One-Pot Hydrothermal Synthesis, Characterization, and Desulfurization Performance of ZnFe2O4/AC Composites
ZnFe2O4/AC composites were prepared by the one-pot hydrothermal method using the activated carbon (AC) as a carrier. The synthesis conditions were optimized by a single-factor experiment. The structural, textural, and surface properties of the adsorbent have been comprehensively characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared (FT-IR) spectroscopy, Brunauer–Emmett–Teller (BET) measurements, and X-ray photoelectron spectroscopy (XPS) analysis. The SO2 removal capacities of the composites were investigated via testing the adsorption capacity at the self-made desulfurization equipment. The results show that the adsorption capacity of ZnFe2O4/AC composites is much higher than that of the AC and ZnFe2O4 samples, respectively. The composite overcomes the disadvantages of the traditional sintering, showing a very high desulfurization performance. The breakthrough time was 147 min, and the sulfur adsorption capacity could reach 23.67% in the desulfurization performance test.
In recent years, with the rapid development of economy and change of the energy consumption structure, the sulfur dioxide (SO2) emission increased gradually. SO2 is a poisonous and hazardous gas which could induce the formation of acidic rain potentially, causing a severe threat to the human health [1–3]. Therefore, a variety of methods have been developed for the removal of sulfur dioxide. Generally, the adsorption by activated carbon (AC) is a feasible strategy for desulfuration [4–6]. In this context, numerous studies have tested activated carbon, due to their unique characteristics, for example, large specific surface area, uniform pore size distribution, and amenability to surface function [5, 7, 8]. Recent studies have shown that activated carbon loaded with metal oxide or the surface functionalized with a suitable reagent is more efficient with respect to the catalytic oxidation of SO2 [9–11].
A considerable amount of research has been conducted on the suitability of various activated carbon loaded with metal oxide for SO2 removal [1, 5]. This composite material can adsorb the contaminants more efficiently than AC or metal oxide alone. Embedding metal oxides in a precursor would enhance the gas adsorption capacity of the resulting composite AC [12, 13], since metal oxide could yield the selective sorbent capacity and the high thermal stability at elevated temperatures [14, 15]. Usually, metal oxides are the potential candidates for SO2 removal. Alkali metal oxides or alkaline earth metal oxides have strong affinity to SO2 even in humid gas atmosphere . Recently, Zn-Fe-based sorbents have been attracted an increasing attention for their effective adsorption of SO2 [17, 18]. Lee et al.  have demonstrated that Ce-Fe-Mg-based sorbents showed excellent sulfur removal capacity and regeneration ability. Podwórny et al.  have interpreted the behaviors of MgO-MgFe2O4 spinels in the gaseous environment having high concentration of SO2. However, the composite metal oxides can easily result in the formation of agglomerated particles when contacted with SO2, which strongly limits their capacity. Thus, it is of great significance to incorporate the metal oxide on the surface of activated carbon. The presence of activated carbon as a carrier can improve the dispersability of metal oxides and reduce the extent of metal oxide aggregation, ultimately strengthening the activity of desulfurization.
In this study, the composite metal oxides ZnFe2O4 with activated carbon materials as a substrate by one-pot hydrothermal synthesis were prepared . On the basis of the conventional hydrothermal synthesis method, the compound was obtained under the dispersion effect of the surfactant cetyl trimethyl ammonium bromide, which eventually made the nanometer ZnFe2O4 spinel to be loaded on the activated carbon. Specifically, the developed preparation strategy of composite materials rarely requires the high-temperature calcination to yield an efficient desulfurization. This strategy also can solve the problem of the aggregation of ZnFe2O4, providing a potential industrial application for desulfurization.
2. Materials and Methods
AC supplied by Chinese Medicine Group Chemical Reagent Co., Ltd. (Shanghai, China) was grounded into particles with sizes of 0.35–0.16 mm (40–100 mush) as substrates. Fe(NO3)3·9H2O and Zn(NO3)2·6H2O of analytical grade were purchased from Tianjin Fuchen Chemical Reagent Co., Ltd., China. Cetyl trimethyl ammonium bromide (CTAB, C16H33N+(CH3)3Br−; purity: >98%) was obtained from Shanghai Aladdin biochemical Polytron Technologies Inc, China. N2 and O2 with purity of 99.999% were purchased from Huaxinda Gas Corporation (Fuzhou, China). SO2 was supplied by Guangdong Huate gas Limited by Share Ltd. (Guangdong, China). The simulated flue gas was prepared which contains 1000 ppm SO2, 5% O2, 5% H2O, and balance N2 gas.
