J. Phys. Chem. 1987, 91, 2324-2327
2324
reasonably convincing. For the anomalously low measured values, the postulate of disconnected spheres seems sufficient to accommodate the data. In any event, whatever the ultimate microstructure which emerges from other experiments, it seems clear that the anomalous forces cannot be ignored.
Summary 1 . The interfacial tensions between microemulsion and oil phases have been measured for the three-component microemulsions employing didodecyldimethylammonium bromide as the surfactant. At the maximum surfactant concentration (the A points in the phase diagrams, Figure 2) the interfacial tension increases from 0.059 dyn/cm for hexane to 0.148 dyn/cm for tetradecane. As one moves down the minimal water line toward the oil corner (AB line in Figure l ) , the interfacial tension dedyn/cm near the B point. creases to -6 X 2. The physical properties of these microemulsions are consistent with interconnected water-filled conduits at high surfactant concentration and low water content (the A points). Upon addition of water or traversing the AB line, the conduits transform to w/o microemulsion spheres. 3. The interfacial tension of these inverted microemulsion structures can be understood in terms of the very large attractive
van der Waals forces associated with water-filled conduit cylinders in an oil continuum. Theory shows that, for nonconducting cylinders, the Hamaker constant is 40 times larger than for either spheres or bilayers; for conducting conduits the effect is even larger. Under the influence of these extraordinarily large attractive forces, the microemulsion conduits are drawn together, squeezing out excess oil until the oil-swollen surfactant chains from adjacent conduits come into contact. The change in energy associated with this process permits for the first time the quantitative evaluation of interfacial tension in inverted microemulsion phases. While the model contains a number of simplifying assumptions, the variation of interfacial tension with oil chain length and accompanying the transition from conduits to spheres can be delineated. 4. These initial observations suggest that the strong attractive colloidal forces associated with water-filled conduits in an oil continuum may play a more important role in inverted hexagonal surfactants and phospholipid phases than has been realized.
Acknowledgment. This work was funded by US.Army Grant DAAG29-85-K-0169. Registry No. Didodecyldimethylammonium bromide, 3282-73-3; octane, 110-54-3; decane, 111-65-9; dodecane, 124-18-5; tetradecane, 112-40-3; hexane, 629-59-4.
A Comparative Study on the Msperslon and Carrier-Catalyst Interaction of Molybdenum Oxides Supported on Various Oxides by Electron Spectroscopy for Chemical Analysis Nabin K. Nag* Department of Fuels Engineering, University of Utah, Salt Lake City, Utah 841 12 (Received: August 28, 1986; In Final Form: December 1 , 1986)
A comparative electron spectroscopy for chemical analysis (ESCA) study on the nature of molybdenum oxide phase dispersed on four different supports is reported. From the fwhm and M o ~ BE ~ ~data , ~it is concluded that Mo on 7-A1203is highly dispersed and that at low Mo loadings there is some electron transfer from the support to Mo; however, this effect is not detectable at higher loadings. SiOz shows no electronic effect at all, and TiOz and ZrOz show only mild effects. In general the dispersion, as reflected by Mo to support ESCA peak intensity ratio, tends to decrease at high loadings for all supports. Co is found to augment the dispersion of Mo and to transfer electrons to Mo supported on A1203.
Introduction Molybdenum-based hydroprocessing containing cobalt or nickel as a promoter are always carried on suitable supports in order to achieve high dispersion of the active phase; bulk MoS2 does have hydroprocessing activity, but not as high as supported ones.1.2 In addition to effecting high dispersion, supports sometimes modify the activity and selectivity pattern of the supported phase. This effect stems from electronic interaction between them3 Therefore, an insight into the chemistry between support and the active component is of vital importance in understanding the mechanism of catalyzed reactions, as well as the nature of the active sites of supported catalysts. ESCA, being a truly surface sensitive tool (probing a depth of ca. 2 nm) is a good means of studying this chemistry. It is generally accepted that the oxide precursors of molybdenum sulfide hydroprocessing catalysts remain as a highly dispersed monolayer2 or as patchy monolayer^^*^ on the support surface and this dispersion is dependent on the nature of the suppod; yAlzO3 is highly effective, whereas S i 0 2 is considerably less effective, for dispersing molybdenum oxide.’ ESCA has been applied to study *Present address: Hanhaw/Filtrol Partnership, Filtrol Division, 3250 E. Washington Blvd., Los Angeles, CA 90023.
