Glutamine-assisted synthesis of Cu-doped CeO₂ nanowires with an improved low-temperature CO oxidation activity

Abstract

Cu-doped CeO₂ nanowires with a diameter under 10 nm have been prepared via a glutamine (GLN)-assisted green strategy. The obtained Cu-doped CeO₂ nanowires exhibit outstanding performance as catalyst for CO oxidation, which can completely convert CO at the lower temperature of 90 °C.

---

Recently, as a harmful gas, CO mainly emitted from fuel burning has caused serious environmental pollution. One of the most efficient methods to abate CO is to oxidize CO into CO₂ with the aid of catalysts. Owing to their high catalytic activity, noble metal (Au, Ag, Pd, Pt, Rh) supported catalysts for CO oxidation have attracted more attention.^1–4 However, the high cost and limited resources of noble metals as well as their poor stability have greatly restricted their further applications.^5–8 So it is urgent to develop non-noble metal-based catalysts for CO oxidation, which are abundantly available and cheap. Nowadays researchers have taken a particular interest in the abundant oxygen vacancy defects and high oxygen storage capacity of CeO₂ and CeO₂-based materials, resulting from the facile cycling between Ce³⁺and Ce⁴⁺.^6,8–13 A series of CeO₂-based mixed bimetal oxides have been investigated extensively and exhibited better catalytic performance than pure CeO_2.^8–11,14 Both theoretical calculations and detailed experiments have demonstrated that doping foreign atoms into the CeO₂ structure is an effective approach to achieving high catalytic activity because of their synergetic effects between ceria and foreign atoms in the redox process. The incorporation of heteroatoms can generate more oxygen vacancies by replacing Ce⁴⁺ ion in the CeO₂ lattice, which provokes a higher oxygen mobility and diffusion from the lattice to the interface of heteroatom oxide and ceria.^14–19 So far, more efforts have been made to synthesize doped ceria for CO oxidation. For example, Cu-doped CeO₂ exhibited a relatively low complete conversion temperature of CO oxidation as low as 125 °C.²⁰ Li et al. reported superior catalytic activity of Ni-doped ceria with 100% conversion at 150 °C.²¹ Mono-disperse Mn-doped CeO₂ nanospheres with 7.8 atom% Mn showed great catalytic performance with a CO conversion of 100% at 255 °C.²² Among various possible alternatives, catalysts composed of copper and cerium oxides have displayed prominent characteristics for CO oxidation.^6,20 As far as we know, the morphology of a catalyst, together with its synthetic method and structure, has a significant influence on the catalytic activity. In CO oxidation reaction, some CeO₂ or CeO₂-based materials with special surface structures have been investigated to understand their morphologic effects on the catalytic activity.^15,21–25 For instance, CeO₂ nanotubes have been grown electrochemically using a porous alumina membrane as a template. These CeO₂ nanotubes showed a catalytic activity in CO oxidation, at 200 °C, around 400 times higher than powdered CeO₂.²⁵ Zeng et al.¹⁵ prepared inverse CeO₂–CuO catalysts with different morphologies by the precipitation combined with impregnation method, and the performance tests proved that CeO₂ supported on the CuO with petal morphology presented good catalytic activity, which could totally convert CO to CO₂ within 155–195 °C temperature range. In addition, neodymium uniformly doped CeO₂ nanowires obtained by controlling the redox property of Ce(III)/Ce(IV) in the hydrothermal synthesis demonstrated very high catalytic activity.²⁶

Among these shape-tunable CeO₂ or CeO₂-based nano-materials, one-dimensional nanowires are the most efficient to improve their catalytic activity because they expose rough and active (110) facets, on which oxygen defects are easy to generate.^27,28 These oxygen defects, in turn, facilitate the migration of lattice oxygen for oxidation of adsorbed CO intermediates. Moreover, nanowires with high surface area can provide much more active sites for CO gas. However, it is still a major challenge for preparing semiconductor nanowires with controllable size and doping. Especially, the size of nanowires under 10 nm still remains a bigger challenge.²⁹

