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TiO2 catalyst for NH3-SCR process: A DRIFT study
Journal of the Energy Institute xxx (2018) 1e8
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The deactivation effect of Cl on V/TiO2 catalyst for NH3-SCR process: A DRIFT study Shuai-wei Liu a, b, Rui-tang Guo a, b, c, *, Xiao Sun a, b, Jian Liu a, b, Wei-guo Pan a, b, Zhiling Xin d, Xu Shi a, b, Zhong-yi Wang a, b, Xing-yu Liu a, b, Hao Qin a, b a
School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, 200090, PR China Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai, 200090, PR China Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, PR China d College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai, 200090, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2018 Received in revised form 24 June 2018 Accepted 25 June 2018 Available online xxx
In present study, the poisoning mechanism of Cl on V/TiO2 catalyst for SCR removal of NOx using NH3 as the reductant was explored by in situ DRIFT technique. The experimental results revealed that the existence of Cl did not change the SCR reaction mechanism over V/TiO2 catalyst, which was mainly under the control of both E-R and L-H mechanisms. The deactivation of V/TiO2 catalyst by Cl was mainly resulted from the inhibited adsorption of NH3 and NOx species, accompanied by the decrease of the reactivity of adsorbed NH3 species. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: SCR V/TiO2 catalyst Deactivation Adsorption Surface reaction
1. Introduction NOx emitted from the burning process of coal, oil and gas makes a great contribution to several pollution issues such as acid precipitation and photochemical pollutant [1e4]. As the well-recognized technique for controlling NOx emission, selective catalytic reduction (SCR) of NOx by using NH3 reductant has been fully commercialized [5e8]. For this purpose, V-based catalyst (using W or Mo as the additive) plays the dominant role in the SCR reactor used in industrial fields [9e11]. Due to the complicated compositions of the flue gas, the deactivation effect of SO2 and alkali metals in fly ash on V-based SCR catalyst is unavoidable [12,13]. Besides SO2 and alkali metals, 10e50 ppm HCl also widely exists in coal-fired flue gas [14]. Hou et al. [15] reported that HCl had a promoting effect on V2O5/AC catalyst, while Lisi et al. [16] revealed a deactivation impact of HCl on a commercial V2O5-WO3/TiO2 catalyst. As well known, the adsorption of reactants and their surface reactions play a vital role in NH3-SCR process [17e19], which is affected by the deposited impurities from the flue gas. However, the effect of Cl deposition on the SCR behavior of V-based catalyst is still uncertain. Therefore, the effect of deposited Cl on the adsorption of NOx, NH3 and their surface reactions over V/TiO2 catalyst were investigated on the basis of in situ DRIFT technique, and the deactivation mechanism of Cl species on V/TiO2 would be discussed. 2. Experimental 2.1. Catalyst preparation The fresh V/TiO2 catalyst used in this study was prepared by impregnation method. The commercial TiO2 in anatase (Aladdin Reagent Inc. Shanghai, China) was used as the support. The specific surface area was 54.3 m2/g. After impregnating the support by incipient wetness in an
* Corresponding author. E-mail address: [email protected] (R.-t. Guo). https://doi.org/10.1016/j.joei.2018.06.016 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Please cite this article in press as: Shuai-wei Liu, et al., The deactivation effect of Cl on V/TiO2 catalyst for NH3-SCR process: A DRIFT study, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.016
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aqueous solution of ammonium vanadate (V/Ti molar ratio ¼ 0.12), the solid was dried at 100 C for 12 h, followed by calcination in air at 500 C for 5 h. The Cl-doped V/TiO2 catalyst was prepared by using impregnation method, as used in our previous study [20]. NH4Cl was used as the source of Cl. At first, the fresh V/TiO2 catalyst sample was impregnated by incipient wetness with the solution of NH4Cl (Cl/V molar ratio ¼ 0.25). Then the sample was dried at 100 C for 12 h and calcined at 500 C in air for 5 h. The obtained catalyst sample was denoted as V/TiO2-Cl. 2.2. Characterizations Textural characteristics of the catalysts were investigated by N2 adsorption/desorption at 77K by using a Quantachrome Autosorb-iQ-AG instrument. The specific surface and the pore size distribution were determined based on Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method respectively. