Adsorption performance of SO2 over ZnAl2O4 nanospheres

Journal of Industrial and Engineering Chemistry 41 (2016) 151–157

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Adsorption performance of SO2 over ZnAl2O4 nanospheres Ling Zhao a,*, Sining Bi a, Jiasi Pei a, Xinyong Li b, Ruihong Yu a,*, Ji Zhao a, Christopher J. Martyniuk c a

College of Environment & Resources, Inner Mongolia University, China School of Environmental Science & Technology, Dalian University of Technology, China c College of Veterinary Medicine, University of Florida, United States b

A R T I C L E I N F O

Article history: Received 13 December 2015 Received in revised form 21 April 2016 Accepted 14 July 2016 Available online 25 July 2016 Keywords: ZnAl2O4 SO2 In situ FTIR Surface properties

A B S T R A C T

A type of uniform ZnAl2O4 nanospheres was selectively synthesized via a facile solvothermal method and their size was controlled to be 320–450 nm in diameter. It exhibits excellent SO2 adsorption capacity. Both physical structure and surface basicity were determined to play important roles in SO2 adsorption process. In situ FTIR investigation revealed that adsorbed SO2 molecules formed surface bisulfite, sulfite, and bidentate binuclear sulfate. The CO2-TPD results revealed the SO2 adsorption capacity of the catalysts correlated closely with their basicity sites. The mechanisms for SO2 adsorption and transformation have been discussed in detail. ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

Introduction SO2 emissions from coal-fired power plants have caused significant environmental and human health effects. Stringent environmental regulations limiting atmospheric SO2 emissions encourage the research of more efficient ways to reduce them. From an economical point of view, the better strategy for the control of SO2 emissions from coal-fired power plants is the addition of selective sorbents [1,2]. Mixed oxides have been reported as a good adsorbent for SO2. Based on this concept, variety of sorbent systems are currently either in use or under laboratory investigation for the removal of the SO2 from flue gases [3]. Many studies were devoted to the development of supported metal oxide sorbents that are not only capable of an efficient SO2 removal but also exhibit a large surface area and high textural stability; this has led to the development of alternative sorbent products in terms of oxides, textures and supports [3]. Zinc aluminate, ZnAl2O4, is a rare mineral that belongs to the spinel group having a normal spinel structure. ZnAl2O4 is of particular interest because it presents a unique combination of properties such as high mechanical strength, high thermal and chemical stability, low sintering temperature, high quantum yields and excellent optical properties [4,5]. It is currently being used as

* Corresponding authors. Tel.: +86 471 4991436; fax: +86 471 4991436. E-mail addresses: [email protected] (L. Zhao), [email protected] (R. Yu).

optical material [6], high temperature material [7], catalysts and catalyst support [8]. In recent years, the studies detailing the use of ZnAl2O4 as catalysts or adsorbent materials focus on many topics such as toxic metal removal from water [9,10] and use of ZnAl2O4 as dielectric substance [5]. However, papers that have examined the use of ZnAl2O4 as gas phase adsorbent materials especially for SO2 abatement are limited. The catalytic performance of metal-oxide catalysts depends on their nature, size, shape, and surface area. Their relationship is a critical factor in determining catalytic activity and selectivity [11]. Preparation approaches to catalysts play a critical role in their activity and stability, especially the chemical precursors are often utilized for the preparation of high purity materials [12]. The advanced metal oxide catalysts can be often tailor-made to control the catalytic activity of their surface [11,13]. Recently, several methods have been employed for the synthesis of ZnAl2O4, such as hydrothermal synthesis [14], sol–gel [15], combustion synthesis [9,10] and co-precipitation method [16]. In the present study, we synthesize two types of ZnAl2O4 through different approaches. Their structure and surface are well characterized. In situ Fourier transform infrared spectroscopy (In situ FTIR) study was conducted to obtain more information about ZnAl2O4 surface after its interaction with sulfur dioxide. The differences in their sulfur dioxide sorption capacity are discussed in terms of the structure feature and surface chemistry, which have a significant effect on their sorption capacity.

http://dx.doi.org/10.1016/j.jiec.2016.07.019 1226-086X/ß 2016 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.

