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Urea Adsorption at Rhodium Single-Crystal Electrodes V. Climent, A. Rodes,* J. M. Pe´rez, J. M. Feliu, and A. Aldaz Departamento de Quı´mica Fı´sica, Universidad de Alicante, Apartado 99, E-03080 Alicante, Spain Received May 3, 2000. In Final Form: July 19, 2000 Structural aspects for the adsorption of urea at rhodium electrodes have been studied in perchloric acid solutions by combining electrochemical and in-situ FTIR spectroscopy experiments with single-crystal surfaces. Urea molecules adsorb at the rhodium electrode surface in competition with hydrogen and oxygenated species. Potential-dependent coverage changes, similar to those typically observed with specifically adsorbed anions, have been found to be structure-sensitive. Strong adsorption of urea molecules at the Rh(100) electrode surface allows the isolation of an irreversibly adsorbed layer that can be characterized in urea-free solutions. The infrared spectra collected for each rhodium surface show different adsorption bands for the adsorbed urea molecules that can be related to different orientations and/or adsorption sites. Urea molecules seem to adsorb always at the Rh(100) surface through the two nitrogen atoms with the C-O axis having a nonzero component perpendicular to the electrode surface. Only O-bonded urea molecules are detected at the Rh(111) electrode surface. Potential-dependent changes for the intensity of the C-O stretching band can be understood on the basis of the tilting of the C-O axis at potentials higher than 0.50 V. In the case of the Rh(110) electrode, O-bonded and N-bonded oriented molecules predominate at low and high potentials, respectively. This change in the orientation of the adsorbed urea molecules can be related to the competitive adsorption of oxygenated species.
Introduction The in-situ characterization of adsorbate layers is an important aspect in the study of the adsorption processes taking place at the metal/solution interface.1 The structuresensitive character of some of these processes allows the use of the adsorbate species as a probe for the electrode surface. The adlayers of carbon monoxide,2-4 nitric oxide,3,5,6 and different inorganic and organic anions7 have been studied with this approach at platinum-group electrodes. From suitable combinations of structuresensitive techniques and well-defined electrode surfaces a variety of electrochemical, spectroscopic, and structural data have been obtained.1-7 The study of some other molecules can be envisaged within the same approach. Urea is an enticing choice not only because of its simplicity but also due to its structural relation with other nitrogen-containing molecules. Previous studies on urea adsorption at polycrystalline8-10 and single-crystal 11-19 platinum electrodes have shown the * To whom correspondence should be addressed. Fax: 34.965903537. Phone: 34.965903400 (ext 2602). E-mail: Antonio.
[email protected]. (1) Adsorption of Molecules at Metal Electrodes; Lipkowski, J., Ross, P. N., Eds.; VCH Publishers: New York, 1995. (2) Villegas, I.; Weaver, M. J. J. Chem. Phys. 1994, 101, 1. (3) Weaver, M. J.; Zou, S. Z.; Tang, C. J. Chem. Phys. 1999, 111, 368. (4) Lucas, C. A.; Markovic, N. M.; Ross, P. N. Surf. Sci. 1999, 425, L381. (5) Rodes, A.; Go´mez, R.; Pe´rez, J. M.; Feliu, J. M.; Aldaz, A. Electrochim. Acta 1996, 41, 729. (6) Rodes, A.; Climent, V.; Orts, J. M.; Pe´rez, J. M.; Aldaz, A. Electrochim. Acta 1998, 44, 1077. (7) Iwasita, T.; Nart, F. C. Prog. Surf. Sci. 1997, 55, 271. (8) Horanyi, G.; Inzelt, G.; Rizmayer, E. M. J. Electroanal. Chem. 1979, 98, 105. (9) Bolzan, A. E.; Iwasita, T. Electrochim. Acta 1988, 33, 109. (10) Bezerra, A. C. S.; deSa, E. L.; Nart, F. C. J. Phys. Chem. B 1997, 101, 6443. (11) Rubel, M.; Rhee, C. K.; Wieckowski, A.; Rikvold, P. A. J. Electroanal. Chem. 1991, 315, 301. (12) Rhee, C. K. J. Electrochem. Soc. 1992, 139, 13C. (13) Rikvold, P. A.; Wieckowski, A. Phys. Scr. 1992, T44, 71. (14) Gamboa-Aldeco, M.; Mrozek, P.; Rhee, C. K.; Wieckowski, A.; Rikvold, P. A.; Wang, Q. Surf. Sci. Lett. 1993, 297, L135.
