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CeO2 microspheres
Materials Research Bulletin 61 (2014) 22–25
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Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Controlled synthesis and properties of porous Cu/CeO2 microspheres Xiang Yao 1, Xiaodan Yang 1, Ranbo Yu * , Pengfei Xu, Jun Chen, Xianran Xing Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 September 2014 Received in revised form 22 September 2014 Accepted 27 September 2014 Available online 30 September 2014
Porous Cu/CeO2 microspheres were synthesized in a system of (NH4)2Ce(NO3)6/CuSO4–ethanol (EtOH)/ H2O–N,N-dimethylformanide (DMF) via a low-temperature hydrothermal route as well as a subsequent calcination. DMF played an important role in both the crystallization of Cu-doped Ce(COOH)3 precursor and the formation of the precursor microspheres. The size and morphology control of the precursor could be achieved by adjusting the synthesis factors including acidity and temperature. BET measurement indicated that the Cu/CeO2 microspheres possessed pretty high specific surface areas up to 169 m2 g1 and multiple pore systems with the pore diameter of about 1.0 nm, 4.9 nm, and 5.9 nm, respectively. Although the doping content of Cu was pretty low, the copper ions were successfully incorporated into the fluorite structure of CeO2, which made these porous Cu/CeO2 microspheres showing much improved oxygen storage capacity (OSC). The novel porous Cu/CeO2 microspheres might be a promising catalyst for selective CO oxidation. ã 2014 Published by Elsevier Ltd.
Keywords: A.Composites B. SolvothermalC X-ray diffraction C. Raman spectroscopy
1. Introduction CeO2 has earned intensive interest in the past decade due to its vital role in emerging technologies for environmental and energyrelated applications. With the fluorite-type structure and cycling easily between reduced and oxidized states (Ce3+and Ce4+) it shows superior chemical and physical stability, high oxygen mobility, and high oxygen vacancy concentrations has been probed for many different applications in a variety of fields, such as active component of three-way catalysts (TWC) [1–3], oxygen ion conductors in solid oxide fuel cells [4–6], polishing agents for chemical mechanical planarization (CMP) process [7], and ultraviolet (UV) blocking materials in UV shielding [8]. Recently, nanostructural and porous CeO2 or CeO2-based compounds has attracted special attention due to the improvements in the specific surface area to volume ratio, redox properties, and transport properties with respect to condensed bulk materials [9–17]. Although various nano-sized CeO2 have been obtained, improvement of surface area were not obvious. On the other hand, to synthesize porous CeO2 may improve the surface area much, e.g., synthesis of mesoporous CeO2 could give surface area up to 200 m2 g1 [18]. However, the random products morphology was not desirable for further application. 3D nanostructures with
* Corresponding author. Tel.: +86 10 62332525; fax: +86 10 62332525. E-mail address: [email protected] (R. Yu). These authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.materresbull.2014.09.083 0025-5408/ ã 2014 Published by Elsevier Ltd.
porous structures exhibited an unprecedented performance [19], to build up 3D nanostructures would be a promising route to CeO2 with larger specific surface area and desirable morphologies. Recently, by using glucose and acryl amide as templates mesoporous CeO2 microspheres with uniform flower like morphologies were synthesized, but its surface area is only 92 m2 g1 [20]; by using carbon spheres as hard template ceria hollow spheres with higher specific surface area of 144 m2 g1 was obtained by Wang et al. [21]. Noticeably, so far for the synthetic strategies of CeO2 with improved surface area, templating techniques are necessary, which resulted in not only much more complicated procedure but also high cost. Therefore, a facial route for the growth of CeO2 with both high surface area and uniform morphology is still challenging us. Catalysts based on combinations between metal and cerium oxides shows promising properties for some important chemical engineering process such as selective CO oxidation. Core–shell Au@CeO2 nanocomposites were synthesized by Qi et al. [22], and the submicrospheres showed superior catalytic stability. But noble metal is too expensive, so because the economic advantages, the copper and cerium oxides catalysts have drawn considerable research attention [23,24]. To contribute the porous nanostructures of copper and cerium would be a possible approach to catalysts with high performance. In this work, by utilizing controllable hydrolysis of DMF the assembly of microspherical Cu/Ce(COOH)3 precursors was achieved. And the corresponding porous Cu/CeO2 microspheres could be easily obtained upon calcinations. The obtained Cu/CeO2
X. Yao et al. / Materials Research Bulletin 61 (2014) 22–25
microspheres possessed not only high surface area but also remarkable oxygen storage capability. 2. Experimental
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sample was further purged in pure He for 30 min to remove oxygen from the system and then exposed to (4%) CO + (1%) Ar + He. The accumulated amount of CO2 per gram of catalyst was monitored as a function of time (OSC calculation method in Supporting information) [25].
