1 Introduction

Direct Alcohol Fuel Cells (DAFCs) are attractive as power sources for mobile and portable applications [1, 2]. Ethanol offers an attractive alternative as fuel because it is produced in large quantities from biomass and is much less toxic than methanol. On the other hand, its complete oxidation to CO2 and water is more difficult than that of methanol due to the difficulties in C–C bond breaking and to the formation of CO-intermediates that poison the platinum anode catalysts [3–5]. Thus, its complete oxidation remains a great challenge and the principal products formed are acetaldehyde and/or acetic acid [6, 7]. Thus, new catalyst materials and new catalyst preparation method developments have been one of the major topics studied concerning the electro-oxidation of ethanol [4, 8]. PtSn/C electrocatalyts are claimed to be more active than PtRu/C electrocatalysts for ethanol electro-oxidation and their performance depends greatly on the preparation procedure and Pt:Sn atomic ratio [9–11]. PtSn/C electrocatalysts prepared by an alcohol-reduction process, where Pt was found as face-centred cubic (fcc) and Sn as a SnO2 phase, were very active for methanol and ethanol oxidation [12]. Mann et al. [13] found that a nanoparticulate catalyst containing Pt and SnO2 allowed the partial conversion of ethanol to CO2 and water and the elevation of the operating temperature of direct ethanol fuel cell (DEFC) to 130 °C facilitated the production of CO2 and provided an improvement of current–voltage response.

Recently, we prepared PtSn/CeO2–C electrocatalysts by an alcohol-reduction process in a single step [14], substituting part of the carbon support by CeO2. The material obtained was more active for ethanol oxidation than the analogous PtSn/C catalyst. In this work, the electrochemical oxidation of ethanol on PtSn/CeO2–C electrocatalyst was investigated by in situ FTIR spectroscopy to obtain information about intermediates and reaction products.

2 Experimental

PtSn/C and PtSn/CeO2–C (Pt:Sn atomic ratio 50:50) electrocatalysts were prepared with metal loading of 20 wt% using H2PtCl6 · H2O (Aldrich) and SnCl2 · 2H2O(Aldrich) as metal sources, ethylene glycol (Merck) as solvent and reducing agent and CeO2 (15 wt%) and carbon Vulcan XC72 (85 wt%) as support [12, 15, 16]. In a typical procedure the metal sources were dissolved in ethylene glycol/water (75/25, v/v) and the supports were added. The resulting mixtures were treated in an ultrasonic bath and were refluxed for 3 h under open atmosphere. The mixtures were filtered and the solid was washed with water and dried at 70 °C for 2 h.

Pt:Sn atomic ratios were obtained by EDAX analysis using a scanning electron microscope Philips XL30 with a 20 keV electron beam and equipped with EDAX DX-4 microanalyser. XRD analyses were performed using a Rigaku diffractometer model Multiflex with a CuKα radiation source.

The electrochemical activity of the electrocatalysts was evaluated in 0.5 mol L−1 H2SO4 solution containing 0.5 mol L−1 of ethanol by cyclic voltammetry in a three-electrode cell using a Voltalab Potentiostat PGZ 420. A Reversible Hydrogen Electrode (RHE) was used as reference and vitreous carbon as auxiliary electrode.

The spectroelectrochemical measurements were carried out in a Fourier transform infrared spectrometer Nicolet Nexus 670 with a MCT detector. The electrochemical cell was fitted with a 60º prismatic CaF2. Reflectivities were recorded after applying potentials steps of 50 mV toward more positive potentials (from 0.050 to 1.000 V vs RHE) in 0.5 mol L−1 H2SO4 solution containing 0.5 mol L−1 of ethanol. Each spectrum was the resulted from the co-addition of 128 interferograms. Spectra were calculated as –ΔA = ΔR/R = (RE2 − RE1)/RE1 where the “reference” spectrum, RE1, was that recorded at the initial potential at 0.050 V vs RHE. For the calculations, E2 > E1, so that, a positive absorption band indicates the consumption of species and a negative absorption band means the production of species.

