TiO2 for Selective Catalytic

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Kinetics, Catalysis, and Reaction Engineering

Metal Sulfate Poisoning Effects over MnFe/TiO2 for Selective Catalytic Reduction of NO by NH3 at Low Temperature Tsungyu Lee, and Hsunling Bai Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00511 • Publication Date (Web): 26 Mar 2018 Downloaded from http://pubs.acs.org on March 26, 2018

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Industrial & Engineering Chemistry Research

Metal Sulfate Poisoning Effects over MnFe/TiO2 for Selective Catalytic Reduction of NO by NH3 at Low Temperature Tsungyu Lee and Hsunling Bai∗ Institute of Environmental Engineering, National Chiao Tung University, Hsinchu 300, Taiwan

∗

To whom correspondence should be addressed:

Professor Hsunling Bai Institute of Environmental Engineering National Chiao Tung University 1001 University Rd., Hsinchu 300 TAIWAN Tel: +886-3-5731868 Fax: +886-3-5725958 E-mail: [email protected]

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ABSTRACT The MnFe/TiO2 catalysts were poisoned by metal sulfates and/or ammonium sulfate to understand the SO2 poisoning effect under low-temperature selective catalytic reduction (SCR) of NO with NH3. The results showed that the formation of metal sulfates had a more serious deactivation effect than that of ammonium salts on the MnFe/TiO2 catalysts. After thermal regeneration, the metal sulfates on the catalyst could not be removed so that the NOx conversion was only recovered from 17% to 35%. On the other hand, water washing was capable to remove both ammonium salts and metal sulfates, and the NOx conversion could be recovered to 88% (compared to 99% for the fresh catalyst). The analytical results of the synchrotron-based XRD, BET, NH3-TPD, and XPS revealed that lower crystallinity, lower specific surface area, lower ratio of Mn4+/Mn3+, higher surface acidity, and more chemisorbed oxygen were the main causes for the presence of metal sulfates poisoning, which then resulted in the low NOx conversion at low temperature.

Keywords: Selective Catalytic Reduction (SCR), Synchrotron-based XRD, Metal sulfates, Ammonium sulfate, SO2 poisoning, Low-temperature catalyst.


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1. INTRODUCTION Selective catalytic reduction (SCR) of nitrogen oxides (NOx) by NH3 is one of the most efficient and economic technologies for the removal of NOx from stationary sources. The commercial SCR catalysts generally used TiO2 as the high surface area support and V2O5-WO3 as the active metals.1, 2 However, the conventional SCR catalysts are active only within a narrow and high temperature range of 300-400oC,3, 4 and the toxicity of vanadium also restricts its application.5

Therefore, researchers are interested in the

development of low temperature SCR catalysts, which can be active in 100-200oC and placed downstream of the desulfurization devices and the particulate controllers. Among the low temperature SCR catalysts, the Mn-based metal oxide mixtures (e.g., MnOx-CeOx and MnOx-FeOx, etc.)6-13 loaded on various supports (e.g., Al2O3, TiO2, SiO2 and carbon materials, etc.)14-17 have demonstrated superior SCR activity at low temperatures due to their various types of labile oxygen, which are beneficial to the fulfillment of the catalytic cycle.18, 19 Even after the desulfurization process, there is still residual SO2 in the flue gas. Many researchers indicated that SO2 had a serious poisoning effect on the activity of the catalyst at low temperature.20-24 The superior SCR activity of MnFe/TiO2 in the presence of SO2 was observed as compared to those of MnOx and FeOx alone.8 In addition, the resistance of Mn-oxide to water vapors and SO2 could also be improved by the introduction of Fe-oxide.8 According to the literature results, there are two types of sulfate species causing the deactivation of SCR catalysts.25-28 One is the ammonium salts (ammonium sulfates and/or ammonium bisulfate) forming by the reaction of SO2 with NH3 at low temperature,



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which then deposit on the catalyst surface and block the active sites of catalysts. The other is the metal sulfates species forming from the reaction of the active metal with SO2, which result in inactive metal catalysts in the SCR reaction. The SCR mechanisms have been proposed over Mn-based catalysts, which include Langmuir–Hinshelwood (L-H) mechanism and Eley–Rideal (E-R) mechanism.7, 29, 30 The Eley–Rideal mechanism assumes that gaseous NO directly reacts with an activated ammonia surface complex.31, 32 In this reaction process, SO2 has a beneficial effect on the activity due to the formation of new Brønsted acid sites.33,

