Reaction Pathways for Reduction of Nitrate Ions on Platinum

Electrocatalytic reduction of nitrate ions at a poly crystalline SnCu modified platinum surface by using an H + conducting solid polymer in a sandwich...
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Langmuir 2000, 16, 771-777

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Reaction Pathways for Reduction of Nitrate Ions on Platinum, Rhodium, and Platinum-Rhodium Alloy Electrodes M. C. P. M. da Cunha, J. P. I. De Souza,† and F. C. Nart*,‡ Instituto de Quı´mica de Sa˜ o Carlos, Universidade de Sa˜ o Paulo, C.P. 780, 13560-970 Sa˜ o Carlos, Sa˜ o Paulo, Brazil Received May 25, 1999. In Final Form: August 23, 1999 The reduction of nitrate ions on platinum, rhodium, and platinum-rhodium alloy electrodes has been investigated using differential electrochemical mass spectrometry and in situ FTIR measurements. For 3 M HNO3 concentration it has been found that nitrate starts the reduction with partial N-O bond dissociation and N-N bond formation generating NO and N2O. At potentials lower than 0.2 V the reaction proceeds forming dissolved NH4+. For potentials lower than 0 V the reduction continues via a multiple pathway reaction leading to the nonselective production of N2, NH2OH, and N2H2. On the alloyed electrodes, the production of NO and N2O has been observed in both cathodic and anodic scans, while on pure platinum and rhodium electrodes the reaction has been observed only during the cathodic scan. Contrasting with the pure platinum and rhodium alloys, where the N-O bond break starts forming NO and N2O, on the alloys HNO2 has been observed as the first reaction step. For alloys with higher rhodium composition, like Pt75Rh25, no N2 has been detected for potentials lower than 0 V.

Introduction The increasing interest in the reduction of nitrate is mainly connected with the degradation of radioactive waste and with pollution control,1 although the synthesis of some nitrogen compounds is of interest as well.2 The reduction of nitrate ion has been investigated extensively in the past. The large number of nitrogen compounds, which can be formed from nitrate, makes the study of this reaction difficult. The use of spectroscopic techniques to detect the possible products is of crucial importance to identify the possible reaction routes and the reaction products. Differential electrochemical mass spectrometry (DEMS) measurements were reported recently.3-5 It was found that the reduction products depend strongly on the electrode nature and on the experimental conditions. For platinum electrodes Nishimura et al.3 found N2 and N2O at potentials between 0.5 and 0.0 vs RHE in HNO3 concentrations higher than 0.5 M. In alkaline solutions N2 and NH3 are produced at potentials around 0.7 vs RHE.4 In a recent paper, no gaseous products were observed for the reduction of nitrate on pure platinum electrodes using DEMS experiments for NO3- concentrations of 0.1 M. At platinum and platinum alloys modified with Ge adatoms, the production of N2O was observed as gaseous product, but no attempt to detect nonvolatile products was reported.5 † On leave from the Departamento de Quı´mica da Univesidade Federal do Para´, Bele´m, PA, Brazil. ‡ E-mail: [email protected].

(1) Li, H.; Robertson, D. H.; Chambers, J. Q.; Hobbs, D. T. J. Electrochem. Soc. 1988, 135, 1154-1158. (2) Plieth, W. F. In Bard, A. J. Encyclopedia of Electrochemistry of the Elements Marcel Dekker: New York, 1978; Vol. 8, p 321. (3) Nishimura, K.; Machida, K.; Enyo, M. Electrochim. Acta 1991, 36, 877-880. (4) Wasmus, S.; Vasini, E. J.; Krausa, M.; Mishima, H. T.; Vielstich, W. Electrochim. Acta 1994, 39, 23-31. (5) Gootzen, J. F. E.; Peeters, P. G. J. M.; Dukers, J. M. B.; Lefferts, L.; Visscher, W.; van Veen, J. A. R. J. Electroanal. Chem. 1997, 434, 171-183.

