H3PW12O40 Grafted on CeO2: A High-Performance Catalyst for the

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H3PW12O40 grafted on CeO2: A high performance catalyst for the selective catalytic reduction of NOx with NH3 Yang Geng, Shangchao Xiong, Bo Li, Yong Liao, Xin Xiao, and Shijian Yang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b03947 • Publication Date (Web): 26 Dec 2017 Downloaded from http://pubs.acs.org on December 26, 2017

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Industrial & Engineering Chemistry Research

H3PW12O40 grafted on CeO2: A high performance catalyst for the selective catalytic reduction of NOx with NH3 Yang Geng, Shangchao Xiong, Bo Li, Yong Liao, Xin Xiao, Shijian Yang ∗ Jiangsu Key Laboratory of Chemical Pollution Control and Resources Reuse, School of Environmental and Biological Engineering, Nanjing University of Science and Technology, Nanjing, 210094 P. R. China

∗

Corresponding author phone: 86-18-066068302; E-mail: [email protected] (S. J. Yang). 1

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Abstract: CeO2 showed a poor SCR activity due to its poor ability for NH3 adsorption. To improve the SCR performance of CeO2, tungstophosphoric acid (i.e., HPW, H3PW12O40) with a high acidic strength was grafted on CeO2 by the adsorption of HPW on CeO2 in a HPW solution. The grafting of HPW on the surface of CeO2 was demonstrated by the characterizations of XPS, XRF, TG-DSC and in situ DRIFT spectra. As HPW on HPW/CeO2-500 still retained the Keggin structure, HPW/CeO2-500 exhibited an excellent ability for NH3 adsorption. Both HPW and CeO2 on/in HPW/CeO2-500 played their functions to the greatest limit for the SCR reaction. CeO2 in HPW/CeO2-500 played the role in the activation of adsorbed NH3 and NO, and the grafted HPW on HPW/CeO2-500 acted as the active sites for NH3 adsorption. Therefore, HPW/CeO2-500 showed a superior SCR performance at 200-450 oC and an excellent H2O/SO2 resistance above 300 oC. Keywords: SCR reaction; HPW; grafting; NH3 adsorption; reaction mechanism.

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1. Introduction Nitrogen oxides (including NO and NO2) contribute to a series of environmental problems for example acid rain, photochemical smog, haze and ozone depletion.1-3 Therefore, NOx emission from automobile and industrial combustion of fossil fuels is a serious concern. Selective catalytic reduction (SCR) of NO by NH3 with V2O5/WO3-TiO2 as the catalyst is now the commercial technology to control NOx emission from coal fired plants.

4

Although the severe limit of NOx

emission can be achieved, V2O5-WO3/TiO2 is still not satisfactory due to the low N2 selectivity, the narrow temperature window and the volatilization of toxic vanadium pentoxide to the environment at high temperatures. 5 Therefore, an environment-friendly SCR catalyst with a better N2 selectivity and a broad temperature window should be developed. 6 As ceria (CeO2) with the fluorite structure can release oxygen in a reducing condition while store oxygen in an oxygen-rich environment, 7 it has been widely used in environmental catalysis. However, CeO2 shows a poor SCR activity due to its poor ability for NH3 adsorption.

8, 9

To

improve the SCR performance of CeO2, it was supported on the acidic supports. Previous studies by Zhang et al. have indicated that the NH3-SCR activity of CeO2 was obviously improved after being supported by TiO2.

10-15

Chen et al. claimed that the superior NH3 adsorption capacity and

the accelerative activation of adsorbed NH3 of CeO2-WO3/TiO2 would be beneficial for the NH3 SCR activity.

5, 16, 17

Meanwhile, CeO2 also mixed with other metal oxides to improve the SCR

activity. Shan et al. reported that CeTiOx catalyst with the highly dispersed CeO2 and TiO2 had a broad temperature window.18-20 CeWOx, which showed a good NH3-SCR activity and high GHSV resistance, was reported by Peng et al.

21-24

Geng et al. reported a high efficient Ce0.2W0.1TiOx

oxide catalyst, which was prepared by a homogeneous precipitation method. 25, 26 Now, Ce-based catalysts are the most promising metal oxide catalysts to substitute V2O5-WO3/TiO2 for the abatement of NOx. 27-29 Tungstophosphoric acid (i.e., HPW, H3PW12O40) with the typical Keggin structure exhibits an excellent performance in many acid-catalyzed reactions due to its high acidic strength. 30-33 HPW was once used as the acidic support to improve the SCR activity of CeO2 and the resistance of V2O5/TiO2 to alkali metals. 32, 33 However, CeO2 supported on HPW did not play the functions of 3



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CeO2 and HPW to the greatest limit for the SCR reaction. First, the low BET surface area of HPW would cause to the low BET surface area of the catalyst. Second, the concentration of Ce4+ on the surface would obviously decrease as CeO2 was supported on HPW. Third, some NH3 adsorbed on HPW would not be activated as they were far away from Ce4+ on the surface. In this work, HPW was grafted on CeO2 by the adsorption of HPW on CeO2 in a HPW solution. Both HPW and CeO2 on/in HPW grafted on CeO2 (i.e., HPW/CeO2-500) played their functions to the greatest limit for the SCR reaction. The grafted HPW acted as the acid sites for NH3 adsorption, and CeO2 played the role of the active components for the activation of adsorbed NH3 and NO. Therefore, HPW/CeO2-500 exhibited an excellent SCR performance even with a high GHSV of 120000 cm3 g-1 h-1.

2. Experimental section 2.1 Catalyst preparation Solid HPW (the chemical structure is shown in Figure S1 in the Supporting Information) was obtained from the Sinopharm Chemical Reagent Co., Ltd. CeO2 was obtained from the calcination of Ce(NO3)3·6H2O under air atmosphere at 550 oC for 4 h.

8, 9

HPW/CeO2 was prepared by the

adsorption of HPW on CeO2 in a HPW solution at room temperature. 5.0 g of CeO2 were added into a HPW solution (200 mL, 25 g L-1 and pH=1.9). After stirring at 800 rpm for 12 h, CeO2 nanoparticles in the HPW solution were separated by centrifugation at 3000 rpm for 10 min. Then, the obtained particles were washed with deionized water twice. At last, the particles were calcined at 500 and 700 oC under air for 3 h after drying at 105 °C for 12 h (i.e., HPW/CeO2-500 and HPW/CeO2-700). Meanwhile, V2O5-WO3/TiO2 (5% V2O5 and 10% WO3), Ce/TiO2 (10% CeO2), CeO2-WO3/TiO2 (10% CeO2 and 6% WO3) and CeWOx (Ce:W=1:1) were prepared as comparisons by the impregnation method or the homogeneous precipitation.5, 13, 34, 35 2.2 Catalytic test The catalytic reaction was performed on a fixed-bed quartz tube microreactor.

36, 37

The flow

rate of simulated flue gas was 100 or 200 mL min-1 and the mass of catalyst (40-60 mesh) was 100 mg, resulting in the gas hourly space velocity (GHSV) of 60000 or 120000 cm3 g-1 h-1 (i.e. approximately 85000 or 170000 h-1 for HPW/CeO2-500). The simulated flue gas consisted of 500 4


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ppm of NO, 500 ppm of NH3, 5% O2, 8% H2O (when used), 100 ppm of SO2 (when used) and balance of N2. The concentrations of gaseous N2O, NO, NO2 and NH3 in the outlet were determined online using an infrared gas analyzer (Thermo, IGS Analyzer). The concentrations of gaseous chemical components in the outlet were recorded as the SCR reaction reached the steady state (i.e., the variation of NOx conversion was less than 2% in 20 min). Meanwhile, the tolerance of the SCR reaction over HPW/CeO2-500 to SO2 and H2O was investigated at 300 oC for 12 h. 2.3 Characterization The chemical composition, X-ray diffraction pattern (XRD), BET surface area, Raman spectra, and in situ DRIFT spectra were determined on an X-ray fluorescence analyzer (XRF, ThermoFisher, ARL perform), an X-ray diffractionmeter (Bruker, AXS D8 Advance), a nitrogen adsorption apparatus (Quantachrome, Autosorb-1), a Raman spectrometer (HORIBA, Jobin Yvon LabRAM, ARAMIS) and a Fourier transform infrared spectrometer (FTIR, Nicolet IS 50), respectively. Thermogravimetric and differential thermal analyses (TG-DTA) were performed on a thermal analyzer (Netzsch STA 409PC) under air atmosphere at a heating rate of 10 oC min-1 from room temperature to 900 oC. X-ray photoelectron spectra (XPS) of HPW/CeO2-500/700 and denuded HPW/CeO2-500/700 (which was bombarded with Ar+ ions) were recorded on an X-ray photoelectron spectroscopy (Thermo, ESCALAB 250) with Al Kα (hv=1486.6 eV) as the excitation source and C 1s line at 284.6 eV as the reference for the binding energy calibration. Temperature programmed reduction (H2-TPR) was conducted on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx) at a heating rate of 10 oC min-1 from room temperature to 1000 oC. Temperature programmed desorption of NO and NH3 (i.e., NO-TPD and NH3-TPD) was conducted on the microreactor at a heating rate of 10 oC min-1 from 50 to 700 oC.

