High electrocatalytic activity of carbon-supported Pd@PdO NPs catalysts prepared by a HaNPV virions template

Abstract

A novel Pd@PdO nanoparticles (NPs) catalyst was successfully prepared by a HaNPV virions template in the absence of reducing agents. Transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and electrochemical measurements were used to investigate the structure, composition and the electrocatalytic activity. The prepared Pd@PdO NPs/C has a high electrochemical active surface area, and thus enhances the performance of electric catalytic oxidation towards methanol and ethanol in alkaline medium. Long-term current–time measurements show that the as-prepared catalysts have better stability and higher CO tolerance than the commercial Pd/C catalysts.

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1. Introduction

Direct liquid fuel cells (DLFCs) using methanol and ethanol as fuel have the advantage of being environmental friendly, having high energy conversion and ease of fuel transportation and storage, making them a leader in green energy technology.^1–4 For methanol and ethanol oxidation platinum has been confirmed as the most active and stable of any of the pure metals. However, it is well known that platinum is readily poisoned by carbon monoxide, a by-product of methanol or ethanol electro-oxidation.^5,6 Pd-based catalysts have been considered to be alternatives for Pt catalysts owing to the advantages of relatively lower cost, more abundant resources and better resistance to CO poisoning.^7–11

Recently, Pan and coworkers¹² studied the electrocatalytic performance of Pt nanoparticles electrodeposited on a PdO thin film in a methanol oxidation reaction (MOR) in acidic media, and found that the synergism of the bifunctional mechanism and the electronic effect should be responsible for high CO tolerance of the Pd/PdO catalyst. In this work, we demonstrate the spontaneous formation of small, uniform and highly crystalline Pd@PdO nanoparticles along Helicoverpa armigera nucleopolyhedrosis virus (HaNPVs) biotemplates without external reducing agents. So far, the research on HaNPV is mainly focused on biological control and genetic engineering.^13–16 In this paper, the HaNPV were used as a scaffold for the synthesis of inorganic nanomaterials. The as-prepared Pd@PdO nanoparticles complexes were loaded on carbon black, and exhibited a higher catalytic activity and stability for both methanol and ethanol electric catalytic oxidation than the commercial Pd/C catalyst.

2. Results and discussion

The morphology and structure of the as-prepared samples were characterized by transmission electron microscopy. TEM images allowed a large number of the assembled metal/virus nanostructures to be visualized, and high-magnification TEM images of a rectangular area is shown as insets (Fig. 1), which revealed the detailed structures of the typical complexes. TEM images of the HaNPV virions (without Pd) are shown in Fig. S1 (in the ESI†) and reveal that those virions have no obvious change after adsorption of Pd particles. Further, the obtained metal/virus nanostructures were anchored on carbon black for use as composite catalysts by an ultrasonic-assisted method. The typical TEM images of carbon supported metal/virus shown in Fig. 2 and S2,† demonstrate that metal/virus were uniformly and stably adsorbed on carbon black. It is known that Pd/C materials are non-uniform due to interaction with various centers at the carbon surface and due to specific alignment.^17,18 It is speculated that the groups (such as phenol, ether, lactone, carboxyl and anhydride) on carbon black and the amino, carboxyl, and peptide group on virus rods increase the interaction of the metal/virus with the supporter, making the metal/virus adsorption more stable on carbon black. Meanwhile, the highly regular and symmetrical functionalities exposed to the external surface of HaNPV capsids permit the growth of metal particles in situ with uniform alignment on their surface.

Fig. 1 TEM image showing the HaNPV capsid covered by metal nanoparticles. Inset: high-magnification TEM image of rectangular area.

Fig. 2 High- and low-magnification TEM images of metal/virus supported by carbon black.

