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TiO2 catalyst for selective catalytic reduction of NO with NH3
Applied Surface Science 378 (2016) 513–518
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The deactivation mechanism of Cl on Ce/TiO2 catalyst for selective catalytic reduction of NO with NH3 Ning-zhi Yang a,b , Rui-tang Guo a,b,∗ , Wei-guo Pan a,b,∗ , Qi-lin Chen a,b , Qing-shan Wang 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 29 January 2016 Received in revised form 27 March 2016 Accepted 29 March 2016 Available online 31 March 2016 Keywords: Ce/TiO2 SCR Cl Deactivation TPD DRIFT
a b s t r a c t The poisoning mechanism of Cl on Ce/TiO2 catalyst was investigated based on temperature programmed desorption (TPD) and the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) studies. The results of NH3 -TPD and NO-TPD indicated that the addition of Cl on Ce/TiO2 catalyst would inhibit the adsorption of NH3 species and NOx species on it. As can be seen from the results of in situ DRIFT study, the NH3 -SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl were all followed both the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism. And the decreased adsorption ability of NH3 species and NOx species on the surface of Ce/TiO2 -Cl should be mainly responsible for its low SCR activity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Selective catalytic reduction (SCR) process has been successively used for controlling NOx emitted from stationary sources such as coal-fired power plants in the past several decades [1,2]. As the commercial catalyst used in this process, vanadium-based catalyst still has some drawbacks, including narrow temperature window for NO reduction, high activity for SO2 oxidation to SO3 and the deactivation by SO2 and alkali metals in the flue gas [3–5]. Therefore, developing alternative low-temperature SCR catalyst has drawn much attention in recent years [6–8]. Due to its high oxygen storage-release capacity and excellent redox properties, ceria-based catalyst is regarded as a competitive candidate for NH3 -SCR reaction [9,10]. Recently, CeO2 supported on different materials such as TiO2 [11], Al2 O3 [12,13], SO2− 4 -ZrO2 [14], WO3 -TiO2 [8] has been reported to be of high efficiency for NH3 -SCR reaction. However, there are still some problems associated with ceria-based catalyst, such as the deactivation by the SO2 in the flue gas and the alkali metals, alkali earth metals and heavy metals contained in the fly ash [15–17]. Besides that, it is well known that HCl
∗ 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). http://dx.doi.org/10.1016/j.apsusc.2016.03.211 0169-4332/© 2016 Elsevier B.V. All rights reserved.
is widely present in the flue gases of coal-fired boilers and municipal solid waste incinerators. During the past decades, the effect of HCl on SCR catalyst has been investigated by some researchers. The study of Lisi et al. [18] found that the deactivation of V2 O5 /TiO2 catalyst by HCl was resulted from the partial loss of vanadium oxide, while the formed new acid sites by HCl may restore the activity to some extent. Chang et al. [19] reported that the coexistence of SO2 and HCl had a serious poisoning effect on 1% Rh/Al2 O3 catalyst for the SCR of NO with CO. Park et al. [20] investigated the deactivation mechanism of HCl on Cu ion-exchanged mordenite catalyst, indicating that the deactivation was mainly caused by the evaporation of Cu ions from the framework of mordenite structure. In our previous study [21], the poisoning effect of Cl on Mn/TiO2 catalyst was investigated, however, investigating the deactivation mechanism of Cl on low-temperature catalyst is still in demand. Therefore, in this study, Ce/TiO2 catalyst for selective catalytic reduction of NO with NH3 was prepared by sol-gel method. And the deactivation mechanism of Cl on Ce/TiO2 catalyst was investigated based on the temperature programmed desorption (TPD) and the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) studies.
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2. Experimental 2.1. Catalyst preparation
Ce/TiO2
2.2. Characterization The temperature programmed desorption measurements were carried out on a chemisorption analyzer (Autosorb-iQ-C, Quantachrome Instruments). In each test, about 150 mg sample was used. Prior to TPD experiments, the catalyst samples were pretreated in pure He at 500 ◦ C for 1 h. After that, the samples were saturated in NH3 (or NO) for 1 h, then the gas was switched to He for 0.5 h. Afterwards, TPD was performed by ramping the temperature at a rate 10 ◦ C/ min to 700 ◦ C in a gas flow of He (50 mL/min). And the desorption of NH3 (or NO) was detected by using a thermal conductivity detector (TCD). The in situ DRIFT spectra were recorded on Nicolet iS 50 FTIR equipped with a gas flow system. In the DRIFT cell, the catalyst sample was pretreated at 500 ◦ C in N2 environment for 2 h, and then cooled to 150 ◦ C. The background spectra were recorded in a N2 atmosphere and were subtracted from each sample spectra at the corresponding temperature. The reaction conditions were set as follows: 800 ppm NH3 , or/and 800 ppm NO + 5% O2 , balance with N2 . And the total flow rate of the gas mixture was kept at 300 mL/min.