existence of a reversible and structure-sensitive anionlike behavior. Structural aspects of urea adsorption were first reported by Wieckowski’s group.11-16 The combination of voltammetric,11,12 radiotracer,14 and ex-situ LEED12,14 and AES12,14 experiments showed the existence of an ordered c(2 × 4) urea adlayer strongly adsorbed at the Pt(100) electrode surface with a surface coverage around 0.25. A theoretical description of the formation of this adlayer was given by using a Monte Carlo simulation of a lattice-gas model that accounts for the shape of the voltammetric curves obtained in the urea-containing solutions.13-16 An in-situ FTIR study carried out by our group confirmed later that urea adsorbs at the Pt(100) electrode without dissociation of the C-N bond and showed that the molecule is bonded through the two nitrogen atoms,17,18 as previously suggested by Rikvold et al.15 This spectroscopic study was extended to the urea adlayers formed at the Pt(111) electrode,19 showing that urea adsorbs through only one of the nitrogen atoms at low coverage. However, O-bonded urea molecules predominate at higher potentials for which a saturation coverage of ∼0.45 is attained.19 This coverage value was confirmed by the analysis of the charge density involved in the urea adsorption process as measured from charge-displacement and potential step experiments.19 More recently the adsorption of urea at Au(100) and Au(111) single-crystal electrodes has also been studied by Nakamura et al.20 The in-situ infrared spectra obtained by these authors showed also that the coordination of the urea molecule to this metal is structure-sensitive. Po(15) Rikvold, P. A.; Gamboa-Aldeco, M.; Zhang, J.; Han, M.; Wang, Q.; Richards, H. L.; Wieckowski, A. Surf. Sci. 1995, 335, 389. (16) Rikvold, P. A.; Zhang, J.; Sung, Y. E.; Wieckowski, A. Electrochim. Acta 1996, 41, 2175. (17) Climent, V.; Rodes, A.; Orts, J. M.; Feliu, J. M.; Pe´rez, J. M.; Aldaz, A. Langmuir 1997, 13, 2380. (18) Climent, V.; Go´mez, R.; Herrero, E.; Orts, J. M.; Rodes, A.; Feliu, J. M.Colloid Surf., A 1998, 134, 133. (19) Climent, V.; Rodes, A.; Orts, J. M.; Aldaz, A.; Feliu, J. M. J. Electroanal. Chem. 1999, 461, 65. (20) Nakamura, M.; Song, M. B.; Ito, M. Surf. Sci. 1999, 427-428, 167.
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tential- and time-dependences of the spectra obtained for the Au(100) electrode were related to the kinetics of the lifting of the (5 × 20) surface reconstruction. In this paper we extend the spectroelectrochemical study of urea adsorption to well-defined rhodium electrodes. Rh(100), Rh(111), and Rh(110) single crystals have been prepared, and their behavior in the presence of urea has been analyzed. Voltammetric and spectroscopic results are compared to those previously reported for platinum surfaces. Experimental Section Rhodium single-crystal electrodes were prepared following the method developed by Clavilier.21 Samples with diameters about 2.0 and 4.0 mm were employed for the electrochemical and spectroelectrochemical experiments, respectively. Clean and wellordered surfaces were obtained by flame annealing in a gasoxygen flame and cooling in a H2 + Ar atmosphere.22 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 acid (Merck Suprapur), urea (Merck for molecular biology), and ultrapure water (Millipore MilliQ). Solutions used in the infrared experiments were prepared in deuterium oxide (Sigma) as received. Prior to each experiment, solutions were deaerated by bubbling Ar (L’Air Liquide N50). Charge displacement experiments are described in detail elsewhere.23,24 They involve the recording of the current transient while dosing CO (L’Air Liquide N47) at controlled potentials. Electrode potentials in the voltammetric and charge displacement experiments were measured against a reversible hydrogen electrode (RHE) whereas a Pd/H2 electrode was used in the spectroelectrochemical experiments. All potentials are referred to the RHE scale. Spectroelectrochemical experiments were performed with a Nicolet Magna 850 spectrometer equipped with a ΜCT detector. The spectroelectrochemical cell25 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 values corresponding to the sample and reference single beam spectra, respectively.
Results Curve a in Figure 1 shows the cyclic voltammogram obtained for the Rh(100) electrode in the 0.1 M HClO4 test solution. This curve, together with those shown below for Rh(111) and Rh(110), is similar to the voltammetric profiles previously reported in the literature for rhodium single-crystal electrodes in the same test electrolyte.26-34 As discussed in these papers, the irreversible reduction (21) Clavilier, J.; Armand, D.; Sun, S.; Petit, M. J. Electroanal. Chem. 1986, 205, 267. (22) Clavilier, J.; El Achi, K.; Petit, M.; Rodes, A.; Zamakhchari, M. A. J. Electroanal. Chem. 1990, 295, 333. (23) Clavilier, J.; Albalat, R.; Go´mez, R.; Orts, J. M.; Feliu, J. M. J. Electroanal. Chem. 1993, 360, 325. (24) Orts, J. M.; Go´mez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. Electrochim. Acta 1994, 39, 1519. (25) Iwasita, T.; Nart, F. C.; Vielstich, W. Ber. Bunsen-Ges. Phys. Chem. 1990, 94, 1030. (26) Hourani, M.; Wieckowski, A. J. Electroanal. Chem. 1987, 227, 259. (27) Wasberg, M.; Hourani, M.; Wieckowski, A. J. Electroanal. Chem. 1990, 278, 425. (28) Chang, S.; Weaver, M. J. J. Electroanal. Chem. 1990, 285, 263. (29) Rhee, C. K.; Wasberg, M.; Horanyi, G.; Wieckowski, A. J. Electroanal. Chem. 1990, 291, 281. (30) Yau, S. L.; Gao, X.; Chang, S.; Schardt, B. C.; Weaver, M. J. J. Am. Chem. Soc. 1991, 113, 6049. (31) Rhee, C. K.; Wasberg, M.; Zelenay, P.; Wieckowski, A.Catal. Lett. 1991, 10, 149. (32) Clavilier, J.; Wasberg, M.; Petit, M.; Klein, L. H. J. Electroanal. Chem. 1994, 374, 123. (33) Sung, Y. E.; Thomas, S.; Wieckowski, A. J. Phys. Chem. 1995, 99, 13513.
