Spectroscopic Study of the Nitric Oxide Adlayers Formed from Nitrous

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Spectroscopic Study of the Nitric Oxide Adlayers Formed from Nitrous Acid Solutions on Palladium-Covered Platinum Single-Crystal Electrodes B. A Ä lvarez, A. Rodes,*,† J. M. Pe´rez, and J. M. Feliu Departamento de Quı´mica Fı´sica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain

J. L. Rodrı´guez and E. Pastor Departamento de Quı´mica Fisica, Universidad de La Laguna, E-38071 La Laguna, Tenerife, Spain Received November 9, 1999. In Final Form: February 8, 2000 Palladium multilayers deposited on Pt(111), Pt(100), and Pt(110) single-crystal electrodes have been characterized by using adsorbed nitric oxide as a probe molecule. Nitric oxide adlayers can be easily formed on the palladium-covered electrodes from nitrous acid or NO-saturated solutions and are stable under electrochemical conditions in a wide range of electrode potentials. The in-situ FTIR spectra obtained for the saturated adlayers have been compared with those previously reported in the N-O stretching region for NO adsorbed on bulk palladium single-crystal surfaces both under UHV and electrochemical conditions. A good agreement has been found in all cases regarding the frequency of the bands observed. Changes in the spectra upon partial reduction of the NO adlayer also fits with coverage-dependent changes previously reported. On the basis of all these structure-sensitive characteristics of the vibrational spectra of adsorbed NO, we can conclude that palladium layers are grown epitaxially on the platinum substrate. Thus, the resulting palladium-covered electrodes seem to be a good alternative for the study of any structuredependent process on palladium. The combination of in-situ FTIR and on-line DEMS experiments reported in this paper has provided additional data on the electrochemical behavior of the palladium-covered electrodes in the nitrous acid solution. Nitric oxide, nitrous oxide, and ammonium have been shown to be the main reduction products formed during the reduction of nitrous acid at the palladium electrode surface. Nitrate and hiponitrite anions seem to be also formed from nitrous acid at 0.90 V. It has also been shown that dissolved ammonium is the only product formed during the reductive stripping of adsorbed NO.

Introduction Palladium adlayers with coverages ranging from (sub)monolayers to multilayers can be easily deposited on platinum1-9 and gold10,11 single-crystal substrates with a variety of methods including electrochemical deposition,4,10,11 vapor vacuum deposition,5,6 and the so-called forced deposition method,2,3,7-9 which involves the reduction of dissolved palladium ions with hydrogen. The characterization of the resulting adlayers is a key point for the use of the platinum-covered electrodes as an alternative to bulk palladium single crystal electrodes. The latter are difficult to handle in electrochemical experiments when compared with other metals such as platinum, rhodium, iridium, or gold. First, palladium electrodes cannot be treated by using the flame treatment † Fax: 34.965903537. Phone: 34.965903400 (ext. 2602). E-mail: [email protected].

(1) Attard, G. A.; Bannister, A. J. Electroanal. Chem. 1991, 300, 467. (2) Clavilier, J.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1991, 310, 429. (3) Llorca, M. J.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1993, 351, 299. (4) Inukai, J.; Ito, M. J. Electroanal. Chem. 1993, 358, 307. (5) Attard, G. A.; Price, R.; Al-Akl, A. Electrochim. Acta 1994, 39, 1525. (6) Attard, G. A.; Price, R. Surf. Sci. 1995, 335, 63. (7) Go´mez, R.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. Surf. Sci. 1995, 327, 202. (8) Go´mez, R.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. Surf. Sci. 1995, 344, 85. (9) Go´mez, R. Doctoral Thesis, Universidad de Alicante, 1994. (10) Baldauf, M.; Kolb, D. M. Electrochim. Acta 1993, 38, 2145. (11) Baldauf, M.; Kolb, D. M. J. Phys. Chem. 1996, 100, 11357.

method commonly used with these metals. Ex-situ UHV preparation techniques12-15 or electropolishing16-19 seemed to be the only alternative method to obtain clean and wellordered palladium electrode surfaces. Indirect information on the surface of the palladium electrodes under electrochemical conditions was obtained from the voltammetric behavior of a structure-sensitive processes such as the underpotential deposition of copper.16-19 The same reaction has been used recently to test a new procedure to prepare clean and well-ordered palladium single-crystal electrodes that involves the resistive heating of the samples in a controlled atmosphere.20 Another problem in the use of bulk palladium electrodes comes from the occurrence of the hydrogen absorption reaction which restricts the electrode potential region where other electrochemical processes can be studied. Hydrogen absorption at palladium-covered electrodes is limited, even for palladium multilayers, to a narrow potential region just prior the onset of hydrogen evolution.3,7-9,10 As in the case of bare platinum electrodes, coupled reversible hydrogen/anion (12) Solomun, T. J. Electroanal. Chem. 1989, 261, 229. (13) Berry, G. M.; Bothwell, M. E.; Michelhaugh, S. L.; McBride, J. R.; Soriaga, M. P. J. Chim. Phys. 1991, 88, 159. (14) Soriaga, M. P. Prog. Surf. Sci. 1992, 39, 325. (15) Zei, M. S. Z. Phys. Chem. 1999, 208, 77. (16) Cherchie, T.; Mayer, C. Electrochim. Acta 1988, 33, 341. (17) Zou, S.; Go´mez, R.; Weaver, M. J. Surf. Sci. 1998, 399, 270. (18) Zou, S.; Go´mez, R.; Weaver, M. J. Langmuir 1999, 15, 2931. (19) Zou, S.; Go´mez, R.; Weaver, M. J. J. Electroanal. Chem. (submitted). (20) Cuesta, A.; Kibler, L. A.; Kolb, D. M. J. Electroanal. Chem. 1999, 466, 165.

10.1021/la991473q CCC: $19.00 © 2000 American Chemical Society Published on Web 04/21/2000