XRD measurements were performed to investigate the crystal structures of the prepared composite catalysts. In XRD spectra (Fig. 3a), the peak located at 25.0° is attributed to the (002) diffraction facets of carbon black. The representative diffraction peaks at 40.1°, 46.6°, 68.1° and 82.1° are well assigned to the (111), (200), (220) and (311) planes of the face-centered cubic Pd, respectively.¹⁹ While, the diffraction peaks centered at 33°, 42.2° and 54.7° are matched well with the (101), (111) and (102) planes of the face-centered cubic PdO, respectively. This result confirms that Pd and PdO were formed in the resulting sample. Also, the surface compositions and electronic structures of the samples were further confirmed through XPS analysis. Fig. 3b shows the high-resolution Pd 3d spectra. The two peaks located at 335.8 and 341.1 eV can be assigned to Pd 3d_3/2 and Pd 3d_5/2 of metallic Pd(0), respectively. The peaks at 337.6 and 342.9 eV are consistent with the reported values for Pd(II), confirming the presence of PdO species on the surface of Pd. Interestingly, the synthesized Pd or PdO were prepared without adding any reductant, suggesting that HaNPV virions have an inherent weak reduction effect. Recently, Lim and co-workers^20,21 reported the controlled synthesis of Pd coatings on TMV biotemplates in the absence of reducing agents. Whereafter, Yi’s group²² also prepared palladium nanoparticles (PdNPs) along genetically modified tobacco mosaic virus (TMV) biotemplates without external reducing agents. However, to the best of our knowledge, except TMV, detailed work on the spontaneous formation of Pd using HaNPV have rarely been reported. Here, for the first time, we use the HaNPV virions as biotemplate, which are naturally occurring, to synthesize Pd nanostructures. Here, the obtained sample was marked as Pd@PdO NPs/C. And all these results suggest that the Pd@PdO NPs/C have been successfully produced in the absence of reducing agents.

Fig. 3 XRD patterns of Pd@PdO NPs/C (a) and XPS spectra of Pd@PdO NPs/C (b).

Fig. 4 shows the detailed structure of the metal/virus complexes, which indicates that the nanoparticles are dispersed uniformly on the rod-like virions substrate without aggregation. The diameter of the virus rod is about 50 nm, and the length is about 340 nm. The histogram of the nanoparticle sizes based on 200 particles shown in the inset of Fig. 4a, indicating that the average particle size is about 2.6 ± 0.4 nm in diameter. HRTEM images (Fig. 4c) show the interplanar spacing of the particle lattice is 0.226 nm, corresponding to the (111) lattice spacing of the face centered cubic palladium. Interestingly, partial Pd clusters exhibit an irregular external structure which is marked by the white line, as shown in Fig. 4d, possibly due to the partial surface oxidation of the Pd clusters for forming PdO.²³

Fig. 4 Typical TEM images of Pd@PdO NPs (a and b) and HRTEM images of Pd@PdO clusters (c and d). The particle size distribution is shown as insets in (a).

Fig. 5 shows the cyclic voltammograms of the palladium catalysts in 0.5 M H₂SO₄ solution at the scan rate of 50 mV s⁻¹. There are two pairs of peaks from −0.24 to 0.02 V, which are attributed to the adsorption/desorption of hydrogen in the surface of palladium.²⁴ The electrochemically active surface areas (ECSAs) of both Pd@PdO NPs/C and the commercial Pd/C catalyst were estimated by a calculation of the charge density for the formation of a fully covered Pd(OH)₂ monolayer,²⁵ which was 430 mC cm⁻². The electrochemical active surface area of the Pd@PdO NPs/C modified GCE is calculated to be 33.57 m² g⁻¹, higher than that of the commercial Pd/C (20.4 m² g⁻¹).

Fig. 5 Cyclic voltammograms of the commercial Pd/C catalyst (curve a) and Pd@PdO NPs/C (curve b) modified electrodes in 0.5 M H₂SO₄ at the scan rate of 50 mV s⁻¹.

