Electrocatalytic Activity for CO, MeOH, and EtOH Oxidation on the Surface of Pt-Ru Nanoparticles Supported by Metal Oxide

This paper describes the electrocatalytic activity for CO, MeOH, and EtOH oxidation on the surface of Pt-Ru nanoparticles supported by metal oxide (Nb-TiO2-H) prepared for use in a fuel cell. To prepare Nb-TiO2-supported Pt-Ru nanoparticles, first, the Nb-TiO2 supports were prepared by sol-gel reaction of titanium tetraisopropoxide with a small amount of the niobium ethoxide in polystyrene (PS) colloids. Second, Pt-Ru nanoparticles were then deposited by chemical reduction of the Pt4+ and Ru3+ ions onto Nb-TiO2 supports (Pt-Ru@Nb-TiO2-CS). Nb element was used to reduce electrical resistance to facilitate electron transport during the electrochemical reactions on a fuel cell electrode. Finally, the Pt-Ru@Nb-TiO2-H catalysts were formed by the removal of core-polystyrene ball from Pt-Ru@TiO2-CS at 500◦C. The successfully prepared Pt-Ru electrocatalysts were confirmed via TEM, XPS, and ICP analysis. The electrocatalytic efficiency of Pt-Ru nanoparticles was evaluated via CO, MeOH, and EtOH oxidation for use in a direct methanol fuel cell (DMFC). As a result, the Pt-Ru@Nb-TiO2-H electrodes showed high electrocatalytic activity for the electrooxidation of CO, MeOH, and EtOH.


Introduction
Many researcher efforts have been devoted to improving the catalytic performance of carbon supported Pt-Ru catalysts [1][2][3].In a colloidal method, dispersion and adsorption of catalytic nanoparticles on the surface of carbon supports is done in the presence of protecting agents to avoid aggregation of particles.It should be noted that the protecting agent is likely to reduce the catalytic activities of catalyst particles.In another method known as the impregnation method, a metal precursor is reduced by the carbon supports dispersed in the solution [4][5][6].Carbon supports should be dispersed well without interference of a protecting agent in the suspension.In previous papers [7,8], the Pt-Ru nanoparticles were deposited on various carbon supports using γ-irradiation to use as anode catalysts in a direct methanol fuel cell (DMFC).However, the life time of the electrode was reduced, since the carbon supports were slowly oxidized in the fuel cell, as shown in C + 2H 2 O −→ CO 2 + 4H + + 4e − , E 0 = 0.207 V versus NHE at 25 • C. ( In general, the support materials should possess the following properties: (1) a high surface area for a high level of dispersion of the nanosized catalysts, (2) low electrical resistance to facilitate electron transport during the electrochemical reactions, (3) a pore structure suitable for fuel or oxidant contact and by product release, and (4) strong interaction between the catalyst nanoparticles and the supports.Oxide materials are widely used as a support in heterogeneous catalyst, since they possess those properties.They have inherently higher stability compared to carbon in oxidizing environments [9,10].The use of titanium dioxide support in a fuel cell operation has been of great interest due to its stability, low cost, commercial availability in water, and ease to control size and structure [11,12].The potential applications of hollow nanomaterials, especially TiO 2 hollow (TiO 2 -H) spheres, have been explored in various areas, such as light-trapping, chemical separation, photocatalysts, biomedicine, and optical devices [13,14].Compared to general core-shell nanostructures, TiO 2 -H spheres are exceptional for their special internal cavity, high specific surface area, high mobility, and low density [15][16][17].However, to our knowledge, little work has been reported on the application of hollow oxide spheres as a support, especially in fuel cells.
In this study, the Nb-doped TiO 2 supports were prepared with core-polystyrene and shell-Nb-TiO 2 , (Nb-TiO 2 -CS), for use as fuel cell anode catalysts.The Pt-Ru@Nb-TiO 2 -CS was then obtained by deposition of the Pt-Ru nanoparticles on the surface of the Nb-TiO 2 -CS using γ-irradiation and chemical reducing agents in aqueous solution, respectively.Finally, the Pt-Ru@Nb-TiO 2 -H was fabricated via removal of the core-polystyrene from Nb-TiO 2 -CS at 500 • C. The electrocatalytic activity of Pt-Ru nanoparticles on metal oxide supports was evaluated via CO, methanol, and ethanol oxidation in a 0.5 M H 2 SO 4 electrolyte in order to use for fuel cell anode electrode.