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adsorption/desorption processes gives rise to characteristic and structure-sensitive voltammetric features that can be related to the amount of deposited palladium.3,7-9,10 Different papers have been published in the last years regarding the characterization of the palladium films deposited on platinum electrodes. Attard et al. reported the characterization of vapor-deposited palladium under UHV conditions.5,6 Voltammetric profiles obtained after transferring these samples to the electrochemical cell were similar to those obtained previously by Clavilier et al.2,3 for similar palladium coverages. Other groups have tried to glean information on the palladium films deposited on platinum by using the CO molecule as a probe to test the electrode surface. Inuaki and Ito4 obtained the infrared spectra for CO adsorbed at Pt(100) and Pt(111) singlecrystal electrodes covered with (sub)monolayers of palladium deposited electrochemically. The surface structure of palladium multilayers deposited on Pt(110),7 Pt(100),8 and Pt(111)9 have also been characterized with adsorbed CO. In addition to the spectroscopic characterization of the CO-saturated adlayer, charge displacement experiments in which CO was dosed at different electrode potentials were carried out allowing the discrimination between hydrogen and anion adsorption processes.7-9 CO coverages at saturation were obtained from careful measurements of the charge density involved in their oxidative stripping and found to fit with data reported under UHV conditions for the corresponding Pd(hkl) surface.7-9 The aim of this paper is to report new data on the surface structure of palladium multilayer films deposited on platinum single electrodes by using nitric oxide as an alternative probe molecule. Nitric oxide adlayers formed on different transition metal single-crystal electrodes as platinum,21-30 rhodium,26,29-31 and iridium32,33 have been studied in the last years using a fruitful combination of voltammetric and in-situ FTIR spectroscopic experiments. More recently, the infrared study of NO adsorbed on electropolished Pd(111) and Pd(110) single-crystal electrodes has also been reported.19,30 All these data, together with those derived from the UHV studies of NO adlayers on palladium single-crystal surfaces,34-41 will be compared with the results reported in this paper. As a first approach, we have tested the possibility of forming the NO adlayers from nitrous acid solutions as previously done with (21) Ye, S.; Kita, H. J. Electroanal. Chem. 1993, 346, 489. (22) Rodes, A.; Go´mez, R.; Orts, J. M.; Feliu J. M.; Aldaz, A. J. Electroanal. Chem. 1993, 359, 315. (23) Rodes, A.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Pe´rez, J. M.; Aldaz, A. Langmuir 1995, 11, 3549. (24) Go´mez, R.; Rodes, A.; Orts, J. M.; Feliu, J. M.; Pe´rez, J. M. Surf. Sci. 1995, 342, L1104. (25) Villegas, I.; Go´mez R.; Weaver, M. J. J. Phys. Chem. 1995, 99, 14832. (26) Rodes, A.; Go´mez, R.; Pe´rez, J. M.; Feliu J. M.; Aldaz, A. Electrochim. Acta 1996, 41, 79. (27) Rodes, A.; Climent, V.; Orts, J. M.; Pe´rez; J. M.; Aldaz, A. Electrochim. Acta 1998, 44, 1077. (28) Tang, C.; Zou S.; Weaver, M. J. Surf. Sci. 1998, 412/413, 344. (29) Tang, C.; Zou, S.; Chang, S. C.; Weaver, M. J. J. Electroanal. Chem. 1999, 467, 92. (30) Weaver, M. J.; Zou; S.; Tang. C. J. Chem. Phys. 1999, 111, 368. (31) Go´mez, R.; Rodes, A.; Pe´rez , J.M.; Feliu, J. M. J. Electroanal. Chem. 1995, 393, 123. (32) Go´mez, R.; Weaver, M. J. Langmuir 1998, 14, 2525. (33) Go´mez, R., Weaver, M. J. J. Phys. Chem. B 1998, 102, 3754. (34) Bertolo, M.; Jacobi, K. Surf. Sci. 1990, 226, 207. (35) Wickham D. T.; Banse; B. A.; Koel, B. E. Surf. Sci. 1991, 243, 83. (36) Chen, P. J.; Goodman, D. W. Surf. Sci. 1993, 297, L93. (37) Ramsier, R. D.; Gao, Q.; Neergaard Waltenburg, H.; Lee, K. W.; Nooij, O. W.; Lefferts, L.; Yates, J. T. Surf. Sci. 1994, 320, 209. (38) Jorgensen, S. W.; Canning, N. D. S.; Maddix, R. J. Surf. Sci. 1987, 179, 322. (39) Nyberg, C.; Uvdal, P. Surf. Sci. 1988, 204, 517.

platinum23,26,27 and rhodium26,31 single-crystal electrodes. This has prompted us to study the electrochemical behavior of the palladium-covered electrodes in the nitrous acid solutions. For this purpose we have found useful to combine the voltammetric and in-situ FTIR measurements with on-line DEMS experiments carried out with palladium electrodes under similar experimental conditions. Despite the use of rough electrodes, DEMS experiments in the present paper provide a unique opportunity of detecting gaseous or voltatile reaction products that, as in the case of molecular nitrogen, cannot be detected in the infrared experiments. Combined DEMS and in-situ FTIR experiments have been useful to determine the stoichiometry of the surface reaction involved in the reductive stripping of the NO adlayers. Experimental Section Voltammetric and in-Situ FTIR Experiments. Platinum single-crystal electrodes with diameters about 4.5 mm were prepared by following the method developed by Clavilier.42 Clean and well-ordered surfaces were obtained by flame annealing in a gas-oxygen flame and cooling in a H2 + Ar atmosphere.43 The clean sample was then transferred to the (spectro)electrochemical cell under the protection of a droplet of ultrapure water in equilibrium with both gases. Working solutions were prepared from concentrated perchloric or sulfuric acid (Merck Suprapur) and ultrapure water (Millipore MilliQ). In some of the infrared experiments solutions were prepared in deuterium oxide (Sigma) as received. Nitrous acid solutions were obtained by adding KNO2 (Merck PA) to the acid working solution. Prior to each experiment, solutions were deaerated by bubbling Ar (L’Air Liquide N50). Electrode potentials were measured against a reversible hydrogen electrode (RHE) in the voltammetric experiments whereas a Pd/H2 electrode was used in the spectroelectrochemical experiments. All potentials are referred to the RHE scale. Palladium was deposited from 10-3 M Pd(SO)4 (in 0.5 M H2SO4) by using the so-called forced deposition method which is described in detail elsewhere.2,3 Briefly, the flame-treated platinum electrode was introduced in a H2 + Ar stream with a droplet of the Pd2+ solution attached to its flat-oriented surface. The sample was rinsed with ultrapure water and transferred to the electrochemical cell where the surface is characterized by recording its cyclic voltammogram in a 0.1 M H2SO4 solution. From the concentration of the solution employed in this work, the formation of a palladium multilayer (around 10 layers) is expected. The nitric oxide adlayers were prepared by dipping the electrode for 5 min at open circuit either in a 0.02 M KNO2 + 0.1 M HClO4 solution or in a saturated solution of NO(g) (L’Air Liquide N20) in water. Then the sample was rinsed with ultrapure water and transferred to the (spectro)electrochemical cell. Spectroelectrochemical experiments were performed with a Nicolet Magna 850 spectrometer equipped with a MCT detector. The spectroelectrochemical cell44 was provided with a prismatic CaF2 window beveled at 60°. Unless otherwise stated, the spectra were collected with p-polarized light with a resolution of 8 cm-1. They are presented as the ratio ∆R/R0 ) (R - R0)/R0, where R and R0 are the reflectance corresponding to the sample and reference single-beam spectra, respectively. On-Line DEMS Experiments. DEMS experiments were carried out in a small flow cell containing approximately 2 cm3 of the working solution. This cell was directely attached to the vacuum chamber containing the mass spectrometer (Balzers (40) Date´, M.; Okuyama, H.; Takagi, N.; Nishijima, N.; Taruga, T. Surf. Sci. 1996, 350, 79. (41) Raval, R.; Harrison, M. A.; Haq; S.; King, D. A. Surf. Sci. 1993, 294, 10. (42) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (43) Clavilier, J.; El Achi, K.; Petit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 295, 333. (44) Iwasita, T.; Nart, F. C.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 333.