Figure 1. Cyclic voltammograms recorded for a Rh(100) singlecrystal electrode in a 0.1 M HClO4 solution in the absence (a) and in the presence of 1 mM urea (b,c). Curves b1 and b2 were recorded during the first and second voltammetric cycle whereas curve c corresponds to the stationary voltammogram in the potential region explored. Sweep rate: 50 mV s-1 (the same for all voltammograms).
of perchlorate anions to chloride takes place in the potential region between 0.35 and 0.20 V.29,31-33 Thus, the voltammetric peaks observed during the negativegoing sweep in the low-potential region can be related to the desorption of the resulting chloride anions together with the adsorption of hydrogen. Oxygen adsorption/ desorption processes give rise to characteristic features in the voltammogram at potentials higher than 0.50 V. All these processes are structure-sensitive, giving rise to characteristic curves for each electrode orientation. The addition of urea to the test solution causes significant changes in the adsorption/desorption behavior of the rhodium electrodes. Figure 1 shows curves obtained for the Rh(100) surface in a 1 mM urea + 0.1 M HClO4 solution. Curves b1 and b2 in this figure were recorded during the first and second voltammetric cycles, respectively, whereas curve c corresponds to the stationary voltammogram recorded in the urea-containing solution. The current densities in curves b and c are lower than those measured in the absence of urea (curve a) and decrease steadily with cycling between 0.05 and 0.80 V. It is clear from Figure 1 that some adsorbed species are formed from urea molecules. This adsorbate is blocking the electrode surface for the adsorption processes taking place in the perchloric acid test electrolyte. No additional information about the nature of this species can be derived from this voltammetric experiment. We tried to displace the adsorbed species formed in the urea-containing solution by dosing CO at controlled potentials between 0.10 and 0.50 V. However, these experiments failed to form the saturated CO layer on the Rh(100) electrode, indicating a very strong interaction between the urea adsorbates and the electrode surface. It is worth noting that the adsorbate coming from urea can be isolated at the Rh(100) electrode surface in the absence of urea in the solution phase. The cyclic voltammogram obtained in the (34) Wan, L. J.; Yau, S. L.; Swain, G. M.; Itaya, K. J. Electroanal. Chem. 1995, 381, 105.
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0.1 M HClO4 solution for the urea-covered electrode after immersion in the urea-containing solution at open circuit (not shown) is similar to curve b2 in Figure 1. To ascertain the nature of the adsorbate formed from urea at the Rh(100) electrode, we have performed in-situ FTIR spectroscopy experiments. Series of infrared spectra collected at different potentials for the urea-covered Rh(100) electrode are reported in Figure 2A and B. As in previous works devoted to the adsorption of urea on platinum single-crystal electrodes,17-19 the spectroscopic experiments were carried out in solutions prepared in deuterium oxide. The interference between urea bands in the frequency region between 1700 and 1300 cm-1 and the O-H bending mode of water at ∼1650 cm-1 is avoided under these conditions. It is worth noting that the same voltammetric response was observed for the rhodium electrodes immersed in the urea-containing solution irrespective of the use of H2O or D2O as solvent. The spectra shown in Figure 2A were obtained in the 1 mM urea + 0.1 M HClO4 solutions and are referred to a single-beam spectrum collected at 0 V in the same experiment. Positive-going bands at around 1600 and 1497 cm-1 are clearly observed for the spectrum collected at 0.60 V. These bands, which are also observed in the spectra collected with s-polarized light (not shown), correspond to the CdO and asymmetric CsN stretching modes of free deuterated urea molecules.35-39 As in the case of Pt(100) electrodes, a single negative-going band is observed at frequencies above 1600 cm-1 in the spectra reported in Figure 2A. This band corresponds to the adsorbate molecules present at the electrode surface, as evidenced by its absence in the spectra collected with s-polarized light. We will discuss later the assignment of this band in connection with the orientation of the adsorbate with respect to the electrode surface. However, we can state now than the adsorbate bands correspond to the CdO stretching mode of adsorbed urea molecules. The changes in the intensity of this band at potentials below 0.30 V are mainly related to the progressive stripping of the urea adlayer. This process seems to be completed after holding the electrode potential at 0 V. Note that this potential is lower than that required to remove urea molecules adsorbed on platinum electrodes.17,19 The frequency shift of this band in the spectra collected at different potentials is also observed for the urea-covered Pt(100) electrode and is related both to changes in the urea coverage and to effects of the electrode potential.17 The red shift of the adsorbate band when the electrode potential decreases gives rise to partial overlapping between this band and that at around 1600 cm-1 for the CdO stretching band of dissolved urea. In the experiment corresponding to Figure 2B, the adsorbed layer formed from the urea solution at open circuit was generated and the Rh(100) electrode transferred to the spectroelectrochemical cell. Then the voltammetric response for the sample was recorded between 0.30 and 0.60 V and the infrared spectra were collected at different electrode potentials from 0.60 V down to 0 V. As in the case of the experiment reported in Figure 2A, the spectrum collected at this latter potential was used as the reference spectrum. All the resulting spectra for (35) Yamaguchi, A.; Miyazawa, T.; Shimanouchi, T.; Mizushima, S. Spectrochim. Acta 1957, 10, 170. (36) Saito, Y.; Machida, K.; Uno, Y. Spectrochim. Acta Part A 1971, 27, 991. (37) Derremaux, P.; Vergoten, G.; Lagant, P. J. Comput. Chem. 1990, 11, 560. (38) Vijay, A.; Sathyanarayana, D. N. J. Mol. Struct. 1993, 295, 245. (39) Rousseau, B.; Vanalsenoy, C.; Keuleers, R.; Desseyn, H. O. J. Phys. Chem. A 1998, 102, 6540.
