Bimetallic Pd-Mo nanoalloys supported on Vulcan XC ...

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Bimetallic PdeMo nanoalloys supported on Vulcan XC-72R carbon as anode catalysts for direct alcohol fuel cell Fariba Fathirad a,b,*, Ali Mostafavi a,1, Daryoush Afzali c,2 a

Department of Chemistry, Faculty of Sciences, Shahid Bahonar University of Kerman, Kerman, Iran Young Research Society, Shahid Bahonar University of Kerman, Kerman, Iran c Department of Environment, Institute of Science and High Technology and Environmental Sciences, Graduate University of Advanced Technology, Kerman, Iran b

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abstract

Article history:

Vulcan XC-72R carbon supported Pd and PdeMo alloys of different Pd:Mo atomic ratios

Received 4 August 2016

were prepared by hydrothermal synthesis method. The bimetallic nanoalloys were char-

Received in revised form

acterized by powder X-ray diffractometry and inductively coupled plasma-atomic emission

19 September 2016

spectroscopy to determine their crystal structures and elemental compositions. Alloy

Accepted 20 September 2016

formation of the nanocatalysts was proven by energy dispersive X-ray spectroscopy line

Available online 12 October 2016

profiles using field emission scanning electron microscopy. The performance of asprepared nanocatalysts was evaluated for the reactions of methanol, ethanol, ethylene

Keywords:

glycol and glycerol electrooxidation in alkaline media by cyclic voltammetry, linear sweep

Anode nanocatalyst

voltammetry and chronoamperometric measurements. It was found that bimetallic Pd

Bimetallic PdeMo alloys

eMo/VC catalysts have higher activity due to different electronic structure as compared to

Direct alcohol fuel cell

the monometallic palladium. Also, the Pd3Mo/VC catalyst showed excellent catalytic ac-

Electrooxidation

tivity, high durability and stability which indeed propose it to be as a promising electrocatalyst for future direct alcohol fuel cells. © 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Direct alcohol fuel cells (DAFCs) have attracted considerable interest in their application to alternative power sources for portable electronic devices and electric vehicles [1e3]. Liquid fuels, such as low molecular weight alcohols (methanol, ethanol, ethylene glycol and glycerol) have several advantages compared to pure hydrogen, because they can be easy handled, transported and stored. Furthermore, they have relatively high

energy conversion efficiency, high mass energy density and low-to-zero pollutant emission, comparable to that of gasoline [4,5]. The electrocatalytic reaction of alcohol oxidation in alkaline media is more facile, allowing to use low catalysts loadings and to select a wide range of catalysts [6e8]. Accordingly, Platinum is an interesting candidate for the alcohols oxidation. High costs of the Pt-based electrocatalysts and susceptibility of the catalysts against poisoning of the CO-like intermediates formed during alcohol oxidation are the main barriers to the commercialization of DAFC technology [9,10]. It is therefore

* Corresponding author. Department of Chemistry, Faculty of Sciences, Shahid Bahonar University of Kerman, Kerman, Iran. Fax: þ98 3433257433. E-mail addresses: [email protected] (F. Fathirad), [email protected] (A. Mostafavi), [email protected] (D. Afzali). 1 Fax: þ98 3433257433. 2 Fax: þ98 3433776617. http://dx.doi.org/10.1016/j.ijhydene.2016.09.138 0360-3199/© 2016 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

