Ce-Sn binary oxide catalyst for the selective catalytic reduction of NOx by NH3

Applied Surface Science 428 (2018) 526–533

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Ce-Sn binary oxide catalyst for the selective catalytic reduction of NOx by NH3 Zhiming Liu a,c,∗ , Xu Feng a , Zizheng Zhou a , Yongjun Feng a , Junhua Li b,∗ a b c

State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China State Key Joint Laboratory of Environment Simulation and Pollution Control, School of Environment, Tsinghua University, Beijing 100084, China Beijing Key Laboratory of Energy Environmental Catalysis, Beijing University of Chemical Technology, Beijing 100029, China

a r t i c l e

i n f o

Article history: Received 21 August 2017 Received in revised form 18 September 2017 Accepted 20 September 2017 Available online 21 September 2017 Keywords: Nitrogen oxides NH3 -SCR Ce-Sn catalyst

a b s t r a c t Ce-Sn binary oxide catalysts prepared by the hydrothermal method have been investigated for the selective catalytic reduction (SCR) of NOx with NH3 . Compared with pure CeO2 and SnO2 , Ce-Sn binary oxide catalyst showed significantly higher NH3 -SCR activity. Moreover, Ce-Sn catalyst showed high resistance against H2 O and SO2 . The high catalytic performance of Ce-Sn binary oxide is attributed to the synergetic effect between Ce and Sn species, which not only enhances the redox property of the catalyst but also increases the Lewis acidity, thus promoting the adsorption and activation of NH3 species, which contributes to improving the NH3 -SCR performance. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Nitrogen oxides (NOx ), which can cause a variety of serious environmental issues, such as photochemical smog, acid rain and ozone depletion, have remained as a major source for air pollution [1,2]. The removal of NOx can be carried out by selective catalytic reduction (SCR) of NOx with NH3 (NH3 -SCR), and V2 O5 -WO3 /TiO2 catalyst is widely used for this process [3,4]. However, V2 O5 WO3 /TiO2 catalyst is not satisfactory with respect to the narrow temperature window and the toxicity of vanadium species. Therefore, many efforts have been tried by the researchers to develop efficient and environmentally benign catalyst system for NH3 -SCR of NOx [5–7]. Attracted by the unique abilities of CeO2 , over which the electron transfer can be shifted easily between the reduced and oxidized states (Ce3+ ↔ Ce4+ ) and variable levels of oxygen vacancies can be accommodated, Ce-based catalysts have been explored for the NH3 -SCR of NOx recently [8–10]. MnOx -CeO2 mixed oxide was found to be very active for the reduction of NOx in the low temperature range (100–200 ◦ C) [11,12]. Over Ce-Ta catalyst the strong interaction between Ce and Ta induces the formation of abun-

∗ Corresponding authors. State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, China. E-mail addresses: [email protected] (Z. Liu), [email protected] (J. Li). http://dx.doi.org/10.1016/j.apsusc.2017.09.175 0169-4332/© 2017 Elsevier B.V. All rights reserved.

dant Ce3+ and chemisorbed oxygen species, thus leading to the high SCR activity [13]. He et al. [14] reported that CeO2 -WO3 catalysts prepared by the homogeneous precipitation method exhibited high SCR activities in a wide operating temperature window. In the case of VOx /CeO2 nanorods catalyst [15], although the formed oligomeric, polymeric VOx and CeVO4 on the surface of CeO2 led to the decrease of the reducibility and surface oxygen defect of the catalyst, the existence of CeVO4 enhanced the Brønsted acid sites, thus improving the number of active sites for the reduction of NOx . Over Ce-W [14,16] and Ce-V [15] binary oxide catalysts, the strong interaction between the different metals contributes to improving the NH3 -SCR activity. SnO2 is a semiconducting metal oxide and can form oxygen vacancies [17]. Our previous research found that SnO2 is the active site for the reduction of NOx with propene [18]. Yu et al. [19] found that SnO2 showed a promoting effect on the activity of CeO2 /TiO2 catalysts. The addition of SnO2 to MnOx -CeO2 also resulted in an enhanced low-temperature activity [20], however, the selectivity to N2 of this catalyst needs to be improved. Nbx SnCeO␦ catalyst was found to be active for the low-temperature NH3 -SCR of NOx [21]. DFT calculations demonstrated that the deposition of Sn on CeO2 led to the reduction of Ce4+ to Ce3+ and the activation of surface oxygen [22]. Zhang et al. [23] reported that the interaction between Ce and Sn species is very important for the NH3 -SCR activity of Ce-Sn based catalyst. The present work attempts to develop Ce-Sn binary oxide catalyst by the hydrothermal method, which has been proved to

