J. Phys. Chem. 1991, 95,7453-7459
1453
using hE and u directly derived from suitable reference compounds in order to determine the accurate coordination numbers and the coordination distances. From the EXAFS analysis as well as TEM observation, the Pt core structure (Figure 7a), in which the 42 Pd atoms are on the surface of the cluster particle and 13 Pt atoms are in the core, is presented as a model for the Pd/Pt (4/1) bimetallic clusters. With decreasing in Pd/Pt ratio, the Pd atoms on the surface of the cluster are substituted for the Pt atoms. Then, the Pd/Pt (1/ 1) bimetallic clusters are suggested to have a modified core structure (Figure 8a,d), in which 28 Pt atoms are located both in the core and on the surface connecting directly each other and 27 Pd atoms form three islands on the surface of the cluster particle. 0 = Pt Figure 9. Metallic skeleton of the [Ni,8Pt,(Co)48H]J-cluster.24
structure of the bimetallic clusters. It can be empirically said for the bimetallic carbonyl clusters that the heavier metal atoms are located near the center of the cluster and the lighter metals are near the surface. For example, [Ni38Pt6(Co)aH]5- was reported to have the structure shown in Figure 9, where the heavier Pt atoms are located at the center and the lighter Ni atoms are near the surface.24
Conclusion The structure of the colloidal dispersions of the palladium/ platinum bimetallic clusters protected by poly(N-vinyl-2pyrrolidone) was studied by EXAFS measurement. Because of the similar contribution of Pd-Pd, Pd-Pt, and Pt-Pt coordination distance, the separation of each contribution is difficult. To analyze these contributions, the data analysis was carried out by (24) Heaton, B.T.; ingsllina, P.; Devenish, R.; Humphreys, C. J.; Ceriotti, A.; Longoni, G.; Marchionna, M. J . Chem. Soc., Chem. Commun. 1987,765.
Acknowledgment. We gratefully acknowledge the assistance of Dr.Kouichi Adachi in taking electron micrographs and of Drs. Atsushi Oyama and Masaharu Nomura a t the National Laboratory for High Energy Physics (KEK) for the EXAFS measurements. This study was supported by a Grant-in-Aid for Scientific Research in the Priority Area of “Macromolecular Complexes” (01612002) from the Ministry of Education, Science and Culture, Japan. Glossary x(k) EXAFS oscillation p(E) observed EXAFS data h Planck‘s constant/2r k wavenumber of the photoelectron E photon energy m mass of the electron coordination number of the j t h coordination shell Nj bond length of the j t h coordination shell rj difference between the theoretical and experimental threshA&, old energies Debye-Waller factor of the j t h coordination shell 3 phase shift of the j t h coordination shell 4j(k) amplitude function of the j t h coordination shell Fj(k) amplitude reduction factor reliability factor
2
I n Situ Infrared Spectroscopy of Carbon Monoxide Adsorbed at Iridium( 111)-Aqueous Interfaces: Double-Layer Effects on the Adlayer Structure Xudong Jiang, Si-Chung ChangJ and Michael J. Weaver* Department of Chemistry, Purdue University, West Lofayette, Indiana 47907 (Received: March 26, 1991; In Final Form: May 20, 1991)
Surface infrared spectra in the C-O stretching ( ~ ~frequency 0 ) region are reported for CO adsorbed on ordered Ir( 11 1) in aqueous media, primarily 0.1 M HClO,, as a function of CO coverage, Oca, and electrode potential, E. In contrast to other platinum-group (1 11) surfaces examined hitherto, the adsorption sites were found to be exclusively atop (or near-atop), even at potentials down to -0.8 V vs SCE (in 0.1 M NaCIOl + 1 mM KOH), as discerned from the appearance of a single uco feature at ca. 1960-2060 cm-I. Large (up to ca. 100 cm-I) frequency upshifts are observed at increasing dosed coverages, especially for adsorption in the “hydrogen” potential region (ca. -0.25 to 4 - 0 5 V vs SCE), where fractional CO coverages of up to 0.6 could be obtained. Significantly smaller saturated Bco values, ca. 0.45, were attained for adsorption in the “double-layer”region, 0 . 2 V. Adlayers formed in these two potential regions also exhibit different spectral and electrochemical properties. For example, CO adlayers prepared within the hydrogen region on I r ( l 1 1 ) exhibit extensive island formation during electrooxidative removal, as discerned from the relatively invariant vk0 values, in a similar fashion to adlayers on low-index Pt and Rh surfaces. For adlayers formed in the double-layer region on Ir( 11I), however, CO island formation is rather more limited under these conditions, as seen from a markedly larger dependence which matches more closely that obtained for partial coverages formed by direct dosing with dilute CO solutions.
Introduction A topic of continuing interest in our laboratory concerns the examination of carbon monoxide adsorption on ordered monocrystalline transition-metal surfaces in electrochemical environ‘Present address: Dow Chemical Co., Midland, MI 48667.
0022-3654/91/2095-7453$02.50/0
ments by means of in situ infrared reflection-absorption spectroscopy (IRAS).’+ These studies are motivated in part by the (1) For overviews, see: (a) Chang, s.-C.; Weaver, M. J. J . Phys. Chem. 1991, 95, 5391. (b) Chang, s.-C.; Roth, J. D.;Ho, Y.;Weaver, M. J. J . Electron Spectrosc. Relat. Phenom. 1990, 54155, 1185.