34

On the other hand, the

Langmuir−Hinshelwood mechanism requires the simultaneous adsorption of gaseous NH3 and NO on the surface. Therefore, the SO42− on the active sites of catalyst would compete with NO on the active sites, which cause the decrease in NOx conversion.1, 35 Until now, most studies were focused on the effect of ammonium salts and many researchers indicated the occupation of ammonium salts on the active sites of the catalysts, which resulted in the decrease of NOx conversion at low temperature.10, 20, 21, 36 On the other hand, although the deactivation of the catalyst due to the presence of ammonium salts and metal sulfates have been mentioned, there has been no study attempting to quantify the individual effect of metal sulfates. Jin et al.37 studied SO2 poisoning effects on the SCR activity of Mn-based catalyst at different temperatures (100-200oC). They indicated that the reaction at 200oC had the highest deactivation, thus they suspected that this might be due to the sulfation of active metals. To the authors' best knowledge, there has been no systematic work on distinguishing the poisoning effect of metal sulfates from that of ammonium sulfates for the low temperature SCR reaction. Therefore this study intends to load different amounts


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of metal sulfates or ammonium sulfates on the MnFe/TiO2 catalyst in order to identify the individual effect of metal sulfates and ammonium sulfate. The catalyst poisoning process is also evaluated under regular SCR conditions where both ammonium salts and metal sulfates would form. The fresh and poisoned catalysts are characterized by XRD, XPS, BET, NH3-TPD, and TGA to understand the crystal phase, oxidation state, surface structure and the surface acidity of the catalysts. The correlations between catalysts’ physiochemical properties, amounts of metal sulfates and ammonium salts and the SCR performance are then studied.

2. EXPERIMENTAL SECTION 2.1. Synthesis of MnFe/TiO2 catalysts In a typical procedure, 8 g of titanium hydroxide powder, TiO(OH)2, 11.57 g of ferric nitrate 9-hydrate and 7.13 g of manganese (II) acetate tetrahydrate was first mixed with 76 ml of D.I. water, followed by the addition of ammonia solution (25 wt%) at 60oC until the pH value of the mixture solution reached 10 and a precipitate was formed. The resulting precipitates were filtered and washed thoroughly with D.I. water several times, and dried at 120oC for 12 hours. Finally, the material was calcined at 350oC. The fresh catalyst was named MnFe/TiO2-F, where F indicates fresh catalyst. 2.2. Catalyst poisoning and SCR activity tests Figure 1 shows an overview of the poisoning procedures to obtain the catalysts poisoned with either metal sulfates or ammonium sulfate and with both metal sulfates and ammonium salts. To load the MnFe/TiO2 catalysts with only metal sulfates, fresh catalysts (1.2 g each time) was passed with 150 ppmv SO2 at 150oC for 1~6 hours, respectively, for



obtaining different amounts of metal sulfates. The poisoned catalysts were named as MnFe/TiO2-MX, where M indicated that the catalysts were pre-poisoned by metal sulfates only, and X indicated the poisoning hours. On the other hand, the MnFe/TiO2 catalysts loaded with only ammonium sulfate were prepared by impregnating the catalysts with aqueous (NH4)2SO4 solutions so that the weight percentages of ammonium sulfates on MnFe/TiO2 were theoretically obtained at 2%, 5% and 10% by weight, respectively. The (NH4)2SO4 poisoned catalysts were then dried in air at 120oC for 12 hours, and they were named as MnFe/TiO2-AY, where A indicated that the catalysts were pre-poisoned by ammonium sulfate only, and Y indicated the weight percentage of ammonium sulfate. Then, the SCR activity tests of MnFe/TiO2MX and MnFe/TiO2-AY were performed under 500 ppmv NO, 500 ppmv NH3 and 10 vol. % O2 at 150oC for 3 hours, in which stable NOx conversion efficiencies were observed during the tests. Furthermore, in order to obtain the MnFe/TiO2 catalysts poisoned by both metal sulfates and ammonium salts under regular SCR processes, the fresh catalysts were directly exposed to a gas stream containing SO2 gas. That is, the SCR activity tests were directly performed with the simultaneous presence of 500 ppmv NO, 500 ppmv NH3, 10 vol. % O2 and 150 ppmv SO2 for 1~6 hours, respectively, at 150oC. After the SO2 poisoning test, the catalysts containing both metal sulfates and ammonium salts were named as MnFe/TiO2-PX, where P indicates poisoned under SCR test with the presence of SO2, and X indicates the poisoning hours. The gas hourly space velocity (GHSV) of the activity test was 47,500 h-1 as calculated at the STP condition of 0oC and 1 atm. The catalysts were preheated in the SCR reactor for 90 minutes to ensure that it reached an