2.2. Preparation of Composite Materials
2.2.1. Pretreatment of Activated Carbon
Firstly, activated carbon was completely washed by deionized water three times. Subsequently, it was placed at 120°C drying for 2 h and then cooled at room temperature for further use.
2.2.2. Preparation of ZnFe2O4/AC Composites
ZnFe2O4/AC composites powder was prepared in the presence of CTAB by using the hydrothermal method. Briefly, 5.0 gram of AC, as the substrate, was added in the mixed solution of 100 mL Zn(NO3)2 (0.2 M) and 100 mL Fe(NO3)3 (0.4 M), which was then mixed with 1 g of CTAB to form a homogeneous solution. The mixed solution, with a stoichiometric ratio of 1 : 2 ([Zn] : [Fe]), was stirred magnetically at 273 K for 30 min. Sodium hydroxide solution (6.0 M) was slowly added into the mixture to obtain a solution with a pH of 10.0. This solution was pretreated in an ultrasonic water bath for 30 min and then transferred to a 500 mL Teflon-lined stainless steel autoclave to keep it at 130°C for 24 h in an oven. Subsequently, the autoclave was cooled to room temperature gradually. Finally, the precipitate was centrifuged and washed with deionized water several times to reduce the impurities in the product. The resulting product was dried in a vacuum oven at 75°C for 6 h, marked as ZnFe2O4/AC.
2.3. Characterization of ZnFe2O4/AC Composites
The obtained samples were characterized on a Rigaku D/Max2550VB/PC X-ray powder diffractometer (XRD) with a Cu Kα radiation source and operated at a voltage of 40 kV and a current of 100 mA. Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet FT-170SX instrument using KBr discs in the 400–4000 cm−1 region. Scanning electron microscopy (SEM) was performed with a JEOLJSM-6360 LV microscope. Chemical composition of the particles was determined by energy-dispersive spectrometry (EDS). Specific BET surface area and pore size distribution were determined with an Autosorb-1-C chemisorption/physisorption analyzer (Quantachrome, USA) by nitrogen adsorption at 77.3 K. The surface composition of the ACS-1 sorbent was obtained using a Thermo Scientific ESCALAB 250 Xi spectrometer (Thermo Fisher Scientific) equipped with monochromatic Al Kα (1486.6 eV) X-rays. The X-ray source was operated at 15 kV and 10 mA. The core-level XPS spectra for C1s, O1s, Zn2p, Fe2p, and S2p were measured at a step pace of 0.1 eV. The binding energies were calibrated taking C1s as a standard with a measured typical value of 248.6 eV. Each spectrum was fitted using a Voigt function (mixed Lorentzian–Gaussian) with Shirley backgrounds.
2.4. SO2 Adsorption Tests
The adsorption tests of ZnFe2O4/AC were carried out in a horizontally oriented quartz tube reactor (16.0 mm × 100.0 mm), and the results were shown in Figure 1. In each experiment, ca. 1.0 g of the ZnFe2O4/AC sorbent was packed into the reactor . The adsorption tests were conducted at atmospheric pressure with a temperature of 35 ± 0.5°C and a gas hourly space velocity of 149.28 ± 10 h−1. The flow rates of SO2, O2, H2O, and N2 were precisely controlled by mass flow controllers. The concentration of SO2 was measured by a flue gas analyzer (model PGD3-IR, Wuhan Quartet Photoelectric Technology Co., Ltd.). When the concentration of SO2 exceeded 1000 ppm in the outlet gas, the adsorption tests were terminated immediately. The breakthrough sulfur capacity of the ZnFe2O4/AC sorbent was calculated as follows :where Sc is the breakthrough sulfur capacity of ZnFe2O4/AC (mg of S/g of sorbent), vsp is the gas hourly space velocity (STP) of the simulated flue gas (h−1), Vbed is the volume of the sorbent in the reactor (mL), m is the weight of the sorbent in the reactor (g), M is the atomic weight of sulfur (g·mol−1), Vmol is the molar volume of gas (22.4 L·mol−1 under standard conditions), Cin and Cout are the inlet and outlet total SO2 concentrations (%), and t is the breakthrough time (BT) (h).
2.5. The Load Rate of ZnFe2O4 on ZnFe2O4/AC Composite
The ZnFe2O4/AC composite was calcined at 1000°C for 2 h. The activated carbon was completely burned, and the rest was ZnFe2O4. The ZnFe2O4 load rate is the remaining ZnFe2O4 mass divided by the quality of the burned activated carbon (AC).where R is the ZnFe2O4 load rate on activated carbon per unit mass (g of ZnFe2O4/g of AC), ms is the remaining ZnFe2O4 mass (g), and mb is the quality of the burned activated carbon (g).
