Synthesis of cerium oxide particles via polyelectrolyte controlled nonclassical crystallization for catalytic application

Abstract

A polyelectrolyte controlled nonclassical crystallization method has been used to synthesize cerium oxide particles with tailored morphology. The results reveal that the as-prepared particles exhibit high specific surface area and enhanced oxygen storage capacity.

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Cerium oxide has attracted enormous research interest due to its wide application potential in fields like catalysts,^1–3 glass polishing materials,⁴ UV absorbents,⁵ gas sensors⁶ and luminescent materials.⁷ In the field of catalysts only, CeO₂ can be used in fuel cell technology,⁸ catalytic wet oxidation,⁹ engine exhaust catalysts,¹⁰ NO removal,¹¹ and photocatalytic oxidation of water due to its high oxygen storage capacity (OSC).¹²

Numerous techniques have been developed to synthesize nanosized to microsized cerium oxide particles. In spite of the following common methods like precipitation,¹³ hydrothermal,^14,15 sol–gel,¹⁶ microemulsion,¹⁷ template synthesis¹⁸ and spray-pyrolysis.^19,20 Also, many methods have been developed to prepare CeO₂ with different nanostructures, including nanorods,²¹ nanowires,²² nanotubes,²³ nanocubes,²⁴ microplates²⁵ and other morphological structures.^26–28 The facile and practical methods, however, are still needed for synthesizing cerium oxide particles with tailored characteristics in terms of particle size, morphology, specific surface area and OSC. Such physical properties are critical for the application of cerium oxide particles in the catalytic industry.

Encouraged by the recent achievements demonstrated by the synthesis of calcium carbonate mesocrystals in the presence of polyelectrolyte,^29–31 it is intriguing to investigate the synthesis of cerium oxide particles with tailored morphology and property through this well controlled nonclassical crystallization method. Several works have been reported on cerium oxide particles prepared in the presence of polymers like poly(vinylpyrrolidone),³² poly(ethylene glycol)-block-poly(methacrylic acid),³³ polyethylene,³⁴ poly(acrylic acid)³⁵ and dendrimer.³⁶ However, these polymers were used only as either dispersion stabilizers or surfactant additives. Moreover, these systems did not reveal the working mechanisms of polymer additives on the particle characteristics. Thus the morphology and physical properties of CeO₂ particles are very hard to predict and control via these methods. It is highly desirable to develop a facile and reproducible method to synthesize well-defined CeO₂ particles.

Herein, we describe a methodology based on the nonclassical crystallization of cerium oxide precursors in the presence of polystyrene sulfonate sodium (PSS), which decompose and form cerium oxide after calcination. PSS is simple and easy available anionic polyelectrolyte can be used as structure directing additives.^30,31 The morphology and the size of the precursor particles remained in cerium oxide particles. The cerium oxide particles with high specific surface area and OSC were obtained with the optimized conditions.

In a typical preparation, precursors were crystallized with the gas diffusion technique,³⁷ in which, Ce(NO₃)₃ solution was mixed with PSS solution in a beaker. The final pH value of the mixture was adjusted to 4 by adding nitric acid. The final mixture solution contained 0.5 g L⁻¹ PSS and 0.07 M Ce(NO₃)₃. The beaker and a watch glass of crushed ammonium carbonate powder, both covered with parafilm with holes, were placed at the bottom of the desiccators. After 14 days of reaction, the precipitate precursor particles were collected, rinsed and dried at 60 °C for 8 hours in a vacuum oven. Cerium oxide particles were obtained by calcination of the precursors. The cerium oxide particles were prepared without adding PSS using a similar procedure for comparison.

