Effects of Preparation Conditions on the Performance of Simultaneous

Feb 2, 2016 - and Yaen Wang. †. †. College of Civil and Environmental Engineering, and. ‡. Beijing Key Laboratory of Resource-Oriented Treatment...
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Effects of Preparation Conditions on the Performance of Simultaneous Desulfurization and Denitrification over Ni/Fe Hydrotalcite-like Compounds Xiao Liu,† Honghong Yi,*,†,‡ Xiaolong Tang,†,‡ Yuantao Li,† Baocong Cui,† and Yaen Wang† †

College of Civil and Environmental Engineering, and ‡Beijing Key Laboratory of Resource-Oriented Treatment of Industrial Pollutants, University of Science and Technology Beijing, Beijing 100083, People’s Republic of China ABSTRACT: The effects of a series divalent metals, ratios of metals, and calcination temperature on the performance of simultaneous desulfurization and denitrification over the hydrotalcite-like compound (HTLC) catalyst were investigated, and the removal mechanism of HTLCs was characterized by X-ray powder diffraction (XRD), Brunauer−Emmett−Teller (BET) surface areas, temperature-programmed desorption−mass spectroscopy (TPD−MS), and Fourier transform infrared spectroscopy (FTIR) analyses. The results showed that Ni/Fe HTLCs had the best adsorption property to remove SO2 and NO among the three HTLCs: Ni/Fe, Mg/Fe, and Co/Fe HTLCs. The optimum preparation conditions of the adsorbent were at the Ni/Fe mole ratio of 2 and calcined at 300 °C. The high specific surface area as well as the existence of Fe2O3 phases resulted in excellent NO and SO2 adsorption capacities. The removal rate of SO2 and NO still remained at 95 and 50% when the reaction time had extended to 40 min. A total of 13% CO2 of the simulated flue gas had no significant effect on the adsorption of NO and SO2. The NO adsorption capacity decreased slowly with the increase of the H2O concentration, and a total of 2% H2O enhanced the SO2 adsorption capacity but inhibited the SO2 adsorption slightly when the H2O concentration still increased to 5%. SO2 was chemisorbed on the Ni/Fe HTLCs, and the products were sulfate and sulfite according to the characterization results. However, NO was physisorbed and chemisorbed on the Ni/Fe HTLCs, and the chemisorption products were nitrate and nitrite.

1. INTRODUCTION The consumption of coal, as the main primary energy source, constitutes 66% of the total energy sources in China, and the annual consumption has reached 3 billion in China. Sulfur dioxide (SO2) and nitrogen oxides (NOx) are the main air pollutants of coal-fired flue gas, which lead to acid rain as well as photochemical smog, and are the important precursor in the ash haze of particulate matter of 2.5 μm or less in diameter (PM2.5).1 Consequently, studying the desulfurization and denitrification technologies is of great significance to solve the air pollution problem. By now, the most successful processes for SO2 and NOx reduction are independent operation, such as limestone−gypsum flue gas desulfurization (FGD) and selective catalytic reduction (SCR). The traditional processes occupy a lot of area and are expensive in operational cost, which does not conform to the situation of constructing resource conservation and a circular economy. Therefore, the process for simultaneous reduction of SO2 and NOx is undoubtedly the major development trend. Hydrotalcite-like compounds (HTLCs) have become the potential applications in adsorption area, which are porous materials and have a high surface area.2 Some research progresses of SO2 or NOx reduction over HTLCs have been made between domestic and foreign scholars. Centi et al.3 used the Cu/Al HTLC-derived oxides to adsorb and store SO2 and showed a good result. Cheng et al.4 studied the removal of SO2 by the MgAlFeCu mixed oxides derived from HTLCs, and the total amounts of adsorbed SO2 reached 1.74 g/g in the optimal situation. Basile et al.,5 preparing a series of Pt- and Pt-/Cusupported catalysts by impregnation of Mg/Al HTLCs, demonstrated that the HTLCs had strong storage of NOx © 2016 American Chemical Society