The catalytic activity measurement was performed in a fixed bed reactor (i.d. = 8 mm). The basic simulated flue gas consists of 600 ppm NO, 600 ppm NH3 , 5% O2 and balance Ar. And the GHSV (gas hourly space velocity) was kept at 108, 000 h−1 . The concentrations of NO and NO2 were monitored by a continuous flue gas analyzer (Thermo 60i). And the value of NO conversion could be calculated by the following equation [22]: [NO]out + [NO2 ]out ) × 100% [NO]in + [NO2 ]in
40
20
0
75
100
125
150
175
200
225
o
Reaction temperature( C) Fig. 1. The SCR activities of Ce/TiO2 and Ce/TiO2 -Cl as a function of reaction temperature Reaction conditions: [NO] = [NH3 ] = 600 ppm, [O2 ] = 5%, balance Ar, GHSV = 108,000 h−1 .
Ce/TiO2 Ce/TiO2-Cl 176
190
100
393
284
200
376
269
300
o
400
500
Temperature( C) Fig. 2. NH3 -TPD profiles of the two catalyst samples.
2.3. Catalytic activity test
NO conversion = (1 −
Ce/TiO2-Cl
60
TCD signal(a.u.)
The fresh Ce/TiO2 catalyst was prepared by sol-gel method. The chemicals used to prepare catalyst samples were of analytical grade. Firstly, butyl titanate (0.1 mol), cerium nitrate (0.03 mol), nitric acid (0.1 mol) and deionized water (0.6 mol) were mixed under vigorous stirring at room temperature until the formation of transparent yellow sol. Then the sol was dried at 80 ◦ Cfor 24 h to convert it into xerogel, followed by calcination in air at 500 ◦ C for 5 h to obtain the fresh catalyst. Next then, the Cl-poisoned catalyst was prepared by impregnation method. The fresh catalyst was impregnated in the solution of NH4 Cl with a certain concentration (the molar ratio of Cl: Ce = 0.05). After stirred for 6 h, the mixed solution was dried at 80 ◦ Cfor 24 h, then the solid was calcined at 500 ◦ C for 5 h. The fresh catalyst and the poisoned catalyst were denoted as Ce/TiO2 and Ce/TiO2 -Cl respectively.
NO conversion (%)
80
poisoning effect of Cl on Ce/TiO2 catalyst was much weaker than that reported in our previous study [21]. This could be attributed the relatively low loading amount of Cl in this study. And the deactivation mechanism would be further discussed in the following sections based on the results of TPD and in situ DRIFT studies.
(1)
when calculating NO conversion, N2 O was regarded as unconverted NOx rather than a product. 3. Results and discussion 3.1. SCR activity The SCR activities of the two catalyst samples as a function of reaction temperature are shown in Fig. 1. It was clear that the values of NO conversions over the two catalyst samples increased with increasing reaction temperature. From Fig. 1, it could be seen that the addition of Cl had a poisoning effect on Ce/TiO2 catalyst. But the
3.2. NH3 -TPD analysis The NH3 -TPD profiles of Ce/TiO2 and Ce/TiO2 -Cl catalyst samples are presented in Fig. 2. As can be observed from Fig. 2, there are three desorption peaks in the NH3 -TPD profile of each catalyst sample. The first peak below 200 ◦ C could be assigned to the desorption of physisorbed NH3 and some NH+ bounded to the weak 4 Brønsted acid sites; the other one ranged from 269 to 284 ◦ C may be related to the desorption of NH+ from the strong Brønsted acid 4 sites; and the third one above 300 ◦ C is resulted from the coordinated NH3 bound to the Lewis acid sites [23–25]. Furthermore, the total peak area of NH3 -TPD profile for Ce/TiO2 decreased after the addition of Cl, indicating the decrease of NH3 adsorbance.
N.-z. Yang et al. / Applied Surface Science 378 (2016) 513–518
Ce/TiO2
515
(A)
1180
Ce/TiO2-Cl TCD signal(a.u.)
Absorbance(a.u.)
1677 1606
1431
N2 purge 30 min 30min 10min 5min
1min
100
200
300
400
500
600
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o
Temperature( C)
1400
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-1
Wavenumber(cm )
Fig. 3. NO-TPD profiles of the two catalyst samples.
(B)
3.3. NO-TPD analysis
N2 purge 30 min
Absorbance(a.u.)
The results of NO-TPD analysis are presented in Fig. 3. It is obvious that each NO-TPD profile contains two desorption peaks. The first one at about 120 ◦ C could be attributed to molecularly adsorbed NO [26], and the other peak at about 390 ◦ C could be related to the decomposition of higher thermal stability nitrate species [24]. Similar with the results of NH3 -TPD analysis, the addition of Cl on Ce/TiO2 catalyst seems to have an inhibition effect on NOx adsorption.