The precursors exhibit thin flake-like morphology in the absence of PSS (Fig. 1A). However, the spherical precursor particles are formed in the presence of PSS (Fig. 1B). Cerium oxide particles are obtained after calcination of precursors at 500 °C for 2 hours (Fig. 1C and D). Notably, the morphology of precursors has been reserved after calcination at 500 °C and no clasped particles have been observed. It indicates that the obtained cerium oxide particles inherit the morphology of its precursors. The disappeared carbon peak in the X-ray energy dispersive spectroscopy (EDS) results indicates that the precursors are decomposed and cerium oxide particles are produced via the calcination (Fig. S1†). The disappeared sulfur peak indicates that PSS decomposed after calcination. The results from the particle size distribution measurement clearly reveal that the mean particle size of precursors is 1.86 μm. The mean particle size of the obtained CeO₂ particles decreases to 1.33 μm (Fig. S2†).

Fig. 1 SEM images of precursor (A) and CeO₂ (C) particles prepared in the absence of PSS and precursor (B) and CeO₂ (D) in the presence of 0.5 g L⁻¹ PSS. CeO₂ particles were obtained by calcining precursor particles at 500 °C.

The crystal structure of CeO₂ particles obtained by calcination of the prepared precursors at different temperatures is analyzed by X-ray diffraction (XRD). The results indicate that cerium oxide particles are formed after being calcined at temperatures above 300 °C (Fig. 2). The typical peaks of face-centered-cubic (FCC) structure of cerium oxide are broad when the precursors were calcined at relatively low temperatures (300–500 °C), however, the peaks get sharper and more intense with increasing temperature. Notably, the low intensity peaks ({400}, {331} and {420}) become visible and sharper at higher calcining temperatures. It reveals that the obtained cerium oxide particles have higher crystallinity at higher calcination temperatures than the lower temperatures. The XRD results indicated that the precursors prepared with and without PSS additives are both mixtures of cerium(III) carbonate hydrates (Ce₂(CO₃)₃8H₂O) and cerium(III) basic carbonates hydrates (CeCO₃OH and Ce₂(CO₃)₂(OH)₂H₂O) (Fig. S3†). The basic carbonate precursors have been reported previously.³⁸ The precursors obtained with PSS also contain very fine CeO₂ particles indicated by the broad peaks. This might due to the oxidation and decomposition of cerium(III) hydroxide produced at the earlier stage of the gas diffusion process. There are two different lattice parameters for the orthorhombic cerium carbonate hydrates (Ce₂(CO₃)₃8H₂O) in the precursors obtained without PSS while there is only one in the precursors obtained with PSS addition. The infrared spectroscopy (IR) data indicated that the precursors contain carbonate hydrates (Fig. S4 & Table S1†). The thermogravimetric analysis revealed that the endotherms around 100–200 °C are due to the loss of bulk water and hydrate water. The endotherms around 240–330 °C were assigned to the decomposition of cerium carbonate and basic cerium carbonate.³⁹ It indicated that the precursors are carbonate hydrates, which is consistent with IR and XRD data (Fig. S5†). The XRD patterns of cerium oxide particles obtained in the absence and presence of PSS are the same (Fig. S6†). It reveals that the addition of PSS did not affect the final crystal structure of cerium oxide particles. However, the morphology and crystal structure of these precursors are quite different (Fig. 1).

Fig. 2 XRD patterns of CeO₂ particles obtained by calcination of precursor at different temperature, (A) 60 °C, (B) 300 °C, (C) 500 °C, (D) 700 °C, (E) 900 °C.

