1 Introduction

Nanocatalysts have attracted wide attention because of their characteristics of more surface active site, high stability and large specific surface area. These characteristics exhibit excellent catalytic performance such as high activity and selectivity required in industrial production [1, 2]. The catalysts can be prepared by impregnation [3, 4], chemical deposition [5], sol–gel [6], chemical vapor deposition [7], hydrothermal synthesis [8], microemulsion [9], electrochemical method [10, 11], etc. Among these methods, microemulsion method, i.e., synthesis of particles with high sensitivity, well distributed, and no hard agglomeration [12], is recognized as an important way to prepare nanoparticles. Since Boutonnet [13] firstly synthesized nanometer noble metal, such as platinum, rhodium, palladium and iridium, etc., by microemulsion technique in the end of the 1980s, this technique has attracted a great deal of attention and has been widely used in environmental pollution treatment in recent years.

SO2 pollution resulted from fire coal and fossil fuel has always been an international hot point [14]. The catalytic reduction dry technology for flue gas desulfurization was paid attention by domestic and international scholars since this technology can reduce SO2 to elemental sulfur by CO with low investment and no secondary pollution [15, 16]. CeO2 is a common rare earth metal oxide with special structure and properties [17, 18], exhibiting strong catalytic action in the reduction reaction of SO2 by CO [19–22]. This oxide is also commonly used as a promoter to improve the catalytic performance of catalysts [23, 24]. Additionally, γ-Al2O3 is the most industrially used catalyst support because of its appropriate pore size distribution and large specific surface area. Currently, the research mainly focuses on nanopowder catalysts [25–27]. This type of catalyst is not suitable for the treatment of SO2 in flue gas because the tiny particle can be brought out of the flue system through gas flow before catalytic reaction.

In this study, a series of catalytic CeO2 nanomaterials (nanocatalysts) supported on globular γ-Al2O3 were successfully prepared by microemulsion method and characterized by X-ray diffraction (XRD) and scanning electron microscope (SEM) techniques. Compared with traditional impregnation method, the optimal preparation conditions by microemulsion and the desulfurization activity of catalysts were investigated.

2 Experimental

2.1 Main materials

Ce(NO3)3·6H2O (AR) was purchased from Hunan Rare Earth Metal Material Research Institute, China. Hexadecyltrimethylammonium bromide (CTAB, analytical reagent (AR)) was purchased from Shanghai Bio Science & Technology Co., Ltd, China. Globular γ-Al2O3 with size of 3–5 mm was purchased from Jiangsu Sanji Industrial Co., Ltd., China with Brunauer–Emmett–Teller (BET) surface area of S BET = 280 m2·g−1.

2.2 Catalyst preparation

2.2.1 Microemulsion

A certain volume of Ce(NO3)3·6H2O (1 mol·L−1, AR) was dropped into a certain proportion of cyclohexane/CTAB/butanol system to form colorless and transparent microemulsion. This microemulsion then gradually became canary yellow after adding NH3·H2O (AR) under constant temperature magnetic stirring for 15 min. Next, pretreated γ-Al2O3 supports were added into the well-prepared microemulsion under continuous stirring. 3 h later, γ-Al2O3-supported CeO2 was obtained after demulsification for the above mixing solution by acetone (AR) and suction filtration. After being washed repeatedly by anhydrous alcohol and distilled water, dried at 70 °C for 24 h and calcinated at 500 °C for 4 h, the pale yellow product was obtained.

2.2.2 Impregnation

According to the different loads of CeO2/γ-Al2O3 catalyst, the corresponding stoichiometric ratio of Ce(NO3)3·6H2O was dissolved in distilled water. Thereafter, pretreated γ-Al2O3 supports were added into the above solution for impregnation at 25 °C for 24 h and 70 °C for 7 days. Finally, the products were dried in an oven at 100 °C and calcinated in muffle furnace by temperature programming.

2.3 Catalyst characterization

XRD patterns were obtained by X’Pert Pro X diffractometer (PANalytical B.V. Co.) operated with Cu Kα radiation (40 mA and 40 kV) over 2θ range of 8°–90° with step size of 0.2 (°)·s−1. SEM images were recorded using Quanta 200 electron microscope (FEI Co., Holland) with an accelerating voltage of 200 kV.

