SAPO-34

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Nature identification of Cu active sites in sulfur-fouled Cu/SAPO-34 regeneration Meiqing Shen, Xinhua Li, Jianqiang Wang, Chen Wang, and Jun Wang Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.7b05053 • Publication Date (Web): 21 Feb 2018 Downloaded from http://pubs.acs.org on February 21, 2018

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Industrial & Engineering Chemistry Research

Nature identification of Cu active sites in sulfurfouled Cu/SAPO-34 regeneration Meiqing Shena,c,d, Xinhua Lia, Jianqiang Wanga, Chen Wangb*, Jun Wanga*, a

Key Laboratory for Green Chemical Technology of State Education Ministry, School of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, PR China

b

School of Environmental and Safety Engineering, North University of China, Taiyuan 030051, PR China c

Collaborative Innovation Centre of Chemical Science and Engineering (Tianjin), Tianjin 300072, PR China

d

State Key Laboratory of Engines, Tianjin University, Tianjin 300072, PR China

* Corresponding authors at: School of Chemical Engineering and Environment, North University of China, 3 Xueyuan Road, Taiyuan 030051, China (C. Wang). School of Chemical Engineering and Technology, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, China. Fax: +86 22 27892301 (J. Wang). Fax: +86 15536306098 (C. Wang).

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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

E-mail addresses: [email protected] (C. Wang), [email protected] (J. Wang).

Abstract In order to probe the nature of regeneraion on the sulfated Cu/SAPO-34 selective catalytic reduction catalysts, CHA structure, copper species and NOx conversion were investigated. A sulfated Cu/SAPO-34 catalyst was treated at different temperatures and time. The ammoniasulfate species remove at 500 oC and copper sulfate completely decompose at 750 oC. The NOx conversion of sulfated catalyst gradually recovers with the degree of regeneration increasing. XRD and ex-situ DRIFTs results show thermal treatment presents no effect on framework structure of Cu/SAPO-34. The TPR and EPR results suggest that thermal treatment has influence on number of isolated Cu2+ and CuO instead of coordination environment and redox ability. However, the apparent activation energy of fresh, sulfated and regenerated catalysts for SCR reaction remains the same, and turnover frequencies are also identical. The study reveals that the activity recovery is attributed to the increased number of active sites by sulfate species decomposition and migration effect. Keywords: Regeneration Mechanism; Cu/SAPO-34; Active Sites; Sulfate Decomposition; Copper Migration.

1. Introduction The growing of mobile sources generates high consumption of fossil fuel and thus brings more nitrogen oxides (NOx) and serious air pollution. The alarming change of environmental surroundings give rise to the essential attention on management of fossil fuel waste, especially


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for auto makers. Selective catalytic reduction (SCR) of NOx with NH3 is currently the most efficient commercial technology for NOx abatement. Some traditional NH3-SCR catalysts, like V2O5/WO3-TiO2 and Cu/ZSM-5, are hard to meet the wide temperature range of mobile sources. Recently, Cu/SAPO-34, a low-cost small-pore chabazite (CHA) zeolite, is recognized to be a promising catalyst to deal with NOx because of its good thermal stability, excellent NH3-SCR activity and low N2O formation 1-5. Catalyst poisoning by sulfur oxides has been recognized as a potential issue for its practical application. Researches on sulfur poisoning of Cu/SAPO-34 SCR catalysts show that the sulfur poisoning usually declines the SCR activity seriously below 400 oC because of formation of copper sulfate or ammonia-sulfate on active sites 6-8. The declined SCR activity can hardly meet the emission requirement, the regeneration of poisoned catalysts is necessary to be considered. As yet the regeneration on sulfated Cu/SAPO-34 was only reported in the level of sulfate decomposition under different temperatures. Zhang et al. mainly studied SO2 poisoning impact on Cu/SAPO-34 and preliminarily explored the regeneration performance 9. They found the NOx conversion almost returned to the original SCR performance when the regeneration temperature reached 600 oC, which could assign to the decomposition of CuSO4 or Ce2(SO4)3. Su et al. focused on the identification of sulfate species during sulfation 10 and pointed that the NOx conversion could not fully recover when stable Al2(SO4)3 species formed during sulfation even the regeneration temperature was as high as 650 oC. In addition to those achievements without consensus on regeneration temperature, there is still query regarding possible change in Cu/SAPO-34 active sites, which suggests two different sites in CHA-type zeolites 1, under the real-world conditions (sulfur poisoning and regeneration treatment). Whether recovered Cu2+ ions will change its initial location and reaction mechanism will change after regeneration?



Meanwhile, the query about what is the changing regularity of sulfate, CuO and isolated Cu2+ during regeneration process is also well worth probing. From this point of view, the regeneration process of sulfur-fouled Cu/SAPO-34 is necessary to study. In this work, we report the effect of regeneration conditions on sulfated Cu/SAPO-34 catalysts as function of temperature and time. Using characterization methods, like XRD, ex-situ DRIFTs, we intend to firstly investigate the change of CHA structure during regeneration process. Then, the copper species, like copper sulfate, CuO and isolated Cu2+, of Cu/SAPO-34 can be studied by using TGA, TPR and EPR. Finally, the structureactivity relationship can be obtained combining with kinetic studies. All conclusions will help to understand the regeneration mechanism during thermal treatment and guide to apply Cu/SAPO-34 on diesel after treatment system.