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isothermal reaction temperature of 150oC. During the SCR test, the NO concentrations at the inlet and outlet of the reactor were monitored by a NO/SO2 analyzer (Ultramat 23, SIEMENS). The NO2 concentration was also monitored by a Spectrophotometer FTIR (Bomem MB 104). The NOx conversion is defined by NOx Conversion = 1 −

× 100 %

(1)

2.3. Catalyst regeneration The MnFe/TiO2-P6 catalysts were regenerated by two different methods: thermal regeneration and water regeneration. For the water regeneration method, 1.2 g of MnFe/TiO2-P6 catalyst was washed by flowing with 50 mL of D.I. water for 30 seconds. Then the catalyst was dried at 150oC for an hour. For the thermal regeneration, 1.2 g MnFe/TiO2-P6 catalyst was heated at 350 oC for 3 hours under air environment. The thermo-gravimetric analysis (TGA) was employed to analyze the regenerated catalysts to determine the sulfate amounts. It was conducted with a NETZSCH TG 209 F1 apparatus. The heating program was carried out under an airflow of 10mL/minute with a heating rate of 10oC/minute from room temperature to 900oC. The regenerated catalysts were also tested for their SCR activity. 2.4. Characterization The crystallinity of the catalysts was determined by synchrotron-based XRD analysis (λ = 0.774908 Å), with data gathered in a large Debye−Scherrer camera at the synchrotron beamline BL01C2 of the National Synchrotron Radiation Research Center (NSRRC), Taiwan. The electron storage ring was operated at 1.8 KeV. The diffraction data were collected in the 2θ range between 5o and 80o. The XRD phases of catalyst samples were identified using the Joint Committee on Powder Diffraction Standards (JCPDS) 7 ACS Paragon Plus Environment


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powder data files. The BET specific surface area and pore volume of the catalysts were measured by N2 adsorption at liquid nitrogen temperature using a Micromeritics ASAP2020 instrument. Prior to the N2 adsorption, the samples were degassed at 120oC for 12 hours. The surface areas were determined by a BET equation in 0.05–0.30 partial pressure range. The pore volume distributions were determined by the BJH method from the desorption branch of the isotherms. The

NH3-temperature

programmed

desorption

(NH3-TPD,

Micromeritics

AutoChem II 2920) was used to detect the surface acidity of catalysts. Prior to the TPD experiments, 0.15g samples were pretreated at 200oC in a flow of He (25 mL/minute) for 30 minutes and cooled down to 50oC. Then the samples were exposed to a flow of 15% NH3/He at 50oC for an hour, and then followed by He purged for another hour. Finally, the TPD measurement was carried out from 50 to 900oC at a ramping rate of 10oC/minute. The amount of NH3 desorbed from the catalysts was monitored by the TCD. The X-ray photoelectron spectroscopy (XPS) measurements were performed on a VG Scientific Microlab 350 with Al Kα radiation (1486.6 eV). Binding energies of Mn 2p and O 1s were calibrated using C 1s peak (BE = 284.6 eV) as the standard.

3. RESULTS AND DISCUSSION 3.1. Ammonium salts and metal sulfates on the catalysts Thermo-gravimetric analysis (TGA) was conducted to investigate the amount of sulfate species on the catalysts. Figure 2(a) and Figure 2(b) show the results of differential thermo-gram (DTG) spectra of fresh catalyst (MnFe/TiO2-F) and poisoned


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catalysts of MnFe/TiO2-AY (Y=2, 5 and 10 wt.%), MnFe/TiO2-MX (X=1~6 hours) and MnFe/TiO2-P6. The weight loss profiles of all samples showed four distinct decomposition steps: (1) The weight loss at low temperature (< 200oC) was assigned to water desorption on the catalyst surface; (2) The weight loss located in the temperature range from 200 to 400oC could be attributed to the decomposition of ammonium salts.38, 39

