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Title: Rational Design of Alkali-Resistant NO Reduction CatalystsUsing a Stable Hexagonal V-Doped MoO3 Support for AlkaliTrapping

Authors: Xiaona Liu, Jiayi Gao, Yaxin Chen, Chao Li, Junxiao Chen,Weiye Qu, Xin Chen, Zhen Ma, and Xingfu Tang

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To be cited as: ChemCatChem 10.1002/cctc.201800818

Link to VoR: http://dx.doi.org/10.1002/cctc.201800818

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Rational Design of Alkali-Resistant NO Reduction Catalysts Using

a Stable Hexagonal V-Doped MoO3 Support for Alkali Trapping

Xiaona Liu,[a] Jiayi Gao,[a] Yaxin Chen,[a] Chao Li,[a] Junxiao Chen,[a] Weiye Qu,[a] Xin Chen,[a] Zhen

Ma,[a,b] and Xingfu Tang*[a,b]

Abstract: To avoid alkali poisoning in the selective catalytic

reduction (SCR) of NOx with NH3, it is desirable to design SCR

catalysts with abundant alkali-trapping sites spatially separated from

catalytically active sites. Here we develop such a catalyst with strong

resistance against alkali poisoning by using hexagonal vanadium-

doped MoO3 (V-HMoO) with alkali-trapping tunnels as a catalyst

support. Although pure HMoO also possesses strong alkali

resistance due to size-suitable tunnels, active Mo5+ ions on the

framework make HMoO thermally unstable, and hence pure HMoO

is not able to be used as a suitable support for SCR in a normal

operating temperature window of 280420 oC. We substitute the

active Mo5+ by stable V5+ due to its fully unoccupied electron

configuration to achieve a thermal stable V-HMoO support, as

confirmed by various characterization results. After loading active

V2O5 onto the V-HMoO support, the catalyst shows high SCR activity

with excellent alkali resistance due to the alkali trapping function of

V-HMoO. This work may give a strategy to design stable alkali-

trapping SCR catalysts in general.

Introduction

Selective catalytic reduction (SCR) of NO with NH3 over supported V2O5 catalysts is efficient for controlling NO emissions from coal-fired power plants,[1, 2] but it is different to be applied in industrial glass and cement boilers, steel furnaces, and bio-fueled boilers, because high-concentration alkali metal ions in the stack gases often lead to severe deactivation of such SCR catalysts.[3] Thus, it is desirable to develop SCR catalysts with strong resistance against alkali poisoning for controlling NO emissions especially in China, because some related regulations have been increasingly stringent.

Several ways have been reported for solving the problem of alkali poisoning: a) supports with stronger acidity can be used to interact preferentially with alkali metal ions to protect active species from alkali poisoning;[4-6] b) “basic” transition metal oxides with less Brønsted acid sites can be used as active phases since they are not easily subject to alkali poisoning;[7, 8] c) electrophoresis treatment or washing the alkali-poisoned catalysts with water or diluted H2SO4 can regenerate the alkali-

poisoned catalysts.[2, 9, 10] In addition, we recently established an ion exchange-coordinate model as a guide for developing alkali-resistant SCR catalysts.[11-13] For instance, hexagonal WO3 with strong acidity and abundant tunnels as the alkali-trapping sites (ATSs) has been used as a support for loading V2O5.[14, 15] Another work reported by Wu et al. demonstrated that titanate nanotube (TNT) with the tubular channel also exhibits superior alkali-resistance under the SCR reactions.[16] The advantage of this method is that the captured alkali metals coordinate steadily with oxygen atoms in the tunnels, allowing for the spatially separation of ATSs and the catalytically active sites (CASs). Similarly, hexagonal MoO3 (HMoO) is supposed to be an excellent support for trapping alkalis because of its tunnel structure and stronger acidity.[17] However, its tunnel structure collapses above 350 oC,[18] thereby disabling its capability of trapping alkalis in a normal SCR temperature window of 280420 oC.[19] Hence, it is necessary to enhance the thermal stability of HMoO as an alkali-trapping support.

According to the literature, the transformation of hexagonal VxMo1-xO3 to orthorhombic VxMo1-xO3 occurs at 460 oC in helium.[20-22] Thus, we doped HMoO by V5+ to obtain a more stable V-HMoO support. The hexagonal tunnels of V-HMoO support can trap alkalis and protect the active species from alkali-poisoning. The thermal stability of HMoO can be enhanced, as evidenced by SXRD patterns and TG curves. The XPS data and EXAFS spectra further account for this result from the perspective of electronic state and atomic coordination. The V-HMoO shows excellent alkali-resistance after loading V2O5 in the SCR reaction. This work may give a strategy to design efficient SCR catalysts operating in the alkali-rich stack gases emitted from some industrial boilers.

