coadsorbate vibrational interactions within mixed carbon monoxide-nitric oxide adlayers on ordered...

Journal of Electroanalytical Chemistry 467 (1999) 92–104

Coadsorbate vibrational interactions within mixed carbonmonoxide-nitric oxide adlayers on ordered low-index

platinum-group electrodes�

Catherine Tang a,b, Shouzhong Zou a, Si-Chung Chang b, Michael J. Weaver a,*a Department of Chemistry, Purdue Uni6ersity, West Lafayette, IN 47907, USA

b Department of Chemistry, National Tsinghua Uni6ersity, Hsinchu 30043, Taiwan, ROC

Received 13 August 1998; received in revised form 20 October 1998; accepted 31 October 1998

Abstract

In-situ infrared reflection-absorption spectra are described for saturated mixed carbon monoxide-nitric oxide adlayers incomparison with CO and NO adsorbed separately on selected ordered Pt-group electrodes in aqueous solution in order to assessthe nature of the coadsorbate vibrational interactions and hence the adlayer structure and bonding. Both chemisorbates exhibitpronounced intramolecular vibrational fingerprints (nCO, nNO bands) that are sensitive to the local vibrational environment as wellas the surface bonding geometry, enabling a microscopic-level assessment of the coadsorbate interactions in relation to CO andNO coadsorbed with water. The surfaces selected—Ir(110), Ir(111), Pt(100), Pt(111), Rh(100), Rh(111), and Pd(111)—yieldnear-exclusive molecular (rather than dissociative) NO adsorption, yet exhibit differing coverage-dependent NO, and especiallyCO, spectral and coordinative properties. Progressive displacement of NO by exposing NO-saturated electrodes to dilute COsolutions yielded chiefly molecularly intermixed CO/NO adlayers. This deduction is most straightforward (and quantitative) onIr(110) and Ir(111), facilitated by the exclusively atop-like adsorption of CO and NO, as gleaned from the single nCO and nNO bandfrequencies. Both these features on Ir(110) redshift markedly (by 40–60 cm−1) upon dilution with either the other chemisorbateor with coadsorbed water. Such redshifts, along with the observed suppression of the nNO band intensity in the CO/NO mixtures,arise chiefly from composition-dependent dynamic-dipole coupling within the intermixed dipolar adlayers, and are diagnostic ofmicroscopic coadsorbate structure. Similar nNO redshifts induced by dilution with coadsorbed CO are observed on each of theother surfaces. The composition-dependent analysis is facilitated by the observation of a lone coverage-dependent nNO band,associated probably with multifold NO coordination, in both the absence and presence of coadsorbed CO in most cases. Whilecorresponding effects upon the nCO band frequencies are also observed, the spectra on Pt and Rh surfaces include a pair of nCO

bands with frequencies, ca. 1800–1850 and 2000–2070 cm−1, suggestive of bridging and atop-like CO coordination, respectively.On Rh(100), Rh(111), Pt(111) and especially Pt(100), NO coadsorption induces CO ‘bridging’ to ‘atop’ site switching. This islargely consistent with the formation of molecularly intermixed CO/NO adlayers where the strong preference of NO for multifoldsites shifts CO molecules into neighboring atop-like configurations. On Pt(111), Rh(111), and Pd(111), however, the nCO spectralfingerprints for the CO/NO adlayers suggest the additional presence of segregated compressed ‘CO-rich’ domains. Comparisonsare made also with the behaviour of analogous CO/NO adlayers in ultrahigh vacuum. © 1999 Elsevier Science S.A. All rightsreserved.

Keywords: Platinum-group electrodes; IR spectroscopy; Vibrational interactions; Carbon monoxide; Nitric oxide

� Dedicated to Dr Jean Clavilier on the occasion of his retirementfrom CNRS, and in commemoration of his seminal contributions toelectrochemistry.

* Corresponding author. Tel.: +1-765-4945466; fax: +1-765-4940239.

E-mail address: [email protected] (M.J. Weaver)

1. Introduction

Arguably the most significant advance in electro-chemistry in recent years concerns the development of

0022-0728/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.PII: S 0 0 2 2 -0728 (98 )00421 -5

C. Tang et al. / Journal of Electroanalytical Chemistry 467 (1999) 92–104 93

‘in-situ electrochemical surface science’, involving theexploration of equilibrium and kinetic properties ofordered monocrystalline metal � solution interfaces. Atthe core of these activities, and indeed pivotally respon-sible for them, was the original recognition by Clavilierof the value of flame-annealing tactics for preparingwell-ordered electrode surfaces [1]. As is often the casewith truly seminal discoveries, the remarkable voltam-metric properties of ordered platinum surfaces preparedin this fashion triggered initial controversy, althoughthe demonstration of similar effects obtained with crys-tals prepared and handled in ultrahigh vacuum (UHV)[2] led to a now-universal acceptance of their validity.In turn, the careful and systematic studies of the elec-trochemical properties of such well-ordered transition-metal surfaces by Clavilier and his coworkers havespawned a widespread exploration of their structureand reactivity, utilizing in addition the increasing swathof in-situ microscopic-level techniques which haveemerged within the last decade (for example, [3]). Alsoimportantly, in the midst of these (for some) intoxicat-ing forays into atomic-/molecular-level structural phe-nomena, Clavilier’s continuing demonstrations of theinterpretative power of conventional electrochemicalmeasurements when attained for truly well-defined sys-tems have served to remind all of us of their abidingvalue.

The Purdue group obtained its first-hand experimen-tal introduction to the enticing world of single-crystalelectrochemistry from a close colleague of Clavilier atthe CNRS Laboratorie d’Electrochimie Interfaciale,Antoinette Hamelin. The encouragement for us to ini-tiate in-situ infrared spectroscopic studies of monocrys-talline Pt-group electrodes was also provided ebullientlyby our Mid-West neighbor, Andzrej Wieckowski. Mostimportantly, though, as for numerous other electro-chemists, our ensuing explorations of microscopic-levelphenomena at such ordered metal � solution interfaceshave always been influenced crucially by the rigor andexemplary scientific style of Clavilier.

A central component of our explorations of adsorp-tion at ordered Pt-group metal � solution interfaces uti-lizing infrared reflection–absorption spectroscopy(IRAS) over the last decade has involved carbonmonoxide, and latterly also nitric oxide, adlayers (forexample [4–12]). The work with nitric oxide was sug-gested by earlier IRAS studies by the Alicante group[13–17], with which Jean Clavilier has also been closelyassociated. Major reasons for selecting these simplediatomic chemisorbates include the exquisite sensitivityof their intramolecular vibrational spectra to the sur-face bonding and local intermolecular interactions,along with the opportunities so provided to link theelectrostatic and chemical properties of ordered electro-chemical interfaces with their UHV-based analogs[4,12].