and improved the resistance to SO2. In comparison to the individual removal of SO2 and NO, simultaneous removal has some advantages, such as the operational cost and equipment being less. The research of simultaneous reduction of SO2 and NO by HTLC-derived oxides was minimal. The trivalent iron (Fe3+) and the divalent metals (Ni2+, Mg2+, and Co2+), as component elements of HTLCs, played a significant role of desulfurization and denitrification.6,7 Therefore, we synthesized the Fe-based HTLCs, collocating with Ni, Mg, and Co divalent metal, for the simultaneous reduction of SO2 and NO. Generally, the formula of HTLCs is [M(II)1 − xM(III)x(OH)2]x+(An−)x/n·mH2O. In detail,8 M(II) and M(III) stand for di- and trivalent cations and x is the mole ratio of M(III)/(M(II) + M(III)) with the value ranging from 0.2 to 0.4. In the study, Ni/Fe, Mg/Fe, and Co/Fe HTLCs were prepared by the co-precipitation method and calcined at a certain temperature to obtain the derived oxides. The effects of the divalent metals, M2+/M3+ mole ratio, and calcination temperature were studied, and the removal mechanism of SO2 and NO was researched.

2. EXPERIMENTAL SECTION 2.1. Preparation of HTLCs. The HTLCs were synthesized by the co-precipitation method, from the corresponding nitrate solutions, Ni(NO3)2·6H2O, [Mg(NO3)2·6H2O and Co(NO3)2·6H2O], and Fe(NO3)3·9H2O, and NaOH/Na2CO3 was a precipitant. Solution A was obtained by dissolving 7 g of NaOH and 10.6 g of Na2CO3 in 180 Received: November 9, 2015 Revised: January 25, 2016 Published: February 2, 2016 2295

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where CNO(in) was the air intake concentration [parts per million (ppm)] of NO, CNOx(out) was the outlet concentration (ppm) of NOx (NO and NO2), CSO2 was the air intake concentration (ppm) of SO2, and CSO2(out) was the outlet concentration (ppm) of SO2.

mL of deionized water, while solution B was obtained by dissolving the metal nitrate precursors. Solutions A and B were added dropwise to beaker C with a moderate amount of deionized water under mechanical stirring at room temperature. The pH, around 11, was adjusted by HNO3 of the solution. After the addition was complete, the resulting slurry aged at 50 °C for 8 h. The final precipitate was suction-filtered, washed repeatly until the pH of the filtrate was around 7, and dried at 110 °C overnight. Finally, the sample was calcined at a certain temperature. In all of the cases, the mole ratio of M2+/M3+ was from 1 to 4, while the calcination temperature ranged from 200 to 500 °C. 2.2. Characterization. X-ray powder diffraction (XRD) was performed in a multifunctional X-ray diffractometer using Cu Kα radiation (λ = 0.154 06 nm) at a rate of 10°/min from 2θ = 5° to 90°. The crystalline phases were identified by matching the Joint Committee on Powder Diffraction Standards (JCPDS) files. The temperature-programmed desorption (TPD) experiment was started with a heating rate of 10 °C/min to 800 °C under N2 flow (200 mL/ min), and the outlet concentrations of SO2 and NO were measured with an Extrel mass spectrometer (MAX300-LG, Pittsburgh, PA). The Brunauer−Emmett−Teller (BET) specific surface areas of the adsorbent were analyzed by N2 adsorption using an Autosorb-1-C instrument. The Fourier transform infrared spectroscopy (FTIR) spectra were recorded using the KBr pellet technique on a Thermo spectrometer in the 4000−400 cm−1 wavenumber range. 2.3. Sample Testing. The adsorption capacity of HTLC-derived oxides was tested in a fixed-bed quartz reactor, which was set in a temperature-control instrument, and the reaction temperature was kept at 110 °C. The flowchart of the experiment was shown in Figure 1. The feed gas mixture contained 300 ppm of NO, 300 ppm of SO2,

t

S NO =

SSO2 =

SO2 removal rate (%) =

C NO(in)

CSO2(in) − CSO2(out) CSO2(in)

∫0 m (C0 − Ci)v dt (4)

G −1

where SNO was the saturated adsorption capacity (mmol g ) of NO, SSO2 was the saturated adsorption capacity (mmol g−1) of SO2, tm was the adsorption time (min), C0 was the air intake concentration (ppm), Ci was the outlet concentration (ppm), v was the feed flow (mL min−1), and G was the mass of the adsorbent.

