Facile synthesis of cubical Co₃O₄ supported Au nanocomposites with high activity for the reduction of 4-nitrophenol to 4-aminophenol

Abstract

Cubic Co₃O₄ supported Au nanocomposites have been synthesized via a facile impregnation method. The Au/Co₃O₄ exhibited excellent catalytic activity for the reduction of 4-nitrophenol (TOF = 9.83 min⁻¹). The unique structure and high catalytic performance make the materials highly promising candidates for diverse applications in the area of catalysis.

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Noble metal nanostructures have attracted considerable attention because of their unique optical, catalytic, and electrochemical properties, which make them suitable materials in both fundamental and application research.¹ In particular, gold nanoparticles (Au NPs) have been proven to have an important role in several catalytic processes, including the reduction of nitro-aromatic compounds,² low-temperature CO oxidation,³ and organic synthesis.⁴ However, Au NPs are unstable and easy to aggregate due to their high surface energy, resulting in changes in the shape and size of the NPs during the reaction with the consequent decay in their original catalytic activity.⁵ Efforts have been made to control the aggregation of the Au NPs. For example, Au NPs catalysts with novel core–shell structure have been convinced to increase the dispersity of the Au NPs by uniformly encircling the surface of the core structure.⁶ Also, various substrates including reduced graphene oxide (RGO),⁷ silica,⁸ carbon nanotubes (CNT),⁹ metal oxides¹⁰ are successfully utilized to obtain catalysts with preferable dispersity. Among them, the incorporation of Au NPs and metal oxide supports provides a convincing strategy to hybrid heterogeneous catalysts for chemical transformation.^11,12 It is reported that the synergistic interactions between Au NPs and metal oxide supports contribute to the catalytic activity.¹³ Recently, Co₃O₄, an important transition metal oxide, has drawn attention as readily available, high-surface-area supports in catalytic transformations.¹⁴ Therefore, Au NPs grown on Co₃O₄ is likely to lead to more efficient and more stable nanocatalysts.

4-Nitrophenol (4-NP) is one of the most common water pollutants with high toxicity and carcinogenic character.¹⁵ However, the reduced product, 4-aminophenol (4-AP) is of great importance in the preparation of various analgesic and antipyretic drugs.¹⁶ It is also used enormously as a photographic developer, dyeing agent, corrosion inhibiting agent, etc.¹⁷ Thus, the catalysts are the main factor for the reduction of 4-NP. Up to now, lots of noble^13,18–26 and non-noble^27–29 metals catalysts have been tested for 4-NP reduction, among which most of the catalysts with high performance are still remained considerable challenges. Therefore, development of highly effective and stable catalyst for the reduction of 4-NP under mild condition is highly desirable.

Several metal-loading methods have been reported including surface ion exchange technique,³⁰ intermittent microwave irradiation (IMI),³¹ electrochemical deposition,³² etc. However, continuing efforts are underway to develop simple synthesis methods. In this work, the cubical Co₃O₄ supported Au nanocomposites (NCs) have been synthesized via a simple facile impregnation method. We have synthesized cubical Co₃O₄ in absence of any template, and then the cubical Co₃O₄ was used as supported for Au NPs to load. The as-prepared Au/Co₃O₄ NCs exhibited excellent catalytic activity for the reduction of 4-NP. The detailed experimental process could be found in the ESI.†