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L. Zhao et al. / Journal of Industrial and Engineering Chemistry 41 (2016) 151–157

Experimental Materials All chemicals in this work were analytical grade reagents and used as starting materials without further purification. The ZnAl2O4 nanospheres were obtained via a facile solutionphase synthetic route. Al(NO3)2
9H2O (6 mmol) and Zn(NO3)2
6H2O (3 mmol) were dissolved into ethylene glycol (90 ml). Then a protective agent NH4Ac (45 mmol) was added into the solution under vigorous stirring. Subsequently, the mixture was put into a Teflonlined stainless steel autoclave of 120 ml capacity and sealed and maintained at 180 8C for 24 h. Finally, the system was allowed to cool at room temperature naturally. The obtained black precipitate was collected by filtration, washed with absolute ethanol and distilled water in sequence for several times, and dried in a vacuum drying box at 80 8C overnight. The final product was calcined at 600 8C for 4 h, and labeled as ZnAl2O4 nanospheres. As a comparison, another type of ZnAl2O4 was prepared by the coprecipitation method as follows. An aqueous solution (100 ml) containing Zn(NO3)2
6H2O (0.05 mol) and Al(NO3)2
9H2O (0.1 mol). The mixed metal salt solution and the NaOH (6 mol/l) solution were simultaneously added to a glass reactor and mixed under vigorous mechanical stirring, with the pH maintained between 9 and 10. Then, the mixture was heated at 90 8C for 18 h. The precipitate was washed several times with absolute ethanol and deionized water and dried at 80 8C overnight. The final product was calcined at 600 8C for 4 h, and labeled as ZnAl2O4-CP. Techniques of characterization The obtained catalysts were characterized on a Rigaku D/Max 2550VB/PC X-ray powder diffractometer (XRD) with a Cu Ka radiation source (k = 0.154056 nm), and operated at a voltage of 40 kV and a current of 100 mA. Scanning electron microscopy (SEM) was performed on a JEOLJSM-6360LV microscope. The surface area was determined by Brunauer-Emmet-Teller (BET) N2 gas adsorption-desorption isotherms obtained at 77 K on a Micromeritics ASAP-2000 equipment. Pore size distributions were calculated from the desorption isotherms by the BJH model. The data of Fourier transform infrared spectroscopy (FTIR) were recorded on a BRUKER VERTEX 70 spectrometer from 400 to 4000 cm 1 at room temperature on KBr mulls. X-ray photoelectron spectroscopy (XPS) data were recorded using a PerkinElmer PHI 5600 electron spectrometer using acrochromatic Al Ka radiation (1486.6 eV) with Ar+ sputtering to remove the surface layer of the sample. The CO2-TPD experiments were carried out on a catalyst of 0.2 g under Ar (45 cm3/min). Prior to CO2 adsorption at 473 K, all samples were pretreated at 773 K for 1 h in a flow of Ar. Once the physically adsorbed CO2 was purged off, the CO2-TPD experiments were started from 303 K to 1173 K with a heating rate of 10 K/min under Ar flow (50 ml/min). A mass spectrometer (Hiden HPR20) was used for on-line monitoring of the CO2-TPD effluent gas. The SO2-TPD experiments were carried out on a catalyst of 0.2 g using a mass spectrometer (Hiden HPR20) for on-line monitoring of the SO2-TPD effluent gas. Prior to TPD procedure, the samples were pretreated at 773 K for 1 h in a flow of Ar and cooled down to room temperature. The samples were then exposed to a flow of 200 ppm SO2 + 5 vol.% O2/Ar (50 ml/min) for 1 h, followed by Ar purge for another 1 h. Finally, the temperature was raised to 1173 K in Ar flow (50 ml/min) at the rate of 10 K/min.