A recent segment of these efforts in our laboratoryhas entailed in-situ IRAS examinations of coadsorbedCO/NO adlayers on ordered Pt-group metal electrodes[18–20]. Since both chemisorbates display environment-sensitive yet distinct (and distinguishable) vibrationalfingerprints, the infrared spectra can yield detailed in-formation on the nature of the intermolecular interac-tions. One basic question is the extent to which thecoadsorbed CO/NO adlayers are intermixed molecu-larly (i.e. form a ‘solid solution’) rather than yieldingsegregated CO and NO domains on the electrode sur-face. While such fundamental issues have received at-tention for some analogous CO/NO adlayers atPt-group metal � UHV interfaces [21–28], no informa-tion has been available previously for electrochemicalsystems.

The present paper contains a comparative survey ofcoadsorbed CO/NO adlayers on several low-index Pt-group electrodes in aqueous acidic solution by means ofin-situ IRAS. The metal surfaces considered here—Ir(111), Ir(110), Pt(111), Pt(100), Rh(111), Rh(100), andPd(111)—display an intriguing diversity of CO/NOcoordinative properties. Of particular interest is theextent to which the coadsorbates mutually affect theirsurface bonding and vibrational properties via the for-mation of molecularly intermixed phases, as comparedwith segregated islands. Such information is extractedby comparing the composition-dependent infrared spec-tra with corresponding coverage-dependent data for thepure adsorbates, i.e. coadsorbed with water (or hydro-gen) rather than with each other. The results show howthe nature and extent of the vibrational interactions aresensitive to the metal and crystallographic orientation,and specifically the preferred CO and NO binding sites.

2. Experimental

Details of the in-situ IRAS instrumentation and pro-cedures are partly as outlined in Chang and Weaver [4]and Tang et al. [18,20]. The FTIR spectrometer was aMattson RS-2 instrument, with a custom-built externalreflection compartment containing the narrow-bandMCT detector. The spectral resolution was either 2 or 4cm−1. The metal disk electrodes, mounted on a glassplunger by wrapping with Teflon tape, were pressedagainst the CaF2 window forming the base of thespectroelectrochemical cell so to create the optical thinlayer.

The various single-crystal surfaces, ca. 0.8–1 cmdiameter and 2 mm thick, were procured mostly fromthe Material Preparation Facility at Cornell University.They were oriented at least within 1°, as verified byX-ray diffraction. The Pt, Rh, and Ir crystals wererepolished with diamond paste down to 0.25 mm grainsize. They were then flame annealed to 1500–1700°C,

C. Tang et al. / Journal of Electroanalytical Chemistry 467 (1999) 92–10494

and cooled down prior to electrochemical or spec-troelectrochemical inspection in a fast-flowing H2+Arstream above ultrapure water, followed by water im-mersion (cf. Clavilier et al. [29,30]). Clean transferral tothe cell and/or dosing solutions (vide infra) was facili-tated by retaining a drop of water upon emersion. Thesurface state was checked usually following flame an-nealing by means of cyclic voltammetry (see specificreferences cited below for details). The Pd(111) crystalwas pretreated instead by means of an electropolishingprocedure, and the ensuing surface state checked bymeans of cyclic voltammograms for the formation andremoval of underpotential deposited copper [31] (cf.Zou et al. [32]).

Carbon monoxide and nitric oxide (Matheson Gases)were 99.99 and 99.0% min., respectively, the formerpacked in an aluminum cylinder (this avoiding the ironcarbonyl contaminants often present in CO stored insteel cylinders). Electrolytes were prepared from con-centrated perchloric acid (double distilled, GFS Chemi-cals) using ultrapure water (Millipore Milli Q). Thereference electrode was Ag � AgCl (Bioanalytical Sys-tems), but all electrode potentials are quoted here ver-sus the standard hydrogen electrode (SHE). Allmeasurements were undertaken at room temperature,2391°C.

3. Results and discussion

We consider here IRAS data obtained for CO/NOadlayers on low-index surfaces of the four Pt-groupmetals, iridium, platinum, rhodium and palladium, inturn. This sequence reflects in part their increasingcomplexity of spectral behavior, facilitating stepwiseconsideration of the various adlayer structural factorswhich determine their vibrational properties. The sur-faces are selected as those where the extent of NOadsorbate dissociation is probably small or negligible,at least at moderate or high coverages. For each sys-tem, the coverage-dependent infrared spectra of COand NO adsorbed separately are compared with corre-sponding composition-dependent data for the CO/NOmixtures in order to ascertain how the molecular envi-ronment of one chemisorbate is affected by the pres-ence of the other component. For brevity, emphasis isplaced on broad-based data interpretation; further de-tails, including experimental procedures, can be foundin the references cited.

3.1. Iridium surfaces

We consider iridium surfaces initially in view of thesimplicity of the chemisorbate spectral behavior, facili-tating discussion at the outset of the links betweenvibrational properties and the coadsorbed adlayer

structure. The infrared spectra obtained separately forthe intramolecular stretching vibrations (nCO, nNO) ofboth CO and NO on Ir(111) [11,19,33], Ir(100) [10], andIr(110) [34] in aqueous 0.1 M HClO4 as a function ofadsorbate coverage (uCO, uNO) and electrode potentialhave been the subject of several earlier reports from thislaboratory. The adsorption of NO on flame-annealedIr(100) is partly dissociative, supporting the likely (1×1) (rather than hexagonal reconstructed) nature of thesurface on the basis of comparisons with UHV data[10]. While this complication precludes straightforwardexamination of CO/NO coadsorption on Ir(100), theother two low-index iridium surfaces yield not onlysimple associative NO adsorption, but display alsoremarkably simple coverage-dependent vibrational be-havior for both adsorbates. Thus CO adsorption onboth Ir(111) and (110) exhibits a single nCO bandthroughout the accessible coverage range from ca.2000–2075 cm−1, the frequency upshifting with in-creasing uCO [11,20,33,34]. These nCO blueshifts areassociated chiefly with uCO-dependent dynamic dipole–dipole coupling rather than ‘static chemical’ adsorbateinteractions [35,36], and are strongly suggestive of ex-clusively atop (or near-atop) CO surface coordination[11]. Furthermore, NO adsorption on both Ir(111) and(110) yields a single nNO band over the entire range ofcoverages [20,34,37]. The nNO frequency, ca. 1800–1850cm−1, which blueshifts also with increasing uNO onIr(110), is again suggestive of preferred atop-type coor-dination [19,20,34]. Both the nCO and nNO bandsblueshift also progressively towards higher electrodepotentials typically by ca. 30–60 cm−1 V−1, i.e. displayconventional Stark-tuning behavior for theseadsorbates.

