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Langmuir 2008, 24, 4352-4357
Surface-Enhanced Infrared Absorption Spectroscopic Studies of Adsorbed Nitrate, Nitric Oxide, and Related Compounds 1: Reduction of Adsorbed NO on a Platinum Electrode Kou Nakata,† Akinori Okubo,† Katsuaki Shimazu,*,† Akira Yamakata,‡ Shen Ye,‡ and Masatoshi Osawa‡ Section of Materials Science, Faculty of EnVironmental Earth Science, Hokkaido UniVersity, Sapporo 060-0810, Japan, and Catalysis Research Center, Hokkaido UniVersity, Sapporo 001-0021, Japan ReceiVed NoVember 6, 2007. In Final Form: December 27, 2007 Surface-enhanced infrared absorption spectroscopy (SEIRA) was used to examine the adsorption state of nitrogen monoxide (nitric oxide, NO) and the reduction of the adsorbed species. The SEIRA spectra gave two distinct bands at 1723-1733 and 1575-1607 cm-1 with an additional weak band at 1656-1676 cm-1 at 0.20 V, the frequencies of which are slightly dependent on the surface coverage. The former two bands are attributed to the on-top and bridged NO, respectively. While the on-top NO stably remained on the surface in the potential range of 0.05 -0.60 V, the bridged NO decreased in its intensity with increasing electrode potential. The reduction of the adsorbed NO obeys first-order kinetics with respect to the adsorbed NO. The rate constants are 2.24 ( 0.03 and 0.24 ( 0.09 s-1 at -0.10 V for the on-top and bridged NO, respectively. Tafel slopes obtained from the potential dependence of the rate constant indicate that the rate-determining step is the first electron-transfer process.
Introduction The reduction of nitrate is receiving increased attention from the viewpoint of environmental remediation of groundwater, because nitrate ions cause serious health risks for human beings, such as methemoglobinemia and cancer, by the action and reactivity of nitrite formed from nitrate in ViVo. Electrochemical denitrification is one of the very promising technologies owing to its environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability, and cost effectiveness.1 We recently reported that Sn-Pt and Sn-Pd binary metal electodes show much higher electrocatalytic activity in acidic solution than any other previously reported electrodes.2-6 However, the selectivity of harmless N2, the most desirable product from an environmental viewpoint, was limited to 50% among all products. Therefore, the improvement of N2 selectivity is a very important target in this field. During the reduction of nitrate, the first electron transfer is usually slow. Therefore, the effect of Sn and any other second metal is considered to be the acceleration of this electron-transfer process. However, the product distribution is determined by subsequent reactions after the reduction of nitrate to nitrite. The adsorbed NO is commonly considered to be an intermediate species during the reduction of nitrate. Therefore, it is very important to examine the reactivity of the adsorbed NO species under the conditions where the reduction of nitrate takes place. * To whom correspondence should be addressed. E-mail: shimazu@ ees.hokudai.ac.jp. Telephone: +81-11-706-2276. Fax: +81-11-706-4868. † Section of Materials Science, Faculty of Environmental Earth Science. ‡ Catalysis Research Center. (1) Rajeshwar, K.; Ibanez, J. G. EnVironmental Electrochemistry; Academic Press: San Diego, CA, 1997. (2) Shimazu, K.; Goto, R.; Tada, K. Chem. Lett. 2002, 204. (3) Shimazu, K.; Kawaguchi, T.; Tada, K. J. Electroanal. Chem. 2002, 529, 20. (4) Tada, K.; Kawaguchi, T.; Shimazu, K. J. Electroanal. Chem. 2004, 572, 93. (5) Tada, K.; Shimazu, K. J. Electroanal. Chem. 2005, 577, 303. (6) Shimazu, K.; Goto, R.; Piao, S.; Kayama, Y.; Nakata, K.; Yoshinaga, Y. J. Electroanal. Chem. 2007, 601, 161.
