- Email: [email protected]
Ti-PILC catalysts
JOURNAL OF FUEL CHEMISTRY AND TECHNOLOGY Volume 46, Issue 10, October 2018 Online English edition of the Chinese language journal Cite this article as: J Fuel Chem Technol, 2018, 46(10), 12311239
RESEARCH PAPER
Experimental study on selective catalytic reduction of NO by C3H6 over Fe/Ti-PILC catalysts DONG Shi-lin1, SU Ya-xin1,*, LIU Xin1, LI Qian-cheng1, YUAN Min-hao1, ZHOU Hao2, DENG Wen-yi1 1
School of Environmental Science and Engineering, Donghua University, Shanghai 201620, China;
2
Changzhou Vocational Institute of Engineering, Changzhou 203164, China
Abstract:
Ti-pillared interlayer clay (PILC)-based catalysts ion exchanged with Fe were prepared and used for selective catalytic
reduction of NOx using propylene as the reducing agent under oxygen-rich conditions. The relationship between structure and properties of the catalysts was studied using N2-adsorption/desorption, XRD, UV-vis, H2-TPR, and Py-FTIR. The results show that the prepared 19Fe/Ti-PILC catalyst can achieve complete removal of NO at 400°C, and N2 selectivity can reach over 90% and has better resistance to water vapor and SO2. N2-isothermal adsorption/desorption and XRD results show that structure of montmorillonite is opened, cross-linked pillars are effective, and a large specific surface area and pore volume are formed. UV-vis results show that the denitrification activity of the catalyst is related to content of oligomeric FexOy. Py-FTIR results show that both Lewis acid and Brønsted acid are presented on the catalyst surface. Fe3+ loading into the pillared clay can significantly increase the Lewis acid content. Lewis acid is one of the influencing factors on the denitrification activity of the catalyst. H2-TPR indicates that the catalyst has a strong reduction ability at about 400°C, and reduction ability of the catalyst is mainly represented by the reduction of Fe3+→Fe2+. Key words:
oxygen-rich conditions; pillared clay; selective catalytic reduction; propylene
Nitrogen oxides (NOx) emitted from combustion of fossil fuels and motor vehicle exhaust are one of the major pollutants in the environment. The three-way catalytic (TWC) technology applied to lean-burn gasoline and diesel engines is no longer effective for removal of nitrogen oxides at oxygen-rich and low-temperature conditions. In the early 1990s, Iwamoto et al[1] and Held et al[2] reported that Cu-ZSM-5 molecular sieve catalysts can efficiently catalyze hydrocarbons to reduce NO under oxygen-rich conditions. Since then, the selective catalytic reduction of NO by hydrocarbons (HC-SCR) was widely studied. HC-SCR is one of the promising alternative technologies for controlling NOx emissions in the world today. So far, there are mainly three categories of HC-SCR catalysts studied by researchers, that is., metal oxide catalysts, precious metal catalysts and molecular sieve catalysts[3–6]. However, there are some problems related to application of these catalysts. Although the metal oxide catalyst has a high denitrification activity in the middle and high temperature regions, the activity in low temperature is poor. As to the noble metal catalyst, it has good
low-temperature activity, but its selectivity is not good, and the amount of N2O generated is high; and also the noble metal catalyst is costly. The molecular sieve catalyst has high catalytic activity in the HC-SCR process, but its hydrothermal stability and the ability to resist H2O and SO2 are poor. Pillared interlayered clays (PILC) is a kind of composite porous material with two-dimensional structure similar to synthetic molecular sieves. It has the characteristics of large pore size, large specific surface area, adjustable pore size, simultaneous formation of Brønsted acid and Lewis acid, and better hydrothermal stability in the HC-SCR process, and thus it has been vigorously studied in recent years [7]. Since Vaughan et al[8] has supported active components on the pillared clay as catalysts, various organic and inorganic pillared clays have been synthesized. Yang et al[9] found that Cu2+ exchanged Ti-PILC was more active than Cu-ZSM-5, and its tolerance to H2O and SO2 in flue gas was higher than that of Cu-ZSM-5. Valverde et al[10] investigated the selective catalytic reduction of NO with C3H6 over Cu, Fe, and Ni ion-exchanged Ti-PILCs.
Received: 08-May-2018; Revised: 09-Aug-2018. Foundation items: Supported by the National Natural Science Foundation of China (51278095) and Jiangsu Province Prospective Joint Research Projects (BY2015032-02). Corresponding author. Tel: 021-67792552, E-mail: [email protected]. Copyright 2018, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fig. 1
Effect of different iron loading of Fe/Ti-PILC on the conversion of NO and C3H6 and N2 selectivity (a): NO conversion; (b): C3H6 conversion; (c): N2 selectivity
NO=0.1%, C3H6=0.1%, O2=1%, N2=balance and GHSV=12000 h–1
Fig. 2
Influence of water vapor and SO2 on NO conversion over 19Fe/Ti-PILC catalyst at 400°C GHSV=12000 h–1, total flow=100 mL/min, N2=balance
The results showed that Cu-exchanged Ti-PILC had the highest catalytic activity and good low-temperature activity, and the highest NO conversion rate was 55% at 260°C, whereas the NO conversion rate over Fe and Ni ion-exchanged Ti-PILCs was only 30% during the C3H6-SCR. Lu et al[11] prepared a Cu-Ti-PILC catalyst by ion exchange method and a maximum of 55% NO conversion was achieved at 250°C. These studies show that the pillared clay-supported Cu catalysts have better low temperature activity for SCR-HC, but the current NO conversion rate is relatively low, and the research on the basic physical and chemical properties of these catalysts is not deep. Previous studies have shown that hydrocarbons are able to
effectively and durably reduce NO over metallic iron surface, without being affected by H2O and SO2 in the flue gas[12–15]. However, there is a problem related to the high reaction temperature window and the high NO reduction efficiency can be achieved only when the temperature is above 800°C, so there is still a distance from the actual engineering application. To solve this problem, Qian et al[16] prepared Fe catalysts supported on Alumina-pillared clays (Fe-PILC) by impregnation method and NO reduction efficiency reached 100% at 400°C during the C3H6-SCR. However, the reducing agent in this reaction is excessive and the ratio of NO to C 3H6 is 1:6. In order to make the catalysts more suitable for lean burning conditions, Fe3+ exchanged Ti-pillared clay catalysts,
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fe/Ti-PILCs were prepared by ion exchange method and were evaluated for C3H6-SCR with the ratio of NO to C3H6 as 1:1. The physicochemical properties of the catalyst were characterized by XRD, H2-TPR, UV-vis and other technical means.
1 1.1
Experimental Preparation of catalysts
Synthesis of TiO2-PILC: Prepare 1 mol/L HCl solution at room temperature, then slowly add 6.6 mL of TiCl4 solution into 8 mL HCl; then slowly add deionized water to the mixture and stir vigorously until the final Ti concentration was 0.82 mol/L and the concentration of HCl was 0.11 mol/L. The mixture was continuously stirred for 12 h to obtain a stable and transparent titanium polymerization cation pillared solution. Then, the pillared solution was slowly dropped to the 1% montmorillonite suspension until the ratio of Ti to clay in the solution was 10 mmol/g, then stirring was continued for 24 h, centrifuged, and washed with deionized water three times until there was not any Cl– (detected by AgNO3), then dried at 110°C for 12 h, calcined at 500°C for 2 h to obtain a carrier TiO2-PILC. Preparation of catalyst Fe/Ti-PILC: 1.5 g of TiO2-PILC was weighed and added into 150 mL of 0.05 mol/L Fe(NO3)3 solution, and the mixture was stirred at 70°C for 6 h, centrifuged and separated. It was washed 3 times with deionized water, then dried at 110°C for 12 h, and calcined at 500°C for 2 h. Catalysts with different Fe loadings were obtained by controlling the ion exchange duration and frequency and denoted as wFe/Ti-PILC, where w means the mass fraction of Fe element. 1.2
SCR evaluation
The selective catalytic reduction of NO by C 3H6 over Fe/Ti-PILC was evaluated with a fixed bed quartz tube reactor of internal diameter 6 mm using 0.4 g of the catalyst (24–50 mesh). The simulated flue gas contained 0.1% NO, 0.1% C3H6, 1% O2, 0.02% SO2, and 10% H2O. The total gas flow rate was 100 mL/min, N 2 was the balance gas, and the Table 1 –1
gas hourly space velocity (GHSV) was 12000 h–1. Prior to the experiment, the samples were pretreated for 30 min in N 2 atmosphere at 300°C to remove the adsorbed water vapor and CO2 in the samples and then were cooled to room temperature. The tests were conducted at 150–600°C and the data was recorded after it became steady for at least 30 min at each temperature. The gas components after reaction, e.g. C3H6, NO, NO2 and N2O were detected on-line by a Fourier transform infrared spectrometer (Thermo Nicolet IS10) equipped with a 250 mL gas cell. The conversion rates of NO and C3H6 were calculated by the following equations: c[NO]inlet − c[NO]outlet NO conversion= ×100% (1) c[NO]inlet c[C3 H6 ]inlet − c[C3 H6 ]outlet C3 H6 conversion= ×100% (2) c[C3 H6 ]inlet where c[NO]inlet and c[C3H6]inlet are the concentration of NO and C3H6 at the inlet, c[NO]outlet and c[C3H6]outlet are the concentration of NO and C3H6 at the outlet. The selectivity of N2 is defined as: c[NO]inlet − c[NO]outlet − c[NO2 ]outlet − 2c[N2 O]outlet s N2 = c[NO]inlet − c[NO]outlet ×100% 1.3
(3)
Catalyst characterization
N2 adsorption-desorption characterization: The sample was subjected to mesoporous analysis using a physical adsorption instrument (Mike). The specific surface area of the sample was calculated according to the BET equation. The pore volume, average pore size and pore size distribution of the catalyst were calculated according to the BJH equation. XRD characterization: XRD test of the sample was carried out on a 18 kW target X-ray diffractometer (model D/max-2550VB+) using Cu K as the radiation source, 3°–60° test, scan rate: 2(°)/min, tube voltage: 40 kV, tube current: 200 mA. ICP characterization: The content of specific elements in the sample was tested by plasma spectrometer (Prodigy-ICP), wavelength range: 150–750 nm, resolution: ≤ 0.004 nm, precision (RSD) (n = 10): < 1.0%.