This unusually straightforward spectral behavior alsoencouraged us to undertake studies of CO/NO coad-sorption on both Ir(111) and (110) electrodes. Theformer study is the subject of a detailed report appear-ing elsewhere [20]. Especially since the Ir(110) systemexhibits somewhat similar behavior, we summarize heresalient IRAS data obtained on this surface along withbriefer remarks concerning Ir(111).

Fig. 1(A) shows a sequence of infrared absorbancespectra in the C–O stretching (nCO) region for increas-ing dosed CO coverages on Ir(110) in aqueous 0.1 MHClO4 at 0.35 V versus SHE. These were obtained byexposing the electrode surface to dilute (ca. 10−5 M)CO for variable time periods (B60 s) before formingthe thin layer [11]. The left-hand values beside eachspectrum refer to the ‘fractional coverage’, XCO, nor-malized so that the saturation uCO value, 1.05 [38], istaken as unity. The uCO values were obtained, as usual[4,38,39], from the infrared spectrophotometric assay ofthe CO2 produced (in the thin layer) from adsorbed COelectrooxidation, specifically from the absorbance of the2345 cm−1 asymmetric O–C–O stretching band. A


corresponding set of uNO-dependent nNO spectra, ob-tained also at 0.35 V, is shown in Fig. 1(B). Unlike thenCO data, the nNO spectra were obtained by immersingthe surface in dilute NO solution for 5 to 30 s beforerapid transferal to the spectroelectrochemical cell [20].(This protocol avoided production of trace NO2 andother solution species which are thought to yield spec-tral interferences.) The potential-difference nNO spectraare referenced to that obtained at a lower potential,0.05 V, following NO electroreduction, so to removesolvent interferences and yield ‘absolute’ nNO bands.This procedure [20] is similar to that employed for COexcept that the latter involves irreversible adlayer elec-trooxidation, typically at 0.6–0.8 V. (Unless noted oth-erwise, IRAS data acquisition for the other systemsdiscussed below utilized similar procedures.) Unlikeadsorbed CO, the uNO (and XNO) values are not knownaccurately since no simple spectrophotometric assaycan be undertaken; the (rough) XNO estimates given inFig. 1(B) were obtained instead from the relative nNO

band intensities [20].The coverage-induced blueshifts of the lone nCO and

nNO bands are evident clearly in Fig. 1(A and B). Forcomparison, Fig. 2 shows a sequence of IRAS data forcoadsorbed CO/NO on Ir(110), also at 0.35 V. Thispotential was chosen since adsorbed CO and NO arethen stable towards electrooxidation and electroreduc-

Fig. 2. Infrared spectra for saturated mixed CO/NO adlayers onordered Ir(110) at 0.35 V as a function of adlayer composition, givenas ‘fractional CO coverages’ XCO.

tion, respectively. The voltammetric oxidation of CO ata given coverage is impeded typically by the presence ofcoadsorbed NO, the voltammetric oxidation wave be-ing shifted by 0.1–0.2 V to higher potentials. The effectof coadsorbed CO on the broader voltammograms forNO reduction, however, are less clearcut [18,20]. Thespectra in Fig. 2 were obtained by partial CO displace-ment of the saturated NO adlayer by time-controlledexposure (for B60 s) to dilute (ca. 10−5 M) COsolution before forming the spectral thin layer [20]. TheXCO values for each coadsorbed mixture, again ob-tained from the CO2 spectroelectrochemical data, aregiven alongside each spectrum.

Comparison of Figs. 1 and 2 shows that dilution ofone chemisorbate by addition of the other yields aprogressive frequency redshift as well as intensity atten-uation of the diluted component, in a similar fashion tothat seen in the coverage-dependent spectra of CO andNO alone (i.e. when coadsorbed with water). Thisbehavior is more clearly evident in graphical form,shown in Fig. 3(A and B). The composition-dependentnCO frequencies in the CO/NO mixtures on Ir(110) at0.35 V are plotted (filled circles) as a function of XCO inFig. 3(A); the nCO–XCO plot obtained for dosed COalone (i.e. coadsorbed with water) is shown for com-parison (open circles). The corresponding nNO–XNO

plots obtained in the NO/CO mixtures and for NOalone are given in Fig. 3(B) (filled and open circles,respectively). Note again that the XNO values are onlyapproximate, being obtained from the relative nNO bandintensities at a given coverage in the absence of CO (cf.

Fig. 1. Infrared absorbance spectra (in log10 absorbance units, a.u.)for: (A) increasing CO coverages, and (B) increasing NO coveragesdosed separately on ordered Ir(110) at 0.35 V versus SHE in 0.1MHClO4. Values given beside each spectrum are the ‘fractional cover-ages’ XCO and XNO, normalized to a saturation value of unity (seetext for further details).


Weaver et al. [19]). While the (absolute) saturation NOcoverage is not known accurately, the XNO and XCO

values are related approximately by XNO= (1−XCO)[19,20]. Similar XCO-dependent nCO behavior to Fig.3(A) is seen also on Ir(111), although the comparison

with respect to the nNO component is complicated byNO island formation with coadsorbed water on Ir(111)[20].

As mentioned previously, such coverage-inducedband frequency shifts can contain contributions fromdynamic dipole–dipole coupling, DnD, and ‘static chem-ical’ interaction terms, DnC [40]. The importance ofsuch adsorbate interactions in the present context lies inthe coverage-dependent sensitivity of the former com-ponent to the microscopic adlayer structure. If thecoadsorbed adlayer is intermixed molecularly, dilutionof one chemisorbate component by another having asignificantly different oscillator (‘singleton’) frequencywill yield a progressive attenuation in DnD for theformer as the extent of dipole–dipole coupling betweenthe like oscillators is diminished gradually [18,20]. Onthe other hand, if the coadsorbates yield local ‘segre-gated islands’, consisting of patches containing mostlyclose-packed CO or NO molecules, progressive substi-tution of one chemisorbate by the other will merelyproduce appropriate changes in band intensity, the nCO

and nNO band frequencies remaining largely invariantsince both DnD and DnC are sensitive only to localadsorbate-adsorbate interactions [40]. (Note, however,that the presence of small islands, containing, say, lessthan 50 chemisorbate molecules, can yield appreciablylower intramolecular frequencies since the propagationof dipole–dipole interactions then becomes finite [41].)