Synthesis of Polystyrene as Core-Ball via Surfactant-Free
Emulsion Polymerization.Polystyrene nanoparticles (PS) as core-ball were prepared as follows: potassium persulfate (KPS) was dissolved completely in deionized (D.I.) water with stirring of 350 rpm for 60 min under nitrogen atmosphere.Styrene monomer was added to the above-prepared solution and polymerized at 75 • C for 24 hours under nitrogen atmosphere.

Preparation of the
Pt-Ru@Nb-TiO 2 -H Catalysts.The Nb-TiO 2 supports were prepared by sol-gel method.SDS (0.5 g) as an anchoring agent was dissolved in the prepared PS colloids (10 mL) and stirred for 60 min under nitrogen atmosphere.Titanium tetraisopropoxide (1.2 mL) and niobium ethoxide (45 μL) were dissolved in 25 mL ethanol.The ethanol solution was slowly added to PS colloids, and the polymerization was processed while stirring (350 rpm) at 75 • C for 24 hours under nitrogen atmosphere.
Scheme 1 shows the preparation procedure of Pt-Ru@ Nb-TiO 2 -H catalysts by chemical reduction method and γirradiation.In Method 1, the core-PS and shell-Nb-TiO 2 supports (0.5 g) were well dispersed in 182 mL D.I. water, and pH was adjusted to 9.0 using NaOH.Hydrogen hexachloroplatinate(IV) hydrate (0.21 g) and ruthenium(III) chloride hydrate (0.205 g) were then added to the above-prepared colloids.To reduce the Pt 4+ and Ru 3+ ions, the formalin, as a reducing agent, was added to the above-prepared colloids.After the Pt 4+ and Ru 3+ ions were reduced by ultrasonic irradiation for 60 min, the prepared Pt-Ru@Nb-TiO 2 -CS nanoparticles were filtered (Whatman-2) and dried in a vacuum oven at 50 • C for 8 h.Upon calcination at 500 • C for 4 hrs in air, the Pt-Ru@Nb-TiO 2 -H catalysts were obtained.
In Method 2, Pt-Ru@Nb-TiO 2 -CS nanostructure was prepared by radiolytic reduction of Pt 4+ and Ru 3+ ions in the presence of the core-PS and shell-Nb-TiO 2 .H 2 PtCl 6 xH 2 O (0.21 g) and RuCl 3 xH 2 O (0.205 g) were dissolved in the Nb-TiO 2 colloids (182 mL) that contained 2-propanol (12.0 mL) as the radical scavenger.Nitrogen was bubbled for 30 min through the solution to remove oxygen, and the solution was then irradiated (Co-60 source) under atmospheric pressure and ambient temperature.A total dose of 30 kGy (a dose rate = 6.48 × 10 5 /h) were applied.Pt-Ru@Nb-TiO 2 -CS nanoparticles were filtered (Whatman-2) and dried in a vacuum oven at 50 • C for 8 h.The Pt-Ru@Nb-TiO 2 -H catalysts were also obtained by the method described above.
To evaluate the catalytic efficiency of Pt-Ru@Nb-TiO 2 -H catalysts for the electro-oxidation of CO, MeOH, and EtOH, the Pt-Ru@Nb-TiO 2 -H catalyst electrode was prepared as follows: firstly, the catalytic inks were prepared by mixing of Pt-Ru@Nb-TiO 2 -H catalysts (5.0 mg) and 5% Nafion solution (0.05 mL) and stirred for 24 h.Secondly, the catalytic inks were applied on a glass carbon (0.02 cm 2 ) by wet coating and dried in a vacuum oven at 50 • C under nitrogen gas.The electro-oxidation of CO, MeOH, and EtOH was examined using the Pt-Ru@Nb-TiO 2 -H catalyst electrode, submerged in 0.