3 Results and discussion

As already published for PtSn/C and PtSn/CeO2–C electrocatalysts [14], the Pt:Sn atomic ratios obtained by EDAX were very similar to the nominal ones and from XRD measurements it was observed that Pt (fcc), SnO2 and CeO2 coexist in PtSn/CeO2–C electrocatalyst. The average crystallite sizes of Pt(fcc) phase present in PtSn/C and PtSn/CeO2–C electrocatalysts were very similar (about 2.5 nm) [14].

Figure 1 shows the cyclic voltammograms (CV) of PtSn/C and PtSn/CeO2–C electrocatalysts in acid solution. The cyclic voltammogram for the PtSn/C electrocatalyst do not show a well-defined hydrogen adsorption–desorption region (0.05–0.4 V). The cathodic scan showed one small peak at about 0.8 V that may be attributed to the reduction of oxide species. PtSn/CeO2–C electrocatalyst have a more defined hydrogen adsorption–desorption region than PtSn/C. The charge associated to the double layer for PtSn/CeO2–C was similar to that observed for PtSn/C. This implies that the true active areas of the electrodes are nearly the same, which agrees with the similarity of the particle sizes evaluated from XRD measurements using the Scherrer equation [14]. The two peaks observed around 0.8 V and 1.1 V in the cathodic scan may be attributed to the reduction of PtO, SnO2 and CeO2 species.

Fig. 1
figure 1

Cyclic voltammograms of PtSn/C and PtSn/CeO2–C electrocatalysts in 0.5 mol L−1 H2SO4, sweep rate: 50 mV s−1

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The PtSn/C and PtSn/CeO2–C electrocatalyst performance in ethanol oxidation are shown in Fig. 2. Ethanol electro-oxidation started at around 0.3–0.4 V for both electrocatalysts; however, at higher potential values PtSn/CeO2–C electrocatalyst showed higher current values than PtSn/C. Thus, a dramatic increase in performance was obtained with the addition of CeO2. Similar results were observed by Wang et al. [17] on methanol oxidation using PtRu/C catalyst with a loading of 15 wt% of CeO2.

Fig. 2
figure 2

Cyclic voltammograms of PtSn/C and PtSn/CeO2–C electrocatalysts in 0.5 mol L−1 H2SO4 solution containing 0.5 mol L−1 of ethanol, sweep rate: 50 mV s−1

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In order to obtain information about intermediates and reaction products in situ infrared spectroscopy was performed in a 0.5 mol L−1 H2SO4 solution containing 0.5 mol L−1 C2H5OH. Figure 3 shows the spectra for PtSn/CeO2-C and PtSn/C in the 1,000–3,000 cm−1 spectral range from 0.100 to 1.000 V vs. RHE. These spectra were accumulated at every 50 mV during a cyclic voltammetric experiment run at 1 mV s−1. The reference reflectivity used for the calculation of ΔR/R was taken at 0.050 V vs. RHE. The principal bands of the species from ethanol adsorption and oxidation observed in the FTIR spectra may be assigned [18–20]: an absorption band located close to 2,345 cm−1 reveals the formation of CO2 species. An absorption band close to 1,725 cm−1 attributed to the stretching mode of the carbonyl group (C=O) of acetic acid and/or acetaldehyde coming from the ethanol oxidation. In this case it is difficult to distinguish between the adsorbed carbonyl species from acetic acid or acetaldehyde because C=O bands in these compounds are very close. A broad band located close to 2,600 cm−1 that was assigned to the C–H vibration mode of acetic acid or to the OH stretching mode of the carboxylic group overlapped with the C–H stretching mode of the methyl group. The bands close to 1,390 and 1,270 cm−1 are attributable to the C–O stretch and O–H deformation vibrations in acetic acid while the bands close to 1,370 and 1,100 cm−1 are due to CH3 symmetric deformation and C–H wagging vibration in acetaldehyde and acetic acid.

