carbon monoxide and methanol in acid medium on Pt-Sn catalysts for low-temperature fuel cells: A comparative review of the effect of Pt-Sn structural characteristics

Electrochimica Acta 56 (2010) 1–14

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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta

Review article

The electro-oxidation of carbon monoxide, hydrogen/carbon monoxide and methanol in acid medium on Pt-Sn catalysts for low-temperature fuel cells: A comparative review of the effect of Pt-Sn structural characteristics E. Antolini a,b,∗ , E.R. Gonzalez b a b

Scuola di Scienza dei Materiali, Via 25 aprile 22, 16016 Cogoleto, Genova, Italy Instituto de Química de São Carlos, USP, C. P. 780, São Carlos, SP 13560-970, Brazil

a r t i c l e

i n f o

Article history: Received 30 April 2010 Received in revised form 23 August 2010 Accepted 24 August 2010 Available online 16 September 2010 Keywords: Pt-Sn catalysts CO oxidation Methanol oxidation

a b s t r a c t The electrocatalytic activity for CO, H2 /CO and CH3 OH oxidation of Pt-Sn catalysts has been extensively investigated for a possible use as anode materials for low-temperature fuel cells. This paper presents an overview of the relationship between the structural characteristics of the catalysts (catalyst composition, degree of alloying, presence of oxides) and their electrocatalytic activity for the oxidation of the different fuels. © 2010 Elsevier Ltd. All rights reserved.

Contents 1. 2. 3.

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Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structure of Pt-Sn catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pt-Sn catalysts for CO and H2 /CO electro-oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. A general overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. CO and H2 /CO electro-oxidation on non-alloyed Pt-Sn catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Sn ad-atoms modified Pt . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.2. Pt-SnOx . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. CO and H2 /CO electro-oxidation on Pt3 Sn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Pt3 Sn(h k l) single-crystal electrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Polycrystalline Pt3 Sn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.4. CO and H2 /CO electro-oxidation on partially alloyed Pt-Sn catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Comparison with Pt-Ru catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pt-Sn catalysts for methanol oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. A general overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Methanol oxidation on non-alloyed Pt-Sn catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.1. Methanol oxidation on platinum-tin oxide/hydroxide catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2.2. Methanol oxidation on Pt catalysts modified by zero-valent Sn . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3. Methanol oxidation on Pt3 Sn and Pt-Sn alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.1. Pt3 Sn single crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3.2. Polycrystalline Pt3 Sn and PtSn alloys . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4. Methanol oxidation on partially alloyed Pt-Sn catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.5. Comparison with Pt-Ru catalysts . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1. H2 /CO oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

∗ Corresponding author at: Scuola di Scienza dei Materiali, Via 25 aprile 22, 16016 Cogoleto, Genova, Italy. E-mail address: [email protected] (E. Antolini). 0013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2010.08.077

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5.2. Methanol oxidation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Platinum is commonly used as anode catalyst in lowtemperature fuel cells fuelled with hydrogen (PEMFC) and methanol (DMFC). Because catalysis is a surface effect, the catalyst needs to have the highest possible surface area. So, the active phase is dispersed on a conductive support as carbon. Pure Pt, however, is not the most efficient anodic catalyst for low-temperature fuel cells. Indeed, platinum itself is known to be rapidly poisoned on its surface by the CO present in the reformate gas used as H2 carrier for the cell or by strongly adsorbed species coming from the dissociative adsorption of methanol [1,2]. Efforts to mitigate the poisoning of Pt have been concentrated on the addition of cocatalysts, particularly ruthenium and tin, to platinum. Pt-Sn nanocomposites have been extensively studied as catalysts for the electro-oxidation of hydrogen/carbon monoxide and methanol. A good CO tolerance during H2 oxidation has been shown by Pt-Sn catalysts. Regarding the methanol electrooxidation, instead, controversial results have been reported. In this review we examine the catalytic activity and the relationship of activity to structural characteristics (phase composition) of Pt-Sn catalysts for the electro-oxidation of CO, H2 /CO and CH3 OH in acid medium. The question of the promotional role of Sn atoms is crucial, so we will discuss whether Sn has to be present in metallic form alloyed to Pt or in the oxide form, and whether the activity enhancement produced by Sn atoms has to be ascribed to a bifunctional or a ligand (electronic) effect. A comparison of the catalytic activity of Pt-Sn catalysts with that of the most used Pt-Ru catalysts, both in fundamental and practical studies, is also reported. 2. Structure of Pt-Sn catalysts Platinum and tin form five bimetallic intermetallic phases, Pt3 Sn, PtSn, Pt2 Sn3 , PtSn2 , and PtSn4 , of which fcc Pt3 Sn and hcp PtSn are congruently melting compositions. These intermetallic phases are distinguished by distinct crystalline structures and unique X-ray diffraction (XRD) patterns. Regarding the different values of the lattice parameter of fcc platinum tin alloys present in literature, Kuznetzov et al. [3] asserted that Pt forms nearly all possible alloys with Sn. Then, the shift of the fcc Pt peaks of Pt-Sn catalysts to lower angles than pure Pt but to higher angles than the fcc Pt3 Sn phase should reveal the formation of a solid solution between Pt and Sn, due to the incorporation of Sn in the fcc structure of Pt. Radmilovic et al. [4], instead, attributed the value of the lattice constant found for a commercial carbonsupported Pt:Sn 1.23:1 catalyst (a = 0.3965 nm, between those of Pt3 Sn (a = 0.4000 nm) and Pt (a = 0.3924 nm)) to a mixture of Pt9 Sn (a = 0.3934 nm) [5] and Pt3 Sn phases. Given the near-coincidence of the Pt9 Sn and Pt3 Sn reflections and the particle size broadening, a mixture of Pt9 Sn and stoichiometric Pt3 Sn would produce a diffraction pattern very similar to that of a non-stoichiometric Pt3 Sn phase. Generally, Pt-Sn catalysts are formed by fully alloyed (fcc Pt3 Sn), fully non-alloyed (Pt-/SnOx or Pt-Sn ad-atoms) or partially alloyed (fcc Pt(1−x) Snx alloy, with lattice parameter <0.40 nm, that is, Pt/Sn atomic ratio >3, and SnOx ) platinum-tin structures. The most common type of tin oxide reported in Pt-Sn catalyst is tin dioxide, SnO2 . SnO2 crystallizes with the rutile structure, wherein the tin atoms are 6 coordinate and the oxygen atoms three coordinate [6]. SnO2 is usually regarded as an oxygen-deficient n-type semiconductor

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[7]. Considering the use of Pt-Sn in acidic fuel cells, it is important to remark that SnO2 dissolves in sulfuric acid to give Sn(SO4 )2 [8]. 3. Pt-Sn catalysts for CO and H2 /CO electro-oxidation 3.1. A general overview Platinum is the best pure metal for H2 oxidation, but Pt electrodes have a low activity for CO electro-oxidation [9,10]. On Pt electrodes, CO molecules are electro-oxidized via a surface reaction between adsorbed CO and adsorbed oxygen species [9,10]. Carbon monoxide can be adsorbed on Pt sites at a negative potential, but oxygen species cannot be adsorbed at a potential of less than 0.75 V vs. RHE [11]. Hence the rate-determining step of CO oxidation is the adsorption of oxygen species on the Pt electrode. On Pt, CO oxidation starts at about 0.8 V vs. RHE, at which potential surface platinum atoms adsorb oxygen species sufficiently. The platinum catalyst used for hydrogen oxidation in PEMFCs is rapidly poisoned due to strong CO adsorption on the platinum surface, leading to a significant decrease in fuel cell power output. The presence of 100 ppm of CO in the anode gas decreases the power output of a single PEMFC to only 25% of its value with pure H2 [12]. In an exploratory approach to find improved electrocatalysts, Gotz and Wendt [12] prepared binary and ternary carbon-supported catalysts with the elements Pt and Ru, W, Mo or Sn, respectively, and tested their activity in fuel cell operation at 75 ◦ C with H2 fuel containing 150 ppm CO. Cocatalytic activities were found for all these elements for oxidation of H2 /CO. The cocatalytic activity of tin for anodic oxidation of H2 /CO is usually explained by its activity for dissociative adsorption of water. Moreover, Shubina and Koper [13], by quantum-chemical calculations on the Pt3 Sn(1 1 1) crystal surface, showed that, in contrast to Ru, CO binds only to Pt and not to Sn atoms, whereas OH has an energetic preference for the Sn sites. This implies that Pt-Sn is a good CO oxidation catalyst. In the following of Section 3 we divided the oxidation of CO and H2 /CO on Pt-Sn catalysts in three parts, that is, on non-alloyed Pt-Sn, on Pt3 Sn phase and on partially alloyed Pt(1−x) Snx -SnOx catalysts. 3.2. CO and H2 /CO electro-oxidation on non-alloyed Pt-Sn catalysts 3.2.1. Sn ad-atoms modified Pt Firstly, Motoo et al. [14,15] studied the enhancement of CO electro-oxidation on Pt electrodes by Sn ad-atoms. The polarization curves for CO electro-oxidation on Pt electrodes having a submonolayer amount of Sn ad-atoms indicated that the presence of less than half a monolayer of Sn ad-atoms makes the potential shift to the negative side by more than 0.4 V. On Pt electrodes, a steep rise of the current is observed at 0.84 V vs. RHE, this potential coinciding with that of oxygen adsorption by Pt sites. Thus, the potential at which CO oxidation starts is determined by the potential of oxygen adsorption by Pt sites. Sn ad-atoms adsorb oxygen at a potentials ≥0.45 V vs. RHE, which are more negative compared to that at which surface platinum atoms adsorb oxygen [16]. This means that with the aid of Sn ad-atoms the oxygen can be introduced to the surface at a potential 0.39 (0.84–0.45) V more negative compared to the Pt surface having no Sn ad-atoms. In this enhancement, surface Pt atoms serve as adsorption sites for CO and Sn ad-atoms for O. According to Watanabe et al. [17], the bifunctional mechanism seems to be the most probable mechanism on the electrocatalysis

