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Rapid synthesis of mesoporous ceria–zirconia solid solutions via a novel salt-assisted combustion process
Materials Research Bulletin 41 (2006) 2318–2324 www.elsevier.com/locate/matresbu
Rapid synthesis of mesoporous ceria–zirconia solid solutions via a novel salt-assisted combustion process Weifan Chen a,*, Fengsheng Li a, Jiyi Yu a, Leili Liu b, Hailian Gao a a National Special Superfine Powder Engineering Research Center, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing, Jiangsu 210094, PR China b College of Chemical Engineering, Shandong Institute of Light Industry, Jinan 250100, PR China
Received 21 January 2006; received in revised form 3 April 2006; accepted 17 April 2006 Available online 5 May 2006
Abstract Mesoporous ceria–zirconia solid solutions were prepared by a novel salt-assisted combustion process using ethylene glycol as a fuel and nitrate as an oxidant. The effects of various operating conditions such as the fuel-to-oxidant ratio and the nature and amount of added salt on the characteristics of the products were investigated by X-ray diffraction (XRD) and nitrogen adsorption analysis. Results from transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) showed that the introduction of leachable inert inorganic salt as a hard agglomeration inhibitor into the redox mixture precursor led to the breakup of fractal nanocrystallite agglomerates and the mesoporous structure formed by the loose agglomeration of monodispersed nanoparticles, which was also confirmed by small-angle XRD and nitrogen adsorption analysis. The presence of salt was found to result in a more than 10-fold increase in the specific surface area of the products from 17.34 to 208.17 m2/g at a given molar ratio of ethylene glycol–nitrate. A mechanism scheme was proposed to illustrate the possible formation processes and discuss the role of the salt additives. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; D. Microstructure
1. Introduction Ceria has been widely used as active component in so-called three-way catalysts (TWCs) for automotive exhaust treatment [1]. One of the most important roles of CeO2 in the three-way catalyst is to provide surface active sites and to act as an oxygen storage/transport medium by shifting between Ce3+ and Ce4+ under reductive and oxidizing conditions, respectively. It has been well documented that the introduction of zirconia into ceria not only improves the oxygen storage capacity (OSC) and thermal resistance, but also enhances catalytic activity at lower temperatures by the formation of ceria–zirconia solid solutions [2]. It is important in catalysis to fabricate high surface area and mesoporous materials, which would offer a large number of active sites for carrying out catalytic reactions [3]. Hence, much effort has been focused on the synthesis of high surface area and mesoporous ceria–zirconia solid solutions, including microemulsion method [4], surfactant-assisted approach [5], direct sonochemical route [6], coprecipitation
* Corresponding author. Tel.: +86 25 84315529; fax: +86 25 84315942. E-mail address: [email protected] (W. Chen). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.04.024
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followed by supercritical drying [7], macromolecule surface modified method [8], continuous hydrothermal synthesis in a near-critical water flow reactor [9]. Among the available chemical processes, self-sustaining solution combustion synthesis is characterized by convenient processing, simple experimental setup, time and energy saving and homogeneous products [10]. Therefore, there are many reports about combustion synthesis of nanosized ceria–zirconia solid solution in recent years. Aruna and Patil [11] synthesized ceria–zirconia solid solution with specific surface area in the range of 36–120 m2/g via a solution combustion process using oxalyl dihydrazine and carbohydrazine as the fuel, which are expensive and carcinogenic. Fu and Lin [12] prepared CexZr1xO2 powders with specific surface area in the range of 40–50 m2/g by a microwave-induced combustion process using glycine as the fuel. Lascalea et al. [13] increased the specific surface area of the combustion-derived Ce–Zr mixed oxide to 93 m2/g by a pH-controlled nitrate–glycine gel-combustion process. Potdar et al. [14] prepared nanosized Ce0.75Zr0.25O2 porous powders with a very wide pore size distribution in the range of 2–250 nm and surface area of about 40.0 m2/g via a glycine-nitrate autocombustion process. The synthesized powders have higher surface areas than the powders prepared by conventional solid-state methods [10], however, these methods are not too satisfying with respect to high surface area and feasibility of commercial production. Hence, combustion synthesis of high surface area ceria–zirconia solid solutions employing safety and cheap fuel has been still a challenging issue. Here, we report, for the first time, the synthesis of high surface area mesoporous ceria–zirconia solid solution by the simple introduction of salt in the combustion reaction using ethylene glycol as a novel fuel and nitrate as an oxidant. The results reveal that the introduction of salt into the redox mixture solution results in a drastic increase in surface area and provides a new means to control the properties of the products for the conventional combustion synthesis (CCS). 