Amino acids, as important biomolecules with specific functional groups, can interact with nano-materials by covalent coupling, electrostatic binding and physical adsorption. Thus, more attention has been paid to amino acids assisted synthesis and growth mechanism of micro/nano structured metal oxides with novel morphology, as well as their potential applications. Using L-cysteine as a structure-directing agent, porous NiO hollow microspheres have been prepared via a simple hydrothermal process.³⁰ Zhang et al. reported the synthesis of hierarchical nano-architectures of mesoporous ceria materials in the presence of different amino acids.³¹ In our previous study, we also have prepared pompon-like hierarchical CuO hollow microspheres via a glutamine (GLN)-assisted green strategy.³² The obtained CuO microspheres exhibited excellent performance as anode for lithium ion battery.

In the present work, we have successfully synthesized Cu-doped CeO₂ nanowires with size under 10 nm using GLN as morphology control agent by a facile hydrothermal method for the first time. Interestingly, the average high aspect ratio of the nanowires was more than 500. The obtained Cu-doped CeO₂ nanowires revealed an enhanced catalytic activity in CO low temperature oxidation in contrast with pure CeO₂ synthesized under the same experimental condition.

CuO–CeO₂ composites were obtained from the calcined Ce–Cu binary precursors. The composites prepared in the presence of GLN and in the absence of GLN have a Cu/(Cu + Ce) atom ratio of 20.02% and 40.68% (designed as CeCu_0.20 and CeCu_0.41, respectively), which have been confirmed by inductively coupled plasma spectroscopy (ICP). Fig. 1 shows XRD patterns of the calcined samples. All peaks of the pure CeO₂ prepared in the presence of GLN can be perfectly indexed to the cubic phase CeO₂ (JCPDS no. 43-1002) (Fig. 1a). As shown in Fig. 1b, CeCu_0.20 nanowires prepared with GLN remain the same structure as pure CeO₂ and have no peaks of copper oxides and salts observed. The absence of impurity peaks indicates the good incorporation of Cu into the CeO₂ lattice.¹¹ However, compared with pure CeO₂ prepared under the same condition, CeCu_0.20 nanowires prepared with GLN show a gradual decrease in the peak intensity and a slight increase in the peak width, indicating that the grain size becomes minimal due to the doping of Cu atom into ceria to form nanowires.³³ It indicates that the addition of Cu ions has no obvious influence on the phase structure except the composition. Nevertheless, compared with CeCu_0.20 sample, the diffraction peaks of CeCu_0.41 sample prepared without GLN are indexed to cubic CeO₂ structure (JCPDS no. 43-1002) and monoclinic phase CuO structure (JCPDS no. 5-661) (Fig. 1c). It shows that with the coexistence of Cu ions and Ce ions, GLN plays an important role in the phase structure and composition due to its competitive coordination with Cu ions and Ce ions. In addition, the prepared samples present different colours. As shown in Fig. S1,† pure CeO₂ (Fig. S1a†) is light yellow, while CeCu_0.20 (Fig. S1b†) and CeCu_0.41 (Fig. S1c†) are brown and black, respectively. It suggests that both glutamine and Cu ions play an important role in the phase structure and composition of the prepared samples.

Fig. 1 XRD patterns of calcined samples (a) pure CeO₂ with GLN, (b) CeCu_0.20 nanowires with GLN, (c) CeCu_0.41 without GLN.

Fig. 2a–c show the typical morphologies of pure CeO₂, CeCu_0.41 prepared without GLN and CeCu_0.20 nanowires prepared with GLN, respectively. It is important to note that both pure CeO₂ and CeCu_0.41 samples reveal irregular nanoparticles (Fig. 2a–b). However, as displayed in Fig. 2c, CeCu_0.20 sample prepared in the presence of GLN consists of loose nanowires which interlace to form a three-dimensional network. According to the above analysis, it can be concluded that both glutamine and Cu ions in the reaction system is indispensable for nanowire-like morphology of copper and cerium composite. The nanowire-like structure may enable the catalysts to keep better contact with CO molecules due to its higher surface area. So it would be anticipated that Cu-doped CeCu_0.20 nanowires will exhibit better catalytic performance than pure CeO₂ and CeCu_0.41 nanoparticles. The analysis from the EDS pattern (Fig. 2d) reveals that Cu-doped CeCu_0.20 nanowires contain elements of copper, cerium and oxygen, which confirms the successfully doping of copper into ceria.

