Insight into Deactivation of Commercial SCR Catalyst by Arsenic: An

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New Insight into Deactivation of Commercial SCR Catalyst by Arsenic: an Experiment and DFT Study Yue Peng, Junhua Li, Wenzhe Si, Jinming Luo, Qizhou Dai, Xubiao Luo, Xin Liu, and Jiming Hao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es503486w • Publication Date (Web): 07 Nov 2014 Downloaded from http://pubs.acs.org on November 8, 2014

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Environmental Science & Technology

New Insight into Deactivation of Commercial SCR Catalyst by Arsenic: an Experiment and DFT Study Yue Penga, b, Junhua Li*, a1 Wenzhe Sia, Jinming Luob, Qizhou Daib, Xubiao Luob, Xin Liua, Jiming Haoa a

State Key Joint Laboratory of Environment Simulation and Pollution Control, School of

Environment, Tsinghua University, Beijing, 100084, China b

School of Civil and Environmental Engineering, Georgia Institute of Technology, 800 West

Peachtree Street, Suite 400 F-H, Atlanta, Georgia, 30332-0595, United States

Abstract The fresh and arsenic poisoned V2O5-WO3/TiO2 catalysts are investigated by experiments and DFT calculations for the SCR activity and deactivation mechanism. Poisoned catalyst (1.40 % of arsenic) presents lower NO conversion and more N2O formation than fresh. Stream (5 %) could further decrease the activity of poisoned catalyst above 350 °C. The deactivation is not attributed to the loss of surface area or the phase transformation of TiO2 at certain arsenic content, but due to the coverage of V2O5 cluster and the decrease of surface acidity: the number of Lewis acid sites and the stability of Brønsted acid sites. Large amounts of surface hydroxyl induced by H2O molecule provides more unreactive As-OH groups and gives rise to a further decrease of SCR activity. N2O is mainly from NH3 unselective oxidation at high temperatures, since the reducibility of catalysts and the number of surface-active oxygen are improved by As2O5. Finally, the reaction pathway seems unchanged after poisoning: NH3 adsorbed on both Lewis and Brønsted acid sites are reactive.

* Corresponding author. Tel.: +86 10 62771093; Fax.: +86 10 62771093. E-mail address: [email protected] (J. Li).

ACS Paragon Plus Environment


Graphical Abstract


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1

Introduction

2

Selective catalytic reduction (SCR) with ammonia is a well-proven technique for

3

NO removal in coal-fired power plant and steel sintering boiler.1, 2 In the SCR process,

4

NO is reduced to N2 over a catalyst, which is usually V2O5 supported on TiO2 and

5

works at approximately 350-400 °C.3 The catalytic convertor is often placed right

6

after the vessel to take advantage of the heat from hot exhaust gas. However, the

7

major drawbacks of this arrangement are the facts that the catalytic convertor operates

8

under high temperatures for a long period (thermal aging) and is exposed to high

9

concentrations of fly ash and poisoning elements, including SO2, alkali/alkaline-earth

10

metals and heavy metals (arsenic and lead).4-7 In order to improve catalyst thermal

11

aging resistance and lifetime, WO3 or MoO3 additives are introduced to V2O5/TiO2.

12

These metal oxides enhance catalysts activity and thermal durability by enhancing the

13

number of Brønsted acid sites and stabilizing TiO2 from phase change (anatase to

14

rutile), respectively. In addition, the catalyst is generally robust against SO2 poisoning

15

above 300 °C.8-10 Previous studies proposed the deactivation mechanism of alkali

16

metals: besides the decrease of surface acidity, the loss of reducibility and the

17

accumulation of inactive nitrite/nitrate species might also restrain catalyst activity.11,

18

12

19

washing or electrophoresis methods are quite effective for regenerating the poisoned

20

catalysts.13 Further, honeycomb monolithic catalyst shows good durability to dust and

21

resistance to alkali metals.