0022-3654/87/2091-2324!§01.50/0
the dispersion and surface properties of supported molybdenum o~ide,’-~’#~~ molybdenum sulfide7*14J5-21-24 and Co-promoted molybdenum oxide7,9J9*2S-29,34 and sulfide7*9J9s24*26-3@34 catalysts. In most cases, 7-A1203 has been used as the support, although a comparative surface structural investigation using various other supports could throw light on the cause of specific support effect on the dispersion and activity of the catalysts. Therefore, the present investigation was undertaken with an eye to studying the (1) Gates, B. C.; Katzer, J. R.; Schuit, G. C. A. Chemistry of Cufulytic Processes; McGraw-Hill: New York, 1979; Chapter 5. (2) Massoth, F.E. Adu. Cutul. 1978, 27, 265. (3) For a recent review on metalsupport interaction, consult: Bond, G. C.; Burch, R. In Catalysis-Specialist Periodical Report; Bond, G . C., Webb, G., Eds.; The Royal Society of Chemistry: London, 1983; Vol. 6 , p 27. (4) Hall, W. K. In Proceedings of the 4th Internutionul Conference on the Chemistry and Uses of Molybdenum; Barry, H. F., Mitchell, P. C. H., Eds.; Climax Molybdenum Co.: Ann Arbor, MI, 1982; p 224. (5) Nag, N.K. J. Cutul. 1986, 92, 432 and references therein. (6) Rodrigo, L.; Marcinkowska, K.; Lafrance, C. P.; Roberge, P. C.; Kaliaguine, S . In Proceedings of the 9th Ibero-American Symposium on Cutulysis; Ibero-American Catalysis Society: Lisbon, Portugal, 1984. (7) Massoth, F. E.; MuraliDhar, G.; Shabtai, J. J . Curd. 1984, 88, 53. (8) Zingg, D. S.; Makovsky, L. E.; Tischer, R. E.; Brown, F. R.; Hercules, D. M. J . Phys. Chem. 1980, 84, 2898.
0 1987 American Chemical Society
Carrier-Catalyst Interaction of Supported MO Oxides
The Journal of Physical Chemistry, Vol. 91, No. 9, 1987 .2325
support-catalyst interaction phenomenon and its possible effect on the dispersion of molybdenum oxide using four different supports.
