9730
J. Phys. Chem. 1993,97, 9730-9735
Auger Electron Spectroscopy, Low-Energy Electron Diffraction, and Electrochemistry of Carbon Monoxide on a Pt( 100) Electrode C. K. Rhee, J. M. Feliu,? E. Herrero,? P. Mrozek, and A. Wieckowski' Department of Chemistry, University of Illinois at Urbana-Champaign, Urbana, Illinois 61801 Received: February 12, 1993; In Final Form: July 7, 1993"
We report on carbon monoxide adsorption on the Pt( 100) electrode in CO-saturated perchloric and sulfuric acid solutions studied by a combination of low-energy electron diffraction, Auger electron spectroscopy, and electrochemistry. We have developed an experimental method that produces accurate CO coverages making use of a capacitive current generated due to iodine adsorption at a constant electrode potential. Using this method, we have obtained the saturation CO coverage equal to 0.77 f 0.04 ML, in a good agreement with the quantitative Auger electron spectroscopy result of 0.79 i 0.09 ML. The LEED analysis of the saturation CO structure that corresponds to the 0.77 ML coverage gives rise to the Pt(100)c(4X2) surface structure, as in earlier gas-phase studies of the equivalent surface system. We also present a complete Auger electron spectroscopy characterization of the electrochemical CO adsorbate. The detailed spectroscopic data closely correspond to the previously reported spectral features for carbon monoxide adsorbed on relevant catalytic substrates in the gas phase. We also specify conditions of a safe transfer of the CO-covered platinum electrode from solution to ultrahigh vacuum through which no desorption of CO takes place.
Introduction The eliminationof carbon monoxide from industrialand exhaust gases through its catalytic oxidation to C02 is a process of major importance in relation to energy and environmental problems. For this reason, extensive research on of the gaseous CO adsorption on catalytic surfaces has been carried out. Clearly, carbon monoxide is now a well-known model adsorbate in surface science.2J In electrocatalysis, the focus on surface chemistry of CO arises predominantly from its adverse poisoning role in methanol fuel cell appli~ations.~~5 In an analogy to the gas-phase research: it has been shown that surface CO is bound to the platinum electrode in either the terminal or the bridging config~rations.~-9 The ratio of both types of the CO adsorbate depends on platinum surface crystallography, the CO coverage, and the electrode p~tential.~-~ On the Pt( 111) electrode, our earlier LEED studies have produced results consistent with those obtained under vacuum dosing conditions.10 At high coverage, we have identified the well-known high-packing adlattices: Pt(1 11)(1/3X3)rect and Pt( 11l)c(d3XS)rect. At low coverage, we have found islands possessing internal organization of these high-packing structures. However, the stability of such islands is limited, and a slow structural rearrangement of the Pt( 111)(d3X3)rect to a Pt(1 ll)c(4X2) surface structure occurs.l0 Earlier measurements of the oxidation charge of carbon monoxide adsorbed on the Pt( 100) electrode gave the maximum coverage of 0.85 f 0.05 ML.9,11J2 This coverage is noticeably higher than 0.77 f 0.03 ML obtained in vacuum13 that, in turn, is characteristic of the ~ ( 4 x 2surface ) structure.13J4 Below, we propose a new experimental protocol to evaluate the CO coverages on the Pt( 100) electrode. The developed experimental procedure gave the coverage data that are in a close agreement with those obtained upon CO adsorption in the gas phase and with our quantitative Auger electron spectroscopy data. Except for our vacuum work on the Pt( 111)/CO systemlo the Soriaga et al. report on the CO interactions with Pd( 111),15 and the Scherson et al. research on CO adsorption on Ni( 111),16 there appears to be no other ultrahigh-vacuum (UHV) study of On leave from Departament de Quimica F i s h , Universitat d'Alacant, Spain. * To whom correspondence concerning this article should be sent. *Abstract published in Advance ACS Abstracts, September 1, 1993.
