Langmuir 1992,8, 2317-2323
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Lateral Modification and the Organization of CO-I Mixed Adlattices on Pt(ll1) D. Zurawskit and A. Wieckowski’ Department of Chemistry, School of Chemical Sciences, University of Illinois, Urbana, Illinois 61801 Received December 5, 1991. In Final Form: June 23, 1992 Modification of the electroactivityand organization of the CO adlattice on Pt(ll1)by coadsorbed iodine was investigatedutilizing a combination of in situ voltammetric measurements and ex situ analysis by low energy electron diffraction (LEED). Three solution dosing procedures were used to form the mixed adlattices, all resulting in immiscible domains of CO and iodine. Iodine was found to retard the CO electrooxidationreaction by compressing the CO domains, making them more difficultto nucleatewith oxidant. The overpotentialfor oxidation has been correlated with the extent of compression determined by LEED measurements. In turn, the compression and resulting CO adlattice structure were found to depend on the route by which the mixed adlattice was formed. The presented LEED surface crystallography and electrochemistry analysis offer an insight into long range organization of the mixed CO-I adlattices. Comparison with short-range organization awaits complementary scanning tunneling and atomic force microscopy results.
Introduction The rate of a heterogeneous electrochemical reaction is determined by the complex interplay of several forces acting at the liquid-solid interface. If the reagent is adsorbed to the electrode surface during electron transfer, interactions in the plane of the electrode can have a particularly pronounced effect on the reaction kinetics. For example, the rate of oxidation of carbon monoxide adsorbed on polycrystalline platinum has been found to be a very sensitive function of the mode of attachment and electronic properties of coadsorbed molecules termed “lateral modifiers”.l It was postulated that the type of the CO-modifier structure plays an integral role in the electroactivity of the CO monolayers. This observation led us to investigate the organizational changes in more detail utilizing a combination of low energy electron diffraction and voltammetry on the (111)plane of singlecrystal platinum. In ita broadest sense the objective of this research is to further the understanding of coadsorption systems in electrochemistry. A background for the chosen coadsorbed system’ was established ina previous study of the structure and voltammetry of CO electrosorbed on Pt(ll1) at submonolayer and monolayer coverage^,^?^ in the extensive research of surface iodine structures performed by Hubbard and co-workers,*l0 and in a recent infrared spec-
* To whom the correspondence concerning this paper should be sent. + Present address: Argonne National Laboratory, Argonne, IL 60439. .. (1)Zurawski, D.; Chan, K.; Wieckowski, A.; J. Electroanal. Chem. 1986,210,315; 1987,230,205. (2)Zurawski. D.; Rice, L.; Hourani, M.; Wieckowski, A,; J. Electroa d . Chem. 1987,230,221. (3)Wasberg, M.;Palaikis, L.; Wallen, S.; Kamrath, M.; Wieckowski, A. J. Electroanal. Chem. 1988,256,51. (4)Zurawski, D.; Wasberg, M.; Wieckowski, A. J.Phys. Chem. 1990, 94,2076. (5)Felter, T.E.;Hubbard, T. A. J.Electroanal. Chem.1979,100,473. (6) Stickney, J. L.; Roeasco, S. D.; Schardt, B. C.; Hubbard, A. T. J. Phys. Chem. 1984,88,251. (7) Hubbard, A. T.; Stickney, J. L.; Rosasco, S. D.; Soriaga, M. P.; Song, D.J. Electroanal. Chem. 1983,150,165. (8)Wieckowski, A.; Schardt, B. C.; Rosasco, S. D.; Stickney, J. L.; Hubbard, A. T.Surf. Sci. 1984,146,115. (9)Lu, F.; Salaita, G. N.; Baltruschat, H.; Hubbard, A. T. J. Electroanol. Chem. 1987,222,305. (10)Hubbard, A. T.Chem. Reo. 1988,88,633. ~~
troelectrochemistry study of CO coadsorption on Pt(ll1) by Chang and Weaver et al.ll We draw upon this previous work and report on the electrooxidation processes of surface CO coadsorbed with iodine a t a total saturationmonolayer coverage, but at varied proportions of the monolayer constituents. Our study also aims at obtaining better insight into the replacement process in which CO substitutes chemisorbed iodine on Pt(ll1). Such a substitution is a critical step in the preparation of platinum single crystals for electrochemical measurements.2 Since there is considerable interest in CO structures as they depend on the CO dosing procedures,12-14three types of CO adlattices were made in this study via different dosing procedures. They were subsequently intermixed with iodine to expose the effecta that such procedures have on the CO structure and ita electroactivity. The resulta of this work identify structure/function relationships in the coadsorbed system of mixed CO-I adlattices.
