ARTICLE pubs.acs.org/est
Effects of Surface Features on Sulfur Dioxide Adsorption on Calcined NiAl Hydrotalcite-like Compounds Ling Zhao,† Xinyong Li,*,†,‡ Xie Quan,† and Guohua Chen*,‡ †
Key Laboratory of Industrial Ecology and Environmental Engineering (MOE) and State Key Laboratory of Fine Chemical, School of Environmental Science and Technology, Dalian University of Technology, Dalian, 116024, China ‡ Department of Chemical and Biomolecular Engineering, Hong Kong University of Science & Technology, Clear Water Bay, Kowloon, Hong Kong
bS Supporting Information ABSTRACT: The hydrotalcite-based NiAl mixed oxides were synthesized by coprecipitation and urea hydrolysis approaches and employed for SO2 removal. The samples were well characterized by inductively coupled plasma (ICP) elemental analysis, X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and N2 adsorption/desorption isotherm analyses. The acidbase properties were characterized by pyridine chemisorption and CO2 temperature-programmed desorption (TPD). The calcined NiAlO from the urea method showed excellent SO2 adsorption and its adsorption equilibrium showed a type I isotherm, which significantly improved the adsorption performance for low-concentration SO2. Both the physical structure and the acidicbasic sites were found to play important roles in the SO2 adsorption process. In situ Fourier transform infrared spectroscopy (FTIR) investigation revealed that adsorbed SO2 molecules formed surface bisulfite, sulfite, and bidentate binuclear sulfate. The mechanisms for SO2 adsorption and transformation are discussed in detail.
’ INTRODUCTION SO2 is one of the most dangerous atmospheric pollutants since it contributes directly to acid rain formation and the destruction of the ozone layer. Thus, stringent environmental regulations limiting atmospheric SO2 emissions encourage research on more efficient ways to reduce them. Many new strategies have been proposed.14 However, the most popular and inexpensive method for SO2 removal is the addition of selective sorbents with the fuel.5 The sorbents used frequently mainly include activated carbons,6 activated carbon fibers,7 and fly ash.8 SO2 could be retained via physical adsorption in their small pores. Although these sorbents have a high capacity for SO2 adsorption, their heats of physical adsorption, which can be linked to the strength of the retention process, are usually low. Thus, reactive adsorption of SO2 has become a current research target, which refers to the adsorption enhanced by chemical reactions on the surface of the adsorbents. These reactions may involve reactions of SO2 with the adsorbent surface and/or with oxygen and/or with species introduced to that surface to impose such reactions. Moreover, due to the stronger acid character of SO2, basic oxides have been proposed to trap these molecules. It is important to search for a versatile and efficient adsorbent for SO2 retention or transformation working at both room temperature and higher temperature. In recent years, basic mixed oxides obtained from hydrotalcitelike (HT) compounds exhibit high efficient SO2 activities to reduce them due to their acidbase character and high surface r 2011 American Chemical Society
area.9,10 Hydrotalcite-like compounds belong to a large class of natural and synthetic anionic clays that have received much attention in the past decades.11,12 The general chemical formula of HTs can be expressed as [M1x2þ Mx3þ(OH)2]xþ[Ax/n n 3 nH2O]x, where M2þ is a divalent cation (Mg2þ, Ni2þ, Zn2þ, Co2þ, Fe2þ, ...) and M3þ is a trivalent cation (Al3þ, Fe3þ, Cr3þ, ...). The positive charge of the layers is caused by the inclusion of M3þ cations in a neutral layer of M2þ cations linking by hydroxyl groups. This charge is balanced by A anions with charge n, for instance, CO32, SO42, Cl, OH, or NO3, among others. Calcination of HTs is an alternative to traditional chemical and physical methods for the fabrication of a wide variety of mixed metal oxide nanocomposite materials composed of mainly metal oxides phase and sometimes the corresponding spinel-like phase. The calcination progressively induces dehydration, dehydroxylation, and loss of compensating anions and leads to acidic and basic mixed oxides with a high surface area. The mixed oxides have been shown to be a better adsorbent for SO2. Thus, chemical reactions of SO2 with HT surfaces are of great interest. Sanchez-Cantu et al.12 synthesized a series of binary and ternary HT materials containing copper, nickel, zinc, iron, magnesium, and/or aluminum and elucidated the role of the chemical Received: March 9, 2011 Accepted: May 6, 2011 Revised: April 27, 2011 Published: May 24, 2011 5373
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Figure 1. (a) SEM image of calcined NiAlO-A; (b) SEM image of calcined NiAlO-B; (c) SEM enlarged image of calcined NiAlO-B; (d) N2 adsorption/ desorption isotherms and corresponding pore size distributions of NiAl-HTs, calcined at 773 K. (2) NiAlO-A; (b) NiAlO-B.