In a recent report the reaction of nitrate ions on polycrystalline platinum electrodes was studied using IR spectroscopy.6 It was found that at moderate overpotentials nitrate ions undergo a dissociative adsorption producing adsorbed NO on platinum electrodes. On the other hand, on polycrystalline gold, undissociated nitrate ions are adsorbed doubly coordinated.6 The reduction of nitrate ions on polycrystalline gold causes only one N-O bond to break, generating nitrite ions, which are adsorbed in the Au double layer region.6 On Au(100) single crystal, nitrate ions are adsorbed and no nitrite ions have been observed.7 On gold electrodes the reduction of nitrate is very slow and hardly detectable, but when the gold surface is modified with Cd adatoms, nitrite ions are produced as the reduction product as observed by in situ FTIR spectroscopy.8,9 The remarkable dependence of the nitrate reduction products on the nature of the surface makes this reaction very interesting to electrocatalysis and in a more general way to the electrochemistry of nitrogen compounds. Although this reaction was studied on various electrodes, including the platinum group metals the reaction pathways of nitrate reduction on platinum and platinum group metals are still not clear. The studies of this reaction on platinum and modified platinum have been carried out using DEMS, which is limited to the volatile reaction products. In the present work we investigate the reduction reaction of nitrate ions on electrodeposited platinum, rhodium, and platinum-rhodium alloys. We use DEMS and in situ FTIR measurements in order to identify the reaction products. We found that the combination of these two auxiliary spectroscopic techniques is helpful in following the reaction pathways, since both volatile and (6) da Cunha, M. C. P. M.; Weber, M.; Nart, F. C. J. Electroanal. Chem. 1996, 414, 163-170. (7) Moraes, I. R.; da Cunha, M. C. P. M.; Nart, F. C. J. Braz. Chem. Soc. 1996, 7, 453-460. (8) Xing, X.; Scherson, D. A. J. Electroanal. Chem. 1986 199, 485488. (9) Xing, X.; Scherson, D. A.; Mack, C. J. Electrochem. Soc. 1990, 137, 2166-2175.

10.1021/la990638s CCC: $19.00 © 2000 American Chemical Society Published on Web 11/12/1999

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Table 1. Composition of the Solutions Used for the Electrodeposition and Bulk Composition of the Electrodeposited Electrodes Obtained by EDAX (in atomic %) Pt and Rh salt concn (mM)

EDAX (Pt:Rh)

electrodes

H2PtCl6‚6H2O

RhCl3‚3H2O

DEMS

FTIR

Pt0.90Rh0.10 Pt0.75Rh0.25

20 20

2.22 8.57

90:10 74:26

90:10 75:25

Table 2. Surface Area of the DEMS Electrodes electrode

area (cm-2)

Pt Pt90Rh10 Pt75Rh25 Rh

31.2 73.2 114 74

nonvolatile products are detected. Moreover, even those products that are not stable enough to be detected by in situ FTIR spectroscopy can be detected by DEMS if they are volatile enough to be pumped through the porous electrodes. Experimental Section The DEMS (differential electrochemical mass spectroscopy) experiments were made in a PPT (MKS) quadrupolar mass spectrometer installed in a stainless steel chamber. This chamber was connected to a second stainless steel prechamber adapted to an electrochemical cell through a hole of 2 mm diameter. The MS chamber was pumped with an 80 L s-1 Turbo-Molecular pump and the prechamber was pumped by an 100 L s-1 drag pump connected to a primary membrane pump. The electrochemical cell was made of PTFE with a membrane at the bottom of the cell. Pt and/or Rh were electrodeposited on this PTFE membrane (average thickness 60 µm, mean pore size 0.17 µm, 50% porosity). When open to the electrochemical cell, the pressure of the MS chamber was of ca. 10-6 and that of the prechamber was ca. 10-3. Before each experiment, the working electrodes were subjected to potential cycling for 15 min at 0.05V s-1 in the potential range 0.05-1.0 V. All voltammograms and mass signals were acquired in a 3 M HNO3 + 0.5 M H2SO4 solution, starting at 1.0 V in the cathodic direction and ending at -0.2 V. A BOMEM DA-8 spectrometer with a liquid nitrogen cooled MCT detector was used for the FTIR experiments. The electrochemical cell was made of PTFE with a CaF2 window placed at the bottom of the cell. The FTIR spectra were taken with the electrode pressed onto the CaF2 window in order to obtain a homogeneous layer of the 0.05 M HNO3 + 1 M HF solution. Spectra were taken at the reference, R0 (0.8 V), and the sample potentials, R, with a resolution of 8 cm-1 and 1000 scans. The resulting spectra are presented as the reflectance ratio ∆R/R0. Electrodes. The working electrodes used in the DEMS experiments were electrodeposited on a PTFE membrane recovered with sputtered Au. The composition of each solution for the different alloys is shown in Table 1. The electrodeposited working electrodes were obtained by potentiostatic deposition of Pt, Rh, or Pt-Rh codeposition on Au substrates, in a 1 M HClO4 solution containing the appropriate platinum and rhodium salts, over the course of 5 min at 0.2 V vs RHE. The bulk compositions of the electrodeposited alloys, obtained by energy dispersive analysis of X-ray (EDAX), are listed in Table 1. The electrode areas were determined by the following experiment. CO was adsorbed for 5 min at 0.2 V in each electrode. After this, N2 was bubbled during 10 min to eliminate the CO in the bulk of solution. Then an anodic scan was initialized until 1.1 V and a completed cycle was allowed in order to obtain a typical voltammogram of the electrode in the electrolyte solution. The charge of the CO oxidation was used to calculate the electrode area assuming one molecule per active site of the electrodes. The calculated areas are presented in Table 2. A reversible hydrogen electrode (RHE) in the electrolyte solution and a Pt foil were used as reference and counter electrodes, respectively.

