Introduction

Fuel cell technology has become increasingly significant during recent years because of growing industrialization and related energy consumption, problems with environmental protection as well as limited resources of fossil fuels. Hence, it is important to find new energy sources that are thermally efficient and environmentally friendly. In this context, fuel cells are an alternative to conventional energy devices because of their potentially high thermodynamic efficiency and environmentally benign products [1, 2].

Many organic compounds have been used as fuels for polymer electrolyte membrane fuel cells, but usually methanol [3, 4] and, more recently, ethanol are considered [5–10]. In the latter case, the complete oxidation of ethanol produces as many as 12 electrons, but relative to methanol, the oxidation process is a fairly complex reaction because it requires dissociation of the C–C bond. The literature data show that, at present, the platinum-based catalysts are the most effective systems for adsorptive activation and dissociation of organic molecules [11–13]. However, the activity of Pt for the electrooxidation of ethanol is poor because the reaction rate on Pt is slow and the main products are not CO2 but rather acetaldehyde and acetic acid [14, 15]. In other words, the bare Pt catalyst has effectively no sufficient ability to break the C–C bond. It is reasonable to expect that fabrication of Pt-based multicomponent systems, by appropriate alloying of interfacial modification, may lead to the improvement of electrocatalytic activity during the electrooxidation of ethanol.

Previous studies indicate that the presence of a second transition metal such as Ru, Rh, Ir, Sn, Pd, Zr, Mo, W, or Pb [4, 9, 10, 16–22] into Pt led to the electrooxidation of ethanol at lower potentials than on pure platinum. The sizeable enhancement was observed in the case of carbon-supported or intentionally decorated Pt–Sn catalysts [9, 10, 23–25], but in all cases, the main products formed were still acetic acid and acetaldehyde [26–28]. A report by Souza's group [11] showed that the presence of small amounts of Rh in Pt-based catalysts played an important role in improving the C–C bond cutting, and it tended to increase the ratio of CO2 to acetaldehyde in the oxidation products. On the other hand, the obtained electrocatalyst was not highly efficient for the electrooxidation of ethanol. More recently, Kowal et al. [8, 29] have described a ternary electrocatalyst utilizing PtRh within carbon-supported SnO2; the system has yielded promising results for the electrooxidation of ethanol. On the whole, it seems that a combination of three components (Pt, Rh, and SnO2) tends to improve the oxidation reaction rate [8, 29]. From another point of view, SnO2 seems to provide a good environment for PtRh nanoparticles. What is even more important, bimetallic PtRh nanostructured catalysts seem to be less prone to oxidative degradation than PtSn or even PtRu ones.

Historically, in the case of methanol electrooxidation, the activity of the electrocatalyst was improved by the addition of molybdenum or tungsten in various forms (elements or metal oxides) as a second or third constituent [30–33]. Molybdenum oxide and tungsten oxide have been demonstrated to show a promoting effect on Pt-based catalysts towards the oxidation of CO, thereby mitigating CO poisoning [34–36] that also constitutes a limiting factor during oxidation of methanol [37–39]. The improvement of catalytic activities of systems containing molybdenum and tungsten oxide for CO electrooxidation was also confirmed by theoretical studies [40, 41]. When it comes to oxidation of ethanol, modification of PtSn nanoparticles with polymolybdates or polytungstates [9, 42–44], with tungsten oxide [10] or molybdenum oxide [31], has also resulted in the enhancement of the system's electrocatalytic properties.

It has been recently postulated that metal oxide species stabilize noble metal nanoparticle dispersions [10, 45], and they have the ability to produce OH− surface groups (from electrodissociation of water) that facilitate the CO electrooxidation process [46, 47]. The latter point is important in the context of electrooxidation of small organic molecules.

In this work, the electrocatalytic activity of carbon-supported PtRh nanoparticles (PtRh/C), that have been intentionally modified or admixed with metal oxide species such as MoO3 or WO3, is investigated for the electrooxidation of ethanol. In addition, general physicochemical properties of the proposed catalysts are described.