Z. Liu et al. / Applied Surface Science 428 (2018) 526–533

be a useful method to prepare the mixed oxide catalyst over which there is strong interaction between the different oxides [24,25]. Herein, Ce-Sn binary oxide catalyst prepared by hydrothermal method is demonstrated to be active for the NH3 -SCR of NOx . In addition, this type of catalyst exhibited high resistance against H2 O and SO2 . N2 -adsorption, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2 temperature programmed reduction (H2 -TPR), temperature programmed desorption of NH3 (NH3 -TPD) and in situ diffuse reflectance infrared fourier transform spectroscopy (DRIFTS) have been conducted to reveal the physicochemical properties of Ce-Sn binary oxide catalyst as well as the reaction mechanism. 2. Experimental section 2.1. Catalyst preparation Series of Ce-Sn-Ox oxide catalysts with different Ce/Sn molar ratio (denoted as CeaSnb, where a/b represents the Ce/Sn molar ratio) were prepared by the hydrothermal method. A certain amount of Ce(NO3 )3 ·6H2 O and SnCl4 ·5H2 O were dissolved in deionized water at room temperature and stirred for 1 h, subsequently ammonia solution was added slowly to the above solution with stirring until pH is ca. 10, then the obtained suspension was transferred to a Teflon-sealed autoclave and aged at 120 ◦ C for 48 h. The precipitate was filtered and washed with deionized water. Subsequently the powder was dried at 120 ◦ C for 12 h, and then calcined at 500 ◦ C for 6 h in air. Pure CeO2 and SnO2 catalysts were also prepared as reference samples using the same method as described above. 2.2. Catalytic performance measurement The NH3 -SCR performance was measured in a fixed-bed quartz reactor from 200 to 450 ◦ C. The typical gas component was composed of 500 ppm NO, 500 ppm NH3 , 5% O2 , 5% H2 O (when used), 50 ppm SO2 (when used) and balance N2 . The catalyst used for the test is 0.12 g and the total gas flow rate is 300 cm3 min−1 , yielding a GHSV of 128,000 h−1 . The concentrations of the inlet and outlet gas were analyzed by a chemiluminescence NO/NO2 analyzer (Thermo Scientific, model 42i-HL) and a FTIR gas analyzer (Gasmet FTIR DX4000). The data were recorded when the catalytic reaction practically reached steady-state condition at each temperature. The NOx conversion and N2 selectivity were calculated via the following equations: NOX conversion = N2 selectivity =

[NOx ]in − [NOx ]out × 100% [NOx ]in

[NOx ]in + [NH3 ]in − [NOx ]out − [NH3 ]out − 2[N2 O]out × 100% [NOx ]in + [NH3 ]in − [NOx ]out − [NH3 ]out

527

an ESCALab220i-XL electron spectrometer from VG Scientific, using Mg K␣ radiation at an accelerating power of 300 W, and the binding energy was calibrated by using C 1 s peak (BE = 284.8 eV). H2 -TPR and NH3 -TPD experiments were conducted on a chemisorption analyzer (Micromeritics, ChemiSorb 2720 TPx). Prior to H2 -TPR the sample was first pretreated in a flow of N2 at 400 ◦ C for 1 h, then cooled down to the room temperature. Subsequently, H2 -TPR runs were conducted by heating the sample at a rate of 10 ◦ C min−1 in a flow of 10% H2 /N2 (50 mL min−1 ). In the case of NH3 -TPD, the sample was pretreated at 350 ◦ C under He flow for 1 h, then it was cooled down to 100 ◦ C and saturated with 10%NH3 /He for 1 h, followed by purging with He for 30 min. Finally, the sample was heated from 100 to 550 ◦ C at a rate of 10 ◦ C min−1 in flowing He. In-situ DRIFTS spectra were collected on a FTIR spectrometer (Nicolet NEXUS 6700) with a Harrick IR cell and a MCT/A detector cooled by liquid N2 . Prior to each experiment, the sample was pretreated in a flow of N2 at 400 ◦ C for 1 h then cooled down to the desired temperature. A background spectrum was collected under a flow of N2 and was subtracted from the sample spectra. All the IR spectra were acquired by accumulating 100 scans with a resolution of 4 cm−1 .

