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A simple and effective route for preparation of platinum nanoparticle and its application for electrocatalytic oxidation of methanol and formaldehyde
Journal of Molecular Liquids 212 (2015) 767–774
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
A simple and effective route for preparation of platinum nanoparticle and its application for electrocatalytic oxidation of methanol and formaldehyde Jahan-Bakhsh Raoof a,⁎, Sayed Reza Hosseini b, Sharifeh Rezaee a a b
Electroanalytical Research Laboratory, Department of Analytical Chemistry, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran
a r t i c l e
i n f o
Article history: Received 28 March 2015 Received in revised form 29 September 2015 Accepted 16 October 2015 Available online xxxx Keywords: Galvanic replacement reaction Nickel nanoparticle Platinum nanoparticle Electrocatalytic oxidation
a b s t r a c t In this work, a new and facile method for preparation of platinum nanoparticles (Pt-NPs) at the surface of glassy carbon electrode (GCE) is reported. Firstly, nickel nanoparticles (Ni-NPs) as sacrificial templates are uniformly electrodeposited onto the GCE by using potentiostatic method at a fixed potential (− 1.0 V vs. Ag│AgCl│KCl (3 M)) in 0.5 M H2SO4 solution. Then, Pt-NPs are prepared through a spontaneous and irreversible process via and Ni-NPs. The obtained results indicate that the galvanic replacement reaction (GRR) between PtCl2− 6 Ni-NPs are completely replaced with Pt-NPs in acid solution. The obtained results from cyclic voltammetry and chronoamperometry reveal that the Pt-NPs demonstrate an enhanced electrocatalytic activity for methanol and formaldehyde oxidation. The effect of several parameters such as electrodepositing potential, NiSO4 concentration, electrodeposition and GRR time for methanol oxidation as well as stability of the modified electrode has been investigated. © 2015 Elsevier B.V. All rights reserved.
1. Introduction To date, among the different transition metal particles, platinum (Pt) due to its unique physical and chemical characteristics, including high durability and specific activity is quite noticeable from others as electrocatalyst [1,2]. However, the high cost and limited supply of Pt are limiting factors. So, the use of Pt particles at nano-scale by controlling the morphology and surface structure is an effective way to reduce the amount of the required Pt and enhance the electrochemical activity [3,4]. Since, catalysis is a surface phenomenon; Pt particles with small sizes demonstrate high activity and display excellent resistance against poisoning effect by COads species, which are formed during the methanol oxidation. Formation of multiple Pt-carbon bonds is less at Pt particles with small sizes, in comparison with bulk Pt [5,6]. So, researches have been focused on the exploration of novel synthetic routes to prepare ultrafine nanostructured Pt catalysts. So far, most studies have been focused on the diverse physical and chemical methods for fabrication of fine noble metal nanoparticles [7]. By considering the potential preparation methods for production of the metal NPs, one of the most plausible ways is metal exchange reaction [8,9]. Galvanic replacement reaction (GRR) as one of typical and
⁎ Corresponding author. E-mail address: [email protected] (J.-B. Raoof).
http://dx.doi.org/10.1016/j.molliq.2015.10.031 0167-7322/© 2015 Elsevier B.V. All rights reserved.
convenient route was widely used for the synthesis of metal NPs by applying particles of various transition metals such as Ni, Cu, Fe, and Co [10,11]. This method has attracted much attention due to its simplicity of operation, cost effectiveness and simple equipment. The GRR is a single step and irreversible reaction which is reliant on the difference in standard electrode potentials of the various elements [12]. The spontaneous reaction occurs between the solid metal (i.e., template) and ions of a second metal having higher electrode potential which leads to the deposition of more noble elements and dissolution of the less noble components. Until now, many works have been made towards the synthesis of Pt nanoparticles (Pt-NPs) or designing of Pt-based bimetallic catalysts and Pt nano-materials with different shapes such as nano-cages, nano-tubes and nano-porous films by GRR with different active metals. For example, Qiu et al. [13] fabricated bimetallic Pt–Au thin films with different Pt/Au ratios by GRR with hierarchical Co thin film. Kuang et al. [14] synthesized hollow Pt nano-spheres/carbon nanotube nano-hybrids by using silver NPs. Also, Tegou et al. [15] prepared mixed Pt/Au by GRR with Ni layers. Mohl et al. [16] prepared CuPd and CuPt bimetallic nanotubes by using Cu, Pt and Pd. Xie et al. [17] synthesized Pd@MxCu1 − x (M = Au, Pd and Pt) nano-cages with porous walls and a yolk–shell structure through the GRR. Bansal et al. [18] prepared Ni–Cu nano-porous surface for catalysis by the GRR. Recently, we have made Cu–Pt bimetallic NPs at the surface of poly (8-hydroxyquinoline) film by using Cu as sacrificial and Pt as second
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metal [19]. Our literature survey indicates that the fabrication of the PtNP modified GCE (Pt-NP/MGCE) by using Ni nanoparticles (Ni-NPs) as sacrificial templates has not been reported. Hence, in the present investigation, at first the Ni-NP electrodepositing was performed onto the GCE surface and Pt-NPs were then prepared by GRR and their efficiency towards the electrocatalytic oxidation of methanol was investigated. The experimental data reveal that the Pt-NP/MGCE showed excellent performance in the methanol oxidation and formaldehyde oxidation.
the surface into the electrolyte solution. Also, in the meantime, PtIV is reduced and deposited on the electrode surface. It should be noted that during the GRR, Ni-NPs are dissolved into the solution via a reaction between Ni-NPs with H+ as a competitive reaction (4):
2. Experimental
It is obvious that the displacing metal in comparison with sacrificial metal manifests oxidation state (4:2). So, according to stoichiometry of the redox reaction, during the reaction by deposition of a Pt atom at the electrode surface, two Ni atoms leach in the surrounding aqueous environment. Hence, on the basis of both reactions (3 and 4), all deposited Ni-NPs release from the electrode surface. Afterwards, when the Pt-NP/MGCE was prepared, it was removed and rinsed with distilled water. The geometric surface area (Ag = 0.018 cm2) was used to calculate the current density, except for the comparison of the methanol oxidation and formaldehyde oxidation. All experiments were performed at ambient temperature.