Properties of catalysts with different loadings of Fe
Catalyst
Fe w/(mg·g )
ABET/(m2·g–1)
Pore volume v/(cm3·g–1)
Pore diameter d/nm
Original clay
–
24
0.099
16.44
Ti-PILC
–
203
0.240
4.73
13Fe/Ti-PILC
135.2
166
0.207
4.97
19Fe/Ti-PILC
192.1
190
0.222
5.07
22Fe/Ti-PILC
221.0
179
0.209
4.73
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fig. 3
Adsorption isotherms of nitrogen on different samples at –196°C (a): N2 adsorption/desorption isotherms; (b): BJH pore size distribution
Fig. 4
XRD patterns of the original clay, Ti-PILC and catalysts
a: Ti-PILC; b: 5Fe/Ti-PILC; c: 13Fe/Ti-PILC; d: 19Fe/Ti-PILC; e: 22Fe/Ti-PILC
H2-TPR characterization: 0.1 g of the catalyst was placed in a vertical quartz tube and heated by a temperature-programmed furnace. At the beginning of the experiment, it was pretreated in a N2 atmosphere at 300°C for 30 min, then a mixture of volume fractions of 5% H 2 and 95% N2 was introduced, and the temperature-programmed furnace was heated from 100 to 700°C, and the heating rate was 5°C/min. The exhaust gas was detected by gas chromatography, and the sample was analyzed every 15 min. UV-vis characterization: The absorption spectrum of the sample was tested on an ultraviolet-visible near-infrared spectrometer with a detection wavelength range of 190-800 nm (integral sphere attachment is 190–2600 nm). Pyridine adsorption infrared spectroscopy: The content of acid sites (Lewis acid and Brønsted acid) on the surface of the catalyst was determined by FT-IR Frontie-type pyridine adsorption infrared spectrometer at room temperature. The pyridine adsorption test was carried out at 170 and 300°C.
2
Results and discussion
2.1
Activity evaluation of catalysts
The selective catalytic reduction of NO with C3H6 was
performed on four Fe/Ti-PILC catalysts with different loadings of iron under oxygen-rich conditions. The results are shown in Figure 1, where the experimental data of Cu/Fe-PILCs is from Ye et al[17]. Figure 1(a) indicates NO conversion of each catalyst increased with the reaction temperature from 150 to 600°C. Fe/Ti-PILC was less efficient to reduce NO below 250°C, whereas at 250–400°C NO reduction efficiency increased rapidly. Except for 5Fe/Ti-PILC, the efficiency of other three catalysts has reached 100% at 400°C. At 600°C NO reduction efficiency can be still maintained at its maximum value. Ye et al[17] studied the SCR-C3H6 over Cu-loaded Fe-pillar-sodiumated sepiolite catalysts and found the highest NO conversion rate was 62%. When Ti-PILC was not loaded with Fe, its denitrification efficiency was extremely low, which was not showed here. The results indicate that Fe plays a significant role in the catalytic reaction. Previous studies have shown the SCR-C3H6 of Cu/Ti-PILC reaches the highest NO reduction, only about 55%, at 250–260°C[10,11]. Compared with Cu/Ti-PILC, the SCR-C3H6 reaction temperature of Fe/Ti-PILC was higher, but the NO reduction efficiency was much higher than that of Cu/Ti-PILC catalyst. When Fe loading in the catalyst gradually increased, the NO conversion rate was also increased. When Fe loading reached 19%, the NO conversion rate was the highest. However, as Fe loading continued to increase, NO conversion rate decreased.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fig. 5
H2-TPR profiles of catalysts
Figure 1(b) shows that the C3H6 conversion rate of each catalyst sample increased with increasing temperature, and the conversion rate of C3H6 can reach 99% or more above 400°C. Since hydrocarbons are also present in the NOx pollution source, the catalyst can simultaneously eliminate NO and hydrocarbons. Figure 1(c) shows that the selectivity of N2 can be substantially more than 90% above 250°C. Since there are both water vapor and SO2 in a real coal combustion flue gas and vehicle exhaust, influence of water vapor and SO2 on HC-SCR is also important. It was demonstrated that performance of metal ion-exchanged zeolites would be seriously weakened by H2O and SO2 during the SCR of NO with HC[18–20]. Although Cu-ZSM-5 catalysts have been extensively studied due to its high activity, the catalytic activity is severely inhibited when H2O and SO2 are present. In order to investigate the effect of water vapor and SO2 on the catalytic activity of the present catalysts, the NO reduction experiments were carried out at 400°C in the presence of water vapor and SO2 respectively. The experimental results are shown in Figure 2. After 10% water vapor was introduced, the NO conversion rate decreased by 10%. This is because the water vapor was adsorbed at the active site, leading to the decreased active sites which were involved in the NO reaction, decreasing the NO reduction. After cutting off the water vapor, the NO reduction immediately rebounded, indicating that the catalyst deactivation by water vapor was reversible. Kim et al[21] reported that the main reason for Cu/Ti-PILC to resist to water vapor better than molecular sieve catalysts was that the hydrophobicity of Cu/Ti-PILC was stronger than that of the later. Cu/Ti-PILC catalysts adsorbed less water than molecular sieve catalysts. Similarly, 0.02% SO2 caused the NO reduction efficiency decreased by 12%, but after the SO2 was cut off, the NO reduction efficiency could still recover to over 90%. This indicates that the active sites were regenerated as the SO2 adsorbed on the catalyst surface gradually desorbed. These results suggest that Fe/Ti-PILC is less affected by water vapor and SO2.
Fig. 6
UV-vis adsorption spectra of catalysts
Long et al[22] studied the catalytic activity of different pillared clays and also found that Ti-PILC showed high catalytic activity as well as high tolerance to SO2 as compared to other catalysts, e.g., Al-PILC, Zr-PILC and Cr-PILC, etc. 2.2
N2 adsorption-desorption characterization
Table 1 lists the results of BET specific surface areas, pore volumes and average pore sizes of the catalysts. The specific surface areas and pore volumes of the original clay increased significantly after TiO2 pillaring, indicating that the micro- and meso-pores appeared in the pillared clay, and the increased specific surface areas and pore volumes are the result of successful pillaring. In addition, compared with the carrier Ti-PILC, the specific surface areas and pore volumes of the catalyst with different loadings of Fe decreased to varying degrees after loading the active component Fe, which may be caused by blocking of the pores by Fe species[10]. As compared with 13Fe/Ti-PILC, the specific surface area of the 19Fe/Ti-PILC catalyst increased from 166 to 190 m2/g and the pore volume increased from 0.207 to 0.222 cm3/g, while the pore diameter was nearly same. When Fe loading continued to increase, the specific surface area and pore volume of 22Fe/Ti-PILC decreased, indicating that the appropriate iron content can increase the specific surface area and pore volume of catalysts. When the iron content was too high, Fe2O3 would agglomerate on the surface of the catalyst carrier. The SCR test of catalysts with different Fe loadings showed that 19Fe/Ti-PILC performed the best NO reduction activity. The reason may be mainly due to the larger specific surface area and pore volume of 19Fe/Ti-PILC, which can provide more surface active sites in the selective catalytic reduction process and can facilitate adsorption of reaction gas on surface of the catalyst. Figure 3(a) shows the adsorption/desorption isotherms of nitrogen N2 of the catalysts. The adsorption isotherms of all samples belong to the IV type corresponding to solid mesoporous materials[23].