; (3) The weight loss located in the temperature range of 470~680oC could be attributed

to the decomposition of TiOSO4 (470~580oC)40 and FeSO4 (540~680oC)41; (4) The weight loss at high temperature (> 680oC) was originated from manganese sulfate.42, 43 Figure 2(a) shows the DTG spectra of fresh and MnFe/TiO2-MX catalysts. It can be seen that when poisoned time was increased, the weight loss due to the decomposition of manganese sulfate (680~900oC) over MnFe/TiO2-MX (X=1~6 hours) increased more seriously than that due to other metal sulfates (TiOSO4 and FeSO4) at 470~680oC. This revealed that SO2 tended to react more with manganese instead of with iron or titanium. Besides, the decomposition temperature of manganese sulfate slightly shifted to a higher temperature when the poisoned time was increased. This indicated that higher chemical binding energy between manganese oxide and the sulfates species was formed on the MnFe/TiO2-MX catalysts as poisoned time was increased. The amounts of sulfate species on the fresh and poisoned catalysts are listed in Table 1. The amount of ammonium salts was calculated by weight difference between fresh catalyst (MnFe/TiO2-F) and poisoned catalysts. Therefore, It can be seen that all MnFe/TiO2-MX catalysts had similar and negligible amounts of ammonium salts (-0.3~0.3 wt.%). This can be expected since the poisoning of MnFe/TiO2-MX were proceeded without the presence of NH3 gas, thus ammonium salts could not be formed on the surface



of catalysts. Besides, it can be seen from Table 1 that the amounts of manganese sulfate on MnFe/TiO2-MX were increased from 1.5 to 4.7 wt.%, while the total weight loss of MnFe/TiO2-MX were increased from 4.1 to 7.2 wt.% with increasing poisoned time from 1 to 6 hours. On the other hand, the amounts of TiOSO4 and FeSO4 were in the range of 2.0~2.8 wt.%, which didn’t have a clear variation with increasing the poisoned time. This result demonstrated that the SO2 tended to react with manganese oxides instead of with iron or titanium oxides. Figure 2(b) shows the DTG spectra of MnFe/TiO2-AY and MnFe/TiO2-P6 catalysts. It can be seen from Figure 2(b) that the fresh catalyst (MnFe/TiO2-F) had only one apparent weight loss peak at 50~150 oC, which was due to H2O desorption from the catalyst surface. For MnFe/TiO2-AY (Y=2, 5 and 10 wt.%) and MnFe/TiO2-P6 catalysts, apart from the weight loss due to H2O desorption, the weight loss due to the decomposition of ammonium sulfate (200~400oC) and metal sulfates (470~680 oC; 680~900oC) were also observed. Mao et al.43 studied the thermal decomposition of (NH4)2SO4 in the presence of Mn3O4. Their experimental results indicated that at 200 to 450 oC, (NH4)2SO4 decomposed to form NH3, H2O and SO3, and then SO3 reacted with Mn3O4 to form manganese sulfate (MnSO4). At higher temperatures (680 to 900 oC), MnSO4 further decomposed to form SO2 and O2. Therefore, although MnFe/TiO2-AY catalysts were obtained from pre-loaded with ammonium sulfate only, its weight loss in the peak range of metal sulfate was still inevitable because some of the ammonium salts could be transformed into metal sulfates during the TGA heating process. As observed from Table 1, the weight percentage of ammonium sulfate on MnFe/TiO2-AY (Y=2, 5, 10%) samples were 0.1, 1.2, and 2.7 wt.%, respectively, as


