Controllable synthesis of CeO₂ nanoparticles with different sizes and shapes and their application in NO oxidation

Abstract

This study describes the synthesis of cobalt–ceria catalysts with octahedron and nanosphere shapes via a facile and surfactant-free hydrothermal method. The morphology of the products could be controlled by adjusting the proportion of solvent. The products evolved from nanospheres to octahedra with decreasing ethylene glycol/water volume fraction ratio in the reaction system. The formation process of the nanospheres involved dissolution–recrystallization-assembly and Ostwald ripening processes. Furthermore, the sizes of the obtained ceria–cobalt nanospheres were about 30 to 150 nm; these were composed of many crystallites with sizes of approximately 7 to 9 nm. The size of the ceria–cobalt nanospheres could be controlled by changing the cobalt doping amount in the mixed water–glycol system. These synthesized catalysts were applied for NO oxidation. The catalytic performance is closely related to the oxygen vacancy and the NO adsorption ability. In the low cobalt doping amount, NO adsorption and desorption played an important role in the oxidation process. However, the adsorption and activation O₂ was the key step when the NO adsorption and desorption ability was similar.

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1. Introduction

Controlling the structure and morphology of nano-materials to alter their physicochemical properties has become a current trend in materials science. In recent years, researchers have focused attention on micro- and nano-materials because of their extensive applications in different fields, such as biomedical science,¹ physics,² environmental protection³ and chemistry.⁴ As is known, theories about structure–function relations serve as a guide in the rational synthesis of functional micro- and nano-materials.⁵ The properties and applications of nanostructure materials are influenced profoundly by their morphology, size, structure and chemical composition.^6–8 According to this rule, many methods have been investigated to structurally modify micro- and nano-materials in order to achieve innovative applications. For instance, three-dimensional composites consisting of carbon nanotubes and various metal oxide nanoparticles have been fabricated utilizing vertically aligned carbon nanotube patterns as the template.⁹ Wang and co-workers reported the synthesis of hierarchical multishell Co₃O₄ hollow microspheres based on carbon microspheres as the sacrificial template.¹⁰ Zhong et al. reported a flowerlike iron oxide using tetrabutylammonium bromide as a soft template.¹¹ These template-assisted methods have demonstrated effectiveness in the forming of hierarchical metal oxides with various unique shapes. However, most of the procedures are tedious due to the adding or removing of templates from the reaction system, which makes the specification of the reaction process more challenging. Recently, Ye and co-workers reported a simple solvothermal synthesis of hierarchical Fe₃O₄–Co₃O₄ yolk–shell nanostructures, in which no templates or surfactants were used.¹² Moreover, other metal oxides with special morphologies were also fabricated by the solvothermal method.^13–15 To date, facile synthesis methods with control over shape and size have emerged as a main synthetic methodology for the preparation of functional nanomaterials. Although many metal oxides with various morphologies have been prepared, a fine, controllable and facile synthesis of novel structures still remains a great challenge.

With high abundance, cerium oxide is a technologically important material due to its wide applications as a promoter in three-way catalysts (TWCs) for the elimination of toxic auto-exhaust gases,¹⁶ the low-temperature water-gas shift (WGS) reaction,¹⁷ oxygen sensors,¹⁸ oxygen permeation membrane systems,^19,20 fuel cells²¹ and ultraviolet absorbents.²² The catalytic property of CeO₂ derives from its unique redox properties and its strong oxygen storage and release capability via facile conversion between the Ce³⁺ and Ce⁴⁺ oxidation states. It is widely reported that cobalt oxide based catalysts are potential candidates for NO oxidation. Wen et al. studied a series of La_1−xCe_xCoO₃ perovskite catalysts for nitrogen monoxide oxidation and reported approximately 80% conversion on La_0.8Ce_0.2CoO₃ at 300 °C.²³ TiO₂ and SiO₂ supported Co₃O₄ catalysts were studied for NO oxidation by Irfan et al.²⁴ They reported a maximum of 69% conversion on Co₃O₄/SiO₂ at 300 °C with high space velocity conditions. From the literature, it is known that cobalt based type catalysts show good activity for the oxidation of NO to NO₂. The NO₂ could be captured in water as nitric acid.²⁵ However, there are few reports in the literature about impurity doping-induced control of the morphology growth and phase transformation of CeO₂ materials.