1677
1606
30min 10min 5min
3.4. In situ DRIFT study 3.4.1. NH3 adsorption The DRIFT spectra of Ce/TiO2 and Ce/TiO2 -Cl after they were exposed to NH3 /N2 for different times are illustrated in Fig. 4(A) and (B) respectively. It is well known that NH3 could be adsorbed onto Brønsted acid sites or Lewis acid sites on the surface of metal oxide catalyst. In Fig. 4(A), several bands at 1677, 1606, 1431 and 1180 cm−1 could be observed. The bands at 1677 cm−1 and 1431 cm−1 could be assigned toNH+ on Brønsted acid sites [27,28]; 4 and the bands at 1606 cm−1 and 1180 cm−1 could be assigned to ammonia species coordinated to Lewis acid sites [28,29]. It was noticeable that the band intensities rarely changed after 5 min, indicating that the adsorption equilibrium of NH3 could be accomplished in 5 min. Furthermore, the amount of NH3 species adsorbed on Lewis acid sites were much higher than that adsorbed on Brønsted acid sites, therefore, Ce/TiO2 catalyst was mainly Lewis acidic, agreeing well with the results of Trovarelli [30]. Fig. 4(B) showed the DRIFT spectra of Ce/TiO2 -Cl catalyst after it was exposed to 800 ppm NH3 for various times. From Fig. 4(B), it can be seen that the locations of the bands were similar with that in the DRIFT spectra of Ce/TiO2 , but the intensities were quite different. For example, the intensities of the bands assigned to NH+ 4 on Brønsted acid sites (1677 cm−1 ) and ammonia species coordinated to Lewis acid sites (1606 and 1180 cm−1 ) in the spectra of Ce/TiO2 -Cl is much lower than that in the spectra of Ce/TiO2 . Furthermore, the band at 1431 cm−1 could not be detected in the spectra of Ce/TiO2 -Cl. After the sample was purged with N2 , the reduction of the band intensities of Ce/TiO2 -Cl was much obvious than that of Ce/TiO2 , indicating that the addition of Cl would lead to a weaker adsorption of NH3 species. Based on the comparison between Fig. 4(A) and (B), it could be concluded that the addition of Cl on Ce/TiO2 catalyst would decrease its Brønsted acidity and Lewis acidity.
1180
1min
2000
1800
1600
1400
1200
-1
Wavenumber(cm ) Fig. 4. In situ DRIFT spectra of Ce/TiO2 (A) and Ce/TiO2 -Cl (B) exposed to 800 ppm NH3 for various times and after purging by N2 for 30 min at 200 ◦ C.
3.4.2. NO + O2 adsorption Fig. 5(A) presents the DRIFT spectra of Ce/TiO2 after it was exposed to 800 ppm NO + 5% O2 for various times. As can be seen form Fig. 5(A), the bands at 1612 and 1226 cm−1 could be attributed to bridged nitrate species [27,31], and the band at 1571 cm−1 was assigned to bidentate nitrate [28]. Due to the accumulation of adsorbed NOx species, the band intensities increased with time. Compared with the adsorption process of NH3 species (Fig. 4), it seemed that the adsorption process of NOx species was much slower. And the N2 purge had little effect on the band intensities of NOx species, meaning the strong interaction between NOx species and catalyst surface. The DRIFT spectra of Ce/TiO2 -Cl catalyst exposed to 800 ppm NO + 5% O2 for various times are shown in Fig. 5(B). The spectra were very similar with that of Ce/TiO2 . However, two new bands at 1538 and 1175 cm−1 could be observed, which could be assigned to bidentate nitrate and bridged nitrate respectively [31,32]. In addition, the band intensities in the spectra of Ce/TiO2 -Cl were much lower than that of Ce/TiO2 . Therefore, the addition of Cl on Ce/TiO2 catalyst would also inhibit the adsorption of NOx species on it. 3.4.3. NO + O2 adsorption after NH3 To investigate the reactivity of adsorbed NH3 species in the SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl catalysts, the in situ DRIFT
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1571
(A)
1226
1226
N2 purge
Absorbance(a.u.)
N2 purge 30min
Absorbance(a.u.)
1571 1612
(A)
1612
30min 10min 5min
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1677
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1606
NH3 2000
1800
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Wavenumber(cm )
Wavenumber(cm )
(B)
1571 1612
(B)
1612 1571 1538
1226 N2 purge
Absorbance(a.u.)
Absorbance(a.u.)
1226 1175 N2 purge 30min 30min 10min
30min 10min 5min
5min
1677
2min
1min
2000
1600
1400
1200
2000
1800
-1
Wavenumber(cm ) Fig. 5. In situ DRIFT spectra of Ce/TiO2 (A) and Ce/TiO2 -Cl (B) exposed to 800 ppm NO +5% O2 for various times and after purging by N2 for 30 min at 200 ◦ C.
spectra of the reaction between NO + O2 and the preadsorbed NH3 species at 200 ◦ C were recorded as a function of time, and the results are shown in Fig. 6(A) and (B) respectively. As shown in Fig. 6(A) and (B), several bands belonging to adsorbed NH3 species on the surfaces of Ce/TiO2 and Ce/TiO2 -Cl could be observed. When NO + O2 was introduced, the bands ascribed to absorbed NH3 species diminished, meanwhile, several bands of NOx adsorption species formed. That is to say, both of the coordinated NH3 species bound to Lewis acid sites and NH+ on Brønsted acid sites were all active species in 4 the NH3 -SCR reaction taking place over Ce/TiO2 and Ce/TiO2 -Cl. 3.4.4. NH3 adsorption after NO + O2 Correspondingly, the in situ DRIFT spectra of the reaction between preadsorbed NO + O2 and NH3 over Ce/TiO2 and Ce/TiO2 Cl are illustrated in Fig. 7(A) and (B). After the introduction of NH3 , it could be seen that the bands attributed bridged nitrate species (1612 and 1226 cm−1 ) on Ce/TiO2 and Ce/TiO2 -Cl disappeared quickly, and some bands of NH3 adsorption species adsorbed on Lewis acid sites [29,33] (1606, 1240 and 1180 cm−1 ) appeared, while the band belonging to bidentate nitrate (1571 cm−1 ) nearly remained unchanged. Therefore, only bridged nitrate species could take part in the NH3 -SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl. When Lian et al. [28] studied the NH3 -SCR reaction mechanism on Ce-Mn/TiO2 and Ca–Ce-Mn/TiO2 catalysts, they also found that some species of monodentate nitrate, bridge nitrate and biden-
1180 1606
NH3
1800
1200
-1
1600
1400
1200
-1
Wavenumber(cm ) Fig. 6. In situ DRIFT spectra of NO + O2 reacted with preadsorbed NH3 species on Ce/TiO2 (A) and Ce/TiO2 -Cl (B)at 200 ◦ C.