3. Results 3.1 Characterization 3.1.1 XRD and BET surface area As shown in Figure 1, XRD pattern of HPW after the calcination at 500 oC for 3 h (i.e., HPW-500) corresponded well to that of HPW (JCPDS: 50-0304). It suggests that the Keggin structure of HPW did not destroy after the calcination at 500 oC. However, XRD pattern of 5



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HPW-700 mainly corresponded to WO3 (JCPDS: 41-0369), suggesting that HPW decomposed to WO3 after the calcination at 700 oC. The chemical composition of HPW/CeO2-500, which resulted from the XRF analysis, was listed in Table S1. XRF analysis hints that the amount of HPW adsorbed on CeO2 was very low (approximately 29 µmol g-1). Therefore, XRD patterns of HPW/CeO2 before the calcination, HPW/CeO2-500 and HPW/CeO2-700 all corresponded well to cubic CeO2 (JCPDS: 34-0394) and any other peaks corresponding to W species cannot be clearly observed (Figure 1). The crystal sizes of CeO2 and HPW/CeO2-500, which were calculated from the Scherrer Equation, were both approximately 11 nm. However, that of HPW/CeO2-700 was approximately 16 nm. Therefore, the BET surface area of HPW/CeO2-700 (30.7 m2 g-1) was much less than those of CeO2 and HPW/CeO2-500 (55.4 and 56.4 m2 g-1). 3.1.2 TG-DTA Figure 2 shows TG-DTA curves of HPW, CeO2 and HPW/CeO2 before the calcination under air atmosphere. DTA profile of HPW shows two endothermic peaks at 79 and 201 oC and one exothermic peak at 597 oC (Figure 2a). As the two endothermic peaks corresponded well to the weight losses, they were assigned to the two steps of the dehydration/dehydroxylation of HPW. 38 Meanwhile, the exothermic peak at 597 oC was assigned to the decomposition of HPW to WO3. 38 TG- DTA analyses suggest that the Keggin structure of HPW would not destroy after the calcination at 500 oC, while it would destroy after the calcination at 700 oC. This deduction was demonstrated by the XRD analysis (Figure 1). DTA profile of CeO2 only shows an endothermic peak at approximately 101 oC (Figure 2b). The endothermic peak corresponded well to the weight loss, so it was attributed to the dehydration/dehydroxylation of CeO2. A slight exothermic peak at 688 oC appeared after the adsorption of HPW on CeO2 (Figure 2c), which may be attributed to the decomposition of HPW on CeO2. The shift of the exothermic peak from 597 to 688 oC suggests that the presence of the support of CeO2 may improve the thermal-stability of HPW. Previous studies reported that the support of zirconia improved the thermal-stability of HPW, support of SiO2 decreased its thermal-stability.

40

39

while the

TG-DTA analyses suggest that HPW on

HPW/CeO2-500 retained the Keggin structure, while it on HPW/CeO2-700 decomposed. 6


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3.1.3 Raman spectra Figure 3 shows the Raman spectra of HPW, HPW-500, HPW-700, CeO2, HPW/CeO2 before the calcination, HPW/CeO2-500 and HPW/CeO2-700. HPW showed four characteristic bands at 1006, 989, 232 and 216 cm-1, which were assigned to the Keggin structure of HPW.41,

42

The

characteristic bands at 1006, 989, 232 and 216 cm-1 corresponding to the Keggin structure of HPW can be clearly observed on HPW-500 and any other bands corresponding to WO3 cannot be observed. It suggests that the Keggin structure did not destroy after the calcination at 500 oC, which was consistent with the results of XRD and TG-DTA. However, the characteristic bands at 1006, 989, 232 and 216 cm-1 corresponding to the Keggin structure of HPW cannot be observed on HPW-700 and the characteristic bands at 698 and 801 cm-1 corresponding to WO3 43 appeared. It suggests that the Keggin structure of HPW destroyed after the calcination at 700 oC. CeO2 only showed an obvious characteristic band at 462 cm-1, which was assigned to the symmetrical stretching mode νs(Ce-O) of the CeO8 vibrational unit in the cubic ceria.44 As the amount of HPW adsorbed on CeO2 was very low (approximately 29 µmol g-1), only the characteristic band corresponding to cubic CeO2 (at 462 cm-1) can be observed on HPW/CeO2 before the calcination and any other bands corresponding to HPW cannot be observed. Therefore, the characteristic bands corresponding to HPW or other W species were not observed on both HPW/CeO2-500 and HPW/CeO2-700. 3.1.4 XPS XPS spectra of HPW/CeO2-500/700 and denuded HPW/CeO2-500/700 over the spectral regions of Ce 3d, O 1s and W 4f are shown in Figure 4. The Ce 3d binding energies of HPW/CeO2-500 mainly appeared at approximately 882.5, 885.5, 889.2, 898.7, 901.2, 903.6, 907.5 and 917.2 eV (Figure 4a). The binding energies at 882.5, 889.2, 898.7, 901.2, 907.5 and 917.2 eV were attributed to Ce4+, while those at 885.5 and 903.6 eV were attributed to Ce3+ .8 The binding energies of O 1s mainly appeared at approximately 529.9, 531.5 and 532.6 eV (Figure 4b), which were assigned to O in CeO2, -OH and HPW, respectively.

8, 45, 46

The appearance of the binding

energy at 532.6 eV on HPW/CeO2-500 in the O 1s spectral region demonstrates that HPW adsorbed CeO2 did not decompose after the calcination at 500 oC, which was consistent with the 7



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result of TG-DTA. The binding energies of W 4f mainly appeared at approximately 37.7 and 35.6 eV (Figure 4c), which were assigned to W6+. 45 The binding energies of Ce 3d, O 1s and W 4f on denuded HPW/CeO2-500 (Figures 4d-4f) were similar to those on HPW/CeO2-500. However, the ratio of O in HPW (at approximately 532.6 eV) to O in CeO2 (at approximately 530.0 eV) obviously decreased after the denudation of HPW/CeO2-500 (Figures 4b and 4e). It indicates that the concentration of HPW on HPW/CeO2-500 obviously decreased after the denudation. The binding energies of Ce 3d and W 4f on HPW/CeO2-700 and denuded HPW/CeO2-700 were similar to those on HPW/CeO2-500 and denuded HPW/CeO2-700 (Figure 4). However, the binding energies of O 1s on HPW/CeO2-700 and denuded HPW/CeO2-700 (Figures 4h and 4k) were quite different from those on HPW/CeO2-500 and denuded HPW/CeO2-500 (Figures 4b and 4e). After the calcination at 700 oC, the binding energy at 532.5 eV corresponding to O in HPW can hardly be observed (Figures 4h and 4k). Meanwhile, a new binding energy appeared at approximately 530.6 eV, which was assigned to O in WO3.46,

47

It suggests that most HPW

adsorbed on CeO2 decomposed to WO3 after the calcination at 700 oC, which was consistent with the result of TG-DTA. The

percentages

of

Ce

and

W

species

on

HPW/CeO2-500/700

and

denuded

HPW/CeO2-500/700, which resulted from the XPS analysis (Figure 4), were listed in Table 1. Table 1 and Table S1 show that the percentage of W on HPW/CeO2-500 (3.6%) was much higher than the content of W in HPW/CeO2-500 (2%). Meanwhile, the percentage of W on HPW/CeO2-500 obviously decreased from 3.6% to 1.5% after the denudation. Table 1 also shows that the percentage of W on HPW/CeO2-700 obviously decreased from 6.7% to 3.0% after the denudation. They suggest that W species mainly appeared on the surfaces of both HPW/CeO2-500 and HPW/CeO2-700. 3.1.5 In situ DRIFT spectra Figure 5 shows in situ DRIFT spectra of HPW, HPW-500, HPW-700, CeO2, HPW/CeO2 before the calcination, HPW/CeO2-500 and HPW/CeO2-700. HPW showed four characteristic vibrations at 1101, 1019, 936 and 866 cm-1, which were assigned to vas(P-Oa), vas(W-Od), vas(W-Ob) and 8


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vas(W-Oc) in the Keggin structure, respectively.

