TiO2 catalyst for NH3-SCR reaction by the modification with Al2(SO4)3

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The enhancement of Zn resistance of Mn/TiO2 catalyst for NH3 -SCR reaction by the modification with Al2 (SO4 )3 Shu-ming Liu a,b, Rui-tang Guo a,b,∗, Peng Sun a,b, Shu-xian Wang a,b, Wei-guo Pan a,b,∗, Ming-yuan Li a,b, Shuai-wei Liu a,b, Xiao Sun a,b, Jian Liu a,b a b

School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, PR China Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai, PR China

a r t i c l e

i n f o

Article history: Received 18 April 2017 Revised 16 June 2017 Accepted 16 June 2017 Available online xxx Keywords: SCR Mn/TiO2 catalyst Zn Resistance Al2 (SO4 )3

a b s t r a c t Heavy metals such as Zn have a poisoning effect on the catalyst for selective catalytic reduction of NOx with NH3 . In this study, it was found that the modification of Mn/TiO2 catalyst with Al2 (SO4 )3 could effectively enhance its Zn resistance. From the results of characterizations including XRD, H2 -TPR, NH3 TPD and XPS, it could be concluded that the lower crystallinity, the higher reducibility, the generation of more acid sites, and the presence of more Mn4+ and chemisorbed oxygen should mainly contribute to the good Zn resistance of Al2 (SO4 )3 -modified Mn/TiO2 catalyst. The results of in situ DRIFT study indicated that the addition of Al2 (SO4 )3 did not change the mechanism of NH3 -SCR reaction over Mn/TiO2 catalyst, which was under the control of both Eley–Rideal and Langmuir–Hinshelwood mechanisms. © 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

1. Introduction Selective catalytic reduction (SCR) process has been widely used for controlling NOx abatement from stationary sources such as coal-fired boilers and municipal solid waste (MSW) incinerators [1–3]. In this system, V-based catalyst (V2 O5 -WO3 /TiO2 , V2 O5 WO3 /TiO2 , etc.) is the mainly commercial catalyst, which exhibits high SCR activity in the temperature range of 30 0–40 0 °C [4,5]. However, there are still some drawbacks associated with this catalyst, including the toxicity of vanadium pentoxide, high conversion of SO2 to SO3 , formation of N2 O at high temperature and the deactivation by alkali metals [6–10]. As correspondence, developing alternative SCR catalyst with environmental-friendly property and high NOx removal efficiency has become a research focus during the past several years. Mn-based catalyst is regarded as a promising low-temperature SCR catalyst due to the presence of types of labile oxygen in MnOx , which could help to complete the catalytic cycle in SCR reaction [11, 12]. Recently, the SCR performances of MnOx supported on different carriers such as TiO2 [13], Al2 O3 [14], SiO2 [15] and CeSiOx [16] have been deeply investigated. Although the SCR reactor using Mn-based catalyst could be set downstream of the electro-

∗

Corresponding authors at: School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, PR China. E-mail addresses: [email protected] (R.-t. Guo), [email protected] (W.-g. Pan).

static precipitator, however, a small amount of chemical poisoning species such as alkali, alkali earth and heavy metals are still present in the flue gas, which would cause the deactivation of Mnbased SCR catalyst, as reported by several groups [17–20]. As a typical heavy metal, Zn is widely present in the flue gas of coal-fired boilers and MSW incinerators. It has been reported that the Zn contents in the fly ashes of coal-fired boilers and MSW incinerators are 3 and 6 g/Kg respectively [21, 22]. In our previous study [23], we found that the modification of Mn/TiO2 catalyst by Nb could enhance its Zn resistance. Previous studies have proven that the addition of solid acid to CeO2 catalyst could greatly enhance its SCR performance [24]. Putluru et al. [25] found that heteropoly acid could improve the K tolerance of V2 O5 /TiO2 catalyst. However, the promotion mechanism of solid acid modification on the resistance of chemical poisoning species is still uncertain. In this study, we used Al2 (SO4 )3 (another solid acid) as the additive of Mn/TiO2 catalyst to enhance its Zn resistance, and the promotion mechanism would be discussed based on the characterization results. 2. Experimental 2.1. Catalyst preparation The Mn/TiO2 catalyst sample used in this study was prepared by sol–gel method with a molar ratio of Mn/Ti of 0.12:1. Detailed procedure was similar with that reported in our previous study [17]. Al2 (SO4 )3 was added to the fresh Mn/TiO2 catalyst sample by

http://dx.doi.org/10.1016/j.jtice.2017.06.039 1876-1070/© 2017 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

