Mechanistic Investigation of the Promotion Effect of Bi Modification on

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Mechanistic Investigation of the Promotion Effect of Bi Modification on the NH-SCR Performance of Ce/TiO Catalyst 3

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Rui-tang Guo, Ming-yuan Li, Peng Sun, Wei-guo Pan, Shu-ming Liu, Jian Liu, Xiao Sun, and Shuai-wei Liu J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b10342 • Publication Date (Web): 21 Nov 2017 Downloaded from http://pubs.acs.org on November 21, 2017

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The Journal of Physical Chemistry

Mechanistic Investigation of the Promotion Effect of Bi Modification on the NH3-SCR Performance of Ce/TiO2 Catalyst

Rui-tang Guoa,b*, Ming-yuan Li a,b, Peng Suna,b, Wei-guo Pana,b*, Shu-ming Liua,b, Jian Liua,b, Xiao Suna,b, Shuai-wei Liua,b a. School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, P. R. China b. Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai, P. R. China c. Ningbo Institute of Technology, Zhejiang University, Ningbo 315100, PR China

Corresponding author1：：Rui-tang Guo; E-mail: [email protected] Corresponding author2：：Wei-guo Pan; E-mail: [email protected]

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Abstract: In this study, Bi was used as the modifier to elevate the performance of Ce/TiO2 catalyst for selective catalytic reduction (SCR) of NOx with NH3. Experimental results indicated that the CeBi/TiO2 catalyst with a Bi/Ce molar ratio of 0.15 exhibited excellent low-temperature SCR performance and SO2/H2O resistance compared with the Ce/TiO2 catalyst. Characterization results revealed that the introduction of proper amount of Bi to Ce/TiO2 catalyst could generate more Ce3+ and chemisorbed oxygen species on its surface, along with the enhanced reducibility and surface acidity. From the results of in situ DRIFT study, the formation of more adsorbed NH3 and NO2 species could be detected, as a result, greatly facilitating the low-temperature NH3-SCR reaction over CeBi/TiO2-0.15 catalyst through the Langmuir−Hinshelwood route.

1. Introduction The combustion of fossil fuels in mobile and stationary sources has emitted a large amount of NOx, as a typical air pollutant, which is deeply involved in the formation of acid rain, photochemical smog and haze.1-3 Previous study has indicated that about 46% NOx emission is from the sources such as coal-fired boilers and municipal solid waste (MSW) incinerators.4 As a reliable and effective method for NOx emission control, the selective catalytic reduction (SCR) of NOx with NH3 in the presence of excess oxygen has been put into industrial application in recent decades.

5,6

In this process, V2O5-WO3 (MO3)/TiO2 catalyst is the most prevalent commercial

catalyst.

7,8

However, the insurmountable drawbacks associated with this catalyst, mainly

including the narrow operation temperature window (300-400 ℃), the toxicity of VOx species, and the deactivation by alkali metals and the drop of N2 selectivity at high temperature, bring about great challenges to its further application in future.

9-15

Therefore, exploring green SCR

2


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catalyst without V species has drawn much interest of many researchers in recent years. Owing to its nontoxicity, high oxygen storage ability and reducibility, CeO2-based SCR catalyst has been focused on great expectations.16-21 Recent studies have revealed that Ce/TiO2 SCR catalyst displayed high SCR activity, 22,23 and the Ce-O-Ti short-range order species with the interaction between Ce and Ti in atomic scale was identified as the active site for Ce/TiO2 catalyst. 24

However, the active temperature window of Ce/TiO2 catalyst lies in the temperature range of

250-350 ℃,

25

which is only a little broader than that of typical V2O5-WO3/TiO2 catalyst.

For

the purpose of avoiding the deactivation of Ce/TiO2 catalyst caused by SO2 and alkali metals, it is of great importance to further enhance the low-temperature SCR activity of Ce/TiO2 catalyst. To further improve the low-temperature SCR activity of Ce/TiO2 catalyst, transition metal modification has been proven to be an appropriate choice. Shu et al.

26

found that the addition of

Fe could enhance the low-temperature SCR activity and SO2 resistance of Ce/TiO2 catalyst. Similar promotion effect has also been observed on other transition metals such as W, Sn, Mo and Zr.27-32 It is well known that Bi-based catalyst has been widely used in photocatalytic pollutant degradation.