composition on the catalytic SO2 removal performance. Centi and Perathoner13 reported that CuAl mixed oxides derived from hydrotalcite precursors showed significantly better SOx trapping performance. Cheng et al.14 focused on a systematic investigation toward the relationship between crystalline structures of MgAlFeCu mixed oxides and their SO2 removal catalytic activity. The mechanism of SOx trapping was studied in detail by Dathe et al.15 Although laboratory investigations have widely studied SO2 reactions on the HT surfaces, to some extent, whereas, large uncertainties remain concerning the whole heterogeneous reaction because the interaction between SO2 and the metal oxides surface is very complicated. Therefore, a better understanding of the overall mechanism of sulfate formation on the metal oxide surface is highly desirable. In situ Fourier transform infrared spectroscopy (FTIR) technique is an effective method for identifying the surface-adsorbed species and their reaction. Herein, this method was employed to investigate the chemical reactions on the solid surface for a better understanding of the mechanisms. In the present study, we synthesize two types of NiAl oxides derived from the hydrotalcite-like precursors through different approaches. Their structure and surface are well characterized. The differences in their sulfur dioxide sorption capacity are discussed in terms of the structure feature and surface chemistry, which have a significant effect not only on the sorption capacity but also on the chemical nature of the surface reaction products.
’ EXPERIMENTAL PROCEDURES Materials. NiAl hydrotalcite were first synthesized by constant-pH coprecipitation and urea hydrolysis methods. The final products were calcined at 773 K for 4 h and labeled as NiAlO-A
and NiAlO-B, respectively. For detailed procedure see the Supporting Information. Techniques of Characterization. The samples were characterized by inductively coupled plasma (ICP), S-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption desorption, X-ray photoelectron spectroscopy (XPS), pyridine chemisorption, and CO2 temperature-programmed desorption (TPD) techniques. For a full description see the Supporting Information. Activity Test. The adsorption equilibrium isotherms of SO2 were analyzed by a volumetric method at 298 and 473 K.16 The method is based on the mole balance of SO2 gas in a closed system. For detailed procedure see the Supporting Information. In Situ FTIR Experiment. The equipment is the same as described in the pyridine chemisorption. In an FTIR cell, the gas flow rate was 200 mL/min. The sample first heated for 60 min at 673 K and then cooled to the desired temperature. All the spectra were recorded at this temperature. The background spectrum was recorded with the flowing of O2 þ N2 and was subtracted from the sample spectrum.
’ RESULTS AND DISCUSSION Characterizations. The XRD patterns of the as-synthesized and calcined NiAl-HT samples are illustrated in Figure S1 (Supporting Information). Before calcination, the two samples exhibit the characteristic features of hydrotalcite-like structure, with narrow, symmetric, strong lines at low 2θ values and weaker, less symmetric lines at high 2θ values. After calcination at 773 K, both samples show four distinctive peaks at 2θ = 37.4°, 43.0°, 63.4°, and 75.2°, which correspond to the NiO (111), (200), (220), and (311) crystal planes (JCPDS 47-1049), respectively. The results indicate that the hydrotalcite layered structure 5374
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Figure 2. XPS spectra of S2p3/2 on the NiAlO samples before (A) and after (B) reaction with SO2 þ O2. (a) NiAlO-A sample; (b) NiAlO-B sample.