3. RESULTS AND DISCUSSION 3.1. Effect of Divalent Metals on Simultaneous Adsorption of SO2 and NO. HTLCs were formed by the anions located between the layers and the positive charged layer. The kinds of divalent metals, partly placed by the trivalent metal, affected the stability and performance of the HTLCs. In this part, the Ni/Fe, Mg/Fe, and Co/Fe HTLCs were synthesized and the adsorption capacities of SO2 and NO were measured. The preparation condition was a M2+/M3+ mole ratio of 2 calcined at 300 °C. We aimed to find out the optimal divalent metal, with the results being plotted in Figure 2. As shown in Figure 2a, the Ni/Fe HTLC-derived oxides showed the best adsorption property of NO. In the first 10 min, the NO removal rate remained at 88% of Ni/Fe HTLC-derived oxides; however, the NO removal rates of Mg/Fe and Co/Fe HTLC-derived oxides were 75 and 40%, respectively. After 10 min, the adsorption activity of NO began to decrease, while the adsorption activity of Ni/Fe HTLC-derived oxides was higher than the others in the whole reaction process. The result may be explained by the higher specific surface and small grain size of the Ni/Fe HTLC-derived oxides.9 In addition, the higher specific surface made NO contact the Ni/Fe HTLC-derived oxides easily, and it was beneficial to the physical and chemical adsorption. After 45 min, the NO removal rate of Ni/Fe HTLC-derived oxides decreased to 15%; however, the others were 10%. As shown in Figure 2b, in the first 30 min, the Ni/Fe HTLC-derived oxides exhibited the longest breakthrough time (100% removal rate) for SO2. However, after 20 min, the SO2 removal rate of Mg/Fe HTLC-derived oxides sharply dropped. The SO2 removal rates of Ni/Fe and Mg/Fe HTLC-derived oxides were 75 and 50%, respectively, when the reaction continued for 60 min. The adsorption capacity of Co/Fe HTLC-derived oxides for SO2 was between the Ni/Fe and Mg/ Fe HTLC-derived oxides. Palomares et al.10 found that the product, cobalt sulfate, was fairly stable, which may cause the SO2 removal rate of Co/Fe HTLC-derived oxides to decrease after 20 min. To select the best divalent metal to synthesize the HTLCs, the adsorption capacities were calculated. Because we mainly studied the simultaneous reduction of SO2 and NO, we selected 60 min as the adsorption time, when NO reached saturation approximately. The adsorption capacities of HTLC-derived oxides were summarized in Table 1. According to Table 1, the Ni/Fe HTLC-derived oxides exhibited the strongest adsorption

5% O2, and balance N2. The feed flow through the reactor was controlled in 200 mL/min using a calibrated mass flow meter, and the gas hourly space velocity was 15 000 h−1. The outlet concentration of the reactor, containing HTLC-derived oxides, was measured by a fuel gas analyzer (Kane KM9106, U.K.). The removal rate of SO2 and NO was obtained by eqs 1 and 2. We selected 60 min as the adsorption time because NO reached saturation approximately, and the saturated adsorption capacities of SO2 and NO were obtained by eqs 3 and 4 C NO(in) − C NOx (out)

(3)

G t

Figure 1. Flowchart of the experiment for SO2 and NO adsorption.

NO removal rate (%) =

∫0 m (C0 − Ci)v dt

× 100% (1)

× 100% (2) 2296

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Figure 2. Effect of divalent metals on the adsorption of SO2 and NO.

Figure 3. Effect of the Ni2+/Fe3+ mole ratio on the adsorption of SO2 and NO.