48

After the calcination of HPW at 500 oC, the

characteristic vibrations at 1101, 1019 and 866 cm-1 corresponding to vas(P-Oa), vas(W-Od) and vas(W-Oc) can be clearly observed. It suggests that the Keggin structure of HPW did not destroy after the calcination at 500 oC, which was consistent with the results of XRD and TG-DTA. However, the band at 936 cm-1 corresponding to vas(W-Ob) shifted to 921 cm-1, which may be related to the dehydration/dehydroxylation of HPW. 46 The characteristic vibrations corresponding to the Keggin structure can hardly be observed on HPW-700 and only the vibration at 1032 cm-1 corresponding to the stretching vibration of W=O double bond 47 can be observed. It suggests that HPW decomposed to WO3 after the calcination at 700 oC, which was consistent with the results of XRD and TG-DTA. CeO2 showed one characteristic vibration at 1054 cm-1, which was assigned to the formation of “carbonate-like” species on ceria due to the chemisorption of CO2 from air. 49 The vibration at 1054 cm-1 corresponding to “carbonate-like” species can be still observed on CeO2 after the adsorption of HPW. Meanwhile, the vibrations at 1101 and 1019 cm-1 corresponding to vas(P-Oa) and vas(W-Od) can be clearly observed. It suggests that HPW adsorbed on CeO2 still retained the Keggin structure. However, the vibration at 921/936 cm-1 corresponding to vas(W-Ob) shifted to 965 cm-1 and the vibration at 866 cm-1 corresponding to vas(W-Oc) shifted to 856 cm-1. They suggest that a weakening of the anion cohesion happened due to the adsorption of HPW on CeO2.39 In situ DRIFT spectrum of HPW/CeO2-500 was close to that of HPW/CeO2 before the calcination. It suggests that HPW on HPW/CeO2-500 still retained the Keggin structure. However, the characteristic vibrations at 1101 and 1019 cm-1 corresponding to the Keggin structure cannot be observed on HPW/CeO2-700 and the band at 978 cm-1 corresponding to vas(W-O-W) 47 can be clearly observed. It suggests that HPW adsorbed on CeO2 decomposed after the calcination at 700 oC. These results were consistent with the results of XPS and TG-DTA analyses. 3.1.6 H2-TPR H2-TPR profile of HPW-500 showed a strong reduction peak at approximately 828 oC (Figure 6), which was assigned to the reduction of W6+.50 H2-TPR profile of CeO2 showed two reduction peaks at 523 and 777 oC, which were assigned to the reduction of surface oxygen species and that 9



of oxygen in bulk, respectively.

8

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XRF analysis (Table S1 in the Supporting Information) hints

that the content of CeO2 in HPW/CeO2-500 was approximately 92%. Then, H2-TPR profile of CeO2 was accordingly subtracted from that of HPW/CeO2-500 (shown in Table S2 in the Supporting Information). Any peaks (including both postive peaks and negative peaks) corresponding to the reduction of CeO2 (at 523 and 777 oC) cannot be observed in the subtracted line. It suggests that the reduction of CeO2 in HPW/CeO2-500 was not obviously influenced by the grafted HPW. As W6+ on HPW/CeO2 can be reduced by Ce3+ on the surface,

51

two reduction

peaks at 565 and 706 oC contained in the H2-TPR profile of HPW/CeO2-500 may be related to the indirect reduction of W6+. However, the reduction peak of HPW/CeO2-700 obviously shifted to higher temperature (Figure 6). It suggests that there may be the solid solution of Ce-W-O in HPW/CeO2-700. The first reduction peaks of CeO2 and HPW/CeO2-500 both appeared on 523 oC, while the first reduction of HPW/CeO2-700 appeared at 639 oC. Therefore, the oxidation abilities of the catalysts increased in the following sequence: HPW/CeO2-700< HPW/CeO2-500= CeO2. 3.1.7 NH3 adsorption and NO adsorption The capacities of HPW-500, HPW-700, CeO2, HPW/CeO2-500 and HPW/CeO2-700 for NH3 adsorption at 50 oC, which resulted from NH3-TPD profiles (Figure 7a), were listed in Table 2. HPW-500 showed a huge ability for NH3 adsorption, while HPW-700 showed a poor ability for NH3 adsorption due to the destruction of the Keggin structure. CeO2 showed a poor ability for NH3 adsorption. The capacity of HPW/CeO2-500 for NH3 adsorption was approximately 2.9 times that of CeO2. It suggests that NH3 adsorption over CeO2 was obviously promoted after the grafting of HPW. As HPW adsorbed on CeO2 decomposed after the calcination at 700 oC (hinted by XPS, TG-DTA and in situ DRIFT spectra), the ability of HPW/CeO2-700 for NH3 adsorption was much less than that of HPW/CeO2-500 (shown in Figure 7a and Table 2). The capacities of HPW-500, HPW-700, CeO2, HPW/CeO2-500 and HPW/CeO2-700 for NO adsorption at 50 oC, which resulted from NO-TPD profiles (Figure 7b), were listed in Table 2. Both HPW-500 and HPW-700 showed poor abilities for NO adsorption. The capacity of HPW/CeO2-500 for NO adsorption was close to that of CeO2. It suggests that the physical

10


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adsorption of NO on CeO2 was hardly restrained after the grafting of HPW. However, the ability of HPW/CeO2-700 for NO adsorption was very poor. In situ DRIFT spectra of the adsorption of NH3 over HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700 at 200 oC were shown in Figure 8a. After the adsorption of NH3, five strong characteristic vibrations appeared over HPW-500 at 1198, 1393, 1434, 1582 and 1667 cm-1. The vibrations at 1198 and 1582 cm-1 were assigned to coordinated NH3 bound to the Lewis acid sites, while the vibrations at 1393, 1434 and 1667 cm-1 were assigned to ionic NH4+ bound to the Brønsted acid sites. 5 The adsorption of NH3 over CeO2 at 200 oC can hardly be observed in the DRIFT spectra (Figure 8a). It also suggests that the ability of CeO2 for NH3 adsorption was very poor. However, both coordinated NH3 bound to the Lewis acid sites (at 1198 and 1582 cm-1) and ionic NH4+ bound to the Brønsted acid sites (at 1419 and 1667 cm-1) appeared on HPW/CeO2-500 after the adsorption of NH3. It suggests that the adsorption of NH3 on CeO2 was notably promoted after the grafting of HPW and NH3 mainly adsorbed on HPW on HPW/CeO2-500. Although the spectrum of the adsorption of NH3 on HPW/CeO2-700 was similar to that on HPW/CeO2-500 (Figure 8a), the intensity of NH3 adsorbed on HPW/CeO2-700 was much less than that adsorbed on HPW/CeO2-500. It also suggests that the ability of HPW/CeO2-700 for NH3 adsorption was much less than that of HPW/CeO2-500. In situ DRIFT spectra of the adsorption of NO+O2 over HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700 at 200 oC were shown in Figure 8b. After the adsorption of NO+O2 at 200 oC, adsorbed NOx species cannot be clearly observed over HPW-500 (Figure 8b). It suggests that NO can hardly adsorb on HPW.

48

After the adsorption of NO+O2 at 200 oC, five characteristic

vibrations at 1558, 1543, 1523, 1271 and 1161 cm-1 appeared on CeO2. The vibration at 1558 cm-1 was assigned to monodentate nitrate, and the vibrations at 1543, 1523, 1271 and 1161 cm-1 were attributed to bidentate nitrate.