Results and Discussion

The poor thermal stability of HMoO originates from active framework Mo5+

with a (t2g)1(eg)0 electronic configuration.[23] HMoO normally has a formula of {H+}[Mo5+Mo6+

5]O18,[24] where {H+} stands for the tunnel H+ and [Mo5+Mo6+

5] stands for the framework Mo5+ and Mo6+. Due to the presence of active framework Mo5+, HMoO can readily convert into more stable -MoO3 in preparation or catalytic processes,[18] concomitant with the change from an active Mo5+

(t2g)1(eg)0 electronic configuration to a stable (t2g)0(eg)0 electronic configuration of the outmost Mo6+ 4d0 orbit. One efficient strategy to avoid that problem is to use stable metal ions with chemical valence of +5 to substitute unstable Mo5+ on the HMoO framework. As evidenced by Beyribey et al.,[25] V5+

can stabilize HMoO due to its stable outmost electronic configuration. Besides, HMoO has strong acidity and strong SO2 tolerance.[26, 27] Herein, we doped HMoO by V5+ to achieve a stable V-HMoO support (Scheme 1), which was then used to make an alkali-resistant SCR catalyst by loading V2O5 onto V-HMoO.

[a] Dr X. Liu, J. Gao, Dr Y. Chen, C. Li, J. Chen, W. Qu, X. Chen, Prof. Dr. Z. Ma, Prof. Dr. X. Tang Department of Environmental Science & Engineering Fudan University Shanghai 200438 (P.R. China) E-mail: tangxf@fudan.edu.cn

[b] Shanghai Institute of Pollution Control & Ecological Security Shanghai 200092 (P.R. China)

Supporting information for this article is given via a link at the end of the document.

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Scheme 1. A strategy for designing a stable alkali-resistant V-HMoO support. The red, blue, and purple balls represent O, Mo, and K atoms, respectively. The blue, yellow and green octahedra represent Mo6+O6, Mo5+O6 and V5+O6, respectively.

Figure 1. SXRD patterns (a) and TG-DTG curves (b) of V-HMoO (red) and HMoO (black). In panel (a): Short vertical lines below the SXRD patterns mark all possible Bragg positions of HMoO (pink) and -MoO3 (blue). Insets: Magnified SXRD patterns of the (100) peak. Peaks due to -MoO3 (). In panel (b): The light blue shade represents the normal SCR temperature window of 280420 oC.

Figure 1a shows the SXRD patterns of V-HMoO and HMoO calcined at 350 oC in air for 3 h. For HMoO, strong peaks are indexed to hexagonal MoO3 (JCPDS 47-0872).[28] Some additional weak peaks of orthorhombic -MoO3 (JCPDS 35-0609) are also discernible,[29] indicating the transformation of a small portion of HMoO to -MoO3 upon calcination at 350 oC. For comparison, V-HMoO calcined at 350 oC preserves the hexagonal MoO3 structure, and no -MoO3 or V2O5 phase can be detected. The (100) peak of HMoO shifts slightly to the lower Bragg angle after the V doping, suggesting that the cell

parameter “a” increases after introducing a small amount of V into hexagonal MoO3 (HMoO).[30]

The SXRD data presented above show that the doping of HMoO by V can improve the thermal stability of HMoO. More information can be obtained from TG-DSC data (Figures 1b and S1). Both HMoO and V-HMoO have a weight loss below 220 oC, due to desorption of water molecules physically adsorbed on the surface.[31] There is a significant weight loss at 350440 oC for HMoO (as seen more clearly by a sharp peak on the DTG and DSC curves), due to the removal of coordinated water molecules and hydrogen from the internal structure when HMoO converts to -MoO3.[18] In contrast, the corresponding weight loss occurs above 450 oC for V-HMoO. A slight weight loss (1%) for V-HMoO at 350450 oC should be associated with the V doping. Overall, the V doping greatly enhances the thermal stability of HMoO.

