Anal. Chem. 1997, 69, 249-252
In Situ Fourier Transform Infrared Spectroscopic Studies of Nitrite Reduction on Platinum Electrodes in Perchloric Acid In Tae Bae, Rachael L. Barbour, and Daniel A. Scherson*
Department of Chemistry, Case Western Reserve University, Cleveland, Ohio 44106-7078
Nitrous oxide generated by the electrochemical reduction of nitrite on a polycrystalline platinum electrode in aqueous 0.1 M HClO4 was monitored by in situ potential difference Fourier transform infrared reflection absorption spectroscopy. An analysis of the intensity of the N2O asymmetric stretch band at 2231 cm-1, A(N2O), monitored during a slow linear potential (E) sweep in the range 0.8 > E > -0.32 V vs SCE, indicated that N2O begins to form at E ∼0.5 V vs SCE, reaches a maximum at E ∼0.05 V, and disappears completely for E < -0.1 V. These observations are in agreement with those made by on-line mass spectrometry (MS) using high-area Pt electrodes in acid electrolytes reported in the literature. In addition, a plot of A(N2O) vs E was found to be directly correlated with that of the integral of the N2O generation rate, as measured by on-line dynamic differential electrochemical MS (DEMS) by Nishimura et al. (Electrochim. Acta 1991, 36, 877) in sulfuric acid solutions. The electrochemical reduction of nitrite (or more properly HNO2) on Pt electrodes in acid electrolytes has received continued attention over the past two decades.1-4 The nature of the products of this multiple electron transfer reaction appears to be a function of pH, the applied potential, and, as may be inferred from more recent studies involving single-crystal electrodes,4 the detailed surface microtopography. In particular, Gadde and Bruckenstein1 examined the reduction of nitrite in 0.1 M HClO4 on polycrystalline (poly) Pt by the rotating ring-disk electrode (RRDE) and online steady state mass spectrometry (MS) techniques. On the basis of the analysis of results obtained from RRDE, acidimetry and gas chromatography, these authors concluded that the products of nitrite reduction under diffusion-limiting current conditions are NH2OH (60.7%), N2O (18.8%), and NH3 (21.6%). In addition, steady state on-line MS measurements involving higharea Pt gas-permeable electrodes, showed maximum N2O collection rates, a quantity proportional to the rate at which this gaseous species is produced, in the range 0.1 < E < 0.4 V vs SCE. Also detected in some of these experiments were NO and N2 derived from the homogeneous phase decomposition of nitrous acid and ammonia and not from a direct electrochemical process. This latter claim was more recently disputed by Nishimura et al.,3 who investigated nitrite reduction in 0.5 M H2SO4 by on-line dynamic (1) Gadde, R. R.; Bruckenstein, S. J. Electroanal. Chem. 1974, 50, 163 and references therein. (2) van der Plas, J. F.; Barendrecht, E. J. R. Neth. Chem. Soc. 1977, 96/5, 133 and references therein. (3) Nishimura, K.; Machida, K.; Enyo, M. Electrochim. Acta 1991, 36, 877. (4) Yeh, S.; Hattori, H.; Kita, H. Ber. Bunsen. Phys. Chem. 1992, 96, 1884. S0003-2700(96)00769-X CCC: $14.00
© 1997 American Chemical Society
differential electrochemical MS (DEMS) and surmised that both NO and N2 are produced by faradaic processes. This paper presents in situ potential difference Fourier transform infrared spectroscopy (PD-FTIRRAS) data for the reduction of nitrite on a solid Pt(poly) electrode in 0.1 M HClO4. It was of particular interest to monitor the amount of dissolved N2O generated during a slow voltammetric sweep to enable direct comparisons with information derived from on-line dynamic DEMS experiments reported in the literature.3 The asymmetric stretch of N2O (ν1) displays a peak with an unusually large absorption cross section centered at 2231 cm-1,5 a region far removed from that in which spectral features due to other potentially contributing species are expected to be observed. These factors allow optimal conditions for enhanced detection sensitivity by in situ PD-FTIRRAS in aqueous electrolytes, as has been fully exploited for carbon dioxide in studies involving its reduction6 and the oxidation of adsorbed CO7 and small organics.8 EXPERIMENTAL SECTION All measurements were performed at room temperature in 0.1 M HClO4 solutions prepared