Please cite this article as: S.-m. Liu et al., The enhancement of Zn resistance of Mn/TiO2 catalyst for NH3 -SCR reaction by the modification with Al2 (SO4 )3 , Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.039

2

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impregnating the sample in a certain amount of Al2 (SO4 )3 solution. The molar ratio of Al/Ti was set as 0.2. Then the mixture was stirred for 6 h, dried at 90 °C for 24 h and calcined at 500 °C in air for 6 h. The Al2 (SO4 )3 loaded Mn/TiO2 catalyst was denoted as MnAl/TiO2 . Next then, Zn was loaded on the two fresh catalyst samples by impregnation method. Zn(NO3 )2 was used as the source of Zn. Firstly, the fresh catalyst sample was impregnated in Zn(NO3 )2 solution (Zn/Mn molar ratio = 1/8). Then the mixture was stirred adequately for 6 h and dried at 90 °C for 24 h. Finally, the solid was calcined at 500 °C in air for 6 h to obtain the Zn-poisoned catalyst sample. The two poisoned catalyst samples were denoted as Mn/TiO2 -Zn and MnAl/TiO2 -Zn respectively.

100 90

MnAl/TiO2

Mn/TiO2

70 60 50

MnAl/TiO2-Zn

40 30 20

2.2. Characterizations

Mn/TiO2-Zn

10 100

150

200

250

300

350

300

350

o

Reaction temperature( C) 98 (B) 96

N2 selectivity (%)

A Quantachrome Autosorb-iQ-AG instrument was used to measure the N2 adsorption–desorption isotherms of the catalyst samples at 77 K. Prior to N2 adsorption, the catalyst sample was degassed at 300 °C. Based on the Brunauer–Emmett–Teller (BET) method, the specific surface area of catalyst sample could be calculated. And the pore size distribution was determined by Barrett– Joyner–Halenda (BJH) method. To investigate the crystal structures of the catalyst samples, X-ray diffraction (XRD) measurement was performed on a Bruker D8 Advance powder diffractometer with CuKα radiation (λ = 0.154056 nm). The surface elements and their states of the catalyst samples were measured by X-ray photoelectron spectroscopy (XPS) with Al Kα X-ray (hν = 1486.6 eV) radiation operated at 150 W (Thermo ESCALAB 250, USA). The binding energy shift due to relative surface charging was calibrated by using C 1 s level at 284.8 eV as the criterion. The morphology of each catalyst sample was observed on a JMF-7500F electron microscope (JEOL Co., Japan) operating at 20 kV and 80 mA. To determine the redox behavior of the catalyst samples, H2 temperature-programmed reduction (H2 -TPR) was performed on a Quantachrome Autosorb-iQ-C chemisorption analyzer using 50 mg catalyst sample. The samples were pretreated in pure N2 for 1 h before TPR experiments. Next then, TPR runs were carried out in 6% H2 /N2 gas flow (30 mL/min) with a linear heating rate of 10 °C/min. NH3 -TPD experiments were also performed on the same chemisorption analyzer. After pretreated in He at 400 °C for 1 h, the catalyst sample was saturated with anhydrous 4% NH3 /He at a flowrate of 30 mL/min for 0.5 h. Desorption experiment was performed by heating the sample in He flow (30 mL/min) from 100 °C to 500 °C with a heating rate of 10 °C/min. The signal of H2 (for TPR test) or NH3 (for TPD test) was monitored by a thermal conductivity detector (TCD). The adsorption behavior of reactants and their surface reactions were investigated by in situ DRIFT study, which was performed on an FTIR spectrometer (Thermo Nicolet iS 50) equipped with a smart collector and an MCT/A detector. At first, the catalyst sample was pretreated in N2 flow (300 mL/min) at 400 °C for 1 h and then cooled down to the desired temperature. The background spectrum was collected in N2 flow and automatically subtracted from the spectrum of the sample. The feeding gas was a mixture of 500 ppm NH3 , or/and 500 ppm NO+5% O2 , N2 balance, with the total flow rate of 300 mL/min. All the DRIFT spectra were recorded by accumulating 100 scans with a resolution of 4 cm−1 .

