Article Cite This: Environ. Sci. Technol. XXXX, XXX, XXX−XXX
pubs.acs.org/est
Tourmaline-Modified FeMnTiOx Catalysts for Improved LowTemperature NH3‑SCR Performance Fei Wang,†,‡,∥ Zhibo Xie,†,‡,∥ Jinsheng Liang,*,†,‡ Baizeng Fang,§ Yu’ang Piao,†,‡ Ming Hao,†,‡ and Zishuo Wang†,‡ †
Downloaded by UNIV OF SOUTHERN INDIANA at 18:45:26:503 on June 03, 2019 from https://pubs.acs.org/doi/10.1021/acs.est.9b02620.
Key Laboratory of Special Functional Materials for Ecological Environment and Information, Hebei University of Technology, Ministry of Education, Tianjin 300130, P. R. China ‡ Institute of Power Source and Ecomaterials Science, Hebei University of Technology, Tianjin 300130, P. R. China § Department of Chemical & Biological Engineering, University of British Columbia, 2360 East Mall, Vancouver, British Columbia V6T 1Z3, Canada S Supporting Information *
ABSTRACT: Low temperature NH3 selective catalytic reduction (NH3-SCR) technology is an efficient and economical strategy for cutting NOx emissions from power-generating equipment. In this study, a novel and highly efficient NH3-SCR catalyst, tourmalinemodified FeMnTiOx is presented, which was synthesized by a simple one-step sol−gel method. We found that the amount of tourmaline has an important impact on the catalytic performance of the modified FeMnTiOx-based catalysts, and the NOx conversion exceeded 80% from 160 to 380 °C with the addition of 5 wt % tourmaline. Compared with the pure FeMnTiOx, the catalytic efficiency at a temperature below 100 °C was increased by nearly 18.9%, and the operation temperature window was broadened significantly. The enhanced catalytic performance of the FeMnTiOx catalyst was mainly attributed to the small spherical nanoparticles structure around the tourmaline powders, resulting in the increased content of Mn3+, Mn4+, and chemical oxygen on the catalytic surface. These as-developed tourmaline-modified FeMnTiOx materials have been demonstrated to be promising as a new type highly efficient low temperature NH3-SCR catalyst. through an impregnation method.16 Chen et al. found that the electron transfer effect between Fe and Mn ions was crucial for accelerating the dehydrogenation of NH3 and oxidation of NO.12 Subsequently, a series of FeMnTiOx catalysts were prepared via coprecipitation,17 sol−gel,18 and other methods, and their operation temperature windows and efficiencies were also improved to some degree. However, the FeMnTiOx catalysts prepared by impregnation or sol−gel methods were often aggregated seriously.19 At a high gas hourly space velocity (GHSV), the tail gas could only contact the active sites of the surface layer, and the active sites aggregated in the interior had little contribution,20,21 thus leading to the catalytic efficiency decrease of the FeMnTiOx at a high GHSV. In addition, the anatase phase was formed along with the emergence of many Mn2+ ions,22,23 which had a negative influence on the conversion of the NOx catalyst. Therefore, it is necessary to explore an economical and efficient method to reduce agglomeration of FeMnTiOx catalyst and accelerate the formation of Mn3+ and Mn4+ during the calcination process.
