Elsevier

Electrochimica Acta

Volume 53, Issue 2, 1 December 2007, Pages 377-389
Electrochimica Acta

Ethanol electrooxidation on novel carbon supported Pt/SnOx/C catalysts with varied Pt:Sn ratio

https://doi.org/10.1016/j.electacta.2007.01.047Get rights and content

Abstract

Novel carbon supported Pt/SnOx/C catalysts with Pt:Sn atomic ratios of 5:5, 6:4, 7:3 and 8:2 were prepared by a modified polyol method and characterized with respect to their structural properties (X-ray diffraction (XRD) and transmission electron microscopy (TEM)), chemical composition (XPS), their electrochemical properties (base voltammetry, COad stripping) and their electrocatalytic activity and selectivity for ethanol oxidation (ethanol oxidation reaction (EOR)). The data show that the Pt/SnOx/C catalysts are composed of Pt and tin oxide nanoparticles with an average Pt particle diameter of about 2 nm. The steady-state activity of the Pt/SnOx/C catalysts towards the EOR decreases with tin content at room temperature, but increases at 80 °C. On all Pt/SnOx/C catalysts, acetic acid and acetaldehyde represent dominant products, CO2 formation contributes 1–3% for both potentiostatic and potentiodynamic reaction conditions. With increasing potential, the acetaldehyde yield decreases and the acetic acid yield increases. The apparent activation energies of the EOR increase with tin content (19–29 kJ mol−1), but are lower than on Pt/C (32 kJ mol−1). The somewhat better performance of the Pt/SnOx/C catalysts compared to alloyed PtSnx/C catalysts is attributed to the presence of both sufficiently large Pt ensembles for ethanol dehydrogenation and C–C bond splitting and of tin oxide for OH generation. Fuel cell measurements performed for comparison largely confirm the results obtained in model studies.

Introduction

For the last decades, research on the electrooxidation of small organic molecules has attracted considerable attention due to the development of direct liquid fuel cells, which requires highly reactive fuels with high energy density and low toxicity. Among the small organic molecules, ethanol is one of the most promising fuels due to its low toxicity, high energy density, biocompatibility and abundant availability [1], [2], [3]. The major challenge for the utilization of ethanol as fuel is its low reactivity in the temperature range technically feasible in the moment. In particular, the cleavage of the C–C bond, which is a pre-condition for the complete oxidation of ethanol to CO2, is highly activated on current electrocatalysts, with incomplete oxidation to acetaldehyde and acetic acid as prevailing reaction pathway. This led to an intense search for novel catalyst materials with high activity for complete ethanol electrooxidation [4], [5], [6]. Following earlier investigations of the ethanol electrooxidation reaction (EOR) which aimed at a fundamental understanding of the EOR process and the elementary reaction steps concentrating on solid Pt or Au electrodes (polycrystalline or single crystal electrodes) as standard model systems [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], later research focused on realistic supported catalysts [5], [15], [16], [18], [19], [20], [21]. The catalytic properties of various different catalysts, in particular of carbon supported Pt alloy catalysts, was explored in model studies [3], [12], [15], [19], [20], [22], [23], [24], [25], [26], focusing on mechanistic details of the ethanol adsorption and reaction process and on the effect of reaction parameters such as ethanol concentration, reaction temperature, catalyst loading on the product distribution, and comparable studies were performed also in fuel cell measurements [3], [5], [20], [22], [23], [27], [28], [29], [30]. Among the investigated catalysts, PtSn/C and PtRu/C catalysts were found to be most active towards the EOR, depending on the respective reaction conditions [3], [5], [20]. It was reported that the overpotential for ethanol electrooxidation could be reduced by 200–300 mV on PtSn catalysts compared to that on Pt [3]. However, the active species and the role of tin in PtSn catalysts still need further investigation.

This contribution presents results of a detailed study on the catalytic properties of a series of carbon supported ‘bimetallic’ Pt-Sn catalysts (with atomic ratios of Pt:Sn of 5:5, 6:4, 7:3, 8:2), where in contrast to previous synthesis routes [26] the second component was not present as alloy, but pre-formed as oxide particles before Pt reduction [31]. This synthesis route reduces the loss of active surface area observed for alloyed PtxSn/C catalysts [31]. The structure and morphology of the catalysts were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM). The electrochemical properties and the active surface area were evaluated by cyclic voltammetry and by pre-adsorbed COad monolayer oxidation (COad stripping). The catalytic properties, including both the EOR activity and the product distribution selectivity, were determined by on-line differential electrochemical mass spectrometry (DEMS) under defined transport conditions, with negligible contributions from transport limitations, under both potentiodynamic and potentiostatic conditions. Finally, the EOR performance of these catalysts as anode catalyst was tested in direct ethanol fuel cell (DEFC) measurements.

Section snippets

Catalyst synthesis

A detailed account of the catalyst preparation procedure is given in ref. [32]. In short, tin(II) chloride dihydrate (SnCl2·2H2O, Liaoning Medical Reagent Co., AR, 98%), chloroplatinic acid hexahydrate (H2PtCl6·6H2O, Shenyang Institute of Noble Metal, 37 wt.% Pt content), ethylene glycol (EG, Shenyang Chemical Reagent Co., 99.9%) and sodium hydroxide (Liaoning Medical Reagent Co., 99%) were used as supplied without further purification. In a typical procedure, 0.2 g SnCl2·2H2O (dissolved in 10 ml

Structural characterization

XRD patterns of the Pt/SnOx/C catalysts are shown in Fig. 1. The diffraction peak at around 25° is attributed to diffraction at the (0 0 2) plane of the hexagonal structure of Vulcan XC-72R carbon. The diffraction peaks at around 39°, 46°, 68° and 81° are due to diffraction at the Pt(1 1 1), (2 0 0), (2 2 0) and (3 1 1) planes, respectively. In addition, some other peaks at around 34° and 52° were also observed, which can be related to tin oxide. The main diffraction peak positions for tin oxide (PCPDF

Discussion

The results of the model studies and fuel cell measurements presented so far can be summarized as follows:

  • (1)

    CO2 contributes more, by around 1%, to the EOR current over the Pt/SnOx/C catalysts than over the alloyed PtSn catalyst [26] under both potentiostatic and potentiodynamic reaction conditions. For the Pt/SnOx/C and the PtSn alloy catalysts, ethanol molecules are more easily dissociated on the former ones, since alloying Sn causes a partial filling of the d band vacancies of Pt, as was

Conclusions

A series of Pt/SnOx/C catalysts with atomic ratios of Pt:Sn = 5:5, 6:4, 7:3 and 8:2 were prepared by a modified polyol method. XRD, TEM and electrochemical characterization confirm that these materials are composed of uniform Pt and SnO2 nanoparticles with mean Pt particle sizes of 2–3 nm. The Pt particle sizes become smaller with increasing tin content. The Pt utilization characterized by the ratio of the active surface area (Sact) and the theoretical surface area (Sthe) decrease slightly, though

Acknowledgements

This work was supported by the Sino-German Center in Beijing and the Deutsche Forschungsgemeinschaft (Be 1201/12-1) and the National Natural Science Foundation of China (Grant Nos. 50575036, 50676093). L. Jiang is grateful for a fellowship from the Alexander von Humboldt Foundation.

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