3. Results and discussion 3.1. NH3 -SCR performance The NH3 -SCR performance of CeO2 , SnO2 and Ce-Sn binary oxide catalysts was compared and the results are shown in Fig. 1. As presented in Fig. 1(a), both pristine CeO2 and SnO2 exhibited poor NH3 -SCR activity. It is evident that Ce-Sn catalysts exhibited significantly higher NH3 -SCR activity than pure CeO2 and SnO2 in the whole temperature region investigated. It is interesting to note that NOx conversion seldom changed as the Ce/Sn molar ratio increased from 1/3 to 3/1. Over 84% of NOx conversion was obtained in the temperature range of 300–450 ◦ C. For Ce-Sn catalysts nearly 100% N2 selectivity was achieved, which is slightly higher than those of pure CeO2 and SnO2 . The above results indicate that the coexistence of Ce and Sn is crucial for the enhanced SCR activity and there is synergetic effect between them.

3.2. Effects of H2 O and SO2 (1) (2)

Where [NOx ]in and [NOx ]out represent the concentration of NOx in the inlet and outlet gas, respectively. Similarly, [NH3 ]in , [NH3 ]out and [N2 O]out are the concentration of NH3 in the inlet gas, outlet gas, and N2 O in the outlet gas, respectively. 2.3. Catalyst characterization N2 adsorption-desorption isotherms were measured by using a Quantachrome Nova Automated Gas Sorption System at a liquid nitrogen temperature. The samples were degassed in a vacuum at 300 ◦ C for 4 h before the N2 physisorption. The specific surface area and the pore volume were calculated by the Brunauer-EmmettTeller (BET) method and the Barrett-Joyner-Halenda (BJH) method, respectively. XRD pattern was collected on an X-ray diffractometer (Bruker D8 ADVANCE) using Cu K␣ radiation. XPS data were recorded on

Since flue gas and diesel exhaust contain H2 O and SO2 , which usually showed poisoning effect in the NH3 -SCR of NOx , understanding of the resistance against H2 O and SO2 is important for the development and application of NH3 -SCR catalysts. The effects of H2 O and SO2 on the activity of Ce1Sn1 catalyst was investigated as a function of time on stream at 350 ◦ C, and the results are shown in Fig. 2. It was found that the NOx conversion is slightly decreased with the introducing of H2 O, and the conversion can recover soon after turning off the H2 O entrance. The competitive adsorption between H2 O and NH3 on the surface of the catalyst could account for the deactivation [26]. The NOx conversion under the co-presence of H2 O and SO2 is almost the same as that with only H2 O in the feeding gas, indicating that there is no synergistic inhibiting effect between H2 O and SO2 on the SCR activity. Compared with the previous reported Ce-Sb catalyst [26], Ce-Sn catalyst is of higher resistance against H2 O and SO2 . Therefore, Ce1Sn1 catalyst is of high tolerance to H2 O and SO2 . Having high SCR activity and strong tolerance to H2 O and SO2 , Ce-Sn oxide catalyst seems to have great potential as an effective NH3 -SCR catalyst.

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Z. Liu et al. / Applied Surface Science 428 (2018) 526–533 Table 1 BET surface area and pore structure results of the different catalysts.