Results and Discussion
3.1.Characterization of Pt-Ru@Nb-TiO 2 -H Catalysts.As mentioned above, the oxide supports for fuel cell electrode have high surface area, low electrical resistance, uniform porous structure, and interaction between catalysts and supports.In particular, surface area and electrical conductivity are very important factors in electrocatalytic reactions.The Nb-TiO 2 powder has been attractive as supports due to its high conductivity [18].The conductivity of the Nb-TiO 2 (∼0.1 Ω −1 cm −1 ) is superior to that of the pure TiO 2 (10 −6 Ω −1 cm −1 ) and commercial Vulcan XC-72 carbon supports.However, the Nb-TiO 2 powder is insufficient as fuel cell supports because of low surface area.To increase surface area, the supports require nanostructure such as coreshell nanostructure or the ordered uniform porous structure.
In a previous paper [19], the PS particles of 450 nm in diameter and poly(styrene-co-styrene sulfonate), PSS, particles of 140-160 nm in diameter were prepared by emulsifier-free emulsion polymerization.The surfaces of the PS and PSS particles were coated with Ag nanoparticles as antimicrobial agents via reduction of Ag ions using γirradiation.In this study, PS particles were used as template for preparing nanostructure supports as shown in Scheme 1. Figure 1 shows SEM images of PS spheres as a template prepared by emulsion-free polymerization.The diameter of the monodispersed PS particles was 450 nm.The surface of the monodispersed PS ball possess hydrophobic properties; therefore, in order to deposit Nb-TiO 2 with hydrophilic properties, anchoring agents such as SDS, poly(Nvinylpyrrolidone), and PVP were used.Figure 2 shows TEM images of the sphere with core-PS and shell-Nb-TiO 2 , PS/Nb-TiO 2 (a), Pt-Ru@Nb-TiO 2 -CS (b), and Pt-Ru@Nb-TiO 2 -H (c) prepared by chemical reduction, as shown in Scheme 1.In Figure 2(a), Nb metals were deposited on the surface of TiO 2 shell wall.As shown in Figures 2(b) and 2(c), the Pt-Ru nanoparticles do not completely appear in TEM images.On the other hand, the nanostructure form (hollow form) with thickness of ∼15 nm was successfully prepared as shown in Figure 2(c).Figure 3 also shows the TEM images of PS/Nb-TiO 2 (a), Pt-Ru@Nb-TiO 2 -CS (b), and Pt-Ru@Nb-TiO 2 -H (c) prepared by γirradiation as shown in Scheme 1.In Figure 3(b), after deposition of Pt-Ru nanoparticle on the surface of Nb-TiO 2 supports using γ-irradiation, the Pt-Ru nanoparticles were well dispersed on the surface of Nb-TiO 2 .However, the Pt-Ru nanoparticles do not appear in TEM image.After calcinations, the patterns of Pt-Ru@Nb-TiO 2 -H catalysts are different compared to that of Pt-Ru@Nb-TiO 2 -H catalysts prepared by chemical reduction.This might be caused by the radiation damage of TiO 2 shell.
As mentioned above, the Pt-Ru nanoparticles cannot clearly be determined by TEM images.For clear evaluation of the existence of Pt-Ru nanoparticles, the core-PS and shell-Nb-TiO 2 supports, PS/Nb-TiO 2 , and Pt-Ru@Nb-TiO 2 -H were analyzed via XPS spectroscopy.Figure 4 shows the XPS data of the PS/Nb-TiO 2 (a) and Pt-Ru@Nb-TiO 2 -H catalysts (b) prepared by chemical reduction.The peaks Nb and Ti element are assigned, as shown in Figure 4(a), and the Pt and Ru elements of the Pt-Ru@Nb-TiO 2 -H prepared by chemical reduction were determined, as shown in Figure 4(b).These results clearly indicate that the Pt-Ru nanoparticles were successfully deposited on the surface of PS/Nb-TiO 2 by chemical reduction method.Figure 5 also shows the XPS spectra of the PS/Nb-TiO 2 nanostructure (a) and Pt-Ru@Nb-TiO 2 -H catalysts (b) prepared by γirradiation method.The peaks Nb and Ti element are also assigned in Figure 5(a), and the Pt and Ru elements of the Pt-Ru@Nb-TiO 2 -H prepared by γ-irradiation were also determined in Figure 5(b).From these results, Pt-Ru nanoparticles were successfully loaded on the surface of PS/Nb-TiO 2 nanostructure by γ-irradiation.on Pt-Ru@Nb-TiO 2 -H catalysts shows very broad patterns compared to that of Pt-Ru nanoparticle on Pt-Ru@Nb-TiO 2 -H catalysts prepared by chemical reduction.The size of Pt-Ru nanoparticle was very small compared to that of Pt-Ru@Nb-TiO 2 catalyst prepared by chemical reduction.Table 1 presents the contents (wt-%) of Pt and Ru element in the Pt-Ru@Nb-TiO 2 -H catalysts prepared by chemical reduction and γ-irradiation.When Pt-Ru alloy nanoparticles are deposited on Nb-TiO 2 supports, γ-irradiation generates slightly higher Pt content than that of Ru content, while chemical reduction produces significantly higher Ru content than that of Pt content.This may be due to the difference in the reduction potential of the two metal ions.The reduction potential of Pt ion, [(PtCl) 4 2− (aq) + 2e − → Pt(s) + 4Cl − (aq), E 0 = 0.73 V], is higher than that of Ru ion.In chemical reduction, a metal ion is reduced by accepting an electron from a reducing agent.On the other hand, when the  produced in aqueous solution affects the yield of Pt-Ru alloy nanoparticles.