Experimental and Adlattice Preparation Procedures The combinedultrahigh vacuum/electrochemistryinstrument has been described in detail else~here.~ One face of a platinum single crystal (Aremco) was polished and oriented to within lo of the (111)crystallographicplane as determined by Laue X-ray diffraction. In preparation for the electrochemicalexperiments, the electrode was cleaned by ion bombardment and subsequent annealing in 3 X lo4 Torr oxygen. Following high-temperature annealing in UHV, the cleanliness and order of the crystal face were verified by Auger and LEED, respectively. The crystal was then transferred to the electrochemical chamber, which was isolated from the UHV chamber and pressurized with argon to prevent contamination from the laboratory atmosphere during introduction of the electrochemical cell. The cell was inserted into the chamber to immerse just the (111)face of the crystal in electrolyte by the meniscus arrangement. A cyclic voltammogram was taken between -0.2 and 0.9 V (vs AgIAgC1 [Cl-I = 1 M) in 0.1 M perchloric acid and compared to the well-known~1k18 (11)Chang, S.-C.; Weaver, M. J. Surf. Sci. 1991,241,11. (12)Palaikis,L.; Zurawski, D.; Hourani, M.; Wieckowski, A. Surf.Sci. 1988,199,183. (13)Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991,95,5391. (14)Feliu, G. J. M.; O h ,J. M.; Fernandex-Vega, A.; Aldaz, A.; Clavilier, J. J.Electroanal. Chem. 1990,296,191. (15)Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J. Electroanol. Chem. 1980,107,205. (16)Clavilier, J.; J. Electroanal. Chem. 1980,107,211.
0743-7463/92/2408-2317$03.00/00 1992 American Chemical Society
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2318 Langmuir, Vol. 8, No. 9, 1992 voltammetric profii of Pt(ll1)to verify that the cleanlineaeand order were maintained during the UHV-aolution transfer. As mentioned in the Introduction, three different methods of producing the carbon monoxide-iodine mixed adlattices were employed. In the f i t two methods, CO was adsorbed from COcontainingelectrolyteand iodinewas subaequentlyadsorbedfrom 1 mM KI in 0.1 M HClOd. Adsorption was performed potentiostatically at 0.15 V vs Ag/AgCI. Partial coverage of CO was obtained either by adsorption from a dilute CO solution or by partial oxidation of the CO monolayer. Consistently,the names for these methods are the dosing method and the ozidation method, respectively; see Results. In the third method, iodine was adsorbed fiit and the resulting adlattice exposed to dilute CO-containingelectrolyte to replace a portion of the iodine by CO.* Below, this method is referred to as the replacement method. Varying CO coverages were obtained by altering the concentration of carbohmonoxide m the electrolyte and its time of interaction with the I-covered electrode surface. Following adsorption,the electrode was rinsed repeatedly with clean electrolyte to remove all traces of free iodine and carbon monoxide. Complete coverage of the surface by the adlattices was verified in a negative-goingvoltammetricscan,whichshowed the absence of all faradaic curreht in the hydrogen adsorption potential region. The adlattices were then either immediately oxidized in a positive-going voltammetric scan or transferred to the UHV chamber, characterized by LEED, then transferred back to the electrochemicalcell for voltammetriccharacterization. As a test for desorption during the U H V exposure, the potential was again scauned through the hydrogen adsorption region upon return of the electrode to solution. As before,’ the post-LEED voltammetric chargesfor electrooxidation of CO and iodine were used to calculate their respective coverages. The coverages are reported as number of adsorbate molecules per platinum site. All electrochemicaland LEED measurementswereconducted at room temperature. The area of the Pt(ll1)plane in contact with the electrolyte was 0.64 cm2. The svpporting electrolyte used in the voltammetric measurements was 0.1 M perchloric acid (reagent grade) in Millipore water (18 MQ).Iodine was dosed from 1mM potassium iodide (reagent grade) in perchloric acid electrolyte. Potentials are reported with respect to a Ag/ AgCl reference electrode with a chloride concentration of 1 M the actual concentration of chloride was 10-2 M to reduce the possibility of chloridecontamination. All current-potential traces were taken at a scan rate of 60 mV/s. Carbon monoxide gas (99.5% minimum) and argon gas (99.999 % minimum) were supplied by Matheson Gas Products. Nitrogen gas (oxygen-free) used for purging theelectrolyte and iodide solutions prior to introduction into the electrochemical cell was supplied by Linde Specialty Gas.