The adsorption states of NO were widely examined in ultrahigh vacuum (UHV) and electrochemical systems by infrared reflection absorption spectroscopy (IRAS) or electron energy loss spectroscopy (EELS). Table 1 summarizes the vibrational frequencies of NO on Pt single crystals such as Pt(111) ,7-14 Pt(100),12,13,15-19 and Pt(110)13,14,20,21 together with those on polycrystalline Pt.11,22 On Pt(111) under UHV conditions, the spectra showed two bands at 1698-1722 and 1475-1519 cm-1.7-11 The former band was observed at the relatively high coverage of >0.1-0.2 and was assigned to the N-O stretching vibration of the on-top NO (atop, linear or terminal NO according to the author’s notation).7,8,14 The latter band at lower wavenumbers was observed at the low coverage of 0.2 1712-172211 100 K 1698-1706 θ ) high, 323 K 1631-163311 θ ) low, 323 K
electrochemical 1665-168012 θ ) sat, 0.3V system
1475-14987
θ ) 0-0.15 14908 100 K
linear,7 atop,14 terminal, or dimer8
300 K
1490-15009 100 K, θ ) low 1485-150010 θ < 0.1 1485-151911 100 K
1395-144012 θ ) 0.3, 0.3 V
170013 0.2-0.75V 1584-168014 0.15-0.4V
assignments
Pt(100) 1805-181015
170013 0.2-0.75 V
3-fold,10,12 bridged7,8
Pt(110)
1630-163515
177020
162020
1582-162612 θ ) 0.3-sat, 0.3V
1761-177014 1582-1600,14 0.3 V 0.2-0.4 V 1712, 0.1-0. 3 V14* 176013 0.2-0.75 V
300 K 230 K, θ ) sat θ ) sat 1596-164116 1796-180921 1595-1613,21 300 K, θ ) 0.07-0.5 1711, 300 K21* 163017
164013 0.2-0.75 V 1609-163818 0.2-0.8 V 1592-162619 θ ) 0.22-0.5, 0.3 V
defect15 in bridged,12 denser phase13 terminal bent16
1440 cm-1 assigned to the bridged or 3-fold NO was not always observed. On Pt(100), only a single band appeared in the wavenumber region of 1582-1641 cm-1 in both the UHV15-17 and electrochemical systems,12,13,18,19 except for the band assigned to NO on the defect sites (1805-1810 cm-1)15 and NO in a denser phase (1700 cm-1).13 The main band on Pt(100) was assigned to the bridged NO or terminal NO with a bent orientation (terminal bent NO).16 On Pt(110), the atop or terminal NO was assigned to the band at 1770-1809 cm-1 in the UHV system20,21 and 1760-1770 cm-1 in the electrochemical system.13,14 The bridged NO was also observed at 1595-1620 cm-1 under UHV conditions.20,21 In 0.1 M HClO4, its band position was at 15821600 cm-1.14 An additional band was also reported between these bands and was assigned to the adsorbed NO on defect sites or NO adsorbed at the long bridge site.14,21 Compared with Pt single crystals, much less information has been reported about the adsorption state of NO on polycrystalline Pt. The band that appeared at around 1650 cm-1 in the UHV system was shifted, broadened, and split into two peaks at 1614 and 1767 cm-1 with an increase in the temperature and coverage at 320 K.11 The adsorbed NO on a polycrystalline Pt electrode was asigned to the band at 1580-1620 cm-1.22 In this paper, we report the infrared spectra of the adsorbed NO measured using surface-enhanced infrared absorption spectroscopy (SEIRAS).23-25 This newly developed method is highly sensitive to the surface and is not affected by the solvent and the solution species. This is a very important advantage particularly for IR spectra of the adsorbed NO because bands usually appear in the frequency range of the H2O bending mode. SEIRAS is also free from any exhaustion and/or accumulation of reactants/products because the SEIRAS cell has a sufficient solution volume. It is also easy to add or replace the solution without any change in the optical alignment. Therefore, we can use the spectrum in a pure electrolyte solution, or a completely NO-free spectrum, as a reference spectrum. It is then expected that accurate spectra of the adsorbed NO with extremely high (23) Osawa, M. Bull. Chem. Soc. Jpn. 1997, 70, 2861. (24) Osawa, M. Top. Appl. Phys. 2001, 81, 163. (25) Osawa, M. In Handbook of Vibrational Spectroscopy; Chalmers, J. M., Griffiths, P. R., Eds.; John Wiley & Sons: Chichester, 2002; Vol. 1, p 785.
