J. Phys. Chem. 1995, 99, 14832-14839
14832
Nitric Oxide as a Probe Adsorbate for Linking Pt(ll1) Electrochemical and Model Ultrahigh-Vacuum Interfaces Using Infrared Spectroscopy Ignacio Villegas, Roberto G6mezJ and Michael J. Weaver” Department of Chemistry, Purdue University, West Lufayette, Indiana 47907 Received: June 9, 1995@
The effects of water coadsorption on nitric oxide adlayers on Pt( 1 11) in ultrahigh vacuum (uhv) are examined with infrared reflection-absorption spectroscopy (IRAS) along with work-function measurements with the objective of relating the uhv-based system to NO chemisorption at the Pt( 111)-aqueous electrochemical interface as studied recently by in-situ IFUS. In contrast to the corresponding (and extensively studied) Pt( 11l)/CO system, solvent coadsorption apparently yields little or no change in the NO surface binding geometry at low as well as saturated chemisorbate coverages, the solvent-induced downshifts (ca. 35-70 cm-I) in the N - 0 stretching ( Y N O ) frequencies being consistent with the occurrence of only an electrostatic Stark effect. This behavior, along with the stability of the electrochemical NO adlayer at relatively high electrode potentials (E), facilitates intercomparison of the surface potentials for the aquated uhv and in-situ interfaces by matching the V N O spectrum for the former with the V N O frequency-E data for the latter interface. This procedure yields an estimate of the “absolute” electrode potential, Ek, of the normal hydrogen electrode equal to 4.9 f 0.1 V. The approximate consistency of this value with some previous estimates of Ek supports the essential validity of the low-temperature uhv-based approach for exploring chemisorbate solvation effects.
Introduction The recent and continuing developments in the in-situ microscopic-level characterization of ordered monocrystalline metal electrodes are forging multidimensional new ties between electrochemistry and ultrahigh vacuum (uhv)-based surface science.’,* This situation encourages the acquisition of common spectroscopic and related information for analogous metal-uhv and electrochemical interfaces with the aim of interrelating their properties. One central issue involves understanding the roles of the additional components present in the so-called electrochemical “double layer”-solvent, ions, and excess electronic charge-in modifying metal-adsorbate structure and bonding. Such effects are expected to be substantial in view of not only the interfacial solvation which is necessarily involved in electrochemical systems but also because of the large and variable electrostatic fields that are commonly generated as a function of the applied electrode potential. One approach which has proven especially useful in this context involves sequential dosing of various components of electrochemical interfaces onto a metal surface in uhv at a suitably low temperature so to avoid significant evaporation. This “uhv electrochemical modeling” tactic, pioneered by Juergen Sass and co-workers in Berlin, provides an intriguing means of exploring synergetic double-layer interactions since the complex multicomponent system can be built up in a stepwise controlled f a ~ h i o n .Our ~ interest in this approach has been encouraged by the prospect of utilizing infrared reflectionabsorption spectroscopy (IRAS) as a molecular probe of structure and bonding in electrochemically relevant uhv-based systems. The value of such vibrational information is enhanced by the applicability of IRAS also to in-situ electrochemical surfaces, thereby enabling detailed behavioral links to be developed between related metal-uhv and metal-solution interfaces.
’
Permanent address: Departament de Quimica Fisica, Universitat d’Alacant, Apartat 99 E-03080 Alacant, Spain. Abstract published in Advance ACS Abstracts, September 15, 1995. @
0022-365419512099- 14832$09.0010
Following our earlier focus on the infrared properties of carbon monoxide adsorbed on monocrystalline Pt-group metals in a q u e ~ u s ~and - ~ nonaqueous media,’ we have recently explored the influence of coadsorbed solvent and solvated cations upon CO adsorbed on Pt( 111) in uhv, utilizing IRAS combined with work-function measurements.8-’0 The results demonstrate the coupled importance of short-range solvation and the surface potential drop engendered by both solvent dipoles and ionic/electronic charges in altering both the CO binding site and vibrational frequencies. Very recently, the Alacant group has demonstrated by means of in-situ IRAS as well as electrochemical measurements that nitric oxide adlayers form readily on low-index Pt electrodes in nitrous acidcontaining electrolytes.’ I Given the substantial information available on the vibrational and other structural properties of NO as well as CO adsorbed on metals in uhv, this opens up the intriguing possibility of exploring such issues also for the former adsorbate. Described herein is an initial examination of the consequences of water coadsorption upon variable-coverage NO adlayers on Pt( 111) in uhv as sensed by IRAS along with work-function measurements. The results display interesting differences to the extensively studied CO case in that solvent coadsorption yields little change in the NO binding geometry, the observed frequency shifts in the N - 0 ( Y N O ) vibration apparently being associated only with electrostatic field (Stark) effects. The combined infraredwork-function results are consistent with corresponding potential-dependent in-situ IRAS data obtained for saturated NO on Pt( 111) in aqueous solution. They also facilitate a quantitative comparison of Y N O frequencies in analogous hydrated-uhv and electrochemical environments as a means of intercomparing surface-potential scales for these two types of metal-solvent interface.
