Urea Adsorption on Platinum Single Crystal Stepped Surfaces

Urea adsorption has been studied at Pt(110) and stepped platinum ... hand, urea molecules bonded through the two nitrogen atoms are detected in the wh...
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Urea Adsorption on Platinum Single Crystal Stepped Surfaces V. Climent,*,† A. Rodes,† R. Albalat,‡ J. Claret,‡ J. M. Feliu,† and A. Aldaz† Departament de Quı´mica Fı´sica, Universitat d’Alacant, Apartat 99, E-03080 Alacant, Spain, and Departament de Quı´mica Fı´sica, Universitat de Barcelona, Marti i Franques, 1, E-08028 Barcelona, Spain Received July 18, 2001. In Final Form: September 5, 2001 Urea adsorption has been studied at Pt(110) and stepped platinum electrodes with orientations vicinal to Pt(111) in the [11 h 0] and [011 h ] zones. In situ infrared spectra and cyclic voltammograms obtained in the urea-containing solutions have been analyzed as a function of the (110) or (100) step densities. At the same time, voltammetric data have been combined with charge displacement experiments in order to determine the potential of zero total charge (pztc) in the presence of urea. The variation of the pztc with the step density is similar to that previously observed for the same surfaces in sulfuric acid solutions thus confirming the anion-like behavior of urea molecules at platinum electrodes. Potential-dependent changes in the bonding of urea at the Pt(110) electrode have been found to be similar to those previously reported for Pt(111), with N-bonded and O-bonding urea predominating at low and high coverages, respectively. The same behavior is observed for stepped surfaces containing (111) terraces and (110) steps. On the other hand, urea molecules bonded through the two nitrogen atoms are detected in the whole coverage range at surfaces with a high density of (100) steps.

1. Introduction Urea adsorption on platinum single-crystal electrodes with different orientations has been the subject of several detailed studies.1-6 These investigations have been recently extended also to gold7 and rhodium8 single-crystal electrodes. One of the main conclusions extracted from these works is the striking resemblance of the electrochemical behavior of urea with that exhibited by specifically adsorbed anions, both organic and inorganic. This result was rather surprising provided that in acidic solutions urea molecules are neutral or even protonated. This behavior led to the inclusion of urea into a group of molecules that exhibit the so-called “anion-like” behavior together with carboxylic acids and some of their derivatives.9 Recent reports have shown that this kind of behavior can also be observed when working with solutions containing some organic cations10,11 although ion-pairing of these cations with adsorbed anions is suggested in this case. The study of the adsorption of both anions and anionlike molecules on platinum and rhodium single-crystal * Corresponding author. E-mail: [email protected]. Fax: 34.965903537. † Universitat d’Alacant. ‡ Universitat de Barcelona. (1) Rubel, M.; Rhee, C. K.; Wieckowski, A.; Rikvold, P. A. J. Electroanal. Chem. 1991, 315, 301. (2) Rhee, C. K. J. Electrochem. Soc. 1992, 139, 13C. (3) Gamboa-Aldeco, M.; Mrozek, P.; Rhee, C. K.; Wieckowski, A.; Rikvold, P. A.; Wang, Q. Surf. Sci. Lett. 1993, 297, L135-L140. (4) Climent, V.; Rodes, A.; Orts, J. M.; Feliu, J. M.; Pe´rez, J. M.; Aldaz, A. Langmuir 1997, 13, 2380. (5) Climent, V.; Rodes, A.; Orts, J. M.; Aldaz, A.; Feliu, J. M. J. Electroanal. Chem. 1999, 461, 65. (6) Climent V.; Go´mez, R.; Orts, J. M.; Rodes, A.; Aldaz, A.; Feliu, J. M. In Interfacial Electrochemistry; Marcel Dekker: New York, 1999; Chapter 26, p 463. (7) Nakamura, M.; Song, M. B.; Ito, M. Surf. Sci. 1999, 427-428, 167. (8) Climent, V.; Rodes, A.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. Langmuir 2000, 16, 10376. (9) Wieckowski, A. In Modern Aspects of Electrochemistry; White, R. E., Bockris, J. O., Conway, B. E., Eds.; Plenum Press: New York, 1990; Vol. 21; p 65.

electrodes can be addressed with a variety of experimental techniques. Among them, our group has developed the so-called CO displacement technique.4,5,8 This technique provides key information in order to unambiguously understand chronocoulometric measurements12 by means of the determination of the potential of zero total charge (pztc). The latter defines the potential value from which the voltammetric current must be integrated in order to extract values of the total charge on the electrode.13,14 On this basis, the origin of the voltammetric currents of hydrogen adsorbing metals can be understood, especially when specific anion adsorption can take place with charge transfer. In this way it was possible to ascribe the anomalous voltammetric wave observed in the voltammogram of Pt(111) in sulfuric acid solutions to the specific adsorption of (bi)sulfate anions.15 This conclusion, obtained with electrochemical methods, is consistent with FTIR spectroscopy16-18 and scanning tunneling microscopy (STM) results.19 The urea adlayers formed on Pt(100),4 Pt(111),5 and Rh(hkl)8 single-crystal electrodes in contact with acidic urea solutions were studied following a similar strategy. Adsorption of urea on Pt(100) electrodes was first ad(10) Pierozynski, B.; Morin, S.; Conway, B. E. J. Electroanal. Chem. 1999, 467, 30. (11) Pierozynski, B.; Zolfaghari, A.; Conway, B. E. Phys. Chem. Chem. Phys. 2001, 3, 469. (12) Go´mez, R.; Feliu, J. M.; Aldaz, A.; Weaver, M. J. Surf. Sci. 1998, 410, 48. (13) Climent, V.; Go´mez, R.; Orts, J. M.; Aldaz, A.; Feliu, J. M. In The Electrochemical Society Proceedings; Korzeniewski, C., Conway, B. E., Eds.; The Electrochemical Society: Pennington, NJ, 1997; Vol. 97-17; p 222. (14) Climent, V.; Go´mez, R.; Feliu, J. M. Electrochim. Acta 1999, 45, 629. (15) Feliu, J. M.; Orts, J. M.; Go´mez, R.; Aldaz, A. J. Electroanal. Chem. 1994, 372, 265. (16) Nart, F. C.; Iwasita, T.; Weber, M. Electrochim. Acta 1994, 39, 961. (17) Faguy, P. W.; Marinkovic, N. S.; Adzic, R. R. J. Electroanal. Chem. 1996, 407, 209. (18) Shingaya, Y.; Ito, M. Surf. Sci. 1997, 386, 34. (19) Funtikov, A. M.; Stimming, U.; Vogel, R. J. Electroanal. Chem. 1997, 428, 147.

