# An Initial Evaluation on the Adsorption of SO2 and NO2 over Porous Fe3O4 Nanoparticles Synthesized by Facile Scalable Method

Authors: Xuan-Manh Pham; Duy Linh Pham; Nguyen Thi Hanh; Tuyet Anh Dang Thi; Le Nhat Thuy Giang; Hoang Thi Phuong; Nguyen Tuan Anh; Hac Thi Nhung; Giang Truong Le; Mai Ha Hoang; Tuyen Van Nguyen
Journal: Journal of Chemistry
 (2019)

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## Abstract

In this study, Fe3O4 nanoparticles used as an adsorbent for the removal of toxic gases were successfully synthesized via the facile coprecipitation method. The Fe3O4 nanoparticles displayed a well-defined morphology with the size of ∼10 nm and a porous structure with a specific area of 115.90 m2/g and a wide range of pore sizes. These nanoparticles exhibited effective adsorption abilities upon the exposure to toxic gases. In particular, the amount of 40.5 mg SO2 and 108.5 mg NO2 was adsorbed in 1 g of Fe3O4 nanoparticles after 60 minutes of exposure, making the Fe3O4 nanoparticles become a promising adsorbent for the removal of toxic gases.


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## Body

## 1. Introduction

Toxic gases that are present during the fires are one of the main causes of fire deaths [1]. Depending on the materials that are burning, some kind of different toxic gases might be produced. These gases may include carbon monoxide (CO), nitrogen dioxide (NO2), sulfur dioxide (SO2), and hydrogen cyanide (HCN) [2]. They prevent cellular respiration, leading these cells to death or causing irritation, and are toxic for the body, at a higher level causing death quickly [3]. In addition to carbon monoxide, the main agent of deaths in the fire, nitrogen dioxide and sulfur dioxide are also known as dangerous agents for the humans’ life. Hence, the removal of these gases from the fire is necessary to protect humans. Nanoparticle powders with a large specific surface area have been proposed for adsorption and cleaning up of fumes and toxic gases generated in fires [4]. The hydroxide, carbonate, bicarbonate, and oxides of metal have attracted wide attention for that because they can be decomposed to CO2 and H2O to extinguish the fire or can be resistant to the fire flame improving the isolating ability of flammable materials with fire and oxygen [5–8]. Among them, the Fe3O4 nanostructured material is a potential candidate due to its stability, high surface area, and easy synthesis [9]. Some researchers have pointed out that Fe3O4 can be an efficient adsorbent for the removal of heavy metal ions [9–11] and inorganic anions [12] from aqueous solutions. However, there have been very few reports using Fe3O4 as an adsorbent for the removal of nitrogen dioxide and sulfur dioxide.The removal of SO2 and NO2 has been also investigated in the field of environment because the amount of these gases released in atmosphere can induce acid rain which is harmful to humans [13, 14]. Active carbon has been known as an effective adsorbent in processes of removing stack gases [15]. Nevertheless, it is difficult to apply active carbon for the removal of SO2 and NO2 in fires due to its flammable nature.In this work, Fe3O4 nanoparticles were prepared by a facile scalable method and applied as an adsorbent for the adsorption of NO2 and SO2. The material was characterized by a number of analytical techniques. The Fe3O4 nanoparticles exhibited good adsorption abilities in the removal of SO2 and NO2.


## 2. Experimental Section

### 2.1. Synthesis of Fe3O4 Nanoparticles

All chemicals were of analytical grade. Iron(III) chloride hexahydrate (FeCl3·6H2O), iron(II) chloride tetrahydrate (FeCl2·4H2O), ammonia solution 25% (NH3·H2O), sulfur dioxide (99.9%), nitrogen dioxide (99.5%), and nitrogen (99.999%) were purchased from Sigma-Aldrich and used without any further purification. The magnetite (Fe3O4) nanoparticles were prepared by the following coprecipitation method [16–18].In brief, FeCl3·6H2O (0.05 mol, 13.52 g) and FeCl2·4H2O (0.03 mol, 5.96 g) were dissolved in 100 mL DI water with a Fe2+/Fe3+ molar ratio of 0.6. Then, the ammonium solution was added dropwise into the solution until pH = 11.0 over a period of 2 h, under vigorous stirring to achieve Fe3O4 nanoparticles with high purity. The obtained precipitate was filtered, washed with DI water, and dried in vacuum at 60°C for 24 h.