(A)

80

NOx conversion (%)

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94 92 90 MnAl/TiO2

88

Mn/TiO2

MnAl/TiO2-Zn

86 84

Mn/TiO2-Zn 100

150

200 250 o Reaction temperature( C)

Fig. 1. (A) NOx conversions and (B) N2 selectivities over the four catalyst samples reaction conditions; [NO] = [NH3 ] = 600 ppm, [O2 ] = 5%, balance Ar, GHSV = 108,0 0 0 h−1 .

experimental run, thus the gas hourly space velocity (GHSV) was about 108,0 0 0 h−1 . The components of the effluent gas, including NO, NO2 , NH3 and N2 O were analyzed by an FTIR spectrometer (Thermo Nicolet iS 50) equipped with a gas cell (0.2 dm3 ). The data was collected after the SCR reaction reached a steady state. Correspondingly, the values of NOx conversion and N2 selectivity could be calculated by:

NOx conversion =

[NOx ]in − [NOx ]out × 100% [NOx ]in

N2 selectivity



=

1−

2[N2 O]out [NOx ]in +[NH3 ]in − [NOx ]out − [NH3 ]out

2.3. Catalytic activity test

3. Results and discussion

The catalytic performance of each catalyst sample was tested in a fixed-bed reactor (i. d. = 8 mm). During the test process, the simulated flue gas contained the following components: 600 ppm NH3 , 600 ppm NO, 5% O2 , balance Ar, with the total flow rate of 1 L/min. About 0.55 cm3 catalyst sample (80–100 mesh) was used in each

3.1. Catalytic performance

(1)

 × 100%

(2)

The NOx conversions over the four catalyst samples are shown in Fig. 1(A). From Fig. 1(A), the modification of Mn/TiO2 catalyst with Al2 (SO4 )3 only has a weak promotion effect on the

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S.-m. Liu et al. / Journal of the Taiwan Institute of Chemical Engineers 000 (2017) 1–8 Table 1 Textural properties of the four catalyst samples. Samples

BET surface area (m2 /g)

Total pore volume (cm3 /g)

Mn/TiO2 MnAl/TiO2 Mn/TiO2 -Zn MnAl/TiO2 -Zn

105.5 110.6 86.3 97.9

0.2112 0.2834 0.1702 0.1936

3

3.3. XPS analysis Average pore diameter (nm) 6.816 5.296 7.209 6.979

NH3 -SCR reaction over it. The NOx conversion over MnAl/TiO2 catalyst is a little higher than that over Mn/TiO2 catalyst. Besides that, MnAl/TiO2 catalyst has much better Zn resistance than Mn/TiO2 catalyst. As presented in Fig. 1(A), the NOx conversion over MnAl/TiO2 catalyst drops about 40% after the addition of Zn, while the NOx conversion over Mn/TiO2 catalyst drops about 70% under the same conditions. Moreover, both Mn/TiO2 and MnAl/TiO2 exhibit the similar N2 selectivities, while the presence of Zn would lead to the drop of N2 selectivity, as illustrated in Fig. 1(B). The drop of N2 selectivity might be originated from the promoted unselective oxidation of NH3 by O2 when Zn was added to the catalyst samples. Liu et al. [26] also indicated that the presence of Zn and P would promote NH3 oxidation and result in a N2 selectivity decrease.