One of the important prerequisites for understanding the

reason why V doping can enhance the thermal stability of

HMoO is to determine the position of the doped V ions. For this

purpose, V-HMoO was characterized by using STEM combined

with EDX mapping (Figures 2a and 2b). As shown in Figure 2a,

V-HMoO has a rod-shaped morphology growing along the [001]

axis,[31] and the inserted model displays the (001) plane section

of hexagonal tunnel with a diameter of ~3.2 Å.[31] No

nanoparticles are observed on the smooth surface of the V-

HMoO rod, whereas V and Mo are well dispersed on the rod

(Figure 2b), indicating that the doped V ions are probably

located in the framework of HMoO, in accordance with the

SXRD results (the inset of Figure 1a). The V/Mo molar ratio is

determined to be ca. 1:12 from the EDX data in Figure S2.

EXAFS data of V-HMoO and a reference V2O5 were used to

further probe the local environment of the doped V ions by

analyzing the k3-weighted V K-edge (Figure 2c). The structural

parameters, obtained by fitting the spectra with theoretical

models, are listed in Table S1. The curve-fitting of R-space and

inverse Fourier transform (FT) spectra are given in Figures S3

and S4, respectively. The (R) k3-weighted EXAFS spectrum of

V-HMoO at the V K-edge is significantly different from that of

V2O5. The first shell of V2O5 can be attributed to the VO bonds

with an average bond length of ~1.83 Å and a coordination

number (CN) of 5 (Table S1), consistent with the literature

data.[32] Note that both the first and the second shells of V-

HMoO are assigned to VO bonds with average bond lengths

of ~1.81 Å (CN of 4) and ~2.35 Å (CN of 2), respectively.[33] The

average VV/O bond length of V2O5 is ~3.00 Å, different from

the average VMo/O bond length of ~3.50 Å (Table S1).[32] The

EXAFS results indicate that the doped V in V-HMoO has a local

environment totally different from that of V in V2O5, and the

doped V may exist in the V-HMoO framework as distorted VO6

octahedra because the CN is 6 for the nearest VO bonds.

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Figure 2. STEM image (a) and EDX mapping (b) of V-HMoO. (c) EXAFS spectra of V2O5 and V-HMoO at the V K-edge with k3 weighting. Inset: V-HMoO model with the tunnel diameter of ~3.2 Å.

To shed light on the role of the framework V for improving the thermal stability of HMoO, the electronic structures of V and Mo in V-HMoO were studied by using XPS (Figure 3 and Table S2). Deconvolution of the Mo 3d XPS of HMoO and V-HMoO (Figure 3a) demonstrates that the molar ratio of Mo5+ decreases by 9% after V doping. By combining the above STEM-EDX mapping and EXAFS results, it is convincing that some active framework Mo5+ ions have been substituted by V ions. Furthermore, the electronic states of the doped V ions were also determined by V 2p XPS of V-HMoO and V2O5 (Figure 3b). Obviously, the framework V species of V-HMoO are dominantly V5+, as judged by the same binding energies of the V 2p peaks of both V-HMoO and V2O5. Owing to the stable outmost (t2g)0(eg)0 electron configuration of V5+ and the unstable (t2g)1(eg)0 configuration of Mo5+, the substitution of active Mo5+ by stable V5+ in the HMoO framework accounts for the enhancement of the thermal stability of V-HMoO.

Figure 3. (a) Mo 3d XPS data of HMoO and V-HMoO. (b) V 2p XPS data of V2O5 and V-HMoO.

We further studied the function of V-HMoO for alkali trapping by loading 1.0 wt% K2O followed by calcination at 350 oC for 12 h. The resulting sample is denoted as KinV-HMoO. XANES and EXAFS spectra can provide useful information about the coordination geometry and local structures of K ions in the KinV-HMoO and K2SO4 (a referfence sample).[34, 35]. Figure 4a displays XANES spectra of KinV-HMoO and K2SO4. A significant difference for the pre-edge absorption peak is observed for both samples, and KinV-HMoO has a strong absorption at the pre-edge region while the absorption is absent for K2SO4 due to the 1s3d dipole-forbidden transition of centro-symmetric octahedral structure, implying that K of KinV-HMoO has a different coordination geometry with respect to that of K2SO4. In addition, no significant difference is observed from the Raman spectra (Figure S5), demonstrating that the structure of V-HMoO is not affected after trapping K ions.

(c)

0 1 2 3 4 50

2

4

6

(R

, k3

)(Å

-4)

R (Å)

V-HMoO V

2O

5

VO

VO

VO

VMo/O

VV/O

V-HMoO Mo 3d

9%

Binding Energy (eV)

Inte

nsity

(a.

u.)