2.1. Reagents and materials NiSO4·6H2O (99%, Fluka) was used in the preparation of Ni deposition solution. H2PtCl6·6H2O (Merck) and NaOH (99%, Merck) were used as received. Methanol and formaldehyde from Merck were of analytical grade. Sulfuric acid (98%, Fluka) was used for the preparation of the supporting electrolyte. K3Fe(CN)6, K4Fe(CN)6 (99%, Fluka) and KCl (99%, Merck) were used for impedance studies. The solvent used in this work for preparation of the samples was double distilled water.
0 2þ − 2NiðsÞ þ PtCl2− 6 ðaqÞ→PtðsÞ þ 2NiðaqÞ þ 6ClðaqÞ
ð3Þ
2þ NiðsÞ þ 2Hþ ðaqÞ →NiðaqÞ þ H2 ðgÞ:
ð4Þ
3. Results and discussion 2.2. Electrochemical measurements
3.1. Electrochemical properties of the modified electrodes
The electrochemical experiments were carried out by using a potentiostat/galvanostat (SAMA 500-C Electrochemical analysis system, Sama, Iran) coupled with a personal computer (PC). Electrochemical impedance spectroscopy (EIS) was performed by an Autolab model PGSTAT 30 with FRA software version 4.9 (Eco Chemie, The Netherlands). The three-electrode system consists of the GCE (1.5 mm in diameter) as working electrode substrate, Ag│AgCl│KCl (3 M) as reference electrode and a Pt wire as an auxiliary electrode. The surface morphology and elemental analysis of the deposits were evaluated by scanning electron microscopy (SEM, model VEGA-Tescan, Razi Metallurgical Research Center, Tehran, Iran) equipped with an energy dispersive spectrometer (EDS).
In order to ascertain the electrochemical response of the prepared modified electrode, after accomplishing Ni-NP electrodeposition, the Ni-NP/MGCE was thoroughly rinsed with distilled water and 15 potential cycles were conducted between 0.2 and 0.8 V in 0.1 M NaOH solution until stable cyclic voltammogram was obtained (Fig. 1A
2.3. Preparation of the Ni nanoparticles Prior to surface modification, the GCE was carefully polished with polishing cloth and alumina slurry until a mirror finish was obtained. To remove the alumina residues that might be trapped at the surface, the polished electrode was placed in ethanol and sonicated for 5 min. Then, the electrode was rinsed thoroughly with distilled water. After that, Ni-NPs were electrodeposited potentiostatically at − 1.0 V vs. Ag│AgCl│KCl (3 M) for 500 s in 0.5 M H2SO4 solution containing 0.5 M NiSO4 according to reaction (1). Also, associated with deposition of Ni-NPs, hydrogen bubbles were released via hydrogen evolution reaction as a competitive reaction according to reaction (2). − Ni2þ ðaqÞ þ 2e →NiðsÞ
ð1Þ
− 2Hþ ðaqÞ þ 2e →H2ðgÞ :
ð2Þ
2.4. Preparation of the Pt-NP/MGCE After Ni-NP electrodeposition, the electrode was immediately immersed in 0.5 M H2SO4 solution containing 5.0 mM H2PtCl6 for 20 min to let GRR between Ni0 and PtIV ions proceeds. Since, the standard reduction potential of the PtIV/Pt0 pair (+0.73 V vs. SHE) is higher than the Ni2+/Ni0 pair (−0.25 V vs. SHE), Ni-NPs can be oxidized by PtCl2− 6 according to following replacement reaction (3) and removed from
Fig. 1. (A) 15th CV signal of the Ni-NP/MGCE in 0.1 M NaOH (solid line) and 0.5 M H2SO4 solution (dotted line) at υ = 50 mV s−1. (B) CV signal of the Pt-NP/MGCE in 0.5 M H2SO4 solution at υ = 50 mV s−1.
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(solid line)). Consecutive cyclic voltammetry (CV) leads to the progressive growth of anodic and cathodic peak current density. As can be seen in this figure, the electrochemical response of the Ni-NP/MGCE exhibited well-defined anodic and cathodic peaks (Ni(III)/Ni(II) redox couple). The redox peak is ascribable to the oxidation of Ni(OH)2 to NiOOH (0.42 V) and reduction of the NiOOH to Ni(OH)2 (0.38 V). The redox process is expressed by the following reaction [20]. NiðOHÞ2 þ OH− →NiOOH þ H2 O þ e− :
ð5Þ
It should be noted that the Ni-NP/MGCE was not stable in acidic medium and when it was placed in 0.5 M H2SO4 solution; Ni-NPs were removed/dissolved from the electrode surface into the solution. According to this reason, the Ni-NP/MGCE has no electrochemical response/ activity in H2SO4 solution (Fig. 1A (dotted line)). Also, in continuous, the electrochemical response of the Pt-NPs was studied in a potential window of − 0.3 to 1.0 V in 0.5 M H2 SO 4 solution and the typical CV signal was presented in Fig. 1B. The whole of CV signal can be readily separated in five potential regions. These typical peaks can be explained as following as: in the lower potential range (I) HER, (II) weakly bonded hydrogen (Hw), (III) strongly bonded hydrogen (Hs) and furthermore in positive potential range (IV) corresponding to the formation and reduction of Pt oxide and potential region (V) is attributed to double layer [21]. Moreover, there is a negligible peak similar to bump between the peaks (I) and (II). According to literature,
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there is a reasonable explanation. The interpretation says that since the appearance of the H3rd is closely related to the start of hydrogen evolution, it can be concluded that the third anodic peak is attributed to oxidation of adsorbed molecular hydrogen which may be the intermediate in hydrogen evolution. This reaction is a slow process and one reasonable cause for this observation can be due to that the molecular hydrogen is sub-surface adsorbed and forms possibly on Pt (110) sites with surface structure, produced by oxidation–reduction cycles. So, forming partial amount of Pt (110) sites with surface structure on a platinized Pt surface presented insignificant and poor current of the H3rd [22]. The peak current of Pt oxide reduction in 0.5 M H2SO4 solution was used for evaluation of the Pt loading at the Pt-NP/MGCE by faradaic law and was estimated at about 1.85 μg cm−2. Furthermore, real surface area (Ar) of the Pt catalyst can be measured according to the following equation: Ar ¼ Q H =Q O
ð6Þ
where QH is the charge consumed for hydrogen adsorption and can be obtained by integrating in the hydrogen adsorption/desorption region. Also, QO has been commonly taken as 210 μC/real cm2. The Ar was obtained at about 0.71 cm2. Also, the roughness factor can be obtained by dividing of Ar to Ag. Hence, the value of the calculated RF is equal to 39.7.
