J. Phys. Chem. C 2010, 114, 6011–6018
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Surface-Enhanced Infrared Absorption Spectroscopic Studies of Adsorbed Nitrate, Nitric Oxide, and Related Compounds. 3. Formation and Reduction of Adsorbed Nitrite at a Platinum Electrode Farhana Rahman Rima,† Kou Nakata,†,‡ Katsuaki Shimazu,*,†,‡ and Masatoshi Osawa§ DiVision of EnVironmental Materials Science, Graduate School of EnVironmental Science, and 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 20, 2009; ReVised Manuscript ReceiVed: February 17, 2010
The formation, potential-dependent structural change, and reduction of adsorbed nitrite at a platinum electrode were examined in 0.1 M HClO4 and 0.1 M NaClO4 by surface-enhanced infrared absorption spectroscopy (SEIRAS). The band assigned to the symmetric NO2 stretch of the nitro form of nitrite appeared at around 1300 cm-1 in both solutions. The potential dependence of the spectra revealed that this adsorbed nitrite is converted to NO adsorbed at on-top, bridge, and defect sites via IR-inactive surface nitrite species. In 0.1 M HClO4, these three adsorbed NO species were also formed during the adsorption process from the solution NO formed by the disproportionation of nitrite. In addition to the reduction via the IR-inactive surface nitrite species, a direct conversion from adsorbed nitrite to adsorbed NO was also suggested in 0.1 M HClO4. 1. Introduction The electrochemical denitrification of nitrate in groundwater and industrial wastes is receiving increased attention because of its environmental compatibility, versatility, energy efficiency, safety, selectivity, amenability, and cost effectiveness.1 Nitrate is known to cause serious health risks for human beings, such as methemoglobinamia and cancer, due to toxic nitrite formed from nitrate in vivo. It is generally believed adsorbed nitrate is reduced to adsorbed nitrite, then further converted to N2 and/or ammonia via adsorbed NO as shown below.2,3 Therefore, it is important to identify the adsorbed nitrite species and to examine its reduction behavior on an electrode surface.
The reduction of nitrite itself has been accomplished as a function of the electrode materials,4-7 crystal orientation on the electrode surface,5,6 and pH of the electrolytes.8-11 However, very limited studies have been conducted as to the adsorption state of nitrite. Therefore, the surface reaction of the nitrite reduction is not well understood. On Au, a weak IR band was observed at 1240 cm-1 in 1 M HF + 0.05 M HNO3, and was assigned to the O-down adsorbed nitrite with a 1-fold or 2-fold coordination formed by the partial reduction of nitrate at negative potentials, while any nitrite band was not observed at Pt under the same conditions.12 Because NO can be formed by * To whom correspondence should be addressed. E-mail: shimazu@ ees.hokudai.ac.jp. Phone: +81-11-706-2276. Fax: +81-11-706-4868. † Division of Environmental Materials Science, Graduate School of Environmental Science, Hokkaido University. ‡ Section of Materials Science, Faculty of Environmental Earth Science, Hokkaido University. § Catalysis Research Center, Hokkaido University.
the disproportionation of nitrite in acidic solutions, nitrite was used as a source of NO in several studies on single-crystal Pt electrodes.13,14 Although nitrite is also expected to be adsorbed directly from the solution, any bands assignable to the adsorbed nitrite were not reported. Therefore, the adsorbed nitrite directly formed from nitrite has never been detected on platinum or any other noble metal electrodes by infrared spectroscopy. In our recent reports,15,16 we examined the formation, reduction, and/or desorption of adsorbed nitrate and NO on Pt using surface-enhanced infrared absorption spectroscopy (SEIRAS). We reported for the first time (1) that the main band at 1547-1568 cm-1 formed from nitrate is assigned to the adsorbed nitrate in contrast to the preceding assignment (adsorbed NO)12,17 and (2) the reduction and/or desorption of the adsorbed nitrate and NO obeys first-order kinetics with respect to the corresponding adsorbed species, and the rate constant increases in the order of bridged NO < adsorbed nitrate < ontop NO. These new findings are due to various advantages of SEIRAS for the examination of adsorbed species such as high sensitivity to the surface species, fast mass transport, and a fast electrochemical response.18 This success encouraged us to examine the adsorption and reduction of nitrite on Pt using SEIRAS. As mentioned above, it is believed that the adsorbed nitrite is an intermediate for the reduction of nitrate. Although the rate-determining step for the reduction of nitrate is considered to be the first electron transfer, the product distribution is determined by subsequent reactions after the reduction of nitrate to nitrite. Therefore, it will be helpful in designing the electrode structure to clarify the surface reaction process for the reduction of nitrite. We recently reported that the Sn-modification of a Pt electrode enhanced an electrocatalytic activity of Pt for the reduction of nitrate.19-21 The activity of Sn-Pt with the Sncoverage of 0.4 reached about 3 mA cm-1 at -0.1 V, which is lower than the highest activity of Sn-Pd (10 mA cm-1),22,23 but higher than that of a Cu-Pd electrode (1 mA cm-1).2 Therefore, the present study will also provide fundamental information in clarifying the second metal effect on the electrocatalytic activity.