atop,14,21 terminal 20
polycrystalline Pt 176711 161411 320 K 320 K
1580-162022 θ ) sat, 0.2-0.5 V
bridged,14,20,21 *long bridge site or defect14,21
quality can be obtained. The cell configuration also allows us to conduct time-resolved measurements as recently demonstrated for the electrooxidation of formic acid to CO2 at a Pt electrode.26-31 Therefore, this method, characterized by fast mass transport and a fast electrochemical response, is suitable for measuring the kinetics of the reduction of the adsorbed NO. We now report the coverage and potential dependences of the SEIRAS spectra of the adsorbed NO on polycrystalline Pt, and the reduction rate of the adsorbed NO. To the best of our knowledge, no infrared spectroscopic study has been reported regarding the reduction of adsorbed NO. Experimental Section Materials. The Pt plating solutions (LECTROLESS PT100 basic solution and reduction reagent solution) were supplied by Electroplating Engineering of Japan. An aqueous solution of 40% NH4F was obtained from Morita Chemical Industries Co., Ltd. PdCl2 was purchased from Wako Pure Chemicals. Nitrogen monoxide (nitric oxide, NO) gas of 99% purity was obtained from Sumitomo Seika Chemicals Co., Ltd. and washed with a 1 M NaOH solution and water immediately before use to mainly remove the nitrogen dioxide. The 50% HF solution of atomic absorption spectrometry grade and all other chemicals of reagent grade, such as HClO4 and NaNO3, were purchased from Kanto Chemicals. The solutions were prepared using Milli-Q water. The prism used for IR measurements in this study was a nondoped Si hemicylinder supplied by Kyoto Pastec Co., Ltd. or Pier Optics Co., Ltd. Preparation of a Thin Pt Film Electrode. A thin Pt film electrode was prepared on the total reflecting plane of a Si hemicylinder prism by an electroless deposition technique.31 The reflecting plane (2.5 cm × 2 cm) of the prism was first polished with alumina (1 µm) followed by ultrasonication in water and acetone, and then treated by a 40% NH4F solution for 1 min to remove the oxide layer. After (26) Samjeske´, G.; Osawa, M. Angew. Chem., Int. Ed. 2005, 44, 5694. (27) Samjeske´, G.; Miki, A.; Ye, S.; Yamakata, A.; Mukouyama, Y.; Okamoto, H.; Osawa, M. J. Phys. Chem. B 2005, 109, 23509. (28) Chen, Y.-X.; Ye, S.; Heinen, M.; Jusys, Z.; Osawa, M.; Behm, R. J. J. Phys. Chem. B 2006, 110, 9534. (29) Samjeske´, G.; Miki, A.; Ye, S.; Osawa, M. J. Phys. Chem. B 2006, 110, 16559. (30) Mukouyama, Y.; Kikuchi, M.; Samjeske´, G.; Osawa, M.; Okamoto, H. J. Phys. Chem. B 2006, 110, 11912. (31) Miki, A.; Ye, S.; Osawa, M. Chem. Commun. 2002, 1500.
4354 Langmuir, Vol. 24, No. 8, 2008
Figure 1. Cyclic voltammogram for the reduction of the adsorbed NO on a Pt/Si prism electrode in a 0.1 M HClO4 (solid curve). The electrode potential was scanned from 0.2 V in the negative direction at the sweep rate of 0.05 V s-1. The dotted curve shows the cyclic voltammogram of a clean Pt/Si prism electrode in 0.1 M HClO4. The adsorbed NO was formed at 0.2 V by bubbling NO gas for 15 min. Before measuring a cyclic voltammogram, the NO-saturated solution was replaced with pure 0.1 M HClO4. depositing Pd on the surface of the reflecting plane by contact with a 0.5% HF solution containing 1 mM PdCl2 for 5 min,32 Pt was deposited on the Pd layer from the mixed Pt plating solution (basic solution/H2O/15 M NH3/reduction reagent solution ) 50/44/5/1 in volume) at 65 °C for 10 min. IR Measurements. Details of the SEIRAS measurements are described elsewhere.23-25,33,34 Briefly, the infrared spectra were recorded using a Bio-Rad FTS60A/896 or FTS575c Fourier transform infrared spectrometer equipped with an HgCdTe detector and a homebuilt single-reflection accessory (incident angle of 70°). The spectrometer was operated with a 4 cm-1 resolution. All spectra are shown in absorbance units. The glass-made electrochemical cell was of the three-compartment type. The counter and reference electrodes were platinized Pt foil and Ag/AgCl (saturated KCl), respectively. Potentials in the text were referred to this reference. An EG&G PARC model 263A potentiostat was used to control the electrode potential. The Pt film electrode was first electrochemically cleaned by cycling the electrode potential in the range from -0.2 to 1.0 V. After the reference spectrum was measured at 0.2 V in 0.1 M HClO4, NO gas was introduced into the electrolyte solution through a tube connected to the cell compartment. IR measurements at 0.2 V were conducted every 5 s until the saturated coverage was obtained. The potential-dependent IR spectra were obtained with a time resolution of 1 s during potential cycling with a sweep rate of 0.02 V s-1. For the reduction of the adsorbed NO, the higher time resolution of 0.20 s was selected to follow the fast reduction.
