Langmuir 1998, 14, 2525-2534
2525
Electrochemical Infrared Studies of Monocrystalline Iridium Surfaces. Part 2: Carbon Monoxide and Nitric Oxide Adsorption on Ir(110) Roberto Go´mez† and Michael J. Weaver* Department of Chemistry, Purdue University, West Lafayette, Indiana 47907 Received October 28, 1997. In Final Form: February 18, 1998 The adsorption of carbon monoxide and nitric oxide on an ordered Ir(110) electrode surface in aqueous 0.1 M HClO4 has been probed by voltammetry together with in-situ infrared reflection-absorption spectroscopy (IRAS). Exclusively atop coordination of both CO and NO is suggested from the relatively high C-O and N-O stretching (νCO, νNO) frequencies observed, 1980-2060 and 1820-1840 cm-1, respectively, that upshift with increasing coverage. Adsorption of NO as well as CO is essentially molecular, with near-unity saturation coverages, as deduced from voltammetry as well as infrared spectrophotometry. The potential-dependent νCO frequencies for the saturated CO adlayer are closely compatible with that for the corresponding Ir(110)/CO interface in ultrahigh vacuum (UHV) once the differences in surface potential are taken into account. In contrast to the case of the latter system, however, the electrochemical Ir(110)/CO interface exhibits a pair of νCO bands at intermediate CO coverages (θCO), suggestive of a difference in substrate-induced adlayer domains in the two environments. Closely similar θCO-dependent νCO spectra and voltammetric oxidation profiles were obtained for adlayers formed by either partial electroxidative stripping from a saturated adlayer or by direct dosing from a dilute CO solution. This unusual behavior indicates that extensive CO “islands” are not formed by partial adlayer electrooxidation, in contrast to the behavior of most ordered low-index Pt-group electrodes, suggesting that the substrate morphology features nanoscale domains rather than large terraces. The νCO and νNO frequencies for saturated adlayers on Ir(110) and (111) are similarly red-shifted from the gas-phase νCO and νNO values. However, the νCO-E and especially the νNO-E dependences (“Stark-tuning” slopes) are markedly larger than the predicted gas-phase values. The larger dνNO/dE values are ascribed to more extensive potentialdependent dπ-2π* back-donation for adsorbed atop NO compared with CO.
Introduction As is well documented,1-3 there has been a recent rapid growth in the monocrystalline atomic- and molecularstructural characterization of metal-solution interfaces, fueled by the emergence of in-situ microscopic-level techniques as well as the availability of reliable procedures for preparing ordered surfaces without the need for ultrahigh vacuum (UHV) methodology. Infrared reflection-absorption spectroscopy (IRAS) has provided vibrational information for a number of molecular adsorbates at such ordered electrochemical interfaces.3,4 The adsorption of carbon monoxide and nitric oxide is of particular interest, not only in view of the sensitivity of their intramolecular vibrational features to the interfacial environment but also because of the rich opportunities for comparison of the surface-bonding properties between analogous electrochemical and metal-UHV systems. The former adsorbate has received the most attention in this regard, although the Alacant group has spearheaded electrochemical IRAS studies of the latter adsorbate on Pt-group transition metals.5 † Permanent Address: Departament de Quı´mica Fı´sica, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain.
(1) For example, see chapters in: (a) Structure of Electrified Interfaces; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1993. (b) Comprehensive Chemical Kinetics; Compton, R. G., Hamnett, A., Eds.; Elsevier: Amsterdam, 1989; Vol. 29. (2) (a) Weaver, M. J.; Gao, X. Annu. Rev. Phys. Chem. 1993, 44, 459. (b) Weaver, M. J. J. Phys. Chem. 1996, 100, 13079. (3) For broad-based reviews of electrochemical IRAS, see: (a) Nichols, R. J. In Adsorption of Molecules at Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1992; Chapter 7. (b) Iwasita, T.; Nart, F. C. In Advances in Electrochemical Science and Engineering; Gerischer, H., Tobias, C. W., Eds.; VCH Publishers: New York, 1995; Vol. 4, p 123. (4) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5391.
We have recently undertaken a systematic examination of the adsorption and catalytic properties of low-index iridium electrodes, utilizing primarily electrochemical IRAS tactics.6 While iridium, as a Pt-group transition metal, is of substantial fundamental significance, the adsorption of CO and NO on ordered Ir surfaces remains relatively unexplored in UHV as well as electrochemical environments. Nevertheless, recent vibrational spectroscopic studies for CO adsorption on Ir(111),7,8 Ir(100),9 and Ir(110)8a,10 indicate the presence of strongly coveragedependent C-O stretching (νCO) frequencies which are consistent, unusually, with near-exclusive terminal (i.e., atop) metal-CO bonding. While vibrational spectral studies of NO are even sparser, being limited so far to Ir(111)11 and Ir(100),12 some propensity for atop surface binding is also evident. (5) (a) Rodes, A.; Go´mez, R.; Perez, J. M.; Feliu, J. M.; Aldaz, A. Electrochim. Acta 1996, 41, 729. (b) Rodes, A.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Pe´rez, J. M.; Aldaz. A. Langmuir 1995, 1, 3549. (c) Go´mez, R.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M. J. Electroanal. Chem. 1995, 393, 123. (d) Go´mez, R.; Rodes, A.; Orts, J. M.; Feliu, J. M.; Pe´rez, J. M. Surf. Sci. 1995, 342, L1104. (6) (a) Go´mez, R.; Weaver, M. J. J. Electroanal. Chem. 1997, 435, 205. (b) Go´mez, R.; Weaver, M. J. J. Phys. Chem., in press. (7) Jiang, X.; Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1991, 95, 7453. (8) (a) Marinova, Ts. S.; Chakarov. Surf. Sci. 1989, 217, 65. (b) Lauterbach, J.; Boyle, R. W.; Schick, M.; Mitchell, W. J.; Meng, B.; Weinberg, W. H. Surf. Sci. 1996, 350, 32. (9) (a) Kisters, G.; Chen, J. G.; Lehwald, S.; Ibach, H. Surf. Sci. 1991, 245, 65. (b) Martin, R.; Gardner, P.; Nalezinski, R.; Tu¨shaus, M.; Bradshaw, A. M. J. Electron Spectrosc. Related Phenom. 1993, 64/65, 619. (10) Lyons, K. J.; Xie, J.; Mitchell, W. J.; Weinberg, W. H. Surf. Sci. 1995, 325, 85. (11) Cornish, J. C. L.; Avery, N. R. Surf. Sci. 1990, 235, 209. (12) Gardner, P.; Martin, R.; Nalezinski, R.; Lamont, C. L. A.; Weaver, M. J.; Bradshaw, A. M. J. Chem. Soc., Faraday Trans. 1995, 91, 3575.