2. Experimental 2.1. Catalyst preparation The ‘one-pot’ method, which is a common method used in our previous studies 7, 11, was chosen to synthesize Cu/SAPO-34 catalysts. The synthesis gel consists of 1 Al2O3, 0.9 P2O5, 0.7 SiO2, 0.2 CuO, 2 Morpholine (MOR), 0.2 Tetraethylenepentamine (TEPA) and 5.69 H2O (molar basis). The sources for Si, P, Al, Cu, templating agent and complexing agent are seen everywhere 5-7, 11. Briefly, the materials are silica sol, orthophosphoric acid, pseudoboehmite, Copper (Ⅱ) sulfate pentahydrate, MOR and TEPA, respectively. Resulted gel was sealed in a 200 ml Teflon-lined stainless steel pressure vessel and heated in an oven at 200 oC under autogenic pressure for 2 days. After the crystallization process, the sediment was separated from the mother liquid via centrifugation, washed by distilled water and then filtrated. Finally, the


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powder was dried at 120 oC in oven for 12 h and calcined in muffle furnace with air at 650 oC for 5 h. In order to make most of CuO migrate to exchange sites, the resulting sample was hydrothermally aged with 10% H2O in air at 750 oC for 6 h and is named as fresh Cu/SAPO-34 (F-Cu).

2.2. Sulfation and regeneration treatment In order to investigate regeneration mechanism and make sure that CHA structure is not affected by SO3/H2SO4, 5 the catalysts in this study were sulfated with SO2-containing streams, in the presence of ammonia. The catalysts were treated at 250 oC with a feed containing 50 ppm SO2, 500 ppm NH3, 500 ppm NO, 5% H2O in air. In this case, the sulfation lasted 16 h with a total sulfur throughput of 68.6 mg S/g catalyst. The sulfated catalysts are denoted as ‘S-N-Cu’, where S stands for SO2 and N for NH3. Then, the sulfated catalysts were treated at 500, 600, 650 and 750 oC with a feed containing 10% H2O in air. The regeneration catalysts are denoted as ‘R-T-t’, where R stands for regeneration processes, T and t for regeneration temperature and time. The description of sulfation conditions and corresponding catalyst nomenclatures are shown in Table 1.

2.3. Catalyst characterization Since the variation of copper species and structure are the main factors to effect SCR performance, the CHA structure variation of regenerated samples need to show. The structure and relative crystallinity of catalysts were measured using XRD spectra. The XRD spectra was collected using X’ Pert Pro diffractometer with nickel-filtered Cu Kα radiation (λ=1.5418 A), operating at 40 kV and 40 mA in the range of 5 - 50o with a step size of 0.01°.



After degassing catalysts at 150 oC for at least 5 h under 0.133 Pa pressure, BET surface areas were measured by N2 adsorption-desorption (F-Sorb 2400 automatic physisorption analyzer, Beijing) at -196 oC. To monitor the sulfate species and structure variation during sulfation, S-N-Cu was treated under NO+O2 because NH3 adsorbed on acid sites (Brønsted and Lewis acid sites) has impact on identification and analysis of ammonia sulfate species and structure. The detailed process of NH3 elimination using NO+O2 titration can be seen in our previous study 7. Exception of S-N-Cu, NO+O2 titration did not perform on other catalysts because of no NH3 remain after thermal treatment in this study. The change of structure and isolated Cu2+ ions under sulfation and regeneration treatments was measured using ex-situ diffuse reflectance infrared Fourier transform spectra (Nicolet 6700 spectrometer). Before each measurement, S-N-Cu was firstly treated under NO+O2 titration to eliminate adsorbed ‘active’ NH3 species on Brønsted and Lewis acid sites 7 and then each sulfated samples were dried at 250 oC for 30 min. The spectra were recorded at 200 oC, and the KBr spectrum under the same condition was used as background. The DRIFT spectra were recorded in the range of 4000 to 650 cm-1 with a resolution of 4 cm-1. Before NH3-TPD experiment, the catalysts were dehydrated at 250 oC for 30 min in 5% O2/N2. When samples were cooled to 100 oC in N2, the samples were purged by 500 ppm NH3/N2 at 100 o

C until NH3 concentration was stable. After that, the samples were purged with N2 to remove

any weakly absorbed NH3 at 100 oC. When NH3 concentration was lower than 10 ppm, the samples were heated from 100 oC to 550 oC at a ramping rate of 10 oC/min.


