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MWNTs nanocomposite and its electrocatalytic performance for bromate determination
Chemical Engineering Journal 200–202 (2012) 32–38
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Fabrication of high dispersion Pd/MWNTs nanocomposite and its electrocatalytic performance for bromate determination Dan-dan Zhou, Liang Ding, Hao Cui, Hao An, Jian-ping Zhai, Qin Li ⇑ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China
h i g h l i g h t s " The Pd/MWNTs nanocomposite with good activity and stability was synthesized.
" The application of Pd/MWNTs for the determination of BrO3 was investigated.
" A BrO3 reduction peak between 0.15 and 0.25 V was obtained by CV measurement. " Linear correlation between the peak intensity and Cbromate (0.1–40 mM) is good.
" The Pd/MWNTs electrode could be employed as an amperometric sensor for BrO3 .
a r t i c l e
i n f o
Article history: Received 23 March 2012 Received in revised form 31 May 2012 Accepted 6 June 2012 Available online 15 June 2012 Keywords: Palladium nanoparticles Electroreduction Multiwall carbon nanotubes Bromate sensor
a b s t r a c t Pd nanoparticles decorated multiwall carbon nanotubes (Pd/MWNTs) catalyst was synthesized by in situ chemical method and its application for the determination of bromate was investigated. The morphology and composition of the Pd/MWNTs catalyst was characterized by Transmission electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The analyses indicate that Pd nanoparticles in metallic-state were homogeneously deposited on MWNTs with an average diameter of 4.5 nm. The electrocatalytic activity of Pd/MWNTs for the reduction of bromate was investigated by cyclic voltammetry (CV). An obvious peak corresponding to the bromate reduction was obtained between 0.15 and 0.25 V. Effects of different factors including scan rate, temperature and initial concentration of bromate ions on bromate determination were studied as well. A good linear relationship between the values of reduction peak and bromate concentration was obtained. Chronoamperometric measurement showed that the Pd/MWNTs modified electrode could be successfully employed as an amperometric sensor for bromate in a wide concentration range (0.1–40 mM) within a short response time (5 s), and the sensitivity of this sensor is 768.08 lA mM1 cm2. The results reported herein indicate that the Pd/MWNTs have a potential application in fabrication of bromate detector. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Bromate, used as a food additive in the production of fermented beverages and fish pastes, has been found in drinking water as disinfection by-products (DBPs) [1]. It has been published that bromate ion can cause renal cell tumors [2]. The International Agency for Research on Cancer has classified it as 2B carcinogen [3]. Bromate is very stable and difficult to be removed from aquatic system. Various kinds of methods have been developed to determinate and eliminate bromate ion in water system [4]. However, second pollution and high consumption of reagents inevitably followed the removal process. Among all those treatment methods, the electrochemical method requires only electricity in operation, ⇑ Corresponding author. Tel./fax: +86 25 83592903. E-mail address: [email protected] (Q. Li). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.06.020
and no second pollution companied. Moreover, catalysts modified electrode can be recycled and thus make it an economic way for disposing bromated wastewater [5]. Utilizing of metal nanoparticles as modified electrode materials has aroused great attentions in recent years [6,7]. Palladium (Pd) is an important part of noble metal group with powerful electrocatalytic activity, which is widely applied as catalyst in heterogeneous reactions [8]. For example, there have been reports of Pd nanoparticles modified electrode for the electroreduction of pentachlorophenol [9], monochloroacetic acid [10], oxygen [11], nitrogen [12], hydrogen [13]. It is reasonable to believe that Pd catalyst is a favorable candidate for bromate determination in water system. However, naked palladium nanoparticles are prone to agglomerate, which greatly restrains their catalytic activity. In order to overcome this technical bottleneck, solid supports which are stable and difficult to react with catalyst are needed, carbon, conducting
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polymer, silica and zeolites appear as good selections [14] That nanometer-sized supported metals show greater catalytic activities than common ones, owing to their excellent physical structure and chemical properties [15]. Due to the novel material and stable physicochemical properties, carbon nanotubes (CNTs) have attracted considerable attentions as support in the field of catalytic materials [16,17]. However, the low chemical reactivity of CNTs prevents their integration with metal materials [18,19]. In order to achieve efficient integration, it is necessary to pretreat the CNTs. Plenty of reports have demonstrated that functional CNTs supported metal nanoparticles catalysts exhibit great catalytic activity, such as Pt [20], Ti [21], Au [22], Ag [23], Sn [24], and Pd [25]. A reasonable Pd loading (5–10 wt.%) presents good catalytic activity, in addition, a high dispersion of palladium nanoparticles on the CNTs is also required because the catalysis reaction occurs only on the surface of the palladium nanoparticles [14]. Therefore, CNTs supported Pd nanoparticles catalysts could be exploited to prepare a novel electrode material for the determination of bromate. However, the electrocatalytic detection and reduction of bromate on Pd/CNTs catalyst has not been reported yet. In this study, palladium/multiwall carbon nanotubes (Pd/ MWNTs) composite was prepared by in situ chemical method. The morphology and composite of catalysts were characterized by Transmission electron microscopy (TEM), Energy dispersive Xray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The Pd/MWNTs modified electrode was utilized to study the determination and reduction of bromate by cyclic voltammetry, multi-potential step measurement, and chronoamperometric measurement. Effects of different factors, including scan rate, temperature and initial concentration of bromate ion were studied as well.
stirred at room temperature for another 2 h to ensure the completion of the reaction. The mixture was then filtered, washed with DD water, and dried in a vacuum oven at 338 K. Finally, this sample was dispersed in 10 mL distilled deionized water under ambient condition.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Electron micrographs of Pd/MWNTs nanomaterial
All the chemicals used in this research were analytical grade and no purification was needed. Potassium bromate (P99.5%), palladium chloride, and potassium borohydride were purchased from Fluka. Multiwall carbon nanotubes (MWNTs, 10–30 lm in length and 8 nm in diameter) were obtained from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences, China. Distilled deionized (DD) water (18.25 MX cm) was employed in all experiments.
