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Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation
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Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation a,** € K. Volkan Ozdokur , Alper Yalın Tatlı a, Buket Yılmaz a, Su¨leyman Koc¸ak b,*, F. Nil Ertas‚ a a b
_ Ege University Faculty of Science, Department of Chemistry, Bornova, 35100, Izmir, Turkey Celal Bayar University, Faculty of Science and Art, Department of Chemistry, 45040, Manisa, Turkey
article info
abstract
Article history:
Manganese and molybdenum mixed oxides were co-deposited in a thin film form by pulsed
Received 12 July 2015
deposition technique on a glassy carbon substrate, and this mixed oxide film was further
Received in revised form
decorated with platinum nanoparticles. Formaldehyde, being a candidate for proton ex-
21 February 2016
change membrane fuel cell applications, was chosen as the test material for the catalytic
Accepted 22 February 2016
activities of the developed surface in alkaline media. The synergetic effect of the mixed
Available online xxx
metal oxide deposit incorporating Pt nanoparticles was verified by using different mol ratios of the corresponding metal ions and applying pulsed deposition conditions and
Keywords:
under optimized conditions and, resultant oxidation peak has shown a significant increase
Pulsed deposition
in the peak current accompanied by the small shift in the peak potential. The modified
Mixed metal oxide
composite electrodes were characterized by SEM, EDX, XPS and EIS.
Manganese oxide
Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Molybdenum oxide Platinum nanoparticle Formaldehyde
Introduction Electrochemical nanotechnologies have witnessed great fundamental advances in the last two decades and hold tremendous potential for electronic, electrochromic, catalytic and analytical applications. A vast number of studies on to the electrode development have been focused on metallic and
carbon based nanomaterials for sensor development, electrocatalysis, and energy storage and conversion studies [1]. Platinum is well-known to exhibit superior catalytic activity on both reactions of hydrogen evolution (HER) and oxygen reduction (ORR). Still, the necessity of cost reduction and improvement of the performance of conventional Ptbased catalysts has led to the development of multicomponent catalysis systems. Currently, nano-sized carbon based
* Corresponding author. Tel.: þ902362013162; fax: þ902362412158. ** Corresponding author. Tel.: þ902323111780; fax: þ902323888294. € E-mail addresses: [email protected] (K.V. Ozdokur), [email protected] (S. Koc¸ak). http://dx.doi.org/10.1016/j.ijhydene.2016.02.127 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. € Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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polymeric composites are very popular for oxygen reduction along with non-noble metal electro-catalysts [2]. Transition metal oxides, in particular, offer a wide range of applications in various fields due to their abundant sources and low cost. Molybdenum and manganese oxides have received a special attention owing to their remarkable electronic, catalytic, and electrochromic properties depending on the synthesis procedure. Electrochemical techniques present advantages over chemical synthesis being more practical and economical way for readily producing large uniform oxide thin films. In addition, the nature of the deposit can also be controlled by changing the deposition parameters and a mixed-valent metal oxide (MeOx) film can be produced by this means on a glassy carbon electrode (GCE) or indium tin oxide (ITO) electrodes. As consistent with the X-ray Photoelectron Spectroscopy (XPS) studies, mixed valent nature of molybdenum oxide (MoOx) deposit including Mo(V) and Mo(VI) has exhibited unique catalytic activity toward nitrite oxidation and ORR as well [3e5]. Further improvement in the catalytic effect can be provided by the combination of hyper d-electronic of noble metals, platinum in particular, with hypo delectronic transition metal oxides [6]. Recent studies in this lab have revealed the synergetic effect of molybdenum [7] or manganese oxide [8] and platinum binary catalyst modified glassy carbon electrode (GCE/MeOx/ Pt) towards ORR in weakly acidic medium. Similar to mixed valent molybdenum oxides, elucidation of the ORR mechanisms of the MnOx has revealed that the couple Mn(III)/Mn(IV) was responsible from the reaction with the O2 adsorbed on the GCE. Electrochemical Pulsed Deposition (PD) technique is currently becoming popular as the pattern of applied potential determines the compositions and hence the morphologies of the MeOx surfaces [9]. PD techniques favor the formation of nucleation sites and hence contribute to a high dispersion of the deposits compared to other methods. Fine nano rods of MnOx deposited by this technique have been reported to display high electrical conductivity and excellent stability [10]. Electrode performance on both anodic and cathodic reactions can be further enhanced by decorating with metallic nanoparticles and carbon nanotubes [11]. Although there are increasing numbers of studies dealing with simultaneous anodic deposition of mixed MeOx, no single data was observed in the literature for their co-deposition by PD technique. Present study describes a procedure for electrochemical co-deposition of MnOx-MoOx by PD technique which was further decorated with Pt nanoparticles (GCE/ MnOx-MoOx/Pt) in pursue of any synergetic effect for electrocatalytic activity. Anodic performance of these novel electrodes was tested upon formaldehyde oxidation which constitutes an intermediate product of methanol oxidation. Formaldehyde (FA) has been assigned as the future candidate for proton exchange membrane (PEM) fuel cells applications [12]. Depending on the electrode material, two mechanisms have been proposed for its electro-oxidation; one is the direct pathway ending up with CO2 formation and another pathway in which the intermediates passivate the active site of catalysts leading the slow reaction kinetics [13]. This reaction has been investigated at various electrode surfaces modified with Pt [14], Pd [15], Au [16], SnO2/Pt [17], Pt/
Fe2O3 [18] and incorporating Pt and Pd metals into a polymeric composite including polypyrrole (PPy) and carbon nanotube (CNT) (PPy/CNT/Pt-Pd) electrodes [19]. Metalemetal oxide nanoparticles are particularly attractive for FA oxidation reaction probably due to their high surface area and size dependent nature [14]. Consequently, development of such catalytic surfaces is an important task for not only fuel cell technology, but also for removal of FA from waste water due to its toxicity. This study presents the early results of the catalytic behavior of mixed transition MeOx deposits decorated with platinum nanoparticles towards formaldehyde oxidation in alkaline media.
Experimental Reagents All chemicals used were of analytical grade and used without any further purification. MnSO4.H20 Na2MoO4 2H2O, NaOH, K4Fe(CN)6, Na2SO4 were purchased from Merck and K2PtCl4 was purchased from Aldrich. All solutions were prepared by using ultrapure water (with 18.2 MU cm resistivity) obtained from MilliPore Q system.