1. INTRODUCTION In recent years, nitrogen oxide (NOx) in industrial tail gas has become one of the most important components of air pollutants.1,2 Low temperature selective catalytic reduction (SCR) denitration is an economical and efficient means for NOx treatment.3−6 Generally, SCR catalysts are required to be active at a temperature below 300 °C, and the catalytic system can be installed after dust removal and desulfurization,7,8 by which the catalyst poisoning by SO2 and dust could be reduced effectively.9,10 Moreover, the flow rate of exhaust gas is relatively slow at low temperature, which profitably increases the contact time between exhaust gas and the catalyst. Accordingly, the catalyst amount and catalytic system area can be reduced significantly. At present, most of the low temperature SCR catalysts such as manganese oxides11,12 have no volatilization and environmental pollution, conforming to the concepts of the economy and the environment. With the incorporation of Ce, Ni, Fe, and other elements, the SO2 resistance and N2 selectivity of MnOx could be further improved.13−15 Among the manganese oxide-based catalysts, the FeMnTiOx oxides as a type of inexpensive and nontoxic catalysts have better SO2 resistance and N2 selectivity, which were first prepared by Yang et al. © XXXX American Chemical Society
Received: May 1, 2019 Accepted: May 17, 2019
A
DOI: 10.1021/acs.est.9b02620 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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BRUKER) in the wavenumber range of 400−4000 cm−1 at a resolution of 4 cm−1. 2.3. Catalytic Activity Measurements. Two g of (40− 100 mesh) catalyst was put into a fixed-bed quartz reactor at different temperatures ranging from 50 to 380 °C. The GSHV was 50 000 h−1 by mixing 800 ppm of NO, 800 ppm of NH3 and 8% O2, and using nitrogen as the equilibrium gas. The NO and NO2 were analyzed by KM940 gas analyzer (Kane, British), and the conversion efficiencies were calculated according to the following formula:
Consequently, the efficiency of the catalyst could be improved at low temperature and high GHSV. As a silicate mineral, tourmaline with spontaneous polarization, negative ion release and far-infrared radiation belongs to trigonal space group, containing B, Al, Na, Fe, Mg, Li, and other elements.24−26 The spontaneous polarity of tourmaline was first performed by Kubo et al. using an impregnation method in the 1980s,27 and it has been applied in the field of catalysts preparation since the 1990s.28 Specifically, it was found that the cluster effect of water molecules caused by hydrogen bonds could be reduced by the far-infrared radiation and spontaneous polarity of tourmaline.29−32 The preparation of SCR catalysts usually involves liquid phase system,33,34 thus tourmaline could play a unique role in the modification of SCR catalysts. However, to the best of our knowledge, there has not been any report on the growth mechanism of the low temperature SCR catalysts modified by tourmaline so far. In this work, tourmaline characterized as the surface electric field and spontaneous polarity was adopted to modify the FeMnTiOx catalysts through one step sol−gel strategy to prevent the agglomeration of FeMnTiOx and form small special structure on the surface of tourmaline, which would propose a novel modification method for SCR catalysts.
X(NOx) =
[C(NO)]in − [C(NO) + C(NO2 )]out × 100% [C(NO)]in
(1)
3. RESULTS AND DISCUSSION 3.1. Catalytic Performance. Figure 1 displays the relationship between NOx conversion of the catalysts and
2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. All the chemical reagents (AR) in the experiments were provided by Aladdin Reagent Incorporation, and the industrial-grade black tourmaline with D50 of 5.9 μm was obtained from the Inner Mongolia Autonomous Region, China. The composition of tourmaline is shown in Table S1 of the Supporting Information (SI). The FeMnTiOx-tourmaline catalysts were prepared by a one-step sol−gel method according to the following procedures and the as-prepared samples were denoted as FeMnTiOx-tn, in which ‘n’ stands for the mass fraction of tourmaline. First, a certain amount of tetrabutyl titanate was dissolved in ethanol to form a transparent solution named as A. Second, deionized water, manganese nitrate solution (50 wt %) and Fe(NO3)3·9H2O were added into ethanol to form solution B. Third, 1−6 wt % tourmaline (corresponding to 0.125, 0.250, 0.375, 0.500, 0.625, and 0.750 g, respectively) was added into solution B based on the theoretical quality of catalyst, and a soliquoid C was acquired through continuous stirring. After that, glacial acetic and concentrated nitric acid were added into the soliquoid C, and then the pH value was adjusted to 1. Next, solution A was added into the soliquoid C to acquire a gel. Finally, the gel was dried at 100 °C, and then calcined at 450 °C for 6 h in air. 2.2. Catalyst Characterization. The specific surface and pore size of the catalysts were measured by N2 adsorption and desorption at 77 K on an Autosorb-iQ instrument from Quantachrome Corporation. The crystal structures of the catalysts were analyzed via X-ray diffraction (XRD) (BRUKER D8 Focus) with a Cu Kα radiation over a 2θ range between 10° and 90°. The morphologies of the catalysts were observed by a scanning electron microscopy (SEM) (FEI Nano SEM450) with energy dispersive spectroscopy (EDAX Genesis Apollo XL), under the accelerating voltage of 1.00 kV and transmission electron microscopy (TEM) (JEOL JEM.2010FEF) under the accelerating voltage of 200 kV. The chemical states of Fe, Mn, and O on the catalyst surfaces were analyzed by X-ray photoelectron spectroscopy (XPS) (ESCALAB 250Xi). In situ DRIFTS of NH3 and NO adsorption were carried out on an infrared spectrometer (TENSORII,
Figure 1. NOx conversion during the NH3-SCR reaction over FeMnTiOx-tourmaline catalysts: t1−6 represents the mass fraction of tourmaline in the catalyst (t1 = 1 wt %; t2 = 2 wt %; t3 = 3 wt %; t4 = 4 wt %; t5 = 5 wt %; t6 = 6 wt %). The points are experimental values and the connecting lines are the fitted curves.