### 2.2. Material Characterization

The specific surface area was determined from N2 adsorption/desorption at 77 K by the Brunauer–Emmett–Teller (BET) method using a Micrometrics surface area analyzer (TriStar II Plus, Micromeritics Instrument Corp, USA). The morphology and particle size were determined by field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan) and particle-size distribution measurement (Litesizer™ 500, Anton Paar Instrument Co. Ltd., Austria). The structure of adsorbents was characterized by high-resolution XRD (D/max Ultima III, Rigaku, Japan). The thermogravimetric analysis (TGA) was determined on TGA-50, Shimadzu, Japan, in an air atmosphere with a heating rate of 5°C·min−1 from room temperature to 700°C. Temperature-programmed reduction (TPR) was carried out under 5% CO in Ar flow (100 cc/min) at a heating rate of 10°C/min, from room temperature to about 900°C using AutoChem™ II 2920 (Micromeritics Instrument Corp, USA).


### 2.3. Adsorption Experiments

For the adsorption study, 1 g of Fe3O4 nanoparticles were exposed to dried SO2 and NO2 at a particular time in the fixed bed reactor under ambient temperature. These gases were mixed with nitrogen as a carrier gas (5% SO2 or NO2 in the N2 atmosphere). The Fe3O4 nanoparticles were pretreated under an Ar flow at 400°C during 2 h and then cooled to the room temperature. After that, the gas stream was supplied with a flow rate of 200 cc/min. The uptake quantities were recorded as a function of time based on a mass change of samples. The presence of the gas phase in contact with the materials was monitored by the FTIR measurement (Thermo Fisher, USA) at 8 cm−1 resolution. In addition, energy-dispersive X-ray spectroscopy (SEM-EDX) (HORIBA-EMAX80; Hitachi High-Technology) was used to evaluate the presence as well as the adsorbed quantity of SO2 and NO2.


## 2.1. Synthesis of Fe3O4 Nanoparticles

All chemicals were of analytical grade. Iron(III) chloride hexahydrate (FeCl3·6H2O), iron(II) chloride tetrahydrate (FeCl2·4H2O), ammonia solution 25% (NH3·H2O), sulfur dioxide (99.9%), nitrogen dioxide (99.5%), and nitrogen (99.999%) were purchased from Sigma-Aldrich and used without any further purification. The magnetite (Fe3O4) nanoparticles were prepared by the following coprecipitation method [16–18].In brief, FeCl3·6H2O (0.05 mol, 13.52 g) and FeCl2·4H2O (0.03 mol, 5.96 g) were dissolved in 100 mL DI water with a Fe2+/Fe3+ molar ratio of 0.6. Then, the ammonium solution was added dropwise into the solution until pH = 11.0 over a period of 2 h, under vigorous stirring to achieve Fe3O4 nanoparticles with high purity. The obtained precipitate was filtered, washed with DI water, and dried in vacuum at 60°C for 24 h.


## 2.2. Material Characterization

The specific surface area was determined from N2 adsorption/desorption at 77 K by the Brunauer–Emmett–Teller (BET) method using a Micrometrics surface area analyzer (TriStar II Plus, Micromeritics Instrument Corp, USA). The morphology and particle size were determined by field emission scanning electron microscope (FE-SEM, S-4800, Hitachi, Japan) and particle-size distribution measurement (Litesizer™ 500, Anton Paar Instrument Co. Ltd., Austria). The structure of adsorbents was characterized by high-resolution XRD (D/max Ultima III, Rigaku, Japan). The thermogravimetric analysis (TGA) was determined on TGA-50, Shimadzu, Japan, in an air atmosphere with a heating rate of 5°C·min−1 from room temperature to 700°C. Temperature-programmed reduction (TPR) was carried out under 5% CO in Ar flow (100 cc/min) at a heating rate of 10°C/min, from room temperature to about 900°C using AutoChem™ II 2920 (Micromeritics Instrument Corp, USA).