Results 1. Exposure of Saturation Coverage CO Structure to Iodine. When the saturation coverage CO structure on the well-ordered Pt(ll1) was exposed to iodidecontaining electrolyte (see Experimental Section), the modification of carbon monoxide oxidation exhibited notably different behavior than on polycrystalline Pt. As shown in Figure 1, curve 1, the simple exposure of a monolayer of CO on polycrystallinePt to iodide-containing electrolyte causes a 110-mV shift in the CO electrooxidation peak potential accompanied by the adsorption of a small amount of iodine1 (01 = 0.1). Voltammetric and Auger experiments show that on Pt(ll1)such an exposure does not result in the uptake of iodine. The voltammetric peak position, charge for CO electrooxidation, and LEED pattern were identical to those obtained for a CO adlattice not exposed to iodide solution. All of the results indicate that iodine exposure has no effect on the saturation coverage CO adlattice on Pt(ll1). (17)Aberdam, D.; Durnnd, R.; Faure, R.; El-Omar, F. Surf. Sci. 1986,
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Figure 1. Electrooxidation of a CO monolayer and CO-iodine mixed monolayers on polycrystalline platinum: 0, voltammogram of the CO monolayer on platinum (AfterCO oxidation, the voltammogram of the clean platinum electrode is retrieved,also labeled as 0‘);I+, eledrooxidation of the CO-I mixed monolayers with decreasing CO to I ratio in the order form 1to 6. CO adsorption was carried out at -0.1 V in 1.0 M HCIO,. Scan rate was 50 mV&.
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clean Pt(ll1) in 0.1 M HC104. Scan rate was 50 mV+. 2. Submonolayer and Monolayer CO Adlattices. A typical electrooxidation wave for submonolayer CO is shown in Figure 2. The submonolayer adlattices exhibit two distinguishable electrooxidation peaks centered at 0.462 and 0.501 V. All partial CO coverages (0.09 I 0co I 0.6)yield a LEED pattern identified as c ( d 3 X 6)rect (Figure 6C in ref 4). The monolayer coverage (8co = 0.7) yields a single voltammetric peak at 0.501 V and a ( 4 3 X 3)rect pattern (Figure 6B in ref 4) identical to that observedlgfor the low temperature saturation coverage of gas phase dosed CO. 3. Mixed Adlattices Formed by the Dosing Method. The voltammetry of mixed adlattices created by dosing first with dilute CO-containing electrolyte to obtain a submonolayer of CO with subsequent dosing of iodine is shown in Figure 3. The oxidation of CO modified in this manner occurs in a single sharp peak throughout the coverage range studied. The potential of this peak (0.4674.477V) lies between those observed for the two oxidation peaka of partial coverages of unmodified CO (Figure 2). There is also a slight positive shift of the CO oxidation peak potential as the ratio of CO to iodine in the adlattice is increased. Representative LEED results for the modified adlattices formed by this type of dosing shown in Figure 4. The
171, 303.
(18)Wagner, F.T.; Ross, P. N. J . Electroanol. Chem. 1988,250,301.