Table 1. Adsorption Capacities of Ni/Fe, Mg/Fe, and Co/Fe HTLC-Derived Oxides adsorption capacity (mmol g−1)

Ni/Fe HTLC-derived oxides

Mg/Fe HTLC-derived oxides

Co/Fe HTLC-derived oxides

SNO SSO2

0.1767 0.3511

0.1174 0.2966

0.1431 0.3243

excellent adsorption capacity. As shown in Figure 3a, the NO removal rate remained at 90% of the Ni/Fe HTLC-derived oxides with the Ni2+/Fe3+ mole ratio of 2 in the first 10 min and the NO removal efficiency can even be kept at 20% after 50 min. The variation trends of the Ni/Fe HTLC-derived oxides with Ni2+/Fe3+ mole ratios of 1, 3, and 4 were similar to the derived oxides with the Ni2+/Fe3+ mole ratio of 2. As shown in Figure 3b, the Ni/Fe HTLC-derived oxides with the Ni2+/Fe3+ mole ratio of 2, exhibited the longest breakthrough time (100% removal rate) for SO2, which was 10 min more than that of the derived oxides with Ni2+/Fe3+ mole ratios of 1 and 3. In combination of the SO2 and NO removal efficiencies, the Ni/ Fe HTLC-derived oxides with the Ni2+/Fe3+ mole ratio of 2 showed excellent adsorption capacity. The adsorption capacities of HTLC-derived oxides with different Ni2+/Fe3+ mole ratios were summarized in Table 2. To study the structure of the Ni/Fe HTLC-derived oxides, the crystalline phase confirmed by XRD of the precursors was plotted in Figure 4. It can be seen that the Ni/Fe HTLCderived oxides with different Ni2+/Fe3+ mole ratios had strong diffraction reflections around 37.4°, 43.0°, and 63.4°, which

capacity. The SO2 and NO adsorption capacities were 0.3511 and 0.1767 mmol g−1, respectively. 3.2. Effect of the M2+/M3+ Mole Ratio on Simultaneous Adsorption of SO2 and NO. The M2+/M3+ mole ratio could influence the purity of the HTLCs and the adsorption activity of the derived oxides. Generally, the HTLCs were synthesized with the M2+/M3+ mole ratio varying from 1 to 5.11,12 To investigate the effect of Ni2+/Fe3+ mole ratios on the adsorption activity and find out the optimal mole ratio, the precursors of the Ni/Fe HTLCs were prepared with Ni2+/Fe3+ mole ratios varying from 1 to 4. The results of the removal rate of SO2 and NO were shown in Figure 3. According to Figure 3, it can be seen that the Ni/Fe HTLCderived oxides with the Ni2+/Fe3+ mole ratio of 2 exhibited 2297

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were calcined at 200, 300, 400, and 500 °C. The results of the removal rate of SO2 and NO were shown in Figure 5.

Table 2. Adsorption Capacities of Ni/Fe HTLC-Derived Oxides with Different Ni2+/Fe3+ Mole Ratios adsorption capacity (mmol g−1)

Ni/Fe (1:1)

Ni/Fe (2:1)

Ni/Fe (3:1)

Ni/Fe (4:1)

SNO SSO2

0.1549 0.3227

0.2050 0.3613

0.1678 0.3365

0.1431 0.3242

Figure 4. XRD patterns of Ni/Fe HTLC precursors with different M2+/M3+ mole ratios: (a) Ni/Fe (1:1), (b) Ni/Fe (2:1), (c) Ni/Fe (3:1), and (d) Ni/Fe (4:1).

were corresponding to the crystal plane of NiO.13 The intensity and sharpness of NiO planes gradually increased with the increase of the Ni2+/Fe3+ mole ratio. What was important was that the XRD pattern of the sample with the Ni2+/Fe3+ mole ratio of 2 displayed a phase that was assigned to the characteristic reflection of Fe2O3 or NiFe2O4. It may be concluded that the existence of Fe2O3 or NiFe2O4 phase enhanced the active sites, which were related to the adsorption of SO2 and NO.14 The increasing adsorption capacities for SO2 and NO were in the following order: Ni/Fe (1:1) < Ni/Fe (4:1) < Ni/Fe (3:1) < Ni/Fe (2:1). The BET surface areas of the Ni/Fe HTLC-derived oxides with Ni2+/Fe3+ mole ratios varying from 1 to 4 and calcined at the same temperature were shown in Table 3. The surface area of Ni/Fe (2:1) HTLC-

Figure 5. Effect of the calcination temperature on the adsorption of SO2 and NO.

