The poisoning effect of PbO and PbCl2 on CeO2-TiO2 catalyst for selective catalytic reduction of NO with NH3

Journal of Colloid and Interface Science 528 (2018) 82–91

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Regular Article

The poisoning effect of PbO and PbCl2 on CeO2-TiO2 catalyst for selective catalytic reduction of NO with NH3 Ye Jiang ⇑, Guitao Liang, Changzhong Bao, Mingyuan Lu, Chengzhen Lai, Weiyun Shi College of Pipeline and Civil Engineering, China University of Petroleum, 66 Changjiang West Road, Qingdao 266580, China

g r a p h i c a l a b s t r a c t

a r t i c l e

i n f o

Article history: Received 10 February 2018 Revised 27 April 2018 Accepted 21 May 2018 Available online 22 May 2018 Keywords: NH3-SCR NO CeO2-TiO2 Pb poisoning Comparison

a b s t r a c t The poisoning effect of PbO and PbCl2 on CeO2-TiO2 catalyst for selective catalytic reduction of NO with NH3 was investigated and compared. Both Pb species could deactivate the CeO2-TiO2 catalyst and PbO had a stronger poisoning effect than PbCl2. From the characterization results of BET, XRD, XPS, NH3-TPD and H2-TPR, it was concluded that the more serious deactivation by PbO could be ascribed to smaller BET surface area, fewer surface Ce3+ and chemisorbed oxygen, stronger interaction between PbO and CeO2-TiO2 catalyst, lower redox properties and surface acidity. The in situ DRIFT study results revealed that the NH3-SCR reaction over CeO2-TiO2 catalyst was governed by both E-R and L-H mechanisms, which wasn’t changed over the Pb-poisoned samples. The greater loss of Brønsted acid sites attributed to fewer surface Ce3+ and more serious inhibition of NO oxidation to NO2 due to fewer surface chemisorbed oxygen were two key factors responsible for more serious deactivation by PbO. Furthermore, the presence of Pb species inhibited the NH3 adsorption on the Lewis acid sites, aggravating the deactivation of CeO2-TiO2 catalyst. Ó 2018 Elsevier Inc. All rights reserved.

1 Introduction Selective catalytic reduction of NOx with NH3 (NH3-SCR) as an effective method for controlling NOx emissions has been widely used in stationary and mobile source applications in recent decades [1–4]. The conventional V2O5-WO3 (or MoO3)/TiO2 SCR ⇑ Corresponding author at: 66 Changjiang West Road, Qingdao, China. E-mail address: [email protected] (Y. Jiang). https://doi.org/10.1016/j.jcis.2018.05.061 0021-9797/Ó 2018 Elsevier Inc. All rights reserved.

catalysts have been commercially applied to stationary combustion sources, especially coal-fired power plants [5,6]. However, some problems existing in these catalysts are inevitable, such as the biological toxicity of vanadium species, the poor tolerance to alkaline metals, the relatively narrow operating temperature range and high N2O formation at high temperature [7]. Cerium-based catalysts are considered as promising candidates for NH3-SCR due to their non-toxicity, excellent oxygen storage-release capacity and remarkable redox properties [7]. Varieties of cerium-based

Y. Jiang et al. / Journal of Colloid and Interface Science 528 (2018) 82–91

catalysts have been synthesized and reported, including Ce-Mn [8], Ce-Ti [9], Ce-W [10], Ce-Mo [11], Ce-Nb [12], Ce-Mn/Ti [13], Ce-P [14] and Ce-Cu/Ti [15], etc. Incineration of municipal solid waste (MSW) for power generation has been developing rapidly, while the NOx emission from MSW incineration plants is in the same magnitude as that from coal-fired power plants [16]. In some countries such as Japan and Sweden, NH3-SCR has been applied in MSW incineration plants because of their strict standards of NOx emission. However, the flue gas from MSW incinerators usually contains numerous heavy metals [17]. As one of the typical heavy metals, lead compound mainly exists in the form of PbO and PbCl2 in the flue gas from coal-fired and MSW incineration power plants [16,18]. It was reported that the concentration of Pb was up to 30 mg/g in dust and 6–40 mg/g in the flue gas before electrostatic precipitators in some MSW incineration plants [19]. Tokarz et al. [20] found that the concentration of Pb on vanadia-based catalysts reached 3350 ppm after the running time of 1908 h in a high-dust waste incinerator. Therefore, it is necessary to make great efforts to investigate the effect of lead compound on SCR catalysts. Several researchers have performed the studies on the effect of Pb on SCR catalysts. Khodayari and Odenbrand [16] proposed that the deactivation of V2O5-WO3/TiO2 catalysts by PbO was mainly ascribed to its competitive adsorption on the active sites. Chen and Yang [21] reported that the poisoning effect of various metal oxides on V2O5/TiO2 catalyst were in the following order: K2O > PbO > Na2O > As2O3 > CaO > P2O5. Guo et al. [22] attributed the deactivation of CeO2-TiO2 catalysts by PbO to enlarged TiO2 nanoparticles and reduced chemisorbed oxygen and surface acidity. Zhou et al. [23] found that the poisoning effect of PbO on Mn-Ce/TiO2 catalyst mainly resulted from decreased NH3 adsorption ascribed to the alteration of acid sites (especially Brønsted acid sites). In our previous studies [17,24], the poisoning effect of PbO and PbCl2 on V2O5/TiO2 catalyst was investigated. It was found that PbCl2 resulted in more serious deactivation of V2O5/TiO2 catalyst, which could be ascribed to smaller BET surface area, lower surface acidity and reducibility over PbCl2-doped V2O5/TiO2 catalyst. However, to our best knowledge, the effect of PbCl2 on CeO2-TiO2 catalysts as well as its comparison with that of PbO has hardly been studied until our present work. In order to clarify the poisoning effect of PbO and PbCl2 on CeO2-TiO2 catalysts, SCR activity measurements and characterizations (including BET, XRD, XPS, H2-TPR, NH3-TPD and in situ DRIFTS) were carried out. The main purpose of this work was to research and compare the substantial changes in CeO2-TiO2 catalysts resulted from the doping of PbO and PbCl2.