Fig. 2. SEM images of the prepared Ni-NP/MGCE (a, b) and Pt-NP/MGCE (c, d).
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3.2. Surface morphology and elemental analysis In order to characterize the prepared electrodes, micrographs of the Ni-NP/MGCE (a, b) and Pt-NP/MGCE (c,d) were investigated by SEM (Fig. 2). Traces (a, b) show that the Ni-NPs with spherical shapes were distributed in the form of extended coverage over a large area on the GCE surface. As shown in trace (c, d), the Pt-NPs can be generated by using complete GRR of the Ni-NPs by Pt-NPs. The Pt-NPs with granular structure and spherical shapes and average sizes at about 80 nm were uniformly spread on the GCE. Fig. 3 presented a typical EDS spectrum for determination of the electrode composition. The EDS gives evidences for the presence of the Ni and Pt in the modified electrode. As can be seen in this figure, Pt and Ni in the modified electrodes are the major elements and C is derived from the GCE. Also, by comparison with (A) and (B), it can be seen that the EDS data are in accordance with obtained results from the SEM. The results reveal that the Ni-NPs are completely replaced with Pt-NPs and no Ni-NPs remain at the final modified electrode. It should be noted that the reason of Au existence is that the specimens are usually coated with an ultra-thin coating of Au before measurements.
Fig. 4. Nyquist plots of impedance measurements of 1.0 mM K3[Fe(CN)6] / K4[Fe(CN)6] + 0.1 M KCl on bare GCE (a) and Pt-NP/MGCE (b).
3.3. EIS investigation EIS is an effective and simple way for evaluating the interface properties of the modified electrodes [23]. All impedance spectra are almost similar in form, composed of a high-frequency semicircle and a lowfrequency straight line. The diameter of the semicircle is assigned to the interfacial charge transfer resistance (Rct) and a straight line is attributed to the diffusion-limited process [24]. Fig. 4 presented the
Nyquist diagrams of the bare GCE (a) and Pt-NP/MGCE (b) in the presence of 1.0 mM K3[Fe(CN)6] / K4[Fe(CN)6] (1:1) + 0.1 M KCl solution. As can be seen in this figure, there is an obvious difference between the diameters of two semicircles. The Pt-NP/MGCE has a very small semicircle domain, implying a much lower Rct than the bare GCE. There are more active sites for easier charge transfer and faradaic reactions at the interface owing to the presence of Pt-NPs. 3.4. Electrocatalytic oxidation of methanol Fig. 5 depicted the CV signals of the methanol oxidation onto the NiNP/MGCE (a) and Pt-NP/MGCE (b) in 0.5 M H2SO4 + 1.36 M CH3OH solution. As can be seen in this figure, the inactivity of the Ni-NP/MGCE for methanol oxidation is in accordance with the observation that Ni-NPs are not stable in H2SO4 medium. Methanol electrooxidation at the Pt surface exhibited the characteristic double CV peaks which appear in the forward and reverse scans [6]. It can be seen from methanol oxidation on the Pt-NP/MGCE that the onset potential is at around 0.15 V and an enormous anodic peak appears approximately at 0.73 V in forward scan which is generated by oxidation of the COads to CO2 (I). The current density was defined as current normalized per Ar and obtained about
Fig. 3. Energy dispersive spectra of the Ni-NP/MGCE (A) and Pt-NP/MGCE (B).
Fig. 5. Electrochemical responses of the Ni-NP/MGCE (a) and Pt-NP/MGCE (b) in 1.36 M CH3OH + 0.5 M H2SO4 solution at υ = 50 mV s−1.
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Scheme 1. A schematic representation of methanol oxidation at the Pt-NP/MGCE.
1.83 mA cm−2. After that, the oxidation peak current density decreases at more positive potential due to the formation of Pt oxides which passives the Pt surface. When the potential scan is reversed, the reduction of the Pt oxide to Pt takes place and clean Pt is available at the surface. Since, the methanol oxidation occurs more easily at the clean Pt surface, therefore the peak current II appears at about 0.53 V in the backward scan which is attributed to the complete oxidation of CH3OH to CO2 (II) [25]. The electrochemical oxidation of methanol was schematically shown in Scheme 1. The important advantage of the Pt particles with nanoscale sizes is high activity and excellent resistance against poisoning effect by COads species, which are formed during the methanol oxidation. This phenomenon is the most important difference of Pt-NPs in comparison with bulk Pt in acid medium. For evaluation of the electrocatalytic activity in methanol oxidation, the onset potential was listed in Table 1 and compared with the some previous works. 3.5. Parameters affecting the electrode modification In order to evaluate the effect of various parameters such as electrodepositing potential, Ni2 + concentration, electrodeposition and GRR times on the methanol oxidation, the amount of peak current density was monitored as an index for finding the optimum conditions and the obtained results were summarized in Table 2. The results indicate that the peak current increases for electrodepositing potential up to Table 1 Comparison of methanol oxidation in H2SO4 solution at the Pt/-NPs/MGCE with some modified electrodes. Electrocatalyst
CH2SO4/M
CCH3OH/M
Eop/V
Ep/V
Ref.
Pt nanorod/Tia Pt-NP/SWCNTa Nano-Pt/MGCEa Pt/MGCEa Pt/SBA-15/MGCEa PtRu/MWCNTb PtNi/CCEa Pt/CCEb Pt/SWCNTa Pt-NP/MGCEb
0.5 1.0 0.5 0.5 0.1 1.0 0.1 0.2 1.0 0.5
2.0 2.0 1.4 0.5 0.05 2.0 0.5 0.15 2.0 1.36
0.36 0.22 0.20 0.20 0.15 0.26 0.15 0.15 0.45 0.15
0.61 0.89 0.7 0.62 0.53 0.75 0.88 0.89 0.64 0.73
[26] [27] [28] [29] [30] [31] [32] [33] [34] This work
All potentials were referred to the (a) SCE and (b) Ag│AgCl│KCl (3 M).