3. Results and Discussion
3.1. Preparation of ZnFe2O4/AC Composite
In the process of preparing the ZnFe2O4/AC composite material by hydrothermal synthesis, the load ratio, ultrasonic time, and drying temperature have severe influence on the adsorption performance for SO2 removal.
3.1.1. Effect of Mass Ratio between Zinc Ferrite and Activated Carbon on Desulfurization
As shown in Figure 2, the mass ratio of zinc ferrite to activated carbon is an important factor affecting the adsorption performance of ZnFe2O4/AC composites on SO2 removal. In order to study the effect of the adsorption performance on SO2 removal through the change of mass ratio (3 : 5, 4 : 5, 5 : 5, 6 : 5, and 7 : 5), the ultrasonic time was set for 60 min and the reaction temperature set at 130°C. The result showed that the sulfur adsorption capacity gradually increased when the mass ratio of zinc ferrite to activated carbon increased from 3 : 5 to 5 : 5. However, the mass ratio reached 6 : 5, and the sulfur capacity of composites gradually declined. Therefore, when the mass ratio was set to 5 : 5, the composite material has the best adsorption performance for SO2 removal with largest sulfur capacity. The main reason was due to the fact that a small amount of zinc ferrite loading causes a low adsorption activity and sulfur capacity. However, at high ZnFe2O4 load, the multilayer coverage phenomenon at the pore structure of activated carbon occurs, resulting in the decreased effective surface area and decreased capacity of the sulfur desulfurization agent.
The ZnFe2O4/AC composite powder, prepared at different mass ratios of zinc ferrite to activated carbon, was also characterized by XRD (Figure 3). XRD patterns showed that ZnFe2O4/AC composite powder had cubic spinel structures, which were consistent with the values reported in the database of ZnFe2O4 (JCPDS 22-1012). The peak intensity reached the highest position at the mass ratio of 5 : 5.
3.1.2. Effect of Ultrasonic Time on Desulfurization Ability of ZnFe2O4/AC Composite
In this case, the experiment controls the condition of the mass ratio of zinc ferrite to activated carbon being at 5 : 5, and the hydrothermal temperature was set at 130°C. The impact of ultrasonic time on desulfurization ability of the ZnFe2O4/AC composite was discussed (Figure 4). Table 1 shows the influence of ultrasonic time on the ZnFe2O4 load ratio.
As shown in Figure 4, without ultrasonic waves, the sulfur capacity reached only 8.85%, while the sulfur capacity of the desulfurizer was enhanced as the ultrasonic duration prolonged. The adsorption capacity of the desulfurizer was enhanced significantly with the effect of ultrasonic waves. When the ultrasonic time was at 60 min, the optimal sulfur capacity of the desulfurizer reached 20.01%. It can be seen that, during the preparation of the desulfurizer, the force generated and hindered the formation of the zinc ferrite crystal with the effect of ultrasonic waves. Therefore, the crystal had preferable dispersability and uniformity. In the presence of ultrasonic waves, the activated component dispersed uniformly on activated carbon and formed the monolayer coverage to enhance the desulfurization ability of the ZnFe2O4/AC composite. As shown in Table 1, the ZnFe2O4 load ratio declined after prolonging the ultrasonic time to 90 and 120 min. Combined with Figure 4, it can be seen that the sulfur capacity of the desulfurizer declined because of the declination in the loading rate efficiency of zinc ferrite.
3.1.3. Effect of Hydrothermal Temperature on Desulfurization Ability of ZnFe2O4/AC Composite
With the condition of the mass ratio of zinc ferrite to activated carbon being at 5 : 5 and the ultrasonic time at 60 min, the impact of hydrothermal temperature on desulfurization ability of the ZnFe2O4/AC composite was discussed (Figure 5).
As shown in Figure 5, the desulfurization ability of the ZnFe2O4/AC composite first increased and then decreased with the increase of hydrothermal temperature. The optimal adsorption capacity of sulfur dioxide reached 20.01% at the hydrothermal temperature of 130°C.
The hydrothermal temperature controlling was essential for the desulfurizer composite to form the desulfurization ability, and it influenced the allocation and growth of the crystal grain. During the hydrothermal reaction process, water was the solvent, and in the sealed pressure vessel, the chemical reaction was proceeded under the condition of high temperature and high pressure. The hydrothermal synthesis reaction was in the molecular level, enhancing the reactivity. The compound with the desulfurization ability generated in the preparation of the desulfurizer composite through a series of reactions. The different hydrothermal temperatures made a big difference in the desulfurization ability of the desulfurizer. Increasing the hydrothermal temperature not only can improve the growth of the zinc ferrite crystal grain but also can decline the efficient specific surface area and enhance the aperture of the desulfurizer.