## 2.3. Adsorption Experiments

For the adsorption study, 1 g of Fe3O4 nanoparticles were exposed to dried SO2 and NO2 at a particular time in the fixed bed reactor under ambient temperature. These gases were mixed with nitrogen as a carrier gas (5% SO2 or NO2 in the N2 atmosphere). The Fe3O4 nanoparticles were pretreated under an Ar flow at 400°C during 2 h and then cooled to the room temperature. After that, the gas stream was supplied with a flow rate of 200 cc/min. The uptake quantities were recorded as a function of time based on a mass change of samples. The presence of the gas phase in contact with the materials was monitored by the FTIR measurement (Thermo Fisher, USA) at 8 cm−1 resolution. In addition, energy-dispersive X-ray spectroscopy (SEM-EDX) (HORIBA-EMAX80; Hitachi High-Technology) was used to evaluate the presence as well as the adsorbed quantity of SO2 and NO2.


## 3. Results and Discussion

### 3.1. Characterization of Fe3O4 Nanoparticles

The porous, interconnected Fe3O4 nanoparticles were synthesized using a facile scalable method. The mixture solution of FeCl3 and FeCl2 was prepared with a Fe2+/Fe3+ molar ratio of 0.6. The used Fe2+/Fe3+ molar ratio is higher than the Fe2+/Fe3+ molar ratio in Fe3O4 because a part of Fe2+ oxidized to Fe3+ will lead to the lower Fe2+/Fe3+ molar ratio. The increase of Fe2+ concentration can attain the ideal Fe2+/Fe3+ molar ratio of ∼0.5. Then, the ammonium solution was added dropwise into the solution until pH = 11.0 to achieve Fe3O4 nanoparticles with high purity. Figure 1 shows the X-ray diffraction (XRD) patterns of the synthesized Fe3O4. All peaks were indexed to cubic Fe3O4 (JCPDS: 01-088-0315) without any impurity phase. However, γ-Fe2O3 has the same crystal structure and a lattice spacing as Fe3O4. To distinguish Fe3O4 from γ-Fe2O3, the d value of lattice spacing of (311) was calculated by Bragg’s equation. The d-spacing value (311) of Fe3O4 was calculated to be 2.5249 Å, similar to the value of 2.52516 Å (standard cubic Fe3O4: 01-088-0315). As reported by Iida et al. [18], the standard d-spacing value (311) of γ-Fe2O3 is 2.5177 Å (γ-Fe2O3, JCPDS: 00-039-1346). It can infer that the phase of the obtained material is completely matched with the phase of the standard cubic Fe3O4.Figure 1
XRD patterns of synthesized Fe3O4 nanoparticles.Moreover, TPRCO profiles of synthesized Fe3O4 further proved that the obtained material was pure Fe3O4. As shown in Figure 2, TPRCO profiles illustrated two distinct peaks of the reduction process, corresponding to two-step reduction of Fe3O4 ⟶ FeO and FeO ⟶ Fe. There was no peak of the Fe2O3 ⟶ Fe3O4 reduction process, referring that there was no presence of Fe2O3 in the synthesized material. The reduction temperature of the two-step reduction was approximately 285°C (Fe3O4 ⟶ FeO) and 595°C (FeO ⟶ Fe). These reduction reactions can be described as follows [19–21]:(1)Fe3O4+CO⟶3FeO+CO2FeO+CO⟶Fe+CO2Figure 2