2.1. Porous Cu/CeO2 microspheres All the chemicals were purchased from Beijing chemical reagent company and used without further purification. The synthesis was performed in a system of (NH4)2Ce(NO3)6/CuSO4– EtOH/H2O–DMF in a molar ratio of 1:100:72. In a typical experiment, ammonium cerium nitrate and copper sulfate was dissolved into the mix solvent of EtOH/H2O under magnetic stirring for at least 30 min, followed by addition of DMF to form a transparent solution. The solution was aged under hydrothermal conditions at 100–150 C for 4–6 days in a Teflon-lined stainless steel autoclave. After washed thoroughly followed by drying at 60 C, the microspherical Cu/Ce(COOH)3 precursor was obtained. The precursors were further calcined at 400 C for 3 h using a temperature programmed muffle furnace with a heating rate of 1 C min1, the porous Cu/CeO2 microspheres were finally obtained. 2.2. Characterization The phases and purity of the products were examined by X-ray powder diffraction (XRD) performed on M21XVHF22 X-ray diffractometer (Japan) with Cu Ka radiation (l = 1.5418 Å). The morphology of the products was observed by a field emission scanning electron microscope (LEO1530) equipped with an energy dispersive X-ray spectrometer (EDS). Raman scattering measurements were performed on a JY-T64000 Raman spectrometer (JY Company, France) under backscattering geometry. The nitrogen adsorption–desorption isotherms at 77 K were measured using a quantachorme instruments AUTOSOR8-1C powders adsorption analyzer. The OSC measurements were carried out at 500 C. The measurements were carried out in a flow reactor system equipped with solenoid valves for rapid introduction of (4%) CO + (1%) Ar + He or (2%) O2 + (1%) Ar + He pluses. Typically, 30 mg powders were loaded into a 1.0 cm i.d. quartz tube reactor and a total gas flow rate of 300 cm3 min1 was employed. The signals of the outlet gas were detected by an on-line quadrupole mass spectrometer (Omnistar 200). Prior to CO step measurements, the sample was heated in (2%) O2 + (1%) Ar + He at 773 K for at least 20 min. The
3. Results and discussion 3.1. X-ray diffraction experiments The phase purity and crystal structure of the precursors were examined by XRD. The precursors without Cu doping was examed for comparison. The XRD patterns of the two samples (Fig. 1) shows that they are both pure and of the same phase. All the peaks could be readily indexed as the rhombohedral cerium formate (ICSD No.: 069333, space group: R3m). For the precursor with Cu doping, the right-shift of the main peak at around 2u = 29 could be observed. This indicated that Cu(II) was incorporated in to the structure of cerium formate, and the right-shift of the peaks was because the smaller ionic diameter of Cu(II) than Ce(IV). To get the final Cu/CeO2 products, the precursors would undergo the calcination. The calcination temperature was determined according to the thermogravimetric (TG) analysis of the precursor (See the Supporting information Fig. S1). The patterns of the products calcined at 400 C can be indexed as a pure face-centered cubic phase as CeO2 (PDF No.: 43-1002, space group: Fm3m) (Fig. 1c). When the samples were further treated at 1000 C well crystallized products could be obtained, and obviously right-shift of the XRD peaks could be detected for the Cu-doped CeO2 (Fig. S2). 3.2. Raman spectra analysis Additionally, more evidence for the successful incorporation of copper ions into the CeO2 structure could be drawn from the Raman spectra of the samples (Fig. 2). The main peak at 464 cm1could be assigned as the T2g vibration of the face-centered cubic fluorite CeO2. With the Cu incorporated into the structure the T2g peak shift slightly [26]. Simultaneously, peaks at 550–600 cm1and 850 cm1could be detected for the Cu-doped CeO2, which might be attributed to the oxygen vacancy resulted from the Cu doping [27–29]. The increase of the oxygen vacancy will enhance the oxygen storage capacity of the products, and the catalytic activity as well.