The microstructure of the obtained samples was characterized by transmission electron microscopy (TEM) and high-angle annular dark-field scanning TEM (HAADF-STEM). As shown in Fig. 1a and b, the cubical Co₃O₄ nanocrystals were synthesized successfully with an average length of 20.4 nm (Fig. S1a†). The TEM and HAADF-STEM images of Au/Co₃O₄ NCs (Fig. 1c and d) show that most of the Au NPs are well dispersed on the surface of cubical Co₃O₄ nanocrystals with the size of about 3.1 nm (Fig. S1b†). A close examination of the catalysts by HRTEM (Fig. S2†), the d-spacing of the particle lattice is ∼0.236 nm, which is consistent with the (111) plane of cubic Au (JCPDS no. 04-0784). The corresponding energy-dispersive X-ray (EDX) spectrum (Fig. 1e) proves the existence of the Au, Co elements. The phase structure and purity of Au/Co₃O₄ NCs were characterized by X-ray diffraction (XRD) (Fig. 1f). The diffraction peaks (2θ) of Co₃O₄ at 19.0°, 31.2°, 36.8°, 38.5°, 55.7°, 59.4°, 65.2° are ascribed to the (111), (220), (311), (222), (400), (422), (544), (440) planes of pure Co₃O₄, which are consistent with the peak positions of Co₃O₄ in JCPDS card no. 42-1467. Similar diffraction peaks are also observed in the Au/Co₃O₄ NCs, indicating that the structures of Co₃O₄ were preserved. However, no characteristic peaks belong to Au can be observed in the XRD patterns which is probably due to the fact that the Au loading of Au/Co₃O₄ NCs is too low to fit the test limit of XRD detection. Moreover, the Au content in Au/Co₃O₄ NCs measured by using ICP analysis was 4.9 wt%, which was quite close to the initial amount of Au.

Fig. 1 TEM images of (a and b) Co₃O₄, (c) Au/Co₃O₄; (d) HAAD-STEM image of Au/Co₃O₄; (e) EDX analysis for the marked area in (d); (f) powder X-ray diffraction patterns for Co₃O₄ and Au/Co₃O₄.

To better understand the composition of Au/Co₃O₄ NCs, we further carried out XPS analysis. Fig. S4† shows the XPS survey scan spectrum of Au/Co₃O₄ NCs. The survey scan spectrum reveals that Au is the only element detected except the Co₃O₄ elements (Co, O). This is well consistent with the EDX result. The high resolution Au 4f XPS spectrum shows two prominent peaks at 87.6 and 83.3 eV, which can be readily assigned to 4f_5/2 and 4f_7/2 of Au(0), respectively (Fig. 2a).³³ Fig. 2b shows the peaks of Co 2p. The peak appeared at 780.1 eV for Co 2p_3/2 and 795.9 eV for Co 2p_1/2.³⁴ The Co²⁺ peaks were observed at 797.3 and 781.7 eV, while the Co³⁺ peaks were observed at 795.4 and 779.8 eV. The peaks appeared at 786.0 and 802.7 eV were satellite peaks of Co²⁺. The Fig. 2c and d show the H₂-TPR profiles of Co₃O₄ and Au/Co₃O₄. It can be found that the Co₃O₄ shows two reduction peaks, are at 305 °C and 363 °C, respectively. The first low temperature peak (α) can be ascribed to the reduction of Co³⁺ to Co²⁺, whereas the second high temperature peak (β) can be assigned to the reduction of Co²⁺ to Co⁰.³⁵ However, the α and β peaks of Au/Co₃O₄ appeared at the higher temperature (347 °C and 449 °C) compared to Co₃O₄, indicating a much stronger interaction of Au with Co₃O₄ support in Au/Co₃O₄ NCs.

Fig. 2 XPS spectra of Au/Co₃O₄ nanocomposites sample. (a) Au 4f, (b) Co 2p peaks; H₂-TPR profiles for the (c) Co₃O₄ and (d) Au/Co₃O₄.