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Figure 1. Cyclic voltammograms obtained in a 0.1 M H2SO4 solution for Pt(hkl) electrodes covered with a multilayer of adsorbed palladium. Sweep rate: 20 mV s-1. QMG 112) with a Faraday cup detector. Mass spectrometric cyclic voltammograms (MSCVs) were recorded simultaneously with cyclic voltammograms. In this way, gaseous and volatile species formed or consumed during electrochemical processes can be detected on line. The working electrode in the DEMS experiments was a porous palladium layer sputtered on a microporous ethylene-tetrafluoroethylene copolymer membrane (Scimat 200/40/60). The geometric area of the electrode was 0.64 cm2, and the real area, ca. 19 cm2. A palladium wire served as counter electrode, and a RHE was used as reference electrode. The working electrode was activated by potential cycling between 0.40 and 1.45 V at 0.01 V s-1. Additional experimental details are described elsewhere.45,46

Results and Discussion 1. Electrochemical Behavior of the PalladiumCovered Platinum Electrodes in the Nitrous Acid Solution. 1.1. Cyclic Voltammetry. Figure 1 show voltammetric curves characteristic of palladium-covered Pt(111), Pt(100), and Pt(110) electrodes. These curves have been described in detail in previous papers where the voltammetric response of palladium-covered platinum single-crystal electrodes with different palladium coverages was discussed.2,3,7-9 In this way, it is worth noting here the absence in the curves reported in Figure 1 of any voltammetric feature corresponding to the adsorption of hydrogen or anionic species on the underlying platinum substrate thus showing that the electrode surface is fully covered with palladium atoms. The curves reported in Figure 1 for the Pd/Pt(111) and Pd/Pt(100) surfaces do not show the dominant voltammetric features at 0.22 and 0.17 V that characterize the adsorption of hydrogen and anions on a monolayer of palladium atoms deposited on Pt(111) and Pt(100) electrodes, respectively. Instead, the reported voltammograms show peaks at 0.290 V for Pt(111) and 0.31 V for Pt(100) (both values being measured in the positive-going scan). These peaks are characteristic for the formation of multilayers of palladium atoms.3,8,9 The same conclusion can be derived from the voltammetric profile shown in Figure 1C for the palladium-covered Pt(110) electrode.3,7 The voltammetric curves shown in Figure 2 correspond to the first voltammetric cycle for the palladium-covered Pt(hkl) electrodes recorded just after the immersion at 0.90 V in a 0.02 M KNO2 + 0.1 M HClO4 solution. Reduction currents observed in these curves can be related to the (45) Pastor, E.; Castro, C.; Rodriguez, J. L.; Gonza´lez, S. J. Electroanal. Chem. 1996, 404, 77. (46) Bittins-Cattaneo, B.; Cattaneo, E.; Ko¨nigshoven, P.; Vielstich, W. In Electroanalytical Chemistry: A Series of Advances; Bard, A. J. Ed.; Marcel Dekker: New York, 1991; Vol. 17, p 181.

Figure 2. First voltammetric cycle in a 0.02 M KNO2 + 0.1 M HClO4 solution for the palladium-covered Pt(hkl) electrodes. Sweep rate: 5 mV s-1.

reduction of nitrous acid or any of the species that could be formed from nitrous acid at the immersion potential (see below). It is worth noting that noticeable oxidation currents are measured when immersing the electrode at potentials slightly more positive than 0.90 V. Even if the study of these oxidation processes is out of the scope of this paper, we can recall here that the formation of nitrate anions has been detected in this potential region in the case of bare platinum single-crystal electrodes.23 Regarding the reduction currents reported in Figure 2, it is expected that the reduction of nitrous acid takes place below 0.90 V with different reduction products being formed as the electrode potential is lowered. The formation of nitric oxide,47,48 nitrous oxide,23,47-49 nitrogen,48 and, finally, hydroxylamine47,49,50 and ammonium47,50 has been proposed to take place under similar conditions in the case of polycrystalline47-50 and single-crystal23 platinum electrodes. Techniques employed in these studies included in-situ FTIR23,49 and on-line DEMS48 as well as classical electrochemical measurements.47,50 We have to recall here the complexity of the chemistry of nitrogen compounds,51 which is reflected in their electrochemical behavior.52 In addition to the species mentioned above, other intermedi(47) Gadde, R. R.; Bruckenstein, S. J. Electroanal. Chem. 1974, 50, 163. (48) Nishimura, K.; Machida, K.; Enyo, M. Electrochim. Acta 1991, 36, 877. (49) Bae, I. T.; Barbour, R. L.; Scherson, D. A. Anal. Chem. 1997, 69, 249. (50) Schmidt, G.; Lobeck, M. Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 189. (51) Jones, K. In Comprehensive Inorganic Chemistry; Bailar, J. C., Emele´us, H. J., Nyholm R., Trotman-Dickenson, A. F., Eds.; Pergamon Press: Oxford, U.K., 1973; Vol. 2, Chapter 19. . (52) Plieth, W. J. In Encyclopedia of the Electrochemistry of the Elements; Bard, A. J., Ed.; Marcel Dekker: New York, 1978; Vol. 8, Chapter 5.

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Figure 3. Series of FTIR spectra collected in a 0.02 M KNO2 + 0.1 M HClO4 solution in D2O for the Pd/Pt(111) electrode during a potential sweep at 5 mV s-1 from 0.90 V (immersion potential) down to 0 V (see text for details).

ate products as hyponitrous acid (or its anion), nitrate anions, and different nitrogen oxides are involved in different chemical and electrochemical equilibria.52 Another point to be considered in this study is the presence of adsorbates at the electrode surface. Adsorbed nitric oxide was detected at platinum and rhodium single-crystal electrodes surface in contact with the nitrous acid solution by in-situ FTIR experiments.23,26,31 1.2. In-Situ FTIR Experiments. To ascertain the nature of both dissolved and adsorbed species formed in the nitrous acid solution at each electrode potential we have carried out parallel in-situ FTIR experiments. Spectra were collected during a slow potential sweep at 5 mV s-1 from 0.90 V (the immersion potential) down to 0 V. It must be kept in mind that fitting the electrode surface against the CaF2 window takes approximately 2 min including the time needed to be sure that a constant baseline is reached before triggering the potential scan. Spectra were obtained by coadding 52 consecutive interferograms which needed 5 s to be collected. Thus the resulting spectrum represents an average of the metal/ eletrolyte interface on a 25 mV interval. Finally, each spectrum was referred to that collected at the immersion potential. Figures 3-5 show some of the spectra obtained under these conditions with the palladium-covered Pt(111), Pt(100), and Pt(110) electrodes, respectively. These experiments were performed in D2O solutions in order to avoid interferences from the O-H bending band which usually obscures the spectral region around 1600 cm-1. The spectra reported in Figures 3-5 show some common features independently of the substrate orientation. Positive-going bands (corresponding to species present at the reference potential and being consumed as the electrode potential is lowered) appear at around 1440 and 1340 cm-1. On the other hand negative-going bands (corresponding to the species formed at the corresponding sample potential) are observed at 2228 and 1288 cm-1.

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Figure 4. As in Figure 3 for the Pd/Pt(100) electrode.

Figure 5. As in Figure 3 for the Pd/Pt(110) electrode.

All these bands were observed irrespective of the polarization of the infrared beam, thus corresponding mainly to vibrational modes of dissolved species. The same bands were also observed in experiments in which solutions were prepared in H2O. Additional positive- and negative-going bands are observed in Figures 3-5 in the spectral region between 1800 and 1450 cm-1. These bands appear at characteristic frequencies for each electrode orientation and cannot observed if the spectra are collected with s-polarized light. In addition, the negative-going bands observed in this region are red-shifted as the electrode


