Preparation of Metal-Incorporated SAPO-34 catalysts and their Catalytic Performance in Selective Catalytic Reduction of Nitric Oxide

Metal-incorporated SAPO-34 catalysts were prepared by one-step hydrothermal method. Effects of various parameters including the types of metal ions, Cu2+ sources, structure directing agents (SDAs), hydrothermal temperature and time on catalyst activity of the metal-incorporated SAPO-34 catalysts were investigated. Three types of metal-incorporated SAPO-34 catalysts (Cu/SAPO-34, Fe/SAPO-34 and Mn/SAPO-34) were successfully obtained. Compared with Fe/SAPO-34 and Mn/SAPO-34 catalysts, Cu/SAPO-34 catalyst revealed complete cubic-like microstructure, wide active temperature window and high conversion rate in selective catalytic reduction (SCR) of nitric oxide. After defined Cu2+ was the favorable active site of SAPO-34 catalyst, the effects of four Cu2+ sources on SCR performance were further investigated, and found Cu2+ sources did not produce significant influence on nitric oxide conversion rates. SDAs determined the formation of Cu/SAPO-34 catalyst, and the Cu/SAPO-34 catalyst adopting tetraethylenepentamine (TEPA) as SDA could maintain higher crystal integrity and active sites, which were in favor of the SCR reaction. Moreover, hydrothermal temperature and time had great influences on the formation of Cu/SAPO-34 catalyst. When the hydrothermal temperature was higher than 150 oC and the hydrothermal time was longer than 3 days, the Cu/SAPO-34 catalyst with a cubic-like structure and high catalyst activity could be obtained.


Introduction
Nitrogen oxides (NO x ), as one of the major atmospheric pollutants, are mainly generated in the combustion of fossil fuels and biofuels, which have given rise to serious environmental problems including greenhouse effect, ozone depletion, acid rain 1-5 . At present, selective catalytic reduction (SCR) has been applied as one of the most effective methods on NO x removal in flue gas [6][7][8] . Compared with noble metal or oxides catalysts, SAPO-34 catalysts demonstrate high NO x conversion efficient and arise widespread concern. However, SAPO-34 catalysts are extremely sensitive to humid environment, which easily causes serious structure deterioration because of the irreversible hydrolysis of bridge hydroxyl group [9][10][11] .
Recently, great efforts have been carried out to improve the SCR activity by doping transition metals into SAPO-34 catalysts. According to previous reports, Fe-incorporated catalyst exhibited enhanced NO x conversion at high temperature [12][13][14][15] . Mn-incorporated catalyst was also developed to improve low-temperature SCR activity [16][17][18] . Besides, introducting Cu 2+ into SAPO-34 catalysts could increase theirs low-temperature hydrothermal stability owing to the enhanced stabilization of negative Si-O-Al connections 19 . Among the metal-incorporated catalysts, Cu-incorporated catalysts demonstrated excellent potential owing to their low price, high catalytic activity, wide temperature range 20,21 .
Currently, Cu/SAPO-34 catalysts could be fabricated by three methods: impregnation, ion-exchange, and one-pot methods. Generally, the distribution of copper species is closely related to the synthesis method 22,23 . Specially, ionexchange and one-pot methods were more favorable due to more active sites generation and less structure damages 13,24,25 . Wang et al. 26 adopted ion-exchange method to fabricate Cu/ SAPO-34 catalysts, which demonstrated Cu species existing as isolated ions were active sites for the SCR reaction. Fickel et al. 20 also utilized ion-exchange method to fabricate Cu/SAPO-34 catalysts. Their work demonstrated Cu/SAPO-34 catalysts with high SCR activity could be achieved even after steaming. Meanwhile, Liu et al. 27 reported their work on Cu/ SAPO-34 catalysts obtained by ion-exchange method. They believed that the migration of surface CuO clusters into the ion-exchanged sites as isolated Cu 2+ improved the activity at high temperatures, which suppressed the competitive oxidation of NH 3 . Cortés-Reyes et al. 28 synthesized Cu/SAPO-34 catalysts in a one-pot process, and found Cu 2+ existed in the interior of the cavities. Martínez-Franco et al. 29 obtained Cu/SAPO-34 catalysts by adopting the co-directing SDA, and concluded that it was easy to control the Cu 2+ loading in the Cu/SAPO-34 catalysts. Tang et al. 30 investigated the effect of Cu 2+ loading on the SCR activity of Cu/SAPO-34 synthesized by one-pot method.
Generally, ion-exchange method contains several complex steps, which needs long production period and *e-mail: chengwensong@dlmu.edu.cn produces a large amount of wastewater. In contrast, the one-pot method requires a simple hydrothermal synthesis, which is more economical and efficient preparation technology for Cu/SAPO-34. Inspired from previous researches, here, we fabricated metal-incorporated SAPO-34 catalysts by one-step hydrothermal synthesis, investigated the effects of various parameters including the types of metal ions, copper ion sources, SDAs, hydrothermal temperature and time on catalyst activity of metal-incorporated SAPO-34 catalysts, characterized the morphology, structure, and chemical composition of metal-incorporated SAPO-34 catalysts by SEM, XRD, XPS and N 2 adsorption techniques.