completely collapsed, and only a new NiO crystalline phase formed at 773 K. It is similar to the phenomenon reported earlier that when a hydrotalcite MgAl is calcined between 673 and 1073 K, only the MgO phase is detected.17 No diffraction peaks corresponding to crystalline Al phase can be observed, owing to the good dispersion of Al species in the oxide matrices. The mean crystallite sizes are estimated as 36.5 and 7.3 nm for NiAlOA and NiAlO-B, respectively (see the Supporting Information). Therefore, it is concluded that the NiAl mixed oxides derived from HTs through urea hydrolysis could possess better crystallinity and smaller crystallite size than those synthesized by the coprecipitation method. The Ni/Al atomic ratios in the calcined samples are calculated from ICP results, as listed in Table S1 (Supporting Information). No obvious difference was found between the Ni/Al atomic ratios in the calcined samples and that in the starting mixed solution, suggesting that the hydrotalcite-like NiAl compounds can be successfully prepared. This is in good accordance with the XRD results of the corresponding precursors. SEM analysis of NiAlO-A (see Figure 1a) reveals that the sample is made up of agglomerates of irregular morphology and the particle size varies widely. By contrast, NiAlO-B (Figure 1b) shows a flowerlike hierarchical structure of 12 μm in size. The structure is composed of densely packed irregular nanoflakes, with the size of ca.1030 nm estimated from enlarged SEM image (Figure 1c). The difference between the two samples mainly lies in the capsular morphology. The N2 adsorption/desorption isotherms of the NiAlO-A and NiAlO-B samples show that both samples are of type IV, which characterizes mesoporous solids, as shown in Figure 1d. It seems that there are two steps in the adsorption path of the NiAlO-B sample, the first one at approximately P/P0 = 0.5 and the second one at approximately 0.75, while the path in the coprecipitated NiAlO-A sample was a continuous one. The hysteresis loop for the calcined coprecipitated HT sample, NiAlO-A, is of type H3, which, in agreement with the morphology of coprecipitated HT materials, is usually given by adsorbents containing slit-shaped pores.18 The two steps observed in the hysteresis loops of the NiAlO-B sample indicate a bimodal pore size distribution as shown in Figure 1d (inset). In the case of NiAlO-B, the pore size population observed on the solid is meaningful because smaller pores are predominant over larger ones. Specific surface areas, pore volume, and average pore size of calcined solids are reported in Table S1 (Supporting Information). It is important to note the NiAlO-B sample possesses high specific surface area and total pore volume (143 m2/g and 0.40 cm3/g), while the NiAlO-A shows only 45 m2/g surface area and 0.24 cm3/g total pore volume. Therefore, the comparison shows that the low surface
area of NiAlO-A can be explained according to its larger and irregular crystal size. Moreover, for the NiAlO-B sample, the two average pore diameters corresponding to the well-defined bimodal pore size distribution are 4.3 and 11 nm, respectively, while the average pore diameter of NiAlO-A is about 21.6 nm. Adsorption of SO2. Figure S2 (Supporting Information) shows the adsorption isotherms of SO2 onto NiAlO-A and NiAlO-B samples at 298 and 473 K. The NiAlO-A sample exhibits a type IV adsorption isotherm of SO2, which means the adsorption capacity is low at the lower P/P0 region (P/P0 < 0.01). By contrast, the SO2 adsorption isotherm of NiAlO-B sample is of type I and has a higher adsorption capacity in the region of P/P0 > 0.01. The reason for this change is that a substantial percentage of pores for NiAlO-B is now in the quasi-micropore region (4.3 nm), and molecules may be adsorbed following the micropore-filling mechanism before