3.2. BET, SEM and XRD analysis The BET values of the four catalyst samples are summarized in Table 1. As can be seen from Table 1, the introduction of Al2 (SO4 )3 into Mn/TiO2 catalyst leads to a slight increase of BET surface area and total pore volume, which is beneficial to the adsorption of reactant species. On the contrary, an obvious drop of BET surface and total pore volume could be observed for the two poisoned catalyst samples, which might be due to the surface shielding/loading by Zn species. To observe the surface micrograph of the four catalyst samples, SEM analysis was performed and the results are illustrated in Fig. 2. From Fig. 2, it could be found that the addition of Al2 (SO4 )3 on Mn/TiO2 catalyst makes its surface become loose, indicating the decrease of aggregation degree, which is beneficial to increase its surface area. On the contrary, a remarkable aggregation could be found on the surfaces of the two poisoned catalyst samples, but there are still more defects on the surface of MnAl/TiO2 -K catalyst, which coincides with its relatively higher surface area than that of Mn/TiO2 -K catalyst. The crystal structures of the four catalyst samples were determined by XRD analysis, and the XRD spectra of them are presented in Fig. 3. Apparently, only the diffraction peaks of anatase phase TiO2 could be detected, indicating that Mn, Al and Zn species are mainly in amorphous state. It is clear that the peak intensities in the spectrum of MnAl/TiO2 catalyst are much lower than that in the spectrum of Mn/TiO2 catalyst, which suggests the decreased crystallinity of Mn/TiO2 catalyst after the loading of Al2 (SO4 )3 . This result might be originated from the strong interaction among Mn, Al and Ti species. After the loading of Zn, the diffraction peaks in the spectrum of the two poisoned catalyst samples become stronger than that in the corresponding fresh samples. This phenomenon should be caused by the gradual formation of large TiO2 particles during the Zn loading process. As a consequence, the Znpoisoned catalyst samples would sinter [27], leading to the decrease of their BET surface areas. Thus the addition of Zn would lead to the growth of TiO2 crystal, similar effect has also been reported by other studies [28, 29].

The compositions and chemical states of surface elements of the four catalyst samples are determined by XPS analysis, and the results are given in Table 2 and Fig. 4 respectively. The Mn 2p3/2 XPS spectra of the four catalyst samples are illustrated in Fig. 4(A). After a peak-fitting deconvolution, the spectra could be separated into 3 peaks corresponding to Mn2+ , Mn3+ and Mn4+ respectively [30–32]. From Table 2, the molar ratio of Mn4+ /(Mn2+ +Mn3+ +Mn4+ ) of Mn/TiO2 and MnAl/TiO2 are 21.9% and 27.2% respectively. Combined with the surface Mn atomic concentrations of them, the surface Mn4+ concentrations of the two catalyst samples could be obtained, as summarized in Table 2. According to previous studies, the Mn species with high valence state are more active in redox action [33, 34]. And the presence of more Mn4+ could promote the oxidation of NO to NO2 , as a result, facilitating the SCR reaction through “fast SCR” pathway: NO+NO2 +2NH3 =2N2 +3H2 O. Thus the presence of more Mn4+ species on the surface of MnAl/TiO2 catalyst is helpful to enhance its SCR activity, especially in low-temperature range [35]. As contrast, the addition of Zn to the two fresh catalyst samples lead to the decrease of surface Mn4+ concentration, as shown in Table 2, which is in accordance of the bad SCR performances of Mn/TiO2 -Zn and MnAl/TiO2 -Zn. The O 1s XPS spectra of the four catalyst samples are exhibited in Fig. 4(B), which could be fitted into two peaks: the one at lower binding energy (529.0–530.0 eV) could be assigned to lattice oxygen (denoted as Oα ), and the other one at higher binding energy (531.3–532.0 eV) belongs to surface chemisorbed oxygen species (denoted as Oβ ), including O22− and O − in defect-oxide or hydroxyl-like group, etc. [36–38]. As listed in Table 2, the Oβ concentration of MnAl/TiO2 catalyst is higher than that of Mn/TiO2 catalyst. It is well recognized that surface chemisorbed oxygen is favorable to NO oxidation due to its higher mobility than that of lattice oxygen [39]. Correspondingly, the “fast SCR” reaction over MnAl/TiO2 catalyst is accelerated. Furthermore, the decreased Oβ concentrations over the two poisoned catalyst samples should be partly responsible for their low SCR activities. It should be noticed that the Oβ concentration over MnAl/TiO2 -Zn catalyst is still 21.2 at.%, which is in consistent with its relatively higher SCR activity than that of Mn/TiO2 -Zn catalyst.

