Electrocatalytic Properties of Nitrous Oxide and Its Voltammetric

Anal. Chem. 1998, 70, 2181-2187

Electrocatalytic Properties of Nitrous Oxide and Its Voltammetric Detection at Palladium Electrodeposited on a Glassy Carbon Electrode Baoxing Wang and Xiao-yuan Li*

Department of Chemistry, The Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong

The electrochemical behavior of nitrous oxide (N2O) on the surface of electrodeposited palladium (Pd) was investigated by voltammetry in aqueous supporting electrolytes. It was found that N2O is weakly adsorbed on a Pd/ GC electrode and can be reduced electrocatalytically with a high efficiency. The reduction product is not adsorbed on the Pd/GC electrode in aqueous neutral and alkaline solutions. However, it is difficult for N2O to be reduced on the same electrode in acidic electrolyte because hydrogen adsorption competes for the adsorption sites. The concentration of N2O can be quantitatively determined using the electrocatalytic reduction current of N2O at the Pd/GC electrode. This electrode provides a sensitive probe for the electrochemical detection of N2O with a low detection limit and a wide linear response range from 24.3 µM to 1.94 mM. A mechanism is proposed to account for the electrocatalytic reduction of N2O on the Pd/GC electrode, which adequately explains our experimental observations. Nitrous oxide (N2O), traditionally known as laughing gas, is a nerve-stimulating molecule. It is commonly employed in the field of radiation chemistry in aqueous media as a scavenger for hydrated electrons.1,2 It was proposed recently as a possible candidate for the direct product of nitric oxide (NO) synthase in biology.3 N2O has been widely employed as both an anesthetic and an analgesic agent for 100 years.4 It has been adopted as an anesthetic carrier gas on anesthetic machines used in clinical practice. A main application of N2O as an analgesic agent is in obstetric medicine, where it is commonly known as Entonox,5 usually administered during labor. It has also been used in medicine as a tracer gas to measure blood flow6 and lung volume7 (1) Conway, B. E. In Modern Aspects of Electrochemistry; Conway, B. E., Bockris, J. O., Eds.; Plenum Press: New York, 1972; p 83 and references citied therein. (2) Frumkin, A. N.; Petrii, O. A.; Damaskin, B. B. In Comprehensive Treatise in Electrochemistry, Bockris, J. O., Conway, B. E., Yeager, E., Eds.; Plenum: New York, 1980; Vol. 1. (3) Schmid, H. H. H. W.; Hofmann, H.; Schindler, U.; Shutenko, Z. S.; Cunningham, D. D.; Feelisch, M. Proc. Natl. Acad. Sci. U.S.A. 1996, 93, 14492. (4) Eger, E. I., II. Nitrious Oxide/N2O; Edward Arnold: London, 1985. (5) Grant, W. J. Medical Gases: Their Properties and Uses; HM+M Publishers: Aylesbury, 1978. (6) Zeidifard, E.; Godfrey, S.; Davies, E. E. J. Appl. Physiol. 1976, 41, 433. S0003-2700(97)00601-X CCC: $15.00 Published on Web 04/17/1998