0 199 1 American Chemical Society
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The Journal of Physical Chemisrry, Vol. 95, No, 19, 1991
newly emerging opportunities to explore adsorbate properties on crystallographically ordered electrochemical surfaces at a level previously exclusive to metal-ultrahigh vacuum (uhv) systems.' We have focused attention initially on platinum and rhodium, primarily because of the availability of reliable surface pretreatment procedures applicable to in situ electrochemical measurements, along with the extensive vibrational spectroscopic and other characterization afforded to these surfaces in uhv. Comparisons between electrochemical IRAS data as a function of CO coverage (dc0) and electrode potential with corresponding measurements for the metal-uhv systems can provide a detailed picture of the influence of the electrochemical double-layer components upon the adlayer structure.' In order to understand surface chemical trends in the adsorptive and catalytic properties, it is clearly desirable to expand such studies to other monocrystalline metals. Of the platinum-group metals of particular catalytic interest, remarkably little information is available for the 5d group VI11 neighbor of platinum, iridium. The Ir( 1 11) surface is stable toward reconstruction and yields ordered CO adlayer structures in uhv as discerned by low-energy electron diffraction (LEED)? Although the LEED patterns have been interpreted in terms of adlayer structures involving bridging and threefold hollow, as well as terminal (i.e., atop), CO binding sites, recent electron energy loss spectroscopic (EELS) data indicate that only atop coordination occurs throughout the entire coverage range.* We report here findings from an initial IRAS study of the adsorption and electrooxidation of CO on Ir( 1 1 1) in aqueous, primarily acidic, electrolytes. In contrast to platinum and rhodium surfaces, purely atop CO coordination is encountered over the entire accessible range of Oc0 and electrode potential. A primary objective of this study is to explore the manner and extent to which the double-layer environment can influence the C O adlayer structure. An interesting sensitivity is observed of the CO adsorbate structure and consequent electrooxidation kinetics to the conditions, including the electrode potential, at which the adlayer is initially formed.
Experimental Section Details of the electrochemical IRAS measurements are largely as provided in refs 3a and 9. The infrared spectrometer was an IBM (Bruker) IR-98-4A Fourier transform instrument, with a Globar light source and either an InSb or MCT, liquid N2cooled, detector (Infrared Associates). The InSb detector was chosen so as to yield optimal signal-to-noise for the terminal C-0 stretching (w0)band, located well above the detector cutoff frequency of ca. 1820 cm-'. The MCT detector was utilized to investigate the possible presence of lower frequency (bridge-bound) uco features. The spectral resolution was usually f 4 cm-I. The optical arrangement involved the use of an electrochemical thin layer formed by pushing the electrode up to the CaF, win do^.^ The infrared beam was incident at about 60° to the surface normal, with the window beveled a t the same angle so as to minimize refraction at the a i d a F 2 interface and hence maximize the angle of incidence at the electrode-aqueous interface.3a (2) '(a) Leung. L.-W. H.; Wieckowski, A.; Weaver, M. J. J. Phys. Chem. 1988,92.6985. (b) Chang, S.-C.; Leung, L.-W. H.; Weaver, M. J. J . Phys. Chem. 1989, 93, 5341. (3) (a) Chang, S.-C.; Weaver, M. J. J . Chem. Phys. 1990,92,4582. (b) Chang, S.-C.; Weaver. M. J. J. Phys. Chem. 1990,94,5095. (c) Chang, S.-C.; Weaver, M.J. Surf.Sei. 1990, 230, 222. (4) (a) Leung, L.-W. H.; Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1989.90.7426. (b) Chang, S . C ; Weaver, M. J. J. ElectrwnaI. Chem. 1990, 285, 263. (5) Chang, S.-C.; Weaver, M.J. Surf, Sei. 1990, 238, 142. (6) (a) Chang, S.-C.; Weaver. M.J. Surj. Sei. 1991,241, 11. (b) Chang, S.-C.; Roth, J. D.;Weaver, M. J. Surf.Sci. 1991, 244, 113. (c) Roth, J. D.; Chang, S.-C.; Weaver, M. J. J . Electroanal. Chem. 1990, 288, 285. (7) (a) Comrie, C. M.; Weinberg, W. H.J. Chem. Phys. 1976, 64, 250. (b) KUppers. J.; Plagge, A. J. Vue. Sei.Technol. 1976, 13, 259. (c) Hagen, D. 1.; Nieuwenhuys, B. E.; Rovida, G.; Somorjai,G. A. Surf. Sei. 1976,57, 632. (8) Marinova, T. S.; Chakarov, D. V. Surf. Sci. 1989, 217, 65. ( 9 ) (a) Corrigen. D. S.;Weaver, M.J. J. PhyJ. Chem. 1986,90,5300. (b) Corrigan, D. S.;Weaver, M. J. J. Electroanal. Chem. 1988, 241, 143.
Jiang et al.
i-
-
850 -0.05 2048
0.13
0.14
k 3 xici' a.u.