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desired to develop low cost non-platinum catalysts with comparable or improved kinetics for anodic electrooxidation and better resistance to CO poisoning. Palladium and palladium based alloys have demonstrated high activity in electrooxidation of alcohols in alkaline media, and therefore can be used as less expensive analogs of platinum [11e14]. Furthermore, it has been reported that the addition of non-noble metals to palladium can facilitate and improve alcohol electrooxidation [15e19]. In the field of heterogeneous catalysis, searching for proper support materials is another effective strategy in view of reducing noble metal loading and enhancing catalyst performance. Carbon materials with various structures and shapes, such as carbon nanofibers, active carbon and carbon nanotubes have been used as electrocatalyst supports [20e23]. No materials other than carbon have the essential properties such as electronic conductivity, corrosion resistance, and low cost required for the commercialization of fuel cells. Vulcan XC-72R carbon black (VC) provides excellent conductivity in a range of applications at relatively low loading levels and is typically easier to disperse compared to other conductive grades of carbon black [24]. Inert surface of carbon support requires suitable chemical modification in order to enhance a favorable interaction with catalyst particles such as those of noble metals. One method of chemical modification is functionalization of the carbon surface which can be carried out in both the gas and liquid phases [25,26]. Acid treatment has been shown to introduce oxygen atoms on the carbon surface which can be found as components of surface oxygen complexes or functionalities. On a relative scale, the surface oxygen functionalities such as carboxylic, anhydride, phenolic and carbonyl possess varying degrees of acidic character [27]. It has been well established in the literatures that surface oxygen groups, which form anchoring sites for metallic precursors as well as for metals, determine the properties of activated carbon as a catalyst support material. The acidic groups on the surface decrease the hydrophobicity of the carbon, leading to accessibility of the surface to aqueous metal precursors, while the less acidic groups increase the interaction of the metal precursor or the metal particle with the support and, as a consequence, minimize the sintering propensity of metal on carbon [28]. We outline herein a process for preparing PdeMo bimetallic nanoparticles supported on pretreatment Vulcan XC-72R carbon (PdeMo/VC) by hydrothermal method. The alloys containing different amounts of Mo were characterized in terms of morphology, crystal structure and chemical components. PdeMo alloy catalysts on multiwall carbon nanotubes have been reported previously as catalytically improved anode for methanol oxidation [29]. In this work, the performance of

PdeMo alloy catalysts on VC was evaluated by cyclic voltammetry and chronoamperometric measurements for the reactions of methanol, ethanol, ethylene glycol and glycerol electrooxidation in alkaline media.

Material and methods Chemicals Vulcan XC-72R carbon powder (VC) was purchased from Cabot Corporation (USA). Palladium (II) acetate (Pd(CH3COO)2), ammonium molybdate ((NH4)2MoO4), methanol, ethanol, glycerol, ethylene glycol (EG) and sodium citrate were purchased from Merck (Darmstadt, Germany)).

Apparatus Product X-ray diffraction (XRD) data was recorded by a Rigaku D-max C III, X-ray diffractometer using Ni-filtered Cu Ka radiation. The morphology and composition of the asprepared nanocatalysts were analyzed with field emission scanning electron microscope (FESEM) (CARL ZEISS-AURIGA 60 microscope, Jena, Germany) which was equipped with an energy-dispersive X-ray analyzer (EDX). All electrochemical measurements were carried out with an Autolab potentiostat/galvanostat (PGSTAT 101, Eco Chemie, Netherlands). The experimental conditions were controlled with Nova 2.0 software. A conventional three-electrode cell was used at 22 ± 1 C. An Ag/AgCl/KClstd (3.0 M) electrode and a platinum wire were used as the reference and auxiliary electrodes, respectively.

Preparation of pretreated Vulcan XC-72R Vulcan XC-72R carbon was chemically treatment to synthesize nanoparticles with the best distribution and size and, as a consequence, to maximize catalytic activity. The chemical pretreatment of Vulcan XC-72R carbon powder was carried out using Senthil Kumar method [20]. 1 g carbon powder was dispersed in a round bottom flask with 1000 mL of 5% nitric acid, 0.07 M phosphoric acid and 0.2 M potassium hydroxide in the deionized water. The mixture was refluxed for 16 h at 120 C. Treated VC was filtered and washed several times with a continuous flow of deionized water to obtain neutral pH and then dried in vacuo at 110 C for 12 h.