(19) Ertl, G.; Neumann, M.; Streit, K. M. Surf. Sci. 1977, 64, 393.
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pattern of Figure 4A corresponds to an adlattice with a CO coverage of 0.24 and is associated with the voltammetry shown in Figure 3A. An adlattice with a CO coverage of 0.50 (voltammetry,Figure 3B) gave the patterns shown in Figure 4B,C. The high-energy (54.7 eV) patterns of these two mixed adlattices (Figure 4C) and the low energy pattern of the 0.24 CO coverage adlattice (Figure 4A), though weak, exhibit 4 7 X d 7 characteristics, yielding an inverse spot distance of d 7 when normalized to the 1 X 1spots of platinum. The lower energy (35.7 eV) pattern of the 0.50 CO coverage adlattice (Figure 4B), by comparison with patterns obtained previously for CO adlattices? can most readily be identified as a CO-like pattern with dimensions intermediate between c ( d 3 X 5)rect and ( 4 3 x 3)rect. It is noteworthy that the 0.50 CO coverage adlattice exhibits a CO-like pattern at the lower electron beam energy and an iodine-like pattern a t the higher electron beam energy. Prior LEED studies of adlattices containing either only iodine or only CO showed that their characteristicspatterns are visible over the entire 22-67 eV energy range, suggestingan altered intensity-energy dependence for the iodine and CO when in the mixed adlattice. 4. Mixed Adlattices Formed by the Oxidation Method. The voltammetry of mixed adlattices created by partially oxidizing a monolayer of CO and then dosing with iodine is shown in Figure 5. The followingtrends in voltammetric behavior with varying coverage of CO were observed. At low coverage (6~0 = 0.24) the modified adlattice is oxidized in two overlapping peaks centered at 0.501 and 0.510 V (Figure 5A). Intermediate coverages (8co = 0.40 to 0.48) are oxidized in a peak at 0.495 f 0.003 V, referred to as peak I (Figure5B-D). A t higher coverages (8co = 0.54 and 0.55) the modified CO is oxidized in two overlapping peaks; a minor portion is oxidized in peak I and the rest in a peak at 0.510 f 0.003 V, referred to as peak I1 (Figure 5E).
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Figure4. Low energy electron diffraction patterns of CO-iodine mixed adlattices formed by the dosing method on Pt(ll1): (A, top) OCO = 0.24;E = 34.7 eV; (B, middle) OCO = 0.50, E = 35.7 eV; (C, bottom) OCO = 0.24, E = 54.7 eV (identical pattern seen for eco = 0.50).
The observed trend in the voltammetric behavior with increasing coverage is indicative of increasing occupation by a more easily oxidized CO adlattice until it has reached saturation, followed by the growth of a less easily oxidized adlattice a t the expense of the first. The potentials of peaks I and I1fall within the scattering range of potentials seen4for oxidation of a saturation coverage of unmodified CO (0.501 f 0.010 V). This coincidence of oxidation potentials may indicate that the CO molecules in the un-
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2320 Langmuir, VoZ. 8, No. 9, 1992
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Figure5. Elechooxidationof CO-iodine mixed adlattices formed by the oxidation method on Pt(ll1): (A) 8co = 0.24, immediately after dosing; (B)8co = 0.40, after LEED; (C)8co = 0.45, immediately after dosing; (D) 8co = 0.48, after LEED; (E) 8co = 0.54, immediately after dosing. Scan rate was 50 mV.s-l.