Apparatus Electrochemical measurements were carried out by using Autolab PGSTAT 204 and EIS measurements Autolab PGSTAT 128N voltammetric analyzer, equipped with conventional three electrode system consisting of Ag/AgCl (sat. KCl) as the reference electrode, Pt wire (99.99%) as the counter electrode and GCE (BASi, 3 mm diameter, geometric surface area 0.0707 cm2) as the working electrode. The SEM images were recorded by using FEI Quanta 250 SEM. XPS surface chemical analyses were performed with a Thermo Electron K-Alpha spectrometer using a monochromatic Al K X-ray source.
Electrode modification Under optimal conditions, 20.0 mL of the supporting electrolyte (0.1 M Na2SO4) containing 2.0 103 M MnSO4 and 8.0 102 M Na2MoO4 solution mixture was pipetted into a voltammetric cell and deaerated with N2 gas for 5 min. Freshly polished GCE was immersed into the cell and Ag/AgCl and Pt electrodes were connected to complete the three electrode system. Pulsed deposition procedure was applied by sequentially holding the potential at 0.25 V for 5 s and then at 1.05 V for 5 s for 100 times as described in a former study [20]. The fabricated mixed metal oxide electrode, i.e. GCE/MnOx-MoOx, was rinsed with pure water and transferred to another cell containing 2 103 M PtCl2 4 solution for decorating the surface with platinum nanoparticles. The same PD procedure was applied for 20 cycles for fabricating the GCE/MnOx-MoOx/ Pt electrode and then, the electrode was rinsed and transferred into 0.1 M NaOH solutions spiked with FA standard solution to be 0.060 M. Cyclic voltammograms have been recorded by scanning the potential at a rate of 50 mV s1 between 0.5 and 0.7 V.
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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2,5e+5
Results and discussion
O1s 2,0e+5
Characterization of the surface
C1s Counts / s
Initial studies were devoted to the characterization of the binary metal oxide surfaces decorated with Pt nanoparticles according to the PD procedure given above. Then, the catalytic activity of the electrode towards formaldehyde oxidation was tested in alkaline media to reveal any potential for its future use in fuel cell.
1,0e+5
5,0e+4
Mo3d
0,0 1200
The backscattering SEM images of the electrodes have revealed a rather irregular distribution of mixed oxides with dandelion-like appearance of MoOx with needle-like MnOx rods forming a structure quite resembling the actual dandelion flower as given in Fig. 1a and b. Clearly, electrochemical PD technique has provided homogenous distribution of Pt nanoparticles with a radius less than 100 nm (Fig. 1c). The SEM-EDS data has revealed the metal content of the film as given inset (Fig. 1d). As shown in Fig. 2, the XPS survey scan spectra of GCE/ MoOx-MnOx/Pt mix films are observed at Mn(2p) (646 eV), O(1s) (532 eV) and Mo(3d) (232 eV). The Mo(3d) spectrum presents a
Mn2p3
Na1s
1,5e+5
1000
800
600
400
200
0
Binding Energy / eV
Fig. 2 e XPS survey scan spectra of the GCE/MoOx-MnOx/Pt.
doublet at 235.87 and 232.97 eV corresponding to the 3d3/2 and 3d5/2 states. These peaks are symmetrical and the ratio of their peak areas (3d5/2/3d3/2) was calculated as 3:2 [5,20]. All molybdenum ions in the oxide are in the Mo(VI) and Mo(V) oxidation state. The Mn(2p) spectrum presents a triplet at 646.00, 654.33 and 642.59 eV. Overall data indicate the incorporation of Mo into the Mn oxide at the formation of MoOxMnOx mixed film [6]. No signal was observed for Pt particles in
Fig. 1 e SEM images for (a, b) GCE/MoOx-MnOx, c) GCE/MoOx-MnOx/Pt, and EDX spectra of d) GCE/MoOx-MnOx, e) GCE/MoOxMnOx/Pt. € Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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Fig. 2 probably due to their trace level on the binary oxide film. In fact, their presence has been approved by the EDX spectroscopy (Fig. 1e). Electrochemical Impedance Spectroscopy (EIS) measurements are proven to be useful for the characterization of electrode processes and complex interfaces. The charge transfer resistance (Rct) values of the fabricated surfaces were determined by using EIS measurements. For this purpose, 10.0 mL of 0.1 M NaNO3 solution containing 5 mM [Fe(CN)6]3/ 4 was pipetted into the cell and subsequently Nyquist curves plotted at a frequency range from 0.05 to 50,000 Hz for the bare GCE, GCE/MoOx-MnOx and GCE/MoOx-MnOx/Pt electrodes. As shown in Fig. 3, the data was fitted to an equivalent electrical circuit which includes the solution resistance (R1), charge transfer resistance (Rct), Warburg impedance (W) and the capacitance of the double layer (Cdl). A large semicircle (380 U) was obtained for the bare GCE as predicted due to its relatively high resistance for electron transfer (Fig. 3a). Upon PD modification of the electrode with mixed metal oxide film, much smaller semicircle corresponding to 102 U was obtained for GCE/MoOx-MnOx (Fig. 3b). The enhancement in the conductivity of the surface can be attributed to the structural alterations with pulsed deposition along with the synergetic interaction between the mixed valent metal oxides. Further decoration with Pt nanoparticles (Fig. 3c) has leaded a drastic decrease in the Rct down to 1 U.
Electrocatalytic activity of the GCE/MnOx-MoOx/Pt electrode is expected to be dependent on the deposition mode and the cell composition as well. Hence, the synergetic effect of the mixed metal oxide deposit was verified by using different mol ratios of the corresponding metal ions and applying PD conditions as given above. These electrodes were inserted into 0.1 M NaOH solution spiked with standard solution of FA to be 0.060 M and then, cyclic voltammograms were recorded. Fig. 4a shows the dependence of oxidation peak current at
Electrode performance towards formaldehyde oxidation In aqueous media, FA mainly exists in the hydrated form and its solvation product, methylene glycol (CH2(OH)2), ionizes in alkaline solution since it has a pKa of 12.8 [21]. Therefore, the influence of pH on the oxidation signal of FA was investigated by using 0.1 M HClO4, BR buffers at pH 4.0, 7.0 and 10.0 and 0.1 M NaOH solutions and corresponding peak currents were recorded as 0.07, 0.10, 0.28, 0.43 and 1.27 mA, respectively. This increase in the peak current was accompanied by the shift in the peak potential in this pH range starting from 0.62 to 0.17 V indicating a pH-dependent oxidation reaction. Since the ionized form is electroactive, further experiments were carried out in 0.1 M NaOH solutions.