Fig. 2 SEM images of (a) pure CeO₂, (b) CeCu_0.41 nanoparticles and (c) CeCu_0.20 nanowires; (d) EDS pattern of CeCu_0.20 nanowires.

Raman spectroscopy is an important measurement tool to investigate the change in the local structure resulted from the incorporation of impurities.³⁴ Fig. 3 shows the Raman spectra of pure CeO₂ and CeCu_0.20 nanowires. As displayed in Fig. 3a, both pure CeO₂ nanoparticles and CeCu_0.20 nanowire exhibit the Raman peak at about 460 cm⁻¹, which can be assigned to the F2g vibration mode of the cubic fluorite lattice.³⁵ The Raman peak intensity for the CeCu_0.20 nanowires significantly reduced (Fig. 3b). It might be related to the increase of disorder in the fluorite structure and cell contraction due to partial substitution of Ce for Cu or existence of oxygen vacancies.³⁶ In addition, CeCu_0.20 nanowires exhibit one more peak at 610 cm⁻¹, which is otherwise absent in the pure CeO₂, further indicating the formation of additional oxygen vacancies on the lattice site Ce⁴⁺ ion after doping of Cu²⁺.³⁷

Fig. 3 Raman spectra of (a) pure CeO₂; (b) CeCu_0.20 nanowires.

The microstructural details of Cu-doped CeCu_0.20 nanowires were further investigated by TEM. CeCu_0.20 nanowires with a diameter of 3–8 nm were be clearly observed in Fig. 4a. The selected area electron diffraction (SAED) pattern in the inset demonstrates that CeCu_0.20 nanowires are polycrystalline structure. Combined the TEM results with the SEM results, the average high aspect ratio of CeCu_0.20 nanowires is calculated to be more than 500. The typical nitrogen adsorption–desorption isotherm of CeCu_0.20 nanowires is illustrated in Fig. 4b. The isotherm with a hysteresis loop in the high relative pressure range of 0.5–1.0 belongs to the type-IV adsorption–desorption isotherm with H3-type hysteresis, indicating the presence of mesopores within the sample caused by the thermal treatment of precursor.³⁸ The BET surface area is calculated to be 101.11 m² g⁻¹, which is much larger than that of purchased pure CeO₂ nanoparticles.³⁹ It is speculated that the nanowire-like structure with higher surface area might provide more interface area for the catalyst and CO gases so as to improve the catalytic activity. Furthermore, oxygen defects are easy to form on nanowires,^27,28 which in turn promote the migration of lattice oxygen for oxidation of adsorbed CO intermediates, implying excellent catalytic performance for nanowires.

Fig. 4 (a) TEM image of CeCu_0.20 nanowires, the inset is SAED pattern of CeCu_0.20 nanowires; (b) N₂ adsorption–desorption isothermal curve of CeCu_0.20 nanowires.

XPS analysis was carried out to further confirm the composition of the calcined CeCu_0.20 nanowires. Elements of Cu, O, Ce, and C co-exist in the CeCu_0.20 nanowires from the survey spectrum (Fig. 5a), where C1s peak is attributed to the residual carbon from the sample and adventitious hydrocarbon from the XPS instrument itself. This result is in good agreement with the EDS data (Fig. 3d). As displayed in Fig. 5b, the presence of the shake-up peaks and a higher Cu 2p_3/2 binding energy at 933.36 eV indicate the existence of Cu(II) species in the CeCu_0.20 composite.¹¹ From Fig. 5c, the peaks centered at 897.73 eV and 916.23 eV can be ascribed to Ce⁴⁺ and those centered at 881.93 eV and 900.43 eV can be ascribed to Ce³⁺, suggesting that Ce atom has two valence states.^11,14

Fig. 5 XPS spectra of calcined CeCu_0.20 nanowires (a) the survey; (b) Cu 2p; (c) Ce 3d.