I
I
2100
2000
u/crfi'
1
I
2100
2000
V/Cfl
Figure 1. Infrared absorbance spectra in terminal C-O stretching region for irreversibly adsorbed CO layers on ordered Ir( I 1 1) in 0.1 M HCIO, at -0.25 V vs SCE for coverage values) ,0( as indicated. Spctra in (A)
were obtained by progressive electrooxidativestripping (0.5-s potential pulses to 0.5 V) from a saturated (eco = 0.6) layer and in (B) by dosing for variable time periods (ca.1-10 min) from dilute (-2 X M) CO solutions. Each spectrum in (A) was acquired by means of 30 interferometer scans, subtracted from which was a similar set obtained subsequently at 0.5 V after CO electrooxidationwas complete. Spectra in (B) each involved acquiring 50 scans. Coverage values were obtained from absorbance of product solution C02 band at 2343 cm-l (see text for further details). The Ir( 11 1) crystal (8 mm diameter, 4 mm thick) was purchased from the Material Preparation Facility at Cornell University. It was oriented within *lo, as verified by X-ray backdiffraction. The surface pretreatment procedures, including hydrogen-air flame annealing, gas-phase iodine adsorption during cooling, and subsequent in situ removal with adsorbed CO, were similar to those for R h ( l l 1 ) as described in ref 4a (cf. ref 10). This entailed replacing the iodine by CO at a relatively negative potential, -0.6 V, in an alkaline electrolyte. Iridium (1 11) surfaces exhibiting identical spectral and electrochemical behavior were obtained, however, by replacing the iodine by CO at a less negative potential, -0.25 V, in 0.1 M HC10, as for Pt(l1 l).h Perchloric acid and sulfuric acid (both doubly distilled) were obtained from GFS Chemicals. Water was purified by means of a Milli-Q UF Plus system (Millipore Co.). Electrode potentials are quoted versus the saturated calomel electrode (SCE), and all measurements were performed at room temperature, 23 f 1 O C .
Results Similarly to some previous electrochemical IRAS studies of C O at ordered low-index platinum- and rhodium-aqueous interfaces in our l a b o r a t ~ r y , the ) ~ overall strategy pursued here involves obtaining uco spectra at Ir(ll1)acidic aqueous interfaces for sequences of CO coverages and electrode potentials. The CO adlayers were formed primarily at two electrode potentials, -0.25 and 0.05 V versus SCE; these values lie within the so-called "hydrogen" and "double-layer" regions in 0.1 M HCIO,, where the primary coadsorbed species are atomic hydrogen and water, respectively. (10) Hourani, M.; Wieckowski. A. J. Electrwnul. Chem. 1987,227,259.
CO Adsorbed at Ir( 11 1)-Aqueous Interfaces
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 1455
In all of the infrared spectra obtained here, the C O bands are located exclusively in the frequency region 1950-2100 cm-I, characteristic of terminal C O in electrochemical environments. Figure 1 contains representative infrared absorbance spectra for CO adlayers at the various coverages indicated on Ir(ll1) in 0.1 M HCIOl at -0.25 V. The left-hand set (Figure 1A) was obtained by forming a saturated CO layer at -0.25 V by bubbling in CO for 1 min, and then purging with nitrogen to remove the solution CO before forming the thin layer. The saturation coverage, 0.60 i 0.05, was determined from the absorbance of the 2343-cm-l C02 band formed by complete C O electrooxidation (cf. ref 2). As usual, Oc0 refers to the ratio of adsorbed CO molecules to iridium atoms (1 3 7 X IOI5 atoms cm-2) present on the presumed unreconstructed surface. A similar Om value, 0.65, was obtained from the faradaic charge, 335 (f20) pC cm-*, contained under the voltammetric wave for C O electrooxidation (vide infra). Whereas the bottom spectrum in Figure 1A refers to this saturation coverage, the upward spectral sequence corresponds to the progressively diminishing coverages as indicated. These spectra were obtained by acquiring sets of 30 interferometer scans at -0.25 V, stepping the potential to 0.5 V for about 0.5 s between each set so as to electrooxidize a further portion of the adsorbed C O each time. Subtracted from each set is a corresponding spectrum obtained upon completion of the CO electrooxidation, so to remove spectral interferences arising from the solvent and so on.3a The values of the intermediate 0, values were determined from the absorbance of the 2343-cm-I solution C02band that also appears in these potential-difference spectra, relative to the value where Oco = 0.60. (This procedure provides relative Oco values that are reliable to a few percent, given the high absorptivity of the 2343-cm-l feature.) The coverage-dependent spectral set in Figure 1B was obtained instead by exposing the clean Ir( 11 1) surface at -0.25 V to a lower C O solution concentration (-2 X M) in 0.1 M HC10, for varying time periods (ca. 1-10 min). Each spectrum shown was obtained by acquiring 50 interferometer scans a t -0.25 V and subtracting another set obtained subsequently at 0.5 V following complete CO electrooxidation. Again, the CO coverages indicated were obtained from the corresponding absorbances of the 2343cm-l COSband appearing in these spectra. Similarly to the behavior of CO adlayers on low-index platinum and rhodium electrodes reported previously, marked differences are evident in the coverage-dependent vco spectra under such CO "electrooxidative stripping" and "direct dosing" conditions shown in Figure 1, A and B, respectively. While in the former case, the peak frequency of