the first adsorption isotherm of H20 was measured a t 25 OC up to a relative pressure of about 0.7, the sample was exposed to +
Tsuyama National College of Technology. Okayama University.
0743-746318612402-0824$01.50/0
saturated H 2 0 vapor for 48 h at 25 "C to complete the surface hydration, and degassed a t the same temperature for 10 h in a vacuum of torr, and then the second adsorption isotherm was measured repeatedly on the same sample at temperatures of 10, 18, and 25 "C. The adsorption equilibrium was attained after a fairly long time in a low-pressure region of the first adsorption process: it took over 1h for the three samples other than OG-1000, while for the latter the equilibrium was accomplished after 3 h. At higher pressures, however, it took about 40 min on every sample. In the second adsorption process, every sample was equilibrated more easily, i.e., within 30 min. The adsorption isotherm was measured volumetrically by a conventional adsorption apparatus, equipped with an oil manometer. Determination of Surface Compounds. After the completion of the second adsorption isotherm, the gas content of samples was measured by the successive ignition loss m e t h ~ d .In ~ other words, the amount of gas expelled by heating samples at every 100 "C interval of temperature from room temperature to 1000 "C was determined volumetrically. The evolved gas contained five kinds of gases, H20,C02,CO, CHI, and H2. Each component was determined by combining two techniques, the gas chromatographic analysis and the volumetric gas analysisa6The amount of acidic surface functional groups was determined by titrating samples with four kinds of bases, NaOC2H5,NaOH, Na2C0,, and NaHCO3,Ioas described in the previous work.6
Results The electron micrograph of t h e oxidized graphite sample, OG-25, is shown in Figure 2, where t h e micrograph of t h e original graphite, G-25, is also added for a comparison. From Figure 2, i t is found that t h e partially burnt-off graphite gives rise to indented edges along t h e periphery of particles and small holes on t h e basal plane which has been considered to be fairly inert toward oxidation." The (1)Young, G.J.; Chessick, J. J.; Healey, F. H.; Zettlemoyer, A. C. J . Phys. Chem. 1954,58, 313. (2) Healey, F. H.; Yu, Y. F.; Chessick, J. J. J . Phys. Chem. 1955, 59, 399. (3) Puri, B. R.; Murari, K.; Singh, D. D. J . Phys. Chem. 1961, 65, 37. (4) Dubinin, M. M.; Serpinsky, V. V. Carbon 1981, 19, 402. (5) Barton, S. S.; Koresh, J. E. J. Chem. SOC.,Faraday Trans. 1 1983,
79,1147. (6) Morimoto, T.; Miura, K. Langmucr 1985, 1 , 658. (7) Morimoto, T.; Miura, K. LangmiLir 1986, 2, 43. (8)Leine, N.R.; Vastola, F. J.; Walker, P. L., Jr. J. Phys. Chem. 1963, 67, 2030. ( 9 ) Nagao, M.; Morishige, K.; 'l'akeshita, T.; Morimoto, T. Bull Chem. SOC.J p n . 1974, 47, 2107. (10) Boehm, H. P.; Diehl, E.; Heck, Lt .; Sappok, R. Angew. Chem., Int. Ed. Engl. 1964, 3, 669.
C 1986 American Chemical Society
Langmuir, Val. 2, No. 6, 1986 825
Adsorption Sites /or Water on Graphite
0
Figure 3. Surface gas content on oxidized Eraphite: Hz0 (A); C02lo):CO (a): CH, (ohH, (0).Solid and broken lines indicate gas content on OG and G, respectively
< H20, COP< CO < HP6 I t is seen from Figure 3 that the content of every kind of gas is reduced by the oxidation treatment. It should be noted that CH, was not detected from OG-25, though a considerable amount was found from G-25. The content of H,O decreases monotonically with rising temperature, while the content curves on H,, CO, and CO, reveal a flat part, which implies that the latter gases are not generated until a threshold temperature is reached. In addition, H, is not expelled from OG-25 below 800 "C,while it is evolved from G-25 when ignited above 700 "C. Moreover, the content of H, on OG-25 is extremely small, Le., 1/5 compared with that on G-25. The amount of CO on G-25, evolved from 25 to 600 O C , is found to have disappeared through the oxidation treatment, which leads to a constant content of CO on OG-25 in this temperature range. The content of CO, on G-25 decreases monotonically with increasing temperature, but that on OG-25 remains constant, with about one-half of the original amount on G-25, till 400 O C , and then decreases. Also,the evolution peak of COz,which corresponds to the point where the greatest decrement is observed in the surface content, shifts to a higher temperature region, 500-600 "C, by the oxidation treatment. The adsorption isotherms of H20 on the OG samples me illustrated in Figure 4, where the adsorbed amount is expressed in cm3 (STP)/mZon the basis of the N2area. The result in Figure 4 demonstrates that the first and second adsorption isotherms differ from each other. As discussed in the previous paper: it is understood that this difference is due to the fact that the chemisorptionof HzO is involved in the first adsorption process and that the rate of chemisorption varies from sample to sample: the chemisorption on OG-25 is fairly slow, but that on OG-400 is fast, and the rate on OG-700 becomes again slow. A very irregular chemical reaction process is suggested to occur during the course of the first adsorption measurement of H,O on OG-1000: steps appear in the first adsorption isotherm. After each sample was equilibrated with saturated H,O vapor and then evacuated a t room temperature, all the second adsorption isotherms on the OG-samples give the shape of the type I1 according to Brunauer's so in the order CH,
Figure 1. Rotary quartz tube used for the oxidation treatment.