10.1021/la011122n CCC: $20.00 © 2001 American Chemical Society Published on Web 11/27/2001

Urea Adsorption on Platinum

dressed by Wieckowski’s group.1-3 The potential dependence of the urea coverage, its saturation value, and the structure of the saturated urea adlayer on this electrode was deduced by the combination of voltammetry,1-3 radiotracer,3 and ex situ low-energy electron diffraction (LEED), and Auger electron spectroscopy (AES)2,3 experiments. These data were combined with a theoretical description of urea adsorption on the Pt(100) electrode by using a Monte Carlo simulation of a lattice-gas model.20-22 The overall picture was complemented by our group by using the CO displacement technique and in situ FTIR measurements.4 The CO displacement experiments showed that the adsorption of urea is associated with the transfer of two electrons from the urea molecule to the metal surface. From these data it was suggested that urea deprotonates upon adsorption. Recent density functional theory (DFT) calculations indicate that the resulting ureyline adsorbate is more stable than the undissociated urea molecule and occupies preferently bridge hollow sites.23 On the other hand, the analysis of the FTIR measurements shows that the adsorbate is bonded to the Pt(100) surface through both nitrogen atoms, with the CO bond nearly perpendicular to the metal surface. The urea adlayer on Pt(111) was also investigated.5 The spectroscopic study showed that urea is bonded to the surface through only one of the nitrogen atoms at low potentials. However, O-bonded urea molecules predominate at higher potentials where a coulometrically determined24,25 saturation coverage of ca. 0.45 is attained.5 Nowadays, the significant dependence of the kinetic and thermodynamic properties of the anion adsorption processes on the surface geometry is well-known. In this way, it has been usually pointed out the importance of the effect exerted by the presence of steps on the electrode surface on the anion adsorption process.26,27 More recently, the dependence of the pztc of platinum stepped surfaces has been investigated by means of the CO displacement technique and its relationship with the step density has been established, both in the presence and in the absence of specifically adsorbed anions.14,28 In this paper we have extended these studies to the characterization of urea adlayers as a model molecule that exhibits anion-like behavior, adsorbed on Pt(111) vicinal surfaces, both in the [11 h 0] and [011 h ] zones, i.e., with (111) and (100) step symmetries, respectively. The CO displacement has been used, in combination with the cyclic voltammetry, for the determination of the pztc of the stepped surfaces in the presence of urea. Following the same approach previously employed with the platinum basal planes, in situ FTIR spectroscopy has been used to get further information about the adsorption geometry of urea on the high Miller indexed surfaces. (20) Rikvold, P. A.; Wieckowski, A. Phys. Scr. 1992, T44, 71. (21) Rikvold, P. A.; Gamboa-Aldeco, M.; Zhang, J.; Han, M.; Wang, Q.; Richards, H. L.; Wieckowski, A. Surf. Sci. 1995, 335, 389. (22) Rikvold, P. A.; Zhang, J.; Sung, Y. E.; Wieckowski, A. Electrochim. Acta 1996, 41, 2175. (23) Garcia-Hernandez, M.; Birkenheuer, U.; Hu, A. G.; Illas, F.; Rosch, N. Surf. Sci. 2001, 471, 151. (24) Lipkowski, J.; Stolberg, L. In Frontiers in Electrochemistry; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers, Inc.: New York, 1992; p 171. (25) Savich, W.; Sun, S. G.; Lipkowski, J.; Wieckowski, A. J. Electroanal. Chem. 1995, 388, 233. (26) Markovic, N. M.; Marinkovic, N. S.; Adzic, R. R. J. Electroanal. Chem. 1988, 241, 309. (27) Ross, P. N. J. Chim. Phys. 1991, 88, 1353. (28) Go´mez, R.; Climent, V.; Feliu, J. M.; Weaver, M. J. J. Phys. Chem. B 2000, 104, 597.