E. Antolini, E.R. Gonzalez / Electrochimica Acta 56 (2010) 1–14

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by oxygen adsorbing ad-atoms for the CO oxidation on platinum electrodes. For CO electro-oxidation, the catalytic activity at a Pt electrode modified by Sn ad-atoms attains a maximum at the coverage of 0.45 (the ratio of the number of Sn ad-atoms to that of surface Pt atoms). The primary factor of CO oxidation enhancement on platinum electrodes by Sn ad-atoms is the onset potential of oxygen adsorption by Sn ad-atoms. The secondary factor is the rate of the oxygen adsorption or that of surface reaction between the oxygen and CO adsorbed by platinum sites. In contrast to the bifunctional mechanism, from structural and electrochemical studies of the adsorption of Sn on Pt (3 3 2) and (1 1 1), the group of Baltruschat and co-workers [18,19] inferred that Sn influences CO molecules in its neighborhood electronically. This leads to a disappearance of the infrared (IR) band from CO adsorbed in the hollow site at high Sn coverages and to higher population of the weakly adsorbed state of CO for all Sn-modified surfaces, i.e. a relative increase of the amount of CO oxidized at low potentials. In addition to this electronic effect, they added that Sn also exerts a co-catalytic effect at low Sn coverages on that part of CO which is adsorbed at a larger distance from Sn due to a bifunctional mechanism. 3.2.2. Pt-SnOx Platinized tin oxide has significantly higher catalytic activity for CO oxidation at low temperatures than either Pt or SnOx alone [20]. The activity for CO oxidation of Pt-SnOx catalysts increases with Pt loading until a maximum activity is reached at about 17 wt% Pt [21]. A reductive pretreatment enhances the activity of Pt-SnOx catalysts relative to no pretreatment or to pretreatment with O2 or an inert gas [22]. The observed synergism is well documented but there is no general agreement concerning its origin. There are various studies of CO oxidation on Pd-SnO2 and Pt-SnO2 catalysts and several explanations of the observed synergism are given. Bond et al. [23] proposed a bifunctional mechanism based on the spillover of both carbon monoxide and oxygen from the noble metal to tin dioxide. Sheintuch et al. [24] also consider a bifunctional mechanism but restrict the spillover to CO. Schryer et al. [25] ascribed the activity enhancement for CO oxidation to the presence of a PtSn alloy. Gangal et al. [26] formulated the hypothesis that the adsorption of the reactants leads to a local temperature increase of the platinum particles, promoting the reaction on adjacent SnO2 sites. Grass and Lintz [27] showed that the synergism in the oxidation of CO on Pt-SnO2 catalysts is mainly due to the adsorption of oxygen on the oxide surface. Subsequently, it migrates to the reaction sites situated at the border between oxide and noble metal particles. However, neither the experimental results nor the model calculations allowed the localization of the reactive sites, so it was not possible to distinguish between the three-phase boundary line platinum-oxide-gas and the adjacent sites. Thus, the possibility of oxygen spillover from tin dioxide onto platinum cannot be excluded but is not necessary to explain the observed results. Crabb et al. [28] studied the CO electro-oxidation on carbon-supported Pt-Sn catalysts prepared using surface organometallic chemistry (SOMC). By XRD and X-ray photoelectron spectroscopy (XPS) analyses they observed the presence of a stable tin oxide at the surface of the platinum crystallites. CO stripping measurements showed promotion of the electro-oxidation of the adsorbed CO monolayer on 0.5 (theoretical surface coverage of platinum surface sites by tin) Pt-SnOx /C compared to Pt/C. The onset potential for CO oxidation decreased from 0.7 V vs. RHE for the Pt/C to 0.24 V vs. RHE in the case of Pt-SnOx /C. The wider oxidation envelope may be interpreted as a contribution at lower potential due to electrooxidation of weakly adsorbed CO on Pt-Sn. As will discuss in the next paragraph, the presence of strongly and weakly adsorbed carbon monoxide on Pt and Pt3 Sn alloy single-crystal surfaces at high surface coverages of carbon monoxide has been reported [29,30].

Fig. 1. CO stripping cyclic voltammetry of Pt-SnOx /C electrodes prepared using SOMC in 1 M H2 SO4 at 25 ◦ C (sweep rate = 10 mV s−1 ). Cycle 1 (—) after CO adsorption at 10 mV, cycle 2 (- - -) after removal of CO by oxidative stripping. Reprinted from Ref. [28], copyright 2000, with permission from The Electrochemical Society.

As no evidence of a pre-peak to the main CO oxidation peak for Pt/C at lower potentials which could be attributed to weakly adsorbed CO was observed, the presence of tin appears to induce the formation of weakly and strongly adsorbed CO sites. Wang et al. [30] reported that the weakly bonded CO adsorbed on Pt sites adjacent to Sn atoms on well-defined Pt3 Sn alloy is an active species, which reacts at lower overpotential with oxygenated species such as OH adsorbed on the tin. The similarity with the results of Crabb et al. [28] is interesting, given that the work of Wang et al. [30] was carried out on well-characterized Pt3 Sn alloy surfaces, whereas no evidence of alloying was found by Crabb et al. [28]. To study the effect of tin coverage on CO oxidation, a range of catalysts with different nominal Sn coverages were prepared using SOMC [28]. The cyclic voltammetry (CV) curves are shown in Fig. 1 from Ref. [28]. Again, a decrease in the onset potential for CO electro-oxidation was observed for all the Pt-SnOx /C catalysts. Comparison of the CO electro-oxidation profiles showed relatively little changes until 1 Pt-SnOx /C, where a significant decrease in the onset potential (to about 0.1 V) was observed. The addition of small amounts of tin has the effect of decreasing both the onset and peak potentials significantly. However, further addition of tin does not affect the potentials greatly. This suggests that a small amount of tin is enough to give good promotional effects. The relative intensity of the CO oxidation peaks did however appear to change on tin addition. It would appear that the shoulder attributed to oxidation weakly adsorbed carbon monoxide increases with respect to the total area on increasing the tin coverage. H2 and CO chemisorption results indicated a decrease in accessible platinum sites with an increase in the tin content. Addition of a small amount of tin resulted in

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Fig. 2. Performance of PEMFCs with Pt/SnO2 and Pt/C as anode catalysts. Platinum loading 1 mg cm−2 for both electrodes. Cathode: Pt/C, cell temperature: 75 ◦ C. Reproduced from Ref. [32], copyright 2006, with permission from Elsevier.

a sharp decrease in the volume of hydrogen or carbon monoxide adsorbed. Further addition caused a further decrease in dispersion until a plateau was reached at about 10% dispersion. The group of Eguchi and co-workers [31,32] prepared tin oxide supported platinum catalysts, Pt/SnO2 , by the impregnation method. The XRD pattern showed the presence of metallic Pt and SnO2 . The Pt/SnO2 catalysts showed higher electrocatalytic activity for CO oxidation with lower onset potential compared to Pt/C, although the binding energy of Pt 4f showed no change. This result indicated that the Sn(II)/Sn(IV) redox couple should promote the CO oxidation, according to the bifunctional mechanism as: Pt-COads + SnO2 -OHads → CO2 + H+ + e−

(1)

The performance of a PEMFC with Pt/SnO2 as anode catalyst was compared with that of a PEMFC with Pt/C. Fig. 2 from Ref. [32] shows the I–V characteristics of PEMFCs with different anode catalysts. In the absence of CO, at a given current, the potential was higher for the cell with Pt/C than for that with Pt/SnO2 . This difference was ascribed to the higher surface area and dispersion of Pt on carbon support. The I–V curves of both cells were deteriorated with a supply of CO (100 ppm) with H2 . The performance decay of the cell with Pt/SnO2 , however, was smaller than that of the cell with Pt/C. 3.3. CO and H2 /CO electro-oxidation on Pt3 Sn 3.3.1. Pt3 Sn(h k l) single-crystal electrodes The most part of the studies regarding the oxidation of CO and H2 /CO on the Pt3 Sn alloy phase were carried out on Pt3 Sn(h k l) single-crystal electrodes, showing that the electrochemical oxidation of carbon monoxide in acid electrolyte is a structure-sensitive reaction, in which the (1 1 1) surface (25 at% Sn) has the highest activity. Gasteiger et al. [33] investigated the electrochemical oxidation of H2 , CO and dilute mixtures of CO in H2 on a wellcharacterized clean annealed surface of Pt3 Sn(1 1 0) single-crystal electrode with a Sn surface concentration of ∼20 at% using the rotating disk electrode (RDE) technique in 0.5 M H2 SO4 at 62 ◦ C. The potential for the onset of CO oxidation on this surface was nearly 0.5 V lower than on pure Pt and approximately 0.15 V lower than on any Pt-Ru alloy. The potential for the onset of H2 oxidation in the presence of 0.1–2% CO coincides with the potential for the onset of CO oxidation on the alloy surface. The polarization curve for the oxidation of these gas mixtures was shifted cathodically by ∼0.3 V

with respect to that on pure Pt. In a following work, Gasteiger et al. [34] studied the kinetics of the electrochemical oxidation of CO and H2 /CO mixtures (0.1 and 2% CO) in H2 SO4 at 25–62 ◦ C on different surfaces of the ordered single-crystal Pt3 Sn alloy. Clean annealed and sputtered-cleaned but not-annealed surfaces of (1 1 0) and (1 1 1) orientation were investigated. A remarkable difference in activity was observed between the annealed (1 1 1) surface and the sputtered but not-annealed (1 1 0) surface, with both surfaces having the same nominal surface composition, 20–25 at% Sn, but different local structures. The onset potential for CO oxidation on the (1 1 1) surface was shifted cathodically by 0.13 V relative to that on the sputtered (1 1 0) surface, and was remarkably close to 0 V vs. RHE. The repulsive interaction from the “crowding” of CO on the Pt sites on the surface is the reason why Pt3 Sn(1 1 1) is even more active for the oxidation of dissolved CO than the (1 1 0) surface. Relative to pure Pt surfaces (of any crystal structure), the potential shift was more than 0.5 V, corresponding to a catalytic activity higher by more than four orders of magnitude. Comparable shifts were observed for the oxidation of H2 /CO mixtures. Both the structure sensitivity and the high catalytic activity of the Pt3 Sn surface were attributed to an adsorbed state of CO unique to this alloy and occurred at relatively high coverage (>0.9 CO/Pt) on the (1 1 1) surface. In the same way, Wang et al. [30] reported the formation of a unique state of adsorbed CO (COads ) on the Pt3 Sn(1 1 0) surface, which is not the same state of COads as occurs on either Pt-Ru or pure Pt surfaces. It is also not a state of COads which is produced by methanol dehydrogenation. Indeed, there are two different forms of COads : one which is oxidized at lower overpotentials, in the socalled pre-oxidation region, characterized as a weakly adsorbed state, and a strongly adsorbed state, which is oxidized at higher overpotentials. The characterization as weakly adsorbed refers to the high coverage state in which the adsorption energy (enthalpy of adsorption) is reduced due to the repulsive COads –COads interaction. The state is unique in the sense that a significant fraction of COads is oxidized at a much lower (∼400 mV) potential than the rest of the COads , a phenomenon that does not occur on any other Pt and Pt-alloy surfaces examined in the same way. This CO state is only formed at high coverages by direct adsorption from dissolved CO. On the pure Pt surfaces there are no states of CO that desorb in the temperature range of −23 to 27 ◦ C, but on the Pt3 Sn surfaces ca. 10–30% of the CO desorbs in this temperature range, with the (1 1 1) surface having the largest fraction in this state. This low-temperature state is populated only after the high temperature states are filled and required relatively high doses of CO. The heat of adsorption of the low-temperature state was estimated to be 70, about 20 kJ mol−1 lower than the heat of adsorption at saturation on the comparable Pt surface, and is an adsorption energy that would be considered “weak chemisorption”. The polarization curves for CO electro-oxidation on Pt, Ru, Pt-Ru and Pt3 Sn(1 1 0) surfaces at ambient temperature and 1600 rpm are shown in Fig. 3 from Ref. [30]. This figure clearly shows the dramatically higher rate of oxidation of CO on Pt3 Sn(1 1 0) surface than on Pt, Ru, or any Pt-Ru alloy surface. From catalytic activity and structure sensitivity studies, it was reported that Pt3 Sn(h k l) single crystals have the highest catalytic activity for CO oxidation ever found at the electrochemical interface in acid solutions [34,35]. It has been proposed that the significant enhancement produced by alloying Pt with Sn atoms can be ascribed to a combination of both bifunctional and ligand (electronic) effects [36]. According to the bifunctional mechanism, the alloying component Sn adsorbs oxygenated species (multiple oxygenated ligands or simply OHads ), which can then react in the Langmuir–Hinshelwood type reaction with the adsorbed CO on Pt atoms [34,35]: Pt-COads + Sn-OHads → CO2 + H+ + e−