The morphology of catalysts was obtained by TEM. Fig. 1a shows a representative TEM image of the oxidized multiwall carbon nanotubes (o-MWNTs). The surface of the carbon nanotubes is smooth. It is well known that acidic purification of MWNTs introduces a large number of carboxylic acids moieties (–COOH) onto the pristine defect sites located on the surface of MWNTs. As shown in Fig. 1b, a mass of spherical nanoparticles, with an average diameter of 4.5 nm and particle size range of 3–9 nm (Fig. 1c), is uniformly deposited on MWNTs. Fig. 1d shows an intense palladium peak at 3.25 keV, demonstrating that Pd nanoparticles were loaded on the multiwall carbon nanotubes microparticles, and the amount of Pd loading is 8.35 wt.%. X-ray photoelectron spectroscopy (XPS) analysis was performed to obtain the electronic state of different element in Pd/MWNTs catalyst. As shown in Fig. 1e, obvious peaks of C 1 s, O 1 s and Pd 3d are observed in the XPS spectrum of Pd/MWNTs nanocomposite. The enlarged XPS spectrum is illustrated in Fig. 1f, which reveals the presence of Pd 3d5/2 and 3d3/2 peaks at binding energies of 334.6 and 339.9 eV, respectively. These binding energy data are in accordance with the data reported for metallic Pd [27]. Combined with the analysis of TEM, EDX and XPS, the metallic-state Pd nanoparticles have deposited on the MWNTs successfully, and the interaction between MWNTs and Pd nanoparticles is strong chemisorption [28].
2.2. Pretreatment of MWNTs Pristine MWNTs were pretreated in a hot mixture solution of concentrated HNO3 and H2SO4 (1:3 by volume) for 4 h under refluxing condition to remove impurities and generate surface functional groups. Then the mixture was isolated by filtration, rinsed thoroughly with DD water, and dried in an oven at 338 K. Pretreatment of MWNTs surface restraints the poisoning caused by foreign impurities, and generates functional groups which could improve the electrocatalytic activity and dispersibility [26]. Surface-oxidized MWNTs (o-MWNTs) samples were stored at room temperature. 2.3. Synthesis of Pd/MWNTs composite o-MWNTs (10 mg) were dispersed in 10 ml distilled deionized water. Then, PdCl2 aqueous solution was added. The mixture took ultrasound treatment for 20 min in order to obtain a uniform solution. Later 0.1 M KBH4 was slowly added into the solution under ultrasonic treatment for 20 min, then the reaction mixture was
2.4. Electrochemical measurement Model CHI 840B electrochemical workstation was employed for cyclic voltammetry characterization, multi-potential step measurement, and chronoamperometry. A three-electrode cell was used in the electrochemical measurements. The glassy carbon (GC) electrode was used as the working electrode. A platinum foil (1 cm2) was used as counter-electrode and the reference-electrode was saturated potassium chloride electrode. The glassy carbon electrode was first polished with a slurry of 0.5 lm alumina and rinsed with distilled deionized water, then 5 lL Pd/MWNTs mixture solution was dropped onto GC electrode, and dried at 338 K under drying oven. 2.5. Characterization The particle size and morphology of the catalyst were determined by Transmission electron microscopy (TEM). TEM observations were carried out on a JEOL JEM-200CX transmission electron microscope operated at an accelerated voltage of 200 keV. The composition of the catalyst was analyzed through Energy dispersive Xray spectroscopy (EDX, Hitachi S-3400N II). The X-ray photoelectron spectroscopy (XPS) was analyzed on a Thermo ESCALAB 250 spectrometer with an Al Ka X-ray source (1486.6 eV). The sample was kept on a sample holder under a vacuum of about 107 Pa.
3.2. Effects of Pd nanoparticles in the catalyst on bromate electroreduction The electrochemical response of hydrogen has been extensively used to investigate the redox property of metal nanoparticles
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Fig. 1. (a) TEM of o-MWNTs, (b)TEM of Pd/MWNTs, (c) the size distribution histograms of Pd nanoparticles deposited on MWNTs, (d) EDX data of Pd/MWNTs catalyst, (e) XPS spectra of Pd/MWNTs composite and (f) the palladium 3d region of XPS.
modified electrode. The electrochemical reaction of hydrogen atom in the tests can be divided into two steps. Firstly, the hydrogen ions are absorbed onto the surface of modified electrode, and this phenomenon is called ‘‘dissolution adsorption mechanism’’ [29]. Then the absorbed hydrogen ions diffuse into the inside of metal nanoparticles bulk and the redox reaction occurs. It is well known that the easy agglomeration of naked palladium nanoparticles restrains their electrochemical activity. Besides, the activity of naked palladium is lower than that of the palladium with supporters [30,31]. The cyclic voltammograms of MWNTs and Pd/ MWNTs catalysts at the scan rate of 50 mV s1 in 0.5 M H2SO4 solution are compared in Fig. 2. Due to the absorptive function and ionic exchange of pure MWNTs, it presents certain redox activity (Fig. 2a). As shown in Fig. 2b, the reduction peaks at 0.4 V and 0.35 V correspond to the reduction of absorbed hydrogen ions and the desorption of hydrogen inside the catalyst, respectively [29]. In the positive scan direction, the peak at 0.15 V corresponds to the absorption of hydrogen ions. Meanwhile, it can be observed that the area of ‘‘redox region’’ of the Pd/MWNTs catalyst is larger than that of pure MWNTs. This could be attributed to the good dispersity and decoration of palladium nanoparticles on the surface of MWNTs,
Fig. 2. Cyclic voltammograms of (a) MWNTs and (b) Pd/MWNTs catalysts in 0.5 M H2SO4 solution at the scan rate of 50 mV s1.
leading to the promotion of the catalytic activity due to the catalytic performance of palladium for C–C bond forming [14].