−1.0 V, NiSO4 concentration up to 0.5 M, electrodeposition time up to 500 s and GRR time up to 20 min and drop afterwards. 3.6. Electrocatalytic oxidation of formaldehyde Formaldehyde has become more interesting with the possibility of its application in liquid fuel cells. The reasons are related to its facility of handling, storage and generally high energy density. The electrochemical oxidation of formaldehyde is important for full understanding of methanol oxidation as a product of partial oxidation of methanol [35,36]. Fig. 6 illustrates the CV signals of the Ni-NP/ MGCE (a) and Pt-NP/MGCE (b) in 0.5 M H2SO4 + 0.32 M formaldehyde. The observation indicates that the Ni-NPs are not stable in 0.5 M H2SO4 solution. Hence, Ni-NP/MGCE gave no signal for formaldehyde oxidation and this indicates that the electrode has no electrocatalytic activity and acts similar to the bare GCE (Fig. 6a). As can be seen from trace (b), formaldehyde oxidation at the Pt-NP/ MGCE starts at about 0.25 V with low current and by sweeping to more positive potentials, the peak current reaches to a maximum at about 0.65 V which is assigned to the oxidation of COads to CO 2 (peak I) [37]. After that, the oxidation peak current decreases at more positive potentials due to the formation of the Pt oxide which causes passivity of the Pt surface. When the potential scan is reversed, the reduction of Pt oxides to Pt take place and clean Pt is available. So, a large anodic peak is observed at about 0.4 V (peak II). This peak is attributed to the complete oxidation of formaldehyde to CO2 [38]. It was well accepted that the electrooxidation of formaldehyde on traditional Pt catalyst follows a dual parallel pathways. In the direct
Table 2 Effect of several parameters on the peak current density of methanol oxidation in 0.5 M H2SO4 + 0.82 M CH3OH. Ed/V j/mA cm−2 CNiSO4/M j/mA cm−2 td/s j/mA cm−2 tr/min j/mA cm−2
−0.9 2.83 0.1 6.74 100 7.86 10 15.50
−1.0 6.74 0.3 12.88 300 20.44 15 35.38
−1.1 5.33 0.5 20.44 500 35.38 20 47.22
−1.2 3.65 0.8 14.55 700 28.55 25 38.5
−1.3 1.35 1.0 10.26 900 16.11 30 24.33
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surface (dehydration pathway, Scheme 2). The poisonous COads are removed via a reaction involving OHads species originated from water dissociation. Indeed, this process is very intricate, as higher potentials are required for water activation on the Pt surface [41]. Therefore, the electrode surface will be blocked by large amounts of COads and inhibit further adsorption of the other formaldehyde molecules. The important advantage of the Pt particles with nanoscale sizes is higher activity and superior resistance against poisoning effect by COads species. For evaluation the electrocatalytic activity, the onset oxidation potential and peak potential were listed in Table 3 and electrooxidation herein has been compared with some previous works. 3.7. Stability study
Fig. 6. CV signals of the Ni-NP/MGCE (a) and Pt-NP/MGCE (b) in 0.32 M HCHO + 0.5 M H2SO4 solution at υ = 50 mV s−1.
route, formaldehyde is converted to CO2 via short-lived intermediate (dehydrogenation pathway, Scheme 2) [39,40]. In the indirect path, formaldehyde is oxidized to CO ads which is adsorbed on the Pt
For further evaluation of the catalytic activity and catalyst stability, chronoamperometric response of the Pt-NP/MGCE was recorded at peak potential values in continuous operation for methanol and formaldehyde (Fig. 7). As can be seen in this figure, current density has slightly changed and decreases gradually with time passing. The observed data reveal that the Pt-NP/MGCE has good poisoning tolerance against generated intermediates due to the Pt NPs and their fine dispersions. The Pt-NPs with small diameters provide more active sites and accessible surface area for methanol oxidation and formaldehyde oxidation.
Scheme 2. Schematic representation of formaldehyde oxidation on the Pt-NP/MGCE.
Table 3 Comparison of electrocatalytic oxidation of formaldehyde at the Pt-NP/MGCE with some modified electrodes at υ = 50 mV s−1. Modified electrode
Electrolyte
CFormaldehyde (M)
Eop/V
Ep/V
Ref.
Pt–Pd/CNT Single crystal disk of Pt (111) Pt–Pd/PPy-CNT Pt/carbon–ceramic Pt–Pd/Nf/MGCE Pt/SWCNT/PANI Pt/nano-PDAN/MGCE Pt/PAANI/MWCNTs/MGCE Pt-NP/MGCE
0.5 M H2SO4 0.1 M HClO4 0.5 M H2SO4 0.1 M H2SO4 0.1 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4
0.50 0.50 0.50 0.75 0.001 0.26 0.26 0.50 0.32
0.30 0.20 0.20 0.20 0.30 0.30 0.20 0.20 0.29
0.62 0.70 0.64 0.85 0.58 0.66 0.75 0.72 0.69
[39] [40] [42] [43] [44] [45] [46] [47] This work
All potentials were referred to SCE.
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Fig. 7. Chronoamperogram of the Pt-NP/MGCE at 0.5 M H2SO4 solution in the presence of (A) 1.36 M CH3OH at Ep = 0.73 V and (B) 0.32 M formaldehyde at Ep = 0.65 V.
4. Conclusion In this work, we have proposed a simple and quick method for preparation of the nano-scale Pt particles via GRR with Ni-NPs. The obtained results from SEM and EDS revealed the complete replacement between sacrificial metal (Ni) and noble metal (Pt). The Pt-NP/MGCE was used for electrocatalytic oxidation of methanol and formaldehyde in H2SO4 solution. It was observed that the Pt-NP/MGCE was capable to catalyze effectively these small organic molecules. The satisfactory activity is exactly connected to the Pt nanostructures, which provides more active sites and high accessible surface areas. The electrocatalysis is affected by various parameters in the modification steps.