However, these metals are highly mobile and soluble in a water form, and water

22

Arsenic (As) is a serious poison for commercial SCR catalyst from stationary

23

sources, where it is presented as As2O3 in gas phase of power plant in concentrations

24

between 1 µg/m3 and 10 mg/m3.14, 15 As2O3 diffuses into catalyst surface and adsorbed

25

on both active (mainly V2O5) and non-active sites (TiO2 support), resulting in the


1


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1

replacement of hydroxyl groups by forming new As hydroxyls. When the catalyst is

2

poisoned by sufficient amounts of As, As2O3 is oxidized to As2O5 by surface

3

superoxide species, and neither CO nor NH3 is adsorbed on Lewis acid sites of

4

poisoned catalyst.16-18 Gao et al. used Pb as a probe atom to reveal the deactivation on

5

V2O5/TiO2 by density functional theory (DFT) calculations. The poisoning

6

mechanism was attributed to the coverage of vanadium active sites by Pb and the loss

7

of surface acidity.19, 20 However, few works have been published to systematically

8

elucidate the influences on surface acidity, reducibility and reaction pathway for

9

poisoned catalysts. The objective of this work is to clarify the poisoning mechanism

10

of As on a commercial V2O5-WO3/TiO2 catalyst.

11

Experiment and Calculation

12

Catalysts preparation and activity measurement

13

The investigated monolith catalyst (fresh) is V2O5-WO3/TiO2 with SO42-, SiO2 and

14

CaO (shown in supporting information, SI, Table S1). The deactivated catalyst

15

(poisoned) is the same catalyst, which is obtained from a power plant in southwest

16

China after running in actual working conditions. Both fresh and poisoned catalysts

17

are crushed and sieved within 40-60 meshes for activity measurement.

18

Activity is performed in fixed-bed quartz reactor (inner diameter of 5 mm) using

19

100 mg catalyst. The feed gas contains 500 ppm NO, 500 ppm NH3, 3 % O2, 5 % H2O

20

(when used) for SCR reactions and 500 ppm NH3, 3 % O2 for NH3 oxidations. The

21

balance is N2. The concentrations of NO, NO2, N2O, NH3, and H2O are continually

22

monitored by an FTIR spectrometer (MultiGas TM 2030 FTIR).

23

Catalysts characterizations

24

BET surface area is carried out with a Micromeritics ASAP 2020 apparatus. The

25

elements contents are characterized by both ICP with an IRIS Intrepid II XSP


2

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1

apparatus (Thermo Fisher Scientific Inc.) and XRF with XRF DEX-LE from

2

SHIMADZU. X-ray photoelectron spectroscopy (XPS) is performed with an

3

ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα

4

radiations. The binding energy is referenced to the C1s line at 284.8 eV.

5

Temperature programmed reduction (TPR) of H2 is performed on a chemisorption

6

analyzer (Micromeritics, ChemiSorb 2720 TPx) under 10 % H2/Ar gas flow (50

7

mL/min) at a rate of 10 °C/min up to 900 °C. Temperature programmed desorption

8

(TPD) of NH3 is performed in a fixed-bed quartz reactor. Before the test, each sample

9

(100 mg) is pretreated in N2 (200 mL/min) gas at 350 °C. The sample is purged under

10

NH3 at room temperature. After isothermal desorption under N2 gas flow (200

11

mL/min) at 100 °C, the temperature is elevated to 600 °C at a rate of 10 °C/min. The

12

concentrations of NH3 are monitored by MultiGas TM 2030 FTIR.

13

In-situ IR spectra are recorded on a Fourier transform infrared spectrometer (FTIR,

14

Nicolet NEXUS 870) equipped with the Harrick IR cell and an MCT detector cooled

15

by liquid N2. The catalyst is first heated to 350 °C under N2 at a total flow rate of 100

16

mL/min for 1 hour to remove adsorbed impurities. The background spectrum is

17

collected in a flowing N2 atmosphere and subtracted from the sample spectra. The IR

18

spectra are recorded by accumulating 32 scans and the resolution is 4 cm-1.

19

Model selection and DFT calculation

20

Bulk V2O5 is also active and selective to produce N2, and the V6O20H10 cluster

21

could be selected as modeled surface. The cluster model exposing V2O5 (010) plane,

22

has been used to investigate reaction mechanism of SCR reaction and NH3

23

oxidation.21-23 The model is constructed by cutting V6O20 moiety from a V2O5 (010)

24

surface and the dangling bonds are saturated by using H atoms as terminators. To

25

further study the interaction between As and V on support, we also calculate V2O5,


3


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1

As2O5 doped on TiO2 (001) plane.24, 25 The TiO2 (001) model is constructed by cutting

2

TiO2 anatase and (3×3) supercell of slab model. A vacuum gap of 15 Å is employed

3

to separate subsequent slabs (Ti36O72). The slab thickness is optimized according to

4

previous TiO2 (001) slab results, in which a 3-layer slab model with the bottom layer

5

fixed to the bulk parameters is sufficient.26, 27 For surface relaxation, no symmetry is

6

used and a dipole correction is included (Fig. S1).