On this basis, the nCO–XCO and nNO–XNO behaviorevident in Fig. 3(A) and (B), respectively, indicatesclearly that largely intermixed, rather than segregated,CO/NO adlayers are present on Ir(110). Most point-edly, the substantial (ca. 60 cm−1) redshifts of both thenCO and nNO wavenumbers observed upon diluting theCO and NO chemisorbate by the other component(solid circles) as well as by water (open circles) areentirely inconsistent with the segregated island model,yet in harmony with the behavior expected for a molec-ularly intermixed layer. Thus the random dilution, say,of the CO adlayer by NO will result in a progressivedecrease in the extent of dynamic coupling between theC–O dipoles as they become surrounded increasinglyby N–O dipoles since the latter have a markedly (ca.250 cm−1) different oscillator frequency. The similarlylarge (ca. 60 cm−1) uCO-induced nCO blueshifts ob-served for CO on Ir(110) (and Ir(111)) at 300 K inUHV have been shown by isotopic substitution experi-ments to be due predominantly to dynamic dipolecoupling (i.e. the DnD component), the ‘static chemical’term DnC (having a positive sign also) being aboutfourfold smaller [35,36]. (Unfortunately, the high costof C18O prevented such an analysis being undertakenfor the electrochemical system, the frequency shifts withthe cheaper isotope 13CO being insufficient to yieldresolved bands [33]). However, given that the ca. 60cm−1 nCO redshifts seen upon diluting CO with water

Fig. 3. (A) Peak frequency of the C–O stretching vibration (nCO)plotted against the fractional-coverage for CO adsorbed in the ab-sence (open circles) and presence (filled circles) of coadsorbed NO onordered Ir(110) at 0.35 V. (B) As in A, but for NO in the absence(open circles) and presence (filled circles) of coadsorbed CO.


on Ir(110) (open circles, Fig. 3(A)) are by implicationalso due chiefly to attenuation of DnD, the comparableshifts obtained by progressive NO replacement (solidcircles, Fig. 3(A)) arise almost certainly from the samesource. Admittedly, the uCO-dependent nCO frequenciesobserved with coadsorbed NO as well as with water areaffected probably also by the DnC component. Indeed,it is possible that the smaller XCO-dependent nCO fre-quency shifts observed with coadsorbed NO rather thanwater partly reflect differences in the DnC instead of theDnD component. Thus the DnC term for the saturatedCO/NO mixtures should be affected by the presence ofthrough-metal CO–NO interactions at low XCO com-pared with CO–CO interactions at high XCO. Given theclosely related nature of the surface bonding for COand NO, it is plausible that the XCO-dependent DnC

term for the CO/NO adlayers is smaller than for CO/water mixtures. Similar factors may contribute also tothe dissimilarities in the nNO frequency-XNO plots seenfor NO coadsorbed with CO versus water (filled andopen circles, Fig. 3(B)).

Another line of evidence favoring strongly the occur-rence of molecular CO/NO intermixing on both Ir(110)and Ir(111) involves the observation of infrared band‘intensity transfer’ [19,20]. This effect, another manifes-tation of dynamic dipole coupling, entails the ‘transfer’of band absorbance from spatially juxtaposed oscilla-tors having different singleton frequencies, where thehigher-wavenumber partner gains intensity at the ex-pense of the lower-wavenumber oscillator. The phe-nomenon is most commonly encountered in isotopicmixtures, such as 13CO/12CO, where the moderate (ca.45 cm−1) difference in singleton frequencies yields alarge (several-fold) transfer of band intensity in favor ofthe higher-wavenumber component [40]. This effect canbe considered to arise physically from the more efficientdynamic screening of the lower-frequency oscillator byneighboring higher-frequency components than viceversa [40]. Although not recognized widely, substantialband-intensity effects are both predicted and observedto occur even for oscillators having widely separated(say, 200–500 cm−1) singleton frequencies, ns, if theyhave suitably large dynamic dipole moments, and arespatially juxtaposed [19,20,42]. We have shown earlierthat such intensity-transfer effects can account also forapparent discrepancies between the CO occupancy inatop and multifold sites in compressed adlayer struc-tures on Pt(111), as deduced from scanning tunnelingmicroscopy (STM), and the relative nCO IRAS bandintensities for these coordination geometries, which fea-ture similarly large (ca. 300 cm−1) ns differences [8,42].Given the ns differences between CO and NO oscillatorson both Ir(110) and (111), ca. 250 cm−1, then, theoccurrence of such intensity transfer provides addi-tional direct evidence of coadsorbate molecularintermixing.

Fig. 4. Effect on infrared spectra for mixed CO/NO adlayers onordered Ir(111) and Ir(110) of electrooxidatively removing the COcomponent. A and B are for XCO=0.7 and 0.25, respectively, onIr(111) at 0.45 V. C and D are for XCO=0.75 and 0.25, respectively,on Ir(110) at 0.35 V.

This point is illustrated in Fig. 4(A) and (B), whichcompares nNO spectral bands for saturated NO/COmixtures on Ir(111) at 0.45 V before and after oxidativeremoval of the CO component (lower and upper spec-tral partners, respectively) [19]. The adlayer composi-tions in Fig. 4(A and B) are XCO=0.70 and 0.25,respectively. The former spectral pair show a marked(3–4 fold) enhancement of the nNO band intensity (at1820–1840 cm−1) upon removing the CO band compo-nent at (2070 cm−1) by pulse oxidation at 0.75 V. Thismarked band absorbance change is approximately con-sistent with theoretical predictions of the suppression ofthe nNO band intensity in random CO-rich mixtures,where each NO molecule will be surrounded chiefly byCOs [19]. The spectral pair in Fig. 4(B) exhibit a similarnNO band suppression, although as expected the effect ismilder because the NO molecules occupy a larger frac-tion of the binding sites in the mixed NO/CO adlayer(XNO=0.75).

Fig. 4(C) and (D) show corresponding IRAS spectralpairs for saturated NO/CO adlayers on Ir(110) at 0.35V, again both before and after oxidative removal of theCO component, and for XNO=0.25 and 0.75, respec-tively. The more dilute NO adlayer on Ir(110) (C)shows a similar (although smaller) extent of band inten-sity suppression to the corresponding Ir(111) case (A).The higher-XNO example (D) again exhibits a weakereffect. The smaller degree of intensity transfer observedfor the NO/CO adlayers on Ir(110) compared withIr(111) may be due to the likely (331) reconstructed


nature of the former surface [36], yielding weaker cou-pling from the anticipated non-parallel NO and COoriented dipoles. Significantly, however, the composi-tion dependence of nNO band intensity suppression inthe NO/CO mixtures on both surfaces is in reasonableaccord with the theoretical predictions [19].