Figure 1. (a) Voltammogram and DEMS measurements of electrodeposited Pt in 3 M HNO3 + 0.5 M H2SO4 solution. Cathodic scan started at 1.0V at 0.01 V s-1, one cycle. The currents were normalized relative to the area of the electrode. (b) Similar experiment as in (a), but recording the ion current for the m/z 29 and 33, corresponding to hydrazine and hydroxilamine.

For FTIR experiments the alloy electrodes were electrodeposited using a method similar to that outlined for the electrodes used in DEMS experiments. The substrate was a smooth gold electrode polished mechanically to a mirror surface with alumina. The compositions of the solution and the conditions of electrodeposition are the same as those used for DEMS electrodes. The alloy bulk compositions obtained by EDAX are shown in Table 1.

Reaction Pathways for Reduction of Nitrate Ions

Figure 2. Voltammogram and DEMS measurements of electrodeposited Rh in 3 M HNO3 + 0.5 M H2SO4 solution. Cathodic scan started at 1.0 V at 0.01 V s-1, one cycle. The currents were normalized relative to the area of the electrode.

Results and Discussion NO3- Reduction at Pure Platinum and Rhodium Electrodes: DEMS Measurements. Gaseous reaction products from the reduction of nitrate on platinum and platinum-rhodium alloys were observed only for NO3-

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concentrations higher than 3 M. This result contrasts with previous observations, where gaseous products were detected from a painted Pt electrode.2 The cyclic voltammograms recorded during the reduction of NO3- on porous platinum electrodes are shown in the upper panels of parts a and b of Figure 1, for two different experiments. In the lower panels of parts a and b of Figure 1 the observed m/z signal for the products of the nitrate cathodic decomposition are shown. The reduction of the NO3- on platinum starts at 0.5 V in 0.5 M H2SO4 and 3 M HNO3. The cathodic current rises up to 0.2 V; a shoulder is observed at 0.25 V and increases again, going through a maximum at 0.1 V, showing clearly at least two electrochemical processes (Figure 1a inset and Figure 1b upper panel). At potentials lower than 0.0 V the current rises very sharply, due to the production of molecular hydrogen, along with the nitrate reduction products. The masses during the first step in the NO3- reduction are m/z 30 and 44. N2O has a main peak at m/z 44. The m/z 30 arises from N2O fragmentation at the mass spectrometer (about 47% of the m/z 44) and production of NO. It is important to note that m/z 30 and 44 start to increase at 0.5 V, coinciding with the electrochemical current observed in the cyclic voltammogram. Additionally, the ion current presents a maximum at 0.3 V, coinciding with the shoulder of the electrochemical current in the cathodic scan of the cyclic voltammogram. The ion current approaches the background at 0.2 V. For potentials lower than 0.0 V the m/z 30 and 44 signals increase again. The mass signal m/z and 28 (N2), 29 (N2H2),10 and m/z 33 (NH2OH) start to be produced around 0 V as well. The m/z 29 and 33 are shown separately in Figure 1b for more clarity. Similar results are presented in Figure 3 for porous Rh electrodes (with the exception of m/z 29 and 33). It is interesting to note that on Rh electrodes the cathodic current due to the NO3- reduction starts to increase at ca.

Figure 3. FTIR spectra of electrodeposited Pt in 0.05 M HNO3 + 1 M HF solution obtained with p-polarized light. For each experiment, 1000 scans with a resolution of 8 cm-1 were used. Reference potential was 1.2 V.

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Figure 4. (a) Transmittance spectra of 10 mM NaN3 in H2O and in 1 M HF solution. (b) Transmittance spectrum of NH4+ in 1 M HF aqueous solution.