2 Experiments 2.1. Catalysts preparation The CeO2-TiO2 catalyst with CeO2/TiO2 mass ratio of 20:100 was prepared by sol-gel method. Tetra-n-butyl titanate (34 ml) and anhydrous ethanol (136 ml) were firstly mixed uniformly to form A solution. Then cerium nitrate (4.010 g), anhydrous ethanol (34 ml), nitric acid (6.8 ml) and deionized water (34 ml) were mixed adequately to form B solution. After that, A was added into B using a peristaltic pump with a speed of 2 ml/min, followed by stirring for 3 h to form a yellowish transparent sol. Then the sol was dried at 80 °C for 24 h and calcined at 500 °C for 5 h to form CeO2-TiO2 catalyst, which was denoted as CT. The PbO or PbCl2-doped CT samples were prepared by impregnation method. CT was impregnated with an aqueous solution containing the appropriate amount of Pb(NO3)2 or PbCl2, followed by heating in water bath at 80 °C with a continuous stirring to form

83

pastes. Then the pastes were impregnated at room temperature for 24 h and dried overnight at 110 °C. Additionally, the PbO poisoning samples needed to be calcined at 500 °C for 5 h. The samples were denoted as PbOx-CT or PbCl2 x-CT, where x represented the molar ratio of Pb to Ce. 2.2. Catalytic activity tests The catalytic activity tests were performed in a fixed bed quartz reactor (i.d. = 8 mm) at atmospheric pressure with the gas hourly space velocity (GHSV) of 90000 h
1. The simulated gas consisted of 1000 ppm NO, 1000 ppm NH3, 3 vol% O2 and N2 as balance gas with the total flow rate of 500 ml/min. The concentrations of various outlet and inlet gas were monitored using a flue gas analyzer (TESTO350, Germany). Furthermore, the catalytic oxidation of NO to NO2 was also measured in similar conditions without NH3 included in the simulated gas. 2.3. Catalysts characterization The BET measurements were performed by N2 adsorption at 77 K using an ASAP 2020 Plus Phys sorption (Micromeritics, ASAP 2020, USA). The crystallographic texture was performed on an X-ray diffractometer (Rigaku, D/max-2200/PC) between 20° and 80° at a step rate of 5° min
1 using Cu Ka (k = 0.15418 nm) radiation. X-ray photoelectron spectroscopy (XPS) analysis was measured on an electron spectrometer (Thermo ESCALAB 250, USA) using 300 W Al Ka radiations. The binding energy was calibrated by the carbon deposit C1S BE value at 284.8 eV. Temperature programmed reduction of H2 (H2-TPR) was carried out using a quartz U-tube reactor with the H2 signal detected by a thermal conductivity detector (TCD). The catalysts (0.1 g) were firstly pretreated in a steam of Ar at 500 °C for 1 h. Then a H2-Ar mixture (10 vol% H2) at a flow rate of 30 ml/min was used as the reductant. The reduction temperature was raised from room temperature to 800 °C with a rate of 10 °C/min and H2 consumption was recorded simultaneously. Temperature programmed desorption of ammonia (NH3-TPD) was also performed using the quartz U-tube reactor. After preheated in a stream of He at 500 °C for 1 h, the catalysts (0.1 g) were saturated with pure ammonia at room temperature at a flow rate of 30 ml/min for 40 min, followed by purging by He. Then the catalysts were heated up to 700 °C at a rate of 10 °C/min and NH3 signal was detected. The in situ DRIFT study was performed on a FTIR spectrometer (Thermo Nicolet iS 50) with the MCT detector. The catalysts were pretreated in Ar for 2 h at 400 °C and cooled down to 50 °C. In the cooling process, the background spectra were recorded and subtracted from the spectra of catalysts. The total flow rate was 300 ml/min, including 600 ppm NH3 or/and 600 ppm NO + 5%O2 with N2 as the balance gas. The DRIFT spectra were recorded by accumulating 32 scans with a resolution of 4 cm
1. 3 Results and discussion 3.1. Catalytic performance The NO conversion of fresh and Pb-doped CT catalysts as a function of reaction temperature were shown in Fig. 1. It could be seen that CT showed an excellent catalytic activity and 90.6–97.5% NO conversion was obtained in the temperature range from 275 °C to 450 °C. As was shown in Fig. 1(a), the NO conversion decreased remarkably with the increasing amount of PbO doped over CT. The

