Received date: July 19, 2016; Accepted date: August 03, 2016; Published date: August 04, 2016
Citation: Yulin Li, Han X, Guo Y, Hou Y, Huan Z, et al. (2016) Selective Catalytic Reduction of NOx over Fe/TiO2 Prepared by F127-Template Method at Mid- Temperature. J Environ Anal Chem 3: 185. doi:10.4172/2380-2391.1000185
Copyright: © 2016 Yulin Li, et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
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A series of catalysts based on Fe/TiO2 were synthesized by template, impregnation, and co-precipitation methods for mid-temperature selective catalytic reduction of NO with NH3. The samples were characterized by Brunauer-Emmett- Teller (BET), X-ray diffraction (XRD), temperature programmed desorption (TPD) and Diffuse reflectance infrared Fourier transform spectra (DRIFTS). Among these catalysts, the sample prepared by template method exhibited the best catalytic performance. Analyses indicated that large specific surface area, good dispersion of active phase, and stronger NH4+ adsorption on Brønsted acid sites might be the main reasons for the high catalytic performance of Fe/TiO2 prepared by template method.
NH3-SCR; Fe/TiO2; Template method; Mid-temperature
Nitrogen oxides (NOx), emitted from stationary and mobile sources, is one of the major atmospheric pollutants, which can cause a series of environmental problems such as acid rain, photochemical smog and ozone depletion . Selective catalytic reduction (SCR) of NO with NH3 is currently proved to be the most commonly and effective technology to remove NOx in the flue gas from stationary sources. In the technology, V2O5/TiO2 promoted by WO3 or MoO3 is the most commercially used catalyst , which exhibits excellent catalytic performance within the temperature range of 330-400°C. However, some drawbacks such as a narrow operating temperature window, dust block, heavy metal deactivated, and biological toxicity caused by vanadium species limit its application [3,4]. Hence, lots of efforts have been devoted to develop novel vanadium-free catalysts with the advantages of wide temperature usable range, high catalytic performance and friendly to environment.
In recent years, Fe-based catalysts have attracted great interest for its good catalytic ability in medium temperature, which can be applied to the purification of flue gas at the range of 150-280°C, such as coke oven flue gas, and glass furnace gas. Several Fe-based catalysts have been developed to employ in the application, such as Fe2O3-TiO2 , Fe2O3- Pillared clay catalyst , and Fe-ZSM-5 catalyst  prepared by solution impregnation , co-precipitation  and ion-exchange methods . Moreover, to further improve the catalytic activity, metal oxides doping (cerium, copper, manganese and the like) was proposed to modify the Fe-based catalysts [11-13]. Although these catalysts showed high denitration efficiency, the still low mid-temperature catalytic activity, complex and costly preparation methods have hindered their largescale applications.
Template method, as one of the most effective pathways to tune the physical and chemical properties of catalysts, has been used in many catalyst preparation processes. In the processes, template would adjust the pore structure and promote the dispersion extent of active phase [14-16], thereby enhancing the catalytic activity of catalysts. However, much less attention has been focused on template method for the preparation of Fe-based catalysts for SCR at mid-temperature. Anionic surfactant Pluronic F127 is a three-block copolymer and could release anions in aqueous solution , which can be an efficient template agent. Therefore, in this work, a novel simple method towards Fe/TiO2 catalyst with high catalytic activity used for NH3-SCR in the mid-temperature range (150-280°C) was synthesized by template method using Pluronic F127 (PEO100PPO70PEO100) as template. For comparison, the catalytic performance of Fe/TiO2 catalysts synthesized by impregnation and coprecipitation methods was also evaluated under the same conditions.
Template method (t): All chemicals used were analytical grade. Firstly, 0.4 mmol Pluronic F127 was dissolved into 0.1 mol ethanol at room temperature. Secondly, 0.1 mol tetraethyl orthotitanate and 9 ml CH3COOH were added into the above solution under vigorous stirring until a transparent yellow solution was formed. Thirdly, a solution with 5.8 g Fe(NO3)3.9H2O in 9 ml deionized water was then added. After stirring for 2 h, the resultant was placed at room temperature for 3 days to form a gelatum. Finally, the gelatum was dried at 110°C overnight and calcined at 450°C in air for 3 h at a heating rate of 2°C/ min. Before using, the catalyst (Fe/TiO2 (t)) was crushed and sieved to 40-60 mesh. The used nominal load of Fe in the catalyst was 10 wt. % (Fe/TiO2=10 wt. %, the same below).
Impregnation method (i): Iron-doped titania was prepared by an aqueous incipient wetness impregnation method. 0.06 mol TiO2 was impregnated by equal volume Fe(NO3)3 solution with 3.6 g of Fe(NO3)3.9H2O. After stirring for 2 h at 80°C the sample was dried at 110°C for 6 h and calcined at 50°C in air for 3 h. Finally, the catalyst of Fe/TiO2(i) was crushed and sieved to 40-60 mesh.