Results and Discussion Cyclic Voltammograms of the Dissolved and Adsorbed NO. The cyclic voltammogram of a Pt thin film electrode on a Si prism in the pure 0.1 M HClO4 solution shows typical characteristics of the clean polycrystalline Pt electrode (Figure 1, dotted curve). In the presence of NO in the solution, the reduction and oxidation current flowed at potentials below 0.05 V and above 0.60 V, respectively (not shown). Figure 1 shows cyclic voltammograms taken after the replacement of the NOsaturated solution with pure 0.1 M HClO4, during which the electrode potential was held at 0.2 V. When the potential was scanned in the negative direction from 0.2 V, the cathodic wave due the redution of the adsorbed NO was observed in the potential range of 0.05 to -0.20 V. On the reverse scan from the negative potential end, the oxidation current in the so-called hydrogen region was lower than that in pure 0.1 M HClO4, showing that (32) Karmalkar, S.; Banerjee, J. J. Electrochem. Soc. 1999, 146, 580. (33) Ataka, K.; Yotsuyanagi, T.; Osawa, M. J. Phys. Chem. 1996, 100, 10664. (34) Osawa, M.; Ataka, K.; Yoshii, K.; Yotsuyanagi, T. J. Electron Spectrosc. Relat. Phenom. 1993, 371, 64/65.
Nakata et al.
the reduction of the adsorbed NO still took place in this region. The current observed during the subsequent scans almost traced that obtained in 0.1 M HClO4. This indicates that the reduction of the adsorbed NO was almost complete during the first potential cycle to 0.2 V via the negative potential end and that the reduction product was not an adsorbed species (if so, a reversible reduction/ oxidation process would take place during the potential cycle). The charge consumed during this cycle for the reduction of the adsorbed NO can be obtained by subtracting the charge due to the formation and desorption of the adsorbed hydrogen from the total charge. The value is -2140 µC cm-2. Assuming a fiveelectron reduction to form ammonium ions,18 the adsorption amount is 4.44 × 10-9 mol cm-2. This corresponds to the surface coverage of 0.47 (the coverage is defined by the ratio of the adsorbed NO to the surface Pt atoms, taking into account the surface roughness of this electrode of 4.3 estimated from the hydrogen adsorption). SEIRA Spectra of the Adsorbed NO. SEIRA spectra were recorded every 5 s during the adsorption of NO at 0.2 V on a polycrystalline Pt electrode. Positive bands appeared in the region of 1500-1900 cm-1 (Figure 2), showing the adsorption of NO.35 It took about 300 s until the bands started increasing. This is due to the relatively large dead volume in a NaOH trap for the removal of NO2 contained as an impurity in the NO gas. At approximately 800 s, both the band intensity and position were unchanged. There are two main bands at about 1730 and 1605 cm-1. There also exists an additional band as a shoulder between them. These bands were labeled as bands 1, 2, and 3 from the higher wavenumber. To determine the band position and intensity of these three bands, the spectra were deconvoluted into the component bands using a Gaussian function.36 A typical deconvoluted spectrum is shown in Figure 2b. The determined band position and intensity are plotted versus the total intesity in Figure 3, in which these intensities are given as relative intensities to the total intensity at 900 s, which correspond to the surface coverages. In the relative total intensity range of g0.40, bands 1 and 2 appeared at 1730 and 1662 cm-1, respectively, independent of the total intensity, although the position of band 2 was somewhat higher when the total intensity was low. On the other hand, band 3 shifted to the higher wavenumber with the increase in the relative total intensity. At the maximum intensity, the band position reached 1603 cm-1, while it was 1575 cm-1 at very low total intensity. It should be noted that all three bands simultaneously appear and the ratio between them remained almost constant over the entire range of coverage; the average ratio for about 30 independent experiments is 52 ( 9 (standard deviation), 5 ( 4, and 43 ( 10% for bands 1, 