tate nitrate were not active in SCR reaction. Thus the active NOx species in SCR reaction may be greatly dependent on the kind of SCR catalyst. 3.5. Reaction mechanism To further understand the NH3 -SCR reaction mechanism over Ce/TiO2 and Ce/TiO2 -Cl, the effect of some operation conditions including NO inlet concentration and NH3 inlet concentration on the SCR activities of the two catalyst samples were investigated based on the method described in [34]. During the activity test process, the concentrations of NO and NH3 were kept the same to ensure the enough SCR reaction. If the NH3 -SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl is mainly controlled by the Eley–Rideal mechanism, the NO conversions over Ce/TiO2 and Ce/TiO2 -Cl should not decrease with the increase of NO concentration from 600 ppm to 1800 ppm. On the other hand, if the NH3 -SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl is mainly controlled by the Langmuir–Hinshelwood mechanism, the NO conversions over Ce/TiO2 and Ce/TiO2 -Cl should halve after the NO inlet concentration was doubled. As can be seen from Fig. 8, NO conversion decreased with increasing NO concentration. In addition, the NO concentration did not halve when the concentration of NO increases from 600 ppm to 1200 ppm. Therefore, the SCR reaction
N.-z. Yang et al. / Applied Surface Science 378 (2016) 513–518 100
(A) 1606
(A)
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NO conversion (%)
N2 purge
Absorbance(a.u.)
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5min 1612
2min
60
[NO]=600 ppm [NO]=1200 ppm [NO]=1800 ppm
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NO+O2
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Reaction temperature( C)
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(B)
(B)
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NO conversion (%)
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Absorbance(a.u.)
N2 purge 30min 10min 5min
1612
[NO]=600 ppm [NO]=1200 ppm [NO]=1800 ppm
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1226
NO+O2
0 60
2000
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1200
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o
Reaction temperature( C)
-1
Wavenumber(cm ) Fig. 7. In situ DRIFT spectra of NH3 reacted with preadsorbed NO + O2 species on Ce/TiO2 (A) and Ce/TiO2 -Cl (B) at 200 ◦ C.
over Ce/TiO2 and Ce/TiO2 -Cl obeys both the Eley-Rideal mechanism and the Langmuir–Hinshelwood mechanism. Based on previous reports and the results of this study, the NH3 -SCR reaction mechanism over Ce/TiO2 and Ce/TiO2 -Cl could be described as follows [35,36]:
Fig. 8. Influence of NO inlet concentration on NO conversion over Ce/TiO2 catalyst (A) and Ce/TiO2 -Cl catalyst (B). Ce/TiO2 Cl catalyst (B) Reaction conditions: 䊏, [NO] = [NH3 ] = 600 ppm; 䊉, [NO] = [NH3 ] = 1200 ppm; , [NO] = [NH3 ] = 1800 ppm; [O2 ] = 5%, balance Ar, GHSV = 108,000 h−1 .
NO(g) + O−* → NO2 (a) Ce4+
(1) E-R mechanism: Firstly, gaseous NH3 is absorbed on Lewis acid sites to form coordinated NH3 . After the dehydration by labile oxygen, the absorbed NH3 is converted into NH2 species. After that, NH2 reacts with gaseous NO to form NH2 NO, which is finally decomposed into N2 and H2 O. The whole reaction process could be written as: Ce4+
NH3 (g) → NH3 (a) over Lewis acid sites
(1)
O2 (g) → 2O(a)
(2)
NH3 (a) + O(a) → NH2 (a) + OH(a)
(3)
NO(g) + NH2 (a) → NH2 NO(a)
(4)
NH2 NO(a) → N2 (g) + H2 O
(5)
(2) L-H mechanism: The adsorbed NOx species reacts with the adsorbed NH3 species to form N2 and H2 O: O2 (g) → 2O−* (surface active sites)
(6)
(7)
NH3 (g) → NH3 (a) over Lewis acid sites
(8)
2NH3 (a) + NO2 (a) + NO(g) → 2N2 + 3H2 O
(9)
4. Conclusions The addition of Cl had a poisoning effect on Ce/TiO2 catalyst for low-temperature SCR reaction of NO by NH3 . In this study, TPD and the in situ DRIFT studies were performed to investigate the deactivation mechanism. The decreased adsorption capacities of NH3 species and NO species could be found from the results of TPD analysis. From the results of DRIFT study, it was found that the NH3 -SCR reactions on Ce/TiO2 and Ce/TiO2 -Cl catalyst samples were all governed by the combination of the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism. The addition of Cl on Ce/TiO2 not only inhibited the adsorption of NH3 species, but also inhibited the adsorption of NOx species, as a result, leading to the deactivation of Ce/TiO2 .