(2)

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Fig. 3. Potentiodynamic CO oxidation current on the sputtered Pt3 Sn(1 1 0) compared with those measured on sputter-cleaned Pt, Ru and Pt-Ru (ca. 50% Ru). All the measurements were performed with a rotating disk electrode in 0.5 M H2 SO4 saturated with CO at 25 ◦ C at a rotation rate of 1600 rpm. The insert provides a magnification of the low current density region, showing the positive-going sweeps for the four surfaces. Reproduced from Ref. [30], copyright 1996, with permission from Elsevier.

The theoretical study done by Shubina and Koper [13] is consistent with this interpretation, i.e., COads does not bind to the Sn, and OHads clearly shows a preference for the Sn sites. The ligand effect, where the alloying component (Sn atoms) may alter the electronic properties of catalytically active metal (Pt atoms), is a consequence of the strong intermetallic bonding between Pt and Sn. The change in the electronic properties of Pt leads to the formation of the so-called “weakly” adsorbed state of COads on Pt sites [30,34]. While the interaction of COads with Pt is unambiguous, the Pt–OHads interaction is more uncertain. Although at low overpotentials oxygenated species are exclusively adsorbed on Sn sites, due to the intermetallic bonding with Sn, the adsorption of OH may be enhanced on Pt sites. Indeed, Sn atoms located below outermost Pt sites may affect different adsorption properties of Pt atoms. This effect arises from the direct correlation between the work function and the potential of zero charge (pzc), i.e., the local pzc of Pt atoms near the Sn atoms is charged positively with the respect to the Sn-unmodified Pt atoms. This would result in promoted OH adsorption at Pt sites (or at the bridge sites between Pt and Sn atoms) at a lower potential than on pure Pt surfaces.The binding site occupancy of COads as well as the onset of CO oxidation on Pt3 Sn surfaces was determined by FTIR and compared to the corresponding Pt single crystals [36]. In contrast to the near invariant band of atop linearly bonded COads (COL ) on Pt(1 1 1), changes in band-shape (splitting of the band) and frequency (increase in the frequency mode) were clearly visible on the Pt3 Sn(1 1 1) surface. Splitting of the bands originates from two different forms of adsorbed CO. More reactive form, with enhanced dipole–dipole coupling and higher frequency, was assigned as the weakly adsorbed CO which is compressed into the islands/patches because of exclusion from Sn sites. For Pt3 Sn(1 1 0) surface neither splitting of COL bond nor “high-frequency” COL was found. In fact the frequencies of COL adsorbed on Pt3 Sn(1 1 0) were lower than for Pt3 Sn(1 1 1) surface. In contrast to classical electrochemical measurements (i.e., I vs. E curves), FTIR data unambiguously showed that the onset of CO oxidation (CO2 band development) starts as low as at E ≈ 0.1 V on both Pt3 Sn surfaces. However, turnover frequency (TOF) of CO oxidation on the Pt3 Sn(1 1 1) surface was higher than on the Pt3 Sn(1 1 0)–(3 × 1) surface. This difference in reactivity arises from the fundamentally different morphology of the two surfaces. Depending on the preparation procedure the same

5

single-crystal Pt3 Sn(1 1 0) produces two different surface symmetries (3 × 1) and (2 × 1), where the later one has about 50 at.% of Sn on the outermost atomic layer. Arrangement of the Pt and Sn surface atoms in (2 × 1) symmetry disables formation of weakly adsorbed CO, which makes this surface the least active in comparison to the other Pt3 Sn surfaces. To explain the line shape of the CO bands on Pt3 Sn(1 1 1), Stamenkovic et al. [35] suggested that, in addition to alloying effects, other factors, such as intermolecular repulsion between coadsorbed CO and OH species, are controlling segregation of CO into cluster domains where the local CO coverage is different from the coverage expected for the CO–CO interaction on an unmodified Pt(1 1 1) surface. Dupont et al. [37] demonstrated the efficiency of Pt3 Sn alloy surfaces toward CO oxidation from first-principles theory. Oxidation kinetics based on atomistic density-functional theory calculations showed that the Pt3 Sn surface alloy exhibits a promising catalytic activity for fuel cells. At room temperature, the corresponding rate outperforms the activity of Pt(1 1 1) by several orders of magnitude. According to the oxidation pathways, the activation barriers are actually lower on Pt3 Sn(1 1 1) and Pt3 Sn/Pt(1 1 1) surfaces than on Pt(1 1 1). Among the energy contributions, a correlation is evidenced between the decrease of the barrier and the strengthening of the attractive interaction energy between CO and O moieties. The presence of tin modifies also the symmetry of the transition states which are composed of a CO adsorbate on a Pt near-top position and an atomic O adsorption on an asymmetric mixed PtSn bridge site. Along the reaction pathways, a CO2 chemisorbed surface intermediate is obtained on all the surfaces. These results are supported by a thorough vibrational analysis including the coupling with the surface phonons which reveals the existence of a stretching frequency between the metal substrate and the CO2 molecule. 3.3.2. Polycrystalline Pt3 Sn Compared to Pt3 Sn single crystals, there are few studies on the H2 /CO oxidation on carbon-supported polycrystalline Pt3 Sn, commonly used as a fuel cell catalyst. Shmidt et al. [38] investigated the electro-oxidation of CO contaminated H2 fuel (100 ppm—2%) over a carbon-supported Pt3 Sn catalyst. Measurements under fuel cell relevant conditions (steady state, continuous gas flow, 60 ◦ C) showed a significantly improved CO tolerance as compared to stateof-the-art Pt and Pt0.5 Ru0.5 anode catalysts, with an overpotential of 230 mV for 250 ppm and only 170 mV for 100 ppm CO/H2 at a mass-specific current density of 0.5 A mgmetal −1 , respectively. Lee et al. [39] investigated the electrocatalysis of CO tolerance in the hydrogen oxidation reaction (HOR) for Pt/C, Pt3 Sn/C and PtRu/C electrocatalysts. Both half and single PEMFC polarization characteristics were studied at several temperatures and CO partial pressures. The strongest effect of the CO partial pressure on the fuel cell performance occurred for the Pt/C catalyst. They proposed that the CO adsorption step occurs predominantly through a displacement path for Pt3 Sn/C and through a free site attack path for CO on both Pt/C and Pt-Ru/C. The data are more consistent with the participation of linear (Pt3 Sn/C, and Pt-Ru/C [T ≤ 55 ◦ C]) and bridged bonded adsorbed CO on Pt/C and Pt-Ru/C [T ≥ 70 ◦ C]. The CO oxidation process occurs at different potentials depending on the nature of the electrode material. The oxidation of CO by the alloy catalysts is not the only contributor to CO tolerance. Changes in the thermodynamics and the kinetics of the CO adsorption process, induced by the alloy catalysts, also contribute to CO tolerance. They observed that the onset of CO oxidation occurs at different potentials depending on the nature of the electrode material. Independently of temperature, the onset of this reaction occurs first for Pt3 Sn/C followed by Pt-Ru/C and finally for Pt/C. In a next work, Camara et al. [40] by kinetic analyses showed that the CO poisoning effect on Pt/C, Pt-Ru/C, and Pt3 Sn/C catalysts occurs through a free Pt site attack mechanism, involving bridge- and linear-bonded

