Langmuir 1996,11,3549-3553
3549
In Situ FTIR Spectroscopy Characterization of the NO Adlayers Formed at Platinum Single Crystal Electrodes in Contact with Acidic Solutions of Nitrite A. Rodes," R. Gbmez, J. M. Orts, J. M. Feliu, J. M. Perez, and A. Aldaz Departament de Quimica Fisica, Universitat d'Alacant, Apartat 99, E-03080 Alacant, Spain Received February 23, 1995. In Final Form: June 9, 1995@ Adsorbed NO has been detected by in situ FTIR spectroscopy on Pt(lOO),P t ( l l l ) ,and Pt(ll0)electrodes in contact with acidic solutions of nitrite. NO remains adsorbed on the platinum surface at potentials between 0.40 and 0.95 V, forming adlayers whose spectral properties are similar to those previously observed under ultrahigh vacuum conditions for NO dosed in the gas phase at high coverages. The N-0 stretching mode appears at around 1700and 1760cm-l for the Pt(ll1)and F't(ll0) electrodes, respectively. In the case of Pt(lOO),two different bands have been observed at 1640 and 1700 cm-'. This latter band can be related with the existence of a denser adlayer which seems to be favored at the higher potentials in the range explored. As in the case of adsorbed CO, an increase in the electrode potential causes an upward shift of the N - 0 stretching frequency of the adsorbed molecule. This shift is less important in the case of the Pt(ll0) surface. NzO has been detected as an intermediate during nitrous acid reduction. Oxidation to nitrate takes place at potentials above 1.10 V.
Introduction The electrochemical reduction of nitrite a t polycrystalline platinum electrodes has been the subject of different papers reporting results obtained in a ~ i d l or - ~alkaline4 media. The extension of these studies to well-defined platinum single-crystal electrodes has only been undertaken In this way, the reduction of nitrite in basic solutions was found to be structure sensitive, giving rise to different voltammetric profiles and reduction products for each basal orientation of p l a t i n ~ m .Inci~ dentally, the existence of irreversibly adsorbed species was put in evidence for Pt(111)6 and Pt(100) surfaces' in contact with acid solutions. In both cases, nitric oxide was thought to be the actual ad~orbate.~,' A confirmation of this proposal would be of great importance from both fundamental and applied points ofview. Firstly, it would be proved that nitric oxide adlayers can be easily generated a t the platinum electrode surface. Secondly, this would constitute the first stage in assessing whether the NO adsorbate plays a role in the electrochemical behavior of small nitrogen compounds. There are no detailed studies concerning the characterization of adsorbed nitric oxide or related species under electrochemical condition^.^^^ This could be achieved by using in situ infrared spectroscopy techniques, which have been proved to be very effective in providing information about the structure and composition of the metaVsolution interface a t a molecular level. The aim of this paper is the application of in situ FTIR spectroscopy to the identification of the adsorbed species formed a t platinum Abstract published inAdvance ACSAbstracts, August 1,1995. (1)Schmid, G.; Lobeck, M. A. Ber. Bunsenges. Phys. Chem. 1969,73,
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189. (2)Gadde, R. R.; Bruckenstein, S. J.Electroanal. Chem. 1974,50, 163. (3)Nishimura, K.;Machida, K.; Enyo, M. Electrochim. Acta 1991, 36,877. (4) Wasmus, S.; Vasini, E. J.; Krausa, M.; Mishima, H. T.; Vielstich, W.Electrochim. Acta 1994,39,23. (5) Ye, S.; Hattori, H.; Kita, H. Ber. Bunsenges. Phys. Chem. 1992, 96,1884. ( 6 )Ye, S.; Kita, H. J.Electroanal. Chem. 1993,346,489. (7)Rodes, A.; G6mez, R.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1993,359,315. (8)Solomun, T. J. Electroanul. Chem. 1986,199,443. (9)Wilke, T.; Gao, X.; Takoudis, G.; Weaver, M. J. Langmuir 1991, 7,714.