The catalytic performance of the as-synthesized cubical Co₃O₄ and Au/Co₃O₄ NCs were examined in the catalytic reduction of 4-NP in the presence of excess NaBH₄ at room temperature. The reaction process was monitored by UV-vis spectrometry. As seen in Fig. 3a, pure 4-NP solution exhibits a strong absorption peak at 317 nm under neutral or acidic conditions, and its color is pale yellow. The adsorption peak of 4-NP immediately red shifts from 317 to 400 nm upon the addition of NaBH₄ solution, and the color of the solution changed from pale yellow to bright yellow owing to the formation of 4-NP ion under the alkaline conditions. The reaction does not proceed for a couple of days even with a large excess of NaBH₄ in the absence of the catalyst,³⁶ this indicates that the use of NaBH₄ alone did not affect the catalytic reaction. Fig. 3c shows the changes in the time-dependent UV-vis spectra during the catalyzed reduction of 4-NP by Au/Co₃O₄ NCs. It can be seen clearly from the spectra that the intensity of the absorbance band at 400 nm successively decreased as the reaction progressed and vanished within 10.7 min, while a new absorption peak at 300 nm appeared and increased. These results indicate the complete reduction of the –NO₂ group of 4-NP to an –NH₂ group. However, in the presence of the cubical Co₃O₄ nanocrystals (see Fig. 3b), only 12% reduction efficiency was observed after a reaction time of 15 min. The above results indicate that the Au/Co₃O₄ NCs exhibited considerably higher catalytic activity for the reduction of 4-NP to 4-AP with NaBH₄ as the hydrogen donor, compared with the bare Co₃O₄ nanocrystals. The linear relationships between ln(C_t/C₀) and the reaction time (t) are obtained in the reduction reaction catalyzed by the Au/Co₃O₄ NCs catalyst (Fig. 3d), where C_t and C₀ are the 4-NP concentration at time t and 0, respectively. The catalytic reduction reaction conforms to the pseudo-first-order kinetics. The action rate constants k, calculated from the rate equation ln(C_t/C₀) = kt, is 0.005 s⁻¹. The turn over frequency (TOF) of Au/Co₃O₄ NCs catalyst is 9.83 min⁻¹, calculated by the moles of 4-NP reduced per mole of Au per consumed time under the present reaction conditions. As shown in Table 1, in comparison with other Au based catalysts, Au/Co₃O₄ NCs catalyst showed a higher catalytic efficiency than most of other Au based catalysts for the reduction of 4-NP, such as Au/TiO₂ (Au: 10.7 wt%),²⁶ graphene/PDA-Au (Au: 3.2 wt%),⁷ etc. Although it showed a relatively lower value than SiO₂@Fe₃O₄–C@Au¹⁸ and Fe₃O₄@SiO₂-LBL-Au²³ (Table 1), the Au/Co₃O₄ NCs catalyst has the advantages of simple synthetic procedure and mild preparation conditions. The recyclability of catalytic reduction was carried out for the Au/Co₃O₄ NCs for five successive cycles (Fig. S5†). The catalytic activity of Au/Co₃O₄ NCs remained almost unchanged after running five cycles. The Au concentration in the filtrate after removing Au/Co₃O₄ NCs catalysts was measured to be 0.318 ppm by ICP, indicating that most of Au NPs still remain on Co₃O₄ after the cycle test. The good stability of the Au/Co₃O₄ NCs can be confirmed by TEM characterization. In contrast to the fresh synthesized Au/Co₃O₄ NCs sample, the TEM image of Au/Co₃O₄ NCs after five runs of durability test exhibits that the Au nanoparticles are still well dispersed on the Co₃O₄ and no obvious agglomeration is observed (see Fig. S6 of the ESI†).

Fig. 3 UV-vis spectra measured after various time periods, showing 4-nitrophenol reduction (a) with and without NaBH₄; (b) in the presence of the Co₃O₄, and (c) in the presence of the Au/Co₃O₄. (d) Plots of ln(C₀/C_t) against the reaction time of the reduction of 4-NP over Au/Co₃O₄ NCs.

Table 1 Comparison of catalytic activity by Au based nanocatalysts for the reduction of 4-nitrophenol

Catalysts	4-NP (μmol)	Time (min)	Au content (μmol)	TOF (min^−1)	Ref.
SiO2@Fe3O4–C@Au	0.5	3.3	8.63 × 10^−3	17.4	19
Fe3O4@SiO2-LBL-Au	0.5	15	2.48 × 10^−3	13.4	23
Au/Co3O4	4	10.7	3.8 × 10^−2	9.83	This work
Fe3O4@PDA-Au	0.85	19	1.09 × 10^−2	4.1	13
Au-PMMA	1.35	10	8.8 × 10^−2	1.53	20
Graphene/PDA-Au	1	13	2 × 10^−1	0.38	7
Au/TiO2	0.3	16	0.26 × 10^3	0.34	26
Au/graphene	0.28	12	1.2 × 10^−1	0.19	24
Fe3O4@SiO2–Au@mSiO2	0.5	15	3.35 × 10^−1	0.1	22
Au@hollow silica	0.24	25	2 × 10^−1	0.048	6
Au-ECCG-CF	1	30	1	0.033	18
Fe3O4–Au	0.4	10	1.9	0.02	21