Moreover, the relative atomic percentages of Cu and Ce identified at the different etching times (0 min, 2 min, 6 min, 11 min) of CeCu_0.20 nanowires are shown in Table S1.† The Cu/Ce relative atom ratio is calculated from the narrow regions scans of Ce 3d and Cu 2p by the atomic sensitivity factors, where Shirley mode is used for subtraction of the spectral background the shakeup contribution to the intensity of the main peak has been also taken into account. With the increase of etching time, the relative atomic content ratio of Cu/(Ce + Cu) is almost kept unchanged from the surface to the interior of CuCe_0.20 nanowires, which is in good accordance with the result of ICP analysis. It can be interred that Cu species may uniformly distribute into CuCe_0.20 nanowires.

To investigate the growth process of CeCu_0.20 nanowires, time-dependent experiments have been done and the precursors obtained at different growth stages were examined by SEM. As shown in Fig. 6a, the obtained sample at early stage (3 h) presented irregular pseudo-spherical structure aggregated by nanoparticles. When the reaction time was prolonged to 6 h (Fig. 6b), a small part of these irregular microspheres started to dissolve. As the reaction proceeded, nanowires came into being (Fig. 6c). When the reaction time was 12 h, irregular microspheres completely changed into uniform nanowires (Fig. 6d). But with the further prolongation of reaction time, nanowires began to aggregate into bundles, as shown in Fig. 7e.

Fig. 6 SEM images of as-synthesized precursors obtained at different times (a) 3 h; (b) 6 h; (c) 9 h; (d) 12 h; (e) 15 h.

Fig. 7 UV-vis diffuse reflectance spectra (DRS) (a) pure CeO₂; (b) CeCu_0.20 nanowires.

On the basis of the above experimental results, the possible formation mechanism of CeCu_0.20 nanowires is proposed and illustrated in Scheme 1. Firstly, at the initial stage of hydrothermal reaction, under the control of GLN, irregularly pseudo-spherical structure comes into being by the assembly of Cu²⁺ and Ce³⁺ ions which are evenly dispersed in the solution. Subsequently, pseudo-spheres begin to dissolve into small nanoparticles with the further prolongation of reaction. Then, chain-like architectures are formed through the oriented attachment of the obtained small nanoparticles. As the reaction proceeds, CeCu_0.20 nanowires with three-dimensional network structure turn up.

Scheme 1 Schematic illustration of the probable formation mechanism of CeCu_0.20 nanowires.

UV-vis diffuse reflectance spectra (DRS) of pure CeO₂ and CeCu_0.20 nanowires are shown in Fig. 7. It can be seen that CeCu_0.20 nanowires have a red shift and an increasing absorbance in the visible region. The direct band gap energy can be estimated by the Tauc plots of (αhν)² against photon energy (hγ), as shown in Fig. 8a and b. It is found that the band gap energies for pure CeO₂ and CeCu_0.20 nanowires are 3.14 eV and 3.11 eV, respectively. The red shift of band gap for CeCu_0.20 nanowires can be explained by the shape effect prevailing over the quantum size effect, which is favorable to the formation of self-trapped excitation from the free excitation ascribed to the coexistence of rich oxygen vacancies among the gaps.⁴⁰

Fig. 8 Plots of (αhν)² as a function of photon energy for (a) pure CeO₂; (b) CeCu_0.20 nanowires.

The prepared CeCu_0.20 nanowires were tested for CO oxidation under a reaction stream with a gas composition of 1% CO and fresh air balanced composition from air generator. Fig. 9 shows the CO conversion temperature on pure CeO₂ and CeCu_0.20 nanowire catalysts. It can be clearly seen that for the first run, CeCu_0.20 nanowire catalyst exhibits the lower complete conversion temperature (T 100%) of CO oxidation as low as 90 °C, but the pure CeO₂ catalyst has no catalytic activity before 180 °C. Moreover, the prepared CeCu 0.20 catalyst still keeps good activity and exhibits 99% CO conversion at 90 °C for a second run, which confirms good stability of the systems. It provides a big evidence that there is a strong synergetic effect between Cu and Ce species in the CeCu_0.20 nanowire composite in the redox process. The existence of Cu can generate more oxygen vacancies by replacing Ce⁴⁺ ion. Meanwhile, the nanowire structure makes oxygen defects form more easily, which can in turn promote the migration of lattice oxygen for oxidation of adsorbed CO intermediates. Furthermore, the high surface area of nanowires with size under 10 nm can provide more active sites for CO. All the above factors, such as Cu doping, the large surface area, and the easy formation of oxygen defects on nanowires, are responsible for the outstanding catalytic oxidation performance of CeCu_0.20 nanowires.