2, and 3, respectively. As far as we know, there are only a couple of papers that report the IR spectra of NO adsorbed on polycrystalline Pt11,22 as described above. In 0.1 M HClO4, only a single band was observed at 1580-1620 cm-1,22 while two absorption bands at 1767 and 1614 cm-1 have been reported in the ultrahigh vacuum system.11 As summarized in the Introduction, two types of adsorption states are typically considered, that is, the on-top (or atop, terminal) and bridged (or 3-fold) NO. The band position generally shifted in the negative direction in the order of on-top > bridged > 3-fold.15 The actual band position also depends on the crystallographic plane. Thus, the band due to the on-top NO (35) The positive band at 1800 cm-1 is due to adsorbed CO. It is hard to remove a trace amount of the adsorbed CO. It is believed that the origin is CO2 in air and/or solvents used for cleaning the cell (Kunimatsu, K.; Senzaki, T.; Samjeske´, G.; Tsushima, M.; Osawa, M. Electrochim. Acta 2007, 52, 5715). When the amount of adsorbed CO is greater in the reference spectrum than in the sample spectra, this band is observed negatively. (36) The peak fitting was performed using the Peak Fitting Module of Origine (OriginLab Corporation).
Adsorbed NO Reduction on a Pt Electrode
Langmuir, Vol. 24, No. 8, 2008 4355
Figure 2. (a) Time course of the SEIRA spectra for NO adsorption on the polycrystalline Pt electrode at 0.2 V in 0.1 M HClO4. (b) Deconvoluted spectrum for the spectrum observed at 900 s.
Figure 4. Potential dependence of SEIRA spectra for the adsorbed NO in 0.1 M HClO4. The electrode potential was scanned from 0.2 V to (a) positive and (b) negative potentials at the sweep rate of 0.02 V s-1. Figure 3. (a) Band positions and (b) band intensities for band 1 (square), band 2 (circle), and band 3 (triangle) as a function of the normalized total intensity. The present data were obtained from the spectra shown in Figure 2. Both the band intensities and the total intensity were normalized to the total intensity at 900 s.
appears at 1665-1722 cm-1 on Pt(111)7-13 and 1760-1809 cm-1 on Pt(110).13,14,20,21 The bridged NO gives bands at 13951519, 1582-1641, and 1582-1620 cm-1 for Pt(111),7-12 Pt(100),12,13,15,16,18,19 and Pt(110),14,20,21 respectively. Therefore, bands 1 and 3 can be assigned to the N-O stretching vibration of the on-top and bridged NO, respectively. The broader bands (the average values of the full width at half-maximum (fwhm) are 110 and 70 cm-1 for bands 1 and 3, respectively) compared to those for the single crystal planes (the average fwhm is about 34 cm-1) are probably due to the coexistence of various crystallographic planes. Band 2 is hard to assign. Because this band is between the bands assigned to the on-top and bridged NO, it may be reasonable to assign it to the intermediate adsorption states such as NO adsorbed at the long bridge site as proposed
for the adsorption state on Pt(110), the terminal bent NO because the decrease in the linearity (Pt-N-O) typically caused the decrease in the wavenumber,15 and the NO adsorbed on defect sites.14,21 It should be noted that this band is within the wavenumber range where the bending vibration mode of water molecules appears. However, it is not due to the water, because the band increased with the increase in the adsorption amount. Potential Dependence of the SEIRA Spectrum of the Adsorbed NO. The potential dependence of the SEIRA spectrum was examined as follows. First, the electrode surface was covered with the adsorbed NO at 0.20 V. After the solution was purged by Ar gas for 15 min to remove the dissolved NO, the spectra were obtained every 1 s during the potential cycling at a sweep rate of 0.02 V s-1. The electrode potential was scanned from 0.20 V to either positive or negative potentials. Figure 4 shows the SEIRA spectra obtained in the positive and negative direction scans.35 All bands almost linearly shifted to higher wavenumbers with increasing potential with slopes of 73, 64, and 61 cm-1 V-1
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Nakata et al.
