Catalytic properties of Pd nanoparticles supported on Cu₂O microspheres for hydrogen peroxide electroreduction

Abstract

Cuprous oxide (Cu₂O) were synthesized by a wet-chemical approach in aqueous solution and Pd electrocatalysts supported on cuporus oxide (Pd/Cu₂O) and Vulcan XC-72 (Pd/C) were synthesized both via modified sodium borohydride reduction method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry (CV) methods were used to characterize the surface morphology and composition of the Cu₂O nanoparticles and the catalysts. Linear sweep voltammetry (LSV) and chronoamperometry (CA) were employed to evaluate the activity and stability of the catalysts. The results showed that the Cu₂O nanoparticles were spherical with a smooth surface and the average diameter was about 600 nm. Pd nanoparticles decorated the surface of the Cu₂O nanospheres and agglomerated slightly and the average size of the Pd nanoparticles was about 10 nm. A certain amount of copper oxide and metallic Cu were formed during the preparation of the Pd/Cu₂O catalyst. The Pd catalyst supported on cuporus oxide (Pd/Cu₂O) showed a superior performance to the catalyst supported on Vulcan XC-72, which was attributed to two reasons. One was that the Pd/Cu₂O catalyst had a larger surface area and the other was that all of the support material components, Cu₂O itself, copper oxide and the metallic Cu formed in the process of catalyst preparation, had activity for the H₂O₂ electroreduction reaction to a certain extent.

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1. Introduction

Fuel cells using hydrogen peroxide as the oxidant, such as metal semi-fuel cells (MSFCs)^1–5 and direct borohydride fuel cells (DBFCs),^6–9 with the merits of high energy density, the capability to operate in an air-free environment (space and underwater), easy storage and distribution of both the fuel and the oxidant,^2,10,11 have received more attention recently.

The activity of electrocatalysts for H₂O₂ reduction directly influences the performance of fuel cells using H₂O₂ as the oxidant, so many studies of the catalysts of H₂O₂ electroreduction have been reported; these include: (1) noble metals, such as platinum, palladium, iridium, gold, silver and their alloys;^1,5,11–16 (2) macrocycle complexes of transition metals, such as Co porphyrin¹⁷ and Cu triazine complexes;^18,19 (3) transition metal oxides, such as cobalt oxides,^10,20,21 uranium oxide²² and ferric oxides.²³ Among these catalysts, noble metal catalysts have the higher activity for the H₂O₂ reduction reaction, although meanwhile, catalytic decomposition of the hydrogen peroxide occurs. Among them, noble metal Pd-based catalysts, such as Pd–Ir,²⁴ Pd–Ag¹² and Pd–Ru,²⁵ showed higher activity and better selectivity. These electrocatalysts are usually deposited on the support material to increase the surface area and reduce sintering effects. Carbon is a common choice for supporting nanosized electrocatalyst particles in low temperature fuel cells.¹⁴ However, carbon materials only serve to support the noble metal nanoparticles and not to improve the activity of the composite catalyst. Therefore, more attention has been paid to the preparation of composites made between noble metals and other catalytic materials. A few studies using oxide material as an active or promoting support have been reported in this regard; examples include Pt/CuO²⁶ and Pd/Co₃O₄.²⁷ This can further enhance the performance of the catalyst and also improve the efficiency of the noble metals.

Both oxides of copper, CuO^13,28 and Cu₂O,²⁹ have shown certain activity for the H₂O₂ reduction reaction, however, they have low electric conductivity and catalytic performance. Many reports^26,30–32 have revealed that these oxides decorated with metal particles could allow for an increase in the conductivity and a further improved performance. In this study, we prepared Cu₂O nanopowders and Pd nanoparticles supported on cuprous oxide and investigated the promotion by the support on the performance of Pd-based catalysts on hydrogen peroxide electroreduction.

