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The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR
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Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
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The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR
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Yinming Fana , Wei Linga , Bichun Huanga,b,* , Lifu Donga , Chenglong Yua , Hongxia Xic
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a
School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China c School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China b
A R T I C L E I N F O
Article history: Received 11 January 2017 Received in revised form 30 June 2017 Accepted 3 July 2017 Available online xxx Keywords: Mn-Ce/CeAPSO-34 NH3-SCR Synergistic effect SO2 tolerance DFT
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A B S T R A C T
Mn-Ce/CeAPSO-34 was prepared, in which cerium was incorporated in the SAPO-34 framework through using a one-step hydrothermal method, while manganese and cerium were supported on the surface by the Ethanol dispersion method. The conversion of NOx in selective catalytic reaction with NH3(NH3-SCR) over Mn-Ce/CeAPSO-34 catalyst reached nearly 97.7% at 140 C and remained about 100% in the temperature range of 160–240 C. Besides, the synergistic effects of cerium in the framework and the surface on the resistance to SO2 and H2O in low-temperature NH3-SCR were investigated. The results demonstrated that a strong synergistic effect existed in Mn-Ce/CeAPSO-34 catalyst, which showed outstanding SO2 tolerance and H2O resistance. At the same time, the structural properties and possible metal-support interaction of the catalysts were characterized by XRD, SEM, H2-TPR, XPS and TG-DSC. According to the characterization results, Ce could inhibit the deposition of NH4HSO4 on the surface of the catalysts and preferred to react with SO2, hence protecting the manganese active sites. Meanwhile, the theoretical results of DFT calculations suggested that Ce site supported on the surface neighbored by Ce site in the framework was more capable of reacting with SO2. © 2017 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Introduction With the advantages of high stability and efficiency, selective catalytic reduction (SCR) is the dominant technology to remove the nitrogen oxides from stationary sources [1,2], while catalysts are the crucial factor that can directly determinate the system efficiency for the SCR technology. However, with a relatively narrow and high temperature window of 300–400 C, (V2O5– WO3(MoO3)/TiO2), the most commercially used catalyst, is easy to be deactivated by SO2 and H2O, which could lead to the toxicity of vanadium to the environment [3,4]. Thus, there is a strong motivation to develop new catalysts that are active at relatively low temperatures and resistant to SO2 and H2O. Some transition
* Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China. Fax: +86 20 39380519. E-mail address: [email protected] (B. Huang).
metal oxides (Ce, Fe, Cu and Mn) supported on that different commercial carriers have been found with high activities at low temperature [5–9]. Among these metal oxides, Mn-based catalysts such as nano-MnOx [10], MnOx/TiO2 [11], MnOx/MWCNTs [7,12], MnOx/Graphene [13], MnOx/ZSM-5 [14] and MnOx/USY [15] exhibit a relatively higher activity in low-temperature SCR reactions. However, the vulnerability of these catalysts to the severe deactivation by SO2 and H2O makes them unsuitable for industrial application. In general, SO2 deactivates the catalysts through the deposition of ammonium sulfates that can generate pore plugging and blockage of active sites, or the formation of metal sulfites/sulfates that can lead to irreversible loss of active sites [16–18]. Besides, H2O inhibits the SCR reaction by the competitive adsorption with NH3 on the Lewis sites [19,20]. Concerning the resistance to SO2 and H2O, much more attention has been paid to adding extra elements (Cu [21,22], Sn [23], Ti [24], Fe [25,26], Zr [27], Ce [28–31] and Mo [32]) as modified material to the SCR catalysts. Among them, Ce with better redox ability,
http://dx.doi.org/10.1016/j.jiec.2017.07.003 1226-086X/© 2017 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 1. XRD patterns of CeAPSO-34.
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excellent oxygen storage capacity and environmental friendliness has enjoyed an extensive application. After investigating the effects of adding Ce on Mn/TiO2 at low temperatures, Jin et al. [16,33] found that Mn-Ce/TiO2 had a significantly higher NO conversion than Mn/TiO2 in the presence of SO2. In addition, he discovered that adding Ce loading on Mn/TiO2 could greatly inhibit the deposition of ammonium sulfates and prevent the formation of metal sulfites/sulfates. Based on the findings of Shu et al. [34], who researched Ce-Fe/TiO2, Ce was sulfated preferentially in SO2containing gases, which caused the increase in the amount of surface active oxygen species and the formation of surface hydroxyls to supply more acid sites. In the research conducted by Shu, two factors that played an important role in the good SO2 resistance of Ce-Fe/TiO2 were considered. Although Ce modification seemed to be more hopeful, with its different deactivation and promotion effects from other metal oxides on catalytic activity in the presence of SO2, it could not be ignored that the severe deactivation by SO2 existed permanently in Ce-modified catalysts at low temperature. Therefore, the activity of Ce-modified catalysts in the presence of SO2 and H2O requires being further improved. It is known that SAPO-34 molecular sieve with adjustable surface acidity and superior specific area properties has been widely used as a carrier in NH3-SCR [35,36]. In this work, cerium was incorporated in the SAPO-34 framework by a one-step hydrothermal method and supported on the surface of the SAPO-34 by the Ethanol dispersion method, respectively. Besides, manganese was also scattered on the surface of the catalyst with the Ethanol dispersion method to improve the low-temperature NH3-SCR activity. In addition, to investigate the synergistic effects of cerium existing in the framework and the surface, Mn/SAPO-34, Mn/CeAPSO-34 and Mn-Ce/SAPO-34 were prepared. Besides, XRD, SEM, H2-TPR, XPS and TG-DSC were employed to fully investigate the structure and the possible metal-support interaction between the catalysts in the presence of SO2 and H2O at low temperature in NH3-SCR. In addition, DFT calculations were applied to research the synergistic effect existing between the cerium in the framework and the surface.
Experimental and calculation
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Catalyst preparation
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CeAPSO-34 was in situ synthesized by a one-step hydrothermal method. Diisopropylamine (DIPA, 99 wt.%, Aladdin) was used as a template, cerium acetate hydrate (99.9 wt.%, Aladdin) as the metal source, phosphoric acid (85 wt.%, Aldrich), pseudoboehmite (68 wt.%, PetroChina, China) and colloidal silica (30 wt.%, Aldrich) as the phosphorus source, aluminum source and silicon source, respectively. The molar ratio used was 1.0P2O5:0.8Al2O3:0.2SiO2:3.0DIPA:0.3Ce:50H2O in the crystallization solution. The synthesis steps were as follows: cerium acetate hydrate was added to a select quantity of distilled water and fully stirred until the cerium acetate hydrate was completely dissolved. Pseudoboehmite was then added to form a homogeneous gel by stirring. Subsequently, colloidal silica was added to the gel with stirring.
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Fig. 2. SEM image of CeAPSO-34.
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 3. EDS spectra of CeAPSO-34.