0.4 V, about 0.1 V higher negative potential than the platinum electrode. This reveals the superior catalytic property of the platinum electrode for nitrate reduction. The two very distinct processes detected on platinum are not observed for rhodium. This is probably due to the fact that the first process starts at higher overpotential and is thus overlapped with the second process, which starts at the same potential as observed for platinum electrodes. The onset of production of m/z 30 and 44 is the same and is practically coincident with the onset of the cathodic current observed in the voltammogram (upper panel). For m/z 30 and 44, a maximum is observed at 0.3 V, and approaches the background level after 0.2 V, as for platinum electrodes. For potentials more negative than 0.0 V N2 production begins, as evidenced by the m/z 28 signal (Figure 2 second panel). At the same point the production of NO and N2O starts again, shown by the rise of the m/z 30 and 44 (Figure 2 third and fourth panels). For rhodium electrodes both m/z 30 and 44 signals have the same range of values around 6.8 × 10-16 A, indicating that the m/z 30 correspond not only to the N2O fragmentation but also to NO. In the case of the platinum electrode, the m/z 30 is about 30% of the m/z 44, which would correspond only to the N2O fragmentation, but as we will see below, the production of dissolved NO is clearly detected by in situ FTIR (see Figure 3a). Therefore, the production of NO cannot be ruled out based on the DEMS measurements for the platinum electrodes. On platinum electrodes (Figure 1, upper panels of parts a and b), two processes are clearly distinguished in the cyclic voltammograms. The first process takes place between 0.5 and 0.2 V and is clearly distinguished by the shoulder in the cathodic current observed at 0.20 V. At the same time, we have observed that the m/z 30 and 44 signals (Figure 1a third and fourth panels), corresponding to N2O and NO, on platinum electrodes, decrease after 0.3 V and almost reach the background at 0.2 V, while the (10) Actually the N2H2 has the main m/z peak at 32. Unfortunately in our case the m/z 32 presents a very large background and only a tiny signal at m/z 32 has been detected. The m/z 29 cannot be assigned to any oxygenated species and is probably the main fragment of N2H2, leading to the (N2H)+ ion, which has been clearly detected.

electrochemical cathodic current continues to rise. This result implies that after 0.2 V another product must be formed in order to maintain the reduction reaction revealed by the electrochemical current. Obviously the reaction product generated at potentials below 0.2 V must be some nitrogen compound with an oxidation state lower than +1. There are many possible nitrogen compounds with the nitrogen oxidation state lower than +1: N2O, NO- or its dimer [N2O2]2-, N3-, NH2OH, N2H4, N2H62+, and NH4+. Since N2O ceases to be produced, this species can be ruled out. Hydrazine (m/z ) 29)10 and hydroxilamine (m/z ) 33) have been detected with DEMS only for potentials lower than 0.0 V (see Figure 1b) and can be ruled out as well. Thus, the production of nonvolatile species must account for the increase in cathodic current observed below 0.2. In this case, the use of in situ FTIR spectroscopy is important for the identification of the soluble compounds. NO3- Reduction at Pure Platinum and Rhodium Electrodes: in Situ FTIR Spectra. We have performed in situ FTIR to try to identify possible soluble nonvolatile products generated during the NO3- reduction on the platinum electrode. The spectra obtained during the reduction of nitrate in acid media are plotted in Figure 3. The spectra were obtained at different potentials. The reference potential was taken at 1.2 V, thus above the reversible potential of the NO3- reduction. For potentials between 0.9 and 0.8 there is one band at 1871 cm-1, which can be assigned to dissolved NO.12 At 0.7 V one feature starts at 2232 cm-1, a band typical of N2O.12 N2O ceases to be produced at 0.3 V, and at potentials lower than 0.2, one band at 1462 cm-1 starts and increases with increasing negative potential. The potential range for production of N2O is in good agreement with the DEMS measurements. In the range 1350-1380 cm-1 there are large nitrate fluctuations, probably due to the high nitrate concentration (3 M HNO3) used in the experiments. (11) Buchholtz, J. R.; Powell, R. E. J. Am. Chem. Soc. 1963, 85, 509511. (12) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 3rd ed.; John Wiley & Sons: New York, 1978.

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Figure 6. Voltammogram and DEMS measurements of electrodeposited Pt75Rh25 in 3 M HNO3 + 0.5 M H2SO4 solution. Cathodic scan started at 1.0 V at 0.01 V s-1, one cycle. The currents were normalized relative to the area of the electrode.