3.4. H2 -TPR analysis The redox behavior of the four catalyst samples are investigated by H2 -TPR analysis, and the results are shown in Fig. 5. It is clear that there are three reduction peaks in the profile of each catalyst sample. For the profile of Mn/TiO2 catalyst, the three reduction peaks appear at 366, 456 and 614 °C, which could be assigned to MnO2 →Mn2 O3 , Mn2 O3 →Mn3 O4 and Mn3 O4 →MnO respectively [40–42]. After the modification with Al2 (SO4 )3 , the reduction peaks move to lower temperature, as shown in the profile of MnAl/TiO2 catalyst. These features indicate the enhanced mobility of surface oxygen after the addition of Al2 (SO4 )3 [43], which should be resulted from the presence of more chemisorbed oxygen, as discussed in Section 3.3. Moreover, the peak area in the profile of MnAl/TiO2 catalyst is larger than that in the profile of Mn/TiO2 catalyst. All these features indicate that the modification with Al2 (SO4 )3 could greatly increase the redox ability of Mn/TiO2 catalyst, which is favorable to enhance its SCR activity and Zn resistance. For the two poisoned catalyst samples, it can be seen that the peaks of Mn4+ →Mn3+ in their profiles become smaller compared with that in the profiles of the corresponding fresh catalyst samples, indicating the decrease of Mn4+ species, as also reflected by the results of XPS analysis.

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Fig. 2. SEM images of (A) Mn/TiO2 , (B) MnAl/TiO2 , (C) Mn/TiO2 -K, (D) MnAl/TiO2 -K.

Table 2 Elemental surface analysis of the four catalyst samples (by XPS). Samples

Mn (at.%)

O (at.%)

Mn4+ /(Mn2+ +Mn3+ +Mn4+ ) (%)

Oβ / (Oα +Oβ )

Mn4+ (at.%)

Oβ (at.%)

Mn/TiO2 MnAl/TiO2 Mn/TiO2 -Zn MnAl/TiO2 -Zn

7.28 7.98 6.33 6.01

67.95 67.36 67.69 67.83

21.9 27.2 15.6 18.7

34.2 38.0 28.1 31.3

1.59 2.17 0.99 1.12

23.2 25.6 19.0 21.2

3.5. NH3 -TPD analysis Adsorption of NH3 on catalyst surface plays an important aspect in NH3 -SCR reaction [44,16], which is greatly dependent on the surface acidity of SCR catalyst. Thus NH3 -TPD analysis was performed to investigate the surface acidities of the four catalyst samples, and the results are demonstrated in Fig. 6. From Fig. 6, a continuous desorption peak from 100 to 500 °C could be detected in the TPD profile of each catalyst sample, suggesting the presence of adsorbed NH3 species with different thermal stability. It should be noticed that the peak intensity and the peak area in the profile of MnAl/TiO2 catalyst are higher than that in the profile of Mn/TiO2 catalyst, especially in the higher temperature range. Thus the addition of Al2 (SO4 )3 to Mn/TiO2 catalyst not only generates some new acid sites for NH3 adsorption, but also enhances the thermal stability of adsorbed NH3 species. The surface acidities of the four catalyst samples are listed in Table 3. A great decrease of surface acidity could be observed for the two poisoned catalyst samples, suggesting that the presence of Zn would seriously inhibit the adsorption of NH3 species.

Table 3 Surface acidities of the four catalyst samples. Samples Mn/TiO2 MnAl/TiO2 Mn/TiO2 -Zn MnAl/TiO2 -Zn

Surface acidity (mmol/g) 0.530 0.627 0.231 0.297

3.6. In situ DRIFT study To further understand the effect of Al2 (SO4 )3 modification on the SCR reaction over Mn/TiO2 catalyst, in situ DRIFT study was performed to identify the adsorbed species, the intermediates and the reaction pathway under different conditions.

