The electrocatalytical reduction of m-nitrophenol on palladium nanoparticles modified glassy carbon electrodes

Electrochimica Acta 58 (2011) 399–405

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The electrocatalytical reduction of m-nitrophenol on palladium nanoparticles modified glassy carbon electrodes Qiaofang Shi, Guowang Diao ∗ College of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou 225002, People’s Republic of China

a r t i c l e

i n f o

Article history: Received 24 June 2011 Received in revised form 21 September 2011 Accepted 22 September 2011 Available online 1 October 2011 Keywords: Palladium nanoparticles Electrodeposition m-Nitrophenol Electrocatalytic reduction Modified electrodes

a b s t r a c t Palladium nanoparticles modified glassy carbon electrodes (Pd/GC) were prepared via the electrodeposition of palladium on a glassy carbon (GC) electrode using cyclic voltammetry in different sweeping potential ranges. The scanning electron microscope images of palladium particles on the GC electrodes indicate that palladium particles with diameters of 20–50 nm were homogeneously dispersed on the GC electrode at the optimal deposition conditions, which can effectively catalyze the reduction of mnitrophenol in aqueous solutions, but their catalytic activities are strongly related to the deposition conditions of Pd. The X-ray photoelectron spectroscopy spectra of the Pd/GC electrode confirmed that 37.1% Pd was contained in the surface composition of the Pd/GC electrode. The cyclic voltammograms of the Pd/GC electrode in the solution of m-nitrophenol show that the reduction peak of m-nitrophenol shifts towards the more positive potentials, accompanied with an increase in the peak current compared to the bare GC electrode. The electrocatalytic activity of the Pd/GC electrode is affected by pH values of the solution. In addition, the electrolysis of m-nitrophenol under a constant potential indicates that the reduction current of m-nitrophenol on the Pd/GC electrode is approximately 20 times larger than that on the bare GC electrode. © 2011 Elsevier Ltd. All rights reserved.

1. Introduction Nitrophenols are important chemical materials, which are widely used to manufacture explosives, drugs, insecticides and dyes, and also used for the corrosion inhibitors of woods and rubber chemicals [1]. During the processes of manufacture and use, nitrophenols with wastewater were discharged into the environment to cause contamination due to their potential toxicity. Nitrophenols are the difficultly degraded organic compounds, and hence are seriously environmental pollutants [2,3]. To treat the nitrophenolic contamination, several methods have been proposed, such as biological processes [4–7], wet oxidation and ozonization [8], and UV-photodegradation using UV/H2 O2 , UV/TiO2 , UV/H2 O2 /Fe3+ and Cu2 O/TiO2 systems [9–12]. Recently, the electrochemical method has received growing interest due to its simplicity, rapidity, stability and environment-friendly [13–20]. Silvester et al. reported the electrochemical reduction mechanism of 4-nitrophenol in the room temperature ionic liquid [C4 dmim][N(Tf2 ) [19]; and Casella and Contursi reported the electrochemical reduction of nitrophenols on silver globular particles electrodeposited under pulsed potential conditions [20].

∗ Corresponding author. Tel.: +86 514 7975436; fax: +86 514 7975244. E-mail address: [email protected] (G. Diao). 0013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2011.09.064

Noble metals of Rh, Pt, Pd, etc. are widely recognized as being important heterogeneous catalysts and electrocatalysts for many reactions and especially used in fuel cells [21,22]. Metal nanoparticles have received considerable attention due to their unique physicochemical properties different from bulk metals [23–26] and large specific surface area. Macanás et al. studied the electrocatalytic reduction of nitrophenol on the polymeric hollow fiber membranes containing catalytic metal nanoparticles [27]. Wang et al. studied the electrocatalytic oxidation of formaldehyde on platinum well-dispersed into single-wall carbon nanotube/polyaniline composite film [28]. Shangguan et al. studied the electrocatalytic oxidation of formaldehyde on a glass carbon electrode with palladium nanoparticles by electrochemical deposition [29]. Gao et al. used palladium electroplated on a carbon nanotube to catalyze the oxidation of formaldehyde and they found that this electrode has a rather high electrocatalytic activity to formaldehyde oxidation [30]. Wang and Li studied the electrocatalytic properties of nitrous oxide at palladium electrodeposited on a glassy carbon electrode [31]. Casella studied the electrocatalytic oxidation of oxalic acid on a palladium-based modified glassy carbon electrode in acidic medium [32]. Considering the fact that nitroaromatic compounds were, on the average, 500-fold more toxic than their corresponding aromatic amine analogs [33], and nano-palladium has high electrocatalytic ability, therefore, in this work we reported the preparation of the palladium nanoparticles modified glassy carbon electrodes