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measured by TGA. These amounts were much lower than the theoretical loaded amounts of ammonium sulfate. However, the total weight loss of MnFe/TiO2-AY (Y=2, 5, 10%) samples were 1.9, 4.6, and 8.5 wt.%, respectively, which were about the same as the theoretical amount of ammonium sulfate impregnated on the catalysts. Thus it indicated that in the TGA heating process, the ammonium salts could transform a significant amount into metal sulfates during TGA analysis. 3.2. MnFe/TiO2-F activity in the presence of H2O and SO2 In the following discussion on NOx conversion, it is noted that the FTIR results revealed that if only NO was presented (without NH3), then at the reaction temperature of 150oC the fresh catalyst would oxidize about 9% of the NO to NO2. However for the SCR test (with NH3), the results showed that the outlet concentrations of the NO2 were 0 ppm for all tests. Therefore the NOx conversion is in fact the same as the NO conversion. And it is possible that during the low temperature SCR reaction, the NO2 formed was completely react with NH3. In the low temperature SCR process, the catalysts are usually deactivated by water vapor (H2O) and residual SO2 in the flue gases. Therefore, it is necessary to investigate the resistance of the catalysts to H2O and SO2 at low temperature. Figure 3 compares the NOx conversion over MnFe/TiO2-F at 150°C in the presence of 5 vol% H2O and 150 ppm SO2. It is seen that the NOx conversion of MnFe/TiO2-F was decreased from 99% to 88% after moisture was introduced into the flue gases. The NOx conversion remained stable at around 88% during the test period. Then after the moisture was removed from the flue gases, the NOx conversion restored to its original value. On the other hand, the effect of SO2 on SCR activity of MnFe/TiO2-F was more



severe as shown in Figure 3. When 150 ppm SO2 was introduced, the NOx conversion efficiency of MnFe/TiO2-F quickly decreases to 18% after 6 hours SO2 poisoning. And when both H2O and SO2 were introduced into the flue gas, a sharp decline of the NOx conversion from 99% to 12% was observed within 6 hours. The NOx conversion did not recover even when SO2 was removed from the flue gas. The catalyst deactivation might be due to that H2O vapors would compete with NO and NH3 on the active sites, which cause the decrease in NOx conversion.44, 45 On the other hand, SO2 could be adsorbed by metal oxides on the catalysts and produced metal sulfates. In addition, some ammonium salts (such as NH4HSO4 and (NH4)2SO4) would be formed by the reaction between SO2 and NH3. The formed metal sulfates and ammonium salts would occupy active sites on the surface of catalysts and gradually deactivate the catalysts.25-28 Moreover, the above results indicated that the effect of H2O was reversible while SO2 had irreversible poisoning effects on the activity of the MnFe/TiO2-F catalyst under low temperature SCR conditions. 3.3. SCR performance tests and catalyst regeneration Figure 4 shows the results of NOx conversions as a function of poisoned time over the MnFe/TiO2-MX and MnFe/TiO2-PX. It can see that during the first hour of poisoned test, these two poisoned catalysts demonstrated over 98% NOx conversions. Then only 40% NOx conversion was achieved over the MnFe/TiO2-M catalyst after 2 hours, while almost 90% NOx conversion was still remained using the MnFe/TiO2-P catalyst under the same reaction time. The result showed that NOx conversion of MnFe/TiO2-M catalyst (with metal sulfate) decreased non-linearly fast with poisoning time and reached to a plateau. On the other hand, the NOx conversion of MnFe/TiO2-P seemed to decrease


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linearly and relatively slowly with poisoning time. This indicated that under the same SO2 exposure, the catalyst poisoned with only metal sulfates (MnFe/TiO2-M) had more serious drawback on the NOx conversion than the catalyst poisoned with both ammonium salts and metal sulfates (MnFe/TiO2-P). Figure 5 shows the NOx conversions of MnFe/TiO2-AY, MnFe/TiO2-P6 as well as the regenerated catalysts of MnFe/TiO2-P6 in the NH3-SCR reaction. It revealed that the NOx conversion of MnFe/TiO2-A5% and MnFe/TiO2-P6 were 86% and 17%, respectively. One can see from Table 1 that the weight losses of ammonium salts (200~400oC) over MnFe/TiO2-A5% and MnFe/TiO2-P6 were similar (1.2 wt.%). This indicated that the presence of ammonium sulfate in the MnFe/TiO2-P6 catalyst had negligible effect on the NOx conversion. The deactivation of MnFe/TiO2-P6 should be mainly due to the presence of metal sulfates. Wang et al.10 and Tang et al.46 regenerated their catalysts after SO2 poisoning via the thermal regeneration method. They found that SCR activity could be recovered from 90% to its initial level of 99%. Jin et al.37, 39 demonstrated the regeneration of catalysts by water washing and showed that the ammonium salts could be washed away easily and the SCR efficiency was recoverable. Moreover, Sheng et al.47 used water washing under both with and without ultrasonic vibration to regenerate the deactivated catalysts. Their result showed that catalytic activity could be almost restored to its original value by water washing with ultrasonic vibration. The above literatures were only focused on recovery of SCR efficiency without discussion of the poisoned species on the catalyst surface. And it would be interesting to investigate the effect of regeneration methods on the removal of both manganese sulfate and ammonium salts in this study.