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis which performed on Leeman Profile spec apparatus was used to obtain the actual concentrations of elements in the catalysts. Based on in situ DRIFT technique, the adsorption behavior of reactant species and their surface reactions were investigated, which was performed on an FTIR spectrometer (Thermo Nicolet iS 50) with a mercury cadmium telluride (MCT) detector. In the DRIFT cell with a KBr window, the catalyst sample was pretreated at 400 C in N2 for 1 h to eliminate the influence of moisture and other adsorbents, then it was cooled to the desired temperature. The background was collected with the flowing of N2 and automatically subtracted from the sample spectrum at the corresponding temperature. During the DRIFT test process, the feeding gas was a mixture of 600 ppm NH3, or/and 600 ppm NOþ5% O2, balance N2, and the total flowrate was set as 300 mL/min. When recording the DRIFT spectra, 100 scans were made and averaged to yield a spectrum with the resolution of 4 cm1. 2.3. Activity test The SCR activities of the two catalyst samples were tested in a fixed-bed reactor (i. d. ¼ 8 mm). In each experimental run, about 0.55 cm3 catalyst sample (80e100 mesh) was used for activity test. And the simulated flue gas was consisted of 600 ppm NO, 600 ppm NH3, 5% O2,
Fig. 1. (A) NOx conversions over the two catalyst samples at different temperature. (B) N2 selectivities of different catalyst samples. Reaction conditions: [NO] ¼ [NH3] ¼ 600 ppm, [O2] ¼ 5%, balance Ar, GHSV ¼ 108, 000 h1 (B) N2 selectivities of different catalyst samples.
Please cite this article in press as: Shuai-wei Liu, et al., The deactivation effect of Cl on V/TiO2 catalyst for NH3-SCR process: A DRIFT study, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.016
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balance Ar. The flowrate of the simulated flue gas was maintained at 1 L/min, thus the corresponding gas hourly space velocity (GHSV) was 108, 000 h1. The concentrations of NO, NO2, NH3 and N2O in the inlet and outlet gas streams were continuously monitored by a FTIR spectrometer (Thermo Nicolet iS 50) equipped with a 0.2 dm3 gas cell. The values of NOx conversion and N2 selectivity could be calculated by:
NOx conversion ¼
½NOx in ½NOx out 100% ½NOx in
N2 selectivity ¼
1
(1)
2½N2 Oout ½NOxin þ ½NH3 in ½NOxout ½NH3 out
100%
(2)
3. Results and discussion 3.1. SCR activity Fig. 1(A) presents the SCR activities of the two catalyst samples as a function of reaction temperature. It could be seen that the NOx conversion over the fresh V/TiO2 catalyst increased with reaction temperature firstly. When the reaction temperature was over 300 C, the NOx conversion values nearly kept stable. The similar trend was also observed on the V/TiO2-Cl catalyst. Apparently, the addition of Cl on V/ TiO2 catalyst had a poisoning effect on it, an activity drop from 88.8% to 70.2% could be detected at 250 C. And the Cl-doping results in a drastic decrease of N2 selectivity, as shown in Fig. 1(B). The deactivation mechanism would be discussed based on the results of in situ DRIFT study. 3.2. BET and ICP analysis The BET surface areas of the two catalysts are listed in Table 1. It is clear that the BET surface area and total pore volume have a decrease after the addition of Cl, which could be owing to the shielding effect of Cl species [21]. Table 2 summarizes the chemical compositions of the two catalysts. From Table 2, it could be seen the element concentrations stay constant with the theoretical contents obtained by molar ratio.
Table 1 Textural properties of the two catalyst samples. Samples
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
V/TiO2 V/TiO2-Cl
11.065 10.441
0.0514 0.0487
3.802 3.796
Table 2 Analysis of chemical compositions of the two catalyst samples. Samples
V/TiO2 V/TiO2-Cl
concentration from ICP (wt.%) V
Ti
Cl
3.25 3.14
21.22 21.03
/ 1.07
Fig. 2. In situ DTIFT spectra of NH3 adsorption over the two catalyst samples at 250 C.
Please cite this article in press as: Shuai-wei Liu, et al., The deactivation effect of Cl on V/TiO2 catalyst for NH3-SCR process: A DRIFT study, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.016
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Fig. 3. In situ DTIFT spectra of NOx adsorption over the two catalyst samples at 250 C.
Fig. 4. In situ DRIFT spectra of the reaction between NOx and the preadsorbed NH3 species over (A) V/TiO2; (B) V/TiO2-Cl at 250 C.
