Available online at www.sciencedirect.com CHEM. RES. CHIN€SE UNIVERSITIES 2008,24(6), 782-785 Article ID 1005-9040~2008~-06-782-04 ScienceDirect

Electrochemical Oxidation of Ammonia on Ir Anode in Potential Fixed Electrochemical Sensor

WAN Yi-ping', LUO Peng', CAI Chen-xin', XIE Lei2 and LU Tian-hong" 1. Department of Chemistiy, Nanjing Normal Universiw, Nanjing 21 0097, P: R. China;

2. RAE Engineering Centel: RAE Systems Inc., Shanghai 201821, P R. China

Abstract Ir catalyst possesses a good electrocatalytic activity and selectivity for the oxidation of NH3 and/or NH40H at Ir anode in the potential fixed electrochemical sensor with the neutral solution. Owing to the same electrochemical behavior of NH, and NH40H in a NaC104 solution, NH40H can be used instead of NH, for the experimental convenience. It was found that the potential of the oxidation peak of NH40H at the Ir/GC electrode in NaC104 solutions is at about 0.85 V, and the current density of the oxidation peak of NH40H is linearly proportional to the concentration of NH40H. The electrocatalytic oxidation of NH40H is diffusion-controlled. Especially, Ir has no electrocatalytic activity for the CO oxidation, illustrating that CO does not interfere in the measurement of NH40H and the potential fixed electrochemical NH3 sensor with the neutral solution, and the anodic Ir catalyst possesses a good selectivity. Therefore, Ir may have practical application in the potential fixed electrochemical NH, sensor with the neutral solution. Keywords Iridium; Potential fixed electrochemical sensor; NH3; NH40H; Neutral solution

1 Introduction

NH3 is a valuable chemical product and material due to its numerous applications in the environmental protection, clinical diagnosis, industrial processes, food processing, and power plants"-51. However, it is a colorless gas with a special odor and is very harmful to the human body. The investigation results show that N H 3 irritates the respiratory system, skin, and eyes of human, resulting in the vomit and headachef6]. Therefore, the effective methods for monitoring NH3 have been demanded for the atmospheric environ- mental measurements and control. The numerous ef- forts have been directed to developing NH3 sensors[31. Among the various sensors, electrochemical sensors possess some advantages, such as their miniaturization, low cost, rapid response, their inherent high sensitivity, and selecti~ity[~'~~. Therefore, electrochemical NH3 sensors are worth to develop.

Since 1960s, the electrocatalytic oxidation of N H 3 has been investigated. However, most of the investigations were carried out in alkaline solutions because the electrocatalytic oxidation of NH3 can be applied in the alkaline fuel cells and the electrocata- lytic oxidation rate of NH3 is high in alkaline solu-

*Corresponding author. E-mail: tianhonglu@263 .net Received February 13, 2008; accepted May 4, 2008.

191 . Later on, the electrocatalytic oxidation of NH3 was studied with a view to develop an electro- chemical NH3 sensorf2] and for the treatment of waste water"'] . Kim et al."'] suggested that N H 3 in basic solution was oxidized through a direct electrolytic reaction. Mishima et al.['] has found that Ir is the most active catalyst in alkaline solution, but it is easy to deactivate. de Vooys et aLi3] have investigated the electrocatalytic activity of Pt, Pd, Rh, Ru, Ir, Cu, Ag, and Au electrodes for N H 3 oxidation in alkaline solu- tions. It was found that Pt and Ir show the steady-state activity. Endo et al. [11,'21 have compared the electro- catalytic activities of the Pt-based composite catalysts, Pt-Me(Me=Ir, Ru, or Ni) for NH3 oxidation in KOH solutions and found that alloying in the Pt-Ir binary system may lead to the increase in its electrocatalytic activity for NH3 oxidation due to a kind of cooperative interaction between Pt and Ir. However, to our know- ledge, almost no investigations in the neutral solution were reported.

Actually, alkaline solutions are not suitable for the electrochemical NH3 sensors because the alkaline solution in the sensor would adsorb COZ from air through the gas diffusion electrode and then carbonate

Supported by RAE Engineering Center, RAE Systems Inc. Fund, China.

Copyright 0 2008, Jilin University. Published by Elsevier Limited. All rights reserved.

No.6 HAN Yi-ping et al. 783

is formed. The change in the alkalinity of the solution would cause the decrease in the sensor performance. Moreover, carbonate easily crystallizes and deposits on the electrode due to its low solubility. This would lead to the destruction of the electrode structure and the invalidation of the sensor.

Based on the comparison of the electrocatalytic activities and selectivities of several metal catalysts, such as Pt, Ir, Pd, and so on for NH3 oxidation, it was found that among these catalysts, Ir has the best per- formance for NH3 oxidation. Therefore, in this article, the electrocatalytic activity and selectivity of Ir cata- lyst for NH3 oxidation in the neutral solution is re- ported. It would lay the strong foundation of the po- tential fixed electrochemical NH3 sensor with the neu- tral solution.