7

All the calculations are based on DFT and performed using Material Studio 5.5

8

modeling DMol3.28, 29 Double-numerical-quality basis set with polarization functions

9

(DNP) and GGA-PBE are used for all calculations.30, 31 Core electrons are treated

10

with DFT semicore-pseudo potentials (DSPP).32 Spin polarization is also applied and

11

the real space cutoff radius is maintained as 4.2 Å. The adsorption energy (Ead) of

12

NH3 molecule is calculated as follows:

13

Ead=Esurface+Eammonia-Eammonia/surface

(1)

14

Esurface is the energy of surface, Eammonia is the energy of an isolated NH3 molecule

15

and Eammonia/surface is the total energy of the same molecule adsorbed on surface. Note

16

that a positive value for Ead suggests a stable adsorption.

17 18

Net charge of atoms during NH3 adsorption is calculated as follows: Net charge=ESP chargeafter-ESP chargebefore

(2)

19

ESP chargeafter is the charge of atoms after NH3 adsorbed and ESP chargebefore is

20

the charge of atoms before adsorption or gaseous NH3 molecules. Note that a positive

21

value for net charge suggests the loss of electrons.

22

Results and Discussion

23

Catalyst structure and SCR performance

24

Table 1 summarizes the properties of fresh and poisoned catalysts. Compared with

25

poisoned catalyst, BET surface area of fresh decreased slightly and the average pore


4

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1

diameter are improved, which is attributed to inevitable thermal aging effect.

2

Nevertheless, the differences are not apparent. It seems that the change of surface area

3

or pore diameter does not play a key role during poisoning process. The As loading is

4

a little higher on the surface than that in the bulk, and only As5+ cations are found in

5

XPS spectra (Fig. S2), implying the enrichment of As2O5 on the catalyst surface.16

6

Other additives remnants during catalysts procedure, especially alkali metals, are also

7

observed by XRF (Table S1). The contents of K2O, Na2O and CaO of the two samples

8

are nearly the same. Therefore the influence of alkali metals on catalyst activity can

9

be neglected in this study and the catalyst can be defined as merely poisoned by As.

10

The crystal structures of catalysts are obtained by XRD and Raman spectra (Fig. S3,

11

S4). Only anatase TiO2 phase is present on both XRD and Raman spectra, and both

12

the W-O-W (794 cm-1) and W=O (980 cm-1) vibrational modes are observed on the

13

Raman spectra.33, 34 The results indicate that As2O5 at certain amount (1.40 % in the

14

bulk and 1.82 % on the surface) does not change the crystal phase of TiO2 anatase or

15

surface WO3 species.

16

Fig. 1(a) shows the differences of NO conversion and N2O formation between

17

fresh and poisoned catalysts under a gas hourly space velocity (GHSV) of 120,000

18

mL/g·h. Fresh catalyst shows higher activity than poisoned catalyst: nearly 85 % of

19

NO conversion is acquired for fresh at 450 °C, poisoned catalyst only yields 60 % of

20

NO conversion. The N2O concentrations for poisoned sample are higher and increase

21

fast above 400 °C. As an important trace gas, N2O could cause global warming and

22

stratospheric ozone depletion.35 In order to study the origin of N2O, NH3 oxidation are

23

studied at 450 °C, with 500 ppm of the inlet NH3 (Fig. 1(b)). NO2 is not detected over

24

the two catalysts and N2 concentrations from NH3 oxidation are calculated by

25

subtracting the concentrations of N2O and NO from inlet NH3. Poisoned catalyst


5


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1

yields the formation of more N2, N2O and NO than fresh catalyst. The N2O is mainly

2

originated from unselective NH3 oxidation under O2-rich flow at high temperatures36

3

and could be improved by As poisoning, i.e., As2O5 not only reduce the SCR activity,

4

but generates more N2O.

5

Stream is always present in the flue gas, and the catalyst activity temporarily

6

decreases from the competitive adsorption of H2O with NH3 on surface acid sites.

7

Once H2O is removed, the activity will be enhanced.37, 38 Fig. 1(c) presents the NO

8

conversion of catalysts under at a higher GHSV (240,000 mL/g·h) in 5 % stream.