Table 3. BET Surface Area of Ni/Fe HTLC-Derived Oxides with Different Ni2+/Fe3+ Mole Ratios sample

Ni/Fe (1:1)

Ni/Fe (2:1)

Ni/Fe (3:1)

Ni/Fe (4:1)

specific surface area

184.4

199.4

200.3

166.9

As shown in Figure 5, it was apparent that the Ni/Fe (2:1) HTLCs calcined at different temperatures exhibited diverse adsorption capacities. The Ni/Fe (2:1) HTLCs calcined at the temperature of 300 °C showed the best adsorption capacity of SO2 and NO. As in Figure 5a, the NO removal efficiency of the four samples decreased rapidly with the reaction time. However, the NO removal efficiency of the Ni/Fe (2:1) HTLCs calcined at the temperature of 300 °C was higher than that of the others. As shown in Figure 5b, the increasing adsorption capacities for SO2 were in the following order: Ni/ Fe (200 °C) < Ni/Fe (500 °C) < Ni/Fe (uncalcined) < Ni/Fe (400 °C) < Ni/Fe (300 °C). The adsorption difference of Ni/Fe HTLCs calcined at different temperatures may be caused by the surface area. In the process of calcination, the crystal water and interlayer anions volatilized and the calcined products reconstruct the original layered structure, resulting in a high surface area and small crystal size.13 Calcined at 300 °C, the crystal water and

derived oxides was similar to that of Ni/Fe (3:1) HTLCderived oxides and was bigger than those of Ni/Fe (1:1) and Ni/Fe (4:1) HTLC-derived oxides. The excellent adsorption capacity of Ni/Fe (2:1) HTLC-derived oxides may result from the high specific surface area as well as the existence of Fe2O3 or NiFe2O4 phase. 3.3. Effect of the Calcination Temperature on Simultaneous Adsorption of SO2 and NO. For the HTLCs calcined at a certain temperature, the crystal water will be lost and the interlayer anions will be removed.15 To obtain the optimal calcination temperature of the Ni/Fe HTLCs with the Ni2+/Fe3+ mole ratio of 2, the dried samples 2298