2. Experimental procedure

2.1. Reagents

Copper sulfate (CuSO₄·5H₂O), glucose monohydrate (C₆H₁₂O₆) and anhydrous ethanol (C₂H₅OH) were purchased from Tianjin Kemiou Chemical Reagent Co. Ltd, and PdCl₂, NaBH₄, NaOH, H₂SO₄, and H₂O₂ (30 wt%) were supplied by Tianjin Tianda Chemical Preparation Co. Ltd. Vulcan XC-72(C) was obtained from Cabot Corp. with a specific surface area (BET) of 254 m² g⁻¹. Nafion solution (5 wt%) was purchased from DuPont Corp. All chemicals were analytical grade and were used as-received without further purification. Ultrapure water (Millipore, 18 MΩ cm) was used throughout the study.

2.2. Catalyst preparation

The preparation of Cu₂O nanoparticles was carried out as follows: 0.2 mol CuSO₄ was dissolved in 200 mL of deionized water. Then 200 mL of a 2 mol L⁻¹ NaOH aqueous solution was added dropwise at a speed of 2 mL min⁻¹ to the above CuSO₄ solution under constant stirring. A blue precipitate of Cu(OH)₂ was produced, and 200 mL of a 2.5 mol L⁻¹ glucose solution was added to the above suspension, and stirred constantly at 80 °C for 1 h. Then the brick red precipitates were produced and centrifuged, rinsed five times using deionized water and rinsed two times using anhydrous ethanol, then dried in a vacuous desiccator at 65 °C for 24 h.

The Pd/Cu₂O catalysts with a Pd metal-loading of 20 wt% were obtained using the chemical reduction method. First, the pH of a H₂PdCl₄ solution was adjusted to 8–9 using the NaOH solution. Then, 60 mg of Cu₂O nanopowder was added into the mixture and treated in an ultrasonic bath. 15 mL of a 1 mol L⁻¹ NaBH₄ solution was added drop by drop and stirred for 4 h. Finally, the mixtures were filtered, washed and dried under vacuum. To compare the characteristics of the electrocatalysts, 20 wt% Pd/C catalysts were prepared through the same procedure using Vulcan XC-72 as the support material.

2.3. Characterization

The crystal phase of the synthesized nanoparticles and the catalysts were identified by X-ray diffraction (XRD, D8FOCUS) using CuKα radiation (2 kV rotating anode, λ = 0.154056 nm). The samples were scanned from 20° to 90° at a scanning rate of 0.02° s⁻¹. The microstructures and morphologies of the Pd/Cu₂O catalysts were determined using scanning electron microscopy (SEM, JSM-6480) and transmission electron microscopy (TEM, H-7650). X-ray photoelectron spectroscopy (XPS) measurements were carried out on a Thermo ESCALAB 250 with Al Kα radiation as the X-ray source and the binding energy was calibrated by means of the C1s peak energy of 284.8 eV. The curves were fitted by using the XPSPEAK41 software.

2.4. Electrochemical characterizations

Electrochemical experiments were carried out in a conventional three-electrode electrochemical cell using a glassy-carbon electrode (d = 5 mm) as the working electrode, a carbon rod as the auxiliary electrode, and a saturated calomel electrode (SCE) as the reference electrode. The coating of catalyst onto the glassy-carbon electrode was carried out as follows: the glassy-carbon electrode was polished with alumina powder down to 0.3 μm, washed with ultrapure water and dried in air completely. 5 mg of the catalyst was mixed with 2 mL of ultrapure water. The mixture was homogenized in an ultrasonic bath for 30 min to obtain a catalyst suspension. 15 μL of the catalyst suspension was spread onto the surface of the glassy-carbon electrode with a loading of approximately 200 μg cm⁻² and dried in air at room temperature for 2 h. Then, 5 μL of Nafion solution was applied to the glassy-carbon electrode and dried in air in order to prevent the catalyst from falling apart. Thus a thin layer of catalyst was fixed onto the surface of the glassy-carbon electrode.