Experimental

All chemicals obtained were commercial materials of analytical grade purity. The PtRh/Vulcan nanoparticles (20% on Vulcan XC-72, Pt/Rh–4:1) were from BASF Fuel Cell Inc. Solutions were prepared using doubly distilled and subsequently de-ionized (Millipore Milli-Q) water. Argon was used to de-aerate the solutions and to keep an oxygen-free atmosphere over the solution during the measurements. Morphology of the catalytic particles was monitored using a LIBRA 120 transmission electron microscope (TEM) operating at 120 kV. Samples for TEM measurements were prepared by placing a drop (1–2 μl) of the solution of carbon-supported nanoparticles onto Fromvar film grids (Agar Scientific) and, later, by subjecting them to drying on 400-mesh Cu. Moreover, the morphology and the composition of catalytic films were assessed using a Nova 200 NanoSEM high-resolution scanning electron microscope equipped with energy dispersive X-ray spectrometry (EDS, Genesis XM4) analyzer. X-ray diffraction (XRD) patterns of the catalysts were obtained with a Bruker D8 Discover system operated with a Cu lamp (1.54 Å) and Vantec (linear) detector (k = 1.5406 Å).

All electrochemical measurements were performed using CH Instruments 750 A workstations in a three-electrode configuration. The reference electrode was a K2SO4 saturated mercury/mercury sulfate electrode (Hg/Hg2SO4), and a carbon rod was the counter electrode. All potentials in the present work were recalculated and expressed versus the reversible hydrogen electrode (RHE). As a rule, glassy carbon electrode was the base for the working electrode (geometric area, 0.071 cm2). The catalyst layer was fabricated through the modification of the glassy carbon by immobilization of PtRh/Vulcan nanoparticles. To prepare the solution of tungstic acid or molybdic acid, the suitable aqueous solution of 0.05 mol dm−3 Na2WO4 or Na2MoO4 was passed through a proton exchange resin, Dowex 50 WX2-200. The suspension of Vulcan-supported PtRh nanoparticles (PtRh/Vulcan) was prepared as follows: a known amount (10 mg) of PtRh/Vulcan nanoparticles was dispersed in 2 cm3 of 0.05 mol dm−3 aqueous solution of tungstic acid or molybdic acid. The suspension was mixed using magnetic stirring for 24 h and centrifuged. Then the supernatant solution was replaced with water, and a stable suspension of WO3 or MoO3-modified PtRh/Vulcan nanoparticles were produced. In the final suspension, the approximate molar ratio of Pt to tungsten oxide or molybdenum oxide (dehydrated) was 1 to 1. To immobilize catalytic nanoparticles, a 5-μl aliquot of the suspension was dropped using a micropipette on the glassy carbon electrode surface. The resulting layer was air-dried for 30 min at room temperature (22 °C). Then 2 μl of Nafion (0.02% alcoholic solution) was dropped on top of the modified glassy carbon electrode. The Nafion film was sufficiently stable to fix the modified and unmodified catalytic nanoparticles on the electrode substrate in the supporting electrolyte. As a rule, the catalytic electrodes were conditioned through the application of 25 complete oxidation/reduction cycles at 50 mV s−1 between 0 and 0.8 V in 0.5 mol dm−3 H2SO4. The total loading of PtRh nanoparticles was approximately 160 μg cm−2.

The CO-stripping measurements were performed in 0.5 mol dm−3 H2SO4 electrolyte using the glassy carbon substrate onto which surface the appropriate catalyst was introduced. The electrolyte was first de-oxygenated by purging argon for 30 min. Subsequently, a few consecutive voltammetric scans (at 50 mV s−1) were recorded in the potential range from 0.0 to 0.8 V. To saturate the solution with CO gas, pure CO (from Air Liquide) was bubbled through the electrolyte for 10 min. The actual CO-adsorption step (on the surface of catalytic Pt nanocenters) was achieved upon application of the potential of 0.1 V for 5 min after which the electrolyte was purged with argon for 30 min under open-circuit conditions to remove dissolved CO. As a rule, three cyclic voltammetric scans (at 10 mV s−1) were recorded in the potential range from 0.0 to 0.9 V. Most of the experiments using these catalysts were performed two to three times with freshly prepared electrodes to ensure the reproducibility of electrode preparation and performance.

