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TiO2 catalyst for NH3-SCR of NO
Journal of the Taiwan Institute of Chemical Engineers 64 (2016) 116–123
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The promotion effect of Co doping on the K resistance of Mn/TiO2 catalyst for NH3 -SCR of NO Qi-lin Chen a,b, Rui-tang Guo a,b,∗, Qing-shan Wang a,b, Wei-guo Pan a,b,∗, Ning-zhi Yang a,b, Chen-zi Lu a,b, Shu-xian Wang 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 24 November 2015 Revised 30 January 2016 Accepted 24 March 2016 Available online 12 April 2016 Keywords: Mn/TiO2 catalyst Cobalt Potassium Resistance Characterization
a b s t r a c t In this study, it was found that the doping of Co on Mn/TiO2 catalyst could enhance its K resistance. The characterization results showed that the modification of Mn/TiO2 catalyst by Co could increase its reducibility and the adsorption of NH3 and NOx species on it. From the results of in situ DRIFT study, the NH3 -SCR reaction over Mn/TiO2 catalyst obeyed the Langmuir–Hinshelwood mechanism. And the addition of K on Mn–Co/TiO2 had little inhibition effect on NOx adsorption. These features made Mn–Co/TiO2 catalyst show better K resistance than Mn/TiO2 . © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In recent several decades, selective catalytic reduction (SCR) process has been put into industrial application for controlling NOx emitted from stationary sources such as coal-fired power plants [1,2]. In this process, the anatase TiO2 supported V2 O5 –WO3 or V2 O5 –MoO3 oxides are the most widely used catalyst [3,4]. However, this catalyst still has several drawbacks, including the adoption of toxic vanadia, the narrow activity window (30 0–40 0 °C) and the deactivation by alkali metals in the fly ash [5–7]. If the SCR reactor is located in the downstream of electrostatic precipitator and wet flue gas desulfurization (WFGD) scrubber, these problems of vanadium-based catalyst mentioned above may be avoid. Due to the fact that the flue gas temperature after desulfurization is generally below 200 °C, therefore, developing new low-temperature SCR catalyst working at 100–250 °C may be a promising way. Manganese oxides, as an environmental-friendly catalyst, are of excellent low-temperature SCR activity [8–10]. It has been proven that the presence of various types of labile oxygen on the surface of manganese oxides plays an important role in the catalytic cycle of SCR reaction [11,12]. In order to enhance the SCR activity and SO2 resistance of manganese-based catalyst, several transition metals (such as Ni, Ce, Cu, Fe, Co, etc.) have been used to mod-
ify the manganese-based catalyst [7,13–17]. However, manganesebased SCR catalyst still suffers the deactivation by alkali metals contained in the fly ash [18–22]. The alkali metals (mainly Na and K) in the fly ash have a serious poisoning effect on SCR catalyst. The loss of reducibility, the decrease of surface acidity and the plug of catalysts channels are usually regarded as the main factors leading to the deactivation of SCR catalyst by alkali metals. Recently, the addition of some transition metals (including Ce, Sb and Nb) have been reported to be an effective method to enhance the resistance of SCR catalyst to alkali metal poisoning [23,24]. It has been reported that Co3+ species in cobalt oxides could facilitate the adsorption of NH3 and enhance the SCR activity [25]; nevertheless, the effect of Co addition on the resistance of Mn/TiO2 catalyst to alkali metals poisoning has not been investigated. In this work, Mn/TiO2 and Mn–Co/TiO2 catalysts were prepared by sol–gel method and then poisoned by K. And the main purpose of this work is to investigate the mechanism of Co addition on the K resistance of Mn/TiO2 catalyst based on the experimental and characterization results. 2. Experimental 2.1. Catalyst preparation
∗
Corresponding authors at: School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, PR China. Tel./fax: +86 2165430410. E-mail addresses: [email protected] (R.-t. Guo), [email protected] (W.-g. Pan).
Mn/TiO2 catalyst was prepared with sol–gel method. Butyl titanate (0.1 mol), deionized water (1.9 mol), nitric acid (0.1 mol),
http://dx.doi.org/10.1016/j.jtice.2016.03.045 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Q.-l. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 64 (2016) 116–123
a
Mn/TiO
100
Mn-Co/TiO 1/8K-Mn/TiO
90
1/8K-Mn-Co/TiO
80
1/4K-Mn/TiO 1/4K-Mn-Co/TiO
70
NO conversion (%)
anhydrous ethanol (2.9 mol) and manganese nitrate (0.02 mol) were mixed under fully stirring at room temperature to yield a yellowish transparent sol. After dried at 80 °C for 24 h, the sol would convert into a xerogel. Then the xerogel was calcined at 500 °C for 5 h to obtain the Mn/TiO2 catalyst. Based on the same method, Mn–Co/TiO2 catalyst was prepared. And cobaltous nitrate was used as the precursor of Co. For Mn–Co/TiO2 , the molar ratio of (Mn + Co)/Ti was set as 0.2:1, and the molar ratio of Mn/Co was set as 2:1. The K-poisoned catalyst was prepared with impregnation method. Potassium (K) was added by impregnating the Mn/TiO2 catalyst (or Mn–Co/TiO2 catalyst) in KNO3 solution with the required concentration. Thereafter, the mixture was stirred for 5 h, dried at 80 °C for 24 h and calcined at 500 °C for 5 h . The Kpoisoned catalyst samples were denoted as xK–Mn/TiO2 and xK– Mn–Co/TiO2 respectively, where x represents the molar ratio of K/Mn (or K/(Mn + Co)).
117
1/2K-Mn/TiO 1/2K-Mn-Co/TiO
60 50 40 30 20 10 0 80
100
120
140
160
180
200
220
240
260
220
240
260
Reaction temperature ( C)
2.2. Characterizations The textural properties of the catalyst samples were measured by N2 adsorption at 77 K using a Quantachrome Autosorb-iQ-AG instrument. The specific surface area and the pore size distribution were determined by the Brunauer–Emmett–Teller (BET) method and the Barrett–Joyner–Halenda (BJH) method respectively. Powder XRD was performed on an X-ray diffractometer (XRD) with CuKα radiation (Bruker D8 Advance, Germany). X-ray photoelectron spectroscopy with Al Kα X-ray (hν = 1486.6 eV) radiation (XPS: Thermo ESCALAB 250, USA) was used to study the chemical states for all elements on the catalyst surface. The shift of the binding energy was calibrated internally by a carbon deposit C1s binding energy at 284.6 eV. The temperature programmed reduction with hydrogen (H2 -TPR) and the temperature programmed desorption of NH3 (NH3 -TPD) were performed on a Quantachrome Autosorb-iQC chemisorption analyzer to study the redox ability and surface acidity of the catalyst samples respectively. The signal of H2 or NH3 was monitored with a thermal conductivity detector (TCD). The in situ DRIFT spectra were collected by a FTIR spectrometer (Thermo, Nicolet iS 50) with an in situ diffuse reflectance cell and a DTGS detector cooled by liquid N2 . Prior to each experiment, the sample was pretreated at 500 °C in a 10% O2 /N2 environment for 2 h, then it was cooled down to the desired reaction temperature (150◦ C). The background spectrum was recorded with the flowing of N2 and subtracted from the sample spectrum. The experimental conditions were set as: 300 mL/min total flow rate, 800 ppm of NH3 or/and 800 ppm of NO + 5 vol% O2 , and N2 balance. 2.3. Activity test The catalytic activity measurements were carried out on a fixed-bed quartz microreactor (8 mm i.d.) using about 0.55 cm3 catalyst sample of 80–100 mesh. The simulated flue gas stream contained 600 ppm NO, 600 ppm NH3 , 5% O2 , and Ar as the balance. The total flow rate of the simulated flue gas was 1 L/min (GHSV = 108,0 0 0 h−1 ). The reaction temperature range for catalytic activity test was from 100 to 240 °C. The concentrations of NO and NO2 in the inlet and outlet gases were measured by a NO– NO2 –NOx analyzer (Thermo, Model 42 i-HL); meanwhile, the formation of N2 O was monitored using a Gasmet FTIR gas analyzer (DX-40 0 0). The NO conversion and the N2 selectivity can be calculated by the following equations:
NO conversion = ([NO]in − [NO]out )/[NO]in × 100%
(1)
N2 selectivity (%)
b 100
95
Mn/TiO
90
Mn-Co/TiO 1/4K-Mn/TiO 1/4K-Mn-Co/TiO
85 80
100
120
140
160
180
200
Reaction temperature ( C) Fig. 1. (A) NO conversion as a function of reaction temperature over different catalysts; (B) N2 selectivity over different catalyst samples.