Figure 6. Spectral change during the potential step experiment from 0.2 V to -0.15 V in 0.1 M HClO4 purged by Ar gas after the adsorbed NO layer was formed.
Figure 5. Potential dependences of (a) the band position and (b) the integrated peak intensity for band 1 (square), band 2 (circle), and band 3 (triangle), which were obtained from the spectra shown in Figure 4. The integrated area was normalized to the total band intensity at 0.2 V. The solid circle in (b) shows the relative total intensity.
for bands 1, 2, and 3, respectively, as shown in Figure 5a. This behavior is a typical characteristic of the adsorbed species. A similar band shift (36-95 cm-1 V-1) was reported for the adsorbed NO.12,18,19 Generally, the potential-induced shift has been discussed in terms of the Stark effect due to the electric field at the electrode/electrolyte interface,12 the back-donation of the charge from the metal to the adsorbed species,37,38 and the change in the lateral interaction between the adsorbed species.37,38 The intensity of band 1 remained almost unchanged in the potential range from 0.05 to 0.60 V versus Ag/AgCl (Figure 5b), which is consistent with the results reported in the literature.12,18 However, the intensity of band 3 decreased with increasing electrode potential, showing that the bridged NO is less stable at positive potentials. At negative potentials, the intensities of both bands 1 and 3 decreased with decreasing electrode potential, showing the reduction of the adsorbed NO as supported by the cyclic voltammogram displayed in Figure 1. The potentials at which the intensity started decreasing are 0.02 and -0.1 V for bands 1 and 3, respectively. This suggests that the on-top NO is reduced easier than the bridged NO. At potentials more positive than 0.6 V, the intensity has a tendency to decrease with increasing electrode potential, showing the oxidation of the adsorbed NO. Kinetics of the Reduction of Adsorbed NO. To determine the reaction rate for the reduction of the adsorbed NO species, the SEIRA spectra were recorded with a time resolution of 0.2 s during the potential step experiments from 0.2 V to the desired potentials in 0.1 M HClO4 purged by Ar gas after the adsorbed layer of NO was formed on the electrode surface. A typical result is shown in Figure 6, in which the potential was stepped to -0.15 V. Both main bands decreased with time, showing the reduction of the adsorbed NO. Immediately after the potential step, band 3 shifted to the lower wavenumbers as expected from the potential dependence of the band position (Figure 5a). No new band appeared upon the reduction, showing that the adsorbed NO is converted to the solution or gaseous species. According (37) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107. (38) Chabal, Y. J. Surf. Sci. Rep. 1988, 8, 211.
Figure 7. Time courses of relative intensities for the reduction of the (a) on-top NO and (b) bridged NO at 0 V (open circle), -0.05 V (solid circle), -0.1 V (open square), -0.15 V (solid square), and -0.2 V (triangle). The band intensities were normalized to the corresponding intensities at 0.2 V. The solid curves show the best-fit curves based on eq 4.
to the literature,18 the most probable product is the ammonium ion. Figure 7 shows the time course of the band intensities normalized to those at 0.2 V immediately before the potential step for bands 1 (a) and 3 (b). The band intensities exponentially decrease with time, t. However, the intensities do not decay to zero unless very negative potentials are applied. This indicates the supply of the adsorbed NO from solution. Although the NOsaturated solution was purged by Ar gas, it is hard to completely remove NO from the solution. Thus, a very small amount of NO remains in the solution. Actually, the NO bands increased when the electrode potential was held at 0.2 V for about several minutes
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Langmuir, Vol. 24, No. 8, 2008 4357
Table 2. Rate Constants for the Reduction of the On-Top NO and Bridged NO rate constants/s-1 electrode potential/ (V vs Ag/AgCl)
on-top NO (1730 cm-1)a
0 -0.05 -0.10 -0.15 -0.20
0.36 ( 0.04 1.10 ( 0.43 2.24 ( 0.03 6.04 ( 0.62
a
bridged NO (1603 cm-1)a 0.07 ( 0.01 0.24 ( 0.09 0.60 ( 0.17 2.11 ( 0.67
Wavenumbers at 0.2 V.