single-crystal electrodes in contact with nitrous acid solutions. Even if the determination of the adsorption site symmetry based only on N-0 stretching frequencies could be considered just as indicative, comparison of the spectra obtained a t the electrodelelectrolyte interface with those reported under ultrahigh vacuum conditions can be a fruitful approach. Vibrational spectroscopy studies of NO adsorbed at well-defined surfaces under ultrahigh vacuum conditions10-18led to the conclusion that the NO molecule could be used as a probe to test surface morphology.18 The absorption band frequencies observed for NO adsorbed on platinum substrates range from 1400 to 1800 cm-I depending on both the surface orientation and coverage.10-18 High coverages favor surface overlayers, giving absorption bands at high frequencies. Several authors have interpreted these data by applying different models. A comparison of the experimental spectra with those of inorganic nitrosyl complexes19leads to a multiple-adsorption-site model where each band is assigned to a particular adsorption site.17 In this way, bands at the lower frequencies (between 1400 and 1500 cm-l for Pt(lll)14J7 and around 1630 cm-l for Pt(110)lZ) have been ascribed to NO adsorbed on bridge sites. The absorption bands at the higher frequencies (around 1700 cm-l for Pt(lll)14J7 and 1760 cm-I for Pt(110)12)have been assigned to linearly bound terminal NO. Within this model, the single band which appears a t around 1640 cm-l for a (1x 1)Pt(100) surface can be related to the NO molecule adsorbed at a n on-top site in a bent configuration.1°J6 This configuration, first observed in nitrosyl c o m p l e ~ e sseems , ~ ~ to be favored by the 4-fold symmetry of the substrate.20 (10)Pirug, G.; Bonzel, H. P.; Hopster, H.; Ibach, H. J.Chem. Ph.ys. 1979,71,553. (11)Ibach, H.; Lewhald, S. Surf. Sci. 1978,76, 1. (12)Gorte, R. J.;Gland, J. L. Su$. Sci. 1981,102,348. (13)DUM. D.S.:Severson, M. W.: Hvlden, J. L.;Overend, J.J.Catal. 1982,78,225. (14)Hayden, B. E. Surf. Sci. 1983,131,419. (15)Gardner, P.;Tiishaus, M.; Martin, R.; Bradshaw, A. M. Su$. Sci. 1990,240,112. (16) Gardner, P.; Tiishaus, M.; Martin, R.; Bradshaw, A. M. Vacuum 1990,41,304. (17)Agrawal, V. K.;Trenary, M. Surf. Sci. 1991,259,116. (18)Hollins, P.Su$. Sci. Rep. 1992, 16, 5 1 and references cited therein.
0743-746319512411-3549$09.00/0 0 1995 American Chemical Society
Rodes et al.
3550 Langmuir, Vol. 11, No. 9, 1995 Other models do not necessarily imply changes in the mode of bonding of adsorbed NO as its coverage varies. In this way, the formation of NO dimers has been invoked to explain the absorption bands a t high coverages.l' Finally, Dunn et al.13 suggested that the coveragedependent shifts in the spectrum of NO adsorbed on platinum were due to strong short-range interactions between adjacent NO molecules. This suggestion is in agreement with recent scanned energy mode photoelectron diffraction experiments which pointed that a common adsorption site exists in the whole coverage range for the N O M ( 111)21 system despite the observation of distinct N-0 vibration frequencies.
Experimental Section Platinum single-crystal electrodes were prepared following the method developed by Clavilier et a1.22 Their diameter was about 2 mm for samples used in the voltammetric experiments, while those employed for in situ FTIR experiments were about 4.5mm in diameter. In all cases, clean and well-orderedsurfaces were obtained after flame annealing and subsequent cooling in an oxygen-free atmo~phere.~~ Test solutions were 0.1 M HC104. They were prepared from concentrated acid (Merck Suprapur) and ultrapure water from a MilliporeMilli-Qsystem. Prior to each experiment, the working solution was deaerated by bubbling Ar (N50). After checking the cleanliness of the system, KNOZ (Merck Pro Analysi) was added up to reach the desired concentration (between 1and 20 mM). Potentials were measured against a reversible hydrogen electrode. Spectroelectrochemical experiments were performed with a Nicolet 5PC spectrometer equipped with a MCT detector. The spectroelectrochemicalcell, which has been described in detail elsewhere,24was provided with a CaFz window beveled at 60". The optical setup allowed the experiments to be performed with the cell placed out of the IR chamber in a vertical configuration. This facilitates the voltammetric controlofthe system both before and after the collection of the spectra. Unless otherwise stated, p-polarized light was used. Spectra were collected with a resolution of 8 cm-l, and they are presented as the ratio RIRo where R and Ro represent the reflectance spectra at the sample and reference potentials, respectively.