2.4 Catalyst activity evaluation

In the catalytic reduction reaction of SO2 by CO (\({\text{CO }} + {\text{ SO}}_{{\text{2}}}\xrightarrow{{{\text{catalyst}}}}{\text{CO}}_{2} + {\text{S}}\)), catalyst activity of CeO2/γ-Al2O3 catalysts with different loads was investigated. The reaction device mainly consisted of temperature control system, fixed bed reactor, gas concentration detection system and mixed gas chamber. The temperature was controlled by thermocouple in the tubular furnace, the reactor was made of quartz material with height of 80 cm and diameter of 2.5 cm, and the analysis of the ingoing and outgoing stream was performed using non-dispersive infrared analyzer (SHIMADZU Co., Japan) with SOA-7000 for SO2 detection and CGT-7000 for CO2 and CO detection.

Before the experiment, nitrogen should be purged into the whole gas circuit for 30 min, and then reacted gases (10 vol% SO2 + 25 vol% CO + 65 vol% N2) with a flow rate at 1 L·min−1 were passed in the reaction device. All the runs were performed using 15 g catalyst. The conversion (X) of SO2 (X can also denote desulfurization efficiency of catalysts) was calculated by the following equation:

$$X\left( {{\text{SO}}_{ 2} } \right) = \left( {\left( {\left[ {{\text{SO}}_{ 2} } \right]_{\text{in}} - \left[ {{\text{SO}}_{ 2} } \right]_{\text{out}} } \right)/\left[ {{\text{SO}}_{ 2} } \right]_{\text{in}} } \right) \times 100\,\%$$
(1)

where [SO2]in is the inlet SO2 concentration and [SO2]out is the outlet SO2 concentration.

3 Results and discussion

3.1 Preparation condition of catalyst by microemulsion

3.1.1 Effect of reaction temperature

According to the method listed in Sect. 2.2.2, CeO2 particles were prepared without adding γ-Al2O3 and acetone. Figure 1 shows SEM images of CeO2 prepared at different reaction temperatures. CeO2 particles prepared at 30 °C are tiny and well distributed (Fig. 1a). This is mainly because the growth and nucleation rate of nanoparticles are affected by reaction temperature [28]. On the one hand, movement of surface active agent and its encapsulated water droplets aggravate with temperature increasing. This not only destroys microemulsion structure but also leads to the increase in particle size and distribution [28]. On the other hand, the number of reactive crystal nucleus and velocity of molecular motion improve with temperature increasing. This causes the decrease in particle size because the nucleation rate is faster than the growth rate. Under a comprehensive consideration, the optimum synthetic temperature is 30 °C.

Fig. 1
figure 1

SEM images of CeO2 prepared at different temperatures: a 20 °C, b 30 °C, and c 40 °C

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3.1.2 Effect of W value

W value is expressed as the ratio of [H2O]/[surface active agent]. The particle size of CeO2/γ-Al2O3 and radius of water droplets can be affected by W value. Figure 2 displays SEM images of CeO2/γ-Al2O3 under different W values. When W value is 3.5 or 7.0, the particle is fine with the average diameter of less than 100 nm. And the particle distribution shown in Fig. 2b (W = 7.0) is better than that in Fig. 2a (W = 3.5). Particle diameter increases when W value is 10.5 (Fig. 2c). As W value increases to 14.0, radius of water droplets increases and can be easily ruptured. This will result in uneven distribution as well as agglomeration phenomenon of particles (Fig. 2d). Therefore, the optimum W value for synthesis of CeO2/γ-Al2O3 particle is 7.0.

Fig. 2
figure 2

SEM images of CeO2/γ-Al2O3 prepared at different W values: a W = 3.5, b W = 7.0, c W = 10.5, and d W = 14.0

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3.1.3 Effect of demulsification method

The way of adding acetone (deemulsifying agent) can affect the agglomeration degree of CeO2/γ-Al2O3 particles. Figure 3 reveals SEM image of CeO2/γ-Al2O3 prepared by different demulsification methods. The methods can be classified into two types: adding acetone slowly by burette (demulsification slowly, Fig. 3b) and adding acetone for only once (demulsification rapidly, as shown in Fig. 3a). Particles prepared by the former are better dispersed and less agglomerated than the ones by the latter. The main reason is that by the latter way, CeO2 particle can detach from the encapsulation of CTAB rapidly and then agglomerate together before being absorbed onto γ-Al2O3. Thus, the optimum demulsification method is adding acetone slowly.

