Facile morphology control of 3D porous CeO₂ for CO oxidation

Abstract

3D porous CeO₂ with various morphologies was successfully synthesized via a facile precipitation using glycine as the soft bio-template. During the synthesis, it was demonstrated that the morphology of CeO₂ depended on the molar ratio of reactants. Furthermore, the catalytic performance towards CO oxidation of the as-synthesized CeO₂ with different morphologies was investigated. CeO₂ with a bowknot shape showed excellent catalytic performance, giving complete CO conversion at 370 °C, due to its properties of much higher oxygen vacancies, loosely packed pore structure and larger specific surface area.

Different morphologies of CeO₂ were obtained via a green and facial method, which realized CO complete conversion at 370 °C.

Introduction

CeO₂ has the most widespread applications in catalysis owing to its excellent oxygen storage capacity.^1,2 It has been proved that the morphology of CeO₂ could influence its catalytic performance to a certain extent. Specifically, the 3D structure of CeO₂ possesses remarkable advantages compared to 2D or 1D CeO₂ samples.³ Thus, it is of great significance to obtain 3D porous CeO₂ materials with various shapes.

Numerous methods for synthesis of 3D porous CeO₂ with different morphologies have been developed, including spray pyrolysis,⁴ precipitation methods,^5,6 combustion synthesis,⁷ hydrothermal methods,^8–10etc. However, among these methods, hazardous reagents and a series of energy consuming and tedious manipulations are usually needed. Again, it is difficult to obtain different morphologies of CeO₂ in the same preparation route. Hence, it is still a challenge to develop a facile process to synthesize 3D porous CeO₂ with different types of morphologies.

The bio-template method is one of the common synthesis strategies to prepare porous materials.¹¹ To date, diatom,¹² butterfly,¹³ wood,¹⁴ leaf,¹⁵ cotton¹⁶ and pollen¹⁷etc. are used as hard biotemplate to prepare kinds of porous materials. Qian reported that porous CeO₂ was obtained through a bio-template of lotus pollen.¹⁸ However, this method was difficult to tune the pore structure and morphology of CeO₂, for the reason that the structure of the templates was natural and inherent. Amine acid, with the special structure of –NH₂ and –COOH, has been used in fabricating porous materials.^19,20 Among the numerous amine acids, glycine is the simplest in the structure. Moreover, Guo reported that amine acid with 2 carbon atoms (such as glycine) led to more loosely packed material, while the amine acid with more complicated structure and more carbons usually resulted in the close-packed products.²⁰ The loosely packed material owns more slit holes, which could increase the ability of mass transfer in catalysis.

Herein, a simple and soft bio-template of glycine was chosen to direct the growth of CeO₂ by a milder precipitation method in this work. We obtained 3D porous CeO₂ (A-CeO₂) with different morphologies by facilely changing the preparation conditions. Furthermore, the model reaction of CO oxidation was used to evaluate the influence of the structure of CeO₂.

Experimental

Urea, glycine, anhydrous ethanol, and cerium nitrate (Ce(NO₃)₃·6H₂O) were analytical grade, and used as received. A-CeO₂ was prepared as following: 3.50 g Ce(NO₃)₃·6H₂O and 1.44 g urea were dissolved in 200 mL deionized water under vigorous stirring at room temperature, then 1.20 g glycine was added. After stirring for another 1 h, the solution was transferred to drying oven heated at 100 °C for certain time. Finally, the sample was dried and calcined at 500 °C in air. The obtained product is denoted as A-CeO₂ (3 : 2 : 1), the proportion in brackets is the molar ratio of urea, glycine and cerium nitrate. Different molar ratio of reactants for A-CeO₂ was prepared under the same reaction condition. The CeO₂ bulk was prepared by calcining Ce(NO₃)₃·6H₂O at 600 °C directly.

Functional groups of A-CeO₂ samples were investigated by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700, USA). The crystal phases were obtained on X-ray powder diffraction (XRD, RigakuUltima IV, Japan). The UV-vis diffuse reflectance spectral (UV-vis DRS, Varian Cary 5000, USA) characterization was employed to calculate the band gap energy of samples. The thermo-gravimetric analysis (TGA, SDT Q600, USA) performed the thermal behavior of A-CeO₂. Morphologies of the samples were taken by scanning electron microscope (SEM, ZEISS SIGMA, Germany) and TEM (TECNAI F30, USA). Surface area and pore were characterized by N₂ adsorption/desorption measurements (BET, Tristar II3020, USA), pore size was calculated by the Barrett–Joyner–Halenda (BJH) model. Raman analysis (Renishaw, INvia) for detecting the oxygen vacancy was performed using a 532 nm excitation laser. The Ce valence state was detected by XPS (Physical Electronics, USA) using a monochromated Al Kα (1486.7 eV) as X-ray source in which the binding energy was calibrated with C 1s 284.8 eV. The H₂-TPR was used to illustrate the reducibility of samples.