33-36

Nevertheless, Bi was not used as the additive to enhance the performance of

SCR catalyst to our best knowledge. Thereafter, a series of CeBi/TiO2 catalyst samples were fabricated and used in NH3-SCR reaction, which presented much higher SCR activity and better resistance to SO2/H2O than Ce/TiO2 catalyst. Moreover, the physico-chemical properties of these samples were characterized to understand the promotion mechanism of Bi modifier on Ce/TiO2 catalyst. 2. Experimental 2.1 Catalyst preparation 3



Based on the coprecipitation method, 37 the catalyst samples used in this study were prepared. When preparing Ce/TiO2 catalyst, cerium nitrate and titanium sulfate were dissolved in deionized water with a Ce/Ti molar ratio of 1:4, then the solution of ammonia (3 moL/L) was added to the solution dropwisely under vigorous stirring at room temperature until the solution pH value reached 11. Then the mixed solution was aged for 3 h to ensure sufficient precipitation, followed by filtered and washed thoroughly with deionized water. The obtained solid was dried at 100 ℃ for 12 h and then calcined in air at 500 ℃ for 5 h to obtain the final Ce/TiO2 catalyst. Similarly, the Bi-doped Ce/TiO2 catalyst samples were prepared by the same method, and bismuth nitrate was used as the precursor of Bi. Correspondingly, the obtained Be-doped Ce/TiO2 catalyst samples were denoted as CeBi/TiO2-x, where x represented the molar ratio of Bi/Ce. Moreover, a commercial V2O5-WO3/TiO2 catalyst (denotes as VW/TiO2) supplied by Shengwang Inc. Zhejiang, China) was used as the counterpart. 2.2 Characterizations The textural properties of the catalyst samples were evaluated based on N2 adsorption at -196 ℃ on a Quantachrome Autosorb-iQ-AG instrument. The specific surface area and the pore size distribution were calculated by using the Brunauer–Emmett–Teller (BET) method and the Barrett– Joyner–Halenda (BJH) method respectively. The crystal phases of the catalyst samples were identified on a Bruker D8 Advance powder diffractometer with CuKα radiation (λ=0.154056 nm). The data was collected in the scattering angle range of 20-80°, with a step size of 0.02°. To obtain the information of surface elements and their chemical states, X-ray photoelectron spectroscopy (XPS) analysis with Al Kα X-ray (hν= 1486.6 eV) was operated on a Thermal 4


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ESCALAB 250 spectrometer. Binding energy shift was calibrated by using C 1s (B. E. =284.8 eV) as the standard. Hydrogen-temperature programmed reduction (H2-TPR) was performed on a Quantachrome Autosorb-iQ-C chemisorption analyzer. Firstly, the catalyst sample (50 mg) was pretreated in pure N2 for 1h, then it was cooled to room temperature. After that the H2-TPR experiment was carried out by ramping the temperature from 100℃ to 800℃ in a gas stream of 5% H2/N2 (30 mL/min) with a constant heating rate of 10 ℃/min. Temperature-programmed desorption of NH3 (NH3-TPD) was also carried out on the same chemisorption analyzer. Before the TPD experiment, the catalyst sample (10 mg) was pretreated at 400 ℃ for 1 h, then it was saturated with NH3 in the gas flow of 4% NH3/He (30 mL/min) for 30 min at room temperature. During the desorption experiment, the catalyst sample was heated from 50 ℃ to 500 ℃ with a linear heating rate of 10 ℃/min. Both for the H2-TPR test and the NH3-TPD test, the signal of H2 or NH3 was recorded by a thermal conductivity detector (TCD). In situ DRIFT study was performed on a FTIR spectrometer (Thermo Nicolet iS 50) with a smart collector and an MCT/A detector cooled by liquid nitrogen. In the DRIFT cell (KBr window) connected with a gas flow system, the catalyst sample was pretreated at 400 ℃ for 0.5 h to remove the adsorbed impurities. Next then, the sample was cooled to the desired temperature to get a background spectrum, which was automatically subtracted from the sample spectra. When performing the DRIFT experiments, the gas components were controlled as follows: 600 ppm NH3, or/and 600 ppm NO+5% O2, balance N2, with the total flow rate of 300 mL/min. All the DRIFT spectra were recorded with an accumulation of 100 scans at 4 cm-1 resolution. 2.3 Catalytic activity test 5



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The SCR activity test for each catalyst sample was evaluated in a fixed-bed quartz flow reactor (i. d.=8 mm) under the following reaction conditions: 600 ppm NH3, 600 ppm NO, 5% O2, 100 ppm SO2 (when used), 5% H2O (when used), balance Ar. All the gases were supplied by gas cylinders, and the flowrate of each gas was precisely controlled by mass flow controller (MFC, Qixing Huangchuang Co., China). The water vapor was added by using an injection pump (LSP01-1A, LongerPump Inc) and an evaporator. In each experimental run, about 0.55 cm3 catalyst sample (80-100 mesh) was used for activity test. The total flowrate of the simulated flue gas was 1L/min, so the GHSV was 108, 000 h-1. The components of the effluent gas, including NO, NO2, N2O and NH3 were continuously measured by the FTIR spectrometer (Thermo Nicolet iS 50) connected with a gas cell of 0.2 dm3 volume. During the activity test process, the SCR reaction could reach a steady state after about 1h. Then the DRIFT spectra were collected for calculating the values of NOx conversion and N2 selectivity: NOx conversion = N selectivity = !1 − [$]

[ ] [ ] [ ]

× 100%

[# ]

%[&' ] [$] [&' ]

