Ultrasonic-spray-assisted synthesis of metal oxide hollow/mesoporous microspheres for catalytic CO oxidation

Abstract

Transition metal oxides with hollow or mesoporous microstructures represent a unique class of heterogeneous catalysts or catalyst supports with many fascinating features such as the large surface areas, tunable pore sizes and volumes, multitude of compositions, and ease of functionalization. In this work, we demonstrated a general ultrasonic-spray-assisted synthesis of various transition metal oxide hollow/mesoporous microspheres, using only low-cost commercial metal chlorides as precursors and water as solvent. As typical examples, five mono-component metal oxides (CeO₂, α-Fe₂O₃, Co₃O₄, SnO₂ and TiO₂) hollow or mesoporous microspheres were prepared and their formation mechanisms were discussed. The catalytic carbon monoxide (CO) oxidation indicated that these metal oxide products exhibited improved catalytic activities than some of those reported previously, due to their tiny crystalline grains, large pore volume and specific surface area. The universality of this synthetic method was also demonstrated in producing multi-component and multi-functional metal oxides with hollow/mesoporous structures. This work therefore provides an industrial level producing method for the scalable preparation of various metal oxide hollow/mesoporous microspheres which are expected to be used as high-performance catalysts or catalyst supports in more chemical reactions.

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Introduction

Transition metal oxides with hollow or mesoporous micro/nanostructures have attracted considerable attention for their potential applications in catalysis,^1,2 energy conversion and storage,^3–6 drug delivery,^7,8 and chemical sensors.^9,10 Especially, they may represent a superior class of high performance catalysts or catalyst supports for CO oxidation because they could offer the large surface areas and interconnected pore channels, which could enhance the distribution of the catalytically active sites to efficiently adsorb CO and O₂ molecules and minimize diffusion barriers of the gas molecules during catalyst reaction.^11–15 Therefore, the transition metal oxide catalysts with hollow or mesoporous microspherical morphologies have sparked broad interest owing to their low cost, environmental friendliness, outstanding thermal stability, and easy to use. In this regard, a high-yield preparation method that is capable of generally producing hollow or mesoporous microspherical catalysts of transition metal oxide is undoubtedly attractive.

Since the discovery of mesoporous silicates based on amphiphilic supramolecular templates,¹⁶ the use of surfactant as templates to organize mesoporous micro/nanostructures has been explored over a wide range of conditions. Poly(alkylene oxide) block copolymers have predominately been employed as the structure-directing agents for the formation of mesostructured metal oxides through casting, film coating or aerosol spray.^17–19 Stucky and co-workers²⁰ have developed a poly(alkylene oxide) block copolymer templating strategy for the synthesis of non-silica based mesostructures, mainly metal oxides, and they used Pluronic P-123 as structure-directing agents in non-aqueous solutions for organizing the network-forming metal-oxide species, using inorganic salts as precursors. On the other hand, previous studies have demonstrated that amphiphilic block copolymers could form spherical micelles in appropriate solvents, to function as templates for the formation of hollow microspheres. However, the morphology of the micelles was affected by various factors, such as the composition of the block copolymers, copolymer concentration, and temperature,^21,22 leading poor reproducibility of these methods for producing hollow microspheres.

Compared with the solution-reaction methods, the aerosol spray assisted synthesis has distinct advantages of facility, large-scale production, high purity, good reproducibility and continuous production.^17,23,24 The detailed descriptions about this technique can be found in several excellent reviews.^17,25,26 Recently, Kuai and co-workers²⁷ synthesized a variety of mesoporous metal oxides by the approach combining aerosol spray pyrolysis of low-cost nitrate salts together with ethanol solvent evaporation-induced assembly of poly(alkylene oxide) block copolymer. Most of the aerosol spray syntheses reported previously were carried out using metal alkoxides as inorganic precursors²⁸ or in non-aqueous solution.^29,30 The use of metal alkoxides or non-aqueous solvents would raise the cost and add pollution to the environment.

In this work, we demonstrated the ultrasonic-spray-assisted synthesis of various transition metal oxide hollow or mesoporous microspheres, using the low-cost commercial metal chlorides as precursors and deionized water as solvent. Typically, five hollow or mesoporous mono-component metal oxides (CeO₂, α-Fe₂O₃, Co₃O₄, SnO₂ and TiO₂) were prepared and their catalytic potentials for CO oxidation was identified. The results indicated that these hollow/mesoporous metal oxide microspheres showed favorable catalytic activities. The universality of this synthetic method was also demonstrated in producing multi-component and multi-functional metal oxides with hollow/mesoporous structures.