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2. Experimental Section Single-crystal electrodes were prepared from small Pt beads obtained by melting 0.5 mm diameter Pt wires (99.99%). Samples with diameters about 2.0 and 4.0 mm were employed for the electrochemical and spectroelectrochemical experiments, respectively. The facets present on the surface of the beads were used to select the desired orientation within (3 min of arc. The electrodes were fixed, cut, and polished as described previously.29 Before each experiment the electrodes were flame annealed, cooled in H2 + Ar atmosphere, and quenched with water in equilibrium with the same mixture of gases. It was found that this treatment leads to better ordered surfaces.30-32 In the same way, this cooling procedure, when applied to Pt(110), leads to electrode surfaces whose voltammetric profiles are similar to those ascribed to the (1 × 1) surface structure.33 Platinum-stepped surfaces were subsequently polarized in the hydrogen evolution potential region (-0.06 V) for 60 s. Sharper voltammetric profiles, suggesting a higher degree of order, are obtained under these conditions, especially for surfaces with (110) steps.28 Working solutions (0.1 M HClO4) were prepared from concentrated perchloric acid (Merck Suprapur) and ultrapure water (Millipore Milli Q) to which urea (Merck for molecular biology) was added to reach the desired concentration. Solutions used in the infrared experiments were prepared in deuterium oxide (Sigma) as received. The working solutions were deaerated by bubbling Ar (Air liquide, N50) for 15 min. Charge displacement experiments at constant potential were performed as described elsewhere by dosing CO (Air liquide, N48).34,35 A wave signal generator (EG&G PARC 175), a potentiostat (Amel 551), and a X-Y-t recorder (Phillips PM 8133) were arranged in the conventional way. The cell was a conventional two-compartment glass cell with an additional inlet for dosing CO gas. Especial care was taken to avoid the presence of atmospheric oxygen in the vicinity of the meniscus between the electrode and the solution. Potentials were measured and are quoted versus the reversible hydrogen reference electrode (RHE). A coiled Pt wire immersed in the working electrolyte was used as counter electrode. All experiments were performed at room temperature. Spectroelectrochemical experiments were performed with a Nicolet Magna 850 spectrometer equipped with a ΜCT detector. The spectroelectrochemical cell was provided with a prismatic CaF2 window bevelled 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/Ro ) (R - Ro)/Ro, where R and Ro are the reflectance values corresponding to the single beam spectra obtained at the sample and reference potentials, respectively.

3. Results and Discussion 3.1. Pt(110) in Comparison to the Other Basal Planes. Figure 1 shows the cyclic voltammograms corresponding to the three platinum basal planes in 0.1 M perchloric acid solutions containing 1 mM urea. As previously described in refs 1, 4, and 5 the presence of urea in the perchloric acid solution exerts significant changes in the voltammetric profiles characterizing these electrode surfaces. In the three cases there is a shift of some of the adsorption states toward less-positive potentials. The most significant effect is observed with the Pt(100) surface, for which the broad features characterizing (29) Clavilier, J.; Armand, D.; Sun, S.-G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (30) Clavilier, J.; El Achi, K.; Rodes, A. J. Electroanal. Chem. 1989, 272, 253. (31) Herrero, E.; Orts, J. M.; Aldaz, A.; Feliu, J. M. Surf. Sci. 1999, 440, 259. (32) Kibler, L. A.; Cuesta, A.; Kleinert, M.; Kolb, D. M. J. Electroanal. Chem. 2000, 484, 73. (33) Markovic, N. M.; Grgur, B. N.; Lucas, C. A.; Ross, P. N. Surf. Sci. 1997, 384, L805. (34) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M. J. Electroanal. Chem. 1993, 360, 325. (35) Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. Electrochim. Acta 1994, 39, 1519.

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Figure 1. Cyclic voltammograms obtained for platinum single-crystal electrodes in contact with 1 mM urea + 0.1 M HClO4 solution: (A) Pt(100); (B) Pt(111); (C) Pt(110). Dashed lines correspond to the voltammograms obtained in the urea-free solution.

the voltammetric profile of this surface in perchloric acid solutions are replaced by a pair of sharp voltammetric peaks around 0.23 V. The voltammetric charge integrated under these peaks amounts to ca. 310 µC cm-2. The excess of charge observed when this value is compared with that corresponding to the hydrogen monolayer attained at the lower potential limit (209 µC cm-2) was ascribed to the urea adsorption. This assumption, which is consistent with the displaced charge by adsorbing CO at potentials higher than the peak, i.e., -101 µC cm-2 at 0.45 V,4 implies that the urea adsorption process takes place with a transfer of charge of two electrons per urea molecule assuming a saturation coverage of 0.25 urea molecules per platinum surface atom.2,3 In the case of Pt(111), the hydrogen adsorption states on this surface, located between 0.06 and 0.35 V, remain almost unaltered in the presence of urea in solution. On the other hand, the adsorption states at the higher potentials, between 0.60 and 0.90 V, usually ascribed to adsorption of oxygenated species, are replaced by the urea adsorption states, located at lower potentials, i.e., between 0.35 and 0.60 V. The charge under these states amounts to ca. 115 µC cm-2.5 This charge has been interpreted as the consequence of the transfer of one electron per adsorbed urea molecule together with a saturation coverage of ca. 0.45.5 This coverage value was consistent with the charge density involved in the urea adsorption process as measured both from chargedisplacement experiments and from the thermodynamic analysis24,25 of potential step experiments.5 The voltammogram corresponding to the Pt(110) surface exhibits a similar behavior to that described above for the other platinum basal planes: the presence of urea results in a shift of the adsorption states toward lower potentials. However, this shift is not so considerable as that observed with the other basal planes, especially for the peak at lower potentials. The adsorption states are also sharper in the presence of urea. In this case, the charge density under the peaks (between 0.06 and 0.36 V) amounts to ca. 220 µC cm-2, after double layer correction. Similarly to what happens with other electrolytes, this charge density is larger than the value corresponding to the interchange of one electron per surface platinum atom in the unreconstructed (1 × 1) surface (149 µC cm-2). Hence, the voltammetric measurements on the Pt(110) surface also

Figure 2. Comparison of the opposite of the displaced charges at different potentials (squares) with the integration of the voltammetric profiles obtained before CO dosing (solid line, curve b) and after CO oxidation (dashed line, curve d). The corresponding voltammograms are also given for comparison in curves a and c, respectively. The inset shows the CO stripping and the succesive voltammetric curves recorded between 0.06 and 0.80 V prior to the recording of curve c.

point toward an oxidative adsorption of urea on this surface. To confirm this observation, CO displacement experiments have been performed on this surface as reported in Figure 2. Curve a in this figure corresponds to the cyclic voltammogram of the flame-treated surface. Full squares in the same figure correspond to the opposite of the displaced charged at different electrode potentials. The charge displaced at 0.1 V, i.e., around 155 µC cm-2 is consistent with previous results published for this surface in different electrolytes36,37 and agrees also with (36) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1992, 330, 489.