( × 100%

（1）（2）

Moreover, the catalytic activity for NO oxidation to NO2 over each catalyst sample was also measured under the same reaction conditions except that NH3 was not present in the simulated flue gas. 3. Results and discussion 3.1 SCR performance Figure1 (A) illustrates the NOx conversion over the five catalyst samples at different temperature. For Bi/TiO2 catalyst, the SCR activity was quite low, which was lower than 40% in the whole experimental temperature range. Ce/TiO2 catalyst exhibited much higher SCR activity 6


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than Bi/TiO2 catalyst, but its SCR performance in low-temperature range (< 200 ℃) was not satisfactory. After the introduction of Bi species into Ce/TiO2 catalyst, the enhanced SCR activity could be detected over each Bi-doped Ce/TiO2 catalyst sample. It seemed that the doping amount of Bi was vital to the SCR activities to Bi-doped Ce/TiO2 catalyst samples. An excellent low-temperature SCR activity could be detected on CeBi/TiO2-0.15 catalyst, which was much higher than the other catalyst samples. The SCR activities of commercial VW/TiO2 catalyst was also exhibited in Figure1(A). NOx conversion over VW/TiO2 catalyst was similar with that over CeBi/TiO2-0.1 and CeBi/TiO2-0.2 catalysts, which was also lower than that of CeBi/TiO2-0.15 catalyst. Moreover, a slight decrease of NOx conversion could be seen on the activity curves at higher temperature (>300 ℃), which should be owing to the oxidation of NH3.37 Besides that, the addition of Bi also had an improving effect on N2 selectivity of Ce/TiO2 catalyst, as displayed in Figure 1(B). The N2 selectivity of CeBi/TiO2-0.15 catalyst was also superior to that of all the other samples.

7



(A) 100 Ce/TiO2

NOx conversion(%)

80

CeBi/Ti-0.1 CeBi/Ti-0.15 CeBi/Ti-0.2 Bi/TiO2

60

VW/TiO2

40

20 100

150

200 250 300 o Reaction temperature( C)

350

400

350

400

100 (B) 98 96 94 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

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92 90 88

Ce/TiO2

86

CeBi/Ti-0.1 CeBi/Ti-0.15 CeBi/Ti-0.2 Bi/TiO2

84 82

VW/TiO2

80 100

150


Figure1 (A) NOx conversion; (B) N2 selectivities over different catalyst samples as a function of reaction temperature Reaction conditions: [NO] = [NH3] =600 ppm, [O2] =5%, balance Ar, GHSV=108, 000 h-1 3.2 SO2 and H2O tolerance It is generally known that SO2 and H2O contained in the flue gas have a deactivation effect on 8


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SCR catalyst. 38-40 Effect of Bi modification on the SO2 and H2O tolerances of Ce/TiO2, VW/TiO2 and CeBi/TiO2-0.15 are shown in Figure2. Obviously, the presence of 100 ppm SO2 and 5% H2O had a distinct deactivation effect on Ce/TiO2 catalyst, an activity drop from about 24% to about 13.6% in 13 h under these conditions. Similar poisoning effect of SO2 and H2O over commercial VW/TiO2 catalyst could also be observed. As contrast, the NOx conversion over CeBi/TiO2-0.15 catalyst kept over 50% during the SO2 and H2O tolerance experiment. And the NOx conversion over CeBi/TiO2-0.15 catalyst nearly recovers to its initial value after the cut of SO2 and H2O. Therefore, the modification of Ce/TiO2 catalyst with Bi could effectively enhance its SO2 and H2O tolerance, which might be originated from the inhibited formation of sulfate species over CeBi/TiO2-0.15 catalyst. The good resistance to SO2 and H2O of CeBi/TiO2-0.15 catalyst was conducive to its application in industrial field.

60 CeBi/Ti-0.15 50 NOx conversion (%)

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


40 30

VW/TiO2

20

Ce/TiO2

10

SO2+H2O off

SO2+H2O on

0 0

4

8

12 Time (h)

16

20

24

Figure2 SO2 and H2O tolerances of the three catalyst samples at 150 ℃ Reaction conditions; [NH3] = [NO] =600 ppm, [SO2] =100 ppm, [O2] = [H2O] =5%, balance Ar, GHSV=108, 000 h-1 9



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3.3 BET and XRD analysis The textural properties of different catalyst samples are summarized in Table1. From Table1, CeBi/TiO2-0.15 catalyst exhibited the largest specific surface area and pore volume among the five catalyst samples. Thus the modification of Ce/TiO2 catalyst by Bi could promote the dispersion of Ce and Ti species in the CeBi/TiO2 catalyst samples, leading to the increase of specific surface area. Correspondingly, the reactant species could be easily adsorbed on the surface of CeBi/TiO2-0.15 catalyst and take part in the NH3-SCR reaction. Table1. Textural properties of different catalyst samples Samples

BET surface area (m2/g)

Pore volume (cm3/g)

Pore diameter (nm)