N2 selectivity = ([NO]in + [NH3 ]in − [NO2 ]out − 2[N2 O]out )/
([NO]in + [NH3 ]in ) × 100%
(2)
To investigate the oxidation of NO over the catalyst samples, the similar experimental conditions were adopted, in which NH3 was not used as the reactant gas. NO conversion to could be determined by:
NO conversion to NO2 = [NO2 ]out /[NOx ]out × 100%
(3)
3. Results and discussion 3.1. Catalytic performance The SCR activities of the catalyst samples as a function of reaction temperature are presented in Fig. 1(A). As can be seen from Fig. 1(A), Mn/TiO2 and Mn–Co/TiO2 are of similar catalytic activities. And the NO conversion increases with increasing reaction temperature in the whole experimental temperature range for each catalyst sample. For the two fresh catalyst samples of Mn/TiO2 and Mn–Co/TiO2 , a sharp decrease of SCR activity could be observed after the addition of K. And the poisoning effect of K increases with increasing K loading amount. It is obvious that Mn–Co/TiO2
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25
A---anatase TiO2 B--- rutile TiO2 A B
B
15
10
A
A
Intensity ( a.u.)
NO conversion (%)
20
Mn-Co/TiO2 Mn/TiO2 B A
Mn-Co/TiO
5
Mn/TiO
B
1/4K-Mn/TiO
1/4K-Mn/TiO2
1/4K-Mn-Co/TiO
0
80
120
160 200 Reaction temperature( C)
240
A
1/4K-Mn-Co/TiO2 B B
20
30
40
50
60
70
80
2 ( °)
Fig. 2. NO oxidation to NO2 over the four catalyst samples.
Fig. 3. XRD patterns of the four catalyst samples.
has a better K resistance than Mn/TiO2 . For instance, when the reaction temperature is 200 °C , the NO conversions on Mn/TiO2 and Mn–Co/TiO2 are all about 98%, while the corresponding values on 1/4K–Mn/TiO2 and 1/4K–Mn–Co/TiO2 are 62% and 46%, respectively. The values of N2 selectivity over the different catalyst samples are shown in Fig. 1(B). It is clear that the N2 selectivity decreases with increasing temperature, which should be resulted from the partial oxidation of NH3 at elevated temperature. As can be seen from Fig. 1(B), the two poisoned catalyst samples show lower N2 selectivities compared with the fresh samples. A similar trend was also found by Peng et al. [26,27].
decrease sharply. This may be caused by the block of microspores or the agglomeration of catalyst during the doping process of K [29]. Fig. 3 shows the XRD patterns of the catalyst samples. For all the catalyst samples, only anatase TiO2 phase and rutile TiO2 phase could be detected in the XRD spectra, indicating that the species of Mn, Co and K are in amorphous structure or highly dispersed on the surface of TiO2 carrier. Mn/TiO2 and Mn–Co/TiO2 are of the similar crystallinity. And the addition of K makes the XRD peak intensities of Mn/TiO2 and Mn–Co/TiO2 increase greatly; therefore, the addition of K would increase the crystallinity. A similar result has also been observed by Jiang et al. [30] when they studied the poisoning effect of K on V/TiO2 catalyst. The high XRD peak intensities of 1/4K–Mn/TiO2 and 1/4K–Mn–Co/TiO2 mean the formation of large TiO2 particles, which would lead to the sintering of the catalyst samples and cause the decrease of specific area, as listed in Table 1. While some other researchers found that the addition of K has little effect on the crystalline phase of SCR catalyst [31,32]. The different results may be attributed to the different K contents of the catalyst samples used in different studies. In addition, a diffraction peak of anatase TiO2 in the XRD spectra of the two poisoned catalyst samples moves to a higher 2θ value compared to that in the XRD spectra of the fresh catalyst samples, which may be resulted from the lattice contraction [33].
3.2. Activity of NO oxidation to NO2 The oxidation activities of NO to NO2 over the four catalyst samples as a function of reaction temperature are shown in Fig. 2. It is obvious that the activities of NO oxidation over the two fresh catalyst samples are higher than that over the two poisoned catalyst samples. Moreover, the addition of Co to Mn/TiO2 catalyst could enhance its NO oxidation activity. According to the study of Long and Yang [28], the presence of NO2 could greatly enhance the SCR activity of Fe-ZSM-5 and the reaction rate of NH3 and the mixture of NO and NO2 was much higher than that with NO alone. Therefore, the high activity for NO oxidation to NO2 over Mn–Co/TiO2 catalyst is also helpful to enhance its K resistance. 3.3. BET and XRD analysis The results of BET analysis are summarized in Table 1. From Table 1, it can be observed that the BET surface area and the pore volume of Mn–Co/TiO2 are larger than that of Mn/TiO2 , which may be resulted from the strong interaction among Mn, Co and Ti. After the addition of K, the BET surface areas and the pore volumes of Table 1 Textural properties of different catalysts.
Samples
BET surface area (m2 /g)
Total pore volume (cm3 /g)
Average pore diameter (nm)
Mn/TiO2 Mn–Co/TiO2 1/4K–Mn/TiO2 1/4K–Mn–Co/TiO2
125.80 133.70 33.06 46.02
0.15 0.17 0.05 0.06
1.92 1.70 1.81 1.91
3.4. H2 -TPR analysis H2 -TPR analysis was performed to investigate the redox behaviors of the catalyst samples, and the results are illustrated in Fig. 4. As shown in Fig. 4, there are two reduction peaks in the profile of Mn/TiO2 catalyst, the first one at about 375 °C could be assigned to a combination of MnO2 →Mn2 O3 and Mn2 O3 →Mn3 O4 [9,34]; and the second one at about 450 °C may be related to Mn3 O4 →MnO [9,34]. For Mn–Co/TiO2 catalyst, three reduction peaks at about 340 °C, 410 °C and 680 °C could be observed, the first one can be attributed to the three reduction process of MnO2 →Mn2 O3 , the second one could be assigned to the combination of Mn2 O3 →Mn3 O4 and Co3+ →Co2+ [34,35]. The third one may be associated to the reduction of the surface capping oxygen species [36]. For the two poisoned catalyst samples, the reduction peaks in their profiles are of the similar nature with that in the profiles of the corresponding fresh catalyst samples. It is well recognized that the reduction peak temperature is an indication of reducibility; lower reduction peak means stronger reducibility [37].
Q.-l. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 64 (2016) 116–123
H.C.=5.42mmol/g
Mn-Co/TiO2
Co
119
Co
2+
3+
Mn/TiO2
H.C.=5.39mmol/g
1/4K-Mn-Co/TiO2
H.C.=5.28mmol/g
Intensity (a.u.)
TCD signal(a.u.)
3+
[Co ]=1.73 at.%
Mn-Co/TiO2
H.C.=4.67mmol/g 1/4K-Mn/TiO2 100
200
300
400
500
600
700
800
900
o
Temperature( C)
3.5. XPS analysis XPS analysis was performed to identify the surface nature and element concentrations. The XPS spectra of Mn 2p3/2 of the catalysts are shown in Fig. 5. According to previous studies [38–40], the
Mn
4+
3+
Mn-Co/TiO2
4+
Intensity (a.u.)
2+
Mn
[Mn ]=4.56at. %
Mn/TiO2
4+
[Mn ]=4.46at. %
1/4K-Mn-Co/TiO2
4+
[Mn ]=3.03at. %
1/4K-Mn/TiO2
4+
[Mn ]=2.42at. % 644
642
785
780
775
770
Fig. 6. Co 2p XPS spectra of Mn–Co/TiO2 and 1/4K–Mn–Co/TiO2 .
Therefore, it may be concluded that the doping of Co on Mn/TiO2 catalyst leads to an increase of its reducibility. From Fig. 4, it can be seen that the reduction peak temperatures of the two fresh catalyst samples move to a higher value after the addition of K, indicating the stabilization of Mn species, which makes them less reducible. In addition, the decrease of catalyst reducibility by the addition of K could also be reflected by the values of H2 consumption (H.C.), as shown in Fig. 4. It is clear that the H2 -consumption of 1/4K–Mn/TiO2 is much lower than that of the other three samples. The reducibilities of the four catalyst samples are in the following sequence: Mn–Co/TiO2 > Mn/TiO2 > 1/4K–Mn–Co/TiO2 > 1/4K– Mn/TiO2 . So the high reducibility of Mn–Co/TiO2 should also play an important role for its excellent resistance to K-poisoning.