9

Catalytic activity decreases at higher GHSV compared to Fig. 1(a), and H2O further

10

restrains NO conversion above 300 °C. While the activity of fresh catalyst under

11

stream is not severe, but the activity of poisoned sample significantly decreases: only

12

35 % of NO conversion is observed at 350 °C, compared with 55 % of NO conversion

13

for fresh catalyst at the same condition. The results reveal there might be a further

14

inhibition effect between As and stream at relatively high temperatures. Therefore,

15

chemical characterizations and DFT calculations are then employed to elucidate the

16

loss of SCR activity, the increase of N2O formation and the further deactivation under

17

H2O by arsenic poisoning. N2O formations are also further studied for different

18

poisoned catalyst and for different test conditions (Fig. S5). The results show that the

19

increase of N2O during SCR process is from arsenic poisoning at high temperatures.

20

Reducibility and surface acidity

21

The XPS spectra of O 1s for fresh and poisoned catalysts are shown in Fig. 2(a).

22

The O 1s bands is fitted into two independent peaks, referred to as the stable oxygen

23

in lattice at 530.2 eV (Oβ) and the surface-active oxygen at 531.9 eV (Oα),39, 40 where

24

Oα is more active in oxidations due to its higher mobility than Oβ. The fractions of Oα

25

are 20.1 % and 23.7 % for fresh and poisoned respectively, suggesting that surface-


6

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1

active oxygen species could be improved by As. Wei et al. studied the role of surface

2

oxygen on TiO2 anatase (101) by DFT calculation and proposed that O2 could easily

3

bond to TiO2 surface, creating superoxide or hydroperoxyl radical species by As5+

4

cations.18 Fig. 2(b) presents the H2-TPR profiles of fresh and poisoned catalysts in the

5

range of 200-900 °C. Two reduction peaks are obtained on fresh catalyst. The peak at

6

494 °C can be attributed to the reduction of V5+ to V0 and W6+ to W4+, a peak at

7

792 °C can be attributed to the reduction of W4+ to W0, and a sharp peak for poisoned

8

catalyst at 416 °C, which could be assigned to the reduction of As5+ to As0.41, 42 The

9

onset reduction temperature shifts to lower temperature for poisoned catalyst, and

10

peaks of tungsten reduction are nearly unaffected (792 °C) or are overlapped (494 °C)

11

by the large reduction peak. Surface As5+ cations increase the reducibility and the

12

number of active surface oxygen. This might be one of the factors improving of N2O

13

formation.

14

The NH3-TPD profiles of fresh and poisoned catalysts are performed below 600 °C

15

(Fig. S6). Relative weak (266 °C) and strong (360 °C) acidities can be identified on

16

the TPD curves based on the temperature regions. The total acidity apparently

17

decreases after poisoning, including partial weak acidity and most of strong acidity.

18

So as to study the types of surface acid sites, in-situ FTIR spectra of NH3

19

adsorption/desorption behaviors are carried out over the two catalysts from 100 °C to

20

350 °C (Fig. 3). Both Lewis and Brønsted acid sites stably bond to the surface of

21

catalyst at 100 °C. Lewis acid sites display broad bands in the 1150-1300 cm-1 as

22

δs(NH3) mode and a relative weak peak at 1603 cm-1 (fresh) or 1607 cm-1 (poisoned)

23

as δas(NH3) mode, whereas Brønsted acid sites present a broad band in the 1390-1480

24

cm-1 as δas(NH4+) mode and a peak at 1670 cm-1 as δs(NH4+) mode.2, 43-45 To diminish

25

the influence of IR absorbance, absorbance intensity is set to 3.00 at 100 °C by


7


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1

calibrating the angle of incident light when collecting the background spectra.

2

Brønsted acid sites seem to be improved after As doping at 100 °C, while Lewis acid

3

sites decrease by comparing the intensity of peaks at 1603 and 1607 cm-1. Arsenic

4

atoms block surface Lewis acid sites (V=O) and provide new Brønsted acid sites (As-

5

OH) of vanadia-based catalyst. Previous studies also ascertained the formation of As-

6

OH groups for WO3(MoO3)/TiO2 catalyst.17 However, whether or not As-OH groups

7

are active has not been demonstrated. With increased the temperature, Lewis acid

8

sites of poisoned catalyst decrease rapidly and nearly disappear (1607 cm-1) at 300 °C,

9

while with respect to fresh catalyst, the band of Lewis acidity (1603 cm-1) is still