from ultrapure quality perchloric acid (Ultrex, Baker) and ultrapurified water generated by a modified Gilmont pyrodistillation system. An ellipsoidally shaped (poly)Pt foil (area 1.7 cm2) cast in Kel-F was used as a working electrode, and a Pt coil and a saturated calomel (SCE) as counter and reference electrodes, respectively. All electrochemical and spectroelectrochemical experiments were conducted in 20 mM NaNO2 (Aldrich)/0.1 M HClO4 or in N2O (Matheson, UHP)-saturated/0.1 M HClO4 solutions employing a CaF2 dove prism FTIRRAS cell and instrumentation described in detail elsewhere.9 In situ FTIRRAS spectra were collected continuously during a linear sweep at 1 mV/s initiated either at the open circuit potential, in the case of the nitrite solution (∼0.62 V), or at 1.2 V for the N2O saturated media, down to the onset of hydrogen evolution at ∼-0.3 V. The data are displayed as normalized potential difference (PD) spectra, i.e., -∆R/R () [R(Eref) - R(E)]/R(Eref)) vs wavenumber (cm-1), where R(Eref) is the spectrum obtained at (5) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons: New York, 1986. (6) Nikolic, B. Z.; Huang, H.; Gervasio, D.; Lin, A.; Fierro, C. A.; Adzic, R. R.; Yeager, E. B. J. Electroanal. Chem. 1990, 295, 415. (7) Chang, S.-C.; Leung, L-W. H.; Weaver, M. J. J. Phys. Chem. 1989, 93, 5341 and references therein. (8) (a) Parsons R.; VanderNoot, T. J. Electroanal. Chem. 1988, 257, 9. (b) Iwasita-Vielstich, T. In Advances in Electrochemical Science and Engineering; Tobias, C., Gerischer, H. Eds.; Wiley: New York, 1990; Vol. 1. (9) Huang, H.; Zhao, M.; Xing, X.; Bae, I. T.; Scherson, D. J. Electroanal. Chem. 1990, 293, 279.
Analytical Chemistry, Vol. 69, No. 2, January 15, 1997 249
Figure 1. Cyclic voltammetry curves for Pt(poly) in 0.1 M HClO4 before (A) and after addition of NaNO2 to a concentration of 20 mM (B) and in N2O-saturated 0.1 M HClO4 (C). Electrode area 1.7 cm2; scan rate 50 mV/s (see text for details).
the initial reference potential, Eref, R(E) is the average of 200 interferometric scans recorded over a potential interval ∆E of ∼80 mV, and E represents the average potential value over that section of the sweep. RESULTS AND DISCUSSION Figure 1 shows cyclic voltammetry curves for Pt(poly) recorded at 50 mV/s in 0.1 M HClO4 before (panel A) and after addition of NaNO2 to a concentration of 20 mM (panel B) and in N2O-saturated 0.1 M HClO4 (panel C). The data were recorded in the FTIRRAS cell just prior to spectral acquisition (for nitrite and N2O reduction) with the Pt electrode placed far away from the CaF2 prism window to avoid distortions due to nonuniform current distribution. All features in panel A in this figure are characteristic of clean Pt(poly) in this medium. In agreement with work reported by other authors,10 the rates of nitrite oxidation on Pt in acid electrolytes are very high, a factor that accounts for the sharp increase in the current for E > 0.75 V vs SCE. In contrast, nitrite reduction is kinetically hindered, as evidenced by the very low currents in the region 0.4-0.6 V. Much faster rates for this latter process were observed only at higher overpotentials, yielding two broad voltammetric peaks centered at about 0.25 and -0.07 V. In fact, Pt(disk)-Pt(ring) RRDE studies1 have shown that, in the range of moderate to high rotating rates, HNO2 reduction in 0.1 M HClO4 proceeds under diffusionlimiting conditions for E e -0.22 V vs SCE. A cyclic voltammogram very similar to that shown in panel B (Figure 1), was reported by Nishimura et al.3 for stagnant 12 mM NaNO2/0.5 M H2SO4 solutions on a porous Pt electrode recorded during acquisition of on-line dynamic DEMS data. A series of normalized PD-FTIRRAS spectra obtained in the region ∼2125-2325 cm-1, arranged in decreasing values of E (top 0.55 V; bottom -0.24 V, see caption for intermediate potentials), using the spectrum obtained at Eref ) 0.62 V as a reference, are shown in panel A (left), Figure 2. These data were collected during a linear sweep in the negative direction (at a rate of 1 mV/s) initiated at Eref, with the electrode pressed against the window (see panel B, Figure 2). The integrated area of N2O (ν1) centered at 2231 cm-1, A(N2O), provides a measure of the amount (10) See, for example: Figure 1 in ref 1.