In our current work, a series of catalysts with different cobalt-doped concentrations and ethylene glycol/water ratios were successfully synthesized by a hydrothermal process. The crystalline structure, the surface area, the surface electronic states and the adsorption capacity of reactant gas were investigated by XRD, TEM, BET surface area analysis, XPS, H₂-TPR and NO-TPD. Excellent catalytic performance was obtained at 74.8% over C6-_10:115 catalyst. The goal was to establish the composition–structure–property relationships, which would provide more insight into the design and rationalization of practical catalysts.

2. Experimental section

2.1 Preparation of the catalysts

All starting reagents were purchased from Sinopharm Chemical Reagent Company (Shanghai, China) and were used without further purification. Deionized water was used throughout.

Synthesis of CeO₂ nanospheres. 0.8682 g cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) was dissolved in 120 mL ethylene glycol, and then 5 mL deionized water was added to the above solution. After continuous stirring for 30 min, the clear solution was transferred into a Teflon-lined autoclave of 200 mL capacity and heated for 16 h at 180 °C. When the autoclave was cooled to room temperature, the yellow products were collected and washed with deionized water and absolute alcohol three times sequentially. Finally, the products were dried at 70 °C overnight and then calcined at 400 °C for 2 h. The above product was labelled as C1.

Synthesis of cobalt-doped CeO₂ nanospheres. 0.8682 g cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) was dissolved in 120 mL ethylene glycol; then, 5 mL deionized water was added to the above solution, and different amounts of CoCl₂·6H₂O (0.0238, 0.0476, 0.0952, 0.1428, 0.1904 and 0.238 g) were added; the corresponding molar ratios of Co : Ce are 0.05, 0.1, 0.2, 0.25, 0.3, 0.4 and 0.5. After continuous stirring for 30 min, the clear solution was transferred into a Teflon-lined autoclave of 200 mL capacity and heated for 16 h at 180 °C. When the autoclave was cooled to room temperature, the gray products were collected and washed with deionized water and absolute alcohol three times sequentially. Finally, the products were dried at 70 °C overnight and then calcined at 400 °C for 2 h. The above products with different cobalt-doping concentrations were labelled as C2–C7.

Synthesis of C6 with different ethylene glycol/water (volume/volume) ratios. 0.8682 g cerium nitrate hexahydrate (Ce(NO₃)₃·6H₂O) and 0.1904 g CoCl₂·6H₂O were dissolved in 125 mL solution (the ratio of ethylene glycol/water is 125EG, 10 : 115, 20 : 105, 60 : 65, 105 : 20, 125H₂O). After continuous stirring for 30 min, the clear solution was transferred into a Teflon-lined autoclave of 200 mL capacity and heated for 16 h at 180 °C. When the autoclave was cooled to room temperature, the gray products were collected and washed with deionized water and absolute alcohol three times sequentially. Finally, the products were dried at 70 °C overnight and then calcined at 400 °C for 2 h. The above products with different ratios of ethylene glycol/water were labelled as C6-_125EG, C6-_10:115, C6-_20:105, C6-_60:65, C6-_105:20 and C6-_125H2O.

2.2 Sample characterization

The powder XRD patterns were recorded on a Beijing Purkinje general instrument XD-3 X-ray diffraction using Cu-Kα radiation at 36 kV and 20 mA (2θ from 5° to 80°). The scanning speed is 8° min⁻¹ and the step value is 0.04°.

Laser Raman Spectra (LRS) were recorded on a Renishaw Invia Raman Microscope with Ar⁺ radiation (514 nm). The laser light was focused onto the samples by using a microscope equipped with a 6100 objective lens.