2. Experimental All chemicals and reagents used were high-purity commercially available Ce(NO3)36H2O (99.5%), Zr(NO3)45H2O (99.5%), ethylene glycol (99%), NaCl (99.9%), KCl (99.9%), and LiCl (99.9%), and were used as received. According to the stoichiometric ratios of Ce0.75Zr0.25O2, the required ethylene glycol–nitrate molar ratios and the desired amount of salt addictives, the proper amount of cerium nitrate, zirconium nitrate, ethylene glycol and salt addictives were dissolved in a minimum volume of deionized water in a quartz beaker to obtain transparent solution, the resulting solution was evaporated on a hot plate adjusted at 110 8C. At this stage, the viscous liquids swelled, followed by the evolution of brown gases, and self-propagating solution combustion slowly occurred, yielding loose light yellow powder. To remove salt, the as-burned powder was boiled in deionized water, filtered and washed with deionized water and ethanol. Finally, the product was dried in an oven at 80 8C for 2 h. Wide-angle and small-angle X-ray powder diffraction (XRD) patterns were obtained using a Bruker Advance X-ray D8 diffractometer, with Cu Ka radiation. The crystallite sizes were calculated from line broadening of the (1 1 1) XRD peak by Sherrer’s formula: D = 0.89l/bcos u (scanning at a rate of 0.5 8/min). Transmission electron micrographs (TEMs) and high-resolution transmission electron micrographs (HRTEMs) were, respectively, taken with a JEOL TEM-200CX microscope and a JEOL TEM-2010. The specific surface area and pore size distribution of the powders were measured with a COULTER SA 3100 analyzer using the multipoint Brunauer, Emmett, and Teller (BET) adsorption technique. The particle sizes were estimated from the formula: DBET = 6/rSBET. 3. Results and discussion It has been well documented that the conventional solution combustion synthesis method often produces nanocrystallite agglomerates which are nanometer in crystallite size calculated by the Scherrer’s formula [10–15]. As the nanocrystallites are agglomerated/sintered together and virtually inseparable, the obtained products do not show high specific surface areas corresponding to their small crystallite sizes. Recently, we found that the introduction of a soluble salt into redox mixture solution breaks up fractal nanocrystallite agglomerates into nanoparticles and prevent nanoparticles from forming hard agglomeration during the combustion process. We denote the method as salt-assisted combustion synthesis (SCS) in comparison to the conventional combustion synthesis, i.e. salt-free combustion synthesis. According to the principle of propellant chemistry [16], for stoichiometric redox reaction between a fuel and an oxidizer, the ratio of the net oxidizing valency of the metal nitrate to the net reducing valency of the fuel should be
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unity. Assumed that in the case of ethylene glycol–nitrate combustion, primarily N2, CO2, and H2O are evolved as the gaseous products, the salt-assisted stoichiometric redox combustion reaction can be expressed as follows: 6CeðNO3 Þ3 6H2 O þ 2ZrðNO3 Þ4 5H2 O þ 13HOCH2 CH2 OH þ O2 þ xNaCl ¼ 8Ce0:75 Zr0:25 O2 þ 13N2 þ 26CO2 þ 85H2 O þ xNaCl In our work, the fuel-to-oxidant molar ratio (which is hereafter referred to as EG/NO3) and amount of added salt (which is hereafter expressed as salt to metal ion molar ratio, SALT/M for short) were varied systematically to investigate the effect of EG/NO3 and SALT/M on characteristics of the as-prepared powders. The experimental results reveal that the auto-propagating combustion reaction occurs for a limited range of fuel-to-oxidant molar ratio, depending on the nature of the fuel and the amount of the added salt. In the case of ethylene glycol–nitrate combustion reaction in the absence of salt, it was experimentally observed that the auto-propagating combustion reaction ceases to occur if EG/NO3 is below 1/4. On the basis of the concept of propellant chemistry [16], EG/NO3 = 1/2 corresponds to the situation of stoichiometric ratio, whereas EG/NO3 below 1/2 is considered as fuel-deficient ratio, and EG/ NO3 above 1/2 is regarded as and fuel-rich ratio. The effect of different EG/NO3 on the properties of Ce0.75Zr0.25O2 prepared via the CCS and SCS processes are summarized in Table 1. The results show that with increasing EG/NO3, BET surface specific areas decrease and crystallite sizes increase. It is remarkable that in the case of equal EG/NO3, an addition of NaCl into the redox mixture solution greatly enhances BET specific surface area from 17.34 to 173.00 m2/g, which is much higher than BET surface areas of Ce0.75Zr0.25O2 prepared via other solution combustion routes [11–14], and reduces