Fig. 3 e Nyquist curves obtained for a) bare GCE b) GCE/ MoOx-MnOx, c) GCE/MoOx-MnOx/Pt.
Fig. 4 e Dependence of FA oxidation peak current on a) Mn:Mo mole ratio and the effect of number of cycle in the potential range for b) MnOx-MoOx and c) Platinum deposition.
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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2þ 0.17 V on the electrode modified in different MoO2 4 /Mn mol ratio in the cell. Further studies were conducted with 40:1 ratio since the best signal was observed. The number of repetitive cycles used in the PD constitutes another critical parameter since it determines the thickness of the deposit. Fig. 4b shows the influence of cycle number used in deposition step of mixed metal oxide film on the FA oxidation signal. The PD was affected at 0.25 V and subsequently at 1.05 V for 5 s each for a cycle range of 10e150. In the further stage, 0.01 M HCl solution containing 2 103 M PtCl2 4 was utilized and 20 cycles were applied in the same potential range. Then, the prepared electrodes were inserted into 0.1 M NaOH solution spiked with 0.060 M FA for recording the cyclic voltammograms. As consistent with Fig. 4b, best results were obtained with 50 cycles for mixed metal oxide deposition. Similarly, the effect of the cycle number used in PD of Pt nanoparticles on the FA signal was examined. It is known that these particles tend to aggregate on the surface eventually converting to bulk deposit for prolonged depositions. Therefore, it is necessary to optimize their size by tuning the electrochemical deposition conditions. As it is evident from Fig. 4c, the oxidation peak current of 0.060 M FA has given a maximum in 20 cycles and then, decreased for further cycle numbers as described above. Under optimized conditions, the kinetics of the FA oxidation process on the GCE/MoOx-MnOx/Pt electrode was examined by means of Tafel plots and by comparing the binary metal oxide electrode. The effect of the cell temperature on the oxidation process was explored at 10, 20 and 30 C by plotting the potential to cover the mixed controlled region against logarithm value of the current (Fig. 5). A linear relationship was observed for GCE/MoOx-MnOx/Pt electrode in the potential range of 0.30 and 0.48 V vs. Ag/AgCl (sat. KCl). On the other hand, in the absence of Pt nanoparticles the binary GCE/MoOx-MnOx electrode has displayed a curved relationship in a rather narrow potential range. Tafel slopes for FA oxidation in the presence and absence Pt nanoparticles on the electrode surfaces are given in Table 1. Thus, it can be deduced from the slope values that the GCE/MoOx-MnOx/Pt electrode
shows a remarkable catalytic activity towards FA electrooxidation [22]. Cyclic voltammograms overlaid in Fig. 6 show the progress in the catalytic performance of all the electrodes studied. The inset on the top left clearly shows that bare GCE exhibits a flat response for FA (Fig. 6a) but, upon modification the electrode surface with PD deposition of mixed metal oxide, a small oxidation peak was observed for FA around 0.2 V followed by another wide peak at 0.2 V (Fig. 6b). In the back scan, a reverse peak specific to the oxidation of an intermediate species was observed at 0.3 V. To clarify the effect of each step in the modification process, the GCE decorated with Pt nanoparticles alone was also tested. An oxidation peak around 0.2 V has appeared with a reverse peak at the same potential (Fig. 6c). Much higher signals were observed for GCE/MoOx/Pt electrode (Fig. 6d) and a similar but, relatively larger response was gained by GCE/ MnOx/Pt (Fig. 6e) electrodes. The best signal, on the other hand, was obtained with mixed metal oxides decorated with Pt nanoparticles and the signal was doubled in comparison to the GCE/Pt electrode indicating high catalytic activity of the modified surface. Another parameter to reveal the catalytic activity apart from the peak current is the position of the peak. As can be deduced from the Fig. 6, the peak potentials have been found about the same for GCE/Pt and GCE/MoOx/Pt electrodes but, for by GCE/MnOx/Pt and GCE/MoOx-MnOx/Pt electrodes both anodic and reverse cathodic peaks have been positioned at slightly more positive potentials (Fig. 6f). For comparison with the previous studies, the oxidation potential of FA at various electrodes reported in the literature has been listed in Table 2. As can be followed from the table, the surfaces modified with nanoparticles of noble metals such as Pt and Pd exhibit a catalytic effect towards FA oxidation where the peak is positioned in positive potentials except Pdmodified TiO2 electrode which gives an oxidation peak at 0.123 V. This FA oxidation peak was observed 0.17 V with GCE/MnOx-MoOx/Pt electrode developed in the present study. Thus, GCE/MnOx-MoOx/Pt electrode showed superior catalytic effect on FA oxidation not only in terms of peak current but also in terms of oxidation potential. As pointed out above, two mechanisms have been proposed for FA oxidation depending on the electrode material. Safavi et al. [29] have reported that electrode fouling intermediate, i.e. adsorbed CO, requires higher electrode potential to be further oxidized into CO2 and therefore FA oxidation at low potentials is inhibited. In direct mechanism, FA is directly oxidized via short-lived intermediate to the final product of CO2. In addition, reverse peaks appear in the anodic scan indicating the contribution of both mechanisms of FA
Table 1 e Tafel slopes of FA for electro-catalyst at the different temperatures. Temperature ( C)
Tafel slope GCE/MoOx-MnOx GCE/MoOx-MnOx/Pt
Fig. 5 e Tafel plots drawn for GCE/MnOx-MoOx and for GCE/ MnOx-MoOx/Pt electrodes at different temperatures (scan rate: 3 mV¡1).