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potential becomes less positive. All these features are typical for adsorbed species. The appearance of the adsorbate bands in the same N-O stretching region as nitrosyl complexes53 strongly suggests that they correspond to nitric oxide adsorbed on the palladium layer. Adsorbed NO can originate from either the decomposition or the partial reduction of nitrous acid. In the next section we will discuss the details concerning the adsorbate bands in Figures 3-5 together with new results obtained in experiments where the NO adlayers where isolated and characterized in a nitrous acid-free solution. Now we will focus on the origin of the solution bands presented above. As stated before, bands appearing at ca. 1440 and 1340 cm-1 correspond to species present at the thin layer between the window and the electrode surface at the immersion potential (0.90 V). From the composition of the solution one could expect bands in the spectra coming from nitrous acid, nitrite anions, or any product formed from these species at 0.90 V. Main bands for nitrous acid should appear at around 1670 and 1275 cm-1 for the νNO and δOH modes, respectively.53 The latter should show a strong isotopic efect in the D2O solution due to the exchange of hydrogen by deuterium atoms which is facilitated by the acid-base equilibrium between nitrous acid, nitrite anions, and the solvent itself. On the other hand, a band at around 1265 cm-1 should be observed for the asymmetric νONO mode of nitrite anions.53 None of these bands can be observed in the spectra reported in Figures 3-5. Thus it can be concluded that the consumption bands at 1440 and 1340 cm-1 originate from species formed from nitrous acid at the immersion potential. It seems that the presence of palladium atoms at the outmost layer of the electrode surface is at the origin of the formation of these species since the corresponding bands were not observed for the bare platinum electrodes.23,49 One of the electrode reactions that can take place in the nitrous acid solution is the decomposition of nitrous acid to form nitric oxide and nitrate anions:

3HNO2 T 2NO + NO3- + H3O+

(1)

This is a slow process in solution52 that could be favored by the presence of palladium atoms at the electrode surface. The formation of adsorbed NO at 0.90 V is evident from the observation of positive bands between 1800 and 1600 cm-1 in Figures 3-5. Thus we expect to have a significant amount of dissolved NO in this potential region as it will be confirmed later by the on-line DEMS experiments. According to reaction 1, we can tentatively relate the band at 1340 cm-1 in Figures 3-5 to the presence of dissolved nitrate anions since the frequency of this band fits well with the asymmetric N-O stretching of free nitrate anions.53 One could also consider the assignment of the band at 1440 cm-1 to the νNO stretch of 2-fold adsorbed nitrate anions as done by da Cunha et al.54 when studying the adsorption of nitrate anions on polycrystalline gold electrodes. However, we must consider the existence of one dissolved species to explain the observation of the same band in the spectra collected with s-polarized light. As a tentative assignment, we can relate the band at 1440 cm-1 to the N-O stretching of hyponitrite anions, N2O2-.53 This species was detected by Bezerra et al.55 as a product of the oxidation of urea in neutral solutions. Hiponitrite (53) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds; Wiley: New York, (a) 1978 and (b) 1986. (54) da Cunha, M. C. P. M.; Weber, M.; Nart, F. C. J. Electroanal. Chem. 1996, 414, 163. (55) Bezerra, A. C. S.; de Sa´, E. L.; Nart, F. C. J. Phys. Chem. B 1997, 101, 6443.

anions could be formed in our case as a reduction product of nitrous acid

2HNO2 + 2H+ + 4e- T N2O22- + 2H2O

(2)

2NO + 2e- T N2O22-

(3)

or NO.

The latter process seems less probable at 0.90 V due to its less positive equilibrium potential.52 The presence of hiponitrite anions would be consistent with the results obtained in previous studies where this species is proposed as an intermediate in the reduction of both nitrous acid and nitric oxide.52 The negative-going bands observed at 2228 and 1288 cm-1 in the spectra reported in Figures 3-5 can be easily assigned to the N-N and N-O stretching mode of dissolved nitrous oxide, respectively.53 As mentioned above, this species was also detected when studying the reduction of nitrous acid on platinum single-crystal23 and polycrystalline48,49 electrodes. N2O bands also appeared during the reduction of nitrous acid on iridium singlecrystal electrodes.32 From the intensity of the bands in the spectra in Figures 3-5 it can be stated that N2O starts to be formed around 0.70 V for palladium-covered Pt(100) and around 0.50 V for the palladium layers on Pt(111) and Pt(110). In all cases, the amount of N2O increases as the electrode potential is lowered. No bands corresponding to any reduction product other than nitrous oxide can be detected in Figures 3-5. More reduced nitrogenated species include molecular nitrogen, hydroxylamine, and ammonium ions. N2 is not detectable by infrared spectroscopic measurements. Regarding hydroxylamine and ammonium ions, their main bands are expected to be mixed with the δOD band of D2O in the experiments reported in Figures 3-5. We carried out additional experiments in water solutions, but no differences were observed in the spectra with respect to those obtained in D2O solutions. Bands for ammonium and hydroxylamine should appear in the region between 1400 and 1450 cm-1 for experiments carried out in water solutions,53 thus overlapping with the positive-going band at 1440 cm-1. The formation of ammonium ions as the only reduction product of adsorbed NO will be confirmed later in experiments carried out in the absence of nitrous acid (see below). 1.3. DEMS Experiments. Additional on-line DEMS experiments have been performed with palladium electrodes in order to complete the picture steaming from the in-situ FTIR experiments described above. DEMS is a useful technique for the on-line detection of gaseous or volatile species being formed or consumed during an electrochemical reaction. This is specially interesting in the case of nitrogen which cannot be detected spectroscopically in the infrared region. Due to the fact that a rough palladium electrode has to be used in the DEMS experiments, we have checked first the behavior of a palladium-covered polycrystalline platinum electrode in the nitrous acid solution. The cyclic voltammogram obtained in the 0.1 M H2SO4 test electrolyte for this electrode was similar to that previously reported by Llorca et al.,3 thus proving that polycrystalline platinum can also be covered with a palladium multilayer. Both the voltammetric curve and the infrared spectra (not shown here) obtained in the nitrous acid solution were similar to those reported respectively in Figure 2 and Figures 3-5 for well-defined platinum substrates except for the intensity and frequency of the bands corresponding to

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Figure 6. (A) (M) Cyclic voltammogram for a DEMS palladium electrode in a 0.01 M KNO2 + 0.1 M HClO4 solution at 10 mV s-1 and (---) voltammetric profile for the same electrode in the perchloric acid test electrolyte. (B) Simultaneously recorded mass signals for m/z ) 44 (N2O), 30 (NO + NOx) and 28 (N2 + N2O) in the 0.01 M KNO2 + 0.1 M HClO4 solution. For details, see text.

adsorbed NO. We can conclude from this behavior that the same reactions are taking place irrespective of the electrode surface structure. Figure 6A shows a stable cyclic voltammogram obtained during the DEMS experiment with a palladium electrode in the 0.01 M KNO2 + 0.1 M HClO4 solution (solid line). Cathodic currents are measured for potentials lower than 0.65 V which are higher than those observed in the voltammogram obtained in the absence of nitrous acid (dashed line). Simultaneously, several potential-dependent mass signals were recorded. The MSCVs for m/z ) 44, 30, and 28 can be seen in Figure 6B. The mass signal for m/z ) 44 is assigned to the formation of N2O ([N2O]•+). The onset for the production of this species is around 0.55 V during the negative sweep, i.e., just in the region where reduction currents are apparent in the cyclic voltammogram. The MSCV develops a maximum at 0.37 V, with a small shoulder at 0.22 V, during the negative-going scan, whereas the signal decays to the background level at less positive potentials and in the subsequent positive-going sweep. The response for m/z ) 28 shown in Figure 6B could be related to the detection of [N2]•+ coming from the evolution of molecular nitrogen. However, a fragment of N2O also contributes to this signal. Considering the fragmentation probabilities for the latter compound and the parallelism between both MSCVs for m/z ) 28 and 44 at potentials lower than 0.60 V, the formation of N2 does not seem to be important in this potential range. Finally, the potential-dependent mass signal observed for m/z ) 30 is considered. NO and all nitrogen oxides, including N2O, contribute to this m/z ratio. According to this fact and taking into account the fragmentation expected for N2O, the intensity of the signal observed at potentials below 0.60 V can also be explained in terms of the formation of nitrous oxide as we did before for the