crystallite size from 5.04 to 3.14 nm, showing that NaCl plays a vital role in forming high specific area surface ceria–zirconia solid solutions via a solution combustion process. The possible mechanism is to be discussed later. As is shown in Table 2, in the case of equal EG/NO3, an addition of NaCl into the reaction mixture solution enhances BET specific surface area from 17.34 to 208.17 m2/g and crystallite size from 5.04 to 5.49 nm with increasing NaCl/M from 0 to 2, whereas particle sizes decrease from 47.99 to 4.00 nm. Comparing crystallite size with particle size of the samples (a), (b), and (c), it can be concluded that each particle obtained via the CCS process consists of multiple nanocrystallites and that the SCS process can break up the nanocrystallite agglomerates and form porous structure, which can be clearly seen in Fig. 1. It is notable, however, that while the introduction of eutectic mixture KCl–LiCl into the redox precursor solution greatly increases BET specific surface area from 17.34 to 166.39 m2/g, BET specific surface area drops from 166.39 to 97.76 m2/g as the added amount of eutectic mixture KCl–LiCl increases, which may be due to the fact that eutectic mixture KCl–LiCl (K/Li = 0.82/1.18) melts at 355 8C far below 801 8C, the melting point of NaCl. Since salt and eutectic mixtures differ widely in melting temperature, solubility and chemical characteristics, the salt type and added salt amount have distinct effect on the characteristics of the resultants in the SCS process. The further investigation is under way. Fig. 1 shows TEM and HRTEM images of Ce0.75Zr0.25O2 samples prepared via the CCS route and the SCS process. Fig. 1(a-1) shows the typical morphology of the particles obtained in the CCS process, i.e. the three-dimensional network fractal. TEM observations at higher magnification showed that these fractals consist of tightly bundled spherical nanoparticles, as shown in Fig. 1(a-2). The ring-type diffraction patterns inset in Fig. 1(a-1) and (b-1) are indexed to polycrystalline Ce0.75Zr0.25O2. The HRTEM image [Fig. 1(b-2)] further reveals clear lattice fringes and no interspace between nanocrystallites. In sharp contrast, Ce0.75Zr0.25O2 samples prepared via the SCS process present the disordered wormhole-like mesoporous morphology, as shown in Fig. 1(b-1). By means of TEM observations at higher magnification shown in Fig. 1(b-2), it is clear that the mesoporous structures of the Ce0.75Zr0.25O2 sample obtained via the SCS process were formed by the loose agglomeration of spherical nanoparticles with particle size in Table 1 Effect of different EG/NO3 on the properties of Ce0.75Zr0.25O2 samples prepared via the CCS and SCS processes Sample
NaCl/M (molar ratio)
EG/NO3 (molar ratio)
BET surface area (m2/g)
Crystallite size (nm)
Particle size (nm)
a b c d e f
0 0 0 2/3 2/3 2/3
1/2 8/13 12/13 1/2 8/13 12/13
17.34 16.98 9.12 173.00 127.50 57.32
5.04 10.41 16.92 3.14 6.44 7.97
47.99 49.00 90.10 4.81 6.53 14.52
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Table 2 Effect of nature and amount of added salt on the properties of the combustion-prepared Ce0.75Zr0.25O2 samples at the EG/NO3 molar ratio of 1/2 Sample
Salt/M (molar ratio)
BET surface area (m2/g)
Crystallite size (nm)
Particle size (nm)
a b c d e f
0 2/3 2 2/3 4/3 2
17.34 173.00 208.17 166.39 145.16 97.76
5.04 5.14 5.49 5.34 5.38 5.42
47.99 4.81 4.00 5.00 5.73 8.51
The salt used in the samples (b) and (c) is NaCl, whereas that used in the samples (d), (e), and (f) is eutectic mixture KCl–LiCl with molar ratio of K/ Li = 0.82/1.18.
the range of 5–7 nm, which is in good agreement with crystallite size calculated using the Scherrer’s equation (listed in Table 2). The HRTEM image [Fig. 1(b-3)] further confirms the presence of the mesopores (black arrows are pointing to these regions) in between nanoparticles with visible lattice fringes. To further determine the presence of mesoporous structure, the small-angle XRD analysis was conducted. As shown in Fig. 2, the small-angle XRD patterns (traces A and B) for the Ce0.75Zr0.25O2 products obtained via the SCS process with different amount of added NaCl contain a sharp diffraction peak centered at 2u = 0.78, indicating the presence of a mesoporous structure [17]. The inset (traces C and D) in Fig. 2 is the wide-angle XRD patterns of the Ce0.75Zr0.25O2 products. All X-ray diffraction peaks in the inset of Fig. 2 can be indexed to a typical cubic fluorite structure. After the comparison of traces C and D, it can be clearly observed that trace C shows a more perfect phase than trace D due to the narrower diffraction peaks and the appearance of such weak diffraction peaks as (1 1 2), (2 2 2),
Fig. 1. Ce0.75Zr0.25O2 samples (a) and (b) are, respectively, prepared via: (a) the CCS process of EG/NO3 = 1/2, and (b) the SCS process of NaCl/ M = 2 and EG/NO3 = 1/2. TEM images of the samples (a) and (b): (a-1), (b-1) at lower magnification, inset: A selected area electron diffraction pattern; (a-2), (b-2) at higher magnification. HRTEM images of the samples (a) and (b): (a-3) and (b-3).