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The weight change of the sulfated catalysts was examined with a METTLER TOLEDO thermal gravimetric analyzer (TGA) using about 15 mg of sample. Before TGA measurements, all catalysts were dehydrated at 100 oC in oven. The sample was first heated from room temperature to 150 oC at 10 oC/min in a gas flow containing N2 (47.5 ml/min) and O2 (2.5 ml/min) and kept at 150 oC for 30 min. The temperature was then increased from 150 to 800 oC at a rate of 10 oC/min in the same gas stream. Temperature Programmed Reduction (TPR) experiments were performed in a U-shaped tubular quartz reactor. Prior to reduction, the samples (100 mg) were first treated at 250 oC under a flow of 30 ml/min 5 % O2/N2 and kept for 30 minutes. Then, the samples were cooled down to room temperature following by purging in N2 with a flow of 30 ml/min. Finally, the samples were measured in a flow of 5% H2/N2 (10 ml/min-1) from 30 oC to 900 oC at a ramping rate of 10 o

C/min. S-N-Cu was not involved in this experiments because decomposition of ammonia-sulfate

affects TCD signal. The location and content of isolated Cu2+ ions were collected using the electron paramagnetic resonance (EPR) spectra (Bruker ESP320). Bruker ESP320E software was used for data analysis. The samples were treated in 20% O2/N2 at 250 oC for 1 day before test, and then sealed into a quartz tube for characterization. X-band (ν=9.78 GHz) spectra were recorded at -150 oC with the magnetic field sweeping from 2000 to 4000 Gauss. The content of isolated Cu2+ was calculated from double integration of the EPR spectra using copper sulfate solutions as a reference.

2.4. SCR activity and kinetics test The SCR activity was measured in a quartz tube reactor. The mixture (0.1 g catalyst with 0.9 g quartz sand in 60-80 mesh) was fixed in the quartz tube with quartz wool on both sides. A



YUDIAN 808P controller with a K-type thermocouple was used to control the temperature and the feed gases were controlled by Beijing Metron S49-33M/MT mass flow controllers. Prior to SCR experiment, a catalyst was heated up to 250 oC and remain for 30 min under a flow of 5% O2 in N2. The reactant gas mixture consists of 500 ppm NOx, 500 ppm NH3 and 5% O2, 7% CO2, 3% H2O balanced by N2, with a gas hourly space velocity (GHSV) of ~72,000 h-1 (1,000 ml/min). The activity was measured in a temperature range of 100 - 600 oC at a 50 oC interval. Data were continuously collected after the system has been stabilized for at least 1 h at each temperature. Concentration of emitted gases (NO, NO2, N2O, NH3 and H2O) was recorded by FTIR (MKS 2030). The NOx conversion is calculated based on Equation 1. NOX Conversion ( %) =

NOxinlet − NOxoutlet ×100% NOxinlet

(1)

Where NOx = NO+ NO2 The kinetic tests was collected in a differential tube reactor. The mixture (25 mg catalyst with 125 mg quartz sand in 80-100 mesh) was also fixed in the tube with quartz wool on both sides. A gas hourly space velocity (GHSV) was 432,000 h-1, which ensure the reaction system are free of external and internal diffusion control. The test was measured in the absence of H2O and CO2 because the presence of H2O and CO2 has weak effect on SCR activity over Cu/SAPO-34 especially at low temperature based on our previous work 2. Before test, a sample was pre-treated in the same way as for activity measurement. The data was collected in the region that the conversion of NOx was under 20% at a 20 oC interval. The NH3-SCR reaction rates and turnover frequency (TOF) are calculated based on Equation 2 and Equation 3.


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Rate  molNOX

X NOX [ %] × FNOX  LNOx ⋅ min −1  ⋅ gcatal ⋅ S  = mcatal [ g ] × 60  s ⋅ min −1  × 22.4  L ⋅ mol−1  ×100 −1

(2)

Where, X NO is NOx conversion, FNO is volumetric flow rate of NOx , mcatal is catalyst weight. X

X

X NOx [ %] × FNOx  LNOx ⋅ min −1  TOF  s  = 60  s ⋅ min −1  × 22.4  L ⋅ mol −1  ×100 × [ number of Active Sites ][ mol ] -1

(3)

Where, X NO is NOx conversion, FNOx is volumetric flow rate of NOx , [Number of active sites] is the amount of isolated Cu2+ sites measured by EPR. X

3. Results 3.1. Effects of regeneration treatment on Cu/SAPO-34 structure

3.1.1. XRD and BET To examine the impact of regeneration conditions on the structure of Cu/SAPO-34 catalyst, XRD experiments were performed. As shown in Figure 1a, all catalysts have typical chabazite (CHA) structure 12. Moreover, the relative crystallinity of all catalysts is similar, as shown in Figure 1b. Table 1 compares the BET surface areas of fresh, sulfated and regenerated catalysts. A 20% reduction in surface area was found on S-N-Cu relative to the fresh, which shows same result with our previous study 7. When S-N-Cu further went through regeneration treatments, BET surface area recovers gradually. Notably, the surface areas of R-750-12 is similar with that of FCu.