The CO oxidation was carried out under atmospheric pressure in a fixed bed reactor containing 0.1 g catalyst. The reactor stream was composition of CO (1%), O₂ (1%) and Ar (98%) with the total flow rate of 20 000 mL h⁻¹ g_cat⁻¹. The reactants and products analysis were investigated by gas chromatography (GC 9560) with a thermal conductivity detector.

Results and discussion

A-CeO₂ was prepared as illustrated in Fig. 1. Ce(NO₃)₃·6H₂O, urea and glycine were dissolved in deionized water under vigorous stirring at room temperature for a certain time, then the solution was transferred to drying oven heated at 100 °C. Finally, the sample was dried and calcined at 500 °C (according to Fig. S1†) in air. The preparation was easy to implement, and we could obtain various morphologies of A-CeO₂ by simply changing the molar ratio of reactants (urea, glycine and cerium nitrate). As shown in Fig. 2, A-CeO₂ with about 10 μm spindle structure and a few spherical particles were obtained without introducing glycine (Fig. 2a). As the increase of glycine, the spherical particles disappeared (Fig. 2b), further increasing mole ratio of glycine, the morphology of A-CeO₂ changed to bowknot (Fig. 2c) and bulk shape (Fig. 2d). Simply, the existence of glycine brought more uniform structure of A-CeO₂, and the amount of glycine influenced the morphology of A-CeO₂. The amount of urea and Ce precursor also affected the structure of A-CeO₂. A-CeO₂ (1.5 : 2 : 1) showed the prism shape (Fig. 2e), A-CeO₂ (6 : 2 : 1) performed small dumbbell shape (Fig. 2f), and A-CeO₂ (3 : 2 : 2) presented dumbbell shape (Fig. 2g). In Fig. 3, Enlarged Fig. 2 and TEM of special samples showed that different shapes owned different stack forms. It was no obvious stack when without glycine (Fig. 3a1 and A1). A-CeO₂ (3 : 2 : 1) showed the stack form of sticks that was linked by balls. However, A-CeO₂ (3 : 2 : 2) piled in prismatic forms.

Fig. 1. Scheme of the synthesis of A-CeO₂.

Fig. 2. SEM of A-CeO₂ with different reactants ((a) A-CeO₂ (3 : 0 : 1); (b) A-CeO₂ (3 : 1 : 1); (c) A-CeO₂ (3 : 2 : 1); (d) A-CeO₂ (3 : 4 : 1); (e) A-CeO₂ (1.5 : 2 : 1); (f) A-CeO₂ (6 : 2 : 1); (g) A-CeO₂ (3 : 2 : 2) the proportion in brackets is the molar ratio of urea, glycine and cerium nitrate).

Fig. 3. SEM and TEM of special A-CeO₂ ((a1 and A1) A-CeO₂ (3 : 0 : 1); (a2 and A2) A-CeO₂ (3 : 2 : 1); (a3 and A3) A-CeO₂ (3 : 2 : 2) the proportion in brackets is the molar ratio of urea, glycine and cerium nitrate).

As shown in Fig. 4a, the prepared A-CeO₂ had the same structure of cubic fluorite with face centered as CeO₂ (JCPDS file no. 34-0394).² Meanwhile, there were no other crystalline byproducts and obvious difference in characteristic diffraction peaks could be observed, confirming the high purity among various morphologies. In FTIR spectra (Fig. 4b), different morphologies of A-CeO₂ showed basically consistence, but the obtained A-CeO₂ exhibited a –NH flexural vibration at about 1522 cm⁻¹ compared with CeO₂, indicating that the addition of glycine could residual N element. Nitrogen adsorption–desorption isotherms of special samples (Fig. 5a) corresponded to the type IV curves with H3 hysteresis loop, that reflected the A-CeO₂ owned the feature of slit mesoporous (2–4 nm, Fig. 5b). And, the samples exhibited similar pore size distribution rule. Different morphologies presented different specific surface areas (Table 1), and A-CeO₂ (3 : 2 : 1) owned the larger specific surface areas of 85 m² g⁻¹. Hence, the 3D porous A-CeO₂ with tuning morphology was successfully obtained via a milder process. The whole preparation process is more economic than the previous report by Mitchell and coworkers.²¹

Fig. 4. XRD pattern (a) and FTIR spectra (b) of different A-CeO₂ samples.

Fig. 5. N₂ adsorption–desorption isotherms (a) and corresponding pore size distribution curves (b) of different A-CeO₂ with different ratio of reactant.

The properties and performance of A-CeO₂.