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3.3. In situ DRIFT study 3.3.1. NH3 adsorption Previous studies had indicated that the adsorption of NH3 was a vital step for NH3-SCR reaction [22e24]. To understand the nature of adsorbed NH3 species over V/TiO2 and V/TiO2-Cl catalysts, the DRIFT spectra of NH3 adsorption over the two catalyst samples at 250 C were recorded and the results are illustrated in Fig. 2. Four bands appeared in the DRIFT spectrum of each catalyst sample, which could be ascribed 1 to coordinated NH3 bound to Lewis acid sites (1605 and 1234 cm1) and NHþ 4 species on Brønsted acid sites (1667 and 1418 cm ) 1 respectively [25e29]. It was clear that the intensity of the band at 1418 cm was much higher than that of other bands, suggesting that the surface acidities of the two catalyst samples were mainly in the form of Brønsted acid sites. After the addition of Cl, a distinct band intensity drop was visible on the DRIFT spectrum of V/TiO2-Cl. Thus the loading of Cl on V/TiO2 catalyst was unfavorable to the adsorption of NH3 species on it. 3.3.2. NOx adsorption The DRIFT spectra of NOx adsorption over the two catalyst samples at 250 C are implied in Fig. 3. From Fig. 3, four bands of adsorbed NOx species were present in the DRIFT spectra of the two catalyst samples, including bridged nitrate (1600 and 1217 cm1), bidentate nitrate (1564 cm1), MNO2 nitro compounds (1357 cm1) [25,30e32]. Our previous study found that the addition of Na or K on Ce/TiO2 catalyst could generate new active sites for NOx adsorption [33]. However, this phenomenon was not observed in this study, which might be owing to the weak alkalinity of Cl species. It was noticeable that the band intensities in the spectrum of V/TiO2-Cl catalyst were much lower than that in the spectrum of the fresh V/TiO2 catalyst, revealing the inhibited adsorption NOx species after the introduction of Cl on V/TiO2 catalyst. 3.3.3. Reaction between NOx species and the preadsorbed NH3 species To identify the role of absorbed NH3 species in the NH3-SCR reactions over the two catalyst samples, the DRIFT spectra of the reaction between NOx and the preadsorbed NH3 species at 250 C were recorded as time, as exhibited in Fig. 4. From Fig. 4 (A), several bands (1605,
Fig. 5. In situ DRIFT spectra of the reaction between NH3 and the preadsorbed NOx species over (A) V/TiO2; (B) V/TiO2-Cl at 250 C.
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1418 and 1234 cm1) appeared in the DRIFT spectrum of V/TiO2 catalyst after the pretreatment with NH3 for 30 min. After the introduction of NO þ O2, all these bands became weaker gradually and completely vanished after 10 min. Meanwhile, two bands of adsorbed NOx species came out and grew with time, indicating that the adsorbed NH3 species were replaced by the adsorbed NOx species. To compare the reactivity of adsorbed NH3 species over Brønsted acid sites and Lewis acid sites, the change of normalized intensities of the three bands at 1605, 1418 and 1234 cm1 in the spectra of V/TiO2 and V/TiO2-Cl at 250 C were recorded as a function of time, as shown in Fig. 6. For the spectra of V/TiO2 catalyst, it could be seen that the reactivity of NH3 species over Leiws acid sites was higher than that of the NH3 species over Brønsted acid sites. For the Cl-poisoned catalyst samples, all the adsorbed NH3 species were of similar reactivity. Obviously, all the NH3 species adsorbed on Brønsted and Lewis acid sites were involved in the NH3-SCR reaction over V/TiO2 catalyst, suggesting that EleyeRideal (E-R) mechanism was applicable for it [34]. For the reaction between NOx species and the preadsorbed NH3 species over V/TiO2-Cl catalyst, similar variation trend reappeared in its DRIFT spectra (Fig. 4 (B)), which also suggested the presence of E-R mechanism for the NH3-SCR reaction over V/TiO2-Cl catalyst. However, it should be noticed that some bands of adsorbed NH3 species were still visible in the spectrum of V/TiO2-Cl catalyst after the introduction of NO þ O2 for 10 min (Fig. 4 (B)), meaning that the addition of Cl on