© 1998 American Chemical Society

because of its biological inertness when used in low concentrations. The application of N2O is not only limited to medicine. In the food industry, it is utilized as a propellant for pressurized containers containing food-stuffs, such as whipped cream.8 However, today, because N2O is now considered as a potential destroyer of the ozone layer, it has gained new scientific and political importance as a greenhouse gas.9-11 Calculations show that the causes of the increase in atmospheric N2O concentration remain an area of scientific uncertainty. Therefore, it is desirable to detect the concentration of N2O in these fields ranging from clinical practice and the food industry to the whole global circumstance. N2O can be directly assayed by three nonelectrochemical strategies.12-14 First, the conventional method of assaying N2O has been via infrared analysis, but this approach is only suited for gaseous analysis and requires expensive apparatus. Second, although gas chromatography for N2O detection can be fit either for liquid or gaseous samples, this method is static, and, therefore, dynamic N2O assay is not possible by using this approach. Third, refractometry can be applied to detect N2O in the gaseous phase, but this is a slow method of analysis and is suitable only for steady-state gas analysis in medicine with current instrument technology. It is, therefore, necessary to develop a sensitive, fast, inexpensive, and user-friendly analytical method to achieve the in vivo real-time detection of N2O. Electrochemical-based sensors provide a promising proach to achieve the goal14 since they proven to be universally applicable in medicine, the food industry, and pollution monitoring and environmental control. Electrochemical reduction of N2O to nitrogen (N2), as depicted below, is favored thermodynamically: (7) Hahn, C. E. W.; Black, A. M. S.; Barton, S. A.; Scott, I. J. Appl. Physiol. 1993, 75 (4), 1863. (8) Simpler, C. A.; Webster, R. C. Industrial Users of Nitrious OxidesNo Laughing Matter. In Clinical Anaesthesia: Nitrous Oxide 1/1964; Eastwood, D. W., Ed.; Davies Co.: Philadelphia, 1964. (9) Rasmussen, R. A.; Khalil, M. A. K. Science 1986, 232, 1624. (10) Averill, B. A. Chem. Rev. 1996, 96, 2951. (11) Bouwman, A. M. In The Global Source of Distribution of Nitrous Oxide; van Amstel, A. R., Ed.; Proceedings of the International IPCC Workshop on Methane and Nitrous Oxide; Netherlands, 1993. (12) Sugg, B. R.; Palayiwa, E.; Davies, W. L.; Jackon, R. McGraghan, T.; Shadbolt, P.; Weller, S. J.; Hahn, C. E. W. Br. J. Aneasth. 1988, 61, 484. (13) Jackon, R.; Palayiwa, E.; Sugg, B. R.; Hahn, C. E. W.; Weller, S. J.; Davies, W. L. Med. Biol. Eng. Comput. 1988, 26, 516. (14) McPeak, H.; Hahn, C. E. W. J. Electroanal. Chem. 1997, 427, 179.

Analytical Chemistry, Vol. 70, No. 10, May 15, 1998 2181

N2O + 2H+ + 2e- f N2 + H2O, E° ) +1.77 V

Although it is known that kinetic factors such as the adsorption rate on the electrode surface and the reactivity with adsorbed hydrogen are important for the reduction of N2O,15 the detailed mechanism is still poorly understood. These kinetic factors are strongly affected by the electrode material, surface conditions, and acidity of the electrolyte solutions. There has not been much work reported so far on the electrochemical reactions of N2O.15-22 The reduction of N2O on Pt was studied in alkaline solution by Johnson and Sawyer.16 They concluded that a clean Pt surface was necessary and that N2O was reduced via the oxide reduction of the oxidized surface. Extension of this work to the acidic solutions and the first study on the reduction of N2O on single-crystal Pt surface by Parsons and co-workers suggested that N2O reacts with the adsorbed hydrogen.17,19 It was proposed that the adsorbed hydrogen plays a catalytic role in the reduction of N2O to N2 and that more strongly adsorbed hydrogen appeared to be more reactive in this process than the weakly adsorbed ones. It has been a matter of controversy whether N2O is actually reduced electrocatalytically by the hydrogens adsorbed on the noble metal (Pt, Pd, Rh, Ir) electrodes. A systematic study on the electrochemical reduction of N2O at various metal electrodes was carried out recently by current-potential curve measurements and by the determination of the reduction products in order to clarify the detailed mechanism of electrocatalysis.22 In the present study, we report our observation of several new electrochemical phenomena on the reduction of N2O on Pd electrodeposited on glassy carbon in a wide range of pHs. The electrochemical process of N2O on the Pd/GC electrode is also discussed on the basis of our observation. We demonstrate that the electrocatalytic reduction of N2O at Pd/GC electrode can be used for its quantitatively analytical detection in aqueous neutral and alkaline solutions. However, our main intention was to build a sensitive probe for the detection of N2O in aqueous neutral media. EXPERIMENTAL SECTION Reagents. K2PdCl6 was obtained from Aldrich and used as received. Phosphate buffer solution of pH 7.0 (PBS, RiedeldeHae¨n) and citrate buffer solution (CBS, Riedel-deHae¨n) were employed as the supporting electrolytes. Water was purified by passage through a Milli-Q purification system. Lecture bottle high-purity N2O was obtained from Boc Gases. All other reagents were of analytical grade and were used without further purification. Standard saturated N2O solutions were prepared by bubbling N2O gas through oxygen-free buffer solutions or through a doubly (15) Van der Stegen, J. H. G.; Visscher, W.; Hoogland, J. G. Electrochem. Technol. 1966, 4, 564. (16) Johnson, K. E.; Sawyer, D. T. J. Electroanal. Chem. 1974, 49, 95. (17) Ebert, H.; Parsons, R.; Ritzoulis, G.; VanderNoot, T. J. Electroanal. Chem. 1989, 264, 181. (18) Furuya, N.; Yoshida, H. J. Electroanal. Chem. 1991, 303, 271. (19) Ritzoulis, G. J. Electroanal. Chem. 1992, 327, 209. (20) Ahmadi, A.; Bracey, E.; Evans, R. W.; Attard, G. J. Electroanal. Chem. 1993, 350, 297. (21) Attard, G.; Ahmadi, A. J. Electroanal. Chem. 1995, 389, 175. (22) Kudo, A.; Mine, A. J. Electroanal. Chem. 1996, 408, 267.