temperature ranging from 50 to 380 °C, with the catalytic data collected in an interval of every 30 °C. It can be seen that the NOx conversion efficiency of the unmodified FeMnTiOx catalyst is ca. 70% at 200 °C and reaches the maximum (ca. 81%) at 350 °C. The catalyst has more than 80% efficiency at 250−380 °C, while the de-NOx efficiency at 120−170 °C is lower than 70%, and the conversion of NOx decreases at 380 °C due to the oxidation of NH3. With the addition of tourmaline, the catalytic efficiency increases nonlinearly, showing an initial increase trend followed by a slight decrease after reaching the maximum. At the optimal addition amount of 5 wt %, the NOx conversion of the catalyst has been significantly improved, namely, the catalytic efficiency can reach ca. 80% and 92% at 160 and 300 °C, respectively. Compared with the pure FeMnTiOx, NOx conversion efficiency is increased ca. 18.9% at 100 °C, and the temperature of the 80% efficiency is decreased by 90 °C, indicating that the catalytic efficiency has been enhanced with a broader operation temperature window due to the tourmaline addition. B
DOI: 10.1021/acs.est.9b02620 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Environmental Science & Technology 3.2. XRD Analysis. Figures 2 and S1 (SI) show the XRD patterns for the FeMnTiOx-tourmaline catalysts with various
Generally, NO conversion increases with the increasing specific surface area of the as-developed catalysts, as shown in Figure 1. However, it is also observed that there are other factors which also have important impacts on the improvement in the catalytic activity. This finding is fully supported by a simple calculation shown as follows. Let us take the FeMnTiOx-t3 catalyst as an example. This catalyst has a specific surface area of 92.61 m2/g, corresponding to an increase of ca. 13.9% compared with the unmodified one. It is interesting to note that the modified catalyst (i.e., FeMnTiOxt3) shows a NO conversion of ca. 58.2% at 80 °C (Figure 1), which is ca. 28.5% higher than that (i.e., 45.3%) of the unmodified FeMnTiOx, implying that about half of the contribution results from other factors rather than the increased specific surface area. These factors will be discussed later in this work. 3.4. Microstructure Analysis. To further analyze the microstructures of the catalysts, SEM and TEM analysis of the FeMnTiOx and FeMnTiOx-t5 were performed. Figure S2 shows the SEM image and EDS mapping of the FeMnTiOx, and Figure S3 shows the SEM images and EDS mapping of the FeMnTiOx-t5 catalysts. It is evident that the morphology of the catalysts has changed significantly after the tourmaline addition. The diameter of the unmodified FeMnTiOx is about 1−2 μm, and there is no obvious MnOx and FeOx grain distribution. It is clear from the EDS mappings that the Fe and Mn elements are uniformly distributed in TiO2 to form a solid solution of FeMnTiOx, which is consistent with the XRD result of the FeMnTiOx. As for the FeMnTiOx-t5 catalyst, spherical nanoparticles have been developed around the tourmaline powders, and the size of each spherical particle is below 100 nm. In addition, the EDS mappings also reveal that the spherical catalyst nanoparticles are distributed uniformly around tourmaline powders, and the component consistence can also be maintained. The blue spot (i.e., spot A) in Figure S3 as an EDS scanning point shows the peaks of Al and Si, which confirms the existence of tourmaline. That is to say, a small amount of amorphous FeMnTiOx can be formed on the tourmaline surface, which leads to the weakening of the Al and Si EDS peaks. Comparing Figures S3 and S2, it is found that the aggregation of the sol particles will be dispersed effectively under the infrared radiation of tourmaline, which leads to the increase of nucleation sites in the sol system. In the process of calcination, a large number of nanoparticles can be obtained along with