Specific surface areas of the different catalysts were determined by N₂ adsorption–desorption measurements at −196 °C by employing the Brunauer–Emmett–Teller (BET) method (Gold App V-sorb 2800p), and the pore volumes and pore sizes of the samples were calculated by the Barrett–Joyner–Halenda (BJH) method. The pore volume was estimated from the amount of adsorbed nitrogen at a relative pressure of 0.99. Prior to N₂ adsorption, the sample was outgassed at 200 °C for 12 h to desorb moisture adsorbed on the surface and inside the porous network.

The micromorphology of the catalysts was examined on a JEOL JEM-2100 transmission electron microscope (TEM) and the sample was deposited on a copper mesh by means of dipcoating. The acceleration voltage was 200 kV.

The metal contents of the catalysts were determined by X-ray fluorescence multi-element analysis (XRF) on a Bruker AXS S4 Explorer.

X-ray photoelectron spectroscopy (XPS) was performed on a Thermo ESCALAB 250 (USA) apparatus with Al Kα X-ray (hν = 1486.6 eV) radiation operated at 150 W to investigate the surface atomic concentrations and the oxidation state distribution of the elements in the samples. The samples were compensated for charging with a low-energy electron beam, and the peak of C 1s (binding energy = 284.4 eV) was used to correct for sample charging. This reference gave BE values with an accuracy of ±0.1 eV. Also, the atomic surface ratios of the corresponding species were given with an accuracy of ±0.1%.

Temperature-programmed desorption (TPD) was carried out on automated chemisorption analyzer (Quantachrome Instruments). About 200 mg sample was used. After NO saturation over 1 h, the gas was switched to He for 0.5 h. Subsequently, TPD was performed by ramping the temperature at 10 °C min⁻¹ to 750 °C in He (70 mL min⁻¹). Desorption of NO was detected by a thermal conductivity detector (TCD).

Hydrogen temperature programmed reduction (H₂-TPR) was performed in a quartz U-tube reactor on an automated chemisorption analyzer (Quantachrome Instruments) by the GC method. About 100 mg of sample was pretreated in an N₂ stream at 600 °C for 0.5 h. As the sample was cooled to 50 °C, N₂ was switched to H₂–N₂ mixture gas (10% H₂, v/v) at a flow rate of 70 mL min⁻¹. A cold trap was used to remove water during TPR. H₂-TPR was performed by heating the sample from 50 to 750 °C; at the same time, the consumption of H₂ was detected by a thermal conductivity detector (TCD).

2.3 Catalytic testing

The catalytic oxidation of NO was performed in a fixed-bed flow microreactor under atmospheric pressure. Typically, 300 mg sample (sieve fraction of 40–60 mesh) was placed in a quartz reactor (6.8 mm i.d.); the reactant gas mixture (390 ppm NO, 8% O₂, N₂ balance) was fed to the reactor with a total flow rate of 100 mL min⁻¹, corresponding to a gas hour space velocity (GHSV) of 35 400 h⁻¹. The steady-state tests were conducted isothermally every 25 °C from 200 to 400 °C, and the gas products (after 90 min reaction) were analyzed by a Ecom-JZKN flue gas analyzer (Germany). The NO conversion is defined as:

NO conversion = (NO_in − NO_out)/NO_in × 100%.