TPRCO profile for Fe3O4 nanoparticles, a heating rate of 5°C/min.According to these reports [19–21], the reduction of Fe3O4 by carbon monoxide starts in the temperature range of ∼509–807°C, which was higher than the reduction temperature of Fe3O4 in our study. The lower reaction temperature can be explained due to nanometer-sized Fe3O4, reacting with carbon monoxide more easily than crystalline Fe3O4.The morphology of Fe3O4 was observed using FE-SEM. As shown in Figure 3(a), Fe3O4 exhibited a uniform granular shape with nanoparticles interconnected to each other. The Fe3O4 nanoparticles have a diameter of ∼10 nm. Besides, the interconnected Fe3O4 nanoparticles formed void spaces which intercalated among Fe3O4 nanoparticles as a porous structure. This unique structure plays an important role: it enables better the access of toxic gases to the inside of material, which can improve the adsorption capacity of materials [22]. Furthermore, using Scherrer’s equation, the size of Fe3O4 particles was calculated to be ∼9.8 nm based on the XRD results (Figure 1), suggesting the similar results between XRD and FE-SEM measurement. Figure 3(b) further affirmed the uniform of Fe3O4 nanoparticles with the narrow particle-size distribution and the nanosize of Fe3O4 [23]. The particle-size distribution of Fe3O4 changes from 7 to 13 nm, revealing the good uniform of Fe3O4 nanoparticles. The nanometer-sized Fe3O4 was expecting to achieve a high specific area. The porous structure of Fe3O4 was characterized using N2 adsorption-desorption isotherms. As shown in Figure 3(c), isotherm adsorption of Fe3O4 depicted typical type-IV (multilayer adsorption) that has a broad range of pore sizes and the Barrett–Joyner–Halenda (BJH) pore size distribution (inset) of the Fe3O4. Via BET and BJH analysis, the specific area, average pore diameter, and pore volume of Fe3O4 were determined to be approximately 115.90 m2/g, 10.63 nm, and 0.30 cm3/g, respectively. The relatively high pore size of the Fe3O4 particles allow the access of toxic gases inside materials easily. Moreover, the large surface area can facilitate the adsorption of stack gases at the material surface/gases interface [24]. The TGA has proceeded in an air atmosphere with a heating rate of 5°C/min from room temperature to 700°C to evaluate the purity of Fe3O4 materials again, as shown in Figure 3(d). The weight gain near 100°C was attributed to the oxidation of Fe3O4 to γ-Fe2O3. Interestingly, the oxidation temperature in our study is lower than the previous study [25]. This behaviour might be due to the high surface area of the nanometer-sized Fe3O4, making them react with oxygen more easily than crystalline Fe3O4. Nevertheless, the temperature near 100°C was also the evaporation temperature of moisture from the sample. Because the loss weight owing to the removal of moisture was lower than the weight gain due to the oxidation, the mass change in TG data just shows the mass gain. The content of moisture and Fe3O4 in the obtained sample was estimated to be 2.21 and 97.79%, respectively (Figure 3(d)).Figure 3
(a) FE-SEM image; (b) particle-size distribution; (c) N2 adsorption/desorption isotherms; (d) thermogravimetric analysis (TGA) of synthesized Fe3O4. Inset in Figure 3(c) shows the pore size distribution calculated from the Barrett–Joyner–Halenda (BJH) formula of synthesized Fe3O4.
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### 3.2. Adsorption Ability of Fe3O4 Nanoparticles

