pubs.acs.org/Langmuir © 2010 American Chemical Society
Electrochemical Oxidation of Hydroxylamine on Gold in Aqueous Acidic Electrolytes: An in Situ SERS Investigation Denis R. M. Godoi,‡ Youjiang Chen,† Huanfeng Zhu,† and Daniel Scherson*,† †
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106, and ‡Departamento de Fı´sico-Quı´mica, Instituto de Quı´mica, Unesp - Univ Estadual Paulista, Araraquara, 14801-970 CP 355, Brazil Received May 25, 2010. Revised Manuscript Received September 1, 2010 The electrooxidation of hydroxylamine (HAM) on roughened Au electrodes has been examined in aqueous buffered electrolytes (pH 3) using in situ surface-enhanced Raman scattering (SERS). Two distinct spectral features were observed at potentials, E, within the range in which HAM oxidation was found to ensue, centered at 803 cm-1 for 0.55 < E < 0.8 V and at 826 cm-1 for 1.0 < E < 1.40 V versus SCE, attributed, respectively, to adsorbed nitrite and adsorbed NO2. Similar experiments performed in solutions containing nitrite instead of HAM under otherwise identical conditions displayed only the peak ascribed to adsorbed nitrite over the range of 0.1 < E < 0.8 V versus SCE with no additional features at higher potentials. These observations strongly suggest that under the conditions selected for these studies the oxidation of HAM on Au proceeds at least in part through a pathway that does not involve nitrite as a solution-phase intermediate.
Introduction The electrochemical oxidation of hydroxylamine (HAM) in aqueous electrolytes has been the subject of rather extensive investigations in recent years.1 Attention has been focused almost exclusively on Pt electrodes,2-7 including, more recently, welldefined single-crystal surfaces.5-7 Considerable insight into the mechanism of this reaction has been gained from in situ Fourier transform infrared spectroscopy and online mass spectrometry studies;2,4-7 however, little is known regarding the reactivity of this important chemical on other metal electrode surfaces. This lack of information stems, in part, from reports in the literature claiming that HAM oxidation does not proceed on common electrode materials such as gold or carbon in acidic electrolytes over the pH range of 0.6-5.6, a behavior attributed to the lack of affinity of HAM for these surfaces.3 In the course of an investigation aimed at elucidating the products of nitrate and nitrite reduction on Au surfaces promoted by Cd underpotential deposition (UPD),8 we found, in stark contrast to such claims, that the onset for the oxidation of HAM on Au electrodes in buffered solutions of pH 3 and 4 occurs at potentials well within the double-layer region, yielding voltammetric curves characterized by two consecutive waves. In fact, a single oxidation wave was also reported in phosphate buffer solutions by Li and Lin9 (4.0 e pH e 10.0) and later by Kannan and Abraham John10 (5.2 e pH e 9.2) in their voltammetric studies. *Corresponding author. E-mail:
[email protected]. (1) Rosca, V.; Duca, M.; de Groot, M. T.; Koper, M. T. M. Chem. Rev. 2009, 109, 2209. (2) Karabinas, P.; Wolter, O.; Heitbaum, J. Ber. Bunsen-Ges. Phys. Chem. Chem. Phys. 1984, 88, 1191 and references therein. (3) Piela, B.; Wrona, P. K. J. Electrochem. Soc. 2004, 151, E69. (4) Rosca, V.; Beltramo, G. L.; Koper, M. T. M. J. Electroanal. Chem. 2004, 566, 53. (5) Rosca, V.; Beltramo, G. L.; Koper, M. T. M. J. Phys. Chem. B 2004, 108, 8294. (6) Rosca, V.; Koper, M. T. M. J. Phys. Chem. B 2005, 109, 16750. (7) Wonders, A. H.; Housmans, T. H. M.; Rosca, V.; Koper, M. T. M. J. Appl. Electrochem. 2006, 36, 1215. (8) Chen, Y. J.; Zhu, H. F.; Rasmussen, M.; Scherson, D. J. Phys. Chem. Lett. 2010, 1, 1907–1911. (9) Li, J.; Lin, X Sens. Actuators, B 2007, 126, 527–535. (10) Kannan, P.; Abraham John, S. Anal. Chim. Acta 2010, 663, 158–164.
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This letter presents in situ surface-enhanced Raman scattering (SERS) spectra for roughened Au electrodes in well-buffered solutions (pH 3) over the potential range in which HAM undergoes electrochemical oxidation.