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Fig. 5. H2 -TPR profiles of the four catalyst samples. Fig. 3. XRD spectra of the four catalyst samples.

Fig. 6. NH3 -TPD profiles of the four catalyst samples.

Fig. 4. (A) Mn 2p

3/2

and (B) O 1s XPS spectra of the four catalyst samples.

3.6.1. NH3 adsorption The DRIFT spectra of NH3 adsorption over the four catalyst samples are presented in Fig. 7. The bands at 1700 and 1447 cm−1 could be assigned to NH+ 4 species on Brønsted acid sites, and the bands at 1602 and 1178 cm−1 belong to NH3 species bound to Lewis acid sites [45–50]. It can be seen that the band intensities in the spectrum of MnAl/TiO2 catalyst are higher than that in the spectrum of Mn/TiO2 catalyst, meaning the adsorption of more NH3 species over MnAl/TiO2 catalyst. Noticeably, the band at 1602 cm−1 in the spectrum of MnAl/TiO2 catalyst is much stronger than the corresponding bond in the spectrum of Mn/TiO2 catalyst, implying the formation of more Lewis acid sites. And the study of Gu et al. [51] also observed the similar phenomenon on the SO2 pretreated CeO2 catalyst. According to previous study, the decomposition temperature of Al2 (SO4 )3 is above 770 °C [52], so SO24− species are still present on the surface of MnAl/TiO2 catalyst. Moreover, the formed surface SO24− could react with Zn to form zinc sulfate, which has a protection effect on the active species. On the other side, the addition of Zn would lead to the drop of NH3 species adsorption amount, as proven by the decreased band intensities in the spectra of the two poisoned catalyst samples. Thus the results of DRIFT study agree well with that of NH3 -TPD analysis.

Please cite this article as: S.-m. Liu et al., The enhancement of Zn resistance of Mn/TiO2 catalyst for NH3 -SCR reaction by the modification with Al2 (SO4 )3 , Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.039

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Fig. 7. In situ DRIFT spectra of NH3 adsorption over the four catalyst samples at 200 °C. Reaction conditions: [NH3 ] = 500 ppm, balance N2 , total flow rate = 300 mL/min.

Fig. 9. In situ DRIFT spectra of the reaction between NOx and the preadsorbed NH3 species over (A) Mn/TiO2 and (B) MnAl/TiO2 at 200 °C. Reaction conditions: [NO] = [NH3 ] = 500 ppm, [O2 ] = 5%, balance N2 , total flow rate = 300 mL/min.

Fig. 8. In situ DRIFT spectra of NO+O2 co-adsorption over the four catalyst samples at 200 °C. Reaction conditions: [NO] = 500 ppm, [O2 ] = 5%, balance N2 , total flow rate = 300 mL/min.

3.6.2. NO+O2 co-adsorption The DRIFT spectra of NO+O2 co-adsorption over the four catalyst samples are illustrated in Fig. 8. There are three bands of adsorbed NOx species could be found, including adsorbed NO2 species (1611 cm−1 ) [53,54], bidentate nitrate (1547 cm−1 ) [55] and monodentate nitrate (1258 cm−1 ) [55,56]. It is noticeable that the band intensities in the spectrum of MnAl/TiO2 catalyst are higher than that in the spectrum of Mn/TiO2 catalyst. Thus the addition of Al2 (SO4 )3 could promote the adsorption of NOx species. In stark contrast, the band intensities in the two poisoned catalyst samples are much lower than that in the spectra of the fresh catalyst samples, indicating the suppressed NOx adsorption by Zn. From the greatly dropped intensities of the band at 1611 cm−1 in the spectra of the two poisoned catalyst samples, it may be concluded that the loading of Zn could inhibit the oxidation of NO to NO2 . So the results of DRIFT study are in good accordance with that of XPS analysis.