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The possible mechanism of catalytic reduction of 4-NP to 4-AP by NaBH₄ on the Au/Co₃O₄ NCs is shown in Scheme 1.³⁷ Based upon the obtained characteristic and catalytic results, we thus believe that the improved catalytic performance of this nanocomposite may be due to the chemical interaction between the Co₃O₄ nanocrystals and the Au NPs found by the H₂-TPR measurement, increasing the rate of electron transfer across the interface of the Co₃O₄–Au. The fast electron transfer in turn can increase the local electron concentration and facilitate efficient electron transfer from the BH₄⁻ donor to the acceptor molecules of 4-NP through the nanocatalyst, and the reduction reaction was thus achieved with a superior catalytic activity.

Scheme 1 Schematic illustration of the mechanism for catalytic reduction of 4-NP to 4-AP by the Au/Co₃O₄ NCs.

In summary, the cubical Co₃O₄ supported Au NCs have been synthesized successfully via a facile impregnation method. The as-prepared Au/Co₃O₄ NCs exhibited excellent catalytic activity for the reduction of 4-nitrophenol with a turn over frequency of 9.83 min⁻¹ at room temperature, which was higher than most of the reported values for the same reaction using Au-based catalysts. The significant enhancement of the catalytic activity could be ascribed to the interaction between the cubical Co₃O₄ and Au nanostructures. The unique structure, durability and high catalytic performance make these materials highly promising candidates for diverse applications in the area of catalysis.

Acknowledgements

The authors gratefully acknowledge the financial support from the NSFC (21363011) and the Project of NSF of Yunnan Province (2013FB094).