3. Results and discussion

3.1 Structural analysis

The crystal structures of the samples were characterized by XRD and Raman analysis. As shown in Fig. 1a, the diffraction peaks of C1–C7 illustrated that they could be well indexed to a pure phase of face-centered cubic ceria (JCPDS no. 34-0394). The synthesized samples maintained a long-range pure fluorite cubic structure. The Raman spectra of C1–C7 are also shown in Fig. 1b. For C1 sample (pure ceria), the observed main Raman band around 461 cm⁻¹ was related to the triply degenerate F_2g Raman active mode of the CeO₂ fluorite structure, which was the only one allowed in first order; this is supported by the XRD results.²⁶ For the C2–C7 samples, a typical characteristic in the main F_2g mode band, which differed from that of pure ceria, could be described as follows: a red shift appeared because of lattice defects.^27–29 The XRD spectra of C6-_125EG, C6-_10:115, C6-_20:105, C6-_60:65, C6-_105:20 and C6-_125H2O samples are shown in Fig. 1c. The peak intensity tended to increase and the FWHM decreased with increasing H₂O amount except for C6-_10:115. This results indicated that the crystallinity of the sample improved and the particle size increased. The particle sizes of C6-_125EG, C6-_10:115, C6-_20:105, C6-_60:65, C6-_105:20 and C6-_125H2O are displayed in Table 1. Among all the samples, C6-_10:115 showed the smallest grain size. The Raman spectra of these samples are also shown in Fig. 1d. All these samples had one obvious band at 461 cm⁻¹. In addition, all catalysts showed no peaks of the crystalline cobalt oxides, indicating that the cobalt oxide species were highly dispersed and/or present as small clusters and/or formed solid solutions which were difficult to detect due to the limitations of XRD.

Fig. 1 XRD (a and c) and Raman spectra (b and d) of C1–C7 and C6-_125EG, C6-_10:115, C6-_20:105, C6-_60:65, C6-_105:20 and C6-_125H2O samples.

Table 1 Grain sizes of C6 with different ethylene glycol/water (volume/volume) ratios

Sample	C6-125EG	C6-10:115	C6-20:105	C6-60:65	C6-105:20	C6-125H2O
Grain size (nm)	7.6	6.9	8.2	16.5	18.6	57.9

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Table 2 The surface area and pore structure parameters of C6 with different ethylene glycol/water (volume/volume) ratios

Sample	C6-125EG	C6-10:115	C6-20:105	C6-60:65	C6-105:20	C6-125H2O
BET (m^2 g^−1)	60	95	45	6	4	2
Pore size (nm)	13.8	10.8	6.1	5.0	3.9	2.1
Pore volume (cm^3 g^−1)	0.143	0.085	0.042	0.016	0.009	0.005

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The morphologies and crystal sizes of the synthesized samples were analyzed by TEM, as shown in Fig. 2. In a typical solvothermal process, the diameters of the CeO₂ microspheres were influenced by the concentration of starting materials and the reaction time.^30,31 Under the temperature of 180 °C, the diameters of the microspheres were observed to be ca. 70 nm, ca. 100 nm, ca. 150 nm, ca. 45 nm, ca. 80 nm, ca. 30 nm and ca. 125 nm for C1–C7, respectively (Fig. 2a–g). At first, the microsphere diameters increased with increasing cobalt amount. Secondly, the diameters decreased to ca. 45 nm with the continuous increase of the cobalt concentration. Lastly, the diameter underwent an increase → decrease → increase process. However, the complex nature of the growth process made it difficult to obtain a straightforward correlation between the particle diameter and the changes in precursor concentration. The formation process of the nanospheres could be well understood by the two-stage growth model, in which nanosized crystalline precursors were nucleated first in supersaturated solution and then initially formed small particles that aggregated into larger secondary particles.^32–34

Fig. 2 TEM images of C1 to C7 (a–g), (scale bar: a–e, g = 200 nm, f = 100 nm).

To investigate the effect of the reaction time on the formation of the final product, the C2 samples at different growth stages (4, 8, 12, 16, 24 and 32 h) were examined thoroughly by TEM. When the reaction time reached 4 h, the main crystallite size was ca. 70 nm (Fig. S1a†). As the reaction time was prolonged to 8 h, the main crystallite size of the spheres increased (ca. 115 nm) (Fig. S1b†). Once the primary ceria–cobalt nanocrystals were formed, they initially tended to form solid spheres, driven by the minimization of the total energy of the system. A simple plausible mechanism based on Ostwald ripening (crystallites grow at the expense of smaller ones) could explain this phenomenon. When the hydrothermal times were 12, 16, 24 and 32 h, the main crystallite sizes were ca. 73, 29, 120 and 130 nm (Fig. S1c–f†), respectively. This could probably be attributed to the dissolution–recrystallization of the nanoparticles composed of the solid nanospheres and the self-assembly of the new nanoparticles, as schematically illustrated in Fig. 3.