Experimental Section The uhv-based experiments were performed in a stainless steel chamber maintained at a base pressure of 5 x Torr using turbo, ion, and titanium sublimation pumps.8 The chamber is 0 1995 American Chemical Society
Nitric Oxide as an Electrochemical Probe Adsorbate equipped with facilities for Auger spectroscopy (AES), lowenergy electron diffraction (LEED), and temperature-programmed desorption (TPD) mass spectroscopy. In addition, a Mattson RS- 1000 Fourier transform instrument equipped with a globar light source and a narrow-band MCT detector was used for IRAS measurements. The p-polarized light beam was reflected from the sample at an 80"-85" angle via KBr windows in the chamber. The spectral resolution was f 2 cm-I. All spectra are reported in a difference format, using the clean surface as a reference. Also, a Delta Phi Elektronik Kelvin probe enabled the measurement of work-function (a)changes with respect to that of the clean Pt( 111). Both NO- and D20induced @ changes were measured in real time during background dosing and were accurate to 10 meV. Nitric oxide (Matheson) was dosed by increasing its partial pressure inside the chamber to an appropriate value between 5 x and 1 x lo-* Torr using a leak valve. The solvent dosing line was filled with D20 vapor generated in a uhvcompatible glass ampule attached to the line and containing the liquid. Gaseous impurities were removed via repeated cycles of freezing, pumping, and thawing. Accurate stepwise increase of the water dosage was achieved by line-of-sight dosing while maintaining the base pressure inside the chamber virtually unchanged. In contrast, continuous dosage during work-function measurements was achieved by back filling the chamber with 1x Torr of D20. As detailed elsewhere,8bTPD measurements enabled the calibration of the solvent dosage for both procedures. The 9 mm diameter Pt( 111) crystals employed for both the uhv and in-situ electrochemical experiments were purchased from the Material Preparation Facility at Comell. They were oriented within 1" as verified by X-ray backdiffraction. Preparation of the Pt(111) surface in uhv involved periodic sputtering with 1 kV Ar+ at 900 K followed by annealing to 1200 K. Carbon impurities were removed before each set of experiments by heating the sample to 600 K in a 3 x lo-' Torr of 0 2 atmosphere. All uhv-based measurements were undertaken at 90 K unless otherwise specified. The Pt( 111) crystal used for in-situ electrochemical IRAS was pretreated by high-temperature flame annealing'2 and cooled in a fast-flowing stream of hydrogen and argon, followed by immersion in ultrapure water saturated with these gases. The surface was then transferred to the spectroelectrochemical cell protected by a drop of water. As usual, cyclic voltammetry was used to check the surface state.I2 The experimental procedures involved in the in-situ IRAS measurements were largely as described p r e v i o ~ s l y . ~The ~ spectrometer was a Bruker (IBM) IR98-4A Fourier transform instrument equipped with a globar light source and a narrow-band MCT detector. The spectral resolution was f 4 cm-I. The electrolytes were prepared from doubly-distilled HC104 (GFS Chemicals) and reagent-grade NaN02 (EM Science), along with ultrapure water from a Millipore system. Electrode potentials were measured against a saturated calomel electrode but are quoted here versus the normal hydrogen electrode (NHE). The in-situ electrochemical IRAS measurements were performed at 23 f 1 "C.
Results and Discussion Pt(ll1)-NO in uhv. As for our previous studies of the Pt( 111)-CO, water system,* the basic tactics employed here involve monitoring the work-function change and IRAS spectra as a function of the NO and water dosages onto initially clean Pt( 111). Combined with a knowledge of the absolute work function for clean P t ( l l l ) , 5.9-6.0 eV,I3 the Kelvin probe measurements yield a values (and hence effective surface potentials) for each surface composition. Besides yielding
J. Phys. Chem., Vol. 99, No. 40, 1995 14833
DOSAGE (L)
' h
0
2
4
COVERAGE (ML)
Figure 1. Changes in work function, A@, induced increasing NO dosages (in langmuirs) onto clean Pt(ll1) and (B, bottom) increasing coverages of deuterated water (in monolayers referenced to metal atomic density) onto either saturated NO adlayer or clean surface. as indicated.
insight into the spatial surface-potential profiles, these data enable the infrared spectra to be compared directly with corresponding potential-dependent IRAS measurements for the in-situ electrochemical i n t e r f a ~ e . ~ . ~ Figure 1A shows CP changes as a function of the NO dosage (in langmuirs) onto initially clean Pt(ll1) at 90 K. The morphology of this plot is closely similar to that obtained previously at 120 K by Kiskinova et aLl4 These authors calibrated their NO dosages, yielding coverages, by means of low-energy electron diffraction (LEED) measurements. Comparison of our @-dosage data with their a'-&" plot (where 6 N O is the fractional NO coverage) enables at least rough @NO values to be estimated for our dosage conditions. Kiskinova et al. detected from X-ray photoelectron spectroscopy (XPS) the presence of two adsorption states. The first, found exclusively for 6 N O ~ 0 . 1 5corresponds , to the rising segment of the a'-@,?O curve, with the second adsorption state found increasingly at higher coverages up to the saturation value of ca. 0.55, which coincides with the ensuing falling @-ON0 segment. We consider below the implications of these results to the IRAS data. As before,8aA@ measurements were also made as a function of water dosage. Figure 1B shows a pair of such curves, with the water exposures, ,e, given as monolayers referenced to the Pt( 111) atomic density. (Note that the so-called water bilayer corresponds to 6, = */3 on this scale.8b) The A@ values are referenced to the clean Pt( 111) surface. The top curve shows the A@-& trace for water dosage onto a saturated NO adlayer, whereas the lower curve refers to water dosage onto initially clean Pt( 111). (Note that deuterated water was used here to facilitate examination of its surface infrared properties as detailed below.) Similarly to the corresponding behavior with adsorbed C0,8athe presence of a saturated NO adlayer is seen to attenuate
14834 J. Phys. Chem., Vol. 99, No. 40, 1995
Villegas et al.