Experimental section

Materials

All chemicals used for synthesis were obtained from chemical companies and used without further purification. Cerium(III) chloride heptahydrate (CeCl₃·7H₂O), iron(III) chloride anhydrous (FeCl₃), cobalt(III) chloride hexahydrate (CoCl₂·6H₂O), tin(IV) chloride pentahydrate (SnCl₄·5H₂O), and titanium(III) chloride (TiCl₃) solution (17 wt%) were purchased from Aladdin reagent Co. Pluronic F127 was purchased from Sigma-Aldrich Co. Purified water was provided by a Millipore Milli-Q water purification system.

Preparation of hollow/mesoporous metal oxide microspheres

In a typical synthesis, the precursor solution was prepared first by successively dissolving a certain amount of Pluronic F127 (0.25 g) and metal chloride (2 mmol) in deionized water (30 mL) with vigorous stirring. The resultant transparent solution was used as a precursor for the ultrasonic aerosol spray process. The precursor solution was then transferred to a household ultrasonic humidifier (1.7 MHz, 30 W) for aerosol generation. The generated mist was carried by an atmospheric pressure that was generated by a mechanical vacuum pump into a tube furnace with a heating zone set at 500 °C. The produced powder was collected on a filter paper. The collected powders were then calcined at 400 °C (ramp rate: 1 °C min⁻¹) in air for 4 h to obtain the final metal oxide hollow or mesoporous microspheres.

The magnetic Fe₃O₄/CeO₂ hollow microspheres and Fe₃O₄/TiO₂ mesoporous microspheres were prepared by thermally treating the α-Fe₂O₃/CeO₂ and α-Fe₂O₃/TiO₂ products at 400 °C for 4 h in a gas mixture of H₂ and N₂ at 5 : 95 (v/v). The preparation of α-Fe₂O₃/CeO₂ and α-Fe₂O₃/TiO₂ products was similar to that of the monocomponent metal oxides hollow/mesoporous microspheres except that two types of metal chlorides were used. FeCl₂·4H₂O (1 mmol) and CeCl₃·7H₂O (1 mmol) were used to prepare α-Fe₂O₃/CeO₂ hollow microspheres. FeCl₂·4H₂O (1 mmol) and TiCl₃ solution (17 wt%, 0.9 mL) were used to prepare α-Fe₂O₃/TiO₂ mesoporous microspheres.

Characterization

The products were examined by powder X-ray diffraction (XRD, Smart Lab Diffractometer, 40 kV, 40 mA), scanning electron microscope (SEM, FEI QF400), transmission electron microscope (TEM, FEI TS12, 120 kV). Nitrogen adsorption/desorption isotherms were measured on a Micromeritics TriStar 3000 system at the liquid-nitrogen temperature. The specific surface area and pore size distribution were analyzed by N₂ according to the Brunauer–Emmett–Teller and Barrett–Joyner–Halenda model, respectively. Magnetic properties at room temperature were measured by a physical property measurement system (PPMS, Quantum Design 6000).

Catalytic CO oxidation

The catalytic performance of the as-prepared metal oxide hollow/mesoporous microspheres for CO oxidation was conducted in a quartz-tube plug flow reactor using 50 mg catalyst in a mixed gas of 1 vol% CO and 99 vol% dried air at a flow rate of 40 mL min⁻¹ corresponding to a gas hourly space velocity (GHSV) of 48 000 mL h⁻¹ g⁻¹. The catalyst was heated to the desired temperatures at a rate of 2 °C min⁻¹ and then kept for 30 min until the catalytic reaction reached a steady state. Then, the composition of effluent gas was analyzed by a portable intelligent professional CO₂ detector (Keernuo Gt901, Shenzhen Keernuo technology CO., LTD). The CO conversion was calculated from the change in CO₂ concentration of the inlet and outlet gases.