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100

Table 1 BET measurements of PbO-CT and PbCl2-CT.

CT PbO 0.05-CT

NO conversion (%)

PbO0.1-CT

80

PbO 0.5-CT

Samples

x

BET surface area (m2/g)

Total pore volume (cm3/g)

Average pore size (nm)

PbOx-CT

0 0.05 0.1 0.5 1

120.15 105.65 102.50 86.94 78.23

0.253 0.218 0.216 0.194 0.166

6.84 6.59 6.82 7.31 6.88

PbCl2x-CT

0 0.05 0.1 0.5 1

120.15 119.01 118.59 107.31 88.72

0.253 0.294 0.294 0.253 0.217

6.84 8.10 8.09 7.68 7.74

PbO 1.0-CT

60 40 20 0 150

200

250

300

350

400

450

500

Temperature (°C)

the worse catalytic activity of poisoning catalysts to some extent but it wasn’t the dominate factor. The X-ray diffraction patterns of all catalysts were presented in Fig. 2. As was shown in Fig. 2, no diffraction peaks of CeO2 species were observed in all samples, indicating CeO2 species were highly dispersed in both fresh and Pb-poisoned CT catalysts. For PbOdoped catalysts, the intensities of anatase TiO2 diffraction peaks decreased slightly with the increasing loadings of PbO. And a new diffraction peak ascribed to lead titanium (PbTiO3) could be observed for PbO1.0-CT. These results manifested that PbO might intercalate into the TiO2 lattice and there existed a strong

(a) 100

CT PbCl2 0.05-CT

NO conversion (%)

PbCl2 0.1-CT

80

PbCl2 0.5-CT PbCl2 1.0-CT

60 40 20

anatase-TiO2 lead titanium oxide-PbTiO3

0 150

200

250

300

350

400

450

PbO

500 CT

(b) Fig. 1. NO conversion of PbOx-CT (a) and PbCl2 x-CT (b). Reaction condition: [NO] = [NH3] = 1000 ppm, [O2] = 3%, N2 balance, total flow rate = 500 ml/min, GHSV = 90000 h
1.

Intensity(a.u.)

Temperature (°C)

PbO0.05-CT

PbO0.1-CT

PbO0.5-CT

catalysts almost lost their catalytic activity when x exceeded 0.5. As for PbCl2-doped catalysts, it was clear that the NO conversion showed a different variation trend from that over PbO-doped catalysts. PbCl2 had a slight effect on the catalytic activity of CT at the temperatures lower than 350 °C. On the contrary, obvious deactivation could be observed with the increasing amount of PbCl2 in the temperature range from 350 °C to 500 °C. From the above, it can be concluded that the introduction of both PbO and PbCl2 inhibited the catalytic activity of CT. The poisoning effect of PbO was severer compared with that of PbCl2.

The textural properties (including BET surface area, total pore volume and average pore diameter) of fresh and Pb-poisoned CT samples were summarized in Table 1. As was shown in Table 1, the increasing loadings of PbO caused a great decrease in BET surface area and total pore volume of CT. PbCl2-CT catalysts had a similar variation trend but a slower decreasing level compared with that of PbO-CT catalysts, which was in good agreement with the catalytic activity (shown in Fig. 1). Large BET surface area was believed to be an important factor to improve the adsorptiondesorption behavior of gases on catalyst surface [25]. However, the average pore diameter didn’t show the same variation trend as BET surface area and total pore volume. Therefore, decreased BET surface area and total pore volume might be responsible for

10

20

30

40

50

60

70

2 (°)

(a) anatase-TiO2 lead dichloride-PbCl2 CT

Intensity(a.u.)

3.2. Textural characteristics of catalysts (BET and XRD)

PbO1.0-CT

PbCl2 0.05-CT PbCl2 0.1-CT

PbCl2 0.5-CT

PbCl2 1.0-CT

10

20

30

40

50

60

2 (°)

(b) Fig. 2. XRD patterns of PbOx-CT (a) and PbCl2 x-CT (b).