52, 53

After the adsorption of NO+O2 at 200 oC, two characteristic

vibrations at 1606 and 1578 cm-1 appeared on HPW/CeO2-500, which were assigned to monodentate nitrite. 53 It suggests that adsorbed NOx species on CeO2 transformed from nitrate to nitrite after the grafting of HPW. Although the spectrum of the adsorption of NO+O2 on HPW/CeO2-700 was similar to that on HPW/CeO2-500, the intensity of adsorbed NOx on 11



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HPW/CeO2-700 was much less than that on HPW/CeO2-500 (Figure 8b). It also suggests that the ability of HPW/CeO2-700 for NOx adsorption was much less than that of HPW/CeO2-500. 3.2 SCR reaction HPW-500 showed a poor SCR activity and NOx conversion was lower than 20% at 150-450 oC (Figure 9a). Meanwhile, CeO2 also showed a poor SCR activity and NOx conversion was lower than 55% at 150-450 oC, which was consistent with the results of other studies.

8, 9

However,

HPW/CeO2-500 showed an excellent SCR activity and NOx conversion was higher than 90% at 200-450 oC. Meanwhile, little N2O formed during NO reduction over HPW/CeO2-500, resulting in an excellent N2 selectivity (Figure 9b). However, HPW/CeO2-700 only showed a moderate SCR activity, which was much less than HPW/CeO2-500. Figures S3 and S4 in the Supporting Information show that the SCR activity of HPW/CeO2-500 was much better than those of commercial V2O5-WO3/TiO2,

35

other Ce based catalysts (for example Ce/TiO2,

13

CeO2-WO3/TiO2 5 and CeWOx 34) and the composite catalyst of CeO2 and HPW. 33 H2O and SO2 are inevitable in the flue gas, which often exhibit a remarkable inhibition on the SCR reaction.13 Therefore, the effect of H2O and SO2 on the SCR reaction over HPW/CeO2-500 was investigated (shown in Figure 9c). Although H2O and SO2 showed a notable inhibition on NO reduction over HPW/CeO2-500, HPW/CeO2-500 still exhibited an excellent SCR activity (NOx conversion >90%) at 300-450 oC in the presence of H2O and SO2 with a high GHSV of 60000 cm3 g-1 h-1 and NOx conversion did not decrease remarkably in the 12 h test. It indicates that HPW/CeO2-500 had an excellent resistance to H2O and SO2 for the NH3-SCR reaction above 300 o

C. Furthermore, Figure S3c in the Supporting Information shows that the temperature window of

HPW/CeO2-500 for the SCR reaction in the presence of H2O and SO2 was much broader than those of CeWOx, 34 Ce/TiO2 13 and CeO2-WO3/TiO2. 5 3.3 Transient reaction study 3.3.1 HPW-500 After the adsorption of NH3 at 200 oC, HPW-500 was mainly covered by coordinated NH3 bound to the Lewis acid sites (1198 and 1582 cm-1) and ionic NH4+ bound to the Brønsted acid sites (1434, 1393 and 1671 cm-1). After the further introduction of NO+O2 for 20 min, adsorbed 12


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NH3 species still existed and their intensities did not decrease (Figure 10a). It suggests that the reaction of adsorbed NH3 species with gaseous NO (i.e. the Eley-Rideal mechanism) cannot contribute to NO reduction over HPW-500. After the adsorption of NO+O2 at 200 oC, adsorbed NOx species cannot be observed over HPW-500 (Figure 10b). It suggests that NO cannot adsorb on HPW-500. 48 Therefore, the reaction of adsorbed NOx species with adsorbed NH3 species (i.e. the Langmuir-Hinshelwood mechanism) cannot contribute to NO reduction over HPW-500. 3.3.2 CeO2 After the adsorption of NH3 at 200 oC, adsorbed NH3 species can hardly be observed on CeO2 (Figure 10c). Therefore, the reaction of adsorbed NH3 species with gaseous NO (i.e. the Eley-Rideal mechanism) may not contribute to NO reduction over CeO2. After the adsorption of NO+O2 at 200 oC, CeO2 was mainly covered by monodentate nitrate (at 1558 cm-1) and bidentate nitrate (at 1543, 1523, 1271 and 1161 cm-1).

52

After the further

introduction of NH3, the vibration at 1558 cm-1 corresponding to monodentate nitrate rapidly diminished. It suggests that the reaction between monodentate nitrate and adsorbed NH3 species (i.e. the Langmuir-Hinshelwood mechanism) can contribute to NO reduction over CeO2. However, the vibrations at 1543, 1523, 1271 and 1161 cm-1 corresponding to bidentate nitrate still existed and their intensities did not decrease after the introduction of NO+O2 for 20 min (Figure 10d). It suggests that the reaction between bidentate nitrate and adsorbed NH3 species cannot contribute to NO reduction over CeO2. 3.3.3 HPW/CeO2-500 After the adsorption of NH3 at 200 oC, coordinated NH3 (at 1198 and 1582 cm-1) and ionic NH4+ (1419 and 1667 cm-1) both appeared over HPW/CeO2-500. After the further introduction of NO+O2, both coordinated NH3 and ionic NH4+ gradually diminished before the appearance of adsorbed NOx species (Figure 10e). It suggests that the reaction of adsorbed NH3 species with gaseous NO (i.e., the Eley-Rideal mechanism) can contribute to NO reduction over HPW/CeO2-500. At last, HPW/CeO2-500 was mainly covered by monodentate nitrite (at 1606 and 1578 cm-1). 13



Page 14 of 38

After the adsorption of NO+O2 at 200 oC, HPW/CeO2-500 was mainly covered by monodentate nitrite (at 1606 and 1578 cm-1). Monodentate nitrite on HPW/CeO2-500 rapidly diminished after the further introduction of NH3 (Figure 10f). It suggests that the reaction of adsorbed monodentate nitrite with adsorbed NH3 species (i.e., the Langmuir-Hinshelwood mechanism) can contribute to NO reduction over HPW/CeO2-500. At last, HPW/CeO2-500 was mainly covered by coordinated NH3 (at 1198 and 1582 cm-1) and ionic NH4+ (at 1419 and 1667 cm-1).

4. Discussion 4.1 Reaction mechanism In situ DRIFT spectra demonstrate that both the Langmuir-Hinshelwood mechanism and the Eley-Rideal mechanism can contribute to NO reduction over HPW/CeO2-500 and HPW/CeO2-700, which was similar to NO reduction over Ce/TiO2, CeO2-WO3/TiO2 and CeWOx.5,

13, 34

NO

reduction through the Eley-Rideal mechanism can be described as follows: 8, 53 N H 3 (g ) → N H 3 (a d )

(1)

NH3(ad) + Ce4+ =O → NH2 +Ce3+ -OH

(2)

→ N 2 +H 2O

(3)

N O (g ) + N H

2

1 1 Ce 3+ -OH+ O 2 → Ce 4+ =O+ H 2 O 4 2

(4)

The SCR reaction generally started with the adsorption of gaseous NH3 on the surface (i.e., Reaction 1).

36

Then, adsorbed NH3 species (including coordinated NH3 and ionic NH4+) were

activated by Ce4+ on the surface to NH2 (i.e., Reaction 2). At last, gaseous NO was reduced by NH2 on the surface to N2 (i.e., Reaction 3). Reaction 4 was the regeneration of Ce4+ on the surface. Meanwhile, NO reduction through the Langmuir-Hinshelwood mechanism can be approximately described as follows: 8, 53 (1)

N H 3 (g ) → N H 3 (a d )

NO

(g )

(5)

→ N O (a d )

NO(ad) + Ce4+ =O → Ce3+ -NOx

(6)

14


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Ce3+ -NOx + NH3(ad) → Ce3+ -NOx NH3 → Ce3+ -OH+N2 +H2O

(7)

Reaction 5 was the physical adsorption of gaseous NO on the surface. Physically adsorbed NO was then activated by Ce4+ on the surface to adsorbed NOx (i.e., nitrite or nitrate). At last, adsorbed NOx reacted with adsorbed NH3 species to NH4NOx (i.e., Reaction 7), which then decomposed to N2. According to Reaction 3, the rate of the SCR reaction through the Eley-Rideal mechanism (i.e., δE-R) can be described as: 13, 54

δ E-R = -

d[NO(g) ]

dt

= k1[NH2 ][NO(g) ]