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adsorbed CO. For all catalysts, the onset of CO oxidation occurs via the bridge-bonded species, but for Pt-Ru/C and Pt3 Sn/C, the reaction starts at smaller potentials. Under this condition, the HOR currents are generated on the vacancies of a CO adsorbed layer created when some of the bridge-bonded CO molecules are oxidized. The linearly adsorbed CO is oxidized at higher overpotentials, leading to an increase of the holes on the CO layer and thus of the rate of the HOR. Wang and Hsing [41] developed an impedance model based on three state-variables for the kinetics and mechanistic investigation of H2 /CO electro-oxidation on Pt/C, Pt-Ru/C and Pt3 Sn/C catalysts. The simulation results showed that the reversing of the impedance pattern of the II and III quadrants is due to the change of rate-determining step from CO oxidation to CO adsorption. The agreement between experiments and the simulation gave rise to the opinion that the adsorbed OH species is responsible for the CO oxidation. The same model has also been utilized to differentiate the reaction mechanisms for the Pt-Ru/C and Pt3 Sn/C systems. They concluded that the promoted OH generation is the primary reason for enhanced activity toward CO oxidation on the Pt-Ru/C. The high activity of the Pt3 Sn/C system toward H2 /CO oxidation, instead, is due to the combination of the promoted OH generation, exclusion of CO on Sn sites and minimization of CO adsorption caused by the intermetallic bonding. 3.4. CO and H2 /CO electro-oxidation on partially alloyed Pt-Sn catalysts The catalytic activity of three different carbon-supported Pt-Sn catalysts for the anodic oxidation of hydrogen, carbon monoxide, and H2 /CO mixtures was correlated with their bimetallic microstructure by Arenz et al. [42]. These catalysts differ primarily by having differing amounts of Pt, Pt3 Sn, PtSn, and SnO2 phase nanoparticles distributed on the carbon support [4]. The catalysts had nominal Pt/Sn atomic ratios of 1:1 and 3:1. The 1:1 catalyst heat-treated at 500 ◦ C contained a Pt-rich fcc alloy phase and tetragonal SnO2 . The 1:1 sample heat-treated at 900 ◦ C consisted of a stoichiometric hcp PtSn phase, a nearly stoichiometric fcc Pt3 Sn phase and tetragonal SnO2 . The 3:1 catalyst reduced in H2 at 270 ◦ C was composed entirely of the stoichiometric Pt3 Sn cubic phase. The mass activity for CO and H2 /CO oxidation was proportional to the amount of Pt3 Sn phase in the catalyst. There is no clear evidence that either the PtSn phase or Pt-SnO2 clusters contribute any significant activity for H2 /CO or CO oxidation in these catalysts. The presence of Sn in the surface thus allows for the continuous oxidation of the CO at a potential where OHads does not form on Pt. The relatively large amounts of unalloyed SnO2 on two of the catalysts suggest that, not only Sn atoms but also SnO2 (or Sn hydro-oxides) adjacent to Pt may provide a bifunctional effect as well. However, the rate of CO oxidation at low potential appears to scale with the amount of Pt3 Sn phase present in the catalyst, implying that reaction (1) does not contribute in any significant way to the total activity of these Pt-Sn catalysts. Lim et al. [43] prepared highly dispersed and nano-sized Pt-Sn/C electrocatalysts in three Pt:Sn atomic ratios (3:1, 1:1 and 1:3) by borohydride reduction and subsequent hydrothermal treatment. XRD patterns clearly indicated the presence of both crystalline fcc Pt(1−x) Snx alloy and SnO2 phases. The activities of the Pt-Sn/C and commercial Pt/C catalysts for CO oxidation were investigated by CO stripping voltammetry in 0.5 M H2 SO4 at 50 mV s−1 . The onset potential and maximum peak of CO oxidations were lower for the Pt-Sn/C catalysts (0.23 and 0.75 V vs. SHE, respectively) compared to the commercial Pt/C catalyst (0.64 and 0.86 V vs. SHE, respectively). The range of CO oxidation of the Pt-Sn/C catalysts was wider than that of the commercial Pt/C catalyst. Moreover, the onset potential of CO oxidation was the same for all the Pt-Sn/C catalysts in spite of different amounts of Sn content. The higher performance under a pure H2 and CO-containing

H2 gases in the single-cell and the better durability under a 0.5 M H2 SO4 solution in accelerated durability tests of the Pt-Sn/C (3:1) catalyst than the commercial Pt/C catalyst was ascribed to the coexistence of PtSn alloys and Sn oxides. No comparison with Pt-SnO2 or Pt3 Sn alloy phase, however, was made. 3.5. Comparison with Pt-Ru catalysts Among different Pt-based materials, platinum-ruthenium is considered the catalysts with the highest CO tolerance. In PEMFCs, Pt-Ru catalysts are much more tolerant to CO poisoning than pure Pt catalysts [44–46]. A direct comparison of Pt-Ru/C and PtSn/C as anode catalysts in PEMFCs fuelled with H2 /CO was carried out by Gotz and Wendt [12] and Lee et al. [39]. Gotz and Wendt [12] compared non-alloyed Pt-Ru/C (1:1) and Pt-Sn/C (1:1) catalysts: the cell with Pt-Ru/C as anode material considerably better performed than that with Pt-Sn. Lee et al. [39] compared alloyed Pt-Ru/C (1:1) and Pt3 Sn/C catalysts: in PEMFC at 85 ◦ C, for different amount of CO, the CO tolerance of Pt-Ru/C was slightly better than that of Pt3 Sn/C. However, RDE measurements of CO and H2 /CO oxidation indicated a superior activity for CO and H2 /CO oxidation of Pt3 Sn alloy than Pt-Ru alloy [30,33,38]. Wang et al. [30] found that the CO oxidation rate on Pt3 Sn(1 1 0) surface is dramatically higher than on Pt, Ru, or any Pt-Ru alloy surface. Gasteiger et al. [33] report results on the electro-oxidation of CO, and H2 /CO mixtures on a sputteredcleaned Pt3 Sn alloy electrode. They found that Pt3 Sn was more active than any Pt-Ru alloy for CO oxidation. The onset potential for CO oxidation on Pt3 Sn surface was ∼0.15 V more negative than on pure Ru, which is the most active surface in the Pt/Ru alloy system if CO is supplied continuously to the electrode surface, namely, in RDE studies and under fuel cell operating conditions. The polarization of a Pt3 Sn electrode at 62 ◦ C oxidizing a H2 /CO gas mixture containing 0.1% CO was reduced by ∼0.35 V with respect to pure Pt and by 0.10 V with respect to Pt-Ru. Schmidt et al. [38] studied the electro-oxidation of CO contaminated H2 feed (100 ppm—2%) over a Pt3 Sn/C catalyst, applying a newly developed thin-film RDE method for the measurement of high surface area catalysts. Measurements under fuel cell relevant conditions (steady state, continuous gas flow, 60 ◦ C) showed a significantly improved CO tolerance as compared to state-of-the-art Pt and Pt0.5 Ru0.5 anode catalysts, with an overpotential of 230 mV for 250 ppm and only 170 mV for 100 ppm CO/H2 at a mass-specific current density of 0.5 A/mgmetal , respectively. 4. Pt-Sn catalysts for methanol oxidation 4.1. A general overview The anodic oxidation of methanol on platinum metal electrodes in acidic electrolytes is a catalytic reaction producing CO2 and six electrons per CH3 OH molecule. The overall reaction for methanol can therefore be formulated as: CH3 OH + H2 O → CO2 + 6H+ + 6e−

(3)

The thermodynamic potential is 0.04 V vs. RHE, that is, very near to the hydrogen electrode potential. However the adsorbed intermediates of the reaction are difficult to be oxidized causing an overpotential of several hundred millivolts until reasonable reaction rates can be observed [2]. Studies of the methanol oxidation reaction (MOR) on bare Pt have shown that the oxidation of this alcohol is inhibited by poison formation and the poison has been identified as a –CO or a –COH species [1,47–49]. The –COH species have been suggested to be a detectable intermediate in the formation of –CO on Pt [50]. The initial adsorption steps comprise the formation of Pt–O and Pt–C bonds by ␣-H and ˇ-H elimination.

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Successive H abstractions by the Pt surface lead to the formation of –COH, then to –CO, in the linear or bridge conformations. Generally, on Pt-Sn surfaces there is a lower onset for oxidation currents in the oxidation of CH3 OH when compared to bare Pt surfaces, attributed to earlier poison oxidation by the presence of excess O at the surface, supplied by Sn species. The presence of Sn atoms near to Pt, however, can have negative effects on methanol adsorption and dehydrogenation. Pt-Sn nanocomposites have been prepared by a variety of electrochemical or chemical methods, and inconsistencies in catalyst performance have been reported. Indeed, as can be seen in detail in the following paragraphs of this section, an enhanced catalytic activity of Pt-Sn catalysts for methanol electro-oxidation, in contrast to no/negligible enhancement of the MOR rate over Pt-Sn catalysts, was reported. For example, Frelink et al. [51] investigated the effect of Sn on methanol oxidation in H2 SO4 using electrodeposited Pt and carbon-supported Pt, and observed that the preparation has a considerable influence, as the Sn effects range from a small increase to a decrease in methanol oxidation activity. Pt was electrodeposited on a smooth Pt electrode from an H2 PtCl6 solution, then, Sn was added by immersing the electrode in SnO/KOH at 80 ◦ C. In the low potential region the MOR activity was a factor 10 larger in the presence of Sn. Measurements of the activity as a function of Sn surface coverage showed that high Sn coverages (>50%) give a decrease in activity, whereas Sn coverages below 50% give an increase in activity. Three different methods were used to prepare carbon-supported Pt-Sn catalysts. (1) Sn was added to Pt/C catalyst by immersion of the electrode in a solution of either SnO/KOH at 80 ◦ C or an aqueous solution of SnCl4 at 23 ◦ C. The amount of Sn on the electrode varied with the adsorption time. Two Sn coverages, 13 and 26%, were investigated. In the potential region between −0.2 and 0 V vs. MSE the MOR activity increased, and the catalyst with 13% Sn showed the highest activity. At higher potentials, however, the activity was lower than that of pure Pt/C. (2) Pt-Sn sol was prepared by adding SnCl4 to a H2 PtCl6 solution, in a stoichiometry of Pt:Sn = 10:l, then the Pt-Sn sol was adsorbed on the carbon support. XPS measurement showed the presence of mainly Pt metal, and Sn only in the oxide form. The Pt:Sn ratio was found to be 8:1. The MOR activity on Pt-Sn/C was about a factor 2 higher than on the Sn-free catalyst. (3) Impregnated Pt-Sn/C was prepared by mixing the carbon support with a dissolved 5:1 PtSn chloride complex. Formaldehyde was then added as a reducing agent. By XPS measurements the ratio of Pt:Sn was found to be 4:3, implying a very strong Sn enrichment of the surface. The MOR activity was lower than that of the Sn free catalyst. For all the catalysts, the optimum Sn surface coverage was found to be low; of the order of 10%. Indeed, as a secondary effect of Sn is to hinder the methanol adsorption, the optimal Sn coverage must be low. As in the case of CO electro-oxidation, we have separated the Pt-Sn catalysts for methanol electro-oxidation in non-alloyed, fully alloyed and partially alloyed catalysts. 4.2. Methanol oxidation on non-alloyed Pt-Sn catalysts 4.2.1. Methanol oxidation on platinum-tin oxide/hydroxide catalysts Firstly, Cathro [52] found that electrodeposited platinum-tin mixtures have enhanced activity toward the oxidation of methanol, formaldehyde, and formic acid when compared with platinum black. This enhancement was attributed to a redox mechanism, with the tin being assumed to be present in an oxidic form. On the basis of the values of the standard potential of the redox couple Sn(OH)2 /Sn(OH)4 (Eo = 0.075 V vs. SHE at 25 ◦ C), he suggested that a direct redox reaction of Sn(OH)4 with the strongly adsorbed residue from methanol takes place. The reduced oxide is then reoxidized electrolytically. Katayama [53] carried out methanol