The CO/NO adlayers on Ir(110) and (111) thereforeconstitute unusually simple systems where adsorptionof both partners occurs exclusively at a similar (atop ornear-atop) coordination site, and where replacement ofsaturated NO by CO apparently involves a near-ran-dom spatial substitution of the former by the lattermolecules. These conditions, along with the readilydistinguishable nCO and nNO spectral fingerprints, facili-tate a quantitative analysis of the coadsorbate vibra-tional interactions and molecular adlayer structure.Each of the remaining Pt-group surfaces consideredhere displays somewhat more complex adlayer struc-tural (and hence spectral) behavior, resulting from thepresence of adsorbate apparently in more than onesurface coordination geometry, together with differ-ences in the binding-site preferences of CO and NO.While these factors tend to make the spectral analysismore complex and less quantitative, they reflect thestructural richness and diversity encountered for thiscoadsorbate pair.

3.2. Platinum surfaces

We consider here Pt(100) and Pt(111). Similarly toIr(110), NO adsorption on the former surface yields asingle nNO band at ca. 1560–1640 cm−1 throughout thecoverage range, which blueshifts by 40 cm−1 or so upto saturation [18]. In contrast, however, the nCO spectralfingerprint on Pt(100) is more complicated, a pair ofbands at ca. 1800–1850 cm−1 and 2030–2060 cm−1

being obtained, attributable to bridging and atop COcoordination, respectively, with relative intensities thatdepend markedly on both the coverage and electrodepotential [43]. Especially given that the relatively lowband frequencies for NO on Pt(100) are suggestive ofpreferential multifold coordination, the influence ofcoadsorbed NO on the CO atop/multifold site occu-pancy as gleaned from the infrared spectra is antici-pated to shed additional light on the nature of theCO/NO interactions. Since we have described this sys-tem elsewhere recently [18], the presentation here isrestricted to a brief synopsis.

With respect to the nNO spectral fingerprint, theCO/NO adlayer properties on Pt(100) are reminiscentof those on Ir(110) and Ir(111). Thus dilution of satu-rated NO by CO on Pt(100) yields a progressive red-shift in the nNO frequency, the XNO-dependent extent ofthese shifts (ca. 50 cm−1) being similar to those ob-served for NO adsorbed with water (see Fig. 6 of Tanget al. [18]). By analogy to the aforementioned behavior

of the iridium systems, this finding is indicative of thepresence of CO/NO intermixing, the CO acting toattenuate progressively the dynamic dipole couplingbetween the NO molecules. A similar observation, withthe same interpretation, has been made for the progres-sive replacement of NO by CO on Pt(100) in UHV at300 K [25].

Furthermore, the nNO band absorbance is suppressedin the mixed NO/CO adlayers on the Pt(100) electrodeby intensity transfer to the higher-wavenumber nCO

mode(s) [18] in a comparable fashion to that notedabove on Ir(110) and Ir(111). This point is illustrated inFig. 5(A), which shows a pair of infrared spectra forNO/CO on Pt(100) at 0.5 V in 0.1 M HClO4 before andafter electrooxidative removal of CO at 0.8 V (cf. Fig.4). The CO coverage is not known accurately since thedeuterated water used as solvent (to avoid interferencein the nNO band region) vitiated accurate assay of theCO2 product band at 2345 cm−1 because of overlapwith the broad O–D stretching feature. Nevertheless,the adlayer is dilute in NO (XNO:0.25), as ascertainedpartly from the nNO band intensity [18]. Removal of thecoadsorbed CO, as evident from the disappearance ofthe atop and bridging nCO features in the upper spec-trum, yields a substantial (3–4 fold) increase in the nNO

band intensity (at 1582 cm−1). While it is not known ifthis nNO band suppression in the CO-rich mixed adlayerresults primarily from the bridging or atop CO (seen atca. 1855 and 2050 cm−1, respectively), the observationprovides strong support to the contention that theCO/NO adlayer is largely intermixed.

Fig. 5. Representative infrared spectra for mixed CO/NO adlayers onordered Pt(100) at 0.5 V, showing the effect of: (A) electrooxidativeremoval of CO for XNO�0.25, and (B) electroreductive removal ofNO for XCO�0.3.


The other significant vibrational property of thePt(100) system, already alluded to, concerns the influ-ence of coadsorbed NO on the CO coordination ge-ometry. This aspect is exemplified in Fig. 5(B), whichcontains a pair of spectra again obtained at 0.5 V, butnow for an dilute CO adlayer (XCO:0.3) both before(lower) and after (upper) NO removal, by pulse elec-troreduction at 0.05 V. Significantly, the presence of theexcess NO coadsorbate is seen to diminish substantiallythe fraction of CO present in the (likely) bridging site,shifting the chemisorbate into atop sites. This is gleanedfrom the marked decrease in the ca. 1830–1840 cm−1

band intensity and the appearance of a strong 2035cm−1 feature, both induced by the coadsorbed NO [18].As anticipated, the upper spectrum is essentially thesame as that observed upon CO dosing to low orintermediate coverages in the absence of NO ontoPt(100) at 0.5 V. This NO-induced CO site-switching isnot observed on Pt(100) in UHV, since the chemisorbedCO yields only a single nCO band, suggestive of prefer-ential atop (or near-atop) coordination, in the absenceas well as the presence of NO [25].

The most plausible interpretation is to invoke NO-in-duced CO site switching from bridging to atop bindinggeometries, again within a molecularly intermixed ad-layer, induced by the strong preference of the latteradsorbate for multifold surface coordination. Alterna-tively, however, it is feasible that the nCO spectralfingerprint in Fig. 5(B) reflects instead the presence ofsegregated CO patches, compressed to form locally highcoverages by surrounding NO molecules. This possibil-ity is suggested by the similarity in the nCO spectralfingerprint at lower XCO in Fig. 5(B) with spectra forhigh average CO coverages, such as the example in Fig.5(A). However, the substantially (ca. 15 cm−1) down-shifted frequencies for both the atop and bridging nCO

bands in Fig. 5(B) compared to (A) argue against thispossibility. If present, the local ‘CO-rich’ patches mustbe small (probably containing fewer than, say, 15–25CO molecules) in order to account for the lower nCO

frequencies in comparison with high-uCO pure CO ad-layers. (As alluded to above, this deduction is based onnumerical simulations of dipole coupling-inducedblueshifts as a function of island size [41].) Conse-quently, then, we deduce that the CO/NO adlayer onPt(100) is largely molecularly intermixed, although it isconceivable that small (probably sub-nanoscale) CO-rich patches are also present.