Figure 5. (a) Voltammogram and ion DEMS measurements of electrodeposited Pt90Rh10 in 3 M HNO3 + 0.5 M H2SO4 solution. Cathodic scan started at 1.0V at 0.01 V s-1, one cycle. The currents were normalized relative to the area of the electrode. (b) The same as in (a) but recording the ion current for m/z 29 and 33.

It is interesting to note that nitrate at a lower concentration (0.05 M) reacts to yield products having spectroscopic features at 1580 and 1450 cm-1 6 but does not produce detectable amounts of volatile products. The

band at 1580 cm-1 has been assigned to adsorbed NO at low coverages.6 The 1462 cm-1 band is also observed with s-polarized light (data not shown), which indicates that this feature corresponds to dissolved species only. As suggested by Heckner based on pure electrochemical measurement,13 it is likely that the further reduction of NO will produce H2N2O2, or NO-. The spectrum of H2N2O2 presents the strongest feature at 1031 cm-1 14 and therefore cannot be the responsible for the 1462 cm-1 band. The other possibility is that the NO is reduced to NO- and does not dimerize. However, in very acidic solutions, like in the present case, it is very unlikely that the NO- would be dissociated. HNO would be pumped to the mass spectrometer and m/z 30 should be observed in the DEMS measurements. Moreover, the matrix NO- spectrum shows one band at 1346-1376 cm-1.12 Shifts of the wavenumber to higher wavenumber values are expected for dissolved ions when compared to matrix isolated species spectra, but the change from 1376 to 1462 cm-1 is rather large to assign this feature unequivocally to dissolved NO-. Other possible nitrogen compounds with an oxidation state less than +1 would be N3- or NH4+. The transmittance FTIR spectra of N3- and NH4+ in aqueous HF solutions are shown in parts a and b of Figure 4, respectively. N3presents the main band at 2049 cm-1, much above the values observed for the in situ spectra. NH4+ ions present a band at 1470 cm-1, which is very close to the band observed for the in situ spectra. Therefore we conclude that below 0.2 V, but still before 0 V, there is N-H bond formation producing NH4+ ions as reduction product. The reduction reaction of nitrate on polycrystalline electrodeposited platinum electrodes follows distinct (13) Heckner, H. N. J. Electroanal. Chem. 1977, 83, 51-63. (14) McGraw, G. E.; Bernitt, D. L.; Hisatsune, I. C. Spectrochim. Acta 1967, 23A, 25.

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Figure 7. FTIR spectra obtained during the reduction of nitrate on electrodeposited Pt90Rh10 in 0.05 M HNO3 + 1 M HF solution obtained with p-polarized light. For each measurement 1000 scans with a resolution of 8 cm-1 were used. Reference potential was 0.8 V.

pathways, depending on the electrode potential. For potentials between 0.6 and 0.1 V the reaction follows a partial N-O bond breaking and N-N coupling reaction leading to the production of NO and N2O. For potentials below 0.2 V N-H bond formation occurs. For potentials lower than 0 V a nonselective partial N-O bond breaking, N-N coupling and N-H bond formation takes place. The in situ FTIR spectra for pure electrodeposited rhodium electrodes (data not shown) show the production of N2O starting at 0.6 V and disappearing at 0.1 V and the production of the NH4+ (band at ca. 1470 cm-1) starting at 0 V. NO3- Reduction on Pt-Rh Alloys Electrodes. The effect of alloying platinum and rhodium from simultaneous electrodeposition of both metals has been evaluated for three different alloy compositions. Since only marginal changes in product distribution have been detected, not all data will be shown here. In Figure 5 the cyclic voltammetry (upper panels) and mass ion current results for Pt90Rh10 are shown (Figure 5a lower panels and Figure 5b). The cyclic voltammogram is very similar to that of pure platinum for the reduction of NO3-. The results for the m/z 30 and 44 show similar qualitative trends as those observed for pure platinum and pure rhodium. The production of NO and N2O starts at 0.55 V, practically at the same value as the onset of the cathodic current in the voltammogram. NO and N2O are formed during the cathodic and the anodic cycle, in contrast to the case of pure platinum and pure rhodium electrodes, on which NO and N2O are formed only during the cathodic scan. As in the case of the pure electrodes, below 0.0 V the production of N2 is observed, together with the simultaneous production of NO and N2O and the onset of m/z 29 and 33 (Figure 5b). These signals can be assigned to