V/TiO2 catalyst would weaken the reactivity of adsorbed NH3 species on it. 3.3.4. Reaction between NH3 species and the preadsorbed NOx species From the other side, the reaction between NH3 species and the preadsorbed NOx species was also investigated by in situ DRIFT study. And the DRIFT spectra are shown in Fig. 5. Evidently, the exposure to NO þ O2 generated two weak bands of adsorbed NOx species (1598 and 1217 cm1) in the DRIFT spectra of V/TiO2 catalyst (Fig. 5 (A)), and the introduction of NH3 into DRIFT cell quickly consumed the adsorbed NOx species in 2 min, accompanied by the formation and accumulation of adsorbed NH3 species with time, which could be reflected by the appearance and growth of NH3 adsorption bands. The DRIFT spectra of this process for V/TiO2-Cl catalyst were basically similar with that of the fresh V/TiO2 catalyst. The consumption of the preadsorbed NOx species over V/TiO2 and V/TiO2-Cl catalysts at 250 C are shown in Fig. 7. From Fig. 7 (A), it could be found that the reactivity of bridged nitrate was higher than that of bidentate nitrate. Similar trend was also visible for the adsorbed NOx over V/TiO2-Cl catalyst (Fig. 7 (B)). Thus all the adsorbed NOx species were active in the NH3-SCR reactions over the two catalyst samples, implying the existence of Langmuir- Hinshelwood (L-H) mechanism [35e37]. Our previous study [30] and the study of Liu et al. [27] indicated that not all the adsorbed NOx species over Ce/TiO2 and Ce-Mn/TiO2 catalysts could participate in the NH3-SCR reactions over them. Thus the role of adsorbed NOx species in NH3-SCR reaction was greatly dependent on the type of SCR catalyst.
Fig. 6. Consumption of preadsorbed NH3 species over (A) V/TiO2 and (B) V/TiO2-Cl at 250 Cafter the introduction of NO þ O2.
Please cite this article in press as: Shuai-wei Liu, et al., The deactivation effect of Cl on V/TiO2 catalyst for NH3-SCR process: A DRIFT study, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.016
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Fig. 7. Consumption of preadsorbed NOx species over (A) V/TiO2 and (B) V/TiO2-Cl at 250 C after the introduction of NH3.
Combined with the results of section 3.3.3 and section 3.3.4, the conclusion could be drawn that the addition of Cl on V/TiO2 catalyst did not change the NH3-SCR reaction mechanism over it, which was the coexistence of E-R and L-H routes. And the same reaction mechanism had also been found for the NH3-SCR reaction over V2O5 catalyst supported on activated semi-coke [38]. Therefore, the deactivation of V/TiO2-Cl catalyst might be originated from the suppressed adsorption of NH3 and NOx species, along with the dropped reactivity of adsorbed NH3 species. 4. Conclusions The deactivation mechanism of Cl on V/TiO2 catalyst for selective catalytic reduction of NOx with NH3 was investigated based on the in situ DRIFT study. Characterization results revealed the inhibited adsorption of NH3 and NOx species over the poisoned catalyst sample; moreover, the addition of Cl on V/TiO2 catalyst would lead to the reactivity drop of adsorbed NH3 species. With all these features, a SCR activity decrease could be detected on V/TiO2 catalyst after the loading of Cl. Acknowledgments This work was financially supported by the National Key R&D Program of China (2018YFB0605002). References [1] G. Qi, R.T. Yang, Performance and kinetics study for low-temperature SCR of NO with NH3 over MnOx-CeO2 catalyst, J. Catal. 217 (2003) 434e441. [2] P. Sun, R. Guo, S. Liu, S. Wang, W. Pan, M. Li, The enhanced performance of MnOx catalyst for NH3-SCR reaction by the modification with Eu, Appl. Catal., A 531 (2017) 129e138. [3] W. Cha, S.H. Ehrmanc, J. Jurng, Surface phenomenon of CeO2-added V2O5/TiO2 catalyst based chemical vapor condensation (CVC) for enhanced selective catalytic reduction at low temperatures, Chem. Eng. J. 304 (2016) 72e78. [4] J. Zhao, Y. Song, W. Lam, W. Liu, Y. Liu, Y. Zhang, et al., Solar radiation transfer and performance analysis of an optimum photovoltaic/thermal system, Energy Convers. Manag. 52 (2011) 1343e1353.