modified and modified CO adlattices are in identical or very similar environments. However, the presence of two separate peaks for the oxidation of one mixed adlattice, as shown in Figure 5E, indicates that two distinctly different forms of CO exist. Comparison of the peak position and width of an adlattice which was subjected to UHV characterization (Figure 5B) to one of similar coverage which was oxidized immediately after dosing (Figure 5C)is favorable. In the majority of cases, the UHV exposure did not effect the voltammetric properties of the mixed adlattices or cause desorption of either component (verified in a voltammetric scan through the hydrogen adsorption region). LEED patterns of mixed adlattices formed by partial oxidation of a monolayer of CO followed by dosing with iodine are shown in Figure 6. They have been identified as (A) ( 4 7 X d 7 ) and (B, C) ( 4 3 X 3)rect and are associated with the voltammetric profiles of Figure 5B (8co = 0.40) and Figure 5D (8co = 0.48),respectively. A notable feature of the LEED results is the appearance of a ( d 7 X 4 7 ) pattern, characteristic of an iodine ad-
Figure6. Low energy electron diffractionpatterns of CO-iodine mixed adlattices on Pt(ll1) formed by the oxidation method: (A, top) 8co = 0.40, E = 33.5 eV; (B,middle) 8co = 0.48, E = 35.7 eV; (C,bottom) 8co = 0.48, E = 52.3 eV.
lattice with a coverage of 0.43, a t a relatively high coverage of CO (8co = 0.40). A real space structure which could give rise to this result comprises ordered islands of iodine and disordered CO domains or CO domains much smaller than the coherence area of the electron beam,20giving rise to the high background observed in the LEED pattern. Another explanation for the absence of spots arising from (20)Ert1,G.; Kuppers,J.LowEnergyElectronsundSuTface C h e h t r y ; Verlag Chemie: Weinheim, FRG,1974; Chapter 9.
CO-I Mixed Adlattices on & ( I l l )
CO is that the intensity-energy profile for ita adlattice is at a minimum at this beam energy (35.7 eV). Such an explanation again invokes an alteration of the intensityenergy behavior as compared to the unmodified adlattice. Another notable feature of these results is the observation of a ( 4 3 X 3)rect pattern, characteristic of the saturation structure of CO, at a coverage (8co = 0.48) far below saturation (8co = 0.7). At this coverage the unmodified adlattice assumes a c ( d 3 X 5)rect configuration? This observation indicates that the presence of iodine compressed CO into a higher coverage structure. These LEED results are consistent with the voltammetric behavior, which shows the CO to be oxidized at the same potential as the saturation coverage unmodified adlattice. 5. Mixed Adlattices Formed by the Replacement Method. As shown previously,2 carbon monoxide replaces iodine adsorbed on Pt(ll1). If the replacement is performed in the double layer region of potentials with a dilute solution of CO M), it is possible to remove only a portion of the iodine. The resulting adlattice saturates the surface and contains both iodine and CO in proportions depending on the concentration of CO used for the replacement and ita time of interaction with the electrode. Voltammetricresults for adlattices formed in this manner are shown in Figure 7 in order of increasing coverage by CO. The modified CO is oxidized in at least three overlapping peaks located at 0.488,0.516, and 0.525V, the first two denoted as peaks I and 11, respectively. The potentials of peaks I and 11coincide with those of peaks I and I1 for the mixed adlattices formed by the oxidation method discussed in the previous section. Peak I1generally dominates the oxidation profiles and its fraction of the total CO oxidation charge increases with coverage until it is the only peak present at a CO coverage of 0.48. The mixed adlattice formed by replacement of iodine by carbon monoxide with a CO coverage of 0.48 (voltammetry, Figure 7D) yielded a ( 4 7X 4 7 )iodine-like pattern with a high background intensity. As was also noted for an adlattice formed by the oxidation method, a significant feature of this result is the appearance a pattern characteristic of the 0.43 coverage iodine adlattice even with a large amount of CO present (8co = 0.48). Discussion On the basis of previous work it is known that the CO oxidation data are consistent with a =reactantpair" mechanism in which the rate-determining step is the formation of the activated complex between adsorbed CO and surface oxidant of the Pt,, type (ref 27 and references therein). To account for the lateral modification of CO by coadsorbed iodine on polycrystalline platinum, we have earlier postulated that the 110-mV shift in the CO electrooxidation peak potential (Figure 1)resulted from the compression of the CO domain into islands of a more compact structure.' That is, the higher electrooxidation overpotential is associated with a structure which, due to ita higher packing density of CO molecules, is more difficult to nucleate with the Pt,, oxidant. Recalling that the CO monolayers reported in ref 1were obtained upon CO dosing in the hydrogen range of potentials, we may notice that this interpretation correlates well with conclusions of the infrared spectroscopy work by Kunimatsu et al.21*22 Namely, it was suggested that carbon monoxide dosed in (21) Kunimatsu, K.; Seki, H.; Golden, W.G.; Gordon, J. G., 11;Philpott, M. R. Langmuir 1986,2,464. (22) Kunimatsu, K.; Gordon, W. G.; Seki, H.; Philpott, M. R. Langmuir 1986, 1, 245.