the terminal C-0 stretching band, vk0, decreases only by ca. 12 cm-l from Oco = 0.60 to 0.10 (Figure lA), substantial (ca. 80 cm-l) vc0 diminutions are obtained under these conditions in the latter case (Figure 1B). Marked increases in the bandwidth are also observed with decreasing coverage under dosing conditions at -0.25 V, from a full width at half-maximum (fwhm) of 17 cm-l at Oco = 0.60 to ca. 45 cm-' at Oco = 0.13. In contrast, the fwhm was virtually independent of Oco under stripping conditions, although a substantial low-frequency "tail" is evident at lower coverages (Figure 1A). For spectra obtained for diminishing coverages formed by slow adlayer electrooxidation a t ca. -0.25 V, this lower frequency component is more clearly resolved as a separate band. Significantly different behavior, however, was obtained for CO adsorbed at potentials positive of 0 V, Le., in the "double-layer" region. Figure 2 shows representative coverage-dependent sets of vco spectra for CO adlayers on Ir(l11) in 0.1 M HClO, obtained under stripping and dosing conditions as in Figure 1, but for an adsorption potential of 0.05 V. There are two marked differences between the data in Figures 1 and 2. First, the saturation CO coverage, @&, at 0.05 V, deduced to be 0.45 (i0.05) from the C 0 2 product absorbance and voltammetric electrooxidation charge as before, is substantially lower than the value, 0.6, obtained at -0.25 V. A further examination revealed that e& values close to 0.6 were obtained in 0.1 M HCIO, for dosing potentials from -0.05 to -0.25 V, with @& 0.45 for potentials from ca. 0.02 to the onset of CO electrooxidation, ca. 0.2 V. The
3
0.10
a a33
ZeZ7
0.20
i"i-2 x 1 6 a. ~ u.
I
I
2100
190(
v / cm? Figure 2. As in Figure 1, but for adsorption at 0.05 V vs SCE. Spectra in (B) each involved acquiring 40 scans.
e&
latter lower values are unlikely to be due to incipient adsorbate electrooxidation since the coverage remained invariant with time following removal of CO from solution. A second marked difference between the properties of CO adlayers formed at -0.25 and 0.05 V is to be found in the Bcodependent spectral behavior. While the large vko-Oco dependencies seen for dosed adlayers at -0.25 V (Figure 1 B) are also seen at 0.05 V (Figure 2B), the behavior observed under "stripping" conditions is entirely different. In contrast to adlayers formed at -0.25 V (Figure lA), large (up to ca.50 cm-I) decreases in vk0 are observed as Oco decreases for adlayers prepared at 0.05 V (Figure 2A). In this regard, then, the corresponding Oco-dependent spectra under electrooxidative stripping and dosing conditions in Figure 2A,B, respectively, are unusually similar. Closer inspection of Figure 2A, however, shows that a shoulder at higher frequencies to the main vco peak is retained as Bco is decreased by electrooxidative stripping at 0.05 V; this feature is absent in the corresponding vm-Bco data obtained by direct dosing in Figure 2B. These vko-Oco data are displayed in graphical form in Figure 3. The circles and triangles refer to adlayers formed in 0.1 M HCIO, at -0.25 and 0.05 V, respectively; the filled and open symbols represent coverages produced by electrooxidative stripping and direct dosing, respectively. An attempt was also made to evaluate the component of the observed Oco-induced vbo upshifts associated with dynamic dipole-dipole coupling, AvD. This entails diluting the I2CO adlayer with I3CO, so to attenuate the AvD component, by dosing with various 12CO/13C0 mixtures (cf. ref 3a). Unfortunately, however, the dipole coupling is sufficiently strong (AvD is large) so to yield only a single vco band for the 12CO/13C0mixtures, vitiating the quantitative evaluation of AvD. Also of interest here is the effect of altering the electrode potential upon the uc0 spectra at a given adsorbate coverage. Data from this type of experiment are illustrated in Figure 4 for a pair of coverages formed by CO dosing at -0.25 V. The two spectral columns contain sequences of potential-dependent spectra, each commencing at -0.25 V, referring to coverages of 0.55 (A) and 0.24 (B). These spectra were obtained by sweeping the potential at 2 mV s-' from -0.25 V; each spectrum involved acquiring 30 interferometer scans (consuming ca. 20 s), subtracted from which was a similar set of scans acquired after complete CO electro-
7456 The Journal of Physical Chemistry, Vol. 95, No. 19, 1991
Jiang et al.
0.30 0.24
0.16
1
0.4
0.2
0
0.6
8,
further details.
12200
I
v / m1
2600
I 2100 ,
'4./6% 0.07
Figure 3. Peak frequencies, vbo, for the terminal C-O stretching band versus the CO coverage, Om, on ordered Ir( 1 11) in 0.1 M HClO, at -0.25 V (circles) and at 0.05 V (triangles), obtained under electrooxidative stripping (filled symbols, solid traces) and "dosing" conditions (open symbols, dashed traces). See caption to Figure 1 and text for
I
v / CnT'
I
1900
Figure 4. Squences of infrared absorbance spcctra in terminal C-O stretching region for irreversibly adsorbed CO on Ir(ll1) in 0.1 M
HCIO,, obtained during positive-going potential sweep at 2 mV s-I from Each spectrum involved acquiring 30 interferometer scans (consuming ca. 20 s), subtracted from which was a similar set of scans acquired after complete CO electrooxidation. The CO coverages, 0.55 (A) and 0.24 (B), were obtained by dosing at -0.25 V with ca. 2 X lW5 M solution CO. Potentials indicated alongside each spectrum are average values during the spectral acquisition.