Table 1 e Data of ICP-AES and EDX compositional analyses. Sample

Loading amount (mg) OAc salts of

Pd/VC Pd-black Pd3Mo PdMo PdMo3

Pd

Mo

42.19 e 42.19 42.19 42.19

e e 12.28 36.84 110.52

Carbon support

80.00 e 104.04 152.12 296.40

Yield (wt.%) (ICP-AES) Pd:Mo

Atomic % (ICP-AES)

Total content wt% (ICP-AES)

Average atomic % (FESEM-EDX)

19.72 98.91 13.65:5.28 9.48:10.39 4.08:14.10

100:0 e 72.6:27.4 50.3:49.7 25.9:74.1

18.28

e e 76.6:23.4 52.2:47.8 25.2:74.8

18.93 19.87 18.18


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ratios and ethylene glycol as reducing agent. All catalysts were prepared with 20 wt% metal loading on support. First, 4 mL aqueous solution containing appropriate amounts of precursors was mixed with 50 mL of EG. 0.1 g of sodium citrate was added to this solution mixture and pH was adjusted to 9. Appropriate amount of pretreated carbon support were added into the above solution and sonicated for 30 min. Then the solution was transferred into an autoclave container, sealed and treated at 160 C for 5 h in oven. The resulting suspension was filtered, washed first with deionized water and then with ethanol. The obtained sediment was dried in vacuo for 12 h at room temperature. Pd/VC was prepared with same procedure in the absence of ammonium molybdate.

Substrate preparation

Fig. 1 e XRD patterns of Pd/VC and PdeMo/VC catalysts.

Nanocatalyst synthesis PdeMo/VC bimetallic alloy catalysts were prepared by hydrothermal method. Palladium acetate and ammonium molybdate were used as metal precursors in controlled molar

A glassy carbon (GC) working electrode (∅3, surface area 0.0314 cm2) was used as substrate. The electrochemical analysis of the synthesized nanocatalysts was performed with pre-modification of GC electrode. First, the GC surface was polished carefully with alumina powder, followed by ultrasonication in ethanol and deionized water several times. Then, 5 mg of nanocatalyst was dispersed in 0.99 mL of water and ethanol (1:1) mixture and 10 mL of 0.5 wt% Nafion solution ultrasonically for 15 min. Finally, 5 mL of the solution was dropped on the surface of electrode and dried in the air before electrochemical experiments. The electrochemical analyses were carried out in alkaline media deaerated by ultrapure N2 for 5 min before measurements.

Fig. 2 e FESEM-EDX of PdMo3/VC (a, d), PdMo/VC (b, e), Pd3Mo/VC (c, f) catalysts.

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Fig. 3 e CV curves of Pd/VC and PdeMo/VC catalysts in 1 M KOH at scan rate of 10 mV s¡1.

Results and discussion Compositional and structural analysis PdeMo bimetallic nanoparticles were synthesized using hydrothermal method where EG acted as the reducing agent. In each reaction, measured amounts of palladium acetate and ammonium molybdate were used to make 20 wt% of the metal (Pd þ Mo) content in the final product in the desired stoichiometric ratio. The compositions analyzed by ICP-AES and

EDX agree with each other and show that the reactions are almost quantitative (Table 1). According to the amounts used in the nanocatalyst synthesis, anode coating and based on the obtained results, the loading of catalyst, metal content (Pd þ Mo) and palladium on the anode surface were calculated 0.80, 0.15 and 0.11 mg cm2, respectively. Fig. 1 shows the XRD patterns of PdeMo/VC catalysts with different ratios of Pd:Mo. For comparison, the XRD pattern of Pd/VC is also displayed. The pattern of Pd/VC shows a broad peak at 2q~ 25 , which is assigned to the (002) plane of the carbon support and a series of peaks at 2q~ 40 , 46 , 68 and 82 which are assigned to the (111), (220), (322) and (431) planes, respectively, for the cubic lattice of Pd (JCPDS, Card No. 870638, Fm-3m). The patterns of PdnZn/VC samples show similar characteristics, but the peaks are slightly shifted to higher values of 2q with the increase of the Mo content, which indicates a contraction of the lattice and the formation of alloy [30,31]. The absence of any peak of the Mo element or its compounds for PdMo and Pd3Mo phases in the XRD patterns suggests that samples are free from macroscopic phase segregation [32]. However, EDX spectra of the PdeMo/VC catalyst clearly show the presence of Mo (Fig. 2). In molybdenum rich catalysts, PdMo3, two different phases, PdMo, and MoO3 were observed. The average crystallite size for the catalysts are calculated based on the (111) diffraction peak using Scherrer equation: dXRD ¼ 0:9 lCuKa/b2q$cosqmax

(1)

Fig. 4 e CV curves of Pd/VC and PdeMo/VC catalysts vs. 1 M methanol (a), 1 M ethanol (b), 0.1 M EG (c) and 0.1 M glycerol (d) in 1 M KOH at scan rate of 10 mV s¡1.