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electro-oxidation on a platinum-tin oxide electrode in an aqueous acidic solution. Conversely to Cathro [52], he asserted that the redox coupling of ionic tins is not likely to act toward the MOR enhancement, but rather the redox coupling of Pt0 /Pt2+ or Pt2+ /Pt4+ . Indeed, XPS data for a Pt-SnOx electrode showed the presence of ionic Pt2+ and Pt4+ species in the catalyst. This result was interpreted in terms of a stabilization of platinum oxides in the presence of SnOx . Platinum orbitals are occupied by OH or O bonds and are not available to adsorb organic residues. Platinum stabilized in ionic forms is likely to enhance the catalytic activity of Pt-SnOx , and the action of redox coupling of Pt0 /Pt2+ or Pt2+ /Pt4+ takes a part in the mechanism of the electrooxidation of methanol. The assertion of Katayama [53] was supported by Aramata et al. [54], which studied the catalytic activities of Rh-Sn oxide, Ir-Sn oxide and Pt-Sn oxide. Tin oxide showed a pronounced enhancement of the MOR activity on Pt-Sn oxide with respect to Pt itself in acidic solutions, but not in alkaline solutions. In the case of Rh-Sn oxide, tin oxide had a negative catalytic activity effect, with respect to Rh, and had no effect in the case of Ir-Sn oxide, with respect to Ir. This difference was correlated to the mediator action of platinum species at oxidative states of Pt4+ and Pt2+ with Pt0 in Pt-Sn oxide, since such platinum states are stabilized by tin oxide in acidic solution. Sobkowski et al. [55] electrodeposited tin in the underpotential condition (upd) from a tin(IV) solution on a platinum surface. They found that tin is not adsorbed in the form of ad-atoms but rather as divalent hydroxy- or sulphate complexes. Enhancement of the electrocatalytic oxidation of chemisorbed species derived from methanol was observed in the potential range from 0.4 to 0.8 V vs. SHE. Bittins-Cattaneo and Iwasita [56] used on line mass spectroscopy to study the interactions between adsorbed tin and methanol adsorbate on Pt electrodes. In agreement with Sobkowski et al. [55], they reported that the catalytic effects of adsorbed tin on platinum upon methanol oxidation seem to be due to the presence of Sn(II) species. Taking into account the electronic configuration of Sn(II) and Pt it seems quite possible that Sn(II) forms a hydroxycomplex like Sn(OH)+ which should offer oxygen atoms for the oxidation of methanol adsorbate to CO2 . In the presence of coadsorbed tin, methanol adsorbate oxidation occurs at potentials 0.15 V lower than on pure platinum. Hable and Wrighton [57] investigated the methanol oxidation at polyaniline-coated electrodes modified with Pt and Pt-Sn particles in aqueous H2 SO4 at 25 ◦ C. XPS analysis showed that Pt is present mainly as Pt(0) and Sn is present in an oxidized state. The activity for methanol oxidation on Pt-Sn particles was higher than that on Pt alone. The most active catalysts typically showed surface ratios of Pt:Sn between 8:1 and 2:l. The onset potential of methanol oxidation on Pt and Pt-Sn was at about +0.2 and 0 V vs. SCE, respectively, and at +0.4 V vs. SCE the peak current for the electrode with Pt-Sn was nearly 4 times higher than for the electrode with Pt alone. Above +0.5 V vs. SCE, the activity for methanol oxidation is inhibited compared to Pt alone. In contrast, the electrodes containing Pt alone typically showed a peak in activity for methanol oxidation at +0.75 V vs. SCE. The same research group [58] studied the electrocatalytic oxidation of small carbohydrate fuels at Pt-Sn-modified electrodes. Gold and glass carbon electrodes were modified with binary Pt-Sn catalyst particles by cycling the electrodes in a solution containing Pt(IV) and Sn(IV) complexes. XPS surface analysis of the Pt-Sn catalysts showed that Sn is present in an oxidized state on the surface, while Pt is fully reduced. The Sn content varied with sample depth. On the surface, the Pt:Sn ratio was approximately 3:1, while in the bulk the Pt:Sn ratio increased to 9:1. Methanol was the least reactive primary alcohol from the series studied in this work. On bare Pt, no oxidation of methanol, formaldehyde, or formic acid occurred without poison formation. In contrast, on Pt-Sn, poison formation seems to depend strongly on the structure of the fuel molecule. Oxidation of formaldehyde on Pt-Sn was very efficient, due to hydration; for-

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mation of poison is slow compared to the rate of conversion of formaldehyde to formic acid and/or CO2 . Generally, hydration of the aldehyde group likely plays a key role in the electrocatalytic oxidation of aldehydes on Pt-Sn: hydration leads to conversion of the aldehyde group to a diol, resulting in a more efficient oxidation on Pt-Sn. They remarked the large differences in the onset potential for methanol, formaldehyde and formic acid oxidation. Formaldehyde was oxidized most readily on Pt-Sn surfaces. Formic acid was also readily oxidized. Methanol, on the other hand, is the least reactive among the three molecules in the path to CO2 formation. Hence, the first 2e− oxidation, the conversion of methanol to formaldehyde, is the efficiency-determining step in the oxidation of methanol to CO2 . The elementary step which determines the rate might be either C–H bond breaking or C–O bond formation. De Oliveira et al. [59] studied the electro-oxidation of methanol on Pt-My Ox (M = Sn, Mo, Os or W) electrodes prepared by the decomposition of polymeric precursors on Ti plates. CV and chronoamperometry (CA) results showed that the Pt-Sny Ox , PtWy Ox and Pt-Moy Ox electrodes promote the oxidation of methanol from potentials of about 400 mV. The electrodes containing tin presented the highest electroactivity. The same research group [60] prepared electrodes through the electrodeposition of platinum micro particles on SnO2 thin films in order to verify the application of this system as a catalyst for the electro-oxidation of methanol. The employed oxides provide a good matrix for the dispersion of platinum particles since they present high roughness. The maximum electro-oxidation current was attained for a platinum content of approximately 600 ␮g cm−2 . CA results showed that the current values obtained for the electro-oxidation of methanol were up to 10 times higher than the current values obtained with platinized platinum under the same conditions. Such increase was attributed to different factors. One is a structural effect similar to that observed when one compares the activities of smooth Pt with that of Pt platinized electrodes. The higher dispersion of Pt particles can lead to small particles and, consequently, to a large number of highly active surface sites that favor the adsorption of OH species at lower potentials. In this case, the oxide film would only function as an efficient matrix for Pt particles dispersion. Another reason was associated to the nature of the oxide film itself and, in this case, two main effects can be taken into account, that is, tin can promote (1) the oxidation of weakly bonded CO, and (2) the formation of OHads , which can combine with adsorbed CO to remove these latter species from the surface. Finally, in a recent work, Cui et al. [61] prepared Pt and Pt-SnO2 catalysts supported on graphitized mesoporous carbon, and investigated their activity for methanol oxidation. CV curves of these catalysts toward methanol oxidation showed that, after tin oxide addition, the MOR activity of Pt is much enhanced compared to that of Pt alone, meaning that the SnO2 assists platinum in methanol electro-oxidation, although the Pt loading amount of the former is only half of the latter. 4.2.2. Methanol oxidation on Pt catalysts modified by zero-valent Sn Conflicting results have been reported in literature regarding the methanol oxidation on Pt catalysts modified by zero-valent Sn. Firstly, to elucidate the mechanism of the enhanced methanol oxidation on Pt-Sn electrodes, Janssen and Moolhuysen [62,63] prepared three types of Pt-Sn electrodes, that is, immersion-type, electro-codeposited and alloyed Pt-Sn electrodes. All these Pt-Sn electrodes showed on average a 50-fold increased activity per unit area as compared with pure platinum. By CV measurements carried out on all these electrodes, they demonstrated that their surfaces are identical on an atomic scale and that the tin atoms are present in the zero-valent state at the platinum surface at potentials where methanol oxidation takes place. Furthermore they showed that the zero-valent tin atoms influence the adsorption properties of the

platinum atoms. Changes in the adsorption properties of platinum due to a strong interaction with adsorbed tin are responsible for stronger adsorption of H2 O, presumably in the form of OH. On this basis, as well as on the basis that no tin redox peaks were observed in the voltammograms and that on Pt-Sn methanol oxidation takes place with an order of less than one, they concluded that the enhancement in methanol oxidation rate on Pt-Sn is brought about not by a redox but by an adsorption mechanism. Watanabe at al. [64] developed a simplified and versatile preparation method of the electrode having high specific surface area platinum with a well-defined tin coverage. It consists of underpotential deposition of tin ad-atoms and subsequent anodic treatment resulting in uniform dispersion of the Sn ad-atoms over all the platinum clusters in the catalyst layer of the electrode. The resulting electrode showed a remarkable enhancement in the specific activity over the pure platinum black electrode by a factor of 100 for methanol oxidation, of more than 1000 for formaldehyde oxidation and of 250 for formic acid oxidation, respectively. This method has the advantage that a much higher specific surface area of catalyst is obtained, as well as an optimum composition, compared with the electrochemical co-deposition or immersion methods proposed previously. The question was raised whether the Sn ad-atoms are able to remain on the surface of the electrode for a long period at operational conditions. After operating continuously for periods exceeding 20 h at 0.4 V vs. RHE in the methanol cell, they observed almost no reduction of the ad-atom coverage. More recently, Wei et al. [65] studied the electrochemical oxidation of methanol on underpotentially deposited-ruthenium-modified platinum electrode (upd-Ru/Pt) and on underpotentially deposited-tin-modified platinum electrode (upd-Sn/Pt). The electro-oxidation of methanol on the upd-Sn/Pt electrode is shown in Fig. 4, together with that on Pt and upd-Ru/Pt electrodes for comparison. The submonolayers of upd-Ru and upd-Sn on a Pt electrode increased the rate of methanol electro-oxidation several times than on a pure Pt electrode. Methanol oxidation on the upd-Sn/Pt electrode shifted toward an even more negative potential than that on upd-Ru/Pt. The effects of tin were sensible to the potential range. In a potential range from 0 to 0.22 V vs. a saturated-potassium-chloride silver chloride electrode (SSCE), the current of methanol oxidation on upd-Sn/Pt with underpotential deposition of 100 s was larger than that on upd-Ru/Pt. It suggests that the upd-Sn ad-atoms are more favorable to enhance the methanol electro-oxidation in the lower potential range than the upd-Ru ad-atoms. Over the potential of 0.22 V vs. SSCE, the low coverage of upd-Sn deposits on the Pt surface was favorable for methanol electro-oxidation. The enhancement effect of upd-Sn ad-atoms for the MOR disappeared as the electrode potential is beyond a certain value. Conversely the above reported results, Beden et al. [66], Campbell and Parsons [67] and Haner and Ross [68] found that Sn ad-atoms decrease the electrocatalytic activity of pure platinum. Beden et al. [66] performed a voltammetric study of the influence of deposited tin on the activity for methanol electro-oxidation of Pt. Sn ad-atoms decreased the MOR activity of platinum, and an inhibiting effect in some concentration ranges of the precursor salt, or in some potential ranges of the electrode was observed. The oxygen adsorption was practically unmodified by upd of Sn ad-atoms from the more dilute solutions. Tin ad-atoms also had an inhibiting effect on the methanol adsorption. Campbell and Parsons [67] investigated the effect of adsorbing submonolayers of tin onto single-crystal, polycrystalline and dispersed platinum electrodes on the oxidation of formic acid and methanol. They found that the oxidation of formic acid and methanol was inhibited by all adsorbed coverages of tin on all electrodes. The adsorbed tin was more stable on carbonsupported Pt than on the smooth electrodes but inhibition was also found here. Haner and Ross [68] modified single-crystal faces of pure Pt by electrodeposited/adsorbed tin, state. A decrease in the