Notes and references

1. T. K. Sau, A. L. Rogach, F. Jackel, T. A. Klar and J. Feldmann, Adv. Mater., 2010, 22, 1805–1825.
2. J. John, E. Gravel, A. Hagege, H. Y. Li, T. Gacoin and E. Doris, Angew. Chem., Int. Ed., 2011, 123, 7675–7678.
3. M. Haruta, N. Yamada, T. Kobayashi and S. Iijima, J. Catal., 1989, 115, 301–309.
4. G. J. Hutchings, Top. Catal., 2008, 48, 55–59.
5. Z. Jin, M. Xiao, Z. Bao, P. Wang and J. Wang, Angew. Chem., Int. Ed., 2012, 51, 6406–6410.
6. S. Wu, C. Tseng, Y. Lin, C. Lin, Y. Hung and C. Mou, J. Mater. Chem., 2011, 21, 789–794.
7. J. Luo, N. Zhang, R. Liu and X. Y. Liu, RSC Adv., 2014, 4, 64816–64824.
8. J. Li, C. Liu and Y. Liu, J. Mater. Chem., 2012, 22, 8426–8430.
9. C. Fernandes, C. Pereira, A. Guedes, S. L. H. Rebelo and C. Freire, Appl. Catal., A, 2014, 486, 150–158.
10. C. Chen, C. Y. Nan, D. S. Wang, Q. Su, H. H. Duan, X. W. Liu, L. S. Zhang, D. Chu, W. G. Song, Q. Peng and Y. D. Li, Angew. Chem., Int. Ed., 2011, 123, 3809–3813.
11. B. Kong, J. Tang, C. Selomulya, W. Li, J. Wei, Y. Fang, Y. C. Wang, G. F. Zheng and D. Y. Zhao, J. Am. Chem. Soc., 2014, 136, 6822–6825.
12. Z. Jin, M. D. Xiao, Z. H. Bao, P. Wang and J. F. Wang, Angew. Chem., Int. Ed., 2012, 51, 1–6.
13. T. Zeng, X. Zhang, H. Niu, Y. Ma, W. Li and Y. Cai, Appl. Catal., B, 2013, 134–135, 293–310.
14. L. Pan, J. Tang and F. W. Wang, Mater. Res. Bull., 2013, 48, 2648–2653.
15. S. Saha, A. Pal, S. Kundu, S. Basu and T. Pal, Langmuir, 2010, 26, 2885–2893.
16. W. J. Liu, D. R. Sun, J. L. Fu, R. S. Yuan and Z. H. Li, RSC Adv., 2014, 4, 11003–11011.
17. R. Xiong, C. H. Lu, Y. R. Wang, Z. H. Zhou and X. X. Zhang, J. Mater. Chem. A, 2013, 1, 14910–14918.
18. H. Wu, X. Huang, M. Gao, X. Liao and B. Shi, Green Chem., 2011, 13, 651–658.
19. T. Zeng, X. Zhang, S. Wang, Y. Ma, H. Niu and Y. Cai, J. Mater. Chem. A, 2013, 1, 11641–11647.
20. K. Kuroda, T. Ishida and M. Haruta, J. Mol. Catal., 2009, 298, 7–11.
21. F. Lin and R. Doong, J. Phys. Chem. C, 2011, 115, 6591–6598.
22. Y. Deng, Y. Cai, Z. Sun, J. Liu, C. Liu, J. Wei, W. Li, C. Liu, Y. Wang and D. Zhao, J. Am. Chem. Soc., 2010, 132, 8466–8473.
23. Y. Zhu, J. Shen, K. Zhou, C. Chen, X. Yang and C. Li, J. Phys. Chem. C, 2011, 115, 1614–1619.
24. J. Li, C. Liu and Y. Liu, J. Mater. Chem., 2012, 22, 8426–8430.
25. Y. Shi, X. L. Zhang, G. Feng, X. S. Chen and Z. H. Lu, Ceram. Int., 2015, 41, 14660–14667.
26. Y. G. Hao, X. K. Shao, B. X. Lin, L. Y. Hu and T. Wang, Mater. Sci. Semicond. Process., 2015, 40, 621–630.
27. N. Sahiner, H. Ozay, O. Ozay and N. Aktas, Appl. Catal., B, 2010, 101, 137–143.
28. J. L. Yang, X. P. Shen, Z. Y. Ji, H. Zhou, G. X. Zhu and K. M. Chen, Appl. Surf. Sci., 2014, 316, 575–581.
29. M. Nemanashi and R. Meijboom, Interface Sci., 2013, 389, 260–267.
30. K. Yasuda and Y. Nishimura, Mater. Chem. Phys., 2003, 82, 921–928.
31. Z. Q. Tian, S. P. Jiang, Y. M. Liang and P. K. Shen, J. Phys. Chem. B, 2006, 110, 5343–5350.
32. B. M. Quinn, C. Dekker and G. S. Lemay, J. Am. Chem. Soc., 2005, 127, 6146–6147.
33. Y. G. Hao, X. K. Shao, B. X. Lin, L. Y. Hu and T. Wang, Mater. Sci. Semicond. Process., 2015, 40, 620–630.
34. Y. Yamada, K. Yano, Q. Xu and S. Fukuzumi, J. Phys. Chem. C, 2010, 114, 16456–16462.
35. W. J. Xue, X. Y. Zhang, P. Li, Z. T. Liu, Z. P. Hao and C. Y. Ma, Acta Physiol., 2011, 27, 1730–1736.
36. R. Ma, Y. J. Kim, D. A. Reddy and T. K. Kimn, Ceram. Int., 2015, 41, 12432–12438.
37. X. D. Wu, C. H. Lu, Z. H. Zhou, G. P. Yuan, R. Xiong and X. X. Zhang, Environ. Sci.: Nano, 2014, 1, 71–79.

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Footnote

† Electronic supplementary information (ESI) available: Materials, methods, ESI figures and table. See DOI: 10.1039/c6ra00183a

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