2182 Analytical Chemistry, Vol. 70, No. 10, May 15, 1998

distilled and deionized water for 30 min after it was bubbled with pure nitrogen for 30 min to remove oxygen. The standard solutions were made fresh right before each experiment and were kept in glass flasks with rubber septums during experiments. Dilutions of the saturated solution were made using deoxygenated and deionized water. The saturation concentration of N2O in water at 25 °C is ∼2.43 × 10-2 M.23 Instrumentation. The electrochemical experiments were performed using a BAS 100B electrochemical analyzer, monitored and recorded with an IBM PC. All experiments were carried out in a single-compartment cell. A Pt disk (1 mm diameter) was used as the counter electrode, a saturated calomel electrode (SCE) as the reference electrode, and a glassy carbon (GC) disk electrode (3 mm diameter) and a Pd/GC electrode as the working electrodes. All potentials were measured and recorded versus the SCE. All solutions were deaerated with nitrogen for at least 10 min prior to the electrochemical experiments unless otherwise stated. A nitrogen atmosphere was maintained over the solution during the experiment. Prior to electrodeposition of Pd, the GC electrode was polished on two fine polishing pads (0.5- and 0.1µm Al2O3 particles, respectively). The polished GC electrode was then sonicated in distilled water and ethanol for 5 min each. Electrodeposition of Palladium on GC Electrode. A very stable electrodeposited Pd film was formed on the GC electrode immersed in a freshly prepared neutral modifier solution (1 mM K2PdCl6 and PBS, pH 7.0) by cycling the potential from +0.80 to -0.25 V or in an aqueous acidic deposition solution (1 mM K2PdCl6 and 0.5 M H2SO4) by cycling the potential from +0.30 to -0.60 V at a scan rate of 20 mV/s. The prepared Pd film displays a well-behaved cyclic voltammogram with the characteristic peaks of the formation and reduction of oxide layer, as well as the hydrogen adsorption-desorption in aqueous weak acidic, neutral, or alkaline solutions, respectively. The amount of electrodeposited Pd was controlled by the number of the cycles. RESULTS AND DISCUSSION Electrocatalytic Reduction of N2O at Pd/GC Electrode. The electrocatalytic activity of Pd/GC electrode toward the N2O reduction in aqueous media was studied and compared with that on a bare GC electrode. Figure 1 shows the cyclic voltammetric responses at the bare GC electrode in PBS (pH 7.0) under a nitrogen atmosphere. A featureless background current was obtained in the potential range of -1.20 to +1.0 V in the blank PBS (the dotted line). Upon the addition of N2O to a concentration of 0.73 mM (the solid line), a small, broad reduction wave centered at about -0.80 V emerges. The cathodic current increases evidently at potential more negative than -1.1 V. No oxidation peak was observed in this potential range. These observations indicate that the oxidation or reduction of N2O at the bare GC electrode is very difficult. The N2O reduction is totally irreversible at a bare GC electrode, and it takes place at a high overpotential (about -0.90 V) with low efficiency. However, its reductive behavior at a Pd/GC electrode is quite different from that at the bare GC electrode. Curve a in Figure 2 shows a typical cyclic voltammetric curve of the Pd/GC electrode in PBS (pH 7.0). The characteristic peaks (23) Gevantman, L. H. in Handbook of Chemistry and Physics, 76th ed.; Lide, D. R., Ed.; CRC Press: Boca Raton, FL, 1995; p 6-3.