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Please cite this article in press as: Shuai-wei Liu, et al., The deactivation effect of Cl on V/TiO2 catalyst for NH3-SCR process: A DRIFT study, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.016
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The deactivation effect of Cl on V/TiO2 catalyst for NH3-SCR process: A DRIFT study Shuai-wei Liu a, b, Rui-tang Guo a, b, c, *, Xiao Sun a, b, Jian Liu a, b, Wei-guo Pan a, b, Zhiling Xin d, Xu Shi a, b, Zhong-yi Wang a, b, Xing-yu Liu a, b, Hao Qin a, b a
School of Energy Source and Mechanical Engineering, Shanghai University of Electric Power, Shanghai, 200090, PR China Shanghai Engineering Research Center of Power Generation Environment Protection, Shanghai, 200090, PR China Shanghai Institute of Pollution Control and Ecological Security, Shanghai, 200092, PR China d College of Environmental and Chemical Engineering, Shanghai University of Electric Power, Shanghai, 200090, PR China b c
a r t i c l e i n f o
a b s t r a c t
Article history: Received 10 February 2018 Received in revised form 24 June 2018 Accepted 25 June 2018 Available online xxx
In present study, the poisoning mechanism of Cl on V/TiO2 catalyst for SCR removal of NOx using NH3 as the reductant was explored by in situ DRIFT technique. The experimental results revealed that the existence of Cl did not change the SCR reaction mechanism over V/TiO2 catalyst, which was mainly under the control of both E-R and L-H mechanisms. The deactivation of V/TiO2 catalyst by Cl was mainly resulted from the inhibited adsorption of NH3 and NOx species, accompanied by the decrease of the reactivity of adsorbed NH3 species. © 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
Keywords: SCR V/TiO2 catalyst Deactivation Adsorption Surface reaction
1. Introduction NOx emitted from the burning process of coal, oil and gas makes a great contribution to several pollution issues such as acid precipitation and photochemical pollutant [1e4]. As the well-recognized technique for controlling NOx emission, selective catalytic reduction (SCR) of NOx by using NH3 reductant has been fully commercialized [5e8]. For this purpose, V-based catalyst (using W or Mo as the additive) plays the dominant role in the SCR reactor used in industrial fields [9e11]. Due to the complicated compositions of the flue gas, the deactivation effect of SO2 and alkali metals in fly ash on V-based SCR catalyst is unavoidable [12,13]. Besides SO2 and alkali metals, 10e50 ppm HCl also widely exists in coal-fired flue gas [14]. Hou et al. [15] reported that HCl had a promoting effect on V2O5/AC catalyst, while Lisi et al. [16] revealed a deactivation impact of HCl on a commercial V2O5-WO3/TiO2 catalyst. As well known, the adsorption of reactants and their surface reactions play a vital role in NH3-SCR process [17e19], which is affected by the deposited impurities from the flue gas. However, the effect of Cl deposition on the SCR behavior of V-based catalyst is still uncertain. Therefore, the effect of deposited Cl on the adsorption of NOx, NH3 and their surface reactions over V/TiO2 catalyst were investigated on the basis of in situ DRIFT technique, and the deactivation mechanism of Cl species on V/TiO2 would be discussed. 2. Experimental 2.1. Catalyst preparation The fresh V/TiO2 catalyst used in this study was prepared by impregnation method. The commercial TiO2 in anatase (Aladdin Reagent Inc. Shanghai, China) was used as the support. The specific surface area was 54.3 m2/g. After impregnating the support by incipient wetness in an
* Corresponding author. E-mail address: [email protected] (R.-t. Guo). https://doi.org/10.1016/j.joei.2018.06.016 1743-9671/© 2018 Energy Institute. Published by Elsevier Ltd. All rights reserved.