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signal at m/z ) 28. However, the contributions observed between 0.90 and 0.60 V could be related to the formation and subsequent reduction of NO. Changes in the signal for m/z ) 30 detected at more positive potentials can be ascribed to the further oxidation of nitrous acid or NO. In summary, DEMS results shows the formation of NO and N2O as the main volatile electroreduction products formed at palladium electrodes in the nitrous acid solution. The formation of small amounts of nitrogen cannot be excluded. It has to be recalled once more that DEMS allows the detection of volatile or gaseous compounds, and therefore, the production of ionic species as hyponitrite and ammonium cannot be followed. In this sense it is noteworthy that all the signals in Figure 6 that can be related to the production of N2O are clearly diminishing for potentials lower than 0.2 V suggesting that this species is further reduced in this potential region. 2. Characterization of the Nitric Oxide Adlayers Formed on the Palladium-Covered Electrodes. 2.1. Isolation of the Adlayer and Identification of Its Reduction Products. Spectra reported in Figures 3-5 have shown the formation of nitric oxide adlayers at the palladium-covered platinum single-crystal electrodes in the nitrous acid solution. As in the case of the adlayers formed on platinum electrodes,24,26,27 those formed on the palladium film can be easily isolated and further characterized in the absence of nitrous acid. The experiment involves the preparation of the palladium-modified surface, its characterization in the 0.1 M H2SO4 test electrolyte (see Figure 1), and the immersion in a 0.02 M KNO2 + 0.1 M HClO4 solution for 5 min. Then the electrode was rinsed with ultrapure water and brought back to the (spectro)electrochemical cell. In the voltammetric experiment reported in Figure 7 for a Pd/Pt(111) electrode, a cyclic voltammogram was recorded first between 0.5 and 1.0 V (curve a). The blocking of the onset for the oxygen adsorption process at the palladium film indicates that some stable adsorbed species was formed at the electrode surface. This adsorbate is not desorbed in the potential region explored. Opening the potential window down to -0.02 V (curve b) shows that the electrode surface remains blocked at potentials around 0.20 V as evidenced by the absence of any voltammetric feature in the potential region where hydrogen adsorption/anion desorption usually take place on the Pd/Pt(111) electrode (see again Figure 1). From our previous experience with NO adlayers on platinum electrodes, we can expect that the NO adlayer could be stripped from the electrode surface at potentials below 0.20 V. This process was achieved for platinum electrodes in a single potential sweep down to the onset of hydrogen evolution as far as the potential was swept at slow enough scan rates (typically lower than 10 mV s-1). A clear voltammetric peak was observed in these experiments involving both the stripping of adsorbed NO and the formation of the corresponding hydrogen adlayer at the lower potential limit of the potential excursion.24,26,27 Curve b in Figure 7 shows only a small peak at 0.06 V and reduction currents coming from the removal of the adlayer at lower potentials that cannot be distinguished from the hydrogen evolution reaction. The latter process is evidenced by the high hydrogen oxidation currents observed between 0 and 0.20 V in the subsequent positive-going sweep (curve c). On the other hand, the recovery of free palladium sites for hydrogen and anion adsorption is evident from the appearance of the typical oxidation peak at 0.290 V (involving the desorption of adsorbed hydrogen and the adsorption of (bi)sulfate anions). It is also clear from Figure 7 that palladium atoms are again accessible for oxygen adsorption at potentials above 0.85 V.


Langmuir, Vol. 16, No. 10, 2000 4701

Figure 8. Cyclic voltammogram (A) and MSCVs (B) recorded during the stripping of the NO adlayer formed at open circuit on a palladium electrode from a 1 mM NaNO2 + 0.1 M HClO4 solution. Test solution: 0.1 M HClO4. Sweep rate: 10 mV s-1.

Figure 7. Voltammetric obtained during an experiment involving the isolation and the reductive stripping of a nitric oxide adlayer at the palladium-covered Pt(111) single-crystal electrode: (a) NO-covered electrode; (b) reductive stripping of the NO adlayer; (c) recovery of the clean electrode surface. Sweep rates: 20 mV s-1 for curves a and c; 2 mV s-1 for curve b. Test solution: 0.1 M H2SO4. The inset shows the in-situ FTIR spectrum for the stripping of the NO adlayer formed at the palladium-covered Pt(111) electrode. Sample potential: 0.50 V: Reference potential: 0 V. A total of 100 interferograms were collected at each potential. Test solution: 0.1 M HClO4 in H2O.

The experiment discussed above shows how adsorbates formed from the nitrous acid solution can be isolated and reductively stripped from the palladium-covered electrode surface. No main changes on the palladium adlayer seem to take place after this experiment. The comparison of the voltammetric charges below the peak at 0.29 V in the voltammograms for Pd/Pt(111) in Figures 1 and 7 indicates that the stripping of the adlayer is not totally achieved under the experimental conditions used. However, we can estimate from these charges that less than 5% of the palladium sites remain blocked after a single sweep down to -0.02 V. Recovery of the palladium surface sites can be improved by holding the electrode potential at around 0 V for a few minutes as described below for the in-situ FTIR experiments. We can obtain more information about the composition of the adlayer formed from the nitrous acid solution and the nature of the products formed during its reductive stripping by performing additional FTIRS and DEMS experiments. The spectrum shown in the inset in Figure 7 was obtained under the same conditions used in the voltammetric experiment reported in the same figure. A sample spectrum was collected at 0.50 V for the adlayer formed after dipping the palladium-covered Pt(111) surface in the nitrous acid solution and transferring the sample to the spectroelectrochemical cell containing a 0.1 M HClO4 solution prepared in H2O. Deuterium oxide was not used in this experiment just to allow the detection of ammonium ions even if the spectral region around 1600

cm-1 can be perturbed by uncompensated water during the potential step. After the collection of the sample spectrum, the electrode was polarized at 0 V for a few minutes and then the reference spectrum was collected. Despite of the distorted baseline of the spectra shown in Figure 7, clear-cut bands can be observed at 1735 and 1462 cm-1. The negative-going band at 1735 cm-1 proves the existence of a NO adlayer on the electrode surface. The detailed analysis of this band and those appearing for the palladium-covered Pt(100) and Pt(110) electrodes will be done below on the basis of the (unperturbed) spectra obtained in D2O solutions. The positive-going band at 1462 cm-1 in Figure 7 corresponds to the product of the NO reduction reaction. Its frequency fits with that of the δNH mode of dissolved NH4+.53 No bands appear at 2228 cm-1 in the spectrum thus discarding the formation of N2O. To conclude that ammonium is the only product formed in this reaction we have to discard the formation of molecular nitrogen. This cannot be done on the basis of the spectroscopic results shown in Figure 7. Some indication can be found in the DEMS experiment reported in Figure 8 for a rough palladium electrode. An adsorption experiment at open circuit was performed with the flow cell as follows. After activation of the electrode in the 0.1 M HClO4 test electrolyte, open circuit conditions were established and a 1 mM NaNO2 + 0.1 M HClO4 solution was introduced in the cell. This solution was kept in the cell during 5 min and then completely replaced by the test electrolyte. Afterward, the potential was swithched on at the open circuit potential (about 0.90 V). The cathodic stripping of the adsorbate was performed in a potential sweep down to 0.05 V while recording simultaneously the corresponding voltammetric current and MSCVs for selected m/z values. The absence of any potential-dependent signal for m/z ) 44 and 28 (see Figure 8B) shows that neither N2O nor N2 is formed during the stripping of the NO adlayer. The same conclusion was reached by Gootzen et al.56 for the reductive stripping of NO adsorbed on a platinum electrode. The absence of N2O in the (56) Gootzen, J. F. E.; van Hardeveld, R. M.; Viesser, W.; van Santen, R. A.; van Veen, J. A. R. Recl. Trav. Chim. Pays-Bas 1996, 115, 480.