Electrochemical measurements were performed on a PARSTAT 2273 Potentiostats-Electrochemistry Workstation (Ametek Corp.). Cyclic voltammetry measurements (CV) were conducted at the potential between −0.3 V and 1.2 V in 0.1 mol L⁻¹ H₂SO₄ solution. Linear scan voltammetry (LSV) was carried out in 0.1 mol L⁻¹ H₂SO₄ solution contain H₂O₂ at various concentrations. The scan rate of the above two procedures was 50 mV s⁻¹ for both and the procedures were both repeated 5 times to obtain stable results. Chronoamperometry (CA) was performed in a 0.1 mol L⁻¹ H₂SO₄ + 0.5 mol L⁻¹ H₂O₂ solution for 1800 s at 0 V. The electrolyte solutions were saturated by bubbling through high purity nitrogen for 20 min before taking the measurements, and they were thereby protected with the nitrogen during potential cycling. All potentials in this paper are reported versus the saturated calomel electrode (SCE).

3. Results and discussion

3.1. Catalyst characterization

XRD measurements were carried out to obtain the structural information for the prepared Cu₂O nanopowders and the two catalysts obtained from different catalyst carriers. Fig. 1 shows the XRD patterns of the Pd/Cu₂O and Pd/C catalysts, and the insert shows the XRD patterns of the Cu₂O nanopowders. From the insert, peaks were observed at about 2θ = 29.8°, 36.7°, 42.5°, 61.5° and 73.7°, which were characteristic of the cubic Cu₂O phase (JCPDS Card no. 65-3288). From the pattern of the Pd/C catalyst, the diffraction peaks at about 39.9°, 46.4°, 67.8° and 81.6° are attributed to the Pd (111), Pd (200), Pd (220) and Pd (311) planes of the face-centered cubic (fcc) phase (JCPDS Card no. 65-2867) and the peak at about 25° was associated with the Vulcan XC-72 support material. From the pattern of the Pd/Cu₂O catalyst, besides the cubic Cu₂O phase and the Pd fcc phase, some unexpected characteristic peaks are observed. The peak at 38.8° is attributed to the CuO (111) planes (JCPDS Card no. 65-2309) and the peaks at 43.3° and 50.4° can be indexed to the (111) and (200) planes of the Cu crystal structure (JCPDS Card no. 04-0836). The results demonstrate that some Cu⁺ ions have been oxidized to Cu²⁺ ions and, at the same time, others have been reduced to metallic Cu by NaBH₄ during the preparation of the Pd/Cu₂O catalyst.

Fig. 1 XRD patterns of the Pd/Cu₂O and Pd/C samples. The inset shows the XRD pattern of Cu₂O nanopowders.

Fig. 2(a) and (b) show the SEM images of the Cu₂O samples at different magnifications, respectively. It can be seen that the Cu₂O particles prepared are spherical in shape with a smooth surface and also display high dispersity; the average diameter of the spheres is about 600 nm. Fig. 2(c) shows a TEM image of the Pd/Cu₂O catalyst and Fig. 2(d) shows the corresponding high magnification image. Pd nanoparticles were observed to be decorating the surface of the Cu₂O nanospheres and could be seen to be slightly agglomerated together to form larger clusters; the average size of the Pd nanoparticles is about 10 nm. It was confirmed from the SEM and TEM images that Pd/Cu₂O composite microspheres with a core–shell structure were obtained.

Fig. 2 (a) and (b) SEM images of Cu₂O nanopowders. (c) and (d) TEM images of Pd/Cu₂O catalysts.