2 Experimental

IrClynH2O was purchased from Alfa Aesar. 0.02% NH3 was obtained from Nanjing Special Gas Company. All other chemicals were of analytical grade and used as received without further purifica- tion.

Electrochemical measurements were performed by means of a CHI 600 electrochemical analyzer and a conventional three-electrode electrochemical cell at (30*1) "C. A Pt plate was used as the auxiliary elec- trode. The Ag/AgCl electrode was used as the refe- rence electrode and all potentials were quoted with respect to the Ag/AgCl electrode. The working elec- trode was prepared as follows. A glassy carbon(GC) electrode was polished with 0.3 and 0.05 pm A1203 sequentially and washed. Then, Ir was electrodepo- sited at -0.6 V on the GC electrode for 250 s in 2 mmol/L IrCI3+0.4 mol/L NaC104 solution. After washing, the working electrode was obtained and ex- pressed as the Ir/GC-1 electrode. After the Ir/GC-1 electrode was cyclically scanned between -0.9 and 0.2 V in 2 mmol/L IrC13+0.4 mol/L NaC104 solution until a stable cyclic voltammogram was obtained, the working electrode was obtained via washing and ex- pressed as the Ir/GC-2 electrode.

The electrocatalytic activities of the Ir/GC elec- trodes for NH3 oxidation were measured in 0.4 mol/L NaC104 solution with the different concentrations of NH3. Two methods were used to introduce NH3 into the solution: (1) NH3 was directly introduced into the solution from a cylinder with 0.02%(volume fraction)

NH3 through bubbling method. (2) The different vo- lumes of 0.15 moVL NH40H solution were added into the solution.

Oxygen was purged from the solution by bub- bling with N2 for 15 min prior to each electrochemical measurement.

3 Results and Discussion

Fig.1 shows the cyclic voltammograms of 0.4 mol/L NaC104 solutions without and with bubbling O.O2%(volume fraction) NH3 for 300 s or 0.003 moVL NH40H at the Ir/GC-2 electrode. It can be seen from Fig.1 that both oxidation peaks of NH3 and NH40H are located at 0.82 V. The current densities of the oxi- dation peaks of NH3 and NH40H are different due to their different concentrations in the solution. The above results demonstrate that the cyclic voltammetric behavior of NH3 and NH40H is the same. Thus, NH40H can be used instead of NH3 for the experi- mental convenience because the concentration of NH40H can be easily and accurately controlled. Then, the relationship between the concentration and re- sponse current can be easily and accurately obtained. Therefore, in the following experiments, NH40H was used.

1.0

0.8 Scan rate: 100 mV/s

I I I I I

0.0 0.2 0 4 0 6 0.8 1.0 1.2 .FN(w. Ag/AgCl)

Fig.1 Cyclic voltammograms of 0.4 mol/L NaCI04 solution without@) and with(b) bubbling 0.02% NH3 for 300 s and 0.003 mol/L NH40H(c) at Ir/GC-2 electrode

Fig.2 presents the cyclic voltammograms of 0.009 mol/L NH40H in 0.4 mol/L NaC104 solution at the Ir/GC-1 and Ir/GC-2 electrodes. It can be observed from Fig.2 that the potential and the current density of the oxidation peaks of NH40H at the Ir/GC-1 elec- trode are 0.95 V and 1.51 mA/cm2, respectively(Fig.2, curve a), whereas at the Ir/GC-2 electrode, they are 0.85 V and 3.13 mA/cm2, respectively(Fig.2, curve 6). It clearly demonstrates that the electrocatalytic activi- ty of the Ir/GC-2 electrode for the NH40H oxidation is higher than that of the IrIGC-1 electrode. This is because the Ir particles become small after the cyclic

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voltammetric treatment. Thus, in the following experiments, the Ir/GC-2 electrode was used.

- - - - - - -

3.5 I I Scan rate: 100 mV/s A

-0.5 I I I I I I I I 0.0 0.2 0.4 0.6 0.8 1.0 1.2

EN(vs. AgiAgCl) Cyclic voltammograms of 0.009 moVL NH40H in 0.4 moVL NaC104 solution at IrlGC-l(a) and Ir/GC3(b) electrodes

Fig.3(A) displays the cyclic voltammograms of the different concentrations of NH40H in 0.4 mol/L NaC104 solutions at the Ir/GC-2 electrode. It can be observed from Fig.3(A) that the onset potentials of the

Fig.2

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0.0 0.2 0.4 0.6 0.8 1.0 1.2 EN(vs. Ag/AgCI)

oxidation of NH40H are the same, 0.5 V, for the different concentrations of NH40H, but the peak po- tentials are slightly changed. When the concentrations of NH40H are 0.0046, 0.009, 0.013; 0.017, and 0.02 mol/L, the peak potentials are 0.80, 0.82, 0.835, 0.85, and 0.865 V, respectively, and the peak current densi- ties are 1.77, 3.50, 5.00, 6.24, and 7.38 mAkm2, re- spectively.