(8)

Where, k1, [NH2] and [NO(g)] were the kinetic constant of Reaction 3, NH2 concentration on the surface and gaseous NO concentration, respectively. The rate of NH2 formation (i.e. Reaction 2) can be described as: 13, 54

d[NH 2 ] = k 2 [ NH 3( ad ) ][Ce 4+ =O ] dt

(9)

Where, k2, [NH3(ad)] and [Ce4+=O] were the kinetic constant of Reaction 2, and the concentrations of NH3 adsorbed and Ce4+ on the surface, respectively. According to Reaction 7, the rate of the SCR reaction through the Langmuir-Hinshelwood mechanism (i.e., δE-R) can be described as: (10)

δ L -H = k 3 [ C e 3 + -N O x N H 3 ]

Where, k3 and [Ce3+-NOxNH3] were the rate constant of NH4NOx decomposition and the concentration of NH3NOx on the surface, respectively. The rate of NH4NOx formation (i.e. Reaction 7) can be described as: d[Ce 3+ -N O x N H 3 ] = k 4 [Ce 3+ -N O x ][ NH 3( ad ) ] dt

(11)

Where, k4 and [Ce3+-NOx] were the rate constant of Reaction 7 and the concentration of adsorbed NOx on the surface, respectively. 4.2 Synergistic effect of HPW and CeO2

15



Equations 8 and 9 suggest that δE-R was mainly dependent on the concentration of NH3 adsorbed, the concentration of Ce4+ on the surface and k2 (i.e. the oxidation ability of the catalyst). Equations 10 and 11 suggest that δL-H was mainly dependent on the concentration of NH3 adsorbed and the concentration of NOx adsorbed. Although CeO2 had an excellent oxidation ability (shown in Figure 6) and there were many Ce4+ cations on CeO2, the concentration of NH3 adsorbed on CeO2 was very low (shown in Figure 8a). Although the concentration of NH3 adsorbed on HPW-500 was very high (shown in Figure 8a), the oxidation ability of HPW-500 was very poor (hinted by the TPR analysis in Figure 6). Hinted by Equations 8 and 9, δE-R of both HPW-500 and CeO2 were very low. Although the concentration of NH3 adsorbed on HPW-500 was very high (shown in Figure 8a), NOx can hardly adsorb on HPW-500 (shown in Figure 8b).

48

Although there was some NOx adsorbed on CeO2

(shown in Figure 8b), the concentration of NH3 species adsorbed on CeO2 was very low (shown in Figure 8a). Hinted by Equations 10 and 11, δL-H of both HPW-500 and CeO2 were very low. As δE-R and δL-H of both HPW-500 and CeO2 were very low, the SCR reaction rates of HPW-500 and CeO2 were both very low, resulting in their poor SCR activities. HPW/CeO2-500 showed an excellent oxidation ability, which was close to that of CeO2 (Figure 6). Meanwhile, HPW/CeO2-500 had a superior capacity for NH3 adsorption (Figure 7a). Furthermore, the concentration of Ce4+ on HPW/CeO2-500 was very high (22.9%). Hinted by Equations 8 and 9, δE-R of HPW/CeO2-500 was very high. Although the adsorption of NOx over CeO2 was restrained after the grafting of HPW (Figure 8b), all adsorbed NOx on HPW/CeO2-500 can take part in the SCR reaction. Meanwhile, the concentration of NH3 adsorbed on HPW/CeO2-500 was very high. Hinted by Equations 10-11, δL-H of HPW/CeO2-500 was very high. As both δE-R and δL-H of HPW/CeO2-500 were very high, the SCR reaction rate of HPW/CeO2-500 was very high, resulting in a superior SCR performance. Both the oxidation ability and the concentration of NH3 adsorbed of/on HPW/CeO2-700 were much less than those of/on HPW/CeO2-500 (hinted by Figure 6 and Table 2). Meanwhile, the concentration of Ce4+ on HPW/CeO2-700 was slightly less than that on HPW/CeO2-500 (Table 1). Hinted by Equations 8 and 9, δE-R of HPW/CeO2-700 was much less than that of HPW/CeO2-500. 16


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Figure 8b suggests that the concentration of NOx adsorbed on HPW/CeO2-700 was much less that on HPW/CeO2-500. Meanwhile, the concentration of NH3 adsorbed on HPW/CeO2-700 was much less that on HPW/CeO2-500 (Table 2 and Figure 8a). Hinted by Equations 10 and 11, δL-H of HPW/CeO2-700 was much less than that of HPW/CeO2-700. As both δL-H and δE-R of HPW/CeO2-700 were much less than those of HPW/CeO2-500, the SCR activity of HPW/CeO2-500 was much better than that of HPW/CeO2-700 (shown in Figure 9a).

5. Conclusion HPW/CeO2-500, which was prepared by the adsorption of HPW on CeO2 in a HPW solution, was developed as a high performance SCR catalyst. HPW on HPW/CeO2-500 still retained the Keggin structure, so HPW/CeO2-500 showed an excellent ability for NH3 adsorption. Both HPW and CeO2 on/in HPW/CeO2-500 played their functions to the greatest limit for the SCR reaction. CeO2 in HPW/CeO2-500 played the role in the activation of adsorbed NH3 and NO, and the grafted HPW on HPW/CeO2-500 acted as the active sites for NH3 adsorption. Therefore, HPW/CeO2-500 showed a superior SCR activity at 200-450 oC and an excellent resistance to H2O and SO2 for the NH3-SCR reaction above 300 oC.

17



Acknowledgements: This study was financially supported by the National Natural Science Fund of China (Grant Nos. 21777070 and 41372044) and the Natural Science Fund of Jiangsu Province (Grant No. BK20150036).

Supporting Information The Supporting Information is available free of charge on the ACS Publications Website (DIO: XXXX), and includes the chemical structure of HPW, the comparison of the SCR performance of HPW/CeO2-500 with V2O5-WO3/TiO2, CeWOx, Ce/TiO2, CeO2-WO3/TiO2 and the composite catalyst of HPW and CeO2, the subtraction of H2-TPR profile of CeO2 from that of HPW/CeO2-500, TEM images of CeO2 and HPW/CeO2-500, NH3 oxidation and NO oxidation over HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700, and in situ DRIFT spectra of the transient reaction over HPW/CeO2-700.

18


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References: (1) Qi, G. S.; Yang, R. T. Low-temperature selective catalytic reduction of NO with NH3 over iron and manganese oxides supported on titania. Appl. Catal. B-Environ 2003, 44, 217-225. (2) Boningari, T.; Smirniotis, P. G. Impact of nitrogen oxides on the environment and human health: Mn-based materials for the NOx abatement. Curr. Opin. Chem. Eng. 2016, 13, 133-141. (3) Li, J.; Chang, H.; Ma, L.; Hao, J.; Yang, R. T. Low-temperature selective catalytic reduction of NOx with NH3 over metal oxide and zeolite catalysts: A review. Catal. Today 2011, 175, 147-156. (4) Topsoe, N. Y. Mechanism of the selective catalytic reduction of nitric oxide by ammonia elucidated by in situ online Fourier transformation infrared spectroscopy Science 1994, 265, 1217-1219. (5) Chen, L. A.; Li, J. H.; Ge, M. F. DRIFT study on cerium-tungsten/titiania catalyst for selective catalytic reduction of NOx with NH3. Environ. Sci. Technol. 2010, 44, 9590-9596. (6) Mou, X. L.; Zhang, B. S.; Li, Y.; Yao, L. D.; Wei, X. J.; Su, D. S.; Shen, W. J. Rod-shaped Fe2O3 as an efficient catalyst for the selective reduction of nitrogen oxide by ammonia. Angew. Chem. Int. Edit. 2012, 51, 2989-2993. (7) Cen, W.; Liu, Y.; Wu, Z.; Wang, H.; Weng, X. A theoretic insight into the catalytic activity promotion of CeO2 surfaces by Mn doping. Phys. Chem. Chem. Phys. 2012, 14, 5769-5777. (8) Yang, S.; Guo, Y.; Chang, H.; Ma, L.; Peng, Y.; Qu, Z.; Yan, N.; Wang, C.; Li, J. Novel effect of SO2 on the SCR reaction over CeO2: Mechanism and significance. Appl. Catal. B-environ 2013, 136–137, 19-28. (9) Gu, T.; Liu, Y.; Weng, X.; Wang, H.; Wu, Z. The enhanced performance of ceria with surface sulfation for selective catalytic reduction of NO by NH3. Catal. Commun. 2010, 12, 310-313. (10) Zhang, L.; Li, L. L.; Cao, Y.; Yao, X. J.; Ge, C. Y.; Gao, F.; Deng, Y.; Tang, C. J.; Dong, L. Getting insight into the influence of SO2 on TiO2/CeO2 for the selective catalytic reduction of NO by NH3. Appl. Catal. B-environ 2015, 165, 589-598. (11) Xu, W. Q.; He, H.; Yu, Y. B. Deactivation of a Ce/TiO2 catalyst by SO2 in the selective catalytic reduction of NO by NH3. J. Phys. Chem. C 2009, 113, 4426-4432. (12) Shu, Y.; Sun, H.; Quan, X.; Chen, S. O. Enhancement of catalytic activity over the 19