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Accordingly, Pd/MWNTs modified electrode has high electrocatalytic activity. To study its electrochemical performance, cyclic voltammograms of Pd/MWNTs catalyst at the scan rate of 50 mV s1 in 0.5 M H2SO4 solution without and with 10 mM bromate were recorded and the results are shown in Fig. 3. An obvious cathodic peak (marked by arrow) can be observed in the presence of bromate (solid line), which is attributed to the reduction of bromate. The above analyses reveal that the electroreduction of bromate occurs within the potential range of 0.25–0.1 V. Additionally, the oxidation peak at 0.4 V and 0.9 V is attributed to the absorption of bromate ions and oxidation of Pd nanoparticles. Therefore, a controlled potential region of 0.6–0.6 V was employed in the following tests, so as to avoid Pd oxidation while maintaining the catalytic activity of metallic state Pd. 3.3. Electroanalytical characterization of Pd/MWNTs catalyst Generally, electrocatalytic reduction of bromate needs high overpotential. The Pd/MWNTs modified electrode can lower the potential energy needed for bromate reduction. In order to study the mechanisms of bromate electroreduction, we performed the cyclic voltammograms under different scan rates, temperatures and initial concentrations of bromate ions. Fig. 4 shows the cyclic voltammograms of the Pd/MWNTs catalyst in 0.5 M H2SO4 solution with 10 mM KBrO3 at different scan rates. Bromate reduction peak potentials shift negatively and the corresponding peak currents increase with the increase of the scan rate, and the relationship between the bromate reduction peak current ip and the square root of scan rate v1/2 is shown in the inset in Fig. 4a. A linear relation can be obtained in a range of 10– 100 mV s1 and the R2 is 0.9821. The results demonstrate that the bromate electroreduction on Pd/MWNTs modified electrode is a diffusion-controlled process [14]. Additionally, the absolute values of peak potential Ep increase linearly with the increase of the logarithm of scan rate log (v) (Fig. 4b). This indicates that the electroreduction of bromate is a completely irreversible electrode process. For the irreversible electron-transfer electrode process, the slope of the linearity between Ep and log (v) can be obtained from the following equation [32]:
K¼
RT 2F ana
ð1Þ
where a means the coefficient of electron transfer and na stands for electrons number involved in the rate determined step. Hence the slope of Ep vs. log (v) in Fig. 4b is determined to be 64 mV, and naa is worked out to be 0.21. Besides, the relation between scan rate (v) and reduction peak current (ip) can be expressed as follows:
Fig. 3. Cyclic voltammograms of Pd/MWNTs catalyst in 0.5 M H2SO4 solution (dash line) and in 0.5 M H2SO4 solution with 10 mM KBrO3 (solid line) at the scan rate of 50 mV s1.
Fig. 4. (a) Cyclic voltammograms of Pd/MWNTs modified electrode in 0.5 M H2SO4 solution with 10 mM KBrO3 at different scan rates of 10, 20, 30, 40, 50, 60, 70 and 100 mV s1 (pointed by arrow), the inset in this picture shows the plotting of the corresponding peak current vs. scan rate. (b) Bromate reduction peak potential vs. the logarithm of scan rate.
ip ¼ ð0:4958 103 ÞnF 3=2 ðRTÞ1=2 ðana Þ1=2 ACD1=2 v 1=2
ð2Þ
where D presents the diffusion coefficient, A stands for the real surface area of the working electrode, and it was obtained that D = 2.826 105 cm2 s1 [33]. Then the number of the electron transfer (n = 6.28) could be calculated, and the overall reduction reac tion of bromate can be written as BrO 3 þ H þ 6e ! Br þ H2 O. The high stability of Br is one of the main reasons why the overall reaction is totally irreversible. Consulting the reported bromate electroreduction mechanism on metal electrode and the result obtained in our test, we proposed the bromate reduction mechanism on Pd/MWNTs interface and the scheme of the reaction is shown in Fig. 5. In the oxidative process (positive scan), H+ and BrO 3 are first absorbed onto the surface of the Pd nanoparticles. Then, in the reductive process (negative scan) (inset in Fig. 5), the absorbed BrO 3 is reduced to Br while electrons transfer through Pd nanoparticles. An obvious bromate reduction peak (peak 1) was found, which can be used for bromate determination. Then, the residual oxygen reacts with the absorbed hydrogen ions, and water molecules are produced (peak 2). Finally, water molecule and Br are desorbed from the surface of Pd nanoparticles. Temperature is also one of the most important factors affecting the reaction rate. The effect of temperature on the electrocatalytic performance of Pd/MWNTs modified electrode was investigated in the range of 278–318 K by cyclic voltammetry and the results are depicted in Fig. 6. As shown in Fig. 6, the rising of temperature leads to a shift of the reduction peak towards more negative potential and the peak currents increase simultaneously. The more
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Fig. 5. Proposed mechanism for the electroreduction of bromate on Pd/MWNTs modified electrode in acidic solution. Inset shows the cyclic voltammogram of Pd/ MWNTs modified electrode in 0.5 M H2SO4 solution with 10 mM KBrO3.
feasibility of electroreduction and the increase of charge transfer rate at electrode/electrolyte interface at elevated temperatures could be responsible for this phenomenon. Additionally, a linear relationship between temperature and reduction current can be observed in the insert. These results indicate that bromate electroreduction is an endothermic reaction. In order to study the feasibility of bromate detection by using Pd/MWNTs modified electrode, cyclic voltammograms were obtained at the scan rate of 50 mV s1 in 0.5 M H2SO4 solution with different concentrations of bromate. It can be observed that adding different amounts of bromate to the cell caused a regular change in the cyclic voltammograms. Increasing the concentration of bromate resulted in a decrease in reduction peak potential and a concomitant increase in corresponding peak current. As can be seen from the inset in Fig. 7, the reduction peak current, as a function of the bromate concentration, produces a linear line with a correlation coefficient of 0.9939 in the range of 1–30 mM bromate. The Pd/MWNTs modified electrode is sensitive to bromate ions. Therefore, from a practical point of view, the Pd/MWNTs modified electrode can be applied for the quantitative determination of bromate.