70

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Ti2p

458.8

464.6

Intensity (a.u.)

interaction between PbO and TiO2, resulting in the expansion of TiO2 lattice [26]. However, the intensities decrease of anatase TiO2 diffraction peaks wasn’t observed over PbCl2-doped catalysts. This indicated that there was no or just a weak interaction between PbCl2 and TiO2, which was in good agreement with the activity results. Interestingly, there were no diffraction peaks of PbO or PbCl2 detected when x was low. The reason may be that it was very dilute for CT at low Pb concentrations, resulting in highly dispersed state of Pb species. With the increasing loadings of Pb species, PbO or PbCl2 would gather and form crystallized structure. As a result, the diffraction peaks of PbO (x = 1.0) and PbCl2 (x = 0.5) were detected. A similar result was obtained on arsenic poisoning CeW/Ti catalysts [26]. Therefore, the stronger interaction between PbO and TiO2 over PbO-CT might lead to its worse catalytic activity than PbCl2-CT.

458.5 CT 464.3 PbCl21.0-CT

458.4 464.2

PbO1.0-CT

470

468

466

464

462

460

458

456

454

Binding energy (eV)

3.3. Surface analysis of catalysts (XPS)

(a)

CT PbO1.0-CT PbCl2 1.0-CT

u' u

u'''

Intensity (a.u.)

v'''

v'

u''

CT

v v''

PbO1.0-CT

PbCl21.0-CT

930

920

910

900

890

880

870

Binding energy (eV)

(b) O1s

O O

Oγ CT

PbO1.0-CT

PbCl2 1.0-CT

536

534

532 530 Binding energy (eV)

528

(c) Fig. 3. XPS spectra of Ti 2p (a), Ce 3d (b) and O 1s (c) of CT, PbO1.0-CT and PbCl2 1.0-CT.

Table 2 Surface atomic concentration of different catalysts. Samples

Ce 3d

Intensity (a.u.)

In order to get a better understanding about the chemical states and the relative proportion of all elements on the catalysts surface, XPS characterization of some samples (CT, PbO1.0-CT and PbCl21.0-CT) was carried out. The surface atomic concentration was summarized in Table 2. After the introduction of PbO and PbCl2, the concentration of Ce decreased from 26.03 at.% to 15.20 at.% and 18.31 at.%, respectively. This variation trend was exactly in the same sequence with the catalytic activity. It can be deduced that lower surface concentration of Ce might result in lower SCR reaction capacity, leading to the worse catalytic activity. Fig. 3(a) presented the XPS spectra of Ti 2p. For CT, the BE values of Ti 2p1/2 and Ti 2p3/2 were 464.6 eV and 458.8 eV, respectively. It could be deduced that Ti existed in the form of Ti4+ rather than Ti3+, which was consistent with the reported values [17,27]. After the introduction of PbCl2 and PbO, the BE shifted to lower values and decreased by 0.4 eV for PbO1.0-CT and 0.3 eV for PbCl2 1.0-CT, respectively. The Ti 2p BE values of Ti3+ was 1.8 eV lower than those of Ti4+ [27]. Therefore, Ti still existed in the form of Ti4+ with the presence of PbCl2 and PbO. It was also observed that the Ti 2p XPS peaks became broader and lower for PbCl2-CT and this variation trend was more obvious after the introduction of PbO. This might be ascribed to the stronger interaction between PbO and TiO2 compared with that between PbCl2 and TiO2. This agreed well with the XRD results. Furthermore, the Ti 2p3/2 BE value of PbTiO3 was about 458.4 eV according to the handbook of XPS [28], as observed in Fig. 3(a). Therefore, it was possible that the significant broadening and lowering of Ti 2p XPS peaks for PbO1.0-CT was originated from the formation of PbTiO3. Similar phenomenon was observed on PbCl2-doped V2O5/TiO2 catalyst in our previous study [17]. Fig. 3(b) showed the XPS spectra of Ce 3d. The peaks denoted as ‘‘v” and ‘‘u” corresponded to Ce3d5/2 spin-orbit components and Ce3d3/2 spin-orbit components respectively [26]. It could be deconvoluted into eight overlapped peaks: the bands v’ and u’ can be attributed to surface Ce3+ corresponding to the 3d104f1 initial electronic state, while the other bands can be attributed to surface Ce4+ corresponding to the 3d104f0 initial electronic state [29,30]. Apparently, the peaks intensities of Ce 3d decreased with the presence of

Metal content (at. %) Ce

Ti

O

Pb

Cl

26.03 15.20 18.31

33.23 18.51 21.28

33.21 23.46 28.92

– 13.43 8.40

– – 4.15

PbO and PbCl2. In comparison, PbO caused a more obvious decrease than PbCl2, which agreed well with surface atomic concentration results above. The ratios of Ce3+/(Ce3+ + Ce4+) could be calculated according to the area integral of the peaks on behalf of Ce3+ and Ce4+. For CT, the ratio of Ce3+/(Ce3+ + Ce4+) was 40.01%. However,