Figure 1. Cyclic voltammograms of a bare GC electrode in blank PBS (a) and PBS containing 0 (‚‚‚) and 0.73 mM N2O (s). Scan rate, 50 mV/s.

Figure 3. Cyclic voltammograms of the Pd/GC electrode in the CBS of pH 2.0 (A) and 3.0 (B) in the absence (‚‚‚) and presence (s) of 0.73 mM N2O. Scan rate, 50 mV/s.

Figure 2. Cyclic voltammograms of Pd/GC electrode in the N2saturated (a) and N2O-saturated (b) PBS (pH 7.0). Scan rate, 50 mV/s.

due to the reversible adsorption-desorption of hydrogen in the region from -0.65 to -0.30 V and the oxide formation and reduction in the region from -0.20 to +0.80 V can be clearly seen, indicating that the hydrogen adsorption sites are available on Pd/ GC electrode surface. Curve b in Figure 2 shows the responses of N2O at the Pd/GC electrode at 50 mV/s in the N2O-saturated PBS (pH 7.0) between +0.80 and -0.65 V. In the cathodic process, a big reduction peak appears at -0.55 V where the hydrogen adsorption takes place. The peak current of the reductive wave starts to increase at ∼0.0 V. In addition, the potential of the oxide reduction shifts slightly to a more negative position, and its current increases slightly. In the anodic process, the corresponding hydrogen desorption peak completely disappears, and no other change was observed. It should be noted that the reduction of N2O seems to be catalyzed by the hydrogen

adsorbed on the Pd/GC electrodes. We attribute this big reduction peak to the catalytic reduction of N2O at the Pd/GC electrode, because the cathodic current at -0.55 V increases with the increase of N2O concentration in PBS and the reduction of N2O does not occur at this potential at the bare GC electrode, as mentioned above. Electrochemical Properties of N2O at Pd/GC Electrode in Different pH Buffers. The acidity of the electrolyte solution has a marked effect on the electrochemical behavior of Pd/GC electrode. Our experimental results indicated that the Pd at the Pd/GC electrode can be easily oxidized to the high-valent Pd(II,III,IV) in strong acidic buffers and can be removed from the Pd/GC electrode surface to the bulk electrolyte solution. In addition, since Pd has a strong tendency to adsorb hydrogen, the potential scan at the acidic pHs (pH 1.0-4.0) was limited to a range that contains the potentials of hydrogen evolution and the formation of the oxide so as to avoid the formation of the soluble Pd species. Figure 3 shows the voltammetric curves of Pd/GC electrode in the buffers of pH 2.0 (Figure 3A) and 3.0 (Figure 3B) in the absence and presence of 0.73 mM N2O, respectively. The hydrogen adsorption-desorption region contains some weak structure which is not well-defined and is unstable and, therefore, is difficult to characterize. Upon the addition of N2O to the buffers of pH 2.0 and 3.0, no pronounced changes were observed between the curves in Figure 3A and B, indicating that N2O reduction at the Pd/GC electrode is not efficient at acidic conditions. However, if the pH of the buffer is raised to 5.0 and 6.0, the Pd/GC electrode becomes stable in a wide potential range and Analytical Chemistry, Vol. 70, No. 10, May 15, 1998

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Figure 5. Cyclic voltammograms of the Pd/GC electrode in the PBS of pH 7.0 (A) and in the CBS of pH 8.0 (B) in the absence (‚‚‚) and presence (s) of 0.73 mM N2O. Scan rate, 50 mV/s.

displays well-defined hydrogen adsorption and desorption waves (Figure 4A,B). The addition of N2O to the two buffers leads to the increase of the reduction currents at the same potentials as that of hydrogen adsorption at the Pd/GC electrode. As shown in Figure 4, the catalytic reduction current of N2O in the buffer of pH 6.0 is markedly larger than that in the buffer of pH 5.0. The main catalytic reduction of N2O appeared at the potential related to the strongly adsorbed hydrogen at pH 6.0 but the weakly adsorbed hydrogen at pH 5.0. When the pH of the buffer is increased further to 7.0 and 8.0, respectively, the potential separations of hydrogen adsorptiondesorption peaks increase slightly in comparison with the cases of pH 5.0 and 6.0 (Figure 5A,B). However, when 150 µL of 2.43 × 10-2 M N2O standard solution was added into the two buffers (5 mL), respectively, a large reduction peak appeared at the same potential as that of hydrogen adsorption, and, meanwhile, the currents of hydrogen desorption peaks decreased evidently indicating that the Pd/GC electrode has a strong electrocatalytic effect on the reduction of N2O under these conditions. The catalytic current of N2O at pH 7.0 is larger than that at pH 8.0, but the half-peak width of the reduction wave at pH 7.0 is smaller than that at pH 8.0. At pH 9.0 and 10.0, the separation between hydrogen adsorption-desorption potentials increases further (Figure 6A,B). Some new and interesting electrochemical behaviors were observed after the addition of N2O to the two buffers, respectively. In the buffer of pH 9.0, two reduction peaks appear at -0.47 and -0.57 V, respectively, and they overlap with each other significantly. In