0022-3654/93/2097-9730$04.00/0
the solid/liquid CO. The results reported in this paper concern the structure, reactivity, and surface coordination of CO on the Pt( 100) electrode. Using Auger electron spectroscopy (AES) and low electron energy diffraction (LEED),1°J7 we have found that the electrochemical CO adsorbate under saturation coverage yields the Pt( lOO)c(4X2)-CO surface structure. This is thesame structure as earlier obtained during gas-phase adsorption of CO. The current results corroborate our earlier conclusions that the solution-dosed and gas-phase-dosedCO on platinum are arranged in the same surface configuration.10 In a broader perspective, the gas-phase and electrochemistry comparison practiced in this work may allow one to develop diagnostic clues on the similarities and differences between solid/liquid and solidlgas interfaces. This, in turn, may unveil the role of solvent and the electrode potential in formation of adsorbate layers in electrochemistry.
Experimental Section An ultrahigh-vacuum instrument combined with an electrochemical cell in the same apparatus has previously been described.10J7 The Pt(100) electrode was prepared by orienting a platinum single crystal (Aremco) to the desired (100) crystallographic plane. The orientation was made to within l o as determined by Laue X-ray diffraction. The crystal was polished with successively finer grades of diamond paste down to 0.25-pm grade (Buehler) and positioned in the UHV chamber. Thecrystal surface was routinely cleaned by argon ion bombardment at 5 X Torr of argon and annealed in 3 X 10-8Torr of oxygen. After the surface preparation was completed,thecrystal was transferred to the electrochemicalchamber that was promptly isolated from the UHV and pressurized with argon. This allowed us to introduce the electrochemical cell to the chamber for voltammetric characterization of the electrode and for the CO adsorption. Only the (100) plane was exposed to the electrolytic solution via a meniscus configuration. AES measurements were carried out in a differentiated mode with 2-eV modulation amplitude using the Perkin Elmer PHI10-155 CMA instrument. A low electron energy of 800 eV at 6 MAwas used, which helped with minimizing effects of electron beam damage on the CO adsorbate.ls The 3 eV/s sweeping rate and 30 ms time constant were used in the measurements. The quantitative analysis of the CO coverage was done using a unique calibration procedure developed in this laboratory. Namely, we 0 1993 American Chemical Society
CO on a Pt(100) Electrode
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9731 150
a
1001
-150 ~~
~
100
0
200
300
400
500
600
'
I
-0.40
0.00
0.40
0.80
E N (Ag/AgCI)
Energy/eV 150
b
1
b
Y L
-100 - 5 7
-150
-0.40 0
100
200
300
400
500
0.00
0.40
0.80
600
EnergyIeV. Figure 1. Secondary electron spectrum of the Pt( 100) crystal using 800eV primary electrons: (a) clean platinumsurface;(b) after CO adsorption in 0.1 M H2S04 solution. made use of a thick urea overlayer obtained on the same Pt( 100) electrode by its immersion in 2 M aqueous solution of urea and drying in vacuum. The evacuation was followed by the AES measurements of the urea carbon signals. This signal is next used to calibrate the number of carbon atoms per one surface site in the CO adsorbate. We will publish the details of this procedure elsewhere. l9 Carbon monoxide gas (99.5% minimum purity) and argon gas (99.999% minimum purity) were supplied by Matheson Gas Products and oxygen-free nitrogen by Linde Specialty Gas. Millipore water (18 MQ)was exclusively used. Other chemicals were double distilled HC104 (GFS), ultrapure H2S04 (Baker), and reagent grade iodine (Baker). All measurements were conducted at room temperature. The area of the Pt(100) plane was 0.53 cm2. Potentials are given with respect to a Ag/AgCl reference electrode, with a chloride concentration of 1 M. All current-potential profiles were taken at a scan rate of 50 mV/s.