The catalytic activity of the Pd@PdO NPs/C towards methanol and ethanol was studied in alkaline medium by a cyclic voltammetry (CV) measurement technique. The commercial Pd/C was also investigated for comparison. Fig. 6 shows the catalytic performance modified electrodes for ethanol and methanol oxidation. The oxidation currents were normalized to ECSAs. Fig. 6a shows the specific activity of the catalysts towards ethanol electrooxidation, sweeping from −1.0 V to 0.4 V at a scan rate of 50 mV s⁻¹. The oxidation peak current density of the as-prepared Pd@PdO NPs/C is 6.4 mA cm⁻², 6.4 times higher than that of the commercial Pd/C (1.0 mA cm⁻²). The results are also better than for Pd nanocrystals supported on C₆₀ nanorods,²⁶ demonstrating that the synthesized materials are more efficient toward ethanol oxidation. The onset potential of ethanol oxidation is at −0.7 V, which is significantly more negative than the commercial Pd/C catalyst with a value of −0.58 V, indicating the enhancement in the kinetics of the ethanol electrooxidation reaction. Generally, the current peak ratio of I_f to I_b (the forward anodic peak current and the backward anodic peak current), could be used to describe the catalyst tolerance to the carbonaceous species on the electrode surface.²⁷ A higher ratio indicates more effective removal of the poisoning species on the catalyst surface. Here, the I_f/I_b ratio for the Pd@PdO NPs/C is 1.15 higher than that of the commercial Pd/C catalyst (0.67), showing a better tolerance to the intermediate carbonaceous species. The stability of Pd@PdO NPs/C toward ethanol electrooxidation was investigated by a long-term current–time measurement technique. As it can be learned from Fig. 6b that the current decay for the reaction on the Pd@PdO NPs/C is significantly slower than that of the commercial Pd/C catalysts, indicating a better tolerance toward ethanol electrooxidation. After 2000 s, the residue current of Pd@PdO NPs/C is 0.4 mA cm⁻², much higher than that of commercial Pd/C (0.1 mA cm⁻²).

Fig. 6 (a) Specific activity towards ethanol electrooxidation in a solution containing 0.5 M ethanol and 1 M KOH at 50 mV s⁻¹, (b) current–time curve recorded at −0.3 V. (c) Specific activity towards methanol electrooxidation in a solution containing 1 M methanol and 1 M KOH at 50 mV s⁻¹, (d) current–time curve recorded at −0.3 V.

Similarly, electrooxidation towards methanol was carried out in 1 M KOH and 1 M methanol aqueous solutions. The forward anodic peak of Pd@PdO NPs/C is 2.5 times higher than that of the commercial Pd/C as illustrated in Fig. 6c, indicating that the as-prepared Pd@PdO NPs/C possess an excellent catalytic activity. It can also be noted from Fig. 6d that the current decay for the reaction on the Pd@PdO NPs/C is significantly slower than that of commercial Pd/C. The results from both activity and stability studies indicate that Pd@PdO NPs/C possess outstanding catalytic performance for ethanol and methanol electrooxidation in alkaline media.

The excellent electrocatalytic performance of Pd@PdO NPs/C for ethanol and methanol oxidation can be explained from the following aspects. On one hand, it has been reported that introducing some metal oxides, such as CeO₂,²⁸ SnO₂,²⁹ RuO₂ (ref. 30) and ZrO₂,³¹ into the catalyst of platinum can greatly diminish the CO poisoning effect, leading to an increased electrochemical performance. Very recently, Ding’s group³² found that PdO on the catalysis of Pt_xPd_y can promote the catalytic activity toward methanol oxidation reaction. Jiang’s group³³ has also found that the addition of oxides like NiO, Co₃O₄ and Mn₃O₄ can promote catalytic activity and stability of the Pd/C electrocatalysts for the alcohol electrooxidation. It was interpreted that the surface oxide species on our prepared Pd@PdO NPs/C can donate oxygen to promote CO to CO₂ oxidation and release the active sites. On the other hand, based on a previous report,¹¹ probably, the evolved hydrogen gas or atoms have reacted with PdO to generate fresh Pd atoms, making the electrochemical oxidation process easier. The freshly prepared Pd atoms or nanoparticles have much higher electrocatalytic activity toward ethanol and methanol oxidation reaction as compared to the obsolete Pd nanoparticles, which probably may explain the fact that our prepared Pd@PdO NPs/C has better electrocatalysis than the commercial Pd/C catalyst. Consequently, Pd@PdO NPs/C catalyst showed a better electrocatalytic ability and higher CO tolerance than the commercial Pd/C catalysts.

3. Conclusion

In conclusion, we have successfully synthesized carbon-supported Pd@PdO composite nanoparticles on rod-like HaNPV virions by a facile method without any additional reducing agent. XRD and XPS results reveal the spontaneous formation of small Pd nanoparticles coexisting with PdO. The obtained Pd@PdO NPs are well dispersed on the carbon supports. The results from electrochemical measurements show that the Pd@PdO NPs/C catalyst exhibits superior electrocatalytic performance towards ethanol and methanol electrooxidation. It can be inferred that the presence of PdO may be responsible for the greatly enhanced electrooxidation and the CO tolerance. The facile preparation of Pd@PdO NPs/C with enhanced catalytic activities makes it a promising candidate anodic electrocatalyst for the future direct fuel cells.