10 20 30
0.064 0.075 0.099
0.196 0.207 0.228
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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Conclusion
Fig. 6 e Cyclic voltammograms recorded in 0.1 M NaOH in the presence of 0.0602 M FA at a) the bare GCE, b) GCE/ MoOx-MnOx, c) GCE/Pt, d) GCE/MoOx/Pt, e) GCE/MnOx/Pt, f) GCE/MoOx-MnOx/Pt electrodes with scan rate of 50 mV s¡1.
This study presents the early results of the catalytic behavior of mixed deposits of transition metal oxides decorated with platinum nanoparticles towards formaldehyde oxidation in alkaline media to reveal any potential for its future use in fuel cell. In addition to the synergetic effect of co-deposited mixed valent metal oxides, the electrochemical pulsed deposition mode has provided a better catalytic surface formation and the GCE/MoOx-MnOx/Pt electrode has displayed the highest peak currents catalytic activity for FA oxidation among the electrodes studied as the signal was doubled in comparison to the GCE/Pt electrode. The peak potentials obtained with the fabricated electrode was also compared with those obtained in the former studies in the literature and it was revealed that electrodes modified with nanoparticles of copper and nickel composites have displayed oxidation at positive potentials more than 0.6 V where the noble metallic nanoparticles such as Pt and Pd exhibit a catalytic effect towards FA oxidation
Table 2 e Comparison of the oxidation peak potentials of formaldehyde with other modified electrodes. Electrode
Supporting electrolyte
Ni/poly (o-toluidine)/Triton X-100 film modified carbon nanotube paste electrode Ni/poly (m-toluidine)/CTAB modified carbon paste electrode Cu-poly (2-aminodiphenylamine) composite Pt/poly (2-Methoxyaniline)-sodium dodecyl sulfate composite electrode Pt nanoparticles ion-implanted-modified ITO electrode Pd nanowire arrays electrode Hollow porous Pd nanoparticles on a GCE Pd nanoparticles on carbon ionic liquid electrode Pd-modified TiO2 electrode GCE/MnOx-MoOx/Pt
0.1 0.1 0.2 0.5 0.1 0.1 0.1 0.1 0.1 0.1
M NaOH M NaOH M NaOH M H2SO4 M NaOH M KOH M KOH M NaOH M NaOH M NaOH
Peak potential
Reference
0.70 V 0.70 V 0.63 V 0.24 V 0.128 V 0.05 V 0.04 V 0.03 V 0.123 V 0.17 V
[23] [24] [25] [26] [27] [28] [15] [29] [30] This study
oxidation in the overall process. On the other hand, the increased reaction rates are attributed to the inhibition of CO adsorption on the electrode surface [29] during the FA oxidation. Faster kinetics revealed by Tafel plots suggests that GCE/ MoOx-MnOx/Pt electrode surface favors the mechanism in direct path. Therefore, reverse peak formation along with the higher slopes obtained from Tafel curves supporters that both mechanisms are involved.
Stability of the electrode The long term stability of modified electrode material is an important factor for practical applications. To evaluate the stability of the GCE/MoOx-MnOx/Pt, a chronoamperometric experiment was carried with 0.1 M NaOH solution containing 0.060 M FA out at 0.1 V Fig. 7 shows that as the potential was applied, the anodic current has rapidly diminished in the first 50 s and then a plateau was observed around 0.2 mA in the time span of 1000 s. The plot of current versus t1/2 derived from the data of chronoamperogram shows that the GCE/ MoOx-MnOx/Pt electrode possesses excellent long-term stability toward FA oxidation [31].
Fig. 7 e Chronoamperometric curves recorded at a) GCE/ MoOx-MnOx/Pt and b) GCE/MoOx-MnOx electrodes in 0.1 M NaOH containing 0.060 M FA. Plot of I vs. t¡1/2 is given in inset. where the peak is positioned at less positive potentials. Thus, GCE/MnOx-MoOx/Pt electrode showed superior catalytic
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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effect on FA oxidation not only in terms of peak current but also in terms of oxidation potential. Also, the electrode possesses excellent long-term stability toward FA oxidation.
Acknowledgment Authors would like to thank to Scientific and Technological Research Council of Turkey TUBITAK 2209 project for financial support.
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[15] Guo Y, Xu Y-T, Gao G-H, Wang T, Zhao B, Fu X-Z, et al. Electro-oxidation of formaldehyde and methanol over hollow porous palladium nanoparticles with enhanced catalytic activity. Catal Commun 2015;58:40e5. [16] Li H-F, Zhang N, Chen P, Luo M-F, Lu J-Q. High surface area Au/CeO2 catalysts for low temperature formaldehyde oxidation. Appl Catal B 2011;110:279e85. [17] Villullas HM, Mattos-Costa FI, Nascente PAP, Bulhoes LOS. Anodic oxidation of formaldehyde on Pt-modified SnO2 thin film electrodes prepared by a solegel method. Electrochim Acta 2004;49(22e23):3909e16. [18] An N, Wu P, Li S, Jia M, Zhang W. Catalytic oxidation of formaldehyde over PtiFe(2)O(3) catalysts prepared by different method. Appl Surf Sci 2013;285:805e9. [19] Selvaraj V, Alagar M, Kumar KS. Synthesis and characterization of metal nanoparticles-decorated PPY-CNT composite and their electrocatalytic oxidation of formic acid and formaldehyde for fuel cell applications. Appl Catal B 2007;75(1e2):129e38. [20] Nakayama M, Tanaka A, Sato Y, Tonosaki T, Ogura K. Electrodeposition of manganese and molybdenum mixed oxide thin films and their charge storage properties. Langmuir 2005;21(13):5907e13. [21] Ojani R, Raoof J-B, Safshekan S. Electrocatalytic oxidation of formaldehyde on nickel modified ionic liquid carbon paste electrode as a simple and efficient electrode. J Appl Electrochem 2012;42(2):81e7. [22] Sun W, Sun G, Qin B, Xin Q. A fuel-cell-type sensor for detection of formaldehyde in aqueous solution. Sens Actuators B 2007;128:193e8. [23] Raoof J-B, Ojani R, Abdi S, Hosseini SR. Highly improved electrooxidation of formaldehyde on nickel/poly (otoluidine)/Triton X-100 film modified carbon nanotube paste electrode. Int J Hydrogen Energy 2012;37:2137e46. [24] Raoof JB, Karimi MA, Hosseini SR, Mangelizade S. Enhanced electrocatalytic activity of nickel particles electrodeposited onto poly (m-toluidine) film prepared in presence of CTAB surfactant on carbon paste electrode for formaldehyde oxidation in alkaline medium. Int J Hydrogen Energy 2011;36:13281e7. [25] Ojani R, Raoof JB, Ahmady-Khanghah Y, Safshekan S. Copper-poly (2-aminodiphenylamine) composite as catalyst for electrocatalytic oxidation of formaldehyde in alkaline media. Int J Hydrogen Energy 2013;38:5457e63. [26] Raoof J-B, Hosseini SR, Rezaee S. Preparation of Pt/poly (2Methoxyaniline)-sodium dodecyl sulfate composite and its application for electrocatalytic oxidation of methanol and formaldehyde. Electrochim Acta 2014;141:340e8. [27] Yu Y, Wang T, Fu Y, Su W, Hu J. Platinum nanoparticles ionimplanted-modified indium tin oxide electrode for electrocatalytic oxidation of formaldehyde. Int J Hydrogen Energy 2014;39:17617e21. [28] Zhang Y, Zhang M, Cai Z, Chen M, Cheng F. A novel electrochemical sensor for formaldehyde based on palladium nanowire arrays electrode in alkaline media. Electrochim Acta 2012;68:172e7. [29] Safavi A, Maleki N, Farjami F, Farjami E. Electrocatalytic oxidation of formaldehyde on palladium nanoparticles electrodeposited on carbon ionic liquid composite electrode. J Electroanal Chem 2009;626:75e9. [30] Yi Q, Niu F, Yu W. Pd-modified TiO2 electrode for electrochemical oxidation of hydrazine, formaldehyde and glucose. Thin Solid Films 2011;519:3155e61. [31] Ojani R, Raoof J-B, Safshekan S. Photoinduced deposition of palladium nanoparticles on TiO2 nanotube electrode and investigation of its capability for formaldehyde oxidation. Electrochim Acta 2014;138:468e75.