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the hydrogen region of potentials is oxidized randomly throughout the adlattice, whereas the oxidation of CO adsorbed in the double layer occurs at the edges of islands. This difference in oxidation mechanism was cited as the basis for the higher oxidation overpotential observed for the double layer dosed CO. The peak potential for the electrooxidation of CO adsorbed on polycrystalline platinum in the double layer region coincides with that observed for iodine-modified CO1. Evidently, the edgetype oxidation takes places in the two cases, one unmodified (double layer dosing) and one iodine-modified (hydrogenpotentialdosing). Interpreting their data, Kunimatau et aL21 have attributed the difference in the oxidation mechanism to morphologicalchanges of the polycrystalline surface induced by the interaction of CO with platinum in the hydrogen region which favors the formation of bridged bonded CO. Apparently, the introduction of iodine reversed or compensates for these morphological changes, causing the CO to be oxidized at the higher overpotential associated with island formation.
2322 Langmuir, Vol. 8, No. 9, 1992
Under the dosing conditions23used in the present study on Pt(lll),the CO oxidation peak position is independent of adsorption potential.12 Therefore, as opposed to polycrystalline platinum, morphological alteration of the surface resulting in different forms of the CO adlattice does not appear to occur on the more stable single crystal surface. Accordingly, the modification of CO electrooxidation on Pt(ll1) by iodine cannot be attributed to a reversal of these effects and the dramatic shift in peak potential for the polycrystalline system is not observed. From this discussion, as well as from the earlier r e ~ e a r c h , l *we ~ *may ~ ~ , conclude ~~ that the overpotential for CO electrooxidation is a measure of the availability of the oxidant to CO within the CO adlattice. The decreased availability in the more compact structures arises from the fact that the access of water molecules to the catalytic Pt sites, on which the water dischargesto form the oxidant, is impeded by the tightly spaced CO molecules. Recall that at all partial coveragesthe unmodified CO is oxidized in a s lit voltammetric response (Figure 2) and displays a c( 3 X 5)rect LEED pattern. The formation of islands has been invoked4in order to explain the appearance of this pattern a t coverages far below that necessary to saturate the surface with this structure (8co = 0.6).19*24-26 The low potential peak in this split voltammetric response has been attributed to CO in low density c(4 x 2) domains and the high potential peak to high density fault line CO’s within these c ( d 3 X 5)rect islands? The single CO electrooxidation peak observed when the mixed (modified) adlattices wereobtained by the dosing method (see Results and Figures 3 and 4) indicates that all CO molecules within the adlattice are approximately equivalent in their accessibility to oxidant. This strongly suggests that there is no penetration of iodine into the CO islands. As a result, immiscible domains of CO and iodine are present on the surface. The oxidation potential of this single peak, between the two peaks observed for unmodified CO, signifies that it is more difficult to nucleate such an adlattice with oxidant than the c(4 X 2) domains but less difficult than the densely-packedfault line CO. Evidently, introduction of iodine leaves the c ( d 3 X 5)rect islands intact but causes compression and reorganization of the adlattice into a form with the above mentioned characteristics. Representative LEED patterns for adlattices formed by the dosing method, shown in Figure 4, support this interpretation. The dimensions of the unit cell of a typical iodinemodified CO adlattice formed by the dosing method can be determined fromthe LEED pattern of Figure 4B. Figure 8showsthe correspondingreal space unit cell and a possible surface structure, which has been denoted c ( d 3 X 11)rect. I t is composed of c(4 X 2) domains which are out of phase with respect to each other along two perpendicular axes. The resulting structure contains singly spaced and doubly spaced fault lines as a consequence of phase shifts in the directions depicted as vertical and horizontal, respectively. The coverage (0 = 0.64) and spacing of CO molecules in the proposed structure are intermediate between c ( d 3 X 5)rectand ( 4 3X 3)rect. Therefore, based on the accessibility of oxidant to the CO molecules, the new adlattice is expected to be oxidized in a potential region intermediate between the two structures as is observed in the voltammetric data. This observation provides a first evidence of its kind that structural compression of a reagent, oxidized via the nucleation-
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(23) Gutierrez, C.; Caram, J. A. J. Electroanul. Chem. 1991,308,321. (24) Biberian, J. P.; Van Hove, M. A. Surf. Sci. 1984,138,361. (25)Avery, N. R.J. Chem. Phys. 1981, 74,4202. (26) Kiskinova, M. Szabo,A.; Yates, J. T., Jr. Surf. Sci. 1988,205,215.