4. Experimental

4.1. Materials

The inclusion bodies used in this experiment were purchased from Henan Jiyuan Baiyun Industrial Co. Ltd. Sodium tetrachloropalladate (Na₂PdCl₄) was purchased from Sigma Co. Ltd. All the aqueous solutions were prepared with distilled water.

4.2. The collection of the HaNPV virions

The inclusion bodies were purified using several centrifugation cycles (4000 rpm for 20–30 min). The deposits were successively re-suspended in 5% (v/v) acetone, 1% (w/w) SDS and phosphate chloride buffer (pH 7.4), followed by washing twice with redistilled water. The final suspension of inclusion bodies was lyophilized.

The highly purified inclusion bodies lyophilized powder were dissolved in 400 μL of DAS (0.3 mol L⁻¹ Na₂CO₃, 0.5 mol L⁻¹ NaCl and 0.03 mol L⁻¹ EDTA) in 2 mL centrifuge tube to form 10 mg mL⁻¹ solution and placed for 30 min, then extracted by 100 μL chloroform and centrifuged at 6000 rpm. The supernatant was centrifuged at 28 000 rpm for 1 h, and the pellets were re-suspended in phosphate chloride buffer to form HaNPV virions solution.

4.3. Synthesis of Pd@PdO NPs/C

200 μL of Na₂PdCl₄ stock solution (10 mM) was added to 600 μL of HaNPV virions solution, and the mixtures were adjusted to pH 7, and then incubated at room temperature for 24 h. Subsequently, 1.9 mg carbon was added and sonicated for 15 min to form a homogenous solution. Finally, the products were collected by centrifugation at 6000 rpm and washed several times with water. Finally, the Pd@PdO NPs/C were obtained.

4.4. Characterization

X-ray diffraction (XRD) patterns of the samples were recorded on a D/max-2500/PC diffractometer. Transmission electron microscopy (TEM) were performed on an Ht-7700 system at 120 kV accelerating voltage. The HRTEM and selected-area electron diffraction (SAED) experiments were obtained using a JEM-2010 TEM operating at 200 kV carried out to examine the crystallinity of the products. XPS measurements were performed on a PHI QUANTERA-II SXM X-ray Photon-electron Spectroscopy.

4.5. Electrochemical measurements

Electrochemical experiments were carried out on a CHI 832C electrochemical workstation, containing a traditional three-electrode cell, including the saturated calomel electrode (SCE) as the reference electrode, platinum electrode as counter electrode, and a modified glassy carbon electrode (GCE) as the working electrode. For preparation of the Pd@PdO NPs/C modified electrode, 8 μL of the samples was loaded on the surface of GCE and dried under an infrared lamp.

Acknowledgements

The authors acknowledge the financial support from the National Natural Science Foundation of China (Grant No. 21371149) and the Natural Science Foundation of Hebei (Grant No. B2016203498, 11965152D) and Research Fund for the Doctoral Program of Higher Education of China (Grant 20131333110010).