the simultaneous growth of nucleated sites. In order to further explore the effect of tourmaline on the grain growth of the catalyst, the TEM bright-field (BF) analysis was adopted. As shown in Figure 3, the grain of FeMnTiOx catalyst is sintered together to form large particles, and the grain size is ca. 10 nm. After the addition of tourmaline, the growth mechanism of FeMnTiOx has been changed obviously, and the agglomeration phenomenon is almost eliminated during the sintering stage. The spherical particles around the tourmaline are actually a number of well dispersed catalyst grains. In addition, a number of spheroidal catalyst particles are well dispersed around the tourmaline, and the catalyst particles with the grain size of about 7.5 nm become significantly smaller after adding tourmaline, confirming that the nucleation sites are increased and the grain growth is limited after the addition of tourmaline powders. 3.5. Effect of Tourmaline on the Growth Mechanism of FeMnTiOx Catalyst. The effect of tourmaline on the growth mechanism of FeMnTiOx catalyst can be discussed
Figure 2. XRD patterns of the FeMnTiOx-tourmaline catalysts with various tourmaline contents.
tourmaline contents. The peaks of the catalysts can be indexed to the anatase TiO2 phase, while no FeOx or MnOx diffraction peaks can be found. This clearly indicates their high dispersion and low crystallization. In addition, the diffraction peaks of (101) and (122) of tourmaline become more pronounced along with the increase of tourmaline addition. The peaks from the FeMnTiOx and FeMnTiOx-t1−6 catalysts are shifted to high angles compared to those from the pure anatase, indicating that Fe3+ and Mn4+ could enter into the crystal lattice of TiOx and replace the cation sites of Ti4+. Moreover, the peaks of FeMnTiOx-t1−6 move to a higher degree than the pure FeMnTiOx, which means there are more Fe3+ and Mn4+ that are dissolved into the TiOx lattice. This may suggest that the water molecular vibration and hydrogen bonds interruption are increased by the effect of farinfrared radiation of tourmaline. Therefore, the ions in the sol are dispersed more evenly, leading to the fact that more Fe and Mn ions enter into the TiOx lattice during the subsequent calcination process. When metal ions are dissolved into TiOx, a large number of defects containing the surface oxygen vacancies are formed, which is easy to absorb O2 from the gas to form surface chemical oxygen and ultimately beneficial to the oxidation of NO and the dehydrogenation of NH3.37 3.3. BET Analysis. Table S2 shows the specific surface area, pore volume and average pore size of the FeMnTiOxtourmaline catalysts. The specific surface area of the unmodified FeMnTiOx is 81.31 m2/g, while the tourmalinemodified catalysts show a rising trend with the increase in the added amount of tourmaline up to 5 wt %. When the tourmaline addition is 5 wt %, the specific surface area reaches 105.18 m2/g, which is ca. 29.4% higher than that of the pure FeMnTiOx. The average pore size of 10.15 nm is observed for the catalyst FeMnTiOx-t5, corresponding to a decrease of ca. 30.8% in comparison with the pure FeMnTiOx. Moreover, when the tourmaline addition is increased to 6 wt %, the surface area of the catalyst starts to decrease due to the relatively smaller surface area of tourmaline, which will affect the overall specific surface area of the catalyst. C