Results and Discussion Characterization of the Clean Pt(100) Electrode. After 10 min of sputtering and 30 min annealing, we obtained a LEED pattern that was characteristicof a clean, reconstructed pt( 100)(5x20) surface.13JoJ1 Auger electron spectra taken with the same surface did not show peaks other than those characteristic of clean paltinum (Figure la). The open circuit potential measured directly after immersion of the sample in sulfuric acid was 0.53 V and was 0.63 V in perchloric acid. Cyclic voltammograms in 0.1 M perchloric and 0.5 M sulfuric acids are presented in Figure 2.9922 The electrochemical quality of the Pt(100) surface may be assessed by comparing our voltammetric data with previous results obtained with the flame-
W (Ag/AgCI) Figure 2. Cyclic voltammetry of the Pt(100) electrode in (a) 0.1 M HC104 and (b) 0.5 M HzS04.
annealed platinum electrode of the same orientation.22.23 The comparison, using a stable voltammogram for the vacuumprepared surface, reveals that the main voltammetric features are hydrogen adsorption-desorption peaks at 0.06 V (Figure 2a). This indicates a high level of surface order. However, the maximum at -0.005 V is also seen which shows that the (100) electrode surface is not defect-free. The defects are most probably concentrated in the grain boundaries and are not induced by oxygen adsorption.z2 Likewise, the overlap of consecutive voltammetric profiles upon cycling in the potential range of the single-crystal stability proves that the surface is clean and free of uncontrolled solution impurities. Adsorption and Oxidation of CO. The electrode potential for carbon monoxide adsorption chosen for this study was -0.200 V. Higher potentials were avoided in order to reduce the possibility of surface CO oxidation upon electrode emersion to the gas phase or during system evacuation. After the adsorption phase of the experiment was completed, the electrode was rinsed repeatedly with electrolyte containing no CO to remove all traces of the unbound CO solute. The adsorbate was either immediately oxidized in a positive-going voltammetric scan (Figure 3a,b) or transferred to the UHV chamber for LEED and AES characterization (Figure 3c). Following the voltammetric oxidation of adsorbed CO and the scan reversal before the platinum oxidation threshold, the voltammograms showed a total recovery of the surface properties of the clean Pt( 100) electrode. This ensured the absence of contamination during the whole proce~s.2~ After CO dosing at -0.200 V and rinsing with clean supporting electrolyte,the CO oxidation charge obtained via anodic stripping process in the clean sulfuric acid solutions between 0.340 and 0.670 V was equal to 488 pC cm-2 (Table I, first row). Because the position of the potential of zero charge (pzc) for Pt(100) is uncertain, there is no a priori knowledgehow such a total oxidative charge should be corrected for the double-layer charging. In the
Rhee et al.
9732 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 1 .oo
TABLE I: Total CO Stripping Charge (p), Stripping Charge after the Double-Layer Correction (Qco, See Appendix), and CO Coverage (0) Obtained from oca'
a
0.7 5
eo(pC/cm2)
Po(pC/cm2)
488 f 20 470 f 30
322 f 20 314 f 30
8co (ML)b 0.77 f 0.04 0.75 f 0.07
500 f 30
335 f 30
0.79 f 0.07
0.5 M H2SO4 0.5 M H2SO4 after UHV
0.50
Q
E
0.1 M HC104
0
6 E \
.-
Carbon monoxide was dosed at -0.200 V in electrolytes specified in the first column of the table. 1 ML corresponds to the number of platinum surface sites per surface area unit (1.3 X 1015atoms cm-2 for Pt(100)).
0.25
0.00
- 0.2 5 -0.40
0.00
0.40
0.80
W (AgIAgCI) 31
1
6
E
1
'
-1 -0.40
I
I
0.00
0.40
I
e
o 0
0.80
o
e
o
.
0
00
000 0
cy
00 0
0 0 0
.
0
0
e
o
000
00
E N (AglAgCI)
e
0
e o e o .