S0743-7463(97)01169-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/11/1998
2526 Langmuir, Vol. 14, No. 9, 1998
Go´ mez and Weaver
Described herein is a combined voltammetric and insitu IRAS study of CO and NO adsorption on Ir(110) in acidic aqueous media. The vibrational findings for the former adsorbate provide an interesting comparison both with corresponding results on Ir(110) in UHV10 and for related Pt-group electrochemical surfaces. The electrochemical properties of Ir(110)-aqueous interfaces have received previous attention.13-17 While the surface structural implications of the present vibrational spectral results are largely tentative, they nonetheless point to some unusual adsorptive properties of the Ir(110) electrochemical interface. Experimental Section The Ir(110) single crystal (9.5 mm in diameter, 2-mm thick) was purchased from the Material Preparation Facility at Cornell University. It was oriented within (1°, as verified by X-ray diffraction. The surface was repolished with diamond paste down to the grain size 1/4 µm. The final treatment was flame annealing as to order the “disturbed” layer formed during the polishing process. This was done by heating the electrode in an oxygen/ gas flame at 1500-1700 °C for 15 min. Further annealing did not alter the final voltammetric profile. Before each experiment, the Ir(110) crystal was flame-annealed again at 1500-1700 °C for 3-5 s and cooled in a fast-flowing stream of hydrogen and argon, followed by immersion in ultrapure water saturated with these gases (cf., ref 18).18 The surface was then transferred either to a conventional electrochemical setup or to the spectroelectrochemical cell protected by a drop of water. Taking advantage of the sensitivity of the voltammetric profile to the surface structure,13,14,16 we used cyclic voltammetry to assess empirically the surface state. The 0.1 M HClO4 electrolyte was prepared from concentrated perchloric acid (GFS Chemicals, Double Distilled) using ultrapure water (Millipore Milli Q) and degassed by bubbling argon for at least 10 min. Carbon monoxide-containing solutions were prepared by bubbling the pure gas (AIRCO, grade 2.3) through a previously deaerated solution. To obtain NO adlayers, we employed two different media: either acidic solutions of KNO2 (EM Science)5 or NO-containing solutions prepared from the pure gas (NO, AIRCO, grade 2.3). Details of the experimental IRAS measurements are largely as described previously.19,20 The infrared spectrometer was an IBM (Brucker) IR-98-4A Fourier transform instrument, with a Globar light source and a liquid-N2-cooled narrow-band MCT detector (Infrared Associates). The spectral resolution was (4 cm-1. The optical arrangement involved the use of an electrochemical thin layer formed by pushing the disk electrode up to the CaF2 window. The infrared beam was incident at about 60° to the surface normal, with the window beveled at the same angle. Potentials are quoted versus a saturated calomel electrode (SCE). All experiments were performed at room temperature, 23 ( 1 °C.
Results and Discussion Cyclic Voltammetry in Perchloric Acid Electrolyte. The dashed line in Figure 1 is a typical cyclic voltammogram obtained at 50 mV s-1 for Ir(110) in 0.1 M HClO4 under continuous cycling (“steady-state”) conditions. The sharp voltammetric features below 0 V are associated with hydrogen adsorption/desorption. The noticeable nonsymmetric appearance of these peaks is quite different from that observed on low-index Pt (13) Motoo, S.; Furuya, N. J. Electroanal. Chem. 1984, 167, 309. (14) Motoo, S.; Furuya, N. J. Electroanal. Chem. 1984, 181, 301. (15) Motoo, S.; Furuya, N. J. Electroanal. Chem. 1986, 197, 209. (16) Furuya, N.; Koide, S. Surf. Sci. 1990, 226, 221. (17) Hoshi, N.; Uchida, T.; Mizimura, T.; Hori, Y. J. Electroanal. Chem. 1995, 381, 261. (18) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245. (19) Chang, S.-C.; Weaver, M. J. J. Chem. Phys. 1990, 92, 4582. (20) Corrigan, D. S.; Weaver, M. J. J. Phys. Chem. 1986, 90, 5300.
Figure 1. Anodic-cathodic cyclic voltammograms obtained at 50 mV s-1 for clean Ir(110) in 0.1 M HClO4 (dashed trace) and after formation of an irreversibly adsorbed CO layer at -0.25 V (traces a-c).
electrodes21 yet reminiscent of the voltammetric behavior of ordered rhodium surfaces in perchloric acid,22-24 including Rh(110).25 The latter behavior has been identified by Wieckowski and co-workers as being due to the irreversible electroreduction of ClO4- to Cl-.22-24 Further evidence that this redox process is occurring also on Ir(110) is shown in Figure 2. The electrode was contacted with the 0.1 M HClO4 electrolyte at the negative potential limit, -0.27 V, followed by the initially positivegoing voltammogram shown by the solid trace. Significantly, the anodic removal of hydrogen is followed by a cathodic peak at -0.05 V also observed during this half cycle. This is consistent with the occurrence of perchlorate reduction once the adsorbed hydrogen is largely removed. The next voltammetric cycle (broken line, Figure 2) shows a smaller reduction peak, as expected since the continued reduction of perchlorate will be inhibited by the adsorption of the chloride product, which should form especially toward more positive potentials. The occurrence of perchlorate reduction also complicates the evaluation of the hydrogen adsorption/desorption charge. This value, 210 µC cm2 after correction for the double-layer contribution, probably includes the desorption/adsorption of chloride and possibly other species, such (21) (a) Clavilier, J. J. Electroanal. Chem. 1980, 107, 211. (b) Rodes, A.; Zamakhchari, M. A.; El Achi, K.; Clavilier, J. J. Electroanal. Chem. 1991, 305, 115. (c) Go´mez, R.; Clavilier, J. J. Electroanal. Chem. 1993, 354, 189. (22) Wasberg, M.; Hourani, M.; Wieckowski, A. J. Electroanal. Chem. 1990, 278, 425. (23) Rhee, C. K.; Wasberg, M.; Horanyi, G.; Wieckowski, A. J. Electrochem. Soc. 1992, 139, 147. (24) Rhee, C. K.; Wasberg, M.; Zelenay, P.; Wieckowski, A. Catal. Lett. 1991, 10, 149. (25) Go´mez, R. Tesi Doctoral, Universitat d’Alicant, Spain, 1994.