Results and discussion

The ultrasonic-spray-assisted synthesis process of the metal oxide hollow or mesoporous microspheres is demonstrated in Scheme 1. The setup is similar with the previously reported one,²⁷ including an ultrasonic humidifier, a tube furnace, and a product collector. The precursor solution containing Pluronic F127 and metal chloride is sprayed by an ultrasonic humidifier, and the generated mist is pumped into the tube furnace which is preheated to 500 °C. The process that the mist goes through the furnace lasts only several seconds. This process is a “droplet-to-particle” aerosol spray hydrolysis-diffusion route.²³ The metal chloride in the spray droplets hydrolyzes to generate tiny colloidal particles which diffuse outwards due to the action of surface tension to form a solidified shell, as the water evaporates meanwhile. If the hydrolysis-generated tiny nanoparticles in the interior of the droplets can diffuse fast to the solidified shell before the solvent evaporates completely, the hollow spheres are finally formed with the tiny nanoparticles as building blocks. The formation mechanism of the hollow microspheres seems to be the coffee ring effect.^31,32 Otherwise, if the evaporation of water is faster than the diffusion of the hydrolysis-generated nanoparticles in the interior of the spray droplets, the mesoporous microspheres are finally formed by assembly of the small nanoparticles with the assistance of F127. The products collected on the filter paper at this stage are amorphous because of the short residence time in the furnace. The collected products are then calcined at 400 °C in air for 4 h to remove the residual organic species. Finally, the metal oxide hollow or mesoporous microspheres assembled with the crystallized nanoparticles are obtained.

Scheme 1 Schematic representation of the ultrasonic-spray-assisted synthesis of hollow or mesoporous metal oxide microspheres.

Table 1 lists the typical metal oxide products prepared by the present method and their porous properties measured by N₂ adsorption/desorption. The results demonstrate that this method can prepare various metal oxide products by the use of low-price metal chlorides as precursors. The as-obtained metal oxide products possess high specific surface area and favorable porous features, which would play an important role in the fundamental study of advanced catalysts for diverse reactions. The phase and microstructures of the metal oxide products were characterized by powder X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). XRD patterns in Fig. 1a, d, g, j, and m identify these products as fluorite-structured cubic CeO₂ (JCPDS No. 04-0593), rhombohedral α-Fe₂O₃ (hematite, JCPDS No. 72-469), cubic Co₃O₄ (JCPDS No. 80-1545), tetragonal SnO₂ (cassiterite, JCPDS No. 41-1445), and tetragonal TiO₂ (anatase, JCPDS No. 89-4921), respectively. From the SEM and TEM images, most of the metal oxides present spherical shapes with diameters ranging from 200 nm to 2 μm, which are mainly resulted from the droplet diameter. Among them, CeO₂ and α-Fe₂O₃ are hollow microspheres with a little larger size. SnO₂ and TiO₂ are solid microspheres with diameters ranging from 200 nm to 1.5 μm, and their mesoporous structures can be seen from TEM images. Co₃O₄ is the hollow microspheres having folds on their shells which are assembled by the loosely packed nanoparticles. These metal oxide hollow/mesoporous microspheres present good dispersion, and no agglomeration is observed even after the 500 °C high-temperature calcination.

Table 1 Preparation conditions and porous properties of the metal oxide products

Product	Precursor	Furnace temperature (°C)	Surface area (m^2 g^−1)	Pore volume (cm^3 g^−1)
CeO2	CeCl3·7H2O	500	75.77	0.16
α-Fe2O3	FeCl3	500	49.33	0.15
Co3O4	CoCl2·6H2O	500	37.55	0.22
SnO2	SnCl4·5H2O	500	104.32	0.21
TiO2	TiCl3 solution (17 wt%)	500	92.78	0.14

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Fig. 1 XRD patterns, SEM and TEM images of the metal oxide hollow/mesoporous microspheres. (a–c) CeO₂, (d–f) α-Fe₂O₃, (g–i) Co₃O₄, (j–l) SnO₂, (m–o) TiO₂.

The specific surface areas and pore size distributions of these metal oxide hollow/mesoporous microspheres are characterized by N₂ adsorption–desorption measurements. Fig. 2 shows the N₂ adsorption–desorption isotherms and the Barrett–Joyner–Halenda (BJH) plots obtained from N₂-adsorption measurements. The Brunauer–Emmett–Teller (BET) surface areas and the BJH pore sizes and volumes of these metal oxide hollow/mesoporous microspheres are summarized in Table 1. The isotherm features of these samples can be assigned to type IV, which demonstrates the mesoporous structures in these samples. The BET surface areas of the as-obtained CeO₂, α-Fe₂O₃, Co₃O₄, SnO₂, and TiO₂ microspheres are 75.77, 49.33, 37.55, 104.32 and 92.78 m² g⁻¹, respectively. These metal oxide materials exhibit much higher surface areas than the bulk porous metal oxides prepared by an improved sol–gel method.^33,34 The BJH adsorption cumulative volumes of pores between 1.7 and 300 nm diameter for the as-obtained CeO₂, α-Fe₂O₃, Co₃O₄, SnO₂, and TiO₂ microspheres are 0.16, 0.15, 0.22, 0.21, 0.14 cm³ g⁻¹, respectively. The pores in these metal oxides are generally disordered. The hollow microspheres (CeO₂, α-Fe₂O₃, and Co₃O₄) show broad BJH pore size distributions, which are mainly caused by loosely packed shells or hollow interiors. The solid mesoporous microspheres (SnO₂ and TiO₂) exhibit narrow BJH pore size distributions, with their BJH adsorption average pore diameters of 6.8 nm and 5.6 nm, respectively.