3.2. Characterization of ZnFe2O4/AC Composite
3.2.1. SEM and EDS Studies
Figure 6 shows the SEM images of AC (a), ZnFe2O4 (b), ZnFe2O4/AC composite material at low magnification (c), and ZnFe2O4/AC composite material at high magnification (d). For ZnFe2O4/AC composite material, the diameter range was 20-50 nm, which implied that it was uniform distributed diameter sphere. This kind of rough structure was beneficial to improve the adsorption performance and also to the transmission of SO2 gas on the adsorbent surface [13, 17].
The EDS analysis of the fresh ZnFe2O4/AC composite material sample and sulfated sample is shown in Figure 7. The spectrum clearly indicates the presence of C, Zn, Fe, and O elements in ZnFe2O4/AC composite material powder. The determined atomic ratio of Zn to Fe in the powder was 1 : 2.02, which was in close proximity to the theoretical ratio in ZnFe2O4. As shown in Figure 7(b), S element appeared in the EDS spectrum of the sulfated sample; thus, it proved that the SO2 was adsorbed on the surface composite materials.
3.2.2. FT-IR and XRD Studies
The FT-IR spectra of the activated carbon sample and ZnFe2O4/AC composite in the range 4000–400 cm−1 are shown in Figure 8. Compared with ZnFe2O4 (a) and activated carbon (b), the FT-IR spectra of ZnFe2O4/AC composites (c) retain the original basic characteristic absorption peak. The spectrum clearly shows a broad absorption around 3400 cm−1, which was a characteristic stretching vibration of hydroxylate (O–H). Peaks localized at 2920 cm−1 and 2849 cm−1 are assigned to asymmetrical and symmetrical vibration of methylene (–CH2–), respectively. Apart from the original characteristics of the activated carbon absorption peak at ZnFe2O4/AC (c), there are two obvious characteristic peaks in the range of 600–400 nm as same as the peak at ZnFe2O4 (a), corresponding to the wave numbers 559 cm−1 and 425 cm−1, which are considered to the infrared spectrum of ferrite composite compounds. The broad and intense band at 559 cm−1 was ascribed to the stretching of Zn–O bond in tetrahedral cations. The broad and intense band at 425 cm−1 was attributed to the stretching of Fe–O bond in octahedral cations. In addition to these characteristic peaks, it can be seen that there was no other impurity peak in zinc ferrite.
X-ray diffraction patterns for activated carbon, ZnFe2O4, and ZnFe2O4/AC system were presented in Figure 9. The activated carbon was presented an undefined structure, which has been pretreated. The zinc ferrite spinel has been loaded on the activated carbon when compared with ZnFe2O4 and ZnFe2O4/AC. These diffraction lines provide clear evidence of the formation of single-phase cubic spine structure. All the diffraction peaks were indexed to the cubic structured literature (JCPDS card number 89-1012, with α = 8.433 Å) .
3.2.3. BET Analyses
Figure 10 shows the N2 adsorption/desorption isotherms of AC, ZnFe2O4, and ZnFe2O4/AC. Table 2 shows the specific surface area, pore volume, and average pore diameter of the samples. The profiles reveal that the adsorption curves of AC and ZnFe2O4/AC are similar. The isotherm of modified activated carbon exhibits a high Type I adsorption/desorption shape with an H4 hysteresis loop in the IUPAC classification. ZnFe2O4 shows a Type IV isotherm and an H3 hysteresis loop. The y-axis of the three materials at the low-pressure end indicates that the material has stronger force with nitrogen. When the micropores are present, the adsorption curve was Type I or IV due to the strong adsorption potential, and the pressure end was mostly due to nitrogen condensate accumulation in the material channel, showing a steady trend. It’s high-pressure side that is related with the degree of particle accumulation, which can explain the particles may not be uniform through the Type I in the rise of High-pressure side. It can be seen from the graph that the adsorption capacity increases slowly in the low-pressure stage. At this time, the N2 molecules adsorb on the inner surface of the mesopores in a single layer to multiple layers. N2 adsorption was the amount of physical adsorption, and the size of the specific surface area corresponding to the modified activated carbon adsorption on the N2 decreased with the corresponding reduction in the surface area [7, 17]. Table 2 shows that AC has a specific surface area of 1273.6 m2·g−1 and a pore size of 2.48 nm. The specific surface area of ZnFe2O4/AC was reduced to 435.9 m2·g−1. After the load modification, the adsorption curve appeared to change the position of the hysteresis loop. The position of P/P0 was changed from 0.45∼1.0 to 0.30∼1.0, which belonged to the micropore adsorption before the hysteresis loop. Mesoporous adsorption after the hysteresis loop indicated that the pore size was increased after modification. Table 2 shows that the pore size increased by about 1 nm, which may be caused by the problem of the accumulation and density of samples during the composite modification process. The metal oxide blocks some of the pores, reduces pore volume, and increases the average pore size .