they enter the pore body. Because the pore opening is smaller than the pore body, molecules will have difficulty escaping once they have been adsorbed onto the pore body. Furthermore, the sample NiAlO-B got the highest amount of SO2 (0.65 mmol/g) at 298 K. The amount lowered down to 0.57 mmol/g at 473 K but was still remarkably higher than that of NiAlO-A at either temperature. These results demonstrate that NiAlO-B should be a better adsorbent for SO2 abatement. X-ray Photoelectron Spectra of NiAlO samples Surfaces. XPS scans recorded in the S2p region taken from NiAlO samples are shown in Figure 2. No S signals (Figure 2A) can be identified, indicating that there are no sulfur species before the reaction. Two species are observed on the surface after reaction in the S2p region with characteristics S2p doublets (S2p3/2 and S2p1/2) present in a 2:1 ratio and with an energy difference of 1.2 eV. The first species can be assigned to surface-bound sulfite with an S2p3/2 binding energy of 168.3 eV (for NiAlO-A) and 168.7 (for NiAlO-B), respectively. The other new species, appearing at an S2p3/2 value of 169.5 eV (for NiAlO-A) and 170 eV (for NiAlOB), have been ascribed to the sulfate species. The values we observe for the sulfite and sulfate binding energy were within the range of values reported in the literature.19,20 For NiAlO-A sample, the relative concentration ratio of [SO32] to [SO42] is 71:29, which is calculated from the peak area of the corresponding species on the XPS spectra. This result indicates that 71% of the total gas-phase SO2 adsorbed on NiAlO-A surface was converted to SO32. The rest (only 29%) existed in the form of the species SO42. For NiAlO-B sample, the relative concentration ratio of [SO32] to [SO42] is 27:73. It is seen clearly that the adsorbed SO42 species on NiAlO-B surface is higher than that on NiAlO-A, which demonstrates that the NiAlO-B sample is more favorable for the conversion of SO2 to be SO42 as compared to NiAlO-A. 5375
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Environmental Science & Technology The O 1s spectra (Figure S3, Supporting Information) are fitted with two peak contributions. The peaks at 530531 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.22,23 Surface Acidity. Pyridine is one of the most widely used basic probe molecules for surface acidity characterization.24 Owing to the sensitivity of its vibrational modes to the nature and strength of surface acid groups, pyridine allows Lewis and Brønsted acid sites to be probed and distinguished from each other and the acid strength of the former to be correlated with the polarizing power of the cation.25 FTIR spectra of pyridine chemisorbed on the samples evacuated at different temperatures are presented in Figure 3. Both the fresh NiAlO-A and NiAlO-B samples (Figure 3a) display peaks around 1446, 1487, 1576, and 1605 cm1, corresponding to pyridine coordinated on Lewis acid sites. It appears that the acidity of NiAlO-B is much higher compared with that of NiAlO-A. A significant decrease in the intensities of the above bands is observed with an increase in temperature, indicating the partial desorption of pyridine. Moreover, no pyridinium species, pyHþ, at 1545 cm1 are detected. This feature indicates that the Brønsted acid sites are very weak for both samples. After reaction with SO2 þ O2, the concentration of Lewis acid sites of the used samples was obviously stronger at different temperatures
Figure 3. FTIR spectra of calcined NiAlO-A and NiAlO-B samples after adsorption of pyridine and desorption at different temperatures: (a) before and (b) after reaction with SO2 þ O2.Curves 13 denote the desorption of pyridine on NiAlO-A at room temperature (RT), 373 K, and 573 K, respectively; curves 46 denote the desorption of pyridine on NiAlO-B at RT, 373 K, and 573 K, respectively.