Results and discussion

Structure and morphology PtRh/Vulcan catalysts

To get some insight into the structure and morphology of the PtRh/Vulcan nanoparticles modified with molybdenum and tungsten oxides, the samples were examined by XRD (Fig. 1). The first broad diffraction peak at 25.0°, which appeared in all the XRD patterns, corresponded to the hexagonal carbon support [48]. In the case of unmodified PtRh/Vulcan nanoparticles (Fig. 1a), three characteristic diffraction peaks appear, which are all clearly broadened, indicating a very small average particle size [49, 50]. Moreover, for all samples, we had no direct evidence for the presence of diffraction peaks that could be attributed to rhodium (PCPDF 05-0685) because they are characterized by diffraction peak positions and crystalline structures [29, 51] similar to platinum (PCPDF 04-0802). These XRD data do not allow clear identification of the phase composition of the metal. The diffractogram of the MoO3-modified PtRh/Vulcan electrocatalyst (Fig. 1b) showed three peaks that appeared almost at the same position as for unmodified PtRh/Vulcan nanoparticles, but no diffraction peaks characteristic of MoO3 were observed (PCPDF 5-0508). Indeed, the pattern of the MoO3/Vulcan sample that had been prepared in the same way showed only the broad diffraction peaks corresponding to the carbon support materials (Fig. 1, inset D). It is reasonable to expect that, in both cases, molybdenum species did not form crystalline aggregates [52]. In the case of WO3-modifed PtRh/Vulcan catalysts, the signals characteristic of WO3 were clearly visible in the XRD pattern (Fig. 1c) (PCPDF 43-0679). The WO3 phase had characteristic peaks attributable to monoclinic WO3. The observation is in accord with earlier reports [53, 54]. The diffraction peaks in the WO3-modified PtRh/Vulcan nanoparticles (Fig. 1c) were narrower in contrast to those in the unmodified and MoO3-modified PtRh/Vulcan electrocatalyst where the diffraction peaks (Fig. 1a and b) were broadened.

Fig. 1
figure 1

XRD images of PtRh/Vulcan (a), MoO3-modifed PtRh/Vulcan (b), WO3-modifed PtRh/Vulcan (c), and MoO3-modifed Vulcan (d)

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In order to get more information about the particle size and the distribution of nanoparticles on the carbon material, TEM analysis was performed. Figure 2 shows the TEM images for PtRh/Vulcan, MoO3-modified PtRh/Vulcan, and WO3-modified PtRh/Vulcan nanoparticles. Low-magnification images show that approximately spherical bimetallic nanoparticles are in all cases uniformly dispersed on the surface of carbon (Vulcan XC-72R) supports, and the distribution lies in a narrow particle size range. Histograms of the particle size distribution (not shown), which reflect analyses of several different portions of the catalysts, are also consistent with a uniform distribution of these catalysts. As can be seen in Fig. 2, both the obtained materials have small particle sizes narrowly distributed in the range ca. 2–6 nm.

Fig. 2
figure 2

Low-magnification TEM images of PtRh/Vulcan (a), MoO3-modifed PtRh/Vulcan (b), and WO3-modifed PtRh/Vulcan (c) nanoparticles

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Figure 3 shows SEM images and EDS spectra of the electrocatalytic materials (a) PtRh/Vulcan, (b) MoO3-modified PtRh/Vulcan, and (c) WO3-modified PtRh/Vulcan nanoparticles following deposition on glassy carbon substrates and finally covered by the thin layer of Nafion. In all cases, the particles are primarily distributed in the range 50 to 100 nm, which is attributed to Vulcan (carbon) support; the white small spots (diameters below 10 nm) originate from PtRh nanoparticles, which are approximately spherical and uniformly distributed. The SEM measurements are in good agreement with TEM results. Figure 3 also shows the EDS for which peaks are assigned to W, Pt, Rh, and Mo. It is clear that W and Mo exist in the metal oxide-modified catalytic material at approximately 3 at% and 4 at% levels, respectively. To validate our EDS-based estimations, molar contents of metal oxides (MoO3, WO3) have been addressed by using Raman spectrometry with an internal standard (KNO3) as proposed by Hercules [55]. The obtained results allow me to state with a large degree of certainty that, in both cases, the molar contents MoO3 and WO3 (relative to Pt/Rh) have not been larger than 10%.