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Fig. 1. SEM images (A) a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V, (B) a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.80 V and (C) cyclic voltammograms of m-nitrophenol on different Pd/GC electrodes, the Pd/GC electrode prepared in different sweeping potential ranges, curve 1: the range of −0.25 to 0.40 V; curve 2: the range of −0.25 to 0.50 V; curve 3: the range of −0.40 to 0.40 V; curve 4: the range of −0.25 to 0.60 V; curve 5: the range of −0.25 to 0.80 V. The solution consisting of 3.0 mM m-nitrophenol and 0.2 M phosphate buffer of pH 7.0, at a scan rate of 50 mV s−1 .

(Pd/GC), the electrocatalytic reduction of m-nitrophenol on the Pd/GC electrode in aqueous solutions, and the influence of pH on the electrocatalytic activity of the Pd/GC electrodes. It was found that this electrode has a rather high electrocatalytic activity to the reduction of m-nitrophenol.

X-ray source (1486.6 eV). All binding energies were referenced to C1s neutral carbon peak at 284.6 eV. 3. Results and discussion

2. Experimental

3.1. Preparation of palladium nanoparticles modified glassy carbon electrodes and characterization

m-Nitrophenol, palladium chloride, sodium dihydrogen phosphate, sodium hydrogen phosphate and other reagents are of analytical reagent grade. Doubly distilled water was used to prepare all solutions. A glassy carbon (GC) electrode (3 mm diameter) was polished with alumina slurry of 3 ␮m diameter on a polishing cloth and then sonicated in a distilled water bath for 10 min before use. A three-electrode cell with a GC or Pd/GC working electrode, platinum wire counter electrode and a saturated calomel reference electrode (SCE) was used as an electrolytic cell. All potentials were measured and recorded versus the SCE. Its temperature was controlled at 25 ± 0.1 ◦ C using a CS501-SP thermostat. All electrochemical experiments were carried out on a CHI 660C electrochemical workstation. The tested solutions were deaerated by passing through nitrogen for 15 min before electrochemical experiments, and a continuous flow of nitrogen was maintained over the sample solution during the experiments. A Philips XL-30 scanning electron microscope (SEM) was used to measure the SEM images of palladium particles deposited on a GC electrode. The X-ray photoelectron spectroscopy (XPS) spectra of Pd/GC electrodes were carried out on a Thermo ESCALAB 250 spectrometer with an Al K␣

Palladium electrodeposition was performed by cyclic voltammetry between different potential ranges for ten cycles in a solution containing 1.1 mM PdCl2 and hydrochloric acid. A GC electrode was used for the deposition of palladium. The scan rate was set at 25 mV s−1 . Fig. 1A and B shows the SEM images of a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V, and Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.80 V. Clearly, particles are obviously observed in Fig. 1A, which is quite different from that in Fig. 1B, indicating that palladium was deposited on the GC surface. Fig. 1A shows that most of particles with diameters of 20–50 nm are homogeneously dispersed on the GC surface. However, only a large Pd agglomeration (0.2–0.4 ␮m) occurs in Fig. 1B. The above results indicate that the images of palladium particles deposited on the GC surface are strongly affected by the sweeping potential range. The possible reasons for this are that when the applied potential is over 0.40 V, the palladium particles formed at the beginning deposition were oxidized to palladium oxide on the GC surface, and oxygen would be produced as the potential scanned to the high anodic potentials that was adsorbed on the GC surface, which hindered the deposition of palladium.

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Pd3d5/2

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Fig. 2. XPS spectra (A) a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V, (B) XPS spectra of Pd based on (A) and (C) a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.80 V (D) XPS spectra of Pd based on (C).