Fig. 3 Schematic of the formation of nanospheres in the whole synthetic process.

The water concentration in the reaction system had a great influence on the morphology of the obtained samples. Three typical samples were carefully studied by HRTEM. The typical TEM image of C6-_10:115 shown in Fig. 4a reveals microparticles with diameters ranging from 130 to 240 nm. Based on the observation from the HRTEM images, it could be concluded that the large microparticles were comprised of many small particles with crystallite sizes of 6 to 8 nm. As shown in the red dotted line in Fig. 4b, there are clear voids with diameters of 6 to 8 nm in the small particles, revealing the mesostructure of the nanospheres. The crystallite size estimated by the Scherrer equation from the width of the strongest (111) line in the XRD pattern was 6.9 nm, which corresponded well with the crystallite size measured from the HRTEM micrographs. The interplanar spacing of the mainly exposed planes were calculated to be 0.313 nm, corresponding to the (111) plane of CeO₂ (Fig. 4b). Trace amounts of (200) planes from the nanoparticles could also be detected. Fig. 4d and f show the selected area electron diffraction (SAED) patterns of the microparticles. It can be seen that the diffraction patterns were neither the typical polycrystal rings nor the typical single crystal dots. The patterns here were elongated dots, which implied that the large cluster was comprised of many small nanocrystals which attached to each other with orientations in different directions. In other words, these small nanocrystals were displayed in a disordered state. Interestingly, the microparticles comprised two different shapes; one was a nanosphere (Fig. 4c), and the other was a nanocube (Fig. 4e). It is obvious that the dots in Fig. 4f are closer to each other. This tendency indicated that the small nanocrystals in the nanocubes had more organized orientations than in the nanospheres. Eventually, the dots in Fig. 4f connected to each other and formed the typical polycrystal rings. These results showed that the nanospheres transformed into nanocubes through the Ostwald ripening process.

Fig. 4 The TEM (a, c and e), HRTEM (b) and selected area electron diffraction (SAED) (d and f) images of C6-_10:115.

As observed from Fig. 5a and b, the nanoparticles of C6-_105:20 were uniformly distributed and attached to each other. The lattice fringes can be clearly observed. After the calculation of the space of the lattice fringes, it was deduced that the dominant lattice fringes at 0.313 nm correspond to the (111) planes. Fig. 5d shows a typical TEM image of the uniform nanooctahedra. A large grain particle was the essential structure. No secondary structure existed, which is similar to C6-_105:20 but different from C6-_10:115. The nanooctahedron edge length sharply increased to about 230 nm, which was much larger than the calculated value (57.9 nm) by XRD. Therefore, we believe that a high concentration of EG could lead to aggregation of the nanoparticles. SAED analysis indicates that the nanooctahedron displayed clear (111) planes with a fringe spacing of 0.313 nm (Fig. 5f).

Fig. 5 TEM and HRTEM images of C6-_105:20 (a–c) and C6-_125H2O (d–f), (scale bar: a = 1 μm, b = 0.5 μm, c = 5 nm, d = 0.5 μm, e = 100 nm and f = 2 nm).

To further investigate the porosity and specific surface area of the samples, N₂ adsorption–desorption measurements were carried out at −196 °C. Fig. 6a shows the adsorption–desorption isotherms of C6 with different ethylene glycol/water volume fraction ratios. The isotherms reveal that there is much less adsorption in the low pressure region. All the samples show type IV isotherms. The hysteresis loops are H3 type for C6-_125EG, C6-_10:115 and C6-_20:105. For C6-_60:65, C6-_105:20 and C6-_125H2O, the hysteresis loops are H4 type. The H3 hysteresis loops indicated that these samples contained mesopores with narrow silt-like shapes.³⁵ However, the H4 hysteresis loops illustrated that the samples demonstrated microporosity and mesoporosity. As observed in the different samples, the isotherms are different because of the surface heterogeneity; the particle size varies from one to another.