Results and Discussion The voltammograms in Figures 1-3 show the electrochemical behavior of &(loo), Pt(lll), and P t ( l l 0 ) electrodes in a 1mMKNOz 0.1 M HClOl solution. Reduction currents can be observed in all cases for potentials below 0.6-0.7 V, giving rise to well-marked maxima between 0.1 and 0.2 V. These currents are mainly related to the irreversible reduction of nitrous acid in solution, which leads to different products depending on the electrode ~ o t e n t i a l . l -In ~ this way, NO was identified as the first reduction product for potentials between 1.0 and 0.6 V.293 N2O appeared between 0.8 and 0.2 V,293while N2 was detected by Nishimura et al.3 between 0.5 and 0.1 V. At lower potentials, ammonium and hydroxylamine are the main reduction products.1,2 The reduction of the adsorbed species coming from nitrous acid also takes place during the negative-going
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(19)Nakamoto, K.Infrared and Raman Spectra of Inorganic and Coordination Compounds; John Wiley & Sons: New York, 1986. (20)Albert, M.R.; Yates, J . T., Jr. The Surface Scientist's Guide to Organometallic Chemistry; American Chemical Society: Washington, DC, 1987. (21)h e n s i o , M.C.; Woodruff, D. P.; Robinson, A. W.; Schindler, K. M.; Gardner, P.; Ricken, D.; Bradshaw, A. M.; Conesa, J. C.; GonzalezElipe, A. R. J. Vac. Sci. Techml., A 1992, 10, 2445. (22)Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986,205,267. (23)Rodes, A.; Zamakhchari, M. A.; El Achi, K.; Clavilier, J. J. Electroanal. Chem. 1991,305, 115. (24)Iwasita, T.;Nart, F. C.; Vielstich, W. Ber.Bunsenges. Phys. Chem. 1990,94, 1030.
gL 1121E"
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b
w 2200
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wavenumber/cm-' Figure 1. (A, top) Voltammogram obtained in a 1mM KNOz + 0.1 M HClO4 solution with a Pt(100)electrode. Sweep rate = 10 mV s-l (the same for all voltammograms). (B, bottom) Potential difference spectra obtained in a 2 mM KNOz 0.1 M HClO4 solution with a Pt(100) electrode. Sample potential = 0.95 V; reference potential = 0.65 V; 2000 interferograms were collected at each potential with the electrode potential being switched every 200 scans: (a)p-polarized radiation; (b) s-polarized radiation.
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sweep of voltammograms shown in Figures 1-3. This adsorbed species has been isolated in the case of Pt(100),7 and its reductive stripping shown to take place in a sharp peakaround0.2V. InthecaseofPt(lll)andPt(llO), the reduction of the adsorbed species starts at higher potentials coinciding with the broad reduction peaks observed between 0.4 and 0.2 V in Figures 2 and 3.25 The presence of adsorbed species at the Pt(ll1) surface is also related with the couple of peaks observed between 0.75 and 1.05 V. These peaks were ascribed by Ye and Kitaa to the existence of a HNOdNO surface redox couple. At potentials higher than 1.05 V, nitrous acid oxidizes to NO3-.' The results reported in refs 6 and 7 have demonstrated the formation of irreversibly adsorbed species when Pt(ll1) and Pt(100) electrode surfaces are put in contact with acidic nitrite solutions. These species are reductively stripped below 0.4 V. They can also be oxidized, albeit remaining adsorbed between 0.95 and 1.10 V. In both cases, adsorbed NO has been proposed to be the stable (25)G6mez, R.;Rodes, A.; Orts, J. M.; PBrez, J. M.; Feliu, J. M. Surf Sci. Lett., submitted.
NO Adlayers Formed at Platinum Single Crystal Electrodes
Langmuir, Vol. 11, No. 9, 1995 3551
1780
a
1707 o m '
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2200
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Figure 2. Voltammogram and potential difference spectrum obtained with a Pt(ll1) electrode in a 1 mM KNOz + 0.1 M HClO4 solution. Sameconditionsas in Figure 1. The spectrum was obtained with p-polarized light.