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it decreased to 33.77% and 35.23% for PbO1.0-CT and PbCl2 1.0-CT, respectively, which was in the same sequence with catalytic activity. This reason might be that some oxygen vacancies were taken up due to the introduction of PbO or PbCl2, inhibiting the transformation from Ce4+ to Ce3+. Ce3+ had a positive effect on the catalytic activity due to its good ability to create charge imbalance, form oxygen vacancies and unsaturated chemical bond on catalysts surface, which would help improve the surface chemisorbed oxygen [31–34]. Therefore, fewer Ce3+ over PbO1.0-CT was an important factor to cause its worse catalytic activity. As was shown in Fig. 3(c), the spectra of O1s could be decomposed into three distinct types of oxygen. The bands at 528.5– 529 eV, 529.5–530 eV and 530.5–531 eV can be assigned to lattice oxygen (Oa), surface chemisorbed oxygen (Ob) and oxygen in hydroxyl or surface adsorbed water species (Oc), respectively [22,35]. Chemisorbed oxygen (Ob) plays a key role in SCR reactions due to its higher mobility [36–38]. It was clear that the peak intensity of Ob decreased after the doping of PbO and PbCl2. The decrease level over PbO1.0-CT was higher than that of PbCl21.0-CT. The ratios of Ob/(Oa + Ob + Oc) of three samples were obtained by the area integral of curves. It decreased from 21% to 18.94% for PbO1.0-CT and 19.75% for PbCl2 1.0-CT. These results agreed well with the catalytic activity results. Thus fewer Ob over PbO1.0-CT was also responsible to its worse catalytic activity. 3.4. Redox properties of catalysts (H2-TPR) The redox properties are regarded as an important factor in NH3-SCR reaction using Ce-based catalysts [39,40]. Therefore, H2-TPR of three samples (CT, PbO1.0-CT and PbCl2 1.0-CT) was performed to ensure the effect of PbO and PbCl2 on the redox properties of CT, as shown in Fig. 4. For CT, a wide reduction peak centered at 510 °C was observed and the onset temperature was 225 °C, which was attributed to the reduction process from surface Ce4+ to Ce3+ [7,26]. After the introduction of PbO and PbCl2, the onset temperature shifted towards a higher one (275 °C). This suggested that the reduction of surface Ce4+ to Ce3+ was inhibited. Additionally, the main reduction temperature decreased to 445 °C for PbO1.0-CT and 456 °C for PbCl2 1.0-CT. This manifested the presence of more Ce4+ on the surface of CT after the introduction of Pb species. In comparison, PbO caused a higher increase of Ce4+ than PbCl2, which was in line with the XPS results. Furthermore, the relative reduction area and H2 consumption was calculated with the results presented in Table 3. It could be observed that both the relative reduction area and H2 consumption decreased with the presence of PbO and PbCl2. This confirmed that

Table 3 Relative reduction area and H2 consumption of different catalysts. Samples

Relative Reduction Area (a.u.)

H2 Consumption (ml/g)

CT PbO1.0-CT PbCl2 1.0-CT

5982.80 4770.47 4982.93

107.27 85.53 89.34

the reduction of surface Ce4+ to Ce3+ was inhibited accompanied by the increased Ce4+ after the introduction of Pb species. Interestingly, a broad reduction peak centered at 645 °C was observed for PbCl2 1.0-CT, while it didn’t occur for PbO1.0-CT. Combined with XRD results, it might result from the different interaction between Pb species and CT. 3.5. Surface acidity of catalysts (NH3-TPD) NH3-TPD technique was used to study the effect of PbO and PbCl2 on the surface acidity of CT and the results were shown in Fig. 5. All the profiles displayed two distinct desorption peaks due to NH3 desorption on two acid sites with different thermal stability. The peak ranging from 150 to 250 °C and the other one ranging from 550 to 650 °C were ascribed to NH3 desorption on the weak (Brønsted acid sites) and strong acid sites (Lewis acid sites), respectively [31,41]. It was obvious that Brønsted acid sites were the main acid sites for CT, as reflected from the peaks intensities. For two poisoning catalysts, the intensities of two desorption peaks decreased enormously, indicating the great loss of two acid sites (especially the Brønsted acid sites). Zhou et al. [23] found the similar result when they studied the poisoning effect of PbO on Mn-Ce/ TiO2 catalyst. In comparison, PbO caused a greater loss of acid sites than PbCl2, which agreed well with the catalytic results. Interestingly, the desorption peak due to Brønsted acid sites shifted to lower temperature with the presence of PbCl2 and PbO, indicating there existed the interaction between Pb species and acid sites. From the above analysis, it could be concluded that the greater loss of acid sites (especially Brønsted acid sites) over PbO1.0-CT contributed to its worse catalytic activity than PbCl2 1.0-CT. 3.6. Catalytic oxidation of NO to NO2 It is believed that the NO oxidation to NO2 has a positive effect on the performance of SCR catalysts [42,43]. Therefore, the activities of NO oxidation to NO2 over three catalyst samples were tested with the results shown in Fig. 6. The activities of NO

CT PbCl21.0-CT

CT PbO1.0-CT

PbO 1.0-CT

445

TCD signal (a.u.)