adsorption on sites having lower coordination numbers (doubly briding, atop, or asymmetric sites) in spite of the different NO dosage (I) situation for the Ni( 11l)/NO system. One argument for this is 1485. 0.3 that that two VNO features are more widely separated, by 200A 240 cm-’, on Pt(ll1) than is the case on Ni(ll1) (35-120 0:6 cm-1).20 In addition, the individual band frequencies are markedly more coverage-dependent in the latter case. In retrospect, both these infrared spectral observations for Ni( 111)/ NO are in reasonable harmony with the diffraction results given that the higher-coverage structures feature NO molecules that have inequivalent site-site distances and hence interadsorbate interactions. Such inequivalent interactions can account for the observation of multiple VNO bands, especially in the presence of substantial dipole-dipole coupling and other @No-dependent interactions as suggested by the &,-induced VNO frequency shifts. In contrast, one is more hard pressed to account for the observed V N O spectra for the Pt( 11l)/NO system in terms of such environmental factors without postulating the presence of NO in an extra binding site toward high coverages. An additional, albeit subsidiary, argument notes that the closely 2000 1600 1600 1400 1200 1000 related adsorbate CO binds exclusively in threefold-hollow sites Frequency (cm-’) on N i ( l l l ) , yet in both atop and doubly bridging sites on Pt(lll).I9 There is some reason, then, to anticipate that NO Figure 2. Infrared spectra in N - 0 stretching (YNO) region for also binds in a lower-symmetry site toward high coverages on increasing dosages (in langmuirs) of NO, as indicated, onto clean Pt(ll1) at 90 K. Pt( 111). Also worthy of comment is the observed marked attenuation markedly the water-induced CP decreases (Figure 1B). This in the ca. 1500 cm-’ band intensity toward higher dosages where effect can be understood qualitatively in terms of the NO adlayer the 1715 cm-’ features grows in (Figure 2). While this behavior essentially denying access of the postdosed water to the metal might be construed as indicating removal of the threefold-hollow surface.8 Closely comparable CP decreases upon water dosage NO under these conditions, attention should be drawn toward were obtained when maintaining the Pt( 111) crystal over the the distorting effects of dynamic dipole-dipole coupling. Thus, temperature range 90- 140 K, the upper bound being determined model calculations undertaken for Pt(111)/CO demonstrate that by water evaporation under uhv conditionssb (but see below). substantial “intensity transfer” from the lower-to-higherOf central interest here are the infrared spectral responses to frequency band partner can occur for close-packed adlayers even such combined variations in the NO and water dosages. Figure when the band frequencies are considerably different, by say 2 shows a sequence of IRAS data in the 1000-2100 cm-’ ca. 200 cm-’ as is the case This theoretical prediction region, encompassing the N - 0 stretching ( V N O ) vibrations, for is consistent with in-situ electrochemical IRAS data together increasing NO dosages (as noted) onto initially clean Pt( 111). with a knowledge of the real-space adlayer structures from inTwo major VNO features are evident, the fist appearing at 1485situ scanning tunneling microscopy (STM).6.21 Given the 1500 cm-I and dominating for dosages below -1 langmuir magnitude of the intensity-transfer effect for adsorbates such (corresponding to @NO I 0.1) and the second at higher as NO and CO having large dynamic dipole moments, it is quite frequencies, ca. 1715 cm-’, which dominates above ca. 2 feasible that there remains significant or even substantial langmuirs (i.e., for @NO 2 0.2). These findings are in accordance occupancy of threefold-hollow sites toward saturation NO with earlier vibrational studies using IRAS or electron energy coverages on Pt( 111). loss spectroscopy (EELS).i5,’6 Effects of Water Coadsorption on Infrared Spectra. The traditional interpretation of such spectra, based on Having evaluated the coverage-dependent nature of NO adsorp“characteristic frequencies” involving comparison with bulktion alone, we are now placed to consider the effect of water phase nitrosyl complexes, assigns the ca. 1500 and 1700 cm-’ features to twofold bridging and atop NO, respecti~ely.’~?’~ coadsorption. Figure 3A displays the effects on the VNO spectra of increasing progressively the deuterated water coverage (as Several recent diffraction-based and related structural studies, indicated) by postdosing onto a saturated NO adlayer, whereas however, have seriously called into question the validity of such Figure 4A-C shows corresponding spectra for low and interassignments. Specifically, the adsorption of NO on Ni( 11l)I7 mediate NO coverages. Examining the former case first, water has been shown to occur exclusively in threefold-hollow sites addition is seen to yield a gradual attenuation of the intense throughout the entire coverage range (0 < @NO 5 0.5),’7,’seven 1714 cm-’ band, being accompanied by the growth of a broader though two distinct and @No-dependentVNO bands are observed feature at a significantly lower frequency, 1678- 1680 cm-I. in a qualitatively similar fashion to the present Pt(111) case. A The band replacement is largely complete by a water coverage dynamical LEED study of the Pt( 111)LNO system also places (e,) of 2.5. the adsorbate in threefold-hollow sites for the ordered p(2 x 2) structure corresponding to @NO = 0.25, although the diffuse This “titration-like” response is closely reminiscent of that nature of the LEED pattems at higher coverages precluded a observed for water dosage onto saturated Pt( 11l)/CO adlayers.s binding-site analysis under these conditions.Is As detailed earlier, the behavior is consistent with the presence Consequently, then, we have some confidence in assigning of hydrogen-bonded water clusters even at low solvent dosages, the lower-coverage 1485-1500 cm-’ VNO feature in Figure 2 so that the chemisorbed adlayer experiences microscopically to threefold-hollow NO. In the light of the present evidence, large “hydrated” and “anhydrous” regions. The NO adsorbate the coordination geometry of the higher-coverage 1700- 1715 situated within these “islands” will then yield a separate (Le., cm-’ feature is uncertain: one is tempted to assign it to uncoupled) spectral fingerprint similar to those for the fully T
1
I
Nitric Oxide as an Electrochemical Probe Adsorbate
J. Phys. Chem., Vol. 99, No. 40, 1995 14835 0.3 1 NO t I YL D20
6 I NO t I U D20
-1714 coverage (a)
Coverage (la)
.