Coprecipitation method (c): Iron-doped titania was also prepared by coprecipitation method. 0.08 mol titanyl sulfate and 4.5 g Fe(NO3)3.9H2O were dissolved completely in 80 ml water, and tammonia water was then gradually added to the solution with stirring for 2 h. The obtained product was filtered and washed with deionized water and then dried at 110°C, finally the catalyst of Fe/TiO2(c) was crushed and sieved to 40-60 mesh.
Characterization of the sample
The textural structures of the samples were measured by N2- adsorption at 77 K in a Micromeritics ASAP2020 system. The specific surface area was determined by the Brunauer-Emmett-Teller (BET) method, and the pore size distributions were calculated by BJH method.
X-ray diffraction (XRD) was performed to characterize the power by an X-ray diffractometer (D8 ADVANCE), operating at 40 kv and 45 mA using Cu-Kα radiation. Diffraction patterns were recorded over a 2θ range of 20-80° using a step size of 0.02°.
The morphology of the catalyst was observed by scanning electron microscope (SEM, JSM-7001F, JEOL, and Japan).
Diffuse reflectance infrared Fourier transform spectra (DRIFTS) were collected from 800 to 4000 cm-1 by a Bruker Tensor 27 instrument equipped with a MCT detector. The spectra were obtained at the resolution of 8 cm-1 with 100 scans in Kubelka-Munk mode.
NH3 temperature programmed desorption (NH3-TPD) experiment was carried out under a total flow rate of 50 ml/min. Before experiment, the samples were pretreated in pure N2 at 300°C for 1 h. NH3 adsorption was performed at 180°C. Desorption was carried out by heating samples from 180°C to 600°C, and the NH3 was continuously monitored using a portable FT-IR gas analyzer.
Catalytic activity test
The activity measurement for the selective catalytic reduction of NO by NH3 was carried out in a fixed-bed reactor under a simulated flue gas containing 500 ppm NO, 500 ppm NH3, 6.5% O2, N2 balance. The total gas flow rate was maintained at 400 cm3/ min over 2.0 g of the catalyst, corresponding to the GHSV of 12000 h-1. For the reaction, the desired temperature was controlled by a programmable temperature controller. The NOx concentration at the inlet and outlet of the reactor were analyzed continuously by a flue gas analyzer (KM9106 Quintox, Kane International Limited).
Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=6 vol %, N2 balance, total flow rate=400 ml/min, GHSV=12000 h-1
The SCR performances of synthesized samples for NH3-SCR as a function of reaction temperature from 120-300°C are presented in Figure 1. The NO conversion of the three catalysts increased with the rise of temperature and followed the order Fe/TiO2(t)>Fe/TiO2(i)>Fe/ TiO2(c). Although the highest NO conversion of these three catalysts was observed at 300°C, the Fe/TiO2(t) showed the best SCR activity, with NO conversion of higher than 90% at the temperature window in the range of 175-300°C. The results indicate that the preparation method of the samples was a crucial factor, and template method is superior to others.
The BET surface area, pore volume, mesopore area and pore size of Fe/TiO2 catalysts are summarized in Table 1. It could be found that the surface area of Fe/TiO2(t) is the largest, and that of Fe/TiO2(c) is smallest, and the trend is same with the mesopore area. The pore size distributions are shown in Figure 2, it can be observed that the pore diameter varied among the different samples, and the porosity of Fe/ TiO2 is mainly made up of mesopores. The pore diameter of Fe/TiO2 (i) mainly distributed at 2-50 nm, and that is why it has the biggest average pore diameter. The most pore size of Fe/TiO2 (t) and Fe/TiO2(c) are about 5 nm and 10 nm respectively. While Fe/TiO2 (t) catalyst shows a relatively larger surface area, which is attribute to the addition of Pluronic F127 that could adjust pore structure . All these results indicated that the textural parameters of Fe/TiO2 had been greatly influenced by mesopore structures, and optimal pore structure of Fe/ TiO2 could provide larger specific surface area. In addition, it can be noted that the variation of surface area was in consistent with catalytic performance, and that is probably that larger surface area can adsorb more of NH3, which could facilitate the reaction of NH3- SCR .
|Catalysts||BET surface area (m2g-1)||Mesopore ABET (m2g-1)||Pore volume (cm3g-1)||Average porediameter (nm)|
Table 1: Textural properties of samples.
XRD and SEM
The XRD patterns of the samples for each method were shown in Figure 3. For all the Fe/TiO2 samples, the distinctive obvious diffraction peaks around 25°, 38°, 48°, 54° and 62° corresponding to (101), (004), (200), (105) and (204) of crystal phases of anatase TiO2 (JCPDS 21- 1272) were detected. Besides the anatase phase, several diffraction peaks assigned to Fe2O3 (JCPDS 33-0664) were also detected in Fe/ TiO2(i) and Fe/TiO2(c). While, the Fe2O3 diffraction peaks could not be detected in the Fe/TiO2(t) sample, although the Fe contents are same as other samples. The result may be due to the great interaction between TiO2 and iron oxides, and thus iron oxides could be dispersed well in the Fe/TiO2(t) sample.