We have also examined the corresponding Pt(111)system. The adsorption of CO and NO on Pt(111) is ina sense a step more complex than on Pt(100) since bothspecies yield infrared spectra featuring a pair of bandsat widely separated frequencies, indicative of multiplebinding geometries for NO as well as CO. Representa-tive coverage-dependent spectra for CO and NO dosedseparately on Pt(111) in 0.1 M HClO4 are shown in Fig.

Fig. 6. Infrared spectra for: (A) increasing CO coverages, and (B)increasing NO coverages, dosed separately on ordered Pt(111) at 0.45and 0.5 V, respectively.

6(A) and (B), respectively. The adsorption potential forCO, 0.45 V, is slightly lower than for NO, 0.5 V, sinceCO electrooxidation commences close to the lattervalue. However, the nCO frequencies should be onlyslightly affected (1–2 cm−1) by the 50 mV lower poten-tial. As before, the values indicated beside each COspectrum are the ‘fractional coverages’ XCO, the satura-tion value of uCO (i.e. where XCO=1) being about 0.65[38,39]. The form of the coverage-dependent nCO spec-tra on Pt(111) in 0.1 M HClO4 has been discussed indetail elsewhere [4,5,8]. In contrast to the UHV-basedsystem which exhibits only atop binding at low uCO [44],both atop and bridging bands are observed in aqueoussolution throughout the accessible coverage range [4].This difference has been traced to the effect of coad-sorbed water molecules on the basis of ‘UHV electro-chemical modeling’ studies involving sequential dosingof double-layer components at low temperature [45,46].While the high-uCO nCO spectral appearance, featuring apredominant atop and a weaker bridging band (Fig.6(A)), is ostensibly similar to that observed on cleanPt(111) in UHV [47], the spatial adlayer structuralarrangements are distinctly different, as deduced di-rectly by in-situ STM for the electrochemical system [8].

The coverage-dependent nCO spectra for NO onPt(111) electrodes have a similar form as IRAS data forthe UHV system [48–50], exhibiting a low-frequencyband (at 1400–1450 cm−1) at low uNO, with a higher-frequency feature (at 1650–1700 cm−1) becoming pre-


dominant towards high coverages (Fig. 6(B)). Indeed,the saturation-coverage NO spectra on Pt(111), as wellas Pt(100), electrodes in aqueous solution exhibit nNO

frequencies that are closely similar to those observedfor the corresponding interfaces in UHV when thecomparison is made at equivalent surface potentials[37]. Such correspondences, which mirror behaviornoted previously for CO adlayers [8,51–53], indicate anintrinsic similarity in the NO adlayer binding in theaqueous electrochemical and UHV environments.While the higher-frequency band for NO on Pt(111),seen exclusively at saturation coverage, has been as-signed usually to atop NO [48], a dynamical low-energyelectron diffraction (LEED) study of the ordered (2×2) structure obtained at saturation indicates instead thepresence of threefold-hollow NO [54]. This assignment,however, leaves unanswered the structural identity ofthe lower-frequency nNO band observed at smallercoverages.

The lower spectra in Fig. 7(A) and (B) show twoexamples of mixed CO/NO IRAS data on Pt(111) in 0.1M HClO4 at 0.5 V, prior to the removal of CO (A) andNO (B) by pulse electrooxidation and electroreduction,respectively. As for Pt(100), D2O solvent was utilized soto avoid spectral interferences in the nNO region. Thisprocedure, coupled with the multiple infrared bandsobserved for both chemisorbates, precludes an accurateestimate of the mixed-adlayer compositions. Neverthe-less, based on the nCO and nNO band intensities weestimate that roughly XCO�0.4 and 0.6 in Fig. 7(A)and (B), respectively. The appearance of the nCO and

nNO bands in the CO/NO adlayers (Fig. 7(A) and (B)) issomewhat similar to those for comparable coverages ofCO and NO present separately, i.e. with coadsorbedwater. Thus both the nCO bands assigned to atop andbridging CO are evident in the lower spectra in bothFig. 7(A) and (B), along with the higher- and lower-fre-quency nNO partners observed for pure NO adlayers atlow and intermediate coverages. Moreover, the bandfrequencies in the CO/NO mixtures are not greatlydifferent from those seen at comparable coverages inthe pure CO and NO adlayers. The latter point isparticularly evident when comparing the form of theNO and CO adlayers in Fig. 7(A) and (B), respectively,prior to and following the selective removal of the othercoadsorbate. Thus the frequencies of both nCO bands inFig. 7(B) remain largely unaffected by NO removal,and a similar observation applies to the higher-fre-quency nNO band in Fig. 7(A). Nevertheless, the lower-frequency nNO feature in Fig. 7(A) is seen to beblueshifted significantly (by 15 cm−1) upon COremoval.

The interpretation of such spectral behavior in termsof the alternative ‘molecularly intermixed’ and ‘segre-gated island’ models considered above is not entirelystraightforward. As in the Ir(110) and Pt(100) systemsalready discussed, the occurrence of molecular inter-mixing would be expected to yield band frequencyredshifts upon dilution of one chemisorbate by theother. To some extent, such effects are evident uponcomparing Figs. 6 and 7. Thus the higher- frequencynNO band is redshifted significantly (by ca. 15 and 20cm−1) in the NO/CO mixtures (Fig. 7) compared withthe saturated pure NO adlayer (Fig. 6(B)) and a similareffect is seen for the bridging nCO band. While the atopnCO band undergoes only small (510 cm−1) redshiftsunder these conditions, this band in the pure CO ad-layer exhibits similar shifts at moderate coverages andabove (Fig. 6(A)). The latter insensitivity of the atopnCO frequency to coverage provides direct evidence ofCO island formation with coadsorbed water on Pt(111)[4].