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Finally, the diisopropylamine was slowly added into the gel mixture and stirred. The resulting gel was transferred to a 100 mL autoclave with a Teflonliner and statically crystallized for 48 h at 200 C. The crystalline products was filtered and washed with distilled water and dried at 110 C overnight. Then, the samples were calcined at 550 C for 6 h to remove the template. For comparison, SAPO-34 was also prepared according to the abovementioned methods. Mn/CeAPSO-34 and Mn-Ce/CeAPSO-34 were prepared successfully by the Ethanol dispersion method in which manganese nitrate (50 wt.% in H2O, Aladdin) and cerium nitrate (99.5 wt.%, Aladdin) used as precursor, respectively. The detailed process was described in previous study [37]. For comparison, Mn/SAPO-34 and Mn-Ce/ SAPO-34 were also prepared according to the above-mentioned methods.
catalysts were used in the test. The feed gas was composed of 0.08% NO, 0.08% NH3, 0.01% SO2 (when added), 10% H2O (when added) and 5.0% O2 balanced by Ar. The total flow rate was 600 mL/min, corresponding to a gas hourly space velocity (GHSV) of 40,000 h1. The inlet and outlet concentrations of NO/NO2 were analyzed by an on-line chemiluminescence NO–NO2–NOx analyzer (Thermal Scientific, model 42i-HL). The activity data were recorded when the reactions practically reached the steady state condition at each temperature. The reaction temperature was controlled from 80 to 240 C with an isotherm step of 20 C. The heating rate was about 5 C/min. The concentrations of NH3 and N2O were monitored by a gas analyzer (Nicolet 6700 FT-IR) to use the least square method to remove the influence of other components in the gas flow under the help of TQ analyst software. The NOx conversion and the N2 selectivity are calculated as follows:
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Catalytic evaluations
NOx conversion ¼
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The evaluation reactions were carried out in a fixed-bed quartz continuous flow reactor under an atmospheric pressure. 500 mg
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½NOx inlet ½NOx outlet 100% ½NOx inlet
Fig. 4. Effect of the calcination temperature and the loading of Mn on SCR activity of Mn/CeAPSO-34.
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 5. XRD patterns of Mn/CeAPSO-34 with different calcination temperature and the loading of Mn.
Fig. 6. H2-TPR profiles of Mn/CeAPSO-34 with different calcination temperature.
N2 selectivity ¼ 1 100% 124 125 126
2½N2 Ooutlet ½NOx inlet þ ½NH3 inlet ½NOx outlet ½NH3 outlet
Catalyst characterization
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X-ray powder diffraction (XRD) patterns were recorded on a D8 Advace diffractometer (Bruker, Germany) with Cu Ka radiation (40 kV, 40 mA). Data was collected between 2u = 10–90 with 0.02 steps and the XRD phases were identified by comparison with the reference data files from Joint Committee on Power Diffraction Standards (JCPDS). Scanning electron microscope (SEM) combined with an energy dispersive X-ray attachment (EDS) were used to
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where [NOx]inlet and [NH3]inlet correspond to the inlet concentration (ppm) of NOx and NH3, respectively. [NOx]outlet, [NH3]outlet and [N2O]outlet correspond to the outlet concentration (ppm) of NOx, NH3 and N2O, respectively.
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Fig. 7. Effect of the different loading of Ce on SCR activity of Mn-Ce/CeAPSO-34.
Fig. 8. XRD patterns of Mn-Ce/CeAPSO-34 with the different loading of Ce. 135 136 137 138 139 140 141 142 143 144 145 146 147
analyze the surface morphology and elemental composition of the catalysts. Hydrogen temperature-programmed reduction (H2-TPR) of the catalyst was carried out on a Auto Chem II (Micromeritics, USA). The samples were first flushed with Ar (30 mL/min) at 350 C for 1 h. After that, the experiments were carried out at a heating rate of 10 C/min from 60 C to 700 C in 10% H2/Ar (30 mL/min). The H2 consumption was recorded by a TCD detector. The chemical state and surface composition of the catalysts was analyzed by Xray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos, UK). All spectra were acquired at a basic pressure 3.33 106 Torr with Al Ka radiation (hv = 1253.6 eV) at 15 kV. The binding energy calibration was performed using the C 1 s peak in the background as the reference energy (284.6 eV). Thermo gravimetric analysis
(TG) and differential scanning calorimetry (DSC) analysis for the catalysts were carried out simultaneously in a static N2 atmosphere using a Netzsch STA 409 instrument. 15 mg of each sample was analyzed between 30 and 800 C at a rate of 10 C/min.
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Computational models and details
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All the calculations were based on DFT and performed by Modeling Dmol3 package [38,39]. SAPO-34 (1 1 1) model was selected in this study and SAPO-34 (111) face was choose to calculation the adsorption energy. Mn3O4 and CeO2@Ce2O3 crystal molecular were build for active component. All the geometry optimization were performed by local density approximation
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Fig. 9. SEM images of Mn/CeAPSO-34 (a, b) and Mn-Ce/CeAPSO-34 (c, d).
Fig. 10. NOx conversions of the catalysts.
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functional LDA-PWC, and the self-consistent field energy was set to 2.0 105 Ha. The adsorption energy are calculated as follows:
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Ead = Egas + Esurface Egas@surface
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where Esurface, Egas and Egas@surface correspond to the energy of surface, an isolated gas molecule and the same molecule adsorbed on the surface, respectively.
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Results and discussion
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In situ syntheses of CeAPSO-34
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The amount of the template and the content of cerium are important factors for the one-step hydrothermal procedure of CeAPSO-34. Based on the results of XRD in Fig. 1, the sample
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Fig. 11. Effects of SO2 and H2O on SCR activity of the catalysts reaction conditions: 0.08% NO, 0.08% NH3, 5% O2, 10% H2O (when added), 0.01% SO2 (when added), 180 C, Ar balance.
Fig. 12. XRD patterns of catalysts after reaction in the presence of SO2 and H2O. 169 170 171
showed high crystallinity and low impurities in the CeAPSO34 molecular sieves when DIPA/P2O5 ratio was 3.0 and Ce/P2O5 ratio was 0.3. The characteristic diffraction peaks of the CeAPSO-
34 at 2u = 9.7, 13.1, 16.2 , 19.3 , 20.9 , 26.2 and 31.4 were similar to those of the synthesized and commercial SAPO-34 samples. At the same time, the typical cubic-like structure of the CeAPSO-
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Fig. 13. H2-TPR profiles of catalysts after reaction in the presence of SO2 and H2O. 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198
34 was clearly identified by the SEM (Fig. 2), which was affirmed by corresponding XRD analysis. In addition, compared with the synthesized SAPO-34, the peaks of the CeAPSO-34 presented a slight shift. With the consideration of the EDS analysis in Fig. 3, the insertion of Ce into the SAPO-34 framework probably resulted in the unit cell size expansion.