Characterization
The crystal structures of catalysts were analyzed by X-ray diffraction (XRD) via a computer controlled diffractometer (Rigaku D/max-2400) with Cu Kα radiation (λ = 1.54178 Å) at the tube voltage of 40 kV and the current of 100 mA. The pore structure properties of catalysts were investigated by N 2 adsorption-desorption measurements (Quantachrome autosorb-iQ 2 Analyzer). The catalysts were degassed at 180 o C for 10 h under vacuum before measurements. The morphology of catalysts were examined by scanning electron microscope (SEM) (Philips XL-30FEG EDX). The elemental composition of catalysts was carried out by X-ray photoelectron spectroscopy (XPS) (Thermo SCIENTIFIC ESCALAB 250 spectrometer). The SCR activity of catalysts were measured in a fixed-bed quartz reactor. The schematic diagram of catalytic activity measurement was shown in Figure 2, and the detailed test process could refer to our previous paper 31 .

SAPO-34 catalyst demonstrated similar characteristics with
Cu/SAPO-34 catalyst, but indicated slightly weak peak intensities 33,34 . Subsequently, the degree of crystallinity was calculated by dividing the area of crystalline peaks by the total area under the diffraction curve (crystalline plus amorphous peaks). The degrees of crystallinity for these catalysts were 85.4% for SAPO-34, 80.94% for Fe/SAPO-34, 88.32% for Cu/SAPO-34 respectively, which further supported the above discussion. However, as for Mn/SAPO-34 catalyst, the XRD pattern indicated a predominance of amorphous material. The calculated crystallinity of Mn/SAPO-34 was only 32.58%, which was much lower than that of SAPO-34 (85.4%), implying the destruction of the catalyst structure after the introduction of Mn 2+ in SAPO-34.
The SEM images shown in Figure 4 presented the morphology of these catalysts. All of the three metalincorporated SAPO-34 samples were cubic-like catalysts. Cu/SAPO-34 catalyst showed relatively intact microstructure with cubic-like morphology, which were similar with SAPO-34. However, for Fe/SAPO-34 and Mn/SAPO-34 catalysts, they all demonstrated larger size than SAPO-34. We also observed some broken crystal fragments appeared on the surface of Fe/SAPO-34 catalyst and even more broken cubic-like structure existed in Mn/SAPO-34 catalyst, which were responsible for the decrease of crystallinity, agreeing with the XRD analysis. Figure 5 provided the nitric oxide conversion rate of the three catalysts. It displayed that when using Cu and Fe species as active centers, the resultant catalyst indicated high catalytic activity for nitric oxide conversion than SAPO-34 and Mn/SAPO-34 catalyst. The active temperature window of Cu/SAPO-34 catalyst was wider (150-550 °C), while the Fe/SAPO-34 catalyst was slightly narrower (200-450 °C), and the conversion rate of the Cu/SAPO-34 catalyst remained above 80% during 450-600 °C. Based on the above analysis, it was concluded that loading copper ions could greatly improve the crystallinity and active temperature windows compared with other metal ions. Therefore, copper ions were selected as the catalytic active center of SAPO-34.