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the value expected from the displacement of a monolayer of hydrogen adsorbed on the surface at this potential, according to the process

Pt-H + CO f Pt-CO + H+ + e The experimental value for the displaced charge at 0.100 V, qdis(0.100 V), can be used for the integration of curve a in Figure 2 according to the equation

q)

|j(E)|dE - qdis(0.1 V) r

E ∫0.1V

(1)

where j(E) is the voltammetric current and r is the sweep rate. This equation relies on the good agreement of the difference between the displaced charges at two different potentials with the voltammetric charge density integrated between the same potential limits, which has been shown for a great number of systems in previous studies.13,36,37 The integrated charge values are shown in Figure 2, curve b, where they can be compared with the opposite of the displaced charges at different electrode potentials. This comparison has to be made by taking into account that the voltammetric and CO displacement experiments deal with the same amount of adsorbed species but with opposite processes, i.e., voltammetric adsorption and COinduced desorption.13,36,37 As it can be seen in Figure 2, the integrated charge is zero for potentials around 190 ( 10 mV which defines the pztc value for the Pt(110) electrode in the urea-containing solution. Negative q values correspond to the hydrogen-covered surface whereas positive q values reflect the presence of adsorbed urea at the electrode surface at potentials above the pztc. This is in agreement with the recording of reduction currents (i.e., negative displaced charges) during the CO-displacement experiments at potentials higher than those at which the voltammetric peaks are observed (0.300 V) according to reaction

Pt-ureaads + nH+ + ne + CO f Pt-CO + urea Taking into account the infrared spectra reported below, and similarly to that previously stated for urea adsorbed at the Pt(111) electrode,5 n is most likely equal to 1. On the other hand, the displaced charges amount to -25 and -50 µC cm-2 at 0.20 and 0.30 V, respectively. Note that the opposite of the displaced charge at 0.30 V is significantly lower than the integrated charge at the same potential as plotted in Figure 2, curve b. Also, we have found that after CO oxidation the Pt(110) surface does not recover exactly its initial state, as recorded before the CO dosing experiment. This was evidenced by the slightly different voltammetric profile recorded after the oxidative CO stripping from the surface (see inset and curve c in Figure 2). A similar behavior has been observed when working with Pt(110) electrodes in acidic solutions containing urea-related compounds such as oxamide and oxamic acid38 and, in a much lower extent, for the same surface in contact with both 0.1 M HClO436 and 0.5 M H2SO4.37 Even if the study of this hypothetical surface modification is beyond the scope of this paper it is worth recalling here that the sharpness of the main voltammetric peak and the charge density below the adsorption states above 0.20 V have been related to the existence of either (37) Clavilier, J.; Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A. in The Electrochemical Society Proceedings; Conway, B. E., Jerkiewicz, G., Eds.; The Electrochemical Society: Pennington, NJ, 1994; Vol. 94-21; p 167. (38) Thornes, D.; Climent, V.; Herrero, E.; Feliu, J. M. In preparation.

Figure 3. In situ FTIR spectra collected at different potentials for an urea-covered Pt(110) electrode in a 1 mM urea + 0.1 M HClO4 solution prepared in D2O: (A) p-polarized light; (B) s-polarized light. Reference potential: 0.05 V. 500 interferograms were collected at each potential.

(1 × 1) or (1 × 2) structures in the case of Pt(110) surfaces obtained under different cooling conditions.33 Thus, the different voltammetric profiles before and after CO oxidation in the presence of urea could be tentatively related to different proportions of (1 × 2) and (1 × 1) patches, whose ratio can be modified during this process. The good agreement between the value of the displaced charge at 0.10 V and that measured at the same potential in the absence of urea suggests that any possible surface modification would take place after CO displacement, most likely when CO is stripped in the urea-containing solution. Since the latter observations could cast some doubts about the reliability of the pztc value obtained for the Pt(110) electrode from the integration of curve a in Figure 2, we have also used eq 1 to integrate the voltammetric curve obtained after the stripping of the CO adlayer formed in the urea-containing solution (curve c in Figure 2). The resulting charge values are plotted in curve d. The main difference between this charge curve and that obtained for the voltammogram obtained before CO dosing (curve b) are the lower charge values obtained at potentials above 0.30 V. This is consistent with the lower currents observed in this potential region. However, it has to be noted that the pztc values derived from both curves c and d in Figure 2 are reasonably the same around 0.19 V. To obtain more information about the nature of the adsorbed species on the Pt(110) surface from urea containing solutions, we have performed in situ FTIR spectroscopic experiments. A set of spectra obtained at different potentials is shown in Figure 3. As in previous works devoted to the adsorption of urea on platinum singlecrystal electrodes, the spectroscopic experiments were carried out in solutions prepared in deuterium oxide in order to avoid the interference between the bands of urea in the frequency region between 1700 and 1300 cm-1 and