Fig. 6 N₂ adsorption–desorption isotherms (a) and pore size distributions (b) of C6 with different ethylene glycol/water (volume/volume) ratios.

The corresponding pore size distributions in the mesoporous region, shown in Fig. 6b, were calculated by the BJH method. The pore size distribution curves suggested that all the samples had defined mesoporosities. The pore size at the maximum probability of the catalysts was 13.8, 10.8, 6.1, 5.0, 3.9 and 2.1 nm for C6-_125EG, C6-_10:115, C6-_20:105, C6-_60:65, C6-_105:20 and C6-_125H2O, respectively. The different surface area, porosity and pore volume in these samples indicated that the amount of EG played an important role in the synthetic process.

3.2 The possible formation mechanism

On the basis of previous literature reports^36–39 and the above-mentioned experiments, we proposed a new mechanism for the formation of cobalt-assisted CeO₂ nanospheres. This consists of dissolution–recrystallization-assembly and Ostwald ripening processes. In the first stage, Ce³⁺ ions hydrolyzed incipiently; they were oxidized to Ce⁴⁺ by NO₃⁻ in the acidic environment after a short solvothermal time, which could be observed by the colour change of the clear solution from colorless to yellow. Then, in the second stage, the initially formed small particles self-assembled into larger secondary particles. Both the literature⁴⁰ and our own experiments show that ethylene glycol plays an important role in microsphere formation. Ethylene glycol is a strong reducing agent with a relatively high boiling point⁴¹ and has been widely used in the glycol process to provide monodisperse fine metal or metal oxide nanoparticles. Meanwhile, ethylene glycol, with high viscosity, acted as an inhibitor which could adsorb on the surface of small nuclei and further prevent the growth of the nuclei.⁴¹ Furthermore, the high water concentration in the glycol solvent would generate more aggregation of small particles. This is due to the decrease of OHCH₂CH₂OH adsorption and the increase of H₂O adsorption on the surface of the small particles. As a result, with the decrease of the ethylene glycol amount in the reaction solution, the morphology of the samples changed from microspheres to octahedra. The poor dispersity of the octahedra was due to the lack of protection of EG. On the other hand, ethylene glycol could coordinate to cobalt ions to form the complexes in the reaction solution, which also hindered the combination of cobalt ions and water molecules.^15,42 In order to verify our speculation, XPS and XRF were performed in order to further illuminate the composition of the elements existing in the samples. Table 3 lists the Co/(Co + Ce) atomic ratios derived from XPS and XRF analysis. The results show that the surface Co/(Co + Ce) ratio in C6-_10:115 was higher than that in C6-_105:20. Interestingly, the Co/(Co + Ce) ratio in C6–_125H2O was the same as that in C6-_105:20. Furthermore, the surface Co/(Co + Ce) ratios were smaller than in the bulk, indicative of surface Ce enrichment and Co deficiency. The dispersion of the cobalt phase is heterogeneous. This phenomenon indicates that cobalt ions more readily combine with ethylene glycol compared with water molecules in the water–glycol system. Without ethylene glycol, cobalt ions could also combine with H₂O to form complexes in the reaction solution.

Table 3 Characterization results from XPS and XRF

Sample	Atomic ratio (%)
	XPS			XRF
	Ce^3+/(Ce^3+ + Ce^4+)	Oβ/(Oβ + Oα)	Co/(Co + Ce)	Co/(Co + Ce)
C6-10:115	20.19	51.2	6.7	6.3
C6-105:20	19.30	44.6	2.0	1.8
C6-125H2O	17.72	41.5	2.0	1.9

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3.3 H₂-TPR reduction properties