Fig. 1C shows the cyclic voltammogram of m-nitrophenol on the different electrodes, in a solution consisting of 3.0 mM mnitrophenol and 0.2 M phosphate buffer with pH 7.0. Curve 1 is the cyclic voltammogram of m-nitrophenol on a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V, in which a reduction peak is at −0.581 V. Curve 2 is the cyclic voltammogram of m-nitrophenol on a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.50 V, in which a reduction peak appears at −0.595 V that is more negative than that on curve 1. Curve 3 is the cyclic voltammogram of m-nitrophenol on a Pd/GC electrode prepared in a sweeping potential range of −0.40 to 0.40 V, in which a reduction peak appears at −0.599 V. Curve 4 is the cyclic voltammogram of m-nitrophenol on a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.60 V, in which a reduction peak appears at −0.639 V. Curve 5 is the cyclic voltammogram of m-nitrophenol on a Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.80 V, in which a reduction peak appears at −0.738 V. This result demonstrates that the Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V can effectively catalyze the reduction of m-nitrophenol. A reduction peak at −0.581 V appears on curve 1, which shifts towards a positive potential by 157 mV compared to the Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.80 V and moreover its peak current is much larger than that on curve 5. These results demonstrate that Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V has a marked catalytic ability to the reduction of m-nitrophenol. The results from Fig. 1C show that the catalytic activity of the Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V is much better than that prepared in other sweeping potential ranges because the palladium particle sizes of the former is much smaller than those of the latter. On the basis of the above results, the Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V will be used in the following experiments.

To confirm deposition of palladium on the GC electrode, the XPS measurements of the Pd/GC electrodes were performed. Fig. 2A and C shows the XPS spectrum of the Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V and Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.80 V. It is clearly, Pd and C were detected in the Pd/GC electrode surface in Fig. 2A and 37.1% Pd is contained in the electrode. In Fig. 2B, two peaks at 335.2 and 340.7 eV are attributed to Pd3d5/2 and Pd3d3/2 , which are characteristic peaks of 3d5/2 and 3d3/2 for Pd0 . In Fig. 2D, two main peaks at 338.1 and 343.3 eV are attributed to Pd3d5/2 and Pd3d3/2 , which are characteristic peaks of 3d5/2 and 3d3/2 for Pd4+ . Two shoulders are observed at the side of each main peak, and their binding energies are less than those of two main peaks, thus, these two shoulders are attributed to Pd2+ . A possible reason for this is that the palladium particles formed at the beginning deposition were oxidized to PdO when the applied potential is higher than 0.4 V, and then further oxidized to PdO2 as the potential scans towards more positive potentials. It was found that only 0.9% Pd is contained in the Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.80 V. This is a main reason why the electrocatalytic activity of the Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.40 V is much higher than that of the Pd/GC electrode prepared in a sweeping potential range of −0.25 to 0.80 V. 3.2. Electrocatalytic reduction of m-nitrophenol Curves 1 and 2 in Fig. 3A are the cyclic voltammograms of the GC and Pd/GC electrodes in 0.2 M phosphate buffer of pH 7.0, respectively. There are no redox peaks on curve 1, indicating that no electroactive species are oxidized and reduced on the bare GC electrode in the experimental potential range. However, a pair of redox peaks is observed on curve 2, which is caused by the adsorption and desorption of hydrogen on palladium [34].

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200

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Fig. 3. Cyclic voltammograms (A) in 0.2 M phosphate buffer of pH 7.0, curves: (1) a bare GC electrode, (2) a Pd/GC electrode. (B) The solution consisting of m-nitrophenol and 0.2 M phosphate buffer, curves: (1) a GC electrode in 1.5 mM m-nitrophenol, (2) a Pd/GC electrode in 1.0 mM m-nitrophenol, (3) a Pd/GC electrode in 1.5 mM m-nitrophenol, at a scan rate of 50 mV s−1 .