10

observed at 350 °C. The thermal stability of Brønsted acid sites under 300 °C is

11

slightly enhanced by As2O5 but they vanish merely at 350 °C. Combined with NH3-

12

TPD results, the stability of surface acid sites could be inferred. Lewis acid sites

13

significantly loss after arsenic poisoning and parts of surface As atoms provide weak

14

Brønsted acid sites. The strength of Lewis acid sites at both low and high temperature

15

decreases, i.e. the adsorptions of NH3 on V=O groups become weak, whereas the

16

stability of Brønsted acid sites though slightly increases on As-OH groups at low

17

temperature, but rapidly loses above 300 °C. Furthermore, a weak peak at 1539 cm-1

18

occurs only on the poisoned catalyst in 200-350 °C. This peak could be attributed to

19

the δ(NH2-) mode,43 which could be served as a probe to the oxidation ability of

20

catalysts, formed by reaction between adsorbed NH3 and surface-active oxygen above

21

200 °C. The formation of this species shows a good accordance with improved

22

oxidation ability of As2O5 on catalyst surface.

23

DFT calculations

24

The optimized structures of acid sites, poisoned sites and the corresponding

25

adsorptions of NH3 molecule on V2O5 cluster are displayed in Fig. 4, the Ead and net


8

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1

charges of surface atoms are summarized in Table 2. Both Lewis and Brønsted acid

2

sites are stably constructed as previous reports by other groups,19, 21, 22 the bond length

3

of V=O and V-O-V are 1.6 and 1.8 Å, respectively, and the distance of O to H is

4

nearly 1.0 Å. Once As atom is doped on Lewis acid sites (Fig. 4(c)), it covered the

5

regions forming by four V=O groups and the bond length of As to O becomes longer

6

(3.1 Å). With respect to Brønsted acid sites, As slightly moves against H atom. Fig.

7

4(e) and (f) shows NH3 adsorptions on fresh models. The Ead is 0.67 and 2.13 eV for

8

Lewis and Brønsted acid sites respectively. By checking the net charges of V and N

9

atoms, the adsorption of NH3 on Lewis acid sites is weak, while some electrons (0.14)

10

transfer from N to H atoms, as a covalent band on Brønsted acid sites. When the NH3

11

molecule is adsorbed on poisoned Lewis acid sites (Fig. 4(e)), a larger Ead is obtained,

12

indicating a more stable adsorbed species. But the net charge of N is only 0.01 and the

13

net charge of As is 0.20, indicating the adsorption of NH3 on As atom is unfavorable

14

and some electrons of As shift to O atom (-0.08). For the NH3 adsorption on Brønsted

15

acid sites (Fig. 4(f)), the Ead for NH3 adsorption is higher (3.37 eV) than that on clean

16

Brønsted acid sites, and the net charge of N was 0.58, which is larger than the

17

adsorption structure (0.14) in Fig. 4(f). The NH3 adsorption of on Brønsted acid sites

18

could be strengthened by As atom. The results show a good accordance with the data

19

of in-situ FTIR spectra.

20

Fig. 5 gives the optimized structure of V2O5 and As2O5 cluster on TiO2 (001) plane.

21

The vanadia dimer (V/Ti) is modeled as having a 4-fold coordination (three V-O

22

bonds and a V=O bond), with two adjacent vanadium sites in a similar configuration

23

to that found on the (010) plane of bulk V2O5 and V6O20 cluster. As2O5 cluster stably

24

bonds to the (001) plane (As/Ti) with the same coordination with V2O5. Further,

25

As2O5 and V2O5 bond together atop of the (001) plane (AsVTi), indicating the two


9


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1

clusters have similar bond length and structures. To study the influence of As to V2O5

2

cluster, we have calculated the project density of states (PDOS) of As and V orbitals

3

for the three models. The valence band of As in AsV/Ti shows nearly unchanged

4

compared with that in As/Ti, while the bottom of conduction band slightly shifts to

5

lower energy region (at 3.2 eV). These results suggest that V2O5 improves the

6

reactivity of surface As2O5, agrees with the TPR results. Therefore, As doped on

7

vanadia-based catalyst provides new surface acid sites and As2O5 itself is promoted

8

by V2O5 for its reactivity on the TiO2 surface.

9

Deactivation mechanism of arsenic

10

The physical properties of poisoned catalyst, including the surface area, pore

11

diameter and crystal phase are similar as those of fresh catalyst at certain As content.