3.2.4. XPS Characterization
Figure 11 shows that the surface elemental composition and valence of ZnFe2O4/AC were characterized by XPS. The binding energy of C1s was 284.5 eV, which was assigned to graphite carbon. Figure 11(a) shows that the prepared samples contain peaks of Zn, Fe, O, C, and N. Figure 11(b) shows that the binding energies at 1021.4 eV and 1044.4 eV, respectively, correspond to Zn 2p3/2 and Zn 2p1/2, which means that the main valence of zinc was +2 on the surface of the sample. Figure 11(c) shows that the Fe element contains two more obvious peaks at 710.9 eV and 724.4 eV corresponding to Fe2+ and Fe3+ corresponding to the peaks of Fe 2p3/2 and Fe 2p1/2. The oxidation state of iron coexists with the mixed phase because there was a weak peak at the binding energy of 719.1 eV, which coincides with the reported Fe3+ compound. The proportion of the two elements can be seen, and the material is ZnFe2O4.
The S2p peak appeared in the XPS high-resolution energy spectrum after the treatment of ZnFe2O4/AC with SO2. To analyze these spectra in more detail and to apply literature data for comparison, we deconvoluted the S2p lines into two components. The comparison with literature data allows unambiguous assignment of these S2p3/2 features to sulfite (165.8–167.5) and sulfate (168.9–171.1) species . Figure 11(d) shows that the binding energies of the peak of sulfur were 166.7 eV and 168.9 eV, which are in good agreement with SO32− and SO42− states, respectively. Therefore, the ZnFe2O4/AC composite surface contains a certain amount of both sulfite and sulfate species after the adsorption of SO2, which further indicates that the chemical adsorption took place .
3.3. Adsorption of SO2
Figure 12 shows the SO2 adsorption abilities of AC, ZnFe2O4 particles, and the ZnFe2O4/AC sample at the self-made desulfurization equipment. It was seen that the adsorption capacity of the ZnFe2O4/AC composite is much higher than that of the AC and ZnFe2O4 samples. This suggests that the ZnFe2O4/AC composite should be a better adsorbent for SO2 abatement. The pure ZnFe2O4 was mainly in the form of powder, which was easy to agglomerate and adversely affected its desulfurization performance . The presence of activated carbon as a carrier can improve the dispersing property of ZnFe2O4, reduce the size of ZnFe2O4 aggregates, and improve the desulfurization activity of ZnFe2O4. The adsorption capacity was consistent with the surface area, pore size distribution, and active principal part of the adsorbent. A substantial percentage of pores for the ZnFe2O4/AC composite was now in the quasi-micropore region (below 5 nm), and molecules may be adsorbed following the micropore-filling mechanism before they enter the pore body. Because the pore opening was smaller than the pore body, molecules will have difficulty in escaping once they have been adsorbed into the pore body. But the SO2 removal capacity of the AC was only 5.32%, which indicated that the physical adsorption of AC on the SO2 was smaller. The SO2 removal effect of the composite was mainly composed of activated carbon and surface-supported ZnFe2O4.
In this paper, ZnFe2O4/AC was successfully prepared by hydrothermal synthesis using activated carbon as the carrier. The best preparation conditions were as follows: the ratio of ZnFe2O4 to AC was 1 : 1, ultrasonic time was 60 min, and the hydrothermal reaction temperature was maintained at 130°C. ZnFe2O4/AC was characterized by SEM, FT-IR, XRD, EDS, BET, and XPS. The results concluded that ZnFe2O4/AC has a rough structure, much higher surface area, and pore volume. The structure of ZnFe2O4/AC should be a better adsorbent for SO2 abatement. The results from XPS reveal that sulfite and sulfate species can be formed on the surface of ZnFe2O4/AC. Compared with ZnFe2O4 and AC, the desulfurization performance of ZnFe2O4/AC has been greatly improved showing the synergistic effect. The product of the breakdown time was 147 min, and the sulfur capacity was 20.01% in the desulfurization performance test.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
The authors gratefully acknowledge the Science and Technology Project of Cooperation in Universities and Colleges in Fujian Province (no. 2014H6013) and Technical Innovation Items of Enterprises in Fujian Province for the financial support.
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