the buffer of pH 10.0, two well-defined reduction peaks of N2O at the Pd/GC electrode appeared at -0.48 and -0.61 V, respectively. The potential of the second reduction peak at -0.61 V, was exactly the same as the potential of hydrogen adsorption (see Figure 6B). The height of the first reduction peak, which was attributed to the reduction of N2O, was larger than that of the second one as can be seen from Figure 6A,B. Figure 7A,B depicts the cyclic voltammograms of the Pd/GC electrode in 0.01 and 0.1 M NaOH, respectively, in the absence (dotted line) and presence (solid line) of 0.73 mM N2O. It is evident that hydrogen adsorption-desorption waves in these alkaline solutions are smaller than their counterparts in the buffers mentioned above. Upon the addition of N2O to the electrolytes, the onset of catalytic reduction of N2O at the Pd/GC electrode begins just after the reduction of the oxide layer at the Pd/GC electrode, as can be seen from Figures 5 and 6. The reduction of N2O occurred mainly in the double-layer region of the Pd/GC electrode. From the above experimental results, obtained in electrolytes of different pHs, we can reach the following conclusions: (i) The more acidic the electrolyte solution is, the more difficult is the reduction of N2O at Pd/GC electrode. However, the catalytic reduction of N2O can be nicely conducted in neutral and alkaline electrolytes. (ii) The catalytic reduction of N2O occurs at the same potentials as that of hydrogen adsorption in the acidic and neutral solutions. It occurs at more positive potential than those of hydrogen adsorption in alkaline solutions.



(iii) The hysteresis of N2O reduction currents, as mentioned in other reports,20,21 was not observed under our experimental conditions. (iv) The catalytic reduction of N2O always takes place after the reduction of the oxide layer. (v) No catalytic oxidation process of N2O at the Pd/GC electrode was observed in anodic sweeps. (vi) The reduction potential of N2O at the Pd/GC electrode does not change significantly in the pH range from 7.0 to 13.0. The reduction of N2O at Pt electrodes (including single-crystal Pt electrode) via adsorbed hydrogen in acidic electrolytes was demonstrated by several recent works.17-19 The adsorbed hydrogen was considered as an essential catalytic intermediate in the reaction N2O(ad) + H(ad) f N2(g) + OH-(ad). In contrast, Attard and co-workers20 believed that N2O decomposes into N2 and oxygen adsorbed on the polycrystalline Pd electrode surfaces. They suggested that the blocking effect of anion adsorption on the electrode, in competition with that of N2O adsorption, is an important factor on the basis of their detailed investigation of N2O reduction in 0.1 and 0.01 M H2SO4, as well as in 0.1 M HClO4. Kudo and Mine22 reported that a Pd electrode has a low overpotential for H2 evolution and that it reduces N2O to N2 with almost 100% Faradaic efficiency and produces a negligible amount of H2 in 0.3 M K2SO4. These observations were attributed to the high reactivity of the adsorbed and/or nascent hydrogen with N2O adsorbed on the Pd electrode surface because atomic hydrogen can react with N2O before it desorbs as H2 molecules. From the observations described above, it was concluded that the adsorbed

Figure 7. Cyclic voltammograms of the Pd/GC electrode in the 0.01 M NaOH (A) and 0.1 M NaOH (B) in the absence (‚‚‚) and presence (s) of 0.73 mM N2O. Scan rate, 50 mV/s.

hydrogens take part in the N2O reduction, and the following mechanism was proposed for N2O reduction at a Pt electrode:

H+ + e- f H(ad) 2H(ad) + N2O(ad) f N2 + H2O N2O + e- f N2O- (ad) N2O- (ad) f N2 + O- (ad) H(ad) + N2O- (ad) f N2 + OHH(ad) + O- (ad) f OHH+ + N2O- (ad) + e- f N2 + OH-

From our experimental results, N2O is clearly not a scavenger of the adsorbed hydrogen on the Pd/GC electrode surfaces. Adsorbed hydrogens do not play an essential and catalytic role in the N2O reduction. This conclusion is particularly true in alkaline electrolytes. The shape of the N2O reduction current can be accounted for as a function of the potential, without the need to involve hydrogen as a catalytic intermediate. On the contrary, the electrosorbed hydrogen is an inhibitor of the reaction, since Analytical Chemistry, Vol. 70, No. 10, May 15, 1998

2185

Table 1. Electrocatalytic Parameters of N2O Reduction at Different pHs pH 2.0 EN2O/V Icat/µA EH(ads)/V

-0.15

3.0

5.0

6.0

7.0

8.0

9.0

10.0

12.0

13.0

-0.21

-0.32 1.0 -0.32

-0.43 4.0 -0.39

-0.45 14.5 -0.45

-0.50 12.4 -0.51

-0.47 16.0 -0.57

-0.48 17.0 -0.61

-0.49 16.8 -0.69

-0.58 14.5 -0.74

it blocks N2O adsorption-decomposition sites. The ease with which N2O can be displaced from the surface is consistent with the extremely weak chemisorption bond of about 20 kJ mol-1 formed between the adsorbed N2O and transition metal surfaces.24 The electrocatalytic activity of N2O reduction at the Pd/GC electrode depends on the ability to generate empty sites for the adsorption of N2O. This implies that one of the adsorbed substrates must be displaced at first by N2O during the electrocatalytic process. Since this adsorption step represents a competition process among all adsorbable species present in the solution. including N2O, for available sites on the Pd/GC electrode surface, the nature of the solvent and other chemical species present in the solution can play a very significant role in the electrocatalytic process. We believe that the electrochemical process of N2O reduction on Pd is similar to that on Pt proposed by Attard and co-workers.20 First of all, the adsorption of H and N2O at the Pd/GC electrode surface is a competitive process. Second, the hydrogen adsorption is potential-dependent, but that of the N2O is not. When the pH value of the electrolytes is less than 7.0, the adsorption sites on Pd surface are mainly occupied by the hydrogen due to its low adsorption potential. As a consequence, only a small reduction current was observed for N2O reduction (Figures 3 and 4). However, when pH is between 7.0 and 8.0, the hydrogen and N2O have a similar adsorption potential, and they compete with equal chance for the adsorption sites on the Pd/GC electrode surface. Therefore, a large catalytic reduction current associated with N2O was observed, and the current overlaps with the hydrogen adsorption peak. At pH 9.0, the adsorption potential of hydrogen is more negative than that of N2O. The adsorption sites are covered predominantly by N2O molecules at first. These adsorbed N2O molecules are electrocatalytically reduced and the products moved from the electrode surfaces into the bulk solution during the cathodic scan. Then, the adsorption sites left by the reduced N2O are occupied by hydrogens, which are reduced later at a more negative potential. Therefore, two large reduction peaks, corresponding to the N2O reduction and hydrogen reduction, respectively, were observed after the reduction peak of the oxide layer. But, why was not the N2O reduced catalytically after hydrogen adsorption peaks? This is probably because, when the adsorbed H+ at the Pd/GC electrode is reduced to hydrogen, the adsorption sites are still occupied (by H) before evolution of H2, and N2O is unable to displace the adsorbed H in the potential region. On the other hand, we observed that the potential of the catalytic reduction of N2O at the Pd/GC electrode does not change much with the pH from pH 7.0 to 13.0. In contrast, the potential of hydrogen adsorption shifts cathodically for ∼60 mV with the increase of every pH unit, as depicted in Figure 8 and Table 1. (24) Avery, N. R. Surf. Sci. 1983, 131, 501.


Figure 8. The plot of the potentials of hydrogen adsorption at Pd/ GC electrode versus the pH values of the electrolyte solutions.