CO and NO Adsorption on Ir(110)
Figure 2. Cyclic voltammogram obtained at 50 mV s-1 for Ir(110) after immersion in 0.1 M HClO4 at -0.27 V (solid trace), followed by a second positive potential-going sweep (broken trace).
as OH-. The voltammetric profiles for the large Ir(110) surface examined here are virtually identical to that obtained for the smaller “bead” crystals in refs 14 and 17, which were prepared and pretreated by means of the Clavilier method.21a Voltammetric Oxidation of CO Adlayers. The solid trace ab shown in Figure 1 is a voltammetric profile at 50 mV s-1 corresponding largely to the oxidation of a saturated CO adlayer, formed at -0.25 V by sparging CO followed by argon purging. The oxidation peak b has a shape reminiscent of CO electrooxidation on Rh(110).25,28c The curve segment c following CO removal again exhibits asymmetric characteristics consistent with the involvement of perchlorate electroreduction. The observed total charge density obtained by integrating the current-potential profile throughout segment b (between 0 and 0.7 V) for CO electrooxidation, Qob, is 670 µC cm2. This charge density together with the atomic density of (unreconstructed) Ir(110), 9.6 × 1014 atom cm-2, yields an estimate of the fractional CO coverage, θCO ) 2.2, which is unreasonably high, especially in comparison with the value obtained by spectroelectrochemistry of 1.0 ((0.05) (vide infra). However, it is necessary to correct the total observed “anodic” charge for the various adsorbed Faradaic and non-Faradaic processes that are coupled to the adsorbed CO electrooxidation.26-29 Specifically, the desired faradaic charge density for CO electrooxidation, Qfar CO, can be extracted from Qob measured between a (26) Chang, S.-C.; Ho, Y.; Weaver, M. J. J. Electrochem. Soc. 1992, 139, 147. (27) Weaver, M. J.; Chang, S.-C.; Leung, L.-W. H.; Jiang, X.; Rubel, M.; Szklarczyk, M.; Wieckowski, A. J. Electroanal. Chem. 1992, 327, 247. (28) (a) Go´mez, R.; Rodes, A.; Perez, J. M.; Feliu, J. M.; Aldaz, A. Surf. Sci. 1995, 327, 202. (b) Go´mez, R.; Rodes, A.; Perez, J. M.; Feliu, J. M.; Aldaz, A. Surf. Sci. 1995, 344, 85. (c) Go´mez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci., in press. (29) (a) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1992, 330, 489. (b) Feliu, J. M.; Orts, J. M.; Go´mez, R.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1994, 372, 265.
Langmuir, Vol. 14, No. 9, 1998 2527
Figure 3. Cyclic voltammograms obtained at 50 mV s-1 for submonolayer CO adlayers on Ir(110) formed by (A) partial electrooxidative removal of a saturated adlayer and (B) dosing from a dilute (caa 2 × 10-5 M) CO solution.
suitable pair of potentials Ei and E+ by noting that28c
Qfar CO ) Qtot - Qdis - Qb
(1)
Here Qdis is the reverse of the “displacement” charge density measured at a given initial electrode potential Ei upon adsorbing the CO adlayer and the last term is the “background” charge density, Qb , consumed between Ei and the “anodic limit” potential E+ in the absence of the adsorbed CO. The Qdis value, 80 µC cm2, was obtained at 0 V by integrating the current flowing upon saturation CO adsorption by gentle CO sparging.29 The Qb value, 260 µC cm-2, obtained from the positive-going currentpotential trace in the absence of CO, constitutes an even larger correction to Qtot. The resulting QCOfar value, 330 µC cm-2, when combined with the (1×1)-Ir(110) packing density noted above, yields a corrected θCO value of 1.05. Significantly, this estimate is close to the aforementioned spectrophotometric value; the residual disparity may be due to an uncertainty in Qb arising from slight perchlorate reduction. Good agreement between the spectrophotometric and corrected Coulometric values of θCO for Ptgroup transition metals is generally obtained when the latter are evaluated by means of eq 1.28c Various voltammetric experiments were also undertaken to examine the electrooxidation of subsaturated CO adlayers on Ir(110). Following earlier work from this laboratory with other low-index Pt,19,30,31 Rh,31,32 and Ir surfaces,7 we examined the voltammetry of adlayers having various CO coverages formed by either (A) partial prior “electrooxidative stripping” of saturated irreversibly adsorbed CO or (B) slow dosing (up to a desired coverage) by using dilute (2 × 10-5 M) CO solutions. In contrast to the behavior of low-index Pt and Rh surfaces, both these procedures yielded relatively broad CO oxidation waves, as exemplified in Figure 3A and B, respectively. Thus, (30) (a) Chang, S.-C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 5095. (b) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 230, 222. (31) (a) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (b) Chang, S.-C.; Roth, J. D.; Weaver, M. J. Surf. Sci. 1991, 244, 113. (32) Chang, S.-C.; Weaver, M. J. J. Electroanal Chem. 1990, 285, 263.