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation a,** € K. Volkan Ozdokur , Alper Yalın Tatlı a, Buket Yılmaz a, Su¨leyman Koc¸ak b,*, F. Nil Ertas‚ a a b
_ Ege University Faculty of Science, Department of Chemistry, Bornova, 35100, Izmir, Turkey Celal Bayar University, Faculty of Science and Art, Department of Chemistry, 45040, Manisa, Turkey
article info
abstract
Article history:
Manganese and molybdenum mixed oxides were co-deposited in a thin film form by pulsed
Received 12 July 2015
deposition technique on a glassy carbon substrate, and this mixed oxide film was further
Received in revised form
decorated with platinum nanoparticles. Formaldehyde, being a candidate for proton ex-
21 February 2016
change membrane fuel cell applications, was chosen as the test material for the catalytic
Accepted 22 February 2016
activities of the developed surface in alkaline media. The synergetic effect of the mixed
Available online xxx
metal oxide deposit incorporating Pt nanoparticles was verified by using different mol ratios of the corresponding metal ions and applying pulsed deposition conditions and
Keywords:
under optimized conditions and, resultant oxidation peak has shown a significant increase
Pulsed deposition
in the peak current accompanied by the small shift in the peak potential. The modified
Mixed metal oxide
composite electrodes were characterized by SEM, EDX, XPS and EIS.
Manganese oxide
Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.
Molybdenum oxide Platinum nanoparticle Formaldehyde
Introduction Electrochemical nanotechnologies have witnessed great fundamental advances in the last two decades and hold tremendous potential for electronic, electrochromic, catalytic and analytical applications. A vast number of studies on to the electrode development have been focused on metallic and
carbon based nanomaterials for sensor development, electrocatalysis, and energy storage and conversion studies [1]. Platinum is well-known to exhibit superior catalytic activity on both reactions of hydrogen evolution (HER) and oxygen reduction (ORR). Still, the necessity of cost reduction and improvement of the performance of conventional Ptbased catalysts has led to the development of multicomponent catalysis systems. Currently, nano-sized carbon based
* Corresponding author. Tel.: þ902362013162; fax: þ902362412158. ** Corresponding author. Tel.: þ902323111780; fax: þ902323888294. € E-mail addresses: [email protected] (K.V. Ozdokur), [email protected] (S. Koc¸ak). http://dx.doi.org/10.1016/j.ijhydene.2016.02.127 0360-3199/Copyright © 2016, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. € Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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polymeric composites are very popular for oxygen reduction along with non-noble metal electro-catalysts [2]. Transition metal oxides, in particular, offer a wide range of applications in various fields due to their abundant sources and low cost. Molybdenum and manganese oxides have received a special attention owing to their remarkable electronic, catalytic, and electrochromic properties depending on the synthesis procedure. Electrochemical techniques present advantages over chemical synthesis being more practical and economical way for readily producing large uniform oxide thin films. In addition, the nature of the deposit can also be controlled by changing the deposition parameters and a mixed-valent metal oxide (MeOx) film can be produced by this means on a glassy carbon electrode (GCE) or indium tin oxide (ITO) electrodes. As consistent with the X-ray Photoelectron Spectroscopy (XPS) studies, mixed valent nature of molybdenum oxide (MoOx) deposit including Mo(V) and Mo(VI) has exhibited unique catalytic activity toward nitrite oxidation and ORR as well [3e5]. Further improvement in the catalytic effect can be provided by the combination of hyper d-electronic of noble metals, platinum in particular, with hypo delectronic transition metal oxides [6]. Recent studies in this lab have revealed the synergetic effect of molybdenum [7] or manganese oxide [8] and platinum binary catalyst modified glassy carbon electrode (GCE/MeOx/ Pt) towards ORR in weakly acidic medium. Similar to mixed valent molybdenum oxides, elucidation of the ORR mechanisms of the MnOx has revealed that the couple Mn(III)/Mn(IV) was responsible from the reaction with the O2 adsorbed on the GCE. Electrochemical Pulsed Deposition (PD) technique is currently becoming popular as the pattern of applied potential determines the compositions and hence the morphologies of the MeOx surfaces [9]. PD techniques favor the formation of nucleation sites and hence contribute to a high dispersion of the deposits compared to other methods. Fine nano rods of MnOx deposited by this technique have been reported to display high electrical conductivity and excellent stability [10]. Electrode performance on both anodic and cathodic reactions can be further enhanced by decorating with metallic nanoparticles and carbon nanotubes [11]. Although there are increasing numbers of studies dealing with simultaneous anodic deposition of mixed MeOx, no single data was observed in the literature for their co-deposition by PD technique. Present study describes a procedure for electrochemical co-deposition of MnOx-MoOx by PD technique which was further decorated with Pt nanoparticles (GCE/ MnOx-MoOx/Pt) in pursue of any synergetic effect for electrocatalytic activity. Anodic performance of these novel electrodes was tested upon formaldehyde oxidation which constitutes an intermediate product of methanol oxidation. Formaldehyde (FA) has been assigned as the future candidate for proton exchange membrane (PEM) fuel cells applications [12]. Depending on the electrode material, two mechanisms have been proposed for its electro-oxidation; one is the direct pathway ending up with CO2 formation and another pathway in which the intermediates passivate the active site of catalysts leading the slow reaction kinetics [13]. This reaction has been investigated at various electrode surfaces modified with Pt [14], Pd [15], Au [16], SnO2/Pt [17], Pt/
Fe2O3 [18] and incorporating Pt and Pd metals into a polymeric composite including polypyrrole (PPy) and carbon nanotube (CNT) (PPy/CNT/Pt-Pd) electrodes [19]. Metalemetal oxide nanoparticles are particularly attractive for FA oxidation reaction probably due to their high surface area and size dependent nature [14]. Consequently, development of such catalytic surfaces is an important task for not only fuel cell technology, but also for removal of FA from waste water due to its toxicity. This study presents the early results of the catalytic behavior of mixed transition MeOx deposits decorated with platinum nanoparticles towards formaldehyde oxidation in alkaline media.