Mn
790
Binding energy (eV)
Fig. 4. H2 -TPR profiles of the four catalyst samples.
646
1/4K-Mn-Co/TiO2
3+
[Co ]=1.02 at.%
640
638
Binding energy (eV) Fig. 5. Mn 2p3/2 XPS spectra of the four catalyst samples.
636
Mn 2p3/2 XPS spectra can be deconvoluted into three peaks corresponding to Mn2+ , Mn3+ and Mn4+ respectively. So the results of XPS analysis are in line with that of H2 -TPR analysis. Based on the results of XPS analysis, we can obtain the surface atomic concentration of Mn4+ on the surface of each catalyst sample, as shown in Fig. 5. From Fig. 5, it can be seen that the surface atomic concentration of Mn4+ on the surface of Mn–Co/TiO2 is higher than that on the surface of Mn/TiO2 ; as for the two poisoned catalyst samples, the surface Mn4+ concentration of 1/4K–Mn–Co/TiO2 is still higher than that of Mn/TiO2 . It has been proven that Mn4+ plays a crucial role in the redox cycle of SCR reaction [7], combined with the H2 -TPR results, the high surface Mn4+ concentration of Mn– Co/TiO2 should be the main origin of its high reducibility, which is helpful to enhance its low-temperature SCR activity. In addition, Mn4+ could promote the oxidation of NO to NO2 , which also facilitates the SCR reaction [41]. Fig. 6 shows the Co 2p XPS spectra of Mn–Co/TiO2 and 1/4K– Mn–Co/TiO2 . After a peak-fitting deconvolution, the Co 2p spectra can be separated into two peaks: Co2+ and Co3+ [35,42]. And the surface Co3+ concentrations of each catalyst were also calculated and shown in Fig. 6. It is clear that the surface Co3+ concentration of the fresh catalyst is higher than that of the poisoned catalyst. It is reported that Co3+ ions could facilitate the adsorption of NH3 [25], so the decrease of surface Co3+ concentration of 1/4K– Mn–Co/TiO2 agrees well with its relatively low surface acidity, as reflected by the results of NH3 -TPD analysis. Fig. 7 exhibits the O 1s XPS spectra of the catalysts. The O 1s peak can be fitted into two peaks: the peak at lower binding energies (528.7–530.9 eV) is attributed to lattice oxygen (Oα ); and the other one at higher binding energies (531.4–532.5 eV) is related to the surface adsorbed oxygen (Oβ ) [41]. From the results of XPS analysis, we can calculate the concentrations of surface adsorbed oxygen of each catalyst sample, as shown in Fig. 7. Fig. 7 implies that Mn–Co/TiO2 catalyst is of the highest surface adsorbed oxygen concentration among the four catalyst samples. For the two-poisoned catalyst samples, the Oβ concentrations of them are notably lower than that of the fresh catalyst samples. But the Oβ concentration of 1/4K–Mn–Co/TiO2 is still higher than that of 1/4K–Mn/TiO2 . As the most active oxygen species in SCR reaction, the surface adsorbed oxygen can promote the oxidation of NO to NO2 , as a result, facilitating the “fast SCR” reaction [41,43]. Based on the results of XPS analysis, the conclusion may be drawn that the high concentrations of Mn4+ , Co3+ and Oβ should also
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O
O
1600 1685
1545
[O ]=30.73 at.%
1430 Mn/TiO2
[O ]=29.65 at.%
1/4K-Mn-Co/TiO2
[O ]=25.42 at.%
Absorbance(a.u.)
Intensity (a.u.)
Mn-Co/TiO2
1200
Mn-Co/TiO2 Mn/TiO2 1/4K-Mn-Co/TiO2 1/4K-Mn/TiO2
1/4K-Mn/TiO2 536
[O ]=19.77 at.%
534
532
530
528
526
2000
1800
Binding energy (eV)
be partly responsible for the good K-resistance of Mn–Co/TiO2 catalyst. 3.6. NH3 -TPD analysis The decrease of both number and strength of the acid sites on the surface of SCR catalyst is regarded as one of the main reasons for the catalyst deactivation by alkali metals [44–48]. Therefore, the surface acidities of the catalyst samples were investigated by NH3 -TPD analysis, and the results are shown in Fig. 8. As shown in Fig. 8, the peak tops in the NH3 -TPD profiles locate in the temperature range of 121–132 °C, 228–263 °C and 310–350 °C respectively. The three kinds of desorption peaks can be attributed to the desorption of physisorbed NH3 and some NH+ 4 bounded to the weak Brønsted acid sites; the NH+ desorbed from the strong Brønsted 4 acid sites; and the desorption of NH3 from Lewis acid sites [49,50]. Noticeably, the total area of the desorption peaks in the profile of Mn–Co/TiO2 is distinctly larger than that of the desorption peaks in the profile of Mn/TiO2 . Thus the introduction of Co into Mn/TiO2 could enhance its surface acidity. After the addition of K, the des-
350
TCD signal(a.u.)
310
Mn-Co/TiO2
Mn/TiO2
235 121
1/4K-Mn/TiO2
263 132
1/4K-Mn-Co/TiO2 100
200
300
400 o
Temperature ( C) Fig. 8. NH3 -TPD profiles of the four catalyst samples.
-1
1400
1200
Fig. 9. In situ DRIFT spectra of NH3 adsorption on the four catalyst samples.
Fig. 7. O 1s XPS spectra of the four catalyst samples.
228
1600
Wavenumber(cm )
500
orption peak area decreases significantly, which suggests that K has an inhibition effect on the adsorption of NH3 . And the NH3 adsorption capacity of 1/4K–Mn–Co/TiO2 is still higher than that of 1/4K–Mn/TiO2 , therefore, the strong surface acidity of Mn–Co/TiO2 should also partly contribute to its good K-resistance. 3.7. In situ DRIFT study To further understand the surface acidity of the four catalyst samples, the in situ DRIFT spectra of NH3 adsorption on them were recorded at 150 °C, and the results are shown in Fig. 9. Fig. 9 shows the presence of Lewis acid sites (1600 and 1200 cm−1 ) [51,52] and −1 NH+ 4 species on Brønsted acid sites (1685, 1430 cm ) on the catalyst samples [53,54]. It is clear that NH3 species adsorbed on Lewis acid sites is much more than that adsorbed on Brønsted acid sites. As can be seen from Fig. 9, the band intensities of Lewis acid sites and Brønsted acid sites in the spectrum of Mn–Co/TiO2 are higher than that in the spectrum of Mn/TiO2 . Thus the doping of Co could enhance the surface acidity, which is in accordance with the results of NH3 -TPD. And the band at 1545 cm−1 could be attributed to NH2 species [54]. It is the product of partial oxidation of adsorbed NH3 species on catalyst surface. The formation of NH2 is regarded as the key step in NH3 -SCR reaction [55]. And the relatively higher intensity of the band at 1545 cm−1 the spectrum of Mn–Co/TiO2 indicates a strong generation of amide on this catalyst. After the addition of K, a sharp decrease of band intensities could be observed, indicating that the great decrease of surface acidity of the two fresh catalyst samples, which is in accordance with the results of NH3 -TPD analysis results. The DRIFT spectra of NO+O2 adsorption on the four catalyst samples are illustrated in Fig. 10. From Fig. 10, several bands assigned to different nitrate species could be found, including monodentate nitrate (1540, 1483, and 1260 cm−1 ) and bridging nitrate (1605 cm−1 ) [56–58]. Compared with Mn/TiO2 , Mn–Co/TiO2 shows larger amount of adsorbed NOx species, as shown in Fig. 10. It can be seen from Fig. 10, the addition of K has a strong inhibition effect on NOx adsorption for Mn/TiO2 catalyst; as for Mn–Co/TiO2 catalyst, the doping of K has little effect on NOx adsorption. To determine the role of adsorbed NH3 species in the SCR reaction over Mn/TiO2 and Mn–Co/TiO2 catalysts, the in situ DRIFT spectra of reaction between preadsorbed NH3 species and NO + O2 at 150 °C as a function of time are shown in Fig. 11(A) and (B). It is obvious that the surfaces of Mn/TiO2 and Mn–Co/TiO2
Q.-l. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 64 (2016) 116–123
121
a
1605
1605
1540
1515 1260 1200
N2 purging
1483 1260 Absorbance(a.u.)