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aqueous solution of ammonium vanadate (V/Ti molar ratio ¼ 0.12), the solid was dried at 100 C for 12 h, followed by calcination in air at 500 C for 5 h. The Cl-doped V/TiO2 catalyst was prepared by using impregnation method, as used in our previous study [20]. NH4Cl was used as the source of Cl. At first, the fresh V/TiO2 catalyst sample was impregnated by incipient wetness with the solution of NH4Cl (Cl/V molar ratio ¼ 0.25). Then the sample was dried at 100 C for 12 h and calcined at 500 C in air for 5 h. The obtained catalyst sample was denoted as V/TiO2-Cl. 2.2. Characterizations Textural characteristics of the catalysts were investigated by N2 adsorption/desorption at 77K by using a Quantachrome Autosorb-iQ-AG instrument. The specific surface and the pore size distribution were determined based on Brunauer-Emmett-Teller (BET) method and Barrett-Joyner-Halenda (BJH) method respectively. Inductively coupled plasma optical emission spectroscopy (ICP-OES) analysis which performed on Leeman Profile spec apparatus was used to obtain the actual concentrations of elements in the catalysts. Based on in situ DRIFT technique, the adsorption behavior of reactant species and their surface reactions were investigated, which was performed on an FTIR spectrometer (Thermo Nicolet iS 50) with a mercury cadmium telluride (MCT) detector. In the DRIFT cell with a KBr window, the catalyst sample was pretreated at 400 C in N2 for 1 h to eliminate the influence of moisture and other adsorbents, then it was cooled to the desired temperature. The background was collected with the flowing of N2 and automatically subtracted from the sample spectrum at the corresponding temperature. During the DRIFT test process, the feeding gas was a mixture of 600 ppm NH3, or/and 600 ppm NOþ5% O2, balance N2, and the total flowrate was set as 300 mL/min. When recording the DRIFT spectra, 100 scans were made and averaged to yield a spectrum with the resolution of 4 cm1. 2.3. Activity test The SCR activities of the two catalyst samples were tested in a fixed-bed reactor (i. d. ¼ 8 mm). In each experimental run, about 0.55 cm3 catalyst sample (80e100 mesh) was used for activity test. And the simulated flue gas was consisted of 600 ppm NO, 600 ppm NH3, 5% O2,
Fig. 1. (A) NOx conversions over the two catalyst samples at different temperature. (B) N2 selectivities of different catalyst samples. Reaction conditions: [NO] ¼ [NH3] ¼ 600 ppm, [O2] ¼ 5%, balance Ar, GHSV ¼ 108, 000 h1 (B) N2 selectivities of different catalyst samples.
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balance Ar. The flowrate of the simulated flue gas was maintained at 1 L/min, thus the corresponding gas hourly space velocity (GHSV) was 108, 000 h1. The concentrations of NO, NO2, NH3 and N2O in the inlet and outlet gas streams were continuously monitored by a FTIR spectrometer (Thermo Nicolet iS 50) equipped with a 0.2 dm3 gas cell. The values of NOx conversion and N2 selectivity could be calculated by:
NOx conversion ¼
½NOx in ½NOx out 100% ½NOx in
N2 selectivity ¼
1
(1)
2½N2 Oout ½NOxin þ ½NH3 in ½NOxout ½NH3 out
100%
(2)
3. Results and discussion 3.1. SCR activity Fig. 1(A) presents the SCR activities of the two catalyst samples as a function of reaction temperature. It could be seen that the NOx conversion over the fresh V/TiO2 catalyst increased with reaction temperature firstly. When the reaction temperature was over 300 C, the NOx conversion values nearly kept stable. The similar trend was also observed on the V/TiO2-Cl catalyst. Apparently, the addition of Cl on V/ TiO2 catalyst had a poisoning effect on it, an activity drop from 88.8% to 70.2% could be detected at 250 C. And the Cl-doping results in a drastic decrease of N2 selectivity, as shown in Fig. 1(B). The deactivation mechanism would be discussed based on the results of in situ DRIFT study. 3.2. BET and ICP analysis The BET surface areas of the two catalysts are listed in Table 1. It is clear that the BET surface area and total pore volume have a decrease after the addition of Cl, which could be owing to the shielding effect of Cl species [21]. Table 2 summarizes the chemical compositions of the two catalysts. From Table 2, it could be seen the element concentrations stay constant with the theoretical contents obtained by molar ratio.
Table 1 Textural properties of the two catalyst samples. Samples
BET surface area (m2/g)
Total pore volume (cm3/g)
Average pore diameter (nm)
V/TiO2 V/TiO2-Cl
11.065 10.441
0.0514 0.0487
3.802 3.796
Table 2 Analysis of chemical compositions of the two catalyst samples. Samples
V/TiO2 V/TiO2-Cl
concentration from ICP (wt.%) V
Ti
Cl
3.25 3.14
21.22 21.03
/ 1.07
Fig. 2. In situ DTIFT spectra of NH3 adsorption over the two catalyst samples at 250 C.
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Fig. 3. In situ DTIFT spectra of NOx adsorption over the two catalyst samples at 250 C.