Fig.3(B) shows the plot of the current density vs. the concentration of NH40H at 0.85 V. It can be ob- served from Fig.3(B) that there is a good linear rela- tionship between the current density and the concen- tration of NH40H at 0.85 V. Its related coefficient, R is 0.99882. Therefore, when Ir was used as the catalyst in the potential fixed electrochemical sensor, only one standard concentration of NH3 is needed as standard gas to demarcate the sensor.

8

6 h

5 4 4

7 E

2

5 10 15 20 10' [NH,OH]/(mol.L-')

Fig3 Cyclic voltammograms of O(a), 0.0046(b), O.O09(c), O.O13(d), O.O17(e), 0.02v) m o l n NH40H in 0.4 mol/L NaC104 solutions at Ir/GC-2 electrode(A) and plot of current density at 0.85 V obtained from Fig3(A) vs. the concentra- tion of NH40H(B)

Fig.4(A) displays the cyclic voltammograms of 0.02 molL NH40H in 0.4 m o m NaC104 solutions at the Ir/GC-2 electrode and different scan rates. It can be observed from Fig.4(A) that the potentials of the oxidation peak of NH40H are not changed with the

8

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I I I I I I 0.0 0.2 0.4 0.6 0.8 1.0 1.2

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scan rate and all of them are located at 0.85 V. The plot of current density at 0.85 V obtained from Fig.4(A) is linearly proportional to the square root of the scan rate.

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Fig. Cyclic voltammograms of 0.02 moUL NH40H in 0. . mol/L NaC104 solution at the Ir/GC-2 electrode and scan rates of 100(a), 75(b), 50(c), 25(d), 10(e), and 5v) mV/s(A) and plot of the current density at 0.85 V obtained from FigA(A) vs. the square root of the scan rate(B)

This indicates that the electrochemical reaction is diffusion-controlled. This is the reason for the good linear relationship between the current density and the concentration of NH40H.

Fig.5 shows the cyclic voltammograms of 0.02 mol/L NH40H in 0.4 mol/L NaC104 solution without and with the saturated CO at the Ir/GC-2 electrode. It can be observed from Fig.5 that only the oxidation

ling et ~ l . 785 No.6 HAN Yi-I:

peaks of NH40H appear, but no oxidation peak of CO exists in either cyclic voltammograms, indicating that Ir has no electrocatalytic activity for the CO oxidation and thus, CO does not interfere in the measurement of NH40H. Although the current densities of the oxida- tion peaks of NH40H in the absence and the presence of the saturated CO are different, it is not due to the interference but due to the part of NH40H being removed from the solution by bubbling the solution with CO.

8 r Scan rate: 100 mV/s

6 -

1 1 I I I I I I

EN(vs. Ag/AgCI) 0 0 0.2 0.4 0.6 0.8 1.0 1.2

Fig.5 Cyclic voltammograms of 0.02 moUL NH40H in 0.4 mol/L NaC104 solution without@) and with@) the saturated CO at the Ir/GC-2 electrode

Fig.6 shows the cyclic voltammograms of 0.4 mol/L NaC104 solution without and with the saturated CO at the Ir/GC electrode. It was found that both the cyclic voltammograms are similar and no CO oxida- tion peak was observed. This further demonstrates that CO cannot undergo the electrocatalytic oxidation at the Ir/GC electrode and Ir has no electrocatalytic ac- tivity for the CO oxidation. Therefore, CO does not interfere in the measurement of NH40H at the Ir elec- trode. Usually, CO easily interferes in the measure- ment of NH3 in the electrochemical NH3 sensor. Thus, the above results illustrated that the Ir catalyst has a good selectivity for the measurement of NH40H.

0.6 I I

-0.61 I I I , I

0.0 0.2 0.4 0.6 0.8 1.0 E N ( w . Ag/AgCI)

Cyclic voltammograms of 0.4 mol/L NaC104 solution without@) and with@) saturated CO at the IrlGC-2 electrode

Figd

4 Conclusions

For the experimental convenience, NH40H is used instead of NH3 because of the same electro-

chemical behavior of NH3 and NH40H in the neutral solution.

The potential of the oxidation peak of NH40H at the Ir/GC electrode in NaC104 solutions are at about 0.85 V and the current density of the oxidation peak of NH40H is linearly proportional to the concentration of NH40H. The electrocatalytic oxidation of NH40H is diffusion-controlled.

Ir has no electrocatalytic activity for the CO oxi- dation. Thus, CO does not interfere in the measure- ment of NH40H and the potential fixed electrochemi- cal NH3 sensor with the neutral solution, and the anodic Ir catalyst possesses good selectivity. There- fore, Ir may have the practical application in the po- tential fixed electrochemical NH3 sensor with the neu- tral solution.

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