6 6
Figure 4. (a) Pt( lOO)-CO LEED pattern after CO adsorption in 0.5 M H2SO4 at -0.200 V. Beam energy 94.9 eV. (b) Diagram of the LEED
E
1
pattern shown in (a).
'
-1 -0.40
I
I
I
0.00
0.40
0.80
E N (AgIAgCI)
Figure 3. Oxidation of CO adsorbed on the Pt( 100)electrode in (a) 0.1 M HClO4, (b) 0.5 M HzS04, and (c) 0.5 M H2S04 after exposure to UHV (see text).
Appendix, we describe a method that employs iodine adsorption process to estimate thedouble-layer correction. As shown therein, the correctionto be subtracted from the total CO oxidation charge is 166 pC cm-2. This gives the net CO electrooxidation charge of 322 pC cm-2. Using the stoichiometry of two electrons per molecule in the CO oxidation process, the latter result implies a
CO coverageof 0.77f 0.04 monolayer (ML). The corresponding value for perchloric acid is 0.79 f 0.07 ML (Table I, third row). Within the experimental scatter this is the same coverage as that obtained with the gaseous CO adsorbed on the Pt( 100) surface.l3 Carbon monoxide adsorption on the Pt(100) electrode gave rise to the LEED pattern shown in Figure 4. This pattern correspondsto the Pt( lOO)c(4X2)-CO surface s t r u ~ t u r eThe .~~~~~ LEED spot distribution shows that the Pt(100)(5X20) reconstruction was lifted to produce the Pt( 111)(1Xl) substrate, as expected from the earlier ~ o r k . 1 3Thec(4X2) ~ ~ ~ ~ ~LEED ~ pattern was found reproducible throughout this study except for the spot intensity vs background ratio that varied somewhat among the measurements. Within the Pt( 1OO)c(4X2)-CO geometry, the total CO coverageshould be a multiple of the 0.25 ML coverage.20 Our coulometric data show that the multiple (within the experimental error) is equal to 3. The Pt(lOO)c(4X2)-&o = 0.75 ML structure has already been reported after carbon
CO on a Pt( 100) Electrode monooxide adsorption in the gas phase.13~21 Therefore, for this (100) surface orientation and for the chosen value of the electrode potential of CO adsorption, we report on a clear similarity, if not identity, between the surface configuration of the vacuum and electrochemical CO adsorbate."J This is the same conclusion as we have previously arrived at with the (1 11) crystallographic orientation of platinum.10 Control of Emersion Process. In a number of control measurements the CO-covered Pt( 100) electrode was prepared at -0.200 V, transferred to the UHV for a period of approximately 20 min, and transferred back to the electrochemical cell for the voltammetric characterization. Table I (second row) contains data on the electrooxidationcharge of the chemisorbed CO and on the CO coverage obtained from the charge after correction for thedouble layer charge; see Appendix. In sulfuricacid solution the mean CO coverage was 0.75 f 0.07 (Table I). This is the same coverage (within the experimental error) as that obtained in this study before the UHV exposure. In perchloric acid, the postevacuationcoverage was lower than thevalue obtained before the UHV exposure. However, the ~ ( 4 x 2 LEED ) pattern was consistently observed with a varying amount of the background intensity. We conclude that some desorption of CO was taking place. The desorption most likely occurredvia oxidation of surface CO by perchlorate vapor during the Pt-CO sample reimmersion. Therefore,the electrolytethat guarantees a successful, desorptionfree outcomeof the electrochemistry-vacuum transfer experiment with the CO adsorbate on the Pt( 100)electrode is a dilute sulfuric acid rather than perchloric acid. AES Analysis of the Electrochemical CO: Comparison with Other CO Systems. The secondary electron spectrum of a clean Pt(100) surface taken at a primary electron beam energy