As mentioned above, if large segregated islands arebeing formed in the CO/NO adlayer one would expectthat the nCO and/or nNO spectral fingerprints would becomparable to those observed for the pure saturatedadlayers. That is clearly not the case here for NO, sincethe mixed-adlayer nNO spectra exhibit the ca. 1400cm−1 feature which is absent in the saturated NO case,along with redshifted nNO frequencies as already noted(Fig. 7). However, the intensity ratio of the atop versusbridging nCO band in the CO/NO mixtures (Fig. 7) isreminiscent of the spectra for saturated CO, but quitedifferent to those for lower CO coverages (Fig. 6(A)).This behavior is suggestive of compression of CO intosmall islands by surrounding NO chemisorbate. Ifpresent, however, the CO islands must be small (proba-

Fig. 7. Representative infrared spectra for mixed CO/NO adlayers onordered Pt(111) at 0.5 V, showing the effect of: (A) electrooxidativeremoval of CO for XNO�0.6, and (B) electroreductive removal ofNO for XCO�0.6.


Fig. 8. Infrared spectra for: (A) increasing CO coverages, and (B)increasing NO coverages dosed separately on ordered Rh(100) at0.45 V.

M HClO4. We have described earlier the uCO-dependentproperties of the Rh(100)/CO system [55]. Unlike COadsorption on clean Rh(100) in UHV where atop bind-ing predominates [56], the aqueous electrochemical sys-tem exhibits an apparently bridging nCO feature at ca.1850–1940 cm−1, which blueshifts markedly with in-creasing XCO, accompanied by an atop-like band at2010–2030 cm−1 which emerges for XCO\0.5. The NOadlayer displays simple spectral behavior, consisting ofa single nNO band at 1600–1690 cm−1 which alsoblueshifts markedly with increasing XNO. As for thePt(100) and Pt(111) systems, NO adsorption on Rh(100)yields nNO frequencies which are compatible with thebehavior in UHV [57] once the differences in surfacepotential are taken into account [37]. Although the NObinding site(s) has apparently not been determined ex-perimentally, theoretical considerations suggest againmultifold coordination [58].

A pair of spectra obtained for mixed NO/CO adlayerson Rh(100) at 0.45 V are shown for comparison in Fig.9. Similarly to Figs. 5 and 7, the spectral pair in Fig.9(A) and (B) show the effects of the selective removal ofCO and NO, respectively. Again, the use of deuteratedwater obliged by the low nNO frequencies precluded anaccurate assessment of the XCO values. On the basis ofthe relative nCO and nNO band intensities, estimates ofXCO in Fig. 9(A) and (B) are roughly 0.5 and 0.6,respectively. Essentially the same spectra for the CO/NO mixtures on Rh(100) were obtained by partialreplacement of either saturated NO or CO adlayers inthe cell prior to thin-layer formation by a dilute solu-

bly containing less than 50 COs) to account for theredshifted atop and bridging nCO frequencies in theCO/NO mixtures compared to the pure saturated COcase [41]. Moreover, such partial ‘bridging to atop’ COsite switching is consistent also with the presence of alargely intermixed CO/NO adlayer in that the preferen-tially multifold-bound NO would be expected to shiftadjacent CO molecules into atop or similar adsorptiongeometries. On the other hand, the nNO band intensitiesin Fig. 7(A) are not increased greatly upon CO removal.The absence of marked intensity suppression for the nNO

band in the NO/CO adlayer is therefore suggestive ofnon-random adsorption also, possibly involving NO-rich regions. Overall, then, the spectral behavior of thePt(111) CO/NO adlayer system is consistent with thepresence of some molecular intermixing, although smallsegregated islands may well be present, especially forCO.

3.3. Rhodium and palladium surfaces

Compared to iridium and platinum, rhodium andpalladium electrodes favor multifold versus atop COcoordination to a greater extent [5,6,32]. It is thereforeof interest to ascertain the manner and extent to whichthe CO/NO adlayer exhibits chemisorbate site switchingas well as dipole–coupling interactions. Fig. 8(A) and(B) show representative coverage-dependent spectra forCO and NO obtained on Rh(100), both at 0.45 V in 0.1

Fig. 9. Representative infrared spectra for mixed CO/NO adlayers onordered Rh(100) at 0.45 V, showing the effect of: (A) electrooxidativeremoval of CO for XNO�0.5, and (B) electroreductive removal ofNO for XCO�0.6.


tion of the other component. This property differs fromCO/NO adlayers on Pt(100), where saturated CO wasfound not to undergo partial replacement upon NOdosing [18].

Similarly to the systems already described, the behav-ior of the nNO band is indicative of the occurrence ofmolecular intermixing. Thus the nNO frequency in theNO/CO mixtures is redshifted markedly (by up to 50cm−1) compared to that for pure saturated NO onRh(100). Moreover, the removal of coadsorbed COleads to an increase in the nNO band intensity, asexemplified in Fig. 9(A), again indicative of nNO bandsuppression in the mixed adlayer. The appearance ofthe mixed-adlayer nCO bands is interesting. While aweaker feature at 2010–2020 cm−1 is observed, sugges-tive of atop binding, an apparent bridging nCO band at1880–1900 cm−1 predominates in the CO/NO mixtures(Fig. 9). The frequencies of the latter are markedly (upto 60 cm−1) lower than that for the saturated pure COlayer, exhibiting in some cases even greater redshiftsthan those observed for CO coadsorbed with water.

The most likely explanation for the nCO behavior isagain to invoke CO/NO molecular intermixing, themarked redshifts seen for the ‘bridging’ nCO band upondilution with NO being due to progressive attenuationof uCO-induced dipole–dipole coupling. The retentionof the apparent bridging nCO geometry in the CO/NOmixtures, however, differs from the behavior on Pt(100)and Pt(111), and presumably reflects a strong prefer-ence for bridging CO coordination on Rh(100) that isevident also from the spectra for CO coadsorbed withwater (Fig. 8(A)). Nevertheless, the additional presenceof the ‘atop’ nCO band in the CO/NO mixtures onRh(100) also suggests the occurrence of partial CO sitetransfer induced by neighboring multifold NOchemisorbate.