Figure4 shows the adsorption abilities of Fe3O4 after exposing to NO2 in conditions of the study. As shown in Figure 4(a), the intensities of the IR features after 15 minutes exposure to NO2 start to change, suggesting that the amount of NO2 was adsorbed on the surface of Fe3O4. The adsorption of NO2 at room temperature leads to an intensity increase of the 1386 cm−1 band, corresponding to the band of nitrate (NO3−) [26]. The adsorption mechanism of NO2 on the Fe3O4 surface was reported by Eltouny et al. [27] via the following equations:(2)2NO2gas⟶2NO2adsorbed2e−+2NO2adsorbed⟶2NO2−adsorbedNO2−adsorbed+NO2−adsorbed⟶NO3−adsorbed+NOgas+e−NO2−adsorbed+NO2gas⟶NO3−adsorbed+NOgasFigure 4
(a) FTIR of Fe3O4 samples after exposing to NO2 gas at the particular time; (b) NO2 adsorption on Fe3O4 samples; (c) SEM-EDX spectra; (d) FE-SEM image of Fe3O4 after 60 min of exposure to NO2. Inset in Figure 4(b) shows the XRD pattern of Fe3O4 after 60 min of exposure to NO2.
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(b)
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(d)The intensity of this peak increases as the time exposing to NO2 gas increases, suggesting that the larger amount of NO2 was adsorbed. The uptake of NO2 by the Fe3O4 nanoparticles was shown in Figure 4(b). The amount of NO2 adsorbed by the Fe3O4 reached the saturation level after 60 minutes of exposure. The mass of NO2 adsorbed by the Fe3O4 after 60 minutes of exposure was recorded at 108.5 mg/g of Fe3O4. The high adsorption values of the Fe3O4 sample were owing to its larger pore volume and high surface area. Furthermore, the presence and mass of NO2 in the Fe3O4 sample were also measured by SEM-EDX analysis. As shown in Figure 4(c), the signal of N, Fe, and O elements was detected in the Fe3O4 sample exposing to NO2 after 60 minutes. Based on the intensity of the N element in the sample, %wt. NO2 in the sample can be, respectively, calculated to be ∼9.79 %wt., corresponding to the NO2 adsorption of ∼108.6 mg/g of Fe3O4. This result was similar to the adsorption mass of NO2 by the Fe3O4 in Figure 4(b). The morphology of the Fe3O4 sample after 60 minutes of exposure to NO2 was analyzed using the FE-SEM measurement. There was no change in the morphology of Fe3O4 nanoparticles before (Figure 3(a)) and after (Figure 4(d)) exposing to the NO2 gas environment, referring to the stable structure of the Fe3O4 sample. Furthermore, the XRD pattern of Fe3O4 after exposing with NO2 (Figure 4(b) inset) did not show any change, compared with the XRD pattern of Fe3O4 (Figure 1). The good stable structure can retain the adsorption ability after the regeneration and repeating the NO2/Fe3O4 adsorption many times.The adsorption abilities of Fe3O4 were further examined with the SO2 gas. Figure 5 shows the adsorption ability of Fe3O4 with SO2. Figure 5(a) shows the FTIR spectra obtained after exposing the Fe3O4 sample to SO2 at different times. The signal at the band of ∼1400 and ∼1120 cm−1, corresponding to the species of physical adsorption of SO2, was detected [15]. However, the intensities of SO2 species were low, suggesting that the small amount of SO2 was adsorbed on the surface of the Fe3O4 sample. The adsorbed amount of SO2 was displayed in Figure 5(b). The mass of SO2 adsorbed by the Fe3O4 was recorded to 40.5 mg/g of Fe3O4 at the saturation level after 60 minutes of exposure. Moreover, the presence and the adsorption mass of SO2 in the Fe3O4 sample was evaluated by SEM-EDX analysis (Figure 5(c)). The signal of the S element was found to be 1.95 %wt. of SO2 in the sample, corresponding to 3.90 SO2 %wt. in the sample after 60 minutes of exposure. This quantity was calculated to ∼40.6 mg/g of Fe3O4, similar to the obtained adsorption of Fe3O4 in Figure 5(b). The surface of the Fe3O4 sample after 60 minutes of exposure with SO2 was analyzed using FE-SEM measurement. As shown in Figure 5(d), the highly porous structure of Fe3O4 was sustained, and no significant change was realized (compared with Figure 3(a)). Moreover, the phase state of Fe3O4 (Figure 5(b) inset) after the adsorption also remains, demonstrating the stable-phase state and porous structure. This structure was promising to be recycled and continuously adsorbs gases many times.Figure 5
(a) FTIR of Fe3O4 samples after exposing to SO2 gas at the particular time; (b) SO2 adsorption on Fe3O4 samples; (c) SEM-EDX spectra; (d) FE-SEM image of Fe3O4 after 60 min of exposure to SO2. Inset in Figure 5(b) shows the XRD pattern of Fe3O4 after 60 min of exposure to SO2.
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(b)
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## 3.1. Characterization of Fe3O4 Nanoparticles