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experiment reported in Figure 8 is consistent with the spectroscopic results reported in Figure 7 whereas that of N2 could be assumed to be also true for the stripping of the NO adlayers in the case of the palladium films deposited on well-ordered platinum single-crystal electrodes. 2.2. Spectroscopic Characterization of the NO Adlayers. The adsorption experiment described above for the palladium-covered Pt(111) electrode has been also performed to check the formation of adsorbed NO on the palladium films deposited on Pt(100) and Pt(110) substrates. Similar conclusions have been reached regarding the formation of strongly bonded adlayers, in the nitrous acid solution. Spectra were collected for these adlayers, and bands were found in the N-O stretching region of adsorbed NO. These experiments were performed in D2O in order to have unperturbed spectra in the region where these bands appear. Figures 9-11 show sets of spectra collected at different sample potentials between 0.90 and 0 V for the NO adlayers formed on the palladium-modified electrodes. These spectra will be discussed in the following and compared with those obtained for NO adsorbed on bulk palladium single-crystal surfaces both under electrochemical and UHV conditions. Before proceeding with this discussion, we should make a general remark about the assignment of the N-O stretching bands observed for the adsorbed NO layer. In most of the UHV papers cited here, bands were assigned to specific adsorption sites on the basis of the comparison between their frequencies and those obtained for nitrosyl coordination compounds. In this way, bands with frequencies above 1700 cm-1 were typically related to atop NO molecules and those at lower frequencies to bridge bonded NO. However, this approach has been showed to lead to wrong assignments for NO adsorbed on different metals.57-60 For example, only 3-fold adsorbed NO was found at the Pt(111) surface59 despite the observation of coverage-dependent bands in the spectral region between 1500 and 1800 cm-1 which were previously assigned to atop and bridge-bonded NO.61,62 In the case of NO adsorbed on Pd(111), recent periodic density-functional calculations63 indicate that atop and 3-fold hollow molecules coexist for the (2 × 2)-3NO (θNO ) 0.75) adlayer. The N-O stretching frequencies calculated in this study account for the experimental values that were previously related to the existence of atop and bridge species.34-36 The same type of study applied to NO adsorbed on Pd(100) concluded that bridge sites are the most stable for a c(2 × 2) adlayer (θNO ) 0.5).64 A LEED pattern corresponding to the same structure was observed by Jorgensen et al.38 at 300 K together with a single band at 1720 cm-1 that was assigned to atop NO molecules. Because of these uncertainties, we will avoid relating the frequency of the observed bands in the spectra of adsorbed NO to a given adsorption site. Instead, we will just take each spectrum as a signature (57) (a) Asensio, M. C.; Woddruff, D. P.; Robinson, A. W.; Schinler, K. M.; Gardner, P.; Ricken, D.; Bradshaw, A. M.; Conesa, J. C.; Gonza´lezElipe, A. R. J. Vac. Sci. Technol. A 1992, 10, 2445; (b) Chem. Phys. Lett. 1992, 192, 259. (58) Aminpirooz, S.; Schmalz, A.; Becker; L.; Haase, J. Phys. Rev. B 1992, 45, 6337. (59) Materer, N.; Barbieri, A.; Gardin, D.; Starke, U.; Batteas, J. D.; Van Hove, M. A.; Somorjai, G. A. Phys. Rev. B 1993, 48, 2859. (60) Kim, Y. J.; Thevuthasan, S.; Herman, G. S.; Peden, C. H. F.; Chambers, S. A.; Belton, D. N.; Pemana, H. Surf. Sci. 1996, 359, 269. (61) Hayden, B. E. Surf. Sci. 1983, 131, 419. (62) Agrawal, V. K.; Trenary, M. Surf. Sci. 1991, 259, 116. (63) Loffreda, D.; Simon, D.; Sautet, P. Chem. Phys. Lett. 1998, 291, 15. (64) Loffreda, D.; Simon, D.; Sautet, P. J. Chem. Phys. 1998, 108, 6447.

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Figure 9. In-situ FTIR spectra collected at different sample potential for the NO adlayer formed at the Pd/Pt(111) electrode. Reference potential: 0 V. A total of 100 interferograms were collected at each potential. Test solution: 0.1 M HClO4 in D2O.

of the NO adlayer formed on each palladium surface at a given coverage. The reader is referred to the original papers and to a recent comparison of the data available for NO adsorbed at different metal/electrolyte and metal/ vacuum interfaces where the question of the adsorption site for adsorbed NO is addressed.30 2.2.1. NO Adsorbed on the Pd/Pt(111) Electrode. Figure 9 shows the spectra collected at different potentials for NO adsorbed on the Pd/Pt(111) electrode. The spectrum obtained at 0.90 V shows N-O stretching bands that appear at 1743 and 1543 cm-1. Both bands shift to lower wavenumbers with constant intensities as the electrode potential decreases in the region between 0.90 and 0.50 V. The tuning rate for the high-frequency band is ca. 40 cm-1 V-1 in this potential region. At potentials around 0.30 V the intensity of the band above 1700 cm-1 diminishes whereas that of the low-frequency band increases. These changes take place together with a sudden red shift of the band at around 1550 cm-1. At potentials between 0.20 and 0.10 V no band is observed above 1600 cm-1. The low-frequency band it is shifted toward lower wavenumbers as the electrode potential is decreased in this potential region. These changes in the spectra are opposite to those found for the vibrational spectra reported for increasing coverages of NO adsorbed on Pd(111) surfaces under UHV conditions.34-37 This fact suggests that changes in the spectra reported in Figure 8A for potentials below 0.40 V are related to the partial stripping of the adlayer in this potential region. This process is not completely achieved until potentials around 0 V are maintained for a few minutes. It is worth noting that Weaver et al. reported always a single band (that above 1700 cm-1) in the spectra obtained for NO adsorbed on bulk Pd(111) electrodes irrespective of NO coverage.19,30 Partial coverages in these experiments where obtained by controlling the time of adsorption in the NO-saturated solution. The authors related the difference between the