We also examined mixed CO/NO adlayers onRh(111). The nCO spectral appearance is complicatedsomewhat by an inability to electrooxidize adsorbedCO entirely on ordered Rh(111). Indeed, this propertyis diagnostic of well-ordered Rh(111) [5,38]. Neverthe-less, the behavior of both the nCO and nNO bands in thecoadsorbed compared with the single chemisorbate en-vironments on Rh(111) is once again indicative ofmolecular intermixing. Thus the nNO band observed inthe pure NO adlayer, blueshifting with increasing XNO

from ca. 1500 to 1580 cm−1 (at 0.5 V), appears simi-larly in the spectra for the NO/CO adlayers, withfrequencies that redshift markedly upon dilution withCO. The adsorption of CO alone on Rh(111) at higherpotentials (\0 V) features a predominant atop band at2000–2030 cm−1 (at 0.5 V) along with a weaker featureat 1800–1820 cm−1, evident at higher coverages[5,7,59]. The saturated adlayer structure, (2×2)–3CO(uCO=0.75), has been deduced from combined STM/IRAS measurements to contain near-atop and multi-

fold CO; this converts reversibly to a structurefeaturing predominantly bridging coordination belowabout 0 V [7]. The ‘atop’ nCO spectral fingerprint pre-dominates also in the mixed CO/NO adlayers onRh(111), dilution with NO again yielding progressive(up to ca. 40 cm−1) redshifts, comparable to that seenwith coadsorbed water. An interesting difference withthe Rh(100) surface is the elimination of the ‘bridging’nCO band on Rh(111) in the presence of coadsorbedNO. This behavior reflects the smaller propensity forCO bridging coordination on Rh(111) compared withRh(100), so that the (presumably) multifold NO is ableto shift the CO in the intermixed adlayer entirely intoatop-like sites.

Lastly, we have examined CO/NO adlayers onPd(111). This complex system will be described else-where as part of a broader examination of adsorptionon Pd(111) and Pd(100) [60]; we restrict discussion hereto brief comments. The adsorption of NO separately onPd(111) in aqueous solution is relatively simple, yield-ing one main nNO band around 1700–1750 cm−1,blueshifting as usual with increasing XNO. In the CO/NO adlayer on Pd(111), the nNO band is redshiftedupon increasing dilution with CO, in a similar fashionto NO adsorption alone. Once again, this behavior isindicative of intermixed NO adsorption. The nCO be-havior in the mixed adlayers, however, is more complexand indicates the presence of both segregated CO andintermixed CO/NO domains. The latter is suggestedfrom the presence of a nCO feature around 1860–1880cm−1, similar to that obtained for low-uCO pure COadlayers. The CO/NO adlayers also exhibit a pair ofnCO bands around 1920 and 1970 cm−1 (at 0.65 V),with frequencies and relative intensities that are remi-niscent of spectra for saturated CO adlayers onPd(111), but markedly different from those for lowercoverages. This behavior, therefore, suggests the addi-tional presence of segregated CO islands [60].

3.4. O6erall adlayer structural implications

Although the foregoing presentation is in large partonly qualitative in nature, the IRAS data nonethelessprovide intriguing insight into the fundamental aspectsof vibrational interactions within chemisorbate adlayersat ordered electrochemical interfaces as well as thestructure of specific CO/NO systems. Several generalconclusions are noteworthy. Firstly, the coadsorbateinfrared properties signal the occurrence of molecularCO/NO intermixing in most instances. This is mostclearly the case on Ir(110) (and Ir(111)), where thepresence of a single (and probably common) bindingsite for CO and NO facilitates the data analysis, al-though similar intermixing is also evident on Pt(100),Rh(100) and Rh(111). Secondly, the propensity forCO/NO molecular intermixing in some cases results in


NO-induced alterations in the CO site occupancy frommultifold to atop coordination. This is most strikinglyevident on Pt(100), where the bridging CO coordinationpreferred at lower coverages with coadsorbed water isreplaced by chiefly atop CO binding in the presence ofNO. Similar, yet milder, site-switching effects are ob-served on Rh(100) and Rh(111). Thirdly, while the NOsurface coordination appears to be largely unaffectedby CO, the intermixed coadsorbate induces markedalterations in the nNO band properties associated withdynamic dipole coupling. The manifestations ofdipole–dipole coupling include nNO band suppressionvia intensity transfer as well as attenuation of thecoupling-induced blueshifts in the nNO (and nCO) bandfrequencies. These vibrational properties provide valu-able diagnoses of molecular-level chemisorbate inter-mixing. Lastly, while such CO/NO intermixing isapparent to some extent in all the systems describedhere, small segregated CO (and possibly NO) islandsalso appear to be present on Pt(111) and Pd(111).

It is of obvious interest to consider the likely factorsthat are responsible for the CO/NO intermixing andcoadsorbate site-switching, especially in comparisonwith related findings for UHV-based systems. On ther-modynamic grounds, intermixed adlayers should beformed when the interaction energy between the twomolecular components, oAB, is more favorable (or lessunfavorable) than the interactions oAA and oBB withinthe corresponding pure adlayers. In the case wherethese interaction terms are similar, intermixing wouldstill be anticipated from entropic considerations [21].Reliable information on the relative values of oAA, oBB,and oAB for CO/NO coadsorption is sparse. Thermaldesorption data from Pt(111) in UHV suggest the pres-ence of weakly attractive CO/NO interactions [23]. Thecoadsorption of NO with CO on Pt(100) in UHV wasobserved to increase the desorption temperature of thelatter, again implying attractive coadsorbate interac-tions [24].

As already mentioned, IRAS data for the latter UHVsystem indicates clearly the presence of CO/NO inter-mixing [25], similarly to the electrochemical CO/NOadlayer on Pt(100). Displacement of NO by CO onRh(111) in UHV is found also from vibrational spec-troscopy to yield intermixed adlayers [26]. While thereverse displacement process yielded CO islands atlower temperatures, heating to 250 K results again inmolecular intermixing [26]. Indeed, the formation ofmolecularly intermixed CO/NO adlayers has been de-duced by means of IRAS for several other surfaces inUHV, including all three low-index palladium faces[27,28]. Apparent NO-induced CO site-switching frombridging to atop geometries has also been noted onPd(110) [27]. The latter phenomenon, as well as theprevalence of CO/NO intermixing, has been suggestedto be due to the synergetic influence on surface bonding

by adjacent CO and NO molecules, whereby coordina-tion of the former and latter species involves a greaterdegree of metal-chemisorbate s-donation and p back-donation, respectively [27,28].

More generally, however, the nature of short-rangeinteractions even between such simple chemisorbates,involving ‘through-metal’ as well as ‘through-space’terms, are not understood satisfactorily at present [61].Adlayer intermolecular interactions are, of course, ofwidespread significance, especially for chemisorption inelectrochemical systems which necessarily involvescoadsorption with the solvent in addition to otherspecies. The development of a reasonably broad-basedunderstanding of interactions within coadsorbed molec-ular systems, especially at ordered interfaces, is there-fore extremely desirable.

Acknowledgements

CT is grateful to the National Science Council ofTaiwan, ROC, for financial aid. This work is supportedby the US National Science Foundation.

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