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Fig. 2. Powder XRD patterns of the Ce0.75Zr0.25O2 samples prepared at the same EG/NO3 ratio of 1/2 and the different NaCl/M ratio: traces A and C are, respectively, small-angle and wide-angle XRD patterns for NaCl/M = 2; traces B and D are small-angle and wide-angle XRD patterns for NaCl/M = 2/3.
which may be explained by the fact that the melt salt is capable of accelerating the material formation kinetics and improving the product crystallinity [18]. The variation of textural properties with the amount of added NaCl was also investigated by N2 sorption analyses. Fig. 3 shows the nitrogen adsorption–desorption isotherms of Ce0.75Zr0.25O2 solid solutions prepared at different NaCl/M molar ratios. The isotherms reveal adsorption and desorption, indicative of 3D intersection of a solid porous structure [19]. The porosity of these mesoporous Ce0.75Zr0.25O2 gradually increased with an increase in NaCl content, as evident by the increase in volume adsorbed at same pressure. The BJH desorption pore size distribution plots of Ce0.75Zr0.25O2 samples prepared at different NaCl/M molar ratio are inset in Fig. 3, which indicate that the amount of added NaCl has a great effect on the porosity and the pore size distribution of Ce0.75Zr0.25O2 samples obtained via the
Fig. 3. N2 adsorption–desorption isotherms and BJH desorption pore size distribution plots (inset) of Ce0.75Zr0.25O2 samples prepared under the different conditions: (a) (&,&) NaCl/M = 2/3, EG/NO3 = 1/2; (b) (~,~) NaCl/M = 2, EG/NO3 = 1/2.
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Fig. 4. Schematic diagram of the possible formation processes of mesoporous Ce0.75Zr0.25O2 in the salt-assisted solution combustion synthesis.
SCS process. It is evident that an increase in the NaCl/M molar ratio from 2/3 to 2 varies the BJH desorption pore size distribution from the range of 5–20 nm to the range of 5–50 nm. The possible formation processes of the wormhole-like mesoporous Ce0.75Zr0.25O2 in the salt-assisted ethylene glycol–nitrate combustion process are depicted in Fig. 4. As is well known, in the process of solvent evaporation, the solute concentration will eventually reach a supersaturated state and begin to nucleate and precipitate especially on the crystal seeds such as some impurities. Since the loss of solvent occurs at the solution surface, it is here that the salt will be in highest concentration. In our SCS process, the nature of the salt precipitation and its location are intimately connected to the prevention of the particles from sintering and the generation of the porous network. Since the self-propagating solution combustion reaction releases large amount of heat in a very short time, resulting in the instant high temperature of the reaction system, the salt precipitation in situ is completed in an instant to form a thin layer of salt crust on the surface of the newly formed nanoparticles with DG < 0 [20]. After the rapid cooling, the salt-coated Ce0.75Zr0.25O2 nanoparticles are trapped into the salt matrix, since the frozen salt matrix is no longer able to move, which prevents the re-agglomeration of the newly formed crystallites during combustion reaction and stabilize the derived nanoparticles [21]. Followed by removing the soluble salt by aqueous wash and drying, due to the spontaneous tendency of agglomeration, nanoparticles aggregates loosely, resulting in the formation of interparticle mesopore structure. 4. Conclusions We demonstrate in this paper the fabrication of the high surface area mesoporous ceria–zirconia solid solutions via the facile introduction of salt in the solution combustion synthesis process. We find that the instant salt precipitation in situ inhibits the formation of hard agglomerates and sintering of naonocrystalites during the combustion synthesis and results in drastic increase in surface area. As an inorganic template, salt addictives offers the advantage of being recyclable, inexpensive, thermally stable, and readily removed by aqueous wash. In view of the large number of salt additives available, the approach would be expected to offer abundant opportunities in materials research. In summary, the introduction of salt into the redox mixture solution provides a novel and effective strategy to tailor the materials properties for the conventional combustion synthesis process. Acknowledgments The present research work described in this paper was supported by the National Natural Science Foundation of China. The authors wish to thank Prof. Yongxiu Li for his valuable advice and Prof. Xiaoheng Liu for his assistance with HRTEM and XRD characterization. References [1] K.C. Taylor, in: J.R. Andersonand, M. Boudart (Eds.), Catalysis, Science and Technology, Springer Verlag, Berlin, 1984, p. 119. [2] T. Masui, Y. Peng, K.I. Machida, G.Y. Adachi, Chem. Mater. 10 (1998) 4005.