Experimental Section Catalysts. An incipient wetting technique, comprised of impregnation of support with solutions containing stoichiometric amounts of (NH4)6M07024-4H20(Mallinkrodt, Analytical Reagent > 99.9%) to achieve the desired Mo loading, was used to prepare the catalysts. The sequence of the steps followed was as follows. The support oxide, calcined at 500 OC for 16 h, was wetted with the molybdate solution (pH 8 f 0.1) by dropwise addition and stirring. The volume of the solution matched the total pore volume of the support taken. After it was oven-dried a t 120 OC overnight, the sample was calcined at 500 OC for 16 h. For the Co-Mo samples, the oven-dry 8 Mo/Al (indicating 8 wt 5% of Mo on A1203) sample was impregnated by Co(N03)z.6H20 solution to give 3 wt % of Co (designated as 3Co8Mo/A1). In two other variations, the pH of the ammonium molybdate solution was adjusted to 1.0 f 0.1 and was used to impregnate 7-A1203(i) by the pore-filling method; and (ii) by the adsorption of molybdate ions from the s o l ~ t i o n .Bulk ~ MOO, was prepared by heating ammonium molybdate at 500 OC for 16 h. The industrial catalyst was Ketjenfine (ca.200 mz g-l). SO2-, Ti02-, and Zr02-supported catalysts were prepared by impregnation with ammonium molybdate solutions at pH 8. The supports were A1203(Ketjen, 205 m2 g-I); Si02(Ketjen, 670 mz g-l, 1.1 mL g-I); Ti02 (Harshaw, 205 m2 g-l, 0.37 mL g-l); Z r 0 2 (home-made, 85 m2 g-I, 0.2 mL g-l). ESCA. A Hewlett-Packard machine (Model 5950B, A1 K a monochromatic X-rays, 1486 eV; the analyzer pressure chamber was less than lo-* Torr) with computer facility for data processing including curve deconvolution, peak area integration, and curve fitting was used for ESCA experiments. An electron floodgun was used for the compensation of charge stemming from the insulating properties, if any, of the samples. The electron gun current was set in predetermined values so as to keep the sample charging at a minimum. Finely ground sample was dusted on double-stick Scotch tape placed on a brass sample holder. The tape was covered by a gold mask with a rectangular central opening on which X-rays could be focussed. Up to 50 scans were taken depending on the concentration of Mo on the supports. Cls binding energy (BE), 284.6 eV (adventitious carbon), was taken as the reference for obtaining other BE'S. Results and Discussion The ESCA spectra of some representative samples containing Mo alone are shown in Figure 1. The M o ~ , BE , ~ and fwhm (full width at half-maximum) values relative to various catalysts are given in Table I. The specific influence of a particular support on the dispersion of supported molybdena, as reflected by the broadening of the peaks (compared with unsupported bulk M a 3 ) is apparent in Figure 1. In the case of Mo/A1203, the Mojd doublet is quite broad (compared with MOO, doublet) and the resolution of the component peaks is rather poor. The resolution and an attendant sharpness of the doublet improves considerably as one passes from Mo/Si02 to Mo/ZrOz to Mo/Ti02 in that order. In fact the M03d doublet of Mo/Ti02 is observed to be as well resolved as the M03d doublet of bulk Moo3. This trend, as indicated by the shape alone of the spectra, is confirmed by the fwhm data displayed in Table I. It is observed that Mo/A1203 samples have the highest fwhm values, ranging from 2.40 to 1.88 eV, and Mo/Si02 have values between 1.81 and 1.65 eV. On the other hand, M o / Z r 0 2 and Mo/Ti02 have fwhm values much lower, ranging from 1.54 to 1.45 eV and from 1.31 to 1.25 eV respectively, than that of Mo/A1203 and Mo/SiOZ samples. It is important to note that Mo/Ti02 catalysts have fwhm values very close to that of bulk MOO,, whereas Mo/ZrOz catalysts have only slightly higher values. The broadening of Mo ESCA peaks has been attributed to various factors including (i) the presence of more than one type of Mo(V1) species with different chemical characteristics which