coverage-dependent changes reported in their paper and those observed under UHV to the existence of compressed NO islands induced by water coadsorption.19,30 Another difference between the results presented here and those reported by Weaver et al. concerns the existence of the small absorption band at around 1550 cm-1 in the spectra collected for the saturated NO adlayer. As we pointed out before, higher NO coverage in UHV experiments results in vibrational spectra for Pd(111) in which the band above 1700 cm-1 dominates the spectra while the intensity of the band at lower wavenumbers decreases. The spectrum for the highest NO coverage seems to be that reported by Chen and Goodman36 for a (2 × 2) ordered structure (θNO ) 0.75) in which an intense band at 1758 cm-1 predominates over a small, but detectable, band at around 1550 cm-1. It is possible that the observation of this band in our experiments is related to the existence of a slightly less dense adlayer. In fact, we cannot determine whether the absolute NO coverage in our experiments is close to 0.75 due to the overlapping between the NO stripping and hydrogen evolution processes (see the discussion of the experiment reported in Figure 7). Weaver et al.19,30 mentioned that higher adsorption times were needed to saturate their palladium single-crystal electrodes with NO in comparison with platinum or rhodium surfaces. In the present study we have found no differences in the spectra for the isolated NO adlayer on Pd/Pt(111) when the adsorption time was increased or when we used a NO saturated solution instead of nitrous acid to generate the NO adlayer. However, some indication about the effect of small differencies in the NO coverage near saturation can be found in the spectra reported in Figure 3 for the Pd/Pt(111) electrode in the presence of nitrous acid. These conditions can lead to NO coverages slightly higher than those reached for the isolated NO adlayer as it happens in the case of adsorbed CO in a CO-saturated solution.65 Spectra shown in Figure 3 for potentials between 0.80 and 0.40 V show changes in the spectral region around 1700 cm-1 which originates from the potential-dependent frequency shift of the highfrequency band characteristic of high NO coverages. It must be reminded that the spectra reported in Figure 3 are referred to the single beam spectrum collected at 0.90 V. However, no bands are observed between 1600 and 1500 cm-1 in agreement with the hypothesis of higher coverages being attained in the presence of nitrous acid. Negative-going bands between 1600 and 1450 cm-1 are only observed at potentials below 0.2 V in the presence of nitrous acid. Note that these bands are still observed in the spectrum collected at potentials around 0 V, for which the isolated NO adlayers characterized in the experiments reported in Figures 7 and 9 were completely removed from the surface. This difference is also related with the formation of denser NO adlayers when nitrous acid is present in the working solution. 2.2.2. NO Adsorbed on the Pd/Pt(100) Electrode. Figures 10A,B shows spectra for NO adsorbed on the palladium-covered Pt(100) electrode. The spectrum in Figure 10A was obtained after generating the NO adlayer by dipping the electrode in a nitrous acid solution. This spectrum shows a main band at 1670 cm-1 with an additional and hardly distinguible feature at 1732 cm-1. This behavior is in contrast with the observation of two positive-going bands with similar intensities at 1740 and 1671 cm-1 in the spectra reported in Figure 4 for the same surface but in the presence of nitrous acid. This difference (65) Rodes, A.; Go´mez, R.; Feliu, J. M.; Weaver, M. J. Langmuir 2000, 16, 811.

Langmuir, Vol. 16, No. 10, 2000 4703

Figure 10. In-situ FTIR spectra collected at different sample potential for the NO adlayer formed at the Pd/Pt(100) electrode: (A) from a nitrous acid solution; (B) from a saturated NO(g) solution. Conditions are the same as in Figure 9.

can be understood on the basis of the coverage-dependent changes reported under UHV conditions for the vibrational spectra of adsorbed NO on Pd(100) surfaces.38-40 EELS spectra for the NO-saturated Pd(100) surface at 100 K show a main band in the region between 1720 and 1790 cm-1, the exact value varying from one paper to the other.38-40 Annealing of the sample at 400 K led to changes in the spectra with the former band being substituted by a band in the region between 1510 and 1560 cm-1.38-40 Nitric oxide coverages were estimated to be 0.65 for the saturated Pd(100) surface at 100 K and 0.25 after annealing at 400 K.40 Jorgensen et al.38 obtained the spectra for different NO exposures up to the saturation of the surface and monitored the changes in the spectra. The band at ca. 1510 cm-1 appeared first at low coverage being substituted by a band at 1720 cm-1 when saturation was reached. The two bands coexisted at intermediate coverages. Changes in the frequency of these bands are expected to take place with increasing NO coverages. Nevertheless, these changes are difficult to follow in the spectra reported in the mentioned papers due to the low resolution of the EELS experiments (between 50 and 90 cm-1). The low intensity of the band at 1732 cm-1 reported in Figure 10A suggests the isolation of a low-coverage NO adlayer from the nitrous acid solution whereas higher NO coverages seems to be reached in the presence of nitrous acid. In an attempt at isolating more dense NO adlayers, we tried (as mentioned before for the Pt(111) substrate) to generate them from a NO-saturated solution. The resulting spectra are reported in Figure 10B. Those at potentials positive to the onset of the stripping of the NO adlayer show bands at 1733 and 1663 cm-1 which are similar to those observed in the presence of nitrous acid. This is the first time that we found a significant difference between the isolated adlayers formed from nitrous acid or NO solutions. The observed behavior points out that

4704


Figure 11. In-situ FTIR spectra collected at different sample potential for the NO adlayer formed at the Pd/Pt(110) electrode. Conditions as in Figure 9.

NO adsorption is specially slow for the palladium-covered Pt(100) and high NO concentrations are needed in solution to approach saturation of the surface. However, the comparison between the spectra reported in Figure 10B with the EELS spectra described above suggests that an intermediate NO coverage (between 0.25 and 0.65) was achieved in our experiments. The spectra reported in Figure 10B for different sample potentials show that the two bands appearing in the spectra are shifted toward lower wavenumber as the electrode potential decreases. Tuning rates measured in the potential range between 0.90 and 0.40 V were 33 and 63 cm-1 V-1 for the high- and low-frequency bands, respectively. In the case of the low NO coverage corresponding to the spectrum reported in Figure 10A, the tuning rate for the band at 1670 cm-1 was ca. 50 cm-1 V-1. At potentials below 0.30 V, the intensities of both bands in Figure 10B decrease with that above 1700 cm-1 disappearing first. A new absorption band develops at 1489 cm-1 in the spectrum collected at 0.30 V. No adsorbate bands are detected after polarizing the electrode at 0.10 V, showing that adsorbed NO can be completely stripped at this potential. 2.2.3. NO Adsorbed on the Pd/Pt(110) Electrode. Figure 11 shows the spectra obtained for the NO adlayer formed at the palladium-covered Pt(110) electrode. A main absorption band appears at 1760 cm-1 with a low-intensity feature at 1672 cm-1 in the spectrum collected at 0.90 V. Both bands are red shifted as the electrode potential decreases. Tuning rates in the potential region of constant coverage are 60 and 80 cm-1 V-1, respectively. Partial reduction of the adlayer makes both bands vanish at potentials below 0.40 V with the band at higher wavenumbers disappearing first. A new band is observed at 1492 cm-1 in the spectrum collected at 0.10 V just prior to the complete stripping of the adlayer. To our knowledge there is only one paper dealing with the infrared characterization of NO adsorbed on Pd(110)

AÄ lvarez et al.