3.1.2. ex-situ DRIFTs Figure 2 shows the ex-situ DRIFTs results of the fresh, sulfated sample with those of regenerated ones. The IR bands in 3500 - 3800 cm-1 region are related to the stretching vibration modes of OH groups (-OH). Two stronger bands at 3614 and 3598 cm-1 can be assigned to the Brønsted OH groups as (Si-O(H)-Al). The band at 896 cm-1 is associated with an internal asymmetric framework vibration perturbed by copper cations, which also generates Lewis acid sites 11. Figure 2b shows the peak intensity when the absorbance is converted to Kubelka-Munk (K-M) units. The intensity of band assigned to Brønsted OH groups keep same on all catalysts, which shows sulfur poisoning and regeneration treatments have no impact on Brønsted OH groups over Cu/SAPO-34. This result agrees with our XRD results. Meanwhile, the intensity of band at 896 cm-1 firstly decreases as F-Cu sulfated in poisoning feed, and then returns to that of fresh one after sulfated catalyst treated at 750 oC.

3.1.3. NH3-TPD Figure 3 shows the NH3-TPD results of fresh, sulfated and regenerated samples. There are two distinct regions at 160-170 oC and 330-360 oC for all the samples and the two regions can be deconvoluted into three peaks due to the asymmetry of the second peak (the detailed deconvolution can be found in our previous study 13). There are three acid sites presented on the Cu/SAPO-34 catalysts (~170, 270 and 330 oC). To reveal the special NH3 species adsorbed on Cu/SAPO-34, in-situ DRIFTs were performed on F-Cu following stepwise NH3 desorption with an increase of temperature (Figure S1). Combing with DRIFTs results, the desorption peak centered at ~170 oC is assigned to external hydroxyl groups (P–OH, Si–OH, and Al–OH). The


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peak at ~270 oC is attributed to moderate Brønsted acid sites (Si-O(H)-Al) and the peak at 330 o

C is assigned to strong Brønsted acid sites and Lewis acid sites (Si-O(H)-Al and isolated Cu2+

ions). S-N-Cu shows more amount of NH3 at higher temperature (centered at ~ 380 oC) compared with that of F-Cu, which is induced by the decomposition of ammonia-sulfate species. The regenerated catalysts show similar NH3 profiles and storage with that of F-Cu, illustrating that regeneration process have less impact on Brønsted acid site. This is consistent with our XRD and ex-situ DRIFTs results.

3.2 Effects of regeneration treatment on copper species over Cu/SAPO-34

3.2.1. TGA measurements Figure 4 compares the TGA profiles of the fresh and sulfated Cu/SAPO-34 catalysts with those of regenerated catalysts. Sample weight loss is found in three temperature regimes. The first weight loss occurs below 300 oC, which can be assigned to water evaporation 14. The second weight loss is found between 300 and 500 oC, and the weight loss decreases in the order: S-N-Cu (2.43 %) > R-500-0.5 (0.8 %) > R-600-0.5 (0.48 %) > R-600-4 (0.45 %) > R-650-4 (0.37 %) > R-750-4 (0.35 %) > R-750-8 (0.32 %) > R-750-12 (0.3 %). The third weight loss is found between 500 and 800 oC, and the weight loss decreases in the order: R-500-0.5 (0.77 %) > S-NCu (0.68 %) > R-600-0.5 (0.54 %) > R-600-4 (0.38%) > R-650-4 (0.19 %) > R-750-4 (0 %) ≈ R750-8 (0 %) ≈ R-750-12 (0 %).



3.2.2. TPR results To obtain more information about the copper species, H2-TPR was carried out on the fresh, sulfated and regenerated catalysts and the results are shown in Figure 5a. For fresh sample, three peaks centered at 270, 310 and 700 oC represent the reduction of isolated Cu2+ to Cu+, the reduction of CuO to Cu0 and the reduction of Cu+ to Cu0, respectively 14-19. The sulfated samples present three peaks above 400 oC (~ 400, 480 and 570 oC), which can be attributed to the reduction of the deactivated copper species and close to those obtained from the reduction of CuSO4.5H2O (Figure S2). This result was also observed by Kartheuser et al. 20. In addition, the peak above 700 oC on sulfated and regenerated catalysts can be attributed to the reduction of sulfate species based on our former study 6. Figure 5b shows the H2 reduction area of different copper species. The amount of isolated Cu2+ increases with an increase of regeneration temperature and time. The isolated Cu2+ ions contents on R-750-8 and R-750-12 are similar with that of fresh one. Meanwhile, the amount of copper sulfate declines as the degree of regeneration increasing. The amount of CuO increases until the sulfated catalyst treated at 600 oC for 4 h. When the treated temperature and time continue to raise, CuO contents gradually decline until no H2 peak assigned to reduction of CuO to Cu appears, as shown in Figure 5a.

3.2.3. EPR results Figure 6a shows the EPR spectra of the Cu/SAPO-34 catalysts under different conditions. The EPR spectra show that the Cu2+ species of the sulfated and regenerated catalysts have the same coordination environment, which locates inside, within or close to the face of 6-membered rings, as the non-sulfated catalyst (F-Cu) with same g// and A// values 2. Figure 6b compares the number


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of isolated Cu2+ ions on the Cu/SAPO-34 catalysts from EPR. The number of isolated Cu2+ ions decreases in the following order: R-750-12 > R-750-8 > F-Cu > R-750-4 > R-650-4 > R-600-4 > R-600-0.5 > R-500-0.5 > S-N-Cu.