3.6.3. Reaction between NOx and preadsorbed NH3 species To investigate the reactivity of the adsorbed NH3 species, the DRIFT spectra of the reaction between NOx and preadsorbed NH3 species over the two fresh catalyst samples are shown in Fig. 9. As can be seen from Fig. 9(A), several bands of adsorbed NH3 species (1602, 1447 and 1178 cm−1 ) appeared after the pretreatment with NH3 . After the introduction of NO+O2 , nearly all the bands disappeared in 2 min, indicating the high reactivity of adsorbed NH3 species. Moreover, several bands of adsorbed NOx species appeared after the exhaustion of adsorbed NH3 species, suggesting that the adsorbed NH3 species were replaced by surface nitrate species. From Fig. 9(B), it is obvious that the DRIFT spectra of the reaction between NOx and preadsorbed NH3 species over MnAl/TiO2 catalyst are very similar with that over Mn/TiO2 catalyst. All the adsorbed NH3 species could participate in the NH3 SCR reaction. Thus the Eley–Rideal (E–R) mechanism is suitable for the NH3 -SCR reactions over the two fresh catalyst samples [56]. Although more adsorbed NH3 species is present over MnAl/TiO2 catalyst, the adsorbed NH3 species still vanished basically in about 2 min.

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4. Conclusions Zn has a poisoning effect on Mn/TiO2 catalyst for selective catalytic reduction of NOx with NH3 . In this study, it was found that the modification of Mn/TiO2 catalyst with Al2 (SO4 )3 could greatly enhance its Zn resistance. From the characterization results, it may be concluded that the modification of Mn/TiO2 catalyst with Al2 (SO4 )3 could restrain the crystallization of Mn/TiO2 catalyst and increase its reducibility and surface acidity, accompanied by the formation of more Mn4+ and surface chemisorbed oxygen. The results of in situ DRIFT study indicated that the addition of Al2 (SO4 )3 to Mn/TiO2 catalyst only promote the adsorption of NH3 and NOx species on it, but not vary the NH3 -SCR reaction mechanism over it, that is, the combination of E–R and L–H mechanisms. Acknowledgments This work was supported by the Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800). Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2017.06.039. References

Fig. 10. In situ DRIFT spectra of the reaction between NH3 and the preadsorbed NOx species over (A) Mn/TiO2 and (B) MnAl/TiO2 at 200 °C. Reaction conditions: [NO] = [NH3 ] = 500 ppm, [O2 ] = 5%, balance N2 , total flow rate = 300 mL/min.

3.6.4. Reaction between NH3 and preadsorbed NOx species On the other side, the DRIFT spectra of the reaction between NH3 and preadsorbed NOx species over the two fresh catalyst samples are shown in Fig. 10. As presented in Fig. 10(A), the pretreatment of Mn/TiO2 catalyst by NO+O2 generated some adsorbed NOx species on its surface, as reflected by the three bands at 1611, 1549 and 1271 cm−1 respectively. And the bands of adsorbed NOx species vanished quickly after the introduction of NH3 for 2 min, revealing the high reactivity of adsorbed NOx species. Next then, several bands of adsorbed NH3 species came into being. For MnAl/TiO2 catalyst, the variation trend of its DRIFT spectra was basically similar with that of the DRIFT spectra of Mn/TiO2 catalyst (Fig. 10(B)). All the adsorbed NOx species are active in the NH3 -SCR reaction, thus the Langmuir– Hinshelwood (L–H) mechanism is also applicable for the NH3 -SCR reactions over Mn/TiO2 and MnAl/TiO2 catalysts [57]. Combined with the results of Sections 3.5.3 and 3.5.4, it seems that the modification of Mn/TiO2 catalyst does not change the mechanism of the NH3 -SCR reaction over it, which is under the control of both E–R and L–H mechanisms. Similar reaction mechanism is also reported in our recent studies [58, 59]. Similar results are also proven by the kinetic study, as given in the supplementary materials.

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Please cite this article as: S.-m. Liu et al., The enhancement of Zn resistance of Mn/TiO2 catalyst for NH3 -SCR reaction by the modification with Al2 (SO4 )3 , Journal of the Taiwan Institute of Chemical Engineers (2017), http://dx.doi.org/10.1016/j.jtice.2017.06.039