Fig. 7. Cyclic voltammograms of Pd/MWNTs modified electrode in 0.5 M H2SO4 solution with addition of different amounts of KBrO3 (1, 3, 5, 7, 9, 13, 15, 17, 23, 25 and 30 mM as pointed by the arrow) at the scan rate of 50 mV s1. Inset in the picture shows the plotting of the corresponding reduction peak current vs. initial concentration of bromate ion.
The effect of applied potential on the reduction current by Pd/ MWNTs catalyst was explored by the multi-potential step measurement and the results are summarized in Fig. 8. As obtained in Fig. 8, the amount of reduction current is determined by the
applied potential. There is no reduction current in the sulfuric acid system in the region of 0.2 to 0.2 V in Fig. 8a, indicating that electroreduction reaction does not occur in this potential region. Moving the applied potential to a further negative value (i.e.0.3 V) led to an increase in the reduction current, which is caused by the electroreduction of absorbed hydrogen ions. As shown in Fig. 8b, the steady-state reduction current increases constantly as the applied potential decreases from 0.15 to 0 V. When shifting the potential towards negative value of 0 to 0.25 V, the responsive current which is attributed to the reduction of bromate remains unchanged. Subsequently, the increase of the reduction current again at 0.3 V is owned to the beginning of hydrogen ions electroreduction. On the basis of the analyses above, an applied potential of 0.1 V was chosen for chronoamperometry. To investigate the stability of Pd/MWNTs catalyst (stored under ambient condition for 2 months), the chronoamperometry measurement at a potential of 0.1 V in 0.5 M H2SO4 solution with 10 mM KBrO3 is shown in Fig. 9. Slight fluctuation of reduction current can be observed, which is ascribed to the bit exfoliation of palladium nanoparticles in the catalyst. Besides, the general reduction current decreases and reaches a steady state subsequently. The Pd/ MWNTs catalyst exhibits high electrocatalytic activity, and the constant current demonstrates its satisfying stability. Through the analysis of the I–t curve, it can be concluded that bromate reduction products may not poison the Pd/MWNTs catalyst and the nature of Pd/MWNTs catalyst does not change, indicating that
Fig. 6. Cyclic voltammograms of Pd/MWNTs modified electrode in 0.5 M H2SO4 solution with 10 mM KBrO3 at different temperatures (278, 288, 298, 308 and 318 K) at the scan rate of 50 mV s1. Inset shows plot of the corresponding peak current vs. the electrolyte temperature.
Fig. 8. The multi-potential step curves of the Pd/MWNTs catalyst (a) in 0.5 M H2SO4 solution and (b) in 0.5 M H2SO4 solution with 10 mM KBrO3. Step decrease: 0.05 V; step time: 10 s.
3.4. Determination of bromate by Pd/MWNTs catalyst
D.-d. Zhou et al. / Chemical Engineering Journal 200–202 (2012) 32–38
Fig. 9. Chronoamperometric curve of Pd/MWNTs catalyst in 0.5 M H2SO4 solution with 10 mM KBrO3 at 0.1 V.
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and was applied for bromate determination. The TEM, EDX and XPS analyses indicated that the application of functional MWNTs as template material could restrain the aggregation of Pd nanoparticles, resulting in the uniform deposition of Pd nanoparticles in metallic-state. During the reductive process, bromate reduction peak was observed, and the reduction process occurred in the region of 0.15 to 0.25 V. The electrochemical analyses of the different effect factors showed that bromate electroreduction process is a completely irreversible electrode process controlled by mass transfer. The relation between the reduction peak intensity and bromate concentration agrees with a linear behavior, with a correlation coefficient of 0.9939. The Pd/MWNTs modified electrode was used to fabricate a simple electrochemical sensor, and it was found that bromate could be detected in a wide linear range (0.1–40 mM) within a short response time (5 s). The above results validate a great application prospect of Pd/MWNTs catalyst for constructing a high performance bromate detector. Acknowledgements This work was funded by the Natural Science Foundation of China (No. 51008154), the Jiangsu Natural Science Foundation (No. SBK201022682), the Fundamental Research Funds for the Central Universities (No. 1112021101), the Scientific Research Foundation of Graduate School of Nanjing University (No. 2012CL19). References
Fig. 10. Chronoamperometric measurements at a Pd/MWNTs modified electrode obtained in 0.5 M H2SO4 solution with increasing concentration of bromate ions in 2 mM step (except 0.1 mM, 0.5 mM, begin with 1 mM). Inset in this picture shows plotting of the responsive current vs. the corresponding concentration of bromate ion.
the Pd/MWNTs modified electrode can be employed for longtime electroreduction of bromate. To evaluate the electrochemical sensing performance of the Pd/ MWNTs catalyst, the amperometric behaviors of the modified electrode toward the bromate reduction under potentiostatic condition (applied potential is 0.1 V) are investigated. Typical chronoamperometric profiles (I–t curves) recorded can be seen in Fig. 10. In addition, a responsive reduction current can be obtained promptly considering the fact that the electrolyte concentration changes within a short response time (5 s). The fast response is attributed to the short penetration depth of bromate ion and the highly active film of the modified electrode. The diagram of the responsive current vs. the electrolyte concentration is shown in the inset in Fig. 10. A linear relationship can be observed in the concentration range of 0.1–40 mM, and the correlation coefficient is >0.999. Additionally, the sensitivity of the sensor obtained from the slope (inset in Fig. 10) of the linear relationship of the calibration curve is 768.08 lA mM1 cm2, which is as good as the reported literature [34]. Therefore, the Pd/MWNTs modified electrode is suitable for amperometric determination of bromate in this concentration region. This suggests Pd/MWNTs catalyst as a valid candidate for bromated detection.