Binding Lnorgy, ev
Figure 1. MoMESCA spectra of various samples: (A) 8Mo/A120,; (B) 8Mo/Si02; ( C ) 8Mo/Zr02; (D) 8Mo/Ti02; and (E) bulk MOO,. TABLE I: ESCA Binding Energy (BE) and Full Width at Half-Maximum (fwhm) Data of Various Samples
loading, support or catalyst y-A1203
wt % Mo Co 2 4 6
8 Si02
Ti02
Zr02
12 2 4 6 8 12 2 4 6 8 12 2 4 6
8
YAl203" y-Al20:
y-Ai2OSe ~-A1203~ MoO,(bulk)'
12 8
8 8 8
3 3 3 3
M03d5,2
BE/eV
fwhm,teV
232.5 232.8 233.0 233.1 233.2 233.2 233.2 233.3 233.1 233.2 233.6 233.7 233.7 233.6 233.5 232.7 232.8 232.6 232.8 232.7 232.9 232.6 232.4 232.5 233.2
2.45 2.40 2.20 1.91 1.88 1.81 1.79 1.65 1.66 1.74 1.30 1.31 1.30 1.27 1.25 1.54 1.53 1.50 1.45 1.48 2.70 2.80 2.60 2.60 1.30
Ketjen 3Co8Mo/Al2O3. Mo/A1203 prepared by impregnation of A120, with ammonium molybdate at pH 8. 'M0/Al20, prepared by impregnation of A1203 with ammonium molybdate at pH 1. dMo/ A1203prepared by adsorption of molybdate ions at pH 1. Prepared by thermal decomposition of (NH,),Mo,02,.4H20. /Precision *O. 10 eV. g Precision &O. 15 eV. cannot be discerned by ESCA;" (ii) electron transfer between Mo and support (metal-support interaction),10,'2and (iii) dif-
2326 The Journal of Physical Chemistry, Vol. 91, No. 9, 1987
tS
0.4
0
0
4
8
12
W t % Mo on Support
Figure 2. ESCA intensity ratio, IM,,/Is (molybdenum to support metal), as a function of Mo loading on support; (A) Mo/A1203; (B) Mo/TiO,; (C) Mo/Zr02; and (D) Mo/SiO,.
ferential charging stemming from poor conductivity of the samples.* In the present context, however, the contribution of the last factor to the peak broadening was kept at minimum by applying the electron gun. The peak broadening is, therefore, considered due mostly to the other two factors. It is well documented in the l i t e r a t ~ r e at , ~ least for A120,supported Mo catalysts, that at low loadings, the dominant molybdenum oxide species that remain in highly dispersed state have tetrahedral coordination. However, as the loading increases, other species with octahedral coordination form with increasing rate as a function of Mo loading and eventually a bulky MOO, phase is formed at sufficiently high loading^.^^,^^ Therefore, the failure
(9) Patterson, T. A.; Carver, J. C.; Leyden, D. E.; Hercules, D. M. J . Phys. Chem. 1976,80, 1700. (10) Armour, A. W.; Mitchell, P. C. H.; Folkesson, B.; Larson, R. J. Less-Common Met. 1974, 36, 361. (11) Ratnasamy, P. J. Catal. 1975, 40, 137. (12) Miller, A. W.; Atkinson, W.; Barber, M.; Swift, P. J. Catal. 1971, 22, 140. (13) Okamoto, Y; Imanaka, T; Teranishi, S. J. Phys. Chem. 1981, 85, 3798. (14) Okamoto, Y.; Tomoika, H; Katoh, Y.; Imanaka, T.; Teranishi, S. J. Phys. Chem. 1980.84, 1833. (15) Okamoto, Y.; Tomioka, H.; Imanaka, T.; Teranishi, S. J. Catal. 1980, 66, 93. (16) Kerkhof, F. P. J. M.; Moulijn, J. A. J . Phys. Chem. 1979,83, 1612. (17) Defosse, C. J. Electron Spectrosc. Relat. Phenom. 1981, 23, 157. (18) Rodrigo, L.; Marinkowska, K.; Adont, A.; Roberge, P. C.; Kaliaguine, S.; Stencel, J. M.; Makovsky, L. E.; Diehl, J. R. J. Phys. Chem. 1986 90,2690. (19) Dufresne, P.; Payen, E.; Grimblot, J.; Bonnelle, J. P. J. Phys. Chem. 1981, 85, 2344. (20) Edmonds, T.; Mitchell, P. C. H. J. Catal. 1980, 64, 431. (21) Angevine, P. J.; Vartuli, J. C.; Delgass, W. N. Proc. Znt. Congr.Catal. 6th, 1976; 1977, 611. (22) Brown, F. R.; Makovsky, L. E.; Rhee, K. H. J. Cazal. 1977,50, 385. (23) Muralidhar, G.; Concha, B. E.; Bartholomew, G. L.; Bartholomew, C. H. J. Catal. 1984, 89, 274. (24) Hercules, D. M.; Klein, J. C. In Applied Electron Spectroscopy for Chemical Analysis; Windwai, H., Ho, F.