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239 Table 2
Percentage of the area of the sub-bands by quantitative analysis of the UV-vis spectra
Catalyst
Percentage/% I1/%
I2/%
I3/%
5Fe/Ti-PILC
70
14
16
13Fe/Ti-PILC
56
23
21
19Fe/Ti-PILC
41
29
30
22Fe/Ti-PILC
39
24
37
I1, I2, and I3 represent isolated Fe3+ species, small FexOy oligomers, and Fe2O3 particles species, respectively
The shape of the hysteresis loop formed by separation of adsorption and desorption branches is related to structure of the pores in the material. The adsorption and desorption branch of the TiO2 carrier are almost parallel to each other. The hysteresis loops for Ti-PILC are H4 type, which indicates the presence of narrow slit-like pores of both microporous and mesoporous scale in the catalyst[24]. After the active component Fe was loaded, the adsorption isotherms of each catalyst were close to those of Ti-PILC, indicating that the Ti-PILC structure was not changed and destroyed after loading Fe. In addition, under small relative pressure the N2 gas was strongly adsorbed by micropores in the catalysts. When p/p0 was 0.44, the adsorption and desorption were almost completely closed due to condensation in the capillary of the catalysts. Figure 3(b) presents the pore size distribution, showing a narrow unimodal shape and the pore diameter is uniformly distributed in the catalyst. In addition, compared with Ti-PILC, after loading Fe the number of mesopores within 4–8 nm reduced and the mesopores were mainly in 3.0–4.0 nm, indicating that the specific surface area and pore volume of the catalyst were reduced after Fe was loaded. 2.3
XRD characterization results
Figure 4 is the X-ray diffraction patterns of montmorillonite, TiO2 support and the catalysts. Chmielarz et al[25] proposed that the distance between each structural unit layer (including the thickness of a structural unit layer) was called the basic spacing and was marked as d(001). This characteristic peak generally appeared near 9°is the diffraction spectrum of montmorillonite. The two-dimensional crystal plane diffraction peak of montmorillonite appeared at 15°and 35°. Figure 4(a) illustrates that the diffraction peak of the d(001) crystal plane of montmorillonite appeared at 8.9°, and the two-dimensional crystal plane diffraction peak of montmorillonite appeared at 17.8°. In addition, diffraction peaks of quartz impurities and white silica impurities appeared at 26.6°and 27.9°[26]. Analysis of the Ti-PILC diffraction spectrum shows that after montmorillonite was pillared, the characteristic diffraction
peak d(001) shifted to 6.2°toward the small angle direction, and the basal spacing of the montmorillonite increased to 1.41 nm. This indicates that the layered structure of montmorillonite was stretched and the pillaring effect was effective. In addition, the two-dimensional (hk) crystal plane diffraction characteristic peak of the clay layer structure still appeared on the XRD pattern of the Ti-PILC sample, indicating that the pillaring process did not destroy its structure. Furthermore, the characteristic peaks of independent anatase TiO2 appeared at 25.14°, 37.89°, 48.98°and 53.92°. Figure 4(b) indicates that when Fe loading was low, the active component iron species was well dispersed, and there was no diffraction characteristic peak of iron oxide in the spectrum. This demonstrates that the larger specific surface area provided by the Ti-PILC support was sufficient to disperse the active component. When Fe loading increased, Fe2O3 characteristic peaks appeared at 33.2°, 35.7°and 49.5°in 19Fe/Ti-PILC and 22Fe/Ti-PILC. 2.4
H2-TPR characterization
In order to characterize reduction ability of the catalysts, H2 temperature programmed reduction (H2-TPR) tests were carried out on catalysts with different loadings of Fe (Figure 5). The spectra of Fe/Ti-PILC catalysts with different Fe loadings were similar. All samples showed two distinct reduction peaks between 250 and 650°C, namely a low temperature (around 370°C) peak with large area and a high temperature (around 600°C) peak of a small area. For assignment of the reduction peak, Long et al[22] studied H2-TPR of Fe-Ti-PILC, and considered that the peak around 400°C corresponded to reduction of Fe3+→Fe2+. Ye et al[17] studied H2-TPR of Cu-loaded Fe-pillar-sodiumated sepiolite and concluded that the peak at 580°C corresponded to reduction of Fe3O4→FeO. Oliveira et al[27] studied H2-TPR of -Fe2O3 and claimed that the peaks appearing at 650 and 740°C corresponded to reduction of Fe3+→FeO and Fe2+→FeO, respectively. In this study the two reduction peaks belong to reduction of Fe2O3→Fe3O4 and Fe3O4→FeO. In addition, shape of the low temperature peak of the catalyst was wider than that of the high temperature peak, indicating that reduction of Fe/Ti-PILC was mainly represented by the reduction of Fe3+→Fe2+. Moreover, compared to other catalysts, 19Fe/Ti-PILC catalyst had the largest reduction peak area and consumed the largest amount of hydrogen at 400°C, which indicates that the catalyst contained a large amount of Fe 2O3. So it has the largest reduction ability at about 400°C, which was consistent with the results of SCR experiment. The high temperature peak of the 19Fe/Ti-PILC catalyst appeared at around 535°C, moving 65°C toward low temperature as compared to 600°C in the other three catalysts.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fig. 7
Py-IR spectra of catalysts adsorbed pyridine (a): 170°C desorption; (b): 300°C desorption
Table 3 Brønsted and Lewis acid content of different catalysts Sample
2.5
40°C desorption /(μmol·g–1) B
L
170°C desorption /(μmol·g–1) B
L
300°C desorption /(μmol·g–1) B
L
Ti-PILC
9.99
382.61
4.05
127.14
2.78
83.83
5Fe/Ti-PILC
46.84
398.59
31.06
197.24
19.36
133.96
13Fe/Ti-PILC
13.96
636.18
10.34
189.85
5.17
122.80
19Fe/Ti-PILC
7.17
688.40
37.83
311.51
24.94
234.59
UV-vis characterization
UV-vis DRS is a common method for studying type and distribution of Fe species in Fe/Ti-PILC samples. Kumar et al[28] and Brandenberger et al[29] reported that the wavelength of absorption peak in 200–300 nm and 300–400 nm were attributed to free Fe3+ species and small oligomer FexOy clusters respectively, while the wavelength above 400 nm were assigned to Fe2O3 particles on surface of the catalyst. In order to calculate the relative amount of different Fe species, the UV-vis spectra of Fe/Ti-PILC catalysts were subjected to peak resolution and fitting, as is shown in Figure 6. There are six peaks in the UV-vis spectrum for Fe/Ti-PILC catalysts, noted respectively as peak I, II, III, IV, V, and VI. The first two peaks are attributed to isolated Fe3+ in tetrahedral sites and higher coordination octahedral sites, respectively. Peak III is small oligonuclear FexOy, while peak IV, V and VI are due to Fe2O3 nanoparticles on the catalyst surface. Besides, through the quantitative analysis of ultraviolet absorption spectra, percentage of the fitted spectrum area of different iron species in the total absorption spectrum area is calculated, and the results are shown in Table 2. I1, I2, and I3 represented the fitted peak area (%) of (I and II), III and (IV, V and VI), respectively. Isolated Fe3+ are the most abundant species in the four catalysts, accounting for 70%, 56%, 41% and 39% of the total Fe species respectively. With increase of iron loading, the content of isolated Fe3+ species decreased gradually, whereas small oligonuclear FexOy and Fe2O3 clusters increased. This
demonstrates that iron species tended to agglomerate when more iron species were introduced into the pillared clay. In addition, among the three catalysts, the content of FexOy ferrite oligomers on 19Fe/Ti-PILC catalyst was the highest. In combination with the best activity of 19Fe/Ti-PILC, it may be concluded it is due to more iron oligomeric species FexOy. 2.6
Pyridine absorption infrared spectroscopy (Py-IR)
The contents of Brønsted and Lewis acid in the catalyst samples were qualitatively and quantitatively analyzed by pyridine adsorption infrared spectroscopy (Py-FTIR) experiments. The carrier Ti-PILC and three typical samples were selected for Py-FTIR experiments at room temperature, 170 and 300°C. The results are shown in Figure 7. There were four absorption peaks for the catalysts, e.g., 1445, 1489, 1575 and 1606 cm–1, respectively. Datka et al[30] showed that the PyL absorption peak formed by the pyridine molecule absorbed at the Lewis acid center appeared at 1440–1460 and 1600–1635 cm–1. The PyH+ absorption peak formed by the pyridine molecule absorbed at the Brønsted acid center appeared in 1535–1550 cm–1. Therefore, the absorption peaks appearing at 1445 and the 1606 cm–1 were the PyL absorption peak. The absorption peak appearing at 1575 cm–1 was the PyH+ absorption peak. The absorption peak at 1489 cm–1 was a mixed one of PyL and PyH+. Sultana et al[31] also showed that the band at 1450 cm–1 corresponded to Brønsted acid, the band at 1450 and 1590–1620 cm–1 corresponded to Lewis acid, and the band at 1490 cm–1 corresponded to Brønsted acid and Lewis acid.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Table 3 shows results of quantitative analysis of the acids for the four samples. Fe-free carrier Ti-PILC contained both Lewis acid and Brønsted acid centers, but the content of Brønsted acid was lower than that of the Lewis acid. Chmielarz et al[32] considered that the acid sites on the pillared clay were mainly located on surface of clay and column, and the aluminoxy octahedron and the siloxane tetrahedron in the clay were Brønsted acidic sites. So the Brønsted acid was mainly located in the edge of the montmorillonite grains, while the Lewis acid was produced mainly due to the presence of aluminum in the montmorillonite octahedral structure. In addition, Toledo-Antonio et al[33] also claimed that pure TiO2 was a typical Lewis acidic site on Ti-PILC catalyst. When Fe was loaded, the amount of Brønsted and Lewis acid in the catalyst increased, which was probably due to the presence of the transition metal ion Fe3+ embedded in the clay structure. The content of Lewis acid in 19Fe/Ti-PILC was higher than that of 13Fe/Ti-PILC. The best activity of 19Fe/Ti-PILC may be related to higher content of Lewis acid, while 5Fe/Ti-PILC performed the worst in SCR activity test and its Lewis acid content was the least.