TCD signal (a.u.)

PbCl2 1.0-CT

645

510 456

275

220 612

170

225 158

200

300

400

500

600

700

Temperature (°C) Fig. 4. H2-TPR profiles of three samples.

800

100

200

300

400

500

600

Temperature (°C) Fig. 5. NH3-TPD profiles of three samples.

700

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NO oxidation to NO2 (%)

90 80

1675

CT PbO1.0-CT

1580

1221

1433

1073

350

PbCl21.0-CT 300

Absorbance (a.u.)

100

70 60 50 40 30

250 200 150 100 50

20 10 0 150

200

250 300 Temperature (°C)

350

400

2000

1800

1600

(a)

1605

1421 1384

1600

1400

1170

Absorbance (a.u.)

300 250 200 150 100 50

2000

1800

1200

1000

-1

Wavenumber (cm ) (b) 1675

1580

1248

1430

350

1052

300

Absorbance (a.u.)

3.7.1. NH3 adsorption The in situ DRIFT spectra of NH3 adsorption over different catalysts (CT, PbO1.0-CT and PbCl2 1.0-CT) were presented in Fig. 7. As was shown in Fig. 7(a), several adsorption bands attributed to NH3 species could be detected over CT, including NH+4 species on Brønsted acid sites (1675, 1433 cm
1) and NH3 species coordinated to Lewis acid sites (1580, 1221 and 1073 cm
1) [42,45–48]. It could be observed that the intensities of the bands ascribed to Brønsted acid sites decreased markedly with increasing temperature, especially the band at 1433 cm
1, which nearly disappeared at 250 °C. In comparison, the bands due to Lewis acid sites were still present even at 350 °C, indicating that the NH3 species absorbed on Lewis acid sites are more thermally stable than that absorbed on Brønsted acid sites [49]. For PbCl2 1.0-CT, a similar spectra of NH3 adsorption was recorded, as shown in Fig. 7(c). The intensities of all the bands decreased compared with that of CT, especially the bands due to Brønsted acid sites. For PbO1.0-CT, only the bands due to Lewis acid sites could be observed with very weak intensities. These results manifested that the addition of PbO and PbCl2 would lead to the loss of surface acid sites (especially the Brønsted acid sites). In comparison, PbO resulted in a greater loss of Brønsted acid sites than PbCl2. These results agreed well with the NH3-TPD analysis. Shu et al. [50] pointed out that more Ce3+ was helpful to the formation of Brønsted acid sites for NH3 adsorption. Combined with XPS results, the Brønsted acid sites mainly resulted from surface Ce3+ while surface Ce4+ was the main active sites supplying Lewis acid sites, which agreed well with other researches [7,49]. Therefore, the greater loss of Brønsted acid sites derived from surface Ce3+ over PbO1.0-CT was a key factor responsible for its worse catalytic activity than PbCl21.0-CT.

1000

Wavenumber (cm )

350

3.7. In situ DRIFT study

1200 -1

Fig. 6. NO oxidation to NO2 of three samples. Reaction condition: [NO] = 1000 ppm, [O2] = 3%, N2 balance, total flow rate = 500 ml/min, GHSV = 90000 h
1.

oxidation to NO2 over three samples were in the following order: CT > PbCl2 1.0-CT > PbO1.0-CT, which was consistent with the SCR performance in Fig. 1. The result demonstrated that the doping of Pb species inhibited the oxidation of NO to NO2 over CT. Guo et al. [44] also found the inhibiting effect of Pb and Zn on the NO oxidation to NO2 over Mn/TiO2 catalyst along with the decreased SCR performance. This could be ascribed to decreased chemisorbed oxygen after the introduction of Pb species [44], which agreed well with the XPS analysis.

1400

250 200 150 100

2000

50

1800

1600

1400

1200

1000

-1

Wavenumber (cm ) (c) Fig. 7. DRIFT spectra of NH3 adsorption over (a) CT, (b) PbO1.0-CT and (c) PbCl2 1.0-CT.

3.7.2. NO+O2 co-adsorption The in situ DRIFT spectra of NO+O2 co-adsorption over three samples were shown in Fig. 8. For CT (Fig. 8(a)), several bands owing to different NOx species could be detected, including weak adsorbed NO2 (1620 and 1600 cm
1) [51,52], monodentate nitrites

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Y. Jiang et al. / Journal of Colloid and Interface Science 528 (2018) 82–91

PbCl21.0-CT, the DRIFT spectra were very similar to that of CT and the new bands at 1508, 1282 and 1235 cm
1 were ascribed to bidentate nitrates [54]. However, the intensities of all the bands decreased, especially the bands ascribed to adsorbed NO2, indicating the NO oxidation to NO2 was highly suppressed. Fig. 8(b) presented the DRIFT spectra of PbO1.0-CT. Compared with PbCl2 1.0-CT, PbO1.0-CT showed a more obvious intensities decrease of the bands at the whole temperature range. These results suggested that the introduction of PbO and PbCl2 would inhibit the adsorption of NOx species (especially adsorbed NO2) over CT, while PbO showed more serious inhibition effect than PbCl2, which agreed well with the catalytic activity.