Figure 2. In-situ FTIR spectra collected at different potentials for an urea-covered Rh(100) electrode (A) in the 1 mM urea + 0.1 M HClO4 solution (100 interferograms were collected at each potential) and (B) in a 0.1 M HClO4 solution after the irreversible adsorption of urea (200 interferograms collected at each potential). Reference potential: 0 V.
Urea Adsorption at Rhodium Single-Crystal Electrodes
Figure 3. Cyclic voltammograms for a Rh(111) single-crystal electrode in a 0.1 M HClO4 solution: (a) in the absence of urea; (b) in the presence of 1 mM urea.
potentials higher than 0.30 V show clear-cut negativegoing bands with their frequencies shifting from 1638 to 1621 cm-1 when the electrode potential is changed from 0.80 to 0.30 V, respectively. The band appears in the same frequency range as that observed in Figure 2A. From these observations it can be stated that the same adsorbate species is formed in both cases. It can be concluded from the spectra in Figure 2B that the desorption of the adlayer starts when the electrode is held at potentials lower than 0.20 V. Dissolved urea molecules released upon desorption of the adlayer diffuse out of the thin layer of solution sampled by the infrared beam. This process makes difficult the detection of dissolved urea molecules at the reference potential under the present experimental conditions. Figure 3 shows the cyclic voltammogram obtained for a Rh(111) electrode in a 1 mM urea + 0.1 M HClO4 solution (curve b). This curve can be compared in the same figure with that recorded in the absence of urea (curve a). As discussed before for the Rh(100) surface, oxygen adsorption at high potentials is blocked, suggesting the presence of adsorbed urea molecules in this potential range. On the other hand, the reduction peak observed at 0.13 V in the absence of urea is sharpened and shifted toward less positive potentials in the urea-containing solution. The adsorption of urea at potentials above 0.15 V will be confirmed by the spectroscopic data reported below. Thus, a fraction of the charge associated with the voltammetric peaks observed below 0.30 V can be related to the adsorption/desorption of urea. The total charge density measured between 0.05 and 0.30 V amounts to ∼300 µC cm-2. The charge related to hydrogen adsorption at the lower limit of this potential region can be estimated by recording the transient current flowing during the potentiostatic displacement of the adsorbed species by adsorbing CO. The integrated charge density amounted to +198 µC cm-2 at 0.10 V. This value is similar to that recorded for the Rh(111) electrode at the same potential in a 0.5 M H2SO4 solution,40 suggesting that the same (40) Go´mez, R. Tesi Doctoral. Universitat d'Alacant, 1994.
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Figure 4. In-situ FTIR spectra collected at different potentials for a urea-covered Rh(111) electrode in a 1 mM urea + 0.1 M HClO4 solution: (A) p-polarized light; (B) s-polarized light. 500 interferograms were collected at each potential.
processes (mainly the desorption of a hydrogen adlayer) are taking place at this potential, irrespective of the test electrolyte
Rh(111)-H + CO + H2O f Rh(111)-CO + H3O+ + e- (1) On the other hand, reduction transient currents are observed when dosing CO at 0.30 V in the urea-containing solution, giving rise to a measured charge density of -109 µC cm-2 that can be adscribed to the reductive desorption of urea. This process could be written in a general way as follows
Rh(111)-(urea)z+ad + CO + mH3O+ + ne- f Rh(111)-CO + urea + mH2O (2) Note that the sum of the absolute values of the charge densities displaced at 0.10 and 0.30 V accounts for the overall voltammetric charge measured in the region between these two potential limits. Figure 4A and B shows series of spectra collected at different potentials for the Rh(111) electrode in the ureacontaining solution. These series were obtained with pand s-polarized light, respectively. The reference spectrum was collected under the same conditions at 0.05 V, that is, for a urea-free surface. Negative-going bands appearing at potentials higher than 0.15 V in Figure 4A correspond to different vibrational modes of the adsorbed urea molecules. These bands overlap with the absorption bands coming from dissolved urea at the reference potential. Solution bands are better observed in the spectra obtained with s-polarized light (Figure 4B), for which the adsorbate bands are absent.The increase of the intensity of these bands when the electrode potential is raised reflects the existence of increasing urea coverages. As a main difference from the spectra reported above for the Rh(100) electrode, two adsorbate bands are observed at around 1575 and 1397 cm-1. The intensities of these two bands increase in the potential region up to 0.50 V. At more
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Figure 5. Cyclic voltammograms for a Rh(110) single-crystal electrode in a 0.1 M HClO4 solution in the absence of urea (a) and in the presence of 1 mM urea (curves b and c).