Curve 1 in Fig. 3B is the cyclic voltammogram of the bare GC electrode in 1.5 mM m-nitrophenol of pH 7.0, in which a reduction peak at −0.74 V appears on curve 1. Clearly, this peak is caused by the reduction of m-nitrophenol on the bare GC electrode. Curves 2 and 3 in Fig. 3B are the cyclic voltammograms of the Pd/GC electrode in 1.0 and 1.5 mM m-nitrophenol of pH 7.0, respectively, in which a new reduction peak at −0.58 V is observed on curves 2 and 3. Comparison with curve 2 in Fig. 3A, this new peak is most possibly attributed to the reduction of m-nitrophenol on the Pd/GC electrode. The reduction peak current at −0.58 V on curve 3 is larger than that on curve 2 in Fig. 3B because of an increase in the concentration of m-nitrophenol. This result confirms that the new reduction peak at −0.58 V on curves 2 and 3 is caused by the reduction of m-nitrophenol on the Pd/GC electrode. In Fig. 3B, the reduction peak potential of m-nitrophenol on the Pd/GC electrode is more positive than that on the bare GC electrode (curve 1), and its reduction current on Pd/GC electrode is larger than that on the bare GC electrode (curve 1). Both results are characteristic of the electrocatalytic reduction for cyclic voltammetry. Silva Luz et al. reported the reduction of 4-nitrophenol at a lithium tetracyanoethylenide (LiTCNE) modified glassy carbon electrode in the aqueous solution of pH 4.5 [18], in which the reduction peak was at −0.7 V vs. SCE that is more negative than the Pd/GC electrode in Fig. 3B by about 0.12 V. Sun et al. reported the electrocatalytic reduction of 4-nitrophenol at room temperature ionic liquid modified electrode in phosphate buffer of pH 7.0 [35], in which the reduction peak were at −0.808 V vs. SCE. Silvester et al. reported the electrochemical reduction of 4-nitrophenol in the room temperature ionic liquid

0

[C4 dmim][N(Tf)2 ] [19]. There were three reductive peaks (two irreversible and one reversible) at −1.03 V, −1.31 V, and −1.79 V vs. Ag, respectively. Casella et al. reported the electrochemical reduction of 4-nitrophenol on silver globular particles electrodeposited under pulsed potential conditions, in which the reduction peak were less negative than the unmodified GC electrode, however, its reduction peak current is lower than that on the bare GC electrode [20]. From above discussion and our results, the electrocatalytic performance of the Pd/GC electrode used in this work is better than those electrodes as mentioned. Fig. 4A shows the linear sweep voltammograms of the Pd/GC electrode at different scan rates, in a solution containing 3 mM m-nitrophenol with pH 7.0. As can be seen in Fig. 4A, the peak current increases with increasing the scan rate accompanied with the slight shift of the peak potential towards the negative potential direction, which is caused by the irreversible electrode reaction of m-nitrophenol. Based on the data in Fig. 4A, the peak current versus the square root of sweep rate is shown in Fig. 4B, in which a straight line with correlation coefficient of 0.9986 is obtained, indicating that the reduction of m-nitrophenol on the Pd/GC electrode is controlled by mass transfer. The result in Fig. 3B indicates that the reduction peak current of m-nitrophenol on the Pd/GC electrode is related to the concentration of m-nitrophenol. This gives us a hint that the concentration of m-nitrophenol would be determined based on the reduction peak current. To simplicity, linear sweep voltammetry was used to determine the relationship between the reduction peak current and the concentration of m-nitrophenol. Fig. 5A shows a portion of the

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v /(mV/s)

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Fig. 4. (A) linear sweep voltammograms of m-nitrophenol on a Pd/GC electrode at various scan rates, curves: (1) 10, (2) 20, (3) 40, (4) 60, (5) 80, (6) 120, (7) 160 mV s−1 in a solution consisting of 3.0 mM m-nitrophenol and 0.2 M phosphate buffer of pH 7.0, (B) A plot of the reduction peak current versus square root of sweep rate based on the result shown in Fig. 4A.

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Fig. 5. (A) Linear sweep voltammograms of a Pd/GC electrode in different concentrations of m-nitrophenol in 0.2 M phosphate buffer of pH 7.0, at a scan rate of 50 mV s−1 . (B) The reduction peak current as a function of the concentration of m-nitrophenol.

linear sweep voltammograms from 0.20 to 5.00 mM m-nitrophenol. Based on the linear sweep voltammograms, the reduction peak current as a function of the concentration of m-nitrophenol is shown in Fig. 5B, in which a straight line with correlation coefficient of 0.996 is obtained in a concentration range of 0.20–5.00 mM m-nitrophenol. Therefore, the Pd/GC electrode can be used to determine the concentration of m-nitrophenol in this concentration range. The reproducibility of the Pd/GC electrode was examined that its catalytic activity decreased by about 4% based on the reduction peak current on the cyclic voltammogram for ten-time measurements, however, the newly prepared electrodes have a good reproducibility. We observed a slow decay of the catalytic activity of the Pd/GC electrode after the prolonged electrolysis, which is attributable to the adsorption of the products of nitrophenol reduction on the surface of Pd/GC electrode.