iron-modified Ce/TiO2 catalyst for selective catalytic reduction of NOx with ammonia. J. Phys. Chem. C 2012, 116, 25319-25327. (13) Xiao, X.; Xiong, S. C.; Shi, Y. J.; Shan, W. P.; Yang, S. J. Effect of H2O and SO2 on the selective catalytic reduction of NO with NH3 over Ce/TiO2 catalyst: Mechanism and kinetic study. J. Phys. Chem. C 2016, 120, 1066-1076. (14) Gao, X.; Jiang, Y.; Fu, Y.; Zhong, Y.; Luo, Z.; Cen, K. Preparation and characterization of CeO2/TiO2 catalysts for selective catalytic reduction of NO with NH3. Catal. Commun. 2010, 11, 465-469. (15) Chaisuk, C.; Wehatoranawee, A.; Preampiyawat, S.; Netiphat, S.; Shotipruk, A.; Mekasuwandumrong, O. Preparation and characterization of CeO2/TiO2 nanoparticles by flame spray pyrolysis. Ceram. Int. 2011, 37, 1459-1463. (16) Chen, L. A.; Li, J. H.; Ge, M. F.; Zhu, R. H. Enhanced activity of tungsten modified CeO2/TiO2 for selective catalytic reduction of NOx with ammonia. Catal. Today 2010, 153, 77-83. (17) Boningari, T.; Somogyvari, A.; Smirniotis, P. G. Ce-based catalysts for the selective catalytic reduction of NOx in the presence of excess oxygen and simulated diesel engine exhaust conditions. Ind. Eng. Chem. Res. 2017, 56, 5483−5494. (18) Li, P.; Xin, Y.; Li, Q.; Wang, Z.; Zhang, Z.; Zheng, L. Ce-Ti amorphous oxides for selective catalytic reduction of NO with NH3: Confirmation of Ce-O-Ti active sites. Environ. Sci. Technol. 2012, 46, 9600-9605. (19) Gao, X.; Jiang, Y.; Zhong, Y.; Luo, Z.; Cen, K. The activity and characterization of CeO2-TiO2 catalysts prepared by the sol–gel method for selective catalytic reduction of NO with NH3. J. Hazard. Mater. 2010, 174, 734-739. (20) Shan, W.; Liu, F.; He, H.; Shi, X.; Zhang, C. The remarkable improvement of a Ce-Ti based catalyst for NOx abatement prepared by a homogeneous precipitation method. Chemcatchem 2011, 3, 1286-1289. (21) Shan, W.; Liu, F.; Yu, Y.; He, H.; Deng, C. L.; Zi, X. Y. High-efficiency reduction of NOx emission from diesel exhaust using a CeWOx catalyst. Catal. Commun. 2015, 59, 226-228. (22) Shan, W. P.; Liu, F. D.; He, H.; Shi, X. Y.; Zhang, C. B. Novel cerium-tungsten mixed oxide 20


Page 20 of 38

Page 21 of 38 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


catalyst for the selective catalytic reduction of NOx with NH3. Chem. Commun. 2011, 47, 8046-8048. (23) Peng, Y.; Li, J. H.; Chen, L.; Chen, J. H.; Han, J.; Zhang, H.; Han, W. Alkali metal poisoning of a CeO2-WO3 catalyst used in the selective catalytic reduction of NOx with NH3: An experimental and theoretical study. Environ. Sci. Technol. 2012, 46, 2864-2869. (24) Shan, W.; Liu, F.; Yu, Y.; He, H. The use of ceria for the selective catalytic reduction of NOx with NH3. Chinese. J. Catal. 2014, 35, 1251-1259. (25) Shan, W. P.; Liu, F. D.; He, H.; Shi, X. Y.; Zhang, C. B. A superior Ce-W-Ti mixed oxide catalyst for the selective catalytic reduction of NOx with NH3. Appl. Catal. B-environ 2012, 115, 100-106. (26) Geng, Y.; Shan, W. P.; Xiong, S. C.; Liao, Y.; Yang, S. J.; Liu, F. D. Effect of CeO2 for a high-efficiency CeO2/WO3-TiO2 catalyst on N2O formation in NH3-SCR: a kinetic study. Catal. Sci. Technol. 2016, 6, 3149-3155. (27) Liu, Z. M.; Zhang, S. X.; Li, J. H.; Ma, L. L. Promoting effect of MoO3 on the NOx reduction by NH3 over CeO2/TiO2 catalyst studied with in situ DRIFTS. Appl. Catal. B-environ 2014, 144, 90-95. (28) Liu, Z. M.; Zhang, S. X.; Li, J. H.; Zhu, J. Z.; Ma, L. L. Novel V2O5-CeO2/TiO2 catalyst with low vanadium loading for the selective catalytic reduction of NOx by NH3. Appl. Catal. B-environ 2014, 158, 11-19. (29) Liu, Z. M.; Zhu, J. Z.; Zhang, S. X.; Ma, L. L.; Woo, S. I. Selective catalytic reduction of NOx by NH3 over MoO3-promoted CeO2/TiO2 catalyst. Catal. Commun. 2014, 46, 90-93. (30) Rao, P. M.; Wolfson, A.; Kababya, S.; Vega, S.; Landau, M. Immobilization of molecular H3 PW12O40 heteropolyacid catalyst in alumina-grafted silica-gel and mesostructured SBA-15 silica matrices. J. Catal. 2005, 232, 210-225. (31) Essayem, N.; Holmqvist, A.; Gayraud, P.; Vedrine, J.; Taarit, Y. B. In situ FTIR studies of the protonic sites of H3PW12O40 and its acidic cesium salts MxH3-xPW12O40. J. Catal. 2001, 197, 273-280. (32) Putluru, S. S. R.; Jensen, A. D.; Riisager, A.; Fehrmann, R. Heteropoly acid promoted 21



V2O5/TiO2 catalysts for NO abatement with ammonia in alkali containing flue gases. Catal. Sci. Technol. 2011, 1, 631-637. (33) Weng, X.; Dai, X.; Zeng, Q.; Liu, Y.; Wu, Z. DRIFT studies on promotion mechanism of H3PW12O40 in selective catalytic reduction of NO with NH3. J. Colloid Interf. Sci. 2016, 461, 9-14. (34) Shan, W. P.; Geng, Y.; Chen, X. L.; Huang, N.; Liu, F. D.; Yang, S. J. A highly efficient CeWOx catalyst for the selective catalytic reduction of NOx with NH3. Catal. Sci. Technol. 2016, 6, 1195-1200. (35) Xiong, S. C.; Xiao, X.; Liao, Y.; Dang, H.; Shan, W. P.; Yang, S. J. Global kinetic study of NO reduction by NH3 over V2O5-WO3/TiO2: Relationship between the SCR performance and the key factors. Ind. Eng. Chem. Res. 2015, 54, 11011-11023. (36) Yang, S. J.; Li, J. H.; Wang, C. Z.; Chen, J. H.; Ma, L.; Chang, H. Z.; Chen, L.; Peng, Y.; Yan, N. Q. Fe-Ti spinel for the selective catalytic reduction of NO with NH3: Mechanism and structure-activity relationship. Appl. Catal. B-environ 2012, 117, 73-80. (37) Yang, S.; Wang, C.; Li, J.; Yan, N.; Ma, L.; Chang, H. Low temperature selective catalytic reduction of NO with NH3 over Mn-Fe spinel: Performance, mechanism and kinetic study. Appl. Catal. B-environ 2011, 110, 71-80. (38) Li, B.; Liu, Z.; Liu, J.; Zhou, Z.; Gao, X.; Pang, X.; Sheng, H. Preparation, characterization and application in deep catalytic ODS of the mesoporous silica pillared clay incorporated with phosphotungstic acid. J. Colloid Interf. Sci. 2011, 362, 450-456. (39) Lopez-Salinas, E.; Hernandez-Cortez, J. G.; Schifter, I.; Torres-Garcia, E.; Navarrete, J.; Gutierrez-Carrillo, A.; Lopez, T.; Lottici, P. P.; Bersani, D. Thermal stability of 12-tungstophosphoric acid supported on zirconia. Appl. Catal. A-gen 2000, 193, 215-225. (40) Zhu, K. K.; He, H. Y.; Xie, S. H.; Zhang, X.; Zhou, W. Z.; Jin, S. L.; Yue, B. Crystalline WO3 nanowires synthesized by templating method. Chem. Phys. Lett. 2003, 377, 317-321. (41) Rocchiccioli-Deltcheff, C.; Fournier, M.; Franck, R.; Thouvenot, R. Vibrational investigations of polyoxometalates. 2. Evidence for anion-anion interactions in molybdenum (VI) and tungsten (VI) compounds related to the Keggin structure. Inorg. Chem. 1983, 22, 207-216. 22