Fig. 3
figure 3

Scanning electron microscopy and EDS analysis of a PtRh/Vulcan, b MoO3-modifed PtRh/Vulcan, and c WO3-modifed PtRh/Vulcan nanoparticles

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Electrochemical behavior in the absence and presence of ethanol

Typical cyclic voltammetric curves obtained for the PtRh/Vulcan, MoO3-modifed PtRh/Vulcan, and WO3-modifed PtRh/Vulcan nanoparticles deposited on glassy carbon electrode from 0.5 mol dm−3 sulfuric acid are shown in Fig. 4. The voltammograms for unmodified PtRh/Vulcan display prominent single peaks in the hydrogen adsorption region (0.0–0.4 V vs. RHE) [56]. These are probably related to hydrogen adsorption/desorption on an intermetallic alloy phase. The currents in the double-layer region between 0.4 and 0.8 V vs. RHE are significant. This behavior is characteristic of carbon-supported binary electrocatalysts containing transition metals [57]. Cyclic voltammetry of the MoO3-modified PtRh/Vulcan system shows similar behavior to that described above for both hydrogen absorption/desorption and for currents in the double-layer region between 0.5 and 0.8 V. However, in the potential range from 0.3 to 0.5 V, an additional peak appears (Fig. 4; A, curve b). For comparison, the cyclic voltammogram of MoO3-modified Vulcan support materials introduced onto the glassy carbon surface is also provided (Fig. 4, B). It is interesting to note that the peaks are at about 0.4 V (Fig. 4; A, curve b) for MoO3-modified PtRh/Vulcan, i.e., at potentials less negative than the analogous peak current observed on bare MoO3/Vulcan (Fig. 4, B). The peak, which is observed at about 0.4 V (Fig. 4; A, curve b), is in agreement with earlier reports describing redox processes involving Mo(V) and Mo(VI) [40, 58–61]. Moreover, the presence of MoO3 leads to an increase in the voltammetric peaks for hydrogen adsorption and desorption, which are presumably due to the hydrogen spillover effect within the hydrogen molybdenum bronze. This phenomenon has been attributed to the interaction between the platinum active sites and molybdenum oxides [37]. By comparing the results of PtRh/Vulcan (Fig. 4; A, curve a) to those of WO3-modifed PtRh/Vulcan materials (Fig. 4; A, curve c), one can observe that the modification by WO3 results in the development of two peaks in the hydrogen adsorption/desorption region. The new peak appears at a higher potential. It perhaps originates from the adsorption of tungsten oxide (WO3), but this conjecture is not readily confirmed by voltammetry because the formation of hydrogen tungsten bronze and the reversal of WO3 overlap the hydrogen absorption and desorption peaks. A clear distinction between these two catalytic systems is that the hydrogen adsorption/desorption region is higher for WO3-modified PtRh/Vulcan than that for the PtRh/Vulcan catalyst.

Fig. 4
figure 4

A Cyclic voltammetric responses of PtRh/Vulcan (a), MoO3-modifed PtRh/Vulcan (b), and WO3-modifed PtRh/Vulcan (c) catalytic systems. B Cyclic voltammetric responses of MoO3-modifed Vulcan. Electrolyte, 0.5 mol dm−3 H2SO4. Scan rate, 10 mV s−1

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The stability of both obtained catalytic materials has been studied under 1-h chronoamperometric and repetitive (50 cycles) voltammetric measurements in the presence of ethanol (for simplicity, not shown here). The results are consistent with the view that PtRh/Vulcan nanoparticles exhibit good stability (8%) in the 0–900-mV (vs. RHE) potential range. Consequently, this potential range has been further utilized during electrochemical measurements for WO3- or MoO3-modified PtRh/Vulcan nanoparticles. The potential practical problem concerns the limited stability of MoO3 in the proposed electrocatalytic system due to possible dissolution of the MoO3 from the surface of carbon-supported PtRh nanoparticles. In the literature, the stability of MoO3 is still controversial [62–67]. However, we have not observed any time-dependent deactivation effect that may imply dissolution of MoO3. In the case of WO3-modified PtRh/Vulcan nanoparticles, no dissolution of the metal oxide is expected. During chronoamperometric measurements pursued for 1 h, effective decreases of steady-state catalytic currents have never exceeded 12% in the case of both tungsten and molybdenum oxides. At this stage, practical stability of the system is rather low, but the present data have fundamental importance. Knowing the possibility of forming more robust WO3 structures at temperatures exceeding 100 °C, further research along this line is necessary.