positive potentials, the intensity of the band at 1575 cm-1 decreases whereas the band at 1397 cm-1 is replaced by a new band at ∼1369 cm-1. Figures 5 and 6 show results for the adsorption of urea at a Rh(110) electrode. The voltammetric curves obtained for this surface in the 1 mM urea + 0.1 M HClO4 (curves b and c) and 0.1 M HClO4 (curve a) solutions are reported in Figure 5. Voltammetric currents after immersion in the urea solution decrease with cycling between 0.05 and 0.70 V. Integrated charge density values ranged from 300 µC cm-2 in the first cycle (curve b) to 216 µC cm-2 in the stationary voltammogram (curve c). This behavior suggests the formation of some irreversibly adsorbed adlayer at the electrode surface. On the other hand, since the voltammetric charge under curve c in Figure 5 is still comparable to that measured in the same potential region for the Rh(110) electrode in the urea-free solution (202 µC cm-2), it can be concluded that the urea adsorption/ desorption process is contributing to some extent to the voltammetric current. In this way, it seems that urea adsorbs at potentials just positive of the nearly symmetric couple of voltammetric peaks at 0.15 V. These peaks can be related to the competitive urea and hydrogen coadsorption. On the other hand, the lower voltammetric currents observed at potentials higher than 0.40 V in the presence of urea indicate the blocking of the rhodium sites for the adsorption of oxygenated species. The spectra obtained at different potentials for the Rh(110) electrode in the 1 mM urea + 0.1 M HClO4 solution are shown in Figure 6A and B. The series collected with s-polarized light in Figure 6B shows the typical bands at 1605 and 1497 cm-1 for the CdO stretching and the asymmetric C-N stretching of dissolved deuterated urea. The highest intensities of these bands are observed for potentials between 0.40 and 0.50 V. Negative-going bands for the adsorbed urea molecules appear at 1548 and 1380 cm-1 for potentials below 0.50 V. The spectrum collected at this latter potential shows a new adsorbate band at 1636 cm-1. The intensity of this band increases, and its frequency is blue-shifted in the potential region between 0.50 and 0.70 V. These changes are paralleled by the decrease of the bands at ∼1570 and ∼1380 cm-1 and the appearance of a new band at 1338 cm-1. Finally, the
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Figure 6. In-situ FTIR spectra collected at different potentials for a urea-covered Rh(110) electrode in a 1 mM urea + 0.1 M HClO4 solution: (A) p-polarized light; (B) s-polarized light. 500 interferograms were collected at each potential.
intensities of all the adsorbate bands decrease at potentials above 0.70 V. Discussion and Conclusions The results obtained from the electrochemical and spectroscopic experiments described above can be used to shed some light on the structural aspects of urea adsorption at rhodium electrodes. Changes in the voltammetric profile obtained for each rhodium single-crystal electrode upon the addition of urea to the perchloric acid solution can be related to the formation of an adsorbed species. This adsorbate is strongly adsorbed on the Rh(100) electrode surface and blocks most of the surface sites for the processes that take place at the electrode surface in the perchloric acid solution. On the other hand, adsorption of urea takes place at the Rh(111) electrode surface in competition with hydrogen and oxygen species in a reversible way, similar to that observed for different specifically adsorbed anions. A mixed behavior is observed for the Rh(110) electrode. It is important to note that the formation of the adlayer from dissolved urea molecules is a reversible process in the sense that undissociated urea molecules are released when the adlayer is desorbed at low potentials. The irreversible adsorption process detected for Rh(100) and, to a lower extent, for Rh(110) electrodes does not imply the dissociation of the C-N bonds of the urea molecule. The in-situ infrared spectra obtained in the presence of urea can be used to confirm the chemical nature of these adsorbed species and to gain additional information about their bonding geometries. The infrared spectra previously reported for urea coordination compounds can be taken as a guide for this purpose. Penland et al.41 first proposed the use of the frequencies for the CdO and CsN stretching modes of urea in a given coordination complex as an indication of its bonding to the metal either through the oxygen or through one of the nitrogen atoms. In this way, CsO stretching frequencies observed in some cases at (41) Penland, R. B.; Mizushima, S.; Curran, C.; Quagliano, J. V. J. Am. Chem. Soc. 1957, 79, 1575.
Urea Adsorption at Rhodium Single-Crystal Electrodes
wavenumbers higher than those for the same mode of the free urea molecules were related to bonding through the nitrogen atom. This implies a reinforcement of the doublebond character of the CsO bond, which is paralleled by a weakening of the CsN bond, as witnessed by the red shift of the corresponding CsN stretching band.41 The opposite behavior, that is, negative and positive shifts of the CsO and CsN stretching bands, respectively, was related to bonding through the oxygen atom.41 The same approach was used to discriminate between O- and N-bonded urea in different coordination compounds.42-45 The first conclusion we can derive from the comparison of the spectra reported in this paper and those for the urea coordination compounds mentioned above is that adsorbed urea can account for all the adsorbate bands observed for the rhodium single-crystal electrode surfaces. Bands above 1500 cm-1 can be assigned to the C-O stretching mode of adsorbed urea while bands appearing at lower wavenumbers can be associated to the asymmetric C-N stretching mode for the same species. At the same time, we can suggest that the observation of different bands for the adsorbate at the Rh(100), Rh(111), and Rh(110) electrodes is related to the existence of different adsorption geometries for adsorbed urea. In the case of the Rh(100) surface a single adsorbate band was observed irrespective of the electrode potential. Since its frequency lies above 1600 cm-1, we can state that this band corresponds to the C-O stretch of N-bonded urea. Additional information about the bonding of the urea molecule at the Rh(100) electrode can be derived from the absence of any adsorbate band in the asymmetric C-N stretching region. This fact can be explained by assuming that the latter mode gives rise to a change in the dipole moment of the adsorbed urea molecule that is parallel to the electrode surface. This would be the case if the axis defined by the nitrogen atoms of the adsorbate was parallel to the electrode surface. Thus, we can conclude that the urea molecule is bonded to the Rh(100) surface through its two nitrogen atoms.