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Rapid synthesis of mesoporous ceria–zirconia solid solutions via a novel salt-assisted combustion process Weifan Chen a,*, Fengsheng Li a, Jiyi Yu a, Leili Liu b, Hailian Gao a a National Special Superfine Powder Engineering Research Center, Nanjing University of Science and Technology, Xiaolingwei 200, Nanjing, Jiangsu 210094, PR China b College of Chemical Engineering, Shandong Institute of Light Industry, Jinan 250100, PR China
Received 21 January 2006; received in revised form 3 April 2006; accepted 17 April 2006 Available online 5 May 2006
Abstract Mesoporous ceria–zirconia solid solutions were prepared by a novel salt-assisted combustion process using ethylene glycol as a fuel and nitrate as an oxidant. The effects of various operating conditions such as the fuel-to-oxidant ratio and the nature and amount of added salt on the characteristics of the products were investigated by X-ray diffraction (XRD) and nitrogen adsorption analysis. Results from transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) showed that the introduction of leachable inert inorganic salt as a hard agglomeration inhibitor into the redox mixture precursor led to the breakup of fractal nanocrystallite agglomerates and the mesoporous structure formed by the loose agglomeration of monodispersed nanoparticles, which was also confirmed by small-angle XRD and nitrogen adsorption analysis. The presence of salt was found to result in a more than 10-fold increase in the specific surface area of the products from 17.34 to 208.17 m2/g at a given molar ratio of ethylene glycol–nitrate. A mechanism scheme was proposed to illustrate the possible formation processes and discuss the role of the salt additives. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Oxides; B. Chemical synthesis; D. Microstructure
1. Introduction Ceria has been widely used as active component in so-called three-way catalysts (TWCs) for automotive exhaust treatment [1]. One of the most important roles of CeO2 in the three-way catalyst is to provide surface active sites and to act as an oxygen storage/transport medium by shifting between Ce3+ and Ce4+ under reductive and oxidizing conditions, respectively. It has been well documented that the introduction of zirconia into ceria not only improves the oxygen storage capacity (OSC) and thermal resistance, but also enhances catalytic activity at lower temperatures by the formation of ceria–zirconia solid solutions [2]. It is important in catalysis to fabricate high surface area and mesoporous materials, which would offer a large number of active sites for carrying out catalytic reactions [3]. Hence, much effort has been focused on the synthesis of high surface area and mesoporous ceria–zirconia solid solutions, including microemulsion method [4], surfactant-assisted approach [5], direct sonochemical route [6], coprecipitation
* Corresponding author. Tel.: +86 25 84315529; fax: +86 25 84315942. E-mail address: [email protected] (W. Chen). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.04.024
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followed by supercritical drying [7], macromolecule surface modified method [8], continuous hydrothermal synthesis in a near-critical water flow reactor [9]. Among the available chemical processes, self-sustaining solution combustion synthesis is characterized by convenient processing, simple experimental setup, time and energy saving and homogeneous products [10]. Therefore, there are many reports about combustion synthesis of nanosized ceria–zirconia solid solution in recent years. Aruna and Patil [11] synthesized ceria–zirconia solid solution with specific surface area in the range of 36–120 m2/g via a solution combustion process using oxalyl dihydrazine and carbohydrazine as the fuel, which are expensive and carcinogenic. Fu and Lin [12] prepared CexZr1xO2 powders with specific surface area in the range of 40–50 m2/g by a microwave-induced combustion process using glycine as the fuel. Lascalea et al. [13] increased the specific surface area of the combustion-derived Ce–Zr mixed oxide to 93 m2/g by a pH-controlled nitrate–glycine gel-combustion process. Potdar et al. [14] prepared nanosized Ce0.75Zr0.25O2 porous powders with a very wide pore size distribution in the range of 2–250 nm and surface area of about 40.0 m2/g via a glycine-nitrate autocombustion process. The synthesized powders have higher surface areas than the powders prepared by conventional solid-state methods [10], however, these methods are not too satisfying with respect to high surface area and feasibility of commercial production. Hence, combustion synthesis of high surface area ceria–zirconia solid solutions employing safety and cheap fuel has been still a challenging issue. Here, we report, for the first time, the synthesis of high surface area mesoporous ceria–zirconia solid solution by the simple introduction of salt in the combustion reaction using ethylene glycol as a novel fuel and nitrate as an oxidant. The results reveal that the introduction of salt into the redox mixture solution results in a drastic increase in surface area and provides a new means to control the properties of the products for the conventional combustion synthesis (CCS). 