250 Analytical Chemistry, Vol. 69, No. 2, January 15, 1997
Figure 2. (A) Series of normalized potential difference spectra (-∆R/R vs wavenumber, Eref ) 0.62 V) in the regions 2125-2325 (left) and 1350-1550 cm-1 (right), collected during a linear potential scan in the negative direction initiated at Eref in a 20 mM NaNO2/0.1 M HClO4 solution at a rate of 1 mV/s (see text for details). From the top: 0.55, 0.48, 0.40, 0.32, 0.24, 0.16, 0.08, 0.00, -0.08, -0.16, and -0.24 V vs SCE. These potentials represent averages over a specific section of the scan. (B) Linear sweep voltammogram obtained during acquisition of the in situ FTIRRAS spectral data in (A). (C) Integrated intensity of the N2O asymmetric stretch band at 2231 cm-1 A(N2O) (left ordinate) vs potential based on the data in (A) (open circles), and the integral of the DEMS N2O signal I(DEMS) determined from the data of Nishimura et al.3 (solid curve). The maximum in the A(N2O) and I(DEMS) signals were assumed to be the same.
of N2O trapped between the electrode and the window during the sweep. A plot of A(N2O) vs E (see open circles in panel C, Figure 2) indicates that N2O is first detected at E ∼0.5 V vs SCE, reaches a maximum at E ∼0.07 V, and disappears completely for potentials more negative than ∼-0.1 V. These data are in qualitative agreement with those obtained by on-line steady state MS by Gadde and Bruckenstein1 in 0.4 mM KNO2/0.1 M HClO4, and on-line dynamic DEMS at 1 mV/s by Nishimura et al.3 in 12 mM NaNO2/0.5 M H2SO4 solutions. From a more general perspective, on-line DEMS provides a measure of the rate at which a given gas phase product is generated, whereas the signal obtained by in situ FTIRRAS is proportional to the total amount of that gas accumulated in the thin layer during the experiment. On this basis, and provided
Figure 3. (A) Series of normalized potential difference spectra (-∆R/R vs wavenumber, Eref ) 0.50 V) in the regions 2125-2325 (left) and 1350-1550 cm-1 (right) collected during a linear potential scan in the negative direction initiated at Eref in a nitrite-free N2Osaturated 0.1 M HClO4 solution at a rate of 1mV/s (see text for details). From the top: 0.46, 0.38, 0.30, 0.22, 0.15, 0.07, 0.00, -0.08, -0.16, -0.24, and -0.32 V vs SCE (B) Linear sweep voltammogram obtained during acquisition of the in situ FTIRRAS spectral data in (A). (C) A(N2O) - A(N2O)E)-0.24 V, where the subscript indicates the potential value at which the reference integrated intensity was calculated.
the amount of nitrite in the thin layer is not fully depleted during the measurements, A(N2O) in panel C, Figure 2, should be proportional to the integral of the dynamic DEMS signal I(DEMS) as a function of the applied potential. The required condition is fulfilled in this case, as judged by the large currents observed at potentials E < 0.0 V in panel B, Figure 2. The A(N2O)-I(DEMS) correspondence is illustrated in panel C, Figure 2, where the I(DEMS) vs E curve (see solid thick curve) was determined from the data of Nishimura et al.,3 assuming a common value for the maxima of the A(N2O) and I(DEMS) signals. According to the dynamic on-line DEMS data, the rate of N2O generation for E less than ∼0.0 V drops to negligible levels. This region coincides with the disappearance of the N2O signal in the PD-FTIRRAS spectra (vide infra). Additional insight into the reaction mechanism of nitrite reduction could be obtained by conducting essentially identical in situ FTIRRAS experiments in nitrite-free, N2O-saturated, 0.1 M
HClO4. Cyclic voltammograms in this medium were recorded with the electrode both removed from (panel C, Figure 1) and pressed against the window during spectral acquisition (panel B, Figure 3). The corresponding series of normalized PD-FTIRRAS spectra using R(Eref ) 0.50 V) as a reference, are given in panel A, Figure 3. As would be expected from the electrochemical data, the plot of A(N2O) - A(N2O)E)-0.24 vs E (see panel C in this figure) shows a monotonic decrease in the spectroscopic signal starting at ∼0.35 V, associated with the onset of N2O reduction within the thin layer. This value is in agreement with that reported elsewhere.1 However, no significant changes in intensity could be observed for E < -0.1 V, despite the fact that the current flowing through the cell in this potential range was not negligible. Two factors are mostly likely responsible for this phenomenon: (i) the much lower rates of N2O reduction in the weakly adsorbed hydrogen region (see panel C, Figure 1), as extensively studied by Ebert et al.11 on single crystal Pt, and (ii) a slow and steady diffusion of N2O into the thin layer, i.e., diffusional coupling or leakage, which would maintain