Page 22 of 38

Page 23 of 38 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


(42) Gao, R.; Zhu, Q.; Dai, W.-L.; Fan, K. A green process for the epoxidation of dicyclopentadiene with aqueous H2O2 over highly efficient and stable HPW-NH2-SBA-15. RSC Adv. 2012, 2, 6087-6093. (43) Boulova, M.; Lucazeau, G. Crystallite nanosize effect on the structural transitions of WO3 studied by Raman spectroscopy. J. Solid State Chem. 2002, 167, 425-434. (44) Lofberg, A.; Guerrero-Caballero, J.; Kane, T.; Rubbens, A.; Jalowiecki-Duhamel, L. Ni/CeO2 based catalysts as oxygen vectors for the chemical looping dry reforming of methane for syngas production. Appl. Catal. B-environ 2017, 212, 159-174. (45) Ladera, R. M.; Ojeda, M.; Fierro, J. L. G.; Rojas, S. TiO2-supported heteropoly acid catalysts for dehydration of methanol to dimethyl ether: relevance of dispersion and support interaction. Catal. Sci. Technol. 2015, 5, 484-491. (46) Jalil, P. A.; Faiz, M.; Tabet, N.; Hamdan, N.; Hussain, Z. A study of the stability of tungstophosphoric acid, H3PW12O40, using synchrotron XPS, XANES, hexane cracking, XRD, and IR spectroscopy. J. Catal. 2003, 217, 292-297. (47) Tocchetto, A.; Glisenti, A. Study of the interaction between simple molecules and WSn-based oxide catalysts. 1. The Case of WO3 powders. Langmuir 2000, 16, 6173-6182. (48) Herring, A. M.; McCormick, R. L. In situ infrared study of the absorption of nitric oxide by 12-tungstophosphoric acid. J. Phys. Chem. B 1998, 102, 3175-3184. (49) Du, X.; Dong, L.; Li, C.; Liang, Y.; Chen, Y. Diffuse reflectance infrared Fourier transform and Raman spectroscopic studies of MoO3 dispersed on CeO2 support. Langmuir 1999, 15, 1693-1697. (50) Wan, Q.; Duan, L.; He, K.; Li, J. Removal of gaseous elemental mercury over a CeO2-WO3/TiO2 nanocomposite in simulated coal-fired flue gas. Chem. Eng. J. 2011, 170, 512-517. (51) Yang, S. J.; Wang, C. Z.; Ma, L.; Peng, Y.; Qu, Z.; Yan, N. Q.; Chen, J. H.; Chang, H. Z.; Li, J. H. Substitution of WO3 in V2O5/WO3-TiO2 by Fe2O3 for selective catalytic reduction of NO with NH3. Catal. Sci. Technol. 2013, 3, 161-168. (52) Hadjiivanov, K. I. Identification of neutral and charged NxOy surface species by IR 23



spectroscopy. Catal. Rev.-Sci. Eng. 2000, 42, 71-144. (53) Busca, G.; Lietti, L.; Ramis, G.; Berti, F. Chemical and mechanistic aspects of the selective catalytic reduction of NOx by ammonia over oxide catalysts: A review. Appl. Catal. B-environ 1998, 18, 1-36. (54) Yang, S.; Qi, F.; Xiong, S.; Dang, H.; Liao, Y.; Wong, P. K.; Li, J. MnOx supported on Fe-Ti spinel: A novel Mn based low temperature SCR catalyst with a high N2 selectivity. Appl. Catal. B-environ 2016, 181, 570-580.

24


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Table 1 Percentages of Ce and W species on HPW/CeO2-500/700 and denuded HPW/CeO2-500/700 /% Ce

W6+

Ce3+

Ce4+

HPW/CeO2-500

6.7

22.9

3.6

denuded HPW/CeO2-500

15.5

18.6

1.5

HPW/CeO2-700

4.3

20.8

6.7


10.9

20.3

3.0

25



Page 26 of 38

Table 2 Capacities of HPW-500, HPW-700, CeO2, HPW/CeO2-500 and HPW/CeO2-700 for NH3 and NOx adsorption at 50 oC

/µmol g-1 NH3

NOx

HPW-500

702

23

HPW-700

54

8

CeO2

79

41

HPW/CeO2-500

232

39

HPW/CeO2-700

128

11

26


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Figure captions Figure 1 XRD patterns of HPW, HPW-500, HPW-700, CeO2, HPW/CeO2 before the calcination, HPW/CeO2-500, HPW/CeO2-700 and WO3. Figure 2 TG-DTA curves under air atmosphere of: (a), HPW; (b), CeO2; (c), HPW/CeO2 before the calcination. Figure 3 Raman spectra of HPW, HPW-500, HPW-700, CeO2, HPW/CeO2 before the calcination, HPW/CeO2-500 and HPW/CeO2-700. Figure 4 XPS spectra of HPW/CeO2-500 in the spectral regions of (a), Ce 3d; (b), O 1s; (c), W 4f; XPS spectra of denuded HPW/CeO2-500 in the spectral regions of (d), Ce 3d; (e), O 1s; (f), W 4f; XPS spectra of HPW/CeO2-700 in the spectral regions of (g), Ce 3d; (h), O 1s; (i), W 4f; XPS spectra of denuded HPW/CeO2-700 in the spectral regions of (j), Ce 3d; (k), O 1s; (l), W 4f. Figure 5 DRIFT spectra of HPW, HPW-500, HPW-700, CeO2, HPW/CeO2 before the calcination, HPW/CeO2-500 and HPW/CeO2-700. Figure 6 H2-TPR profiles of HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700. Figure 7 TPD profiles of HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700: (a), NH3; (b), NOx. Figure 8 (a), DRIFT spectra of the adsorption of NH3 over HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700 at 200 oC; (b), DRIFT spectra of the adsorption of NO+O2 over HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700 at 200 oC. Figure 9 SCR performance of HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700: (a), NOx conversion; (b), N2 selectivity. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=5%, catalyst mass=100 mg, total flow rate=200 mL min-1 and GHSV=120000 cm3 g-1 h-1. (c), Effect of H2O and SO2 on NO reduction over HPW/CeO2-500. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=5%, [H2O]=8%, [SO2]=100 ppm, catalyst mass=100 mg, total flow rate=100 mL min-1 and GHSV=60000 cm3 g-1 h-1. Figure 10 (a), DRIFT spectra taken at 200 oC upon passing NO+O2 over NH3 presorbed HPW/CeO2-500; (b), DRIFT spectra taken at 200 oC upon passing NH3 over NO+O2 presorbed HPW/CeO2-500. 27



(110)

(222) (332) (220) (400) (431)

(543)(651)

HPW

HPW-500 HPW-700 (111)

(220) (311)

CeO2

HPW/CeO2 before the calcination HPW/CeO2-500 HPW/CeO2-700 (110) (001)

10

20

WO3(JCPDS: 41-0369)

(111) (200)

30

40

50

60

70

80

2θ/degree

Figure 1 XRD patterns of HPW, HPW-500, HPW-700, CeO2, HPW/CeO2 before the calcination, HPW/CeO2-500, HPW/CeO2-700 and WO3.