-L., Eds.; Wiley: New York, 1982; Chapter 8. ( 2 5 ) Payen, E.; Barbillat, J.; Grimblot, J.; Bonnelle, J. P. Spectrosc. Lett. 1978, I I, 997. (26) Chin, R. L.; Hercules, D. M. J. Phys. Chem. 1982, 86, 3079. (27) Cimino, A.; DeAngelis, B. A. J . Catal. 1975, 36, 11. (28) Okamoto, Y.; Imanaka, T.; Teranishi, S. J. Catal. 1980, 66, 448. (29) Grimblot, J.; Bonnelle, J. F.; Beaufils, J. P. J. Electron Spectrosc. 1976, 8, 437. (30) Brinnen, J. S.; Armstrong, W. D. J . Catal. 1978, 54, 57. (31) Alstrup, I.; Chorkendorff, I.; Candia, R.; Clausen, B. S.;Topsoe, H. J. Catal. 1982, 77, 397. (32) Declerck-Grimee, R. I.; Canesson, D.; Friedman, R. M.; Fripiat, J. J. J . Phys. Chem. 1978,82, 885, 889. (33) Brown, J. R.; Ternan, M. Ind. Eng. Chem. Prod. Res. Deu. 1984, 23, 557. (34) For a recent review consult Barr, T. L. In Practical Surface Analysis; Briggs, D., Seah, M. P., Eds.; Wiley: New York, 1983; Chapter 8, p 335. (35) Giordano, N.; Bart, J. C. G.; Castellan, A.; Martinotti, G. J . Caral. 1975, 36, 81.
1
238
234
230
Bindlng Energy, eV
Figure 3. M03d ESCA spectra of various CoMo/A1203 samples: (A) pH 8, Mo impregnation; (B) Ketjen catalyst; (C) pH 1, Mo impregnation; (D) pH 7, Mo adsorption from solution; (E) bulk Moo3.
on the part of the techniques to resolve the small energy differences associated with different molybdenum oxide species is partly responsible for the peak broadening. Accordingly, the lack of broadening in Mo/Ti02 and only marginal broadening in Mo/ ZrOz catalysts indicate that the dispersed molybdenum oxide phases on T i 0 2 and Z r 0 2 have more uniform geometrical and chemical characteristics, as opposed to that on A1203and Si02. The electron-transfer process between support and supported phase can be studied by noting changes in BE's. In the case of Mo/A1203, the BE's of the samples containing low amounts of Mo are lower than that of bulk MOO,. However, following a distinct increasing trend as a function of Mo loading, the BE reaches the value of 233.2, which is the same for bulk MOO,. N o such trend is noted for the other catalysts. This observation is in accord with the previously mentioned fact that the nature and distribution of molybdenum oxide species on A1203change appreciably with loading, and that the ouerall chemical as well as geometrical characteristics of Alz03-supported molybdenum oxides with high Mo loadings are very close to that of bulk MOO,. Therefore, an electronic interaction between Mo and A1203, particularly at the low loading region, is strongly indicated by the BE data. The Mo BE'S of Mo/Si02 samples remain invariant over the whole range of loading and virtually the same as that of bulk MOO,; no electronic interaction is therefore indicated in this case. As a matter of fact Si02-supported molybdenum oxides have been reported to show XRD lines of MOO, phase even at very low loadings;' this indicates a poor or lack of dispersion and absence of support-catalysts interaction. In the case of Mo/Ti02 and Mo/ZrOz catalysts, no change in BE as a function of Mo loading is observed. However, the magnitudes of the average BE of these catalysts, 233.6 f 0.07 and 232.7 f 0.07 eV for Mo/Ti02 and Mo/ZrO,, respectively, show appreciable shift from that of bulk M o o 3 (233.2 eV). Therefore, electron transfer from support to Mo in Mo/Zr02 and from Mo to support in Mo/Ti02 catalysts is indicated by the results. It is worth remembering that in the above discussion it has been assumed that relaxation and other complicating effects that cause chemical shifts are absent. The ESCA peak intensity ratio, ZMo(3d,,2)/ZS (where S stands for AlZp,Si2p,Ti2P3,2,or Zr3d,,2),for different series of catalysts (36) Medema, J.; Van Stam, C.; De Beer, V. J. H.; Konings, A. J. A,; Koningsberger, D. C. J . Catal. 1978, 53, 386.