Ce/TiO2 CeBi/TiO2-0.1 CeBi/TiO2-0.15 CeBi/TiO2-0.2 Bi/TiO2

174.2 183.7 197.6 190.3 151.6

0.766 0.928 0.937 0.649 0.553

6.559 6.338 5.617 9.602 9.341

XRD patterns of the five catalyst samples are presented in FigureS1. From FigureS1, no diffraction peak could be detected in the XRD pattern of each catalyst sample, suggesting that Ce, Bi and Ti species were well dispersed and all the catalyst samples mainly existed in amorphous state, indicating that there were strong interactions among Ce, Bi and Ti species.41 Thus all the catalyst samples exhibited large specific surface area, as listed in Table1. 3.4 XPS analysis The surface elements and their chemical states over Ce/TiO2 and CeBi/TiO2-0.15 catalysts were identified by XPS analysis. And the results are given in Figure3 and Table2. As shown in Figure3(A), the XPS spectra of Ce 3d was composed of two multiplets (labeled u and v), which represented the Ce 3d3/2 and Ce 3d5/2 states respectively.42 According to previous studies,

43-45

the six peaks denoted as u, u'', u''', v, v'' and v''' could be attributed to Ce4+; and the 10


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other two peaks of u' and v' could be assigned to the characteristic peaks of Ce3+. From the surface area ratios of corresponding characteristic peaks, the relative contents of Ce3+/Ce for the five catalyst samples could be obtained, as listed in Table2. The results indicated that the ratio of Ce3+/Ce of CeBi/TiO2-0.15 was much higher than that of Ce/TiO2 catalyst, in good accordance with their low-temperature SCR performances (≤250 ℃). The enrichment of Ce3+ species was favorable to the formation of oxygen defects, as a result, promoting the redox transformation between Ce3+ and Ce4+: Ce2O3+1/2O2↔CeO2.

42

Moreover, the presence of more Ce3+ species

could generate more charge imbalance and unsaturated chemical bonds, as a result, accelerating the oxidation of NO to NO2. 46,47 Accordingly, the NH3-SCR reaction could be enhanced through the “fast SCR” route: 2NH3+NO+NO2=2N2+3H2O.

48

So the existence of more Ce3+ species

should be an important reason for the excellent low-temperature SCR activity of CeBi/TiO2-0.15 catalyst. The O 1s XPS spectra of Ce/TiO2 and CeBi/TiO2-0.15 catalysts are implied in Figure3 (B), it contained two kinds of oxygen species: lattice oxygen (B. E. =529.0-530.0 eV, labeled by Oα) and chemisorbed oxygen (B. E.=531.0-531.9 eV, labeled by Oβ). 48,49 From the results of XPS analysis, the concentrations of chemisorbed oxygen over the five catalyst samples could be obtained, as summarized in Table2. It could be seen that the Oβ concentrations over Ce/TiO2 and CeBi/TiO2-0.15 catalysts were 22.76 at. % and 25.45 at.% respectively. In addition, CeBi/TiO2-0.15 catalyst possessed the highest Oβ concentration among the five catalyst samples. It was well recognized that chemisorbed oxygen was the most active oxygen species in NH3-SCR reaction, which could easily exchange with the gas oxygen and surface-adsorbed oxygen molecules.50 Therefore, the possession of more chemisorbed oxygen species over CeBi/TiO2-0.15 11



catalyst was also beneficial to the NH3-SCR reaction due to the promoted NO oxidation and the subsequent “fast SCR” reaction, as a consequence, facilitating the low-temperature SCR reaction over it. Moreover, the atomic Bi/(Ce+Ti) ratios for the three CeBi/TiO2 catalyst samples were also calculated and listed in Table 2. Noticeable, the Bi/(Ce+Ti) ratio of each catalyst sample was much higher than the corresponding theoretical value, suggesting that Bi species were concentrated on catalyst surface. The Ti 2p XPS spectra of the Ce/TiO2 and CeBi/TiO2-0.15 catalysts are exhibited in Figure3 (C), which contained two double peaks (Ti 2p1/2 and Ti 2p3/2). Compared with the XPS spectra of Ce/TiO2 catalyst, a binding energy shift to lower value could be found in the XPS spectra of CeBi/TiO2-0.15 catalyst, which further confirmed the strong interaction between Ce and Ti species.

(A) 3d3/2

3d5/2

Ce/TiO2

Intensity (a.u)

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

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u'''

u''

u'

u

v'''

v''

v' v

CeBi/TiO2-0.15

920

915

910

905 900 895 Binding energy (eV)

12


890

885

880

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(B) Oα

Intensity (a.u)

Oβ

Ce/TiO2

Oα Oβ

CeBi/TiO2-0.15

534

532

530 Binding energy (eV)

(C)

528

526

Ti 2p3/2

Ti 2p1/2

Intensity (a.u)

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


Ce/TiO2

CeBi/TiO2-0.15

468

466

464

462 460 Binding energy (eV)

458

456

454

Figure3 (A) Ce 3d (B) O 1s and (C) Ti 2p XPS spectra of Ce/TiO2 and CeBi/TiO2-0.15 catalysts