Absorbance(a.u.)
30min
Mn-Co/TiO2 Mn/TiO2
20min
1540 1483
10min 5min
1/4K-Mn-Co/TiO2
2min
1/4K-Mn/TiO2
NOx 2000
1800
1600
-1
1400
1200
Wavenumber(cm ) 2000
1800
1600
1400
1200
-1
Wavenumber(cm )
b
1515 1605
Fig. 10. In situ DRIFT spectra of NO + O2 adsorption on the four catalyst samples.
1260 1200
pure N2
a Absorbance(a.u.)
1280
1540 1605
Absorbance(a.u.)
N2 purging 30min
30min 20min 1540
10min 5min 2min
20min 1600 1200
10min 5min
2000
1545
1800
1600
1400
1200
-1
Wavenumber(cm )
2min NH3 2000
NOx
1430 Fig. 12. In situ DRIFT spectra of NH3 reacted with preadsorbed NO + O2 species on Mn/TiO2 (A) and Mn–Co/TiO2 (B).
1800
1600
1400
1200
-1
Wavenumber(cm )
b
1540 1605 1280
Absorbance(a.u.)
N2 purging 30min 20min 10min 1200
5min 2min
1600 1545
NH3 2000
1800
1430
1600
1400
1200
-1
Wavenumber(cm ) Fig. 11. In situ DRIFT spectra of NO + O2 reacted with preadsorbed NH3 species on Mn/TiO2 (A) and Mn–Co/TiO2 (B).
catalysts were covered by several NH3 species after exposure to NH3 . After the gas mixture of NO + O2 was introduced, the bands of NH3 species vanished, meanwhile, several bands of NOx species appeared. Therefore, both the NH3 bound to Lewis acid sites and NH+ 4 bound to Brønsted acid sites are active species in the NH3 SCR reaction over Mn/TiO2 and Mn–Co/TiO2 catalysts. On the other hand, the role of adsorbed NOx species in the NH3 -SCR reaction over Mn/TiO2 and Mn–Co/TiO2 catalysts was also investigated by DRIFT study. And the DRIFT spectra of reaction between preadsorbed NOx species and NH3 are shown in Fig. 12(A) and (B). When NH3 was introduced, the bands of NOx species decreased gradually with time. Therefore, the adsorbed NOx species could also take part in the NH3 -SCR reaction, but the activity of adsorbed NOx species are lower than that of adsorbed NH3 species. Thus the NH3 -SCR reaction over Mn/TiO2 and Mn–Co/TiO2 catalysts is controlled by Langmuir–Hinshelwood mechanism, in which the adsorbed NH3 species react with the adsorbed NOx species to form N2 and H2 O. Therefore, the promoted adsorption of NH3 species and NOx species on the surface of Mn–Co/TiO2 catalyst could greatly enhance its K resistance. As shown in Fig. 11, the addition of K on Mn–Co/TiO2 nearly has no inhibition effect on the adsorption of NOx species, which is also beneficial to its K resistance.
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4. Conclusions The effect of Co doping on the K-resistance performance of Mn/TiO2 SCR catalyst was investigated in this study. The experimental results showed that Mn–Co/TiO2 had better K-resistance than Mn/TiO2 . The results H2 -TPR and NH3 -TPD showed that Mn–Co/TiO2 exhibits stronger reducing ability and surface acidity. From the results of XPS analysis, we find that the presence of higher concentrations of Mn4+ and Co3+ on the surface of Mn–Co/TiO2 , which resulting in its good redox property and high NH3 adsorption capacity. Moreover, the presence of rich surface adsorbed oxygen greatly enhances the “fast SCR” reaction. And the results of DRIFT study indicated that the NH3 -SCR reaction over Mn/TiO2 catalyst and Mn–Co/TiO2 catalysts followed the Langmuir–Hinshelwood mechanism. It seemed that the adsorption of NH3 species and NOx species on Mn/TiO2 catalyst was greatly inhibited by the loading of K, while the adsorption capacity of NOx species on Mn–Co/TiO2 catalyst rarely changed after the addition of K. These good features could give a suitable explanation for the excellent resistance of Mn–Co/TiO2 catalyst to K-poisoning. Acknowledgments This work was supported by the National Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800). References [1] Janssens TVW, Falsig H, Lundegaard LF, Vennestrøm PNR, Rasmussen SB, Moses PG, et al. A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia. ACS Catal 2015;5:2832. [2] Kacimi M, Ziyad M, Liotta LF. Cu on amorphous AlPO4 : preparation, characterization and catalytic activity in NO reduction by CO in presence of oxygen. Catal Today 2015;241:151. [3] Foo R, Vazhnova T, Lukyanov DB, Millington P, Collier J, Rajaram R, et al. Formation of reactive Lewis acid sites on Fe/WO3 –ZrO2 catalysts for higher temperature SCR applications. Appl Catal B: Environ 2015;162:174. [4] Fang C, Shi L, Hu H, Zhang J, Zhang D. Rational design of 3D hierarchical foam-like Fe2 O3 @CuOx monolith catalysts for selective catalytic reduction of NO with NH3 . RSC Adv 2015;5:11013. [5] Lu X, Song C, Jia S, Tong Z, Tang X, Teng Y. Low-temperature selective catalytic reduction of NOx with NH3 over cerium and manganese oxides supported on TiO2 –graphene. Chem Eng J 2015;260:776. [6] Putluru SSR, Schill L, Jensen AD, Siret B, Tabaries F, Fehrmann R. Mn/TiO2 and Mn–Fe/TiO2 catalysts synthesized by deposition precipitation – promising for selective catalytic reduction of NO with NH3 at low temperatures. Appl Catal B: Environ 2015;165:628. [7] Thirupathi B, Smirniotis PG. Nickel-doped Mn/TiO2 as an efficient catalyst for the low-temperature SCR of NO with NH3 : catalytic evaluation and characterizations. J Catal 2012;288:74. [8] Wang X, Zheng Y, Xu Z, Liu X, Zhang Y. Low-temperature selective catalytic reduction of NO over MnOx /CNTs catalysts: effect of thermal treatment condition. Catal Commun 2014;50:34. [9] Maitarad P, Zhang D, Gao R, Shi L, Li H, Huang L, et al. Combination of experimental and theoretical investigations of MnOx /Ce0.9 Zr0.1 O2 nanorods for selective catalytic reduction of NO with ammonia. J Phys Chem C 2013;117:9999. [10] Pourkhalil M, Moghaddam AZ, Rashidi A, Towfighi J, Mortazavi Y. Preparation of highly active manganese oxides supported on functionalized MWNTs for low temperature NOx reduction with NH3 . Appl Surf Sci 2013;279:250. [11] Wallin M, Forser S, Thormählen P, Skoglundh M. Screening of TiO2 -supported catalysts for selective NOx reduction with ammonia. Ind Eng Chem Res 2004;43:7723. [12] Park TS, Jeong SK, Hong SH, Hong SC. Selective catalytic reduction of nitrogen oxides with NH3 over natural manganese ore at low temperature. Ind Eng Chem Res 2001;40:4491. [13] Wu Z, Jin R, Liu Y, Wang H. Ceria modified MnOx /TiO2 as a superior catalyst for NO reduction with NH3 at low-temperature. Catal Commun 2008;9:2217. [14] Thirupathi B, Smirniotis PG. Co-doping a metal (Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) on Mn/TiO2 catalyst and its effect on the selective reduction of NO with NH3 at low-temperatures. Appl Catal B: Environ 2011;110:195. [15] Kang M, Park ED, Kim JM, Yie JE. Cu–Mn mixed oxides for low temperature NO reduction with NH3 . Catal Today 2006;111:236. [16] Jiang B, Li Z, Li S. Mechanism study of the promotional effect of O2 on low-temperature SCR reaction on Fe–Mn/TiO2 by DRIFT. Chem Eng J 2013;225:52. [17] Qiu M, Zhan S, Yu H, Zhu D, Wang S. Facile preparation of ordered mesoporous MnCo2 O4 for low-temperature selective catalytic reduction of NO with NH3 . Nanoscale 2015;7:2568.