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9

ratio. With more tin, the activity at platinum becomes less. Because another effect of tin is to impede methanol adsorption, the optimal tin surface composition must be low. 4.3. Methanol oxidation on Pt3 Sn and Pt-Sn alloys

Fig. 4. (1) Methanol electro-oxidation in 0.5 M H2 SO4 + 0.5 M CH3 OH at a sweep rate of 5 mV s−1 on a pure Pt electrode (a), a upd-Ru/Pt electrode formed in 0.5 M H2 SO4 containing 5 mM RuCl3 at Eupd = 0.58 V with Qupd = 10 mC cm−2 (b), and upd-Sn/Pt electrodes formed in 0.5 M H2 SO4 containing 10−5 M SnSO4 at Eupd = 0.172 V hold for 10 s (c) and for 100 s (d). (2) The potential window of 1 from 0 to 0.25 V. Reproduced from Ref. [65], copyright 2006, with permission from the American Chemical Society.

rate of reaction occurred with the addition of tin. The methanol oxidation rate decreased as the amount of irreversibly adsorbed tin increased. To explain the effect of tin ad-atoms on methanol oxidation, Ishikawa et al. [69] carried out a relativistic density-functional study of the elementary steps of CH3 OH oxidation on pure platinum and mixed Pt-Sn metals. Cluster models of Ptn Sn10−n were used to simulate the metal surfaces. They found that the presence of tin in the cluster weakens the platinum—methanol bond. The same is true for other adsorbates (CH2 OH, CHOH and CHO). On the (Pt3 )(Sn4 Pt3 ) cluster, with the highest Sn/Pt ratio, the adsorption energy on Pt is decreased dramatically in comparison to adsorption on pure platinum. Similar trends are found in other mixed cluster species. Tin is a poor electron acceptor: the tin site in (Pt3 )(Pt4 Sn3 ) does not adsorb CH3 OH (Eads = 0). The dehydrogenation of CH2 OH on (Pt3 )(Sn2 Pt5 ) has an activation energy of 0.75 eV, 0.2 eV higher than that on pure platinum. On (Pt3 )(Sn4 Pt3 ), the dehydrogenation of CH2 OH has even higher activation energy (0.84 eV). The dehydrogenation energies of CH3 OH and CH2 OH on the cluster also become endothermic. The results indicated that a Pt-Sn surface with a high atomic percentage of tin is not conducive to CH3 OH dissociation. Hence, the optimal tin surface coverage must be low. The dissociation of H2 O at a tin site in Pt-Sn is only slightly more favorable than at a platinum site. The presence of Sn atoms reduces the Pt–CO bond strength substantially, indicating the existence of a ‘ligand effect’. They concluded that the activity of a platinum site in mixed Pt-Sn for CH3 OH dissociation should vary somewhat with atomic Sn/Pt

4.3.1. Pt3 Sn single crystals Haner and Ross [68] studied the geometric and electronic effect of tin atoms on the platinum surface by using single-crystal faces of the ordered Pt3 Sn alloy. They found that none of the alloy surfaces were more effective catalysts than any of the pure platinum surfaces and that alloying platinum with tin to any extent significantly reduced the activity. They proposed that the effect of tin is primarily an electronic effect. The voltammetry data support the conclusion that there is a very strong “ligand effect” on the way methanol adsorbs on the Pt surface due alloying the Pt with Sn, but this effect is not beneficial for catalysis. Wang et al. [30] investigated the activity of the Pt3 Sn(1 1 0) single-crystal surface toward methanol oxidation in acid solution. A small enhancement of methanol oxidation was observed in long time potential step measurements. The steady-state activity of the Pt3 Sn(1 1 0) surface showed a factor of 3 enhancement (which is small compared with that of Pt-Ru alloys) for methanol oxidation over the pure Pt, but for times smaller than ca. 10 s. the Pt3 Sn(1 1 0) surface was less active than Pt, consistent with that reported with potentiodynamic measurements [68]. The other low index surfaces of Pt3 Sn showed no enhancement over Pt even at long times. On the other hand, as previously reported, Pt3 Sn is very active for the oxidation of dissolved CO, with an onset potential approximately 300 mV lower than the most active Pt-Ru alloy surface, and more than 500 mV lower than on polycrystalline Pt. The apparently paradoxical results can be explained in terms of a unique state of COads on this surface, which is only formed at high coverages by direct adsorption from dissolved CO and is not formed by the dehydrogenation of methanol, since the multiple Pt atom sites needed to dehydrogenate methanol are blocked by COads at low coverage. 4.3.2. Polycrystalline Pt3 Sn and PtSn alloys A poor stability of polycrystalline Pt-Sn alloys in acid media, which can positively influence their MOR activity, has been reported in literature. Janssen and Moolhuysen [62,63] submitted polycrystalline PtSn (50:50) and Pt3 Sn alloys to repetitive potential cycling (20–100 cycles between 0 and +1.6 V vs. SHE) in a H2 SO4 /CH3 OH solution. The samples initially showed large corrosion currents. After one cycle the corrosion current already decreased to low values but polarization curves for methanol oxidation were recorded after prolonged cycling. The alloy catalysts showed high MOR activity, similar to that of Pt-Sn catalysts prepared by the immersion technique and by electrodeposition. Moreover, their tin coverages were also very much the same, indicating that they have similar surfaces. Beden et al. [66] carried out a repetitive potential cycling between −0.6 and 0.6 V vs. MSE on Pt-Sn in a H2 SO4 /CH3 OH solution. The evolution of the voltammograms showed that the Pt-Sn alloys are not at all stable, and that Sn is preferentially dissolved in the electrolyte solution. After about 100 cycles, the voltammogram was like that of pure platinum. Moreover, the surface area decreased during the continuous potential cycling, which can certainly be attributed to a superficial rearrangement of the platinum deposit. The change of the oxidation current of methanol during potential cycling showed that, when the electrode surface content in Sn decreases, due to its preferential dissolution in the electrolyte, the oxidation current increases, whereas the anodic polarization curve is shifted to more anodic potentials. Liu et al. [70] prepared polycrystalline Pt3 Sn nanoparticles with controlled size and narrow size distribution via polyalcohol reduction of platinum acetylacetonate and tin acetylacetonate in trioctylamine. The

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carbon-supported Pt3 Sn particles exhibited a high activity for both CH3 OH and CO oxidation; no comparison, however, was made with pure Pt. By comparison of the voltammogram of Pt3 Sn particles in 1.0 M H2 SO4 solution after 100 scan cycles between −240 and 500 mV vs. SCE with the voltammogram acquired in the first scan little changes were observed, suggesting that the Pt3 Sn catalysts are structurally and chemically stable in the acid environment when the applied potential is lower than 500 mV. However, when the positive potential limit was extended to 1000 mV vs. SCE, the distinct hydrogen adsorption/desorption peaks characteristic of pure Pt were observed due to the dissolution of Sn on Pt3 Sn catalyst surface. Thus, the alloyed particles are only stable in acid environment when the applied potential is lower than 500 mV. 4.4. Methanol oxidation on partially alloyed Pt-Sn catalysts Many papers reported methanol oxidation on partially alloyed Pt-Sn catalysts, commonly formed by a fcc Pt(1−x) Snx solid solution, but also by the Pt3 Sn alloy phase, and tin oxide. Honma and Toda [71] investigated the temperature dependence of methanol oxidation for Pt and Pt-Sn catalysts in the temperature range from 25 to 140 ◦ C. The nominal Pt:Sn atomic composition was 1:1. XRD analysis showed the presence of the Pt3 Sn alloy phase. As a consequence non-alloyed tin has to be present in the catalyst either as Sn(0) or SnOx . In all the temperature range investigated, PtSn catalysts exhibited electrocatalytic properties superior to those of Pt. The MOR current densities at 0.1 V (vs. Ag/AgCl) of Pt and Pt-Sn samples increased almost linearly with the temperature up to 140 ◦ C. The MOR current densities of Pt-Sn at 140 and 25 ◦ C were about five times and eight times larger than that on pure Pt, respectively. The MOR onset potential decreased with increase of temperature both for Pt and Pt-Sn samples, and in particular, the onset potential for Pt-Sn became remarkably smaller when increasing the temperature to 140 ◦ C. Zhou et al. [72] studied DMFCs employing carbon-supported Pt, Pt-Sn and Pt-Ru as anode catalysts, respectively. The atomic ratio of Pt to Ru or Sn was = 1:1. XRD analysis indicated the formation of a fcc Pt(1−x) Snx solid solution with a lattice parameter of 0.3987 nm. Comparing nominal (Pt/Sn = 1) and XRD (Pt/Sn > 3/1) compositions, the presence of a large amount of non-alloyed Sn has to be inferred. They found that the addition of Ru or Sn to the Pt considerably enhances the electrooxidation of methanol, and that Pt-Ru/C is more suitable for use as anode catalyst in DMFCs than Pt-Sn/C. Zheng et al. [73] prepared Pt-Sn bimetallic nanoparticles by a hydrothermal method. The atomic Pt:Sn ratio of the Pt-Sn catalyst by inductively coupled plasma–atomic emission spectroscopy (ICP–AES) was 1:1.85. XRD analysis showed the formation of the Pt3 Sn alloy phase. XPS spectrum showed two peaks: one at 485.7 eV and other at 487.4 eV, attributed to the presence zero-valent Sn and to oxidized species of Sn(II/IV), respectively. Taking account of the XPS, XRD and the ICP-AES data, they concluded that a fraction of Sn would be Sn(0) alloyed with Pt, and the remaining Sn would be Sn(II/IV) oxides bounded to the support. Electrocatalytic oxidation of methanol at the Pt-Sn nanoparticles showed remarkably enhanced activity and lifetime compared with that at Pt nanoclusters. The Pt-Sn nanoparticles promoted the oxidation of methanol by lowering its overpotential and enlarging its peak current. The catalytic lifetime of Pt-Sn nanoparticles was 4 times higher than that of Pt nanoclusters. The effect of Sn content in partially alloyed Pt-Sn catalysts on the MOR activity has been reported in different papers [43,74–76]. In all these works, XRD analysis indicated the formation of fcc Pt(1−x) Snx solid solutions. Sn content in the Pt(1−x) Snx alloy increased directly with the amount of Sn precursor, but only part of the tin present in the catalysts was in an alloyed form. The presence of tin oxide was also reported. Colmati et al. [74] prepared Pt-Sn catalysts