while the high temperature signal corresponded to the reduction of Mn2O3 to MnO. According to the H2-TPR patterns, the reduction peaks firstly shifted to the low temperature region and then to the relatively higher region with the increase of the calcination temperature. The results conformed well to the NH3-SCR activity of Mn/CeAPSO-34 at different calcination temperatures. Fig. 7 presented the performance of Mn-Ce/CeAPSO-34 in NO reduction. The catalyst presented the best NH3-SCR activity and high N2 selectively at low temperature when Ce/Mn ratio was 0.4, and the NOx conversion over the catalyst reached nearly 97.7% at 140 C and remained about 100% in the temperature range of 160– 240 C. The XRD patterns (Fig. 8) of Mn-Ce/CeAPSO-34 demonstrated that no characteristic peaks of any Ce compounds were detected, which was similar to the results of Mn/CeAPSO-34. With the consideration of the SEM images (Fig. 9), it can be concluded that the manganese and cerium supported on the surface of CeAPSO-34 with scattered distribution had little effect on the key active sites of CeAPSO-34 in terms of the SO2 and H2O resistance. Mn/SAPO-34 and Mn-Ce/SAPO-34 were prepared for comparison, and the NH3-SCR activity of the catalysts was also investigated, with the results shown in Fig. 10. According to the results, a moderate amount of cerium supported on the catalysts could improve the monolithic catalyst activity to a certain degree, yet not significantly. Nevertheless, it had exerted little influence on the activity of the catalysts when cerium existed in the framework, proving that manganese species served as the dominant active
NH3-SCR activity and characteristics Mn/CeAPSO-34 was prepared by the Ethanol dispersion method. As shown in Fig. 4, when the calcination temperature was 400 C and the loading of Mn was 15 wt.%, the best NH3-SCR activity of the catalyst was observed in the reaction temperature range of 80–240 C. Moreover, the NOx conversion over the catalyst reached nearly 95% at 140 C and remained about 100% in the temperature range of 160–240 C. The XRD profiles (Fig. 5) of Mn/ CeAPSO-34 remained almost unchanged after introducing Mn, which indicated that the structure of CeAPSO-34 was not changed by the loading of Mn. Meanwhile, no characteristic peaks of any Mn compounds were observed, indicating that Mn was well dispersed in the catalysts or the formed compounds were not big enough to be detected. Fig. 6 provided the H2-TPR curves for estimating the reducibility of Mn/CeAPSO-34 catalysts at different calcination temperatures. The catalysts presented two well defined reduction peaks, in which the reduction process of MnO2 to Mn2O3 was suggested as the reduction process for the low temperature peak, Table 1 Surface atomic concentration of catalysts before and after reaction with SO2. Sample
Atomic fraction/% Al
Si
P
C
O
Mn
Ce
S
N
Mn/SAPO-34 Mn/SAPO-34-SO2 Mn/CeAPSO-34 Mn/CeAPSO-34-SO2 Mn-Ce/SAPO-34 Mn-Ce/SAPO-34-SO2 Mn-Ce/CeAPSO-34 Mn-Ce/CeAPSO34-SO2
7.15 7.21 6.02 6.18 6.59 7.01 6.01 6.22
0.79 0.82 0.74 0.70 0.76 0.79 0.70 0.73
8.38 8.27 7.65 7.51 7.84 8.05 7.57 7.62
34.17 42.58 24.64 35.75 15.29 29.04 12.56 27.26
39.32 32.49 48.46 40.25 52.49 44.68 54.67 46.88
10.19 1.04 10.25 2.09 10.32 3.07 10.35 4.28
– – 2.24 1.18 6.71 1.56 8.14 1.84
– 2.84 – 2.08 – 1.74 – 1.39
– 4.75 – 4.26 – 4.06 – 3.78
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 14. Mn 2p and O 1s XPS spectra of catalysts before and after reaction in the presence of SO2.
Fig. 15. Ce 3d XPS spectra of catalysts before and after reaction in the presence of SO2.
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Fig. 16. TG and DSC curves of catalysts after reaction in the presence of SO2 (a) Mn/SAPO-34 (b) Mn/CeAPSO-34 (c) Mn-Ce/SAPO-34 (d) Mn-Ce/CeAPSO-34. 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254
sites. To investigate the synergistic effects of cerium incorporated in the framework and the surface of SAPO-34 in the presence of SO2 and H2O at low temperature, the small difference of the catalyst activity could be ignored. Resistance to SO2 and H2O and characteristics The effects of SO2 and H2O on the catalysts’ SCR activity at 180 C were exhibited in Fig. 11. In the presence of 0.01% SO2, 10% H2O or 0.01% SO2 + 10% H2O, the NO conversion for Mn-Ce/CeAPSO34 (decreased to 88%, 89.08% and 65.45%) was much higher than that for Mn-Ce/SAPO-34 (decreased to 67%, 76.94%, and 42.16%), Mn/CeAPSO-34 (decreased to 56%, 68.1% and 40.29%) and Mn/ SAPO-34 (decreased to 16.9%, 36% and 13.28%). In terms of the catalysts, the activity followed the sequence of Mn-Ce/CeAPSO-34 > Mn-Ce/SAPO-34 > Mn/CeAPSO-34 > Mn/SAPO-34, indicating that cerium could dramatically improve the catalysts’ resistance to SO2 and H2O. In addition, Mn-Ce/CeAPSO-34 exhibited the best activity of SO2 and H2O tolerance, which clearly indicated that a strong synergistic effect existed between the cerium in the framework and the surface. Therefore, it can be known that the synergistic effect was not a simple overlap of single factors but a complex phenomenon. The SO2 and H2O tolerance of Mn-Ce/SAPO34 was better than that of Mn/CeAPSO-34, possibly because the cerium supported on the surface of catalysts led to more exposure of Ce compounds to SO2 and H2O-containing gases. As a consequence, the reaction with SO2 and H2O became much more easily and then the manganese active sites were protected. After the SO2 was removed, the NO conversions of the four catalysts could not recover to the previous level, implying that the deactivation by SO2 was irreversible, which might be due to the formation of metal sulfates or the deposition of ammonium
sulfates. In comparison, after the removal of H2O, the activity could recover to its original level, indicating that H2O inhibition to the SCR reaction was reversible via the competitive adsorption with NH3 on acid sites [40]. According to Fig. 12, which displayed the XRD patterns of the poisoned catalysts, the peaks over Mn/SAPO-34 occurring at 23.2 and 33 could be assigned to the formed phase of NH4HSO4 (PDF#35-1500). Then, it can be deduced that the newly formed crystal phases of NH4HSO4 resulted in the decrease of SCR activity. However, there were no detectable peaks of NH4HSO4 for the used Mn/CeAPSO-34, Mn-Ce/SAPO-34 and Mn-Ce/CeAPSO-34, which might be because that NH4HSO4 formed might exist in amorphous species or had not reached the detection limitation. The results demonstrated that cerium presenting in the framework and the surface could greatly inhibit the deposition of NH4HSO4. The H2TPR curves of the poisoned catalysts were illustrated in Fig. 13, and the results suggested that two reduction peaks were observed in the used Mn-Ce/CeAPSO-34 and Mn-Ce/SAPO-34, while only one was presented in the used Mn/CeAPSO-34 and Mn/SAPO-34. For the poisoned catalysts, the low-temperature reduction peak position was in the sequence of Mn/SAPO-34 > Mn/CeAPSO34 > Mn-Ce/SAPO-34 > Mn-Ce/CeAPSO-34. The position of the reduction peak was related to the oxidation capacity, which was a reason for the anti-SO2 and H2O-poisoning effect of cerium doping in the framework and the surface of catalysts. Meanwhile, the H2-TPR results were conformed to the NH3-SCR activity of the samples. The atomic surface compositions by XPS analysis were given in Table 1. Besides, it was noted that the surface content of manganese on the fresh catalysts showed a slight increase with the incorporation of cerium oxides in the framework and the surface. This was in accordance with the result in Fig. 10 that
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 17. Optimized structures of (A) Mn/SAPO-34 (B) Mn/CeAPSO-34 (C) Mn-Ce/SAPO-34 (D) Mn-Ce/CeAPSO-34. The red balls are oxygen, purple balls are manganese, white Q7 balls are cerium, yellow balls are silicon, pink balls are aluminum, light pink balls are phosphorus. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304