With this bonding geometry, the symmetric C-N stretching mode would fulfill the surface selection rule. However, the corresponding band cannot be observed under the present experimental conditions, since its frequency (∼1020 cm-1 (ref 43)) lies in a spectral region dominated by the symmetric ClO4- stretching mode. Even if some tilting of the N-CO-N plane with respect to the surface normal cannot be discarded from the spectra reported here, the observation of a strong C-O stretching band suggests that the direction defined by the C-O axis is almost perpendicular to the electrode surface. As previously discussed for urea adsorbed on Pt(100) electrodes, the loss of two hydrogen atoms upon urea adsorption can be tentatively assumed.17 According to this (42) Srivastava, P. C.; Aravindakshan, C. Z. Phys. Chem. 1983, 264, 61. (43) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination compounds; John Wiley & Sons: New York, 1986. (44) Theophanides, T.; Harvey, P. D. Coord. Chem. Rev. 1987, 76, 237. (45) Woon, T. C.; Wickramasinghe, W. A.; Fairlie, D. P. Inorg. Chem. 1993, 32, 2190.
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Figure 7. Plot against the electrode potential of the band frequencies of urea molecules adsorbed on the rhodium single crystal electrodes: (A) Rh(100) [(a) in the urea-containing solution; (b) irreversibly adsorbed urea]; (B) Rh(111); (C) Rh(110).
hypothesis, z in eq 2 (written for Rh(100)) would be equal to 0 whereas m and n would be 2. We can extend the comparison between the behavior of urea adsorbed on Rh(100) and Pt(100) surfaces to the analysis of the effect of the electrode potential on the C-O stretching frequency. Figure 7A shows a plot of the band frequencies measured in the spectra reported in Figure 2A and B as a function of the electrode potential. Plots are linear in both cases in the potential region between 0.30 and 0.70 V with the same tuning rate of 63 cm-1 V-1. Slightly lower wavenumbers for the band observed in the urea-free solution at a given potential can be rationalized by considering the existence of a lower urea coverage when compared with that reached in the presence of urea. The effect of coverage on the adsorbate band via lateral coupling can also be invoked to explain the sudden decrease of the C-O stretching frequency as a consequence of partial desorption at potentials lower than 0.30 V. The plot of the C-O stretching frequency in the case of urea adsorbed on Pt(100) electrodes showed a linear behavior with respect to electrode potential in a region where the urea coverage was found to be constant. This seems also to be the case for the Rh(100) electrode in the potential range between 0.30 and 0.70 V, as evidenced by the constant intensity of the adsorbate band in the spectra shown in Figure 2B or from the similar intensities of the C-N stretching band for dissolved urea in the spectra at 0.30 and 0.60 V in Figure 2A. The potential-dependent frequency shift at constant coverage can be related either to an electrochemical Stark effect or to changes in the distribution of the intramolecular electronic distribution. Namely, an additional reinforcement of the C-O bond can be expected when retrieving electron density from the oxygen atom of adsorbed urea when the electrode charge becomes positive by increasing the electrode potential. On the contrary, increasing the surface electronic density would decrease the double-bond character of the C-O bond and give rise to lower band frequencies as the electrode potential decreases. Two related aspects can be discussed regarding the results reported in this paper for the adsorption of urea at the Rh(111) electrode. These are the effects of the electrode potential on both the urea coverage and the bonding geometry. A first qualitative approach to the role of the electrode potential on the urea coverage is given by the integration of the bands corresponding to the depletion of the solution urea molecules upon adsorption. Figure 8 shows a plot of the intensity of the bands at 1605 cm-1 for
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Figure 8. Potential-dependent changes for adsorbed urea coverage at the Rh(111) electrode in the 1 mM urea + 0.1 M HClO4 solution. (a) Voltammetric current density recorded during the positive-going sweep. (b) Integrated intensity values for the C-O stretching band at 1605 cm-1 measured in the spectra reported in Figure 4B. (b) Charge density associated with urea adsorption as calculated from the integration of the voltammetric current in the potential region between 0.30 and 0.70 V (see text for details).