Fig. 1. XRD patterns of the microspherical precursors (a and b) and calcined products (c).
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X. Yao et al. / Materials Research Bulletin 61 (2014) 22–25
Fig. 2. Raman spectra of (a) porous CeO2 microspheres (b) porous Cu/CeO2 microspheres.
Fig. 4. N2 adsorption–desorption isotherms of the as-synthesized Cu/CeO2 microspheres.
3.3. FE-SEM micrograph analysis The morphology and structure of the precursors and final products were characterized by using the field emission scanning electron microscope (FE-SEM). The as-synthesized precursor possesses spherical morphology with the diameter range of 120–250 mm, upon calcination at 400 C the precursor transformed into fluorite-type structure Cu/CeO2 spheres (Fig. 3). EDS analysis of the Cu/CeO2 sample indicated that the amount of Cu was very little, and the Cu/Ce ratio was 1:24.5 (see Supporting information Table S1). In this synthesis, the DMF plays a crucial role in the formation of the microspheres. In aqueous DMF might hydrolyze into formic acid and dimethylamine as following equation: Hþ
C3 H7 ON þ H2 O! C2 H7 N þ HCOOH
(1)
And the acid may accelerated this hydrolysis process. The formic acid would react with cerium sources to form Ce(COOH)3, and dimethylamine might act as structure-directing agent to help the microspheres assembly. 3.4. N2 adsorption–desorption isotherms The N2 adsorption–desorption isotherms of the Cu/CeO2 microspheres was given as Fig. 4. The BJH pore size distribution curve was shown in the inset. The BET surface areas of Cu/CeO2
microspheres calculated from this isotherm was 169 m2 g1, which is similar to that of the pure CeO2 microspheres (about 170 m2 g1) synthesized under the same conditions. The multistage pore size distribution indicated that the main bimodal pore diameter is around 4.9 nm and 5.9 nm, respectively. Further investigation of the micro porosity indicated that the Cu/CeO2 microspheres also possessed micropores of about 1.0 nm (Fig. S3). 3.5. The total oxygen storage capacity The total oxygen storage capacity (OSC) can express the maximum OSC and contains information about the overall reducibility of the solid. At 500 C the OSC values of the as-synthesized Cu/CeO2 microspheres is 1846 mmol[O]/g, which is much higher than those of the pure CeO2 microspheres (1169 mmol[O]/g) synthesized under the same conditions and the commercial CeO2 powder (616 mmol[O]/g). Since the incorporation of Cu to CeO2 did not result in the obvious increase of the surface area, it could be drawn that the substitution of Ce4+ with Cu2+ may result in more oxygen defects in the CeO2 microspheres, which make the sample showed much increased OSC in spite of a tiny doping content of Cu. So the sharply improvement of the OSC might be attributed to the formation of the microspherical morphology of CeO2 as well as the incorporation of Cu. It is reported that the surface oxygen vacancies of catalysts might
Fig. 3. FE-SEM image of the porous Cu/CeO2 microspheres, inset is the surface image of the Cu/CeO2 microspheres.