305 306 307 308 309
cerium slightly strengthened the catalysts activity, and this is also conducive to investigating the synergistic effects of cerium with respect to the resistance to SO2 and H2O. Meanwhile, Mn-Ce/ CeAPSO-34 had the highest surface concentration of cerium, indicating that more cerium was available to participate in NH3SCR with the addition of SO2 and H2O. Compared with the fresh catalysts, the manganese concentration in the four used catalysts all declined to a certain extent and the sulfur and nitrogen atoms could be detected after the reaction under SO2 flow. The used Mn/ SAPO-34 showed the highest level in the decreasing manganese concentration and the increasing sulfur and nitrogen concentration, followed by Mn/CeAPSO-34, Mn-Ce/SAPO-34 and Mn-Ce/ CeAPSO-34 in sequence. According to the results, the sulfating of the active sites was inhibited in some ways after the cerium was incorporated in the framework and the surface of catalysts. It can be seen from Fig. 14(a) and (b), which presents the XPS spectra for Mn 2p, Mn 2p spectra had two peaks, which are Mn 2p1/ 2 (653 eV) and Mn 2p3/2 (642 eV), respectively. Through peakfitting deconvolutions, three peaks appeared in the Mn 2p3/2 spectra, respectively, Mn2+ (639.7–640.5 eV), Mn3+ (641.0– 641.5 eV) and Mn4+ (643.2–644.0 eV). It is worthwhile of norticing that the relative surface content of Mn3+ and Mn4+ over the four fresh catalysts was much higher than that of Mn2+. Based on the Table 2 Adsorption energies of SO2 on catalysts. Sample
Mn/SAPO-34 Mn/CeAPSO-34 Mn-Ce/SAPO-34 Mn-Ce/CeAPSO-34
Ead (SO2) (eV) Mn1
Mn2
Mn3
Ce1
Ce2
Ce3
Ce4
Ce5
4.02 3.92 4.01 3.90
4.15 4.13 3.87 3.80
4.15 4.11 4.09 4.05
– 5.92 – 6.01
– 5.76 – 6.64
– – 6.14 6.82
– – 6.05 6.07
– – 6.22 6.20
literatures [41,42], the Mn3+ and Mn4+ species could contribute to enhancing NH3-SCR activity at low temperature. The surface content of manganese species was in a way decreased after the addition of SO2. Besides, the calculated amount of Mn3+ and Mn4+ species on the four used catalysts was in the sequence of Mn-Ce/ CeAPSO-34 > Mn-Ce/SAPO-34 > Mn/CeAPSO-34 > Mn/SAPO-34, correlating well with the result that the sulfating of manganese was somewhat inhibited after incorporating cerium oxides in the framework and the surface. The O 1s XPS pattern (Fig. 14(c) and (d)) showed that two types of surface oxygen existed in the catalysts. With reference to the literatures [43,44], the binding energy of 529.1–529.5 eV corresponded to the lattice oxygen (Oa), while that of 531.0–531.5 eV was ascribed to the chemisorbed oxygen (Ob).Obviously, the Ob species exhibited the larger peak areas for all samples, and the amount of Ob species on the four fresh and used catalysts followed the sequence of Mn-Ce/CeAPSO-34 > Mn-Ce/SAPO-34 > Mn/ CeAPSO-34 > Mn/SAPO-34. Furthermore, it has been testified that Ob species, with their higher mobility, were more active than Oa species, one reason for which referred to that the presence of cerium oxides in the framework and the surface led to the content increase of Ob species. As a result, the NH3-SCR activity in the presence of SO2 could be further improved. It can be known from the Ce 3d XPS pattern (Fig. 15) that Ce coexisted as Ce4+and Ce3+. According to the peak-fitting deconvolutions, seven peaks appeared in the Ce 3d spectra [45], which also showed that much more Ce4+ species existed on the surface of the catalysts. In comparison with the fresh catalysts, the content of cerium species decreased in spent catalysts, and the Ce4+ peaks (886.2 eV) in the used catalysts moved to 887.5 eV. Considering the literatures [46,47], the Ce4+ peaks of the catalysts shifted towards a higher binding energy position after the SCR with SO2 due to the sulfating of the cerium. These results suggested that the cerium
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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oxides incorporated in the framework and the surface preferred to react with SO2 to form sulfate species in sulfur atmosphere, thereby, to some degree, inhibiting the formation of manganese sulfate species and protecting the activity of manganese active sites. The TG-DSC curves of the four used catalysts were exhibited in Fig. 16. One major weight loss emerging at about 80–110 C could be ascribed to the evaporation of water in the used catalysts. In terms of the used Mn/SAPO-34 catalyst, there was one other relatively obvious weight loss at 217 C, which was caused by the decomposition of NH4HSO4 [16,48]. The results were consistent with the XRD results of the used catalysts. Additionally, the TG-DSC results further confirmed that cerium oxides presenting in the framework and the surface could inhibit the formation of NH4HSO4 on the catalyst surface during the SCR process with SO2. This was one of the reasons accounting for the catalysts’ strong resistance to SO2 poisoning.
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DFT study
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The models of Mn/SAPO-34, Mn/CeAPSO-34, Mn-Ce/SAPO34 and Mn-Ce/CeAPSO-34 were established to research the synergistic effect existing between the cerium oxides in the framework and the surface. The optimized geometric structure for all the catalyst models was shown in Fig. 17. According to the surface exposed by Mn and Ce cations in different environments, the surface cations were labeled as Mn1, Mn2, Mn3 as well as Ce1, Ce2, Ce3, Ce4, Ce5. Then, the adsorption energy of SO2 molecule adsorbed on the surface of all catalysts was analyzed and the results were presented in Table 2. It was noted that the Ead (SO2) of Ce sites was greater than that of Mn sites, and the Ead (SO2) of Ce site supported on the surface neighbored by Ce site in the framework was greater than that of other Ce sites. At the same time, the Ead (SO2) of the Mn sites neighbored by Ce sites was lower than that of other Mn sites. It is already known that the positive value of Ead demonstrates a strong interaction between the adsorption surface and gas. Therefore, these results indicated that compared with the Mn sites, SO2 was more capable of occupying the Ce sites, especially the Ce site supported on the surface neighbored by Ce site existing in the framework, which prevented the manganese active sites from being sulfated to some extent. Besides, the theoretical results were in well line with the NH3-SCR activity in the presence of SO2.
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Conclusions
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The low-temperature NH3-SCR activity of Mn-Ce/CeAPSO34 catalyst was significantly increased in the presence of SO2 and H2O. According to the characterization results, the sulfating of the manganese active sites and the deposition of NH4HSO4 on the surface of the catalysts were to some extent inhibited after the cerium oxides were incorporated in the framework and the surface. Regarding these catalysts, the activity in the presence of SO2 and H2O was in the sequence of Mn-Ce/ CeAPSO-34 > Mn-Ce/SAPO-34 > Mn/CeAPSO-34 > Mn/SAPO-34. The results indicated that a strong synergistic effect existed between the cerium in the framework and the surface, and the synergistic effect was not a simple overlap of single factors yet a complex phenomenon. Furthermore, the theoretical results implied that the synergistic effect could be attributed to the [-Ce-O-Ce-]units, indicating that the Ce site supported on the surface neighbored by Ce site in the framework would be beneficial to the reaction with SO2.
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Acknowledgments The research was financially supported by the National Natural Science Foundation of China (NSFC-51478191) and Science and Technology Project of Guangdong Province (2014A020216003).