Fig. 4. In situ DRIFT spectra of the reaction between NOx and the preadsorbed NH3 species over (A) V/TiO2; (B) V/TiO2-Cl at 250 C.
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3.3. In situ DRIFT study 3.3.1. NH3 adsorption Previous studies had indicated that the adsorption of NH3 was a vital step for NH3-SCR reaction [22e24]. To understand the nature of adsorbed NH3 species over V/TiO2 and V/TiO2-Cl catalysts, the DRIFT spectra of NH3 adsorption over the two catalyst samples at 250 C were recorded and the results are illustrated in Fig. 2. Four bands appeared in the DRIFT spectrum of each catalyst sample, which could be ascribed 1 to coordinated NH3 bound to Lewis acid sites (1605 and 1234 cm1) and NHþ 4 species on Brønsted acid sites (1667 and 1418 cm ) 1 respectively [25e29]. It was clear that the intensity of the band at 1418 cm was much higher than that of other bands, suggesting that the surface acidities of the two catalyst samples were mainly in the form of Brønsted acid sites. After the addition of Cl, a distinct band intensity drop was visible on the DRIFT spectrum of V/TiO2-Cl. Thus the loading of Cl on V/TiO2 catalyst was unfavorable to the adsorption of NH3 species on it. 3.3.2. NOx adsorption The DRIFT spectra of NOx adsorption over the two catalyst samples at 250 C are implied in Fig. 3. From Fig. 3, four bands of adsorbed NOx species were present in the DRIFT spectra of the two catalyst samples, including bridged nitrate (1600 and 1217 cm1), bidentate nitrate (1564 cm1), MNO2 nitro compounds (1357 cm1) [25,30e32]. Our previous study found that the addition of Na or K on Ce/TiO2 catalyst could generate new active sites for NOx adsorption [33]. However, this phenomenon was not observed in this study, which might be owing to the weak alkalinity of Cl species. It was noticeable that the band intensities in the spectrum of V/TiO2-Cl catalyst were much lower than that in the spectrum of the fresh V/TiO2 catalyst, revealing the inhibited adsorption NOx species after the introduction of Cl on V/TiO2 catalyst. 3.3.3. Reaction between NOx species and the preadsorbed NH3 species To identify the role of absorbed NH3 species in the NH3-SCR reactions over the two catalyst samples, the DRIFT spectra of the reaction between NOx and the preadsorbed NH3 species at 250 C were recorded as time, as exhibited in Fig. 4. From Fig. 4 (A), several bands (1605,
Fig. 5. In situ DRIFT spectra of the reaction between NH3 and the preadsorbed NOx species over (A) V/TiO2; (B) V/TiO2-Cl at 250 C.
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1418 and 1234 cm1) appeared in the DRIFT spectrum of V/TiO2 catalyst after the pretreatment with NH3 for 30 min. After the introduction of NO þ O2, all these bands became weaker gradually and completely vanished after 10 min. Meanwhile, two bands of adsorbed NOx species came out and grew with time, indicating that the adsorbed NH3 species were replaced by the adsorbed NOx species. To compare the reactivity of adsorbed NH3 species over Brønsted acid sites and Lewis acid sites, the change of normalized intensities of the three bands at 1605, 1418 and 1234 cm1 in the spectra of V/TiO2 and V/TiO2-Cl at 250 C were recorded as a function of time, as shown in Fig. 6. For the spectra of V/TiO2 catalyst, it could be seen that the reactivity of NH3 species over Leiws acid sites was higher than that of the NH3 species over Brønsted acid sites. For the Cl-poisoned catalyst samples, all the adsorbed NH3 species were of similar reactivity. Obviously, all the NH3 species adsorbed on Brønsted and Lewis acid sites were involved in the NH3-SCR reaction over V/TiO2 catalyst, suggesting that EleyeRideal (E-R) mechanism was applicable for it [34]. For the reaction between NOx species and the preadsorbed NH3 species over V/TiO2-Cl catalyst, similar variation trend reappeared in its DRIFT spectra (Fig. 4 (B)), which also suggested the presence of E-R mechanism for the NH3-SCR reaction over V/TiO2-Cl catalyst. However, it should be noticed that some bands of adsorbed NH3 species were still visible in the spectrum of V/TiO2-Cl catalyst after the introduction of NO þ O2 for 10 min (Fig. 4 (B)), meaning