Selective Catalytic Reduction of NO by NH3 on CeO

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Selective Catalytic Reduction of NO by NH3 on CeO2-MOx (M= Ti, Si, Al) Dual Composite Catalysts: Impact of Surface Acidity Lei Zhang, Jingfang Sun, Lulu Li, Shumin Ran, Genxiang Li, Chuanjiang Li, Chengyan Ge, and Lin Dong Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03652 • Publication Date (Web): 19 Dec 2017 Downloaded from http://pubs.acs.org on December 19, 2017

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Industrial & Engineering Chemistry Research

Selective Catalytic Reduction of NO by NH3 on CeO2-MOx (M= Ti, Si, Al) Dual Composite Catalysts: Impact of Surface Acidity Lei Zhang†, Jingfang Sun‡, Lulu Li‡, Shumin Ran†, Genxiang Li†, Chuanjiang Li †, Chengyan Ge§, Lin Dong*,‡ †

School of Environmental and Chemical Engineering, Chongqing Three Gorges University,

Chongqing 404000, People's Republic of China ‡

Key Laboratory of Mesoscopic Chemistry of MOE, School of Chemistry and Chemical

Engineering, Nanjing University, Nanjing 210093, People's Republic of China §

School of Chemistry and Chemical Engineering, Yancheng institute of Technology, Yancheng

224051, People's Republic of China

Corresponding author: Email: [email protected]; Tel: +86-25-83592290. 1

ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT: A series of ceria based dual composite oxides were prepared via a co-precipitation method, and we focus on studying the relationships between the catalytic activity and physicochemical properties of ceria based catalysts. It is found that catalytic activity followed the order of CeTi > CeSi > CeAl > pure CeO2, X-ray diffraction (XRD) indicates that all the ceria based dual composite oxides were the cubic fluorite-type phase of CeO2, the NH3 adsorption-desorption results reveal that the NH3 adsorption stability and amount of CeO2 was improved by modification with TiO2, SiO2 or Al2O3, and the order of surface acidity was in agreement with the order of catalytic activity. Combined with other characterizations, we suppose that surface acidity is a decisive factor to influence on the catalytic activity of ceria based catalysts in the NH3-SCR reaction.

Keywords: Ceria based catalyst; NH3-SCR; Acidic additive; In situ DRIFT

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1. INTRODUCTION NOx pollutant emissions result in some environmental issues. The selective catalytic reduction NO by NH3 (NH3-SCR) is an effective technology to reduce NOx elimination, especially for coal-fired plant. The commercial vanadia based catalyst has been widely used for the stationary sources. However, this catalyst has several defects, such as the toxicity of active component V2O5 and narrow operation temperature window.1–4 To overcome these drawbacks, it is necessary to develop some new NH3-SCR catalysts. Ceria (CeO2), as functional rare-earth oxide, has been widely investigated in catalytic elimination fields including in three-way catalysis (TWC), NH3-SCR, CO oxidation, preferential oxidation of CO in H2 and soot combustion mainly due to its high oxygen storage-release efficiency.5–7 It is reported that adding foreign metal oxide to CeO2 can improve the physicochemical properties and catalytic performance of ceria based catalysts.1 Nowadays ceria based catalysts have received increasing attention in NH3-SCR reaction. Although pure CeO2 displays poor NH3-SCR activity, ceria based dual composite oxides mainly including CeO2-WO3,8–10 CeO2-TiO211–13 display much excellent NH3-SCR activity at medium and high temperature. Introducing metal oxide to CeO2 can modulate the surface acidity and redox ability of ceria based catalysts, which can change NH3 adsorption and NO oxidation process.8,14,15 It is considered that TiO2, SiO2 and Al2O3, as low cost materials, are acidic oxides,16,17 which are used to adjust the surface acidity of CeO2. As reported elsewhere, the interaction between CeO2 and Al2O3 or SiO2 could promote the NH3-SCR activity of Ce/Al and Ce/Si catalysts.18–20 To the best of our knowledge, there are no comparative studies about the catalytic regularity of ceria based catalysts induced by modification with TiO2, SiO2 and Al2O3. However, comparative study is important for us to understand some basic principles for ceria based composite oxides in NH3-SCR reaction. In the present work, a series of CeO2-MOx (M = Ti, Si, Al) dual composite oxides were prepared via a co-precipitation method, and the physicochemical properties of catalysts were characterized by multiple methods. We systematically 3