from pH 6.0 to 7.9; the reduction peak potential of m-nitrophenol on the bare GC electrode shifts towards negative potential direction slightly with increasing pH in Fig. 7B. However, the relationship between the reduction peak current and pH on the Pd/GC electrode is different from that on the bare GC electrode, in the former the reduction peak current increases pronouncedly from pH 5.5 to 7.0, and then decreases from pH 7.0 to 7.9. This difference is caused by palladium nanoparticles on the GC electrode. The results in Fig. 7 demonstrate that the reduction peak current on the Pd/GC electrode is larger than that on the bare GC electrode, and the reduction peak potential of m-nitrophenol on the Pd/GC electrode is more positive than that on the bare GC electrode at each corresponding pH, which confirm that the Pd/GC electrode has a evident electrocatalytic ability to the reduction of m-nitrophenol.

3.3. The influence of pH on the electrocatalytic reduction of m-nitrophenol

On the basis of the results shown in Fig. 7A, a maximum current for the reduction of m-nitrophenol on the Pd/GC electrode occurs at pH 7.0. Thus, a solution containing 3.0 mM m-nitrophenol with pH 7.0 was used to be electrolyzed at different potentials. Fig. 8A and B shows the i–t curves measured at −0.522 and −0.622 V, respectively. Fig. 8A shows that the current of the GC electrode is very small and changes slightly with time, indicating that m-nitrophenol was hardly reduced on the GC electrode at this potential; however, the current on the Pd/GC electrode changes quickly within 60 s and then decreases slowly with time. The beginning current on the Pd/GC electrode is approximately 20 times larger than that on the GC electrode. The i–t curves in Fig. 8B are identical in shapes

The reduction of m-nitrophenol needs participation of protons. Therefore, pH should affect the electrochemical reduction of mnitrophenol. Fig. 6A and 6B show the cyclic voltammograms of m-nitrophenol on the bare GC and Pd/GC electrodes at different pH values, respectively. Based on the results in Fig. 6, the reduction peak current and the reduction peak potential as a function of pH are shown in Fig. 7A and 7B, respectively. Fig. 7A shows that the reduction peak current of m-nitrophenol on the bare GC electrode increases slightly from pH 5.5 to 6.0, and then decreases slowly

3.4. Electrolysis of m-nitrophenol at given potentials

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Fig. 6. Effect of pH on the cyclic voltammograms, (A) a bare GC electrode, (B) a Pd/GC electrode, the solution consisting of 1.5 mM m-nitrophenol and 0.2 M phosphate, at a scan rate of 50 mV s−1 .

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-40

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Fig. 7. (A) The relationship between the reduction peak current and pH. (B) The relationship between the reduction peak potential and pH, based on the results shown in Fig. 6.

to those in Fig. 8A. The beginning current on the Pd/GC electrode is approximately 12 times larger than that on the GC electrode in Fig. 8B. Clearly, the catalytic effectiveness of the Pd/GC electrode at −0.622 V is less than that at −0.522 V. This is mainly attributed to that the reduction current of m-nitrophenol on the GC electrode at −0.522 V is very small because m-nitrophenol is hardly reduced at this potential. The results from the constant potential electrolysis also demonstrate that Pd/GC electrode can effectively catalyze the reduction of m-nitrophenol. As can be seen in Fig. 8A and B, the reduction current of m-nitrophenol on the Pd/GC electrode is always much higher than that on the bare GC electrode over the reaction time, indicating that the Pd/GC electrode has a high catalytic ability to m-nitrophenol reduction during the electrolysis process. 3.5. Approaching the reduction mechanism of m-nitrophenol Among nitrophenols, the reduction of 4-nitrophenol was widely studied in different media and at various electrodes. The reduction mechanism is more complex. Silvester et al. reported the reduction of 4-nitrophenol in the ionic liquid [C4 dmim][N(Tf)2 ] on a Au microelectrode, in which there are three reduction peaks on the cyclic voltammogram [19]. Amatore et al. [36] and Morales-Morales et al. [37] presented the reaction mechanisms of nitrophenols containing several steps with free radicals in organic media. Forryan et al. reported the cyclic voltammogram of 4-nitrophenol in DMF (0.2 M TBAP) at a gold electrode, in which there are three reduction peaks at the potentials more negative than −1.0 V (vs. Ag) [38]. However, Laviron and Roullier reported the reduction of 4nitrophenol to 4-aminophenol [39]; Sun et al. reported the direct