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Contents lists available at ScienceDirect
Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
A simple and effective route for preparation of platinum nanoparticle and its application for electrocatalytic oxidation of methanol and formaldehyde Jahan-Bakhsh Raoof a,⁎, Sayed Reza Hosseini b, Sharifeh Rezaee a a b
Electroanalytical Research Laboratory, Department of Analytical Chemistry, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran Nanochemistry Research Laboratory, Faculty of Chemistry, University of Mazandaran, 47416-95447 Babolsar, Iran
a r t i c l e
i n f o
Article history: Received 28 March 2015 Received in revised form 29 September 2015 Accepted 16 October 2015 Available online xxxx Keywords: Galvanic replacement reaction Nickel nanoparticle Platinum nanoparticle Electrocatalytic oxidation
a b s t r a c t In this work, a new and facile method for preparation of platinum nanoparticles (Pt-NPs) at the surface of glassy carbon electrode (GCE) is reported. Firstly, nickel nanoparticles (Ni-NPs) as sacrificial templates are uniformly electrodeposited onto the GCE by using potentiostatic method at a fixed potential (− 1.0 V vs. Ag│AgCl│KCl (3 M)) in 0.5 M H2SO4 solution. Then, Pt-NPs are prepared through a spontaneous and irreversible process via and Ni-NPs. The obtained results indicate that the galvanic replacement reaction (GRR) between PtCl2− 6 Ni-NPs are completely replaced with Pt-NPs in acid solution. The obtained results from cyclic voltammetry and chronoamperometry reveal that the Pt-NPs demonstrate an enhanced electrocatalytic activity for methanol and formaldehyde oxidation. The effect of several parameters such as electrodepositing potential, NiSO4 concentration, electrodeposition and GRR time for methanol oxidation as well as stability of the modified electrode has been investigated. © 2015 Elsevier B.V. All rights reserved.
1. Introduction To date, among the different transition metal particles, platinum (Pt) due to its unique physical and chemical characteristics, including high durability and specific activity is quite noticeable from others as electrocatalyst [1,2]. However, the high cost and limited supply of Pt are limiting factors. So, the use of Pt particles at nano-scale by controlling the morphology and surface structure is an effective way to reduce the amount of the required Pt and enhance the electrochemical activity [3,4]. Since, catalysis is a surface phenomenon; Pt particles with small sizes demonstrate high activity and display excellent resistance against poisoning effect by COads species, which are formed during the methanol oxidation. Formation of multiple Pt-carbon bonds is less at Pt particles with small sizes, in comparison with bulk Pt [5,6]. So, researches have been focused on the exploration of novel synthetic routes to prepare ultrafine nanostructured Pt catalysts. So far, most studies have been focused on the diverse physical and chemical methods for fabrication of fine noble metal nanoparticles [7]. By considering the potential preparation methods for production of the metal NPs, one of the most plausible ways is metal exchange reaction [8,9]. Galvanic replacement reaction (GRR) as one of typical and
⁎ Corresponding author. E-mail address: [email protected] (J.-B. Raoof).
http://dx.doi.org/10.1016/j.molliq.2015.10.031 0167-7322/© 2015 Elsevier B.V. All rights reserved.
convenient route was widely used for the synthesis of metal NPs by applying particles of various transition metals such as Ni, Cu, Fe, and Co [10,11]. This method has attracted much attention due to its simplicity of operation, cost effectiveness and simple equipment. The GRR is a single step and irreversible reaction which is reliant on the difference in standard electrode potentials of the various elements [12]. The spontaneous reaction occurs between the solid metal (i.e., template) and ions of a second metal having higher electrode potential which leads to the deposition of more noble elements and dissolution of the less noble components. Until now, many works have been made towards the synthesis of Pt nanoparticles (Pt-NPs) or designing of Pt-based bimetallic catalysts and Pt nano-materials with different shapes such as nano-cages, nano-tubes and nano-porous films by GRR with different active metals. For example, Qiu et al. [13] fabricated bimetallic Pt–Au thin films with different Pt/Au ratios by GRR with hierarchical Co thin film. Kuang et al. [14] synthesized hollow Pt nano-spheres/carbon nanotube nano-hybrids by using silver NPs. Also, Tegou et al. [15] prepared mixed Pt/Au by GRR with Ni layers. Mohl et al. [16] prepared CuPd and CuPt bimetallic nanotubes by using Cu, Pt and Pd. Xie et al. [17] synthesized Pd@MxCu1 − x (M = Au, Pd and Pt) nano-cages with porous walls and a yolk–shell structure through the GRR. Bansal et al. [18] prepared Ni–Cu nano-porous surface for catalysis by the GRR. Recently, we have made Cu–Pt bimetallic NPs at the surface of poly (8-hydroxyquinoline) film by using Cu as sacrificial and Pt as second
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metal [19]. Our literature survey indicates that the fabrication of the PtNP modified GCE (Pt-NP/MGCE) by using Ni nanoparticles (Ni-NPs) as sacrificial templates has not been reported. Hence, in the present investigation, at first the Ni-NP electrodepositing was performed onto the GCE surface and Pt-NPs were then prepared by GRR and their efficiency towards the electrocatalytic oxidation of methanol was investigated. The experimental data reveal that the Pt-NP/MGCE showed excellent performance in the methanol oxidation and formaldehyde oxidation.
the surface into the electrolyte solution. Also, in the meantime, PtIV is reduced and deposited on the electrode surface. It should be noted that during the GRR, Ni-NPs are dissolved into the solution via a reaction between Ni-NPs with H+ as a competitive reaction (4):
2. Experimental
It is obvious that the displacing metal in comparison with sacrificial metal manifests oxidation state (4:2). So, according to stoichiometry of the redox reaction, during the reaction by deposition of a Pt atom at the electrode surface, two Ni atoms leach in the surrounding aqueous environment. Hence, on the basis of both reactions (3 and 4), all deposited Ni-NPs release from the electrode surface. Afterwards, when the Pt-NP/MGCE was prepared, it was removed and rinsed with distilled water. The geometric surface area (Ag = 0.018 cm2) was used to calculate the current density, except for the comparison of the methanol oxidation and formaldehyde oxidation. All experiments were performed at ambient temperature.