References

1. R. Larsen, S. Ha, J. Zakzeski and R. I. Masel, J. Power Sources, 2006, 15, 78–84.
2. W. He, H. Jiang, Y. Zhou, S. Yang, X. Xue and Z. Zou, Carbon, 2012, 50, 265–274.
3. S. Y. Shen, T. S. Zhao, J. B. Xu and Y. S. Li, J. Power Sources, 2010, 195, 1001–1006.
4. A. Brouzgou, A. Podias and P. Tsiakaras, J. Appl. Electrochem., 2013, 43, 119–136.
5. Z. X. Wu, Y. Y. Lv, Y. Y. Xia, P. A. Webley and D. Y. Zhao, J. Am. Chem. Soc., 2012, 134, 2236–2245.
6. Y. Y. Li, W. Gao, L. Y. Ci, C. M. Wang and M. A. Pulickel, Carbon, 2010, 48, 1124–1130.
7. S. Tang, S. Vongehr, Z. Zheng, H. Ren and X. Meng, Nanotechnology, 2012, 23, 255–606.
8. Q. Gao, M. R. Gao, J. W. Liu, M. Y. Chen, C. H. Cui and H. H. Li, Nanoscale, 2013, 5, 3202–3207.
9. Y. Lu, A. Eyssler, E. H. Otal, S. K. Matam, O. Brunko and A. Weidenkaff, Catal. Today, 2013, 208, 42–47.
10. Y. H. Chin, C. Buda, M. Neurock and E. Iglesia, J. Am. Chem. Soc., 2013, 135, 15425–15442.
11. K. Q. Ding, Y. Li, Y. B. Zhao, L. Liu, H. B. Gu and L. K. Liu, et al., Electrochim. Acta, 2014, 149, 186–192.
12. C. S. Chen and F. M. Pan, J. Power Sources, 2012, 208, 9–17.
13. L. H. Lua, L. K. Nielsen and S. Reid, Biol. Control, 2003, 26, 57–67.
14. G. H. Bergold and W. E. Ripper, Nature, 1957, 180, 764–765.
15. X. Chen, F. W. J. Ijkel, R. Tarchini, X. Sun, H. Sandbrink and H. Wang, et al., J. Gen. Virol., 2001, 82, 241–257.
16. L. H. Lua, M. Pedrini, S. Reid, A. Robertson and D. E. Tribe, J. Gen. Virol., 2002, 83, 947–957.
17. E. O. Pentsak, A. S. Kashin, M. V. Polynski, K. O. Kvashnina, P. Glatzel and V. P. Ananikov, Chem. Sci., 2015, 6, 3302–3313.
18. Z. Bai, Q. Zhang, J. Lv, S. Chao, L. Yang and J. Qiao, Electrochim. Acta, 2015, 177, 107–112.
19. A. J. Wang, F. F. Li, Z. Y. Bai and J. J. Feng, Electrochim. Acta, 2012, 85, 685–692.
20. J. S. Lim, S. M. Kim, S. Y. Lee, E. A. Stach, J. N. Culver and M. T. Harris, Nano Lett., 2010, 10, 3863–3867.
21. J. S. Lim, S. M. Kim, S. Y. Lee, E. A. Stach, J. N. Culver and M. T. Harris, J. Colloid Interface Sci., 2011, 356, 31–36.
22. C. X. Yang, C. H. Choi, C. S. Lee and H. Yi, ACS Nano, 2013, 7, 5032–5044.
23. B. J. Jiang, S. Z. Song, J. Q. Wang, Y. Xie, W. Y. Chu and H. F. Li, Nano Res., 2014, 7, 1280–1290.
24. W. Hong, Y. X. Fang, J. Wang and E. Wang, J. Power Sources, 2014, 248, 553–559.
25. L. Xiao, L. Zhuang, Y. Liu, J. Lu and H. Abruna, J. Am. Chem. Soc., 2009, 131, 602–608.
26. H. R. Barzegar, G. Z. Hu, C. Larsen, X. E. Jia, L. Edman and T. Wagberg, Carbon, 2014, 73, 34–40.
27. P. C. Su, H. S. Chen, T. Y. Chen, C. W. Liu, C. H. Lee and J. F. Lee, Int. J. Hydrogen Energy, 2013, 38, 4474–4482.
28. J. Xi, J. Wang, L. Yu, X. Qiu and L. Chen, Chem. Commun., 2007, 16, 1656–1658.
29. L. H. Jiang, G. Q. Sun, H. Zhou, S. G. Sun, Q. Wang and S. Y. Yan, J. Phys. Chem. B, 2005, 109, 8774–8778.
30. L. Cao, F. Scheiba, C. Roth, F. Schweiger, C. Cremers and U. Stimming, et al., Angew. Chem., Int. Ed., 2006, 45, 5315–5319.
31. Y. X. Bai, J. J. Wu, J. Y. Xi, J. S. Wang, W. T. Zhu, L. Q. Chen and X. P. Qiu, Electrochem. Commun., 2005, 7, 1087–1090.
32. K. Q. Ding, Y. H. Wang, H. W. Yang, C. B. Zheng, Y. L. Cao and H. G. Wei, et al., Electrochim. Acta, 2013, 100, 147–156.
33. J. Lu, S. Lu, D. Wang, M. Yang, Z. Liu, C. Xu and S. P. Jiang, Electrochim. Acta, 2009, 54, 5486–5491.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra14283d

‡ These authors contributed equally to this work.

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