4. Conclusion In this work, Pd/MWNTs catalyst with favorable activity and stability was successfully prepared by in situ synthesis method
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Chemical Engineering Journal journal homepage: www.elsevier.com/locate/cej
Fabrication of high dispersion Pd/MWNTs nanocomposite and its electrocatalytic performance for bromate determination Dan-dan Zhou, Liang Ding, Hao Cui, Hao An, Jian-ping Zhai, Qin Li ⇑ State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210046, PR China
h i g h l i g h t s " The Pd/MWNTs nanocomposite with good activity and stability was synthesized.
" The application of Pd/MWNTs for the determination of BrO3 was investigated.
" A BrO3 reduction peak between 0.15 and 0.25 V was obtained by CV measurement. " Linear correlation between the peak intensity and Cbromate (0.1–40 mM) is good.
" The Pd/MWNTs electrode could be employed as an amperometric sensor for BrO3 .
a r t i c l e
i n f o
Article history: Received 23 March 2012 Received in revised form 31 May 2012 Accepted 6 June 2012 Available online 15 June 2012 Keywords: Palladium nanoparticles Electroreduction Multiwall carbon nanotubes Bromate sensor
a b s t r a c t Pd nanoparticles decorated multiwall carbon nanotubes (Pd/MWNTs) catalyst was synthesized by in situ chemical method and its application for the determination of bromate was investigated. The morphology and composition of the Pd/MWNTs catalyst was characterized by Transmission electron microscopy (TEM), Energy dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). The analyses indicate that Pd nanoparticles in metallic-state were homogeneously deposited on MWNTs with an average diameter of 4.5 nm. The electrocatalytic activity of Pd/MWNTs for the reduction of bromate was investigated by cyclic voltammetry (CV). An obvious peak corresponding to the bromate reduction was obtained between 0.15 and 0.25 V. Effects of different factors including scan rate, temperature and initial concentration of bromate ions on bromate determination were studied as well. A good linear relationship between the values of reduction peak and bromate concentration was obtained. Chronoamperometric measurement showed that the Pd/MWNTs modified electrode could be successfully employed as an amperometric sensor for bromate in a wide concentration range (0.1–40 mM) within a short response time (5 s), and the sensitivity of this sensor is 768.08 lA mM1 cm2. The results reported herein indicate that the Pd/MWNTs have a potential application in fabrication of bromate detector. Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction Bromate, used as a food additive in the production of fermented beverages and fish pastes, has been found in drinking water as disinfection by-products (DBPs) [1]. It has been published that bromate ion can cause renal cell tumors [2]. The International Agency for Research on Cancer has classified it as 2B carcinogen [3]. Bromate is very stable and difficult to be removed from aquatic system. Various kinds of methods have been developed to determinate and eliminate bromate ion in water system [4]. However, second pollution and high consumption of reagents inevitably followed the removal process. Among all those treatment methods, the electrochemical method requires only electricity in operation, ⇑ Corresponding author. Tel./fax: +86 25 83592903. E-mail address: [email protected] (Q. Li). 1385-8947/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cej.2012.06.020
and no second pollution companied. Moreover, catalysts modified electrode can be recycled and thus make it an economic way for disposing bromated wastewater [5]. Utilizing of metal nanoparticles as modified electrode materials has aroused great attentions in recent years [6,7]. Palladium (Pd) is an important part of noble metal group with powerful electrocatalytic activity, which is widely applied as catalyst in heterogeneous reactions [8]. For example, there have been reports of Pd nanoparticles modified electrode for the electroreduction of pentachlorophenol [9], monochloroacetic acid [10], oxygen [11], nitrogen [12], hydrogen [13]. It is reasonable to believe that Pd catalyst is a favorable candidate for bromate determination in water system. However, naked palladium nanoparticles are prone to agglomerate, which greatly restrains their catalytic activity. In order to overcome this technical bottleneck, solid supports which are stable and difficult to react with catalyst are needed, carbon, conducting
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polymer, silica and zeolites appear as good selections [14] That nanometer-sized supported metals show greater catalytic activities than common ones, owing to their excellent physical structure and chemical properties [15]. Due to the novel material and stable physicochemical properties, carbon nanotubes (CNTs) have attracted considerable attentions as support in the field of catalytic materials [16,17]. However, the low chemical reactivity of CNTs prevents their integration with metal materials [18,19]. In order to achieve efficient integration, it is necessary to pretreat the CNTs. Plenty of reports have demonstrated that functional CNTs supported metal nanoparticles catalysts exhibit great catalytic activity, such as Pt [20], Ti [21], Au [22], Ag [23], Sn [24], and Pd [25]. A reasonable Pd loading (5–10 wt.%) presents good catalytic activity, in addition, a high dispersion of palladium nanoparticles on the CNTs is also required because the catalysis reaction occurs only on the surface of the palladium nanoparticles [14]. Therefore, CNTs supported Pd nanoparticles catalysts could be exploited to prepare a novel electrode material for the determination of bromate. However, the electrocatalytic detection and reduction of bromate on Pd/CNTs catalyst has not been reported yet. In this study, palladium/multiwall carbon nanotubes (Pd/ MWNTs) composite was prepared by in situ chemical method. The morphology and composite of catalysts were characterized by Transmission electron microscopy (TEM), Energy dispersive Xray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS). The Pd/MWNTs modified electrode was utilized to study the determination and reduction of bromate by cyclic voltammetry, multi-potential step measurement, and chronoamperometric measurement. Effects of different factors, including scan rate, temperature and initial concentration of bromate ion were studied as well.
stirred at room temperature for another 2 h to ensure the completion of the reaction. The mixture was then filtered, washed with DD water, and dried in a vacuum oven at 338 K. Finally, this sample was dispersed in 10 mL distilled deionized water under ambient condition.
2. Experimental
3. Results and discussion
2.1. Materials
3.1. Electron micrographs of Pd/MWNTs nanomaterial
All the chemicals used in this research were analytical grade and no purification was needed. Potassium bromate (P99.5%), palladium chloride, and potassium borohydride were purchased from Fluka. Multiwall carbon nanotubes (MWNTs, 10–30 lm in length and 8 nm in diameter) were obtained from Chengdu Organic Chemicals Co., Ltd., Chinese Academy of Sciences, China. Distilled deionized (DD) water (18.25 MX cm) was employed in all experiments.