Figure 2. Scanning electinn micrographs of graphite: (a) G-25; (b) OG-25.
specific surface area of graphite is reduced by this treatment from 8.63 i 0.20 m2/g on G-25 to 7.36 i 0.32 m2/g on OG-25. This is probably due t o the fact that fine graphite particles were consumed by the burning treae ment. Figure 3 shows the variation in the surface content of four kinds of gases, H,O, COa CO, and Ha, on OG-25 as a function of heating temperature, and also here the surface content on G-25 is cited for a comparison. In this calculation, the amount of each gas is assumed to be zero on the surface heated at loo0 "C, and the surface content is expressed in the number of molecules per nanometer squared on the basis of the N, area. At first sight, this figure shows that the surface content on OG-25 increases in the order H2 < HaO, C02 < CO, while on G-25 i t does
~~
(11) Hennig, G. R.In Chemistry and Physics of Carbon; Walker.
L..Jr., Ed.; Mareell Dekkeer: New York. 1966, Vol. 2, p 1.
P.
(12) Brunsuer. S.; Deming, L. S.; Deming. W.E.;Teller, E.J . Am. Chem. Soe. 1940,62, 1123.
826 Langmuir, Vol. 2, No. 6, 1986
Miura and Morimoto
Closed circle, first
Figure 4.
adsorption Table I. Monolayer Volume of Water and Surface Content of Gases on Samples" surface content OG-25 OG-400 OG-700 OG-1000 G-25
0.889 0.880 0.637 0.544 1.740
0.858 0.759 (0.394) 0.256 (0.002) 0.236 (0.000) 1.710
0.937 0.853 (0.812) 0.108 (0.004) 0.097 (0.000) 1.644
1.289 1.288 (1.274) 1.360 (1.235) 0.270 (0.000) 1.952
0.000 0.000 (0.000) 0.293 (0.000) 0.049 (0.000) 1.233
0.486 0.486 (0.486) 0.486 (0.486) 0.332 (0.000) 2.294
Expressed in molecules/nm' on the basis of N2 area.
having a fairly large knee compared with that on the original graphite samples G.6 In Table I, the monolayer volume of H 2 0 , V,, and the surface content of each gas evolved are listed, where the V , value was computed by applying the BET method to the second adsorption isotherms. The surface content in the bracket represents the value obtained when measured just after the pretreatment of the sample at a given temperature. When the samples were exposed to H 2 0 vapor during the first adsorption process, a small amount of surface compounds were found to be regenerated by means of the successive ignition loss method. As a result, it was found that they are decomposed over the whole range of heating temperature and that the same kinds of gases are evolved as those found before the H 2 0 adsorption process. Thus, the amount of each component is added to the value in the bracket in Table I, and the sum of them is listed outside the bracket; the final value is directly related to the surface on which the adsorption of H 2 0 occurs in the second adsorption process. Table I indicates that the V , value of H 2 0 on the OG samples decreases when the pretreatment temperature is raised. The depression in the V , value is not so large compared with that on the G samples: the V , values on OG-25 and OG-1000 are 0.889 and 0.544 molecules/nm2, respectively, while those on G-25 and G-lo00 are 1.740 and 0.305 molecules/nm2,respectively.6 On the other hand, the amounts of COz, H,O, and CO expelled from the regenerated surface compounds on the OG samples are characteristically large compared with those on the G samples. The amount of H,O, C02, and CH, generated from the regenerated compounds on the OG samples decreases with increasing ignition temperature, while that of H, and CO increases, as in the cases of the G samples.6 Unexpectedly, the CH,-desorbing compounds are regenerated on both OG-700 and OG-1000 through a chemical
1
OO
0.2
0.4
0.6
0.8
1.o
1
Amount of adsorbed water /molecules.nm2
Figure 5. Isosteric heat of adsorption of H20, qst,on oxidized OG-25; ( 0 )OG-400; ( 0 )OG-700; ( 0 )OG-1000. graphite: (0) Arrows indicate V , (solid line), H 2 0 content (broken line), and C 0 2 content (dotted line).