2.1. Reagents and materials NiSO4·6H2O (99%, Fluka) was used in the preparation of Ni deposition solution. H2PtCl6·6H2O (Merck) and NaOH (99%, Merck) were used as received. Methanol and formaldehyde from Merck were of analytical grade. Sulfuric acid (98%, Fluka) was used for the preparation of the supporting electrolyte. K3Fe(CN)6, K4Fe(CN)6 (99%, Fluka) and KCl (99%, Merck) were used for impedance studies. The solvent used in this work for preparation of the samples was double distilled water.
0 2þ − 2NiðsÞ þ PtCl2− 6 ðaqÞ→PtðsÞ þ 2NiðaqÞ þ 6ClðaqÞ
ð3Þ
2þ NiðsÞ þ 2Hþ ðaqÞ →NiðaqÞ þ H2 ðgÞ:
ð4Þ
3. Results and discussion 2.2. Electrochemical measurements
3.1. Electrochemical properties of the modified electrodes
The electrochemical experiments were carried out by using a potentiostat/galvanostat (SAMA 500-C Electrochemical analysis system, Sama, Iran) coupled with a personal computer (PC). Electrochemical impedance spectroscopy (EIS) was performed by an Autolab model PGSTAT 30 with FRA software version 4.9 (Eco Chemie, The Netherlands). The three-electrode system consists of the GCE (1.5 mm in diameter) as working electrode substrate, Ag│AgCl│KCl (3 M) as reference electrode and a Pt wire as an auxiliary electrode. The surface morphology and elemental analysis of the deposits were evaluated by scanning electron microscopy (SEM, model VEGA-Tescan, Razi Metallurgical Research Center, Tehran, Iran) equipped with an energy dispersive spectrometer (EDS).
In order to ascertain the electrochemical response of the prepared modified electrode, after accomplishing Ni-NP electrodeposition, the Ni-NP/MGCE was thoroughly rinsed with distilled water and 15 potential cycles were conducted between 0.2 and 0.8 V in 0.1 M NaOH solution until stable cyclic voltammogram was obtained (Fig. 1A
2.3. Preparation of the Ni nanoparticles Prior to surface modification, the GCE was carefully polished with polishing cloth and alumina slurry until a mirror finish was obtained. To remove the alumina residues that might be trapped at the surface, the polished electrode was placed in ethanol and sonicated for 5 min. Then, the electrode was rinsed thoroughly with distilled water. After that, Ni-NPs were electrodeposited potentiostatically at − 1.0 V vs. Ag│AgCl│KCl (3 M) for 500 s in 0.5 M H2SO4 solution containing 0.5 M NiSO4 according to reaction (1). Also, associated with deposition of Ni-NPs, hydrogen bubbles were released via hydrogen evolution reaction as a competitive reaction according to reaction (2). − Ni2þ ðaqÞ þ 2e →NiðsÞ
ð1Þ
− 2Hþ ðaqÞ þ 2e →H2ðgÞ :
ð2Þ
2.4. Preparation of the Pt-NP/MGCE After Ni-NP electrodeposition, the electrode was immediately immersed in 0.5 M H2SO4 solution containing 5.0 mM H2PtCl6 for 20 min to let GRR between Ni0 and PtIV ions proceeds. Since, the standard reduction potential of the PtIV/Pt0 pair (+0.73 V vs. SHE) is higher than the Ni2+/Ni0 pair (−0.25 V vs. SHE), Ni-NPs can be oxidized by PtCl2− 6 according to following replacement reaction (3) and removed from
Fig. 1. (A) 15th CV signal of the Ni-NP/MGCE in 0.1 M NaOH (solid line) and 0.5 M H2SO4 solution (dotted line) at υ = 50 mV s−1. (B) CV signal of the Pt-NP/MGCE in 0.5 M H2SO4 solution at υ = 50 mV s−1.
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(solid line)). Consecutive cyclic voltammetry (CV) leads to the progressive growth of anodic and cathodic peak current density. As can be seen in this figure, the electrochemical response of the Ni-NP/MGCE exhibited well-defined anodic and cathodic peaks (Ni(III)/Ni(II) redox couple). The redox peak is ascribable to the oxidation of Ni(OH)2 to NiOOH (0.42 V) and reduction of the NiOOH to Ni(OH)2 (0.38 V). The redox process is expressed by the following reaction [20]. NiðOHÞ2 þ OH− →NiOOH þ H2 O þ e− :
ð5Þ
It should be noted that the Ni-NP/MGCE was not stable in acidic medium and when it was placed in 0.5 M H2SO4 solution; Ni-NPs were removed/dissolved from the electrode surface into the solution. According to this reason, the Ni-NP/MGCE has no electrochemical response/ activity in H2SO4 solution (Fig. 1A (dotted line)). Also, in continuous, the electrochemical response of the Pt-NPs was studied in a potential window of − 0.3 to 1.0 V in 0.5 M H2 SO 4 solution and the typical CV signal was presented in Fig. 1B. The whole of CV signal can be readily separated in five potential regions. These typical peaks can be explained as following as: in the lower potential range (I) HER, (II) weakly bonded hydrogen (Hw), (III) strongly bonded hydrogen (Hs) and furthermore in positive potential range (IV) corresponding to the formation and reduction of Pt oxide and potential region (V) is attributed to double layer [21]. Moreover, there is a negligible peak similar to bump between the peaks (I) and (II). According to literature,
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there is a reasonable explanation. The interpretation says that since the appearance of the H3rd is closely related to the start of hydrogen evolution, it can be concluded that the third anodic peak is attributed to oxidation of adsorbed molecular hydrogen which may be the intermediate in hydrogen evolution. This reaction is a slow process and one reasonable cause for this observation can be due to that the molecular hydrogen is sub-surface adsorbed and forms possibly on Pt (110) sites with surface structure, produced by oxidation–reduction cycles. So, forming partial amount of Pt (110) sites with surface structure on a platinized Pt surface presented insignificant and poor current of the H3rd [22]. The peak current of Pt oxide reduction in 0.5 M H2SO4 solution was used for evaluation of the Pt loading at the Pt-NP/MGCE by faradaic law and was estimated at about 1.85 μg cm−2. Furthermore, real surface area (Ar) of the Pt catalyst can be measured according to the following equation: Ar ¼ Q H =Q O
ð6Þ
where QH is the charge consumed for hydrogen adsorption and can be obtained by integrating in the hydrogen adsorption/desorption region. Also, QO has been commonly taken as 210 μC/real cm2. The Ar was obtained at about 0.71 cm2. Also, the roughness factor can be obtained by dividing of Ar to Ag. Hence, the value of the calculated RF is equal to 39.7.