The morphology of catalysts was obtained by TEM. Fig. 1a shows a representative TEM image of the oxidized multiwall carbon nanotubes (o-MWNTs). The surface of the carbon nanotubes is smooth. It is well known that acidic purification of MWNTs introduces a large number of carboxylic acids moieties (–COOH) onto the pristine defect sites located on the surface of MWNTs. As shown in Fig. 1b, a mass of spherical nanoparticles, with an average diameter of 4.5 nm and particle size range of 3–9 nm (Fig. 1c), is uniformly deposited on MWNTs. Fig. 1d shows an intense palladium peak at 3.25 keV, demonstrating that Pd nanoparticles were loaded on the multiwall carbon nanotubes microparticles, and the amount of Pd loading is 8.35 wt.%. X-ray photoelectron spectroscopy (XPS) analysis was performed to obtain the electronic state of different element in Pd/MWNTs catalyst. As shown in Fig. 1e, obvious peaks of C 1 s, O 1 s and Pd 3d are observed in the XPS spectrum of Pd/MWNTs nanocomposite. The enlarged XPS spectrum is illustrated in Fig. 1f, which reveals the presence of Pd 3d5/2 and 3d3/2 peaks at binding energies of 334.6 and 339.9 eV, respectively. These binding energy data are in accordance with the data reported for metallic Pd [27]. Combined with the analysis of TEM, EDX and XPS, the metallic-state Pd nanoparticles have deposited on the MWNTs successfully, and the interaction between MWNTs and Pd nanoparticles is strong chemisorption [28].
2.2. Pretreatment of MWNTs Pristine MWNTs were pretreated in a hot mixture solution of concentrated HNO3 and H2SO4 (1:3 by volume) for 4 h under refluxing condition to remove impurities and generate surface functional groups. Then the mixture was isolated by filtration, rinsed thoroughly with DD water, and dried in an oven at 338 K. Pretreatment of MWNTs surface restraints the poisoning caused by foreign impurities, and generates functional groups which could improve the electrocatalytic activity and dispersibility [26]. Surface-oxidized MWNTs (o-MWNTs) samples were stored at room temperature. 2.3. Synthesis of Pd/MWNTs composite o-MWNTs (10 mg) were dispersed in 10 ml distilled deionized water. Then, PdCl2 aqueous solution was added. The mixture took ultrasound treatment for 20 min in order to obtain a uniform solution. Later 0.1 M KBH4 was slowly added into the solution under ultrasonic treatment for 20 min, then the reaction mixture was
2.4. Electrochemical measurement Model CHI 840B electrochemical workstation was employed for cyclic voltammetry characterization, multi-potential step measurement, and chronoamperometry. A three-electrode cell was used in the electrochemical measurements. The glassy carbon (GC) electrode was used as the working electrode. A platinum foil (1 cm2) was used as counter-electrode and the reference-electrode was saturated potassium chloride electrode. The glassy carbon electrode was first polished with a slurry of 0.5 lm alumina and rinsed with distilled deionized water, then 5 lL Pd/MWNTs mixture solution was dropped onto GC electrode, and dried at 338 K under drying oven. 2.5. Characterization The particle size and morphology of the catalyst were determined by Transmission electron microscopy (TEM). TEM observations were carried out on a JEOL JEM-200CX transmission electron microscope operated at an accelerated voltage of 200 keV. The composition of the catalyst was analyzed through Energy dispersive Xray spectroscopy (EDX, Hitachi S-3400N II). The X-ray photoelectron spectroscopy (XPS) was analyzed on a Thermo ESCALAB 250 spectrometer with an Al Ka X-ray source (1486.6 eV). The sample was kept on a sample holder under a vacuum of about 107 Pa.
3.2. Effects of Pd nanoparticles in the catalyst on bromate electroreduction The electrochemical response of hydrogen has been extensively used to investigate the redox property of metal nanoparticles
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Fig. 1. (a) TEM of o-MWNTs, (b)TEM of Pd/MWNTs, (c) the size distribution histograms of Pd nanoparticles deposited on MWNTs, (d) EDX data of Pd/MWNTs catalyst, (e) XPS spectra of Pd/MWNTs composite and (f) the palladium 3d region of XPS.
modified electrode. The electrochemical reaction of hydrogen atom in the tests can be divided into two steps. Firstly, the hydrogen ions are absorbed onto the surface of modified electrode, and this phenomenon is called ‘‘dissolution adsorption mechanism’’ [29]. Then the absorbed hydrogen ions diffuse into the inside of metal nanoparticles bulk and the redox reaction occurs. It is well known that the easy agglomeration of naked palladium nanoparticles restrains their electrochemical activity. Besides, the activity of naked palladium is lower than that of the palladium with supporters [30,31]. The cyclic voltammograms of MWNTs and Pd/ MWNTs catalysts at the scan rate of 50 mV s1 in 0.5 M H2SO4 solution are compared in Fig. 2. Due to the absorptive function and ionic exchange of pure MWNTs, it presents certain redox activity (Fig. 2a). As shown in Fig. 2b, the reduction peaks at 0.4 V and 0.35 V correspond to the reduction of absorbed hydrogen ions and the desorption of hydrogen inside the catalyst, respectively [29]. In the positive scan direction, the peak at 0.15 V corresponds to the absorption of hydrogen ions. Meanwhile, it can be observed that the area of ‘‘redox region’’ of the Pd/MWNTs catalyst is larger than that of pure MWNTs. This could be attributed to the good dispersity and decoration of palladium nanoparticles on the surface of MWNTs,
Fig. 2. Cyclic voltammograms of (a) MWNTs and (b) Pd/MWNTs catalysts in 0.5 M H2SO4 solution at the scan rate of 50 mV s1.
leading to the promotion of the catalytic activity due to the catalytic performance of palladium for C–C bond forming [14].