DOI: 10.1021/acs.est.9b02620 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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caused by the spontaneous polarization and far-infrared radiation properties of tourmaline powders.25,41 In order to verify this viewpoint, the surface tensions of the sol before and after adding tourmaline were measured with the KRUSS surface tension tester. As shown in Figure S4, the surface tension of the pure sol increased continuously with the stirring time, while the surface tension of sol decreased first and then increased after adding tourmaline, but was significantly lower than that of the pure sol system. Therefore, the ΔG* will be reduced. After adding tourmaline, the thermal vibration of the sol particles could be enhanced, and the agglomeration effect of the sol particles will be reduced subsequently. Besides, the diffusion activation energy of the sol particles will be reduced in the nucleation process. The both impacts from the enhanced thermal vibration and reduced diffusion activation energy points lead to the nucleation increase in the sol−gel system after adding tourmaline. Moreover, the existence of a large number of nucleation sites will restrict significantly the growth of the grain, leading to the formation of nanosphere catalyst with a special morphology. In the sol−gel system, the crystal growth is mainly caused by diffusion, which is in accordance with the Fick diffusion law, as shown in eq 3 in which R is particle radius, D is diffusion coefficient, ρ∞ and ρs are boundary conditions, and v is advance rate.
Figure 3. TEM micrographs of the FeMnTiOx and FeMnTiOx-t5.
from the nucleation and growth, respectively. Figure 4 shows the agglomeration of the sol particles in the sol system before
R = Dv(ρ∞ − ρs )t 1/2
(3)
Supposing each grain is grown independently, the crystal growth is mainly effected by the diffusion coefficient (D) and time (t). The diffusion coefficient could be reduced with the increase of molecular vibration caused by the spontaneous polarization and far-infrared radiation properties, and as a result, the grain size is decreased after the addition of tourmaline. Obviously, it is necessary to further examine if the valence state of the surface elements of the catalyst will be affected with the change of the catalyst microstructures. 3.6. XPS Analysis. In order to further confirm the species and valence states on the surface of catalysts, XPS tests of FeMnTiOx and FeMnTiOx-t5 were carried out, as shown in Figure 5. Figure 5A displays the Mn 2p XPS spectra for the FeMnTiOx and the FeMnTiOx-t5, which can be fitted by three different Mn species: Mn2+, Mn3+, and Mn4+. Generally, MnO2 and Mn2O3 show high SCR catalytic efficiency and N2 selectivity.35−37 By calculating the peak area and the proportion of each Mn valence state, the proportions of Mn valence states from high to low are Mn4+, Mn3+, and Mn2+. Before and after adding tourmaline, the total proportions of Mn4+ and Mn3+ is ca. 66.5%, and 73.3%, and the total ration of Mn3+ and Mn4+ is increased nearly 7% after tourmaline addition, which means tourmaline makes a significant contribution to the generation of Mn3+ and Mn4+ ions. Thus, it can be concluded that the vibration and migration rates of Mn2+ ions in the precursor are increased by the pyroelectricity and far-infrared radiation of tourmaline, and their contact probabilities with O2 will be enhanced during the calcination process. Therefore, the proportions of Mn3+ and Mn4+ in the catalyst are increased, and the NOx conversion of the catalyst can be improved after tourmaline addition. Figure 5B shows the XPS O 1s spectra for the FeMnTiOx and FeMnTiOx-t5, which can be fitted by two different O species: Oα and Oβ. According to previous reports,35,37 the
Figure 4. Mechanistic illustration of tourmaline in the preparation of materials: (a) before tourmaline addition, and (b) after tourmaline addition.
and after adding tourmaline. For the sol−gel method the nucleation can take place in the sol stage, which conforms to the nucleation formula in the liquid phase as shown below, in which N is nucleation number, K is proportional factor, ΔG* is nucleation energy, Q is diffusion activation energy, T is nucleation temperature, and k is the Boltzmann constant. During the above process, both ΔG* and Q can be affected by the spontaneous polarization and far-infrared radiation properties of tourmaline powders.