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Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc
The deactivation mechanism of Cl on Ce/TiO2 catalyst for selective catalytic reduction of NO with NH3 Ning-zhi Yang a,b , Rui-tang Guo a,b,∗ , Wei-guo Pan a,b,∗ , Qi-lin Chen a,b , Qing-shan Wang 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 29 January 2016 Received in revised form 27 March 2016 Accepted 29 March 2016 Available online 31 March 2016 Keywords: Ce/TiO2 SCR Cl Deactivation TPD DRIFT
a b s t r a c t The poisoning mechanism of Cl on Ce/TiO2 catalyst was investigated based on temperature programmed desorption (TPD) and the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) studies. The results of NH3 -TPD and NO-TPD indicated that the addition of Cl on Ce/TiO2 catalyst would inhibit the adsorption of NH3 species and NOx species on it. As can be seen from the results of in situ DRIFT study, the NH3 -SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl were all followed both the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism. And the decreased adsorption ability of NH3 species and NOx species on the surface of Ce/TiO2 -Cl should be mainly responsible for its low SCR activity. © 2016 Elsevier B.V. All rights reserved.
1. Introduction Selective catalytic reduction (SCR) process has been successively used for controlling NOx emitted from stationary sources such as coal-fired power plants in the past several decades [1,2]. As the commercial catalyst used in this process, vanadium-based catalyst still has some drawbacks, including narrow temperature window for NO reduction, high activity for SO2 oxidation to SO3 and the deactivation by SO2 and alkali metals in the flue gas [3–5]. Therefore, developing alternative low-temperature SCR catalyst has drawn much attention in recent years [6–8]. Due to its high oxygen storage-release capacity and excellent redox properties, ceria-based catalyst is regarded as a competitive candidate for NH3 -SCR reaction [9,10]. Recently, CeO2 supported on different materials such as TiO2 [11], Al2 O3 [12,13], SO2− 4 -ZrO2 [14], WO3 -TiO2 [8] has been reported to be of high efficiency for NH3 -SCR reaction. However, there are still some problems associated with ceria-based catalyst, such as the deactivation by the SO2 in the flue gas and the alkali metals, alkali earth metals and heavy metals contained in the fly ash [15–17]. Besides that, it is well known that HCl
∗ 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). http://dx.doi.org/10.1016/j.apsusc.2016.03.211 0169-4332/© 2016 Elsevier B.V. All rights reserved.
is widely present in the flue gases of coal-fired boilers and municipal solid waste incinerators. During the past decades, the effect of HCl on SCR catalyst has been investigated by some researchers. The study of Lisi et al. [18] found that the deactivation of V2 O5 /TiO2 catalyst by HCl was resulted from the partial loss of vanadium oxide, while the formed new acid sites by HCl may restore the activity to some extent. Chang et al. [19] reported that the coexistence of SO2 and HCl had a serious poisoning effect on 1% Rh/Al2 O3 catalyst for the SCR of NO with CO. Park et al. [20] investigated the deactivation mechanism of HCl on Cu ion-exchanged mordenite catalyst, indicating that the deactivation was mainly caused by the evaporation of Cu ions from the framework of mordenite structure. In our previous study [21], the poisoning effect of Cl on Mn/TiO2 catalyst was investigated, however, investigating the deactivation mechanism of Cl on low-temperature catalyst is still in demand. Therefore, in this study, Ce/TiO2 catalyst for selective catalytic reduction of NO with NH3 was prepared by sol-gel method. And the deactivation mechanism of Cl on Ce/TiO2 catalyst was investigated based on the temperature programmed desorption (TPD) and the in situ diffuse reflectance infrared transform spectroscopy (DRIFT) studies.
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2. Experimental 2.1. Catalyst preparation
Ce/TiO2
2.2. Characterization The temperature programmed desorption measurements were carried out on a chemisorption analyzer (Autosorb-iQ-C, Quantachrome Instruments). In each test, about 150 mg sample was used. Prior to TPD experiments, the catalyst samples were pretreated in pure He at 500 ◦ C for 1 h. After that, the samples were saturated in NH3 (or NO) for 1 h, then the gas was switched to He for 0.5 h. Afterwards, TPD was performed by ramping the temperature at a rate 10 ◦ C/ min to 700 ◦ C in a gas flow of He (50 mL/min). And the desorption of NH3 (or NO) was detected by using a thermal conductivity detector (TCD). The in situ DRIFT spectra were recorded on Nicolet iS 50 FTIR equipped with a gas flow system. In the DRIFT cell, the catalyst sample was pretreated at 500 ◦ C in N2 environment for 2 h, and then cooled to 150 ◦ C. The background spectra were recorded in a N2 atmosphere and were subtracted from each sample spectra at the corresponding temperature. The reaction conditions were set as follows: 800 ppm NH3 , or/and 800 ppm NO + 5% O2 , balance with N2 . And the total flow rate of the gas mixture was kept at 300 mL/min.
The catalytic activity measurement was performed in a fixed bed reactor (i.d. = 8 mm). The basic simulated flue gas consists of 600 ppm NO, 600 ppm NH3 , 5% O2 and balance Ar. And the GHSV (gas hourly space velocity) was kept at 108, 000 h−1 . The concentrations of NO and NO2 were monitored by a continuous flue gas analyzer (Thermo 60i). And the value of NO conversion could be calculated by the following equation [22]: [NO]out + [NO2 ]out ) × 100% [NO]in + [NO2 ]in
40
20
0
75
100
125
150
175
200
225
o
Reaction temperature( C) Fig. 1. The SCR activities of Ce/TiO2 and Ce/TiO2 -Cl as a function of reaction temperature Reaction conditions: [NO] = [NH3 ] = 600 ppm, [O2 ] = 5%, balance Ar, GHSV = 108,000 h−1 .