13



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Table2. XPS results of Ce/TiO2 and CeBi/TiO2-0.15 catalysts Samples

Ce

O

Bi

Ti

Ce3+/Ce

Oβ/O

Bi/(Ce+Ti)

Ce3+

Oβ

(at. %)

(at. %)

(at. %)

(at. %)

(%)

(%)

(%)

(at. %)

(at. %)

Ce/TiO2

5.23

70.45

/

24.32

26.50

32.31

/

1.39

22.76

CeBi/TiO2-0.1

5.15

72.39

1.07

21.45

28.56

33.34

4.02

1.47

24.13

CeBi/TiO2-0.15

5.00

72.23

1.11

21.66

31.19

35.24

4.16

1.56

25.45

CeBi/TiO2-0.2

4.43

73.18

1.95

20.44

30.23

32.25

7.84

1.34

23.60

Bi/TiO2

/

68.25

6.21

25.54

/

15.99

0.243

/

15.99

3.5 H2-TPR analysis As well recognized, the NOx removal performance of a SCR catalyst was greatly dependent on its redox property, especially for low-temperature SCR reaction.

51

The redox abilities of

different catalyst samples were determined by H2-TPR analysis, and the results are displayed in Figure4. Obviously, there were one or two reduction peaks in the profile of each catalyst sample. For the profile of Ce/TiO2 catalyst, the first peak and the second peak could be ascribed to the reduction of surface CeO2 and bulk CeO2 respectively. 50, 52-54 Only a peak appeared at 513 ℃ in the profiles of Bi/TiO2 catalyst, which could be attributed to the reduction of Bi2O3.55 Compared with that in the profile of Ce/TiO2 catalyst, the surface CeO2 reduction peaks in the profiles of Bi-doped catalyst samples moved to lower temperature, suggesting the increase of reducibility. It was noteworthy that CeBi/TiO2-0.15 catalyst exhibited the lowest reduction temperature and largest peak area, revealing its strong reducibility and high oxygen storage ability, which was in consistent with its excellent low-temperature SCR activity.

14


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536 Ce/TiO2

TCD signal (a.u.)

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


439

CeBi/TiO2-0.1

588

593

390 504

CeBi/TiO2-0.15

405

CeBi/TiO2-0.2

100

200

513

375

Bi/TiO2

300

400 500o Temperature ( C)

600

700

800

Figure4 H2-TPR profiles of different catalyst samples 3.6 NO oxidation NO oxidation to NO2 could greatly improve the low-temperature SCR activity due to the promoted “fast SCR” reaction.56 Therefore, the activities for NO oxidation over Ce/TiO2, Bi/TiO2 and CeBi/TiO2-0.15 catalysts were also tested, as illustrated in FigureS2. Apparently, the NO conversions over Ce/TiO2 and Bi/TiO2 catalysts were very low, which were less than 20% in the whole experimental temperature range. In sharp contrast, NO conversion over CeBi/TiO2-0.15 catalyst was much higher than that over the other two catalyst samples, which should be resulted from the enrichment of Ce3+ and surface chemisorbed oxygen species. Therefore, it was expected that the addition of proper amount of Bi could enhance the oxidation activities of NO to NO2, thereby enhancing its low-temperature SCR activity. 3.7 NH3-TPD analysis As pointed out by Topsøe et al., 57 the adsorbed NH3 species played a pivotal role in NH3-SCR 15



reaction, which was closely related to the surface acidity of SCR catalyst. Based on NH3-TPD technique, the surface acidities of different catalyst samples were determined, as shown in Figure5. As revealed by the continuous desorption peak from 100 ℃ to 450 ℃ in the profile of each catalyst sample, a variety of acid sites with different thermal stability were present on the surface of each catalyst sample. The broad desorption peak could be ascribed to the desorption of physisorbed NH3 and some NH+% species from weak Brønsted acid sites (100-200 ℃), the desorbed NH+% species from strong Brønsted acid sites (200-300 ℃) and the desorption of NH3 species bound to Lewis acid sites (>300 ℃) respectively. 58, 59 From the low peak intensity, it was evident that Bi/TiO2 catalyst only possessed weak surface acidity. And the addition of Bi on Ce/TiO2 catalyst could generate more surface acid sites, as proven by the increased peak intensities in the profiles of CeBi/TiO2 catalyst samples. Similar conclusion could also be drawn from the results of quantitative calculation, as listed in Table3. The surface acidity of CeBi/TiO2-0.15 catalyst was much stronger than that of the rest samples.

16


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Ce/TiO2 CeBi/TiO2-0.1

TCD signal (a.u.)