J. Phys. Chem. 1987, 91, 2327-2332 is plotted as a function of Mo loading in Figure 2. It is observed in general for all catalysts that Mo dispersion, as reflected by the intensity ratio, remains virtually constant at low loadings, but it shows a decreasing trend at the high loading region. In the case of Mo/Zr02, a sharp fall in dispersion is observed beyond 8% Mo loading. These results, therefore, suggest that a t higher loadings the dispersion of molybdenum oxide is lower than that at lower loadings. This decrease in dispersion might be due to the formation of bulky Moo3-like species at higher loading^.^*^^*^^ The decline in the IMo/Isratio at higher loadings, especially the sharp fall in the ratio for M o / Z r 0 2 (Figure 2C), may be partly due to the migration of Mo into the support lattice and not due to agglomeration, which would be reflected in the BE and fwhm data in Table I. It is important to note that the curves in Figure 2 do not allow the comparison of relative dispersion of Mo between different supports because the ordinate indicates the ratio of the integrated peak areas rather than the atomic ratio of Mo to support. The promotional effect of Co on the hydrodesulfurization activity of Mo-based catalysts is well-known.’*2Several efforts have been made to understand the origin of this effect,’V2 which has been attributed mainly to (i) some electronic interaction3’f8 between Co and Mo, leading to enhanced intrinsic activity of the active Mo sites and (ii) increased dispersion’ of the Mo phase by Co. In order to throw some light on this phenomenon, some Co-Mo/Alz03 catalysts were studied by ESCA. The spectra and the BE data are displayed in Figure 3 and Table I, respectively. From Figure 3 it is observed that all the Co-Mo catalysts show spectra which are characteristic of highly dispersed molybdenum oxide. Slight variation in the BE’S may be due to the difference in the preparative methods. The fwhm values (Table I) are higher than that of the 8% Mo-only sample. The Co-Mo catalysts, irrespective of the method of their preparation, have higher dis-
2327
persion of Mo, as compared with the parent Mo-only catalysts. The peak intensity ratio (IMo/IN)of all four Co-Mo catalysts (see the box in Figure 2) has almost the same magnitude, which is much higher than that of the 8Mo/AI2O3 sample containing no Co. This is a clear manifestation of enhanced dispersion of the molybdenum oxide phase by Co. Also, the M03d~ BE values of the promoted catalysts (except for the Ketjen cataiyst) are found (Table I) to be appreciably lower (by ca. 0.4 eV) than that of the unpromoted 8Mo/A1203 catalyst. This strongly indicates a transfer of electrons from Co to Mo. It appears, therefore, that the promotional effect of Co is due to both enhanced dispersion of Mo and electron transfer from Co to Mo. In fact, this type of electron transfer has been predicted from quantum chemical c a l c ~ l a t i o n sand , ~ ~experimentally observed from IR band shifts of adsorbed nitric oxide38 recently. However, partial coverage of the alumina surface by Co as a possible contribution to higher IMo/IA, ratio for the Co-Mo/Alz03 catalysts cannot be ruled out. Based on the BE and fwhm data, as well as the consideration of the related electronic effects, it may be concluded that the Mmupport interaction is the greatest for the A1203support, and the least for the S i 0 2 support, whereas it is intermediate between these two extremes for the T i 0 2 and Z r 0 2 supports. This electronic interaction may be related to the relative degree of dispersion of Mo on the various supports. Co is found to enhance the Mo/Al ESCA intensity ratio, indicating an increase in the dispersion of Mo on A1203.