-0.25 V.
oxidation, at 0.52 V. The potentials indicated beside each spectrum are the average values. These "single potential alteration infrared" (SPAIR)spectragbshow vm band frequencies that shift with increasing potential to an extent that depends upon Oco; the
I
I
I
-0.20
I
I
I
0
I
1
L
0.20
E/V vs SCE
Figure 5. Peak frequency of the terminal C-O stretching band, dm, on ordered Ir( 11 1) in 0.1 M HCIO, vs the electrode potential, E, for various fixed CO coverages, Oca, as indicated, formed initially at -0.25 V by solution CO dosing (as in Figure 4). Dashed trace segments refer to potentials after the onset of CO electrooxidation.
data in Figure 4A,B yield duko/dE slopes of 21 and 43 cm-I V-I, respectively. Figure 5 shows a sequence of ua-E plots extracted from spectra such as in Figure 4 for a more comprehensive series of coverages (as indicated) formed by CO dosing at -0.25 V. For each coverage examined, an essentially linear dependence of the vco frequency upon the electrode potential E is obtained at potentials prior to the onset of CO electrooxidation (=0.2 V). The duko/dE slope (often termed the "Stark tuning rate") increases from ca.20 cm-' V-] at saturation coverage (OCO = 0.6) to ca. 50 cm-I V-l at low values (eco Ls 0.1). While a comparable Om-dependent trend has been seen for ordered low-index platinum and rhodium surfaces? the absolute values of the Stark tuning rate at high coverages are noticeably smaller on Ir(ll1) than those obtained under similar conditions on Pt(ll1) and Rh( 11 l).s Related SPAIRS measurements undertaken for CO adsorbed within the double-layer region revealed an irreversible potential-induced change in the adlayer structure. Figure 6 shows a typical SPAIR spectral sequence for a near-saturated CO coverage, 0.38, formed a t 0.05 V. These spectra were obtained similarly to those in Figure 4, except that the potential sweep was initially negative-going from 0.05 V, being reversed after the potential reached 4 - 2 5 V. Inspection of Figure 6 shows that when the potential changed from 0.05 to -0.25 V, in addition to the expected vko downshift the vco band intensity increased by ca. 35% while the fwhm decreased from 26 to 22 cm-I. (Note that 8co remains unchanged throughout the potential excursion, prior to the onset of CO electrooxidation at ca. 0.2 V). A group of uco-E plots extracted from spectra such as in Figure 6, for three different coverages (as indicated) formed by CO dosing at 0.05 V, are shown in Figure 7. In each case, the uko values at each potential during the subsequent positive-going potential sweep are significantly (3-5 cm-I) below those obtained for the initial negative-going scan. The former um values are closer to, although still 5-10 cm-' above, those obtained for a given coverage following CO dosing in the hydrogen region. This finding therefore indicates that shifting the potential into the hydrogen region creates irreversible structural changes in the adlayer structure formed in the
CO Adsorbed at Ir( 1 1 1)-Aqueous Interfaces
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 1451
,,.../,
!,. .. i
-0.2
,
,
:,,..
:.....:::
:.,
,
I
0
0.2
E/V
M
0.4 SCE
0.6
-0.2
0 E/V
0.2 M
0.4
0.6
SCE
Figure 8. Anodic-cathodic cyclic voltammograms at 50 mV s-l for electrooxidationof irreversibly adsorbed CO on Ir( 11 1) in 0.1 M HCIO, for adlayers formed under (A) 'electrooxidative stripping" and (B) "direct dosing" conditions. Dotted traces in both (A) and (B) are cor-
responding voltammograms obtained following complete electrooxidative removal of adsorbed CO. Key to CO coverages, determined from the faradaic charge under the anodic voltammetric wave: (A) a, 19 = 0.65; b, 0.51; c, 0.42; d, 0.30. (B) a, 0.65; b, 0.50; c, 0.42; d, 0.28.
I , , , , , , V/Cili' Figure 6. Similar to Figure 4, but for CO dosing at 0.05 V,so that 6,
= 0.38. Also, the 2 mV s-' potential sweep was initially negative-going, the sweep direction being reversed at -0.25 V.