Fig. 9 Oxidation temperature on pure CeO₂ and CeCu_0.20 nanowires.

In conclusion, the presented results suggest a bio-inspired approach for the preparation of Cu-doped CeO₂ nanowires with a diameter under 10 nm and with a high aspect ratio more than 500 using GLN as structure directing agent. The addition of GLN and the reaction time for 12 h as well as the copper atom play a critical role in the formation of CeCu_0.20 nanowires. The obtained Cu-doped CeO₂ nanowires exhibit high catalytic activity and good stability for CO oxidation, superior to that of most reported Cu-doping CeO₂ materials. And the excellent catalytic performance of Cu-doped CeO₂ nanowires might be ascribed to the unique structure characters. On one hand, the morphology of nanowires with size under 10 nm is conducive to the formation of oxygen defects and possess high surface area which can provide much more active sites for CO gas. On the other hand, the doping of Cu atoms into CeO₂ lattice can provide more oxygen vacancies in the redox process. This amino acids-assisted synthetic strategy might also be extended to synthesis of other nanostructured CeO₂ based composites for use in CO conversion.

Acknowledgements

This work was financially supported by 973 Program (Grant 2011BE091101) and the Major Research plan of the National Natural Science Foundation of China (Grant 91227107), which are gratefully acknowledged.