Experimental Section All measurements were performed at room temperature (23 °C) in deareated aqueous 0.1 M phosphate buffer (pH 3) containing 0.1 M NaClO4, to be referred to hereafter as the base electrolyte, in the presence of either hydroxylamine (HAM) or nitrite. (See below for other details.) In situ SERS spectra were recorded using a commercial quartz cuvette (Spectrosil far-UV quartz, Starna) as the main electrochemical cell compartment. The Au working electrode was a disk encased in Teflon of a projected area of ca. 0.022 cm2, and the counter electrode was a Au foil (6 13 mm2 and 0.15 mm in thickness). A commercial saturated calomel electrode (SCE, Radiometer Analytical, REF401) placed in an external compartment and connected to the cell with PTFE tubing served as the reference electrode. For the in situ SERS experiments, the Au disk was first polished using silicon carbide grinding paper (Microcut P4000, Buehler), followed by sonication in ultrapure water for 5 min. Subsequently, the surface was roughened electrochemically by the method reported by Gao,11 which involves 25 oxidation-reduction cycles in aqueous 0.1 M KCl between -0.3 and þ1.25 V versus SCE at sweep rates of 1 and 0.5 V s-1 for scans toward positive and negative values, respectively. Following each linear scan, the potential was held at the positive or negative limit for 1.2 and 30 s, respectively. This specific protocol rendered Au electrodes displaying optimal SERS activity in terms of peak intensities and overall stability. Raman spectra were acquired with a Sentinel CHROMEX (Bruker) system using a 70 mW, 785 nm laser. The power at the sample was estimated to be about 15 mW. The electrode potential was controlled by a Pine bipotentiostat (model AFRDE4). In situ SERS spectra of roughened Au electrodes were collected for 60 s at fixed potentials both in ascending and descending order in increments of 0.05 V. After each acquisition, the potential was scanned to the next value and held there for ca. 30 s and a new (11) Gao, P.; Gosztola, D.; Leung, L. W. H.; Weaver, M. J. J. Electroanal. Chem. 1987, 233, 211–222.
Published on Web 09/20/2010
DOI: 10.1021/la102117d
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Figure 1. Cyclic voltammograms for a roughened Au electrode in a solution of 1 mM NH2OH (thick line) or 1 mM NH2OH 3 HCl (dashed line) in phosphate buffer (pH 3.0) in 0.1 M NaClO4 recorded at a scan rate of 50 mV/s in the same cell in which the in situ SERS measurements were performed. Also shown for comparison is the voltammogram of a smooth Au electrode in the neat base electrolyte previously acquired in a conventional cell before (thin curve). spectrum was recorded. Except for hydroxylamine (Aldrich, 99.999% pure), all other chemicals (i.e., H3PO4, NaH2PO4 3 H2O, Na2HPO4 3 7H2O (Fisher), and NaClO4 3 H2O (Aldrich, puriss ACS reagent)) were analytical-reagent grade and used without any further purification. All solutions were prepared with ultrapure water (18.3 MΩ cm, EASYpure UV system, Barnstead) and deaerated with Ar or nitrogen before use.
Results and Discussion Shown in Figure 1 (thick line) is a cyclic voltammogram recorded at a scan rate of 50 mV/s for a roughened Au electrode in a 1 mM NH2OH (HAM) solution in the base electrolyte recorded in the same cell where the in situ SERS measurements were performed. A cursory inspection of these data reveals at least two well-defined peaks with onset potentials at ca. 0.35 and 0.7 V versus SCE, respectively, and two additional features of much lower amplitude centered at ca. 0.6 and 1.0 V. On the basis of a comparison with the voltammogram for a smooth Au electrode in the neat base electrolyte (i.e., devoid of HAM (thin line, Figure 1)) recorded in a conventional cell, the first three voltammetric feature peaks are found in the so-called double layer of Au, and the onset potential for the fourth, a rather minor peak, is very close to that associated with Au oxide formation (i.e., 1 V versus SCE). Insight into the mechanism of HAM oxidation was obtained from the analysis of a series of in situ SERS spectra recorded for a roughened Au electrode in the base electrolyte containing 1 mM HAM over the wavelength range of 400-2300 cm-1 as a function of the applied potential. Data were collected in sequence in both ascending and descending order (details in Experimental Section) in increments of 0.05 V starting from the lower potential, Elow = 0.4 V, up to the upper potential, Eup = 1.2 V versus SCE and subsequently in the reverse direction. Attention will be focused in what follows only on those peaks associated with the presence of HAM in solution, which were found to be in the range of 750835 cm-1. It should be stressed that several potential-dependent features, the origin of which have not as yet been elucidated, were also observed in a wider spectral region (not shown here) in the 15712 DOI: 10.1021/la102117d
Figure 2. Series of in situ SERS spectra for a roughened Au electrode in the base electrolyte containing 1 mM NH2OH recorded in potential increments of 0.05 V starting from E = 0.4 V up to E = 1.2 V (A) and for a subsequent series acquired at potentials in the opposite direction (B).