the mole balance of SO2 gas in a closed system. UHP grade SO2 gas was used for the measurement. For each test, the amount of the ZnAl2O4 adsorbent was 0.1 g. In situ FTIR experiment FTIR spectra were recorded on a VERTEX 70-FTIR equipped with a smart collector. The catalyst (approximately 20 mg) was prepared by pressing it into a pellet and then secured inside the infrared cell, which consists of two KBr windows and a sample holder for the catalyst pellet. The infrared cell is connected to a vacuum chamber through a Teflon tube and a glass gas manifold with ports for gas introduction and a vacuometer. Prior to each experiment, the catalyst was first heated for 60 min at 673 K, and then cooled to the desired temperature. The background spectrum was recorded with the flowing of O2 + N2 and was subtracted from the catalyst spectrum. All spectra were measured with a resolution of 4 cm 1 and with an accumulation of 100 scans. All the measurements were repeated at least twice.

Results and discussion Characterizations The XRD patterns of the fresh ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts are illustrated in Fig. 1. Both samples exhibit the characteristic peaks at 2u of 31.28, 36.88, 44.78, 55.68, 59.38 and 65.28, which correspond to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) crystallographic nucleation planes of ZnAl2O4 spinel phase (JCPDS 82-1043), respectively [18]. The XRD pattern shows that the product is single-phase and that the only peaks detected were the characteristic peaks of the cubic phase ZnAl2O4. These results confirm that the ZnAl2O4 catalysts obtained in our work are of very high purity. The average particle size was calculated by X ray diffraction line broadening using the Debye-Scherrer equation [19], d = (0.9l)/(h1/2cos u), where d is the grain size; l is the wavelength of the X-ray (Cu Ka, 0.15418 nm); u is the diffraction angle of the peak; and h1/2 is the full-width at half-height of the peaks. The average particle sizes calculated using the most intense peak (3 1 1) are estimated as 9.9 and 14.6 nm for ZnAl2O4 nanospheres and ZnAl2O4-CP, respectively (see Table 1). Therefore, it is concluded that the ZnAl2O4 nanospheres through a facile solvothermal method could possess better crystallinity and smaller crystallite size than ZnAl2O4-CP sample synthesized by the [(Fig._1)TD$IG] coprecipitation method.

Activity test The adsorption equilibrium isotherms of SO2 were analyzed using a volumetric method at 298 K [17]. The method is based on

Fig. 1. XRD patterns of the as-synthesized ZnAl2O4 nanospheres and ZnAl2O4-CP samples. (a) ZnAl2O4-CP sample; (b) ZnAl2O4 nanospheres.

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Table 1 Structural properties of the ZnAl2O4 nanospheres and the ZnAl2O4-CP samples. Sample

Zn/Al atomic ratio

ZnAl2O4-CP ZnAl2O4 nanospheres a b c d

[(Fig._2)TD$IG]

e

In solution

In solid

1:2 1:2

1:1.91 1:1.94

Crystalline sizeb (nm)

SBETc (m2/g)

Total volumed (cm3/g)

Mean pore sizee (nm)

14.6 9.9

56.4 136.3

0.06 0.36

6.8 22.5

a

Calculated from ICP analysis. Calculated by the Scherrer equation. BET surface area calculated from the linear part of the BET plot. Single-point total pore volume of pores at P/P0 = 0.99. Estimated using the desorption branch of the isotherm.

Fig. 2. SEM images of the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts. (a) ZnAl2O4-CP sample; (b) ZnAl2O4 nanospheres.