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In combination with the experimental results, the SO2 and NO adsorption capacities demonstrated that the HTLCderived oxides had advantage in the simultaneous removal of SO2 and NO. Meanwhile, the activated carbon was a common material to remove SO2 and NO and was modified in some study progresses.17−19 Although the SO2 and NO adsorption properties of the modified commercial activated carbon were good, the outstanding question was that the inhibitory effect of SO2 on NO was very serious, which resulted in the low NO adsorption capacity. The effect of simultaneous removal was not ideal when the NO removal efficiency decreased a lot, even when the SO2 removal efficiency was very high.20 In contrast, the inhibitory effect of SO2 on NO was not very serious for HTLCs in our present study. In future studies, the SO2 and NO adsorption capacities of Ni/Fe HTLCs can be enhanced by optimizing the preparation conditions. 3.4. Effect of CO 2 and H 2 O Contents on the Simultaneous Adsorption of SO2 and NO. In the actual flue gas, there was certain concentrations of CO2 and moisture. Considering the experimental need and industrial condition, the effect of moisture and CO2 on the simultaneous adsorption of SO2 and NO was worth studying.21 To simulate the flue gas, the feed gas mixture contained 300 ppm of NO, 300 ppm of SO2, 5% O2, 13% CO2, and balance N2 and the moisture content was adjusted to 0, 2, and 5%, which was added to the flue gas by the evaporator. In this part, we selected the derived oxides, the Ni/Fe HTLCs with the Ni2+/Fe3+ mole ratio of 2 and calcined at 300 °C, as the adsorbent. The results of the removal rate of SO2 and NO were shown in Figure 7. As shown in Figure 7, with the addition of only 13% CO2, the NO and SO2 removal efficiencies had no significant change compared to the absence of CO2. With the increase of the H2O concentration, the NO removal efficiency decreased slowly. The water molecules may occupy the active position, which resulted in the inhibition of NO adsorption.22 When the moisture concentration was 2%, the SO2 adsorption capacity increased a little. The reason may be that SO2 reacted with H2O and promoted the SO2 chemical adsorption,23 and the promotion effect was greater than the competitive adsorption of the water molecule. However, with the moisture concentration increased to 5%, the efficiency of desulfurization decreased slightly. It may be that the inhibition effect of competitive adsorption was the primary effect. The NO and SO2 adsorption capacities of HTLC-derived oxides were summarized in Table 6. 3.5. Analysis of the Adsorption Mechanism. To find out the adsorption process of SO2 and NO, the TPD−mass spectroscopy (MS) measurement was carried out. The adsorption saturation sample, Ni/Fe (2:1) HTLCs calcined at 300 °C, was heated by TPD, and the outlet concentrations of SO2 and NO were measured, with the results shown in Figure 8. As shown in Figure 8a, with the temperature increasing, NO was rapidly desorbed and the three desorption peaks appeared at 160, 320, and 570 °C. According to the previous studies,24,25 the highest appeared at 160 °C, which corresponded to the physisorption. The other two NO desorption peaks may correspond to the chemisorption, with the decomposition of nitrate and nitrite.26 The NO2 desorption peaks appeared at 160 and 350 °C, which may indicate that some NO was oxidized in the adsorption process. As shown in Figure 8b, SO2 was detected when the temperature increased to 500 °C and the desorption peak appeared at 590 °C. It indicated that most

interlayer anions volatilized completely and without particles sintering or agglomerating. When the calcination temperature was higher than 300 °C, the adsorption efficiencies of SO2 and NO decreased because the phenomenon of metal oxide particle agglomeration may result in the decrease of the surface area and even the formation of the spinel structure.16 The BET surface areas of the Ni/Fe (2:1) HTLC-derived oxides calcined at 200, 300, 400, and 500 °C were shown in Table 4. The surface area of the Ni/Fe (2:1) HTLC-derived oxides calcined at 300 °C was higher than that of the others. Table 4. BET Surface Area of Ni/Fe (2:1) HTLC-Derived Oxides Calcined at Different Temperatures sample specific surface area (m2/g)

calcined at 200 °C

calcined at 300 °C

calcined at 400 °C

calcined at 500 °C

38.2

176.9

154.3

99.0

Besides, the formation of metal oxides in the process of calcination may increase the active adsorption point. The XRD phase of Ni/Fe HTLC-derived oxides calcined at different temperatures was reported in Figure 6. The diffraction

Figure 6. XRD patterns of Ni/Fe (2:1) HTLC-derived oxides calcined at different temperatures: (a) calcined at 200 °C, (b) calcined at 300 °C, (c) calcined at 400 °C, and (d) calcined at 500 °C.

reflections around 2θ = 37.4°, 43.0°, and 63.4°, which corresponded to the crystal plane of NiO,13 and the sharp and symmetric reflection of Fe2O3 appeared in the XRD phase of Ni/Fe HTLC-derived oxides calcined at 300 °C. Through the investigation of the preparation conditions, the high specific surface area as well as the existence of the Fe2O3 phase resulted in the excellent NO and SO2 adsorption capacities. The adsorption capacities of HTLC-derived oxides calcined at different temperatures were summarized in Table 5. Table 5. Adsorption Capacities of Ni/Fe (2:1) HTLCDerived Oxides Calcined at Different Temperatures adsorption capacity (mmol g−1)

uncalcined

SNO SSO2

0.1465 0.3149

calcined calcined at 200 °C at 300 °C 0.1398 0.3524

0.2290 0.3722

calcined calcined at 400 °C at 500 °C 0.1559 0.3483

0.1457 0.3479 2299

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Figure 8. TPD spectrogram of Ni/Fe HTLCs.