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(Figure 3b). This is a direct indication that some chemical reaction occurred in the process, which has to be considered in derivation of the adsorption mechanism. A detailed explanation is given below. Surface Basicity. The CO2 TPD profiles of NiAlO-A and NiAlO-B samples obtained at 773 K before and after reaction with SO2 are given in Figure S4 (Supporting Information). All samples showed three CO2 desorption peaks, which could be assigned to the weak, moderate, and strong basic sites, respectively. For metal oxides, the weak basic sites were associated with OH groups, the moderate basic sites were related to MO pairs, and the strong basic sites were ascribed to the coordinatively unsaturated O2 ions.26 In a higher temperature region, a large amount of CO2 evolution centered at 800 and 1000 K took place on NiAlO-B, while a trace amount was observed on NiAlO-A. This contrast strongly suggests that NiAlO-B sample has more total amounts of the moderate and strong basic sites than NiAlOA. Noticeably, the amounts of basic sites of the two samples were remarkably decreased after reaction with SO2 þ O2. The consumption of surface basic sites means that the reaction between SO2 and surface basic sites must have occurred. In Situ FTIR Spectra of SO2 Adsorption on the NiAlO Samples. To investigate the interaction between SO2 þ O2 and NiAlO samples, in situ FTIR experiments were conducted to figure out the adsorption pathways and to analyze the nature of the sulfate species on the NiAlO sample. Each sample was exposed to a flow of SO2/O2/N2 (200 ppm SO2, 5% O2, and N2 as balance) at 298 and 473 K for 10 min, and then the inlet and outlet were closed. The adsorption behaviors of the samples at 298 and 473 K are similar to each other; thus only the results of the two samples at 473 K are chosen as the representative, as illustrated in Figure 4. The broad bands centered at 3388 cm1 (for NiAlO-A) and 3353 cm1 (for NiAlO-B) are assigned to the ν(OH) stretching of adsorbed H2O.27,28 It is evident that the intensities of the negative peaks at 3740 cm1 (for NiAlO-A) and 3710 cm1 (for NiAlO-B) increased with time. These bands are attributed to the vibrations of the surface hydroxyl species (OH).27,28 The negative features indicate either consumption of hydroxyl groups from the surface or that the hydroxyl groups are involved in hydrogen bonding during the reaction. Several new bands at 1203, 1127, 1090, 1050, and 970 cm1 (for NiAlOA) and at 1448, 1225, 1139, 1050, and 975 cm1 (for NiAlO-B) were detected, and the intensity increased with time until the surface was saturated. All of these peaks are also assigned to chemisorbed SO2 since they did not disappear after evacuation was carried out. The bands at 1050, 970, and 975 cm1 could be
Figure 4. Dynamic changes of in situ FTIR spectra for NiAlO at 473 K after reaction with SO2 þ O2 in closed system. (a) NiAlO-A sample; (b) NiAlO-B sample. 5376
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Environmental Science & Technology assigned to the stretching motion of surface-coordinated bisulfite and/or sulfite. The results agree well with other infrared studies reported previously.2931 John et al.29 investigated SO2 adsorption on γ-Al2O3 particles and found that the broad band centered at 1060 cm1 was assigned to strongly adsorbed SO2 identified as a sulfite species. Martin et al. 30 studied reaction of SO2 on CaO particles. They observed a broad absorption band centered at 950 cm1 and assigned this band to the ν3 and ν1 stretching modes of surface sulfite. Schoonheydt and Lunsford31 also reported surface sulfite absorption bands at 975 and 1040 cm1 following reaction of SO2 on MgO at room temperature. Additionally, according to previous studies,30,32 the bands at 1203, 1127, and 1090 cm1 (for NiAlO-A), and 1225, 1139, and 1050 cm1 (for NiAlO-B) 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.33 According to the theory, there were two infrared sulfate vibrations that were accessible to FTIR investigation, which were the nondegenerate symmetric stretching ν1 band and the triply degenerate asymmetric stretching ν3 band. When the bidentate sulfate complex was formed on the surface of the sample, the ν3 band would split into three bands between 1250 and 1050 cm1. 