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Figure 8. Proposed model for c ( d 3 X 1l)rect CO structure in
CO-I mixed adlattices formed by the dosing method on Pt(ll1). A saturation coverage of this structure would have 0.64 CO/Pt. The relative sizes of the Pt atoms and CO molecules are shown to scale. growth mechanisms,27 can contribute to a measurable deceleration of the observed overall reaction rate reflected by an increased overpotential for the reaction. The mixed adlattices formed by the oxidation and replacement methods (see Results) exhibit similar voltammetric behavior, Figures 5 and 7. Both types of adlattices are oxidized in two peaks (peaks I and 11) at approximately the same potentials, though the coverage dependence of the areas of peaks I and I1 differs significantly for the two dosing methods. Peaks I and I1 fall within the scattering range of potentials observed for the electrooxidation of a saturation coverage of unmodified C0.4 Inspection of the two voltammograms taken for the same CO coverage (8co = 0.48, Figures 5D and 7D) and integration of the anodic charge in the 0.9-1.3 V region shows that the adlattice formed by oxidation contains a significantly larger amount of iodine. Since the two adlattices occupy the same surface area, the CO domains formed by replacement of iodine must be more compact than the domains formed by oxidation in order to accomodate more iodine. The former adlattice is oxidized in peak I1 and the latter in peak I; therefore peak I1a t the more positive potential is associatedwith themore compact adlattice and peak I a t the less positive potential with the less compact adlattice. The CO adlattice structures responsible for peaks I and I1 can be identified from the LEED patterns obtained for adlattices formed by the dosing and replacement methods. LEED patterns have been presented for three adlattices which exhibit CO electrooxidation at the potential of peak I, Figure 6. A (d7X d7)iodine-like pattern (Figure 6A) was observed for an adlattice formed by the dosing method with a CO coverage of 0.40. At a higher coverage of CO, 0.48, and CO-like pattern (Figure 6B,C) was seen for an adlattice also formed by the dosing method. The latter two patterns are characteristic of a ( d 3 X 3)rect adlattice, identified previously for the saturation coverage of unmodified C0.4 However, significant differences between the present patterns and that observed for the unmodified adlattice are apparent. Figure 6B,C exhibit broad spots and a high background in comparison with the saturation coverage pattern for unmodified CO (ref 4, Figure 6B). Among the several types of adlattice imperfections which may be responsible for these LEED features are antiphase domains and random adatoms or vacancies in the adlat(27) McCallum, C.; Pletcher, D. J. Electroanal. Chem. 1976,20,277.