Electrocatalytic Efficiency of CO, MeOH, and EtOH
Oxidation on the Surface of Pt-Ru Nanoparticles on Nb-TiO 2 -H Supports.The efficiency of these catalysts toward the electrochemical oxidation of carbon monoxide (CO) was tested.Figure 7 presents the cyclic voltammograms (CVs) of electro-oxidation of CO oxidation on the Pt-Ru@Nb-TiO 2 -H catalyst electrodes.Peak of stripping CO could be seen at 0.8 V for the Pt-Ru@Nb-TiO 2 -H catalyst electrodes prepared by chemical reduction (a) and γ-irradiation (b).These peaks signify that CO oxidation is energetically favorable at these electrodes.The electrochemically active specific area (SEAS) of the catalysts was calculated by using the charges deduced from the CV of CO adsorption and desorption electro-oxidation process, and using the following equation where Q CO is the charge for CO desorption electro-oxidation in microcolumb (μC), G represents the summation of Pt + Ru metal loading (μg) on the electrode, and 420 is the charge required to oxidize a monolayer of CO on the catalysts in μC cm −2 .The electrochemical SEASs are 58 and 36 m 2 g −1 for the Pt-Ru@Nb-TiO 2 -H catalyst prepared by chemical reduction (a) and γ-irradiation (b), respectively.It may be considered that the higher SEAS for Pt-Ru@Nb-TiO 2 -H catalyst is obtained due to the smaller particle size, even distribution, and the large loading of Pt-Ru alloy nanoparticles on the surface of Nb-TiO 2 supports.Figure 8 presents the CVs recorded at the Pt-Ru@Nb-TiO 2 -H catalyst electrodes for electro-oxidation of methanol ).We conclude that a higher content of Pt(wt%) in Pt-Ru@Nb-TiO 2 -H catalysts (γ-irradiation) determines the catalytic efficiency towards methanol oxidation.However, as one can see in Figure 3(c), the TiO 2 supports was destroyed during γ-irradiation.Therefore, the chemical reduction assisted with ultrasonic irradiation for the deposition of Pt-Ru nanoparticles onto Nb-TiO 2 supports was better than that of γ-irradiation method as stability of metal oxide supports.Figure 9 shows the CVs for EtOH oxidation with various concentrations onto the surface of Pt-Ru nanoparticles deposited with Nb-TiO 2 supports in 0.5 M H 2 SO 4 electrolyte.The peak current values at 0.8 V (versus Ag/AgCl) corresponding to the oxidation of ethanol.The maximum catalytic efficiency was appeared in 0.5 M EtOH concentration.As a result, the prepared Pt-Ru nanoparticles onto metal oxide supports can be used in fuel cell anode electrode.

Conclusion
This study describes the preparation of Pt-Ru@Nb-TiO 2 -H catalysts by chemical reduction assisted with ultrasonic irradiation and γ-irradiation for fuel cell anode electrode.The conclusion was as follows.
(1) The Pt-Ru@Nb-TiO 2 -H catalyst was successfully prepared by chemical reduction assisted with ultrasonic irradiation and γ-irradiation.
(2) The size, morphology, and composition of Pt-Ru@ Nb-TiO 2 -H catalysts were determined via TEM, XRD, and elemental analysis.(3) The Pt-Ru@TiO 2 -H electrodes showed the high electrocatalytic activity for electro-oxidation of CO, MeOH, and EtOH.As a result, the catalyst prepared by chemical reduction assisted with ultrasonic irradiation can be used in a fuel cell electrode.

Figure 1 :
Figure 1: SEM images of PS spheres as template prepared by emulsion-free polymerization.

Figure 6 presents
XRD patterns of the Pt-Ru@Nb-TiO 2 -H catalysts prepared by Method 1 (a) and by Method 2 (b).

Table 1 :
Contents of the containing elements in Pt-Ru@Nb-TiO 2 -H prepared by Method 1 and 2 analyzed by ICP.Pt-Ru alloy nanoparticle is deposited on Nb-TiO 2 support by γ-irradiation irradiation, the hydrated electron (e aq − ) is generated in aqueous solution.This hydrated electron may hold lower reduction potential, so the Pt ions are quickly reduced to that of Ru ions.The hydrated electron