that the addition of Cl on V/TiO2 catalyst would weaken the reactivity of adsorbed NH3 species on it. 3.3.4. Reaction between NH3 species and the preadsorbed NOx species From the other side, the reaction between NH3 species and the preadsorbed NOx species was also investigated by in situ DRIFT study. And the DRIFT spectra are shown in Fig. 5. Evidently, the exposure to NO þ O2 generated two weak bands of adsorbed NOx species (1598 and 1217 cm1) in the DRIFT spectra of V/TiO2 catalyst (Fig. 5 (A)), and the introduction of NH3 into DRIFT cell quickly consumed the adsorbed NOx species in 2 min, accompanied by the formation and accumulation of adsorbed NH3 species with time, which could be reflected by the appearance and growth of NH3 adsorption bands. The DRIFT spectra of this process for V/TiO2-Cl catalyst were basically similar with that of the fresh V/TiO2 catalyst. The consumption of the preadsorbed NOx species over V/TiO2 and V/TiO2-Cl catalysts at 250 C are shown in Fig. 7. From Fig. 7 (A), it could be found that the reactivity of bridged nitrate was higher than that of bidentate nitrate. Similar trend was also visible for the adsorbed NOx over V/TiO2-Cl catalyst (Fig. 7 (B)). Thus all the adsorbed NOx species were active in the NH3-SCR reactions over the two catalyst samples, implying the existence of Langmuir- Hinshelwood (L-H) mechanism [35e37]. Our previous study [30] and the study of Liu et al. [27] indicated that not all the adsorbed NOx species over Ce/TiO2 and Ce-Mn/TiO2 catalysts could participate in the NH3-SCR reactions over them. Thus the role of adsorbed NOx species in NH3-SCR reaction was greatly dependent on the type of SCR catalyst.
Fig. 6. Consumption of preadsorbed NH3 species over (A) V/TiO2 and (B) V/TiO2-Cl at 250 Cafter the introduction of NO þ O2.
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Fig. 7. Consumption of preadsorbed NOx species over (A) V/TiO2 and (B) V/TiO2-Cl at 250 C after the introduction of NH3.
Combined with the results of section 3.3.3 and section 3.3.4, the conclusion could be drawn that the addition of Cl on V/TiO2 catalyst did not change the NH3-SCR reaction mechanism over it, which was the coexistence of E-R and L-H routes. And the same reaction mechanism had also been found for the NH3-SCR reaction over V2O5 catalyst supported on activated semi-coke [38]. Therefore, the deactivation of V/TiO2-Cl catalyst might be originated from the suppressed adsorption of NH3 and NOx species, along with the dropped reactivity of adsorbed NH3 species. 4. Conclusions The deactivation mechanism of Cl on V/TiO2 catalyst for selective catalytic reduction of NOx with NH3 was investigated based on the in situ DRIFT study. Characterization results revealed the inhibited adsorption of NH3 and NOx species over the poisoned catalyst sample; moreover, the addition of Cl on V/TiO2 catalyst would lead to the reactivity drop of adsorbed NH3 species. With all these features, a SCR activity decrease could be detected on V/TiO2 catalyst after the loading of Cl. Acknowledgments This work was financially supported by the National Key R&D Program of China (2018YFB0605002). References [1] G. Qi, R.T. Yang, Performance and kinetics study for low-temperature SCR of NO with NH3 over MnOx-CeO2 catalyst, J. Catal. 217 (2003) 434e441. [2] P. Sun, R. Guo, S. Liu, S. Wang, W. Pan, M. Li, The enhanced performance of MnOx catalyst for NH3-SCR reaction by the modification with Eu, Appl. Catal., A 531 (2017) 129e138. [3] W. Cha, S.H. Ehrmanc, J. Jurng, Surface phenomenon of CeO2-added V2O5/TiO2 catalyst based chemical vapor condensation (CVC) for enhanced selective catalytic reduction at low temperatures, Chem. Eng. J. 304 (2016) 72e78. [4] J. Zhao, Y. Song, W. Lam, W. Liu, Y. Liu, Y. Zhang, et al., Solar radiation transfer and performance analysis of an optimum photovoltaic/thermal system, Energy Convers. Manag. 52 (2011) 1343e1353.
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Please cite this article in press as: Shuai-wei Liu, et al., The deactivation effect of Cl on V/TiO2 catalyst for NH3-SCR process: A DRIFT study, Journal of the Energy Institute (2018), https://doi.org/10.1016/j.joei.2018.06.016