0
1487,
o
~
c ,hv
0.3
0.3
1416,
2000
2000
lab0
1600
1400
1200
1600 1400 1200 Frequency (cm-l)
1000
T
\
1.8
-
I 2000
2600
1000
0.6 L NO t I U D20
p,\\ I
1200
1000
Frequency (cm-1) 6 L NO t I YL D20
1800 1600 1400 Frequency (cm-')
2ioo
2iOO
2000
1800
I
11.2LN0t1MLD20
Frequency (cm-') Figure 3. Infrared spectra in (A, top) N - 0 stretching and (B, bottom)
deuterated water 0-D stretching regions for increasing coverages of DzO,as indicated, onto a saturated NO adlayer on Pt( 111) at 90 K. hydrated or anhydrous surface, the proportion of the two spectral features thereby being essentially proportional to their relative integrated areas. Further evidence supporting this notion is provided from the spectral region encompassing the 0-D stretching (VOD) vibration, the set corresponding to Figure 3A being shown in Figure 3B. The VOD spectra show the hallmarks of strong hydrogen bonding even at low water exposures (6, Il), in particular the broad downshifted feature at 2500 cm-' (Figure 3B). These VOD spectral features are very similar to those obtained for D20 dosing onto saturated CO adlayers at 95 K and are consistent with hydrophobic chemisorbate-water interactiow8 Another similarity in the infrared spectral response to water dosing onto saturated NO and CO adlayers concerns the behavior at varying temperatures. For example, dosing water onto a saturated NO adlayer at 140 K rather than 90 K yielded a similar "titration-like'' response of the VNO bands, yet markedly 15) was required for complete surface more water (6,
-
2000
1800 1600 1400 Frequency (cm-')
1200
1000
Figure 4. As for Figure 3A, but for lower predosages of NO, equal to (A) 0.3, (B) 0.6, and ( C ) 1.2 langmuirs.
hydration, and the hydrated V N O feature appeared at slightly lower frequencies, ca. 1675 cm-' (cf. ref 8b). The VOD spectral
Villegas et al.
14836 J. Phys. Chem., Vol. 99, No. 40, 1995
region exhibited considerable “structure”, consistent with the presence of “polycrystalline” rather than amorphous ice in a similar fashion to the behavior with CO adlayers (see Figure 5 of ref 8b). Also, heating the surface following water postdosing at 90 K yields a sharpening and slight downshifting of the hydrated YNO band by ca. 140 K, the original anhydrous V N O spectrum being recovered upon further heating to 170 K so to desorb the water. The former findings are consistent with the formation of polycrystalline nanoscale “chunks”, or even pillars, of ice at higher temperatures, so that a substantial number of equivalent water monolayers are required in order to entirely cover the surface. While this temperature-dependent water dosage behavior might be viewed as an unwelcome complication from the perspective of uhv-based electrochemical modeling, the results point to an advantage of employing lower-temperature (ca. 90 K) dosing in that such omate polycrystalline water structuring is evidently muted under these conditions. The effects of water postdosing onto low and intermediate NO coverages, exemplified in Figure 4A-C, are particularly insightful. The first of this trio, Figure 4A, refers to a very low NO dosage, 0.3 langmuir (corresponding to 8 N O I0.05), where the spectrum contains only a single YNO band at 1487 cm-’ due to threefold-hollow NO. Addition of water yields removal of this feature even at the lowest dosage, 8, = 0.3, and its replacement by a broader downshifted YNO band at 1416 cm-’ (Figure 4A). The second spectral set, Figure 4B, also refers to a low NO dosage, 0.6 langmuir, but where the higherfrequency band component (at 1703 cm-’) for the anhydrous NO layer has begun to be discemible. Again, addition of low water dosages, 8, x 0.6, replaces the ca. 1500 cm-’ band by a broadened downshifted feature, now at 1424 cm-’. Interestingly, the weaker higher-frequency YNO component also apparently undergoes a comparable downshift, to 1643 cm-I, and the relative band intensities of these higher- and lower-frequency partners remain largely unchanged upon water addition. (Note that the broad feature evident at around 1200 cm-’ arises from a D20 scissoring vibration.) The last spectral set, Figure 4C, refers to a sufficiently high NO dosage, 1.2 langmuirs, so that the ca. 1700 cm-‘ YNO component is now dominant. Again, water addition yields comparable downshifts of both the 1709 and 1493 cm-I bands, to 1655 and 1425 cm-I, respectively, with the relative band intensities remaining largely unaltered. In summary, then, water coadsorption at low or intermediate coverages yields marked (ca. 60-70 cm-’) frequency downshifts of both the lower- and higher-wavenumber YNO bands, but with essential retention of the relative band intensities. A somewhat smaller, ca. 35 cm-I, downshift of the then-dominant higher-frequency YNO feature is obtained for water dosage onto a saturated NO adlayer. Following previous discussions of the related