2528 Langmuir, Vol. 14, No. 9, 1998
Figure 4. Infrared spectra in the C-O stretching (νCO) region for coverage-dependent CO adlayers on Ir(110) in 0.1 M HClO4 at -0.25 V formed by (A) partial electrooxidative removal of a saturated adlayer or (B) dosing from a dilute (ca. 2 × 10-5 M) CO solution. Fractional CO coverages (θCO) indicated beside each spectrum were obtained by spectrophotometry (see text for details).
the cyclic voltammogram labeled b in Figure 3A, obtained immediately following the partial adlayer electrooxidation seen in a, is broadened in a fashion similar to that for voltammogram b shown in Figure 3B. Surprisingly, the peak current for the oxidation of initially lower CO coverages is situated at 0.1-0.2 V higher potentials than that for the saturated adlayer. As detailed further below, this behavior indicates that CO adlayer electrooxidation on Ir(110) does not occur primarily via extensive CO island formation, in contrast to the situation on low-index Pt and Rh electrodes.30-32 Infrared Spectra of CO Adlayers. Of central interest here is the infrared spectral form in the CO stretching (νCO) region of CO adlayers on Ir(110) as a function of adsorbate coverage, θCO, and electrode potential, E. Typical sets of coverage-dependent νCO absorbance spectra obtained in 0.1 M HClO4 at -0.25 V following either progressive electrooxidative stripping or direct dosing from dilute (2 × 10-5 M) CO solutions are shown in Figure 4A and B, respectively. Each spectrum involved acquiring 100 interferograms at -0.25 V, subtracted from which was a similar set obtained at 0.47 V following complete CO electrooxidation, so to remove solvent and other spectral interferences in the usual fashion.19 The θCO values indicated beside each spectrum were obtained, as usual,19,27 from the intensity of the 2343-cm-1 band arising from the thin-layer CO2 product. The required calibration for this procedure was extracted from the corresponding CO2 band intensity for saturated adlayer electrooxidation on Pt(110) (for which θCO ≈ 1.0).27 Note again that the saturated θCO value observed on Ir(110), 1.0 ((0.05), is consistent with the above voltammetric estimate after
Go´ mez and Weaver
Figure 5. Electrode potential-dependent νCO spectra for three CO coverages, as indicated, formed by CO dosing on Ir(110) in 0.1 M HClO4.
the appropriate Faradaic and non-Faradaic corrections have been applied to the latter. The saturated CO adlayer displays a single νCO band near 2060 cm-1 (Figure 4), comparable to that obtained on Ir(111) in 0.1 M HClO4 and ascribed to atop CO coordination.7 The corresponding IRAS spectrum for saturated CO on Ir(110) in UHV shows a single νCO band at 2086 cm-1, again attributed to atop binding.10 Indeed, low-index Ir surfaces in general show an apparently strong propensity for atop coordination on this basis,7-10 although recent diffraction data for some metal-UHV systems emphasize the need for caution when assigning binding sites merely on the basis of vibrational frequencies.33 Examination of the spectra for varying coverages in Figure 4 reveals substantial θCO-induced changes under both (A) stripping and (B) dosing conditions, including the appearance of a pair of νCO bands at intermediate coverages. Before considering these νCO data further, however, it is useful to consider also the effect of the electrode potential. Figure 5 shows E-dependent νCO spectra for three different CO coverages formed by dilute CO-solution dosing, obtained for the progressively increasing potentials indicated. In each case, the νCO frequencies upshift linearly with increasing potential until above approximately 0.2 V, where slow adlayer electrooxidation commences on the spectral time scale (∼1 min). Significantly, the νCO-E band shapes are largely (33) (a) Asensio, M. C.; Woodruff, D. P.; Robinson, A. W.; Schindler, K.-M.; Gardner, P.; Ricken, D.; Bradshaw, A. M.; Conesa, J. C.; GonzalezElipe, A. R. Chem. Phys. Lett. 1992, 192, 259. (b) Aminpirooz, S.; Schmalz, A.; Becker, L.; Haase, J. Phys. Rev. B 1992, 45, 6337. (c) Mapledoram, L. D.; Wander, A.; King, D. A. Chem. Phys. Lett. 1993, 208, 409. (d) Schindler, K.-M.; Hofmann, Ph.; Weiss, K.-U.; Dippel, R.; Gardner, P.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P.; Davila, M. E.; Asensio, M. C.; Consesa, J. C.; Gonzalez-Elipe, A. R. J. Electron. Spectrosc. Related Phenom. 1993, 64/65, 75.
CO and NO Adsorption on Ir(110)
Figure 6. Plots of νCO frequency versus electrode potential for dosed CO adlayers on Ir(110) in 0.1 M HClO4 at the various coverages indicated. The pairs of points for the saturated adlayer (θCO ) 1.0) refer to the absence and presence of solution CO (open squares and filled circles, respectively).
independent of potential prior to the onset of CO electrooxidation. As seen in Figure 6, the νCO-E slopes increase from 23 cm-1 V-1 for θCO ≈ 1.0 to about 65 cm-1 V-1 at θCO ≈ 0.1. A similar progressive enlargement of such “Stark-tuning” slopes with decreasing CO coverage has been observed on other Pt-group transition metals.7,19,30-32,34 Interestingly, the pair of νCO bands seen at intermediate coverages display significantly different νCO-E slopes, the values for the lower-frequency component being about 2-fold larger than those for the higherfrequency partner (Figure 5). This behavior suggests that the former band arises from adlayer domains having lower local coverages than those of the latter component, thereby accounting for the observed combination of smaller νCO and larger dνCO/dE values (also see below). As discussed previously,30-32,34-36 the combination of such νCO-E data with differences in the surface potentials for the electrochemical interfaces and the corresponding metal-UHV systems enables the observed dissimilarities in νCO frequencies between these two environments arising from Stark tuning to be assessed. Any additional wavenumber disparities signal likely differences in adlayer structure or other effects. (Note that the Stark-tuning effect at a given θCO value is chiefly dependent only on the surface potential rather than the nature of the solvent or other specific environmental factors.34-36) The work function, Φ, for the CO-saturated Ir(110)-UHV interface is about 5.7 eV,37 and the “conversion constant”, Ek, between the electrode potential (vs SCE) and the work (34) Weaver, M. J. Appl. Surf. Sci. 1993, 67, 147. (35) (a) Chang, S.-C.; Jiang, X.; Roth, J. D.; Weaver, M. J. J. Phys. Chem. 1991, 95, 5378. (b) Jiang, X.; Weaver, M. J. Surf. Sci. 1992, 275, 237. (36) (a) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 101, 5842. (b) Villegas, I.; Weaver, M. J. J. Phys. Chem. B 1997, 101, 10166. (37) The Φ value for clean Ir(110) in UHV is about 5.5 eV,38 and saturation CO adsorption increases Φ by 0.2 eV.39