3.3 Effects of regeneration treatment on NH3-SCR reaction over Cu/SAPO-34 3.3.1. Catalyst activity To understand how regeneration treatment affects the catalytic activity, the NH3-SCR reaction was carried out and the results are shown in Figure 7. Compared to F-Cu, the NOx conversions of the sulfated catalysts decrease between 100 and 400 oC. The results agree with our previous study 7. When the sulfated catalyst underwent regeneration treatment, the NOx conversion gradually recovers as the regeneration temperature and time increasing. Notably, R-750-8 and R750-12 show similar NOx conversion with that of F-Cu. The 7 ppm N2O is found on R-750-12 at 600 oC, and the other sample with different regeneration treatment decreases the N2O formation by 1 to 2 ppm (Figure 7b).

3.3.2. Kinetics study Figure 8 shows the Arrhenius plots for the SCR reaction over the fresh, sulfated and regenerated catalysts. All the catalysts show the similar apparent activation energies (Ea). According to the Equation of Arrhenius, the only difference parameter in rate constant is the preexponential factor, which decreases in the following order: R-750-12 > R-750-8 > F-Cu > R750-4 > R-650-4 > R-600-4 > R-600-0.5 > R-500-0.5 > S-N-Cu.



4. Discussion 4.1. Effects of regeneration treatment on structure of Cu/SAPO-34 catalysts Our XRD and ex-situ DRIFTs results of F-Cu and S-N-Cu illustrate that SO2 poisoning with NH3 do not influence CHA structure of Cu/SAPO-34. This conclusion is consistent with other lectures 8, 9. Besides, XRD results also suggest that regeneration treatment has no impact on the CHA structure. This suggestion is confirmed by ex-situ DRIFTs results, which show the intensity of Si-O(H)-Al bonds (3625 and 3600 cm-1) keep same upon different regeneration treatments. The NH3-TPD result also reflects that the Si-O(H)-Al bonds don’t change because NH3 profiles and acidity are same among regenerated catalysts and fresh sample. Our study reveals that regeneration process do not further affect the CHA structure as sulfate species decomposition (the detailed interpretation about sulfate decomposition are seen in Section 4.2). Since regeneration conditions make no difference among F-Cu and regenerated catalysts on CHA structure, it is reasonable believed that the SCR performance could be influenced by variation of copper species (the detailed discussion about SCR performance and copper species are shown in section 4.3).

4.2. Effects of regeneration treatment on Cu species of Cu/SAPO-34 Our TGA result of S-N-Cu suggests that ammonia-sulfate species and copper sulfate co-exist on Cu/SAPO-34 after SO2 and NH3 sulfation, as evidenced by same DTG temperature ranges between sulfate species on S-N-Cu and references (ammonium sulfate and copper sulfate pentahydrate) in Figure S3. According to our pervious study about the effect of different sulfate species on Cu/SAPO-34 7, ammonia-sulfate species and sulfate formed on Cu2+ ions as Cu2+2(NH3)-SO42- and CuSO4 during sulfation, respectively. Since the thermal stability of ammonia-


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sulfate is lower than that of copper sulfate (~420 vs. 730 oC), ammonia-sulfate fully removes when regeneration temperature is 500 oC as supported by DTG results in Figure S3a. Meanwhile, the poisoned Cu2+ ions affected by ammonia-sulfate recover, which is justified by our EPR results. Combining with the similar NH3 oxidation results of S-N-Cu and F-Cu in Figure S4 (CuO is the active sites for NH3 oxidation 21 and the oxidation tests suggest the similar CuO contents between these two samples), it reveals that the decomposition product of ammoniasulfate species make poisoned isolated Cu2+ ions directly recover when regeneration temperature is 500 oC. When temperature is higher than 500 oC, only copper sulfate remains on Cu/SAPO-34 as also shown in Figure S3a. Copper sulfate gradually decomposes as temperature continues increasing and completely remove when temperature reaches at 750 oC. The sulfated Cu/SAPO-34 catalysts need such high regenerated temperature (750 oC) because of the property of copper sulfate with higher thermal stability. The change of copper sulfate content is further confirmed by our H2TPR results, which show the area of H2 reduction peak between 400 and 600 oC assigned to copper sulfate declines until disappearing. This reveals that decomposition of copper sulfate is more favorable when a sulfated Cu/SAPO-34 catalyst is treated at higher temperature. Moreover, regeneration time is also proved to be critical for copper sulfate decomposition. At 600 oC, little copper sulfate was found when treated under long time (4 vs. 0.5 h). The possible reactions in the process of copper sulfate decomposition can be achieved after H2-TPR results analysis (Figure 5). According to Figure 5, the amount of isolated Cu2+ increases slightly, but content of CuO increases rapidly when regeneration temperature is below 600 oC. Since the decomposition product of copper sulfate is CuO as supported by Beevers et al. 22, it is reasonable believed that the dominant reaction is copper sulfate decomposition at this