Pt( 11l)/CO s y ~ t e m ,these ~ , ~ water-induced effects may be usefully considered to be due to (a) alterations in the electrostatic field sensed by the chemisorbed NO (the so-called Stark effect) and/or (b) changes in the NO surface coordination geometry. There is strong evidence that the effects of water and other solvents on the CO chemisorbate are influenced importantly by both (a) and (b).8,y Thus, solvent coadsorption onto dilute CO adlayers on Pt(ll1) yields a near-complete removal of the otherwise dominant YCO band at ca. 2100 cm-’ due to atop CO and the appearance of a substantially (ca. 300 cm-I) downshifted and weaker YCO feature. This behavior is consistent with the solventinduced shifting of the chemisorbed CO from atop to multifold (bridging or possibly threefold-hollow) sites, which can be understood in terms of a dipole-induced stabilization of the latter
by molecularly juxtaposed CO/solvent coadsorption.8.y At high CO coverages, such solvent-induced site transfer is incomplete, markedly smaller YCO frequency downshifts being observed that are consistent with the presence of only the Stark effect (a).89 Independent evidence for the stabilization of multifold versus atop sites in the electrochemical Pt(lll)/CO system has been obtained, in particular, from in-situ STM.6 Furthermore, such binding-site shifts for CO are anticipated as a direct consequence of the decreases in surface potential that attend water coadsorption since this situation should encourage dn 2n* backbonding which is more prevalent for multifold versus atop coordination.8a This qualitative chemical idea is also supported by some theoretical molecular-orbital calculations.** In contrast, the present results suggest that water coadsorption yields little or no change in the NO adsorption geometry. Two lines of evidence support this notion. First, as detailed further below, the YNO frequency downshifts induced by water dosing are consistent with the corresponding decreases in surface potential, thereby indicating that factor (a) can largely account for the observed changes. Second, the lack of significant waterinduced alterations in the relative intensities of the higher- and lower-frequency bands when both are present suggests that little or no adsorbate site transfer is taking place. This contrasting behavior of adsorbed NO and CQ is perhaps unsurprising given that the preferred binding site for the former (Le., that found at low coverages) is threefold hollow, rather than atop as for the latter. Consequently, any increased preference for multifold coordination brought about by water adsorption should not alter the NO binding site. Most importantly in the present context, this relatively straightforward behavior facilitates infrared spectral comparisons with the in-situ electrochemical system. We next summarize pertinent spectral results for the latter, enabling this link to be developed in quantitative terms. Pt(ll1)-NO Electrochemical Interfaces. As already mentioned, the Alacant group has recently reported in-situ infrared spectra for Pt(ll1) and other metal electrodes in nitrous acid electrolytes that demonstrate conclusively the formation of stable NO adlayers.” While the adsorbate undergoes oxidation to NO3- and reduction to form ammonium cations, it is stable from ca. 0.4 to 1.0 V vs NHE on Pt( 111). This adlayer stability at high potentials contrasts the situation for CO, which readily oxidizes to C02 at markedly lower potentials, and facilitates further the comparison with the uhv-based interface (vide infra). Figure 5 shows a sequence of electrode potential-dependent in-situ infrared spectra obtained here for a saturated NO adlayer on Pt( 111) in contact with aqueous 0.1 M HC104 containing 2 mM NaN02. (The saturated adlayer was selected here for comparison with the uhv system since the coverage is then most readily controlled.) Rather than employ the usual potentialmodulation IRAS technique, these spectra were obtained by means of the “single potential alteration infrared” (SPAIR) procedure which we have utilized previously, for example, to examine adsorbed C0.23 Specifically here, this entailed acquiring a set of interferometer scans (50) at the sequence of “sample” potentials indicated beside each spectrum in Figure 5 , adjusting the potential sequentially to lower values, before stepping to 0.18 V to record the “reference” spectrum. At this last potential, adsorbed NO is reduced irreversibly to ammonium cations. The advantage of this SPAIR sequence is that the NO adsorbate is present only at each sample potential, so that a (positive-going) unipolar V N O band appears in Figure 5 , along with a negativegoing feature at a well-separated frequency (ca. 1480 cm-’) associated with an N h + bending vibration.24 Aside from confirming that is indeed the reduction product, this tactic
-.