Effects of copper ion sources
After defined copper ions were the favorable active site of SAPO-34 catalyst, we further investigated the relationship between different Cu 2+ sources (Cu(COOH) 2 , CuSO 4 , CuCl 2 , Cu(NO 3 ) 2 , and) and SCR performance. It was obvious that the characteristic peaks were observed in all of the four catalysts synthesized at 150 o C for 7 days (Figure 6), which agreed with those reported in the literatures 30,35 , and indicated Cu/SAPO-34 catalysts with the four types of Cu 2+ sources were all successfully obtained. There were no significant different on diffraction peaks derived from different copper ion sources, which implied that the crystalline structure of the catalysts were almost not affected by the copper ion sources. The degrees of crystallinity for these four catalysts were also very close, which were 88.32%, 87.94%, 86.87%, 87.49% for Cu(COOH) 2 , CuSO 4 , CuCl 2 , Cu(NO 3 ) 2 as Cu 2+ sources, respectively. SEM images further confirmed the result from XRD analysis. Although different copper ion sources were adopted, the morphologies of these resultant catalysts were similar, and they all showed cubic-like microstructures (Figure 7).
The XPS wide scan spectrum of the Cu/SAPO-34 catalyst was presented in Figure 8a. Al, Si, Cu, P, C, N and O were detected in, displaying the obtained catalyst was SAPO-34 structure containing Cu 2+ . Figure 8b presented the high resolution spectrum of Cu 2p, which displayed two shake-up satellite peaks at 935.6 and 955.3 eV, corresponding to Cu 2p 3/2 and Cu 2p 1/2 , respectively. Generally, these two peaks were used as a characteristic to determine copper ions, implying that copper ions had been successfully loaded in SAPO-34 catalyst and mainly existed as the Cu 2+ state 36,37 .
The result of nitrogen adsorption-desorption isotherms of Cu/SAPO-34 was showed in Figure 9. All catalysts demonstrated typical IV curves with the hysteresis loop. The BET surface area ranged from 232.49-250.29 m 2 /g, especially, the largest BET surface area was derived from Cu(CH 3 COO) 2 with 250.29 m 2 /g and performed strong adsorption and desorption capacity.   sources. It can observed that the effects of Cu 2+ sources on the nitric oxide conversion rates of the resultant Cu/SAPO-34 catalysts were not significant. Combined with the above analysis on their morphologies and nitrogen adsorptiondesorption characteristics, Cu(CH 3 COO) 2 was recommended as the candidate Cu 2+ source for the subsequent experiments.