Ce/TiO2 Ce/TiO2-Cl 176
190
100
393
284
200
376
269
300
o
400
500
Temperature( C) Fig. 2. NH3 -TPD profiles of the two catalyst samples.
2.3. Catalytic activity test
NO conversion = (1 −
Ce/TiO2-Cl
60
TCD signal(a.u.)
The fresh Ce/TiO2 catalyst was prepared by sol-gel method. The chemicals used to prepare catalyst samples were of analytical grade. Firstly, butyl titanate (0.1 mol), cerium nitrate (0.03 mol), nitric acid (0.1 mol) and deionized water (0.6 mol) were mixed under vigorous stirring at room temperature until the formation of transparent yellow sol. Then the sol was dried at 80 ◦ Cfor 24 h to convert it into xerogel, followed by calcination in air at 500 ◦ C for 5 h to obtain the fresh catalyst. Next then, the Cl-poisoned catalyst was prepared by impregnation method. The fresh catalyst was impregnated in the solution of NH4 Cl with a certain concentration (the molar ratio of Cl: Ce = 0.05). After stirred for 6 h, the mixed solution was dried at 80 ◦ Cfor 24 h, then the solid was calcined at 500 ◦ C for 5 h. The fresh catalyst and the poisoned catalyst were denoted as Ce/TiO2 and Ce/TiO2 -Cl respectively.
NO conversion (%)
80
poisoning effect of Cl on Ce/TiO2 catalyst was much weaker than that reported in our previous study [21]. This could be attributed the relatively low loading amount of Cl in this study. And the deactivation mechanism would be further discussed in the following sections based on the results of TPD and in situ DRIFT studies.
(1)
when calculating NO conversion, N2 O was regarded as unconverted NOx rather than a product. 3. Results and discussion 3.1. SCR activity The SCR activities of the two catalyst samples as a function of reaction temperature are shown in Fig. 1. It was clear that the values of NO conversions over the two catalyst samples increased with increasing reaction temperature. From Fig. 1, it could be seen that the addition of Cl had a poisoning effect on Ce/TiO2 catalyst. But the
3.2. NH3 -TPD analysis The NH3 -TPD profiles of Ce/TiO2 and Ce/TiO2 -Cl catalyst samples are presented in Fig. 2. As can be observed from Fig. 2, there are three desorption peaks in the NH3 -TPD profile of each catalyst sample. The first peak below 200 ◦ C could be assigned to the desorption of physisorbed NH3 and some NH+ bounded to the weak 4 Brønsted acid sites; the other one ranged from 269 to 284 ◦ C may be related to the desorption of NH+ from the strong Brønsted acid 4 sites; and the third one above 300 ◦ C is resulted from the coordinated NH3 bound to the Lewis acid sites [23–25]. Furthermore, the total peak area of NH3 -TPD profile for Ce/TiO2 decreased after the addition of Cl, indicating the decrease of NH3 adsorbance.
N.-z. Yang et al. / Applied Surface Science 378 (2016) 513–518
Ce/TiO2
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(A)
1180
Ce/TiO2-Cl TCD signal(a.u.)
Absorbance(a.u.)
1677 1606
1431
N2 purge 30 min 30min 10min 5min
1min
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400
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600
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o
Temperature( C)
1400
1200
-1
Wavenumber(cm )
Fig. 3. NO-TPD profiles of the two catalyst samples.
(B)
3.3. NO-TPD analysis
N2 purge 30 min
Absorbance(a.u.)
The results of NO-TPD analysis are presented in Fig. 3. It is obvious that each NO-TPD profile contains two desorption peaks. The first one at about 120 ◦ C could be attributed to molecularly adsorbed NO [26], and the other peak at about 390 ◦ C could be related to the decomposition of higher thermal stability nitrate species [24]. Similar with the results of NH3 -TPD analysis, the addition of Cl on Ce/TiO2 catalyst seems to have an inhibition effect on NOx adsorption.