Fig. 3
figure 3

In situ FTIR spectra of the species resulting from the oxidation of ethanol in 0.5 mol L−1 H2SO4 + 0.5 mol L−1 ethanol on a PtSn/C and b PtSn/CeO2–C electrocatalysts at potentials varying from 0.100 to 1.000 V vs. RHE. RERef taken at 0.050 V

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The FTIR spectra recorded for in situ ethanol oxidation on PtSn/C (Fig. 3a) showed a band at 2,340 cm−1 (due to CO2 formation) that was detected from 0.500 to 1.000 V and its intensity increased with increase in potential. A broad band at 2,600 cm−1 and the absorption band at 1,270 cm−1 (attributed to acetic acid formation) were also detected at potentials higher than 0.450 V and their intensities increased with increase in potential. The absorption band close to 1,725 cm−1 (expected for acetaldehyde and/or acetic acid formation) was detected from 0.300–0.350 V and its intensity also increased with increase in potential. It can be also observed that acetaldehyde and/or acetic acid were formed preferentially to CO2. The complete electro-oxidation of ethanol to CO2 releases 12 electrons per molecule, while the partial oxidation to acetaldehyde and acetic acid releases only two and four electrons per molecule, respectively.

The FTIR spectra obtained for the ethanol oxidation on PtSn/CeO2–C electrode (Fig. 3b) showed that the band at 2,340 cm−1 (due to CO2 formation) and the bands at 2,600 and 1,270 cm−1 (associated to acetic acid formation) were detected in the potential range from 0.500 to 1.000 V. A surprising observation was the non clear observation of the band at 1,725 cm−1 in the spectra (attributed to the formation of acetaldehyde and/or acetic acid) at all potentials. The above observations infer that CO2 and acetic acid are the main final products formed on PtSn/CeO2–C.

These results suggest that on PtSn/C electrocatalyst the ethanol molecule is initially adsorbed on the Pt surface (step 1) followed by two possible mechanistic routes (see Scheme 1). Thus, α-C–H bond breaking and simultaneous molecular rearrangement occurs to form acetaldehyde (step 2a). The adsorbed acetaldehyde can be desorbed to the solution or it can be further oxidized to acetic acid (step 3). This step occurs through the bifunctional mechanism where the SnO2 provides oxygenated species in order to oxidize acetaldehyde into acetic acid. On PtSn/CeO2–C electrocatalyst the formation of acetic acid may be explained by the same route (step 1, 2a and 3); however, the result regarding the absence of the band at 1,725 cm−1 in the spectra, suggests that as soon as the acetaldehyde was formed it was rapidly transformed into acetic acid. In this case, the bifunctional mechanism is favored by the beneficial synergistic effect of SnO2 and CeO2, since cerium oxide is well known in heterogeneous catalysis as a good source of oxygenated species [21]. Experiments to verify this last supposition are currently in progress. The formation of CO2 species may be explained by C–C bond breaking of the adsorbed acetaldehyde molecule (step 4) as soon as it is formed, leading to the formation of two intermediate adsorbed species, CO and a CHx fragment, that are further oxidized to CO2 (step 5). The formation of CO2 as product may also occur by C–C bond breaking of the adsorbed ethanol (steps 1, 2b and 5).

Scheme 1
scheme 1

Scheme of ethanol oxidation on Pt–Sn electrocatalysts

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4 Conclusions

PtSn/CeO2–C electrocatalysts prepared by an alcohol-reduction process exhibited higher performance for ethanol oxidation than PtSn/C electrocatalyst. FTIR studies for ethanol oxidation on PtSn/C electrocatalyst showed that acetaldehyde and acetic acid were the principal products formed. On PtSn/CeO2–C the principal final products formed were CO2 and acetic acid. This delivers more electrons per ethanol molecule and explains the highest voltammetric current values obtained for this catalyst. Thus, the addition of CeO2 to PtSn/C electrocatalyst leads to a more complete oxidation of the ethanol molecule. These features should be investigated using other in situ techniques to identify the mechanisms occurring. Also, experiments using these electrocatalysts in real conditions (single direct ethanol fuel cell) and the identification of the products formed under these conditions are currently in progress.