2. Experimental All chemicals and reagents used were high-purity commercially available Ce(NO3)36H2O (99.5%), Zr(NO3)45H2O (99.5%), ethylene glycol (99%), NaCl (99.9%), KCl (99.9%), and LiCl (99.9%), and were used as received. According to the stoichiometric ratios of Ce0.75Zr0.25O2, the required ethylene glycol–nitrate molar ratios and the desired amount of salt addictives, the proper amount of cerium nitrate, zirconium nitrate, ethylene glycol and salt addictives were dissolved in a minimum volume of deionized water in a quartz beaker to obtain transparent solution, the resulting solution was evaporated on a hot plate adjusted at 110 8C. At this stage, the viscous liquids swelled, followed by the evolution of brown gases, and self-propagating solution combustion slowly occurred, yielding loose light yellow powder. To remove salt, the as-burned powder was boiled in deionized water, filtered and washed with deionized water and ethanol. Finally, the product was dried in an oven at 80 8C for 2 h. Wide-angle and small-angle X-ray powder diffraction (XRD) patterns were obtained using a Bruker Advance X-ray D8 diffractometer, with Cu Ka radiation. The crystallite sizes were calculated from line broadening of the (1 1 1) XRD peak by Sherrer’s formula: D = 0.89l/bcos u (scanning at a rate of 0.5 8/min). Transmission electron micrographs (TEMs) and high-resolution transmission electron micrographs (HRTEMs) were, respectively, taken with a JEOL TEM-200CX microscope and a JEOL TEM-2010. The specific surface area and pore size distribution of the powders were measured with a COULTER SA 3100 analyzer using the multipoint Brunauer, Emmett, and Teller (BET) adsorption technique. The particle sizes were estimated from the formula: DBET = 6/rSBET. 3. Results and discussion It has been well documented that the conventional solution combustion synthesis method often produces nanocrystallite agglomerates which are nanometer in crystallite size calculated by the Scherrer’s formula [10–15]. As the nanocrystallites are agglomerated/sintered together and virtually inseparable, the obtained products do not show high specific surface areas corresponding to their small crystallite sizes. Recently, we found that the introduction of a soluble salt into redox mixture solution breaks up fractal nanocrystallite agglomerates into nanoparticles and prevent nanoparticles from forming hard agglomeration during the combustion process. We denote the method as salt-assisted combustion synthesis (SCS) in comparison to the conventional combustion synthesis, i.e. salt-free combustion synthesis. According to the principle of propellant chemistry [16], for stoichiometric redox reaction between a fuel and an oxidizer, the ratio of the net oxidizing valency of the metal nitrate to the net reducing valency of the fuel should be
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unity. Assumed that in the case of ethylene glycol–nitrate combustion, primarily N2, CO2, and H2O are evolved as the gaseous products, the salt-assisted stoichiometric redox combustion reaction can be expressed as follows: 6CeðNO3 Þ3 6H2 O þ 2ZrðNO3 Þ4 5H2 O þ 13HOCH2 CH2 OH þ O2 þ xNaCl ¼ 8Ce0:75 Zr0:25 O2 þ 13N2 þ 26CO2 þ 85H2 O þ xNaCl In our work, the fuel-to-oxidant molar ratio (which is hereafter referred to as EG/NO3) and amount of added salt (which is hereafter expressed as salt to metal ion molar ratio, SALT/M for short) were varied systematically to investigate the effect of EG/NO3 and SALT/M on characteristics of the as-prepared powders. The experimental results reveal that the auto-propagating combustion reaction occurs for a limited range of fuel-to-oxidant molar ratio, depending on the nature of the fuel and the amount of the added salt. In the case of ethylene glycol–nitrate combustion reaction in the absence of salt, it was experimentally observed that the auto-propagating combustion reaction ceases to occur if EG/NO3 is below 1/4. On the basis of the concept of propellant chemistry [16], EG/NO3 = 1/2 corresponds to the situation of stoichiometric ratio, whereas EG/NO3 below 1/2 is considered as fuel-deficient ratio, and EG/ NO3 above 1/2 is regarded as and fuel-rich ratio. The effect of different EG/NO3 on the properties of Ce0.75Zr0.25O2 prepared via the CCS and SCS processes are summarized in Table 1. The results show that with increasing EG/NO3, BET surface specific areas decrease and crystallite sizes increase. It is remarkable that in the case of equal EG/NO3, an addition of NaCl into the redox mixture solution greatly enhances BET specific surface area from 17.34 to 173.00 m2/g, which is much higher than BET surface areas of Ce0.75Zr0.25O2 prepared via other solution combustion routes [11–14], and reduces crystallite size from 5.04 to 3.14 nm, showing that