m+
Nitric Oxide as an Electrochemical Probe Adsorbate
J. Phys. Chem., Vol. 99, No. 40,1995 14837 replaced by the higher-frequency (1670- 1695 cm-I) feature toward intermediate and high ON0 values.’ I C
Quantitative Spectral Comparisons between uhv and Electrochemical Interfaces. The most straightforward comparison between the present infrared spectra for the analogous Pt( 111)-uhv and aqueous electrochemical interfaces entails ascertaining the electrode potential, EM, at which the V N O frequency for the latter matches that for the former. Given that complete aquation of the Pt(l1 l)/NO-uhv interface yields V N O % 1680 f 2 cm-I (Figure 2A), this electrode potential is read from Figure 6 to be EM = 0.5 f 0.05 V vs NHE. The workfunction data for the aquated NO-saturated Pt( 111)-uhv interface in Figure 1B (upper trace) provides the corresponding “absolute” potential for this interface, Q M / e (where e is the electronic charge). While this A@-6, plot does not exhibit an entirely level plateau even at high water coverages (6, 4), a satisfactory value of QMle, 5.4 V, can be deduced by combining these data with the Q value for clean Pt(lll), 5.9 eV, mentioned above. Following Trasatti,26one can relate the electrode potential, EM,of an electrochemical interface with the work function, a“, of the same system (Le., on the vacuum scale) by3,26
-
(
1 0
‘
1700
I
’
1500
1
0
v/cm-‘ Figure 5. In-situ infrared spectra for saturated NO adlayer on Pt( 111) in aqueous 0.1 M HClO4 containing 2 mM NaN02 at “sample” potentials (vs NHE) as indicated. “Reference” spectrum was recorded at 0.18 V. (See text for spectral acquisition details.) 1705
-,
1 -
I
1675
0:4
0.5
0.6
0.1
0.8
0.9
EIV vs NHE Figure 6. Plot of V N O frequency for saturated NO adlayer on Pt( 111) in aqueous 0.1 M HC104 versus the electrode potential, extracted from infrared spectral data in Figure 5.
avoids the partial interference from bipolar YNO band components which is prevalent when using potential modulation, resulting in part from the large (ca. 30 cm-I) V N O bandwidth. The resulting plot of the YNO frequency obtained from these spectra as a function of the electrode potential, E (vs NHE), is given in Figure 6. While there is a significant “break” in the v ~ 0 - E plot at about 0.65 V, a monotonic dependence is nonetheless evident. Similarly to carbon monoxide and other chemisorbates, the VNO frequency exhibits a marked dependence on the applied electrode potential, dvcddE 30-60 cm-‘ V i , which can be understood in terms of an electrostatic-field (Stark) effect and/or on the basis of potential (or charge)-dependent metal-adsorbate bonding.25 Most significantly for the present purposes, the V N O frequency thereby provides an independent monitor of variations in the local potential drop within the electrochemical inner layer. The coverage of the saturated NO adlayer has been evaluated by faradaic electrochemistry to be about 0.5,’’ consistent with the estimate for the uhv interface mentioned above. Similarly to the uhv-based Pt( 111)/NO, water system considered above, an V N O band at 1410-1445 cm-’ is observed for the in-situ electrochemical interface at low ‘NO coverages, being largely
EM = a‘/e -
(1)
where Ek is the so-called “absolute potential” of the reference electrode, Le., referring to an electron transferred from the metal into vacuum (but close to the solution-vacuum interface). The appropriate value of Ek has been the subject of considerable debate as well as un~ertainty.~.*~ Essentially two groups of values, one around 4.45 V and an alternative, higher, cluster around 4.8 V, have been advocated on experimental and conceptual grounds.27 There is good reason to anticipate that the hydrated Pt(ll1) surface in uhv provides a reasonable facsimile of the uncharged electrochemical interface, that is, at the so-called potential of zero charge (pzc). (While the actual electrochemical system additionally contains electrolyte ions, the net ionic as well as electronic interfacial charge is zero at the pzc.) Inserting the values of EM and #Mle noted above (0.5 and 5.4 V) into eq 1 yields Ek X 4.9 f 0.1 v. This estimate of the “absolute potential” of the NHE is closely compatible with the higher cluster of values noted above. These values were derived primarily from experiments involving electrode emersion into uhv.26 While such strategies would appear to provide a direct route for evaluating Ek, they suffer from the removal of interfacial water which attends ambienttemperature transfer in u h ~ Indeed, .~ some experiments involving “wet emersion” of gold and platinum electrodes into a watersaturated atmosphere, where double-layer solvation probably remains intact in the gas phase, yield QM values that are up to 0.3 eV lower for emersed compared to the in-situ electrode surfaces.2* This finding suggests that the higher (ca. 4.8 V) Ek values extracted from “dry emersion” experiments may reflect the absence of interfacial solvation, given that water decreases QM typically by ca. 0.5-1 eV (cf. Figure lB).3 Nevertheless, given that the present infrared spectral comparison refers to fully solvated (albeit low-temperature) uhv interfaces, the deduction of a relatively high (4.9 V) Ek value might be construed as supporting the validity of such estimates. We would prefer, however, to refrain from making such a claim. Thus, it is possible that the water-induced alterations both in the overall surface potential drop, affecting the QM values, and in the local electrostatic field which influences the V N O frequencies are subtly different in the present uhv-based and in-situ electrochemical environments, resulting in part from the sub-