-0.20
0 E/V
V I
0.20 SCE
Flpre 7. Similar to Figure 5, but for irreversibly adsorbed CO formed at 0.05 V by solution CO dosing to the three coverages indicated. Also, the potential was swept initially negative at 2 mV s-' to -0.25 V before
reversing. double-layer region so to resemble more closely the structures created by initial dosing within the former potential region. While most infrared spectral data were obtained in 0.1 M HCIO,, some measurements were undertaken in 0.5 M H2S04 to check the possible presence of specific anion effects. Broadly speaking, virtually identical yc0 spectra were dbtained in these two electrolytes, as a function of both electrode potential and adsorbate coverage. One significant behavioral difference between 0.1 M HC104 and 0.5 M H2S04,however, is that even smaller
saturated C O coverages, P & H 0.3, were obtained within the double-layer region in the latter electrolyte. Some spectral measurements were also undertaken in alkaline electrolytes, such as 0.1 M NaC10, 1 mM KOH, so to expand the negative range of electrode potentials over which CO adsorption could be examined prior to the onset of cathodic hydrogen evolution (cf. ref 5). In contrast to Pt( 11 1) and especially Rh( 111),5 exclusively atop C O coordination persisted under these conditions, even to ca. -0.8 V vs SCE,as discerned from the observation of only a terminal uco band downshifted in frequency from those exhibited in Figure 5 by an extent concordant with the decreases in electrode potential. Given the observed marked effects of the electrochemical double layer and formation conditions upon the CO adlayer structure as discerned from infrared spectroscopy, it is of interest to examine how such differences influence the voltammetric oxidation of the adlayers. As in earlier studies,'*'b it is instructive to examine the voltammograms for various CO coverages formed by prior partial electrooxidative stripping versus direct dosing. Figure 8A contains some representativevoltammograms at 50 mV E' in 0.1 M HClO, for a series of initial 6, values (as indicated in the figure caption) prepared by partial stripping from a saturated adlayer at -0.25 V, and Figure 8B displays corresponding data for adlayers formed by direct dosing. The dotted trace in both figures is the anodic-cathodic voltammogram obtained in the absence of adsorbed CO. The Bco-dependent morphology of the voltammograms in Figure 8A,B shows some similarities in that the onset of CO electrooxidation shifts to lower overpotentials as Bco decreases in the same fashion for adlayers formed under stripping as well as dosing conditions. Nonetheless, the "stripped" adlayers (Figure 8A) tend to retain a voltammetric wave component at higher overpotentials to a greater extent than the adlayers having the same initial Bco but formed by direct CO dosing (Figure 8B). Also of interest here is the comparison between the voltammetric behavior of adlayers formed in the hydrogen region, such as in Figure 8, and those prepared in the double-layer region. Some pertinent data of this type are shown in Figure 9A, again using 50 mV s-l potential sweeps for irreversibly adsorbed CO on Ir( 11 1) in 0.1 M HCIO,. The solid and dashed-dotted traces refer to CO adlayers formed at -0.25 and 0.05V, respectively. Besides the larger faradaic charge contained under the former wave, reflecting a higher value, the voltammetric wave shapes and oxidation potentials for these two adlayers are clearly different. The dashed curve refers to a saturated adlayer formed at 0.05 V (as for the dashed-dotted trace), but swept first into the hydrogen region to -0.25V before reversing the potential scan in the positive direction.
+
7458 The Journal of Physical Chemistry, Vol. 95, No. 19, 199'1
A
1
T
-0.2
0
0.2
E/V
vs SCE
0.4
0.6
-0.2
0 E/V
0.2
0.4
1
0.6
vs SCE
Figure 9. Cyclic voltammograms at 50 mV s-I for elcctrooxidation of irreversibly adsorbed CO on Ir( 1 1 1) in (A) 0.1 M HCIO, and (B) 0.5
M H$O4. Solid traces: CO adlayers formed in the hydrogen potential region (-0.25 V). Dashed-dotted traces: CO adlayers formed in the double-layer region (0.05 V in (A) and 0.08 V in (B)), with potential sweep in the positive direction. Dashed tram: CO adlayers formed in the double-layer region, potential sweep initially in the negative direction. Dotted traces: typical anodic-cathodic voltammograms obtained in the absence of CO. The CO saturation coverage, derived from the faradaic charge under voltammetric wave, is 0.65 for CO adlayers formed in the hydrogen region and 0.50 and 0.30 for CO adlayen formed in the double-layer region in 0.1 M HCIO, and 0.5 M H2S04,respectively. As for the um spectra presented above, this potential excursion into the hydrogen region has a substantial influence on the form of the ensuing voltammetric oxidation wave. Interestingly, comparison of the dashed trace in Figure 9A with voltammograms for adlayers having comparable coverages, but formed initially in the hydrogen region (e.g., curves b and c in Figure 8A), reveals marked similarities. It therefore appears that the irreversible structural changes wrought upon CO adlayers on Ir( 111) by shifting the potential from the double layer to the hydrogen region, clearly evident from the infrared spectra, also have a counterpart in the corresponding voltammetry. Similar voltammetric behavior to that observed in 0.1 M HCIO,, as in Figure 9A, is also seen in 0.5 M H2S04,as displayed in Figure 9B. The voltammogram observed in 0.5 M H2S04 alone (dotted curve in Figure 9B), most characteristically in the hydrogen region, is closely similar to that reported in ref 1 1. One more feature of the voltammograms shown in Figure 9 is worthy of comment here. Sweeping the potential into the hydrogen region for saturated adlayers formed at 0.05 V yields significant hydrogen adsorption, as discerned from the voltammetric hydrogen peaks (dashed traces, Figure 9A.B). This finding indicates that the irreversible structural transformation of the adlayer occurring under these conditions is accompanied by hydrogen coadsorption, as might be expected given the higher @, values obtained within the hydrogen region since ec0 remains unchanged during the potential excursion.