X-ray photoelectron spectroscopy (XPS) was used to determine the surface composition and possible electronic interactions of the Cu₂O nanopowders and the Pd/Cu₂O catalysts. A typical survey XPS spectrum shown in Fig. 3(a) reveals that both samples are composed of Cu and O, while an additional peak assigned to Pd appears in the XPS spectrum of Pd/Cu₂O, which indicates the presence of Pd species on Cu₂O. Fig. 3(b) presents the high resolution XPS scan spectra of Pd 3d. As shown in Fig. 3(b), the Pd 3d signal of the Pd/Cu₂O catalyst can be fitted to two pairs of doubles: Pd 3d3/2 (340.5 eV), Pd 3d5/2 (335.2 eV) and Pd 3d3/2 (342.2 eV), Pd 3d5/2 (337.0 eV), which can be assigned to Pd⁰ and Pd^IIO species, respectively.³³ Fig. 3(c) and (d) show the Cu 2p XPS scan spectra of the Cu₂O and Pd/Cu₂O samples, respectively. The fitting of the Cu 2p spectrum (Fig. 3(c)) shows peaks at 932.5 eV and 952.5 eV, which correspond to the Cu 2p3/2 and Cu 2p1/2 of the Cu₂O,³⁴ respectively. As shown in Fig. 3(d), the peak fit of the Cu 2p3/2 core level for the Pd/Cu₂O catalyst revealed two binding energy states at 932.0 and 942.0 eV, which were assigned to Cu₂O and CuO, respectively.^33,35 Moreover, a satellite signal at 934.4 eV was also assigned to CuO.³⁶ It is obvious that a considerable amount of CuO exists on the surface of the Pd/Cu₂O catalyst, resulting in the presence of the characteristic peak of CuO (111) in the XRD pattern of the Pd/Cu₂O catalyst. Although some Cu⁰ should also exist on the catalyst surface, it is hard to discriminate from Cu₂O due to their similar binding energy. One can distinguish them from the position of their LMM-2 auger transition in the XPS spectra which are at about 568 eV and 570 eV for Cu and Cu₂O, respectively.³⁷ From Fig. 3(e), it can be seen that there is a peak at about 570 eV in the XPS spectra of the Cu₂O samples, thus a certain conclusion can be made that the 932.0 eV peak in Fig. 3(c) is related to Cu₂O. However, the peak at 569 eV in the XPS spectra of the Pd/Cu₂O samples revealed two binding energy states at 968.0 and 970.0 eV, which are attributed to Cu⁰ and Cu⁺ ions, respectively. This illustrates that a certain quantity of the metallic Cu also exists on the surface of the Pd/Cu₂O catalyst, which is in agreement with the discussed XRD results.

Fig. 3 (a) XPS survey spectra of the Cu₂O and Pd/Cu₂O samples. (b) Pd 3d scan spectra of the Pd/Cu₂O samples. (c) and (d) show the Cu 2p scan spectra of the Cu₂O and Pd/Cu₂O samples. (e) Cu LMM-2 auger transition of the Cu₂O and Pd/Cu₂O samples.

3.2. Electrochemical measurements

Fig. 4 shows cyclic voltammograms (CV) of the Pd/Cu₂O and Pd/C catalysts in a 0.1 mol L⁻¹ H₂SO₄ solution, and the insert shows the CV of the Cu₂O nanopowder. It can be seen from the insert that a pair of nearly reversible redox peaks were observed at about −0.2 V and 0.1 V (versus SCE) which can be attributed to the reduction of the CuO/Cu₂O redox couple.²⁹ The shape of the voltammetry curves of the Pd/Cu₂O and Pd/C catalysts are very similar and both samples exhibit a hydrogen adsorption/desorption peak at −0.3–0 V and a Pd-oxide formation/reduction peak at 0.5–1.2 V. The reduction of the palladium oxide of the Pd/Cu₂O or Pd/C catalyst shows a well-defined cathodic peak between 0.1 V and 0.5 V. However, the anodic peak at 0.3 V in the Pd/Cu₂O catalyst, which was not seen in the Pd/C sample, indicates the oxidation of a Cu–O species.³³ This is due to the existence of a certain quantity of the metallic Cu on the surface of the Pd/Cu₂O catalyst, which is consistent with the findings of XRD and XPS analysis. The electrochemically active surface area (EASA) of the catalysts can be calculated from the charge obtained from the cathodic peak between 0.1 V and 0.5 V. It is assumed that a monolayer of PdO was formed and its reduction charge value is 405 μC cm⁻².³⁸ Such estimated EASA values of the Pd/Cu₂O and Pd/C catalysts are 53.3 m² g_Pd⁻¹ and 30 m² g_Pd⁻¹, respectively. This indicated that the Pd/Cu₂O electrode has a larger surface area than the Pd/C electrode.

Fig. 4 Cyclic voltammograms of Pd/C and Pd/Cu₂O catalysts in a 0.1 mol L⁻¹ H₂SO₄ solution at a scan rate of 50 mV s⁻¹.