Fig. 2. SEM images of the prepared Ni-NP/MGCE (a, b) and Pt-NP/MGCE (c, d).
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3.2. Surface morphology and elemental analysis In order to characterize the prepared electrodes, micrographs of the Ni-NP/MGCE (a, b) and Pt-NP/MGCE (c,d) were investigated by SEM (Fig. 2). Traces (a, b) show that the Ni-NPs with spherical shapes were distributed in the form of extended coverage over a large area on the GCE surface. As shown in trace (c, d), the Pt-NPs can be generated by using complete GRR of the Ni-NPs by Pt-NPs. The Pt-NPs with granular structure and spherical shapes and average sizes at about 80 nm were uniformly spread on the GCE. Fig. 3 presented a typical EDS spectrum for determination of the electrode composition. The EDS gives evidences for the presence of the Ni and Pt in the modified electrode. As can be seen in this figure, Pt and Ni in the modified electrodes are the major elements and C is derived from the GCE. Also, by comparison with (A) and (B), it can be seen that the EDS data are in accordance with obtained results from the SEM. The results reveal that the Ni-NPs are completely replaced with Pt-NPs and no Ni-NPs remain at the final modified electrode. It should be noted that the reason of Au existence is that the specimens are usually coated with an ultra-thin coating of Au before measurements.
Fig. 4. Nyquist plots of impedance measurements of 1.0 mM K3[Fe(CN)6] / K4[Fe(CN)6] + 0.1 M KCl on bare GCE (a) and Pt-NP/MGCE (b).
3.3. EIS investigation EIS is an effective and simple way for evaluating the interface properties of the modified electrodes [23]. All impedance spectra are almost similar in form, composed of a high-frequency semicircle and a lowfrequency straight line. The diameter of the semicircle is assigned to the interfacial charge transfer resistance (Rct) and a straight line is attributed to the diffusion-limited process [24]. Fig. 4 presented the
Nyquist diagrams of the bare GCE (a) and Pt-NP/MGCE (b) in the presence of 1.0 mM K3[Fe(CN)6] / K4[Fe(CN)6] (1:1) + 0.1 M KCl solution. As can be seen in this figure, there is an obvious difference between the diameters of two semicircles. The Pt-NP/MGCE has a very small semicircle domain, implying a much lower Rct than the bare GCE. There are more active sites for easier charge transfer and faradaic reactions at the interface owing to the presence of Pt-NPs. 3.4. Electrocatalytic oxidation of methanol Fig. 5 depicted the CV signals of the methanol oxidation onto the NiNP/MGCE (a) and Pt-NP/MGCE (b) in 0.5 M H2SO4 + 1.36 M CH3OH solution. As can be seen in this figure, the inactivity of the Ni-NP/MGCE for methanol oxidation is in accordance with the observation that Ni-NPs are not stable in H2SO4 medium. Methanol electrooxidation at the Pt surface exhibited the characteristic double CV peaks which appear in the forward and reverse scans [6]. It can be seen from methanol oxidation on the Pt-NP/MGCE that the onset potential is at around 0.15 V and an enormous anodic peak appears approximately at 0.73 V in forward scan which is generated by oxidation of the COads to CO2 (I). The current density was defined as current normalized per Ar and obtained about
Fig. 3. Energy dispersive spectra of the Ni-NP/MGCE (A) and Pt-NP/MGCE (B).
Fig. 5. Electrochemical responses of the Ni-NP/MGCE (a) and Pt-NP/MGCE (b) in 1.36 M CH3OH + 0.5 M H2SO4 solution at υ = 50 mV s−1.
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771
Scheme 1. A schematic representation of methanol oxidation at the Pt-NP/MGCE.
1.83 mA cm−2. After that, the oxidation peak current density decreases at more positive potential due to the formation of Pt oxides which passives the Pt surface. When the potential scan is reversed, the reduction of the Pt oxide to Pt takes place and clean Pt is available at the surface. Since, the methanol oxidation occurs more easily at the clean Pt surface, therefore the peak current II appears at about 0.53 V in the backward scan which is attributed to the complete oxidation of CH3OH to CO2 (II) [25]. The electrochemical oxidation of methanol was schematically shown in Scheme 1. The important advantage of the Pt particles with nanoscale sizes is high activity and excellent resistance against poisoning effect by COads species, which are formed during the methanol oxidation. This phenomenon is the most important difference of Pt-NPs in comparison with bulk Pt in acid medium. For evaluation of the electrocatalytic activity in methanol oxidation, the onset potential was listed in Table 1 and compared with the some previous works. 3.5. Parameters affecting the electrode modification In order to evaluate the effect of various parameters such as electrodepositing potential, Ni2 + concentration, electrodeposition and GRR times on the methanol oxidation, the amount of peak current density was monitored as an index for finding the optimum conditions and the obtained results were summarized in Table 2. The results indicate that the peak current increases for electrodepositing potential up to Table 1 Comparison of methanol oxidation in H2SO4 solution at the Pt/-NPs/MGCE with some modified electrodes. Electrocatalyst
CH2SO4/M
CCH3OH/M
Eop/V
Ep/V
Ref.
Pt nanorod/Tia Pt-NP/SWCNTa Nano-Pt/MGCEa Pt/MGCEa Pt/SBA-15/MGCEa PtRu/MWCNTb PtNi/CCEa Pt/CCEb Pt/SWCNTa Pt-NP/MGCEb
0.5 1.0 0.5 0.5 0.1 1.0 0.1 0.2 1.0 0.5
2.0 2.0 1.4 0.5 0.05 2.0 0.5 0.15 2.0 1.36
0.36 0.22 0.20 0.20 0.15 0.26 0.15 0.15 0.45 0.15
0.61 0.89 0.7 0.62 0.53 0.75 0.88 0.89 0.64 0.73
[26] [27] [28] [29] [30] [31] [32] [33] [34] This work
All potentials were referred to the (a) SCE and (b) Ag│AgCl│KCl (3 M).