the spectra reported in Figure 4B. The voltammetric current density measured during the positive-going sweep is given in the same figure just for comparison (curve a). It can be seen in Figure 8 that urea adsorption starts at potentials around 0.20 V and the urea coverage increases steadily at more positive potentials. Partial desorption of the urea adlayer takes place at potentials above 0.80 V and can be related to the coadsorption of oxygenated species. The intensities of the solution bands in Figure 4B are proportional to the urea coverage reached at each electrode potential, but we cannot calculate these surface coverages directly from the spectra. What we can do, however, is to estimate the charge corresponding to urea adsorption at a given potential, qurea(E), from the charge displaced at 0.30 V, qd(0.3V), normalized by the ratio of the intensities of the band at 1605 cm-1, I1605(E) and I1605(0.3V),
I1605(E) qurea(E) ) |qd(0.3V)| I1605(0.3V)
(3)
The validity of this relation relies on the assumption that qd(0.3V), which amounts to -109 µC‚cm-2 (see above), originates only from the desorption of the urea adlayer formed at this potential. In other words, we assume that no adsorbed chloride is present at the electrode surface at this potential because adsorbed urea is blocking the electrode surface for the perchlorate reduction process. This assumption seems reasonable on the basis of the absence of a net reduction current in the cyclic voltammograms recorded in the presence of urea between 0.20 and 0.35 V, even at low sweep rates down to 2 mV‚s-1. Since the charge density related to urea adsorption at 0.30 V is directly available from the charge displacement experiments described above, it also seems possible to calculate the charge density related to the urea adsorption process at a given potential, E, as follows
qurea(E) ) qurea(0.3V) +
E ∫0.3V
|j(E)| dE - qdl r
(4)
In this equation j(E) is the voltametric current density, r is the sweep rate, and qdl is the double-layer charge between 0.30 V and E. The observation of a nearly constant current region between 0.30 and 0.70 V in the positivegoing sweep of the cyclic voltammogram suggests that no significant increase of the urea coverage takes place in this potential region. This conclusion does not agree with the plot for the integrated band intensity in Figure 8 that shows clearly that the maximum urea coverage is reached at potentials around 0.70-0.80 V. Furthermore, if the current density observed between 0.30 and 0.70 V is ascribed only to the double-layer charging process, the corresponding double-layer capacity would be too high (around 200 µF cm-2). Thus, we can assume that some faradaic process should take place in this potential region. From the spectroscopic results described above, this process could be identified as the adsorption of urea. Thus, changes in the urea coverage between 0.30 and 0.70 V could be calculated from the integration of the voltammetric current in this potential region if we assume that no other process is contributing to the observed current. To do this it is necessary to estimate the double-layer capacity between 0.30 and 0.70 V for the Rh(111) electrode in the perchloric acid solution, but the adsorption of oxygenated species avoids such an estimation. As a rough approximation we can assume that the double-layer capacity is the same as that measured in a 0.5 M H2SO4 solution, that is, around 70 µF cm-2. Charge densities for urea adsorption calculated with this qdl value are plotted in Figure 8, curve b (solid line). This plot shows that the estimated value for the double-layer contribution gives charge densities for urea adsorption that are consistent with the potential-dependent changes in the integrated intensities in the spectra reported in Figure 4B. As shown in Figure 8, the maximum urea coverage attained at 0.70 V would correspond to a charge density value of 170 µC cm-2. Note that the charge density corresponding to the transfer of one electron per surface atom on a (1 × 1) Rh(111) surface amounts to 258 µC cm-2. To determine absolute coverage values for adsorbed urea, it is necessary to make additional assumptions about the number of electrons exchanged per adsorbed urea molecule. Previous studies devoted to urea adsorption at Pt(100) single-crystal electrodes concluded that two electrons are transferred per adsorbed urea molecule.17 This result was obtained from the comparison between the charge density corresponding to the adsorption of the saturated urea adlayer 17 and the absolute coverage obtained from radiotracer and ex-situ AES and LEED experiments.14 On the other hand, the thermodynamical analysis of the charge density involved in the formation of the urea adlayer at Pt(111) electrodes in potential-step experiments suggested that one electron per adsorbed urea molecule was exchanged in this case.19 This difference between the adsorption behavior of urea at Pt(100) and Pt(111) electrodes could be understood on the basis of the different bonding, as shown from the in-situ infrared spectra: through the two nitrogen atoms for Pt(100) and either through one nitrogen atom or through the oxygen atom for Pt(111). At present we have no direct data about either the absolute coverage or the oxidation state for urea at the Rh(111) electrode. We can only indicate from the charge calculated above that urea coverage would range from 0.42 at 0.30 V to 0.66 at 0.70 V if one electron was exchanged per adsorbed urea molecule. On the other hand, the transfer of two electrons per adsorbed urea would imply coverages ranging from 0.21 to 0.33 in the same potential region. It must be kept in mind that the coverage
Urea Adsorption at Rhodium Single-Crystal Electrodes
value for urea at 0.30 V is based on the charge density measured during the potentiostatic adsorption of CO at this potential. However, values at higher potentials rely either on the integration of the infrared bands (for which the area is determined with an error that could be as high as a 10%) or on the integration of the cyclic voltammograms. In this latter case the correction of the doublelayer contribution is just tentative. Besides, the existence of some contribution from the adsorption of oxygenated species to the voltammetric current cannot be completely discarded. The number of electrons exchanged upon urea adsorption on Rh(111) could be inferred from the urea bonding geometry suggested by the corresponding spectra for adsorbed urea. A first inspection of these spectra in Figure 4A suggests that this bonding geometry depends on the electrode potential. This potential-dependent behavior can be visualized by plotting the frequencies for the different adsorbate bands observed for Rh(111) as a function of the electrode potential. Such a plot is reported in Figure 7B. The absence of adsorbate bands above 1600 cm-1 in the spectra reported in Figure 4A suggests that urea molecules do not adsorb on the Rh(111) through the nitrogen atoms irrespective of the electrode potential. Instead, the band at 1575 cm-1 indicates the presence of O-bonded urea molecules. This band is always observed with other adsorbate bands, located in the region between 1300 and 1400 cm-1. These bands can be related to the asymmetric C-N stretching mode. If this assignment is correct, the surface selection rule implies that the change in the dynamic dipole related to this vibrational mode has a nonzero component perpendicular to the electrode surface. In other words, the Rh-O-C angle is always different from 180°.