hydrazine and hydroxylamine, respectively, as in the case of the pure electrodes. After increasing the rhodium content in the alloy, the cyclic voltammogram is more similar to that of pure rhodium, as can be seen in Figure 6 for the alloy Pt75Rh25. The production of NO and N2O is also observed starting at 0.5 V as in the case of Pt90Rh10 or pure Pt. The 1465 cm-1 band has been observed for the alloys as can be seen in Figure 7 (for Pt90Rh10), but starts at ca. 0 V, as in the case of pure rhodium electrodes. For Pt90Rh10 a feature at 1272 cm-1, typical of HNO2, can be observed when the potential reaches 0.7 V. The HNO2 production decreases with the increasing Rh content in the alloy and is practically undetectable for the Pt75Rh25 (spectra not shown). The on line DEMS and in situ FTIR measurements bring to light two points that need to be understood. The first point is the lack of NO infrared signal for platinumrhodium alloys and the accompanying absence of N2O bands in the in situ FTIR spectra at potentials lower than 0.0 V. These observations can be explained by the fact that in the time scale of the in situ FTIR measurements, NO (for potentials between 0.7 and 0.2 V) and N2O (at potentials below 0.0 V) are further reduced. In the case of DEMS measurements these products are promptly pumped to the quadrupole chamber where they can be detected. The second point to be addressed is the differences between the present results and results previously published.3 While in the present work the gaseous products were observed only for 3 M HNO3, previous results show a large signal of N2 for 0.1 M HNO3. This also contrasts with the reduction products observed here, which can be identified as N2O and NO. The only way to reconcile the different results is to take into account the differences in

Reaction Pathways for Reduction of Nitrate Ions

electrodes preparation methods. In this study the electrodes were prepared by electrodeposition of platinum onto a sputtered gold substrate, while in the previous study painting a platinum powder onto the PTFE membrane made the electrodes. Nevertheless, more measurements will be needed to clarify this point. Finally, the use of rough electrodes, as obtained from electrodeposition, prevented the identification of adsorbed intermediates. Future works using smooth single-crystal electrodes are planned to try to identify possible stable intermediates, responsible for the N-N and N-H coupling reaction. Conclusions The reaction products of nitrate reduction were followed by DEMS and in situ FTIR measurements. The reaction starts producing NO followed by N2O on platinum electrodes. A mixture of N2O and NO is produced at rhodium and at Pt-Rh alloy electrodes, revealing a general trend on the platinum, rhodium and platinum-rhodium alloys. In summary, the reduction reaction starts with partial N-O bond breaking and N-N coupling, generating N2O as the primary reaction product below 0.4 V. The gaseous reaction products reach a maximum at about 0.3 V for all electrodes, although the electrochemical cathodic current rises further, indicating the production of another soluble product, which has been identified as dissolved NH4+ by in situ FTIR spectroscopy. Further reduction at potentials between 0.2 and 0.0 V leads mainly to N-H bond formation with the production of NH4+ ions. At potentials lower than 0 V, where hydrogen starts to be evolved from the electrode, N2H2, NH2OH, and N2, together with NH4+, N2O, and NO, are observed, indicating a multiple pathway, nonselective reaction. The curious

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point in the NO3- reduction is the lack of N-N coupling between 0.1 and 0 V, where only NH4+ has been detected. For potentials more negative than 0 V, it is clear that partial N-O bond retention occurs, leading to the formation of NO, N2O, and NH2OH. The reasons for these two unexpected pathways are not clear from this study and more experiments are planned to clarify this point. The main difference between the pure electrodes and the alloy electrodes is the detection of HNO2 as the first reaction step for Pt90Rh10 alloy, followed by the simultaneous production of NO and N2O and later NH4+. Lowering the platinum content in an alloy leads to a decrease in the production of HNO2. N2 has been detected only for very negative potentials in all electrodes. The second important difference between pure and alloy electrodes is that in the case of the alloys electrodes, the production of N2O and NO is observed for the cathodic and anodic scans, although in the anodic scan the ion current for these two products is smaller. It implies that the reduction reaction during the cathodic scan leaves the surface less poisoned than in the case of the pure electrodes. Rhodium electrodes are much less catalyticaly active than pure platinum electrodes, as can be seen through the normalized ion current for the masses 44 and 30, which are 1 order of magnitude less than those observed for platinum. In the alloy, a high percentage of rhodium increases the cathodic current related to the production of NO (see cyclic voltammogram and mass signal of Figure 5). Acknowledgment. FAPESP, CNPq, and CAPES from Brazil supported this work. LA990638S