base electrolyte and thus are unrelated to the presence of HAM in solution. As clearly shown in panel A of Figure 2, in situ SERS data acquired for potentials in ascending order (i.e., Elow f Eup) revealed a band centered at 803 cm-1 emerging at ca. E=0.5 V, which is about 0.1 to 0.15 V more positive than the onset potential for HAM oxidation in Figure 1 (thick line). This feature gained in intensity for more positive potentials, reaching a maximum at 0.65 V, and subsequently began to decrease to later disappear at ca. 0.8 V. On the basis of data reported in the literature,12,13 this spectral feature can be attributed to the bending mode of adsorbed nitrite. As the potential reached ca. 1.0 V, a value that corresponds closely to the onset of oxide formation on Au (Figure 1), a new spectral feature began to appear at 826 cm-1. As pointed out by Weaver et al.,14 a band at the same energy is found in the SERS spectrum of certain transition metals upon sequential O2/NO dosing15 as well as in the high-resolution electron energy loss spectra of Pd(111) exposed to NO2 in ultrahigh vacuum environments reported by Wickham et al.,16 which was ascribed tentatively to the bending mode of adsorbed NO2. As the potential sequence was reversed (i.e., Eup f Elow), the feature at 826 cm-1 persisted down to about 0.80 V and that due to adsorbed nitrite reappeared at ca. 0.7 V, later losing much of its intensity at ca. 0.55 V (panel B, Figure 2). A more quantitative analysis of the data presented in Figure 2 is provided by a plot of the normalized integrated intensity of the spectral features observed as a function of the applied potential shown in Figure 3 (right ordinate). Also displayed in this (12) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 6th ed.; Wiley: Hoboken, NJ, 2009. (13) Evans, A. R.; Fitchen, D. B. Phys. Rev. B 1970, 2, 1074. (14) Zou, S. Z.; Gomez, R.; Weaver, M. J. Langmuir 1997, 13, 6713. (15) Wilke, T.; Gao, X. P.; Takoudis, C. G.; Weaver, M. J. Langmuir 1991, 7, 714. (16) Wickham, D. T.; Banse, B. A.; Koel, B. E. Surf. Sci. 1991, 243, 83.
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Figure 3. Plots of the normalized integrated intensities of the peaks ascribed to adsorbed nitrite and adsorbed NO2 as a function of the applied potential during HAM oxidation on a roughened Au electrode (right ordinate). Also shown (solid line) is the linear scan voltammogram for the roughened Au electrode in the same solution as in the caption of Figure 2 at 50 mV/s. See the text for details.
Figure (left ordinate) is the linear scan voltammogram recorded in the same electrolyte at a scan rate of 50 mV/s. As evidenced from these data, the disappearance of the band ascribed to adsorbed nitrite virtually coincides with the onset of the second oxidation peak centered at 0.8 V versus SCE in the linear voltammetric scan. Further insight into this phenomenon was obtained from separate in situ SERS experiments collected in the same base electrolyte containing 1 mM NaNO2 instead of HAM. As shown in Figure 4, the same peak at around 803 cm-1 was found in the potential range of 0.3 < E < 0.75 V versus SCE. A comparison between the normalized integrated intensity of this feature and the cyclic voltammetry recorded in the same solution (Figure 5) clearly indicates that the extent of nitrite adsorption increases with the applied potential within the double-layer region but gradually disappears as the oxidation of nitrite begins. Contrary to the results obtained with HAM, however, no band due to adsorbed NO2 was detected in solutions containing only nitrite. This finding is rather surprising because the oxidation of nitrite under conditions similar to those employed for these experiments is known to generate solution-phase NO2 as an intermediate17 and strongly suggests that the mechanism of HAM oxidation proceeds at least in part via a pathway that does not involve solutionphase nitrite. Virtually identical experiments were performed by replacing HAM by NH2OH 3 HCl under otherwise identical conditions. As shown in Figure 1 (dotted line), the presence of chloride shifted the onset potential of the voltammetric peaks ascribed to HAM oxidation toward more positive values and led to a large increase in the intensity of the most positive peak. Most importantly, however, and in contrast to the behavior found in Figure 2 for HAM, no evidence was obtained for the presence of any spectral features attributable to HAM-derived adsorbates. This behavior is due in all likelihood to the strong affinity of chloride for Au, which, as shown by the data collected in this work, would displace the more weakly adsorbed nitrite or NO2. Measurements (17) Xing, X. K.; Scherson, D. A. Anal. Chem. 1988, 60, 1468.
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Figure 4. Series of SERS spectra recorded in situ for a roughened Au electrode in the base electrolyte containing 1 mM sodium nitrite recorded in potential increments of 0.05 V from E = 0.4 V up to E = 1.20 (A) and for a subsequent series for potentials in the negative direction (B).
Figure 5. Plots of the normalized integrated intensity of the peak ascribed to adsorbed nitrite as a function of the applied potential during nitrite oxidation on a roughened Au electrode (scattered symbols, right ordinate) on the basis of the data in Figure 4. Also shown by solid lines is the cyclic voltammogram for the roughened Au electrode in the same solution as in the caption of Figure 4 at 50 mV/s (left ordinate).
involving other spectroscopic techniques including FTIR and reflectance spectroscopy are now in progress in our laboratories to gain further insight into this interesting redox process. Acknowledgment. This work was supported by the NSF. D.R.M.G. thanks the Capes Foundation within the Ministry of Education, Brazil (proc. 0643-09-2) for a fellowship.
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