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


CeBi/TiO2-0.15

CeBi/TiO2-0.2

Bi/TiO2

100

200

300 o Temperature ( C)

400

500

Figure5 NH3-TPD profiles of different catalyst samples Table3. Surface acidities of different catalyst samples Samples

Surface acidity (mmoL/g)

Ce/TiO2 CeBi/TiO2-0.1 CeBi/TiO2-0.15 CeBi/TiO2-0.2 Bi/TiO2

0.183 0.249 0.309 0.228 0.123

3.8 In situ DRIFT study 3.8.1 DRIFT spectra of NH3 adsorption To further identify the nature of surface acid sites on Ce/TiO2 and CeBi/TiO2-0.15 catalyst samples, the DRIFT spectra of NH3 adsorption over them at different temperature were recorded, and the results are presented in Figure6. Some bands in the range of 1100-1700 cm-1 could be detected. The two bands at 1653 and 1432 cm-1 could be attributed to NH+% species on Brønsted acid sites, and the bands at 1600, 1312 and 1167 cm-1 belonged to coordinated NH3 species bound 17



to Lewis acid sites. 60-64 Due to the low thermal stability of adsorbed NH+% species, the two bands at 1653 and 1432 cm-1 quickly vanished when the temperature reached 200 ℃. On the contrary, the NH3 species adsorbed on Lewis acid sites were more stable, which was still visible at 350 ℃. For the spectra of NH3 adsorption over CeBi/TiO2-0.15 catalyst (Figure6 (B)), the situation was very similar. However, the intensities of the bands increased obviously compared with that of the bands in the DRIFT spectra of Ce/TiO2 catalyst, suggesting the enhanced adsorption of NH3 species over CeBi/TiO2-0.15 catalyst. Thus the results of DRIFT study agreed well with that of NH3-TPD analysis. The modification of Ce/TiO2 catalyst by Bi could form more acid sites on its surface, including both Brønsted and Lewis acid sites. For the spectra of NH3 adsorption over Bi/TiO2 catalyst at different temperature (Figure S4), four bands of adsorbed NH3 species were visible, which could be ascribed to NH+% species on Brønsted acid sites (1637, 1477 and 1392 cm-1), NH3 species bound to Lewis acid sites (1600 and 1167 cm-1).60,62,65-68 Similar with the bands in the DRIFT spectra of the other two catalyst samples, all the bands became weaker with increasing temperature. From the low band intensities in the DRIFT spectra of Bi/TiO2 catalyst, it could be concluded that the adsorbed NH3 species over Bi/TiO2 catalyst were less that that over Ce/TiO2 and CeBi/TiO2-0.15 catalysts, as also indicated by the results of NH3-TPD analysis.

18


Page 18 of 40

Page 19 of 40

(A)

1167 1600

o

350 C

Absorbance(a.u.)

1312

o

300 C o

250 C o

200 C o

150 C o

100 C o

50 C 1653

0.1

1432

2000

1800

1600 1400 -1 Wavenumber(cm )

1200

(B)

1167 1305

1600

o

350 C o

Absorbance(a.u.)

300 C o

250 C o

200 C o

150 C o

100 C o

50 C 1656 1445

0.1

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


2000

1800


1200

Figure6 In situ DRIFT spectra of NH3 adsorption over (A) Ce/TiO2 and (B) CeBi/TiO2-0.15 catalysts at different temperature 3.8.2 DRIFT spectra of NO+O2 co-adsorption NOx adsorbed species over Ce/TiO2 and CeBi/TiO2-0.15 were also measured by in situ 19



Page 20 of 40

DRIFT technique at various temperatures, and the results are shown in Figure7. From Figure7 (A), several bands at 1610, 1564, 1298 and 1233 cm-1 were observed in the DRIFT spectra of Ce/TiO2 catalyst. The band at 1610 cm-1 could be assigned to adsorbed NO2 species; the band at 1564 and 1298 cm-1 should be originated from bidentate nitrate and monodentate nitrate respectively; the other band at 1233 cm-1 could be ascribed to bridged nitrite 60,69-71 All the band became weaker with increasing temperature, indicating the desorption of adsorbed NOx species at high temperature. Similar effect had also reported by Chen et al.

72

The band at 1298 cm-1 nearly

vanished at 250 ℃, which should be owing to the low thermal stability of monodentate nitrate species. The DRIFT spectra of NO+O2 co-adsorption over CeBi/TO2-0.15 catalyst were basically similar with that over Ce/TiO2 catalyst, as shown in Figure7 (B). Compared with that in Figure7 (A), a slight increase of band intensities could be found in Figure7 (B), revealing the promoted adsorption of NOx species over CeBi/TiO2-0.15 catalyst. From the DRIFT spectra of NO+O2 co-adsorption over Bi/TiO2 catalyst, the adsorbed NOx species over it could be attributed to adsorbed NO2 (1608 cm-1), bidentate nitrate (1576 and 1530 cm-1) and monodentate nitrate (1269 cm-1).70,73 It was noticeable that the band intensities of the DRIFT spectra of Bi/TiO2 catalyst was much lower than that of the DRIFT spectra of Ce/TiO2 and CeBi/TiO2-0.15 catalysts, meaning the poor ability of Bi/TiO2 catalyst for NOx adsorption.

20


Page 21 of 40

(A) 1610

1233

1564 o

350 C o

Absorbance(a.u.)