Experimental Reagents All chemicals used were of analytical grade and used without any further purification. MnSO4.H20 Na2MoO4 2H2O, NaOH, K4Fe(CN)6, Na2SO4 were purchased from Merck and K2PtCl4 was purchased from Aldrich. All solutions were prepared by using ultrapure water (with 18.2 MU cm resistivity) obtained from MilliPore Q system.
Apparatus Electrochemical measurements were carried out by using Autolab PGSTAT 204 and EIS measurements Autolab PGSTAT 128N voltammetric analyzer, equipped with conventional three electrode system consisting of Ag/AgCl (sat. KCl) as the reference electrode, Pt wire (99.99%) as the counter electrode and GCE (BASi, 3 mm diameter, geometric surface area 0.0707 cm2) as the working electrode. The SEM images were recorded by using FEI Quanta 250 SEM. XPS surface chemical analyses were performed with a Thermo Electron K-Alpha spectrometer using a monochromatic Al K X-ray source.
Electrode modification Under optimal conditions, 20.0 mL of the supporting electrolyte (0.1 M Na2SO4) containing 2.0 103 M MnSO4 and 8.0 102 M Na2MoO4 solution mixture was pipetted into a voltammetric cell and deaerated with N2 gas for 5 min. Freshly polished GCE was immersed into the cell and Ag/AgCl and Pt electrodes were connected to complete the three electrode system. Pulsed deposition procedure was applied by sequentially holding the potential at 0.25 V for 5 s and then at 1.05 V for 5 s for 100 times as described in a former study [20]. The fabricated mixed metal oxide electrode, i.e. GCE/MnOx-MoOx, was rinsed with pure water and transferred to another cell containing 2 103 M PtCl2 4 solution for decorating the surface with platinum nanoparticles. The same PD procedure was applied for 20 cycles for fabricating the GCE/MnOx-MoOx/ Pt electrode and then, the electrode was rinsed and transferred into 0.1 M NaOH solutions spiked with FA standard solution to be 0.060 M. Cyclic voltammograms have been recorded by scanning the potential at a rate of 50 mV s1 between 0.5 and 0.7 V.
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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2,5e+5
Results and discussion
O1s 2,0e+5
Characterization of the surface
C1s Counts / s
Initial studies were devoted to the characterization of the binary metal oxide surfaces decorated with Pt nanoparticles according to the PD procedure given above. Then, the catalytic activity of the electrode towards formaldehyde oxidation was tested in alkaline media to reveal any potential for its future use in fuel cell.
1,0e+5
5,0e+4
Mo3d
0,0 1200
The backscattering SEM images of the electrodes have revealed a rather irregular distribution of mixed oxides with dandelion-like appearance of MoOx with needle-like MnOx rods forming a structure quite resembling the actual dandelion flower as given in Fig. 1a and b. Clearly, electrochemical PD technique has provided homogenous distribution of Pt nanoparticles with a radius less than 100 nm (Fig. 1c). The SEM-EDS data has revealed the metal content of the film as given inset (Fig. 1d). As shown in Fig. 2, the XPS survey scan spectra of GCE/ MoOx-MnOx/Pt mix films are observed at Mn(2p) (646 eV), O(1s) (532 eV) and Mo(3d) (232 eV). The Mo(3d) spectrum presents a
Mn2p3
Na1s
1,5e+5
1000
800
600
400
200
0
Binding Energy / eV
Fig. 2 e XPS survey scan spectra of the GCE/MoOx-MnOx/Pt.