after removal of the adsorbed NO at -0.2 V. Therefore, it is necessary to take into consideration the adsorption process in addition to the reductive desorption of the adsorbed NO. The reaction scheme would be described as follows: kad
kred
NO(soln) 98 NO(a) 98 products
(1)
the reaction rate of the adsorbed NO is expressed by
-
dΓ ) kredΓ - kadCNO(Γ0 - Γ) dt
(2)
where Γ0 and Γ are the adsorption amounts of NO at t ) 0 and t ) t, respectively. Because the adsorption time at 0.2 V was sufficiently long, Γ0 is considered to be the maximum adsorption amount. The rate constants, kred and kad, have units of s-1. The concentration of NO in the solution, CNO, is assumed to be unchanged during the reduction. Under steady-state conditions (-dΓ/dt ) 0), we obtain
kredΓs ) kadCNO(Γ0 - Γs)
(3)
where Γs is the adsorption amount of NO in the steady state. The combination of eq 2 with eq 3 and integration give the following equation.
( ) {
Γs Γ Γs 1 ) + 1exp -kred t Γ0 Γ0 Γ0 1 - (Γs/Γ0)
}
(4)
Assuming that the band intensity is proportional to coverage, Γ/ Γ0 is equal to the normalized intensity shown in Figure 7. The curve-fitting was performed using kred and Γs as parameters. The best-fit curves are shown by the solid curves in Figure 7. The average values of kred at various potentials for several independent experiments are summarized in Table 2. The rate constants for the on-top and bridged NO increased with the decrease in the applied potential as expected for the reduction reaction. In addition, the rate constant for the on-top NO at -0.1 V is 1 order of magnitude greater than that for the bridged NO. It should be noted that the high kred value cannot be accurately determined due to the fact that a sufficient number of data points were not particularly obtained during the initial stage of the reduction; the sampling interval of 0.2 s becomes identical to the half-life time at kred ) 3.5 s-1. For this reason, the kred value for the on-top NO at -0.2 V was not determined. The adsorbed NO species producing band 2 shows rate constants greater than those of the
Figure 8. Electrode potential versus logarithm of kred plot for the (a) on-top NO (open circle) and (b) bridged NO (solid circle).
other two adsorbed NOs. However, the band intensity was often so small that an accurate rate constant could not be determined. Figure 8 shows the potential dependence of the rate constant, which is essentially equivalent to the so-called Tafel plot. Tafel slopes were determined to be 0.122 and 0.104 V decade-1 for the on-top and bridged NOs, respectively. These values are close to 0.119 V decade-1. Therefore, it is concluded that the ratedetermining step for the reduction of the on-top and bridged NO is the first electron-transfer process.
Summary SEIRA spectra of the adsorbed NO on a polycrystalline Pt electrode in 0.1 M HClO4 were obtained for the first time. At a high surface coverage, two distinct bands were observed at 1730 and 1603 cm-1 at 0.2 V versus Ag/AgCl (sat. KCl) and were assigned to the on-top and bridged NOs, respectively. Particularly, the band due to the on-top NO on the polycrystalline Pt electrodes was first observed in this paper, although both adsorbed NOs were reported in the ultrahigh vacuum system.11 As described above, the reference state of the SEIRA spectra was set in a completely NO-free electrolyte solution. Therefore, it becomes possible to acquire spectra of higher quality and higher reliability using SEIRA, compared to conventional IRAS. We consider that the new observations of the on-top NO on a polycrystalline Pt electrode are due to these advantages of SEIRA spectroscopy. In this paper, we also first determine the surface reaction rate for the reduction of the adsorbed NOs. The first-order rate constant of the on-top NO is about 10 times higher than that of the bridged NO in the potential range of -0.05 to -0.15 V. Both rate constants increased with decreasing electrode potential as expected for the reduction reaction. Also, the slopes of the electrode potential versus the logarithm of the rate constant indicate that the first electron transfer is the rate-determining step for the reduction of the adsorbed NO on the polycrystalline Pt electrode surfaces. Acknowledgment. This work was partially supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Science, Sports and Culture, Japan (Nos. 17350075 and 18350038), the Northern Advancement Center for Science & Technology, and the Japan Science and Technology Agency. LA703394Z