temperature range (500-600 oC). When the regeneration temperature raises continuously (600 < T < 750 oC), the amount of CuO declines. Meanwhile, there is consistent growth in amount of isolated Cu2+ and sustained reduction in content of copper sulfate. This phenomenon indicates that the migration reaction from CuO to isolated Cu2+ ions also involved in regeneration process. For R-750-4, copper sulfate disappears and the amount of isolated Cu2+ and CuO are consistent with that of F-Cu, revealing that the reaction rates of copper sulfate decomposition and CuO migration are similar. For R-750-8 and R-750-12, it can be concluded that copper sulfate decomposes completely and CuO migration is the only reaction with regeneration time increase continuously. Combining with the changing regularity of copper sulfate, CuO and isolated Cu2+ contents above 500 oC, it shows that an increase of Cu2+ ions contents during regeneration process is due to copper sulfate decomposition and migration effect. Whether the property (coordination environment and redox ability) of recovered Cu2+ ions keeps same after regeneration process? This question can be answered by EPR and H2-TPR results. As shown in Figure 5a and Figure 6a, the coordination environment and redox ability of isolated Cu2+ ions on regenerated catalysts are same with that of F-Cu, illustrating that the property of recovered Cu2+ keeps same after regeneration.

4.3. Effects of regeneration treatment on NH3-SCR activity According to our NH3-TPD results (Figure 3), the acid contents of the sulfated and regenerated catalysts are similar to that of Cu-F. What's more, the property of Cu2+ on these samples are also same with F-Cu, as shown in TPR and EPR results. The changing factor is the content of isolated Cu2+ ions, as shown in EPR results (Figure 6b). Since the apparent activation energies obtained on the sulfated and regenerated catalysts are the same, the turnover frequencies


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(TOFs) can be calculated based on the isolated Cu2+ ions. Figure 9 shows the TOFs are identical for all catalysts and this result strongly supports the nature of activity variation is due to the changed number of isolated Cu2+ sites. To further investigate the effect of regeneration treatment on NH3-SCR performance, it is essential to correlate the increased contents of Cu2+ with the reduced amount of sulfur when S-N-Cu is treated at different regeneration conditions and the results are shown in Figure 10 (the calculation about the reduced amount of sulfur can be seen in Supporting Information). When temperature reached 500 oC, ammonia-sulfate species decompose and make poisoned Cu2+ recover. When the temperature continued to raise, the regeneration process doesn’t involve a simple decomposition reaction from CuSO4 to isolated Cu2+, but decomposition (CuSO4 to CuO) and migration (CuO to isolated Cu2+) reactions. Figure 10 also confirms that the rate of migration reaction increases as the slope gets closer to zero when regeneration temperature gradually goes up.

4.4 Regeneration mechanism over Cu/SAPO-34 A possible process mechanism of regeneration is proposed, as shown in Scheme 1. During the sulfation process, ammonia-sulfate and copper sulfate were formed on isolated Cu2+ ions. Correspondingly, during the regeneration process, the ammonia-sulfate formed on Cu2+ sites directly removed, which make Cu2+ sites recovery. Meanwhile, copper sulfate firstly decomposed to CuO, and finally transformed to Cu2+ sites under regeneration treatment. With high hydrothermal stability and good regeneration characteristic, Cu/SAPO-34 catalyst, a good candidate, could be applied on application with DPF in after treatment systems when SO2 presented in exhaust. Besides, to make sure the sulfate species decomposition and copper



migration reactions occur completely, the appropriate temperature and time need to be focused in regeneration process.

5. Conclusions The regeneration of NH3-SCR activity over sulfated Cu/SAPO-34 was investigated under various thermal conditions. Thermal treatment has no impact on CHA structure, but influences the amount of copper species. Ammonia-sulfate remove firstly at 500 oC and make poisoned Cu2+ ions directly recover. And then copper sulfate decomposes gradually until temperature goes up to 750 oC (500-750 oC). During copper sulfate decomposition, migration reaction from CuO to isolated Cu2+ ions always presents as the amount of CuO declines until disappearing and isolated Cu2+ increase continuously when temperature is higher than 600 oC. After thermal treatment, the recovered Cu2+ ions show the same property with that of fresh catalyst. Combining with the kinetic study, the regeneration process increases the SCR reaction rate of the regenerated Cu/SAPO-34 catalysts by increasing the number of isolated Cu2+ sites. Our study demonstrates that the appropriate temperature and time need to take into consider in regeneration process and Cu/SAPO-34 with good regeneration characteristic can be applied on the application with DPF in after treatment systems when SO2 existed in the exhaust. Associated Content The Supporting Information consist of the detailed information about DRIFTS upon NH3 desorption over Cu/SAPO-34; H2-TPR results of copper sulfate pentahydrate; DTG results of catalysts; NH3 oxidation tests and the calculation process of sulfur content.