CO-I Mixed Adlattices on P t ( l l 1 ) tice.20*28 It can be proposed that the modified CO oxidized in peak I (Figure 5 ) is adsorbed in antiphase ( 4 3 X 3)rect domains containing considerable defects such as vacancies. These defects may facilitate the oxidation mechanism causing the adlattice to be oxidized on the negative edge of the scattering range of potentials observed for the oxidation of the unmodified ( 4 3 X 3)rect adlattice. A model for the organization of mixed CO-iodine adlattices formed by the oxidation method also invokes the presence of immiscible domains of CO and iodine. The dimensions of the CO domains are coverage dependent; at a coverage of 0.40 the domain size is much smaller than the coherence width of the LEED electron beam and diffraction spots from adsorbed CO are not evident. However, at a coverage of 0.48 and above the domain size becomes significantcompared to the coherence width such that a CO pattern appear^.^ At coverages below 0.48 the CO is adsorbed in defective ( d 3 X 3)rect domains. On the basis of the voltammetric data, the defective CO domains reach their maximum occupation of the mixed adlattice at a CO coverage between 0.48 and 0.54. Above this coverage more compact domains of CO are formed at the expense of the defective domains. As mentioned previously, the predominance of either peak I or peak I1 in the voltammetry of a particular coverage of CO depends on the route by which the adlattice was formed. With the oxidation method, peak I is the dominant peak at CO coverages below 0.54; above this coverage peak I1 is the dominant peak although peak I is still present. In contrast, peak I1 dominates the voltammetric profiles of all the mixed adlattices formed by replacement, with peak I completely absent at a CO coverage of 0.48. The voltammetric trends indicate that the oxidation method favors defective domains of CO and the replacement method favors the relatively well-ordered structure. It has been postulated above that the ( 4 3 X 3)rect structure formed by the oxidation method results from the compression of c ( d 3 X 5)rect domains of CO. It appears that such compression causes the formation of a defective adlattice. The formation of relatively well-ordered CO domains during the partial replacement of iodine by CO indicates that the iodine replacement by carbon monoxide involves nucleation and growth of CO islands within the iodine adlattice. As mentioned in the Introduction, the I-CO replacement has not only theoretical but also experimental (28) Ross,P. N.; Wagner, F. T. In Advances in Electrochemistry and EZectrochemicalEngineering; Gerischer, H., Ed.; Wiley New York,1984; Vol. 13, Chapter 2.
Langmuir, Vol. 8, No. 9, 1992 2323 significance since the replacement process is a key ingredient of the method of a single crystal preparation developed in this laboratory?
Conclusions The combined voltammetric/LEED investigations of the CO-iodine mixed adlattices on Pt(ll1) showed that iodine modifies the electroactivity of CO by altering ita organization on the electrode surface. All three methods for creating the adlattices result in immiscible domains of CO and iodine. Iodine effecta the electroactivity of CO by compressing the CO domains, with the observed overpotential for the oxidation reaction dependent on the extent of compression. The extent of compression in turn depends on the route by which the adlattice was formed. The c ( d 3 X 5)rect CO domains formed for submonolayer coverages of CO dosed from dilute solutions are compreased into a c ( d 3 X 1l)rect structure by the coadsorbed iodine. This structure has a packing density intermediate between that for the c ( d 3 X 5)rect and ( 4 3 X 3)rect structures of unmodified CO and this has been cited as the basis for ita intermediate oxidation potential. Domains formed by partial oxidation of a monolayer of CO are compressed to a larger extent into a ( 4 3 X 3)rect structure. LEED showed that these ( 4 3 X 3)rect domains contain a considerable number of imperfections. A ( 4 3 X 3)rect CO structure was also formed when replacing iodine by CO; however it is well-ordered compared to the ( 4 3 X 3)rect structure formed by the oxidation method. The voltammetricresults indicate that both the defective and well-ordered domains are present in the mixed adlattices formed by oxidation and replacement, with the wellordered domains being more difficult to oxidize. In this work, evidence was provided that structural compression of a reagent, oxidized via the nucleationgrowth mechanism, can reduce the overall reaction rate. Earlier data obtained with anodic stripping of underpotentially deposited metals,6y7 likewise occurring via the nucleation-growth mechanism, point to an opposite trend. That is, the more compressed structures are oxidized first and the loose ones are oxidized at more positive potentiale. Evidently, the fact that a surface oxidant is needed in the former case, and not in the latter, makes the significant mechanistic difference which accounts for the resulta of this work.
Acknowledgment. The support of the National Science Foundation (NSF-MRL-89-20538) and Air Force Office of ScientificResearch (AFOSR-89-0368)is gratefully acknowledged.