investigated the influence of texture, surface element component, redox property and surface acidity on the catalytic performance of ceria based composite oxides. Meanwhile, we hope that this study can guide us to prepare some cost-effective and high performance ceria based catalysts, which are applied to the medium and high temperature NH3-SCR reaction. 2. EXPERIMENTAL SECTION 2.1. Sample preparation CeO2-MOx (M = Ti, Si, Al) composite oxides (Ce:M = 1:1 molar ratio) were prepared by a co-precipitation method. The desired quantities of Ce(NO3)·6H2O as ceria source was dissolved in excess distilled water to form solution. Tetrabutyl titanate, tetraethyl orthosilicate and Al(NO3)·9H2O were used as titanium, silicon and alumina source, respectively. The corresponding desired quantities of M source was dissolved in distilled water (for tetrabutyl titanate, using ethanol and water (molar ratio is C2H5OH : H2O = 3:1) as solvent). The above Ce and M solutions were mixed together under stirring, and then precipitated by dripping ammonium hydroxide until the pH = 9~10. Filtrating and washing the obtained precipitates a few times by distilled water, and subsequently the precipitates were dried in air at 110 ºC for 12 h and further calcined at 500 ºC for 4 h in flowing air. In addition, pure CeO2 was obtained by directly calcining Ce(NO3)3·6H2O at 500 ºC for 4 h in flowing air. The prepared samples were CeO2-TiO2 (CeTi), CeO2-SiO2 (CeSi) and CeO2-Al2O3 (CeAl). Otherwise, the commercial vanadium based catalyst (denoted as VBC) was provided by Wuxi Weifu International Trade Co.,Ltd. (China). 2.2. Characterization The data of specific surface area were obtained by a Micrometrics ASAP-2020 analyzer in the 77 K liquid N2. The sample (about 0.1 g) was degassed in a N2/He mixture at 300 ºC for 4 h before nitrogen adsorption measurement. XRD patterns were measured by a Philips X′pert Pro diffractometer with Ni-filtered CuKα1 radiation (0.15408 nm), and the operation voltage and current of X-ray tube were 40 kV and 40 mA, respectively. 4


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X-ray photoelectron spectroscopy (XPS) data were recorded by a PHI 5000 Versa Probe high performance electron spectrometer with Al Kα radiation. The sample was pretreated in an ultra-high vacuum chamber ( CeSi > CeAl > pure CeO2. In addition, the SO2 resistance of pure CeO2 and CeM samples was investigated at 300 ºC by introducing 200 ppm SO2 in NH3-SCR condition. As shown in Figure 2c, for pure CeO2, when SO2 was introduced into the reaction feed, NO conversion was improved from ca. 45% to ca. 90%. As reported elsewhere,21–23 the pre-sulfation of pure CeO2 was beneficial to promoting its NO conversion. When introducing metal oxide to CeO2, the NO conversion of CeM samples could keep ca. 90% during 25 h testing in the presence of SO2, which indicates that ceria based dual composite oxides exhibited excellent SO2 resistance performance. In addition, H2O and H2O + SO2 resistance for CeM samples were investigated at 300 ºC. As shown in Figure 2, only adding H2O to reaction feed, NO conversion just slightly dropped and could keep steady during 12 h testing. When H2O + SO2 were added, NO conversion would continuously decrease during another 12 h testing. However, when H2O + SO2 were cut off, NO conversion would recover to the level, which suggests the deactivation induced by H2O + SO2 is reversible for CeM samples. 100

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Figure 2. NO conversion (a), N2 selectivity (b), SO2 tolerence testing at 300 ºC (c) and H2O and H2O + SO2 tolerence testing at 300 ºC (d) for CeM samples.

3.2. XRD and BET results Figure 3 shows the XRD patterns of CeM samples, and all the diffraction peaks of CeM samples could be assigned to the cubic fluorite-type phase of CeO2 (JCPDS 34-0394), and no TiO2, SiO2 and Al2O3 phases could be observed, indicating that TiO2, SiO2 and Al2O3 were dispersive or amorphous. Meanwhile, the peak intensity of CeM samples gradually decreased by the order of pure CeO2 > CeTi > CeAl > CeSi, indicating that the crystallization of CeM samples got worse by the order of pure CeO2 > CeTi > CeAl > CeSi. Meanwhile, the peaks of CeM samples got broader compared with those of pure CeO2, which indicates that the particle size of CeO2 in CeM samples was smaller than the particle size of the pure CeO2 sample according to Debye-Scherrer equation.24,25 Table 1 displays that CeM samples had larger specific surface area than pure CeO2. Interestingly, though CeSi and CeAl had larger specific surface area than CeTi, CeTi exhibited much higher NO elimination efficiency than CeSi and CeAl. Thus, except specific surface area, other factors can effect on the catalytic performance of CeM samples.