The Zn/Al atomic ratios in the catalysts are calculated from ICP results, as listed in Table 1. For these samples, no obvious differences were found for Zn/Al atomic ratios in the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts before and after calcination, respectively, suggesting that ZnAl2O4 spinel in our work can be successfully prepared. This is in good accordance with the XRD results of the corresponding precursors (see Fig. 1). The texture of ZnAl2O4-CP and ZnAl2O4 nanospheres catalysts can be observed via SEM images presented in Fig. 2. It can be seen that the ZnAl2O4-CP (see Fig. 2a) consists of microparticles with irregular morphology, and the particle size varies in a wide range which increased from 500 nm to 5 mm. This is similar to the results discussed in previous studies [20]. By contrast, the ZnAl2O4 nanospheres consist of nanoparticles exhibiting a spherical morphology with the diameters ranging from 320 nm to 450 nm (Fig. 2b). Moreover, the surface of ZnAl2O4 nanospheres are much more smooth than that for the ZnAl2O4-CP catalyst. The origin of the morphological difference between the two types of samples most likely lies in their respective preparation method. The textural parameters of the catalysts, such as BET specific surface area and pore volume are reported in Table 1. It is important to note the ZnAl2O4 nanospheres possess high specific surface area and total pore volume (136.3 m2/g and 0.36 cm3/g), while the ZnAl2O4-CP sample shows only 56.4 m2/g of specific surface area and 0.06 cm3/g of total pore volume. It is well known that the specific surface area could vary depending upon the crystallite size and shape. Therefore, the low specific surface area of ZnAl2O4-CP could be explained according to its larger and irregular crystal size, as observed in SEM analysis (see Fig. 2a). The ZnAl2O4 nanospheres have higher specific surface area and total pore volume, which is important in catalytic materials determining the accessibility of reactant molecules. Moreover, the average pore diameter of ZnAl2O4-CP and ZnAl2O4 nanosphere are about 22.5 and 6.8 nm, respectively. The FTIR spectra of ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts are shown in Fig. 3. The intensive peaks at 3450 and

1640 cm 1 could be attributed to the v (OH) stretching and d (OH) bending vibrations of hydroxyl groups. The peaks at 675 cm 1 are assigned to the vibrations of M–O (M = Zn and Al). Table 2 displays the XPS results of the two samples. The peaks around 1047 and 1023 eV could be attributed to Zn 2p1/2 and Zn 2p3/2, which show that Zn is in the form of Zn2+. The binding energy values of Al 2p and Al 2s at 79 and 123 eV, respectively, indicate that Al is in the form of Al3+ (Table 2). The O 1s spectra (Fig. 4) are fitted with two peak contributions. The peaks around 530.5 eV are due to the surface lattice oxygen of metal oxides (O2 ). The peaks around 532.0 eV belong most likely to the adsorbed oxygen or the surface hydroxyl species [21]. Some researchers have reported that O2 and surface hydroxyl are active oxygen species and play critical roles in oxidation reaction [22]. It is seen clearly that the

[(Fig._3)TD$IG]

Fig. 3. FTIR spectra of the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts. (a) ZnAl2O4-CP sample; (b) ZnAl2O4 nanospheres.

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Table 2 The XPS analysis results of the ZnAl2O4 nanospheres and the ZnAl2O4-CP samples. Sample

Bindingenergy (eV)

Surface atomic ratio Zn:Al:O

Zn 2p3/2

Zn 2p1/2

Al 2p

Al 2s

O 1s

C 1s

ZnAl2O4-CP ZnAl2O4 nanospheres

1047.1 1047.8

1022.9 1023.5

78.1 79.2

123.7 123.1

534.2 535.7

285 285

1:2.06:3.97 1:2.14:3.81

[(Fig._5)TD$IG] surface lattice oxygen (O2 ) on ZnAl2O4 nanospheres surface calculated from the peak area is higher than that on ZnAl2O4-CP. The results demonstrate that the structure of nanospheres could improve the ZnAl2O4 catalyst to promote more surface lattice oxygen species, thus could influence the catalytic activity for SO2 abatement. No contaminant species were observed within the sensitivity of the technique. Only adsorbed carbon peak with C1s at 285 eV on the surface was present in the samples. Table 2 gives the results from XPS analysis indicating the atomic ratio of elements Zn/Al/O = 1:2.14:3.81 (for ZnAl2O4 nanospheres) and 1:2.06:3.97 (for ZnAl2O4-CP), and they are in reasonable agreement with the percentages deduced from the molecular formula of ZnAl2O4 catalysts (Zn/Al/O = 1:2:4). These results are consistent with the previous literature data [23]. Adsorption of SO2 To evaluate the ability of the catalysts to adsorb sulfur dioxide the adsorption isotherms experiments were conducted. Fig. 5 presents the adsorption isotherms of SO2 at 298 K onto the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts. Both of the isotherms are of type I isotherm, which means the adsorption capacity is increased at lower P/P0 region and have an adsorption plateau at higher pressures. This plateau is generally associated with saturation. Meantime, significant difference was observed. It is seen that the adsorption capacity of ZnAl2O4 nanospheres (1.32 mmol/g) is remarkably higher than that of the ZnAl2O4-CP sample (0.56 mmol/g). This suggests that the ZnAl2O4 nanospheres should be a better adsorbent for SO2 abatement. Surface basicity Temperature-programmed desorption of CO2 (CO2-TPD) was carried out to investigate the basic properties of catalysts. The CO2TPD profiles of ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts before and after reaction with SO2 are given in Fig. 6. Though it is