238 236 234 232 230

18%

HMoO Mo6+

Mo5+

528 525 522 519 516

V2O

5

V-HMoO

V 2p

Inte

nsity

(a.

u.)

Binding Energy (eV)

10

1x

(a)

(b)

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Figure 4. XANES (a) and EXAFS (b) spectra of KinV-HMoO and K2SO4 at the K K-edge with k2 weighting. Insets: (a) Spectral fitting edges, the pink shade shows the 1s3d transition. (b) Models: the purple and red balls represent K and O atoms, respectively.

The local environment of K ions was determined by EXAFS. The corresponding structural parameters for fitting the spectra with theoretical models are listed in Table S3. The curve-fitting of R-space and inverse FT spectra are displayed in Figure S6. The first shell of KinV-HMoO can be ascribed to the K−O bonds with an average bond length of ~2.91 Å and CN of 6, consistent with the literature.[33] The K ions are located in the V-HMoO tunnels to form a KO6 motif with a D3d symmetry, as modeled in the inset of Figure 4b. As expected, the first shell of K2SO4 has an average bond length of ~2.52 Å and CN of 6 in KO6 octahedra with an Oh symmetry.[15, 34] The different geometric symmetries of K ions of KinV-HMoO and K2SO4 result in the different X-ray absorption characteristics in the XANES spectra (Figure 4a). Hence, the XANES and EXAFS data demonstrate that the K ions have been trapped in the V-HMoO tunnels. The SXRD patterns in Figure S7 can also show that the K ions have been trapped in the tunnels; there is no dispersed K2SO4 crystals.

Figure 5. XNO over the different catalysts with or without 1.0 wt% K2O loading. Reaction conditions: 500 ppm NO, 500 ppm NH3, 3.0 vol% O2, balanced N2, and GHSV of 66, 000 h−1.

Finally, we used the stable alkali-resistant V-HMoO support for developing SCR catalysts by loading active V2O5 on V-HMoO.[14, 15] SCR experiments were conducted to investigate the resistance against alkali poisoning. As shown in Figure 5, HMoO shows catalytic activity with 30% NO conversion (XNO) at 350 oC. In contrast, HMoO with 1.0 wt% K2O loading is totally inactive. The reason may be that K2O poisons the catalyst. In fact, HMoO has turned into -MoO3 (without a tunnel structure to trap K) during the calcination of HMoO and K2SO4 mixture at 350 oC for 12 h (Figure S8). V-HMoO is more active than HMoO, showing 42% XNO at 350 oC. The enhancement in catalytic activity implies that the framework V ions may provide some CASs. V-HMoO with 1.0 wt% K2O loading is slightly less active than V-HMoO, showing 32% XNO at 350 oC. Combined with the data in Figure S9, the slight decrease in catalytic activity should not be ascribed to a structural change of V-HMoO (Figure S8). Instead, that should be attributed to the affecting of framework V ions by K+ in the tunnel. The NH3-SCR activity and alkali resistance performance of the V-HMoO supports with different contents of doped V are provided in Figure S10. When V-HMoO (h-V0.1Mo0.9O3 in this case) is used as a support to load active V2O5, the catalyst V2O5/V-HMoO is very active for SCR, achieving 100% XNO at a lower temperature of 300 oC. The XNO is slightly lower (90% at 300 oC) when V2O5/V-HMoO is loaded with 1.0 wt% K2O, due to poisoning of the framework V ions by K+ in the tunnel. For comparison, a conventional V2O5/WO3-TiO2 catalyst without such a tunnel structure for trapping alkalis is dramatically deactivated by K2O (Figure S11).

Conclusions

In conclusion, the hexagonal tunnel structure of HMoO is stabilized by doping V5+. The resulting V2O5/V-HMoO catalyst is active for SCR. In addition, the catalyst can withstand alkali-poisoning because it contains HMoO tunnels for trapping alkalis. Such a SCR catalyst with both CASs and ATSs should be

200 250 300 350

0

20

40

60

80

100

V-HMoO K

inV-HMoO

HMoO K

inHMoO

V2O

5/V-HMoO

KinV

2O

5/V-HMoO

XN

O (

%)

T (oC)

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promising for treating NO emissions from some industrial stack gases containing alkalis.

Experimental Section

Materials.