−1.0 V, NiSO4 concentration up to 0.5 M, electrodeposition time up to 500 s and GRR time up to 20 min and drop afterwards. 3.6. Electrocatalytic oxidation of formaldehyde Formaldehyde has become more interesting with the possibility of its application in liquid fuel cells. The reasons are related to its facility of handling, storage and generally high energy density. The electrochemical oxidation of formaldehyde is important for full understanding of methanol oxidation as a product of partial oxidation of methanol [35,36]. Fig. 6 illustrates the CV signals of the Ni-NP/ MGCE (a) and Pt-NP/MGCE (b) in 0.5 M H2SO4 + 0.32 M formaldehyde. The observation indicates that the Ni-NPs are not stable in 0.5 M H2SO4 solution. Hence, Ni-NP/MGCE gave no signal for formaldehyde oxidation and this indicates that the electrode has no electrocatalytic activity and acts similar to the bare GCE (Fig. 6a). As can be seen from trace (b), formaldehyde oxidation at the Pt-NP/ MGCE starts at about 0.25 V with low current and by sweeping to more positive potentials, the peak current reaches to a maximum at about 0.65 V which is assigned to the oxidation of COads to CO 2 (peak I) [37]. After that, the oxidation peak current decreases at more positive potentials due to the formation of the Pt oxide which causes passivity of the Pt surface. When the potential scan is reversed, the reduction of Pt oxides to Pt take place and clean Pt is available. So, a large anodic peak is observed at about 0.4 V (peak II). This peak is attributed to the complete oxidation of formaldehyde to CO2 [38]. It was well accepted that the electrooxidation of formaldehyde on traditional Pt catalyst follows a dual parallel pathways. In the direct
Table 2 Effect of several parameters on the peak current density of methanol oxidation in 0.5 M H2SO4 + 0.82 M CH3OH. Ed/V j/mA cm−2 CNiSO4/M j/mA cm−2 td/s j/mA cm−2 tr/min j/mA cm−2
−0.9 2.83 0.1 6.74 100 7.86 10 15.50
−1.0 6.74 0.3 12.88 300 20.44 15 35.38
−1.1 5.33 0.5 20.44 500 35.38 20 47.22
−1.2 3.65 0.8 14.55 700 28.55 25 38.5
−1.3 1.35 1.0 10.26 900 16.11 30 24.33
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surface (dehydration pathway, Scheme 2). The poisonous COads are removed via a reaction involving OHads species originated from water dissociation. Indeed, this process is very intricate, as higher potentials are required for water activation on the Pt surface [41]. Therefore, the electrode surface will be blocked by large amounts of COads and inhibit further adsorption of the other formaldehyde molecules. The important advantage of the Pt particles with nanoscale sizes is higher activity and superior resistance against poisoning effect by COads species. For evaluation the electrocatalytic activity, the onset oxidation potential and peak potential were listed in Table 3 and electrooxidation herein has been compared with some previous works. 3.7. Stability study
Fig. 6. CV signals of the Ni-NP/MGCE (a) and Pt-NP/MGCE (b) in 0.32 M HCHO + 0.5 M H2SO4 solution at υ = 50 mV s−1.
route, formaldehyde is converted to CO2 via short-lived intermediate (dehydrogenation pathway, Scheme 2) [39,40]. In the indirect path, formaldehyde is oxidized to CO ads which is adsorbed on the Pt
For further evaluation of the catalytic activity and catalyst stability, chronoamperometric response of the Pt-NP/MGCE was recorded at peak potential values in continuous operation for methanol and formaldehyde (Fig. 7). As can be seen in this figure, current density has slightly changed and decreases gradually with time passing. The observed data reveal that the Pt-NP/MGCE has good poisoning tolerance against generated intermediates due to the Pt NPs and their fine dispersions. The Pt-NPs with small diameters provide more active sites and accessible surface area for methanol oxidation and formaldehyde oxidation.
Scheme 2. Schematic representation of formaldehyde oxidation on the Pt-NP/MGCE.
Table 3 Comparison of electrocatalytic oxidation of formaldehyde at the Pt-NP/MGCE with some modified electrodes at υ = 50 mV s−1. Modified electrode
Electrolyte
CFormaldehyde (M)
Eop/V
Ep/V
Ref.
Pt–Pd/CNT Single crystal disk of Pt (111) Pt–Pd/PPy-CNT Pt/carbon–ceramic Pt–Pd/Nf/MGCE Pt/SWCNT/PANI Pt/nano-PDAN/MGCE Pt/PAANI/MWCNTs/MGCE Pt-NP/MGCE
0.5 M H2SO4 0.1 M HClO4 0.5 M H2SO4 0.1 M H2SO4 0.1 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4 0.5 M H2SO4
0.50 0.50 0.50 0.75 0.001 0.26 0.26 0.50 0.32
0.30 0.20 0.20 0.20 0.30 0.30 0.20 0.20 0.29
0.62 0.70 0.64 0.85 0.58 0.66 0.75 0.72 0.69
[39] [40] [42] [43] [44] [45] [46] [47] This work
All potentials were referred to SCE.
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Fig. 7. Chronoamperogram of the Pt-NP/MGCE at 0.5 M H2SO4 solution in the presence of (A) 1.36 M CH3OH at Ep = 0.73 V and (B) 0.32 M formaldehyde at Ep = 0.65 V.
4. Conclusion In this work, we have proposed a simple and quick method for preparation of the nano-scale Pt particles via GRR with Ni-NPs. The obtained results from SEM and EDS revealed the complete replacement between sacrificial metal (Ni) and noble metal (Pt). The Pt-NP/MGCE was used for electrocatalytic oxidation of methanol and formaldehyde in H2SO4 solution. It was observed that the Pt-NP/MGCE was capable to catalyze effectively these small organic molecules. The satisfactory activity is exactly connected to the Pt nanostructures, which provides more active sites and high accessible surface areas. The electrocatalysis is affected by various parameters in the modification steps.
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