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the O-H bending mode of water around 1650 cm-1. The reference potential was chosen as 0.05 V, a potential negative enough to ensure the absence of contributions corresponding to adsorbed urea in the reference spectrum. A rather complicated set of bands between 1400 and 1700 cm-1 in the spectra collected at potentials located between 0.3 V and the onset of the surface oxidation indicate the presence of adsorbed urea in this potential region. These complex spectra are the result of the overlapping of negative-going bands, corresponding to vibrations in the adsorbed urea molecules, and positive-going bands. The latter are originated as a consequence of the depletion of dissolved urea in the thin-layer film after its adsorption when the potential is stepped from the reference to the sample potential. The frequencies of the positive-going bands, at 1609 and 1500 cm-1, which are better observed in the spectra collected with s-polarized light (Figure 3B), are in accordance with the C-O and asymmetric C-N stretching vibrations of deuterated urea in solution.39,40 Regarding the negative-going bands observed in the spectra of Figure 3A at potentials higher than 0.3 V, two different potential regions can be distinguished. At potentials below 0.6 V, the spectra show a clear band at 1627 cm-1, that changes into a band at 1580 cm-1 for potentials higher than 0.6 V. A similar change in the frequency of the band of the adsorbed urea with the applied potential was also observed on Pt(111) electrodes.5 This behavior was interpreted by considering a change in the mode of bonding of the urea molecules to the surface. In the lower potential range (and also lower coverage) the urea molecules would be bonded to the surface through one of the nitrogen atoms. This bonding geometry would cause a reinforcement of the CO bond, resulting in a blue shift of the corresponding vibrational mode. Conversely, at higher potentials (and urea coverage) the adsorption would take place through the oxygen atom, resulting in a weakening of the CO bond and a displacement of the band center toward lower wavenumbers. This interpretation parallels that usually done for interpreting the spectra of urea coordination compounds.41 The spectra also show characteristic features at wavenumbers around 1500 cm-1 that can be assigned to the asymmetric CN stretching of adsorbed urea molecules. The observation of a band in this region of the spectra indicates that the adsorbed molecule is tilted with respect to the direction perpendicular to the surface in such a way that the surface selection rule is satisfied. However, the overlapping between the adsorbate bands with the solution band at 1500 cm-1 does not enable us to perform a clear analysis of this part of the spectra. The evolution of urea coverage with the applied potential can be followed from the integrated intensity of the urea consumption solution band at 1609 cm-1, corresponding to the CO vibration mode. This has been made in Figure 4. It may be observed that urea adsorption is clearly detected at potentials around 0.20 V and reaches a plateau at around 0.3-0.4 V, then a 30% increase is produced at higher potentials. It is noteworthy that this increase is related to the voltammetric feature observed in the voltammogram (Figure 2) in the same potential region. This also agrees with the results given by charge displacement experiments, in the sense that urea adsorption (39) Yamaguchi, A.; Miyazawa, T.; Shimanouchi, T.; Mizushima, S. Spectrochim. Acta 1957, 10, 170. (40) Dobado, J. A.; Molina, J.; Portal, D. J. Phys. Chem. A 1998, 102, 778. (41) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination compounds, 4th ed.; John Wiley & Sons: New York, 1986.

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Figure 4. Plot of the integrated intensity of the solution band at 1609 cm-1 measured from the spectra in Figure 3B collected with s-polarized light as a function of the electrode potential.

Figure 5. Positive-going voltammetric profiles corresponding to four different Pt(S)[n(111) × (111)] stepped surfaces in 0.1 M HClO4 + 1 mM urea (solid line) and 0.1 M HClO4 (dashed lines). Sweep rate: 50 mV/s. It is also shown the integrated curve that gives the value of the total charge at each potential. Solid circles correspond to the experimental values obtained for the total charge through CO displacement. The empty circle indicates Eq ) 0.

takes place with charge transfer: the charging curves also show an increase in charge density in this potential region. 3.2. Stepped Surfaces Vicinal to Pt(111). Figures 5 and 6 show cyclic voltammograms obtained in a 1 mM urea + 0.1 M HClO4 solution with some selected stepped

Urea Adsorption on Platinum

Figure 6. As Figure 5 but for Pt(S)[n(111) × (100)] stepped surfaces.

surfaces vicinal to Pt(111) in the [11 h 0] and [011 h ] zones, respectively. The voltammograms recorded in the absence of urea for the same surfaces are also given for comparison. It has been reported that these stepped surface electrodes maintain their nominal terrace-step structure, Pt(S)[n(111) × (111)] and Pt(S)[n(111) × (100)], if the flame annealing procedure is carried out under appropriate conditions.30,31,42,43 From the effect of increasing step density on the voltammetric profiles reported in Figures 5 and 6 for the urea-free solution, distinct contribution from step and terrace sites can be identified. The introduction of the surface steps is reflected in the sharp voltammetric peak around 0.12 and 0.27 V for the (111) and (100) step symmetries, respectively, as inferred from the increase of the charge density under the corresponding peaks with the increase of the step density. After comparison with Figure 1 it is also evident that the contribution of (111) steps strongly resembles that of the Pt(110) basal plane. This observation is consistent with the appearance of (110) surface sites at the junction between the (111) terrace and step planes so that the Pt(S)[n(111) × (111)] surface structure could also be noted as Pt(S)[(n - 1)(111) × (110)].30,43 More evidence of this (110) type reactivity will be given below. The similarity between the voltammetric profiles with and without urea in solution at potentials lower than 0.35 V allows the current recorded in this potential region to be assigned mainly to hydrogen adsorption/desorption processes. However, it is clearly observed that the step-related features are shifted toward less-positive potentials as compared to that observed in perchloric acid solution (dashed lines in Figures 5 and 6). In a similar way, adsorption states at potentials higher (42) Rodes, A.; El Achi, K.; Zamakhchari, M. A.; Clavilier, J. J. Electroanal. Chem. 1990, 284, 245. (43) Clavilier, J.; El Achi, K.; Rodes, A. Chem. Phys. 1990, 141, 1.