Fig. 5. Adsorption equilibrium isotherms of SO2 in the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts at 298 K.

hard to define a clear boundary between different desorption regions, the CO2 desorption profile of each sample can be divided into three regions, indicating three types of basic sites with a certain amount of basicity on the surface of samples. Region I from 380 to 450 K is weak basic sites, region II in the temperature range of 850–920 K, corresponding to the moderately strong basic sites, and region III in the range of 920–1100 K, which is attributed to the strong basic sites. For metal oxides, the weak basic sites were associated with the OH groups, the moderate basic sites were related to the M–O pairs and the strong basic sites were ascribed to the coordinative unsaturated O2 ions [24].

[(Fig._6)TD$IG]

[(Fig._4)TD$IG]

Fig. 4. XPS spectra of O1s on the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts.

Fig. 6. CO2-TPD profiles of the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts before and after reaction with SO2. (Curves a and b denote CO2-TPD profiles of the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts before reaction with SO2, respectively; curves c and d denote CO2-TPD profiles of the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts after reaction with SO2, respectively.).

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As can be seen from the Fig. 6, a large amount of CO2 evolution centered at 850 and 970 K took place on ZnAl2O4 nanospheres (a curve), while a trace amount was observed on ZnAl2O4-CP catalyst (b curve). This contrast strongly suggests that ZnAl2O4 nanospheres have more total amounts of the moderate and strong basic sites than ZnAl2O4-CP catalyst. Furthermore, the amounts of basic sites of the two samples were remarkably decreased after reaction with SO2 + O2 (curves of c and d). The consumption of surface basic sites means that the reaction between SO2 and surface basic sites must have occurred. A number of different groups have investigated that strong SO2 adsorption could occur at basic sites [25,26] Karge et al. determined that the interaction of SO2 with basic sites on the surface of g-Al2O3 led to the formation of chemisorbed SO2 [25]. Pacchioni et al observed that MgO(1 0 0), which had more basic sites, may provide favorable sites for sulphate formation. It was suggested that sulfite could form by interaction of the sulfur atom in SO2 with two surface fivecoordinated O2 anions [26]. For the ZnAl2O4 catalysts, the sites for SO2 adsorption are expected to associate with OH , Zn–O, Al–O, or O2 . TPD analysis The SO2-TPD experiments for ZnAl2O4 nanospheres and ZnAl2O4-CP catalyst are illustrated in Fig. 7 after exposure to 200 ppm SO2 + 5 vol.% O2 in He for 1 h at 298 K. The SO2 (m/z = 64) signal was monitored. In order to understand the surface species better, the TPD graph was fitted using Lorenz and Gaussian curves [27] as shown in Fig. 7 with a correlation coefficient of 0.991 (for ZnAl2O4-CP) and 0.993 (for ZnAl2O4 nanospheres). There are four distinct peaks centered around 480, 620, 910 and 1000 K for ZnAl2O4-CP (465, 635, 970 and 1150 K for ZnAl2O4 nanospheres), respectively. The peaks around 460–480 K were assigned to the desorption of molecular SO2 from ZnAl2O4 surface [28]. It is seen clearly that the adsorbed SO2 on ZnAl2O4 nanospheres surface is greatly higher than that on ZnAl2O4-CP catalyst on the basis of the peak area. Moreover, two overlapping SO2 desorption features were observed between 900 and 1150 K. According to the previous literature [29], the higher temperature peak centered around 1000–1150 K should be derived from the thermally stable sulfate compounds formed on the Zn–O site, while the lower temperature peak (around 900 K) should be attributed to SO2 decomposed from the sulfate species formed on the Al–O site. The TPD results provide clear evidence for the formation of sulfate species by the reaction of ZnAl2O4 catalyst with SO2 + O2. It is seen clearly that the sulfate species on ZnAl2O4 nanospheres surface is greatly higher than that on ZnAl2O4-CP catalyst, which demonstrates that the ZnAl2O4