CeO SnO

CeO2 SnO2 Ce1Sn1 Ce1Sn3 Ce3Sn1

2 2

Ce1Sn1 Ce1Sn3 Ce3Sn1

BET(m2 /g)

Pore volume(cm3 /g)

Pore size(nm)

46 25 63 49 61

0.14 0.09 0.34 0.27 0.30

12.36 14.75 21.27 22.26 19.62

CeO 2 SnO 2 CeO 2

0 200

250

300

350

400

450 Ce3Sn1

Temperature (ºC ) Ce1Sn1

(b)

Ce1Sn3

CeO SnO

SnO 2 2 2

10

Ce1Sn1 Ce1Sn3 Ce3Sn1

20

30

40

50

60

Fig. 3. XRD patterns of CeO2 , SnO2 and Ce-Sn binary oxide catalysts.

3.3. Catalyst characterization 0 200

250

300

350

Temperature (

400

450

)

Fig. 1. NH3 -SCR activity (a) and N2 selectivity (b) of CeO2 , SnO2 and CeSn binary oxide catalysts. Reaction conditions: [NO] = [NH3 ] = 500 ppm, [O2 ] = 5%, GHSV = 128,000 h−1 .

80

60

on

off

40

3.3.1. BET and XRD The BET surface area, pore volume and pore diameter of the studied catalysts are summarized in Table 1. Compared with CeO2 and SnO2 , Ce-Sn binary oxide catalysts possessed larger surface area, which indicates that the co-presence of CeO2 and SnO2 induced the structural modification of the samples. Fig. 3 displayed the XRD patterns of CeO2 , SnO2 and Ce-Sn binary oxide catalysts. The typical peaks assigned to CeO2 [14] and SnO2 [27] were observed over CeO2 and SnO2 catalysts, respectively. Over Ce3Sn1 catalyst the intensity of the peak assigned to CeO2 is decreased due to the introduction of SnO2 , and the peak ascribed to SnO2 is absent. Compared with pure CeO2 , the peaks related to CeO2 over Ce3Sn1 catalyst is shifted to high angle direction slightly, suggesting that Sn4+ has been successfully incorporated into the lattice of CeO2 to form uniform solid solution (containing −Ce4+ -O-Sn4+ -species). With the increasing the molar ratio of Sn/Ce, the peak related to SnO2 appeared, and the peak ascribed to CeO2 were broadened and shifted to a higher angle further. The peak shift indicates that over the Ce-Sn binary oxide catalysts the strong interaction between Ce and Sn species existed.

H 2O

20

H 2O+SO

2

0

Fig. 2. Effects of H2 O and SO2 on the activity of Ce1Sn1 catalyst at 350 ◦ C. Reaction conditions: [NO] = [NH3 ] = 500 ppm, [O2 ] = 5%, [H2 O] = 5%, [SO2 ] = 50 ppm, GHSV = 128,000 h−1 .

3.3.2. XPS XPS analysis was conducted to reveal the chemical state of the active metal in the catalysts. The XPS spectra of Ce 3d, which was decomposed into eight components, are illustrated in Fig. 4(a). The sub-bands denoted as u’ and v’ represent the 3d10 4f1 initial electronic state of Ce3+ , and the other six bands labeled as u, u”, u”’, v, v” and v”’ related to the 3d10 4f0 initial electronic state of Ce4+ [24,28]. The ratio of Ce3+ over Ce-Sn binary oxide catalyst is higher than that of the pure CeO2 . This fact indicates that the introduction of Sn leads to the conversion of Ce4+ to Ce3+ . Fig. 4(b) displayed the Sn3d XPS spectra. For SnO2 , the binding energies of Sn 3d5/2 and Sn 3d3/2 are centered at 495.2 and 486.9 eV, respectively. It is noted that

Z. Liu et al. / Applied Surface Science 428 (2018) 526–533

(a)

529

3+

Ce

Ce 4+ Ce U'''

U

CeO 2

U''

3+

ratio

V''' V

U' V''

V'

Ce3Sn1

14.2%

Ce1Sn1

24.5% 20.1%

Ce1Sn3 29.0%

920

910

900

890

880

Binding energy (eV)

(b)

Sn3d 3/2

Sn3d 5/2 486.9

495.2

SnO 2 494. 9

486. 4

Ce1Sn3 Ce1Sn1 Ce3Sn1

500

498

496

494

492

490

488

486

484

482

Fig. 4. Ce3d (a) and Sn3d (b) XPS spectra of different catalysts.