14838 J. Phys. Chem., Vol. 99, No. 40, 1995 stantially lower temperatures utilized for the former experiments. Such a notion finds some indirect support in the occurrence of a sizable inflection in the in-situ v ~ 0 - Eplot at ca. 0.65 V vs NHE (Figure 6). This inflection may well coincide with the pzc given that the ionic double-layer composition will switch from cation- to anion-like at lower and higher potentials, respectively. The inferred pzc value, 0.65 V, for the in-situ Pt( 11l)/NO electrochemical interface is slightly higher than that, 0.5 V, deduced above by matching the VNO-E plot with the observed Y N O frequency for the aquated F’t(l1 l)/NO-uhv interface. Inserting the former EM estimate rather than the spectrally deduced value into eq 1 yields a significantly (ca. 0.15-0.2 V) lower E k estimate, about 4.7 V. Probably more significantly, however, the present infrared spectral comparison supports the essential validity of the (perhaps controversial!) notion3 that the uhv-based “electrochemical modeling” experiments provide molecular-level insight into interfacial solvation effects which are of direct relevance, and therefore of crucial value, to in-situ electrochemical systems. We therefore consider now some implications of the water dosage-dependent infrared spectral changes seen for the Pt( 111)-uhv interface in this regard. Further Implications of uhv Infrared Spectra. We mentioned above, but have not yet justified, that the water-induced downshifts in the V N O frequencies are compatible with the occurrence of only the electrostatic Stark effect (a). A reasonable estimate of (a) can be obtained from the V N O frequencypotential dependence shown in Figure 6. While the data refer to a ion-, rather than (solvent) dipole-, induced Stark effect, their compatibility has been demonstrated for chemisorbed CO.9” The ca. 35 cm-’ V N O frequency downshift induced by water on a saturated NO adlayer (Figure 3A) along with the corresponding @ downshift (0.4-0.5 eV, Figure 1B) is closely comparable to the in-situ VNO-Edependence seen for the in-situ electrochemical interface. Similar AvNo-A@ responses, ca. 60-70 cm-I eV-’, are extracted from corresponding 10w-0No data (such as in Figure 4A), bearing in mind that essentially the same waterinduced A@ response seen on clean Pt( 111) (Figure 1B) also applies at low NO coverages. Another interesting facet of the present results concerns the effects of water dosing upon the infrared band intensities, especially in view of the above evidence that the NO binding site(s) remain largely unaltered. At least at low chemisorbate coverages in these circumstances, one expects the V N O band intensities to be diminished somewhat by dielectric screening, resulting from the significant electronic polarizability of the surrounding solvent molecule^.^^.^^ We have recently discussed the influence of dosing various solvents upon the infrared band intensities for variable-coverage CO adlayers on Pt( 111) in these terms, although the situation is complicated by the marked effects of solvent coadsorption upon the CO binding site.9b Inspection of the corresponding data for water coadsorption onto NO reveals the occurrence of significant if modest changes in band intensity. Thus at low 0 N O values, where only the threefold-hollow band is observed, water dosing is seen to diminish the integrated Y N O band intensity, I, by about 1S - 2 fold (e.g., Figure 4A). These changes are accompanied by substantial increases in the bandwidth, from 6-7 to 25 cm-I, which can be understood in terms of inhomogeneous band broadening associated with adsorbate solvation (cf. CO case, ref 8). Comparison with the theoretical predictions is hampered somewhat by uncertainties in some parameters, such as the dynamic dipole moment (vibrational polarizability) of the adsorbed NO, a”.However, presuming that the a, values for NO and CO are not greatly different,3’ the model calculations
Villegas et al. given for the latter in ref 9b should roughly apply here. These predict for water coadsorption an intensity attenuation factor, I& of ca. compatible with the above observation for adsorbed NO. One might expect that the degree of band intensity attenuation would be milder for saturated NO adlayers, given that the postdosed water will then be excluded from direct contact with the metal surface. Indeed, little band attenuation is seen under these circumstances, although the presence of water again yields substantial band broadening (Figure 3A).
Concluding Remarks Even though the preceding study forms only an abbreviated survey of solvent coadsorption effects on chemisorbed NO, the results uncover some interesting differences with the muchstudied Pt( 111)/CO chemisorbate system which has proven so informative for relating the infrared spectral properties of electrochemical to metal-uhv interfaces. In particular, the apparent retention of the NO binding site(s) upon solvent addition allows the coadsorbate Stark effect to be explored in a more quantitative fashion than is feasible for the Pt( 111)/CO system. As a consequence, it would be worthwhile to explore these effects on a broader front, including the examination of other solvent coadsorbates and variations in the interfacial ionic charge, as reported recently for Pt( 11l)/C0.8,9 While probably unspectacular in themselves, such studies should contribute significantly toward cementing our understanding of environmental effects upon structure and bonding at electrochemical interfaces in a fashion that can be linked directly to the simpler conventional uhv-based systems.