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The promotion effect of Co doping on the K resistance of Mn/TiO2 catalyst for NH3 -SCR of NO Qi-lin Chen a,b, Rui-tang Guo a,b,∗, Qing-shan Wang a,b, Wei-guo Pan a,b,∗, Ning-zhi Yang a,b, Chen-zi Lu a,b, Shu-xian Wang 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 24 November 2015 Revised 30 January 2016 Accepted 24 March 2016 Available online 12 April 2016 Keywords: Mn/TiO2 catalyst Cobalt Potassium Resistance Characterization
a b s t r a c t In this study, it was found that the doping of Co on Mn/TiO2 catalyst could enhance its K resistance. The characterization results showed that the modification of Mn/TiO2 catalyst by Co could increase its reducibility and the adsorption of NH3 and NOx species on it. From the results of in situ DRIFT study, the NH3 -SCR reaction over Mn/TiO2 catalyst obeyed the Langmuir–Hinshelwood mechanism. And the addition of K on Mn–Co/TiO2 had little inhibition effect on NOx adsorption. These features made Mn–Co/TiO2 catalyst show better K resistance than Mn/TiO2 . © 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In recent several decades, selective catalytic reduction (SCR) process has been put into industrial application for controlling NOx emitted from stationary sources such as coal-fired power plants [1,2]. In this process, the anatase TiO2 supported V2 O5 –WO3 or V2 O5 –MoO3 oxides are the most widely used catalyst [3,4]. However, this catalyst still has several drawbacks, including the adoption of toxic vanadia, the narrow activity window (30 0–40 0 °C) and the deactivation by alkali metals in the fly ash [5–7]. If the SCR reactor is located in the downstream of electrostatic precipitator and wet flue gas desulfurization (WFGD) scrubber, these problems of vanadium-based catalyst mentioned above may be avoid. Due to the fact that the flue gas temperature after desulfurization is generally below 200 °C, therefore, developing new low-temperature SCR catalyst working at 100–250 °C may be a promising way. Manganese oxides, as an environmental-friendly catalyst, are of excellent low-temperature SCR activity [8–10]. It has been proven that the presence of various types of labile oxygen on the surface of manganese oxides plays an important role in the catalytic cycle of SCR reaction [11,12]. In order to enhance the SCR activity and SO2 resistance of manganese-based catalyst, several transition metals (such as Ni, Ce, Cu, Fe, Co, etc.) have been used to mod-
ify the manganese-based catalyst [7,13–17]. However, manganesebased SCR catalyst still suffers the deactivation by alkali metals contained in the fly ash [18–22]. The alkali metals (mainly Na and K) in the fly ash have a serious poisoning effect on SCR catalyst. The loss of reducibility, the decrease of surface acidity and the plug of catalysts channels are usually regarded as the main factors leading to the deactivation of SCR catalyst by alkali metals. Recently, the addition of some transition metals (including Ce, Sb and Nb) have been reported to be an effective method to enhance the resistance of SCR catalyst to alkali metal poisoning [23,24]. It has been reported that Co3+ species in cobalt oxides could facilitate the adsorption of NH3 and enhance the SCR activity [25]; nevertheless, the effect of Co addition on the resistance of Mn/TiO2 catalyst to alkali metals poisoning has not been investigated. In this work, Mn/TiO2 and Mn–Co/TiO2 catalysts were prepared by sol–gel method and then poisoned by K. And the main purpose of this work is to investigate the mechanism of Co addition on the K resistance of Mn/TiO2 catalyst based on the experimental and characterization results. 2. Experimental 2.1. Catalyst preparation
∗
Corresponding authors at: School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, PR China. Tel./fax: +86 2165430410. E-mail addresses: [email protected] (R.-t. Guo), [email protected] (W.-g. Pan).
Mn/TiO2 catalyst was prepared with sol–gel method. Butyl titanate (0.1 mol), deionized water (1.9 mol), nitric acid (0.1 mol),
http://dx.doi.org/10.1016/j.jtice.2016.03.045 1876-1070/© 2016 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Q.-l. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 64 (2016) 116–123
a
Mn/TiO
100
Mn-Co/TiO 1/8K-Mn/TiO
90
1/8K-Mn-Co/TiO
80
1/4K-Mn/TiO 1/4K-Mn-Co/TiO
70
NO conversion (%)
anhydrous ethanol (2.9 mol) and manganese nitrate (0.02 mol) were mixed under fully stirring at room temperature to yield a yellowish transparent sol. After dried at 80 °C for 24 h, the sol would convert into a xerogel. Then the xerogel was calcined at 500 °C for 5 h to obtain the Mn/TiO2 catalyst. Based on the same method, Mn–Co/TiO2 catalyst was prepared. And cobaltous nitrate was used as the precursor of Co. For Mn–Co/TiO2 , the molar ratio of (Mn + Co)/Ti was set as 0.2:1, and the molar ratio of Mn/Co was set as 2:1. The K-poisoned catalyst was prepared with impregnation method. Potassium (K) was added by impregnating the Mn/TiO2 catalyst (or Mn–Co/TiO2 catalyst) in KNO3 solution with the required concentration. Thereafter, the mixture was stirred for 5 h, dried at 80 °C for 24 h and calcined at 500 °C for 5 h . The Kpoisoned catalyst samples were denoted as xK–Mn/TiO2 and xK– Mn–Co/TiO2 respectively, where x represents the molar ratio of K/Mn (or K/(Mn + Co)).
117
1/2K-Mn/TiO 1/2K-Mn-Co/TiO
60 50 40 30 20 10 0 80
100
120
140
160
180
200
220
240
260
220
240
260
Reaction temperature ( C)
2.2. Characterizations The textural properties of the catalyst samples were measured by N2 adsorption at 77 K using a Quantachrome Autosorb-iQ-AG instrument. The specific surface area and the pore size distribution were determined by the Brunauer–Emmett–Teller (BET) method and the Barrett–Joyner–Halenda (BJH) method respectively. Powder XRD was performed on an X-ray diffractometer (XRD) with CuKα radiation (Bruker D8 Advance, Germany). X-ray photoelectron spectroscopy with Al Kα X-ray (hν = 1486.6 eV) radiation (XPS: Thermo ESCALAB 250, USA) was used to study the chemical states for all elements on the catalyst surface. The shift of the binding energy was calibrated internally by a carbon deposit C1s binding energy at 284.6 eV. The temperature programmed reduction with hydrogen (H2 -TPR) and the temperature programmed desorption of NH3 (NH3 -TPD) were performed on a Quantachrome Autosorb-iQC chemisorption analyzer to study the redox ability and surface acidity of the catalyst samples respectively. The signal of H2 or NH3 was monitored with a thermal conductivity detector (TCD). The in situ DRIFT spectra were collected by a FTIR spectrometer (Thermo, Nicolet iS 50) with an in situ diffuse reflectance cell and a DTGS detector cooled by liquid N2 . Prior to each experiment, the sample was pretreated at 500 °C in a 10% O2 /N2 environment for 2 h, then it was cooled down to the desired reaction temperature (150◦ C). The background spectrum was recorded with the flowing of N2 and subtracted from the sample spectrum. The experimental conditions were set as: 300 mL/min total flow rate, 800 ppm of NH3 or/and 800 ppm of NO + 5 vol% O2 , and N2 balance. 2.3. Activity test The catalytic activity measurements were carried out on a fixed-bed quartz microreactor (8 mm i.d.) using about 0.55 cm3 catalyst sample of 80–100 mesh. The simulated flue gas stream contained 600 ppm NO, 600 ppm NH3 , 5% O2 , and Ar as the balance. The total flow rate of the simulated flue gas was 1 L/min (GHSV = 108,0 0 0 h−1 ). The reaction temperature range for catalytic activity test was from 100 to 240 °C. The concentrations of NO and NO2 in the inlet and outlet gases were measured by a NO– NO2 –NOx analyzer (Thermo, Model 42 i-HL); meanwhile, the formation of N2 O was monitored using a Gasmet FTIR gas analyzer (DX-40 0 0). The NO conversion and the N2 selectivity can be calculated by the following equations:
NO conversion = ([NO]in − [NO]out )/[NO]in × 100%
(1)
N2 selectivity (%)
b 100
95
Mn/TiO
90
Mn-Co/TiO 1/4K-Mn/TiO 1/4K-Mn-Co/TiO
85 80
100
120
140
160
180
200
Reaction temperature ( C) Fig. 1. (A) NO conversion as a function of reaction temperature over different catalysts; (B) N2 selectivity over different catalyst samples.