The porous, interconnected Fe3O4 nanoparticles were synthesized using a facile scalable method. The mixture solution of FeCl3 and FeCl2 was prepared with a Fe2+/Fe3+ molar ratio of 0.6. The used Fe2+/Fe3+ molar ratio is higher than the Fe2+/Fe3+ molar ratio in Fe3O4 because a part of Fe2+ oxidized to Fe3+ will lead to the lower Fe2+/Fe3+ molar ratio. The increase of Fe2+ concentration can attain the ideal Fe2+/Fe3+ molar ratio of ∼0.5. Then, the ammonium solution was added dropwise into the solution until pH = 11.0 to achieve Fe3O4 nanoparticles with high purity. Figure 1 shows the X-ray diffraction (XRD) patterns of the synthesized Fe3O4. All peaks were indexed to cubic Fe3O4 (JCPDS: 01-088-0315) without any impurity phase. However, γ-Fe2O3 has the same crystal structure and a lattice spacing as Fe3O4. To distinguish Fe3O4 from γ-Fe2O3, the d value of lattice spacing of (311) was calculated by Bragg’s equation. The d-spacing value (311) of Fe3O4 was calculated to be 2.5249 Å, similar to the value of 2.52516 Å (standard cubic Fe3O4: 01-088-0315). As reported by Iida et al. [18], the standard d-spacing value (311) of γ-Fe2O3 is 2.5177 Å (γ-Fe2O3, JCPDS: 00-039-1346). It can infer that the phase of the obtained material is completely matched with the phase of the standard cubic Fe3O4.Figure 1
XRD patterns of synthesized Fe3O4 nanoparticles.Moreover, TPRCO profiles of synthesized Fe3O4 further proved that the obtained material was pure Fe3O4. As shown in Figure 2, TPRCO profiles illustrated two distinct peaks of the reduction process, corresponding to two-step reduction of Fe3O4 ⟶ FeO and FeO ⟶ Fe. There was no peak of the Fe2O3 ⟶ Fe3O4 reduction process, referring that there was no presence of Fe2O3 in the synthesized material. The reduction temperature of the two-step reduction was approximately 285°C (Fe3O4 ⟶ FeO) and 595°C (FeO ⟶ Fe). These reduction reactions can be described as follows [19–21]:(1)Fe3O4+CO⟶3FeO+CO2FeO+CO⟶Fe+CO2Figure 2
TPRCO profile for Fe3O4 nanoparticles, a heating rate of 5°C/min.According to these reports [19–21], the reduction of Fe3O4 by carbon monoxide starts in the temperature range of ∼509–807°C, which was higher than the reduction temperature of Fe3O4 in our study. The lower reaction temperature can be explained due to nanometer-sized Fe3O4, reacting with carbon monoxide more easily than crystalline Fe3O4.The morphology of Fe3O4 was observed using FE-SEM. As shown in Figure 3(a), Fe3O4 exhibited a uniform granular shape with nanoparticles interconnected to each other. The Fe3O4 nanoparticles have a diameter of ∼10 nm. Besides, the interconnected Fe3O4 nanoparticles formed void spaces which intercalated among Fe3O4 nanoparticles as a porous structure. This