28


Page 28 of 38

HPW

96

597

201

79

0 -1

88

-3

84

-4

0

101

100 200 300 400 500 600 700 800

100 -1 98

-2

96

-3

94

-4 100 200 300 400 500 600 700 800

o

o

Temperature/ C

Temperature/ C

a

102

b

HPW/CeO2 before the calcination

1 0

100

688

-1 98 -2 96

-3

Heat flow/mW mg

101

Weight/%

1

-1

-2

CeO2

-1

92

102

Heat flow/mW mg

Weight/%

100

Heat flow/mW mg -1

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


Weight/%

Page 29 of 38

-4

94 100 200 300 400 500 600 700 800 o

Temperature/ C

c Figure 2 TG-DTA curves under air atmosphere of: (a), HPW; (b), CeO2; (c), HPW/CeO2 before the calcination.

29



1006 216 232

989 HPW HPW-500

801

698

HPW-700

462

CeO2 HPW/CeO2 before the calcination

HPW/CeO2-500 HPW/CeO2-700

200

400

600

800

1000

-1

Raman shifit/cm

Figure 3 Raman spectra of HPW, HPW-500, HPW-700, CeO2, HPW/CeO2 before the calcination, HPW/CeO2-500 and HPW/CeO2-700.

30


Page 30 of 38

Page 31 of 38 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


HPW/CeO2-500

Ce 3d

917.2 907.5 903.6

910

901.2 898.7

900

O 1s

532.6

529.9

885.5 882.5

890

880

536

534

37.7

532

530

528

40

Binding Energy/eV

a

38

Ce 3d

36

34

Binding Energy/eV

b

denuded HPW/CeO2-500 903.7

W 4f

531.5

Binding Energy/eV

917.2 907.5

HPW/CeO2-500 35.6

889.2

920

HPW/CeO2-500

c


O 1s

W 4f


530.0

885.5 901.2

531.5

889.1

882.4

898.8

532.7 35.8 37.9

920

910

900

890

880

536

534

Binding Energy/eV

532

d

528

40

38

Ce 3d

901.1 898.5 907.5 903.5

34

f

HPW/CeO2-700

O 1s

HPW/CeO2-700

W 4f 35.7

529.8 885.3 889.2

36

Binding Energy/eV

e

HPW/CeO2-700 917.0

530

Binding Energy/eV

37.8

882.5 530.5 531.5

920

910

900

890

880

536

Binding Energy/eV

534

532

901.0 903.8 916.9

Ce 3d

38

36

34

i


O 1s


W 4f 35.5

898.8 885.6

529.6

37.6

882.2

907.5

910

40

h

530.6

889.1

920

528

Binding Energy/eV

g denuded HPW/CeO2-700

530

Binding Energy/eV

900

890

531.5

880

536

534

532

530

528

40

38

36

Binding Energy/eV

Binding Energy/eV

Binding Energy/eV

j

k

l

34

Figure 4 XPS spectra of HPW/CeO2-500 in the spectral regions of (a), Ce 3d; (b), O 1s; (c), W 4f; XPS spectra of denuded HPW/CeO2-500 in the spectral regions of (d), Ce 3d; (e), O 1s; (f), W 4f; XPS spectra of HPW/CeO2-700 in the spectral regions of (g), Ce 3d; (h), O 1s; (i), W 4f; XPS spectra of denuded HPW/CeO2-700 in the spectral regions of (j), Ce 3d; (k), O 1s; (l), W 4f. 31


866

936

1101

HPW

1054 1032

921

HPW-500 HPW-700

965

856

CeO2 1019

1101

HPW/CeO2 before the calcination

HPW/CeO2-500 HPW/CeO2-700

978

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

1019


1200

1100

1000

900 -1

800

Wavenumber/cm

Figure 5 DRIFT spectra of HPW, HPW-500, HPW-700, CeO2, HPW/CeO2 before the calcination, HPW/CeO2-500 and HPW/CeO2-700.

32


Page 32 of 38

Page 33 of 38 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


828

0.1

777

HPW-500×0.1

523 706

523

777

HPW/CeO2-500

565

826 639

200

400

CeO2

600

800

HPW/CeO2-700

1000

o

Temperature/ C

Figure 6 H2-TPR profiles of HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700.

33



HPW-700

CeO2

100

HPW/CeO2-500

NOx concentration/ppm

200

HPW/CeO2-700 NH3 concentration/ppm

NH3 concentration/ppm

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

150 100

HPW-500

1000 800 600 400 200 0 100

200

300

400

500

600

o

Temperature/ C

50

HPW-500

HPW-700

HPW/CeO2-500

80

Page 34 of 38

CeO2

HPW/CeO2-700

60 40 20 0

0 100

200

300

400

o

500

600

100

200

300

400

o

500

600

Temperature/ C

Temperature/ C

a b Figure 7 TPD profiles of HPW-500, HPW-700, CeO2, HPW/CeO2-500 and HPW/CeO2-700: (a), NH3; (b), NOx.

34


0.1

CeO2

HPW/CeO2-700

1606 1578

1198

1419

1582

HPW/CeO2-700

-1

CeO2 HPW/CeO2-500

HPW/CeO2-500

1800 1700 1600 1500 1400 1300 1200 1100

1161

1271

1558 1523

HPW-500

HPW-500×0.5

1543

1434 1393

1198

1582

0.2

1667

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


1667

Page 35 of 38

1800 1700 1600 1500 1400 1300 1200 1100 -1

Wavenumber/cm

Wavenumber/cm

a

b

Figure 8 (a), DRIFT spectra of the adsorption of NH3 over HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700 at 200 oC; (b), DRIFT spectra of the adsorption of NO+O2 over HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700 at 200 oC.

35



HPW-500

100

N2 selectivity/%

NOx conversion/%

HPW/CeO2-500

CeO2

HPW/CeO2-700

100 80 60 40 20

80 60 40 20

HPW-500

0 150

200

250

300

350

400

150

450

CeO2

HPW/CeO2-500

HPW/CeO2-700

0

200

250

300

350 o

400

450

Temperature/ C

o

Temperature/ C

a

b

SCR SCR+ H2O+ SO2

80 100

60

NOx conversion/%

NOx conversion/%

100

40 20

100

80 60

80 H2O and SO2 on

40

40

20

20 o

0

NOx

300 C 0

0

H2O and SO2 off 60

2

4

N2

6

N2 selectivity/%

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

Page 36 of 38

0 8

10

12

Time/h

150

200

250

300

350

400

450

o

Temperature/ C

c Figure 9 SCR performance of HPW-500, CeO2, HPW/CeO2-500 and HPW/CeO2-700: (a), NOx conversion; (b), N2 selectivity. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=5%, catalyst mass=100 mg, total flow rate=200 mL min-1 and GHSV=120000 cm3 g-1 h-1. (c), Effect of H2O and SO2 on NO reduction over HPW/CeO2-500. Reaction conditions: [NH3]=[NO]=500 ppm, [O2]=5%, [H2O]=8%, [SO2]=100 ppm, catalyst mass=100 mg, total flow rate=100 mL min-1 and GHSV=60000 cm3 g-1 h-1.

36


Page 37 of 38 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


a

b

c

d

e

f

Figure 10 (a), DRIFT spectra taken at 200 oC upon passing NO+O2 over NH3 presorbed HPW-500; (b), DRIFT spectra taken at 200 oC upon passing NH3 over NO+O2 presorbed HPW-500; (c), DRIFT spectra taken at 200 oC upon passing NO+O2 over NH3 presorbed CeO2; (d), DRIFT spectra taken at 200 oC upon passing NH3 over NO+O2 presorbed CeO2; (e), DRIFT spectra taken at 200 oC upon passing NO+O2 over NH3 presorbed HPW/CeO2-500; (f), DRIFT spectra taken at 200 oC upon passing NH3 over NO+O2 presorbed HPW/CeO2-500.

37



For Table of Contents Only

38


Page 38 of 38

H3PW12O40 Grafted on CeO2: A High-Performance Catalyst for the

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