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Figure 7. Plot of the pztc corresponding to Pt(S)[n(111) × (111)] (O, b) and Pt(S)[n(111) × (100)] (0, 9) as a function of the step density. Filled symbols stand for the pztc in the ureacontaining solution whereas open symbols correspond to the data obtained in the urea-free 0.1 M HClO4.

than 0.35 V corresponding to (111) terrace sites shift toward lower potential values as a consequence of the addition of urea to the solution. Charge displacement experiments have been carried out with these surfaces, and the resulting displaced charges at 0.10 V have been plotted in Figures 5 and 6, together with the integrated voltammetric charge, according to expression 1. The measured values of displaced charges at 0.10 V are the same, within the experimental error, to those obtained in 0.1 M perchloric acid, or 0.5 M sulfuric acid.14,28 By use of these values of displaced charge, the corresponding pztc values in urea-containing solutions can be estimated as described above for the Pt(110) electrode. From the obtained data it may be concluded that there is a displacement of the pztc toward lower potential values when the step density (numer of steps per centimeter) is increased. This behavior, which is very similar to that found in the previously reported studies carried out in perchloric14 and sulfuric acid28 solutions, is better observed in the plot of Figure 7. The pztc values obtained in perchloric acid are also included in Figure 7, to show the similarities between urea-containing solutions and an electrolyte solution of the same pH in which specific anion adsorption is not expected to take place. Note that the data for the Pt(110) electrode have been included in the plot for the Pt(S)[n(111) × (111)] surfaces since the former can be considered as the limiting stepped surface with n ) 2. The plots for the two series of stepped surfaces show that the pztc first decreases almost linearly with the increase of the number of steps both in the presence and in the absence of urea. However, when the step density is further increased, a plateau is attained, even reversing the tendency at the higher step densities. Figure 7 also shows that the presence of urea shifts toward less positive potentials the pztc values for the Pt(S)[n(111) × (100)] surfaces with n < 9 when compared with the corresponding values in the urea-free solution. This behavior is related

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Figure 8. In situ FTIR spectra collected at different potentials for an urea-covered Pt(331) ≡ Pt(S)[3(111) × (111)] electrode in a 1 mM urea + 0.1 M HClO4 solution prepared in D2O: (A) p-polarized light; (B) s-polarized light. Reference potential: 0.05 V. 500 interferograms were collected at each potential.

to the increasing adsorption of urea on the (100) steps which compensates the effect of hydrogen adsorption at potentials around 0.26 V which also increases as witnessed by the increasing intensity of the corresponding voltammetric peak in Figure 6. This makes the pztc value to be almost constant around the corresponding peak potential, i.e., 0.26 V. This behavior is similar to that observed when the pztc values obtained for the same surfaces in sulfuric and perchloric acid solutions 14,28 are compared. All these results stress the anion-like behavior of urea. Similarly to the strategy followed with the platinum single crystal basal planes, we have performed in situ FTIR measurements also with the platinum-stepped surfaces in contact with urea-containing solutions. Figures8 and 9 show spectra collected with p-polarized light for three selected electrodes having a high density of steps either with (111) symmetry (Pt(331)) or (100) symmetry (Pt(322) and Pt(311)). Additional spectra for the Pt(322) and Pt(311) electrodes have also been collected with s-polarized light (not shown) to help in the assignment the adsorbate bands. The spectra shown for Pt(331) and Pt(322) in Figures 8A and 9A, respectively, are both qualitatively very similar to those described above for Pt(110) and also to those previously reported for the Pt(111) electrode.5 Namely, a band at ca. 1630 cm-1 is observed at potentials lower than 0.6 V, which is transformed into a band at ca. 1580 cm-1 at potentials higher than 0.6 V. Also bands in the region around 1500 cm-1 are observed, overlapped with the positive band at 1500 cm-1, that can be assigned to the asymmetric CN stretching of adsorbed urea molecules. The result obtained in the case of Pt(331) ≡ Pt[3(111) × (111)] is quite logical because the junction of (111) terrace sites and (111) monatomic step sites defines a (110) site. In this way, this surface is better represented as Pt(331)

Climent et al.

Figure 9. As in Figure 8A but for (A) Pt(322) ≡ Pt(S)[5(111) × (100)] electrode and (B) Pt(311) ≡ Pt(S)[2(111) × (100)] electrode. 400 and 200 interferograms collected at each potential, for A and B, respectively.

≡ Pt[2(111) × (110)]. Since in both basal planes a similar spectroscopic response was obtained, the change in the bonding mode of urea could be expected to be the same as that reported in both limiting cases. The absence of bands corresponding to adsorbed urea molecules at potentials lower than 0.30 V is probably due to the overlapping between negative- (whose band center is shifted with the potential) and positive-going bands. Indeed, consumption bands can be observed at potentials between 0.15 and 0.3 V in the spectra collected with s-polarized light (Figure 8B), indicating that urea adsorption begins in this potential region. However, the extrapolation of this result to stepped surfaces with lower step density is not possible because the intensity of the bands falls below the detection limit. Even if the voltammetric peak at ca. 0.12 V in Figure 5 suggests that urea adsorbs at step sites, the analysis of the voltammetric charges (see below) suggests that the amount of adsorbed urea is not significant unless potentials higher than 0.34 V are applied. In the case of Pt(322) ≡ Pt[5(111) × (100)] it appears that the surface reactivity mainly reflects that of the fiveatom-wide terraces since the spectrum collected at 0.30 V does not show significant urea adsorption on the step sites (see Figure 9A). To point out the influence of the (100) steps, a stepped surface containing higher step density is required. Figure 9B shows spectra obtained under similar conditions with a Pt(311) electrode surface. This surface corresponds to the turning point between the Pt(S)[n(111) × (100)] and Pt(S)[n(100) × (111)] sets of surfaces and contains the highest step density attainable in the [01 h 1] zone. These spectra differ from those obtained with the other stepped surfaces in the absence of the band around 1580 cm-1 that appears at the higher potentials in the case of Pt(331) and Pt(322). Incidentally, the band around 1500 cm-1 assigned to the asymmetric stretching