1600

Absorbance (a.u.)

1273

1470

350

1154

300 250 200 150 100 50 1620

2000

1800

1600

1400

1200

1000

-1

Wavenumber (cm )

(a)

1590

1450

1290

1052

Absorbance (a.u.)

350 300 250 200 150 100 50

2000

1800

1600

1400

1200

1000

-1

3.7.3. Reaction between NOX and pre-adsorbed NH3 species In order to better understand the role of adsorbed NH3 during the SCR process, the in situ DRIFT spectra of the reaction between NO+O2 and pre-adsorbed NH3 were carried out. The samples were firstly pretreated by 600 ppm NH3/N2 for 30 min at 300 °C, followed by purged by N2 for 15 min. Then 600 ppm NO and 5% O2/ Ar were introduced into the DRIFT cell with the DRIFT spectra recorded as the function of time. As shown in Fig. 9(a), several bands ascribed to adsorbed NH3 (1675, 1580 and 1156 cm
1) could be detected after exposed to NH3 for 30 min. After the introduction of NO+O2, the intensities of these bands decreased markedly within 2 min. All the bands almost disappeared after 5 min. Then several bands due to NOx species (1600, 1438 and 1254 cm
1) were recorded with the reaction process going on. Therefore, it could be deduced that both the NH3 species adsorbed on Brønsted acid sites and Lewis acid sites were active in the SCR reaction process over CT. Therefore, the NH3-SCR reactions over CT are under the control of Eley-Rideal (E-R) mechanism [49]. For two poisoning samples, a similar variation trend could be observed, as shown in Fig. 9(b) and (c). Combined with the DRIFT spectra of NH3 adsorption, the introduction of PbO and PbCl2 would inhibit the adsorption of NH3 species over CT, yet the reactivity of adsorbed NH3 wasn’t suppressed during the SCR process.

Wavenumber (cm )

(b) 1235 1617

Absorbance (a.u.)

350

2000

1282

1508

1154

300 250 200 150 100 50

1800

1600

1400

1200

1000

-1

Wavenumber (cm )

(c) Fig. 8. DRIFT spectra of NO+O2 co-adsorption over (a) CT, (b) PbO1.0-CT and (c) PbCl2 1.0-CT.

(1470 cm
1 and 1154 cm
1) [53] and bidentate nitrates (1273 cm
1) [54]. Apparently, the NOx species adsorbed on the surface of CT mainly exited in the form of adsorbed NO2. As for

3.7.4. Reaction between NH3 and pre-adsorbed NOX species In this experiment, the function of pre-adsorbed NOx in the SCR process was researched based on the in situ DRIFT study. All the samples were pretreated by 600 ppm NO and 5% O2/N2 for 30 min at 300 °C. Then the samples were purged by N2 for 15 min, followed by the introduction of 600 ppm NH3/Ar to the DRIFT cell. The DRIFT spectra were recorded, as shown in Fig. 10. As shown in Fig. 10(a), several bands (1601, 1470 and 1228 cm
1) due to adsorbed NOx were observed in the DRIFT spectra of CT when exposed to NO for 30 min. It was obvious that all the bands vanished within 2 min after NH3 was introduced to the DRIFT cell, indicating the high reactivity of pre-adsorbed NOx over fresh CT during the SCR process. This indicated that all adsorbed NOx over CT were active during the SCR process. These results suggested that the NH3-SCR reactions over CT were also governed by LangmuirHinshelwood (L-H) mechanism, in which adsorbed NOx species reacted with adsorbed NH3 species [50]. For two poisoning samples, as presented in Fig. 10(b) and (c), similar DRIFT spectra were recorded during the experiment process, suggesting that the introduction of PbO and PbCl2 didn’t change the reactivity of adsorbed NOx in spite of the decreased adsorption amount of them. 3.8. Poisoning mechanism The in situ DRIFT study revealed that the NH3-SCR reaction over CT was governed by both E-R (Eqs. (1)–(3) and L-H mechanism (Eqs. (4)–(6), which was not changed with the presence of PbO and PbCl2.

89

Y. Jiang et al. / Journal of Colloid and Interface Science 528 (2018) 82–91

1600

30min

1438

1704

1254

1590 1252

30min

Absorbance (a.u.)

Absorbance (a.u.)

20min

10min

5min

NO+O2 2min

20min 10min 5min NH3 2min NO+O2 2min

1601

NH3 30min

1675

2000

1800

1580

1156

1600

1400

1200

2000

1000

1800

1600

(a)

1590

1000

1200

1000

1472

1273

30min

1200

Wavenumber (cm ) (a)

-1

1600

1400 -1

Wavenumber (cm )

1290

Absorbance (a.u.)