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 three split ν3 bands were between 1050 and 1250 cm1. If the bidentate mononuclear sulfate was formed, the bands would shift to higher wavenumbers.33 Taking all of this together, we propose here the main production of SO2 adsorption was the bidentate binuclear sulfate. The peak located at 1050 cm1 could be attributed to the cooperative effects of surface sulfite and sulfate. Finally, the band at 1448 cm1 was previously found by Watson and Ozkan,34 but the band assignment was not given. According to previous studies,35 this band could be assigned to the SO3 species. For sample NiAlO-A shown in Figure 4a, the intensity ratio of 970 and 1050 cm1 is even stronger than that of 1090, 1127, and 1203 cm1 in terms of peak area, which implies a larger quantity of surface sulfite than sulfate on NiAlO-A. The result can be very well correlated with XPS analysis (see Figure 2a). Meanwhile, by comparing Figure 4b with Figure 4a, it should be noted that the total amount of surface SO32, HSO3, and SO42 species of sample NiAlO-B is much larger than that of sample NiAlO-A on the basis of the peak area. Possible Mechanism of Reactions of SO2 on NiAlO Samples. The adsorption process of SO2 is cooperative physisorption and chemisorption. The physisorption capacity is determined by the surface area and number of pores on the adsorbent substrate. Mixed oxides prepared from hydrotalcites have higher surface area and they are rich in pores. This may be one reason why calcined hydrotalcites have a better adsorption capacity for SO2. In our work, the sample NiAlO-B synthesized by the urea method could have more SO2 molecules adsorbed due to its larger specific surface area and total pore volume as well as smaller crystallite size compared with NiAlO-A. These results are distinct from the previous report,12 in which the SOx reduction over the catalysts was not assigned to their textural properties (except for the MgAl). As mentioned above, SO2 reacted with calcined NiAlO samples and produced a larger fraction of surface-coordinated sulfite/bisulfite and bidentate binuclear sulfate. The chemisorption capacity is linked to the surface acidicbasic sites on mixed oxides with an effect of predominant basicity. The
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early study36 elaborated that adsorption of SO2 on Lewis acid sites (coordinately unsaturated aluminum atoms) resulted in weakly adsorbed SO2. Adsorption of SO2 on Lewis base sites (exposed oxygen atoms), followed by rearrangement where the sulfite species attaches to aluminum through the sulfur atom, resulted in chemisorbed sulfite. Thus, we propose the following possible mechanism for the reaction of SO2 on NiAlO samples. As the adsorbed SO2 on the surfaces of metal oxides could exist in more than one state, there could be several possible reaction pathways: SO2 ðgÞ þ ½OH -MðsÞ f ½HSO3 -MðsÞ 2½OH -MðsÞ þ SO2 ðgÞ f ½SO3 2 -MðsÞ þ H2 O SO2 ðgÞ þ ½O2 -MðsÞ f ½SO3 2 -MðsÞ
ð1Þ ð2Þ ð3Þ
2½HSO3 -MðsÞ þ O2 þ 2OH f 2½SO4 2 -MðsÞ þ 2H2 O ð4Þ 2½SO3 2 -MðsÞ þ O2 f 2½SO4 2 -MðsÞ
ð5Þ
2
OH is adsorbed hydroxyl group and O is lattice oxygen atom. The direct interaction of SO2 with hydroxyl groups could yield bisulfite or sulfite on the surface. In situ FTIR results have provided evidence for the formation of adsorbed bisulfite and sulfite and the direct consumption of hydroxyl groups (Figure 4). More O2 would become available because of OH consumption, and it would be expected that reaction 3 should increase SO2 adsorption. This could well explain the obtained in situ FTIR spectra, in which the bands of adsorbed SO2 species continued increasing after the OH groups had stopped loss. The O2 in the gas phase are prone to be adsorbed on the surface and form surface-active oxygen. The adsorbed bisulfite and sulfite could be further oxidized by the active oxygen to form sulfate. Moreover, the redox property of NiAlO (mainly Ni species) makes sulfate formation reasonable. The reduction of Ni species