reaction when the samples are exposed to saturated HzO vapor at room temperature, though they were once destroyed completely by the burning treatment. The qst value calculated by applying the Clausius-Clapeyron equation to the second adsorption isotherms is given in Figure 5, where the adsorbed amount is expressed in the number of molecules per unit area. The horizontal broken line in this figure indicates the heat of liquefaction of H,O at 25 "C, H L . On OG-25, the qst value decreases with increasing amount of adsorbed H,O and approaches the HL level at 0.25 molecules/nm2; then it is kept at the HLlevel. The same feature is observed on OG-400 and OG-700, but the break point on these curves slightly shifts to a smaller amount of adsorbed H,O. On OG-1000, the qst value drops steeply, crosses the HL level, attains a minimum value of 28.9 kJ/mol near 0 = 0.3, and then increases gradually to the HLlevel. A similar tendency has
Adsorption Sites for Water on Graphite
Langmuir, Vol. 2, No. 6, 1986 827
T a b l e 11. A m o u n t s of Acidic S u r f a c e Oxides acidic surface oxidesa carboxyl, sample OG-25 OG-400 OG-700 OG-1000 G-25
HzO, COz 0.32 0.00 0.00 0.00 1.12
lactone, COz 0.00 0.00
phenol,
0.00 0.00 0.28
carbonyl,
HzO, CO
CO
1.34 0.34 0.22 0.31 0.94
0.46 0.41 0.30 0.04 1.02
total 2.12 0.75 0.52 0.35 3.36
Expressed in groups/nm* on the basis of Nz area.
been observed on the other graphite samples, G-10006and HG-1000,7 the latter being prepared by the 1000 "C treatment of the autoclave-treated graphite. The amount of the acidic surface functional groups is listed in Table 11, which was determined by titrating samples with four kinds of bases and expressed in the number of groups per nanometer squared on the basis of the N2 area. The lactone groups could not be found on OG-25, though they were present on G-25. The carboxyl groups could not be detected after heating the sample at 400 "C. The OG samples have more phenol groups than the G6 and HG samples7treated under the same condition. However, the concentration of carbonyl groups on the OG samples is less than one-half of the value on the G and HG samples, and especially it is extremely small on OG-1000.
Discussion Pyrolysis of Acidic Surface Oxides. In the previous
paper^,^^^ the amount of HzO and C02 evolved by heating graphite samples has been related to the concentration of surface oxides determined by the titration of the samples with different kinds of bases. As a result, it was concluded that the evolution of H 2 0 and C 0 2 can be most satisfactorily explained by the following sequence of the decomposition reaction^.'^-'^ COOH
+ OH COOH 20H
-
-+
lactone
C02+ H 2 0
(1)
C02
(2)
HzO C02
(3)
(4)
In Figure 6, the surface content of H 2 0 and COPon the OG samples is plotted against the amount of the two kinds of gases calculated from the concentration of the surface oxides according to the reaction sequence (1)-(3). Here, reaction 4 is omitted, since the OG samples have no lactones. As discussed in the previous paper, if the 1:l relationship holds between the two sets of data, it can be understood that all the acidic surface oxides determined by the titration have been decomposed into two kinds of gases, HzO and C02, according to reactions 1-3. However, the result in Figure 6 shows that the experimentally determined amount of H 2 0 and COz is always larger than (13)Garten, V. A.;Weiss, D. E.; Willis, J. B. Aust. J. Chem. 1967, 10, 295. (14)Rivin, D. Rubber Chem. Technol. 1963, 36, 729. (15)Boehm, H. P. Angew. Chem., Int. E d . Engl. 1966,5, 533. (16)Lang, F.M.; de Noblet, M.; Donnet, J. B.; Lahaye, J.; Papirer, E. Carbon 1967,5, 47. (17)Coltharp, M.T.; Hackerman, N. J. Phys. Chem. 1968, 72, 1171. (18)Barton, S.S.;Boulton, G. L.; Harrison, B. H. Carbon 1972, 10, 395. (19)Tremblay, G.; Vastola, F. J.; Walker, P. L., Jr. Carbon 1978,16, 35.