1677
1606
30min 10min 5min
3.4. In situ DRIFT study 3.4.1. NH3 adsorption The DRIFT spectra of Ce/TiO2 and Ce/TiO2 -Cl after they were exposed to NH3 /N2 for different times are illustrated in Fig. 4(A) and (B) respectively. It is well known that NH3 could be adsorbed onto Brønsted acid sites or Lewis acid sites on the surface of metal oxide catalyst. In Fig. 4(A), several bands at 1677, 1606, 1431 and 1180 cm−1 could be observed. The bands at 1677 cm−1 and 1431 cm−1 could be assigned toNH+ on Brønsted acid sites [27,28]; 4 and the bands at 1606 cm−1 and 1180 cm−1 could be assigned to ammonia species coordinated to Lewis acid sites [28,29]. It was noticeable that the band intensities rarely changed after 5 min, indicating that the adsorption equilibrium of NH3 could be accomplished in 5 min. Furthermore, the amount of NH3 species adsorbed on Lewis acid sites were much higher than that adsorbed on Brønsted acid sites, therefore, Ce/TiO2 catalyst was mainly Lewis acidic, agreeing well with the results of Trovarelli [30]. Fig. 4(B) showed the DRIFT spectra of Ce/TiO2 -Cl catalyst after it was exposed to 800 ppm NH3 for various times. From Fig. 4(B), it can be seen that the locations of the bands were similar with that in the DRIFT spectra of Ce/TiO2 , but the intensities were quite different. For example, the intensities of the bands assigned to NH+ 4 on Brønsted acid sites (1677 cm−1 ) and ammonia species coordinated to Lewis acid sites (1606 and 1180 cm−1 ) in the spectra of Ce/TiO2 -Cl is much lower than that in the spectra of Ce/TiO2 . Furthermore, the band at 1431 cm−1 could not be detected in the spectra of Ce/TiO2 -Cl. After the sample was purged with N2 , the reduction of the band intensities of Ce/TiO2 -Cl was much obvious than that of Ce/TiO2 , indicating that the addition of Cl would lead to a weaker adsorption of NH3 species. Based on the comparison between Fig. 4(A) and (B), it could be concluded that the addition of Cl on Ce/TiO2 catalyst would decrease its Brønsted acidity and Lewis acidity.
1180
1min
2000
1800
1600
1400
1200
-1
Wavenumber(cm ) Fig. 4. In situ DRIFT spectra of Ce/TiO2 (A) and Ce/TiO2 -Cl (B) exposed to 800 ppm NH3 for various times and after purging by N2 for 30 min at 200 ◦ C.
3.4.2. NO + O2 adsorption Fig. 5(A) presents the DRIFT spectra of Ce/TiO2 after it was exposed to 800 ppm NO + 5% O2 for various times. As can be seen form Fig. 5(A), the bands at 1612 and 1226 cm−1 could be attributed to bridged nitrate species [27,31], and the band at 1571 cm−1 was assigned to bidentate nitrate [28]. Due to the accumulation of adsorbed NOx species, the band intensities increased with time. Compared with the adsorption process of NH3 species (Fig. 4), it seemed that the adsorption process of NOx species was much slower. And the N2 purge had little effect on the band intensities of NOx species, meaning the strong interaction between NOx species and catalyst surface. The DRIFT spectra of Ce/TiO2 -Cl catalyst exposed to 800 ppm NO + 5% O2 for various times are shown in Fig. 5(B). The spectra were very similar with that of Ce/TiO2 . However, two new bands at 1538 and 1175 cm−1 could be observed, which could be assigned to bidentate nitrate and bridged nitrate respectively [31,32]. In addition, the band intensities in the spectra of Ce/TiO2 -Cl were much lower than that of Ce/TiO2 . Therefore, the addition of Cl on Ce/TiO2 catalyst would also inhibit the adsorption of NOx species on it. 3.4.3. NO + O2 adsorption after NH3 To investigate the reactivity of adsorbed NH3 species in the SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl catalysts, the in situ DRIFT
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1571
(A)
1226
1226
N2 purge
Absorbance(a.u.)
N2 purge 30min
Absorbance(a.u.)
1571 1612
(A)
1612
30min 10min 5min
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1677
5min 2min
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1606
NH3 2000
1800
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Wavenumber(cm )
Wavenumber(cm )
(B)
1571 1612
(B)
1612 1571 1538
1226 N2 purge
Absorbance(a.u.)
Absorbance(a.u.)
1226 1175 N2 purge 30min 30min 10min
30min 10min 5min
5min
1677
2min
1min
2000
1600
1400
1200
2000
1800
-1
Wavenumber(cm ) Fig. 5. In situ DRIFT spectra of Ce/TiO2 (A) and Ce/TiO2 -Cl (B) exposed to 800 ppm NO +5% O2 for various times and after purging by N2 for 30 min at 200 ◦ C.
spectra of the reaction between NO + O2 and the preadsorbed NH3 species at 200 ◦ C were recorded as a function of time, and the results are shown in Fig. 6(A) and (B) respectively. As shown in Fig. 6(A) and (B), several bands belonging to adsorbed NH3 species on the surfaces of Ce/TiO2 and Ce/TiO2 -Cl could be observed. When NO + O2 was introduced, the bands ascribed to absorbed NH3 species diminished, meanwhile, several bands of NOx adsorption species formed. That is to say, both of the coordinated NH3 species bound to Lewis acid sites and NH+ on Brønsted acid sites were all active species in 4 the NH3 -SCR reaction taking place over Ce/TiO2 and Ce/TiO2 -Cl. 3.4.4. NH3 adsorption after NO + O2 Correspondingly, the in situ DRIFT spectra of the reaction between preadsorbed NO + O2 and NH3 over Ce/TiO2 and Ce/TiO2 Cl are illustrated in Fig. 7(A) and (B). After the introduction of NH3 , it could be seen that the bands attributed bridged nitrate species (1612 and 1226 cm−1 ) on Ce/TiO2 and Ce/TiO2 -Cl disappeared quickly, and some bands of NH3 adsorption species adsorbed on Lewis acid sites [29,33] (1606, 1240 and 1180 cm−1 ) appeared, while the band belonging to bidentate nitrate (1571 cm−1 ) nearly remained unchanged. Therefore, only bridged nitrate species could take part in the NH3 -SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl. When Lian et al. [28] studied the NH3 -SCR reaction mechanism on Ce-Mn/TiO2 and Ca–Ce-Mn/TiO2 catalysts, they also found that some species of monodentate nitrate, bridge nitrate and biden-
1180 1606
NH3
1800
1200
-1
1600
1400
1200
-1
Wavenumber(cm ) Fig. 6. In situ DRIFT spectra of NO + O2 reacted with preadsorbed NH3 species on Ce/TiO2 (A) and Ce/TiO2 -Cl (B)at 200 ◦ C.