Note that the same conclusion was derived from the infrared spectra obtained for urea adsorbed on the Pt(111) electrode at potentials higher than 0.70 V.19 From the similar bonding geometry for urea adsorbed at Pt(111) and Rh(111) electrodes we could conclude that the same number of electrons, that is, one electron per adsorbed urea molecule, is exchanged in both cases. As previously done in the case of platinum, we can assume that deprotonation takes place upon urea adsorption,19 which implies in this case that m and n are equal to 1 in eq 2. Both the changes in the frequency of the adsorbate band in the C-N stretching region and the potential-dependent intensity of the C-O stretching band suggest some change in the orientation of the urea molecules. The latter seems to be different in the low- and high-potential regions for which the C-N stretching band is observed at 1400 or 1350 cm-1, respectively. Note that both bands are observed in the potential region between 0.40 and 0.60 V, suggesting the coexistence of two different kinds of adsorbed urea molecules. This conclusion is consistent with the opposite sign for the slopes of the corresponding ν-E plots in this potential region in Figure 7B. The negative slope for the band at ∼1400 cm-1 indicates a decreasing coverage of the corresponding species. On the contrary, the shift toward higher wavenumbers of the band at ∼1350 cm-1 suggests that the amount of the related adsorbate is increasing between 0.40 and 0.80 V. Note that the
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Figure 9. Plot of the integrated intensity values for the C-O stretching band measured in the spectra reported in Figure 6B for urea adsorbed at the Rh(110) electrode.
development of this band takes place at the same time as urea coverage increases and the intensity of the C-O stretching band decreases. This behavior strongly suggests that the change in the orientation of the adsorbed urea molecules is probably a tilt of the C-O axis related to an increase in the compactness of the adlayer. As mentioned above, adsorption of urea at the Rh(110) has some similarities with the processes described for Rh(100) and Rh(111) electrodes. First, there is a slow accumulation of urea at the electrode surface. However, a significant fraction of the surface remains available for the reversible adsorption-desorption of urea, which is controlled by the electrode potential. The spectra shown for the urea-covered Rh(110) electrode in Figure 6A and the corresponding plot in Figure 7C for the frequencies of the different bands observed in these spectra suggest the existence of changes in the bonding geometry depending of the electrode potential. The adsorbate band at 1563 cm-1 in the spectrum obtained at 0.40 V can be assigned to O-bonded urea molecules. The small band at 1380 cm-1 in this spectrum could be related to the corresponding C-N stretching band. Note that the spectra obtained with s-polarized light suggest that the maximum urea coverage is attained in the potential region between 0.40 and 0.50 V (Figure 9). As a difference from the behavior of the Rh(111) surface, the observation of adsorbate bands at frequencies higher than 1600 cm-1 suggests the existence of N-bonded urea molecules in the high-potential region. This change from O-bonded to N-bonded molecules takes place together with a shift of the C-N stretching band from 1380 to 1338 cm-1. This shift is in agreement with the interpretation of Pendland41 that predicts a weakening of the C-N bond when the C-O bond is strengthened in the N-bonded molecule. On the other hand, the observation of the C-N stretching band at 1380 cm-1 indicates that the urea molecule is bonded through only one of the nitrogen atoms, to satisfy the surface selection rule.
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The spectra in Figure 6 and the plot of the intensities of the consumption bands in Figure 9 suggest that changes in bonding geometry and coverage for urea adsorbed at the Rh(110) are related in a different way from that discussed in the case of Rh(111). The change from O-bonded to N-bonded molecules takes place now with a decrease in the urea coverage and could be related with the competitive adsorption of oxygenated species at potentials above 0.50 V.
Climent et al.
infrared spectra suggest a tilting of the C-O axis in the higher potential region where the maximum urea coverage is attained. (4) The spectra obtained for the Rh(110) electrode in the urea-containing solution indicate the existence of O-bonded urea molecules at potentials below 0.40 V which are substituted by N-bonded molecules at higher potentials where the competitive adsorption of oxygenated species takes place.
Summary The main spectroelectrochemical results obtained for the urea adlayers formed at rhodium single-crystal electrodes can be summarized as follows: (1) Urea adsorption at rhodium electrodes is a reversible process with respect to the electrode potential and takes place without breaking of the C-N bond. (2) Urea adsorption at the Rh(100) electrode surface takes place through the two nitrogen atoms irrespective of the electrode potential and urea coverage. (3) Only O-bonded urea molecules are detected at the Rh(111) electrode. Potential-dependent changes of the
Acknowledgment. V.C. thanks the Conselleria de Cultura, Educacio´ I Cie`ncia de la Generalitat Valenciana for the award of a doctoral grant. The authors are grateful for the financial support afforded by the DGICYT through Contract PB96-0409. 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. LA000639G
