Elsevier

Journal of Power Sources

Volume 278, 15 March 2015, Pages 639-644
Journal of Power Sources

Enhanced methanol oxidation activity and stability of Pt particles anchored on carbon-doped TiO2 nanocoating support

https://doi.org/10.1016/j.jpowsour.2014.12.096Get rights and content

Highlights

  • Pt particles are well dispersed on TiO2–C support.

  • Strong metal-support interactions exist between Pt particles and TiO2–C support.

  • Pt/TiO2–C catalyst exhibits enhanced catalytic performance for methanol oxidation.

Abstract

In this work, carbon-doped TiO2 nanocoating (TiO2–C) was prepared by a sol–gel process and employed as the support of Pt nanoparticles for methanol oxidation reaction (MOR). The obtained Pt/TiO2–C catalyst was characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. XRD characterization shows that the average crystallite sizes of Pt particles and TiO2–C support are 2.7 and 6.5 nm, respectively. TEM characterizations show that Pt particles are highly dispersed on TiO2 nanocoating, which preserves its nanoscale structure without no apparent sintering after carbon doping. XPS characterization shows that the Pt particles anchored on TiO2–C exhibit positively shifted binding energies of Pt 4f. Cyclic voltammetry (CV) and chronoamperometry (CA) characterizations show that TiO2–C has a greatly enhanced electrical conductivity and Pt/TiO2–C catalyst has better electrocatalytic activity and stability than Pt/C catalyst for MOR, which could be attributed to the high dispersion of Pt particles on TiO2–C support, the strong metal-support interactions between Pt particles and TiO2–C support, and the rich active –OH species on TiO2–C support.

Introduction

Recently, methanol oxidation reaction (MOR) has attracted great attention due to the promising applications of direct methanol fuel cells (DMFCs) to automobile industries and portable electronics [1], [2]. It is well-established that the complete MOR is a complex multistep reaction that requires active sites for methanol adsorption and dehydrogenation as well as sites for supplying oxygen-containing species for the oxidation of the carbonaceous intermediates formed during adsorption and dehydrogenation [3]. Despite recent advances, however, the activity and the durability of the MOR electrocatalysts are still the major problems that hinder the large-scale commercialization of DMFCs [4]. Pt particles supported on high-surface-area carbon (Pt/C) are the most popular electrocatalysts for MOR. However, the Pt/C catalyst could be easily poisoned by the adsorbed CO-like intermediate species generated during methanol oxidation process. It is reported that the adsorbed CO could be oxidized with the aid of oxygen-containing species such as -OH formed on the Pt surface (Eq. (1)).Pt-(CO)ads + −OHads → Pt + CO2 + H++e

However, the generation of oxygen-containing species on the Pt surface only takes place at higher potentials (>0.7 V vs. RHE) [3], which creates a significant overpotential for MOR and at the same time causes the heavy poisoning of Pt catalyst by CO-like intermediate species. In addition, in DMFC environment the carbon support degrades over time due to the electrochemical corrosion of carbon [5], which could cause the migration and agglomeration of Pt nanoparticles, thus decreasing the activity and stability of Pt/C catalyst.

To mitigate the poisoning of Pt catalyst, modifying Pt catalyst by incorporating a second metal and using metal oxide as the support of Pt catalyst are the two most widely employed strategies. Second metals such as Ru [6], [7], Ni [3], [8], Ti [9], Co [10], V [9] and Pd [11] have been suggested and metal oxides such as TiO2 [1], [12], WO3 [13] and SnO2 [14] have also been demonstrated to serve the same purpose. The enhanced MOR performance on Pt catalyst modified by the incorporation of a second metal or metal oxide (M) can be rationalized by the so-called bifunctional mechanism: Pt provides active sites for methanol adsorption and dehydrogenation while M supplies oxygen-containing species for the oxidation of the CO-like intermediate species adsorbed on Pt surface [1], [13], [15]. In addition, the electronic (ligand) effect has also been postulated, which assumes that M can modify the electronic properties of Pt by changing the electron density of states of the d-band and the Fermi level energy. This electronic modification could destabilize the interactions between the CO molecules and the Pt surface and hence weaken the Pt–CO bonding and mitigate the poisoning of Pt [3].