the peaks of Ce-Sn catalysts shifted to a lower binding energy compared to those of pure SnO2 , indicating that excess electrons existed around Sn over the Ce-Sn catalyst, which is induced via the redox equilibrium of 2Ce4+ + Sn2+ ↔ 2Ce3+ + Sn4+ [27,29]. This fact further demonstrated the existence of the synergetic effect between Ce and Sn. The spectra of O 1s were also investigated and illustrated in Fig. 5. By performing a peak-fitting deconvolution two types of oxygen species are identified. The peak at higher binding energy (531–533 eV) was considered to be the surface adsorbed oxygen species (O␣ ), and the band at lower binding energy (528.5–531 eV) was ascribed to the lattice oxygen species (O␤ ) [10,25]. In the case of Ce-Sn catalyst, the binding energy of O␤ is located between those of pure CeO2 and SnO2 , further demonstrating that there is strong interaction between Ce and Sn species. It is evident that relative concentration ratio of O␣ over Ce-Sn catalyst is higher than those of pure CeO2 and SnO2 , indicating that the surface oxygen species is increased due to the co-presence of CeO2 and SnO2 . The intro-

duction of Sn induces the transformation of Ce4+ to Ce3+ , which could create oxygen vacancies and unsaturated chemical bonds over the catalyst surface, thus leading to an increase of the surface chemisorbed oxygen. The abundant surface absorbed oxygen was proposed to be beneficial for the NH3 -SCR of NOx [30,31]. 3.3.3. H2 -TPR Since the redox property was a key factor determining the activity of NH3 -SCR catalyst, H2 -TPR was conducted to compare the reducibility of CeO2 , SnO2 and Ce-Sn binary oxide catalysts and the obtained profiles were illustrated in Fig. 6. Pure SnO2 catalyst showed a reduction peak at 515 ◦ C, which could be due to the reduction of Sn4+ [32]. In the case of CeO2 , two reduction peaks appeared at around 461 and 780 ◦ C. The former peak is probably ascribed to the reduction of the surface oxygen of ceria (Ce4+ -O-Ce4+ ) and the second one is related to the reduction of Ce4+ to Ce3+ [24,33]. Interestingly, Ce1Sn3 catalyst exhibited a single peak centering around 565 ◦ C, which could be due to the co-reduction of surface species

530

Z. Liu et al. / Applied Surface Science 428 (2018) 526–533

O

O ratio

O CeO 2

27.3%

Ce3Sn1

61.1%

Ce1Sn1

51.0%

Ce1Sn3

56.7% O

O

SnO 2

536

43.1%

534

532

530

528

526

Fig. 5. XPS spectra of O1s for CeO2 , SnO2 and Ce-Sn binary oxide catalysts.

3.4. In-situ DRIFTS studies

515

565

SnO 2 Ce1Sn3

473

Ce1Sn1 Ce3Sn1 CeO 2

200

287 665

425

400

Fig. 7. NH3 -TPD profiles of CeO2 , SnO2 and Ce-Sn binary oxide catalysts.

600

800

Fig. 6. H2 -TPR profiles of CeO2 , SnO2 and Ce-Sn binary oxide catalysts.

including Ce4+ and Sn4+ . The lower reduction temperature compared with CeO2 indicates that the presence of SnO2 makes Ce4+ more reducible. With increasing the ratio of Ce/Sn, the reduction peak is shifted to lower temperatures, and over Ce3Sn1 catalyst the reduction temperature is 287 ◦ C. This fact indicates that over Ce1Sn1 and Ce3Sn1 catalysts the strong interaction between Ce and Sn makes both Ce4+ and Sn4+ become more reducible. Therefore, H2 -TPR results also demonstrated that there existed a synergetic effect between Ce and Sn, which effectively promoting the redox properties of the catalyst. 3.3.4. NH3 -TPD Fig. 7 illustrates the NH3 -TPD profiles of CeO2 , SnO2 and Ce-Sn catalysts. Little NH3 was desorbed from SnO2 . Over CeO2 a NH3 desorption peak appeared at 426 ◦ C, which is ascribed to strong acid sites [13]. It is obvious that over Ce-Sn binary oxide catalyst the area of the NH3 desorption peak related to strong acid sites is higher than that of CeO2 , and Ce1Sn3 possesses the highest strong acid sites. Over Ce1Sn1 and Ce3Sn1 catalysts, the NH3 desorption peak related to weak acid sites can also be observed. Therefore, the co-existence of Ce and Sn species resulted in enhanced surface acidity of the catalyst, which is beneficial for the improvement of the NH3 -SCR activity [5].