Figure 5 shows representative cyclic voltammograms obtained in the presence of 0.5 M ethanol at a glassy carbon electrode modified with PtRh/Vulcan (Fig. 5; A, curve a), MoO3-modified PtRh/Vulcan (Fig. 5; A, curve b), and WO3-modifed PtRh/Vulcan (Fig. 5; A, curve c). The shapes of curves are typical for the electrooxidation reactions of organic alcohols; specifically, there are two peaks in both forward and reverse scans as reported in the literature [56, 68–70]. Most likely, the first (least positive) anodic peak reflects primarily the oxidation of ethanol to CO2; in a given scan, there may be some contribution to this current from incompletely oxidized carbonaceous species formed during the prior cycle. What is more important is that the onset potential for ethanol electrooxidation is decreased in comparison to that at bare PtRh/Vulcan by modification of the catalyst with MoO3 or WO3 by ca. 100 or 70 mV (Fig. 5, A), respectively. In both cases, the current densities are higher over the entire potential range, confirming the beneficial effect of these metal oxides on the electrocatalytic oxidation of ethanol. The observation can be interpreted in terms of the activation role of either MoO3 or WO3 on the interfacial water molecules at potentials lower than those expected for unmodified PtRh/Vulcan electrocatalysts [10]. In other words, the presence of MoO3 or WO3 on PtRh/Vulcan nanoparticles tends to enhance electrooxidation of ethanol in comparison to unmodified PtRh/Vulcan electrocatalysts.

Fig. 5
figure 5

A Cyclic voltammetric responses for oxidation of 0.5 mol dm−3 ethanol at PtRh/Vulcan (a), MoO3-modifed PtRh/Vulcan (b), and WO3-modifed PtRh/Vulcan (c) catalysts. B LSV responses for the oxidation of 0.5 mol dm−3 ethanol at PtRh/Vulcan (a), MoO3-modifed PtRh/Vulcan (b), and WO3-modifed PtRh/Vulcan (c) catalyst. Electrolyte, 0.5 mol dm−3 H2SO4. Scan rate, 10 mV s−1

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The inset to Fig. 5 (Fig. 5, B) illustrates background-subtracted linear scan voltammograms (LSVs) for 0.5 M ethanol in 0.5 M H2SO4 at glassy carbon electrodes modified with PtRh/Vulcan (Fig. 5; B, curve a), MoO3-modified PtRh/Vulcan (Fig. 5; B, curve b), and WO3-modifed PtRh/Vulcan (Fig. 5; B, curve c). The voltammograms show well-defined peaks for the anodic processes, thereby allowing refinement of the values of the potentials for the onset of ethanol oxidation. The onset potentials at bare PtRh/Vulcan nanoparticles and at MoO3-modified and WO3-modified PtRh/Vulcan nanoparticles were 0.41, 0.36, and 0.30 V, respectively, which again illustrates the enhancement of the catalytic activity for the MoO3 system even in comparison to the WO3-modified system and, particularly, to unmodified nanoparticles.

Activities of these electrodes towards the electrocatalytic oxidation of ethanol were compared by chronoamperometry at two applied potentials and with current densities recorded using the geometric areas of the electrodes (Fig. 6). In agreement with the cyclic voltammetry described above, the catalytic current density obtained, when the PtRh/Vulcan nanoparticles were modified with MoO3, was increased at both 0.3 and 0.4 V in comparison to WO3-modified and unmodified electrocatalysts. At 0.3 V, the current was only developed when a metal oxide was present, which is consistent with the onset potential observed during the cyclic voltammetric oxidation of ethanol. At 0.4 V, all three electrode systems yielded current with the sensitivities in the order MoO3-modified > WO3-modified > unmodified PtRh/Vulcan nanoparticles. In all cases, at 0.4 V, steady-state currents were developed in the range 5–10 min. It seems that the presence of metal (Mo or W) oxo species in the vicinity of PtRh nanoparticles does not preclude the ethanol adsorption (activation) step. Further, the appearance of steady-state amperometric currents implies the existence of a balance between the rates of liberation of the active sites by oxidative desorption of intermediate species that otherwise poison these sites (e.g., CO, CH x , CH3CHO, and CH3COOH) and the adsorption of ethanol at these catalytic sites. It is known [34, 38, 71–73] that the metal oxide (MoO3 or WO3)-containing environment at the electrocatalytic interface activates interfacial water at a lower potential (provide –OH groups on the oxide surface). This process is likely responsible for facilitating the electrooxidation of the surface poisoning intermediate species, especially CO, which frees these sites for the oxidation of new ethanol molecules.