Figure 6 shows the DTG spectra of poisoned MnFe/TiO2-P6 as well as the regenerated MnFe/TiO2-P6 catalysts after thermal regeneration and water regeneration. It can be seen that the thermal regeneration could remove ammonium salts from the catalyst, but it still remained a significant amount of manganese sulfate on the catalyst surface. On the other hand, the water washing regeneration could remove all ammonium salts and most of the manganese sulfate. The amounts of ammonium salts and manganese sulfate presented after thermal regeneration and water washing regeneration can be seen more clearly in Table 1. One can see that the thermal regeneration could remove ammonium salts to a negligible amount (0.3 wt.%), but it could only remove about ~15% of manganese sulfate and still leave 3.5 wt.% of manganese sulfate on the catalyst. After thermal regeneration, the NOx conversion was only recovered from 17% to 35% as can be observed from Figure 5. Compared with the thermal regeneration, the water washing showed good regeneration ability for the deactivated MnFe/TiO2-P6 catalysts, it could remove all ammonium salts and leave only 0.8 wt.% of manganese sulfate. And the NOx conversion after water washing could be recovered back to 88%. 3.4. Crystallinity of catalysts The crystal phases of the fresh and poisoned catalysts were revealed by the synchrotron-based XRD results shown in Figure 7. The peaks located at 25.4, 37.8, 48.0 and 54.5 corresponded to the (101), (004), (200) and (105 and 211) planes of the anatase phase (JCPDS 21-1272), the peaks located at 25.14°, 29.20°, 32.67°, 36.34°, 38.12°, 44.35°, 50.98°, 60.10°, 64.85° corresponded to the (112), (202), (220), (213), (004), (400), (332), (404), (440) planes of the Mn3O4 (JCPDS 75-1560),48 and the peaks located at


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30.1°, 35.5°, 43.1°, 53.4°, 57.0° and 62.6° corresponded to the (220), (311), (400), (422), (511), and (440) planes of the Fe3O4 (JCPDS 19-629).49 The XRD patterns obtained from the synchrotron had much better resolution as compared to the XRD instruments regularly used in the literature for the analysis of SCR catalysts where the crystallinity of the metal catalyst was usually un-observable due to the well disperse of the active metals.50 In this study as depicted in Figure 7, the crystalline phases of the active metals can be seen more clearly from the synchrotron-based XRD pattern. One can observe that MnFe/TiO2-F catalyst had the highest peaks of Mn3O4 (32.67° and 60.10°) and Fe3O4 (35.5°). When increasing the poisoned time, the peaks of Mn3O4 and Fe3O4 gradually decrease for MnFe/TiO2-M catalysts, indicating the formation of metal sulfates would lead to a decrease in Mn3O4 and Fe3O4 crystallinity. Furthermore, even after 6 hours poisoning, the XRD pattern didn’t show peaks assigned to MnSO4 phase (2θ = 18.04°, 18.29°, 25.39°, 25.95°, 26.62°, 28.39°, 34.37°, and 34.78° (JCPDs 070230)).51 This might be due to that the synchrotron-based XRD was still not sensitive enough to identify a small amount of MnSO4 crystal, or the MnSO4 was in amorphous phase. On the other hand, the MnFe/TiO2-P6 catalyst showed clear peaks of ammonium salts, which were located at 21.2°and 26.3°.52,53 And the peaks of Mn3O4 and Fe3O4 on the MnFe/TiO2-P6 catalyst were clearer than those on the MnFe/TiO2-M6 catalysts. Therefore, metal sulfates might lead to decrease in the crystallinity of Mn3O4 and Fe3O4, which could be part of the reasons for the more significant decrease in the NOx conversion in Figure 4 as compared to the MnFe/TiO2-P6 catalyst. 3.5. Specific surface area of catalysts The specific surface areas (SBET) of the samples from the N2 physisorption



measurement are summarized in Table 1. There are several researchers indicated that higher specific surface area could provide more active sites to enhance SCR efficiency and inhibit SO2 poisoning.16,