Acknowledgment. Thanks are extended to the referees for their valuable suggestions to improve this paper and Harshaw/Filtrol Partnership (Filtrol Division) for bearing the publication costs. (37)Harris, S.Polyhedron 1986, 5, 151. (38)Topsoe, Nan-Yu; Topsoe, H. Bull. SOC.Chim. Belg. 1981,90, 1311.
I n Situ Radiochemical Characterization of Adsorbates at Smooth Electrode Surfaces Elizabeth K. Krauskopf, Kyle Chan, and Andrzej Wieckowski* Department of Chemistry, University of Illinois, Urbana, Illinois 61801 (Received: October 20, 1986)
A radioelectrochemistry instrument has been constructed in order to characterize adsorbates on smooth electrodes, in situ. The instrument is described, and preliminary results are discussed. A smooth, polycrystalline electrode is pressed against a glass scintillator detector in order to reduce the solution background count rate to manageable levels. After the linearity of the system was confirmed by using NaHI4CO3in solution, the kinetics of HI4COOH adsorption and the potential dependence M, the sensitivity of this technique of [2,6-14C]pyridineadsorption were measured. At a bulk surfactant concentration of is about 1% of a monolayer. Adsorption kinetics of processes occurring on a time scale of seconds can be measured. This technique is expected to complement both in situ and ex situ surface electrochemistry spectroscopies.
Introduction of solid-fiquid meapplicationof radiotracer metho& for interfaces began in 1930 with Joliot’s experiments on the electrodepition of poloniUm.1 progressin combining radiochemistry with eldrxhemistry was accelerated as electrochemists switched their emphasis from mercury electrodes to solid electrodes, which created the need for measuring the surface concentrations of adsorbates a t solid surfaces. Three laboratories in the USA, Germany, and Soviet Union, headed by J. O’M. Bockris,2 K. S ~ h w a b eand , ~ V. E. Kazarinov? respectively, should be praised (1) Joliot, F. J . Chim. Phys. Phys.-Chim. Biol 1930, 27, 119. (2) Blomgren, E.A.; Bockris, J. O’M. Nature (London) 1960, 186,305. Wroblows, H.; Green, M. Elecfrochim. Acta 1963, 8,679. (3) Schwabe, K.; Weissmantel, Ch. Z . Phys. Chem. (Leipzig) 1960,215, 48.
for making pioneering contributions to modern interfacial studies using radiochemistry. In the early 1970s, two other European groups596joined the earlier investigators. Progress in the field spanning the first reports to the present has been r e v i e ~ e d . ~ , ~ Application of radioisotopes in gas-phase surface science has also been described.9 (4) Kazarinov, V. E. Elecktrokhimiya 1966, 2, 1170. Kazarinov, V.E.; Tysyachnaya, G. J.; Andreev, V. N. J. Electroanal. Chem. 1975, 65,391. (5)Horanyi, G.; Solt, F.; Nagy, F. J. Electroanal. Chem. 1971, 31, 87. (6)Sobkowski, J.; Wieckowski, A. J . Electroanal. Chem. 1972, 34, 185. (7)Horanyi, G. Elecfrochim. Acta 1980, 25, 43. (8)Kazarinov, V. E.;Andreev, V. N. In Comprehensive Treafise of Electrochemistry; Yeager, E., Bockris, J. O M . , Conway, B. E., Sarangapani, S.,Eds.; Plenum: New York, 1984;Vol. 9. (9) Davis, S.M.; Gordon, B. E.: Press, M.; Somorjai, J . Vac. Sci. Technol. 1981, 19, 231.
0022-3654/87/2091-2327$OlSO/O 0 1987 American Chemical Society