X. Yao et al. / Materials Research Bulletin 61 (2014) 22–25
increase as the content of Cu increased [30], so the OSC values might also increase with the content of Cu increase in the limited amount of Cu-doping. However, under the present synthetic condition, only a tiny amount of Cu could be incorporated into the CeO2 structure, so far obtaining CeO2 microspheres with higher Cu-doping content has not been achieved yet. Further study on CeO2 microspheres with controlled Cu doping content might bright more inspiring results on OSC and related properties. 4. Conclusions In summary, Cu/CeO2 microspheres with high surface area and large pore volume were synthesized through a simple route. DMF and its hydrolysis in the reaction system contributed to both the crystallization of precursor and the assembly of its microspheres. The incorporation of copper ions into the fluorite ceria structure made the final porous Cu/CeO2 microspheres express pretty high specific surface area and OSC values. The synthesis would open an approach to metal oxides with high specific surface area and potential catalytic application. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No. 51072020,21271021, 21031005). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2014.09.083. References [1] E. Aneggi, C. de Leitenburg, G. Dolcetti, et al., Diesel soot combustion activity of ceria promoted with alkali metals, Catal. Today 136 (1) (2008) 3–10. [2] G. Chen, F. Rosei, D. Ma, Interfacial reaction-directed synthesis of Ce–Mn binary oxide nanotubes and their applications in CO oxidation and water treatment, Adv. Funct. Mater. 22 (18) (2012) 3914–3920. [3] J. Kašpar, P. Fornasiero, M. Graziani, Use of CeO2-based oxides in the three-way catalysis, Catal. Today 50 (2) (1999) 285–298. [4] Z. Zhan, S.A. Barnett, An octane-fueled solid oxide fuel cell, Science 308 (5723) (2005) 844–847. [5] M. Balaguer, V.B. Vert, L. Navarrete, et al., SOFC composite cathodes based on LSM and co-doped cerias (Ce0.8Gd0.1 X0.1O2d, X = Gd, Cr, Mg, Bi, Ce), J. Power Sources 223 (2013) 214–220. [6] P. Jasinski, T. Suzuki, H.U. Anderson, Nanocrystalline undoped ceria oxygen sensor, Sens. Actuators B 95 (1) (2003) 73–77. [7] Y. Xijuan, X. Pingbo, S. Qingde, Size-dependent optical properties of nanocrystalline CeO2: Er obtained by combustion synthesis, Phys. Chem. Chem. Phys. 3 (23) (2001) 5266–5269. [8] R. Si, Y.W. Zhang, L.P. You, et al., Self-organized monolayer of nanosized ceria colloids stabilized by poly(vinylpyrrolidone), J. Phys. Chem. B 110 (12) (2006) 5994–6000.
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[9] A. Corma, P. Atienzar, H. Garcia, et al., Hierarchically mesostructured doped CeO2 with potential for solar-cell use, Nat. Mater. 3 (6) (2004) 394–397. [10] S.C. Laha, R. Ryoo, Synthesis of thermally stable mesoporous cerium oxide with nanocrystalline frameworks using mesoporous silica templates, Chem. Commun. 17 (2003) 2138–2139. [11] W. Liu, X. Liu, L. Feng, et al., The synthesis of CeO2 nanospheres with different hollowness and size induced by copper doping, Nanoscale 6 (18) (2014) 10693–10700. [12] D.M. Lyons, K.M. Ryan, M.A. Morris, Preparation of ordered mesoporous ceria with enhanced thermal stability, J. Mater. Chem. 12 (4) (2002) 1207–1212. [13] K. Harada, T. Oishi, S. Hamamoto, et al., Lattice oxygen activity in Pr- and La-doped CeO2 for low-temperature soot oxidation, J. Phys. Chem. C 118 (1) (2013) 559–568. [14] S. Yang, L. Gao, Controlled synthesis and self-assembly of CeO2 nanocubes, J. Am. Chem. Soc. 128 (29) (2006) 9330–9331. [15] L. Yan, X. Xing, R. Yu, et al., Facile alcohothermal synthesis of large-scale ceria nanowires with organic surfactant assistance, Phys. B: Condens. Matter 390 (1) (2007) 59–64. [16] L. Yan, R. Yu, J. Chen, et al., Template-free hydrothermal synthesis of CeO2 nano-octahedrons and nanorods: investigation of the morphology evolution, Cryst. Growth Des. 8 (5) (2008) 1474–1477. [17] R. Yu, L. Yan, P. Zheng, et al., Controlled synthesis of CeO2 flower-like and well-aligned nanorod hierarchical architectures by a phosphate-assisted hydrothermal route, J. Phys. Chem. C 112 (50) (2008) 19896–19900. [18] D. Terribile, A. Trovarelli, J. Llorca, et al., The synthesis and characterization of mesoporous high-surface area ceria prepared using a hybrid organic/inorganic route, J. Catal. 