Langmuir, Vol. 14, No. 9, 1998 2529
function (i.e. vs vacuum) scales is around 4.9 ((0.2) V.40 Extrapolating the present νCO data for the saturated CO adlayer on Ir(110) (Figure 6) to the appropriate electrode potential, approximately 0.8 V versus SCE, yields a νCO value of 2080 cm-1. The proximity of this value to the measured νCO frequency for a saturated adlayer on Ir(110) in UHV, 2086 cm-1,10 suggests strongly that the CO surface binding within a saturated adlayer (θCO ) 1.0) in the electrochemical and UHV environments at 300 K is closely similar. Markedly different behavior between the electrochemical and UHV systems, however, is evident upon comparing the IRAS data at varying CO coverages. In the UHV case, only a single νCO band is obtained throughout the full θCO range examined from 0.007 to 1.0, although the νCO frequency upshifts markedly, from about 2000 to 2086 cm-1, over this coverage span.10 This behavior has been attributed to the presence of only atop (or similar) CO binding, with the θCO-induced frequency upshifts being due primarily to dipole-dipole coupling.10 (The presence of other binding geometries should lead to additional bands, at least at lower coverages where “intensity borrowing” dipole coupling effects, which can obscure the appearance of additional oscillator frequencies in spectra, should be less prevalent.) The simple θCO-dependent spectral behavior observed in UHV is perhaps surprising, since Ir(110) is now known to reconstruct to form not a missing-row (1 × 2) substrate pattern but rather a microfaceted (331) structure consisting of two-atom-wide (111) terraces.10,42 This substrate structure remains upon CO adsorption even at 300 K.10 Although a single, albeit broader, νCO band at around 1980-2000 cm-1 is also observed at low θCO values (ca. 0.1) at the Ir(110)-aqueous interface, as already mentioned a pair of νCO features are clearly observed upon CO dosing to form intermediate coverages (0.4 < θCO < 0.6) (Figure 4B). Consequently, while the νCO frequencies are roughly compatible with the corresponding UHV data (upon extrapolating the νCO-E data as outlined above), the intermediate-coverage spectral fingerprint is clearly different. The observation of multiple νCO bands is reminiscent of that observed for CO dosing at the Pt(110)-aqueous 0.1 M HClO4 interface; the latter was surmised to be due to the adsorbate-induced removal of (1 × 2) reconstruction.30b The occurrence of CO-induced changes in substrate structure so to yield distinct nanoscale (or larger) domains at intermediate coverages may also account for the present findings. However, the precise microscopic structure produced on Ir(110) by the flameannealing hydrogen-cooling procedure is not known at present, so that further speculation along these lines is unwarranted. Nonetheless, an intriguing as well as unusual feature of the present Ir(110)/CO system is that electrooxidative stripping yields largely similar θCO-dependent νCO behavior to that seen upon direct CO dosing, including the observation of a pair of νCO bands at intermediate coverages (Figure 4). These similarities are emphasized further in the plots of νCO frequencies versus θCO at -0.25 V for both the “CO dosing” and “electrooxidative stripping” conditions (38) Nieuwenhuys, B. E.; Meijer, D. T.; Sachtler, W. M. H. Surf. Sci. 1973, 40, 125. (39) Nieuwenhuys, B. E. Surf. Sci. 1981, 105, 505. (40) This value arises by selecting an “average” estimate of Ek, 4.6 V vs normal hydrogen electrode (NHE); note that Ek estimates on this reference scale vary from about 4.4 to 4.85 V.4,36,41 (41) Wagner, F. T. Chapter 9 in ref 1a. (42) (a) Koch, R.; Borbonous, M.; Haase, O.; Rieder, K. H. Phys. Rev. Lett. 1991, 67, 3416. (b) Avrin, W. F.; Merrill, R. P. Surf. Sci. 1992, 274, 231.
2530 Langmuir, Vol. 14, No. 9, 1998
Figure 7. Plots of νCO frequency versus coverage for CO adlayers on Ir(110) in 0.1 M HClO4 formed by either partial electrooxidative stripping or direct CO dosing to the desired θCO value, as indicated.
shown in Figure 7. This behavior contrasts markedly with the θCO-dependent νCO spectra seen for electrooxidative stripping conditions on low-index Pt and Rh electrodes, which exhibit νCO frequencies and relative band intensities that remain largely unchanged as θCO is decreased.19,30-32 The latter pattern is indicative of the formation of substantial CO “islands” during electrooxidative removal, a reaction occurring only at domain edges, so that the local environment of most chemisorbate molecules (and hence the collective CO vibrational properties) is largely unaffected even upon reaching relatively low average θCO values. However, there are several other ordered Pt-group electrodes where CO electrooxidative stripping initiated from saturated adlayers yields substantial νCO frequency decreases and changes in the infrared band profile, including Ir(111) in the “double-layer” region (i.e. above 0 V),7 Pt(100),30a,31b and Pd(110).43 In the latter two examples, the νCO spectral changes develop slowly (in approximately a few minutes) upon forming electrooxidatively the lower-coverage adlayers. These effects are likely due to adlayer island dissipation caused by the presence of limited-size (nanoscale) substrate terrace domains and in the Pd(110) case by the generation of a (1 × 2) missing-row reconstruction.43 While the lack of spatial structural information for the present Ir(110) substrate precludes detailed assessments, it is tempting to surmise that related nanoscale phenomena may occur in the present case. Noteworthy in this context are the closely similar voltammetric profiles observed for subsaturated CO adlayers formed under electrooxidative stripping and direct-dosing conditions (Figure 3). This behavior is entirely consistent with the correspondingly similar θCO-dependent νCO spectral fingerprints summarized in Figures 4 and 7. It is nonetheless interesting to also examine sequences of νCO spectra obtained during slow adlayer electrooxidation. An example is shown in Figure 8. Each spectrum was obtained while the potential was held for 30 s at each of the progressively more positive values indicated. Unlike (43) Zou, S.; Go´mez, R.; Weaver, M. J. Surf. Sci. 1998, 399, 270.