Figure 6. Relation between a m o u n t of gas evolved by igniting graphite a n d that calculated from decomposition reactions: (0) H20; (v)COz. Solid line indicates 1:l relationship.
the calculated one, as in the cases of other graphite sample~.~ It ,is~ more striking to see that the amount of C02 deviates remarkably from the 1:l relationship compared with the amount of H20. This suggests that non-acidic COz-desorbing oxides20,21 exist in abundance on the OG samples. Adsorption Sites for Water on Graphite. The V , value of HzO as well as the content of HzO and C 0 2 are indicated by arrows in Figure 5. When the pretreatment temperature of the oxidized graphite is raised, the qst curve falls down more steeply and approaches the HL level at smaller coverage of HzO. In the extreme case of OG-1000, the qatcurve reveals a distinct minimum, at which the qst value lies below the HLlevel. The same feature was observed in the cases of the G6 and HG ~ a m p l e s . ~ As described above, the content of H 2 0 and C 0 2 on OG-1000 comes from regenerated surface oxides (Table I). Therefore, it is reasonable to infer that the adsorbed H 2 0 molecules interact with the regenerated surface oxides. The amount of adsorbed H 2 0 at which the qst curve crosses the HL level is approximately equal to the C 0 2 content, and the HzOcontent is still larger than the C02 content. Therefore, it can be considered that the adsorption of HzO occurs on the pair sites of the COz- and HzO-desorbing oxides until the crossing point in the qst curve is attained, as discussed in the cases of G-1W6and HG-1000.7 In spite of the variety of COz content, 0.035, 0.011, and 0.097 molecules/nm2 on three samples, G-1000,6 HG-1000,7and OG-1000, respectively, the crossing point of the qst curve with the HL level agrees well with the COz content on each sample. After these sites are completely occupied by the adsorbed HzO molecules, the adsorption of H 2 0 will take place on the oxide sites which can expel only HzO on pyrolysis, and further adsorption onto the preadsorbed H 2 0 molecules will result in the formation of clusters. A t the last stage, the qst value increases gradually toward the HL value, as discussed in the cases of G-1000 and HG-1000. It can be pointed out that the amount of adsorbed H20 at the qst minimum is smaller than the HzO content. This may be due to the fact that part of the H20-desorbing oxides on OG-1000 is located on the surface where it does not play a role as the adsorption sites for HzO. The qst curve on OG-700 has a break point, at which the amount of adsorbed H 2 0 is approximately equal to the H,O content, and the qat value almost coincides with the (20)Puri, B. R.; Sharma, G. K.; Sharma, S. K. J. Indian Chem. SOC. 1967, 44, 64.
(21)Voll, M.;Boehm, H. P. Carbon 1970, 8, 741.
Langmuir 1986, 2, 828-835
828
H L value. As shown in Figure 5, the shape of the qst curve on OG-400 and OG-25 is very similar t o t h a t on OG-700, t h e only difference being t h e fact t h a t t h e break point slightly shifts to higher coverage of H 2 0 from OG-700 to OG-400 and further to OG-25. Admittedly, there is a great difference between the qst curves on OG-700 and OG-1000. T h e reason why such a difference appears can be attributed to the fact that the CO content on OG-700 is as much as those on OG-25 a n d OG-400, in contrast t o a smaller value on OG-1000 (Table I). Walker and Janov have reported t h a t the CO-desorbing oxides on Graphon act as the sites for adsorption of HzO, which give the heat of adsorption similar to t h e HL value.22 Thus, the H,O
molecules will be adsorbed on the CO-desorbing oxides of OG-25, OG-400, a n d OG-700 after t h e sites, such as the C02- and HpO-desorbing oxides, have been completely covered with H,O.
Acknowledgment. We thank Professor Shigeharu Kittaka of Okayama University of Science for his great help in the electron microscopic observation. We also thank the Ministry of Education (Japan) for the financial support granted for this research. Registry No. H20, 7732-18-5; graphite, 7782-42-5. ( 2 2 ) Walker, P. L., Jr.; Janov, J. J. Colloid Interface SCL.1968,28,449.