10.1021/jp911027q 2010 American Chemical Society Published on Web 03/05/2010
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J. Phys. Chem. C, Vol. 114, No. 13, 2010
2. Experimental Section 2.1. Preparation of a Thin Pt Film Electrode. A thin Pt film electrode was deposited on the total reflecting plane of a nondoped Si hemicylinder prism (Pier Optics Co., Ltd.) by an electroless deposition technique.24 The readily oxidized reflecting plane of the Si prism was activated for metal deposition by contact with 40% NH4F (Morita Chemicals Industries Co., Ltd.) for 1 min after successively polishing with 1 µm alumina followed by ultrasonication in acetone and water. Pd was first deposited by contacting the prism with a 0.5% HF solution containing 1 mM PdCl2, which was prepared from a 50% HF solution of atomic absorption spectrometry grade, PdCl2 of reagent grade (Wako Pure Chemicals), and Milli-Q water, for 3 min; Pt was then deposited on the Pd layer by dropping a mixture of Pt plating solutions (Electroplating Engineering of Japan, LECTROLESS PT100 basic solution:H2O:15 M NH3: reduction reagent solution ) 50:44:5:1 in volume) at 60-70 °C for 10 min. 2.2. Electrochemical and IR Measurements. A flow cell made of polychlorotrifluoroethylene (PCTFE) was used for the electrochemical and infrared measurements. Both a platinized Pt foil counter electrode and a Ag/AgCl (saturated KCl) reference electrode were assembled in the cell. The Pt-coated prism was attached to the cell via an O-ring. The electrode was electrically connected to a TOHO model PS-7 potentiostat with a TOHO model FG-02 function generator or BAS model CV50W potentiostat via a copper foil inserted between the cell body and the edge of the electrode (prism). Prior to the measurements, the cell was cleaned in a mixed acid (HNO3 + H2SO4) in order to minimize any organic contaminants, followed by sufficient rinsing with Milli-Q water. The cell was connected to solution reservoirs via a 4-way valve. The solution in the cell (volume: 1.8 mL) was easily replaced with another solution by switching the valve during potential control of the electrode. In the solution reservoirs, 0.1 M HClO4, 0.1 M NaClO4, and those solutions containing nitrite, which were freshly prepared from reagent grade chemicals and Milli-Q water, were stored, and then sufficiently deaerated by flowing Ar gas of 5 N purity into the reserviors prior to use. The Pt film electrode was electrochemically cleaned by cycling the electrode potential in the range from -0.2 to 1.2 V in 0.1 M HClO4. All SEIRA spectra were measured with a Bio-Rad FTS60A/ 896 Fourier transform infrared spectrometer equipped with an HgCdTe detector and a home-built single-reflection accessory (incident angle of 70°).18 The spectral resolution was 4 cm-1. The reference spectrum was measured at fixed electrode potentials in pure electrolyte solutions: 0.7 V in 0.1 M HClO4, or 0.5 V in 0.1 M NaClO4. The IR spectra during the adsorption of nitrite were collected every 1 s until saturated coverage was obtained. The potential dependence of the IR spectra was examined with a time resolution of 1 s during the potential sweep at the sweep rate of 0.02 V s-1 after the replacement of the nitrite solution with a pure electrolyte solution. All spectra are shown in absorbance units. 3. Results and Discussion 3.1. In an Acidic Solution. 3.1.1. Cyclic Voltammograms. Figure 1 shows the cyclic voltammograms of a Pt thin film electrode in 0.1 M HClO4. The cyclic voltammogram immediately after the electrochemical cleaning (Figure 1, solid curve) is typical for a polycrystalline Pt electrode, showing the surface cleanliness of the electrode. After a mixed solution of 0.01 M NaNO2 and 0.1 M HClO4 was introduced into the cell
Rima et al.
Figure 1. Cyclic voltammograms of a Pt electrode in 0.1 M HClO4 before (solid curve) and after (dashed and dotted curves) the formation of the adsorbed nitrite. The adsorbed nitrite was formed by the introduction of 0.01 M NaNO2 + 0.1 M HClO4 at 0.7 V. The cyclic voltammograms were recorded at the scan rate of 0.02 V s-1 after the replacement of the nitrite solution with a fresh 0.1 M HClO4 solution. No solution replacement was conducted between the first (dashed curve) and the second (dotted curve) cycles after the adsorption.
at 0.7 V to form the adsorbed species, 40 mL of a 0.1 M HClO4 solution was flowed. Cyclic voltammograms were then measured. A slightly greater reduction current compared to that for a clean Pt electrode was observed in the potential range of 0.5-0.1 V during the first negative-going scan starting at 0.7 V, showing the reduction of the adsorbed species. In a more negative potential region, a further clear reduction wave was observed at -0.08 V. These results show that the reduction of the adsorbed species took place in two stages. The cathodic charges due to the reduction of the adsorbed species are 338 and 2170 µC cm-2 in the potential ranges of 0.5-0.1 and