178 (1) (1998) 299–308. [19] S. Zhao, H. Yin, L. Du, Three dimensional N-doped graphene/PtRu nanoparticle hybrids as high performance anode for direct methanol fuel cells, J. Mater. Chem. A 2 (11) (2014) 3719–3724. [20] C. Sun, J. Sun, G. Xiao, et al., Mesoscale organization of nearly monodisperse flowerlike ceria microspheres, J. Phys. Chem. B 110 (27) (2006) 13445–13452. [21] S. Wang, J. Zhang, J. Jiang, et al., Porous ceria hollow microspheres: synthesis and characterization, Microporous Mesoporous Mater. 123 (1) (2009) 349–353. [22] J. Qi, J. Chen, G. Li, et al., Facile synthesis of core–shell Au@CeO2 nanocomposites with remarkably enhanced catalytic activity for CO oxidation, Energy Environ. Sci. 5 (10) (2012) 8937–8941. [23] G. Sedmak, S. Ho9 cevar, J. Levec, Kinetics of selective CO oxidation in excess of H2 over the nanostructured Cu0.1Ce0.9O2y catalyst, J. Catal. 213 (2) (2003) 135–150. [24] J. Tian, W. Na, H. Wang, et al., Preparation and characterization of Cu-doped mesoporous CeO2 for CO oxidation, J. Cent. South Univ. 21 (2014) 482–486. [25] M. Shen, X. Wang, Y. An, et al., Dynamic oxygen storage capacity measurements on ceria-based material, J. Rare Earths (English Edition) 25 (1) (2007) 48–52. [26] X. Wang, J.A. Rodriguez, J.C. Hanson, et al., Unusual physical and chemical properties of Cu in Ce1xCuxO2 oxides, J. Phys. Chem. B 109 (42) (2005) 19595–19603. [27] H.Y. Zhu, T. Hirata, Y. Muramatsu, Phase separation in 12 mol% ceria-doped zirconia induced by heat treatment in H2 and Ar, J. Am. Ceram. Soc. 75 (10) (1992) 2843–2848. [28] K. Krishna, A. Bueno-López, M. Makkee, et al., Potential rare earth modified CeO2 catalysts for soot oxidation: I. Characterisation and catalytic activity with O2, Appl. Catal. B: Environ. 75 (3) (2007) 189–200. [29] B.M. Reddy, G. Thrimurthulu, P. Saikia, et al., Silica supported ceria and ceria– zirconia nanocomposite oxides for selective dehydration of 4-methylpentan2-ol, J. Mol. Catal. A: Chem. 275 (1) (2007) 167–173. [30] D. Zhang, Y. Qian, L. Shi, Cu-doped CeO2 spheres: synthesis, characterization, and catalytic activity, Catal. Commun. 26 (2012) 164–168.
Contents lists available at ScienceDirect
Materials Research Bulletin journal homepage: www.elsevier.com/locate/matresbu
Short communication
Controlled synthesis and properties of porous Cu/CeO2 microspheres Xiang Yao 1, Xiaodan Yang 1, Ranbo Yu * , Pengfei Xu, Jun Chen, Xianran Xing Department of Physical Chemistry, University of Science and Technology Beijing, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 1 September 2014 Received in revised form 22 September 2014 Accepted 27 September 2014 Available online 30 September 2014
Porous Cu/CeO2 microspheres were synthesized in a system of (NH4)2Ce(NO3)6/CuSO4–ethanol (EtOH)/ H2O–N,N-dimethylformanide (DMF) via a low-temperature hydrothermal route as well as a subsequent calcination. DMF played an important role in both the crystallization of Cu-doped Ce(COOH)3 precursor and the formation of the precursor microspheres. The size and morphology control of the precursor could be achieved by adjusting the synthesis factors including acidity and temperature. BET measurement indicated that the Cu/CeO2 microspheres possessed pretty high specific surface areas up to 169 m2 g1 and multiple pore systems with the pore diameter of about 1.0 nm, 4.9 nm, and 5.9 nm, respectively. Although the doping content of Cu was pretty low, the copper ions were successfully incorporated into the fluorite structure of CeO2, which made these porous Cu/CeO2 microspheres showing much improved oxygen storage capacity (OSC). The novel porous Cu/CeO2 microspheres might be a promising catalyst for selective CO oxidation. ã 2014 Published by Elsevier Ltd.