54-56

Yang et al.57,

58

indicated that ammonia gas is mainly

adsorbed on the support of SCR catalyst in the form of ionic NH4+ and coordinated NH3. The reactive monodentate nitrate on active metal could react with two neighboring NH4+ on the support to form intermediate species, which could further react with gaseous or weakly adsorbed NO to form N2. The BET results shown in Table 1 demonstrated that as compared to the fresh catalyst of 140 m2/g, the specific surface areas of MnFe/TiO2-A2%, A5% and A10% decreased to 134, 124 and 107 m2/g, respectively. On the other hand, the MnFe/TiO2-M6 had a lower specific surface area of 70 m2/g as compared to that of MnFe/TiO2-P6 (90 m2/g), which was roughly equal to that of the MnFe/TiO2-M4 (89 m2/g). This indicated that the deactivation due to ammonium sulfate was that it gave rise to the physical blockage of pore, which caused the decrease in specific surface area.59 On the other hand, the SO2 poisoning on the active metals would transform metal oxides into metal sulfates. Compared with the metal oxides, metal sulfates had lower specific surface area. For example, the specific surface area of MnSO4 and Mn3O4 were 4.3±1 and 75 m2/g, respectively, as demonstrated by Husar60 and Tang et al.61 This may explain why the metal sulfates had a more serious decrease in the specific surface area than depositing ammonium salts on the catalyst surface. 3.6. NH3-TPD The NH3-TPD analysis was conducted to investigate the surface acidity on the catalysts, with results displayed in Figure 8. The NH3 desorption profiles of all samples


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had three distinct desorption temperature ranges: (1) the NH3 desorption peaks centered at low temperature (< 200oC) were assigned to physical desorption, (2) desorption peaks located in the temperature range from 200 to 500oC were attributed to the Brønsted acid sites, and (3) the NH3 desorption peaks centered at high temperature (> 500oC) were usually originated from Lewis acid sites.62, 63 Wang et al.64 and Gong et al.65 employed NH3-TPD to study the ammonia storage mechanisms. They found that the Brønsted acid sites act as an NH3 reservoir that supplies additional NH3 via migration to the Lewis acid sites for the SCR reaction. Moreover, Dumesic et al.22 used NH3-TPD to investigate the kinetic of SCR. Their result indicated that the SCR reaction scheme involves adsorption of ammonia on Brønsted acid sites, follows by activation of ammonia via reaction with redox sites. This activated form of ammonia reacts with gaseous or weakly adsorbed NO, which produces N2 and H2O and leads to partial reduction of the catalyst. The above results indicated that the Brønsted acid sites could store NH3, and it could improve the SCR reaction. The NH3-TPD results shown in Figure 8 demonstrated that MnFe/TiO2-MX and MnFe/TiO2-P6 had more Brønsted acid sites as compared with those of MnFe/TiO2-F. Thus it is realized that the formation of SO42− on the catalyst surface increased the NH3 adsorption and the amounts of Brønsted acid sites. However, the desorption peaks of TPD at 700-900 oC were somewhat overlapped with the decomposition temperatures of bulk MnSO4 (670-900 oC) (see Figure 2(a) for the DTG profiles). The TPD peak temperatures were only slightly higher than the DTG temperatures. Therefore the TPD desorption peaks could also be due to the decomposition of metal sulfates in addition to the presence of Lewis acid sites. In order to verify this, a blank TPD was performed for the MnFe/TiO2-



M6 catalyst which used He as the adsorption/desorption gas instead of using NH3 gas. The result showed that the He-TPD result also showed a desorption peak at 700-900oC with about 85% integration area of that of the NH3-TPD result. Thus this suggested that the TPD desorption peak above 700 oC could possibly be due to both the Lewis acid sites and the decomposition of metal sulfate. However, the NOx conversions of the MnFe/TiO2-MX and MnFe/TiO2-P6 catalysts were decreased rather than increased. Thus it was possible that the SCR reaction of Mnbase catalyst mainly followed the Langmuir−Hinshelwood mechanism52-54 instead of the Eley–Rideal mechanism31,

32

in the low temperature range (

TiO2 for Selective Catalytic

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