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of CN bonds in the adsorbed urea molecules is absent in the spectra obtained with the Pt(311) surface. Only the band at 1660 cm-1 corresponding to the CO stretching of adsorbed urea molecules is observed in the spectra collected with this surface in the whole potential range. Thus, the resulting behavior is very similar to that obtained with the Pt(100) basal plane.4 This suggests that the urea molecules are preferentially bonded through the nitrogen atom in the whole potential range studied as far as the amount of (100) sites present on the surface become dominant. As a final comment, urea adsorption is clearly observed at 0.3 V, thus suggesting a greater adsorption of the molecule on this electrode in comparison to those previously described. Taking into account that the pztc of this electrode is quite similar to that of Pt(322), the reported behavior points out the special properties that appear when high step density electrode surfaces are considered. 3.3. Analysis of the Components of the Voltammetric Charge of Stepped Surfaces. The changes in the voltammograms of Figures 5 and 6 as a consequence of the introduction of steps into the ordered Pt(111) surface allow the assignment of the different voltammetric features to adsorption processes on step and terrace sites. The voltammetric peaks at 0.12 and 0.27 V in Figures 5 and 6, respectively, can be assigned to adsorption processes in the step sites, as deduced from the increase of its height as the step density in increased. In the same way, the remaining voltammetric charge below 0.34 V could be assigned, under this assumption, to hydrogen adsorption on terrace sites. Taking into account the values of the corresponding pztc’s and the FTIR experiments, the voltammetric features at potentials higher than 0.34 would correspond to urea adsorption on terrace sites, with charge transfer. To go further in this analysis, the voltammetric charge corresponding to each of these processes can be considered in more detail and compared with the charges corresponding to the step and terrace site densities predicted by the hard-sphere model of the stepped surfaces.42,43 According to this model, the theoretical charge density that could be expected as a result of the interchange of one electron per Pt step atom is given by the expressions42,43

qstep111 )

e 1 2e cos β ) N111 cos β (2a) d 2 3 d n3

qstep100 )

2e 1 e cos β ) N100 cos β (2b) d 31/2d2 n - 1 3

1/2 2

(

)

(

)

where n is the number of atomic rows in the terrace, d is the atomic diameter of platinum, e is the elementary charge of the electron, β is the angle between the planes of the terrace and the stepped surface, and N is the step density. The superscripts 111 and 100 refer to the symmetry of the step for the surfaces in the [11 h 0] and [011 h ] zones, respectively. Note that the former expresions would hold for the saturation of step sites with hydrogen. Similarly, the charge corresponding to the adsorption of hydrogen on terrace sites is given by42

qterrace100 ) θmax

2e (n - 1) cos β ) 3 d n-1 3 2e θ N100 cos β (3) qPt(111) 3d max 1/2 2

( [

)

]

for the Pt(S)[n(111) × (100)] surfaces. The factor θmax takes into account that the hydrogen saturation coverage on the Pt(111) ordered surface is not equal to unity (θmax = 0.66)35 whereas

qPt(111) ) θmax

2e 31/2d2

(4)

stands for the hydrogen charge on the Pt(111) surface (N ) 0 and β ) 0), which amounts to 160 µC cm-2. As a first approximation, the maximum hydrogen coverage on terrace sites has been considered as independent of the number of atomic rows in the terrace, although this assumption is expected to fail at very short terrace width. To obtain a similar expression for the Pt(S)[n(111) × (111)] stepped surfaces, it is necessary to take into account that the intersection of the two (111) sites defines a (110) site and the surface structure can be also noted as Pt(S)[(n 1)(111) × (110)]. Hence, it is possible to obtain two different equations, depending on the symmetry of the step site that is assumed43

qterrace111 ) θmax

2e (n - 1) cos β ) 3 d n-2 3 2e θ N100 cos β (5a) qPt(111) 3d max 1/2 2

(

)

[

qterrace110 ) θmax

]

2e (n - 2) cos β ) 3 d n-2 3 4e qPt(111) θ N110 cos β (5b) 3d max 1/2 2

[

(

)

]

These equations correspond to lines b and c in Figure 10A. Note that the step density is the same irrespective of the (111) or (110) step symmetry for the surfaces in the [11 h 0] zone (N111 ) N110) and thus eq 2 holds in both cases to calculate the corresponding charge densities. The comparison between the experimental and the hardsphere charge density values has been done in the plots of Figure 10. In these graphs, the straight lines correspond to the expected charges from the hard-sphere model for steps (a) and terraces (b or c) when plotting q/cos β vs the step density. Experimental charge densities corresponding to adsorption at step and terrace sites below 0.34 V have been integrated from the voltammograms of Figures 5 and 6. A reasonable straight baseline has been used to separate both contributions, as indicated in the first voltammogram of Figures 5 and 6. Figure 10A shows that there is a good agreement between the experimental charges corresponding to adsorption at step sites (filled circles) and those predicted from the model for stepped surfaces in the [11h 0] zone with n > 4. This gives support to the assignment of adsorption sites described at the beginning of the discussion section and suggests that urea molecules do not adsorb significantly with charge transfer on step sites at potentials lower than 0.34 V. The small shift of the peak potential toward lower values, observed in the voltammograms recorded in urea-containing solutions (Figure 5), would not be related with significant charge contributions related to urea adsorption. Strictly speaking, the previous analysis and conclusions cannot be extended to stepped surfaces with shorter terraces. This could be due to the uncertainty in the deconvolution of the charge corresponding to adsorption on terrace and step sites. Deviations of the experimental values from those predicted by the model

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Climent et al.