Fig. 2 N₂ adsorption–desorption isotherm plots and BJH pore size distributions of the products. (a and b) CeO₂, (c and d) α-Fe₂O₃, (e and f) Co₃O₄, (g and h) SnO₂, and (i and j) TiO₂.

The present ultrasonic-spray-assisted approach can also handily produce multi-component metal oxide hollow/mesoporous microspheres, by choosing two or more different types of metal chlorides and simply dissolving them together in the precursor solutions. Therefore, the metal oxide hollow/mesoporous microspheres with particular functionalities can be readily designed and prepared. In our study, the magnetic multi-component metal oxides of Fe₃O₄/CeO₂ hollow microspheres and Fe₃O₄/TiO₂ mesoporous microspheres are prepared, as typical examples. The α-Fe₂O₃/CeO₂ and α-Fe₂O₃/TiO₂ composite metal oxides were prepared first, which is similar to that of the mono-component metal oxides hollow/mesoporous microspheres except that two types of metal chlorides were used. The magnetic Fe₃O₄/CeO₂ hollow microspheres and Fe₃O₄/TiO₂ mesoporous microspheres were obtained by thermally treating the α-Fe₂O₃/CeO₂ and α-Fe₂O₃/TiO₂ products at 400 °C for 4 h in a gas mixture of H₂ and N₂ at 5 : 95 (v/v). The XRD pattern (Fig. 3a) of the Fe₃O₄/CeO₂ sample presents two sets of diffraction peaks which can be assigned to the cubic Fe₃O₄ (magnetite, JCPDS No. 75-1610) and the fluorite-structured cubic CeO₂ (JCPDS No. 04-0593), respectively. The diffraction peaks of both Fe₃O₄ and CeO₂ are evidently observed. The SEM image (Fig. 3b) reveals that the Fe₃O₄/CeO₂ composite metal oxide is hollow microspheres with diameters ranging from 500 nm to 2 μm, similar with the morphology of CeO₂ sample. The XRD pattern (Fig. 3d) of the Fe₃O₄/TiO₂ sample also presents two sets of diffraction peaks which belong to the cubic magnetite Fe₃O₄ and the tetragonal TiO₂ (anatase, JCPDS No. 89-4921), respectively. The diffraction peaks of TiO₂ component are very weak. The SEM image (Fig. 3e) of this sample shows that the Fe₃O₄/TiO₂ hybrid metal oxide is solid microspheres with diameters ranging from 500 nm to 1.5 μm, similar with the morphology of TiO₂ sample. To verify the magnetic properties of the Fe₃O₄/CeO₂ and Fe₃O₄/TiO₂ hybrid metal oxides, the relationship between the applied magnetic field (H) and the induced magnetization (M) was studied using vibrating-sample magnetometer (VSM) at room temperature and the magnetization curves of Fe₃O₄/CeO₂ and Fe₃O₄/TiO₂ are shown in Fig. 3c and f, respectively. By increasing the applied magnetic field from 0 to 7000 (Oe), the magnetizations dramatically increased, and the magnetic saturation (Ms) values of Fe₃O₄/CeO₂ and Fe₃O₄/TiO₂ are 8.9 and 3.3 emu g⁻¹, respectively. Narrow magnetic hysteresis loop with extremely small coercivity and remanence values indicated the super-paramagnetic properties of the two samples. The magnetic separability of the two hybrid metal oxides was examined by placing a magnet beside the glass bottle containing the suspension. Obviously, the brown powder in water can be easily collected by the magnet. This offers a simple way for efficiently separating magnetic catalyst from a suspension system by applying external magnetic field.

Fig. 3 (a) XRD pattern, (b) SEM image, and (c) magnetization curves of the Fe₃O₄/CeO₂ hollow microspheres; (d) XRD pattern, (e) SEM image, and (f) magnetization curves of the Fe₃O₄/TiO₂ mesoporous microspheres. Insert pictures show the products were easily dispersed in water (left) and easy separation by magnet (right).