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Acknowledgements The authors are grateful to the financial support from the National Key Research and Development program (2017YFC0211302), National Natural Science Foundation of China (No 21676195) and the Science Fund of State Key Laboratory of Engine Reliability (skler-201714).

References (1) Gao, F.; Washton, N. M.; Wang, Y.; Kollar, M.; Szanyi, J.; Peden, C. H. F. Effects of Si/Al Ratio on Cu/SSZ-13 NH3-SCR Catalysts: Implications for the Active Cu Species and the Roles of Brønsted Acidity. J. Catal. 2015, 331, 25-38. (2) Xue, J.; Wang. X.; Qi, G.; Wang, J.; Shen, M.; Li, W. Characterization of Copper Species over Cu/SAPO-34 in Selective Catalytic Reduction of NOx with Ammonia: Relationships between Active Cu Sites and De-NOx Performance at Low Temperature. J. Catal. 2013, 297, 56-64. (3) Fickel, D.W.; Addio, E.D.; Lauterbacha, J.A.; Lobo, R.F. The Ammonia Selective Catalytic Reduction Activity of Copper-exchanged Small-pore Zeolites. Appl. Catal. B. 2011, 102, 441-448. (4) Wang, D.; Zhang, L.; Kamasamudram, K.; Epling, W.S. In Situ-DRIFTS Study of Selective Catalytic Reduction of NOx by NH3 over Cu-Exchanged SAPO-34. ACS Catal. 2013, 3, 871-881. (5) Kumar, A.; Smith, M. A.; Kamasamudram, K.; Currier, N. W.; An, H., Yezerets, A. Impact of Different Forms of Feed Sulfur on Small-pore Cu-zeolite SCR Catalyst. Catal. Today. 2014, 231, 75-82.



(6) Shen, M.; Wen, H., Hao, T.; Yu, T.; Fan, D.; Wang, J.; Li, W.; Wang, J. Deactivation Mechanism of SO2 on Cu/SAPO-34 NH3-SCR Catalysts: Structure and Active Cu2+. Catal. Sci. Technol. 2015, 5, 1741-1749. (7) Wang, C.; Wang, J.; Wang, J.; Yu, T.; Shen, M.; Wang, W.; Li, W. The Effect of Sulfate Species on the Activity of NH3-SCR over Cu/SAPO-34. Appl. Catal. B. 2017, 204, 239-249. (8) Brookshear, D.W.; Nam, J.; Nguyen, K.; Toops, T.J.; Binder, A. Impact of Sulfation and Desulfation on NOx Reduction Using Cu-chabazite SCR Catalysts. Catal. Today. 2015, 258, 359-366. (9) Zhang, L.; Wang, D.; Liu, Y.; Kamasamudram, K.; Li, J.; Epling, W. SO2 Poisoning Impact on the NH3-SCR Reaction over a Commercial Cu-SAPO-34 SCR Catalyst. Appl. Catal. B. 2014, 156-157, 371-377. (10) Su, W.; Li, Z.; Zhang, Y.; Meng, C.; Li, J. Identification of Sulfate Species and Their Influence on SCR Performance of Cu/CHA Catalyst. Catal. Sci. Technol. 2017, 7, 15231528. (11) Wang, J.; Fan, D.; Yu, T.; Wang, J.; Hao, T.; Hu, X.; Shen, M. Improvement of Lowtemperature Hydrothermal Stability of Cu/SAPO-34 Catalysts by Cu2+ Species. J. Catal. 2015, 322, 84-90. (12) Buchholz, A.; Wang, W.; Xu, M.; Arnold, A.; Hunger, M. Thermal Stability and Dehydroxylation of Brønsted Acid Sites in Silicoaluminophosphates H-SAPO-11, H-SAPO18, H-SAPO-31, and H-SAPO-34 Investigated by Multi-nuclear Solid-state NMR Spectroscopy. Micro. Meso. Mater. 2002, 56, 267-278.