of 800 eV is shown in Figure 1a. The major Pt Auger electron transitions are 64, 150, 168, 237, and 251 eV. The interesting features of this spectrum are the loss peaks appearing at kinetic energies 480,462, and 449 eV. The first two losses are due to N~(4dsp) and N4(4d3/2) electrons of platinum excited into empty states above the Fermi level. These spectral features correspond to the energy losses of around 316 and 333 eV below the primary electron energy, in agreement with the literature data.26 The third loss peak, 30 eV below the Ns ionization peak, is due to a single plasmon loss (the free electron Pt plasmon energy is 30.2 eV).z7 As mentioned in Experimental Section, the quantitativeanalysis of the CO coveragewas done using a thick urea overlayer deposited on the same Pt( 100) electrode in 2 M aqueous urea solution (and dried in vacuum).19 The coverage obtained via this method is 0.79 0.09, in a very good agreement with the coulometricresult; see above. The AES analysis of the Pt(lOO)c(4X2)-0co structure (the ideal coverage, 0, equal to 0.75 ML) reveals that the carbon spectrum consists of three Auger electron transitions (Figure lb). The energies of these transitions are compared in Table I1 with the experimental literature data for Mo(CO)6 carbonyl as well as for CO chemisorbed on several metal surfaces. The compiled data were obtained from either direct or differentiated electron spectra,18~28-3~ and the peak energies are referred to the Fermi level in most of the c a ~ e s . ~The ~ -relative ~ ~ energy positions are given with respect to peak no. 3 (Table 11), which will be further denoted as peak 3. The shape of the Auger electron spectrum located among 260 and 275 eV is a characteristic fingerprint of carbon in chemisorbed CO and in metal carbonyls.29 These carbon features are very sensitive to the electron beam.'* We have found that peak 3 at 262.6 eV (Figure lb, inset) rapidly disappeared upon irradiation by the electron beam and the Auger electron spectrum of carbon evolved toward the final shape (Figure lb), where the graphitic Auger electron transition around 273 eV is dominant. Since the previously unoccupied 2u orbital of CO is filled with metal d electron to screen the core hole, the interpretation of the AES lines of CO cannot conclusively be performed using the molecular orbital scheme approximation.29 Consequently, Baker
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9733 TABLE Ik Carbon Auger Electron Energies (eV) in Carbon Monoxide on Metal Surfaces and in Mo(C0)6 peak no. after ref 30 substrate 1 2 3 4 MO(CO)~ gas26 264.0 261.0 255.0 249.0 energy dif -6.0 -9.0 0.0 6.0
co/cu(ii1)29
energy dif CO/Cu( 100)30 energy dif CO/Ni(l 1l)3O energy dif CO/Ni( 100)3' energy dif CO/Pt(ply)'* energy dif co/Pt(ii1)29 energy dif
279.5 -9.5 276.7 -9.7
279.5
-11.0
CO/Pt( 11 1)32*33
280.0
energy dif CO/Pt(ll1)3' energy dif CO/Pt(loo).~ energy dif
-10.0 275.6 -11.0 212.9 -10.3
275.5 -5.5 213.6 -6.4 274.4 -7.1 215.1 -6.7 212.0 -8.0 215.5 -1.0 273.1 -8.5
270.0 0.0 267.3 0.0 267.3 0.0 269.0
0.0 264.0 0.0 268.5 0.0 270.0 0.0 264.6
0.0 262.6 0.0
26 1.O 9.0 258.0 9.3 260.2 6.9 260.0 9.0 259.5 9.0 260.0 10.0 257.5 7.1 254.3
8.3