the concentration within, at a small, but, nevertheless, finite steady state value. In this case, the measured currents may be assumed to originate predominantly from N2O reduction in the edge region of the electrode. Based on the in situ FTIRRAS spectroscopic data in panel C, Figure 3, the onset of N2O reduction is more positive than the potential at which A(N2O) begins to decrease in the nitritecontaining solutions (panel C, Figure 2). This effect is consistent with the on-line dynamic DEMS data for nitrite reduction,3 which shows that the onset for N2 generation, derived from the reduction of N2O, occurs at a potential at which N2O is produced at a constant rate, i.e., within the rising section of the A(N2O) vs E plot in panel C, Figure 2 (0.5-0.07 V). In other words, over a region of ∼100-150 mV, the reduction of nitrite can yield N2 via a sequential mechanism that involves N2O as an intermediate, i.e.,
2HNO2 + 2H+ + 2e- f N2O + 2H2O N2O + 2H+ + 2e- f N2 + H2O At more negative potentials, DEMS shows small amounts of N2 without detectable signals due to N2O,3 suggesting that, during nitrite reduction in this range, N2O is rapidly consumed before it leaves the solution phase. The ability of Pt in 0.1 M HClO4 to reduce N2O in the presence of nitrite at sufficiently negative potentials, first noted by Gadde and Bruckenstein,1 accounts for the disappearance of N2O accumulated in the thin layer or, equivalently, the sharp decrease in A(N2O) (panel C, Figure 2) during the scan in the negative direction. As was mentioned in the introduction, Gadde and Bruckenstein concluded on the basis of various analytical techniques that N2O is formed even at very negative potentials (-0.22 V vs SCE). However, the residence time of species within the convoluted structure of the high-area electrodes involved in the MS measurements is expected to be much longer than that on smooth surfaces. Hence, under such conditions, reaction intermediates, which would otherwise escape into the bulk solution in a forced convection arrangement, such as a conventional RDE, could react (11) Ebert, H.; Parsons, R.; Ritzoulis, G.; VanderNoot, T. J. Electroanal. Chem. 1989, 264, 181.
Analytical Chemistry, Vol. 69, No. 2, January 15, 1997
251
further to form other products. In fact, the N2O collection rates reported in ref 1 at the lowest potential examined (E ) -0.1 V) had dropped by over 75% from the maximum value observed over the full voltage range. On-line MS (or DEMS) can only detect gas phase species present in the highly convoluted high-area Pt electrode. Since no N2O or N2 (or NO) were found by Nishimura et al.3 in a region just positive to hydrogen evolution, the species produced therein exhibit very low volatility. A clear positive-pointing feature at 1458 cm-1 was found in the in situ PD-FTIRRAS for nitrite reduction in the region in which atomic hydrogen is adsorbed on the Pt surface [see panel A (right), Figure 2] but not in the corresponding spectra of N2O reduction [see panel A (right), Figure 3]. These results support the view that the reduction of nitrite (HNO2) at very negative potentials can proceed via a multielectron transfer process to yield hydroxylamine as a product (NH δ mode for NH2OH-acid adduct occurs at 1460 cm-1)12 as reported in ref 1,
more negative peak current maximum for nitrite reduction at -0.1 V (panel B, Figure 1) and that at this potential strongly adsorbed hydrogen on Pt reaches saturation coverage. Additional spectroelectrochemical experiments involving Pt single-crystal surfaces will be required, however, before the roles of strongly and weakly adsorbed hydrogen in the mechanistic pathways of this complex electrode process could be more clearly established. In summary, a direct correlation has been found between in situ PD-FTIRRAS and dynamic DEMS results for the generation of nitrous oxide by the reduction of nitrite on Pt in acid solutions as a function of the applied potential. Although rather stringent conditions must be met to make valid comparisons, the analysis herein provided establishes a semiquantitative link between results obtained by these two very different techniques. ACKNOWLEDGMENT This work was supported by ARPA Contract NOOO14-92-J1848.
HNO2 + 5e- + 5H+ f NH2OH + H2O It is noteworthy that the peak current maximum for N2O reduction (panel C, Figure 1) coincides almost precisely with the (12) Pouchert, C. J. The Aldrich Library of Infrared Spectra, 3rd ed.; Aldrich Chemical Co., Inc.: Milwaukee, WI, 1981.
252
Analytical Chemistry, Vol. 69, No. 2, January 15, 1997
Received for review July 30, 1996. Accepted October 18, 1996.X AC960769N X
Abstract published in Advance ACS Abstracts, December 1, 1996.