Keywords: A.Composites B. SolvothermalC X-ray diffraction C. Raman spectroscopy
1. Introduction CeO2 has earned intensive interest in the past decade due to its vital role in emerging technologies for environmental and energyrelated applications. With the fluorite-type structure and cycling easily between reduced and oxidized states (Ce3+and Ce4+) it shows superior chemical and physical stability, high oxygen mobility, and high oxygen vacancy concentrations has been probed for many different applications in a variety of fields, such as active component of three-way catalysts (TWC) [1–3], oxygen ion conductors in solid oxide fuel cells [4–6], polishing agents for chemical mechanical planarization (CMP) process [7], and ultraviolet (UV) blocking materials in UV shielding [8]. Recently, nanostructural and porous CeO2 or CeO2-based compounds has attracted special attention due to the improvements in the specific surface area to volume ratio, redox properties, and transport properties with respect to condensed bulk materials [9–17]. Although various nano-sized CeO2 have been obtained, improvement of surface area were not obvious. On the other hand, to synthesize porous CeO2 may improve the surface area much, e.g., synthesis of mesoporous CeO2 could give surface area up to 200 m2 g1 [18]. However, the random products morphology was not desirable for further application. 3D nanostructures with
* Corresponding author. Tel.: +86 10 62332525; fax: +86 10 62332525. E-mail address: [email protected] (R. Yu). These authors contributed equally to this work.
1
http://dx.doi.org/10.1016/j.materresbull.2014.09.083 0025-5408/ ã 2014 Published by Elsevier Ltd.
porous structures exhibited an unprecedented performance [19], to build up 3D nanostructures would be a promising route to CeO2 with larger specific surface area and desirable morphologies. Recently, by using glucose and acryl amide as templates mesoporous CeO2 microspheres with uniform flower like morphologies were synthesized, but its surface area is only 92 m2 g1 [20]; by using carbon spheres as hard template ceria hollow spheres with higher specific surface area of 144 m2 g1 was obtained by Wang et al. [21]. Noticeably, so far for the synthetic strategies of CeO2 with improved surface area, templating techniques are necessary, which resulted in not only much more complicated procedure but also high cost. Therefore, a facial route for the growth of CeO2 with both high surface area and uniform morphology is still challenging us. Catalysts based on combinations between metal and cerium oxides shows promising properties for some important chemical engineering process such as selective CO oxidation. Core–shell Au@CeO2 nanocomposites were synthesized by Qi et al. [22], and the submicrospheres showed superior catalytic stability. But noble metal is too expensive, so because the economic advantages, the copper and cerium oxides catalysts have drawn considerable research attention [23,24]. To contribute the porous nanostructures of copper and cerium would be a possible approach to catalysts with high performance. In this work, by utilizing controllable hydrolysis of DMF the assembly of microspherical Cu/Ce(COOH)3 precursors was achieved. And the corresponding porous Cu/CeO2 microspheres could be easily obtained upon calcinations. The obtained Cu/CeO2
X. Yao et al. / Materials Research Bulletin 61 (2014) 22–25
microspheres possessed not only high surface area but also remarkable oxygen storage capability. 2. Experimental
23
sample was further purged in pure He for 30 min to remove oxygen from the system and then exposed to (4%) CO + (1%) Ar + He. The accumulated amount of CO2 per gram of catalyst was monitored as a function of time (OSC calculation method in Supporting information) [25].