et al. have suggested that the electrons come from below the Fermi level, resulting in AES peaks 1 and 2 around 280 and 275 eV, respectively (Table 11), and have nonintramolecular origin.29 In turn, Netzer and Matthew argued that whereas peak 2 is of the intramolecular origin, the intense peak at 280 eV involves states within the metal conduction band.3213~This is corroborated by Auger electron spectra of gas-phase carbonyls of Stucky et al. and their companion theoretical considerationswhere NiCO was employed as a model molecule.28 The information on the Auger electron transitions of carbon monoxide adsorbed on the Pt( 100) surface in gas phase is not yet available. The present analysis, aimed at assigning the spectral features observed in this study to the previously published ones, is made using AES data on CO adsorption on other than Pt(100) metal substrates and on an organometallic compound, Mo(CO)~ (Table 11). Theanalysisshowsthat therelativeAESpeakenergies fall into three different classes. The transition marked as peak 1 is around 10 eV above peak 3 and the transition represented by peak 2 appears close to 7 eV above peak 3, while the AES transition denoted as peak 4 is around 8 eV below peak 3. The transition observed presently at 272.9 eV is 10.3 eV above peak 3. This suggests the (5u/ 1u,2u-d) final two-hole configuration as for the CO/Pt( 111) surfacesystem.32.33 The most recent results for the latter system show four, instead of three, distinct peaks, but peak 2 has the lowest intensity.34 The relative positions of peaks 1,3, and 4 in the spectra presented here are in accord with the platinum/CO data of ref 34. Peak 2 is not found in Figure lb, presumably because of its low intensity and a limited CMA spectrometer energy resolution. The assignment of peaks 3 and 4 in CO/Pt( 100) is not definite, but the real situation cannot be much different from that for CO/Ni( 100)describing 3 as (5u,5u), (5u,lu), (ls,lu) and 4 as (4u,4u) final two-hole configuration.31 Peaks 3 and 4 are, most likely, of the intramolecular origin. On the basis of the above analysis, we conclude that there is a clear similarity in the spectral appearance of the chemisorbed CO from the gas phase and obtained under electrochemical conditions. Pt(100)c(2X4)-Bco = 0.75 Comparison between Electrochemical and Gas-Phase W i g . According to Biberian et al.,14 the general notation of Pt( lOO)c(2X4)-B~o= 0.75 may embody four different structures that differ as to the intimate details of thecoordinationby the bridge- and terminal-bondedCO molecules (Figure 5). Earlier in situ infrared spectroelectrochemistry measurements are particularly instrumental in providing the link between our structural data in Figure 4 and the four entries in Figure 5. Namely, the infrared data show that the population of the terminal CO is predominant over the bridge-bonded p0pulation.~-9Since in (a) and (c) the main species is the bridge-
9134 The Journal of Physical Chemistry, Vol. 97, No. 38, 1993
Rhee et al. arrived at above, the estimate remains well within the uncertainty limits of this analysis. Moreover, the bridge/terminal infrared intensity ratio for the gas-phase-dosed CO may be obtained based upon the data of ref 6. The integration of the infrared spectra presented in the quoted work-for dosing to the unreconstructed Pt(100) surface-gives the intensity ratio of 0.23. This latter value is also very close to the value of 0.3 obtained by Norton et al.,35 who used the high-resolution electron energy loss spectroscopy and the same CO dosing conditions as those of ref 6. These gas-phase-dosing values are surprisingly close to the Weaver in situ electrochemicalbridge/terminal ratio.9 Since we may assume that carbon monoxide dissolved in electrolytic solution is adsorbed on the unreconstructed Pt( 100) surface, we may conclude that the surface configuration of CO adsorbed at the threshold of hydrogen evolution is the same as that observed after the gasphase adsorption to the Pt(100) substrate. As shown above, the unit cell that for this configuration is Pt( 100)c(4X2)-B~o= 0.75, and the experimentally measured CO coverage is 0.77 f 0.04 molecules per platinum surface site.