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Accordingly, Pd/MWNTs modified electrode has high electrocatalytic activity. To study its electrochemical performance, cyclic voltammograms of Pd/MWNTs catalyst at the scan rate of 50 mV s1 in 0.5 M H2SO4 solution without and with 10 mM bromate were recorded and the results are shown in Fig. 3. An obvious cathodic peak (marked by arrow) can be observed in the presence of bromate (solid line), which is attributed to the reduction of bromate. The above analyses reveal that the electroreduction of bromate occurs within the potential range of 0.25–0.1 V. Additionally, the oxidation peak at 0.4 V and 0.9 V is attributed to the absorption of bromate ions and oxidation of Pd nanoparticles. Therefore, a controlled potential region of 0.6–0.6 V was employed in the following tests, so as to avoid Pd oxidation while maintaining the catalytic activity of metallic state Pd. 3.3. Electroanalytical characterization of Pd/MWNTs catalyst Generally, electrocatalytic reduction of bromate needs high overpotential. The Pd/MWNTs modified electrode can lower the potential energy needed for bromate reduction. In order to study the mechanisms of bromate electroreduction, we performed the cyclic voltammograms under different scan rates, temperatures and initial concentrations of bromate ions. Fig. 4 shows the cyclic voltammograms of the Pd/MWNTs catalyst in 0.5 M H2SO4 solution with 10 mM KBrO3 at different scan rates. Bromate reduction peak potentials shift negatively and the corresponding peak currents increase with the increase of the scan rate, and the relationship between the bromate reduction peak current ip and the square root of scan rate v1/2 is shown in the inset in Fig. 4a. A linear relation can be obtained in a range of 10– 100 mV s1 and the R2 is 0.9821. The results demonstrate that the bromate electroreduction on Pd/MWNTs modified electrode is a diffusion-controlled process [14]. Additionally, the absolute values of peak potential Ep increase linearly with the increase of the logarithm of scan rate log (v) (Fig. 4b). This indicates that the electroreduction of bromate is a completely irreversible electrode process. For the irreversible electron-transfer electrode process, the slope of the linearity between Ep and log (v) can be obtained from the following equation [32]:
K¼
RT 2F ana
ð1Þ
where a means the coefficient of electron transfer and na stands for electrons number involved in the rate determined step. Hence the slope of Ep vs. log (v) in Fig. 4b is determined to be 64 mV, and naa is worked out to be 0.21. Besides, the relation between scan rate (v) and reduction peak current (ip) can be expressed as follows:
Fig. 3. Cyclic voltammograms of Pd/MWNTs catalyst in 0.5 M H2SO4 solution (dash line) and in 0.5 M H2SO4 solution with 10 mM KBrO3 (solid line) at the scan rate of 50 mV s1.
Fig. 4. (a) Cyclic voltammograms of Pd/MWNTs modified electrode in 0.5 M H2SO4 solution with 10 mM KBrO3 at different scan rates of 10, 20, 30, 40, 50, 60, 70 and 100 mV s1 (pointed by arrow), the inset in this picture shows the plotting of the corresponding peak current vs. scan rate. (b) Bromate reduction peak potential vs. the logarithm of scan rate.
ip ¼ ð0:4958 103 ÞnF 3=2 ðRTÞ1=2 ðana Þ1=2 ACD1=2 v 1=2
ð2Þ
where D presents the diffusion coefficient, A stands for the real surface area of the working electrode, and it was obtained that D = 2.826 105 cm2 s1 [33]. Then the number of the electron transfer (n = 6.28) could be calculated, and the overall reduction reac tion of bromate can be written as BrO 3 þ H þ 6e ! Br þ H2 O. The high stability of Br is one of the main reasons why the overall reaction is totally irreversible. Consulting the reported bromate electroreduction mechanism on metal electrode and the result obtained in our test, we proposed the bromate reduction mechanism on Pd/MWNTs interface and the scheme of the reaction is shown in Fig. 5. In the oxidative process (positive scan), H+ and BrO 3 are first absorbed onto the surface of the Pd nanoparticles. Then, in the reductive process (negative scan) (inset in Fig. 5), the absorbed BrO 3 is reduced to Br while electrons transfer through Pd nanoparticles. An obvious bromate reduction peak (peak 1) was found, which can be used for bromate determination. Then, the residual oxygen reacts with the absorbed hydrogen ions, and water molecules are produced (peak 2). Finally, water molecule and Br are desorbed from the surface of Pd nanoparticles. Temperature is also one of the most important factors affecting the reaction rate. The effect of temperature on the electrocatalytic performance of Pd/MWNTs modified electrode was investigated in the range of 278–318 K by cyclic voltammetry and the results are depicted in Fig. 6. As shown in Fig. 6, the rising of temperature leads to a shift of the reduction peak towards more negative potential and the peak currents increase simultaneously. The more
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Fig. 5. Proposed mechanism for the electroreduction of bromate on Pd/MWNTs modified electrode in acidic solution. Inset shows the cyclic voltammogram of Pd/ MWNTs modified electrode in 0.5 M H2SO4 solution with 10 mM KBrO3.
feasibility of electroreduction and the increase of charge transfer rate at electrode/electrolyte interface at elevated temperatures could be responsible for this phenomenon. Additionally, a linear relationship between temperature and reduction current can be observed in the insert. These results indicate that bromate electroreduction is an endothermic reaction. In order to study the feasibility of bromate detection by using Pd/MWNTs modified electrode, cyclic voltammograms were obtained at the scan rate of 50 mV s1 in 0.5 M H2SO4 solution with different concentrations of bromate. It can be observed that adding different amounts of bromate to the cell caused a regular change in the cyclic voltammograms. Increasing the concentration of bromate resulted in a decrease in reduction peak potential and a concomitant increase in corresponding peak current. As can be seen from the inset in Fig. 7, the reduction peak current, as a function of the bromate concentration, produces a linear line with a correlation coefficient of 0.9939 in the range of 1–30 mM bromate. The Pd/MWNTs modified electrode is sensitive to bromate ions. Therefore, from a practical point of view, the Pd/MWNTs modified electrode can be applied for the quantitative determination of bromate.