[(Fig._7)TD$IG]

155

nanospheres exhibit more favorable for the conversion of SO2 to be SO42 as compared to ZnAl2O4-CP catalyst. In addition, on the basis of previous studies [30], we suggest that the peaks centered at 630 K are associated with the decomposition of surface sulfite. These TPD results are also the proofs for the adsorption isotherms of SO2 analysis reported previously (Fig. 5). In situ FTIR spectra of SO2 adsorption on the ZnAl2O4 samples To investigate the interaction between SO2 + O2 and these catalysts, in situ FTIR experiments were conducted to figure out the adsorption pathways and to analyze the nature of the sulfate species on the catalysts. Each sample was exposed to a flow of SO2/ O2/N2 (200 ppm SO2, 5%O2, and N2 as balance) at 298 K for 80 min respectively, and then the evacuation was carried out. As shown in Fig. 8, FTIR peaks resulting for sulfur dioxide adsorption can be identified at 1201, 1092, 1046, 960 cm 1 (for ZnAl2O4-CP), and 1223, 1156, 1039, 951 cm 1 (for ZnAl2O4 nanospheres), and the intensity increased with time until the surface is saturated. All of these peaks are also assigned to chemisorbed SO2 since they did not disappear after evacuation was carried out. The bands at 1046, 1039, 960 and 951 cm 1 could be assigned to the stretching motion of surface-coordinated bisulfite and/or sulfite. The results agree well with other infrared studies reported previously [31–36]. Schoonheydt and Lunsford [31] reported surface sulfite absorption bands at 975 and 1040 cm 1 following reaction of SO2 on MgO at room temperature. John et al. [32] and Goodman et al. [33] investigated SO2 adsorption on g-Al2O3 particles and found that the broad band centered at 1060 cm 1 was assigned to strongly adsorbed SO2 identified as a sulfite species. Additionally, according to previous studies [34,35], the bands at 1201, 1092 cm 1 (for ZnAl2O4-CP), and 1223, 1156 cm 1 (for ZnAl2O4 nanospheres) match well with the stretching motion of adsorbed sulfate on the surface of the samples. The relationship between the symmetry of sulfate complexes and their infrared spectra has been well established [36]. According to the theory, there were two infrared sulfate vibrations that were accessible to FTIR investigation, which were the nondegenerate symmetric stretching n1 band and the triply degenerate asymmetric stretching n3 band. When the bidentate sulfate complex was formed on the surface of the sample, the n3 band would split into several bands between 1250 and 1050 cm 1. More specifically, the bidentate sulfate could still be divided into the bidentate binuclear sulfate and the bidentate mononuclear sulfate. In the case of a bidentate binuclear sulfate, the typical split n3 bands were between 1050 and 1250 cm 1. If the bidentate mononuclear sulfate was formed, the bands would shift to higher wavenumbers [36]. Therefore, we propose that

Fig. 7. SO2-TPD profiles of the ZnAl2O4 nanospheres and ZnAl2O4-CP catalysts. (a) ZnAl2O4-CP; (b) ZnAl2O4 nanospheres.