adsorbed species in the potential range between 0.5 and 0.95 V.6s7 This potential range coincides with a nearly flat region in the voltammograms in Figures 1-3. This would allow the detection of adsorbed NO by a n in situ spectroscopic technique in the presence of nitrous acid and without any strong interference from solution processes. In situ IR spectroscopy experiments were carried out in 2 mM KNO2 0.1 M HClOd solutions. After recording the voltammetric response of the flame-annealed electrode between 0.65 and 0.95 V,the electrode surface was pushed against the window. Spectra were collected alternatively at 0.65 and 0.95 V and then coadded in order to cancel any time-dependent baseline shifi. Spectrum a in Figure 1 is the result of referring the single-beam spectrum obtained at 0.95 V (sample potential) to that collected at 0.65 V (reference potential) for a R(100) electrode. A negative-going band appears at 1697 cm-' together with a bipolar band centered at 1635cm-'. These features are absent in the spectrum obtained under the same experimental conditions with s-polarized light (spectrum b), thus proving that they are unambiguously related to the presence of adsorbed species. The frequencies reported above are typical of N-0 stretching for NO adsorbed at R(100) as deduced from gas-phase experiments.1°J6J6 For temperatures between 90 and 300 K,a n absorption band a t 1641cm-l is characteristic ofthe ~ ( 4 x 2 )
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Figure 3. Voltammogram (A, top) and spectra (B,bottom) obtained with a Pt(ll0) electrode in a 1 mM KN02 0.1 M HC104 solution. (a) Potential difference spectrum obtained under the same experimental conditions as those in Figures 1 and 2: p-polarizedlight. (b)Spectrumobtained at 0.8V(samp1e potential) referred to that obtained at 0.1 V (referencepotential): p-polarized light; 200 interferograms collected at each potential. (c) The same as spectrum b but with s-polarized light.
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NO overlayer (0 = 0.5) formed on a (1x1) R(100) substrate.15 At higher coverages, which may be reached at temperatures below 300 K, a disordered phase characterized by a NO stretching frequency of 1680 cm-' is formed.15 These two characteristic bands can be related to those observed in the spectrum in Figure 1. The bipolar shape of the band centered a t 1635cm-' can be understood as a n effect of the electrode potential on the vibration frequency for NO adsorbate which remains adsorbed a t both the reference and sample potentials. The appearance of the band a t 1697 cm-l implies the existence of a denser phase which is preferentially formed at the higher potentials in the range explored (see below). The agreement between the spectral features observed for nitric oxide adsorbed on a P t ( l O O ) ( l x 1)surface in ultrahigh vacuum and those reported here for the well-ordered R(100)/electrolyte interface might be expected since the latter has been shown to be unreconstructed throughout the potential range used in this work.26 In addition, it is (26) "idswell, I. M.; MarkoviC, N. M.; Ross, P.N. Phys. Rev. Lett. 1993, 71,1601.
Rodes et al.
3552 Langmuir, Vol. 11, No. 9, 1995 well-known that the adsorption of nitric oxide above 210 K lifts the hexagonal reconstruction characteristic of the clean Pt( 100) surface under ultrahigh vacuum condition~.'~J~ The spectrum obtained with a Pt(ll1)electrode in the same solution and under the same polarization conditions as that for Pt(100) is shown in Figure 2. In this case, a bipolar band appears around 1700 cm-'. This frequency range is that for the N - 0 stretching band observed a t high NO coverages with Pt(ll1)~ u r f a c e s . ~ ~ JAs ~ Jin~ J ~ the case of Pt(lOO),the bipolar shape of the band results from the existence of an effect of the electrode potential on the band frequency. The absence of bands between 1400 and 1500 cm-', characteristic of NO adsorbed at low coverage^,^^,^^^^^," indicates that the coverage attained in the experiment reported in Figure 2 is close to that corresponding to surface saturation. In contrast with the spectra obtained for Pt(ll1)and Pt(1001, no clearcut bands appear in the potential difference spectra obtained for R ( l l 0 )(spectrum a in Figure 3). Only a broad feature can be observed around 1800 cm-l. NO adsorbed on P t ( l l 0 ) gives rise to absorption bands at 1760 and 1630 cm-' which have been assigned to the presence of linear and multibonded NO, respectively.12 The band a t higher wavenumbers predominates at coverages near saturation.12 Two hypotheses could explain the absence of these bands in the potential difference spectrum shown in Figure 3: either NO is not adsorbed under potentiostatic conditions on the P t ( l l 0 ) electrode or the potential-dependent shift of the N - 0 stretching band for adsorbed NO is not high enough to result in a distinguishable band in the potential difference spectra. The latter would be favored if bands are broad. To elucidate between both possibilities, the spectrum obtained a t a given potential where NO is adsorbed should be referred to that of the bare surface. As mentioned above, the