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Figure 3. XRD patterns of CeM samples. Table1 Surface component, acid amount and specific surface area of CeM samples. Samples

Atomic ratio (%) Ce3+/(Ce3++Ce4+)

O'/(O'+O'')

Acid content (a.u.)

Specific surface area (m2·g–1)

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25.7 53.5 56.2 23.1

4946 10887 15770 23384

74.0 133.6 241.1 89.5

CeO2 CeAl CeSi CeTi

3.3. XPS results Surface component and chemical state of elements of ceria based samples were investigated by XPS. Figure 4a shows that Ce 3d spectra were fitted by Gaussian-Lorentz curves and were composed of two multiplets labelled as u for 3d3/2 and 3d5/2 for v. It is reported that the bands of u' and v' could be ascribed to the primary photoemission of Ce3+, and the other six bands labeled as u''', v''', u'', v'', u, and v could be assigned to Ce4+.11,21 Thus Ce3+ and Ce4+ could be found in all the samples. As listed in Table 1, the Ce3+ atomic concentration of CeM samples was higher than that of pure CeO2. It is reported that Ce3+ content increased with the particle size of CeO2 decreasing.26. In addition, Raman results (in Figure S1 and Table S1) indicates that CeM samples had higher relative quantity of oxygen vacancies than CeO2, and it is reported that higher oxygen vacancies concentration, higher Ce3+ content in ceria based material.27,28 Thus the Ce3+ relative content of CeM was higher than that of pure CeO2. As shown in Figure 4b for O1s spectra, the O' bands could be ascribed to lattice oxygen and the O'' band represented surface adsorbed oxygen, 9



including active oxygen species (O- or O22-) or hydroxyl-like group.13 Table 1 shows that the O'/(O' + O'') values of CeSi and CeAl were much higher than those of pure CeO2 and CeTi, it is possible that more hydroxyl-like groups were formed over CeSi and CeAl according to the latter DRIFT results. (b) O1s O

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Figure 4. XPS results for CeM samples: Ce 3d (a) and O1s (b).

3.4. H2-TPR results H2-TPR was conducted to investigate the redox property of ceria based catalysts, and the results were shown in Figure 5. The low temperature reduction peak (LT peak) in 300–600 ºC range could be assigned to the reduction of surface and sub-surface CeO2, while the high temperature reduction peak (HT peak) over 600 ºC could be due to the reduction of bulk CeO2.11 It is reported that there are two oxygen storage sites including in surface and sub-surface ceria in ceria based catalysts, which exhibit different oxygen storage processes in different reaction conditions and probably have influence on catalytic performance.29 However, combined with the analysis of Table S2 and Figure S2 in the supporting information, we propose that the reduction content of surface and sub-surface CeO2 is not the main factor to result in the difference of catalytic activity between CeO2 and CeTi catalysts in NH3-SCR reaction. Compared with pure CeO2, the area of LT peak increased and the area of HT peak decreased for CeM samples, which indicates that CeM samples had more surface CeO2 because of the increase of specific surface area and the decrease of particle size. It is worth noting that the peak temperature of LT peak shifted to higher temperature following 10


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the order of CeO2 ≈ CeTi < CeSi < CeAl, which indicates that the reducibility of surface CeO2 decreased by the order of CeO2 ≈ CeTi > CeSi > CeAl. Catalytic performance evaluation in Figure 2a found that the order of NO conversion below 250 ºC was CeTi > CeSi > CeAl. Thus, the activity at relatively lower temperature was related with the reducibility of surface and sub-surface CeO2. At the same time, the NO + O2 reaction of CeM samples was conducted and the results of NO conversion to NO2 were shown in Figure S3, the order of NO conversion to NO2 was CeTi > CeSi > CeAl when reaction temperature was below 350 ºC, which was in good agreement with the order of low temperature NH3-SCR activity of CeM samples. It is reported that NO2 could promote fast SCR reaction at relatively lower temperature and improve catalytic performance at low temperature.30–33. In addition, the surface reducibility of pure CeO2 was better than that of CeM samples, however, the low temperature activity of CeM samples was superior to that of pure CeO2, which suggests that other factor could influence on catalytic activity.

Intensity(a.u.)

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CeTi

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CeO2 200

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Figure 5. H2–TPR results of CeM samples.