2.1. Porous Cu/CeO2 microspheres All the chemicals were purchased from Beijing chemical reagent company and used without further purification. The synthesis was performed in a system of (NH4)2Ce(NO3)6/CuSO4– EtOH/H2O–DMF in a molar ratio of 1:100:72. In a typical experiment, ammonium cerium nitrate and copper sulfate was dissolved into the mix solvent of EtOH/H2O under magnetic stirring for at least 30 min, followed by addition of DMF to form a transparent solution. The solution was aged under hydrothermal conditions at 100–150 C for 4–6 days in a Teflon-lined stainless steel autoclave. After washed thoroughly followed by drying at 60 C, the microspherical Cu/Ce(COOH)3 precursor was obtained. The precursors were further calcined at 400 C for 3 h using a temperature programmed muffle furnace with a heating rate of 1 C min1, the porous Cu/CeO2 microspheres were finally obtained. 2.2. Characterization The phases and purity of the products were examined by X-ray powder diffraction (XRD) performed on M21XVHF22 X-ray diffractometer (Japan) with Cu Ka radiation (l = 1.5418 Å). The morphology of the products was observed by a field emission scanning electron microscope (LEO1530) equipped with an energy dispersive X-ray spectrometer (EDS). Raman scattering measurements were performed on a JY-T64000 Raman spectrometer (JY Company, France) under backscattering geometry. The nitrogen adsorption–desorption isotherms at 77 K were measured using a quantachorme instruments AUTOSOR8-1C powders adsorption analyzer. The OSC measurements were carried out at 500 C. The measurements were carried out in a flow reactor system equipped with solenoid valves for rapid introduction of (4%) CO + (1%) Ar + He or (2%) O2 + (1%) Ar + He pluses. Typically, 30 mg powders were loaded into a 1.0 cm i.d. quartz tube reactor and a total gas flow rate of 300 cm3 min1 was employed. The signals of the outlet gas were detected by an on-line quadrupole mass spectrometer (Omnistar 200). Prior to CO step measurements, the sample was heated in (2%) O2 + (1%) Ar + He at 773 K for at least 20 min. The
3. Results and discussion 3.1. X-ray diffraction experiments The phase purity and crystal structure of the precursors were examined by XRD. The precursors without Cu doping was examed for comparison. The XRD patterns of the two samples (Fig. 1) shows that they are both pure and of the same phase. All the peaks could be readily indexed as the rhombohedral cerium formate (ICSD No.: 069333, space group: R3m). For the precursor with Cu doping, the right-shift of the main peak at around 2u = 29 could be observed. This indicated that Cu(II) was incorporated in to the structure of cerium formate, and the right-shift of the peaks was because the smaller ionic diameter of Cu(II) than Ce(IV). To get the final Cu/CeO2 products, the precursors would undergo the calcination. The calcination temperature was determined according to the thermogravimetric (TG) analysis of the precursor (See the Supporting information Fig. S1). The patterns of the products calcined at 400 C can be indexed as a pure face-centered cubic phase as CeO2 (PDF No.: 43-1002, space group: Fm3m) (Fig. 1c). When the samples were further treated at 1000 C well crystallized products could be obtained, and obviously right-shift of the XRD peaks could be detected for the Cu-doped CeO2 (Fig. S2). 3.2. Raman spectra analysis Additionally, more evidence for the successful incorporation of copper ions into the CeO2 structure could be drawn from the Raman spectra of the samples (Fig. 2). The main peak at 464 cm1could be assigned as the T2g vibration of the face-centered cubic fluorite CeO2. With the Cu incorporated into the structure the T2g peak shift slightly [26]. Simultaneously, peaks at 550–600 cm1and 850 cm1could be detected for the Cu-doped CeO2, which might be attributed to the oxygen vacancy resulted from the Cu doping [27–29]. The increase of the oxygen vacancy will enhance the oxygen storage capacity of the products, and the catalytic activity as well.
Fig. 1. XRD patterns of the microspherical precursors (a and b) and calcined products (c).
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Fig. 2. Raman spectra of (a) porous CeO2 microspheres (b) porous Cu/CeO2 microspheres.
Fig. 4. N2 adsorption–desorption isotherms of the as-synthesized Cu/CeO2 microspheres.