doublet at 235.87 and 232.97 eV corresponding to the 3d3/2 and 3d5/2 states. These peaks are symmetrical and the ratio of their peak areas (3d5/2/3d3/2) was calculated as 3:2 [5,20]. All molybdenum ions in the oxide are in the Mo(VI) and Mo(V) oxidation state. The Mn(2p) spectrum presents a triplet at 646.00, 654.33 and 642.59 eV. Overall data indicate the incorporation of Mo into the Mn oxide at the formation of MoOxMnOx mixed film [6]. No signal was observed for Pt particles in
Fig. 1 e SEM images for (a, b) GCE/MoOx-MnOx, c) GCE/MoOx-MnOx/Pt, and EDX spectra of d) GCE/MoOx-MnOx, e) GCE/MoOxMnOx/Pt. € Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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Fig. 2 probably due to their trace level on the binary oxide film. In fact, their presence has been approved by the EDX spectroscopy (Fig. 1e). Electrochemical Impedance Spectroscopy (EIS) measurements are proven to be useful for the characterization of electrode processes and complex interfaces. The charge transfer resistance (Rct) values of the fabricated surfaces were determined by using EIS measurements. For this purpose, 10.0 mL of 0.1 M NaNO3 solution containing 5 mM [Fe(CN)6]3/ 4 was pipetted into the cell and subsequently Nyquist curves plotted at a frequency range from 0.05 to 50,000 Hz for the bare GCE, GCE/MoOx-MnOx and GCE/MoOx-MnOx/Pt electrodes. As shown in Fig. 3, the data was fitted to an equivalent electrical circuit which includes the solution resistance (R1), charge transfer resistance (Rct), Warburg impedance (W) and the capacitance of the double layer (Cdl). A large semicircle (380 U) was obtained for the bare GCE as predicted due to its relatively high resistance for electron transfer (Fig. 3a). Upon PD modification of the electrode with mixed metal oxide film, much smaller semicircle corresponding to 102 U was obtained for GCE/MoOx-MnOx (Fig. 3b). The enhancement in the conductivity of the surface can be attributed to the structural alterations with pulsed deposition along with the synergetic interaction between the mixed valent metal oxides. Further decoration with Pt nanoparticles (Fig. 3c) has leaded a drastic decrease in the Rct down to 1 U.
Electrocatalytic activity of the GCE/MnOx-MoOx/Pt electrode is expected to be dependent on the deposition mode and the cell composition as well. Hence, the synergetic effect of the mixed metal oxide deposit was verified by using different mol ratios of the corresponding metal ions and applying PD conditions as given above. These electrodes were inserted into 0.1 M NaOH solution spiked with standard solution of FA to be 0.060 M and then, cyclic voltammograms were recorded. Fig. 4a shows the dependence of oxidation peak current at
Electrode performance towards formaldehyde oxidation In aqueous media, FA mainly exists in the hydrated form and its solvation product, methylene glycol (CH2(OH)2), ionizes in alkaline solution since it has a pKa of 12.8 [21]. Therefore, the influence of pH on the oxidation signal of FA was investigated by using 0.1 M HClO4, BR buffers at pH 4.0, 7.0 and 10.0 and 0.1 M NaOH solutions and corresponding peak currents were recorded as 0.07, 0.10, 0.28, 0.43 and 1.27 mA, respectively. This increase in the peak current was accompanied by the shift in the peak potential in this pH range starting from 0.62 to 0.17 V indicating a pH-dependent oxidation reaction. Since the ionized form is electroactive, further experiments were carried out in 0.1 M NaOH solutions.
Fig. 3 e Nyquist curves obtained for a) bare GCE b) GCE/ MoOx-MnOx, c) GCE/MoOx-MnOx/Pt.
Fig. 4 e Dependence of FA oxidation peak current on a) Mn:Mo mole ratio and the effect of number of cycle in the potential range for b) MnOx-MoOx and c) Platinum deposition.
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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2þ 0.17 V on the electrode modified in different MoO2 4 /Mn mol ratio in the cell. Further studies were conducted with 40:1 ratio since the best signal was observed. The number of repetitive cycles used in the PD constitutes another critical parameter since it determines the thickness of the deposit. Fig. 4b shows the influence of cycle number used in deposition step of mixed metal oxide film on the FA oxidation signal. The PD was affected at 0.25 V and subsequently at 1.05 V for 5 s each for a cycle range of 10e150. In the further stage, 0.01 M HCl solution containing 2 103 M PtCl2 4 was utilized and 20 cycles were applied in the same potential range. Then, the prepared electrodes were inserted into 0.1 M NaOH solution spiked with 0.060 M FA for recording the cyclic voltammograms. As consistent with Fig. 4b, best results were obtained with 50 cycles for mixed metal oxide deposition. Similarly, the effect of the cycle number used in PD of Pt nanoparticles on the FA signal was examined. It is known that these particles tend to aggregate on the surface eventually converting to bulk deposit for prolonged depositions. Therefore, it is necessary to optimize their size by tuning the electrochemical deposition conditions. As it is evident from Fig. 4c, the oxidation peak current of 0.060 M FA has given a maximum in 20 cycles and then, decreased for further cycle numbers as described above. Under optimized conditions, the kinetics of the FA oxidation process on the GCE/MoOx-MnOx/Pt electrode was examined by means of Tafel plots and by comparing the binary metal oxide electrode. The effect of the cell temperature on the oxidation process was explored at 10, 20 and 30 C by plotting the potential to cover the mixed controlled region against logarithm value of the current (Fig. 5). A linear relationship was observed for GCE/MoOx-MnOx/Pt electrode in the potential range of 0.30 and 0.48 V vs. Ag/AgCl (sat. KCl). On the other hand, in the absence of Pt nanoparticles the binary GCE/MoOx-MnOx electrode has displayed a curved relationship in a rather narrow potential range. Tafel slopes for FA oxidation in the presence and absence Pt nanoparticles on the electrode surfaces are given in Table 1. Thus, it can be deduced from the slope values that the GCE/MoOx-MnOx/Pt electrode
shows a remarkable catalytic activity towards FA electrooxidation [22]. Cyclic voltammograms overlaid in Fig. 6 show the progress in the catalytic performance of all the electrodes studied. The inset on the top left clearly shows that bare GCE exhibits a flat response for FA (Fig. 6a) but, upon modification the electrode surface with PD deposition of mixed metal oxide, a small oxidation peak was observed for FA around 0.2 V followed by another wide peak at 0.2 V (Fig. 6b). In the back scan, a reverse peak specific to the oxidation of an intermediate species was observed at 0.3 V. To clarify the effect of each step in the modification process, the GCE decorated with Pt nanoparticles alone was also tested. An oxidation peak around 0.2 V has appeared with a reverse peak at the same potential (Fig. 6c). Much higher signals were observed for GCE/MoOx/Pt electrode (Fig. 6d) and a similar but, relatively larger response was gained by GCE/ MnOx/Pt (Fig. 6e) electrodes. The best signal, on the other hand, was obtained with mixed metal oxides decorated with Pt nanoparticles and the signal was doubled in comparison to the GCE/Pt electrode indicating high catalytic activity of the modified surface. Another parameter to reveal the catalytic activity apart from the peak current is the position of the peak. As can be deduced from the Fig. 6, the peak potentials have been found about the same for GCE/Pt and GCE/MoOx/Pt electrodes but, for by GCE/MnOx/Pt and GCE/MoOx-MnOx/Pt electrodes both anodic and reverse cathodic peaks have been positioned at slightly more positive potentials (Fig. 6f). For comparison with the previous studies, the oxidation potential of FA at various electrodes reported in the literature has been listed in Table 2. As can be followed from the table, the surfaces modified with nanoparticles of noble metals such as Pt and Pd exhibit a catalytic effect towards FA oxidation where the peak is positioned in positive potentials except Pdmodified TiO2 electrode which gives an oxidation peak at 0.123 V. This FA oxidation peak was observed 0.17 V with GCE/MnOx-MoOx/Pt electrode developed in the present study. Thus, GCE/MnOx-MoOx/Pt electrode showed superior catalytic effect on FA oxidation not only in terms of peak current but also in terms of oxidation potential. As pointed out above, two mechanisms have been proposed for FA oxidation depending on the electrode material. Safavi et al. [29] have reported that electrode fouling intermediate, i.e. adsorbed CO, requires higher electrode potential to be further oxidized into CO2 and therefore FA oxidation at low potentials is inhibited. In direct mechanism, FA is directly oxidized via short-lived intermediate to the final product of CO2. In addition, reverse peaks appear in the anodic scan indicating the contribution of both mechanisms of FA
Table 1 e Tafel slopes of FA for electro-catalyst at the different temperatures. Temperature ( C)
Tafel slope GCE/MoOx-MnOx GCE/MoOx-MnOx/Pt
Fig. 5 e Tafel plots drawn for GCE/MnOx-MoOx and for GCE/ MnOx-MoOx/Pt electrodes at different temperatures (scan rate: 3 mV¡1).