Fig. 5 shows linear sweep voltammograms of the Pd/Cu₂O and Pd/C catalysts in N₂-saturated 0.1 mol L⁻¹ H₂SO₄ solutions with various H₂O₂ concentrations at a scan rate of 50 mV s⁻¹. It can be seen that the current density for hydrogen peroxide electroreduction on the two catalysts increases with the increase in concentration of H₂O₂ and the current density on the Pd/Cu₂O electrode is higher than that on the Pd/C electrode, which indicates that the catalytic activity of the Pd/Cu₂O electrode is higher than that of the Pd/C electrode. There are two reasons leading to the higher catalytic activity of the Pd/Cu₂O catalyst. One is that the Pd/Cu₂O electrode has a larger specific surface and the other may be that both the support Cu₂O itself²⁹ and the two other products formed during the preparation of the Pd/Cu₂O catalyst, including CuO^13,28 and the metal Cu,³⁹ show a certain activity for the action of H₂O₂ reduction. It should be noted that the noble metal Pd has the higher catalytic activity on H₂O₂ reduction in an acid medium.¹¹ There have been several studies on the electroreduction action of hydrogen peroxide on Cu and its oxides surfaces in different pH solutions.^13,18,39,40 It has been shown that the catalysis in this reaction is brought about by the Cu(II)/Cu(I) and Cu(I)/Cu(0) couples. As shown in eqn (1)–(4), Cu with the higher valence in the couples, such as Cu(II) and Cu(I), were firstly reduced electrochemically to Cu with the low valence, such as Cu(I) and Cu(0), which reacted chemically with H₂O₂ and resulted in H₂O₂ reverting to OH⁻ and in the regeneration of the catalyst.

2CuO + H₂O + 2e → Cu₂O + 2OH⁻ (1)

Cu₂O + H₂O + 2e → 2Cu + 2OH⁻ (2)

Cu₂O + H₂O₂ → 2CuO + H₂O (3)

2Cu + H₂O₂ → Cu₂O + H₂O (4)

Fig. 5 Linear sweep voltammograms of the Pd/C and Pd/Cu₂O catalysts in a 0.1 mol L⁻¹ H₂SO₄ solution contain H₂O₂ at different concentrations at a scan rate of 50 mV s⁻¹.

Chronoamperometry tests for the hydrogen peroxide electroreduction reaction on the Pd/Cu₂O and Pd/C catalysts were performed at 0 V for 1800 s. As is shown in Fig. 6, the two catalysts exhibit excellent stability although the current density on the two catalysts slightly decreases during the test because of the depletion of H₂O₂ near the electrode surface. The current density for the hydrogen peroxide electroreduction reaction of the Pd/Cu₂O catalyst (about 38 mA cm⁻²) is higher than that of Pd/C (about 17 mA cm⁻²). This agrees well with the linear sweep voltammetry study. The higher current density can be attributed to the higher active surface area and the promoting effect of the supports to the Pd catalyst for the hydrogen peroxide electroreduction reaction.

Fig. 6 Chronoamperometry of Pd/C and Pd/Cu₂O catalysts in a 0.1 mol L⁻¹ H₂SO₄ + 0.5 mol L⁻¹ H₂O₂ solution at a scan rate of 50 mV s⁻¹.

4. Conclusions

In this work, cuprous oxide nanoparticles were synthesized by a wet-chemical approach in aqueous solution and the Pd catalysts supported on Cu₂O and C were prepared by means of a modified sodium borohydride reduction method. As a result, the Cu₂O nanosphere was cubical and the size was about 600 nm. Palladium nanoparticles were decorated on Cu₂O as metal with a face-centered cubic structure; the particle size was about 10 nm. Metallic Cu and CuO were produced during the synthesis of the Pd/Cu₂O composites. The reduction reaction of hydrogen peroxide on the Pd/Cu₂O catalyst has good stability and a higher current density. This was due to the fact that (1) the Pd/Cu₂O catalyst has the larger surface area; (2) the support Cu₂O and the metallic Cu and CuO produced in the preparation process improved the activity of Pd towards the H₂O₂ reduction reaction. Using this method, catalysts are prepared in aqueous solution easily with high catalytic activity and at a lower cost. The method therefore has great potential in industrial applications.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (21003070; 21463017) and the Natural Science Foundation of Inner Mongolia, China (2012MS0208).

Notes and references

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