ji ΔG* + Q zyz N = K × expjjj− zz kT (2) k { In the above equation, ΔG* is related to the liquid degree of under cooling and surface tension, and the surface tension will be decreased with the fracture of hydrogen bonds in the sol
D
DOI: 10.1021/acs.est.9b02620 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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solutions, resulting in a closer contact between Fe3+ and Ti4+, which is caused by the size decrease of the sol particles in the sol phase after the addition of tourmaline. 3.7. In Situ FTIR Analysis. In order to explore the changes of NH3 and NO adsorbed on the catalysts surface after tourmaline addition, FTIR spectra of NH3 and NO adsorption were recorded at 150 °C and shown in Figure S6 and Figure 6,
Figure 6. In situ DRIFTS spectra of FeMnTiOx and FeMnTiOx-t5 during the NO adsorption at 150 °C. Reaction conditions: 0.08% NO, 5% O2, N2 as balance, and total gas flow rate of 100 mL/min.
respectively. In Figure S6, the characteristic peaks at 1405 and 1765 cm−1 are ascribed to NH4+ linked to Brønsted acid sites of catalysts,11 and those at 1255, 1598, and 1640 cm−1 are considered to be the coordinated NH3 bound to the Lewis acid sites of catalysts.40 Moreover, the adsorption peaks of NH3 and NH4+ are enhanced by the tourmaline addition, and a new N− H vibration peak appears at 1255 cm−1. Combined with the change of morphology and tourmaline characteristics, the contact probability between the catalyst and gas could be enhanced by the catalyst nanoparticles around the tourmaline powders, leading to the enhancement of in situ infrared peak intensity. Figure 6 displays the FTIR spectra of NO + O2 adsorption with N2 as the balance at 150 °C for 2 h, and the surface adsorption species are mainly bridging, monodentate and bidentate nitrate. The peaks at 1255 and 1295 cm−1, at 1395 and 1557 cm−1, and at 1625 cm−1 correspond to monodentate nitrate,41−44 bidentate nitrate,45,46 and bridging nitrate,47,48 respectively. After tourmaline addition, the characteristic peaks of nitrate are increased, which can be attributed to the chemical oxygen enhancement on the catalyst surface. Comparing Figure S6 with Figure 6, it can be seen that the adsorption effects of NH3 and NO can be enhanced significantly due to the catalyst nanoparticles around the tourmaline fine powders, and the new adsorption species appear on the surface of catalysts by the spontaneous polarization of tourmaline. The FTIR spectrum with preadsorbed NH3 exposed to NO + O2 at 150 °C were introduced to study the reaction mechanism of FeMnTiOx-t5 (Figure 7). It can be found that both NH3 and NH4+ species are involved in the SCR reaction,
Figure 5. Fitted profiles of (A) Mn 2p and (B) O 1s of the MnTiOx and FeMnTiOx-t5 catalysts.
peaks at low binding energy (Oα) and high binding energy (Oβ) represent the lattice oxygen and the chemical oxygen, respectively. The ratio increase of Oβ/(Oα + Oβ) is about 10% after tourmaline addition, which is calculated from the peaks area and the proportions of Oα and Oβ. The reason could be due to that 100 nm-sized catalysts nanoparticles have been formed around the tourmaline fine powders, which leads to more surface chemical oxygen on the catalysts surface. In most of the SCR reactions, surface chemical oxygen can exchange with adsorbed gaseous oxygen or oxygen molecules easily,38,39 which promotes the oxidation of NO as the limiting step in SCR. Besides, the number of Brønsted acid sites will be enhanced with the increase of surface chemical oxygen, leading to the improvement of NH3 adsorption capacity. Moreover, the XPS spectra of Fe 2p is shown in Figure S5, and the peaks at low binding energy and high binding energy represent the Fe3+ 2p3/2 and Fe3+ 2p1/2, respectively. It can be found that the valence of Fe in the catalyst is stable after adding tourmaline, and there is no peak of Fe2+ in the catalyst. In addition, the peak of Fe shifts slightly to the high energy region after the addition of tourmaline, which could be attributed to Ti4+ as a higher positive charge group attracting around the Fe atom. Obviously, the above test further confirms the XRD and TEM results. Fe and Ti form more solid E
DOI: 10.1021/acs.est.9b02620 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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Figure 8. Illustration diagram of the catalysis toward NH3-SCR.