electrocatalytic reduction of 4-nitrophenol at an ionic liquid (BPPF6 ) modified carbon paste electrode in the phosphate buffer of pH 7.0 [35]. On basis of the cyclic voltammograms of m-nitrophenol on the Pd/GC electrode in Fig. 3B, there are two reduction peaks at −0.74 and −0.58 V. The former is attributed to the hydrogen adsorption on a Pd/GC electrode [34], and the latter is caused by the reduction of m-nitrophenol. In addition, the in situ chemical-ESR (electron spin resonance) measurement of the electrolysis of m-nitrophenol in a solution consisting of 3 mM m-nitrophenol and 0.2 M phosphate buffer of pH 7.0 was carried out at constant potentials from −0.40 to −0.80 V (vs. Ag/AgCl with a saturated KCl solution). However, no ESR signals were observed during the electrolytic process. However, MoralesMorales et al. reported that no ESR signal was observed near the first reduction peak, nevertheless, an intense ESR spectrum generated at the more negative potential of −2.1V vs. Fc+ /Fc [37]. In this work, when the applied potential was over −0.80 V, hydrogen bubbles were formed at the working electrode. The reduction of m-nitrophenol on the Pd/GC electrode is irreversible, therefore the following equation is used here [40]: 1/2

ip = (2.99 × 105 )n(˛na )1/2 A C0∗ D0 1/2 where ip is the reduction peak current, ˛ is the electron transfer coefficient, na is the number of electrons in involved in the ratedetermining step, D0 is the diffusion coefficient of the electroactive species and the other symbols have their usual meaning. According to equation: ˛na = 47.7/(Ep − Ep/2 ) [40], values of ˛na were calculated to be 1.01 for the irreversible reduction of m-nitrophenol,

0

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Fig. 8. Electrolysis of m-nitrophenol on a Pd/GC electrode under constant potentials: (A) −0.552 V, (B) −0.622 V, in a solution consisting of 3.0 mM m-nitrophenol and 0.2 M phosphate buffer of pH 7.0.

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using a concentration of m-nitrophenol of 3 mM, and its diffusion coefficient is 9.17 × 10−6 cm2 s−1 [41]. The number of total electrons was calculated to be 4.04, indicating that the reduction of m-nitrophenol in the aqueous solution is four-electron reaction, which is coincident with reports earlier [42–44]. According to the above results, the reduction of m-nitrophenol is suggested as follows: R-NO2 + e− → R-NO2 −

(slow)

R-NO2 − + 3e− + 4H+ → R-NHOH + H2 O (fast) This reaction mechanism is similar to the reduction of 4nitrophenol [18]. 4. Conclusion The palladium nanoparticles modified on glassy carbon electrodes were prepared using cyclic voltammetry. Their catalytic activities are strongly depending on the sweeping potential range. The electrocatalytic reduction of m-nitrophenol in aqueous solutions was carried out using cyclic voltammetry and potentiostatic method. The cyclic voltammograms of the Pd/GC electrode in the m-nitrophenol solutions indicate that the reduction peak potential of m-nitrophenol shifts towards the more positive potentials and the reduction peak current increases compared to those on the bare GC electrode. The electrolysis of m-nitrophenol under a constant potential also indicates that the reduction current on the Pd/GC electrode is much higher than that on the bare GC electrode. Therefore, the Pd/GC electrode has an effectively electrocatalytic ability to the reduction of m-nitrophenol in the aqueous solutions, which is expected that this electrode can be used for the decomposition of nitrophenols and can be used to determine m-nitrophenol in a given concentration range. The Pd/GC electrode can directly catalyze the reduction of m-nitrophenol in the aqueous solution. The stability of Pd/GC electrode is expected to be improved in our future work. Acknowledgements The authors acknowledged the financial support from the National Natural Science Foundation of China (Grant Nos. 20973151, 20901065), the Specialized Research Fund for the Doctoral Program of Higher Education (SRFDP, 20093250110001), the Foundation of Jiangsu Provincial Key Program of Physical Chemistry in Yangzhou University. References [1] M.R.H. Poden, S.K. Bhattacharya, M. Qu, Water Res. 29 (1995) 391. [2] H.E. Wise, P.D. Fahrenthold, Environ. Sci. Technol. 15 (1981) 1292.

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