NaCl plays a vital role in forming high specific area surface ceria–zirconia solid solutions via a solution combustion process. The possible mechanism is to be discussed later. As is shown in Table 2, in the case of equal EG/NO3, an addition of NaCl into the reaction mixture solution enhances BET specific surface area from 17.34 to 208.17 m2/g and crystallite size from 5.04 to 5.49 nm with increasing NaCl/M from 0 to 2, whereas particle sizes decrease from 47.99 to 4.00 nm. Comparing crystallite size with particle size of the samples (a), (b), and (c), it can be concluded that each particle obtained via the CCS process consists of multiple nanocrystallites and that the SCS process can break up the nanocrystallite agglomerates and form porous structure, which can be clearly seen in Fig. 1. It is notable, however, that while the introduction of eutectic mixture KCl–LiCl into the redox precursor solution greatly increases BET specific surface area from 17.34 to 166.39 m2/g, BET specific surface area drops from 166.39 to 97.76 m2/g as the added amount of eutectic mixture KCl–LiCl increases, which may be due to the fact that eutectic mixture KCl–LiCl (K/Li = 0.82/1.18) melts at 355 8C far below 801 8C, the melting point of NaCl. Since salt and eutectic mixtures differ widely in melting temperature, solubility and chemical characteristics, the salt type and added salt amount have distinct effect on the characteristics of the resultants in the SCS process. The further investigation is under way. Fig. 1 shows TEM and HRTEM images of Ce0.75Zr0.25O2 samples prepared via the CCS route and the SCS process. Fig. 1(a-1) shows the typical morphology of the particles obtained in the CCS process, i.e. the three-dimensional network fractal. TEM observations at higher magnification showed that these fractals consist of tightly bundled spherical nanoparticles, as shown in Fig. 1(a-2). The ring-type diffraction patterns inset in Fig. 1(a-1) and (b-1) are indexed to polycrystalline Ce0.75Zr0.25O2. The HRTEM image [Fig. 1(b-2)] further reveals clear lattice fringes and no interspace between nanocrystallites. In sharp contrast, Ce0.75Zr0.25O2 samples prepared via the SCS process present the disordered wormhole-like mesoporous morphology, as shown in Fig. 1(b-1). By means of TEM observations at higher magnification shown in Fig. 1(b-2), it is clear that the mesoporous structures of the Ce0.75Zr0.25O2 sample obtained via the SCS process were formed by the loose agglomeration of spherical nanoparticles with particle size in Table 1 Effect of different EG/NO3 on the properties of Ce0.75Zr0.25O2 samples prepared via the CCS and SCS processes Sample
NaCl/M (molar ratio)
EG/NO3 (molar ratio)
BET surface area (m2/g)
Crystallite size (nm)
Particle size (nm)
a b c d e f
0 0 0 2/3 2/3 2/3
1/2 8/13 12/13 1/2 8/13 12/13
17.34 16.98 9.12 173.00 127.50 57.32
5.04 10.41 16.92 3.14 6.44 7.97
47.99 49.00 90.10 4.81 6.53 14.52
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Table 2 Effect of nature and amount of added salt on the properties of the combustion-prepared Ce0.75Zr0.25O2 samples at the EG/NO3 molar ratio of 1/2 Sample
Salt/M (molar ratio)
BET surface area (m2/g)
Crystallite size (nm)
Particle size (nm)
a b c d e f
0 2/3 2 2/3 4/3 2
17.34 173.00 208.17 166.39 145.16 97.76
5.04 5.14 5.49 5.34 5.38 5.42
47.99 4.81 4.00 5.00 5.73 8.51
The salt used in the samples (b) and (c) is NaCl, whereas that used in the samples (d), (e), and (f) is eutectic mixture KCl–LiCl with molar ratio of K/ Li = 0.82/1.18.
the range of 5–7 nm, which is in good agreement with crystallite size calculated using the Scherrer’s equation (listed in Table 2). The HRTEM image [Fig. 1(b-3)] further confirms the presence of the mesopores (black arrows are pointing to these regions) in between nanoparticles with visible lattice fringes. To further determine the presence of mesoporous structure, the small-angle XRD analysis was conducted. As shown in Fig. 2, the small-angle XRD patterns (traces A and B) for the Ce0.75Zr0.25O2 products obtained via the SCS process with different amount of added NaCl contain a sharp diffraction peak centered at 2u = 0.78, indicating the presence of a mesoporous structure [17]. The inset (traces C and D) in Fig. 2 is the wide-angle XRD patterns of the Ce0.75Zr0.25O2 products. All X-ray diffraction peaks in the inset of Fig. 2 can be indexed to a typical cubic fluorite structure. After the comparison of traces C and D, it can be clearly observed that trace C shows a more perfect phase than trace D due to the narrower diffraction peaks and the appearance of such weak diffraction peaks as (1 1 2), (2 2 2),
Fig. 1. Ce0.75Zr0.25O2 samples (a) and (b) are, respectively, prepared via: (a) the CCS process of EG/NO3 = 1/2, and (b) the SCS process of NaCl/ M = 2 and EG/NO3 = 1/2. TEM images of the samples (a) and (b): (a-1), (b-1) at lower magnification, inset: A selected area electron diffraction pattern; (a-2), (b-2) at higher magnification. HRTEM images of the samples (a) and (b): (a-3) and (b-3).