3 Conclusions
and DeNOx activity of Cu-ZSM-5 catalysts by variation of OH/Cu2+. Catal Today, 2012, 197(1): 214–227. [6] Kumar P A, Reddy M P, Ju Lk, Hyun-Sook B, Phil H H. Low temperature propylene SCR of NO by copper alumina catalyst. J Mol Catal A: Chem, 2008, 291(1/2): 66–74. [7] Long R Q, Chang M T, Yang R T. Enhancement of activities by sulfation on Fe-exchanged TiO2-pillared clay for selective catalytic reduction of NO by ammonia. Appl Catal B: Environ, 2001, 33(2): 97–107. [8] Vaughan D E W, Lussier R J, Magee J S. Pillared interlayered clay materials useful as catalysts and sorbents: CA, US4176090. 1979. [9] Yang R T, Tharappiwattananon N, Long R Q. Ion-exchanged pillared clays for selective catalytic reduction of NO by ethylene in the presence of oxygen. Appl Catal B: Environ, 1998, 19(3/4): 289–304. [10] Valverde J L, Lucas A D, Sánchez P, Dorado F, Romero A. Cation exchanged and impregnated ti-pillared clays for selective catalytic reduction of NOx by propylene. Appl Catal B: Environ, 2003, 43(1): 43–56. [11] Lu G, Li X Y, Qu Z P, Zhao Q D, Zhao L, Chen G H. Copper-ion exchanged Ti-pillared clays for selective catalytic reduction of NO by propylene. Chem Eng J, 2011, 168(3): 1128–1133.
Montmorillonite was used as raw material to prepare Titanium dioxide pillared clay (Ti-PILC) catalyst and the specific area and pore volume of the catalysts increased obviously after the pillaring process with TiO2. Fe was supported on Ti-PILCs by ion exchange method to prepare Fe/Ti-PILC catalysts and used for SCR-C3H6. Under oxygen-rich conditions, 19Fe/Ti-PILC achieved complete removal of NO at 400°C, and always remained the highest activity at 400–600°C. In addition, the catalyst had good resistance to water vapor and SO2, and the NO reduction can still reach 90% in the presence of water vapor and SO 2.
References
[12] Dou Y F, Su Y X, Lu Z X, Zhou H, Deng W Y. Experimental study of NO reduction by ethane over iron. J Fuel Chem Technol, 2015, 43(10): 1273–1280. [13] Liang J Q, Su Y X, Zhou H, Deng W Y. Performance and mechanism of NO reduction by iron combined with propene. J Fuel Chem Technol, 2016, 44(8): 977–984. [14] Su Y X, Su A L, Ren L M, Deng W Y. Effect of SO2 on NO reduction by methane over iron. J Fuel Chem Technol, 2014, 42(3): 377–384. [15] Zhou H, Su Y X, Qi Y Z, Lu Z X, Deng W Y. Effect of water vapor on NO reduction by methane over iron. J Fuel Chem Technol, 2014, 42(11): 1378–1386. [16] Qian W Y, Su Y X, Yang X, Yuan M H, Deng W Y, Zhao B T. Experimental study on selective catalytic reduction of NO with
[1] Iwamoto M, Yahiro H, Yu U Y. Selective reduction of NO by lower hydrocarbons in the presence of O2 and SO2 over copper ion-exchanged zeolites. Catal, 1990, 32(6): 430–433. [2] Held W, Koenig A, Richter T, Puppe L. Catalytic NOx reduction in net oxidizing exhaust gas. SAE Trans, 1990, 99(4): 209–216. [3] Zhou H, Su Y X, Deng W Y, Zhong F C. A review of HC-SCR over metal oxides-based catalysts. Environ Sci Technol, 2016, 39(1): 93–100. [4] Wang Q Y, Wen Y B, Dong X F, Lin W M. Crosslinked clay synthesis and its application in selective catalytic reduction of NOx with C3H6. J Chem Eng Chin Univ, 2006, 20(4): 598–603. [5] Yashnik S A, Salnikov A V, Vasenin N T, Anufrienko V F, Ismagilov Z R. Regulation of the copper-oxide cluster structure
propene over iron based catalysts supported on aluminum pillared clays. J Fuel Chem Technol, 2017, 45(12): 1499–1507. [17] Ye Q, Yan L N, Huo F F, Wang H P, Cheng S Y, Kang T F. Cu-supported on Fe-pillared sepiolite: Characterization and selective catalytic reduction(SCR)of NO by propene. Acta Chim Sin, 2011, 69(13): 1524–1532. [18] Yang T T, Bi H T, Cheng X. Effects of O2, CO2 and H2O on NOx adsorption and selective catalytic red uction over Fe/ZSM-5. Appl Catal B: Environ, 2011, 102(1/2): 163–171. [19] Martínez-Hernández A, Fuentes G A. Redistribution of cobalt species in Co-ZSM-5 during operation under wet conditions in the reduction of NOx by propane. Appl Catal B: Environ, 2005, 57(3): 167–174.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239 [20] Komvokis V G, Iliopoulou E F, Vasalos I A, Triantafyllidis K S, Marshall C L. Development of optimized Cu-ZSM-5 deNOx
adsorption of contaminants in water. Appl Clay Sci, 2003, 22(4): 169–177.
catalytic materials both for HC-SCR applications and as FCC
[28] Kumar M S, Schwidder M, Grünert W, Bruckner A. On the
catalytic additives. Appl Catal A: Environ, 2007, 325(2):
nature of different iron sites and their catalytic role in Fe-ZSM-5
345–352.
DeNOx, catalysts: New insights by a combined EPR and
[21] Kim B S, Lee S H, Park Y T, Ham S W, Chae H J, Nam I S. Selective catalytic reduction of NOx, by propene over copper-exchanged pillared clays. Korean J Chem Eng, 2001, 18(5): 704–710. [22] Long R Q, Yang R T. Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts. Appl Catal B: Environ, 2000, 24(1): 13–21. [23] Gregg S J, Sing K S W. Adsorption, Surface Area and Porosity. New York: Academic Press Inc, 1982.
UV/VIS spectroscopic approach. J Catal, 2004, 227(2): 384–397. [29] Brandenberger S, Kröcher O, Wokaun A, Tissler A, Althoff R. The role of Brønsted acidity in the selective catalytic reduction of NO with ammonia over Fe-ZSM-5. J Catal, 2009, 268(2): 297–306. [30] Datka J, Turek A M, Jehng J M, Wachs I E. Acidic properties of supported niobium oxide catalysts: An infrared spectroscopy investigation. J Catal, 1992, 135(135): 186–199.
[24] Long R Q, Yang R T. Selective catalytic reduction of nitrogen
[31] Sultana A, Haneda M, Fujitani T, Hamada H. Influence of Al2O3
oxides by ammonia over Fe3+-exchanged TiO2-pillared clay
support on the activity of Ag/Al2O3 catalysts for SCR of NO
catalysts. J Catal, 1999, 186(2): 254–268.
with decane. Catal Lett, 2007, 114(1): 96–102.
[25] Chmielarz L, Piwowarska Z, Kuśtrowski P, Węgrzyn A, Gil B,
[32] Chmielarz L, Piwowarska Z, Kuṡtrowskip, Węgrzyn A, Gil B,
Kowalczyk A, Dudek B, Dziembaj R, Michalik M. Comparison
Kowalczyk A, Dudek B, Dziembaj R, Michalik M. Comparison
study of titania pillared interlayered clays and porous clay
study of titania pillared interlayered clays and porous clay
heterostructures modified with copper and iron as catalysts of
heterostructures modified with copper and iron as catalysts of
the DeNOx process. Appl Clay Sci, 2011, 53(2): 164–173.
the DeNOx process. Appl Clay Sci, 2011, 53(2): 164–173.
[26] Shen B X, Ma H Q, Yang X Y, Yao Y. Study on preparation,
[33] Toledo-Antonio J A, Cortés-Jácome M A, Navarrete J,
characterization and de-NO activity of Mn-CeOx/Ti-PILC. J
Angeles-Chavez C, Lopez-Salinas E, Rendon-Rivera A.
Fuel Chem Technol, 2012, 40(5): 615–620.
Morphology induced CO, pyridine and lutidine adsorption sites
[27] Oliveira L C A, Rios R V R A, Fabris J D, Sapag K, Garg V K, Lago R M. Clay-iron oxide magnetic composites for the
on TiO2: Nanoparticles, nanotubes and nanofibers. Catal Today, 2010, 155(3/4): 247–254.