Go´ mez and Weaver
Figure 8. Potential-dependent νCO spectra obtained for an initially saturated CO adlayer on Ir(110) during slow adsorbate electrooxidation (see text).
the sequence shown in Figure 4A, which entailed returning to -0.26 V after each partial oxidation at 0.25-0.30 V, the high-frequency νCO band near 2060 cm-1 [which is diagnostic of locally close-packed CO on Ir(110)] is partly retained during adlayer removal, although a component lying toward progressively lower frequencies becomes increasingly evident (Figure 8). This pattern indicates that some local CO islands can be formed during electrooxidation, although large interfacial regions contain CO molecules at much lower local packing densities, possibly even randomly decorating the surface. Nonetheless, the occurrence of substantial CO island dissipation even during adlayer oxidation for the present system sets it apart from the behavior on Pt(100) and Pd(110), where the appearance of lower-frequency νCO spectral features occurs only slowly (over a few minutes or longer) following partial electrooxidative removal.30a,31b,43 Returning to comparisons between the present electrochemical Ir(110)/CO interface and the corresponding UHV system, another behavioral difference concerns the θCO-dependent integrated band intensities, Ai. In the UHV environment, Ai increases with coverage only to θCO ≈ 0.5, the band intensities then decreasing slightly up to saturation (θCO ≈ 1).10 This behavior can be understood, at least qualitatively, in terms of increasing dielectric screening by neighboring CO molecules toward higher θCO values, the effect yielding progressively lower effective infrared absorptivities and therefore offsetting the effect of increasing adsorbate density on the band intensity.44 The occurrence of coverage-induced adsorbate ordering may also contribute to the observed nonmonotonic AiθCO dependence at the Ir(110)-UHV interface. In the electrochemical Ir(110)/CO system, however, the Ai-θCO dependence was found to be approximately linear throughout the coverage range, even though the νCO band morphology is more complex than that for the UHV case. Similarly linear Ai-coverage dependences have also been observed for CO adsorption in other Pt-group electrochemical interfaces (e.g., ref 19). This behavior can be (44) (a) Hollins, P.; Pritchard, J. Prog. Surf. Sci. 1985, 19, 275. (b) Luo, J. S.; Tobin, R. G.; Lambert, D. K. Chem. Phys. Lett. 1993, 204, 445.
CO and NO Adsorption on Ir(110)
Langmuir, Vol. 14, No. 9, 1998 2531
HClO4. Cyclic voltammograms obtained within the region 0-0.6 V were essentially featureless, consistent with the presence of an NO monolayer in a fashion similar to that for CO. However, sweeping above 0.6 V yielded a slow electrooxidative removal, as shown in the anodic voltammetric segment at 1 mV s-1, given by trace a in Figure 9. Following this cycle, most of the charge (∼90%) associated with hydrogen adsorption-desorption on Ir(110) is recovered (segment b in Figure 9), showing that the NO adlayer electrooxidation was largely completed within segment a. The anodic charge density apparently associated with NO oxidation, as estimated by the area bounded by trace a and the ensuing negative-going sweep in Figure 9, is 410 µC cm-2. Assuming that the anodic process is (vide infra)5
NOad + 6H2O f NO3- + 4H3O+ + 3e-
Figure 9. Anodic voltammetric behavior for an irreversibly adsorbed NO adlayer on Ir(110) in 0.1 M HClO4: trace a, oxidation of NO adlayer, followed by return (negative potentialgoing) sweep at 1 mV s-1; trace b, ensuing cyclic voltammogram at 50 mV s-1 in hydrogen adsorption-desorption region.
ascribed to electronic dielectric screening by surrounding solvent as well as CO molecules, so that the effect occurs to a roughly comparable extent throughout the range of adsorbate coverages. Indeed, direct evidence favoring such an explanation has been obtained by “electrochemical modeling” experiments in UHV, where infrared band intensities for C-O and N-O stretching vibrations are seen to be attenuated upon the addition of solvent molecules, especially at lower CO and NO coverages.45 Such UHV-based measurements also show that the vibrational band widths increase upon interfacial solvation, especially at lower adsorbate coverages.46,47 The wider νCO bands seen here as well as in other electrochemical systems in comparison with the bandwidths observed at the solvent-free UHV interfaces can therefore be ascribed to inhomogeneous band broadening engendered by adsorbate solvation.45,46 Voltammetry of NO Adlayers. As in the case of CO, prior to describing IRAS results for NO adlayers on Ir(110), it is desirable to examine the voltammetric characteristics. As reported earlier for other ordered Pt-group electrodes, adsorbed NO is known to form spontaneously upon immersion in acidic nitrite as well as in NO-sparged solutions.5 We have shown this also to be the case on low-index iridium electrodes.6b An example of the anodic characterization of NO on Ir(110) is shown in Figure 9. The freshly pretreated Ir(110) surface was first immersed for ca. 30 s in 0.1 M HClO4 + 0.1 M NaNO2. After it was rinsed with ultrapure water, the surface was transferred to an electrochemical cell containing Ar-sparged 0.1 M (45) (a) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Langmuir 1995, 11, 2777. (b) Villegas, I.; Go´mez, R.; Weaver, M. J. J. Phys. Chem. 1995, 99, 14832. (46) (a) Kizhakevariam, N.; Jiang, X.; Weaver, M. J. J. Chem. Phys. 1994, 100, 6750. (b) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Surf. Sci. 1995, 336, 37. (c) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. J. Phys. Chem. 1995, 99, 7677. (47) Ibbotson, D. E.; Wittrig, T. S.; Weinberg, W. H. Surf. Sci. 1981, 110, 294.