electrode is expected to be free of adsorbed species other than adsorbed hydrogen for potentials around 0.1 V. This potential has been chosen to collect the reference spectra in the presence of nitrous acid. In the experiments correspondingto spectra b and c in Figure 3, the flame-treated P t ( l l 0 ) electrode was immersed in the test solution a t 0.8 V and a spectrum was recorded a t this potential. Then, the electrode potential was stepped at 0.1 V to collect the reference spectrum. A broad band centered a t 1760 cm-' appears in the spectrum obtained with p-polarized light (spectrum b) which is not observed if the radiation is s-polarized (spectrum c). This fact indicates that the absorption band in spectrum b is related to an adsorbed species. The coincidence between the frequency of this band and that previously reported under ultrahigh vacuum conditions for NO-saturated P t ( l l 0 ) surfaced2 suggests that this adsorbate exists in the Pt( 1lO)/electrode interface a t relatively high coverage. The results described above have allowed the identification of adsorbed NO as the species formed on the platinum single-crystal electrodes when immersed in a nitrous acid solution. In situ FTIR experiments have also been performed in order to monitor the electrode reactions taking place in different potential regions when HNOz is present in the working solution. In these experiments, the electrode was immersed a t 0.8 V in a 20 mM KNOZ + 0.1 M HC104 solution. This concentration was chosen in order to facilitate the detection of intermediate species. After collecting a reference spectrum, the electrode potential was stepped either to lower or higher potentials a t which the corresponding sample spectra were recorded. The spectra reported in Figure 4 are those obtained in the case of the Pt(100) electrode. Some interesting features are observed when the electrode potential is lowered down
0.70 V
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Figure 4. (A, top) Spectra obtained with a Pt(100) electrode at different potentials below 0.8 V in a 20 mM KNOz 0.1 M HC104 solution. Potentials refer to the sample potential. Reference potential = 0.8 V, 200 interferograms collected at each potential with p-polarized light. (B,bottom) Spectra obtained with a Pt(100)electrode at different potentials above 0.8 V in a 20 mM KNOz 0.1 M HClO4 solution. Potentials refer to the sample potential. Reference potential = 0.8 V, 200 interferogramscollected at each potential with p-polarized light. to 0.1 V (Figure 4A). First, a well-marked positive band appears a t around 1700 cm-' for all the spectra referred to that obtained a t 0.8 V. The frequency of this band is similar to that of the high-frequency band observed in the spectrum shown in Figure 1. As in this latter case, it is possible to relate this band with the high-coveragefeature observed under ultrahigh vacuum.15 The band a t 1700 cm-' seems to be predominant at 0.8 V under the present experimentalconditions. Changing the electrodepotential toward less positive values causes the appearance of the band characteristic of the ordered NO adlayer a t around 1630 cm-'. This band shifts t o lower frequencies as the electrode potential decreases,while its intensity increases suddenly a t 0.5 V. This latter fact indicates the formation of an additional amount of adsorbed NO coming from the reduction of nitrous acid at this potential. No bands for nitric oxide in solution, which should appear a t 1880 have been observed. This fact indicatesthat, under the present experimental conditions, the formation of NO from nitrous acid is a process restricted to the electrode
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Langmuir, Vol. 11, No. 9, 1995 3553
NO Adlayers Formed at Platinum Single Crystal Electrode'S
denser, less ordered adlayer, observed in ultrahigh vacuum.lS Under our experimental conditions, the formation of this structure for the NO adlayer is potentialdependent and might be favored by the presence ofnitrous acid in the test solution. Anyway,the multiple-adsorptionsite model, which would imply the existence of NO adsorbed on on-top sites with either a linear or a bent configuration, cannot be ruled out. Within the same model, linearly bonded NO would be the species giving rise to the absorption bands observed with the Pt(ll1) and P t ( l l 0 ) electrodes. InthecaseofbothPt(l11)andPt(100),thebipolar shape ofthe absorption bands in the potential difference spectra clearly reflects a n effect of the electrode potential on the frequency of the N-0 stretching mode, which is qualitatively similar to that observed in the case of adsorbed C0.27 The same explanation, i.e., either a n Stark effect or a back-bonding mechanism, could be invoked in both cases. It is difficult to discern a t this stage between these two possibilities, but it is clear that the existence of one electron in the 2n* orbital of the free NO molecule should make more severe the effect of back-donation from the electrode surface than in the case of adsorbed CO. In this way, back-donation can be a t the origin of the substantial downward shift of the N-0 stretching frequency observed upon