provides the trapping of electrons that is necessary for the oxidation of SO2 to surface sulfate. In the presence of oxygen, the reduced Ni species can then be oxidized again. It is obvious that the surface basicity sites are important for SO2 chemisorption. A number of different groups have investigated that strong SO2 adsorption could occur at basic sites.3739 Karge and Dalla Lana38 further developed this theory. They determined that the interaction of SO2 with basic sites on the surface of γ-Al2O3 led to the formation of chemisorbed SO2. Also, Pacchioni et al.39 observed that MgO(100), which had more basic sites, may provide favorable sites for sulfate formation. It was suggested that sulfite could form by interaction of the sulfur atom in SO2 with two surface five-coordinated O2 anions. For NiAl mixed oxides from hydrotalcites, which have a structure similar to MgO, the sites for SO2 adsorption are expected to associate with OH, NiO, AlO, or O2. In our study, the more efficient adsorption of SO2 on the NiAlO-B sample is linked to higher aciditybasicity strength as compared to NiAlO-A. When the sulfate or sulfite was formed, SdO has a covalent double bond and has a much stronger affinity to electrons as compared with that of a simple metal oxide; hence, the Lewis acid strength of metal ions becomes substantially stronger by the inductive effect of SdO in the complex. A possible model for the change in surface acidity is shown in Figure 5. The enhanced acidity can be well explained by the 5377
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Figure 5. One possible model for the change in surface acidity.
results of pyridine chemisorption (Figure 3), which makes acid strength become stronger after adsorption of SO2. In contrast, the basic sites of the samples became less after reaction with SO2 þ O2 since they were consumed to form sulfite and sulfate. Finally, when the O2 anions are consumed, O2 in the gas phase can compensate so that the oxidation reaction can continue until the surface is fully covered by surface SO32, HSO3, and SO42 species.
’ ASSOCIATED CONTENT
bS
Supporting Information. Additional text describing materials preparation and characterization of samples; one table listing chemical composition, crystalline size, and structural parameters of calcined NiAlO; and four figures showing XRD, adsorption equilibrium isotherms of SO2, XPS spectra, and CO2TPD profiles for calcined NiAlO samples. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ852-2358-7138 (G.C.); þ86-411-8470-7733 (X.L.). Fax: þ852-2358-0054 (G.C.); þ86-411-8470-8084 (X.L.). E-mail:
[email protected] (G.C);
[email protected] (X.L.).
’ ACKNOWLEDGMENT This work was supported financially by the National Nature Science Foundation of China (20877013, NSFC-RGC 21061160495), the National High Technology Research and Development Program of China (863 Program) (2010AA064902), the Major State Basic Research Development Program of China (973 Program) (2007CB613306), and the Excellent Talents Program of Liaoning Provincial University (LR2010090). ’ REFERENCES (1) Hu, G.; Sun, Z.; Gao, H. Novel process of simultaneous removal of SO2 and NO2 by sodium humate solution. Environ. Sci. Technol. 2010, 44, 6712–6717. (2) Raju, T.; Chung, S. J.; Moon, I. S. Novel process for simultaneous removal of NOx and SO2 from simulated flue gas by using a sustainable Ag(I)/Ag(II) redox mediator. Environ. Sci. Technol. 2008, 42, 7464– 7469. (3) Wang, C.; Liu, H.; Li, X. Z.; Shi, J. Y.; Ouyang, G.; Peng, M.; Jiang, C. C.; Cui., H. N. A new concept of desulfurization: the electrochemically driven and green conversion of SO2 to NaHSO4 in aqueous solution. Environ. Sci. Technol. 2008, 42, 8585–8590. (4) Li, Y. Z.; Dong, H. L.; Li, Y.; Xu, X. C. Simultaneous removal of SO2 and trace As2O3 from flue gas: mechanism, kinetics study, and effect of main gases on arsenic capture. Environ. Sci. Technol. 2007, 41, 2894–2900. (5) Tseng, H. H.; Wey, M. Y.; Liang, Y. S.; Chen, K. H. Catalytic removal of SO2, NO and HCl from incineration flue gas over activated carbon-supported metal oxides. Carbon 2003, 41, 1079–1085.
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