Figure 10. Plot of the deconvoluted charge densities corresponding to adsorption states at potentials lower than 0.34 V on terrace (squares) and step (circles) sites, as a function of the step density. Open squares correspond to charge corresponding to adsorption on terrace sites prior to double layer correction. The solid lines correspond to the predicted values for the step (a) and terrace (b) charge densities from the hard-sphere model of the (A) Pt(S)[n(111) × (111)] and (B) Pt(S)[n(111) × (100)] stepped surfaces. Dashed line (c) correspond to the terrace charge density for the Pt(S)[(n - 1)(111) × (110)] surface structure.

are observed at lower step densities for the surfaces in the [011h ] zone (Figure 10B). These deviations can be explained by considering that urea adsorption with charge transfer is contributing somewhat to the charge under the peak previously attributed to hydrogen adsorption on (100) steps. Once again, the shift of the hydrogen adsorption energy on step sites, as reflected in the small shift of the peak potential in Figure 6, is a consequence of the presence of urea in solution. Obviously, the former analysis could not discard some interaction of urea molecules with step sites without charge transfer. It has to be recalled here that the spectroscopic results shown in Figures 8 and 9 indicate that urea adsorption on Pt(331) and Pt(311) starts at potentials significantly lower than 0.3 V. To measure the charge corresponding to adsorption on terrace sites from the voltammetric current density below 0.34 V, a correction to subtract the double layer charging contribution should be considered. Classically, this contribution is estimated by assuming a constant value of double layer capacity in the whole range of potential. This value is usually estimated from the voltammetric current in a potential range where only capacitative current is expected. Such a potential region is clearly observed in the voltammograms of the stepped surfaces in 0.1 M HClO4 (dashed lines in Figures 5 and 6). However, this double layer capacity value is hardly obtained in the voltammograms recorded in the urea-containing solution. An alternative approach is to plot the overall charge values, without any double layer correction. These uncorrected values, which have been plotted in Figure 10 (open squares), lie parallel to the straight lines corresponding to eqs 3 and 5b. This suggests that a constant double layer correction should be applied in order to obtain the hydrogen adsorption on each surface. The final values, obtained after this uniform double layer correction is performed, are in good agreement with the values predicted by the step-terrace model for surfaces n > 4 and validates the assumptions made when deducing eqs 3 and 5. Deviation of the charges attributed to hydrogen adsorption on terrace sites observed for the surfaces with shorter terraces can

be explained considering that θmax deviates from the constant value corresponding to the Pt(111) ordered surface. Another important feature of the plot of Figure 10A is that the experimental charges corresponding to the hydrogen adsorption on terrace sites of the stepped surfaces in the [110] zone agree better with the expected charges from the consideration of a (110) step symmetry (line c). This is not unexpected, since similar results have been reported previously for different sulfuric acid solutions.43 A similar analysis can be done with the charge corresponding to the adsorption states adscribed to urea adsorption on terrace site, i.e., those appearing at potentials higher than 0.34 V in the voltammograms reported in Figures 5 and 6. Figure 11A shows plots of this charge as a function of the charge corresponding to the interchange of one electron per terrace platinum atom (as given by eqs 3 and 5b with θmax ) 1) for the stepped surfaces with (100) (open symbols) and (110) (filled symbols) steps, respectively. The resulting plots show a linear relation between both magnitudes, which reflects a nearly constant coverage of urea on the (111) terraces irrespective of their width. This is better observed in the plot of Figure 11B, where the urea coverage, calculated as the ratio between the urea charge and the nominal charge corresponding to one electron per terrace site, has been plotted as a function of the step density. A nearly constant value of coverage is obtained for terrace width higher than five atoms, although a small decrease is observed at higher step densities. 4. Conclusions This work completes previous studies on the adsorption of urea at platinum single crystal electrode surfaces with additional data regarding the electrochemical behavior of this molecule at Pt(110) and stepped surfaces vicinal to Pt(111). Results reported in this paper are mainly consistent with the anion-like character of urea adsorption.

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Figure 11. (A) Plot of the charge corresponding to adsorption states at potentials between 0.35 and 0.7 V, attributed to urea adsorption on terrace sites, as a function of the theoretical terrace charges given by the hard-sphere model for the Pt(S)[n(111) × (111)] stepped surfaces (filled symbols) and Pt(S)[n(111) × (100)] stepped surfaces (open symbols). (B) Urea coverage on terraces as a function of the step density, same symbols as in (A).

This is witnessed, for example, by the effect of increasing the step density on the shift of the pztc in the presence of urea which is similar to that previously reported in sulfuric acid solutions. As in the case of (bi)sulfate anions, the presence of steps increases the adsorption of urea at lower potentials as shown in the infrared spectra obtained for surfaces with a high density of steps. On the other hand, adsorption of urea at (111) terraces takes place at potentials higher than 0.34 V with charge transfer, probably related to the deprotonation of the adsorbate, and with a nearly constant coverage, irrespective of the terrace width. This latter behavior, which is not observed in the case of (bi)sulfate anions, confirms previous observations of Wieckowski’s group with electrochemically perturbed Pt(111) electrodes. Another piece of information presented in this paper is related with the potential-dependent changes in the bonding of urea at platinum electrodes. Spectra collected

for the Pt(110) electrode are similar to those previously reported for Pt(111) and suggest the existence of N-bonded urea molecules at low coverages whereas O-bonded species predominate at high coverages. The same behavior is observed for stepped surfaces containing (111) terraces and (110) steps. On the other hand, urea bonded through the two nitrogen atoms is detected in the whole coverage range at Pt(311) surfaces, which contains a high density of (100) steps. Acknowledgment. Financial support from DGI Project No. BQU2000-0240 is gratefully acknowledged. The authors are also grateful to the Conselleria d’Educacio´ i Cie`ncia de la Generalitat Valenciana for the funds for the purchase of the FTIR facility. LA011122N