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Fig. 5 e LSV curves of Pd3Mo/VC catalyst in different scan rates vs. (a) 1 M methanol:10, 20,40, 50, 75, 100 mV s¡1, (b) 1 M ethanol: 10, 20, 30, 50, 60, 70, 90 mV s¡1 (c) 0.1 M EG: 10, 20, 40, 50, 60, 80 mV s¡1 and (d) 0.1 M glycerol: 10, 20, 40, 60, 70, 80, 90, 100 mV s¡1. Inset: the plot of peak current density vs. square root of scan rates.

where d is the average particle size (nm), l is the wavelength of the X-ray used (1.54056 A ), q is the angle at the maximum of the peak (rad), b2q is the width of the peak at half height in radians. The calculated average crystallite sizes are 8.1, 6.9, 7.8 and 5.9 nm for the Pd/VC, PdMo3/VC, PdMo/C and Pd3Mo/VC catalysts, respectively. The morphology of PdeMo/VC nanocatalysts was characterized by FESEM imaging (Fig. 2). The favorable morphology of the Pd/C and PdeMo/C nanocatalysts could be attributed to the well-controlled and homogenous nucleation and growth generated by the ethylene glycol as a reducing agent. The white visible spots on the images are believed to be NPs which were deposited on the Vulcan carbon support with compact and granulated structure. However, in order to obtain the suitable scientific data of the NPs on the support, EDX investigations were carried out.

areas of PdeMo/VC are reduced from that of Pd/C. Reduction of active surface area upon alloy formation may be partly due to the down shift of the Pd d-band center, which weakens binding energy of adatoms [29,33]. The activity of the PdeMo/VC catalysts for alcohol electrooxidation in alkaline electrolyte was tested and compared to the activity of monometallic Pd/VC. As shown in Fig. 4(aed), for all alcohols, PdMo/VC and PdMo3/VC show approximately similar current density, which is higher than

Electrochemical measurements Cyclic voltammetry The cyclic voltammograms of the Pd/VC, PdMo3/VC, PdMo/VC, and Pd3Mo/VC catalysts were first investigated in 1 M KOH solution, and are shown in Fig. 3. The observed peaks in the CV potential range of 1.0 to 0.6 were ascribed to adsorption and desorption of OH on the surface of catalyst. In addition to the reverse scan of CV curves, reduction peak of palladium oxide was observed at ~ 0.4 V. A large reduction peak of palladium oxides indicates a markedly increased active surface area of the electrocatalysts. The electrochemical surface

Fig. 6 e Chronoamperometric curves of Pd3Mo/VC catalyst in electrooxidation of different alcohols at ¡0.3 V (vs. Ag/ AgCl/KClstd).

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Table 2 e Comparison of the proposed catalyst with other literature catalysts. Catalyst

Oxidative compound

Pd loading (mg cm2)

Activity (mA cm2)

Durability (T) (s)

Durability (J) (mA cm2)

PdIn3/SiO2

Methanol Ethanol Ethylene glycol Glycerol Glycerol Methanol Ethanol Methanol Methanol Ethanol Ethylene glycol Glycerol