RESEARCH PAPER
Experimental study on selective catalytic reduction of NO by C3H6 over Fe/Ti-PILC catalysts DONG Shi-lin1, SU Ya-xin1,*, LIU Xin1, LI Qian-cheng1, YUAN Min-hao1, ZHOU Hao2, DENG Wen-yi1 1
School of Environmental Science and Engineering, Donghua University, Shanghai 201620, China;
2
Changzhou Vocational Institute of Engineering, Changzhou 203164, China
Abstract:
Ti-pillared interlayer clay (PILC)-based catalysts ion exchanged with Fe were prepared and used for selective catalytic
reduction of NOx using propylene as the reducing agent under oxygen-rich conditions. The relationship between structure and properties of the catalysts was studied using N2-adsorption/desorption, XRD, UV-vis, H2-TPR, and Py-FTIR. The results show that the prepared 19Fe/Ti-PILC catalyst can achieve complete removal of NO at 400°C, and N2 selectivity can reach over 90% and has better resistance to water vapor and SO2. N2-isothermal adsorption/desorption and XRD results show that structure of montmorillonite is opened, cross-linked pillars are effective, and a large specific surface area and pore volume are formed. UV-vis results show that the denitrification activity of the catalyst is related to content of oligomeric FexOy. Py-FTIR results show that both Lewis acid and Brønsted acid are presented on the catalyst surface. Fe3+ loading into the pillared clay can significantly increase the Lewis acid content. Lewis acid is one of the influencing factors on the denitrification activity of the catalyst. H2-TPR indicates that the catalyst has a strong reduction ability at about 400°C, and reduction ability of the catalyst is mainly represented by the reduction of Fe3+→Fe2+. Key words:
oxygen-rich conditions; pillared clay; selective catalytic reduction; propylene
Nitrogen oxides (NOx) emitted from combustion of fossil fuels and motor vehicle exhaust are one of the major pollutants in the environment. The three-way catalytic (TWC) technology applied to lean-burn gasoline and diesel engines is no longer effective for removal of nitrogen oxides at oxygen-rich and low-temperature conditions. In the early 1990s, Iwamoto et al[1] and Held et al[2] reported that Cu-ZSM-5 molecular sieve catalysts can efficiently catalyze hydrocarbons to reduce NO under oxygen-rich conditions. Since then, the selective catalytic reduction of NO by hydrocarbons (HC-SCR) was widely studied. HC-SCR is one of the promising alternative technologies for controlling NOx emissions in the world today. So far, there are mainly three categories of HC-SCR catalysts studied by researchers, that is., metal oxide catalysts, precious metal catalysts and molecular sieve catalysts[3–6]. However, there are some problems related to application of these catalysts. Although the metal oxide catalyst has a high denitrification activity in the middle and high temperature regions, the activity in low temperature is poor. As to the noble metal catalyst, it has good
low-temperature activity, but its selectivity is not good, and the amount of N2O generated is high; and also the noble metal catalyst is costly. The molecular sieve catalyst has high catalytic activity in the HC-SCR process, but its hydrothermal stability and the ability to resist H2O and SO2 are poor. Pillared interlayered clays (PILC) is a kind of composite porous material with two-dimensional structure similar to synthetic molecular sieves. It has the characteristics of large pore size, large specific surface area, adjustable pore size, simultaneous formation of Brønsted acid and Lewis acid, and better hydrothermal stability in the HC-SCR process, and thus it has been vigorously studied in recent years [7]. Since Vaughan et al[8] has supported active components on the pillared clay as catalysts, various organic and inorganic pillared clays have been synthesized. Yang et al[9] found that Cu2+ exchanged Ti-PILC was more active than Cu-ZSM-5, and its tolerance to H2O and SO2 in flue gas was higher than that of Cu-ZSM-5. Valverde et al[10] investigated the selective catalytic reduction of NO with C3H6 over Cu, Fe, and Ni ion-exchanged Ti-PILCs.
Received: 08-May-2018; Revised: 09-Aug-2018. Foundation items: Supported by the National Natural Science Foundation of China (51278095) and Jiangsu Province Prospective Joint Research Projects (BY2015032-02). Corresponding author. Tel: 021-67792552, E-mail: [email protected]. Copyright 2018, Institute of Coal Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fig. 1
Effect of different iron loading of Fe/Ti-PILC on the conversion of NO and C3H6 and N2 selectivity (a): NO conversion; (b): C3H6 conversion; (c): N2 selectivity
NO=0.1%, C3H6=0.1%, O2=1%, N2=balance and GHSV=12000 h–1
Fig. 2
Influence of water vapor and SO2 on NO conversion over 19Fe/Ti-PILC catalyst at 400°C GHSV=12000 h–1, total flow=100 mL/min, N2=balance
The results showed that Cu-exchanged Ti-PILC had the highest catalytic activity and good low-temperature activity, and the highest NO conversion rate was 55% at 260°C, whereas the NO conversion rate over Fe and Ni ion-exchanged Ti-PILCs was only 30% during the C3H6-SCR. Lu et al[11] prepared a Cu-Ti-PILC catalyst by ion exchange method and a maximum of 55% NO conversion was achieved at 250°C. These studies show that the pillared clay-supported Cu catalysts have better low temperature activity for SCR-HC, but the current NO conversion rate is relatively low, and the research on the basic physical and chemical properties of these catalysts is not deep. Previous studies have shown that hydrocarbons are able to
effectively and durably reduce NO over metallic iron surface, without being affected by H2O and SO2 in the flue gas[12–15]. However, there is a problem related to the high reaction temperature window and the high NO reduction efficiency can be achieved only when the temperature is above 800°C, so there is still a distance from the actual engineering application. To solve this problem, Qian et al[16] prepared Fe catalysts supported on Alumina-pillared clays (Fe-PILC) by impregnation method and NO reduction efficiency reached 100% at 400°C during the C3H6-SCR. However, the reducing agent in this reaction is excessive and the ratio of NO to C 3H6 is 1:6. In order to make the catalysts more suitable for lean burning conditions, Fe3+ exchanged Ti-pillared clay catalysts,
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fe/Ti-PILCs were prepared by ion exchange method and were evaluated for C3H6-SCR with the ratio of NO to C3H6 as 1:1. The physicochemical properties of the catalyst were characterized by XRD, H2-TPR, UV-vis and other technical means.
1 1.1
Experimental Preparation of catalysts
Synthesis of TiO2-PILC: Prepare 1 mol/L HCl solution at room temperature, then slowly add 6.6 mL of TiCl4 solution into 8 mL HCl; then slowly add deionized water to the mixture and stir vigorously until the final Ti concentration was 0.82 mol/L and the concentration of HCl was 0.11 mol/L. The mixture was continuously stirred for 12 h to obtain a stable and transparent titanium polymerization cation pillared solution. Then, the pillared solution was slowly dropped to the 1% montmorillonite suspension until the ratio of Ti to clay in the solution was 10 mmol/g, then stirring was continued for 24 h, centrifuged, and washed with deionized water three times until there was not any Cl– (detected by AgNO3), then dried at 110°C for 12 h, calcined at 500°C for 2 h to obtain a carrier TiO2-PILC. Preparation of catalyst Fe/Ti-PILC: 1.5 g of TiO2-PILC was weighed and added into 150 mL of 0.05 mol/L Fe(NO3)3 solution, and the mixture was stirred at 70°C for 6 h, centrifuged and separated. It was washed 3 times with deionized water, then dried at 110°C for 12 h, and calcined at 500°C for 2 h. Catalysts with different Fe loadings were obtained by controlling the ion exchange duration and frequency and denoted as wFe/Ti-PILC, where w means the mass fraction of Fe element. 1.2
SCR evaluation
The selective catalytic reduction of NO by C 3H6 over Fe/Ti-PILC was evaluated with a fixed bed quartz tube reactor of internal diameter 6 mm using 0.4 g of the catalyst (24–50 mesh). The simulated flue gas contained 0.1% NO, 0.1% C3H6, 1% O2, 0.02% SO2, and 10% H2O. The total gas flow rate was 100 mL/min, N 2 was the balance gas, and the Table 1 –1
gas hourly space velocity (GHSV) was 12000 h–1. Prior to the experiment, the samples were pretreated for 30 min in N 2 atmosphere at 300°C to remove the adsorbed water vapor and CO2 in the samples and then were cooled to room temperature. The tests were conducted at 150–600°C and the data was recorded after it became steady for at least 30 min at each temperature. The gas components after reaction, e.g. C3H6, NO, NO2 and N2O were detected on-line by a Fourier transform infrared spectrometer (Thermo Nicolet IS10) equipped with a 250 mL gas cell. The conversion rates of NO and C3H6 were calculated by the following equations: c[NO]inlet − c[NO]outlet NO conversion= ×100% (1) c[NO]inlet c[C3 H6 ]inlet − c[C3 H6 ]outlet C3 H6 conversion= ×100% (2) c[C3 H6 ]inlet where c[NO]inlet and c[C3H6]inlet are the concentration of NO and C3H6 at the inlet, c[NO]outlet and c[C3H6]outlet are the concentration of NO and C3H6 at the outlet. The selectivity of N2 is defined as: c[NO]inlet − c[NO]outlet − c[NO2 ]outlet − 2c[N2 O]outlet s N2 = c[NO]inlet − c[NO]outlet ×100% 1.3
(3)
Catalyst characterization
N2 adsorption-desorption characterization: The sample was subjected to mesoporous analysis using a physical adsorption instrument (Mike). The specific surface area of the sample was calculated according to the BET equation. The pore volume, average pore size and pore size distribution of the catalyst were calculated according to the BJH equation. XRD characterization: XRD test of the sample was carried out on a 18 kW target X-ray diffractometer (model D/max-2550VB+) using Cu K as the radiation source, 3°–60° test, scan rate: 2(°)/min, tube voltage: 40 kV, tube current: 200 mA. ICP characterization: The content of specific elements in the sample was tested by plasma spectrometer (Prodigy-ICP), wavelength range: 150–750 nm, resolution: ≤ 0.004 nm, precision (RSD) (n = 10): < 1.0%.