Conclusions
C) d) Figure 5. Surface structures of Pt(lOO)c(2X4)-Bm = 0.75 ML after Bibcrian et al." I
5 1
0
-10
-15
t/s
Figure6. Time-dependentelectriccurrentgenerated due to water/iodine displacementon the Pt( 100) surface in 0.1 M H2S04 at El potential (see
Appendix). bonded CO (four bridging CO molecules in the unit cell vs two on-topCO), thestructures (a) and (c) cannot portraytheadsorbate observed in this work. The symmetric and asymmetric types of the bridge-bonded CO (Figure 5) have also been dealt with by the in situ surface infrared investigations. Apparently, for the ( 100) platinum substrates characteristic of the long-range surface order, the bridge-bonded CO is adsorbed in a symmetric configuration.s In view of the well-ordered character of our electrode, this earlier information allows us to assign the ~ ( 4 x 2 ) LEED pattern to the structure (b) of the Biberian diagram14in Figure 5. In this structure, there are 33% of the CO molecules in the symmetric bridge-bonded configuration and 67% of the CO molecules in the terminal coordination, with the 0.5 ratio of the bridging/terminal CO populations. Weaver et al. have found that the ratio between the integrated infrared intensities of the bridge- and terminal-bonded CO under saturation conditions at -0.25 V (vs SCE) is 0.3.9 Applying the infrared absorptivity ratio of the bridge/terminal CO bands suggested in ref 9, q,/et = 0 . 4 4 5 , to the ratio of the integrated infrared intensities of this reference gives the ratio of the bridge/ terminal populations of 0.6-0.75. While higher than the 0.5 value
1. The voltammograms obtained for the Pt(100) electrode prepared by the vacuum treatment (this work) and the flame annealing meth0d*2*~3 are basically the same. The voltammetry may be used for the CO coverage calculation provided that a double-layer correction is made as explained in the Appendix. 2. As referenced to various CO-metal-surface systems and a relevant organometallic compound, the adsorbate obtained in the CO saturated solutions, and emersed to the UHV for surface analysis, has the necessary AES characteristics of surface CO. 3. The coverage of carbon monoxide adsorbed on the Pt( 100) electrode in sulfuric or perchloric acid solutions at -0.200 V obtained by the voltammetry is0.77 f 0.04. This can be compared with the Auger electron spectroscopyvalue of 0.79 f 0.09. These values correspond to the ideal Pt(lOO)c(4X2)-Bco = 0.75 unit cell (LEED). This is the same structure, and coverage, as obtained via dosing of gaseous CO to the Pt(100) substrate (and studied in the UHV). 4. On the basis of the infrared spectra of ref 9, the ratio of the bridge/terminal populationsis estimated to be 0.6-0.75. Given theuncertaintyofthe analysis,thisvalueisclose tothe0.5 expected from the adsorbate modeling.14 We therefore conclude that the population ratio of the bridge-bonded and terminal-bonded CO obtained in solution is in the category of the ideal 0.33:0.67 ratio in the Biberian diagram (Figure 5b). 5 . When a dilute sulfuricacid is used as supporting electrolyte, the solid/liquid surface CO is stable during the solution-UHVsolution transfer. Discernible desorption was observed when perchloric acid was used.