(2)
i.e., a three-electron reaction, allows us to estimate the fractional NO coverage, θNO, in an analogous manner to that for adsorbed CO, as outlined above. This procedure yields a θNO value of 0.9, similar to that (1.0) found for saturated NO on Ir(110) at temperatures up to 300 K in UHV.47 The NO adlayer on Ir(110) can also be electroreduced upon sweeping (again at slow rates) to negative potentials, ca. 0 to -0.25 V. The reaction now probably involves five electrons (vide infra):5
NOad + 6H3O+ + 5e- f NH4+ + H2O
(3)
A complication in this case is that the voltammetric charge has to be corrected for several concurrent processes, especially hydrogen adsorption, upon removing the adsorbed NO. The charge density corrected along these lines (cf. eq 1) is about 690 µC cm-2, again yielding a θNO estimate close to 0.9. Adlayers formed by exposure of Ir(110) to NO-saturated solutions yielded slightly higher, although similar, coverages (θNO ≈ 1). Significantly, the concordance between the anodically and cathodically determined θNO estimates, as well as with the UHV-based value, gives us confidence in their approximate validity. Infrared Spectra of NO Adlayers. Given that NO adlayer electrooxidation commences only at high potentials, where some disordering of Ir(110) is expected from surface-oxide formation, most infrared spectra for adsorbed NO were obtained by employing a “reference” spectrum obtained after adlayer reduction. Figure 10 shows typical infrared absorbance spectra for NO on Ir(110) in 0.1 M HClO4 + 2 mM NaNO2, obtained by acquiring 50 interferometer scans at the set of decreasing electrode potentials indicated, followed by another 50 scans at -0.25 V, the latter being subtracted from the former so as to remove the solvent and other spectral interferences as before. The spectra show a single positive-going band which upshifts in frequency from about 1815 to 1835 cm-1 with increasing potential, the intensity decreasing below 0 V. This band is assigned to the N-O stretch (νNO) of adsorbed NO. Although there are no previous reports of NO vibrational spectra on Ir(110) in either electrochemical or UHV environments, the present νNO band can tentatively be attributed to atop-bonded NO. Similarly high νNO frequencies have been observed for NO adsorbed on hexagonally reconstructed Ir(100) and given the same assignment.12 No νNO bands below 1800 cm-1 were observed here on Ir(110), as would be anticipated for
2532 Langmuir, Vol. 14, No. 9, 1998
Go´ mez and Weaver
Figure 11. Coverage-dependent νNO spectra obtained at 0.34 V for irreversibly adsorbed NO on Ir(11) in 0.1 M HClO4, formed by solution NO dosing at 0.54 V (see text). Figure 10. Potential-dependent infrared specta for an NO layer formed in nitrite-containing solution, after transfer to 0.1 M HClO4. The positive-going component is the νNO band; the negative-going component arises from ammonium cations formed after stepping to -0.25 V to reduce the NO adlayer (see text).
multifold coordinated NO.48 (Although interferences from the water bending mode around 1600-1700 cm-1 can obscure the detection of adsorbate bands in this frequency range, the absence of νNO features at such lower frequencies was confirmed by performing some IRAS experiments in deuterated solvent.) The occurrence of NO adlayer reduction to NH4+ by -0.25 V, as proposed in eq 3, is also supported by the potential-difference spectra in Figure 10 in that a negative-going feature (i.e., present at the reference potential, -0.25 V) is clearly evident at 1450 cm-1. This band is readily assigned to a NH4+ bending vibration.49 Even though the presence of the relatively intense νNO band indicates that NO adsorption is primarily molecular in nature, the possibility should be considered that some NO dissociation to form adsorbed N and O fragments is also taking place. Indeed, our recent study of NO adsorption on polycrystalline iridium film electrodes by utilizing surface-enhanced Raman spectroscopy (SERS) along with IRAS indicated the presence of chemisorbed oxygen as well as molecularly adsorbed NO, the former identified from a low-frequency metal-oxygen SERS vibration.50 While we certainly cannot rule out the occurrence of some NO dissociation (vide infra), it is likely to be relatively minor under the present conditions. Evidence supporting this assertion includes not only the aforementioned selfconsistent estimates of saturated θNO values extracted from anodic and cathodic voltammetry but also estimates obtained from the observed presence of a well-defined νNO band at around 1780-1820 cm-1 even at low NO coverages, where extensive adsorbate dissociation is most likely. (48) Similarly to (if even more so than for) the case for adsorbed CO, one needs to bear in mind the demonstrated dangers of assigning binding sites for NO purely on the basis of the νCO frequency values.33 (49) Corrigan, D. S.; Weaver, M. J. J. Electroanal. Chem. 1988, 241, 143. (50) Zou, S.; Go´mez, R.; Weaver, M. J. Langmuir, 1997, 13, 6713.
Representative νNO spectra obtained at 0.34 V for irreversibly adsorbed NO (i.e., following exposure to NOcontaining solution and electrode transfer to 0.1 M HClO4) are shown in Figure 11. The lower NO coverages were accessed by dosing from dilute (3 × 10-5 M) NO solutions for controlled time periods (30 s to 5 min). A shift toward lower νNO frequencies is clearly evident at lower θNO values, probably arising from diminishing dipole-dipole coupling in a similar fashion to that for the CO adlayers. Note also that the νNO band broadens toward lower θNO values, again similar to the case for CO, probably due to solvation and variations in the local bonding environment. Examination of the νNO frequency-potential (νNO-E) dependence as a function of θNO was limited by the lack of quantitative information on the subsaturated NO coverages. Nevertheless, the νNO-E slopes were found to increase with decreasing adsorbate coverages in a fashion similar to that for adsorbed CO (Figure 6). The value for a saturated NO adlayer is 35 cm-1 V-1, increasing to about 60 cm-1 V-1 at low θNO values. Identical θNO-dependent spectra were obtained for irreversibly adsorbed adlayers found by partial electroreductive NO removal. (Note that the partial electrooxidative procedure is less desirable here, since the resulting surface will be disordered by oxide formation.) Potential-difference νNO infrared spectra were nevertheless obtained on Ir(110) in HNO2-containing electrolyte by using a high reference potential, ca. 1.15 V, so as to electrooxidize entirely the NO layer. Similar E-dependent νNO spectra were obtained as in Figure 10 in the approximate range 0-0.8 V. A negative-going band centered around 1380 cm-1 was also observed, consistent with the infrared bending vibration for nitrate,51 confirming the anodic reaction proposed in eq 2 above. Comparable νNO spectra were also obtained in NOsaturated 0.1 M HClO4 electrolyte. A representative potential-dependent set of IRAS data is shown in Figure 12, again employing a negative reference potential (-0.26 V) so as to electroreductively remove the NO adlayer. As expected, the negative-going NH4+ band is again evident (51) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 5th ed., Part B; Wiley: New York, 1997; p 87-9.