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(13) Wang, J.; Yu, T.; Wang, X.; Qi, G.; Xue, J.; Shen, M.; Li, W. The Influence of Silicon on the Catalytic Properties of Cu/SAPO-34 for NOx Reduction by Ammonia-SCR. Appl. Catal. B. 2012, 127, 137-147. (14) Wang, L.; Gaudet, J.R.; Li, W.; Weng, D. Migration of Cu Species in Cu/SAPO-34 During Hydrothermal Aging. J. Catal. 2013, 306, 68-77. (15) Richter, M.; Fait, M.J.G.; Eckelt, R.; Schneider, M.; Radnik, J.; Heidemann, D.; Fricke, R. Gas-phase Carbonylation of Methanol to Dimethyl Carbonate on Chloride-free Cuprecipitated Zeolite Y at Normal Pressure. J. Catal. 2007, 245, 11-24. (16) Torre-Abreu, C.; Henriques, C.; Ribeiro, F.R.; Dehalay, G.; Ribeiro, M.F. Selective Catalytic Reduction of NO on Copper-exchanged Zeolites: the Role of the Structure of the Zeolite in the Nature of Copper-active Sites. Catal. Today. 1999, 54, 407-418. (17) Herman, R.G.; Lunsford, J.H.; Beyer, H.; Jacobs, P.A.; Uytterhoeven, J.B. Redox Behavior of Transition-metal Ions in Zeolites. 1. Study of Reversibility of Hydrogen Reduction of Copper of Copper Y Zeolites. J. Phys. Chem. 1975, 79, 2388-2394. (18) Beyer, H.; Jacobs, P.A.; Uytterhoeven, J.B. Redox Behaviour of Transition Metal Ions in Zeolites. Part 2.—Kinetic Study of the Reduction and Reoxidation of silver-Y Zeolites. J. Chem. Soc. Faraday Trans. 1976, 72, 674-685. (19) Berthomieu, D.; Delahay, G. Recent Advances in CuI/IIY: Experiments and Modeling. Catal. Rev. 2006, 48, 269-313. (20) Kartheuser, B.; Hodnett, B.K.; Riva, A.; Centi, G.; Matralis, H.; Ruwet, M.; Grange, P.; Passarini, N. Temperature-Programmed Reduction and X-ray Photoelectron Spectroscopy of Copper Oxide on Alumina following Exposure to Sulfur Dioxide and Oxygen. Ind. Eng. Chem. Res. 1991, 30, 2105-2113.



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Figure/Table Captions Table 1 Sulfation conditions and BET surface areas Figure 1. XRD patterns (a) and relative crystallinity (b) of fresh, sulfated and regenerated Cu/SAPO-34 catalysts. Figure 2. The ex-situ DRIFTs results (a) and the change of Brønsted and Lewis acid sites peak intensity over catalysts based on K-M function (b). Figure 3. NH3-TPD results (a) and acidity (b) of fresh, sulfated and regenerated catalysts. Figure 4. TGA data of fresh, sulfated and regenerated catalysts. Figure 5. H2-TPR profiles of fresh, sulfated and regenerated samples (a); the peak area of different species estimated based on the TPR spectra (b). Figure 6. EPR spectra of fresh, sulfated and regenerated catalysts (a); the amount of isolated Cu2+ ions in fresh and sulfated catalysts estimated based on the EPR spectra (b). Figure 7. NOx conversion as a function of reaction temperature over catalysts (a), N2O formation during NH3-SCR reaction on the catalysts (b). The reaction was carried out with a feed containing 500ppm NO, 500ppm NH3, 5% O2, 7% CO2, 3% H2O, balance N2 and with GHSV=72000 h-1. Figure 8. Arrhenius plots of the SCR reaction rates over fresh, sulfated and regenerated samples. Figure 9. TOFs results over fresh, sulfated and regenerated catalysts. Figure 10. The relationship between the increased isolated Cu2+ ions and the reduced amount of sulfur on catalysts. Scheme 1 The mechanism of regeneration during thermal treatment.




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Table 1 Sulfation conditions and BET surface areas Samples’ Names

Sulfation/Regeneration Conditions

BET surface areas (m2/g)

F-Cu

—

640

S-N-Cu

250 oC 50 ppm SO2+ 500 ppm NH3+ 500

510

ppm NOx + 5 % H2O, in air R-500-0.5

500 oC 10% H2O in air for 0.5 h

576

R-600-0.5

600 oC 10% H2O in air for 0.5 h

578

R-600-4

600 oC 10% H2O in air for 4 h

592

R-650-4


609

R-750-4


611

R-750-8


614

R-750-12


625

Total flow rate: 1000 ml/min.



Figure 1. XRD patterns (a) and relative crystallinity (b) of fresh, sulfated and regenerated Cu/SAPO-34 catalysts.


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Figure 2. The ex-situ DRIFTs results (a) and the change of Brønsted and Lewis acid sites peak intensity over catalysts based on K-M function (b).



Figure 3. NH3-TPD results (a) and acidity (b) of fresh, sulfated and regenerated catalysts.


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Figure 4. TGA data of fresh, sulfated and regenerated catalysts.



Figure 5. H2-TPR profiles of fresh, sulfated and regenerated samples (a); the peak area of different species estimated based on the TPR spectra (b).


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Figure 6. EPR spectra of fresh, sulfated and regenerated catalysts (a); the amount of isolated Cu2+ ions in fresh and sulfated catalysts estimated based on the EPR spectra (b).



Figure 7. NOx conversion as a function of reaction temperature over catalysts (a), N2O formation during NH3-SCR reaction on the catalysts (b). The reaction was carried out with a feed containing 500ppm NO, 500ppm NH3, 5% O2, 7% CO2, 3% H2O, balance N2 and with GHSV=72,000 h-1.


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Figure 8. Arrhenius plots of the SCR reaction rates over fresh, sulfated and regenerated samples.



Figure 9. TOFs results over fresh, sulfated and regenerated catalysts.


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Figure 10. The relationship between the increased isolated Cu2+ ions and the reduced amount of sulfur on catalysts.



Scheme 1 The mechanism of regeneration during thermal treatment.


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The mechanism of regeneration 139x108mm (300 x 300 DPI)


SAPO-34

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