Properties of catalysts with different loadings of Fe
Catalyst
Fe w/(mg·g )
ABET/(m2·g–1)
Pore volume v/(cm3·g–1)
Pore diameter d/nm
Original clay
–
24
0.099
16.44
Ti-PILC
–
203
0.240
4.73
13Fe/Ti-PILC
135.2
166
0.207
4.97
19Fe/Ti-PILC
192.1
190
0.222
5.07
22Fe/Ti-PILC
221.0
179
0.209
4.73
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fig. 3
Adsorption isotherms of nitrogen on different samples at –196°C (a): N2 adsorption/desorption isotherms; (b): BJH pore size distribution
Fig. 4
XRD patterns of the original clay, Ti-PILC and catalysts
a: Ti-PILC; b: 5Fe/Ti-PILC; c: 13Fe/Ti-PILC; d: 19Fe/Ti-PILC; e: 22Fe/Ti-PILC
H2-TPR characterization: 0.1 g of the catalyst was placed in a vertical quartz tube and heated by a temperature-programmed furnace. At the beginning of the experiment, it was pretreated in a N2 atmosphere at 300°C for 30 min, then a mixture of volume fractions of 5% H 2 and 95% N2 was introduced, and the temperature-programmed furnace was heated from 100 to 700°C, and the heating rate was 5°C/min. The exhaust gas was detected by gas chromatography, and the sample was analyzed every 15 min. UV-vis characterization: The absorption spectrum of the sample was tested on an ultraviolet-visible near-infrared spectrometer with a detection wavelength range of 190-800 nm (integral sphere attachment is 190–2600 nm). Pyridine adsorption infrared spectroscopy: The content of acid sites (Lewis acid and Brønsted acid) on the surface of the catalyst was determined by FT-IR Frontie-type pyridine adsorption infrared spectrometer at room temperature. The pyridine adsorption test was carried out at 170 and 300°C.
2
Results and discussion
2.1
Activity evaluation of catalysts
The selective catalytic reduction of NO with C3H6 was
performed on four Fe/Ti-PILC catalysts with different loadings of iron under oxygen-rich conditions. The results are shown in Figure 1, where the experimental data of Cu/Fe-PILCs is from Ye et al[17]. Figure 1(a) indicates NO conversion of each catalyst increased with the reaction temperature from 150 to 600°C. Fe/Ti-PILC was less efficient to reduce NO below 250°C, whereas at 250–400°C NO reduction efficiency increased rapidly. Except for 5Fe/Ti-PILC, the efficiency of other three catalysts has reached 100% at 400°C. At 600°C NO reduction efficiency can be still maintained at its maximum value. Ye et al[17] studied the SCR-C3H6 over Cu-loaded Fe-pillar-sodiumated sepiolite catalysts and found the highest NO conversion rate was 62%. When Ti-PILC was not loaded with Fe, its denitrification efficiency was extremely low, which was not showed here. The results indicate that Fe plays a significant role in the catalytic reaction. Previous studies have shown the SCR-C3H6 of Cu/Ti-PILC reaches the highest NO reduction, only about 55%, at 250–260°C[10,11]. Compared with Cu/Ti-PILC, the SCR-C3H6 reaction temperature of Fe/Ti-PILC was higher, but the NO reduction efficiency was much higher than that of Cu/Ti-PILC catalyst. When Fe loading in the catalyst gradually increased, the NO conversion rate was also increased. When Fe loading reached 19%, the NO conversion rate was the highest. However, as Fe loading continued to increase, NO conversion rate decreased.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fig. 5
H2-TPR profiles of catalysts
Figure 1(b) shows that the C3H6 conversion rate of each catalyst sample increased with increasing temperature, and the conversion rate of C3H6 can reach 99% or more above 400°C. Since hydrocarbons are also present in the NOx pollution source, the catalyst can simultaneously eliminate NO and hydrocarbons. Figure 1(c) shows that the selectivity of N2 can be substantially more than 90% above 250°C. Since there are both water vapor and SO2 in a real coal combustion flue gas and vehicle exhaust, influence of water vapor and SO2 on HC-SCR is also important. It was demonstrated that performance of metal ion-exchanged zeolites would be seriously weakened by H2O and SO2 during the SCR of NO with HC[18–20]. Although Cu-ZSM-5 catalysts have been extensively studied due to its high activity, the catalytic activity is severely inhibited when H2O and SO2 are present. In order to investigate the effect of water vapor and SO2 on the catalytic activity of the present catalysts, the NO reduction experiments were carried out at 400°C in the presence of water vapor and SO2 respectively. The experimental results are shown in Figure 2. After 10% water vapor was introduced, the NO conversion rate decreased by 10%. This is because the water vapor was adsorbed at the active site, leading to the decreased active sites which were involved in the NO reaction, decreasing the NO reduction. After cutting off the water vapor, the NO reduction immediately rebounded, indicating that the catalyst deactivation by water vapor was reversible. Kim et al[21] reported that the main reason for Cu/Ti-PILC to resist to water vapor better than molecular sieve catalysts was that the hydrophobicity of Cu/Ti-PILC was stronger than that of the later. Cu/Ti-PILC catalysts adsorbed less water than molecular sieve catalysts. Similarly, 0.02% SO2 caused the NO reduction efficiency decreased by 12%, but after the SO2 was cut off, the NO reduction efficiency could still recover to over 90%. This indicates that the active sites were regenerated as the SO2 adsorbed on the catalyst surface gradually desorbed. These results suggest that Fe/Ti-PILC is less affected by water vapor and SO2.
Fig. 6
UV-vis adsorption spectra of catalysts
Long et al[22] studied the catalytic activity of different pillared clays and also found that Ti-PILC showed high catalytic activity as well as high tolerance to SO2 as compared to other catalysts, e.g., Al-PILC, Zr-PILC and Cr-PILC, etc. 2.2
N2 adsorption-desorption characterization
Table 1 lists the results of BET specific surface areas, pore volumes and average pore sizes of the catalysts. The specific surface areas and pore volumes of the original clay increased significantly after TiO2 pillaring, indicating that the micro- and meso-pores appeared in the pillared clay, and the increased specific surface areas and pore volumes are the result of successful pillaring. In addition, compared with the carrier Ti-PILC, the specific surface areas and pore volumes of the catalyst with different loadings of Fe decreased to varying degrees after loading the active component Fe, which may be caused by blocking of the pores by Fe species[10]. As compared with 13Fe/Ti-PILC, the specific surface area of the 19Fe/Ti-PILC catalyst increased from 166 to 190 m2/g and the pore volume increased from 0.207 to 0.222 cm3/g, while the pore diameter was nearly same. When Fe loading continued to increase, the specific surface area and pore volume of 22Fe/Ti-PILC decreased, indicating that the appropriate iron content can increase the specific surface area and pore volume of catalysts. When the iron content was too high, Fe2O3 would agglomerate on the surface of the catalyst carrier. The SCR test of catalysts with different Fe loadings showed that 19Fe/Ti-PILC performed the best NO reduction activity. The reason may be mainly due to the larger specific surface area and pore volume of 19Fe/Ti-PILC, which can provide more surface active sites in the selective catalytic reduction process and can facilitate adsorption of reaction gas on surface of the catalyst. Figure 3(a) shows the adsorption/desorption isotherms of nitrogen N2 of the catalysts. The adsorption isotherms of all samples belong to the IV type corresponding to solid mesoporous materials[23].