Structure and Composition of a Platinum(ll1) Surface as a Function of pH and Electrode Potential in Aqueous Bromide Solutions Ghaleb N. Salaita, Donald A. Stern, Frank Lu, Helmut Baltruschat, Bruce C. Schardt,? John L. Stickney,* Manuel P. Soriaga,§ Douglas G. Frank, and Arthur T. Hubbard* Department of Chemistry, University o f California, Santa Barbara, California 93106 Received J u n e 23, 1986 Studies are reported in which surface layers formed by immersion of a well-defined Pt(ll1) electrode surface into aqueous CaBrz solutions were characterized with regard to structure, composition, and reactivity by means of low-energy electron diffraction (LEED), Auger electron spectroscopy, and linear-scan voltammetry. Voltammetry revealed a series of potential-dependent adsorption processes. Comparison of voltammetric curves with surface analysis data (Auger) and structural data (LEED) permitted identification of adsorbed layer structural transitions and adsorption/desorption processes responsible for the electrochemical behavior. Halogen adsorption was stronger from acidic than from alkaline solutions and was stronger a t potentials in midrange than at positive or negative extremes. Br atoms were the principal adsorbed form of halogen and underwent reductive desorption to Br- anions beginning at -0.1 V (vs. Ag/AgCl reference). A sharp voltammetric peak occurred at the onset of Br reductive desorption, corresponding to a structural transition within the halogen layer from ( 3 x 3 ) to (4x4) hexagonal close packing. In alkaline Br- solutions adsorption of oxygen species gives rise to a prominent reversible voltammetric peak (oxygen packing density, Bo = 0.5; LEED pattern, P t ( l l l ) ( l x l ) with a diffuse oxygenous overlayer), followed by an irreversible process which disordered the Pt surface. Br was not strongly adsorbed from alkaline solutions due primarily to strong competition from oxygenous adsorbates. Retention of water by the surface from vacuum correlated with Cap+retention and varied from 5 to 15 water molecules per Cap+cation, being largest at alkaline pH. In contrast, K+ ions, being less strongly hydrated, did not retain detectable amounts of water under vacuum. The Pt surface was hydrophilic toward bromide solutions over the full range of pH and potential.
Introduction In a preceding paper1 we reported t h a t ordered layers, adlattices, were formed when a P t ( l l 1 ) surface was immersed into aaueous solutions of KCN, KSCN, KHS, KI, or KBr. Relative affinities of various cations for retention a t such surfaces were explored.2 T h e p H dependence of ionization of t h e adlattices derived from KCN or KSCN solutions a t Pt(ll1) was found to reflect the structure and intermolecular interactions within the l a t t i ~ e . ~Evidence ,~
* T o whom correspondence should be addressed. Present address: Department of Chemistry, University of Cincinnati, Cincinnati, OH 45221. Present address: Department of Chemistry, Purdue University, West Lafayette, IN 47405. Present address: Department of Chemistry, University of Georgia, Athens, GA 30602. 'Present address: Department of Chemistry, Texas A&M University, College Station, T X 77843.
*
0743-7463/86/2402-0828$01.50/0
for strong adsorption of Br- onio polycrystalline Pt, for instance r a d i ~ c h e m i c a l , ~e ,l ~l i p s o m e t r i ~ ,and ~ ~ ~electroChemical d a h g has been reported. (1) Stickney, J. L.; Rosasco, S. D.; Salaita, G. N.; Hubbard, A. T. Langmuir 1985, I , 66. (2) Rosasco, S. D.; Stickney, J. L.; Salaita, G. N.; Frank, D. G.; Katekaru, J. y.; Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Hubbard, A. T. Electroanal. Chem. 1985, 188, 95. (3) Schardt, B. C.; Stickney, J. L.; Stern, D. A,; Frank, D. G.; Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. M.; Soriaga, M. P.; Hubbard, A. T. Inorg. Chem. 1985,24, 1419. (4) Frank, D. G.; Katekaru, J. Y.; Rosasco, S. D.; Salaita, G. N.; Schardt, B. C.; Soriaga, M. P.; Stern, D. A.; Stickney, J. L.; Hubbard, A. T. Langmuir 1985, I , 587. ( 5 ) Petrii, 0. A.; Frumkin, A. N.; Shchigorev, I. G. Elektrokhimiya . , , " a
IYIU,
II
,,,A
0,4uu.
(6) Kazarinov, V. E.; Petrii, 0. A.; Topolev, V. V.; Vosev, A. V. Elektrokhimiya 1971, 1365, (7) Hyde, P. J.; Maggiore, C. J.; Redondo, A.; Srinivasan, S.; Gottesfeld. S. J . Electroanal. Chem. 1985. 186. 267. (8) Hyde, P. J.; Gottesfeld, S. Surf. Scl 1985, 149, 601.
0 1986 American Chemical Society