12

However, significant changes of acidity and reducibility by As2O5 hinder the NO

13

conversions and improve the N2O formations. The loss of SCR activity is due to the

14

decrease of Lewis acid sites and unstable Brønsted acid sites (As-OH) at high

15

temperatures. The co-suppression of As and H2O above 300 °C is attributed to the

16

competitive adsorption of H2O on the surface As2O5. Once H2O bonds to acid sites

17

closing to As atom, more As-OH groups is produced, and the deep inhibition effect

18

occurs. N2O is generated from the unselective oxidation of NH3 resulting from the

19

improvement of reducibility and surface-active oxygen by As.

20

Reaction mechanism over both catalysts was carried out by transient in-situ FTIR

21

spectra at 250 °C and 350 °C (Fig. S7-S9). The key pathways (Eley-Rideal) and

22

intermediate species were not changed over poisoned catalyst compared to fresh

23

V2O5-WO3/TiO2: NH3 adsorbed on both Lewis and Brønsted acid sites were active

24

with gaseous NO above 250 °C. Arsenic improved the NH3 oxidation and produced

25

more N2O at high temperatures.


10

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1 2 3

Notes The authors declare no competing financial interest.

Acknowledgement

4

The authors gratefully acknowledge the financial supports from National Natural

5

Science Foundation of China (21325731, 21407088 and 21221004), and National

6

High-Tech Research and the Development (863) Program of China (2013AA065401)

7

and the International Postdoctoral Exchange Fellowship Program of China

8

(20130032). The authors appreciate support from the Brook Byers Institute for

9

Sustainable Systems, Hightower Chair and Georgia Research Alliance at Georgia

10

Institute of Technology and CPI YUANDA Environmental-Protection Engineering

11

Co., Ltd.

12

Supporting Information Available: Some related tables and figures are shown

13

in the supporting information. This information is available free of charge via the

14

Internet at http://pubs.acs.org/.


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Table 1. Physical parameters of fresh and poisoned catalysts DPore SA (m2/g) （nm） fresh 52.7 113.9 poisoned 52.0 115.4 a acquired from XRF results b acquired from ICP results c acquired from XPS spectra

As contenta 0 1.51 %

As contentb 0 1.40 %

surface Asc 0 1.82 %

surface SO42-c 1.43 % 1.42 %


V contentb 0.66 % 0.53 %

W+Ti+Si contenta 97.1 % 95.4 %

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Table 2. Ead and net charges (ESP charge) during the NH3 adsorption based on Fig. 4 freshL Ead (eV) 0.67 net charge of As net charge of N -0.26 net charge of Oa a the O atom boned between As and V

freshB 2.13 0.14 -

poisonedL 1.17 0.20 0.01 -0.08


poisonedB 3.37 0.29 0.58 -0.02

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Fig. 1. Comparisons on the SCR performance of fresh and poisoned catalysts. Reaction conditions: catalyst mass=100 mg, (a) [NO]=500 ppm, [NH3]=500 ppm, [O2]=3 %, total flow rate=200 mL/min, GHSV=120,000 mL/g·h, (b) [NH3]=500 ppm, [O2]=3 %, total flow rate=200 mL/min, GHSV=120,000 mL/g·h at 450 °C, (c) [NO]=500 ppm, [NH3]=500 ppm, [O2]=3 %, [H2O]=5 %, total flow rate=400 mL/min, GHSV=240,000 mL/g·h.


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Page 21 of 24


Fig. 2. (a) XPS spectra of O 1s and (b) H2-TPR profiles of fresh and poisoned catalysts.


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Fig. 3. In-situ FTIR spectra of NH3-TPD of (a) fresh and (b) poisoned catalysts.


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Fig. 4. Optimized V2O5 cluster structures of (a) Lewis acid sites, (b) Brønsted acid sites, (c) poisoned Lewis acid sites, and (d) poisoned Brønsted acid sites and (e)-(h) the corresponding NH3 adsorption configurations. Vanadium atoms are green, oxygen atoms are red, arsenic atoms are purple, nitrogen atoms are blue and hydrogen atoms are white.


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Fig. 5. Optimized structures of V2O5, As2O5 clusters doped on TiO2 (001) plane and projected density of state (PDOS) of As and V orbitals on the slab models.


22

Insight into Deactivation of Commercial SCR Catalyst by Arsenic: An

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