NO adsorption.lo-l8 A n accurate quantitative determination of the potential-induced shift of the N-0 stretching frequency requires the isolation of the adsorbed species in order to eliminate any effect (changes in surface coverage) coming from the presence of nitrous acid in the electrolyte. Work is in progress to accomplish this requirement. It is noteworthy that the effect ofthe electrode potential on the N-0 stretching frequency is much lower for NO adsorbed on the P t ( l l 0 ) electrode than in the case of Pt(ll1)and Pt(100). It is also interesting to remark the broadness of the absorption band observed for the NOcovered Pt(110) surface. Inhomogeneous broadening2' caused by a lack of order in the NO adlayer could be a t the origin of this observation. There is no voltammetric evidence indicating that the underlying Pt(110)substrate BforPt(100)werealsoperformedwithPt(lll)andPt(llO). could be disturbed upon nitric oxide a d ~ o r p t i o n . ~ ~ The formation of nitrate anions at potentials higher than Finally, the spectroscopic investigation reported in this 1.0V was always observed, as in Figure 4A,but the amount work has confirmed some aspects of both the oxidation of NzO formed during the rediction of nitrous acid seems and reduction reactions of nitrous acid previously shown to be below the detection limit under the present experiby various authors. In this way, N20 has been detected mental conditions for Pt(ll1)and Pt(ll0). The spectra as a n intermediate during the reduction of nitrous acid obtained for these surfaces at potentials where Nod. was between 0.6 and 0.4 V on Pt(100). At potentials above stable showed only the potential dependence of the bands ataround 1760cm-lforPt(llO) and 1 7 0 0 ~ m - ~ f o r P t ( l l l ) . 1.10 V, both adsorbed NO and nitrous acid in solution oxidize to nitrate. Conclusions Acknowledgment. The financial support of the The main conclusion derived from this work is that NO,& DGICYT through Contracts PB93-0944 and UE94-003 1 is formed on the three basal planes of platinum from acidic is gratefully acknowledged. We thank the Conselleria solutions of nitrite. For each electrode surface, the in situ d'Educaci6 i C i h c i a de la Generalitat Valenciana for the FTIR spectra show bands whose frequencies are similar funds for the purchase of the FTIR facility. Dr.T. Iwasita to those obtained under ultrahigh vacuum conditions for is acknowledged for help and advice in the improvement NO adsorbed a t high coverages.1°-18 NO remains adsorbed of the optical setup used in the FTIR experiments. at the platinudacid solution interface with a relatively high surface coverage for potentials between 0.4 and 0.95 LA9501360 V. In the case of Pt(100) and in addition to the absorption band at 1640 cm-l, we have detected a high-frequency (27) Chang, S.C.; Weaver, M. J. Surf. Sci. 1990,238,142. (28) Hoftinann, F.M. Sufi. Sci. Rep. 1983,3,107. band at around 1700 cm-l which may be related with a surface and, thereby, the amount of dissolved NO is negligible. Between 0.6 and 0.5 V, a new band develops a t 2227 cm-', which is again absent in the spectra corresponding to potentials lower than 0.3 V. This band can be ascribed to the N-N stretching of N2019 and indicates the formation of this species as a n intermediate in the overall reduction process of nitrous acid. The N-0 stretching of N20, which should appear a t around 1300 cm-l,19 has not been clearly observed probably because its intensity is below the detection limit under the present experimental conditions. In any case, the detection of NzO as a n intermediate during the electrochemical reduction of nitrous acid is in agreement with the results previously reported for polycrystalline platinum elect r o d e ~ . ~Finally, ,~ it is noteworthy that the spectrum obtained at 0.1 V does not show any band corresponding to either adsorbed or solution species. Unfortunately, we were not able to detect the final reduction products (probably ammonium). Figure 4B shows the spectra obtained with Pt(100) as the electrode potential was stepped from 0.8 to 1.2 V in the 20 mM KNOz 0.1 M HC104 solution. All the spectra for potentials equal or higher than 0.9 V show the same positive band a t 1693 cm-l which appeared also in Figure 4A. This band corresponds to the NO adlayer formed a t 0.8 V (reference potential) which disappears at higher potentials. Partial oxidation of this adlayer could explain the appearance of the negative band at 1653 cm-l in the spectrum obtained a t 1.0 V inasmuch as this lower frequency feature seems to be associated to a less compact NO a d 1 a ~ e r . lAt ~ 1.10 V, no adsorbed species could be detected, whereas a broad negative band centered at 1370 cm-l appears. The intensity of this feature increases as the electrode potential does, whereas its frequency fits with one of the N - 0 stretching modes of free nitrate anions.lg This agreement suggests that nitrate anions are mainly formed when nitrous acid oxidizes. The oxidation of adsorbed nitric oxide is believed to lead to the same final product. Experiments similar to those reported in Figure 4A and
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