H₂-TPR tests were performed to study the reducibility of C6 with different ethylene glycol/water volume fraction ratios. As shown in Fig. 7, three well-defined reduction regions are observed, which are the low temperature region (peak α), the medium temperature region (peak β) and the high temperature region (peak γ), respectively. At the low-temperature range below 280 °C, peak α was attributed to the reduction of the adsorbed oxygen; peak β at medium temperature was attributed to the reduction of Co³⁺ to Co²⁺ and Co²⁺ to Co.⁴³ Peak γ at the high-temperature region could be attributed to the reduction of surface Ce⁴⁺.^44,45 Based on the integrated peak areas shown in Table S2,† it can be clearly seen that the peak area of α is much larger in C6-_125EG, C6-_10:115 and C6-_20:105, indicating the existence of a larger amount of oxygen vacancies. Moreover, the temperature of the reduction peak β shifted to a lower temperature with increasing amount of EG. In addition, the peak area of β also increased, indicating an increased amount of cobalt. It is worth mentioning that the β peak of C6-_10:115 was the largest among all the catalysts. Thus, the TPR analysis clearly demonstrated a decreased reduction temperature and overall increased H₂ consumption. This substantiated the increased reducibility of the Co–Ce catalyst due to the improved catalyst–support interaction.

Fig. 7 H₂-TPR profiles of C6 with different ethylene glycol/water (volume/volume) ratios.

3.4 X-ray photoelectron spectroscopy (XPS) results

XPS was performed in order to further illuminate the surface composition and the chemical state of the elements existing in the catalysts. The deconvoluted photoelectron spectra of Ce 3d and O 1s are shown in Fig. 8. The Ce 3d core level spectra (Fig. 8a) of all the samples were deconvoluted into eight contributions. The spin–orbit splitting of Ce 3d_5/2 and Ce 3d_3/2 was 18.5 eV for all the samples, which was in good agreement with the literature.⁴⁶ The u′′′ satellite peak at about 916.5 eV was the fingerprint of the Ce⁴⁺ state, and its high intensity and area suggested that the majority of the ceria was in the form of Ce⁴⁺. After deconvolution, the appearance of bands labelled u′ and v′ were typical for Ce³⁺ ions, which suggested that both oxidation states, Ce⁴⁺ and Ce³⁺, coexist on the surfaces of the samples.^44,46 The relative intensities of u′ to v′ to the whole eight bands were 20.19%, 19.30% and 17.72%, referring to C6-_10:115, C6-_105:20 and C6-_125H2O, respectively, which is shown in Table 3.

Fig. 8 XPS spectra Ce 3d (a) and O 1s (b) of C6-_10:115, C6-_105:20 and C6-_125H2O.

The O 1s spectra were mainly composed of two components (Fig. 8b). The high-resolution spectrum for the O 1s ionization features was numerically fitted with the Gaussian features representing the primary O 1s ionization feature and the chemically shifted O 1s features from chemisorbed surface species such as O₂⁻ and O⁻. The strong band O_α (around 529 eV) was attributed to the characteristic oxygen peak of the metal oxides,⁴⁷ while the shoulder O_β with higher binding energy (around 531 eV) was the result of chemisorbed oxygen. Obviously, the ratios of chemisorbed oxygen to the whole type of oxygen in C6-_10:115 were the highest (Table 3), which indicates that 10 mL water in the water–glycol system was beneficial for the formation of oxygen vacancies on the oxide surface. This conclusion was in accordance with the results of H₂-TPR. This phenomenon is related to the catalytic oxidation activity.