3.5. In situ DRIFT and NH3-TPD NH3 adsorption and activation were investigated by NH3 adsorption in situ DRIFT. As shown in Figure 6, for pure CeO2, the bands at 1574, 1291, 1143 and 1061 cm–1 were assigned to the NH3 adsorption at Lewis acid sites, and the bands at 1670, 1460 and 1392 cm–1 were ascribed to the NH3 adsorption at Brönsted acid sites.34 When temperature was over 200 ºC, a new band at 1540 cm–1 due to nitrate species existed,35 which implies that the adsorbed NH3 was oxidized at higher temperature. 11



The NH3 adsorption behavior of CeM samples was different with pure CeO2. Some new bands existed on CeM samples. For CeSi, the band at 1599 cm–1 due to Lewis acid sites and the band at 1451 cm–1 due to Brönsted acid sites appeared, for CeAl, the band of Lewis acid sites shifted to 1595 cm–1 and bands of Brönsted acid sites shifted to 1445 and 1417 cm–1. In addition, the band intensity of Brönsted acid sites obviously increased on CeSi and CeAl compared with pure CeO2. It is reported that Brönsted acid sites were provided by surface hydroxyl groups (M-OH).36–38 XPS results have proved that the ratio of surface oxygen species on CeSi and CeAl increased. Combined with the NH3 adsorption of CeSi and CeAl, we deduce that the surface oxygen species were mainly hydroxyl groups on CeSi and CeAl, and thus the hydroxyl groups could provide more Brönsted acid sites compared with pure CeO2. While for CeTi, NH3 was strongly adsorbed at Lewis acid sites (1152 and 1602 cm–1) and was only weakly adsorbed at Brönsted acid sites (1441 and 1667 cm–1) below 150 ºC. With temperature increasing, the intensity of bands decreased for all samples. In order to reduce the interference of nitrate signal, we chose bands in 3100–3400 cm–1 range assigned to N-H vibrations of adsorbed NH3 to illustrate the NH3 adsorption stability of CeM sampels.36,39 The bands of N-H vibrations totally disappeared at 250, 300, 350 and 400 ºC on pure CeO2, CeAl, CeSi and CeTi, respectively, which indicates that the stability order of NH3 adsorption was CeTi > CeSi > CeAl > CeO2. CeO2

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Figure 6. In situ DRIFT NH3 adsorption-desorption spectra of CeM samples as a function of temperature.

Surface acid content was measured by NH3-TPD. Figure 7 shows that a broad peak from 100 to 400 ºC existed in all samples, and the peak temperature of NH3 desorption was followed by the order of CeTi > CeSi > CeAl > CeO2, which indicates that NH3 adsorption stability was in accordance with the DRIFT results. In addition, the relative content of surface acid sites was calculated by peak areas in NH3-TPD curves and listed in Table 1. The order of relative content was CeTi > CeSi > CeAl > CeO2. Interestingly, though the specific surface area of CeSi or CeAl was much larger than that of CeTi, the content of acid sites on CeTi was the highest.

CeTi

Intensity (a.u.)

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Figure 7. NH3-TPD results of CeM samples.

3.6. Discussion Combined with the above characterizations, the relationship between catalytic activity and physicochemical properties of ceria based composite oxides was plotted 13



in Figure 8. The order of activity was in good agreement with the order of surface acidity including the adsorption intensity and amount of NH3, which suggests that surface acidity played an important role in NH3-SCR reaction. The literature10,40,41 has pointed out that the CeO2 catalysts modified by solid acids exhibit excellent catalytic performance in the NH3-SCR reaction. The reported acidic additives can be classified by two categories: metal oxide and acid radical. The representation of metal oxide is WO3 corresponding to WO3-CeO2 catalyst, and the representation of acid radical is SO42– corresponding to sulfated CeO2 catalyst. It is reported that WO3 and SO42– additives can greatly improve the surface acidity of CeO2, and even WO3-CeO2 and sulfated CeO2 catalyst have a very similar activity window in the range of 200–450 º C.10,41 In addition, both the modification of WO3 and SO42– can inhibit the redox ability of surface CeO2,9,21 just like the results of TiO2, SiO2 and Al2O3 in this study. Meanwhile, Figure 8 shows that the order of activity was inconsistent with the order of redox ability, thus all the results indicate that the redox ability of ceria based catalysts is not a decisive factor for NH3-SCR catalytic performance, especially for the ceria based catalysts used for NH3-SCR reaction at medium and high temperature. However, properly modulating the redox property of ceria based catalysts is conductive to enhancing its low temperature catalytic activity, such as using ultra-low copper to modify TiO2/CeO2 catalyst or adding manganese to WO3-CeO2 catalyst.42,43 Based on the above results and analysis, acidic additives such as TiO2, SiO2, Al2O3, WO3 and SO42– can greatly improve the NH3-SCR catalytic performance of CeO2. In addition, adding proper amount of the third oxide, which has more excellent redox ability than CeO2, to the CeO2-MOx dual composite oxides can promote their low temperature catalytic activity. Thus, to further satisfy the requirement of NO elimination at low temperature, we can use the third oxide to modulate the low temperature catalytic activity of CeO2-MOx dual composite oxides.