Fig. 6
figure 6

Current–time responses at 0.3 V (a) and 0.4 V (b) for the oxidation of 0.5 mol dm−3 ethanol at bare PtRh/Vulcan (a), MoO3-modified PtRh/Vulcan (b), and WO3-modified PtRh/Vulcan (c) catalysts. Insert (C) chronoamperometric profiles of the prepared WO3 and MoO3 catalysts for ethanol oxidation recorded at 0.3 V. Electrolyte, 0.5 mol dm−3 H2SO4

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CO-stripping diagnostic experiments

To get some insight into the systems' abilities to remove (through oxidation) CO adsorbates, CO-stripping voltammetric experiments were performed at glassy carbon electrodes modified with PtRh/Vulcan, MoO3-modified PtRh/Vulcan, and WO3-modified PtRh/Vulcan catalysts (Fig. 7). The upper potential limit was chosen to avoid the over-oxidation of the surface and irreversible damage to the electrocatalyst structure. As shown in Fig. 7, significant differences in the onset potentials and peak potentials for CO oxidation during the first anodic cycles recorded using the MoO3-modified system in relation to the WO3-modified and unmodified catalysts were observed. Moreover, in all cases, CO blocks the hydrogen adsorption–desorption region which confirms that the surface is fully covered by CO. The position of the main CO-stripping (oxidation) peak for MoO3-modified PtRh/Vulcan and WO3-modified PtRh/Vulcan catalysts appeared at 0.66 and 0.70 V, respectively. The fact that the peak was only observed during the first anodic cycle implied that all adsorbed CO was oxidized and removed from the surface under such conditions. At the MoO3-modified system, in comparison to the WO3-modified catalyst, the CO-oxidation peak shifted negatively by 80 and 40 mV, respectively, versus the main COads electrooxidation peak (E = 0.74 V), appearing at the bare PtRh/Vulcan catalyst. The obtained results were in agreement with previous observations concerning enhancement of activities of Pt and PtSn catalysts by modification or admixing with selected transition metal oxides [31, 73–75].

Fig. 7
figure 7

CO stripping voltammograms recorded at 10 mV s−1 in 0.5 mol dm−3 H2SO4 for the PtRh/Vulcan (a), MoO3-modifed PtRh/Vulcan (b), and WO3-modifed PtRh/Vulcan (c) catalysts. CO adsorption was done at 0.1 V. The solid curve shows the first cycles, and the dotted curve shows the second cycles

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The electrochemically active surface area (S A) was calculated assuming a monolayer of adsorbed CO on the modified PtRh/Vulcan catalysts and measuring the charge required to oxidize this monolayer. The latter value was calculated by integrating CO-stripping peaks and assuming that the coulombic charge was 420 μC cm−2 [41, 75]. The S A values for the catalysts were 45, 44, and 42 m2 g−1 for bare PtRh/Vulcan, MoO3-modified PtRh/Vulcan, and WO3-modified PtRh/Vulcan, respectively. CO-stripping voltammetric investigations show that metal oxide species block only a small fraction of the electrochemically active PtRh particle surface area.

Conclusions

We have demonstrated an improvement in carbon-supported PtRh nanoparticles for the electrocatalytic oxidation of ethanol by adsorption of metal oxides (MoO3 or WO3) thereon. The existence these metal oxide layers on PtRh/Vulcan nanoparticles yields higher catalytic currents for the oxidation of ethanol under voltammetric and chronoamperometric conditions and a lower potential for the onset for this process. The increased catalytic efficiency shall be related to improved removal of poisoning species such as COads from the active sites. So far, there is no evidence for enhanced C–C bond splitting necessary for the oxidation of ethanol. The activation effect may also involve direct specific interactions (chemical or electronic) between metal oxides and Pt or Rh sites. It cannot be excluded that interactions between platinum or rhodium and molybdenum oxide or tungsten oxide may result in changes in the surface electronic structures and thus in changes in the adsorption energies of carbon monoxide or even ethanol on these surfaces [9]. Further research is along this line.