The texture properties of these CeO₂ particles are investigated. N₂ isotherms exhibit the type IV isotherm with H4 hysteresis loop according to the classification of adsorption isotherms (Fig. 3A).⁴⁰ The shapes of hysteresis loops have often been identified with specific pore structures. The H4 type of hysteresis loop is often associated with narrow slit-like pores. In this case the type IV isotherms with H4 loops reveal that CeO₂ particles produced in the presence and absence of PSS are both mesoporous with slit-like pores. The calculated pore size distribution curves reveal that both CeO₂ mesopores have narrow distribution, with the maximum pore radius of 1.69 nm in the presence of PSS and 1.68 nm in the absence of PSS (Fig. 3B & Table 1). The pore volume increases 10% while the BET surface area is 24 m² g⁻¹ higher in the presence of PSS than in the absence of PSS (Table 1). In comparison with the CeO₂ particles prepared without PSS, the CeO₂ particles prepared with PSS exhibit bigger pore size, bigger pore volume and higher surface area, which indicates that the presence of PSS enhances the mesoporous structure during the synthesis procedure. It is proposed that nanocrystallines of the precursors rearrange, self-assemble and form spherical particles with slit-like mesoporous structure with the aid of PSS. Apparently, the CeO₂ particles prepared in the absence of PSS also exhibit mesoporous properties with smaller pore size, which is attributed to the random aggregation of those flake-like particles. The effects of PSS concentration on the morphology of particles were studied. The results reveal that 0.5 g L⁻¹ is the best concentration to obtain the spherical morphology. The particles with hemispherical and plate-like shape were obtained at low PSS concentration of 0.1 g L⁻¹, while a high PSS concentration of 1.0 g L⁻¹ causes particle aggregation and results in the irregular shapes (Fig. 4).

Fig. 3 (A) N₂ adsorption isotherms and (B) pore size distribution curve of CeO₂ particles prepared in the absence and presence of PSS.

Table 1 Physical characteristics of CeO₂ particles obtained in the absence and in the presence of PSS

	CeO2	CeO2^a
a Prepared in the presence of 0.5 g L^−1 PSS.
Specific surface area (m^2 g^−1)	80.10	104.10
Volume of pores (mL g^−1)	0.10	0.11
Radius of pores (nm)	1.68	1.69
Oxygen storage capacity (μmol g^−1)	329	449

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Fig. 4 SEM images of precursor particles prepared in the solution contain (A) 0.1 g L⁻¹, (B) 0.5 g L⁻¹ and (C) 1.0 g L⁻¹ PSS.

The oxygen storage capacity (OSC) is a very important factor when the particles are considered to be used as three-way catalysts. The OSC measurement is carried out at 200 °C using the oxygen pulse gas chromatographic technique. The samples were reduced in H₂ at 550 °C and then pure N₂ was flowed to cool the temperature to 200 °C. Then O₂ was pulse injected at an interval of 2 min via six channel valves. A thermal conductivity detector (TCD) detector was used to collect the signal of the non-absorbed O₂ (Fig. S7†). Then the pulse injection stop after the sample was saturated with oxygen. OSC is calculated by dividing the molar amount of uptake oxygen by the mass of sample. The result revealed that the OSC of CeO₂ particles prepared with PSS dramatically increased to 449 μmol g⁻¹ from the value of 329 μmol g⁻¹ for the CeO₂ particles prepared without PSS (Table 1). The enhanced oxygen storage capacity could probably be attributed to the unique mesoporous structure with slit-like pores which helps to increase surface area and oxygen binding sites. At the same time, the larger pore volume and pore size offers a short diffusion path, which allows for efficient oxygen transportation.

Conclusions

In summary, we have developed a facile synthesis of cerium oxide particles using the nonclassical crystallization methodology in the presence of PSS. The spherical and mesoporous CeO₂ particles are obtained after the calcination of precursors, which are formed after the addition of ammonium carbonate powder via the gas diffusion technique. PSS is used as an important additive in determining the morphology of the precursors, which remains in CeO₂ particles. The results suggest that the precursors nanocrystallines are self-assembled with the aid of PSS in a nonclassical crystallization way. The obtained CeO₂ particles are mesoporous with slit-like pores which provide high surface area and pore volume, both of which are favourable for catalysts. The enhanced OSC promises that these CeO₂ particles could be applied as three-way catalysts.

Acknowledgements

This work is supported by The National Outstanding Youth Fund project (5104526) and The National Natural Science Fund project (201066010).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: The preparation and characterization of CeO₂ particles. See DOI: 10.1039/c3ra44698k

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