N2 selectivity = ([NO]in + [NH3 ]in − [NO2 ]out − 2[N2 O]out )/
([NO]in + [NH3 ]in ) × 100%
(2)
To investigate the oxidation of NO over the catalyst samples, the similar experimental conditions were adopted, in which NH3 was not used as the reactant gas. NO conversion to could be determined by:
NO conversion to NO2 = [NO2 ]out /[NOx ]out × 100%
(3)
3. Results and discussion 3.1. Catalytic performance The SCR activities of the catalyst samples as a function of reaction temperature are presented in Fig. 1(A). As can be seen from Fig. 1(A), Mn/TiO2 and Mn–Co/TiO2 are of similar catalytic activities. And the NO conversion increases with increasing reaction temperature in the whole experimental temperature range for each catalyst sample. For the two fresh catalyst samples of Mn/TiO2 and Mn–Co/TiO2 , a sharp decrease of SCR activity could be observed after the addition of K. And the poisoning effect of K increases with increasing K loading amount. It is obvious that Mn–Co/TiO2
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25
A---anatase TiO2 B--- rutile TiO2 A B
B
15
10
A
A
Intensity ( a.u.)
NO conversion (%)
20
Mn-Co/TiO2 Mn/TiO2 B A
Mn-Co/TiO
5
Mn/TiO
B
1/4K-Mn/TiO
1/4K-Mn/TiO2
1/4K-Mn-Co/TiO
0
80
120
160 200 Reaction temperature( C)
240
A
1/4K-Mn-Co/TiO2 B B
20
30
40
50
60
70
80
2 ( °)
Fig. 2. NO oxidation to NO2 over the four catalyst samples.
Fig. 3. XRD patterns of the four catalyst samples.
has a better K resistance than Mn/TiO2 . For instance, when the reaction temperature is 200 °C , the NO conversions on Mn/TiO2 and Mn–Co/TiO2 are all about 98%, while the corresponding values on 1/4K–Mn/TiO2 and 1/4K–Mn–Co/TiO2 are 62% and 46%, respectively. The values of N2 selectivity over the different catalyst samples are shown in Fig. 1(B). It is clear that the N2 selectivity decreases with increasing temperature, which should be resulted from the partial oxidation of NH3 at elevated temperature. As can be seen from Fig. 1(B), the two poisoned catalyst samples show lower N2 selectivities compared with the fresh samples. A similar trend was also found by Peng et al. [26,27].
decrease sharply. This may be caused by the block of microspores or the agglomeration of catalyst during the doping process of K [29]. Fig. 3 shows the XRD patterns of the catalyst samples. For all the catalyst samples, only anatase TiO2 phase and rutile TiO2 phase could be detected in the XRD spectra, indicating that the species of Mn, Co and K are in amorphous structure or highly dispersed on the surface of TiO2 carrier. Mn/TiO2 and Mn–Co/TiO2 are of the similar crystallinity. And the addition of K makes the XRD peak intensities of Mn/TiO2 and Mn–Co/TiO2 increase greatly; therefore, the addition of K would increase the crystallinity. A similar result has also been observed by Jiang et al. [30] when they studied the poisoning effect of K on V/TiO2 catalyst. The high XRD peak intensities of 1/4K–Mn/TiO2 and 1/4K–Mn–Co/TiO2 mean the formation of large TiO2 particles, which would lead to the sintering of the catalyst samples and cause the decrease of specific area, as listed in Table 1. While some other researchers found that the addition of K has little effect on the crystalline phase of SCR catalyst [31,32]. The different results may be attributed to the different K contents of the catalyst samples used in different studies. In addition, a diffraction peak of anatase TiO2 in the XRD spectra of the two poisoned catalyst samples moves to a higher 2θ value compared to that in the XRD spectra of the fresh catalyst samples, which may be resulted from the lattice contraction [33].
3.2. Activity of NO oxidation to NO2 The oxidation activities of NO to NO2 over the four catalyst samples as a function of reaction temperature are shown in Fig. 2. It is obvious that the activities of NO oxidation over the two fresh catalyst samples are higher than that over the two poisoned catalyst samples. Moreover, the addition of Co to Mn/TiO2 catalyst could enhance its NO oxidation activity. According to the study of Long and Yang [28], the presence of NO2 could greatly enhance the SCR activity of Fe-ZSM-5 and the reaction rate of NH3 and the mixture of NO and NO2 was much higher than that with NO alone. Therefore, the high activity for NO oxidation to NO2 over Mn–Co/TiO2 catalyst is also helpful to enhance its K resistance. 3.3. BET and XRD analysis The results of BET analysis are summarized in Table 1. From Table 1, it can be observed that the BET surface area and the pore volume of Mn–Co/TiO2 are larger than that of Mn/TiO2 , which may be resulted from the strong interaction among Mn, Co and Ti. After the addition of K, the BET surface areas and the pore volumes of Table 1 Textural properties of different catalysts.
Samples
BET surface area (m2 /g)
Total pore volume (cm3 /g)
Average pore diameter (nm)
Mn/TiO2 Mn–Co/TiO2 1/4K–Mn/TiO2 1/4K–Mn–Co/TiO2
125.80 133.70 33.06 46.02
0.15 0.17 0.05 0.06
1.92 1.70 1.81 1.91
3.4. H2 -TPR analysis H2 -TPR analysis was performed to investigate the redox behaviors of the catalyst samples, and the results are illustrated in Fig. 4. As shown in Fig. 4, there are two reduction peaks in the profile of Mn/TiO2 catalyst, the first one at about 375 °C could be assigned to a combination of MnO2 →Mn2 O3 and Mn2 O3 →Mn3 O4 [9,34]; and the second one at about 450 °C may be related to Mn3 O4 →MnO [9,34]. For Mn–Co/TiO2 catalyst, three reduction peaks at about 340 °C, 410 °C and 680 °C could be observed, the first one can be attributed to the three reduction process of MnO2 →Mn2 O3 , the second one could be assigned to the combination of Mn2 O3 →Mn3 O4 and Co3+ →Co2+ [34,35]. The third one may be associated to the reduction of the surface capping oxygen species [36]. For the two poisoned catalyst samples, the reduction peaks in their profiles are of the similar nature with that in the profiles of the corresponding fresh catalyst samples. It is well recognized that the reduction peak temperature is an indication of reducibility; lower reduction peak means stronger reducibility [37].
Q.-l. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 64 (2016) 116–123
H.C.=5.42mmol/g
Mn-Co/TiO2
Co
119
Co
2+
3+
Mn/TiO2
H.C.=5.39mmol/g
1/4K-Mn-Co/TiO2
H.C.=5.28mmol/g
Intensity (a.u.)
TCD signal(a.u.)
3+
[Co ]=1.73 at.%
Mn-Co/TiO2
H.C.=4.67mmol/g 1/4K-Mn/TiO2 100
200
300
400
500
600
700
800
900
o
Temperature( C)
3.5. XPS analysis XPS analysis was performed to identify the surface nature and element concentrations. The XPS spectra of Mn 2p3/2 of the catalysts are shown in Fig. 5. According to previous studies [38–40], the
Mn
4+
3+
Mn-Co/TiO2
4+
Intensity (a.u.)
2+
Mn
[Mn ]=4.56at. %
Mn/TiO2
4+
[Mn ]=4.46at. %
1/4K-Mn-Co/TiO2
4+
[Mn ]=3.03at. %
1/4K-Mn/TiO2
4+
[Mn ]=2.42at. % 644
642
785
780
775
770
Fig. 6. Co 2p XPS spectra of Mn–Co/TiO2 and 1/4K–Mn–Co/TiO2 .
Therefore, it may be concluded that the doping of Co on Mn/TiO2 catalyst leads to an increase of its reducibility. From Fig. 4, it can be seen that the reduction peak temperatures of the two fresh catalyst samples move to a higher value after the addition of K, indicating the stabilization of Mn species, which makes them less reducible. In addition, the decrease of catalyst reducibility by the addition of K could also be reflected by the values of H2 consumption (H.C.), as shown in Fig. 4. It is clear that the H2 -consumption of 1/4K–Mn/TiO2 is much lower than that of the other three samples. The reducibilities of the four catalyst samples are in the following sequence: Mn–Co/TiO2 > Mn/TiO2 > 1/4K–Mn–Co/TiO2 > 1/4K– Mn/TiO2 . So the high reducibility of Mn–Co/TiO2 should also play an important role for its excellent resistance to K-poisoning.