Fig. 3
figure 3

SEM images of CeO2/γ-Al2O3 prepared by different demulsification methods: a rapid demulsification and b slow demulsification

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3.2 Catalyst characterizations

Based on the analysis of Sect. 3.1, the optimal condition for synthesizing CeO2/γ-Al2O3 catalyst by microemulsion is temperature of 30 °C, W value of 7.0, and slow demulsification. A series of nano-CeO2/γ-Al2O3 catalysts prepared under this optimal conditions were named as 2CeO2/γ-Al2O3, 4CeO2/γ-Al2O3, 6CeO2/γ-Al2O3, 8CeO2/γ-Al2O3, and 10CeO2/γ-Al2O3 (the numbers before CeO2 denote the mass ratios in the nanoparticles).

3.2.1 XRD

XRD patterns of nano-CeO2/γ-Al2O3 catalysts are portrayed in Fig. 4. The catalysts with different proportions of CeO2 exhibit similar XRD patterns. In addition, the crystal peaks in Fig. 4 match with (111), (200), (220), and (311) crystal planes of CeO2 (JCPDS No. 03-065-5923) besides the characteristic peak of Al2O3 (JCPDS No. 00-004-0880). The results suggest that CeO2 crystal phase is contained in the prepared catalysts.

Fig. 4
figure 4

XRD patterns of different catalysts

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The grain size was calculated by Debye–Scherrer formula:

$$D = K\lambda /\beta {\cos} {\theta}$$
(2)

where D is grain size, K is shape factor, λ is wavelength, β is full width at half maximum (FWHM), and θ is diffraction angle. The results are summarized in Table 1, indicating that the catalysts prepared by microemulsion can be controlled in nanosize.

Table 1 Calculated results of grain size
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3.2.2 SEM

SEM images of 2CeO2/γ-Al2O3 catalysts prepared by impregnation and microemulsion are presented in Fig. 5. According to Fig. 5a, particles prepared by traditional impregnation method exhibit uneven distribution and serious agglomeration which can easily plug the pore of support. This significantly affects the performance of CeO2/γ-Al2O3 catalyst. Compared with those by impregnation method, the CeO2/γ-Al2O3 particles synthesized by microemulsion under the optimum condition are well distributed with less agglomeration (Fig. 5b). Additionally, the average diameter of particles occurred in Fig. 5a is obviously larger than that in Fig. 5b. The former cannot reach nanoscale, and the latter is less than 100 nm. The above difference reveals that the diameter of active substances can be effectively controlled by microemulsion method.

Fig. 5
figure 5

SEM images of 2CeO2/γ-Al2O3 catalysts prepared by different methods: a impregnation and b microemulsion

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3.3 Catalyst desulfurization activity

3.3.1 Activation temperature

To determine the optimum activation temperature, reaction temperature was controlled at 350, 450, 550, and 600 °C by gradually temperature-programmed of tubular furnace. Figure 6 shows desulfurization activation curves of 2CeO2/γ-Al2O3. This can be divided into four stages. (1) When temperature is below 400 °C, SO2 concentration decreases rapidly because of the early absorption of catalyst. CO2 has not been detected in the outlet. (2) Low concentration CO2 can be detected in the outlet as the temperature is enhanced to 450 °C. At the same time, SO2 concentration begins to increase due to desorption on the catalyst. (3) SO2 concentration decreases continually as a result of chemical adsorption effect of catalyst when temperature increases from 450 to 500 °C. (4) When temperature is up to 600 °C, elemental sulfur generates and adheres to the tube wall. Moreover, conversion rate of SO2 is steadily maintained around 68 %. This suggests that catalytic reaction has been entered into the activation stage (this stage denotes that active materials on CeO2/γ-Al2O3 react with CO and SO2). Therefore, the optimum activation temperature is 600 °C.

Fig. 6
figure 6

Desulfurization activation curves of 2CeO2/γ-Al2O3

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3.3.2 Average desulfurization rate

After nano-CeO2/γ-Al2O3 was activated at 600 °C for 2 h, operation of decreasing temperature to room temperature and then increasing temperature again were implemented. From the two stages of decreasing and increasing temperature, catalytic reduction action of nano-CeO2/γ-Al2O3 on SO2 was researched.