3.4.1. Adsorption of NH3 Fig. 8 presented the DRIFT spectra of NH3 adsorption on CeO2 , SnO2 and Ce1Sn1 catalysts at different temperatures. As shown in Fig. 8(a), over CeO2 catalyst the bands respectively ascribed to the deformation species of adsorbed NH3 (1592 and 1288 cm−1 ) [26], amid (NH2 ) species(1556 and 1527 cm−1 ) [34,35], NH4 + species on Brønsted acid sites (1467 and 1417 cm−1 ) [10], the intermediates of ammonia oxidation(1388 and 1355 cm−1 ) [36], coordinated NH3 on Lewis acid sites (1132 cm−1 ) [25,37] and the N H stretching vibration modes of the coordinated NH3 (3363 and 3239 cm−1 ) [38] are observed. Above 200 ◦ C the peak ascribed to NH3 on Lewis acid sites disappear, only the bands related to NH2 species, NH4 + species on Brønsted acid sites and the intermediates of ammonia oxidation can be observed. In the case of pure SnO2 , the adsorption of NH3 is very weak and only one peak ascribed to NH3 on Lewis acid sites (1238 cm−1 ) [39] is observed. From Fig. 8(c), it can be seen that the intensities of the peaks ascribed to the adsorbed NH3 on Lewis acid sites is higher than those observed on CeO2 and SnO2 catalysts. The amount of Lewis acid sites played a significant role in the reduction of NOx with NH3 , and more Lewis acid sites contribute to the enhanced NH3 -SCR activity [10]. Even above 300 ◦ C the noticeable peaks assigned to NH3 on Lewis acid sites, NH4 + species on Brønsted acid sites and the intermediates of ammonia oxidation are still observed. This is in accordance with NH3 -TPD results. The coexistence of Ce and Sn leads to more NH3 adsorbed and activated, which is significantly beneficial to NH3 -SCR reaction. 3.4.2. Co-adsorption of NO + O2 The DRIFT spectra of NO + O2 adsorption over CeO2 , SnO2 and Ce1Sn1 catalysts at different temperatures are shown in Fig. 9. Over CeO2 catalyst several bands at 1693, 1608, 1570, 1535, 1442, 1352, 1273, 1238, 1218 and 1101 cm−1 were detected (see Fig. 9(a)), which are respectively assigned to N2 O4 (1693 cm−1 ) [40], monodentate nitrite(1608 cm−1 ) [41], bidentate nitrate(1570 and 1535 cm−1 ) [25,42], trans-N2 O2 2− species(1442 and 1101 cm−1 ) [26,34], cis-N2 O2 2− species(1352 cm−1 ) [26], chelating nitrite (1273 cm−1 ) [25] and bridged nitrate(1238 and 1218 cm−1 ) [5,26]. As shown in Fig. 9(b), the adsorption of NO + O2 over SnO2 is very weak. Only four bands at 1519, 1432, 1362 and 1334 cm−1 appeared, which are respectively due to bidentate nitrate(1519 cm−1 ) [43], trans-N2 O2 2− species(1432 cm−1 ) [26], cis-N2 O2 2− species(1362 and 1334 cm−1 ) [26]. In the case of Ce1Sn1 catalyst(see Fig. 9(c)), the DRIFT spectra of NO + O2 adsorption are

200 150

1400

0.1

-1

1132

150 100 1442

1288

200

2000

1200

1800

1600

1400

1200 -1

Wavenumber (cm

)

(b)

(b) 1519

1238

0.5

350 300

350

250

)

1362 1334

Wavenumber (cm

250

1432

1600

1417 1388

3363 3239

1592 1556 1527

100

300

1273 1238

250

Absorbance (a.u.)