10 20 30
0.064 0.075 0.099
0.196 0.207 0.228
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
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Conclusion
Fig. 6 e Cyclic voltammograms recorded in 0.1 M NaOH in the presence of 0.0602 M FA at a) the bare GCE, b) GCE/ MoOx-MnOx, c) GCE/Pt, d) GCE/MoOx/Pt, e) GCE/MnOx/Pt, f) GCE/MoOx-MnOx/Pt electrodes with scan rate of 50 mV s¡1.
This study presents the early results of the catalytic behavior of mixed deposits of transition metal oxides decorated with platinum nanoparticles towards formaldehyde oxidation in alkaline media to reveal any potential for its future use in fuel cell. In addition to the synergetic effect of co-deposited mixed valent metal oxides, the electrochemical pulsed deposition mode has provided a better catalytic surface formation and the GCE/MoOx-MnOx/Pt electrode has displayed the highest peak currents catalytic activity for FA oxidation among the electrodes studied as the signal was doubled in comparison to the GCE/Pt electrode. The peak potentials obtained with the fabricated electrode was also compared with those obtained in the former studies in the literature and it was revealed that electrodes modified with nanoparticles of copper and nickel composites have displayed oxidation at positive potentials more than 0.6 V where the noble metallic nanoparticles such as Pt and Pd exhibit a catalytic effect towards FA oxidation
Table 2 e Comparison of the oxidation peak potentials of formaldehyde with other modified electrodes. Electrode
Supporting electrolyte
Ni/poly (o-toluidine)/Triton X-100 film modified carbon nanotube paste electrode Ni/poly (m-toluidine)/CTAB modified carbon paste electrode Cu-poly (2-aminodiphenylamine) composite Pt/poly (2-Methoxyaniline)-sodium dodecyl sulfate composite electrode Pt nanoparticles ion-implanted-modified ITO electrode Pd nanowire arrays electrode Hollow porous Pd nanoparticles on a GCE Pd nanoparticles on carbon ionic liquid electrode Pd-modified TiO2 electrode GCE/MnOx-MoOx/Pt
0.1 0.1 0.2 0.5 0.1 0.1 0.1 0.1 0.1 0.1
M NaOH M NaOH M NaOH M H2SO4 M NaOH M KOH M KOH M NaOH M NaOH M NaOH
Peak potential
Reference
0.70 V 0.70 V 0.63 V 0.24 V 0.128 V 0.05 V 0.04 V 0.03 V 0.123 V 0.17 V
[23] [24] [25] [26] [27] [28] [15] [29] [30] This study
oxidation in the overall process. On the other hand, the increased reaction rates are attributed to the inhibition of CO adsorption on the electrode surface [29] during the FA oxidation. Faster kinetics revealed by Tafel plots suggests that GCE/ MoOx-MnOx/Pt electrode surface favors the mechanism in direct path. Therefore, reverse peak formation along with the higher slopes obtained from Tafel curves supporters that both mechanisms are involved.
Stability of the electrode The long term stability of modified electrode material is an important factor for practical applications. To evaluate the stability of the GCE/MoOx-MnOx/Pt, a chronoamperometric experiment was carried with 0.1 M NaOH solution containing 0.060 M FA out at 0.1 V Fig. 7 shows that as the potential was applied, the anodic current has rapidly diminished in the first 50 s and then a plateau was observed around 0.2 mA in the time span of 1000 s. The plot of current versus t1/2 derived from the data of chronoamperogram shows that the GCE/ MoOx-MnOx/Pt electrode possesses excellent long-term stability toward FA oxidation [31].
Fig. 7 e Chronoamperometric curves recorded at a) GCE/ MoOx-MnOx/Pt and b) GCE/MoOx-MnOx electrodes in 0.1 M NaOH containing 0.060 M FA. Plot of I vs. t¡1/2 is given in inset. where the peak is positioned at less positive potentials. Thus, GCE/MnOx-MoOx/Pt electrode showed superior catalytic
€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 6 ) 1 e7
effect on FA oxidation not only in terms of peak current but also in terms of oxidation potential. Also, the electrode possesses excellent long-term stability toward FA oxidation.
Acknowledgment Authors would like to thank to Scientific and Technological Research Council of Turkey TUBITAK 2209 project for financial support.
references
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€ Please cite this article in press as: Ozdokur KV, et al., Development of pulsed deposited manganese and molybdenum oxide surfaces decorated with platinum nanoparticles and their catalytic application for formaldehyde oxidation, International Journal of Hydrogen Energy (2016), http://dx.doi.org/10.1016/j.ijhydene.2016.02.127