Samples	E g (eV)	Area RCe–O/area F2g (%)	S BET (m2 g−1)	T 100 (°C)
CeO2	2.67	5.91	—	620
A-CeO2 (3 : 0 : 1)	2.63	6.88	78	480
A-CeO2 (3 : 2 : 1)	2.48	8.74	85	370
A-CeO2 (3 : 2 : 2)	2.47	7.38	70	480

As we all know that oxygen vacancies dominate the electronic and chemical properties of CeO₂.² In this work, Raman (Fig. 6a) and XPS (Fig. 6b and S3†) were used to exhibit the concentration of oxygen vacancies. According to the report,²² the typical Raman shift at 460 cm⁻¹ and 600 cm⁻¹ can be assigned to F2g vibration of the fluorite-type structure and oxygen vacancies. More importantly, the integral area ratios of oxygen vacancy peak (R_Ce–O) and F2g reflect the oxygen vacancy concentration. As shown in Table 1, A-CeO₂ (3 : 2 : 1) had the larger oxygen vacancy concentration, and relatively narrower energy gap (E_g) (Fig. S2† and Table 1) further verified the higher oxygen vacancy concentration of A-CeO₂ (3 : 2 : 1). The XPS spectra were used to study the valence state of Ce/oxygen vacancies of CeO₂ and etc. As shown in Fig. 6b and S3,† the Ce 3d spectrum of samples showed the appearance of the v, v′′, v′′′, u, u′′ and u′′′, which were attributed to Ce⁴⁺. The v₀, v₁, u₀ and u₁ peaks were characteristic peaks of Ce³⁺.²³ Moreover, the Ce³⁺/Ce⁴⁺ of CeO₂, A-CeO₂ (3 : 0 : 1), A-CeO₂ (3 : 2 : 1) and A-CeO₂ (3 : 2 : 2) were 0.837, 0.857, 0.888 and 0.873 respectively. It was known that the higher Ce³⁺ concentration in nanoparticles was correlated with higher oxygen vacancy, so the obtained A-CeO₂ owned more oxygen vacancy than CeO₂, and A-CeO₂ (3 : 2 : 1) exhibited the highest oxygen vacancy concentration, that coincided with the results of the Raman spectra.

Fig. 6. Raman spectra (a) of CeO₂ and different A-CeO₂ with different ratio of reactant and Ce 3d core level XPS pattern (b) of A-CeO₂ (3 : 2 : 1).

The H₂-TPR profiles (Fig. 7) showed that all the samples owned a broad profile centered around 500 °C corresponding to the consumption of surface oxygen species and a high-temperature reduction peak above 800 °C arose from bulk reduction.²³ The peak characteristic of surface oxygen species at lower temperature of A-CeO₂ indicated that the synthesized nanomaterials possessed relatively stronger surface activity, and A-CeO₂ (3 : 2 : 1) exhibited the strongest surface activity.

Fig. 7. The H₂-TPR profiles of CeO₂ and different A-CeO₂ with different ratio of reactant.

The formation process of typical A-CeO₂ was investigated by FTIR, SEM and XRD. Fig. S4a† showed the FTIR spectra of the reactants and products. It could be inferred that glycine acted as a complexing agent in the reaction. Urea was supposed to be a precipitant due to the phenomenon of more A-CeO₂ formation as increasing the dosage of urea (Table S1†). The molar ratio of reactants could influence the dispersion and hydrolysis rate of Ce³⁺, and then affect the morphology of A-CeO₂. The adsorbed Ce³⁺ hydrolyzed on the surface of glycine, and then the sphere CeCO₃OH (Fig. S4b,† orthorhombic structure, JCPDS file no. 41-0013) was formed. In order to further verify the sphere structure, addition of ethanol in the solvent to prevent from the hydrolysis of Ce³⁺, and then decrease the formation rate of A-CeO₂,³ the obtained sample was sphere nanoparticle (Fig. S5†). Moreover, the shape of A-CeO₂ changed from sphere, bowknot to bulk over time (Fig. S6†). Above all, we supposed the schematic diagram as shown in Fig. 8.

Fig. 8. Schematic illustration for the formation process of A-CeO₂.

The catalytic effect for CO oxidation of typical A-CeO₂ with different morphologies was investigated. Fig. 9 showed that the catalytic performance was highly dependent on the shape of A-CeO₂, and the complete CO conversion temperature of A-CeO₂ (3 : 2 : 1) was 370 °C, which was much lower than others especially the CeO₂ (620 °C) in bulk (Fig. S7†). The catalytic performance was also better than the reported commercial CeO₂ powers, CeO₂ nanowires and 3D ordered macroporous CeO₂.^5,6,24 The A-CeO₂ (3 : 2 : 1) with bowknot shape showed the best catalytic effect might be due to its outstanding features: higher oxygen vacancy (Table 1) for improving oxygen mobility,²⁴ more loosely packed porous structure (Fig. 4d) and larger specific surface area (Table 1), which could intensify mass transfer.⁷

Fig. 9. CO oxidation evaluation of different CeO₂.

Conclusions

In summary, a series of A-CeO₂ with different morphologies, namely spindle, bowknot, dumbbell and prism, have been successfully fabricated via a milder precipitation route assisted by glycine. The catalytic performance for CO oxidation of the as-synthesized A-CeO₂ is strongly dependent on the shape of A-CeO₂. Owing to the outstanding properties, A-CeO₂ (3 : 2 : 1) with bowknot shape possesses the excellent catalytic activity with the complete CO conversion temperature of 370 °C. The facile and eco-friendly synthesis method of A-CeO₂ may provide reference for the preparation of other 3D porous metal oxides. The prepared CeO₂ with different microstructures is promising for the future practical application in catalysis.

Conflicts of interest

There are no conflicts to declare.