The catalytic performances of the as-obtained hollow/mesoporous metal oxide microspheres were investigated by catalytic CO oxidation reaction. Fig. 4 shows the conversion of CO oxidation over the five catalysts as a function of reaction temperature, and the reaction temperatures for the 10%, 50%, 100% CO conversion. The ignition temperatures, corresponding to 10% CO conversion, for Co₃O₄, CeO₂, α-Fe₂O₃, SnO₂, and TiO₂ microspheres are 115 °C, 144 °C, 175 °C, 233 °C, and 241 °C, respectively. The Co₃O₄ hollow microspheres exhibit the highest catalytic activity. Moreover, the Co₃O₄ hollow microspheres have a half-conversion temperature of 169 °C, which is smaller than that of CeO₂ hollow microspheres (192 °C), α-Fe₂O₃ hollow microspheres (231 °C), SnO₂ mesoporous microspheres (274 °C), and TiO₂ mesoporous microspheres (276 °C), respectively. Furthermore, the reaction temperature for 100% CO conversion over the Co₃O₄, CeO₂, α-Fe₂O₃, SnO₂, and TiO₂ catalysts are 260 °C, 280 °C, 320 °C, 360 °C, and 380 °C, respectively. Over-all, the sequence of catalytic activity expressed by CO conversion is Co₃O₄ hollow microspheres > CeO₂ hollow microspheres > α-Fe₂O₃ hollow microspheres > SnO₂ mesoporous microspheres > TiO₂ mesoporous microspheres. Compared with several of the corresponding metal oxide catalysts reported previously, the present catalysts exhibit very favorable catalytic performance. For example, the reaction temperature (260 °C) for 100% CO conversion over the present Co₃O₄ hollow microspheres is lower than the previously reported ultrathin Co₃O₄ nanowires with about 3 nm diameter which achieved over 80% CO conversion at 260 °C.³⁵ The present α-Fe₂O₃ hollow microspheres also show a higher catalytic activity than those of the previously reported porous α-Fe₂O₃ nanorods with a length of ca. 300 nm and a diameter of ca. 50 nm³⁶ and the α-Fe₂O₃ thin films synthesized by a pulsed-spray evaporation chemical vapor deposition (PSE-CVD).³⁷ Moreover, the catalytic activity of the present SnO₂ mesoporous microspheres is nearly comparable to that of the 1.9 nm SnO₂ nanosheets reported previously, and higher than that of the SnO₂ nanoparticles with an average size of approximately 3 nm.³⁸ The mechanism of catalytic activity of the present hollow/mesoporous metal oxides for CO oxidation is accepted as an absorption and oxidation process of CO.^39,40 The hollow/mesoporous metal oxide microspheres can provide more surface active sites and oxygen species because their tiny crystalline grains, large pore volumes and specific surface areas, which is favorable to improve the catalytic activity. Therefore, the results strongly highlight the great potentials of these metal oxide hollow/mesoporous microspheres as catalysts in CO oxidation.

Fig. 4 The catalytic performances of the as-obtained metal oxide microspheres. (a) Catalytic CO conversion vs. reaction temperatures curves, (b) 3D histogram of the 10%, 50%, 100% CO conversion vs. reaction temperatures.

Conclusions

In summary, we demonstrated a general ultrasonic-spray-assisted synthesis of various hollow/mesoporous transition metal oxide microspheres, using the low-cost commercial metal chlorides as precursors and water as solvent. Five hollow or mesoporous metal oxides (CeO₂, α-Fe₂O₃, Co₃O₄, SnO₂ and TiO₂) were prepared typically. The microstructures and porous features were characterized. Moreover, these hollow/mesoporous metal oxides show improved catalytic activities for CO oxidation than some of those reported previously. The universality of the present method was also demonstrated in handily producing multi-component and multi-functional metal oxides with hollow or mesoporous structures. As a result, this work not only provides a facile and scalable strategy for fabricating various metal oxide hollow or mesoporous microspheres but also demonstrates that the products are of great potential to be high-performance catalysts or catalyst supports in heterogeneous catalysis.

Acknowledgements

This work is supported by the National Natural Science Foundation of China (21471004), China Postdoctoral Science Foundation of Special Funding (2015T80644), and the Excellent Youth Talents Support Plan in Colleges and Universities of Anhui Province. We thank Dr Zhenhua Sun of Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences for his help in some sample characterizations.

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