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Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
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The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR
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Yinming Fana , Wei Linga , Bichun Huanga,b,* , Lifu Donga , Chenglong Yua , Hongxia Xic
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a
School of Environment and Energy, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China Guangdong Provincial Key Laboratory of Atmospheric Environment and Pollution Control, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China c School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou Higher Education Mega Centre, Guangzhou 510006, PR China b
A R T I C L E I N F O
Article history: Received 11 January 2017 Received in revised form 30 June 2017 Accepted 3 July 2017 Available online xxx Keywords: Mn-Ce/CeAPSO-34 NH3-SCR Synergistic effect SO2 tolerance DFT
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A B S T R A C T
Mn-Ce/CeAPSO-34 was prepared, in which cerium was incorporated in the SAPO-34 framework through using a one-step hydrothermal method, while manganese and cerium were supported on the surface by the Ethanol dispersion method. The conversion of NOx in selective catalytic reaction with NH3(NH3-SCR) over Mn-Ce/CeAPSO-34 catalyst reached nearly 97.7% at 140 C and remained about 100% in the temperature range of 160–240 C. Besides, the synergistic effects of cerium in the framework and the surface on the resistance to SO2 and H2O in low-temperature NH3-SCR were investigated. The results demonstrated that a strong synergistic effect existed in Mn-Ce/CeAPSO-34 catalyst, which showed outstanding SO2 tolerance and H2O resistance. At the same time, the structural properties and possible metal-support interaction of the catalysts were characterized by XRD, SEM, H2-TPR, XPS and TG-DSC. According to the characterization results, Ce could inhibit the deposition of NH4HSO4 on the surface of the catalysts and preferred to react with SO2, hence protecting the manganese active sites. Meanwhile, the theoretical results of DFT calculations suggested that Ce site supported on the surface neighbored by Ce site in the framework was more capable of reacting with SO2. © 2017 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Introduction With the advantages of high stability and efficiency, selective catalytic reduction (SCR) is the dominant technology to remove the nitrogen oxides from stationary sources [1,2], while catalysts are the crucial factor that can directly determinate the system efficiency for the SCR technology. However, with a relatively narrow and high temperature window of 300–400 C, (V2O5– WO3(MoO3)/TiO2), the most commercially used catalyst, is easy to be deactivated by SO2 and H2O, which could lead to the toxicity of vanadium to the environment [3,4]. Thus, there is a strong motivation to develop new catalysts that are active at relatively low temperatures and resistant to SO2 and H2O. Some transition
* Corresponding author at: School of Environment and Energy, South China University of Technology, Guangzhou 510006, PR China. Fax: +86 20 39380519. E-mail address: [email protected] (B. Huang).
metal oxides (Ce, Fe, Cu and Mn) supported on that different commercial carriers have been found with high activities at low temperature [5–9]. Among these metal oxides, Mn-based catalysts such as nano-MnOx [10], MnOx/TiO2 [11], MnOx/MWCNTs [7,12], MnOx/Graphene [13], MnOx/ZSM-5 [14] and MnOx/USY [15] exhibit a relatively higher activity in low-temperature SCR reactions. However, the vulnerability of these catalysts to the severe deactivation by SO2 and H2O makes them unsuitable for industrial application. In general, SO2 deactivates the catalysts through the deposition of ammonium sulfates that can generate pore plugging and blockage of active sites, or the formation of metal sulfites/sulfates that can lead to irreversible loss of active sites [16–18]. Besides, H2O inhibits the SCR reaction by the competitive adsorption with NH3 on the Lewis sites [19,20]. Concerning the resistance to SO2 and H2O, much more attention has been paid to adding extra elements (Cu [21,22], Sn [23], Ti [24], Fe [25,26], Zr [27], Ce [28–31] and Mo [32]) as modified material to the SCR catalysts. Among them, Ce with better redox ability,
http://dx.doi.org/10.1016/j.jiec.2017.07.003 1226-086X/© 2017 Published by Elsevier B.V. on behalf of The Korean Society of Industrial and Engineering Chemistry.
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 1. XRD patterns of CeAPSO-34.
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excellent oxygen storage capacity and environmental friendliness has enjoyed an extensive application. After investigating the effects of adding Ce on Mn/TiO2 at low temperatures, Jin et al. [16,33] found that Mn-Ce/TiO2 had a significantly higher NO conversion than Mn/TiO2 in the presence of SO2. In addition, he discovered that adding Ce loading on Mn/TiO2 could greatly inhibit the deposition of ammonium sulfates and prevent the formation of metal sulfites/sulfates. Based on the findings of Shu et al. [34], who researched Ce-Fe/TiO2, Ce was sulfated preferentially in SO2containing gases, which caused the increase in the amount of surface active oxygen species and the formation of surface hydroxyls to supply more acid sites. In the research conducted by Shu, two factors that played an important role in the good SO2 resistance of Ce-Fe/TiO2 were considered. Although Ce modification seemed to be more hopeful, with its different deactivation and promotion effects from other metal oxides on catalytic activity in the presence of SO2, it could not be ignored that the severe deactivation by SO2 existed permanently in Ce-modified catalysts at low temperature. Therefore, the activity of Ce-modified catalysts in the presence of SO2 and H2O requires being further improved. It is known that SAPO-34 molecular sieve with adjustable surface acidity and superior specific area properties has been widely used as a carrier in NH3-SCR [35,36]. In this work, cerium was incorporated in the SAPO-34 framework by a one-step hydrothermal method and supported on the surface of the SAPO-34 by the Ethanol dispersion method, respectively. Besides, manganese was also scattered on the surface of the catalyst with the Ethanol dispersion method to improve the low-temperature NH3-SCR activity. In addition, to investigate the synergistic effects of cerium existing in the framework and the surface, Mn/SAPO-34, Mn/CeAPSO-34 and Mn-Ce/SAPO-34 were prepared. Besides, XRD, SEM, H2-TPR, XPS and TG-DSC were employed to fully investigate the structure and the possible metal-support interaction between the catalysts in the presence of SO2 and H2O at low temperature in NH3-SCR. In addition, DFT calculations were applied to research the synergistic effect existing between the cerium in the framework and the surface.
Experimental and calculation
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Catalyst preparation
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CeAPSO-34 was in situ synthesized by a one-step hydrothermal method. Diisopropylamine (DIPA, 99 wt.%, Aladdin) was used as a template, cerium acetate hydrate (99.9 wt.%, Aladdin) as the metal source, phosphoric acid (85 wt.%, Aldrich), pseudoboehmite (68 wt.%, PetroChina, China) and colloidal silica (30 wt.%, Aldrich) as the phosphorus source, aluminum source and silicon source, respectively. The molar ratio used was 1.0P2O5:0.8Al2O3:0.2SiO2:3.0DIPA:0.3Ce:50H2O in the crystallization solution. The synthesis steps were as follows: cerium acetate hydrate was added to a select quantity of distilled water and fully stirred until the cerium acetate hydrate was completely dissolved. Pseudoboehmite was then added to form a homogeneous gel by stirring. Subsequently, colloidal silica was added to the gel with stirring.
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Fig. 2. SEM image of CeAPSO-34.
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 3. EDS spectra of CeAPSO-34.