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Fig. 2. Powder XRD patterns of the Ce0.75Zr0.25O2 samples prepared at the same EG/NO3 ratio of 1/2 and the different NaCl/M ratio: traces A and C are, respectively, small-angle and wide-angle XRD patterns for NaCl/M = 2; traces B and D are small-angle and wide-angle XRD patterns for NaCl/M = 2/3.
which may be explained by the fact that the melt salt is capable of accelerating the material formation kinetics and improving the product crystallinity [18]. The variation of textural properties with the amount of added NaCl was also investigated by N2 sorption analyses. Fig. 3 shows the nitrogen adsorption–desorption isotherms of Ce0.75Zr0.25O2 solid solutions prepared at different NaCl/M molar ratios. The isotherms reveal adsorption and desorption, indicative of 3D intersection of a solid porous structure [19]. The porosity of these mesoporous Ce0.75Zr0.25O2 gradually increased with an increase in NaCl content, as evident by the increase in volume adsorbed at same pressure. The BJH desorption pore size distribution plots of Ce0.75Zr0.25O2 samples prepared at different NaCl/M molar ratio are inset in Fig. 3, which indicate that the amount of added NaCl has a great effect on the porosity and the pore size distribution of Ce0.75Zr0.25O2 samples obtained via the
Fig. 3. N2 adsorption–desorption isotherms and BJH desorption pore size distribution plots (inset) of Ce0.75Zr0.25O2 samples prepared under the different conditions: (a) (&,&) NaCl/M = 2/3, EG/NO3 = 1/2; (b) (~,~) NaCl/M = 2, EG/NO3 = 1/2.
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Fig. 4. Schematic diagram of the possible formation processes of mesoporous Ce0.75Zr0.25O2 in the salt-assisted solution combustion synthesis.
SCS process. It is evident that an increase in the NaCl/M molar ratio from 2/3 to 2 varies the BJH desorption pore size distribution from the range of 5–20 nm to the range of 5–50 nm. The possible formation processes of the wormhole-like mesoporous Ce0.75Zr0.25O2 in the salt-assisted ethylene glycol–nitrate combustion process are depicted in Fig. 4. As is well known, in the process of solvent evaporation, the solute concentration will eventually reach a supersaturated state and begin to nucleate and precipitate especially on the crystal seeds such as some impurities. Since the loss of solvent occurs at the solution surface, it is here that the salt will be in highest concentration. In our SCS process, the nature of the salt precipitation and its location are intimately connected to the prevention of the particles from sintering and the generation of the porous network. Since the self-propagating solution combustion reaction releases large amount of heat in a very short time, resulting in the instant high temperature of the reaction system, the salt precipitation in situ is completed in an instant to form a thin layer of salt crust on the surface of the newly formed nanoparticles with DG < 0 [20]. After the rapid cooling, the salt-coated Ce0.75Zr0.25O2 nanoparticles are trapped into the salt matrix, since the frozen salt matrix is no longer able to move, which prevents the re-agglomeration of the newly formed crystallites during combustion reaction and stabilize the derived nanoparticles [21]. Followed by removing the soluble salt by aqueous wash and drying, due to the spontaneous tendency of agglomeration, nanoparticles aggregates loosely, resulting in the formation of interparticle mesopore structure. 4. Conclusions We demonstrate in this paper the fabrication of the high surface area mesoporous ceria–zirconia solid solutions via the facile introduction of salt in the solution combustion synthesis process. We find that the instant salt precipitation in situ inhibits the formation of hard agglomerates and sintering of naonocrystalites during the combustion synthesis and results in drastic increase in surface area. As an inorganic template, salt addictives offers the advantage of being recyclable, inexpensive, thermally stable, and readily removed by aqueous wash. In view of the large number of salt additives available, the approach would be expected to offer abundant opportunities in materials research. In summary, the introduction of salt into the redox mixture solution provides a novel and effective strategy to tailor the materials properties for the conventional combustion synthesis process. Acknowledgments The present research work described in this paper was supported by the National Natural Science Foundation of China. The authors wish to thank Prof. Yongxiu Li for his valuable advice and Prof. Xiaoheng Liu for his assistance with HRTEM and XRD characterization. References [1] K.C. Taylor, in: J.R. Andersonand, M. Boudart (Eds.), Catalysis, Science and Technology, Springer Verlag, Berlin, 1984, p. 119. [2] T. Masui, Y. Peng, K.I. Machida, G.Y. Adachi, Chem. Mater. 10 (1998) 4005.
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