Mn
790
Binding energy (eV)
Fig. 4. H2 -TPR profiles of the four catalyst samples.
646
1/4K-Mn-Co/TiO2
3+
[Co ]=1.02 at.%
640
638
Binding energy (eV) Fig. 5. Mn 2p3/2 XPS spectra of the four catalyst samples.
636
Mn 2p3/2 XPS spectra can be deconvoluted into three peaks corresponding to Mn2+ , Mn3+ and Mn4+ respectively. So the results of XPS analysis are in line with that of H2 -TPR analysis. Based on the results of XPS analysis, we can obtain the surface atomic concentration of Mn4+ on the surface of each catalyst sample, as shown in Fig. 5. From Fig. 5, it can be seen that the surface atomic concentration of Mn4+ on the surface of Mn–Co/TiO2 is higher than that on the surface of Mn/TiO2 ; as for the two poisoned catalyst samples, the surface Mn4+ concentration of 1/4K–Mn–Co/TiO2 is still higher than that of Mn/TiO2 . It has been proven that Mn4+ plays a crucial role in the redox cycle of SCR reaction [7], combined with the H2 -TPR results, the high surface Mn4+ concentration of Mn– Co/TiO2 should be the main origin of its high reducibility, which is helpful to enhance its low-temperature SCR activity. In addition, Mn4+ could promote the oxidation of NO to NO2 , which also facilitates the SCR reaction [41]. Fig. 6 shows the Co 2p XPS spectra of Mn–Co/TiO2 and 1/4K– Mn–Co/TiO2 . After a peak-fitting deconvolution, the Co 2p spectra can be separated into two peaks: Co2+ and Co3+ [35,42]. And the surface Co3+ concentrations of each catalyst were also calculated and shown in Fig. 6. It is clear that the surface Co3+ concentration of the fresh catalyst is higher than that of the poisoned catalyst. It is reported that Co3+ ions could facilitate the adsorption of NH3 [25], so the decrease of surface Co3+ concentration of 1/4K– Mn–Co/TiO2 agrees well with its relatively low surface acidity, as reflected by the results of NH3 -TPD analysis. Fig. 7 exhibits the O 1s XPS spectra of the catalysts. The O 1s peak can be fitted into two peaks: the peak at lower binding energies (528.7–530.9 eV) is attributed to lattice oxygen (Oα ); and the other one at higher binding energies (531.4–532.5 eV) is related to the surface adsorbed oxygen (Oβ ) [41]. From the results of XPS analysis, we can calculate the concentrations of surface adsorbed oxygen of each catalyst sample, as shown in Fig. 7. Fig. 7 implies that Mn–Co/TiO2 catalyst is of the highest surface adsorbed oxygen concentration among the four catalyst samples. For the two-poisoned catalyst samples, the Oβ concentrations of them are notably lower than that of the fresh catalyst samples. But the Oβ concentration of 1/4K–Mn–Co/TiO2 is still higher than that of 1/4K–Mn/TiO2 . As the most active oxygen species in SCR reaction, the surface adsorbed oxygen can promote the oxidation of NO to NO2 , as a result, facilitating the “fast SCR” reaction [41,43]. Based on the results of XPS analysis, the conclusion may be drawn that the high concentrations of Mn4+ , Co3+ and Oβ should also
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O
O
1600 1685
1545
[O ]=30.73 at.%
1430 Mn/TiO2
[O ]=29.65 at.%
1/4K-Mn-Co/TiO2
[O ]=25.42 at.%
Absorbance(a.u.)
Intensity (a.u.)
Mn-Co/TiO2
1200
Mn-Co/TiO2 Mn/TiO2 1/4K-Mn-Co/TiO2 1/4K-Mn/TiO2
1/4K-Mn/TiO2 536
[O ]=19.77 at.%
534
532
530
528
526
2000
1800
Binding energy (eV)
be partly responsible for the good K-resistance of Mn–Co/TiO2 catalyst. 3.6. NH3 -TPD analysis The decrease of both number and strength of the acid sites on the surface of SCR catalyst is regarded as one of the main reasons for the catalyst deactivation by alkali metals [44–48]. Therefore, the surface acidities of the catalyst samples were investigated by NH3 -TPD analysis, and the results are shown in Fig. 8. As shown in Fig. 8, the peak tops in the NH3 -TPD profiles locate in the temperature range of 121–132 °C, 228–263 °C and 310–350 °C respectively. The three kinds of desorption peaks can be attributed to the desorption of physisorbed NH3 and some NH+ 4 bounded to the weak Brønsted acid sites; the NH+ desorbed from the strong Brønsted 4 acid sites; and the desorption of NH3 from Lewis acid sites [49,50]. Noticeably, the total area of the desorption peaks in the profile of Mn–Co/TiO2 is distinctly larger than that of the desorption peaks in the profile of Mn/TiO2 . Thus the introduction of Co into Mn/TiO2 could enhance its surface acidity. After the addition of K, the des-
350
TCD signal(a.u.)
310
Mn-Co/TiO2
Mn/TiO2
235 121
1/4K-Mn/TiO2
263 132
1/4K-Mn-Co/TiO2 100
200
300
400 o
Temperature ( C) Fig. 8. NH3 -TPD profiles of the four catalyst samples.
-1
1400
1200
Fig. 9. In situ DRIFT spectra of NH3 adsorption on the four catalyst samples.
Fig. 7. O 1s XPS spectra of the four catalyst samples.
228
1600
Wavenumber(cm )
500
orption peak area decreases significantly, which suggests that K has an inhibition effect on the adsorption of NH3 . And the NH3 adsorption capacity of 1/4K–Mn–Co/TiO2 is still higher than that of 1/4K–Mn/TiO2 , therefore, the strong surface acidity of Mn–Co/TiO2 should also partly contribute to its good K-resistance. 3.7. In situ DRIFT study To further understand the surface acidity of the four catalyst samples, the in situ DRIFT spectra of NH3 adsorption on them were recorded at 150 °C, and the results are shown in Fig. 9. Fig. 9 shows the presence of Lewis acid sites (1600 and 1200 cm−1 ) [51,52] and −1 NH+ 4 species on Brønsted acid sites (1685, 1430 cm ) on the catalyst samples [53,54]. It is clear that NH3 species adsorbed on Lewis acid sites is much more than that adsorbed on Brønsted acid sites. As can be seen from Fig. 9, the band intensities of Lewis acid sites and Brønsted acid sites in the spectrum of Mn–Co/TiO2 are higher than that in the spectrum of Mn/TiO2 . Thus the doping of Co could enhance the surface acidity, which is in accordance with the results of NH3 -TPD. And the band at 1545 cm−1 could be attributed to NH2 species [54]. It is the product of partial oxidation of adsorbed NH3 species on catalyst surface. The formation of NH2 is regarded as the key step in NH3 -SCR reaction [55]. And the relatively higher intensity of the band at 1545 cm−1 the spectrum of Mn–Co/TiO2 indicates a strong generation of amide on this catalyst. After the addition of K, a sharp decrease of band intensities could be observed, indicating that the great decrease of surface acidity of the two fresh catalyst samples, which is in accordance with the results of NH3 -TPD analysis results. The DRIFT spectra of NO+O2 adsorption on the four catalyst samples are illustrated in Fig. 10. From Fig. 10, several bands assigned to different nitrate species could be found, including monodentate nitrate (1540, 1483, and 1260 cm−1 ) and bridging nitrate (1605 cm−1 ) [56–58]. Compared with Mn/TiO2 , Mn–Co/TiO2 shows larger amount of adsorbed NOx species, as shown in Fig. 10. It can be seen from Fig. 10, the addition of K has a strong inhibition effect on NOx adsorption for Mn/TiO2 catalyst; as for Mn–Co/TiO2 catalyst, the doping of K has little effect on NOx adsorption. To determine the role of adsorbed NH3 species in the SCR reaction over Mn/TiO2 and Mn–Co/TiO2 catalysts, the in situ DRIFT spectra of reaction between preadsorbed NH3 species and NO + O2 at 150 °C as a function of time are shown in Fig. 11(A) and (B). It is obvious that the surfaces of Mn/TiO2 and Mn–Co/TiO2
Q.-l. Chen et al. / Journal of the Taiwan Institute of Chemical Engineers 64 (2016) 116–123
121
a
1605
1605
1540
1515 1260 1200
N2 purging
1483 1260 Absorbance(a.u.)