Effects of SDAs
As we known, during the crystallization process, the SDAs play important roles on the physicochemical properties of the synthesized catalysts because of their structure-directing, charge-compensating and space-filling roles 34 . For investigating the effects of SDAs on the performance of catalysts, three SDAs (TEPA, TETA, and DETA) were investigated. XRD patterns of Cu/SAPO-34 catalysts synthesized by the three SDAs at 150 o C for 7 days were showed in Figure 11. Obviously, the Cu/SAPO-34 catalyst synthesized by DETA did not demonstrate the typical characteristic peaks of SAPO-34, which meant that DETA was easy to produce structure defects in the formation of Cu/SAPO-34. However, the Cu/SAPO-34 catalysts adopted TEPA and TETA as SDAs exhibited the chabazite phase of SAPO-34, and the former had a stronger peak intensities, which might be attributed to its good crystal structure. The degrees of crystallinity for the three catalysts were calculated as 88.32%, 83.65%, and 77.16% for TEPA, TETA, and DETA as SDAs, respectively, which agreed with the above analysis. Figure 12 showed the SEM images of Cu/SAPO-34 catalysts synthesized by the three SDAs. Obviously, the integrity of cubic-like structure of these catalysts was in the order: TEPA>TETA>DETA, and the Cu/SAPO-34 catalyst obtained by using TEPA maintained higher crystal integrity, thus might possess more microspores and active  sites, which produced higher SCR catalytic activity 38 . However, as for the Cu/SAPO-34 catalyst prepared from DETA, the incomplete cubic-like structure was observed, which might suppress the diffusion of reactants, declining its catalytic activity.
N 2 adsorption-desorption isotherms were presented in Figure 13. They all demonstrated typical type IV curves, and the corresponding BET surface area of DETA, TETA and TEPA were 40.18 m 2 /g, 224.89 m 2 /g and 250.29 m 2 /g, respectively. The result indicated TEPA had the largest surface areas with strong adsorption and desorption ability for SCR reaction. Figure 14 presented the nitric oxide conversion rates of the three Cu/SAPO-34 catalysts. When using DETA as SDA, the catalytic activity was extremely unstable throughout the temperature range, and the conversion rate of nitric oxide was significantly low than other two catalysts due to its broken crystal structure. The nitric oxide conversion rates of the Cu/SAPO-34 catalysts obtained by TEPA and TETA as SDAs were rather close with active temperature window range from 150-450 o C. The difference between them was reflected in their catalytic activity above 450 o C, and the reduction of catalytic activity of the former was less than that of the latter.
Sum up, the microstructure and morphology of SAPO-34 varied with the use of different SDAs because SDAs could affect Si coordination structure in SAPO-34 39 , which further influenced Cu species distribution in the resultant Cu/ SAPO-34 catalyst. Accordingly, the Cu/SAPO-34 catalyst derived from TEPA possessed the largest surface area, demonstrating highest NOx conversion, which agreed with previous results on large specific surface area favored high catalytic activity 40 . Therefore, it was the optimal choice to adopt TEPA as SDA.

Effects of hydrothermal temperature
For studying the influence of hydrothermal temperature on the structure, morphology, and nitric oxide conversion rate of Cu/SAPO-34 catalysts synthesized at 7 days, XRD   analysis was firstly carried out. The crystallinity of Cu/ SAPO-34 was improved as the hydrothermal temperature increased from 100 to 180 o C, and slightly declined at 200 o C (Figure 15). The degrees of crystallinity for these catalysts demonstrated similar trend, which were 30.14%, 39.47%, 88.32%, 88.54%, and 87.62% for the hydrothermal temperature of 100 o C, 120 o C, 150 o C, 180 o C, and 200 o C, respectively. Especially, for the Cu/SAPO-34 samples obtained at 100 and 120 o C, all diffraction peaks did not appear and an amorphous phase was found, which indicated the low hydrothermal temperature could not meet the requirement to synthesize Cu/SAPO-34.
As shown in Figure 16, at low hydrothermal temperature, the synthetic material was irregular fragment crystals, which were basically amorphous substances. The result meant that when the hydrothermal temperature was too low, it was hard to form cubic-like Cu/SAPO-34 catalyst. When increasing the hydrothermal temperature to above 150 o C, the cubic-like Cu/SAPO-34 catalyst started to be formed. The surface of the catalyst was smoother and there was almost no debris crystal attached. Further improved to 200 o C, the crystal size of the catalyst became slight bigger than those catalysts synthesized at other hydrothermal temperatures, which might be attributed to that the crystallization rate of the catalysts were fast at high hydrothermal temperature 41 . Figure 17 showed nitric oxide conversion rate of Cu/SAPO-34 catalysts obtained at various hydrothermal temperatures. Obviously, when the temperature reached 150 °C, the active temperature window was 150-550 °C, and the removal rate of nitric oxide almost reached 100% at 150-450 °C. Therefore, the hydrothermal temperature of 150 °C was recommended for nitric oxide removal in this work.