under UHV conditions. In this paper, Raval et al.41 reported the infrared spectra obtained for different NO dosings at 180 and 300 K. The spectrum obtained at 300 K for the highly covered surface (θNO > 0.5) showed absorption bands at 1734 and 1669 cm-1. Those collected for coverages lower than 0.5 are characterized by bands at ca. 1630 and 1505 cm-1. Typical coverage dependencies are observed in the high- and low-coverage regimes, i.e., bans shifting upward and the high-frequency band becoming dominant in the spectra as the NO coverage increases. It is not clear from this paper which are the saturation coverage and relative intensity of the bands at 1734 and 1669 cm-1 for the saturated surface at room temperature. The authors invoke the existence of an adsorbate-induced (1 × 3) reconstruction of the Pd(110) surface, kinetically hindered at low temperatures, to explain the complex temperature and coverage dependencies of the infrared spectra. The existence of such an adsorbate-induced reconstruction of the Pd(110) substrate was also proposed in the case of carbon monoxide both under UHV66 and electrochemical17 conditions. Independently of the existence of these reconstruction processes, we can confirm here that the N-O stretching bands observed in the spectra in Figure 11 follow the trends observed under UHV conditions for the NO-covered Pd(110) at room at room temperature.41 We can also compare the spectra reported above for the palladium-covered Pt(110) with those obtained by Weaver et al. for a bulk Pd(110) electrode.30 These authors found a good agreement between their spectra for adsorbed NO and the UHV data except for the absence of bands below 1600 cm-1 in the low-coverage regime. The spectra reported by the Purdue group for the NO-saturated electrode show only the band at 1760 cm-1 with a tuning rate of 52 cm-1 V-1. As mentioned above, the tuning rate found for the same band in our spectra is 60 cm-1 V-1, only slightly above this value. As in the case of the palladium-covered Pt(111) electrode, the observation of the small feature at lower wavenumbers (which is difficult to detect in the spectra collected in the presence of nitrous acid (see Figure 5)) can be related to small differences in the NO coverage for the saturated adlayer. 2.3. Epitaxial Growth of the Palladium Adlayers. The spectroscopic results reported in this paper are consistent with the epitaxial growth of the palladium films on the platinum single substrates. As in previous studies where the CO molecule was used to probe the surface of palladium multilayers,4,7-9 the structural information derived from the infrared spectra of the adsorbed layer is mainly related to the local arrangement of surface atoms defining adsorption sites which seem to be the same for the studied palladium films and for the palladium singlecrystal surfaces. This does not imply necessarily the existence of long-range order on a palladium multilayer free of surface defects or the growth of the palladium film in a perfect layer-by-layer mode. The former information can only be derived indirectly from the spectrocopic data when distinctive bands are related to the formation of ordered adsorbate structures requiring the existence of two-dimensional order in the underlying substrate. A wellknown example is the observation of the 3-fold hollow CO band for the (2 × 2)-3CO adlayer that characterize the saturated CO adlayer on Pt(111) electrodes.65 The epitaxial growth of the palladium multilayers deduced here from the infrared spectra of adsorbed NO complements the previous observation of two-dimensional (66) Raval, R.; Haq, S.; M. A. Harrison; Blyholder, G.; King, D. A. Chem. Phys. Lett. 1990, 167, 629.


palladium islands for coverages below the monolayer. This result was reported by Inukai and Ito4 using the distinctive frequencies C-O stretching frequencies observed when CO is adsorbed on platinum and palladium atoms. A similar behavior has been observed in our laboratory for adsorbed NO.67 As mentioned above, the existence of (1 × 1) palladium islands was also suggested by Attard et al.5,6 on the basis of LEED and AES results for palladium films evaporated under UHV conditions. Attard et al. also concluded from the coverage dependence of the palladium Auger emission lines that the palladium layers grew layerby-layer at the Pt(111) and Pt(100) (1 × 1) substrates. This growth mode is also known as the Frank-van der Merwe growth mode.68 Recent in-situ surface X-ray diffraction experiments carried out by Markovic et al.69 for electrodeposited palladium films on the same substrates are basically in agreement with this conclusion. In this way, Markovic et al. found a flat epitaxial palladium film on both Pt(111) and Pt(100) electrodes when the voltammetric features previously assigned to the formation of the palladium monolayer were fully developed. However, these authors found that, even if the palladium deposit keeps on growing in registry with the substrate, the second layer of palladium atoms was not completed before the deposition of third and next layers started. Thus the surfaces obtained for palladium coverages corresponding to the voltammetric features previously related to the deposition of the second palladium layer are not strictily flat. Markovic et al. observed that the roughness of the surface first increased when depositing more palladium onto the palladium monolayer but later decreased again in such a way that a quite flat surface was reached for the palladium multilayers (ca. 10 layers thick). The resulting palladium multilayer films deposited onto Pt(111) and Pt(100) substrates, which are similar to those investigated in the present study, can be compared well with those for bulk Pd(111) and Pd(100) samples, respectively. It can be assumed that the same conclusion could be extended to the films grown on the Pt(110) electrode surface. Conclusions The first main conclusion derived from this study is that stable NO adlayers can be easily formed just by dipping the palladium-covered electrodes either in a nitrous acid or NO-saturated aqueous solution. These adlayers can be transferred through an oxygen-containing atmosphere and characterized in a test electrolyte solution in the absence of nitrous acid or NO. Voltammetric experiments have shown that the reductive stripping of adsorbed NO takes place at less positive potentials than in the case of the bare platinum electrodes thus providing a wider potential region where the adlayer can be characterized spectroscopically. Parallel DEMS and FTIR (67) Rodes, A.; A Ä lvarez, B.; Feliu, J. M. Manuscript in preparation. (68) Budevski, E.; Staikov, G.; Lorenz, W. J. Electrochemical Phase Formation and Growth. An Introduction to the Initial States of Metal Deposition; VCH: Weinheim, Germany, 1996. (69) Markovic, N. M.; Lucas, C. A.; Climent, V.; Ross, P. N. To be submitted for publication.

Langmuir, Vol. 16, No. 10, 2000 4705

experiments reported in this paper have confirmed dissolved ammonium as the only detectable product coming from the stripping of the NO adlayer. The formation of intermediate reduction products such as nitrous oxide or molecular nitrogen can be discarded. The knowledge of the stoichiometry for the stripping reaction (which involves the exchange of 5 electrons/adsorbed NO molecule) is a key point for the determination of the absolute NO coverage. Unfortunately, this electrochemical method for the determination of the NO coverage, which was successfully employed in the case of platinum and rhodium single-crystal electrodes, could not be used for the palladium-covered electrodes studied in this work. The interaction between the outmost palladium atoms and the NO molecules is so strong that the stripping of the NO adlayer overlaps with the hydrogen evolution reaction. Thus, the charge involved in the reduction of adsorbed NO to dissolved ammonium cannot be measured. The spectra obtained for the palladium-covered electrodes show in all cases N-O stretching bands that are different from those characteristic for the saturated NO adlayers on the platinum substrate. At the same time, the main features in the FTIR spectra obtained for the Pd/Pt(hkl) electrodes have been shown to compare well with those previously reported for NO adlayers on bulk palladium single crystals. The latter were available for the three basal orientations of palladium under UHV conditions and also for Pd(111) and Pd(110) electrodes. Distinctive features in the spectra for the NO adlayer can be related to the orientation of the electrode surface. The comparison between the palladium films and the welldefined palladium single-crystal surface included the spectra for the NO-saturated adlayers and the coveragedependent changes of the spectra. We have not performed a detailed study on this latter point by dosing NO at different coverages (which will be addressed in a future paper), but we have obtained equivalent information through the changes observed during the partial stripping of the adlayers. The ensemble of the spectroscopic data reported in this paper is consistent with the epitaxial growth of the palladium films on the platinum substrates. This conclusion confirms the possibility of using the palladium-covered platinum single-crystal electrodes for the study of the structural aspects of the electrochemical reactivity of palladium. Acknowledgment. The authors acknowledge fruitful discussions with Dr. N. M. Markovic about his research on the surface characterization of the palladium layers grown on platinum single crystal electrodes. Prof. M. J. Weaver is acknowledged for providing the manuscripts of some of his papers dealing with NO adsorption on palladium single-crystal electrodes before their publication. The authors are grateful for the financial support afforded by the DGICYT through Contract PB96-0409 and the Gobierno de Canarias through Contract PI1999/070. The funds provided by the Conselleria de Cultura, Educacio´ i Ciencia de la Generalitat Valenciana for the purchase of the FTIR facility are also acknowledged. LA991473Q

Spectroscopic Study of the Nitric Oxide Adlayers Formed from Nitrous

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