[(Fig._8)TD$IG]

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Fig. 8. Dynamic changes of in situ FTIR spectra for ZnAl2O4 at 298 K after reation with SO2+O2. (a) ZnAl2O4-CP; (b) ZnAl2O4 nanospheres.

bidentate binuclear sulfate was the main production of SO2 adsorption on the ZnAl2O4 samples. For ZnAl2O4-CP sample shown in Fig. 8a, the intensity ratio of 960 and 1046 cm 1 is even stronger than that of 1092 and 1201 cm 1 in terms of peak area, which implies a larger quantity of surface sulfite than sulfate on ZnAl2O4CP catalyst. Meanwhile, by comparing Fig. 8b with Fig. 8a, it should be noted that the total amount of surface SO32 , HSO3 and SO42 species of sample ZnAl2O4 nanospheres is remarkably larger than that of ZnAl2O4-CP sample based on the peak area. The result can be very well correlated with the adsorption isotherms of SO2 analysis (see Fig. 5). The possible mechanism of the reactions of SO2 on the ZnAl2O4 samples The adsorption process of SO2 is the cooperative physisorption and chemisorption. The physisorption capacity is determined by the surface area and number of pores on the adsorbent substrate. As shown in Table 2, the ZnAl2O4 nanospheres have relatively high surface areas and they are rich in pores. The results are similar to that of other catalysts as proposed in the literature [18]. The chemisorption capacity is related with the oxides surface basicity sites. This is quite understandable because these basic sites are involved as the reactive sites in this reaction. Pacchioni et al. [26] conducted a theoretical study of the adsorption and reaction of SO2 on clean, completely dehydroxylated MgO surfaces. It was suggested that sulfite could be formed by interaction of the sulphur atom in SO2 with two surface five-coordinated O2 anions. Chen et al. [37] observed that the formation of the surface

[(Fig._9)TD$IG]

coordinated sulfite could be due to the interaction of SO2 with fourcoordinated O2 anions while MgO particles reacted with SO2. For ZnAl2O4 samples, the sites for SO2 adsorption are expected to associate with O2 –Zn2+ (or Al3+) sites. During SO2 sorption, the acidic SO2 reacts with basic O2 sites depending on their coordination (Fig. 9). Oxygen atoms located at edges and corners of the crystal planes which have stronger basicity than oxygen atoms in basal planes, are more important for SO2 adsorption. Thus, we propose the following possible mechanism of the reaction of SO2 on ZnAl2O4 catalysts. At first SO2 could be weakly adsorbed on the surface of the ZnAl2O4 catalysts, while SO2 could interact with the surface hydroxides or O2 anions on the surface of the ZnAl2O4 catalysts, forming the surface-coordinated SO32 / HSO3 and SO42 . In situ FTIR results have provided the evidence for the formation of adsorbed SO32 /HSO3 and SO42 (Fig. 8). When the O2 anions are consumed, O2 in the gas-phase can supplement it so that the oxidation reaction can continue until the surface is fully covered by surface SO32 , HSO3 and SO42 species.

Conclusions A type of uniform ZnAl2O4 nanospheres with excellent SO2 adsorption capacity could be selectively synthesized via a facile solvothermal method. Compared with the ZnAl2O4 prepared by the coprecipitation method, ZnAl2O4 nanospheres are controllable to be 320–450 nm in diameter and have observably higher surface area and pore volume. These results suggest that the ZnAl2O4 nanospheres should be a better adsorbent for SO2 abatement. The CO2-TPD results revealed the SO2 adsorption capacity of the catalysts correlated closely with their basicity sites. The stronger basicity, the better adsorption performance of SO2 the catalyst exhibits. In situ FTIR revealed that surface bisulfite, sulfite and bidentate binuclear sulfate were formed on ZnAl2O4 catalysts as the products of reaction with SO2 + O2. This work provides a fundamental understanding on the mechanism of SO2 catalytic oxidation over ZnAl2O4 catalysts. Further studies to explore the performances for SO2 removal by catalytic oxidation under real environmental conditions are under progress. Acknowledgements

Fig. 9. The basic sites are important for SO2 adsorption.

This study is supported by the National Natural Science Foundation of China (Nos. 21347001, 21567018), Inner Mongolia Natural Science Foundation (No. 2013MS0203), Research Fund for

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