Figure 7 displays the conversion (X) of SO2 with temperature decreasing. Desulfurization rates of the activated 2CeO2/γ-Al2O3, 4CeO2/γ-Al2O3, 6CeO2/γ-Al2O3, 8CeO2/γ-Al2O3, and 10CeO2/γ-Al2O3 catalysts do not decrease rapidly with temperature decreasing, and basically keep in the range of ±4 %, indicating that the activity of prepared catalysts maintains stable in a certain temperature range.

Fig. 7
figure 7

Conversion of SO2 with temperature decreasing

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The conversion (X) of SO2 with temperature increasing is shown in Fig. 8. Firstly, catalytic reduction action of 6CeO2/γ-Al2O3 catalyst prepared by microemulsion on SO2 is the best. The conversion rate can reach up to 80 % when temperature is higher than 550 °C. Moreover, in the load range from 2 % to 6 %, desulfurization efficiency of catalysts prepared by microemulsion improves with the increase in load amount at corresponding temperature. However, while the load amount increases to 8 %, desulfurization efficiency decreases because the increase in active materials amount on the catalyst causes the uneven distribution as well as agglomeration phenomenon of active constituents.

Fig. 8
figure 8

Conversion of SO2 with temperature increasing by microemulsion method (A) and impregnation method (B)

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Secondly, at the same temperature, desulfurization efficiency of catalysts with the same load prepared by microemulsion is obviously higher than that by the traditional impregnation method. This further demonstrates the effect of the microemulsion method.

3.3.3 Mechanism analysis

There are generally two kinds of mechanism models, i.e., (1) redox mechanism [29]: reduction of SO2 by CO to elemental sulfur and CO2 can be completed over oxygen vacancies on the surface of catalyst, which is promoted by oxygen transfer; (2) intermediate product mechanism [30]: metal active substance of catalyst can be activated to metal sulfides (active phase) and then react with CO to form COS with better reducibility, which finally reduces SO2 to elemental sulfur.

Figure 9 shows XRD patterns of 6CeO2/γ-Al2O3 prepared by microemulsion. By comparing the wave peaks of 6CeO2/γ-Al2O3 before and after desulfurization reaction in Fig. 9, it can be seen that only CeO2 instead of sulfide is observed before and after reaction. Furthermore, abundant lattice oxygen is considered to be contained in the structure of CeO2 [29]. Therefore, catalytic reduction mechanism of SO2 over nano-CeO2/γ-Al2O3 follows redox mechanism. These reactions are inferred as follows:

$$2 {\text{CeO}}_{ 2} \to {\text{Ce}}_{ 2} {\text{O}}_{ 3} \square {\text{ + O}}$$
(3)
$${\text{CO* + O}} \to {\text{CO}}_{ 2}$$
(4)
$${\text{CO}}_{ 2} *\to {\text{CO}}_{ 2} { + *}$$
(5)
$${\text{SO}}_{ 2} * *\to {\text{SO** + O}}$$
(6)
$${\text{SO**}} \to {\text{S + O + **}}$$
(7)
$${\text{Ce}}_{ 2} {\text{O}}_{ 3} \square {\text{ + O}} \to 2 {\text{CeO}}_{ 2}$$
(8)

where * and ** are active adsorption sites of CO and SO2 in CeO2, respectively, O is lattice oxygen, and □ is vacancy of lattice oxygen.

Fig. 9
figure 9

XRD patterns of 6CeO2/γ-Al2O3 before desulfurization reaction and after desulfurization reaction

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4 Conclusion

A series of γ-Al2O3-supported CeO2 nanocatalysts were successfully prepared by microemulsion method, employed for the removal of SO2 and exhibiting high activity in the reaction compared with traditional impregnation method. The desulfurization rate of these nanocatalysts maintains stable with the temperature decreasing from 600 to 450 °C. Moreover, with the temperature increasing from 350 to 600 °C, the desulfurization rate of 6CeO2/γ-Al2O3 catalyst is the best, reaching up to 80 % when temperature is higher than 550 °C. The catalytic reduction mechanism of SO2 over nano-CeO2/γ-Al2O3 follows redox mechanism. From the facts mentioned above, these nano-CeO2/γ-Al2O3 catalysts prepared by microemulsion method may find potential applications in the catalytic reduction of SO2.