unique structure plays an important role: it enables better the access of toxic gases to the inside of material, which can improve the adsorption capacity of materials [22]. Furthermore, using Scherrer’s equation, the size of Fe3O4 particles was calculated to be ∼9.8 nm based on the XRD results (Figure 1), suggesting the similar results between XRD and FE-SEM measurement. Figure 3(b) further affirmed the uniform of Fe3O4 nanoparticles with the narrow particle-size distribution and the nanosize of Fe3O4 [23]. The particle-size distribution of Fe3O4 changes from 7 to 13 nm, revealing the good uniform of Fe3O4 nanoparticles. The nanometer-sized Fe3O4 was expecting to achieve a high specific area. The porous structure of Fe3O4 was characterized using N2 adsorption-desorption isotherms. As shown in Figure 3(c), isotherm adsorption of Fe3O4 depicted typical type-IV (multilayer adsorption) that has a broad range of pore sizes and the Barrett–Joyner–Halenda (BJH) pore size distribution (inset) of the Fe3O4. Via BET and BJH analysis, the specific area, average pore diameter, and pore volume of Fe3O4 were determined to be approximately 115.90 m2/g, 10.63 nm, and 0.30 cm3/g, respectively. The relatively high pore size of the Fe3O4 particles allow the access of toxic gases inside materials easily. Moreover, the large surface area can facilitate the adsorption of stack gases at the material surface/gases interface [24]. The TGA has proceeded in an air atmosphere with a heating rate of 5°C/min from room temperature to 700°C to evaluate the purity of Fe3O4 materials again, as shown in Figure 3(d). The weight gain near 100°C was attributed to the oxidation of Fe3O4 to γ-Fe2O3. Interestingly, the oxidation temperature in our study is lower than the previous study [25]. This behaviour might be due to the high surface area of the nanometer-sized Fe3O4, making them react with oxygen more easily than crystalline Fe3O4. Nevertheless, the temperature near 100°C was also the evaporation temperature of moisture from the sample. Because the loss weight owing to the removal of moisture was lower than the weight gain due to the oxidation, the mass change in TG data just shows the mass gain. The content of moisture and Fe3O4 in the obtained sample was estimated to be 2.21 and 97.79%, respectively (Figure 3(d)).Figure 3
(a) FE-SEM image; (b) particle-size distribution; (c) N2 adsorption/desorption isotherms; (d) thermogravimetric analysis (TGA) of synthesized Fe3O4. Inset in Figure 3(c) shows the pore size distribution calculated from the Barrett–Joyner–Halenda (BJH) formula of synthesized Fe3O4.
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## 3.2. Adsorption Ability of Fe3O4 Nanoparticles