Acknowledgment. R.G. gratefully acknowledges the Ministry of Education and Science (Spain) and U.S.I.A. for the award of a MECmulbright Postdoctoral Fellowship. This work is supported by the National Science Foundation. References and Notes (1) For a recent review, see: Weaver, M. J.; Gao, X. Annu. Rev. Phys. Chem. 1993, 44, 459. (2) Lipkowski, J., Ross, P. N., Eds. Structure of Electr$ed Interfaces; VCH Publishers: New York, 1993. (3) For an erudite review, see: Wagner, F. T. In ref 2, Chapter 9. (4) Chang, S.-C.; Weaver, M. J. J . Phys. Chem. 1991, 95, 5391. ( 5 ) (a) Chang, S.-C.; Weaver, M. J. Surf. Sci. 1990, 238, 142. (b) Chang, S.-C.; Weaver, M. J. J . Chem. Phys. 1990, 92, 4582. (6) Villegas, I.; Weaver, M. J. J . Chem. Phys. 1994, 101, 1648. (7) (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. (8) (a) Kizhakevariam, N.; Jiang, X.; Weaver, M. J. J . Chem. Phys. 1994, 100, 6750. (b) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. J . Phys. Chem. 1995, 99, 7677. (9) (a) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Surf. Sci. 1995, 336, 37. (b) Kizhakevariam, N.; Villegas, I.; Weaver, M. J. Langmuir 1995, I I, 2777. (10) (a) Villegas, I.; Kizhakevariam, N.; Weaver, M. J. Surf. Sci. 1995, 335, 300. (b) Villegas, I.; Weaver, M. J. J . Chem. Phys. 1995, 103, 2295. ( c ) Villegas, I.; Weaver, M. J. Electrochim. Acta, in press. Manuscripts in preparation. (11) (a) Rodes, A.; Gbmez, R.; Orts, J. M.; Feliu, J. M.; Ptrez, J. M.; Aldaz, A. Langmuir, in press. (b) Rodes, A,; Gbmez, R.; Ptrez, J. M.; Feliu, J. M.; Aldaz, A. Electrochim. Acta, in press. (c) Gbmez, R.; Rodes, A.; Orts, J. M.; Feliu, J. M.; PCrez, J. M. Surf: Sci., submitted. (12) Clavilier, J.; Faure, R.; Guinet, G.; Durand, R. J . Electroanal. Chem. 1980, 107, 205. ( 1 3 ) Rotemund, H. H.; Jakubith, S.; Kubala, S.; von Oertzen, A,; Ertl, G. J . Electron Spectrosc. Relai. Phenom. 1990, 52, 8 11. (14) Kiskinova, M.; Pirug, G.; Bonzel, H. P. Surf. Sci. 1984, 136, 285. (15) (a) Hayden, B. E. Surf. Sci. 1983, 131, 419. (b) Dunn, D. S.; Severson, M. W.; Hylden, J. L.; Overend, J. J . Catal. 1982, 78, 225. (c) Agrawal, V. K.; Trenary, M. Surf. Sci. 1991, 259, 116. (16) (a) Gland, J. L.; Sexton, B. A. Surf. Sci. 1980,94,355. (b) Bamam, M. E.; Koel, B. E.; Carter, E. A. Surf. Sci. 1989, 219, 467. (17) (a) Asensio, M. C.; Woodruff, D. P.; Robinson, A. W.; Schindler, K.-M.; Gardner, P.; Ricken, D.; Bridshaw, A. M.; Conesa, J. C.; Gonzhlez-
J. Phys. Chem., Vol. 99, No. 40, 1995 14839
Nitric Oxide as an Electrochemical Probe Adsorbate Elipe, A. R. Chem. Phys. Len. 1992,192,259. (b) Aminpirooz, S.; Schmalz, A.; Becker, L.; Haase, J. Phys. Rev. B 1992,45,6337. (c) Asensio, M. C.; Woodruff, D. P.; Robinson, A. W.; Schindler, K.-M.; Bradshaw, A. M.; Conesa, J. C.; Gonzfilez-Elipe,A. R. J. Vac. Sci. Technol. A 1992,10,2445. (18) Materer, N.; Barbieri, A.; Gardin, D.; Starke, U.; Batteas, J. D.; Van Hove, M. A.; Somorjai, G. A. Surf. Sci. 1994,303,319;Phys. Rev. B 1993, 48, 2859. (19) Schindler, K.-M.; Hofmann, Ph.; Weiss, K.-U.; Gardner, P.; Fritzsche, V.; Bradshaw, A. M.; Woodruff, D. P.; Davila, M. E.; Asensio, M. C.; Conesa, J. C.; Gonzaez-Elipe, A. R. J . Electron Spectrosc. Relat. Phenom. 1993, 64/65, 75. (20) Erley, W. Surf. Sci. 1988, 205, L771. 121) Severson. M. W.: Stuhlmann. C.: Villeeas. I.: Weaver. M. J. J . Chem.'Phys., in press. 122) Mehandru, S. P.: Anderson, A. B. J. Phvs. Chem. 1989, 93, 2044. (23) Comgan, D. S.; Leung, L.-W. H.; Weaver, M. J. Anal. Chem. 1987, 59, 2252. (24) Corrigan, D. S.; Weaver, M. J. J . Electroanal. Chem. 1988, 241, 143. (25) For example, see: (a) Lambert, D. K. J. Chem. Phys. 1988, 89, 3847. (b) Weaver, M. J. Appl. SurJ Sci. 1993, 67, 147. I
(26) Trasatti, S. J . Electroanal. Chem. 1983, 150, 1; 1982, 139, 1. (27) For example, see: (a) Hansen, W. N.; Hansen, G. J. ACS Symp. Ser. 1988, No. 378, 166. (b) Trasatti, S. Electrochim. Acta 1991,36, 1659. (28) Samec, Z.; Johnson, B. W.; Doblhofer, K. Surf. Sci. 1992, 264, 440. (29) (a) Persson, B. N. J.; Ryberg, R. Phys. Rev. B 1981,24,6954. (b) Persson, B. N. J.; Liebsch, A. Surf. Sci. 1981, 110, 356. (30) Luo, J. S.; Tobin, R. G.; Lambert, D. K. Chem. Phys. Lett. 1993, 204, 445. (31) In ref 15a. both the vibrational and electronic polarizabilities for NO are estimated to be similar to those for CO on P t ( l l l ) , although a smaller value of a, for NO, 0.075 A3, compared to CO (ca. 0.2 A3), is deduced in ref 32 from band intensities on Ni( 111). If the latter is correct, the estimated extent of water-induced screening upon the YNO band intensity would be somewhat smaller than the ca. twofold value obtained for CO on Pt( 111) in ref 9b. (32) Erley, W.; Persson, B. N. J. Surf. Sci. 1989, 218, 494.
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