form N2 and H2O. Consequently, the catalytic efficiency can be promoted with the increase of Mn3+, Mn4+, and chemical oxygen after the tourmaline addition, and the surface electric field formed by spontaneous polarization of tourmaline. Therefore, the rate-limiting steps in the low temperature SCR reaction, NH3 dehydrogenation, and NO oxidation can be improved.
Figure 7. In situ DRIFTS spectra of FeMnTiOx-t5 during transient NO exposure after NH3 adsorption at 150 °C. Reaction conditions: 0.08% NO, 5% O2, N2 as balance, and total gas flow rate 100 mL/min.
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and the decreasing rate of NH4+ is faster than that of NH3. In addition, there is no adsorption peak of nitrates when NO is introduced. Compared to Figure S7, it can be seen that the peak of nitrate is not decreased with the continuous introduction of NH3, instead, new NH3 adsorption peaks appeared. Therefore, the main reaction model of FeMnTiOx-t5 is the E-R mechanism, and the Brønsted acid active sites play a leading role. The NO2 generated by NO and O2 on the surface oxygen of the catalyst, reacts with NH4+ or NH3 in acid sites, in order to form NH4NO2 and NH2NO2 and then decomposes into N2 and H2O. During the process, the adsorption process of NO and NH3 can be enhanced by the surface electric field of tourmaline, and the dehydrogenation of NH3 and oxidation of NO will be improved with the increase of Mn3+, Mn4+, and chemical oxygen content. 3.8. NH3-TPD Analysis. In order to further study the effect of tourmaline on the adsorption capacity and surface acid sites of FeMnTiOx, temperature-programmed desorption (TPD) tests of FeMnTiOx and FeMnTiOx-t5 were carried out by NH3 chemisorption method.49−51 It can be seen from Figure S8 that the adsorption characteristics of the two catalysts are similar. The desorption peak at 125 °C represents the physical adsorption of NH3 on the catalyst surface, and the desorption peak at 300 °C represents the chemical adsorption. After adding tourmaline, both physical and chemical adsorption abilities of FeMnTiOx were improved. The reason can be attributed to the improved morphology of FeMnTiOx and the enhancement of high valent manganese and surface oxygen content, which have been completely confirmed by TEM and XPS test results. As a result, the adsorption capacity and surface acid concentration of the FeMnTiOx have been improved. 3.9. Catalytic Mechanism. According to the analyses from SEM, TEM, and FTIR, physical properties and catalytic mechanism of FeMnTiOx-tourmaline are shown in Figure 8. It is believed that the contact probability between the catalysts and NO or NH3 can be improved through the surface electric field formed by the tourmaline, and NO can be efficiently oxidized into NO2, which then reacts with NH2 and NH4+ to
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.est.9b02620. Composition and content of tourmaline, XRD patterns, BET data, SEM images, EDS mapping, surface tension, fitting profile, in situ DRIFTS spectra, and NH3-TPD profiles (Tables S1 and S2 and Figures S1−S8) (PDF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Baizeng Fang: 0000-0001-5855-7766 Author Contributions ∥
F.W. and Z.X. contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was financially supported by National Key R&D Program of China (No: 2017YFB0310802), National Natural Science Foundation of China (No: 51874115), Introduced Overseas Scholars Program of Hebei province, China (No: C201808) and Excellent Young Scientist Foundation of Hebei province, China (No: E2018202241).
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REFERENCES
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DOI: 10.1021/acs.est.9b02620 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.est.9b02620 Environ. Sci. Technol. XXXX, XXX, XXX−XXX