Absorbance(a.u.)
30min
Mn-Co/TiO2 Mn/TiO2
20min
1540 1483
10min 5min
1/4K-Mn-Co/TiO2
2min
1/4K-Mn/TiO2
NOx 2000
1800
1600
-1
1400
1200
Wavenumber(cm ) 2000
1800
1600
1400
1200
-1
Wavenumber(cm )
b
1515 1605
Fig. 10. In situ DRIFT spectra of NO + O2 adsorption on the four catalyst samples.
1260 1200
pure N2
a Absorbance(a.u.)
1280
1540 1605
Absorbance(a.u.)
N2 purging 30min
30min 20min 1540
10min 5min 2min
20min 1600 1200
10min 5min
2000
1545
1800
1600
1400
1200
-1
Wavenumber(cm )
2min NH3 2000
NOx
1430 Fig. 12. In situ DRIFT spectra of NH3 reacted with preadsorbed NO + O2 species on Mn/TiO2 (A) and Mn–Co/TiO2 (B).
1800
1600
1400
1200
-1
Wavenumber(cm )
b
1540 1605 1280
Absorbance(a.u.)
N2 purging 30min 20min 10min 1200
5min 2min
1600 1545
NH3 2000
1800
1430
1600
1400
1200
-1
Wavenumber(cm ) Fig. 11. In situ DRIFT spectra of NO + O2 reacted with preadsorbed NH3 species on Mn/TiO2 (A) and Mn–Co/TiO2 (B).
catalysts were covered by several NH3 species after exposure to NH3 . After the gas mixture of NO + O2 was introduced, the bands of NH3 species vanished, meanwhile, several bands of NOx species appeared. Therefore, both the NH3 bound to Lewis acid sites and NH+ 4 bound to Brønsted acid sites are active species in the NH3 SCR reaction over Mn/TiO2 and Mn–Co/TiO2 catalysts. On the other hand, the role of adsorbed NOx species in the NH3 -SCR reaction over Mn/TiO2 and Mn–Co/TiO2 catalysts was also investigated by DRIFT study. And the DRIFT spectra of reaction between preadsorbed NOx species and NH3 are shown in Fig. 12(A) and (B). When NH3 was introduced, the bands of NOx species decreased gradually with time. Therefore, the adsorbed NOx species could also take part in the NH3 -SCR reaction, but the activity of adsorbed NOx species are lower than that of adsorbed NH3 species. Thus the NH3 -SCR reaction over Mn/TiO2 and Mn–Co/TiO2 catalysts is controlled by Langmuir–Hinshelwood mechanism, in which the adsorbed NH3 species react with the adsorbed NOx species to form N2 and H2 O. Therefore, the promoted adsorption of NH3 species and NOx species on the surface of Mn–Co/TiO2 catalyst could greatly enhance its K resistance. As shown in Fig. 11, the addition of K on Mn–Co/TiO2 nearly has no inhibition effect on the adsorption of NOx species, which is also beneficial to its K resistance.
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4. Conclusions The effect of Co doping on the K-resistance performance of Mn/TiO2 SCR catalyst was investigated in this study. The experimental results showed that Mn–Co/TiO2 had better K-resistance than Mn/TiO2 . The results H2 -TPR and NH3 -TPD showed that Mn–Co/TiO2 exhibits stronger reducing ability and surface acidity. From the results of XPS analysis, we find that the presence of higher concentrations of Mn4+ and Co3+ on the surface of Mn–Co/TiO2 , which resulting in its good redox property and high NH3 adsorption capacity. Moreover, the presence of rich surface adsorbed oxygen greatly enhances the “fast SCR” reaction. And the results of DRIFT study indicated that the NH3 -SCR reaction over Mn/TiO2 catalyst and Mn–Co/TiO2 catalysts followed the Langmuir–Hinshelwood mechanism. It seemed that the adsorption of NH3 species and NOx species on Mn/TiO2 catalyst was greatly inhibited by the loading of K, while the adsorption capacity of NOx species on Mn–Co/TiO2 catalyst rarely changed after the addition of K. These good features could give a suitable explanation for the excellent resistance of Mn–Co/TiO2 catalyst to K-poisoning. Acknowledgments This work was supported by the National Natural Science Foundation of China (21546014) and the Natural Science Foundation of Shanghai, China (14ZR1417800). References [1] Janssens TVW, Falsig H, Lundegaard LF, Vennestrøm PNR, Rasmussen SB, Moses PG, et al. A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia. ACS Catal 2015;5:2832. [2] Kacimi M, Ziyad M, Liotta LF. Cu on amorphous AlPO4 : preparation, characterization and catalytic activity in NO reduction by CO in presence of oxygen. Catal Today 2015;241:151. [3] Foo R, Vazhnova T, Lukyanov DB, Millington P, Collier J, Rajaram R, et al. Formation of reactive Lewis acid sites on Fe/WO3 –ZrO2 catalysts for higher temperature SCR applications. Appl Catal B: Environ 2015;162:174. [4] Fang C, Shi L, Hu H, Zhang J, Zhang D. Rational design of 3D hierarchical foam-like Fe2 O3 @CuOx monolith catalysts for selective catalytic reduction of NO with NH3 . RSC Adv 2015;5:11013. [5] Lu X, Song C, Jia S, Tong Z, Tang X, Teng Y. Low-temperature selective catalytic reduction of NOx with NH3 over cerium and manganese oxides supported on TiO2 –graphene. Chem Eng J 2015;260:776. [6] Putluru SSR, Schill L, Jensen AD, Siret B, Tabaries F, Fehrmann R. Mn/TiO2 and Mn–Fe/TiO2 catalysts synthesized by deposition precipitation – promising for selective catalytic reduction of NO with NH3 at low temperatures. Appl Catal B: Environ 2015;165:628. [7] Thirupathi B, Smirniotis PG. Nickel-doped Mn/TiO2 as an efficient catalyst for the low-temperature SCR of NO with NH3 : catalytic evaluation and characterizations. J Catal 2012;288:74. [8] Wang X, Zheng Y, Xu Z, Liu X, Zhang Y. Low-temperature selective catalytic reduction of NO over MnOx /CNTs catalysts: effect of thermal treatment condition. Catal Commun 2014;50:34. [9] Maitarad P, Zhang D, Gao R, Shi L, Li H, Huang L, et al. Combination of experimental and theoretical investigations of MnOx /Ce0.9 Zr0.1 O2 nanorods for selective catalytic reduction of NO with ammonia. J Phys Chem C 2013;117:9999. [10] Pourkhalil M, Moghaddam AZ, Rashidi A, Towfighi J, Mortazavi Y. Preparation of highly active manganese oxides supported on functionalized MWNTs for low temperature NOx reduction with NH3 . Appl Surf Sci 2013;279:250. [11] Wallin M, Forser S, Thormählen P, Skoglundh M. Screening of TiO2 -supported catalysts for selective NOx reduction with ammonia. Ind Eng Chem Res 2004;43:7723. [12] Park TS, Jeong SK, Hong SH, Hong SC. Selective catalytic reduction of nitrogen oxides with NH3 over natural manganese ore at low temperature. Ind Eng Chem Res 2001;40:4491. [13] Wu Z, Jin R, Liu Y, Wang H. Ceria modified MnOx /TiO2 as a superior catalyst for NO reduction with NH3 at low-temperature. Catal Commun 2008;9:2217. [14] Thirupathi B, Smirniotis PG. Co-doping a metal (Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) on Mn/TiO2 catalyst and its effect on the selective reduction of NO with NH3 at low-temperatures. Appl Catal B: Environ 2011;110:195. [15] Kang M, Park ED, Kim JM, Yie JE. Cu–Mn mixed oxides for low temperature NO reduction with NH3 . Catal Today 2006;111:236. [16] Jiang B, Li Z, Li S. Mechanism study of the promotional effect of O2 on low-temperature SCR reaction on Fe–Mn/TiO2 by DRIFT. Chem Eng J 2013;225:52. [17] Qiu M, Zhan S, Yu H, Zhu D, Wang S. Facile preparation of ordered mesoporous MnCo2 O4 for low-temperature selective catalytic reduction of NO with NH3 . Nanoscale 2015;7:2568.
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