In addition, it is noted that the anatase peaks of Fe/TiO2(i) were sharper than the other two, indicating that the crystalline degree and size on the sample increased according to Scherrer’s formula. The result of SEM (Figure 4) imagine also showed that the agglomeration and sintering of fine particles are existed on the surface of the Fe/TiO2(i), which could contribute to the increase of grain size. The large particle was not conducive to the dispersion of active phase in the catalyst, thereby leading to a decrease of NO removal efficiency. According to Scherrer’s formula, larger half peak width of Fe/TiO2(t) and Fe/ TiO2(c) implied smaller grain sizes. By comparison, the particle size of Fe/TiO2(t) is the smallest in these three catalysts which could be also identified by SEM, which was beneficial to SCR process .
To investigate the NH3 adsorption on the catalysts at 180°C, NH3-TPD analysis was performed and the results were presented in Figure 5. All of the samples exhibited a broad NH3 desorption peak in the temperature range of 180-450°C, which was attributed to NH3 desorbed by weak and medium acid sites , which implied that the Fe/TiO2 had both Lewis acid and Brønsted acid. However, the peak areas of various catalysts were extremely different, indicting the different acid-site density or the number of each acid-site . The quantitative estimation of acid sites is shown in Figure 6, and it followed the order Fe/TiO2(t)>Fe/TiO2(c)>Fe/TiO2 (i). It is well know that the chemisorbed NH3 molecules were crucial roles in the SCR reaction according to Eley-Rideal mechanism . In the case of Fe/TiO2(t), the peak intensity started to rise at temperature 180°C, and the adsorption capacity was 0.18 mmol/g, which was two times higher than that of Fe/ TiO2(i). It could be attributed to the larger specific surface area, optimal pore structure , and the interaction between TiO2 and iron oxides  during calcination process for the removal of template. It is worth noting that the variation of adsorption capacity was not consistent with the catalytic performance, suggesting that there also other factors affecting SCR reaction.
To further explore the Brønsted acid and Lewis acid on the surface of the catalysts, NH3 adsorption was performed with a DRIFT spectrometer at 180°C. NH3 (500 ppm) was injected firstly for 30 min. After N2 purging, the adsorbed species were examined as shown in Figure 7. Several bands in the ranges of 1000-1700 cm-1 and 3300- 3700 cm-1 were observed over these samples. The bands at 1441 cm-1 and 1640 cm-1 were attributed to the symmetric and asymmetric bending vibration of N-H bond in NH4+ chemisorbed on Brønsted acid sites, respectively [23,24]. While the band at 1136 and 1347 cm-1 were the symmetric bending vibrations of N-H bond in coordinate NH3 chemisorbed on Lewis acid . In the N-H stretching vibration region, bands were found at 3339 and 3452 cm-1. Some negative bands around 3610 and 3754 cm-1 were also found, which could be assigned to hydroxyl consumption due to the interaction with NH3 to form NH4+ [26,27]. The obtained results also implied that both Lewis acid sites and Brønsted acid sites are existed on the surface of all samples as NH3-TPD. For Fe/TiO2(c), few peaks can be detected except an obvious adsorption of the NH4+ (1640 cm-1) corresponding to Brønsted acid sites and this could account for its poor catalytic efficiency. The main reason for this result was possibly related to the small surface area, which leading to the less exposed adsorption sites in the catalyst. In contrast, stable acid sites could be found on Fe/TiO2(t) and Fe/TiO2(i), and simultaneously, it seemed that Brønsted acid was stronger than Lewis acid over the both samples, but the activity of Fe/TiO2(t) with the stronger NH4+ adsorbed on the Brønsted acid sites tended to be higher than Fe/TiO2(i). In addition, the intensity of peaks at 3610 cm-1 on Fe/TiO2(t) increased obviously. The reason may be ascribed to the preparation process: after calcination at 500°C, the template of F127 was turned into gas and exhaust, while the hydroxyl group on isolate active phase was difficult to be removed . It is important to note that a band appeared at 1553 cm-1, which did not belong to Brønsted or Lewis acid sites. Studies [8,25] suggested that it could be attributed to amid (-NH2) species, which played a key role in SCR reaction, corresponding to the higher catalytic activity of Fe/TiO2(t) and Fe/TiO2(i) than Fe/ TiO2(c). In a conclusion, the Brønsted and Lewis acid sites both existed in the samples, but the former is dominant. The stronger Brønsted acid sites should be a mainly reason for the enhanced catalytic activity of Fe/ TiO2(t), especially during mid-temperature.
Catalysts based on Fe/TiO2 were synthesized by three different methods. Among the catalysts, the Fe/TiO2 prepared using F127 as template showed excellent catalytic activity for NO removal at medtemperature. About 90% catalytic efficiency could be obtained over Fe/ TiO2 (t). Through the BET, XRD, SEM, TPD and DRIFT characteristic analysis, different physical structure and surface properties over various catalysts were observed. Large specific surface area, good dispersion of of active phase, and stronger NH4+ adsorption assigned to Brønsted acid sites might be the main reasons for the highest catalytic performance of Fe/TiO2 (t).
This work was supported by the Natural Science Foundation China (21177136 and 21106174), the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA07030300), and the International Cooperation of Shanxi Province (2012081019).
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