Fig. 5. Polarization curves and power density curves in single DMFC with Pt90 Sn10 /C and Pt75 Sn25 /C prepared by FAM and commercial Pt/C and Pt75 Sn25 /C by E-TEK as anode electrocatalysts for methanol oxidation at 70 ◦ C and 1 atm. O2 pressure (a) and at 90 ◦ C and 3 atm. O2 pressure (b) using a 2 mol L−1 methanol solution. Anode metal loading 0.4 mg cm−2 . Cathode 20 wt% Pt/C, Pt loading 0.4 mg cm−2 . () Pt/C E-TEK; ( ) Pt90 Sn10 /C; ( ) Pt75 Sn25 /C; (♦) Pt75 Sn25 /C E-TEK. Reproduced from Ref. [74], copyright 2005, with permission from Elsevier.

with different Sn content by the formic acid method (FAM). The onset potential of methanol oxidation of all the Pt-Sn catalysts was lower than pure Pt, but increased with Sn content in the alloy. The DMFC polarization and power density curves for Pt90 Sn10 /C and Pt75 Sn25 /C by FAM and for commercial Pt/C and Pt75 Sn25 /C are shown in Fig. 5, where it can be seen that, notwithstanding the Pt loading on anode catalyst layer was lower (0.37 mg cm−2 for Pt90 Sn10 /C and 0.33 mg cm−2 for Pt75 Sn25 /C against 0.4 mg cm−2 for Pt/C) and the metal particle size was higher (3.3 nm for Pt90 Sn10 /C and 3.9 nm for Pt75 Sn25 /C against 2.9 nm for Pt/C) than Pt/C, the performance in terms of geometric area of the cell with Pt90 Sn10 /C and Pt75 Sn25 /C by FAM was better than that with Pt/C, particularly at low current density. It is important to remark the effect of the current density. As can clearly be observed in Fig. 6 (cell potential vs. lattice constant plot), at low current density (0.01 A cm−2 ) the potential of the cells with Sn-containing catalysts was more than 100 mV higher than Pt. At 0.64 A cm−2 , instead the cell potential decreased with the content of Sn in the catalyst. This result can be ascribed to the poor methanol adsorption/dehydrogenation due to the increase of Sn content in the Pt(1−x) Snx alloy. Indeed, at low current density, a low amount of methanol is required for the cell operation, then the CH3 OH adsorption/dehydrogenation is less important, and the rate of the MOR is determined by the COads oxidation. With increasing methanol expend, the methanol adsorption/dehydrogenation becomes the determining step of the MOR. The maximum power density, instead, results from an optimum balance of CH3 OH adsorption and CO oxidation. While the lowest onset potential for MOR was obtained by the Pt90 Sn10 /C electrocatalyst, in this case the best performance (power density per geometric area and maximum power density per Pt active area was attained by the cell with the Pt75 Sn25 /C catalyst by the

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Fig. 6. Cell potential at various current densities vs. Pt-Sn lattice parameter. Current densities: () 0.01 A cm−2 ; ( ) 0.40 A cm−2 ; ( ) 0.64 A cm−2 . Reproduced from Ref. [74], copyright 2005, with permission from Elsevier.

FAM. The high performance of Pt75 Sn25 /C by the FAM is due to the optimal mixing of Sn content, degree of alloying and particle size. According to the authors, the poor performance of Pt75 Sn25 /C by E-Tek, instead, results from the high degree of alloying of Pt and Sn, which hinder methanol adsorption, owing to a too large Pt-Pt bond distance and/or a decrease in the Pt d-band vacancies. Liu et al. [75] prepared carbon-supported Pt-Sn nanoparticles by a microwave-assisted polyol process. The electro-oxidation of methanol on these catalysts was investigated at room temperature by CV and CA. The peak current density of methanol oxidation increased in the order Pt65 Sn35 /C > Pt80 Sn20 /C > Pt/C > Pt50 Sn50 /C. Pt35 Sn65 /C was a relatively inactive catalyst toward methanol oxidation (there was virtually no anodic peak). CA measurements showed that for the Pt/C catalyst the current decayed continuously even after 1 h, whereas, among all Pt-Sn/C catalysts, the Pt65 Sn35 /C catalyst was able to maintain the highest current density for over 1 h. Kim et al. [76] prepared a series of carbon-supported bimetallic Pt-Sn catalysts for the electro-oxidation of C1 -C3 alcohols (i.e., methanol (C1 ), ethanol (C2 ), and 1-propanol (C3 )) with different Pt:Sn atomic ratios using the borohydride reduction method (BM) combined with freeze-drying procedure at room temperature. They found that addition of Sn into Pt leads to substantial enhancements in the catalytic activity for the electro-oxidation of alcohols. The peak current densities were improved for methanol, ethanol, and 1-propanol, respectively, over all Pt-Sn/C catalysts compared to the Pt/C catalyst, suggesting that additions of Sn into Pt can significantly improve the reaction rate regardless of the Sn contents up to 50%. Interestingly, the activity enhancements with the addition of Sn become much larger by 2.5-, 3.5- and 6.0-folds, considering the best composition of Pt and Sn to give maximum activity like Pt3 Sn1 /C, Pt2 Sn1 /C, and Pt3 Sn2 /C for methanol, ethanol, and 1propanol, respectively. Lim et al. [43] synthesized highly dispersed Pt-Sn/C electrocatalysts by the BM and subsequent hydrothermal treatment. In a H2 SO4 /CH3 OH solution at 40 ◦ C, the Pt3 Sn1 /C catalyst showed the highest activity for methanol oxidation, owing to the slight increase in Pt lattice parameter favorable to methanol adsorption and the presence of Sn oxide in the vicinity of Pt particles. Especially, the area normalized current density at 0.6 V for the Pt3 Sn1 /C catalyst was about 3-fold higher than that of the commercial Pt/C catalyst. To evaluate the optimum Sn content, we have plotted the Pt-Sn to Pt MOR activity ratio against Sn content, using data from different works [43,75,76]. As shown in Fig. 7, according to both a Gaussian and a Lorentzian distribution, the maximum MOR activity is at a Sn/Pt atomic ratio of 0.40 (Pt:Sn = 2.5:1). The lattice parameter, instead, almost linearly increases with Sn content in the catalyst.

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Fig. 7. Dependence of the lattice parameter and the Pt-Sn to Pt MOR activity ratio against Sn content in the catalyst, obtained using data from different works [43,75,76]. The full lines represent the Lorentzian fit of all the MOR activity data. Full symbols: MOR activity; open symbols: lattice parameter. Squares: Lim et al. [43]; circles: Liu et al. [75]; triangles: Kim et al. [76].

On the basis of the model of Antolini and Gonzalez [82], based on the presence in partially alloyed Pt-Sn/C catalysts of Pt, Pt3 Sn alloy and SnOx , and on the independent activity for ethanol oxidation of Pt3 Sn and Pt-SnOx , and considering that the amount of Pt3 Sn phase linearly depends on the lattice parameter, the presence of a maximum in the MOR activity vs. Sn content plot can be explained by the positive effect of SnOx on the MOR activity of Pt, and by the negative effects of the Pt3 Sn phase (poor MOR activity) and SnOx (reduction of available surface Pt active sites), both the negative effects increasing with the increase of Sn content in the catalyst. 4.5. Comparison with Pt-Ru catalysts Firstly, it has to be remarked that, while alloyed, partially alloyed and non-alloyed Pt-Ru catalysts all present a high MOR activity for different Ru contents, in the case of Pt-Sn catalysts the MOR activity strongly depends on their structural characteristics and Sn content. Generally, comparison between Pt-Ru and Pt-Sn catalysts indicated that the former are more active for methanol oxidation, either in non-alloyed [12], partially alloyed [72,77], metal-adsorbed [78,79] or single-crystal [30] form. Some works, however, reported a higher activity of Pt-Sn catalysts for methanol oxidation than that of Pt-Ru [65,80,81]. In these works, Pt-Sn catalysts were in form of either Sn ad-atom modified Pt, partially alloyed Pt-Sn/C or non-alloyed Pt-SnO2 . Wei et al. [65] found that in a potential range from 0 to 0.22 V vs. SSCE, the current of methanol oxidation on upd-Sn/Pt with underpotential deposition of 100 s was larger than that on upd-Ru/Pt. Oliveira Neto et al. [80] prepared partially alloyed Pt-Ru/C and Pt-Sn/C electrocatalysts by the alcohol reduction process. By chronoamperometry measurements using a thin porous coating technique they found that the Pt-Sn/C electrocatalyst has higher activity for methanol oxidation at room temperature compared to Pt-Ru/C. Liu et al. [81] prepared carbon-supported Pt, PtSnO2 and PtRu particles were prepared by As evidenced by XPS, most Pt, Sn and Ru atoms in the nanoparticles were Pt(0), Sn(IV) and Ru(0). Among three catalysts, preliminary tests on a single cell of a DMFC indicated PtSnO2 /C as the best anode catalyst. 5. Conclusions Commonly, the catalytic activity of Pt-Sn catalysts is compared with that of Pt and Pt-Ru. The activity of Pt-Sn for H2 /CO oxidation is always higher than that of Pt. Compared to Pt-Ru, the activity for

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Table 1 Electrochemical activity and catalytic effects for the oxidation of CO, H2 /CO and CH3 OH of Pt-Sn catalyst with different structures. Fuel

Catalyst

Activity

Effect

References

Sn ad-atom modified Pt

High

[14–19]

Pt/SnOx

High

Pt3 Sn

Very high

Bifunctional and electronic effects. Oxidation of adsorbed CO at lower potentials as compared to Pt/C. Weakly bonded CO adsorbed on Pt sites adjacent to Sn atoms Bifunctional effect. Oxidation of adsorbed CO at lower potentials as compared to Pt/C Bifunctional and electronic effects. Weakly bonded CO adsorbed on Pt sites adjacent to Sn atoms Bifunctional and electronic effects.