To address the issue of carbon support corrosion, there have been many efforts in searching for oxidation-resistant non-carbon catalyst supports. Metal oxides as catalyst supports of DMFCs have attracted great attention because of their higher corrosion resistance, strong interactions with metal catalyst particles which could prevent the agglomeration of metal particles and produce an electronic effect on metal particles, and abundant hydroxyl groups which could help remove the adsorbed CO-like intermediate species [1], [14], [16], [17]. Among the many metal oxides, TiO2 has attracted great attention due to its low cost, environmental friendliness and great corrosion resistance in fuel cell environment. In addition, TiO2-based supports may also provide catalytic advantages for the electrochemical oxidation of methanol because anatase TiO2 is an active photocatalyst for the destruction of organic compounds [18], [19]. Unfortunately, replacing carbon with traditional TiO2 support is not possible due to its low electrical conductivity. To increase the conductivity of TiO2, reducing TiO2 into sub-stoichiometric TinO2n−1 [20] and doping TiO2 with other element such as Nb [21] and C [22] are the two most effective methods. However, both methods involve high temperature treatment, which could increase the particle size of TiO2 and thus decrease its specific surface area. In addition, since TiO2 undergoes a phase transition from anatase to the less catalytically active rutile near 700 °C, the high-temperature treatment may also reduce some catalytic promotion of methanol oxidation by the support [18].

To mitigate the poisoning of Pt and corrosion of carbon support, in this work, carbon-doped TiO2 nanocoating on carbon black (CB) was prepared by a sol–gel process and employed as the support to anchor Pt particles for MOR. The carbon-doped TiO2 nanocoating possesses several merits as the support of Pt catalyst for MOR. First, Pt particles were directly anchored on TiO2, which could fully utilize the possible strong metal-support interactions (SMSI) [23], [24] and the possible electronic metal-support interactions (EMSI) [14] between Pt and TiO2 for MOR. Second, the electrical conductivity of TiO2 could be enhanced due to the carbon doping treatment. Third, the nanoscale TiO2 coated on CB could still have a relatively large specific surface area because the high-surface-area CB could provide a good backbone for the nanoscale TiO2 coating during carbon doping treatment and the TiO2 nanocoating only takes up a small amount of total mass of the support. Fourth, CB coated by TiO2 nanocoating could solve the problem of electrochemical corrosion of carbon. As a result, the carbon-doped TiO2 nanocoating exhibits an exciting and desirable nanostructure, which makes it an effective replacement for CB in the role of catalyst support for DMFCs. Compared with the Pt particles anchored on CB, the Pt particles highly anchored on carbon-doped TiO2 nanocoating support exhibit a significantly improved electrocatalytic activity and durability toward MOR. It is proposed that the improved performance is the result of the synergistic effect from the interactions between Pt and TiO2.

Section snippets

Preparation of carbon-doped TiO2 nanocoating

CB (Vulcan XC-72) was ultrasonically functionalized by H2SO4/HNO3 (v/v = 3:1) at 60 °C for 1 h, followed by thorough washing with ultrapure water and drying overnight. A typical process for preparing the carbon-doped TiO2 nanocoating is as follows. First, 100 mg of the functionalized CB was well dispersed in a solution of 10 mL of ethanol, 500 μL of benzyl alcohol and 500 μL of ultrapure water with the aid of ultrasonication and stirring. Then 100 μL of titanium isopropoxide dissolved in 5 mL

Results and discussion

Fig. 1 shows the typical TEM images of TiO2 nanocoating before (a) and after (b) carbon doping. From Fig. 1a it can be seen that CB is well coated by a nanoscale TiO2 nanocoating. The possible mechanism for TiO2 nanocoating formation on CB is as follows. Benzyl alcohol could adsorb on CB via π–π interaction between the aromatic CB surface and the benzyl ring of benzyl alcohol. The hydroxyl groups of benzyl alcohol would then coordinate with Ti and may further induce condensation to form a

Conclusions

In summary, carbon-doped TiO2 nanocoating was prepared by a sol–gel process and employed as the support to anchor Pt particles for MOR. Physical characterizations show that the TiO2 nanocoating with a greatly enhanced electrical conductivity still preserves its nanoscale structure without no apparent sintering after carbon doping. The highly dispersed Pt particles anchored on TiO2–C exhibit positively shifted binding energies of Pt 4f. Electrochemical characterizations show that the Pt/TiO2–C

Acknowledgments

The present study was supported by the Nature Science Foundation of China (21306144) and the Educational Commission of Hubei Province of China (Q20131511). Yuan-Hang Qin acknowledges the fellowship from the China Scholarship Council.

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