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Finally, the diisopropylamine was slowly added into the gel mixture and stirred. The resulting gel was transferred to a 100 mL autoclave with a Teflonliner and statically crystallized for 48 h at 200 C. The crystalline products was filtered and washed with distilled water and dried at 110 C overnight. Then, the samples were calcined at 550 C for 6 h to remove the template. For comparison, SAPO-34 was also prepared according to the abovementioned methods. Mn/CeAPSO-34 and Mn-Ce/CeAPSO-34 were prepared successfully by the Ethanol dispersion method in which manganese nitrate (50 wt.% in H2O, Aladdin) and cerium nitrate (99.5 wt.%, Aladdin) used as precursor, respectively. The detailed process was described in previous study [37]. For comparison, Mn/SAPO-34 and Mn-Ce/ SAPO-34 were also prepared according to the above-mentioned methods.
catalysts were used in the test. The feed gas was composed of 0.08% NO, 0.08% NH3, 0.01% SO2 (when added), 10% H2O (when added) and 5.0% O2 balanced by Ar. The total flow rate was 600 mL/min, corresponding to a gas hourly space velocity (GHSV) of 40,000 h1. The inlet and outlet concentrations of NO/NO2 were analyzed by an on-line chemiluminescence NO–NO2–NOx analyzer (Thermal Scientific, model 42i-HL). The activity data were recorded when the reactions practically reached the steady state condition at each temperature. The reaction temperature was controlled from 80 to 240 C with an isotherm step of 20 C. The heating rate was about 5 C/min. The concentrations of NH3 and N2O were monitored by a gas analyzer (Nicolet 6700 FT-IR) to use the least square method to remove the influence of other components in the gas flow under the help of TQ analyst software. The NOx conversion and the N2 selectivity are calculated as follows:
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Catalytic evaluations
NOx conversion ¼
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The evaluation reactions were carried out in a fixed-bed quartz continuous flow reactor under an atmospheric pressure. 500 mg
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½NOx inlet ½NOx outlet 100% ½NOx inlet
Fig. 4. Effect of the calcination temperature and the loading of Mn on SCR activity of Mn/CeAPSO-34.
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 5. XRD patterns of Mn/CeAPSO-34 with different calcination temperature and the loading of Mn.
Fig. 6. H2-TPR profiles of Mn/CeAPSO-34 with different calcination temperature.
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2½N2 Ooutlet ½NOx inlet þ ½NH3 inlet ½NOx outlet ½NH3 outlet
Catalyst characterization
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X-ray powder diffraction (XRD) patterns were recorded on a D8 Advace diffractometer (Bruker, Germany) with Cu Ka radiation (40 kV, 40 mA). Data was collected between 2u = 10–90 with 0.02 steps and the XRD phases were identified by comparison with the reference data files from Joint Committee on Power Diffraction Standards (JCPDS). Scanning electron microscope (SEM) combined with an energy dispersive X-ray attachment (EDS) were used to
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where [NOx]inlet and [NH3]inlet correspond to the inlet concentration (ppm) of NOx and NH3, respectively. [NOx]outlet, [NH3]outlet and [N2O]outlet correspond to the outlet concentration (ppm) of NOx, NH3 and N2O, respectively.
Please cite this article in press as: Y. Fan, et al., The synergistic effects of cerium presence in the framework and the surface resistance to SO2 and H2O in NH3-SCR, J. Ind. Eng. Chem. (2017), http://dx.doi.org/10.1016/j.jiec.2017.07.003
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Fig. 7. Effect of the different loading of Ce on SCR activity of Mn-Ce/CeAPSO-34.
Fig. 8. XRD patterns of Mn-Ce/CeAPSO-34 with the different loading of Ce. 135 136 137 138 139 140 141 142 143 144 145 146 147
analyze the surface morphology and elemental composition of the catalysts. Hydrogen temperature-programmed reduction (H2-TPR) of the catalyst was carried out on a Auto Chem II (Micromeritics, USA). The samples were first flushed with Ar (30 mL/min) at 350 C for 1 h. After that, the experiments were carried out at a heating rate of 10 C/min from 60 C to 700 C in 10% H2/Ar (30 mL/min). The H2 consumption was recorded by a TCD detector. The chemical state and surface composition of the catalysts was analyzed by Xray photoelectron spectroscopy (XPS, Axis Ultra DLD, Kratos, UK). All spectra were acquired at a basic pressure 3.33 106 Torr with Al Ka radiation (hv = 1253.6 eV) at 15 kV. The binding energy calibration was performed using the C 1 s peak in the background as the reference energy (284.6 eV). Thermo gravimetric analysis
(TG) and differential scanning calorimetry (DSC) analysis for the catalysts were carried out simultaneously in a static N2 atmosphere using a Netzsch STA 409 instrument. 15 mg of each sample was analyzed between 30 and 800 C at a rate of 10 C/min.
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Computational models and details
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All the calculations were based on DFT and performed by Modeling Dmol3 package [38,39]. SAPO-34 (1 1 1) model was selected in this study and SAPO-34 (111) face was choose to calculation the adsorption energy. Mn3O4 and CeO2@Ce2O3 crystal molecular were build for active component. All the geometry optimization were performed by local density approximation
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Fig. 9. SEM images of Mn/CeAPSO-34 (a, b) and Mn-Ce/CeAPSO-34 (c, d).
Fig. 10. NOx conversions of the catalysts.
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functional LDA-PWC, and the self-consistent field energy was set to 2.0 105 Ha. The adsorption energy are calculated as follows:
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Ead = Egas + Esurface Egas@surface
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where Esurface, Egas and Egas@surface correspond to the energy of surface, an isolated gas molecule and the same molecule adsorbed on the surface, respectively.
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Results and discussion
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In situ syntheses of CeAPSO-34
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The amount of the template and the content of cerium are important factors for the one-step hydrothermal procedure of CeAPSO-34. Based on the results of XRD in Fig. 1, the sample
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Fig. 11. Effects of SO2 and H2O on SCR activity of the catalysts reaction conditions: 0.08% NO, 0.08% NH3, 5% O2, 10% H2O (when added), 0.01% SO2 (when added), 180 C, Ar balance.
Fig. 12. XRD patterns of catalysts after reaction in the presence of SO2 and H2O. 169 170 171
showed high crystallinity and low impurities in the CeAPSO34 molecular sieves when DIPA/P2O5 ratio was 3.0 and Ce/P2O5 ratio was 0.3. The characteristic diffraction peaks of the CeAPSO-
34 at 2u = 9.7, 13.1, 16.2 , 19.3 , 20.9 , 26.2 and 31.4 were similar to those of the synthesized and commercial SAPO-34 samples. At the same time, the typical cubic-like structure of the CeAPSO-
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Fig. 13. H2-TPR profiles of catalysts after reaction in the presence of SO2 and H2O. 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198
34 was clearly identified by the SEM (Fig. 2), which was affirmed by corresponding XRD analysis. In addition, compared with the synthesized SAPO-34, the peaks of the CeAPSO-34 presented a slight shift. With the consideration of the EDS analysis in Fig. 3, the insertion of Ce into the SAPO-34 framework probably resulted in the unit cell size expansion.