3.5 Catalytic performance

Ceria (CeO₂) is one of the most reactive rare-earth oxides and attracts much attention due to its high oxygen storage capacity.⁴⁸ Here, the NO conversion reaction model was adopted to evaluate the catalytic properties of our prepared samples. Fig. 9 shows the catalytic activity profiles of the samples with different cobalt doping amounts and water–glycol ratios. As shown in Fig. 9a, C6 showed the best catalytic performance and C1 had the lowest activity. The low catalytic performance of C1 demonstrated that the cobalt acted as the active site in the NO oxidation process. In addition, we compared C6 with 9.6 wt% Co/CeO₂ prepared by wet impregnation.⁴⁹ The results indicated that C6 had a conversion of 68.2% at 275 °C, which was much higher than the reported reference (≈30% at 270 °C). The catalytic performances for different water–glycol ratios are shown in Fig. 9b. It can be obviously seen that the catalytic performance of our samples was sharply improved by increasing the amount of EG in the system. Both C6-_10:115 and C6-_125EG reached their best NO conversions at a relatively low temperature of 300 °C. C6-_20:105 reached the maximum conversion, 67.7% of NO conversion at 325 °C. The NO conversions of C6-_60:65, C6-_105:20 and C6-_125H2O were lower than 18%, and they continued to increase with increasing temperature. The NO conversion began to drastically increase at 225 °C for C6-_10:115, C6-_125EG and C6-_20:105. Among all these samples, C6-_10:115 exhibited the highest NO conversion, which reached 74.8% at 300 °C. As shown in Table 2, C6-_125EG, C6-_10:115 and C6-_20:105 have larger surface areas, while C6-_60:65, C6-_105:20 and C6-_125H2O have surface areas of only 2 to 6 m² g⁻¹. Thus, the surface area data could provide evidence to explain the different catalytic properties among our products. Due to the small surface area of C6-_60:65, C6-_105:20 and C6-_125H2O, the reactant gases could not be adsorbed sufficiently to make adequate contact with the catalysts. The NO-TPD profiles of three typical samples of C2, C6 and C6-_10:115 are shown in Fig. S2.† The results indicated that C6 and C6-_10:115 had higher amounts of strongly adsorbed species, i.e., nitrates. Nitrates were the important intermediate products which could react with NO⁺; the ion pair NO⁺NO₃⁻ could decompose to two NO₂ molecules.⁵⁰ From Fig. S2,† the abilities of surface strongly adsorbed species were similar between C6 and C6-_10:115, but stronger than C2. It could be found that the content of nitrates in C6 and C6-_10:115 was similar; however the ratio of O_β was higher in C6-_10:115. Also, the NO conversion over C6-_10:115 was relatively higher. Hence, it could be concluded that the adsorption and activation of O₂ were the key step when the NO adsorption and desorption ability was similar. Interestingly, the ratios of O_β in C2 and C6 were similar; the catalytic performance was low due to the low cobalt doping amount. This phenomenon indicated that the NO adsorption and desorption ability was a crucial factor when the cobalt amount was low.

Fig. 9 Catalytic performance of different Co³⁺ doping amounts (a) and different water–glycol ratios (b).

4. Conclusion

In summary, a serious of samples with different cobalt doping amounts and different water–glycol ratios has been successfully developed by a one-step solvothermal approach. C6-_10:115 exhibited the highest NO oxidation performance, with a NO conversion efficiency as high as 74.8% at 300 °C. The NO conversion capacity was determined by the NO adsorption ability and the amount of oxygen vacancies. The shape of the sample could be controlled by tuning the water–glycol ratio. Based on the SAED pattern of C6-_10:115, the nanospheres transformed into nanocubes through an Ostwald ripening process. The formation process of the nanospheres involved dissolution–recrystallization-assembly and Ostwald ripening processes. We hope these findings will provide a rational preparation method for novel CeO₂-based catalysts with desirable morphologies and sizes, as well as other functional nanomaterials.

Acknowledgements

This work was financially supported by the Assembly Foundation of The Industry and Information Ministry of the People's Republic of China 2012 (543), the National Natural Science Foundation of China (51408309 and 51578288), Science and Technology Support Program of Jiangsu Province (BE2014713), Natural Science Foundation of Jiangsu Province (BK20140777), Industry-Academia Cooperation Innovation Fund Projects of Jiangsu Province (BY2014004-10), Science and technology project of Nanjing (201306012), Jiangsu Province Scientific and Technological Achievements into a Special Fund Project (BA2015062), A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education of Jiangsu Higher Education Institutions.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra04678a

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