Figure 7. Effect of CO2 and H2O on the adsorption of SO2 and NO.

are attributed to SO42−, which indicated the chemisorption of SO2 and corresponded to the result of TPD.29,30 According to the analysis of FTIR spectra of the fresh and used samples, it was apparent that the product of chemisorption was NO3− and SO42− and H2O molecules were adsorbed during the reaction.

SO2 was chemisorbed and generated sulfate and sulfite on the Ni/Fe HTLC-derived oxides. Guo et al.27 studied the adsorption of SO2 and NO on activated carbon, and the results showed that most SO2 was chemisorbed and the desorption peak appeared in the high-temperature section. To investigate the adsorption product and conclude the possible adsorption process, the FTIR measurement was carried out, with the FTIR spectra shown in Figure 9. In comparison of the fresh and used sample FTIR spectra, it was obvious that the broad band observed at 3410 cm−1 of the used sample was attributed to the OH stretching, attributed to the presence of the hydroxyl of absorbed water.13 Similarly, the band observed at 1620 cm−1 can be attributed to the vibration of H2O molecules.28 The band observed at 1370 cm−1 is attributed to NO 3 − , and it indicated that NO was chemisorbed.29 The bands observed at 970 and 1140 cm−1

4. CONCLUSION The increasing adsorption capacities for SO2 and NO of three kinds of HTLCs, synthesized by the co-precipitation method, were in the following order: Mg/Fe < Co/Fe < Ni/Fe, as a result of a higher specific surface and smaller grain size of the Ni/Fe HTLC-derived oxides. With the Ni2+/Fe3+ mole ratio of 2, calcined at 300 °C, the XRD of the Ni/Fe HTLC-derived oxides showed the appearance of the Fe2O3 phase, which may enhance the active sites of adsorption. With the calcination temperature of 300 °C, the Ni/Fe (2:1) HTLC-derived oxides showed the best adsorption capacity because the crystal water

Table 6. Adsorption Capacities of Ni/Fe (2:1) HTLC-Derived Oxides with the CO2 and H2O Presence adsorption capacity (mmol g−1)

0% CO2 + 0% H2O

13% CO2 + 0% H2O

13% CO2 + 2% H2O

13% CO2 + 5% H2O

SNO SSO2

0.2290 0.3722

0.2119 0.3705

0.2029 0.3744

0.1980 0.3530

2300

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Figure 9. FTIR spectra of Ni/Fe HTLCs.

and interlayer anions volatilized completely and without particles sintering or agglomerating and the specific surface area reached 176.9 m2/g. In addition, the inhibitory effect of SO2 on NO was not very serious for HTLCs in our present study, and the adsorption capacity may be enhanced in future research. A total of 13% CO2 of the simulated flue gas had no significant effect on the adsorption of NO and SO2. The NO adsorption capacity decreased slowly with the increase of the H2O concentration, and a total of 2% H2O enhanced the SO2 adsorption capacity but inhibited the SO2 adsorption slightly when the H2O concentration still increased to 5%. SO2 was chemisorbed and generated sulfate and sulfite on the Ni/Fe HTLC-derived oxides. However, the physisiorption and chemisorption existed in the process of NO adsorption, and the products of chemisorption were nitrate and nitrite.

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AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-010-62332747. E-mail: yhhtxl@163. com. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work was supported by the National Natural Science Foundation of China (21577006 and 21077047), the Basic Scientific Research Business Expenses Special Funds Project of Central University (FRF-TP-13-041), and the New Century Excellent Talents in University (NCET-12-0776).

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DOI: 10.1021/acs.energyfuels.5b02645 Energy Fuels 2016, 30, 2295−2301