300 C o

250 C o

200 C o

150 C o

100 C o

50 C

0.1

1298

2000

1800


1200

(B) 1560 1610

1235

o

Absorbance(a.u.)

350 C o

300 C o

250 C o

200 C o

150 C o

100 C o

50 C

1286

0.1

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


2000

1800


1200

Figure7 In situ DRIFT spectra of NO+O2 co-adsorption over (A) Ce/TiO2 and (B)CeBi/TiO2-0.15 catalysts at different temperature 3.8.3 Reaction between NOx and the preadsorbed NH3 species 21



The catalyst sample was first pretreated with 600 ppm NH3/N2 for 30 min, followed by purged by pure N2 for 15 min. Then 600 ppm NO+5% O2/N2 was introduced into the IR cell at 150 ℃, and the DRIFT spectra were recorded with time and illustrated in Figure8. As noted above, several bands of adsorbed NH3 species appeared in the DRIFT spectra of Ce/TiO2 catalyst (Figure8 (A)), including 1600, 1432 and 1167 cm-1. With the introduction of NO+O2, all these bands quickly disappeared in 2 min. At the same time, the characteristic bands of adsorbed NOx species appeared at 1608, 1559 and 1230 cm-l, meaning that the adsorbed NH3 species were exhausted and the surface of Ce/TiO2 catalyst was covered by adsorbed NOx species. The results indicated that all the adsorbed NH3 species were involved in the NH3-SCR reaction over Ce/TiO2 catalyst. Similar reaction process reappeared in the DRIFT spectra of CeBi/TiO2-0.15 catalyst under the same reaction conditions. Therefore, the NH3-SCR reactions over the two catalyst samples was under the rule of Eley−Rideal (E-R) mechanism.1, 72,74,75 For the DRIFT spectra of the reaction between NOx and preadsorbed NH3 species (Figure S6), the trend was similar, revealing the presence of E-R mechanism for the NH3-SCR reaction over it.

22


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Page 23 of 40

(A) 1608

1559

1230

Absorbance(a.u.)

30min 10min 5min NO+O2 2min NH3 30min 1167

0.1

1600

1312

1432

2000

1800

(B)


1200

1561 1605

Absorbance(a.u.)

1230

30min 10min 5min NO+O2 2min

1167

NH3 30min 1600

0.1

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


2000

1800


1305

1200

Figure 8. DRIFT spectra of the reaction between NOx and the preadsorbed NH3 species over (A) Ce/TiO2 and (B) CeBi/TiO2-0.15 catalysts at 150 ℃ 3.8.4 Reaction between NH3 and the preadsorbed NOx species From another side, NH3 and NOx species were introduced to the IR cell in the reversed order 23



to investigate the role of preadsorbed NOx species in the NH3-SCR reactions over Ce/TiO2 and CeBi/TiO2-0.15 catalysts, and the DRIFT spectra of this process were exhibited in Figure9. From Figure9 (A), the pretreatment of Ce/TiO2 catalyst with NO+O2 generated several kinds of adsorbed NOx species, as reflected by the bands at 1610, 1564, 1298 and 1233 cm-1. After the introduction of NH3, most of the bands quickly vanished in 2 min, accompanied by the formation and accumulation of adsorbed NH3 species with time. However, not all the preadsorbed NOx species could participate in the NH3-SCR reaction over Ce/TiO2 catalyst. The band at 1564 cm-1 kept unchanged in the reaction process, thus bidentate nitrate was inert in the SCR reaction over Ce/TiO2 catalyst. Liu et al.

62

also indicated that bidentate nitrate was the spectator in the

NH3-SCR reaction over MnCe/TiO2 catalyst. And the inertia of bidentate nitrate in the SCR reaction over Ce/TiO2 catalyst was also pointed out in our previous study.76 For the reaction between NH3 and preadsorbed NOx species over CeBi/TiO2-0.15 catalyst, the trend was basically the same. Therefore, most of the adsorbed NOx species were active in the NH3-SCR reactions over Ce/TiO2 and CeBi/TiO2-0.15 catalysts, implying the existence of Langmuir−Hinshelwood (L-H) mechanism.74 Similar situation was also applicable to the DRIFT spectra of the reaction between NH3 and the preadsorbed NOx species over Bi/TiO2 catalyst (Figure S7), not all the adsorbed NOx species could be concerned in the NH3-SCR reaction over it, which was still under the control of L-H mechanism.

24


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Page 25 of 40

(A) 1564 1593 1245

1183

Absorbance(a.u.)

30min 10min 5min NH3 2min

NO+O2 30min

1610 1233

0.1

1298

2000

1800


1200

(B) 1560 1590 1248

Absorbance(a.u.)