Therefore, we conclude that N2O is not a scavenger of the adsorbed hydrogen and that the reduction current is attributable solely to the electrocatalytic reduction and decomposition of N2O on metal sites. The electrochemical behavior of N2O at the Pd/ GC electrode in our experiments cannot be explained by the reaction mechanism proposed for N2O reduction on Pt surfaces by Attard and co-workers20 and Kudo and co-worker.22 We therefore propose the following mechanism for the catalytic reduction of N2O on the Pd/GC electrode:

Pd* + N2O S Pd-N2O (dominant reaction when pH > 7.0) Pd* + H+ + e- S Pd-H (dominant reaction when pH < 7.0)

}

competitive adsorptions

Pd-N2O + 2e- f Pd-O2- + N2 (slow) Pd-O2- + 2H+ f Pd* + H2O (fast) Pd* + H+ + e- S Pd-H

where Pd* denotes an N2O catalytic reduction and decomposition site on the Pd/GC electrode surface. Quantitative Determination of N2O. As has been shown above, the pH value of electrolyte has a strong effect on the catalytic reduction of N2O on the Pd/GC electrode. The Pd/GC electrode displays the best electrocatalytic activity on the N2O

V) and the concentration of N2O. As shown in Figure 9, the catalytic current of N2O is linearly proportional to the concentration of N2O from 2.43 × 10-5 to 1.94 × 10-3 M (inset). The electrocatalytic transformation of N2O into N2 without byproducts warrants a stoichiometric redox reaction and may explain the excellent proportionality of the catalytic current with the substrate concentration over more than 2 orders of magnitude. On the other hand, the adsorption of N2O on the Pd/GC electrode is reversible at room temperature, and its fragmentation intermediates on Pd/GC electrode are completely unstable (e.g., Pd-O2-) and removed quickly from the electrode surface. This is also an important aspect for the excellent linear relationship between the catalytic current and the N2O concentration observed over more than 2 orders of magnitude.

Figure 9. Cyclic voltammograms of the Pd/GC electrode in PBS (pH 7.0) containing (1) 0, (2) 4.86 × 10-5, (3) 9.72 × 10-5, (4) 1.94 × 10-4, (5) 2.92 × 10-4, (6) 5.35 × 10-4, (7) 7.78 × 10-4, (8) 9.72 × 10-4, (9) 1.17 × 10-3, (10) 1.36 × 10-3, (11) 1.55 × 10-3, and (12) 1.75 × 10-3 M N2O, respectively. Scan rate, 50 mV/s. The inset shows the linear relationship between catalytic current of N2O and the concentration of N2O.

reduction in the pH range from 7.0 to 10.0. The main aim of our present work is to develop a sensitive method for N2O detection in biosamples, which requires the analysis of N2O in aqueous neutral buffer solution. Therefore, we chose the phosphate buffer solution of pH 7.0 as the supporting electrolyte in the following investigation. Moreover, the catalytic reduction peak of N2O overlaps completely with the hydrogen adsorption peak and forms a single, very well-defined reduction peak. We can, therefore, measure accurately the catalytic current generated by N2O reduction. Figure 9 shows the cyclic voltammetric curves of the Pd/GC electrode in PBS (pH 7.0) containing different concentrations of N2O at a scan rate of 50 mV/s. The inset shows the linear relationship between the catalytic currents at -0.45 V (the catalytic current is defined as the magnitude of cathodic current at -0.45

CONCLUSIONS In this paper, we studied the electrochemical reduction of N2O at Pd electrodeposited on GC electrode in aqueous acidic, neutral, and alkaline solutions. The electrocatalytic reduction of N2O always takes place after the reduction of the oxide layer and before the hydrogen evolution potential is reached. Also, it is strongly affected by the acidity of the electrolyte. In acidic electrolytes, the electrocatalytic reduction of N2O is blocked by the adsorption of hydrogen on the Pd/GC electrode; i.e., N2O is such a weak adsorbent that it loses the competition for the adsorption sites to discharged protons at low pH. We also demonstrated the possible application of our Pd/GC electrode in the determination of N2O. The linear range of N2O determination is from 24.3 µM to 1.94 mM when using the cyclic voltammetric technique. The mechanism of the catalytic electrochemical reactions of N2O at the Pd/ GC electrode is discussed in detail on the basis of our experimental observations. ACKNOWLEDGMENT We acknowledge the support of this project by Research Grant Council of Hong Kong and by Hong Kong University of Science and Technology. Received for review June 11, 1997. Accepted February 15, 1998. AC970601H

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Electrocatalytic Properties of Nitrous Oxide and Its Voltammetric

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