5 Effects of hydrothermal time
The influences of hydrothermal time on the structure, morphology, and removal efficiency of nitric oxide of Cu/SAPO-34 catalyst synthesized at 150 o C were further   evaluated. Obviously, short hydrothermal time (1 day) led to the produce of Cu/SAPO-34 catalyst with poor crystallinity (Figure 18), which was calculated as 79.06%. Improving the hydrothermal time to 3 days, the stronger peak intensities were indicated, suggesting the formation Cu/SAPO-34 catalyst with good crystallinity (85.77%). Extending the hydrothermal time to 7 days, the peak intensities were further increased, the degrees of crystallinity was improved to 88.32%. The SEM images provided by Figure 19 also approved the conclusion from XRD analysis. At 1 day of hydrothermal time, incomplete cubic-like structure was observed. When the hydrothermal reaction exceeded 3 days, the synthesized Cu/SAPO-34 catalysts revealed relatively complete cubic-like morphology. Figure 20 presented the nitric oxide conversion rates of Cu/SAPO-34 catalysts synthesized at the hydrothermal time of 1, 3, and 7 days, respectively. Obviously, the nitric oxide conversion rates of the three Cu/SAPO-34 catalysts were improved dramatically with the increase of reaction temperature below 150 °C, and then stayed stable. As the reaction temperature exceeded 350 °C, the nitric oxide conversion rate began to decline for the Cu/SAPO-34 catalyst synthesized at short hydrothermal time (1 day). However, for the two Cu/SAPO-34 catalysts synthesized at 3 and 7 days, almost the same trends on nitric oxide conversion rates were observed before 500 o C, however, slight high nitric oxide conversion rates were observed due to its good crystallinity at long hydrothermal time.
Four kinds of reported Cu/SAPO-34 catalysts (ionexchange method and one-pot method) were used for comparison with our catalysts. Wang et al. 26 reported the ion-exchanged Cu/SAPO-34 catalyst, which demonstrated low catalyst activity (about 20% -70% of nitric oxide conversion rate) below 350 o C, and even at the high reaction temperature range of 350-550 o C, the nitric oxide conversion rates only remained at about 70-80%. Liu et al. 27 presented their work on ion-exchanged Cu/ SAPO-34 catalyst, and the catalyst indicated high catalyst activity (above 80% of nitric oxide conversion rate) at reaction temperature range of 200-350 o C. As for "one-pot" synthesized Cu/SAPAO-34 catalysts, they usually demonstrated higher catalyst activity than those obtained by ion-exchange method. For example, Martínez-Franco et al. 29 fabricated high activity Cu/ SAPAO-34 catalyst via one-pot method. The resultant catalyst showed excellent catalyst activity (above 80% of nitric oxide conversion rate) at the whole reaction temperature range (200-500 o C), and especially at 250-400 o C, nitric oxide conversion rate reached 100%. Tang et al. 30 got high activity Cu/SAPAO-34 catalyst via one-pot method, and nitric oxide conversion rates were above 80% at reaction temperature of 200-600 °C. In this work, we also obtained the Cu/SAPAO-34 catalyst by one-pot hydrothermal method, its catalyst activity was close to that obtained by Martínez-Franco et al. 29 by one-pot method, and higher than those fabricated by Tang et al. 30 via one-pot method, and Wang et al. 26 and Liu et al. 27 via ion-exchange method.

Conclusion
In summary, we have successfully fabricated various metal-incorporated SAPO-34 catalysts via one-step hydrothermal synthesis. Among them, Cu/SAPO-34 catalysts demonstrated complete microstructure with cubic morphology, wide temperature window, and high nitric oxide conversion rate. Subsequently, A series of Cu/SAPO-34 catalysts were  synthesized using four kinds of copper salts (CuSO 4 , Cu(NO 3 ) 2 , CuCl 2 , and Cu(COOH) 2 ) as Cu 2+ sources, and found that Cu 2+ sources did not produce significant influence on the nitric oxide conversion rates of the resultant Cu/SAPO-34 catalyst. SDAs had an important impact on properties of the synthesized Cu/SAPO-34 catalysts, and the Cu/SAPO-34 catalyst adopting TEPA as SDA could maintain higher crystal integrity and active sites, which were benefit to the SCR reaction. Furthermore, hydrothermal temperature and time had significant influences on the formation of Cu/SAPO-34 catalyst. When the hydrothermal temperature was higher than 150 o C and the hydrothermal time was longer than 3 days, the Cu/SAPO-34 catalyst with a cubic-like structure and high catalyst activity could be obtained.