0.200

32.00 68.00 120.0 90.00 102.7 1.500 90.00 59.60 71.20 121.2 234.8 182.6

5000

1.0 4.1 6.1 2.0 25.0 0.015 20 e 5.00 10.0 25.0 17.0

PdCo(3)/Au Mo30@Pd70/MVCNT Pd7Ir/VC Pd7Rh2/GO Pd3Mo/VC

e 0.003 0.128 0.600 0.110

the monometallic nanocatalyst. Pd3Mo/VC catalyst has significantly higher activity than other catalysts for methanol, ethanol, ethylene glycol, and glycerol with peak activities of 71.2, 121.2, 234.8, 182.6 mA cm2 respectively. These results are supported by the active surface results of CV studies. Theoretical calculations and experimental data in literatures [34] demonstrated that, Pd-M alloys undergo phase segregations, in which the noble metal Pd migrates to the surface forming a pure Pd overlayer on the bulk alloys. The electronic structures of the metal overlayers can alter significantly upon bonding with the substrate, and in turn, their catalytic properties can change. we note that alloying Pd with Mo lowers the d-band position of the Pd overlayer (according to Fig. 3) and, therefore, may modify the activity significantly by inducing strain and electron redistribution between the substrate and the overlayer [35]. Furthermore, the promotion effect of Pd3Mo/VC catalyst compared to other ratio of bimetallic alloys could be due to the less content of Mo and smaller size of catalyst that effects on electronic structures. The reverse effect, were overlapped for PdMo/VC and PdMo3/VC, therefore lead to similar activity for alcohol electrooxidation.

Linear sweep voltammetry For investigation of Pd3Mo/VC reaction mechanism, the effect of scan rate on the electrocatalytic oxidation of alcohols was studied. As can be seen in Fig. 5, the oxidation potential of LSV peaks shifted to more positive potentials with increasing of scan rate in all alcohols, confirming the kinetic limitation in the reaction. Also, plots of peak height (J) vs. the square root of scan rate (n1/2) were linear in the range of different scan rates, suggesting diffusion controlled process rather than surface controlled.

Chronoamperometric measurements A useful method to investigation of the electrochemical activity and stability of catalysts in fuel cells is chronoamperometric measurements. The curves were recorded in 1.0 mol L1 KOH in presence of each alcohol for 5400 s in 0.3 V vs. Ag/AgCl, KCl std. As shown in Fig. 6, the current densities of Pd3Mo/VC catalyst for all alcohols decayed relatively rapid at the initial and then reach a constant state. A higher current density is an indication of better electroactivity. After an

1000 1000 3600 e 5400

Ref [10]

[14] [29] [36] [37] This work

initial drop in performance further activity was stabilized for all alcohols in a rather high current density.

Comparison of proposed nanocatalyst with other reported nanocatalysts A comparison of the electrocatalytic performance of Pd3Mo/ VC catalyst developed in this study with other nanocatalysts for the electrooxidation of alcohols is presented in Table 2. The present nanocatalyst is more effective as compared to all of the others. The Pd3Mo/VC catalyst show excellent catalytic activity, high durability and stability which indeed propose it to be as a promising electrocatalyst for future direct alcohol fuel cells.

Conclusion A series of active PdeMo catalysts supported on Vulcan XC72R carbon were synthesized by the hydrothermal method at pH ¼ 9. These bimetallic samples were compared to the monometallic Pd/VC catalysts for the electrooxidation of methanol, ethanol, ethylene glycol and glycerol in alkaline media. We have found that the catalytic activities of bimetallic alloys is higher than monometallic Pd. The modified electronic structure in Pd could significantly influence on adsorption of ethanol, methanol, ethylene glycol and glycerol over the catalyst. The electronic structure of the Pd overlayer can alter significantly upon bonding with the substrate and, in turn, its catalytic properties can change. We note that alloying Pd with Mo lowers the d-band position of the noblemetal overlayer and therefore, modifies the activity significantly by inducing strain and electron redistribution between the substrate and the overlayer. The amounts of Mo and catalyst size are also two key parameters for optimum performance of the synthesized catalysts. Pd3Mo/VC catalyst has a higher activity compared to other catalysts due to smaller size and less Mo content. PdMo/VC catalyst has less Mo content and larger size than PdMo3/VC. Therefore two catalyst activities are approximately similar. It's worth mentioning that the Pd3Mo/VC catalyst presents the highest catalytic activity and durability, therefore is a potential anode catalyst for DAFC.


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