(NH4)6Mo7O24·4H2O, HNO3, NH4VO3, and K2SO4 were purchased from Sinopharm Chemical Regent (Shanghai, China). (NH4)6H2W12O39·nH2O were purchased from Alfa Aesar. TiO2 was purchased from Acros Organics. All chemicals were used without further purification. Deionized water was used in the preparation of samples.

Catalyst Preparation.

HMoO and V-HMoO were synthesized by hydrothermal method. For preparing HMoO, 2.0 mmol (NH4)6Mo7O244H2O were dissolved in deionized water and then the pH of the solution was regulated to 1.0 by adding HNO3 dropwise. The aqueous solution was transferred into a Teflon lined stainless autoclave (100 mL) and kept at 120 oC for 12 h. The system was cooled to room temperature. The slurry obtained was then filtered, washed with deionized water, and dried at 80 oC for 12 h. The resultant powder was calcined at 350 oC for 3 h. V-HMoO was prepared using the same procedure except that 1.4 mmol NH4VO3 and 1.8 mmol (NH4)6Mo7O24·4H2O were used instead of 2.0 mmol (NH4)6Mo7O24·4H2O.

V2O5/V-HMoO was prepared by dispersing 0.2 g V2O5 (prepared by calcining NH4VO3 at 500 oC) and 2.0 g V-HMoO in deionized water, The slurry was stirred continuously for the evaporation of water. The samples were dried at 80 oC and calcined at 400 oC. For comparison, a conventional V2O5/WO3-TiO2 catalyst with 3.0 wt% V2O5 and 9.0 wt% WO3 was synthesized by wet impregnation. To further loading potassium ions, samples were impregnated with a K2SO4 solution. Catalysts with different amounts of K+ loading were obtained after being dried at 80 oC and calcined at 350 oC for 12 h. The amount of K+ loading is expressed in terms of mass fraction of K2O with respect to catalyst.

Catalytic Evaluation.

NH3-SCR activity measurements were performed in a fixed-bed quartz reactor (inner diameter 8 mm) under atmospheric pressure. The feed gas contained 500 ppm NO, 500 ppm NH3, 3.0 vol % O2, and balanced N2. The total flow rate was 500 mL min−1 and 0.5 g sample (40−60 mesh) was used. The gas hourly space velocity (GHSV) was 66,000 h-1. Data were recorded by a temperature-programmed mode at a ramp of 5 oC min-1. The concentration of NO in the outlet was continually monitored by a Fourier transform infrared spectrometer (FTIR, Thermo Scientific Antaris IGS analyzer).

Catalyst Characterization.

Synchrotron X-ray diffraction (SXRD) patterns were obtained at BL14B of the Shanghai Synchrotron Radiation Facility (SSRF) at a wavelength of 0.6886 Å. The thermogravimetry (TG) curves were determined by a thermal analyzer (TG-DSC perkin STA800) with a ramp rate of 10 oC min-

1 in air and the DTG was the corresponding first derivative of TG. Scanning transmission electron microscopy (STEM) and X-ray spectroscopy (EDX) mapping were obtained on a JEM 2100F transmission electron microscope. XANES and EXAFS spectra of V K-edge were measured at BL14W of the SSRF. XANES and EXAFS

spectra of K K-edge of the samples were measured at BL4B7A of the Beijing Synchrotron Radiation Facility (BSRF). Data analyses were conducted by using the IFEFFIT 1.2.11 software package. X-ray photoelectron spectroscopy (XPS) analysis was performed on American Thermo Fischer equipped with a monochromatic Al Kα source (Kα = 1486.6 eV) and a charge neutralizer. The binding energies of Mo, V, K were calibrated according to the binding energy of C 1s at 284.6 eV. The curve fitting was operated by using XPSPEAK 4.1 with a Shirley background.

Acknowledgements

This work was financially supported by the NSFC (21477023 and 21777030). The SXRD patterns and X-ray absorption spectra covering XANES and EXAFS spectra were measured at the SSRF and BSRF.

Keywords: substitute • hexagonal V-HMoO • thermal stability •

NH3-SCR • alkali-trapping

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Entry for the Table of Contents

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Xiaona Liu, Jiayi Gao, Yaxin Chen, Chao Li, Junxiao Chen, Weiye Qu Xin Chen, Zhen Ma, and Xingfu Tang*

Page No.1 – Page No.6 Rational Design of Alkali-Resistant NO Reduction Catalysts Using a Stable Hexagonal V-Doped MoO3 Support for Alkali Trapping

10.1002/cctc.201800818

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