Discussion The present results exhibit several noteworthy features that differ significantly from those observed previously for CO adsorption on Pt( 11l)"s5 and Rh( 11 l)".*5 electrodes. First, the Ir( 1 1 1) surface binds CO exclusively in atop sites throughout the range of coverages and electrode potentials (even down to -0.8 V vs SCE) that are accessed here. By contrast, Rh( 1 1 1) exhibits predominantly bridging CO coordination at such low potentials,w and multifold as well as atop CO binding is prevalent on Pt(1 1 As noted above, the occurrence of exclusively atop CO coordination on I r ( l 1 1 ) in uhv has been observed by means of EELS,in both the absence and presence of predosed hydrogen.s Earlier LEED data for CO dosed on clean Ir( I 1 I ) show the presence of two distinct diffraction patterns, having the symmetries I),3a75
(1 1) Motoo, S.;Furuya, N. J. Elecrroanal. Chem. 1986, 197, 209.
Jiang et al. and (2d3X2d3)R3Oo (13, = 0.6).' ( d 3 x d 3 ) R 3 0 ° (0, = These data have been interpreted in terms of CO adlayer structures containing predominantly threefold hollow coordination. Such deductions appear to be incorrect in light of the vibrational spectroscopic data obtained subsequently? At least the lower coverage, Bc0 = LEED pattern is clearly consistent with CO coordination purely in atop as well as in multifold sites. Assignment of the higher coverage (eco N 0.6) structure is less straightforward. Since the effective diameter of upright CO is significantly larger than the atomic diameter of iridium (2.72 A) as well as other Pt-group transition metals, CO is unlikely to pack purely in symmetric atop sites at higher coverages, ec0 k Nevertheless, CO can still bind in asymmetric ('off-center") atop sites with relatively little cost to the metal-CO binding energy (and with only a minor effect on the uco frequency),13J4 yet achieving a substantial diminution in the repulsive CO-CO interactions. [An example of a structure containing such asymmetric atop CO, for Rh( 11 1)/CO(0CO=0.75), has been demonstrated in both uhvI2and electrochemical envir~nments.'~]Indeed, recent Monte Carlo simulations for (1 11) surfaces suggest that CO adlayer structures containing exclusively near-atop coordination can be formed at high coverages (dCo N 0.5) if the atop binding energy is favored relative to those for alternative bridging geometries by a t least 2.4 kcal m01-l.I~ Such a strong preference for atop binding is indeed predicted on the basis of molecular orbital (MO) theory for Ir( 11 1) throughout the range of surface potentials accessed in the uhv and the present electrochemical environment.I6J7 These MO calculations also account for the smaller propensity for atop versus bridging coordination observed for Pt(ll1) and especially Rh(111).I6 While ordered C O adlayer structures may not be formed on Ir( 11 1) in the electrochemical environment as occur in uhv, the attainment of coverages as high as ca. 0.6 as are observed here, featuring exclusively near-atop CO, is therefore not surprising on the basis of our present understanding of surface bonding. Presumably, appropriate shifts in the CO binding site from symmetric to asymmetric atop positions toward high coverages can yield an acceptable degree of repulsive CO-CO interactions. The close proximity of adjacent CO's in the high-coverage adlayer structures is indeed indicated from the unusually large (up to 100 cm-l) uk0 upshifts observed with increasing dosed Oc0 (Figures 1-3). This is especially the case for adsorption within the hydrogen region, where the highest Oc0 values (ca. 0.6) are obtained. It is conceivable that the preponderance of atop C O binding may arise in part from a disordered state of the present Ir( 11 1) surface. However, preliminary atomic resolution scanning tunneling microscopy measurements of the Ir(1 l l ) crystal in air following the flame annealing/iodine pretreatment noted above display images indicating the presence of a well-ordered iodine adlayer. Left unexplained, however, are the marked differences in the CO adlayer structures on Ir( 11 1) formed by dosing in the double-layer and hydrogen potential regions. Although some differences in the saturation coverages and potential-dependent uLo values have been noted for adlayers formed under these two conditions on Pt( 11 l),2a,3athe present results are much more notable in this regard. One can speculate that the significantly (2540%) lower values obtained by dosing within the double-layer region result from the need to accommodate coadsorbed water in the CO adlayer structure. The concomitant uptake of coadsorbed hydrogen seen by shifting the potential positive of ca. (12) For example, see: Van Hove, M.A.; Koestner, K. J.; Frost, J. C.; Somorjai, G.A. Surf. Sci. 1983, 129, 482. (13) Schweizer, E.;Persson, B. N. J.; Tiishaus, M.; Hoge, D.; Bradshaw, A. M. Surf.Sci. 1989, 213, 49. (14) Persson, B. N. J.; Tllshaus, M.; Bradshaw, A. M. J . Chem. Phys. 1990, 92, 5034. (IS) Yau, S.-L.;Gao, X.;Chang. S.-C.; Schardt, 6 . C.; Weaver, M.J. J . Am. Chem. Soc. 1991, 113, 6049. (16) Chang. S.-C.; Weaver, M.J.; Anderson, A. B. Unpublished results. (17) These calculations utilize the *atom superposition and electron delocalization molecular orbital" (ASED-MO) method developed by Anderson." (18) See,for example: Anderson, A. B. J . Elecrroand. Chem. 1990, 280, 31.