1180

30min 10min 5min NH3 2min

NO+O2 30min

1610 1235 1286

0.1

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


2000

1800


1200

Figure 9. DRIFT spectra of the reaction between NH3 and the preadsorbed NOx species over (A) Ce/TiO2 and (B) CeBi/TiO2-0.15 catalysts at 150℃ 3.9 Identification of the reaction mechanism 3.9.1 Kinetics study 25



Page 26 of 40

As previously described, both E-R and L-H mechanisms were applicable to the NH3-SCR reactions over Ce/TiO2 and CeBi/TiO2-0.15 catalysts. Thus the total SCR reaction rate could be written as: , = ,-. + ,0&

(3)

where r was the total SCR reaction rate, ,-. and ,0& were the SCR reaction rates through E-R and L-H routes respectively. For a NH3-SCR reaction through E-R route, the reaction took place between adsorbed NH3 species and gaseous NO, so the reaction with respect to NH3 and NO should be 0 and 1 respectively. In the NH3-SCR reaction through L-H route, adsorbed NOx reacted with adsorbed NH3 species, thus the reaction orders with respect to both NO and NH3 were all 0. Therefore, ,-. could be described by: ,-. = 12 [NO]

(4)

Thus Equation (3) could be expressed by: , = 12 [NO] + ,0&

(5)

Based on Equation (5), effect of NO inlet concentration on SCR reaction ration could be obtained, as presented in FigureS3. To meet the condition of differential reactor model, a large GHSV value of 600, 000 mL·g-1·h-1 was used for SCR activity test. From FigureS3, a good linear relationship could be observed (r2>0.97), further confirming the coexistence of E-R and L-H mechanisms for the NH3-SCR reactions over the two catalyst samples. 3.9.2 Reaction mechanism By virtue of the method proposed by Yang et al.,77 the contributions of E-R and L-H mechanisms on the NH3-SCR reactions over Ce/TiO2 and CeBi/TiO2-0.15 catalysts could be 26


Page 27 of 40 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


obtained, as shown in Figure10. Here the SCR reaction rate was calculated by a normalization based on SBET. And the ratio of CL-H/ (CE-R++CL-H) represented the contribution of L-H mechanism to the NH3-SCR reaction. From Figure 10(A) and Figure10 (B), it was noticeable that the addition of Bi on Ce/TiO2 catalyst could distinctly enhance the contribution of L-H mechanism to the NH3-SCR reaction over it, which should be resulted from the promoted NO oxidation and the formation of more adsorbed NH3 species on CeBi/TiO2-0.15 catalyst. The corresponding reaction routes could be expressed by:

74

According to the study of Savara et al.,

78

NH+ NO could

decompose to N2 and H2O under 100 ℃, which is a vital step for low-temperature SCR reaction. NO + O (g) → NO (a) 9:;
? NH8(a) (Lewis acid sites)

(7)

NO (a) + 2NH8 (a) + NO(g)→ 2N +3H O

(8)

9:'
? NH+% (a) (Brønsted acid sites) NH+% (a) + e + NO (a) → NH+ NO (a) → N + 2H O

27


(9) (10)


6

1.0 (A)

L-H mechanism E-R mechanism

0.8

4 0.6

0.4 2

CL-H/(CL-H+CE-R)

Normalized reaction -9 -1 2 rate (10 mol.s .m )

CL-H/(CL-H+CE-R)

0.2

0

0.0 100

150

200 250 300 o Reaction temperature ( C)

350

400

1.0

6 L-H mechanism E-R mechanism

CL-H/(CL-H+CE-R)

0.8

4 0.6

0.4 2

CL-H/(CL-H+CE-R)

(B)

Normalized reaction -9 -1 2 rate (10 mol.s .m )

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 28 of 40

0.2

0.0

0 100

150


350

400

Figure10 Contributions of E-R and L-H mechanisms to the NH3-SCR reactions over (A) Ce/TiO2 and (B) CeBi/TiO2-0.15 catalysts Reaction conditions: [NO] = [NH3] =600 ppm, [O2] =5%, balance Ar, GHSV=600, 000 mL·g-1·h-1 4. Conclusions In this study, the experimental results indicated that the modification of Ce/TiO2 catalyst with 28


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proper amount of Bi (Bi/Ce molar ratio=0.15) could effectively enhance its low-temperature SCR performance and the tolerance to SO2/H2O. After the introduction of Bi to Ce/TiO2 catalyst, characterization results revealed that the generation of more Ce3+ and surface chemisorbed oxygen species. Moreover, more adsorbed NH3 species with high activity were detected on the surface of CeBi/TiO2-0.15 catalyst. All these features were propitious to the NH3-SCR reaction over CeBi/TiO2-0.15 catalyst through L-H pathway, leading to its excellent SCR performance in the low-temperature range. Supporting Information XRD patterns, NO oxidation activities, SCR reaction rates at different NO and NH3 inlet concentrations, NH3 adsorption DRIFT spectra over Bi/TiO2, NO+O2 co-adsorption DRIFT spectra over Bi/TiO2, DRIFT spectra of NO+O2 reacted with preadsorbed NH3 species over Bi/TiO2 at 150 ℃, DRIFT spectra of NH3 reacted with preadsorbed NOx species over Bi/TiO2 at 150 ℃ Acknowledgments This work was supported by the Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800).

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