350

300

1800

1101

1693

1355

1467

350

4000 3500 2000

(a)

0.5

1218

(a)

1608 1570 1535

0.1

531

1352

Z. Liu et al. / Applied Surface Science 428 (2018) 526–533

300

200

250

150

200

100

150 100

350

350

300

300

250

250

200 150

200

100

100

1200 -1

)

(c) 1693

1373

1461

3239

0.5

1153

1558 1600

1238

150

3363

1800

1400

1218

(c)

4000 3500 2000

1600

Wavenumber (cm

)

1168

0.1

-1

1800

1273

Wavenumber (cm

2000

1200

1352

1400

1461

1600

1608 1570 1535

1800

1249

4000 3500 2000

1400

1200

)

2000

1800

1600

1400

1200 -1

Fig. 8. DRIFT spectra of NH3 adsorption on CeO2 (a), SnO2 (b) and Ce1Sn1 (c) catalysts at different temperatures.

Fig. 9. DRIFT spectra of NO + O2 adsorption on CeO2 (a), SnO2 (b) and Ce1Sn1 (c) catalysts at different temperatures.

similar to those of CeOx catalyst, however, the intensities of the bands become weaker. As shown in NH3 -TPD and DRIFTS spectra of NH3 adsorption, the introduction of SnO2 to CeO2 resulted in more acid sites formed, thus reducing the alkali sites where nitrate species adsorbed. Therefore, the co-presence of Ce and Sn suppressed the adsorption of NOx species.

3.4.3. Reactivity of surface adsorbed species In order to investigate the reactivity of adsorbed NOx species in the SCR reaction on Ce1Sn1 catalyst, the dynamic change of in-situ DRIFT spectra of the reaction between preadsorbed NOx and NH3 at 350 ◦ C is recorded. As presented in Fig. 10, no obvious decrease of the intensities of the peaks ascribed to N2 O4 (1693 cm−1 ), bidentate nitrate(1570 and 1535 cm−1 ), trans-N2 O2 2− species(1470 cm−1 ),

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Z. Liu et al. / Applied Surface Science 428 (2018) 526–533

1273 1215 1362

1596 1570 1535 1470

NH 3 60min

1693

3313

0.1

NH 3 30min NH 3 20min NH 3 10min NH 3 5min NH 3 2min NO+O 2 60min

4000 3500 2000

1800

1600

1400 -1

1200

)

1206

1273

1373

1461

NO+O 2 60min

1693

0.1

1601 1570 1535

Fig. 10. Dynamic changes of the in situ DRIFT spectra in a flow of NH3 over Ce1Sn1 catalyst pre-exposed to NO + O2 for 60 min followed by N2 purging for 30 min at 350 ◦ C.