Acknowledgment. This work was supported by the National Science Foundation under Grant DMR 89-20538, administrated by the Materials Research Laboratory, Urbana. C. K. Rhee acknowledges support by the Air Force Office of Scientific Research (AFOSR-89-0368). J. M. Feliu acknowledges financial support from the C.C.E.C. of the Generalitat Valenciana for a 2 month stay in the University of Illinois. E. Herrero is also indebted to the C.C.E.C. of the Generalitat Valenciana for the award of a F.P.I. grant. Appendix Double-LayerCorrectionwith Iodine Adsorption. The process of carbon monoxide adsorption causes a significant modification of the double-layer structure of the electrode,24and the integral capacity of the electrode-before and after CO adsorption-is not the same. After electrooxidation of adsorbed CO the state of the electrode may return to the CO-free situation. The CO removal is associated with readsorption of water molecules and anions that were initially displaced by the CO adsorption. In
The Journal of Physical Chemistry, Vol. 97, No. 38, 1993 9735
CO on a Pt(100) Electrode order to determine the CO coverage via coulometry, the capacitive charge associated with the replacement of CO by water and anions must be known and be subtracted from the total CO electrooxidation charge. Only such a corrected, purely faradaic current can be used for the coverage determination. As mentioned in Results and Discussion, the electrodeproperties of the clean Pt(100) electrode fully recover after CO oxidation. This means that the double-layer structure and its charge are completely restored at the end of the oxidative-desorption scan. Therefore, the overall charge measured (@O) is qco = Qco + Q"'
(1)
In order to obtain the faradaic component of the total charge ( P o ) , the double-layer charging contribution (@I) must be subtracted from the total, Po= qco- @I. To obtain the doublelayer contribution, two states are to be considered. The initial state at the onset of the CO oxidation, Qi, is characterized by a potential Ei and an integral double-layer capacity Po:
Qi= (Ei- Epz:O)@O
(2)
where EprcCois the potential of zero charge for the CO-covered electrode. The final state-at the end of the CO oxidation-has a potential E, and a double-layer capacity equal to that of the clean surface:
Q, = ( E , - EpzcC'Can)Kc'can
(3) The double-layer correction which has to be applied in this case is24
Q"' = Q,- Qi = (E, - Epz~*ean)Kc'can - (Ei- Epz:o)@o
(4)
In eq 4, both the Kclmn and KCO values are known from the measurements and equal to 260 and 14 pF cm-2, respectively. The difference in the double-layer capacity between the COcovered and the clean electrode can yield the value of @I. The values of the potential of zero charge EpzcC1ean and EpzcCo are unknown and are not easily accessible in a direct manner. Below, we describe a method based upon the considerations in ref 24 that yields an reliable estimate of @I. Namely, we evaluate Qdl by using the charge released upon iodine adsorption at potential which coincides with the end of the voltammetric oxidation of carbon monoxide (E,). We believe that this method may be applicable to many other similar situations where the capacitive terms need to be known. In the measurements carried out to obtain the double-layer correction, the electrode potential was fixed at the upper limit of thecostrippingprocess. Agasmixtureof I2 N2wasintroduced into the cell atmosphere, and through dissolution in the layer of electrolyte adjacent to the electrode, iodine was brought to the electrode surface. It is well-known that iodine adsorbs readily on platinum and easily displaces adsorbed water molecules. We have recorded current transients associated with such a water/ iodine displacement (Figure 6). The shape of the current-time transient shown in this figure depends on the I2 + N2 gas flow rate. The charge obtained from the current-time integration, Ql, is the required estimate of the @I assuming that iodine is adsorbed at Ef as a neutral specie^.^^,^' We can now define the initial and the final states characterized by Ef, Kc1-n and Ef, R,respectively:
+
Q' = ( E f- EpzJ)K'- ( E , - EpzcClean)Kclcan
(5) From this equation, ( E f- EpzcCIWn)Kclmncan be calculated and substituted into eq 3. Therefore
Q"' = ( E , - EP,')K1 - Q' - (Ei - Epz:o)@o
(6)
Several values are known in this equation. The value of Q1, measured from the iodine displacement experiments, is -166 f 10 pC cm-2. (The negative value corresponds to a reduction charge.) The values of KCO and R are estimated from the current
obtained voltammetrically in the double-layer region of electrodes covered by carbon monoxide and iodine. This gives approximately 14 and 6 pF cm-2 for P and Kr, respectively. Furthermore, E, is 0.67 V and Ei is 0.20 V. Substituting these values into eq 5 gives
Q"' = 166 + [6(0.67 - Epd)- 14(0.20 - EPz,"")] pC cm-' (7) The small values of the electrode capacity when it is covered by iodine and carbon monoxide make the two corrections negligible with respect to the main value of Q1= 166 f 10 pC cm-2. In fact, this assessment holds irrespective of any realistic shifts in the pzc from EpzfClan caused by carbon monoxide or iodine adsorption. Therefore, the correction for the overall stripping CO charges is 166 i 10 pC cm-2.
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