tate nitrate were not active in SCR reaction. Thus the active NOx species in SCR reaction may be greatly dependent on the kind of SCR catalyst. 3.5. Reaction mechanism To further understand the NH3 -SCR reaction mechanism over Ce/TiO2 and Ce/TiO2 -Cl, the effect of some operation conditions including NO inlet concentration and NH3 inlet concentration on the SCR activities of the two catalyst samples were investigated based on the method described in [34]. During the activity test process, the concentrations of NO and NH3 were kept the same to ensure the enough SCR reaction. If the NH3 -SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl is mainly controlled by the Eley–Rideal mechanism, the NO conversions over Ce/TiO2 and Ce/TiO2 -Cl should not decrease with the increase of NO concentration from 600 ppm to 1800 ppm. On the other hand, if the NH3 -SCR reaction over Ce/TiO2 and Ce/TiO2 -Cl is mainly controlled by the Langmuir–Hinshelwood mechanism, the NO conversions over Ce/TiO2 and Ce/TiO2 -Cl should halve after the NO inlet concentration was doubled. As can be seen from Fig. 8, NO conversion decreased with increasing NO concentration. In addition, the NO concentration did not halve when the concentration of NO increases from 600 ppm to 1200 ppm. Therefore, the SCR reaction
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(A) 1606
(A)
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NO conversion (%)
N2 purge
Absorbance(a.u.)
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5min 1612
2min
60
[NO]=600 ppm [NO]=1200 ppm [NO]=1800 ppm
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NO+O2
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Reaction temperature( C)
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(B)
(B)
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NO conversion (%)
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Absorbance(a.u.)
N2 purge 30min 10min 5min
1612
[NO]=600 ppm [NO]=1200 ppm [NO]=1800 ppm
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1226
NO+O2
0 60
2000
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1000
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o
Reaction temperature( C)
-1
Wavenumber(cm ) Fig. 7. In situ DRIFT spectra of NH3 reacted with preadsorbed NO + O2 species on Ce/TiO2 (A) and Ce/TiO2 -Cl (B) at 200 ◦ C.
over Ce/TiO2 and Ce/TiO2 -Cl obeys both the Eley-Rideal mechanism and the Langmuir–Hinshelwood mechanism. Based on previous reports and the results of this study, the NH3 -SCR reaction mechanism over Ce/TiO2 and Ce/TiO2 -Cl could be described as follows [35,36]:
Fig. 8. Influence of NO inlet concentration on NO conversion over Ce/TiO2 catalyst (A) and Ce/TiO2 -Cl catalyst (B). Ce/TiO2 Cl catalyst (B) Reaction conditions: 䊏, [NO] = [NH3 ] = 600 ppm; 䊉, [NO] = [NH3 ] = 1200 ppm; , [NO] = [NH3 ] = 1800 ppm; [O2 ] = 5%, balance Ar, GHSV = 108,000 h−1 .
NO(g) + O−* → NO2 (a) Ce4+
(1) E-R mechanism: Firstly, gaseous NH3 is absorbed on Lewis acid sites to form coordinated NH3 . After the dehydration by labile oxygen, the absorbed NH3 is converted into NH2 species. After that, NH2 reacts with gaseous NO to form NH2 NO, which is finally decomposed into N2 and H2 O. The whole reaction process could be written as: Ce4+
NH3 (g) → NH3 (a) over Lewis acid sites
(1)
O2 (g) → 2O(a)
(2)
NH3 (a) + O(a) → NH2 (a) + OH(a)
(3)
NO(g) + NH2 (a) → NH2 NO(a)
(4)
NH2 NO(a) → N2 (g) + H2 O
(5)
(2) L-H mechanism: The adsorbed NOx species reacts with the adsorbed NH3 species to form N2 and H2 O: O2 (g) → 2O−* (surface active sites)
(6)
(7)
NH3 (g) → NH3 (a) over Lewis acid sites
(8)
2NH3 (a) + NO2 (a) + NO(g) → 2N2 + 3H2 O
(9)
4. Conclusions The addition of Cl had a poisoning effect on Ce/TiO2 catalyst for low-temperature SCR reaction of NO by NH3 . In this study, TPD and the in situ DRIFT studies were performed to investigate the deactivation mechanism. The decreased adsorption capacities of NH3 species and NO species could be found from the results of TPD analysis. From the results of DRIFT study, it was found that the NH3 -SCR reactions on Ce/TiO2 and Ce/TiO2 -Cl catalyst samples were all governed by the combination of the Eley-Rideal mechanism and the Langmuir-Hinshelwood mechanism. The addition of Cl on Ce/TiO2 not only inhibited the adsorption of NH3 species, but also inhibited the adsorption of NOx species, as a result, leading to the deactivation of Ce/TiO2 .
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