Fig. 7. Cyclic voltammograms of Pd/MWNTs modified electrode in 0.5 M H2SO4 solution with addition of different amounts of KBrO3 (1, 3, 5, 7, 9, 13, 15, 17, 23, 25 and 30 mM as pointed by the arrow) at the scan rate of 50 mV s1. Inset in the picture shows the plotting of the corresponding reduction peak current vs. initial concentration of bromate ion.
The effect of applied potential on the reduction current by Pd/ MWNTs catalyst was explored by the multi-potential step measurement and the results are summarized in Fig. 8. As obtained in Fig. 8, the amount of reduction current is determined by the
applied potential. There is no reduction current in the sulfuric acid system in the region of 0.2 to 0.2 V in Fig. 8a, indicating that electroreduction reaction does not occur in this potential region. Moving the applied potential to a further negative value (i.e.0.3 V) led to an increase in the reduction current, which is caused by the electroreduction of absorbed hydrogen ions. As shown in Fig. 8b, the steady-state reduction current increases constantly as the applied potential decreases from 0.15 to 0 V. When shifting the potential towards negative value of 0 to 0.25 V, the responsive current which is attributed to the reduction of bromate remains unchanged. Subsequently, the increase of the reduction current again at 0.3 V is owned to the beginning of hydrogen ions electroreduction. On the basis of the analyses above, an applied potential of 0.1 V was chosen for chronoamperometry. To investigate the stability of Pd/MWNTs catalyst (stored under ambient condition for 2 months), the chronoamperometry measurement at a potential of 0.1 V in 0.5 M H2SO4 solution with 10 mM KBrO3 is shown in Fig. 9. Slight fluctuation of reduction current can be observed, which is ascribed to the bit exfoliation of palladium nanoparticles in the catalyst. Besides, the general reduction current decreases and reaches a steady state subsequently. The Pd/ MWNTs catalyst exhibits high electrocatalytic activity, and the constant current demonstrates its satisfying stability. Through the analysis of the I–t curve, it can be concluded that bromate reduction products may not poison the Pd/MWNTs catalyst and the nature of Pd/MWNTs catalyst does not change, indicating that
Fig. 6. Cyclic voltammograms of Pd/MWNTs modified electrode in 0.5 M H2SO4 solution with 10 mM KBrO3 at different temperatures (278, 288, 298, 308 and 318 K) at the scan rate of 50 mV s1. Inset shows plot of the corresponding peak current vs. the electrolyte temperature.
Fig. 8. The multi-potential step curves of the Pd/MWNTs catalyst (a) in 0.5 M H2SO4 solution and (b) in 0.5 M H2SO4 solution with 10 mM KBrO3. Step decrease: 0.05 V; step time: 10 s.
3.4. Determination of bromate by Pd/MWNTs catalyst
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Fig. 9. Chronoamperometric curve of Pd/MWNTs catalyst in 0.5 M H2SO4 solution with 10 mM KBrO3 at 0.1 V.
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and was applied for bromate determination. The TEM, EDX and XPS analyses indicated that the application of functional MWNTs as template material could restrain the aggregation of Pd nanoparticles, resulting in the uniform deposition of Pd nanoparticles in metallic-state. During the reductive process, bromate reduction peak was observed, and the reduction process occurred in the region of 0.15 to 0.25 V. The electrochemical analyses of the different effect factors showed that bromate electroreduction process is a completely irreversible electrode process controlled by mass transfer. The relation between the reduction peak intensity and bromate concentration agrees with a linear behavior, with a correlation coefficient of 0.9939. The Pd/MWNTs modified electrode was used to fabricate a simple electrochemical sensor, and it was found that bromate could be detected in a wide linear range (0.1–40 mM) within a short response time (5 s). The above results validate a great application prospect of Pd/MWNTs catalyst for constructing a high performance bromate detector. Acknowledgements This work was funded by the Natural Science Foundation of China (No. 51008154), the Jiangsu Natural Science Foundation (No. SBK201022682), the Fundamental Research Funds for the Central Universities (No. 1112021101), the Scientific Research Foundation of Graduate School of Nanjing University (No. 2012CL19). References
Fig. 10. Chronoamperometric measurements at a Pd/MWNTs modified electrode obtained in 0.5 M H2SO4 solution with increasing concentration of bromate ions in 2 mM step (except 0.1 mM, 0.5 mM, begin with 1 mM). Inset in this picture shows plotting of the responsive current vs. the corresponding concentration of bromate ion.
the Pd/MWNTs modified electrode can be employed for longtime electroreduction of bromate. To evaluate the electrochemical sensing performance of the Pd/ MWNTs catalyst, the amperometric behaviors of the modified electrode toward the bromate reduction under potentiostatic condition (applied potential is 0.1 V) are investigated. Typical chronoamperometric profiles (I–t curves) recorded can be seen in Fig. 10. In addition, a responsive reduction current can be obtained promptly considering the fact that the electrolyte concentration changes within a short response time (5 s). The fast response is attributed to the short penetration depth of bromate ion and the highly active film of the modified electrode. The diagram of the responsive current vs. the electrolyte concentration is shown in the inset in Fig. 10. A linear relationship can be observed in the concentration range of 0.1–40 mM, and the correlation coefficient is >0.999. Additionally, the sensitivity of the sensor obtained from the slope (inset in Fig. 10) of the linear relationship of the calibration curve is 768.08 lA mM1 cm2, which is as good as the reported literature [34]. Therefore, the Pd/MWNTs modified electrode is suitable for amperometric determination of bromate in this concentration region. This suggests Pd/MWNTs catalyst as a valid candidate for bromated detection.
4. Conclusion In this work, Pd/MWNTs catalyst with favorable activity and stability was successfully prepared by in situ synthesis method
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