Notes and references

1. J. Xu, T. White and P. Li, J. Am. Chem. Soc., 2010, 132, 10398.
2. M. M. Han, X. J. Wang and Y. N. Shen, J. Phys. Chem. C, 2010, 114, 793.
3. Y. F. Han, Z. Y. Zhong, K. Ramesh, F. X. Chen and L. W. Chen, J. Phys. Chem. C, 2007, 111, 3163.
4. Z. L. Wu, S. H. Zhou and H. G. Zhu, J. Phys. Chem. C, 2009, 113, 3726.
5. A. A. Herzing, C. J. Kiely, A. F. Carley, P. Landon and G. J. Hutchings, Science, 2008, 321, 1331.
6. X. W. Xie, Y. Li, Z. Q. Liu, M. Haruta and W. J. Shen, Nature, 2009, 458, 746.
7. F. Yang, J. Graciani, J. Evans, P. Liu, J. Hrbek, J. F. Sanz and J. A. Rodriguez, J. Am. Chem. Soc., 2011, 133, 3444.
8. C. G. Maciel, L. P. R. Profeti and E. M. Assaf, J. Power Sources, 2011, 196, 747.
9. R. D. Monte and J. Kaspar, J. Mater. Chem., 2005, 15, 633.
10. C. W. Sun, H. Li and L. Q. Chen, Energy Environ. Sci., 2012, 5, 8475.
11. J. W. Qin, J. F. Lu, M. H. Cao and C. W. Hu, Nanoscale, 2010, 2, 2739.
12. Y. F. Su, Z. C. Tang, W. L. Han, P. Zhang, Y. Song and G. X. Lu, CrystEngComm, 2014, 16, 5189.
13. B. C. Liu, Y. Liu, C. Y. Li, W. T. Hu, P. Jing, Q. Wang and J. Zhang, Appl. Catal., B, 2012, 127, 47.
14. F. Yang, J. J. Wei, W. Liu, J. X. Guo and Y. Z. Yang, J. Mater. Chem. A, 2014, 2, 5662.
15. S. H. Zeng, W. L. Zhang and N. Liu, Catal. Lett., 2013, 143, 1018.
16. A. Hornes, A. B. Hungria and P. Bera, J. Am. Chem. Soc., 2010, 132, 34.
17. A. L. Camara, A. Kubacka and Z. Schay, J. Power Sources, 2011, 196, 4364.
18. Z. Hu and H. Metiu, J. Phys. Chem. C, 2011, 115, 17898.
19. I. Yeriskin and M. Nolan, J. Chem. Phys., 2009, 131, 244702.
20. A. Martinez-Arias, M. Fernández-García, J. Soria and J. C. Conesa, J. Catal., 1999, 182, 367.
21. T. Y. Li, G. L. Xiang, J. Zhuang and X. Wang, Chem. Commun., 2011, 47, 6060.
22. X. Y. Zhang, J. J. Wei, H. X. Yang, X. F. Liu, W. Liu, C. Zhang and Y. Z. Yang, Eur. J. Inorg. Chem., 2013, 25, 4443.
23. A. B. Kehoe, D. O. Scanlon and G. W. Watson, Chem. Mater., 2011, 23, 4464.
24. Z. L. Wu, M. J. Li, J. Howe, H. M. Meyer and S. H. Overbury, Langmuir, 2010, 26, 16595.
25. L. Gonzalez-Rovira, J. M. Sanchez-Amaya, M. Lopez-Haro, E. del Rio, A. B. Hungria, P. Midgley, J. J. Calvino, S. Bernal and F. J. Botana, Nano Lett., 2009, 9, 1395.
26. J. Ke, J. W. Xiao, W. Zhu, H. C. Liu, R. Si, Y. W. Zhang and C. H. Yan, J. Am. Chem. Soc., 2013, 135, 15191.
27. Z. Wu, M. Li and S. H. Overbury, J. Catal., 2012, 285, 61.
28. H. X. Mai, L. D. Sun, Y. W. Zhang, R. Si, W. Feng, H. P. Zhang, H. C. Liu and C. H. Yan, J. Phys. Chem. B, 2005, 109, 24380.
29. K. Tang, J. Zhang, W. Yan, Z. Li, Y. Wang, W. Yang, Z. Xie, T. Sun and H. J. Fuchs, J. Am. Chem. Soc., 2008, 130, 2676.
30. D. Xie, Q. M. Su, Z. M. Dong, J. Zhang and G. H. Du, CrystEngComm, 2013, 15, 8314.
31. G. J. Zhang, Z. R. Shen, M. Liu, C. H. Guo, P. C. Sun, Z. Y. Yuan, B. H. Li, D. T. Ding and T. H. Chen, J. Phys. Chem. B, 2006, 110, 25782.
32. J. Wang, Y. C. Liu, S. Y. Wang, X. T. Guo and Y. P. Liu, J. Mater. Chem. A, 2014, 2, 1224.
33. M. D. Hernandez-Alonso, A. B. Hungria and A. Martinez-Arias, Phys. Chem. Chem. Phys., 2004, 6, 3524.
34. Y. J. Lee, G. H. He, A. J. Akey and R. Si, J. Am. Chem. Soc., 2011, 133, 12952.
35. M. Guo, J. Q. Lu, Y. N. Wu, Y. J. Wang and M. F. Luo, Langmuir, 2011, 27, 3872.
36. J. Guzman, S. Carrettin and A. Corma, J. Am. Chem. Soc., 2005, 127, 3286.
37. T. Taniguchi, T. Watanebe, S. Ichinohe, M. Yoshimura, K. Katsumata, K. Okadaa and N. Matsushita, Nanoscale, 2010, 2, 1426–1428.
38. J. C. Groen, L. A. Peffer and J. A. Ramirez, Microporous Mesoporous Mater., 2003, 60, 1.
39. C. W. Sun, J. Sun and G. L. Xiao, J. Phys. Chem. B, 2006, 110, 13445.
40. Q. S. Xie, Y. Zhao, H. Z. Guo, A. Lu, X. X. Zhang, L. S. Wang, M. S. Chen and D. L. Peng, ACS Appl. Mater. Interfaces, 2014, 6, 421.

---

Footnote

† Electronic supplementary information (ESI) available: Experimental details and Fig. S1. See DOI: 10.1039/c4ra16556j

---