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239 Table 2
Percentage of the area of the sub-bands by quantitative analysis of the UV-vis spectra
Catalyst
Percentage/% I1/%
I2/%
I3/%
5Fe/Ti-PILC
70
14
16
13Fe/Ti-PILC
56
23
21
19Fe/Ti-PILC
41
29
30
22Fe/Ti-PILC
39
24
37
I1, I2, and I3 represent isolated Fe3+ species, small FexOy oligomers, and Fe2O3 particles species, respectively
The shape of the hysteresis loop formed by separation of adsorption and desorption branches is related to structure of the pores in the material. The adsorption and desorption branch of the TiO2 carrier are almost parallel to each other. The hysteresis loops for Ti-PILC are H4 type, which indicates the presence of narrow slit-like pores of both microporous and mesoporous scale in the catalyst[24]. After the active component Fe was loaded, the adsorption isotherms of each catalyst were close to those of Ti-PILC, indicating that the Ti-PILC structure was not changed and destroyed after loading Fe. In addition, under small relative pressure the N2 gas was strongly adsorbed by micropores in the catalysts. When p/p0 was 0.44, the adsorption and desorption were almost completely closed due to condensation in the capillary of the catalysts. Figure 3(b) presents the pore size distribution, showing a narrow unimodal shape and the pore diameter is uniformly distributed in the catalyst. In addition, compared with Ti-PILC, after loading Fe the number of mesopores within 4–8 nm reduced and the mesopores were mainly in 3.0–4.0 nm, indicating that the specific surface area and pore volume of the catalyst were reduced after Fe was loaded. 2.3
XRD characterization results
Figure 4 is the X-ray diffraction patterns of montmorillonite, TiO2 support and the catalysts. Chmielarz et al[25] proposed that the distance between each structural unit layer (including the thickness of a structural unit layer) was called the basic spacing and was marked as d(001). This characteristic peak generally appeared near 9°is the diffraction spectrum of montmorillonite. The two-dimensional crystal plane diffraction peak of montmorillonite appeared at 15°and 35°. Figure 4(a) illustrates that the diffraction peak of the d(001) crystal plane of montmorillonite appeared at 8.9°, and the two-dimensional crystal plane diffraction peak of montmorillonite appeared at 17.8°. In addition, diffraction peaks of quartz impurities and white silica impurities appeared at 26.6°and 27.9°[26]. Analysis of the Ti-PILC diffraction spectrum shows that after montmorillonite was pillared, the characteristic diffraction
peak d(001) shifted to 6.2°toward the small angle direction, and the basal spacing of the montmorillonite increased to 1.41 nm. This indicates that the layered structure of montmorillonite was stretched and the pillaring effect was effective. In addition, the two-dimensional (hk) crystal plane diffraction characteristic peak of the clay layer structure still appeared on the XRD pattern of the Ti-PILC sample, indicating that the pillaring process did not destroy its structure. Furthermore, the characteristic peaks of independent anatase TiO2 appeared at 25.14°, 37.89°, 48.98°and 53.92°. Figure 4(b) indicates that when Fe loading was low, the active component iron species was well dispersed, and there was no diffraction characteristic peak of iron oxide in the spectrum. This demonstrates that the larger specific surface area provided by the Ti-PILC support was sufficient to disperse the active component. When Fe loading increased, Fe2O3 characteristic peaks appeared at 33.2°, 35.7°and 49.5°in 19Fe/Ti-PILC and 22Fe/Ti-PILC. 2.4
H2-TPR characterization
In order to characterize reduction ability of the catalysts, H2 temperature programmed reduction (H2-TPR) tests were carried out on catalysts with different loadings of Fe (Figure 5). The spectra of Fe/Ti-PILC catalysts with different Fe loadings were similar. All samples showed two distinct reduction peaks between 250 and 650°C, namely a low temperature (around 370°C) peak with large area and a high temperature (around 600°C) peak of a small area. For assignment of the reduction peak, Long et al[22] studied H2-TPR of Fe-Ti-PILC, and considered that the peak around 400°C corresponded to reduction of Fe3+→Fe2+. Ye et al[17] studied H2-TPR of Cu-loaded Fe-pillar-sodiumated sepiolite and concluded that the peak at 580°C corresponded to reduction of Fe3O4→FeO. Oliveira et al[27] studied H2-TPR of -Fe2O3 and claimed that the peaks appearing at 650 and 740°C corresponded to reduction of Fe3+→FeO and Fe2+→FeO, respectively. In this study the two reduction peaks belong to reduction of Fe2O3→Fe3O4 and Fe3O4→FeO. In addition, shape of the low temperature peak of the catalyst was wider than that of the high temperature peak, indicating that reduction of Fe/Ti-PILC was mainly represented by the reduction of Fe3+→Fe2+. Moreover, compared to other catalysts, 19Fe/Ti-PILC catalyst had the largest reduction peak area and consumed the largest amount of hydrogen at 400°C, which indicates that the catalyst contained a large amount of Fe 2O3. So it has the largest reduction ability at about 400°C, which was consistent with the results of SCR experiment. The high temperature peak of the 19Fe/Ti-PILC catalyst appeared at around 535°C, moving 65°C toward low temperature as compared to 600°C in the other three catalysts.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Fig. 7
Py-IR spectra of catalysts adsorbed pyridine (a): 170°C desorption; (b): 300°C desorption
Table 3 Brønsted and Lewis acid content of different catalysts Sample
2.5
40°C desorption /(μmol·g–1) B
L
170°C desorption /(μmol·g–1) B
L
300°C desorption /(μmol·g–1) B
L
Ti-PILC
9.99
382.61
4.05
127.14
2.78
83.83
5Fe/Ti-PILC
46.84
398.59
31.06
197.24
19.36
133.96
13Fe/Ti-PILC
13.96
636.18
10.34
189.85
5.17
122.80
19Fe/Ti-PILC
7.17
688.40
37.83
311.51
24.94
234.59
UV-vis characterization
UV-vis DRS is a common method for studying type and distribution of Fe species in Fe/Ti-PILC samples. Kumar et al[28] and Brandenberger et al[29] reported that the wavelength of absorption peak in 200–300 nm and 300–400 nm were attributed to free Fe3+ species and small oligomer FexOy clusters respectively, while the wavelength above 400 nm were assigned to Fe2O3 particles on surface of the catalyst. In order to calculate the relative amount of different Fe species, the UV-vis spectra of Fe/Ti-PILC catalysts were subjected to peak resolution and fitting, as is shown in Figure 6. There are six peaks in the UV-vis spectrum for Fe/Ti-PILC catalysts, noted respectively as peak I, II, III, IV, V, and VI. The first two peaks are attributed to isolated Fe3+ in tetrahedral sites and higher coordination octahedral sites, respectively. Peak III is small oligonuclear FexOy, while peak IV, V and VI are due to Fe2O3 nanoparticles on the catalyst surface. Besides, through the quantitative analysis of ultraviolet absorption spectra, percentage of the fitted spectrum area of different iron species in the total absorption spectrum area is calculated, and the results are shown in Table 2. I1, I2, and I3 represented the fitted peak area (%) of (I and II), III and (IV, V and VI), respectively. Isolated Fe3+ are the most abundant species in the four catalysts, accounting for 70%, 56%, 41% and 39% of the total Fe species respectively. With increase of iron loading, the content of isolated Fe3+ species decreased gradually, whereas small oligonuclear FexOy and Fe2O3 clusters increased. This
demonstrates that iron species tended to agglomerate when more iron species were introduced into the pillared clay. In addition, among the three catalysts, the content of FexOy ferrite oligomers on 19Fe/Ti-PILC catalyst was the highest. In combination with the best activity of 19Fe/Ti-PILC, it may be concluded it is due to more iron oligomeric species FexOy. 2.6
Pyridine absorption infrared spectroscopy (Py-IR)
The contents of Brønsted and Lewis acid in the catalyst samples were qualitatively and quantitatively analyzed by pyridine adsorption infrared spectroscopy (Py-FTIR) experiments. The carrier Ti-PILC and three typical samples were selected for Py-FTIR experiments at room temperature, 170 and 300°C. The results are shown in Figure 7. There were four absorption peaks for the catalysts, e.g., 1445, 1489, 1575 and 1606 cm–1, respectively. Datka et al[30] showed that the PyL absorption peak formed by the pyridine molecule absorbed at the Lewis acid center appeared at 1440–1460 and 1600–1635 cm–1. The PyH+ absorption peak formed by the pyridine molecule absorbed at the Brønsted acid center appeared in 1535–1550 cm–1. Therefore, the absorption peaks appearing at 1445 and the 1606 cm–1 were the PyL absorption peak. The absorption peak appearing at 1575 cm–1 was the PyH+ absorption peak. The absorption peak at 1489 cm–1 was a mixed one of PyL and PyH+. Sultana et al[31] also showed that the band at 1450 cm–1 corresponded to Brønsted acid, the band at 1450 and 1590–1620 cm–1 corresponded to Lewis acid, and the band at 1490 cm–1 corresponded to Brønsted acid and Lewis acid.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239
Table 3 shows results of quantitative analysis of the acids for the four samples. Fe-free carrier Ti-PILC contained both Lewis acid and Brønsted acid centers, but the content of Brønsted acid was lower than that of the Lewis acid. Chmielarz et al[32] considered that the acid sites on the pillared clay were mainly located on surface of clay and column, and the aluminoxy octahedron and the siloxane tetrahedron in the clay were Brønsted acidic sites. So the Brønsted acid was mainly located in the edge of the montmorillonite grains, while the Lewis acid was produced mainly due to the presence of aluminum in the montmorillonite octahedral structure. In addition, Toledo-Antonio et al[33] also claimed that pure TiO2 was a typical Lewis acidic site on Ti-PILC catalyst. When Fe was loaded, the amount of Brønsted and Lewis acid in the catalyst increased, which was probably due to the presence of the transition metal ion Fe3+ embedded in the clay structure. The content of Lewis acid in 19Fe/Ti-PILC was higher than that of 13Fe/Ti-PILC. The best activity of 19Fe/Ti-PILC may be related to higher content of Lewis acid, while 5Fe/Ti-PILC performed the worst in SCR activity test and its Lewis acid content was the least.