CO and H2 /CO

CH3 OH

Partially alloyed Pt-Sn

High

Pt/SnOx

Moderate/high (low potentials and low Sn content)

Non-alloyed Sn(0)

Moderate/high (low potentials and low Sn coverage) Poor Poor

Pt3 Sn single crystal

Polycrystalline Pt3 Sn Partially alloyed Pt-Sn

Moderate (by Sn dissolution) High (optimum Sn/Pt: ratio 0.4; optimum lattice parameter 0.396 nm)

H2 /CO oxidation of Pt3 Sn is higher by RDE measurements at room temperature, while in PEMFCs the activity of Pt-Sn is slightly lower (Pt3 Sn) [39] or considerably lower (non-alloyed Pt-Sn [12]. In the case of methanol oxidation, instead, the activity of Pt-Sn is always lower than that of Pt-Ru, but can be higher or lower than that of Pt, depending on structural characteristic of Pt-Sn. The catalytic activities of Pt-Sn catalysts with different structural characteristics for the oxidation of CO, H2 /CO and CH3 OH are summarized in Table 1. Different effects of alloyed and non-alloyed Sn on the catalytic activity of Pt are reported, depending on the fuel. 5.1. H2 /CO oxidation Pt3 Sn presents a very good activity for CO oxidation. Both the structure sensitivity and the high catalytic activity of the Pt3 Sn surface were attributed to an adsorbed state of CO unique to this alloy and occurred at relatively high coverage (>0.9 CO/Pt) on the Pt3 Sn surfaces [30,34–36]. Pt/SnOx and Sn ad-atoms modified Pt also show good activity for CO oxidation. In the case of non-alloyed Pt-Sn the enhanced activity for CO oxidation has been mainly ascribed to a bifunctional effect, but the formation of weakly bonded CO adsorbed on Pt sites adjacent to Sn atoms is also reported [17,18,28,31]. In addition to the improved activity for CO oxidation, Pt-Sn catalysts are extremely active for H2 oxidation [42]. 5.2. Methanol oxidation While on bare Pt, no oxidation of methanol, formaldehyde, or formic acid occurs without poison formation, on Pt-Sn, with Sn either as Sn(0) or in an oxidized state, poison formation seems to depend strongly on the structure of the fuel molecule. Indeed, the two intermediates along the path of oxidation to CO2 , formaldehyde and formic acid, are oxidized on Pt-Sn at very negative potentials compared to methanol. Thus, the conversion of methanol to formaldehyde is the efficiency-determining step in the oxidation of CH3 OH to CO2 . Oxidation of formaldehyde on Pt-Sn is very efficient, due to his hydration in the methylene glycol form. Formic acid was also readily oxidized on Pt-Sn surfaces. Methanol, on the other hand, is the least reactive among the three molecules in the path to CO2 formation. Thus, in the stepwise oxidation of CH3 OH to CO2 , the first 2e− oxidation step, i.e., the conversion of methanol to formaldehyde (as methylene glycol), is the rate-determining

Oxidation of adsorbed CO and CHO at lower potentials as compared to Pt/C. No effect on methanol adsorption and C–H bond dissociation. Strong adsorption of H2 O, presumably in the form of OH.

[20–32] [30,33–41]

[42,43] [52–61]

[62–65]

Inhibiting effect on the methanol adsorption Oxidation of adsorbed CO and CHO at lower potentials as compared to Pt/C. Low methanol adsorption and C–H bond dissociation

[66–69] [30,68]

Synergic effect of alloyed and non-alloyed Sn.

[43,71–76]

[62,63,66]

step. There is a significant structural difference between methanol, formaldehyde and formic acid. Oxidation of methanol to CO2 requires the addition of one O atom, besides the loss of six H atoms. However, formaldehyde exists 99.99% hydrated in aqueous solutions in the form of a diol, methylene glycol (H2 C–(OH)2 ). Thus, the two necessary C–O bonds that will constitute the final product CO2 are already present in both formaldehyde (as two seminal C–OH bonds) and HCOOH (as one C O bond and one C–OH bond) [58]. Generally, the MOR activity of non-alloyed Pt-Sn catalysts is higher than that of Pt at low potentials and for low Sn contents in the catalyst (in the order of 10 at%). Controversial results of methanol oxidation on Sn ad-atoms modified Pt, however, are reported in literature. Comparison of literature data is complicated by many factors which influence the electrocatalytic processes, namely, the state of the electrode surface (smooth or rough with various roughness factors, polycrystalline or monocrystalline), the kind of supporting electrolyte (different energies of ad-atom adsorption on platinum owing to competition with anion adsorption and complex formation in the bulk of the solution), the concentration of the adsorbed substance (self-inhibition), the sequence of addition of the fuel and promotor to the supporting electrolyte (preadsorption of one component influences the adsorption of the second one), the range of polarization potentials (the initially formed chemisorbed products can be reduced by hydrogen adsorbed on the electrode surface), etc. Moreover, to further complicate the evaluation of the MOR activity of the Sn ad-atoms modified Pt, that is, whether this arises from the operation of a cyclic Sn(II)/Sn(IV) redox system or from modification of the platinum surface by Sn(0), in some cases the presence of oxidized tin in Sn ad-atoms modified Pt catalysts was reported [83,84]. Wang et al. [30] showed that Sn adsorbed on Pt from solution has a higher activity for methanol oxidation than Pt3 Sn, the latter actually being less active than Pt. This apparently paradoxical result can be explain in terms of the need to balance the adsorption of methanol on Pt sites and the oxidative removal of COads . At 25 ◦ C the enhancement is maximized at a very low coverage of Sn, consistent with an absence of adsorption of methanol on Sn ad-atoms and blocking effect of Sn on the Pt ensemble needed for dehydrogenation. The surface concentration of Sn in Pt3 Sn is too high for methanol dehydrogenation. Besides the issue of Sn coverage, Wang et al. [30] also argued that COads formed from dehydrogenation of methanol also has an unique inhibiting effect on the rate of methanol oxidation and is not the same as COads formed during

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the oxidation of COad formed from adsorption in a solution saturated with CO. In particular, there are differences in the nature (coverages) of COads produced from the two sources: the coverage from methanol dehydrogenation is much lower, thus it forms the strongly adsorbed state of COads which is a relatively inactive state. Unlike Pt3 Sn single crystals, polycrystalline Pt3 Sn showed a high MOR activity, due mainly to catalyst corrosion [59,63]. Indeed, the change of the oxidation current of methanol during potential cycling showed that, when the electrode surface content in Sn decreases, due to its preferential dissolution in the electrolyte, the oxidation current increases. Moreover, differences between the characteristics of single-crystal and polycrystalline Pt3 Sn are reported by Arenz et al. [42]. Vibrational spectroscopy of adsorbed CO by in situ FTIR showed no peak splitting of the a-top CO on the polycrystalline Pt3Sn. Consequently, no formation of the compressed CO adlayer characteristic for the Pt3 Sn(1 1 1) electrode [35] occurred. It would appear that the formation of this compressed adlayer requires larger Pt3 Sn(1 1 1) facets that are formed on the nanoparticles in carbon-supported Pt3Sn catalysts. Thus, in addition to Sn dissolution, this feature could explain the different catalytic behaviour for methanol oxidation of single-crystal and polycrystalline Pt3 Sn. Regarding the partially alloyed Pt-Sn catalysts, the presence of a maximum in the MOR activity vs. Sn content plot can be explained by the positive effect of SnOx on the MOR activity of Pt, and by the negative effects of the Pt3 Sn phase (poor MOR activity) and SnOx (reduction of available surface Pt active sites), both the negative effects increasing with the increase of Sn content in the catalyst. The Sn/Pt atomic ratio of 0.40 seems the best compromise between the different catalytic effects of Pt3 Sn and SnOx . Summarizing, compared to most used Pt-Ru catalysts, in general Pt-Sn catalysts present a lower catalytic activity both for H2 /CO oxidation in PEMFCs, although a higher activity for CO and H2 /CO oxidation of Pt3 Sn than Pt-Ru was observed by RDE measurements, and for methanol oxidation. For CO and H2 /CO oxidation alloyed Pt3 Sn catalysts present a very high activity, but, to compete with the state-of-the-art Pt-Ru PEMFC anode, future works should be addressed to development of Pt3 Sn/C catalysts with higher CO tolerance than Pt-Ru also in PEMFCs, as observed by RDE measurements. In the case of methanol oxidation, the best performance has been shown by non-alloyed Pt-SnOx . Because the most active form of the catalyst is a mixture of Pt and SnOx phases rather than a Pt3 Sn alloy phase in which Pt and Sn are intimate neighbors, synthesis methods have to be developed, which maximize the area of junction between Pt and SnOx , to obtain a high activity for methanol oxidation. Acknowledgment The authors thank the Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Proc. 310151/2008-2) for financial assistance to the project. References [1] R. Parsons, T. Vandernoot, J. Electroanal. Chem. 257 (1988) 9. [2] X.H. Xia, T. Iwasita, F. Ge, W. Vielstich, Electrochim. Acta 41 (1996) 711. [3] V.I. Kuznetsov, A.S. Belyi, E.N. Yurchenko, M.D. Smolikov, M.T. Protasova, E.V. Zatolokina, V.K. Duplayakin, J. Catal. 99 (1986) 159. [4] V. Radmilovic, T.J. Richardson, S.J. Chen, P.N. Ross, J. Catal. 232 (2005) 199. [5] I.R. Harris, M. Norman, A.W. Brayant, J. Less-Common Met. 16 (1968) 427. [6] N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon, Oxford, 1984. [7] L. Smart, E.A. Moore, Solid State Chemistry: An Introduction, CRC Press, 2005. [8] A.F. Holleman, E. Wiberg, Inorganic Chemistry, Academic Press, San Diego, 2001. [9] B.J. Piersma, E. Gileadi, in: J.O.M. Bockris (Ed.), Modern aspects of electrochemistry, vol. 4, Plenum Press, New York, 1966. [10] W. Vielstich, Fuel Cells, Wiley-Interscience, London, 1970.

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