NO+O 2 30min NO+O 2 20min NO+O 2 10min NO+O 2 5min NO+O 2 2min

4000 3500 2000

1800

1600

1400

1168

3329

1249

NH 3 60min

1200

it can be seen that over Ce-Sn catalyst Sn4+ is incorporated into the lattice of CeO2 , resulting in the strong interaction between Ce and Sn species. XPS results showed that over Ce-Sn binary oxide catalysts there are two redox couples (Ce4+ /Ce3+ and Sn4+ /Sn2+ ). The strong interaction between Ce and Sn can promote the electron transfer by the redox cycle of 2Ce4 + Sn2+ ↔ 2Ce3+ + Sn4+ . It has been reported that the redox property is closely related to the catalytic cycle [44,45]. The redox property of Ce-Sn binary oxide catalyst is improved as shown in H2 -TPR profiles due to the promoted electron transfer between Ce and Sn species. Previous research demonstrated that both the redox property and the surface acidity of the catalyst played important roles in NH3 -SCR activity [5,25]. As shown in H2 -TPR and NH3 -TPD profiles (see Figs. 6 and 7), for the three Ce-Sn catalysts, the redox property is decreased in the order of Ce3Sn1 > Ce1Sn1 > Ce1Sn3, whereas the surface acidity is decreased in the reverse order of Ce1Sn3 > Ce1Sn1 > Ce3Sn1. Both the redox property and surface acidity, collectively, explain the similar NH3 -SCR activity of these three Ce-Sn catalysts (see Fig. 1). Over Ce-Sn catalyst, the adsorbed NOx is not reactive in the NH3 -SCR of NOx (see Fig. 10). While over Ce-Ta catalyst [13], both bridging and bidentate nitrate species can react with the coordinated NH3 on the Lewis acid sites. This fact indicates that the combined metal with Ce can affect the reactivity of adsorbed NOx species. Previous research demonstrated that the adsorption and activation NH3 is a key step in the NH3 -SCR of NOx [5,30]. The electron transfer would contribute to the adsorption and activation of NH3 [24,25]. Compared with pure CeO2 and SnO2 , the synergetic effect between Ce and Sn over Ce-Sn catalyst promotes the adsorption of NH3 on Lewis acid sites (see Fig. 8), which is very reactive for the reduction of NOx (see Fig. 11). Lietti et al. [47] also reported that the adsorbed NH3 on the Lewis acid sites is reactive and it react with gas-phase NO to form N2 . Since the adsorbed nitrate species over Ce-Sn catalyst is not reactive, the NH3 -SCR reaction over Ce-Sn binary oxide catalyst mainly followed the Eley-Rideal mechanism, in which adsorbed NH3 species reacts with gaseous NO to form N2 and H2 O. The synergetic effect between Ce and Sn not only enhances the redox property of the catalyst but also increases the Lewis acidity, both of which contribute to improving the NH3 -SCR performance. 4. Conclusions

Fig. 11. Dynamic changes of the in situ DRIFT spectra in a flow of NO + O2 over Ce1Sn1 catalyst pre-exposed to NH3 for 60 min followed by N2 purging for 30 min at 350 ◦ C.

cis-N2 O2 2− species(1362 cm−1 ), chelating nitrite (1273 cm−1 ) and bridged nitrate(1215 cm−1 ) is observed after exposing NH3 flow even for 60 min, suggesting that they are spectator species in the NH3 -SCR reaction. The intensity of the band assigned to monodentate nitrite(1596 cm−1 ) was slowly decreased and the band disappeared until 60 min. The reactivity of adsorbed NH3 species in SCR reaction on Ce1Sn1 catalyst was also investigated by the transient study at 350 ◦ C, and the results are shown in Fig. 11. Switching the gas to NO + O2 results in the decrease of the intensities of all bands assigned to ammonia species. The peaks due to adsorbed NH3 on Lewis acid sites(1249 and 1168 cm−1 ) vanished in 5 min, and those assigned to NH4 + species on Brønsted acid sites (1461 cm−1 ) and the intermediates of ammonia oxidation(1373 cm−1 ) also disappeared as the reaction going on. Meanwhile some new bands ascribed to NOx species appeared. This fact indicates that the adsorbed NH3 on Lewis acid sites are very reactive in the NH3 -SCR of NOx . The strong interaction between different metal oxides is beneficial for the activity enhancement [25,44–46]. From the XRD analysis

Novel Ce-Sn binary oxide catalyst for the selective catalytic reduction of NOx has been developed. The binary oxide catalyst exhibited much higher activity than pure CeO2 and SnO2 . Moreover, Ce-Sn catalyst exhibited high tolerance to H2 O and SO2 , which is very promising for practical applications in the control of NOx emissions. The synergetic effect between Ce and Sn is responsible for the high NH3 -SCR performance of Ce-Sn catalyst. In-situ DRIFTS studies revealed that the co-presence of Ce and Sn not only inhibited the adsorption of inactive NOx species, but also promotes the formation of reactive NH3 on Lewis acid sites, which can react with gaseous NO to form N2 and H2 O following the Eley-Rideal mechanism. Acknowledgements This research was financially supported by the National Natural Science Foundation of China (21377010, 21677008, 21611130170) and the Beijing Municipal Natural Science Foundation (8162030). References [1] Z. Liu, S.I. Woo, Recent advances in catalytic DeNOx science and technology, Catal. Rev. Sci. Eng. 48 (2006) 43–89.

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