3 Conclusions
and DeNOx activity of Cu-ZSM-5 catalysts by variation of OH/Cu2+. Catal Today, 2012, 197(1): 214–227. [6] Kumar P A, Reddy M P, Ju Lk, Hyun-Sook B, Phil H H. Low temperature propylene SCR of NO by copper alumina catalyst. J Mol Catal A: Chem, 2008, 291(1/2): 66–74. [7] Long R Q, Chang M T, Yang R T. Enhancement of activities by sulfation on Fe-exchanged TiO2-pillared clay for selective catalytic reduction of NO by ammonia. Appl Catal B: Environ, 2001, 33(2): 97–107. [8] Vaughan D E W, Lussier R J, Magee J S. Pillared interlayered clay materials useful as catalysts and sorbents: CA, US4176090. 1979. [9] Yang R T, Tharappiwattananon N, Long R Q. Ion-exchanged pillared clays for selective catalytic reduction of NO by ethylene in the presence of oxygen. Appl Catal B: Environ, 1998, 19(3/4): 289–304. [10] Valverde J L, Lucas A D, Sánchez P, Dorado F, Romero A. Cation exchanged and impregnated ti-pillared clays for selective catalytic reduction of NOx by propylene. Appl Catal B: Environ, 2003, 43(1): 43–56. [11] Lu G, Li X Y, Qu Z P, Zhao Q D, Zhao L, Chen G H. Copper-ion exchanged Ti-pillared clays for selective catalytic reduction of NO by propylene. Chem Eng J, 2011, 168(3): 1128–1133.
Montmorillonite was used as raw material to prepare Titanium dioxide pillared clay (Ti-PILC) catalyst and the specific area and pore volume of the catalysts increased obviously after the pillaring process with TiO2. Fe was supported on Ti-PILCs by ion exchange method to prepare Fe/Ti-PILC catalysts and used for SCR-C3H6. Under oxygen-rich conditions, 19Fe/Ti-PILC achieved complete removal of NO at 400°C, and always remained the highest activity at 400–600°C. In addition, the catalyst had good resistance to water vapor and SO2, and the NO reduction can still reach 90% in the presence of water vapor and SO 2.
References
[12] Dou Y F, Su Y X, Lu Z X, Zhou H, Deng W Y. Experimental study of NO reduction by ethane over iron. J Fuel Chem Technol, 2015, 43(10): 1273–1280. [13] Liang J Q, Su Y X, Zhou H, Deng W Y. Performance and mechanism of NO reduction by iron combined with propene. J Fuel Chem Technol, 2016, 44(8): 977–984. [14] Su Y X, Su A L, Ren L M, Deng W Y. Effect of SO2 on NO reduction by methane over iron. J Fuel Chem Technol, 2014, 42(3): 377–384. [15] Zhou H, Su Y X, Qi Y Z, Lu Z X, Deng W Y. Effect of water vapor on NO reduction by methane over iron. J Fuel Chem Technol, 2014, 42(11): 1378–1386. [16] Qian W Y, Su Y X, Yang X, Yuan M H, Deng W Y, Zhao B T. Experimental study on selective catalytic reduction of NO with
[1] Iwamoto M, Yahiro H, Yu U Y. Selective reduction of NO by lower hydrocarbons in the presence of O2 and SO2 over copper ion-exchanged zeolites. Catal, 1990, 32(6): 430–433. [2] Held W, Koenig A, Richter T, Puppe L. Catalytic NOx reduction in net oxidizing exhaust gas. SAE Trans, 1990, 99(4): 209–216. [3] Zhou H, Su Y X, Deng W Y, Zhong F C. A review of HC-SCR over metal oxides-based catalysts. Environ Sci Technol, 2016, 39(1): 93–100. [4] Wang Q Y, Wen Y B, Dong X F, Lin W M. Crosslinked clay synthesis and its application in selective catalytic reduction of NOx with C3H6. J Chem Eng Chin Univ, 2006, 20(4): 598–603. [5] Yashnik S A, Salnikov A V, Vasenin N T, Anufrienko V F, Ismagilov Z R. Regulation of the copper-oxide cluster structure
propene over iron based catalysts supported on aluminum pillared clays. J Fuel Chem Technol, 2017, 45(12): 1499–1507. [17] Ye Q, Yan L N, Huo F F, Wang H P, Cheng S Y, Kang T F. Cu-supported on Fe-pillared sepiolite: Characterization and selective catalytic reduction(SCR)of NO by propene. Acta Chim Sin, 2011, 69(13): 1524–1532. [18] Yang T T, Bi H T, Cheng X. Effects of O2, CO2 and H2O on NOx adsorption and selective catalytic red uction over Fe/ZSM-5. Appl Catal B: Environ, 2011, 102(1/2): 163–171. [19] Martínez-Hernández A, Fuentes G A. Redistribution of cobalt species in Co-ZSM-5 during operation under wet conditions in the reduction of NOx by propane. Appl Catal B: Environ, 2005, 57(3): 167–174.
DONG Shi-lin et al / Journal of Fuel Chemistry and Technology, 2018, 46(10): 12311239 [20] Komvokis V G, Iliopoulou E F, Vasalos I A, Triantafyllidis K S, Marshall C L. Development of optimized Cu-ZSM-5 deNOx
adsorption of contaminants in water. Appl Clay Sci, 2003, 22(4): 169–177.
catalytic materials both for HC-SCR applications and as FCC
[28] Kumar M S, Schwidder M, Grünert W, Bruckner A. On the
catalytic additives. Appl Catal A: Environ, 2007, 325(2):
nature of different iron sites and their catalytic role in Fe-ZSM-5
345–352.
DeNOx, catalysts: New insights by a combined EPR and
[21] Kim B S, Lee S H, Park Y T, Ham S W, Chae H J, Nam I S. Selective catalytic reduction of NOx, by propene over copper-exchanged pillared clays. Korean J Chem Eng, 2001, 18(5): 704–710. [22] Long R Q, Yang R T. Selective catalytic reduction of NO with ammonia over V2O5 doped TiO2 pillared clay catalysts. Appl Catal B: Environ, 2000, 24(1): 13–21. [23] Gregg S J, Sing K S W. Adsorption, Surface Area and Porosity. New York: Academic Press Inc, 1982.
UV/VIS spectroscopic approach. J Catal, 2004, 227(2): 384–397. [29] Brandenberger S, Kröcher O, Wokaun A, Tissler A, Althoff R. The role of Brønsted acidity in the selective catalytic reduction of NO with ammonia over Fe-ZSM-5. J Catal, 2009, 268(2): 297–306. [30] Datka J, Turek A M, Jehng J M, Wachs I E. Acidic properties of supported niobium oxide catalysts: An infrared spectroscopy investigation. J Catal, 1992, 135(135): 186–199.
[24] Long R Q, Yang R T. Selective catalytic reduction of nitrogen
[31] Sultana A, Haneda M, Fujitani T, Hamada H. Influence of Al2O3
oxides by ammonia over Fe3+-exchanged TiO2-pillared clay
support on the activity of Ag/Al2O3 catalysts for SCR of NO
catalysts. J Catal, 1999, 186(2): 254–268.
with decane. Catal Lett, 2007, 114(1): 96–102.
[25] Chmielarz L, Piwowarska Z, Kuśtrowski P, Węgrzyn A, Gil B,
[32] Chmielarz L, Piwowarska Z, Kuṡtrowskip, Węgrzyn A, Gil B,
Kowalczyk A, Dudek B, Dziembaj R, Michalik M. Comparison
Kowalczyk A, Dudek B, Dziembaj R, Michalik M. Comparison
study of titania pillared interlayered clays and porous clay
study of titania pillared interlayered clays and porous clay
heterostructures modified with copper and iron as catalysts of
heterostructures modified with copper and iron as catalysts of
the DeNOx process. Appl Clay Sci, 2011, 53(2): 164–173.
the DeNOx process. Appl Clay Sci, 2011, 53(2): 164–173.
[26] Shen B X, Ma H Q, Yang X Y, Yao Y. Study on preparation,
[33] Toledo-Antonio J A, Cortés-Jácome M A, Navarrete J,
characterization and de-NO activity of Mn-CeOx/Ti-PILC. J
Angeles-Chavez C, Lopez-Salinas E, Rendon-Rivera A.
Fuel Chem Technol, 2012, 40(5): 615–620.
Morphology induced CO, pyridine and lutidine adsorption sites
[27] Oliveira L C A, Rios R V R A, Fabris J D, Sapag K, Garg V K, Lago R M. Clay-iron oxide magnetic composites for the
on TiO2: Nanoparticles, nanotubes and nanofibers. Catal Today, 2010, 155(3/4): 247–254.