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activity window (NO conversion>80%) 3+ 4+ Ce /Ce ratio (XPS) NH3 adsorption intensity ( DRIFT) specific surface area (BET) surface acid content (NH3-TPD) redox ability (H2-TPR)

Relative intensity

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CeTi

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CeO2

Figure 8. Relationship between NH3-SCR catalytic activity and physicochemical properties of CeM samples.

4. CONCLUSIONS Pure CeO2 and CeO2-MOx dual composite oxides were prepared for NH3-SCR reaction and the added metal oxide could promote the catalytic activity of CeO2. Combined with multiple methods of characterization, the modification of CeO2 with metal oxide (TiO2, SiO2 and Al2O3) could promote the adsorption stability and adsorption content of NH3, which is important for improving the catalytic performance of CeO2-MOx catalysts at medium and high temperature, and the obtained results also suggest that the order of activity was in well agreement with the order of surface acidity for the ceria based dual composite catalysts. Thus, using a proper acidic additive to modify CeO2 is an efficient way to prepare a ceria based catalyst applied to the NH3-SCR reaction. AUTHOR INFORMATION *

Corresponding author. Tel: +86-25-83592290; Fax: +86-25-83317761; E-mail:

[email protected] ACKNOWLEDGEMENT This work was supported by the National Natural Science Foundation of China (21607019, 21607122); the Open Project Program of Jiangsu Key Laboratory of Vehicle Emissions Control (OVEC013); the Talent Introduction Project of Chongqing 15



Three Gorges University (20151114). ASSOCIATED CONTENT Supporting Information Further additional information about the Raman spectra of pure CeO2 and CeM samples (Figure S1 and Table S1), NO conversion results for CeO2 and CeTi samples (Figure S2), the integral area data of H2-TPR peaks for CeO2 and CeTi samples (Table S2), and the catalytic activity of NO oxidation reaction over CeM samples (Figure S3). This material is available free of charge via the Internet at http://pubs.acs.org. REFERENCES (1) Tang, C. J.; Zhang, H. L.; Dong, L. Ceria-based catalysts for low-temperature selective catalytic reduction of NO with NH3. Catal. Sci. Technol. 2016, 6, 1248–1264. (2) Shan, W. P.; Liu, F. D.; Yu, Y. B.; He, H. The use of ceria for the selective catalytic reduction of NOx with NH3. Chin. J. Catal. 2014, 35, 1251–1259. (3) Brandenberger, S.; Kröcher, O.; Wokaun, A.; Tissler, A.; Althoff, R. The state of the art in selective catalytic reduction of NOx by ammonia using metal-exchanged zeolite catalysts. Catal. Rev. 2008, 50, 492–531. (4) Wang, J. H.; Zhao, H. W.; Haller, G.; Li, Y. D. Recent advances in the selective catalytic reduction of NOx with NH3 on Cu-Chabazite catalysts. Appl. Catal. B 2017, 202, 346–354. (5) Sun, C. W.; Li, H.; Chen, L. Q. Nanostructured ceria-based materials: synthesis, properties, and applications. Energy Environ. Sci. 2012, 5, 8475–8508. (6) Trovarelli, A. Catalytic properties of ceria and CeO2-containing materials. Catal. Rev. 1996, 38, 439–520. (7) Zhang, D. S.; Du, X. J.; Shi, L. Y.; Gao, R. H. Shape-controlled synthesis and catalytic application of ceria nanomaterials. Dalton Trans. 2012, 41, 14455–14475. (8) Chen, L.; Li, J. H.; Ge, M. F.; Ma, L.; Chang, H. Z. Mechanism of selective catalytic reduction of NOx with NH3 over CeO2-WO3 catalysts. Chin. J. Catal. 2011, 32, 836–841. 16


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For Table of Contents Only activity window (NO conversion>80%) 3+ 4+ Ce /Ce ratio (XPS) NH3 adsorption intensity ( DRIFT) specific surface area (BET) surface acid content (NH3-TPD) redox ability (H2-TPR)

Relative intensity

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


CeTi

CeSi

CeAl

CeO2

Relationship between the catalytic activity and physicochemical properties of CeO2-MOx (M = Ti, Si, Al) dual composite oxides in the selective catalytic reduction NO by NH3

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Selective Catalytic Reduction of NO by NH3 on CeO

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