while the high temperature signal corresponded to the reduction of Mn2O3 to MnO. According to the H2-TPR patterns, the reduction peaks firstly shifted to the low temperature region and then to the relatively higher region with the increase of the calcination temperature. The results conformed well to the NH3-SCR activity of Mn/CeAPSO-34 at different calcination temperatures. Fig. 7 presented the performance of Mn-Ce/CeAPSO-34 in NO reduction. The catalyst presented the best NH3-SCR activity and high N2 selectively at low temperature when Ce/Mn ratio was 0.4, and the NOx conversion over the catalyst reached nearly 97.7% at 140 C and remained about 100% in the temperature range of 160– 240 C. The XRD patterns (Fig. 8) of Mn-Ce/CeAPSO-34 demonstrated that no characteristic peaks of any Ce compounds were detected, which was similar to the results of Mn/CeAPSO-34. With the consideration of the SEM images (Fig. 9), it can be concluded that the manganese and cerium supported on the surface of CeAPSO-34 with scattered distribution had little effect on the key active sites of CeAPSO-34 in terms of the SO2 and H2O resistance. Mn/SAPO-34 and Mn-Ce/SAPO-34 were prepared for comparison, and the NH3-SCR activity of the catalysts was also investigated, with the results shown in Fig. 10. According to the results, a moderate amount of cerium supported on the catalysts could improve the monolithic catalyst activity to a certain degree, yet not significantly. Nevertheless, it had exerted little influence on the activity of the catalysts when cerium existed in the framework, proving that manganese species served as the dominant active
NH3-SCR activity and characteristics Mn/CeAPSO-34 was prepared by the Ethanol dispersion method. As shown in Fig. 4, when the calcination temperature was 400 C and the loading of Mn was 15 wt.%, the best NH3-SCR activity of the catalyst was observed in the reaction temperature range of 80–240 C. Moreover, the NOx conversion over the catalyst reached nearly 95% at 140 C and remained about 100% in the temperature range of 160–240 C. The XRD profiles (Fig. 5) of Mn/ CeAPSO-34 remained almost unchanged after introducing Mn, which indicated that the structure of CeAPSO-34 was not changed by the loading of Mn. Meanwhile, no characteristic peaks of any Mn compounds were observed, indicating that Mn was well dispersed in the catalysts or the formed compounds were not big enough to be detected. Fig. 6 provided the H2-TPR curves for estimating the reducibility of Mn/CeAPSO-34 catalysts at different calcination temperatures. The catalysts presented two well defined reduction peaks, in which the reduction process of MnO2 to Mn2O3 was suggested as the reduction process for the low temperature peak, Table 1 Surface atomic concentration of catalysts before and after reaction with SO2. Sample
Atomic fraction/% Al
Si
P
C
O
Mn
Ce
S
N
Mn/SAPO-34 Mn/SAPO-34-SO2 Mn/CeAPSO-34 Mn/CeAPSO-34-SO2 Mn-Ce/SAPO-34 Mn-Ce/SAPO-34-SO2 Mn-Ce/CeAPSO-34 Mn-Ce/CeAPSO34-SO2
7.15 7.21 6.02 6.18 6.59 7.01 6.01 6.22
0.79 0.82 0.74 0.70 0.76 0.79 0.70 0.73
8.38 8.27 7.65 7.51 7.84 8.05 7.57 7.62
34.17 42.58 24.64 35.75 15.29 29.04 12.56 27.26
39.32 32.49 48.46 40.25 52.49 44.68 54.67 46.88
10.19 1.04 10.25 2.09 10.32 3.07 10.35 4.28
– – 2.24 1.18 6.71 1.56 8.14 1.84
– 2.84 – 2.08 – 1.74 – 1.39
– 4.75 – 4.26 – 4.06 – 3.78
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Fig. 14. Mn 2p and O 1s XPS spectra of catalysts before and after reaction in the presence of SO2.
Fig. 15. Ce 3d XPS spectra of catalysts before and after reaction in the presence of SO2.
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Fig. 16. TG and DSC curves of catalysts after reaction in the presence of SO2 (a) Mn/SAPO-34 (b) Mn/CeAPSO-34 (c) Mn-Ce/SAPO-34 (d) Mn-Ce/CeAPSO-34. 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254
sites. To investigate the synergistic effects of cerium incorporated in the framework and the surface of SAPO-34 in the presence of SO2 and H2O at low temperature, the small difference of the catalyst activity could be ignored. Resistance to SO2 and H2O and characteristics The effects of SO2 and H2O on the catalysts’ SCR activity at 180 C were exhibited in Fig. 11. In the presence of 0.01% SO2, 10% H2O or 0.01% SO2 + 10% H2O, the NO conversion for Mn-Ce/CeAPSO34 (decreased to 88%, 89.08% and 65.45%) was much higher than that for Mn-Ce/SAPO-34 (decreased to 67%, 76.94%, and 42.16%), Mn/CeAPSO-34 (decreased to 56%, 68.1% and 40.29%) and Mn/ SAPO-34 (decreased to 16.9%, 36% and 13.28%). In terms of the catalysts, the activity followed the sequence of Mn-Ce/CeAPSO-34 > Mn-Ce/SAPO-34 > Mn/CeAPSO-34 > Mn/SAPO-34, indicating that cerium could dramatically improve the catalysts’ resistance to SO2 and H2O. In addition, Mn-Ce/CeAPSO-34 exhibited the best activity of SO2 and H2O tolerance, which clearly indicated that a strong synergistic effect existed between the cerium in the framework and the surface. Therefore, it can be known that the synergistic effect was not a simple overlap of single factors but a complex phenomenon. The SO2 and H2O tolerance of Mn-Ce/SAPO34 was better than that of Mn/CeAPSO-34, possibly because the cerium supported on the surface of catalysts led to more exposure of Ce compounds to SO2 and H2O-containing gases. As a consequence, the reaction with SO2 and H2O became much more easily and then the manganese active sites were protected. After the SO2 was removed, the NO conversions of the four catalysts could not recover to the previous level, implying that the deactivation by SO2 was irreversible, which might be due to the formation of metal sulfates or the deposition of ammonium
sulfates. In comparison, after the removal of H2O, the activity could recover to its original level, indicating that H2O inhibition to the SCR reaction was reversible via the competitive adsorption with NH3 on acid sites [40]. According to Fig. 12, which displayed the XRD patterns of the poisoned catalysts, the peaks over Mn/SAPO-34 occurring at 23.2 and 33 could be assigned to the formed phase of NH4HSO4 (PDF#35-1500). Then, it can be deduced that the newly formed crystal phases of NH4HSO4 resulted in the decrease of SCR activity. However, there were no detectable peaks of NH4HSO4 for the used Mn/CeAPSO-34, Mn-Ce/SAPO-34 and Mn-Ce/CeAPSO-34, which might be because that NH4HSO4 formed might exist in amorphous species or had not reached the detection limitation. The results demonstrated that cerium presenting in the framework and the surface could greatly inhibit the deposition of NH4HSO4. The H2TPR curves of the poisoned catalysts were illustrated in Fig. 13, and the results suggested that two reduction peaks were observed in the used Mn-Ce/CeAPSO-34 and Mn-Ce/SAPO-34, while only one was presented in the used Mn/CeAPSO-34 and Mn/SAPO-34. For the poisoned catalysts, the low-temperature reduction peak position was in the sequence of Mn/SAPO-34 > Mn/CeAPSO34 > Mn-Ce/SAPO-34 > Mn-Ce/CeAPSO-34. The position of the reduction peak was related to the oxidation capacity, which was a reason for the anti-SO2 and H2O-poisoning effect of cerium doping in the framework and the surface of catalysts. Meanwhile, the H2-TPR results were conformed to the NH3-SCR activity of the samples. The atomic surface compositions by XPS analysis were given in Table 1. Besides, it was noted that the surface content of manganese on the fresh catalysts showed a slight increase with the incorporation of cerium oxides in the framework and the surface. This was in accordance with the result in Fig. 10 that
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Fig. 17. Optimized structures of (A) Mn/SAPO-34 (B) Mn/CeAPSO-34 (C) Mn-Ce/SAPO-34 (D) Mn-Ce/CeAPSO-34. The red balls are oxygen, purple balls are manganese, white Q7 balls are cerium, yellow balls are silicon, pink balls are aluminum, light pink balls are phosphorus. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.) 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304