CO Adsorbed at Ir( 1 1 1)-Aqueous Interfaces
The Journal of Physical Chemistry, Vol. 95, No. 19, 1991 1459
A qualitatively similar, yet more dramatic, effect of CO island 0 V in the presence of this saturated layer, as discerned from dissipation is evident during electrooxidative removal of adlayers Figure 9A, also provides evidence of the “openness” of the CO formed within the double-layer region (Figure 2A). The observed adlayer structure formed within the double-layer region. similarity between the Oa-dependent vko.values obtained for such The significant alterations in the spectral characteristics of this adlayers under stripping and dosing conditions (Figures 2A,B and dosed adlayer that result from such potential alterations into the 3) indicates that CO islands, if formed, dissipate rapidly during hydrogen region, specifically the unexpected vCofrequency deelectrooxidation. This unusual behavior may well be connected creases along with increases in band intensity (Figures 6 and 7), with the “openn adlayer structure as deduced above. The greater show clearly that marked structural changes are wrought in this stability of CO islands within adlayers formed in the hydrogen manner even at fixed Oco (0.45). A possible explanation for these region is also reasonable on this basis, given the initially more spectral changes is that the CO adsorbate resides in more asymmetric atop sites, and therefore is more tilted, in the presence of densely packed CO structure. The formation of CO islands during electrooxidative adlayer coadsorbed water rather than hydrogen. In this less vertical adsorbate configuration, the band absorbance will be diminished removal is also anticipated to be manifested in the voltammetric on the basis of the usual infrared surface selection rules, and the For CO adlayers at intermediate initial coverages vco frequency might be expected to increase since the extent of formed by partial electrooxidative stripping on low-index Pt dr27r’ back-donation should be reduced. Both these qualitative surfaces, the voltammetric wave shape and oxidation potentials predictions are in accordance with the observations. change little with Oco, suggesting that the kinetics are limited by Although the detailed factors responsible for these varying reaction at the edges of large CO is lend^.^ For adlayers at intermediate initial coverages formed by direct CO dosing, however, double-layer effects upon the CO adlayer structure are far from understood, it is nevertheless of interest to examine more closely the voltammetric waves broaden and shift to lower overpotentials corresponding differences in the CO properties during electroas Oco decreases. The latter result reflects an increasingly facile reaction between CO and the water (or OH) oxidant as the CO oxidation. An interesting feature of the present results in this regard concerns the Oco-dependent uco frequencies and band adlayer becomes less densely p a ~ k e d .Inspection ~ of Figure 8A,B reveals that the latter type of behavior is observed on Ir( 1 1 1) for shapes under “electrooxidative stripping” conditions. In some respects, the Oco-dependent spectra for adlayers formed at 4 - 2 5 adlayers formed by prior partial electrooxidativestripping (Figure V (Figure 1A) are reminiscent of those reported previously for 8A) as well as by direct CO dosing (Figure 8B). This finding low-index platinum and rhodium surface^.^*^*^^ Thus, the peak provides further evidence of the lower stability of CO islands on frequency, vko, undergoes only minor (ca. 10 cm-I) decreases with Ir( 11 l), even for adlayers formed within the hydrogen region, diminishing Oco under these circumstances (Figures 1A and 3), compared with adlayers on low-index Pt and Rh surfaces. Overall, then, CO adlayers on Ir( 111) display unusual structural contrasting the very marked (ca. 100 cm-I) Bco-induced vk0 decreases observed under “direct dosing” conditions (Figures 1B and and electrochemical features in comparison with those on Pt and Rh surfaces, including a surprising sensitivity to the conditions 3). These differences have been ascribed to the formation of extensive CO islands during electrooxidation, so that high local of their formation. It is tempting to attribute such “double-layCO coverages are maintained even at low uoeruge Oco v a l ~ e s . ~ ? ~ er-sensitive” ~ behavior generally to the observed overwhelming The substantial frequency upshifts observed as a result of such preference for atop rather than multisite CO coordination on dense CO adlayer packing have been traced primarily to the effects Ir( 11 l), in that this circumstance may well restrict severely the of dynamic dipole-dipole coupling on the basis of experiments range and flexibility of the possible adlayer structures and the with 12CO/’3C0mixture^.^+^^ manner in which they are formed. Further interpretation, however, The formation of adsorbate islands during CO electrooxidation will require additional microscopic-level information. One inon Ir( 11 1) implicates a mechanism whereby reaction occurs at triguing possibility is in situ scanning tunneling microscopy, esthe edges of close-packed CO domains, since then an adjacent pecially given the recent demonstration that atomic-resolution CO adsorption site is available for the water and/or hydroxyl coadlayer structures can be obtained for Pt and Rh surface^.'^.'^ reactant.3AbFurther scrutiny of the present results, however, shows Such experiments, along with an examination of Ir (100) and (1 10) that significant dissipation of these islands occurs during elecsurfaces, will be underway shortly. trooxidative removal. This is evidenced by the substantial “tail” Acknowledgment. The work is supported by the National toward lower frequencies that appears at lower coverages in Figure Science Foundation. 1A, indicating that partial adsorbate removal yields regions of less densely packed CO, comparable to those formed under direct dosing conditions (Figure 1 B). (19) Yau, S.-L. Ph.D. Thesis, Purdue University, 1990.