3.3. FE-SEM micrograph analysis The morphology and structure of the precursors and final products were characterized by using the field emission scanning electron microscope (FE-SEM). The as-synthesized precursor possesses spherical morphology with the diameter range of 120–250 mm, upon calcination at 400 C the precursor transformed into fluorite-type structure Cu/CeO2 spheres (Fig. 3). EDS analysis of the Cu/CeO2 sample indicated that the amount of Cu was very little, and the Cu/Ce ratio was 1:24.5 (see Supporting information Table S1). In this synthesis, the DMF plays a crucial role in the formation of the microspheres. In aqueous DMF might hydrolyze into formic acid and dimethylamine as following equation: Hþ
C3 H7 ON þ H2 O! C2 H7 N þ HCOOH
(1)
And the acid may accelerated this hydrolysis process. The formic acid would react with cerium sources to form Ce(COOH)3, and dimethylamine might act as structure-directing agent to help the microspheres assembly. 3.4. N2 adsorption–desorption isotherms The N2 adsorption–desorption isotherms of the Cu/CeO2 microspheres was given as Fig. 4. The BJH pore size distribution curve was shown in the inset. The BET surface areas of Cu/CeO2
microspheres calculated from this isotherm was 169 m2 g1, which is similar to that of the pure CeO2 microspheres (about 170 m2 g1) synthesized under the same conditions. The multistage pore size distribution indicated that the main bimodal pore diameter is around 4.9 nm and 5.9 nm, respectively. Further investigation of the micro porosity indicated that the Cu/CeO2 microspheres also possessed micropores of about 1.0 nm (Fig. S3). 3.5. The total oxygen storage capacity The total oxygen storage capacity (OSC) can express the maximum OSC and contains information about the overall reducibility of the solid. At 500 C the OSC values of the as-synthesized Cu/CeO2 microspheres is 1846 mmol[O]/g, which is much higher than those of the pure CeO2 microspheres (1169 mmol[O]/g) synthesized under the same conditions and the commercial CeO2 powder (616 mmol[O]/g). Since the incorporation of Cu to CeO2 did not result in the obvious increase of the surface area, it could be drawn that the substitution of Ce4+ with Cu2+ may result in more oxygen defects in the CeO2 microspheres, which make the sample showed much increased OSC in spite of a tiny doping content of Cu. So the sharply improvement of the OSC might be attributed to the formation of the microspherical morphology of CeO2 as well as the incorporation of Cu. It is reported that the surface oxygen vacancies of catalysts might
Fig. 3. FE-SEM image of the porous Cu/CeO2 microspheres, inset is the surface image of the Cu/CeO2 microspheres.
X. Yao et al. / Materials Research Bulletin 61 (2014) 22–25
increase as the content of Cu increased [30], so the OSC values might also increase with the content of Cu increase in the limited amount of Cu-doping. However, under the present synthetic condition, only a tiny amount of Cu could be incorporated into the CeO2 structure, so far obtaining CeO2 microspheres with higher Cu-doping content has not been achieved yet. Further study on CeO2 microspheres with controlled Cu doping content might bright more inspiring results on OSC and related properties. 4. Conclusions In summary, Cu/CeO2 microspheres with high surface area and large pore volume were synthesized through a simple route. DMF and its hydrolysis in the reaction system contributed to both the crystallization of precursor and the assembly of its microspheres. The incorporation of copper ions into the fluorite ceria structure made the final porous Cu/CeO2 microspheres express pretty high specific surface area and OSC values. The synthesis would open an approach to metal oxides with high specific surface area and potential catalytic application. Acknowledgment This work was financially supported by the National Natural Science Foundation of China (No. 51072020,21271021, 21031005). Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.materresbull.2014.09.083. References [1] E. Aneggi, C. de Leitenburg, G. Dolcetti, et al., Diesel soot combustion activity of ceria promoted with alkali metals, Catal. Today 136 (1) (2008) 3–10. [2] G. Chen, F. Rosei, D. Ma, Interfacial reaction-directed synthesis of Ce–Mn binary oxide nanotubes and their applications in CO oxidation and water treatment, Adv. Funct. Mater. 22 (18) (2012) 3914–3920. [3] J. Kašpar, P. Fornasiero, M. Graziani, Use of CeO2-based oxides in the three-way catalysis, Catal. Today 50 (2) (1999) 285–298. [4] Z. Zhan, S.A. Barnett, An octane-fueled solid oxide fuel cell, Science 308 (5723) (2005) 844–847. [5] M. Balaguer, V.B. Vert, L. Navarrete, et al., SOFC composite cathodes based on LSM and co-doped cerias (Ce0.8Gd0.1 X0.1O2d, X = Gd, Cr, Mg, Bi, Ce), J. Power Sources 223 (2013) 214–220. [6] P. Jasinski, T. Suzuki, H.U. Anderson, Nanocrystalline undoped ceria oxygen sensor, Sens. Actuators B 95 (1) (2003) 73–77. [7] Y. Xijuan, X. Pingbo, S. Qingde, Size-dependent optical properties of nanocrystalline CeO2: Er obtained by combustion synthesis, Phys. Chem. Chem. Phys. 3 (23) (2001) 5266–5269. [8] R. Si, Y.W. Zhang, L.P. You, et al., Self-organized monolayer of nanosized ceria colloids stabilized by poly(vinylpyrrolidone), J. Phys. Chem. B 110 (12) (2006) 5994–6000.
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