30min

20min

Absorbance (a.u.)

1228

1470

10min 5min

NO+O2 2min

20min 10min 5min NH3 2min NO+O2 30min 1450

1275

NH3 30min 1605

1384

1250

2000

2000

1800

1600

1400

1200

1800

1600

1000

1400 -1

Wavenumber (cm ) (b)

-1

Wavenumber (cm )

(b) 1709

1608

1421

1248

20min

Absorbance (a.u.)

1605 1458

1154

Absorbance (a.u.)

30min

30min

10min 5min

20min 10min 5min NH3 2min NO+O2 30min 1617

NO+O2 2min

1508

1154

NH3 30min 1675

1590

2000

1423

2000

1800

1600

1400

1200

1800

1600

1400

1200

1000

-1

Wavenumber (cm ) (c)

1000

-1

Wavenumber (cm )

(c)

Fig. 10. DRIFT spectra of the reaction between NH3 and pre-adsorbed NOx species over (a) CT, (b) PbO1.0-CT and (c) PbCl2 1.0-CT.

Fig. 9. DRIFT spectra of the reaction between NO+O2 and pre-adsorbed NH3 species over (a) CT, (b) PbO1.0-CT and (c) PbCl2 1.0-CT. Ce3

þ

NH3 ðaÞ ƒƒƒƒ! NHþ4 ðaÞ

ð1Þ

NHþ4 ðaÞ ƒƒƒƒ! NH2 ðaÞ þ 2Hþ þ 2e

Surface O

ð2Þ

NH2 ðaÞ þ NOðgÞ ! NH2 NOðaÞ ! N2 þ H2 O

ð3Þ

NOðgÞ þ O ðsurfÞ ! NO2 ðaÞ Ce4

þ

ð4Þ

NH3 ðgÞ ƒƒƒƒ! NH3 ðaÞ

ð5Þ

NH3 ðaÞ þ NO2 ðaÞ þ NOðgÞ ! N2 þ H2 O

ð6Þ

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Y. Jiang et al. / Journal of Colloid and Interface Science 528 (2018) 82–91

N O

H

N

N

H

E-R+L-H mechanism

H H

mechanisms. Future research should be focused on the enhanced resistance to Pb species of CT, which will accelerate the industrial application of CT.

O

N

O

H

Acknowledgments

O

NH3

NO+O2

B

L

Ce33+ C

Ce44+

NO2

References

TiO2 NO+O2

NH3

NO2

Pb B

L

Ce33+ C

Ce44+ C

Strong effect

This work was supported by the National Natural Science Foundation of China (No. 51506226), Natural Science Foundation of Shandong Province (No. ZR2015EM010), ‘‘the Fundamental Research Funds for the Central Universities” (No. 15CX05005A) and the scholarship from China Scholarship Council, China (CSC) (No. 201706455013).

TiO2 Weak effect

Fig. 11. Schematic diagram of Pb poisoning on CT.

From the XPS results, the introduction of Pb species inhibited the transformation of surface Ce4+ to Ce3+, resulting in the great loss of Brønsted acid sites. This inhibited both Eqs. (1) and (2) through E-R mechanism. Considering that the Brønsted acid sites served as the main acid sites (as reflected by the NH3-TPD results), the great loss of Brønsted acid sites due to decreased Ce3+ was a key factor accounting for the poisoning effect of Pb species on CT. On the other hand, remarkable decrease of adsorbed NO2 was observed with the presence of Pb species (shown in Fig. 7), indicating the reaction (4) was suppressed over the surface of CT. Combined with the XPS result, it might be ascribed to the decreased surface chemisorbed oxygen. As a result, reaction (6) through L-H mechanism was significantly inhibited. Furthermore, the NH3 adsorption on the Lewis acid sites was also inhibited by Pb additives (shown in Fig. 6), which aggravated the deactivation of CT. Based on the analysis above, a possible poisoning schematic diagram of Pb species on CT was proposed, as was shown in Fig. 11.

4. Conclusion PbO resulted in a stronger poisoning effect on CT than PbCl2. The characterization results revealed that the more serious deactivation by PbO could be ascribed to smaller BET surface area, fewer surface Ce3+ and chemisorbed oxygen, stronger interaction between PbO and CT, lower redox properties and surface acidity. Combined the XPS, NO oxidation and in situ DRIFT results, the great loss of Brønsted acid sites ascribed to decreased surface Ce3+ and the inhibited NO oxidation to NO2 due to decreased surface chemisorbed oxygen were two key factors responsible for deactivation by Pb species. The NH3 adsorption on the Lewis acid sites was also inhibited with the presence of Pb species, aggravating the deactivation of CT. Furthermore, the NH3-SCR reaction mechanism wasn’t changed by Pb species, which was a combination of E-R and L-H

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