CO and NO Adsorption on Ir(110)
Langmuir, Vol. 14, No. 9, 1998 2533 Table 1. Vibrational Frequencies and Stark-Tuning Slopes for Adsorbed CO and NO on Low-Index Iridium Surfaces environment gas phase Ir(111), 0 V vs SCE Ir(111), UHV Ir(110) 0 V vs SCE Ir(110), UHV
νCO (cm-1)
νNO (cm-1)
dνCO/dE (cm-1 V-1)
dνNO/dE (cm-1 V-1)
2145a 2070c,f,g 1980c,f,g 2090e,h 2645e,h 2060c,j 1990c,j 2085e,k 2010e,k
1877a 1825c,f 1820c,f 1860e,i
12b 20d,f 50d,f
8b 50d,f 50d,f
1828c,j 1785c,j
23d,j 65d,j
35d,j 60d,j
a Observed vibrational frequency for gas-phase molecule, from tabulation in ref 57. b Calculated Stark-tuning slope for gas-phase molecule, extracted from frequency-electrostatic field slope (4.29 × 10-7 and 2.77 × 10-7 cm-1 V-1 for CO and NO, respectively58) by using eq 4, assuming that the molecular length, di, is 3.5 Å (see text).34 c Observed (or extrapolated) frequency for saturated and low-coverage (θ ∼ 0.1) CO or NO adlayer (upper and lower values, respectively) at iridium-aqueous interface at 0 V versus SCE. d Observed band frequency-potential slope for saturated and lowcoverage (θ ∼ 0.1) CO or NO adlayer at iridium-aqueous interface. e Observed frequency for saturated or low-coverage (θ ∼ 0.1) CO or NO adlayer at iridium-UHV interface at 300 K. f Go´mez, R. Unpublished results g Reference7. h Reference 8b. i Reference 11. j This work. k Reference 10.
Figure 12. Potential-dependent infrared spectra on Ir(110) in NO-saturated 0.1 M HClO4, obtained at the decreasing potentials indicated followed by NO reduction (to NH4+) at -0.26 V. The positive-going component at approximately 1820 cm-1 is the νNO vibration from the NO adlayer; the components at 2230 and 1285 cm-1 are due to solution N2O (see text). The negative-going component at 1450 cm-1 is due to the NH4+ reduction product.
(cf. Figure 10). The potential-dependent νNO behavior is largely as before, although the band frequencies and intensities are slightly higher than those in Figure 10, consistent with a higher θNO value (vide supra). The NO adlayer is also more resistant to electroreduction, as seen from the negative-going νNO band, which is indicative of the survival of some adsorbed NO even at the reference potential. A more interesting difference with the HNO2 electrolyte case, however, is the additional appearance of a strong sharp positive-going band at 2230 cm-1 along with a barely discernible feature at 1285 cm-1 (Figure 12). These bands are readily assigned to the N-N and N-O stretching vibrations of nitrous oxide (N2O).52 This molecule is unlikely to be adsorbed; N2O is desorbed entirely from Ir(111) by 120 K in UHV.11 Most likely, then, N2O is present in the thin-layer solution. The potential-difference IRAS data indicate the presence of N2O only above 0.1 V. The onset of N2O reduction at this potential on Ir(110) was confirmed by voltammetry of N2O-saturated 0.1 M HClO4 solutions. Interestingly, while N2O electroreduction proceeds apace at potentials below 0.1 and -0.05 V on Ir(110) and Ir(111), respectively, it is virtually electroinactive on Ir(100).53 The surface-structure sensitivity of N2O reduction on monocrystalline Pt-group electrodes has been noted previously and examined in some detail.54 Comparison of Adsorbed CO and NO Vibrational Properties. The above deduction that both adsorbed CO (52) Reference 51, Part A, p 166. (53) Go´mez, R. Unpublished results. (54) (a) Ebert, H.; Parsons, R.; Ritzoulis, G.; Vandernoot, T. J. Electroanal. Chem. 1989, 264, 181. (b) Ahmadi, A.; Bracey, E.; Evans, R. W.; Attard, G. A. J. Electroanal. Chem. 1993, 350, 297. (c) Attard, G. A.; Ahmadi, A. J. Electroanal Chem. 1995, 389, 175.
and NO adsorb molecularly at the ordered Ir(110)aqueous interface exclusively in an atop (or near-atop) configuration to yield closely similar (unity) saturation coverages offers an interesting opportunity to compare the vibrational properties of these related diatomic chemisorbates in essentially the same physical as well as chemical-bonding environments. Indeed, a similar situation applies to Ir(111)-aqueous interfaces in that coordination of both CO and NO again appears to be exclusively atop and associative at room temperature.7,55,56 These electrochemical IRAS results on Ir(111) are further corroborated by similar vibrational observations for CO8b and NO11 on Ir(111) in UHV at ambient temperatures, although IRAS data are only available for the former system.8b Such coordinative simplicity on the (111) as well as (110) faces appears to be unique to iridium. In view of this apparently favorable situation, then, we now consider briefly the electrode potential- (or electrostatic field-) dependent infrared properties of CO and NO adlayers on Ir(110) and (111) in relation to some expectations from simple vibrational and bonding models. A summary of some pertinent vibrational information for adsorbed CO and NO adlayers on Ir(111) and (110) in both electrochemical and UHV environments is given in Table 1. In the former case, the νCO and νNO frequencies evaluated at a fixed arbitrary potential (0 V vs SCE) are listed along with the corresponding average dνCO/dE and dνNO/dE values over the range of potentials where these adlayers are stable (vide supra). The upper and lower entries listed for each system in Table 1 refer to saturated (θ ) 1.0) and low-coverage (θ ∼ 0.1) adlayers, respectively. While the coverages of the saturated adlayers are most well defined, facilitating comparisons between the electrochemical and UHV systems (vide supra), the lowcoverage vibrational data are less influenced by adsorbateadsorbate interactions. Quantitative comparisons between the νNO band frequencies in the electrochemical and UHV environments are thwarted by a lack of work-function (55) Go´mez, R. Unpublished results. (56) The behavior of NO on unreconstructed Ir(100) in electrochemical as well as UHV environments12 appears to be complicated by adsorbate dissociation. An account of the former system is given elsewhere.6b.
2534 Langmuir, Vol. 14, No. 9, 1998
Go´ mez and Weaver
data in the latter case. However, saturation atop NO adsorption might be expected to yield similarly small ∆Φ values (say