Figure4 shows the adsorption abilities of Fe3O4 after exposing to NO2 in conditions of the study. As shown in Figure 4(a), the intensities of the IR features after 15 minutes exposure to NO2 start to change, suggesting that the amount of NO2 was adsorbed on the surface of Fe3O4. The adsorption of NO2 at room temperature leads to an intensity increase of the 1386 cm−1 band, corresponding to the band of nitrate (NO3−) [26]. The adsorption mechanism of NO2 on the Fe3O4 surface was reported by Eltouny et al. [27] via the following equations:(2)2NO2gas⟶2NO2adsorbed2e−+2NO2adsorbed⟶2NO2−adsorbedNO2−adsorbed+NO2−adsorbed⟶NO3−adsorbed+NOgas+e−NO2−adsorbed+NO2gas⟶NO3−adsorbed+NOgasFigure 4
(a) FTIR of Fe3O4 samples after exposing to NO2 gas at the particular time; (b) NO2 adsorption on Fe3O4 samples; (c) SEM-EDX spectra; (d) FE-SEM image of Fe3O4 after 60 min of exposure to NO2. Inset in Figure 4(b) shows the XRD pattern of Fe3O4 after 60 min of exposure to NO2.
(a)
(b)
(c)
(d)The intensity of this peak increases as the time exposing to NO2 gas increases, suggesting that the larger amount of NO2 was adsorbed. The uptake of NO2 by the Fe3O4 nanoparticles was shown in Figure 4(b). The amount of NO2 adsorbed by the Fe3O4 reached the saturation level after 60 minutes of exposure. The mass of NO2 adsorbed by the Fe3O4 after 60 minutes of exposure was recorded at 108.5 mg/g of Fe3O4. The high adsorption values of the Fe3O4 sample were owing to its larger pore volume and high surface area. Furthermore, the presence and mass of NO2 in the Fe3O4 sample were also measured by SEM-EDX analysis. As shown in Figure 4(c), the signal of N, Fe, and O elements was detected in the Fe3O4 sample exposing to NO2 after 60 minutes. Based on the intensity of the N element in the sample, %wt. NO2 in the sample can be, respectively, calculated to be ∼9.79 %wt., corresponding to the NO2 adsorption of ∼108.6 mg/g of Fe3O4. This result was similar to the adsorption mass of NO2 by the Fe3O4 in Figure 4(b). The morphology of the Fe3O4 sample after 60 minutes of exposure to NO2 was analyzed using the FE-SEM measurement. There was no change in the morphology of Fe3O4 nanoparticles before (Figure 3(a)) and after (Figure 4(d)) exposing to the NO2 gas environment, referring to the stable structure of the Fe3O4 sample. Furthermore, the XRD pattern of Fe3O4 after exposing with NO2 (Figure 4(b) inset) did not show any change, compared with the XRD pattern of Fe3O4 (Figure 1). The good stable structure can retain the adsorption ability after the regeneration and repeating the NO2/Fe3O4 adsorption many times.The adsorption abilities of Fe3O4 were further examined with the SO2 gas. Figure 5 shows the adsorption ability of Fe3O4 with SO2. Figure 5(a) shows the FTIR spectra obtained after exposing the Fe3O4 sample to SO2 at different times. The signal at the band of ∼1400 and ∼1120 cm−1, corresponding to the species of physical adsorption of SO2, was detected [15]. However, the intensities of SO2 species were low, suggesting that the small amount of SO2 was adsorbed on the surface of the Fe3O4 sample. The adsorbed amount of SO2 was displayed in Figure 5(b). The mass of SO2 adsorbed by the Fe3O4 was recorded to 40.5 mg/g of Fe3O4 at the saturation level after 60 minutes of exposure. Moreover, the presence and the adsorption mass of SO2 in the Fe3O4 sample was evaluated by SEM-EDX analysis (Figure 5(c)). The signal of the S element was found to be 1.95 %wt. of SO2 in the sample, corresponding to 3.90 SO2 %wt. in the sample after 60 minutes of exposure. This quantity was calculated to ∼40.6 mg/g of Fe3O4, similar to the obtained adsorption of Fe3O4 in Figure 5(b). The surface of the Fe3O4 sample after 60 minutes of exposure with SO2 was analyzed using FE-SEM measurement. As shown in Figure 5(d), the highly porous structure of Fe3O4 was sustained, and no significant change was realized (compared with Figure 3(a)). Moreover, the phase state of Fe3O4 (Figure 5(b) inset) after the adsorption also remains, demonstrating the stable-phase state and porous structure. This structure was promising to be recycled and continuously adsorbs gases many times.Figure 5
(a) FTIR of Fe3O4 samples after exposing to SO2 gas at the particular time; (b) SO2 adsorption on Fe3O4 samples; (c) SEM-EDX spectra; (d) FE-SEM image of Fe3O4 after 60 min of exposure to SO2. Inset in Figure 5(b) shows the XRD pattern of Fe3O4 after 60 min of exposure to SO2.
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## 4. Conclusions

Fe3O4 nanoparticles have successfully been synthesized via the facile scalable method, in which Fe3O4 nanoparticles well achieved a porous structure with the high specific area and purity. The Fe3O4 nanoparticles exhibited good adsorption with toxic gases. The surface of the Fe3O4 nanoparticles adsorbed high amounts of SO2 and NO2 of 40.5 mg/g of Fe3O4 and 108.5 mg/g of Fe3O4, respectively, at a saturation state after 60 minutes of exposure. The high adsorption performance was attributed to the high porosity and specific areas, which enabled the better diffusion and adsorption of toxic gases to the surface of the porous material. The resultant structure of Fe3O4 after exposing to the toxic gases would be the good adsorbent, which can be regenerated and adsorb gases many times.



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*Source: 9742826-2019-06-12.xml*