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cerium slightly strengthened the catalysts activity, and this is also conducive to investigating the synergistic effects of cerium with respect to the resistance to SO2 and H2O. Meanwhile, Mn-Ce/ CeAPSO-34 had the highest surface concentration of cerium, indicating that more cerium was available to participate in NH3SCR with the addition of SO2 and H2O. Compared with the fresh catalysts, the manganese concentration in the four used catalysts all declined to a certain extent and the sulfur and nitrogen atoms could be detected after the reaction under SO2 flow. The used Mn/ SAPO-34 showed the highest level in the decreasing manganese concentration and the increasing sulfur and nitrogen concentration, followed by Mn/CeAPSO-34, Mn-Ce/SAPO-34 and Mn-Ce/ CeAPSO-34 in sequence. According to the results, the sulfating of the active sites was inhibited in some ways after the cerium was incorporated in the framework and the surface of catalysts. It can be seen from Fig. 14(a) and (b), which presents the XPS spectra for Mn 2p, Mn 2p spectra had two peaks, which are Mn 2p1/ 2 (653 eV) and Mn 2p3/2 (642 eV), respectively. Through peakfitting deconvolutions, three peaks appeared in the Mn 2p3/2 spectra, respectively, Mn2+ (639.7–640.5 eV), Mn3+ (641.0– 641.5 eV) and Mn4+ (643.2–644.0 eV). It is worthwhile of norticing that the relative surface content of Mn3+ and Mn4+ over the four fresh catalysts was much higher than that of Mn2+. Based on the Table 2 Adsorption energies of SO2 on catalysts. Sample
Mn/SAPO-34 Mn/CeAPSO-34 Mn-Ce/SAPO-34 Mn-Ce/CeAPSO-34
Ead (SO2) (eV) Mn1
Mn2
Mn3
Ce1
Ce2
Ce3
Ce4
Ce5
4.02 3.92 4.01 3.90
4.15 4.13 3.87 3.80
4.15 4.11 4.09 4.05
– 5.92 – 6.01
– 5.76 – 6.64
– – 6.14 6.82
– – 6.05 6.07
– – 6.22 6.20
literatures [41,42], the Mn3+ and Mn4+ species could contribute to enhancing NH3-SCR activity at low temperature. The surface content of manganese species was in a way decreased after the addition of SO2. Besides, the calculated amount of Mn3+ and Mn4+ species on the four used catalysts was in the sequence of Mn-Ce/ CeAPSO-34 > Mn-Ce/SAPO-34 > Mn/CeAPSO-34 > Mn/SAPO-34, correlating well with the result that the sulfating of manganese was somewhat inhibited after incorporating cerium oxides in the framework and the surface. The O 1s XPS pattern (Fig. 14(c) and (d)) showed that two types of surface oxygen existed in the catalysts. With reference to the literatures [43,44], the binding energy of 529.1–529.5 eV corresponded to the lattice oxygen (Oa), while that of 531.0–531.5 eV was ascribed to the chemisorbed oxygen (Ob).Obviously, the Ob species exhibited the larger peak areas for all samples, and the amount of Ob species on the four fresh and used catalysts followed the sequence of Mn-Ce/CeAPSO-34 > Mn-Ce/SAPO-34 > Mn/ CeAPSO-34 > Mn/SAPO-34. Furthermore, it has been testified that Ob species, with their higher mobility, were more active than Oa species, one reason for which referred to that the presence of cerium oxides in the framework and the surface led to the content increase of Ob species. As a result, the NH3-SCR activity in the presence of SO2 could be further improved. It can be known from the Ce 3d XPS pattern (Fig. 15) that Ce coexisted as Ce4+and Ce3+. According to the peak-fitting deconvolutions, seven peaks appeared in the Ce 3d spectra [45], which also showed that much more Ce4+ species existed on the surface of the catalysts. In comparison with the fresh catalysts, the content of cerium species decreased in spent catalysts, and the Ce4+ peaks (886.2 eV) in the used catalysts moved to 887.5 eV. Considering the literatures [46,47], the Ce4+ peaks of the catalysts shifted towards a higher binding energy position after the SCR with SO2 due to the sulfating of the cerium. These results suggested that the cerium
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oxides incorporated in the framework and the surface preferred to react with SO2 to form sulfate species in sulfur atmosphere, thereby, to some degree, inhibiting the formation of manganese sulfate species and protecting the activity of manganese active sites. The TG-DSC curves of the four used catalysts were exhibited in Fig. 16. One major weight loss emerging at about 80–110 C could be ascribed to the evaporation of water in the used catalysts. In terms of the used Mn/SAPO-34 catalyst, there was one other relatively obvious weight loss at 217 C, which was caused by the decomposition of NH4HSO4 [16,48]. The results were consistent with the XRD results of the used catalysts. Additionally, the TG-DSC results further confirmed that cerium oxides presenting in the framework and the surface could inhibit the formation of NH4HSO4 on the catalyst surface during the SCR process with SO2. This was one of the reasons accounting for the catalysts’ strong resistance to SO2 poisoning.
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DFT study
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The models of Mn/SAPO-34, Mn/CeAPSO-34, Mn-Ce/SAPO34 and Mn-Ce/CeAPSO-34 were established to research the synergistic effect existing between the cerium oxides in the framework and the surface. The optimized geometric structure for all the catalyst models was shown in Fig. 17. According to the surface exposed by Mn and Ce cations in different environments, the surface cations were labeled as Mn1, Mn2, Mn3 as well as Ce1, Ce2, Ce3, Ce4, Ce5. Then, the adsorption energy of SO2 molecule adsorbed on the surface of all catalysts was analyzed and the results were presented in Table 2. It was noted that the Ead (SO2) of Ce sites was greater than that of Mn sites, and the Ead (SO2) of Ce site supported on the surface neighbored by Ce site in the framework was greater than that of other Ce sites. At the same time, the Ead (SO2) of the Mn sites neighbored by Ce sites was lower than that of other Mn sites. It is already known that the positive value of Ead demonstrates a strong interaction between the adsorption surface and gas. Therefore, these results indicated that compared with the Mn sites, SO2 was more capable of occupying the Ce sites, especially the Ce site supported on the surface neighbored by Ce site existing in the framework, which prevented the manganese active sites from being sulfated to some extent. Besides, the theoretical results were in well line with the NH3-SCR activity in the presence of SO2.
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Conclusions
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The low-temperature NH3-SCR activity of Mn-Ce/CeAPSO34 catalyst was significantly increased in the presence of SO2 and H2O. According to the characterization results, the sulfating of the manganese active sites and the deposition of NH4HSO4 on the surface of the catalysts were to some extent inhibited after the cerium oxides were incorporated in the framework and the surface. Regarding these catalysts, the activity in the presence of SO2 and H2O was in the sequence of Mn-Ce/ CeAPSO-34 > Mn-Ce/SAPO-34 > Mn/CeAPSO-34 > Mn/SAPO-34. The results indicated that a strong synergistic effect existed between the cerium in the framework and the surface, and the synergistic effect was not a simple overlap of single factors yet a complex phenomenon. Furthermore, the theoretical results implied that the synergistic effect could be attributed to the [-Ce-O-Ce-]units, indicating that the Ce site supported on the surface neighbored by Ce site in the framework would be beneficial to the reaction with SO2.
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Acknowledgments The research was financially supported by the National Natural Science Foundation of China (NSFC-51478191) and Science and Technology Project of Guangdong Province (2014A020216003).
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